Concomitance of octamolybdate isomers in metastable crystal structures isolated using homoleptic Co^II/Co^III complexes as structure-directing templates

Abstract

Exploiting the kinetic domain provided by hydrothermal conditions, it was possible to isolate three transient metastable crystal structures, each bearing concomitant pairs of octamolybdate isomers, namely, α–β, γ–β or βcs–βgp stabilized by distinctive homoleptic [Co(bpy)3] n+ (bpy = 2,2′-bipyridine; n = 2 or 3) cations generated in situ: [Co(bpy)3]4[(α-Mo8O26)(β-Mo8O26)]·5H2O (1), [NH4][Co(bpy)3][(γ-Mo8O26)0.5(β-Mo8O26)0.5]·4H2O (2) and [Co(bpy)3]2[(βcs-Mo8O26)0.5(βgp-Mo8O26)]·12H2O (3). Solid 1 with the space group P21/n and unit-cell parameters a = 22.160 (6), b = 14.209 (3), c = 24.641 (4) Å, β = 99.10 (2)° and V = 7661 (3) Å3 resulted in the same crystal structure as that synthesized previously under different conditions by Sun et al. [J. Mol. Struct. (2005), 741, 149–153]. Factors directing the reaction, such as product composition and phase stability, were monitored by analysis of the PXRD patterns of the bulk solids obtained under different experimental conditions. The relative proportions of the mixed phases 1–3 or their stabilization are highly dependent on the initial Co:Mo molar ratio and the reaction temperature. In particular, an increase in temperature induces the transformation of 1–3 into more thermodynamically stable phases formed by one-dimensional coordination polymers [Co(bpy)2(β-Mo8O26)0.5] n (4) and [(MoO3)(bpy)] n (5). The crystal structures of 1–3 correspond to molecular salts self-assembled by C—H...O—Mo, C...H and H...H intermolecular contacts. A Hirshfeld surface analysis for 1 showed that the C...H and H...H interactions represent an average of 51.8% of the total cation–cation intermolecular contacts. In contrast, these interactions are vastly reduced in 2 (23.0%) and 3 (average 28.5% for both isomers). EPR experiments indicated that the crystal structures of 1 and 3 are paramagnetic, and that for 2 is diamagnetic. The paramagnetism of 3 stems from the in situ formation of [Co(bpy)3]3+ in a high-spin configuration. The structure-directing properties of the [Co(bpy)3] n+ cations in the isolation and self-assembly of concomitant octamolybdate isomers are also described from the viewpoint of crystal engineering.

Sorption of Arsenate, Selenate, and Molybdate on the Barite (001) Surface

“…We postulate that the observation of Mo(VI) sorption on the barite surface is related to the chemistry of molybdate, which is largely distinct from those of arsenate and selenate. Molybdate has a high propensity to polymerize/oligomerize in the solution, especially at low pH and high molybdate concentration. , The presumable adsorption of molybdate oligomers with sizes ranging from 5.9 to 7.3 Å (the longest Mo–Mo distance in the oligomers of Mo 6 O 21 6– , Mo 7 O 24 6– , and Mo 8 O 28 8– ) − is consistent with the wide distribution of Mo(VI) above the surface (from 2 to 15 Å above the surface, Figure ). In contrast, our solution calculations show negligible oligomeric molybdate species (Table S6), which suggests that the adsorption of molybdate oligomers formed in the solution is unlikely.…”