Reactions of trans,trans-(C6F5)(p-tol3P)2Pt(CC)4Pt(Pp-tol3)2(C6F5) and diphosphines Ar2P(CH2)mPAr2 yield trans,trans-(C6F5)(Ar2P(CH2)mPAr2)Pt(CC)4Pt(Ar2P(CH2)mPAr2)(C6F5), 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 degrees -189.9 degrees or more than a half twist. Crystal structures of 7-9, which have shorter sp3 chains, exhibit nonhelical conformations. Reaction of the corresponding Pt(CC)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.

Reactions of trans-(C6F5)(Ph2P(CH2)m'CH=CH2)2PtCl (1; m' = a, 6; b, 7; c, 8; d, 9; e, 10) and H(CC)2H (HNEt2, cat. CuI) give trans-(C6F5)(Ph2P(CH2)m'CH=CH2)2Pt(CC)2H (3a-e, 80-95%). Oxidative homocouplings of 3a-d under Hay conditions (O2, cat. CuCl/TMEDA, acetone) yield trans,trans-(C6F5)(Ph2P(CH2)m'CH=CH2)2Pt(CC)4Pt(Ph2P(CH2)m'CH=CH2)2(C6F5) (4a-d, 64-84%). Treatment of 3c-e with excess HCCSiEt3 under Hay conditions gives trans-(C6F5)(Ph2P(CH2)m'CH=CH2)2Pt(CC)3SiEt3 (56-73%). Homocouplings (n-Bu4N+ F-, Me3SiCl, Hay conditions) afford trans,trans-(C6F5)(Ph2P(CH2)m'CH=CH2)2Pt(CC)6Pt(Ph2P(CH2)m'CH=CH2)2(C6F5) (13c-e, 59-64%). Reactions of 4a-d and 13c-e with Grubbs' catalyst, followed by hydrogenation, give mixtures of trans,trans-(C6F5)(Ph2P(CH2)mPPh2)Pt(CC)nPt(Ph2P(CH2)mPPh2)(C6F5) with termini-spanning diphosphines and trans,trans-(C6F5)(Ph2P(CH2)mPPh2)Pt(CC)nPt(Ph2P(CH2)mPPh2)(C6F5) with trans-spanning diphosphines (m = 2m' + 2; n = 4, 6). The latter (n = 4) are independently synthesized by similar metatheses/hydrogenations of 1a-d to give trans-(C6F5)(Ph2P(CH2)mPPh2)PtCl (49-59%), followed by analogous introductions of (CC)4 chains (66-77%). Crystal structures of complexes with termini-spanning diphosphines show sp3 chains with both double-helical (m/n = 20/4) and nonhelical (m/n = 20/6) conformations, and highly shielded sp chains. The sp3 chains of complexes with trans-spanning diphosphines exhibit double half-clamshell conformations. The dynamic properties of both classes of molecules are analyzed in detail.

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