Experimental and Computational Studies of the Reaction of Carbon

Article pubs.acs.org/Organometallics

Experimental and Computational Studies of the Reaction of Carbon Dioxide with Pincer-Supported Nickel and Palladium Hydrides Hee-Won Suh,† Timothy J. Schmeier,† Nilay Hazari,*,† Richard A. Kemp,*,‡,§ and Michael K. Takase† †

The Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131, United States § Sandia National Laboratories, Albuquerque, New Mexico, United States ‡

S Supporting Information *

ABSTRACT: A series of Ni(II) and Pd(II) hydrides supported by PNP and PCP ligands, including iPr2PNP(CH3)PdH (iPr2PNP(CH3) = N(2-PiPr2-4-MeC6H3)2), iPr2 PNP (CH3) NiH, iPr2 PNP (F) PdH (iPr2PNP(F) = N(2-PiPr2-4-C6H3F)2), CyPhPNPPdH (CyPhPNP = N(2-P(Cy)(Ph)-4-MeC6H3)2), tBu2PCPPdH (tBu2PCP = 2,6C6H3(CH2PtBu2)2), tBu2PCPNiH, Cy2PCPPdH (Cy2PCP = 2,6C6H3(CH2PCy2)2), and Cy2PCPNiH, were prepared using literature methods. In addition, the new Ni and Pd hydrides Cy2PSiPMH (M = Ni, Pd; Cy2PSiP = Si(Me)(2-PCy2-C6H4)2) supported by PSiP ligands were synthesized. The analogous metal hydride complexes supported by the Ph2PSiP ligand (Ph2PSiP = Si(Me)(2-PPh2-C6H4)2) could not be prepared. Instead, the Ni(0) and Pd(0) η2-silane complexes Ph2PSiHPM(PPh3) (M = Ni, Pd; Ph2PSiHP = (H)Si(Me)(2-PPh2-C6H4)2), which have been proposed to be in equilibrium with Ph2PSiPMH (M = Ni, Pd) and PPh3, were prepared. Facile carbon dioxide insertion into the metal−hydride bond to form the metal formate complexes tBu2PCPM-OC(O)H (M = Ni, Pd) or Cy2PCPM-OC(O)H (M = Ni, Pd) was observed for PCP-supported species, and a similar reaction was observed for Cy2PSiP-supported hydrides to form Cy2PSiPMOC(O)H (M = Ni, Pd). No reaction with carbon dioxide was observed for any complexes supported by PNP ligands. The η2-silane complex Ph2PSiHPPd(PPh3) reacted rapidly with carbon dioxide to give Ph2PSiPPd-OC(O)H and PPh3, while the corresponding Ni complex Ph2PSiHPNi(PPh3) did not react with carbon dioxide. DFT calculations indicate that carbon dioxide insertion is thermodynamically favorable for PSiP- and PCP-supported hydrides because the strong trans influence of the anionic carbon donor destabilizes the metal−hydride bond. In contrast, carbon dioxide insertion is thermodynamically unfavorable for the PNP-supported species. In the case of the η2-silane complexes, carbon dioxide insertion is thermodynamically favorable for Pd and unfavorable for Ni. This is because the equilibrium between the metal hydride and PPh3 and the η2-silane complex more strongly favors the metal hydride for Pd than for Ni. In the cases of metal hydrides, the thermodynamic favorability of carbon dioxide insertion can be predicted from the natural bond orbital charge on the hydride. The pathway for carbon dioxide insertion into the metal hydride is concerted and features a four-centered transition state. The energy of the transition state for carbon dioxide insertion decreases as the trans influence of the anionic donor of the pincer ligand increases.

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INTRODUCTION The decline in the world’s petroleum reserves and concerns about the environmental consequences of fossil fuel use has led to a search for alternative carbon sources.1 Carbon dioxide is a particularly attractive feedstock, due to its high abundance, low cost and toxicity, and relative ease of transport.1,2 However, the catalytic conversion of carbon dioxide is complicated by its high kinetic and thermodynamic stabilities. It has been demonstrated that carbon dioxide can bind to a variety of different transitionmetal centers, which in some cases weakens the strong CO double bonds, facilitating its conversion into value-added products.3 Another method for activation and subsequent transformation involves the insertion of carbon dioxide into metal− element bonds (for example, M−H, M−C, M−O, and M−P bonds), which is generally controlled by thermodynamic factors.3,4 This reaction is especially important for late-transitionmetal complexes, where the insertion of carbon dioxide into a © 2012 American Chemical Society

metal−element (M−E) bond generates a metal carboxylate product (M−OC(O)E), which contains a relatively weak M−O bond and can therefore undergo further transformation. In particular, the insertion of carbon dioxide into Ir and Ni hydrides supported by pincer ligands is believed to be a key step in several catalytic cycles for the conversion of carbon dioxide into more valuable products. Both Nozaki's and our group have developed highly active Ir(III) catalysts, with different ancillary PNP pincer ligands, that can be used for the thermal hydrogenation of carbon dioxide to formic acid.5 Brookhart and Meyer have recently developed a related complex for the electrocatalytic production of formic acid.6 In these systems, the first step in catalysis is proposed to be the insertion of carbon dioxide into an Ir hydride to generate an O-bound formate. Similarly, Received: September 4, 2012 Published: November 5, 2012 8225

dx.doi.org/10.1021/om3008597 | Organometallics 2012, 31, 8225−8236

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Figure 1. PNP-, PCP-, and PSiP-supported Pd and Ni hydrides prepared in this work.11−16,20

the first step in the catalytic cycle proposed by Guan and coworkers for the reduction of carbon dioxide to methanol using a stoichiometric quantity of borane is proposed to involve the insertion of carbon dioxide into a Ni(II) hydride, supported by a POCOP pincer ligand.7 Conversely, Iwasawa and co-workers recently described a Pd-based system for the catalytic hydrocarboxylation of allenes with carbon dioxide in the presence of AlEt3.8 The Pd catalyst features an unusual PSiP pincer ligand, and at one stage in the catalytic cycle a Pd(II) hydride supported by a PSiP ligand is believed to be in equilibrium with a Pd(0) species containing an η2-coordinated Si−H bond. In order for productive catalysis to occur, allene insertion into the Pd(II) hydride needs to be faster than carbon dioxide insertion. It is possible that a greater understanding of the mechanism of carbon dioxide insertion into pincer-supported transition-metal hydrides will lead to more efficient catalysts for all of these systems. Over the last two decades there have been several reports of carbon dioxide insertion into well-defined Ni(II) and Pd(II) hydrides.7a,9 In an important early work, Darensbourg and co-workers reported that carbon dioxide insertion was observed only into systems with a relatively weak Ni−H bond in complexes of the type trans-HNi(X)(PCy3)2 (X = H, Me, Ph, CF3, SC6H5, S-4-C6H4CH3, OC6H5).9b Subsequently, both Guan and co-workers and our group have studied carbon dioxide insertion into Ni(II) hydrides supported by POCOP and PCP pincer ligands, respectively, and proposed that the mechanism of insertion involves a four-center, metal-containing transition state.7,9e,f Recently, we described the first isolated example of the insertion of carbon dioxide into a pincer-supported Ir(III) hydride complex.5b,10 We proposed that the reaction proceeds via initial nucleophilic attack of the hydride on electrophilic carbon dioxide, with no direct involvement from the metal. Furthermore,

a simple computational model was introduced for predicting the thermodynamic favorability of carbon dioxide insertion into six-coordinate Ir hydrides, which was based on the natural bond orbital (NBO) charge on the hydride. As the NBO charge on the hydride decreased (indicating that the negative charge on the hydride increased), the thermodynamic favorability of carbon dioxide insertion increased. Here, the reaction of carbon dioxide with a number of Ni(II) and Pd(II) hydrides supported by PCP, PNP, and PSiP pincer ligands has been tested. We find that insertion only occurs when there is a strong trans-influence ligand trans to the hydride and that our model based on NBO charge can be extended to the Ni and Pd systems. The mechanism of insertion has been modeled using density functional theory (DFT), which provides insight into the requirements for decreasing the barrier for the insertion of carbon dioxide.

