Zinc and Magnesium Catalysts for the Hydrosilylation of Carbon

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Zinc and Magnesium Catalysts for the Hydrosilylation of Carbon Dioxide Michael Rauch and Gerard Parkin* Department of Chemistry, Columbia University, New York, New York 10027, United States S Supporting Information *

employ non-precious metals, occur at room temperature, and may be modified to control the level of reduction. With respect to developing hydrosilylation catalysts that are based on non-precious metals, we recently reported the first example of zinc-catalyzed hydrosilylation of CO2. Specifically, we demonstrated that the tris(2-pyridylthio)methyl zinc hydride complex, [κ3-Tptm]ZnH, is an effective catalyst for the hydrosilylation of CO2 to the silyl formate, HCO2Si(OEt)3, on a multigram scale.11,12 Seeking to develop further this chemistry, we have examined the use of the structurally related tris[(1-isopropyl i benzimidazol-2-yl)dimethylsilyl]methyl ligand, [TismPr Benz],13 to afford active catalysts for hydrosilylation.i In this regard, the terminal zinc hydride complex, [κ3-TismPr Benz]ZnH (1), may be obtained via the sequence illustrated in Scheme 2, and has been structurally characterized by X-ray diffraction (Figure 1).14

ABSTRACT: The terminal zinc and magnesium hydride i i compounds, [κ3-TismPr Benz]ZnH and [TismPr Benz]MgH, which feature the tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl ligand, react with B(C6F5)3 to afford i the ion pairs, {[TismPr Benz]M}[HB(C6F5)3] (M = Zn, Mg), which are rare examples of these metals in trigonal monopyramidal coordination environments.i Significantly, in combination with B(C6F5)3, {[TismPr Benz]M}[HB(C6F5)3] generates catalytic systems for the hydrosilylation of CO2 by R3SiH to afford sequentially the bis(silyl)acetal, H2C(OSiR3)2, and CH4 (R3SiH = PhSiH3, Et3SiH, and Ph3SiH). In contrast to many other catalysts for these transformations, both the zinc and magnesium catalytic systems are active at room temperature, and the latter provides the first example of catalytic hydrosilylation of CO2 involving a magnesium compound. Also of note, the selectivity of the catalytic systems may be controlled by the nature of the silane, with PhSiH3 favoring CH4, and Ph3SiH favoring the bis(silyl)acetal, H2C(OSiPh3)2.

Scheme 2

T

he efficient utilization of carbon dioxide as a renewable C1 source for the synthesis of value-added organic chemicals and fuels is not only of intrinsic value, but also offers potential for abating the increasing levels of carbon dioxide in the atmosphere.1,2 However, CO2 is thermodynamically very stable and kinetically resistant to many transformations, which presents a major impediment to achieving this objective. Significant effort is, therefore, underway to develop catalysts that are capable of effecting the functionalization of CO2.1 One approach focuses on hydrosilylation, a transformation that has several advantages, namely: (i) addition of a Si−H bond to CO2 is thermodynamically more favorable than is the addition of the H−H bond, (ii) inexpensive and environmentally benign hydrosilanes are readily available,3,4 and (iii) hydrosilylation of CO2 can proceed in a stepwise manner to afford a series of products with different carbon oxidation levels, which include silyl formates, silyl acetals, methoxy silanes, and methane (Scheme 1).3,5 However, despite

i

Although [κ3-TismPr Benz]ZnH alone is not a catalyst for the hydrosilylation of CO2 by PhSiH3, a catalytic system for the reduction of CO2 to CH4 is obtained in the presence of B(C6F5)3 (Table 1). More interesting, however, is the observation that the i corresponding magnesium hydride complex, [TismPr Benz]MgH (2),13b in the presence of B(C6F5)3, provides a much more active catalytic system (Table 1),15 and also represents the first example of catalytic hydrosilylation of CO2 by a magnesium compound.16 While combinations of B(C6F5)3 with other metal compounds have been reported to serve as catalysts for hydrosilylation of CO2,9a-f it is noteworthy that the zinc and magnesium systems are active at room temperature, and utilize earth-abundant and nontoxic main group metals. The magnesium system is also robust and may be recycled multiple times without significant loss in activity. The ability to control selectively the formation of the various CO2 reduction products with different carbon oxidation levels (Scheme 1) is of considerable significance since each possesses unique reactivity. Therefore, we have investigated the use of different silanes in both the zinc and magnesium systems, and have observed that whereas PhSiH3 forms selectively CH4, Ph3SiH

