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” Fe 0 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‐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‐ i Pr 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 FeN 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 ].