A BASIS-SET OF RE6SE8 CLUSTER BUILDING-BLOCKS AND DEMONSTRATION OF THEIR LINKING CAPABILITY - DIRECTED SYNTHESIS OF AN RE12SE16 DICLUSTER

A previously developed solid-state route (dimensional reduction) has l ed to the first molecular clusters containing the face-capped octahedr al [Re-6(mu(3)-Q)(8)](2+) core (Q = S, Se). Among these is the protona ted cluster [Re6Se7(SeH)I-6](3-), whose substitution reactions have be en examined in an effort to create a set of site-differentiated precur sors for forming multicluster assemblies. Reaction with Et(3)P under r eflux with different conditions of reactant mole ratio and time afford s the clusters fac-[Re6Se8(PEt(3))(3)I-3](1-) (2), trans- (3) and cis- [Re6Se6(PEt(3))(4)I-2] (4), [Re6Se8(PEt(3))(5)I](1+) (5), and [Re6Se8( PEt(3))(6)](2+) (6). The synthesis of fully substituted 6 in quantitat ive yield required the forcing conditions of 20 equiv of phosphine in refluxing DMF for 3 days. Reaction of 4 with AgBF4 in dichloromethane/ acetonitrile gave cis-[Re6Se8(PEt(3))(4)(MeCN)(2)](2+) (7); a similar reaction of 5 yielded [Re6Se8(PEt(3))(5)(MeCN)](2+) (8) and that of [R e6Se7(SeH)I-6](3-) led to fully substituted [Re6Se8(MeCN)(6)](2+) (9). The structures of clusters 2-9 as Bu(4)N(+) (2), iodide (5), SbF6- (7 ), or BF4- (6, 8, 9) salts were proven by X-ray crystallography. No si gnificant variance was observed in the face-capped geometry of the cor e when bound by iodine, phosphine, and nitrile ligands alone or in com bination. Terminal Re-P/I/N bond lengths were similarly independent of coligands. The combination of ligands of different lability in cluste rs 2-9 should make them of considerable utility in producing linked cl uster assemblies/materials wherein labile ligands are displaced by bri dging ligands. The simplest of these reactions, the direct coupling of two [Re6Se8](2+) cluster cores, is demonstrated here. When thermolyze d at 180 degrees C for 24 h under dynamic vacuum, the compounds [8]X(2 ) (X = BF4-, SbF6-) lose acetonitrile and condense to form two new com pounds containing the dicluster [Re12Se16(PEt(3))(10)](4+) (10), whose structure has been established by X-ray methods. This cluster is comp osed of two [Re6Se8](2+) portions which are centrosymmetrically connec ted by two Re-(mu(4)-Se) bonds to form a rhomboidal Re2Se2 unit. The b ridge bonds are ca. 0.1 Angstrom longer than the Re(mu(3)-Se) bonds of the remainder of the core; they have the same value as in the bridgin g rhombs present in the two-dimensional phase Re6Se8Cl2, which is a pr ecursor solid to molecular [Re6Se8](2+) clusters by dimensional reduct ion. The synthesis of 10 is directed by the structure of 8, which can give only one logical product upon deligation of acetonitrile. The syn thesis concept is potentially extendable to other types of oligomerize d clusters.