Stepwise Synthesis , Hydrogen-Bonded Supramolecular Structure , and Magnetic Property of a Co – Mn Heterodinuclear Complex

A cobalt(III)–manganese(II) heterometallic dinuclear complex, [Mn{Co(μ-Himn)3}Cl2(CH3OH)], was prepared by a metalloligand approach. X-ray crystallographic analysis indicated that the metalloligand [Co(Himn)3] underwent mer/fac geometrical isomerization upon coordination to a Mn ion. Owing to the non-coordinating N–H bonds in the [Co(Himn)3] moiety, the heterodinuclear complex exhibited hydrogen bond interactions with the Cl− ligand of the neighboring complex to construct two-dimensional hydrogen-bond networks. The bond distances around the Mn center and the χMT value at 300 K indicate that the Mn center is in a divalent state. The temperature dependence of the χMT product and field dependence of the magnetization showed the isotropic nature of the MnII center.


Introduction
Discrete cobalt and manganese complexes have been attracting much attention owing to their fascinating magnetic properties such as single-molecule magnet behavior [1][2][3][4][5].Heteronuclear complexes are strong candidates for such a property because of the magnetic coupling between different paramagnetic metal ions [6].On the other hand, the combination of paramagnetic and diamagnetic ions is also a nice approach to prepare a single-molecule magnet with a single magnetic center (single-ion magnet) [7][8][9].One of the advantages of single-ion magnets is facile control of the coordination geometry and crystal field to achieve a large negative axial zero-field splitting, which results in a high spin-reversal barrier.Sørensen et al. reported an excellent approach in which a C 4 symmetric diamagnetic metalloligand was employed to prepare D 4d symmetric Ln III single-ion magnets [8,9].In our previous study, we reported a tris-bidentate-chelate cobalt(III) complex as a tridentate metalloligand to obtain a Co III -Co II -Co III mixed-valent complex [4].The Co II ion in this complex takes ideal D 3d symmetry owing to the rigid C 3 symmetric metalloligand and intermolecular hydrogen bonding interactions.This stepwise reaction strategy can be applied to other first-row d-block metal ions to obtain heterometallic complexes.To extend our study to a heterometallic complex, we report the synthesis and crystal structure of a Co-Mn dinuclear complex using a tris-bidentate cobalt(III) complex as a metalloligand.

Synthesis and Characterization
The tris-bidentate-chelate metalloligand, [Co III (Himn) 3 ], was synthesized by the reaction of CoCl 2 •6H 2 O and Himn − in a 1:3 ratio in methanol in air (Scheme 1).The methanol solution of the metalloligand was added to a methanolic solution of MnCl 2 •6H 2 O under a N 2 atmosphere to afford brown crystals of the Co-Mn heterometallic complex.The obtained crystalline product was stable in air.

