SYNTHESIS, STRUCTURE, AND REACTIVITY OF CHIRAL RHENIUM ALKYNE COMPLEXES OF THE FORMULA [(ETA-5-C5H5)RE(NO)(PPH3)(RCCR')]+BF4-

Reactions of [(eta-5-C5H5)Re(NO)(PPh3)(ClC6H5)]+BF4- and RC = CR' (R = R' = H (a), CH3 (b), CH2CH3 (c), C6H5 (d); R/R' = C(CH3)3/H (e)) give alkyne complexes [(eta-5-C5H5)Re(NO)(PPh3)(RC = CR')]+BF4- (4a-e; 73-97%). A crystal structure of 4c.ClCH2CH2Cl (orthorhombic, Pbca; a = 15.724 (1) angstrom, b = 18.593 (2) angstrom, c = 22.128 (2) angstrom, Z = 8) shows a 3-hexyne ligand conformation that directs the ethyl substituents syn and anti to the PPh3 ligand. The C = C bond is only slightly longer (1.24 (1) angstrom) than in free alkynes, but the propargylic carbons are bent back from the C = C axis (angle C = C-C = 143.4 (8), 147 (1)-degrees). NMR data show high barriers to alkyne ligand rotation (4b,c: DELTA-G double-ended-dagger (180-degrees-C) greater-than-or-equal-to 22 kcal/mol). Difference H-1 NOE and decoupling experiments enable detailed assignments of NMR resonances. Reactions of 4a,e and t-BuO-K+ give acetylide complexes (eta-5-C5H5)Re(NO)(PPh3)(C = CR') (5a,e; 74-97%). Complex 4e rearranges (80-degrees-C, C6H5Cl, 1 h) to vinylidene complex [(eta-5-C5H5)Re(NO)(PPh3)(= C = CHC(CH3)3)]+BF4-, a sample of which can be independently prepared from 5e and HBF4.O(CH2CH3)2 (92%). The physical and chemical properties of 4a-e are compared to those of other alkyne complexes.