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-( tBu 2 PCH 2 ) 2 C 5 H 3 N) and PONOP (2,6-( tBu 2 PO) 2 C 5 H 3 N) pincer ligands are described: [M(PNP)(biph)][BAr F 4 ] (M = Rh, 1a; Ir, 1b; biph = 2,2'-biphenyl; Ar F = 3,5-(CF 3 ) 2 C 6 H 3 ) and [M(PONOP)(biph)][BAr F 4 ] (M = Rh, 2a; Ir, 2b). These complexes are structurally dynamic in solution, exhibiting pseudorotation of the biph ligand on the 1 H NMR time scale (Δ G ⧧ ca. 60 kJ mol -1 ) and, in the case of the flexible PNP complexes, undergoing interconversion between helical and puckered pincer ligand conformations (Δ G ⧧ 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.