One-Dimensional Single-Chain Molecular Magnet with a Cross

Article pubs.acs.org/JPCC

One-Dimensional Single-Chain Molecular Magnet with a CrossLinked π−π Coordination Network [{CoII(Δ)CoII(Λ)}(ox)2(phen)2]n Pramod Bhatt,* Nidhi Thakur, M. D. Mukadam, Sher Singh Meena, and S. M. Yusuf Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India ABSTRACT: A one-dimensional single-chain molecular magnet [{CoII(Δ)CoII(Λ)}(ox)2(phen)2]n with a cross-linked π−π coordination network has been synthesized hydrothermally and investigated for its structural and magnetic properties. Oxalate (ox) ligands bridge the paramagnetic CoII metal ions, whereas phenanthroline (phen) ligands exhibit π−π coordination networks. The compound has been thoroughly investigated using room-temperature X-ray and neutron diffraction, infrared spectroscopy, dc magnetization, and reverse Monte Carlo (RMC) simulation methods. Rietveld refinement of the powder X-ray and neutron diffraction patterns at room temperature confirms the single-phase formation of the compound in the monoclinic structure with a space group P21. Structural analysis reveals that the compound assembles in the form of a one-dimensional zigzag chainlike structure. Chains consist of two asymmetric Δ- and Λ-CoII ions and a π−π coordinating network bridged by ox and phen ligands, respectively. The chain lies in the crystallographic ac plane, infinite in length with the absence of interchain π−π overlap. dc magnetization data fitted over the high-temperature regime (T > 5 K) using the Ising chain model reveals the one-dimensional magnetic nature of the compound with alternately spaced magnetic CoII sites with two different Landé g factors and exchange coupling constant values. Different Landé g factors (2.5 and 2.1) and negative exchange coupling constant values (−108 and −20 K) at two alternating CoII sites indicate a ferrimagnetic behavior of this chain compound. In the low-temperature regime (T < 5 K) the compound exhibits spontaneous magnetization and hysteresis at 1.6 K, due to the occurrence of long-range threedimensional (3-D) magnetic ordering. A prominent clustering of the C atoms around the Co atoms is ascertained by RMC analysis of diffuse scattering in the neutron diffraction pattern. The pair correlations distances between Co−Co, Co−C, Co−N, and Co−O atoms are found to be 5.0, 2.2, 2.5, and 2.4 Å, respectively, for the compound.

■

INTRODUCTION Research in the field of molecular magnetism1−3 is rapidly expanding with an aim to design and develop new magnetic materials with tunable physical properties. Molecular magnets can be synthesized by modifying the ligands, building blocks, paramagnetic metal ions, preparation methods, etc., in organic synthesis.4−9 Various forms of molecular magnets include organometallics,10 charge-transfer magnets,11 single-molecule magnets (SMMs),12 single-chain magnets (SCMs),13 spin clusters, etc.14 SCMs are magnetically isolated chains with finite magnetization even in the absence of an applied magnetic field. Discovery of the first SCM15 [CoII(hfac)2(NITphOMe)] (hfac = hexafluoroacetylacetone, NITPhOMe = 4′-methoxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) by Gatteschi in 2001 accelerated immense activities toward the design and development of SCMs.13,16−20 These chain magnets possess many interesting magnetic properties such as spontaneous magnetization,21 slow magnetic relaxation,22 large hysteresis,23,24 and possible high magnetic transition temperatures. In addition, chain-type compounds provide unique opportunities to investigate fundamental aspects of magnetic intra/interactions, superexchange interactions, and magneto−structural correlations. However, the recent motivation behind constructing SCM is to increase the transition and © 2014 American Chemical Society

blocking temperatures to fulfill the requirement for their future electronic application as molecular memory devices or recording media. 20 Few applications of such class of compounds have been proposed in high-density magnetic storage media25 or quantum bits (qbits) in quantum computing26 owing to quantum tunneling of their magnetization.27 Basic requirements for synthesis of SCMs are (i) large Ising anisotropy of the magnetic ions and (ii) very weak interchain interactions. The large magnetic anisotropy leads to the origin of the slow magnetic relaxation, whereas weak interchain interaction controls the low dimensional nature of magnetic ordering in the compound. Therefore, various types of architectures including 0-D, 1-D, 2-D, and 3-D structures have been synthesized using appropriate metal centers or controlling the interchain interaction by varying the hydrogenbonding interaction28 and π−π coordination networks interaction.21 Keeping in mind the above requirements of designing SCMs, we used anisotropic CoII ions as a paramagnetic spin centers, bridged by the ox ligand. It has been reported that anisotropy affects the magnetic behavior of SCMs.29 For example, if isotropic ions are employed, isotropic Received: December 30, 2013 Published: January 2, 2014 1864

dx.doi.org/10.1021/jp411302d | J. Phys. Chem. C 2014, 118, 1864−1872

The Journal of Physical Chemistry C

Article

C, 51.8; H, 2.4; N, 8.6. Found: Co, 17.8; C, 50.3; H, 3.0; N, 9.2. X-ray diffraction (XRD) measurement is performed at room temperature in a Bragg−Brentano geometry using a Rigaku diffractometer over a Q range of 0.3−5.0 Å−1 using Cu Kα radiation. For further structural characterization, infrared (IR) spectra were recorded in the range of 400−4000 cm−1 by loading the samples in a KBr pellet on a Bruker VERTEX 80v Fourier transform infrared (FTIR) spectrometer. The neutron diffraction pattern is recorded using a wavelength of λ = 1.249 Å at 300 K on the five linear position-sensitive detectors (PSD) based powder diffractometer-II at the Dhruva reactor, Trombay, India. Magnetization measurements were carried out using a Cryogenic Ltd., U.K., make commercial vibrating sample magnetometer as a function of both temperature and magnetic field. Temperature-dependent magnetization measurements were carried out in zero-field-cooled (ZFC) and field-cooled (FC) conditions down to 1.5 K. Hysteresis curves were recorded down to 1.6 K.

