Transition metal coordination compounds with Schiff bases continue to attract the attention of researchers because of the easier synthesis, the stability of the molecular structures and the large diverse properties observed  -  . Indeed catalytic    , optical  , magnetic   , fluorescent   and biological activities     are reported. The complexes derived from the first row of transition metals can give several models of metalloenzymes   and metalloproteins   . Heptacoordinated manganese (II) and cobalt (II) structures have also attracted a great deal of interest from chemists in recent years     . These kind of structures are present in enzymes: catalase   , ribonucleotide manganese reductase  and arginase  . In addition to their interests in biology, these complexes are likely to give very interesting magnetic properties. In the literature, dinuclear compounds of manganese or cobalt which exhibit ferromagnetic or antiferromagnetic interactions are largely reported    . In recent years, we have prepared different symmetrical Schiff base types from the condensation of carbono or thiocarbono hydrazide and aldehydes or ketones which are subsequently used to prepare dinuclear or tetranuclear metal complexes    . In the absence of any particular precautions, the reaction which is a nucleophilic addition of the primary amine on the carbonyl group, proceeds on both arms of the carbonohydrazide. By controlling the conditions of the reaction we prepared a ligand from the monocondensation of carbonohydrazide and acetyl pyridine or pyridine carbaldehyde. The topologies of these dissymmetric ligands lead it to act in tridentate or tetradentate fashions leading fascinating structures. In this context, we report the isolation and complete characterization of this ligand and its Mn(II) and Co(II) complexes in which the metal centers are heptacoordinated as showed by crystal X-ray diffraction analysis. The structure of the formed crystals has been by single X-ray diffraction analysis and the data have been deposited in CCDC Cambridge crystallographic data centre.
2.1. Material and Physical Measurement
Carbonohydrazide (98%), 2-acetylpyridine (99%), 2-pyridinecarboxaldehyde (99%), MnCl2∙4H2O (99%), Co(NO3)2∙6H2O (99.99%) and analytical grade methanol were supplied from Aldrich. All chemicals and solvents were used directly without further purification. Elemental analyses of C, H and N were recorded on a VxRio EL Instrument. The FAB mass spectra were recorded using a Micromass Autospec spectrometer using 3-nitrobenzylalcohol as the matrix. Infrared spectra were obtained on an FTIR Spectrum Two of Perkin Elmer spectrometer in the 4000 - 400 cm−1 region. The UV-Visible spectra was recorded on a Perkin Elmer Lambda UV-Vis spectrophotometer. The molar conductance of 1 × 10−3 M in DMSO solutions of the metal complexes was measured at 25˚C using a WTW LF-330 conductivity meter with a WTW conductivity cell. Room temperature magnetic susceptibilities of the powdered samples were measured using a Johnson Mattey scientific magnetic susceptibility balance (Calibrant: Hg [Co(SCN)4]). The voltammetric measurements were recorded using a Palm Sens3 type potentiostat controlled by PSTrace software. A glassy carbon working electrode with a radius of 2 ± 0.1 mm, a stainless steel wire as a counter-electrode and Ag/AgCl electrode as a reference were used. Cyclic voltammetry experiments were performed in acetonitrile solution 0.1 M of tetrabutylammonium hexafluorophosphate (TBAHFP) as a supporting electrolyte.
2.2. Synthesis of the Ligand 1-(1-(Pyridin-2-yl)Ethylidene)Carbonohydrazide: H4L1
To a solution of carbonohydrazide (3.0 g, 0.333 mmol) in a mixture of 10 mL of distillate water and 30 mL of methanol was added dropwise a solution of 2-acetylpyridine (4.038 g, 0.330 mmol) in 10 mL of methanol. The mixture was stirred under reflux for 4 hours. A white precipitate appears gradually. On cooling, the precipitate was isolated by filtration and successively washed with 2 × 10 mL of hot methanol and dried under P4O10. The filtrated was stored at 4˚COn standing for one week crystals suitable for X-ray analysis were isolated. MP: 222˚C. Yield: 86.4%. Analytical for C8H11N5O: Calc (found) %C = 49.73 (49.43); %H = 5.74 (5.78); %N = 36.25 (36.21). IR (υ, cm−1): 3306; 3086; 1671; 1629; 1578; 1506; 1466, 1141. 1H NMR (dmso-d6, δ (ppm)): 2.36 (s, 3H, -CH3); 4.12 (s, 2H, -NH2); 7.32 - 8.51 (m, 4H, HPy); 8.19 (s, 1H, -(C=O)-NH-NH2); 9.64 (s, 1H, -(C=O)-NH-(C=N)-). 13C NMR (dmso-d6, δ (ppm)):157.32 (CH3-C=N-); 155.30 (C=O); 148.37 (Cipso); 145.45 (CPy); 136.43 (CPy); 123.47 (CPy); 120.13 (CPy); 11.03 (-CH3).
