Preparation, structures, and redox and emission characteristics of the isothiocyanate complexes of hexarhenium(III) clusters [Re6(μ3-E)8(NCS)6]4- (E = S, Se)

Hexarhenium(III) complexes with terminal isothiocyanate ligands, {(n-C4H9)(4)N}(4)[Re-6(mu(3)-S)(8)(NCS)(6)] (1) and (L)(4)-[Re-6(mu(3)-Se)(8)(NCS)(6)] (L+ = PPN+ (2a), (n-C4H9)(4)N+ (2b)), have been prepared by three different methods. Complex 1 was prepared by the reaction of {(n-C4H9)(4)N}(4)[Re-6(mu(3)-S)(8)Cl-6] with molten KSCN at 200 degreesC, while 2b was obtained by refluxing the chlorobenzene-DMF (2:1 v/v) solution of [Re-6(mu(3)-Se)(8)(CH3CN)(6)](SbF6)(2) and [(n-C4H9)(4)N]SCN. The [Re-6(mu(3)-Se)(8)(NCS)(6)](4-) anion was also obtained from a mixture of Cs-2[Re-6(mu(3)-Se)(8)Br-4] 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 P2(1)/n (No. 14), a = 14.464(7) A, b = 22.059(6) Angstrom, c = 16.642(8) Angstrom, beta = 113.62(3)degrees, V = 4864(3) Angstrom(3), Z = 2; 2a.4DMF, C162H144N14O4P8S6Se8Re6, triclinic, space group P (1) over bar (No. 2), a 15,263(2) Angstrom, b = 16.429(2) Angstrom, c = 17.111(3) Angstrom, alpha = 84.07(1)degrees, beta = 84.95(1)degrees, gamma = 74.21(1)degrees, 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 Re-6(III)/(Re5ReIV)-Re-III process in acetonitrile are +0.84 and +0.70 V vs. Ag/AgCl for [Re-6(mu(3)-S)(8)(NCS)(6)](4-) and [Re-6(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)/mus), 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)(4)N}(4)[Re-6(mu(3)-S)(8)Cl-6] are also reported.