Structural Diversity in Ni Chain Coordination Polymers: Synthesis, Structures, Isomerism and Magnetism

Synthesis, crystal structure and thermal properties of catena-poly[[bis-(4-methyl-pyridine)-nickel(II)]-di-μ-thio-cyanato], which shows an alternating all-trans and cis-cis-trans-coordination of the NiS2Np 2Nt 2 octa-hedra (p = 4-methyl-pyridine, t = thio-cyanate)

The title compound, [Ni(NCS)2(C6H7N)2] n , was prepared by the reaction of Ni(NCS)2 with 4-methyl-pyridine in water. Its asymmetric unit consists of two crystallographically independent NiII cations, of which one is located on a twofold rotational axis whereas the second occupies a center of inversion, two independent thio-cyanate anions and two independent 4-methyl-pyridine co-ligands in general positions. Each NiII cation is octa-hedrally coordinated by two 4-methyl-pyridine coligands as well as two N- and two S-bonded thio-cyanate anions. One of the cations shows an all-trans, the other a cis-cis-trans configuration. The metal centers are linked by pairs of μ-1,3-bridging thio-cyanate anions into [101] chains. X-ray powder diffraction shows that a pure crystalline phase has been obtained and thermogravimetry coupled to differential thermoanalysis reveals that the title compound loses half of the 4-methyl-pyridine coligands and transforms into Ni(NCS)2(C6H7N). Nearly pure samples of this compound can be obtained by thermal annealing and a Rietveld refinement demonstrated that it is isotypic to its recently reported Cd analog [Neumann et al., (2020 ▸). CrystEngComm. 22, 184-194] In its crystal structure, the metal cations are linked by one μ-1,3(N,S)- and one μ-1,3,3(N,S,S)-bridging thio-cyanate anion into single chains that condense via the μ-1,3,3(N,S,S)-bridging anionic ligands into double chains.

Synthesis, crystal structure and thermal properties of a new polymorphic modification of diiso-thio-cyanato-tetra-kis-(4-methyl-pyridine)cobalt(II)

The title compound, [Co(NCS)2(C6H7N)4] or Co(NCS)2(4-methyl-pyridine)4, was prepared by the reaction of Co(NCS)2 with 4-methyl-pyridine in water and is isotypic to one of the polymorphs of Ni(NCS)2(4-methyl-pyridine)4 [Kerr & Williams (1977 ▸). Acta Cryst. B33, 3589-3592 and Soldatov et al. (2004 ▸). Cryst. Growth Des. 4, 1185-1194]. Comparison of the experimental X-ray powder pattern with that calculated from the single-crystal data proves that a pure phase has been obtained. The asymmetric unit consists of one CoII cation, two crystallographically independent thio-cyanate anions and four independent 4-meth-yl-pyridine ligands, all located in general positions. The CoII cations are sixfold coordinated to two terminally N-bonded thio-cyanate anions and four 4-methyl-pyridine coligands within slightly distorted octa-hedra. Between the complexes, a number of weak C-H⋯N and C-H⋯S contacts are found. This structure represent a polymorphic modification of Co(NCS)2(4-methyl-pyridine)4 already reported in the CCD [Harris et al. (2003 ▸). NASA Technical Reports, 211890]. In contrast to this form, the crystal structure of the new polymorph shows a denser packing, indicating that it is thermodynamically stable at least at low temperatures. Thermogravimetric and differential thermoanalysis reveal that the title compound starts to decomposes at about 100°C and that the coligands are removed in separate steps without any sign of a polymorphic transition before decomposition.

Synthesis, crystal structure and properties of poly[(μ-2-methyl-pyridine N-oxide-κ2O:O)bis-(μ-thio-cyanato-κ2N:S)cobalt(II)]

