Efficient Reduction of Carbon Dioxide to Methanol Equivalents

Jan 24, 2017 - Dimeric magnesium(I) β-diketiminates: a new class of quasi-universal reducing agent. Cameron Jones. Nature Reviews Chemistry 2017 1, ...
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Efficient Reduction of Carbon Dioxide to Methanol Equivalents Catalyzed by Two-Coordinate Amido−Germanium(II) and −Tin(II) Hydride Complexes Terrance J. Hadlington,† Christos E. Kefalidis,‡ Laurent Maron,*,‡ and Cameron Jones*,† †

Monash Centre for Catalysis, School of Chemistry, Monash University, P.O. Box 23, Melbourne, Victoria 3800, Australia Université de Toulouse et CNRS, INSA, UPS, UMR 5215, LPCNO, 135 Avenue de Rangueil, F-31077 Toulouse, France

‡

S Supporting Information *

ABSTRACT: The bulky amido−germanium(II) and −tin(II) hydride complexes, L†EH [E = Ge or Sn; L† = -N(Ar†) (SiPri3); Ar† = C6H2Pri{C(H)Ph2}2-4,2,6], which are two-coordinate in solution, are shown to be efficient and highly selective “singlesite” catalysts for the reduction of CO2 to methanol equivalents (MeOBR2), using HBpin or HBcat as the hydrogen source. L†SnH is the most active non-transition metal catalyst yet reported for such reductions, yielding turnover frequencies of up to 1188 h−1 at room temperature. Computational studies have identified two thermodynamically and kinetically viable catalytic pathways by which these reductions may operate. Spectroscopic investigations have identified several reaction intermediates, which leads to the conclusion that one of these reaction pathways predominates in the experimental situation. Stoichiometric reactivity studies have shed further light on the reaction mechanisms in operation and indicate that the involvement of the second reaction pathway cannot be ruled out. This study highlights the potential of relatively cheap, main group complexes as viable alternatives to transition metal-based systems in the catalytic transformation of small molecules. KEYWORDS: CO2, reduction, catalysis, germanium, tin, low-coordinate

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h−1). In addition, a handful of dual-site frustrated Lewis pair (FLP) catalysts and related compounds (both non-metal- and main group metal-based) have been employed as catalysts for the borane reduction of CO2 to methanol equivalents.6,11 These reactions typically also require elevated temperatures and/or CO2 pressures to proceed efficiently. The most active of these is the dual-site non-metal system, C6H4(PPh2)(Bcat)-1,2 (cat = catecholato), which catalyzes (at 0.33 mol %) the reduction of CO2 by BH3(SMe2) at 60 °C with a TOF of 853 h−1.11a For the sake of comparison, the most active of any homogeneous catalyst used for the reduction of CO2 to a methanol equivalent is a palladium thiolate system, which was very recently reported to yield TOFs of up to 1780 h−1 when HBcat was used as the hydrogen source.7a It would be quite beneficial to develop new, more efficient main group element-based catalysts for the reductive hydroboration of CO2, which could be used as cheaper and less toxic alternatives to transition metal catalysts in this area.12 Moreover, expanding the ranks of such catalysts may lead to a better understanding of the mechanisms by which they operate. This is important as, at present, there is sparse

INTRODUCTION The rising levels of carbon dioxide in Earth’s atmosphere, and the climatic problems associated with this,1 have led to a rapid escalation of research activities aimed at the catalytic conversion of CO2 into value-added chemicals and fuels.2 By and large, these efforts have been directed at utilizing CO2 as a C1 feedstock for the synthesis of, for example, methane, formic acid, formaldehyde, CO, and methanol.2,3 While heterogeneously and electrochemically catalyzed processes are of considerable importance in this realm,4 excellent progress has been made using transition metal complexes as homogeneous catalysts for such transformations over the past decade.5−7 In contrast, main group metal and non-metal compounds had been little used as homogeneous catalysts for the reduction of CO2 until 2010.6,8 In this respect, of most relevance to this study are reductions of CO2 to methanol equivalents, MeOBR2, using boranes, HBR2, as hydrogen sources. To the best of our knowledge, there are only two reports of such reductions (using HBpin, where pin = pinacolato) being catalyzed at a single site in main group metal complexes, viz., the β-diketiminato s- and p-block compounds, ( D i p Nacnac)Ga(H)(Bu t ), 9 and (DipNacnac)M(THF)n(μ-H)B(C6F5)3 (M = Mg, and n = 0; M = Ca, and n = 1).10 However, in those cases, high catalyst loadings (10 mol %) and elevated temperatures (60 °C) were required to achieve low turnover frequencies (2.5, 0.07, and 0.1 © 2017 American Chemical Society

