Self-Assembly of {2}-Metallacryptands and {2}-Metallacryptates

Heterometallic clusters based on an uncommon asymmetric "V-shaped" [Fe3+(μ-OR)Ln3+(μ-OR)2Fe3+]6+ (Ln = Gd, Tb, Dy, Ho) structural core and the investigation of the slow relaxation of the magnetization behaviour of the [Fe2Dy] analogue

The synthesis, crystal structures, Mössbauer spectra and variable temperature dc and ac magnetic susceptibility studies of a new family of trinuclear heterometallic Fe³⁺/Ln³⁺ complexes, [Fe₂Ln(PhCO₂)₃((py)₂CO₂)((py)₂C(OMe)O)₂(NO₃)Cl] (Ln = Gd (1/Gd), Tb (1/Tb), Dy (1/Dy), and Ho (1/Ho)), where (py)₂CO₂^2- and (py)₂C(OMe)O^- are the anions of the gem-diol and hemiketal derivatives of di-2-pyridyl ketone, are reported. Compounds 1/Ln are based on an asymmetric "V-shaped" [Fe³⁺(μ-OR)Ln(μ-OR)₂Fe³⁺]⁶⁺ structural core formed from the connection of the two terminal Fe³⁺ centers to the central Ln³⁺ ion either through one or two alkoxide groups originating from the alkoxide-type bridging ligands. Direct current magnetic susceptibility studies reveal the presence of weak antiferromagnetic interactions between the Fe³⁺ ions. Alternating current magnetic studies indicate the presence of a slow-magnetic relaxation process in 1/Dy with an energy barrier U_eff = 6.7 (±0.3) K and a pre-exponential factor, τ₀ = 2.2 (±0.4) × 10^-7 s. The electronic, magnetic and relaxation properties of the complexes were further monitored by variable temperature ⁵⁷Fe Mössbauer spectroscopy. At T > 80 K the spectra from the complexes comprise two quadrupole doublets the hyperfine parameters of which reflect the distinct coordination environment of the two Fe³⁺ terminal sites. At T < 20 K, the Mössbauer spectra for 1/Dy are affected by magnetic relaxation effects. At 1.5 K, the spectrum of 1/Dy comprises well defined magnetic sextets indicating relaxation times slower than the characteristic time of the Mössbauer technique (10^-7 s) in agreement with the dynamic magnetic measurements. 1/Gd exhibits broad unresolved magnetic sextets at 1.5 K indicating that the spin relaxation time is of the order of the Mössbauer characteristic time at this temperature. For 1/Tb, 1/Ho the Mössbauer spectra exhibit slight broadening even at the lowest available temperature consistent with magnetic relaxation times less than 10^-7 s.

A new ZnII metallocryptand with unprecedented diflexure helix induced by V-shaped diimidazole building blocks

Taking advantage of V-shaped ligands, a Zn^II metallocryptand, namely {[Zn₂(didp)₂(m-bdc)₂]}_n, (1) [didp = 2,8-di(1H-imidazol-1-yl)-dibenzothiophene and m-H₂bdc = isophthalic acid], has been hydrothermally synthesized. Single-crystal X-ray diffraction analysis reveals a 26-membered butterfly-type metallomacrocycle [Zn₂(didp)₂]. One m-bdc^2- ligand bridges [Zn₂(didp)₂] units to form a laterally non-symmetric [Zn₂(didp)₂(m-bdc)] metallocryptand with an exo-exo conformation. Another crystallographically independent m-bdc^2- functions as a secondary synthon to bridge discrete metallocryptands into a 1D zigzag chain architecture. Undoubtedly, the choice of two matched ligands in this work is crucial for metallocryptand construction and structure expansion. Interestingly, a rare helical chain with two flexures in one single L and/or R strand is observed. Another important feature is the C-O...π interactions, by which the dimensionality extension of (1) can be induced. Fluorescence measurements and density functional theory (DFT) calculations illustrate that the emission of (1) can probably be attributed to ligand-to-ligand charge transfer (LLCT).

Self-assembly of bis-β-diketone-based [M22] dinuclear platforms into 2-dimensional coordination polymers

Bis-β-diketone [M₂L₂] dinuclear platforms self-assemble into 2-dimensional coordination polymers.

Endohedral dynamics of push-pull rotor-functionalized cages

A series of [Pd2L4] coordination cages featuring endohedral functionalities in central backbone positions was synthesized. Although attached via C[double bond, length as m-dash]C double bonds, the substituents behave as molecular rotors. This is explained by their pronounced donor-acceptor character which lowers rotational barriers and allows for electronic control over the spinning rates inside the cage. The dynamic behaviour of the free ligands, assembled cages and host-guest complexes is compared with the aid of NMR experiments, X-ray structure analysis and molecular modelling.

