Synthesis, Structures and Properties of Cobalt Thiocyanate Coordination Compounds with 4-(hydroxymethyl)pyridine as Co-ligand

Reaction of Co(NCS) 2 with 4-(hydroxymethyl)pyridine (hmpy) leads to the formation of six new coordination compounds with the composition [Co(NCS) 2 (hmpy)) 4 ] (1), [Co(NCS) 2 (hmpy) 4 ] ˆ H 2 O (1-H 2 O), [Co(NCS) 2 (hmpy) 2 (EtOH) 2 ] (2), [Co(NCS) 2 (hmpy) 2 (H 2 O) 2 ] (3), [Co(NCS) 2 (hmpy) 2 ] n ¨4 H 2 O (4) and [Co(NCS) 2 (hmpy) 2 ] n (5). They were characterized by single crystal and powder X-ray diffraction experiments, thermal and elemental analysis, IR and magnetic measurements. Compound 1 and 1-H 2 O form discrete complexes, in which the Co(II) cations are octahedrally coordinated by two terminal thiocyanato anions and four 4-(hydroxymethyl)pyridine ligands. Discrete complexes were also observed for compounds 2 and 3 where two of the hmpy ligands were substituted by solvent, either water (3) or ethanol (2). In contrast, in compounds 4 and 5, the Co(II) cations are linked into chains by bridging 4-(hydroxymethyl)pyridine ligands. The phase purity was checked with X-ray powder diffraction. Thermogravimetric measurements showed that compound 3 transforms into 5 upon heating, whereas the back transformation occurs upon resolvation. Magnetic measurements did not show any magnetic exchange via the hmpy ligand for compound 5.

Metal thiocyanates with 4-(hydroxymethyl)pyridine (hmpy) are of particular interest as hmpy is known to act as terminal ligand, coordinating mainly via the pyridine N atom to the metal centers.
It is noted, that a few coordination compounds with this ligand were recently reported, which showed only coordination by the pyridine N atom except for two different metal complexes [48][49][50][51][52][53].
Co(II) thiocyanate coordination compounds are of general interest because whenever Co(II) cations form polymeric chains with thiocyanato anions and are additionally coordinated by terminal N-bonded co-ligands, a slow relaxation of the magnetization might be observed, which can be traced back to the relaxation of single chains [44][45][46][47].In this context, it is noted that for [Cd(N 3 ) 2 (4-(hydroxymethyl)pyridine) 2 ] n a crystal structure is found that is close to that of our desired compound [52].In this compound, the Cd cations are linked by alternating µ-1,3 and µ-1,1 anionic azide anions into chains and are additionally coordinated by only N-bonded 4-(hydroxymethyl)pyridine ligands.However, a µ-1,1 coordination of the thiocyanate anions is very rare and therefore, for the desired compound, if it exists, one would expect alternating chains of only µ-1,3 bridging anionic ligands.Therefore, Co(NCS) 2 was reacted in different molar ratios with 4-(hydroxymethyl)pyridine in several solvents.Five different coordination complexes were obtained, which were characterized by single crystal and X-ray powder diffraction (XRPD), thermal analysis, magnetic measurements and IR spectroscopy.

