An examination of C-H bond activation by cationic TpMe2Ir(III) complexes

Synthesis and characterization of a set of iridium(III) compounds utilizing the sterically encumbering hydridotris(3,5-dimethylpyrazolyl)borate ligand (Tp(Me2)) has been performed, and a comparison with the corresponding Cp* complexes is presented. Tp(Me2)(PMe3)Ir(Me)-OTf (7, OTf = O3SCF3) was synthesized and found to be unreactive toward a variety of C-H bonds, in contrast to the behavior of Cp*(PMe3)Ir(Me)OTf (1). We have rationalized this lack of reactivity in terms of an electronic effect rather than a steric effect imposed by the Tp(Me2) ligand. Metathesis of the triflate ligand with the BArf (BArf = B[3,5-C6H3(CF3)(2)](4)) anion under a N-2 atmosphere produces the dinitrogen complex [Tp(Me2)(PMe3)Ir(N-2)(Me)][BArf] (3-N-2). Compound 3-N-2 is indeed reactive toward dative ligands, H-2, and hydrocarbons, activating the C-H bonds of benzene and aldehydes but not those of saturated hydrocarbons. A rationale for the difference in behavior between the Cp* and Tp(Me2) systems with respect to C-H activation is presented.