High-resolution ESI Research Articles

Synthetic and structural investigation of [PdBr2(CNR)2] (R = Cy, Xyl)

Reaction of cis-dichloro-bis-(cyclohexylisocyanide)palladium(II) ([PdCl2(CNCy)2], (1) or cis-dichloro-bis-(2,6-dimethylphenylisocyanide)palladium(II) ([PdCl2(CNXyl)2], (2) with excess of potassium bromide in acetone furnished complexes trans-dibromo-bis-(cyclohexylisocyanide)palladium(II) ([PdBr2(CNCy)2], (3) or trans-dibromo-bis-(2,6-dimethylphenylisocyanide)palladium(II) ([PdBr2(CNXyl)2], (4), respectively. Both compounds were characterized by elemental analyses (C, H, N), high resolution ESI+-MS, IR, 1H and 13C{1H} NMR spectroscopies, and their crystals were analyzed by a single-crystal and powder X-ray diffraction methods. To rationalize the dependence of the structure of 1–4 on type of halogen ligands, the Gibbs free energies of corresponding cis- and trans-isomers of 1–4 were estimated at the DFT B3LYP level of theory.

Synthesis and biological activity of new 18β-glycyrrhetinic acid derivatives

In an attempt to find out new, potent and safe anti-inflammatory molecules and as a contribution in the chemistry of triterpenes, a series of 18β-glycyrrhetinic acid (GTA) derivatives (4a–j, 5a–e, 7–9, 11–13) were synthesized and evaluated as anti-inflammatory agents using carrageenan induced rat paw edema method. The synthesized derivatives proved superior anti-inflammatory activity to GTA. Moreover, some of the produced derivatives demonstrated higher effect than prednisolone and indomethacin. This remarkable anti-inflammatory effect was combined with no detrimental effect on the gastrointestinal tract (GIT) of the test rats. All of the synthesized compounds were characterized by NMR spectroscopy and high-resolution ESI mass spectrometry.

Structures of cell-wall phosphate-containing glycopolymers of Bifidobacterium longum BIM B-476-D

Glycopolymers with oligosaccharyl phosphate repeats of two types and ribitol and glycerol teichoic acids were isolated from cell wall of Bifidobacterium longum BIM B-476-D by stepwise extraction with 10% CCl3CO2H. The following structures of the glycopolymers were established by sugar analysis, selective cleavage with aq 2% HOAc, dephosphorylation with 48% HF, 2D NMR spectroscopy, and high-resolution ESI MS: [Display omitted] The ribitol teichoic acid also contains minor d-alanine, whose position was not determined.

Six New Phenolic Glycosides and a New Ceramide from the Flowers of Wedelia biflora and Their Cytotoxicity Against Some Cancer Cell Lines

The plant Wedelia biflora has been used in traditional medicine in India and Vietnam to treat various symptoms. However, the chemical composition of its flowers remains mostly unknown. Therefore, we now report the isolation and structural elucidation of six new phenolic glycosides {wedebicoside A - F (1-6)} and one new ceramide [wedebiceramide (9)], together with six known compounds, 1-O-(2',4'-diangeloyloxy-beta-D-fucopyranosyl)-6-hydroxythymol (7), 1-O-[2',4'-diangeloyloxy-3'-(3"-angeloyloxy-beta-D-fucopyranosyl)-beta-D-fucopyransyl]-6-hydroxythymol (8), anhydrosecoisolariciresinol (10), friedeline (11), epifriedelanol (12) and stigmasterol (13) from the flowers of Wedelia biflora. Their structures were established by 1D and 2D NMR spectroscopy, as well as by high resolution ESI-MS analysis and comparison with literature data. The cytotoxic activities against HeLa, MCF-7 and NCI-H460 were evaluated on some purified compounds at the concentration of 100 microg/mL. Compounds 1, 2, 3 and 5 showed strong cytotoxic activities against three surveyed cancer cell lines. Consequently, this study elucidated the phytochemical composition of W. biflora, as well as the potential use of some of the new compounds against some cancers.