Heisenberg chains are to be expected, whereas if anisotropic metal ions are used, Ising chain-type compounds are expected.29 Thus, a large single-ion magnetic anisotropy of the CoII ion could lead to attractive magnetic characteristics, such as spin canting, hysteresis, and slow magnetic relaxation. Simultaneously, a change in the π−π stacking interactions may also lead to variation in interchain interactions. Therefore, various ligands, such as hydroxyl,30 cyano,16 azido,31 oxalato/ oxamate,32 and carboxylate,33 have most commonly been employed as bridging ligands as they can efficiently transmit magnetic coupling. In the present study, we used the ox ligand, which has a bis-chelating coordinating ability and remarkable capability to transmit electronic effects when acting as a bridge between two paramagnetic centers because of its smaller length of ∼1−2 Å. In addition, the symmetry and antisymmetry of ox play an important role in synthesizing new types of molecular magnets, whereas being a π-functional ligand, the phen molecule is expected to allow π−π interactions among these molecules, which may give rise to short-range or, long-range magnetic ordering and, consequently, emerging new materials with distinct magnetic properties. Thus, we synthesized the oxand phen-based molecular magnet [{Co II(Δ)Co II (Λ)}(ox)2(phen)2]n using the hydrothermal synthesis method and investigated its structural and magnetic properties. Since the magnetic properties are mainly interconnected to the structural properties, which depend on the types of order or disorder (dynamic, static, and subsitutional) present in the materials and hence a deeper understanding of local structure, order and disorder at the atomic scale is also highly desirable. In addition, characterization of structural order or disorder at the molecular level is an increasingly important challenge in the field of magnetism. Therefore, a detailed structural disorder investigation has been done by carrying out a reverse Monte Carlo (RMC)34 simulation on the powder neutron diffraction data. Refinement of experimental diffraction data using the standard method like Rietveld35,36 does not have the ability to analyze the structural disorder/diffuse scattering component. However, the RMC method gives both the qualitative and the quantitative information of the local structure in the compound. Here, we mainly focused on the pair correlation and arrangement between atoms in the [{CoII(Δ)CoII(Λ)}(ox)2(phen)2]n compound.

■

RESULTS AND DISCUSSION A. X-ray/Neutron Diffraction and Crystal Structure Study. Figure 1 presents the Rietveld-refined (using the

■

EXPERIMENTAL SECTION Compound [{CoII(Δ)CoII(Λ)}(ox)2(phen)2]n has been synthesized using the hydrothermal synthesis method as reported earlier in the literature.21,24 Chemicals used for synthesis are reagent grade from Sigma-Aldrich and Alfa Aesar with high purity (99.9% pure). The hydrothermal reaction was carried out in a 250 mL Teflon-lined vessel at 180 °C in water. A reaction mixture of CoCl2 (0.5193 g, 0.2 mmol), oxalic acid (0.756 g, 0.3 mmol), 1,10-phenanthroline monohydrate (0.792 g, 0.2 mmol), H2O (160 mL, 440 mol) in molar ratios of 2:3:2:4400, and 4 mL of CsOH was sealed in a 250 mL Teflonlined stainless-steel autoclave, heated to 180 °C within 3 h at a rate of ∼60 °C/h, kept at 180 °C for 96 h, cooled to 70 °C at a rate of 6 °C/h, and then left to cool to room temperature. The precipitate was filtered and washed many times with ethanol (EtOH) and finally allowed to dry in air. This synthesis was reproducible, and the yield of the compound was calculated and found to be ∼30%. Values (in wt %) obtained from elemental analysis are summarized here. Anal. Calcd for Co2C28H16N4O8: Co, 18.1;

Figure 1. Rietveld-refined room-temperature XRD (a) and neutron diffraction (b) patterns of the compound. (a, inset) Magnified view of the XRD pattern. Open circles and solid lines indicate the observed and calculated patterns, respectively. Solid line at the bottom shows the difference between observed and calculated patterns. Vertical lines at the bottom show the position of allowed Bragg peaks. (hkl) values of selected Bragg peaks are also marked.

FULLPROF program)36 room-temperature X-ray and neutron powder diffraction patterns of [{Co I I (Δ)Co I I (Λ)}(ox)2(phen)2]n. Refinements show that the compound is in a single-crystalline phase with a monoclinic structure of space group P21. Important structural parameters of atoms, such as atomic coordinates, lattice constant, bond length, bond angles, and site occupancies, derived from Rietveld analysis of X-ray diffraction, are shown in Table 1. The compound contains a 1865



Article

Table 1. Structural Parameters for a Few Atoms Obtained from Rietveld Refinement of the X-ray Powder Diffraction Pattern of [{CoII(Δ)CoII(Λ)}x(ox)2(phen)2]na atom Co1 Co2 O1 O2 O3 O4 O5 O6 O7 O8 N1 N2 N3 N4 C1 C2

x 0.135 0.610 0.230 0.320 0.497 0.423 0.803 −0.057 0.723 0.006 0.062 0.224 0.700 0.551 0.312 0.343 formula MW lattice parameters a b c α=γ β

a

y (4) (3) (4) (5) (8) (7) (5) (3) (1) (9) (7) (6) (5) (8) (2) (8)

0.961 (8) 0.981 (7) 1.052 (1) 0.887 (1) 0.882 (7) 1.056 (7) 0.893 (9) 1.053 (2) 1.050 (3) 0.886 (1) 0.896 (4) 1.054 (5) 1.074 (5) 0.911 (5) 1.130 (1)) 1.195 (8) C28H16Co2N4O8 654.3

z

occupancy

bond length (Å)

0.391(6) 0.100 (7) 0.250 (9) 0.358 (7) 0.222 (4) 0.138 (3) 0.127 (5) 0.370 (7) 0.255 (6) 0.263 (7) 0.562 (3) 0.557 (7) −0.041 (1) −0.041 (1) 0.558 (2) 0.666 (7)

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Co1−O1 2.1(3) Co1−O2: 2.0(3) Co1−O3 3.92(5) Co1−O4 3.96(8) Co −O6 2.14(9) Co1−O8 2.0(3) Co1−N1 2.08(3) Co1−N2 2.23(1)

