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2006, 110, 12-15 Published on Web 12/09/2005
Competing Magnetic Interactions in a Dinuclear Ni(II) Complex: Antiferromagnetic O-H‚‚‚O Moiety and Ferromagnetic N3- Ligand Sumana Sarkar,† Ayan Datta,‡ Amrita Mondal,† Deepak Chopra,§ Joan Ribas,| Kajal Krishna Rajak,*,† Sairam S. M,‡ and Swapan K Pati*,‡ Inorganic Chemistry Section, Department of Chemistry, JadaVpur UniVersity, Kolkata 700032, India, Theoretical Sciences Unit, Jawaharlal Nehru Center for AdVcanced Scientific Research, Jakkur P. O., Bangalore, India, Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, 560012, India, and Department de Quimica Inorganica, UniVersitat de Barcelona, Diagonal 6487, 08028 Barcelona, Spain ReceiVed: October 6, 2005; In Final Form: NoVember 16, 2005
Synthesis, structural characteristics, magnetic studies and DFT calculations in Ni(II) dinuclear complexes containing two bridging N3- and an O-H‚‚‚O linkage reveal the existence of ferromagnetic interactions between Ni(II) centers via N3- ligands and antiferromagntic interactions through the H-bonded moiety. The overall magnetic behavior of the system depends on the delicate balance between these two competing interactions.
The synthesis and characterizaton of new dinuclear transitionmetal complexes is an active area of research for potential applications in single-molecular magnets (SMM).1,2 Microscopic understanding of the magneto-structural correlation depends crucially on the superexchange coupling (J) between the spins of unpaired electrons located at metal atoms and connected through the bridging ligands (L) or interacting units.3,4 A huge amount of literature is known for the dependence of magnitude and sign of J on the M-L-M angle and M-L bond lengths.5 The Goodenough-Kanamori rules, in particular, serve as very important “rules of thumb” for the design of novel magnetic materials with different bridging ligands.6 However, for systems possessing interactions between M centers through weak Hbonding interactions such as O-H‚‚‚O, N-H‚‚‚O, and N-H‚ ‚‚N, such correlations are unavailable.7 This is primarily due to the unavailability of such crystals in the literature and the complexity of the nature of various H-bonding interactions. Interestingly, such weak molecular electrostatic interactions have led to the rapid development of novel structures in the area of supramolecular chemistry through “crystal-engineering”.8 In this communication, we specifically ask whether such supramolecular forces exhibit superexchange behavior and, if so, how such “superexchange-through-supramolecular-synthon” controls the overall magnetic behavior of a molecular system. The NNNO-coordinating tripodal ligand, {N-(4-methyl-2hydroxybenzyl)-N-(2-pyridylmethyl)}-N′,N′-dimethylethylenediamine (HL), has been used in this present work and it has been synthesized by reported procedure.9 To a methanolic solution (10 mL) of nickel(II) perchlorate hexahydrate (0.183 * Corresponding authors. (Rajak) E-mail:
[email protected]; (Pati) E-mail:
[email protected], Fax: +91-080-846766; Tel: +91-080-8462750. † Jadavpur University, Kolkata. ‡ Jawaharlal Nehru Center for Advanced Scientific Research. § Indian Institute of Science. | Universitat de Barcelona.
10.1021/jp0556963 CCC: $33.50
g, 0.5 mmol), HL (0.150 g, 0.5 mmol) and sodium azide (0.046 g, 0.7 mmol) were added. The resulting pale blue solution was stirred for 0.5 h at room temperature. Slow evaporation of the solution yielded pale blue crystalline product The IR spectrum of the complex display a stong peak near 2000 cm-1 corresponding to azide stretch and the stretching frequency of the hydrogen bonded OH group occurs as a broad peak near 2980 cm-1. Structural analysis shows that compound 1 crystallizes in ionic form. Crystal data for 1: C36H49ClN12Ni2O6, Mr ) 898.74, orthorhombic, space group Pbcn, a ) 16.345(4), b ) 17.334(4), c ) 14.973(3) Å, β ) 90°, V ) 4242.2(17) Å3, Z ) 4, dc ) 1.432 Mgm3, µ(Mo-KR) ) 1.011 mm-1, F(000) ) 1912, crystal size: 0.30 × 0.30 × 0.20 mm. A total of 31629 reflections were measured, 4604 reflections were unique (Rint ) 0.0707). T ) 293(2) K, 1.84° < θ < 30.05°, -22 e h e 21, -18 e k e 19, -20 e l 21, R1 ) 0.1057, wR2 ) 0.1991 (I > 2σ(I)), R1 ) 0.1434, wR2 ) 0.2139 (all data), GOF ) 1.283, largest difference peak and hole ) 0.838 and -0.568 e Å-3. Single