Distinct water-exchange mechanisms for trinuclear transition-metal clusters.

Mechanisms for water exchange from the bioxo-capped M-M-bonded trinuclear clusters, [M3(mu3-O)2(mu-O2CCH3)6(OH2)3]2+ [M = Mo(IV) and W(IV)], were investigated using high-pressure 17O NMR and compared to our previous work on a similar Rh(III) trimer. Reaction rates decrease by more than a factor of 2 when pressure is increased from 6 to 250 MPa for the Mo(IV) trimer, while exchange rates increase by less than a factor of 1.2 (10-229 MPa) for the W(IV) trimer. From the pressure dependence of the reaction rate, activation volumes (DeltaV()) were calculated to be DeltaV() = +8.0 (+/-0.4) cm(3) mol(-1) and DeltaV = -1.9 (+/-0.2) cm(3) mol(-1) for the Mo(IV) cluster and W(IV) cluster, respectively, which is the largest difference ( approximately 10 cm(3) mol(-1)) in activation volumes for any pair of 4d-5d (and 3d-4d) transition metal species located within the same group of the periodic table. If we interpret these activation volumes in terms of Swaddle's semiempirical model, which he established for simple octahedral monomers (Associative (A) = DeltaV approximately -13; Interchange (I) = DeltaV approximately 0; or Dissociative (D) = DeltaV approximately +13), our results suggest that water exchange follows a dissociative-interchange (Id) mechanism for the Mo(IV) cluster and an associative-interchange (Ia) activation mode for the W(IV) trimer. These volumes exhibit a unique changeover in the water-exchange mechanism despite considerable similarities in molecular structure and reactivity. This changeover could provide a standard for computational simulations of ligand-exchange pathways in molecules that are more complicated than monomers.