& Single-Molecule Magnets Probing the Origin of Magnetic Anisotropy in a Dinuclear {Mn III Cu II } Single-Molecule Magnet: The Role of Exchange Anisotropy Saurabh Kumar Singh and Gopalan Rajaraman* [a] Abstract: Using ab initio calculations all the components of the magnetic anisotropy in a dinuclear [Mn III Cu II Cl(5-Br- sap) 2 (MeOH)] single-molecule magnet (SMM) have been computed. These calculations reveal that apart from the single-ion anisotropy, the exchange anisotropy also plays a crucial role in determining the sign as well as the magni- tude of the cluster anisotropy. Developed magneto-struc- tural correlations suggest that a large ferromagnetic ex- change can in fact reduce the ground-state anisotropy, which is an integral component in the design of SMMs. Interest in polynuclear clusters has increased tremendously since the discovery of single-molecule magnetic (SMM) behav- ior in a {Mn 12 } cluster. [1] Since then, a number of potential ap- plications for SMMs have been proposed including information storage devices and solid-state Q bits in quantum computing. [2] The proposed potential applications are related to the bistabili- ty observed at very low temperatures due to slow relaxation of magnetization reversal. It has been established that the relaxa- tion of magnetization in SMMs is due to an energy barrier re- lated to spin ground state S and negative axial anisotropy D of the ground state (j D j S 2 for an integer spin ground state). En- hancing the energy barrier for SMMs is vital for future develop- ments, and this has been the primary goal for many research- ers working in this area. [3] It is a known fact that increasing S and/or D is expected to raise the barrier height; however, there are no rational approaches available to fine tune these parameters to date. [4] Even though individually the S [5] and the D [6] parameters have been raised significantly, the barrier height has not been raised considerably for transition-metal complexes. [4c] Over the years, a significant development has been done in theoretical and experimental side to understand the factors, which are likely to control the ground state spin S, [7] and at many instances the magnetic anisotropy is suggested as the most important parameter over the total spin to increase the barrier height. [8] Some path-breaking discoveries with a very large D values include report on a monomeric Ni II complex [6a] possessing D value as high as 250 cm 1 and a monomeric Co II SMM having D value of 70 cm 1 . [6b] However, such large D values for di/polynuclear complexes are unfounded. Under- standing the microscopic origin of ZFS parameter in di/poly- nuclear complexes hold the key to the future success, because experimental, [9, 10] as well as theoretical, studies [11a–c,12] targeting to probe the origin of single-ion and cluster anisotropies are relatively scarce. The factors that control D in polynuclear clus- ters are poorly understood, because there are many competing components, such as single-ion and exchange anisotropy. [9a] By using DFT/ab initio methods, D and E parameters of several mononuclear complexes and some SMMs [13] have been com- puted, and for mononuclear complexes some efforts has been undertaken to probe the origin of ZFS. [11a–c, 14] Recently, Neese and co-workers articulated that effort to understand the pa- rameters involved in maximizing the anisotropy of small nucle- arity clusters is crucial for future success in this area. [15] Despite numerous theoretical studies on ZFS of mononuclear com- plexes, studies on polynuclear complexes are uncommon and particularly how the single-ion anisotropy transforms in the cluster framework is rather limited. Herein, we address some of these issues and studied in detail the anisotropy of a ferromagnetically coupled dinuclear {Mn III Cu II } SMM [16] by using state-of-the-art ab initio calculations to offer insights on D and also provide some information to enhance the energy barrier. Synthesis, structure, and magnetic properties of the dinuclear {Mn III Cu II } complex [Mn III Cu II Cl(5-Br- sap) 2 (MeOH)] (1, 5-Br-sap = 5-bromo-2-salicylideneamino-1- proanol; see Figure 1 a) has been reported earlier. [16] Complex 1 was thoroughly characterized by d.c., a.c., magnetization and high-field (HF) EPR spectroscopy. [16, 17] It has been charac- terized as an SMM and single-crystal HF-EPR studies and low- temperature magnetization data estimate the axial zero-field splitting D 5/2 as 1.81 cm 1 , whereas the rhombic parameter E is assumed to be zero. The effective energy barrier (U eff ) in 1 was estimated from the relaxation studies to be 10.5 K con- firming the SMM characteristic of the complex 1. [16] Besides, 1 has also been thoroughly investigated by low-temperature NMR experiments, [18] spin-filtering and spin-transport studies. [19] In complex 1, the Mn III ion is in an elongated tetragonal en- vironment, whereas the Cu II ion was found to be in a square- planar environment having two m-alkoxo bridges, which [a] S. K. Singh, Dr. G. Rajaraman Department of Chemistry, Indian Institute of Technology Bombay, Mumbai 400076 (India) E-mail : rajaraman@chem.iitb.ac.in Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304357, containing DFT computed ener- gies, spin-density plots, CASSCF- and NEVPT2-computed D and E values, ex- citation analysis, CASSCF computed d orbitals and their splitting pattern in different environment. Chem. Eur. J. 2014, 20,1–6  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 && These are not the final page numbers! ÞÞ Communication DOI: 10.1002/chem.201304357