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

Abstract

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)<inf>2</inf>](PF<inf>6</inf>)<inf>2</inf> (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)](PF<inf>6</inf>)<inf>2</inf> (2) or neutral [FeL¹(N<inf>3</inf>)<inf>2</inf>] (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)<inf>2</inf>](PF<inf>6</inf>)<inf>3</inf> (4) has been isolated after chemical oxidation. SQUID and Mössbauer data have shown an S = ¹/<inf>2</inf> 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 {C<inf>2</inf>N<inf>2</inf>} hybrid ligand L¹ allowed for useful comparison with the related iron(II) complex that has a macrocyclic {C<inf>4</inf>} tetracarbene ligand. Chemical Equation Presented).