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(2){Mo(IV)(3)O(4)(im)(3)[Mo(VI)O(3)(Hcit)](2)}·3im·4H(2)O (1), (Him)(2){Mo(IV)(3)SO(3)(im)(3)[Mo(VI)O(3)(Hcit)](2)}·im·6H(2)O (2), molybdenum(v) bipyridine homocitrate trans-[(Mo(V)O)(2)O(H(2)homocit)(2)(bpy)(2)]·4H(2)O (3) and molybdenum(vi) citrate (Et(4)N)[Mo(VI)O(2)Cl(H(2)cit)]·H(2)O (4) (H(4)cit = citric acid, H(4)homocit = homocitric acid, im = imidazole and bpy = 2,2′-bipyridine) with different oxidation states were prepared. 1 and 2 are the coupling products of [Mo(VI)O(3)(Hcit)](3−) anions and incomplete cubane units [Mo(IV)(3)O(4)](4+) ([Mo(IV)(3)SO(3)](4+)) 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 (1)H and (13)C NMR spectroscopies and cyclic voltammograms, showing no decomposition of the species.