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

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 (ΔH^⧧ = 15.1 kcal mol^–1, and ΔS^⧧ = −4.7 cal mol^–1 K^–1). 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 1–3. Mössbauer data reflect the asymmetric electronic situation that results from the strongly covalent Fe–C^NHC bonds in the basal plane constituted by the macrocyclic ligand L¹. Oxidation of 1 has been monitored by UV–vis spectro-electro chemistry, and the resulting iron­(III) complex [FeL¹(MeCN)₂]­(PF₆)₃ (4) has been isolated after chemical oxidation. SQUID and Mössbauer 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’s 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.