Effect of metal cations [Li+, Na+, K+, Be2+, Mg2+, and Ca2+] on the structure of 2‐(3′‐hydroxy‐2′‐pyridyl)benzoxazole: A theoretical investigation

Density functional theory calculations were performed at the B3LYP/6‐311++G(d,p) level to systematically explore the geometrical multiplicity and binding strength for the complexes formed by alkaline and alkaline earth metal cations, viz. Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺, and Ca²⁺ (Mⁿ⁺, hereinafter), with 2‐(3′‐hydroxy‐2′‐pyridyl)benzoxazole. A total of 60 initial structures were designed and optimized, of which 51 optimized structures were found, which could be divided into two different types: monodentate complexes and bidentate complexes. In the cation‐heteroatom complex, bidentate binding is generally stronger than monodentate binding, and of which the bidentate binding with five‐membered ring structure has the strongest interaction. Energy decomposition revealed that the total binding energies mainly come from electrostatic interaction for alkaline metal ion complexes and orbital interaction energy for alkaline earth metal ion complex. In addition, the electron localization function analysis show that only the Be? O and Be? N bond are covalent character, and others are ionic character. © 2012 Wiley Periodicals, Inc.     

Proton‐Assisted Hydrogen Activation on Polyhedral Cations

The treatment of [1,1‐(PR₃)₂‐3‐(Py)‐closo‐1,2‐RhSB₉H₈] (PR₃=PMe₃ ( 2 ) or PPh₃ and PMe₃ ( 3 ); Py=pyridine) with triflic acid (TfOH) affords [1,3‐μ‐(H)‐1,1‐(PR₃)₂‐3‐(Py)‐1,2‐RhSB₉H₈]⁺ (PR₃=PMe₃ ( 4 ) or PMe₃ and PPh₃ ( 5 )). These products result from the protonation of the 11‐vertex closo‐cages along the Rh(1)? B(3) edge. These unusual cationic rhodathiaboranes are stable in solution and in the solid state and they have been fully characterized by multinuclear NMR spectroscopy. In addition, compound 5 was characterized by single‐crystal X‐ray diffraction. One remarkable feature in these structures is the presence of three {Rh(PPh₃)(PMe₃)}‐to‐{ηⁿ‐SB₉H₈(Py)} (n=4 or 5) conformers in the unit cell, thus giving an uncommon case of conformational isomerism. [1,1‐(PPh₃)₂‐3‐(Py)‐closo‐1,2‐RhSB₉H₈] ( 1 ), that is, the bis‐PPh₃‐ligated analogue of compounds 2 and 3 , is also protonated by TfOH, but, in marked contrast, the resulting cation, [1,3‐μ‐(H)‐1,1‐(PPh₃)₂‐3‐(Py)‐1,2‐RhSB₉H₈]⁺ ( 6 ), is attacked by a triflate anion with the release of a PPh₃ ligand and the formation of [8,8‐(OTf)(PPh₃)‐9‐(Py)‐nido‐8,7‐RhSB₉H₉] ( 9 ). The result is an equilibrium that involves cationic species 6 , neutral OTf‐ligated compound 9 , and [HPPh₃]⁺, which is formed upon protonation of the released PPh₃ ligand. The resulting ionic system reacts readily with H₂ to give cationic species [8,8,8‐(H)(PPh₃)₂‐9‐(Py)‐nido‐8,7‐RhSB₉H₉]⁺ ( 7 ). This reactivity is markedly higher than that previously found for compound 1 and it introduces a new example of proton‐assisted H₂ activation that occurs on a polyhedral boron‐containing compound.     

