Mononuclear Iron(II) Dicarbonyls Derived from NNS Ligands – Structural Models Related to a “Pre‐Acyl” Active Site of Mono‐Iron (Hmd) Hydrogenase

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We report the syntheses and characterization of dicarbonyliron complexes derived from tridentate, ortho‐substituted Schiff base pyridine/thioether ligands (RNNS). Metalation reactions of RNNS (R = CH3, OCH3) at low temperature (–78 °C) with [Fe(CO)4(Br)2] afforded the desired complexes [(RNNS)Fe(CO)2Br]Br (2‐COBr, 3‐COBr). Reactions under similar conditions with more sterically demanding ligands [R = quinoline (Q), ClPh] afforded complex salts of the form [(RNNS)Fe(CO)2Br][Fe(CO)3(Br)3] (4‐COFe and 5‐COFe, respectively). Alternatively, the metalation of the RNNS ligands (for all R ≠ H) with [Fe(CO)4(Br)2] in Et2O at room temperature reliably affords the complex species of type [(RNNS)Fe(CO)2Br][Fe(CO)3(Br)3] (2‐COFe, 3‐COFe, 5‐COFe). The metalation reactions of RNNS at only moderately low temperatures (–20 to 0 °C) result in the loss of CO to form the corresponding trigonal‐bipyramidal iron(II) dibromide species of type [(RNNS)FeBr2] (2‐Br, 4‐Br, 5‐Br; μeff ≈ 5.3 μB, S = 2). The IR spectrum of each dicarbonyl cation exhibits two ν(CO) stretches at ≈ 2070 and 2030 cm–1. Low‐temperature 1H NMR spectroscopy measurements of 2‐CO to 5‐CO in CD3CN (–35 to 5 °C) revealed sharp resonances in the diamagnetic region. Under dark conditions, each dicarbonyl species is relatively stable (<10 % loss of CO, 1–2 h). However, photolysis revealed varying extents of photostability (stability rank: R = OMe > Me ≈ Q > ClPh). An examination of the structural parameters reveals that higher photostabilities correlate with shorter Fe–C(O) bond lengths, which are induced by variation of the ortho substituent of the pyridine ring. DFT calculations along the putative photolysis pathway revealed that the bulky ligand substituent (in 5‐CO) destabilizes the monocarbonyl intermediate, and this is a likely explanation for its more rapid rate of CO photodissociation. Relevance to a possible “apo‐active site” of mono‐iron hydrogenase (pre‐acyl formation) is discussed.