Hyperbranched polymers with thermoresponsive property highly sensitive to ions

The salt effects on the water solubility of thermoresponsive hyperbranched polyethylenimine and polyamidoamine possessing large amounts of isobutyramide terminal groups (HPEI-IBAm and HPAMAM-IBAm) were studied systematically. Eight anions with sodium as the counterion and ten cations with chloride as the counterion were used to measure the anion and cation effects on the cloud point temperature (T(cp)) of these dendritic polymers in water. It was found that the T(cp) of these dendritic polymers was much more sensitive to the addition of salts than that of the traditional thermoresponsive linear polymers. At low anion concentration, the electrostatic interaction between anions and the positively charged groups of these polymers was dominant, resulting in the unusual anion effect on the T(cp) of these polymers in water, including (1) T(cp) of these dendritic polymers decreasing nonlinearly with the increase of kosmotropic anion concentration; (2) the chaotropic anions showing abnormal salting-out property at low salt concentration and the stronger chaotropes having much pronounced salting-out ability; (3) anti-Hofmeister ordering at low salt concentration. At moderate to high salt concentration, the specific ranking of these anions in reducing the T(cp) of HPEI-IBAm and HPAMAM-IBAm polymers was PO(4)(3-) > CO(3)(2-) > SO(4)(2-) > S(2)O(3)(2-) > F(-) > Cl(-) > Br(-) > I(-), in accordance with the well-known Hofmeister series. At moderate to high salt concentration, the specific ranking order of inorganic cations in reducing the T(cp) of HPEI-IBAm polymer was Sr(2+) ≈ Ba(2+) > Na(+) ≈ K(+) ≈ Rb(+) > Cs(+) > NH(4)(+) ≈ Ca(2+) > Li(+) ≈ Mg(2+). This sequence was only partially similar to the typical Hofmeister cation series, whereas at low salt concentration the cation effect on T(cp) of the dendritic polymer was insignificant and no obvious specific ranking order could be found.     

Subtle role of polyatomic anions in molecular construction: structures and properties of AgX bearing 2,4'-thiobis(pyridine) (X(-) = NO(3)(-), BF(4)(-), ClO(4)(-), PF(6)(-), CF(3)CO(2)(-), and CF(3)SO(3)(-))

Studies on the subtle effects and roles of polyatomic anions in the self-assembly of a series of AgX complexes with 2,4'-Py(2)S (X(-) = NO(3)(-), BF(4)(-), ClO(4)(-), PF(6)(-), CF(3)CO(2)(-), and CF(3)SO(3)(-); 2,4'-Py(2)S = 2,4'-thiobis(pyridine)) have been carried out. The formation of products appears to be primarily associated with a suitable combination of the skewed conformers of 2,4'-Py(2)S and a variety of coordination geometries of Ag(I) ions. The molecular construction via self-assembly is delicately dependent upon the nature of the anions. Coordinating anions afford the 1:1 adducts [Ag(2,4'-Py(2)S)X] (X(-) = NO(3)(-) and CF(3)CO(2)(-)), whereas noncoordinating anions form the 3:4 adducts [Ag(3)(2,4'-Py(2)S)(4)]X(3) (X(-) = ClO(4)(-) and PF(6)(-)). Each structure seems to be constructed by competition between pi-pi interactions of 2,4'-Py(2)S spacers vs Ag.X interactions. For ClO(4)(-) and PF(6)(-), an anion-free network consisting of linear Ag(I) and trigonal Ag(I) in a 1:2 ratio has been obtained whereas, for the coordinating anions NO(3)(-) and CF(3)CO(2)(-), an anion-bridged helix sheet and an anion-bridged cyclic dimer chain, respectively, have been assembled. For a moderately coordinating anion, CF(3)SO(3)(-), the 3:4 adduct [Ag(3)(2,4'-Py(2)S)(4)](CF(3)SO(3))(3) has been obtained similarly to the noncoordinating anions, but its structure is a double strand via both face-to-face (pi-pi) stackings and Ag.Ag interactions, in contrast to the noncoordinating anions. The anion exchanges of [Ag(3)(2,4'-Py(2)S)(4)]X(3) (X(-) = BF(4)(-), ClO(4)(-), and PF(6)(-)) with BF(4)(-), ClO(4)(-), and PF(6)(-) in aqueous media indicate that a [BF(4)(-)] analogue is isostructural with [Ag(3)(2,4'-Py(2)S)(4)]X(3) (X(-) = ClO(4)(-) and PF(6)(-)). Furthermore, the anion exchangeability for the noncoordinating anion compounds and the X-ray data for the coordinating anion compounds establish the coordinating order to be NO(3)(-) > CF(3)CO(2)(-) > CF(3)SO(3)(-) > PF(6)(-) > ClO(4)(-) > BF(4)(-).     