Received: November 21, 2016 Revised: January 19, 2017 Published: January 24, 2017 1853

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ACS Catalysis experimental mechanistic information available for main group compounds that catalyze CO2 reductions. Types of compounds that we believed might prove to be effective in this regard are the germanium(II) and −tin(II) hydride complexes, 1 and 2, respectively (Figure 1), which we have shown to be two-

Table 1. Secondary Borane Reduction of CO2, Catalyzed by 1 or 2 catalysta

loading (mol %)

BR2

time (h)b

yield (%)c

TOF (h−1)d

1 1 2 2 2

10 1 10 1 1

Bpin Bpin Bpin Bpin Bcat

24 48 1.2 6.6 0.08

>99 93 >99 >99 >99

0.4 2.1 8 14.5 1188

a

Catalyst 2 can also be generated in situ using the precatalyst L†SnOBut. bAll reactions performed in d6-benzene at 20 °C under ∼2 bar of CO2 using 1 equiv of HBpin. cObtained by integration of product signals in 1H NMR spectra vs signals for HBR2 and a tetramethylsilane internal standard. dAverage value for complete reaction.

Figure 1. Catalysts used in this study.

coordinate in solution, because of their extremely bulky amide ligands13,14 and to possess an empty p orbital at the metal center. This leads to the electrophilic compounds being powerful and selective reagents for the hydrometalation of a range of unsaturated substrates (e.g., alkenes,15 aldehydes, and ketones16). Furthermore, the low-coordinate nature of both compounds has led to them finding use as extremely active catalysts for the hydroboration of unactivated ketones and aldehydes.16 Because of this, it seemed reasonable that they could also catalyze the reduction of CO2 to methanol equivalents, using boranes. Further impetus to investigate this possibility came from the fact that experimental stoichiometric,17 and computationally predicted catalytic,18 reductions of CO2 to methanol, mediated by higher-coordinate germanium(II) hydrides [e.g., (DipNacnac)GeH], have recently been reported. Here we show that 1 and 2 are efficient and highly selective catalysts for the reductive hydroboration of CO2 to MeOBR2 compounds and that 2 is by far the most active nontransition metal catalyst yet described for such reductions. The mechanisms of these reactions were investigated in detail using spectroscopic and computational analyses, in addition to stoichiometric reactivity studies.

amounts of the likely initial CO2 insertion products, viz., the metal formates L†E(κ2-O,O′-O2CH) [E = Ge for 3 and Sn for 4 (see below)], (pinBO)2CH2 5,19 and the final products (pinB)2O and MeOBpin. During the early stages of the reactions, the amount of (pinBO)2CH2 present increased at the expense of the formate, while as the reaction progressed, (pinBO)2CH2 was lost as more (pinB)2O and MeOBpin were formed. These experimental observations imply that 3 or 4 and 5 are intermediates in the CO2 reduction reactions. It is of note that these compounds are similar to previously proposed intermediates in related reductions mediated by main group systems.10,17 1 or 2

CO2 + 3HBR 2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ MeOBR 2 + (R 2B)2 O (BR 2 = HBpin or HBcat)

(1)

To increase the rate of the reduction reactions, the more potent borane, HBcat, was used in place of HBpin. In the case of the reduction catalyzed by 1, the product mixture was not as clean as that obtained using HBpin and contained a number of unidentifiable species, in addition to (catB)2O and MeOBcat. However, when a 1 mol % loading of catalyst 2 was used, the reaction was extremely clean and was complete within 5 min, giving rise to a remarkable, and reproducible, TOF of 1188 h−1. This far exceeds any reported value for the hydroboration of CO2, yielding methanol equivalents, catalyzed by non-transition metal species.6 Furthermore, this includes situations in which more reactive boranes (e.g., BH3 and 9-BBN) and higher temperatures are employed.6 In our case, the reactions are so rapid that possible intermediates could not be observed spectroscopically. Computational Studies. To gain an understanding of the mechanisms of the reductions mentioned above, computational studies (ωB97xD) were employed to probe the profile of the reaction between CO2 and HBpin, catalyzed by 1, as a test case. It should be mentioned that a variety of pathways were identified for this reaction, though the majority were found to lead to reaction dead ends or encountered one or more transition states en route to the final products, (pinB)2O and MeOBpin, that were considered to have energies too high for the pathways to be viable (see the Supporting Information). With that said, two routes that had similar energy barriers to their progression and were deemed viable were uncovered. Schematic representations of the catalytic cycles are depicted in Scheme 1, while the energies of the reaction profiles can be found in Figures 2 and 3. The first thing to mention about the calculated catalytic cycles is that reaction pathways very similar to both of those

