REGULAR ARTICLE
Iron(III) and cyano-bridged dinuclear copper(II) complexes:
synthesis, structures and magnetic property of the copper(II) complex
GOUTAM MAHATAaand ANANGAMOHAN PANJAa,b,*
aDepartment of Chemistry, Gokhale Memorial Girls’ College, 1/1 Harish Mukherjee Road, Kolkata 700 020, India
bDepartment of Chemistry, Panskura Banamali College, Panskura 721 152, India E-mail: goutammahata1977@gmail.com; ampanja@yahoo.co.in
MS received 5 April 2020; revised 23 May 2020; accepted 26 May 2020
Abstract. This report describes the synthesis and structural characterizations of three new complexes;
[Na3Fe2(bbp)(Hbbp)(CN)6(H2O)9]4H2O (1), (Bu4N)[Fe(bbp)2]3CH3OH2H2O (2), and a cyano-bridged dinuclear copper(II) complex, [Cu2(tren)2(CN)][Fe(bbp)2]36CH3OH4H2O (3), where H2bbp = bis(2-ben- zimidazolyl)pyridine and tren = tris(2-aminoethyl)amine. X-ray crystallography reveals that the geometry of iron(III) centers in the complex anions of 1–3 is an octahedral, while it is a trigonal bipyramidal around copper(II) center in the complex cation of3. Both bbp ligands meridionally coordinate iron(III) centers in the complex cations of 2 and 3, while three cyanide ions and bbp (or Hbbp) ligand occupied either of the meridional positions around the metal center in1. In the dinuclear complex cation of3, the Cu(II)-CN-Cu(II) bridging region is strictly linear as dictated by the symmetry with CuCu separation of 5.084 (7) A˚ . Variable temperature magnetic susceptibility study shows that the cyano-bridge mediates the antiferromagnetic cou- pling between copper(II) centers withJvalue of-110 K in3.
Keywords. Fe(III) and Cu(II) complexes; cyano-bridged dinuclear Cu(II); crystal structures; magnetic study.
1. Introduction
Transition metal cyanide complexes have attracted considerable interest due to their extraordinary variety of chemical and physical properties including color, magnetism, conductivity and hydrogen storage activ- ity.1–3 Recently, magnetic interactions between para- magnetic metal ions through the cyanide bridges have been extensively investigated.4 In particular, the cya- nide-bridged 3D bimetallic assemblies of Prussian- blue-type compounds have attracted great attention owing to the fact that many of them exhibit a long- range magnetic ordering above room temperature, photo-responsive behavior and slow relaxation of the magnetization.4,5 It is well known that interaction between the metal centers through cyanide groups in an end-to-end fashion is the basis of all the interesting physical properties of such compounds. Therefore, dinuclear complexes in which a single cyanide ion bridges the metal centers can be served as model
compounds for the study of exchange interaction between the metal centers.6 Although a large number of cyanide-bridged 1D, 2D and 3D complexes were synthesized and their magneto-structural correlations have been established, simple M–CN–M(M0) com- plexes containing one cyanide linkage were not much explored.7 The simplest example in this regard is a dinuclear copper(II) complex with only one localized electron per metal center. It has been found that such complexes exhibit significant variation in the strength of antiferromagnetic coupling between two copper(II) centers depending upon the relative disposition of the bridging cyanide group and the type, and geometry of the metal centers.7,8In the complexes with tetradentate tripodal ligands, two trigonal bipyramidal (TBP) copper(II) centers are bridged by a cyanide at an axial coordination site and therefore, stronger antiferro- magnetic interaction is expected in such complexes as the lobe of dz2
orbital of the copper(II) ion containing unpaired electron is directed towards therorbitals of the cyanide ion, resulting in a greater overlap with the
*For correspondence
https://doi.org/10.1007/s12039-020-01807-zSadhana(0123456789().,-volV)FT3](0123456789().,-volV)
