Hydrogen and Dihydrogen Bonds in the Reactions of Metal Hydrides

Review pubs.acs.org/CR

Hydrogen and Dihydrogen Bonds in the Reactions of Metal Hydrides Natalia V. Belkova, Lina M. Epstein, Oleg A. Filippov, and Elena S. Shubina* A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov Street 28, 119991 Moscow, Russia ABSTRACT: The dihydrogen bondan interaction between a transition-metal or main-group hydride (MH) and a protic hydrogen moiety (HX)is arguably the most intriguing type of hydrogen bond. It was discovered in the mid-1990s and has been intensively explored since then. Herein, we collate up-to-date experimental and computational studies of the structural, energetic, and spectroscopic parameters and natures of dihydrogen-bonded complexes of the form MH···HX, as such species are now known for a wide variety of hydrido compounds. Being a weak interaction, dihydrogen bonding entails the lengthening of the participating bonds as well as their polarization (repolarization) as a result of electron density redistribution. Thus, the formation of a dihydrogen bond allows for the activation of both the MH and XH bonds in one step, facilitating proton transfer and preparing these bonds for further transformations. The implications of dihydrogen bonding in different stoichiometric and catalytic reactions, such as hydrogen exchange, alcoholysis and aminolysis, hydrogen evolution, hydrogenation, and dehydrogenation, are discussed.

CONTENTS 1. Introduction 2. Spectral Criteria, Structural Parameters, and Energetics of Dihydrogen Bonds 2.1. Spectral Criteria of Dihydrogen Bonding 2.1.1. NMR Spectroscopy 2.1.2. IR Spectroscopy 2.1.3. Intermolecular H···H Stretching Vibration 2.2. Crystallographic Structural Data 2.3. Theoretical Insight into Dihydrogen Bonding 2.3.1. Energy Decomposition Analysis and Electronic Changes 2.3.2. Multifurcated Dihydrogen Bonds 2.3.3. Role of Transition-Metal Atom in Dihydrogen Bond 2.3.4. Isotope Effects in Dihydrogen Bonds 2.4. Dihydrogen Bond Thermodynamics and Hydride Ligand Basicity 2.5. Effects of Metal Atom and Ligands on Hydride Basicity 2.6. Structural Reorganization and Repolarization Caused by Hydrogen Bonding 3. Dihydrogen-Bonded Complexes as Intermediates of Proton Transfer to Hydrides 3.1. General Considerations of the Reaction Mechanism 3.2. Transition-Metal Hydrides: Proton-Transfer Dichotomy {[M(η2-H2)]+ vs [M(H)2]+}; Ion Pairing 3.3. Transition-Metal Hydrides: Proton Transfer and H2 Evolution

© 2016 American Chemical Society

3.4. Hydrogen Bonding and Protonation of Hydrides with Pendant Nitrogen Centers 3.5. Main-Group Hydrides: Concerted Proton Transfer and H2 Evolution 3.6. Dihydrogen Bonding and H2 Activation by Frustrated Lewis Pairs 3.7. Dihydrogen Bonding and Proton Transfer to Transition-Metal Tetrahydroborates 3.8. Kinetics of Dihydrogen-Bond-Mediated Proton Transfer 3.9. Proton/Hydride and Isotope (H/D) Exchange 4. MH···Y Hydrogen Bonds and Proton Transfer from Transition-Metal Hydrides 4.1. Transition-Metal Hydrides as Proton Donors in Hydrogen Bonds 4.2. Dihydrogen Bonding between Two Hydride Complexes MH···HM′ and Proton Transfer 4.3. Reactions through MH···Y Bonds 5. Solvent Effects 6. Implications of Dihydrogen Bonding and Proton Transfer in Different Processes 6.1. Stoichiometric Reactions 6.2. Catalytic Processes 7. Conclusions Author Information Corresponding Author Notes Biographies Acknowledgments Abbreviations

8546 8546 8546 8546 8547 8547 8547 8549 8550 8550 8551 8552 8553 8554 8555 8555 8555

8558 8560 8563 8564 8565 8567 8568 8568 8570 8571 8572 8573 8573 8574 8576 8576 8576 8576 8576 8577 8577

8556 Special Issue: Metal Hydrides

8557

Received: February 5, 2016 Published: June 10, 2016 8545

DOI: 10.1021/acs.chemrev.6b00091 Chem. Rev. 2016, 116, 8545−8587

Chemical Reviews References

Review

by 2−4 ppm.16,17,27 Because free and hydrogen-bonded molecules are in rapid exchange on the NMR time scale in the temperature range available for common organic solvents, all NMR parameters depend on the position of the hydrogenbonding equilibrium. Such a spectral picture is indicative of the formation of intra- or intermolecular hydrogen bond and has been observed for both organic and organometallic/inorganic bases.27 Resonances of other nuclei, such as 31P for phosphorus ligands,13,28−30 are also sensitive to hydrogen bond formation. In some cases, such resonances confirm the involvement of a corresponding ligand, such such as carbonyl (13C)31 or Nheterocycles (15N),32,33 in hydrogen bonding. The involvement of metal hydrides in hydrogen bonding, that is, the formation of dihydrogen bonds of the type MH··· HX, can be confirmed by the changes in the parameters of the hydride resonance. A high-field shift of the hydride atom signal by 0.1−0.8 ppm and a 1.5−3-fold decrease of its longitudinal relaxation time (T1min) both serve as evidence for dihydrogen bond formation.13,23,34,35 Measurements of T1 relaxation times indicated the existence of intramolecular N−H···HM hydrogen bonding in the complexes [(η5C5H4(CH2)nNMe2H+)RuH(L2)]BF4 (n = 2, 3) for L2 = dppm36 and (PPh3)237 but not for L2 = (P(OPh)3)2.38 The T1min changes for intermolecular dihydrogen bonds have been analyzed using WH(CO)2(NO)(PR3)2,13 ReH2(CO)(NO)(PR3)2,34,39 and (CP3)RuH(CO)240 as the hydride species. Because of the above-mentioned rapid exchange of free and hydrogen-bonded species, the chemical shift and T1min value of a hydride resonance in 1H NMR spectra can be considered as weighted averages between those of free and dihydrogenbonded hydrides. Thus, the shift (ΔδMH) and the decrease of T1min of the hydride resonances depend on the factors affecting the position of the MH + HX ↔ MH···HX equilibrium and the value of its constant KHB: the proton-donor amount and strength, the temperature, and the solvent.13,30,34,40−45 The use of the Freon mixture CDF3/CDF2Cl as a solvent made it possible to reach a slow exchange regime for the system Cp*ReH(CO)(NO)/R F OH at 96 K and to observe decoalescence of the averaged hydride resonance into two ReH signals of free and DHB hydrides.46 An important advantage of 1H NMR spectroscopy is the possibility for estimating H···H distances from spin−lattice T1 relaxation measurements.23 Short HM···HX contacts cause strong homonuclear dipolar coupling that provides an additional contribution to nuclear dipole−dipole relaxation (eq 1)

8577

1. INTRODUCTION The dihydrogen bondan interaction between a transitionmetal or main-group hydride (MH) and a protic hydrogen moiety (HX)was suggested almost 50 years ago.1,2 Studies of the spectra and reactivities of boron hydrides L·BH3 (L = Py, Me2NH Me3N, Et3N, Et3P) and Me3NBH2X (X = Cl, Br, I) in the 1960−1970s1−6 suggested that BH3 and BH2 groups can act as proton acceptors in hydrogen bonds despite the lack of lone pairs or π electrons. However, this novel idea was not appreciated at the time and remained unfairly forgotten. Much later, looking for bifunctional hydrides as potential hydrogenation catalysts, Milstein and co-workers reported the structure of IrH(OH)(PMe3)4.7 However, the H···H distance of 2.40(1) Å determined by neutron diffraction was considered “too long” for a normal hydrogen bond. Just a few years later, in the mid-1990s, the existence of intra- and intermolecular proton−hydride hydrogen bonds was confirmed by different methods.8−14 This intriguing type of hydrogen bond became widely recognized in a short time and has now been extensively explored. The structural, energetic, and spectroscopic parameters of dihydrogen-bonded complexes MH···HX have been studied both experimentally and computationally for a wide variety of hydrides ranging from very simple species such as LiH or BeH2 to group 13 hydrides and complex transitionmetal hydrido compounds. Developments in the area have been periodically reviewed.15−23 Dihydrogen bonds play a role in, among other aspects of chemistry, crystal packing, potential hydrogen-storage materials, and organometallic reaction mechanisms. The latter is a subject of our own research, and herein, we present a modern view of various structural, energetic, and spectroscopic aspects of these unconventional hydrogen bonds revealed both experimentally and computationally for a wide variety of transition-metal and main-group hydrides. Particular attention is paid to their roles in the reactions of proton transfer and H2 evolution, as well as to the implications of these steps in catalytic reactions such as hydrogenation and dehydrogenation. 2. SPECTRAL CRITERIA, STRUCTURAL PARAMETERS, AND ENERGETICS OF DIHYDROGEN BONDS “The hydrogen bond is an attractive interaction between a hydrogen atom from a molecule or a molecular fragment X−H, in which X is more electronegative than H, and an atom or a group of atoms in the same or a different molecule in which there is evidence of bond formation”.24 Certain structural, electronic, spectroscopic, and energetic criteria and characteristics have been deduced for both conventional hydrogen bonds and dihydrogen bonds. They can be obtained experimentally using various physical methods or computationally. Ideally, the two types of approaches should be combined.

rH ··· H (Å) = 5.815(T1min /ν)1/6

(1)

Originally proposed for η -H2 complexes, this method has been used by many researchers to determine both intra-8,9,11,12,48 and intermolecular13,42,49−53 H···H distances. An enhancement of H−H exchange coupling, JHa−Hb, between adjacent hydride sites in polyhydrides54 was suggested35,41 as another sensor for the strength of dihydrogen bonding, but it did not find wide application.28 As originally shown in the example of the ruthenium trihydride complex Cp*RuH3(PCy3) interacting with various proton donors,41 the JHa−Hb constant increases progressively with proton-donor concentration and strength proportionally to the average hydride chemical shifts δHa and δHb. Evidence for the occurrence of dihydrogen bonding can be obtained by 1D nuclear Overhauser effect (NOE) spectroscopy13 or currently by 2D 1H nuclear Overhauser enhancement spectroscopy (NOESY). For example, the intermolecular 2

2.1. Spectral Criteria of Dihydrogen Bonding

Spectroscopic [variable-temperature infrared (VT-IR) and nuclear magnetic resonance (VT-NMR)] studies demonstrate the existence of dihydrogen bonding in the solid state and in solution. The spectral criteria of dihydrogen bond (DHB) formation are readily determined for hydrides of main-group elements and transition metals.17,25,26 2.1.1. NMR Spectroscopy. In NMR spectra, hydrogen bonding shifts the 1H resonance of HX moieties to lower field 8546

47


Chemical Reviews

Review

proton−hydride interaction of [HRe2(CO)9]− and TFE was confirmed by a 2D NOESY experiment at 221 K, which showed a cross-peak between the hydride signal (at −7.90 ppm) and the OH resonance (at 2.95 ppm).42 2.1.2. IR Spectroscopy. IR spectroscopy, because of its short time scale, allows for the detection of separate absorptions for free and hydrogen-bonded species. The formation of a dihydrogen bond of the type MH···HX, as for any hydrogen bond of weak to medium strength, entails the appearance of a new, much wider and more intense band, νXHbonded, of the proton donor with a band shift, ΔνXH = νXHbonded − νXHfree, of up to −450 cm−1. These changes are related to the elongation of the proton-donating HX bond. In some cases, a shift of the νXH stretching vibration to higher frequencies (the so-called blue shift) is observed as a result of HX bond contraction.55 Such a blue shift was predicted for dihydrogen bonds formed by F3CH and F3SiH with H Be−X, HMg−X, and H4Si (X = H, F, Cl, CH3)56 and for NH···HB dihydrogen bonds in the complex of BH3NH3 with HNO.57 However, as for conventional hydrogen bonds XH···Y, examples of blue-shifted dihydrogen bonds are rare; for the majority of dihydrogen bonds, the νXH bands shift to lower frequencies. Evidence that the hydride ligand is a hydrogen-bonding site is provided by the appearance of a new low-frequency νMH or νEH band (or shoulder), νbonded MH . In addition, the stretching vibration bands of other ligands (e.g., CO, NO), νL, appear at higher frequencies as the hydride donicity is attenuated by its participation in dihydrogen bonding.13,39,40,46,49−51,58 In a given solvent, the band shifts, Δν = νbonded − νfree, induced by hydrogen bond formation depend on the interaction strength, increasing with the acidity of HX.13,59,60 When the bandwidths of the free and hydrogen-bonded species are rather high, they can overlap. In this case, the maximum position of the resulting envelope depends on the equilibrium shift (as in NMR spectra), as was observed, for example, for (CP3)Re(CO)2H··· HX61 and (CP3)Ru(CO)H2···HX.40 Otherwise, the protondonor concentration and temperature affect only the intensity ratio of the νfree and νbonded bands, allowing for the estimation of the hydrogen-bond-formation constants KHB.13,50,60 The lower-frequency shift of νMH bands observed experimentally as evidence of dihydrogen bond formation has been confirmed by numerous density functional theory (DFT) calculations, which explained this effect by the elongation of MH bond. Theoretical calculations have also shown that spectral criteria developed during the studies of monohydrides can be applied to polyhydrides as well.62−64 2.1.3. Intermolecular H···H Stretching Vibration. Observation of the H···H stretching vibration (νHH) in IR spectra is direct experimental evidence of dihydrogen bond formation. Stretching vibrations of hydrogen bonds XH···Y, νHY, are located below 600 cm−1. Analysis of this region is rarely an easy task, and for this reason, νHH vibrations are rarely analyzed even theoretically. The vibrational modes of dihydrogen bonding were analyzed for MoH(CO)2(NO)(PH3)2 interacting with HF.65 They have also been reported for several simpler DHB complexes, including BeH2···HCN,66 MH···HX (M = Na, Li; X = F, Cl, Br),67 MH2···HC (M = Zn, Cd),68 and AlH···HRgF (Rg = Ar, Kr).69 Detailed normal-coordinate analysis of IR spectra in the lowfrequency range was performed for dihydrogen-bonded complexes MH4−·HOR (M = B, Al, Ga; ROH = CH3OH, CF3CH2OH).70 The assignment of the intermolecular H···H

stretching mode (νσ) having a high characteristic potential energy distribution (PED) of 81−100% was made. Experimental measurements on the DHB complexes of BH4− with three alcohols [ROH = FCH 2 CH 2 OH, CF 3 CH 2 OH, (CF3)2CHOH] in CH2Cl2 solution revealed a broad νσ band, confirming the predicted assignment of νσ and the influence of the acid strength.70 They showed the correlation of the DHB enthalpy values, ΔHDHB, obtained from experiment, and with the calculated H···H distances (Table 1). Table 1. Spectral (ΔνOH, νσ), Thermodynamic (−ΔHexp), and Structural (rH···H) Parameters of DHB Complexes of Boron Tetrahydride with Different Proton Donorsa ROH

νσ in CH2Cl2 (cm−1)

ΔνOH (cm−1)

−ΔHexp in CH2Cl2 (kcal/mol)

rH···H (Å)

CFH2CH2OH CF3CH2OH (CF3)2CHOH

318 324 362

247 290 402

4.6 5.2 6.5

1.63 1.55 1.46

a

Reprinted with permission from ref 70. Copyright 2008 American Chemical Society.

Thus, spectral (NMR, IR) studies provide reliable evidence of DHB formation. Moreover, the resulting information provides an experimental basis for theoretical considerations (see below). The upfield shift of a hydride resonance in 1H NMR spectra suggests increased shielding of this nucleus resulting from the shift of the electron density and the more negative charge on the hydride ligand involved in the dihydrogen bond. The increase of the exchange coupling in polyhydrides also suggests partial charge transfer from the metal hydride to the hydrogen-bond donor.35 The lowfrequency shifts and increase in intensity of the νXH and νMH bands observed in IR spectra of dihydrogen-bonded complexes suggest elongation and additional polarization of the interacting bonds, MH and HX. 2.2. Crystallographic Structural Data

To date, there is only one neutron diffraction structure of a specifically chosen dihydrogen-bonded transition-metal hydride, namely, the ReH5(PPh3)3·indole complex (Figure 1).71 A Cambridge Structural Database (CSD)72 search for

Figure 1. Neutron diffraction crystal structure of ReH5(PPh3)3·indole complex. Data from ref 71. 8547


Chemical Reviews

Review

interactions (90−130°).74,75 The latter imply the presence of three-center−two-electron bonding between a filled σXH orbital and an empty orbital of the metal and are characterized by relatively short MH distances (1.8−2.3 Å).75 Among the 117 DHB structures analyzed, those with relatively short M···H distances ( HD > DH > DD.159,160 Isotope substitution also affects the geometry of hydrogenbonded complexes, which is referred to as a geometric hydrogen-bond isotope effect or the Ubbelohde effect.161 It should be noted that, historically, the X···Y distance is taken as

Figure 8. Optimized structures of PH 3 model of [(η 5 C5H4(CH2)2NHMe2)RuH(PPh3)2]+.

Ru moiety suggests that this is a primary interaction. Its importance is further supported by AIM analysis,156 which reveals the NH···HRu bond path to be highly bent toward the metal. The proposed mechanism for proton/hydride exchange in this complex does not involve a proton transfer within the dihydrogen bond, but rather involves rotation of the aminocyclopentadienyl ligand and protonation of the metal through another hydrogen-bonded intermediate with a very similar NH···Ru interaction (Figure 8, right). Finally, (C5H4OH)Re(H)(NO)(PiPr3) crystallizes as a hydrogen-bonded dimer with two intermolecular OH···Re bonds (Figure 9).76 The proton-exchange equilibria in this complex and its cyclohexyl congener proceed through trans8552


Chemical Reviews

Review

the indicator of the Ubbelohde effect in crystallography because of difficulties in determining the H-atom position, and the resulting phenomenon is called a secondary isotope effect. The distance between heteroatoms X···Y becomes shorter with increasing isotope mass; that is, in XHδ+···δ−HY, it was found to be longest in the Hδ+···δ−H complex and shortest in the Tδ+···δ−T isotopomer for the FH···HBH2, HCCH···HLi, and H3NH+···HBeH systems.162 The dihydrogen bond distance itself follows the opposite trend: The Hδ+···δ−H distance is shorter than the Dδ+···δ−D and Tδ+···δ−T distances.159,162,163 A more recent study of the above-mentioned NH4+/BeH2 dihydrogen-bonded system by a multicomponent molecular orbital method163 showed that substitution for hydridic hydrogen gives Hδ+···δ−D and Hδ+···δ−T distances that are shorter than the Hδ+···δ−H distance, whereas substitution for the proton makes the Dδ+···δ−H and Tδ+···δ−H distances longer than the Hδ+···δ−H distance. The authors attributed this phenomenon to the charge distribution: Heavier hydrogen isotopes are always more electron-rich than lighter hydrogen isotopes, and hydridic hydrogen becomes even more electron rich as a result of DHB formation. Therefore, the shortest and strongest DHB is Hδ+···δ−T, whereas the Tδ+···δ−H analogue is the longest and weakest. Finally, we note that the results obtained by two groups159,160,163 are the same for the homoisotopomers (HH, DD, TT) but give different orders of energy and distance for the heteroisotopomers.

Figure 10. Dependence of the dihydrogen-bond-formation enthalpy, −ΔHDHB, on the proton-donor strength, Pi(HX). HX = iPrOH, MeOH, CFH 2 CH 2 OH, CF 3 CH 2 OH, (CF 3 ) 2 CHOH, and (CF3)3COH. Experimental data from refs 40, 49, 51, 148, and 149.

hydrogen-bond-formation enthalpy derived from the computed νHX frequencies [ΔHtheor(Δν)] by application of the ΔH/Δν correlation (eq 2). Thus, perfect agreement of the Ej values calculated using ΔHtheor(Δν) and ΔHexp was found for BH··· HX bonds formed by DMAB [Ej(BH) = 0.62]149 and (Ph3P)2Cu(η2-BH4) [Ej(BH) = 0.91].148 This approach also works for proton-donating ability Pi, as was shown for the example of the NH group of DMAB in NH···Y complexes [Pi(NH) = 0.45].149 Dihydrogen bonds are of medium to weak strength (experimentally determined ΔHDHB values are typically lower than −8 kcal·mol−1), and their structural, electronic and spectroscopic features are similar to those of classical hydrogen bonds of the same energies. The hydride−proton distance is a good indicator of the interaction strength, with r(Hδ−···Hδ+) being shorter for stronger bonds.98,115,128,169 Computational data also reveal linear correlations between the dihydrogen bond enthalpy, ΔHDHB, and Hδ−···Hδ+ distance for close-tolinear MH···HX bonds (Figure 11). Experimentally derieved ΔHDHB values (estimated from IR measurements) and H···H distances (estimated from changes

2.4. Dihydrogen Bond Thermodynamics and Hydride Ligand Basicity

The formation enthalpies (ΔHHB) and entropies (ΔSHB) of DHBs are readily determined by the van’t Hoff method (with the temperature dependence of formation constants obtained from IR or NMR data).63 Several correlations have been developed to estimate the enthalpies of intermolecular hydrogen bonds, ΔHHB, using changes in the IR band positions (ΔνXH) and intensities (ΔAXH) (eqs 2−4, ΔHHB in kcal·mol−1) ΔHHB = −18ΔνHX /(ΔνHX + 720)

(2)

ΔHHB = −0.30ΔνHX1/2

(3)

ΔHHB = −2.9ΔAHX1/2 = − 2.9(Abonded1/2 − A free1/2 ) (4)

We have shown13,16,164 that these correlations, which were originally proposed for classical hydrogen bonds of organic acids and bases,165,166 work well for dihydrogen bonding. The formation enthalpy of any hydrogen bond, ΔHHB, depends on the partner (see, e.g., Figure 10) and solvent, so a relation has been developed that takes the proton-donor and proton-acceptor abilities of acids and bases into account167

Ej = ΔHij/(ΔH11Pi)

(5)

where Pi and Ej characterize the proton-donor and protonacceptor properties of acid and base, respectively (both are invariable), in a hydrogen bond and ΔH11 is the hydrogenbonding enthalpy for a standard pair PhOH−Et2O (P1 = E1 = 1.00). These parameters, namely, the acidity factor Pi and basicity factor Ej, are defined independent of the partners and media.167 Equation 5 was shown to be applicable to dihydrogen bonding MH···HX, allowing for the characterization of the proton-accepting abilities of hydride ligands.13,16,17,168 Interestingly, the basicity factors Ej can be estimated using the

Figure 11. Correlation between the DHB enthalpy values, ΔHDHB, and H···H distances computed for DHB complexes of different hydrides. Data from refs 62, 100, 110, 127, 140, 147−149, 169, and 170. 8553


Chemical Reviews

Review

in the relaxation time, T1min of the hydride ligand) are far less numerous. For a few hydrides these data were obtained in the same low-polarity solvent CH2Cl2 (CD2Cl2), allowing the same ΔHDHB versus rH···H correlation to be built (Figure 12).44

Table 3. Basicity Factors of Hydride Ligands Ordered by Position of the Core Metal in the Periodic Table Ej

compound

ref(s)

Group 5 Cp2NbH3 (tBuC5H4)2TaH3

0.93 1.10a

62

Group 6 WH(CO)2(NO)(PPh3)2 WH(CO)2(NO)(POiPr3)2 WH(CO)2(NO)(PEt3)2 WH(CO)2(NO)(PMe3)2 Cp*MoH(CO)(PMe3)2 Cp*MoH(PMe3)3 Cp*MoH3(dppe) Cp*WH3(dppe)c ReH7(dppe) ReH5(PPh3)3 [HRe2(CO)9]− ReH2(CO)NO(PMe3)2 (CP3)Re(CO)2H (NP3)ReH3c

Figure 12. Correlation between the enthalpy of DHB formation (ΔH°DHB; derived from IR data) and the H···H distance (determined from NMR data on T1min) for complexes of fluorinated alcohols [TFE = CF3CH2OH, HFIP = (CF3)2CHOH, PFTB = (CF3)3COH] in dichloromethane. Data from refs 40, 44, 49, and 52.

TpRuH(dppe) CpRuH(CO)(PCy3) CpRuH(dppe) (CP3)RuH2(CO) RuH2(dppm)2 (CP3)RuH(BH4) Cp*FeH(dppe) Cp*RuH(dppe) Cp*OsH(dppe)c (PP3)FeH2 (PP3)RuH2 (PP3)OsH2

2.5. Effects of Metal Atom and Ligands on Hydride Basicity

The Ej values of hydride ligands vary from 0.5 to 1.7 (Table 3). They depend on the metal atom and the ligand environment and can be similar to those of organic bases such as CH3CN (Ej = 0.75) and Et2O (1.0), even reaching the values of strong bases such as dimethyl sulfoxide (DMSO; 1.27), Py (1.53), and Et3N (1.7).167 The basicity factors Ej of hydride ligands increase on going down a group in the periodic table. For example, for isostructural metal hydrides, Ej increases in the order (PP3)FeH2 < (PP3)RuH2 < (PP3)OsH2, Cp*Fe(dppe)H < Cp*Ru(dppe)H < Cp*Os(dppe)H, Cp*MoH3(dppe) < Cp*WH3(dppe). The electronic and steric effects of ligands determine the electron density on the metal core and affect the protonaccepting properties of the hydride ligand. Thus, the protonaccepting ability of ruthenium hydrides increases from TpRuH(dppe) to CpRuH(dppe) and then to Cp*RuH(dppe). Substitution of one PMe3 ligand in Cp*MoH(PMe3)3 for a carbonyl group decreases the Ej value of the hydride by ca. 0.1 unit. In the series of tungsten hydrides WH(CO)2(NO)(PR3)2, the Ej value increases in the order PPh3 < P(OiPr)3 < PEt3 < PMe3, as one would expect considering the phosphine cone angles and basicities.13 For the related rhenium hydrides ReH2(CO)(NO)(PR3)2, the increase of the steric bulk on going from PMe3 to PiPr3 causes a switch of the hydrogenbonding site from the hydride in ReH2(CO)(NO)(PMe3)2 (Ej = 0.80) to the NO group in ReH2(CO)(NO)(PiPr3)2 (Ej = 0.60).58 The coligands present at the metal site can impose strong trans effects on the hydride basicity and dihydrogen bond strength. As was shown for intramolecular NH···HIr dihydrogen bonds in the series of 2-aminopyridine iridium complexes [IrH2(Y)(2-C6H4NH2)(PPh3)2]n+ (n = 0, 1), the DHBs strengthen in the order F < Cl < Br < I < CN < CO < H.48 The most electropositive atom H is the best σ donor in the series, weakening the MH bond trans to it and inducing a

(tBuPCP)NiH (tBuPCP)PdH BH3NHMe2 [B12H12] 2− [B10H10] 2− BH4− (PPh3)2CuBH4 (PP3)RuH(BH4) BH3NEt3 AlH3NEt3 [AlH4]− [GaH4]−

0.70 0.72 0.87 0.91 (1.28)b 1.36 1.43 1.73 Group 7 0.54d 0.63d 0.64 0.80 0.97 1.44 Group 8 0.81 1.02 1.21 1.39 1.40 1.43a 1.34 1.39 1.47 1.12 1.33 1.67 Group 10 0.69 0.89 Group 13 0.62 0.63 0.83 1.25 (1.26)b 0.91 0.98 0.53 1.00 (1.35)b 1.35 (1.33)b

13 13 13 13 31 172 64 152 173 173 42 58 49 175 44 50 43 40 171 59 52 53 51 51 51 174 176 149 177 177 100, 164 148 147 164 178 100 100

Unpublished data. bCalculated from the theoretical data on ΔHOH. Bifurcated interaction with the main interaction being from the metal. d Calculated from experimental ΔH values in the original work. a c

shift in electron density toward the trans H ligand, increasing the partial negative charge on the hydride ligand and making the NH···HIr bond stronger.48 The different trans effects of NO and CO groups in the case of ReH2(CO)(NO)(PR3)2 hydrides179 determine the preference for an intermolecular dihydrogen bonding to the hydride ligand trans to NO group.34,39,58 Coordination of a BH4− anion to a transitionmetal center decreases its proton-accepting ability; for example, 8554


Chemical Reviews

Review

Table 4. Computed Complexation Energies and MH/OH Bond Polarizations in Hydrogen- and Dihydrogen-Bonded Complexes

ΔEcompl (kcal·mol−1) Δpol(MH) (%) Δpol(OH) (%)

Me3N·TFE

Cp*FeH(dppe)·TFE

CpMoH(CO)3·NMe3

N···HO

FeH···HO

N···HMo

−10.7

−8.2 9.1 −2.8

−9.2 −4.4

−3.6

compare BH4− and (PPh3)2CuBH4 (Table 3). The Ej values are lowest for the neutral borohydrides BH3NHMe2 and BH3NEt3.

sections, we discuss various aspects of the proton-transfer reaction itself and the processes in which they are involved.

