Arsenic(III) halide complexes with acyclic and macrocyclic thio- and selenoether coligands: synthesis and structural properties

The preparations of the new complexes [AsBr(3)[MeS(CH(2))(2)SMe]], [AsX(3)([9]aneS(3))] (X = Cl, Br or I; [9]aneS(3) = 1,4,7-trithiacyclononane), [AsCl(3)([14]aneS(4))] ([14]aneS(4) = 1,4,8,11-tetrathiacyclotetradecane), [AsX(3)([8]aneSe(2))] ([8]aneSe(2) = 1,5-diselenacyclooctane), [(AsX(3))(2)([16]aneSe(4))] ([16]aneSe(4) = 1,5,9,13-tetraselenacyclohexadecane), and [(AsBr(3))(2)([24]aneSe(6))] ([24]aneSe(6) = 1,5,9,13,17,21-hexaselenacyclotetracosane) are described. These are obtained from direct reaction of the appropriate AsX(3) and 1 mol equiv of the thio- or selenoether ligand in anhydrous CH(2)Cl(2) (or thf for X = I) solution. The products have been characterized by microanalysis and IR and (1)H NMR spectroscopy. In solution they are extensively dissociated, reflecting the weak Lewis acidity of AsX(3). Reaction of AsX(3) with MeSe(CH(2))(2)SeMe or MeC(CH(2)EMe)(3) (E = S or Se) gave only oils. Treatment of PCl(3) or PBr(3) with Me(2)S, MeE(CH(2))(2)EMe, or [9]aneS(3) failed to give solid complexes, and there was no evidence from NMR spectroscopy for any adduct formation in solution. The crystal structures of the first series of thioether and selenoether complexes of As(III) are described: [AsBr(3)[MeS(CH(2))(2)SMe]], C(4)H(10)AsBr(3)S(2), a = 10.2818(6) A, b = 7.8014(5) A, c = 14.503(1) A, beta = 102.9330(2) degrees, monoclinic, P2(1)/c, Z = 4; [AsI(3)[MeS(CH(2))(2)SMe]], C(4)H(10)AsI(3)S(2), a = 9.1528(1) A, b = 11.5622(2) A, c = 12.0939(2) A, beta = 93.863(1) degrees, monoclinic, P2(1)()/n, Z = 4; [AsCl(3)([9]aneS(3))], C(6)H(12)AsCl(3)S(3), a = 17.520(4) A, b = 17.520(4) A, c = 16.790(7) A, tetragonal, I4(1)cd, Z = 16; [AsCl(3)([14]aneS(4))], C(10)H(20)AsCl(3)S(4), a = 13.5942(2) A, b = 7.7007(1) A, c = 18.1270(3) A, beta = 111.1662(5) degrees, monoclinic, P2(1)()/n, Z = 4; [(AsCl(3))(2)([16]aneSe(4))], C(12)H(24)As(2)Cl(6)Se(4), a = 9.764(3) A, b = 13.164(1) A, c = 10.627(2) A, beta = 114.90(1) degrees, monoclinic, P2(1)()/n, Z = 2; [(AsBr(3))(2)([16]aneSe(4))], C(12)H(24)As(2)Br(6)Se(4), a = 10.1220(1) A, b = 13.4494(2) A, c = 10.5125(2) A, beta = 113.49(2) degrees, monoclinic, P2(1)()/n, Z = 2. [AsBr(3)[MeS(CH(2))(2)SMe]] and [AsI(3)[MeS(CH(2))(2)SMe]] reveal discrete mu(2)-halo As(2)X(6) dimeric structures involving distorted octahedral As(III), with the dithioether ligand chelating. [AsCl(3)([9]aneS(3))] adopts a discrete molecular distorted octahedral geometry with the thioether behaving as a weakly coordinated fac-capping ligand. [AsCl(3)([14]aneS(4))] forms an infinite sheet involving two mu(2)-chloro ligands on each As but bridging to two distinct As centers. Each macrocycle coordinates to two adjacent As centers via one S atom, giving a cis-octahedral Cl(4)S(2) donor set at As(III). The structures of [(AsCl(3))(2)([16]aneSe(4))] and [(AsBr(3))(2)([16]aneSe(4))] adopt 2-dimensional sheet structures with mu(2)-dihalo As(2)X(6) dimers cross-linked by mu(4)-tetraselenoether macrocycles, giving a disorted cis-X(4)Se(2) donor set at each As center. These species are compared with their antimony(III) and bismuth(III) analogues where appropriate.     

