Isocyanide Activation by Phosphinimide-Ligated Cr(II) Centers

This report describes the synthesis and reactivity of the heteroleptic phosphinimide/phosphinimine Cr(II) complex HL′CrAdHMDS, which was obtained by incomplete protonolysis of Cr(HMDS)2(THF)2 in the presence of the bisphosphinimine pro-ligand H2L′Ad. Addition of common ligands such as N-heterocyclic carbenes and isocyanides displaces the remaining HMDS ligand concomitant with complete deprotonation of the phosphinimine pro-ligand furnishing three- and four-coordinated complexes, respectively. The resulting tetrahedral chromium(II) bis(isocyanide) species was found to adopt an unusual S = 1 spin state at room temperature. Under certain reaction conditions, one coordinated isocyanide ligand was found to insert into a chromium-phosphinimide bond. The described compounds are characterized by single-crystal X-ray diffraction and solution-phase spectroscopy, and the mechanism for isocyanide insertion was investigated by DFT calculations.