Metal- and Ligand-Centered Reactivity of meta-Carboranyl-Backbone Pincer Complexes of Rhodium

We report the synthesis of the chelating phosphinite-arm carboranyl POBOP-H (POBOP = 1,7-OP­(i-Pr)2-m-carboranyl) ligand precursor, preparation of its rhodium complexes, and their reactivity in oxidative addition/reductive elimination reactions. The oxidative addition of iodobenzene to the low-valent (POBOP)­Rh­(PPh3) resulted in the selective formation of the 16-electron complex (POBOP)­Rh­(Ph)­(I), featuring a highly strained exohedral rhodium–boron bond. The complex (POBOP)­Rh­(Ph)­(I) is the first example of a B-carboranyl aryl metal complex, which is a proposed intermediate in metal-promoted B–C coupling reactions. The complex (POBOP)­Rh­(Ph)­(I) was selectively and directly converted, in the presence of acetonitrile, to (POB­(BPh)­OP)­Rh­(H)­(I)­(CH3CN) (POB­(BPh)­OP = 1,7-OP­(i-Pr)2-2-Ph-m-carboranyl) through unprecedented cascade reductive elimination of the phenyl-B-carboranyl and the oxidative addition of a vicinal B–H bond of the boron cluster to the metal center, exhibiting both metal- and cluster-centered reactivity.