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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Metal Hydride Vibrations: The Trans Effect of the Hydride David Schnieders,†,‡ Brian T. H. Tsui,§ Molly M. H. Sung,§ Mark R. Bortolus,⊥ Gary J. Schrobilgen,⊥ Johannes Neugebauer,*,†,‡ and Robert H. Morris*,§ †

Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstraße 40, 48149 Münster, Germany Center for Multiscale Theory and Computation, Corrensstraße 40, 48149 Münster, Germany ⊥ Department of Chemistry, McMaster University, Hamilton, Ontario L8S 4M1, Canada § Department of Chemistry, University of Toronto, 80 St. George Street Toronto, Ontario M5S 3H6, Canada Downloaded via NOTTINGHAM TRENT UNIV on August 28, 2019 at 18:58:33 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: trans-Dihydride complexes are important in many homogeneous catalytic processes. Here vibrational spectroscopy and density functional theory (DFT) methods are used for the first time to reveal that 4d and 5d metals transmit more effectively than the 3d metals influence of the ligand trans to the hydride and also couple the motions of the trans-hydrides more effectively. This property of the metal is linked to higher hydride reactivity. The IR and Raman spectra of trans-FeH2(dppm)2, trans-RuH2(PPh(OEt)2)4, and mer-IrH3(PiPr2CH2pyCH2PiPr2) provide M−H force constants and H−M− H interaction force constants that increase as FeII < RuII < IrIII. DFT methods are used to determine, for the first time, the effect of the metal ion (MnI, ReI, FeII, RuII, OsII, CoIII, RhIII, IrIII, PtIV) and ligands on the gap in wavenumbers between the symmetric νsymH−M−H and antisymmetric νasymH−M−H vibrational modes of hydrides that are mutually trans in d6 octahedral complexes. The magnitude of this gap reflects the degree of coupling of, or interaction between, these modes, and this is shown to be a distinctive property of the metal ion. The more polarizable 4d and 5d metal ions are found to have an average gap of 246 cm−1, while the 3d metals have only 90 cm−1. This has been verified experimentally for 3d, 4d, and 5d transition-metal transdihydrides, where both the IR and Raman spectra have been measured: trans-RuH2(PPh(OEt)2)4 (from the literature) and trans-FeH2(PPh2CH2PPh2)2 and mer-IrH3(PiPr2CH2pyCH2PiPr2) (this work). Because the 4d and 5d metal ions tend to be better catalysts for the hydrogenation of substrates with polar bonds, this gap may be a fundamental determinant of the kinetic hydricity of the catalyst. Finding the magnitude of this gap and a new estimate of the large hydride trans-effect (Δνt −235 cm−1) allows us to improve the simple equation reported previously, which allows a better estimate of νM−H.

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INTRODUCTION

simple equation to estimate the wavenumber of the metal hydride stretch (Figure 1b, eq 1). The interesting aspect of this equation is that the use of a parameter for the trans ligand Lt, but not the cis ligands, gives a good estimate of the M−H stretching wavenumber. This was established by fitting the equation to a large set of literature data.25 The difficulty arises because at least two force constants are needed to describe the normal stretching modes of the H−[M]−H units: that of the M−H bond, k1, and that of the interaction across the metal between the trans M−H bonds, k12 (Figure 1c),28−30 but the magnitudes of these constants and their dependence on the metal are unknown. For this reason, the use of a single Δνt parameter for the hydride ligand, which has the largest trans-influence with a Δνt value estimated at −290 cm−1, resulted in poor agreement between

This investigation of the vibrational properties of octahedral transition-metal complexes with trans-dihydride ligands was sparked by two observations. The first is the nature of the transition state for the attack of a hydride on electrophiles such as the carbon of aldehydes, ketones, esters, imines, or carbon dioxide (CO2) or the proton of alcohols by highly active homogeneous catalysts based on FeII,1−12 RuII,12−19 OsII,12,16,20 CoIII,21 and IrIII 12,22−24 trans-dihydrides (M in Figure 1a). Here the movement of the hydride ligands at the transition state corresponds to an antisymmetric stretching mode, with the attacking hydride moving away from the metal and toward the electrophile, while the trans-hydride is moving toward the metal.7,18,25−27 The second observation is the difficulty in finding a parameter Δνt to describe the influence of a hydride trans to a hydride compared to that of other ligands trans to a hydride when using a © XXXX American Chemical Society

Received: July 29, 2019

A

DOI: 10.1021/acs.inorgchem.9b02302 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

quantities for a trans-dihydride complex. The complex transRuH2(C6H5P(OC2H5)2)4 has a νasymH−M−H of 1643 cm−1 from the IR spectrum and a νsymH−M−H of 1886 cm−1 from the Raman spectrum, resulting in a gap of around 240 cm−1.31 Assuming that the uncoupled vibration has a wavenumber halfway between the antisymmetric and symmetric vibrations, a correction of 120 cm−1 was proposed to account for the gap observed in the trans-hydride vibrations on the basis of this single example (Figure 2).25 However, k12 does depend on the metal (see below). In fact, the estimated wavenumber (using eq 1) for the CO 2 reduction catalyst system merIrH3(PiPr2CH2pyCH2PiPr2) of 1840 cm−1 is about 160 cm−1 higher than the experimentally measured νasymH−M−H value of 1678 cm−1,23 indicating a possible change in the coupling constants between ruthenium and iridium (the CH2pyCH2 backbone of the ligand is derived from picoline). Therefore, it is important to obtain the missing information about the coupling between the two modes on different transition-metal systems in order to improve predictions on these complexes and identify new properties of d6 metal ions. In this study, we will use computational methods to generate the gaps between the antisymmetric and symmetric dihydride vibrational wavenumbers. Furthermore, we will take a detailed look at the nature of the vibrations. While there is mention in the literature of anharmonicity in M−H stretching vibrations,32−38 it is shown here that the antisymmetrical H−M−H vibration at low amplitudes is perfectly harmonic because of the compensation effect of one hydride moving in while the other moves out. Also, we tackle an unanswered question in the literature: does a hydrogen bonded to a transition metal vibrate out as a hydrogen atom, proton, or hydride? In the case of octahedral transdihydride complexes of the metal ions examined, the hydrogen leaving, at least at low vibrational amplitudes, develops a negative charge, while the trans-hydrogen loses its negative charge, an effective transmission of charge through the metal, which retains a constant charge. We show for the first time that the 4d and 5d metals display, on average, a 20% larger negative charge gradient along the antisymmetric stretch than the 3d metals. This might contribute to the well-known high catalytic activity involving hydride transfer of the trans-dihydrides of RuII,

