Reaction of manganese (II) thiocyanate with 2,5-dimethylpyrazine leads to the formation of three new coordination compounds of compositions Mn(NCS)2(2,5-dimethylpyrazine)2(H2O)2 (1), Mn(NCS)2(H2O)2(MeOH)2-tris(2,5-dimethylpyrazine) solvate (2), and Mn(NCS)2 (H2O)4-tetrakis(2,5-dimethylpyrazine) solvate (3) that were characterized by single crystal X-ray diffraction. In their crystal structures, the Mn(II) cations are sixfold coordinated by two terminally N-bonded thiocyanato anions and two water molecules as well as by two 2,5-dimethylpyrazine ligands (1), two ethanol (2), or two water molecules (3), within slightly distorted octahedra. In compounds 2 and 3, additional 2,5-dimethylpyrazine ligands are located in the cavities of the structures as solvate molecules. X-ray powder diffraction has shown that compounds 1 and 2 cannot be prepared as pure phases and that batches of compound 3 contain only minor traces of a contamination. Differential thermoanalysis and thermogravimetry have revealed that upon heating of compound 3, an intermediate of composition Mn(NCS)2(2,5-dimethylpyrazine) (4) is formed, which cannot be obtained from solution. To mimic the structure of 4, single crystals of a Cd compound of the same composition (5) were prepared from the liquid phase, and single crystal X-ray analysis has shown that it is isotypic to 4.
Investigations on the synthesis, structures, and magnetic properties of new coordination polymers are still a major activity in coordination chemistry [1–11]. In this context, compounds in which paramagnetic metal cations are linked by “small-sized” anionic ligands that can mediate magnetic exchange are of special interest. Even if most of such compounds are based on, e.g. carboxylates or azides, thio- or selenocyanato coordination compounds are also of interest because in dependence on the coordination mode of the anionic ligand and the topology of the coordination network and they can show a variety of different magnetic properties [12–32].
In our own investigations, we are especially interested in compounds in which the paramagnetic metal cations are coordinated additionally by N-donor co-ligands and are linked by pairs of thiocyanato anions into chains because, depending on the nature of the metal cation and the organic co-ligand, different magnetic properties including a slow relaxation of the magnetization are observed [33–40]. Because the terminal coordination of the anionic ligands is energetically favored, the compounds with bridging thiocyanato ligands are more difficult to prepare. In these cases, the compounds with a bridging coordination can sometimes be obtained by thermal decomposition of simple discrete complexes of compositions Mn(NCS)2(L1)4 or Mn(NCS)2(L1)2(L2)2 (L1 = N-donor co-ligand; L2 = O-donor co-ligand as, e.g. H2O, MeOH, EtOH), in which the metal cations are coordinated by terminally N-bonded anionic ligands.
In this context, we became interested in 2,5-dimethylpyrazine as a co-ligand, and we investigated if new Mn(II) coordination polymers with μ-1,3-bridging thiocyanato anions are directly available from solution or if simple precursors can be obtained that might transform into the desired compounds by simple heating. Only one structure of a Mn coordination compound with this ligand has previously been reported, in which the co-ligand is located in the cavities of the structure and thus does not participate in metal coordination . Here we report on the results of our investigations.
2 Results and discussion
2.1 Crystal and molecular structures
If Co(NCS)2 is reacted with 2,5-dimethylpyrazine in water, methanol, or ethanol, single crystals of three new compounds of compositions Mn(NCS)2(2,5-dimethylpyrazine)2(H2O)2 (1), Mn(NCS)2(H2O)2(MeOH)2-tris (2,5-dimethylpyrazine) solvate (2), and Mn(NCS)2 (H2O)4-tris(2,5-dimethylpyrazine) solvate (3) are obtained.
2.1.1 Crystal and molecular structure of Mn(NCS)2 (2,5-dimethylpyrazine)2(H2O)2 (1)
Compound 1 crystallizes in the monoclinic space group C2/c with four formula units in the unit cell. The asymmetric unit consists of one manganese (II) cation located on a twofold rotation axis and one thiocyanato anion, one water molecule, and one 2,5-dimethylpyrazine ligand in general positions (Fig. 1a). The manganese (II) cations are coordinated by two N-bonded thiocyanato anions, two water ligands, and two 2,5-dimethylpyrazine ligands in a slightly distorted octahedral geometry forming discrete complexes. The Mn–N distances range from 2.1382(12) to 2.4000(10) Å, whereas the Mn–O distances amount to 2.2162(10) Å (Table 1). The bond angles range from 79.60(6) to 102.98(4)° as well as from 165.42 to 166.56(5)°. In the crystal structure, the discrete complexes are linked by intermolecular O–H···N hydrogen bonding between the water H atoms and the non-coordinating N atom of the 2,5-dimethylpyrazine ligand of a neighboring complex into layers that are parallel to the bc plane (Fig. 1b and Table 2).
