Experimental Characterization of the Hydride 1H Shielding Tensors for

Article pubs.acs.org/JPCA

Experimental Characterization of the Hydride 1H Shielding Tensors for HIrX2(PR3)2 and HRhCl2(PR3)2: Extremely Shielded Hydride Protons with Unusually Large Magnetic Shielding Anisotropies Piotr Garbacz,†,‡ Victor V. Terskikh,§ Michael J. Ferguson,† Guy M. Bernard,† Mariusz Kędziorek,‡ and Roderick E. Wasylishen*,† †

Gunning/Lemieux Chemistry Centre, University of Alberta, Edmonton, Alberta T6G 2G2, Canada Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland § Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada ‡

S Supporting Information *

ABSTRACT: The hydride proton magnetic shielding tensors for a series of iridium(III) and rhodium(III) complexes are determined. Although it has long been known that hydridic protons for transition-metal hydrides are often extremely shielded, this is the first experimental determination of the shielding tensors for such complexes. Isolating the 1H NMR signal for a hydride proton requires careful experimental strategies because the spectra are generally dominated by ligand 1H signals. We show that this can be accomplished for complexes containing as many as 66 ligand protons by substituting the latter with deuterium and by using hyperbolic secant pulses to selectively irradiate the hydride proton signal. We also demonstrate that the quality of the results is improved by performing experiments at the highest practical magnetic field (21.14 T for the work presented here). The hydride protons for iridium hydride complexes HIrX2(PR3)2 (X = Cl, Br, or I; R = isopropyl, cyclohexyl) are highly shielded with isotropic chemical shifts of approximately −50 ppm and are also highly anisotropic, with spans (=δ11 − δ33) ranging from 85.1 to 110.7 ppm. The hydridic protons for related rhodium complexes HRhCl2(PR3)2 also have unusual magnetic shielding properties with chemical shifts and spans of approximately −32 and 85 ppm, respectively. Relativistic density functional theory computations were performed to determine the orientation of the principal components of the hydride proton shielding tensors and to provide insights into the origin of these highly anisotropic shielding tensors. The results of our computations agree well with experiment, and our conclusions concerning the importance of relativistic effects support those recently reported by Kaupp and co-workers.

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were predicted. In 1996, Ziegler and co-workers15 carried out DFT-GIAO calculations on several low-valent transition-metal hydrides that qualitatively supported the interpretation of Buckingham and Stephens but yielded shielding anisotropies that were much smaller in magnitude and more realistic. Both models predicted that it is the large positive values of σ⊥para that are responsible for the “hydridic” shifts (i.e., positive magnetic shielding and negative chemical shifts relative to tetramethylsilane (TMS)). The nuclear magnetic properties of nuclei in the vicinity of heavy atoms (i.e., the hydride protons in the present study) may be susceptible to relativistic effects.16−19 Recent relativistic four-component DFT calculations by Kaupp and coworkers20 demonstrate that for 4d and 5d complexes, there are sizable spin−orbital contributions that further contribute to the shielding perpendicular to the M−H bond. Jameson and de Dios have recently discussed negative hydride chemical shifts in their annual review of the literature on magnetic shielding.21 The other classic example of a positive σpara contribution to

INTRODUCTION Because the M−H moiety in transition-metal hydrides is particularly labile,1 these complexes play a central role in catalytic inorganic and organometallic chemistry. For example, many industrial processes in the petrochemical industry, such as alkene hydrogenation, rely on the involvement of the M−H moiety.1 NMR spectroscopy as well as neutron and X-ray diffraction has played, and continues to play, an essential role in elucidating the structure of these compounds.2−6 Since the early days of NMR spectroscopy, chemists have been intrigued by the negative chemical shifts characteristic of hydrogen nuclei bonded to transition metals. Typically, these 1H chemical shifts fall in the range of −5 to −20 ppm, but values as large as −50 ppm are known.2,7−9 In the early literature, it was argued that electron density from the metal center results in diamagnetic shielding of the M−H proton and that this is the source of the “high-field” shift;10 however, Buckingham and Stephens11,12 presented a model based on Ramsey’s theory13,14 that indicates that the paramagnetic contribution to the shielding, σpara, is in fact positive. Furthermore, very large and seemingly unrealistic values of the shielding anisotropy, (σ∥ − σ⊥) ≈ −500 ppm, © 2014 American Chemical Society