2.3. Synthesis of the Ligand 1-(Pyridin-2-Ylmethylene)Carbonohydrazide: H4L2
To a solution of carbonohydrazide (3.0 g, 0.333 mmol) in a mixture of 10 mL of distillate water and 30 mL of methanol was added dropwise a solution of 2-pyridinecarboxaldehyde (3.5672 g, 0.330 mmol) in 10 mL of methanol. The mixture was stirred at reflux for 4 hours. After cooling to room temperature, the orange solution was stored in the refrigerator and a precipitate appears overnight. The compound was isolated by filtration and washed successively with 2 x 10 mL of cold methanol and 2 × 10 mL of diethyl ether before being dried over P4O10. MP: 165˚C - 167˚C. Yield: 62.1%. Analytical for C7H9N5O: Calc (found) %C = 46.92 (46.90); %H = 5.06 (5.04); %N = 39.09 (39.05). IR (υ, cm−1): 3304; 3197; 1675; 1634; 1609; 1585; 1519; 1494, 1151. 1H NMR (dmso-d6, δ (ppm)): 4.08 (s, 2H, -NH2); 7.30 - 8.52 (m, 4H, HPy); 8.21 (s, 1H, H-C=N-NH-C=O); 10.67 (s, 1H, -(C=O)-NH-NH2-). 13C NMR (dmso-d6, δ (ppm)): 156.27 (CH3-C=N-); 155.500 (C=O); 148.79 (Cipso); 140.46 (CPy); 136.47 (CPy); 123.54 (CPy); 119.50 (CPy) (Scheme 1).
2.4. Synthesis of the Complexes
2.4.1. [Mn2(H4L1)2(H2O)4]∙Cl4 (1)
To a suspension of L1H4 (0.193 g, 1 mmol) in 10 ml of methanol was added MnCl2∙4H2O (0.1979 g, 1 mmol). The resulting yellowish mixture was stirred for 30 min. The resulting yellow suspension was stirred at room temperature during one hour. The suspension was filtered and the filtrate was left at room temperature. On standing for one week crystals suitable for X-ray analysis were isolated.
Scheme 1. Preparation of the ligands and the complexes.
Yield: 74.6 (%). SM (ESI-TOF) (m/z): 194.1 [(H5L)]+; 361 [Mn(H4L)Cl(H2O)3Na]2+; 440 [Mn(H4L)(H3L)]3+; 602.97 [Mn2(H4L)2(Cl)(H2O)4]3+. IR (cm−1): 3276 cm−1; 3186 cm−1; 3142 cm−1; 3070 cm−1; 1658 cm−1; 1595 cm−1; 1557 cm−1; 1470 cm−1; 1362 cm−1; 1185 cm−1; 1154 cm−1; 1027 cm−1; 829 cm−1. UV-vis (Solution, DMF, nm): 219; 357; 421. µeff = 7.72 µB. Λ (Ω−1∙cm2∙mol−1, DMF): 292.