Synthesis and Characterization
The tris-bidentate-chelate metalloligand, [Co III (Himn)3], was synthesized by the reaction of CoCl2• 6H2O and Himn − in a 1:3 ratio in methanol in air (Scheme 1).The methanol solution of the metalloligand was added to a methanolic solution of MnCl2• 6H2O under a N2 atmosphere to afford brown crystals of the Co-Mn heterometallic complex.The obtained crystalline product was stable in air.
Scheme 1. Synthetic procedure of the Co-Mn heterodinuclear complex.The metalloligand [Co III (Himn)3] undergoes mer to fac isomerization upon coordination to a Mn II ion.
The heterometallic dinuclear complex, [Mn{Co(µ -Himn)3}Cl2(CH3OH)]• 0.78CH3OH• 1.26H2O, was characterized by single-crystal X-ray analysis.The molecular structure and crystallographic information are shown in Figure 1 and Table 1, respectively.In the crystal, the metalloligand, [Co(Himn)3], coordinated to a Mn center via phenolate-O atoms as a tridenate ligand with the fac geometry.As the as-synthesized metalloligand takes the mer configuration in 100% yield, the metalloligand underwent mer/fac isomerization upon addition of Mn II ions [10].This facile isomerization is presumably because of the partial formation of labile [Co II (Himn)3] − owing to the [Co(Himn)3] 0/− redox equilibrium induced by Mn II ions [4].Thus, we succeeded in designing a Co-Mn heterometallic complex by the metalloligand approach.
In the heterometallic complex, the Co center took N3O3 octahedral coordination geometry with an average bond distance of ca.1.90 Å , which is consistent with those of analogous Co III complexes (Table 2) [4,10].On the other hand, the geometry of the hexacoordinated Mn center was rather ambiguous.Such ambiguous geometry is a characteristic of Mn II complexes, which lack crystal field stabilization.The oxidation states of the metal centers were confirmed to be Co III and Mn II by bond valence sum analysis [11].A positional disorder was observed at the methyl group of the coordinating methanol molecule with occupancy of ca.0.5 (C28 and C28B).Furthermore, considerable disorder was observed for solvent molecules of crystallization.It is to be noted that the SQUEEZE program [17]was employed for structural refinement to treat the heavy disorder, and 10 electrons, which corresponds to a water molecule, in a 35 Å 3 void were removed.Such heavy disorder of the solvent molecules is illustrative of the fluid nature of the solvent-accessible void.Scheme 1. Synthetic procedure of the Co-Mn heterodinuclear complex.The metalloligand [Co III (Himn) 3 ] undergoes mer to fac isomerization upon coordination to a Mn II ion.
The heterometallic dinuclear complex, [Mn{Co(µ-Himn) 3 }Cl 2 (CH 3 OH)]•0.78CH3 OH•1.26H 2 O, was characterized by single-crystal X-ray analysis.The molecular structure and crystallographic information are shown in Figure 1 and Table 1, respectively.In the crystal, the metalloligand, [Co(Himn) 3 ], coordinated to a Mn center via phenolate-O atoms as a tridenate ligand with the fac geometry.As the as-synthesized metalloligand takes the mer configuration in 100% yield, the metalloligand underwent mer/fac isomerization upon addition of Mn II ions [10].This facile isomerization is presumably because of the partial formation of labile [Co II (Himn) 3 ] − owing to the [Co(Himn) 3 ] 0/− redox equilibrium induced by Mn II ions [4].Thus, we succeeded in designing a Co-Mn heterometallic complex by the metalloligand approach.
In the heterometallic complex, the Co center took N 3 O 3 octahedral coordination geometry with an average bond distance of ca.1.90 Å, which is consistent with those of analogous Co III complexes (Table 2) [4,10].On the other hand, the geometry of the hexacoordinated Mn center was rather ambiguous.Such ambiguous geometry is a characteristic of Mn II complexes, which lack crystal field stabilization.The oxidation states of the metal centers were confirmed to be Co III and Mn II by bond valence sum analysis [11].A positional disorder was observed at the methyl group of the coordinating methanol molecule with occupancy of ca.0.5 (C28 and C28B).Furthermore, considerable disorder was observed for solvent molecules of crystallization.It is to be noted that the SQUEEZE program [12] was employed for structural refinement to treat the heavy disorder, and 10 electrons, which corresponds to a water molecule, in a 35 Å 3 void were removed.Such heavy disorder of the solvent molecules is illustrative of the fluid nature of the solvent-accessible void.
)) 0.0533 wR 2 (F 2 : all data) 0.1282  The nonsymmetric coordination geometry of the Mn center resulted in the formation of a supramolecular sheet structure in the ab plane.As mentioned above, many solvent molecules of crystallization were observed in this crystal.These molecules are located between the hydrogen-bonded sheets-where a one-dimensional solvent-accessible void is formed-in which these disordered molecules are located.The shape of the void (534 Å 3 /cell) in the packing diagram is shown in Figure 3.
The nonsymmetric coordination geometry of the Mn center resulted in the formation of a supramolecular sheet structure in the ab plane.As mentioned above, many solvent molecules of crystallization were observed in this crystal.These molecules are located between the hydrogenbonded sheets-where a one-dimensional solvent-accessible void is formed-in which these disordered molecules are located.The shape of the void (534 Å 3 /cell) in the packing diagram is shown in Figure 3.

Magnetic Properties
To investigate the magnetic properties, the temperature dependence of the χMT product and field dependence of the magnetization for [Mn II {Co III (µ -Himn)3}Cl2(CH3OH)]• 0.78CH3OH• 1.26H2O were measured (Figure 4).The χMT value at 300 K was 4.39 cm 3 K mol −1 , which is consistent with the spin-only value of 4.375 cm 3 K mol −1 for the S = 5/2 system.Upon cooling down to 5 K, almost no change in the χMT value was observed, which is typical Curie-Weiss behavior.In the magnetization vs. field plot, the magnetization is nearly saturated in the presence of a dc field of 5 T.These magnetic properties are characteristic of the isotropic Mn II center without magnetic coupling.To determine the g-factor, the temperature dependence of the magnetization and field dependence of the magnetization were simultaneously fitted with the following spin Hamiltonian using the PHI program [12].

H = gβSH.
Both of the data were well-fitted by the PHI program without considering zero-field splitting and intermolecular interactions.The g-factor was determined to be 2.00.

Magnetic Properties
To investigate the magnetic properties, the temperature dependence of the χ M T product and field dependence of the magnetization for [Mn II {Co III (µ-Himn) 3 }Cl 2 (CH 3 OH)]•0.78CH3 OH•1.26H 2 O were measured (Figure 4).The χ M T value at 300 K was 4.39 cm 3 K mol −1 , which is consistent with the spin-only value of 4.375 cm 3 K mol −1 for the S = 5/2 system.Upon cooling down to 5 K, almost no change in the χ M T value was observed, which is typical Curie-Weiss behavior.In the magnetization vs. field plot, the magnetization is nearly saturated in the presence of a dc field of 5 T.These magnetic properties are characteristic of the isotropic Mn II center without magnetic coupling.To determine the g-factor, the temperature dependence of the magnetization and field dependence of the magnetization were simultaneously fitted with the following spin Hamiltonian using the PHI program [13].
Both of the data were well-fitted by the PHI program without considering zero-field splitting and intermolecular interactions.The g-factor was determined to be 2.00.