Assignments of the Pfr-Pr FTIR difference spectrum of cyanobacterial phytochrome Cph1 using 15N and 13C isotopically labeled phycocyanobilin chromophore

The reversible red and far-red light-induced transitions of cyanobacterial phytochrome Cph1 from Synechocystis PCC 6803 were investigated by Fourier transform infrared (FTIR) difference spectroscopy. High-quality light-induced Pfr-Pr difference FTIR spectra were recorded for the 58 kDa N-terminal domain of Cph1 by repetitive photochemical cycling and signal averaging. The Pfr-Pr difference spectra in H(2)O and D(2)O were very similar to those previously reported for full-length 85 kDa Cph1.(1) Published assignments were extended by analysis of the effects of (13)C and (15)N isotope substitutions at selected sites in the phycocyanobilin chromophore and by (15)N global labeling of the protein. The Pfr-Pr difference spectra were dominated by an amide I peak/trough at 1653 cm(-1)(+)/1631 cm(-1)(-) and a smaller amide II band at 1554 cm(-1). Labeling effects allowed specific chromophore assignments for the C(1)=O (1736 cm(-1)(-)/1724 cm(-1)(+)) and C(19)=O (1704 cm(-1)(-)) carbonyl vibrations, C=C vibrations at 1589 cm(-1)(+), and bands at 1537(-), 1512(+), 1491(-), 1163(+), 1151(-), 1134(+), 1109(-), and 1072(-) cm(-1) that must involve chromophore C-N bonds. A variety of additional changes were insensitive to isotope labeling of the chromophore. Effects of (15)N labeling of the protein were used to tentatively assign some of these to specific amino acid changes. Those insensitive to (15)N labeling included a protonated aspartic or glutamic acid at 1734 cm(-1)(-)/1722 cm(-1)(+) and a cysteine at 2575 cm(-1)(+)/2557 cm(-1)(-). Bands sensitive to (15)N protein labeling at 1487 cm(-1)(+)/1502 cm(-1)(-) might arise from trytophan and bands at 1261 cm(-1)(+)/1244 cm(-1)(-) and 1107 cm(-1)(-)/1095 cm(-1)(+) might arise from a histidine environment or protonation change. These assignments are discussed in light of the 15Z-E photoisomerization model of phototransformation and the associated protein conformational changes.     

Correlating axial and equatorial ligand field effects to the single-molecule magnet performances of a family of dysprosium bis-methanediide complexes