2.4.2. [Co(H4L2)(NO3)(H2O)2]∙(NO3) (2)
To a suspension of L2H4 (0.1792 g, 1 mmol) in 10 ml of methanol was added Co(NO3)2∙6H2O (0.2910 g, 1 mmol) dissolved in 5 mL of methanol. The resulting brown mixture was stirred for 60 min at room temperature. The resulting solution was filtered and the filtrate was left at room temperature. On standing for one week crystals suitable for X-ray analysis were isolated. Yield: 36.8 (%). IR (cm−1): 3227 cm−1; 1637 cm−1; 1578 cm−1; 1540 cm−1; 1455 cm−1; 1402 cm−1; 1380, 1294 cm−1; 1205 cm−1; 1116 cm−1; 1026 cm−1; 936 cm−1. UV-vis (Solution, DMF, nm): 225; 321; 421; 530. µeff = 4.99 µB. Λ (Ω−1∙cm2∙mol−1, DMF): 83.1.
2.5. Crystal Structure Determination and Refinement
Crystals suitable for X-diffraction, of the reported compounds, were grown by slow evaporation of MeOH solution of the complex. Details of the X-rays crystal structure solution and refinement are given in Table 1. Diffraction data were collected using an ENRAF NONIUS Kappa CCD diffractometer with graphite monochromatized MoKα radiation (λ = 0.71073 Å). All data were corrected for Lorentz and polarization effects. No absorption correction was applied. Complex scattering factors were taken from the program package SHELXTL  . The structures were solved by direct methods which revealed the position of all non-hydrogen atoms. All the structures were refined on F2 by a full-matrix least-squares procedure using anisotropic displacement parameters for all non-hydrogen atoms  . The hydrogen atoms of water molecules and NH groups were located in the Fourier difference maps and refined. Others H atoms (CH and CH3 groups) were geometrically optimized and refined as riding model by AFIX instructions. Molecular graphics were generated using ORTEP-3  .
3. Results and Discussion
3.1. General Study
The infrared spectrum of each molecule ligand shows the presence of two bands in the range 3310 cm−1 and 3080 cm−1 attributable to the vibrations υN-H of the ligand. The vibration bands relative to υC=O and υC=N are respectively located at 1671 cm−1 and 1634 cm−1 for H4L1 and 1675 cm−1 and 1639 cm−1 for H4L2. Bands assignable υC=N and υC=C of the pyridine ring are located between 1460 cm−1 and 1590 cm−1  . The vibration of the N-N bond are pointed at ca.1146 cm−1 for both ligands. The 1H NMR spectra of the ligands recorded in dmso-d6 and show similar signals. The difference is due to the presence of a methyl group on the imino carbon atom for L1H4 instead of imino hydrogenatom as for L2H4. The
Table 1. Crystal data and data collection details of the complexes (1) and (2).
presence of a singlet signal incorporating two protons attributable to NH2 at 4.08 ppm for both spectra in indicative of the mono condensation of the carbono hydrazide with the ketoprecursors. The 13C NMR spectra are similar except the presence of a signal at 11.03 ppm relative to the presence of a methyl group in H4L1. The FAB mass spectrometry of the compound [Mn2(H4L1)2(H2O)4]∙Cl4 gave clear evidence of fragmentation of the complex. The parent ion expected at 710.16 amu was not observed. The observed mass/charge ratio of 602.97 amu, 440 amu, 361 amu and 194.10 amu compared to the calculated exact mass of 603.08 amu, 440.12 amu, 360.63 amu and 194.21 amu are respectively due to the fragments [Mn2(H4L)2(Cl)(H2O)4]3+, [Mn(H4L)(H3L)]3+, [Mn(H4L)Cl(H2O)3Na]2+, and [(H5L)]+. Upon coordination the υC=O and the υC=N were strongly shifted to low frequencies in both spectra. For the Mn(II) complex the υC=O and the υC=N are respectively pointed at 1658 cm−1 and 1595 cm−1 while the spectrum of the Co(II) complex show the υC=O and the υC=N band at 1637 