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 C-H activation occurs in the presence of a (internal) base, thus resulting in the new cyclometalated complex [Rh(I) (CO)(κ(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 [D1 ]formic acid. The addition of HBF4 to 3 leads to rapid net protonolysis of the Rh-C bond to produce [Rh(I) (CO)(κ(3) -P,N,(C-H)-1)] (4). This species features a rare aryl C-H 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)(κ(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)(κ(3) -P,N,C-1)] prior to C-C reductive elimination. The Rh(III) -trans-diiodide complex [Rh(I) (CO)(I)2 (κ(3) -P,N,C-1)] (6) has been structurally characterized as a model compound for this elusive intermediate.