Characterization of N‐Boc/Fmoc/Z‐N′‐formyl‐gem‐diaminoalkyl derivatives using electrospray ionization multi‐stage mass spectrometry

N‐Boc/Fmoc/Z‐N′‐formyl‐gem‐diaminoalkyl derivatives, intermediates particularly useful in the synthesis of partially modified retro‐inverso peptides, have been characterized by both positive and negative ion electrospray ionization (ESI) ion‐trap multi‐stage mass spectrometry (MSⁿ). The MS² collision induced dissociation (CID) spectra of the sodium adduct of the formamides derived from the corresponding N‐Fmoc/Z‐amino acids, dipeptide and tripeptide acids show the [M + Na‐NH₂CHO]⁺ ion, arising from the loss of formamide, as the base peak. Differently, the MS² CID spectra of [M + Na]⁺ ion of all the N‐Boc derivatives yield the abundant [M + Na‐C₄H₈]⁺ and [M + Na‐Boc + H]⁺ ions because of the loss of isobutylene and CO₂ from the Boc protecting function. Useful information on the type of amino acids and their sequence in the N‐protected dipeptidyl and tripeptidyl‐N′‐formamides is provided by MS² and subsequent MSⁿ experiments on the respective precursor ions. The negative ion ESI mass spectra of these oligomers show, in addition to [M‐H]^?, [M + HCOO]^? and [M + Cl]^? ions, the presence of in‐source CID fragment ions deriving from the involvement of the N‐protecting group. Furthermore, MSⁿ spectra of [M + Cl]^? ion of N‐protected dipeptide and tripeptide derivatives show characteristic fragmentations that are useful for determining the nature of the C‐terminal gem‐diamino residue. The present paper represents an initial attempt to study the ESI‐MS behavior of these important intermediates and lays the groundwork for structural‐based studies on more complex partially modified retro‐inverso peptides. Copyright © 2013 John Wiley & Sons, Ltd.     

Preparation and Characterization of Dinuclear Chromium(III) Complexes of a Hexadentate Tetraaza‐Dithiophenolate Ligand

The ability of the tetraaza‐dithiophenolate ligand H₂L² (H₂L² = N,N′‐Bis‐[2‐thio‐3‐aminomethyl‐5‐tert‐butyl‐benzyl]propane‐1,3‐diamine) to form dinuclear chromium(III) complexes has been examined. Reaction of Cr^IICl₂ with H₂L² in methanol in the presence of base followed by air‐oxidation afforded cis,cis‐[(L²)Cr^III₂(μ‐OH)(Cl)₂]⁺ ( 1a ) and trans,trans‐[(L²)Cr^III₂(μ‐OH)(Cl)₂]⁺ ( 1b ). Both compounds contain a confacial bioctahedral N₂ClCr^III(μ‐SR)₂(μ‐OH)Cr^IIIClN₂ core. The isomers differ in the mutual orientation of the coligands and the conformation of the supporting ligand. In 1a both Cl^? ligands are cis to the bridging OH function. In 1b they are in trans‐positions. Reaction of the hydroxo‐bridged complexes with HCl yielded the chloro‐bridged cations cis,cis‐[(L²)Cr^III₂(μ‐Cl)(Cl)₂]⁺ ( 2a ) and trans,trans‐[(L²)Cr^III₂(μ‐Cl)(Cl)₂]Cl ( 2b ), respectively. These bridge substitutions proceed with retention of the structures of the parent complexes 1a and 1b .     

Calix[4]pyrrole‐Based Heteroditopic Ion‐Pair Receptor That Displays Anion‐Modulated,Cation‐Binding Behavior

