Tautomerism of a compartmental Schiff base ligand and characterization of a new heterometallic CuII–DyIII complex — Synthesis, structure and magnetic properties

Inorganic Chemistry Communications 52 (2015) 64–68

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Tautomerism of a compartmental Schiff base ligand and characterization of a new heterometallic CuII–DyIII complex — Synthesis, structure and magnetic properties B. Cristóvão a,⁎, B. Miroslaw b a b

Department of General and Coordination Chemistry, Maria Curie-Sklodowska University, Maria Curie-Sklodowska sq. 2, 20-031 Lublin, Poland Department of Crystallography, Maria Curie-Sklodowska University, Maria Curie-Sklodowska sq. 3, 20-031 Lublin, Poland

a r t i c l e

i n f o

Article history: Received 30 October 2014 Received in revised form 3 December 2014 Accepted 24 December 2014 Available online 27 December 2014 Keywords: Schiff base Tautomerism 3d–4f complex Ferromagnetic interaction

a b s t r a c t A crystal structure of a compartmental ligand N,N′-bis(2,3-dihydroxybenzylidene)-1,3-diaminopropane (H4L = C17H18N2O4) (1) containing N2O2-inner and O4-outer coordination sites and a characterization of its novel diphenoxo-bridged discrete dinuclear complex [CuDy(H2L)(MeOH)(NO3)3]·2MeOH (2) are reported. The Schiff base ligand 1 crystallizes in the orthorhombic P212121 space group with a molecule in a bent conformation. The compound at 100 and 293 K displays the keto-enol tautomerism with the equilibrium in both temperatures shifted with a different degree towards the zwitterionic keto-amino form. The quantum chemical calculations showed preferences for enol-imino form in a gas phase and for keto-amine in solutions. The keto-amino tautomer is stabilized by intermolecular interactions. The complex 2 crystallizes in the triclinic P-1 space group as a dinuclear compound with CuIIDyIII core. The Dy(III) ion is nine-coordinated whereas the coordination number of Cu(II) is five. The temperature dependence of the magnetic susceptibility and the field-dependent magnetization indicated that the interaction between Cu(II) and Dy(III) metal centers in 2 is ferromagnetic. © 2014 Elsevier B.V. All rights reserved.

In recent years, the design, synthesis and investigation of 3d–4f coordination compounds are of great interest for their interesting structures and functional properties as magnetic, catalytic, optical and electronic materials [1–8]. The structures of these complexes depend on many factors, such as coordination geometries of the metal ions, nature of the ligands, counter anions, ratio of using reagents, and kind of the solvent [4, 7,9–11]. Studies of 3d–4f compounds mainly concentrated on the correlations between molecular structure and magnetic properties. However, the magnetic properties of heteronuclear complexes are still difficult to predict due to various factors such as thermal population of the Stark components of LnIII, the interactions between 3d–4f ions, the stereochemistry of the complexes, and masking effects [1,2,4,5,10,12–16]. The polydentate salen type Schiff bases containing suitable ligating donor sets are very effective in synthesis of heteronuclear complexes [1–10]. Salicyladimines as free ligands exhibit photo- or thermochromic properties. It was concluded that the way of molecular packing in the lattice is a key factor determining the type of possible chromic properties. The thermochromic crystals are usually built of planar molecules tightly packed in columns, whereas photochromic ones are formed by non-planar molecules arranged more loosely [17–20]. The presence of ortho hydroxyl group in such molecules enables the formation of intramolecular hydrogen bonding (O\H⋯N and O⋯H\N) and is responsible ⁎ Corresponding author. E-mail address: [email protected] (B. Cristóvão).

http://dx.doi.org/10.1016/j.inoche.2014.12.019 1387-7003/© 2014 Elsevier B.V. All rights reserved.

