Anton M. Prokhorov

Brightly Luminescent Pt(II) Pincer Complexes with a Sterically Demanding Carboranyl-Phenylpyridine Ligand: A New Material Class for Diverse Optoelectronic Applications

A series of three Pt(II) complexes with a doubly cyclometalating terdentate ligand L1, L1H2 = 3,6-bis(p-anizolyl)-2-carboranyl-pyridine, and diethyl sulfide (1), triphenylphosphine (2), and t-butylisonitrile (3) as ancillary ligands were synthesized. X-ray diffraction studies of 1 and 2 show a coordination of the L1 ligand in a C-N-C mode in which the bulky and rigid o-carborane fragment is cyclometalated via a C atom. Importantly, no close intermolecular Pt-Pt contacts occur with this ligand type. The new Pt(II) pincer complexes display very high luminescence quantum yields at decay times of several tens of μs even in solution under ambient conditions. On the basis of the low-temperature (T = 1.3 K) emission decay behavior, the emission is assigned to a ligand centered triplet excited state (3)LC with small (1,3)MLCT admixtures. Because the phosphorescence is effectively quenched by molecular oxygen, optical sensors operating in a wide range of oxygen pressure can be developed. Owing to the very high luminescence quantum yields, the new materials might also become attractive as emitter materials for diverse optoelectronic applications.

Emissive metallomesogens based on 2-phenylpyridine complexes of iridium(III).

Preparation of Ir(III) complexes using anisotropic 2,5-di(4-alkoxyphenyl)pyridine ligands leads to emissive, liquid-crystalline complexes containing bound Cl and dimethyl sulfoxide. Using analogous poly(alkoxy) ligands allows the preparation of bis(2-phenylpyridine)iridium(III) acac complexes, which are also mesomorphic. The observation of liquid crystallinity in octahedral complexes of this type is without precedent.

Phosphorescent Mesomorphic Dyads Based on Tetraacetylethane Complexes of Iridium(III)

Complexes of iridium and platinum with aromatic ligands are of interest as emissive materials owing to their high spin–orbit coupling constants, which can promote phosphorescence from triplet states, a process that is otherwise formally forbidden. They are particularly relevant to organic light-emitting diode (OLED) technology, where the combination of charges leads to a surplus of triplet to singlet states, although their application in areas such as bioimaging and sensing have also received recent significant attention. Incorporation of emissive complexes can raise the maximum internal efficiency of an OLED to 100% by promoting triplet emission. 4] Thompson and co-workers have shown that the bis(2phenylpyridine)iridium(III) chromophore is a very effective triplet emitter, the physical properties of which make it suitable for incorporation into devices, and the emission characteristics of which can be readily tuned through substitution of the ligands. To add the property of liquid crystallinity to emissive materials is also of interest, for the ordered state of the molecules in the different mesophases suggests an improved pathway for the transport of charge carriers, whereas certain mesophases offer the possibility of polarized emission. We have reported liquid-crystalline Pt(II) phosphor compounds based on 1,3-di(2-pyridyl)benzene ligands that form columnar phases, whereas those based on extended 2,5-diphenylpyridines show nematic and SmA phases with very high photoluminescent quantum yields. More recently and following a report of a luminescent, ionic, liquid-crystalline complex of Ir(III), we reported the first example of a charge-neutral, luminescent, mesogenic complex of Ir(III) based on a hexacatenar phenylpyridine (ppy) ligand, with acetylacetonate (acac) as an ancillary ligand (1a). In the same study, a related complex (1b) based on a tetracatenar ligand was found to be highly emissive (F = 50%) but not liquid-crystalline, whereas the di-m-chloro dimer, 2, of the same ligand was liquid-crystalline, but scarcely emissive (F< 1%; Figure 1).

The amidine rearrangement in 5-amino-6-aryl-1,2,4-triazine-4-oxides initiated by hydroxylamine

Abstract Addition of hydroxylamine at the 3 position of 6-aryl-5-amino-1,2,4-triazine-4-oxides initiates the amidine rearrangement resulting in 6-aryl-5-hydroxylamine-1,2,4-triazines, as confirmed by an experiment with 15 N-labeling.

SNH reactions of pyrazine N-oxides and 1,2,4-triazine 4-oxides with CH-active compounds

Nucleophilic substitution of hydrogen in pyrazine N-oxides under the action of CH-active compounds requires activation with acylating agents. This activation facilitates aromatization of intermediate σH adducts via elimination of the acid residue to form substituted pyrazines. More electrophilic 1,2,4-triazine 4-oxides react with carbanions derived from CH-active compounds without additional activation according to a scheme, which has previously been unknown for azine N-oxides. This scheme involves aromatization of σH adducts through elimination of water by the E1cb mechanism. The reaction products occur in DMSO-d6 solutions predominantly as 6-methylene-1,6-dihydropyrazines and 5-methylene-4,5-dihydro-1,2,4-triazines.

Cyclohexanedione Bisaminals as Intermediates for Cyclen, Homocyclen, and Cyclam Synthesis

Abstract A new easy‐to‐run route to cyclen, homocyclen, and cyclam is proposed, based on the cyclization with dibromo‐ or ditosyloxy‐derivatives of bisaminal intermediates obtained by condensation of the appropriate linear tetraamine with cyclohexanedione. In the cyclization step, the use of cesium carbonate instead of potassium carbonate as proton trapper caused a remarkable increase of yields.

Synthesis and mesomorphism of modified 2,5-Di(4-dodecoxyphen-1-yl)-c-cyclopentene-pyridines

ABSTRACT Herein, we report on a new series of mesomorphic compounds synthesised by directional functionalisation of 2,5-di(4-dodecoxyphen-1-yl-c-cyclopentene-pyridine) on the c-cyclopentene moiety. The first functionalisation of the starting compound gave rise to a racemic product modified with a hydroxyl group on the fused cyclopentene ring. Further oxidation of this alcohol to a prochiral ketone and subsequent enantioselective reduction back to the alcohol afforded a new chiral alcohol. Further, the hydroxyl group of the chiral alcohol could be substituted by fluorine in a SN2 reaction, leading to a chiral compound with enantiomeric excess (ee) of 66% and chiral nematic liquid-crystalline (LC) phase. GRAPHICAL ABSTRACT

Nucleophilic substitution or dipolar 1,3-cycloaddition in reactions of cyanamide with 4-arylpyrimidine 1-oxides

Pyrimidine 1-oxides with cyanamide afforded 2-ureidopyrimidines as the result of the nucleophilic substitution of hydrogen, whereas the formation of similar 2-trichloroacetylaminopyrimidines occurs as dipolar 1,3-cycloaddition of the same oxides to trichloroacetonitrile under much more drastic conditions and in lower yields.

Nucleophilic substitution of hydrogen in the reaction of 1,2,4-triazin-4-oxides with cyanamide

It was shown that cyanamide can successfully be used in reactions of nucleophilic substitution of hydrogen with 1,2,4-triazin-4-oxides in the presence of a base to give 5-cyanoimino-1,2,4-triazines. It was found by13C NMR spectroscopy that these compounds and their alkylation products at the cyclic nitrogen atom exist in the form of 5-cyanoimino-2,5-dihydro-1,2,4-triazines.

Triazines and Tetrazines

Abstract This review covers the work published in the calendar year 2013. Novel reaction chemistry and new ring synthetic methods for 1,2,3-triazines, 1,2,4-triazines, 1,3,5-triazines, and tetrazines are reviewed.