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

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posted on 2019-05-21, 19:06 authored by Thomas M. Hood, Baptiste Leforestier, Matthew R. Gyton, Adrian B. Chaplin
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)]­[BArF4] (M = Rh, 1a; Ir, 1b; biph = 2,2′-biphenyl; ArF = 3,5-(CF3)2C6H3) and [M­(PONOP)­(biph)]­[BArF4] (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.