for the keto-enol tautomerism which accounts for the formation of either enol-imino or keto-amino forms [17–20]. Both tautomers can coexist in the crystalline state causing the positional disorder of the hydrogen atom involved in the intramolecular hydrogen bonding O⋯H⋯N [21]. The planarity of the molecule facilitates the proton transition along the hydrogen bond path. The energy of an internal proton transfer in such systems is only few kcal·mol−1 and may occur easily in both directions in the ground state [17,22]. This process is also associated with a change in the π-electron density distribution within the molecule [17–20]. Flores-Leonar et al. studied asymmetric o-hydroxyaldehydes and determined that the keto form is less stable in vacuo than the enol form because of the decreasing of aromaticity of the aryl ring, whereas the polarity of solvent stabilizes the keto form in the solution [23]. Our quantum chemical calculations performed at B3LYP/6–311 + G(d,p) showed that also in the case of N,N′-bis(2,3-dihydroxybenzylidene)-1,3diaminopropane (1) slightly more favorable energetically form in the gas phase was the enol-imino one (ΔE = 3.014 kcal·mol−1) and in solutions of various polarity and protic properties (MeOH, CHCl3 and DMSO) the keto-amino tautomer had always lower energy (Table S1). Additionally, the free energies of solvation and induced dipole moments were calculated for both tautomers in each of the selected solvents (Table S1). Here, we report the molecular and crystal structure of N,N′-bis(2,3dihydroxybenzylidene)-1,3-diaminopropane (1) (C17H18N2O4) abbreviated as H4L at 100 K (1a) and 293 K (1b) (Fig. 1). The experimental details and single crystal structural analyses data are given in the electronic

B. Cristóvão, B. Miroslaw / Inorganic Chemistry Communications 52 (2015) 64–68

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Fig. 1. Molecular structure of the Schiff base ligand (1) at 100 K.

Supplementary material (crystal data and structure refinement are presented in Table S2, selected bond lengths in Table S3, and hydrogen bonds in Table S4). The crystal packing of 1 with the top and side view at the folded chains running along the c crystallographic axis is presented in Figs. S1 and S2. The 1 crystallizes in the orthorhombic system, space group P212121 (Tables S2, S3). The molecule has a non-planar conformation. The crystals are yellow at 100 K (1a) and orange at the room

temperature (1b). To locate the hydrogen atoms attached to the O1/N1 and O2/N2 atoms we computed the slant plane Fourier electron density contour maps for O1⋯N1 and O2⋯N2 regions (Fig. 2) [24]. The electron density distribution analysis suggested the positional disorder with predominant occupancy for H-atom at the nitrogen for both parts of the Schiff base molecule. The site occupancy factors for H atoms attached to N1/O1 and N2/O2 atoms were 1/0 and 0.56/0.44 at room temperature

Fig. 2. Slant plane Fourier electron density contour maps calculated in the planes of the N1/O1 and N2/O2 atoms.

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B. Cristóvão, B. Miroslaw / Inorganic Chemistry Communications 52 (2015) 64–68

