Ligand Substitution Reactions of [Re₆S₈Br₆]⁴^-: A Basis Set of Re₆S₈ Clusters for Building Multicluster Assemblies

Abstract

Access to the molecular solution chemistry of face-capped octahedral cluster cores of the type [Re6(μ3-Q)8]2+ (Q = S, Se) has recently been realized in this laboratory by the protocol of dimensional reduction of solid frameworks containing bridged cores. A new series of triethylphosphine-ligated hexanuclear rhenium clusters has been prepared from the starting cluster [Re6S8Br6]4- by ligand substitution of terminal bromide ligands using excess Et3P in DMF at elevated temperatures. These clusters are of the general formula [Re6S8(PEt3)nBr6-n](n-4)+ (n = 2−6). The following compounds were synthesized: (Bu4N)2[cis-Re6S8(PEt3)2Br4] (in limited quantity), (Bu4N)2[trans-Re6S8(PEt3)2Br4], (Bu4N)[mer-Re6S8(PEt3)3Br3], (Bu4N)[fac-Re6S8(PEt3)3Br3], cis-[Re6S8(PEt3)4Br2], trans-[Re6S8(PEt3)4Br2], [Re6S8(PEt3)5Br]Br, and [Re6S8(PEt3)6]Br2. All clusters exhibit characteristic 31P NMR spectra, and all except (Bu4N)[fac-Re6S8(PEt3)3Br3] have been characterized by X-ray crystallography. The [Re6S8]2+ core is analogous to the [Re6Se8]2+ core also under investigation in this laboratory, but it has the advantage of increased thermal control over the substitution process. Both disubstituted isomers, cis- and trans-[Re6S8(PEt3)2Br4]2-, are accessible with the [Re6S8]2+ core but not with [Re6Se8]2+. This set of clusters, together with [Re6Se8(PEt3)nI6-n](n-4)+ (n = 3−6) prepared earlier, constitute an array of building blocks for the potential synthesis of multicluster assemblies by halide ligand substitution with bridging ligands or with ligands subject to removal by thermolysis.