O1−Co1−O2 O1−Co1−O3 O1−Co1−O4 O1−Co1−O6 O1−Co1−O8 O1−Co1−N1 O1−Co1−N2

79.1(18) 59.0(3) 20.7(6) 87.14(19) 95.9(10) 166.4 (3) 92.3 (3)

Co2−O1 Co2−O3 Co2−O4 Co2−O5 Co2−O7 Co2−N3 Co2−N4

O1−Co2−O3 O1−Co2−O4 O1−Co2−O5 O1−Co2−O7 O1−Co2−N3 O1−Co2−N4

58.5(14) 19 (10) 144.5(8) 90.3(2) 119(3) 105.1(3)

3.95(5) 2.12(1) 2.02(8) 2.12(9) 2.06(5) 2.11(6) 2.17(1)

bond angles (deg)

9.07(5) Å 13.6(8) Å 10.17(9) Å 90° 92.4(7)°

x, y, and z denote the fractional coordinates.

Figure 2. Crystal structure of [{CoII(Δ)CoII(Λ)}(ox)2(phen)2]n compound prepared using ORTEP. (a) Single repeating unit of the chain. Color coding for atoms: Co (green), N (magenta), C (blue), O (red). (b and c) Infinite 1-D chain-like structure of the compound. Symmetry for the monoclinic structure: (1) x, y, z; (2) −x, y + 1/2, −z.

are different in terms of their bond lengths and bond angles with respect to each other as seen in Table 1. Thus, both CoII ions are asymmetric in atomic nature; therefore, there are two locally distinct CoII ions [CoII(Δ) and CoII(Λ)] in the compound (Figure 2a). The distance between the two phen

chain-like structure in which two crystallographically distinct CoII ions are bridged by the ox ligand as shown in Figure 2. CoII ions are separated by each other with an intermetallic distance of ∼5.34 Å. Though both CoII ions (Co1 and Co2) are connected with similar atomic surroundings at both sites, they 1866



Article

ligands is found to be ∼14.8 Å. The repetition of a single chain unit results into an infinite 1-D zigzag chain of type [phen− CoII(Δ)−ox−CoII(Λ)−phen] as shown in Figure 2b and 2c. Chains are well separated from each other with intermetallic distances of ∼8.5 and 10.8 Å, suggesting a negligible interchain magnetic interaction. Distances between the two nearest phen groups within the interchain and intrachain direction are found to be ∼4.9 and 10.4 Å, respectively, suggesting the possibility of a weak π−π magnetic interaction along the interchain. The room-temperature neutron diffraction pattern (recorded at 1.249 Å wavelength) is also consistent with the results derived from the X-ray diffraction pattern for the compound. B. IR Study. An IR spectral study is carried out on the compound to find any characteristic stretching frequencies in the absorption spectrum. Figure 3 shows the infrared spectra of

bands.42 Thus, the intense bands at ∼1500, 1100, and ∼730 cm−1 relative to their nearest neighbors suggest high-spin states of the CoII metal ions in the compound. C. dc Magnetization Study. Field-cooled (FC) and zerofield-cooled (ZFC) magnetization versus temperature (T) curves for [{CoII(Δ)CoII(Λ)}(ox)2(phen)2]n under an external applied magnetic field of 200 and 50 Oe are shown in Figure 4a

Figure 4. Field-cooled (FC) and zero-field-cooled (ZFC) magnetization versus temperature (T) curve of [{CoII(Δ)CoII(Λ)}(ox)2(phen)2]n compound under an external applied magnetic field of 200 (a) and (b) 50 Oe. (b) Magnetic data shown down to a temperature of ∼4.9 K; (inset) magnetic transition at a low temperature of ∼3 K.

and 4b, respectively. Magnetization data show a broad hump over the temperature range of ∼18−35 K for both fields. This broad hump in the magnetization curves has been previously ascribed to the presence of an antiferromagnetic short-range spin correlation in the chain compounds.44,45 The χT vs T (χ is the dc magnetic susceptibility) curve at 50 Oe is shown in Figure 5a. The χT vs T curves fall down with decreasing temperature. This kind of behavior is a characteristic of ferrimagnetic materials.5,46 The temperature dependence of the inverse of the susceptibility (χ−1) shown in Figure 5b is fitted by a straight line using the Curie−Weiss law

Figure 3. Room-temperature IR spectrum for [{CoII(Δ)CoII(Λ)}(ox)2(phen)2]n over the range of 400−4000 cm−1 (a) and in the magnified range of 400−1800 cm−1 (b).

the [{CoII(Δ)CoII(Λ)}(ox)2(phen)2]n compound at room temperature. The observed characteristic peaks due to CO, CN, and CC bonds confirm the presence of ox and phen ligands. In general, the frequency of the ring vibrations of the uncoordinated ox is observed at 1621 cm−1 which shifts to 1674 cm−1, confirming coordination of the ox ligand to the CoII metal center.37,38 IR bands in the 1750−1250 and 855−400 cm−l regions are mainly dominated by the ox and phen ligands, respectively.39 The magnified view of the IR spectra (1800−400 cm−l) of the compound is shown in Figure 3b. Characteristic absorption bands found at 1674, 1608.5, 1517.8, 1425.2, 1145.6, 852.4, 798.4, 719.0, and 486.0 cm−1 are attributed to CO, CC, CN, CC−H, and OCO bonds of ox and phen ligands. Observed peaks are comparable with previously reported IR frequencies of ox- and phen-based compounds.40−43 IR stretching frequencies can also be used to ascertain the spin states of the metal ions. The low-spin−highspin transition results in an increase of intensities of the C−C and C−N bands relative to the intensities of the nearest

χ=

C T − θp

where C is the Curie constant and θp is the paramagnetic Curie temperature. The negative value of θp (−22.1 K) indicates that the interaction between the two CoII ions is antiferromagnetic in nature. The spin-only effective paramagnetic moment (μeff) of 6.07 μB/fu has been obtained using the formula (3CkB/ NA)1/2μB ≈ (8C)1/2μB, where NA is Avogadro’s number and kB is the Boltzmann constant. The theoretically expected (spin only) value of μeff are calculated using the formula (μeff)2 = Σ[g2{n·S(S + 1)}]μB2, where g is the gyromagnetic ratio (∼2), n is the number of magnetic ions with spin S in the formula unit, and the summation Σ runs over all magnetic ions in the formula unit. The theoretically calculated value of spin-only μeff 1867



Article

Figure 6. Fitting of experimental magnetization (M) vs temperature (T) data for temperature T > 5 K with the 1-D Ising chain model considering the parallel and perpendiculars contribution of magnetization for the [{CoII(Δ)CoII(Λ)}(ox)2(phen)2]n compound.

