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+)

Alternative improved methods for the preparation of [Mo3S4(H2O)(9)](4+), [Mo4S4(H2O)(12)](5+) and [Mo7S8(H2O)(18)](8+) are described from polymeric {Mo3S7Br4}(x) (obtained by heating together the elements), via water soluble [Mo3S7Br6](2-). Abstraction of neutral sulfur with phosphines and aquation of terminal ligands in dilute acids gives [Mo3S4(H2O)(9)](4+) in yields of up to 85%. The single cube [Mo4S4(H2O)(12)](5+), and corner-shared double cube [Mo7S8(H2O)(18)](8+), are obtained by treating [Mo3S4(H2O)(9)](4)+ with different reductants. The best procedure for [Mo7S8(H2O)(18)](8+) is with hypophosphorous acid (H3PO2), when yields of up to 20% are obtained. Yields of [Mo4S4(H2O)(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 [Mo3S4(H2O)(9)](4+) following reduction. Surprisingly, the more direct approach for the preparation of [Mo7S8(H2O)(18)](4+), involving addition of [Mo3S4(H2O)(9)](4+) to [Mo4S4(H2O)(12)](4+), gives only approximate to 3% yield, most likely due to the inertness of [Mo4S4(H2O)(12)](4+). The procedures described are effective also for [Mo3Se7Br6](2-), and in exploratory studies the double cube [Mo7Se8(H2O)(18)](8+) has been prepared for the first time. The kinetics of substitution of H2O on [Mo7S8(H2O)(18)]8+ by NCS- have been investigated, and two stages identified. One of these is a [NCS-]-dependent equilibration, with rate constants (25 degrees C) for formation k(f) = 0.173 M-1 s(-1) and aquation k(aq) = 0.20 x 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 approximate to 1.5 x 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 degrees C independent of [H+] in the range 0.87-2.00 M, I = 2.00 M (Lipts). The stoichiometry indicates higher than expected consumption (approximate to 7 equivalents) of [Co(dipic)(2)](-). A mechanism involving transient formation of [Mo7S8(H2O)(18)](9+), which fragments to [Mo3S4(H2O)(9)](4+) and [Mo4S4(H2O)(12)](5+), is proposed. In separate experiments the latter was found to be oxidised by [Co(dipic)(2)](-) yielding first [Mo4S4(H2O)(12)](6+), which itself fragments with formation of [Mo3S4(H2O)(9)](4+), the Mo-2(v) dimer [Mo2O2S2(H2O)(6)](2+), and other products.