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

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.