Molybdenum citrate towards the protonation of FeMo-co in nitrogenase

The structure of FeMo-cofactor (FeMo-co) in nitrogenase has been clarified as MoFe7S9C[ R -(H)homocit](Hhis)(cys) (H4homocit = homocitrate, Hhis = histidine, Hcys = cysteine), where homocitrate chelates to Mo(III) via α-alkoxy/α-hydroxy and α-carboxy groups. Recent model comparisons and theoretical calculations suggested a hydroxy coordinated homocitrate in FeMo-co, which may play an important role in storing and transferring hydrogen source for the reduction of substrate. Herein, two molybdenum citrates [MoIV2O(Hcit)2(tpy)2]·3H2O ( 1 ) (H4cit = citrate, tpy = α,α,α-terpyridine) and (H2tpy)2[MoVI2O5(Hcit)2]·7.5H2O ( 2 ) have been obtained via hydrothermal reactions under the reduction of hydrazine hydrochloride in acidic condition. 1 and 2 were fully characterized by elemental analyses, IR, UV-vis, EPR and 13C NMR spectra, bond valence calculations and X-ray single crystal diffractions. Structural analysis indicates that 1 is a binuclear complex, which seven coordination sites are occupied by one μ2-O atom, three nitrogen atoms of terpyridine, three oxygen atoms from citrate. Citrate chelates to Mo via α-alkoxy, α-carboxy and β-carboxy groups, the uncoordinated β-carboxylic acid group interacts with water molecules through strong hydrogen bonds. 2 is a common binuclear Mo(VI) complex counterbalanced by protonated terpyridine cations. The coordination of citrates in 2 is similar to that of 1 . 13C NMR spectrum indicates that 2 partially dissociates in solution. We have also analyzed Mo–O distances of 1 and 2 , as well as previously reported molybdenum α-hydroxycarboxylates. It is found that trans -effect of Mo=O group, protonation of α-alkoxy group and oxidation state of Mo are the three major factors that affect the distances between Mo and coordinated atoms from α-hydroxycarboxylates. Mo–O distances are elongated about 0.05−0.13 A from trans -effect, while this effect for α-carboxy group is stronger than that of α-alkoxy group. Mo–O (α-hydroxy) distance is about 0.11 A longer than that of Mo–O (α-alkoxy) due to protonation effect. Mo–O (α-alkoxy/α-carboxy) distances show negative correlation with oxidation state of Mo. Linear fit gives a MoIII–O (α-carboxy) distance that is close to those of wild-type FeMo-co and citrate-substituted cofactor of variant Mo-nitrogenase, while the calculated MoIII–O (α-alkoxy) distance is much shorter. The difference is well in agreement with the protonation effect. This result gives a quantitative conclusion for the protonation mode of homocitrate in FeMo-co. A new protonated model is also deduced for citrate-substituted cofactor of variant Mo-nitrogenase. It seems more structural data of model compounds should be included for statistical analysis, especially those complexes in low valences.