DOI: 10.1021/cg900975y
Self-Assembly of Multidecker NiII Clusters from Preformed Ni4 Decks Cong-Min Ji,† Hui-Juan Yang,† Chong-Chao Zhao,† Vassilis Tangoulis,‡ Ai-Li Cui,† and Hui-Zhong Kou*,†
2009, Vol. 9 4607–4609
†
Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China, and Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece
‡
Received August 15, 2009; Revised Manuscript Received September 18, 2009
ABSTRACT: In a basic solution of Ni(ClO4)2 3 6H2O and pyridine-2-amidoxime (H2pyaox), tetranuclear [Ni4(Hpyaox)2(pyaox)2]2þ
fragments are formed that self-assemble to generate single-, double-, or triple-decker clusters [Ni4(Hpyaox)2(pyaox)2(Him)4](ClO4)2 (1), [Ni8(Hpyaox)4(pyaox)4(H2O)4](ClO4)2 3 H2O 3 CH3CN (2), [Ni12(Hpyaox)6(pyaox)6(H2O)2(N3)2](ClO4)4 3 8H2O 3 CH3CN (3). Magnetic studies show that the compounds possess high-spin ground states.
Coordination bonds are less stable than the covalent bonds, and it is comparatively easy for coordination bonds to rearrange eventually generating a new compound. This coordination-driven self-assembly strategy has led to ample coordination compounds and currently remains a powerful way of constructing new complexes (especially polynuclear ones) in the field of coordination chemistry.1 Some thus-obtained polynuclear compounds display exciting molecular structures and show significant applications in disease treatment, catalysis, gas adsorption, molecular recognition, and magnetism.1 Among the assembled “multidecker” polynuclear complexes, most are triple-decker sandwiches with the organic ligands as decks.2 Herein we report novel coordination-driven self-assembly of a rare family of single-, double-, and triple-decker NiII complexes, where each deck is composed of the same tetranuclear cationic units [Ni4(Hpyaox)2(pyaox)2]2þ (H2pyaox = pyridine2-amidoxime3). The synthetic strategy is based on the fact that single-decker [Ni4]2þ fragments are present in the solution confirmed by ESI-MS (388.13 m/e for [Ni4(Hpyaox)2(pyaox)2]2þ, Scheme 1 and Figure S1 in the Supporting Information). Compared to analogous pyridine-2-aldoxime (Hpao), which led to a number of polynuclear complexes with interesting molecular structure and magnetic properties,4-6 only one polynuclear complex [Ni12(Hpyaox)6(pyaox)6(MeOH)2Cl2]Cl4 3 5MeOH (4)7 based on H2pyaox has been prepared so far. With the usual antiferromagnetic coupling in oxime-bridged NiII compounds,5 the observation of ferromagnetic coupling in complex 4 presses for the elucidation of factors that affect magnetism. Complexes 1-3 are additional examples with ferromagnetic interaction, and magneto-structural correlation studies show that the N-O bond distance is the dominant factor. Mixing equimolar Ni(ClO4)2 3 6H2O and H2pyaox in MeCNH2O gave a red brown solution when the pH value of the solution was adjusted to 9-10 using ammonia. Complexes 1-3 can be isolated from the solution in the presence of imidazole (for 1), additional water (for 2) or sodium azide (for 3), respectively.8 X-ray crystallography9 reveals that complexes 1-3 have the novel multidecker molecular structure (Figure 1) with each deck made up of two square planar NiII ions and two octahedral NiII ions, which are connected by two Hpyaox- and two pyaox2ligands (Scheme 1). Therefore, they represent a series of new compounds showing the rare decker-by-decker assembly. The coordination sphere of the octahedral NiII ions in each deck is completed by two axial coordinations from imidazole nitrogen atoms, oxime oxygen atoms, oxime nitrogen atoms, or water oxygen atoms in the complexes. *Corresponding author. r 2009 American Chemical Society
Scheme 1. Drawings of the Single-Decker [Ni4(Hpyaox)2(pyaox)2]2þ Fragment Showing the Connectivity Pattern and the Arrangement around the NiII Ions in Complexes 1-3
For the double-decker compound 2, two decks are linked by four “pillars” (two Ni-Noxime and two Ni-Ooxime bonds), and significantly the Ni-Noxime bonds have been rarely observed in the oxime complexes.7,10 To satisfy the coordination, the [Ni4]2þ decks deviate from planarity, different from that in compound 1. Similarly, adjacent decks in the triple-decker complex 3 are linked by two Ni-Noxime and two Ni-Ooxime bonds. In addition, there are two terminal azido ligands and water molecules on four octahedral NiII ions of two peripheral decks. Complex 3 is isostructural to complex 4.7 The oxime N-O bonds of doubly deprotonated pyaox2ligands are generally larger than 1.40 A˚. The Nioct-N-O-Nioct torsion angles are in the range 0.8-21.1. The adjacent octahedral Ni 3 3 3 Ni separations through the diatomic N-O bridges are 3.938(6) for 1, 3.984(3) for 2 and 3.885 and 3.949 A˚ for 3. Although rich hydrogen-bonding interaction is present in the complexes, the intermolecular octahedral Ni 3 3 3 Ni distances are remote (9.097 for 1, 6.829 for 2, and 10.277 A˚ for 3). Magnetic susceptibility measurements of complexes 1-3 show similar magnetic properties (Figure 2): the χmT values increases steadily with the decrease of temperature, typical of the presence of overall ferromagnetic coupling. Because the planar four-coordinate NiII ions are diamagnetic, the magnetism should be originated from the six-coordinate NiII ions in the complexes. The room temperature χmT values are higher than the calculated spin-only values for two (compound 1), four (compound 2) and six (compound 3) uncoupled octahedral NiII ions with g = 2.0, which should be due to the presence of intermetallic ferromagnetic Published on Web 09/24/2009
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Crystal Growth & Design, Vol. 9, No. 11, 2009 Scheme 2. Magnetic Superexchange Pathways in Complex 3a
a The broken hollow bonds represent the long Ni-N bonds. For the double-decker complex 2, two NiII ions on the right should be removed. For the single-decker complex 1, only two NiII ions within a deck are involved.
