Hydrothermal Synthesis and Crystal Structure of [{Cu(2,2-bpy)2}2Mo8O26]: a beta-octamolybdate Cluster Covalently Bonded to Two {Cu(2,2-bpy)2}2+ Coordination Complexes via Bridging oxo Groups

The many examples of R7(Z)X12 cluster phases for R=rare-earth metal, Z=interstitial element, X = Cl, Br, I, occur in a characteristic rhombohedral structure R3+[R6(Z)X12] in which the seventh R atom is situated in a trigonal antiprismatic Xi site between halide-interbridged clusters in an anionic network that can be described as [La6(Os)]Xi6Xi-a6Xa-i6]. The first example of a substituted derivative, NaLa6(Os)I12, has been achieved via exploratory synthetic reactions in neat systems near 850°C. The structure has been refined in space group R (No. 148), a=16.214(4) ?, c=11.172(4) ? , R/Rw(F)=3.5/3.9%. The cluster is geometrically very close to an ideal octahedron in shape, showing a trigonal elongation of only 0.026(3) ? and La–Os–La angles of 90±0.37(4)°. The La–I and Na–I distances are likewise very similar and substantially ideal. This regularity is unexpected in view of both the clusterʹs apparent 15-cluster-based electron configuration with an expected nominal t31u HOMO and the behavior of many other cluster structures with incomplete electronic structures. Furthermore, the phase inexplicably exhibits only a TIP term in magnetic susceptibility, ~3.4×10-4 emu mol-1, for which no explanation is currently available.