Iron Complexes of a Macrocyclic N-Heterocyclic Carbene/Pyridine Hybrid Ligand

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• 作者：Iris Klawitter ; Markus R. Anneser ; Sebastian Dechert ; Steffen Meyer ; Serhiy Demeshko ; Stefan Haslinger ; Alexander P枚thig ; Fritz E. K眉hn ; Franc Meyer
• 刊名：Organometallics
• 出版年：2015
• 出版时间：June 22, 2015
• 年：2015
• 卷：34
• 期：12
• 页码：2819-2825
• 全文大小：402K
• ISSN：1520-6041

文摘

The tetradentate ligand system L¹ combines two N-heterocyclic carbene (NHC) and two pyridine donor functions in a macrocyclic scaffold. Its iron(II) complex [FeL¹(MeCN)₂](PF₆)₂ (1) has been synthesized and fully characterized. The macrocyclic ligand in 1 is puckered and shows a significant barrier for ring inversion (螖H^{猝?/sup> = 15.1 kcal mol鈥?, and 螖S猝?/sup> = 鈭?.7 cal mol鈥? K鈥?). Axial ligands in 1 can be readily substituted to give heteroleptic [FeL1(CO)(MeCN)](PF₆)₂ (2) or neutral [FeL1(N₃)₂] (3). The strong ligand field of the NHC/pyridine hybrid ligand imparts low-spin states (S = 0) for all iron(II) complexes 1鈥?. M枚ssbauer data reflect the asymmetric electronic situation that results from the strongly covalent Fe鈥揅NHC bonds in the basal plane constituted by the macrocyclic ligand L1. Oxidation of 1 has been monitored by UV鈥搗is spectro-electro chemistry, and the resulting iron(III) complex [FeL1(MeCN)₂](PF₆)₃ (4) has been isolated after chemical oxidation. SQUID and M枚ssbauer data have shown an S = 1/₂ ground state for 4, and X-ray crystallographic analyses of 1 and 4 revealed that structural parameters of the {FeL1} core are basically invariant with respect to changes in the metal ion鈥檚 oxidation state. Density functional theory calculations support the experimental findings. The combined structural, spectroscopic, and electrochemical data for 1 with its {C₂N₂} hybrid ligand L1 allowed for useful comparison with the related iron(II) complex that has a macrocyclic {C₄} tetracarbene ligand.}