An assessment of oxime-oximato hydrogen bonding: A structural and thermodynamic study of copper(II) complexes

Abstract

Copper(II) complexes formed by the diamine dioxime ligand 4,4,9,9- tetramethyl-5,8-diazadodecane-2,11-dione dioxime (H₂dddo) and its O-methyl ether (Hddmo) and bis-O-methyl ether (dddm) have been studied. The log K (potentiometric) and ΔH (calorimetric) values are reported for formation of the four-coordinate complexes [Cu(Hddmo)]²⁺ and [Cu(dddm)]²⁺ and for the deprotonation of [Cu(Hddmo)]²⁺ to give [Cu(ddmo)]⁺ at 25°C, I = 0.1 M NaCl. (log K = 12.11 ± 0.04, 9.10±0.02 and -6.76±0.09; ΔH= -43±2, -17.9±0.4 and +21.9±0.9 kJ mol^-1 respectively.) Comparison with data for [Cu(H₂dddo)]²⁺ and [Cu(Hdddo)]⁺ affords information about the relative contributions of (i) oxime deprotonation (c. 10^5.6), and (ii) oxime oximato hydrogen bonding (c. 10^3.5), to the stability of these (non-conjugated) dioxime complexes. The formation of intramolecular (oxime-oximato) hydrogen bonds is favoured by a positive ΔS; intermolecular bonds (oximato-solvent) give rise to a negative ΔS.    

A single-crystal X-ray molecular structure determination is reported for the complex Cu(dddm)-(ClO₄)₂. In the solid state the complex cation is square pyramidal with one perchlorate ion weakly coordinated in the apical position and the four nitrogen donor atoms forming a distorted square plane about the copper atom. Steric effects associated with O-methyl substitution are assessed. Steric hindrance between adjacent =NOR groups does not increase measurably when =NOH is replaced by =NOMe.