Synthesis and stabilities of mononuclear iron (II) dicarbonyls derived from neutral NNS ligands : structural models of the apo-active site of mono-iron (Hmd) hydrogenase

Herein is described the syntheses and characterization of iron dicarbonyl complexes derived from tridentate ortho-substituted Schiff base pyridine/thioether ligands ([subscript R]NNS) and (Fe(CO)₄(Br)₂). Two general synthetic routes are reported for the isolation of such iron carbonyls. First, metalation of [subscript R]NNS ligands within Et₂O at room temperature affords the complex species of type (([subscript R]NNS)Fe(CO)₂Br)(Fe(CO)₃(Br)₃). Alternatively, metalations of [subscript R]NNS (R = CH₃, OCH₃) at low temperature (-78 °C) in CH₂Cl₂/MeCN mixtures with (Fe(CO)₄(Br)₂) afford complexes of formula ([subscript R]NNS)Fe(CO)₂Br)Br. Reactions under similar conditions with more sterically demanding ligands (R = quinoline (Q), chlorophenyl (ClPh)) afford a complex salt of the form (([subscript R]NNS)Fe(CO)₂Br)(Fe(CO)₃(Br)₃). Complexes have been characterized using X-ray crystallography, IR spectroscopy and low temperature ¹H NMR spectroscopy. Photostability of the complexes has been studied using UV-vis and solution IR spectroscopies. Relevance to the apo-active site of mono-iron hydrogenase is discussed.