Carbene Transfer from a Pyridine Dipyrrolide Iron–Carbene Complex: Reversible Migration of a Diphenylcarbene Ligand into an Iron–Nitrogen Bond

The reaction between the square-planar, paramagnetic iron–carbene complex (MesPDPPh)­Fe­(CPh2) (where [MesPDPPh]2– is the doubly deprotonated form of 2,6-bis­(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)­pyridine) and two equivalents of carbon monoxide or 2,6-dimethylphenylisocyanide, CNdmp, resulted in carbene insertion into a Fe–Npyrrole bond of the MesPDPPh ligand. Crystallographic studies established complete scission of the Fe–Npyrrole bond in the resulting low-spin FeII complexes. Experiments using 1-adamantylisocyanide, CN1Ad, demonstrated that binding of two exogenous ligands was required to promote carbene migration and allowed the isolation of diamagnetic (MesPDPPh)­Fe­(CPh2)­(CN1Ad). Computational studies by density functional theory (DFT) and complete-active-space self-consistent field (CASSCF) calculations support that (MesPDPPh)­Fe­(CPh2)­(CN1Ad) possesses a similar electronic structure to porphyrin iron carbenes, best described as containing a neutral Fischer-type carbene coordinated to a low-spin FeII center. DFT calculations also suggested that carbene migration is triggered by an increase in carbene electrophilicity upon binding of a second isocyanide ligand. Thermolysis of the carbene insertion product obtained following the addition of CNdmp to (MesPDPPh)­Fe­(CPh2) revealed that the insertion is fully reversible and provided the ketenimine N-(2,2-diphenylethenylidene)-2,6-dimethylbenzenamine as the major product.