Multifrequency cw-EPR and DFT Studies of an Apparent Compressed Octahedral Cu(II) Complex

The syntheses and single-crystal X-ray structures of the mononuclear complexes [Cu­(bmet)]­(ClO₄)₂·H₂O, [Cu­(bmet)]­Br₂·2MeCN, and [Zn­(bmet)]­(ClO₄)₂·H₂O (bmet = <i>N</i>,<i>N</i>′-bis­(2,2′-bipyridin-6-ylmethyl)­ethane-1,2-diamine) are described. All three complexes feature a central metal ion bound to all six N atoms of the bmet ligand, which displays a meridional-facial-facial-meridional (<i>mffm</i>) configuration. The three complexes show one N–M–N axis to be significantly shorter than the others in agreement with an apparent compressed octahedral geometry. The X-ray structures of a single crystal of [Cu­(bmet)]­(ClO₄)₂·0.375H₂O resolved from data recorded at different temperatures display no remarkable structural modifications. However, they all display both as a powder and, in solution, an axial <i>g</i>₁ > g₂ ≳ <i>g</i>₃ > g_<i>e</i> electron paramagnetic resonance (EPR) pattern at low temperature, which is indicative of tetragonally elongated octahedra, while at room temperature the Q-band EPR spectra display a more rhombic <i>g</i>₁ ≳ <i>g</i>₂ > g₃ > g_<i>e</i> pattern. The fully density functional theory optimized structure of the Cu^II complexes displays significant structural modifications only along one N_imine–M–N_amine axis resulting in an elongated octahedral structure. Furthermore, the EPR parameters predicted from this structure are comparable to those determined experimentally from the axial EPR signal recorded at low temperature, consistent with the unpaired electron residing mainly in the {3d_<i>x</i>²_{–<i>y</i>²}} orbital. The structural and electronic properties of [Cu­(bmet)]²⁺ are different from those in other previously described dynamic Jahn–Teller systems. We propose that these data can be rationalized by a dynamic Jahn–Teller effect perturbed by the strain of the hexadentate bmet ligand