CRYSTAL GROWTH & DESIGN
Oxamido-Bridged Bimetallic Complexes Involving Nitronyl Nitroxide Radical Ligands: Crystal Structure and Magnetic Behavior
2005 VOL. 5, NO. 2 783-787
Zhi-Liang Liu,† Li-Cun Li,†,‡ Dai-Zheng Liao,† Zong-Hui Jiang,*,† and Shi-Ping Yan† Department of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China, and State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, People’s Republic of China Received July 6, 2004;
Revised Manuscript Received September 27, 2004
ABSTRACT: Two novel dinuclear copper(II) complexes containing four spin carriers have been synthesized and characterized structurally and magnetically. These complexes are formulated as [Cu2(apox)(NIT-4Py)2](ClO4)2‚2H2O (1) and [Cu2(apox)(NIT-2Py)2](ClO4)2 (2), where apox ) N,N′-bis(3-aminopropyl) oxamido, NIT-4Py ) 2-(4′-pyridyl)4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, and NIT-2Py ) 2-(2′-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl3-oxide. The structure of complexes 1 and 2 consists of centrosymmetric trans oxamido-bridged copper(II) binuclear units. In complex 1, the coordination geometry of the copper atom is distorted square plane and the radical ligands (NIT-4Py) coordinate to the copper atoms via the nitrogen atoms of the pyridine ring, whereas in complex 2, the coordination geometry around each copper atom is distorted square pyramidal and the apical position is occupied by an oxygen atom of the radical ligand. Magnetic analyses indicate that both compounds 1 and 2 exhibit strong antiferromagnetic coupling between copper(II) ions through the oxamido bridge and there are ferro- and antiferromagnetic exchange interactions between copper(II) ions and radical ligands for compounds 2 and 1, respectively. Introduction The design and synthesis of new molecular-based magnetic materials have attracted intense study over recent years.1-4 The major research aims in this field are on one hand the chemical design of molecular assemblies that exhibit a spontaneous magnetization and on the other hand the rationalization of magnetostructural correlation.5 Oxamido-bridged bimetallic complexes and radical-metal complexes are two active research fields of designing and synthesizing new molecular magnetic materials.6-10 As is well-known, the oxamido-bridged ligand N,N′bis(3-aminopropyl) oxamide (H2apox) can adopt cis or trans conformation when it coordinates to metal ions.11 Up to now, trans oxamido-bridged dinuclear copper(II) complexes (Chart 1) with different terminal ligands such as bpy, N3-, and NCO- have been considerably investigated in structural and magnetic characterization.12-14 On the other hand, important research results have been obtained by the metal-radical approach in which the stable organic nitroxide radicals are used to coordinate to paramagnetic metal ions.15-17 Therefore, combining the two synthetic approaches, namely, assembling oxamido-bridged bimetallic complexes by using organic radical spin carriers as terminal ligands, should result in novel heterospin molecular magnetic materials. In this paper, we report two novel compounds which are oxamido-bridged bimetallic complexes involving nitronyl nitroxide radical ligands, [Cu2(apox)(NIT-4Py)2](ClO4)2‚2H2O (1) and [Cu2(apox)(NIT-2Py)2](ClO4)2 (2), * To whom correspondence should be addressed. † Nankai University. ‡ Chinese Academy of Sciences.