Figure 1. (a) Reaction coordinate for an attack by trans-dihydride complexes on electrophiles such as ketones, CO2, and imines where [M] represents the metal ion and the four cis ligands. (b) Equation to estimate the νM−H wavenumber by adding contributions from the metal ion, ν0, the trans ligand, Δνt, and the charge on the complex, Δνn. (c) Equations that describe the antisymmetric IR-active normal mode νasym (eq 2) and the Raman-active normal mode νsym (eq 3), where mY and mX are the masses of the atoms, k1 is the M−H bond force constant, and k12 is the H−M−H interaction force constant.

the νasymH−M−H values that were calculated using eq 1 and the 40 experimental values reported to date. These two observations raise two questions: (1) What are the factors, such as the nature of the metal ion and the cis ligands, that determine the interaction (expressed by k12 or the size of the gap νsymH−M−H−νasymH−M−H) between the trans-hydrides? (2) Is there a relationship between the interaction described by k12 and the rate of reaction of hydride addition to substrates in reactions catalyzed by trans-dihydrides? If so, this is valuable information when considering catalyst design. The equations for the physics of the stretching vibrations for systems like H[M]H were described by Herzberg28 for simple linear symmetrical triatomics (Figure 1) such as CO2, including those that describe their fundamental frequencies upon isotopic substitution. In order to obtain the magnitudes of k1 and k12, both the Raman and IR frequencies need to measured. However, there is only one report of the measurement of both of these

Figure 2. Coupling of the metal hydride vibration into the antisymmetric (bottom) and symmetric (top) trans-dihydride vibrations on the example of the trans-RuH2(C6H5P(OC2H5)2)4 complex. B


Article

Inorganic Chemistry Table 1. Vibrational Data for the Representative Hydrides νsymH−M−H, cm−1 II

Fe RuII IrIII

a

1822 1886b 1982a (solid)

νasymH−M−H, cm−1

gap, cm−1

k1, mdyn Å−1

k12, mdyn Å−1

a

113 243 304−357

1.81 1.82 1.93−1.98

0.15 0.28 0.39−0.34

1709 1643b 1625a (solid)a1678c (KBr)

This work. bReference 31. cReference 23. There is also an Ir−H mode at 2131 cm−1 for hydride trans to pyridyl.

a

potentials48 were employed in the case of rhenium, osmium, iridium, and platinum. All normal modes and frequencies were calculated using the harmonic oscillator approximation. While the model complexes studied here are small enough to be routinely calculated using complete vibrational analysis, we point out that it is possible to obtain the targeted quantities in a highly efficient manner by using the mode-tracking algorithm,49 as implemented in the AKIRA program of the MoViPac suite.50 An example is given in the Supporting Information (SI). Symmetric trans-dihydride vibrations were corrected by employing a Morse potential approximation, where the transition from the ground state to the first excited state is given by eq 4.51

OsII, RhIII, and IrIII, which have the highest negative charge gradients. While kinetic isotope effects in hydride chemistry have been examined,39,40 we reveal here the dramatic effect on this charge gradient by simply substituting a hydrogen trans to the hydride of interest with a deuterium.

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EXPERIMENTAL SECTION

All manipulations were conducted under an argon atmosphere using standard Schlenk-line or glovebox techniques. IR spectra were recorded on a Bruker ALPHA Fourier transform infrared (FT-IR) spectrometer with a platinum diamond attenuated-total-reflectance (ATR) attachment, and absorption band positions are reported uncorrected, in reciprocal centimeters. The Raman spectra were recorded at room temperature on a Bruker RFS 100 FT-Raman spectrometer using 1064 nm excitation, 300 mW laser power, and ±0.5 cm−1 resolution as previously described.41 A total of 1000 scans were collected on a sample sealed under argon in a glass capillary. 1H and 31P{1H} NMR spectra were recorded on an Agilent VnmrS 400 MHz spectrometer with an AutoX Probe. mer-IrH2Cl(PiPr2CH2pyCH2PiPr2) was prepared as described in the literature.23 trans-Fe(H)2(dppm)2. Using a modification of a literature procedure,42 anhydrous FeCl3 (0.45 g, 2.8 mmol) was dissolved in cold tetrahydrofuran (THF; 15 mL) to afford a dark-green solution. To this was added a solution of 1,2-bis(diphenylphosphino)methane (dppm; 2.1 g, 5.5 mmol) in benzene (15 mL). A suspension of LiAlH4 (0.2 g, 5.3 mmol) in THF (10 mL) was added dropwise with stirring over a 5 min period with vigorous evolution of gas. The color changed from dark green to orange to dark brown over the course of the addition. The mixture was filtered over a plug of Celite, and the filtrate was left undisturbed at −30 °C for 40 h. The red crystals that formed were collected on a sintered-glass frit, washed with ether (10 mL) and benzene (10 mL), and dried under reduced pressure. Yield: 245 mg, 11%. IR (ATR, cm−1): ν 1709 (m, Fe−H). Raman (cm−1): ν 1822 (s, Fe−H). 1H NMR (400 MHz, CH2Cl2): δ −7.35 (Fe−H, broad). 31 1 P{ H} NMR (162 MHz, CH2Cl2): δ 26.1 (Fe−P, d, 27 Hz, extreme second-order pattern). trans-Fe(D)2(dppm)2. The compound was synthesized following a procedure similar to that for trans-Fe(H)2(dppm)2. Yield: 24 mg, 7%. IR (ATR, cm−1): ν 1240 (m, Fe−D). Raman (cm−1): ν 1305 (s, Fe− D). 31P{1H} NMR (162 MHz, CH2Cl2): δ 24.5 (Fe−P, t, 7 Hz, extreme second-order pattern). mer-Ir(H)3(PiPr2CH2pyCH2PiPr2). Using a modification of a literature procedure,23 a Schlenk tube was charged with Ir(H)2Cl(PiPr2CH2pyCH2PiPr2) (144 mg, 0.125 mmol) dissolved in THF (10 mL). To this was added a suspension of NaH (450 mg, 18.75 mmol) in THF (5 mL) in one portion. The Schlenk tube was sealed, and the mixture was allowed to react for 24 h. The color changed from colorless to light yellow. The mixture was filtered through a plug of Celite, and the solvent was removed under reduced pressure. The yellow powder was recrystallized from THF/pentane at −30 °C. The pale-yellow crystals that formed were collected on a sintered-glass frit, washed with cold pentane (5 mL), and dried under reduced pressure. Yield: 98 mg, 73%. IR (ATR, cm−1): ν 2129 (m, Ir−H), 1625 (s, Ir−H). Raman (cm−1): ν 2129 (m, Ir−H), 1982 (s, Ir−H).