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RESULTS AND DISCUSSION Reaction of Pincer-Supported Pd and Ni Hydrides with Carbon Dioxide. The series of PNP-, PCP-, and PSiPsupported Pd and Ni hydrides shown in Figure 1 was prepared. The compounds i P r 2 PNP ( C H 3 ) PdH, 11 i P r 2 PNP ( C H 3 ) NiH, 12 iPr2 PNP(F)PdH,11 CyPhPNPPdH,13 tBu2PCPPdH,14 tBu2PCPNiH,15 Cy2PCPPdH,16 and Cy2PCPNiH15 were prepared using literature methods. The new Cy2PSiP-supported hydride Cy2PSiPNiH was synthesized through the reaction of Cy2PSiPNiCl17 with LiEt3BH in THF, a synthetic route that has previously been used to synthesize PCP-supported Ni and Pd hydrides.15,16 The 1H NMR spectrum of Cy2PSiPNiH clearly established the presence of a Ni hydride, as a triplet integrating to one proton was observed at δ −3.50 ppm. The corresponding reaction of Cy2PSiPPdCl17 with LiEt3BH in THF gave one 8226


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clean product by 1H and 31P NMR spectroscopy, which is most likely Cy2PSiPPdH, although no clear hydride signal (below δ 0 ppm) was observed by 1H NMR spectroscopy. Turculet and co-workers have previously demonstrated that the reaction of Cy2PSiPPtCl with LiEt3BH generates the formally Pt(0) species Cy2PSiHPPt, where the hydride has been delivered to the silicon atom of the ligand, and an η2-Si−H group coordinates to Pt.18 In Turculet’s η2-silane complex Cy2PSiHPPt, the proton bound to silicon was observed at δ 5.48 ppm, with no resonances observed below δ 0 ppm.18 In the case of our reaction between Cy2PSiPPdCl and LiEt3BH, no resonances were observed between δ 4 and 6 ppm in the 1H NMR spectrum. One possible explanation for the lack of clear diagnostic Pd−H or Si−H signals is that both Cy2PSiPPdH and the Pd(0) species Cy2PSiHPPd are present in rapid equilibrium in solution. Low-temperature 1H and 31P NMR spectra of Cy2PSiPPdH only display one set of resonances; therefore, there is little evidence to support this hypothesis. The solid-state structure of the product from the reaction of Cy2PSiPPdCl and LiEt3BH was elucidated by X-ray crystallography and is consistent with Cy2 PSiPPdH (Figure 2). However, it should be noted that,

in Ph2PSiPPdCl (see the Supporting Information), although there is a significant increase in the Pd−Si bond length. In Cy2 PSiPPdH, the Pd−Si bond distances are 2.3300(15) and 2.3309(15) Ǻ , whereas in Ph2PSiPPdCl it is 2.2889(20) Ǻ . This is consistent with the higher trans influence of a hydride ligand in comparison with a chloride ligand, and this large difference is unlikely to be caused by a change in the substituent on the phosphines. Overall, apart from the absence of a clear hydride peak in the 1H NMR spectrum all of our data are consistent with our assignment of the complex as Cy2PSiPPdH. Computational evidence (vide infra) also suggests that the Cy2PSiPPdH structure is preferred over the Pd(0) isomer. Interestingly, the reaction of complexes of type Ph2PSiPMCl (M = Ni (see the Experimental Section for procedure), Pd19) with a variety of different hydride sources resulted in complex mixtures of products which could not be identified. Presumably, the extra steric bulk provided by the cyclohexyl substituents on the phosphines in Ph2PSiPMH (M = Ni, Pd) stabilize the metal hydrides from decomposition. It has been proposed that the Pd(0) and Ni(0) species Ph2PSiHPM(PPh3), supported by the Ph2 PSiHP ligand that features an η2-Si−H interaction with the metal center, are in equilibrium with the Pd(II) and Ni(II) hydrides Ph2 PSiPMH and free PPh3.20 The complexes Ph2PSiHPPd(PPh3) and Ph2PSiHPNi(PPh3) were prepared using literature methods as potential sources of the metal hydrides Ph2PSiPPdH and Ph2PSiPNiH, respectively.20 Our family of pincer hydride complexes was screened for reactivity with carbon dioxide. Under 1 atm of carbon dioxide at room temperature, all of the PCP-supported complexes rapidly insert carbon dioxide to form O-bound formate complexes (eq 1).

This is consistent with our previous results for tBu2PCPNiH9e and Wendt’s studies for tBu2PCPPdH9c (although Wendt utilized 4 atm of carbon dioxide and the O-bound formate product was not isolated). For all PCP-supported systems, the rate of insertion was too fast to be measured, even at low temperature. Similarly, rapid insertion of carbon dioxide into the metal hydride to form an O-bound formate was observed when the Cy2PSiPsupported hydrides Cy2PSiPNiH and Cy2PSiPPdH were placed under 1 atm of carbon dioxide (eq 2). All of the new PCP and Cy2 PSiP formate complexes were fully characterized. In contrast to the rapid reaction between the PCP- and Cy2PSiP-supported

Figure 2. Ortep21 drawing of Cy2PSiPPdH at the 50% probability level. Selected hydrogen atoms and solvent of crystallization are omitted for clarity. There are two independent molecules in the asymmetric unit; only one molecule is shown. Selected bond lengths (Ǻ ) and angles (deg): Pd(1)−P(1) = 2.2628(14), Pd(1)−P(2) = 2.2661(14), Pd(1)− Si(1) = 2.3300(15); P(1)−Pd(1)−P(2) = 160.27(5), P(1)−Pd(1)− Si(1) = 84.24(5), P(2)−Pd(1)−Si(1) = 85.07(5), Pd(2)−P(3) = 2.2651(14), Pd(2)−P(4) = 2.2654(15), Pd(2)−Si(2) = 2.3309(15), P(3)−Pd(2)−P(4) = 159.70(6), P(3)−Pd(2)−Si(2) = 85.01(5), P(4)−Pd(2)−Si(2) = 84.36(5), 159.70(6).

although the hydride was located in the difference Fourier map, due to the high level of residual electron density around the Pd center the assignment of the hydrogen atom cannot be guaranteed. The PSiP ligand binds in a tridentate meridional fashion, typical of a Pd(II) square-planar complex. The P(1)− Pd(1)−P(2) bond angles are 160.27(5) and 159.70(6)°, and the atoms P(1), Pd(1), P(2), and Si(1) are almost coplanar. The bond lengths and angles in Cy2PSiPPdH are similar to those 8227


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hydrides and carbon dioxide, no reaction was observed between any PNP-supported Ni or Pd hydrides and carbon dioxide, even at elevated temperature. When Ph2PSiHPPd(PPh3) was reacted with carbon dioxide at room temperature, instantaneous formation of the O-bound formate Ph2PSiPPd-OC(O)H, which was fully characterized, and PPh3 were observed (eq 3). However, no reaction was

observed between Ph2PSiHPNi(PPh3) and carbon dioxide, even at elevated temperature. Our hypothesis for this difference in reactivity (which is explored further computationally, vide infra) is that the η2-silane species is significantly more favored than the M(II) hydride for Ni in comparison with Pd. As a result, the insertion of carbon dioxide is only thermodynamically favorable for Pd. X-ray Structures of Pincer-Supported Complexes. The compounds Cy2PCPNi-OC(O)H, Cy2PCPPd-OC(O)H (which was crystallized as a mixture containing approximately 12% Cy2 PCPPdCl), tBu2PCPPd-OC(O)H, Cy2PSiPNi-OC(O)H, and Cy2 PSiPPd-OC(O)H were characterized by X-ray crystallography (Figures 3−7). While there are several previous

Figure 4. Ortep21 drawing of Cy2PCPPd-OC(O)H at the 50% probability level. The structure contains 12.6(10)% Cy2PCPPdCl, which is omitted along with selected hydrogen atoms for clarity. There is disorder in the formate ligand, which occupies two different sites. The second site is denoted using the label A. There is also disorder in one of the cyclohexyl groups, which along with the disorder in the formate is not shown. Selected bond lengths (Ǻ ) and angles (deg): Pd(1)−P(1) = 2.2968(13), Pd(1)−P(2) = 2.2940(14), Pd(1)−O(1) = 2.103(14), Pd(1)−O(1A) = 2.100(10), Pd(1)−C(2) = 2.025(5), C(1)−O(1) = 1.28(2), C(1A)−O(1A) = 1.27(2), C(1)−O(2) = 1.24(3), C(1A)−O(2A) = 1.25(2); P(1)−Pd(1)−C(2) = 83.96(14), P(1)−Pd(1)−P(2) = 164.00(5), P(1)−Pd(1)−O(1) = 97.9(8), P(1)−Pd(1)−O(1A) = 90.2(13), C(2)−Pd(1)−O(1) = 167.3(7), C(2)−Pd(1)−O(1A) = 167.8(7).

