Role of (1,3) {Cu-Cu} Interaction on the Magneto-Caloric Effect of Trinuclear {CuII-GdIII-CuII} Complexes: Combined DFT and Experimental Studies

Molecular refrigeration is found to be of great interest in the field of coordination chemistry, and Gel ion based complexes are particularly attractive, as they exhibit a large magneto-caloric effect (MCE). As the magnetic coupling in Gd-III clusters is difficult to control, other avenues to enhance the MCE values have been explored and incorporation of 3d metal ions in the cluster aggregation with Gd-III yielding {3d-Gd} clusters are targeted. Among the transition-metal ions, the Cull ion is particularly attractive, as it does not possess any anisotropy, and in this regard, several di- and polynuclear {Cu-Gd} clusters are reported to yield attractive MCE values. While the role of near-neighbor {Cu-Gd} interactions in the MCE has been explored in detail, how the next nearest-neighbor interaction influences the MCE has not been explored. To explore the importance of next-nearest-neighbor (1,3) {Cu-Cu) interaction, we have undertaken detailed density functional studies on five trinuclear {Cu-II-Ud(III)-Cu-II} complexes that are reported in the literature. In addition, we also report the synthesis and magnetic and EPR studies of a novel complex [(CuSALen)(2)Gd(NO3)(3)} (6; where SALen is N,N'-ethylenebis(salicylaldiminato)). Both magnetic and EPR studies reveal an S = 9/2 ground state for 6 with a very small zero-field splitting parameter (+0.01 cm(-1)), which aid in the achievement of a large MCE value for this molecule. Magnetization data collected for 6 yield a magnetic entropy change (-Delta S-m) of 17 J Kg(-1) K-1 at 3.5 K by employing a 7 T magnetic field. Our calculations on all six complexes reveal that {Cu-Gd} exchange is ferromagnetic in nature, while the next-nearest-neighbor {Cu-Cu} exchange is found to vary from a weak ferromagnetic to a moderate antiferromagnetic interaction. In all of the cases studied, simulated susceptibility data are in excellent agreement with the experimental data, offering confidence in the computed] values. In addition, we have developed a mechanism of magnetic coupling for {Cu-II-Gd-III-Cu-II} trinuclear complexes, where the role of formally empty 5d, 6s, and 6p orbitals of Gel ion is established. In particular, our studies reveal that the next-nearest-neighbor {Cu-Cu} interaction is strongly correlated to Cu-Gd-Cu angle, with both smaller and larger angles yielding stronger antiferromagnetic exchange. The antiferromagnetic {Cu-Cu} interaction diminishes the gap between the ground S = 9/2 state and first excited S = 7/2 state, leading to enhancement of MCE values. In contrast to the general belief that weak interactions are desired for large MCE, our study advocates targeting a stronger antiferromagnetic {Cu-Cu} interaction to obtain larger MCE values in this class of clusters.