Three Chiral Cyanide-Bridged Cr – Cu Complexes : Synthesis , Crystal Structures and Magnetic Properties

Two trans-dicyanidochromium(III)-containing building blocks and one chiral copper(II) compound have been employed to assemble cyanide-bridged heterometallic complexes, resulting in three chiral cyanide-bridged Cr(III)–Cu(II) complexes, {[Cu(L)2Cr(L)(CN)2]ClO4}2 · CH3OH · H2O (1a, L1 = (S,S)-1,2-diaminocyclohexane, H2L =1,2-bis(pyridine-2-carboxamido)benzene), {[Cu(L)2Cr(L)(CN)2]ClO4}2 · CH3OH · H2O (1b, L2 = (R,R)-1,2-diaminocyclohexane) {[Cu(L)2Cr(L)(CN)2][Cr(L)(CN)2]} · CH3OH · 2H2O (2), (H2L = 1,2-bis(pyridine-2-carboxamido)-4-chlorobenzene). All the three complexes have been characterized by elemental analysis, IR spectroscopy and X-ray structure determination. Single-crystal X-ray diffraction analysis shows that the two enantiomeric complexes 1a, 1b and the complex 2 belong to cyanide-bridged cationic binuclear structure type with ClO4 or the anionic cyanide building block as balance anion for complexes 1a, 1b or 2, respectively. Investigation of the magnetic properties of the complexes 1a and 2 reveals the weak ferromagnetic coupling between the neighboring Cr(III) and Cu(II) ions through the bridging cyanide group.


Introduction
2][3][4] During the process of the synthesis of the new magnetic complexes, the choice of magnetic spin carriers, bridging bonds and coordination ligands plays a very important role on the structure and the functional property of the target magnetic complexes.4][35][36][37][38] Studies have shown that these types of cyanide-containing precursors were good choices for assembling cyanide bridged bimetallic magnetic complexes with different structures, such as multinuclear, nanomolecular and one-dimension-al chains, and interesting magnetic properties.On the other hand, in the research field of functional molecular magnetic materials, the design and synthesis of chiral magnetic materials are of great significance for the basic research of magnetic induction second harmonic generation (MSHG) and magnetic chiral dichroism (MCHD) and their possible applications in a variety of new technologies.][41][42][43][44][45][46] In order to find new chiral molecular magnetic complexes and further enrich the low-dimensional cyanide bridged trans-dicyano-based compounds, we investigated the reactions of trans-dicyanidochromium(III) precursors with chiral organic amine copper compounds (Scheme 1) and obtained three new cyanide-bridged chiral Cr(III)-Cu(II) complexes, including the two enantiomeric complexes . This paper will mainly concern the synthesis, crystal structures and magnetic properties for the above three complexes.
Caution! KCN is hypertoxic and hazardous.Perchlorate salts of metal complexes with organic ligands are potentially explosive.These chemicals should be handled in small quantities with great care.

Preparation of the Complexes 1a, 1b and 2
All three complexes were prepared using similar procedure.Therefore, a representative method for preparation of the complex 1a is described herein.
Complex 1a was prepared by the following procedures: The acetonitrile solution (10 mL) formed in situ by [Cu(ClO 4 ) 2 ] • 6H 2 O (36.5 mg, 0.1 mmol) and L 1 (22.8 mg, 0.2 mmol) was added slowly to a solution containing K[Cr(L 3 )(CN) 2 ] (91.6 mg, 0.20 mmol) dissolved in a mixture of methanol and water (8 mL: 2 mL).The mixture was stirred only for one minute at room temperature and filtered at once to remove any insoluble material, and then the filtrate was allowed to evaporate slowly without disturbance for about one week.The dark-orange crystals generated suitable for X-ray diffraction were collected by filtration, washed with cool methanol, and dried in air.Yield:

3. X-ray Data Collection and Structure Refinement
Crystal data of three complexes were collected by using single-crystals with suitable dimensions on an Oxford Diffraction Gemini E diffractometer with MoKα radiation (λ = 0.71073 Å) at room temperature, and the collected frames were integrated by using the preliminary cell-orientation matrix.The structures of these three complexes were solved by direct method and expanded using Fourier difference techniques with the SHELXTL-97 program package. 48All non-hydrogen atoms were readily located Scheme 1.The starting materials used for synthesizing the three complexes.

Experimental
Elemental analyses of carbon, hydrogen, and nitrogen were carried out with an Elementary Vario El.The infrared spectroscopy on KBr pellets was performed on a Magna-IR 750 spectrophotometer in the 4000-400cm -1 region.Variable-temperature magnetic susceptibilities for the reported complexes were performed on a Quantum Design MPMS SQUID magnetometer.The experimental susceptibilities were corrected for the diamagnetism of the constituent atoms (Pascal's tables).