r orbitals of the cyanide ion. In contrast, a weaker antiferromagnetic coupling would be predicted since the equatorial disposition of the cyanide results in reduced overlap of the r orbitals of the bridging cyanide and dz2 orbital of copper(II) ions.9 Moreover, strength of the antiferromagnetic coupling is influ- enced by separation of two copper(II) ions and the angle subtended by the metal centers and bridging cyanide. Therefore, it could be worthy to work with similar ligand systems with variable counter anions which may lead to similar geometrical features with varying degrees of structural distortions. This would be helpful to understand precisely the magneto-struc- tural behavior of cyanide-bridged dinuclear copper(II) complexes, and the interest of which would have been shifted to application of this knowledge into the structures of polynuclear clusters and higher dimen- sionality with complex magnetic behaviors. In our previous work, a novel mer-[Fe(bbp)(CN)3]2- build- ing block was developed to produce hetero-dinuclear system, that showed interesting optical and magnetic properties induced by an intramolecular metal-to- metal electron transfer triggered and modulated by the protonation–deprotonation at bbp/H2bbp ligand (Scheme1).7a Therefore, we envisioned that such ligand having different degrees of protonation ability may give us opportunity to produce diverse coordi- nation compounds following electroneutrality princi- pal. Moreover, the mer-iron(III) tricyanide complex could be utilized to afford different heterometallic compounds. In continuation to our previous work, in this paper, we report the syntheses and structures of three new complexes, a symmetric mer-iron(III) tri- cyanide precursor, [Na3Fe2(bbp)(Hbbp)(CN)6(H2-
O)9]4H2O (1); (Bu4N)[Fe(bbp)2]3CH3OH2H2O (2);
and a cyanide bridged dinuclear copper(II) complex, [Cu2(tren)2(CN)][Fe(bbp)2]36CH3OH4H2O (3), where H2bbp = bis(2-benzimidazolyl)pyridine and tren = tris(2-aminoethyl)amine. The most striking feature of the complex 3 is that it is strictly linear in the bridging region which is not observed as yet for
this class of compounds. Magnetic property of com- plex 3 has been described.
2. Experimental
2.1 Materials and physical measurements
All the reagent grade chemicals and solvents were purchased from commercial sources and were used as received. Bis(2-benzimidazolyl)pyridine (H2bbp) was prepared according to literature method.10 Elemental analysis for C, H and N were performed following the classical Pregl-Dumas technique on a Thermo Fisher Flash EA1112. FTIR spectra were recorded in the range of 400–4000 cm-1on a Nicolet 750 Magna-IR spectrometer using KBr pellets. Variable temperature magnetic susceptibility measurements of polycrys- talline samples were carried out using a Quantum Design MPMS SQUID magnetometer. Experimental data were corrected for the sample holder and for the diamagnetic contribution of the sample. Caution!
Cyanide salts are very toxic and, therefore, appropriate precaution must be taken before handling.
2.2 Synthesis of
[Na3Fe2(bbp)(Hbbp)(CN)6(H2O)9]4H2O (1)
A solution of H2bbp (3.13 g, 10 mmol) in 20 mL of methanol was added to a solution of FeCl36H2O (2.70 g, 10 mmol) in 20 mL of methanol. The result- ing red solution was refluxed for 1 h followed by the addition of NaCN (2.205 g, 47 mmol) in 40 mL of water. The mixture was refluxed for further 8 h during that time the solution turned dark blue. The solvent was removed by rotary evaporator and the blue mass was extracted with methanol. Addition of ether to the solution afforded blue powder, which was collected by filtration, washed with ether and dried under vacuum.
Single crystals suitable for x-ray analysis were obtained from the slow evaporation of1in acetonitrile
Scheme 1. Protonation–deprotonation equilibria of bis(2-benzimidazolyl)pyridine (H2bbp) ligand.
at ambient temperature over a period of one week.
Yield: 5.178 g, 87%. Anal. Calcd. C44H49N16Na3O13- Fe2: C, 44.39; H, 4.15; N, 18.82. Found: C, 44.60; H, 4.24; N, 18.51. IR (KBr, cm-1): 3421 br (mOH);
1611 m (mC=N); 2113 s, 2106sh (mC:N).