Scheme 1

the utility of the hydrosilylation of CO2, most examples of hydrosilylation employ precious metal catalysts6−8 at elevated temperatures, and control of the reduction level is challenging.9,10 Therefore, we report here catalytic systems for hydrosilylation that © XXXX American Chemical Society

Received: October 9, 2017

A

DOI: 10.1021/jacs.7b10776 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society Scheme 3

i

Figure 1. Molecular structures of [κ3-TismPr Benz]ZnH (1) (left) and i {[TismPr Benz]Zn}+ (4) (right, counterion omitted for clarity).

Table 1. Catalytic Hydrosilylation of CO2 catalysta

R3SiH

product

[Mg] (0.5%)/[B] (2.5%)c

PhSiH3

CH4

[Mg] (0.5%)/[B] (2.5%)c

Ph3SiH

H2C(OSiPh3)2

[Zn] (2.5%)/[B] (5.0%)e [Zn] (2.0%)/[B] (10.0%)e

PhSiH3 Ph3SiH

CH4 H2C(OSiPh3)2

a

i

i

TONb (TOF/h−1) 542, 416d (38.7, 29.7)d 83, 178d (1.2, 2.5)d 117 (0.8) 12 (0.1) i

[Mg] = [TismPr Benz]MgH or [TismPr Benz]MgMe; [Zn] = {[TismPr Benz]Zn}[HB(C6F5)3]; [B] = B(C6F5)3. bNumber of Si−H bonds consumed i per metal. cC6D6. dValues for [TismPr Benz]MgH listed first. eC6D5Br.

affords selectively the bis(silyl)acetal, H2C(OSiPh3)2 (Table 1).17,18 The formation of H2C(OSiPh3)2 is of particular significance because the selective reduction of CO2 to the formaldehyde level19,20 has little precedence by comparison to its reduction to methane.21 Formaldehyde is a widely used industrial chemical, with a consumption of more than 30 million tons per year, and is principally obtained by the oxidation of methanol.22 As such, the synthesis of formaldehyde and its derivatives from CO2 provides an alternative approach for developing this area of chemistry.22a,23 With respect to the selectivity exhibited by Ph3SiH, it is pertinent to note that although Ph3SiH has not been widely used for the i hydrosilylation of CO2,8b,9a the magnesium catalyst [TismPr Benz]MgH/B(C6F5)3 is both more active and more selective than the non-precious metal catalyst composed of (L3)Zr(CH2Ph)2/B(C6F5)3. Specifically, whereas the magnesium catalyst may achieve a TOF of 2.5 h−1 and form selectively the acetal, H2C(OSiPh3)2, the zirconium catalyst has a TOF of 0.13 h−1 and affords a ca. 2:1 mixture of H2C(OSiPh3)2 and (Ph3Si)2O, with the latter resulting from reduction of the CO2 to methane.9a,24 In terms of the mechanism of the catalytic cycle, a plausible i sequence emanating from [TismPr Benz]MH involves the initial insertioni of CO2 into the M−H bond to afford a formate species, [TismPr Benz]M(O2CH), as illustrated for the magnesium system in Scheme 3. Indeed,i 1H NMR spectroscopy demonstrates i that both [κ3-TismPr Benz]ZnH25 and [TismPr Benz]MgH react rapidly i with CO2, and the magnesium formate complex, [TismPr Benz]Mg(κ2-O2CH) (3), has been structurally characterized by X-ray diffraction (Figure 2).26 However, despite this observation, additional studies suggest that the direct insertion of CO2 into the M−H bond is not part of the catalytic cycle. Specifically, the magnesium and zinc hydride compounds, i i [κ3-TismPr Benz]ZnH and [TismPr Benz]MgH, also undergo rapid PriBenz hydride abstraction by B(C6F ) to afford {[Tism ]Zn}i 5 3 [HB(C6F5)3] (4) and {[TismPr Benz]Mg}[HB(C 6F5)3] (5), respeci tively. The molecular structures of {[TismPr Benz]M}[HB(C6F5)3]

i

Figure 2. Molecular structures of {[TismPri Benz]Mg}+ (5) (top left, counterion omitted for clarity), [TismPr Benz]Mg(κ2-O2CH) (3) i (bottom left), and [TismPr Benz]MgOC(H)OB(C6F5)3 (6) (right).

