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

Molecular refrigeration is found to be of great interest in the field of coordination chemistry, and Gd^III 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 Cu^II 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-Gd^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)₂Gd(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 (-ΔS_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 J 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 Gd^III 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.