Syntheses, homeomorphic and configurational isomerizations, and structures of macrocyclic aliphatic dibridgehead...

The diphosphine complexes cis- or trans-[upper bond 1 start]PtCl₂(P((CH₂) )₃P[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((CH₂) )₃)P (3b-e, 91-71%). These "empty cages" react with PdCl₂ or PtCl₂ sources to afford trans-[upper bond 1 start]MCl₂(P((CH₂) )₃P[upper bond 1 end]). Low temperature ³¹P 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: ΔH ^‡ 7.3/8.2 kcal mol^-1, ΔS ^‡ -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·2BH₃. The configurational stabilities of in,out-3b, c, e preclude phosphorus inversion in the interconversion of in,in and out,out isomers. Low temperature ³¹P NMR spectra of in,out-3b, c reveal degenerate in,out/out,in homeomorphic isomerizations (ΔG ^‡ _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·2BH₃ crystallize in out,out conformations, the former with a cycloalkane solvent guest inside.