crystals for X-ray diffraction studies were obtained by slow evaporation of methanolic solution. The X-ray intensity data were measured at 293 K on Bruker AXS SMART APEX CCD diffractometer (Mo-KR, λ ) 0.71073 Å). The detector was placed at a distance of 6.03 cm from the crystal. A total of 606 frames were collected with a scan width of 0.3° in different settings of φ. The data were reduced in SAINTPLUS and empirical absorption correction was applied using the SADABS package.10 Metal atoms were located by direct methods and the rest of the non-hydrogen atoms emerged from successive Fourier synthesis. The structure was refined by full matrix least-squares procedure on F2. All non-hydrogen atoms were refined anisotropically. The amine hydrogen atoms were directly located in difference Fourier maps, and the remaining hydrogen atoms were included in calculated positions. Calculations were per© 2006 American Chemical Society
Letters
Figure 1. Structure of cationic part of 1 with atom labeling scheme. Selected bond distances (Å) and angles (°): Ni(1)-N(1), 2.385(5); Ni(1)-N(2), 1.995(5); Ni(1)-N(3), 1.986(4); Ni(1)-N(4), 2.000(4); Ni(1)-N(4)#1, 2.387(5); Ni(1)-O(1), 1.926(4); O(1)‚‚‚H(1)A, 1.249(5); Ni‚‚‚Ni, 3.271(5), N(4)-Ni(1)-N(4)#1, 82.61(19); Ni(1)-N(4)-Ni(1)#1, 96.00(19); O(1)-H(1)A-O(1)#1, 165.41(9).
Figure 2. Top panel: The variation of magnetic susceptibility (open circles) as a function of temperature. The Curie-Weiss law fit for χ at high temperature (filled diamonds). The inset shows the effective magnetic moment (triangles) as a function of temperature. Lower panel: Magnetization as a function of the applied field (expt). The calculated M from Brillouin functions for S ) 1 and S ) 2 are also shown.
formed using the SHELXTL V5.03 12 program package.11 (CCDC code: 273627). A pertinent feature of the structure is that the two phenoxide groups are in cis position, and these groups are linked by symmetrical O‚‚‚H‚‚‚O hydrogen bonding (O‚‚‚O 2.477(6) Å). Structure of the cationic part is shown in Figure 1. All the Ni-N bond distances are usual and fall in the range 1.986(4)-2.387(5) Å. The Ni-O bond length is 1.926(4) Å. The magnetic properties of complex 1 were carried out in a SQUID magnetometer down to 2 K. After subtracting the diamagnetic contribution, the temperature dependence of the molar susceptibility, χM, is plotted in Figure 2 (measured at 0.1 T). χM increases from 0.00977 cm3 mol-1 at 300 K to 1.14 cm3 mol-1 at 2 K. This is indicative of weak internuclear ferromagnetic exchange between two Ni(II) centers. For a quantitative estimation of the ferromagnetic coupling between the Ni(II) ions, the susceptibility data is fitted to an expression of the type ΣSz〈Sz2〉 exp(-ESz/kBT)/ΣSz exp(-ESz/kBT), where Sz ) 0; -1, 0, 1, and -2,-1,0,+1,+2 for the S values of 0, 1, and 2, respectively. The energy for each Sz state (ESz) is calculated using the
J. Phys. Chem. B, Vol. 110, No. 1, 2006 13 Heisenberg spin-Hamiltonian including the single-ion anisotropy parameter D as H ) -JS1‚S2+D(Sz2-S(S+1)/3). The best fit yields J ) +5.76 K and D ) +3.66 K. The fit is shown as a dashed line in Figure 2. The small positive value of J indeed suggests weak ferromagnetic interactions in the sample. A Curie-Weiss law fit for the high-temperature data (T ) 180 K to 300 K) to a form χ ) [C/(T-Θ)] fits very well down to T ) 90 K and gives a value of C ) 2.87 and θ ) +5.73 K, same as the J value derived above. Study of the variation of the effective magnetic moment (µeff) with temperature (Figure 2, top panel, inset) shows that at high temperature, µeff saturates to 4.74 mB, quite close to the spin-only value of two Ni2+ ions. In Figure 2 (lower panel), we also plot the variation of the magnetization with magnetic field at T ) 2 K. Magnetization increases with the increase in the field strength, indicating that the excited spin states are significantly populated at such a low T. To confirm this, two independent Brillouin functions, one with S ) 1 and another with S ) 2 are plotted. As can be seen, the M(H) curve corresponds to a spin moment greater than a single Ni2+ ion (S ) 1), but less than the moment derived from completely polarized dimer of Ni2+ (S ) 2). Such a partial polarization is due to very weak couplings between the magnetic centers. The ferromagnetic coupling is characteristic for the polynuclear Ni(II) complex