ANHYDROUS MONOMERIC GLYOXAL PYROLYTIC GENERATION AND STABILIZATION BY THE CHIRAL RHENIUM FRAGMENT [ETA(5)-C5H5)RE(NO)(PPH3)]+

Reaction of [(eta5-C5H5)Re(NO)(PPh3)(ClCH2Cl)]+BF4- (5+BF4-) and a CH3OH/H2O glyoxal solution gives the pi complex [(eta5-C5H5)Re(NO)(PPh3)(eta2-O=ChCH=O)]+BF4- (1+BF4-) in 17% yield as a 95:5 mixture of RS,SR/RR,SS diastereomers. The trimeric dihydrate of glyoxal and P2O5 are pyrolyzed under vacuum at 160-degrees-C. Pure anhydrous monomeric glyoxal can be trapped at -80-degrees-C, as assayed by H-1 and C-13 NMR. Subsequent reaction with 5+BF4- gives, 1+BF4- (33 %; 95:5 RS,SR/RR,SS). The NMR and IR properties of 1+BF4- are studied in detail, especially with regard to the free and bound O=CH moieties. A H-1 NOE experiment suggests that an s-cis O=C-C=O conformer dominates in solution. The crystal structure of (RS,SR)-1+BF4-also shows an s-cis conformer, with free/bound O=C bond lengths of 1.206(7)/1.331(5) A. Reaction of 1+BF4- and NaI in CH3CN gives 1+I- (75%), which when placed in CH30H gives (eta5-C5H5)Re(NO)(PPh3)(I) (69%). Reaction of 1+BF4- and CD3CN at 80-degrees-C gives [(eta5-C5H5)Re(NO)(PPh3)(NCCD3)]+BF47. However, in none of these substitutions is free glyoxal detected. Well-defined additions to the glyoxal ligand could not be effected.