−20 K and ga, gb = 2.5, 2.1) obtained from the fitting of the magnetization data show negative j1 and j2 values and different Landé g factors using the parallel contribution of magnetization. The negative j1 and j2 values and different Landé g factors suggest a 1-D ferrimagnetic nature of the compound down to 5 K.24,45 Results are similar to other 1-D ferrimagnetc compounds reported so far for bimetallic complexes characterized by Ising models of chain magnets with different g values at different sites.44,45,47,48 In addition, as the temperature is further decreased, the compound shows a strong rise in magnetization below ∼3 K as depicted in the inset of Figure 4b. This strong enhancement in magnetization could arise from the long-range magnetic ordering as a result of interchain π−π interaction through phen ligands. Figure 7 shows the magnetization (M) as a function of applied magnetic field (H) for [{Co II (Δ)Co II (Λ)}-

Figure 5. χT versus temperature (T) curve (a) under an external applied magnetic field of 50 Oe for the compound [{CoII(Δ)CoII(Λ)}(ox)2(phen)2]n. (b) Inverse susceptibility (χ−1) vs temperature (T) curve fitted using the Curie−Weiss law.

is found to be 5.47 μB/fu, assuming both CoII ions in their highspin states (i.e., S = 3/2). The difference in the experimentally observed and theoretically calculated value of μeff could be due to deviation in the Landé g factor value. In order to explain the low-dimensional nature of the compound, magnetization data have been analyzed in the hightemperature regime (T > 5 K) using a one-dimensional Ising chain model considering two different pathways of magnetic interactions (j1 and j2) and Landé g factors (ga and gb) as follows

where j1 and j2 refer to two different exchange interaction pathways shown by the dotted and solid lines with two different Landé g factors ga and gb. Since the compound is in polycrystalline form, magnetization data were then fitted by taking into account the contribution from both parallel and perpendicular susceptibilities44 χ = (1/3)(χparallel + 2χperpendicular) considering two limiting conditions of χ.

Figure 7. Magnetization (M) vs field (H) curves for [{CoII(Δ)CoII(Λ)}(ox)2(phen)2]n up to ±50 kOe field at various temperatures. (Inset) Magnified view of the hystereses curves. Compound shows coercivity at 1.6 K.

⎛ NμB2 ⎞ 2 χparallel = ⎜ ⎟{g exp(J+ /2kT ) + g−2 exp(−J+ /2kT )} ⎝ 2kT ⎠ +

(ox)2(phen)2]n at various temperatures. M vs H curves do not show any saturation under an applied magnetic field of ±50 kOe. The compound does not possess any coercivity down to 5 K; however, a coercivity of ∼0.13 kOe (inset of Figure 7) has been observed at 1.6 K due to long-range magnetic ordering. This suggests that the compound behaves like a 3-D ferrimagnetic at 1.6 K. The 3-D ferrimagnet ordering of the compound can also be understood from the superexchange interaction between the CoII ions through the bridged ox ligand. A similar kind of superexchange interaction has been previously explained for the [{Fe(Δ)Fe(Λ)}1−x{Cr(Δ)Cr(Λ)}x(ox)2(phen)2]n compound.24 Since both CoII ions are in their high-spin states, all electrons are arranged in their

/cosh(J− /2kT )

and ⎛ Ng 2B2 ⎞ ⎟{tanh(J+ /4kT ) + (J+ /4kT )} χperpendicular = ⎜ ⎝ 2J ⎠ sech2(J /4kT )

Here g± = (ga ± gb)/2 and J± = (j1 ± j2)/2. Figure 6 shows the fitting of the magnetization curve down to 5 K using the Ising chain model assuming both parallel and perpendicular components of susceptibility. The parameters (j1, j2 = −108, 1868



Article

magnetic orbital t2g (dxy, dyz, and dzx) and eg (dz2 and dx2−y2) with an electronic configuration of t2g5eg2 (S = 3/2). The exchange constant for antiferromagnetic coupling can arise due to a nonorthogonality (overlap) of their magnetic orbitals, i.e., the superexchange interaction is antiferromagnetic (AFM) only with ions of similar orbital symmetries.49 D. Reverse Monte Carlo Simulation Study. Detailed characterization of local structural disorder at the molecular level has been investigated using the RMC simulation. In RMC modeling, a structural model of atoms is generated and tried to match with that generated from experimental diffraction data. Atoms are placed in a supercell of a crystalline unit cell with periodic boundary conditions. The cell geometry, size, and positions of atoms in coordinates are known as the configuration. Parameters obtained from Rietveld analysis of the neutron diffraction pattern have been used as an input file for the initial configuration. The pair correlation function {gcal ij (r)} is then calculated for this initial configuration cell using the following expression gijcal (r )

=

ordering in the present case), which are consistent with the experimental neutron diffraction data. Figure 8 shows the fitted

Figure 8. RMC-calculated total scattering contribution of the roomtemperature neutron diffraction pattern of [{CoII(Δ)CoII(Λ)}(ox)2(phen)2]n. Experimental neutron diffraction data are shown by the open circles, whereas RMC-calculated total scattering and nuclear diffuse scattering are shown by the thick line and open triangles, respectively. Solid line at the bottom of the curves shows the difference between the observed and the RMC-calculated total intensities.

nijcal(r ) 4πr 2drρ(r )

(1)

