Synthesis and third-order nonlinear optical properties of [Mo₃(μ₃-S)(μ₂- S₂)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.