Review pubs.acs.org/CR
Dihydrogen Complexation Robert H. Crabtree* Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520-8107, United States ABSTRACT: Dihydrogen complexation with retention of the H−H bond, once an exotic concept, has by now appeared in a very wide range of contexts. Three structural types are currently recognized: Kubas dihydrogen, stretched dihydrogen, and compressed dihydrides. These can be difficult to distinguish, hence the development of a number of novel spectroscopic methods for doing so, mainly based on NMR spectroscopy. Three important reactivity patterns are identified: proton loss, oxidative addition, and dissociation, each of which often contributes to larger reaction schemes, as in homogeneous hydroformylation. Main group examples are beginning to appear, although here it is mainly by computational studies that the relevant structures can be identified. Enzymes such as the hydrogenases and nitrogenases are also proposed to involve these structures.
CONTENTS 1. 2. 3. 4.
Introduction Types of Dihydrogen Complex Early Work Spectroscopy and Structure 4.1. NMR Specroscopy 4.2. Neutron Diffraction 4.3. Inelastic Neutron Scattering (INS) 4.4. Vibrational Spectroscopy 5. Computational Work 6. Reactivity 6.1. Deprotonation and Proton Transfer 6.2. Oxidative Addition 6.3. Dissociation 6.4. Catalysis 6.5. Hydrogen Storage 7. Main Group Elements 8. Dihydrogen Complexation in Enzymes and Model Complexes 9. Dihydrogen Bonding and Hydrogen Bonding 10. Anion and Ligand Effects 11. Catalysis 12. Ignoble Metals 13. Conclusion and Outlook Author Information Corresponding Author Notes Biography Acknowledgments References
or as transient species present at some point along a reaction coordinate. Examples from the Main Group and in enzymes can now be included. Indeed, whenever dihydrogen is either a reactant or a product in a transformation, there can be a few cases where a dihydrogen complex is not a transient at some point on the reaction pathway. Dihydrogen complexation is best characterized in isolable transition metal dihydrogen complexes, where an H2 molecule is bound side-on to the metal. A significant degree of the H−H bonding is usually retained as in the celebrated discovery example, the Kubas complex, W(H2)(CO)3L2 (1) {eq 1, L = PiPr3 (1a) or P(C6H11}3) (1b)}.1 This binding mode leads to an elongation of the H−H distance (dHH = 0.81 Å in 1a) compared with that of free H2 (dHH = 0.74 Å). In most such complexes, often termed ’Kubas-type’, the resulting dHH typically falls in the range 0.80−1.0 Å. In less common cases, much longer dHH distances are seen that fall in the range of 1.0−1.6 Å, but, for reasons discussed below, these are not usually considered as full blooded dihydrogen complexes. Instead, those having dHH distances at the lower end of the 1.1−1.6 Å range are often termed stretched dihydrogen complexes, implying that only a small amount of HH bonding remains. Examples at the upper end of the dHH range are termed ’compressed dihydrides’, implying that no significant HH bonding remains and that the structure is the result of a distortion in the coordination geometry. The appropriate ranges for the three classes, Kubas, stretched dihydrogen, and compressed dihydride, are subject to discussion, but these are commonly considered to be Kubas, ≤ 1 Å; stretched, 1.0−1.25 Å; compressed dihydride, > 1.25 Å. The term nonclassical hydride is usually applied to the whole range of dHH from 0.74−1.6 Å. Numerous reviews of the field have appeared.2−9 Dihydrogen complexation of the Kubas type has been the most extensively
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1. INTRODUCTION The field of dihydrogen complexation began with transition metal examples where the complex could be directly observed. We can now see that these comprise only a small part of the full range of possibilities. In numerous cases, computational chemistry has suggested such complexes either as intermediates © 2016 American Chemical Society
Special Issue: Metal Hydrides Received: January 16, 2016 Published: March 14, 2016 8750
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2. TYPES OF DIHYDROGEN COMPLEX The Kubas7 complex itself (1) is the archetype for the majority of isolable examples. The best evidence8 for the retention of the H−H bond came from the neutron diffraction data and seeing a substantial 1JHD coupling in the 1H NMR of the HD analogue. In free HD, 1JHD takes the value of 43.2 Hz, but in typical (HD) complexes, values of ∼15−35 Hz are common (e.g., 33.5 Hz for 1a). This is the result of a significant amount of H−H σ bonding character remaining on binding of H2, given that classical M(H)(D) species have much lower 1JHD values, typically ∼2 Hz. The LnM(H2) interaction is best described7 as a combination of H2(σ) to metal direct donation to a suitable empty metal dσ orbital, augmented by back-donation from a filled metal dπ orbital to the H2(σ*) (Figure 1). The H2 bonding pair
reviewed,2−6 most notably in the definitive 2001 monograph by Kubas7 himself, who has also written a personal account8 of the discovery. The present review therefore emphasizes recent advances, although earlier work is selectively included to give context. Dihydrogen complexation is also important conceptually because H2 is the simplest possible stable σ-bonded molecule and constitutes the archetype for metal binding of XH σ bonds in general (X = C, Si···). The latter also bind in a similar way to (H2), as fully discussed elsewhere.7,9 The Kubas-type ligand is usually represented as (η2-H2) but in what follows we use (H−H) for specifying this particular class of ligand. Likewise, (H)2 represents a classical dihydride. This LnM(H2) interaction (where LnM represents a generalized metal complex fragment) is important because it greatly activates the dihydrogen molecule, particularly for the following three processes, as detailed in section 5. In type (i) proton transfer from the (H−H) can occur to an external base; depending on the extent of (H−H) activation, this can even be a very weak base. Likewise, in the reverse process, a proton often protonates a metal hydride to give an (H−H) intermediate, which may either be stable or go on to other species such as the dihydride LnM(H)2. In type (ii), related to the type (i) process, a proton can also transfer from (H−H) to an adjacent internal base, for example to an adjacent MH, MR (R = alkyl), or to an MNR2 group. Proton transfer to MH typically leads to fluxional behavior, where the (H−H) exchanges on the NMR time scale with an adjacent terminal MH. On reversibility arguments, a classical hydride complex should be able to undergo fluxional exchange via an (H−H) species as intermediate or transition state. Proton transfer to MR usually releases RH from the metal and leaves behind an MH bond and thus constitutes the widely occurring and important sigma-CAM10 or σ bond metathesis11 reaction pathways. Proton transfer from an (H−H) intermediate to an adjacent MNR2 group can lead to formation of HMNHR2 in a heterolytic H2 activation such as occurs in the Noyori12 asymmetric hydrogenation catalyst. In type (iii), if the metal can undergo a +2e redox change, oxidative addition of (H−H) can give a dihydride, LnM(H)2. This also provides an alternate mechanism for fluxional (H−H)/hydride exchange. Isolation of stable dihydrogen complexes permits close experimental study, including spectroscopic properties and determination of the dHH in specific structures. Where dihydrogen complexes are only transients on a reaction trajectory, they are not normally experimentally observable. This is where computation13,14 (section 4) shows its power by documenting such transients. Even Main Group elements have been found to form dihydrogen complexes as transient species in reactions that involve dihydrogen.15,16 Thus, we have an ’iceberg’ effect where the greater part of the topic is hidden from view.
Figure 1. Bonding model adopted for dihydrogen complexation in 1: direct donation from H2(σ) to an empty M(dσ) orbital (ML σ bond, right) accompanied by back bonding from a filled M(dπ) orbital to an empty H2(σ*) (ML π bond, left). The d-orbitals are shown in schematic form omitting the lobes that are not involved in bonding.
