Synthesis and NMR spectral investigations of cycloocta-1,5-dieneiridium(I) complexes containing substituted benzimidazoles-Anomalous shielding of <i>iso-</i>propyl protons in [IrCl(COD)( <i>i</i>-PrbzlH)]

Abstract

The bridge splitting reaction of the dimer [lr(<i>µ</i>-Cl)(COD)]₂, where COD is cycloocta-1,5-diene, with 2-substituted benzimidazoles (R-bzlH) in acetone affords mononuclear complexes of the formula [IrCI(COD)(R-bzlH)] [R=H, methyl (Me), ethyl (Et), propyl(<i>n</i>-Pr or <i>i</i>-Pr), butyl(<i>n</i>-Bu) or 2'-quinolyl (Qnl)].The presence of triethylamine in the system drives the reaction towards trimerisation to provide complexes of the type [Ir(COD)(<i>µ</i>-R-bzl)]₃(R=H, Me, Et, n-Pr or n-Bu). Treatment of the mononuclear complexes with PPh₃ in presence of NaCIO₄ produces cationic species of the type [Ir(COD)(PPh₃)(R-bzlH)]CIO_4.The ¹H and ¹³CNMR spectral features of [lrCI(COD)(<i>i</i>-PrbzlH)] suggest the presence of conformers in solution. The preferred orientation of the isopropyl group in one of the conformers, apparently caused by steric effect, is manifested in the anomalous shielding of the methyl protons which resonate at <i>δ</i> - 0.28 ppm. This seems to be the first obervation of an unusual behaviour of isopropyl substituent of the N-heterocycle in an organometallic complex. The ³¹P NMR spectra of the cationic complexes display a downfield coordination-induced shift of δ 18-22 ppm for PPh₃. The ¹H{¹H} correlated spectra suggest a trimeric structure for [lr(COD)(<i>µ</i>-R-bzl)]_n in chloroform. Displacement of COD by CO yields cis-dicarbonyl derivatives of the composition [Ir(CO)₂(<i>µ</i>-R-bzl)]_n.