The title compound, [Co(NCS)2(C6H7NO)]n or Co(NCS)2(2-methyl-pyridine N-oxide), was prepared by the reaction of Co(NCS)2 and 2-methyl-pyridine N-oxide in methanol. All crystals obtained by this procedure show reticular pseudo-merohedric twinning, but after recrystallization, one crystal was found that had a minor component with only a very few overlapping reflections. The asymmetric unit consists of one CoII cation, two thio-cyanate anions and one 2-methyl-pyridine N-oxide coligand in general positions. The CoII cations are octa-hedrally coordinated by two O-bonding 2-methyl-pyridine N-oxide ligands, as well as two S- and two N-bonding thio-cyanate anions, and are connected via μ-1,3(N,S)-bridging thio-cyanate anions into chains that are linked by μ-1,1(O,O) bridging coligands into layers. No pronounced directional inter-molecular inter-actions are observed between the layers. The 2-methyl-pyridine coligand is disordered over two orientations and was refined using a split model with restraints. Powder X-ray diffraction (PXRD) indicates that a pure sample was obtained and IR spectroscopy confirms that bridging thio-cyanate anions are present. Thermogravimetry and differential thermoanalysis (TG-DTA) shows one poorly resolved mass loss in the TG curve that is accompanied by an exothermic and an endothermic signal in the DTA curve, which indicate the decomposition of the 2-methyl-pyridine N-oxide coligands.

Synthesis, crystal structure and properties of chlorido-tetra-kis-(pyridine-3-carbo-nitrile)-thio-cyanato-iron(II)

Reaction of FeCl2·4H2O with KSCN and 3-cyano-pyridine (pyridine-3-carbo-nitrile) in ethanol accidentally leads to the formation of single crystals of Fe(NCS)(Cl)(3-cyano-pyridine)4 or [FeCl(NCS)(C6H4N2)4]. The asymmetric unit of this compound consists of one FeII cation, one chloride and one thio-cyanate anion that are located on a fourfold rotation axis as well as of one 3-cyano-pyridine coligand in a general position. The FeII cations are sixfold coordinated by one chloride anion and one terminally N-bonding thio-cyanate anion in trans-positions and four 3-cyano-pyridine coligands that coordinate via the pyridine N atom to the FeII cations. The complexes are arranged in columns with the chloride anions, with the thio-cyanate anions always oriented in the same direction, which shows the non-centrosymmetry of this structure. No pronounced inter-molecular inter-actions are observed between the complexes. Initially, FeCl2 and KSCN were reacted in a 1:2 ratio, which lead to a sample that contains the title compound as the major phase together with a small amount of an unknown crystalline phase, as proven by powder X-ray diffraction (PXRD). If FeCl2 and KSCN is reacted in a 1:1 ratio, the title compound is obtained as a nearly pure phase. IR investigations reveal that the CN stretching vibration for the thio-cyanate anion is observed at 2074 cm-1, and that of the cyano group at 2238 cm-1, which also proves that the anionic ligands are only terminally bonded and that the cyano group is not involved in the metal coordination. Measurements with thermogravimetry and differential thermoanalysis reveal that the title compound decomposes at 169°C when heated at a rate of 4°C min-1 and that the 3-cyano-pyridine ligands are emitted in two separate poorly resolved steps. After the first step, an inter-mediate compound with the composition Fe(NCS)(Cl)(3-cyano-pyridine)2 of unknown structure is formed, for which the CN stretching vibration of the thio-cyanate anion is observed at 2025 cm-1, whereas the CN stretching vibration of the cyano group remain constant. This strongly indicates that the FeII cations are linked by μ-1,3-bridg-ing thio-cyanate anions into chains or layers.

Comparison of the crystal structures of the low- and high-temperature forms of bis-[4-(di-methyl-amino)-pyridine]-dithio-cyanato-cobalt(II)

Single crystals of the high-temperature form I of [Co(NCS)2(DMAP)2] (DMAP = 4-di-methyl-amino-pyridine, C7H10N2) were obtained accidentally by the reaction of Co(NCS)2 with DMAP at slightly elevated temperatures under kinetic control. This modification crystallizes in the monoclinic space group P21/m and is isotypic with the corresponding Zn compound. The asymmetric unit consists of one crystallographically independent Co cation and two crystallographically independent thio-cyanate anions that are located on a crystallographic mirror plane and one DMAP ligand (general position). In its crystal structure the discrete complexes are linked by C-H⋯S hydrogen bonds into a three-dimensional network. For comparison, the crystal structure of the known low-temperature form II, which is already thermodynamically stable at room temperature, was redetermined at the same temperature. In this polymorph the complexes are connected by C-H⋯S and C-H⋯N hydrogen bonds into a three-dimensional network. At 100 K the density of the high-temperature form I (ρ = 1.462 g cm-3) is higher than that of the low-temperature form II (ρ = 1.457 g cm-3), which is in contrast to the values determined by XRPD at room temperature. Therefore, these two forms represent an exception to the Kitaigorodskii density rule, for which extensive inter-molecular hydrogen bonding in form II might be responsible.