Treatment of the new methanediide–methanide complex [Dy(SCS)(SCSH)(THF)] (1Dy, SCS = {C(PPh₂S)₂}²⁻) with alkali metal alkyls and auxillary ethers produces the bis-methanediide complexes [Dy(SCS)₂][Dy(SCS)₂(K(DME)₂)₂] (2Dy), [Dy(SCS)₂][Na(DME)₃] (3Dy) and [Dy(SCS)₂][K(2,2,2-cryptand)] (4Dy). For further comparisons, the bis-methanediide complex [Dy(NCN)₂][K(DB18C6)(THF)(toluene)] (5Dy, NCN = {C(PPh₂NSiMe₃)₂}²⁻, DB18C6 = dibenzo-18-crown-6 ether) was prepared. Magnetic susceptibility experiments reveal slow relaxation of the magnetisation for 2Dy–5Dy, with open magnetic hysteresis up to 14, 12, 15, and 12 K, respectively (∼14 Oe s⁻¹). Fitting the alternating current magnetic susceptibility data for 2Dy–5Dy gives energy barriers to magnetic relaxation (U_eff) of 1069(129)/1160(21), 1015(32), 1109(70), and 757(39) K, respectively, thus 2Dy–4Dy join a privileged group of SMMs with U_eff values of ∼1000 K and greater with magnetic hysteresis at temperatures >10 K. These structurally similar Dy-components permit systematic correlation of the effects of axial and equatorial ligand fields on single-molecule magnet performance. For 2Dy–4Dy, the Dy-components can be grouped into 2Dy–cation/4Dy and 2Dy–anion/3Dy, where the former have almost linear C Dy C units with short average Dy C distances, and the latter have more bent C Dy C units with longer average Dy C bonds. Both U_eff and hysteresis temperature are superior for the former pair compared to the latter pair as predicted, supporting the hypothesis that a more linear axial ligand field with shorter M–L distances produces enhanced SMM properties. Comparison with 5Dy demonstrates unusually clear-cut examples of: (i) weakening the equatorial ligand field results in enhancement of the SMM performance of a monometallic system; (ii) a positive correlation between U_eff barrier and axial linearity in structurally comparable systems.

Studies on equatorial donor and C Dy C angle variation effects on energy barriers to the slow relaxation of magnetisation are reported.     

Use of U-shaped donor-bridge-acceptor molecules to study electron tunneling through nonbonded contacts

A systematic determination of electronic coupling matrix elements in U-shaped molecules is demonstrated. The unique architecture of these systems allows for the determination of the electronic coupling through a pendant molecular moiety that resides between the donor and acceptor groups; this moiety quantifies the efficiency of electron tunneling through nonbonded contacts. Experimental electron-transfer rate constants and reaction free energies are used to calibrate a molecular-based model that describes the solvation energy. This approach makes it possible to experimentally determine electronic couplings and compare them with computational values.     

Metal ion scrambling in hexanuclear M(6)(Et(2)NCO(2))(12) complexes (M = Co,Mg). Synthesis,solid state structure,and solution dynamics of heteronuclear Co(n)Mg(6-n)(Et(2)NCO(2))(12) complexes

Heteronuclear diethylcarbamato complexes of the form Co(n)()Mg(6)(-)(n)()(Et(2)NCO(2))(12) were prepared from the isostructural homonuclear precursors Mg(6)(Et(2)NCO(2))(12), 1, and Co(6)(Et(2)NCO(2))(12), 2, via a solvothermal methodology. Two materials were selected for single-crystal X-ray diffraction analysis: Co(1.6)Mg(4.4)(Et(2)NCO(2))(12) and Co(2.7)Mg(3.3)(Et(2)NCO(2))(12). Both compounds crystallize in the orthorhombic space group Ccca, as do 1 and 2. The molecular structure is best described as two trinuclear M(3) units cross-linked by diethylcarbamate ligands and twisted about one another, so that the complex has overall D(2) symmetry and is chiral. Each trinuclear unit consists of two terminal pentacoordinate metal ions and one central hexacoordinate metal ion. The X-ray diffraction data were unambiguous that the Co(2+) ions migrate exclusively to the pentacoordinate sites in the heteronuclear complexes, thus demonstrating that metal ion scrambling at the molecular level must occur. The composition of individual crystals can be continuously varied for Co(2+) mole fractions chi(Co) < 0.5, and the a and c unit cell distances are linearly related to chi(Co). This indicates that the compounds behave as solid solutions. There appears to be either a chemical or crystallographic phenomenon inherent in the synthetic methodology that prevents isolation of heteronuclear materials having chi(Co) > 0.5. Solution electronic spectroscopy and molecular weight measurements show that 2 can dissociate in chloroform and cyclohexane solution to give a dimeric complex 2'. This behavior contrasts with the stability of 1 in solution, as shown by NMR. The kinetic rate profile for formation of Co(n)Mg(6-n)(Et(2)NCO(2))(12) reveals saturation kinetics and is consistent with direct attack by 2' on 1 to give the heteronuclear complex via a higher nuclearity intermediate. This study illustrates a general method for the preparation of solids based on heteronuclear Werner-type complexes of the M(6)(Et(2)NCO(2))(12) structure type, and the mechanism by which such compounds can be formed from isostructural homonuclear precursors.     

