Theoretical study of oxidative addition to platinum metal complexes: VI. Complex formation of Rh(I), Pd(II), Ir(I), and Pt(II) with tridentate N- and P-donor fac-chelating ligands as a tool for controlling activity of metal complexes in oxidative methane addition

Abstract

An approach was formulated to designing tridentate chelating ligands L ensuring stabilization of high-valence platinum metal hydridoalkyl complexes [(Me)M(L)(H)Cl] (M = Rh, Pd+, Ir, and Pt+) and thermodynamically more favorable oxidative methane addition to the corresponding low-valence complexes [M(L)Cl]. The approach is based on diminishing strain in ligand framework and in chelate rings formed by the ligand, on using strong-field ligands, and also on minimizing deformation of metal coordination entity in the course of the reaction with methane. Within the framework of a nonempirical molecular-orbital method full geometry optimization was performed for the [M(L)Cl] and [(Me)M(L)(H)Cl] species containing eight N- and P-donor ligands L fitting to a greater or lesser extent the above-mentioned requirements: tris(iminomethyl)-methane, cyclopropane-1,2,3-triamine, 1,3,5-triazacyclohexane, cyclohexane-1,3,5-triamine, 1,4,7-triazacyclonona-2,5,8-triene, cyclonona-1,4,7-triene-3,6,9-triamine, tris(2-phosphavinyl)methane, and cyclonona-1,4,7-triene-3,6,9-triphosphine. By calculations at the MP2 and B3LYP levels with allowance for electron correlation it was found that the energies of the reactions of the [M(L)Cl] complexes with methane vary by almost 38 kcal/mol (for example, from -19 to +19 kcal/mol for M = Pd+). A relationship between the deformation of chelate rings, the dentacity of ligand L in the [M(L)Cl] species, and the efficiency of the ligand in stabilization of high-valence methal hydridoalkyl complexes was established. The existence of complexes [(Me)M(L) · (H)Cl] with L = cyclohexane-1,3,5-triamine and 1,4,7-triazacyclonona-2,5,8-triene, moderately stable at M = Pd(IV) and stable at M = Pt(IV) and Ir(III), and also of stable alkylhydridorhodium(III) complexes was predicted for the first time. Characteristics of the frontier orbitals of complexes [M(L)CI]p, [CpMCl]q, and [MCl2(PH3)2]q (M = Rh, Ir, q = -1, p = 0; M = Pd, Pt, q = 0, p = +1) with distorted and undistorted coordination entities were compared.