Preparation, Structures, and Redox and Emission Characteristics of the Isothiocyanate Complexes of Hexarhenium(III) Clusters [Re6 ((mu)3-E)8(NCS)6]^4- (E = S, Se)

Hexarhenium(III) complexes with terminal isothiocyanate ligands, {(n-C4H9)4N}4[Re6((mu)3-S)8(NCS)6] (1) and (L)4[Re6((mu)3-Se)8 (NCS)6] (L^+ = PPN^+ (2a), (n-C4H9)4N^+ (2b)), have been prepared by three different methods. Complex 1 was prepared by the reaction of {(n-C4H9)4N}4[Re6(3-S)8Cl6] with molten KSCN at 200 C, while 2b was obtained by refluxing the chlorobenzene-DMF (2:1 v/v) solution of [Re6((mu)3-Se)8(CH3CN)6](SbF6)2 and [(n-C4H9)4N]SCN. The [Re6((mu)3-Se)8(NCS)6]^4- anion was also obtained from a mixture of Cs2[Re6(3-Se)8Br4] and KSCN in C2H5OH by a mechanochemical activation at room temperature for 20 h and isolated as 2a. The X-ray structures of 1 and 2a·4DMF have been determined (1, C70H144N10S14Re6, monoclinic, space group P21/n (No. 14), a = 14.464(7) (angstrom), b = 22.059(6) (angstrom), c = 16.642(8) (angstrom), (beta)= 113.62(3), V = 4864(3) (angstrom)^3, Z = 2; 2a·4DMF, C162H144N14O4P8S6Se8Re6, triclinic, space group P (No. 2), a = 15.263(2) (angstrom), b = 16.429(2) (angstrom), c = 17.111(3) (angstrom),(alpha)= 84.07(1),(beta)= 84.95(1),(gamma) = 74.21(1), V = 4098.3(8) (angstrom)^3, Z = 1). All the NCS- ligands in both complexes are coordinated to the metal center via nitrogen site with the Re-N distances in the range of 2.07-2.13 (angstrom). The redox potentials of the reversible ReIII6/ReIII5ReIV process in acetonitrile are +0.84 and +0.70 V vs. Ag/AgCl for [Re6((mu)3-S)8 (NCS)6]^4- and [Re6((mu)3-Se)8(NCS)6]^4-, respectively, which are the most positive among the known hexarhenium complexes with six terminal anionic ligands. The complexes show strong red luminescence with the emission maxima ((lambda)max/nm), lifetimes ((tau) em/(mu)s), and quantum yields ((phi)em) being 745 and 715, 10.4 and 11.8, and 0.091 and 0.15 for 1 and 2b, respectively, in acetonitrile. The data reasonably well fit in the energy-gap plots of other hexarhenium(III) complexes. The temperature dependence of the emission spectra and (tau)em of 1 and {(n-C4H9)4N}4[Re6((mu)3-S)8Cl6] are also reported.