is only very weakly basic, so its involvement is insufficient on its own to permit formation of an isolable (H−H) complex back-donation is required for this to happen. The requirement for back-donation means that the vast majority of stable (H−H) complexes involve π basic metal fragments: d0 and high valent metals are therefore not expected to bind H2 in a stable way. For a number of stable LnM(H−H) complexes, LnM(N2) or LnM(CO) analogues are also known;7 in common with H2, both N2 and CO also require significant back-donation for stable binding. The bonding scheme of Figure 1 is a simple extension17 of the Dewar−Chatt picture that has long been applied to metal-alkene binding, but in the present case, H2 σ and σ* take on the donor and acceptor roles previously assigned to CC π and π* in LnM(C2H4). Ironically, Dewar18 himself felt that σ bonds should not be able to form stable sigma complexes. Steric effects can play a role where bulky coligands protect the (H−H) binding site from disturbance, hence the common occurrence of tricyclohexyl- and triisopropylphosphines in the field. On the other hand, the (H−H) ligand itself is probably too small to be directly affected by steric crowding, although full oxidative addition of M(H−H) to M(H)2 would add one unit to the coordination number and thus increase the crowding of the coligands and alter the coordination geometry adopted. This effect could tend to inhibit the oxidative addition, although no specific examples seem so far to have been suggested. Complex 1 has a high trans effect ligand, in this case CO, trans to the (H−H) binding site. This feature appears in many other (H−H) complexes and is consistent with the expectations of the antisymbiotic effect19 by which a high trans effect ligand is expected to prefer a low trans effect ligand such as (H−H) trans to itself. Complex 1 also shares another feature common to the majority of (H−H) complexes: a d6 octahedral structure. This is by far the largest category of (H−H) 8751
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change in the system, replacing L = P(iPr)3 by PMe3 in eq 1, is sufficient to tip the equilibrium entirely in favor of the dihydride, presumably for steric reasons.7 Unlike the case of eq 1 where the equilibration is slow on the NMR time scale at low temperature, and where both tautomers give separate NMR signals, dihydrogen-hydride exchange can be much faster. In [Ir(H−H)H2(PPh3)2(NHC)]+ (NHC = N,N′-dimethylbenzimidazole-2-ylidene), where the fluxional exchange barrier is calculated to be less than 5 kcal/mol, no decoalescence of the M−H and (H−H) NMR signals is ever seen.33 Less common is case b, where a stretched (or elongated) dihydrogen complex is seen, a topic fully reviewed by Heinekey, Lledós, and Lluch.34 Instead of a tautomeric mixture, we have a single structure with dHH in the range 1−1.25 Å;4,9,35 a situation represented here as (H···H). The term ’stretched dihydrogen’ sometimes ’elongated’was originally introduced to cover this relatively rare group. In a majority of cases we have a doubleminimum potential energy surface (PES) with classical and nonclassical species in equilibrium, and only less commonly do we see a single-minimum PES with a stretched dihydrogen as the stable ground state form. Thus, if the barrier for the oxidative addition is high, we would expect a tautomeric equilibrium for this situation of intermediate back-donation, while if the barrier is very low or nonexistent, then a stretched form can result. Stretched H2 complexes are highly sensitive to weak interactions.36 For example, IrH(H···H)Cl2(PiPr3)2 is a stretched dihydrogen hydride in the crystal (dHH 1.11 Å by n diffraction) but a classical trihydride in solution. The difference is thought to be due to intermolecular Ir−Cl···H−Ir hydrogen bonding in the solid. [Cp*Mo(CO)H2(PMe3)2]BF4, although unstable to loss of H2 above 230 K, provides another case where the equilibrium between the dihydride and dihydrogen products is so finely balanced that a slight change, in this case of solvent, affects the outcome. The classical dihydride is formed from the monohydride and HBF4 in CH2Cl2, while the (H−H) complex is formed from the same reactants in THF. Computational work suggests that the change of solvent affects the ion pairing of the salt. Relatively small changes of the ligand set can also alter the balance between classical and nonclassical forms of a polyhydride. For example, Parkin and co-workers37 have shown that although the molybdocene trihydride series of general type [Cp2MoH3]+ are well-known classical trihydrides, their ansa analogues where the two Cp rings are linked by Me2Si bridges take on a nonclassical structure, as in [{Me2Si(C5H4)2}MoH(H−H)]+. Consistent with the usual properties of (H−H) complexes, the same ansa effect also enhances the acidity of the hydride as well as the tendency of the complex to lose H2.
complexes, but a few are known with other configurations. Some isolable d2 e.g, [Cp′2Nb(H−H)(PMe2Ph)]OTf,20 d4 e.g, [Mo(H···H)(PMe3)2(NMe)(C6H4{NSiMe3}2)]OTf21 and d8 e.g., [Pd(PCP)(H−H)]+ (PCP = monoanionic pincer ligand)22 complexes are known, but, no doubt because of their weak back donor power, d10 cases are still rare, although a d10 Ni−(H−H) adduct has been detected23 as an intermediate in H−H oxidative addition across a Ni−B bond. Other examples have been found in a low temperature matrix, as in d10 Pd(H−H)n (n = 1, 2 and 3)24 or d4 WH4(H−H)4.25 A number have appeared in gas phase work, as in d10 (H−H)CuF26 or d10 (H−H)AgCl (dHH = 0.795 Å by APF-D calculations).27 There is also good evidence for formation of f 7 Cp*2Eu(H−H) as a labile species28 in solution, even though back-donation is lacking in this case.29 Even if the formation of isolable H2 complexes cannot occur, weak binding may still be adequate for promoting reactivity via transient species or at low temperature in hydrogen storage30 applications. The bonding in LnM(H−H) largely determines its reactivity pattern. If (H−H) (σ) to metal direct donation is dominant, the (H−H) ends up being little perturbed in the complex versus the free molecule. Even so, the (H−H) is strongly activated for proton loss (section 5). This would apply to weak π donor LnM fragments, such as would be expected for higher oxidation states, with first row metals, and for complexes with a high cationic charge31 or having an Ln ligand set that includes strong π acceptors, as in 1. This also applies when the ligand trans to the (H−H) site has a high trans effect. The H−H distance in this extensive class of compound is typically 0.8−1 Å, only modestly changed from free H2 (dHH = 0.74 Å). Vibrational spectral studies on 1 show that the force constant for the H−H stretch, 1.3 mdyn/Å, is less than 25% of that in free H2, suggesting that H−H bond weakening is already well along the path of the reaction coordinate for oxidative addition,32 even if the change in dHH from free H2 is only 0.07 Å. Less common are complexes having a greater degree of backdonation,14 in which case the H−H bonds tend to elongate or even break altogether. This would be expected for strong π donor LnM fragments, that are more likely in lower oxidation states, for third row metals, and in neutral complexes with a strong donor Ln ligand set; the L ligand trans to the (H···H) site now tends to have a lower trans effect than in the prior case. With a strong back donor LnM fragment, three alternative processes tend to occur. Either a) we see a tautomeric equilibrium between the (H−H) form and the classical oxidative addition product, or b) the H···H bond elongates beyond 1 Å while retaining some H···H interaction, at least until dHH reaches ∼1.25 Å, or c) oxidative addition leads to a compressed dihydride or a classical hydride. The more common case, a, is exemplified by the Kubas complex itself, the (H−H) (1) and dihydride (2) forms being in equilibrium (eq 1) with the exchange being slow enough for the two tautomers to appear independently in the proton NMR spectrum at low temperature. There must be many cases analogous to eq 17 where one or other tautomer is present in amounts that are kinetically significant, but observationally undetectable, and in which the tautomerism may therefore remain unsuspected. For 1b itself, ΔG† for eq 1 is 16 ± 2 kcal/mol,7 consistent with the requirement that the ligands undergo a substantial rearrangement during the reaction; this picture suggests that the intrinsic barrier for H2 oxidative addition is low and that ligand rearrangement is the hard part of the process. A small
3. EARLY WORK Before Kubas7 securely characterized complex 1, a number of complexes later shown to be H2 complexes were already recognized as unusual. For example, Aresta’s38 “FeH4(PEtPh2)3″ (3) of 1971, then assumed to be an Fe(IV) complex, showed a puzzling IR band at 2380−2400 cm−1 that disappeared on formation either of FeH2(PEtPh2)3 or of FeH2(N2)(PEtPh2)3. After much later reformulation of the supposed Fe(IV) complex as an Fe(II) dihydrogen complex,39 this band was reassigned to the Fe(H−H) stretch in Fe(H−H)H2(PEtPh2)3, the vibration no doubt being coupled to the adjacent Fe−H bonds leading to enhanced band intensity. In their 1976 report on “RuH4(PPh3)3″ (4), Ashworth and Singleton40 were clearly puzzled both by the easy loss of H2 from 4 and by the breadth 8752
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above: exchange of (H−H) for N2 in the Morris case with formation of [Fe(N2)H(dppe)2]+ by slow reaction with N2.
of the hydride resonance in the proton NMR spectrum. The authors even speculated, although without any definite evidence, that an H−H bond might be present in 4. In our own much later 1986 study,39 both 3 and 4 were indeed identified as H2 complexes, mainly relying on the abnormally fast T1 relaxation of the hydride signal, an NMR relaxation method to be discussed in more detail in section 4.1 below. The same method showed that OsH4(P{p-C6H4Me}3)3 is a classical Os(IV) tetrahydride, the role of the tritolyl- rather than triphenylphosphine only being to faciliate measurements by enhancing solubility.