Crystal Structures
2.2.1.Co(NCS) 2 (4-(hydroxymethyl)pyridine) 4 (1) and Co(NCS) 2 (4-(hydroxymethyl) Compound 1 crystallizes in the orthorhombic space group P2 1 2 1 2 1 with four formula units in the unit cell (Table 1).The asymmetric unit consists of one cobalt cation, two thiocyanate anions and four 4-(hydroxymethyl)pyridine ligands lying on general positions (Figure 1a and Figure S1 in the Supplemental Material).The CoN 6 octahedra are slightly distorted and the metal nitrogen distances are in the range of 2.099(3) to 2.179(2) Å and the angles are in the range of 88.06 (9) to 92.66 (9) ˝and from 176.27 (9) to 178.42 (10) ˝(Table S1 in the Supplementary Material).The discrete complexes are connected by intermolecular O-H¨¨¨S hydrogen bonds between the H atom of the hydroxyl group and the thiocyanato S atom into layers that are parallel to the ab plane (Figure 1b and Table S2 in the Supplementary Material).
For compound 1-H 2 O, no single crystals were obtained.The powder pattern of this compound is similar to the one from the corresponding Ni compound, which was recently reported [53].Rietveld analysis of 1-H 2 O (see Experimental Section, Table 1 and Figure S2 in the Supplementary Material) confirmed that both complexes are isostructural.1-H 2 O crystallizes in the cubic space group Pn-3n with six formula units in the unit cell.The crystal structure consists, similar to compound 1, of discrete complexes, in which the Co(II) cations are coordinated by two N-bonded thiocyanate anions and four 4-hydroxypyridine ligands in an octahedral fashion (Figure S3 in the Supplementary Material).The crystal packing led to the formation of voids in which additional water is embedded.1-H 2 O contains roughly 2 water molecules per sum formula, determined from XRPD.The water shows a similar disorder as in the isostructural Ni-complex (two orientations, both on special positions, threefold rotoinversion and three-fold axis, respectively).The presence of water was confirmed by thermogravimetric analysis.special positions, threefold rotoinversion and three-fold axis, respectively).The presence of water was confirmed by thermogravimetric analysis.Compound 2 crystallizes in the monoclinic space group C2/c with four formula units in the unit cell.The Co(II) cations are located on two-fold rotation axes, whereas complex 3 crystallizes in the orthorhombic space group Pbca with Z = 4 and the cobalt cations located on inversion centers (Table 1).
Although compounds 2 and 3 show a six-fold coordination of the cations, the coordination is different: In compound 2 the anionic ligands are trans coordinated, whereas the N-donor co-ligands and the ethanol molecules are cis coordinated (Figure 2a).In contrast, in compound 3 all ligands are trans coordinated, which is somewhat surprising because the ethanol molecules might occupy more space than water molecules (Figure 3b).The cobalt nitrogen distances in compound 2 are in the range of 2.078(3) to 2.158(2) Å while the cobalt oxygen distances are around 2.078(3) Å with angles ranging from 86.26 (12) to 91.90(10)° and from 175.51 (9) to 176.8(10)° (Table S3 in the Supplementary Material).For compound 3, the cobalt nitrogen distances are in the range of 2.085(3) Å to 2.180(3) Å and the cobalt oxygen distances are 2.101(2) Å with bonding angles in the range of 86.80 (10) to 91.20 (10)° and of 180 (Table S3 in the Supplementary Material).
In  Compound 2 crystallizes in the monoclinic space group C2/c with four formula units in the unit cell.The Co(II) cations are located on two-fold rotation axes, whereas complex 3 crystallizes in the orthorhombic space group Pbca with Z = 4 and the cobalt cations located on inversion centers (Table 1).
Although compounds 2 and 3 show a six-fold coordination of the cations, the coordination is different: In compound 2 the anionic ligands are trans coordinated, whereas the N-donor co-ligands and the ethanol molecules are cis coordinated (Figure 2a).In contrast, in compound 3 all ligands are trans coordinated, which is somewhat surprising because the ethanol molecules might occupy more space than water molecules (Figure 3b).The cobalt nitrogen distances in compound 2 are in the range of 2.078(3) to 2.158(2) Å while the cobalt oxygen distances are around 2.078(3) Å with angles ranging from 86.26 (12) to 91.90 (10) ˝and from 175.51(9) to 176.8(10) ˝(Table S3 in the Supplementary Material).For compound 3, the cobalt nitrogen distances are in the range of 2.085(3) Å to 2.180(3) Å and the cobalt oxygen distances are 2.101(2) Å with bonding angles in the range of 86.80 (10) to 91.20 (10) ˝and of 180 (Table S3 in the Supplementary Material).
In compound 2 the discrete complexes are connected by intermolecular O-H¨¨¨S hydrogen bonding between the H atom of the methanol group of the 4-(hydroxymethyl)pyridine ligand and the thiocyanato S atom of a neighbored complex into chains along the crystallographic b axis.These chains are further connected into layers by intermolecular O-H¨¨¨O hydrogen bonding between the hydroxyl H atom of the ethanol molecules of one complex and the hydroxyl O atom of a neighboring complex (Figure 3a and Table S4 in the Supplementary Material).hydroxyl H atom of the ethanol molecules of one complex and the hydroxyl O atom of a neighboring complex (Figure 3a and Table S4 in the Supplementary Material).
In compound 3, the complexes are connected by intermolecular O-H⋅⋅⋅O hydrogen bonding between the methanol group of the 4-(hydroxymethyl)pyridine ligand and the water molecule into chains, that are further linked into layers by additional intermolecular O-H⋅⋅⋅S hydrogen bonding between the H atoms of the water molecules and the thiocyanate S atoms of neighbored complexes (Figure 3: bottom and Table S4 in the Supplementary Material).

[Co(NCS)2(4-(hydroxymethyl)pyridine)2]n•4 H2O (4)
Compound 4 crystallizes in the triclinic space group 1 P with 1 formula unit in the unit cell (Table 1).The Co(II) cation lies on a special position and is octahedrally coordinated by four µ-1,6 bridging 4-(hydroxymethyl)pyridine ligands and two terminally N bonded thiocyanate anions (Figure 4a and Figure S6 in the Supplementary Material).The cobalt nitrogen distances are in the range of 2.078(2) to 2.1603(18) Å and the cobalt oxygen distances amount to 2.1412(15) Å with bonding angles in the range of 88.18 (7) to 91.82 (7)° and 180° (Table S5 in the Supplementary Material).The crystal structure consists of four water molecules per sum formula, located between the 1D polymers (Figure 4).In compound 3, the complexes are connected by intermolecular O-H¨¨¨O hydrogen bonding between the methanol group of the 4-(hydroxymethyl)pyridine ligand and the water molecule into chains, that are further linked into layers by additional intermolecular O-H¨¨¨S hydrogen bonding between the H atoms of the water molecules and the thiocyanate S atoms of neighbored complexes (Figure 3: bottom and Table S4 in the Supplementary Material).

