Sven H. Zottnick

Deprotonation of a Hydridoborate Anion

The first deprotonation of a borohydride anion was achieved by treatment of [BH(CN)3 ]- with strong non-nucleophilic bases, which resulted in the formation of alkali-metal salts of the tricyanoborate dianion B(CN)32- in up to 97 % yield and 99.5 % purity. [BH(CN)3 ]- is less acidic than (Me3 Si)2 NH but a stronger acid than iPr2 NH. Less sterically hindered, more nucleophilic bases such as PhLi and MeLi mostly attack a CN group under formation of imine dianions [RC(N)B(CN)3 ]2- , which can be hydrolyzed to ketones of the [RC(O)B(CN)3 ]- type. The boron-centered nucleophile B(CN)32- reacts with CO2 and CN+ reagents to give salts of the [B(CN)3 CO2 ]2- dianion and the tetracyanoborate anion [B(CN)4 ]- , respectively, in excellent yields.

Near-Infrared Luminescence and Inner Filter Effects of Lanthanide Coordination Polymers with 1,2-Di(4-pyridyl)ethylene

A series of 12 lanthanide coordination polymers was synthesized from anhydrous LnCl3 and 1,2-di(4-pyridyl)ethylene (dpe) under solvothermal conditions in either thiazole (thz) or pyridine (py). The reactions yielded ∞1[Ln2Cl6(dpe)2(thz)4]·dpe with Ln = Ce (1), Nd (2), ∞1[LnCl3 (dpe)(py)2]·(dpe/py) with Ln = Gd (3), Er (4), and ∞1[LnCl3(dpe) (thz)2](dpe/thz) with Ln = Sm (5), Gd (6), Tb (7), Dy (8), Er (9), Yb (10), as well as ∞1[HoCl3(dpe)(thz)2]·thz (11) and ∞2[La2Cl6(dpe)3(py)2]·dpe (12). One-dimensional coordination polymers (CPs) and a two-dimensional network of five different constitutions are formed by connection of LnCl3 units via dpe molecules. As free ligand, dpe shows an excimer effect that is reduced in the coordination polymers. In addition, dipyridylethylene proves to be a suitable sensitizer for the photoluminescence of lanthanides in the near-infrared region (NIR) only. Thereby, dpe differs from the related ligand 1,2-di(4-pyridyl)ethane. For the compounds presented, four different luminesc...

Anhydrous, Homoleptic Lanthanide Frameworks with the Pentafluoroethyltricyanoborate Anion

Pentafluoroethyltricyanoborate frameworks of rare-earth metal ions of the general formula [Ln{C2F5B(CN)3}3(OH2)n] (Ln = La, Eu, Ho; n = 0, 3; [Ln13(OH2)n]) were synthesized using the oxonium salt (H3O)[C2F5B(CN)3] ((H3O)1) and lanthanide chlorides LnCl3·nH2O as starting compounds. Single-crystals of ∞3[La{C2F5B(CN)3}3] (∞3[La13]) are obtained from the room temperature ionic liquid (RTIL) [EMIm]1 using either a ionothermal approach or by recrystallization of anhydrous microcrystalline [La13] that is obtained from reactions in aqueous media after drying in a vacuum. Removal of water from [Ln13(OH2)3] (Ln = Eu, Ho) to give microcrystalline ∞3[Ln13] is achieved in a vacuum at elevated temperatures. All compounds were characterized by vibrational and NMR spectroscopy, thermogravimetry, and elemental analysis. The structures of the three-dimensional coordination polymers ∞3[Eu13(OH2)3] and ∞3[La13] were elucidated by single-crystal X-ray-diffraction. According to powder diffraction studies on anhydrous ∞3[Ln13] (Ln = La, Eu, Ho), the three compounds are isotypic. A study of the photoluminescence properties reveals that both Eu3+ compounds, [Eu13] and [Eu13(OH2)3], are strongly luminescent, the emission of the anhydrous framework being significantly more intense than the one of the hydrate. The Eu-compounds benefit from a sensitizer effect of the anion. In contrast, the Ho-containing framework ∞3[Ho13] exhibits separate chromophores and a strong reabsorption of the fluorescence by the Ho3+ ions.