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E(1) − E(0) = v harm −

v harm 2 2De

(4)

vharm is the frequency obtained from the harmonic oscillator model, and De is the dissociation energy. The dissociation energy De is calculated as the dissociation into two hydrogen atoms, De = E(M) + 2E(H•) − E(MH2), in order to avoid a large sensitivity on solvent effects and therefore allow a vacuum treatment. We are aware that this is a quite simplified correction for anharmonicity effects and that more sophisticated approaches are known.52−54 However, our approach allows one to routinely estimate anharmonicity effects for symmetric trans-dihydride complexes of larger size and produces results well within the accuracy targeted by the estimate formula that is to be extended in this work (see the SI for more details). As shown in section 2 and the SI, antisymmetric trans-dihydride vibrations follow the harmonic oscillator model to a large extent. Therefore, they are not corrected using the formula above. Coordinates for the potential curves, Ri′, were generated by distorting the equilibrium geometry R along the (Cartesian) normal modes Qci obtained from the harmonic oscillator model (eq 5).

R i′ = R + aQ ic

(5)

Potential energy curves were then obtained by running single-point calculations on these structures.

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RESULTS AND DISCUSSION Antisymmetric−Symmetric Wavenumber Gaps from Experiment. We have measured the Raman and IR spectra of the oxygen-sensitive hydrides trans-FeH2(dppm)2 and merIr(H)3(PiPr2CH2pyCH2PiPr2) as representatives of 3d and 5d metal complexes to go along with the existing data for the transRuH2(C6H5P(OC2H5)2)4 complex (Table 1). The absorptions are very intense in each case, with the iridium complex displaying two intense absorptions in the IR at 2129 cm−1 for H−Ir trans to the pyridyl nitrogen and 1982 cm−1 for νasymH−Ir−H and two intense Raman lines at 2129 cm−1 for H−Ir trans to the nitrogen and 1982 cm−1 for νsymH−Ir−H. Equation 1 with ν0 for IrIII of 2130 cm−1 and Δνt for nitrogen donors allows an estimate of the wavenumber for H−Ir trans to the nitrogen vibration as 2130 − 5 = 2125 cm−1 in excellent agreement with the experiment.25 Thus, the previously reported vibration at 2131 cm−1 (KBr) can now be assigned to the H−Ir−N mode.23 The position of νasymH−Ir−H is sensitive to the measurement medium because the KBr sample gave a literature value of 1678 cm−1, while the solid film on the ATR crystal gave 1625 cm−1.

COMPUTATIONAL DETAILS

All calculations were performed using Turbomole v7.2.1.43 The B3LYP functional as implemented in Turbomole44−46 was used in combination with the def2-TZVP47 basis set. The ECP-28 effective core potentials48 were used for ruthenium and rhodium, and the ECP-60 effective core C


Article

Inorganic Chemistry

For PMe3 and PH3, OsII ∼ FeII > RuII, while for PF3 and CO, FeII > OsII > RuII. These trends follow the ν0 values and agree approximately with eq 1 after the splitting Δsplit of Figure 1 (and see below) is applied. It is intriguing that RuII has the lowest νasymH−M−H of the group 8 series here and in the experimental data of Table 1. This supports a qualitative link, as illustrated in Figure 1 between IR spectroscopy and the kinetics of hydride transfer (see also below). For PF3 > CO > PH3 ∼ PMe3, eq 1 in Figure 1 implies that there should be no cis ligand dependence on νM−H. However, the ligand sets in the model complexes [MH2(PMe3)4]n and [MH2(PF3)4]n have extremes of electron donor and acceptor power, respectively, resulting in a range of νM−H for the same metal ion of 1851−1817 cm−1 for PtIV to 1784−1542 cm−1 for MnI. Equation 1 was based on data from compounds of intermediate reactivity, with mixed ligands for the most part that are readily synthesized and studied; when extremely donating or withdrawing donor sets are present, this simple equation is likely to break down and the effect of cis ligands will need to be taken into consideration. νsymH−M−H after correction (Table S10) follows the same metal-ion-dependent trends across a row as the νasymH−M−H just discussed: CoIII > FeII > MnI and PtIV > IrIII > OsII > ReI. However, the trend down the rows can only be understood once a correction for Δsplit is made (see below): ReI > MnI; OsII > RuII > FeII; IrIII > RhIII > CoIII. Again there is a large cis ligand dependence with PF3 ∼ CO > PH3 ∼ PMe3 except for PtIV, where it is almost reversed: PMe3 > PH3 ∼ PF3 > CO. Here M(d) → L(σ*) or M(d) → L(p) backbonding to the π-acid ligands is minimal, resulting in a different ordering in this case. Antisymmetric−Symmetric Wavenumber Gaps from DFT Calculations. The resulting wavenumber gaps between the antisymmetric and symmetric vibrations, νsym−νasym, are summarized in Table 2 as values and as a “heat map”, with the