Synthesis, crystal structure and thermal properties of poly[bis-[μ-3-(amino-meth-yl)pyridine-κ2 N:N']bis(thio-cyanato-κN)manganese(II)]

The reaction of Mn(NCS)2 with a stoichiometric amount of 3-(amino-meth-yl)pyridine in ethanol led to the formation of the title compound, [Mn(NCS)2(C6H8N2)2] n , which is isotypic to its Zn, Co and Cd analogues. The manganese cation is located on a centre of inversion and is octa-hedrally coordinated in an all-trans configuration by two terminal N-bonded thio-cyanate anions as well as four 3-(amino-meth-yl)pyridine co-ligands, of which two coordinate with the pyridine N atom and two with the amino N atom. The 3-(amino-meth-yl)pyridine co-ligands connect the MnII cations into layers extending parallel to (10). These layers are further connected into a three-dimensional network by relatively strong inter-molecular N-H⋯S hydrogen bonding. Comparison of the experimental X-ray powder diffraction pattern with the calculated pattern on the basis of single-crystal data proves the formation of a pure crystalline phase. IR measurements showed the CN stretching vibration of the thio-cyanate anions at 2067 cm-1, which is in agreement with the presence of terminally N-bonded anionic ligands. TG-DTA measurements revealed that the title compound decomposes at about 500 K.

Synthesis, characterization and thermal behaviour of solid phases in the quasi-ternary system Mg(SCN)2-H2O-THF

Mg(SCN)2·4H2O can be converted into previously unknown compounds Mg(SCN)2·(4 - x) H2O·xTHF with x = 0, 2 and 4 by multiple recrystallization in tetrahydrofuran (THF). The phases were characterized by infrared spectroscopy (IR), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), and their crystal structures were solved from X-ray powder diffraction (XRPD) data. In the crystal structures isolated Mg(NCS)2(H2O)4-x(THF)x units form layered motifs. The thermal behavior of Mg(SCN)2·4H2O and Mg(SCN)2·4THF was investigated by temperature dependent in situ XRPD, where Mg(SCN)2·4THF was found to acquire a room temperature (α-form) and high temperature modification (β-form). The phase transformation is associated with an order-disorder transition of the THF molecules and with a reversion of the stacking order of the layered motifs. Further heating eventually leads to the formation of Mg(SCN)2·2THF. There thiocyanate related sulfur atoms fill the voids in the coordination sphere of magnesium, which leads to the formation of one dimensional electroneutral ∞[Mg(NCS)2/2(SCN)2/2(THF)2] chains. All investigated Mg(SCN)2·(4 - x) H2O·xTHF phases exhibit a remarkable anisotropic thermal expansion, and Mg(SCN)2·4H2O and Mg(SCN)2·2THF were found to show both positive and negative thermal expansion coefficients.

Thermodynamically metastable chain and stable layered Co(NCS)2 coordination polymers: thermodynamic relations and magnetic properties

Reaction of Co(NCS)2 with 4-bromopyridine leads to the formation of discrete complexes with the composition Co(NCS)2(4-bromopyridine)4·(CH3CN)0.67 (1), Co(NCS)2(4-bromopyridine)2(H2O)2 (2), Co(NCS)2(4-bromopyridine)2(CH3OH)2 (3) and Co(NCS)2(4-bromopyridine)2(CH3CN)2 (4). Upon heating compounds 2 and 4 transform into a crystalline product with the composition Co(NCS)2(4-bromopyridine)2 (5-I) that also can easily be obtained from solution. In this compound, the Co cations are linked by single μ-1,3-bridging thiocyanate anions into layers. Thermal decomposition of 3 leads to a second isomer (5-II), which is thermodynamically metastable and can also be synthesized from solution under kinetic control. In contrast to 5-I, the Co cations are linked by pairs of anionic ligands into linear chains. The magnetic exchange is very weak in 5-I, but much stronger and ferromagnetic along the linear chains in 5-II. AF ordering in 5-II is reached at 3.05 K, and magnetic relaxation is observed at the metamagnetic transition with an Arrhenius barrier of 17.1(3) cm-1. Ab initio computational studies reveal a different type of magnetic anisotropy to be present in the two crystallographically - independent Co centers in 5-II.

