CuII-Azide Polynuclear Complexes of Three Different Building

Article pubs.acs.org/crystal

CuII-Azide Polynuclear Complexes of Three Different Building Clusters with the Same Schiff-Base Ligand: Synthesis, Structures, Magnetic Behavior, and Density Functional Theory Studies Published as part of the Crystal Growth & Design virtual special issue IYCr 2014 - Celebrating the International Year of Crystallography Sandip Mukherjee* and Partha Sarathi Mukherjee* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560012, India S Supporting Information *

ABSTRACT: Three copper-azido complexes [Cu4(N3)8(L1)2(MeOH)2]n (1), [Cu4(N3)8(L1)2] (2), and [Cu5(N3)10(L1)2]n (3) [L1 is the imine resulting from the condensation of pyridine-2-carboxaldehyde with 2-(2-pyridyl)ethylamine] have been synthesized using lower molar equivalents of the Schiff base ligand with Cu(NO3)2·3H2O and an excess of NaN3. Single crystal X-ray structures show that the basic unit of the complexes 1 and 2 contains CuII4 building blocks; however, they have distinct basic and overall structures due to a small change in the bridging mode of the peripheral pair of copper atoms in the linear tetranuclear structures. Interestingly, these changes are the result of changing the solvent system (MeOH/H2O to EtOH/H2O) used for the synthesis, without changing the proportions of the components (metal to ligand ratio 2:1). Using even lower proportions of the ligand, another unique complex was isolated with CuII5 building units, forming a two-dimensional complex (3). Magnetic susceptibility measurements over a wide range of temperature exhibit the presence of both antiferromagnetic (very weak) and ferromagnetic exchanges within the tetranuclear unit structures. Density functional theory calculations (using B3LYP functional, and two different basis sets) have been performed on the complexes 1 and 2 to provide a qualitative theoretical interpretation of their overall magnetic behavior.

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INTRODUCTION The research on molecular magnetic materials, in which clusters or extended polynuclear complexes are built using paramagnetic metal-ions and bridging ligands, have flourished over the last few decades. The goal is not only to understand the wide variety of magnetic exchange mechanisms but also to find suitable systems for technological applications, especially in information storage and processing. These assemblies of metalions also provide the opportunity for tuning their quantum magnetic properties, because of the sensitivity of the spin exchange interactions on bonding and bridging geometries.1 Short anionic bridging ligands (or functional groups), containing one to three atoms (alkoxo, aryloxo, hydroxo, cyanido, oxalato, dicyanamido, azido, pyrazolato, carboxylato, oximato, etc.), are generally preferred in this field of research because they are capable of mediating strong magnetic © 2014 American Chemical Society

exchange between paramagnetic metal-ions or inducing bulk magnetic ordering.2 For understanding the governing factors of the overall magnetic behavior of molecule based magnetic systems, recently the theoretical study (density functional theory (DFT), ab initio) of exchange interactions between paramagnetic metal centers has also proved to be very fruitful.3 The versatility of bonding modes of the azido anion (triatomic pseudohalide) makes it a preferred choice as a bridging ligand in transition metal-ion polynuclear complexes.4−11 Intricate networks are generated when more than one of these bonding modes coexist in the same complex. The sign and magnitudes of the spin exchange mediated by this ion Received: May 26, 2014 Revised: June 27, 2014 Published: July 14, 2014 4177

dx.doi.org/10.1021/cg500764g | Cryst. Growth Des. 2014, 14, 4177−4186

Crystal Growth & Design

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one-dimnsional (1D) structure are very different, and these differences are the result of changing the solvent mixture from MeOH/H2O to EtOH/H2O. Although we have previously reported structures like complex 2, the topology of 1 is unprecedented. After decreasing the molar ratio of the ligand further, we obtained another unique complex (3) with pentanuclear basic structure. Although it is not possible to give an explanation, or provide a bottom-up building mechanism for the formation of these complexes, they are very important in the field of molecular magnetism for constructing new kinds of magnetic exchange systems for experimental and theoretical studies.

have also been widely studied both experimentally and theoretically. In the end-on (EO) bridging mode, the sign and strength of the magnetic exchange transmitted by the azido anion can be correlated to the bridging angle and the bond distances. For copper-azido exchange systems, below the cutoff angle of ∼108°, the exchange is generally ferromagnetic, while it is antiferromagnetic above this angle and the magnitudes increase almost in a linear fashion as the angle deviates from the cutoff angle. The end-to-end (EE) mode mediates antiferromagnetically with very few exceptions.4−11 However, this “versatility” of the bridging modes also implies that it is generally not possible to predict the bridging network of the final assembly. This serendipitous nature of the assembly process means no real control over the structure of the final product, and thus the ligand is not suitable for building predesigned assemblies of interest. However, it also provides opportunities of obtaining assemblies which would otherwise be impossible to design through a bottom-up approach. In fact, with a judicious choice of the auxiliary blocking or bridging coligands suitable systems for magnetic studies can be obtained, even with the limitations of serendipity. In our previous studies (and also a few other groups), we have illustrated the effect of the relative molar quantities of CuII ion and the chelating bidentate and tridentate ligands, on the structure and magnetic properties of the neutral Cu-azido systems.10 To provide more coordination sites for azido anion (because it will give the azido ions the opportunity to explore the possibility of new bridging modes), we can use lower molar equivalents of the chelating neutral coligands. We found that even small changes in the structures (the organic backbone) of these coligands resulted in completely different structural motifs. So far we have studied different sets of coligands that form many different kinds of assemblies. Here we report how the same neutral tridentate Schiff base ligand can be used to synthesize different copper-azido magnetic-exchange systems using different reaction conditions (solvents, molar ratios). In this article we present the synthesis, structure, magnetic properties, and DFT studies of [Cu4(N3)8(L1)2(MeOH)2]n (1), [Cu4(N3)8(L1)2] (2), and [Cu5(N3)10(L1)2]n (3) [L1 is the Schiff base resulting from the condensation of pyridine-2carboxaldehyde with 2-(2-pyridyl)ethylamine, Scheme 1]. Interestingly, although the metal to ligand ratio in complexes 1 and 2 is the same, the basic tetranuclear and overall extended

