σ-borane and dihydroborate complexes of ruthenium

Room temperature reaction of the bis(dihydrogen) complex RuH2(H2)2(PCy3)2 (1) with excess pinacol borane (HBpin) generates the novel complex RuH[(μ-H)2Bpin](σ-HBpin)(PCy3)2 (2) by loss of dihydrogen. Complex 2 was characterized spectroscopically and by X-ray crystallography. It contains two pinacolborane moieties coordinated in a different fashion, one as a dihydroborate (B-H distances : 1.58(3) and 1.47(3) Å) and the other as a σ-borane (B-H distance: 1.35(3) Å). In addition, reaction of 1 with one equiv of HBpin yields total conversion to a new complex tentatively formulated as RuH[(μ-H)2Bpin](H2)(PCy3)2 (3) on the basis of NMR data. In the presence of excess HBpin, 3 is converted to 2. Furthermore, under an atmosphere of dihydrogen, a C7D8 solution of 2 rapidly converts to 3 and finally regenerates 1 over a much longer period. Thus, complex 3 is an intermediate in the formation of 2 from 1. In these processes the borane is eliminated as HBpin later hydrolyzed to BpinOBpin. Selective hydroboration of ethylene (3 bar) into C2H5Bpin is achieved using 1 or 2 as catalyst precursors in toluene, whereas in THF, competitive formation of the vinylborane C2H3Bpin (56% under 20 bar of C2H4) can be favored. Copyright © 2002 American Chemical Society.