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

By the solvothermal reaction under acidic conditions of Cu(NO)·3HO, NaCO 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-κO,O)copper(II)]-μ-oxalato-κO,O:O]], {(CHN)[Cu(CO))]} or {(Hppz)[Cu(CO)]}, (I), and the discrete ionic complex 4,4'-(ethane-1,2-diyl)dipyridinium bis(oxalato-κO,O)copper(II), (CHN)[Cu(CO)] or (Hbpa)[Cu(CO)], (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(CO)] units for (I) and (II) are stabilized by Hppz and Hbpa 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(CO)(bpa)] 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.