4. SPECTROSCOPY AND STRUCTURE Identification of dihydrogen ligands in isolable complexes remains a challenge in part because conventional X-ray diffraction is not often able to securely differentiate between classical and nonclassical structures in polyhydrides, H atoms being only weakly diffracting. Indeed, some dihydrogen complexes no doubt lie undiscovered in the literature, wrongly assumed to be classical hydrides. A battery of spectroscopic techniques have therefore been developed for making the needed structural distinction, notably methods based on solution NMR spectroscopy. 4.1. NMR Specroscopy
NMR spectroscopy has become the standard first approach in the identification of dihydrogen complexes. A number of measurements help to distinguish between the LnM(H−H) and LnM(H)2 structures for a dihydride (Ln represents the n ancillary ligands), where the dHH of the dihydrogen is assumed to lie in the range found for the great majority of cases, say 2), where fluxionality usually makes measurement of the 1JHD either unreliable or impossible. In such a case, dipole−dipole relaxation provides a useful criterion. The T1 value for the (H−H) resonance in a diamagnetic dihydrogen complex is short, far shorter than that for any other proton in a typical complex, making it a good qualitative indicator of nonclassical (H−H) character. The short dHH in an (H−H) ligand leads to a greatly enhanced dipole−dipole relaxation rate because this DD relaxation depends on the inverse sixth power of the dHH. Detailed interpretation of the T1 data to derive the dHH is complicated because several other factors48,49 influence the T1 value. The magnetic field dependence is easiest to allow for, because the T1 scales with the spectrometer frequency, but the temperature of the measurement is an important variable as well because this alters the rotational motion of the complex. Knowledge of the rotational correlation time of the complex is necessary to derive dHH, but this is not normally available. We proposed that this difficulty could be avoided if the minimum T1 value with variation of the temperature were determined experimentally. At this temperature, the relaxation rate is maximal and the equations relating the T1 to dHH are much simplified because the rotational correlation time now becomes 0.63/2πν (ν = spectrometer frequency). The minimum T1 values for typical complexes fortunately tend to occur in a conveniently accessible temperature in the range of −20° to −80°. The dHH values derived from T1 min studies corresponded closely, but not perfectly, with dHH values independently determined by other methods, such as crystallography and 8753
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1
JHD of the (HD) analogue. This deviation was traced by Morris50 to the rotational motion of the (H−H) about the M(H2) axis, a factor not considered in our original data analysis. As discussed in detail by Morris,50,51 when (H−H) rotation is considered, the dHH needs to be corrected by multiplying it by a correction factor, C, which takes the value of 1.0 for no (H−H) rotation but becomes 0.79 for fast rotation. In the absence of direct knowledge of the true rotation rate, a value of C = 0.9 seemed appropriate to give an approximate dHH. In another Ru dihydrogen complex, the X-ray structural data gave an H−H distance of 0.75(7) Å, seemingly too short. T1 data gave a much more reasonable 0.90 Å but only assuming fast Ru(H2) rotation. Interpretation of the T1 data has normally assumed such a fast rotation regime, but as we have seen above20 this does not always apply. Morris has compared the T1-derived dHH values for a very extensive series of complexes with those from 1JHD measurements, and where these are also available, from coupling data or crystallography. The data was most consistent with an intermediate rotation regime for many of the cases studied. Beyond simple rotation, two other motional regimes were also considered by Morris: (i) torsional oscillation of the H2 in a 2-fold potential energy surface and (ii) the (H−H) hydrogens undergoing rapid 90° hops between sites of unequal population in a potential surface with 4-fold symmetry. Of the 73 dihydrogen complexes examined in detail in this context, fast rotation was proposed for 32 of them, six complexes had slow internal H2 motion, and in 35 cases either torsional libration or fast hopping had to be considered. The Morris51−53 papers give full details. A recent example33 involves complexes 6 and 7 (L = PPh3), in which the classical trihydride, 6, has a T1(min) of 350 ms, while the highly fluxional dihydrogen dihydride complex, 7, has a T1(min) of 30 ms at 500 MHz. This way of estimating dHH assumes fast rotation about the M(H2) bond, as is true for most (H−H) complexes, but where rotation is slow, substantially different dHH values apply, as has been fully reviewed by Morris.52,53
from the tricarbonyl. The 1JHD of the Cr(HD) analogue, 33.4 Hz, translates to a calculated 0.86 Å for dHH. The T1(min) of 14 ms at 205 K brackets this value by giving a dHH of 0.78 Å, assuming fast (H−H) rotation or 0.99 Å if slow. As a result of recent work, complexes at the long dHH end of the range previously considered stretched (H···H) complexes are now generally reassigned to the class of compressed dihydrides, symbolized here as (H∼H) complexes. The ∼ symbolizes a compressed spring rather than a bond. Kubas uses the latter term for the range 1.2−1.5 Å.7 On the basis of their computational results, Devarajan and Ess14 prefer 0.8−1.0 Å for standard (H−H) complexes and 1.0−1.2 Å for stretched examples and reserve the term compressed dihydride for the range 1.2−1.6 Å. Compressed dihydrides can be thought of as structures that would normally be transients on the distant approach part of the trajectory for oxidative addition but now frozen out as ground state structures by an interplay of the usual suspects, electronic and steric factors. Lluch, Lledós, Heinekey58,59 (LLH) and co-workers have persuasively argued that the different types of dihydrogen complex can be experimentally distinguished from differences in the temperature dependence of 1JHD. For the Kubas type with short dHH, the 1JHD value is essentially constant with temperature, but in the stretched cases up to about 1.25 Å, 1JHD rises with increasing temperature, consistent with an increase in H−H bonding with temperature. Beyond ∼1.25 Å, 1JHD is still temperature-dependent, but the situation is the opposite in that the 1JHD now falls and the dihydride character now increases with increasing temperature. For example, 1JHD rises for the stretched complex [Cp*Ru(dppm)(H···H)]+, where dHH is 1.10 Å (n-diff).47,60 In an example of a compressed dihydride, [Cp*Ir(dmpm)(H∼H)]+ where dHH is 1.49 Å (from T1), 1JHD falls with temperature rise, the system becoming more dihydride-like.61,62 Theoretical study has identified two minima on the PES for this complex, one being an (H−H) complex and the other a dihydride with a dHH of 1.63 Å; a full dynamics study validates the origin of the temperature variation in 1 JHD. In classical dihydrides, in contrast, 1JHD is both small and temperature independent. Another example of a stretched dihydrogen series showing a big temperature dependence of 1 JHD as well as a dihydrogen-dihydride equilibrium is [(p-XPOCOP)IrH2] (10, X = MeO, Me, F, H, and C6F5).63
Another technical problem that sometimes affects the T1 is the presence of metal nuclei that contribute to the relaxation of the hydride protons. This is negligible for most d block metals but not for Nb, V, Re, Mn, Co, and Ta, where inclusion of the metal−hydrogen dipole−dipole relaxation improves the accuracy of the T1 method.54 The d8 complex [Rh(HD)(PONOP)]BArF4 (8) shows an elaborate multiplet in the proton NMR, in which a 1JHD of 30.1 Hz can be identified. The other couplings present are 1JRhH and 2JPH with values of 27.5 Hz and 4.5 Hz. The T1(min) of 33 ms for the (H−H) isotopomer is also consistent with the proposed formulation. In contrast, moving to Ir, known for its enhanced back-donation versus Rh, the analogous [Ir(HD)(PONOP)]BArF4 has a T1(min) of 873 ms, identifying it as a classical dihydride.55 A related d8 pincer complex [RhI(HD)(PNP)] (9) has a T1(min) of 39 ms at −40° and a 1JHD of 18 ms corresponding to a stretched (H···H) complex with a dHH of 1.13 Å.56 Egbert and Heinekey57 have synthesized [(arene)M(CO)2(H−H)] complexes (M = Cr, Mo, W) by photolytic CO extrusion
The pronounced temperature variability of 1JHD for stretched dihydrogen and compressed dihydride complexes is consistent with a flat, anharmonic PES associated with the H−H stretch. The differences in sign of the temperature variation of the 1JHD is associated with the sense of the anharmonicity of the PES. For the stretched dihydrogen case, the PES is flatter as the system moves toward shorter dHH, while for the compressed 8754
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dihydride case, the PES is flatter as the system moves toward longer dHH. Thus, as higher vibrational levels are populated by thermal excitation, the average dHH moves to higher or lower values, depending on the situation. Because of the lower zero point energies for the heavier hydrogen isotopes, higher vibrational levels are expected to be occupied, leading to the previously predicted36,58,59,64 temperature shift of the dHD, dHT, and dDT as is indeed observed (Figure 2).47 However, it is not
have essentially equivalent H to D ratios from LnM(H−H)pHq cases where the (H−H) and H sites are expected to have both different intrinsic chemical shifts and different equilibrium H to D ratios. The latter difference arises from the high vibrational frequency of the (H−H) leading to a higher zero point energy difference between H and D in the (H−H) site. The result is a temperature-dependent preference of the heavy isotope for the stronger bond. With a partially deuterated sample, say of a fluxional dihydrogen hydride, the IPR effect manifests itself in the form of temperature-dependent, distinct resonances for the d0, d1, and d2 isotopomers in the proton spectrum. On the other hand, if the molecule is a fluxional classical trihydride, no IPR shifts are observed. The classical and nonclassical cases can thus be distinguished. Quantitative analysis of the data can give the chemical shifts of the (H−H) and H sites and the ΔE for isotope fractionation between the sites. IPR effects have been seen in cis-[Ir(H)(H−H)(S2CH)(PCy3)2][BF4] and [Ru(H)(H−H)(PPh3)2(dipy)][OTf], both of which are borderline stretched complexes (dHH = 1.05 Å).67,68 From quadrupolar coupling constant (DQCC) parameters, determined from solid-state 2H NMR data of a number of D2 complexes, combined with DFT calculations, Bakhmutov69 has proposed a criterion for telling rapidly spinning (H−H) ligands from nonrotating ones. Occasionally, very anomalous, temperature-dependent couplings can be seen in the NMR spectra of di- or polyhydrides, as well as extreme second order behavior. This results from a pairwise quantum mechanical exchange of the two protons in a double-well potential.70 If their true origin is not recognized, these data could be misinterpreted in terms of the presence of (H−H) ligands. Quantum mechanical exchange coupling (QMEC), although a relatively rare phenomenon, can be the origin of the effect. Work by the groups of Chaudret70 and Heinekey71 led to the discovery of temperature-dependent H,H′ coupling values that can greatly exceed 250 Hz, thus being far higher than would ever be expected for a typical 1 JHH′ coupling in a nonclassical hydride. A good example is [CpIrH3(PPh3)]+ where the coupling is 900 Hz at 170 K. Very low temperatures are often required for full analysis of QMEC data, prompting the need for low temperature solvents, in this case CDFCl2.72 [Cp*RuH3(P(pyr)3)] (pyr =1-pyrrolyl) is even more extreme with a coupling of 3000 Hz at 180 K.73 The effect is thought to result from quantum mechanical exchange between pairs of inequivalent protons that are relatively close in space, although formation of an (H−H) state is not required74 so the phenomenon is not of direct interest in the present context except as a warning that large temperature-dependent coupling can occur without any need for invoking the presence of (H−H) complexes either as ground states or as intermediate states on an exchange pathway. As an example of a case of QMEC in which the data was problematic to interpret and structural misassignments became possible, the NMR data from the osmium complex 11 was originally interpreted as implicating a stretched Os(II) dihydrogen complex 11a (L = PiPr3). Later reanalysis carried the NMR measurements to much lower temperature to confirm QMEC and suggested that an Os(IV) compressed dihydride complex 11b is the true structure.75 Most (H−H) complexes show rapid rotation about the M(H−H) bond, implying that back-donation can occur to stabilize the intermediate rotational geometries throughout the rotation process. Such low barriers are typically quantified by inelastic neutron scattering, as described in section 4.3 below.
