Towards multistranded molecular wires: Syntheses, structures, and reactivities of tetraplatinum bis(polyynediyl) complexes with Pt-Cx-Pt-(P(CH2)3P)2-Pt-Cx-Pt-(P(CH2)3P)2 cores (x = 4, 6, 8)

Abstract

Reactions of trans,trans-(C₆F₅)(p-tol₃P)₂Pt(CC)_nPt(Pp-tol₃)₂(C₆F₅) (PtC_xPt; x = 2n) and the 1,3-diphosphine Ph₂P(CH₂)₃PPh₂ (2.5 equiv) give the tetraplatinum complexes trans, trans,trans,trans-(C₆F₅)Pt(CC)_nPt(C₆F₅)(PPh₂(CH₂)₃Ph₂P)₂(C₆F₅)Pt(CC)_nPt(C₆F₅)(PPh₂(CH₂)₃Ph₂P)₂ ([Pt′C_xPt′]₂; x = 4/6/8, 39%/95%/81%). Crystal structures of [Pt′C₈Pt′]₂ and two solvates of [Pt′C₆Pt′]₂ are determined. These confirm that each diphosphine spans two platinum atoms from different Pt(CC)_nPt linkages, as opposed to (1) the 1,2-diphosphine Ph₂P(CH₂)₂PPh₂, which under similar conditions with PtC₈Pt affords the diplatinum bis(chelate) cis,cis-(PPh₂(CH₂)₂Ph₂P)(C₆F₅)Pt(CC)₄Pt(C₆F₅)(PPh₂(CH₂)₂Ph₂P) (73%) or (2) α,ω-diphosphines with longer methylene chains, which span the platinum termini. The formulation [Pt′C₄Pt′]₂ is supported by a reaction with PEt₃ (10 equiv) to give trans,trans-(C₆F₅)(Et₃P)₂Pt(CC)₂Pt(PEt₃)₂(C₆F₅). In [Pt′C₈Pt′]₂ and one solvate of [Pt′C₆Pt′]₂, the chains cross at 77.2°–87.7° angles, with the closest interchain carbon–carbon distances (3.27–3.61 Å) less than the sum of the van-der-Waals radii. In the other solvate of [Pt′C₆Pt′]₂, the chains are essentially parallel, and the separation is much greater (4.96 Å). UV-visible spectra show no special electronic interactions. However, cyclic voltammograms indicate irreversible oxidations, in contrast to the partially reversible oxidations of PtC₆Pt and PtC₈Pt. The initially formed radical cations are proposed to undergo rapid chain–chain coupling. The new complexes decompose without melting above 185 °C. With [Pt′C₈Pt′]₂, IR spectra indicate the formation of a new CC-rich substance.