The stiffness of the M−H bond, as signaled by the magnitude of k1, increases as FeII ∼ RuII < IrIII, while the splitting, given by the gap size or k12, increases as FeII < RuII < IrIII. The gap for metal deuteride vibrations is also of interest, so we collected vibrational spectra for one example, transFeD2(dppm)2, which was prepared by the reaction of FeCl2(dppm)2 with LiAlD4. The Raman mode νsymD−Fe−D appeared at 1305 cm−1. Herzberg reports (using eq 3 of Figure 1) that for isotopic substitution of a YXY system to YiXYi, where Yi is an isotope, νsymYi−X−Yi should appear at νsymYi−X−Yi(mY/ mYi)1/2. Therefore, νsymD−Fe−D is expected to be at νsymH−Fe−H(1/ 2)1/2 = 1288 cm−1, in fair agreement with the experiment considering that D[M]D systems are not exactly symmetrical triatomics. Differences could be due to anharmonicity, solidstate effects on the vibrations, or even distortions of the octahedron from 180° angles. The antisymmetric stretch νsymD−Fe−D, by use of eq 3 in Figure 1, is expected to be at 1228 cm−1, close to the experimental value of 1240 cm−1. The gap from the experiments is 65 cm−1, while the gap from the calculations is 60 cm−1 (1288−1228). In general, it can be shown that the gap for D[M]D will be close to (gapH[M]H/21/2) by use of eqs 2 and 3 because mX (the atomic mass of a transition metal) is much greater than my, the atomic mass of deuterium or hydrogen. Antisymmetric and Symmetric Wavenumbers from Density Functional Theory (DFT) Calculations. Given the challenges of the synthesis and study of oxygen-sensitive, highly reactive dihydrides, we turned to computational methods to increase our knowledge of trans H−M−H interactions. In order to obtain information about the coupling of the antisymmetric and symmetric vibrations, the current calculations focus on octahedral model complexes of the d6 transition metals with monodentate ligands, as indicated in Figure 3. This included

Table 2. Wavenumber Gaps between the Antisymmetric and Symmetric Vibrations of trans-Hydride Complexes of Different Transition Metals Containing Different Cis Ligandsa Figure 3. Model complexes set up for the study of the antisymmetric and symmetric trans-dihydride vibrations.

complexes of MnI, FeII, CoIII, RuII, RhIII, ReI, OsII, IrIII, and PtIV. These are the metal ions where experimental IR vibrational data are available for νM−H or where trans-dihydrides are known to exist, and eq 1 was fit to the data of many complexes with monodentate ligands. Furthermore, the effect of ligands cis to the hydrides is studied by using complexes of the type trans[MH2(PMe3)4]n, trans-[MH2(PH3)4]n, trans-[MH2(PF3)4]n, and trans-[MH2(CO)4]n in order to cover a wide range of donor and π-acceptor strengths. The charge n of the complex is determined by the oxidation state of the metal minus 2 in order to account for the presence of the two hydrides. The wavenumbers for the antisymmetric and symmetric H− M−H modes are provided in Tables S8 and S9, respectively. The symmetric vibration wavenumbers, after an anharmonicity correction, are provided in Table S10. The νasymH−M−H have the following trends: CoIII > FeII > MnI and PtIV > IrIII > OsII > ReI. These trends are mirrored by the ν0 values published for use in eq 1 in Figure 1.25

a

In reciprocal centimeters. bAverage for each metal ion.

largest gaps in red and the smallest in green. It is clear that the 4d and 5d metals cause larger gaps than the 3d metals, as we observed experimentally. Upon going across the rows from MnI to CoIII and from ReI to IrIII, the gaps increase continuously. In contrast, RuII and RhIII show almost the same gaps with averaged values of 249 and 245 cm−1. Moving on from IrIII to PtIV, the trend of increasing gaps along a row is disrupted, and the gap decreases instead by an average of 50 cm−1. D


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Inorganic Chemistry

asym Table 3. Classes of trans-Dihydrides with νobs H−M−H Values and Their Force Constantsa

Going down the individual transition-metal groups also reveals a trend: moving from FeII to RuII and from CoIII to RhIII results in large increases in the gap by 152 and 129 cm−1 on average, while the changes from RuII to OsII and from RhIII to IrIII are significantly smaller, with averaged values of 14 and 32 cm−1. It is not trivial to give a chemically intuitive reason for these trends because they are a result of large changes in the kinetic and electrostatic contributions, which cancel each other out to produce such subtle effects. In general, the effect of cis-ligands seems to be connected to their electron-withdrawing properties: the more electronwithdrawing PF3 ligands decrease the gaps, while PH3 and PMe3 show similar results. CO is expected to decrease the gaps because of its π-accepting properties; however, coupling of the metal hydride vibrations with the closely lying CO stretching vibrations seems to increase the gaps in some cases. The gaps for the ruthenium complexes of 269, 260, 211, and 256 cm−1 are very close to the gap of 240 cm−1 reported for trans-RuH2(C6H5P(OEt)2)4. Considering that there are two electronegative oxygen atoms on the phosphorus atoms of the ligands in that compound, the gap should be located somewhere between our PMe3 and PF3 systems; this is reproduced by our model. Similarly, the gap found by IR and Raman spectroscopy for trans-FeH2(PPh2CH2PPh2)2 is 113 cm−1, in good agreement for the gaps calculated for trans-FeH2(PMe3)4 (104 cm−1) and trans-FeH2(PH3)4 (106 cm−1) (Table 2). However, a previous DFT study of iridium(III) trans-dihydride complexes55 reported νsymH−M−H in the range 1992−2184 cm−1 and νasymH−M−H in the range 1527−1817 cm−1. We notice that the gaps for these compounds range from 365 to 467 cm−1 and are hence significantly larger than our expected gap of 246 cm−1. Most of this difference can probably be traced back to neglect of the anharmonicity for νsymH−M−H in that study and the different functional used. Further small deviations probably arise from the difference in the ligand structures; our model calculations considered only monodentate ligand complexes. The equations for the harmonic frequencies of a linear symmetric molecule along with the gap values of Table 2 allow calculation of the M−H force constants k1 and the H−M−H interaction force constants k12 starting with the observed asym νobs H−M−H (see the SI). Table 3 lists the range of force constants so obtained for classes of complexes for which experimental asym νobs H−M−H values are available. As expected, the iron(II) and cobalt(III) complexes with weaker M−H bonds have smaller k1 values than the neutral osmium and iridium complexes. The neutral ruthenium complexes have intermediate k1 values. Anionic hydride complexes have lower k1 values than the neutral and cationic complexes. The smaller gaps for the 3d metal complexes are reflected in the much smaller interaction force constants k12 than those of the 4d and 5d metal complexes, which transmit this cooperative effect more efficiently. Force constants from Table 3 along with the equations for the frequencies of linear symmetric molecules can be used to predict the positions of the symmetric and asymmetric D−M−D vibrations. Where experimental data are available (Table 4), the agreement is good. Improvement of Equation 1. The simple eq 1 was therefore modified by a parameter for the splitting, Δνsplit, to account for the vibrational modes of the 39 trans-dihydrides where data are available (eq 6), and eq 7 provides an estimate of the antisymmetric stretch of the trans-deuteride (see also Tables S5 and S6 and Figure S2). It was also discovered during the