where apox stands for N,N′-bis(3-aminopropyl) oxamido, NIT-4Py stands for 2-(4′-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, and NIT-2Py stands for 2-(2′-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3oxide. We describe successively the synthesis, crystal structures, and magnetic properties of these two complexes. To the best of our knowledge, these complexes are new examples using organic spin carriers as terminal ligands in oxamido-bridged bimetallic complexes. Experimental Section Preparation of [Cu2(apox)(NIT-4Py)2](ClO4)2‚2H2O (1). Compound 1 was prepared by the addition of 10 mL of methanol solution of NIT-4Py (46.8 mg, 2 mmol)18 to a 10 mL aqueous-methanol (1:1 ratio) solution obtained by mixing Cu(ClO4)2‚6H2O (37.0 mg, 1 mmol) and Cu(apox) (26.4 mg, 1 mmol).19 The mixture was further stirred for 30 min and filtered. The blue filtrate was allowed to stand at room temperature for 2 weeks. Dark-blue crystals were obtained. Yield, 67%. Anal. Calcd for C32H52Cl2Cu2N10O16: C, 37.25; H, 5.04; N, 13.58. Found: C, 37.64; H, 4.83; N, 13.85%. Preparation of [Cu2(apox)(NIT-2Py)2](ClO4)2 (2). Compound 2 was prepared by a way similar to that of 1 but with NIT-2Py instead of NIT-4Py. Yield, 49%. Anal. Calcd for C32H48Cl2Cu2N10O14: C, 38.60; H, 4.82; N, 14.07. Found: C, 38.28; H, 4.86; N, 13.85%. Physical Measurements. Elemental analyses for carbon, hydrogen, and nitrogen were carried out on a Perkin-Elmer elemental analyzer, model 240. The infrared spectrum was
10.1021/cg0497851 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/29/2005
Crystal Growth & Design, Vol. 5, No. 2, 2005
Liu et al.
Figure 1. ORTEP drawing of the cation in complex 1, [Cu2(apox)(NIT-4Py)2]2+. Thermal ellipsoids are drawn at the 40% probability level. Table 1. Crystal Data and Structure Refinement Summary for Compounds 1 and 2 formula formula weight crystal system space group unit cell parameters a (Å) b (Å) c (Å) β (°) V (Å3) Z Dcalc (g/cm3) scan range F(000) measured reflns µ/mm-1 temperature/K parameters refined no. of reflns collected no. of independent reflns R (int) goodness of fit R wR
C32H52Cl2Cu2N10O16 1030.82 monoclinic P2(1)/n
C32H48Cl2Cu2N10O14 994.75 monoclinic P2(1)/c
13.8503(16) 12.4517(14) 14.1060(17) 119.247(2) 2122.6(4) 2 1.613 1.74 < θ < 25.03° 1068 -31 e h e 33 -14 e k e 16 -28 e I e 25 1.209 298(2) 288 8634
7.69940(10) 12.4931(3) 22.1419(4) 95.8210(10) 2118.83(7) 2 1.553 1.85 < θ < 25.04° 1020 -9 e h e 9 -9 e k e 14 -21 e I e 26 1.205 293(2) 271 7392
0.0253 1.020 0.0335 0.0826
0.0469 1.164 0.0568 0.1465
taken on a Nicolet 5DX FT-IR spectrophotometer using KBr pellets. The electronic spectrum was taken on a Shimadzu UV-2401 PC spectrophotometer in MeOH. Variable-temperature magnetic susceptibilities were measured on a Maglab system2000 magnetometer in the temperature range 4-300 K, with an applied field of 1 T. Diamagnetic corrections were made with Pascal’s constants for all the constituent atoms. X-ray Analysis Structure Determination. The X-ray single-crystal data for both compounds were collected at room temperature on a computer-controlled Bruker SMART 1000 (for compound 1) and Siemens SMART (for compound 2) diffractometer equipped with graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å). The crystallographic data are listed in Table 1. The structures were solved using the direct method by the SHELXS-97 program.20 The H atoms were assigned with common isotropic displacement factors and included in the final refinement by use of geometrical restraints. A fullmatrix least-squares refinement on F2 was carried out using the SHELXL-97 package of programs.21
Results and Discussion Crystal Structures. (a) [Cu2(apox)(NIT-4Py)2](ClO4)2‚2H2O (1). Compound 1 consists of a binuclear complex cation [Cu2(apox)(NIT-4Py)2]2+, two solvent water molecules, and two ClO4- ions. An ORTEP drawing of the cation is shown in Figure 1. The cation is a centrosymmetric trans oxamido-bridged copper(II)
Figure 2. ORTEP drawing of the cation in complex 2, [Cu2(apox)(NIT-2Py)2]2+. Thermal ellipsoids are drawn at the 40% probability level. Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for Compound 1a Cu1-N1 Cu1-N2 N1-C16#1 C17-C16#1
Bond Lengths 1.968(2) Cu1-O1 1.989(2) Cu1-N3 1.294(3) O1-C16 1.503(5)
N(1)-Cu(1)-O(1) O(1)-Cu(1)-N(2) O(1)-Cu(1)-N(3) O(1)-C(16)-C(16)#1 N(1)#1-C(16)-C(16)#1
1.974(19) 2.022(2) 1.278(3)
Bond Angles 83.71(8) N(1)-Cu(1)-N(2) 177.81(9) N(1)-Cu(1)-N(3) 87.43(8) N(2)-Cu(1)-N(3) 117.5(3) O(1)-C(16)-N(1)#1 114.5(3)
94.91(9) 170.81(9) 94.02(9) 128.0(2)
a Symmetry transformations used to generate equivalent atoms: #1 -x, -y + 1, -z.