General Consideration
All chemicals were used as received without further purification.

Single-Crystal X-ray Crystallography
X-ray diffraction data were obtained at 90(2) K using a Bruker SMART APEX diffractometer system with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å).A single crystal was mounted with a glass capillary and flash-cooled with a cold nitrogen gas stream.Data were processed using Bruker APEX III software packages.Absorption correction was applied using empirical methods [14].Structures were solved using SHELXT software packages and refined on F 2 (with all independent reflections) using a SHELXL software package [15,16].In the structural refinement, all C-H atoms were located using a riding model and refined isotropically.The positions of the N-H and O-H atoms were determined from difference-Fourier maps and freely refined except for the N5-H5A atom.The N5-H5 atom was located using a riding model because it could not be located from the difference-Fourier maps.Some low-angle reflections were omitted from the refinement because these reflections were likely to be affected by the beam stopper.Reflections due to solvent disorder located in the void spaces between the two-dimensional sheet structure were treated using the SQUEEZE program [17].Here, 10 electrons in the 35 Å 3 /void were removed.The crystallographic data were deposited with the Cambridge Crystallographic Data Centre: Deposition number CCDC-1887480.These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge, CB2 1EZ, UK; Fax, +44-1223-336033; email: deposit@ccdc.cam.ac.uk).

General Consideration
All chemicals were used as received without further purification.

Synthetic Method
The ligand, 2-(2-imidazolinyl)phenol (H 2 imn), was prepared according to a previously reported method [14].To a methanol solution (10 mL) of CoCl 2 •6H 2 O (24.2 mg, 0.1 mmol) and H 2 imn (47.1 mg, 0.3 mmol) was slowly added a methanol solution of KO t Bu (32.8 mg, 0.3 mmol).The reaction mixture was stirred overnight at room temperature in air and then evaporated to dryness.The resultant green residue was dissolved in methanol (5 mL), and the insoluble white residue was removed by filtration.The filtrate was degassed by bubbling N 2 gas, and then a 5 mL methanol solution of MnCl 2 •6H 2 O (18.0 mg, 0.1 mmol) was added.The reaction mixture was allowed to stand at room temperature under a N 2 atmosphere for two weeks, and brown crystals were obtained.Yield:

Single-Crystal X-ray Crystallography
X-ray diffraction data were obtained at 90(2) K using a Bruker SMART APEX diffractometer system with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å).A single crystal was mounted with a glass capillary and flash-cooled with a cold nitrogen gas stream.Data were processed using Bruker APEX III software packages.Absorption correction was applied using empirical methods [15].Structures were solved using SHELXT software packages and refined on F 2 (with all independent reflections) using a SHELXL software package [16,17].13 In the structural refinement, all C-H atoms were located using a riding model and refined isotropically.The positions of the N-H and O-H atoms were determined from difference-Fourier maps and freely refined except for the N5-H5A atom.The N5-H5 atom was located using a riding model because it could not be located from the difference-Fourier maps.Some low-angle reflections were omitted from the refinement because these reflections were likely to be affected by the beam stopper.Reflections due to solvent disorder located in the void spaces between the two-dimensional sheet structure were treated using the SQUEEZE program [17].Here, 10 electrons in the 35 Å 3 /void were removed.The crystallographic data were deposited with the Cambridge Crystallographic Data Centre: Deposition number CCDC-1887480.These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge, CB2 1EZ, UK; Fax, +44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk).

Figure 1 .
Figure 1.The molecular structure of [Mn{Co(µ-Himn) 3 }Cl 2 (CH 3 OH)] (50% probability levels).Hydrogen atoms have been omitted for clarity.The disordered C atom of the coordinating methanol molecule is shown at the two possible positions (C28 and C28B).

Figure 4 .
Figure 4. (a) Temperature dependence of the χMT product of [Mn II {Co III (µ -Himn)3}Cl2(CH3OH)] in the presence of a 0.3 T static field and an χM vs. T plot below 20 K (inset).(b) Field dependence of the magnetization of [Mn II {Co III (µ -Himn)3}Cl2(CH3OH)] at 1.9 K.The solid lines correspond to the fit using the PHI program.The magnetization fit is the same as fitting with a Brillouin function.

Figure 4 .
Figure 4. (a) Temperature dependence of the χ M T product of [Mn II {Co III (µ-Himn) 3 }Cl 2 (CH 3 OH)] in the presence of a 0.3 T static field and an χ M vs. T plot below 20 K (inset).(b) Field dependence of the magnetization of [Mn II {Co III (µ-Himn) 3 }Cl 2 (CH 3 OH)] at 1.9 K.The solid lines correspond to the fit using the PHI program.The magnetization fit is the same as fitting with a Brillouin function.