1. General Procedures and Materials
All the reactions were carried out under an air atmosphere and all chemicals and solvents used were reagent Chen et al.: Three Chiral Cyanide-Bridged Cr-Cu Complexes: ... and refined anisotropically.In ligand L 4 in complex 2 chlorine atoms Cl1 and Cl2 were refined as disordered over two positions with 0.80:0.20 and 0.50:0.50occupancy ratios, respectively.Hydrogen atoms were assigned isotropic displacement coefficients U(H) = 1.2U(C) or 1.5U(C) and their coordinates were allowed to ride on their respective carbon atoms or nitrogen atoms using SHELXL-97 except of the solvent H atoms.For the latter, they were refined isotropically with fixed U values and the DFIX command was used to rationalize the bond parameter.CCDC 1861738-1861740 for these three complexes contain the supplementary crystallographic data for this paper, which can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif.Details of the crystal parameters, data collection, and refinement of complexes 1-2 are summarized in Table 1.

1. Synthesis and General Characterization
4][35][36][37][38] The relatively large planar pyridinecarboxamide ligand at the equatorial position can not only effectively lower the dimensionality of the resulted complex, but also weaken the supramolecular intermolecular magnetic interactions.With this in mind and also for the purpose of the preparation of chiral magnetic complexes, we investigated the reactions of trans-dicyanidochromium(III) with chiral amine copper(II) compounds and obtained three new chiral cyanide-bridged Cr(III)-Cu(II) complexes.The different balance anions, i.e.ClO 4 -for complexes 1a, 1b and the cyanometallate for complexes 2 indicate that the structure of the cyanide precursor has some effects on the structure of the cyanide-bridged complex formed.The three cyanide-bridged complexes have been characterized by IR spectroscopy.In the IR spectra of 1a and 1b, two sharp peaks due to the cyanide-stretching vibration were observed at about 2125-2130 and 2155-2160 cm -1 , respectively, indicating the presence of bridging and non-bridging cyanide ligands in these complexes.The strong broad peak centered at about 1100 cm -1 for these two complexes is attributed to the free ClO 4 -anion.To confirm the optical activity and enantiomeric nature, the circular dichroism (CD) spectrum were measured in KBr pellets for complexes 1-2.The CD spectrum of 1a and 1b exhibit positive and negative Cotton effect at the same wavelengths (Figure . 1).

2. Crystal structures of complexes 1a, 1b and 2
Some important structural parameters for complexes 1a, 1b and 2 are collected in Table 2.The perspective view of the enantiomeric structure of complexes 1a and 1b is demonstrated in Figure 2. The cell packing diagram of the complex 1a is given in Figure 3, which is similar to that for the complex 1b.For the complex 2, its cationic binuclear structure and the cell packing diagram are shown in Figures 4 and 5, respectively.all the reported complexes, each cyanide-containing building block acting as a monodentate ligand through one of its two trans cyanide groups connects the Cu(II) ion with the other cyanide group as terminal.The Cr(III) ion The complexes 1a and 1b as a pair of enantiomer, containing Cr 2 Cu 2 unit in the unit cell with a dimer structure, crystallize in monoclinic cell setting with the non-central space group P2 1 , while complex 2 crystallizes in triclinic cell setting with the non-central space group P1.All the three complexes are with the similar cationic cyanide-bridged binuclear structure and the different balance anion, i.e. the free ClO 4 -for complexes 1a, 1b and the free cyanide building block for complex 2. The distances between the O atom of the ClO 4 -ion and the Cu(II) ion is about 3.273 Å, indicating the existed weak interaction.In    All the H atoms and the solvent molecules have been omitted for clarity.
Chen et al.: Three Chiral Cyanide-Bridged Cr-Cu Complexes: ... is six-coordinates with four equatorial nitrogen atoms from the pyridinecarboxamide ligand and two carbon atoms from the two cyanide groups with a trans position, so that forming a slightly distorted octahedral geometry, which can be proven by the bond parameters around the Cr(III) ion (Table 2).The Cr-C≡N bond angles in these three complexes are in a comparatively narrow range of 165.6(7)°-178.8(7)°,showing almost the linear conformation for these three atoms.
The Cu atom in complexes 1a, 1b and 2 is five-coordinated by a N 5 unit, in which four N atoms come from the two chiral amine ligands and the additional one N atom from the bridging cyanide group.The Cu atom is only out of the plane formed by four N atoms 0.16(2), 0.17(4) and 0.058(2) Å toward to the fifth coordinated N cyanido atom in these three complexes, indicating that these five atoms are almost located in a plane.The average Cu-N amine bond lengths in complexes 1a, 1b and 2 are 2.009, 2.008 and 2.014 Å, respectively, obviously shorter than the Cu-N cy- anido bond length with the values of 2.346, 2.344 and 2.324 Å, clearly showing the markedly distorted square pyramid surrounding of the Cu(II) ion.Additionally, it should be pointed out that there exists conspicuous difference for the C≡N-Cu bond angles in these three complexes.The C≡N-Cu bond angle in complexes 1a and 1b are only 139.0(9)°, 141.7(7)°, respectively, while the corresponding angle in the complexes 2 is obviously larger than those in complexes 1a and 1b with value 159.2(10)°.The intramolecular Cr(III)-Cu(II) separation through bridging cyanide group are 5.076, 5.098 and 5.412 Å for these three complexes, which are obviously shorter than the shortest intermolecular metal-metal distance with the values of 6.519, 7.543, 8.108 and 7.511 Å in the three complexes, respectively.