binuclear unit, where the NIT-4Py radicals coordinate to the copper(II) ions via the nitrogen atoms of the pyridyl rings. The coordination geometry of the copper atom is distorted square plane with an oxygen atom and a nitrogen atom from the amide, a nitrogen atom from the amine group, and a nitrogen atom from the pyridine ring of the radical ligand. The deviation of the copper(II) ion from the least-squares plane through N1, N2, N3, and O1 is 0.052 Å. The Cu-N bond distances are 1.968(2), 1.989(2), and 2.022(2) Å for N(1), N(2), and N(3), respectively. The Cu(1)-N(3) bond length is somewhat larger than that of the former two; however, the Cu(1)-N(3) bond length of 2.022(2) Å is typical of those found in copper(II) complexes with pyridylsubstituted nitroxide radicals.22-24 The ON-C-NO moiety is not coplanar with the pyridine ring, and the dihedral angle is 10.1°. The shortest contact between uncoordinated nitroxide groups [O(2)- - -O′(2)(1 - x, 1 - y, -z)] is 3.930 Å, and the closest distance of the two intramolecular NO groups [O(2)- - -O(2A)] via the oxamido bridge is 16.722 Å. The Cu...Cu distance through the oxamido bridge is 5.252 Å. The selected bond lengths and angles are given in Table 2. (b) [Cu2(apox)(NIT-2Py)2](ClO4)2 (2). Compound 2 consists of the binuclear complex cation [Cu2(apox) (NIT-2Py)2]2+ and two ClO4- ions. An ORTEP drawing of the cation is shown in Figure 2. The structure of the cation is centrosymmetric trans oxamido-bridged copper(II) binuclear. The coordination geometry around each copper atom is distorted square pyramidal: CuN3O2
Oxamido-Bridged Bimetallic Complexes
Crystal Growth & Design, Vol. 5, No. 2, 2005 785
Table 3. Selected Bond Lengths (Å) and Bond Angles (deg) for Compound 2a Cu-N(4) Cu-N(5) Cu-O(1) O(2)-N(3) C(13)-C(13)#1
Bond Lengths 1.957(4) Cu-O(3)#1 2.002(4) Cu-N(1) 2.406(4) O(1)-N(2) 1.276(5) O(3)#1-C(13) 1.523(10)
1.972(3) 2.049(4) 1.281(5) 1.272(6)
Bond Angles N(4)-Cu-O(3)#1 83.85(15) N(4)-Cu-N(5) 96.19(18) O(3)#1-Cu-N(5) 170.14(18) N(4)-Cu-N(1) 166.46(18) O(3)#1-Cu-N(1) 85.03(15) N(5)-Cu-N(1) 96.09(18) N(4)-Cu-O(1) 95.37(16) O(3)#1-Cu-O(1) 104.30(14) N(5)-Cu-O(1) 85.53(17) N(1)-Cu-O(1) 79.93(15) O(3)-C(13)-N(4) 129.0(5) O(3)-C(13)-C(13)#1 117.4(5) N(4)-C(13)-C(13)#1 113.6(5) a Symmetry transformations used to generate equivalent atoms: #1 -x + 1, -y + 1, -z + 1.
