Christian Näther

Highly Efficient Reversible Z−E Photoisomerization of a Bridged Azobenzene with Visible Light through Resolved S1(nπ*) Absorption Bands

The reversible Z-E photoswitching properties of the (Z) and (E) isomers of the severely constrained bridged azobenzene derivative 5,6-dihydrodibenzo[c,g][1,2]diazocine (1) were investigated quantitatively by UV/vis absorption spectroscopy in solution in n-hexane. In contrast to normal azobenzene (AB), 1 has well separated S(1)(n pi*) absorption bands, peaking at lambda(Z) = 404 nm and lambda(E) = 490 nm. Using light at lambda = 385 nm, it was found that 1Z can be switched to 1E with very high efficiency, Gamma = 92 +/- 3%. Conversely, 1E can be switched back to 1Z using light at lambda = 520 nm with approximately 100% yield. The measured quantum yields are Phi(Z-->E) = 72 +/- 4% and Phi(E-->Z) = 50 +/- 10%. The thermal lifetime of the (E) isomer is 4.5 +/- 0.1 h at 28.5 degrees C. The observed photochromic and photoswitching properties of 1 are much more favorable than those for normal AB, making our title compound a promising candidate for interesting applications as a molecular photoswitch especially at low temperatures. The severe constraints by the ethylenic bridge apparently do not hinder but favor the Z-E photoisomerization reactions.

Self-Assembly of 2,8,14,20-Tetraisobutyl-5,11,17,23-tetrahydroxyresorc[4]arene

We report herein the observation of a hexameric structure of a hydroxyresorc[4]arene in the solid state, enclosing a large interior space. This artificial molecular container is stabilized only by hydrogen bonds. The tendency to form aggregates in solution is demonstrated mainly by means of ESI-MS methods.

Electronic Structure of Six-Coordinate Iron(III)−Porphyrin NO Adducts: The Elusive Iron(III)−NO(radical) State and Its Influence on the Properties of These Complexes

This paper investigates the interaction between five-coordinate ferric hemes with bound axial imidazole ligands and nitric oxide (NO). The corresponding model complex, [Fe(TPP)(MI)(NO)](BF4) (MI = 1-methylimidazole), is studied using vibrational spectroscopy coupled to normal coordinate analysis and density functional theory (DFT) calculations. In particular, nuclear resonance vibrational spectroscopy is used to identify the Fe-N(O) stretching vibration. The results reveal the usual Fe(II)-NO(+) ground state for this complex, which is characterized by strong Fe-NO and N-O bonds, with Fe-NO and N-O force constants of 3.92 and 15.18 mdyn/A, respectively. This is related to two strong pi back-bonds between Fe(II) and NO(+). The alternative ground state, low-spin Fe(III)-NO(radical) (S = 0), is then investigated. DFT calculations show that this state exists as a stable minimum at a surprisingly low energy of only approximately 1-3 kcal/mol above the Fe(II)-NO(+) ground state. In addition, the Fe(II)-NO(+) potential energy surface (PES) crosses the low-spin Fe(III)-NO(radical) energy surface at a very small elongation (only 0.05-0.1 A) of the Fe-NO bond from the equilibrium distance. This implies that ferric heme nitrosyls with the latter ground state might exist, particularly with axial thiolate (cysteinate) coordination as observed in P450-type enzymes. Importantly, the low-spin Fe(III)-NO(radical) state has very different properties than the Fe(II)-NO(+) state. Specifically, the Fe-NO and N-O bonds are distinctively weaker, showing Fe-NO and N-O force constants of only 2.26 and 13.72 mdyn/A, respectively. The PES calculations further reveal that the thermodynamic weakness of the Fe-NO bond in ferric heme nitrosyls is an intrinsic feature that relates to the properties of the high-spin Fe(III)-NO(radical) (S = 2) state that appears at low energy and is dissociative with respect to the Fe-NO bond. Altogether, release of NO from a six-coordinate ferric heme nitrosyl requires the system to pass through at least three different electronic states, a process that is remarkably complex and also unprecedented for transition-metal nitrosyls. These findings have implications not only for heme nitrosyls but also for group-8 transition-metal(III) nitrosyls in general.