Au103(SR)45, Au104(SR)45, Au104(SR)46 and Au105(SR)46 nanoclusters

High resolution ESI mass spectrometry of the "22 kDa" nanocluster reveals the presence of a mixture containing Au103(SR)45, Au104(SR)45, Au104(SR)46, and Au105(SR)46 nanoclusters, where R = -CH2CH2Ph. MALDI TOF MS data confirm the purity of the sample and a UV-vis spectrum shows minor features. Au102(SC6H5COOH)44, whose XRD crystal structure was recently reported, is not observed. This is due to ligand effects, because the 102 : 44 composition is produced using aromatic ligands. However, the 103-, 104- and 105-atom nanoclusters, protected by -SCH2CH2Ph and -SC6H13 ligands, are at or near 58 electron shell closing.

A Palladium(II) Center Activates Nitrile Ligands toward 1,3-Dipolar Cycloaddition of Nitrones Substantially More than the Corresponding Platinum(II) Center

Palladium(II)-coordinated NCR(1) (R(1) = Et (1), NMe(2) (2), Ph (3)) species react smoothly with acyclic nitrones such as the ketonitrones Ph(2)C═N(O)R(4) (R(4) = p-MeC(6)H(4) (4), p-ClC(6)H(4) (5)) and the aldonitrone p-MeC(6)H(4)CH═N(O)Me (6) in the corresponding nitrile media. This reaction proceeds as a consecutive two-step intermolecular cycloaddition to give the mono- and bis-2,3-dihydro-1,2,4-oxadiazole complexes [PdCl(2)(R(1)CN){N(a)═C(R(1))ON(R(4))C(b)(R(2)R(3))}]((a-b)) (7a-13a; R(2), R(3) = Ph; R(4) = C(6)H(4)Me-p, R(1) = Et (7), NMe(2) (8), Ph (9); R(4) = C(6)H(4)Cl-p, R(1) = Et (10), NMe(2) (11), Ph (12); R(2) = H, R(3) = C(6)H(4)Me-p, R(4) = Me, R(1) = NMe(2) (13)) and [PdCl(2){N(a)═C(R(1))ON(R(4))C(b)(R(2)R(3))}(2)]((a-b)) (7b-13b), respectively. Inspection of the obtained data and their comparison with the previous results indicate that the Pd(II) centers provide substantially greater activation of RCN ligands toward the 1,3-dipolar cycloaddition than the relevant Pt(II) centers. The palladium(II)-mediated 1,3-dipolar cycloaddition of ketonitrones to nitriles is reversible. All complexes were characterized by elemental analyses (C, H, N), high-resolution ESI-MS, and IR and (1)H and (13)C{(1)H} NMR spectroscopy. The structure of trans-7b was determined by single-crystal X-ray diffraction. Metal-free 5-NR'(2)-2,3-dihydro-1,2,4-oxadiazoles (7c-13c) were liberated from the corresponding (2,3-dihydro-1,2,4-oxadiazole)(2)Pd(II) complexes by treatment with 1,2-(diphenylphosphino)ethane, and the heterocycles were characterized by high-resolution ESI(+)-MS and (1)H and (13)C{(1)H} spectroscopy.

Synthesis and Characterization of cis-(RNC)2PtII Species Useful as Synthons for Generation of Various (Aminocarbene)PtII Complexes

