Metamagnetism and long range ordering in μ-1,3 bridging transition metal thiocyanato coordination polymers

Selective Crystallization of Linkage Isomers, [RhIII(NCS)(SCN)5]3- and [RhIII(SCN)6]3-, to Investigate Structural Trans Influence and Thermal Stability

Linkage isomers of homoleptic complexes, [RhIII(SCN)6]3- and [RhIII(NCS)(SCN)5]3-, formed in aqueous solution were successfully separated by employing methyltriphenylphosphonium (MePPh3+) and 1-ethylquinolinium (EtQu+) ions as countercations, respectively. The single-crystal X-ray analysis of (MePPh3)3[RhIII(SCN)6] (1) indicated that all of the SCN- ligands coordinate to the RhIII ion by S atoms with an octahedral symmetry, where the average bond length of Rh-S is 2.374(7) Å. On the other hand, the RhIII ion of (EtQu)3[RhIII(NCS)(SCN)5]·H2O (2) is coordinated by five S atoms and one N atom of the SCN- ligands with a C4v symmetry. Structural trans influence was observed in the shorter bond length of Rh-S at the trans position of Rh-N. The Rh-S bond length is 2.3398(13) Å significantly shorter than those of 1 by ca. 0.04 Å, although DFT calculations based on the crystal structures indicated that the effective bond order of Rh-N is higher than those of Rh-S. Thermal stability examination by thermogravimetric and differential thermal analyses (TG/DTA) and IR spectroscopy indicated that the linkage isomerization of [RhIII(SCN)6]3- to [RhIII(NCS)(SCN)5]3- proceeded after melting around 174 °C. These results clearly indicate that [RhIII(NCS)(SCN)5]3- is thermodynamically more stable than [RhIII(SCN)6]3- in solid states, although further linkage isomerization hardly occurs.

Photosynthesis of a Dihydroimidazopyridine Chelate Shines Light on the Reactions of a Photoactivated Iron(III) Complex with O2

The high-spin (S = ⁵/₂) meridional diastereoisomer of [Fe^III(tpena)]²⁺ (tpena = N,N,N'-tris(2-pyridylmethyl)ethylendiamine-N'-acetate), mer-[Fe(tpena)]²⁺, undergoes photolytic CO₂ release to produce an iron(II) intermediate of a radical dihydroimidazopyridine ligand (L^•). The structure of this unprecedented transient iron(II)(L^•) complex is supported by UV-vis and Mössbauer spectroscopies, DFT calculations, as well as the X-ray structural characterization of an μ-oxo iron(III) complex of the oxidized derivative of L^•, namely, [Fe^III₂O(Cl)₂(L⁺)₂](ClO₄)₄(MeCN)₂ (L⁺ = 2-(2-(bis(pyridin-2-ylmethyl)amino)ethyl)-2,3-dihydro-1H-imidazo[1,5-a]pyridin-4-ium). [Fe^III₂O(Cl)₂(L⁺)₂]⁴⁺ is obtained only in the absence of O₂. Under aerobic conditions, O₂ will intercept the iron(II)(L^•) complex to form a putative Fe(III)-alkylperoxide complex which cascades to an iron(II) complex of SBPy3 (SBPy3 = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-aldimine). Thus, through different oxidative pathways, the unknown ligand L⁺ or SBPy3 forms by loss of a one-carbon-atom or a two-carbon-atom unit, respectively, from the glycyl arm of tpena. Acceleration of the photodecarboxylation step is achieved by addition of thiocyanate because of transient formation of a more photoreactive NCS^- adduct of [Fe(tpena)]²⁺. This has allowed for kinetic observation of the reaction of [Fe^II(L^•)]²⁺ with O₂ which is, unexpectedly, promoted also by light. We propose that this corresponds to the energy needed for the conversion of the ring-closed radical ligand L^• to a ring-opened tautomer to allow for O₂ insertion between the C and Fe atoms of the iron(II) complex.

Strengthening the Magnetic Interactions in Pseudobinary First-Row Transition Metal Thiocyanates, M(NCS)2

Understanding the effect of chemical composition on the strength of magnetic interactions is key to the design of magnets with high operating temperatures. The magnetic divalent first-row transition metal (TM) thiocyanates are a class of chemically simple layered molecular frameworks. Here, we report two new members of the family, manganese(II) thiocyanate, Mn(NCS)₂, and iron(II) thiocyanate, Fe(NCS)₂. Using magnetic susceptibility measurements on these materials and on cobalt(II) thiocyanate and nickel(II) thiocyanate, Co(NCS)₂ and Ni(NCS)₂, respectively, we identify significantly stronger net antiferromagnetic interactions between the earlier TM ions-a decrease in the Weiss constant, θ, from 29 K for Ni(NCS)₂ to -115 K for Mn(NCS)₂-a consequence of more diffuse 3d orbitals, increased orbital overlap, and increasing numbers of unpaired t_2g electrons. We elucidate the magnetic structures of these materials: Mn(NCS)₂, Fe(NCS)₂, and Co(NCS)₂ order into the same antiferromagnetic commensurate ground state, while Ni(NCS)₂ adopts a ground state structure consisting of ferromagnetically ordered layers stacked antiferromagnetically. We show that significantly stronger exchange interactions can be realized in these thiocyanate frameworks by using earlier TMs.

