Two new transition metal thiocyanate coordination polymers with the composition [Co(NCS)(4-vinylpyridine)] (1) and [Co(NCS)(4-benzoylpyridine)] (2) were synthesized and their crystal structures were determined. In both compounds the Co cations are octahedrally coordinated by two trans-coordinating 4-vinyl- or 4-benzoylpyridine co-ligands and four μ-1,3-bridging thiocyanato anions and linked into chains by the anionic ligands. While in 1 the N and the S atoms of the thiocyanate anions are also in trans-configuration, in 2 they are in cis-configuration. A detailed magnetic study showed that the intra-chain ferromagnetic coupling is slightly stronger for 2 than for 1, and that the chains in both compounds are weekly antiferromagnetically coupled. Both compounds show a long range magnetic ordering transition at T = 3.9 K for 1 and T = 3.7 K for 2, which is confirmed by specific heat measurements. They also show a metamagnetic transition at a critical field of 450 Oe (1) and 350 Oe (2), respectively. Below T1 and 2 exhibit magnetic relaxations resembling relaxations of single chains. The exchange constants obtained from magnetic and specific heat data are in good accordance with those obtained from constrained DFT calculations carried out on isolated model systems. The ab initio calculations allowed us to find the principal directions of anisotropy.

Abstract. In this report a rational route to coordination polymers that can show cooperative magnetic phenomena is presented. In this approach compounds based on transition metal cations, small sized terminal N-bonded anionic ligands and additional neutral N-donor coligands are heated, which lead to the formation of intermediates, in which the metal cations are linked by the anionic ligands. Predominantly, the use of this method for the synthesis of bridged thio-and selenocyanato coordination compounds is described in this article but it can also be extended for the preparation of other compounds. In most cases the intermediates are formed in very pure form and in quantitative yields. Thus, compounds, which are not or at least very difficult to obtain, can be prepared if the synthesis is performed in solution. This is especially valid for thio-and selenocyanato coordination compounds, which mostly prefer terminal bonding instead of bridging co-

The reaction of nickel thiocyanate with pyrazine in a 1:2 ratio leads to the new ligand-rich 1:2 (ratio metal/ligand) compound [Ni(SCN) 2 (pyrazine) 2 ] n (1), in which the metal atoms are coordinated by four N atoms of pyrazine ligands and two N atoms of thiocyanate anions in a slightly distorted octahedral arrangement. If an excess of the metal thiocyanate is used and the reaction is performed under solvothermal conditions, single crystals of the ligand-deficient 1:1 compounds [M(SCN) 2 (pyrazine) 2 ] n [M = Fe (2I), Co (3), Ni (4)] are obtained. Compound 2I is isotypic with [Mn(SCN) 2 (pyrazine)] n but different to 3 and 4, which are also isotypic. Investigations on the synthesis of these compounds reveal that only compound 4 can be prepared phase-pure in solution; all other ligand-deficient compounds are always contaminated with large amounts of the corresponding ligand-rich coordination polymers. In the crystal structures, the metal atoms

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Reaction of Ni(NCS) with 4-aminopyridine in different solvents leads to the formation of compounds with the compositions Ni(NCS)(4-aminopyridine) (1), Ni(NCS)(4-aminopyridine)(HO) (2), [Ni(NCS)(4-aminopyridine)(MeCN)]·MeCN (3), and [Ni(NCS)(4-aminopyridine)] (5-LT). Compounds 1, 2, and 3 form discrete complexes, with octahedral metal coordination. In 5-LT the Ni cations are linked by single thiocyanate anions into chains, which are further connected into layers by half of the 4-aminopyridine coligands. Upon heating, 1 transforms into an isomer of 5-LT with a 1D structure (5-HT), that on further heating forms a more condensed chain compound [Ni(NCS)(4-aminopyridine)] (6) that shows a very unusual chain topology. If 3 is heated, a further compound with the composition Ni(NCS)(4-aminopyridine) (4) is formed, which presumably is a dimer and which on further heating transforms into 6 via 5-HT as intermediate. Further investigations reveal that 5-LT and 5-HT are related by enantiotropism, with 5-LT being the thermodynamic stable form at room-temperature. Magnetic and specific heat measurements reveal ferromagnetic exchange through thiocyanate bridges and magnetic ordering due to antiferromagnetic interchain interactions at 5.30(5) K and 8.2(2) K for 5-LT and 6, respectively. Consecutive metamagnetic transitions in the spin ladder compound 6 are due to dipolar interchain interactions. A convenient formula for susceptibility of the ferromagnetic Heisenberg chain of isotropic spins S = 1 is proposed, based on numerical DMRG calculations, and used to determine exchange constants.

