Monodisperse Molecular Models for the sp Carbon Allotrope Carbyne; Syntheses, Structures, and Properties of Diplatinum Polyynediyl Complexes with PtC₂₀Pt to PtC₅₂Pt Linkages

Abstract

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-(C 6 F 5 )(p-tol 3 P) 2 Pt(C�C) n SiEt 3 (n = 4−6, PtC x Si (x = 2n)) with n-Bu 4 N + F − /Me 3 SiCl followed by excess tetrayne H(C�C) 4 SiEt 3 (HC 8 Si) and then CuCl/TMEDA and O 2 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, 28,32,36,48,. These are carefully characterized by 13 C 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.

Reactions of Platinum Terminal Polyynyl Complexes trans-(C6F5)(p-tol3P)2Pt(C≡C)nH (n = 2–4) and n-BuLi, Generation of Functional Equivalents of Pt(C≡C)nLi Species, and Derivatization with Organic and Inorganic Electrophiles

“…Individually, the crystal structures of PtC 4 Me , PtC 6 Me , and PtC 8 Me are routine, with bond lengths, bond angles, and arene/arene stacking interactions similar to many other alkynyl and polyynyl adducts of trans -(C 6 F 5 )­( p -tol 3 P) 2 Pt. , In accordance with computational predictions and experimental analyses, the Pt–C bond lengths contract from 2.009(2) to 1.983(3) Å. In principle, the carbon–carbon bond lengths should also exhibit monotonic trends, but as is often the case with lighter atoms, the ESD values are too high for rigorous conclusions.…”

“…All instrumentation and characterization protocols were identical to those in recent full papers in this series. ,, These are summarized, together with chemical sourcing and purification, in the Supporting Information. All reactions were conducted under dry inert atmospheres using conventional Schlenk techniques, but workups were carried out in air.…”

“…For some time, we have been studying the syntheses, structures, reactivities, and electronic properties of bimetallic polyynediyl complexes of the formula L y M­(CC) n ML y . − One objective has been to access adducts of very long sp carbon chains, for example with n > 25 (MC >50 M). Parallel studies of extended polyynes with organic end groups have been carried out by other researchers. , All parties have sought models for the elusive polymeric sp carbon allotrope, carbyne, and probed numerous physical properties as a function of chain length.…”

“…Synthetic efforts have utilized various homocoupling and heterocoupling reactions of terminal alkynes. Our most recent endeavors have involved platinum­(II) end group of the formula (C 6 F 5 )­( p -tol 3 P) 2 Pt …”

Impact of Halogen Termination and Chain Length on π-Electron Conjugation and Vibrational Properties of Halogen-Terminated Polyynes

“…Despite these remarkable properties, the application of sp-carbon chains is limited due to their poor stability. Indeed, they suffer from ozonolysis processes, are sensitive to heating and light irradiation, and can undergo cross-linking reactions between adjacent chains, leading to a rearrangement into more stable sp 2 structures. − Among different adopted strategies to stabilize these compounds, the chemical synthesis of sp-carbon chains with bulky terminations is one of the most investigated routes, − producing up to gram-scale stable solid-state samples of sp-carbon chains. ,,− …”