A new 3-D Ni 2+ coordination polymer constructed from C 2 O 4 2− and N 2 H 4 : Synthesis, structure and magnetic property

Effect of pH on the charge-assisted hydrogen-bonded assembly of the anionic [Cu(oxalate)2]2- building unit and N,N'-ditopic cations

By the solvothermal reaction under acidic conditions of Cu(NO₃)₂·3H₂O, Na₂C₂O₄ and the N,N'-ditopic organic coligands 1-(pyridin-4-yl)piperazine (ppz) and 1,2-bis(pyridin-4-yl)ethane (bpa), two novel anionic copper(II) coordination compounds were obtained, namely the one-dimensional coordination polymer catena-poly[4-(pyridin-1-ium-4-yl)piperazin-1-ium [[(oxalato-κ²O¹,O²)copper(II)]-μ-oxalato-κ³O¹,O²:O^1']], {(C₉H₁₅N₃)[Cu(C₂O₄)₂)]}_n or {(H₂ppz)[Cu(C₂O₄)₂]}_n, (I), and the discrete ionic complex 4,4'-(ethane-1,2-diyl)dipyridinium bis(oxalato-κ²O¹,O²)copper(II), (C₁₂H₁₄N₂)[Cu(C₂O₄)₂] or (H₂bpa)[Cu(C₂O₄)₂], (II). The products were characterized by single-crystal X-ray diffraction, elemental analysis, powder X-ray diffraction, thermogravimetric analyses and UV and IR spectroscopic techniques. The [Cu(C₂O₄)₂]^2- units for (I) and (II) are stabilized by H₂ppz²⁺ and H₂bpa²⁺ cations, respectively, via charge-assisted hydrogen bonds. Also, a study of the pH-controlled synthesis of this system shows that (I) was obtained at pH values of 2-4. When using bpa, a two-dimensional square-grid network of [Cu(C₂O₄)(bpa)]_n was obtained at a pH of 4. This indicates that the pH of the reaction also plays a key role in the structural assembly and coordination abilities of oxalate and N,N'-ditopic coligands.