Solid state synthesis, structures and redox properties of the new [Mo6Bri7TeiBra6]3− and [Mo6Bri7SeiBra6]3− octahedral cluster units: Crystallochemistry of the Rb2+xMo6Bri8−xYixBra6 series (x=0.5 for Y=Te; 0.25⩽x⩽0.7 for Y=Se) and Rb2Mo6Br14

The solid state synthesis and the structure of the Rb2Mo6Bri8Bra6 bromide (C2/c, image, image, image, image, image, image; image) and Rb2+xMo6Bri8−xYixBra6 chalcohalides built up from molybdenum octahedral clusters (image for image; image for image) 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, image, image, image; image) and Rb2.7Mo6Br13.3Se0.7 (Pn-3, image, image, image; image). On the other hand, in the case of tellurium (data: Rb2.5Mo6Br13.5Te0.5, Pn-3, image, image, image; image) 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 (image, 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 A2W6Br14 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 image vs. SCE for [Mo6Br13Se]3− whilst a similar oxidation process centred at 0.38 V is observed for [Mo6Br13Te]3−.