Reactions of
trans,
trans-(C
6F
5)(
p-tol
3P)
2Pt(C
![](/images/entities/tbd1.gif)
C)
4Pt(P
p-tol
3)
2(C
6F
5) and diphosphines Ar
2P(CH
2)
mPAr
2 yield
ja0716103a10001">trans,trans-(C6F5)(Ar2ja071610300000">P(CH2)mPAr2)ja071610300001">Pt(C
C)4Pt(Ar2P(CH2)mPAr2)(C6F5), in which the platinumatoms are spanned via an sp and two sp
3 carbon chains (Ar/
m =
3, Ph/14, 87%;
4,
p-tol/14, 91%;
5,
p-C
6H
4-
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 shieldthe sp chain at approximately the van der Waals distance, with both enantiomers in the unit cell. Theplatinum square planes define angles of 196.6
![](/images/entities/deg.gif)
-189.9
![](/images/entities/deg.gif)
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(C
![](/images/entities/tbd1.gif)
C)
6Pt complex and Ph
2P(CH
2)
18PPh
2 gives an analogous adduct (27%). The crystal structure showstwo independent molecules, one helical and the other not. Low-temperature NMR data suggest that theenantiomeric helical conformations of
3-5 rapidly interconvert in solution. Cyclic voltammograms of
3-5show more reversible oxidations than model compounds lacking bridging sp
3 chains. These are the onlydouble-helical molecules that do not feature bonding interactions between the helix strands, or covalentbonds to templates dispersed throughout the strands, or any type of encoding. The driving force for helixformation is analyzed.