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EXPERIMENTAL SECTION

Materials. Cu(NO3)2·3H2O, NaN3, pyridine-2-carboxaldehyde, 2(2-pyridyl)ethylamine were obtained from commercial sources and were used without further purification. Physical Measurements. Elemental analyses of C, H, and N were performed using a PerkinElmer 240C elemental analyzer. IR spectra were recorded as KBr pellets using a Magna 750 FT-IR spectrophotometer. The powder X-ray diffraction (XRD) data were collected using a D8 Advance X-ray diffractometer to verify the phase purity of these complexes (Figure S1, Supporting Information). The measurements of variable-temperature magnetic susceptibility were carried out on a Quantum Design MPMS-XL5 SQUID magnetometer. Susceptibility data were collected using an external magnetic field of 0.2 T for all the complexes in the temperature range of 2−300 K. The experimental susceptibility data were corrected for diamagnetism (using Pascal’s tables).12 Magnetizations of all the complexes were also measured in the field ranging from −5 to +5 T at 2 K, but no hysteresis loop was observed. Caution! Although we did not experience any problems with the compounds reported in this work, azido complexes of metal ions in the presence of organic ligands are potentially explosive. Only a small amount of material should be prepared, and it should be handled with care. Synthesis of the Complex [Cu4(N3)8(L1)2(MeOH)2]n (1). A 5 mL methanolic solution of pyridine-2-carboxaldehyde (1 mmol, 107 mg) and 2-(2-pyridyl)ethylamine (1 mmol, 122 mg) was refluxed for 15 min and was added slowly (hot) to a hot methanolic solution (10 mL) of Cu(NO3)2·3H2O (2 mmol, 484 mg). After this mixture was stirred and heated at 50 °C for 5 min, a hot aqueous solution of NaN3 (20 mmol; 1300 mg) dissolved in 5 mL of water was added slowly. The mixture was stirred for 15 min (at 50 °C) and filtered. Rectangular green crystals of 1 were obtained in 48 h from the filtrate. Isolated yield: ∼70%. Anal. Calcd for 1, C28H34N30O2Cu4: C, 31.23; H, 3.18; N, 39.02. Found: C, 31.19; H, 3.47; N, 39.36. IR (KBr, cm−1): 2030, 2049, and 2075 for the azido groups. Synthesis of the Complex [Cu4(N3)8(L1)2]n (2). A 5 mL ethanolic solution of pyridine-2-carboxaldehyde (1 mmol, 107 mg) and 2-(2pyridyl)ethylamine (1 mmol, 122 mg) was refluxed for 15 min and was added slowly (hot) to a hot ethanolic solution (10 mL) of Cu(NO3)2· 3H2O (2 mmol, 484 mg). After stirring and heating this mixture at 50 °C for 5 min, a hot aqueous solution of NaN3 (20 mmol; 1300 mg) dissolved in 5 mL of water was added slowly. The mixture was stirred for 15 min (at 50 °C) and filtered. Block-shaped dark green crystals of 2 were obtained in 48 h from the filtrate. Isolated yield: ∼60%. Anal. Calcd for 2, C26H26N30Cu4: C, 30.83; H, 2.59; N, 41.49. Found: C, 30.75; H, 2.76; N, 41.58. IR (KBr, cm−1): 2029, 2051, and 2082 for the azido groups. Synthesis of the Complex [Cu5(N3)10(L1)2]n (3). A 5 mL methanolic solution of pyridine-2-carboxaldehyde (0.67 mmol, 72 mg) and 2-(2-pyridyl)ethylamine (0.67 mmol, 82 mg) was refluxed for 15 min and was added slowly (hot) to a hot methanolic solution (10 mL) of Cu(NO3)2·3H2O (2 mmol, 484 mg). After this mixture was stirred and heated at 50 °C for 5 min, a hot aqueous solution of NaN3 (20 mmol; 1300 mg) dissolved in 5 mL of water was added slowly. The mixture was stirred for 15 min (at 50 °C) and filtered. Rectangular brown crystals of 3 were obtained in 48 h from the filtrate. Isolated

