sp Carbon Chains Surrounded by sp³ Carbon Double Helices:  Coordination-Driven Self-Assembly of Wirelike Pt(C⋮C)_nPt Moieties That Are Spanned by Two P(CH₂)_mP Linkages

Reactions of trans,trans-(C₆F₅)(p-tol₃P)₂Pt(C⋮C)₄Pt(Pp-tol₃)₂(C₆F₅) and diphosphines Ar₂P(CH₂)_mPAr₂ yield in which the platinum atoms are spanned via an sp and two sp³ carbon chains (Ar/m = 3, Ph/14, 87%; 4, p-tol/14, 91%; 5, p-C₆H₄-t-Bu/14, 77%; 7, Ph/10, 80%; 8, Ph/11, 80%; 9, Ph/12, 36%; only oligomers form for m > 14). Crystal structures of 3−5 show that the sp³ chains adopt chiral double-helical conformations that shield the sp chain at approximately the van der Waals distance, with both enantiomers in the unit cell. The platinum square planes define angles of 196.6°−189.9° or more than a half twist. Crystal structures of 7−9, which have shorter sp³ chains, exhibit nonhelical conformations. Reaction of the corresponding Pt(C⋮C)₆Pt complex and Ph₂P(CH₂)₁₈PPh₂ gives an analogous adduct (27%). The crystal structure shows two independent molecules, one helical and the other not. Low-temperature NMR data suggest that the enantiomeric helical conformations of 3−5 rapidly interconvert in solution. Cyclic voltammograms of 3−5 show more reversible oxidations than model compounds lacking bridging sp³ chains. These are the only double-helical molecules that do not feature bonding interactions between the helix strands, or covalent bonds to templates dispersed throughout the strands, or any type of encoding. The driving force for helix formation is analyzed.