Sequential Migrations between Boron and Rhodium Centres: A Cooperative Process between Rhodium and a Mono-Substituted Borohydride Unit

The sodium salt of a mono-substituted borohydride anion containing a 2-mercaptopyridyl unit (mp) is reported herein. This compound has been coordinated to a rhodium(I) center providing the complex, [Rh{κ3-H,H,S-H3B(mp)}(NBD)] (1) (where NBD = 2,5-norbornadiene) in which the boron based ligand is coordinated to the rhodium center via the thione donor and two of the B‒H bonds of the BH3 unit. Reaction of complex 1 with carbon monoxide results in the activation of the complex leading to the product of a formal intramolecular hydroboration reaction where the NBD unit has, in effect, inserted into one of the B‒H bonds. Three complexes have been prepared in which the newly formed norbornenyl unit (nbe) is located at the boron center, [Rh{κ3-H,H,S-H2B(nbe)(mp)}(CO)2] (2), [Rh{κ3-H,H,S-H2B(nbe)(mp)}(CO)(PCy3)] (3) and [Rh{κ3-H,H,S-H2B(nbe)(mp)}(CO)(PPh3)] (4). The identities of the three complexes have been confirmed by spectroscopic and analytical techniques. Further confirmation has been obtained via structural characterization of 3. Studies have confirmed that the reactivity occurs at the metal center. A metal-ligand cooperative mechanism, involving initial migration of hydride from boron to metal center, has been postulated for the formation of the new complexes based on previous investigations. The newly formed norbornenyl unit then migrates from metal center to boron.