Figure 2. Bond distances and uncertainties in [Cp*Ru(dppm)(H···H)]+ as a function of temperature: HD, triangles, HT, circles; DT, squares. Reprinted with permission from J. Am. Chem. Soc. 2001, 123, 2085. Copyright 2001 American Chemical Society.
clear how far the LLH categories can be usefully applied to the not uncommon case of polyhydrides where 1JHD is normally unavailable as a result of fluxionality (e.g., [ReH5(H∼H)(PPh3)2] dHH = 1.357 Å {n diffraction}] and where we are reduced to applying the distance criterion and referring to a compressed polyhydride. If so, Figure 3 gives an indication of the nomenclature.
Figure 3. Current state of nomenclature in the field. The dHH ranges given for the divisions are indicative only and not precise values.
Current practice thus identifies three groups of nonclassical hydrides. The most numerous, Kubas-type complexes, typically having dHH below 1.0 Å. Those with dHH from ∼1.0 to ∼1.25 Å are considered true stretched dihydrogen cases, but ones with dHH > ∼1.25 Å are better thought of as compressed dihydrides, a distinction ideally based on NMR and computational data, not just dHH. All three classes can be considered nonclassical65 in that they form a continuum of structures that do not follow the classical, pre-Kubas expectation for the exclusive occurrence of well separated terminal hydrides. Useful information can still be extracted from NMR data even when the fluxionality is so fast that distinct (H−H) and hydride peaks cannot be distinguished at any accessible temperature. Isotopic perturbation of resonance (IPR)66 that can arise in partially deuterated polyhydrides can be applied to distinguish between classical polyhydrides where all H sites 8755
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A 1999 NMR study by Caulton81 suggested that there were three hydrogens, not just one, and the T1 study gave a value of 26 ms that suggested a nonclassical structure. The complex was therefore reformulated as [OsClH(H−H)(PPh3)3] but neither 1 JHD nor T1(min) were available because of fluxionality and low solubility, so the dHH could not be estimated. The definitive 2005 neutron diffraction study by Jia and Bau82 showed that [OsClH(H∼H)(PPh3)3] is in fact a compressed dihydride complex with one short dHH of 1.48(2) Å. The third hydride is adjacent to, and coplanar with, the (H∼H) group with the closest dHH being 1.67(2) Å. The full story of this structure therefore took 30 years to be told and involved three different structural proposals. Of course, it cannot be excluded that the structure differs between solution and the solid state. The same study by Jia and Bau82 also compared the structural data for a number of selected hydrides in order to map out a Bürgi−Dunitz83 reaction trajectory for the oxidative addition of H2 to a transition metal, each structure being considered as a way station along the reaction path. Including the stretched (H···H) examples means the change in dHH along the pathway is very great, almost every intermediate distance between free H2 and a classical dihydride being apparent in one or other member of the series. Paramagnetic dihydrogen complexes are rare because characterization of any (H−H) complexation is complicated without NMR data, but one example, [MoCp†(PMe3)2H(H∼H)] (Cp† = 1,2,4-C5H2{t-Bu}3), has been identified crystallographically by neutron diffraction (dHH = 1.36(6) Å). This case illustrates the limited applicability of the oxidation state concept and the consequent dn assigment to stretched dihydrogen and compressed dihydride complexes, because if the complex is considered as a trihydride, we have a d2 Mo(IV) species, but if as a dihydrogen complex, albeit stretched, we have a d4 Mo(II) complex.84
Much higher rotation barriers, in the range accessible to NMR spectroscopy, can occur in d2 complexes where no backdonation is possible if the H2 undergoes a 90° rotation since only one filled dπ orbital is available for back-donation. Such is the case for [Cp′2Nb(H−H)(PMe2Ph)]OTf, for example, where H2 rotation, with a rotation barrier of 11 kcal/mol, is frozen on the NMR time scale at 203 K as shown by the HD complex having two resonances in the 1H NMR spectrum, one for the endo and the other for the exo rotational isomer.20 4.2. Neutron Diffraction
The key references in this field are the two reviews:76,77 one from 1981 by Teller and Bau followed by an update by Bau in 1997. These illustrate the essential role played by neutron diffraction in verifying the short H−H bond distances in a number of critical cases and, by extension, validating the spectroscopic methods for characterizing such complexes in general. The librational motion of the bound (H−H) molecule does pose problems, however, because it can make the standard ellipsoidal model of the nuclear positional motion invalid. When the true banana-shaped distribution of nuclear positions resulting from (H−H) libration is modeled by ellipsoids, the resulting apparent H−H distance is artifactually short. The data analysis can be modified to take account of this problem, however. For example, the apparent dHH of 1.08(3) Å for [Cp*Ru(H−H)dppm]BF4 became 1.10(2) Å after the librational correction was applied,36 so no great error was involved, at least in this case. The Kubas complex 1a proved to have a dHH of 0.82(1) Å typical of the Dewar−Chatt78 bonding model with weak back-donation. On the other hand, [Re(H∼H)H5{P(pCH3C6H4)3}2] was the first recognized nonclassical polyhydride with a dHH of 1.357(7) Å77 and thus falls into the compressed polyhydride (H∼H) range. Somewhat longer distances are borderline classical, as in [OsH6{PPh(i-Pr)2}2] with a dHH of 1.650(6) Å, for example.77 An example of a recent n-diffraction study, the pincer complex, 12, has a bis-dihydrogen hydride structure with dHH values of 0.839(4) Å and 0.839(8) Å, where both (H−H) ligands are trans to a high trans effect ligand, H or aryl. The larger uncertainty for the second value was associated with facile rotation of this (H−H) molecule. Additional data collected at 100, 150, and 200 K permitted an analysis of the libration of this (H−H) with the result that an estimate of the rotational barrier (1.8 kcal/mol) could be obtained. This study also served to verify the computationally predicted structure, which closely matched the experimental one.79 In a classic example of the difficulty in characterizing nonclassical hydrides, only finally resolved by n-diffraction, a compound said to be [OsClH(PPh3)3] was reported in 1974.80
4.3. Inelastic Neutron Scattering (INS)
In contrast to the elastically scattered neutrons that give structural data, the neutrons useful in the INS technique undergo an energy change on interacting with atomic nuclei in the sample. This energy shift, which can be measured, reflects the energy deposited in, or received from, a nuclear motion such as a bond vibration in the sample. For H2 complexes, H2 rotation can be probed in this way. INS is very sensitive to the height of the H2 rotational barrier in nonclassical hydrides because the rotational tunnel splitting of the ground state has an exponential dependence on the barrier height.85 Notably for our purpose, the range of rotational energy barriers accessible by INS, typically from as low as 0.5 up to a few kcal/mol, is well below the range available from any other experimental technique, leaving aside the n-diffraction study mentioned in section 4.2. INS work often includes a computational aspect, as in the study by Hall, Eckert, and colleagues on [IrXH2(H−H)(P{iPr}3)2] (X = Cl, Br, I) which was directed to understanding the mechanism of hydride fluxionality. The two lowest energy processes proved to be (H−H) rotation and (H−H) oxidative addition. In particular, the calculated (0.4−0.7 kcal/mol) and experimental INS (0.5−1.0 kcal/mol) rotation barriers were in good agreement.86 INS has also been applied to materials, for example in assessing (H−H) motion in at the cobalt site of a cobalt aluminophosphate catalyst87 or studying (H−H) absorption by a Cu(II) MOF.88 Unfortunately, a ’barrier gap’ still exists between those low barriers that can be quantified by 8756
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INS and the much higher ones, say > ∼10 kcal/mol, that can be quantified from NMR coalescence data.