cm−1 and 1578 cm−1. These shifts are indicative of the involvement of the carbono oxygen atom and the nitrogen atom of the azomethine in the complexation with Mn(II) and Co(II) in both cases   . In both spectrum broad absorption bands appear at 3276 cm−1 and 3227 cm−1 for the complexes of Mn(II) and Co(II) respectively. These bands are due to the presence of a coordinated water molecule  . Additional bands due to the nitrate moieties appears. The sharp and intense band at 1380 cm−1 is attributed to the free nitrate group. The three absorption bands occurring at 1455 cm−1, 1294 cm−1 and 1026 cm−1 respectively attributable to υ1 (υN=O), υ5 (υasNO2) and υ2 (υsNO2) clearly identify a nitrate group coordinated in bidentate fashion  . In fact the separation Δυ = υ1 –υ5 is used as criterion of differentiation of the different modes of coordination of the nitrate moiety. The Δυ value of 161 cm−1 is in accordance with the presence of bidentate nitrate group  . In the UV-vis spectra of the monohydrazone ligands two bands are observed at 233 and 318 nm for H4L1 and at 235 and 315 nm for H4L2. For both spectra the first band is attributable to the n → π* transitions in aromatic rings while the second band is assignable to the n → π* transition. The electronic spectra of the manganese complex in DMF shows three main bands at 219, 357 and 421 nm and the cobalt (II) complex in DMF exhibits three main bands around 225, 321 and 421 nm and a shoulder around 530 nm. By comparing these bands to the absorption in the spectra of the free ligand, bathochromic shifts is observed for the maxima bands of the free ligand. The band observed at 357 nm for Mn(II) complex and the band pointed at 321 nm for Co(II) complex are assigned to the aromatic ring transition π → π* and the other bands are attributed to the n → π* transitions of C=N chromophore. The values of the conductance of the two complexes in DMF are 192 Ω−1∙cm2∙mol−1 for the manganese (II) complex which is 2:1 electrolyte and 83.1 Ω−1∙cm2∙mol−1 for the cobalt (II) complex which is 1:1 electrolyte in nature  . The magnetic susceptibility measurements of the complexes were performed at room temperature. The value of 7.72 µB for the manganese complex is less than the expected value of ca.10 µB for two Mn(II) ions, indicating the presence of two metal atoms per molecule which are in weak antiferromagnetic coupling interaction. For the complex of cobalt (II) the magnetic moment value of 4.99 µB is in the range expected for a mononuclear hepta coordinated cobalt (II) complex  .
3.2. Description of the Structures
Suitable single-crystals for X-ray diffraction of the Mn(II) and Co(II) complexes were obtained by slow solvent evaporation at room temperature. Crystal data, collection and refinement parameters are listed in Table 1. Selected bond lengths and angles are summarized in Table 2. Hydrogen bond data for complexes 1 and 2 are gathered in Table 3. Figures 1-4 displays the asymmetric unit and the packing diagrams of the two compounds.
Table 2. Selected bond lengths (Å) and angles (˚) for the complexes (1) and (2).
Symmetry code: (i) −x + 1, −y + 1, −z + 1.
Table 3. Hydrogen bonds for the complexes (1) and (2).
Symmetry codes: (i) −x, −y + 1, −z + 1; (ii) x, y + 1, z; (iii) −x + 1, −y + 1, −z; (iv) −x + 1, −y, −z + 1; (v) −x + 1, −y + 1, −z + 1; (vi) −x,1−y,1−z; (vii) 1 − x, −y, 2 − z.
Figure 1. ORTEP plot (30% probability ellipsoids) showing the molecule structure of the complex 1.
Figure 2. Hydrogen bonds in complex 1.
Figure 3. ORTEP plot (30% probability ellipsoids) showing the molecule structure of the complex 2.
Figure 4. Hydrogen bonds in complex 2.