a

class

k1

k12

FeH2(PRAr2)2(PR2Ar)(CO) FeH2(PRAr2)4 FeH2(PRAr2)2(PR2Ar)2 FeH2(PR2Ar)4 CoH3(PRAr2)3 CoH3(PAr3)3 [RuH3(PR3)2(CO)]− RuH2(PR2Ar)4 RuH2(PX3)4 RuH2(PAr3)2(NR3)2 [OsH3(PR3)2(CO)]− [IrH4(PR3)2]− IrH3(PR3)2(py) IrH2(PRAr2)2(NR3)(H) IrH2(PR3)2(Cl)(R) IrH3(PAr3)3 IrH2(PR3)2(CO)(Ar) IrH2(PR2Ar)2(CO)(R) IrH3(PAr3)2(CO) [IrH2(PRAr2)4]+

1.7 1.81 1.82 1.83 1.88 1.91 1.71−1.73 1.78 1.83 2.12 1.95−1.97 1.94−2.03 1.95 2.01 2.05 2.10−2.12 2.10−2.12 2.14 2.19 2.15

0.13 0.15 0.13 0.13 0.15 0.17 0.26−0.27 0.27 0.27 0.3 0.29 0.30−0.31 0.38 0.31 0.31 0.32 0.32 0.32 0.32 0.32

In millidynes per angstrom.

Table 4. Data for Deuterated Complexes with a transDihydride Unit As Calculated by Formulae 2 and 3, Figure 1 complex

asym νobs DMD

asym νcalc DMD

sym νcalc DMD

gap

CoD3(PEtPh2)3 CoD3(PPh3)3 IrD3(CO)(PPh3)2 FeD2(dppm)2

1260 1263 1278 1240

1247 1254 1269 1228

1309 1325 1458 1288

62 71 189 61

course of refitting of the data to the new equation that the 3d metal complexes have values of Δνt that are diminished by a factor Ct ≈ 0.6 relative to the 4d and 5d metals (Ct = 1); this factor significantly improves the overall fit of all of the data for metal hydride and deuteride stretching wavenumbers versus that calculated by the use of eqs 6 and 7 (see Table S7 and Figure S3). The scaled down Δνt for 3d metal complexes is another reflection of the poorer transmission of cooperative effects across the metal compared to the 4d and 5d metals: t t n 0 vHcalc −M−H = C Δν + Δν + ν ± Δνsplit

(6)

and t t n 0 vDcalc −M−D = (C Δν + Δν + Δν − Δνsplit)

Aw + 4 2A w + 4 (7)

with Aw being the atomic mass of the most abundant metal isotope. Equation 7 is derived with the assumption that the H−M−H and D−M−D units in the octahedral systems that are considered here behave in a fashion similar to a linear, symmetric triatomic molecule (see the SI). The splitting is subtracted to give the antisymmetric trans-dihydride vibration and added to get the symmetric trans-dihydride vibration. Furthermore, the value of Δνt, which reflects the trans-effect of the hydride for 4d and 5d metals, changed from −29025 to −235 cm−1, while that of the 3d metals changed from −290 to −235 × E


Article

Inorganic Chemistry 0.6 = −141 cm−1. In order to maintain simplicity, the gaps were averaged over the transition-metal groups, giving the Δνsplit values summarized in Table 5.

used for the harmonic oscillator approximation hold; i.e., the potential close to the minimum can be described by a parabolic form. Of course, at larger distances, the potential will distort from the harmonic potential because the Pauli-repulsion contributions to the potential energy will outweigh other terms. An exponential form is expected in these regions. Behavior of Atomic Charges along the Normal Modes. In order to investigate whether a hydride or a hydrogen atom will dissociate along the normal modes of the trans-dihydride vibrations, atomic charges along the normal modes were investigated. Figure 5 shows the natural charges of the atoms involved in the two vibrations along the respective normal modes, referenced to the natural charges in the equilibrium structure. Along the antisymmetric vibration, we can see that the leaving hydride, H2, is increasing in negative charge in a linear fashion. This shows that it actually picks up negative charge in order to leave as a hydride. The hydride moving closer to the metal center shows a linear decrease of negative charge with a similar slope. The metal center itself shows only a small change of charge. This reveals that, during the antisymmetric vibration, one hydride loses charge to the metal center while moving toward it, leading to a similar increase in the charge of the leaving hydride, as indicated schematically in Figure 6. Therefore, the electronic charge is formally moved from one hydride to the other. This observation of a metal acting as an “electron shuttle” is not unlike the way we think about metal conductors, where we often conceive of a string of electrons being pushed through a metal wire. The symmetric vibration also shows a linear increase in negative charge with increasing distance, now for both hydrides. The slope is similar for both hydrides and is about half as large as the slope in the case of the antisymmetric vibration. In this case, the metal center decreases in negative charge, showing that the metal center donates electron density to the two leaving hydrides. This linearity is observed for every model complex studied here. In order to generate a descriptor that is suitable to compare different complexes, we calculated the gradient of the change in the natural charge along these vibrational modes. However, because the amplitudes of the normal modes might change for different systems, we did not use the derivative with respect to

Table 5. Effect of the Row of the Metal on the Averaged Value 1 of the Splitting Δνsplit ( 2 Δνgap) between the Antisymmetric and Symmetric Stretches for Use in Equation 6 nd