[Co
Compound 4 crystallizes in the triclinic space group P1 with 1 formula unit in the unit cell (Table 1).The Co(II) cation lies on a special position and is octahedrally coordinated by four µ-1,6 bridging 4-(hydroxymethyl)pyridine ligands and two terminally N bonded thiocyanate anions (Figure 4a and Figure S6 in the Supplementary Material).The cobalt nitrogen distances are in the range of 2.078(2) to 2.1603(18) Å and the cobalt oxygen distances amount to 2.1412(15) Å with bonding angles in the range of 88.18 (7) to 91.82 (7) ˝and 180 ˝(Table S5 in the Supplementary Material).The crystal structure consists of four water molecules per sum formula, located between the 1D polymers (Figure 4).
The Co(II) cations are linked by pairs of the 4-(hydroxymethyl)pyridine ligands into chains that elongate in the direction of the crystallographic a axis (Figure 4a).These chains are further linked via hydrogen bonding to the solvate water molecules.Intermolecular O-H¨¨¨S hydrogen bonds are observed between H atoms from water molecules or from the hydroxyl group of the 4-(hydroxymethyl)pyridine ligand and the thiocyanate S atoms of neighboring chains (Figure 4b).The water molecules are also linked to the hydroxyl group by intermolecular O-H¨¨¨O hydrogen bonding (Figure 4b and Table S6 in the Supplementary Material).The Co(II) cations are linked by pairs of the 4-(hydroxymethyl)pyridine ligands into chains that elongate in the direction of the crystallographic a axis (Figure 4a).These chains are further linked via hydrogen bonding to the solvate water molecules.Intermolecular O-H⋅⋅⋅S hydrogen bonds are observed between H atoms from water molecules or from the hydroxyl group of the 4-(hydroxymethyl)pyridine ligand and the thiocyanate S atoms of neighboring chains (Figure 4b).The water molecules are also linked to the hydroxyl group by intermolecular O-H⋅⋅⋅O hydrogen bonding (Figure 4b and Table S6 in the Supplementary Material).No single crystals were obtained for compound 5. Its crystal structure was determined from XRPD data by refining the recently reported, isostructural Ni-complex (see Experimental Section, Table 1 and Figure S7 in the Supplementary Material) [53].[Co(NCS)2(hmpy)2] crystallizes in the monoclinic space group P21/c with four formula units in the unit cell and all atoms lying on general positions.The Co(II) cations are coordinated by one terminal and two bridging thiocyanate anions groups as well as one terminal and two bridging 4-(hydroxymethyl)pyridine ligands and show a slightly distorted octahedral coordination geometry (Figure 5).Half of the thiocyanate anions are connecting two neighboring cobalt ions, which are further connected by the ligand hmpy to form a zig zag polymer along the [100] direction.
Based on the crystal structure, X-ray powder patterns were calculated and compared with the experimental pattern.Careful inspection of the measured powder pattern revealed that most of the compounds were obtained as pure phase, except for some batches of compounds 2 and 4 (Figures S8-S13 in the Supplementary Material).In both cases, some additional, weak reflections appeared in the measured pattern, indicating not further characterized impurities.This is not really surprising  No single crystals were obtained for compound 5. Its crystal structure was determined from XRPD data by refining the recently reported, isostructural Ni-complex (see Experimental Section, Table 1 and Figure S7 in the Supplementary Material) [53].[Co(NCS) 2 (hmpy) 2 ] crystallizes in the monoclinic space group P2 1 /c with four formula units in the unit cell and all atoms lying on general positions.The Co(II) cations are coordinated by one terminal and two bridging thiocyanate anions groups as well as one terminal and two bridging 4-(hydroxymethyl)pyridine ligands and show a slightly distorted octahedral coordination geometry (Figure 5).Half of the thiocyanate anions are connecting two neighboring cobalt ions, which are further connected by the ligand hmpy to form a zig zag polymer along the [100] direction.
because so many related compounds were obtained, so it is assumed that they exist in equilibrium in solution.Moreover, compound 4 was found to be quite unstable and easily loses the water molecules.This is also the case for the ethanol solvate 2 which loses some of the solvent even at room-temperature.