Nested-channel for on-demand alternation between electrospray ionization regimes

On-demand electrospray ionization from different liquid channels in the same emitter was realized using filamented capillary and gas phase charge supply. The solution sub-channel was formed when back-filling solution to the emitter tip by capillary action along the filament. Gas phase charge carriers were used to trigger electrospray ionization from the solution meniscus at the tip. The meniscus at the tip opening may be fully filled or partially empty to generate electrospray ionization in main-channel regime and sub-channel regime, respectively. For emitters with 4 μm tip opening, the two nested electrospray (nested-ESI) channels accommodated ESI flow rates ranging from 50 pL min⁻¹ to 150 nL min⁻¹. The platform enabled on-demand regime alternations within one sample run, in which the sub-channel regime generated smaller charged droplets. Ionization efficiencies for saccharides, glycopeptide, and proteins were enhanced in the sub-channel regime. Non-specific salt adducts were reduced and identified by regime alternation. Surprisingly, the sub-channel regime produced more uniform responses for a peptide mixture whose relative ionization efficiencies were insensitive to ESI conditions in previous picoelectrospray study. The nested channels also allowed effective washing of emitter tip for multiple sampling and analysis operations.

Nested electrospray ionization alternates on-demand between microscale main-channel and nanscale sub-channels.     

Controlling the outcome of an N-alkylation reaction by using N-oxide functional groups

Covalent modifiers of proteins are of importance in chemical proteomics, an emerging chemical technology used to assign protein function. In this study, high-field (1)H NMR techniques were used to analyze the reaction of the bioactive compound, 2,3-bis(bromomethyl)quinoxaline 1,4-dioxide, with amines (a model system for proteins containing nitrogen-based nucleophiles). Unexpectedly, the results show that a double nucleophilic substitution reaction involving 2 equiv of the amine is preferred to an intramolecular cyclization pathway. A direct comparison with the reaction carried out on a substrate lacking the N-oxide functional groups is also provided. X-ray crystal structures and computational studies are used to rationalize the observed differences in reactivity between the two systems.     

X-ray photoelectron spectroscopic evidence of distinctive underpotential deposition states of Ag and Cu on Pt substrates

The underpotential deposition (u.p.d.) of copper and silver ions on platinum electrodes has been studied using X-ray photoelectron spectroscopy (x.p.s.). A significant chemical shift of the x.p.s. binding energies of the u.p.d. copper and silver atoms relative to bulk copper and silver metal is obtained. Analogous spectral results are obtained for in situ vapor deposition of copper and silver atoms on clean platinum substrates which reveal the metallic nature of the u.p.d. species.     

CsOH-promoted epoxide ring-opening with phosphines: mild and efficient synthesis of monohydroxyphosphines

A mild and convenient synthesis of monohydroxyphosphines has been achieved by epoxide ring-opening using primary or secondary phosphines in the presence of cesium hydroxide, 4 Å molecular sieves and DMF at room temperature. These reaction conditions were found to be highly regio- and stereoselective producing various monohydroxyphosphines exclusively in moderate to high yields.     

The role of intersystem crossing steps in singlet oxygen chemistry and photo-oxidations

Singlet oxygen chemistry and photo-oxidation reactions, in general, often require one or more critical reaction steps that involve an intersystem crossing from a singlet state to a triplet state or vice versa. This paper considers two important intersystem crossing mechanisms, electron spin-electron orbit (spin-orbit) coupling and electron spin-nuclear spin (spin-spin) coupling, and how they may be involved : (1) in the deactivation of ¹O₂ to ³O₂ ; (2) in the thermal catalytic conversion of ³O₂ to ¹O₂; and (3) in the fragmentation of aromatic endoperoxides to yield O₂ and an aromatic substrate.