Water Reduction and Dihydrogen Addition in Aqueous Conditions With ansa-Phosphinoborane |
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| Authors: | Dr Kristina Sorochkina Dr Konstantin Chernichenko Prof?Dr Vladimir V Zhivonitko Dr Martin Nieger Prof?Dr Timo Repo |
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| Institution: | 1. Department of Chemistry, University of Helsinki, A. I. Virtasen aukio 1, 00014 Helsinki, Finland;2. NMR Research Unit, University of Oulu, P.O. Box 3000, 90014 Oulu, Finland |
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| Abstract: | Ortho-phenylene-bridged phosphinoborane (2,6-Cl2Ph)2B-C6H4-PCy2 1 was synthesized in three steps from commercially available starting materials. 1 reacts with H2 or H2O under mild conditions to form corresponding zwitterionic phosphonium borates 1-H2 or 1-H2O . NMR studies revealed both reactions to be remarkably reversible. Thus, when exposed to H2, 1-H2O partially converts to 1-H2 even in the presence of multiple equivalents of water in the solution. The addition of parahydrogen to 1 leads to nuclear spin hyperpolarization both in dry and hydrous solvents, confirming the dissociation of 1-H2O to free 1 . These observations were supported by computational studies indicating that the formation of 1-H2 and 1-H2O from 1 are thermodynamically favored. Unexpectedly, 1-H2O can release molecular hydrogen to form phosphine oxide 1-O . Kinetic, mechanistic, and computational (DFT) studies were used to elucidate the unique “umpolung” water reduction mechanism. |
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| Keywords: | DFT frustrated Lewis pairs hydrogen activation mechanism umpolung of proton water reduction |
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