with azido bridging ligand in end-on, EO (1,1) coordination mode. Almost all reported dinuclear complexes show a [Ni-(N3)2-Ni] planar unit with an inversion center, where the Ni-N-Ni angle is close to 101-104°, with ferromagnetic coupling, J, lying between +25 and +50 K, approximately.12 Complex 1 is different from most of the reported similar complexes for several reasons: the Ni-NNi-N core is not planar and the azido bridging ligands give a boat configuration, when usually this conformation is found to be in chair form. Furthermore, the Ni-N-Ni angles are comparatively lower (close to 95°). These features explain why the J value is the lowest reported so far for these Ni(II) dinuclear complexes with two azido as bridging ligands. Apart from these two distorted ferromagnetic N3- ligands, the complex shares additional interaction through the O-H‚‚‚ O moiety. Such O-H‚‚‚O moieties are known in the literature to be weakly antiferromagntic.6 In analogy to the two EO azide linkages, the O-H‚‚‚O linkage can be regarded as magnetically equivalent to an end-to-end (EE) azide ligand, well-known to be an antiferromagnetic ligand. This compound thus exhibits a unique ratio (2:1) of ferromagnetic to antiferromagnetic interactions. For a detailed analysis of such a very interesting phenomenon in a single molecule, we have performed a detailed computational study based on density functional theory (DFT) within the broken symmetry (BS) formalism.13 The broken symmetry approach, along with electron correlations at the B3LYP level, has been widely accepted in binuclear systems to investigate magnetic properties.14 Due to the octahedral arrangement of the two Ni(II) centers, the eg orbital on each Ni atom has two unpaired electrons corresponding to integer spin, S ) 1, and the overall multiplicity of the ground state is M ) 2S + 1 ) 5 (pentate). The low-spin (triplet) and lowest spin (singlet) states are found to be higher in energy compared to the ferromagnetic highest spin state (see Supporting Information for the energies). The energies for each of these three states are calculated at the UB3LYP/LANL2DZ level in the G03 suite of programs with BS calculations for the singlet state.15 Note that the calculations are performed on the full molecular structure as retrieved from the crystallographic coordinates.
14 J. Phys. Chem. B, Vol. 110, No. 1, 2006
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Figure 4. Schematic representation of antiferromagnetic coupling through an H-bonded ligand. Note that the strongest antiferromagnetic interactions (bonding MOs) are for the cases when X ) Y and when the H-atom is placed equally between the electronegative centers. Figure 3. Variation in the magnitude of the coupling constant for (a) 2N3- + O-H‚‚‚O linkages, (b) 1 N3- + O-H‚‚‚O linkages, (c) only O-H‚‚‚O linkage, (d) only N-H‚‚‚N linkage, and (e) only N-H‚‚‚O linkages.
With the Hamiltonian corresponding to Heisenberg term together with single-ion anisotropy parameter discussed above, it is fairly simple to derive three equations for the calculation of J and D: E(singlet, BS) ) +2J, E(triplet) ) +J + 2D/3, and E(pentate) ) -J + 2D (see Supporting Information for derivation of the equations). Solving these equations, we get: J ) +9.20 K and D ) +1.43 K which are quite close to the experimental values. Note that the experimental J and D correspond to the full three-dimensional system, while the calculations are performed on the dimer molecular structure, and so the discrepancy. The molecular structure suggests that the exchange parameter, J, so obtained arises primarily due to three superexchange terms. For a more quantitative understanding for the role of each exchange interaction, we sequencially remove the two N3- ligands. Removal of the first N3- [N(4)#1-N(5)#1-N(6)#1] leads to a decrease in the magnitude of J from +9.20 K to +2.90 K. For test, from the whole structure, keeping the first, we removed the second N3- ligand, for which the magnitude of J is calculated as +3.10 K. This suggests that both the azide linkages contribute almost equally toward the ferromagnetic coupling between the two Ni(II) atoms. However, when both the azide linkages are removed, the magnitude of J is calculated as -3.25 K. Thus, the O-H‚‚‚O moiety by itself shows a dominant antiferromagnetic interaction between the metal centers. This