The effect of N-alkyl chain-length on water binding to the copper(II) coordination sphere in bis[N,N-dialkyl-L-α-alaninato]copper(II) complexes

The role of N-alkyl chain-length in binding or release of water molecules to the copper(II) coordination sphere in bis[N,N-dialkyl-L-alpha-alaninato]copper(II) complexes (N-alkyl: methyl-, ethyl-, propyl-) was studied by EPR and UV/VIS spectroscopies. Several hydrogen bond-making and non-hydrogen bond-making organic solvents with different capacities to dissolve water were used. In the alaninato complexes the amino-acid side-chains (-CH3) are too short to form intramolecular Van der Waals contacts. Thus, the effect of water binding to the copper(II) coordination sphere of the complex on the solute-solvent interactions can be studied separately from the intramolecular conformational effects of the amino acid side-chains. Water bound at the copper(II) ion in Cu(DMA)(2) was found to be essential for the stability of the molecule. In Cu(DEA)(2), apical water can easily exchange with water molecules from the solvent in both hydrogen bond-making and non-hydrogen bond-making solvents. Apical water in Cu(DPA)(2) can be exchanged with solvent molecules in non-hydrogen bond-making solvents like chloroform, while in the hydrogen bond-making solvents like methanol, apical water is screened from the solvent by the N-propyl chains. (C) 1999 Elsevier Science Ltd. All rights reserved.