Density functional theory investigations of the electronic structure and magnetic properties of transition metal MII/GdIII complexes (M = Co and Cu)

A relativistic DFT study of two mixed M^II and Gd^III bimetallic complexes with the following general formula: LMGd(μ-O₂CCH₃)(O₂CCH₃)₂ with (M = Co and Cu) and (L = 6,6′-((1E,1′E)-((2,2-Dimethylpropane1,3-diyl)bis(azanylylidene))bis(methanylylidene))bis(2-methoxyphenol) has been carried out. The computations have been conducted at the scalar relativistic ZORA/B3LYP/TZP level. First, a good agreement between the optimized geometries and the X-ray ones has been obtained. The computation of the magnetic exchange coupling constants using the Broken Symmetry approach (BS) at the same level of theory has been done. Using their fully optimized geometries we obtained the following coupling constants, indicating a ferromagnetic character for both complexes, J = +0.26 cm⁻¹ and J = +1.49 cm⁻¹ respectively, values close to the experimental results, namely J = +0.22 cm⁻¹ and J = +2.6 cm⁻¹. The electronic structures have been rationalized considering the MO diagrams, the distributions of atomic spin densities, in conjunction with Natural Population Analysis (NPA) and Multipole Derived Charge (MDC) analysis. The analysis of the spin populations indicates the localization of the seven 4f unpaired electrons of the Gd(III) ion on the metal center, whereas a significant delocalization of the 3d electrons of the M(II) ion occurs. The driving role of the Cu-O-Gd angle on the magnetic behavior has been investigated. It is found that the actual angle value i.e. 103.44° insures the highest values of the bond orders within the magnetic core, the core oxygen spin populations reaching their maximum value, favoring the superexchange mechanism; concomitantly, the highest value of the coupling constant is obtained.