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RESULTS AND DISCUSSION Catalytic Studies. The reduction of CO2 to methanol equivalents, using either HBpin or the more reactive HBcat as a hydrogen source, proceeded according to the overall reaction depicted in eq 1. In the case of HBpin, and as found previously for the catalytic hydroboration of aldehydes and ketones,16 germanium(II) hydride 1 as a catalyst is less active than its more electrophilic tin(II) counterpart, 2. At a 10 mol % loading of 1, the CO2 reduction reaction is complete after 24 h at 20 °C, yielding a TOF of 0.4 h−1 (see Table 1). Decreasing the catalyst loading marginally incresed the TOF, but the reaction was slower and the yield reduced. In contrast, when 2 was employed as the catalyst at a 10 mol % loading, the TOF at reaction completion was more than an order of magnitude higher at 8 h−1. Decreasing the loading to 1 mol % yielded an overall TOF of 14.5 h−1, which is significantly greater than that achieved by the previously most active single-site main group metal catalyst, (DipNacnac)Ga(H)(But).9 Importantly, the reactions catalyzed by both 1 and 2 were typically extremely clean, yielding MeOBpin and (pinB)2O in greater than 99% yield upon completion, with no other identifiable boron-containing products. However, following the reactions by 1H and 11B{1H} NMR spectroscopy did shed light on implied intermediates in the reaction. In both cases, and at various stages of the reactions, the mixtures contained differing 1854

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acid/Lewis base activation of CO2.6 Both possibilities were considered here, with the first one calculated as being operative. With regard to the second case, we explored the reaction of CO2 with two molecules of ambiphilic 1, i.e., one molecule acting as a Lewis acid through its empty p orbital and one as a Lewis base through its Ge lone pair. In addition, the hydrogermylation of CO2 by the Ge−Ge bonded dimer of 1, viz., L†(H)GeGe(H)L† (i.e., as the compound exists in the solid state), was investigated, but no realistic pathway could be found in the last two cases.20 Once formate 3 is formed, it undergoes a cycloaddition reaction with HBpin across one GeO bond to give sixmembered transition state TSC‑D. This then leads to int-E, which can been described as an adduct of pinBC(O)(H) with complex 1. After a low-energy hydrogen migration to the electrophilic carbon of the formate fragment, complex 6 is formed.21 The possibility that the reaction between 3 and HBpin instead involved a formal σ-bond metathesis reaction, yielding free 1 and pinBC(O)(H), which then react together to give 6, was also examined. Although similar reactivity has been suggested for main group-catalyzed CO2 reductions in the past,10 calculations here show it to be a reaction dead end.22 Once formed, intermediate 6 can proceed to the final products via two similar pathways. In route A (Scheme 1), it undergoes a σ-bond metathesis reaction with HBpin to regenerate 1 and give (pinBO)2CH2 5. Given the near equivalent enthalpies of the products and reactants of this stage of the reaction, they could be in equilibrium with each other. Compounds 1 and (pinBO)2CH2 then react in the rate-determining step (ΔH⧧ = 36.0 kcal/mol), yielding MeOBpin and the experimentally observed (see below) germanium(II) borate ester, 7. The latter readily reacts with the final equivalent of HBpin to give catalyst, 1, and the diboroxane product, (pinB)2O. This seems to be the most dominant pathway that is operational in the experimental situation, as 3, 5, and products MeOBpin and (pinB)2O were all spectroscopically observed, and their concentrations shown to be interdependent, during the course of the reaction. The rather large kinetic barrier to the formation of (pinBO)2CH2 5 is consistent with the slow rate of the overall reaction (TOF = 0.4 h−1). The alternate catalytic cycle (route B, Scheme 1) involves elimination of formaldehyde from 6, to give 7, as the ratedetermining step. The barrier to this process (ΔH⧧ = 36.7 kcal/ mol) is only slightly higher than that for route A. Intermediate 7 in route B undergoes a σ-bond metathesis reaction with