|Mn–N (NCS)||Mn–N (co-ligand)||Mn–O|
Within these layers pairs of 2,5-dimethylpyrazine, ligands are stacked into dimers that are located around centers of inversion, which is indicative of π···π interactions. These layers are additionally linked by intermolecular O–H···S hydrogen bonds between the second water H atoms and thiocyanato S atoms into a three-dimensional network (Fig. 1b and Table 2).
2.1.2 Crystal and molecular structure of the Mn(NCS)2(H2O)2(MeOH)2 tris(2,5-dimethylpyrazine) solvate (2)
Compound 2 crystallizes in the triclinic space group P1̅ with two formula units in the unit cell. The asymmetric unit consists of one Mn(II) cation two water, two methanol molecules and two thiocyanato anions as well as two 2,5-dimethylpyrazine ligands in general positions. One additional 2,5-dimethylpyrazine ligand is located around a center of inversion (Fig. 2a).
The Mn(II) cations are coordinated by two thiocyanato anions, two water, and two methanol molecules in a slightly distorted octahedral geometry. The Mn–N distances are in the range of 2.1541(15) to 2.1940(14) Å and the Mn–O distances ranges from 2.1409(12) to 2.2366(12) Å (Table 1). The bond angles are between 83.18(5) and 99.89(6)° and between 170.46 and 170.99(5)°, respectively.
The discrete complexes are connected by intermolecular O–H···S hydrogen bonds between one water H atom and the thiocyanato S atom into chains that elongate in the direction of the crystallographic a axis (Fig. 2b and Table 2). There are cavities in the structure where uncoordinated 2,5-dimethylpyrazine ligands are located. These molecules are involved in intermolecular O–H···N hydrogen bonding between the water and the methanol O–H H atoms and the 2,5-dimethlypyrazine N atoms connecting the discrete complexes into a three-dimensional hydrogen-bonded network (Fig. 2b and Table 2).
2.1.3 Crystal and molecular structure of the Mn(NCS)2(H2O)4 tetrakis(2,5-dimethylpyrazine) solvate (3)
Compound 3 crystallizes in the triclinic space group P1̅ with one formula unit in the unit cell. In the crystal structure, two thiocyanato ligands and four water molecules are coordinated to the Mn(II) cations, forming a slightly distorted octahedron (Fig. 3a). The Mn–N distances amount to 2.2029(13) Å, and the Mn–O distances are 2.1740(11) and 2.2246(11) Å, with bond angles ranging from 87.67(4) to 92.33(4)° as well as 180° (Table 1). The asymmetric unit consists of a Mn(II) cation that is located on a center of inversion and two water ligands as well as two thiocyanato anions that occupy general positions. There are three additional and uncoordinated 2,5-dimethylpyrazine molecules, of which two are located around centers of inversion (Fig. 3a).
In contrast to compounds 1 and 2, no intermolecular O–H···S hydrogen bonds are observed. As in compound 1, there are O–H···N hydrogen bonds between the water H atoms and the 2,5-dimethylpyrazine N atoms to give chains in the direction of the crystallographic b axis (Fig. 3b). Within these chains, centrosymmetric 2,5-dimethylpyrazine dimers are observed with parallel orientation, indicative of π···π interactions (Fig. 3b). These chains are linked by additional O–H···N hydrogen bonds of 2,5-dimethylpyrazine molecules into a three-dimensional network. The 2,5-dimethylpyrazine ligands that are involved in this interaction form columns that elongate in the direction of the crystallographic a axis (Fig. 3b).
It is noteworthy that in two of the three structures, the 2,5-dimethylpyrazine ligands do not participate in the metal coordination and that they only act as solvate molecules to form a dense packing. A similar situation is found in the Mn coordination compound mentioned in the introduction, which indicates that 2,5-dimethylpyrazine is a weakly coordinating ligand . Coordination to the N atoms is hindered because of the neighboring bulky methyl groups.