Received: November 19, 2013 Revised: January 24, 2014 Published: January 24, 2014 1203

dx.doi.org/10.1021/jp411378j | J. Phys. Chem. A 2014, 118, 1203−1212

The Journal of Physical Chemistry A

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shielding is for the fluorine nuclei of Cl−F, where the paramagnetic term (perpendicular) is positive;22 likewise, for 19 F in Br−F and I−F, the paramagnetic term is also positive.23 Given the importance of NMR spectroscopy in characterizing transition-metal hydrides and the extensive literature on their isotropic proton chemical shifts, it is somewhat surprising that there is no reliable experimental data on 1H nuclear magnetic shielding tensors for hydrides. Here, we present experimental results for several closely related iridium(III) and rhodium(III) hydrides, specifically HIrX2[P(cyclohexyl)3]2, where X = Cl, Br, and I; HIrX2[P(isopropyl)3]2, where X = Cl and I; and HRh(III)Cl2(PR3)2, where R = isopropyl or cyclohexyl. Our experimental 1H chemical shift tensors are compared with those calculated using zeroth-order regular approximation (ZORA) DFT24−26 and those reported for HIrCl2[P(CH3)3]2 and HRhCl2[P(CH3)3]2 by Kaupp and coworkers.20 In addition, we demonstrate the utility of hyperbolic secant (HS) pulses27−29 in characterizing the 1H magnetic shielding tensors for hydrides, such as the iridium and rhodium hydrides considered in this work. Finally, we present X-ray crystallographic data for HRhCl2[P(i-Pr-d7)3]2 and HIrCl2[P(iPr-d7)3]2, which confirm earlier data that indicates that these complexes are five-coordinate square-pyramidal structures with the hydride at the apex of the pyramid and that the two phosphine ligands are trans to one another as are the two chlorine atoms.30−32

packed in NMR rotors in an air atmosphere. Products were characterized using solution-phase 1H and 31P NMR spectroscopy. Elemental analyses were carried out for HIrCl2[P(i-Pr)3]2 and HRhCl2[P(i-Pr)3]2. Further details of the preparation and characterization procedures are available in the Supporting Information. The structures of HIrCl2[P(i-Pr-d7)3]2 and HRhCl2[P(i-Prd7)3]2 were determined using single-crystal X-ray diffraction. Selected crystal and structural data are available in the Supporting Information. To verify that the unusually shielded hydride proton of HIrCl2[P(Cy-d11)3]2 was not a consequence of paramagnetism, its bulk magnetic susceptibility was determined using a Johnson Mathey magnetic susceptibility balance. These measurements indicate that the susceptibility for this complex is −7.44 × 10−9 m3 kg−1 and hence that the compound is diamagnetic. See the Supporting Information for more details. Solid-State NMR Spectroscopy. Preliminary 1H NMR spectra were acquired on a Bruker Avance 500 MHz NMR spectrometer equipped with a 2.5 mm MAS probe (University of Alberta). Magic angle spinning (MAS) measurements were performed at spinning frequencies of 4.5, 6.0, and 7.5 kHz. A background suppression pulse sequence (90−180−180°),39,40 which relies on the fact that the background and sample nuclei experience different 90° hard pulses, was used. For a fixed power level, the 90° pulse was 4 μs for the protons of water within the coil but 25 μs for the probe background signal. This indicates a background suppression factor of approximately 15. Other acquisition parameters were 16 K scans, a 50 ms FID acquisition time, and a 0.1 s recycle delay. Measurements were also carried out at 900 MHz using a Bruker Avance II NMR spectrometer equipped with a 2.5 mm H/X MAS probe (National Ultrahigh-Field NMR Facility for Solids in Ottawa, Canada). MAS measurements were performed at nine different spinning frequencies ranging from 4.0 to 18.0 kHz; to avoid experimental artifacts, the magic angle was carefully optimized. To eliminate background 1H NMR signals, a rotor-synchronized solid-echo41 pulse sequence (90°−τ−180°−τ−ACQ, where τ was set such that the rotor was at the same position in its cycle at the midpoint of each pulse) was used. In addition, a pulse sequence that uses HS pulses to selectively excite the hydride proton was used for spectral editing.42,43 The HS pulse was generally applied to the low-frequency third-order spinning sideband of the isotropic hydride resonance. Typical acquisition parameters were HS pulse durations of 0.7−1.5 ms at a power level of 26.4 kHz, 256 scans, 33 ms FID acquisition times, 5 s recycle delays, and 3 μs 90° pulses. In all experiments, the stability of the spinning frequency was better than ±10 Hz. All experimental chemical shifts are reported relative to that for neat liquid TMS (δiso = 0.0 ppm) in a rotor inclined at the magic angle relative to the applied magnetic field. Experimental principal components of the 1H chemical shift tensor were obtained by analyzing the intensities of spinning sidebands as described by Herzfeld and Berger;44 this method is based on the procedure reported in 1979 by Maricq and Waugh.45 Iterative fitting of the intensities was carried out using the program WSOLIDS.46 The number of sidebands included in the simulations varied between 6 and 18. The reported principal components are average values obtained from analyses of the various spectra, which also yielded the standard deviations.