IR Spectroscopy
All compounds were measured by IR-spectroscopy, to investigate if the coordination mode of the anionic ligands can be determined from the value of the asymmetric CN stretching vibration.Usually for compounds with terminally N-bonded thiocyanato anions a value below 2100 cm −1 is expected, whereas for µ-1,3 bridging thiocyanato anions this vibration should be observed above 2100 cm −1 [38].It is noted that for some compounds these regions overlap and a definite decision cannot be made.This is especially the case for discrete complexes with terminally N-bonded anionic ligands, where the metal centers are additionally coordinated by O-donor ligands like, e.g., water.In this case this value is usually shifted to higher wavenumbers [30].
However, for compounds 1 and 1-H2O the value of the CN stretching vibration is observed at 2073 cm −1 and 2084/2095 cm −1 indicating an N-terminal coordination of the thiocyanato anions, which is in agreement with the crystal structure (Figures S14 and S15 in the Supplementary Material).For the solvate complexes 2 and 3, the CN stretching vibration is observed at 2090 and 2115 cm −1 and at 2098 and 2111 cm −1 , which is at the border line between the values expected for terminal and bridging NCS ligands (Figure S16 and S17 in the Supplementary Material).However, as mentioned above they are shifted to higher values, because in both compounds the Co(II) cations are additionally coordinated by oxygen atoms from the hydroxyl group.For compound 4, the CN vibration occurs at 2092 cm −1 , which is reasonable because the cations are coordinated by terminal anions and only N-bonded 4-(hydroxymethyl)pyridine ligands (Figure S18 in the Supplementary Material).Finally, for compound 5, the CN vibrations are found at 2115, 2092 and 2098 cm −1 , which again is consistent with the results of the structure determination showing both terminal and bridging NCS ligands (Figure S19 in the Supplementary Material).Based on the crystal structure, X-ray powder patterns were calculated and compared with the experimental pattern.Careful inspection of the measured powder pattern revealed that most of the compounds were obtained as pure phase, except for some batches of compounds 2 and 4 (Figures S8-S13 in the Supplementary Material).In both cases, some additional, weak reflections appeared in the measured pattern, indicating not further characterized impurities.This is not really surprising because so many related compounds were obtained, so it is assumed that they exist in equilibrium in solution.Moreover, compound 4 was found to be quite unstable and easily loses the water molecules.This is also the case for the ethanol solvate 2 which loses some of the solvent even at room-temperature.

IR Spectroscopy
All compounds were measured by IR-spectroscopy, to investigate if the coordination mode of the anionic ligands can be determined from the value of the asymmetric CN stretching vibration.Usually for compounds with terminally N-bonded thiocyanato anions a value below 2100 cm ´1 is expected, whereas for µ-1,3 bridging thiocyanato anions this vibration should be observed above 2100 cm ´1 [38].It is noted that for some compounds these regions overlap and a definite decision cannot be made.This is especially the case for discrete complexes with terminally N-bonded anionic ligands, where the metal centers are additionally coordinated by O-donor ligands like, e.g., water.In this case this value is usually shifted to higher wavenumbers [30].
However, for compounds 1 and 1-H 2 O the value of the CN stretching vibration is observed at 2073 cm ´1 and 2084/2095 cm ´1 indicating an N-terminal coordination of the thiocyanato anions, which is in agreement with the crystal structure (Figures S14 and S15 in the Supplementary Material).For the solvate complexes 2 and 3, the CN stretching vibration is observed at 2090 and 2115 cm bridging NCS ligands (Figures S16 and S17 in the Supplementary Material).However, as mentioned above they are shifted to higher values, because in both compounds the Co(II) cations are additionally coordinated by oxygen atoms from the hydroxyl group.For compound 4, the CN vibration occurs at 2092 cm ´1, which is reasonable because the cations are coordinated by terminal anions and only N-bonded 4-(hydroxymethyl)pyridine ligands (Figure S18 in the Supplementary Material).Finally, for compound 5, the CN vibrations are found at 2115, 2092 and 2098 cm ´1, which again is consistent with the results of the structure determination showing both terminal and bridging NCS ligands (Figure S19 in the Supplementary Material).
It is noted that all five compounds show very broad bands above 3000 cm ´1, which belong to the O-H stretching-vibration of the hmpy ligand.

Thermoanalytical Measurements
To investigate the thermal properties of the compounds, measurements using simultaneously differential thermoanalysis and thermogravimetry (DTA-TG) were performed.In this context it was checked if a different, e.g., metastable modification of [Co(NCS) 2 (4-(hydroxymethyl)pyridine) 2 ] n can be obtained as recently reported for other ligands [22,24,30].
Compound 1 shows two very poorly resolved mass steps in the TG curve upon heating, which are accompanied by endothermic signals in the DTA curve (Figure 6 and Figure S20 in the Supplementary Material).The experimental mass loss of the first TG step (∆m (exp) = 39%) is in reasonable agreement with the calculated mass loss assuming the loss of two 4-(hydroxymethyl)pyridine ligands (∆m (calc) = 36%).The thermogravimetric curve of 1-H 2 O looks similar, except that a further mass loss is observed at lower temperatures, which is associated with the removal of water molecules, which are located in the crystal cavities (Figure 6 and Figure S20 in the Supplementary Material).In order to check whether the water removal leads to compound 1, as it was recently reported for the isostructural Ni-complex [53], the measurement was repeated and interrupted after the first mass loss.XRPD investigations showed, that a phase of poor crystallinity was obtained, which could neither be identified nor indexed.It is noted that all five compounds show very broad bands above 3000 cm −1 , which belong to the O-H stretching-vibration of the hmpy ligand.

