Imidazole to NHC rearrangements at molybdenum centers: an experimental and theoretical study

Both manganese and rhenium complexes of the type [M(bipy)(CO)(3)(N-RIm)](+) (bipy=2,2'-bipyridine) undergo deprotonation of the central CH group of the N-alkylimidazole (N-RIm) ligand when treated with a strong base. However, the outcome of the reaction is very different for either metal. For Mn, the addition of the equimolar amount of an acid to the product of the deprotonation affords an N-heterocyclic carbene (NHC) complex, whereas for Re, once the deprotonation of the central imidazole CH group has occurred, the bipy ligand undergoes a nucleophilic attack on an ortho carbon, affording the C-C coupling product. The extension of these studies to pseudo-octahedral [Mo(η(3)-allyl)(bipy)(CO)(2)(N-RIm)](+) complexes has allowed us to isolate cationic NHC complexes (Mn(I)-type behavior), as well as their neutral imidazol-2-yl precursors. Theoretical studies of the reaction mechanisms using DFT computations were carried out on the deprotonation of [Mn(bipy)(CO)(3)(N-PhIm)](+), [Re(bipy)(CO)(3) (N-MesIm)](+), and [Mo(η(3)-C(4)H(7))(bipy)(CO)(2) (N-MesIm)](+) complexes (Mes=mesityl) at the B3LYP/6-31G(d) (LANL2DZ for Mn, Re, and Mo) level of theory. Our results explain why different products have been found experimentally for Mn, Mo, and Re complexes. For Re, the process leading to a C-C coupling product is clearly more favored than those forming an imidazol-2-yl product. In contrast, for Mn and Mo complexes, the lower stabilizing interaction between the central imidazole and ortho bipy C atoms, along with the higher lability of the ligands, make the formation of an NHC-type product kinetically more accessible, in good agreement with experimental findings.     

Ligand binding inside the cavities of lacunar and saddle-shaped cyclidene complexes: molecular mechanics and molecular dynamics studies

Cobalt(II) complexes with tetradentate macrocyclic cyclidene ligands are known to coordinate one additional axial base molecule, leaving the sixth vacant coordination site at the metal ion available for small ligand (e.g., O2) binding. Molecular mechanics and molecular dynamics simulations provide a microscopic view of 1-methylimidazole (MeIm) binding within the cavities of several lacunar (bridged) and saddle-shaped (unbridged) cyclidenes and uncover the roles of the bridges and the walls of the clefts in steric protection of the cobalt(II) coordination site. Short bridges (C3 and C6) prevent inside-the-cavity MeIm binding because of severe ligand distortions leading to high-energy penalties (58 and 25 kcal/mol, respectively), while long bridges (C8 and C12) flip away from the MeIm binding site, allowing for penalty-free MeIm inclusion. In the unbridged saddle-shaped complex, there is no energy difference between inside- and outside-the-cavity MeIm binding. The preferential existence of the coordinatively unsaturated, five-coordinate species Co(unbrCyc)(MeIm)2+ should therefore be explained by electronic, rather than steric, factors. Molecular dynamics and free energy simulations reveal the presence of a weak (ca. 4 kcal/mol in the gas phase and ca. 2 kcal/mol in methanol solution) noncovalent MeIm binding site at the entrance of the cleft of cobalt(II) unbridged cyclidene, at a distance of about 4 A from the metal ion. The macrocycle geometry remains undistorted at such large Co-N(MeIm) separations, while the cavity opens up by 0.9 A upon covalent MeIm binding (Co-N(MeIm) distance of 2 A). An increase in macrocycle strain energy upon MeIm inclusion is compensated by favorable nonbonded interactions between the incoming base and the walls of the unbridged cyclidene.     

