Comparison of hydroxycarboxylato 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]·im·H2O (1), Na2[Mo3SO3(R,S-lact)3(im)3]·10H2O (2), and [Mo6O10(R,S-lact)2(im)10]·16H2O (3) have been isolated and characterized (H2glyc = glycolic acid, H2lact = lactic acid, im = imidazole). α-Alkoxy coordination with molybdenum [Mo–Oα-alkoxy 1.993(7)av A] in 1 and 2 showed obvious differences to their counterpart with α-hydroxy coordination [MoIV3S4(PPh3)3(Hlact)2(lact)] [2.204(4)av A] 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–Oav 2.272 A), which can serve as indirect evidence for the protonation of homocitrate in FeMo-co. The C–OHα-hydroxy bonds were longer than the short C–Oα-alkoxy bonds. Trinuclear Mo3SO3 cores were stabilized by imidazoles and/or α-hydroxycarboxylates, whereas only two glycolates were present in 1. α-Hydroxycarboxylates in 1 and 2 acted as bidentate ligands of Mo(IV) atoms through α-alkoxy and α-carboxy groups, while the imidazoles coordinated monodentately with nitrogen atoms. The lactates in 3 coordinated with Mo(IV) atoms through two oxygen atoms of α-carboxy groups, leaving the α-hydroxy group free. Furthermore, novel hexanuclear oxomolybdenum(V) malate Na6[(Mo2O4)3(mal)4]·5H2O (4) was also isolated (H3mal = malic acid). Solid-state and solution 13C NMR resonances of carbon atoms in α-alkoxy groups appeared in a high-field region (71.6, 77.4 ppm), indicating that α-alkoxy groups were easy to protonate.