Christian Heering

Bifunctional pyrazolate–carboxylate ligands for isoreticular cobalt and zinc MOF-5 analogs with magnetic analysis of the {Co4(μ4-O)} node

The ditopic ligands 3,5-dimethyl-pyrazolate-4-carboxylate, –Me2pzCO2–, and 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoate, –Me2pzC6H4CO2–, combine a pyrazolate and carboxylate functionality in axial orientation and lead to porous cobalt or zinc azolate–carboxylate frameworks that have the same cubic pcu-a topology and {M4(μ4-O)} nodes (M = Co, Zn) as MOF-5 and other IRMOFs. The microporous networks [M4(μ4-O)(Me2pzCO2)3] (M = Co, Zn) with the short linker exhibit a solvent-induced gate effect, evidenced by gas desorption hysteresis due to small pore apertures of 2.8 A diameter together with small amounts of high-boiling solvent remaining in the activated samples. For [Co4(μ4-O)(Me2pzCO2)3], the low-pressure H2 storage capacity (1.7 wt%, 1 bar , 77 K) is higher than for MOF-5, and the CO2 uptake of 20.8 wt% puts it among the top MOFs for low-pressure CO2 sorption even though the BET surface is less than 1000 m2 g−1. The analysis of the magnetic properties of [Co4(μ4-O)(Me2pzCO2)3] takes into account the distribution of tetrahedra resulting from the disorder of the pyrazolate–carboxylate linker. An antiferromagnetic coupling observed for [Co4(μ4-O)(Me2pzCO2)3] arises from the interactions of the cobalt(II) ions through the combined μ4-O + syn–syn carboxylate and μ4-O + pyrazolate bridges.

Cytotoxic 14-Membered Macrolides from a Mangrove-Derived Endophytic Fungus, Pestalotiopsis microspora

Seven new 14-membered macrolides, pestalotioprolides C (2), D-H (4-8), and 7-O-methylnigrosporolide (3), together with four known analogues, pestalotioprolide B (1), seiricuprolide (9), nigrosporolide (10), and 4,7-dihydroxy-13-tetradeca-2,5,8-trienolide (11), were isolated from the mangrove-derived endophytic fungus Pestalotiopsis microspora. Their structures were elucidated by analysis of NMR and MS data and by comparison with literature data. Single-crystal X-ray diffraction analysis was used to confirm the absolute configurations of 1, 2, and 10, while Moshers method and the TDDFT-ECD approach were applied to determine the absolute configurations of 5 and 6. Compounds 3-6 showed significant cytotoxicity against the murine lymphoma cell line L5178Y with IC50 values of 0.7, 5.6, 3.4, and 3.9 μM, respectively, while compound 5 showed potent activity against the human ovarian cancer cell line A2780 with an IC50 value of 1.2 μM. Structure-activity relationships are discussed. Coculture of P. microspora with Streptomyces lividans caused a roughly 10-fold enhanced accumulation of compounds 5 and 6 compared to axenic fungal control.

A fluorite isoreticular series of porous framework complexes with tetrahedral ligands: new opportunities for azolate PCPs

Three porous coordination polymers with a simplified formula of [M4(μ4-Cl)L2]·nGuest (M = Cu, Cd) based on two adamantane-derived tetrahedral tetrazolate ligands were synthesized. Along with one known representative, the row of compounds establishes a new isoreticular series with fluorite as the underlying net.

Syntheses, structures and properties of group 12 element (Zn, Cd, Hg) coordination polymers with a mixed-functional phosphonate-biphenyl-carboxylate linker