Cis-Dichlorobis(tert-butylisocyanide)platinum(II), 4, and cis-dichlorobis(cyclohexylisocyanide)platinum(II), 5, were prepared by treatment of dichlorobis(propiononitrile)platinum(II), 1, with tert-butylisocyanide (2) or cyclohexylisocyanide (3). The crystal structures of 4 and 5 were determined by a single crystal X-ray diffraction. The title compounds crystalize in the triclinic space group P $$ \overline{1} $$ [a = 12.1551(3), b = 12.2025(5), c = 20.9187(7) A, α = 99.3024(15), β = 99.671(2), γ = 90.515(2)°] for 4, and in the monoclinic space group P21/n [a = 12.0396(12), b = 9.5448(9), c = 13.7759(15) A, β = 96.698(8)°] for 5. In both structures, the Pt atoms exhibit slightly distorted square planar coordination geometry, and the isocyanide ligands are mutually in the cis-position. The fragments C–N–C–Pt in both complexes are almost linear, and the CN bonds of the isocyanide moiety [C1–N1 1.156(11) and C6–N2 1.136(11) A (for 4), C1–N1 1.150(4) and C8–N2 1.144(3) A (for 5)] are within the typical range for the CN triple bonds values. In addition, both compounds were characterized by elemental analyses (C, H, N), high resolution ESI+-MS, IR, 1H and 13C{1H} NMR spectroscopies. Bis-isocyanide platinum(II) complexes were prepared via a new procedure and fully characterized by several analytical techniques, including NMR and X-ray diffraction.

Oxidation of cyclohexane by transition-metal complexes with biomimetic ligands

This work reports the catalytic activity, in homogeneous phase, of transition-metal complexes of the first-row (V(IV), Mn(III), Fe(III) Co(III) and Cu(II)) with biomimetic Schiff base ligands with N2O2 coordination sphere, as well as an N4 (Fe(II)), in the room-temperature oxidation of cyclohexane using environmentally benign reagents: hydrogen peroxide (30wt%) as the oxygen source and acetonitrile as the solvent. Nitric acid is also used as promoter of the oxidation reaction. The structure of the ligands is confirmed by FTIR, 1H NMR and high-resolution ESI mass spectrometry. The corresponding transition metal complexes are characterized by elemental analysis, high resolution ESI mass spectrometry, FTIR and UV–vis. Cyclohexanone and cyclohexanol are the main products of the oxidation of cyclohexane, obtained when the following complexes are used as homogeneous catalysts in only 1mol% based on the substrate: VO(IV), Fe(III) and Cu(II) complexes with the N2O2 Schiff base, new Fe(II) complex with the Schiff base with N4 coordination sphere and commercial [VO(acac)2] with O4 coordination sphere. The Fe(III) complex with N2O2 Schiff base ligand ([Fe(salhd)Cl]) is the homogeneous catalyst with highest activity, which could be further enhanced by the addition of methyl electron donating groups to the N2O2 Schiff base aldehyde fragment (reaching 46% oxygenate yields and 45 turnover numbers). Cyclooctane and n-hexane could also be oxidized to the corresponding ketones and alcohols with higher turnover numbers than cyclohexane by the Fe(III) complex with N2O2 Schiff base ligand.

Synthesis, characterization and cytotoxicity studies of platinum(II) complexes with amino acid ligands in various coordination modes

Reactions of [Pt(CO3)(PPh3)2]·CH2Cl2 (1) with non-substituted and alkyl substituted amino acids, NH(R)CH(R′)CO2H (R/R′=H/Me, L1; H/iPr, L2; H/CH2CHMe2, L3; Me/H, L4; Et/H, L5), in the presence of Tl[PF6] in methanol afforded with liberation of CO2 the formation of platinum(II) complexes of the type [Pt(PPh3)2{NHR–CHR′–C(O)O-κN,κO}][PF6] (R/R′=H/Me, 2; H/iPr, 3; H/CH2CHMe2, 4; Me/H, 5; Et/H, 6). Single-crystal X-ray diffraction analysis of complex 4 exhibited a square-planar coordination of the platinum atom having coordinated two triphenylphosphane ligands and a deprotonated κN,κO-coordinated leucine ligand (L3−H). On varying the pKa value of the amino group, platinum(II) complexes with different coordination modes of amino acid ligands were obtained. Thus, treatment of complex 1 with N-acetyl l-alanine (L6), possessing a comparatively highly acidic NH proton, in 1:1 ratio in methanol resulted in the formation of [Pt(PPh3)2{N(COMe)–CHMe–C(O)O-κN,κO}] (7), while reacting N-phenyl glycine (L7) having a moderately acidic NH proton with complex 1 afforded a mixture of complexes [Pt(PPh3)2{NPh–CH2–C(O)O-κN,κO}] (8) and [Pt(PPh3)2{NHPh–CH2–C(O)O-κO}2] (10). Treatment of complex 1 with two equivalents of L6/L7 in dichloromethane resulted in the formation of [Pt(PPh3)2{NHR–CHR′–C(O)O-κO}2] (R/R′=COMe/Me, 9; Ph/H, 10). An analogous reactivity was observed for l-lactic acid on treating with complex 1 in 1:1 and 2:1 ratio resulting in [Pt(PPh3)2{O–CHMe–C(O)O-κO,κO′}] (11) and [Pt(PPh3)2{HO–CHMe–C(O)O-κO}2] (12). The identities of all complexes have been proven by NMR (1H, 13C, 31P) spectroscopic and high-resolution ESI mass-spectrometric investigations. In vitro cytotoxicity studies against human tumor cell lines (8505C, A2780, HeLa, SW480, and MCF-7) showed the highest activities for the neutral complex 7. Furthermore, complexes 7 and 9 against the A2780 cell line induced an apoptotic mode of cell death, which was further supported by morphological investigation and DNA laddering. Cell cycle perturbation studies showed that both complexes induced faster cell death than cisplatin.