2.6. Structural Reorganization and Repolarization Caused by Hydrogen Bonding

3.1. General Considerations of the Reaction Mechanism

The intermediacy of intra- or intermolecular DHBs in the proton transfer from OH and NH acids to H ligands of transition-metal hydrides is key to the formation of nonclassical di- or polyhydrides (Scheme 4).189 The reversibility of the

Two important effects are recognized for all types of hydrogen bonds: hyperconjugation and rehybridization.121,180−183 Bent’s rule relates the orbital hybridization and electronegativity of substituents, with more electronegative substituents preferring hybrid orbitals having lower s character. Therefore, upon HB formation, the s character of spn hybrid XH orbitals increases, the electron density at hydrogen decreases, and the XH bond becomes stronger. Hyperconjugation arises from the transfer of electron density from the base to the antibonding σ*XH orbital and leads to the opposite effect, namely, a weakening of the XH bond. Superposition of these two contributions could explain the counterintuitive behavior of hydrogen bonds: Charge transfer takes place from a base to an acid HX, but at the same time, the charge on the HX atom becomes more positive. In the case of medium to strong hydrogen bonds, the effect of hyperconjugation outweighs rehybridization, leading to XH bond lengthening. The effect on the XH bond of proton donors was found to be the same for any type of hydrogen bond.57,121,183−188 The hydridic part of a dihydrogen bond, that is, the EH or MH bond, undergoes an intuitive and opposite change. Upon DHB formation, the s character of the E atom decreases, and the HE atom becomes more negatively charged.121,185,186 The ZH bond polarization, pol(ZH), defined as the amount of electron density on a H atom relative to the total electron density on the σZH orbital, is a convenient numerical characteristic of charge distribution within ZH bonds. It changes in the opposite directions when H becomes more electronegative (more hydridic) and more electropositive (more protic). In the case of transition-metal hydrides, the amount of electron density at the “hydride” hydrogen, that is, the MH bond polarization [pol(MH), in %], depends on the metal and the supporting ligands. The σMH orbital is centered mainly on hydrogen, which typically has over 45% of the total electron density. In dihydrogen-bonded complexes of Cp*MH(dppe) species with CF3CH2OH, pol(MH) increases, whereas pol(OH) decreases, reflecting the increase of the alcohol acidity and MH bond hydricity.119 Hence, DHB formation results in a change in the polarizations of XH and MH bonds (see Table 4), preparing both for subsequent transformations.

Scheme 4. General Mechanism of Proton Transfer through Dihydrogen Bonding

proton-transfer step has been shown; the equilibrium shifts toward the protonation products upon cooling or increasing the HX concentration and strength. The important property of many systems studied is the slow exchange between the DHB and (η2-H2) species on the NMR time scale, with these signals typically being well-resolved from one another. Still, the proton transfer in both directions is fast enough to allow the observation of exchange cross-peaks in a 2D EXSY map between the signal of the proton-transfer product and the averaged resonance of the MH/MH···HX mixture and sometimes even the broad XH signal.42,43 The protonation of neutral metal hydrides was studied in low-polarity solvents, where the conjugate base of the protonating agent should remain in the proximity of the η2H2 cation as a tight ion pair that can subsequently transform into a solvent-separated ion pair and then free ions (Scheme 4).190 The formation of homoconjugate [M-(η2-H2)]+···[X··· H···X]− ion pairs (X = OTf,191,192 CF3COO,50,60,170 RFO, ArO117,170,193) has been observed spectroscopically, and their role in the stabilization of [M-(η2-H2)]+ species was assessed theoretically.60,62,117,170 In the case of weaker HX acids (X = CF3COO, RFO, ArO), hydrogen bonding between the η2-H2 ligand and the counteranion provides additional stabilization to these ion pairs, as demonstrated for CpRuH(CO)(PCy3),50,60 PP3MH2,51,117 Cp*MH(dppe),52,170 and other complexes by IR and UV−visible spectroscopy. For example, in the case of [CpRu(η2-H2)(CO)(PCy3)]+, the νCO band appears at a frequency depending on X = ORF, with the band shift increasing with the anion basicity and, thus, with the [M(η2H2)]+···[XHX]− hydrogen-bond strength.60 DFT calculations have shown that the formation of homoconjugated ions reduces the basicity of X− and prevents deprotonation of the dihydrogen complex.31,60,62,64 Also, a strong [X···H···X]− hydrogen bond provides additional stabilization to the proton-transfer product through delocalization of the counteranion negative charge. Interestingly, when both nonclassical (dihydrogen) and classical (polyhydride) isomers of the cation exist, the higher acidity of the coordinated dihydrogen gives the stronger hydrogen bond to the

3. DIHYDROGEN-BONDED COMPLEXES AS INTERMEDIATES OF PROTON TRANSFER TO HYDRIDES Formation of a dihydrogen bond MH···HX to a transitionmetal or main-group-element hydride is the first incipient step of proton transfer to these compounds. In the following 8555


Chemical Reviews

Review

homoconjugate anion instead of the classical polyhydride.64 This stronger interaction protects the [M(η2-H2)]+ species from further transformation, which could be isomerization into dihydride or H2 loss (Scheme 5). Thus, the temperature at

substantially lowers the proton-transfer barrier.31,60,64,170 The transition states of proton transfer (Figure 13) feature almost fully formed HH bonds (HH distance of ca. 0.9 Å), where the H2 unit is bonded simultaneously to the [M]+ unit and the homoconjugate anion (MH and HO distances of ca. 1.6− 1.9 and 1.3−1.5 Å, respectively).31,60,64,170 Indeed, it was found experimentally that a second proton-donor molecule might be necessary at an earlier stage of the reaction, namely, the protontransfer step itself. The protonation of CpHW(CO)2(PMe3) by PhNH3+ takes place only in the presence of the conjugate base PhNH2.197 Several kinetic studies have also shown the involvement of a second proton-donor molecule in the protonation and suggested that the reaction occurs through a late transition state (vide infra; section 3.7.).

Scheme 5

which the rearrangement of [Cp*Fe(η2-H2)(dppe)]+ into trans[Cp*Fe(H)2(dppe)]+ is observed depends on the counteranion, being greatest when X− is most basic.59 Similar effects were observed for [(κ3-NP3)IrH4]+ [κ3-NP3 = κ3-P,P,PN(CH2CH2PPh2)3], which is highly fluxional on the NMR time scale when obtained by (κ3-NP3)IrH3 protonation with HBF4·OEt2, but gives well-resolved hydride signals when the counterion is [RFO···HORF]− [RFOH = (CF3)2CHOH].194 DFT calculations suggest that this effect originates from the stabilization of [(κ3-NP3)Ir(η2-H2)(H)2]+ species through the interaction with the homoconjugated anion.194 In the case of trans-[FeH(η2-H2)(dppe)2]+, deprotonation by NEt3 is faster for ion pairs formed by the dihydrogen and the BF4 or PF6 anion, proceeding through FeH2···N and Fe H···HN hydrogen bonds.195 In all cases, the anions are in the proximity of the η2-H2 ligand, but the bulky phenyls of BPh4− block the dihydrogen ligand and hinder the deprotonation. Formation of the homoconjugate anion [TfO···HOTf]− shifts the equilibrium between trans-[M(η2-H2)(CN)(L2)2]+ and trans-[M(H)(CNH)(L2)2]+ (M = Fe, Ru; L2 = dppm, dppe, dppp) toward the former, showing that a delicate balance of classical (TfOH···OTf, CN-H···OTf) and nonclassical [M(η2H2)···OTf] hydrogen bonds can influence the relative stability of tautomers in solution.192 In the case of the terminal hydride [HFe2(adtNH)(CO)2(dppv)2]+, stronger bonding of the protonated amine to BF4− in comparison to BArF4− favors the ammonium tautomer, shifting the ammonium/amine hydride equilibrium to the left.196 On the other side of the proton-transfer equilibrium, the involvement of the second proton-donor molecule leads to the cooperative enhancement of the dihydrogen-bond strength and

3.2. Transition-Metal Hydrides: Proton-Transfer Dichotomy {[M(η2-H2)]+ vs [M(H)2]+}; Ion Pairing

Nonclassical η2-H2 complexes are often formed as kinetic protonation products, transforming to classical dihydride isomers upon warming (pathway a, Scheme 6). A classic Scheme 6

example is the aforementioned low-temperature protonation of Cp*FeH(dppe) first described by Hamon et al.198 Later, we showed that the [M(η2-H2)]+ → trans-[M(H)2]+ isomerization occurs as a direct intramolecular rearrangement.59,170,199 The same mechanism has been established for the protonation of the ruthenium analogue Cp*RuH(dppe) (Scheme 7),52 as well as of Cp*MoH3(dppe).64,153 However, in the latter case, the formation of the cationic dihydrido dihydrogen complex [Cp*Mo(H)2(η2-H2)(dppe)]+ was shown only theoretically; the transformation into the experimentally observed tetrahydride [Cp*Mo(H)4(dppe)]+ features a very low barrier.64 When the M···HO interaction has a large influence on dihydrogen bonding, direct proton transfer to the metal becomes possible, such that a classical hydride is afforded without the formation of an η2-H2 intermediate (pathway b, Scheme 6).53,152,175,200 The spectroscopic data obtained for

Figure 13. Optimized structures of the transition state and ion-pair product for proton transfer from two CF3COOH molecules to CpRuH(CO)(PH3). Data from ref 60. 8556


Chemical Reviews

Review

Scheme 7

Scheme 8

Cp*WH3(dppe)152,153 and (κ4-NP3)ReH3 [κ4-NP3 = κ3P,P,P,N-N(CH2CH2PPh2)3]175 interacting with fluorinated alcohols evidence the formation of hydrogen bonding to the metal, whereas DFT calculations on Cp*WH3(dppe)·HORF suggested the formation of bifurcated dihydrogen-bonded complexes featuring simultaneous HM···HO and M···HO interactions but no η2-H2 species on the reaction pathway.152 In a similar situation, low-temperature proton transfer within the bifurcated dihydrogen-bonded complex Cp*OsH(dppe)· HORF yields cis dihydride [Cp*Os(H)2(dppe)]+, which transforms into the trans isomer above 230 K.53,200 A number of complexes give dihydrogen−dihydride mixtures as thermodynamic products of proton transfer.201−204 In such cases, the question arises whether the dihydride is formed by simple isomerization of a nonclassical intermediate or by deprotonation of the latter with subsequent proton transfer to the metal through an M···HX hydrogen-bonded intermediate (Scheme 6). Thus, protonation of CpRuH(dppe) by strong acids at room temperature yields a 1:2 mixture of [CpRu(η2H2)(dppe)]+ and trans-[CpRu(H)2(dppe)]+,205,206 whereas the dihydrogen complex is the sole protonation product at 213 K.207 According to experiment and calculations, the lowtemperature proton transfer leads through dihydrogen bonding to the cationic dihydrogen complex, whose transformation to dihydride is reversible and involves deprotonation to the initial hydride with subsequent formation of Ru···HX hydrogen bond and proton transfer to the metal site (Scheme 8).43 However, as this is energetically less favorable, the hydrogen bond to the metal atom (Ru···HX) could not be observed experimentally. The substitution of Cp for Cp* lowers the hydride basicity and diminishes the steric hindrance at the metal, allowing better access of the proton donor in the anti position. As a result, the two proton-accepting sites, namely, the hydride ligand and metal atom, compete with each other, and direct proton transfer to the metal yielding the trans-dihydride becomes operative in the case of the Cp complex. The possibility of anti proton transfer to the metal has been suggested for the formation of trans-[Cp*Os(H)2(CO)2]+ from [Cp*Os(η2-H2)(CO)2]+.208 Thus, the dihydrogen−dihydride dichotomy is strongly affected by the ligand environment: Changes in the steric and electronic properties upon substitution of Cp* by Cp induce very significant qualitative changes in the reactivity of the hydride complex. The reversibility of trans-[CpRu(H)2(dppe)]+ formation is important for its operation as an ionic hydrogenation catalyst,209 such

that both nominally [Ru(η2-H2)]+ and [Ru(H)2]+ species can transfer a proton to the product. 3.3. Transition-Metal Hydrides: Proton Transfer and H2 Evolution

The natures of the metal and the supporting ligands definitely play a major role in determining the stability of η2-H2 complexes. A delicate balance between σ(H2)-to-dσ*(M) donation and dπ-to-σ*(H2) back-donation determines the possibility of oxidative addition of H2 and transformation of the η2-H2 species into classical dihydrides.210,211 In addition to these aspects related to the metal electron density, the strength of the cation−anion interaction is also key (vide supra). When the metal is relatively electron-poor and back-donation is not strong enough, H2 liberation can occur (Scheme 9).30,40,212−214 Scheme 9

Interestingly, this reaction is also further governed by the choice of anion. Obtained by protonation of (CP3)ReH(CO) 2 , 49 Re(CO)H 2 (NO)(PR 3 ) 2 , 58 and CpRuH(CO)(PCy3),50,60 the corresponding [M(η2-H2)]+X− complexes could be isolated as BF4− salts, but lose H2 when X− is more basic (RO−, RCOO−). Similarly, trans-[MH(CN)(depe)2] (M = Fe, Ru, Os) complexes give relatively stable trans-[M(η2H2)(CN)(depe)2]+ monocations when protonated with 1 equiv of HBF4 but vigorously lose H2 when triflic acid is used.192 The tetrahydrido complex [Cp*Mo(dppe)H4]+ is unstable as a BF4− salt215 but appeared to be stable when generated in benzene/toluene with the use of excess CF3COOH.154 This is the most striking example of the profound effect that the solvent and acid choice can have and can be interpreted in terms of the different H-bonding abilities and coordinating properties of anions further fine-tuned by solvent polarity.154 Sometimes, M(η2-H2) complexes are so unstable that they are not observed experimentally even at low temperatures. In this case, dihydrogen bond formation is directly followed by H2 evolution yielding MX species. Such is the case for reactions of WH(CO)2(NO)(PR3)2 with weak acids,13,216 for which the dihydrogen complex is found theoretically as a local minimum.65 An analogous situation is the intramolecular transformation of dihydrogen-bonded aminopyridine iridium hydride (Scheme 10).12 In the case of WH(CO)2(NO)(PR3)2, the reaction rate does not depend on the anion basicity 8557


Chemical Reviews

Review

C 5 H 4 (CH 2 ) n NHMe 2 )RuH(dppm)] + but not for [(η 5 C5H4(CH2)nNHMe2)RuH(PPh3)2]+.36,37 The presence of all species depicted in Scheme 9 was confirmed experimentally and theoretically for several (arene)RuHn(κ1-P-R2PHet)m systems (arene = Cp*, p-cymene; n = 2, 3; m = 1, 2) bearing phosphines with noncoordinated nitrogen centers (Het = pyridine, quinoline, 2-N-methylimidazolyl, etc.).28,220,221 The protonation occurs initially at the heterocycle N atom, yielding compounds stabilized by hydrogen (NH+···X) or dihydrogen (NH+···HRu) bonding. Such adducts are stable only at low temperatures, above which proton transfer from the NH group to the hydride ligand proceeds, eventually resulting in H2 evolution and the formation of thermally stable [(arene)RuH n−1 (κ 2 -P,NR2PHet)m]+ species. The same reaction mechanism was proposed for the protonation of RuClH(κ1-PPh2py)2(κ2-P,NPPh2py).222 The reverse reaction, namely, activation of dihydrogen by [CpRu(PTA)2X] (X = Cl, H) in protic media,223,224 was studied theoretically, taking into account a cluster of three hydrogen-bonded molecules (H2O)3.225 Intramolecular proton transfer from coordinated H2 to a nitrogen atom of PTA takes place through such a water chain to afford the monohydride [CpRu(PTA){PTA(H)}H]+ (Scheme 12), the proposed active catalytic species in the biphasic hydrogenation of benzylideneacetone using [CpRu(PTA)2H].224 A rotation of the protonated PTA ligand during the HH heterolysis step gives an isomer stabilized by a hydrogen bond to the metal, with proton transfer through the hydrogen-bonded water chain affording trans-[CpRu(PTA)2(H)2]+.225 A similar mechanism involving a cooperative chain of methanol molecules connecting a pyridine substituent and a WH moiety was invoked to account for the acceleration of H/D exchange with excess CD3OD observed for HW(CO)2(NO)(PPh2py)2 in comparison to HW(CO)2(NO)(PPh3)2.226 Other closely related complexes include diphosphines with incorporated pendant amines, which can also serve as proton relays that accelerate intra- and intermolecular proton mobility and H2 heterolytic splitting/release.227,228 Inspired by bimetallic nickel−iron or diiron hydrogenase enzymes, studies of these species are motivated by the production or oxidation of hydrogen for energy storage and use.229−231 Highly relevant to the topic of this review are the processes of proton transfer between a metal center, a hydride, and a pendant amine, as well as the influence of hydrogen bonding on the structure and reactivity of the species involved. Thus, DFT calculations indicate a hydrogen bond between Ni0 and the NH group of the protonated ligand in isomers A and B of [Ni(PR2NR′2H)2]2+ complexes (Scheme 13). The strength of this interaction is ∼7 kcal·mol−1 with significant d(Ni) → σ*(NH) donation232 and with such a hydrogen bond preceding the observed proton exchange in [Ni(PR2NR′2H)2]2+. Stronger NH···N hydrogen bonds with an estimated free energy of about 9−10 kcal·mol−1 in the pinched moiety of isomers B and C result in higher

Scheme 10

(coordination ability) but does correlate with the HX strength,216 suggesting that proton transfer is rate-determining. The same is true for the reaction of [W3Q4H3(dmpe)3]+ (Q = S, Se) and [W 3 PdS 4 H 3 (dmpe) 3 (CO)]+ with excess acid.217−219 The T1 relaxation data obtained for the hydride signal of [W3Se4H3(dmpe)3]+ in the presence of acid support dihydrogen bond formation as the first reaction step.217 The dihydrogen complex was neither observed by NMR spectroscopy nor found as a local minimum in DFT calculations; the coordination of H2 to the metal appeared to be very weak even in the transition state (TS; Figure 14).219

Figure 14. Optimized geometry of the transition state of protonation of [W3PdS4H3(dmpe)3(CO)]+ by two HCl molecules. Data from ref 219.

3.4. Hydrogen Bonding and Protonation of Hydrides with Pendant Nitrogen Centers

Complexes with intramolecular NH···HM hydrogen bonds can be obtained by protonation of pendant amino groups in the neutral hydrides [η5-C5H4(CH2)nNMe2]RuH(L2) [n = 2, 3; L2 = dppm,36 (PPh3)237]. In contrast to the PPh3 species, which feature NH···Ru hydrogen bonds (Figure 8), the dppm derivatives lose H2 slowly to afford the new salt [(η5,κ1-Cp-N)Ru(dppm)](BPh4) with a chelating amino cyclopentadienyl ligand. Such a product suggests that proton transfer occurs in the dihydrogen bond, leading to the monocationic dihydrogen complex [(η5-Cp-N)Ru(η2-H2)(dppm)]+ as a H2 evolution intermediate. Further support for this reaction mechanism (Scheme 11) is provided by the reverse H2 uptake and H/D exchange with D 2 O observed for [(η 5 Scheme 11

8558


Chemical Reviews

Review

Scheme 12. Intramolecular Proton Transfer from Coordinated H2 of [CpRu(PTA)2(η2-H2)]+ to a Nitrogen Atom of the PTA Ligand and Then to a Metal Atom through a Hydrogen-Bonded Water Chaina

a

Data from ref 225.

Scheme 13a

methylpyrrolidine) are rate-limiting steps of electrocatalytic H2 oxidation.233 Similar steps were invoked to rationalize the oxidation of (CpC5F4N)Fe(PtBu2NBn2)(H) under Ar atmosphere,234 a process in which first the proton is transferred intramolecularly from oxidized FeIII to the pendant amine and then intermolecular proton transfer to the parent iron(II) hydride yields [Fe(η2-H2)]+, whose formation is followed by rapid loss of H2 (Scheme 14). Added bases such as Nmethylpyrrolidine replace the neutral hydride in Scheme 14 in deprotonation of the oxidized complex such that the net reaction involves two-electron oxidation yielding H+ and [(CpC5F4N)Fe(PtBu2NBn2)]+ instead of hydride ligand oxidation to form H2 in the absence of an exogenous base. Interestingly, the dihydrogen ligand in the complexes [(CpC5F4N)Fe(P2RNR′)(η2-H2)]+ (R = Ph, Et, tBu; R′ = Me, t Bu) undergoes rapid proton/hydride exchange through reversible heterolytic cleavage. The resulting dihydrogenbonded proton−hydride species [M(H)(PRNR′(H))]+ are less than 5 kcal·mol−1 more energetic than [M(η2-H2)]+, with the estimated barrier between the two being less than 6.8 kcal· mol−1.233,235 The balance of the intramolecular proton-transfer equilibrium strongly depends on the dihydrogen acidity/basicity of the hydride and nitrogen centers determined by the nature of the substituents in the diphosphine and Cp ligands. The complexes [CpFe(PPh2NBn2)(H2)]+236 and [(CpC5F4N)Fe(PEt2NMe)(H2)]+233 exist as nonclassical [M(η2-H2)]+ species, whereas the use of PtBu2NtBu2 in the latter complex234,235 allows isolation of a N-protonated isomer featuring a very short N H···HFe dihydrogen bond [1.489(10) Å] (Figure 15). In the same way, reversible heterolytic cleavage of H 2 upon

a

Adapted with permission from ref 232. Copyright 2011 American Chemical Society.

kinetic barriers and account for the lack of intramolecular exchange of this proton with the endo proton in isomer B. In related work, combined experimental and computational studies showed that steric repulsion precludes direct intermolecular deprotonation of iron(III) hydrides [(CpC5F4N)Fe(PRNMePR)(H)]+ (R = Ph, Et) by an exogenous base. Indeed, intramolecular deprotonation by the pendant amine and subsequent deprotonation by an exogenous base (N8559


Chemical Reviews

Review

Scheme 14a

178°], in proton transfer to tetrahydroborate.100 The stabilizing role of dihydrogen bonding in the transition state of the thermal decomposition of LiBH4/triethanolamine in the solid state was shown.243 No intermediates other than DHB complexes were observed by spectral methods for the reaction of heavier tetrahydrides MH 4 − (M = Al, Ga) with alcohols.100,140 According to DFT calculations,100 the TS structures can be described as M(η2-H2) complexes similar to those of transition metals. The M → H2 back-donation stabilizing dihydrogen complexes of transition metals is so weak in some cases that the existence of such complexes on the reaction pathway was confirmed only theoretically.31,62,64,65,216 For the same reason, such complexes in main-group hydrides are almost unknown. So far, the existence of weakly bound dihydrogen species MH3(η2-H2) has been observed only by IR matrix isolation for M = B, Ga, Al.244−246 The boron dihydrogen complex exists at higher temperatures (25 K)246 than those of Ga and Al (4 K)244,245 because of the tighter coordination of the HH ligand. Thus, the computational analysis of main-group hydrides shows that proton transfer, H2 elimination, and formation of the alkoxo product occur after dihydrogen bonding in a concerted way (Scheme 15).100,247−249 The single exception is the reaction between CF3OH and BH4−, for which BH3(η2H2)···OR is found as a local minimum formed by protonation of the hydride (see Figure 16).100

a


coordination to [(PPh2NBn2)Mn(CO)]+ leads to the formation of a manganese hydride and a protonated amine that undergo very fast H+/H− exchange.237

Scheme 15. Mechanism of the Reaction of Main-Group Metal Hydrides with Proton Donors

3.5. Main-Group Hydrides: Concerted Proton Transfer and H2 Evolution

The mechanism by which reactions of main-group metal hydrides with proton donors proceed is similar to that of transition-metal hydrides. For example, both spectroscopic and computational investigations70,164,238−242 provide evidence for the intermediacy of the monodentate dihydrogen-bonded complex of BH4− with various HX proton donors, ROH··· HBH3 [r(HO···HB) = 1.351−1.654 Å, ∠OH···HB = 165−

The simultaneous presence of an acidic functionality in the DMAB molecule determines its ability to form diverse hydrogen-bonded adducts and directs DMAB reactivity.149 Computational analyses suggest a cyclic structure for the active intermediate of proton transfer from OH acids, with this structure featuring both BH···H(O) and NH···O(H) bonds

Figure 15. Molecular structures of (left) [(CpC5F4N)Fe(P2EtNMe)(η2-H2)]+ and (right) [(CpC5F4N)FeH(P2tBuN2tBuH)]+ determined by X-ray and neutron diffraction, respectively.233,235 BAr4 anions have been omitted for clarity. Left structure reprinted with permission from ref 233. Copyright 2014 American Chemical Society. 8560


Chemical Reviews

Review

Figure 16. DFT energy profile for the reaction of BH4− with CF3OH in the gas phase. Adapted with permission from ref 100. Copyright 2006 American Chemical Society.

(Figure 17). In noncoordinating solvents, the suggested reaction pathways (Scheme 16) include concerted proton

Scheme 16. Possible Reaction Pathways of DMAB Alcoholysisa

Figure 17. M06-optimized geometry of cyclic hydrogen-bonded complex DMAB·HOMe. Reproduced with permission from ref 149. Copyright 2015 American Chemical Society.

transfer within hydrogen-bonded cyclic species and H2 evolution (TSBHNH, blue arrow) or dissociation of a B−N bond (TSB−N, red arrow). The latter becomes more feasible in coordinating media, where the solvent molecule promotes B− N dissociation (Scheme 16, green arrow). These data provide a plausible explanation for the observed solvent effect, as the reaction is significantly faster in MeCN, Me2CO, and tetrahydrofuran (THF) [with a half-life of ca. 4 h for (CF3)2CHOH in acetone] than in CH2Cl2 and C6H5F (2 weeks).149 The proton-accepting ability of AlH3NMe3 (trimethylamine alane, TMAA) is about twice that of Et3NBH3 (Ej values equal to 1.0178 and 0.53,164 respectively). Accordingly, the observa-

a


tion and characterization of DHB complexes of the former was possible only with weak proton donors, such as NH and CH acids, because proton transfer to TMAA occurs readily even at 8561


Chemical Reviews

Review

Figure 18. Geometries of DHB complexes (top) and corresponding transition states (bottom) for the reaction of TMAA with CH3OH and (CH3OH)2. Hydrogen atoms of NMe3 removed for clarity. Data from ref 178.

low temperatures.178 Unexpectedly, H2 elimination from dihydrogen-bonded complexes yielding Al-OR/Al-NR2 species was found to proceed more easily than that with more basic anion BH4− (Ej = 1.25).100 The results of DFT calculations showed that the increased Lewis acidity of the aluminum center relative to the boron analogue gives rise to two reaction pathways yielding the same reaction product through quite similar M(η2-H2)-like transition states (Figure 18).178 The similarity of the two TSs (TSDHB and TSLewis, Figure 18) indicates that nucleophilic attack of oxygen on aluminum and proton transfer to hydride are synchronous, leading directly to H2 elimination and formation of the alkoxo product. Notably, despite the lower proton-accepting ability and weaker DHB complex of TMAA in comparison to those of AlH4− (values of −5.6 and −10.7 kcal·mol−1, respectively, for AlH···HOCH3), the energy barrier is considerably lower for the former (17.4 kcal·mol−1 with methanol, TSDHB) than for AlH4− (27.3 kcal· mol−1) because of the presence of a stronger Al···O interaction in the TMAA system. The second ROH molecule acts as a bifunctional catalyst, transferring a proton to the hydride and accepting a proton from another ROH molecule (TScycl, Figure 18). This structure allows better stabilization of the TMAA/ (HOCH3)2 adduct and decreases the reaction barrier relative to that proceeding through the Lewis complex. The formation of these complexes explains the greater reactivity of TMAA with

proton donors even though its proton-accepting ability is lower than that of BH4−. In a similar manner, the coordination of a THF or tetrahydropyran (THP) molecule to the Al center of bis(NHC)AlH[Fe(CO)4] is the first step of facile CH bond activation of ethers proposed on the basis of DFT calculations (Figure 19).250 It brings α-CH and HAl in contact, allowing for H2 formation and elimination to afford bis(NHC)Al(2-cyclo-OC 4 H 7 )[Fe(CO) 4 ] and bis(NHC)Al(2-cycloOC5H9)[Fe(CO)4] upon reaction of bis(NHC)Al(Br)[Fe(CO)4] with KH in THF and THP, respectively. The lower nucleophilicity of the gallium atom means that, in contrast, the stability of bis(NHC)GaH[Fe(CO)4] in ethereal solvents allows isolation and characterization.250 The importance of alcohol coordination to one aluminum center, along with concomitant formation of the dihydrogen bond, that is, simultaneous Al···O and OH···HAl interactions, was shown in the reaction of pyrazolate-bridged dialuminum μhydride with diphenylmethanol. Such a cyclic intermediate (Scheme 17) is stabilized by 12.7 kcal·mol−1 but evolves H2 to form an alkoxo-bridged complex with a barrier of only 4.2 kcal· mol−1.251 8562


Chemical Reviews

Review

Scheme 18. Intra- and Intermolecular Dihydrogen Bonds in Tetramethylpyperidine-Based FLP Systems Characterized by Neutron Diffraction

Figure 19. Reaction-energy profile for CH activation by bis(NHC)AlHFe(CO)4 derived from DFT calculations. Data from ref 250.