A new ditopic ion‐pair receptor 1 was designed, synthesized, and characterized. Detailed binding studies served to confirm that this receptor binds fluoride and chloride ions (studied as their tetraalkylammonium salts) and forms stable 1:1 complexes in CDCl₃. Treatment of the halide‐ion complexes of 1 with Group I and II metal ions (Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, and Ca²⁺; studied as their perchlorate salts in CD₃CN) revealed unique interactions that were found to depend on both the choice of the added cation and the precomplexed anion. In the case of the fluoride complex [ 1? F]^? (preformed as the tetrabutylammonium (TBA⁺) complex), little evidence of interaction with the K⁺ ion was seen. In contrast, when this same complex (i.e., [ 1? F]^? as the TBA⁺ salt) was treated with the Li⁺ or Na⁺ ions, complete decomplexation of the receptor‐bound fluoride ion was observed. In sharp contrast to what was seen with Li⁺, Na⁺, and K⁺, treating complex [ 1? F]^? with the Cs⁺ ion gave rise to a stable, receptor‐bound ion‐pair complex [Cs ?1? F] that contains the Cs⁺ ion complexed within the cup‐like cavity of the calix[4]pyrrole, which in turn was stabilized in its cone conformation. Different complexation behavior was observed in the case of the chloride complex [ 1? Cl]^?. In this case, no appreciable interaction was observed with Na⁺ or K⁺. In addition, treating [ 1? Cl]^? with Li⁺ produces a tightly hydrated dimeric ion‐pair complex [ 1? LiCl(H₂O)]₂ in which two Li⁺ ions are bound to the crown moiety of the two receptors. In analogy to what was seen in the case of [ 1? F]^?, exposure of [ 1? Cl]^? to the Cs⁺ ion gives rise to an ion‐pair complex [Cs ?1? Cl] in which the cation is bound within the cup of the calix[4]pyrrole. Different complexation modes were also observed when the binding of the fluoride ion was studied by using the tetramethylammonium and tetraethylammonium salts.     

Chirale Halbsandwich‐Pentamethylcyclopentadienyl‐Rhodium(III)‐ und Iridium(III)‐Komplexe mit Schiff‐Basen aus Salicylaldehyd und α‐Aminosäureestern [1]

Chiral Half‐sandwich Pentamethylcyclopentadienyl Rhodium(III) and Iridium(III) Complexes with Schiff Bases from Salicylaldehyde and α‐Amino Acid Esters [1] A series of diastereoisomeric half‐sandwich complexes with Schiff bases from salicylaldehyde and L‐α‐amino acid esters including chiral metal atoms, [(η⁵‐C₅H₅)(Cl)M(N,O‐Schiff base)], has been obtained from chloro bridged complexes [(η⁵‐C₅Me₅)(Cl)M(μ‐Cl)]₂ (M = Rh, Ir). Abstraction of chloride from these complexes with Ag[BF₄] or Ag[SO₃CF₃] affords the highly sensitive compounds [(η⁵‐C₅Me₅)M(N,O‐Schiff base]⁺X^? (M = Rh, Ir; X = BF₄, CF₃SO₃) to which PPh₃ can be added under formation of [(η⁵‐C₅Me₅)M(PPh₃)(N,O‐Schiff base)]⁺X^?. The diastereoisomeric ratio of the complexes ( 1 ‐ 7 and 11 ‐ 12 ) has been determined from NMR spectra.     

Gas‐phase fragmentation reactions of protonated benzofuran‐ and dihydrobenzofuran‐type neolignans investigated by accurate‐mass electrospray ionization tandem mass spectrometry

We have investigated gas‐phase fragmentation reactions of protonated benzofuran neolignans (BNs) and dihydrobenzofuran neolignans (DBNs) by accurate‐mass electrospray ionization tandem and multiple‐stage (MSⁿ) mass spectrometry combined with thermochemical data estimated by Computational Chemistry. Most of the protonated compounds fragment into product ions B ([M + H–MeOH]⁺), C ([ B –MeOH]⁺), D ([ C –CO]⁺), and E ([ D –CO]⁺) upon collision‐induced dissociation (CID). However, we identified a series of diagnostic ions and associated them with specific structural features. In the case of compounds displaying an acetoxy group at C‐4, product ion C produces diagnostic ions K ([ C –C₂H₂O]⁺), L ([ K –CO]⁺), and P ([ L –CO]⁺). Formation of product ions H ([ D –H₂O]⁺) and M ([ H –CO]⁺) is associated with the hydroxyl group at C‐3 and C‐3′, whereas product ions N ([ D –MeOH]⁺) and O ([ N –MeOH]⁺) indicate a methoxyl group at the same positions. Finally, product ions F ([ A –C₂H₂O]⁺), Q ([ A –C₃H₆O₂]⁺), I ([ A –C₆H₆O]⁺), and J ([ I –MeOH]⁺) for DBNs and product ion G ([ B –C₂H₂O]⁺) for BNs diagnose a saturated bond between C‐7′ and C‐8′. We used these structure‐fragmentation relationships in combination with deuterium exchange experiments, MSⁿ data, and Computational Chemistry to elucidate the gas‐phase fragmentation pathways of these compounds. These results could help to elucidate DBN and BN metabolites in in vivo and in vitro studies on the basis of electrospray ionization ESI‐CID‐MS/MS data only.     