Second-sphere interaction of anions with a weakly binding metal complex host: probing the effect of counteranions

The reaction of [Re(OTf)(CO)5] with N-methylimidazole (MeIm) afforded [Re(CO)3(MeIm)3]OTf (1). The reactions of 1 with KPF6, NaBPh4 and NaBAr'4 (Ar' = 3,5-bis(trifluoromethyl)phenyl) afforded [Re(CO)3(MeIm)3]PF6 (2) [Re(CO)3(MeIm)3]BPh4 (3) and [Re(CO)3(MeIm)3]BAr'4 (4) respectively. An analogous reaction using N-phenylimidazole (PhIm) yielded [Re(CO)3(PhIm)3]BAr'4 (7). These new compounds were characterized by IR and NMR, and the structures of 1 and 2 were determined by X-ray diffraction. Compounds [Re(CO)3(MeIm)3]2[PtCl6] (5), [Re(CO)3(MeIm)3][HSO4] (6), [Re(CO)3(PhIm)3][Br] (8) and [Re(CO)3(PhIm)3][NO3] (9) were crystallized from equimolar mixtures of either 4 or 7 and the tetrabutylammonium salt of the corresponding anion, and their structures were determined by X-ray diffraction. The solution behavior of 1-4, 7 toward several anions was studied spectroscopically, including the quantitative determination of binding constants by 1H NMR. The cationic tris(imidazole)complexes are stable against imidazole-by-anion substitution, and the main hydrogen bonding interactions involve the imidazole NC(H)N groups. The binding constants for compounds 1-4 with several external anions follow the order 1<2<3<4, indicating that the strength of the cationic complex-counteranion interaction follows the order OTf(-) > PF6(-) > BPh4(-) > BAr'4(-).     

Oxidation of the tris(carbene)borate complex PhB(MeIm)3MnI(CO)3 to MnIV[PhB(MeIm)3]2(OTf)2

Reaction of the tris(carbene)borate ligand PhB(MeIm)3- with [Mn(CO)3(tBuCN)Br]2 leads to the manganese(I) tricarbonyl complex PhB(MeIm)3Mn(CO)3. In contrast to related complexes that are air-stable, PhB(MeIm)3Mn(CO)3 is O2-sensitive and is converted to a homoleptic MnIV complex. IR and cyclic voltammetry measurements of these complexes establish the exceptionally strong donating nature of the tris(carbene)borate ligand.     

I-substituted imidazolemanganese(II) complexes

Summary A series of manganese(II) spin-free complexes of the type: MnL_n Cl₂ [L =N-Methylimidazole (MeIm), n = 1,2,3,6; L =_N-ethylimidazole (EtIm), n = 4], MnL_nBr₂ (L = MeIm, n = 2, 4, 6; L = EtIm, n = 4, 6), MnL_nI₂ (L = MeIm, n = 4, 6; L = EtIm, n = 6), MnL_n(NCS)₂ (L = MeIm, n = 3, 4; L = EtIm, n = 4), MnL₆X₂ (L = MeIm, EtIm; X = NO₃, ClO₄, BF₄); (Me₄N)(MnLCl₃)(L = MeIm, EtIm); (Et₄N)[Mn(MeIm)Br₃] and Mn(MeIm)₂(N₃)₂ · 5 H₂O were prepared and characterized. The complexes have either octahedral, distorted octahedral, polymeric octahedral or tetrahedral structures.     

Characterization of imidazolate-bridged dinuclear and mononuclear hydroperoxo complexes