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 of octa-kis-(4-meth-oxy-pyridinium) bis-(4-meth-oxy-pyridine-κN)tetra-kis-(thio-cyanato-κN)ferrate(III) bis-[(4-meth-oxypyri-dine-κN)pentakis-(thio-cyanato-κN)ferrate(III)] hexa-kis-(thio-cyanato-κN)ferrate(III) with iron in three different octa-hedral coordination environments

The crystal structure of the title salt, (C6H8NO)8[Fe(NCS)4(C6H7NO)2][Fe(NCS)5(C6H7NO)]2[Fe(NCS)6], comprises three negatively charged octa-hedral FeIII complexes with different coordination environments in which the FeIII atoms are coordinated by a different number of thio-cyanate anions and 4-meth-oxy-pyridine ligands. Charge balance is achieved by 4-meth-oxy-pyridinium cations. The asymmetric unit consists of three FeIII cations, one of which is located on a centre of inversion, one on a twofold rotation axis and one in a general position, and ten thio-cyanate anions, two 4-meth-oxy-pyridine ligands and 4-meth-oxy-pyridinium cations (one of which is disordered over two sets of sites). Beside to Coulombic inter-actions between organic cations and the ferrate(III) anions, weak N-H⋯S hydrogen-bonding inter-actions involving the pyridinium N-H groups of the cations and the thio-cyanate S atoms of the complex anions are mainly responsible for the cohesion of the crystal structure.

Crystal structure of bis-(pyridine-4-carbo-thio-amide-κN1)bis-(thio-cyanato-κN)cobalt(II) methanol monosolvate

The asymmetric unit of the title compound, [Co(NCS)2(C6H6NS)4]·CH3OH, consists of one cobalt(II) cation, two thio-cyanate anions, four pyridine-4-carbo-thio-amide ligands and one methanol mol-ecule that are located in general positions. The CoII cations are coordinated by two terminal N-bonding thio-cyanate anions and four N-bonding pyridine-4-carbo-thio-amide ligands, resulting in discrete and slightly distorted octa-hedral complexes. These complexes are linked into a three-dimensional network via inter-molecular N-H⋯S hydrogen bonding between the amino H atoms and the thio-cyanate and pyridine-4-carbo-thio-amide S atoms. From this arrangement, channels are formed in which the methanol solvate mol-ecules are embedded and linked to the host structure by inter-molecular O-H⋯S and N-H⋯O hydrogen bonding.

Crystal structure of di-aqua-bis-(4-cyano-pyridine-κN)bis-(thio-cyanato-κN)iron(II) 4-cyano-pyridine disolvate

The asymmetric unit of the title compound, [Fe(NCS)2(C6H4N2)2(H2O)2]·2C6H4N2, comprises one FeII cation occupying an inversion centre as well as one thio-cyanate anion, one water mol-ecule and two 4-cyano-pyridine mol-ecules in general positions. The iron cations are coordinated by two N-bonded thiocyanate anions, two (pyridine)N-bonded 4-cyano-pyridine ligands and two water mol-ecules into discrete complexes. The resulting coordination polyhedron can be described as a slightly distorted octa-hedron. The discrete complexes are connected through centrosymmetric pairs of (pyridine)C-H⋯N(cyano) hydrogen bonds into chains that are further linked into a three-dimensional network through inter-molecular O-H⋯N hydrogen bonds involving the 4-cyano-pyridine solvent mol-ecules.

Transition Metal Thiocyanate Complexes of Picolylcyanoacetamides

A variety of transition metal complexes involving picolylcyanoacetamides (pica = NCCH2CONH-R; R = 2-picolyl- (2pica), 3-picolyl- (3pica), 4-picolyl- (4pica)) and thiocyanate have been synthesised and their solid-state structures have been determined. The complexes were all obtained from reactions between the corresponding metals salts and pica ligands with sodium thiocyanate under ambient conditions. Both 3pica and 4pica coordinate to the metal solely through the nitrogen atom of the picolyl group and form discrete tetrahedral [M(NCS)2(pica)2] (3pica; M = Mn, Zn; 4pica; M = Co) and octahedral [M(NCS)2(3pica)4] (M = Co, Fe, Ni) complexes. In addition, one-dimensional N,S-thiocyanate-bridged coordination polymers poly-[M(µ-NCS)2(pica)2] (3pica; M = Cd; 4pica; M = Co, Cd) were obtained. The ligand 2pica gave the discrete octahedral complexes [Co(NCS)2(2pica)2] and [Cd(NO3)2(2pica)2] in which 2pica chelates in a bidentate fashion through its picolyl and carbonyl groups. Magnetic susceptibility measurements on the cobalt(ii) complexes were performed and showed short-range antiferromagnetic coupling for the [Co(NCS)2(4pica)2]n 1D polymer.