Coexistence of Metamagnetism and Slow Relaxation of the Magnetization in a Cobalt Thiocyanate 2D Coordination Network

Recently, strategies for the design of coordination polymers, hybrid compounds, or metal–organic frameworks (MOFs) that show cooperative magnetic phenomena have become of increasing interest. Because of their great potential for possible applications as storage materials or in molecular electronics, 1D materials with a large magnetic anisotropy, slow relaxation of the magnetization M, and a hysteresis of molecular origin, for example, “single-chain magnets” (SCMs) are of special interest. Moreover, for future applications multifunctional materials are needed, in which different physical properties can be tuned or switched as a function of external parameters. These criteria also apply to metamagnetic compounds, which show different magnetic properties below and above a critical field HC. [1c,4] Unfortunately, because of strong interchain interactions most of these compounds show only 3D ordering above HC. [5] Therefore, only a very few metamagnetic coordination compounds have been reported in which slow relaxation of the magnetization is observed. 6] In our research we have developed an alternative method for the synthesis of compounds that show cooperative magnetic interactions. In this approach transition-metal coordination compounds with terminally bound anions and neutral co-ligands are heated leading to a stepwise removal of the co-ligands and the formation of intermediates with bridging anions and modified magnetic interactions. We have found that a large number of different compounds can be prepared by this route and that the dimensionality of the networks can easily be adjusted. In this context we have reported on the directed synthesis of a compound that shows SCM behavior. Such a behavior usually occurs only in 1D coordination networks, but should, in principle, also be observed in 2D networks if the magnetic chains are separated by magnetically inactive ligands. To investigate this possibility, precursor compounds based on cobalt(II) thiocyanate and the bidentate co-ligand 1,2-bis(4-pyridyl)ethylene (bpe) were prepared, and the intermediates formed by thermal decomposition were characterized for their magnetic properties. The reaction of Co(NCS)2 with an excess of bpe leads to the formation of [Co(NCS)2(bpe)(bpe)]n (1). [10] In its crystal structure the cobalt cations are octahedrally coordinated by four bpe ligands and two terminal N-bonded thiocyanato anions (Figure 1, top). The metal cations are linked by the bpe ligands into chains that are further connected by the coligands into layers. This arrangement leads to the formation of cavities in which additional bpe ligands are trapped. In further experiments using slightly different reaction conditions the hydrate [Co(NCS)2(bpe)2(H2O)2] [10] (2) could be obtained, in which the cobalt(II) cations are surrounded by two bpe ligands, two water molecules, and two terminal N-bonded thiocyanato anions in an octahedral coordination environment (Figure 1, bottom). These complexes are linked into layers by O H···N hydrogen bonds. Compounds 1 and 2 represent potential precursors for the preparation of liganddeficient compounds and thus, were investigated by thermoanalytical methods. On heating compound 1, a single mass step is observed, which leads to the formation of [Co(NCS)2(bpe)]n (4). [10] If the hydrate 2 is heated, two mass steps are observed corresponding to the formation of the anhydrate 3 in the first step, which transforms into compound 4 in the second step (see Supporting Information). Based on this information, single crystals of 4 were prepared using hydrothermal conditions. In the crystal structure of 4 the cobalt cations are octahedrally coordinated by two Sand two N-bonded thiocyanato anions as well as two N-bonded bpe ligands. The cations are linked into chains by m-1,3 bridging thiocyanato anions, which are further connected into layers by the bpe ligands (Figure 1, middle). Magnetic measurements on all the compounds show significant differences between the ligand-rich precursors 1– 3 and the ligand-deficient compound 4. In compounds 1–3 the thiocyanato anions are only terminal N-bonded, so that paramagnetic behavior is observed (see Supporting Information). On cooling, decreasing cm T values are observed until at about 25 K from which point increasing cm T values are observed which decrease again at approximately 4 K. A small magnetic exchange through the bpe ligands cannot be completely excluded. In contrast, for 4 a ferromagnetic coupling is observed between neighbored Co centers at HDC = 1 kOe (DC = direct current). Moreover, in the hysteresis curve a step is observed indicating metamagnetic behavior (Figure 2). Magnetic measurements at HDC = 0.1 kOe show antiferromagnetic behavior (Figure 3). Additional field dependent alternating current (AC) measurements using an external static field (HDC = 2 kOe, HAC = 10 Oe) show a transition from antiferromagnetic to ferromagnetic behavior at H>HC [*] Dipl.-Chem. S. W hlert, Dipl.-Chem. J. Boeckmann, Dr. M. Wriedt, Prof. Dr. C. N ther Institut f r Anorganische Chemie Christian-Albrechts-Universit t zu Kiel Max-Eyth-Strasse 2, 24118 Kiel (Germany) E-mail: cnaether@ac.uni-kiel.de