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EXPERIMENTAL SECTION Sample Preparation. Iridium(III) chloride hydrate (54.5% Ir) and iridium(III) bromide hydrate (35.9% Ir) were purchased from Strem Chemicals Inc. Rhodium(III) chloride hydrate (39% Rh), hexachloroiridium(IV) acid hydrate (36.8% Ir), sodium iodide, hydrochloric acid (38%), hydrobromic acid (48%), tri(isopropyl)phosphine (95%), tricyclohexylphosphine (98%), and organic solvents were purchased from SigmaAldrich. Deuterium-labeled phosphines were obtained from CDN Isotopes (Montreal, Canada) as 1:1 carbon disulfide complexes, that is, (tricyclohexyl-d33)phosphine·CS2 (98.4% 2 H) and tri(isopropyl-d21)phosphine·CS2 (98% 2H). Tri(isopropyl-d21)phosphine and (tricyclohexyl-d33)phosphine were prepared from the carbon disulfide complexes as described in the literature.33 Tri-isopropylphosphine is pyrophoric and must be handled in an inert atmosphere at all times (e.g., a Schlenk tube). All organic solvents, that is, 2propanol, toluene, and acetone, were degassed at least three times by freeze−pump−thaw cycles. Other materials with the exception of P(i-Pr)3·CS2 and PCy3·CS2 were used without further purification. Syntheses of the iridium(III) and rhodium(III) hydride complexes were carried out in a Schlenk system under argon (Air Products, 99.9%). HIrCl2[P(i-Pr)3]2,34 HIrI2[P(i-Pr)3]2,35 HIrCl2(PCy3)2,36 HRhCl2[P(i-Pr)3]2,37 and HRhCl2(PCy3)237 were prepared according to literature methods. HIrBr2(PCy3)2 was prepared following an alternate method for the synthesis of HIrCl2(PCy3)2,38 using IrBr3 as a starting material instead of IrCl3. HIrI2(PCy3)2 was synthesized in the same way as that described for HIrI2[P(i-Pr)3]2;35 however, longer reflux times were necessary for the preparation of the PCy3 complex. The methods used to synthesize the deuterium-labeled phosphine ligand complexes were analogous to those used for the unlabeled complexes. All complexes were sufficiently stable in air that they could be separated from solvents by filtration and 1204



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Quantum Chemical Computations. DFT calculations of the hydride proton shielding of the iridium and rhodium complexes were carried out using the Amsterdam Density Functional (ADF) program.47−49 Relativistic effects were included using ZORA24−26,50,51 with spin−orbit coupling.52 Calculations of proton shielding tensors for HIrCl2(PMe3)2 were performed using both the TZ2P and QZ4P basis sets provided with the ADF software package; the PBE0 hybrid functional,53 based on the generalized gradient approximation of Perdew, Burke, and Ernzerhof (PBE),54,55 was used for all calculations. Because the principal components of the magnetic shielding tensor obtained using these two basis sets were the same within ±0.5 ppm, the TZ2P basis set was used for other calculations reported here. Calculated hydrogen magnetic shielding values were converted to chemical shifts according to δ(hydride) = σ(TMS) − σ(hydride), where the shielding of the protons of TMS was taken as 31.3 ppm. We obtained the latter value from DFT calculations using the same level of theory that we used for calculations on the transition-metal hydrides. The geometry for TMS was taken from an optimized structure reported by Gajda and Katrusiak.56 Temperature and zero-point vibration corrections were neglected in all of our calculations as they are relatively small compared to the effects discussed here (vide infra). Finally, we would like to point out that the experimental value that we recently obtained for the absolute magnetic shielding constant of hydrogen nuclei in gaseous TMS was 30.783 (7) ppm,57 within 0.5 ppm of the value used here. In obtaining calculated chemical shift tensor principal components for the hydride protons of the transitionmetal hydrides (vide infra), we felt it better to be consistent and use the calculated value for the reference compound, σ(1H TMS (g)) = 31.3 ppm, and for the transition-metal hydride (i.e., gas-phase values for isolated molecules). Note that under MAS conditions, the bulk magnetic susceptibility effect vanishes.58 Optimized structures were obtained with ADF47−49 using ZORA24−26 (including spin−orbit coupling)52 and the TZ2P basis; initial atomic coordinates for the optimizations were those reported for HIrCl2(PMe3)2 and HRhCl2(PMe3)2 by Kaupp and co-workers.20 Atomic coordinates for the optimized structures are summarized in the Supporting Information. The geometry of the hydride proton neighborhood of HIrCl 2 (PH 3 ) 2 , HIrCl 2 (PMe 3 ) 2 , HRhCl 2 (PH 3 ) 2 , and HRhCl2(PMe3)2 was in agreement with structures of HIrCl2[P(i-Pr-d7)3]2 and HRhCl2[P(i-Pr-d7)3]2 determined by X-ray crystallography (vide infra).

