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.