Effects of Thiolate Ligation in Monoiron Hydrogenase (Hmd): Stability of the {Fe(CO)₂}²⁺ Core with NNS Ligands

Abstract

In this work, we report the effects of NNS-thiolate ligands and nuclearity (monomer, dimer) on the stability of iron complexes related to the active site of monoiron hydrogenase (Hmd). A thermally stable iron(II) dicarbonyl motif is the core feature of the active site, but the coordination features that lead to this property have not been independently evaluated for their contributions to the {Fe(CO)} stability. As such, non-bulky and bulky benzothiazoline ligands (thiolate precursors) were synthesized and their iron(II) complexes characterized. The use of non-bulky thiolate ligands and low-temperature crystallizations result in isolation of the dimeric species [(NNS)Fe(CO)(I)] (1), [(NNS)Fe(CO)(I)] (2), and [(NNS)Fe(CO)(I)] (3), which exhibit dimerization via thiolato (μ-S) bridges. In one particular case (unsubstituted NNS ligand), the pathway of decarbonylation and oxidation from 1 was crystallographically elucidated, via isolation of the half-bis-ligated monocarbonyl dimer [(NNS)Fe(CO)]I (4) and the fully decarbonylated and oxidized mononuclear [(NNS)Fe]I (5). The transformations of dicarbonyl complexes (1, 2, and 3) to monocarbonyl complexes (4, 6, and 7) were monitored by UV/vis, demonstrating that 1 and 3 exhibit longer t (80 and 75 min, respectively) than 2 (30 min), which is attributed to distortion of the ligand backbone. Density functional theory calculations of isolated complexes and putative intermediates were used to corroborate the experimentally observed IR spectra. Finally, dimerization was prevented using a bulky ligand featuring a 2,6-dimethylphenyl substituent, which affords mononuclear iron dicarbonyl complex, [(NNS)Fe(CO)Br] (8), identified by IR and NMR spectroscopies. The dicarbonyl complex decomposes to the decarbonylated [(NNS)Fe] (9) within minutes at room temperature. Overall, the work herein demonstrates that the thiolate moiety does not impart thermal stability to the {Fe(CO)} unit formed in the active site, further indicating the importance of the organometallic Fe-C(acyl) bond in the enzyme.

Development of an FeII Complex Exhibiting Intermolecular Proton Shifting Coupled Spin Transition

“…[24] In contrast, at 250 K, the average bond lengths were 1.985(1) Å (FeÀ N) Avg and 2.299(5) Å (FeÀ S) Avg , which are characteristic of LS Fe II compounds. [25] These results are consistent with the magnetic properties and indicate the occurrence of ST at the Fe II center.…”

“…The average metal‐to‐ligand bond lengths at 275 K are 2.177(2) Å for Fe−N and 2.474(1) Å for Fe−S, which are consistent with those of a typical HS Fe II complex [24] . In contrast, at 250 K, the average bond lengths were 1.985(1) Å (Fe−N) Avg and 2.299(5) Å (Fe−S) Avg , which are characteristic of LS Fe II compounds [25] . These results are consistent with the magnetic properties and indicate the occurrence of ST at the Fe II center.…”

Development of an FeII Complex Exhibiting Intermolecular Proton Shifting Coupled Spin Transition

“…[24] In contrast, at 250 K, the average bond lengths were 1.985(1) Å (FeÀ N) Avg and 2.299(5) Å (FeÀ S) Avg , which are characteristic of LS Fe II compounds. [25] These results are consistent with the magnetic properties and indicate the occurrence of ST at the Fe II center. Difference Fourier maps (DFMs) are useful analytical tools for visualizing changes in proton positions.…”

Palladium(II) halide complexes of N,N,S-tridentate Schiff bases: synthesis, structural studies in solution and solid state, and analysis of Hirshfeld surfaces

“…[6] In the same vein, an anionic Schiff obtained by the opening-ring of a thiazoline ring showed a k 2 NkS-tridentate coordinate mode in the octahedral complexes of Fe(II), that have been proposed for the study of reactivity of new models of metalloenzymes such as the hydrogenase. [5,7] Regarding to halogen Pd(II) complexes of analogue N,S-polydentate Schiff bases, some bromo-and chloro complexes containing five-membered chelate rings with a  2 NS-tridentate coordinate mode have been reported. [8,9,10] In some  2 NS-tricoordinate Pd(II) complexes of analogue sulfur Schiff bases, the antimicrobial and antitumor activity has been described.…”