Crystal structure of bis-(N,N'-di-methyl-thio-urea-κS)bis-(thio-cyanato-κN)cobalt(II)

During systematic investigations on the synthesis of coordination polymers with Co(NCS)2 involving different thio-urea derivatives as coligands, crystals of the title compound Co(NCS)2(N,N'-di-methyl-thio-urea)2, or [Co(C3H8N2S)2(NCS)2], were obtained. These crystals were non-merohedric twins and therefore, a twin refinement using data in HKLF-5 format was performed. In the crystal structure of this compound, the CoII cations are coordinated by two N-terminally bonded thio-cyanate anions as well as two S-bonding N,N'-di-methyl-thio-urea mol-ecules, forming two crystallographically independent discrete complexes each with a strongly distorted tetra-hedral geometry. An intricate network of inter-molecular N-H⋯S and C-H⋯S hydrogen bonds can be found between the complexes. The thermogravimetric curve of the title compound shows two discrete steps in which all coligand mol-ecules have been emitted, which is also accompanied by partial decomposition of the cobalt thio-cyanate. If the measurement is stopped after the first mass loss, only broad reflections of CoS can be found in the XRPD pattern of the residue, which proves that this compound decomposes completely upon heating. However, at lower temperatures an endothermic signal can be found in the DTA and DSC curve, which corresponds to melting, as proven by thermomicroscopy.

Variation of the Chain Geometry in Isomeric 1D Co(NCS)2 Coordination Polymers and Their Influence on the Magnetic Properties

Two different isomers of [Co(NCS)₂(4-chloropyridine)₂]_n (3C and 3L) were synthesized from solution and by thermal decomposition of Co(NCS)₂(4-chloropyridine)₂(H₂O)₂ (2), which show a different metal coordination leading to corrugated chains in 3C and to linear chains in 3L. Solvent mediated conversion experiments prove that 3C is thermodynamically stable at room temperature where 3L is metastable. Magnetic measurements reveal that the magnetic exchange in 3L is comparable to that observed for previously reported related chain compounds, whereas in 3C with corrugated chains, the ferromagnetic interaction within the chains is strongly suppressed. The magnetic ordering takes place at 2.85 and 0.89 K, for 3L and 3C, respectively, based on specific heat measurements. For 3L the field dependence of magnetic relaxations in antiferromagnetically ordered ferromagnetic chains is presented. In addition, 3L is investigated by FD-FT THz-EPR spectroscopy, revealing a ground to first excited state energy gap of 14.0 cm^-1. Broken-symmetry DFT calculations for 3C and 3L indicate a ferromagnetic intrachain interaction. Ab initio CASSCF/CASPT2/RASSI-SO computational studies reveal significantly different single-ion anisotropies for the crystallographically independent cobalt(II) centers in 3C and 3L. Together with the geometry of the chains this explains the magnetic properties of 3C and 3L. The ab initio results also explain the weaker exchange interaction in 3C and 3L as compared to previously reported [Co(NCS)₂(L)₂]_n compounds with linear chains.

Crystal structure, synthesis and thermal properties of bis-(4-benzoyl-pyridine-κN)bis-(iso-thio-cyanato-κN)bis-(methanol-κN)iron(II)

In the crystal structure of the title compound, [Fe(NCS)2(C12H9NO)2(CH4O)2], the FeII cations are octa-hedrally coordinated by two N atoms of 4-benzoyl-pyridine ligands, two N atoms of two terminal iso-thio-cyanate anions and two methanol mol-ecules into discrete complexes that are located on centres of inversion. These complexes are linked via inter-molecular O-H⋯O hydrogen bonds between the methanol O-H H atoms and the carbonyl O atoms of the 4-benzoyl-pyridine ligands, forming layers parallel to (101). Powder X-ray diffraction proved that a pure sample was obtained but that this compound is unstable and transforms into an unknown crystalline phase within several weeks. However, the solvent mol-ecules can be removed by heating in a thermobalance, which for the aged sample as well as the title compound leads to the formation of a compound with the composition Fe(NCS)2(4-benzoyl-pyridine)2, which exhibits a powder pattern that is similar to that of Mn(NCS)2(4-benzoyl-pyridine)2.

