Spectroscopic and reactivity studies of diastereomeric iridium(III) hydride complexes of the chelating tritertiary phosphine ligand Cyttp

Abstract

The chelating tritertiary phosphine C6H5P(CH2CH2CH2P(C6, H11)2)2 (Cyttp) in octahedral six-coordinate iridium(III) complexes [IrH2Z(Cyttp](0 or 1+) (Z = Cl, CO, CH3CN, and related ligands) is meridionally bound. The conformation of the Cyttp ligand is such that the phenyl group of the central phosphorus lies out of the meridional plane. This leads to diastereomers in which the Z group is syn or anti to the phenyl group in question. These diastereomers were readily characterized by a general nuclear Overhauser effect difference NMR technique involving an intensity change to the apical-hydrido absorbance syn-to-phenyl upon irradiation of phenyl-ortho protons. A single diastereomer, cis-anti-mer-[IrH2Cl(Cyttp)] (6a), was prepared by (a) refluxing [IrCl(COD)]2 and Cyttp in ethanol or (b) treatment of [IrCl(COD)(Cyttp)] in benzene with excess dihydrogen. The stereochemical course for ligand substitution reactions on 6a was determined using the Cyttp phenyl group as a stereochemical label. Of six chloro-ligand metathesis reactions performed on cis-anti-mer-[IrH2Cl(Cyttp)] (6a), four gave mixtures of the two cis-syn-mer-/cis-anti-mer-[IrH2Z(Cyttp)](0 or 1+) diastereomers (Z = CH3CN, CN, PMe2Ph, and P(OCH2)3CEt), one gave inversion only (the cis-syn-mer compound (Z = CO)), and only one gave retention (cis-anti-mer-[IrH2I(Cyttp)]. A dissociative mechanism is proposed for the substitution reactions. Loss of stereochemical integrity in 6a was also observed in other reactions. Insertions of CS2 into the iridium–meridonal-hydride bond and SnCl2 into the iridium–chlorine bond produced mixtures of cis-syn-mer/cis-anti-mer diastereomers. Electrophiles (C7H7+ in the presence of CH3CN, I2, HCl, and HOAc in the presence of TI+) attack only the meridonal hydride to yield various ratios of geometric and (or) syn/anti isomers. Trends in the relative chemical shifts of the hydride protons and 31P nuclei in syn versus anti isomers are noted and rationalized.