Scheme 1. Five Neutral Coligands Used for Constructing Copper-Azido Clustersa

a 1

L for the present study. 4178



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yield: ∼70%. Anal. Calcd for 3, C26H26N36Cu5: C, 26.91; H, 2.26; N, 43.45. Found: C, 26.82; H, 2.35; N, 43.53. IR (KBr, cm−1): 2028, 2059, and 2085 for the azido groups. X-ray Crystallographic Data Collection and Refinements. Single crystal X-ray data for all the four complexes were collected on a Bruker SMART APEX CCD diffractometer using the SMART/SAINT software.13 Intensity data were collected using graphite-monochromatized Mo Kα radiation (0.71073 Å) at 293 K. The structures were solved by direct methods using the SHELXL-201314 program incorporated into WinGX.15 Empirical absorption corrections were applied with SADABS.16 All non-hydrogen atoms were refined with anisotropic displacement coefficients. The hydrogen atoms bonded to carbon were included in geometric positions and given thermal parameters equivalent to 1.2 times those of the atom to which they were attached. Crystallographic data and refinement parameters are given in Table 1, and important interatomic distances and angles are given in Table 2.

contribution.24 The use of the nonprojected energy of the brokensymmetry solution as the energy of the low spin state within the DFT framework provides more or less satisfactory results avoiding the cancellation of the nondynamic correlation effects.25 The broken symmetry approach along with electron correlations at the B3LYP level has been widely used to investigate magnetic properties in a large number of magnetic systems.3,17−20 For each model complex, we performed two calculations using LanL2DZ and TZVP basis sets for all the atoms. A quadratic convergence method was employed in the self-consistent field (SCF) process.26 The J values were calculated using the following equation:

Jij = (E BS − E HS)/(2SiSj + Sj) where EBS = energy of the broken symmetry singlet state, and EHS = energy of the triplet state and Si = Sj = 1/2 (for Cu2+).18

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RESULTS AND DISCUSSION Synthesis. All the three complexes were obtained from the reaction of Cu(NO3)2·3H2O and lower molar equivalents of the Schiff base ligand L1, formed in situ, with excess of NaN3 in an alcohol (MeOH/EtOH) and water mixture. The excess of NaN3 prevents the immediate precipitation of Cu(N3)2 and allows crystallization of multidimensional compounds containing the desired coligands via self-assembly of the smaller units.10 Complexes 1 and 2 were both synthesized using a half molar equivalent of the tridentate ligand. However, changing the solvent mixture from MeOH/H2O (1) to EtOH/H2O (2) significantly changed the basic and overall 1D structure of the complex. For complex 1, the crystal structure contains two noncoordinated MeOH molecules (which do not seem to have any direct effect on the structure of the complex) per CuII4 units, but there are no solvent molecules in the structure of complex 2. In complex 3 the ligand to metal ratio is 2:5; however, this complex can only be synthesized in pure form if the ligand to metal ratio used for synthesis is 2:6. But these smaller ratios do not give any crystalline complexes from the ethanolic solvent mixture. Intense and broad multiple infrared absorptions of azido stretching vibrations in the range from 2025 to 2090 cm−1 are consistent with the presence of various bonding modes of the bridging azido ligands. Structure Description of [Cu4(N3)8(L1)2(MeOH)2]n (1). This complex crystallizes in the monoclinic space group P21/n revealing a 1D arrangement consisting of tetranuclear building units (Figure 1). The asymmetric units consist of two metal atoms, one tridentate L1 ligand, four azido anions, and one noncoordinated MeOH molecule (for which the H atoms could not be located). The neutral tridentate ligand L1 binds with three coordination sites on one of the CuII atoms (Cu1, with a square pyramidal geometry), and the other metal atom (Cu2, also with a square pyramidal geometry) has only azido anions in its coordination sphere. The symmetry of the crystal allows two Cu2 atoms to link together by azido bridges to form the basic tetranuclear unit, in which the Cu1 atoms are at the periphery. Cu1 has three nitrogen atoms from the blocking ligand L1 and one μ1,1 nitrogen atom of an azido group (joining it with the adjacent Cu2 atom in end-on fashion) in its basal coordination sites [Cu1−Nbas, bond lengths range from 1.962(5) to 2.034(4) Å]. The apical nitrogen atom is provided by one μ1,3 azido [Cu1−N12, 2.383(6) Å] group that links Cu1 to a Cu2 atom in a neighboring tetranuclear unit. In the basal plane of Cu2, there are two μ1,1 nitrogen atoms (with the same label N13) from two end-on azido bridges (which joins it to a neighboring Cu2 atom within the unit), a nitrogen atom from a

Table 1. Crystallographic Data and Refinement Parameters for 1−3. empirical formula fw T (K) crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z ρcalcd (g cm−3) μ (Mo Kα) (mm−1) λ/Å F(000) collected reflns unique reflns GOF (F2) R1a wR2b a

1

2

3

C14H13N15OCu2 534.47 293(2) monoclinic P21/n 7.5931(7) 24.381(3) 11.3978(11) 90.00 101.229(3) 90.00 2069.6(4) 4 1.715 2.098

C13H13N15Cu2 506.46 293(2) monoclinic P21/n 11.861(4) 9.023(3) 18.170(6) 90.00 96.143(11) 90.00 1933(1) 4 1.740 2.237

C26H26N36Cu5 1160.53 293(2) monoclinic P21/c 12.635(4) 12.919(4) 13.354(4) 90.00 98.516(17) 90.00 2155(1) 2 1.788 2.498

0.710 73 1072 33558 4759 1.075 0.0695 0.1484

0.710 73 1016 27338 3380 1.047 0.0402 0.0924

0.710 73 1158 30838 3787 1.024 0.0730 0.1095

R1 = Σ||F0| − |Fc||/ Σ|F0|. bwR2 = [Σ{w(F02 − Fc2)2}/Σ{w(F02)2}]1/2.

