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Establishing the Hydride Donor Abilities of Main Group Hydrides Zachariah M

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Establishing the Hydride Donor Abilities of Main Group Hydrides Zachariah M Article pubs.acs.org/Organometallics Establishing the Hydride Donor Abilities of Main Group Hydrides Zachariah M. Heiden* and A. Paige Lathem Department of Chemistry, Washington State University, Pullman, Washington 99164, United States *S Supporting Information ABSTRACT: Interest in reductions with main group hydrides has been reinvigorated with the discovery of frustrated Lewis pairs. Computational analysis showed that the borohydride of the commonly used Lewis acid B(C6F5)3 was determined to be 15 kcal/mol less reducing than borohydride ([BH ]−), 22 4 − kcal/mol less reducing than aluminum hydride ([AlH4] ), and 41 kcal/mol less reducing than superhydride ([HBEt ]−). In − 3 addition to [HB(C6F5)3] , a hydride donor ability scale with an estimated error of ∼3 kcal/mol includes 132 main group hydrides with gradually changing reducing capabilities spanning 160 kcal/mol. The scale includes representatives from organosilanes, organogermanes, organostannanes, borohydrides, boranes, aluminum hydrides, NADH analogues, and CH hydride donors. The large variety of reducing agents and the wide span of the scale (ranging from 0.5 to 160 kcal/mol in acetonitrile) make the scale a useful tool for the future design of metal-based or main group reducing agents. ■ INTRODUCTION noncoordinating base.10 DuBois and co-workers have worked fi Sodium borohydride has become the most important hydride- extensively in this eld, employing the hydride donor ability of metal hydride complexes to better understand the activation of based reductant used on the industrial scale, with a market 11 1 dihydrogen with metals. Using the pK for the deprotonation share exceeding 50%. Some of the key drivers governing its a of a metal hydride (eq 1), the two-electron oxidation of a metal interest are that it is the least expensive commercially available complex (eq 2), and the reduction of a proton to a hydride (eq metal hydride (on a hydride equivalent basis), it is safe with 3), the hydride donor ability of a metal hydride can be regard to storage and use, the industrial implementation estimated (eq 4).11 requires no or limited equipment investment, the ease of workup (boron salts), solvents such as water and methanol can ++ LnnMH→+ L M HGK °= 1.37(p a ) (1) be employed, and both chemo- and diastereoselectivity can be achieved.1 Although [BH ]− is the reducing agent of choice 4 L M→+ L M2+− 2eGE °=° 46.1[ (II/0)] from an industrial standpoint, several other main group nn (2) hydrides have been widely used in the reduction of organic +− − +− 2 − H+→ 2e HGE °=−° 46.1[ (H /H )] (3) substrates, as [BH4] is not the ideal reducing agent for every chemical reaction. Main group hydrides are primarily used as sum: stoichiometric reagents in both industrial and academic applications, but with the advent of frustrated Lewis pairs L MH++−→+ L M2 H (FLPs)3 the possibility of catalytic main group reducing agents nn has been realized. The emphasis on chemical reactions avoiding GKE°=1.37(pa ) + 46.1[ ° (II/0)] + 79.6 (4) precious metals has become a concern of the pharmaceutical − industry due to increased restrictions on trace-metal Utilizing eqs 1 4, DuBois and co-workers have been able to impurities.4 The discovery of catalytic main group reducing show metal hydrides with hydride donor abilities ranging from agents opens up the possibility of their use in reductions on the 26 to 89 kcal/mol (see Supporting Information, Table S4). industrial scale. Chase and co-workers have recently been able Although, these values are helpful in the determination of H2 to show that the use of a chemical scavenger promotes the activation, they provide little insight into the capability of these catalytic hydrogenation of imines using a sterically bulky borane metalhydridecomplexesasreducingagentsinorganic at catalyst loadings as low as 0.5 mol %.5 Although the use of reactions. This discrepancy becomes problematic, as evaluating main group Lewis acids to promote the catalytic reduction of the reducing capacity of metal hydrides versus main group organic substrates seems promising, their reducing capacity for hydrides, which are widely used in organic and industrial organic substrates has not been quantified beyond the reductions, would greatly aid in the design of metal-based and − exploration of substrate scope.6 9 metal-free reduction catalysts. Interest in the quantification of hydride transfer from elemental hydrides has originated through the investigation of Received: November 14, 2014 the heterolytic activation of H2 with a metal complex and a Published: May 6, 2015 © 2015 American Chemical Society 1818 DOI: 10.1021/om5011512 Organometallics 2015, 34, 1818−1827 Organometallics Article The use of the thermodynamic cycle described in eqs 1−4 Geometry optimizations were undertaken using the M06-2X/6- can be very helpful in describing the hydride donor abilty of 31G(d,p) level of theory in the gas phase.27 The M06-2X transition metal complexes, but becomes more problematic in functional has been shown to be