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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

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posted on 04.07.2007, 00:00 by Jürgen Stahl, Wolfgang Mohr, Laura de Quadras, Thomas B. Peters, James C. Bohling, José Miguel Martín-Alvarez, Gareth R. Owen, Frank Hampel, John A. Gladysz
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 35 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 79, 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 35 rapidly interconvert in solution. Cyclic voltammograms of 35 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.