2.3 Synthesis of
(Bu4N)[Fe(bbp)2]3CH3OH2H2O (2)
To a solution of H2bbp (0.626 g, 2 mmol) in 20 mL of methanol, added FeCl36H2O (0.270 g, 1 mmol) in 20 mL methanol with stirring and the resulting red solution was refluxed for 30 min. Aqueous solution of NaOH (0.160 g, 4 mmol), 30 mL, was added to the solution and the reflux was continued for another 1 hr, meanwhile color of the solution changed to dark blue.
The resulting solution was filtered and concentrated to about 30 mL. Solid Bu4NBr (3.22 g, 10 mmol) was added to the filtrate, resulting in an instantaneous precipitation of 1 as a blue microcrystalline solid, which was filtered, washed with water and dried under vacuum. Crops of crystals suitable for x-ray diffraction were obtained from slow evaporation of 1:1 (v/v) mixture of MeOH–H2O solvent. Yield: 0.945 g, 90%.
Anal. Calcd. for C57H74N11O5Fe: C, 65.26; H, 7.11; N, 14.69. Found: C, 65.38; H, 7.22; N, 14.49. IR (KBr, cm-1): 3462 br (mNH, OH); 2964 m, 2931 m, 2863 m(mC–H); 1609 s (mC=N).
2.4 Synthesis of
[Cu2(tren)2(CN)][Fe(bbp)2]36CH3OH4H2O (3) 2.4a Procedure A: Solid Cu(NO3)23H2O (0.096 g, 0.4 mmol) and tren (58.50 mg, 0.4 mmol) were com- bined in 40 mL of CH3OH with stirring for 10 min.
Treatment of this mixture with 1(0.358 g, 0.3 mmol) in 20 mL of methanol afforded a dark-blue solution, which was also stirred for further 10 min at ambient temperature and allowed to stand for crystallization.
Dark-blue block crystals suitable for x-ray diffraction were obtained from slow evaporation after several days (yield: 0.229 g, 42%). Anal. Calcd. for C133-
H132N39O10Cu2Fe3: C, 58.48; H, 4.87; N, 20.00.
Found: C, 58.67; H, 4.92; N, 19.89. IR (KBr, cm-1):
3297 br (mNH); 1611 m (mC=N); 2172 m (mC:N).
2.4b Procedure B: To a stirring solution of Cu(NO3)2-
3H2O (0.096 g, 0.4 mmol) and tris(aminoethyl)amine (58.50 mg, 0.4 mmol) in 20 mL of methanol was added sodium cyanide (0.024 g, 0.2 mmol) dissolved in 10 mL of water. Finally, a solution of 2 (0.6 g, 0.6 mmol) in 40 mL of methanol was mixed with the solution and was
stirred well for further 20 min. The solution was left standing for a few days, and the dark blue crystals that formed were collected and washed with water-methanol.
Yield: 0.393 g, 72%.
2.5 X-ray crystallography
X-ray diffraction data of suitable crystals of1–3were collected on a Nonius Kappa CCD diffractometer equipped with graphite monochromatized Mo-Ka radiation (k = 0.71073 A˚ ). Crystals were mounted on a glass fiber using N-Paratone oil and cooled in situ using an Oxford Cryostream 600 Series to 150 K for data collection. Intensity data were integrated by DENZO-SMN and scaled with SCALEPACK.11 Sev- eral scans inuandxdirections were made to increase the number of redundant reflections, which were averaged in the refinement cycles. The structures were solved by the direct methods and all the non-hydrogen atoms were refined with anisotropic thermal parame- ters by a full-matrix least squares based on F2 using the SHELX-2018 program.12 The distance restraint was applied between nitrogen and oxygen atoms of lattice methanol molecules in 2 as they were unsta- ble during refinement. Moreover, the bridging cyanide group in complex cation of [Cu2(tren)2CN]3?in3was refined adopting a disordered model with each site occupied by 50% of both carbon and nitrogen atoms to make anisotropic displacement parameters acceptable.
