N‑Substituted Derivatives of the Azadithiolate Cofactor from the [FeFe] Hydrogenases: Stability and Complexation

• Published in: Inorganic chemistry Vol. 54; no. 12; pp. 5717 - 5724
• Main Authors: Angamuthu, Raja, Chen, Chi-Shian, Cochrane, Tyler R, Gray, Danielle L, Schilter, David, Ulloa, Olbelina A, Rauchfuss, Thomas B
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 15.06.2015
• Subjects: Catalysis; Chemistry Techniques, Synthetic; Coenzymes - chemical synthesis; Coenzymes - chemistry; Crystallography, X-Ray; Hydrogenase - chemistry; Hydrolysis; Iron-Sulfur Proteins - chemistry; Magnetic Resonance Spectroscopy; Molecular Structure; Nickel - chemistry; Organometallic Compounds - chemistry; Sulfhydryl Compounds - chemistry
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.5b00290

Experiments are described that probe the stability of N-substituted derivatives of the azadithiolate cofactor recently confirmed in the [FeFe] hydrogenases (Berggren, G., et al. Nature 2013, 499, 66). Acid-catalyzed hydrolysis of bis­(thioester) BnN­(CH2SAc)2 gives [BnNCH2SCH2]2 rather than azadithiol BnN­(CH2SH)2. Treatment of BnN­(CH2SAc)2 with NaOtBu generates BnN­(CH2SNa)2, which was trapped with NiCl2(diphos) (diphos = 1,2-C2H4(PR2)2; R = Ph (dppe) and Cy (dcpe)) to give fully characterized complexes Ni­[(SCH2)2­NBn]­(diphos). The related N-aryl derivative Ni­[(SCH2)2­NC6H4Cl]­(diphos) was prepared analogously from 4-ClC6H4N­(CH2SAc)2, NaOtBu, and NiCl2(dppe). Crystallographic analysis confirmed that these rare nonbridging [adtR]2– complexes feature distorted square planar Ni centers. The analogue Pd­[(SCH2)2­NBn]­(dppe) was also prepared. 31P NMR analysis indicates that Ni­[(SCH2)2­NBn]­(dppe) has basicity comparable to typical amines. As shown by cyclic voltammetry, the couple [M­[(SCH2)2­NBn]­(dppe)]+/0 is reversible near −2.0 V versus Fc+/0. The wave shifts to −1.78 V upon N-protonation. In the presence of CF3CO2H, Ni­[(SCH2)2­NBn]­(dppe) catalyzes hydrogen evolution at rate of 22 s–1 in the acid-independent regime, at room temperature in CH2Cl2 solution. In contrast to the instability of RN­(CH2SH)2 (R = alkyl, aryl), the dithiol of tosylamide TsN­(CH2SH)2 proved sufficiently stable to allow full characterization. This dithiol reacts with Fe3(CO)12 and, in the presence of base, NiCl2(dppe) to give Fe2[(SCH2)2­NTs]­(CO)6 and Ni­[(SCH2)2­NTs]­(dppe), respectively.