Synthesis and electrochemical characterization of hexanuclear platinum bis-pseudohalides.

The hexanuclear phosphido-bridged dication [Pt6(μ-PBu(t)2)4(CO)6](2+), (1)(2+), reacts under mild conditions with pseudo-halide anions (CN(-), NCO(-), NCS(-)) to afford the corresponding neutral bis-substituted clusters Pt6(μ-PBu(t)2)4(CO)4X2 (2, X = CN; 3, X = SCN; 4, X = NCO). The reaction with sodium azide affords 4, which may arise from the formation of the intermediate bis-azido derivative Pt6(μ-PBu(t)2)4(CO)4(N3)2, 5, and CO. These react rapidly with each other affording 4 and N2. Cluster 5 was alternatively prepared as a stable compound by reacting with NaN3 the neutral cluster Pt6(μ-PBu(t)2)4(CO)4(OTf)2, 14, which contains two weakly bonded triflate anions. As expected, 5 reacts with carbon monoxide (1 atm) affording cluster 4 instantaneously and quantitatively. The new pseudo-halide clusters 2-5, which are interesting potential precursors of cluster-containing molecular frameworks, have been characterized by IR and multinuclear NMR spectroscopy. The solid state structures of clusters 2-4 have also been studied by single-crystal X-ray diffractometry. The main features of the molecular structures remain similar to those of many related congeners described previously; in addition, an interesting reversible phase transition was observed in the crystal structure of cluster 3. As their known congeners, the clusters undergo two reversible one-electron reductions and an irreversible oxidation. An excellent linear correlation was found between the redox potentials of the cathodic processes and Lever's ligand parameter E(L).