Probing chemical reduction in a cobalt(III) complex as a viable route for the inhibition of the 20S proteasome

In this paper we evaluate the viability of ligand dissociation in the cobalt complex [CoIII(L1)2]ClO4 (1) following biological reduction. We performed detailed electrochemical characterization of 1 in several solvents, along with spectroelectrochemical and chemical reduction to monitor the phenolate-to-cobalt(III) LMCT band observed at 440nm in the UV–Vis region in presence and absence of a sacrificial reductant. DFT calculations were performed to confirm the nature of this band. Species 1 displays no signs of ligand protonation at pH 3 over a period of 24h while probing its stability in solution. Spectrophotometric monitoring at pH 3 in presence of ascorbic acid shows clearly a decrease of the LMCT band, implying that reduction of the metal center has taken place.The products of chemical reduction were analyzed by high resolution ESI+ mass spectrometry and support a mechanism in which biological reduction leads to ligand dissociation.

Coordination-driven self-assembly of polyoxometalates into discrete supramolecular triangles

Pd(II)-directed self-assembly of a 3-pyridyl grafted Lindqvist hexavanadate led to the formation of a unique trimeric species, as confirmed by a variety of techniques, including pulsed-field gradient NMR spectroscopy and high-resolution ESI mass spectrometry.

Facile and Reversible 1,3-Dipolar Cycloaddition of Aryl Ketonitrones to Platinum(II)-Bound Nitriles: Synthetic, Structural, and Theoretical Studies

The reaction between trans-[PtCl2(NCR)2] (R = Et 1, NMe22, NEt23, NC5H104) and the acyclic triaryl ketonitrones Ph2C═N(O)C6H4R′-p (R′ = H 5, Me 6, Cl 7, OMe 8) proceeds as a facile and consecutive two-step intermolecular cycloaddition to give the mono-cycloaddition products trans-[PtCl2(NCR){Na═C(R)ON(C6H4R′-p)CbPh2}](a−b) (R/R′ = Et/H 9, Et/Me 10, Et/Cl 11, Et/OMe 12, NMe2/H 13, NMe2/Me 14, NMe2/Cl 15, NMe2/OMe 16, NEt2/H 17, NEt2/Me 18, NEt2/Cl 19, NEt2/OMe 20, NC5H10/H 21, NC5H10/Me 22, NC5H10/Cl 23, NC5H10/OMe 24) and then the bis-2,3-dihydro-1,2,4-oxadiazole complexes trans-[PtCl2{Na═C(R)ON(C6H4R′-p)CbPh2}2](a−b) (R/R′ = Et/H 25, Et/Me 26, Et/Cl 27, Et/OMe 28, NMe2/H 29, NMe2/Me 30, NMe2/Cl 31, NMe2/OMe 32, NEt2/H 33, NEt2/Me 34, NEt2/Cl 35, NEt2/OMe 36, NC5H10/H 37, NC5H10/Me 38, NC5H10/Cl 39, NC5H10/OMe 40). The ketonitrones Ph2C═N(O)C6H4R′-p were found to be unexpectedly much more reactive toward the platinum(II)-bound nitriles than the related aldonitrones p-R‴C6H4CH═N(O)R″ (R′′ = Me, Ph; R‴ = H, Me), and the difference in the reactivity in 1,3-dipolar cycloaddition (DCA) of the keto- and aldonitrones was interpreted by theoretical calculations and was explained in terms of the orbital arguments as a result of the increase of the HOMOnitrone energy from aldo- to ketonitrones. The first example of the reversibility in metal-mediated DCA of nitrones to nitriles was observed, and this phenomenon, as follows from the performed theoretical study, is justified by the thermodynamic instability of the PtII-bound 3,3-diaryl-2,3-dihydro-1,2,4-oxadiazoles. Metal-free C5-diphenyl-2,3-dihydro-1,2,4-oxadiazoles 42 and 43 were liberated from corresponding (oxadiazole)2PtII complexes 26 and 30 by treatment with excess NaCN, and these heterocycles were characterized by high-resolution ESI+-MS and 1H and 13C{1H} NMR spectroscopies.

