Sequential Migrations between Boron and Rhodium Centers: A Cooperative Process between Rhodium and a Monosubstituted Borohydride Unit.

The sodium salt of a monosubstituted borohydride anion containing a 2-mercaptopyridyl unit (mp) is reported herein. This compound was coordinated to a rhodium(I) center providing the complex [Rh{κ3 -H,H,S-H3 B(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 were prepared in which the newly formed norbornenyl unit (nbe) is located at the boron center, namely, [Rh{κ3 -H,H,S-H2 B(nbe)(mp)}(CO)2 ] (2), [Rh{κ3 -H,H,S-H2 B(nbe)(mp)}(CO)(PCy3 )] (3), and [Rh{κ3 -H,H,S-H2 B(nbe)(mp)}(CO)(PPh3 )] (4). The identities of the three complexes were confirmed by spectroscopic and analytical techniques. Further confirmation was obtained via structural characterization of 3. Studies confirmed that the reactivity occurs at the metal center. A metal-ligand cooperative mechanism, involving initial migration of hydride from boron to metal center, was postulated for the formation of the new complexes based on previous investigations. The newly formed norbornenyl unit then migrates from metal center to boron.