SYNTHESIS OF DINUCLEAR RHENIUM(VI) OXO SULFIDO COMPOUNDS

Reaction of the d1-d1 dimer syn-[Re(mu-O)O(CH2CMe3)2]2 in pyridine with 1 equiv of H2S at 0-degrees-C for 2 h gave syn-Re2(mu-O)(mu-S)O2(CH2CMe3)4 in 86% yield. With 2 equiv of H2S at 23-degrees-C, a mixture of golden yellow syn-[Re(mu-S)O(CH2CMe3)2]2 and red anti-[Re(mu-S)O(CH2CMe3)2]2 was produced (total yield 76%). The red anti isomer was isolated from the mixture by low-temperature fractional crystallization from toluene; crystals of the isomers were also separated mechanically. Gentle heating (40-degrees-C) of anti-[Re(mu-S)O(CH2CMe3)2]2 in pyridine gave syn-[Re(mu-S)O(CH2CMe3)2]2 in 88% yield. The isomerization was reversible in pyridine solvent. In NMR experiments, anti-[Re(mu-S)O(CH2CMe3)2]2 was converted to the syn isomer in benzene-d6 at 23-degrees-C in 2 weeks, but the isomerization was accompanied by decomposition. Addition of a trace of bipyridine to the benzene-d6 solution resulted in a faster (2 days at 23-degrees-C) and cleaner (> 90% yield by H-1 NMR integration) anti to syn isomerization. A d1 Re(O)(S)(CH2CMe3)2 intermediate in the isomerization is proposed. The syn and anti isomers of [Re(mu-S)O(CH2CMe3)2]2 were characterized by X-ray crystallography. In each case the rhenium centers have approximately square-pyramidal geometries with the oxo ligands in the apical positions. The syn isomer has virtual C2v symmetry and the anti C2h symmetry. The Re-Re distances are 2.759 (3) angstrom (syn) and 2.871 (2) angstrom (anti). Crystal data for the syn form of Re2S2O2C20H44 at-20 (1) degrees-C: monoclinic, space group C2, a = 23.82 (1) angstrom, b = 5.788 (5) angstrom, c = 19.124 (8) angstrom, beta = 98.47 (2)-degrees, Z = 2. Crystal data for the anti form of Re2S2O2C20H44 at -79 (1)-degrees-C: monoclinic, space group P21/n, a = 13.453 (3) angstrom, b = 6.234 (2) angstrom, c = 14.949 (3) angstrom, beta = 94.50 (2)-degrees, Z = 2.