Synthesis and structure of [M3(μ3-Se)(μ-SeS)3]4+ core compounds (M = Mo, W): a less-common type of linkage isomerism for the μ-SSe ligand

Abstract

The M₃(μ₃-Se)(μ-Se)₃⁴⁺ clusters (written also as M₃Se₄⁴⁺) show a strong tendency to add sulfur to the μ-Se bridges giving new chalcogen-rich clusters with M₃(μ₃-Se)(μ-SeS)₃⁴⁺ cores i.e. Mo₃Se₄S₃⁴⁺. They are isolated as dithiophosphate complexes [M₃(μ₃-Se)(μ-SeS)₃((RO)₂PS₂)₃]Cl (1a, M = Mo, R = Et; 1b, R = i-Pr; 2a, M = W, R = Et, 2b, R = i-Pr) upon treatment of solutions of [M₃Se₄(H₂O)₉]⁴⁺ in 2 M HCl with a large excess of P₄S₁₀/ROH reagent. The structures of 1a and 2a have been determined by single crystal X-ray analysis. The sulfur atom forms a part of the μ-SeS ligand, which is asymmetrically coordinated to two metal atoms, so that the sulfurs are almost coplanar with the M₃ triangle (equatorial positions) and the Se atoms are strongly out of the plane (axial positions). This represents a rare kind of linkage isomerism since the opposite bridging mode is observed for the SeS ligand in the clusters M₃(μ₃-S)(μ-SeS)₃⁴⁺ in which the Se is equatorial and the S occupies the axial positions. As an alternative the pure dithiophosphate salts KS₂P(OR)₂ (R = Et, i-Pr) do not lead to sulfur addition and allow isolation of [Mo₃Se₄((RO)₂PS₂)₃(μ-CH₃COO)(C₅H₅N)] (3) upon recrystallyzation from a CH₃CN–CH₃COOH–pyridine mixture. Both 3 and its sulfur analogue abstract sulfur from propylene sulfide at room temperature to give Mo₃Se₄S₃⁴⁺ and Mo₃S₇⁴⁺ clusters, respectively.