Molecules that Turn Themselves Inside-Out: Tuning in/out Equilibria and Homeomorphic Isomerization in Macrobicyclic Dibridgehead Diphosphines P((CH ) ) P Newly Accessible by Earth-Abundant-Metal Templates.

Photolyses of trans-Fe(CO) (P((CH ) ) P) (n=10 (a), 12 (b), 14 (c), 16 (d), 18 (e)) in the presence of PMe provide the first economical and scalable route to macrobicyclic dibridgehead diphosphines P((CH ) ) P (1). These are isolated as mixtures of in,in/out,out isomers that equilibrate with degenerate in,out/out,in isomers at 150 °C via pyramidal inversion at phosphorus. For the entire series, VT P NMR data establish or bound K , rates, and activation parameters for a variety of phenomena, many of which involve homeomorphic isomerizations, topological processes by which certain molecules can turn themselves inside out (e. g., in,in⇌out,out). This provides the first detailed mapping of such trends in homologous series of aliphatic bicyclic compounds XE((CH ) ) EX with any type of bridgehead. Isomeric diborane adducts 1 a,d ⋅ 2BH are also characterized. Crystal structures of out,out-1 a and in,in-1 a ⋅ 2BH aid isomer assignments and reveal unusual cage conformations.