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)2}2+ 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)2Fe2(CO)2(I)2] (1), [(NPhNS)2Fe2(CO)2(I)2] (2), and [(MeNNS)2Fe2(CO)2(I)2] (3), which exhibit dimerization via thiolato (μ2-S)2 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)3Fe2(CO)]I (4) and the fully decarbonylated and oxidized mononuclear [(NNS)2Fe]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 t1/2 (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, [(NPhNSDMPh)Fe(CO)2Br] (8), identified by IR and NMR spectroscopies. The dicarbonyl complex decomposes to the decarbonylated [(NPhNSDMPh)2Fe] (9) within minutes at room temperature. Overall, the work herein demonstrates that the thiolate moiety does not impart thermal stability to the {Fe(CO)2}2+ unit formed in the active site, further indicating the importance of the organometallic Fe-C(acyl) bond in the enzyme.