IR analysis of the carboxylate forms in structurally determined [CuII(κ3-L)2] species isolated from different acidic solutions

Several copper(II) coordination compounds with the tridentate κ³-<em>N,N,O</em> bisligand (L = bis(3,5-dimethylpyrazol-1-yl)acetate or its molecular acid form HL = Hbdmpza) were synthesized from different starting metal salts in appropriate acidic water solutions as reported. The XRD single crystal structural analysis reveals neutral acidic ligand form in [Cu(HL)₂]·2(HSO₄) (<strong>1</strong>) and [Cu(HL)₂]·2Cl (<strong>2</strong>), while ionic in [Cu(L)₂]·(CH₃COOH)·(H₂O) (<strong>3</strong>), and [Cu(L)₂]·2(H₂O) (<strong>4</strong>). The molecular structures of <strong>2</strong> and <strong>3 </strong>are first reported herein, thus enabling a clearer insight for the IR - structural analysis. Two carboxylate C-O bond lengths ranges, namely differing 1.29/1.22 Ǻ for <strong>2</strong>, and the same 1.24/1.24 Ǻ for <strong>3</strong>, respectively, are in agreement with the single(longer)/double(shorter) character of both bonds for the neutral carboxylic HL in <strong>2</strong>, and the same character of both bonds for the carboxylate anionic L^- in <strong>3</strong>, as seen for the related <strong>1 </strong>(HL) and <strong>4 </strong>(L^-). The most distinguished IR spectra difference for the molecular (HL) / ionic (L^-) is at the 1700 cm^-1 carboxylate band. The C=O free ligand (HL) double bond position at 1740 cm^-1 is most similar with 1702 cm^-1 in <strong>1</strong>,<strong> </strong>while a split band at 1697, 1665 cm^-1 is seen for <strong>2</strong>. On the other hand, the anionic asymmetric carboxylate stretching IR band (<strong>`</strong>n_as(COO⁻)) for <strong>3</strong> (1642 cm^-1) and <strong>4 </strong>(1635 cm^-1) is found at lower energy region. Thus, the additional band within the same region, as seen only for <strong>3 </strong>at 1716 cm^-1, is assigned to the network neutral acetic acid C=O double bond.