examples of crystallographically characterized monomeric Ni formates,7a,9e,f,22 to the best of our knowledge, the Pd complexes represent only the second, third, and fourth crystallographically characterized Pd formates23 and are the first to be prepared directly from carbon dioxide. The structures of Cy2 PCPNi-OC(O)H and Cy2PCPPd-OC(O)H are essentially isostructural, and the metal center is in the distorted-squareplanar geometry, typical of many PCP-supported complexes.24 Interestingly, the Ni−C and Ni−O bond lengths in Cy2PCPNiOC(O)H are the same within crystallographic error (Ni(1)− O(1) = 1.931(5) Ǻ , Ni(1)−O(1A) = 1.923(16) Ǻ , and Ni(1)− C(2) = 1.933(3) Ǻ ), whereas in the Pd complex Cy2PCPPdOC(O)H the Pd−C bond length is significantly shorter than the Pd−O bond length (Pd(1)−O(1) = 2.103(14) Å, Pd(1)O(1A) = 2.100(10) Å, and Pd(1)−C(2) = 2.025(5) Ǻ ). Although the size difference between Ni and Pd may cause the PCP ligand to bind more tightly to Pd and shorten the Pd−C bond length relative to the Pd−O bond length, a contributing factor could be that the Ni−O bond is stronger than the Pd− O bond and exerts a greater trans influence. A similar trend is observed when comparing the bond lengths in tBu2PCPPdOC(O)H with those in the previously crystallographically characterized complex tBu2PCPNi-OC(O)H.9e Overall the structure of tBu2PCPPd-OC(O)H is similar to that of Cy2PCPPd-OC(O)H and the small change in the phosphine substituents does not greatly affect the geometry of the complexes. In the case of Cy2PSiPPd-OC(O)H, the Pd−O bond length (Pd(1)−O(1) = 2.197(3) Ǻ ) is significantly longer than that in Cy2 PCPPd-OC(O)H or tBu2PCPPd-OC(O)H, presumably

Figure 3. Ortep21 drawing of Cy2PCPNi-OC(O)H at the 50% probability level. Selected hydrogen atoms are omitted for clarity. There is disorder in the formate ligand, which occupies two different sites. The second site is denoted using the label A. There is also disorder in one of the cyclohexyl groups, which along with the disorder in the formate is not shown. Selected bond lengths (Ǻ ) and angles (deg): Ni(1)−P(1) = 2.1938(10), Ni(1)−P(2) = 2.1963(10), Ni(1)− O(1) = 1.931(5), Ni(1)−O(1A) = 1.923(16), Ni(1)−C(2) = 1.933(3), C(1)−O(1) = 1.223(7), C(1A)−O(1A) = 1.216(16), C(1)−O(2) = 1.249(17), C(1A)−O(2A) = 1.24(2); P(1)−Ni(1)− C(2) = 85.85(10), P(2)−Ni(1)−C(2) = 84.89(11), P(1)−Ni(1)− P(2) = 164.98(4), P(1)−Ni(1)−O(1) = 84.89(11), P(2)−Ni(1)− O(1) = 94.5(8), C(2)−Ni(1)−O(1) = 168.3(6). 8228


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Figure 7. Ortep21 drawing of Cy2PSiPPd-OC(O)H at the 50% probability level. There is disorder in the formate ligand, which occupies two different sites. The second site is denoted using the label A. Only the major component is shown. Selected hydrogen atoms are omitted for clarity. Selected bond lengths (Ǻ ) and angles (deg): Pd(1)−P(1) = 2.3235(19), Pd(1)−P(2) = 2.2577(19), Pd(1)−O(1) = 2.218(3), Pd(1)−Si(1) = 2.2272(18), C(1)−O(1) = 1.254(6), C(1)−O(2) = 1.216(7), Pd(1A)−P(1) = 2.313(3), Pd(1A)−P(2) = 2.374(3), Pd(1A)−O(1A) = 2.192(6), Pd(1A)−Si(1A) = 2.384(3), C(1A)−O(1A) = 1.257(8), C(1A)−O(2A) = 1.248(8); P(1)−Pd(1)− Si(1) = 85.72(6), P(2)−Pd(1)−Si(1) = 84.95(6), P(1)−Pd(1)−P(2) = 168.09(8), P(1)−Pd(1)−O(1) = 98.40(11), P(2)−Pd(1)−O(1) = 89.64(11), Si(1)−Pd(1)−O(1) = 169.71(12), P(1)−Pd(1A)−Si(1) = 82.46(9), P(2)−Pd(1A)−Si(1) = 79.06(9), P(1)−Pd(1A)−P(2) = 152.89(11), P(1)−Pd(1A)−O(1A) = 99.7(2), P(2)−Pd(1A)−O(1A) = 102.2(2), Si(1)−Pd(1A)−O(1A) = 169.8(3).

Figure 5. Ortep21 drawing of tBu2PCPPd-OC(O)H at the 50% probability level. Selected hydrogen atoms are omitted for clarity. The molecule is located on a 2-fold rotation axis. Symmetry-related positions are denoted using the letter A. Selected bond lengths (Ǻ ) and angles (deg): Pd(1)−P(1) = 2.3351(7), Pd(1)−P(1A) = 2.3351(7), Pd(1)−O(1) = 2.196(4), Pd(1)−C(11) = 2.033(4), C(1)−O(1) = 1.263(15), C(1)−O(2) = 1.20(2); P(1)−Pd(1)−C(11) = 83.092(16), P(1)−Pd(1)−P(1A) = 166.18(3), P(1)−Pd(1)−O(1) = 97.04(12), P(1)−Pd(1)−O(1A) = 96.15(12), C(11)−Pd(1)−O(1) = 162.77(14).

reflecting the stronger trans influence of the PSiP ligand in comparison with that of the PCP framework. Similarly, for Cy2PSiPNiOC(O)H, the Ni−O bond length (Ni(1)−O(1) = 1.9680(15) Ǻ) is longer than that in Cy2PCPNi-OC(O)H. In addition, the angle between the phosphine ligands and the metal center reflect that the geometry around the metal center is distorted further away from a typical square-planar geometry for the PSiP ligand bound to Pd in comparison with the PCP ligand (in Cy2PSiPPd-OC(O)H, P(1)−Pd(1)−P(2) = 162.82(3)°, whereas in Cy2PCPPd-OC(O)H, P(1)−Pd(1)−P(2) = 164.00(5)° and in tBu2PCPPd-OC(O)H, P(1)−Pd(1)−P(1A) = 166.18(3)°). A similar effect has been noticed in other Pd and Pt systems supported by the PSiP ligand, although crystallographically characterized examples are still relatively rare.8,17,18,25 It appears that this trend does not hold for Ni, as the P(1)−Ni(1)−P(2) bond angle in Cy2PCPNi-OC(O)H is 164.98(4)°, which is almost identical with the P(1)−Ni(1)− P(2) bond angle in Cy2PSiPNi-OC(O)H of 165.16(2)°. As with the PCP-supported complexes, it appears that the Ni− O bond is stronger in PSiP-supported complexes in comparison with the Pd−O bond. The Ni(1)−O(1) bond length in Cy2 PSiPNi-OC(O)H is approximately 0.25 Ǻ shorter than the Ni(1)−Si(1) bond length, while the Pd(1)−O(1) bond length in Cy2PSiPPd-OC(O)H is only 0.08 Ǻ shorter than the Pd(1)− Si(1) bond length. Computational Studies of the Reaction of Pd and Ni Hydrides with Carbon Dioxide. In order to understand the reactivity of pincer-supported hydrides with carbon dioxide,

Figure 6. Ortep21 drawing of Cy2PSiPNi-OC(O)H at the 50% probability level. Selected hydrogen atoms are omitted for clarity. Selected bond lengths (Ǻ ) and angles (deg): Ni(1)−P(1) = 2.1807(6), Ni(1)− P(2) = 2.1953(6), Ni(1)−O(1) = 1.9680(15), Ni(1)−Si(1) = 2.2201(6), C(1)−O(1) = 1.257(3), C(1)−O(2) = 1.219(3); P(1)− Ni(1)−Si(1) = 83.97(2), P(2)−Ni(1)−Si(1) = 85.84(2), P(1)− Ni(1)−P(2) = 165.16(2), P(1)−Ni(1)−O(1) = 91.45(5), P(2)− Ni(1)−O(1) = 97.32(5), Si(1)−Ni(1)−O(1) = 171.89(5). 8229


