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

摘要

Oxo and thiomolybdenum(iv/vi) imidazole hydrocitrates K-2{MoIV3O(4)(im)(3)[(MoO3)-O-VI(Hcit)](2)}3im 4H(2)O (1), (Him)(2){MoIV3SO(3)(im)(3)[(MoO3)-O-VI(Hcit)](2)}im6H(2)O (2), molybdenum(v) bipyridine homocitrate trans-[((MoO)-O-V)(2)O(H(2)homocit)(2)(bpy)(2)]4H(2)O (3) and molybdenum(vi) citrate (Et4N)[(MoO2Cl)-O-VI(H(2)cit)]H2O (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 [(MoO3)-O-VI(Hcit)](3-) anions and incomplete cubane units [MoIV3O(4)](4+) ([MoIV3SO(3)](4+)) with monodentate imidazoles, respectively, where tridentate citrates coordinate with -hydroxy, -carboxy and -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 -hydroxy groups, while 3 and 4 with higher oxidation states of +5 and +6 are deprotonated with -alkoxy group even under strong acidic condition, respectively. This shows the relationship between the oxidation state and protonation of the -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 -alkoxy and monodentate -carboxy groups. Molybdenum(vi) citrate 4 is only protonated with coordinated and uncoordinated -carboxy groups. The solution behaviours of 1 and 2 are discussed based on H-1 and C-13 NMR spectroscopies and cyclic voltammograms, showing no decomposition of the species.