Synthesis and properties of chiral rhenium pentatetraenylidene complexes of the formula [(η5-C5Me5)Re(NO)(PPh3)(CCCCCAr2)]+BF4− (CAr2=9-fluorenylidene)

The sequential reaction of (η5-C5Me5)Re(NO)(PPh3)(CCCCH) with n-BuLi (one equivalent), 9-fluorenone (OCAr2; 1.5 equivalents), and Me3O+BF4− (3.0 equivalents) gives thermally labile (η5-C5Me5)Re(NO)(PPh3)(CCCCC(OMe)Ar2) (2a) as an orange–red powder in 69% yield. Analogous reactions with 2,7-dichloro- and 2,7-dibromofluorenone give the corresponding complexes 2b,c (68–71%). Treatment of 2a–c with BF3·OEt2 (one equivalent) gave the title complexes (3a–c) as labile dark blue powders (79–83%) that exhibited diagnostic cumulenic IR bands νCCC (1993–1987, 1902–1894 cm−1) and ReC 13C-NMR signals (256.9 ppm, JCP=10.8 Hz), and richly featured UV–vis spectra. Attempts to prepare stabile derivatives of 3a–c (additions of phosphines, amines, TCNE; anion metatheses) invariably gave a multitude of products.