Changing the Charge: Electrostatic Effects in Pd-Catalyzed Cross-Coupling

A stable dianionic 14-electron Pd(0) complex supported by monoanionic carboranyl phosphines is reported. This complex rapidly undergoes the oxidative addition of Cl-C6H5 at room temperature and is a competent catalyst for Kumada cross-coupling. The isosteric PdL2 complex, supported by neutral o-carboranyl phosphines, does not display the same reactivity. The high reactivity of the dianionic Pd(0) complex toward chloroarenes can be explained by electrostatic effects that promote both formation of monophosphine-ligated LPd0 and stabilization of the transition state during oxidative addition. This mode of stabilization is distinct from the well-known π-arene interactions of biaryl phosphines, in that it occurs both on and off cycle.