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Manganese Silyl Dihydride Complexes: A Spectroscopic, Crystallographic, and Computational Study of Nonclassical Silicate and Hydrosilane Hydride Isomers

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posted on 24.05.2019, 17:03 by Jeffrey S. Price, David J. H. Emslie, Bob Berno
Manganese silyl dihydride complexes [(dmpe)2MnH2(SiHR2)] {R = Ph (3a), R = Et (3b)} and [(dmpe)2MnH2(SiH2R)] {R = Ph (4a), R = nBu (4b)} were generated by exposure of silylene hydride complexes [(dmpe)2MnH­(=SiR2)] (1a: R = Ph, 1b: R = Et) and disilyl hydride complexes [(dmpe)2MnH­(SiH2R)2] (2a: R = Ph, 2b: R = nBu), respectively, to H2 at room temperature. In solution, 3a,b and 4a,b exist as an equilibrium mixture of a central isomer with a meridional H–Si–H arrangement of the silyl and hydride ligands {this isomer may be considered to contain an η3-coordinated silicate (H2SiR3) anion}, and a transHSi isomer with trans-disposed hydride and nonclassical hydrosilane ligands (the latter is the result of significant but incomplete hydrosilane oxidative addition). Additionally, DFT calculations indicate the thermodynamic accessibility of lateralH2 and transH2 isomers with cis- and trans-disposed silyl and dihydrogen ligands, respectively. Compounds 3a and 4a crystallized as the central isomer, whereas 4b crystallized as the transHSi isomer. Bonding in the central and transHSi isomers of 3a,b and 4a,b was further investigated through 29Si_edited 1H–1H COSY solution NMR experiments to determine both the sign and magnitude of J29Si,1H coupling (negative and positive values of J29Si,1H are indicative of dominant 1- and 2-bond coupling, respectively). These experiments afforded J29Si,1H coupling constants of −47 Hz for η3-(H2SiR3) in the central isomer of 3b (calcd −40 to −47 for 3a,b and 4a,b), −38 to −54 Hz for η2-(R3Si–H) in the transHSi isomer of 3a,b and 4a,b (calcd −26 to −47 Hz), and 5 to 9 Hz for the terminal manganese hydride ligand in the transHSi isomer of 3b and 4a,b (calcd 12–14 Hz for 3a,b and 4a,b), experimentally supporting the nonclassical nature of bonding in the central and transHSi isomers.