3.2.1. Complex 1
The [Mn2(H4L1)2(H2O)4]∙4Cl complex crystallizes in the triclinic system with the space group P-1. The view of the structure of the complex 1 is depicted in Figure 1. The asymmetric unit contains two manganese (II) atoms, two organic ligand molecules, four coordinated water molecules and four free chloride anions. Each of the ligand molecules acts in tetradentate fashion through one nitrogen atom from the pyridine, one imino nitrogen atom, one nitrogen atom amino and one carbonyl oxygen atom which acts as bridge between the two manganese (II) atoms. Each of the two metal ions Mn(II) is heptacoordinated and the coordination polyhedron of the Mn(II) center is best described as a distorted pentagonal bipyramid with a MnN3O4 chromophore. Each equatorial planes around each Mn(II) atom is occupied by a pyridine nitrogen atom, an imino nitrogen atom, one oxygen atom from the same ligand, one carbonyl bridged oxygen atom and one amino nitrogen atom from another ligand molecule. The apical positions are occupied by two oxygen atoms of the coordinated water molecules. The angles formed between the metal center and the ligand donor atoms in equatorial position are different from the ideal angle for a regular pentagon (72˚) [N5i-Mn1-N4i = 68.80 (4)˚, O1i-Mn1-N4i = 66.56 (4)˚, N5i-Mn1-N1 = 92.20 (4)˚, O1-Mn1-N1 = 67.72 (4)˚, O1-Mn1-O1i = 65.78 (4)˚] with a sum of 361.1˚  . The apical angle formed by the water oxygen atoms in axial positions around the manganese (II) atom [O2w-Mn1-O3w = 169.82 (5)˚] signiﬁcantly deviate from the ideal value of 180˚ (Table 2). The angles Mn1-O1-Mn1i and O1-Mn-O1i are respectively 114.22 (4)˚ and 65.78 (4)˚. For the Mn atom the apical position is occupied by the water oxygen atoms Ow2 and OW3 with 2.1606 (11) Å and 2.1454 (11) Å distance. These distances are shorter than those of the equatorial bonds [Mn1-O1 = 2.2135 (10) Å, Mn1-N5i = 2.2968 (12) Å, Mn1-O1i = 2.3219 (9) Å, Mn1-N4i = 2.3247 (11) Å and Mn1-N1 = 2.5223 (13) Å] (Table 2) and are comparable to those found in similar complex  . The interatomic distance O1-C1 of 1.2433 (16) Å is close proximity to that observed in the structure of the free ligand which is of the order of 1.231 (3) Å. This fact is indicative of double character bond upon complexation with the Mn(II) ion. Each mononuclear Mn(II) molecule complex is connected to another complex molecule by hydrogen bonding interactions leading to the formation of a supramolecular chain structure propagating along the a axis. The chains are connected by hydrogen bonding interaction of type N-H…Cl, C-H…Cl, Ow-H…Cl, and C-H…N. The uncoordinated chloride anion acts via hydrogen bonds as bridge between the chains leading to the formation of a 3D supramolecular structure (Figure 2, Table 3).
3.2.2. Complex 2
The compound [Co(H4LD)(H2O)2(NO3)]∙NO3 crystallizes in the triclinic system with a P-1 space group. The view of the structure of the complex 2 is depicted in Figure 3. The asymmetric unit is composed by a cobalt (II) atom coordinated to one organic ligand molecule, one bidentated coordinated nitrate anion, two coordinated water molecules and one uncoordinated nitrate ion. The ligand acts in tridentate manner through the oxygen atom of the carbonyl moiety, the imino nitrogen atom and the pyridine nitrogen atom. The cobalt ion is situated in a N2O5 core and the environment is best described as pentagonal bipyramid geometry (Figure 3). The equatorial plane around the Co(II) ion is occupied by the three donor atoms from the organic ligand molecule and the two oxygen atoms of the coordinated nitrate group while the apical positions are occupied by the two oxygen atoms of the coordinated water molecules. The angles formed between the equatorial atoms and the Co(II) ion are: N0aa-Co1-O1 = 71.50 (8)˚, N0aa-Co1-N1 = 74.30 (8)˚, N1-Co1-O6 = 55.96 (8)˚, O5-Co1-O1 = 75.71 (8)˚ and are different from the ideal angle value of 72˚ for a pentagonal basal plan  . The angle O4-Co1-O3 formed between the apical atoms and the Co(II) ion deviated from the ideal value of 180˚ with a value of 175.31 (8)˚. The sum of the angles around Co(II) is 360.12˚ and is very close to the ideal sum of 360˚. These deviations are indicative of a distortion of geometry around the cobalt center which is best described as a distorted pentagonal bipyramid. The equatorial distances [Co1-O1 = 2.267 (2) Å, Co1-O5 = 2.212 (2) Å, Co1-O6 = 2.298 (3) Å, Co1-N1 = 2.192 (2) Å and Co1-N0aa = 2.119 (2) Å] are longer that of the axial distances [Co1-O3 = 2.104 (2) Å, Co1-O4 = 2.092 (2) Å] (Table 2) and are comparable to the values reported for a seven coordinated cobalt(II) complex  .