5. COMPUTATIONAL WORK Much of the existing computational work is associated with specific experimental studies, as already discussed in the relevant sections, but some pure computation has also been published. The key reference13 to the early work is a Chemical Review from 2000 that discusses the theoretical tools available for understanding the bonding in (H−H) complexes. For example, energy decomposition analysis, the extended transition state decomposition scheme, and charge decomposition analysis all point to the importance of back-donation to H2 σ* in weakening the H−H bond.13 This makes the important point that H, although one of the hardest atoms to locate experimentally, is one of the easiest to treat computationally. This thought led to a powerful DFT computational method in which H atoms can be located with high reliability even in complex structures. The technique extends beyond mononuclear species to clusters of substantial size, where X-ray crystallography is completely powerless. In one of the most recent examples, 14 hydride ligands were located by Balcells and colleagues97 in [Ir6(IMe)8(CO)2(H)14]2+, a polyhydride cluster formed as a deactivation product from a [Ir(IMe)2(CO)2]+-catalyzed glycerol dehydrogenation to form lactic acid, where the seven distinct types of hydrides proved to be either terminal or bridging without any nonclassical hydride ligands being present. A useful concept for predicting the type of hydride to be expected for a given metal fragment LnM has been suggested.98 On this model, an LnM(H−H) complex is considered as resulting from H2 interacting with the singlet state of LnM, while the classical LnM(H)2 dihydride is considered as arising from the triplet state of the LnM fragment. The S/T energy gap should thus be a key factor in deciding which type of complex is formed. Additional factors have to be considered, such as the dissociation energies De(MH) and De(MH2). The treatment the series of complexes M(CO)n(PH3)(5‑n) (where M = Cr, Mo, W; n = 0, 3, 5) gave the result, shown in Figure 4, that different
4.4. Vibrational Spectroscopy
Since the band intensity in infrared spectroscopy depends on the dipole moment change during the vibration, free H2 is IR silent, but bound M(H−H) can gain some intensity either because the M−H bonds have some dipolar character or by coupling to other vibrations, such as an adjacent terminal H or CO. Even so, the resulting IR bands tend to be weak, and the technique has thus not been a major source of information. In the best-studied7 case of the Kubas complex, four of five expected modes have been observed in the IR: νHH at 2690, νMH2(asymm.) at 1575, νMH2(symm.) at 975, and δMH2(in plane.) at 1575 cm−1. The fifth mode, δMH2(out of plane.) at ∼640 cm−1, comes out of INS data. IR data do have a role in identifying (H−H) complexes in some cases, as in the matrix isolation work by Poliakoff and Turner89 on Fe(CO)(NO)2(H−H) and Co(CO)2 (NO)(H−H) from UV photolysis of Fe(CO)2(NO)2 and Co(CO)3(NO) in liquid xenon at −104 °C under 10−20 atm of H2 or on cis(C2H4)M(CO)4(H−H) similarly generated.90 DFT work is now often combined with the matrix isolation work, as in the case of Pd(H−H) with νHH at 2971 cm−1 and dHH(calc) = 0.854 Å as well as Pd(H−H)n (n = 2,3). Numerous other metals form similar complexes in a matrix, such as Cr(H−H)H2, Cr(H−H)2H2, and Cu(H−H)H.91,92 Gas phase IR spectroscopy has identified numerous (H−H) complexes of main group and transition metal cations of the type M(H−H)+ (M = Li, B, Na, Mg, Al, Cr, Mn, Zn, Ag).93 In the Mn complex, studied in particular detail, the (H−H) is very weakly bound and minimally perturbed from free H2. It has a rotationally resolved series of νHH bands from 4022 to 4078 cm−1, a dHH of 0.768 Å, and an M−H2 centroid distance of 2.51 Å; this is well outside the range seen in isolable H2 complexes and the binding energy is a mere 1.9 kcal/mol, so this seems to be best considered94 as a van der Waals rather than a Kubas complex. Vibrational studies are also relevant to H2 absorption to metal ions in MOFs. In the HKUST-1 MOF, for example, H2 binding to Cu(II) is associated with a band assigned to νHH at 4100 cm−1.95 The band intensity in Raman spectroscopy has the advantage of depending on the polarizability change during the vibration, hence application to (H−H) complexes might seem promising. In reality, however, the absorption of energy from the required laser irradiation often decomposes the complex, preventing the collection of any useful data. The Kubas complex is one of the rare cases of an isolable complex where data was successfully obtained, although the authors report their experimental difficulties as follows: Despite the use of low power (ca. 1 mW) and cooling of the sample to 77 K, partial decomposition slowly took place when the sample was illuminated by the laser beam during the course of the experiments. Surprisingly, the rate of decomposition was higher at 77 K than at 298 K (possibly due to a sample phase change), so spectra were recorded at room temperature. The IR, Raman, and INS data were all successfully interpreted in the context of a normal-mode analysis that provides unambiguous assignments for all metal−hydrogen stretching and bending frequencies.32 The equilibrium isotope effect is inverse, with KH2/KD2 = 0.78 at 300 K, as a result of zero point effects.96
Figure 4. An approach to predicting the type of hydride to be expected, where the energy difference between the classical and nonclassical structures is related to key bond energies and the singlet−triplet energy gap of the metal fragment. Reprinted with permission from Organometallics 1998, 17, 4932−4939. Copyright 1998 American Chemical Society.
regions of the plot contain the three types of outcome: a classical dihydride, a dihydrogen complex, and a fluxional equilibrium between the two forms. 8757
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strong acid character, explaining the occurrence of such reactions as shown in eq 3. WMe6 being a 12e complex, no difficulty is expected in binding a small, 2e ligand. Once transiently bound, these H2 ligands could release protons that serve to liberate methane from the complex, as well as provide the classical hydride ligands in the product. Six repetitions complete the process.101 WMe6 + 2Pi Pr3 + 6H 2 → WH6(Pi Pr3)2 + 6MeH
(3)
The same type of reactivity has been seen in a number of cases since that time. The most recent such result comes from Okuda’s group102 and involves a Ca(II) silyl complex LCa(SiPh3) (L = 1,4,7,10-tetramethyltetraazacyclododecane) that undergoes hydrogenolysis at 60 °C to give a cationic calcium hydride cluster complex [(LCa)3(μ-H)2]+. This seems to imply that a transient H2 complex is formed even in this main group case, where such a Kubas interaction could not have been safely predicted. In an example from the Evans group,103 the sterically crowded Cp*3Sm was hydrogenolyzed to form [(Cp*2Sm)2(μ-H)2] and Cp*H. In neither Sm(III) nor Ca(II) is oxidative addition of H2 allowed because these elements are already at their maximal oxidation state, but an (H−H) interaction is still allowed because binding of H2 involves no change of oxidation state. Where similar hydrogenolyses occur for transition metals not in their highest oxidation state, deprotonation of an (H−H) complex or transient cannot safely be proposed without more definite evidence because it is always possible that oxidative addition occurs to give a transient dihydride that rapidly reacts by reductive elimination with the alkyl to give the final product. Computation can be suggestive in such cases but it cannot safely go beyond the specific pathways that have been tested, and it is always possible that a lower energy pathway could have been missed. On occasion, a transition metal hydrogenolysis proceeds in a case where a dihydrogen complex is specifically detected prior to the MC cleavage step. It is tempting to postulate a proton transfer to the carbon atom in such a case, but oxidative addition/reductive elimination could still occur via a transient dihydride. An example where this possibility cannot be eliminated is shown nearby (eq 4).104 The dihydrogen hydride is fluxional so it is no objection to the hydrogenolysis path to note that the H2 ligand is trans to the Ir−C bond to be broken. The H2 could first transfer a proton to the adjacent terminal hydride to form an H2 ligand in the cis position, able to subsequently transfer a proton to the aryl group. Once the Ir−C cleavage occurs, the substituted pyridine departs, giving way to the solvent, Me2CO.
Figure 5. Calculations for the [CpRu(H2PCH2PH2)(H−H)]+ model compound show the flat, anharmonic PES for this dihydrogen complex. Contours in kcal/mol. Reprinted with permission from J. Am. Chem. Soc. 1997, 119, 9840. Copyright 1997 American Chemical Society.
The potential energy surface (Figure 5) for the model complex [CpRu(H2PCH2PH2)(H−H)]+ shows a broad, flat, and very anharmonic region ’East Southeast’ of the global minimum, suggesting that the complex is able to sample a wide range of structures at room temperature, accounting for the temperature dependence of 1JHD in related dihydrogen complexes, as discussed in section 4.1.64 The valley leading ’North’ in the same figure corresponds to H2 dissociation. Molecular dynamics calculations have been applied to (PH3)3MH4, (M= Os, Ru, and Fe) by Morrison and Reilly.99 The results not only show the anharmonicity of the vibrations involving the hydrogen atoms but also model a fluxional exchange process for the iron complex in which one H2 ligand oxidatively adds to give a new dihydride unit while a pair of terminal hydrides reductively eliminates to form a new H2 ligand.
6. REACTIVITY This topic divides into three main classes: 1) deprotonation and proton transfer; 2) oxidative addition; and 3) dissociation. All these steps can contribute to catalytic activity, an application that has been the topic of a detailed and comprehensive review by Esteruelas and Oro.100 6.1. Deprotonation and Proton Transfer
The Dewar−Chatt bonding model17 of Figure 1 suggests that in cases where direct donation is dominant and the backdonation component is minimal, the (H−H) tends to attain a net positive charge favorable for proton loss. This implies that the activation of the (H−H) for this process could still be strong where the M(H2) bond strength is, if not negligible, at least very weak. Such is probably the case for the reaction of H2 with d0 metals where no stable (H−H) complex is to be expected, but binding can still occur as a transient. Given that many stable H2 complexes already have pKas well into negative territory, in the case of such a transient we can also expect 8758
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suggested that the proton could transfer via the metal center, understandable since the amido N was trans to the H2 binding site. Once the H−H bond had broken, but before the N−H bond had formed, this transfer, if intramolecular, required that the proton was bound to the Ni, thus implying the presence of a transient species with Ni(IV) character along the reaction pathway, although this species was not a local minimum and cannot therefore be considered as a true intermediate.31
Even in cases where a classical dihydride is the last observed precursor to the hydrogenolysis, we cannot exclude transient H2 formation, at least in cases where the metal has a viable oxidation state two units lower. An apparent case of this process comes from Esteruelas, Larramona and Oñate,105 where an Os(IV) dihydride reacts with 1,1-dimethylallene to give an Os(II) complex of iPrCHCH2. DFT work indicates that after a partial reductive elimination to give a compressed dihydride (dHH = 1.386 Å), proton transfer to the central carbon of the coordinated allene occurs in the rate-determining step. In a number of reports, reviewed by Jia, Lin, and Lau,106 dihydrogen complexes have been identified as the key intermediates in hydrogenation catalysis. For example, Cr(CO)6 is a photocatalyst for the hydrogenation of norbornadiene, where proton transfer from the (H−H) ligand of [Cr(H−H)(η4-nbd)(CO)3] to the alkene is supported by IR and NMR studies. In a related case, Poliakoff and co-workers107 have invoked (H−H) intermediates in the formation of dimethyl succinate via hydrogenation of coordinated dimethyl fumarate (= L) in [Fe(CO)4L], in a polyethylene matrix and stimulated by UV irradiation. Perhaps the most important example of a pathway involving proton transfer from (H−H) is the classic Co2(CO)8-catalyzed hydroformylation that converts RCHCH2, CO and H2 to RCH2CH2CHO. The key product-forming step, illustrated by eq 5, involves proton transfer from the (H−H) to the cobalt acyl group. This has now been authenticated13,108,109 by computational work from the 1990s. Remarkably, however, Milton Orchin (1914−2013) correctly guessed at the nature of this step as early as 1972 in his review110 of Co hydroformylation. In this work, he even invoked a Chatt−Dewar bonding scheme for M(H−H) similar to the one shown in Figure 1, although without any proof nor any comment about the possibility of isolating such complexes in the future.