is clearly seen in Figure 3, where the ferromagnetic interaction between the Ni(II) centers decreases with reduction in the number of azide linkages. The role of the O-H‚‚‚O linkage in antiferromagnetic coupling is also qualitatively observed in the spin density and molecular orbitals plots (see Supporting Information). For a quantitative estimation for the role of a H-bonded pair in the antiferromagnetic coupling, we substitute the O-H‚‚‚O interaction for the compound by N-H‚‚‚N and N-H‚‚‚O moieties and perform calculations for the magnitude of J in the compounds without the azide linkages. An N-H‚‚‚N group leads to an antiferromagnetic coupling of J ) -2.68 K, while an N-H‚‚‚O group leads to weak antiferromagnetic coupling of -0.92 K. Thus, symmetric H-bonded systems with an H-atom being placed between two atoms of same electronegativity as that for O-H‚‚‚O and N-H‚‚‚N lead to stronger antiferromagnetic coupling compared to an H-atom being placed between two asymmetric potentials such as O and N atoms. This is easy to follow since an H-atom being placed in the middle of two nonmagnetic electronegative centers (O or N) will lead to equal sharing of its single electron. This, in turn, gives a greater overlap of the filled orbitals (of O or N) between them as well
as with the metal centers and thus a more facile antiferromagnetic coupling. This is schematically shown in Figure 4, where the most favorable bonding leading to a antiferromagnrtic pairing occurs for symmetric H-bonded pairs. In conclusion, we have synthesized and characterized a Ni(II) dimer, where a delicate balance between two opposite exchange interactions exists. Stronger antiferromagnetic coupling is observed for symmetric pairs such as O-H‚‚‚O and N-H‚‚‚N compared to asymmetric N-H‚‚‚O hydrogen bonds. Our analysis of such a delicate force as supramolecular superexchange would serve as a rule-of-thumb for design of new and novel magnetic materials. Acknowledgment. S.K.P. and K.R. acknowledge support from CSIR and DST, India. J.R. thanks the Spanish Government (Grant BQU2003/00539). S.S.M. thanks CSIR for JRF. Supporting Information Available: Crystallographic information file (CIF), derivation of expressions for J, spin density plots, and atomic orbital contributions to frontier orbitals. This material is available free of charge via Internet at http:// pubs.acs.org. References and Notes (1) Magnetism: Molecules to Materials; Miller, J. S.; Drillon, M., Eds.; Wiley-VCH: New York, 2001-2003; Vol. 1-4. (2) Kahn, O. Molecular Magnetism; VCH Publishers: New York, 1993. (3) Ruiz, E.; Alvarez, S. ChemPhysChem 2005, 6, 1094. (4) Rodriguez, J. H.; McCusker, J. K. J. Chem. Phys. 2002, 116, 6253. (5) Hay, P. J.; Thibeault, J. C.; Hoffmann, R. J. Am. Chem. Soc. 1975, 97, 4884. (6) Goodenough, J. B. Magnetism and the Chemical Bond; Wiley: New York, 1966. (7) (a) Desplanches, C.; Ruiz, E.; Fortea, A. R.; Alvarez, S. J. Am. Chem. Soc. 2002, 124, 5197. (b) Figgis, B. N.; Kennedy, B. J.; Murrey, K. S.; Reynolds, P. A.; Wright, S. Austr. J. Chem. 1982, 35, 1807. (8) (a) Scheiner, S. Hydrogen Bonding: A Theoretical PerprectiVe; Oxford University Press: Oxford, 1997. (b) Desiraju, G. R., Steiner, T. The Weak Hydrogen Bond in Structural Chemistry and Biology, Oxford University Press: Oxford, 1999. (9) Kannappan, R.; Mahalakshmy, R.; Rajendiram, T. M.; Venkatesan, R.; Sambasiva Rao, P. Proc. Indian Acad. Sci. (Chem. Sci.) 2003, 115, 1. (10) SMART, SAINT, SADABS, XPREP, SHELXTL.; Bruker AXS Inc.: Madison, WI, 1998. (11) Sheldrick, G. M. SHELXTL, version 5.03; Siemens Analytical Instruments, Inc., Madison, WI, 1994. (12) (a) Ribas, J.; Escuer, A.; Monfort, M.; Vicente, R.; Cortes, R.; Lezama, L.; Rojo, T. Coord. Chem. ReV. 1999, 193-195, 1027. (b) Cortes, R.; Lezama, L.; Mautner, F. A.; Rojo, T. Molecule-Based Magnetic Materials; Turnbull, M. M., Sugimoto, T., Thompson, L. K. Eds.; ACS Symposium Series 644, 1996; Chapter 12. (13) (a) Noodleman, L. J. Chem. Phys. 1981, 74, 5737. (b) Ramirez, A. P. Annu. ReV. Mater. 1994, 24, 453. (c) Magnetic systems with competing interactions (frustrated spin systems); Diep, H. T., Ed.; World Scientific: Singapore, 1994. (d) Ramirez, A. P. Handbook on Magnetic Materials;
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J. Phys. Chem. B, Vol. 110, No. 1, 2006 15 Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