(M = Zn, Mg) have been determined by X-ray diffraction (Figures 1 and 2),27 which demonstrates that the compounds possess uncommon trigonal monopyramidal28,29 geometriesi and exist as ion pairs. In this regard, the structure of {[TismPr Benz]Mg}[HB(C6F5)3] is in marked contrast to that of the related compound, [ToM]MgHB(C6F5)3, which does not exist as an ion pair but rather possesses a distinct Mg−H−B interaction.30 Furthermore, the magnesium center of [ToM]MgHB(C6F5)3 exhibits close contacts with two of the fluoride substituents (2.15 and 2.19 Å), such that the compound possesses a distorted octahedral geometry. A similar coordination mode of the anion [HB(C6F5)3]− is also observed in 31,32 [HC{RCNDipp} The ability of 2]MgHB(C6F5)3 derivatives. i the [TismPr Benz] ligand to support a trigonal monopyramidal geoi metry in {[TismPr Benz]M}[HB(C6F5)3] and inhibit approach of the counterion is undoubtedly a consequence of its sterically demanding nature, as evidenced by the large values of its crystallographic cone angle (334°, Zn; 329°, Mg)33 and its percent buried volume (82.5%, Zn; 80.4%, Mg).34 For comparison, the cone angle (252°) and percent buried volume (59.2%) of the [ToM] ligand in [ToM]MgHB(C6F5)3i are much smaller than the corresponding i values for the [TismPr Benz] ligand in {[TismPr Benz]Mg}[HB(C6F5)3]. Despite the i similar nature of the magnesium and zinc ion pairs, {[TismPr Benz]M}[HB(C6F5)3], the two compounds differ in their reactivity towards CO2.35 Specifically, whereas the magnesium complex reacts rapidly with CO2 (1 atm) to afford the fori matoborate derivative, [TismPr Benz]MgOC(H)OB(C6F5)3 (6),36 as illustrated in Scheme 3, the zinc compound does not form an observable product under the same conditions. The magnesium B

DOI: 10.1021/jacs.7b10776 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society i

room temperature, but requires elevated temperatures.10b,40,46 Thus, iti is evident that the three benzimidazole arms of [TismPr Benz] serve a critical function by providing a cylindrical i cavity about the metal center of {[TismPr Benz]M}+ that is sufficient to inhibit association of the [HB(C6F5)3]− counterion, but still allow access of CO2, thereby enabling activation of the moleculei towards nucleophilic attack. To a certain extent, the {[TismPr Benz]M}[HB(C6F5)3] ion pair may be considered to resemble a frustrated Lewis pair,47 in which an active site is created between a cationic metal center and a reactive anionic component; the analogy, however, requires the Lewis base function to be provided by the pair of electrons in the B−H bond.8d,9b In summary, the terminal zinc and magnesium hydride comi i pounds, [κ3-TismPr Benz]ZnH and [TismPr Benz]MgH, react with i B(C6F5)3 to afford the ion pairs, {[TismPr Benz]M}[HB(C6F5)3], which, in combination with B(C6F5)3, generate catalytic systems that are capable of room temperature hydrosilylation of CO2 by i R3SiH. Significantly, {[TismPr Benz]Mg}[HB(C6F5)3]/B(C6F5)3 provides the first example of a catalytic system for hydrosilylation of CO2 that involves a magnesium compound. Also of note, the selectivity of the systems may be controlled by the silane, such that PhSiH3 forms selectively CH4, while Ph3SiH affords selectively the bis(silyl)acetal, H2C(OSiPh3)2.

compound, [TismPr Benz]MgOC(H)OB(C6F5)3, is also obtained i upon addition of B(C6F5)3 to the formate derivative, [TismPr Benz]Mg(O2CH), and structural characterization by X-ray diffraction demonstrates that the formate moiety bridges the magnesium and boron centers (Figure 2).37,38 With irespect to the potential role of the formatoborate species, [TismPr Benz]MgOC(H)OB(C6F5)3, in the catalytic cycle, a pertinent observation is that it does not alone catalyze the hydrosilylation of CO2 with PhSiHi3. In the presence of additional B(C6F5)3, however, [Tismi Pr Benz]MgOC(H)OB(C6F5)3 and PhSiH3 regenerate {[TismPr Benz]Mg}[HB(C6F5)3] and thereby enable catalytic turnover. Consideration of the literature indicates that additional B(C6F5)3 can have an impact on the catalytic system by two means, namely: (i) B(C6F5)3 activates the silicon of R3SiH towards nucleophilic attack via formation of an incipient adduct R3SiHB(C6F5)3,39 and (ii) B(C6F5)3 removes the formatoborate anion, [HCO2B(C6F5)3]−, in the form of the formato bis(borate) anion, [HC{OB(C6F5)3}2]−.10b,40 The essential features of a possible catalytic cycle for the hydrosilylation of CO2,8c,9a,e,f which incorporate the reactivity described above, are summarized in Scheme 4. The initial steps involve the

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Scheme 4

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b10776.