Figure 1. Structure of complexes 1-3 (hydrogen atoms are not shown for clarity).
Figure 2. Temperature dependence of χmT for complexes 1-3 in an applied field of 2 kOe. The solid lines represent the best fit.
coupling and the high g value (gNi > 2.0) for octahedral NiII in the compounds. The decrease at low temperatures should be due to the zero-field splitting (ZFS) effect of the high-spin ground state, Zeeman effect, and/or intercluster antiferromagnetic interaction. To evaluate the strength of NiII-NiII magnetic coupling, we performed fits to the magnetic susceptibilities (above 20 K in order to preclude the intermolecular magnetic interaction and ZFS) using the MAGPACK program11 based on the following = -2J1(S^Ni2S^Ni3 þ S^Ni2S^Ni5 þ S^Ni2AS^Ni3A þ Hamiltonian H ^ ^ S Ni2AS Ni5A) - 2J2(S^Ni2S^Ni2A þ S^Ni3S^Ni5A þ S^Ni3AS^Ni5 þ S^Ni3 = -2J1(S^Ni2AS^Ni5A þ S^Ni2A þ S^Ni2S^Ni3A) for complex 3 and H S^Ni2S^Ni3) - 2J2(S^Ni2S^Ni2A þ S^Ni3S^Ni5A þ S^Ni3S^Ni2A) for complex 2,
as shown in Scheme 2.7 The best fit for complex 3 gave the parameters g = 2.36, J1 = 7.88 cm-1, and J2 = 0.24 cm-1 with the agreement factor of R = Σ[(χmT)obsd - (χmT)calcd]2/ Σ(χmT)obsd2 = 2.7 10-5. The satisfactory fit for complex 2 gave the parameters of g = 2.19, J1 = 11.47 cm-1, and J2 = 3.10 cm-1 (R = 4.9 10-5). The magnetic susceptibilities (5-300 K) of single-decker complex 1 can be fitted by the expressions derived from the isotropic = exchange spin Hamiltonian12 for dimeric NiII complexes H -2JS^NiS^Ni0 - DNi (S^Niz2 þ S^Ni0 z2) - gβH(S^Ni þ S^Ni0 )zJ0 S^zÆS^zæ), giving the parameters of g = 2.16(1), J = 1.9(1) cm-1, DNi = -4.0(5) cm-1, zJ0 = -0.06(3) cm-1 (R = 1.3 10-6). Magnetization data measured at 2 K for complexes 1 and 2 and 5 K for complex 3 show that the curves are in good agreement with that calculated with Anisofit 2.0 software13 for the highest possible ground spin state of S = 2, 4, and 6, respectively (see Figure S4 in the Supporting Information). These results corroborate the presence of ferromagnetic coupling through the N-O bridges. Although the negative zfs D parameters (-0.18 to -1.21 cm-1) were obtained from the fitting, no SMM property has been observed in complexes 1-3 based on the static AC magnetic susceptibility measurements. Intermolecular magnetic coupling is likely responsible for this.7 It is accepted that the torsion angle of Ni-N-O-Ni and the bond distances of bridging N-O groups are two main factors that affect magnetism. Complex 1 represents a good example for magneto-structural correlation study because it has the smallest torsion angle (2.8) of Ni-N-O-Ni among the known oximatobridged Ni(II) complexes. Complex [NiL’(pao)3Ni]ClO4 consisting of remarkably distorted Ni-N-O-Ni (41.3-44.3) linkages exhibit antiferromagnetic coupling.14 The short N-O bond distances (1.326(3)-1.338(3) A˚) in it may be the reason for the antiferromagnetic exchange. The unexpected ferromagnetic coupling in complex 1 should be unambiguously assigned to the long N-O bond distances (1.413(10) A˚). It is noteworthy that the ferromagnetically coupled complexes 1-4 have the long bridging N-O bond distances of more than 1.39 A˚, which is likely to be the approximate boundary between antiferromagnetic and ferromagnetic for NiII complexes. Obviously, further examples should be prepared to prove this estimation. Additionally, DFT calculations should be an interesting effort to help understand the mechanism of the magnetic coupling in the ferromagnetic oxime-bridged Ni(II) compounds. Work along this line is in progress in our laboratory. In conclusion, multidecker NiII clusters have been isolated from the solutions with dispersed [Ni4]2þ decks. Magneto-structural studies indicate that the bridging N-O bond distances are
Communication
Crystal Growth & Design, Vol. 9, No. 11, 2009
responsible for the Ni(II) 3 3 3 Ni(II) ferromagnetic exchange through N-O bridges. Acknowledgment. The authors acknowledge the financial support of the National Natural Science Foundation of China (Projects 20671055 and 20771065). Supporting Information Available: ES-MS spectra of the Ni(ClO4)2 3 6H2O-H2pyaox (molar ratio = 1:1) in an alkaline H2O-MeOH-MeCN solution (pH 9-10); field dependence of magnetization for complexes 1-3; top view of the multidecker structure for complexes 1-3 (PDF). X-ray crystallographic file (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.