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COMPUTATIONAL METHODOLOGY

The following computational methodology was used to calculate the exchange coupling constants in the reported complexes (1−2).17−20 Using the phenomenological Heisenberg Hamiltonian H = −Σ(i>j)JijSiSj (where where Si and Sj are the spin operators of the paramagnetic metal centers i and j; and the Jij parameters are the exchange-coupling constants for the different pairwise interactions between the paramagnetic metal centers of the molecule) to describe the exchange coupling between each pair of transition-metal ions, the full Hamiltonian matrix for the system can be constructed. The tetranuclear complexes studied were split into dimers (geometries obtained from the crystal structures), and the exchange coupling value J1 or J2 was obtained by taking into account the energy of two different spin distributions for each of the dimeric model complexes: triplet with S = 1 and singlet with S = 0. The hybrid B3LYP functional21 has been used in all calculations as implemented in Gaussian 03 package,22 mixing the exact Hartree− Fock-type exchange with Becke’s expression for the exchange functional23 and that proposed by Lee−Yang−Parr for the correlation 4179



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Table 2. Selected Bond Distances (Å) and Angles (deg) for 1−3a 1 Cu(1)−N(1) Cu(1)−N(4) Cu(2)−N(7) Cu(2)−N(13)#2 N(1)−Cu(1)−N(2) Cu(1)−N(4)−Cu(2)

2.034(4) 1.962(5) 1.984(6) 2.008(4)

Cu(1)−N(2) Cu(1)−N(12)#1 Cu(2)−N(10)

1.988(4) 2.383(6) 1.957(6)

Cu(1)−N(3) Cu(2)−N(4) Cu(2)−N(13)

79.71(19) 115.64(23)

N(2)−Cu(1)−N(3) Cu(2)−N(13)−Cu(2)#2

2.018(4) 2.399(5) 2.001(4) 92.04(18) 103.31(19)

2 Cu(1)−N(1) Cu(1)−N(4) Cu(2)−N(4) Cu(2)−N(13) N(1)−Cu(1)−N(2) Cu(1)−N(4)−Cu(2)

2.037(3) 1.981(3) 2.625(3) 2.018(3)

Cu(1)−N(2) Cu(1)−N(6)#3 Cu(2)−N(9) Cu(2)−N(13)#4 80.74(14) 129.73(15)

1.979(3) 2.582(4) 1.977(3) 2.010(3) N(2)−Cu(1)−N(3) Cu(2)−N(13)−Cu(2)#4

Cu(1)−N(3) Cu(1)−N(7) Cu(2)−N(10)

2.050(3) 2.486(4) 1.944(4) 92.62(14) 102.65(15)

3 Cu(1)−N(1) Cu(1)−N(4) Cu(2)−N(7) Cu(2)−N(16) Cu(3)−N(13) Cu(3)−N(16)#3 N(1)−Cu(1)−N(2) Cu(1)−N(7)−Cu(2) Cu(2)−N(16)−Cu(3)

2.009(10) 1.963(11) 1.986(14) 2.032(12) 1.969(12) 1.983(10)

Cu(1)−N(2) Cu(1)−N(7) Cu(2)−N(10) Cu(3)−N(6)#5 Cu(3)−N(13)#3

1.982(12) 2.498(14) 1.904(14) 2.587(15) 1.969(12)

Cu(1)−N(3) Cu(2)−N(4)#5 Cu(2)−N(13) Cu(3)−N(6)#6 Cu(3)−N(16)

84.1(6) 120.9(5) 100.7(5)

N(2)−Cu(1)−N(3) Cu(2)−N(13)−Cu(3) Cu(1)−N(4)−Cu(2)#5

101.9(5)

1.992(12) 2.718(14) 2.009(11) 2.587(15) 1.983(10) 94.0(6) 99.3(5)

Symmetry transformations used to generate equivalent atoms: [#1], +x + 1,+y, +z; [#2], −x + 1,−y, −z + 1; [#3], −x,−y + 2, −z; [#4], −x, −y + 1,−z; [#5], +x, −y + 1/2 + 1, +z + 1/2; [#6], −x, +y + 1/2, −z − 1/2. a

Figure 2. Ball and stick representation, showing the 1D arrangement of complex 1. Hydrogen atoms and noncoordinated solvent molecules (MeOH) have been removed for clarity.

Figure 1. Thermal ellipsoid probability plot of the basic unit of 1. Hydrogen atoms and noncoordinated solvent molecules (MeOH) have been removed for clarity. Thermal ellipsoids are at the 25% probability level.

