New procedures for the preparation of [Mo3S4(H2O)9]4+, [Mo4S4(H2O)12]5+ and [Mo7S8(H2O)18]8+ and their Se analogues: redox and substitution studies on the double cube [Mo7S8(H2O)18]8+

Abstract

Alternative improved methods for the preparation of [Mo 3 S 4 (H 2 O) 9 ] 4+ , [Mo 4 S 4 (H 2 O) 12 ] 5+ and [Mo 7 S 8 (H 2 O) 18 ] 8+ are described from polymeric {Mo 3 S 7 Br 4 } x (obtained by heating together the elements), via water soluble [Mo 3 S 7 Br 6 ] 2- . Abstraction of neutral sulfur with phosphines and aquation of terminal ligands in dilute acids gives [Mo 3 S 4 (H 2 O) 9 ] 4+ in yields of up to 85%. The single cube [Mo 4 S 4 (H 2 O) 12 ] 5+ , and corner-shared double cube [Mo 7 S 8 (H 2 O) 18 ] 8+ , are obtained by treating [Mo 3 S 4 (H 2 O) 9 ] 4+ with different reductants. The best procedure for [Mo 7 S 8 (H 2 O) 18 ] 8+ is with hypophosphorous acid (H 3 PO 2 ), when yields of up to 20% are obtained. Yields of [Mo 4 S 4 (H 2 O) 12 ] 5+ are variable and in the range 10–65% depending on the reductant and procedure employed. The reactions provide examples of Mo III and Mo IV –sulfido reassembly from [Mo 3 S 4 (H 2 O) 9 ] 4+ following reduction. Surprisingly, the more direct approach for the preparation of [Mo 7 S 8 (H 2 O) 18 ] 8+ , involving addition of [Mo 3 S 4 (H 2 O) 9 ] 4+ to [Mo 4 S 4 (H 2 O) 12 ] 4+ , gives only ≈3% yield, most likely due to the inertness of [Mo 4 S 4 (H 2 O) 12 ] 4+ . The procedures described are effective also for [Mo 3 Se 7 Br 6 ] 2- , and in exploratory studies the double cube [Mo 7 Se 8 (H 2 O) 18 ] 8+ has been prepared for the first time. The kinetics of substitution of H 2 O on [Mo 7 S 8 (H 2 O) 18 ] 8+ by NCS - have been investigated, and two stages identified. One of these is a [NCS - ]-dependent equilibration, with rate constants (25 °C) for formation k f = 0.173 M -1 s -1 and aquation k aq = 0.20 × 10 -3 s -1 at [H + ] = 1.96 M, I = 2.00 M (Lipts), pts - = p-toluenesulfonate. The other is a [NCS - ]-independent step assigned as isomerisation of the S-bonded thiocyanato product (k ≈1.5 × 10 -3 s -1 ). With [Co(dipic) 2 ] - (dipic = pyridine-2,6-dicarboxylate) as oxidant, second-order kinetics are observed with the rate constant 0.31 M -1 s -1 at 25 °C independent of [H + ] in the range 0.87–2.00 M, I = 2.00 M (Lipts). The stoichiometry indicates higher than expected consumption (≈7 equivalents) of [Co(dipic) 2 ] - . A mechanism involving transient formation of [Mo 7 S 8 (H 2 O) 18 ] 9+ , which fragments to [Mo 3 S 4 (H 2 O) 9 ] 4+ and [Mo 4 S 4 (H 2 O) 12 ] 5+ , is proposed. In separate experiments the latter was found to be oxidised by [Co(dipic) 2 ] - yielding first [Mo 4 S 4 (H 2 O) 12 ] 6+ , which itself fragments with formation of [Mo 3 S 4 (H 2 O) 9 ] 4+ , the Mo V 2 dimer [Mo 2 O 2 S 2 (H 2 O) 6 ] 2+ , and other products.