N-Substituted Derivatives of the Azadithiolate Cofactor from the [FeFe] Hydrogenases: Stability and Connplexation

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 (NaOBu)-Bu-t 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)(2)NBn](diphos). The related N-aryl derivative Ni[(SCH2)(2)NC6H4Cl](diphos) was prepared analogously from 4-ClC6H4N(CH2SAc)(2), (NaOBu)-Bu-t, and NiCl2(dppe). Crystallographic analysis confirmed that these rare nonbridging [adt(R)](2-) complexes feature distorted square planar Ni centers. The analogue Pd[(SCH2)(2)NBn](dppe) was also prepared. P-31 NMR analysis indicates that Ni[(SCH2)(2)NBn](dppe) has basicity comparable to typical amines. As shown by cyclic voltammetry, the couple [M[(SCH2)(2)NBn](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)(2)NBn](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 Fe-3(CO)(12) and, in the presence of base, NiCl2(dppe) to give Fe-2[(SCH2)(2)NTs](CO)(6) and Ni[(SCH2)(2)NTsi(dPPe), respectively.