{gcal ij (r)}

ncal ij (r)

where is the pair correlation function, is the number of atoms of type “i” situated at a distance r and r + dr from a central atom “j”. ρ(r) is the atomic number density at distance r. The partial structure factor is calculated using the Fourier transform of the pair correlation functions {gcal ij (r)} cal Pinitial (Q ) = ρ

∫0

∞

4πr 2{gijcal (r ) − 1}

sin Qr dr Qr

RMCPOW pattern of room-temperature neutron diffraction data for the compound. F(Q), the sum of the scattering amplitudes from all atoms in the reciprocal space at a given Q value, is plotted. The room-temperature neutron diffraction pattern has been fitted well by employing the RMC program. This indicates that inherent diffuse scattering is present in the compound. Generally, the reasons behind diffuse scattering are defects, structural or positional disorder, impurity, and local ordering. However, we rule out the possibility of having impurity and defects in the compound. Therefore, in the present compound, the positional disorder, arising due to the local ordering of atoms, is the prime source of diffuse scattering. The local ordering in the compound can be understood from a structural point of view. It is clearly seen from Figure 2 that the CoII ions are residing at two locally distinct sites [CoII(Δ) and CoII(Λ)] in the compound and are therefore of asymmetric nature. This results in two kinds of periodic arrangement of atoms in the compound: (i) local ordering due to both asymmetric CoII ions and the phen ligand connected to them, i.e., −phen−CoII(Δ) and CoII(Λ)−phen− are different along the intrachains, and (ii) ordering due to repeating of a single asymmetric unit [phen−CoII(Δ)−ox−CoII(Λ)−phen] along the inter- and intrachain. These two kinds of periodic arrangements or orderings of atoms in the compound lead to the short-range ordering which results in diffuse scattering in the compound. The previously discussed magnetization data also confirm the presence of short-range magnetic ordering over the temperature range of ∼18−35 K for the compounds. A study of diffuse X-ray scattering using RMC simulation of the short-range order in ytterbium iodine phthalocyanine has been previously reported.51 Partial pair correlation function analysis provides information about the local crystal structure and is related to the probability of finding the center of an atom at a given distance from the center of another atom. Figure 9 shows plots of partial pair correlation functions g(r) at 300 K for the Co−Co, Co−C, Co−N, and Co−O atoms before (considering the periodic arrangement of atoms, as obtained by Rietveld refinement) and after RMC simulation. It is seen that the Co, C, N, and O atoms are periodically arranged, as obtained from Rietveld

(2)

where Q is the momentum transfer vector. The total structure factor is calculated using cal Sinitial (Q ) =

∑ P cal(Q ) − 1

(3)

where the summation is over all atoms. Then finally the difference between the experimentally observed total structure factor, Sexp(Q), and that determined from the initial configuration, Scal(Q), is calculated using m

χinitial 2 =

∑ z=1

{S cal(Q m) − S exp(Q m)}2 σ2

(4)

where the summation is over m experimental data points and σ is the experimental error. The minimum Qm value used should be larger than or equal to 2π/L, where L is the minimum dimension of the configuration. After calculating χinitial2, atoms are allowed to move randomly in the configuration. However, if any two atoms approach closer than a predefined (cutoff) distance, then the move is rejected and a new atom is chosen for a new move. The new total structure factors, {Scal move(Q)}, and the difference χmove2 is calculated for the configuration of atoms of given moves. If χinitial2 > χmove2, the move is accepted and the new configuration becomes the initial one, and when χmove2 > χinitial2, the move is accepted with the probability of exp{(−(χmove2 − χinitial2)/2}; otherwise it is rejected. The above process is continued until a good fit between the experimental and the calculated structure factor is obtained. In the present study, we used the RMCPOW program34,50 to investigate the diffuse scattering in the neutron diffraction data of the [{CoII(Δ)CoII(Λ)}(ox)2(phen)2]n compound. This program has the ability for developing structural models of crystalline materials with local order or disorder (short-range 1869



Article

Figure 10. Condensed view of the simulated configuration (64 unit cells) as projected into a single unit cell (a) before and (b) after RMC simulation for [{CoII(Δ)CoII(Λ)}(ox)2(phen)2]n. Color coding of atoms: Co1 (red), Co2 (green), N (cyan), O (blue), C (magenta), and H (yellow).

neutron diffraction data we could quantify the diffuse scattering originated due to positional disorder (short-range ordering) of atoms in the compound. This is a first attempt to characterize the disorder phenomena in chain molecular magnets in correlation with their magnetic properties.

■

Figure 9. RMC-simulated partial pair correlation function g(r) with distance (r) for Co−Co, Co−C, Co−N, and Co−O in [{CoII(Δ)CoII(Λ)}(ox)2(phen)2]n at room temperature. (a and e) Pair correlation for Co−Co atoms before and after RMC simulation. (b and f) g(r) vs r for Co−C atoms before and after RMC simulation. Similarly, c and d and g and h show the pair correlation of Co−N and Co−O atoms, respectively, before and after RMC simulations.

CONCLUSION We successfully synthesized a ox- and phen-based onedimensional single-chain molecular magnet [{Co II(Δ)CoII(Λ)}(ox)2(phen)2]n with a cross-linked π−π coordination networks using the hydrothermal method. Rietveld refinement of the powder X-ray diffraction and neutron pattern at room temperature confirms single-phase formation of the compound in the monoclinic structure with space group P21. This singlechain compound exhibits spontaneous magnetization and hysteresis below ∼3 K. The Ising chain model confirms the presence of the one-dimensional magnetic nature of the compound with two different Landé g factors and exchange coupling constant values giving rise to a ferrimagnetic ordering in the compound. Moreover, diffuse scattering in the neutron diffraction pattern has been qualitatively and quantitatively analyzed for the first time for [{Co II (Δ)Co II (Λ)}(ox)2(phen)2]n using RMC simulations. RMC study revealed that the pair correlation distance between Co−C, Co−N, and Co−O atoms is found to be 2.2, 2.5, and 2.4 Å, respectively, suggesting C clustering around Co ions in the compound. A similar study could be very useful for determining the correlation between the structural and the magnetic properties of other single-chain molecular magnets and thus serve as a model system to understand the phenomenon of short- or long-range ordering in chain molecular magnets.