Design, synthesis and photophysical studies of simple fluorescent anion PET sensors using charge neutral thiourea receptors

The synthesis of four fluorescent photoinduced electron transfer (PET) chemosensors 1-4 for anions is described. These are all based on a simple design employing charge neutral aliphatic or aromatic thiourea anion receptors connected to an anthracene fluorophore via a methylene spacer. Here the anion recognition occurred through 1 : 1 hydrogen bonding between the thiourea protons and the anion, as demonstrated by observing the changes in the (1)H NMR in DMSO-d(6) where the two thiourea protons were shifted downfield upon addition of anions. Whereas 1-3 were designed for the detection of anions such as fluoride, acetate or phosphate, 4 was made for the recognition of N-protected amino acids. All the sensors showed 'ideal' behaviour where only the fluorescence emission was quenched upon anion recognition, due to enhanced efficiency of electron transfer quenching from the receptor to the excited state of the fluorophore. By simply varying the nature of the thiourea substituent it was possible to modulate, or tune, the acidity of the thiourea receptor moiety, altering the sensitivity of the anion recognition. For, the anion selectivity and the degree of the fluorescence quenching were in the order of F(-) > AcO(-) > H(2)PO(4)(-), with Cl(-) or Br(-) not being detected.     

The anion sequence in the phase transformation of mesostructures templated by non-ionic block copolymers

A p6m to Ia3d mesophase evolution is achieved by simply adjusting the acidity and/or anion species in the presence of block copolymers; the unusual anion sequence that affects the phase behavior of block copolymer templated mesostructured solids is revealed to be SO(4)(2-)(HSO(4)(-)) > NO(3)(-) > Br(-) > Cl(-).     

Anion coordination complex [Cl⊂Pt(bpt)<Subscript>4</Subscript>]Cl (bpt=<Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>′-bis(3-pyridylmethyl)-2-thiourea)

The anion coordination complex, [Cl?Pt(bpt)₄]Cl (bpt=N,N′-bis(3-pyridylmethyl)-2-thiourea), was synthesized and studied by X-ray crystal structure analysis, NMR and FAB mass spectra. In the solid state, the Pt(bpt)₄ anion receptor adopts a cone conformation to bind the chloride anion through hydrogen bonds and electrostatic interaction in which the four branches of the thiourea ligands bind the chloride anion to form N-H?Cl^? hydrogen bonds (3.49–3.81 Å). The entraped chloride anion is situated above the Pt(II) center at 3.52 Å. Further second-sphere coordination assemby from the Pt(bpt)₄ core with 8 zinc(II) tetraphenylporphyrins (ZnPr) is discussed.     

The benzil-cyanide reaction and its application to the development of a selective cyanide anion indicator

The benzil-cyanide reaction is a cyanide-specific reaction that has been exploited to produce a colorimetric indicator for this toxic anion. This was done by producing a pi-extended analogue of benzil, 7, which is soluble in a 70:30 (v/v) mixture of methanol-water. In this medium, dilute solutions of 7 are yellow but produce colorless products when exposed to low concentrations of cyanide anion (> or = 1.7 microM; added as an aqueous NaCN solution), but no other common anions (e.g., OH(-), F(-), N3(-), benzoate(-), and H2PO4(-)). On the basis of these observations and supporting mechanistic analyses, it is concluded that the modified benzil system 7 is a promising cyanide anion indicator that is attractive in terms of its selectivity, ease-of-use, water compatibility, and the low, naked-eye discernible cyanide detection limit it provides.     

