Rates and activation parameters for solvent exchange with [M(PR3)2-(solvent)2H2]+(M = RhIII or IrIII, R = cyclohexyl or phenyl) cations
Abstract
Using 1H n.m.r. line-broadening and, where possible, stopped-flow Fourier-transform n.m.r., rates and activation parameters have been determined for solvent exchange with complexes of the type [M(PR3)2(solvent)2H2]+(M = RhIII or IrIII; R = phenyl, solvent = MeCN or Me2CO; R = cyclohexyl, solvent = MeCN) in [2H2]-dichloromethane solution. In all cases the co-ordinated solvent molecules are in trans positions with respect to the hydride ligands, and at 298 K the various complexes have rate constants for solvent exchange (k1) which differ almost by a factor of 107; k1(Rh) > k1(lr) because for analogous species ΔH‡(Rh) < ΔH‡(lr), and the acetone solvates are much more labile than the acetonitrile complexes because of lower ΔH‡ values. Values of ΔS‡ are significantly positive (8–32 J K–1 mol–1 when M = RhIII, 25–30 J K–1mol–1 when M = IrIII) in line with a dissociative, D mechanism. Hydrogen-1 n.m.r. spectra of [M(PR3)2(solvent)2H2]+ and [Ir(PR3)2(OCMe2)(L)H2]+ ions (L = MeOH, MeCN, pyridine, styrene, or cyclo-octene) are reported and compared.