Dalton Transactions Dynamic Article Links Cite this: Dalton Trans., 2011, 40, 10897 www.rsc.org/dalton PAPER Density functional studies on dinuclear {Ni II Gd III } and trinuclear {Ni II Gd III Ni II } complexes: magnetic exchange and magneto-structural maps† Saurabh Kumar Singh, Neeraj Kumar Tibrewal and Gopalan Rajaraman* Received 8th April 2011, Accepted 27th June 2011 DOI: 10.1039/c1dt10600g Theoretical calculations using density functional methods have been performed on two dinuclear {Ni II –Gd III } and two trinuclear {Ni II –Gd III –Ni II } complexes having two and three m-OR (R = alkyl or aromatic groups) bridging groups. The different magnetic behaviour, having moderately strong ferromagnetic coupling for complexes having two m-OR groups and weak ferromagnetic coupling for complexes having three m-OR groups, observed experimentally is very well reproduced by the calculations. Additionally, computation of overlap integrals MO and NBO analysis reveals a clear increase in antiferromagnetic contribution to the net exchange for three m-OR bridged {Ni–Gd} dimers and also provides several important clues regarding the mechanism of magnetic coupling. Besides, MO and NBO analysis discloses the role of the empty 5d orbitals of the Gd III ion on the mechanism of magnetic coupling. Magneto-structural correlations for Ni–O–Gd bond angles, Ni–O and Gd–O bond distances, and the Ni–O–Gd–O dihedral angle have been developed and compared with the published experimental {Ni–Gd} structures and their J values indicate that the Ni–O–Gd bond angles play a prominent role in these types of complexes. The computation has then been extended to two trinuclear {Ni II –Gd III –Ni II } complexes and here both the {Ni–Gd} and the {Ni–Ni} interactions have been computed. Our calculations reveal that, for both structures studied, the two {NiGd} interactions are ferromagnetic and are similar in strength. The {Ni–Ni} interaction is antiferromagnetic in nature and our study reveals that its inclusion in fitting the magnetic data is necessary to obtain a reliable set of spin Hamiltonian parameters. Extensive magneto-structural correlations have been developed for the trinuclear complexes and the observed J trend for the trinuclear complex is similar to that of the dinuclear {Ni–Gd} complex. In addition to the structural parameters discussed above, for trinuclear complexes the twist angle between the two Ni–O–Gd planes is also an important parameter which influences the J values. 1. Introduction Molecules which retain magnetisation in the absence of magnetic field are called single molecule magnets (SMMs). 1 They have wide spread potential applications – ranging from high-density information storage devices to solid-state Q-bits in quantum computing. 2,3 Enhancing the barrier height (related to the spin ground state S and axial zero-field splitting D) for the reorientation of magnetisation in SMMs remains one of the primary challenges Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, 400076, India. E-mail: rajaraman@chem.iitb.ac.in; Fax: (+91)22- 2576-7152; Tel: (+91)-22-2576-7183 †Electronic supplementary information (ESI) available: DFT computed energies, structures of all model complexes, computed overlap integrals, and spin densities of important atoms, correlation developed by varying Ni–Ni distance in model 1b, magneto-structural correlations developed for complex 2, simulation of the susceptibility behaviour for complex 3, NBO analysis data and list of other Ni–Gd di and trinuclear complexes reported with their selected structural parameters and experimental J values. See DOI: 10.1039/c1dt10600g in taking these SMMs to end-user applications. During the last decade, hundreds of SMMs containing transition metal atoms have been reported, notably a {Mn 6 III } SMM with a barrier height of 86 K – the largest for any transition metal SMMs known to- date. 4 An alternative strategy to increase the barrier height emerges from a simple idea of employing highly anisotropic ions such as lanthanides in the cluster aggregation. 5 With lanthanides, even simple mononuclear Tb III /Dy III complexes are reported to have SMM characteristics with a large energy barrier for relaxation of magnetisation. 6–9 A recent report of a {Dy 4 III } cluster with a record barrier height of 170 K illustrates the importance of 4f ions in the cluster aggregation. 10 A particular problem associated with clusters containing only 4f elements are the very weak exchange interactions and fast quantum tunnelling of the magnetisation leading to the isolation of SMMs with a large barrier height, a challenging task. To overcome this drawback, 3d metal ions are combined with 4f ions to make {3d–4f} 5 clusters and this strategy had produced several novel SMMs This journal is © The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 10897–10906 | 10897 Published on 15 August 2011. Downloaded by INDIAN INSTITUTE OF TECHNOLOGY BOMBAY on 22/07/2015 05:49:00. View Article Online / Journal Homepage / Table of Contents for this issue