Scheme 1. Two Cycles Calculated for the Reduction of CO2 to MeOBpin, Catalyzed by 1

described here have previously been proposed (but not computationally assessed) for main group metal hydridecatalyzed reductions of CO2 to methanol equivalents.10 Second, the first two steps, leading to germa-/bora-acetal intermediate 6, are common in both cycles. In both, the initial reaction is the hydrogermylation of CO2, giving intermediate int-B (Figure 2), which upon isomerization affords the energetically more stable and experimentally observed (see below) germanium formate, 3 (ΔH = −24.2 kcal/mol), with a low kinetic barrier (ΔH⧧ = 13.5 kcal/mol). As has been shown previously, calculated CO2 reduction mechanisms typically involve an initial nucleophilic attack of a catalyst at the C center of CO2, or dual-site Lewis

Figure 2. Calculated [density functional theory, ωB97xD/6-311+G*(Ge)//SDD(Si)//6-31G**(O,N,C,B,H)] reaction profile for the formation of intermediate 6. 1855

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Figure 3. Two calculated [density functional theory, ωB97xD/6-311+G*(Ge)//SDD(Si)//6-31G**(O,N,C,B,H)] reaction profiles for the formation of MeOBpin and (pinB)2O from intermediate 6 ([Ge] = L†Ge). Route A (top) and route B (bottom). The areas encircled in red are common to both pathways.

absence of HBpin, to give the corresponding formates, 3 and 4, respectively (Scheme 2). These compounds could alternatively be prepared in good yield via reaction of amido group 14 halides, L†EX (E = Ge, and X = Cl; E = Sn, and X = Br) with potassium formate, KOC(H)O. Both 3 and 4 were crystallo-

HBpin to give 1 and (pinB)2O, the former of which hydrogermylates formaldehyde to give the germanium methoxide, 8. Finally, this reacts with HBpin to give the methanol equivalent, MeOBpin, and catalyst 1. Because this is as kinetically viable as route A, the two routes may operate simultaneously, despite the experimental observation of significant quantities of (pinBO)2CH2 (derived from route A) in the catalytic reaction mixture. Although computational limitations did not allow for calculation of the fully optimized reaction profiles (see the Supporting Information) for the reduction of CO2 catalyzed by 2, using either HBpin or HBcat as a hydrogen source, it was ascertained that these profiles were similar to those involving 1, including the rate-determining process that led to (pinBO)2CH2. The approximate barriers to this process (ΔH⧧) were 34 kcal/mol (HBpin) and 19 kcal/mol (HBcat), which are consistent with the significantly enhanced experimental reaction rates (TOFs of 14.5 and 1188 h−1, respectively), relative to that achieved using 1 as the catalyst. Stoichiometric Reactivity Studies. With a view of adding weight to the validity of the catalytic cycles discussed above, a series of stoichiometric reactivity studies involving 1, 2, CO2, and/or HBpin were performed, with some interesting results. First, it was confirmed that 1 and 2 react with CO2, in the

Scheme 2. Synthesis of Compounds 3 and 4 and Proposed Pathways to 8−10 from 3

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Figure 4. Thermal ellipsoid plots (25% probability surface) of (a) compound 4, (b) compound 7, (c) compound 9, and (d) compound 10.

decomposed to a mixture of 10, 1, 3, and another product, believed to be the germanium methoxide, 8. Compounds 1 and 3 could result from a β-hydride elimination process involving the bridging methylene unit of 9 [cf. ambient-temperature reversible β-hydride eliminations from L†Ge(alkyl) compounds15], while 8 could be generated by hydrogermylation of formaldehyde by 1. Unfortunately, efforts to rationally synthesize 8 to confirm its identity in the reaction mixture were not successful. On the other hand, 10 was alternatively synthesized in good yield by treating a solution of the amidodigermyne, L†GeGeL†, with N2O, as we have previously reported for closely related amido-digermynes.24 Although this reactivity is interesting, compound 10 is unreactive toward HBpin. Therefore, the series of reactions leading to it are not likely to be integral to, or even occurring in, the experimental reduction of CO2 by HBpin, in the presence of catalyst 1. As a means of gaining further information about the role of formates 3 and 4 in a catalytic context, toluene solutions of each were reacted with 1 equiv of HBpin, in the absence of CO2, at ambient temperature. These reactions led to one major product as determined by 1H NMR spectroscopic analyses of the mixtures. Concentrating and cooling the reaction solutions afforded good yields of the germanium(II) and −tin(II) borate esters, 7 and 11, respectively (Scheme 3). Both of these were crystallographically characterized (see Figure 4 for the molecular structure of 7) and represent the first structurally authenticated examples of p-block metallo-borate esters. Variable-temperature NMR spectroscopic analyses (−80 to 25 °C, d8-toluene solutions) of the 1:1 reaction mixtures were used in an attempt to identify any intermediates. However, although signals for several species were observed upon warming, and before ultimate conversion to 7 or 11, nothing definitive could be concluded about the nature of the intermediates. With that said, the fact that reactions of formates 3 and 4 with 1 equiv of HBpin gave high spectroscopic yields of the metallo-borate esters, 7 and 11, respectively (presumably

