Hybrids of [FeFe]- and [NiFe]-H₂ase Active Site Models

Abstract

Complexes of the type (diphosphine)­Ni­(μ-SR)2Fe­(CO)3 are investigated with azadithiolate (adt, HN­(CH2S–)2) as the dithiolate. The resulting complexes are hybrid models for the active sites of the [NiFe]- and [FeFe]-hydrogenases. The key complex (dppv)­Ni­(μ-adt)­Fe­(CO)3 (3) was prepared from the complex Ni­[(SCH2)2NCbz]­(dppv), which contains a Cbz-protected adt ligand (Cbz = C­(O)­OCH2Ph, dppv = cis-1,2-(Ph2P)2C2H2). This complex combines with Fe2(CO)9 to give (dppv)­Ni­[(μ-SCH2)2NCbz]­Fe­(CO)3, which is readily deprotected to give 3. Complex 3 undergoes protonation at both Fe and N to give successively [(dppv)­Ni­(μ-adt)­FeH­(CO)3]+ ([H3]+) and [(dppv)­Ni­(μ-adtH)­FeH­(CO)3]2+ ([H3H]2+). The redox properties and dynamics of these complexes resemble previously reported analogues with propanedithiolate. Solutions of [H3]+ readily degrade to [(dppv)­Ni­[(μ-SCH2)2NCH2]­Fe­(CO)3]+ ([4]+), which features a methylene group linking N and Fe. Complex [4]+ can be made in high yield by reaction of [H3]+ with CH2O, and this conversion was also demonstrated with 13CH2O. Complex [4]+ undergoes hydrogenolysis by photochemical reaction with H2 to give [(dppv)­Ni­[(μ-SCH2)2NMe]­FeH­(CO)3]+, the N-methylated analogue of [H3]+. Upon treatment ith Me3O+, [4]+ undergoes quaternization, giving [(dppv)­Ni­[(μ-SCH2)2N­(Me)­CH2]­Fe­(CO)3]2+. In contrast with the lability of [H3]+, the phosphine-substituted derivative [(dppv)­Ni­(μ-adt)­FeH­(CO)2(PPh3)]+ did not degrade. Most complexes were characterized by X-ray crystallography.