Analogies between Metallaboratranes, Triboronates, and Boron Pincer Ligand Complexes

The reaction of [RuRCl(CO)(PPh3)2] (R = C6H5, CHCHPh) with the Yamashita−Nozaki ligand C6H4{N(CH2PPh2)}2BH (dppBH) results in facile elimination of benzene or styrene and formation of the ruthenium [PBP]− pincer complex [Ru{κ3B,P,P′-dppB)Cl(CO)(PPh3)] via a stepwise sequence that recalls the formation of the ruthenaboratrane [Ru(CO)(PPh3){B(mt)3}] (mt = methimazolyl) from [RuRCl(CO)(PPh3)2] and Na[HB(mt)3]. This analogy is explored herein, including observations regarding notionally similar processes for triboronate ligands. Thus, while the major product of the reaction of [Ru(C6H5)Cl(CO)(PPh3)2] with [nBu4N][B3H8] is the phenylruthenatetraborane [B3H8Ru(C6H5)(CO)(PPh3)2], the hydrido complex [B3H8RuH(CO)(PPh3)2] is also obtained, consistent with alcoholic transfer hydrogenation of the putative complex [B3H7Ru(CO)(PPh3)2].