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 led to the first molecular clusters containing the face-capped octahedral [Re-6(mu(3)-Q)(8)](2+) core (Q = S, Se). Among these is the protonated cluster [Re6Se7(SeH)I-6](3-), whose substitution reactions have been examined in an effort to create a set of site-differentiated precursors for forming multicluster assemblies. Reaction with Et(3)P under reflux with different conditions of reactant mole ratio and time affords 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 quantitative 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 [Re6Se7(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 significant variance was observed in the face-capped geometry of the core when bound by iodine, phosphine, and nitrile ligands alone or in combination. Terminal Re-P/I/N bond lengths were similarly independent of coligands. The combination of ligands of different lability in clusters 2-9 should make them of considerable utility in producing linked cluster assemblies/materials wherein labile ligands are displaced by bridging ligands. The simplest of these reactions, the direct coupling of two [Re6Se8](2+) cluster cores, is demonstrated here. When thermolyzed 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 compounds containing the dicluster [Re12Se16(PEt(3))(10)](4+) (10), whose structure has been established by X-ray methods. This cluster is composed of two [Re6Se8](2+) portions which are centrosymmetrically connected by two Re-(mu(4)-Se) bonds to form a rhomboidal Re2Se2 unit. The bridge 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 bridging rhombs present in the two-dimensional phase Re6Se8Cl2, which is a precursor solid to molecular [Re6Se8](2+) clusters by dimensional reduction. The synthesis of 10 is directed by the structure of 8, which can give only one logical product upon deligation of acetonitrile. The synthesis concept is potentially extendable to other types of oligomerized clusters.