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DFT calculations were performed. Instead of modeling the full pincer ligands, a modified framework in which all substituents on the phosphines were replaced by methyl substituents was utilized. The compounds used as models for calculations are shown in Figure 8. The validity of this model was confirmed

clearly thermodynamically favored for both Ni and Pd. Furthermore, comparison (see the Supporting Information) of the optimized structures of Me2PSiPPdH and Me2PSiHPPd with the solid-state structure of Cy2PSiPPdH shows a much closer agreement with the Pd(II) form. This provides further support for our experimental assignment of the structure of Cy2PSiPPdH. Our results show that carbon dioxide insertion into Me2 PNPPdH is significantly thermodynamically uphill, which is in agreement with our experimental result that no insertion was observed into any PNP-supported Pd hydrides. For Me2PNPNiH, carbon dioxide insertion is approximately thermoneutral, which is consistent (within DFT error) with no reaction being observed experimentally. Similarly, for PCP-supported species, insertion into the Ni complex Me2PCPNiH is calculated to be considerably more favorable than insertion into Me2PCPPdH, which was calculated to be thermoneutral (within DFT error). Although we are unable to compare our absolute calculated thermodynamic values with experiment (to verify our calculated values of ΔG°), for all systems a similar trend was observed. Carbon dioxide insertion is more favorable for Ni hydrides in comparison to Pd hydrides with the same ancillary ligand, which is similar to what we had observed previously for Ni and Pd η1-allyl complexes.27 It is proposed that the greater thermodynamic favorability for Ni arises because Ni−O bonds are stronger than Pd−O bonds, due to the metal and ligand orbitals being closer in energy. The X-ray structures of Cy2PCPNiOC(O)H and Cy2PCPPd-OC(O)H provide support for this hypothesis, as the difference between the Ni−C and Ni−O bonds in Cy2PCPNi-OC(O)H is smaller than the difference between the Pd−C and Pd−O bonds in Cy2PCPPd-OC(O)H (vide supra). Interestingly, our observation that carbon dioxide insertion into Ni hydrides is more thermodynamically favorable than insertion into Pd hydrides is opposite to the generally accepted trend that Ni hydrides are less hydridic than Pd hydrides and should be worse H− donors.28 The calculations also show that as the strength of the trans influence of the donor trans to the hydride increases, carbon dioxide insertion becomes more favorable. We had observed a similar trend when computationally exploring the insertion of carbon dioxide into Ir hydrides,5b and Darensbourg had proposed a similar trend for Ni hydrides with phosphine ligands on the basis of experimental data.9b In a fashion analogous to our work with Ir hydrides,5b the favorability of carbon dioxide insertion can be predicted simply by looking at the NBO charge on the hydride. The more negative the value of charge on the hydride, the more favorable carbon dioxide insertion becomes, presumably because a more negative charge on the hydride indicates a weaker ground state M−H bond strength (which can be inferred from the M−H bond length). The success of this model for predicting the favorability of carbon dioxide insertion indicates that in these systems there is no need to consider the bond strength of the M−O bond formed in the product and that the relative destabilization of the metal hydride in the starting material is the most important factor to model.

Figure 8. Structures of different complexes studied using DFT.

by performing selected calculations with tert-butyl groups on the phosphines, and these are reported in the Supporting Information. In general, excellent agreement was observed between calculated and experimental bond lengths and angles, where direct comparison was possible, suggesting that our chosen level of theory is appropriate for modeling the geometries of the complexes. The thermodynamics for the insertion of carbon dioxide into different pincer-supported hydrides to form a κ1-O formate product were calculated, along with the kinetic barriers for the reaction. The values are shown in Table 1, along with the calculated M−H bond Table 1. Calculated ΔG° and ΔG⧧ Values for Carbon Dioxide Insertion into Pincer-Supported Hydride Complexes and Calculated Metal−Hydride Bond Lengths and NBO Charge on Hydride

compd

ΔG° CO2 insertion (kJ mol‑1)a

ΔG⧧ CO2 insertion (kJ mol‑1)a

M−H bond length (Å)

NBO charge on hydride

Me2

PNPNiH Me2 PCPNiHb Me2 PSiPNiHc

−2.5 −20.6 −32.0

113.6 62.3 39.0

1.47 1.52 1.54

−0.094 −0.159 −0.176

Me2

37.0 2.3 −24.5

147.9 72.6 33.8

1.57 1.63 1.67

−0.045 −0.142 −0.176

PNPPdH PCPPdH Me2 PSiPPdHc Me2

a

Solvent effects were calculated using the IEPCM model for benzene. Values previously reported in ref 9e. cEnergy relates to insertion into the Ni(II) or Pd(II) hydride.

b

lengths in the starting hydrides and the calculated NBO charges on the hydrides. Unfortunately, due to the lack of available experimental data on both the thermodynamics of CO2 insertion and the barriers for insertion, we are unable to assess the accuracy of our model in relation to energies; however, we note that we have utilized the same computational methodology in related Ir and Ni systems, where good agreement was observed between experimental and theoretical energies.5b,26 For the PSiP-supported Ni and Pd hydrides Me2PSiPNiH and Me2PSiPPdH, the energy difference between the M(II) isomer with the hydride on the metal and the M(0) isomer with the hydride on Si was calculated (eq 4). The M(II) form is 8230


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The effect of the trans ligand is more pronounced for Pd than for Ni, and the difference in the thermodynamic favorability for carbon dioxide insertion into the strongest (PSiP) and weakest (PNP) trans-influence donor is significantly larger for Pd (the difference is 61.4 kJ mol−1) than for Ni (the difference is 29.5 kJ mol−1). The span of NBO charges on the hydride is also greater for Pd (the range is 0.131) than for Ni (the range is 0.082). Although the NBO charges on Ni and Pd hydrides can be correlated directly to each other to predict the thermodynamic favorability for carbon dioxide insertion, the results are on a scale different from those reported previously for Ir hydrides5b (see the Supporting Information). Thus, the trend line for predicting the favorability of insertion into an Ir hydride cannot be used to predict the energetics of insertion into a Ni or Pd hydride. The calculations predict that carbon dioxide insertion into the PSiP-supported hydride Me2PSiPNiH is highly favorable (ΔG° = −32.0 kJ mol−1), as is insertion into Me2PSiPPdH (ΔG° = −24.5 kJ mol−1). Experimentally, carbon dioxide insertion into both Cy2PSiPNiH and Cy2PSiPPdH was observed, which is consistent with our calculations. However, experimentally insertion into Ph2PSiHPNi(PPh3) did not occur, even though carbon dioxide insertion into Ph2PSiHPPd(PPh3) was facile. Further calculations on the model systems Me2PSiHPNi(PMe3) and Me2PSiHPPd(PMe3) reveal that the formation of the Ni(II) hydride required for carbon dioxide insertion is significantly less favorable than the formation of the corresponding Pd(II) hydride (eq 5). In our model system PMe3 was

utilized as the ancillary ligand, whereas PPh3 was used in the experimental system. This perhaps explains why the calculations predict that the Pd(II) form is favored but experimentally the Pd(0) form is primarily observed by NMR spectroscopy.20 Given that PMe3 is a significantly better donor than PPh3, the formation of Ni(II) and Pd(II) complexes with PPh3 should be even less electronically favored than the calculations predict. Consistent with this hypothesis, calculations performed with PH3 and PCl3 as the ancillary ligand (see the Supporting Information) show that the Ni(0) or Pd(0) forms become more favorable. Unfortunately, we were unable to gain information about the role of sterics in the equilibrium, as DFT calculations on the full experimental systems failed. Overall, these calculations suggest that carbon dioxide insertion into Ph2 PSiHPNi(PPh3) is thermodynamically unfavorable and as a result no reaction is observed experimentally. For all systems that were studied, the lowest energy pathway for carbon dioxide insertion involves a single transition state. The transition states for carbon dioxide insertion into the Ni complexes Me2PNPNiH, Me2PCPNiH,9e and Me2PSiPNiH are shown in Figure 9, and the corresponding transition states for the Pd species are shown in Figure 10, with selected bond distances and angles shown in Tables 2 and 3, respectively. In the transition states, both the C−H and M−O bonds are formed simultaneously in a fashion analogous to the transition state that both our group and that of Guan and co-workers have previously described for Ni complexes.7b,9e The transition states are best described as four-centered and are typical for a 1,2insertion reaction. Although the distances between the metal center and the carbon of carbon dioxide are relatively short (approximately 2.40 Ǻ for Ni and 2.60 Ǻ for Pd), it does not appear that there is any significant orbital interaction between these two atoms, and the short distance may simply be a consequence of the geometry required for the simultaneous formation of the new C−H and M−O bonds. As the trans influence of the donor trans to the hydride increases, the angle at which carbon

Figure 9. Calculated transition states for carbon dioxide insertion into (a)

Me2

Figure 10. Calculated transition states for carbon dioxide insertion into (a) 8231

PNPNiH, (b)

Me2

Me2

PNPPdH, (b)

PCPNiH9e and (c)

Me2

PCPdH, and (c)

Me2

PSiPNiH.

