A Synthetic Breakthrough into an Unanticipated Stability Regime: A Series of Isolable Complexes in which C6, C8, C10, C12, C16, C20, C24, and C28 Polyynediyl Chains Span Two Platinum Atoms

The reaction of trans‐[PtCl(p‐tol){P(p‐tol)3}2] (PtCl) and H(CC)2H (cat. CuI, HNEt2) gives PtC4H (82 %), which can be cross‐coupled with excess HCCSiEt3 (acetone, O2, CuCl/TMEDA; Hay conditions) to yield PtC6Si (77 %). The addition of nBu4N+F− in wet acetone gives PtC6H (84 %), and further addition of ClSiMe3 (F− scavenger) and excess HCCSiEt3 (Hay conditions) yields PtC8Si (23 %). Similar cross‐coupling reactions of PtCxH (generated in situ for x>6) and excess H(CC)2SiEt3 give a) x=4, PtC8Si (29 %), PtC12Si (30 %), and PtC16Si (1 %); b) x=6, PtC10Si (59 %) and PtC14Si (7 %); c) x=8, PtC12Si (42 %); and d) x=10, PtC14Si (20 %). Hay homocoupling reactions of PtC4H, PtC6H, PtC8H, and PtC10H give PtC8Pt, PtC12Pt, PtC16Pt, and PtC20Pt (88–70 %), but PtC12H decomposes too rapidly. However, when PtC12Si and PtC14Si are subjected to Hay conditions, protodesilylation occurs in the presence of the oxidizing agent and PtC24Pt (36 %) and PtC28Pt (51 %) are isolated. Reactions of PtC6H and PtC10H with PtCl (CuI, HNEt2) give PtC6Pt (56 %) and PtC10Pt (84 %). The effect of the chain lengths in PtCxPt upon thermal stabilities (>200 °C for x≤20), IR νCC patterns (progressively more bands), colors (yellow to orange to deep red), UV/Vis spectra (progressively red‐shifted and more intense bands with ε>400 000 M−1 cm−1), redox properties (progressively more difficult oxidations), and NMR spectra (many monotonic trends) are analyzed, including implications for the sp carbon allotrope carbyne. Whereas all other dodecaynes and tetradecaynes rapidly decompose at room temperature, PtC24Pt and PtC28Pt remain stable at >140 °C. Crystal structures of PtCxSi (x=6, 8, 10) and PtCxPt (x=6, 8, 10, 12) have been determined. Shattering the record for the longest isolable polyynes (an example of which is shown here) is made possible by increasingly sophisticated deprotection and oxidative coupling strategies that minimize the decomposition of labile [(L)mM((CC)nH)] intermediates. There is no indication that any type of stability limit has been reached with the title compounds, the PtCxPt distances of which extend to 3.9 nm.