Synthesis and Third-Order Nonlinear Optical Properties of [Mo 3 (u 3 -S)(u 2 -S 2 ) 3 ] 4+ Clusters with Maleonitriledithiolate, Oxalate and Thiocyanate Ligands

Substitution of the bromine ligands in [(n-Bu)4N]2 [Mo3S7Br6] (1) by bidentate ligands such as maleonitriledithiolate (mnt) or oxalate (ox) affords the trinuclear clusters [Mo3S7(mnt)3]2- ([2]2-) and [Mo3S7 (ox)3]2- ([3]2-) in moderate yields. The structures of clusters [2]2- and [3]2- have been determined by single-crystal X-ray diffraction as their tetrabutylammonium salts. Cluster 2 forms dimers by interaction of one sulfur atom of the mnt ligand with the anionic binding site of the cluster anion. The structure of 3 reveals the presence of aggregates between the anionic cluster and a Br- ion. The ESI-MS indicate that these cluster-bromine adducts are also present in solution. Electrochemical studies on complexes 1-3 show two irreversible waves associated with reductive cleavage of the disulfido bridges and one or two quasi-reversible oxidation processes for [Mo3S7(ox)3]2- or [Mo3S7(mnt)3]2-, respectively. No oxidation waves have been observed for compound 1 or for its thiocyanate derivative [(n-Bu)4N]2 [Mo3S7(SCN)6] (4) within the acetonitrile solvent window. The optical limiting properties of the complexes 1-4 have been measured by the Z-scan technique employing 40 ns pulses at 523 nm. Power limiting was observed for clusters 1, 2 and 4, whereas the oxalate derivative 3 was photochemically unstable under our experimental conditions