Trimorphism of Zn(NCS)₂(4-dimethylaminopyridine)₂: Crystal Structures, Thermodynamic Relations, and Comparison with the Co(II) Polymorphs

Abstract

Reaction of Zn(NCS) 2 with 4-dimethylaminopyridine (DMAP) leads to the formation of three polymorphic modifications of Zn(NCS) 2 (DMAP) 2 (I−Zn, II−Zn, and III−Zn), of which only the crystal structure of I−Zn is reported in literature. In all three modifications the Zn(II) cations are tetrahedrally coordinated by two terminal N-bonded thiocyanate anions and two DMAP ligands forming discrete complexes, which differ in the dihedral angle between the DMAP ring planes and the arrangement of the complexes in the crystals. All three modifications show melting, and the thermodynamic relations and the transition behavior were investigated by temperature-dependent X-ray powder diffraction, differential scanning calorimetry, thermomicroscopy, and solvent-mediated conversion experiments. These reveal that I−Zn and II−Zn as well as II−Zn and III−Zn are related by enantiotropism, whereas I−Zn and III−Zn are related by monotropism. The thermodynamic transition temperature between I−Zn and II−Zn is much lower than that between the isotypic Co(II) modifications I−Co and II−Co reported recently.

Spin Transition of an Iron(II) Organoborate Complex in Different Polymorphs and in Vacuum-Deposited Thin Films: Influence of Cooperativity

“…On one hand, one cannot exclude that some batches of 1-II already contain traces of stable polymorph 1-I that can act as nuclei, and on the other hand, such polymorphic transitions can frequently be suppressed using higher heating rates. Similar observations have recently been made for polymorphic modifications of simple Zn complexes …”

“…Elemental analysis gave a composition identical to that of 1-II , indicating that a polymorph of 1-II ( 1-I ) was obtained. In this context, it is noted that Fe­(II) and Zn­(II) as well as Fe­(II) and Co­(II) compounds are frequently isotypic and that in several cases also different polymorphic modifications are observed. − Therefore, to retrieve structural information about the new modification, corresponding compounds with Co ( 3 ) and Zn ( 2 ) were prepared and investigated by single-crystal X-ray diffraction (see below). If the experimental XRPD patterns of 1-I and 3 were compared with that calculated from the single-crystal data of 2 , it became obvious that all compounds are isotypic (Figure S8).…”

“…Similar observations have recently been made for polymorphic modifications of simple Zn complexes. 91 For the Zn (2) and Co (3) compounds, we also performed DSC measurements (Figure S19). Their DSC heating curves do not show a broad endothermic peak, which additionally confirms that the broad endothermic peak observed in the DSC measurement for 1-II corresponds to the SCO transition.…”

Thermodynamically stable and metastable coordination polymers synthesized from solution and the solid state

“…10,[46][47][48] In several cases, polymorphic or isomeric modifications are obtained, which is of importance for the study of structureproperty relationships. [49][50][51] By this reaction, crystalline powders not suitable for single crystal X-ray diffraction (SCXRD) are obtained, but in this case, the corresponding Cd compounds can easily be crystallized from solution (see above) and in several cases they are isotypic to the less chalcophilic metal cations, which allows the retrieval of structural information by Rietveld refinements. 52 Recently, we became especially interested in FeIJII) thiocyanate compounds, which, in contrast to, e.g.…”

Structural Variety in Mn(NCS)2 4-Cyanopyridine Coordination Compounds: Synthesis, Structures, Isomerism, and Magnetic Properties

“…This is likely the reason for the abundance of reports on coordination polymers. − In a large number of cases the metal cations are linked by small-sized ligands, which is of additional advantage in molecular magnetism, because the magnetic exchange can be mediated. − In this context anionic ligands such as cyanide, azide, and thiocyanate are of interest and a great number of these compounds have been reported in the last decades. Concerning the structural properties of such compounds, structure control is difficult to achieve for coordination compounds based on metal thiocyanates, because in contrast to cyanide and azide anions, this anionic ligand can coordinate to metal centers in multiple ways and this might be the reason a relatively large number of polymorphic and isomeric modifications are known. − In the majority of compounds this ligand is only terminally coordinated to the metal centers, but there is an increasing number of compounds in which the metal cations are linked by bridging thiocyanate anions into coordination networks. For transition-metal cations such as Mn­(II), Fe­(II), Co­(II), Ni­(II), and Cd­(II) the most common motif consists of chains, in which the metal cations are octahedrally coordinated and are linked by pairs of anionic ligands − and there are only a few structures reported in the literature, in which the cations are linked by single thiocyanate anions. − In these chains the metal cations can be coordinated differently, leading to either linear or corrugated chains. − In comparison to this, layered thiocyanate networks are very rare and in most cases the transition-metal cations are linked by pairs of anionic ligands into dimers that are further connected by single thiocyanate anions into layers − or they are exclusively linked by single μ-1,3-bridging anions into 2D networks. − To the best of our knowledge, there is only one compound with a 3D thiocyanate network .…”