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

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.

Synthesis, crystal structure, thermal and spectroscopic properties of ZnX2-2-methylpyrazine (X = Cl, Br, I) coordination compounds

Reaction of zinc(II) chloride, bromide and iodide with 2-methylpyrazine (2-Mepyz) leads to the formation of coordination compounds with the composition ZnX2(2-Mepyz)2 (X = Cl; 1-Cl, Br; 1-Br and I; 1-I). In the compounds each Zn cation is tetrahedrally coordinated by two halide anions and two 2-methylpyrazine ligands forming discrete complexes. TG-DTA and temperature dependent PXRD measurements prove that upon heating compounds 1 transform into new compounds with the composition ZnX2(2-Mepyz) (2), that are subsequently converted into compounds with the composition (ZnX2)3(2-Mepyz) (3) upon further heating. It was also found that compounds 2 can be prepared directly in solution. For ZnI2(2-Mepyz) (2-I) crystals were obtained and characterized by single crystal X-ray diffraction, whereas the crystal structures of 2-Cl and 2-Br were determined ab initio from PXRD data. In these compounds the Zn cations are also tetrahedrally coordinated and linked into chains by bridging 2-methylpyrazine ligands. The (ZnX2)3(2-Mepyz) compounds can only be obtained by thermal decomposition, and the products are of poor crystallinity and extremely hygroscopic, which prevented structure determinations.

Synthesis, crystal structure and thermal properties of bis-(aceto-nitrile-κN)bis-(3-bromo-pyridine-κN)bis-(thio-cyanato-κN)cobalt(II)

Single crystals of the title compound, [Co(NCS)₂(C₅H₄BrN)₂(C₂H₃N)₂], were obtained by the reaction of Co(NCS)₂ with 3-bromo-pyridine in aceto-nitrile. The Co^II cations lie on crystallographic inversion centers and are coordinated by two N-bonded thio-cyanate anions, two 3-bromo-pyridine and two aceto-nitrile ligands thereby forming slightly distorted CoN₆ octa-hedra. In the crystal, these complexes are linked by C-H⋯S and C-H⋯N hydrogen bonds into a three-dimensional network. In the direction of the crystallographic b-axis, the complexes are arranged into columns with neighboring 3-bromo-pyridine ligands stacked onto each other, indicating π-π inter-actions. The CN stretching vibration of the thio-cyanate anions is observed at 2066 cm^-1, in agreement with the presence of only N-bonded anionic ligands. TG-DTA measurements reveal that in the first mass loss the aceto-nitrile ligands are removed and that in the second step, half of a 3-bromo-pyridine ligand is lost, leading to the formation of a polymeric compound with the composition [(Co(NCS)₂)₂(C₅H₄BrN)₃] _n already reported in the literature.

Co(NCS)2 Chain Compound with Alternating 5- and 6-Fold Coordination: Influence of Metal Coordination on the Magnetic Properties

The reaction of Co(NCS)₂ with 3-bromopyridine leads to the formation of discrete complexes [Co(NCS)₂(3-bromopyridine)₄] (1), [Co(NCS)₂(3-bromopyridine)₂(H₂O)₂] (2), and [Co(NCS)₂(3-bromopyridine)₂(MeOH)₂] (3) depending on the solvent. Thermogravimetric measurements on 2 and 3 show a transformation into [Co(NCS)₂(3-bromopyridine)₂]_n (4), which upon further heating is converted to [{Co(NCS)₂}₂(3-bromopyridine)₃]_n (5), whereas 1 transforms directly into 5 upon heating. Compound 5 can also be obtained from solution, which is not possible for 4. In 4 and 5, the cobalt(II) cations are linked by pairs of μ-1,3-bridging thiocyanate anions into chains. In compound 4, all cobalt(II) cations are octahedrally coordinated (OC-6), as is usually observed in such compounds, whereas in 5, a previously unkown alternating 5- and 6-fold coordination is observed, leading to vacant octahedral (vOC-5) and octahedral (OC-6) environments, respectively. In contrast to 4, the chains in 5 are very efficiently packed and linked by π···π stacking of the pyridine rings and interchain Co···Br interactions, which is the basis for the formation of this unusual chain. The spin chains in 4 demonstrate ferromagnetic intrachain exchange and much weaker interchain interactions, as is usually observed for such linear chain compounds. In contrast, compound 5 shows almost single-ion-like magnetic susceptibility, but the magnetic ordering temperature deduced from specific heat measurements is twice as high as that in 4, which might originate from π···π stacking and Co···Br interactions between neighboring chains. More importantly, unlike all linear Co(NCS)₂ chain compounds, a dominant antiferromagnetic exchange is observed for 5, which is explained by density functional theory calculations predicting an alternating ferro- and aniferromagnetic exchange within the chains. Theoretical calculations on the two different cobalt(II) ions present in 5 predict an easy-axis anisotropy that is much stronger for the octahedral cobalt(II) ion than for the one with the vacant octahedral coordination, with the magnetic axes of the two ions being canted by an angle of 84°. This almost orthogonal orientation of the easy axis of magnetization for the two cobalt(II) ions is the rationale for the observed non-Ising behavior of 5.