Thermoanalytical Measurements
To investigate the thermal properties of the compounds, measurements using simultaneously differential thermoanalysis and thermogravimetry (DTA-TG) were performed.In this context it was checked if a different, e.g., metastable modification of [Co(NCS)2(4-(hydroxymethyl)pyridine)2]n can be obtained as recently reported for other ligands [22,24,30].
Compound 1 shows two very poorly resolved mass steps in the TG curve upon heating, which are accompanied by endothermic signals in the DTA curve (Figure 6 and Figure S20 in the Supplementary Material).The experimental mass loss of the first TG step (Δm(exp) = 39%) is in reasonable agreement with the calculated mass loss assuming the loss of two 4-(hydroxymethyl)pyridine ligands (Δm(calc) = 36%).The thermogravimetric curve of 1-H2O looks similar, except that a further mass loss is observed at lower temperatures, which is associated with the removal of water molecules, which are located in the crystal cavities (Figure 6 and Figure S20 in the Supplementary Material).In order to check whether the water removal leads to compound 1, as it was recently reported for the isostructural Ni-complex [53], the measurement was repeated and interrupted after the first mass loss.XRPD investigations showed, that a phase of poor crystallinity was obtained, which could neither be identified nor indexed.TG measurements of compound 2 and 3 showed a subsequent mass loss.The first step was associated with a desolvation process (Δm(calc) = 19% for 2 and 8.4% for 3) (Figure 6 and Figure S21 in the Supplementary Material).The calculated and measured Δm for compound 4 deviates, which was attributed to its instability and therefore partial desolvation upon storage (Figure 6 and Figure S22 in the Supplementary Material).TG measurements of compound 2 and 3 showed a subsequent mass loss.The first step was associated with a desolvation process (∆m (calc) = 19% for 2 and 8.4% for 3) (Figure 6 and Figure S21 in the Supplementary Material).The calculated and measured ∆m for compound 4 deviates, which was attributed to its instability and therefore partial desolvation upon storage (Figure 6 and Figure S22 in the Supplementary Material).
To identify the intermediates, which were formed by removal of the 4-(hydroxymethyl)pyridine ligand respectively water or ethanol, the TG measurements were repeated and stopped after the corresponding steps and the isolated residues were investigated by XRPD (Figure 7).The residue of 1 is amorphous, observed by XRPDdata, while compounds 2, 3 and 4 transformed into [Co(NCS)2(4-(hydroxymethyl)pyridine)2]n (5), which was obtained as a pure phase (Figure 7).It is noted that after removal of all ligands in several cases good crystalline powders are observed that consists of Co(NCS)2.

Resolvation
To investigate if compound 1 can be transformed into 1-H2O, a saturated solution of 1 with an excess of solid was stirred in water and the residue was investigated by XRPD (Figure 8).Comparison of the measured powder pattern proofed that 1 completely transforms into 1-H2O.In additional experiments, compound 5 was soaked either in water or ethanol and the compounds were investigated by XRPD afterwards.Compound 5 can transform into the hydrated form 3, after soaking in water for three days (Figure 9), whereas no transformation was observed after the treatment in ethanol.No transformation of compound 5 takes place while kept in an aqueous or EtOH atmosphere for several days.The residue of 1 is amorphous, observed by XRPDdata, while compounds 2, 3 and 4 transformed into [Co(NCS) 2 (4-(hydroxymethyl)pyridine) 2 ] n (5), which was obtained as a pure phase (Figure 7).It is noted that after removal of all ligands in several cases good crystalline powders are observed that consists of Co(NCS) 2 .

Resolvation
To investigate if compound 1 can be transformed into 1-H 2 O, a saturated solution of 1 with an excess of solid was stirred in water and the residue was investigated by XRPD (Figure 8).Comparison of the measured powder pattern proofed that 1 completely transforms into 1-H 2 O.The residue of 1 is amorphous, observed by XRPDdata, while compounds 2, 3 and 4 transformed into [Co(NCS)2(4-(hydroxymethyl)pyridine)2]n (5), which was obtained as a pure phase (Figure 7).It is noted that after removal of all ligands in several cases good crystalline powders are observed that consists of Co(NCS)2.

