Synthesis and reactivity of rhodium and iridium alkene, alkyl and silyl complexes supported by a phenyl-substituted PNP pincer ligand

Abstract

New rhodium and iridium complexes supported by the phenyl-substituted PNP pincer ligand PNP^PhH (HN(2-PPh₂-4-Me-C₆H₃)₂) (1) were synthesized. The reaction of 2 equiv. of 1 with [(COD)IrCl]₂ afforded the coordination complex [(PNP^PhH)Ir(COD)]Cl (2) featuring hydrogen bonding between the N–H group and the chloride anion, as characterized by NMR spectroscopy and X-ray crystallography. Reaction of 1 with [(COE)₂IrCl]₂ or [(COE)₂RhCl]₂ in benzene provided a mixture of complexes including (PNP^PhH)MHCl₂ (M = Ir (4), M = Rh (7)) and (PNP)M(COE) (M = Ir (5), M = Rh (8)). Alkene complexes of the type (PNP^Ph)M(L) (M = Ir, L = COD (3) and COE (5); M = Rh, L = COE (8) and L = ethylene (9)) were synthesized by reaction of (PNP^Ph)Li with the appropriate alkene chloride complexes. Reactions of silanes with 5, 8 or 9 produced silyl hydride complexes (PNP^Ph)MH(SiR₃) (M = Ir, R = Ph (16) and R = Et (17); M = Rh, R = Ph (18), Et (19) and Ph₂Cl (20)) via Si–H oxidative addition. The J_SiH coupling constants for rhodium complexes 18, 19 and 20 were determined to be ca. 35 Hz, while iridium complexes 16 and 17 exhibited coupling constants less than 10 Hz. X-Ray crystal structures of 16 and 18 reveal isostructural complexes featuring a trigonal bipyramidal geometry about iridium with a mer binding of the PNP^Ph ligand. A hydride ligand, located from the Fourier map for 18, has a short contact of 1.83(3) Å with the silicon atom. Oxidative addition of iodomethane to 5 and 8 afforded (PNP^Ph)M(Me)(I)(THF) (M = Rh (14), M = Ir (12)), respectively. Arene C–H activation upon thermolysis of 12 in benzene produced (PNP^Ph)M(Ph)(I)(THF). Iridium silyl iodide complexes (PNP^Ph)IrI(SiR₃) (SiR₃ = SiPh₃ (21), SiH₂Mes (22) and SiH₂Xyl (23)) resulted from addition of organosilanes to 12, via elimination of CH₄.