Article pubs.acs.org/JPCA
Fine-Tuning of Magnetic Properties in Nickel(II) Trinuclear EMACs via Modifications of Equatorial Ligands P. Szarek* and W. Grochala Centre of New Technologies, University of Warsaw, Warsaw, Poland
Downloaded by UNIV OF MANITOBA on September 11, 2015 | http://pubs.acs.org Publication Date (Web): August 25, 2015 | doi: 10.1021/acs.jpca.5b05409
S Supporting Information *
ABSTRACT: The relationship between equatorial ligands structures and magnetic response of [Ni3]6+ extended metal atom chain core has been investigated. The distances between metal ions in Ni metal strings are largely predefined by framework provided through equatorial ligands. The equatorial ligands thus have primary influence on the magnitude of magnetic coupling between terminal high spin centers. Since the σ channel has greatest contribution to J, the variations in Ni−Ni bond lengths have immediate and strong effect on magnetic properties. The secondary, yet important role is played by ligand field strength and nucleophilicity. It has been shown that energy difference between singly occupied σ-type MOs composed of d(z2) of terminal ions and doubly occupied σ-type MO evolved from d(z2) of the central ion in antiferromagnetic state solution is inversely proportional to magnitude of J. Hence, the alignment between energies of d(z2) orbitals on HS and LS centers directly affected by ligand field strength governs the magnetic response. Moreover, the greater basicity of lone pairs coordinating terminal metal atoms correlates with the larger absolute value of magnetic coupling constant.
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INTRODUCTION The metal−organic complexes constitute a model example of rational design of functional materials, especially in the field of nanotechnology, with nanosensors, spintronics or molecular electronic applications. The inorganic (metal) component contributes to such features as, i.e., magnetic or optical properties, while the organic part often serves as a frame for structural diversification, additionally offering a range of polarizabilities and luminescent activity. Proper understanding of structure−activity relationship is essential for new innovative design, utilizing full potential and complexity of these hybrid molecules. The linear 1D chains of transition metal atoms (extended metal atom chain, EMAC) supported on an organic multidentate ligands1−5 are an excellent example of such hybrid materials. The oligopyridylamine ligands, such as dipyridyloamine (dpa), or similar, containing pyridyl and amine nitrogen atoms, have been used in general to synthesize various metal string complexes, reported so far.1−5 Some modifications include pyrazine instead of pyridine rings,6,7 pyridine substituted with N-sulfonyl based groups or acetamide groups,8−10 naphtyridyne groups,11 or rigid 1,9-diazaphenoxazine units.12 Ligand modifications are introduced in order to diversify the transition metal atom constituents via metal− ligand specific interactions, to extend the length of metal chains and to affect the electronic properties of EMACs. In this theoretical work, we investigate design strategy based on modification of ligand field around metal ions in the string, beyond the effect of axial ligands.13−26 Although all the metal ions are coordinated by nitrogen lone-pairs, the ligand field exerted by them, around each next metal ion, is in principle © 2015 American Chemical Society
different. Hence, for transition metal cations prone to spincrossover, the low spin configuration of transition metal ion might be preferentially stabilized over high spin one in some parts of chain (various d-orbital splitting). Recently a new type of ligand has been tested, (4-methylpyridyl)thiazolylamine,27 where the base properties of coordinating N atoms are clearly different from those in dpa. It has been suggested that high spin state of terminal metal atoms in the EMAC chain is the result of five-coordination and that the lack of axial ligands leads to diamagnetic molecule.28 However, later studies9 show that electron attracting substituents attached to amine nitrogen atoms coordinating terminal metal ions can be used to attain high spin state, despite the lack of axial ligands, hence directly through modification of equatorial ligands (using the amine− pyridyl−amine sequence). The Ni(II)-based chain complexes are one of the most extensively studied, by both experimental and theoretical methods.1−5 Among them the trinuclear species are best understood, providing suitable amount of data for comparison, therefore we have chosen the trimetal [Ni3]6+ core in order to examine ligand effects. The nickel(II) EMACs are characterized by two magnetically active terminal centers, while the metal ions in the middle of the chain are diamagnetic (square-planar, four-coordinated), with weak interactions between Ni(II) ions in the chain.6,8,29,30 In the following part, the coordination environment provided by aliphatic amino-nitrogen atoms is Received: June 6, 2015 Revised: August 7, 2015 Published: August 12, 2015 9363
DOI: 10.1021/acs.jpca.5b05409 J. Phys. Chem. A 2015, 119, 9363−9372
Article
The Journal of Physical Chemistry A
popular double-ζ Pople’s 33 and its modifications, 35,36 LANL2DZ,33 and Ahlrichs’s37,38 sets either take much longer convergence times than Def2-SVP or convergence problems have emerged. It turns out that the geometrical parameters of the prototypical Ni3(dpa)4Cl2 complex (for which high quality experimental data are available) obtained using LC-TPSSTPSS are in excellent agreement with the experimental results, although the predicted bond lengths are slightly smaller by the chosen method; this may be compared with B3LYP calculated bond lengths, which are significantly overestimated (refer to Table 1). The magnetic interactions of transition metal complexes, which are of interest here, are commonly described with spincoupling Hamiltonian of Heisenberg−Dirac−van Vleck,40−43 composed of Jij exchange integral, resulting from the overlap of the magnetic orbitals, and measuring the strength of interaction between magnetic centers, with total spin operators Sî and Sĵ :
Downloaded by UNIV OF MANITOBA on September 11, 2015 | http://pubs.acs.org Publication Date (Web): August 25, 2015 | doi: 10.1021/acs.jpca.5b05409
compared with the aromatic amines to discuss the influence of equatorial ligands on the electronic structure and magnetic properties of trinickel metal chains. Second, the deactivating and activating effects of electron donor (EDG) and acceptor groups (EAG) in pyridine ring are analyzed. Finally, the use of other heterocycles i.e. five membered azoles, instead of azines, to modulate electronic structure of metal ion chains through local differences in Lewis basicity, as well as σ donating and π accepting properties of nitrogen atoms, and magnetic superexchange to influence the strength of are considered. The Figure 1 schematically presents the composition of systems under study.
Ĥ = −2 ∑ Jij Sî Sĵ (1)
i