Dinucleating ligands having two metal-binding sites bridged by an imidazolate moiety, Hbdpi, HMe(2)bdpi, and HMe(4)bdpi (Hbdpi = 4,5-bis(di(2-pyridylmethyl)aminomethyl)imidazole, HMe(2)bdpi = 4,5-bis((6-methyl-2-pyridylmethyl)(2-pyridylmethyl)aminomethyl)imidazole, HMe(4)bdpi = 4,5-bis(di(6-methyl-2-pyridylmethyl)aminomethyl)imidazole), have been designed and synthesized as model ligands for copper-zinc superoxide dismutase (Cu,Zn-SOD). The corresponding mononucleating ligands, MeIm(Py)(2), MeIm(Me)(1), and MeIm(Me)(2) (MeIm(Py)(2) = (1-methyl-4-imidazolylmethyl)bis(2-pyridylmethyl)amine, MeIm(Me)(1) = (1-methyl-4-imidazolylmethyl)(6-methyl-2-pyridylmethyl)(2-pyridylmethyl)amine, MeIm(Me)(2) = (1-methyl-4-imidazolyl-methyl)bis(6-methyl-2-pyridylmethyl)amine), have also been synthesized for comparison. The imidazolate-bridged Cu(II)-Cu(II) homodinuclear complexes represented as [Cu(2)(bdpi)(CH(3)CN)(2)](ClO(4))(3).CH(3)CN.3H(2)O (1), [Cu(2)(Me(2)bdpi)(CH(3)CN)(2)](ClO(4))(3) (2), [Cu(2)(Me(4)bdpi)(H(2)O)(2)](ClO(4))(3).4H(2)O (3), a Cu(II)-Zn(II) heterodinuclear complex of the type of [CuZn(bdpi)(CH(3)CN)(2)](ClO(4))(3).2CH(3)CN (4), Cu(II) mononuclear complexes of [Cu(MeIm(Py)(2))(CH(3)CN)](ClO(4))(2).CH(3)CN (5), [Cu(MeIm(Me)(1))(CH(3)CN)](ClO(4))(2)( )()(6), and [Cu(MeIm(Me)(2))(CH(3)CN)](ClO(4))(2)( )()(7) have been synthesized and the structures of complexes 5-7 determined by X-ray crystallography. The complexes 1-7 have a pentacoordinate structure at each metal ion with the imidazolate or 1-methylimidazole nitrogen, two pyridine nitrogens, the tertiary amine nitrogen, and a solvent (CH(3)CN or H(2)O) which can be readily replaced by a substrate. The reactions between complexes 1-7 and hydrogen peroxide (H(2)O(2)) in the presence of a base at -80 degrees C yield green solutions which exhibit intense bands at 360-380 nm, consistent with the generation of hydroperoxo Cu(II) species in all cases. The resonance Raman spectra of all hydroperoxo intermediates at -80 degrees C exhibit a strong resonance-enhanced Raman band at 834-851 cm(-1), which shifts to 788-803 cm(-1) (Deltanu = 46 cm(-1)) when (18)O-labeled H(2)O(2) was used, which are assigned to the O-O stretching frequency of a hydroperoxo ion. The resonance Raman spectra of hydroperoxo adducts of complexes 2 and 6 show two Raman bands at 848 (802) and 834 (788), 851 (805), and 835 (789) cm(-1) (in the case of H(2)(18)O(2), Deltanu = 46 cm(-1)), respectively. The ESR spectra of all hydroperoxo complexes are quite close to those of the parent Cu(II) complexes except 6. The spectrum of 6 exhibits a mixture signal of trigonal-bipyramid and square-pyramid which is consistent with the results of resonance Raman spectrum.     

Production and isolation of ligated metal(IV)‐oxo ions by tandem mass spectrometry

High valent metal(IV)‐oxo species, [M(?O)(MeIm)_n(OAc)]⁺ (M = Mn–Ni, MeIm = 1‐methylimidazole, n = 1–2), which are relevant to biology and oxidative catalysis, were produced and isolated in gas‐phase reactions of the metal(II) precursor ions [M(MeIm)_n(OAc)]⁺ (M = Mn–Zn, n = 1–3) with ozone. The precursor ions [M(MeIm)(OAc)]⁺ and [M(MeIm)₂(OAc)]⁺ were generated via collision‐induced dissociation of the corresponding [M(MeIm)₃(OAc)]⁺ ion. The dependence of ozone reactivity on metal and coordination number is discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Voltammetry of Osmium End‐Labeled Oligodeoxynucleotides at Carbon,Mercury, and Gold Electrodes

Voltammetric behavior of oligodeoxynucleotide (ODN) 5′‐T₄₀ (GAA)₇–3′ end‐labeled with osmium tetroxide,2,2‐bipyridine [Os(VIII)bipy] was compared with Os(VIII)bipy‐base‐ and with Os(VI)bipy‐sugar‐modified thymine ribosides. Cyclic voltammograms of Os(VIII)bipy‐modified ODN at mercury and carbon electrodes were similar but not identical to those of Os(VIII)bipy‐modified thymine riboside. Treatment of the ODN with Os(VI)bipy did not result in the ODN modification, in agreement with the known specificity of the reagent to the sugar cis‐diols. We show that in addition to mercury and carbon electrodes, the gold electrode can be used to detect Os(VIII)bipy‐labeled ODN. Comparison of voltammetric behavior of end‐labeled ODN using three types of electrodes most frequently used in DNA analysis may help to optimize electrochemical DNA sensors.     