Δνsplit (cm−1)

nd

Δνsplit (cm−1)

3d 4d

45 123

5d

123

The use of eq 6 with the appropriate Δνsplit values results in an improved estimate for the dihydride compounds considered in ref 25, from a mean average error from 77 to 38 cm−1, and allows one to estimate the wavenumber of the symmetric transdihydride vibration and the wavenumber of the antisymmetric D−M−D vibration from eq 7, with fair accuracy (see the SI for detailed results). The major sources of discrepancies between the observed and calculated values are the untreated anharmonicity, cation interactions with anionic hydrides, sample medium effects (e.g., Nujol vs KBr), and possible misassignment of the peaks. Nature of the Antisymmetric and Symmetric transDihydride Vibrations. Next, we discuss the nature of the two vibrations considered in this article by investigating the potential curve around the minimum. The potential curves for the antisymmetric and symmetric trans-dihydride vibrations of trans-FeH2(PMe3)4 were generated as indicated in the Computational Details section and are shown in Figure 4. The potential curve of the symmetric vibration shows a clear deviation from the harmonic potential, reflecting its anharmonicity, as is expected for most vibrations, and for hydride vibrations especially. The antisymmetric potential curve, however, shows harmonic behavior to a very good approximation up to quite large distortions, a. This behavior is explained by the symmetry of the vibration: as one hydride moves further away from the metal center, the other moves closer and vice versa. This will generate a symmetrical potential curve. For small distortions aQci , the same assumptions as those

Figure 4. Potential curves of the antisymmetric and symmetric trans-dihydride vibrations of trans-FeH2(PMe3)4 together with the respective harmonic potentials. Δr denotes the distortion of one of the two hydrides incorporated in the coupled vibrations. F


Article

Inorganic Chemistry

Figure 5. Changes of the natural charges obtained from natural population analyses along the antisymmetric and symmetric trans-dihydride vibrations for trans-FeH2(PMe3)4. Shown are the natural charges of the metal and two hydrides in reference to the charge at the equilibrium structure. Δr is the change of the bond length between the metal and hydrogen atom H2 with respect to the equilibrium structure.

ence of the electron densities in M−H bonds, we performed additional test calculations with the CAM-B3LYP functional recommended there. The resulting charge gradients differ by less than 10% from the B3LYP values, and all qualitative trends are preserved. Therefore, we continue using the B3LYP functional for consistency. Note that the results are only listed for one of the hydrides because the charge gradient of the other one has the same absolute value, either with the same (symmetric) or with opposite (antisymmetric) sign. For the antisymmetric vibration, it can be seen that the charge gradient becomes less negative across a row of Table 6, i.e., from MnI to FeII to CoIII. This is not surprising considering the increasing oxidation state of the metal center and therefore the larger tendency to withdraw charge from the hydrides. The trend is the same but not as dramatic as that of the estimated thermodynamic pKaTHF values of [MnH 2 (PMe 3 ) 4 ] − > 60, FeH 2 (PMe 3 ) 4 = 56, and [CoH2(PMe3)4]+ = 26, which reflect the relative stability of the conjugate base complexes.57 Similar trends can be found for the 4d metals. For the 5d metals, a steeper negative gradient from ReI to OsII is observed in most cases, while the transitions OsII to IrIII and IrIII to PtIV reproduce the trend observed before. Going down the individual groups, a large change to more

Figure 6. Schematic depiction of the electron movement during the antisymmetric dihydride vibration.

the vibrational modes. Instead, we used the derivative with respect to the metal−hydride distance (corresponding to the slope of the H2 line in Figure 5), which is more transferable between different systems. This corresponds to the tendency of a catalyst molecule to polarize the metal−hydride bond as the hydride dissociates. Keeping in mind the similarity of the antisymmetric trans-dihydride vibration to the reaction coordinate during hydride-transfer reactions,7,18,25−27 this could have implications for the catalytic activity of a transition-metal complex. Table 6 summarizes these charge gradients for the different model complexes, as obtained with the computational model described in the Computational Details section. Because a recent benchmark56 revealed a pronounced functional depend-

Table 6. Gradient of the Change in the Natural Charge with Respect to the Metal−hydride Distancea

a

In units of elementary charge per angstrom for the leaving hydride along the normal modes of the antisymmetric and symmetric trans-dihydride vibrations for the different model complexes. G


Article

Inorganic Chemistry

Figure 7. Changes of the natural charges obtained from natural population analyses along the antisymmetric vibration for trans-FeH2(PMe3)4 and trans-FeHD(PMe3)4. Shown are the natural charges of the metal and two hydrides or the hydride and deuteride in reference to the charge at the equilibrium structure. Δr is the change of the bond length between the metal and H2 or hydrogen atom with respect to the equilibrium structure.

negative gradients from the 3d metals FeII and CoIII to the 4d metals RuII and RhIII can be observed. For the 5d metals, the charge gradient is again less steep. Interestingly, it is the 4d metals ruthenium and rhodium that show the most negative charge gradients and that also form the most active hydrogenation catalysts,9,13,14,17−19,58−84 although the mechanism of hydride transfer is often different for ruthenium (outer sphere)58,60 and rhodium (inner sphere).85 Looking at the charge gradients of the symmetric vibration, the gradients seem to be much less negative than those for the antisymmetric vibration. This is certainly because of the fact that now both hydrides withdraw negative charge from the metal center instead of one of them donating charge, as is the case in the antisymmetric vibration. As seen for the antisymmetric vibration, the absolute values of the gradients decrease across a row because of the increasing oxidation state of the metal center. Descending an individual group, the gradients also now become less negative. In some cases, the gradients even become positive, indicating that a hydrogen atom or a proton dissociates instead of a hydride. This seems to be the case for the metal that has the highest oxidation state within this study, PtIV, for any of the cis ligands studied here. For the more electron-withdrawing ligands, the metals in oxidation state III also will not dissociate into hydrides, as seen for the trans-[MH2(PF3)4]n- and trans[MH2(CO)4]n-type complexes. In the case of the trans[MH2(PH3)4]n-type complexes, only the IrIII and PtIV complexes show positive charge gradients, with the charge gradients of RhIII and CoIII being very low but still negative. The coupling of the two hydride modes can be suppressed by substituting one of the hydrides with a deuteride. Figure 7 compares the resulting changes in the natural charges with those of the corresponding trans-hydride complex. The donation of charge originating from the deuteride is significantly smaller than that observed for the hydride. As a result, the metal center now shows a significant loss of charge as the hydride dissociates, which, in turn, now takes up negative charge at a slower pace. Again, charge gradients can be calculated in a fashion similar to that for the trans-dihydride compounds above; i.e., the change in the natural charges along the vibrational mode is calculated, and the slope of the plot ΔNPA versus ΔrM−H is determined.

