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

Abstract

Oxo and thiomolybdenum(iv/vi) imidazole hydrocitrates K 2 {MoIV3O 4 (im) 3 [Mo VI O 3 (Hcit)] 2 }·3im·4H 2 O (1), (Him) 2 {MoIV3SO 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 [MoIV3O 4 ] 4+ ([MoIV3SO 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.