Based on single crystal data, X-ray powder diffraction (XRPD) patterns have been calculated and compared with experimental patterns. Despite much effort to obtain pure phases, results show that none of the compounds obtained were pure phases. Additional solvates containing different amounts of solvate molecules formed. However, one batch of compound 3 was obtained that contains only a very small amount of an additional crystalline phase that cannot be identified, and this batch was used for further investigations (Fig. 4).
2.2 Thermoanalytical measurements
In all of our investigations, we have found no hints of the formation of a compound in which the metal cations are linked by the anionic ligands into chains. Therefore, we have investigated compound 3 by simultaneous differential thermoanalysis and thermogravimetry (DTA-TG). It is to be expected that not all of the water molecules and the 2,5-dimethylpyrazine ligands are removed in one step and that intermediates will form, which might correspond to the desired compounds. In this context, it must be kept in mind that even if compound 3 contains a small amount of a contamination, it can be assumed that this phase also consists of a solvate that will lose the solvate molecules on heating and that presumably transforms into the same co-ligand-deficient compound.
DTA-TG of compound 3 shows three mass steps in the TG curve that are accompanied with endothermic events in the DTA curve (Fig. 5). The experimental mass loss of 56.3 % in the first step is in reasonable agreement with that calculated for the removal of all of the water molecules and three of the 2,5-dimethylpyrazine ligands (Δmcalcd. = 58.7 %). Therefore, it can be expected that in the first TG step, a compound of composition Mn(NCS)2(2,5-dimethylpyrazine) (4) is formed, in which the metal cations must be linked by bridging anionic ligands. In the second TG step, the remaining 2,5-dimethylpyrazine molecules are removed, and on further heating, decomposition of Mn(NCS)2 is observed (Fig. 5). A closer look onto the DTG curve indicates that the first TG step consists of additional steps, but heating rate-dependent TG measurements have shown that these events cannot be resolved.
To investigate the compound formed in the first step, a second TG run was performed in which this intermediate was isolated and investigated by infrared (IR) spectroscopy (Fig. 6). For this compound, the asymmetric CN stretching vibration is observed at 2095 cm−1 and is therefore shifted to significantly higher values compared with the precursor. In compound 3, this vibration is observed at 2063 cm−1, which is exactly in the range expected for terminally N-bonded thiocyanato anions [42–44]. In contrast, a value of 2095 cm−1 is a clear indication that μ-1,3-bridging thiocyanato anions are present [42–44].
However, because the structure of compound 4 is unknown, we used an alternative procedure to extract structural information. In a previous work, we have shown that corresponding compounds based on cadmium(II) can be prepared . Because this cation is much more chalcophilic, the compounds with bridging thiocyanato anions are more stable and can easily be crystallized from solution, even if an excess of the co-ligand is used. In several cases, they are isotypic to their paramagnetic counterparts and their structures can be determined by XRPD.
Crystals of a compound of the composition Cd(NCS)2(2,5-dimethylpyrazine) (5) can easily be obtained by the reaction of Cd(NCS)2 and 2,5-dimethylpyrazine. Its structure has been determined by single crystal X-ray diffraction.
2.2.1 Crystal and molecular structure of Cd(NCS)2 (2,5-dimethylpyrazine) (5)
The Cd compound 5 crystallizes in the monoclinic space group P21/c with two formula units in the cell. The Cd(II) cations are coordinated by four thiocyanato anions as well as two 2,5-dimethylpyrazine ligands forming an octahedron with a slightly distorted geometry (Fig. 7a). The bond lengths around the cadmium center are in range of 2.299(3) to 2.508(2) Å for the Cd–N and 2.6696(11) Å for the Cd–S. The bond angles are between 85.45(9) and 94.55(9)° and at 180°.
The asymmetric unit consists of a Cd(II) cation and a 2,5-dimethylpyrazine ligand that are located on centers of inversion and a thiocyanato anion in a general position. The Cd(II) cations are linked by μ-1,3-bridging thiocyanato anions to four different Cd cations forming layers that are located in the bc plane (Fig. 7b). These layers are further linked by μ-1,4-bridging 2,5-dimethylpyrazine ligands forming a three-dimensional coordination network (Fig. 7c). In contrast to the structure of compounds 1–3, neighboring 2,5-dimethylpyrazine ligands are oriented perpendicularly to another. It is noted that a compound previously investigated in our group also shows a similar thiocyanato coordination network .