refinement, and lie over the range up to ∼18 Å in the configuration cell (Figure 9a−d). However, it is evident from Figure 9e that after RMC analysis the distribution of Co−Co pairs has lost its perfectly periodic arrangement but still lies over a distance of ∼18 Å. From RMC analysis, the g(r) functions, depicted in Figure 9f, 9g, and 9h for the C, N, and O, respectively, show confinement of atoms within the short distance. The maximum number of Co−C, Co−N, and Co−O pairs is confined within a distance of ∼2.2, 2.5, and 2.4 Å, respectively, suggesting their clustering. However, clustering is most prominent for the C atoms. The wide distribution of Co− Co pairs (shown in Figure 9e) and confinement of Co−C, Co− N, and Co−O pairs results in positional disorder in this compound. These observations are much more evident in Figure 10, where the distribution of atoms has been shown in the condensed view, as projected into a single unit cell of the simulated configuration cell (64 unit cells). Before RMC simulation, it is seen that all atoms are periodically arranged at crystallographic positions (Figure 10a). However, after RMC (Figure 10b), the atoms are found to have positional disorder as seen from the clustering and wide distribution of atoms. This positional disorder of atoms leads to a diffuse scattering in the compound. Clustering is clearly evident from the distribution of C and H atoms in the two-dimensional plots (Figure 10b). As discussed earlier, periodic arrangements of asymmetric Co atoms and the corresponding phen ligands provide two kinds of ordering of atoms which leads to short-range ordering and diffuse scattering in the compound. The novelty of the work lies in the fact that with the help of RMC simulation of the

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS P.B. would like to thank Dr. A. Das and Mr. P. Jha for neutron diffraction and FT-IR measurements, respectively. 1870



■

Article

(21) Li, L.-L.; Lin, K.-J.; Ho, C.-J.; Sun, C.-P.; Yang, H.-D. A Coordination π−π Framework Exhibits Spontaneous Magnetization. Chem. Commun. 2006, 1286−1288. (22) Dong, D.-P.; Zhang, Y.-J.; Zheng, H.; Zhuang, P.-F.; Zhao, L.; Xu, Y.; Hu, J.; Liu, T.; Duan, C.-Y. Slow Magnetic Relaxation in a Cyano-Bridged Ferromagnetic {FeIIINiII} Alternating Chain. Dalton Trans. 2013, 42, 7693−7698. (23) Ishii, N.; Okamura, Y.; Chiba, S.; Nogami, T.; Ishida, T. Giant Coercivity in a One-Dimensional Cobalt-Radical Coordination Magnet. J. Am. Chem. Soc. 2008, 130, 24−25. (24) Bhatt, P.; Thakur, N.; Meena, S. S.; Mukadam, M. D.; Yusuf, S. M. Hydrothermally Synthesized Oxalate and Phenanthroline Based Ferrimagnetic One-Dimensional Spin Chain Molecular Magnets [{Fe(Δ)Fe(Λ)}1−x{Cr(Δ)Cr(Λ)}x(ox)2(phen)2]n (x = 0, 0.1 and 0.5) With Giant Coercivity of 3.2 T. J. Mater. Chem. C 2013, 1, 6637− 6652. (25) Cavallini, M.; Gomez-Segura, J.; Ruiz-Molina, D.; Massi, M.; Albonetti, C.; Rovira, C.; Veciana, J.; Biscarini, F. Magnetic Information Storage on Polymers by Using Patterned Single-Molecule Magnets. Angew. Chem., Int. Ed. 2005, 44, 888−892. (26) Leuenberger, M. N.; Loss, D. Quantum Computing in Molecular Magnets. Nature 2001, 410, 789−793. (27) Thomas, L.; Lionti, F.; Ballou, R.; Gatteschi, D.; Sessoli, R.; Barbara, B. Macroscopic Quantum Tunnelling of Magnetization in a Single Crystal of Nanomagnets. Nature 1996, 383, 145−147. (28) Maspoch, D.; Catala, L.; Gerbier, P.; Ruiz-Molina, D.; VidalGancedo, J.; Wurst, K.; Rovira, C.; Veciana, J. Radical para-Benzoic Acid Derivates: Transmission of Ferromagnetic Interactions Through Hydrogen Bonds at Long Distances. Chem.-Eur. J 2002, 8, 3635−3645. (29) Sun, H.-L.; Wang, Z.-M.; Gao, S. [M(N3)2(H2O)2]·(bpeado): Unusual Antiferromagnetic Heisenberg Chain (M = Mn) and Ferromagnetic Ising Chain (M = Co) with Large Coercivity and Magnetic Relaxation (bpeado = 1,2-Bis(4-pyridyl)ethane-N,N′-dioxide). Chem.Eur. J. 2009, 15, 1757−1764. (30) Li, X. J.; Wang, X. Y.; Gao, S.; Cao, R. Two Three-Dimensional Metal-Organic Frameworks Containing One-Dimensional Hydroxyl/ Carboxylate Mixed Bridged Metal Chains: Syntheses, Crystal Structures, and Magnetic Properties. Inorg. Chem. 2006, 45, 1508− 1516. (31) Xu, H. B.; Wang, B. W.; Pan, F.; Wang, Z. M.; Gao, S. Stringing Oxo-Centered Trinuclear [MnIII3O] Units Into Single-Chain Magnets with Formate or Azide Linkers. Angew. Chem. 2007, 119, 7532−7536. (32) Pardo, E.; Ruiz-Garcia, R.; Lloret, F.; Faus, J.; Julve, M.; Journaux, Y.; Novak, M. A.; Delgado, F. S.; Ruiz-Perez, C. Ligand Design for Heterobimetallic Single-Chain Magnets: Synthesis, Crystal Structures, and Magnetic Properties of MIICuII (M=Mn, Co) Chains with Sterically Hindered Methyl-Substituted Phenyloxamate Bridging Ligands. Chem.Eur. J. 2007, 13, 2054−2066. (33) Zheng, Y. Z.; Tong, M. L.; Zhang, W. X.; Chen, X. M. Assembling Magnetic Nanowires Into Networks: A Layered CoII Carboxylate Coordination Polymer Exhibiting Single-Chain-Magnet Behavior. Angew. Chem. 2006, 118, 6458−6462. (34) Mellergård, A.; McGreevy, R. L. Reverse Monte Carlo Modelling of Neutron Powder Diffraction Data. Acta Crystallogr., Sect. A 1999, 55, 783−789. (35) Rietveld, H. M. A Profile Refinement Method for Nuclear and Magnetic Structures. J. Appl. Crystallogr. 1969, 2, 65−71. (36) Rodriguez-Carvajal, J. FULLPROF, Nov 2007; http://www.ill. eu/sites/fullprof/. (37) Agwara, M. O.; Ndifon, P. T.; Ndosiri, N. B.; Paboudam, A. G.; Yufanyi, D. M.; Mohamadou, A. Synthesis, Characterisation and Antimicrobial Activities of Cobalt(II),Copper(II) and Zinc(II) MixedLigand Complexes Containing 1,10-Phenanthroline and 2,2′-Bipyridine. Bull. Chem. Soc. Ethiop. 2010, 24, 383−389. (38) Reddy, P. A. N.; Nethaji, M.; Chakravarty, A. R. Synthesis, Crystal Structures and Properties of Ternary Copper(II) Complexes Having 2,2′-Bipyridine and α-Amino Acid Salicylaldiminates as Models for the Type-2 Sites in Copper Oxidases. Inorg. Chim. Acta 2002, 337, 450−458.