syn and anti conformations in 2‐hydroxy‐5‐[(E)‐(4‐nitrophenyl)diazenyl]benzoic acid and two related salts

The crystal structures of 2‐hydroxy‐5‐[(E)‐(4‐nitrophenyl)diazenyl]benzoic acid, C₁₃H₉N₃O₅, (I), ammonium 2‐hydroxy‐5‐[(E)‐phenyldiazenyl]benzoate, NH₄⁺·C₁₃H₉N₂O₃⁻, (II), and sodium 2‐hydroxy‐5‐[(E)‐(4‐nitrophenyl)diazenyl]benzoate trihydrate, Na⁺·C₁₃H₈N₃O₅⁻·3H₂O, (III), have been determined using single‐crystal X‐ray diffraction. In (I) and (III), the phenyldiazenyl and carboxylic acid/carboxylate groups are in an anti orientation with respect to each other, which is in accord with the results of density functional theory (DFT) calculations, whereas in (II), the anion adopts a syn conformation. In (I), molecules form slanted stacks along the [100] direction. In (II), anions form bilayers parallel to (010), the inner part of the bilayers being formed by the benzene rings, with the –OH and –COO⁻ substituents on the bilayer surface. The NH₄⁺ cations in (II) are located between the bilayers and are engaged in numerous N—H...O hydrogen bonds. In (III), anions form layers parallel to (001). Both Na⁺ cations have a distorted octahedral environment, with four octahedra edge‐shared by bridging water O atoms, forming [Na₄(H₂O)₁₂]⁴⁺ units.     

Electron impact,chemical ionization and negative chemical ionization mass spectra,and mass-analysed ion kinetic energy spectrometry–collision-induced dissociation fragmentation pathways of some deuterated 2,4,6-trinitrotoluene (TNT) derivatives

Mass-analysed ion kinetic energy spectrometry (MIKES) with collision-induced dissociation (CID) has been used to study the fragmentation processes of a series of deuterated 2,4,6-trinitrotoluene (TNT) and deuterated 2,4,6-trinitrobenzylchloride (TNTCI) derivatives. Typical fragment ions observed in both groups were due to loss of OR′ (R′ = H or D) and NO. In TNT, additional fragment ibns are due to the loss of R₂′O and 3NO₂, whilst in TNTCI fragment ions are formed by the loss of OCI and R₂′OCI. The TNTCI derivatives did not produce molecular ions. In chemical ionization (Cl) of both groups. MH⁺ ions were observed, with [M – OR′]⁺ fragments in TNT and [M – OCI]⁺ fragments in TNTCI. In negative chemical ionization (NCI) TNT derivatives produced M^?′, [M–R′]^?, [M–OR′]^? and [M–NO]^? ions, while TNTCI derivatives produced [M–R]^?, [M–Cl]^? and [M – NO₂]^? fragment ions without a molecular ion.     

Characterization of five [C4H7O]+ Ion structures. Fragmentation of the 2-pentanone molecular ion

