Regioselective B-Cyclometalation of a Bulky o-Carboranyl Phosphine and the Unexpected Formation of a Dirhodium(II) Complex

The bulky carboranyl monophosphine closo-1,2-B10H10C(H)C((PBu2)-Bu-t) (L) has been prepared in a one-pot procedure from o-carborane. The reaction of [PdCl2(NCPh)(2)] with L rapidly gave the binuclear B-cyclopalladate [Pd-2(mu-Cl)(2)(kappa(2)-L')(2)] (L' = L deprotonated at B3) as a mixture of two diastereoisomers, assigned structures 1a and 1a'. The Cl bridges of 1a/1a' are cleaved by the addition of PEt3 to give the mononuclear [PdCl(kappa(2)-L')(PEt3)] (2) as a single isomer, with the P atoms mutually trans. The metalation occurs at boron positions 3 and 6 in the carborane cluster, and DFT calculations show that the 3,6-borometalate is lower in energy than the isomeric 4,5-borometalate and 2-carbometalate. Treatment of [Rh2Cl2(CO)(4)] with L led to the slow precipitation of the dirhodium(II) species [Rh-2(mu-Cl)(2)(CO)(2)(kappa(2)-L')(2)] (3). The crystal structures of ligand L and complexes 1a, 2, and 3 have been determined.