graphically characterized as monomers with the formate ligands κ2-O,O′-chelating their metal centers (see Figure 4 for the molecular structure of 4). This situation contrasts with that for related complexes incorporating bulky bidentate amide ligands, in which the formate acts as a monodentate ligand to the metal center, e.g., (DipNacnac)EOC(H)O (E = Ge or Sn).23 Also, this thermodynamic stability, favoring the chelated over monodentate isomers, is reflected in the estimated enthalpy energy difference (ΔH = 3.5 kcal/mol) as depicted in Figure 2 (int-B vs 3). The 1H NMR spectra of the pure compounds are identical to those of the reaction components predicted to be metal formates in the catalytic studies described above. Moreover, when samples of 3 or 4 were added to large excesses of HBpin in C6D6 solutions under CO2 atmospheres, the formation of MeOBpin and (pinB)2O proceeded in a fashion similar to that in which the metal hydrides, 1 or 2, were used as catalysts. This gives good evidence that 3 and 4 are, indeed, intermediates in the catalytic cycle(s) in operation. Interestingly, while the reaction of 2 with excess CO2 rapidly, and essentially quantitatively, led to 4, the reaction of 1 with CO2 was slower. In addition to 3, several other products were spectroscopically observed in the reaction mixture (Scheme 2). Two of these, the unprecedented digerma-acetal, 9, and the bis(germylene)ether, 10, were crystallized from the reaction mixture and structurally authenticated (Figure 4). Their 1H NMR spectra were identical to those of two of the components in the reaction mixture resulting from the treatment of 1 with CO2. It seems likely that 9 is formed because the reaction of 1 with CO2 is slow enough that a CO bond of formed 3 can undergo a 1,2-addition reaction with 1. This was confirmed by reacting a 1:1 mixture of 1 and 3, in the absence of CO2 or HBpin, which gave 9 in a moderate isolated yield. Compound 10 could result from elimination of formaldehyde from thermally unstable 9. This possibility was tested by heating a solution of pure 9 in C6D6 for 2 h at 60 °C. The 1 H NMR spectrum of the mixture confirmed that 9 had largely 1857

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ACS Catalysis Crystallographic Crystallographic Crystallographic Crystallographic Crystallographic

Scheme 3. Synthesis of Compounds 7 and 11 and Their Reactivity toward HBpin

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data data data data data

for for for for for

compound compound compound compound compound

4 (CIF) 7 (CIF) 9 (CIF) 10 (CIF) 11 (CIF)

AUTHOR INFORMATION

Corresponding Authors

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

Laurent Maron: 0000-0003-2653-8557 Cameron Jones: 0000-0002-7269-1045

also eliminating formaldehyde), suggests that they proceed as per route B (Scheme 1). If they were to proceed via the sequence in route A, 2 equiv of HBpin would be required for complete conversion to 7 and 11, and MeOBpin would be generated as an observable byproduct. This result is, of course, in contrast to the catalytic studies mentioned above, which were surmised to proceed via route A, on the basis of spectroscopic analyses of the reaction mixtures. However, those were performed in the presence of very large excesses of HBpin, which would certainly favor route A. Whatever the case, 7 and 11 are likely intermediates in both calculated catalytic cycles. Strong evidence of this comes from the fact that when toluene solutions of both were treated with 1 equiv of HBpin, the reactions cleanly and rapidly generated approximately 1:1 mixtures of the metal hydride catalyst, 1 or 2, and the diboroxane, (pinB)2O.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Australian Research Council (grant to C.J.) and the U.S. Air Force Asian Office of Aerospace Research and Development (Grant FA2386-14-14043 to C.J.). Part of this research was undertaken on the MX1 beamline at the Australian Synchrotron (Victoria, Australia). L.M. acknowledges the Alexander von Humboldt Foundation for a grant of experienced researcher, the Chinese Academy of Science for a senior researcher grant, and CAlMip for computational time.

