Synthesis and optical power limiting properties of heteroleptic Mo₃S₇ clusters

Substitution of the halide ligands in (Bu₄N)₂[Mo₃S₇X₆] (X = Cl, Br) by diimine ligands, such as 4,4′-dimethyl-2,2′-bipyridine (dmbpy), 2,2′-bipyridine (bpy) and 1,10-phenanthroline (phen), affords the neutral heteroleptic clusters Mo₃S₇Cl₄(dmbpy) (1), Mo₃S₇Br₄(dmbpy) (2), Mo₃S₇Br₄(bpy) (3), and Mo₃S₇Br₄(phen) (4). Further substitution of the halide ligands in Mo₃S₇Br₄(diimine) clusters by dmit (1,3-dithiole-2-thione-4,5-dithiolate) allows the preparation of the mixed diimine-dithiolene neutral cluster complexes Mo₃S₇(dnbpy)(dmit)₂ (5, dnbpy = 4,4′-dinonyl-2,2′-bipyridine), Mo₃S₇(dcmbpy)(dmit)₂ (6, dcmbpy = 4,4′-dimethoxycarbonyl-2,2′-bipyridine), and Mo₃S₇(dcbpy)(dmit)₂ (7, dcbpy = 2,2′-bipyridine-4,4′-dicarboxylic acid). The optical limiting properties of complexes 1-7 have been assessed by the open-aperture Z-scan technique at 570 nm, employing a nanosecond optical parametric oscillator. In order to investigate the effect of increasing the π-system, complexes 1-4, with the general formula Mo₃S₇X₄(diimine), (X = Cl, Br), were compared to clusters 5-7, containing the dmit ligand. The influence of the metal content on the optical power limiting properties was also investigated by comparing the trinuclear series of complexes prepared herein with the bis(dithiolene) dinuclear cluster (Et₄N)₂[Mo₂O₂S₂(BPyDTS₂)₂], which has been recently prepared by our group. All trinuclear clusters 1-7 are efficient optical limiters (σ_eff > σ₀) with the threshold limiting fluence F_15% decreasing on proceeding from dinuclear to trinuclear clusters and, generally, on extending the π-system.