Cu2−N13−Cu2, are respectively at 115.64(23)° and 103.31(19)°. The torsion angle involving the EE-azido group bridging Cu1 and Cu2 of the neighboring units is [Cu2−N10− N12-Cu1] 74.52°. Structure Description of [Cu4(N3)6(L1)2]n (2). The crystal structure of complex 2 reveals a 1D arrangement of tetranuclear building units (Figures 3 and 4). The asymmetric units consist of two metal atoms, one tridentate L1 ligand, and four azido anions. The tridentate ligand coordinate to one of the CuII atoms (Cu1, having a distorted octahedral geometry), and the other metal (Cu2, having a square pyramidal coordination environment) has only azido anions in its coordination sphere. Similar to complex 1, the symmetry of the crystal allows two Cu2 atoms to join by azido bridges to form the basic tetranuclear unit. Cu1 has three nitrogen atoms from the blocking ligand and one μ1,1 nitrogen atom of an azido group (joining it with the adjacent metal atom in end-on fashion) in its equatorial sites [Cu1−Neq, 1.979(3)−2.050(3) Å]. The axial nitrogen atoms are provided by one μ1,3 azido [Cu1−N7,

μ1,3 azido group (linking to a Cu1 atom in an adjacent unit), and a nitrogen atom from a pendant azido group [Cu2−Nbas, 1.957(6)−2.008(4) Å]. The apical nitrogen on Cu2 is provided by an end-on azido group bridging to an adjacent Cu1 atom within the basic unit [Cu2−N4, 2.399(5) Å]. Thus, Cu1 is bridged to Cu2 by one end-on (EO) azido group, and Cu2 is connected to its adjacent Cu2 by two end-on azido bridges forming the tetranuclear unit. Each tetranuclear unit joins to its neighboring unit by two μ1,3 azido bridges (N10−N11−N12), and the resulting chain formed runs along the crystallographic a axis (Figure 2). Within the Cu4II units the Cu1−Cu2 distance [3.6982(9) Å] is larger than that for the Cu2−Cu2 distance [3.1440(11) Å], while the two nearest metal atoms from the adjacent units (Cu1−Cu2) are separated by 5.4161(11) Å. The two EObridging angles within the tetranuclear unit, Cu1−N4−Cu2 and 4180



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Consequently, in complex 2 (like in complexes I−III), the Cu1−Cu2 distance is ca. 4.2 Å, and the EO-bridging angle is 129.73(15)° (for I−III the corresponding angles are in the range 131−137°). Within the Cu4II units the Cu2−Cu2 distance [3.1445(13) Å] is almost same that for the other three complexes, while the two nearest metal atoms from the adjacent units (Cu1−Cu1) are separated by 5.329(2) Å. The EO-bridging angle within the tetranuclear unit, Cu2−N13−Cu2, measures at 102.65(15)°. The torsion angle involving the EE-azido groups bridging two Cu1 atoms of the neighboring units is [Cu1−N4−N6-Cu1] 49.30°. Including this two complexes (1 and 2), we have now synthesized six complexes with tridentate neutral Schiff-base ligands with a ligand to metal ratio of 1:2. Although we have previously obtained two structures almost identical to complex 2 before, both the basic unit and the overall linking pattern of the cluster units of 1 are unique. The complex [Cu4(N3)8(L5)2]n (IV) is probably the closest in structure to complex 1, in the sense that all the azido bridges within the basic structure of these two complexes are end-on in nature. In all of these six complexes, the most closely related part is the central pair, which in all the cases bonds in an almost identical fashion. Structure Description of [Cu5(N3)10(L1)2]n (3). This complex crystallizes in the monoclinic space group P21/c and has a two-dimensional structure with CuII5 repeating units. This complex has three CuII ions (one with half occupancy), two tridentate L1 ligands, and five azido anions in its asymmetric unit (Figure 5). The Cu1 atom is linked to the tridentate ligand

Figure 3. Thermal ellipsoid probability plot of the basic unit of 2. Hydrogen atoms have been removed for clarity. Thermal ellipsoids are at 25% probability level.

Figure 4. Ball and stick representation of the 1D arrangement of 2. Hydrogen atoms have been removed for clarity.

2.486(4) Å] group and one μ1,1,3 azido group [Cu1−N6, 2.582(4) Å] from an adjacent tetranuclear unit. In the basal plane of Cu2, there are two μ1,1 nitrogen atoms from two endon azido bridges (which joins it to a neighboring Cu2 atom within the same unit), a nitrogen atom from a μ1,3 azido group, and a nitrogen atom from a pendant azido group [Cu2−Nbas, 1.944(4)−2.018(3) Å], while the apical nitrogen is provided by an end-on azido group bridging to an adjacent Cu1 atom [Cu2−N4, 2.625(3) Å]. Thus, Cu1 is bridged to Cu2 by one end-on and one end-to-end azido group, and Cu2 is connected to its adjacent Cu2 by two end-on azido bridges forming the tetranuclear unit. Each unit joins to its neighboring unit by two μ1,1,3 azido bridges and chain formed runs along the crystallographic b axis (Figure 4). In our previous works with similar tridentate ligands (Scheme 1), we reported three complexes: [Cu4(N3)8(L2)2]n (I), [Cu4(N3)8(L3)2]n (II),10k and [Cu4(N3)8(L4)2]n (III),10m having almost identical tetranuclear building units to that of complex 2. However, surprisingly the connectivity in the 1D chain of complex III remarkably differs from those two (I and II) previously reported complexes and 2. In complexes 2, I, and II, the tetranuclear units were joined in a head-to-tail manner (joining the peripheral Cu atoms by two end-to-end azido bridges), while in complex III the units are stacked laterally (peripheral and central Cu atoms of adjacent units joined by two end-on azido bridges). Coexistence of both EO and EE-azido bridges between two copper atoms in these complexes is also very interesting. The EO-azido bridging mode is converging in nature and brings two bridged metal centers very close (ca. 3 Å), while the three-atom EE-azido bridging mode keeps the metals at greater separations (ca. 5 Å). So, for these modes to coexist between two metal atoms, the EO-mode needs to involve at least one longer bond (axial or apical), and the bridging angle also expands.