The hydrogen atoms attached to carbon and nitrogen atoms were included in calculated positions, whereas hydrogen atoms attached to oxygen atoms of lattice methanol and water molecules, except a disordered water molecule over two positions in both 1 and 2, were located on the difference Fourier map and refined with fixed isotropic thermal parameters based on those of parent atoms. It is to be noted that the hydrogen atoms attached to oxygen atom of methanol and water molecules were not stable during refinement cycles and thus their positional parameters were fixed before final refinement. The crystallographic data for com- pounds 1–3 are given in Table1.
3. Results and Discussion
3.1 Synthesis and characterization
Bis(2-benzimidazolyl)pyridine (H2bbp) can be served as a neural tridentate ligand (H2bbp) and can also be deprotonated in basic medium, and therefore, served as an anionic tridentate ligand as shown in Scheme1.
In our previous work, we have demonstrated that
treatment of H2bbp with FeCl3in presence of double- fold excess of NaCN produced the dianionic form of this building block, mer-[Fe(bbp)(CN)3]2-, which was crystallized as a tetrabutylammonium salt.13But in the present study, the same reaction in the presence of only one and half fold excess of NaCN yielded [Na3-
Fe2(bbp)(Hbbp)(CN)6(H2O)9]4H2O(1), in which either of the blocking ligands are in mono-anionic and di-anionic forms. However, complex 1 is soluble in most organic solvents such as acetonitrile, methanol, acetone and N,N-dimethylformamide, making it a promising candidate for a versatile building block.
One-pot reaction of complex 1, copper(II) nitrate tri- hydrate and tripodal tren ligand in 3:4:4 molar ratio produced compound 3, which can be described as a subsequent degradation product from the hydrolytic decomposition of1. Therefore, we aimed to synthesize 3in a straightforward way and for that, complex2was isolated first by the reaction of FeCl3 with H2bbp in 1:2 molar ratio in presence of NaOH for deprotonation of benzimidazole groups. Thereafter, treatment of 2 with copper(II) salt and tren ligand in 3:2:2 molar ratio
in presence of cyanide ion produced3in a much better yield (Scheme 2).
The most important features of the IR spectra of the complexes1and3involve the characteristic stretching frequencies of cyanide ion. The IR spectrum of 1 exhibits the characteristic CN stretching vibration at 2113 s and 2106 sh cm-1 indicative of a low spin iron(III) complex which falls in the same range of other tricyanide precursors reported in the litera- ture.13–18 In addition, bands at 3421 br cm-1 and 1611 m cm-1have also been observed. The relatively broad band at 3421 cm-1 might be due to the water molecules, while band at 1611 cm-1 due to C=N stretching of bbp/Hbbp ligand. The IR spectrum of 3 exhibits only weak band at 2172 cm-1, which is attributable to the asymmetric stretching vibration of bridging cyanides. In addition to the cyanide stretching band, the spectrum shows another broad band at 3298 cm-1, which may be assigned to mO–H of sol- vated methanol and water molecules. The broadening of the peak might be owing to the involvement of those molecules in strong hydrogen-bonding Table 1. Crystal data and structure refinement of1–3.