Me2

PSiPPdH.


Organometallics

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respectively, are better ligands for Ni, which is harder than Pd, whereas the softer Si− donor in PSiP is better suited for Pd. Overall, the calculated barriers for insertion into the PCP- and PSiP-supported Ni and Pd complexes are low, which is consistent with a rapid reaction being observed experimentally. Applications to Catalysis. We believe that the results of this study may be relevant to at least two different catalytic reactions involving carbon dioxide. (i) A crucial step in many systems for the catalytic hydrogenation of carbon dioxide to formic acid is the insertion of carbon dioxide into a metal− hydride bond.5,6,30 Our results suggest that both the thermodynamic favorability and the rate of this insertion can be increased by adding a stronger trans donor opposite the hydride. At this stage Ni and Pd catalysts for carbon dioxide hydrogenation have not been developed, but clearly the step involving carbon dioxide insertion into the metal−hydride bond is facile and the remaining challenges are to liberate the bound formate from the metal and regenerate the hydride. We believe that liberation of the bound formate from Pd-based systems will be easier than for Ni-based systems, as our structural data suggest that the Ni−O bond strength is stronger than the Pd−O bond. (ii) As part of their proposed mechanism for the catalytic carboxylation of allenes,8 Iwasawa and co-workers have suggested that an allene inserts into the Si−H bond of Ph2PSiHPPd(PPh3) to form Ph2 PSiallylPPd(PPh3) (Scheme 1).20a This reaction is proposed to initially involve the insertion of the allene into Ph2PSiPPdH (which along with PPh3 is in equilibrium with Ph2PSiHPPd(PPh3), as described above) to form Ph2PSiPPd(η1-allyl), followed by reductive elimination and coordination of PPh3 to give Ph2PSiallylPPd(PPh3).20a Our studies on carbon dioxide insertion into Ph2PSiHPPd(PPh3) indicate that carbon dioxide insertion will compete with allene insertion, as both reactions proceed at similar rates. Thus, carbon dioxide insertion into the Pd hydride may slow down the rate of allene carboxylation during catalysis. This suggests that it may be possible to achieve more efficient catalysis by using a metal hydride catalyst that does not undergo carbon dioxide insertion. To probe this possibility, all of the PCP and PNP Ni and Pd hydrides prepared as part of this work were reacted with the allene 3methyl-1,2-butadiene. Surprisingly, even at elevated temperatures (80 °C), no insertion of the allene into the metal hydride was observed. DFT calculations indicate that allene insertion into the metal hydride is thermodynamically favorable in all cases (see the Supporting Information), which implies that direct allene insertion is kinetically unfavorable. Therefore, we believe that Ph2PSiHPPd(PPh3) may be able to directly insert allene to form a new Si−C bond, and the unique ability of the Ph2 PSiP ligand to stabilize both Pd(0) and Pd(II) is crucial for catalysis. Further work will look to explore this possibility using Cy2 PSiPNiH and Cy2PSiPPdH as catalysts for allene carboxylation in the presence of phosphines.

Table 2. Calculated Bond Lengths and Angles in the Transition State for CO2 Insertion into Me2PNPNiH, Me2 PCPNiH, and Me2PSiPNiHa bond length or angle

Me2

PNPNiH

Ni(1)−H(1) C(1)−H(1) C(1)−O(2) C(1)−O(1) Ni(1)−O(1) Ni(1)−C(1) Ni(1)−E(1)b Ni(1)−C(1)−O(1) Ni(1)−H(1)−C(1)−O(1)

a b

PCPNiH9e

Me2

1.84 1.20 1.23 1.26 2.34 2.39 1.92 71.9 34.2

Me2

PSiPNiH

1.87 1.20 1.23 1.26 2.37 2.41 1.92 72.7 36.9

1.84 1.20 1.23 1.26 2.45 2.37 2.24 78.4 49.0

All bond lengths are in Ǻ , and all bond and torsion angles are in deg. E = N(1), C(1), Si(1), depending on the identity of the ligand.

Table 3. Calculated Bond Lengths and Angles in the Transition State for CO2 Insertion into Me2PNPPdH, Me2 PCPPdH, and Me2PSiPPdHa bond length or angle Pd(1)−H(1) C(1)−H(1) C(1)−O(2) C(1)−O(1) Pd(1)−O(1) Pd(1)−C(1) Pd(1)−E(1)b Pd(1)−C(1)−O(1) Pd(1)−H(1)−C(1)−O(1)

a b

Me2

PNPPdH 2.05 1.19 1.23 1.26 2.57 2.54 2.06 77.1 51.4

Me2

PCPPdH 2.13 1.18 1.24 1.26 2.64 2.59 2.03 78.6 55.3

Me2

PSiPPdH 2.13 1.18 1.24 1.26 2.79 2.61 2.31 84.5 63.5

All bond lengths are in Ǻ , and all bond and torsion angles are in deg. E = N(1), C(1), Si(1), depending on the identity of the ligand.

dioxide approaches the hydride changes and the metal−oxygen bond length in the transition state increases. This trend is more pronounced for Pd than for Ni, and the Pd(1)−O(1) bond length in the transition state for carbon dioxide insertion into Me2 PNPPdH is 0.22 Ǻ greater than that of Me2PNPNiH. In fact, the transition state for insertion into Me2PSiPPdH starts to move away from that of a typical concerted 1,2-insertion reaction toward the type of transition states that have been proposed for electrophilic attack of carbon dioxide by a nucleophilic metal hydride, a process common for group 6 metal hydrides.29 In general, carbon dioxide insertion into late-transition-metal complexes is a multistep process involving nucleophilic attack of a ligand on electrophilic carbon dioxide,26 and the concerted mechanism described for the pincer-supported group 10 hydrides in this work are unusual. The energies of the transition states for carbon dioxide insertion into the different pincer hydrides are shown in Table 1. Similar to the thermodynamic trends for carbon dioxide insertion, the energy of the transition state decreases as the trans influence of the ligand trans to the hydride increases. The difference between the transition state energies for the strongest and weakest trans-influence donors is 74.6 kJ mol−1 for Ni and 114.1 kJ mol−1 for Pd. The higher trans-influence ligands are better donors to the metal center and are presumably better at stabilizing the transition state, in which some electron density is lost from the metal. For the PNP and PCP ligands, the barriers for insertion into Ni hydrides are lower than the barrier for Pd hydrides, but the converse is true for the PSiP ligand. We suggest that the harder N− and C− donors in PNP and PCP,

■

CONCLUSIONS The reactivity of carbon dioxide with a variety of different pincersupported Ni(II) and Pd(II) hydrides has been investigated. Our experimental and computational studies indicate that the identity of the ligand trans to the hydride is crucial in determining the thermodynamic favorability of carbon dioxide insertion into the metal−hydride bond. This effect is more pronounced for Pd than for Ni. Pincer ligands with strong trans-influence donors trans to the hydride weaken the metal−hydride bond and make carbon dioxide insertion more favorable. As a result, we see carbon 8232