6.2. Oxidative Addition
In conventional oxidative addition of H2, it seems inescapable that a progressive series of (H−H) species are present along the kinetic pathway. In some cases, both the (H−H) and (H)2 species are separately observable as tautomers, as in the Kubas complex itself. Other examples are now known. Iridium pincer complexes [Ir(PCP)H4] and [Ir(POCOP)H4] are important catalyst precursors and have therefore been studied in great detail (PCP = m-C6H3{CH2P(tBu)2}2; POCOP = m-C6H3{OP(tBu)2}2). Solution NMR data is more consistent with a dihydrogen-dihydride structure, but n-diffraction data for the PCP analogue indicates a compressed tetrahydride structure (dHH ∼ 1.49 Å). Computational data shows that the dihydrogen-dihydride and tetrahydride structures are very close in energy. There is an extremely flat PE surface with a very small barrier between the two structures, thus the simplest way of accommodating all the data is to argue for a solution Boltzmann equilibrium sampling the two structures. The temperature independence of the NMR data shows that the two forms must be essentially isoenergetic. Alternatively, the excited vibrational states of the tetrahydride may acquire dihydrogen character as the motion of the H atoms samples the whole lowenergy part of the very flat PE surface.112 The analogous [Co(POCOP)Hx] exists only at low temperatures where both x = 2 and x = 4 forms were detected by NMR spectroscopy. The former is an (H−H) complex and the latter a dihydrogen cis-dihydride. DFT calculations for the tetrahydride located two other minima, a trans-dihydride dihydrogen complex and a bisdihydrogen structure, at 1.6 and 4.8 kcal/mol above the global minimum, but no classical isomer was found.113 This is therefore an area of chemistry where the idea that a compound has one specific structure starts to break down, as is also true for the classic case of CH5+.114 Carreón-Macedo and Harvey115 have examined how spin state changes can affect oxidative addition reactions of H2. In the slow oxidative addition116 of H2 to triplet [W{N(CH2CH2NSiMe3)3}H] to form the classical trihydride [W{N(CH2CH2NSiMe3)3}H3] as a singlet product, they ascribed the slow rate to the high barrier resulting from the crossing between the reactant triplet and the product singlet potential energy surfaces (PESs). At the minimum energy crossing point between the two PESs, the incoming H2 has a minimally lengthened dHH of 0.794 Å and correspondingly long dMH of ∼2.1 Å that has to be compared to the standard dMH of 1.735 Å (av.) in the product trihydride, where the MH bond is fully formed. The transient intermediacy of an (H−H) complex, albeit weakly bound in this case, would of course not have been possible to substantiate in the absence of computational work. Equilibrium isotope effects (EIEs) in (H−H) formation and in the oxidative addition of H2 have been reviewed by Parkin,117 who points out that the situation is much more complex than would arise from the usual simple idea that deuterium prefers the site associated with the highest frequency oscillator. Unusual temperature dependencies are possible, so that the
(CH3CO)Co(H−H)(CO)3 → CH3CHO + HCo(CO)3 (5)
Brookhart and co-workers saw catalysis by water or alcohols of H2 addition to an [Ir(PONOP)CH3] to give the corresponding dihydride (eq 6). This was ascribed to the pathway shown in which the metal first accepts a proton from the alcohol to form a 16e Ir(III) intermediate. This was thought to bind H2, followed by a proton transfer from (H−H) to the RO− counteranion.111 The nonstandard mechanism gives the dihydrido product a trans IrH2 arrangement contrasting with the cis geometry usually seen in concerted oxidative addition.
Caulton and co-workers saw proton transfer from (H−H) to a NiIINR2 nitrogen in their PNP pincer ligand. DFT work 8759
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Proton reduction125 to H2 is of intense current interest in connection with the water splitting problem in the solar fuels area of energy chemistry. Sandhya and Suresh126 have suggested on the basis of DFT calculations that a dihydrogen complex is involved in proton reduction to H2 by a ruthenium(II) PNN pincer complex. This intermediate is thought to be formed by proton transfer to a Ru−H from an adjacent water via an Ru−H···H−O dihydrogen bond (see section 9). Indeed, it seems likely that dihydrogen bonded species occur on the pathway of any reaction of a proton donor with a hydride to give an H2 complex. This would explain why some proton transfers can give a dihydrogen complex as a kinetic product when protonation at the metal is the final thermodynamic product. Such is the case for Cp*Fe(dppe)H, as shown in eq 7.127
same system may have both normal and inverse EIEs in different temperature ranges. These effects are analyzed in terms of the full expression for the EIE that includes terms for the symmetry factor, the mass-moment of inertia, and the excitation energy, as well as the zero-point energy. Even where a dihydrogen complex exists, it may not always be the key reactive intermediate in a catalytic cycle. In a pincerRh catalyzed CO2 reduction by H2, for example, Huang, Fujita, and co-workers118 have shown that although an H2 complex is thermodynamically the more stable intermediate form, oxidative addition to the dihydride is needed for the catalytic reaction to proceed, because only in the latter does the hydride have the needed nucleophilic character required for attack at the electrophilic CO2 carbon. 6.3. Dissociation
Easy loss of H2 from an (H−H) complex is both very common and has been fully reviewed by Basallote and co-workers,119 so it is not treated in detail here. This loss is typically the first step of dissociative substitution in (H−H) complexes, as in a water gas shift reaction cycle catalyzed by M(CO)5 (M = Fe or Ru),120 but a different path occasionally prevails. For example, trans[FeH(H−H)(dppe)2]+ can undergo substitution by initial hemilability of the dppe.119 Going back to the classic case, when H2 dissociates from the Kubas complex, 1b, an agostic species is formed in which a C−H bond from the ligand cyclohexyl group forms an intramolecular sigma complex,7,9 of type LnM(H−C). This binding is labile because introduction of H2 leads back to 1b via displacement of the C−H bond. Kinetic data was obtained for the oxidative addition barrier as well as for association and dissociation of H2, both much easier processes than the oxidative addition, as shown by the relative rates for association, dissociation, and oxidative addition being in the ratio 1200:25:1.7 Thus, given a suitable open or labile site, binding of H2 can be a very easy process, particularly when associated with a minimal degree of ligand rearrangement.