Anion exchange reaction potentials as approximate estimates of the relative thermodynamic stabilities of Mg/Al layered double hydroxides containing different anions

Coatings of hydrotalcite-like nitrate-intercalated Mg/Al layered double hydroxides are electrochemically deposited on a Pt electrode by electrogeneration of base by reduction of a mixed metal nitrate aqueous solution. As-prepared coatings are stable to workup and function as rugged electrodes. The voltammetric response generated by anion exchange of intercalated nitrate for dissolved anions from solution under equilibrium conditions is employed to estimate the thermodynamic stabilities of the Mg/Al layered double hydroxides comprising different anions relative to the nitrate-containing phase. Among monovalent anions, the most stable is the fluoride-containing LDH (ΔG° = -48.7 kJ mol(-1)) relative to the nitrate-containing LDH. The stability in aqueous phase decreases as F(-) > Cl(-) > Br(-) > NO(2)(-) > NO(3)(-), whereas, among divalent anions, SO(4)(2-) (ΔG° = -8.7 kJ mol(-1)) > CO(3)(2-) (ΔG° = 14.3 kJ mol(-1)). The results of monovalent ions match well with the Miyata series, whereas the divalent anion series is at variance with the commonly held belief that carbonate-LDHs are more stable than sulfate-LDHs.     

A variety of spin-crossover behaviors depending on the counter anion: two-dimensional complexes constructed by NH...Cl- hydrogen bonds, [FeIIH3LMe]Cl.X (X = PF6 -, AsF6 -, SbF6 -, CF3SO3 -; H3L(Me) = Tris[2-{[(2-methylimidazol-4-yl)methylidene]amino}ethyl]amine)

A family of spin-crossover (SC) complexes, [Fe(II)H(3)L(Me)]Cl.X (X(-) = PF(6) (-), AsF(6) (-), SbF(6) (-), CF(3)SO(3) (-)), 1-4, has been synthesized, in which H(3)L(Me) denotes the hexadentate N(6) tripod-like ligand tris[2-{[(2-methylimidazol-4-yl)methylidene]amino}ethyl]amine, containing three imidazole groups, with a view to establishing the effect of the counter anion on the SC behavior. These complexes have been found to crystallize in the same monoclinic crystal system with similar cell dimensions. The general crystal structure consists of a two-dimensional (2D) extended network constructed by NH...Cl- hydrogen bonds between Cl- and the imidazole NH groups of three neighboring [Fe(II)H(3)L(Me)]2+ ions, while the anion X exists as an isolated counter anion and occupies the space between the 2D sheets. Magnetic susceptibilities and M?ssbauer spectra have revealed a variety of SC behaviors depending on the counter anion, including a one-step HS<==>(HS + LS)/2 (1, X = PF(6) (-)), a two-step HS<==>(HS + LS)/2<==>LS with a slow thermal relaxation (2, X = AsF(6) (-)), a gradual one-step HS<==>LS (3, X = SbF(6) (-)), and a steep one-step HS<==>LS with hysteresis (4, X = CF(3)SO(3) (-)). The complexes assume the space group P2(1)/n in the HS state, P2(1) in the HS + LS state, and P2(1)/n in the LS state. The Fe-N bond lengths and the N-Fe-N bond angles are indicative of the HS, HS + LS, and LS states. The molecular volumes, V, of the counter anions have been evaluated by quantum-chemical calculations as follows: 53.4 A(3) (BF(4) (-)), 54.4 A(3) (ClO(4) (-)), 73.0 A(3) (PF(6) (-)), 78.5 A(3) (AsF(6) (-)), 88.7 A(3) (SbF(6) (-)), and 86.9 A(3) (CF(3)SO(3) (-)). The size and shape of the counter anion affects the flexible 2D network structure constructed by the hydrogen bonds, leading to modifications of the SC behavior. These estimated relative sizes of the counter anions correlate well with the observed SC behaviors.     