Figure 5. Thermal ellipsoid probability plot of the basic unit of 3. Hydrogen atoms have been removed for clarity. Thermal ellipsoids are at 25% probability level.

and has a square pyramidal geometry, while Cu2 (square pyramidal) and Cu3 (with half occupancy and a distorted octahedral geometry) have only azido groups in their coordination sphere. The basal plane of the Cu1 atom is occupied [Cu1−Nbas, 1.963(11)−2.009(10) Å] by the three nitrogen atoms from the Schiff-base ligand and a μ1,1,3 nitrogen atom from an azido group linking to a neighboring Cu2 and Cu3 atoms from an adjacent cluster unit. The apical position is taken up by a μ1,1 nitrogen atom from an azido group linking to a neighboring Cu2 atom within the basic unit [Cu1−N7, 2.498(14) Å]. The Cu2 atom has three μ1,1 nitrogen atoms from three azido groups (two linking to Cu3 atom and one linking to Cu1 atom in the same basic unit) and a pendant azido group in its basal plane [Cu2−Nbas, 1.904(14)− 4181



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2.032(12) Å], while the apical position is taken up by a μ1,1 nitrogen atom of a μ1,1,3 azido group [Cu2−N4, 2.718(14) Å]. The Cu3 atom has two sets of μ1,1 nitrogen atoms from EOazido groups (linking to two adjacent Cu2 atoms) in its equatorial plane [Cu3−Neq, 1.969(12)−1.983(10) Å], and the axial positions are taken up by the nonbridging nitrogen atoms of two symmetry equivalent μ1,1,3 azido groups [Cu3−Nax, 2.587(15) Å]. So the Cu2 and the Cu3 atoms creates a trinuclear unit (Cu2−Cu3−Cu2) linked by double EO-azido groups, and the Cu2 atoms link to two Cu1 atoms on the periphery to generate a pentanuclear basic structure. Each of these units are linked to four neighboring units through μ1,1 azido groups (linking Cu1 atoms with Cu2 atoms in the adjacent units) and generates a 2D overall structure parallel to the crystallographic bc plane (Figure 6).

Figure 7. Plots of χMT vs T (in the temperature range of 2−300 K) and magnetization (M) vs field (H) [inset] at 2 K for complex 1 [the data corresponds to per CuII4 units]. The red line indicates the fitting using theoretical model (see text).

a single EO-azido group J1 and J2 are not expected to be identical.10k,m,27 A reasonable fit can be obtained for noninteracting tetranuclear units applying the conventional Hamiltonian: H = −J1(S1S2 + S3S4) − J2 S2S3

Considering these two different exchange parameters, the analysis of the experimental susceptibility values has been performed using the following expression: χM = (Ng 2β 2 /3kT )[A /B] Figure 6. Ball and stick representation of the 2D arrangement of 3. Hydrogen atoms have been removed for clarity.

(1)

where A = [30 exp (E1/kT) + 6 exp (E2/kT) + 6 exp (E3/kT) + 6 exp (E4/kT)] and B = [5 exp (E1/kT) + 3 exp (E2/kT) + 3 exp (E3/kT)+ 3 exp (E4/kT) + exp (E5/kT) + exp (E6/kT)].

Within the basic unit the Cu1−Cu2 distance is 3.600(2) Å, and Cu2−Cu3 distance is 3.091(2) Å, and for the adjacent units Cu1−Cu2 distance is 3.909(2) Å. The EO-azido bridging angles within the basic units are 99.3(5)° [Cu1−N4−Cu2], 101.9(5)° [Cu2−N13−Cu3] and 100.7(5)° [Cu2−N16−Cu3, while the angle between adjacent units is 120.9(5)° [Cu1− N7−Cu2].

