Toward metal-capped one-dimensional carbon allotropes:: Wirelike C6-C20 polyynediyl chains that span two redox-active (η5-C5Me5)Re(NO)(PPh3) endgroups

Reaction of the butadiynyl complex (eta(5)-C5Me5)Re(No)(PPh3)(C drop CC drop CH) (ReC4H) with Cu(OAc)(2) (pyridine, 80 degrees C) gives the mu-octatetraynediyl 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 ReC6SiEt3 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 ReC16Re (71-67%). The former is also available from ReC4Cu and BrC drop CC drop CBr (45%), and ReC10Re can be accessed by cross-coupling. ReC6H and ReC8H are 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 (asymptotic 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 shift of the first to thermodynamically less favorable potentials, so that only a single oxidation is observed for ReC20Re), and thermal stabilities (solid-state decompositions at 155 degrees C, ReC20Re, and 178-217 degrees C, lower homologues) are studied in detail.