Figure 1. Proton NMR spectra of a MAS sample of HIrCl2[P(i-Pr)3]2 acquired at 500 MHz (11.75 T) at spinning frequencies up to 35 kHz. The strong peak in the middle of the spectrum originates from triisopropylphosphine ligand protons. The hydride proton signal, when detectable, is indicated by an arrow.

d11)phosphine, as described in the Supporting Information. Proton NMR spectra of HIrCl2[P(Cy-d11)3]2, acquired at 900 and 500 MHz, are shown in Figure 2a and b, respectively. The peaks arising from the hydride proton are clearly visible as a series of spinning sidebands even at relatively slow MAS frequencies. Comparison of 1H NMR spectra acquired at several spinning frequencies indicates that δiso = −50.2 ppm for the hydride proton of this compound. Comparable values were reported for closely related compounds dissolved in solution.7−9 Although the deuteration level for the ligands of this sample is given by the supplier as 98.4%, a significant 1H NMR signal remains from the residual 1H nuclei in the cyclohexyl moiety and possibly from residual 1H nuclei from the probe. Using rotor-synchronized spin−echo and background suppression pulse sequences for spectra obtained at two fields yielded spinning sideband patterns that were analyzed to yield the results listed in the first two columns of Table 1, where we have used the so-called Maryland convention in reporting the chemical shift tensors.59,60 The data in Table 1 indicate that the span, defined as the total breadth of the powder pattern, exceeds 100 ppm, which is unusually large given that 1H chemical shift tensors typically have spans less than 30 ppm (see Table 2).61−75 The data in Table 1 also demonstrate a great benefit of acquiring 1H spectra at the highest possible applied magnetic field; the precision of the data acquired at 21.14 T is 5−10 times greater than that acquired at 11.75 T. To further improve the reliability of our chemical shift tensor data, we took advantage of the highly selective nature of HS

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RESULTS AND DISCUSSION H NMR Spectra. Proton NMR spectra of solid HIrCl2[P(iPr)3]2, acquired at 500 MHz as a function of MAS frequency, are shown in Figure 1. While the hydride proton peak at δ ≈ −51 ppm is visible at spinning frequencies greater than 15 kHz, the spectra are dominated by 1H NMR peaks arising from the 42 1H nuclei associated with the two isopropylphosphine ligands. Because of the numerous strongly dipolar coupled 1H nuclei in this sample, extracting hydride shielding tensor information from these homogeneously broadened NMR spectra is problematic. Even more “NMR challenging” is HIrCl2(PCy3)2 because each molecule contains 66 1H nuclei from the two tricyclohexylphosphine ligands. To minimize interference from the nonhydridic protons, deuterated analogues of the compounds of interest were prepared from tri(isopropyl-d7)phosphine and tri(cyclohexyl1

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Figure 2. Proton NMR spectra of a MAS sample of solid HIrCl2[P(Cy-d11)3]2 acquired at 21.14 T with a spinning frequency of 5 kHz using the spin−echo pulse sequence (a) and at 11.75 T with a spinning frequency of 6 kHz using the DEPTH39,40 pulse sequence (b). The asterisk in (b) indicates a 1H peak arising from an impurity at natural abundance.

Table 1. Summary of Experimental 1H NMR Parametersa for the Hydride Proton of HIrCl2[P(Cy-d11)3]2 11.75 Tb δ11/ppm δ22/ppm δ33/ppm δiso/ppm Ω/ppm κ

10 −53 −108 −50.1 118 −0.07

± ± ± ± ± ±

5 5 5 0.1 5 0.10

21.14 Tc 6.1 −50.8 −105.9 −50.21 112 −0.016

± ± ± ± ± ±

1.1 0.5 0.9 0.03 2.0 0.020

without the application of the HS pulse (upper trace, Figure 3), and then, a second spectrum was obtained under identical conditions except that a HS pulse was applied to one of the spinning sidebands, in this case, the third-order, low-frequency sideband (middle trace, Figure 3); this eliminated the NMR signal from the hydride proton, yielding a spectrum with only NMR signal from the nonhydridic 1H nuclei in the sample. Subtracting the latter spectrum from the first spectrum yields the 1H NMR spectrum of the hydride proton (lower trace, Figure 3). This technique yielded 1H NMR spectra with only minor residual distortions due to nonhydridic 1H nuclei, allowing us to determine the principal components of the chemical shift tensor from an analysis of the spinning sideband pattern; we believe that these values are accurate to better than ±1.0 ppm and are reproducible to better than ±0.5 ppm. Typical 1H NMR spectra obtained as described above using a 900 MHz NMR spectrometer at three different MAS frequencies are shown in Figure 4; data obtained from analyses of these and other spectra are summarized in Table 3. As mentioned in the Experimental Section, several MAS