Scheme 17. Computed Structure of the Dihydrogen-Bonded Cyclic Intermediate in the Reaction of Pyrazolate-Bridged Dialuminum μ-Hydride with Methanola Figure 20. Crystal structure of ansa-aminoborane 1-N-TMPH−CH2− 2-[HB(C6F5)2]C6H4 determined by neutron diffraction.258

acid and base fragments limits their flexibility to accommodate the H···H interaction, which is supported by a shorter internuclear B···N separation of 3.35 Å, compared to the significantly larger one [3.829(3) Å] in the unconstrained intermolecular hydrogenated FLP. An important consequence of the intermolecular FLP flexibility is the linearity of the dihydrogen-bonded NH···H moiety (Figure 21).

a


3.6. Dihydrogen Bonding and H2 Activation by Frustrated Lewis Pairs

As shown above, the reaction of main-group metal hydrides with proton donors leads to irreversible H2 elimination, which is a serious problem for their application as hydrogen-storage materials. The reverse process, namely, heterolytic splitting of dihydrogen, can be realized in the case of “frustrated Lewis pairs” (FLPs). Described for the first time by Stephan and coworkers252 for the example of (C6H2Me3)2P(C6F4)B(C6F5)2, this phenomenon was a subject of intense experimental and theoretical investigation. Several reviews253,254 and a twovolume set of Topics in Current Chemistry255,256 were recently published, and therefore, we only briefly touch on this area here. Formed by a Lewis acid and a Lewis base unable to give a “classical” donor−acceptor bond due to steric hindrance between the components, FLPs readily coordinate small molecules such as H2, CO2, SO2, and N2O.253 In the case of dihydrogen, this leads to heterolytic splitting of H2 yielding a dihydrogen-bonded ion pair.257 Two such systems were characterized by neutron diffraction.258,259 Comparison of these intra- and intermolecular hydrogenated FLPs (Scheme 18) shows that the BH and NH bond lengths in ansaaminoborane 1-N-TMPH−CH2−2-[HB(C6F5)2]C6H4 (1.24 and 1.03 Å, respectively) (Figure 20)258 are comparable to those determined in the tetramethylpiperidinium/[HB(C6F5)2(C6Cl5)]− pair.259 However, the distance between the H atoms is significantly shorter in the intramolecular FLP (1.67 Å) than in the intermolecular one (1.8047 Å). One explanation for this difference could be that the bridge between the Lewis

Figure 21. Neutron structure of [(C6F5)2(C6Cl5)BH][H−TMP] at 100 K. H atoms on the TMP molecule (except for those bound to the N atom) have been omitted for clarity. Data from ref 259.

A theoretical investigation132 of the BHδ−···δ+HP bonds in the ion-pair complexes [(CF3)3BH]−·[HPH3−n(Me)n]+ (n = 0−3) showed that their main features are similar to those of DHBs in neutral pairs and ion-molecular complexes (the linearity of the H···HP moiety, the characteristics of charge transfer, the topological properties of the DHB critical point). In the reaction with H2, the classical Lewis pair (CF3)3B·PH3 exhibits a high barrier and leads to the unstable ion-pair product [(CF3)3BH]−[HPH3]+, in contrast to the lower barrier 8563


Chemical Reviews

Review

models (polarization in the electric field inside the FLP cavity or synergistic electron transfer) are suggested for HH bond activation260,264,266 leading to concerted or stepwise H2 cleavage.264,266,267 Both linear and nonlinear transition-state structures have been obtained in different calculations of concerted HH splitting.261,266 Moreover, the calculations suggest that, by playing with the properties of the Lewis acid part and its basic counterpart, it is possible to switch from concerted to stepwise H 2 activation. 267 Thus, further explorations aiming to clarify the driving forces and mechanisms of H2 activation by FLPs are necessary.

and more stable DHB ion pair formed by the FLP (C6F5)3B· P(tBu)3 (Figure 22).132 These and other computational

3.7. Dihydrogen Bonding and Proton Transfer to Transition-Metal Tetrahydroborates

Coordination of the BH4 anion to a transition metal alters the properties and reactivity of tetrahydroborate. Such complexes find many applications, and a large number of studies of their structural and dynamic properties have been performed.268−270 Thus, coordination to a [(Ph3P)2Cu] fragment lowers the tetrahydroborate basicity relative to that of free BH4−.148 However, the reactivity pattern is preserved: According to the spectroscopic data, DHB formation precedes proton transfer and H2 liberation, which occur in one step even in the presence of CF3CH2OH, yielding the reaction product [{(Ph3P)2Cu}2(μ2,η4-BH4)]+ (Scheme 19). DFT/M06 calculations showed the reaction intermediate to be the bifurcated DHB complex with the alcohol interacting strongly with a terminal BH bond. Additional bonding to the bridging hydride ligand helps to orient the alcohol molecule and assists the subsequent formation of the cyclic transition state (Figure 23). The copper

Figure 22. Potential energy profiles calculated for the heterolytic cleavage of H2 by the classical Lewis pair B(CF3)3·PH3 and the frustrated Lewis pair (C6F5)3B·P(tBu)3. Reproduced with permission from ref132. Copyright 2009 American Chemical Society.

results260−262 strongly support the view that the frustration energy lowers the H2 splitting energy barrier and increases the exothermicity of the reaction. Reliable information on FLP association and the structure in solution was obtained by 19F,1H HOESY, diffusion, and temperature-dependent 19F and 1H NMR studies of PR3· B(C6F5)3.263 The authors found a number of equiprobable orientations of PR3·B(C6F5)3 in solution, with the DFT calculations showing the little difference (only 1 kcal·mol−1) between the two extreme orientations of P and B atoms facing each other and pointing in opposite directions. The low formation constant of K = 0.5 M−1 and the positive free energy value (ΔG = +0.4 kcal·mol−1) obtained by diffusion NMR spectra are characteristic of slightly endoergic association processes. These results suggest that association takes place through weak dispersion rather than residual acid−base interactions.263 The importance of noncovalent dispersion energy contributing to almost the entire interaction energy was confirmed computationally in a systematic study of the intermolecular association of six different Lewis acids and Lewis bases.262 The ability of FLPs to activate H2 has been applied in catalytic hydrogenation where the delivery of proton and hydride to an organic substrate regenerates the FLP.257,264 However, the mechanism of H2 activation by FLPs is still under debate. “Part of the problem is that, under the experimental conditions, several simultaneous processes can take place resulting in misleading kinetic data”,265 which is supposed to be the main experimental tool. Theoretical modeling of the process is also not free of difficulties.265,266 Two different

Figure 23. Directions of atom rearrangements in the transition state of proton transfer to the (Me3P)2Cu(η2-BH4) from CF3OH. Adapted with permission from ref 148. Copyright 2012 American Chemical Society.

atom acts as a Lewis acid in the TS, the structure of which resembles that for the reaction of Me3NAlH3 with two MeOH molecules described above. Such a highly ordered TS is in agreement with the significantly negative activation entropy determined experimentally for the reaction with p-

Scheme 19

8564


Chemical Reviews

Review

Figure 24. X-ray structures of (iPrPNP)FeH(η1-HBH3)(CO) and [{(iPrPNP)FeH(CO)}2(μ2,η1:η1-H2BH2)][BPh4] formed under protic conditions.275 BPh4 anions and THF molecules have been omitted for clarity.

NO2C6H4OH (ΔH‡ = 3.3 ± 0.6 kcal·mol−1 and ΔS‡ = −59 ± 2 cal·mol−1·K−1).148 The proton transfer to (Ph3P)2Cu(η2-BH4) is more facile (has a lower barrier) than that to free BH4−,100 despite the lower basicity of the former. The release of [BH3OR]− and H2 from INT1 (Scheme 19), identified as a local minimum, produces the bis(phosphine)copper moiety that coordinates to another molecule of (Ph3P)2Cu(η2-BH4). This mechanism could be operative in the dimerization of other transition-metal hydroborates.271−274 For example, treatment of (iPrPNP)FeH(η1-HBH3)(CO) with 2,6-lutidinium tetraphenylborate [iPrPNP = κ3-N,P,P-HN(CH2CH2PiPr2)2] gives the dimeric complex [{(iPrPNP)FeH(CO)}2(μ2,η1:η1H2BH2)][BPh4] (Figure 24).275 Both complexes feature intramolecular DHBs between the NH proton of the pincer ligand and the terminal hydrogen atoms of the η1-HBH3 ligand.

Table 5. Dihydrogen Bond Energies (ΔEDHB, kcal·mol−1) and Activation Energies of Proton Transfer (Ea, kcal·mol−1) and Corresponding H···H Distances (rHH, Å) Computed for the Reaction of Boron and Aluminum Hydrides with Different Acids DHB HX

3.8. Kinetics of Dihydrogen-Bond-Mediated Proton Transfer

Hydrogen bond formation is extremely fast and has, particularly in the past, been considered a diffusion-limited step.276 Still, it is the first, incipient, step of proton transfer, and the two steps are intimately related. Indeed, DHB formation enthalpies, ΔH°DHB, are proportional to the enthalpy change accosiated with the proton-transfer step, ΔH°PT.18 The correlation between the basicity factors Ej of hydride ligands and the enthalpy of the overall protonation reaction (Scheme 4) [MH] + HX ↔ [M(η 2 -H2 )] + (ΔH° DHB + ΔH° PT) provides quantitative evidence that the proton transfer is thermodynamically more favorable for more basic hydrides.18 The H···H distance in the TS becomes shorter, and the barrier decreases for stronger DHBs (Table 5). However, there is no common correlation between the two distances, which should be considered separately for each metal/element (Figure 25). That probably reflects the peculiarity of the dependence of the reactivity on the core element. Because the DHBs become stronger with the increase of the proton-donor acidity, the proton-transfer barriers decrease accordingly (Figure 26). Detailed kinetic studies of proton transfer involving metal hydrides are still rare. Nevertheless, the available data suggest the participation of a second proton-donor molecule in the proton-transfer step (Scheme 20).59,152,195 Consequently, the observed rate constant kobs, given by kobs =

K1K 2k[HA]2 1 + K1[HA] + K1K 2[HA]2

≈

K1K 2k[HA]2 1 + K1[HA]

ΔEDHB

H2Oa HFa HCla CH3OHb TFEb CF3OHb

−10.1 −15.5 −13.9 −10.1 −16.0 −22.2

pyrrole CH3OH TFE CF3OH

−3.4 −5.6 −5.7 −8.2

CH3OH TFE CF3OH

−12.0 −19.0 −24.7

CH3OH TFE HFIP PNP

−4.9 −7.6 −9.4 −8.4

TS rHH

AlH4− 2.009 1.371 1.226 1.622 1.513 1.258 Me3NAlH3c 1.870 1.702 1.599 1.478 BH4−b 1.654 1.553 1.351 Me2NHBH3d 1.969 1.933 1.804 1.834

Ea

rHH

27.5 18.4 6.7 27.3 22.5 5.3

0.989 0.917 0.782 0.981 0.893 0.780

30.0 17.4 18.6 15.8

1.160 1.154 1.132 0.873

54.1 48.0 27.9

0.838 0.793 0.755

54.7 48.8 40.7 42.8

0.859 0.823 0.794 0.768

a MP2/6-311++G(d,p).139 bB3LYP/6-311++G(d,p).100 311++G(d,p).178 dM06/6-311++G(d,p).149

c

B3LYP/6-

This model clearly shows the dependence of the observed rate law on the position of hydrogen-bonding equilibrium: When equilibrium constant K1 is small (K1[HA] ≪ 1) and the dominant species is free hydride, the expression simplifies to yield a second-order dependence on [HA],152,218 whereas a first-order behavior is expected if the dominant species is the 1:1 adduct, MH···HA (K1[HA] ≫ 1).59,64,170,277 When two separate NMR resonances are observed for the MH···HA and [M(η2-H2)]+ complexes, the activation free energy for proton transfer, ΔG‡PT, lies in the range of 12−16 kcal·mol−1.40,61,278,279 For relatively slow reactions that can be monitored on the conventional time scale by following the evolution of IR, UV−vis, or NMR spectra, such as for WH(CO)2(NO)(PR3)2,216 WH(CO)(NO)(PMe3)3,280 and CpRuH(CO)(PCy3),60 or using an electrochemical procedure, such as for cis-FeH2(dppe)2281 and (PP3)FeH2,282 the ΔG‡PT

(6)

can be simplified by neglecting the last term in the denominator.18 8565


Chemical Reviews

Review

Scheme 20. Kinetic Model for Proton Transfer to Transition-Metal Hydride Involving Two Proton-Donor Molecules

= 14 kcal·mol−1 and ΔS‡ = −32 cal·mol−1·K−1 were obtained for the H2 evolution reaction in Scheme 10.12 In all of these reactions, the activation entropy is negative, suggesting an associative process with a highly ordered transition state. DFT calculations showed this to be the case; the transition state features a H2 molecule, elongated (“activated”) to different extents and tightly bonded between the metallocomplex cation and anion (Figure 13). As a result of substantial negative ΔS‡ values, the activation free energy ΔG‡PT for the MH···HX → [M(η2-H2)]+[XHX]− transformation increases with temperature, as does the protontransfer rate constant. One must note that positive ΔS‡ values suggest a dissociative transition state, as determined for H2 evolution from the [Cp*MoH4(dppe)]+···[OCOCF3]− ion pair (ΔH‡H2 = 31.8 ± 0.5 kcal·mol−1, ΔS‡H2 = 36 ± 2 cal·mol−1· K−1)154 and from the ruthenium cluster (μH)2(H)2Ru3(CO)8(μ-P(t-Bu)2)2 (ΔH‡H2 = 23.2 ± 0.8 kcal· mol−1, ΔS‡H2 = 3.6 ± 1.4 cal·mol−1·K−1).286 In the former case, the reaction rate does not depend on the solvent, as the rates in benzene, THF, and acetonitrile are identical. The available data allow for the discussion of the evolution of the potential energy profile for dihydrogen-bond-mediated proton transfer. The observation of equilibrium between the DHB complex and the proton-transfer product [M-(η2H2)]+[XHX]− indicates a double-well character of the reaction potential energy profile (Figure 27). For weak dihydrogen bonds, the proton-transfer barrier is too high and cannot be overcome. When the DHB strength increases with changing acid or base, the ion-pair minimum deepens more quickly than the minimum of the molecular DHB complex because of the much stronger electrostatic interaction operating in ionic species. Ultimately, the increase of the acid−base strength leads to the disappearance of the DHB minimum and spontaneous proton transfer.

Figure 25. Relationship between the computed HH distances in the DHB complexes and TSs for aluminum hydrides (squares) and boron hydrides (triangles) (data from Table 5). The circle represents the distance in the bidentate AlH4−···H2O complex (similar to complex f in Scheme 2). The dashed line is the HH distance in a free H2 molecule (0.74 Å).

values vary from 14 to 27 kcal·mol−1 at 298 K with rate constants lower than 101 s−1.18 Faster reactions, investigated using stopped-flow techniques, such as those of Cp*FeH(dppe),59,170 Cp*MH3(dppe) (M = Mo,64 W152), and [W3Q4H3(dmpe)3]+ (Q = S, Se),217,218,277,283 feature ΔG‡PT values between 10 to 15 kcal·mol−1 at 298 K. Variable-temperature kinetic studies allowing for the derivation of the activation enthalpy, ΔH‡PT, and entropy, ΔS‡PT, associated with the proton-transfer step are even more limited (rows 1−3, Table 6). Variable-temperature kinetics and H/D exchange experiments established that the proton transfer within the dihydrogen-bonded complex formed by the OH groups of triethanolamine and the BH4− anion is the ratelimiting step of H2 evolution in the solid state.243 Its activation parameters, ΔH‡ = 20.1 ± 2.4 kcal·mol−1 and ΔS‡ = −16.8 ± 6.2 cal·mol−1·K−1, appear to be comparable to the analogous values reported for the aqueous hydrolysis of BH4− in neutral water (rows 4 and 5, Table 6).284 The analysis of NMR dynamics for partially protonated rhenium285 and tungsten197 hydrides gave similar activation parameters (rows 6−11, Table 6). However, these studies did not take into account dihydrogen bond formation as the first reaction step. This might be the explanation for the positive ΔH‡ and ΔS‡ values determined for proton transfer to ReH2(CO)(NO)(PR3)2 from 2 equiv of CF3COOH,279,285 which usually works as a dimer.64 Activation parameters of ΔH‡

Figure 26. Correlations between the proton-donor (left) acidity factors Pi and (right) pKa values and the activation free energies ΔG‡ (kcal·mol−1) for the protonation of WH(CO)2(NO)(PEt3)2 in hexane,216 Cp*FeH(dppe) in CH2Cl2,59 and WH(CO)(NO)(PMe3)3 in toluene-d8. Adapted with permission from ref 18. Copyright 2010 John Wiley & Sons. 8566


Chemical Reviews

Review

Table 6. Experimentally (IR or NMR) Determined Activation Enthalpies (ΔH‡PT, kcal·mol−1) and Entropies (ΔS‡PT, cal·mol−1· K−1) for Hydride Protonations 1 2 3 4 5 6 7 8 9 10 11

hydride

HX

CpRuH(CO)(PCy3) Cp*FeH(dppe) Cp*MoH3(dppe) BH4− BH4− CpW(CO)2(PMe3)H CpW(CO)2(PMe3)D ReH2(NO)(CO)(PiPr3)2 ReH2(NO)(CO)(POiPr3)2 ReD2(NO)(CO)(POiPr3)2 ReD(PMe3)4(CO)

(CF3)3COH (CF3)2CHOH CF3CH2OH (HOCH2CH2)3N H2O [Me3C6H4NMe2D]+ [Me3C6H4NMe2H]+ CF3COOH CF3COOH CF3COOD CF3COOD

solvent hexane CH2Cl2 CH2Cl2 H2O CD2Cl2 CD2Cl2 toluene-d8 CD2Cl2 toluene CH2Cl2

ΔHDHB

ΔH‡PT

ΔS‡PT

ref(s)

−7.9 −6.5 −6.1

11.0 2.6 12.3 20 20.6 10.7 11.2 7.7 7.4 7.1 6.3

−19 −44.5 −15.7 −17 −22 −35.2 −30.2 −2.3 −5.5 −10.3 −19.2

60 170 64 243 243,284 197 197 285 285 285 285

Figure 28. Qualitative reaction-energy profiles for hydride−proton exchange (weak acids, solid line; medium-strength acids, dotted line) and proton transfer from strong acids (bold line). Adapted with permission from ref 290. Copyright 2012 Elsevier.

Figure 27. Schematic representation of the evolution of the protontransfer energy profiles with acid−base strength.

corresponds to a ΔG‡ value of 24.4 kcal·mol−1, comparable to the activation energies of slow proton-transfer reactions (vide supra, section 3.7.). Water-soluble CpRu(PTA)2H undergoes H/D exchange with neat D2O (t1/2 = 127 min at 25 °C) having ΔH‡ = 16.2 ± 0.5 kcal·mol−1 and ΔS‡ = −22 ± 2 cal·mol−1· K−1.223 No isotope exchange was observed in CD3OD over the course of 2 weeks, reflecting a weaker DHB and a higher activation energy for proton transfer in this solvent. On the other side of the proton-transfer equilibrium, when formation of a [M(η2-H2)]+ complex is essentially quantitative, the dynamic equilibrium involving neutral species still allows isotope exchange. For example, the nonclassical species generated by the low-temperature reaction of Cp*FeH(dppe) with CF3COOD was found to exist as a mixture containing three isotopomeric ligands: HH, HD, and DD.199 In this case, the formation of HH and DD species can only result from the incorporation of H and D into the acid and hydride, respectively. The reaction of Cp*Mo(CO)(PMe3)2H with 10 equiv of CF3COOD yields an (η2-HD) complex, accompanied by Cp*Mo(PMe3)2(CO)D formation due to H/D exchange.214 The latter was the only species observed in the presence of 1 equiv of the deuterated acid, indicating that, under these conditions, the reversible protonation of the molybdenum hydride occurs to a sufficient extent to allow H/D exchange.214 Exchange between the hydride and H+ of the protonated pyridine substituent was observed for [RuClH(κ2-PPh2py){(κ1PPh2py)2H}]CF3CO2 in acetone-d6 with an activation energy, Ea, of 13.6 ± 2.7 kcal·mol−1.222 Rapid intramolecular proton/ hydride exchange with an activation barrier of 12 kcal·mol−1

3.9. Proton/Hydride and Isotope (H/D) Exchange

Hydride/proton and H/D isotope exchange are well documented for transition-metal hydrides. The generally accepted H/D exchange mechanism for transition-metal hydrides involves protonation of the hydride with formation of the η2-HD complex, its rotation, and finally its deprotonation.221,222,285,287−292 According to DFT calculations, TSs for H−/H+ and H/D exchange are seemingly similar to those for proton transfer (Figure 13). Lying at higher energy, the TSs of exchange differ only by longer HH and HO distances, with rHH being in the range of “stretched dihydrogen ligands”.212 Dihydrogen-bonded complexes can be easily envisaged as intermediates of these exchange processes, although these phenomena can occur even when DHB is too weak to be detected experimentally. Indeed, intermolecular hydride− proton or isotope exchange is observed in NMR spectra even when the dihydrogen bond is not strong enough to allow proton transfer and observation of the [M(η2-H2)]+ species. Thus, isotope exchange was observed for Cp*MoH(PMe3)3 and WH(CO)2(NO)(PR3)2 in the presence of CH3OD.172,226,293 These examples correspond to the case when the transition state is too high and the ionic form ([M(η2H2)]+···[X]− ion pair) is too close in energy to the TS to yield a proton-transfer product of sufficient stability (HX1 in Figure 28). For the H/D exchange of Cp2Mo(H)(OTf) with D2O, a kobs value of 1.14·10−4 s−1 at 45 °C was reported,294 which 8567


Chemical Reviews

Review

and 40 h, respectively).300 This difference can be explained in terms of the stronger trans influence of CO, which weakens IrHb···DOD bonding and attenuates reactivity. Interestingly, both the NH2 group and the hydride ligands of cis,trans-RuH2(κ2-2-NH2CH2Py)(PPh3)2 undergo rapid H/D exchange with D2O or CD3OD to give the fully deuterated complex. In the case of (η 6 -p-cymene)RuH[(S,S)NH2CHPhCHPhNTs], only the NH2 group becomes deuterated, not the hydride ligand (Scheme 21). Such differences in the reactivity of the two complexes can be explained by the varying extent of dihydrogen bonding in intermediates of H/D exchange, as well as the lower proton-transfer barrier for cis,trans-RuH2(κ2-2-NH2CH2Py)(PPh3)2 (Figure 31). The mechanism of H/D exchange in [B10H10]2− hydride resembles that of transition-metal hydrides. The process was shown to occur through protonation of the apical hydride ligand to produce an η2-H2 intermediate, with subsequent H H* shift of the anion prior to deprotonation.290 Hydrogen− deuterium exchange and an equilibrium isotope effect appear to be responsible for the observation of various deuterated amine groups ([ND2BH3]− or [NHDBH3]−) in the amido moiety of dihydrogen-bonded Ca(NH2BH3)2·2NH3.302 In contrast, possible H/D exchange within DHB fragments was not taken into account despite “this ubiquitous interaction ... evident in the structures” of partially deuterated alkali-metal amidoboranes MND2BH3.303 Instead, the authors suggest that evolution of H2 instead of HD occurs through homopolar BHδ−···δ−HB bonds. Another interesting example of the different reactivities of NH and BH groups is provided by ansa-aminoborane 1-N-TMPH− CH2−2-[HB(C6F5)2]C6H4 (Figure 20), which is the product of oxidative H2 splitting by the corresponding frustrated Lewis pair compound.258 Dissolved in CH3OD, it is converted completely into ND···HB, whereas reverse exchange is observed in protic solvents. Notably, no H/D scrambling between nitrogen and boron was observed, suggesting that no HD rotation occurs and that the basicity of the BH ligand is low. The inverse kinetic isotope effects (KIEs), kHH/kHD, were determined from measurements of the proton-transfer reaction rates using HX and DX acids (Table 7).281,282,304,305 DFT calculations on Cp*MoH(CO)(PMe 3 ) 2 complexes of (MeOH)3290 showed that the transition state becomes lower in energy upon deuterium introduction. The proton-transfer barrier is also reduced, leading to the inverse kinetic isotope effect observed experimentally for the protonation of transitionmetal hydrides.

was observed for the endo isomer of trans-[HFe(PNHP) (dmpm)(CH3CN)](BPh4)2, which was obtained by ligand protonation with 1 equiv of p-cyanoanilinium tetrafluoroborate in acetone-d6 at −80 °C.295 These activation energy values are similar to those for fast proton transfer (vide supra, section 3.7.), confirming the similarity of the two processes. Kinetic studies of the deuterium-exchange reactions in substituted bis(cyclopentadienyl)zirconium amido hydride compounds (η5-C5Me4H)2Zr(NHR)H (R = tBu, NMe2, Me, H) under 4 atm D2 revealed that the rate of intramolecular exchange is higher than that of the intermolecular process and provided experimental evidence for the intermediacy of an η2H2 complex.296 An equilibrium isotope effect (EIE) of 0.41 was obtained, showing the preference for the MH/ND isotopomer (C5Me4H)2Zr(H)(NDtBu) over MD/NH. An EIE of 0.17− 0.24 at −20 °C was estimated for the [MD(NH)]+ ⇆ [MH(ND)]+ equilibrium of isotopomers of [CpC5F4NFeH(PtBu2NtBu2H)]BArF4,234 a value similar to the EIE of 0.11−0.23 estimated at −20 °C for HD isotopomers of the complex [MnH(CO)(PPh2NBn2H)(bppm)]+ [bppm = (PArF2)2CH2].237 A similar exchange process has been observed for the closely related [CpC6F5FeH(PtBu2NBn2H)]+, [CpFeH(PPh2NBn2H)]+, and [CpFeH(PPh2NPh2H)]+ complexes.236 Interestingly, the intermolecular H/D exchange between amine groups of these complexes was evident from the formation of HD isotopomers upon exposure of [FeH(NH)]+ species to mixtures of H2 and D2.234,236 Although the deuterated species have lower energy than the H isotopomers, the XD/MH isotopomers are energetically favored over the MH/XD isotopomers (Figure 29)234,237,290,297

Figure 29. Qualitative diagram illustrating the origin of the kinetic and equilibrium isotope effects in proton transfer to metal hydrides.

as a result of the larger zero-point energy differences of the X H/X−D bonds compared to the MH/M−D bonds.298 To surmount this difference and allow deuterium incorporation into a metal hydride, an excess of D acid is usually used. The protonation/deprotonation cycle involving the hydride [(η6-p-cymene)RuH(κ2-N,N-dmobpy)](BF4) (dmobpy =4,4′dimethoxy-2,2′-bipyridine)292 and model complex [RhH2Cl(PR3)3]287 was studied as a part of the H/D exchange reactions catalyzed by these complexes.292,299 The protonation of hydride ligands by hydrated H3O+ (H+ solvated by three water molecules) is shown to occur through dihydrogen-bonded adducts, which can easily convert to dihydrogen species (Figure 30). The reverse reaction−deprotonation of η2-H2(HD) − enables the H/D exchange to occur. In the case of the ruthenium complex, this cycle is coupled with transfer hydrogenation of ketones, allowing selective isotope labeling at the Cα of the alcohol products.292 On a related note, the two hydrides in trans-Ir(CO)(Cl)(H)2(TPPTS)2 undergo slow H/D exchange with D2O at different rates (t1/2), with the more shielded hydride Ha trans to Cl exchanging two times faster than Hb trans to CO (t1/2 = 20

4. MH···Y HYDROGEN BONDS AND PROTON TRANSFER FROM TRANSITION-METAL HYDRIDES The reactivity of transition-metal hydrides as a proton source is well appreciated, and the most recent review, by R. Morris, is included in this special issue.306 These processes are mediated by MH···Y hydrogen bonds involving cationic or neutral transition-metal hydrides. 4.1. Transition-Metal Hydrides as Proton Donors in Hydrogen Bonds

The structural parameters and spectroscopic features of hydrogen bonds involving transition-metal hydrides as proton donors, MHδ+···Y, are, in general, similar to those of classical hydrogen bonds.17,25,26 The formation of such hydrogen bonds between a cationic hydride (e.g., [Cp* 2 OsH] + or [WH5(dppe)2]+) and trifluoroacetate anion as a proton 8568


Chemical Reviews

Review

Figure 30. Optimized structures and relative Gibbs free energies in water of the species involved in proton transfer to [(η6-p-cymene)RuH(κ2-N,Ndmobpy)](BF4) and H/D exchange. Distances are in Å. Data from ref 292.

