Synthesis and Structural Dynamics of Five-Coordinate Rh(III) and Ir(III) PNP and PONOP Pincer Complexes.

The synthesis and characterization of a homologous series of five-coordinate rhodium(III) and iridium(III) complexes of PNP (2,6-( tBuPCH)CHN) and PONOP (2,6-( tBuPO)CHN) pincer ligands are described: [M(PNP)(biph)][BAr] (M = Rh, 1a; Ir, 1b; biph = 2,2'-biphenyl; Ar = 3,5-(CF)CH) and [M(PONOP)(biph)][BAr] (M = Rh, 2a; Ir, 2b). These complexes are structurally dynamic in solution, exhibiting pseudorotation of the biph ligand on the H NMR time scale (Δ G ca. 60 kJ mol) and, in the case of the flexible PNP complexes, undergoing interconversion between helical and puckered pincer ligand conformations (Δ G ca. 10 kJ mol). Remarkably, the latter is sufficiently facile that it persists in the solid state, leading to temperature-dependent disorder in the associated X-ray crystal structures. Reaction of 1 and 2 with CO occurs for the iridium congeners 1b and 2b, leading to the formation of sterically congested carbonyl derivatives.