21.14 Td 5.4 −50.8 −105.3 −50.24 110.7 −0.015

± ± ± ± ± ±

0.4 0.4 0.2 0.03 0.6 0.012

a

The Maryland convention59 is used to describe the NMR spectra: isotropic chemical shift, δiso = (δ11 + δ22 + δ33)/3; span, Ω = δ11 − δ33; and, skew, κ = 3(δ22 − δiso)/Ω. bSpectra were recorded with spinning frequencies of 4.5, 6.0, and 7.5 kHz. cSpectra were obtained with six MAS frequencies ranging from 2.5 to 18 kHz. dObtained with selective saturation of the hydride peak using HS pulses; see the text.

pulses.27−29,43 It has previously been shown that if one selectively saturates any one sideband from a spinning sideband pattern arising from a specific nucleus, the complete resonance can be saturated.42,43 First, a 1H NMR spectrum was obtained

Table 2. Typical Spansa of Chemical Shift Tensors for Hydrogen Nuclei61 compounds with C−H bonds

hydrates

acids and acid salts

paramagnetic zirconium halide hydrides

a

examples

Ω/ppma

ref

trans-1,2-diiodoethylene benzene poly(ethylene) cyclohexane Na2Fe(CN)5NO·H2O H2O CaSO4·2H2O (COOH)2; α-oxalic acid C4O4H2; squaric acid H*OOCCH2COOK KHF2 ZrBrH ZrClH

4.5 ∼5 (223 K) 5.9 (77 K) 8.9 (77 K) 21.2 (290 K) 34.2 (173 K) 35.7 17.8 22.4 33.0 48.5 51.3 102.5

62 63, 64 65 65 66 67, 68 69 70 71 72 73 74 74, 75

Span, Ω = δ11 − δ33. 1206



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Figure 3. Suppression of residual proton signals from the tricyclohexylphosphine ligands of HIrCl2[P(Cy-d11)3]2 under MAS conditions at a spinning frequency of 8 kHz (21.14 T). Spectra were recorded using the spin−echo pulse sequence (upper trace), selective saturation of the third sideband of the hydride MAS pattern via HS pulses (middle trace), and the difference between these spectra (i.e., the middle trace subtracted from the upper trace).

DFT Computations. The results of ZORA DFT calculations are presented in Table 3. To maintain computation times within acceptable limits, the P(i-Pr)3 and PCy3 ligands were replaced with trimethylphosphine, PMe3. To verify that this substitution did not have a significant impact on the calculated results, the chemical shift tensor for HIrCl2[P(iPr)3]2 was calculated at the same level of theory as for those obtained with the PMe3 ligands. These results yielded similar calculated principal components, with the greatest deviation, to δ33, of 3.2 ppm. Thus, for the remaining compounds, only the smaller ligand was used for the calculations. Table 3 along with Figure 5a, a plot of experimentally determined versus calculated chemical shift tensor principal components, shows that there is a good linear relationship between calculated and experimental values, with a slope of 1.16 and R2 = 0.998. Considering the necessary approximations in these calculations (e.g., substitution of the tri-isopropylphosphine ligands with trimethylphosphine ligands), the deviation from a perfect-fit slope of 1.0 is considered minor. Another factor possibly affecting the computational results is the fact that the implementation of the ADF program used in this work does not include the response of the first-order exchange−correlation potential on calculated shielding.78 Nevertheless, the DFT calculations reproduce the experimental trends. For example, the calculated spans are greater for the chloroiridium complexes than those for the bromo or iodo complexes, as observed experimentally. Despite significantly different spans depending on the halide substituent, there are only minor variations in the isotropic hydride shielding, a trend that is also observed experimentally. Calculated orientations for the hydride proton shielding tensors

