Transformation of a norbornadiene unit to ethylenylcyclopentene requiring cooperation between boron and rhodium centers

The synthesis of the lithium salt of a bis-substituted borohydride anion containing a phenyl substituent and a 2-mercaptopyridyl unit (mp) is reported herein. This salt has been used as a pro-ligand for the synthesis of rhodium(I) complex, [Rh{κ 3-H,H,S-H 2B(Ph)(mp)}(NBD)] (1) (where NBD = 2,5-norbornadiene). The new boron-based ligand coordinates to the rhodium center via the thione donor and the two B-H bonds of the BPhH 2 unit, with a dihydroborate motif. Reaction of complex 1 with two equivalents of triphenylphosphine leads to an unprecedented rearrangement and transfer of the former norbornadiene ligand to the boron center. The transformation occurs via an initial hydride migration from boron to rhodium center followed by a hydroboration of one of the double bonds. Finally, a ring-opening process occurs involving both boron and rhodium centers leading to an unusual boron-bound ethylenylcyclopentene unit. The product of this reaction was confirmed as [Rh{η 1-S,η 2-B,C-B(Ph)(CHCH 2(C 5H 7)(mp)}(PPh 3) 2] (2). The net result of these transformations is the incorporation of the two hydrogen atoms from the secondary borohydride ligand [BPhH 2(mp)] - into the former norbornadiene unit. The end point positions of these hydrogen atoms were confirmed by deuterium labeling experiments. Complex 2 was further reacted with carbon monoxide to generate [Rh{η 1-S,η 2-B,C-B(Ph)(CHCH 2(C 5H 7)(mp)}(CO)(PPh 3)] (3) via ligand substitution. The new ligand and the three complexes, 1, 2, and 3, have been characterized by spectroscopic techniques as well as by X-ray crystallography. Detailed characterization of 2 and 3 revealed an unusual η 2-B,C coordination mode within these complexes. Further studies have demonstrated that complexes 2 and 3 react with hydrogen gas (or dimethylamine borane as a source of H 2) to generate the hydrogen addition products involving the unprecedented activation of the Rh-η 2-B,C motif. Complexes 2 and 3 were further found to be active catalysts for the hydrogenation of olefins and the dehydrogenation of dimethylamine borane.