Structural and reactivity studies of "pincer" pyridine dicarbene complexes of Fe-0: Experimental and computational comparison of the phosphine and NHC donors

Pincer carbene complexes: Comparison of the electronic structures of pincer Fe0 pyridine bis(imidazol-2-ylidene) and pyridine diphosphine dicarbonyl complexes (see figure) show subtle differences that account for observed spectroscopic trends. Intermolecular CH activation has been observed in Fe pyridine dicarbene complexes. Reduction of [Fe(C-N-C)(Br)2] or [Fe(C-NMe-C)(I)2] with Na/Hg under N2 gave the Fe0 complexes [Fe(C-N-C)(N2)2] and [Fe(C-NMe-C)(N2)2] for which C-N-C=2,6-bis(arylimidazol-2-ylidene)-pyridine, C-NMe-C=2,6-bis(arylimidazol-2-ylidene)-3,5-dimethyl-pyridine and aryl=2,6-iPr2C6H3. Substitution of the coordinated N2 by CO or CN(2,6-xyl), xyl=2,6-dimethylphenyl, took place readily to afford [Fe(C-N-C)(L)2] and [Fe(C-NMe-C)(L)2], L=CO or CN(2,6-xyl). The electronic characteristics of the N-heterocyclic carbene and phosphine donors in the complexes [Fe(C-N-C)(CO)2], [Fe(C-NMe-C)(CO)2] and [Fe(P-N-P)(CO)2], P-N-P=2,6-bis(di-tert-butylphosphinomethyl)-pyridine, have been evaluated by spectroscopic, structural and computational methods. The unexpected reduced Fe0 to CO backbonding in [Fe(C-N-C)(CO)2] and [Fe(C-NMe-C)(CO)2] compared to [Fe(P-N-P)(CO)2] is accounted for by backbonding from the FeNpyridine bond in the imidazol-2-ylidene complexes. Reduction of [Fe(C-N-C)(Br)2] under Ar gave mixtures from which a complex with a metalated pyridine and a dangling imidazolium group was isolated. Photolysis of [Fe(C-N-C)(N2)2] in the presence of benzaldehyde phenylimine gave an ortho-metalated benzaldehyde phenylimine. The FeII complex [Fe(C-N-C)(CO)(Br)2] was prepared by the reaction of [Fe(C-N-C)(Br)2] with CO or reduction of CO2 with [Fe(C-N-C)(N2)2].