frequencies were used to obtain the standard deviations given in Tables 1 and 3. Molecular Structures. Before presenting the results of DFT computations, we briefly discuss the crystal structures of two complexes investigated here, HRhCl2[P(i-Pr-d7)3]2 and HIrCl2[P(i-Pr-d7)3]2. Our results for HRhCl2[P(i-Pr-d7)3]2 are in very good agreement with those first reported by Harlow et al. in 1992.32 The crystals are monoclinic, symmetry P21/c with Rh at an apparent center of inversion because of hydride disorder (50% above and below the coordination plane). Neglecting this disorder, the molecule has an approximate, but not exact, plane of symmetry encompassing the Rh, hydride H, and two P atoms. The Rh−H bond length was refined to 1.40 (4) Å; values ranging from 1.31(8) to 1.581(3) Å have been reported for terminal Rh−H bonds determined from neutron diffraction studies.3 Kaupp and co-workers obtained a Rh−H bond length of 1.505 Å for HRhCl2(PMe3)2 in a computational geometry optimization.20 The crystal structure of HIrCl2[P(i-Pr-d7)3]2 is isostructural with that of HRhCl2[P(i-Pr-d7)3]2. In this case, the Ir−H bond length was restrained to 1.50 Å (see the Supporting Information). This is less than typical values obtained from neutron diffraction, approximately 1.6 Å (see Table 3 of Bau and Drabnis),3 but is comparable to the value of 1.535 Å obtained from a geometry optimization of HIrCl2(PMe3)2 by Kaupp and co-workers.20 Interestingly, the crystal symmetry and unit cell dimensions determined here for HIrCl2[P(i-Prd7)3]2 are similar to those obtained for what was originally (erroneously) assigned as H2IrCl2[P(i-Pr)3]276 and later corrected to HIrCl2[P(i-Pr)3]2.77 1207



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Figure 4. Proton NMR spectra of solid HIrCl2[P(Cy-d11)3]2. In the top trace, a calculated spectrum expected for the hydride proton of a stationary sample is shown. In the lower traces, spectra of MAS samples obtained at the indicated spinning frequencies at 21.14 T are shown; simulated spectra are displayed immediately above the corresponding experimental spectra. The isotropic peak appears at −50.2 ppm.

Table 3. Experimental and Calculated Principal Components of the Chemical Shift Tensors for the Hydride Protons of Iridium and Rhodium Hydride Complexesa δ11 /ppm HIrCl2[P(Cy-d11)3]2 HIrBr2[P(Cy-d11)3]2 HIrI2[P(Cy-d11)3]2 HRhCl2[P(Cy-d11)3]2 HIrCl2[P(i-Pr-d7)3]2 HIrI2[P(i-Pr-d7)3]2 HRhCl2[P(i-Pr-d7)3]2

5.4 1.3 −3.1 12.4 2.7 −5.4 11.6

HIrCl2(PMe3)2 HIrBr2(PMe3)2 HIrI2(PMe3)2 HRhCl2(PMe3)2

6.9 4.3 −3.5 13.1

± ± ± ± ± ± ±

0.4 0.6 1.7 0.7 4.4 1.8 1.1

δ22 /ppm −50.8 −51.6 −55.6 −35.7 −48.7 −54.7 −35.2 −41.7 −44.0 −47.5 −32.2

± ± ± ± ± ± ±

0.4 0.5 1.2 1.0 0.9 1.4 1.0

δ33 /ppm Experimental −105.3 ± 0.2 −100.7 ± 0.6 −88.2 ± 1.2 −73.0 ± 0.5 −107.6 ± 4.2 −90.8 ± 1.3 −73.9 ± 0.7 Calculated −88.9 −84.6 −71.4 −63.3

δiso /ppm −50.24 −50.33 −48.97 −32.11 −51.18 −50.27 −32.49 −41.2 −41.4 −40.8 −27.5

± ± ± ± ± ± ±

0.03 0.04 0.03 0.20 0.08 0.15 0.03

Ω /ppm 110.7 102.0 85.1 85.4 110.3 85.4 85.5 95.8 88.9 67.9 76.4

± ± ± ± ± ± ±

0.6 0.8 2.1 0.9 6.1 2.2 1.3

κ −0.015 −0.037 −0.233 −0.126 0.068 −0.155 −0.095

± ± ± ± ± ± ±

0.012 0.015 0.042 0.036 0.024 0.051 0.036

−0.015 −0.087 −0.296 −0.186

a

Experimental values were obtained from an analysis of the spinning sideband intensities in spectra obtained at 21.14 T. Calculated results are from relativistic DFT calculations. Calculated magnetic shielding was converted to chemical shifts according to δcalc = 31.3 − σcalc; see the text for details.

