Monodisperse Molecular Models for the sp Carbon Allotrope Carbyne; Syntheses, Structures, and Properties of Diplatinum Polyynediyl Complexes with to Linkages.

Extended conjugated polyynes provide models for the elusive sp carbon polymer carbyne, but progress has been hampered by numerous synthetic challenges. Stabilities appear to be enhanced by bulky, electropositive transition-metal endgroups. Reactions of -(CF)(-tolP)Pt(C≡C)SiEt ( = 4-6, ( = 2)) with -BuNF/MeSiCl followed by excess tetrayne H(C≡C)SiEt () and then CuCl/TMEDA and O give the heterocoupling products , , and sometimes higher homologues. The species presumably arise via protodesilylation of under the reaction conditions. Chromatography allows the separation of , , and (from = 4), and ( = 5), or and ( = 6). These and previously reported species are applied in similar oxidative homocouplings, affording the family of diplatinum polyynediyl complexes ( = 20, 24, 28, 32, 36, 40 in 96-34% yields and = 44, 48, 52 in 22-7% yields). These are carefully characterized by C NMR, UV-visible, and Raman spectroscopy and other techniques, with particular attention to behavior as the C chain approaches the macromolecular limit and endgroup effects diminish. The crystal structures of solvates of , , and , which feature the longest sp chains structurally characterized to date, are analyzed in detail. All data support a polyyne electronic structure with a nonzero optical band gap and bond length alternation for carbyne.