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Concomitance of octamolybdate isomers in metastable crystal structures isolated using homoleptic Co<sup>II</sup>/Co<sup>III</sup>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.