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(1) (a) Climate Change 2007: Synthesis Report. Intergovernmental Panel on Climate Change: Geneva, 2007. (b) Lackner, K. S.; Brennan, S.; Matter, J. M.; Park, A.-H. A.; Wright, A.; van der Zwaan, B. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 13156−13162. (2) Aresta, M., Ed. Carbon Dioxide as a Chemical Feedstock; WileyVCH: Weinheim, Germany, 2010. (3) Selected reviews: (a) Liu, Q.; Wu, L.; Jackstell, R.; Beller, M. Nat. Commun. 2015, 6, 5933. (b) Murphy, L. J.; Robertson, K. N.; Kemp, R. A.; Tuononen, H. M.; Clyburne, J. A. C. Chem. Commun. 2015, 51, 3942−3956. (c) Cokoja, M.; Bruckmeier, C.; Rieger, B.; Herrmann, W. A.; Kuhn, F. E. Angew. Chem., Int. Ed. 2011, 50, 8510−8537. (d) Olah, G. A. Angew. Chem., Int. Ed. 2005, 44, 2636−2639. (e) Jessop, P. G.; Ikariya, T.; Noyori, R. Chem. Rev. 1995, 95, 259− 272. (4) Selected reviews: (a) Kortlever, R.; Shen, J.; Schouten, K. J. P.; Calle-Vallejo, F.; Koper, T. M. J. Phys. Chem. Lett. 2015, 6, 4073− 4082. (b) Costentin, C.; Robert, M.; Saveant, J. M. Chem. Soc. Rev. 2013, 42, 2423−2436. (c) Takeda, H.; Ishitani, O. Coord. Chem. Rev. 2010, 254, 346−354. (d) Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. Chem. Soc. Rev. 2009, 38, 89−99. (5) Review: Wang, W.-H.; Himeda, Y.; Muckerman, J. T.; Manbeck, G. F.; Fujita, E. Chem. Rev. 2015, 115, 12936−12973. (6) Review: Chong, C. C.; Kinjo, R. ACS Catal. 2015, 5, 3238−3259. (7) Examples of relevant transition metal-catalyzed CO2 reductions that appeared after refs 5 and 6: (a) Ma, Q.-Q.; Liu, T.; Li, S.; Zhang, J.; Chen, X.; Guan, H. Chem. Commun. 2016, 52, 14262−14265. (b) Lu, Z.; Williams, T. J. ACS Catal. 2016, 6, 6670−6673. (8) Review: Revunova, K.; Nikonov, G. I. Dalton Trans. 2015, 44, 840−866. (9) Abdalla, J. A. B.; Riddlestone, I. M.; Tirfoin, R.; Aldridge, S. Angew. Chem., Int. Ed. 2015, 54, 5098−5102. (10) Anker, M. D.; Arrowsmith, M.; Bellham, P.; Hill, M. S.; KociokKöhn, G.; Liptrot, D. J.; Mahon, M. F.; Weetman, C. Chem. Sci. 2014, 5, 2826−2830. (11) (a) Courtemanche, M.-A.; Légaré, M.-A.; Maron, L.; Fontaine, F.-G. J. Am. Chem. Soc. 2013, 135, 9326−9329. (b) Wang, T.; Stephan, D. W. Chem. - Eur. J. 2014, 20, 3036−3039. (c) Wang, T.; Stephan, D.

CONCLUSIONS In summary, we have shown that bulky amido−germanium(II) and −tin(II) hydride complexes, which are two-coordinate in solution, can be extremely efficient and highly selective singlesite catalysts for the reduction of CO2 to methanol equivalents, using secondary boranes as the hydrogen source. Moreover, the amido−tin(II) hydride is the most active non-transition metal catalyst yet reported for such reductions, yielding TOFs as high as 1188 h−1. Computational studies have identified two thermodynamically and kinetically viable catalytic cycles by which these reductions may operate. Following the course of the reactions using spectroscopic techniques leads to the conclusion that one of these reaction pathways predominates in the experimental situation. A series of related stoichiometric reactions have shed further light on the reaction mechanisms in operation and indicate that involvement of the second reaction pathway cannot be ruled out. As a whole, this study further indicates that relatively cheap main group complexes can rival the practicality of transition metal-based systems in catalytic transformations of small molecules. We continue to explore and develop this area of chemistry.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b03306. Details of the synthesis, characteristic data for all new compounds, and full details and references for the catalysis, crystallographic, and computational studies (PDF) Cartesian coordinates for all calculated compounds, intermediates and transition states (XYZ) 1858

