Solid state synthesis, structures and redox properties of the new [(Mo6Br7TeBr6a)-Te-i-Br-i](3-) and [(Mo6Br7SeBr6a)-Se-i-Br-i](3-) octahedral cluster units: Crystallochemistry of the (Rb2+xMo6Br8-xYxBr6a)-Y-i-Br-i series (x=0.5 for Y=Te; 0.25 <= x <

摘要

The solid state synthesis and the structure of the (Rb2Mo6Br8Br6a)-Br-i bromide (C2/c, a = 20.376(5) angstrom, b = 15.297(5) angstrom, c = 9.757(5) angstrom, beta=115.969(5)degrees, Z = 4, R-1 = 0.037; wR(2) = 0.057) and (Rb2+xMo6Br8-xYxBr6a)-Y-i-Br-i chalcohalides built up from molybdenum octahedral clusters (x = 0.5 for Y = Te; 0.25 <= x <= 0.7 for Y = Se) are presented. The crystallochemistry results are based on combined single-crystal and powder X-ray diffraction investigations from several preparations with different loaded compositions. A continuous solid state solution is evidenced between the two limit compositions Rb2.25Mo6Br13.75Se0.25 (Pn-3, a = 13.999(2) A, Z = 4, R, = 0.039; wR(2) = 0.058) and Rb2.7Mo6Br13.3Se0.7 (Pn-3, a = 14.144(2) angstrom, Z = 4, R-1 = 0.029; wR(2) = 0.048). On the other hand, in the case of tellurium (data: Rb2.5Mo6Br13.5Te0.5, Pn-3, a = 14.0936(5) angstrom, Z = 4, R-1 = 0.028; wR(2) = 0.048) no solid state solution is observed as previously reported for Rb2.5Re6S6.5Cl7.5 and K2.5Re6S6.5Br7.5. The structural features that act in the stabilization of the new [Mo6Br13Y](3-) units (Y = Se, Te) are evidenced and discussed by comparison with those obtained for related compounds in particular in Rb2.5Re6S6.5Cl7.5, K2.5Re6S6.5Br7.5, and A(2)W(6)Br(14) series (A = monovalent cation). The substitution of chalcogen for halogen strongly modifies the intrinsic properties of the M6L14 unit. Indeed, the electrochemistry performed on the [Mo6Br13Y](3-) unit after dissolution of the crude solid samples isolated reveals a new and chemically reversible oxidation process at E-1/2 = 0.46 V vs. SCE for [Mo6Br13Se](3-) whilst a similar oxidation process centred at 0.38 V is observed for [Mo6Br13Te](3-). (c) 2005 Elsevier Inc. All rights reserved.