Resolvation
To investigate if compound 1 can be transformed into 1-H2O, a saturated solution of 1 with an excess of solid was stirred in water and the residue was investigated by XRPD (Figure 8).Comparison of the measured powder pattern proofed that 1 completely transforms into 1-H2O.In additional experiments, compound 5 was soaked either in water or ethanol and the compounds were investigated by XRPD afterwards.Compound 5 can transform into the hydrated form 3, after soaking in water for three days (Figure 9), whereas no transformation was observed after the treatment in ethanol.No transformation of compound 5 takes place while kept in an aqueous or EtOH atmosphere for several days.In additional experiments, compound 5 was soaked either in water or ethanol and the compounds were investigated by XRPD afterwards.Compound 5 can transform into the hydrated form 3, after soaking in water for three days (Figure 9), whereas no transformation was observed after the treatment in ethanol.No transformation of compound 5 takes place while kept in an aqueous or EtOH atmosphere for several days.

Magnetic Investigations
For compound 5, the temperature dependence of the susceptibility was measured at HDC = 1000 Oe.The χM versus T curve shows a steady increase with decreasing temperature, which indicates only paramagnetic behavior (Figure 10).The χMT value decreases on cooling, which is indicative for dominating antiferromagnetic interactions (Figure 10).The analysis of the magnetic data according to the Curie-Weiss law results in a negative Weiss constant of θ = −18.85K and is therefore in agreement with the antiferromagnetic interactions.Calculations of the Curie constant yielded a value of 3.37 cm 3 •K•mol −1 , from which an experimental effective magnetic moment of 5.19 µB was calculated.This value is slightly higher than the theoretical value of 3.87 µB for Co 2+ in a high spin configuration and this can be traced back to the strong spin-orbit coupling for Co(II).AC measurements show a steady increase of the susceptibility in χM′ and no signal in χM′′ as expected for only paramagnetic behavior and this behavior is also indicated by filed dependent magnetic measurements (Figures S23 and S24 in the Supplementary Material).Therefore, this ligand does not mediate strong magnetic exchange, but it cannot be excluded that some magnetic order is observed at very low temperatures.

Magnetic Investigations
For compound 5, the temperature dependence of the susceptibility was measured at H DC = 1000 Oe.The χ M versus T curve shows a steady increase with decreasing temperature, which indicates only paramagnetic behavior (Figure 10).

Magnetic Investigations
For compound 5, the temperature dependence of the susceptibility was measured at HDC = 1000 Oe.The χM versus T curve shows a steady increase with decreasing temperature, which indicates only paramagnetic behavior (Figure 10).The χMT value decreases on cooling, which is indicative for dominating antiferromagnetic interactions (Figure 10).The analysis of the magnetic data according to the Curie-Weiss law results in a negative Weiss constant of θ = −18.85K and is therefore in agreement with the antiferromagnetic interactions.Calculations of the Curie constant yielded a value of 3.37 cm 3 •K•mol −1 , from which an experimental effective magnetic moment of 5.19 µB was calculated.This value is slightly higher than the theoretical value of 3.87 µB for Co 2+ in a high spin configuration and this can be traced back to the strong spin-orbit coupling for Co(II).AC measurements show a steady increase of the susceptibility in χM′ and no signal in χM′′ as expected for only paramagnetic behavior and this behavior is also indicated by filed dependent magnetic measurements (Figures S23 and S24 in the Supplementary The χ M T value decreases on cooling, which is indicative for dominating antiferromagnetic interactions (Figure 10).The analysis of the magnetic data according to the Curie-Weiss law results in a negative Weiss constant of θ = ´18.85K and is therefore in agreement with the antiferromagnetic interactions.Calculations of the Curie constant yielded a value of 3.37 cm 3 ¨K¨mol ´1, from which an experimental effective magnetic moment of 5.19 µ B was calculated.This value is slightly higher than the theoretical value of 3.87 µ B for Co 2+ in a high spin configuration and this can be traced back to the strong spin-orbit coupling for Co(II).AC measurements show a steady increase of the susceptibility in χ M 1 and no signal in χ M 11 as expected for only paramagnetic behavior and this behavior is also indicated by filed dependent magnetic measurements (Figures S23 and S24 in the Supplementary Material).Therefore, this ligand does not mediate strong magnetic exchange, but it cannot be excluded that some magnetic order is observed at very low temperatures.

General
4-(hydroxymethyl)pyridine and Co(NCS) 2 were obtained from Alfa Aesar (Ward Hill, MA, USA).All chemicals and solvents were used without further purification.Crystalline powders of all compounds were prepared by stirring the reactants in the respective solvents at room temperature.The residues were filtered and washed with appropriate solvents and dried in air.The purity of all compounds was checked by XRPD and elemental analysis.

Elemental Analysis
CHNS analysis was performed using a EURO EA elemental analyzer, fabricated by EURO VECTOR Instruments and Software.

IR Spectroscopy
The IR spectra were measured using an ATI Mattson Genesis Series FTIR Spectrometer, control software: WINFIRST, from ATI Mattson (Midland, ON, Canada).