perpendicular to the M−H bond are significant and are, in part, responsible for the highly shielded hydride protons, particularly for the Ir complex. The paramagnetic contribution is only negative for the component of the shielding tensor along the M−H bond, that is, σ11. As predicted by Buckingham and Stephens,11,12 Ziegler and co-workers,15 and, more recently, Kaupp et al.,20 the paramagnetic contribution to the components of the shielding tensor perpendicular to the M− H bond, σ22 and σ33, are positive. Qualitatively, all experimental trends are reproduced by the computations. Some points of interest are as follows:

of the iridium and rhodium complexes are similar, with the direction of least shielding along the M−H bond; that for HIrCl2(PMe3)2 is shown in Figure 5b. Kaupp and co-workers used relativistic four-component DFT calculations to calculate the hydride shielding for some of these compounds;20 calculated isotropic values were close to our experimental values. In Table 4, contributions from the diamagnetic, σdia, paramagnetic, σpara, and spin−orbit, σSO, magnetic shielding mechanisms to the hydride shielding tensors for HRhCl2(PMe3)2 and HIrCl2(PMe3)2 are presented. As reported earlier by Kaupp et al.,20 the spin−orbit contributions 1208



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tensor parallel to the P−Ir−P axis than for those along the X− Ir−X axis, that is, σdia, σpara, and σSO are 5.5, 13.0, and 28.7 ppm, respectively, greater along σ33 than along σ22. Silence of 191/193Ir: Absence of Spin−Spin Coupling to the Hydridic Proton and 31P Nuclei. DFT calculations (ZORA relativistic calculations with the TZ2P basis set)47,48 on HIrCl2(PMe3)2 predict values of 112 and 122 Hz for 1 191 1 J( Ir, H) and 1J(193Ir,1H), respectively. From the optimized Ir−H bond length of 1.539 Å, a direct dipolar coupling of approximately 600 Hz is predicted. However, no indication of spin−spin coupling to either 191Ir (I = 3/2, Ξ = 1.718%, NA = 37.3%) or 193Ir (I = 3/2, Ξ = 1.871%, NA = 62.7%) is observed. This is probably a consequence of self-decoupling62,81−83 of the Ir nuclei due to a large nuclear quadrupolar coupling constant and resultant short iridium T1; calculations at the same level of theory predict CQ(191Ir) = 1.27 GHz and CQ(193Ir) = 1.17 GHz. As confirmed by line shape simulations (not shown), hydridic 1 H peaks would be significantly broader than we observed experimentally if the Ir nuclei were not self-decoupled, a consequence of the residual dipolar coupling81,82 combined with indirect spin−spin coupling. This effect would also be observed in 31P NMR spectra because the 31P nuclei are directly bonded to iridium. Yet, there is no indication of 1J(Ir,31P) in NMR spectra of the latter (not shown). Related examples of self-decoupling of quadrupolar nuclei have been observed previously for 1H and 31P NMR spectroscopy.62,83 Finally, Oldfield and co-workers suggested that broadening of 13C NMR spectra of solid Ir4(CO)12 may be due to residual spin−spin coupling with 191/193Ir;84 however, neither solid-state NMR experiments at 21.14 T nor DFT CASTEP 85 computations (not shown) support this hypothesis.

Figure 5. (a) Plot of experimental versus calculated principal components (ii =11, 22, or 33) of the hydride proton chemical shift tensors for the metal hydride complexes under consideration. The linear fit through the points, δexp = 1.16 × δcalc − 1.51 (R2 = 0.998), is shown. Calculated chemical shifts were obtained according to δcalc = 31.3 − σcalc (see the Experimental Section for more details). (b) Calculated orientation of the hydride proton magnetic shielding tensor in hydridodichlorobis(trimethylphosphine)iridium(III). For clarity, the methyl group hydrogen atoms have been omitted.

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CONCLUSIONS To experimentally characterize hydride 1H shielding tensors in powder samples of transition-metal hydrides with hydrogenrich ligands via solid-state 1H NMR spectroscopy, we find it essential to substitute the ligand protons by deuterium. Furthermore, the utility of the HS pulses to selectively saturate the hydride 1H NMR resonance and not perturb the residual ligand 1H NMR signals allowed us to obtain relatively clean 1H NMR spectra for the hydride proton(s) alone. Analysis of spinning sidebands acquired at several slow MAS frequencies yielded the first reliable experimental characterization of transition-metal hydride proton shielding tensors. The isotropic chemical shifts of the iridium and rhodium hydrides are approximately −50 and −32 ppm, respectively, very similar to

(i) The component of the shielding tensor parallel to the X− Ir−X axis, that is, σ22, shows the least variation with halide substitution, as expected.79,80 For example, for the iridium complexes, substitution of the two chlorine atoms with iodine results in an 8.5 ppm increase in the shielding in the direction of σ11, a 4.8 ppm increase in the direction of σ22, and a 17.1 ppm decrease in the direction of σ33; computational results are comparable (see Table 3). (ii) The origin of the enhanced shielding parallel to the Ir−P bond, σ33, compared to that parallel to the Ir−X bond, σ22, is unclear; however, from the computations summarized in Table 4, it is clear that contributions from all three mechanisms are more positive for the principal component of the shielding