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Experimental details and computational data (PDF) Crystallographic data (CIF)

AUTHOR INFORMATION

Corresponding Author

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rapid reaction iof [TismPr Benz]MH with B(C6F5)3 to afford the ion pair, {[TismPr Benz]M}[HB(C6F5)3],41 which subsequently reacts i with CO2 to afford the formatoborate complex, [TismPr Benz]MOC(H)OB(C6F5)3; in the presence of B(C6F5)3, the latter may exist in i equilibrium with the formato bis(borate) species, {[TismPr Benz]M}[HC{OB(C6F5)3}2].42 The final sequence of the catalytic cycle involves release of the silylformate, HCO2SiR3, and i regeneration of {[TismPr Benz]M}[HB(C6F5)3] via attack of a formate oxygen atom at the silicon of the activated silane, R3SiHB(C6F5)3.43 The reactive formate moiety could be a PriBenz component of either (i) intact [Tism ]MOC(H)OB(C 6F5)3, i (ii) [TismPr Benz]MO2CH, (iii) the dissociated ion, [HCO2B(C6F5)3]−,44 or (iv) the formato bis(borate) anion, [HC{OB(C6F5)3}2]−;10b however, the data available do not allow these possibilities to be distinguished. Following the initial metal-catalyzed hydrosilylation event to release HCO2SiR3, subsequent reduction to CH4 is proposed to proceed via bis(silyl)acetal and methoxy silane intermediates, namely H2C(OSiR3)2 and CH3OSiR3 (Scheme 1).9a-c,10b In this regard, while incipient R3SiHB(C6F5)3 is not an active catalyst for the initial hydrosilylation of CO2,9a-c the ability of R3SiHB(C6F5)3 to achieve sequential reduction of HCO2SiR3 to CH4 via H2C(OSiR3)2 and CH3OSiR3 is precedented.9a-c,45 The important role of the metal center in facilitating nucleophilic attack upon CO2 in the first step of the conversion is highlighted by the fact that the reaction between the 2,2,6,6tetramethylpiperidine derivative, [TPMH][HB(C6F5)3], and CO2 to form [TPMH][HCO2B(C6F5)3] does not occur at

*[email protected] ORCID

Gerard Parkin: 0000-0003-1925-0547 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the U.S. Department of Energy, Office of Basic Energy Sciences (DE-FG02-93ER14339), for support of this research. M.R. acknowledges the National Science Foundation for a Graduate Research Fellowship under Grant No. DGE-16-44869. Professor Clark Landis is thanked for assistance with NBO calculations, and Dr. Serge Ruccolo is thanked for helpful advice. We dedicate this paper to the memory of Professors Gilbert Stork and Ronald Breslow.

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REFERENCES

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DOI: 10.1021/jacs.7b10776 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