Compound 1 2 3
Empirical formula C44H48Fe2N16Na3O13 C57H73FeN11O5 C133H132Cu2Fe3N39O10
Formula weight 1189.65 1048.11 2731.40
Temperature (K) 150 (2) 150 (2) 120 (2)
Wavelength (A˚ ) 0.71073 0.71073 0.71073
Crystal system Monoclinic Orthorhombic Trigonal
Space group C2/c Pna21 R 3
a (A˚ ) 25.603 (7) 17.4174 (2) 23.5102 (3)
b (A˚ ) 15.823 (4) 24.0913 (3) 23.5102 (3)
c(A˚ ) 25.722 (5) 13.2248 (1) 39.4750 (7)
b () 95.69 (2) 90 120
Volume (A˚3) 10369.1 (4) 5549.23 (10) 18895.8 (6)
Z 8 4 6
Dcalc(g cm-3) 1.524 1.255 1.440
Absorption coefficient (mm-1) 0.664 0.329 0.747
F(000) 4904 2232 8514
h Range for data collection () 1.748–27.481 2.487–27.547 2.697–27.468
Limiting indices -33BhB 33, -22BhB 22, -30Bh B30,
-20BkB 20, -31BkB 27, -25Bk B25, -33BlB31 -17BlB17 -51Bl B51
Reflections collected/unique 40744/11822 42972/12674 19180/9595
Independent reflection/Rint 7548/0.0694 11081/0.0457 6689/0.0399
restraints/parameters 2/715 5/681 0/561
Goodness-of-fit onF2 1.040 1.041 1.030
FinalRindices [I[2r(I)] R1= 0.089, R1= 0.0411, R1= 0.0427,
wR2= 0.2556 wR2= 0.1012 wR2= 0.0954
Rindices (all data) R1= 0.1373, R1= 0.0551, R1= 0.0758,
wR2= 0.2991 wR2= 0.1068 wR2= 0.1062
Largest diff. peak/hole (e A˚-3) 1.785/-1.125 0.892/-0.425 1.553/-0.656
interactions for stabilizing the crystal packing in the solid state. Moreover, bands at 3159 cm-1, 2888–3089w cm21 and 1612 s cm-1 can be assigned tomN–H,mC–HandmC=N, respectively. The IR spectrum of complex 2 shows a broad band at 3471 br, 2916–
2873 s and 1609 m cm-1, those can be assigned as
mO–Hof water molecules,mC–Hof tetrabutylammonium ion and mC=N for bbp ligands, respectively.
3.2 Description of the structures
The structures of the complexes 1–3 have been determined by single crystal x-ray diffraction study.
Crystallographic data and details of data collection, and refinement for all the complexes are listed in Table1, and selected bond lengths and angles for1–3 are summarized in Table2.
3.2aStructure of1: X-ray diffraction study reveals that the complex 1 crystallizes in the monoclinic C2/c space group along with some lattice water molecules.
The structure of complex of 1 with the selected atom labeling schemes is depicted in Figure1. The
asymmetric unit consists of two iron(III) centers and three sodium ions. Therefore, from the charge balance point of view, one of the iron(III) coordination sites is monoanionc, while other is dianionic. The origin of this difference is due to different extent of deproto- nation of benzimidazole moieties from H2bbp ligand.
In the monoanioc form, the coordinated tridentate ligand is singly deprotonated (Hbbp), whereas it is doubly deprotonated (bbp) in the dianionic form.
However, the coordination geometry of both the iro- n(III) centers are almost identical and are best described as a slightly distorted octahedral structure, consisting of three nitrogen atoms of the tridentate ligand (Hbbp/bbp) and three cyanide groups, occupy- ing either of the meridional positions of the octahe- dron.13The Fe–C distances vary in the range 1.908(6) A˚ (Fe1–C22) to 1.978(6) A˚ (Fe2–C42), while the Fe–
N bond distances spanning in narrow range of 1.924(4) A˚ (Fe1–N3) to 1.969(4) A˚ (Fe2–N13), and these are in good agreement with those observed in struc- turally characterized low-spin iron(III) cyanide Table 2. Selected bond lengths (A˚ ) and bond angles (8)
for 1.
Distances
Fe1–C20 1.957 (6) Fe1–C21 1.955 (6)
Fe1–C22 1.908 (6) Fe1–N1 1.937 (4)
Fe1–N3 1.924 (4) Fe1–N5 1.950 (4)
Fe2–C42 1.978 (6) Fe2–C43 1.909 (6)
Fe2–C44 1.961 (5) Fe2–N9 1.926 (4)
Fe2–N11 1.941 (4) Fe2–N13 1.969 (4)
Angles
N6–C20–Fe1 175.8 (6) N7–C21–Fe1 178.3 (5) N8–C22–Fe1 178.9 (7) N14–C42–Fe2 177.5 (6) N15–C43–Fe2 176.8 (6) N16–C44–Fe2 173.5 (5)
Figure 1. ORTEP drawing of the complex anion in 1.
Thermal ellipsoids are drawn in 30% probability; sodium ions, solvent molecules and hydrogen atoms have been omitted for clarity.
