Formation of paramagnetic mononuclear iron nitrosyl complexes from diamagnetic di- and tetranuclear iron-sulphur nitrosyls: characterisation by EPR spectroscopy and study of thiolate and nitrosyl ligand exchange reactions

Abstract

The diamagnetic Roussin esters Fe 2(SR) 2(NO) 4 readily underwent exchange with thiols R′SH to yield Fe 2(SR′) 2(NO) 4: the exchange was faster in polar, coordinating solvents where paramagnetic, mononuclear complexes of types [Fe(NO) 2(solvent) 2] + and Fe(NO) 2(SR)(solvent) were formed. With the corresponding thiolate anions RS -, the esters Fe 2(SR) 2(NO) 4 formed the mononuclear complexes [Fe(SR) 2(NO) 2] -, which were fully characterised by EPR spectroscopy for R = H, Me, Et, i-Pr, t-Bu and PhCH 2: assignments of hyperfine couplings were confirmed by use of 15N. With Fe 2(SR) 2(NO) 4 and a different set of thiolate anion, R′S -, in excess, thiol exchange occurred to give [Fe(SR′) 2(NO) 2] -. A mechanism for formation of Fe 2(SR′) 2(NO) 4 from Fe 2(SR) 2(NO) 4 has been proposed. The paramagnetic mononuclear complexes [Fe(SR) 2(NO) 2] were also readily formed from the diamagnetic clusters [Fe 4S 3(NO) 7] - and Fe 4S 4(NO) 4, together with [Fe(SR) 3(NO)] -, and additionally from [Fe(CO) 3NO] -. [Fe(SMe) 2(NO) 2] -. was found to be a precursor of isolable Fe 2(SMe) 2(NO) 4, and [Fe(SH) 2 (NO) 2] - to be the common precursor of both Roussin′s red anion [Fe 2S 2(NO) 4] - and Roussin's black anion [Fe 4S 3 (NO) 7] - interconvertible by appropriate adjustment of pH. The nitrosyl groups in these complexes were freely labile, and mononitrosyliron and dinitrosyliron fragments were readily interconvertible: FE(NO) fragments were favoured by the dimethyldithiocarbamate ligand (Me 2NCS 2) and Fe(NO) 2 fragments by thiolate ligands, RS -, regardless of the origin of the Fe(NO) x ( x = 1,2) fragment: both mono- and dinitrosyliron complexes persisted with [( i-PrO) 2S 2] - as ligand. Isotopic labelling showed the occurrence of rapid exchange of nitrogen between nitrosyl ligands and added nitrite in Fe(NO)(S 2CNMe 2) 2 and [Fe(SR) 2(NO) 2] -