Based on the structural data of compound 5, the XRPD pattern was calculated and compared with that measured for compound 4, which clearly shows that both compounds are isotypic (Fig. 8, top). To additionally prove the identity of compound 4, a Pawley fit was made, from which the unit cell parameters were extracted. Like the Cd compound 5, the Mn compound 4 crystallizes in space group P21/c with a = 7.6179(3), b = 9.6011(5), c = 7.8762(3) Å, β = 112.101(3)°, and V = 533.76(4) Å3. From the difference curve, it is indicated that a pure material might have been obtained. Finally, in the IR spectra of compound 5, the asymmetric CN stretching vibration is observed at 2104 cm−1, which is in the range observed for the Mn intermediate 4 (Fig. 6).
The new Mn compound was also investigated by magnetic susceptibility measurements. The susceptibility continuously increases with decreasing temperature, showing only paramagnetic behavior. The χTversusT curve decreases, indicating dominating antiferromagnetic interactions. Several batches were measured, and according to our measurements, some of them are contaminated with a very small amount of an additional crystalline phase that also shows paramagnetic behavior. However, the occurrence of only paramagnetism is not surprising because other Mn thiocyanato coordination polymers with μ-1,3-bridging anionic ligands show either paramagnetic behavior or antiferromagnetic ordering .
In the present work, new manganese (II) thiocyanato coordination compounds with 2,5-dimethylpyrazine were prepared with the major goal of obtaining compounds in which the Mn cations are linked by the anionic ligands into Mn(NCS)2 chains. Independent of molar stoichiometry and the solvent used, compounds with terminally N-bonded co-ligands were constantly obtained, and in most, the 2,5-dimethylpyrazine molecules do not participate in metal coordination. The N coordination is hindered because of the methyl groups next to the N atom. However, if one of these compounds is heated, a transformation into a new compound of composition Mn(NCS)2(2,5-dimethylpyrazine) is obtained, which is isotypic to the corresponding Cd compound prepared in this work. This shows the importance of the diamagnetic analogues to retrieve structural information on their paramagnetic counterparts. In the crystal structure of the new Mn compound, the metal cations are linked by μ-1,3 bridging thiocyanato anions, and therefore, this coordination corresponds to the desired one. Even if this new phase cannot be obtained in pure form, it demonstrates the potential of thermal decomposition reactions for the synthesis of new coordination polymers that cannot be obtained from the solution phase.
4 Experimental section
4.1 Syntheses of Mn(NCS)2 and Cd(NCS)2
MnSO4·H2O, Cd(SO)4·8/3H2O, and 2,5-dimethylpyrazine were obtained from Merck and Ba(NCS)2 ·3H2O was obtained from Alfa Aesar. Mn(NCS)2 and Cd(NCS)2 were prepared by the reaction of equimolar amounts of MnSO4·H2O and Ba(NCS)2·3H2O in water. The resulting white precipitate of BaSO4 was filtered off, and the filtrate was concentrated to complete dryness, resulting in a white residue of Mn(NCS)2. Cd(NCS)2 was prepared by the reaction of equimolar amounts of CdSO4·8/3H2O and Ba(NCS)2·3H2O in water. The resulting white precipitate of BaSO4 was filtered off, and the filtrate was concentrated to complete dryness, resulting in a white residue of Cd(NCS)2. The purity was proven by XRPD and elemental analysis.
4.1.1 Preparation of Mn(NCS)2(2,5-dimethylpyrazine)2 (H2O)2 (1)
Single crystals suitable for X-ray diffraction were obtained by the reaction of Mn(NCS)2 (25 mg, 0.15 mmol) and 2,5-dimethylpyrazine (33 μL, 0.30 mmol) in 1 mL acetonitrile. Block-shaped single crystals were obtained after a few weeks, but XRPD showed that no pure sample was obtained.
4.1.2 Preparation of Mn(NCS)2(H2O)2(MeOH)2-tris (2,5-dimethylpyrazine) solvate (2)
Single crystals suitable for X-ray diffraction were obtained by the reaction of Mn(NCS)2 (25 mg, 0.15 mmol) and 2,5-dimethylpyrazine (66 μL, 0.60 mmol) in 1 mL methanol. Block-shaped single crystals were obtained after a few weeks, but XRPD showed that no pure sample was obtained.