The [C₄H₇0]⁺ ions [CH₂?CH? C(?OH)CH₃]⁺ (1), [CH₃CH?CH? C(?OH)H]⁺ (2), [CH₂?C(CH₃)C(?OH)H]⁺ (3), [Ch₃CH₂CH₂C?O]⁺ (4) and [(CH₃)₂CHC?O]⁺ (5) have been characterized by their collision-induced dissociation (CID) mass spectra and charge stripping mass spectra. The ions 1–3 were prepared by gas phase protonation of the relevant carbonyl compounds while 4 and 5 were prepared by dissociative electron impact ionization of the appropriate carbonyl compounds. Only 2 and 3 give similar spectra and are difficult to distinguish from each other; the remaining ions can be readily characterized by either their CID mass spectra or their charge stripping mass spectra. The 2-pentanone molecular ion fragments by loss of the C(1) methyl and the C(5) methyl in the ratio 60:40 for metastable ions; at higher internal energies loss of the C(1) methyl becomes more favoured. Metastable ion characteristics, CID mass spectra and charge stripping mass spectra all show that loss of the C(1) methyl leads to formation of the acyl ion 4, while loss of the C(5) methyl leads to formation of protonated vinyl methyl ketone (1). These results are in agreement with the previously proposed potential energy diagram for the [C₅H₁₀O]⁺˙ system.     

Comparison of collision‐ versus electron‐induced dissociation of sodium chloride cluster cations

The collision‐induced dissociation (CID) and electron‐induced dissociation (EID) spectra of the [(NaCl)_m(Na)_n]ⁿ⁺ clusters of sodium chloride have been examined in a hybrid linear ion trap Fourier transform ion cyclotron resonance mass spectrometer. For singly charged cluster ions (n = 1), mass spectra for CID and EID of the precursor exhibit clear differences, which become more pronounced for the larger cluster ions. Whereas CID yields fewer product ions, EID produces all possible [(NaCl)_xNa]⁺ product ions. In the case of doubly charged cluster ions, EID again leads to a larger variety of product ions. In addition, doubly charged product ions have been observed due to loss of neutral NaCl unit(s). For example, EID of [(NaCl)₁₁(Na)₂]²⁺ leads to formation of [(NaCl)₁₀(Na)₂]²⁺, which appears to be the smallest doubly charged cluster of sodium chloride observed experimentally to date. The most abundant product ions in EID spectra are predominantly magic number cluster ions. Finally, [(NaCl)_m(Na)₂]^{+ .} radical cations, formed via capture of low‐energy electrons, fragment via the loss of [(NaCl)_n(Na)] ^. radical neutrals. Copyright © 2008 John Wiley & Sons, Ltd.     

Spectroscopic and electrochemical study for evaluating DNA interaction activity of 4‐(3‐halophenyl)‐6‐(pyridin‐2‐yl)pyrimidin‐2‐amine based piano stool Cp* Rh (III) and Ir (III) complexes

Six novel organometallic half sandwich complexes [(η5‐C₅Me₅)M(L^1–3)Cl]Cl.2H₂O were synthesized using [{(η⁵‐C₅Me₅)M(μ‐Cl)Cl₂], where M = Ir (III)/Rh (III) and L^1–3 = three pyridyl pyrimidine based ligands; and characterized by NMR, Infra‐red spectroscopy, conductance, elemental and thermal analysis. The complex‐DNA binding mode and/or strength evaluated using absorption titration, electrochemical studies and hydrodynamic measurement proposed intercalative binding mode, which was also confirmed by molecular docking study. Differential pulse voltammetry and cyclic voltammetry studies indicated an alteration in oxidation and reduction potentials of complexes (M⁺⁴/M⁺³) in presence of CT‐DNA. The metal complexes can cleave plasmid DNA as proposed in gel electrophoretic analysis. The LC₅₀ values of complexes evaluated on brine shrimp suggested their potent cytotoxic nature.     

3‐Rhoda‐1,2‐diazacyclopentanes: A Series of Novel Metallacycle Complexes Derived From CN Functionalization of Ethylene

