Reactions of manganese silyl dihydride complexes with CO2

Under mild conditions (room temperature, 1.1 atm. of CO2), the silyl dihydride complexes [(dmpe)2MnH2(SiHR2)] {R = Ph (1Ph2); Et (1Et2)} reacted rapidly and quantitatively with carbon dioxide to afford [(dmpe)2MnH2{Si(x1-O2CH)R2)}] {R = Ph (2Ph2); Et (2Et2)}; the products of apparent CO2 insertion into the terminal Si-H bond. In addition, room temperature reactions of [(dmpe)2MnH2(SiH2R)] {R = Ph (1Ph); nBu (1Bu)} with CO2 (1.1 atm.) yielded [(dmpe)2MnH2{Si(x1-O2CH)2R)}] {R = Ph (3Ph); nBu (3Bu)} containing two formate substituents on silicon. The latter reactions proceeded in a stepwise fashion, rapidly forming [(dmpe)2MnH2{SiH(x1-O2CH)R}] {R = Ph (4Ph); nBu (4Bu)} intermediates, which then slowly converted into 3Ph and 3Bu. During the syntheses of 3Ph and 3Bu, significant amounts of H2 and previously reported [(dmpe)2Mn (CO)(x1-O2CH)] (5) were also formed. The reaction of [(dmpe)2MnD2(SiH2nBu)] (d2-1Bu) with CO2 was carried out, yielding d2-3Bu as the major reaction product, predominantly (>95%) as [(dmpe)2MnD2{Si(x1-O2CH)2nBu}] featuring two deuteride ligands. DFT calculations to probe the relative energies of silicate [(dmpe)2Mn(r/3H2SiR3)], trans-hydrosilane/hydride (trans-[(dmpe)2MnH(H-SiR3)]), trans-dihydrogen/silyl (trans-[(dmpe)2Mn (H2)(SiR3)]), and lateral-dihydrogen/silyl (cis-[(dmpe)2Mn(H2)(SiR3)]) isomers of 2Ph2, 3Ph, and 4Ph are also reported; the lowest energy structures of 2Ph2 and 3Ph are those of the silicate isomers, consistent with the NMR spectra obtained for 2R2 and 3R. Also, compound 2Ph2 was isolated, and crystallized as the silicate isomer; the solid state structure of 2Ph2 is qualitatively analogous to that of 1Ph2, but the Mn-Si bond in 2Ph2 is significantly shorter {2.2876(7) angstrom vs 2.3176(3) angstrom}.