Toward Metal-Capped One-Dimensional Carbon Allotropes: Wirelike C₆−C₂₀Polyynediyl Chains That Span Two Redox-Active (η⁵-C₅Me₅)Re(NO)(PPh₃) Endgroups

Abstract

Reaction of the butadiynyl complex (η5-C5Me5)Re(NO)(PPh3)(C⋮CC⋮CH) (ReC4H) with Cu(OAc)2 (pyridine, 80 °C) gives the μ-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⋮CSiEt3 or BrC⋮CC⋮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 °C) gives ReC12Re or ReC16Re (71−67%). The former is also available from ReC4Cu and BrC⋮CC⋮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; ε >190 000 M-1 cm-1), IR/Raman νC⋮C patterns (progressively more bands), 13C NMR chemical shifts (asymptotic limit of 64−67 ppm for ReC⋮C(C⋮C)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 °C, ReC20Re, and 178−217 °C, lower homologues) are studied in detail.