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(7) Papatriantafyllopoulou, C.; Jones, L. F.; Nguyen, T. D.; Matamoros-Salvador, N.; Cunha-Silva, L.; Paz, F. A. A.; Rocha, J.; Evangelisti, M.; Brechin, E. K.; Perlepes, S. P. Dalton Trans. 2008, 3153–3155. (8) Anal. Calcd for C36H38Cl2N20Ni4O12 (1): C, 34.6; H, 3.1; N, 22.4. Found: C, 34.6; H, 3.0; N, 22.6%. IR (KBr, cm-1): 1100 br (ClO4-). Anal. Calcd for C52H56Cl4N26Ni8O30 (2): C, 29.2; H, 2.6; N, 17.1. Found: C, 29.6; H, 2.8; N, 17.5%. IR (KBr, cm-1): 1100 br (ClO4-). C17H19MnN2O6 (2 3 H2O): C, 50.74; H, 4.76; N, 6.97. Anal. Calcd for C74H89Cl4N43Ni12O38 (3): C, 29.3; H, 3.0; N, 19.8. Found: C, 29.6; H, 3.0; N, 19.6%. IR (KBr, cm-1): 2036 s (vs, NdNdN), 1100 br (ClO4-). (9) Crystal data for 1: C36H38Cl2N20Ni4O12 (Mr = 1248.60), triclinic, P1, a = 10.444(2) A˚, b = 11.355(2) A˚, c = 11.697(2) A˚, R = 101.92(3), β = 101.18(3), γ = 112.71(3), V = 1193.1(4) A˚3, Fcalcd = 1.738 g cm-3, μ = 1.748 mm-1, T = 293 K, MoKR, λ = 0.71073 A˚, 9114 measured reflections, 4166 independent reflections, Rint = 0.0868. R1 = 0.0939, wR2 = 0.2131 (I > 2σ(I)) and S = 1.034. Crystal data for 2: C52H56Cl4N26Ni8O30 (Mr = 2136.71), monoclinic, P21/c, a = 15.936(3) A˚, b = 17.524(4) A˚, c = 14.848(3) A˚, β = 110.53(3), V = 3883.0(14) A˚3, Fcalcd = 1.828 g cm-3, μ = 2.132 mm-1, T = 293 K, MoKR, λ = 0.71073 A˚, 35611 measured reflections, 8611 independent reflections, Rint = 0.0526. R1 = 0.0438, wR2 = 0.0891 (I > 2σ(I)) and S = 1.022. Crystal data for 3: C74H89Cl4N43Ni12O38 (Mr = 3035.20), triclinic, P1, a = 14.086(3) A˚, b = 15.411(3) A˚, c = 15.621(3) A˚, R = 60.60(3), β = 77.43(3), γ = 82.97(3), V = 2882.9(10) A˚3, Fcalcd = 1.748 g cm-3, μ = 2.100 mm-1, T = 293 K, MoKa, λ = 0.71073 A˚, 21994 measured reflections, 9859 independent reflections, Rint = 0.0893. R1 = 0.0729, wR2 = 0.1671 (I > 2σ(I)) and S = 1.039. (10) Psomas, G.; Dendrinou-Samara, C.; Alexiou, M.; Tsohos, A.; Raptopoulou, C. P.; Terzis, A.; Kessissoglou, D. P. Inorg. Chem. 1998, 37, 6556–6557. (11) Borras-Almenar, J. J.; Clemente-Juan, J. M.; Coronado, E.; Tsukerblat, B. S. J. Comput. Chem. 2001, 22, 985–991. (12) Nanda, K. K.; Addison, A. W.; Paterson, N.; Sinn, E.; Thompson, L. K.; Sakaguchi, U. Inorg. Chem. 1998, 37, 1028–1036. (13) Shores, M. P.; Sokol, J. J.; Long, J. R. J. Am. Chem. Soc. 2002, 124, 2279–2292. (14) Chaudhuri, P.; Weyhermuller, T.; Wagner, R.; Khanra, S.; Biswas, B.; Bothe, E.; Bill, E. Inorg. Chem. 2007, 46, 9003–9016.