Scheme 2. Route to synthesis of the complexes.
complexes.13–18 The iron(III) centers are well sepa- rated by trinuclear sodium clusters and each of the metal center is connected to a sodium ion through one of the axial cyanides. The deprotonated nitrogen atoms of benzimidazole groups further coordinate nearby sodium ions to construct a 2D polymeric structure of1 in the (101) plane (Figure2). The Fe–C–N bond angles for terminal cyanides are very close to linearity, ranging from 176.8(6) to 178.9(7), while slight deviation from the linearity has been observed for sodium-bound cyanides (173.5(5) and 175.8(6)). The shortest Fe1Fe1 separation in the structure is 7.848(1) A˚ , while for Fe2Fe2 and Fe1Fe2 sepa- rations are 8.839(1) and 9.907(1) A˚ . The polymeric structure of 1 is further stabilized by the strong hydrogen bonding involving benzimidazole nitrogen, terminal cyanide groups and lattice water molecules as well as bypp stacking interaction between the aro- matic rings of the ligands of shortest contact of 3.268 A˚ .
3.2bStructure of 2: An ORTEP drawing of the com- plex anion in 2 is displayed in Figure3, with data collection and structure solution parameters in Table 1 and selected bond distances and angles in Table 3. The complex anion sits on a general position in the orthorhombic space group Pna21and crystallizes with tetrabutylammonium ion and three lattice methanol
and two water molecules. The geometry of the anionic part of2is considered to be a distorted octahedron, in which iron(III) is surrounded by six nitrogen donors from two dianionic bbp ligands, each of them is occupied by a meridional position of the octahedron.
The maximum deviations of bond angles from ideal geometry are observed for N8–Fe1–N10 (81.17(11)) and N6–Fe1–N10 (162.52(11)˚), that are responsible for the major structural distortion in2. The small bite angles of bbp ligands towards Fe1 are the main factors accounting for the distortion. The Fe–N bond distances span in a very narrow range 1.915(3)–1.949(3) A˚ , and they are in good agreement with those observed in structurally characterized similar low-spin iron(III) complexes.13–18The packing diagram of2is stabilized by the strong hydrogen-bonding interaction involving benzimidazole nitrogen atoms and lattice methanol, and water molecules. The C–Hpinteractions among the aromatic ring of bbp ligands provided the addition stability towards the solid-state packing (Figure1 Supplementary Information).
3.2cStructure of3: The crystal structure of complex3 is shown in Figure 4, while bond lengths and bond angles are displayed in Table4. The molecular struc- ture of 3comprises with a cyano-bridged dicopper(II) complex cation, [Cu2(tren)2CN]3?, three iron-based mononuclear anions, [Fe(bbp)2]-, and six lattice Figure 2. 2D polymeric structure of1in the (101) plane.
methanol and four lattice water molecules. Each of the copper(II) ion is five coordinated with almost perfect trigonal bipyramidal geometry as suggested by the Addison parameter s of 1.01.19 Three primary amine nitrogen atoms of tren ligand construct the equatorial plane around the metal center, while one axial position is occupied by the tertiary amine-N and other one by the cyanide ion. Likewise, other crystal structures of similar complexes, the single end-to-end bridging cyano group is disordered about the inversion center, effectively scrambling the carbon and nitrogen atoms.9,20 This group was modelled with one atomic position consisting equal proportions of carbon and nitrogen. However, unlike the most of reported com- plexes, the remarkable feature of the present com- pound is that the bridging region, i.e. Cu–CN–Cu is strictly linear, which resides on the crystallographic
3-fold axis, running along the direction of the c axis.