Strategy for the resolution of a chiral dearomatization agent:[TpRe(CO)(1-methylimidazole)] coordination of alpha-pinene (Tp = hydridotris(pyrazolyl)borate)

A methodology for resolving TpRe(CO)(1-methylimidazole)(eta(2)-benzene) has been developed utilizing (R)-alpha-pinene. Each enantiomer of the [TpRe(CO)(MeIm)] system can be obtained with the enantiomer ratio (er) = 97:3 by taking advantage of differing rates of pinene substitution for the two diastereomers of TpRe(CO)(MeIm)(eta(2)-(R)-alpha-pinene).     

Proton-coupled electron transfer reactions at a heme-propionate in an iron-protoporphyrin-IX model compound

A heme model system has been developed in which the heme-propionate is the only proton donating/accepting site, using protoporphyrin IX-monomethyl esters (PPIX(MME)) and N-methylimidazole (MeIm). Proton-coupled electron transfer (PCET) reactions of these model compounds have been examined in acetonitrile solvent. (PPIX(MME))Fe(III)(MeIm)(2)-propionate (Fe(III)~CO(2)) is readily reduced by the ascorbate derivative 5,6-isopropylidine ascorbate to give (PPIX(MME))Fe(II)(MeIm)(2)-propionic acid (Fe(II)~CO(2)H). An excess of the hydroxylamine TEMPOH or of hydroquinone similarly reduces Fe(III)~CO(2), and TEMPO and benzoquinone oxidize Fe(II)~CO(2)H to return to Fe(III)~CO(2). The measured equilibrium constants, and the determined pK(a) and E(1/2) values, indicate that Fe(II)~CO(2)H has an effective bond dissociation free energy (BDFE) of 67.8 ± 0.6 kcal mol(-1). In these PPIX models, electron transfer occurs at the iron center and proton transfer occurs at the remote heme propionate. According to thermochemical and other arguments, the TEMPOH reaction occurs by concerted proton-electron transfer (CPET), and a similar pathway is indicated for the ascorbate derivative. Based on these results, heme propionates should be considered as potential key components of PCET/CPET active sites in heme proteins.     

Insights into the Halogen Oxidative Addition Reaction to Dinuclear Gold(I) Di(NHC) Complexes

Gold(I) dicarbene complexes [Au₂(MeIm‐Y‐ImMe)₂](PF₆)₂ (Y=CH₂ ( 1 ), (CH₂)₂ ( 2 ), (CH₂)₄ ( 4 ), MeIm=1‐methylimidazol‐2‐ylidene) react with iodine to give the mixed‐valence complex [Au(MeIm‐CH₂‐ImMe)₂AuI₂](PF₆)₂ ( 1 a^I ) and the gold(III) complexes [Au₂I₄(MeIm‐Y‐ImMe)₂](PF₆)₂ ( 2 c^I and 4 c^I ). Reaction of complexes 1 and 2 with an excess of ICl allows the isolation of the tetrachloro gold(III) complexes [Au₂Cl₄(MeIm‐CH₂‐ImMe)₂](PF₆)₂ ( 1 c^Cl ) and [Au₂Cl₄(MeIm‐(CH₂)₂‐ImMe)₂](Cl)₂ ( 2 c^Cl‐Cl ) (as main product); remarkably in the case of complex 2 , the X‐ray molecular structure of the crystals also shows the presence of I‐Au‐Cl mixed‐sphere coordination. The same type of coordination has been observed in the main product of the reaction of complexes 3 or 4 with ICl. The study of the reactivity towards the oxidative addition of halogens to a large series of dinuclear bis(dicarbene) gold(I) complexes has been extended and reviewed. The complexes react with Cl₂, Br₂ and I₂ to give the successive formation of the mixed‐valence gold(I)/gold(III) n a^X and gold(III) n c^X (excluding compound 1 c^I ) complexes. However, complex 3 affords with Cl₂ and Br₂ the gold(II) complex 3 b^X [Au₂X₂(MeIm‐(CH₂)₃‐ImMe)₂](PF₆)₂ (X=Cl, Br), which is the predominant species over compound 3 c^X even in the presence of free halogen. The observed different relative stabilities of the oxidised complexes of compounds 1 and 3 have also been confirmed by DFT calculations.     