REFERENCES

(1) Kahn, O. Molecular Magnetism; VCH: New York, 1993. (2) Mroziński, J. New Trends of Molecular Magnetism. Coord. Chem. Rev. 2005, 249, 2534−2548. (3) Blundell, S. J.; Pratt, F. L. Organic and Molecular Magnets. J. Phys.: Condens. Matter 2004, 16, R771−R828. (4) Bhatt, P.; Yusuf, S. M.; Mukadam, M. D.; Yakhmi, J. V. Enhancement of Curie Temperature in Electrochemically Prepared Crystalline Thin Films of Prussian Blue Analogs KjFeIIk[CrIII(CN)6]l· mH2O. J. Appl. Phys. 2010, 108, 023916. (5) Bhatt, P.; Thakur, N.; Mukadam, M. D.; Meena, S. S.; Yusuf, S. M. Evidence for the Existence of Oxygen Clustering and Understanding of Structural Disorder in Prussian Blue Analogues Molecular Magnet M1.5[Cr(CN)6]·zH2O (M = Fe and Co): Reverse Monte Carlo Simulation and Neutron Diffraction Study. J. Phys. Chem. C 2013, 117, 2676−2687. (6) Kumar, A.; Yusuf, S. M.; Keller, L.; Yakhmi, J. V. Microscopic Understanding of Negative Magnetization in Cu, Mn, and Fe Based Prussian Blue Analogues. Phys. Rev. Lett. 2008, 101, 207206. (7) Bhatt, P.; Kanciurzewska, A.; Carlegrim, E.; Kapilashrami, M.; Belova, L.; Rao, K. V.; Fahlman, M. Ferromagnetism Above Room Temperature in Nickel-Tetracyanoethylene Thin Films. J. Mater. Chem. 2009, 19, 6610−6615. (8) Bhatt, P.; Yusuf, S. M.; Bhatt, R.; Schütz, G. Magnetic Properties of Nanoparticles of Prussian Blue-Based Molecular Magnets M3[Cr(CN)6]2·zH2O (M = Fe, Co, and Ni). Appl. Phys. A: Mater. Sci. Process. 2012, 109, 459−469. (9) Verdaguer, M.; Girolami, G. S. Magnetic Prussian Blue Analogs, in Magnetism: Molecules to Materials V; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2005. (10) Ferlay, S.; Mallah, T.; Ouahes, R.; Veillet, P.; Verdaguer, M. A Room-Temperature Organometallic Magnet Based on Prussian Blue. Nature 1995, 378, 701−703. (11) Miller, J. S.; Epstein, A. J. Molecule-Based MagnetsAn Overview. MRS Bull. 2000, 25, 21−30. (12) Gatteschi, D.; Sessoli, R.; Cornia, A. Single-Molecule Magnets Based on Iron(III) Oxo Clusters. Chem. Commun. 2000, 725−732. (13) Bogani, L.; Vindigni, A.; Sessoli, R.; Gatteschi, D. Single Chain Magnets: Where to From Here? J. Mater. Chem. 2008, 18, 4750−4758. (14) Basler, R.; Boskovic, C.; Chaboussant, G.; Güdel, H. U.; Murrie, M.; Ochsenbein, S. T.; Sieber, A. Molecular Spin Clusters: New Synthetic Approaches and Neutron Scattering Studies. ChemPhysChem 2003, 4, 910−926. (15) Caneschi, A.; Gatteschi, D.; Lalioti, N.; Sangregorio, C.; Sessoli, R.; Venturi, G.; Vindigni, A.; Rettori, A.; Pini, M. G.; Novak, M. A. Cobalt(II)-Nitronyl Nitroxide Chains as Molecular Magnetic Nanowires. Angew. Chem., Int. Ed. 2001, 40, 1760−1763. (16) Toma, L. M.; Lescouëzec, R.; Pasán, J.; Ruiz-Pérez, C.; Vaissermann, J.; Cano, J.; Carrasco, R.; Wernsdorfer, W.; Lloret, F.; Julve, M. [Fe(bpym)(CN)4]−: A New Building Block for Designing Single-Chain Magnets. J. Am. Chem. Soc. 2006, 128, 4842−4853. (17) Miyasaka, H.; Clérac, R.; Mizushima, K.; Sugiura, K.-i.; Yamashita, M.; Wernsdorfer, W.; Coulon, C. [Mn2(saltmen)2Ni(pao)2(L)2](A)2 with L = pyridine, 4-picoline, 4-t-butylpyridine, Nmethylimidazole and A = ClO4−, BF4−, PF6−, ReO4−: A Family of Single-Chain Magnet. Inorg. Chem. 2003, 42, 8203−8213. (18) Ferbinteanu, M.; Miyasaka, H.; Wernsdorfer, W.; Nakata, K.; Sugiura, K.-i.; Yamashita, M.; Coulon, C.; Clérac, R. Single-Chain Magnet (NEt4)[Mn2(5-MeOsalen)2Fe(CN)6] Made of MnIII-FeIIIMnIII Trinuclear Single-Molecule Magnet with an ST = 9/2 Spin Ground State. J. Am. Chem. Soc. 2005, 127, 3090−3099. (19) Clérac, R.; Miyasaka, H.; Yamashita, M.; Coulon, C. Evidence for Single-Chain Magnet Behavior in a MnIII-NiII Chain Designed with High Spin Magnetic Units: A Route to High Temperature Metastable Magnets. J. Am. Chem. Soc. 2002, 124, 12837−12844. (20) Sun, H.-L.; Wang, Z.-M.; Gao, S. Strategies Towards SingleChain Magnets. Coord. Chem. Rev. 2010, 254, 1081−1100. 1871



