Molybdenum imidazole citrate and bipyridine homocitrate in different oxidation states - balance between coordinated α-hydroxy and α-alkoxy groups.

Oxo and thiomolybdenum(iv/vi) imidazole hydrocitrates K{Mo O(im)[MoO(Hcit)]}·3im·4HO (1), (Him){Mo SO(im)[MoO(Hcit)]}·im·6HO (2), molybdenum(v) bipyridine homocitrate -[(MoO)O(Hhomocit)(bpy)]·4HO (3) and molybdenum(vi) citrate (EtN)[MoOCl(Hcit)]·HO (4) (Hcit = citric acid, Hhomocit = homocitric acid, im = imidazole and bpy = 2,2'-bipyridine) with different oxidation states were prepared. 1 and 2 are the coupling products of [MoO(Hcit)] anions and incomplete cubane units [Mo O] ([Mo SO]) with monodentate imidazoles, respectively, where tridentate citrates coordinate with α-hydroxy, α-carboxy and β-carboxy groups, forming pentanuclear skeleton structures. The molybdenum atoms in 1 and 2 show unusual +4 and +6 valences based on charge balances, theoretical bond valence calculations and Mo XPS spectrum. The coordinated citrates in 1 and 2 are protonated with α-hydroxy groups, while 3 and 4 with higher oxidation states of +5 and +6 are deprotonated with α-alkoxy group even under strong acidic condition, respectively. This shows the relationship between the oxidation state and protonation of the α-alkoxy group in citrate or homocitrate, which is related to the protonation state of homocitrate in FeMo-cofactor of nitrogenase. The homocitrate in 3 chelates to molybdenum(v) with bidentate α-alkoxy and monodentate α-carboxy groups. Molybdenum(vi) citrate 4 is only protonated with coordinated and uncoordinated β-carboxy groups. The solution behaviours of 1 and 2 are discussed based on H and C NMR spectroscopies and cyclic voltammograms, showing no decomposition of the species.