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.