Rh‐containing metallacycles, [(TPA)Rh^III(κ²‐(C,N)‐CH₂CH₂(NR)₂‐]Cl; TPA=N,N,N,N‐tris(2‐pyridylmethyl)amine have been accessed through treatment of the Rh^I ethylene complex, [(TPA)Rh(η²‐CH₂CH₂)]Cl ([ 1 ]Cl) with substituted diazenes. We show this methodology to be tolerant of electron‐deficient azo compounds including azo diesters (RCO₂N?NCO₂R; R=Et [ 3 ]Cl, R=iPr [ 4 ]Cl, R=tBu [ 5 ]Cl, and R=Bn [ 6 ]Cl) and a cyclic azo diamide: 4‐phenyl‐1,2,4‐triazole‐3,5‐dione (PTAD), [ 7 ]Cl. The latter complex features two ortho‐fused ring systems and constitutes the first 3‐rhoda‐1,2‐diazabicyclo[3.3.0]octane. Preliminary evidence suggests that these complexes result from N–N coordination followed by insertion of ethylene into a [Rh]?N bond. In terms of reactivity, [ 3 ]Cl and [ 4 ]Cl successfully undergo ring‐opening using p‐toluenesulfonic acid, affording the Rh chlorides, [(TPA)Rh^III(Cl)(κ¹‐(C)‐CH₂CH₂(NCO₂R)(NHCO₂R)]OTs; [ 13 ]OTs and [ 14 ]OTs. Deprotection of [ 5 ]Cl using trifluoroacetic acid was also found to give an ethyl substituted, end‐on coordinated diazene [(TPA)Rh^III(κ²‐(C,N)‐CH₂CH₂(NH)₂‐]⁺ [ 16 ]Cl, a hitherto unreported motif. Treatment of [ 16 ]Cl with acetyl chloride resulted in the bisacetylated adduct [(TPA)Rh^III(κ²‐(C,N)‐CH₂CH₂(NAc)₂‐]⁺, [ 17 ]Cl. Treatment of [ 1 ]Cl with AcN?NAc did not give the Rh?N insertion product, but instead the N,O‐chelated complex [(TPA)Rh^I ( κ²‐(O,N)‐CH₃(CO)(NH)(N?C(CH₃)(OCH?CH₂))]Cl [ 23 ]Cl, presumably through insertion of ethylene into a [Rh]?O bond.     

Five‐coordinate dinuclar cobalt (II), copper (II), zinc (II) and cadmium (II) complexes with 4‐bromo‐N‐(2‐pyridinylmethylene)benzenamine: Synthesis,characterisation and methyl methacrylate polymerization

A series of late transition metal complexes, [(bpma)Co(μ – Cl)Cl] ₂ , [(bpma)Cu(μ – Cl)Cl] ₂ , [(bpma)Zn(μ – Cl)Cl] ₂ and [(bpma)Cd(μ – Br)Br] ₂ (where bpma is 4‐bromo‐N‐((pyridin‐2‐yl)methylene)benzenamine) have been synthesized and structurally characterized. The X‐ray structures of dimeric complexes [(bpma)M(μ – X)X] ₂ (M = Co, Cu and Zn, X = Cl; M = Cd, X = Br) showed a distorted 5‐coordinate trigonal bipyramidal geometry involving two nitrogen atoms of N,N‐bidentate ligand, two bridged and one terminal halogen atoms. The complex [(bpma)Cu(μ – Cl)Cl] ₂ revealed the highest catalytic activity for the polymerisation of methyl methacrylate in the presence of modified methylaluminoxane with an activity of 9.14 × 10⁴ g PMMA/mol·Cu·h at 60 °C and afforded syndiotactic poly (methylmethacrylate) (rr = 0.69).     

One‐Electron Oxidation and Sensitivity of Uric Acid in On‐Line Electrochemistry and in Electrospray Ionization Mass Spectrometry

The sensitivity of detection of uric acid (H₂U) in positive ion mode electrospray ionization mass spectrometry (ESI MS) was enhanced by uric acid oxidation during electrospray ionization. With a carrier solution of pH 6.3>pK_a1=5.4 of H₂U, protonated unoxidized uric acid [H₂U+H]⁺ (m/z 169) was detected together with the protonated uric acid dimer [2H₂U+H]⁺ (m/z 337). The dimer likely forms by 1e^? oxidation of urate (HU^?) followed by rapid radical dimerization. A covalent structure of the dimer was verified by H/D exchange experiments. Efficiency of 2e^?, 2H⁺ oxidation of uric acid is low during ESI in pH 6.3 carrier solution and improves when a low on‐line electrochemical cell voltage is floated on the high voltage of the ES in on‐line electrochemistry ESI MS (EC/ESI MS). The intensity of the uric acid dimer decreases with an increase in the low applied voltage. In a carrier solution with 0.1 M KOH, pH 12.7>pK_a2=9.8 of H₂U, allantoin (Allnt) (MW 158.04), the final 2e^?, 2H⁺ oxidation product of uric acid, was detected as a potassium complex [K(Allnt)+K]⁺ (m/z 235) and the [2H₂U+H]⁺ dimer was not detected. In direct ESI MS analysis of 1000‐fold diluted urine [NaHU+H]⁺ (pK_sp NaHU=4.6) was detected in 40/60 (vol%) water/methanol, 1 mM NH₄Ac, pH ca. 6.3 carrier solution. A new configuration of the ESI MS instrument with a cone‐shaped capillary inlet significantly enhanced sensitivity in ESI and EC/ESI MS measurements of uric acid.     