Our own dihydrogen dihydride complex, [Ir(H−H)H(PPh3)2(NHC)]+, 7, proved to be a catalyst for the hydrogenation of N-heteroarenes. Although normally a hard reaction for a homogeneous catalyst, this catalyst was fully effective at 1 atm H2 and room temperature. The mechanism followed an unexpected outer-sphere path in which the acidic H2 first transfers a proton to the arene nitrogen, followed by hydride transfer to an adjacent ring carbon.33 This means the pathway is in fact a frustrated Lewis Pair process, and, indeed, the reaction is only successful for bulky substrates that are sterically inhibited from binding to Ir. Bullock and colleagues propose a somewhat similar ionic hydrogenation path for ketone reduction at 90 °C and 60 atm by a highly acidic Ru(H−H) intermediate, [Cp*Ru(CO)2(H−H)]+.128 6.5. Hydrogen Storage
Hydrogen storage129,130 is an energy problem of current interest connected with the possibility of a future hydrogen economy in which H2 would have to be reversibly stored to help counter the problem of intermittency in solar and wind power. Numerous H2 storage strategies have been suggested, but the one relevant to the present discussion is adsorption/ desorption of H2 in molecular form on a variety of high surface area materials. For example, carbon nanomaterials have attracted attention in this context, but interaction energies are low.131 To enhance the interaction strength to the required ∼20 kJ/mol, metals have been introduced into the material with the object of forming metal dihydrogen complexes.132 For example, a wide range of transition metals follow this pattern.133 Embedding Co(II) ions in a mesoporous carbon134 or incorporating them in a hydrazine-based coordination polymer135 greatly enhances the storage performance. More surprisingly, even incorporation of the non-d block ion, Ca2+, gives useful adsorption enhancements: Beheshti et al.136 find that Ca ions on B-doped graphene can bind H2 with energies of ∼9 kcal/mol, said to be in a suitable range for reversible storage applications. Theoretical work suggests that the Ca2+ is π-bound to C6 rings of the carbon substrate and that H2 binds to calcium leading to a dHH of ca. 0.84 Å. Much theoretical work has been directed to the design of potential hydrogen storage materials relying on the Kubas interaction, as detailed in a recent study.137
6.4. Catalysis
A key 2005 review by Kubas lays out the main directions in this area.121 Numerous catalytic reactions involve the reaction of H2 with metal complexes, and, in a number of cases, definite evidence has been obtained, either experimental or theoretical, in favor of such intermediates. The Esteruelas−Oro review100 covers early work; some newer cases were noted in earlier sections of this review, but other recent information is now also available. For example, Sakai, Fujita, and co-workers have looked at acceptorless alcohol dehydrogenation by [Cp*Ir(bpyO)(OH2)]+ (bpyO = α,α′-bipyridonate) by DFT. The reaction occurs in three steps: alcohol dehydrogenation, formation of a dihydrogen complex, and H2 dissociation. These results enabled them to develop a cheaper and better catalyst, [(η6-C6Me6)Ru(bpyO)(OH2)].122 In a related case, Le Floch, Sabo-Etienne, and co-workers123 found that a dihydrogen complex made an efficient synthetic precursor for catalytic transfer hydrogenation of ketones, the active site becoming available by dihydrogen dissociation. Formic acid dehydrogenation has attracted much recent attention, and, in one such case, Manca et al. have shown that their [(Ru(N{CH2CH2PPh2}3)Cl2] catalyst cycles via intermediate dihydrogen complexes identified by DFT calculations, as well as by the isolation of [(Ru(N{CH2CH2PPh2}3)Cl(H−H)]+.124 8760
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Many of the available metal−organic frameworks (MOFs) have been successfully tested for H2 storage, and computational work has identified the Kubas interaction as a predominant mode of H2 binding for MOFs with open or labile transition metal sites within the framework. For example a Cu(II) containing MOF of this type was developed by Thomas, Schröder, and co-workers.138 Main group ions were again effective in binding H2 relatively weakly via a combination of electrostatic and dispersive interactions.139 This MOF gas storage area has been fully reviewed recently.140 A V(III)-containing hydrazide gel was considered to store H2 by forming Kubas-type H2 complexes with the metal centers. The reversible storage of 4.04 wt % H2 at 77 K and 85 bar corresponded to a true volumetric adsorption of 80 kg H2/m3 and an excess volumetric adsorption of 60 kg/m3.141 Binding of multiple dihydrogen molecules per metal atom is obviously advantageous in enhancing the weight percent of stored hydrogen relative to sorbent. In this context, small metal clusters have been shown by DFT calculations to be capable of binding multiple H2 molecules.142 For example, the tetrahedral cluster TiAl3 reacts with the first H2 molecule by oxidative addition to give a dihydride with two Ti−H−Al bridges. The Ti atom can then bind up to four additional H2 molecules as (H−H) adducts, and even a fifth then binds weakly. These results were discussed in relation to pertinent experimental work on mixed metal hydride hydrogen storage materials. Similar behavior has been seen for amorphous Cr(III) alkyl hydride and Ti(III) hydride gels.143,144 A problem130 in this area is that in many reports the mass of H2 stored per unit mass of storage material is too low for most practical applications. In some cases, irreversible dihydrogen release is reported without any viable strategy being suggested to accomplish the storage step.
Figure 6. Optimized transition states (B97-D/TZVPP’ level) for H2 activation by (C6F5)2BCH2CH2PMes2. The arrows point in the direction of the transition-state normal mode toward products. Reproduced from Angew. Chem., Int. Ed. 2010, 49, 1402 with permission. Copyright 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Figure 7. Optimized structures of dihydrogen complex H3P···(H−H)B(CF3)3 at the MP2/6-31++G(d,p) level. Reprinted with permission from J. Phys. Chem. A, 2009, 113, 8108. Copyright 2009 American Chemical Society.
7. MAIN GROUP ELEMENTS Main group elements lack the filled dπ orbitals that help stabilize (H−H) bonding, but, as mentioned above, they can be powerful Lewis acids and may be able to bind H2, if more weakly than many of the cases mentioned up to this point. A strong clue that such binding is feasible comes from the chemistry of frustrated Lewis pairs (FLPs). These are bulky acid−base pairs such as t-Bu3P/B(C6F5)3 that are sterically prohibited from forming standard adducts, t-Bu3P−B(C6F5)3 in this case,145 but they can react with H2.146−148 Indeed, in one model for the relevant complex of acid, base, and H2, the H2 is bound not side-on but at least in a slanted conformation to the acid, with the base donating into the σ* orbital of H2, the role that would have been assigned to the filled dπ orbitals in transition metal M(H−H) complexes (Figure 6).149 Gao et al.150 have looked at the problem via a quantum model, H3P/B(CF3)3, of the experimental Bu3P/B(C6F5)3 system and have found a type of H2 binding that is much more side-on to the acid, rather than slanted (Figure 7). The lower steric bulk of the quantum model over the experimental system permits the base to approach closer to the H−H direction without any steric clash with the substituents on boron. The σ-bond of Et3Si−H, a good deal more basic than that of H−H, has now been shown to form an isolable sigma complex with a strongly Lewis acidic borane. The crystal structure securely identifies the bonding mode,151,152 and the adduct reacts with nucleophiles, accounting for the role of this species in metal-free ’frustrated-Lewis-pair’ hydrosilylation reactions.
Wang and Ma153 have looked at SiH2 insertion into H2 with DFT calculations and identified a transition state, 13, that resembles a stretched H2 complex with dHH = 1.137 Å. The geometry is such that the filled sp orbital of the silylene can interact with the H2 σ* and the empty p orbital with the H2 σ orbital in an analogue of the Kubas interaction. The initial (H−H)SiH2 adduct is a local minimum on the PE surface (Figure 8) in which the H2 only interacts with the empty p orbital; the dHH of 0.8 Å implies that the H2 is little changed from the free state.
8. DIHYDROGEN COMPLEXATION IN ENZYMES AND MODEL COMPLEXES The possible relevance of dihydrogen complexation to the active site metal clusters of enzymes such as nitrogenase and hydrogenase had been suggested in a series of reports from as early as 1986,2,7,13,154−158 but advances in the area of mechanism has required much structural, spectroscopic, kinetic, and computational work.159 For example, Siegbahn and coworkers identified an H2 complex as a key intermediate in the Ni−Fe hydrogenase mechanism.160 In the monoiron hydrogenases, an Fe-(H−H) intermediate has been proposed by 8761
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Figure 8. Some selected structures along the intrinsic reaction coordinate (IRC) of the insertion reaction of silylene with H2, calculated at the B3LYP/6-311+G** level. Bond distances are in Å and energies in kcal/mol. Reprinted with permission from J. Organomet. Chem. 2009, 694, 2567. Copyright 2009 Elsevier.
Yang and Hall on computational grounds from DFT data.161 Since there is no experimental evidence available so far, the area of dihydrogen complexation in enzymes remains to be definitively established. Liu, Bullock, and co-authors162 have reported on a hydrogenase model compound in which an Fe(II) dihydrogen complex is deprotonated by a pendant amine with the resultant formation of a Fe−H···H−N dihydrogen bonded tautomer, characterized by neutron diffraction (dHH = 1.489(10) Å). This record short distance indicates the presence of an unusually strong DHB, and the NMR data indicates that there is rapid and reversible (H2) cleavage in this system with estimated lower limit for the exchange rate of 2.2 × 104 s−1 at −80 °C. The same coauthors163 have also applied the same pendant amine ligand to a biomimetic strategy for electrocatalytic oxidation of H2. Their proposed catalytic cycle, shown in Figure 9, provides a very plausible analogy for the hydrogenase mechanism. The electrocatalytic implications of this strategy have also been reviewed very recently.164 Other similar biomimetic pendant base electrocatalysts have also been reported.165,166
with proton donors such as phenols.The reversibility of eq 8 has been verified experimentally in several cases.171,172
The preferential kinetic protonation of terminal metal hydrides by acids AH can be followed by rearrangement of the resulting dihydrogen complex to a thermodynamically more stable classical hydride with the proton now bound to the metal. The formation of an AH···HM dihydrogen bond on the approach trajectory to proton transfer seems the most plausible hypothesis to account for kinetic protonation at MH. In one such case, [CpRu(H−H)(PPh3)2]+ is formed on protonation of the neutral monohydride, even though the complex later irreversibly rearranges on standing to the classical isomer, [CpRu(H)2(PPh3)2]+.173 The same sequence also applies to CpW(CO)2(PMe3)H.174 Dihydrogen complexes and dihydrogen bonded intermediates are also increasingly invoked as catalytic intermediates.175−177 Classical hydrogen bonding is also implicated where (H−H) acts as a proton donor to a weak base. The hydrogen bonding between a BF4− counterion and the (H−H) ligand of the complex, [FeH(H−H)(dppe)2]+, considerably slows proton transfer to external NEt3 suggesting that the base has to displace the counterion to form a hydrogen bond with the H2, needed for unimpeded proton transfer to occur.178 The interaction with BF4− was detected by its effect on the rotational motion of the hydrogen-bonded BF4− counterion, as shown by changes in the T1 of the anion signal in the 19F NMR spectrum.179 Since all these reactions involve the acidification of H2 on binding or basicity of a hydride before protonation, a simple way to estimate the appropriate pKas would be very useful. Morris180 has devised an equation to do this for metal hydrides and dihydrogen complexes that involves accounting for a
9. DIHYDROGEN BONDING AND HYDROGEN BONDING Dihydrogen bonding (DHB) results from an attractive interaction, with an interaction energy comparable with that of a conventional hydrogen bond, between a hydridic hydride and a weak proton acid (eq 8). Although it was reviewed in 2001,167 the most up-to-date and extensive review is by Shubina and colleagues168 to be found in the present special issue, so we only need to mention a few key points here. As also suggested by eq 8, dihydrogen complexes can be proton donors to suitable weak bases. Thus, deprotonation of bound (H−H) by a base, B, often goes via a DHB as intermediate.169 By reversibility arguments both directions in eq 8 are expected to involve hydrogen bonding (HB). In the forward direction, the ’reactant’ in eq 8 contains a HB, while the product contains a DHB, with the reverse applying to the reverse direction. A DHB intermediate of this type has been seen by Berke170 and co-workers in the protonation of W(CO)(H)(NO)(PMe3)3 8762
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Figure 9. Proposed mechanism for a biomimetic electrocatalytic oxidation of H2.163 Reprinted with permission from Tianbiao Liu; Qian Liao; Molly O’Hagan; Elliott B. Hulley; Daniel L. DuBois; R. Morris Bullock; Organometallics 2015, 34, 2747−2764. DOI: 10.1021/om501289f. Copyright 2015 American Chemical Society.
number of independent factors such as the acidity constants for each of the ligands in the conjugate base of the hydride or dihydrogen complex along with a correction for the charge and the periodic row of the transition metal.