Table 4. Calculated Contributions to the Principal Components of the Hydride Proton Magnetic Shielding Tensors for the Rhodium and Iridium Complexesa σdia σpara σso σtot σexp

a

hydride

σ11 /ppm

σ22 /ppm

σ33 /ppm

σiso /ppm

HRhCl2(PMe3)2 HIrCl2(PMe3)2 HRhCl2(PMe3)2 HIrCl2(PMe3)2 HRhCl2(PMe3)2 HIrCl2(PMe3)2 HRhCl2(PMe3)2 HIrCl2(PMe3)2 HRhCl2[P(Cy-d11)3]2 HIrCl2[P(Cy-d11)3]2

47.2 56.2 −26.5 −20.2 −2.6 −11.7 18.1 24.3 18.9 ± 0.7 25.9 ± 0.4

16.2 15.2 29.6 27.1 17.6 30.6 63.4 72.9 67.0 ± 1.0 82.1 ± 0.4

22.6 20.7 43.3 40.1 28.6 59.3 94.5 120.1 104.3 ± 0.5 136.6 ± 0.2

28.6 30.7 15.5 15.7 14.5 26.1 58.6 72.4 63.41 ± 0.20 81.54 ± 0.03

Experimental chemical shift values were converted to magnetic shielding values according to σexp = 31.3 − δexp (see the text). 1209



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the values measured for these compounds in solution.2,7−9 For the two series of iridium(III) halides investigated, the chlorides have the largest spans, approximately 110 ppm, while the iodides have the smallest spans, approximately 85 ppm. In the case of the two rhodium(III) chloride complexes considered in this work, the spans are both 85 ppm, approximately 30% less than the corresponding iridium hydrides. For the HIrX2(PR3)2 and HRhCl2(PR3)2 complexes, ZORA DFT computations reproduce the observed trends. The calculations indicate that the principal components of the hydride hydrogen shielding tensor perpendicular to the metal−hydrogen bond have positive paramagnetic contributions, in agreement with earlier theoretical predictions. The ZORA DFT computations reported here as well as the calculated results reported by Kaupp and coworkers20 indicate that the diamagnetic shielding tensor is very anisotropic with enhanced shielding along the metal−hydrogen bond axis. Most important, the calculations show that relativistic effects are largely responsible for the unusual shielding of the hydride protons in the complexes considered in the present work and that the spin−orbit term is responsible for the significant differences between the principal components of the magnetic shielding tensors perpendicular to the M−H bonds of the Rh and Ir complexes.

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the Natural Sciences and Engineering Research Council of Canada (NSERC) for financial support through the Discovery Grant Program, and we thank NSERC for a Major Resources Support grant. R.E.W. also acknowledges the Canada Research Chairs program and the University of Alberta for research support. Finally, we thank Professor Robert H. Morris (U. of Toronto) for sending R.E.W. a sample of HIrCl2[P(i-Pr)3]2 many years ago, which sparked his interest in this project.

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ASSOCIATED CONTENT

S Supporting Information *

Sample preparation procedures, X-ray diffraction data for HIrCl2[P(i-Pr-d7)3]2 and HRhCl2[P(i-Pr-d7)3]2, including the corresponding crystallographic information files, atomic coordinates used for DFT calculations, 1H NMR spectra of solid samples, measurement of bulk susceptibility for HIrCl2[P(Cyd11)3]2, and description of the apparatus used for the distillation of a small amount of liquid. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 780-492-4336. Fax: 780-492-8231. Notes

The authors declare no competing financial interest. E-mail: [email protected] (P.G.). Victor.Terskikh@ nrc-cnrc.gc.ca (V.V.T.). [email protected] (M.J.F.). [email protected] (G.M.B.). [email protected] (M.K.).

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ACKNOWLEDGMENTS We thank Professor Joe Takats and Dr. Tom Nakashima (U. of Alberta) for their interest in this project and for many helpful suggestions. We are also grateful to Dr. Jason Cooke (U. of Alberta) for obtaining the magnetic susceptibility data discussed herein and to Ms. Michelle Ha (U. of Alberta) for her help with the computational aspects of this work. This project was partially co-operated within the Foundation of Polish Science MPD Programme cofinanced by the European Union European Regional Development Fund (P.G.). Access to the 900 MHz NMR spectrometer was provided by the National Ultrahigh-Field NMR Facility for Solids (Ottawa, Canada), funded by the Canadian Foundation for Innovation, the Ontario Innovation Trust, Recherche Québec, the National Research Council of Canada, and Bruker BioSpin and managed by the University of Ottawa (http://nmr900.ca). R.E.W. thanks 1210



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Experimental Characterization of the Hydride 1H Shielding Tensors for

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