Synthesis and X-ray characterization of the phosphido-carbonyl cluster anions

The [Co(9)P(CO)(21)](2)(-) anion has been isolated from the products of the reaction between Na[Co(CO)(4)] and PCl(5) in tetrahydrofuran at reflux. The structure of the cluster anion [Co(9)P(CO)(21)](2)(-) in its tetraphenylphosphonium salt has been elucidated by X-ray analysis. The crystals are monoclinic, space group P2(1)/n, a = 12.528(3), b = 14.711(5), c = 19.312(6) A, beta = 93.68(2) degrees, Z = 2. Final R = 0.065 for 2300 unique reflections having I > 3sigma(I). The anion, which is disordered about an inversion center, consists of a monocapped square antiprismatic cluster containing an interstitial phosphide and surrounded by 13 terminal and 8 edge-bridging carbonyl ligands. Average values are: Co-Co 2.685 A, and Co-P 2.256 A. The [Co(10)P(CO)(22)](3)(-) anion has been obtained by condensation of the [Co(9)P(CO)(21)](2)(-) anion with [Co(CO)(4)](-) in tetrahydrofuran at reflux. While the [Co(9)P(CO)(21)](2)(-) anion is stable under CO, the [Co(10)P(CO)(22)](3)(-) anion is decomposed to [Co(9)P(CO)(21)](2)(-) and [Co(CO)(4)](-). The benzyltrimethylammonium salt of the [Co(10)P(CO)(22)](3)(-) anion has been studied by X-ray analysis. It gives triclinic crystals, space group P_1, a = 11.452(3), b = 23.510(6), c = 25.606(4) A, alpha = 112.46(1), beta = 95.79(1), gamma = 73.548(2) degrees, Z = 4. Final R = 0.041 for 8600 unique reflections having I > 3sigma(I). There are two independent trianions in the asymmetric unit, both showing similar geometries, consisting of bicapped square antiprismatic clusters with a central P atom, each bearing 10 terminal and 12 edge-bridging carbonyl ligands, 8 of which, bound to the capping metals, are markedly asymmetric. Average values are: Co-Co 2.678 A, and Co-P 2.262 A. Electrochemistry shows that [Co(9)P(CO)(21)](2)(-) and [Co(10)P(CO)(22)](3)(-) in acetonitrile solution undergo either a one-electron oxidation or a two-electron reduction. This latter process appears as a single step in the case of the dianion and as two separated one-electron steps in the case of the trianion. All the processes are accompanied by slow chemical complications, thus testifying that no stable redox congeners exist for these phosphide clusters.     

Negative ion electrospray mass spectra of the maculatin peptides from the tree frogs Litoria genimaculata and Litoria eucnemis

Collision-induced fragmentations of deprotonated maculatin 1 peptides provide significant sequencing information. When the peptide lacks those residues which can fragment through their alpha side chains (e.g. Thr, Ser, Glu and Gln in this study) the basic alpha and beta' backbone cleavages occur from the [Mbond;H](-) anion. When Thr, Ser, Glu and Gln are present, the ease of side-chain fragmentation of these residues is: Thr (loss of MeCHO) > Ser (CH(2)O) > Glu (H(2)O) > Gln (NH(3)). When one of more of these residues is (are) present, the alpha and beta' cleavages often occur from a fragment rather than the [Mbond;H](-) anion, e.g. for Thr, the [(Mbond;H)(-)bond;MeCHO](-) anion. These four residues also initiate gamma backbone cleavage reactions. The relative abundances of peaks resulting from gamma cleavage are Glu > Ser = Thr > Gln for maculatin 1 spectra. An unusual Gln19/Ile17 cyclisation/cleavage reaction occurs in maculatin spectra: the peptide [Mbond;H](-) anion must adopt a helical conformation in order for these two groups to interact. Analogous fragmentations have been reported previously in the negative ion spectra of the caerin 1 peptides.     

