Toward metal-capped one-dimensional carbon allotropes: Wirelike C-6-C-20 polyynediyl chains that span two redox-active (eta(5)-C5Me5)Re(NO)(PPh3) endgroups

Reaction of the butadiynyl complex (eta(5)-C5Me5)Re(No)(PPh3)(C drop CC dro p CH) (ReC4H) with Cu(OAc)(2) (pyridine, 80 degrees C) gives the mu-octatet raynediyl complex ReC8Re (70%). Analogous cross-coupling of ReC4H and ReC2H gives (after chromatography) ReC4Re (14%), ReC6Re (44%), and ReC8Re (15%). Longer sp carbon chains are accessed by reactions of ReC4H with n-BuLi and CuI, which give ReC4Cu. This isolable species is treated in situ with BrC drop CSiEt3 or BrC drop CC drop CSiMe3 (excess EtNH2, THF) to give ReC6SiEt 3 or ReC8SiMe3 (84-77%). Desilylations (wet n-Bu4N+F-) yield ReC6H or ReC8H (88-73%). Then Cu(OAc)(2) (pyridine, 50 degrees C) gives ReC12Re or ReC16R e (71-67%). The former is also available from ReC4Cu and BrC drop CC drop C Br (45%), and ReC10Re can be accessed by cross-coupling. ReC6H and ReC8H ar e similarly converted to ReC10SiR3 (R = Me, Et; 51-26%) and ReC12SiMe3 (43% ). Desilylation of ReC10SiR3 gives labile ReC10H, but only black powder is obtained from ReC12SiMe3. In situ coupling of ReC10H gives ReC20Re (52-34%) , which unlike lower homologues is not obtained in analytically pure form. The effects of chain length upon visible spectra (progressively red-shifted and more intense bands; epsilon > 190 000 M-1 cm(-1)), IR/ Raman nu(C drop C) patterns (progressively more bands),C-13 NMR chemical shifts (asymptoti c limit of 64-67 ppm for ReC drop C((C) under bar drop (C) under bar)(n)), cyclic voltammetry (decreased reversibility of two oxidations; a gradual sh ift of the first to thermodynamically less favorable potentials, so that on ly a single oxidation is observed for ReC20Re), and thermal stabilities (so lid-state decompositions at 155 degrees C, ReC20Re, and 178-217 degrees C, lower homologues) are studied in detail.