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)(κ³-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<inf>1</inf>]formic acid. The addition of HBF<inf>4</inf> to 3 leads to rapid net protonolysis of the Rh-C bond to produce [Rh^I(CO)(κ³-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)(κ²-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(CH<inf>3</inf>)(CO)(I)(κ³-P,N,C-1)] prior to C-C reductive elimination. The Rh^III-trans-diiodide complex [Rh^I(CO)(I)<inf>2</inf>(κ³-P,N,C-1)] (6) has been structurally characterized as a model compound for this elusive intermediate.