one of the more accurate DFT the analysis of main group hydrides. A problem that arises in functionals for main group and FLP chemistry.17,20,28,29 More attempting to use eqs 1−4 for main group hydrides is that most accurate energies were obtained using single-point energy main group hydrides cannot be deprotonated in the presence of calculations at the M06-2X/6-311G++(d,p) level of theory in a base. Also, electrochemical analysis of the parent Lewis acids MeCN, employing the optimized molecular geometries at the − often results only in a one-electron reduction wave,12 14 which M06-2X/6-31G(d,p) level of theory. Acetonitrile was chosen as does not make eq 2 valid for main group hydrides. To remedy the solvent for computational analysis to evaluate the calculated the inability to use a thermodynamic cycle for the hydride donor abilities versus values previously reported for determination of the hydride donor ability of main group metal complexes (see Supporting Information). Although the hydrides, one could examine the equilibrium constant between coordination of acetonitrile to the resulting boranes was a metal complex with a known hydride donor ability and a main considered, CD3CN solutions of HSiEt3 and [lutidinium][HB- group Lewis acid. This technique has been employed in the (C6F5)3] showed no hydride loss over the course of a week and analysis of the hydride donor ability of superhydride were assumed to have no effect on the hydride donor ability of − ([HBEt3] ), where a solution of BEt3 was titrated with the examined main group hydride. A similar assumption is used HRh(dmpe)2, resulting in an estimated hydride donor ability in the determination of the hydride donor ability of metal − 15,16 30−32 of 26 kcal/mol for [HBEt3] . This technique can be hydride complexes. The PCM-UFF solvation model problematic with main group hydrides, as the hydride donor (default for Gaussian 09) was used.27 The IEFPCM-UA0 ability of metal complexes tends to be measured in polar solvation model has been previously employed in the solvents, which are not compatible with highly Lewis acidic determination of hydride donor abilities of transition metal − 30,32 main group complexes. To date, [HBEt3] is the only main complexes, but the PCM-UFF solvation model was found group hydride species where the hydride donor ability has been to give slightly better results in the calculated hydride donor − Δ − 16 experimentally determined. Equation 4 shows the hydride ability of [HBEt3] ( GH = 26 kcal/mol, experimentally); donor ability of metal hydrides; in a similar fashion, eq 5 can be see below. applied to determine the hydride donor ability of main group Common Main Group Hydrides in Organic Reduc- Δ hydrides ( GH−). tions. The most common reducing agents employed in −− industrial scale organic synthesis are NaBH4,NaBH3CN, [Lewis Acid H]→+Δ Lewis Acid H GH− (5) 2 LiAlH4, and NaHBEt3 (superhydride). From a reactivity To investigate the hydride donor ability of main group metal standpoint, superhydride is considered a stronger reducing hydrides and obtain reasonable values to be utilized in agent than lithium aluminum hydride, and lithium aluminum experimental design, computational methods were employed hydride is a stronger reducing agent than borohydride,2,33,34 but with the advantage of avoiding experimental dilemmas as to date, measurement of the reducing power of these main described above. Papaí and co-workers have previously group hydrides has been qualitative as opposed to quantitative. computed the ability of boranes to accept a hydride to aid in In this study, we aimed to quantify the reducing power of 17,18 the understanding of H2 activation with FLPs, but this main group hydrides through computational analysis. To study was limited to the analysis of only 12 boron-based Lewis determine the validity of our computational values, we initially − acids. Krossing and co-workers have very recently investigated determined the hydride donor ability for [HBEt3] , which was the effect of alkoxy substitution on a boron-based Lewis acid computed to be 24 kcal/mol. The computed value of 24 kcal/ both experimentally and computationally, in addition to mol agrees fairly well with the experimentally determined value computing the gas phase hydride, fluoride, chloride, and of 26 kcal/mol,16 which is estimated to have an experimental methyl ion affinity of 34 other main group compounds using error of ±2 kcal/mol. high-level calculations.19 Gilbert (computationally)20 and Piers To further validate the computational model, we compared a (experimentally)21 have probed the effect of fluorine computational versus experimental relative Lewis acidity of a fl substitution on the Lewis acidity of uorinated boranes, but borane of interest versus B(C6F5)3. The ratio of the computed generated contradicting conclusions. To experimentally quanti- hydride donor abilities of the borohydride of interest to − fy the Lewis acidity of newly synthesized Lewis acids, the [HB(C6F5)3] was determined to yield a computational relative Gutmann−Beckett or Childs’ Lewis acidity tests are − Lewis acidity to B(C6F5)3 (see Supporting Information, Table utilized.22 25 Although these tests can provide insight into S2).
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