Theoretical Studies on Trinuclear {(Mn2GdIII)-Gd-III} and Tetranuclear {(Mn2Gd2III)-Gd-III} Clusters: Magnetic Exchange, Mechanism of Magnetic Coupling, Magnetocaloric Effect, and Magneto-Structural Correlations

摘要

Among various applications that are proposed for {3d-4f} clusters, magnetic refrigeration based on the principle of the magnetocaloric effect (MCE) is gaining attention in recent years due to the substantially large MCE values reported for these types of molecules. While various factors play a role in controlling the MCE values, understanding the structural parameters that control the magnetic exchange play a vital role in the development of novel molecules possessing attractive MCE characteristics. In this regard, theoretical tools based on density functional methods are indispensable. In this work, we have employed density functional methods to study the magnetic properties of six {(MnGdIII)-Gd-III} clusters. This comprises a trinuclear complex {(Mn2GdIII)-Gd-III}, [Mn2GdO (Piv)(2)(dmem)(2)(NO3)(3)] (dmem = 2-{[2-(dimethylamino)ethyl]methylamino}ethanol) (1), along with four tetranuclear {(Mn2Gd2III)-Gd-III} complexes, [Mn2Gd2O2(Piv)(8)(HO2CCMe3)(2)(MeOH)(2)] (Piv = 2,2-dimethylpropanoic acid) (2), [Mn2Gd2O2(Piv)(8)(HO2CCMe3)(4)] (3), [Mn2Gd2(OH)(2)(O2CPh)(4)(NO3)(2)(teaH)(2)] (tea = triethanolamine) (4), and [Mn2Gd2(O)(Piv)(2)(hep)(4)(NO3)(4)] (hep = 2-(2-hydroxyethyl)pyridine) (5), and a single-chain compound containing the {(Mn2Gd2III)-Gd-III} core, [Mn(2)Ln(2)(OH)(OMe)(hmp)(4)(NO3)(4)(O3SC6H4CH3)(2)](n) (hmp = 2-hydroxymethylpyridine) (6). Here we have evaluated the exchange interactions between Mn-III and Gd-III ions and Mn-III center dot center dot center dot Mn-III ions in trinuclear as well as tetranuclear complexes. Our DFT-computed exchange interaction (J) values reproduce the experimental susceptibility data well, offering confidence in the estimated J values. Our calculations yield a diverse set of J values among these complexes ranging from weak ferromagnetic to moderate antiferromagnetic {Mn-III center dot center dot center dot Gd-III} coupling. Using orbital overlap and NBO analysis, we have explored the mechanism of magnetic coupling and deciphered