Comparison of hydroxycarboxytato imidazole molybdenum(IV) complexes and nitrogenase protein structures: indirect evidence for the protonation of homocitrato FeMo-cofactors

Glycolato and R,S-lactato imidazole molybdenum(IV) complexes [Mo3SO3(glyc)(2)(im)(5)]center dot im center dot H2O (1), Na-2[Mo3SO3(R,S-lact)(3)(im)(3)]center dot 10H(2)O (2), and [Mo6O10(R,S-lact)(2)(im)(10)]center dot 16H(2)O (3) have been isolated and characterized (H(2)glyc = glycolic acid, H(2)lact = lactic acid, im = imidazole). alpha-Alkoxy coordination with molybdenum [Mo-O alpha-alkoxy 1.993(7)(av) angstrom] in 1 and 2 showed obvious differences to their counterpart with alpha-hydroxy coordination [(Mo3S4)-S-IV(PPh3)(3)(Hlact)(2)(lact)] [2.204(4)(av) angstrom] as shown in M. N. Sokolov, S. A. Adonin, A. V. Virovets, P. A. Abramov, C. Vicent, R. Llusar and V. P. Fedin, Inorg. Chim. Acta, 2013, 395, 11-18. This was also true for the 36 reported structures of FeMo-cofactors in the RCSB protein data bank (Mo-O-av 2.272 angstrom), which can serve as indirect evidence for the protonation of homocitrate in FeMo-co. The C-OH alpha-hydroxybonds were longer than the short C-O alpha-alkoxy bonds. Trinuclear Mo3SO3 cores were stabilized by imidazoles and/or alpha-hydroxycarboxylates, whereas only two glycolates were present in 1. alpha-Hydroxycarboxylates in 1 and 2 acted as bidentate ligands of Mo(IV) atoms through alpha-alkoxy and alpha-carboxy groups, while the imidazoles coordinated monodentately with nitrogen atoms. The lactates in 3 coordinated with Mo(IV) atoms through two oxygen atoms of alpha-carboxy groups, leaving the alpha-hydroxy group free. Furthermore, novel hexanuclear oxomolybdenum(V) malate Na-6[(Mo2O4)(3)(mal)(4)]center dot 5H(2)O (4) was also isolated (H(3)mal = malic acid). Solid-state and solution C-13 NMR resonances of carbon atoms in alpha-alkoxy groups appeared in a high-field region (71.6, 77.4 ppm), indicating that alpha-alkoxy groups were easy to protonate.