sp carbon chains surrounded by sp3 carbon double helices:: Coordination-driven self-assembly of wirelike Pt(CC)nPt moieties that are spanned by two P(CH2)mP linkages

Reactions of trans,trans-(C6F5)(p-tol(3)P)(2)Pt(C C)(4)Pt(Pp-tol(3))(2)(C6F5) and diphosphines Ar2P(CH2)(m)-PAr2 yield trans,trans-(C6F5)(Ar2P(CH2)(m)PAr2)Pt(C C)(4)Pt(Ar2P(CH2)(m)PAr2)(C6F5), in which the platinum atoms are spanned via an sp and two sp(3) 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 sp(3) 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 degrees-189.9 degrees or more than a half twist. Crystal structures of 7-9, which have shorter sp(3) chains, exhibit nonhelical conformations. Reaction of the corresponding Pt(CC)(6)Pt complex and Ph2P(CH2)(18)PPh2 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 sp(3) 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.