E1 = J1 /2 + J2 /4 E2 = −J1 /2 + J2 /4

E3 = −J2 /4 − (J12 + J2 2 )1/2 /2 E4 = −J2 /4 + (J12 + J2 2 )1/2 /2

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MAGNETIC BEHAVIOR Complex 1. The dc magnetic susceptibility measured on a polycrystalline sample of 1 (applied field of 0.2 T) is shown in Figure 7 as both χM vs T and χMT vs T plots (where χM is the molar magnetic susceptibility per CuII4 unit). At room temperature (300 K), the χMT value is 1.68 cm3 K mol−1, which is a little higher than expected for four uncoupled CuII ions (χMT = 0.375 cm3 K mol−1 for an S = 1/2 ion with g = 2.0). The χMT value increases gradually upon lowering the temperature to reach a maximum value of 1.87 cm3 K mol−1 at 22 K. Below this temperature the χMT value decreases to 1.70 cm3 K mol−1 at 2 K. The 1/χM vs T plots (300−25 K) obey the Curie−Weiss law (Figure S2, Supporting Information) with a positive Weiss constant of θ = 5.0(3) K, which along with the nature of the χMT vs T plot indicates a dominant ferromagnetic interaction among the metal ions. The magnetic exchange in the basic centrosymmetric core can be modeled as Cu(S1)-J1-Cu(S2)-J2-Cu(S3)-J1-Cu(S4) and as the two central CuII ions are bridged by double symmetric EO-azido bridges, while the peripheral CuII ions are bridged by

E5 = −J1 /2 − J2 /4 − (4J12 − 2J1J2 + J2 2 )1/2 /2

E6 = −J1 /2 − J2 /4 + (4J12 − 2J1J2 + J2 2 )1/2 /2

The values giving the best fit (2−300 K) are J1 = −0.456(2) cm−1, J2 = +61.8(3) cm−1, and g = 2.075(2) [R = 8.01 × 10−7]. Interacting tetranuclear magnetic cluster model does not give any reasonable fit of the data (with high errors and no fitting of lower temperature susceptibilities), so it can be assumed that intercluster interaction, if any, must be very small. The longer apical bond lengths in all of the CuII ions indicate that the unpaired electron in each of the metal atoms should reside in the basal dx2−y2 magnetic orbital. The Cu−NEO−Cu bond angles for the two central metal atoms is 103.3° (well below the cutoff angle 108°), and all the bridging bonds are “short” (basal bonds directly projected toward the magnetic dx2−y2 orbitals on the metal atoms containing the unpaired electrons). Therefore, a moderately strong ferromagnetic interaction (J2 = +61.8 cm−1) is expected for this pair of CuII ions. The very small magnitude for J1 can be justified by the fact that the only bridge between 4182



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the peripheral CuII ions involves a very long bond linked to the central CuII ion, which is involved in a very strong and direct interaction with its neighboring CuII ion (weakening the Cu1− Cu2 exchange interaction). The EO-azido group with a high bridging angle (115.6°) is expected to exchange antiferromagnetically. Complex 2. Figure 8 shows the temperature dependence of χM and χMT values for complex 2 (where χM is the molar

ions can be justified by the fact that the EO-azido bridge between the peripheral CuII ions involves a very long bond and the bridging angle is 129.7° (way above the cutoff angle), and the EE-azido bridge also (which is expected to exchange antiferromagnetically) involves a longer bond. Complex 3. In Figure 9, the temperature dependence of χM and χMT values for complex 3 (where χM is the molar magnetic

Figure 9. Plots of χMT vs T (in the temperature range of 2−300 K) and magnetization (M) vs field (H) [inset] at 2 K for complex 3 [the data correspond to per CuII5 units].

Figure 8. Plots of χMT vs T (in the temperature range of 2−300 K) and magnetization (M) vs field (H) [inset] at 2 K for complex 2 [the data corresponds to per CuII4 units]. The red line indicates the fitting using theoretical model (see text).

susceptibility per CuII5 unit) has been shown. The room temperature (300 K) χMT value of 2.09 cm3 K mol−1 is slightly higher than expected for five uncoupled CuII ions (expected value 1.875 cm3 K mol−1). On lowering the temperature, the χMT value increases gradually down to about 22 K to reach 2.29 cm3 K mol−1 and then decreases to 2.15 cm3 K mol−1 at 2 K very sharply. The 1/χM vs T plots (300−25 K) obey the Curie− Weiss law (Figure S2, Supporting Information) with a positive Weiss constant of θ = 4.3(3) K. In the pentanuclear basic structure, the three central CuII ions are linked by double EO-azido groups, with bridging angles 101.9 and 100.7°. Therefore, it is expected that the exchange (same for both pair because of symmetry) should be moderately positive (close in magnitude of J2 for 1 and 2). They are also linked by one EE-azido group which involves longer apical and axial bonds, and so the resulting antiferromagnetic interaction, if any, should be very small. The peripheral CuII ions are bridged by a single EO-azido bridge (involving a longer bond) with a bridging angle of 99.3°. So, again a very small ferromagnetic interaction is expected for the magnetic exchange in the peripheral CuII ion pairs and the ground spin state is expected to be S = 5/2. But the equation

magnetic susceptibility per CuII4 unit). The room temperature (300 K) χMT value 1.69 cm3 K mol−1 is again slightly higher than expected for four uncoupled CuII ions and increases gradually on lowering the temperature and reaches a maxima at 30 K (χMT = 1.85 cm3 K mol−1). Below this temperature the χMT value sharply decreases to reach 1.52 cm3 K mol−1 at 2 K. The 1/χM vs T plots (300−25 K) obey the Curie−Weiss law (Figure S2, Supporting Information) with a positive Weiss constant of θ = 4.8(4) K. The nature of the susceptibility plots suggests dominant ferromagnetic exchange among the CuII ions through azido bridges. The symmetry of the structure of the CuII4 cluster molecule suggests that the magnetic exchange can be modeled similar to complex 1 (Scheme 2), and values giving the best fit (9−300 K using eq 1) are J1 = −0.915(3) cm−1, J2 = +71.2(5) cm−1, and g = 2.071(3) [R = 2.45 × 10−6]. The moderately strong positive value of J2 can be justified (as in the case of complex 1) by the involvement in magnetic exchange of two symmetric (all short bonds) EO-azido groups with bridging angle of 102.7° (well below the cutoff angle). The very small antiferromagnetic exchange between the peripheral Scheme 2. Model Complexes Used for Computational Studies

