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

Danopoulos, Andreas A. and Pugh, D. and Smith, Hayley and Sassmannshausen, Jorg (2009) Structural and reactivity studies of "pincer" pyridine dicarbene complexes of Fe-0: Experimental and computational comparison of the phosphine and NHC donors. Chemistry - A European Journal, 15 (22). pp. 5491-5502. ISSN 0947-6539

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Abstract

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].