and NIT-2Py radicals coordinate to copper(II) ions as bidentate through the oxygen atoms of nitroxide groups and the nitrogen atoms of the pyridyl rings. The apical coordination site of the square pyramid is occupied by the oxygen atom O(1) of the nitroxide radical. The basal plane is completed by the carbonyl oxygen O(3A), amide N(4), and amine N(5) nitrogen atoms of the oxamido ligand and a nitrogen N(1) of the pyridine ring. These four atoms are displaced by 0.1387 Å [N(1)] and 0.1470 Å [N(4)] above and 0.1589 Å [O(3A)] and 0.1268 Å [N(5)] below the mean basal plane. The O(3)-Cu-N(5) and N(4)-Cu-N(1) angles are 170.14(18) and 166.46(18)°, respectively. The copper atom is displaced out of the equatorial plane toward the axial oxygen by 0.0275 Å. The two Cu-O bonds are markedly different from each other, the equatorial being 1.972(3) Å and the axial 2.406 Å. The shortest contact between nitroxide groups [O(1)- - -O′(2)(1 + x, y, z)] is 5.089 Å, and the distance of two coordinated NO groups [O(1)- - -O(1A)] is 7.866 Å. The Cu...Cu distance through the oxamido bridge is 5.236 Å. The selected bond lengths and angles are given in Table 3. Spectroscopic Characterization. The relevant features of the IR spectra of compounds 1 and 2 are the presence of a sharp and intense doublet at 3190 and 3125 cm-1 due to the ν(NH2) stretching vibrations and a sharp and intense peak at 1610 cm-1 (amide νCdO).25 The IR spectrum of compound 1 shows a medium absorption band at ∼1370 cm-1 attributed to the stretching vibration of N-O, which suggests that the oxygen atom of the nitroxide moiety of NIT-4Py does not coordinate to metal ions. In compound 2, the stretching vibration absorption band of the N-O of the radical appears at ∼1356 cm-1, which indicates that the oxygen atom of the nitroxide group coordinates to metal ions.26 These results are in agreement with the crystal structures. The electronic absorption spectra of two complexes in MeOH below 450 nm are dominated by intense bands due to intraradical ligand transitions and chargetransfer transitions between metal ion and ligand.18,27-29 In the 500-900 nm region, the spectra of the two complexes show broad d-d bands centered at 615 and 650 nm, respectively. These are attributed to the spin allowed d-d transitions of Cu(II) in a square plane environment.30 Magnetic Properties. The magnetic susceptibilities of the two complexes have been measured in the range
Figure 3. The χm and µeff versus T plots of compound 1; the solid line shows the fitting values.
Figure 4. The χm and µeff versus T plots of compound 2; the solid line shows the fitting values.
of 4-300 K. The variation curves of χm and µeff versus T are shown in Figures 3 and 4 for complexes 1 and 2, respectively. At room temperature, the µeff values of complexes 1 and 2 are 2.92 and 3.00µB, respectively, which are much lower than that expected for four uncorrelated spins (µeff ) 3.46µB). The µeff values of both complexes decrease slowly on lowering the temperature and give a plateau around 90 K. The µeff values of complexes 1 and 2, corresponding to the plateau region, are 2.45 and 2.50µB, respectively, indicating two uncoupled S ) 1/2 spins. This implies strong antiferromagnetic interactions between copper(II) ions through the oxamido bridge. For complex 2, further cooling below 20 K causes a further decrease of the µeff values, pointing to a weak antiferromagnetic coupling at low temperature. To characterize quantitatively the magnetic behavior of the two complexes, the spin Hamiltonian describing the situation of this four-spin system31,32 can be given as H ˆ ) -2J1S ˆ Cu1S ˆ Cu2 - 2J2(S ˆ Cu1S ˆ rad1 + S ˆ Cu2S ˆ rad2), where J1 and J2 stand for the Cu(II)-Cu(II) and Cu(II)-Rad magnetic interactions, respectively. Assuming gCu ) gR ) g and the magnetic data were preliminarily analyzed with the magnetic equation based on the above isotropic Hamiltonian, the expression of the mole susceptibility can be given as below:
Crystal Growth & Design, Vol. 5, No. 2, 2005
Ng2β2 A kT B
Liu et al.