Organometallics

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Scheme 1

Laboratories, Inc. performed the elemental analyses (inert atmosphere). Literature procedures were followed to prepare iPr2PNP(CH3)PdH,11 iPr2 PNP(CH3)NiH,12 iPr2PNP(F)PdH,11 CyPhPNPPdH,13 tBu2PCPPdH,14 tBu2 PCPNiH,15 Cy2PCPPdH,16 Cy2PCPNiH,15 Cy2PSiPPdCl,17 Cy2PSiPNiCl,17 Ph2PSiHP,19 Ph2PSiHPPd(PPh3),20a Ph2PSiHPNi(PPh3),20b Ph2 PSiPPdCl (which was crystallized as part of this work),19 and [Ni(η3allyl)Cl]2.31 X-ray Crystallography. Crystal samples were mounted in MiTeGen polyimide loops with immersion oil. Low-temperature diffraction data (ω scans) were collected on a Rigaku SCXMini diffractometer coupled to a Mercury275R CCD detector with Mo Kα radiation (λ = 0.710 73 Å) for Cy2 PCPNi-OC(O)H, Cy2 PCPPd-OC(O)H, tBu2 PCPPd-OC(O)H, and Ph2PSiPNiCl, on a Rigaku R-AXIS RAPID diffractometer coupled to a R-AXIS RAPID imaging plate detector with Mo Kα radiation (λ = 0.710 73 Å) for Cy2PSiPPdH, Cy2PSiPPdOC(O)H, and Ph2PSiPPdCl, and on a Rigaku MicroMax-007HF diffractometer coupled to a Saturn994+ CCD detector with Cu Kα radiation (λ = 1.541 78 Å) for Cy2PSiPNi-OC(O)H. All structures were solved by direct methods using SHELXS32 and refined against F2 on all data by full-matrix least squares with SHELXL-9733 using established refinement techniques.34 All non-hydrogen atoms were refined anisotropically. All non-hydrogen atoms were refined anisotropically. Unless otherwise noted, hydrogen atoms were included into the model at geometrically calculated positions and refined using a riding model. The isotropic displacement parameters of all hydrogen atoms were fixed to 1.2 times the U value of the atoms they are linked to (1.5 times for methyl groups). All disorders were refined with the help of similarity restraints on the 1,2- and 1,3-distances and anisotropic displacement parameters as well as rigid bond restraints for anisotropic displacement parameters. Details of the crystal and refinement data for Ph2 PSiPNiCl, Ph2PSiPPdCl, Cy2PSiPPdH, Cy2 PCPNi-OC(O)H, Cy2 PCPPd-OC(O)H, tBu2PCPPd-OC(O)H, Cy2PSiPNi-OC(O)H, and Cy2PSiPPd-OC(O)H are given in the Supporting Information. Computational Methods. All calculations were performed with the Gaussian09 Revision A.02 package35 with the hybrid B3LYP functional.36 The basis set was LANL2DZ for Ni and Pd and 6-31G+ +(d,p) for all other atoms. The LANL2DZ pseudopotential was used for Ni and Pd. Full optimization of geometry was performed without any symmetry constraint, followed by analytical computation of the Hessian matrix to identify the nature of the located extrema as minima or transition states. In representative cases internal reaction coordinate (IRC) calculations were performed on transition states to demonstrate that a motion connecting the reactant and product was present. The zero-point, thermal, and entropy corrections were evaluated to compute enthalpies and Gibbs free energies. Entropy effects were calculated in the gas phase at 298 K and 1 atm. Solvent was modeled using the

dioxide insertion into PCP- and PSiP-supported species but do not observe any reaction with PNP-supported complexes. In general, carbon dioxide insertion is more thermodynamically favorable for Ni in comparison with Pd. As with octahedral Ir(III) systems, the thermodynamic favorability of carbon dioxide insertion can be predicted by looking at the NBO charge on the hydride.5b The pathway for carbon dioxide insertion into the Ni(II) and Pd(II) hydrides is unusual, because the reaction proceeds through a single four-membered transition state. In contrast, many other insertions of carbon dioxide into late-transition-metal element bonds proceed through a two-step outer-sphere mechanism, in which a nucleophilic ligand initially attacks electrophilic carbon dioxide.26 The ability of the nondirectional 1s orbital on the hydride to support a four-centered transition state is presumably crucial for stabilizing the concerted transition state. However, even in the Ni(II) and Pd(II) hydride systems as the nucleophilicity of the hydride is increased, there is less metal− oxygen bonding character in the transition state. Overall, we believe that our results will assist in the design of complexes for the catalytic conversion of carbon dioxide.

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EXPERIMENTAL SECTION

General Methods. Experiments were performed under a dinitrogen atmosphere in an M. Braun drybox or using standard Schlenk techniques. (Under standard glovebox conditions purging was not performed between uses of hexane, diethyl ether, benzene and toluene; thus, when any of these solvents were used, traces of all these solvents were in the atmosphere and could be found intermixed in the solvent bottles.) Moisture- and airsensitive liquids were transferred by stainless steel cannula on a Schlenk line or in a drybox. The solvents for air- and moisture-sensitive reactions were dried by passage through a column of activated alumina followed by storage under dinitrogen. All commercial chemicals were used as received except where noted. 1,3-Bis((di-tert-butylphosphino)methyl)benzene was purchased from Strem Chemicals or Santa Cruz Biotech. Anhydrous carbon dioxide was obtained from Airgas Inc. and was not dried prior to use. C6D6 was obtained from Cambridge Isotope Laboratories and dried over sodium metal prior to use. NMR spectra were recorded on Bruker AMX-400 and -500 spectrometers at ambient probe temperatures unless noted. Chemical shifts are reported with respect to residual internal protio solvent for 1H and 13C{1H} NMR spectra and to an external standard for 31P{1H} spectra (85% H3PO4 in H2O at δ 0.0 ppm). IR spectra were measured using a diamond Smart Orbit ATR on a Nicolet 6700 FT-IR instrument. Robertson Microlit 8233