Chemiluminescent decomposition of a dioxetane bearing a 3-(1-cyanoethenyl)phenyl moiety induced by Michael addition of an anion of malonate

A thermally stable dioxetane bearing a 3-(1-cyanoethenyl)phenyl group (1) was synthesized. Michael addition of an anion of malonate (3a,b) to a dioxetane (1) substituted with a 3-(1-cyanoethenyl)phenyl moiety took place to give an intermediary dioxetane bearing a benzylic anion, which decomposes rapidly with accompanying emission of crimson light. When an anion of chloromalonate (3c) was used as a base, intramolecular cyclopropanation of 3c occurred concurrently with the Michael-addition-induced chemiluminescent decomposition.     

Development of cation/anion "interaction" scales for ionic liquids through ESI-MS measurements

Electrospray ionization mass spectrometry applied to ionic liquids allowed the study of loosely bonded supramolecules, originating from these organic salts. Based on the observation that ionic liquids formed cationic [C(q+1)X(q)](+) and anionic [C(q)X(q+1)](-) supramolecular aggregates, we have investigated mixed networks, formed by different cations coordinated to a selected anion or by different anions bonded to a given cation, i.e., [C1...X...C2](+) and [X1...C...X2](-), with the aim to build a scale of the cation-anion interaction strength. The qualitative order of intrinsic bond strength to Br- was found to be the following: [emim](+) > [bmim](+) > [mor1,2](+) > [hmim]+ > [omim](+) > [mor1,4](+) > [bupy](+) > [bpyrr](+) > [picol](+) > [bm(2)im](+) > [TBA](+). Similarly, the interaction energies to 1-butyl-3-methylimidazolium (bmim) species envisaged two classes of anions: species tightly coordinated to the cationic moiety that include CF3COO(-), Br(-), N(CN)2(-), and BF4(-) and anions loosely interacting with the alkylimidazolium species such as OTf(-), PF6(-), and Tf2N(-).     

Molecular Complexes of Crown Ethers, Part 5: Complexes of DB18C6 with Some Acceptors

The UV-Visible spectra of DB18C6 as a donor with TCNE (Tetracyanoethylene), and DDQ (2,3-dichloro-5,6-dicyano-1,4- benzoquinone) as acceptors were studied. Charge transfer spectra were obtained for these systems from which it was possible to calculate the formation constant, Kc. The effects of potassium halides were studied. This study shows that in the presence of the anion there is an electron transfer from the anion to the acceptor. This process is enhanced by the presence of the crown ether, CE. The formation of the anion salt with the acceptor, in the presence of the CE, follows the trend F = I > Br > Cl. It is also indicated in this study that the interaction between DB18C6 and the acceptor follow the trend DDQ > TCNE.     

Intramolecular charge-transfer-induced chemiluminescent decomposition of 1,2-dioxetanes bearing a phenylmethanide anion

Dioxetanes (1) bearing a phenyl moiety substituted with a methylene or methine having an electron-withdrawing group(s) (-CH₂-Ew or -CH(X)-Ew) and dioxetane (2) bearing a 3-(1-cyanoethenyl)phenyl group were synthesized. Treatment of dioxetanes (1) with tetrabutylammonium fluoride (TBAF) caused their decomposition with accompanying emission of light with maximum wavelength at 530-758 nm. The Michael addition of a bis(methoxycarbonyl)methanide anion to dioxetane (2) produced initially an unstable dioxetane bearing a phenylmethanide anion, decomposition of which gave light with maximum wavelength at 710-740 nm. Intramolecular cyclopropanation without decomposition of the dioxetane ring took place concurrently for the Michael reaction-induced decomposition of 2 with the bis(methoxycarbonyl)chloromethanide anion.     