Scheme 21a

Table 7. Experimentally Determineda Kinetic Isotope Effects for Protonation of Transition-Metal Hydrides in THF hydride Cp2WH2 (PP3)FeH2

FeH2(dppe)2

RuH2(dppe)2 [W3S4H3(dmpe)3]+

acid

kHH/kHD

ref

HCl HCl HBr CF3SO3H HCl HBr CF3SO3H HCl CF3COOH HClb

0.39 0.62 0.64 0.45 0.36 0.55 0.21 0.38 0.80 0.97

305 282

281

304 283

Stopped flow with UV−visible detection or cyclic voltammetry. bIn MeCN. a

a

DX = D2O, CD3OD.

the base was confirmed by the lower-frequency shift of νMH bands in the presence of excess base (e.g., ΔνOsH = −25 cm−1).308 Phosphine oxides are excellent bases for hydrogenbond studies, combining high proton-accepting abilities and high hydrogen-bond-formation constants with rather low pKa(YH+) values. The shift of their νPO bands to lower frequencies in the presence of proton donors (ΔνPO = 10−20 cm−1) can be used for the detection of MH···Y bonds in solution308 and the solid state.310 Slightly larger coupling constants, JHa−Hb, in dicationic iridium trihydride than in the monocation (Scheme 22) at similar temperatures have been interpreted as an indication of intramolecular IrH···N hydrogen bonding between the hydride ligands and the piperidyl nitrogen.311 The acidity of metal-hydride bonds is increased by oxidation,312,313 which allows the formation of a weak Mo H···F hydrogen bond in [Cp*Mo(PMe3)3H]PF6, as confirmed by X-ray data and theoretical calculations.172 The participation of an η2-H2 ligand in intermolecular hydrogen bonding with an external neutral base was studied experimentally in the case of trans-[Ru(DMeOPrPE)2H(η2H2)]+ [DMeOPrPE = 1,2-bis(bis(methoxypropyl)-phosphino)ethane].314 1H NMR monitoring revealed a progressive

Figure 31. Structures and relative energies of the species involved in H/D exchange with D2O of the hydride ligand in the model complexes (η 6 -C 6 H 6 )RuH(NH 2 CH 2 CH 2 N Ms ) and cis,trans-RuH 2 (κ 2 -2NH2CH2Py)(PMe3)2. The relative free energies and electronic energies (in parentheses) are given in kcal·mol−1. Data from ref 301.

acceptor causes the νOCO bands to shift by 20−25 cm−1.307−309 Involvement of the metal-hydride bond in the interaction with 8569


Chemical Reviews

Review

300 ms for the low- and high-field resonances, respectively. Xray analysis revealed the O and H atoms to be at a distance [2.33(6) Å] indicative of hydrogen bonding. The first data on hydrogen bonds of the type MHδ+···Y were reported rather recently.317,318 The IR spectroscopic and quantum chemical (DFT/M05) features of these hydrogen bonds were studied in the case of the well-known hydride complexes CpM(CO)3H (M = Mo, W) interacting with organic bases (Py, R3PO, NEt3) and the boron hydride H3BNEt3. The interaction with bases caused the appearance of low-frequency shoulders/bands of νCO and νMH vibrations (ΔνCO = 4−12 cm−1 and ΔνMH = 9−12 cm−1). These are very weak bonds with ΔH values of less than −3.5 kcal·mol−1,317 and the calculated MH···Y distances (2.51−2.48 Å)119,317 are comparable to the CH···Y contacts in hydrogen-bonded complexes.180,319−322 DFT calculations119,317 have shown that the MH bond polarization in CpMH(CO)3 is similar to that in HCF3.180 As expected, the decrease in the MH bond polarization [Δpol(MH), Table 4] caused by hydrogen bonding reflects an increase in positive charge on the metal-bound hydrogen (by 0.053−0.084 units), as expected for a proton donor. However, in the case of MH···N bonds, this change in polarization occurs at substantially lower interaction energies and at notably lower energies of n(Y) -to-σ*(MH) donation than in the complexes of Hal3CH (Hal = F,180 Cl323) with Py and NH3.317 This facile polarization allows for Mδ−−Hδ+ heterolytic splitting. Deprotonation of these hydrides occurs readily, placing the CpMH(CO)3 complexes on par with mediumstrength OH proton donors in terms of the pKa(CH3CN) scale. Compare, for example, the pKa(CH3CN) values of 13.9 and 16.1 in CH3CN for CpMoH(CO)3 and CpWH(CO)3, respectively, to the values of 20.55 for (CF3)3COH, 16.66 for 2,4-dinitrophenol, and 11.00 for 2,4,6-trinitrophenol,324 for example.

Scheme 22

downfield shift (up to 0.170 and 0.225 ppm) of the η2-H2 resonance in the presence of increasing amounts of pyridine Noxide and acetone. A hypsochromic shift of 14 nm was observed in UV−visible spectra employing 4-nitropyridine Noxide as a hydrogen-bond acceptor. This effect has also been observed in the solid state, with the salt [Os(η2-H2)(CH3CN)(dppe)2](BF4)2 featuring a hydrogen bond between a fluorine atom and the dihydrogen ligand with a H···F distance of ca. 2.4 Å (the van der Waals radii of hydrogen and fluorine are 1.2 and 1.5 Å, respectively).192 Hydrogen bonds in neutral transitionmetal hydrides in the solid state were revealed by analysis of the Cambridge Structural Database, which also gave a number of structures with intermolecular MH···O hydrogen bonding, mainly to CO groups.315 Comparison of the distances suggests that the strength of the MH···O hydrogen bonds is similar to that of CH···O hydrogen bonds. An intramolecular hydrogen bond of the type MH···OC involving a neutral hydride complex was shown for WH3(η1OCOMe)(dppe)2 (Scheme 23).316 It causes deshielding of the Scheme 23

4.2. Dihydrogen Bonding between Two Hydride Complexes MH···HM′ and Proton Transfer

The most challenging task in this area of research was to detect a HB between two hydride complexes, one of which would be a proton donor and the other, a proton acceptor. The experimental evidence for such DHBs was provided by VTNMR and VT-IR spectroscopic studies of the pincer hydrides (tBuPCP)MH (M = Ni, Pd) and the tungsten complex

MH···Y hydride ligand, whose 1H NMR resonance occurs at a substantially lower field (δ 2.92 ppm) relative to the resonances of the other hydrides (δ −2.78 ppm). Despite this difference, their T1 relaxation times (298 K) are quite close, being 255 and

Figure 32. (Left) Optimized structure of the DHB adduct between (tBuPCP)NiH and CpWH(CO)3 with selected bond lengths (in Å). Hydrogen atoms of the tBuPCP ligand are omitted for clarity. (Right) Fragment of the molecular graph of the system. Data from ref 174. 8570


Chemical Reviews

Review

Scheme 24

CpWH(CO)3.174,176 The changes observed in 1H NMR and IR spectra were similar to those for the interactions of (tBuPCP)MH with alcohols174,176,325 and of CpWH(CO)3 with bases,317,318 although they were significantly smaller, in accord with their very low DHB formation enthalpies: ΔH°DHB = −1.0 and −1.2 kcal·mol−1 for nickel and palladium, respectively. The computational (DFT/M06) analysis revealed interesting peculiarities of these complexes. The hydridic hydride ligand of (tBuPCP)MH appeared to interact with both the “protic” hydride ligand and one of the Cp ring hydrogens of CpWH(CO)3 (see Figure 32). The length of the HPd···HW bond (2.43 Å) was found to be smaller than the HNi···HW distance (2.58 Å), but the HPd···HCp bond (2.20 Å) was found to be slightly longer in comparison to HNi···HCp (2.16 Å), in agreement with the different energies of these contacts derived by AIM analysis and overall complexation energies. These DHB complexes are stable only at low temperatures and gradually liberate H2 above 230−250 K.174,176 The activation enthalpy and entropy values are substantially higher for the nickel hydride than for the palladium hydride: ΔH‡Ni = 8.7 ± 0.8 kcal·mol−1 and ΔS‡Ni = −41 ± 3 cal·mol−1·K−1 versus ΔH‡Pd = 6.5 ± 1.5 kcal·mol−1 and ΔS‡Pd = −35 ± 6 cal·mol−1· K−1. The reaction mechanism (Scheme 24) suggested on the basis of experimental and theoretical data includes proton transfer as the rate-determining step. The approach of two metal-bound hydrogens affords a H2 molecule bridging the two metal atoms in a μ,η1:1-end-on fashion. The η2-H2 side-on species are more stable, but they are only intermediates evolving H2. According to the X-ray diffraction analysis, the reaction product features a carbonyl ligand bridging the two metal centers in a rather unconventional “isocarbonylic” (μ−κ,C:κ,OCO) mode.

Figure 33. Computed structures of identified intermediates for rhodium−rhenium-catalyzed hydroformylation. Data from ref 326.

pyridine ligand are the proposed reaction steps transforming the (κ2-P,N-L)RhCl(CO) complex into (κ3-P,N,C-L)RhCl(CO) (Figure 34).330 DFT calculations revealed that the

4.3. Reactions through MH···Y Bonds

Figure 34. DFT-calculated free-energy profiles (ΔG298 K in kcal·mol−1) of two possible mechanisms for proton transfer from the ortho-phenyl position to the dearomatized ligand backbone. Methyl groups instead of tert-butyl groups on the phosphane unit were used. Adapted with permission from ref 330. Copyright 2015 John Wiley & Sons.

The “acidic” hydrides CpMH(CO)3 (M = Mo, W) and HM(CO)5 (M = Mn, Re) have been used as parts of bimetallic catalytic systems for the hydroformylation of alkenes.326−328 The Rh4(CO)12 precatalyst was shown to transform into unsaturated Rh(CO)4(RCO), which forms weak hydrogen bonds with the acidic hydrides CpMH(CO)3. Interestingly, in this case, the hydrogen-bonded complex is also stabilized by CO···HM and CO···HCCp interactions,327,328 but in our view, only the more reactive MH bonds are relevant to the reaction. This assertion is supported by the similar activity of Rh(CO)4(RCO)/HM(CO)5 system featuring only the Rh··· HM interaction within reaction intermediates (see Figure 33).326,329 CH activation at the phenyl group and subsequent proton transfer from (L)RhH(CO) hydride to the dearomatized

unfavorable direct proton transfer to the hydride species becomes nearly barrierless when mediated by a tBuOH molecule involved in RhH···OH···C hydrogen bonds. In another example, the oxidation of (κ2-P2N2)MnIH(PP)(CO) is followed by deprotonation of the manganese(II) hydride, which first transfers a proton intramolecularly to the pendant amine nitrogen, which is then deprotonated by an exogeneous base (Scheme 25).331 This intramolecular protontransfer step is a bottleneck of electrocatalytic oxidation of H2 8571


Chemical Reviews

Review

Scheme 25a

a

Reprinted with permission from ref 331. Copyright 2015 American Chemical Society.

of p-nitrophenol (HOAr) in THF is a possible cause of the complete deprotonation of [Cp*WH4(dppe)]BF4 by p-nitrophenolate.152 In this case, only neutral forms are observed, in contrast to the equilibrium mixture of molecular Cp*WH3(dppe)·HOAr and ionic [Cp*WH4(dppe)]+·OAr− hydrogen-bonded species found in toluene.152 The study of solvent effects in the [PP3RuH2]/substituted phenol system117 provided experimental evidence suggesting that, for solvents with comparable polarities, the proton transfer is more facile in a protic solvent (CH3OH) than in an aprotic one (CH3CN). The basis of this effect is the cooperative enhancement of primary hydrogen-bonding interactions imposed by the additional hydrogen bonding of CH3OH with the solutes. The same, but obviously weaker, effect is caused by CH acids such as dichloromethane.117 Interestingly, the rate constants for protonation of the hydride cluster [W3S4H3(dmpe)3]+ correlate with the acid strength (HCl, HBF 4 , CF 3 COOH) in CH 2 Cl 2 218 and anhydrous acetonitrile.283 However, addition of water makes the reaction slower, with the lowest rate being observed for the hydrated proton, which is supposed to be the strongest acid.283 This effect was attributed to the disruption of the H+···solvent hydrogen bonds representing the most important contribution to the activation barrier of the protonation process. Also of interest is the observation that the high polarity of MeCN almost cancels the KIE for [W3S4H3(dmpe)3]+ protonation by HCl (Table 7).283 For CpMH(CO)3 complexes interacting with organic bases, a theoretical study of the solvent effect (hexane, CH2Cl2, THF, and CH3CN) using the conductor-like polarizable continuum model (CPCM) revealed the expected destabilization of the hydrogen-bonded MH···Y complexes and lowering of the deprotonation barrier on going from the gas phase to solution, at which point the proton transfer became thermodynamically more favorable.317 Surprisingly, IR measurements indicated easier proton transfer in nonpolar hexane than in other more polar coordinating solvents. It turned out that, in this case of very weak hydrogen bonds, the specific solvation is particularly important at the hydrogen-bond-formation step. The strength of the MH···Y hydrogen bonds [Y = pyridine, R3PO (R = nC8H17, NMe2), NEt3] is similar to that of MH···S bonds (S = THF or CH3CN, with H-bonding to N or O, respectively). Even with a weak CH acid (SH, Scheme 26), CH2Cl2 forms hydrogen bonds with bases with a strength comparable to that of MH···Y bonds. Hence, these competitive weak interactions lower the activities of either the hydrides or the bases in the main proton-transfer reaction, as depicted in Scheme 26. These

by metallocomplexes that have pendant amines in the diphosphine ligand. The computed activation barrier (ΔG‡) for intramolecular proton transfer from the metal to the pendant amine is 20.4 kcal·mol−1 for [(PPh2NBn2)MnIIH(CO)(bppm)]+ and 21.3 kcal· mol−1 for [(PPh2NMe)MnIIH(CO)(bppm)]+. The high barrier appears to result from both the unfavorable proton transfer from MnH to nitrogen (thermodynamically uphill by 9 kcal· mol−1 for [(PPh2NBn2)MnIIH(CO)(bppm)]+ because of a mismatch of 6.6 pK a units) and the relatively long manganese−nitrogen separation in the MnIIH complexes.331 Much lower ΔG‡ values of 10.7−12.2 kcal·mol−1 were reported for intramolecular proton transfer from the related nickel hydrides [(PCy2NBn2)2NiIIIH]2+.232

5. SOLVENT EFFECTS Solvent properties have a great influence on hydrogen-bond formation and proton-transfer equilibria. The latter is particularly sensitive to medium polarity because of the much stronger electrostatic interaction operating in ionic species. In a sense, the environment exerts a nonspecif ic influence, with higher-polarity solvents assisting the proton transfer. For example, a spectroscopic and theoretical study of proton transfer to (PP3)RuH2 showed a shift of the equilibrium to the ion-pair product [(PP3)RuH(η2-H2)]+···[ArOHOAr]− caused by a gradual increase in medium polarity, leading to the formation of the solvent-separated ions [(PP3)RuH(η2-H2)]+// [ArOHOAr]− at high dielectric permittivity (ε > 20).117 Proton transfer from (CF3)2CHOH or (CF3)3COH to Cp*RuH3(PCy3) yielding [Cp*Ru(H)2(η2-H2)(PCy3)]+[ORF]− was observed at low temperatures because of the high dielectric permittivity of the Freon mixture CDCl 2F/CDF 3 (the dielectric permittivity ε of a 1:1 CHF2Cl/CHF3 mixture increases from about 20 at 170 K to 45 at 95 K), whereas no reaction occurs under similar conditions in toluene.35 Moreover, kinetic data obtained for WH(CO)2(NO)(PR3)2216 and Cp*FeH(dppe)170 revealed the proton-transfer activation free energies ΔG‡ to be systematically higher in hexane (ε = 1.8332) than in the more polar CH2Cl2 (ε = 8.93 at 298 K332) (Figure 26).18 Theoretical studies on the protonation of CpRuH(CO)(PCy3),60 Cp*FeH(dppe),170 and Cp*MoH3(dppe)64 showed a decrease in the activation energy for the conversion of dihydrogen-bonded complexes MH···HX···HX to [M(η2-H2)]+···[XHX]− ion pairs on going from the gas phase to a heptane solution and then further to a CH2Cl2 solution. Specif ic solvent influences arise from, among other effects, the solvent’s ability to serve as a hydrogen-bond donor or acceptor. Hence, the proton-transfer equilibrium positions are very different in THF and CH2Cl2, with the former having a rather high proton-accepting ability (Ej = 1.04) that thwarts the formation of the hydrogen bond of interest and hampers proton transfer.51,117,154 Such highly favorable specific solvation

Scheme 26

8572


Chemical Reviews

Review

(Figure 35) allows for the lowering of the energies of both the intermediates and the transition states.338

secondary effects shift the proton-transfer equilibrium and lead to the counterintuitive hampering of proton transfer upon solvent change from hexane to moderately polar CH2Cl2 or THF. The same scheme is operative for dihydrogen bonding, when Y is a basic metal hydride. Obviously, the coordinating solvents can not only dampen the hydride−proton-donor interaction, but also crucially affect the stability of the ionic products (right-hand part of Scheme 26). For example, when protonation of Cp*Mo(dppe)H3 by trifluoroacetic acid is carried out in nonpolar, weakly coordinating solvents such as benzene or toluene, the excess acid allows formation of the homoconjugate anion, [CF3COO(HOOCCF3)n]−, which increases charge separation and stabilizes Cp*Mo(dppe)H4+ toward H2 loss. In contrast, the hydrogen-bonded ion pair Cp*Mo(dppe)H4+···−OOCCF3 evolves to Cp*Mo(dppe)H2(O2CCF3) through the replacement of H2 by the more basic trifluoroacetate ligand in THF or MeCN, the dihydride product being inert toward excess acid in these solvents.154 The low-temperature protonation of Cp*MoH(CO)(PMe3)2 by HBF4·Et2O produces the η2-H2 complex in THF, but in CH2Cl2, this cation exists as a classical dihydride.214 The explanation for this striking difference was provided by DFT calculations. Both solvents, THF and CH2Cl2, appeared to act as CH-proton donors whose binding to BF4 affects the strength of the cation−anion interaction and, ultimately, the possibility of H2 homolytic cleavage. The assistance of solvent molecules acting as bifunctional catalysts in proton-transfer reactions is supported by strong computational evidence.225,287,292,333−336 Some such systems featuring dihydrogen bonds involving H2O/H3O+ clusters were already mentioned above (Scheme 12, Figure 30, Figure 31). The reversible 1,3-shift of the NH proton to the iridium observed for a series of five-coordinate iridium(I) phosphine complexes with protic N-heterocyclic carbene ligands (Scheme 27) is suggested to proceed by a similar water-assisted proton-

Figure 35. Computed structures of the tertiary hydrogen-bonded complexes (left) NiH22+·H2O·B and (right) NiH+·H2O·BH+ of [Ni(PCy2NBn2H2)2]2+ interacting with water and bases (B = Et3N, aniline). Data from ref 338.

6. IMPLICATIONS OF DIHYDROGEN BONDING AND PROTON TRANSFER IN DIFFERENT PROCESSES 6.1. Stoichiometric Reactions

There are many more examples implicating classical hydrogen and dihydrogen bonding in the reactivity of metal hydrides. Intramolecular NH···HOs bonding in triosmium clusters of the type H2Os3(CO)10L (L = NH2Et, NHEt2, HNCPh2) obtained by ligand addition to H2Os3(CO)10 directs the stereochemistry of the products. Only one isomer is formed, having the amine or imine ligand cis to the terminal hydride, and no reaction takes place in the case of L = NEt3.340,341 Intermolecular dihydrogen bonding between nBu4NBH4 and the HO group of 2-hydroxycycloalkanones was shown to control the stereoselectivity of the reduction reaction.342 In the same manner, dihydrogen H3BH···HN bonding to the pincer ligand in ( R PNP)FeCl 2 (CO) [ R PNP = κ 3 -N,P,P-HN(CH2CH2PR2)2; R = iPr, Cy] is suggested to favor hydride delivery cis to the NH bond, initially yielding a mixture of cis and trans isomers of (RPNP)FeHCl(CO) (Scheme 28) in a 6:1 ratio instead of a statistical 1:1 ratio.275 The trans-NH-FeH isomer appears to be thermodynamically preferred, probably because of a stronger proton−chloride interaction. In the solid state, this complex packs into dimers with an intramolecular Cl···H(N) distance of 2.58(3) Å and a pair of intermolecular Cl···H(N) hydrogen bonds of 2.78(3) Å. Accordingly, the N H vibrations of the starting dichlorides shift to lower frequencies in trans-hydridochlorides.275 Treatment of (RPNP)FeHCl(CO) (Scheme 28) with excess BH4 − gives the aforementioned mononuclear complex (iPrPNP)FeH(η1-HBH3)(CO) (Figure 24). The latter was shown to be an active catalyst for acceptorless alcohol dehydrogenation, although its entry into the catalytic cycle was neither discussed nor studied.343 In aprotic solvents, BH3 removal from MH(BH4) complexes can occur in the presence of an appropriate trapping agent such as PR3,344 aniline,345 or even substrate ketone or aldehyde.345 However, in the presence of alcohol, dihydrogen bonding and proton transfer to

Scheme 27

relay mechanism.337 The experimental value of ΔG‡298 K estimated in the presence of water is 18.3 kcal·mol−1, on par with the barrier calculated for the water-assisted mechanism [ΔG‡298 K(CH2Cl2) = 18.4 kcal·mol−1]. Weaker hydrogen bonding NH···X− with less coordinating anions promotes shift of the equilibrium, with the equilibrium constants increasing in the order Cl < BF4 < PF6 < CB11H12 < BArF4. The electron-donating phosphine ligand further tunes this equilibrium, stabilizing the iridium(III) hydride species. A first-order dependence on water was observed experimentally338 for intermolecular proton exchange between the isomers of the [Ni(PCy2NBn2H2)2]2+ complex, which is the electrocatalyst for H2 oxidation.232,339 Computations suggested that water facilitates the deprotonation of [Ni(PCy2NBn2H2)2]2+ by shuttling the proton between the nickel hydride and the base. The formation of tertiary hydrogen-bonded complexes 8573


Chemical Reviews

Review

Scheme 28a

a

Data from ref 275.

catalytic reaction intermediates that we consider to be somewhat underestimated. One of the first published examples was the catalytic activity of the indenylruthenium hydride (η5-C9H7)Ru(dppm)H for the hydration of nitriles, a reactivity not observed for the chloro analogue (η5-C9H7)Ru(dppm)Cl.355 DFT calculations attributed this reactivity to RuH···HOH dihydrogen bonding in the transition state, which lowers the reaction barrier in the case of the hydride and is not possible in the chloro system. A similar DHB-promoting effect is believed to be responsible for the catalytic activity of the TpRuH(PPh3)(MeCN) complex in acetonitrile hydration, where again the chloro analogue does not show such activity.355 (iPrPNP)FeH(η1-HBH3)(CO) (Figure 24) and related iron complexes catalyze acceptorless alcohol and hemiacetal dehydrogenation,343 as well as the dehydrogenative coupling of alcohols. The reaction mechanism proposed on the basis of experimental data and computational modeling relies on metal−ligand cooperation.343 The dehydrogenation sequence typical for bifunctional catalysts involves intermediates stabilized by dihydrogen (CH···HFe) and hydrogen (NH···O) bonds (Scheme 30). Dihydrogen elimination begins with the formation of a cyclic hydrogen-bonded intermediate that transforms into the Fe(η2-H2) complex before eventually losing H2 and regenerating the hydride amido species (PNP)FeH(CO) (Scheme 30). Taken in the reverse order, these reaction steps should be operative in CO hydrogenation, as was indeed shown for the analogous ruthenium complexes (κ3-PhPNP)RuHCl(CO) and trans-(κ3-PPhNPPh)RuX2(CO) [X = H, Cl; κ3-PhPNP = κ3P,N,P-HN(CH2CH2PPh2)2] catalyzing the hydrogenation of fluorinated esters (Figure 36).356 An ab initio molecular dynamics study335 on the transfer hydrogenation of formaldehyde catalyzed by the model ruthenium complex (η6-C6H6)Ru(κ2-O,N-OCH2CH2NH) in methanol showed the feasibility of a concerted solventmediated mechanism where the transfer of the NH proton to the keto oxygen is mediated by the MeOH molecule (Scheme 31). Addition of H2 to the 16-electron Ru centers and subsequent heterolytic cleavage in diamine and diamine/diphospine model systems (Figure 37)357 confirmed the importance of alcoholic solvent observed for many bifunctional Noyori-, Morris- and Shvo-type catalysts. The alcohol is found to stabilize H2 coordination and lower the activation energy of subsequent rate-determining η2-H2 heterolytic cleavage. In this way, the catalyst becomes ready to coordinate a ketone substrate molecule and perform hydrogenation possibly through an alcohol-assisted cycle (Scheme 31). Cooperative dihydrogen (RuH···HO) and hydrogen (NH···O) bonding also enables the reverse reaction of HH bond formation (Figure 37).357 Experimental catalytic runs confirmed that the increased basicity of the tertiary nitrogen center in the ligand makes the

coordinated BH4 should be invoked as the mechanism of precatalyst activation. The reaction of trans-RuH(η1-BH4)(binap)(1,2-diamine) with the strong acid HBF4 followed by addition of iPrONa/iPrOH in toluene was shown to yield [trans-RuH(binap)(1,2-diamine)(ROH)]+[(RO)(ROH)n]− in which the loosely associated ion pair of the solven to the cation with the alkoxide species exhibits dynamic behavior on the NMR time scale.346,347 In the case of (PP3)RuH(η1-BH4), the formation of the [(PP3)RuHeq(η2-H2)]+ cation is observed not only in the reaction with strong sulfuric acid348 but in the presence of CF3COOH and even of fluorinated alcohols.147 A combined spectroscopic and DFT study suggests that the proton transfer occurs through acid-mediated dissociation of the BHbrRu bond preceded by dihydrogen-bonded intermediates in which the proton donor interacts with either the RuH or η1-BH4 moiety (Scheme 29).147 Hence, in the absence of added acid or Scheme 29. Two Reaction Roots for (PP3)RuH(η1-BH4) Alcoholysis

base, the reaction of the [MH(BH4)] complex with the alcoholic solvent could generate the cationic five-coordinate hydrido ruthenium complex, as well as H2, RO−, and B(OR)3, providing a route to catalytically active species. 6.2. Catalytic Processes

Proton-responsive ligands occupy a significant place in the burgeoning area of transition-metal catalysts supported by noninnocent ligands. Such complexes have been the subjects of several reviews,349−353 including those focused on the electrocatalysis of H2 oxidation and production by hydrogenase models.230,354 Presented here are some examples highlighting the role of hydrogen- and dihydrogen-bonded species as the 8574


Chemical Reviews

Review

Scheme 30. Hydrogen-Bonded Intermediates (Red and Orange Framed) of the MeOH and MeOCH2OH Dehydrogenation and Reaction Pathway for H2 Evolution Computed for the Model Catalyst {N(CH2CH2PMe2)2}FeH(CO) (Blue Framed)a

a


Figure 37. Potential energy surface for MeOH-mediated H2 addition and heterolytic cleavage in the diamine model system. ΔEZPE in kcal· mol−1. Adapted with permission from ref 357. Copyright 2005 American Chemical Society.

Figure 36. Possible catalytic cycle (ΔG298 K, in kcal·mol−1) for the formation of trifluoroacetaldehyde methyl hemiacetal. R = CH3. Reprinted with permission from ref 356. Copyright 2013 American Chemical Society. Hydrogen-bonded intermediates similar to those in Scheme 30 are red framed.