Differential Thermal Analysis and Thermogravimetry
The heating-rate dependent DTA-TG measurements were performed in a nitrogen atmosphere (purity: 5.0) in Al 2 O 3 crucibles using a STA-409CD instrument from Netzsch (Exton, PA, USA).All measurements were performed with a flow rate of 75 mL¨min ´1 and were corrected for buoyancy and current effects.The instrument was calibrated using standard reference materials.

Single-Crystal Structure Analysis
Single-crystal data collections were performed on an imaging plate diffraction system: Stoe IPDS-1 for 4 as well as Stoe IPDS-2 for 2, 3 with MoKα radiation.The structures were solved with Direct Methods using SHELXS-97 and structure refinements were performed using least-squares methods against F 2 using SHELXL-2013.[55] Numerical absorption corrections were applied using X-RED and X-SHAPE of the program package X-Area.All non-hydrogen atoms were refined with anisotropic thermal displacement parameters.All hydrogen atoms were positioned with idealized geometry and were refined isotropic with an U iso (H) = ´1.2U eq (C) (1.5 for methyl H atoms) of the corresponding parent atom using a riding model.The hydroxyl hydrogen atoms were located in the difference Fourier map, their bond lengths were set to ideal values and finally they were refined using a riding model.The disorder of the ethyl group in compound 2 was modeled using a split model.CCDC 1455775 (1), CCDC 1455776 (1-H 2 O), CCDC 1455777 (2), CCDC 1455778 (3), CCDC 1455779 (4) and CCDC 1455780 (5) contain the supplementary crystallographic data for this paper.These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html.

X-Ray Powder Diffraction
The measurements were performed using: (1) a PANalytical X'Pert Pro MPD Reflection Powder Diffraction System with CuKα 1 radiation (λ =1.540598 Å) equipped with a PIXcel semiconductor detector from PANanlytical; (2) a Stoe Transmission Powder Diffraction System (STADI P) with CuKa 1 radiation (λ = 1.540598Å) equipped with a MYTHEN 1K1 detector and a Johannson-type Ge(111) monochromator from STOE & CIE; and (3) a Stoe Stadi-P machine (Mo Ka α radiation, λ = 0.7093 Å), equipped with a MYTHEN 1K dector and a Johannson-type Ge(111) monochromator in Debye Scherrer mode.Rietveld refinements [56] of 1-H 2 O and 5 were performed using TOPAS 5.0.[57].Structure determination of compounds 1-H 2 O and 5 was performed by Rietveld refinements of the corresponding isostructural Ni-complexes (Table 2).The profile function was described in both cases with the fundamental parameter approach [55], while the background was modelled by Chebychev polynomials of 12th and 11th order.The Rietveld refinement for compound 5 was carried out using a rigid body model for describing the ligand hmpy, whereas the individual bond lengths of the thiocyanate group were restrained.For both compound 1-H 2 O and 5, hydrogen atoms were fixed at geometric calculated positions and an overall isotropic thermal displacement parameter was used for all atoms.

Magnetic Measurements
All magnetic measurements were performed using a PPMS (Physical Property Measurement System) from Quantum Design, which was equipped with a 9 Tesla magnet.The data were corrected for core diamagnetism.

Conclusions
In the present contribution, investigations on new cobalt(II) thiocyanato coordination compounds with 4-(hydroxymethyl)pyridine as ligand are reported, with the major goal to prepare a 1D compound in which the Co(II) cations are linked by the anionic ligands into chains.Even if several new compounds were discovered and analyzed with single crystal and powder X-ray diffraction, thermal and elemental analysis, magnetic and IR measurements, most of them consist of simple discrete complexes that are coordinated in part by additional solvent molecules.One of these compounds (5) exhibits a composition that corresponds to that, expected for the desired compound but unfortunately, its crystal structure consists of only Co(NCS) 2 dimers that are linked into chains by the 4-(hydroxymethyl)pyridine ligand.Some of these compounds can be thermally decomposed, which either leads to amorphous products or to the formation of 5.There is no indication for the formation of a further modification with the desired structural features, even if a similar structure is known for the corresponding azido compound with Cd(II) as cation.
Supplementary Materials: The supplementary materials are available online at www.mdpi.com/2073-4352/6/4/38/s1.IR spectra, DTA-TG curves, experimental and calculated X-ray powder patterns as well as tables with selected bond lengths and angles.

Figure 1 .
Figure 1.Crystal structure of 1 with view of the coordination sphere (a) and along the crystallographic a axis with intermolecular hydrogen bonding shown as dashed lines (b).The ORTEP plot is shown in Figure S1 in the Supplementary Material.

Figure 1 .
Figure 1.Crystal structure of 1 with view of the coordination sphere (a) and along the crystallographic a axis with intermolecular hydrogen bonding shown as dashed lines (b).The ORTEP plot is shown in Figure S1 in the Supplementary Material.