Impact of the synthetic approach on the magnetic properties and homogeneity of mixed crystals of tunable layered ferromagnetic coordination polymers

Different synthetic routes were applied to synthesize mixed crystals of [Co_xNi_1−x(NCS)₂(4-tert-butylpyridine)₂]_n that finally results in homogeneous samples for which the critical temperature can be tuned as function of the Co : Ni ratio.

New isomeric Ni(NCS)2 coordination compounds: crystal structures, magnetic properties as well as ex situ and in situ investigations on their synthesis and transition behaviour

Some stable and metastable isomeric Ni(NCS)₂ chain compounds were synthesized and structurally characterized and investigated for their transformation behavior and their magnetic properties using a variety of methods.

Crystal structure of pyridinium tetra-iso-thio-cyanato-dipyridine-chromium(III) pyridine monosolvate

In the crystal structure of the title compound, (C5H6N)[Cr(NCS)4(C5H5N)2]·C5H5N, the CrIII ions are octa-hedrally coordinated by four N-bonding thio-cyanate anions and two pyridine ligands into discrete negatively charged complexes, with the CrIII ion, as well as the two pyridine ligands, located on crystallographic mirror planes. The mean planes of the two pyridine ligands are rotated with respect to each other by 90°. Charge balance is achieved by one protonated pyridine mol-ecule that is hydrogen bonded to one additional pyridine solvent mol-ecule, with both located on crystallographic mirror planes and again rotated by exactly 90°. The pyridinium H atom was refined as disordered between both pyridine N atoms in a 70:30 ratio, leading to a linear N-H⋯N hydrogen bond. In the crystal, discrete complexes are linked by weak C-H⋯S hydrogen bonds into chains that are connected by additional C-H⋯S hydrogen bonding via the pyridinium cations and solvent mol-ecules into layers and finally into a three-dimensional network.

Crystal structure, synthesis and thermal properties of bis-(aceto-nitrile-κN)bis-(4-benzoyl-pyridine-κN)bis-(iso-thio-cyanato-κN)nickel(II)

In the crystal structure of the title com-pound, [Ni(NCS)2(CH3CN)2(C12H9NO)2] or Ni(NCS)2(4-benzoyl-pyridine)2(aceto-nitrile)2, the NiII ions are octa-hedrally coordinated by the N atoms of two thio-cyanate anions, two 4-benzoyl-pyridine ligands and two aceto-nitrile mol-ecules into discrete com-plexes that are located on centres of inversion. In the crystal, the discrete com-plexes are linked by centrosymmetric pairs of weak C-H⋯S hydrogen bonds into chains. Thermogravimetric measurements prove that, upon heating, the title com-plex loses the two aceto-nitrile ligands and transforms into a new crystalline modification of the chain com-pound [Ni(NCS)2(4-benzoyl-pyridine)2], which is different from that of the corresponding CoII, NiII and CdII coordination polymers reported in the literature. IR spectroscopic investigations indicate the presence of bridging thio-cyanate anions but the powder pattern cannot be indexed and, therefore, this structure is unknown.

Crystal structure of bis(4-benzoyl-pyridine-κN)bis(methanol-κO)bis(thio-cyanato-κN)nickel(II) methanol monosolvate

The asymmetric unit of the title compound, [Ni(NCS)2(C12H9NO)2(CH3OH)2]·CH3OH, comprises one NiII cation, two thio-cyanate anions, two 4-benzoyl-pyridine coligands, two coordinating, as well as one non-coordinating methanol mol-ecule. The NiII cation is coordinated by two terminally N-bonded thio-cyanate anions, the N atoms of two 4-benzoyl-pyridine coligands and the O atoms of two methanol ligands within a slightly distorted octa-hedron. Individual complexes are linked by inter-molecular O-H⋯S hydrogen bonding into chains parallel to [010] that are further connected into layers parallel to (10) by C-H⋯S hydrogen bonds. Additional C-H⋯O hydrogen-bonding inter-actions lead to the formation of a three-dimensional network that limits channels extending parallel to [010] in which the non-coordinating methanol mol-ecules are located. They are hydrogen-bonded to the coordinating methanol mol-ecules. X-ray powder diffraction revealed that the compound could not be prepared as a pure phase.