Post-Synthetic Shaping of Porosity and Crystal Structure of Ln-Bipy-MOFs by Thermal Treatment

The reaction of anhydrous lanthanide chlorides together with 4,4′-bipyridine yields the MOFs ∞2[Ln2Cl6(bipy)3]·2bipy, with Ln = Pr − Yb, bipy = 4,4′-bipyridine, and ∞3[La2Cl6(bipy)5]·4bipy. Post-synthetic thermal treatment in combination with different vacuum conditions was successfully used to shape the porosity of the MOFs. In addition to the MOFs microporosity, a tuneable mesoporosity can be implemented depending on the treatment conditions as a surface morphological modification. Furthermore, thermal treatment without vacuum results in several identifiable crystalline high-temperature phases. Instead of collapse of the frameworks upon heating, further aggregation under release of bipy is observed. ∞3[LaCl3(bipy)] and ∞2[Ln3Cl9(bipy)3], with Ln = La, Pr, Sm, and ∞1[Ho2Cl6(bipy)2] were identified and characterized, which can also exhibit luminescence. Besides being released upon heating, the linker 4,4′-bipyridine can undergo activation of C-C bonding in ortho-position leading to the in-situ formation of 4,4′:2′,2′′:4′′,4′′′-quaterpyridine (qtpy). qtpy can thereby function as linker itself, as shown for the formation of the network ∞2[Gd2Cl6(qtpy)2(bipy)2]·bipy. Altogether, the manuscript elaborates the influence of thermal treatment beyond the usual activation procedures reported for MOFs.

Optical isotherms as a fundamental characterization method for gas sensing with luminescent MOFs by comparison of open and dense frameworks

Optical isotherms as a novel fundamental characterization method for MOF sensors are presented by a combination of simultaneous monitoring of sorption processes of different analyte gases (N2, Ar, CO2, and O2) together with in situ photoluminescence spectroscopy. Thereby, a direct correlation of both properties, luminescence and adsorption, is achieved, which provides a direct quantitative access to the effect of the MOF–analyte interaction on the photoluminescence of a MOF system. In addition, changes in equilibration time of the sorption process, temperature dependence and cyclic repetition can be systematically investigated. Thereby, optical isotherms establish a frame of reference for MOF luminescence sorption sensors. The MET MOF system (MET = metal triazolate) was chosen as a porous model candidate. A strong intensity increase of the photoluminescence of the MET-type MOF 3∞[Zn(Tz)2], (Tz− = 1,2,3-triazolate), was achieved by introduction of Mn2+ as a luminescence activator. Statistical replacement of Zn2+ with Mn2+ in 3∞[Zn0.9Mn0.1(Tz)2] retains the original structural microporosity. The obtained optical isotherms were further compared to results from a non-porous, luminescent coordination polymer 3∞[Sr0.95Eu0.05(Im)2] in order to elaborate the novel characterization concept in a broader context, showing that this concept is a fundamental step to achieve quantitative read-out of the sensing signal for both, porous and dense systems.

