Mononuclear, dinuclear, and pentanuclear [{N,S(thiolate)}iron(II)] complexes: Nuclearity control, incorporation of hydroxide bridging ligands, and magnetic behavior

Article abstract of DOI:10.1002/chem.200500156

The mixed N₃S(thiolate) ligand 1-[bis(2-(pyridin-2-yl)ethyl} amino]-2-methylpropane-2-thiol (Py₂SH) was used in the synthesis of four iron(II) complexes: [(Py₂S)FeCl] (1), [(Py₂S)FeBr] (2), [(Py₂S)₄Fe₅^II(μ-OH) ₂]-(BF₄)₄ (3). and [(Py₂S) ₂Fe₂^II(μ-OH)]BF₄ (4). The X-ray structures of 1 and 2 revealed monomeric iron(II)-alkylthiolate complexes with distorted trigonal-bi-pyramidal geometries. The paramagnetic ¹H NMR spectra of 1 and 2 display resonances from δ = -25 ppm to + 100 ppm, consistent with a high-spin iron(II) ion (S=2). Spectral assignments were made on the basis of chemical shift information and T₁ measurements and show the monomeric structures are intact in solution. To provide entry into hydroxide-containing complexes, a novel synthetic method was developed involving strict aprotic conditions and limiting amounts of H₂O. Reaction of Py₂SH with NaH and Fe-(BF₄)₂·6H ₂O under aprotic conditions led to the isolation of the pentanuclear, μ-OH complex 3, which has a novel dimer-of-dimers type structure connected by a central iron atom. Conductivity data on 3 show this structure is retained in CH₂Cl₂. Rational modification of the ligand-to-metal ratio allows control over the nuclearity of the product, yielding the dinuclear complex 4. The X-ray structure of 4 reveals an unprecedented face-sharing, biooctahedral complex with an [S₂O] bridging arrangement. The magnetic properties of 3 and 4 in the range 1.9-300 K were successfully modeled. Dinuclear 4 is antiferromagnetically coupled [J = -18.8(2) cm^-1]. Pentanuclear 3 exhibits ferrimagnetic behavior, with a high-spin ground state of S_T=6, and was best modeled with three different exchange parameters [J= -15.3(2), J′ = -24.7(3), and J″ = -5.36(7) cm^-1]. DFT calculations provided good support for the interpretation of the magnetic properties.