Dmitry N. Kozhevnikov

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.

Synthesis of Cyclometallated Platinum Complexes with Substituted Thienylpyridines and Detailed Characterization of Their Luminescence Properties

Synthesis of various derivatives of 2-(2-thienyl)pyridine via substituted 3-thienyl-1,2,4-triazines is reported. The final step of the synthesis is a transformation of the triazine ring to pyridine in an aza-Diels-Alder-type reaction. The resulting 5-aryl-2-(2-thienyl)pyridines (HL1-HL4) and 5-aryl-2-(2-thienyl)cyclopenteno[c]pyridines (HL5-HL8) (with aryl = phenyl, 4-methoxyphenyl, 2-naphtyl, and 2-thienyl) were used as cyclometallating ligands to prepare a series of eight luminescent platinum complexes of the type [Pt(L)(acac)] (L = cyclometallating ligand, acac = acetylacetonato). X-ray single crystal structures of three complexes of that series, [Pt(L5)(acac)] = [Pt(5-phenyl-2-(2-thienyl)cyclopenteno[c]pyridine)(acac)], [Pt(L6)(acac)] = [Pt(5-(4-methoxy)-2-(2-thienyl)cyclopenteno[c]pyridine)(acac)], and [Pt(L7)(acac)] = [Pt(5-(2-naphtyl)-2-(2-thienyl) cyclopenteno[c]pyridine)(acac)] were determined. Photoluminescence and electronic absorption spectra of the new [Pt(L)(acac)] complexes are reported. For two representative compounds of that series, [Pt(L4)(acac)] and [Pt(L5)(acac)], a detailed photophysical characterization based on highly resolved emission and excitation spectra, as well as on emission decay properties, was carried out. The studies down to low temperature (T = 1.2 K) and up to high magnetic fields (B = 10 T) allowed us to characterize the three individual substates of the emitting triplet state. In particular, it is shown that the lowest triplet states of [Pt(L4)(acac)] and [Pt(L5)(acac)] are largely ligand-centered (LC) of (3)pi pi* character, which experience only weak spin-orbit couplings to higher lying singlet states.

[1,5]Sigmatropic shift of hydrogen in amination of 3-pyrrolidino-1,2,4-triazine 4-oxide

Abstract The reaction of 3-pyrrolidino-1,2,4-triazine 4-oxide with ammonia leads to the product of tele -substitution of pyrrolidine - 5-amino-1,2,4-triazine 4-oxide. Sigmatropic shift of hydrogen postulated for such reactions has been proved by isolation of key intermediates.

Chloromethyl-, dichloromethyl-, and trichloromethyl-1,2,4-triazines and their 4-oxides: method for the synthesis and tele-substitution reactions with C-nucleophiles

A simple procedure was developed for the synthesis of 1,2,4-triazines and their 4-oxides containing the ClCH2, Cl2CH, or CCl3 group at position 3 by cyclization of 2-aryl-2-hydrazono-1-oximinoethanes with the corresponding chloroacetonitriles. The reaction pathway depends on the number of halogen atoms in the acetonitrile used. The reactions with trichloroacetonitrile, monochloroacetonitrile, and dichloroacetonitrile afford 3-trichloromethyl-1,2,4-triazines, 3-chloromethyl-1,2,4-triazine 4-oxides, and a mixture of the corresponding dichloromethyltriazines and their 4-oxides, respectively. The reactions of 3-trichloromethyl-1,2,4-triazines with indoles and phenols are accompanied by tele-substitution with elimination of halogen from the trichloromethyl group to give 5-indolyl- (or 5-hydroxyphenyl)-3-dichloromethyl-1,2,4-triazines.

Deoxygenative vs. Vicarious Nucleophilic Substitution of Hydrogen in Reactions of 1,2,4-Triazine 4-Oxides with α-Halocarbanions

The 3,6-diaryl-1,2,4-triazine 4-oxides 1a−e undergo a nucleophilic substitution of hydrogen with the α-halomethyl aryl sulfones 2, 3 and 7 by two alternative pathways: vicarious nucleophilic substitution (VNS) and/or an intramolecular deoxygenative process. The former pathway is found to dominate in the reaction of 1 with bromomethyl tolyl sulfone (7) yielding the 5-tosylmethyl-1,2,4-triazine 4-oxides 6, while the reaction with the chloromethyl aryl sulfones 2 and 3 leads to the 5-arylsulfonylchloromethyl-1,2,4-triazines 4 and 5, respectively, as the products of deoxygenative substitution. The reaction of 6-phenyl-1,2,4-triazine 4-oxide (1f) with the chloromethyl aryl sulfones 2 and 3 proceeds differently. At low temperature (−75 C) the products of the VNS reaction at position 5 (6f and 12f) are formed. At room temperature the 7-chloro-1-hydroxy-3-phenyl-7-(arylsulfonyl)-1,4,5-triazahepta-1,3,6-trienes 10f and 11f are obtained by addition of the carbanions 2 and 3 in the 3-position, followed by the ring opening of the resulting σ adduct. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

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.

A new route to 6,6′′-dicyano-2,2′:6′,2′′-terpyridines and their complexes with Ni(II)

Abstract A new methodology for the synthesis of functionalised 2,2′:6′,2′′-terpyridine systems is suggested: sequential synthesis of the heterocyclic assemblies based on the 1,2,4-triazine ring, direct introduction of the cyano group in the 1,2,4-triazine ring and the transformation of the latter to a pyridine ring via an aza-Diels–Alder reaction.

Tuning the Excimer Emission of Amphiphilic Platinum(II) Complexes Mediated by Phospholipid Vesicles

Two new amphiphilic platinum(II) complexes, [Pt(2-(4-fluorophenyl)-5-(4-dodecyloxyphenyl)pyridine) (acac)] (Pt-1) and [Pt(2-(4-dodecyloxyphenyl)-5-(thien-2-yl)-c-cyclopentenepyridine) (acac)] (Pt-2), where acac is acetylacetonate, were synthesized and characterized. Apart from conventional phosphorescence of single molecules (ME-monomer emission), complexes Pt-1 and Pt-2 also exhibit excimer emission (EE) when embedded into phospholipid vesicles, that is assigned to emissive Pt-Pt excimers. The EE intensity in vesicular media appeared to depend on the viscosity of the vesicles and the concentration of the embedded complex. Differences in the EE properties of complexes Pt-1 and Pt-2 are correlated with the energies of the π-character frontier orbitals defined by the design of the cyclometalating phenylpyridine ligand. Higher energies of the frontier π-orbitals (HOMO and LUMO) naturally promote stronger π-π interactions, thus obstructing the PtII-PtII interaction.

Triazines, Tetrazines and Fused Ring Polyaza Systems

Abstract The review covers work published in the calendar year 2010. Novel reaction chemistry and new ring synthetic methods for 1,2,3-triazines, 1,2,4-triazines, 1,3,5-triazines, tetrazines, and purines are reviewed.

CCDC 1421707: Experimental Crystal Structure Determination

Related Article: Valery N. Kozhevnikov , Dmitry N. Kozhevnikov , Tatiana V. Nikitina , Vladimir L. Rusinov , Oleg N. Chupakhin , Manfred Zabel , Burkhard Konig|2003|J.Org.Chem.|68|2882|doi:10.1021/jo0267955