Bioinspired co-crystals of Imatinib providing enhanced kinetic solubility

Graphical abstract Figure. No caption available. Abstract Realizing the full potential of co‐crystals enhanced kinetic solubility demands a comprehensive understanding of the mechanisms of dissolution, phase conversion, nucleation and crystal growth, and of the complex interplay between the active pharmaceutical ingredient (API), the coformer and co‐existing forms in aqueous media. One blueprint provided by nature to keep poorly water‐soluble bases in solution is the complexation with phenolic acids. Consequently, we followed a bioinspired strategy for the engineering of co‐crystals of a poorly water‐soluble molecule – Imatinib – with a phenolic acid, syringic acid (SYA). The dynamics of dissolution and solution‐mediated phase transformations were monitored by Nuclear Magnetic Resonance (NMR) spectroscopy, providing mechanistic insights into the 60 fold‐increased long lasting concentrations achieved by the syringate co‐crystals as compared to Imatinib base and Imatinib mesylate. This lasting effect was linked to SYA’s ability to delay the formation and nucleation of Imatinib hydrate – the thermodynamically stable form in aqueous media – through a metastable association of SYA with Imatinib in solution. Results from permeability studies evidenced that SYA did not impact Imatinib’s permeability across membranes while suggesting improved bioavailability through higher kinetic solubility at the biological barriers. These results reflect that some degree of hydrophobicity of the coformer might be key to extend the kinetic solubility of co‐crystals with hydrophobic APIs. Understanding how kinetic supersaturation can be shaped by the selection of an interactive coformer may help achieving the needed performance of new forms of poorly water‐soluble, slowly dissolving APIs.

Lanthanide Coordination Polymers and MOFs Based on the Dicyanodihydridoborate Anion

New lanthanide cyanoborates were synthesized from anhydrous lanthanide chlorides and the acid H[BH2 (CN)2 ] in either acetonitrile or pyridine. Reactions in acetonitrile lead to three-dimensional, anionic metal-organic frameworks (MOFs) 3 ∞ [Ln2 {BH2 (CN)2 }9 ]⋅[Ln(CH3 CN)9 ] (Ln=Ce, Eu, Tb) which incorporate complex cations [Ln(CH3 CN)9 ]3+ in the pores of the framework for charge compensation. In contrast, the reactions in pyridine result in the formation of one-dimensional coordination polymers 1 ∞ [H(py)2 ][LnCl2 {BH2 (CN)2 }2 (py)2 ]⋅0.5 py (Ln=Ce, Pr, py=pyridine) with [H(py)2 ]+ as counter ions for the anionic strand structure. The products show intense photoluminescence, for Ce3+ based on 5d-4f transitions in the blue spectral region, whereas the Eu3+ and Tb3+ compounds exhibit characteristic photoluminescence based on 4f-4f transitions of the respective lanthanide ions. The observed photoluminescence is mainly attributed to a direct excitation of the lanthanide ions and sensitization of the lanthanide ions by the [BH2 (CN)2 ]- anions. These results mark the utilized borate anions as versatile building block for new coordination compounds.

Bis-Salicylatoborate as Versatile Sensitizer for Highly Luminescent Ln-oxoborates from UV to NIR with 4f- and 5d-Participation of the Lanthanides

Several lanthanide bis(salicylato)borates (BSB) have been synthesized from anhydrous Ln-chlorides together with pyridine. The manifold products start with the formation of small complexes, indicated by [ErCl2(py)4(BSB)], on to 1D-polymers 1∞[Ln(BSB)3(py)2], Ln = Y, La - Nd, Sm, with a reduced py content. Along the lanthanide contraction, the formation of 2D-networks of the constitution 2∞[Ln(BSB)3(py)], Ln = Sm, Eu, Tb, Dy, Er, is observed by further release of py. The coordination polymers exhibit intense photoluminescence from the UV to the NIR region by Ln-specific 4f-4f-emission for Nd3+, Sm3+, Eu3+, Tb3+, and Dy3+. Emission is remarkable for Dy3+ (yellow-white) and Nd3+ (NIR) based on sensitizer effects of the [BSB]- anion that shows energy transfer to most lanthanide ions. For Ce3+, participation of 5d-states is observed giving parity-allowed broad band 5d-4f-emission. The bis(salicylato)borate ligand shows fluorescence in the UV with a short lifetime of only 2 ns, which makes the energy transfer in the other Ln-compounds remarkable and marks it as versatile sensitizer for these metal ions.