A ) 10 exp(-E1/kT) + 2 exp(-E2/kT) + 2 exp(-E3/kT) + 2 exp(-E4/kT) (2) B) 5 exp(-E1/kT) + 3[exp(-E2/kT) + exp(-E3/kT) + exp(-E4/kT)] + exp(-E5/kT) + exp(-E6/kT) (3) E1 ) -J2 - J1/2
E2 ) J2 - J1/2
E3 ) J1/2 + (J22 + J12)1/2
E4 ) J1/2 - (J22 + J12)1/2
E5 ) J2 + J1/2 + (4J22 - 2J2J1 + J12)1/2
E6 ) J2 + J1/2 - (4J22 - 2J2J1 + J12)1/2
For complex 1, the best fitting for the experimental data gives J1 ) -192.70 cm-1, J2 ) -11.80 cm-1, g ) 2.01 and the agreement factor R ) ∑[(χm)obs - (χm)calc]2/ ∑[(χm)obs]2 ) 3.49 × 10-5. The results indicate strong antiferromagnetic interaction between copper(II) ions through the oxamido bridge, which is so strong that only the singlet state (S ) 0) of the copper dimer can be populated below 90 K. This strong antiferromagnetic interaction between copper(II) ions via the oxamido group is due to the good overlap of the magnetic orbitals of both copper(II) ions, and the JCu-Cu value is comparable to data reported for other oxamido-bridged copper dimers.19,33 Weak antiferromagnetic coupling between Cu(II) and NIT-4Py can be interpreted well from the X-ray crystal structure analysis. The ON-C-NO group is not coplanar with the pyridine ring, and the dihedral angle is 10.1°, which results in rather small pπ spin density at the pyridyl nitrogen, arising from the spin polarization of the π-electrons by the aminoxyl radical center at position 4 of the pyridyl ring. Therefore, the interaction between the Cu(II) ion and the coordinated radical should be weak.34 These weak antiferromagnetic exchange couplings between metal and radical agree with reported data on complexes in which the NIT-4Py radical coordinates to the copper(II) ion through the nitrogen atom of the pyridyl ring as in this compound.22 For complex 2, however, using an analogous theoretical model as above, the fitting is poor below 20 K. Therefore, the parameter Θ is introduced in eq 1. On the other hand, based on the X-ray crystal structure analysis, the oxygen of the nitroxide radical coordinates with the Cu(II) at the apical position of squarepyramidal. In this case, the unpaired electron of the nitroxide radical delocalizes on the π orbital (five-atom fragment ONCNO), which is orthogonal to the magnetic orbital dx2-y2 of the Cu(II) ion. This coordination mode results in ferromagnetic interaction between Cu(II) and the radical,35 and the value of J2 should be positive. The best fitting for the experimental data gives J1 ) -195.41 cm-1, J2 ) 25.65 cm-1, Θ ) -2.56 K, g ) 2.03, and the agreement factor R ) ∑[(χm)obs - (χm)calc]2/∑[(χm)obs]2 ) 2.02 × 10-3. The results indicate that there is ferro-
magnetic exchange coupling between Cu(II) and the radical and strong antiferromagnetic exchange interaction between Cu(II) and Cu(II). Below 90 K, only the singlet state (S ) 0) of the copper dimer is populated as observed in complex 1. The µeff value decreases sharply under 20 K; this can be ascribed to the magnetic coupling of uncoordinated NO groups through space, intramolecular magnetic coupling of two coordinated NO groups through the oxamido bridge, or both of them. Conclusion Two new heterospin copper(II)-radical complexes have been synthesized and characterized structurally and magnetically. In these two complexes, two copper(II) ions are bridged by the oxamido ligand to form a copper(II) dimer unit and the nitronyl nitroxide radicals coordinate to the copper(II) ions as terminal ligands to result in the four-spin system. The magnetic properties of both complexes are discussed in connection with their crystal structures. This is a new way to prepare heterospin molecular magnetic materials. Acknowledgment. We acknowledge the generous financial support of the National Natural Science Foundation of China (Nos. 20331010 and 20471032). Supporting Information Available: X-ray crystallographic data for compounds 1 and 2 (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.
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