Revision of the O-polysaccharide structure of Yersinia pseudotuberculosis O:1a; confirmation of the function of WbyM as paratosyltransferase

An O-polysaccharide was isolated by mild acid degradation at pH 4.5 of the long-chain lipopolysaccharide of Yersinia pseudotuberculosis PB1 (serotype O:1a) and studied using 2D NMR spectroscopy. It was found to contain two uncommon monosaccharides: paratose (3,6-dideoxy-d-ribo-hexose, Par) in the furanose form and 6-deoxy-d-manno-heptose (d-6dmanHep). The following structure of a branched tetrasaccharide repeat (O-unit) with a disaccharide side chain was established:▪This structure is at variance with the O-polysaccharide structure of Y. pseudotuberculosis O:1a reported earlier (Komandrova, N. A.; Gorshkova, R. P.; Isakov, V. V.; Ovodov, Y. S. Bioorg. Khim.1984, 10, 232–237). A comparative study by high-resolution ESI MS of the short-chain lipopolysaccharides from strain PB1 and a wbyM mutant thereof confirmed the function of wbyM as the paratosyltransferase gene.

Structure of a polysaccharide from Providencia rustigianii O11 containing a novel amide of 2-acetamido-2-deoxygalacturonic acid with l-glutamyl-l-alanine

An acidic polysaccharide was isolated from Providencia rustigianii O11 by the phenol–water extraction. The polysaccharide was cleaved by solvolysis with triflic acid to yield disaccharides with uronic acid derivatives at the non-reducing end. The polysaccharide and the disaccharides were studied by chemical analyses, high-resolution ESI MS, and 2D 1H and 13C NMR spectroscopy, and the following structure of the tetrasaccharide repeating unit of the polysaccharide was established:▪where GalNAcA stands for 2-acetamido-2-deoxygalacturonic acid, GalNAcA6GluAla for N-(2-acetamido-2-deoxygalacturonoyl)-l-glutam-1-yl-l-alanine, QuiNAc4NAcyl for 2-acetamido-4-[(S)-3-hydroxybutanoylamino]-2,4,6-trideoxyglucose (∼75%) or 2,4-diacetamido-2,4,6-trideoxyglucose (∼25%); the d configuration of GalNA and QuiN4N was ascribed tentatively. To the best of our knowledge, this is for the first time that an amide of uronic acid with a dipeptide is found in bacterial polysaccharides.

