Novel Dimerization of Maleic Anhydride at a Mo2 Complex: Phase-Driven Keto/Enol Tautomerism in a Phosphinidenium–Ylide Complex

A novel dimerization of maleic anhydride involving olefinic to carbonyl C–C coupling and an [1,7]-H shift is promoted by a strongly nucleophilic pyramidal phosphinidene complex. The resulting carbene is stabilized by coordination of the P atom of the phosphinidene ligand and undergoes keto/enol tautomerization when going from solution to the solid state. This is accompanied by a strong pyramidalization of the ylidic carbon atom, facilitated by the presence of π···π stacking interactions in the crystal lattice.