Synthesis, structures, and properties of Mn(II) and Cd(II) thiocyanato coordination compounds with 2,5-dimethylpyrazine as co-ligand

Reaction of manganese (II) thiocyanate with 2,5-dimethylpyrazine leads to the formation of three new coordination compounds of compositions Mn(NCS)2(2,5-dimethylpyrazine)2(H2O)2(1), Mn(NCS)2(H2O)2(MeOH)2-tris(2,5-dimethylpyrazine) solvate (2), and Mn(NCS)2(H2O)4-tetrakis(2,5-dimethylpyrazine) solvate (3) that were characterized by single crystal X-ray diffraction. In their crystal structures, the Mn(II) cations are sixfold coordinated by two terminally N-bonded thiocyanato anions and two water molecules as well as by two 2,5-dimethylpyrazine ligands (1), two ethanol (2), or two water molecules (3), within slightly distorted octahedra. In compounds2and3, additional 2,5-dimethylpyrazine ligands are located in the cavities of the structures as solvate molecules. X-ray powder diffraction has shown that compounds1and2cannot be prepared as pure phases and that batches of compound3contain only minor traces of a contamination. Differential thermoanalysis and thermogravimetry have revealed that upon heating of compound3, an intermediate of composition Mn(NCS)2(2,5-dimethylpyrazine) (4) is formed, which cannot be obtained from solution. To mimic the structure of4, single crystals of a Cd compound of the same composition (5) were prepared from the liquid phase, and single crystal X-ray analysis has shown that it is isotypic to4.

Crystal structure of catena-poly[[[bis-(pyridine-4-carbo-thio-amide-κN (1))cadmium]-di-μ-thio-cyanato-κ(2) N:S;κ(2) S:N] methanol disolvate]

The asymmetric unit of the polymeric title compound, {[Cd(NCS)2(C6H6N2S)]·2CH3OH} n , consists of one cadmium(II) cation that is located on a centre of inversion as well as one thio-cyanate anion, one pyridine-4-carbo-thio-amide ligand and one methanol mol-ecule in general positions. The Cd(II) cations are octa-hedrally coordinated by the pyridine N atom of two pyridine-4-carbo-thio-amide ligands and by the S and N atoms of four thio-cyanate anions and are linked into chains along [010] by pairs of anionic ligands. These chains are further linked into layers extending along (201) by inter-molecular N-H⋯O and O-H⋯S hydrogen bonds. One of the amino H atoms of the pyridine-4-carbo-thio-amide ligand is hydrogen-bonded to the O atom of a methanol mol-ecule, and a symmetry-related methanol mol-ecule is the donor group to the S atom of another pyridine-4-carbo-thio-amide ligand whereby each of the pyridine-4-carbo-thio-amide ligands forms two pairs of centrosymmetric N-H⋯S and O-H⋯S hydrogen bonds. The methanol mol-ecules are equally disordered over two orientations.

Synthesis, structures, magnetic, and theoretical investigations of layered Co and Ni thiocyanate coordination polymers

Two new ferromagnetic layered Co(ii) and Ni(ii) thiocyanate coordination polymers were synthesized, structurally characterized and investigated for their magnetic behavior using susceptibility measurements and theoretical methods.

Synthesis, structure and properties of [Co(NCS)2(4-(4-chlorobenzyl)pyridine)2]n, that shows slow magnetic relaxations and a metamagnetic transition

[Co(NCS)₂(4-(4-chlorobenzyl)pyridine)₂]_n shows a metamagnetic behavior and slow relaxations of magnetization indicative of a single chain magnet behavior.

catena-Poly[[bis-(pyridine-κN)nickel(II)]-di-μ-thio-cyanato-κ(2) N:S;κ(2) S:N]

In the title compound, [Ni(NCS)₂(C₅H₅N)₂]_n, the Ni²⁺ cation is coordinated by four thio­cyanate anions (μ-1,3) and two pyridine ligands within a slightly distorted octa­hedral configuration. The Ni—N bond lengths to the pyridine rings are 2.1189 (17) and 2.1241 (17) Å, whereas those to the thiocyanate anions are 2.0299 (18) and 2.0359 Å. The Ni—S bond lengths are 2.5357 (6) and 2.5568 (6) Å. The Ni²⁺ cations are linked by N:S-bridging thio­cyanate ligands into chains extending along [010]. The Ni⋯Ni distance within the chains is 5.5820 (5) Å. The asymmetric unit contains two Ni²⁺ cations of which one is located on a centre of inversion, whereas the second is located on a general position.