Structural and Reactivity Studies of “Pincer” Pyridine Dicarbene Complexes of Fe0: Experimental and Computational Comparison of the Phosphine and NHC Donors

Abstract

Reduction of [Fe(C-N-C)(Br)(2)] or [Fe(C-N(Me)-C)(I)(2)] with Na/Hg under N(2) gave the Fe(0) complexes [Fe(C-N-C)(N(2))(2)] and [Fe(C-N(Me)-C)(N(2))(2)] for which C-N-C = 2,6-bis(arylimidazol-2-ylidene)-pyridine, C-N(Me)-C = 2,6-bis(arylimidazol-2-ylidene)-3,5-dimethyl-pyridine and aryl = 2,6-iPr(2)C(6)H(3). Substitution of the coordinated N(2) 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-N(Me)-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-N(Me)-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 Fe(0) to CO backbonding in [Fe(C-N-C)(CO)(2)] and [Fe(C-N(Me)-C)(CO)(2)] compared to [Fe(P-N-P)(CO)(2)] is accounted for by backbonding from the Fe-N(pyridine) 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)(N(2))(2)] in the presence of benzaldehyde phenylimine gave an ortho-metalated benzaldehyde phenylimine. The Fe(II) 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 CO(2) with [Fe(C-N-C)(N(2))(2)].