Tricaesium tris(oxalato‐κ2O1,O2)chromate(III) dihydrate

The title compound, Cs₃[Cr(C₂O₄)₃]·2H₂O, has been synthesized for the first time and the spatial arrangement of the cations and anions is compared with those of the other members of the alkali metal series. The structure is built up of alternating layers of either the d or l enantiomers of [Cr(oxalate)₃]³⁻. Of note is that the distribution of the [Cr(oxalate)₃]³⁻ enantiomers in the Li⁺, K⁺ and Rb⁺ tris(oxalato)chromates differs from those of the Na⁺ and Cs⁺ tris(oxalato)chromates, and also differs within the corresponding BEDT‐TTF [bis(ethylenedithio)tetrathiafulvalene] conducting salts. The use of tris(oxalato)chromate anions in the crystal engineering of BEDT‐TTF salts is discussed, wherein the salts can be paramagnetic superconductors, semiconductors or metallic proton conductors, depending on whether the counter‐cation is NH₄⁺, H₃O⁺, Li⁺, Na⁺, K⁺, Rb⁺ or Cs⁺. These materials can also be superconducting or semiconducting, depending on the spatial distribution of the d and l enantiomers of [Cr(oxalate)₃]³⁻.     

Construction of (3,8)‐connected three‐dimensional cobalt(II) and copper(II) coordination polymers with 1,3‐bis[(1,2,4‐triazol‐4‐yl)methyl]benzene and benzene‐1,3,5‐tricarboxylate ligands

Coordination polymers (CPs) have been widely studied because of their diverse and adjustable topologies and wide‐ranging applications in luminescence, chemical sensors, magnetism, photocatalysis, gas adsorption and separation. In the present work, two coordination polymers, namely poly[(μ₅‐benzene‐1,3,5‐tricarboxylato‐κ⁶O¹:O^1′:O³:O³:O⁵,O^5′){μ₃‐1,3‐bis[(1,2,4‐triazol‐4‐yl)methyl]benzene‐κ³N:N′:N′′}di‐μ₃‐hydroxido‐dicobalt(II)], [Co₂(C₉H₃O₆)(OH)(C₁₂H₁₂N₆)]_n or [Co₂(btc)(OH)(mtrb)]_n, (1), and poly[[diaquabis(μ₃‐benzene‐1,3,5‐tricarboxylato‐κ³O¹:O³:O⁵)bis{μ₃‐1,3‐bis[(1,2,4‐triazol‐4‐yl)methyl]benzene‐κ³N:N′:N′′}tetra‐μ₃‐hydroxido‐tetracopper(II)] monohydrate], {[Cu₄(C₉H₃O₆)₂(OH)₂(C₁₂H₁₂N₆)₂(H₂O)₂]·H₂O}_n or {[Cu₄(btc)₂(OH)₂(mtrb)₂(H₂O)₂]·H₂O}_n, (2), were synthesized by the hydrothermal method using 1,3‐bis[(1,2,4‐triazol‐4‐yl)methyl]benzene (mtrb) and benzene‐1,3,5‐tricarboxylate (btc^3?). CP (1) exhibits a (3,8)‐coordinated three‐dimensional (3D) network of the 3,8T38 topological type, with a point symbol of {4,5,6}₂{4²·5⁶·6¹⁶·7²·8²}, based on the tetranuclear hydroxide cobalt(II) cluster [Co₄(μ₃‐OH)₂]. CP (2) shows a (3,8)‐coordinated tfz‐d topology, with a point symbol of {4₃}₂{4⁶·6¹⁸·8⁴}, based on the tetranuclear hydroxide copper(II) cluster [Cu₄(μ₃‐OH)₂]. The different (3,8)‐coordinated 3D networks based on tetranuclear hydroxide–metal clusters of (1) and (2) are controlled by the different central metal ions [Co^II for (1) and Cu^II for (2)]. The thermal stabilities and solid‐state optical diffuse‐reflection spectra were measured. The energy band gaps (E_g) obtained for (1) and (2) were 2.72 and 2.29 eV, respectively. CPs (1) and (2) exhibit good photocatalytic degradation of the organic dyes methylene blue (MB) and rhodamine B (RhB) under visible‐light irradiation.     