A Silyliumylidene Cation Stabilized by an Amidinate Ligand and 4‐Dimethylaminopyridine

The synthesis and reactivity of a silyliumylidene cation stabilized by an amidinate ligand and 4‐dimethylaminopyridine (DMAP) are described. The reaction of the amidinate silicon(I) dimer [ L Si:]₂ ( 1 ; L =PhC(NtBu)₂) with one equivalent of N‐trimethylsilyl‐4‐dimethylaminopyridinium triflate [4‐NMe₂C₅H₄NSiMe₃]OTf and two equivalents of DMAP in THF afforded [ L Si(DMAP)]OTf ( 2 ). The ambiphilic character of 2 is demonstrated from its reactivity. Treatment of 2 with 1 in THF afforded the disilylenylsilylium triflate [ L′ ₂( L )Si]OTf ( 3 ; L′ = L Si:) with the displacement of DMAP. The reaction of 2 with [K{HB(iBu)₃}] and elemental sulfur in THF afforded the silylsilylene [ L SiSi(H){(NtBu)₂C(H)Ph}] ( 4 ) and the base‐stabilized silanethionium triflate [ L Si(S)DMAP]OTf ( 5 ), respectively. Compounds 2 , 3 , and 5 have been characterized by X‐ray crystallography.     

The Aqueous Ca2+ System,in Comparison with Zn2+, Fe3 +, and Al3 +: An Ab Initio Molecular Dynamics Study

Herein, we report on the structure and dynamics of the aqueous Ca²⁺ system studied by using ab initio molecular dynamics (AIMD) simulations. Our detailed study revealed the formation of well‐formed hydration shells with characteristics that were significantly different to those of bulk water. To facilitate a robust comparison with state‐of‐the‐art X‐ray absorption fine structure (XAFS) data, we employ a 1st principles MD‐XAFS procedure and directly compare simulated and experimental XAFS spectra. A comparison of the data for the aqueous Ca²⁺ system with those of the recently reported Zn²⁺, Fe³⁺, and Al³⁺ species showed that many of their structural characteristics correlated well with charge density on the cation. Some very important exceptions were found, which indicated a strong sensitivity of the solvent structure towards the cation′s valence electronic structure. Average dipole moments for the 2nd shell of all cations were suppressed relative to bulk water.     

Acetylene Derivatives of Cationic Diazaoxatriangulenes and Diaza [4]Helicenes - Access to Red Emitters and Planar Chiral Stereochemical Traits

Cationic triangulenes, and related helicenes, constitute a rich class of dyes and fluorophores, usually absorbing and emitting light at low energy, in the orange to red domains. Recently, to broaden the scope of applications, regioselective late-stage functionalizations on these core moieties have been developed. For instance, with the introduction of electron-donating groups (EDGs), important bathochromic shifts are observed pushing absorptions towards or in the near-infrared (NIR) spectral domain while emissive properties disappear essentially completely. Herein, to upset this drawback, acetylene derivatives of cationic diazaoxa triangulenes (DAOTA) and [4]helicenes are prepared (16 examples). Contrary to other EDG-functionalized derivatives, C≡C− functionalized products remain broadly fluorescent, with red-shifted absorptions (Δλ_abs up to 25 nm) and emissions (Δλ_em up to 73 nm, Φ_PL up to 51 %). Quite interestingly, a general dynamic stereoisomerism phenomenon is evidenced for the compounds derived from achiral DAOTA cores. At low temperature in ¹H NMR spectroscopy (218 K), N−CH₂ protons become diastereotopic with chemical shifts differences (Δδ) as high as +1.64 ppm. The signal coalescence occurs around 273 K with a barrier of ∼12 kcal mol⁻¹. This phenomenon is due to planar chiral conformations (S_p and R_p configurations), induced by the geometry of the alkyl (n-propyl) side-chains next to the acetylenic substituents. Ion pairing studies with Δ-TRISPHAT anion not only confirm the occurrence of the chiral conformations but evidence a moderate but definite asymmetric induction from the chiral anion onto the cations. Finally, DFT calculations offer a valuable insight on the geometries, the corresponding stereodynamics and also on the very large difference in NMR for some of the diastereotopic protons.     

Diradicals Photogeneration from Chloroaryl-Substituted Carboxylic Acids

With the aim of generating new, thermally inaccessible diradicals, potentially able to induce a double-strand DNA cleavage, the photochemistry of a set of chloroaryl-substituted carboxylic acids in polar media was investigated. The photoheterolytic cleavage of the Ar−Cl bond occurred in each case to form the corresponding triplet phenyl cations. Under basic conditions, the photorelease of the chloride anion was accompanied by an intramolecular electron-transfer from the carboxylate group to the aromatic radical cationic site to give a diradical species. This latter intermediate could then undergo CO₂ loss in a structure-dependent fashion, according to the stability of the resulting diradical, or abstract a hydrogen atom from the medium. In aqueous environment at physiological pH (pH=7.3), both a phenyl cation and a diradical chemistry was observed. The mechanistic scenario and the role of the various intermediates (aryl cations and diradicals) involved in the process was supported by computational analysis.     