Although two copper(II) atoms are crystallographi- cally independent to each other, but they are almost identical in terms of their geometrical features. All C(cyanide)–Cu(II)–N(primary amine) bond angles are identical as they are related to each other by the crystallographic 3-fold axis, which are elevated well above 90o, with associated N11–Cu1–N12 angles reduced well below the ideal geometry (Table 3). As a consequence, the copper(II) ions are significantly displaced by 0.219 A˚ away from the mean equatorial plane towards the axial cyanide ligand, a common feature of these types of TBP Cu(II) complexes.20–29 The CuCu separation along the bridging cyanide is 5.084(7) A˚ , which is somewhat longer than the other reported cyanide-bridged dicopper(II) complexes with tren ligand. The strict linearity in the bridging region in3is the main reason behind the longer metalmetal distance in the present case. The anionic part of the complex 3 is eventually identical as that of 2 and, therefore, the discussion of that part is no longer required. The solid state structure of3is stabilized by a complex network of hydrogen bonding interactions as depicted in Figure2, Supplementary Information.
3.3 Magnetic properties
The magnetic property of complex3has been studied in the temperature range of 1.8–300 K using the external magnetic field of 1000 Oe andvMTvsT plot Figure 3. Molecular structure of complex2 with thermal ellipsoids are drawn in 30% probability and hydrogen atoms, and water molecules have been omitted for clarity.
Table 3. Selected bond lengths (A˚ ) and bond angles () for 2.
Distances
Fe1–N1 1.949 (3) Fe1–N3 1.915 (3)
Fe1–N5 1.946 (3) Fe1–N6 1.943 (3)
Fe1–N8 1.918 (2) Fe1–N10 1.946 (3) Angles
N3–Fe1–N8 179.65 (12) N1–Fe1–N5 162.53 (11) N6–Fe1–N10 162.52 (11) N3–Fe1–N5 81.51 (11) N6–Fe1–N8 81.36 (11) N1–Fe1–N3 81.03 (11) N8–Fe1–N10 81.17 (11)
is depicted in Figure 5. ThevMT product (*1.80 cm3 mol-1K) of3at 300 K is slightly smaller than that of spin-only value of 1.875 cm3 mol-1 K for magneti- cally non-interacting two copper(II) (S = 1/2) and three low spin iron(III) (S = 1/2) ions with g = 2.00, which is indicative of a strong antiferromagnetic coupling between the cyano-bridged copper(II) ions even at room temperature.30,31 However, the vMT decreases gradually with lowering the temperature until *130 K and then exhibits an abrupt decrease to attain a plateau of 1.32 cm3 mol-1 K at 50 K and finally, it decreases again below 15 K. The shape of the curve unambiguously supports the occurrence of strong antiferromagnetic exchange interactions between the paramagnetic copper(II) ions, mediated by the cyanide bridge. The final decrease of vMT
below 15 K could be attributed to the intermolecular antiferromagnetic interaction, and/or the zero-field splitting effect of the iron(III) ions. Least-squares fit- ting of the susceptibility data through a simple Blea- ney–Bowers expression obtained from the Heisenberg-Dirac-Van Vleck Hamiltonian H =-2JSCuII • SCuII
in the temperature range of 1.8–300 K gives g = 1.85(2); J= -110.6(3) K.
The antiferromagnetic interaction between the cop- per(II) ions in 3 is considerably strong and can be partially explained especially by analyzing the geo- metrical features of the binuclear cationic moieties. The main factors that influence the antiferromagnetic inter- action between the copper(II) ions are the position of cyanide group relative to each copper(II) ion, the angle subtended by the two copper(II) ions and the cyanide Figure 4. Perspective view of complex3(30% ellipsoid). All the hydrogen atoms and solvent molecules are omitted for clarity.
Table 4. Selected bond lengths (A˚ ) and bond angles () for3.
