Harald Kelm

Temperature-Induced Spin-Transition in a Low-Spin Cobalt(II) Semiquinonate Complex†

Spin crossover and valence tautomerism are examples of processes that can be utilized as a basis for achieving molecular switches. Whereas the spin-crossover process is characterized by a temperature-, pressure-, or light-induced change of the electronic state of the metal ion to one with a different spin multiplicity, valence tautomerism entails an intramolecular redox reaction between a metal ion and a coordinated ligand, which, in a few instances, is accompanied by a change in the spin state of the metal ion. Various reported low-spin cobalt(III) catecholate complexes, which can be transformed into high-spin cobalt(II) semiquinonate complexes by raising the temperature, provide excellent examples of the latter process. In contrast, spin-crossover chemistry is dominated by octahedral iron(II) complexes with a FeN6 coordination sphere; [2] however, there are only very few known octahedral cobalt(II)-containing spin-crossover complexes. Herein we describe the first cobalt(II) semiquinonate complex that displays spin-crossover properties rather than valence tautomerism. The starting point of our investigation was the olive-green cobalt(III) 3,5-di-tert-butylcatecholate (dbc ) complex [Co(L-N4Me2)(dbc)](BPh4)·0.8MeCN·0.2Et2O (1) containing the dimethyl derivative of the tetraazamacrocyclic ligand 2,11diaza[3.3](2,6)pyridinophane (L-N4Me2) as coligand. This complex was obtained in 42 % yield by oxidation of the red cobalt(II) catecholate complex [Co(L-N4Me2)(dbc)] (prepared in situ from equimolar solutions of cobalt(II) perchlorate, L-N4Me2, and 3,5-di-tert-butylcatecholate) with ferrocenium tetrafluoroborate ([Fe(Cp)2](BF4); Cp = cyclopentadienyl), followed by a metathesis reaction with sodium tetraphenylborate (Scheme 1). In accordance with the description of 1 as a cobalt(III) catecholate complex, solutions and solids of this substance are diamagnetic. X-ray

Oxoanion binding to a cyclic pseudopeptide containing 1,4-disubstituted 1,2,3-triazole moieties.

A macrocyclic pseudopeptide 3 is described featuring three amide groups and three 1,4-disubstituted 1,2,3-triazole units along the ring. This pseudopeptide was designed such that the amide NH groups and the triazole CH groups converge toward the cavity, thus creating an environment well suited for anion recognition. Conformational studies in solution combined with X-ray crystallography confirmed this preorganisation. Solubility of 3 restricted binding studies to organic media such as 5 vol% DMSO/acetone or DMSO/water mixtures with a water content up to 5 vol%. These binding studies demonstrated that 3 binds to a variety of inorganic anions in DMSO/acetone including chloride, nitrate, sulfate, and dihydrogenphosphate anions. In the more competitive DMSO/water mixtures, only affinity to the more strongly coordinating oxoanions is retained. Quantitative binding studies showed that dihydrogen phosphate complexation in DMSO/water involves the dimer of the H2PO4- anion. By contrast, sulfate and hydrogenpyrophosphate complexation involves a stepwise process comprising formation of a 1 : 1 complex followed by a 2R : 1A complex in which two molecules of 3 (R) bind to a single anion (A). While the second binding equilibrium is associated with a much smaller stability constant in comparison with the first one in the case of sulfate complexation, the two binding constants are of similar magnitude in the case of the hydrogenpyrophosphate anion. Formation of the 2R : 1A complex was attributed to the fact that the cavity size and rigidity of 3 prevents saturation of all hydrogen acceptor sites on the anionic guests.