The effects of biologically important divalent and trivalent metal cations on the cyclization step of dopamine autoxidation reaction: A quantum chemical study

Dopamine is the most essential monoaminergic neurotransmitter involved in the pathophysiology of neurodegenerative disorders, and its autoxidation has been recognized as one of the potential trigger factors for dopaminergic neuron loss. The cyclization of dopamine o-quinone was shown to be the irreversible and rate-limiting step of the autoxidation reaction at physiologic pH values. Furthermore, various metal ions such as Al³⁺, Fe³⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺, and Mn²⁺ have been clinically associated with neurodegeneration, especially Parkinsonism and dementia. It has been proposed that these metal ions could increase the rate of the dopamine autoxidation reaction; however, the exact mechanism has not yet been fully understood. Using advanced quantum chemical calculations with the inclusion of solvent effects we showed that except for Mn²⁺, the studied metal cations could form complexes with dopamine o-quinone and significantly increase the dopamine o-quinone cyclization rate in aqueous solution; first, by enabling the cyclization to proceed spontaneously without the attack of the unprotonated amino group by hydroxide ion; second, by decreasing the intrinsic activation energy; and third, by decreasing the free energy of protonated amino group deprotonation. The latter also decreases the protective effect of acidic pH on dopamine autoxidation found in synaptic vesicles. The results are fully consistent with experimental data and provide deeper understanding of the effects of metal cations on the dopamine autoxidation reaction at physiologic pH values.     

Coordination Chemistry of Cyclopropenylidene‐Stabilized Phosphenium Cations: Synthesis and Reactivity of Pd and Pt Complexes

A straightforward synthesis of cyclopropenylidene‐stabilized phosphenium cations 1 a – g through the reaction of [(iPr₂N)₂C₃⁺Cl]BF₄ with secondary phosphines is described. Their donor ability was evaluated by analysis of the CO stretching frequency in Rh complexes [RhCl(CO)L₂](BF₄)₂ and electrochemical methods. The cyclopropenium ring induces a phosphite‐type behavior that can be tuned by the other two substituents attached to the phosphorus atom. Despite of the positive charge that they bear, phosphenium cations 1 a – g still act as two‐electron donor ligands, forming adducts with Pd^II and Pt^II precursors. Conversely, in the presence of Pd⁰ species, an oxidative insertion of the Pd atom into the C_carbene–phosphorus bond takes place, providing dimeric structures in which each Pd atom is bonded to a cyclopropenyl carbene while two dialkyl/diaryl phosphide ligands serve as bridges between the two Pd centers. The catalytic performance of the resulting library of Pt^II complexes was tested; all of the cationic phosphines accelerated the prototype 6‐endo‐dig cyclization of 2‐ethynyl‐1,1′‐biphenyl to afford pentahelicene. The best ligand 1 g was used in the synthesis of two natural products, chrysotoxene and epimedoicarisoside A.     

Facile Synthesis of Phosphaamidines and Phosphaamidinates using Nitrilium Ions as an Imine Synthon

Readily accessible nitrilium triflates are convenient imine building blocks for the expedient synthesis of a novel class of 1,3‐P,N ligands as demonstrated for the reaction with primary phosphanes. This procedure allows variation of all substituents. X‐ray crystal structures are reported for nitrilium ions, phosphaamidines, and three phosphaamidinate complexes. The lithium phosphaamidinate is N coordinated and its reaction with [AuCl(tht)] (tht=tetrahydrothiophene) gives a unique P‐bridged gold trimer, while a P,N‐bidentate complex results from [{RhCl(cod)}₂]. The nitrilium ion methodology allows extension of the 1,3‐P,N motive to bis(imino)phosphanes, which are the neutral phosphorus analogues of the valuable β‐diketiminate ligand.