AbstractIn the search for polydentate sulfur ligands that are able to form water‐soluble iron complexes which can bind nitrogenase relevant molecules, the new pentadentate ligands pyCO2MeS4−H2 [2,6‐bis[2‐mercapto‐3‐(methoxycarbonyl)phenylthio]dimethylpyridine] (1) and pyCO2HS4−H2 [2,6‐bis(2‐mercapto‐3‐carboxyphenylthio)dimethylpyridine] (2) having NS4 donor atom sets and terminal thiolate donors have been synthesized. The starting material was CO2MeS2−H2 (2,3‐dimercapto benzoic acid methyl ester) which was alkylated with 2,6‐bis[(tosyloxy)methyl]pyridine. The problem of specifically achieving regioselective mono‐alkylation of this 1,2‐benzene‐dithiol derivative was solved by carrying out the alkylation of CO2MeS2−H2 at −78 °C in the presence of stoichiometric amounts of a base. Saponification of 1 afforded the carboxylic acid derivative. Coordination of pyCO2MeS42− to FeII in the presence of co‐ligands (L = CO, PMe3) yielded the complexes [Fe(L)(pyCO2MeS4)] where L = CO (5) or PMe3 (4). Upon treatment with NOBF4, complex 5 afforded [Fe(NO)(pyCO2MeS4)]BF4 (7) which could be subsequently converted to the isolable 19 valence electron species [Fe(NO)(pyCO2MeS4)] (8) upon reduction with N2H4. In the absence of potential co‐ligands, coordination of pyCO2MeS42− to FeII afforded the dinuclear complex [Fe(pyCO2MeS4)]2 (6) whilst coordination to NiII gave [Ni(pyCO2MeS4)]x (3). Solubility of these complexes in water could be achieved by replacing the CO2Me groups with CO2H substituents. The ligand pyCO2HS42− afforded the iron complexes [Fe(L)(pyCO2HS4)] [L = CO (10) and PMe3 (12)] and [Fe(NO)(pyCO2HS4)]BF4 (11). Both 10 and 12 could be reversibly deprotonated to give the corresponding water‐soluble salts (NMe4)2[Fe(L)(pyCO2S4)] with L = CO {(NMe4)2 [9]} and PMe3 {(NMe4)2 [13]}. The complexes were characterized by elemental analysis, spectroscopic methods and X‐ray structural determinations. The molecular structure of [Fe(PMe3)(pyCO2HS4)] (12) was found to exhibit inter‐ and intramolecular O−H···O and O−H···S hydrogen bonds which serve as models for proton transfer steps from external sources to the active sites of metal sulfur enzymes. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)