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densities on the bridging EO and EE-azido groups are very low (as the magnitude of the exchange is very low).

derived from the model Cu(S1)-J1-Cu(S2)-J2-Cu(S3)-J2-Cu(S4)J1-Cu(S5) did not provide any reasonable fit of the data. The field-dependent isothermal magnetization plots (0−5 T, 2 K) show that the magnetization value of 1 and 2 steadily increases to reach to ca. 4 Nβ at 5 T (which is expected for a ground state of S = 2 with g = 2, confirming the very low magnitude of the antiferromagnetic interaction between the peripheral CuII ions), while the magnetization values for 3 steadily increases to reach to ca. 5 Nβ (as expected for a ground state of S = 5/2 with g = 2) at 5 T [Figures 7−9]. Theoretical Study. Dinuclear spin-exchange models (to determine the pairwise exchange parameters) were generated for theoretical studies using the crystal structure geometries for the complexes 1 and 2 (Scheme 2). This procedure of splitting the multinuclear complexes into CuII2 units not only saves computational time but also was found to give more accurate results (close to the experimentally fitted values) as compared to the tetranuclear models. Spin-unrestricted DFT calculations were performed on these model complexes, using the Gaussian 03 package at the B3LYP level employing the lanl2dz and tzvp basis sets. The results of the theoretical studies (see the Computational Methodology section for details) in terms of the calculated exchange parameters are summarized in Table 3.

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CONCLUDING REMARKS In conclusion, we have described three new copper-azido complexes with the same neutral tridentate chelating ligand (used in lower molar proportion to the metal ion), obtained under different reaction conditions. Complexes 1 (isolated from MeOH/H2O with an unique new topology) and 2 (isolated from EtOH/H2O) both have the same Schiff base blocking ligand to metal ratio but are very different in their basic and overall extended structures. Again by decreasing the molar proportion of the blocking ligand, we isolated another complex (3) with pentanuclear basic unit and new topology. For complexes 1 and 2, we were able to fit the magnetic susceptibility data, and the resulting exchange parameters were corroborated with DFT (using two different basis functions). The very versatile nature of the azido anion as a bridging ligand prevents us from providing an explanation for the different structures formed under different reaction conditions, and thus it is very difficult to design bottom-up synthetic methods for a target assembly. However, this serendipity provides us with many new topologies to study for their structural and magnetic properties (structure−property correlation). However, we hope that studying these serendipitously formed assemblies will eventually lead to a better understanding of the actual process of assembly formation, which provides the motivation for the study of such systems.

Table 3. Comparison of the Experimental and DFT Studies model

Ji (cm−1)

DFT (lanl2dz)

DFT (tzvp)

from fitting

1A 1B 2A 2B

J1 J2 J1 J2

−0.9 +76.1 −3.3 +84.5

−1.3 +81.1 −4.9 +92.5

−0.456(2) 61.8(3) −0.908(5) 74(1)

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic data in CIF format for complexes 1−3, PXRD patterns, χM vs T data and Curie−Weiss fitting of the 1/ χM vs T data for 1−3. This material is available free of charge via the Internet at http://pubs.acs.org/.

For model complexes 1A and 2A, the singlet states were found to be the ground state (which implies negative exchange parameters, J1), while for model complexes 1B and 2B, the high spin triplet states are the ground spin-states (which implies positive exchange parameters, J2). The calculated exchange parameters are found to be very close to the experimentally fitted exchange values. Figure 10 shows the plots of the spin density distributions corresponding to the ground spin states for complexes 1 and 2. The spin density distributions for triplet ground states for the models 1B and 2B show the predominance of the delocalization mechanism through σ type exchange pathways involving the dx2−y2 magnetic orbitals of the CuII ions and the sp2 hybrid orbitals of the EO-azido nitrogen bridging atoms. The considerably high spin-densities on the EO-azido bridges provide evidence for the moderately strong positive exchange observed experimentally. As expected, for 1A and 2A the spin

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AUTHOR INFORMATION

Corresponding Authors

*(S.M.) E-mail: [email protected]. Fax: 91-80-23601552. Tel: 91-80-22933352. *(P. S. M.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors thank Mr. Bijan Roy for his help in collecting the single-crystal X-ray diffraction data. Authors also thank the Department of Science and Technology (DST), New Delhi, for financial support.

Figure 10. Spin density maps of the ground spin states calculated for model complexes 1A, 1B, 2A, and 2B at B3LYP level (lanl2dz). Positive and negative spin populations are represented as yellow and green surfaces. The isodensity surfaces correspond to a value of 0.01 e/b3. 4184



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