DOI: 10.1021/acscatal.6b03306 ACS Catal. 2017, 7, 1853−1859

Research Article

ACS Catalysis W. Chem. Commun. 2014, 50, 7007−7010. (d) das Neves Gomes, C.; Blondiaux, E.; Thuéry, P.; Cantat, T. Chem. - Eur. J. 2014, 20, 7098− 7106. (e) Blondiaux, E.; Pouessel, J.; Cantat, T. Angew. Chem., Int. Ed. 2014, 53, 12186−12190. (f) Mukherjee, D.; Shirase, S.; Spaniol, T. P.; Mashima, K.; Okuda, J. Chem. Commun. 2016, 52, 13155−13158. (12) Selected recent reviews: (a) Power, P. P. Nature 2010, 463, 171−177. (b) Power, P. P. Acc. Chem. Res. 2011, 44, 627−637. (c) Asay, M.; Jones, C.; Driess, M. Chem. Rev. 2011, 111, 354−396. (d) Yao, S.; Xiong, Y.; Driess, M. Organometallics 2011, 30, 1748− 1767. (e) Martin, D.; Soleilhavoup, M.; Bertrand, G. Chem. Sci. 2011, 2, 389−399. (13) (a) Hadlington, T. J.; Hermann, M.; Li, J.; Frenking, G.; Jones, C. Angew. Chem., Int. Ed. 2013, 52, 10199−10203. (b) Hadlington, T. J.; Schwarze, B.; Izgorodina, E. I.; Jones, C. Chem. Commun. 2015, 51, 6854−6857. (14) Hadlington, T. J.; Jones, C. Chem. Commun. 2014, 50, 2321− 2323. (15) Hadlington, T. J.; Hermann, M.; Frenking, G.; Jones, C. Chem. Sci. 2015, 6, 7249−7257. (16) Hadlington, T. J.; Hermann, M.; Frenking, G.; Jones, C. J. Am. Chem. Soc. 2014, 136, 3028−3031. (17) (a) Tan, G.; Wang, W.; Blom, B.; Driess, M. Dalton Trans. 2014, 43, 6006−6011. (b) Jana, A.; Tavcar, G.; Roesky, H. W.; John, M. Dalton Trans. 2010, 39, 9487−9489. (18) Takagi, N.; Sakaki, S. J. Am. Chem. Soc. 2013, 135, 8955−8965. (19) Bontemps, S.; Vendier, L.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2012, 51, 1671−1674. (20) It was also experimentally determined that L†GeH 1 does not react, or form an adduct with, HBpin. Therefore, it is assumed that 1 is not activated toward reaction with CO2 in the presence of HBpin. (21) Similar reactivity patterns have been proposed, computationally, for CO2/HBcat reduction reactions, catlyzed by nickel pincer complexes: Huang, F.; Zhang, C.; Jiang, J.; Wang, Z. X.; Guan, H. Inorg. Chem. 2011, 50, 3816−3825. (22) The 1,2-addition reaction of HBpin across one CO bond of 3, to give 6, was also examined computationally (ΔH⧧ = 29.2 kcal/mol) but found to be kinetically less accessible (see Figure S4). (23) (a) Jana, A.; Ghoshal, D.; Roesky, H. W.; Objartel, I.; Schwab, G.; Stalke, D. J. Am. Chem. Soc. 2009, 131, 1288−1293. (b) Jana, A.; Roesky, H. W.; Schulzke, C.; Doering, A. Angew. Chem., Int. Ed. 2009, 48, 1106−1109. (c) Choong, S. L.; Woodul, W. D.; Schenk, C.; Stasch, A.; Richards, A. F.; Jones, C. Organometallics 2011, 30, 5543−5550. (24) Li, J.; Hermann, M.; Frenking, G.; Jones, C. Angew. Chem., Int. Ed. 2012, 51, 8611−8614.

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DOI: 10.1021/acscatal.6b03306 ACS Catal. 2017, 7, 1853−1859