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)(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 [D 1 ]formic acid. The addition of HBF 4 to 3 leads to rapid net protonolysis of the RhC bond to produce [Rh I (CO)(κ 3 ‐ P , N ,( C H )‐ 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)(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 (CH 3 )(CO)(I)(κ 3 ‐ P , N , C ‐ 1 )] prior to CC 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.