Iron(II/III) α-hydroxycarboxylates toward the local coordination environments of FeFe-cofactor in Fe‑nitrogenase.

Enantiomerically pure iron(II/III) lactates Λ-FeII(S-Hlact)2(H2O)2 (1), Na[Λ-FeIII(S-Hlact)2(S-lact)]·3.5H2O (2a) and Na[Δ-FeIII(R-Hlact)2(R-lact)]·3.5H2O (2b), along with iron(II/III) citrates (H2pz)2[FeIII2(cit)2(H2O)2]·2H2O (3) and [FeII(Hpz)4]n[FeIII2(cit)2(Hpz)2]n·2n(Hpz)·0.25nH2O (4) (H2lact = lactic acid, H4cit = citric acid, Hpz = pyrazole) have been obtained. In 1 and 2, lactates chelate with iron bidentately through α-hydroxy/α-alkoxy and α-carboxy groups, respectively, forming stable five-membered chelated rings. Strong intermolecular hydrogen bonds have been found between α-hydroxy and α-alkoxy groups, with strong electron delocalization. For 3, citrate chelates one of the iron centers in a tridentate mode via its α-alkoxy, α-carboxy, and β-carboxy groups, leaving protonated pyrazole free. Further substitution of the coordinated water molecule in 3 results in the formation of mixed-valent pyrazole iron(II/III) citrate 4. The iron lactates and citrates show resemblance to the local environment of the active site of homocitrate and imidazole-coordinated FeFe-cofactor in Fe-only nitrogenase. The biological relevance has been discussed in details.