Fluoride-selective colorimetric sensor based on thiourea binding site and anthraquinone reporter

A structurally simple colorimetric sensor, N-4-nitrobenzene-N'-1'-anthraquinone-thiourea (1), for anions was synthesized and characterized by (1)H NMR, ESI mass and IR methods. In acetonitrile, the addition of F(-) changed 1 solution from colorless to yellow. In the presence of other anions such as CH(3)CO(2)(-), H(2)PO(4)(-), HSO(4)(-) and Cl(-), however, the absorption spectrum of 1 was slightly red shifted with no obvious color changes observed. The association constants of anionic complexes followed the order of F(-)>CH(3)CO(2)(-)>H(2)PO(4)(-)>HSO(4)(-)>Cl(-)>Br(-), which was different from the order of anion basicity. AM1 calculation results indicated that the most stable configuration of 1 existed in the Z-E-conformation with a six-membered ring via intramolecular hydrogen bond. This made thiourea moiety of 1 in an unfavorable conformation to bond with oxygen-anionic substrates such as CH(3)CO(2)(-) and H(2)PO(4)(-), thus leading to a high selectivity and sensitivity for the detection of F(-).     

A novel colorimetric sensor of dihydrogen-phosphate based on metal complex between 8-hydroxy quinoline-5-azo-4'-nitrobenzene and cobalt

A novel colorimetric sensor based on 8-hydroxy quinoline-5-azo-4'-nitrobenzene (1) was prepared and used for recognizing anions. 1 and its metal complex (1.Co) were found to show response to anions such as CH(3)CO(2)(-), H(2)PO(4)(-), HSO(4)(-), F(-) and dramatic color changes were observed. The selectivity and sensitivity of 1 and 1.Co for sensing anions were different, which was in the order of CH(3)CO(2)(-)>F(-)>H(2)PO(4)(-)>HSO(4)(-) for 1 and H(2)PO(4)(-)>HSO(4)(-)>CH(3)CO(2)(-) approximately F(-) for 1.Co, respectively. In CH(3)CN, sensor 1.Co exhibited excellent specificity toward H(2)PO(4)(-), and the color variety was dependent on the concentration of H(2)PO(4)(-) which was attributed to anion structure and stability of anionic complex (1-anion), metal complex (1-Co) and inorganic complex (Co-anion).     

pH transients during salt removal in isoelectric trapping separations: a curse revisited

The pH transients that occur during isoelectric trapping separations as a result of the removal of nonampholytic ionic components have been re-examined. Salts containing strong electrolyte anions and cations, both with equal and dissimilar mobilities, have been studied using anodic and cathodic buffering membranes whose pH values were both equidistant and nonequidistant from pH 7. The direction and magnitude of the pH transient (acidic or basic) was found to depend on both the mobilities of the anion and cation (mu(anion)/mu(cation)) and the pH difference between pH 7 and the pH of the buffering membranes (|pH(memb) (anodic) - 7|/|7 - pH(memb) (cathodic)|). When |pH(memb) (anodic) - 7|/|7 - pH(memb) (cathodic)| = 1, mu(anion)/mu(cation)<1 leads to an acidic pH transient, mu(anion)/mu(cation) = 1 eliminates the pH transient and mu(anion)/mu(cation)>1 leads to a basic pH transient. When mu(anion)/mu(cation) = 1, |pH(memb) (anodic) - 7|/|7 - pH(memb) (cathodic)|<1 leads to a basic pH transient, |pH(memb) (anodic) - 7|/|7 - pH(memb) (cathodic)| = 1 eliminates the pH transient and |pH(memb) (anodic) - 7|/|7 - pH(memb) (cathodic)|>1 leads to an acidic pH transient. By selecting appropriate anodic and cathodic buffering membranes to adjust the |pH(memb) (anodic) - 7|/|7 - pH(memb) (cathodic)| value, pH transients caused by dissimilar anion and cation mobilities can be avoided.     

