Heterobimetallic H₂ Addition and Alkene/Alkane Elimination Reactions Related to the Mechanism of E-Selective Alkyne Semihydrogenation

Abstract

Mechanistic aspects of an E-selective alkyne semihydrogenation catalyst are studied through computational modeling and experimental model reactions. We previously communicated the semihydrogenation of diarylalkynes to produce trans-alkenes using the heterobimetallic catalyst (IMes)-AgRuCp(CO) 2 . In this report, we disclose further details on the catalyst decomposition products under catalytic conditions, the mechanism of bimetallic H 2 activation, and the factors affecting selectivity for E-alkene generation. Under hydrogenation conditions, the catalyst decomposition product HRuCp(CO)(IMes) was isolated and characterized. Resubmitting this species to the catalytic conditions did not provide useful hydrogenation catalysis, confirming the presence of a bimetallic mechanism under optimal catalytic conditions. The detailed nature of heterobimetallic H 2 activation was probed by calculating internuclear bond orders, atom/fragment charges, and NBO occupancies as functions of reaction coordinate for a model reaction between (IMe)CuRp and H 2 . The collected results indicate a late transition state involving deprotonation of a Cu(H 2 ) σ-complex by the proximal Rp fragment. Late stages of the reaction profile feature H···H dihydrogen bonding between (IMe)CuH and HRuCp(CO) 2 , indicative of heterolytic H 2 activation. NBO analysis indicates that the key orbital interactions involved in H 2 activation are (a) donation from the filled H 2 σ-orbital into a Cu 4p acceptor orbital and (b) back-donation from a filled Cu−Ru bonding orbital of predominantly Ru 4d character into the empty H 2 σ*-orbital. Experimental support for the previously proposed cascade alkyne → Z-alkene → E-alkene process was provided by stoichiometric reactions between HRuCp(CO) 2 and isolable (IPr)CuR models of catalytic (IMes)AgR intermediates (R = alkenyl, alkyl). The collected experimental results indicate that selectivity for E-alkene generation is dictated by the relative rates of monometallic β-hydride elimination and bimetallic alkane elimination, which are impacted by several structural features of the catalyst. The mechanistic detail provided by these studies will inform the development of second-generation hydrogenation catalysts.

Bimetallic catalysis for C–C and C–X coupling reactions

“…Upon C–H borylation, the Cu/Fe catalyst is regenerated through a binuclear H 2 elimination that proceeds through a highly polar transition state. 102 The bifunctional nature of this series of catalysts is actively being pursued for a variety of other catalytic applications. 21 , 30 …”

Catalytic Reactions by Heterobimetallic Carbonyl Complexes with Polar Metal–Metal Interactions

“…The [Ag-Ru] catalyst was found to exhibit an unusual inverse H2/D2 isotope effect, which was consistent with computational modeling of the proposed mechanism. 22 Crucially several Ag-only and Ru-only complexes were shown to have little or no catalytic activity as well as poor E/Z selectivity under these conditions, 16 highlighting the importance of the heterobinuclear design. In 2019, Norton and co-workers developed the cooperative heterobimetallic Ti/Cr-catalyzed anti-Markovnikov hydrogenation of epoxides.…”

“…CO2, R-X) 10,15 or heterolysis of apolar substrates (e.g. H2) 16 has served to validate the presence of polar metal-metal interactions containing Lewis acidic and Lewis basic fragments. 7 The availability of these cooperative bond activation processes opens opportunities for catalytic reaction discoveries.…”

Mechanistic investigations via DFT support the cooperative heterobimetallic C–H and O–H bond activation across TaIr multiple bonds

“…This can be explained, in part, by the common use of sterically encumbering polyfunctional chelating ligands to drive the assembly of the two metals, which restrains the accessibility of both metal cores, therefore limiting their interest for catalysis. Still, rare examples of heterobimetallic oxidative additions (HBOA) of H-H, [10][11][12][13][14][15] C-X (X = halide, 16,17 H, [18][19][20][21][22] OH, [23][24][25][26] NHR, [23][24][25] ), Si-H 24 or B-H 27 bonds across metal-metal' bonds have emerged in the literature (see Figure 1). These pioneer studies highlight the potential of polarized heterobimetallic metalmetal bonds to promote the cooperative cleavage of strong sigma bonds.…”