Scheme 32

Scheme 31. Snapshots of the Solvated Catalyst Substrate System in Transfer Hydrogenationa

accompany all of the steps of the hydrogenation of the ketone, thereby lowering the activation energies. However, a sixmembered pericyclic transition state remained unaffected. Rather, the protic solvent molecule became involved as the proton source in the release of the hydrogenation product, suggesting that the catalytic reaction might not necessarily proceed through the 16-electron amido complex.360 The same six-membered transition state was computed recently for the asymmetric transfer hydrogenation of acetophenone catalyzed by the amine(imine)diphosphine iron carbonyl (Scheme 33).361 Detailed computational analysis carried out in dielectric continuum, combined with recent experimental observa-

a Reprinted with permission from ref 335. Copyright 2007 American Chemical Society.

catalyst more active, in agreement with the proposed reaction mechanism (intramolecular proton transfer as the ratedetermining step).357−359 Moreover, the hydrogenation in an aprotic solvent (THF) was found to be substantially slower. More recent computational work has taken into account both nonspecific (polarity) and specific (hydrogen-bonding) solvation of the analogous ruthenium catalyst (Scheme 32).360 Hydrogen-bonded alcohol (iPrOH) molecules were shown to 8575


Chemical Reviews

Review

entails lengthening of the participating bonds and their additional polarization (repolarization) due to the redistribution of the electron density. Thus, the formation of a dihydrogen bond allows activation of both the MH and XH bonds in one step, facilitating proton transfer and preparing these reactive fragments for further transformations. Thus, dihydrogen bonds can control hydride structure and reactivity and even affect the selectivity of reactions such as hydrogen exchange, alcoholysis, aminolysis, hydrogen evolution, and double-bond reduction. Well-studied stoichiometric processes of dihydrogen bonding, proton transfer, and H2 evolution have always been thought to be important in transition metals catalyzed hydrogenation and dehydrogenation. Recent mechanistic studies add weight to this argument. Dihydrogen-bonding interactions are similarly important in the topical field of frustrated Lewis pair chemistry that was only briefly mentioned in this review. Dihydrogen bonds have a good combination of strength and directionality, potentially allowing for the rapid self-organization of molecular building blocks into extended regular structures and defining the physical properties of materials, opening a way for the solidstate preparation of new materials. Among the potential applications of this knowledge is the development of hydrogen-storage materials.

Scheme 33

tions,362 has allowed revision of the widely accepted metal− ligand bifunctional mechanism of the catalytic hydrogenation of acetophenone by trans-[RuCl2((S)-binap)((S,S)-dpen)], that is, Noyori’s catalyst.363 Thus, hydride transfer (the enantio- and rate-determining step) still proceeds in an outer-sphere manner as originally suggested; however, only one bond is cleaved (RuH), yielding a NH + ···O −CHPhMe ion pair. The reaction product originates from proton transfer to the generated (R)-1-phenylethoxide anion from the η2-H2 ligand of the cationic Ru complex and not from the NH moiety (Scheme 34).362 This reaction sequence362 strongly resembles that suggested for [(PNP)MH]-catalyzed hydrogenation (Figure 36), unifying the reaction mechanisms for two types of complexes.343,356 Scheme 34. Revised Catalytic Cycle for the Asymmetric Hydrogenation of Acetophenone Catalyzed by trans[RuH2((S)-binap)((S,S)-dpen)]a

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Natalia V. Belkova graduated from M. V. Lomonosov Moscow State University (MSU) and received a Ph.D. degree at INEOS. Her Ph.D. work was awarded an Academia Europeae award for young scientists from Russia in 1997. After a two-year postdoctoral fellowship at St. Jude Children’s Research Hospital (Memphis, TN), she returned to INEOS where she received a D.Sc. degree and is now a leading researcher. In 2016, she was elected a Professor of the Russian Academy of Sciences. Her research interests are in hydrogen bonding in organometallic chemistry and their implications in the mechanisms of reactions involving the migration of hydrogen ions (proton transfer, hydride transfer) and the structure−property relationships of metal complexes.

a


Lina M. Epstein received her Ph.D. degree at MSU and D.Sc. degree at INEOS. In 1985−2000, she was head of the Spectroscopy Group in the laboratory of organometallic chemistry at INEOS. Professor of Chemistry since 1991, she is now leading researcher at INEOS. Her scientific interests are molecular spectroscopy of intermolecular interactions, especially neutral and ionic hydrogen bonding as well their participation in proton-transfer processes, and examination of fundamental problems such as the electronic structure, nature of bonds, and proton-donor or proton-acceptor ability of transition-metal complexes. She has published over 250 papers in Russian and international journals, as well as several book chapters.

7. CONCLUSIONS Hydrogen bonding is one of the most important natural phenomena, at the intersection of chemistry, physics, and biology. The concept and theory of hydrogen bonding were developed throughout the 20th century, yet it is only 20 years ago that hydrogen bonds involving transition-metal hydrides, MHδ−···δ+HX, were discovered. These dihydrogen bonds are very important in hydride chemistry, and presented here are experimental and computational studies of the structural, energetic, and spectroscopic parameters and natures of dihydrogen-bonded complexes of the form MH···HX for a variety of transition-metal hydrides and main-group-element hydrides. Because it is a weak interaction, dihydrogen bonding

Oleg A. Filippov was educated and received his Ph.D. degree at MSU and then joined INEOS in 2002. In 2007−2008, he had a fellowship in Universitat Autònoma de Barcelona in the group of Prof. Agusti ́ Lledós. Now, he is a senior researcher in the laboratory of metal hydrides at INEOS, specializing in computational chemistry and 8576


Chemical Reviews

Review

(8) Park, S.; Ramachandran, R.; Lough, A. J.; Morris, R. H. A New Type of Intramolecular H···H···H Interaction Involving N−H··· H(Ir)...H−N Atoms. Crystal and Molecular Structure of [IrH(η1SC5H4NH)2(η2-SC5H4N)(PCy3)]BF4·0.72CH2Cl2. J. Chem. Soc., Chem. Commun. 1994, 2201−2202. (9) Lough, A. J.; Park, S.; Ramachandran, R.; Morris, R. H. Switching On and Off a New Intramolecular Hydrogen-Hydrogen Interaction and the Heterolytic Splitting of Dihydrogen. Crystal and Molecular Structure of [Ir{H(η1- SC5H4NH)}2(PCy3)]BF4·2.7CH2Cl2. J. Am. Chem. Soc. 1994, 116, 8356−8357. (10) Peris, E.; Lee, J. C.; Crabtree, R. H. Intramolecular N-H X-Ir (X = H, F) Hydrogen Bonding in Metal Complexes. J. Chem. Soc., Chem. Commun. 1994, 2573−2573. (11) Lee, J. C.; Rheingold, A.; Muller, B.; Pregosin, P. S.; Crabtree, R. H. Complexation of an Amide to Iridium via an Iminol Tautomer and Evidence Ir−H···H−O Hydrogen Bond. J. Chem. Soc., Chem. Commun. 1994, 1021−1022. (12) Lee, J. C.; Peris, E.; Rheingold, A.; Crabtree, R. H. An Unusual Type of H···H Interaction: Ir-H···H-O and Ir-H···H-N Hydrogen Bonding and Its Involvement in σ-Bond Metathesis. J. Am. Chem. Soc. 1994, 116, 11014−11019. (13) Shubina, E. S.; Belkova, N. V.; Krylov, A. N.; Vorontsov, E. V.; Epstein, L. M.; Gusev, D. G.; Niedermann, M.; Berke, H. Spectroscopic Evidence for Intermolecular M-H···H-OR Hydrogen Bonding: Interaction of WH(CO)2(NO)L2 Hydrides with Acidic Alcohols. J. Am. Chem. Soc. 1996, 118, 1105−1112. (14) Richardson, T.; de Gala, S.; Crabtree, R. H.; Siegbahn, P. E. M. Unconventional Hydrogen Bonds: Intermolecular B-H···H-N Interactions. J. Am. Chem. Soc. 1995, 117, 12875−12876. (15) Shubina, E. S.; Belkova, N. V.; Epstein, L. M. Novel types of hydrogen bonding with transition metal π-complexes and hydrides. J. Organomet. Chem. 1997, 536−537, 17−29. (16) Epstein, L. M.; Shubina, E. S. New Types of Hydrogen Bonding in Organometallic Chemistry. Coord. Chem. Rev. 2002, 231, 165−181. (17) Belkova, N. V.; Shubina, E. S.; Epstein, L. M. Diverse World of Unconventional Hydrogen Bonds. Acc. Chem. Res. 2005, 38, 624−631. (18) Belkova, N. V.; Epstein, L. M.; Shubina, E. S. Dihydrogen Bonding, Proton Transfer and Beyond: What We Can Learn from Kinetics and Thermodynamics. Eur. J. Inorg. Chem. 2010, 2010, 3555− 3565. (19) Custelcean, R.; Jackson, J. E. Dihydrogen Bonding. Structures, Energetics, and Dynamics. Chem. Rev. 2001, 101, 1963−1980. (20) Crabtree, R. H.; Siegbahn, P. E. M.; Eisenstein, O.; Rheingold, A. L.; Koetzle, T. F. A New Intermolecular Interaction: Unconventional Hydrogen Bonds with Element−Hydride Bonds as Proton Acceptor. Acc. Chem. Res. 1996, 29, 348. (21) Crabtree, R. H. Hydrogen Bonding & Dihydrogen Bonding. In Encyclopedia of Inorganic Chemistry, 2nd ed.; King, R. B., Ed.; John Wiley & Sons: New York, 2006. (22) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 6th ed.; John Wiley & Sons: Hoboken, NJ, 2014. (23) Bakhmutov, V. I. Dihydrogen Bonds: Principles, Experiments and Applications; John Wiley & Sons: Hoboken, NJ, 2008. (24) Arunan, E.; Desiraju, G. R.; Klein, R. A.; Sadlej, J.; Scheiner, S.; Alkorta, I.; Clary, D. C.; Crabtree, R. H.; Dannenberg, J. J.; Hobza, P.; et al. Definition of the Hydrogen Bond (IUPAC Recommendations 2011). Pure Appl. Chem. 2011, 83, 1637−1641. (25) Epstein, L. M.; Belkova, N. V.; Gutsul, E. I.; Shubina, E. S. Spectral Features of Unconventional Hydrogen Bonds and Proton Transfer to Transition Metal Hydrides. Pol. J. Chem. 2003, 77, 1371− 1383. (26) Filippov, O. A.; Belkova, N. V.; Epstein, L. M.; Shubina, E. S. Chemistry of Boron Hydrides Orchestrated by Dihydrogen Bonds. J. Organomet. Chem. 2013, 747, 30−42. (27) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: New York, 1997. (28) Jimenez-Tenorio, M.; Puerta, M. C.; Valerga, P.; Moncho, S.; Ujaque, G.; Lledós, A. Proton-Transfer Reactions to Half-Sandwich Ruthenium Trihydride Complexes Bearing Hemilabile P,N Ligands:

molecular spectroscopy and their application to studies of noncovalent interactions, main-group compounds, and hydrides. Elena S. Shubina was educated and received her Ph.D. degree at MSU and completed a D.Sc. degree at INEOS, where she has been a Professor of Chemistry and the head of the Metal Hydrides Laboratory since 2001. She has been plenary, invited, or keynote lecturer at European and International Conferences on organometallic, coordination, and boron chemistry. She is a member of organizing committees for international conferences on organometallic and coordination chemistry held in Russia and a member of EuCheMS Division on Organometallic Chemistry. Her research interests lie in the fields of physical organoelement chemistry, molecular spectroscopy, noncovalent interactions involving metal complexes, transitionmetal and main-group-element hydrides, and supramolecular compositions. She has published over 150 papers in Russian and international journals, as well as several book chapters.

ACKNOWLEDGMENTS This work was financially supported by the Russian Science Foundation (Grant 14-13-00801). Thanks are expressed to Dr. F. M. Dolgushin (INEOS) for fruitful discussions of crystallographic data. ABBREVIATIONS ArF 3,5-(CF3)2C6H3 bppm κ2-P,P-(PArF2)2CH2331 CP3 κ3-CH3C(CH2CH2PPh2)3 DMAB dimethylamine borane dmpe κ2-P,P-Me2P(CH2)2PMe2 dppe κ2-P,P-Ph2P(CH2)2PPh2 dppm κ2-P,P-(PPh2)2CH2 tBu PCP κ3-2,6-C6H3(CH2PtBu2)2 R P 2NR′ κ2-P,P-coordinated aminodiphosphanes PR2NR′2 κ2-P,P-coordinated cyclic diaminodiphosphanes R PNP κ3-P,N,P-HN(CH2CH2PR2)2 pincer ligands PP3 κ4-P(CH2CH2PPh2)3 PTA 1,3,5-Triaza-7-phosphaadamantane Py pyridine TPPTS P(m-C6H4SO3Na)3 REFERENCES (1) Burg, A. B. Enhancement of P-H Bonding in a Phosphine Monoborane. Inorg. Chem. 1964, 3, 1325−1327. (2) Rudolph, R. W.; Parry, R. W. Fluorophosphine Ligands. IV. The Apparent Base Strengths of Difluorophosphine, Trifluorophosphine, and Phosphine toward the Lewis Acid Borane. J. Am. Chem. Soc. 1967, 89, 1621−1625. (3) Titov, L. V.; Makarova, M. D.; Rosolovskii, V. Y. Guanidinium Borohydride. Dokl. Akad. Nauk SSSR 1968, 180, 381. (4) Brown, M. P.; Heseltine, R. W. Co-ordinated BH3 as a Proton Acceptor Group in Hydrogen Bonding. Chem. Commun. 1968, 1551− 1552. (5) Brown, M. P.; Heseltine, R. W.; Smith, P. A.; Walker, P. J. An Infrared Study of Co-ordinated BH3 and BH2 Groups as Proton Acceptors in Hydrogen Bonding. J. Chem. Soc. A 1970, 410−414. (6) Brown, M. P.; Walker, P. J. Hydrogen Bonds between Coordinated BH3 and BH2 Groups and OH Groups. Thermodynamics of Formation by Infrared Spectroscopy. Spectrochim. Acta, Part A 1974, 30, 1125−1131. (7) Stevens, R. C.; Bau, R.; Milstein, D.; Blum, O.; Koetzle, T. F. Concept of the H(δ+)... H(δ-) Interaction. A Low-Temperature Neutron Diffraction Study of cis-[IrH(OH)(PMe3)4]PF6. J. Chem. Soc., Dalton Trans. 1990, 1429−1432. 8577


Chemical Reviews

Review

Experimental and Density Functional Theory Studies. Inorg. Chem. 2010, 49, 6035−6057. (29) Yandulov, D. V.; Caulton, K. G.; Belkova, N. V.; Shubina, E. S.; Epstein, L. M.; Khoroshun, D. V.; Musaev, D. G.; Morokuma, K. Diminishing π-Stabilization of an Unsaturated Metal Center: Hydrogen Bonding to OsHCl(CO)(PtBu2Me)2. J. Am. Chem. Soc. 1998, 120, 12553−12563. (30) Guilera, G.; McGrady, G. S.; Steed, J. W.; Burchell, R. P. L.; Sirsch, P.; Deeming, A. J. Synthesis and Characterisation of [(triphos)Fe(CO)H2] and its Protonation to a Dihydrogen Complex via an Unconventional Hydrogen-Bonded Intermediate. New J. Chem. 2008, 32, 1573−1581. (31) Dub, P. A.; Filippov, O. A.; Belkova, N. V.; Daran, J.-C.; Epstein, L. M.; Shubina, E. S.; Poli, R. Hydrogen Bonding to Carbonyl Hydride Complex Cp*Mo(PMe3)2(CO)H and its Role in Proton Transfer. Dalton Trans. 2010, 39, 2008−2015. (32) Grotjahn, D. B.; Miranda-Soto, V.; Kragulj, E. J.; Lev, D. A.; Erdogan, G.; Zeng, X.; Cooksy, A. L. Hydrogen-Bond Acceptance of Bifunctional Ligands in an Alkyne−Metal π Complex. J. Am. Chem. Soc. 2008, 130, 20−21. (33) Grotjahn, D. B.; Kraus, J. E.; Amouri, H.; Rager, M.-N.; Cooksy, A. L.; Arita, A. J.; Cortes-Llamas, S. A.; Mallari, A. A.; DiPasquale, A. G.; Moore, C. E.; et al. Study of Secondary Interactions in Cp*Ir Complexes of Imidazolylphosphines Bearing an NH Group. J. Am. Chem. Soc. 2010, 132, 7919−7934. (34) Messmer, A.; Jacobsen, H.; Berke, H. Probing Regioselective Intermolecular Hydrogen Bonding to [Re(CO)H2(NO)(PR3)2] Complexes by NMR Titration and Equilibrium NMR Methodologies. Chem. - Eur. J. 1999, 5, 3341−3349. (35) Gründemann, S.; Ulrich, S.; Limbach, H.-H.; Golubev, N. S.; Denisov, G. S.; Epstein, L. M.; Sabo-Etienne, S.; Chaudret, B. SolventAssisted Reversible Proton Transfer within an Intermolecular Dihydrogen Bond and Characterization of an Unstable Dihydrogen Complex. Inorg. Chem. 1999, 38, 2550−2551. (36) Chu, H. S.; Lau, C. P.; Wong, K. Y.; Wong, W. T. Intramolecular N−H···H−Ru Proton−Hydride Interaction in Ruthenium Complexes with (2-(Dimethylamino)ethyl)cyclopentadienyl and (3-(Dimethylamino)propyl)cyclopentadienyl Ligands. Hydrogenation of CO2 to Formic Acid via the N−H···H−Ru Hydrogen-Bonded Complexes. Organometallics 1998, 17, 2768−2777. (37) Ayllón, J. A.; Sayers, S. F.; Sabo-Etienne, S.; Donnadieu, B.; Chaudret, B.; Clot, E. Proton Transfer in Aminocyclopentadienyl Ruthenium Hydride Complexes. Organometallics 1999, 18, 3981− 3990. (38) Lam, Y. F.; Yin, C.; Yeung, C. H.; Ng, S. M.; Jia, G.; Lau, C. P. Attenuation of Intramolecular Ru−H···H−N Dihydrogen Bonding in Aminocyclopentadienyl Ruthenium Hydride Complexes Containing Phosphite Ligands. Organometallics 2002, 21, 1898−1902. (39) Belkova, N. V.; Shubina, E. S.; Ionidis, A. V.; Epstein, L. M.; Jacobsen, H.; Messmer, A.; Berke, H. Intermolecular Hydrogen Bonding of ReH2(CO)(NO)L2 Hydrides with Perfluoro-tert-butyl Alcohol. Competition between M−H···H−OR and M−NO···H−OR Interactions. Inorg. Chem. 1997, 36, 1522−1525. (40) Bakhmutov, V. I.; Bakhmutova, E. V.; Belkova, N. V.; Bianchini, C.; Epstein, L. M.; Masi, D.; Peruzzini, M.; Shubina, E. S.; Vorontsov, E. V.; Zanobini, F. In-depth NMR and IR Study of the Proton Transfer Equilibrium between [{MeC(CH2PPh2)3}Ru(CO)H2] and Hexafluoroisopropanol. Can. J. Chem. 2001, 79, 479−489. (41) Ayllon, J. A.; Sabo-Etienne, S.; Chaudret, B.; Ulrich, S.; Limbach, H.-H. Modulation of Quantum Mechanical Exchange Couplings in Transition Metal Hydrides through Hydrogen Bonding. Inorg. Chim. Acta 1997, 259, 1−4. (42) Donghi, D.; Beringhelli, T.; D’Alfonso, G.; Mondini, M. NMR Investigation of the Dihydrogen-Bonding and Proton-Transfer Equilibria between the Hydrido Carbonyl Anion [HRe2(CO)9]− and Fluorinated Alcohols. Chem. - Eur. J. 2006, 12, 1016−1025. (43) Silantyev, G. A.; Filippov, O. A.; Tolstoy, P. M.; Belkova, N. V.; Epstein, L. M.; Weisz, K.; Shubina, E. S. Hydrogen Bonding and

Proton Transfer to Ruthenium Hydride Complex CpRuH(dppe): Metal and Hydride Dichotomy. Inorg. Chem. 2013, 52, 1787−1797. (44) Silantyev, G. A.; Kozinets, E. M.; Filippov, O. A.; Yakurnova, E. D.; Belkova, N. V. Dihydrogen Bonding Formed by (hydrido)[hydrotris(pyrazolyl)borato]ruthenium. The Effect of Ligands on the Proton-Withdrawing Ability of Ruthenium Complexes. Russ. Chem. Bull. 2014, 63, 2434−2437. (45) Guari, Y.; Ayllon, J. A.; Sabo-Etienne, S.; Chaudret, B.; Hessen, B. Influence of Hydrogen Bonding on the Spectroscopic Properties and on the Reactivity of Ruthenium Hydrido Dihydrogen Complexes. Inorg. Chem. 1998, 37, 640−644. (46) Shubina, E. S.; Belkova, N. V.; Ionidis, A. V.; Golubev, N. S.; Smirnov, S. N.; Shakh-Mokhammedi, P.; Epstein, L. M. Interaction of Rhenium Hydride (η5-C5Me5)ReH(CO)(NO) with Fluorinated Alcohols. Russ. Chem. Bull. 1997, 46, 1349−1351. (47) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. Assessment of the “T1 Criterion” for Distinguishing between Classical and Nonclassical Transition-Metal Hydrides: Hydride Relaxation Rates in Tris(triarylphosphine)osmium Tetrahydrides and Related Polyhydrides. J. Am. Chem. Soc. 1991, 113, 4173−4184. (48) Peris, E.; Lee, J. C.; Rambo, J. R.; Eisenstein, O.; Crabtree, R. H. Factors Affecting the Strength of N-H···H-Ir Hydrogen Bonds. J. Am. Chem. Soc. 1995, 117, 3485−3491. (49) Shubina, E. S.; Belkova, N. V.; Bakhmutova, E. V.; Vorontsov, E. V.; Bakhmutov, V. I.; Ionidis, A. V.; Bianchini, C.; Marvelli, L.; Peruzzini, M.; Epstein, L. M. In situ IR and NMR Study of the Interactions between Proton Donors and the Re(I) Hydride Complex [{MeC(CH2PPh2)3}Re(CO)2H]. ReH···H Bonding and ProtonTransfer Pathways. Inorg. Chim. Acta 1998, 280, 302−307. (50) Belkova, N. V.; Ionidis, A. V.; Epstein, L. M.; Shubina, E. S.; Gruendemann, S.; Golubev, N. S.; Limbach, H. H. Proton Transfer to CpRuH(CO)(PCy3) Studied by Low-Temperature IR and NMR Spectroscopy. Eur. J. Inorg. Chem. 2001, 2001, 1753−1761. (51) Gutsul, E. I.; Belkova, N. V.; Sverdlov, M. S.; Epstein, L. M.; Shubina, E. S.; Bakhmutov, V. I.; Gribanova, T. N.; Minyaev, R. M.; Bianchini, C.; Peruzzini, M.; et al. Low-Temperature IR and NMR Studies of the Interaction of Group 8 Metal Dihydrides with Alcohols. Chem. - Eur. J. 2003, 9, 2219−2228. (52) Belkova, N. V.; Dub, P. A.; Baya, M.; Houghton, J. Kinetics and Thermodynamics of Proton Transfer to Cp*Ru(dppe)H: via Dihydrogen Bonding and (η2-H2)-complex to the Dihydride. Inorg. Chim. Acta 2007, 360, 149−162. (53) Dub, P. A.; Belkova, N. V.; Filippov, O. A.; Silantyev, G. A.; Daran, J.-C.; Epstein, L. M.; Poli, R.; Shubina, E. S. Protonation of Cp*M(dppe)H Hydrides: Peculiarities of the Osmium Congener. Eur. J. Inorg. Chem. 2010, 2010, 1489−1500. (54) Sabo-Etienne, S.; Chaudret, B. Quantum Mechanical Exchange Coupling in Polyhydride and Dihydrogen Complexes. Chem. Rev. 1998, 98, 2077. (55) Hobza, P.; Havlas, Z. Blue-Shifting Hydrogen Bonds. Chem. Rev. 2000, 100, 4253−4264. (56) Feng, Y.; Zhao, S.-W.; Liu, L.; Wang, J.-T.; Li, X.-S.; Guo, Q.-X. Blue-Shifted Dihydrogen Bonds. J. Phys. Org. Chem. 2004, 17, 1099− 1106. (57) Yang, Y.; Zhang, W. Theoretical Study of N-H···H-B BlueShifted Dihydrogen Bond. J. Mol. Struct.: THEOCHEM 2007, 814, 113−117. (58) Belkova, N. V.; Shubina, E. S.; Gutsul, E. I.; Epstein, L. M.; Eremenko, I. L.; Nefedov, S. E. Structural and Energetic Aspects of Hydrogen Bonding and Proton Transfer to ReH2(CO)(NO)(PR3)2 and ReHCl(CO)(NO)(PMe3)2 by IR and X-ray Studies. J. Organomet. Chem. 2000, 610, 58−70. (59) Belkova, N. V.; Revin, P. O.; Epstein, L. M.; Vorontsov, E. V.; Bakhmutov, V. I.; Shubina, E. S.; Collange, E.; Poli, R. Kinetics and Mechanism of the Proton Transfer to Cp*Fe(dppe)H: Absence of a Direct Protonation at the Metal Site. J. Am. Chem. Soc. 2003, 125, 11106−11115. (60) Belkova, N. V.; Besora, M.; Epstein, L. M.; Lledós, A.; Maseras, F.; Shubina, E. S. Influence of Media and Homoconjugate Pairing on 8578


Chemical Reviews

Review

Transition Metal Hydride Protonation. An IR and DFT Study on Proton Transfer to CpRuH(CO)(PCy3). J. Am. Chem. Soc. 2003, 125, 7715−7725. (61) Belkova, N. V.; Bakhmutova, E. V.; Shubina, E. S.; Bianchini, C.; Peruzzini, M.; Bakhmutov, V. I.; Epstein, L. M. The Energy Profile of Proton Transfer from Bronsted Acids to Terminal Hydrides in Transition Metal Complexes Can Be Estimated by Combining In Situ IR and NMR Spectroscopy. Eur. J. Inorg. Chem. 2000, 2000, 2163− 2165. (62) Bakhmutova, E. V.; Bakhmutov, V. I.; Belkova, N. V.; Besora, M.; Epstein, L. M.; Lledós, A.; Nikonov, G. I.; Shubina, E. S.; Tomas, J.; Vorontsov, E. V. First Investigation of Non-Classical Dihydrogen Bonding between an Early Transition-Metal Hydride and Alcohols: IR, NMR, and DFT Approach. Chem. - Eur. J. 2004, 10, 661−671. (63) Belkova, N. V.; Epstein, L. M.; Filippov, O. A.; Shubina, E. S. IR spectroscopy of hydrides and its application to hydrogen bonding and proton transfer studies. In Spectroscopic Properties of Inorganic and Organometallic Compounds: Techniques, Materials and Applications; Yarwood, J., Douthwaite, R., Duckett, S., Eds.; The Royal Society of Chemistry: London, 2012; Vol. 43, pp 1−28. (64) Belkova, N. V.; Revin, P. O.; Besora, M.; Baya, M.; Epstein, L. M.; Lledós, A.; Poli, R.; Shubina, E. S.; Vorontsov, E. V. Hydrogen Bonding and Proton Transfer to the Trihydride Complex [Cp*MoH3(dppe)]: IR, NMR, and Theoretical Investigations. Eur. J. Inorg. Chem. 2006, 2006, 2192−2209. (65) Orlova, G.; Scheiner, S. Intermolecular MH···HR Bonding in Monohydride Mo and W Complexes. J. Phys. Chem. A 1998, 102, 260−269. (66) McDowell, S. A. C.; Forde, T. S. Isotope Effects in BeH2···HCN. J. Mol. Struct.: THEOCHEM 2003, 624, 109−114. (67) Hugas, D.; Simon, S.; Duran, M. MH···HX Dihydrogen Bond with M = Li, Na and X = F, Cl, Br: A CP-Corrected PES Calculation and an AIM Analysis. Struct. Chem. 2005, 16, 257−263. (68) Solimannejad, M.; Scheiner, S. Theoretical Investigation of the Dihydrogen Bond Linking MH2 with HCCRgF (M = Zn, Cd; Rg = Ar, Kr). J. Phys. Chem. A 2005, 109, 11933−11935. (69) Solimannejad, M.; Boutalib, A. Theoretical Investigation of the Weakly Dihydrogen Bonded Dimers H 2−n Xn AlH···HArF and H2−nXnAlH···HKrF (n = 0−2; X = F, Cl). Chem. Phys. 2006, 320, 275−280. (70) Filippov, O. A.; Tsupreva, V. N.; Epstein, L. M.; Lledós, A.; Shubina, E. S. Intermolecular HH Vibrations of Dihydrogen Bonded Complexes H3EH−···HOR in the Low-Frequency Region: Theory and IR Spectra. J. Phys. Chem. A 2008, 112, 8198−8204. (71) Wessel, J.; Lee, J. C.; Peris, E.; Yap, G. P. A.; Fortin, J. B.; Ricci, J. S.; Sini, G.; Albinati, A.; Koetzle, T. F.; Eisenstein, O.; et al. An Unconventional Intermolecular Three Center N-H···H2Re Hydrogen Bond in Crystalline [ReH5(PPh3)3].indole.C6H6. Angew. Chem., Int. Ed. Engl. 1995, 34, 2507−2509. (72) The Cambridge Structural Database, version 5.36; The Cambridge Crystallographic Data Centre: Cambridge, U.K., Feb 2015. (73) Krogsrud, S.; Toniolo, L.; Croatto, U.; Ibers, J. A. Hydrogenation of Aryldiazenido Complexes. Synthesis and Structure of Trans-hydrido(acetone phenylhydrazone)bis(triphenylphosphine)platinum(II) Tetrafluoroborate. J. Am. Chem. Soc. 1977, 99, 5277− 5284. (74) Yao, W.; Eisenstein, O.; Crabtree, R. H. Interactions between C—H and N—H Bonds and d8 Square Planar Metal Complexes: Hydrogen Bonded or Agostic? Inorg. Chim. Acta 1997, 254, 105−111. (75) Brookhart, M.; Green, M. L. H.; Parkin, G. Agostic Interactions in Transition Metal Compounds. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 6908−6914. (76) Landwehr, A.; Dudle, B.; Fox, T.; Blacque, O.; Berke, H. Bifunctional Rhenium Complexes for the Catalytic Transfer-Hydrogenation Reactions of Ketones and Imines. Chem. - Eur. J. 2012, 18, 5701−5714. (77) Lagaditis, P. O.; Sues, P. E.; Lough, A. J.; Morris, R. H. Exploring the Decomposition Pathways of Iron Asymmetric Transfer Hydrogenation Catalysts. Dalton Trans. 2015, 44, 12119−12127.