LDI and ESI MS as well as low energy CID of a self‐assembling nanorod‐forming fullerene derivative

An amphiphatic fullerene derivative (8-(N-Methyl-Fullero-Pyrrolidinium-1-yl-chloride)-3,6-Dioxaoctan-1-Ammonium Chloride (MFPDAC)), which is of great interest in nanotechnology due to the fact that it forms self-assembling fullerenic nanorods, has been structurally characterized with emphasis to its purity and thermal treatment of a formed nanorod film (on a LDI target) by means of laser desorption/ionization (LDI) coupled with high-resolution curved field reflectron time-of-flight (TOF) mass spectrometry, and by low energy MS/MS as well as in-source fragmentation experiments applying an quadrupole ion trap (QIT) combined with a two-stage reflectron TOF analyzer. The interpretation of LDI results has been supplemented by ESI QIT MS(n) (n = 1-3), as well as high-resolution ESI reflectron TOF mass spectrometric experiments. Based on the experimental data obtained by both desorption/ionization techniques, various types of analyzers and sample treatments, we could completely characterize MFPDAC and further found out that the investigated sample was not entirely free of impurities. Furthermore, the envisaged loss of the derivative sidechain upon the heat treatment in vacuum of the self-assembled nanorod sample film on a metallic substrate could be successfully monitored by LDI MS.

Structural and spectroscopic study of novel Ag(I) metal–organic complexes with dyes – Experimental vs. theoretical methods

Sixteen new metal–organic AgI-compounds of organic dyes are synthesized, isolated spectroscopic and structurally elucidated by means of single crystal and powder X-ray diffraction, high resolution ESI mass spectrometry (MS), electronic absorption (EAs) and vibrational spectroscopy, diffuse reflectance (Ds) and fluorescence (Fs) spectroscopy in condense phases, nuclear-magnetic resonance, ICP, and thermal methods. Quantum chemical DFT calculations, as well as NBO analysis are performed of the novel complexes and ligands. Structurally related model metal–organic chromophores are also studied. The obtained correlation between the theoretical and experimental data marked the studied complexes as promising novel nonlinear optical (NLO) materials.

Novel Reactivity Mode of Metal Diaminocarbenes: Palladium(II)-Mediated Coupling between Acyclic Diaminocarbenes and Isonitriles Leading to Dinuclear Species

Metal-mediated coupling between equimolar amounts of cis-[PdCl2(CNR1)2] [R1 = 2,6-Me2C6H3 (Xyl) 1, 2-Cl,6-Me-C6H32, cyclohexyl (Cy) 3] and H2NC5H3R2N [R2 = H, 2-aminopyridine 4; R2 = NH2, 2,6-diaminopyridine 5] proceeds smoothly for 12 h at 20–25 °C and leads to the diaminocarbene species [PdCl{C(NHC5H3R2N)═N(H)R1}(CNR1)]Cl (6–9). In the reaction of 2 with 5 (1:1 molar ratio), corresponding carbene 10 was detected only by high-resolution ESI+-MS in a mixture with other yet unidentified products. Addition of each of 6–8 to starting 1 or 2 (1:1 molar ratio) in the presence of excess solid K2CO3 in CHCl3 and heating of the reaction mixture for 12 h at 40 °C led to a novel type of dinuclear complexes, 11 (75% isolated yield) and 13 (65%). Similar dinuclear complexes 12 and 14, formed by addition of 7 or 10 to 2, were identified by high-resolution ESI+-MS in the mixture with other species (e.g., 7 and 10). Generation of 11–14 proceeds via a cascade reaction including addition of the amino group of a 2-aminopyridine to the metal-activated isonitrile, ring-closure, and coupling of the derived acyclic diaminocarbene complex with the yet unreacted starting material. Complexes 6–9, 11, and 13 were fully characterized by elemental analyses (C, H, N), high-resolution ESI+-MS, IR, and 1H and 13C{1H} NMR spectroscopies, while 10, 12, and 14 were identified by ESI+-MS. In addition, the structures of five complexes [6, 6a (the latter is a neutral species derived from the deprotonation of 6), 9, 11, and 13] were elucidated by single-crystal X-ray diffraction.

