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.