Mononuclear Fe^II and heterobinuclear Fe^IINi^II thiolate complexes derived from a compartmental hexaazadithiophenolato ligand: synthesis, structure and properties

Compartmental macrocycles have proven to be very successful for the synthesis of heterobinuclear transition metal complexes. We obtained a mixed-ligand FeNi thiolate complex, [FeNiL(µ-OAc)]ClO₄ (3), where L^2- represents a 24-membered hexaaza-dithiophenolato ligand, by metalation of either the mononuclear [Fe^II(H₂L)](ClO₄)₂ (1) or [Ni^II(H₂L)](ClO₄)₂ (2) complex with Ni^II or Fe^II salts in good to moderate yields. However, the homobinuclear [Fe₂L(µ-OAc)]ClO₄ (4), or [Ni₂L(µ-OAc)]ClO₄ (5), complexes are invariably formed as byproducts (≈12-15%). The metal atoms in 3 are bridged at a distance of 3.4588(5) Å by two thiolate residues and one external μ_1,3-acetato ligand to give a bioctahedral N₃Fe^II(μ-(SR)₂(µ-OAc)Ni^IIN₃) core structure. Magnetic susceptibility measurements for 3 show the presence of a high-spin Fe^II (S = 2) ion, which is antiferromagnetically coupled to the Ni^II (S = 1) ion to give a S_total = 1 ground state. Broken symmetry DFT calculations revealed J = -6.88 cm^-1 (H = -JS₁S₂). The integrity and robust nature of complex 3 is maintained in solution as confirmed by ESI mass spectrometry and UV-vis-NIR spectroscopy. Cyclic voltammetry measurements for 3 in MeCN show two electrochemically quasi-reversible redox waves which are assigned to sequential oxidations of Fe^II and Ni^II, respectively (E(Fe^II/III) = +0.30 V, E(Ni^II/III) = +1.29 V vs. SCE). The [Fe^II(H₂L)]²⁺ complex was isolated as a pale-yellow, paramagnetic (μ_eff = 5.41μ_B) perchlorate salt. It comprises a five-coordinated Fe^II ion in a distorted square-pyramidal N₃S₂ coordination environment, and a system of intramolecular N⁺-H⋯S^- hydrogen bonds in the free pocket of the compartmental macrocycle.