DIHYDROGEN - A BETTER LIGAND THAN WATER - IR AND X-RAY EVIDENCE FOR AQUO COORDINATION IN W(CO)3(PR3)2(H2O), THERMODYNAMICS OF H2O VERSUS ETA(2)-H2 BINDING, AND H2O/D2 ISOTOPIC EXCHANGE - IMPLICATIONS ON THE BIOLOGICAL ACTIVATION OF HYDROGEN

Reactions of water with M(CO)3(PR3)2 and M(CO)3(PR3)2(eta2-H-2) (M = Mo, W; R = Cy (cyclohexyl), i-Pr) have been studied in various organic solvents. The products contained reversibly bound H2O, and infrared studies showed nu(OH) modes at widely varying positions. IR of O-18-labeled complexes and H-1 NMR gave no evidence for hydride or hydroxide ligands, indicating that oxidative addition of water did not occur. NMR of the aquo complexes showed rapid exchange between free and coordinated water at 298 K. The aquo complex W(CO)3(P-i-Pr3)2(H2O).THF was isolated from THF and structurally characterized. A long W-O distance of 2.320 (5) angstrom was observed for the reversibly bound H2O ligand, which also undergoes hydrogen-bonding interactions with both lattice THF and a CO on an adjacent molecule. Water was found to instantaneously displace the dihydrogen ligand in W(CO)3(PR3)2(eta2-H-2) in THF solution to give aquo complexes, but in hexane H-2 remained bound under a H-2 atmosphere. Thermodynamic measurements of the equilibrium W(CO)3(PR3)2(H-2) + H2O half arrow right over half arrow left W(CO)3(PR3)2(H2O) + H-2 in THF showed that the DELTAH value for binding was 3-4 kcal/mol higher for H2O. However, a higher entropy change related to hydrogen-bonding interactions between H2O and solvent resulted in DELTAG favoring H-2 coordination at 25-degrees-C by 1-2 kcal/mol. Isotopic exchange of W(CO)3(P-i-Pr3)2(eta2-D2 with H2O under a D2 atmosphere took place in THF, giving W(CO)3(P-i-Pr3)2(D2O). Both the favored binding of H-2 versus H2O and the latter exchange are relevant to the function of H-2-activating enzymes such as hydrogenase. Crystal data for W(CO)3(P-i-Pr3)2)(H2O).THF: space group P2(1)/n, a = 13.554 (2) angstrom, b = 16.417 (5) angstrom, c = 15.059 (4) angstrom, beta = 116.24 (2)-degrees, Z = 4.