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(25) However, the signal associated with formation of a formate i species upon addition of CO2 to [κ3-TismPr Benz]ZnH does not persist in solution for more than 10 min at room temperature. (26) Terminal magnesium formate compounds are not well known, and we are aware of only one other example: Schnitzler, S.; Spaniol, T. P.; Okuda, J. Inorg. Chem. 2016, 55, 12997. For a bridging formate compound, see ref 16a. (27) Note that abstraction of the hydride ligand is accompanied by a significant reduction in the Mg−Catrane distance. (28) Chakrabarti, N.; Sattler, W.; Parkin, G. Polyhedron 2013, 58, 235. (29) For some examples of trigonal monopyramidal zinc centers, see: (a) Ray, M.; Hammes, B. S.; Yap, G. P. A.; Rheingold, A. L.; Borovik, A. S. Inorg. Chem. 1998, 37, 1527. (b) Blacquiere, J. M.; Pegis, M. L.; Raugei, S.; Kaminsky, W.; Forget, A.; Cook, S. A.; Taguchi, T.; Mayer, J. M. Inorg. Chem. 2014, 53, 9242. (30) Lampland, N. L.; Pindwal, A.; Neal, S. R.; Schlauderaff, S.; Ellern, A.; Sadow, A. D. Chem. Sci. 2015, 6, 6901. (31) Anker, M. D.; Arrowsmith, M.; Arrowsmith, R. L.; Hill, M. S.; Mahon, M. F. Inorg. Chem. 2017, 56, 5976. (32) For other examples of magnesium species with B−H and B−F interactions, see: Alves, L. G.; Martins, A. M.; Duarte, M. T. J. Mol. Struct. 2012, 1026, 168. (33) Müller, T. E.; Mingos, D. M. P. Transition Met. Chem. 1995, 20, 533. (34) Falivene, L.; Credendino, R.; Poater, A.; Petta, A.; Serra, L.; Oliva, R.; Scarano, V.; Cavallo, L. Organometallics 2016, 35, 2286. (35) The different reactivities could reflect the dissimilar oxophilicities of Mg (0.6) and Zn (0.2). See: Kepp, K. P. Inorg. Chem. 2016, 55, 9461. i (36) It is worth noting that [TismPr Benz]MgOC(H)OB(C6F5)3 does 13 not undergo facile exchange with CO2 over a period of 1 day, which i indicates that the reaction of {[TismPr Benz]Mg}[HB(C6F5)3] with CO2 is essentially irreversible under these conditions. (37) Computational studies indicate that the most appropriate Lewis i structure for [TismPr Benz]MgOC(H)OB(C6F5)3 is one in which the oxygen atom that is attached to Mg is associated with the CO double bond. See Supporting Information. (38) For examples of LnMOC(H)OB(C6F5)3 derivatives, see: Kather, R.; Lork, E.; Vogt, M.; Beckmann, J. Z. Anorg. Allg. Chem. 2017, 643, 636. See also refs 3b, 8d, and 9b,e,f. (39) (a) Parks, D. J.; Piers, W. E. J. Am. Chem. Soc. 1996, 118, 9440. (b) Parks, D. J.; Blackwell, J. M.; Piers, W. E. J. Org. Chem. 2000, 65, 3090. (40) Ashley, A. E.; Thompson, A. L.; O’Hare, D. Angew. Chem., Int. Ed. 2009, 48, 9839. i (41) Indeed, the methyl compound, [TismPr Benz]MgMe, also generates i {[TismPr Benz]Mg}[HB(C6F5)3] in the presence of R3SiH and B(C6F5)3, i thereby providing a means for [TismPr Benz]MgMe to serve as a precatalyst. For a related example, see ref 30. (42) Species with NMR spectroscopic features (ref 10b) consistent with i {[TismPr Benz]M}[HC{OB(C6F5)3}2] have been observed for both the magnesium and zinc systems. (43) It is worth noting that other systems (for example, refs 9b,d,e) can operate with a 1:1 metal:B(C6F5)3 ratio, which suggests that the formatoborate, LnMOC(H)OB(C6F5)3, is in equilibrium with a mechanistically significant concentration of the formate LnMO2CH and i B(C6F5)3. The necessity for additional B(C6F5)3 for the [TismPr Benz]MOC(H)OB(C6F5)3 system indicates that the B(C6F5)3 coordinates strongly to the formate ligand. (44) For a structurally characterized example of the anion ref 40. [HCO2B(C6F5)3]− that is not coordinated to a metal center, see i (45) Additional pathways involving participation of {[TismPr Benz]M}+ cannot be discounted. (46) Other studies have also shown that insertion into the B−H bond of [HB(C6F5)3]− is more difficult than into that of [HBPh3]− due to a lower hydricity. See ref 3b and the following: (a) Mukherjee, D.; Osseili, H.; Spaniol, T. P.; Okuda, J. J. Am. Chem. Soc. 2016, 138, 10790. (b) Mukherjee, D.; Wiegand, A. K.; Spaniol, T. P.; Okuda, J. Dalton Trans. 2017, 46, 6183. (c) Heiden, Z. M.; Lathem, A. P. Organometallics 2015, 34, 1818. (47) (a) Stephan, D. W. Science 2016, 354, aaf7229. (b) Flynn, S. R.; Wass, D. F. ACS Catal. 2013, 3, 2574. D

DOI: 10.1021/jacs.7b10776 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX