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-CNHC 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. Chemical Equation Presented).