Syntheses, homeomorphic and configurational isomerizations, and structures of macrocyclic aliphatic dibridgehead diphosphines; molecules that turn themselves inside out.

The diphosphine complexes cis - or trans -[upper bond 1 start]PtCl 2 (P((CH 2 ) n ) 3 P[upper bond 1 end]) ( n = b/12, c/14, d/16, e/18) are demetalated by MC[triple bond, length as m-dash]X nucleophiles to give the title compounds (P((CH 2 ) n ) 3 )P (3b-e, 91-71%). These "empty cages" react with PdCl 2 or PtCl 2 sources to afford trans -[upper bond 1 start]MCl 2 (P((CH 2 ) n ) 3 P[upper bond 1 end]). Low temperature 31 P NMR spectra of 3b and c show two rapidly equilibrating species (3b, 86 : 14; 3c, 97 : 3), assigned based upon computational data to in , in (major) and out , out isomers. These interconvert by homeomorphic isomerizations, akin to turning articles of clothing inside out (3b/c: Δ H ‡ 7.3/8.2 kcal mol -1 , Δ S ‡ -19.4/-11.8 eu, minor to major). At 150 °C, 3b, c, e epimerize to (60-51) : (40-49) mixtures of ( in , in / out , out ) :  in , out isomers, which are separated via the bis(borane) adducts 3b, c, e·2BH 3 . The configurational stabilities of in , out -3b, c, e preclude phosphorus inversion in the interconversion of in , in and out , out isomers. Low temperature 31 P NMR spectra of in , out -3b, c reveal degenerate in , out / out , in homeomorphic isomerizations (Δ G ‡ Tc 12.1, 8.5 kcal mol -1 ). When ( in , in / out , out )-3b, c, e are crystallized, out , out isomers are obtained, despite the preference for in , in isomers in solution. The lattice structures are analyzed, and the D 3 symmetry of out , out -3c enables a particularly favorable packing motif. Similarly, ( in , in / out , out )-3c, e·2BH 3 crystallize in out , out conformations, the former with a cycloalkane solvent guest inside.