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 η2-H2 binding, and H2O/D2 isotopic exchange. Implications on the biological activation of hydrogen

Reactions of water with M(CO){sub 3}(PR{sub 3}){sub 2} and M(CO){sub 3}(PR{sub 3}){sub 2}({eta}{sup 2}-H{sub 2}) (M = Mo, W; R= Cy (cyclohexyl), i-pr) have been studied in various organic solvents. The products contained reversibly bound H{sub 2}O, and infrared studies showed v(OH) modes at widely varying positions. IR of {sup 18}O-labeled complexes and {sup 1}H 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(C){sub 3}(P-i-Pr{sub 3}){sub 2}(H{sub 2}O){lg_bullet}THF was isolated from THF and structurally characterized. A long W-O distance of 2.320 (5) A was observed for the reversibly bound H{sub 2}O 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){sub 3}(PR{sub 3}){sub 2}({eta}{sup 2}-H{sub 2}) in THF solution to give aquo complexes, but in hexane H{sub 2} remained bound under a H{sub 2} atmosphere. Thermodymanic measurements of the equilibrium W(CO){sub 3}(PR{sub 3}){sub 2}(H{sub 2}) + H{sub 2}O W(CO){sub 3}(PR{sub 3}){sub 2}(H{sub 2}O) + H{sub 2} in THF showed that the {Delta}H valuemore » for binding was 3-4 kcal/mol higher for H{sub 2}O. However, a higher entropy change related to hydrogen-bonding interactions between H{sub 2}O and solvent resulted in {Delta}G favoring H{sub 2} coordination at 25{degrees}C by 1-2 kcal/mol. Isotopic exchange of W(CO){sub 3}(P-i-Pr{sub 3}){sub 2}({eta}{sup 2}dD{sub 2}) with H{sub 2}O under a D{sub 2} atmosphere took place in THF, giving W(CO){sub 3}(P-i-Pr{sub 3}){sub 2}(D{sub 2}O). Both the favored binding of H{sub 2} versus H{sub 2}O and the latter exchange are relevant to the function of H{sub 2}-activating enzymes such as hydrogenase. 41 refs., 11 figs., 7 tabs.« less