"Catch-Store-release" Strategy for the Stabilization of 4,4'-Methylene Diphenyl Diisocyanate (4,4'-MDI).

4,4'-Methylene diphenyl diisocyanate (MDI) is an industrially crucial compound, being one of the most utilized linkers in the polyurethane industry. However, its long-term stability is limited due to dimerization to form insoluble uretdione. Herein we demonstrate an organometallic "catch-store-release" concept aiming at improving long-term chemical stability of MDI. Treatment of MDI with two equivalents of selected N-heterocyclic carbenes (NHC) forms stable MDI-NHC adducts. Treatment of the adducts with CuCl forms metastable di-Cu(I) complexes, which undergo decomposition to reform MDI (up to 85%), along with Cu-NHC complexes. The yield of reformed MDI can be improved (up to 95%) by the release of the NHC ligands in the form of thiourea, which prevents subsequent MDI dimerization/polymerization by the carbenes. Furthermore, the need to separate MDI from the reaction mixture can be eliminated by the direct reaction of MDI-NHC complexes with alcohols (as model for diols), which form dicarbamate (as model for polyurethane) quantitatively.