(78) Abdur-Rashid, K.; Gusev, D. G.; Landau, S. E.; Lough, A. J.; Morris, R. H. Organizing Chain Structures by Use of Proton−Hydride Bonding. The Single-Crystal X-ray Diffraction Structures of [K(Q)][Os(H)5(PiPr3)2] and [K(Q)][Ir(H)4(PiPr3)2], Q = 18-Crown-6 and 1,10-Diaza-18-crown-6. J. Am. Chem. Soc. 1998, 120, 11826−11827. (79) Dykeman, R. R.; Luska, K. L.; Thibault, M. E.; Jones, M. D.; Schlaf, M.; Khanfar, M.; Taylor, N. J.; Britten, J. F.; Harrington, L. Catalytic Deoxygenation of Terminal-Diols under Acidic Aqueous Conditions by the Ruthenium Complexes [(η6-arene)Ru(X)(N∩N)](OTf)n, X = H2O, H, η6-arene = p-Me-iPr-C6H4, C6Me6, N∩N = bipy, phen, 6,6′-diamino-bipy, 2,9-diamino-phen, n = 1, 2): Influence of the ortho-Amine Substituents on Catalytic Activity. J. Mol. Catal. A: Chem. 2007, 277, 233−251. (80) Spasyuk, D.; Smith, S.; Gusev, D. G. Replacing Phosphorus with Sulfur for the Efficient Hydrogenation of Esters. Angew. Chem., Int. Ed. 2013, 52, 2538−2542. (81) Mebi, C. A.; Frost, B. J. Isomerization of trans-[Ru(PTA)4Cl2] to cis-[Ru(PTA)4Cl2] in Water and Organic Solvent: Revisiting the Chemistry of [Ru(PTA)4Cl2]. Inorg. Chem. 2007, 46, 7115−7120. (82) Landau, S. E.; Groh, K. E.; Lough, A. J.; Morris, R. H. Large Effects of Ion Pairing and Protonic−Hydridic Bonding on the Stereochemistry and Basicity of Crown-, Azacrown-, and Cryptand222-potassium Salts of Anionic Tetrahydride Complexes of Iridium(III). Inorg. Chem. 2002, 41, 2995−3007. (83) Santiso-Quinones, G.; Rodriguez-Lugo, R.; Sacchetti, V.; Grutzmacher, H. Ru(II)-Hydride-Trop Complexes: X-Ray SingleCrystal Determination and Quantum-Chemical Calculations. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 2013, 69, 1421−1426. (84) Patel, B. P.; Yao, W.; Yap, G. P. A.; Rheingold, A. L.; Crabtree, R. H. Re-H···H-N Interactions in the Second-Coordination Sphere of Crystalline [Re(PPh3)2(imidazole)]. Chem. Commun. 1996, 991−992. (85) Eguillor, B.; Esteruelas, M. A.; Fernández, I.; Gómez-Gallego, M.; Lledós, A.; Martín-Ortiz, M.; Oliván, M.; Oñate, E.; Sierra, M. A. Azole Assisted C−H Bond Activation Promoted by an OsmiumPolyhydride: Discerning between N and NH. Organometallics 2015, 34, 1898−1910. (86) Abdur-Rashid, K.; Gusev, D. G.; Lough, A. J.; Morris, R. H. Intermolecular Proton−Hydride Bonding in Ion Pairs: Synthesis and Structural Properties of [K(Q)][MH5(PiPr3)2] (M= Os, Ru; Q= 18crown-6, 1-aza-18-crown-6, 1,10-diaza-18-crown-6). Organometallics 2000, 19, 834−843. (87) Gusev, D. G.; Lough, A. J.; Morris, R. H. New Polyhydride Anions and Proton−Hydride Hydrogen Bonding in Their Ion Pairs. X-ray Crystal Structure Determinations of Q[mer-Os(H)3(CO)(PiPr3)2], Q = [K(18-crown-6)] and Q = [K(1-aza-18-crown-6)]. J. Am. Chem. Soc. 1998, 120, 13138−13147. (88) Scheiner, S. Hydrogen Bonding: A Theoretical Perspective; Oxford University Press: New York, 1997. (89) Gilli, G.; Gilli, P. The Nature of the Hydrogen Bond: Outline of a Comprehensive Hydrogen Bond Theory; Oxford University Press: Oxford, U.K., 2009. (90) Ireta, J.; Neugebauer, J.; Scheffler, M. On the Accuracy of DFT for Describing Hydrogen Bonds: Dependence on the Bond Directionality. J. Phys. Chem. A 2004, 108, 5692−5698. (91) Parthasarathi, R.; Subramanian, V.; Sathyamurthy, N. Hydrogen Bonding without Borders: An Atoms-in-Molecules Perspective. J. Phys. Chem. A 2006, 110, 3349−3351. (92) Majerz, I.; Olovsson, I. Comparison of the Proton-Transfer Paths in Hydrogen Bonds from Theoretical Potential-Energy Surfaces and the Concept of Conservation of Bond Order III. O-H-O Hydrogen Bonds. Phys. Chem. Chem. Phys. 2010, 12, 5462−5467. (93) Orlova, G.; Scheiner, S. Inter- and Intramolecular Hydrogen Bonds with Transition Metal Atoms in Metallocenes of the Iron Subgroup. Organometallics 1998, 17, 4362−4367. (94) Orlova, G.; Scheiner, S.; Kar, T. Activation and Cleavage of H-R Bonds through Intermolecular H - H Bonding upon Reaction of Proton Donors HR with 18-Electron Transition Metal Hydrides. J. Phys. Chem. A 1999, 103, 514−520. 8579


Chemical Reviews

Review

(95) Calhorda, M. J. Weak Hydrogen Bonds: Theoretical Studies. Chem. Commun. 2000, 801−809. (96) Liu, Q.; Hoffmann, R. Theoretical Aspects of a Novel Mode of Hydrogen-Hydrogen Bonding. J. Am. Chem. Soc. 1995, 117, 10108− 10112. (97) Orlova, G.; Scheiner, S. Intermolecular H···H Bonding and Proton Transfer in Semisandwich Re and Ru Complexes. J. Phys. Chem. A 1998, 102, 4813−4818. (98) Grabowski, S. J.; Sokalski, W. A.; Leszczynski, J. How Short Can the H···H Intermolecular Contact Be? New Findings that Reveal the Covalent Nature of Extremely Strong Interactions. J. Phys. Chem. A 2005, 109, 4331−4341. (99) Hugas, D.; Simon, S.; Duran, M.; Fonseca Guerra, C.; Bickelhaupt, F. M. Dihydrogen Bonding: Donor-Acceptor Bonding (AH···HX) versus the H2 Molecule (A-H2-X). Chem. - Eur. J. 2009, 15, 5814−5822. (100) Filippov, O. A.; Filin, A. M.; Tsupreva, V. N.; Belkova, N. V.; Lledós, A.; Ujaque, G.; Epstein, L. M.; Shubina, E. S. Proton-Transfer and H2-Elimination Reactions of Main-Group Hydrides EH4− (E = B, Al, Ga) with Alcohols. Inorg. Chem. 2006, 45, 3086−3096. (101) Estarellas, C.; Frontera, A.; Quinonero, D.; Alkorta, I.; Deya, P. M.; Elguero, J. Energetic vs Synergetic Stability: A Theoretical Study. J. Phys. Chem. A 2009, 113, 3266−3273. (102) Alkorta, I.; Elguero, J.; Foces-Foces, C. Dihydrogen bonds (AH···H-B). Chem. Commun. 1996, 1633−1634. (103) Kulkarni, S. A. Dihydrogen Bonding in Main Group Elements: An Ab Initio Study. J. Phys. Chem. A 1998, 102, 7704−7711. (104) Remko, M. Thermodynamics of Dihydrogen Bonds (A-H···HB). Mol. Phys. 1998, 94, 839−842. (105) Minyaev, R. M.; Gribanova, T. N. Structures and Formation Energies of Bimolecular Complexes MH···HF and HM···FH (M = Cu, Ag, and Au). Russ. J. Inorg. Chem. 2001, 46, 1521−1530. (106) Esrafili, M. D.; Fatehi, P.; Solimannejad, M. Mutual Interplay between Pnicogen Bond and Dihydrogen Bond in HMH···HCN··· PH2X Complexes (M = Be, Mg, Zn; X = H, F, Cl). Comput. Theor. Chem. 2014, 1034, 1−6. (107) Berski, S.; Lundell, J.; Latajka, Z. A Density Functional Study of the Xenon Dihydride-Water Complex. J. Mol. Struct. 2000, 552, 223−232. (108) Lundell, J.; Berski, S.; Latajka, Z. Dihydrogen-Bonded Complexes of Xenon Dihydride with Water: Ab Initio Calculations and Topological Analysis of Electron Localization Function (ELF). Phys. Chem. Chem. Phys. 2000, 2, 5521−5527. (109) Lundell, J.; Pettersson, M. The Dihydrogen-Bonded Complex XeH2-H2O. Phys. Chem. Chem. Phys. 1999, 1, 1691−1697. (110) Solimannejad, M.; Amlashi, L. M.; Alkorta, I.; Elguero, J. XeH2 as a Proton-Accepting Molecule for Dihydrogen Bonded Systems: A Theoretical Study. Chem. Phys. Lett. 2006, 422, 226−229. (111) Grabowski, S. J. What Is the Covalency of Hydrogen Bonding? Chem. Rev. 2011, 111, 2597−2625. (112) Grabowski, J. S., What is Common for Dihydrogen Bond and H···σ InteractionTheoretical Analysis and Experimental Evidences. In Noncovalent Forces; Scheiner, S., Ed.; Challenges and Advances in Computational Chemistry and Physics; Springer International Publishing: Cham, Switzerland, 2015; Vol. 19, Chapter 6, pp 159−187. (113) Cybulski, H.; Pecul, M.; Sadlej, J.; Helgaker, T. Characterization of Dihydrogen-Bonded D-H···H-A Complexes on the Basis of Infrared and Magnetic Resonance Spectroscopic Parameters. J. Chem. Phys. 2003, 119, 5094−5104. (114) Alkorta, I.; Elguero, J.; Grabowski, S. J. How to Determine Whether Intramolecular H···H Interactions Can Be Classified as Dihydrogen Bonds. J. Phys. Chem. A 2008, 112, 2721−2727. (115) Grabowski, S. J. BeH2 as a Proton-Accepting Molecule for Dihydrogen Bonded Systems - Ab Initio Study. J. Mol. Struct. 2000, 553, 151−156. (116) Rauk, A. Orbital Interaction Theory of Organic Chemistry, 2nd ed.; John Wiley & Sons: New York, 2001. (117) Belkova, N. V.; Gribanova, T. N.; Gutsul, E. I.; Minyaev, R. M.; Bianchini, C.; Peruzzini, M.; Zanobini, F.; Shubina, E. S.; Epstein, L.

M. Specific and Non-Specific Influence of the Environment on Dihydrogen Bonding and Proton Transfer to RuH 2 {P(CH2CH2PPh2)3}. J. Mol. Struct. 2007, 844-845, 115−131. (118) Filippov, O. A.; Belkova, N. V.; Epstein, L. M.; Lledós, A.; Shubina, E. S. Directionality of Dihydrogen Bonds: The Role of Transition Metal Atoms. ChemPhysChem 2012, 13, 2677−2687. (119) Filippov, O. A.; Golub, I. E.; Osipova, E. S.; Kirkina, V. A.; Gutsul, E. I.; Belkova, N. V. Activation of M-H Bonds of Transition Metal Hydrides upon Complexation with Acids and Bases. Russ. Chem. Bull. 2014, 63, 2428−2433. (120) Wu, Y.; Feng, L.; Zhang, X. Theoretical Insights into the Properties of the Dihydrogen-Bonded HXH···HC≡CH Complexes (X = Be, Mg, and Ca). J. Mol. Struct.: THEOCHEM 2008, 851, 294−298. (121) Grabowski, S. J. Non-Covalent Interactions - QTAIM and NBO Analysis. J. Mol. Model. 2013, 19, 4713−4721. (122) Zhu, W.-L.; Puah, C. M.; Tan, X.-J.; Jiang, H.-L.; Chen, K.-X. Quantum Chemistry Investigation on the Dihydrogen Bond between Silicane and Ammonium. J. Phys. Chem. A 2001, 105, 426−431. (123) Jensen, F. Introduction to Computational Chemistry, 2nd ed.; Wiley: Chichester, U.K., 2007. (124) Grabowski, S. J.; Sokalski, W. A.; Leszczynski, J. Nature of XH+δ···−δH-Y Dihydrogen Bonds and X-H···σ Interactions. J. Phys. Chem. A 2004, 108, 5823−5830. (125) Kar, T.; Scheiner, S. Comparison between Hydrogen and Dihydrogen Bonds among H3BNH3, H2BNH2, and NH3. J. Chem. Phys. 2003, 119, 1473−1482. (126) Kulkarni, S. A.; Srivastava, A. K. Dihydrogen Bonding in Main Group Elements: A Case Study of Complexes of LiH, BH3, and AlH3 with Third-Row Hydrides. J. Phys. Chem. A 1999, 103, 2836−2842. (127) Cybulski, H.; Tyminska, E.; Sadlej, J. The Properties of Weak and Strong Dihydrogen-Bonded D-H···H-A Complexes. ChemPhysChem 2006, 7, 629−639. (128) Fabiano, E.; Constantin, L. A.; Della Sala, F. Wave Function and Density Functional Theory Studies of Dihydrogen Complexes. J. Chem. Theory Comput. 2014, 10, 3151−3162. (129) Bercaw, J. E.; Chen, G. S.; Labinger, J. A.; Lin, B.-L. Protonolysis of Platinum(II) and Palladium(II) Methyl Complexes: A Combined Experimental and Theoretical Investigation. Organometallics 2010, 29, 4354−4359. (130) Mitoraj, M. P. Bonding in Ammonia Borane: An Analysis Based on the Natural Orbitals for Chemical Valence and the Extended Transition State Method (ETS-NOCV). J. Phys. Chem. A 2011, 115, 14708−14716. (131) Hugas, D.; Guillaumes, L.; Duran, M.; Simon, S. Delocalization Indices for Non-Covalent Interaction: Hydrogen and DiHydrogen Bond. Comput. Theor. Chem. 2012, 998, 113−119. (132) Gao, S.; Wu, W.; Mo, Y. The B-H···H-P Dihydrogen Bonding in Ion Pair Complexes [(CF3)3BH−][HPH3−n(Me)n+] (n = 0−3) and Its Implication in H2 Elimination and Activation Reactions. J. Phys. Chem. A 2009, 113, 8108−8117. (133) Grabowski, S. J.; Sokalski, W. A.; Leszczynski, J. Wide Spectrum of H···H Interactions: van der Waals Contacts, Dihydrogen Bonds and Covalency. Chem. Phys. 2007, 337, 68−76. (134) Oliveira, B. G.; Araujo, R. C. M. U.; Silva, J. J.; Ramos, M. N. A Theoretical Study of Three and Four Proton Donors on Linear HX··· BeH2···HX and Bifurcate BeH2···2HX Trimolecular DihydrogenBonded Complexes with X = CN and NC. Struct. Chem. 2010, 21, 221−228. (135) Oliveira, B. G.; Araujo, R. C. M. U.; Ramos, M. N. Multiple Proton Donors on BeH2···2HCl Trimolecular Dihydrogen-Bonded Complex: Some Theoretical Insights. Struct. Chem. 2008, 19, 665− 670. (136) Oliveira, B. G.; Vasconcellos, M. L. A. A. A B3LYP and QTAIM Study of a New Proton Donor for Dihydrogen Bonds: The Case of the C2H5+···nBeH2 complexes (n = 1 or 2). Struct. Chem. 2009, 20, 897−902. (137) Oliveira, B. G.; Vasconcellos, M. L. A. A. The Ethyl Cation as Proton Donor for Dihydrogen Bonds in the mC2H5+···nMgH2 (m = 1 8580


Chemical Reviews

Review

or 2 and n = 1 or 2) Complex: A Theoretical Study. Inorg. Chem. Commun. 2009, 12, 1142−1144. (138) Zabardasti, A.; Kakanejadi, A.; Moosavi, S.; Bigleri, Z.; Solimannejad, M. Anticooperativity in Dihydrogen Bonded Clusters of Ammonia and BeH42−. J. Mol. Struct.: THEOCHEM 2010, 945, 97− 100. (139) Marincean, S.; Jackson, J. E. Quest for IR-Pumped Reactions in Dihydrogen-Bonded Complexes. J. Phys. Chem. A 2004, 108, 5521− 5526. (140) Belkova, N. V.; Filippov, O. A.; Filin, A. M.; Teplitskaya, L. N.; Shmyrova, Y. V.; Gavrilenko, V. V.; Golubinskaya, L. M.; Bregadze, V. I.; Epstein, L. M.; Shubina, E. S. Interaction of the [GaH4]− Anion with Weak XH Acids - A Spectroscopic and Theoretical Study. Eur. J. Inorg. Chem. 2004, 2004, 3453−3461. (141) Yang, Y.; Liu, Y.; Yang, D.; Li, H.; Jiang, K.; Sun, J. Theoretical Study on the Dehydrogenation Reaction of Dihydrogen Bonded Phenol-Borane-Trimethylamine in the Excited State. Phys. Chem. Chem. Phys. 2015, 17, 32132−32139. (142) Fujii, A.; Patwari, G. N.; Ebata, T.; Mikami, N. Vibrational Spectroscopic Evidence of Unconventional Hydrogen Bonds. Int. J. Mass Spectrom. 2002, 220, 289−312. (143) Naresh Patwari, G.; Ebata, T.; Mikami, N. Gas Phase Dihydrogen Bonded Phenol-Borane-Trimethylamine Complex. J. Chem. Phys. 2001, 114, 8877−8879. (144) Naresh Patwari, G.; Ebata, T.; Mikami, N. Dihydrogen Bonded Phenol-Borane-Dimethylamine Complex: An Experimental and Theoretical Study. J. Chem. Phys. 2002, 116, 6056−6063. (145) Hamada, I.; Yamauchi, K.; Oguchi, T. Structure and Intermolecular Bonding in NaBH4·2H2O: a density functional theory study. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 214119-1− 214119-4. (146) Yamawaki, H.; Fujihisa, H.; Gotoh, Y.; Nakano, S. Formation of LiBH4 Hydrate with Dihydrogen Bonding. J. Alloys Compd. 2012, 541, 111−114. (147) Belkova, N. V.; Bakhmutova-Albert, E. V.; Gutsul, E. I.; Bakhmutov, V. I.; Golub, I. E.; Filippov, O. A.; Epstein, L. M.; Peruzzini, M.; Rossin, A.; Zanobini, F.; et al. Dihydrogen Bonding in Complex (PP3)RuH(η1-BH4) Featuring Two Proton-Accepting Hydride Sites: Experimental and Theoretical Studies. Inorg. Chem. 2014, 53, 1080−1090. (148) Golub, I. E.; Filippov, O. A.; Gutsul, E. I.; Belkova, N. V.; Epstein, L. M.; Rossin, A.; Peruzzini, M.; Shubina, E. S. Dimerization Mechanism of Bis(triphenylphosphine)copper(I) Tetrahydroborate: Proton Transfer via a Dihydrogen Bond. Inorg. Chem. 2012, 51, 6486− 6497. (149) Golub, I. E.; Gulyaeva, E. S.; Filippov, O. A.; Dyadchenko, V. P.; Belkova, N. V.; Epstein, L. M.; Arkhipov, D. E.; Shubina, E. S. Dihydrogen Bond Intermediated Alcoholysis of Dimethylamine− Borane in Nonaqueous Media. J. Phys. Chem. A 2015, 119, 3853− 3868. (150) Johnson, E. R.; Keinan, S.; Mori-Sánchez, P.; Contreras-García, J.; Cohen, A. J.; Yang, W. Revealing Noncovalent Interactions. J. Am. Chem. Soc. 2010, 132, 6498−6506. (151) Contreras-García, J.; Johnson, E. R.; Keinan, S.; Chaudret, R.; Piquemal, J.-P.; Beratan, D. N.; Yang, W. NCIPLOT: A Program for Plotting Noncovalent Interaction Regions. J. Chem. Theory Comput. 2011, 7, 625−632. (152) Belkova, N. V.; Besora, M.; Baya, M.; Dub, P. A.; Epstein, L. M.; Lledós, A.; Poli, R.; Revin, P. O.; Shubina, E. Effect of the Metal Atom Nature on Hydrogen Bonding and Proton Transfer to [Cp*MH3(dppe)]: Tungsten vs. Molybdenum. Chem. - Eur. J. 2008, 14, 9921−9934. (153) Andrieu, J.; Belkova, N. V.; Besora, M.; Collange, E.; Epstein, L. M.; Lledós, A.; Poli, R.; Revin, P. O.; Shubina, E. S.; Vorontsov, E. V. Hydrogen Bonding and Proton Transfer Involving the Trihydride Complexes Cp*MH3(dppe) (M = molybdenum, tungsten) and Fluorinated Alcohols: The Competitive Role of the Hydride Ligands and the Metal. Russ. Chem. Bull. 2003, 52, 2679−2682.

(154) Dub, P. A.; Baya, M.; Houghton, J.; Belkova, N. V.; Daran, J. C.; Poli, R.; Epstein, L. M.; Shubina, E. S. Solvent Control in the Protonation of Cp*Mo(dppe)H3 by CF3COOH. Eur. J. Inorg. Chem. 2007, 2007, 2813−2826. (155) Filippov, O. A.; Kirkina, V. A.; Belkova, N. V.; Stoccoro, S.; Zucca, A.; Babakhina, G. M.; Epstein, L. M.; Shubina, E. S. First Example of Hydrogen Bonding to Platinum Hydride. Z. Phys. Chem. 2013, 227, 869. (156) Shi, F. Theoretical Investigation on the Nature of Intramolecular Interactions in Aminocyclopentadienyl Ruthenium Hydride Complexes. Organometallics 2006, 25, 4034−4037. (157) Bai, W.; Tse, S.; Lee, K.; Sung, H.-Y.; Williams, I.; Lin, Z.; Jia, G. Synthesis and Characterization of MH···HOR Dihydrogen Bonded Ruthenium and Osmium Complexes (η5-C5H4CH2OH)MH(PPh3)2 (M = Ru, Os). Sci. China: Chem. 2014, 57, 1079−1089. (158) McDowell, S. A. C.; Forde, T. S. Isotope Effects in Linear Dihydrogen Bonded Complexes Containing LiH. J. Chem. Phys. 2002, 117, 6032−6037. (159) Hayashi, A.; Shiga, M.; Tachikawa, M. H/D Isotope Effect on the Dihydrogen Bond of NH4+·BeH2 by Ab Initio Path Integral Molecular Dynamics Simulation. J. Chem. Phys. 2006, 125, 204310-1− 204310-7. (160) Hayashi, A.; Shiga, M.; Tachikawa, M. H/D Isotope Effect on the Dihydrogen Bond by Ab Initio Path Integral Molecular Dynamics Simulation. Mol. Simul. 2007, 33, 185−188. (161) Ubbelohde, A. R.; Gallagher, K. J. Acid-Base Effects in Hydrogen Bonds in Crystals. Acta Crystallogr. 1955, 8, 71−83. (162) Nakai, H.; Ikabata, Y.; Tsukamoto, Y.; Imamura, Y.; Miyamoto, K.; Hoshino, M. Isotope Effect in Dihydrogen-Bonded Systems: Application of the Analytical Energy Gradient Method in the Nuclear Orbital Plus Molecular Orbital Theory. Mol. Phys. 2007, 105, 2649− 2657. (163) Udagawa, T.; Tachikawa, M. Why is N···Be Distance of NH3H+···DBeH Shorter than that of NH3D+···HBeH? Paradoxical Geometrical Isotope Effects for Partially Isotope-Substituted Dihydrogen-Bonded Isotopomers. J. Comput. Chem. 2014, 35, 271−274. (164) Epstein, L. M.; Shubina, E. S.; Bakhmutova, E. V.; Saitkulova, L. N.; Bakhmutov, V. I.; Chistyakov, A. L.; Stankevich, I. V. Unusual Hydrogen Bonds with a Hydride Atom in Boron Hydrides Acting as Proton Acceptor. Spectroscopic and Theoretical Studies. Inorg. Chem. 1998, 37, 3013−3017. (165) Iogansen, A. V. Direct Proportionality of the Hydrogen Bonding Energy and the Intensification of the Stretching ν(XH) Vibration in Infrared Spectra. Spectrochim. Acta, Part A 1999, 55, 1585−1612. (166) Iogansen, A. V. Estimation of Medium (Solvent) Influence on Hydrogen Bond Properties by Using Rule of Multiplication of AcidBase Functions of Molecules. Teor. Eksp. Khim. 1971, 7, 312−317. (167) Iogansen, A. V. Rule of Products of Acid-Base Functions of Molecules upon Hydrogen Bonding in CCl4 Solutions. Teor. Eksp. Khim. 1971, 7, 302−311. (168) Epstein, L. M.; Belkova, N. V.; Shubina, E. S. Dihydrogen Bonded Complexes and Proton Transfer to Hydride Ligands by Spectral (IR, NMR) Studies. In Recent Advances in Hydride Chemistry; Peruzzini, M., Poli, R., Eds.; Elsevier: Amsterdam, The Netherlands, 2001; Chapter 14, pp 391−418. (169) Alkorta, I.; Elguero, J.; Mo, O.; Yanez, M.; Del Bene, J. E. Ab Initio Study of the Structural, Energetic, Bonding, and IR Spectroscopic Properties of Complexes with Dihydrogen Bonds. J. Phys. Chem. A 2002, 106, 9325−9330. (170) Belkova, N. V.; Collange, E.; Dub, P.; Epstein, L. M.; Lemenovskii, D. A.; Lledós, A.; Maresca, O.; Maseras, F.; Poli, R.; Revin, P. O.; et al. Experimental and Computational Studies of Hydrogen Bonding and Proton Transfer to Cp*Fe(dppe)H. Chem. Eur. J. 2005, 11, 873−888. (171) Ayllon, J. A.; Gervaux, C.; Sabo-Etienne, S.; Chaudret, B. First NMR Observation of the Intermolecular Dynamic Proton Transfer Equilibrium between a Hydride and Coordinated Dihydrogen: 8581


Chemical Reviews

Review

(dppm)2HRuH···H-OR = [(dppm)2HRu(H2)]+(OR)−. Organometallics 1997, 16, 2000−2002. (172) Baya, M.; Dub, P. A.; Houghton, J.; Daran, J.-C.; Poli, R.; Belkova, N. V.; Shubina, E. S.; Epstein, L. M.; Lledós, A. Investigation of the [Cp*Mo(PMe3)3H]n+ (n = 0, 1) Redox Pair: Dynamic Processes on Very Different Timescales. Inorg. Chem. 2009, 48, 209− 220. (173) Peris, E.; Wessel, J.; Patel, B. P.; Crabtree, R. H. d0 and d2 Polyhydrides as Unconventional Proton Acceptors in Lntermolecular Hydrogen Bonding. J. Chem. Soc., Chem. Commun. 1995, 2175−2176. (174) Levina, V. A.; Rossin, A.; Belkova, N. V.; Chierotti, M. R.; Epstein, L. M.; Filippov, O. A.; Gobetto, R.; Gonsalvi, L.; Lledós, A.; Shubina, E. S.; et al. Acid−Base Interaction between Transition-Metal Hydrides: Dihydrogen Bonding and Dihydrogen Evolution. Angew. Chem., Int. Ed. 2011, 50, 1367−1370. (175) Albinati, A.; Bakhmutov, V. I.; Belkova, N. V.; Bianchini, C.; de los Rios, I.; Epstein, L.; Gutsul, E. I.; Marvelli, L.; Peruzzini, M.; Rossi, R.; et al. Synthesis, Characterization, and Interconversion of the Rhenium Polyhydrides [ReH3(η4-NP3)] and [ReH4(η4-NP3)]+ {NP3 = tris[2-(diphenylphosphanyl)-ethyl]amine}. Eur. J. Inorg. Chem. 2002, 2002, 1530−1539. (176) Osipova, E. S.; Belkova, N. V.; Epstein, L. M.; Filippov, O. A.; Kirkina, V. A.; Titova, E. M.; Rossin, A.; Peruzzini, M.; Shubina, E. S. Dihydrogen Bonding and Proton Transfer from MH vs OH Acids to Group 10 Metal Hydrides ( tBu PCP)MH [ tBu PCP = κ 3 -2,6(tBu2PCH2)2C6H3; M = Ni, Pd]. Eur. J. Inorg. Chem. 2016, 2016, 1415−1424. (177) Shubina, E. S.; Bakhmutova, E. V.; Filin, A. M.; Sivaev, I. B.; Teplitskaya, L. N.; Chistyakov, A. L.; Stankevich, I. V.; Bakhmutov, V. I.; Bregadze, V. I.; Epstein, L. M. Dihydrogen Bonding of DecahydroCloso-Decaborate(2−) and Dodecahydro-Closo-Dodecaborate(2−) Anions with Proton Donors: Experimental and Theoretical Investigation. J. Organomet. Chem. 2002, 657, 155−162. (178) Filippov, O. A.; Tsupreva, V. N.; Golubinskaya, L. M.; Krylova, A. I.; Bregadze, V. I.; Lledós, A.; Epstein, L. M.; Shubina, E. S. ProtonTransfer and H2-Elimination Reactions of Trimethylamine Alane: Role of Dihydrogen Bonding and Lewis Acid-Base Interactions. Inorg. Chem. 2009, 48, 3667−3678. (179) Nietlispach, D.; Bakhmutov, V. I.; Berke, H. Deuterium Quadrupole Coupling Constants and Ionic Bond Character in Transition Metal Hydride Complexes from 2H NMR T1 Relaxation Data in Solution. J. Am. Chem. Soc. 1993, 115, 9191−9195. (180) Alabugin, I. V.; Manoharan, M.; Peabody, S.; Weinhold, F. Electronic Basis of Improper Hydrogen Bonding: A Subtle Balance of Hyperconjugation and Rehybridization. J. Am. Chem. Soc. 2003, 125, 5973−5987. (181) Alabugin, I. V.; Bresch, S.; dos Passos Gomes, G. Orbital Hybridization: A Key Electronic Factor in Control of Structure and Reactivity. J. Phys. Org. Chem. 2015, 28, 147−162. (182) Alabugin, I. V.; Bresch, S.; Manoharan, M. Hybridization Trends for Main Group Elements and Expanding the Bent’s Rule Beyond Carbon: More than Electronegativity. J. Phys. Chem. A 2014, 118, 3663−3677. (183) Galdino de Oliveira, B. Structure, Energy, Vibrational Spectrum, and Bader’s Analysis of π···H Hydrogen Bonds and H−δ··· H+δ Dihydrogen Bonds. Phys. Chem. Chem. Phys. 2013, 15, 37−79. (184) Feng, L.; Bai, F.-Q.; Wu, Y.; Zhang, H.-X. Dihydrogen Bond in C2H4−nCln ··· NaH (n = 0, 1, 2, 3) Complexes: Ab Initio, AIM and NBO Studies. Mol. Phys. 2011, 109, 645−653. (185) Liu, Y. Hydrogen Bonding Characterization of XH2NH2··· HNO (X = B, Al, Ga) Complexes: A Theoretical Investigation. Int. J. Quantum Chem. 2008, 108, 1123−1129. (186) Trung, N. T.; Hue, T. T.; Nguyen, M. T.; Zeegers-Huyskens, T. Theoretical Study of the Interaction between HNZ (Z = O, S) and H2XNH2 (X = B, Al). Conventional and Dihydrogen Bonds. Phys. Chem. Chem. Phys. 2008, 10, 5105−5113. (187) Yu, W.; Lin, Z.; Huang, Z. Coexistence of Dihydrogen, Blueand Red-Shifting Hydrogen Bonds in an Ultrasmall System: Valine. ChemPhysChem 2006, 7, 828−830.

