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

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Abstract

Oxo and thiomolybdenum(IV/VI) imidazole hydrocitrates K2{MoIV3O4(im)3[MoVIO3(Hcit)]2}·3im·4H2O (1), (Him)2{MoIV3SO3(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 [MoIV3O4]4+ ([MoIV3SO3]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.

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

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Supplementary files

Article information


Submitted
04 Nov 2018
Accepted
11 Dec 2018
First published
02 Jan 2019

This article is Open Access

RSC Adv., 2019,9, 519-528
Article type
Paper

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

S. Wang and Z. Zhou, RSC Adv., 2019, 9, 519
DOI: 10.1039/C8RA09134J

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