New Pentadentate Carboxylate-Derivatized Sulfur Ligands Affording Water Soluble Iron Complexes with [Fe(NS4)] Cores that Bind Small Molecules (CO, NO, PMe3) as Co-Ligands

Dioxygen activation by a dinuclear thiolate-ligated Fe(ii) complex

Dioxygen activation by FeII thiolate complexes is relatively rare in biological and chemical systems because the sulfur site is at least as vulnerable as the iron site to oxidative modification. O2 activation by FeII-SR complexes with thiolate bound trans to the O2 binding site generally affords the FeIV[double bond, length as m-dash]O intermediate and oxidized thiolate. On the other hand, O2 activation by Fe(ii)-SR complexes with thiolate bound cis to the O2 binding site generates FeIII-O-FeIII or S-oxygenated complexes. The postulated FeIV[double bond, length as m-dash]O intermediate has only been identified in isopenicillin N synthase recently. We demonstrated here that O2 activation by a dinuclear FeII thiolate-rich complex produces a mononuclear FeIII complex and water with a supply of electron donors. The thiolate is bound cis to the postulated dioxygen binding site, and no FeIII-O-FeIII or S-oxygenated complex was observed. Although we have not detected the transient intermediate by spectroscopic measurements, the FeIV[double bond, length as m-dash]O intermediate is suggested to exist by theoretical calculation, and P-oxidation and hydride-transfer experiments. In addition, an unprecedented FeIII-O2-FeIII complex supported by thiolates was observed during the reaction by using a coldspray ionization time-of-flight mass (CSI-TOF MS) instrument. This is also supported by low-temperature UV-vis measurements. The intramolecular NHO[double bond, length as m-dash]FeIV hydrogen bonding, calculated by DFT, probably fine tunes the O2-activation process for intramolecular hydrogen abstraction, avoiding the S-oxygenation at cis-thiolate.

Redox Interconversion between Cobalt(III) Thiolate and Cobalt(II) Disulfide Compounds

The redox interconversion between Co(III) thiolate and Co(II) disulfide compounds has been investigated experimentally and computationally. Reactions of cobalt(II) salts with disulfide ligand L¹SSL¹ (L¹SSL¹ = di-2-(bis(2-pyridylmethyl)amino)-ethyl disulfide) result in the formation of either the high-spin cobalt(II) disulfide compound [Co^II₂(L¹SSL¹)Cl₄] or a low-spin, octahedral cobalt(III) thiolate compound, such as [Co^III(L¹S)(MeCN)₂](BF₄)₂. Addition of thiocyanate anions to a solution containing the latter compound yielded crystals of [Co^III(L¹S)(NCS)₂]. The addition of chloride ions to a solution of [Co^III(L¹S)(MeCN)₂](BF₄)₂ in acetonitrile results in conversion of the cobalt(III) thiolate compound to the cobalt(II) disulfide compound [Co^II₂(L¹SSL¹)Cl₄], as monitored with UV–vis spectroscopy; subsequent addition of AgBF₄ regenerates the Co(III) compound. Computational studies show that exchange by a chloride anion of the coordinated acetonitrile molecule or thiocyanate anion in compounds [Co^III(L¹S)(MeCN)₂]²⁺ and [Co^III(L¹S)(NCS)₂] induces a change in the character of the highest occupied molecular orbitals, showing a decrease of the contribution of the p orbital on sulfur and an increase of the d orbital on cobalt. As a comparison, the synthesis of iron compounds was undertaken. X-ray crystallography revealed that structure of the dinuclear iron(II) disulfide compound [Fe^II₂(L¹SSL¹)Cl₄] is different from that of cobalt(II) compound [Co^II₂(L¹SSL¹)Cl₄]. In contrast to cobalt, reaction of ligand L¹SSL¹ with [Fe(MeCN)₆](BF₄)₂ did not yield the expected Fe(III) thiolate compound. This work is an unprecedented example of redox interconversion between a high-spin Co(II) disulfide compound and a low-spin Co(III) thiolate compound triggered by the nature of the anion.

Low-spin Co^III−thiolate compounds and high-spin Co^II and Fe^II disulfide compound have been synthesized from reactions of a disulfide ligand with Co^II and Fe^II salts. The redox interconversion between the cobalt compounds has been investigated. It is shown that addition of chloride ions to a solution of the Co^III−thiolate compound results in the formation of the Co^II-disulfide compound, whereas removal of chloride anions from the Co^II disulfide compound regenerates the Co^III−thiolate complex.

Tungsten phosphanylarylthiolato complexes [W{PhP(2-SC6H4)2-kappa3S,S',P} 2] and [W{P(2-SC6H4)3-kappa4S,S',S",P}2]: synthesis, structures and redox chemistry

PhP(2-SHC6H4)2 (PS2H2) reacts with WCl6 with reduction of tungsten to give the air-sensitive tungsten(IV) complex [W{PhP(2-SC6H4)2-kappa(3)S,S',P}2] (1). 1 is oxidised in air to [WO{PhPO(2-SC6H4)2-kappa(3)S,S',O}{PhP(2-SC6H4)2-kappa(3)S,S',P}] (2). The attempted synthesis of 2 by reaction of 1 with iodosobenzene as oxidising agent was unsuccessful. [W{P(2-SC6H4)3-kappa(4)S,S',S",P}2] (3) was formed in the reaction of P(2-SHC6H4)3 (PS3H3) with WCl6. The W(VI) complex 3 contains two PS3(3-) ligands, each coordinated in a tetradentate fashion resulting in a tungsten coordination number of eight. The reaction of 3 with AgBF4 yields the dinuclear tungsten complex [W2{P(2-SC6H4)3-kappa(4)S,S',S",P}3]BF4 (4). Complexes 1-4 were characterised by spectral methods and X-ray structure determination.

Can the semiempirical PM3 scheme describe iron-containing bioinorganic molecules?

A set of iron parameters for use in the semiempirical PM3 method have been developed to allow the structure and redox properties of the active sites of iron-containing proteins to be accurately modeled, focussing on iron-sulfur, iron-heme, and iron-only hydrogenases. Data computed at the B3LYP/6-31G* level for a training set of 60 representative complexes have been employed. A gradient-based optimization algorithm has been used, and important modifications of the core repulsion function have been highlighted. The derived parameters lead in general to good predictions of the structure and energetics of molecules both within and outside the training set, and overcome the extensive deficiencies of a B3LYP/STO-3G model. Particularly encouraging is the success of the parameters in describing [4Fe-4S] cubanes. The derived parameter set provides a starting point should greater accuracy for a more restricted range of compounds be required.

Development of parameter sets for semi-empirical MO calculations of transition metal systems: Iron parameters for iron–sulfur proteins

A semi-empirical parameter set for iron has been developed which is appropriate for the study of iron-sulfur proteins having a single iron atom, by fitting to density-functional theory (DFT) calculations obtained for a series of small models of iron-containing proteins. These parameters are obtained using a modified BFGS optimisation procedure previously used to obtain semi-empirical parameters for the main group elements. The modifications to this procedure for obtaining parameters for transition metal atoms are outlined. In addition to modifications to the semi-empirical core repulsion function, which yield significant improvements in the calculation of molecular structures, compared to the standard core repulsion function, are outlined. The reported parameters are then tested on a set of model complexes containing a variety of ligands and show good agreement with both DFT and experimental data for these species.