(188) Kun, Y.; Liu, Y.; Lü, L. Theoretical Study of N(C)-H···H-B Multi-Dihydrogen Bonds. Chin. Sci. Bull. 2009, 54, 1182−1189. (189) See, e.g., refs 12, 35−37, 40, 42, 49−51, 58, 61, and 171. (190) Macchioni, A. Ion Pairing in Transition-Metal Organometallic Chemistry. Chem. Rev. 2005, 105, 2039−2074. (191) Bullock, R. M.; Song, J.-S.; Szalda, D. J. Protonation of Metal Hydrides by Strong Acids. Formation of an Equilibrium Mixture of Dihydride and Dihydrogen Complexes from Protonation of Cp*Os(CO) 2 H. Structural Characterization of [CpW(CO) 2 (PMe 3 )(H)2]+OTf−. Organometallics 1996, 15, 2504−2516. (192) Fong, T. P.; Forde, C. E.; Lough, A. J.; Morris, R. H.; Rigo, P.; Rocchini, E.; Stephan, T. Synthesis of the Acidic Dihydrogen Complexes trans-[M(H2)(CN)L2]+ and trans-[M(H2)(CNH)L2]2+ where M = Fe, Ru, Os and L = dppm, dppe, dppp, depe, and Dihydrogen Substitution by the Trifluoromethanesulfonate Anion to Give trans-[Ru(OTf)(CN)L2] or trans-[Ru(OTf)(CNH)L2]OTf. J. Chem. Soc., Dalton Trans. 1999, 4475−4486. (193) Gutsul, E. I.; Belkova, N. V.; Babakhina, G. M.; Epstein, L. M.; Shubina, E. S.; Bianchini, C.; Peruzzini, M.; Zanobini, F. How Can the Metal Affect the Proton Transfer to the Dihydrides [{P(CH2CH2PPh2)3}MH2] (M = Fe, Ru, Os)? A Low-Temperature Electronic Spectroscopy Study. Russ. Chem. Bull. 2003, 52, 1204− 1206. (194) Rossin, A.; Gutsul, E. I.; Belkova, N. V.; Epstein, L. M.; Gonsalvi, L.; Lledós, A.; Lyssenko, K. A.; Peruzzini, M.; Shubina, E. S.; Zanobini, F. Mechanistic Studies on the Interaction of [(k3-P,P,PNP3)IrH3] [NP3 = N(CH2CH2PPh2)3] with HBF4 and Fluorinated Alcohols by Combined NMR, IR, and DFT Techniques. Inorg. Chem. 2010, 49, 4343−4354. (195) Basallote, M. G.; Besora, M.; Castillo, C. E.; FernándezTrujillo, M. J.; Lledós, A.; Maseras, F.; Máñez, M. A. Crucial Role of Anions on the Deprotonation of the Cationic Dihydrogen Complex trans-[FeH(η2-H2)(dppe)2]+. J. Am. Chem. Soc. 2007, 129, 6608−6618. (196) Carroll, M. E.; Barton, B. E.; Rauchfuss, T. B.; Carroll, P. J. Synthetic Models for the Active Site of the [FeFe]-Hydrogenase: Catalytic Proton Reduction and the Structure of the Doubly Protonated Intermediate. J. Am. Chem. Soc. 2012, 134, 18843−18852. (197) Papish, E. T.; Rix, F. C.; Spetseris, N.; Norton, J. R.; Williams, R. D. Protonation of CpW(CO)2(PMe3)H: Is the Metal or the Hydride the Kinetic Site? J. Am. Chem. Soc. 2000, 122, 12235−12242. (198) Hamon, P.; Toupet, L.; Hamon, J.-R.; Lapinte, C. Novel Diamagnetic and Paramagnetic Iron(II), Iron(III), and Iron(IV) Classical and Nonclassical Hydrides. X-ray Crystal Structure of [Fe(C5Me5)(dppe)D]PF6. Organometallics 1992, 11, 1429−1431. (199) Baya, M.; Maresca, O.; Poli, R.; Coppel, Y.; Maseras, F.; Lledós, A.; Belkova, N. V.; Dub, P. A.; Epstein, L. M.; Shubina, E. S. Dihydrogen to Dihydride Isomerization Mechanism in [(C5Me5)FeH2(Ph2PCH2CH2PPh2)]+ through the Experimental and Theoretical Analysis of Kinetic Isotope Effects. Inorg. Chem. 2006, 45, 10248− 10262. (200) Dub, P. A.; Belkova, N. V.; Lyssenko, K. A.; Silantyev, G. A.; Epstein, L. M.; Shubina, E. S.; Daran, J.-C.; Poli, R. Synthesis and Protonation Studies of Cp*Os(dppe)H: kinetic vs thermodynamic control. Organometallics 2008, 27, 3307−3311. (201) Chinn, M. S.; Heinekey, D. M. Dihydrogen Complexes of Ruthenium. 2. Kinetic and Thermodynamic Considerations Affecting Product Distribution. J. Am. Chem. Soc. 1990, 112, 5166−5175. (202) Chinn, M. S.; Heinekey, D. M. Synthesis and properties of a series of ruthenium dihydrogen complexes. J. Am. Chem. Soc. 1987, 109, 5865−5867. (203) Esteruelas, M. A.; Gomez, A. V.; Lahoz, F. J.; Lopez, A. M.; Onate, E.; Oro, L. A. Five-Coordinate Complex [RuHCl(CO)(PPri3)2] as a Precursor for the Preparation of New Cyclopentadienylruthenium Compounds Containing Unsaturated η1-Carbon Ligands. Organometallics 1996, 15, 3423−3435. (204) Ontko, A. C.; Houlis, J. F.; Schnabel, R. C.; Roddick, D. M.; Fong, T. P.; Lough, A. J.; Morris, R. H. Protonation and H2 Heterolysis Reactions of Electrophilic (η5-C5R5)Ru(dfepe)(X) (R = 8582


Chemical Reviews

Review

H, Me; X = H, OTf) Complexes. Organometallics 1998, 17, 5467− 5476. (205) Conroy-Lewis, F. M.; Simpson, S. J. Molecular Hydrogen Complexes: The Effect of Steric Factors on the Protonation of the Monohydrides [(η5-C5H5)Ru(Ph2P(CH2)nPPh2)H] (n = 1−3). J. Chem. Soc., Chem. Commun. 1987, 1675−1676. (206) Jia, G.; Morris, R. H. Wide Range of pKa Values of Coordinated Dihydrogen. Synthesis and Properties of Some η2Dihydrogen and Dihydride Complexes of Ruthenium. J. Am. Chem. Soc. 1991, 113, 875−883. (207) Jia, G.; Lough, A. J.; Morris, R. H. Synthesis and the Kinetic and Thermodynamic Acidity of η2-Dihydrogen and Dihydride Complexes of the Type [Ru(C5Me5)H2L2]+. X-ray Crystal Structure Determination of the Complex [Ru(C5Me5)(η2-H2)(PPh2CH2PPh2)]BF4. Organometallics 1992, 11, 161−171. (208) Egbert, J. D.; Bullock, R. M.; Heinekey, D. M. Cationic Dihydrogen/Dihydride Complexes of Osmium: Structure and Dynamics. Organometallics 2007, 26, 2291−2295. (209) Guan, H.; Iimura, M.; Magee, M. P.; Norton, J. R.; Zhu, G. Ruthenium-Catalyzed Ionic Hydrogenation of Iminium Cations. Scope and Mechanism. J. Am. Chem. Soc. 2005, 127, 7805−7814. (210) Heinekey, D. M.; Oldham, W. J. Coordination Chemistry of Dihydrogen. Chem. Rev. 1993, 93, 913−926. (211) Crabtree, R. H. Dihydrogen Complexation. Chem. Rev. 2016, DOI: 10.1021/acs.chemrev.6b00037. (212) Kubas, G. J. Fundamentals of H2 Binding and Reactivity on Transition Metals Underlying Hydrogenase Function and H 2 Production and Storage. Chem. Rev. 2007, 107, 4152−4205. (213) Shin, J. H.; Churchill, D. G.; Parkin, G. Carbonyl Abstraction Reactions of Cp*Mo(PMe3)3H with CO2, (CH2O)n, HCO2H, and MeOH: the Synthesis of Cp*Mo(PMe3)2(CO)H and the Catalytic Decarboxylation of Formic Acid. J. Organomet. Chem. 2002, 642, 9− 15. (214) Dub, P. A.; Belkova, N. V.; Filippov, O. A.; Daran, J.-C.; Epstein, L. M.; Lledós, A.; Shubina, E. S.; Poli, R. Solvent Dependent Dihydrogen/Dihydride Stability for [Cp*Mo(CO) ¬(PMe3)2H2]+[BF4]− Determined by Multiple Solvent···Anion··· Cation Non-Covalent Interactions. Chem. - Eur. J. 2010, 16, 189−201. (215) Pleune, B.; Poli, R.; Fettinger, J. C. Synthesis, Structure, and Protonation Studies of Cp*MH3(dppe) (M = Mo, W). PseudoTrigonal-Prismatic vs Pseudo-Octahedral Structures for Half-Sandwich Group 6 M(VI) Derivatives. Organometallics 1997, 16, 1581−1594. (216) Belkova, N. V.; Epstein, L. M.; Krylova, A. I.; Faerstein, E. G.; Shubina, E. S. Kinetics of Protonation of Tungsten Hydrides WH(CO)2(NO)L2 by Weak OH-Acids. Russ. Chem. Bull. 2007, 56, 870−874. (217) Basallote, M. G.; Estevan, F.; Feliz, M.; Fernández-Trujillo, M. J.; Hoyos, D. A.; Llusar, R.; Uriel, S.; Vicent, C. Synthesis and Structure of the Incomplete Cuboidal Clusters [W3Se4H3(dmpe)3]+, [W3Se4H3(OH) (dmpe)3]+ and [W3Se4(OH)3(dmpe)3]+, and the Mechanism of the Acid-Assisted Substitution of the Coordinated Hydrides. Dalton Trans. 2004, 530−536. (218) Algarra, A. G.; Basallote, M. G.; Feliz, M.; Fernández-Trujillo, M. J.; Llusar, R.; Safont, V. S. New Insights into the Mechanism of Proton Transfer to Hydride Complexes: Kinetic and Theoretical Evidence Showing the Existence of Competitive Pathways for Protonation of the Cluster [W3S4H3(dmpe)3]+ with Acids. Chem. Eur. J. 2006, 12, 1413−1426. (219) Algarra, A. G.; Basallote, M. G.; Feliz, M.; Fernández-Trujillo, M. J.; Llusar, R.; Safont, V. S. The Role of Solvent on the Mechanism of Proton Transfer to Hydride Complexes: The Case of the [W3PdS4H3(dmpe)3(CO)]+ Cubane Cluster. Chem. - Eur. J. 2010, 16, 1613−1623. (220) Espino, G.; Caballero, A.; Manzano, B. R.; Santos, L.; PérezManrique, M.; Moreno, M.; Jalón, F. A. Experimental and Computational Evidence for the Participation of Nonclassical Dihydrogen Species in Proton Transfer Processes on Ru−Arene Complexes with Uncoordinated N Centers. Efficient Catalytic Deuterium Labeling of H2 with CD3OD. Organometallics 2012, 31, 3087−3100.

(221) Jalon, F. A.; Manzano, B. R.; Caballero, A.; Carrion, M. C.; Santos, L.; Espino, G.; Moreno, M. Facile Ru-H2 Heterolytic Activation and Intramolecular Proton Transfer Assisted by Basic NCenters in the Ligands. J. Am. Chem. Soc. 2005, 127, 15364−15365. (222) Caballero, A.; Jalon, F. A. Three-Center Dihydrogen Bond with Fast Interchange between Proton and Hydride: A Very Active Catalyst for D+-H2 exchange. Chem. Commun. 1998, 1879−1880. (223) Frost, B. J.; Mebi, C. A. Aqueous Organometallic Chemistry: Synthesis, Structure, and Reactivity of the Water-Soluble Metal Hydride CpRu(PTA)2H. Organometallics 2004, 23, 5317−5323. (224) Mebi, C. A.; Frost, B. J. Effect of pH on the Biphasic Catalytic Hydrogenation of Benzylidene Acetone Using CpRu(PTA)2H. Organometallics 2005, 24, 2339−2346. (225) Rossin, A.; Gonsalvi, L.; Phillips, A. D.; Maresca, O.; Lledós, A.; Peruzzini, M. Water-Assisted H-H Bond Splitting Mediated by [CpRu(PTA)2Cl] (PTA = 1,3,5-triaza-7-phosphaadamantane). A DFT Analysis. Organometallics 2007, 26, 3289−3296. (226) Baur, J.; Jacobsen, H.; Burger, P.; Artus, G.; Berke, H.; Dahlenburg, L. The Chemistry of New Nitrosyltungsten Complexes with Pyridyl-Functionalized Phosphane Ligands. Eur. J. Inorg. Chem. 2000, 2000, 1411−1422. (227) DuBois, D. L.; Bullock, R. M. Molecular Electrocatalysts for the Oxidation of Hydrogen and the Production of Hydrogen − The Role of Pendant Amines as Proton Relays. Eur. J. Inorg. Chem. 2011, 2011, 1017−1027. (228) Karasik, A. A.; Balueva, A. S.; Musina, E. I.; Sinyashin, O. G. Chelating Cyclic Aminomethylphosphines and Their Transition Metal Complexes as a Promising Basis of Bioinspired Mimetic Catalysts. Mendeleev Commun. 2013, 23, 237−248. (229) Raugei, S.; DuBois, D. L.; Rousseau, R.; Chen, S.; Ho, M.-H.; Bullock, R. M.; Dupuis, M. Toward Molecular Catalysts by Computer. Acc. Chem. Res. 2015, 48, 248−255. (230) Bullock, R. M.; Helm, M. L. Molecular Electrocatalysts for Oxidation of Hydrogen Using Earth-Abundant Metals: Shoving Protons Around with Proton Relays. Acc. Chem. Res. 2015, 48, 2017−2026. (231) Bullock, R. M.; Appel, A. M.; Helm, M. L. Production of Hydrogen by Electrocatalysis: Making the H-H Bond by Combining Protons and Hydrides. Chem. Commun. 2014, 50, 3125−3143. (232) O’Hagan, M.; Shaw, W. J.; Raugei, S.; Chen, S.; Yang, J. Y.; Kilgore, U. J.; DuBois, D. L.; Bullock, R. M. Moving Protons with Pendant Amines: Proton Mobility in a Nickel Catalyst for Oxidation of Hydrogen. J. Am. Chem. Soc. 2011, 133, 14301−14312. (233) Darmon, J. M.; Raugei, S.; Liu, T.; Hulley, E. B.; Weiss, C. J.; Bullock, R. M.; Helm, M. L. Iron Complexes for the Electrocatalytic Oxidation of Hydrogen: Tuning Primary and Secondary Coordination Spheres. ACS Catal. 2014, 4, 1246−1260. (234) Liu, T.; Liao, Q.; O’Hagan, M.; Hulley, E. B.; DuBois, D. L.; Bullock, R. M. Iron Complexes Bearing Diphosphine Ligands with Positioned Pendant Amines as Electrocatalysts for the Oxidation of H2. Organometallics 2015, 34, 2747−2764. (235) Liu, T.; Wang, X.; Hoffmann, C.; DuBois, D. L.; Bullock, R. M. Heterolytic Cleavage of Hydrogen by an Iron Hydrogenase Model: An Fe-H···H-N Dihydrogen Bond Characterized by Neutron Diffraction. Angew. Chem., Int. Ed. 2014, 53, 5300−5304. (236) Liu, T.; Chen, S.; O’Hagan, M. J.; Rakowski DuBois, M.; Bullock, R. M.; DuBois, D. L. Synthesis, Characterization, and Reactivity of Fe Complexes Containing Cyclic Diazadiphosphine Ligands: The Role of the Pendant Base in Heterolytic Cleavage of H2. J. Am. Chem. Soc. 2012, 134, 6257−6272. (237) Hulley, E. B.; Welch, K. D.; Appel, A. M.; DuBois, D. L.; Bullock, R. M. Rapid, Reversible Heterolytic Cleavage of Bound H2. J. Am. Chem. Soc. 2013, 135, 11736−11739. (238) Shubina, E. S.; Bakhmutova, E. V.; Saitkulova, L. N.; Epstein, L. M. Intermolecular Hydrogen Bonds BH···HX in Solution. Mendeleev Commun. 1997, 7, 83−84. (239) Duffin, A. M.; England, A. H.; Schwartz, C. P.; Uejio, J. S.; Dallinger, G. C.; Shih, O.; Prendergast, D.; Saykally, R. J. Electronic 8583


Chemical Reviews

Review

Structure of Aqueous Borohydride: A Potential Hydrogen Storage Medium. Phys. Chem. Chem. Phys. 2011, 13, 17077−17083. (240) Zabardasti, A.; Zare, N.; Arabpour, M. Theoretical Study of Dihydrogen Bonded Clusters of Water with Tetrahydroborate. Struct. Chem. 2011, 22, 691−695. (241) Li, P.; Henkelman, G.; Keith, J. A.; Johnson, J. K. Elucidation of Aqueous Solvent-Mediated Hydrogen-Transfer Reactions by ab Initio Molecular Dynamics and Nudged Elastic-Band Studies of NaBH4 Hydrolysis. J. Phys. Chem. C 2014, 118, 21385−21399. (242) Giammanco, C. H.; Kramer, P. L.; Fayer, M. D. Dynamics of Dihydrogen Bonding in Aqueous Solutions of Sodium Borohydride. J. Phys. Chem. B 2015, 119, 3546−3559. (243) Custelcean, R.; Jackson, J. E. Topochemical Dihydrogen to Covalent Bonding Transformation in LiBH4·TEA: A Mechanistic Study. J. Am. Chem. Soc. 2000, 122, 5251−5257. (244) Wang, X. F.; Andrews, L. Infrared Spectra of Gallium Hydrides in Solid Hydrogen: GaH1,2,3, Ga2H2,4,6, and the GaH2,4− Anions. J. Phys. Chem. A 2003, 107, 11371−11379. (245) Wang, X.; Andrews, L.; Tam, S.; DeRose, M. E.; Fajardo, M. E. Infrared Spectra of Aluminum Hydrides in Solid Hydrogen: Al2H4 and Al2H6. J. Am. Chem. Soc. 2003, 125, 9218−9228. (246) Tague, T. J.; Andrews, L. Reactions of Pulsed-Laser Evaporated Boron Atoms with Hydrogen. Infrared Spectra of Boron Hydride Intermediate Species in Solid Argon. J. Am. Chem. Soc. 1994, 116, 4970−4976. (247) Mykolayivna-Lemishko, K.; Montero-Campillo, M. M.; Mó, O.; Yáñez, M. Behavior of Carboxylic Acids upon Complexation with Beryllium Compounds. J. Phys. Chem. A 2014, 118, 5720−5726. (248) Montero-Campillo, M. M.; Yáñez, M.; Lamsabhi, A. M.; Mó, O. Spontaneous H2 Loss through the Interaction of Squaric Acid Derivatives and BeH2. Chem. - Eur. J. 2014, 20, 5309−5316. (249) Swinnen, S.; Nguyen, V. S.; Sakai, S.; Nguyen, M. T. Calculations Suggest Facile Hydrogen Release from Water Using Boranes and Alanes as Catalysts. Chem. Phys. Lett. 2009, 472, 175− 180. (250) Tan, G.; Szilvási, T.; Inoue, S.; Blom, B.; Driess, M. An Elusive Hydridoaluminum(I) Complex for Facile C−H and C−O Bond Activation of Ethers and Access to Its Isolable Hydridogallium(I) Analogue: Syntheses, Structures, and Theoretical Studies. J. Am. Chem. Soc. 2014, 136, 9732−9742. (251) Yu, Z.; Wittbrodt, J. M.; Xia, A.; Heeg, M. J.; Schlegel, H. B.; Winter, C. H. Hydrogen and Dihydrogen Bonding as Important Features of the Reactivity of the Bridging Hydride in PyrazolateBridged Dialuminum Complexes. Organometallics 2001, 20, 4301− 4303. (252) Welch, G. C.; Juan, R. R. S.; Masuda, J. D.; Stephan, D. W. Reversible, Metal-Free Hydrogen Activation. Science 2006, 314, 1124− 1126. (253) Stephan, D. W.; Erker, G. Frustrated Lewis Pair Chemistry: Development and Perspectives. Angew. Chem., Int. Ed. 2015, 54, 6400−6441. (254) Stephan, D. W. Frustrated Lewis Pairs. J. Am. Chem. Soc. 2015, 137, 10018−10032. (255) Frustrated Lewis Pairs I: Uncovering and Understanding; Erker, G., Stephan, D. W., Eds.; Topics in Current Chemistry; Springer: Berlin, 2013; Vol. 332. (256) Frustrated Lewis Pairs II: Expanding the Scope; Erker, G., Stephan, D. W., Eds.; Topics in Current Chemistry; Springer: Berlin, 2013; Vol. 334. (257) Paradies, J. Metal-Free Hydrogenation of Unsaturated Hydrocarbons Employing Molecular Hydrogen. Angew. Chem., Int. Ed. 2014, 53, 3552−3557. (258) Schulz, F.; Sumerin, V.; Heikkinen, S.; Pedersen, B.; Wang, C.; Atsumi, M.; Leskelae, M.; Repo, T.; Pyykkoe, P.; Petry, W.; et al. Molecular Hydrogen Tweezers: Structure and Mechanisms by Neutron Diffraction, NMR, and Deuterium Labeling Studies in Solid and Solution. J. Am. Chem. Soc. 2011, 133, 20245−20257. (259) Zaher, H.; Ashley, A. E.; Irwin, M.; Thompson, A. L.; Gutmann, M. J.; Kramer, T.; O’Hare, D. Structural and Theoretical

Studies of Intermolecular Dihydrogen Bonding in [(C6F5)2(C6Cl5)B]HH-[TMP]. Chem. Commun. 2013, 49, 9755−9757. (260) Rokob, T. A.; Bakó, I.; Stirling, A.; Hamza, A.; Pápai, I. Reactivity Models of Hydrogen Activation by Frustrated Lewis Pairs: Synergistic Electron Transfers or Polarization by Electric Field? J. Am. Chem. Soc. 2013, 135, 4425−4437. (261) Rokob, T. A.; Hamza, A.; Stirling, A.; Soós, T.; Pápai, I. Turning Frustration into Bond Activation: A Theoretical Mechanistic Study on Heterolytic Hydrogen Splitting by Frustrated Lewis Pairs. Angew. Chem., Int. Ed. 2008, 47, 2435−2438. (262) Becerra, M.; Real-Enriquez, M.; Espinosa-Gavilanes, C.; Zambrano, C. H.; Almeida, R.; Torres, F. J.; Rincón, L. On the Thermodynamic Stability of the Intermolecular Association between Lewis Acids and Lewis Bases: A DFT Study. Theor. Chem. Acc. 2016, 135, 1−11. (263) Rocchigiani, L.; Ciancaleoni, G.; Zuccaccia, C.; Macchioni, A. Probing the Association of Frustrated Phosphine−Borane Lewis Pairs in Solution by NMR Spectroscopy. J. Am. Chem. Soc. 2014, 136, 112− 115. (264) Rokob, T. A.; Hamza, A.; Stirling, A.; Pápai, I. On the Mechanism of B(C6F5)3-Catalyzed Direct Hydrogenation of Imines: Inherent and Thermally Induced Frustration. J. Am. Chem. Soc. 2009, 131, 2029−2036. (265) Mas-Ballesté, R.; Lledós, A. H−H Bond Activation. In Comprehensive Inorganic Chemistry II, 2nd ed.; Reedijk, J., Poeppelmeier, K., Eds.; Elsevier: Amsterdam, The Netherlands, 2013; Vol. 9, pp 727−766. (266) Grimme, S.; Kruse, H.; Goerigk, L.; Erker, G. The Mechanism of Dihydrogen Activation by Frustrated Lewis Pairs Revisited. Angew. Chem., Int. Ed. 2010, 49, 1402−1405. (267) Lu, Z.; Cheng, Z.; Chen, Z.; Weng, L.; Li, Z. H.; Wang, H. Heterolytic Cleavage of Dihydrogen by “Frustrated Lewis Pairs” Comprising Bis(2,4,6-tris(trifluoromethyl)phenyl)borane and Amines: Stepwise versus Concerted Mechanism. Angew. Chem., Int. Ed. 2011, 50, 12227−12231. (268) Marks, T. J.; Kolb, J. R. Covalent Transition Metal, Lanthanide, and Actinide Tetrahydroborate Complexes. Chem. Rev. 1977, 77, 263− 293. (269) Besora, M.; Lledós, A. Coordination Modes and Hydride Exchange Dynamics in Transition Metal Tetrahydroborate Complexes. In Contemporary Metal Boron Chemistry I: Borylenes, Boryls, Borane Sigma-Complexes, and Borohydrides; Marder, T., Lin, Z., Eds.; Structure and Bonding; Springer: Berlin, 2008; Vol. 130, pp 149−202. (270) Makhaev, V. D. Structural and Dynamic Properties of Tetrahydroborate Complexes. Russ. Chem. Rev. 2000, 69, 727−746. (271) Green, B. E.; Kennard, C. H. L.; Smith, G.; James, B. D.; Healy, P. C.; White, A. H. Crystal and Molecular Structure of μTetrahydroborato-bis[bis(triphenylphosphine)copper(I)] Perchlorate. Inorg. Chim. Acta 1984, 81, 147−150. (272) Esteruelas, M. A.; Garcia, M. P.; Lopez, A. M.; Oro, L. A.; Ruiz, N.; Schluenken, C.; Valero, C.; Werner, H. Reactivity of MH(η2H2BH2)(CO)(PiPr3)2 (M = Os, Ru) toward Electrophiles: Synthesis of New Hydridocarbonylosmium(II) and -ruthenium(II) Complexes Containing Triisopropylphosphine as Ligand. Inorg. Chem. 1992, 31, 5580−5587. (273) Hillier, A. C.; Jacobsen, H.; Gusev, D.; Schmalle, H. W.; Berke, H. [{Fe(PEt3)3}2(μ-H)6B][BPh4]: A Complex Containing Octahedral Hypercoordinate Boron. Inorg. Chem. 2001, 40, 6334−6337. (274) Guilera, G.; Sean McGrady, G.; Steed, J. W.; Kaltsoyannis, N. Expanding Metallaborane Chemistry: An Octahedral BH6 Moiety Supported through M-H-B Bridges. New J. Chem. 2004, 28, 444−446. (275) Koehne, I.; Schmeier, T. J.; Bielinski, E. A.; Pan, C. J.; Lagaditis, P. O.; Bernskoetter, W. H.; Takase, M. K.; Würtele, C.; Hazari, N.; Schneider, S. Synthesis and Structure of Six-Coordinate Iron Borohydride Complexes Supported by PNP Ligands. Inorg. Chem. 2014, 53, 2133−2143. (276) Zheng, J.; Fayer, M. D. Hydrogen Bond Lifetimes and Energetics for Solute/Solvent Complexes Studied with 2D-IR 8584