Air‐Stable,Well‐Soluble AI2[Nb6Cl18] Cluster Compounds (AI = Organic Cation): A New Route for Preparation,Single‐Crystal Structures,Properties, and ESI‐Mass Spectra

Five niobium cluster compounds of the A^I₂[Nb₆Cl₁₈] type (A^I = organic cation: [nPr₄N]⁺, [nBu₄N]⁺, [BMIm]⁺, [Ph₄P]⁺, and [PPN]⁺) are obtained through treatment of [Nb₆Cl₁₄(H₂O)₄] · 4H₂O with excess of thionyl chloride in the presence of an organic chloride, A^ICl. Single‐crystal structure studies show that the compounds consist of discrete cations and cluster [Nb₆Cl₁₈]^2– anions. The cluster unit of the hydrated cluster starting material is oxidized by two electrons. Powder diffraction studies and NMR spectroscopic measurements show all compounds to crystallize without co‐crystallized solvent molecules. They are air and water stable. The solubility in organic solvents changes to a great extent on changing the type of cation. The ESI‐MS spectra of [nPr₄N]₂[Nb₆Cl₁₈] and [Ph₄P]₂[Nb₆Cl₁₈] show the pseudomolecular peak of the anionic cluster as well as additional signals, which involve simultaneously chloride mass loss and reduction processes.     

Ring‐opening polymerization of rac‐lactide catalyzed by Al(III) and Zn(II) complexes incorporating Schiff base ligands derived from pyrrole‐2‐carboxaldehyde

A series of Al(III) chloride [LAl‐Cl]; Al(III) alkoxide [LAl‐OR]₂; and Zn(II) [LZn]₂ complexes with Schiff base ligands were obtained. ¹H NMR and X‐ray diffraction studies indicate that [LAl‐Cl] complexes have C_s symmetry and the Al center is penta‐coordinated. The Al(III) alkoxide complex [L⁵Al‐OⁱPr]₂ is a dimer bridged by OⁱPr^? with the Al center in a distorted octahedral environment. Zn complexes [LZn]₂ are double helix dimers with tetra‐coordinated Zn centers. The catalytic activity for the ring‐opening polymerization of rac‐lactide was evaluated. The best activity in this series is shown by the aluminium chloride complex with a flexible three‐carbon bridge; more flexible four‐carbon bridges lower the activity.     

Synthesis,structural and chemosensitivity studies of arene d6 metal complexes having N‐phenyl‐N´‐(pyridyl/pyrimidyl)thiourea derivatives