The new phosphonate-carboxylate ligand from 4-phosphono-biphenyl-4′-carboxylic acid (H2O3P–(C6H4)2–CO2H, H3BPPA) is based on the rigid biphenyl system and is studied toward the coordination behavior of group 12 elements zinc, cadmium and mercury. The crystalline products from hydrothermal syntheses highlight the versatile and different coordination modes with the (partially) deprotonated H3BPPA ligand to give coordination polymeric 3D-[Zn5(μ3-OH)4(μ4-O3P–(C6H4)2–CO2-μ2)2]n (5), 2D-[Zn(μ6-O3P–(C6H4)2–CO2H)]n (6), 3D-[Cd3(μ5-O3P–(C6H4)2–CO2-μ2)(μ6-O3P–(C6H4)2–CO2-μ3)]n (7) and 2D-[Hg(μ3-HO3P–(C6H4)2–CO2H)]n (8). The cobalt complex, 2D-[Co(μ4-O3P–(C6H4)2–CO2H)]n (9) is isostructural to 6. Through additional classic strong carbonyl O–H⋯O hydrogen bonding the dimensionality of the 2D coordination networks increases to 3D supramolecular frameworks. The carboxy-phosphonate ligand shows five different coordination modes which can be described as μ4-O3P–CO2-μ2 (5), μ6-O3P– (6), μ5-O3P–CO2-μ2, μ5-O3P–CO2-μ3 (7), and μ3-O3P– (8), that is, the ligand bridges altogether between 3 to 8 metal atoms with the phosphonate group alone connecting already 3 to 6 metal atoms. Layers of metal–oxygen polyhedra are interconnected via the biphenyl linker, which either coordinates metal atoms with both donor groups or the –COOH end forms tail-to-tail hydrogen bonds to create 3D or 2D coordination networks, respectively. In the flat {MOx} layers in 6 and 7 the Zn and Cd metal nodes represent a honeycomb and an mcm net, respectively. The coordination polyhedra of the Cd atoms in compound 7 were analyzed towards a trigonal-prismatic coordination environment. The complexes are hydrolytically very stable due to their hydrothermal preparation from aqueous solution at 180–200 °C. The compounds could be stored in water or air for months without apparent decomposition. Compounds 5 and 7, where the ligand is fully deprotonated, start to decompose at ∼400 °C. The fluorescence emission spectrum of the ligand, 4, shows an intense peak at 365 nm (λex = 316 nm). The fluorescence emission of the metal complexes 5, 7 and 9 is shifted towards larger wavelengths with values of 417 nm, 415 nm and 410 nm, respectively (λex = 354 nm for 5, λex = 350 nm for 7, λex = 400 nm for 8, λex = 360 nm for 9). In addition, the crystal structures of the H3BPPA ligand precursors 4-iodo-4′-biphenylcarboxylic acid methyl ester, and 4-diethylphosphono-4′-biphenylcarboxylic acid methyl ester are described here for the first time.

Achieving visible light excitation in carbazole-based Eu3+–β-diketonate complexes via molecular engineering

Herein, we present the synthesis, characterization and photophysical properties of a series of Eu3+ complexes prepared with novel carbazole-based fluorinated β-diketones, namely, 4,4,5,5,5-pentafluoro-3-hydroxy-1-(9-phenyl-9H-carbazol-2-yl)pent-2-en-1-one (L1) and 4,4,5,5,5-pentafluoro-3-hydroxy-1-(9-(4-methoxyphenyl)-9H-carbazol-2-yl)pent-2-en-1-one (L2) as primary ligands and a bidentate phosphine oxide molecule, 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene oxide (DDXPO) as ancillary ligand. Using the Sparkle/PM3 model the molecular geometries of the designed complexes are optimized and the luminescent parameters are calculated by the LUMPAC software. The results demonstrated that suitably expanded π-conjugation in the developed Eu3+–carbazole based β-diketonate complexes dramatically red-shifted the excitation maximum to the visible region (λex,max = 420 nm) with an impressive quantum yield (34–42%). The triplet state energy levels of L1 and L2 in the complexes are higher than that of the lowest excited level of Eu3+ ion, 5D0, so the photoluminescence mechanism of the Eu3+ complexes was proposed as a ligand-sensitized luminescence process. The predicted luminescent parameters from the Sparkle/PM3 structures are in agreement with the experimental data, which shows the efficacy of the theoretical models adopted in the present study.

A view on systematic truncation of tetrahedral ligands for coordination polymers

Bis-, tris- and tetrakis(carboxyphenyl)adamantanes were probed for the synthesis of coordination polymers of d-metals, with a successful outcome for Mn, Co, and Cd. Formation of sql, hcb and dia frameworks based on small clusters demonstrates the dominant role of the ligand shape in defining the outcome of crystallization.

Targeted solid phase fermentation of the soil dwelling fungus Gymnascella dankaliensis yields new brominated tyrosine-derived alkaloids

Seven new brominated tyrosine-derived alkaloids, gymnastatins T–Y (1–6) and dankastatin D (7), together with three known likewise brominated analogues gymnastatins I–K (8–10) were isolated from the soil fungus Gymnascella dankaliensis through fermentation on solid rice medium following addition of NaBr. None of these compounds were detected when the fungus was cultured on rice that either lacked NaBr or that contained NaCl instead, indicating a remarkable plasticity of the fungal secondary metabolism. All structures were elucidated on the basis of one and two dimensional NMR spectroscopic analyses and MS data. The absolute configuration of the new gymnastatin T (1) was determined by X-ray crystallographic data. All isolated alkaloids showed potent to moderate cytotoxicity against the L5178Y mouse lymphoma cell line with IC50 values ranging from 0.078 to 14.1 μM.