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

✍ Scribed by Andreas A. Danopoulos; David Pugh; Hayley Smith; Jörg Saßmannshausen

• Publisher: John Wiley and Sons
• Year: 2009
• Tongue: English
• Weight: 392 KB
• Volume: 15
• Category: Article
• ISSN: 0947-6539
• DOI: 10.1002/chem.200900027

✦ Synopsis

Abstract

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 CH activation has been observed in Fe pyridine dicarbene complexes.magnified image

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 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~].