Figure 2 .
Figure 2. View of the discrete complexes in the crystal structure of 2 (a) and 3 (b), showing the coordination sphere of the Co(II) cations.ORTEP plots of 2 and 3 can be found in Figures S4 and S5 in the Supplementary Material.
compound 2 the discrete complexes are connected by intermolecular O-H⋅⋅⋅S hydrogen bonding between the H atom of the methanol group of the 4-(hydroxymethyl)pyridine ligand and the thiocyanato S atom of a neighbored complex into chains along the crystallographic b axis.These chains are further connected into layers by intermolecular O-H•••O hydrogen bonding between the

Figure 2 .
Figure 2. View of the discrete complexes in the crystal structure of 2 (a) and 3 (b), showing the coordination sphere of the Co(II) cations.ORTEP plots of 2 and 3 can be found in Figures S4 and S5 in the Supplementary Material.

Figure 3 .
Figure 3. Crystal structure of 2 with view along the crystallographic b axis (a) and of 3 with view along the crystallographic b axis (b).Intermolecular hydrogen bonding is shown as dashed lines.

Figure 3 .
Figure 3. Crystal structure of 2 with view along the crystallographic b axis (a) and of 3 with view along the crystallographic b axis (b).Intermolecular hydrogen bonding is shown as dashed lines.

Figure 4 .
Figure 4. Crystal structure of 4 with view of the chain (a) and with view along the crystallographic a axis (b).Intermolecular hydrogen bonding shown as dashed lines and an ORTEP plot is shown in Figure S6 in the Supplementary Material.

Figure 4 .
Figure 4. Crystal structure of 4 with view of the chain (a) and with view along the crystallographic a axis (b).Intermolecular hydrogen bonding shown as dashed lines and an ORTEP plot is shown in Figure S6 in the Supplementary Material.

Figure 5 .
Figure 5. Crystal structure of 5 with view of the dimer (a) and the chain (b).

Figure 5 .
Figure 5. Crystal structure of 5 with view of the dimer (a) and the chain (b).

´1
and at 2098 and 2111 cm ´1, which is at the border line between the values expected for terminal and Crystals 2016, 6, 38 8 of 17

Figure 8 .
Figure 8. Calculated XRPD patterns of 1 (A) and 1-H2O (D).The experimental XRPD patterns of 1 (B); and of the product obtained after stirring this compound in water for 2 d (C).

Figure 8 .
Figure 8. Calculated XRPD patterns of 1 (A) and 1-H2O (D).The experimental XRPD patterns of 1 (B); and of the product obtained after stirring this compound in water for 2 d (C).

Figure 8 .
Figure 8. Calculated XRPD patterns of 1 (A) and 1-H 2 O (D).The experimental XRPD patterns of 1 (B); and of the product obtained after stirring this compound in water for 2 d (C).

Crystals 2016, 6 , 38 10 of 16 Figure 9 .
Figure 9. Calculated XRPD patterns of 5 (A) and 3 (D).The experimental XRPD patterns of the residue obtained after the first mass step (B); and of the product obtained after stirring compound 5 in water for 3 d (C).

Figure 10 .
Figure 10.χM and χM T as a function of the temperature for 5 at HDC = 1000 Oe.

Figure 9 .
Figure 9. Calculated XRPD patterns of 5 (A) and 3 (D).The experimental XRPD patterns of the residue obtained after the first mass step (B); and of the product obtained after stirring compound 5 in water for 3 d (C).

Crystals 2016, 6 , 38 10 of 16 Figure 9 .
Figure 9. Calculated XRPD patterns of 5 (A) and 3 (D).The experimental XRPD patterns of the residue obtained after the first mass step (B); and of the product obtained after stirring compound 5 in water for 3 d (C).

Figure 10 .
Figure 10.χM and χM T as a function of the temperature for 5 at HDC = 1000 Oe.

Figure 10 .
Figure 10.χ M and χ M T as a function of the temperature for 5 at H DC = 1000 Oe.

Table 1 .
Selected crystal data and details on the structure determinations for compounds 1, 2, 3 and 4.

Table 1 .
Selected crystal data and details on the structure determinations for compounds 1, 2, 3 and 4.
Compounds 2 and 3 form simple solvate complexes, in which the Co(II) cations are coordinated by two terminally bonded thiocyanate anions, two terminally bonded 4-(hydroxymethyl)pyridine ligands, and two ethanol, respectively, water molecules with a slightly distorted octahedral coordination geometry (Figure2, FiguresS4 and S5in the Supplementary Material).Compounds 2 and 3 form simple solvate complexes, in which the Co(II) cations are coordinated by two terminally bonded thiocyanate anions, two terminally bonded 4-(hydroxymethyl)pyridine ligands, and two ethanol, respectively, water molecules with a slightly distorted octahedral coordination geometry (Figure2, Figures S4 and S5 in the Supplementary Material).

Table 2 .
Selected crystal data and details of the Rietveld refinements for compounds 1-H 2 O and 5.