The reaction of ZnI2 and pyrimidine in acetonitrile results in the formation of the 1:2 compound ZnI2(pyrimidine)2 (1), which consists of discrete tetrahedral building blocks. Slow heating of 1 at 1 degrees C/min leads to its transformation into the ligand-deficient intermediate 1:1 compound ZnI2(pyrimidine) (3), which upon further heating decomposes into the most ligand-deficient 2:1 compound (ZnI2)2(pyrimidine) (4). In contrast, the 2:3 compound (ZnI2)2(pyrimidine)3 (2) is formed as an intermediate by decomposing 1 using a faster heating rate of 8 degrees C/min. Compound 2 consists of oligomeric units in which each ZnI2 unit is coordinated by two iodine atoms and one bridging and one terminal pyrimidine ligand. The crystal structure of compound 3 is built up of ZnI2 units, which are connected by the ligands into chains. For the thermal transformation of 1 into 3 via 2 as the intermediate, a smooth reaction pathway is found in the crystal structure, for which only small translational and rotational changes are needed. The metastable solvated compound (ZnI2)(pyrimidine)(acetonitrile)0.25 (5) consisting of (ZnI2)4(pyrimidine)4 rings is obtained by quenching the reaction of ZnI2 and pyrimidine in acetonitrile using an antisolvent. On heating, 5 decomposes into a new polymorphic 1:1 compound 6, which consists of (ZnI2)(pyrimidine) chains. On further heating, 6 transforms into a third polymorphic 1:1 compound 7, which consists of (ZnI2)3(pyrimidine)3 rings, and finally into the 1:1 compound 3. Solvent-mediated conversion experiments reveal that compounds 1-4 are thermodynamically stable, whereas compounds 5-7 are metastable. Time-dependent crystallization experiments unambiguously show that compound 7 is formed by kinetic control and transforms within minutes into compound 6, which finally transforms into 3. Compound 3 represents the thermodynamically most stable 1:1 modification, whereas compounds 6 and 7 are metastable. The different compounds obtained by thermal decomposition and by crystallization from solution represent a snapshot of the species in solution and thus provide insight into the formation of coordination compounds.

For prednisolone, two polymorphic modifications (forms I and II) and one sesquihydrate were found. Form I crystallizes in the monoclinic space group P2 1 , whereas form II and the sesquihydrate crystallize orthorhombic in space group P2 1 2 1 2 1 . In all forms, the molecules are connected via O-H • • • O hydrogen bonding. Solvent-mediated conversion experiments reveal that form II represents the thermodynamically stable form at room temperature, whereas form I is metastable. This observation could also be independently confirmed by force field calculations. Differential scanning calorimetry (DSC) measurements indicate no polymorphic transformation of one form into the other. The DSC melting point of form I is significantly higher than that of form II, indicating that form I represents the thermodynamically most stable form at higher temperatures and that both modifications behave enantiotropic. This was proven by solvent-mediated crystallization experiments that show that above 130°form II transforms into form I. Dehydration of the sesquihydrate results in the formation of modification I. The 13 C MAS spectra allow distinguishing between all three phases and confirms the crystallographic symmetry. By combining the NMR data with quantum chemical shift calculations on DFT niveau, packing effects were probed.