Bipyridine‐Assisted Assembly of Au Nanoparticles on Cu Nanowires To Enhance the Electrochemical Reduction of CO2

We report a new strategy to prepare a composite catalyst for highly efficient electrochemical CO₂ reduction reaction (CO₂RR). The composite catalyst is made by anchoring Au nanoparticles on Cu nanowires via 4,4′‐bipyridine (bipy). The Au‐bipy‐Cu composite catalyzes the CO₂RR in 0.1 m KHCO₃ with a total Faradaic efficiency (FE) reaching 90.6 % at ?0.9 V to provide C‐products, among which CH₃CHO (25 % FE) dominates the liquid product (HCOO^?, CH₃CHO, and CH₃COO^?) distribution (75 %). The enhanced CO₂RR catalysis demonstrated by Au‐bipy‐Cu originates from its synergistic Au (CO₂ to CO) and Cu (CO to C‐products) catalysis which is further promoted by bipy. The Au‐bipy‐Cu composite represents a new catalyst system for effective CO₂RR conversion to C‐products.     

Absorption, Circular Dichroism, and Luminescence Spectroscopy of Electrogenerated Delta-[Ru(bipy)(3)](+/0/)(-) and Delta-[Os(bipy)(3)](+/0/)(-) (bipy = 2,2'-Bipyridine)

The electronic absorption and circular dichroism (CD) spectra of the complexes produced by the one, two, and three electron reduction of Delta-[Ru(bipy)(3)](2+) and Delta-[Os(bipy)(3)](2+) are reported. The CD spectra give unequivocal proof that the added electrons are localized on individual bipiridine ligands and thus that the complexes are correctly formulated [M(bipy)(2)(bipy(-))](+), [M(bipy)(bipy(-))(2)](0), and [M(bipy(-))(3)](-). The absorption spectra of the triply reduced species [M(bipy(-))(3)](-) (M = Ru, Os) are compared to those of the Fe(II) and Ir(III) analogs. The luminescence spectra of the two triply reduced complexes [Ru(bipy(-))(3)](-) and [Os(bipy(-))(3)](-). are also presented. The MLCT luminescence found in the parent complexes is completely quenched and is replaced by a weak luminescence attributed to the pi(10) --> pi(7) transition of the (coordinated) [bipy](-) ion.     

Rhenium-promoted diastereo- and enantioselective cyclopentannulation reactions: furans as 1,3-propene dipoles

The rhenium furan complexes TpRe(CO)(MeIm)(eta2-2-methylfuran) (1) and TpRe(CO)(MeIm)(eta2-2,5-dmethylfuran) (2) undergo Lewis acid-promoted cyclopentannulation reactions with enones and enals to generate 3-acetylcyclopentene complexes. During the reaction, a rearrangement occurs such that the alpha and beta carbons of the enone are incorporated into the new carbocycle. Treatment of these complexes with an oxidant (H2O2 or silver triflate) liberates the acetylcyclopentene. When a resolved form of the rhenium complex is used, the acetylcyclopentenes can be obtained enantioselectively.     

Mechanistic Studies on the Gas‐Phase Dehydrogenation of Alkanes at Cyclometalated Platinum Complexes

