Magneto-structural correlations in asymmetric oxalato-bridged dicopper(II) complexes with polymethyl-substituted pyrazole ligands

<p>Two oxalato-bridged dinuclear copper(II) complexes, [{Cu(Hdmpz)₃}₂(μ-ox)](ClO₄)₂·2H₂O (<b>1</b>) and [{Cu(Htmpz)₃}₂(μ-ox)](ClO₄)₂·2H₂O (<b>2</b>) (Hdmpz = 3,5-dimethyl-1<i>H</i>-pyrazole and Htmpz = 3,4,5-trimethyl-1<i>H</i>-pyrazole), have been synthesized and structurally and magnetically characterized. The crystal structures of <b>1</b> and <b>2</b> consist of asymmetric bis-bidentate μ-oxalatodicopper(II) complex cations with two short [Cu–O = 1.976(2) (<b>1</b>) and 1.973(2) Å (<b>2</b>)] and two long copper–oxygen bonds [Cu–O = 2.122(2) (<b>1</b>) and 2.110(2) Å (<b>2</b>)]. The environment at each Cu^II ion in <b>1</b> and <b>2</b> is closer to the trigonal bipyramidal geometry than to the square pyramidal [<i>τ</i> = 0.633 (<b>1</b>) and 0.711 (<b>2</b>)]. The magnetic properties of <b>1</b> and <b>2</b> show moderate antiferromagnetic coupling between the Cu^II ions through the oxalato bridge [<i>J</i> = −129 (<b>1</b>) and 161 cm⁻¹ (<b>2</b>); <b>H</b> = −<i>J</i><b>S</b>_<b>1</b> · <b>S</b>_<b>2</b> with <i>S</i>₁ = <i>S</i>₂ = <i>S</i>_Cu = ½]. A thorough magneto-structural analysis has been carried out for the family of asymmetric μ-oxalatodicopper(II) complexes with different terminal ligands to determine the influence on the sign and strength of the magnetic coupling of subtle structural factors, such as the trigonal distortion of the metal geometry and the lengthening or the asymmetry of the metal-oxalato bond distances.