10. ANION AND LIGAND EFFECTS A striking case of a dihydrogen substitution in which the nature of the product depends on the anion chosen comes from Bianchini and co-workers. This involves a solid-state reaction in which H2 replaces a bound N2 in crystalline [(PP3)Co(N2)]X (PP3 = P(CH2CH2PPh2)3; X = BPh4 or PF6). Depending on the nature of X, the product is either the red dihydrogen complex, [(PP3)Co(H2)]PF6, or the white dihydride, [(PP3)Co(H)2]BPh4.181 While the dHH in conventional Kubas type dihydrogen complexes is often sensitive to the nature of the trans ligand, a recent counterexample182 exists where dHH is essentially the same in both cis- and trans-[Cr(CO)4(H2)(PMe3)].
Figure 10. DFT-calculated structure of a proposed intermediate, [(NP3)Ru(η2-H2)(NH2BH3)]+ (NP3 = N(CH2CH2PPh2)3), in amine borane dehydrogenation. Reprinted with permission from ref 184. Copyright 2014 John Wiley & Sons.
11. CATALYSIS In the catalysis area, other recent transfer hydrogenations have involved (H2) complexes. Since these can undergo easy H/D exchange with water, it proved possible to introduce D into the hydrogenation products using D2O as the only deuterium source.183 Dihydrogen complexes can also play a role in the catalytic dehydrogenation of amine boranes. For
example, according to DFT calculations by Rossin, Peruzzini, and co-workers, 184 a key intermediate, [(NP3)Ru(H2)(NH2BH3)]+, has the structure shown in Figure 10. The B−H···(H2) dihydrogen bond, having the relatively short dHH of 1.93 Å, makes this structure particularly interesting and further illustrates the protonic character of the dihydrogen ligand. 8763
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12. IGNOBLE METALS The chemistry of ’ignoble’ metals is receiving increased attention, but this is an area where spin state problems and paramagnetism are more likely. For example, the reaction of [MH(dppe)2]+ (M = Fe, Ru, Os; dppe =1,2-bis(diphenylphosphino)ethane) with H2 to give trans-[MH(H2)(dppe)2]+ was probed by Fultz and co-workers185 who documented a triplet to singlet spin state transition on binding that accounted for the strong binding affinity for M = Fe. A number of Ni(II)(H2) complexes have also been reported recently.186,187 In paramagnetic (H2) complexes, EPR data can provide useful information. Such is the case for (TPB)Co(H2)(TPB = B(o-C6H4PiPr2)3) where Co-(H2) rotation was probed by EPR with H/D isotopic substitution.188,189 The H2 ligand is a free rotor in a 6-fold barrier imposed by the C3 ligand symmetry, unlike the case of a closely related Fe analogue where the H2 is strongly localized and tunnels between energetic minima.
ACKNOWLEDGMENTS The author thanks the US DOE catalysis program for funding our work in this area. REFERENCES (1) Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.; Wasserman, H. J. Characterization of the First Examples of Isolable Molecular Hydrogen Complexes. J. Am. Chem. Soc. 1984, 106, 451− 452. (2) 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. (3) Heinekey, D. M.; Oldham, W. J. Coordination Chemistry of Dihydrogen. Chem. Rev. 1993, 93, 913−926. (4) Jessop, P. G.; Morris, R. H. Reactions of Transition-Metal Dihydrogen Complexes. Coord. Chem. Rev. 1992, 121, 155−284. (5) Szymczak, N. K.; Tyler, D. R. Aspects of Dihydrogen Coordination Chemistry relevant to Reactivity in Aqueous Solution. Coord. Chem. Rev. 2008, 252, 212−230. (6) Crabtree, R. H. Dihydrogen Complexes: Their Structure and Chemistry. Acc. Chem. Res. 1990, 23, 95−101. (7) Kubas, G. J. Metal Dihydrogen and σ-bond Complexes; Kluwer/ Plenum: New York, 2001. (8) Kubas, G. J. Activation of dihydrogen and Coordination of Molecular H2 on Transition Metals. J. Organomet. Chem. 2014, 751, 33−39. (9) Crabtree, R. H. Transition Metal Complexation of Sigma Bonds. Angew. Chem., Int. Ed. Engl. 1993, 32, 789−805. (10) Perutz, R. N.; Sabo-Etienne, S. The Sigma-CAM Mechanism: Sigma Complexes as the basis of Sigma-Bond Metathesis at LateTransition-Metal Centers. Angew. Chem., Int. Ed. 2007, 46, 2578− 2592. (11) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 6th ed.; J. Wiley and Sons: Hoboken, NJ, 2014; p 179. (12) Hashiguchi, S.; Fujii, A.; Takehara, J.; Ikariya, T.; Noyori, R. Asymmetric Transfer Hydrogenation of Aromatic Ketones Catalyzed by Chiral Ruthenium(II) Complexes. J. Am. Chem. Soc. 1995, 117, 7562−7583. (13) Maseras, F.; Lledós, A.; Clot, E.; Eisenstein, O. Transition Metal Polyhydrides: From Qualitative Ideas to Reliable Computational Studies. Chem. Rev. 2000, 100, 601−636. (14) Devarajan, D.; Ess, D. H. Metal-Mediated Dihydrogen Activation. What Determines the Transition-State Geometry? Inorg. Chem. 2012, 51, 6367−6375. (15) Rokob, T. A.; Hamza, A.; Stirling, A.; Papai, 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. (16) Rokob, T. A.; Hamza, A.; Stirling, A.; Soos, T.; Papai, 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. (17) Kubas, G. J. Metal-Dihydrogen and Sigma-Bond Coordination: the Consummate Extension of the Dewar-Chatt-Duncanson Model for Metal-Olefin Bonding. J. Organomet. Chem. 2001, 635, 37−68. (18) Dewar, M. J. S. Olefin Binding to Hg2+ Ions. Bull. Soc. Chim. Fr. 1951, C71−C72. (19) Pearson, R. G. Antisymbiosis and Trans Effect. Inorg. Chem. 1973, 12, 712−713. (20) Jalon, F. A.; Otero, A.; Manzano, B. R.; Villasenor, E.; Chaudret, B. First Observation in a Niobium Complex of the Rotation of a Coordinated H-D Molecule Blocked at the NMR Time-Scale. J. Am. Chem. Soc. 1995, 117, 10123−10124. (21) Cameron, T. M.; Ortiz, C. G.; Ghiviriga, I.; Abboud, K. A.; Boncella, J. M. The Synthesis and Reactivity of a Molybdenum (IV) Stretched-Dihydrogen Complex. J. Am. Chem. Soc. 2002, 124, 922− 923.
13. CONCLUSION AND OUTLOOK Dihydrogen complexation, originally confined to a handful of isolable complexes, has since proved applicable to a very wide range of complexes and reactions. The difficult structural challenges presented by the expansion of the field has led to the development of several experimental and computational methods for their resolution. Unusual phenomena, such as QMEC, have been identified as a byproduct of work in the area. We now recognize that (H−H) species can occur in most reaction pathways involving dihydrogen as reactant or product. The facile loss of a proton from some of these intermediates is an important step that has been implicated in a variety of such reactions. Future developments may be anticipated in the materials chemistry area, where binding modes can be harder to distinguish than in molecular chemistry. If hydrogen storage continues to gain interest, a strong possibility if climate change considerations increasingly influence energy policy choices, then this materials aspect will gain high economic value. Dihydrogen complexation has proved particularly attractive to computational chemists, and we can safely anticipate many future developments in that aspect of the field as well. AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Notes
The author declares no competing financial interest. Biography Bob Crabtree, educated at New College, Oxford with Malcolm Green, did his Ph.D. with Joseph Chatt at Sussex and spent four years in Paris with Hugh Felkin at the CNRS, Gif. At Yale since 1977, he is now a Whitehead Professor. He has been an ACS and RSC organometallic chemistry awardee, Baylor Medallist, Mond lecturer, Kosolapoff awardee, Stauffer Lecturer, has chaired the ACS Inorganic Division, and is the author of an organometallic textbook, now in its sixth edition. Early work on catalytic alkane C−H activation and functionalization was followed by work on H2 complexes, dihydrogen bonding, and catalysis for green and energy chemistry. He is a Fellow of the ACS, RSC, IUPAC, and the American Academy of Arts and Sciences. 8764
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