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After 10 min, 31P NMR spectroscopy showed complete conversion to a new product. The solution was dried in vacuo to give Cy2PCPNiOC(O)H as a pale yellow solid in 93% yield (10 mg, 0.017 mmol). The resulting solid was dissolved in pentane and recrystallized at −30 °C to give pale yellow crystals suitable for X-ray crystallography. IR (ATR, Smart Orbit diamond plate, cm−1): 1616.9 (ν(CO2)), 1313.1 (ν(CO2)). 1H NMR (C6D6, 500.0 MHz): δ 8.64 (1H, t, OCOH, J = 2.5 Hz), 7.05 (1H, t, ArH, J = 7.2 Hz), 6.90 (2H, d, ArH, J = 7.5 Hz), 2.84 (4H, t, ArCH2P, J = 3.8 Hz), 2.54 (4H, d, CyH, J = 7.5 Hz), 1.94 (4H, tt, CyH, J = 12.3 Hz, 3 Hz), 1.76 (4H, d, CyH, J = 12.5 Hz), 1.60 (20H, m, CyH), 1.19 (16H, m, CyH). 13C{1H} NMR (C6D6, 125.8 MHz): δ 167.4, 154.2 (t, J = 17.1 Hz), 153.4 (t, J = 13.0 Hz), 125.7, 122.7 (t, J = 8.7 Hz), 33.9 (t, J = 9.8 Hz), 32.5 (t, J = 13.5 Hz), 29.2, 29.0, 27.8 (t, J = 6.4 Hz), 27.6 (t, J = 4.8 Hz), 26.9. 31 1 P{ H} NMR (C6D6, 135.0 MHz) δ 49.8. Anal. Found (calcd for C33H52NiO2P2): C, 66.2 (65.9); H, 8.8 (8.7). Cy2 PCPPd-OC(O)H. To a solution of Cy2PCPPdH (12 mg, 0.020 mmol) in 500 μL of C6D6 in a J. Young NMR tube was added an excess of 1 atm carbon dioxide via a dual-manifold Schlenk line. After 10 min, 31P NMR spectroscopy showed complete conversion to a new product. The solution was dried in vacuo, and the remaining residue was dissolved in pentane and recrystallized at −30 °C to give Cy2PCPPd-OC(O)H as a colorless solid in 85% yield (11 mg, 0.017 mmol). Colorless crystals suitable for X-ray crystallography were grown from a saturated pentane solution at −30 °C. IR (ATR, Smart Orbit diamond plate, cm−1): 1614.1 (ν(CO2)), 1311.9 (ν(CO2)). 1H NMR (C6D6, 500.0 MHz): δ 9.13 (1H, s, OCOH), 7.09 (1H, t, ArH, J = 7.9 Hz), 7.03 (2H, d, ArH, J = 7.4 Hz), 2.92 (4H, t, ArCH2P, J = 4.1 Hz), 2.35 (4H, d, CyH, J = 12.8 Hz), 2.05 (4H, m, CyH), 1.7 (4H, d, CyH, J =13.0 Hz), 1.57 (16H, m, CyH), 1.38 (4H, m, CyH), 1.23 (4H, m, CyH), 1.10 (8H, m, CyH). 13C{1H} NMR (C6D6, 125.8 MHz): δ 166.9, 156.2, 151.5 (t, J = 11.1 Hz), 125.2, 122.8 (t, J = 10.4 Hz), 34.1 (t, J = 10.7 Hz), 33.3 (t, J = 11.8 Hz), 29.1 (t, J = 2.0 Hz), 28.6, 27.2 (t, J = 6.7 Hz), 27.1 (t, J = 5.2 Hz), 26.5. 31P{H} NMR (C6D6, 135.0 MHz) δ 52.8. Anal. Found (calcd for C33H52O2P2Pd): C, 60.8 (61.0); H, 7.8 (8.0). tBu2 PCPPd-OC(O)H. An excess of 1 atm carbon dioxide was added to a solution of tBu2PCPPdH (10 mg, 0.020 mmol) in 500 μL of C6D6 in a J. Young NMR Tube via a dual-manifold Schlenk line. 31P NMR spectroscopy after 10 min showed complete conversion to a new product. The solution was dried in vacuo to give tBu2PCPPd-OC(O)H as a white solid in 86% yield (9.4 mg, 0.017 mmol). The resulting solid was redissolved in pentane and recrystallized at −30 °C to give tBu2 PCPPd-OC(O)H as a colorless crystalline solid suitable for X-ray crystallography. IR (ATR, Smart Orbit diamond plate, cm−1): 1617.6 (ν(CO2)), 1314.8 (ν(CO2)). 1H NMR (C6D6, 500.0 MHz): δ 9.12 (1H, s, OCOH), 7.02 (1H, t, ArH, J = 7.1 Hz), 6.91 (2H, d, ArH, J = 7.4 Hz), 2.93 (4H, t, PCH2, J = 3.8 Hz), 1.28 (36H, t, PC(CH3)3, J = 6.7 Hz). 13 C{1H} NMR (C6D6, 125.8 MHz): δ 167.9, 155.4, 152.1 (t, J = 10.3 Hz), 125.4, 122.8 (t, J = 10.2 Hz), 35.2 (t, J = 7.2 Hz), 34.1 (t, J = 10.4 Hz), 29.6 (t, J = 3.2 Hz). 31P{H} NMR (C6D6, 135.0 MHz) δ 72.5. Anal. Found (calcd for C25H44O2P2Pd): C, 54.8 (55.1); H, 8.0 (8.1). Cy2 PSiPNi-OC(O)H. To a solution of Cy2PSiPNiH (30 mg, 0.047 mmol) in 500 μL of C6D6 in a J. Young NMR tube at room temperature was added an excess of 1 atm carbon dioxide via a dualmanifold Schlenk line. 31P NMR spectroscopy after 10 min showed complete conversion to a new product. The volatiles were removed under vacuum to give a yellow powder in 96% yield (31 mg, 0.045 mmol). The resulting solids were dissolved in diethyl ether and recrystallized at −30 °C to give CyPSiPNi-OC(O)H as an orange crystalline solid in 67% yield (27 mg, 0.030 mmol). IR (ATR, Smart Orbit diamond plate, cm−1): 1595 (ν(CO2)), 1334 (ν(CO2)). 1H NMR (C6D6, 300 MHz): δ 8.94 (t, 1H, OCOH, J = 1.5 Hz), 7.91 (d, 2H, ArH, J = 7 Hz), 7.36 (d, 2H, ArH, J = 7 Hz), 7.27 (t, 2H, ArH, J = 7 Hz), 7.19 (t, 2H, ArH, J = 7 Hz), 2.65 (d, 2H, CyH, J = 12 Hz), 2.46 (t, 2H, CyH, J = 12 Hz), 2.32 (t, 2H, CyH, J = 12 Hz), 2.16 (d, 2H, CyH, J = 12 Hz), 1.86−0.97 (40H, CyH), 0.73 (s, 3H, SiMe). 13C{1H} NMR (C6D6, 100.6 MHz): δ 168.7, 157.3 (t, J = 26 Hz), 140.9 (t, J = 23 Hz), 132.3 (t, J = 10 Hz), 130.7, 130.2, 128.8,

IEPCM model (benzene) as implemented in Gaussian 09, and NBO analysis was performed using the NBO keyword as implemented in Gaussian 09. The coordinates and energies for optimized structures are given in the Supporting Information. Synthesis and Characterization of Compounds. Ph2PSiPNiCl. A room-temperature solution of (Ph2PSiP)H (50 mg, 0.088 mmol) in 3 mL of benzene was added to a solution of 0.7 equiv of [Ni(η3allyl)Cl]2 (16.7 mg, 0.062 mmol) in 2 mL of benzene. The resulting orange solution was allowed to stand at room temperature overnight with stirring. An orange powder precipitated from the solution. The supernatant was decanted, and the precipitate was washed with 1 mL of benzene and 2 × 1 mL of pentane and dried in vacuo to give Ph2 PSiPNiCl as a pale orange solid in 94% yield (55 mg, 0.083 mmol). The product has poor solubility in benzene, toluene, CH2Cl2, CHCl3, and CH3CN. X-ray-quality crystals were grown by layering (Ph2PSiP)H (50 mg, 0.088 mmol) in 2 mL of benzene with [Ni(η3-allyl)Cl]2 (17.8 mg, 0.066 mmol) in 2 mL of benzene in a vial. The vial was left to sit overnight at room temperature. The formation of orange crystals, which were suitable for X-ray analysis, was observed. 1 H NMR (CDCl3, 500.0 MHz): δ 8.05 (2H, d, ArH, J = 10 Hz), 7.78 (4H, m, ArH), 7.52 (2H, t, ArH, J = 5 Hz), 7.47−7.20 (18 H, m, ArH), 0.32 (3H, s, SiMe). 31P{1H} NMR (CDCl3, 135.0 MHz) δ 43.6. This compound was not characterized further, due to its poor solubility. Cy2 PSiPNiH. One equivalent of LiEt3BH (1 M in THF, 15 μL, 0.015 mmol) was added to Cy2PSiPNiCl (10 mg, 0.015 mmol) in 0.5 mL of THF at room temperature. The reaction mixture was allowed to stand at room temperature for 1 h. The volatiles were removed under vacuum. The resulting brown oil was redissolved in pentane, filtered through Celite, and concentrated. The product was recrystallized at −30 °C to give Cy2PSiPNiH as a yellow powder in 50% yield (41 mg, 0.063 mmol). The compound was thermally unstable; therefore, no elemental analysis was obtained. 1 H NMR (C6D6, 400.0 MHz): δ 8.18 (d, 2H, ArH, J = 7 Hz), 7.47 (br d, 2H, ArH, J = 7 Hz), 7.35 (t, 2H, ArH, J = 7 Hz), 7.24 (t, 2H, ArH, J = 7 Hz), 2.46−2.12 (m, 8H, CyH), 2.22−1.84 (m, 8H, CyH), 1.68−0.94 (m, 28H, CyH), 0.89 (s, 3H, SiMe), −3.50 (t, 1H, Ni−H, J = 46 Hz). 13C{1H} NMR (C6D6, 100.6 MHz): δ 159.8 (t, J = 26 Hz), 145.1 (t, J = 21 Hz), 133.4 (t, J = 10 Hz), 130.3, 129.8, 128. 1, 37.7 (t, J = 11 Hz), 35.3 (t, J = 13 Hz), 32.1 (t, J = 3 Hz), 29.8, 28.9, 28.4, 27.6−27.4 (overlapping resonances), 27.1, 27.0, 26.4, 7.2. 31P{1H} NMR (C6D6, 161.9 MHz): δ 78.5. Cy2 PSiPPdH. To a solution of Cy2PSiPPdCl (80 mg, 0.11 mmol) in 4 mL of THF at room temperature was added LiEt3BH (1 M in THF, 110 μL, 0.11 mmol). The reaction mixture was allowed to stand at room temperature for 1 h, during which time the solution changed color from clear to red-orange. The volatiles were removed under vacuum, and the resulting oil was dissolved in 4 mL of pentane, yielding a red solution and a white precipitate. The solution was filtered through Celite and the filtrate collected. The volatiles were removed under reduced pressure to give crude Cy2PSiPPdH. The crude product was recrystallized twice in concentrated pentane at −30 °C to give Cy2PSiPPdH as a white solid in 69% yield (84 mg, 0.12 mmol). The resulting solid was recrystallized at −30 °C in pentane to give Cy2 PSiPPdH as a colorless crystalline solid suitable for X-ray crystallography. 1 H NMR (C6D6, 400.0 MHz): δ 8.22 (d, 2H, ArH, J = 7 Hz), 7.48 (br d, 2H, ArH, 8 Hz), 7.35 (t, 2H, ArH, J = 7 Hz), 7.22 (t, 2H, ArH, J = 7 Hz), 2.70 (m, 2H, CyH), 2.37 (t, 2H, CyH, J = 12 Hz), 2.20 (m, 4H, CyH), 1.97 (br d, 2H, CyH, J = 12 Hz), 1.87 (d, 4H, CyH, J = 12 Hz), 1.66 (d, 2H, CyH, J = 12 Hz), 1.58−0.98 (28H, CyH), 0.95 (s, 3H, SiMe). 13C{1H} NMR (C6D6, 100.6 MHz): δ 159.3 (t, J = 25 Hz), 143.9 (t, J = 20 Hz), 134.1 (t, J = 11 Hz), 131.0, 130.0, 128.4, 37.5 (t, J = 11 Hz), 37.1 (t, J = 13 Hz), 32.1 (t, J = 4 Hz), 29.3, 28.8 (t, J = 3 Hz), 28.2 (t, J = 3 Hz), 27.5−27.3 (overlapping resonances), 27.0 (t, J = 6 Hz), 26.9, 26.3, 8.55. 31P{1H} NMR (C6D6, 161.9 MHz): δ 81.0. Anal. Found (calcd for C37H56P2PdSi): C, 63.6 (63.7); H, 8.1 (8.1). Cy2 PCPNi-OC(O)H. To a solution of Cy2PCPNiH (10.2 mg, 0.018 mmol) in 500 μL of C6D6 in a J. Young NMR tube was added an excess of 1 atm carbon dioxide via a dual-manifold Schlenk line. 8234


