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)center dot 3H(2)O, Na-2-C2O4 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-kappa O-2(1), O-2)-copper(II)]-mu-oxalato-kappa O-3(1), O-2:O-1']], {(C9H15N3)[Cu(C2O4)(2))]}(n) or {(H(2)ppz)-[ Cu(C2O4)(2)]}(n), (I), and the discrete ionic complex 4,4'-(ethane-1,2-diyl) di-pyridinium bis(oxalato-kappa O-2(1), O-2) copper(II), (C12H14N2)[Cu(C2O4)(2)] or (H(2)bpa)[Cu(C2O4)(2)], (II). The products were characterized by single-crystal X-ray diffraction, elemental analysis, powder X-ray diffraction, thermogravimetric analyses and UVand IR spectroscopic techniques. The [Cu(C2O4)(2)](2-) units for (I) and (II) are stabilized by H(2)ppz(2+) and H(2)bpa(2+) 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.