The Magnetic Properties of Complexes
Figure 6 shows the temperature dependences of magnetic susceptibility of complexes 1a and 2 measured in the temperature range of 2-300 K in the applied field of 2000 Oe.The room temperature c m T values are 2.13 and 4.09 emu K mol -1 for these two complexes, respectively, which are slightly lower than the spin only value of 2.25 emu K mol -1 for one uncoupled Cu(II) (S = 1/2) ion and one Cr(III) (S = 3/2) ion in complex 1a and 4.125 emu K mol -1 for one uncoupled Cu(II) (S = 1/2) ion and two Cr(III) (S = 3/2) ion in complex 2 based on g = 2.00.With the temperature decreasing, the c m T values increases gradually and attains the value of 2.37 and 6.76 emu K mol -1 about 15 K, then decreases sharply to 1.51 and 4.51 emu K mol -1 at 2 K, which indicated the characteristic of ferromagnetic coupling between the cyanide-bridged Cr(III)-Cu(II) center.The magnetic susceptibility for these two complexes conforms well to Curie-Weiss law in the range 2-300 K and give the positive Weiss constant q = 1.38 K and Curie constant C = 2.15 emu K mol -1 for complex 1a and q = 7.01 K and Curie constant C = 4.15 emu K mol -1 for complex 2, further proves the ferromagnetic coupled Cr(III)-Cu(II) through the cyanide bridge.
On the basis of the binuclear model, the magnetic susceptibility of complex 1a can be fitted accordingly by the following expression (1) derived from the isotropic exchange spin Hamilton Ĥ = -2JŜ Cu Ŝ Cr .For complex 2, its magnetic susceptibility has been analyzed based-on also the binuclear model but by introducing the additional isolated Cr(III) ion with the expression (2). (1) (2) By using the above model, the susceptibilities over the temperature range of 2-300 K for these two complexes were simulated, giving the best-fit parameters J = 0.74(2) cm -1 , g = 2.01(2), R = ∑(c obsd T -c cald T) 2 /∑(c obsd T) 2 = 2.30 × 10 -5 for 1a and J = 2.37(2), g = 2.01(8), R = 3.21 × 10 -5 for 2, respectively, which can further proven the weak ferro- magnetic coupling between the Cr(III) ion and Cu(II) ion through the bridging cyanide group.

Conclusion
In summary, three new chiral cyanide-bridged heterobimetallic complexes, in which two of them are a pair of enantiomers, have been designed and successfully synthesized based on the chiral amine copper(II) compounds and the trans-dicyanidochromium(III)-containing building blocks.All the three complexes present the similar cationic cyanide-bridged binuclear structure but with different balanced anion, giving the information that the cyanide precursors with slight structural difference still have some influence on the forming of the target complexes.Investigation over the magnetic properties of the reported complexes reveals that the ferromagnetic coupling between the cyanide-bridged Cr(III)-Cu(II) center.

Acknowledgement
This work was supported by the Natural Science Foundation of China (21671121) and the Natural Science Foundation of Shandong Province (ZR2018BB002).

Figure 2 .
Figure 2. The perspective view of the enantiomeric structure of complexes 1a and 1b.All the H atoms, the balanced anion and the solvent molecules have been omitted for clarity.

Figure 3 .
Figure 3.The cell packing diagram along c axis of the complex 1a.All the H atoms and the solvent molecules have been omitted for clarity.

Figure 4 .
Figure 4. Perspective view of the cationic structure of the complex 2. All the H atoms, the balanced anion and the solvent molecules have been omitted for clarity.

Figure 5 .
Figure 5.The cell packing diagram along b axis of the complex 2.All the H atoms and the solvent molecules have been omitted for clarity.

Figure 6 .
Figure 6.The c m T and c m -1 vs T curves for complexes 1a (left) and 2 (right).