Cycloaddition of alkynes to diimino Mo3S4 cubane-type clusters: a combined experimental and theoretical approach

Abstract

A heterocyclic ligand 4,4′-di-tert-butyl-2,2′-bipyridine (dbbpy) has been coordinated to the Mo₃S₄ cluster unit affording the complex [Mo₃S₄Cl₃(dbbpy)₃]⁺ ([1]⁺) in a one-step ligand-exchange protocol from [Mo₃S₄(tu)₈(H₂O)]Cl₄·4H₂O (tu = thiourea). The new cluster was isolated as [1]PF₆ and [1]Cl salts in high yields and the crystal structure of the latter determined by X-ray analysis. The synthetic procedure was extended to tungsten to afford [W₃S₄Cl₃(dbbpy)₃]⁺ ([2]⁺). Kinetic and NMR studies show that [1]⁺ reacts with several alkynes to form dithiolene species via concerted [3+2] cycloaddition reactions whereas [2]⁺ remains inert under similar conditions. The different rates for the reactions of [1]⁺ are rationalised by computational (DFT) calculations, which show that the more electron-withdrawing the substituents of the alkyne the faster the reaction. The inertness of [2]⁺ is due to the endergonicity of its reactions, which feature ΔG_r values systematically 5–7 kcal mol⁻¹ more positive than for those of [1]⁺.