Peroxomolybdate(vi)–citrate and –malate complex interconversions by pH-dependence. Synthetic, structural and spectroscopic studies

Abstract

The reaction of potassium molybdate(VI) with biologically relevant ligands, citric and malic acids, in the presence of H2O2 was investigated for the effect of pH variations on the product pattern. That with citric acid led to the formation of the monomeric complex K4[MoO(O2)2(cit)].4H2O (1) in the pH range 7-9, and dimer K5[MoO(O2)(2-)(Hcit)H(Hcit)(O2)2OMo].6H2O (2) (H4cit = citric acid) at pH 3-6 through carboxylate-carboxylic acid hydrogen bonding. The relation with the previously identified K4[MoO3(cit)].2H2O (4) and K4[Mo2O5(Hcit)2].4H2O (5) were shown. These and other intermediates were shown to react in the pH range 3-6 to give a more stable species 2; the reaction sequence was demonstrated either by the protonation from 1 or the deprotonation of [MoO(O2)2(H2cit)](2-) (8). Evidence that 2 exists as a dimer in solution is presented. The reaction with (S)-malic acid afforded Delta-K(2n)[MoO(O2)2((S)-Hmal)]n.nH2O (3) (H3mal = malic acid) that was oxidized further to oxalato molybdate (11) by H2O2. The three complexes 1-3 were characterized by elemental analysis, UV, IR and NMR spectroscopies, in addition to the X-ray structural studies that show citrate and malate being coordinated as bidentate ligands via alpha-alkoxyl and alpha-carboxylate groups. The formation of these complexes is dictated by pH and their thermal stabilities varied with the coordinated hydroxycarboxylate ligands.

Synthesis and catalytic epoxidation potential of oxodiperoxo molybdenum(VI) complexes with 2-hydroxybenzohydroxamate and 2-hydroxybenzoate: the crystal structure of PPh4[MoO(O2)2(HBA)]

“…This is consistent with the observation that greater stability of the diperoxo molybdate complexes is attained when the two peroxo groups coordinate in the equatorial plane [15]. The Mo-O (oxo) and Mo-O (peroxo) distances are comparable to corresponding values reported in the literature [16,18,34,35]. The lengthening of the Mo1-O1 (2.442(2) Å ) distance compared to the Mo1-O2 (2.100(2) Å ) bond length reflects the strong trans influence of the oxo ligand.…”

Layered Double Hydroxides Containing Intercalated Zinc Sulfide Nanoparticles: Synthesis and Characterization

“…The coordination chemistry of citrate is rather complex and a variety of mononuclear and polynuclear complexes have been structurally characterized. [33][34][35][36][37] Generally, trianionic HCit 3-is the predominant form above pH 5.8, but it is possible to remove the proton from the hydroxy group to give a tetra-anionic Cit 4-species at higher pH; the pH required is lowered considerably when the citrate is coordinated to a transition-metal cation. On the basis of elemental analysis (see Exp.…”

“…The asymmetric ν as (COO) stretching mode shows a shift to 1596 cm -1 , comparable to the value for LDH-HCit (1593 cm -1 , spectrum not shown). Furthermore, the band at 643 cm -1 (see inset of part c in Figure 3) due to the Zn-O vibration mode [23,47] is no longer detected, and an additional IR absorption band at around 1710 cm -1 , assigned to the free COOH vibration mode, [34,58,59] appears (see part d in Figure 3, inset), suggesting that (at least) one of the carboxylate groups has been protonated. In K 4 [(MoO 2 ) 2 -O(HCit) 2 ]·4 H 2 O, where the central carboxylate group is protonated, the free COOH vibration mode is observed [39] at around 1715 cm -1 .…”

Formations of a nonanuclear peroxo molybdate and its heptanuclear precursor

“…However, more details of isolation and conversion of the peroxo heptamolybdates have been frustrated, probably due to the instability of the system and the slow formation of the anions [8]. Previously, we have reported the pH dependent interconversions of peroxo molybdate(VI)-citrate and -malate complexes [9]. Here we further study the formations of polynuclear peroxo molybdates.…”