Houston Jacqueline R, Richens David T, Casey William H
Department of Chemistry, University of California, Davis, California 95616, USA.
Inorg Chem. 2006 Sep 18;45(19):7962-7. doi: 10.1021/ic0609608.
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.
利用高压(^{17}O)核磁共振研究了生物氧封端的(M - M)键合三核簇([M_3(\mu_3 - O)_2(\mu - O_2CCH_3)_6(OH_2)_3]^{2 + })（(M = Mo(IV))和(W(IV))）的水交换机制，并与我们之前对类似铑（III）三聚体的研究进行了比较。对于(Mo(IV))三聚体，当压力从(6)兆帕增加到(250)兆帕时，反应速率降低超过(2)倍；而对于(W(IV))三聚体，交换速率增加不到(1.2)倍（(10 - 229)兆帕）。根据反应速率对压力的依赖性，计算出(Mo(IV))簇和(W(IV))簇的活化体积((\Delta V^{\ddagger}))分别为(\Delta V^{\ddagger}= + 8.0(±0.4) cm^3 mol^{-1})和(\Delta V^{\ddagger}= - 1.9(±0.2) cm^3 mol^{-1})，这是位于元素周期表同一族中的任何一对(4d - 5d)（以及(3d - 4d)）过渡金属物种活化体积的最大差异（约(10 cm^3 mol^{-1})）。如果我们根据斯沃德尔为简单八面体单体建立的半经验模型（缔合（(A)）(=\Delta V^{\ddagger}\approx - 13)；交换（(I)）(=\Delta V^{\ddagger}\approx 0)；或解离（(D)）(=\Delta V^{\ddagger}\approx + 13)）来解释这些活化体积，我们的结果表明，(Mo(IV))簇的水交换遵循解离 - 交换（(I_d)）机制，而(W(IV))三聚体遵循缔合 - 交换（(I_a)）活化模式。尽管分子结构和反应性有相当大的相似性，但这些体积在水交换机制上呈现出独特的转变。这种转变可以为比单体更复杂的分子中配体交换途径的计算模拟提供一个标准。
