Arene Ruthenium(II) Complexes Bearing the κ-P or κ-P,κ-S Ph2P(CH2)3SPh Ligand

Neutral [Ru(η⁶-arene)Cl₂{Ph₂P(CH₂)₃SPh-κP}] (arene = benzene, indane, 1,2,3,4-tetrahydronaphthalene: 2a, 2c and 2d) and cationic [Ru(η⁶-arene)Cl(Ph₂P(CH₂)₃SPh-κP,κS)]X complexes (arene = mesitylene, 1,4-dihydronaphthalene; X = Cl: 3b, 3e; arene = benzene, mesitylene, indane, 1,2,3,4-tetrahydronaphthalene, and 1,4-dihydronaphthalene; X = PF₆: 4a–4e) complexes were prepared and characterized by elemental analysis, IR, ¹H, ¹³C and ³¹P NMR spectroscopy and also by single-crystal X-ray diffraction analyses. The stability of the complexes has been investigated in DMSO. Complexes have been assessed for their cytotoxic activity against 518A2, 8505C, A253, MCF-7 and SW480 cell lines. Generally, complexes exhibited activity in the lower micromolar range; moreover, they are found to be more active than cisplatin. For the most active ruthenium(II) complex, 4b, bearing mesitylene as ligand, the mechanism of action against 8505C cisplatin resistant cell line was determined. Complex 4b induced apoptosis accompanied by caspase activation.     

Excitation dependent quenching of luminescence in LED phosphors

Droop, the decrease of efficiency with increased power density, became a major topic with InGaN LEDs, after its introduction in 2007. This paper provides insight into droop in localized center luminescence phosphors, exemplified here by Eu²⁺ doped materials. This topic is of increasing importance, as high brightness blue LEDs have reached outputs >1 W/mm². The nonlinearities in phosphor quantum efficiency result in drive‐dependent color point shift and reduction of overall efficiency of phosphor converted white LEDs which utilize Eu²⁺ activated phosphors. The efficiency quenching can be traced back to two processes, well‐known in laser physics, excited state absorption or/and cross relaxation by Foerster/Dexter transfer. Both processes lead to reduction in phosphor efficiency, but they can be differentiated. Understanding the root cause of efficiency quenching opens ways to minimize the practical consequences. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Insights into LiAlH4 Catalyzed Imine Hydrogenation

Commercial LiAlH₄ can be used in catalytic quantities in the hydrogenation of imines to amines with H₂. Combined experimental and theoretical investigations give deeper insight in the mechanism and identifies the most likely catalytic cycle. Activity is lost when Li in LiAlH₄ is exchanged for Na or K. Exchanging Al for B or Ga also led to dramatically reduced activities. This indicates a heterobimetallic mechanism in which cooperation between Li and Al is crucial. Potential intermediates on the catalytic pathway have been isolated from reactions of MAlH₄ (M=Li, Na, K) and different imines. Depending on the imine, double, triple or quadruple imine insertion has been observed. Prolonged reaction of LiAlH₄ with PhC(H)=NtBu led to a side-reaction and gave the double insertion product LiAlH₂[N]₂ ([N]=N(tBu)CH₂Ph) which at higher temperature reacts further by ortho-metallation of the Ph ring. A DFT study led to a number of conclusions. The most likely catalyst for hydrogenation of PhC(H)=NtBu with LiAlH₄ is LiAlH₂[N]₂. Insertion of a third imine via a heterobimetallic transition state has a barrier of +23.2 kcal mol⁻¹ (ΔH). The rate-determining step is hydrogenolysis of LiAlH[N]₃ with H₂ with a barrier of +29.2 kcal mol⁻¹. In agreement with experiment, replacing Li for Na (or K) and Al for B (or Ga) led to higher calculated barriers. Also, the AlH₄⁻ anion showed very high barriers. Calculations support the experimentally observed effects of the imine substituents at C and N: the lowest barriers are calculated for imines with aryl-substituents at C and alkyl-substituents at N.     

Switching the Mechanism of NADH Photooxidation by Supramolecular Interactions

A series of three Ru(II) polypyridine complexes was investigated for the selective photocatalytic oxidation of NAD(P)H to NAD(P)⁺ in water. A combination of (time-resolved) spectroscopic studies and photocatalysis experiments revealed that ligand design can be used to control the mechanism of the photooxidation: For prototypical Ru(II) complexes a ¹O₂ pathway was found. Rudppz ([(tbbpy)₂Ru(dppz)]Cl₂, tbbpy=4,4'-di-tert-butyl-2,2'-bipyridine, dppz=dipyrido[3,2-a:2′,3′-c]phenazine), instead, initiated the cofactor oxidation by electron transfer from NAD(P)H enabled by supramolecular binding between substrate and catalyst. Expulsion of the photoproduct NAD(P)⁺ from the supramolecular binding site in Rudppz allowed very efficient turnover. Therefore, Rudppz permits repetitive selective assembly and oxidative conversion of reduced naturally occurring nicotinamides by recognizing the redox state of the cofactor under formation of H₂O₂ as additional product. This photocatalytic process can fuel discontinuous photobiocatalysis.     

Crystal and Molecular Structure of 3-Methoxy-estra-1,3,5(10)-trien-17β-ol

The crystal structure determination of the title compound was undertaken to determine the conformational flexibility of the steroid skeleton. 3-Methoxy-estra-1,3,5(10)-trien-17β-ol (C₁₉H₂₆O₂) crystallizes in the space group P2₁2₁2₁ with Z = 12, a = 12.589(2), b = 16.274(5), and c = 23.535(6) Å. The structure was solved by direct methods and refined to R = 0.056. Three symmetry independent molecules having different molecular conformations were observed in the crystal. The most flexible region of the molecules is the B ring which adopts following conformations: 7α,8β-half-chair, 8β-sofa, and an intermediate form between half-chair and sofa.