Distances
Cu1–N11 2.033 (3) Cu1–N12 2.097 (2)
Cu2–N13 2.041 (3) Cu2–N14 2.098 (2)
Cu2–N15 1.971 (5) Cu1–C45 1.984 (4)
Fe1–N1 1.940 (2) Fe1–N3 1.915 (2)
Fe1–N5 1.966 (2) Fe1–N6 1.932 (2)
Fe1–N8 1.912 (2) Fe1–N10 1.947 (2)
Angles
C45–Cu1–N11 180.0 N15–Cu2–N13 180.0
N15–C45–Cu1 180.0 C45–N15–Cu2 180.0
N12–Cu1-N12 118.92 (2) N14–Cu2–N14 118.93 (2)
N11–Cu1–N12 83.99 (6) N13–Cu2–N14 84.02 (6)
C45A–Cu1–N12 95.98 (6) N8–Fe1–N10 80.76 (8)
N3–Fe1–N8 178.93 (8) N1–Fe1–N5 162.33 (8)
N6–Fe1–N10 162.42 (8) N3–Fe1–N5 81.02 (8)
N6–Fe1–N8 81.77 (8) N1–Fe1–N3 81.32 (8)
group, and the separation between copper(II) ions. In the present TBP system, the cyanide ligand is located on an axial position, and therefore, the overlap between the dz2orbital of the copper(II) ions and therorbital of the bridging cyanide group becomes effective which gives rise to a moderately strong antiferromagnetic interaction. When the cyanide ion is in the equatorial position of a TBP array, the reduced overlap of the r orbitals of the cyanide ion and dz2 orbitals of the cop- per(II) ions results in weaker antiferromagnetic inter- action between the two copper(II) ions. The strength of the antiferromagnetic exchange is not only influenced by the position of the cyanide ligand, but also by the angle subtended by the two copper(II) ions and the cyanide group and the CuCu separation as well. The main structural feature of3is that it is strictly linear in the bridging region since both the copper(II) centers and bridging cyanide reside on a crystallographic 3-fold axis. Therefore, an effective overlap is expected between therorbital of the cyanide ion and dz2orbital of the copper(II) ion. On the other hand, this ideal geometry incidentally results in a slightly longer CuCu separation of 5.084(7) compared to the similar structure reported earlier, which could give rise in a slightly weaker overlap. However, the higher value of antiferromagnetic exchange coupling constant in the present case supports that the linearity plays the key role for manipulating the magnitude of coupling con- stant over the metalmetal separation.6,9,30,31
4. Conclusions
In summary, we have synthesized and structurally characterized three new complexes, [Na3Fe2(bbp) (Hbbp)(CN)6(H2O)9]4H2O (1), (Bu4N)[Fe(bbp)2]
3CH3OH2H2O (2), and a cyano-bridged dinuclear Cu(II) complex, [Cu2(tren)2(CN)][Fe(bbp)2]36CH3 OH4H2O (3), where H2bbp = bis(2-benzimida- zolyl)pyridine and tren = tris(2-aminoethyl)amine.
X-ray crystallographic studies reveal that the geometry of iron(III) centers in the complex anions of1–3is an octahedral, while it is a trigonal bipyramidal around copper(II) center in the complex cation of3. Both bbp ligands meridionally coordinate iron(III) centers in the complex cations of2 and 3, while three cyanide ions and bbp (or Hbbp) ligand occupied either of the meridional positions in 1. The deprotonated benzimi- dazole nitrogen atom and an axial cyanide ion further connect trinuclear Na3 clusters, leading to the con- struction of 2D polymeric structure of 1. In the dinu- clear complex cation in 3, the Cu–CN–Cu bridging region is strictly linear with CuCu separation of 5.084(7) A˚ in which cyanide ion bridges two cop- per(II) centers through axial coordination. Variable temperature magnetic susceptibility study revealed that the cyano-bridge propagates the antiferromagnetic coupling between the copper(II) centers with J value of -110 K for 3. This coupling constant is typical for axially connected cyano-bridged TBP copper(II) complexes and are larger than those found for related complexes connected at equatorial coordina- tion sites.
5. Supplementary Information (SI)
Crystallographic data for the structural of complexes 1–3 are available athttp://www.ias.ac.in/chemsci.
Acknowledgements
AP gratefully acknowledges the financial support of this work by the CSIR, New Delhi, India (sanction no.
01(2834)/15/EMR-II dated 02/06/15). We would also like to thank Prof. Corine Mathonie`re, Universite´ de Bordeaux, ICMCB, UPR 9048, Pessac F-33600, France and Prof.
Rodolphe Cle´rac, Universite´ Bordeaux, CRPP, UPR 8641, Pessac, F-33600, France for magnetic studies.
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