Iron(III) Metallacryptand and Metallacryptate Assemblies Derived from Aroylbis(N,N-diethylthioureas)

The reaction of isophthaloylbis(N,N-diethylthiourea), H₂L¹, with FeCl₃·6H₂O gives the dinuclear tris-complex [Fe₂(L¹)₃] (5), possessing a cryptand-like structure. A similar reaction with the ligand 2,6-dipicolinoylbis(N,N-diethylthiourea), H₂L², however, results in the formation of the anionic, mononuclear Fe(III) complex [Fe(L²)₂]^- (6), which could be isolated as its "Tl⁺ salt" by the subsequent addition of Tl(NO₃). A tighter view to the solid state structure of the obtained product, however, characterizes compound 6 as a one-dimensional coordination polymer, in which four-coordinate Tl⁺ ions connect the {[Fe(L²)₂]^-} units to infinite chains. When Fe³⁺ ions and Tl⁺ ions are added to H₂L² simultaneously in a one-pot reaction, a different product is obtained: a cationic trinuclear complex of the composition {M⊂[Fe₂(L²)₃]}⁺. It has been isolated as a PF₆^- salt and represents a {2}-metallacryptate with a nine-coordinate Tl⁺ ion in the central void. Structurally related products of the compositions {M⊂[Fe₂(L²)₃]}(PF₆) (M = Na⁺, K⁺, Rb⁺) (8(PF₆)) could be isolated from analogous reactions with alkaline salts instead of Tl(NO₃). {2}-Metallacryptates with larger central voids were synthesized with the ether-spaced aroylbis(N,N-diethylthiourea) H₂L³. The compounds {M⊂[Fe₂(L³)₃]}(PF₆) (M = K⁺, Rb⁺, Tl⁺ or Cs⁺) (9(PF₆)) were prepared by a similar protocol like those with H₂L² with the simultaneous addition of the metal ions to a solution of H₂L³. Due to the larger spacer between the aroylthiourea units, the coordination number of the central M⁺ ions is 12 by six carbonyl and six ether oxygen atoms. All products were characterized by elemental analysis, IR spectroscopy, and X-ray structure analysis. Cyclic voltammetric studies were carried out with the three representative complexes [Fe₂(L¹)₃], {K⊂[Fe₂(L²)₃]}(PF₆), and {K⊂[Fe₂(L³)₃]}(PF₆). The obtained voltammograms indicate the dependence of the redox properties of the oligonuclear systems on the conjugation in the organic backbones of the ligands.

Approaches to the synthesis of optically pure helicates

Methods by which helical bimetallic assemblies may be accessed as single isomers are described and reviewed. While there are a number of notable successes, the conclusion is made that no existing approach satisfies the criteria which will allow these fascinating compounds to emerge from the synthetic research laboratory environment to practical application in e.g. healthcare. For this to be achieved the compounds must be optically pure and non-racemising, soluble and chemically stable in water, readily available on a large scale, chemically diverse and synthetically robust.

Zn(3), Ni(3), and Cu(3) complexes of a novel tricompartmental acyclic ligand

A new tricompartmental acyclic ligand (H(4)L) was prepared and fully characterized. It reacts with zinc, nickel, and copper(II) acetate to yield [Zn(3)L(OAc)(2)].H(2)O (1.H(2)O), [Ni(3)L(OAc)(2)(MeOH)(2)].2MeCN.2MeOH (2.2MeCN.2MeOH), and [Cu(3)L(OAc)(2)].3H(2)O (3.3H(2)O), respectively. 2.2MeCN.2MeOH precipitates as single crystals, which lose the solvates upon drying to give 2. The reactivity of the acetate complexes in a methanol/acetonitrile basic medium in air was investigated. Thus, 1.H(2)O and 2 are unstable in this medium, both suffering partial hydrolysis to produce single crystals of [Zn(3)L(3-Br-5-Cl-Sal)(2)].2MeCN (4.2MeCN; 3-Br-5-Cl-Sal(-) = 3-bromo-5-chlorosalicylaldehydate) and [Ni(3)L(3-Br-5-Cl-Sal)(2)].2MeCN (5.2MeCN) as one of the reaction byproducts, while 3.3H(2)O gives rise to a reaction of ligand displacement, generating crystals of [Cu(3)L(OMe)(2)].MeOH (6.MeOH). Complexes 2.2MeCN.2MeOH, 4.2MeCN, 5.2MeCN, and 6.MeOH were crystallographically characterized, and these studies demonstrate not only the trinucleating ability of the fully deprotonated L(4-) ligand but also its tendency to induce chirality in the isolated compounds. The magnetic characterization of 2, 3.3H(2)O and 6.MeOH shows that the magnetic coupling between adjacent metal ions is weak, with 2 being ferromagnetic and 3.3H(2)O and 6.MeOH antiferromagnetic.