and 0.71/0.29 and 0.59/0.41 at 100 K, respectively. The difference between both halves of the molecule is a consequence of formation in the crystal intermolecular O3\H⋯O2 and O4\H⋯O1 hydrogen bonds with different D⋯A distances (Table S4). The tautomeric equilibrium is shifted more towards the keto form in the part of the molecule, where the D⋯A distance is shorter. It implies that intermolecular interactions contribute to the stabilization of the keto-amino tautomer. At low temperature the thermal motions are smaller, the intermolecular distances decreases and subsequently the hydrogen bonds become stronger, therefore the equilibrium is shifted towards keto-amino tautomer for both fragments of the molecule. This results in stabilizing the keto-amino form. In the crystal the zwitterionic form has elongated C_N bonds, whereas the C\O and C\C(N) bonds are shorter, than the common double C_N and single C\O, C\C bonds. This geometry is closer to the parameters calculated for molecules in solution than in a gas phase that confirms that intermolecular interactions are crucial for stabilization of the ketoamino tautomer. The above bond distances of keto-amino tautomer of 1 correspond to the bond distances found in other salicylaldimine compounds [17,18]. A survey of the Cambridge Structural Database (CSD Version 5.35 with updates May 2014 [25]) revealed only one crystal structure with the above ligand (Refcode: NAMDIP, Zeyrek et al. [11]). During reaction of N,N′-bis(2,3-dihydroxybenzylidene)-1,3-diaminopropane (1) with Cu(CH3COO)2·H2O and Dy(NO3)3·5H2O in hot methanol we have obtained a new heterodinuclear compound [CuDy(H2L)(MeOH)(NO3) 3]·2MeOH (2) in 46% yield (the experimental details and single crystal structural analyses data are given in the electronic Supplementary material). The complex 2 crystallizes in the triclinic P-1 space group (Table S2). The central part of the coordination core in 2 is occupied by Cu(II) and Dy(III) ions (Fig. 3). The Cu(II) ion is present in the inner N1N2O1O2 cavity of the compartmental ligand. The average Cu\N and Cu\O distances are 1.976(4) and 1.948(4) Å, respectively (Table S5). The coordination polyhedron formed around the Cu(II) has a classical distorted square pyramidal geometry with nitrate ion in the apical position. The nine-coordinated oxophilic Dy(III) ion is present in the open, larger space O1O2O3O4 of the Schiff base ligand. The Dy\O bond lengths depend on the chemical nature of the oxygen atoms (hydroxy, nitrate, phenoxo). They vary from 2.321(3) Å for Dy1\O1 (phenoxo) to 2.471(3) Å for Dy1\O13 (nitrate). In the complex 2 the metal ions are bridged via two phenolic oxygen atoms

O1 and O2 from hexadentate Schiff base ligand. The Cu1O1O2Dy1 fragment including two phenoxo oxygen atoms is nearly planar. The dihedral angle between the Cu1O1O2 and Dy1O1O2 planes is equal to 1.2°. The Cu1⋯Dy1 separation is 3.507(6) Å. The dihedral angle between two rings of the Schiff base ligand in the complex 2 is smaller (13.6°) than in the molecule of 1 (45.8° and 47.2° at 100 K and 293 K, respectively). In the crystal structure of 2 there are many intra- as well as intermolecular hydrogen bonds (Table S4). The adjacent molecules are bridged by hydrogen bonds through methanol molecules and are arranged in double chains running along the b crystallographic axis (Fig. S3). Between chains in the crystal structure there are weak C\H⋯O hydrogen bonds. The intermolecular interactions give an extended three-dimensional network but does not lead to short internuclear contacts with the shortest separations between metal ions being observed for Cu⋯Dy 7.245(6) Å. The selected structural parameters of 2 and other diphenoxo-bridged CuII–DyIII complexes [15,26,27] have been presented in Table 1. The geometrical parameters in the coordination environment of the dysprosium center in all the compared complexes are similar. It is worth noticing that the comparison between heterodinuclear complexes of N,N′-bis(2,3-dihydroxybenzylidene)-1,3-diaminopropane obtained by us [CuDy(H2L)(MeOH)(NO3)3]·2MeOH (2) and [CuGd (L)(NO3)3]·Me2CO reported by Zeyrek et al. [11] indicates some structural differences in spite of the very similar ionic radii of the two lanthanide(III) metal ions (0.908 and 0.938 Å for Dy(III) and Gd(III), respectively). The most important structural difference is related to the coordination polyhedron around the copper(II) ion: square pyramid for 2 and square-planar for the latter. There are also differences between the coordination number of Ln(III) being equal to 9 for Dy(III) and 10 for Gd(III), respectively. The magnetic susceptibility of 2 as a function of temperature is shown in Fig. 4. The thermal evolution of χ−1 M obeys the Curie–Weiss law in the whole temperature. At 300 K, the χMT product is equal to 14.44 cm3 mol−1 K which corresponds the value 14.54 cm3 mol−1 K expected for a pair of noninteracting CuII (S = 1/2) and DyIII (4f9, J = 15 / 2, S = 5 / 2, L = 5, 6H15/2) ions. As the temperature is lowered, χMT gradually increases to 18.84 cm3 mol−1 K, indicating the presence of a ferromagnetic interaction between Cu(II) and Dy(III) ions and then abruptly decreases to 11.84 cm3 mol−1 K−1 which may be attributed to zero-field splitting (ZFS) effects and/or intermolecular interactions. Positive value of Weiss 5.2 K constant could additionally confirm

Fig. 3. Molecular structure of the CuII–DyIII complex (2).