Copper(II), nickel(II), cobalt(II) and zinc(II) complexes of 2-[2-(6-methylbenzothiazolyl)azo]-5-dimethylaminobenzoic acid: synthesis,spectral, thermal and molecular modelling studies

Complexes of copper(II), nickel(II), cobalt(II), and zinc(II) with 2-[2-(6-methylbenzothiazolyl)azo]-5-dimethylaminobenzoic acid have been prepared and characterized by elemental analysis, vibrational spectra, magnetic susceptibility measurements, conductance measurements and e.p.r. spectra. Stability constants have been evaluated potentiometrically. Electronic spectra, magnetic susceptibility measurements and molecular modeling studies support a distorted square planar geometry around the metal ions. Vibrational spectra indicate the coordination of the azo group, nitrogen of benzothiazole, the carboxylate anion and the acetate ion on complexation with the metal ion. All complexes are found to be monomers. The stability of the complexes follow the order: copper(II) > nickel(II) > cobalt(II) > zinc(II).     

Microsolvation of the dicyanamide anion: [N(CN)(2)(-)](H(2)O)n (n = 0-12)

Photoelectron spectroscopy is combined with ab initio calculations to study the microsolvation of the dicyanamide anion, N(CN)(2)(-). Photoelectron spectra of [N(CN)(2)(-)](H2O)n (n = 0-12) have been measured at room temperature and also at low temperature for n = 0-4. Vibrationally resolved photoelectron spectra are obtained for N(CN)(2)(-), allowing the electron affinity of the N(CN)2 radical to be determined accurately as 4.135 +/- 0.010 eV. The electron binding energies and the spectral width of the hydrated clusters are observed to increase with the number of water molecules. The first five waters are observed to provide significant stabilization to the solute, whereas the stabilization becomes weaker for n > 5. The spectral width, which carries information about the solvent reorganization upon electron detachment in [N(CN)(2)(-)](H2O)n, levels off for n > 6. Theoretical calculations reveal several close-lying isomers for n = 1 and 2 due to the fact that the N(CN)(2)(-) anion possesses three almost equivalent hydration sites. In all the hydrated clusters, the most stable structures consist of a water cluster solvating one end of the N(CN)(2)(-) anion.     

Mechanistic study on the coupling reaction of aryl bromides with arylboronic acids catalyzed by (iminophosphine)palladium(0) complexes. Detection of a palladium(II) intermediate with a coordinated boron anion

The complexes [Pd(eta2-dmfu)(P-N)] [P-N = 2-(PPh2)C6H4-1-CH=NR, R = C(6)H(4)OMe-4; CHMe2; C6H3Me2-2,6; C6H3(CHMe2)-2,6] react with an excess of BrC6H4R1-4 (R1= CF3; Me) yielding the oxidative addition products [PdBr(C6H4R1-4)(P-N)] at different rates depending on R [C6H4OMe-4 > C6H3(CHMe2)-2,6 > CHMe2 approximately C6H3Me2-2,6] and R1 (CF3> Me). In the presence of K2CO3 and activated olefins (ol = dmfu, fn), the latter compounds react with an excess of 4-R2C6H4B(OH)2 (R2= H, Me, OMe, Cl) to give [Pd(eta2-ol)(P-N)] and the corresponding biaryl through transmetallation and fast reductive elimination. The transmetallation proceeds via a palladium(II) intermediate with an O-bonded boron anion, the formation of which is markedly retarded by increasing the bulkiness of R. The intermediate was isolated for R = CHMe2, R1 = CF3 and R2= H. The boron anion is formulated as a diphenylborinate anion associated with phenylboronic acid and/or as a phenylboronate anion associated with diphenylborinic acid. In general, the oxidative addition proceeds at a lower rate than transmetallation and represents the rate-determining-step in the coupling reaction of aryl bromides with arylboronic acids catalyzed by [Pd(eta2-dmfu)(P-N)].