In the crystal, the complex molecules are linked by hydrogen bonds, giving rise to a three-dimensional network (Figure 4, Table 3). The structure is built up from pentagonal bipyramids around the Co(II) atom, which are assembled in layers parallel to the bc plane. These layers are interconnected by hydrogen bonds. The coordinating axial water molecule points into the interlayer space and acts as a hydrogen-bond donor towards oxygen atom O-NO2 of the coordinated nitrate (Figure 4) O1W-H1WB…ONO2. Within a layer, the molecules are not interconnected by hydrogen bonds.
3.2.3. Electrochemistry Study [Mn2(H4L)2(H2O)2]∙4Cl
The electrochemical properties of the Mn(II) complex was studied by cyclic voltammeter using a graphite carbon working electrode and a Pt-wire auxiliary electrode in acetonitrile using LiClO4 a the supporting electrolyte. The voltammetric parameters were studied in the scan rate interval 100 - 200 mVs−1. In Figure 5 describes the electrochemical behavior of the complex in acetonitrile medium. The cyclic votammogram of the complex [Mn2(H4L)2(H2O)2]∙4Cl was recorded at −3.00 to +3.00 V (vs Ag/AgCl) with a scan rate of 100 mV/s in DMF solution at room temperature. We note one oxidation and two reduction pics and two oxidation (Figure 5). The cathodic wave shows two distinct reductive peaks at ca. −0.9 v and −1.3 V (ΔE = 0.4 V) which correspond to two electrons charge
Figure 5. Cyclic Votammograms of the complex in CH3CN containing 0.1 M [NBu4]∙[PF6]. Working electrode: a 5 mm diameter of graphitecarbon electrode, scan rate: 100 m∙Vs−1.
Figure 6. Cyclic Votammograms of the complex in CH3CN containing 0.1 M [NBu4]∙[PF6] Scan rate: 25, 150, 100, 200, 300 and 500 mVs−1. Working electrode: a 2 mm diameter of graphite carbon electrode.
transfer process Mn(II)Mn(II) → Mn(II)Mn(I) and Mn(II)Mn(I) → Mn(I)Mn(I). The anodic wave shows two oxidative peaks −1.00 and +1.2 V corresponding to the reversible process Mn(I)Mn(I) → Mn(II)Mn(I) and Mn(II)Mn(I) → Mn(II)Mn(II). A reversible transition Mn(II) → Mn(I) had already been observed for a Mn(II) complex in acetonitrile media  . By varying the scans speed the intensities of all the peaks increase with the scanning speed (Figure 6).
We describe here one seven coordinated manganese (II) complexes of the tetradentate ligand 1-(1-(pyridin-2-yl)ethylidene)carbonohydrazide and one seven coordinated cobalt(II) of the tridentate 1-(pyridin-2-ylmethylene)carbonohydrazide. The single crystal structure of the Mn(II) complex 1 as well as of the Co(II) complex 2 has revealed a distorted pentagonal-bipyramidalgeometry. The equatorial plane in complex 1 is occupied by N2O and NO from two tetradentate ligand molecules acting as bridges between two Mn(II) ions. The axial positions are occupied by two coordinated water molecules. In the complex 2, the equatorial plane of the bipyramid is occupied by the tridentate N2O ligand and two oxygen atoms from the coordinated nitrate anion which acts in bidentate chelating fashion, the two apical positions being occupied by two coordinated water molecules. The cyclic voltammetry studies in CH3CN show that the manganese (II) complex 2 shows two electrons charge transfer process.
The authors thank the FONDATION SONATEL for his financial support http://fondationsonatel.sn/.
CCDC-1944387 and 1944386 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/, or by e-mailing firstname.lastname@example.org, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44(0)1223-336033.