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Table 1 The selected structural parameters (bond distances in Å and angles in deg) and the nature of the magnetic exchange interactions of dinuclear CuII–DyIII compounds derived from N,N′-bis(2,3-dihydroxybenzylidene)-1,3-diaminopropane and other diphenoxobridged Schiff base ligands. δ

Compound

Cu\Ophen

Dy\Ophen

Cu⋯Dy

2

1.951(3) 1.944(3) 1.902(2) 1.899(2) 1.956(2) 1.958(2) 1.951(2) 1.983(2)

2.321(3) 2.344(3) 2.318(2) 2.375(6) 2.372(2) 2.362(2) 2.365(2) 3.369(2)

3.507

1.2

F

3.383

4.0

F

3.510

17.3

F

3.618

2.9

F

3 4 5

Exch. inter

δ — dihedral angle between the Ophen–Cu–Ophen and Ophen–Dy–Ophen planes. Exch. inter. — exchange interactions, F — ferromagnetic, 2 — present work. 3 — [CuLn(L1)(NO3)3] (Koner et al. [15]); 4 — [CuII(L)(C3H6O)LnIII(NO3)3] (Ishida et al. [27]). 5 — [CuLn(L)(NO3)2(H2O)3MeOH]NO3·MeOH (Cristóvão et al. [26]).

the ferromagnetic exchange coupling between the metal ions. When the Dy(III) ion is exchange-coupled with a 3d magnetic ion, for instance Cu(II), the temperature dependence of χMT is due to both the thermal population of the rare earth ion(III) Stark components and the Dy(III)–Cu(II) coupling. It follows that it is, in principle, very difficult to determine not only the magnitude of the Dy(III)–Cu(II) interaction, but even its nature. The field dependence of the magnetization M at 2 K for 2 is shown in Fig. 5. The M versus H plot shows a relatively rapid increase of the magnetization at low field in accord with the ferromagnetic interactions between Dy(III) and Cu(II) ions, and a linear increase at high field without achieving the expected saturation value (10 Nβ for the Dy(III) ion and 1 Nβ for the Cu(II) ion). At 5 T the magnetization 2 is equal to 7.30 Nβ. This is due to the crystal field effects on the Dy(III) ion, which remove the 16-fold degeneracy of the 6H15/2 ground state [1,27,28]. As summarized in Table 1, the metal centers in the diphenoxo-bridged CuII–DyIII complexes reported by us and other authors are coupled ferromagnetically. According to the literature the exchange interaction in the CuII–LnIII compounds is governed by the dihedral angle (δ) between the CuO(phenoxo)2 and LnO(phenoxo)2 planes. The higher value of this angle, the weaker coupling between metal centers should be expected. The super-exchange contribution is assumed for complexes with a planar LnO2Cu molecular fragment [15, 16]. The δ values in the 2 is equal to 1.26°, and the bridging moiety is almost planar similar as in the reported compounds with δ = 2.9–4.04.7° [15,26] (in spite of the CuII–DyIII reported by Ishida [27] where the bridging fragment is significantly twisted, δ = 17.3°). In the compound 2 and other CuII–DyIII analogs [15,26] the magnetic exchange interaction is expected to be stronger compared to that in the complex obtained by

II III Fig. 4. Plots of χMT (left) and χ−1 M (right) versus T curves for the Cu –Dy (2). A solid line is the best fit to the Curie–Weiss law.

Fig. 5. Isothermal magnetization of the CuII–DyIII complex (2) at T = 2 K.

