A Synthetic Breakthrough into an Unanticipated Stability Regime:  Readily Isolable Complexes in which C16−C28 Polyynediyl Chains Span Two Platinum Atoms

• Published in: Journal of the American Chemical Society 2005-08, Vol.127 (30), p.10508-10509
• Main Authors: Zheng, Qinglin, Gladysz, J. A
• Format: Article
• Language: English
• Subjects: Chemistry; Exact sciences and technology; Organic chemistry; Organometalloidal and organometallic compounds; Preparations and properties; Transition metals derivatives
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja0534598

The oxidative cross-coupling of trans-(p-tol)(p-tol3P)2Pt(C⋮C)2H (PtC4H) and excess H(C⋮C)2SiEt3 (O2, cat. CuCl, TMEDA, acetone; Hay conditions) gives PtC8Si (29%), PtC12Si (30%), and PtC16Si (1%; Si = SiEt3). The less stable PtC6H is generated in situ from PtC6Si and n-Bu4N+F-; following the addition of ClSiMe3 (F- scavenger) and excess H(C⋮C)2SiEt3, the Hay conditions afford PtC10Si (59%) and PtC14Si (7%). Analogous sequences can be conducted with PtC8Si and PtC10Si. When PtC6Si, PtC8Si, and PtC10Si are similarly reacted in the absence of H(C⋮C)2SiEt3, homocouplings to PtC12Pt (88%), PtC16Pt (70%), and PtC20Pt (72%) occur. However, analogous reactions of PtC12Si and PtC14Si fail, presumably due to the rapid decomposition of PtC12H and PtC14H. Brønsted acidity trends suggest that (C⋮C) n moieties should become better leaving groups with increasing chain length. Thus, when PtC12Si and PtC14Si are subjected to the Hay conditions alone, PtC24Pt (36%) and PtC28Pt (51%) are isolated, with substantial recovery of starting material from the former reaction. Presumably adventitious water and/or other nucleophiles effect desilylation (as also seen in the cross-couplings), after which homocoupling is rapid due to the simultaneous availability of an oxidizing agent. PtC24Pt and PtC28Pt are thermally stable to ≥140 °C, whereas all other known dodecaynes and tetradecaynes rapidly decompose at room temperature. There is every reason to believe that this series can be further extended. UV−visible spectra show progressively red-shifted and more intense bands with ε > 400 000 M-1 cm-1.