In the ion/molecule reactions of the cyclometalated platinum complexes [Pt(L? H)]⁺ (L=2,2′‐bipyridine (bipy), 2‐phenylpyridine (phpy), and 7,8‐benzoquinoline (bq)) with linear and branched alkanes C_nH_2n+2 (n=2–4), the main reaction channels correspond to the eliminations of dihydrogen and the respective alkenes in varying ratios. For all three couples [Pt(L? H)]⁺/C₂H₆, loss of C₂H₄ dominates clearly over H₂ elimination; however, the mechanisms significantly differs for the reactions of the “rollover”‐cyclometalated bipy complex and the classically cyclometalated phpy and bq complexes. While double hydrogen‐atom transfer from C₂H₆ to [Pt(bipy? H)]⁺, followed by ring rotation, gives rise to the formation of [Pt(H)(bipy)]⁺, for the phpy and bq complexes [Pt(L? H)]⁺, the cyclometalated motif is conserved; rather, according to DFT calculations, formation of [Pt(L? H)(H₂)]⁺ as the ionic product accounts for C₂H₄ liberation. In the latter process, [Pt(L? H)(H₂)(C₂H₄)]⁺ (that carries H₂ trans to the nitrogen atom of the heterocyclic ligand) serves, according to DFT calculation, as a precursor from which, due to the electronic peculiarities of the cyclometalated ligand, C₂H₄ rather than H₂ is ejected. For both product‐ion types, [Pt(H)(bipy)]⁺ and [Pt(L? H)(H₂)]⁺ (L=phpy, bq), H₂ loss to close a catalytic dehydrogenation cycle is feasible. In the reactions of [Pt(bipy? H)]⁺ with the higher alkanes C_nH_2n+2 (n=3, 4), H₂ elimination dominates over alkene formation; most probably, this observation is a consequence of the generation of allyl complexes, such as [Pt(C₃H₅)(bipy)]⁺. In the reactions of [Pt(L? H)]⁺ (L=phpy, bq) with propane and n‐butane, the losses of the alkenes and dihydrogen are of comparable intensities. While in the reactions of “rollover”‐cyclometalated [Pt(bipy? H)]⁺ with C_nH_2n+2 (n=2–4) less than 15 % of the generated product ions are formed by C? C bond‐cleavage processes, this value is about 60 % for the reaction with neo‐pentane. The result that C? C bond cleavage gains in importance for this substrate is a consequence of the fact that 1,2‐elimination of two hydrogen atoms is no option; this observation may suggest that in the reactions with the smaller alkanes, 1,1‐ and 1,3‐elimination pathways are only of minor importance.     

Formation of N2O from a nickel nitrosyl: isolation of the cis-[N2O2]2- intermediate

Addition of 2,2'-bipyridine (bipy) to [Ni(NO)(bipy)][PF(6)] (1) results in formation of a rare five-coordinate nickel nitrosyl [Ni(NO)(bipy)(2)][PF(6)] (2). This complex exhibits a bent NO(-) ligand in the solid state. On standing in acetonitrile, 2 furnishes the NO coupled product, [Ni(κ(2)-O(2)N(2))(bipy)] (8) in moderate yield. Subsequent addition of 2 equiv of acetylacetone (H(acac)) to 8 results in formation of [Ni(acac)(2)(bipy)], N(2)O, and H(2)O. Preliminary mechanistic studies suggest that the N-N bond is formed via a bimetallic coupling reaction of two NO(-) ligands.     

Cycloaddition reactions of dihapto-coordinated furans

Complexes of the type [TpRe(CO)(L)(eta(2)-furan)], where Tp = hydridotris(pyrazolyl)borate and L = PMe(3) (1) or (t)BuNC (2), undergo dipolar cycloadditions with TCNE (tetracyanoethylene) to afford 7-oxabicycloheptene complexes 3 and 4, respectively. The corresponding 2-methylfuran complexes (5 and 7) for these L ligands give similar methyloxabicycloheptene complexes (6 and 8), with a diastereomer ratio >20:1 for 8. For L = N-methylimidazole (MeIm, 9), TCNE oxidizes the complex, but cycloadditions can be achieved with DMAD (dimethyl acetylenedicarboxylate) as the electrophile. Three complexes are observed: one in which DMAD undergoes a cycloaddition with the carbonyl ylide form of the complex (10C), and two complexes that are coordination diastereomers where DMAD has undergone a formal [2+2] cycloaddition with the uncoordinated double bond of the 4,5-eta(2)-furan ligand (10B and 10A). With the imidazole complex of 2-methylfuran (11), only the [2+2] products (12B and 12A) are observed. In the case of the 2,5-dimethylfuran complex (L = MeIm, 13), which is formed as a single coordination diastereomer, only one [2+2] product is observed (14), the structure of which was confirmed by X-ray crystallography. Oxidative decomplexation of 14 results in liberation of the free oxabicyclo[3.2.0]heptadiene 15, which can be thermally converted to the corresponding oxepin 16 in 70% yield.     

