Oxidative addition of an aromatic ortho C–H bond of tetraphosphine to asymmetric diiridium(i) centres

Abstract

Reactions of a tetraphosphine, meso-bis{[(diphenylphosphinomethyl)phenyl]phosphino}propane (dpmppp), with [IrCl(cod)]₂ and CO (1 atm) or isocyanide (RNC) in the presence of NH₄PF₆ at 80–100 °C in dichloromethane/acetonitrile/acetone and/or methanol mixed solvents afforded asymmetric diiridium(II) complexes, [Ir₂(H)(Cl)(μ-(dpmppp-H)-κP⁴C)(CO)₃]PF₆ (1) and [Ir₂(H)(μ-(dpmppp-H)-κP⁴C)(RNC)₄)]-(PF₆)₂ (R = 2,6-xylyl (2), 2,4,6-mesityl (3); dpmppp-H = {PPh(o-C₆H₄)CH₂P(Ph)(CH₂)₃P(Ph)CH₂PPh₂}⁻). A similar reaction with ^tBuNC resulted in the formation of a mononuclear Ir^III complex of [Ir(H)(dpmppp-κP³)(^tBuNC)₂](PF₆)₂ (4). Complexes 1–3 were characterized by ESI mass spectrometry, ¹H and ³¹P NMR spectroscopy and X-ray diffraction analyses. They were found to consist of cis/trans-P,P asymmetric Ir^II–Ir^II bonded dinuclear structures derived from oxidative addition of an ortho C–H bond of dpmppp (Ir–Ir = 2.8044(2) Å (1), 2.8569(2) Å (2), and 2.8524(5) Å (3)), resulting in a [IrPCCIr] intermetallic cyclometal-bridge and a terminal hydride. DFT calculations indicated the presence of Ir–Ir, Ir–H, and Ir–C_ortho covalent bonds. Initial stages of the reactions with CO and XylNC at room temperature were investigated by ³¹P{¹H} NMR spectroscopy and found to contain a symmetrical Ir^I dinuclear unit with dpmppp that was readily transformed into 1 and 2 upon heating. The Ir^I₂ intermediate with XylNC, [Ir₂(XylNC)₄(μ-dpmppp)](PF₆)₂ (6), was isolated and characterized by X-ray crystallography and DFT calculations as an electron-deficient 32e⁻ Ir^I₂ species involving a Ir^I→Ir^I dative bond (2.7989(5) Å). The reaction pathways from 6 to 2 were investigated by DFT calculations. The present study suggested that a novel oxidative addition of an ortho C–H bond proceeded on the cis/trans-P,P asymmetric diiridium(I) scaffold supported by the tetraphosphine, dpmppp, which was assumed to be facilitated by dimetal cooperation with switching Ir→Ir dative interactions.