Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 1 / 8 / b / iii / Solution

The frustrated Lewis pair captures carbon dioxide cooperatively: the phosphine lone pair attacks its carbon while a carbonyl oxygen coordinates to aluminium. A five-membered ring closes alongThe new ${\mathrm{C}}_{\mathrm{C}{\mathrm{O}}_2}$ atom is bonded to P, double-bonded to one O and single-bonded to the O bound to Al. P becomes phosphonium-like and the O–Al end completes a zwitterionic carboxylate-like adduct; the original alkene remains intact.

A direct P–C bond to the captured CO${}_2$ carbon accounts for its large ${}^{13}$C–${}^{31}$P coupling. The carbonyl-like chemical shift is consistent with a captured, bent CO${}_2$ unit. Reversibility follows from cleavage of the new P–C and Al–O contacts to regenerate the separated donor and acceptor.