Article

(39) Fujita, J.; Martell, A. E.; Nakamoto, K. Infrared Spectra of Metal Chelate Compounds. VI. A Normal Coordinate Treatment of Oxalato Metal Complexes. J. Chem. Phys. 1962, 36, 324. (40) Kim, J.; Lee, U.; Koo, B. K. 1D Chain Crystal Structure of Copper(II) Oxalate Containing a 4,4′-Bipyridine and 1,10-Phenanthroline Ligands: [Cu2(ox)(4,4′-bpy)(phen)2](NO3)2. Bull. Korean Chem. Soc. 2010, 31, 487−490. (41) Cao, J.; Liu, S.; Cao, R.; Xie, L.; Ren, Y.; Gao, C.; Xu, L. Organic-Inorganic Hybrids Assembled by Bis(undecatungstophosphate) Lanthanates and Dinuclear Copper(II)Oxalate Complexes. Dalton Trans. 2008, 115−120. (42) Gerasimova, T. P.; Katsyuba, S. A. Bipyridine and Phenanthroline IR-Spectral Bands as Indicators of Metal Spin State in Hexacoordinated Complexes of Fe(II), Ni(II) and Co(II). Dalton Trans. 2013, 42, 1787−1797. (43) Reinoso, S.; Vitoria, P.; Gutierrez-Zorrilla, J. M.; Lezama, L.; Madariaga, J. M.; Felices, L. S.; Iturrospe, A. Coexistence of Five Different Copper(II)-Phenanthroline Species in the Crystal Packing of Inorganic-Metalorganic Hybrids Based on Keggin Polyoxometalates and Copper(II)-Phenanthroline-Oxalate Complexes. Inorg. Chem. 2007, 46, 4010−4021. (44) Coronado, E.; Drillon, M.; Nugteren, P. R.; Jongh, L. J. d.; Beltrane, D. Anisotropic Exchange in the Cobalt-Cobalt and CobaltCopper Dinuclear EDTA Hydrate (Co2(EDTA)·cntdot·.6H2O, CoCu(EDTA)·cntdot·6H2O) Bimetallic Ordered Chains. Low Temperature Investigation of the Thermal and Magnetic properties. J. Am. Chem. Soc. 1988, 110, 3907−3913. (45) Ho, C. J.; Her, J. L.; Sun, C. P.; Yang, C. C.; Huang, C. L.; Chou, C. C.; Li, L.-L.; Lin, K. J.; Li, W. H.; Lynn, J. W.; et al. Neutron Diffraction and Specific Heat Studies on the Magnetic Ordering in the [FeII(Δ)FeII(Λ)(ox)2(Phen)2]n Molecular Magnet. Phys. Rev. B 2007, 76, 224417. (46) Thakur, N.; Yusuf, S. M.; Yakhmi, J. V. Structural Disorder in Alkaline Earth Metal Doped BaxMn[Fe(CN)6]2(x+1)/3·zH2O Molecular Magnets: A Reverse Monte Carlo Study. Phys. Chem. Chem. Phys. 2010, 12, 12208−12216. (47) Coronado, E.; Drillon, l. M.; Fuertes, A.; Beltran, D.; Mosset, A.; Galy, J. Structural and Magnetic Study of Tetraaqua(EDTA)Dinickel Dihydrate [Ni2(EDTA)(H2O)4·cntdot·2H2O]. Alternating Lande Factors in a Two-Sublattice 1D System. J. Am. Chem. Soc. 1986, 108, 900−905. (48) Román, P.; Guzmán-Miralles, C.; Luque, A.; Beitia, J. I.; Cano, J.; Lloret, F.; Julve, M.; Alvarez, S. Influence of the Peripheral Ligand Atoms on the Exchange Interaction in Oxalato-Bridged Nickel(II) Complexes: An Orbital Model. Crystal Structures and Magnetic Properties of (H3dien)2[Ni2(ox)5]·12H2O and [Ni2(dien)2(H2O)2(ox)]Cl2. Inorg. Chem. 1996, 35, 3741−3751. (49) Verdaguer, M. Rational Synthesis of Molecular Magnetic Materials:a Tribute to Olivier Kahn. Polyhedron 2001, 20, 1115−1128. (50) Mellergård, A. RMCPOW, Version 2.4; June 3, 2005; http:// www.isis.rl.ac.uk/RMC/rmcpow.htm. (51) Krawczyk, J.; Pietraszko, A.; Lukaszewicz, K. Diffuse X-Ray Scattering and Reverse Monte Carlo Simulation of the Short-Range Order in Ytterbium Iodine Phthalocyanine [YbPc2]I2. Acta Crystallogr., Sect. B 2002, 58, 622−626.

1872


One-Dimensional Single-Chain Molecular Magnet with a Cross

Recommend Documents