Metal-Based Inhibition of Poly(ADP-ribose) Polymerase − The Guardian Angel of DNA

The inhibition activity of a series of anticancer metal complexes based on platinum, ruthenium, and gold metal ions was evaluated on the zinc-finger protein PARP-1, either purified or directly on protein extracts from human breast cancer MCF7 cells. Information on the reactivity of the metal complexes with the PARP-1 zinc-finger domain was obtained by high-resolution ESI FT-ICR mass spectrometry. An excellent correlation between PARP-1 inhibition in protein extracts and the ability of the complexes to bind to the zinc-finger motif (in competition with zinc) was established. The results support a model whereby displacement of zinc from the PARP-1 zinc finger by other metal ions leads to decreased PARP-1 activity. In vitro combination studies of cisplatin with NAMI-A and RAPTA-T on different cancer cell lines (MCF7, A2780, and A2780cisR) showed that, in some cases, a synergistic effect is in operation.

Highly Stereoselective 1,3-Dipolar Cycloaddition of Nitrones to (Nitrile)2PtIISpecies Furnishing Diastereomerically Pure 2,3-Dihydro-1,2,4-oxadiazole Ligands

Diastereomerically pure platinum(II) complexes 4−9 bearing tetrahydro-5,8-methanocyclohexa[3′,2′:4,5][1,3]oxazolo[3,2-b][1,2,4]oxadiazole ligands were generated under mild conditions (CH2Cl2, 20−25 °C, 24 h) by an intermolecular PtII-mediated 1,3-dipolar cycloaddition between enantiomerically pure camphor-derived oxazoline-N-oxides and the coordinated nitriles in the complexes trans-[PtCl2(R′CN)2] (R′ = Et, Ph, NMe2). These species were characterized by elemental analyses (C, H, N), high-resolution ESI+-MS, IR, 1H and 13C NMR spectroscopies, and also X-ray diffraction (for 5, 7, 8·CHCl3, and 9·Me2CO). Free heterocycles 10−13 were liberated as single stereoisomers from the (2,3-dihydro-1,2,4-oxadiazole)2PtII complexes (R = Et, Ph) by treatment with excess NaCN and were characterized by high-resolution ESI+-MS and 1H and 13C NMR spectroscopies.

Unexpectedly efficient activation of push–pull nitriles by a PtII center toward dipolar cycloaddition of Z-nitrones

Pt(II)-coordinated NCNR'(2) species are so highly activated towards 1,3-dipolar cycloaddition (DCA) that they react smoothly with the acyclic nitrones ArCH=N(+)(O(-))R'' (Ar/R'' = C(6)H(4)Me-p/Me; C(6)H(4)OMe-p/CH(2)Ph) in the Z-form. Competitive reactivity study of DCA between trans-[PtCl(2)(NCR)(2)] (R = Ph and NR'(2)) species and the acyclic nitrone 4-MeC(6)H(4)CH=N(+)(O(-))Me demonstrates comparable reactivity of the coordinated NCPh and NCNR'(2), while alkylnitrile ligands do not react with the dipole. The reaction between trans-[PtCl(2)(NCNR'(2))(2)] (R'(2) = Me(2), Et(2), C(5)H(10)) and the nitrones proceed as consecutive two-step intermolecular cycloaddition to give mono-(1a-d) and bis-2,3-dihydro-1,2,4-oxadiazole (2a-d) complexes (Ar/R'' = p-tol/Me: R'(2) = Me(2)a, R'(2) = Et(2)b, R'(2) = C(5)H(10)c; Ar/R'' = p-MeOC(6)H(4)/CH(2)Ph: R'(2) = Me(2)d). All complexes were characterized by elemental analyses (C, H, N), high resolution ESI-MS, IR, (1)H and (13)C{(1)H} NMR spectroscopy. The structures of trans-1b, trans-2a, trans-2c, and trans-2d were determined by single-crystal X-ray diffraction. Metal-free 5-NR'(2)-2,3-dihydro-1,2,4-oxadiazoles 3a-3d were liberated from the corresponding (dihydrooxadiazole)(2)Pt(II) complexes by treatment with excess NaCN and the heterocycles were characterized by high resolution ESI(+)-MS, (1)H and (13)C{(1)H} spectroscopy.