The respective charge gradients of the dissociating hydrides are compared in Table 7. Because, within the Born−Oppenheimer Table 7. Charge Gradients in Units of Elementary Charge per Angstrom for the Antisymmetric Vibrations of trans[MH2(PMe3)4]n and trans-[MHD(PMe3)4]n

a

metal

trans-H−

trans-D−

Δa

MnI FeII CoIII RuII RhIII ReI OsII IrIII PtIV

−0.42 −0.42 −0.37 −0.55 −0.52 −0.47 −0.50 −0.49 −0.43

−0.26 −0.30 −0.21 −0.33 −0.24 −0.31 −0.27 −0.21 −0.13

−0.16 −0.13 −0.17 −0.22 −0.28 −0.16 −0.23 −0.27 −0.30

The difference between the two gradients.

approximation, the deuteride does not differ from the hydride electronically, the difference between the charge gradients is solely a result of decoupling of the modes and the resulting removal of the push−pull mechanism described above. This results in a large reduction of the negative charge gradient in all cases. The effect seems to be especially large for the platinum group metals ruthenium, rhodium, osmium, iridium, and platinum. This analysis could, of course, be used to compare the electronic effects of different trans-ligands. As an example, Table 8 compares the charge gradients of the antisymmetric vibrations of trans-[MHD(PMe3)4]n (resembling the electronic effect of a trans-hydride) and trans-[MHCl(PMe3)4]n. The charge gradients of the trans-chloride compounds show more negative charge gradients for the 3d and 4d metals and less negative charge gradients for the 5d metals. As an attempt to explain this trend, Table 9 shows the charge gradient on the trans-chloride ligand, split into contributions from s- and p-type orbitals. While the 3d metals show the largest s contributions to the charge gradient, the 4d metals show the largest p-type contributions. This indicates a slightly different binding situation within the H


Article

Inorganic Chemistry

the charge gradients grow more negative, the activation energies for the hydride-transfer step decrease, which can be connected to a more nucleophilic hydride. Interestingly, the trans-dihydride complexes show strongly negative charge gradients and especially low activation barriers. This is in line with the findings in this work and the empirical observation of highly reactive trans-dihydride complexes. Most noticeably, both quantities show a maximum for the 4d metal when the groups FeII, RuII, and OsII are compared, followed by the 5d metal and placement of the 3d metal last. Only IrIII shows a qualitatively different behavior. Compared to, for example, FeII, a significantly more negative charge gradient is connected to almost unchanged activation energies. Because the activation energy is a result of several effects within the complex and substrate, of which we are isolating only one, deviations from a simple trend are expected here. Nevertheless, a linear correlation between the charge gradients and activation energies for the complexes shown here can be observed, as shown in Figure 8. There are some quantitative experimental data on the kinetics of hydride transfer from transition-metal hydrides (and none for trans-dihydrides). Darensbourg and co-workers reported that the 5d metal tungsten hydride carbonyl anions transferred hydride to alkyl halides, ketones, and epoxides faster than the 3d metal chromium and iron carbonyl anions and that substitution of CO for P(OMe)3 increased the rate.87,88 Bullock and coworkers, who measured the rate of hydride transfer to the trityl cation from metal hydride complexes, found that the rates decreased with the 4d > 5d > 3d metals (Mo0 > W0 > Cr0; ReI > MnI; RuII > OsII > FeII).89−91 These trends are quite consistent with those of Table 6.

Table 8. Charge Gradients in Units of Elementary Charge per Angstrom for the Antisymmetric Vibrations of trans[MHD(PMe3)4]n and trans-[MHCl(PMe3)4]n metal

trans-D−

trans-Cl−

Δa


−0.260 −0.296 −0.208 −0.330 −0.240 −0.307 −0.275 −0.212 −0.128

−0.352 −0.316 −0.226 −0.368 −0.285 −0.295 −0.268 −0.211 −0.120

0.092 0.019 0.018 0.038 0.044 −0.012 −0.007 −0.001 −0.008

a

The difference between the two gradients.

Table 9. Charge Gradients of the Chloride Ligand in Units of Elementary Charge per Angstrom for the Antisymmetric Vibration Split into s- and p-Type Orbital Contributions metal

s

p


0.02571 0.02522 0.02010 0.01517 0.01302 0.01891 0.01702 0.01270 0.00640

0.02646 0.05491 0.10235 0.11399 0.16345 0.07366 0.11372 0.16121 0.19986

■

resulting dissociated complexes, which might explain the differences in the charge gradient changes. However, this electronic effect seems to be much lower than the effect resulting from the push−pull mechanism. Connection between the Charge Gradients and Reactivity. In order to show that the charge gradients are connected to the reactivity of transition-metal hydride complexes in hydride-transfer reactions, we compare the corresponding activation energies to the charge gradients for a few complexes. We chose the M(H)x(CO)3−x(iPr2PNHPiPr2)type complexes discussed by Wei and Jiao86 where M is MnI, FeII, RuII, OsII, or IrIII and x is chosen to be equal to the oxidation state of the metal, such that the overall complex is neutral in charge. These complexes were chosen specifically because they show similar steric bulk and complex−substrate interactions, thus reducing the influence of other effects within this study. Wei and Jiao reported activation barriers for ester and aldehyde reductions using these catalysts. The corresponding charge gradients and activation energies are summarized in Table 10. The metals MnI, FeII, RuII, and OsII show the expected trend: as

