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:O1']], {(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.