Synthesis of the Novel [W3PdS4H3(dmpe)3(CO)]+ Cubane Cluster and Kinetic Studies on the Substitution of Coordinated Hydrides in Acidic Media

Abstract

Reaction of the incomplete cuboidal [W3S4H3(dmpe)3]+ cluster with a Pd(0) complex under a CO atmosphere produces a rare example of a heterodimetallic hydrido cluster of formula [W3PdS4H3(dmpe)3(CO)]+ ([1]+). There are not significant changes in the W-W bond lengths on going from the trinuclear to the tetranuclear cluster. The average W-W and W-Pd bond distances of 2.769[10] and 2.90[2] A, respectively, are consistent with the presence of single bonds between metal atoms. The heterodimetallic [1]+ complex is easier to oxidize and more difficult to reduce than its trinuclear precursor, which reflects the electron-donating capability of the Pd(CO) fragment. However, mechanistic studies on the reaction of [1]+ with acids show a lower basicity for this complex in comparison with that of its trinuclear precursor, so there is a major electron-density rearrangement within the cluster core upon Pd(CO) coordination. This rearrangement is also reflected in an unusual expansion of the sulfur tetrahedron within the W3PdS4 core with the concomitant elongation of the W-S bond distances by 0.04 A with respect to the analogous bond lengths in the trinuclear precursor. For those thermodynamically favored proton-transfer processes, the reaction mechanism of [1]+ with acids is quite similar to that observed for the incomplete trinuclear cluster, with only small changes in the rate constants. The reaction of [1]+ with HCl in acetonitrile/water mixtures produces [W3PdS4Cl3(dmpe)3(CO)]+ ([2]+) in two kinetically distinguishable steps. Proton transfer occurs in the initial step, in which the W-H bonds are attacked by the acid to yield dihydrogen-bonded adducts that are further attacked by an acetonitrile molecule to give [W3PdS4(CH3CN)3(dmpe)3(CO)]4+ and dihydrogen. The nature of processes involved in the second step are not well-understood with the present data, although it is very likely that these correspond to some secondary processes. In the third resolved step, the coordinated CH3CN ligands in [W3PdS4(CH3CN)3(dmpe)3(CO)]4+ are substituted by Cl- to afford the final [2]+ product. No reaction is observed between [1]+ and HCl in neat acetonitrile, whereas the product of the reaction of [1]+ with HBF4 or Hpts (pts- = p-toluenesulfonate) in this solvent is [W3PdS4(CH3CN)3(dmpe)3(CO)]4+. The reaction occurs in a single kinetic step with a first- (Hpts) or second-order (HBF4) dependence with respect to the acid. The first- and second-order acid dependences can be interpreted through the initial formation of dihydrogen adducts with one or two acid molecules, respectively.