CONCLUSION The interaction force constants and wavenumber gaps that we have determined from the IR and Raman spectra of representative hydrides and by calculation show that the transhydrides influence each other more for the 4d and 5d metals than the 3d metals. This trend can be explained partly on the basis of the “primogenic effect”, where the 4d and 5d orbitals extend further outside of the electron core of the atom compared to the 3d orbitals (which do not have a radial node), resulting in a larger and stronger covalent component to the bonding.92 Shielding of the 5d electrons by the relativistically contracted s orbitals also contributes to this difference for the third-row transition-metal ions. We have calculated gaps between the antisymmetric and symmetric trans-dihydride vibrations of the metals MnI, FeII, CoIII, RuII, RhIII, ReI, OsII, IrIII, and PtIV to extend the data used to fit parameters for estimation of the metal−hydride vibrational wavenumbers presented in ref 57. The splitting of the two vibrations, determined as half of the gap between the symmetric and antisymmetric trans-dihydride stretch vibration, is metaldependent and can be approximated as 45 cm−1 for the 3d metal ions studied and 123 cm−1 for the 4d and 5d metal ions. Furthermore, we studied the influence of different ligands cis to the hydrides, revealing a trend to decreasing splittings for more electron-withdrawing ligands. Although the splitting observed in the model complexes studied here show a larger range (21−69 cm−1 for the 3d metal and 89−146 cm−1 for the 4d and 5d metals), the resulting estimate formula shows good accuracy with a mean absolute deviation of 47 cm−1 for a range of different trans-dihydride compounds. This accuracy is sufficient to confidently interpret IR spectra of relevant compounds, while the formula remains easy to use. The gaps along with a new

Table 10. Comparison of the Charge Gradients to Activation Energies EA for Hydride-Transfer Reactions to Aldehydes and Esters for Complexes Discussed by Wei and Jiao86 EA, kcal mol−1 metal I

Mn FeII RuII OsII IrIII

charge gradient, e Å−1

aldehyde

ester

−0.36 −0.48 −0.58 −0.54 −0.56

15.6 14.1 11.7 13.5 14.3

32.7 29.6 26.8 29.3 29.6 I


Article

Inorganic Chemistry

Figure 8. Activation energy EA for the reduction of aldehydes (left) and esters (right) versus the charge gradients (CG) for different complexes of the type M(H)x(CO)3−x(iPr2PNHPiPr2) [x = 1 (Mn), 2 (Fe, Ru, and Os), and 3 (Ir)], as discussed by Wei and Jiao.86

Δνt(hydride) value of −235 cm−1 were included in a new simplified formula (eq 4), resulting in better agreement with experimentally observed wavenumbers. Furthermore, the information about the wavenumber gaps allows one to estimate the wavenumber of the symmetric trans-dihydride vibration. Elaborating on the nature of the two vibrations, we could show that the antisymmetric vibration is, in fact, very harmonic, while the symmetric vibration shows the expected anharmonic behavior. In order to answer the question of whether a hydride, a hydrogen atom, or a proton is developing along the normal modes of these vibrations, we looked at the change in the natural atomic charges along the normal modes of the two vibrations. We recognized that, within the antisymmetric vibration, the leaving hydride develops a negative charge drawn from the metal center and therefore dissociates as a hydride. Simultaneously, the second hydride donates negative charge to the metal center at the same pace, therefore creating an effective electron transfer from one hydride to another. We isolated the effect of this push− pull mechanism by replacing the trans-hydride with a transdeuteride. The effect seems to be much larger than electronic effects originating from, for example, substituting the transdeuteride with a trans-chloride and seems to be especially strong for the platinum group metals. Note that complexes transRuHCl(diphosphine)(diamine) do not transfer a hydride to a ketone and are not catalysts for ketone hydrogenation, whereas trans-RuH2(diphosphine)(diamine) is a very efficient catalyst.18 The symmetric vibration corresponds to dissociation into two hydrides in most cases. Increasing the oxidation state of the metal center and placing more electron-withdrawing ligands cis to the hydrides result in a dissociation into hydrogen atoms or protons. The changes in the natural atomic charges of the hydrides are very linear. The corresponding gradients show trends that follow chemical intuition and some quantitative hydride-transfer kinetics for other hydride systems, where the order is 4d > 5d > 3d metal. The largest gradients highlight metals forming especially active hydrogenation catalysts, like ruthenium and rhodium. Furthermore, a comparison of the charge gradients and activation barriers in hydrogenation reactions suggests a correlation between the two quantities. The gradients therefore seem to be suitable for characterizing potential hydrogenation catalysts and could help in catalyst design. On this basis, we suggest that 3d metal catalysts, if hydride transfer is the turnover limiting step, should have more donating and smaller ligands than equivalent 4d and 5d metal complexes to compensate for

the inherent difference in the charge gradients. While various spectroscopic and crystallographic methods are available to assess the (ground-state) trans-influence of a hydride,55,93−98 examples of methods such as the one reported here to assess the kinetic trans-effect of hydrides are rare. These ideas will be developed further in future work.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b02302. Choice of computational model, treatment of anharmonicity effects, equations for a linear symmetric triatomic and force constants, improvements to the simple formula in ref 25 including a table listing all of the metal hydride vibrational data, spectra of trans-Fe(H)2(dppm)2, transFe(D)2(dppm)2, and mer-Ir(H)3(PiPr2CH2pyCH2PiPr2), and absolute wavenumbers of the model complexes (PDF) Cartesian coordinates of all of the calculated structures as .xyz files (ZIP)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

David Schnieders: 0000-0002-8042-7930 Brian T. H. Tsui: 0000-0002-9091-330X Molly M. H. Sung: 0000-0002-0874-0922 Mark R. Bortolus: 0000-0002-1671-381X Gary J. Schrobilgen: 0000-0001-5109-6979 Johannes Neugebauer: 0000-0002-8923-4684 Robert H. Morris: 0000-0002-7574-9388 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.S. and J.N. thank the DFG for funding through IRTG 2027. R.H.M. and G.J.S. thank the NSERC for Discovery Grants. M.R.B. and B.T.H.T. thank the Ontario Ministry of Education for a Queen Elizabeth II Graduate Scholarship and Ontario Graduate Scholarship, respectively. J


Article

Inorganic Chemistry

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