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 (η 5 -C 5 Me 5 )Re(NO)(PPh 3 )(C⋮CC⋮CH) ( ReC 4 H ) with Cu(OAc) 2 (pyridine, 80 °C) gives the μ-octatetraynediyl complex ReC 8 Re (70%). Analogous cross-coupling of ReC 4 H and ReC 2 H gives (after chromatography) ReC 4 Re (14%), ReC 6 Re (44%), and ReC 8 Re (15%). Longer sp carbon chains are accessed by reactions of ReC 4 H with n -BuLi and CuI, which give ReC 4 Cu . This isolable species is treated in situ with BrC⋮CSiEt 3 or BrC⋮CC⋮CSiMe 3 (excess EtNH 2, THF) to give ReC 6 SiEt 3 or ReC 8 SiMe 3 (84−77%). Desilylations (wet n -Bu 4 N + F - ) yield ReC 6 H or ReC 8 H (88−73%). Then Cu(OAc) 2 (pyridine, 50 °C) gives ReC 12 Re or ReC 16 Re (71−67%). The former is also available from ReC 4 Cu and BrC⋮CC⋮CBr (45%), and ReC 10 Re can be accessed by cross-coupling. ReC 6 H and ReC 8 H are similarly converted to ReC 10 SiR 3 (R = Me, Et; 51−26%) and ReC 12 SiMe 3 (43%). Desilylation of ReC 10 SiR 3 gives labile ReC 10 H, but only black powder is obtained from ReC 12 SiMe 3 . In situ coupling of ReC 10 H gives ReC 20 Re (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 ν C ⋮ C patterns (progressively more bands), 13 C 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 ReC 20 Re ), and thermal stabilities (solid-state decompositions at 155 °C, ReC 20 Re, and 178−217 °C, lower homologues) are studied in detail.