Ishida et al. [27]. The result obtained for 2 is in a very good agreement with the theoretical model from Kahn et al. [14]. They suggest that for the 4f1–4f 6 configuration of LnIII, angular and spin moments are antiparallel in the 2S + 1LJ free-ion ground state (J = L − S). A parallel alignment of the CuII and LnIII spin moment would lead to an antiparallel alignment of the angular moment, that is to an overall antiferromagnetic interaction, whereas for the 4f 8–4f13 configurations (J = L + S), a parallel alignment of the CuII and LnIII spin moment would result in an overall ferromagnetic interaction. Acknowledgments The access to supercomputing facilities at the Academic Computer Centre CYFRONET, AGH, Krakow, (Grant No. MNiSW/Zeus_lokalnie/ UMCSLublin/015/2013) is gratefully acknowledged. Appendix A. Supplementary material Crystallographic data for 1a, 1b and 2 have been deposited with the Cambridge Crystallographic Data Centre: CCDC 1031524–1031526. This data can be obtained free of charge via www.ccdc.cam.ac.uk/data_ request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.inoche.2014.12.019. References [1] M. Towatari, K. Nishi, T. Fujinami, N. Matsumoto, Y. Sunatsuki, M. Kojima, N. Mochida, T. Ishida, N. Re, Jerzy Mrozinski, Inorg. Chem. 52 (2013) 6160–6178. [2] T. Kajiwara, M. Nakano, K. Takahashi, S. Takaishi, M. Yamashita, Chem. Eur. J. 17 (2011) 196–205. [3] G. Cosquer, F. Pointillart, Y. Le Gal, St. Golhen, O. Cador, L. Ouahab, Chem. Eur. J. 17 (2011) 12502–12511. [4] O. Iasco, G. Novitchi, E. Jeanneau, D. Luneau, Inorg. Chem. 52 (2013) 8723–8731. [5] A. Elmali, Y. Elerman, J. Mol. Struct. 737 (2005) 29–33. [6] L. Xu, Q. Zhang, G. Hou, P. Chen, G. Li, D.l M. Pajerowski, C.L. Dennis, Polyhedron 52 (2013) 91–95. [7] W.-B. Sun, P.-F. Yan, G.-M. Li, J.-W. Zhang, H. Xu, Inorg. Chim. Acta 362 (2009) 1761–1766. [8] X. Yang, D. Schipper, A. Liao, J.M. Stanley, R.A. Jones, B.J. Holliday, Polyhedron 52 (2013) 165–169. [9] Y. Ouyang, C.-Z. Xie, J.-Y. Xu, L. Yu, M.-L. Zhang, D.-Z. Liao, Inorg. Chem. Commun. 27 (2013) 166–170. [10] B. Cristóvão, J. Kłak, B. Miroslaw, J. Coord. Chem. 67 (2014) 2728–2746. [11] C.T. Zeyrek, A. Elmali, Y. Elerman, J. Mol. Struct. 740 (2005) 47–52. [12] M.L. Kahn, C. Mathonière, O. Kahn, Inorg. Chem. 38 (1999) 3692–3697. [13] J.P. Costes, F. Dahan, A. Dupuis, J.P. Laurent, Chem. Eur. J. Inorg. Chem. 4 (1998) 1616–1620. [14] M. Andruh, I. Ramade, E. Codjovi, O. Guillou, O. Kahn, J.C. Trombe, J. Am. Chem. Soc. 115 (1993) 1822–1829. [15] R. Koner, H.-H. Lin, H.-H. Wie, S. Mohanta, Inorg. Chem. 44 (2005) 3524–3536. [16] A. Jana, S. Majumder, L. Carrella, M. Nayak, T. Weyhermueller, S. Dutta, D. Schollmeyer, E. Rentschler, R. Koner, S. Mohanta, Inorg. Chem. 49 (2010) 9012–9025. [17] A. Elmali, M. Kabak, E. Kavlakoglu, Y. Elerman, T.N. Durlua, J. Mol. Struct. 510 (1999) 207–214. [18] Z. Popović, G. Pavlović, D. Matković-Čalogovića, V. Roje, I. Leban, J. Mol. Struct. 615 (2002) 23–31.

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