Influence of anionic sulfonate-containing co-ligands on the solid structures of silver complexes supported by 4,4'-bipyridine bridges

In this paper, ten new silver compounds, namely [Ag(bipy)](L1).H2O (1), [Ag(bipy)](L2).2H2O (2), [Ag2(bipy)2(H2O)2](L3).H2O (3), [Ag(L4)(bipy)].H2O (4), [Ag(L5)(bipy)] (5), [Ag(L6)(bipy)].0.5CH3CN (6), [Ag3(L7)2(bipy)2].2(H2O) (7), [Ag2(L8)(bipy)1.5(H2O)].H2O (8), [Ag2(L9)(bipy)2(H2O)2] (9) and [Ag3(L10)(bipy)2][(bipy)(H2O)2].(H2O)3.5 (10) (where bipy = 4,4'-bipyridine, L1 = 6-amino-1-naphthalenesulfonate anion, L2 = 2-naphthalenesulfonate anion, L3 = sulfosalicylate anion, L4 = p-aminobenzenesulfonate anion, L5 = 4-dimethyaminoazobenzenen-4'-sulfonate anion, L6 = 2,5-dichloro-4-amino-benzenesulfonate anion, L7 = 8-hydroxyquinoline-5-sulfonate anion, L8 = 2-nitroso-1-naphthol-4-sulfonate anion, L9 = 2,6-naphthalenedisulfonate anion and L10 = 1,3,5-naphthalenetrisulfonate anion), have been synthesized and characterized by elemental analyses, IR spectroscopy and X-ray crystallography. In compounds 1-6, Ag(I) centers are linked by bipy ligands to form 1D Ag-bipy chain structures, in which the sulfonate anions of compounds 1-3 act as counter ions. The sulfonate anions of compounds 4 and 5 connect Ag-bipy chains to form 1D double chain structures, respectively. The sulfonate anions of compound 6 connect Ag-bipy chains to form a 2D layer structure. Unexpectedly, compound 7 shows a hinged chain structure, and these chains interlace with each other through hydrogen bonds and pi-pi interactions to generate a 3D structure with channels along the c axis. Compounds 8 and 9 show 1D ladder-like structures. In compound 10, the Ag-bipy chains are connected by sulfonate anions to generate a 3D poly-threaded network, in which an isolated Ag-bipy chain is inserted. The results indicate that the anionic sulfonate-containing co-ligands play an important role in the final structures of the Ag(I) complexes. Additionally, the luminescent properties of these compounds were also studied.     

Derivatisation of peptides with osmium tetroxide, 2,2′-bipyridine: capillary electrophoretic and MALDI-TOF mass spectrometric study

Site-specific chemical modification is a useful technology in characterisation of proteins, but the number of chemical probes of the protein structure reacting with proteins under mild conditions in aqueous solutions is rather limited. Here we studied the reaction of osmium tetroxide, 2,2′-bipyridine (Os,bipy) with several peptides using capillary zone electrophoresis (CZE) and matrix-assisted laser desorption-ionisation-time-of-flight mass spectrometry (MALDI-TOF MS). Both techniques showed formation of a stable complex of Os,bipy with tryptophan residues. In CZE peaks with different migration times and UV-Vis spectra were observed. MALDI-TOF MS showed the formation of a product with characteristic isotopic pattern corresponding to the presence of osmium atom. Oxidation of cysteine and methionine side chains to cysteic acid and methionine sulfone by Os,bipy was detected by CZE and confirmed by MALDI-TOF and post-source decay (PSD) mass spectra. PSD showed specific shifts of molecular weights of the peptides and their fragments after the derivatisation. We believe that Os,bipy may become a useful agent in the characterisation of proteins.     

Nuclear magnetic resonance of Zn(II) complexes with 2,2′-bipyridine

The complexes Zn(bipy)Cl₂ and Zn(bipy)₂Cl₂ as well as 2,2′-bipyridyl in aqueous solution (D₂O) have been examined by the NMR method. The presence of the monocationic bipy D⁺ form in aqueous bipyridyl solution has been found. The changes of chemical shifts of bipyridyl protons for complexes Zn(bipy)₃Cl₂ and Zn(bipy)Cl₂ have confirmed explicitly the essential influence of diamagnetic currents on the NMR spectrum of Zn(bipy)₃Cl₂. The comparison of the spectra of 2,2′-bipyridyl (in CH₃OH) and of Zn(bipy)Cl₂ may also suggest the presence of the nonbonding metal-proton 6 interaction.