Chemical Reviews

Review

Source. Theoretical and Experimental Demonstration of a Ru−H/D+ Exchange as the Key Step. ACS Catal. 2014, 4, 1040−1053. (293) Van Der Zeijden, A. A. H.; Sontag, C.; Bosch, H. W.; Shklover, V.; Berke, H.; Nanz, D.; Von Philipsborn, W. IR, Multinuclear-NMR, and Structural Studies on [WH(CO)2(NO)(PR3)2]: cis-Influence of Phosphorus Ligands on Hydride Character. Helv. Chim. Acta 1991, 74, 1194−1204. (294) Kuo, L. Y.; Weakley, T. J. R.; Awana, K.; Hsia, C. Aqueous “Green” Organometallic Chemistry: Structure of the Molybdocene Monohydride Used for Carbonyl Reduction in Water. Organometallics 2001, 20, 4969−4972. (295) Henry, R. M.; Shoemaker, R. K.; DuBois, D. L.; DuBois, M. R. Pendant Bases as Proton Relays in Iron Hydride and Dihydrogen Complexes. J. Am. Chem. Soc. 2006, 128, 3002−3010. (296) Toomey, H. E.; Pun, D.; Veiros, L. F.; Chirik, P. J. 1,2-Addition versus σ-Bond Metathesis Reactions in Transient Bis(cyclopentadienyl)zirconium Imides: Evidence for a d0 Dihydrogen Complex. Organometallics 2008, 27, 872−879. (297) Calvert, R. B.; Shapley, J. R.; Schultz, A. J.; Williams, J. M.; Suib, S. L.; Stucky, G. D. Equilibrium Isotope Effect on Hydrogen Distribution between Carbon- and Metal-Bound Sites. A Neutron Diffraction Study of Partially Deuterated Decacarbonyldihydridomethylenetriosmium. J. Am. Chem. Soc. 1978, 100, 6240−6241. (298) Gómez-Gallego, M.; Sierra, M. A. Kinetic Isotope Effects in the Study of Organometallic Reaction Mechanisms. Chem. Rev. 2011, 111, 4857−4963. (299) Kovacs, G.; Nadasdi, L.; Laurenczy, G.; Joo, F. Aqueous Organometallic Catalysis. Isotope Exchange Reactions in H2-D2O and D2-H2O Systems Catalyzed by Water-Soluble Rh- and Ru-Phosphine Complexes. Green Chem. 2003, 5, 213−217. (300) Paterniti, D. P.; Roman, P. J.; Atwood, J. D. Activation of Hydrogen on Iridium(I) Centers in Water: Solvent Effects, pH Effects, and Iridium−Hydride Exchange with D2O. Organometallics 1997, 16, 3371−3376. (301) Bai, W.; Lee, K.-H.; Tse, S. K. S.; Chan, K. W.; Lin, Z.; Jia, G. Ruthenium-Catalyzed Deuteration of Alcohols with Deuterium Oxide. Organometallics 2015, 34, 3686−3698. (302) Chua, Y. S.; Li, W.; Shaw, W. J.; Wu, G.; Autrey, T.; Xiong, Z.; Wong, M. W.; Chen, P. Mechanistic Investigation on the Formation and Dehydrogenation of Calcium Amidoborane Ammoniate. ChemSusChem 2012, 5, 927−931. (303) Wolstenholme, D. J.; Titah, J. T.; Che, F. N.; Traboulsee, K. T.; Flogeras, J.; McGrady, G. S. Homopolar Dihydrogen Bonding in Alkali-Metal Amidoboranes and Its Implications for Hydrogen Storage. J. Am. Chem. Soc. 2011, 133, 16598−16604. (304) Basallote, M. G.; Durán, J.; Fernández-Trujillo, M. J.; Máñez, M. A. Mechanisms of Reactions of Dihydrogen Complexes: Formation of trans-[RuH(H2)(dppe)2]+ and Substitution of Coordinated Dihydrogen. Inorg. Chem. 1999, 38, 5067−5071. (305) Henderson, R. A.; Oglieve, K. E. The mechanisms of protonation of [M(η5-C5H5)2H2](M = Mo or W). J. Chem. Soc., Dalton Trans. 1993, 3431−3439. (306) Morris, R. H. Brønsted−Lowry Acid Strength of Metal Hydride and Dihydrogen Complexes. Chem. Rev. 2016, DOI: 10.1021/acs.chemrev.5b00695. (307) Epstein, L. M.; Shubina, E. S.; Krylov, A. N.; Kreindlin, A. Z.; Rybinskaya, M. I. Ionic Hydrogen Bond and Deprotonation of Cationic Osmium Hydride [(η5-C5Me5)2OsH]+A. J. Organomet. Chem. 1993, 460, 87−89. (308) Epstein, L. M.; Shubina, E. S.; Krylov, A. N.; Kreindlin, A. Z.; Rybinskaya, M. I. Interaction between [(η5-C5Me5)2OsH]+PF6− and Nitrogen or Oxygen Bases. Hydrogen Bonds of the Type [OsH]+... B. J. Organomet. Chem. 1993, 447, 277−280. (309) Shubina, E. S.; Krylov, A. N.; Belkova, N. V.; Epstein, L. M.; Borisov, A. P.; Mahaev, V. D. The Role of Hydrogen Bonds Involving the Metal Atom in Protonation of Polyhydrides of Molybdenum and Tungsten of MH4(dppe)2. J. Organomet. Chem. 1995, 493, 275−277. (310) Peris, E.; Crabtree, R. H. Infrared Detection of M−H···OPPh3 Hydrogen Bonds. J. Chem. Soc., Chem. Commun. 1995, 2179−2180.

Vibrational Echo Spectroscopy. J. Am. Chem. Soc. 2007, 129, 4328− 4335. (277) Basallote, M. G.; Durán, J.; Fernández-Trujillo, M. J.; Máñez, M. A. Kinetics of Formation of Dihydrogen Complexes by Protonation of CpRuHL Complexes (L = DPPM, DPPE, 2 PPh3) with HBF4·Et2O in THF and the Failure To Observe Dihydrogen Complexes in the Reactions with Other Acids. Organometallics 2000, 19, 695−698. (278) Belkova, N. V.; Gutsul, E. I.; Shubina, E. S.; Epstein, L. M. Proton Transfer to Organometallic Hydrides via Unconventional Hydrogen Bonding: Problems and Perspectives. Z. Phys. Chem. 2003, 217, 1525−1538. (279) Bakhmutov, V. I. Proton Transfer to Hydride Ligands with Formation of Dihydrogen Complexes: A Physicochemical View. Eur. J. Inorg. Chem. 2005, 2005, 245−255. (280) Avramovic, N.; Hock, J.; Blacque, O.; Fox, T.; Schmalle, H. W.; Berke, H. Hydridic Reactivity of W(CO)(H)(NO)(PMe3)3 Dihydrogen Bonding and H2 Formation with Protic Donors. J. Organomet. Chem. 2010, 695, 382−391. (281) Basallote, M. G.; Duran, J.; Fernández-Trujillo, M. J.; Angeles Máñez, M. Kinetics of Protonation of cis-[FeH2(dppe)2]: Formation of the Dihydrogen Complex trans-[FeH(H2)(dppe)2]+ (dppe = Ph2PCH2CH2PPh2). J. Chem. Soc., Dalton Trans. 1998, 2205−2210. (282) Basallote, M. G.; Durán, J.; Fernández-Trujillo, M. J.; Angeles Máñez, M.; Rodriguez de la Torre, J. Kinetics of Formation of Dihydrogen Complexes: Protonation of cis-[FeH2{P(CH2CH2PPh2)3} with Acids in Tetrahydrofuran. J. Chem. Soc., Dalton Trans. 1998, 745− 750. (283) Basallote, M. G.; Feliz, M.; Fernández-Trujillo, M. J.; Llusar, R.; Safont, V. S.; Uriel, S. Mechanism of the Reaction of the [W3S4H3(dmpe)3]+ Cluster with Acids: Evidence for the AcidPromoted Substitution of Coordinated Hydrides and the Effect of the Attacking Species on the Kinetics of Protonation of the MetalHydride Bonds. Chem. - Eur. J. 2004, 10, 1463−1471. (284) Mesmer, R. E.; Jolly, W. L. The Hydrolysis of Aqueous Hydroborate. Inorg. Chem. 1962, 1, 608−612. (285) Feracin, S.; Buergi, T.; Bakhmutov, V. I.; Eremenko, I.; Vorontsov, E. V.; Vimenits, A. B.; Berke, H. Hydrogen/Hydrogen Exchange and Formation of Dihydrogen Derivatives of Rhenium Hydride Complexes in Acidic Solutions. Organometallics 1994, 13, 4194−4202. (286) Safarowic, F. J.; Bierdeman, D. J.; Keister, J. B. Kinetics and Mechanism of Reversible Oxidative Addition of Hydrogen across the Metal-Metal Bond of (μ-H)2Ru3(CO)8(μ-P(t-Bu)2)2. Steric Promotion of Metal-Metal Bond Cleavage But a CO Dissociative Mechanism. J. Am. Chem. Soc. 1996, 118, 11805−11812. (287) Kovács, G.; Schubert, G.; Joó, F.; Pápai, I. Theoretical Mechanistic Study of Rhodium(I) Phosphine-Catalyzed H/D Exchange Processes in Aqueous Solutions. Organometallics 2005, 24, 3059−3065. (288) Cayuela, E.; Jalon, F. A.; Manzano, B. R.; Espino, G.; Weissensteiner, W.; Mereiter, K. Monocyclopentadienylhydride Derivatives of Ruthenium: Stereoselective Proton Transfer and Proton−Hydride Exchange in an Extremely Short Dihydrogen Bond. J. Am. Chem. Soc. 2004, 126, 7049−7062. (289) Lau, C. P.; Ng, S. M.; Jia, G.; Lin, Z. Some Ruthenium Hydride, Dihydrogen, and Dihydrogen-Bonded Complexes in Catalytic Reactions. Coord. Chem. Rev. 2007, 251, 2223−2237. (290) Filippov, O. A.; Belkova, N. V.; Epstein, L. M.; Lledós, A.; Shubina, E. S. Hydrogen−Deuterium Exchange in Hydride Chemistry: Dihydrogen Bonded Complexes as Key Intermediates. Comput. Theor. Chem. 2012, 998, 129−140. (291) Maseras, F.; Lledós, A.; Clot, E.; Eisenstein, O. Transition Metal Polyhydrides: From Qualitative Ideas to Reliable Computational Studies. Chem. Rev. 2000, 100, 601−636. (292) Carrión, M. C.; Ruiz-Castañeda, M.; Espino, G.; Aliende, C.; Santos, L.; Rodríguez, A. M.; Manzano, B. R.; Jalón, F. A.; Lledós, A. Selective Catalytic Deuterium Labeling of Alcohols during a Transfer Hydrogenation Process of Ketones Using D2O as the Only Deuterium 8585


Chemical Reviews

Review

(311) Mar Abad, M.; Atheaux, I.; Maisonnat, A.; Chaudret, B. Control of Proton Transfer by Hydrogen Bonding in the Protonated Forms of the Binucleophilic Complex [{η5-C5H4CH(CH2)4NMe}Ir(PPh3)H2]. Chem. Commun. 1999, 381−382. (312) Ryan, O. B.; Tilset, M. Oxidation of CpRu(CO)(PMe3)H by 2/3, 1, and 2 Electrons by the Judicious Choice of Reaction Conditions. Generation of a Bridging Hydride via the Reaction between a 17-Electron Metal Hydride Cation Radical and Its Conjugate Base. J. Am. Chem. Soc. 1991, 113, 9554−9561. (313) Poli, R. Paramagnetic Mono- and Polyhydrides of the Transition Metals. In Recent Advances in Hydride Chemistry;; Peruzzini, M., Poli, R., Eds.; Elsevier: Amsterdam, The Netherlands, 2001; Chapter 6 , pp 139−188. (314) Szymczak, N. K.; Zakharov, L. N.; Tyler, D. R. Solution Chemistry of a Water-Soluble η2-H2 Ruthenium Complex: Evidence for Coordinated H2 Acting as a Hydrogen Bond Donor. J. Am. Chem. Soc. 2006, 128, 15830−15835. (315) Braga, D.; Grepioni, F.; Tedesco, E.; Biradha, K.; Desiraju, G. R. Hydrogen Bonding in Organometallic Crystals. 4. M−H- - -O Hydrogen-Bonding Interactions. Organometallics 1996, 15, 2692− 2699. (316) Fairhurst, S. A.; Henderson, R. A.; Hughes, D. L.; Ibrahim, S. K.; Pickett, C. J. An Intramolecular W-H···OC hydrogen bond? Electrosynthesis and X-ray Crystallographic Structure of [WH3(η1OCOMe)(Ph2PCH2CH2PPh2)2]. J. Chem. Soc., Chem. Commun. 1995, 1569−1570. (317) Levina, V. A.; Filippov, O. A.; Gutsul, E. I.; Belkova, N. V.; Epstein, L. M.; Lledós, A.; Shubina, E. S. Neutral Transition Metal Hydrides as Acids in Hydrogen Bonding and Proton Transfer: Media Polarity and Specific Solvation Effects. J. Am. Chem. Soc. 2010, 132, 11234−11246. (318) Belkova, N. V.; Gutsul, E. I.; Filippov, O. A.; Levina, V. A.; Valyaev, D. A.; Epstein, L. M.; Lledós, A.; Shubina, E. S. Intermolecular Hydrogen Bonding between Neutral Transition Metal Hydrides (η5-C5H5)M(CO)3H (M = Mo, W) and Bases. J. Am. Chem. Soc. 2006, 128, 3486−3487. (319) Wang, X.; Zhou, G.; Tian, A.; Wong, N.-B. Ab Initio Investigation on Blue Shift and Red Shift of C−H Stretching Vibrational Frequency in NH3···CHnX4−n (n = 1,3, X = F, Cl, Br, I) Complexes. J. Mol. Struct.: THEOCHEM 2005, 718, 1−7. (320) Scheiner, S. The CH···O hydrogen bond: A historical account. In Theory and Applications of Computational Chemistry; Dykstra, C. E.; Frenking, G.; Kwang, S. K.; Scuseria, G. E., Eds.; Elsevier: Amsterdam, The Netherlands, 2005; Chapter 29, pp 831−857. (321) Gu, Y.; Kar, T.; Scheiner, S. Comparison of the CH···N and CH···O Interactions Involving Substituted Alkanes. J. Mol. Struct. 2000, 552, 17−31. (322) Domagala, M.; Grabowski, S. J. Hydrocarbons as Proton Donors in C-H···N and C-H···S Hydrogen Bonds. Chem. Phys. 2010, 367, 1−6. (323) Li, A. Y.; Wang, S. W. Ab Initio Investigation of Hydrogen Bonds between Pyridine and HCl, CHCl 3 . J. Mol. Struct.: THEOCHEM 2007, 807, 191−199. (324) Kütt, A.; Leito, I.; Kaljurand, I.; Soovaeli, L.; Vlasov, V. M.; Yagupolskii, L. M.; Koppel, I. A. A Comprehensive Self-Consistent Spectrophotometric Acidity Scale of Neutral Broensted Acids in Acetonitrile. J. Org. Chem. 2006, 71, 2829−2838. (325) Gerber, R.; Fox, T.; Frech, C. M. P Csp3P and P Csp2P Palladium(II) Hydride Pincer Complexes: Small Structural DifferenceLarge Effect on Reactivity. Chem. - Eur. J. 2010, 16, 6771−6775. (326) Klähn, M.; Garland, M. V. On the Mechanism of the Catalytic Binuclear Elimination Reaction in Hydroformylation Systems. ACS Catal. 2015, 5, 2301−2316. (327) Li, C.; Gao, F.; Cheng, S.; Tjahjono, M.; van Meurs, M.; Tay, B. Y.; Jacob, C.; Guo, L.; Garland, M. From Stoichiometric to Catalytic Binuclear Elimination in Rh−W Hydroformylations. Identification of Two New Heterobimetallic Intermediates. Organometallics 2011, 30, 4292−4296.

(328) Li, C.; Cheng, S.; Tjahjono, M.; Schreyer, M.; Garland, M. Concurrent Synergism and Inhibition in Bimetallic Catalysis: Catalytic Binuclear Elimination, Solute-Solute Interactions and a HeteroBimetallic Hydrogen-Bonded Complex in Rh-Mo Hydroformylations. J. Am. Chem. Soc. 2010, 132, 4589−4599. (329) Li, C.; Chen, L.; Garland, M. Synchronicity of Mononuclear and Dinuclear Events in Homogeneous Catalysis. Hydroformylation of Cyclopentene Using Rh4(CO)12 and HRe(CO)5 as Precursors. J. Am. Chem. Soc. 2007, 129, 13327−13334. (330) Jongbloed, L. S.; de Bruin, B.; Reek, J. N. H.; Lutz, M.; van der Vlugt, J. I. Facile Synthesis and Versatile Reactivity of an Unusual Cyclometalated Rhodium(I) Pincer Complex. Chem. - Eur. J. 2015, 21, 7297−7305. (331) Hulley, E. B.; Kumar, N.; Raugei, S.; Bullock, R. M. Manganese-Based Molecular Electrocatalysts for Oxidation of Hydrogen. ACS Catal. 2015, 5, 6838−6847. (332) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry; 3 ed.; Wiley-VCH: Weinheim, Germany, 2003. (333) Iron, M. A.; Ben-Ari, E.; Cohen, R.; Milstein, D. Metal-Ligand Cooperation in the Trans Addition of Dihydrogen to a Pincer Ir(I) Complex: A DFT Study. Dalton Trans. 2009, 9433−9439. (334) Díez, J.; Gimeno, J.; Lledós, A.; Suárez, F. J.; Vicent, C. Imidazole Based Ruthenium(IV) Complexes as Highly Efficient Bifunctional Catalysts for the Redox Isomerization of Allylic Alcohols in Aqueous Medium: Water as Cooperating Ligand. ACS Catal. 2012, 2, 2087−2099. (335) Handgraaf, J.-W.; Meijer, E. J. Realistic Modeling of Ruthenium-Catalyzed Transfer Hydrogenation. J. Am. Chem. Soc. 2007, 129, 3099−3103. (336) Jiménez-Tenorio, M.; Puerta, M. C.; Valerga, P.; Ortuño, M. A.; Ujaque, G.; Lledós, A. Counteranion and Solvent Assistance in Ruthenium-Mediated Alkyne to Vinylidene Isomerizations. Inorg. Chem. 2013, 52, 8919−8932. (337) Song, G.; Su, Y.; Periana, R. A.; Crabtree, R. H.; Han, K.; Zhang, H.; Li, X. Anion-Exchange-Triggered 1,3-Shift of an NH Proton to Iridium in Protic N-Heterocyclic Carbenes: HydrogenBonding and Ion-Pairing Effects. Angew. Chem., Int. Ed. 2010, 49, 912−917. (338) Ho, M.-H.; O’Hagan, M.; Dupuis, M.; DuBois, D. L.; Bullock, R. M.; Shaw, W. J.; Raugei, S. Water-Assisted Proton Delivery and Removal in Bio-Inspired Hydrogen Production Catalysts. Dalton Trans. 2015, 44, 10969−10979. (339) Yang, J. Y.; Bullock, R. M.; Shaw, W. J.; Twamley, B.; Fraze, K.; DuBois, M. R.; DuBois, D. L. Mechanistic Insights into Catalytic H2 Oxidation by Ni Complexes Containing a Diphosphine Ligand with a Positioned Amine Base. J. Am. Chem. Soc. 2009, 131, 5935−5945. (340) Aime, S.; Gobetto, R.; Valls, E. Role of Os−H- - -H−N Interactions in Directing the Stereochemistry of Carbonyl Cluster Hydride Derivatives. Organometallics 1997, 16, 5140−5141. (341) Aime, S.; Diana, E.; Gobetto, R.; Milanesio, M.; Valls, E.; Viterbo, D. Structural and Spectroscopic Study of the Dihydrogen Bond in an Imine Triosmium Complex. Organometallics 2002, 21, 50− 57. (342) Gatling, S. C.; Jackson, J. E. Reactivity Control via Dihydrogen Bonding: Diastereoselection in Borohydride Reductions of αHydroxyketones. J. Am. Chem. Soc. 1999, 121, 8655−8656. (343) Chakraborty, S.; Lagaditis, P. O.; Förster, M.; Bielinski, E. A.; Hazari, N.; Holthausen, M. C.; Jones, W. D.; Schneider, S. WellDefined Iron Catalysts for the Acceptorless Reversible Dehydrogenation-Hydrogenation of Alcohols and Ketones. ACS Catal. 2014, 4, 3994−4003. (344) Chantler, V. L.; Chatwin, S. L.; Jazzar, R. F. R.; Mahon, M. F.; Saker, O.; Whittlesey, M. K. Stoichiometric and Catalytic Reactivity of the N-Heterocyclic Carbene Ruthenium Hydride Complexes [Ru(NHC)(L)(CO)HCl] and [Ru(NHC)(L)(CO)H(η2-BH4)] (L = NHC, PPh3). Dalton Trans. 2008, 2603−2614. (345) Langer, R.; Iron, M. A.; Konstantinovski, L.; Diskin-Posner, Y.; Leitus, G.; Ben-David, Y.; Milstein, D. Iron Borohydride Pincer Complexes for the Efficient Hydrogenation of Ketones under Mild, 8586


Chemical Reviews

Review

Base-Free Conditions: Synthesis and Mechanistic Insight. Chem. - Eur. J. 2012, 18, 7196−7209. (346) Sandoval, C. A.; Yamaguchi, Y.; Ohkuma, T.; Kato, K.; Noyori, R. Solution Structures and Behavior of trans-RuH(η1-BH4)(binap)(1,2-diamine) Complexes. Magn. Reson. Chem. 2006, 44, 66−75. (347) Sandoval, C. A.; Ohkuma, T.; Muñiz, K.; Noyori, R. Mechanism of Asymmetric Hydrogenation of Ketones Catalyzed by BINAP/1,2-Diamine−Ruthenium(II) Complexes. J. Am. Chem. Soc. 2003, 125, 13490−13503. (348) Bianchini, C.; Perez, P. J.; Peruzzini, M.; Zanobini, F.; Vacca, A. Classical and Nonclassical Polyhydride Ruthenium(II) Complexes Stabilized by the Tetraphosphine P(CH2CH2PPh2)3. Inorg. Chem. 1991, 30, 279−287. (349) Grützmacher, H. Cooperating Ligands in Catalysis. Angew. Chem., Int. Ed. 2008, 47, 1814−1818. (350) Gunanathan, C.; Milstein, D. Metal−Ligand Cooperation by Aromatization−Dearomatization: A New Paradigm in Bond Activation and “Green” Catalysis. Acc. Chem. Res. 2011, 44, 588−602. (351) Gelman, D.; Musa, S. Coordination Versatility of sp3Hybridized Pincer Ligands toward Ligand−Metal Cooperative Catalysis. ACS Catal. 2012, 2, 2456−2466. (352) Werkmeister, S.; Neumann, J.; Junge, K.; Beller, M. PincerType Complexes for Catalytic (De)Hydrogenation and Transfer (De)Hydrogenation Reactions: Recent Progress. Chem. - Eur. J. 2015, 21, 12226−12250. (353) van der Vlugt, J. I.; Reek, J. N. H. Neutral Tridentate PNP Ligands and Their Hybrid Analogues: Versatile Non-Innocent Scaffolds for Homogeneous Catalysis. Angew. Chem., Int. Ed. 2009, 48, 8832−8846. (354) Rauchfuss, T. B. Diiron Azadithiolates as Models for the [FeFe]-Hydrogenase Active Site and Paradigm for the Role of the Second Coordination Sphere. Acc. Chem. Res. 2015, 48, 2107−2116. (355) Fung, W. K.; Huang, X.; Man, M. L.; Ng, S. M.; Hung, M. Y.; Lin, Z.; Lau, C. P. Dihydrogen-Bond-Promoted Catalysis: Catalytic Hydration of Nitriles with the Indenylruthenium Hydride Complex (η5-C9H7)Ru(dppm)H (dppm = Bis(diphenylphosphino)methane. J. Am. Chem. Soc. 2003, 125, 11539−11544. (356) Otsuka, T.; Ishii, A.; Dub, P. A.; Ikariya, T. Practical Selective Hydrogenation of α-Fluorinated Esters with Bifunctional Pincer-Type Ruthenium(II) Catalysts Leading to Fluorinated Alcohols or Fluoral Hemiacetals. J. Am. Chem. Soc. 2013, 135, 9600−9603. (357) Hedberg, C.; Källström, K.; Arvidsson, P. I.; Brandt, P.; Andersson, P. G. Mechanistic Insights into the Phosphine-Free RuCp*-Diamine-Catalyzed Hydrogenation of Aryl Ketones: Experimental and Theoretical Evidence for an Alcohol-Mediated Dihydrogen Activation. J. Am. Chem. Soc. 2005, 127, 15083−15090. (358) Ito, M.; Sakaguchi, A.; Kobayashi, C.; Ikariya, T. Chemoselective Hydrogenation of Imides Catalyzed by Cp*Ru(PN) Complexes and Its Application to the Asymmetric Synthesis of Paroxetine. J. Am. Chem. Soc. 2007, 129, 290−291. (359) Ito, M.; Hirakawa, M.; Murata, K.; Ikariya, T. Hydrogenation of Aromatic Ketones Catalyzed by (η5-C5(CH3)5)Ru Complexes Bearing Primary Amines. Organometallics 2001, 20, 379−381. (360) Dub, P. A.; Ikariya, T. Quantum Chemical Calculations with the Inclusion of Nonspecific and Specific Solvation: Asymmetric Transfer Hydrogenation with Bifunctional Ruthenium Catalysts. J. Am. Chem. Soc. 2013, 135, 2604−2619. (361) Zuo, W.; Prokopchuk, D. E.; Lough, A. J.; Morris, R. H. Details of the Mechanism of the Asymmetric Transfer Hydrogenation of Acetophenone Using the Amine(imine)diphosphine Iron Precatalyst: The Base Effect and The Enantiodetermining Step. ACS Catal. 2016, 6, 301−314. (362) Dub, P. A.; Henson, N. J.; Martin, R. L.; Gordon, J. C. Unravelling the Mechanism of the Asymmetric Hydrogenation of Acetophenone by [RuX2(diphosphine)(1,2-diamine)] Catalysts. J. Am. Chem. Soc. 2014, 136, 3505−3521. (363) Ohkuma, T.; Ooka, H.; Hashiguchi, S.; Ikariya, T.; Noyori, R. Practical Enantioselective Hydrogenation of Aromatic Ketones. J. Am. Chem. Soc. 1995, 117, 2675−2676. 8587


Hydrogen and Dihydrogen Bonds in the Reactions of Metal Hydrides

Recommend Documents