4.1.3 Preparation of Mn(NCS)2(H2O)4 tetrakis (2,5-dimethylpyrazine) solvate (3)
Single crystals suitable for X-ray diffraction were obtained by the reaction of Mn(NCS)2·4H2O (60 mg, 0.25 mmol) and 2,5-dimethylpyrazine (108 μL, 1.00 mmol) in 1 mL water. Block-shaped single crystals were obtained after a few weeks. A beige crystalline powder was obtained by reacting Mn(NCS)2·4H2O (159 mg, 0.5 mmol) and 2,5-dimethylpyrazine (216 μL, 2.00 mmol) in 1 mL water. XRPD showed that this compound is nearly phase-pure and contains only a very small amount of a different crystalline phase that does not correspond to compounds 1 or 2. – C26H40MnN10O4S2: calcd. C 46.21, H 5.97, N 20.73, S 9.49; found C 47.47, H 6.44, N 21.81, S 11.07 %.
4.1.4 Preparation Cd(NCS)2(2,5-dimethylpyrazine) (5)
Single crystals suitable for X-ray diffraction were obtained by the reaction of Cd(NCS)2 (34 mg, 0.15 mmol) and 2,5-dimethylpyrazine (65 μL, 0.60 mmol) in 1 mL water. Block-shaped single crystals were obtained after a few weeks. A white crystalline powder was obtained by reacting CdCl2 (159 mg, 0.5 mmol) with KNCS (87 mg, 0.9 mmol) and 2,5-dimethylpyrazine (192 μL, 1.75 mmol) in 1 mL water. – C8H8CdN4S2: calcd. C 28.54, H 2.39, N 16.64, S 19.05; found C 28.65, H 2.28, N 16.60, S 19.37 %.
4.2 Elemental analysis
CHNS analysis was performed using a Euro EA elemental analyzer manufactured by Euro Vector Instruments and Software.
4.3 Differential thermal analysis and thermogravimetry
The heating-rate-dependent DTA-TG measurements were performed in a nitrogen atmosphere (purity, 5.0) in Al2O3 crucibles using an STA-409CD instrument from Netzsch. All measurements were performed with a flow rate of 75 mL/min and were corrected for buoyancy and current effects. The instrument was calibrated using standard reference materials.
4.4 Single-crystal structure determinations
Data collections were carried out on an imaging plate diffraction system: Stoe IPDS-1 and Stoe IPDS-2 with MoKα radiation. The structures were solved with Direct Methods using Shelxs-97, and structure refinements were performed against F2 using Shelxl-97 . Numerical absorption correction was applied using the programs X-Red and X-Shape of the program package X-Area [47–49]. All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were positioned with idealized geometry and were refined isotropically with Uiso(H) = −1.2 Ueq(C) (−1.5 for methyl H atoms) using a riding model with C–H = 0.95 Å for 2 and 3 and C–H = 0.93 Å for 1 and 5 for aromatic and C–H = 0.98 Å for 2 and 3 and C–H = 0.96 Å for 1 and 5 for methyl H atoms. The methyl H atoms were allowed to rotate but not to tip. The O–H hydrogen atoms were located in a difference map, their bond lengths were set to ideal values, and finally, they were refined isotropically with Uiso(H) = −1.5 Ueq(O) using a riding model. Details of the structure determination are given in Table 3.
|Refl. with Fo > 4 σ(Fo)||2165||5092||3386||967|
|R1a [Fo > 4 σ(Fo)]||0.0258||0.0369||0.0353||0.0240|
|wR2b (all data)||0.0691||0.0963||0.0896||0.0503|
aR1 = Σ||Fo| − |Fc||/Σ|Fo|; bwR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2, w = [σ2(Fo2) + (AP)2 + BP]−1, where P = (max(Fo2, 0) + 2Fc2)/3; c GoF = S = [Σw(Fo2 − Fc2)2/(nobs − nparam)]1/2.
CCDC 1433519 (5), 1433520 (1), 1433521 (2), and 1433522 (3) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre viahttp://www.ccdc.cam.ac.uk/data_request/cif.
The IR spectra were measured using an ATI Mattson Genesis Series FTIR Spectrometer with Winfirst software from ATI Mattson.
4.6 X-ray powder diffraction
The measurements were performed using (1) a PANalytical X’Pert Pro MPD Reflection Powder Diffraction System with CuKα radiation (λ = 154.0598 pm) equipped with a PIXcel semiconductor detector from PANanlytical and (2) a STOE Transmission Powder Diffraction System (Stadi P) with CuKα radiation that was equipped with a Mythen K1000 detector from STOE & Cie.
Dedicated to: Professor Wolfgang Jeitschko on the occasion of his 80th birthday.
This work was supported by the state of Schleswig-Holstein. We also thank Prof. Dr. Wolfgang Bensch for access to his experimental facilities.
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