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Notes

35.9 (t, J = 10 Hz), 30.5 (t, J = 2 Hz), 29.5, 28.9, 28.8, 27.8−27.44 (overlapping resonances), 26.9, 26.4, 7.36. 31P{1H} NMR (C6D6, 121.5 MHz): δ 50.7. Anal. Found (calcd for C38H56NiO2P2Si): C, 65.0 (65.8); H, 7.9 (8.1). Cy2 PSiPPd-OC(O)H. To a solution of Cy2PSiPPdH (35 mg, 0.050 mmol) in 500 μL of C6D6 in a J. Young NMR tube at room temperature was added an excess of 1 atm carbon dioxide via a dualmanifold Schlenk line. 31P NMR spectroscopy after 10 min showed near-complete conversion to a new product. The volatiles were removed under reduced pressure, and the remaining solids were washed with pentane (2 × 1 mL) to give Cy2PSiPPd-OC(O)H as a white solid in 59% yield (22 mg, 0.030 mmol). IR (ATR, Smart Orbit diamond plate, cm−1): 1603 (ν(CO2)), 1317 (ν(CO2)). 1H NMR (C6D6, 400.0 MHz): δ 9.49 (t, 1H, OCOH, J = 1.5 Hz), 8.00 (d, 2H, ArH, J = 7.5 Hz), 7.40 (br d, 2H, ArH, J = 7.5 Hz), 7.30 (t, 2H, ArH, J = 7 Hz), 7.19 (t, 2H, ArH, J = 7 Hz), 2.67 (t, 2H, CyH, J = 12 Hz), 2.43 (m, 4H, CyH), 2.37 (d, 2H, CyH, J = 13 Hz) 1.81−0.99 (m, 36H, CyH), 0.80 (s, 3H, SiMe). 13C{1H} NMR (C6D6, 100.6 MHz): δ 167.8, 156.7 (t, J = 27 Hz), 140.2 (t, J = 20 Hz), 133.0 (t, J = 12 Hz), 131.4, 130.5, 129.0, 36.3 (t, J = 11 Hz), 36.24 (t, J = 11 Hz), 30.1 (t, J = 3 Hz), 29.3(overlapping resonances), 29.1, 27.6 (t, J = 6 Hz), 27.3 (overlapping resonances), 26.7, 26.2, 8.96. 31 1 P{ H} NMR (C6D6, 161.9 MHz): δ 57.82. Anal. Found (calcd for C38H56O2P2PdSi): C, 60.7 (61.6); H, 7.5 (7.6). Ph2 PSiPPd-OC(O)H. To a solution of Ph2PSiHPPd(PPh3) (30 mg, 0.032 mmol) in 2 mL of benzene at room temperature was added an excess of 1 atm carbon dioxide via a dual-manifold Schlenk line. After 15 min of stirring, the solution was dried in vacuo and washed with diethyl ether (2 × 2 mL) and pentane (2 × 1 mL) to give Ph2PSiPPdOC(O)H as a pale yellow solid in 62% yield (14 mg, 0.02 mmol). The resulting solid was dissolved in diethyl ether and recrystallized to give a colorless crystalline solid suitable for X-ray crystallography. IR (ATR, Smart Orbit diamond plate, cm−1): 1618.7 (ν(CO2)), 1329.5 (ν(CO2)). 1H NMR (C6D6, 500.0 MHz): δ 9.02 (1H, t, OCOH, J = 1.5 Hz), 8.21 (4H, q, ArH, J = 6 Hz), 7.91 (2H, d, ArH, J = 7.5 Hz), 7.63−7.59 (4H, m, ArH), 7.34−7.31 (2H, m, ArH), 7.20− 7.14 (H, m, ArH), 7.08 (4H, t, ArH, J = 7.5 Hz), 7.01 (3H, t, ArH, J = 7.5 Hz), 6.98−6.96 (9H, m, ArH), 0.33 (3H, t, SiMe, J = 1.5 Hz). 31 1 P{ H} NMR (C6D6, 121.5 MHz): δ 46.9. 13C NMR (C6D6, 100.6 MHz): δ 157.5, 153.5 (t, J = 29.5 Hz, 143.4 (t, J = 25.4 Hz), 135.0 (t, J = 7.8 Hz), 134.1 (t, J = 7.0 Hz), 134.2 (s), 133.2 (t, J = 12.8 Hz), 133.8, 133.1 (t, J = 21.7 Hz), 132.4 (t, J = 21.7 Hz), 130.7 (d, J = 2.5 Hz), 130.3, 129.7 (t, J = 3.1 Hz), 129.1 (t, J = 5.2 Hz), 128.6 (t, J = 5.1 Hz), 6.02. Anal. Found (calcd for C38H32O2P2PdSi): C, 62.7 (63.7); H, 4.6 (4.5).

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.H. acknowledges support from the National Science Foundation through Grant CHE-1150826. This work was supported in part by the Yale University Faculty of Arts and Sciences High Performance Computing Facility (and staff). This work was also supported in part by the Department of Energy (DE-FG02-06ER15765) via a grant to R.A.K. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.

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ASSOCIATED CONTENT

S Supporting Information *

CIF files, tables, and figures giving X-ray information for Ph2 PSiPNiCl, Ph2PSiPPdCl, Cy2PSiPPdH, Cy2PCPNi-OC(O)H, Cy2 PCPPd-OC(O)H, tBu2PCPPd-OC(O)H, Cy2PSiPNi-OC(O)H, and Cy2PSiPPd-OC(O)H, cartesian coordinates and energies for optimized structures, a comparison of the calculated structures of Me2PCPNiH, tBu2PCPNiH, Me2PCPPdH, and tBu2 PCPPdH, a comparison of bond lengths and angles in the optimized structures of Me2PSiPPdH and Me2PSiHPPd with the experimental structure of Cy2PSiPPdH, NBO analysis, and calculations on the insertion of allene into metal hydrides. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

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*E-mail: [email protected] (N.H.); [email protected] (R.A.K.). 8235


Organometallics

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Experimental and Computational Studies of the Reaction of Carbon

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