The d⁶ metal complexes of thiourea derivatives were synthesized to investigate its cytotoxicity. Treatment of various N‐phenyl‐N´ pyridyl/pyrimidyl thiourea ligands with half‐sandwich d⁶ metal precursors yielded a series of cationic complexes. Reactions of ligand (L1‐L3) with [(p‐cymene)RuCl₂]₂ and [Cp*MCl₂]₂ (M = Rh/Ir) led to the formation of a series of cationic complexes bearing general formula [(arene)M(L1)к²_(N,S)Cl]⁺, [(arene)M(L2)к²_(N,S)Cl]⁺ and [(arene)M(L3)к²_(N,S)Cl]⁺ [arene = p‐cymene, M = Ru ( 1 , 4 , 7 ); Cp*, M = Rh ( 2 , 5 , 8 ); Cp*, Ir ( 3 , 6 , 9 )]. These compounds were isolated as their chloride salts. X‐ray crystallographic studies of the complexes revealed the coordination of the ligands to the metal in a bidentate chelating N,S‐ manner. Further the cytotoxicity studies of the thiourea derivatives and its complexes evaluated against HCT‐116 (human colorectal cancer), MIA‐PaCa‐2 (human pancreatic cancer) and ARPE‐19 (non‐cancer retinal epithelium) cancer cell lines showed that the thiourea ligands displayed no activity. Upon complexation however, the metal compounds possesses cytotoxicity and whilst potency is less than cisplatin, several complexes exhibited greater selectivity for HCT‐116 or MIA‐PaCa‐2 cells compared to ARPE‐19 cells than cisplatin in vitro. Rhodium complexes of thiourea derivatives were found to be more potent as compared to ruthenium and iridium complexes.     

Gas‐phase fragmentation of metal adducts of alkali‐metal oxalate salts

Upon collisional activation, gaseous metal adducts of lithium, sodium and potassium oxalate salts undergo an expulsion of CO₂, followed by an ejection of CO to generate a product ion that retains all three metals atoms of the precursor. Spectra recorded even at very low collision energies (2 eV) showed peaks for a 44‐Da neutral fragment loss. Density functional theory calculations predicted that the ejection of CO₂ requires less energy than an expulsion of a Na⁺ and that the [Na₃CO₂]⁺ product ion formed in this way bears a planar geometry. Furthermore, spectra of [Na₃C₂O₄]⁺ and [³⁹K₃C₂O₄]⁺ recorded at higher collision energies showed additional peaks at m/z 90 and m/z 122 for the radical cations [Na₂CO₂]^+? and [K₂CO₂]^+?, respectively, which represented a loss of an M^? from the precursor ions. Moreover, [Na₃CO₂]⁺, [³⁹K₃CO₂]⁺ and [Li₃CO₂]⁺ ions also undergo a CO loss to form [M₃O]⁺. Furthermore, product‐ion spectra for [Na₃C₂O₄]⁺ and [³⁹K₃C₂O₄]⁺ recorded at low collision energies showed an unexpected peak at m/z 63 for [Na₂OH]⁺ and m/z 95 for [³⁹K₂OH]⁺, respectively. An additional peak observed at m/z 65 for [Na₂¹⁸OH] ⁺ in the spectrum recorded for [Na₃C₂O₄]⁺, after the addition of some H₂¹⁸O to the collision gas, confirmed that the [Na₂OH] ⁺ ion is formed by an ion–molecule reaction with residual water in the collision cell. Copyright © 2014 John Wiley & Sons, Ltd.     

Antioxidant Pathways and One‐Electron Oxidation of Dopamine and Cysteine in Electrospray and On‐Line Electrochemistry Electrospray Ionization Mass Spectrometry

Dopamine [DA]⁺ (m/z 154), DA dimer [2DA‐H]⁺ (m/z 307) and DA quinone [DAQ]⁺ (m/z 152) are detected in positive ion mode electrospray ionization mass spectrometry (ESI MS) of dopamine in 50/1/49 (vol%) water/acetic acid/methanol. H/D exchange experiments support a covalent structure of DA dimer. Thus, ESI of DA may involve 1e^?, 1H⁺ oxidation processes followed by rapid radical dimerization. The DA quinone signal is low in ESI MS, which indicates a low efficiency of the 2e^?, 2H⁺ oxidation reaction. On‐line electrochemistry ESI MS (EC/ESI MS) with low electrochemical cell voltage floated on high ES voltage increases electrospray current and improves sensitivity for DA. The DA quinone signal increases and DA dimer signal decreases. A new configuration of the ESI MS instrument with a cone‐shaped capillary inlet significantly enhanced sensitivity of ESI and EC/ESI MS measurements. A DA quinone‐cysteine adduct [DAQ+Cys]⁺ was detected in solutions of DA with cysteine (Cys). ESI MS and EC/ESI MS indicate formation of the DA quinone‐cysteine adduct by 1e^? pathway. Oxidation pathways in ESI MS are relevant to biological reactivity of DA and Cys.