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
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abstract
Reaction of the butadiynyl complex (5- C5Me5)Re(NO)(PPh3)(CCCCH) (ReC4H) with Cu(OAc)2 (pyridine, 80 C) gives the -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 BrCCSiEt3 or BrCCCCSiMe3 (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 BrCCCCBr (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; > 190 000 M-1 cm-1), IR/ Raman V(CC) patterns (progressively more bands), 13C NMR chemical shifts (asymptotic limit of 64-67 ppm for ReCC(CC)), 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.
