Abstract
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.
Highlights
Polymorphism and isomerism is a widespread phenomenon in coordination chemistry (Braga & Grepioni, 2000; Moulton & Zaworotko, 2001; Batten et al, 1998; Zhang et al, 2009)
We have recently reported the crystal structure of form II of Co(NCS)2(DMAP)2 (DMAP = 4-dimethylaminopyridine, C7H10N2), which crystallizes as discrete complexes in which the cobalt cations are tetrahedrally coor
Both of them were identified by single crystal X-ray diffraction, which proves that the block-like crystals correspond to the unit cell of the Ni compound, whereas the needle-like crystals correspond to the metastable form I of Co(NCS)2(DMAP)2, which was obviously obtained accidentally under kinetic control
Summary
Polymorphism and isomerism is a widespread phenomenon in coordination chemistry (Braga & Grepioni, 2000; Moulton & Zaworotko, 2001; Batten et al, 1998; Zhang et al, 2009). One class of compounds in which polymorphism and especially isomerism is observed are coordination compounds based on transitionmetal thiocyanates, because this anionic ligand shows several different coordination modes leading to a large structural variability (Bohme et al, 2020; Jochim et al, 2020; Mautner et al, 2018; Neumann et al, 2020a; Wellm et al, 2020a; Werner et al, 2015; Buckingham, 1994; Barnett et al, 2002) In this context, we have recently reported the crystal structure of form II of Co(NCS)2(DMAP) (DMAP = 4-dimethylaminopyridine, C7H10N2), which crystallizes as discrete complexes in which the cobalt cations are tetrahedrally coor-. We present the structure of form II at 100 K, because in our previous work it was measured at 170 K (Neumann et al, 2018b)
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