AQUEOUS ORGANOMETALLIC CHEMISTRY - STRUCTURE AND DYNAMICS IN THE FORMATION OF (ETA(5)-PENTAMETHYLCYCLOPENTADIENYL)RHODIUM AQUA COMPLEXES AS A FUNCTION OF PH

The structures of the (eta(5)-pentamethylcyclopentadienyl)rhodium aqua complexes, as a function of pH, were studied by H-1, C-13, O-17, and 2D NOESY NMR spectroscopic techniques as well as by FAB mass spectrometry and potentiometric titration. The starting complex for our NMR experiments, [Cp*Rh(H2O)(3)](OTf)(2), 1, was structurally characterized by single-crystal X-ray crystallography [130 K, Mo K alpha radiation, lambda = 0.710 73 Angstrom, a = 23.979(9) Angstrom, b = 9.726(4) Angstrom, c = 18.257(6) Angstrom, Z = 8, orthorhombic, space group Pna2(1), 3879 independent reflections, R = 0.0482, R(w) = 0.1062]. Both H-1 and C-13 NMR titration experiments of the starting complex, 1, were performed by dissolving 1 in H2O (D2O) and obtaining spectra from pH 2-14. From pH 2-5 only one Cp* signal(H-1 NMR, 1.57 ppm; C-13 NMR, 5.78 ppm) was observed, which was attributed to 1. As the pH of the solution with 1 was increased from 5 to 7, a dynamic and rapid equilibrium was observed to provide putative [Cp*Rh(mu-OH(H2O](2)(OTf)(2) 2, and [(Cp*Rh)(2)(mu-OH)(3)](OTf/OH), 3; unfortunately, only one H-1 or C-13 NMR signal for Cp*Rh at 1.50 (Cp*) or 5.41 ppm (C-CH3), respectively, was found for the latter two species, with broadening of the signals at pH 5.5-6, indicating that conversion from putative 2 to 3 was very fast on the NMR time scale. As the pH was further increased from 7 to 10, only the H-1 or C-13 NMR signal for 3 was observed at 1.50 or 5.41 ppm, respectively. In addition, starting the equilibrium from 3 (3 reversible arrow 1 via putative 2) within the pH range 14-2 provided similar results. The 2D NOESY NMR exchange phasing experiments at pH 5.8 and 11 showed correlations between the Cp* CH3 groups and the H2O or mu-OH groups attached to Rh and between both Cp* CH3 groups of the Cp*Rh aqua complexes, although separate signals for bulk H2O and mu-OH or H2O ligands bonded to Rh were not observed due to a rapid exchange process. A potentiometric titration study gave further evidence that the conversion of 1 double right arrow 3 via putative 2 occurs rapidly with only one pK(a) of 5.3 being observed, reaffirming the fact that the conversion of 1 double right arrow 3 via putative 2 was extremely fast. The pseudo-first-order rate of conversion of 1 double right arrow 3 at pH 5.8 was measured by an NMR spin population transfer technique to be k(1) = 7.18 s(-1) (1, 0.034 M; T-1 = 1.6 s), while k(-1), 3 double right arrow 1, was found to be 2.93 s(-1) (T-1 = 1.5 s). The equilibrium constant, K-eq, at pH 5.8 for 1 reversible arrow 3 was found to be 353. O-17 NMR studies again showed that H2O molecules bonded to Cp*Rh and those in the bulk solution are in very fast exchange (k > 8150 s(-1)).