Synthesis and NMR properties of dihydrogen-hydride complexes of rhodium and iridium

Abstract

Addition of one equiv. of a tertiary phosphine ligand to toluene solutions of TpM(C$\sb2$H$\sb4)\sb2$ (Tp = hydridotris(1-pyrazolyl)borate; M = Rh, Ir) yields trigonal bipyramidal TPM(PR$\sb3$)(C$\sb2$H$\sb4$) complexes in which the phosphine ligand occupies the axial site and the ethylene ligand lies in the equatorial plane. Hydrogen readily displaces the respective ethylene ligands through thermally accessible square planar (sp) ($\eta\sp2$-Tp)M(PR$\sb3$)(C$\sb2$H$\sb4$) intermediates to form TpM(PR$\sb3$)H$\sb2$ complexes and free ethylene. Chemical evidence for the unobserved sp intermediate is obtained upon reaction of TpIr(PPh$\sb3$)(C$\sb2$H$\sb4$) with excess PPh$\sb3$, which gives an equilibrium mixture of starting material and (N, C$\sp5$, N-Tp)Ir(PPh$\sb3)\sb2$H, formed via cyclometallation of a pyrazolyl arm of the Tp ligand (K$\sb{\rm eq}$ = 0.1). Protonation of TpM(PR$\sb3$)H$\sb2$ complexes affords fluxional dihydrogen-hydride complexes, which reveal only a single hydride resonance at all accessible temperatures in the $\sp1$H NMR spectrum. Short T$\sb1$(min) values of 21-22 ms (Ir) and 7 ms (Rh) indicate an H-H bond length of 0.88-1.11 A in the iridium complexes and 0.73-0.92 A in the rhodium complex depending on the relative rate of H$\sb2$ rotation. In the case of the iridium complexes, partial substitution of the hydride positions with deuterium or tritium results in large temperature dependent isotope shifts and resolvable $J\sb{\rm H-D}$ or $J\sb{\rm H-T}$ coupling. Analysis of the chemical shift and coupling data as a function of temperature is consistent with a preference for the heavy hydrogen isotopes to occupy the hydride rather than the dihydrogen site.