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-(tBu₂PCH₂)₂C₅H₃N) and PONOP (2,6-(tBu₂PO)₂C₅H₃N) pincer ligands are described: [M­(PNP)­(biph)]­[BAr^F₄] (M = Rh, 1a; Ir, 1b; biph = 2,2′-biphenyl; Ar^F = 3,5-(CF₃)₂C₆H₃) and [M­(PONOP)­(biph)]­[BAr^F₄] (M = Rh, 2a; Ir, 2b). These complexes are structurally dynamic in solution, exhibiting pseudorotation of the biph ligand on the ¹H NMR time scale (ΔG^⧧ ca. 60 kJ mol^–1) and, in the case of the flexible PNP complexes, undergoing interconversion between helical and puckered pincer ligand conformations (ΔG^⧧ ca. 10 kJ mol^–1). 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.