SYNTHESES AND ELECTROCHEMISTRY OF SULFUR-BRIDGED INCOMPLETE CUBANE-TYPE MIXED-METAL CLUSTERS OF MOLYBDENUM(IV) AND TUNGSTEN(IV) - X-RAY STRUCTURES OF [MOW2S4(H2O)(9)](CH3C6H4SO3)(4)CENTER-DOT-9H(2)O, [MO2WS4(H2O)(9)](CH3C6H4SO3)(4)CENTER-DOT-9H(2)O, NA-2[MOW2S4(HNTA)(3)]CENTER-DOT-5H(2)O, AND NA-2[MO2WS4(HNTA)(3)]CENTER-DOT-5H(2)O

Reduction of a equimolar amount of (NH4)(2)WS4 and Na-2[Mo2O2S2(cys)(2)].4H(2)O with NaBH4 gives a mixture of sulfur-bridged clusters, chromatographic separation of which gives novel incomplete cubane-type molybdenum-tungsten mixed-metal clusters [MoW2S4(H2O)(9)](4+) and [Mo2WS4(H2O)(9)](4+). From the aqua clusters solid samples [MoW2S4(H2O)9](CH3C6H4SO3)(4).9H2O (MoW(2)aq) and [Mo2WS4(H2O)9] (CH3C6H4SO3)(4).9H2O (Mo(2)Waq) were isolated, and the derivatives Na-2[MoW2S4(Hnta)(3)].5H(2)O (MoW(2)nta), and Na-2[Mo2WS4(Hnta)(3)].5H(2)O (Mo-2-Wnta) were obtained (H(3)nta = nitrilotriacetic acid). Crystal structures of the four clusters were determined. Compound MoW(2)aq crystallized in the triclinic space group P ($) over bar 1 with a 15.303(3) Angstrom, b = 16.682(6) Angstrom, c = 12.055(3) Angstrom, alpha = 96.02(3)degrees, beta = 108.58(2)degrees, gamma = 102.16(2)degrees, V = 2802.1(15) Angstrom(3), Z = 2, R = 4.77%. Compound Mo(2)Waq crystallized in the triclinic space group P ($) over bar 1 with a = 15.327(5) Angstrom, b = 16.704(5) Angstrom, c = 12.057(5) Angstrom, alpha = 95.64(3)degrees beta = 108.82(3)degrees, gamma = 102.41(2)degrees, V = 2806.2(18) Angstrom(3), Z = 2, R = 5.98%. Compound MoW(2)nta crystallized m the monoclinic space group P2(1)/a with a = 21.754(5) Angstrom, b = 12.733(5) Angstrom, c = 13.374(5) Angstrom, beta = 101.95(3)degrees, V = 3624.4(21) Angstrom(3), Z = 4, R = 3.90%. Compound Mo(2)Wnta crystallized in the monoclinic space group P2(1)/a with a = 21.741(5) Angstrom, b = 12.714(5) Angstrom, c = 13.384(6) Angstrom, beta = 102.10(3), V = 3617.3(24) Angstrom(3), Z = 4, R = 4.74%. Molybdenum and tungsten atoms in the four crystals are statistically disordered. Peak positions of the two big bands in each infrared spectrum of the four aqua clusters, [Mo3S4(H2O)9](CH3C6H4SO3)(4).9H(2)O (Mo(3)aq), Mo(2)Waq, MoW(2)aq, and [W3S4(H2O)(9)](CN3C6H4SO3)(4).9H(2)O (W(3)aq), in the 550-400-cm(-1) region shift to lower wavenumbers when the molybdenum atom is replaced by tungsten. The bands are tentatively assigned to nu (metal-OH2) (at higher wavenumber) and nu (metal(3)-S) (at lower wavenumber). Binding energies of molybdenum (3d(3/2) and 3d(5/2)) and tungsten (4f(5/2) and 4f(7/1)) are obtained from XPS spectra of the clusters with Mo3-nWnS4 cores (n = 0-3). The binding energies of Mo in Mo(3)aq and Mo(3)nta change little on the replacement of Mo with W, and those of W in W(3)aq and W(3)nta change little on the replacement of W with Mo, also: these phenomena can be explained by the softness of the bridging sulfurs, which act as a buffer for the electron density changes on the Mo and W atoms. Current-sampled de polarograms and cyclic voltammograms of MoW(2)nta and Mo(2)Wnta show three consecutive one-electron reductive steps in alkaline solution (pH 11.4): E(1/2) (V vs Ag/AgCl) = -0.84, -1.40, -1.78; -0.73, -1.22, -1.66, respectively. These steps correspond to the change of oxidation states of the three metals in each cluster: (IV, IV, IV) --> (IV, IV, III) --> (IV, III, III) --> (III, III, III). Electronic spectra of one-electron reduction products, the oxidation state being (IV, IV, III), obtained by bulk electrolysis of MoW(2)pta and Mo(2)Wnta have been reported for the first time. The half-wave potentials E(1/2) are significantly dependent on the cluster metals. In all the reduction processes the [MonW3-nS4(Hnta)(3)](2-) clusters (n = 0-3) are easily reduced with the increase in the numbers of Mo(n) in the cluster. The proceding reduction center of the molybdenum-tungsten mixed-metal clusters is mainly on the Mo atom(s).