Site-differentiated hexanuclear rhenium(III) cyanide clusters [Re6Se8(PEt3)n(CN)6-n]n-4 (n = 4, 5) and kinetics of solvate ligand exchange on the cubic [Re6Se8]2+ core

The site-differentiated, cyanide-substituted hexanuclear rhenium(III) selenide clusters cis- and trans-[Re6Se8(PEt3)(4-)(CN)(2)] and [Re6Se8(PEt3)(5)(CN)](+) have been prepared from heterogeneous reactions of the corresponding iodo clusters with AgCN in refluxing chloroform. Isolated yields are 68%, 46%, and 64% for cis-[Re6Se8(PEt3)(4)(CN)(2)], trans-[Re6Se8(PEt3)(4)(CN)(2)], and [Re6Se8(PEt3)(5)(CN)](+), respectively. The new compounds are air- and water-stable and are characterized by X-ray diffraction crystallography, P-31 NMR and IR spectroscopies, and FAB mass spectrometry. In related work, the solvent exchange rates of two site-differentiated monosolvate clusters, [Re6Se8(PEt3)(5)(MeCN)](SbF6)(2) and [Re6Se8(PEt3)(5)(Me2SO)](SbF6)(2), in neat solvents were measured by H-1 NMR. These clusters are substitutionally inert; k approximate to 10(-5)-10(-6) s(-1) at 318 K. Activation parameters indicate a dissociative ligand exchange mechanism; DeltaH(#) values obtained from least-squares fitting of temperature-dependent kinetics data exceed RT by a factor of ca. 50 over the temperature range studied. These results demonstrate that the substitutional lability encountered in a previous study of cluster photophysics cannot result from ground-state thermal reactions.