Synthesis, crystal structure and magnetic properties of coordination compounds of Mn(NCS)2 with the 3-bromopyridine ligand

Reactions of Mn(NCS)2 with 3-bromopyridine in acetonitrile lead to the formation of Mn(NCS)2(3-bromopyridine)4 (1) and Mn(NCS)2(3-bromopyridine)2(MeCN)2 (2) that were characterized by single crystal X-ray diffraction. Compounds 1 and 2 consist of discrete complexes, in which the Mn(II) cations are octahedrally coordinated by two trans-N-bonding thiocyanate anions and four pyridine (1) or two pyridine and two acetonitrile ligands (2). Thermoanalytical measurements on 1 and 2 have shown that upon heating half of the 3-bromopyridine co-ligands from 1 or both acetonitrile ligands from 2 are removed leading to a crystalline phase with the composition [Mn(NCS)2(3-bromopyridine)2] n (3-II). From dry n-butanol a phase with the same composition was obtained (3-I) that corresponds to a polymorphic or isomeric form of 3-II. Crystal structure analysis of 3-I shows that in this form the Mn cations are linked by pairs of anionic ligands into linear chains. The results of magnetic measurements on 3-I show antiferromagnetic interactions along the chains and the analysis of the magnetic susceptibility using the Fisher model for chains gave a J value of −5.76(5) K.

Crystal structures of two Co(NCS)2 urotropine coordination compounds with different Co coordinations

In the crystal structure of the title compounds, discrete complexes with different Co coordinations are observed, which are linked by inter­molecular hydrogen bonding into three-dimensional networks.

The reaction of Co(NCS)₂ with urotropine in ethanol leads to the formation of two different compounds, namely, bis­(ethanol-κO)bis­(hexa­methyl­ene­tetra­mine-κN)bis­(thio­cyanato-κN)cobalt(II)–di­aqua-κ²O-bis­(hexa­methyl­ene­tetra­mine-κN)bis­(thio­cyanato-κN)cobalt(II)–ethanol–hexa­methyl­ene­tetra­mine (1.2/0.8/1.6/4), [Co(NCS)₂(C₆H₁₂N₄)₂(C₂H₆O)₂]_1.2·[Co(NCS)₂(C₆H₁₂N₄)₂(H₂O)₂]_0.8·1.6C₂H₆O·4C₆H₁₂N₄, 1, and tris­(ethanol-κO)(hexa­methyl­ene­tetra­mine-κN)bis(thio­cyanato-κN)cobalt(II), [Co(NCS)₂(C₆H₁₂N₄)(C₂H₆O)₃], 2. In the crystal structure of compound 1, two crystallographically independent discrete complexes are observed that are located on centres of inversion. In one of them, the Co cation is octa­hedrally coordinated to two terminal N-bonded thio­cyanate anions, two urotropine ligands and two ethanol mol­ecules, whereas in the second complex 80% of the coordinating ethanol is exchanged by water. Formally, compound 1 is a mixture of two different complexes, i.e. di­aqua­dithio­cyanato­bis­(urotropine)cobalt(II) and di­ethano­ldi­thio­cyanato­bis­(uro­trop­ine)cobalt(II), that contain additional ethanol and urotropine solvate mol­ecules leading to an overall composition of [Co(NCS)₂(urotropine)₂(ethanol)_1.2(H₂O)_0.8·0.8ethanol·4urotropine. Both discrete complexes are linked by inter­molecular O—H⋯O and O—H⋯N hydrogen bonding and additional urotropine solvate mol­ecules into chains, which are further connected into layers. These layers combine into a three-dimensional network by pairs of centrosymmetric inter­molecular C—H⋯S hydrogen bonds. In the crystal structure of compound 2, di­thio­cyanato­(urotropine)tri­ethano­lcobalt(II), the cobalt cation is octa­hedrally coordinated to two terminal N-bonded thio­cyanate anions, one urotropine ligand and three ethanol mol­ecules into discrete complexes, which are located in general positions. These complexes are linked by inter­molecular O—H⋯N hydrogen bonding into layers, which are further connected into a three-dimensional network by inter­molecular C—H⋯S hydrogen bonding.

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.