Metamagnetism and Single-Chain Magnetic Behavior in a Homospin One-Dimensional Iron(II) Coordination Polymer

Reaction of FeCl(2)·4H(2)O with KNCSe and pyridine in ethanol leads to the formation of the discrete complex [Fe(NCSe)(2)(pyridine)(4)] (1) in which the Fe(II) cations are coordinated by two N-terminal-bonded selenocyanato anions and four pyridine co-ligands. Thermal treatment of compound 1 enforces the removal of half of the co-ligands leading to the formation of a ligand-deficient (lacking on neutral co-ligands) intermediate of composition [Fe(NCSe)(2)(pyridine)(2)](n) (2) to which we have found no access in the liquid phase. Compound 2 is obtained only as a microcrystalline powder, but it is isotypic to [Cd(NCSe)(2)(pyridine)(2)](n) and therefore, its structure was determined by Rietveld refinement. In its crystal structure the metal cations are coordinated by two pyridine ligands and four selenocyanato anions and are linked into chains by μ-1,3 bridging anionic ligands. Magnetic measurements on compound 1 show only paramagnetic behavior, whereas for compound 2 an unexpected magnetic behavior is found, which to the best of our knowledge was never observed before for a iron(II) homospin compound. In this compound metamagnetism and single-chain magnetic behavior coexist. The metamagnetic transition between the antiferromagnetically ordered phase and a field-induced ferromagnetic phase of the high-spin iron(II) spin carriers is observed at a transition field H(C) of 1300 Oe and the single-chain magnetic behavior is characterized by a blocking temperature T(B), estimated to be about 5 K.

Cr (en)2SbS3:: the first example of a single SbS3 ligand in a transition-metal-complex, and the crystal structure of Cr (en)3SbS4

Abstract Application of mild solvothermal conditions in the reaction of CrCl3·8H2O, Sb2S3 and S in an aqueous solution of ethylenediamine (en) yielded the two novel thioantimonate compounds Cr (en)2SbS3 (1) and [Cr (en)3]SbS4 (2). The crystal structure of 1 consists of neutral chromium centered complexes, where chromium is chelated by two ethylendiamine molecules and one bidentate SbS 33− group giving a distorted octahedral coordination. Compound 1 is the first example of a transition metal complex with a SbS 33− ligand. Slightly different synthetic conditions lead to the formation of 2, which consists of isolated octahedral coordinated Cr (en)33 cations and tetrahedral SbS 43− anions. The crystal structures were determined and the thermal decomposition was investigated.

Coordination‐Induced Spin Crossover (CISCO) through Axial Bonding of Substituted Pyridines to Nickel–Porphyrins: σ‐Donor versus π‐Acceptor Effects

