Facile Synthesis and Versatile Reactivity of an Unusual Cyclometalated Rhodium(I) Pincer Complex

The synthesis of the reactive PN(C-H) ligand 2-di(tert-butylphosphanomethyl)-6-phenylpyridine (1(H)) and its versatile coordination to a Rh-I center is described. Facile CH activation occurs in the presence of a (internal) base, thus resulting in the new cyclometalated complex [Rh-I(CO)((3)-P,N,C-1)] (3), which has been structurally characterized. The resulting tridentate ligand framework was experimentally and computationally shown to display dual-site proton-responsive reactivity, including reversible cyclometalation. This feature was probed by selective H/D exchange with [D-1]formic acid. The addition of HBF4 to 3 leads to rapid net protonolysis of the RhC bond to produce [Rh-I(CO)((3)-P,N,(CH)-1)] (4). This species features a rare aryl CH agostic interaction in the solid state, as shown by X-ray diffraction studies. The nature of this interaction was also studied computationally. Reaction of 3 with methyl iodide results in rapid and selective ortho-methylation of the phenyl ring, thus generating [Rh-I(CO)((2)-P,N-1(Me))] (5). Variable-temperature NMR spectroscopy indicates the involvement of a Rh-III intermediate through formal oxidative addition to give trans-[Rh-III(CH3)(CO)(I)((3)-P,N,C-1)] prior to CC reductive elimination. The Rh-III-trans-diiodide complex [Rh-I(CO)(I)(2)((3)-P,N,C-1)] (6) has been structurally characterized as a model compound for this elusive intermediate.