Bimetallic Cu/Pd Catalysts with Bridging Aminopyrimidinyl Phosphines for Decarboxylative Cross-Coupling Reactions at Moderate Temperature

A bimetallic catalyst system is presented that enables the decarboxylative cross‐coupling of triflates with carboxylate salts at only 100 °C, which is 70 °C lower than with previous Cu/Pd‐based systems. The new protocol allows the coupling of a broad range of aryl triflates with various substituted 2‐nitrobenzoates in good to excellent yields. The key feature of the catalyst system is a bidentate P,N‐ligand designed to bridge the Pd and Cu centres and thereby facilitating the rate‐determining transmetalation step. Mass spectrometry (ESI‐MS) studies support the ability of the aminopyrimidinyl phosphine to simultaneously coordinate copper and palladium.

The Strong β−CF3 Shielding Effect in Hexafluoroisopropanol and 100 Other Organic Solvents Revisited with 17O NMR Spectroscopy

An 17O NMR spectroscopy survey of more than 100 ubiquitous organic solvents and compounds, including some typical oxofluorinated solvents such as hexafluoroisopropanol, trifluoroethanol, trifluoroacetic acid, and others, is presented with D2O as a reference. A strong alternating α,β−CF3‐substituent chemical shift effect was thus observed. This alternating deshielding–shielding effect is suspected to have a role in the exceptional properties of these oxofluorinated solvents, notably in oxidative cross‐coupling reactions.

Mechanism of the Dehydrogenative Phenothiazination of Phenols

Abstract The straightforward capture of oxidized phenothiazines with phenols under aerobic conditions represents a unique cross‐dehydrogenative C−N bond‐forming reaction in terms of operational simplicity. The mechanism of this cross‐dehydrogenative N‐arylation of phenothiazines with phenols has been the object of debate, particularly regarding the order in which the substrates are oxidized and their potentially radical or cationic nature. Understanding the selective reactivity of phenols for oxidized phenothiazines is one of the key objectives of this study. The reaction mechanism is investigated in detail by utilizing electron paramagnetic resonance spectroscopy, cyclic voltammetry, radical trap experiments, kinetic isotope effects, and solvent effects. Finally, the key reaction steps are calculated by using density functional theory (DFT) and broken‐symmetry open‐shell singlet DFT methods to unravel a unique biradical mechanism for the oxidative phenothiazination of phenols.

Bromidotetra­kis­(2-ethyl-1H-imidazole-κN3)copper(II) bromide

The CuII ion in the title molecular salt, [CuBr(C5H8N2)4]Br, is coordinated in a square-pyramidal geometry by four N atoms of imidazole ligands and one bromide anion in the apical position. In the crystal, the ions are linked by N—H⋯Br hydrogen bonds involving both the coordinating and the free bromide species as acceptors. A C—H⋯Br interaction is also observed. Overall, a three-dimensional network results.

From mononuclear to polynuclear: Copper and zinc complexes obtained from polypyridylamine ligands related to tris(2-pyridylmethyl)-amine (tmpa)

Abstract Copper(I)/(II) complexes and a zinc compound with polypyridylamine ligands (related to the tripodal ligand tris(2-pyridylmethyl)amine, tmpa) were synthesized. Crystallographic characterization was possible for most of the complexes obtained. The different structures of the complexes allowed some insight into the basic understanding of the design of coordination polymers. Depending on ligand, solvent or anion, either mononuclear, dinuclear or polynuclear complexes were obtained. Graphical Abstract

Molecular tectonics: homochiral coordination polymers based on pyridyl-substituted cyclic tetrapeptides

Upon combining enantiopure bis-4-pyridylphenyl-substituted cyclotetrapeptides with HgCl2 and CdCl2, homochiral 1D zigzag coordination polymers were obtained, while the use of a bis-4-pyridyl-substituted cyclotetrapeptide with HgCl2 led to the formation of a different type of 1D coordination network.

tert-Butyl 1-hy­droxy­piperidine-2-carboxyl­ate

The title compound, C10H19NO3, is a disubstituted piperidine bearing substituents in two equatorial positions. One of the substituents is a hydroxy group bound to nitrogen and the second a tert-butyl ester group bound to the carbon next to the endocyclic nitrogen. Enantiomers of the title compound form hydrogen-bridged dimers across a center of inversion.