Cycloaddition of alkynes to diimino Mo3S4 cubane-type clusters: a combined experimental and theoretical approach†
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]+ (+) in a one-step ligand-exchange protocol from [Mo3S4(tu)8(H2O)]Cl4·4H2O (tu = thiourea). The new cluster was isolated as PF6 and 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]+ (+). Kinetic and NMR studies show that + reacts with several alkynes to form dithiolene species via concerted [3+2] cycloaddition reactions whereas + remains inert under similar conditions. The different rates for the reactions of + 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 + is due to the endergonicity of its reactions, which feature ΔGr values systematically 5–7 kcal mol−1 more positive than for those of +.