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Molybdenum imidazole citrate and bipyridine homocitrate in different oxidation states - balance between coordinated
1State Key Laboratory for Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University Xiamen 361005 China zhzhou@xmu.edu.cn +86-592-2183047 +86-592-2184531.
Abstract:
Oxo and thiomolybdenum(iv/vi) imidazole hydrocitrates K2{MoIV 3O4(im)3[MoVIO3(Hcit)]2}·3im·4H2O (1), (Him)2{MoIV 3SO3(im)3[MoVIO3(Hcit)]2}·im·6H2O (2), molybdenum(v) bipyridine homocitrate trans-[(MoVO)2O(H2homocit)2(bpy)2]·4H2O (3) and molybdenum(vi) citrate (Et4N)[MoVIO2Cl(H2cit)]·H2O (4) (H4cit = citric acid, H4homocit = homocitric acid, im = imidazole and bpy = 2,2'-bipyridine) with different oxidation states were prepared. 1 and 2 are the coupling products of [MoVIO3(Hcit)]3- anions and incomplete cubane units [MoIV 3O4]4+ ([MoIV 3SO3]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 1H and 13C NMR spectroscopies and cyclic voltammograms, showing no decomposition of the species.
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