Nickel-porphyrins, with their rigid quadratic planar coordination framework, provide an excellent model to study the coordination-induced spin crossover (CISCO) effect because bonding of one or two axial ligands to the metal center leads to a spin transition from S=0 to S=1. Herein, both equilibrium constants K(1S) and K(2), and for the first time also the corresponding thermodynamic parameters DeltaH(1S), DeltaH(2), DeltaS(1S), and DeltaS(2), are determined for the reaction of a nickel-porphyrin (Ni-tetrakis(pentafluorophenyl)porphyrin) with different 4-substituted pyridines by temperature-dependent NMR spectroscopy. The association constants K(1S) and K(2) are correlated with the basicity of the 4-substituted pyridines (R: OMe>H>CO(2)Et>NO(2)) whereas the DeltaH(1S) values exhibit a completely different order (OMeCO(2)Et>NO(2)). 4-Nitropyridine exhibits the largest binding enthalpy, which, however, is overcompensated by a large negative binding entropy. We attribute the large association enthalpy of nitropyridine with porphyrin to the back donation of electrons from the Ni d(xz) and d(yz) orbitals into the pi orbitals of pyridine, and the negative association entropy to a decrease in vibrational and internal rotation entropy of the more rigid porphyrin-pyridine complex. Back donation for the nitro- and cyanopyridine complexes is also confirmed by IR spectroscopy, and shows a shift of the N-O and C-N vibrations, respectively, to lower wave numbers. X-ray structures of 2:1 complexes with nitro-, cyano-, and dimethylaminopyridine provide further indication of a back donation. A further trend has been observed: the more basic the pyridine the larger is K(1S) relative to K(2). For nitropyridine K(2) is 17 times larger than K(1S) and in the case of methoxypyridine K(2) and K(1S) are almost equal.

[V16Sb4O42(H2O){VO(C6H14N2)2}4]: a terminal expansion to a polyoxovanadate archetype.

The charge-neutral antimonatopolyoxovanadium(IV) cluster [V(IV)16Sb(III)4O42(H2O){V(IV)O(C6H14N2)2}4].10H2O.C6H14N2 was obtained under solvothermal conditions. The central cluster fragment, [V(IV) 16Sb(III)4O42], is a derivative of the [V18O42] archetype and is formed by replacing two VO5 polyhedra by two Sb2O5 units. The {V20Sb4} structure expands the {V16Sb4} motif by the addition of four square-pyramidal, terminal VO(1,2-diaminocyclohexane)2 groups. At low temperatures, the magnetic ground state is characterized by four independent S = 1/2 sites.

Rational design of bridging selenocyanates by thermal decomposition reactions

The powerful use of thermal decomposition reactions as tool for the directed synthesis of a new selenocyanate is described. This method offers a facile access to a large number of new compounds, in which the metal centers are bridged by small anionic linker ligands and therefore, cooperative magnetic phenomena can be expected.

Synthesis, Crystal Structure, and Magnetic Properties of the Coordination Polymer [Fe(NCS)2(1,2-bis(4-pyridyl)-ethylene)]n Showing a Two Step Metamagnetic Transition

Reaction of iron(II) thiocyanate with an excess of trans-1,2-bis(4-pyridyl)-ethylene (bpe) in acetonitrile at room temperature leads to the formation of [Fe(NCS)(2)(bpe)(2)·(bpe)] (1), which is isotypic to its Co(II) analogue. Using slightly different reaction conditions the literature known compound [Fe(NCS)(2)(bpe)(2)(H(2)O)(2)] (2) was obtained as a phase pure material. Simultaneous differential thermoanalysis and thermogravimetry prove that the hydrate 2 transforms into the anhydrate [Fe(NCS)(2)(bpe)(2)] (3), that decomposes on further heating into the new ligand-deficient 1:1 compound of composition [Fe(NCS)(2)(bpe)](n) (4), which can also be obtained directly by thermal decomposition of 1. Further investigations reveal that 4 can also be prepared under solvothermal conditions, and single crystal structure analysis shows that the iron(II) cations are linked via μ-1,3 bridging thiocyanato anions into chains, that are further connected into layers by the bpe ligands. Magnetic measurements, performed on powder samples, prove that 1 and 2 show only Curie-Weiss behavior, whereas in 4 antiferromagnetic ordering with a Néel temperature of 5.0 K is observed. At T < 4.0 K a two-step metamagnetic transition occurs at applied magnetic fields of 1300 and 1775 Oe. The magnetic properties are discussed and compared with those of related compounds.