Monodisperse Molecular Models for the sp Carbon Allotrope Carbyne; Syntheses, Structures, and Properties of Diplatinum Polyynediyl Complexes with PtC20Pt to PtC52Pt 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 trans-(C6F5)­(p-tol3P)2Pt­(CC) n SiEt3 (n = 4–6, PtC x Si (x = 2n)) with n-Bu4N+F–/Me3SiCl followed by excess tetrayne H­(CC)4SiEt3 (HC 8 Si) and then CuCl/TMEDA and O2 give the heterocoupling products PtC x +8 Si, PtC x +16 Si, and sometimes higher homologues. The PtC x +16 Si species presumably arise via protodesilylation of PtC x +8 Si under the reaction conditions. Chromatography allows the separation of PtC 16 Si, PtC 24 Si, and PtC 32 Si (from n = 4), PtC 18 Si and PtC 26 Si (n = 5), or PtC 20 Si and PtC 28 Si (n = 6). These and previously reported species are applied in similar oxidative homocouplings, affording the family of diplatinum polyynediyl complexes PtC x Pt (x = 20, 24, 28, 32, 36, 40 in 96–34% yields and x = 44, 48, 52 in 22–7% yields). These are carefully characterized by 13C NMR, UV–visible, and Raman spectroscopy and other techniques, with particular attention to behavior as the C x chain approaches the macromolecular limit and endgroup effects diminish. The crystal structures of solvates of PtC 20 Pt, PtC 24 Pt, and PtC 26 Si, 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.