Studies on the condensation pathway to and properties of diiron azadithiolate carbonyls

Reaction of Fe-2(SH)(2)(CO)(6) and HCHO, which gives Fe-2[(SCH2)(2)NH](CO)(6) in the presence of NH3, affords the possible intermediate Fe-2(SCH2OH)(2)(CO)(6), which has been characterized crystallographically as its axial-equatorial isomer. Fe-2(SCH2OH)(2)(CO)(6) was shown to react with ammonia and amines to give Fe-2[(SCH2)(2)NR](CO)(6) (R = H, alkyl). Related hemithioacetal intermediates were generated by treatment of Fe-2(SH)(2)(CO)(6) with RC(O)C(O)R (R = H, Ph, 4-F-C6H4) to give cycloadducts. The benzil derivative Fe-2[S2C2(OH)(2)Ph-2](CO)(6), a C-2-symmetric species, was also characterized crystallographically. The acylated azadithiolate Fe-2[(SCH2)(2)NAc](CO)(6) was prepared by reaction of Li2Fe2S2(CO)(6) with (ClCH2)(2)NC(O)Me. DNMR experiments show that the free energies of activation for rotation of the amide bond are the same for Fe-2[(SCH2)(2)NAc](CO)(6) and Fe-2[(SCH2)(2)NAc](CO)(4)(PMe3)(2), which implies that the ligands on the iron centers do not strongly affect the basicity of the nitrogen. As a control, we showed that the thioamide Fe-2[(SCH2)(2)NC(S)Me](CO)(6) does exhibit a significantly higher barrier to rotation, attributable to the increased double-bond character of the N-C(S) bond.