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

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