Electronic communication mediated by a PtI-PtI σ-bond

Abstract

A binuclear complex [Pt2(dppm)2(C≡CFc)2] (1) (dppm = bis(diphenylphosphino)methane, Fc = C5H5Fe≡C5H4, ferrocenyl) containing two redox-active ferrocenylacetylides linked by a PtI-PtI σ-bond is designed to probe electronic communication through a C≡C-Pt-Pt-C≡C linkage. The X-ray crystal structure of the complex shows a Pt-Pt bond distance of 2.7023(2) Å and a Fe-Fe separation of 14.474(2) Å. The cyclic voltammogram (CV) of 1 displays two quasi-reversible one-electron Fc oxidations (Δp ≈ 70 mV at 20 mV s-1, ipc/ipa ≈ 1) separated by 267 ± 10 mV, and accordingly the equilibrium constant for the comproportionation 12+ + 1 ↔ 2 1+ is estimated to be (3.3 ± 1.5) × 104. These suggest considerable electronic communication between the redox centers in the mixed-valence complex 1-. The UV-vis-NIR absorption spectrum of 1 shows an intervalence-charge-transfer band at 11 300 ± 50 cm-1 (∈max = 610 ± 10 M-1 cm-1), and the electronic coupling parameter HAB is estimated to be 190 ± 20 cm-1. Addition of d10 AuICl and AuIBr fragments to the Pt-Pt bond gives rise to two A-frame clusters [Pt2(μ-AuX)(dppm)2(C≡CFc)2] (X = Cl or Br), whose X-ray crystal structures are composed of isosceles triangles of Pt2Au. In contrast to 1, electronic communication through the Pt2Au cluster is very weak, as the CVs of the compounds exhibit no splitting of Fc oxidations. Possible reasons for the shutdown of electronic communication are discussed.