Discrete and polymeric cobalt organophosphates: isolation of a 3-D cobalt phosphate framework exhibiting selective CO2 capture

Abstract

Structurally diverse mononuclear, dinuclear, and tetranuclear cobalt organophosphates and a three-dimensional framework based on a D4R cobalt phosphate are reported. The role of auxiliary ligands in determining the nuclearity of the phosphate clusters has further been established. Reaction of cobalt acetate tetrahydrate with 2,6-di-iso-propylphenylphosphate (dippH₂) in methanol or DMSO in the presence of ancillary N-donor ligands leads to the formation of mononuclear octahedral cobalt phosphate [Co(dippH)₂(py)₄] (1), dinuclear octahedral cobalt phosphates [Co(dipp)(NN)(MeOH)₂]₂·2MeOH (NN = bpy 2; phen 3), tetrahedral cobalt phosphates [Co(dipp)(L)₂]₂·2(MeOH) (L = imz 4; dmpz 5) and symmetric and asymmetric tetranuclear tetrahedral cobalt phosphates [Co(dipp)(2-apy)]₄ (6) and [Co₄(dipp)₄(2-apy)₃(DMSO)]·(DMSO)·(H₂O) (7), in nearly quantitative yields. The use of a linear N-donor ditopic linker, 3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine (dptz), as the ancillary ligand leads to the formation of a robust three dimensional, four-fold interpenetrated network based on the D4R platform, {[Co(dipp)(dptz)_0.5]₄}_n (8), under ambient conditions. The new compounds have been characterized by analytical, thermo-analytical and spectroscopic techniques. Further, the molecular structures of compounds 1–8 have been established using single crystal X-ray diffraction studies. Compound 1 is a mononuclear complex in which the dippH ligands occupy trans-positions around the octahedral cobalt ion. The core structure of compounds 2–5, a Co₂P₂O₄ ring, resembles the S4R (single-4-ring) SBU of zeolites, whereas the Co₄P₄O₁₂ inorganic core found in compounds 6 and 7 resembles the D4R (double-4-ring) SBU. Cobalt organophosphate framework 8 shows significant CO₂ adsorption at 273 K (7.73 wt% at 1 bar and 18.21 wt% at 15.5 bar) with high selectivity to CO₂ uptake over N₂ and H₂ at 273 K. Magnetic studies of these symmetric complexes indicate the presence of weak antiferromagnetic interactions between the metal ions via the phosphate bridging moiety.