Complexation of strontium with triisopropyl ester of dichloromethylene-bis(phosphonic acid) in water and ionic liquids

The complexation of strontium with triisopropyl ester of dichloromethylene-bis(phosphonic acid) (NaL) is examined using ³¹P NMR in water and the hydrophilic ionic liquid 1-butyl-3-methylimidazolium dicyanamide ([BMIM][N(CN)₂]):      

Stability constants of cesium complexes with 18-crown-6 in ionic liquids

The stability constants of the complex[Cs(18C6)]⁺ (18C6 is 18-crown-6 (L)) in N-butylpyridinium methyl sulfate (I) and of the complex [Cs(18C6)₂]⁺ in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (II) were measured by using ¹³³Cs NMR spectroscopy at 23°C. It was found that logK(Cs + L) in solvent I is 1.20±0.13 and logK(CsL + L) in solvent IIis 1.18±0.05. For the complex [Cs(18C6)₂]⁺, the dependence of its stability constant on the temperature in the 23–50°C range was obtained and the enthalpy change in the complexation was determined: ΔH(CsL + L)= ?47 kJ/mol. It was demonstrated that the enthalpy change is favorable for the formation of [Cs(18C6)₂]⁺, while the entropy change hinders the complexation.     

Biocatalytic Oxidation Reactions: A Chemist's Perspective

Oxidation chemistry using enzymes is approaching maturity and practical applicability in organic synthesis. Oxidoreductases (enzymes catalysing redox reactions) enable chemists to perform highly selective and efficient transformations ranging from simple alcohol oxidations to stereoselective halogenations of non‐activated C?H bonds. For many of these reactions, no “classical” chemical counterpart is known. Hence oxidoreductases open up shorter synthesis routes based on a more direct access to the target products. The generally very mild reaction conditions may also reduce the environmental impact of biocatalytic reactions compared to classical counterparts. In this Review, we critically summarise the most important recent developments in the field of biocatalytic oxidation chemistry and identify the most pressing bottlenecks as well as promising solutions.     

Formation and structural characterization of novel polynuclear palladium selenolato complexes [Pd3Se(SePh)3(PPh3)3]Cl and [Pd6Cl2Se4(SePh)2(PPh3)6]

In addition to well-known dinuclear phenylselenolato palladium complexes, the reaction of [PdCl₂(PPh₃)₂] and NaSePh affords small amounts of novel trinuclear and hexanuclear complexes [Pd₃Se(SePh)₃(PPh₃)₃]Cl (1) and [Pd₆Cl₂Se₄(SePh)₂(PPh₃)₆] (2). Complex 1 is triclinic, P1?, a=13.6310(2), b=16.2596(2), c=16.9899(3) Å, α=83.1738(5), β=78.9882(5), γ=78.7635(5)°. Complex 2 is monoclinic, C2/c, a=25.7165(9), b=17.6426(8), c=27.9151(14) Å, β=110.513(2)°. There are no structural forerunners for 1, but the hexanuclear complex 2 is isostructural with [Pd₆Cl₂Te₄(TeR)₂(PPh₃)₆] (R=Ph, C₄H₃S) that have been observed as one of the products in the oxidative addition of R₂Te₂ to [Pd(PPh₃)₄]. Mononuclear palladium complexes may play a significant role as building blocks in the formation of the polynuclear complexes.     

Cofactor-Free,Direct Photoactivation of Enoate Reductases for the Asymmetric Reduction of C=C Bonds

Enoate reductases from the family of old yellow enzymes (OYEs) can catalyze stereoselective trans-hydrogenation of activated C=C bonds. Their application is limited by the necessity for a continuous supply of redox equivalents such as nicotinamide cofactors [NAD(P)H]. Visible light-driven activation of OYEs through NAD(P)H-free, direct transfer of photoexcited electrons from xanthene dyes to the prosthetic flavin moiety is reported. Spectroscopic and electrochemical analyses verified spontaneous association of rose bengal and its derivatives with OYEs. Illumination of a white light-emitting-diode triggered photoreduction of OYEs by xanthene dyes, which facilitated the enantioselective reduction of C=C bonds in the absence of NADH. The photoenzymatic conversion of 2-methylcyclohexenone resulted in enantiopure (ee>99 %) (R)-2-methylcyclohexanone with conversion yields as high as 80–90 %. The turnover frequency was significantly affected by the substitution of halogen atoms in xanthene dyes.     

Stability of crown-ether complexes with alkali-metal ions in ionic liquid-water mixed solvents

Stability constants of sodium and cesium ion complexes with 18-crown-6 (18C6) and dibenzo-18-crown-6 (DB18C6) in N-butyl-4-methyl-pyridinium tetrafluoroborate [BMP][BF₄] aqueous solutions were measured using the ²³Na and ¹³³Cs NMR technique at 23 °C. To the best of our knowledge, the estimated values of stability constants reported in this study are the first such values given for ionic liquid solutions. The cationic exchange between the free and complexed species is rapid, and only formation of the 1:1 complexes [M(18C6)]⁺ and [M(DB18C6)]⁺ (M = Na⁺, Cs⁺) were observed. The complex formation constants demonstrated a strong dependence on the [BMP][BF₄] concentration. For [M(18C6)]⁺, in solutions with a 0.33–0.70 mole fraction of water in [BMP][BF₄], lg K values are found to be more than one unit higher than the lg K values measured in pure aqueous solutions, although no information concerning the influence of [BMP][BF₄] on the complex formation selectivity could be observed. DB18C6 complexes revealed significantly lower stability under the same conditions. An extrapolation to zero water content gave the lg K = 2.42 for [Cs(18C6)]⁺ in [BMP][BF₄]. It was discovered that when added to water, [BMP][BF₄] increases the solubility of crown ethers and decreases the solubility of alkali metal nitrates. Complex formation with crown ethers enhances the solubility of alkali metal salts in [BMP][BF₄].     

Thermodynamics of cesium complexes formation with 18-crown-6 in ionic liquids

Thermodynamic data for cesium complexes formation with 18-crown-6 (18C6, L) [Cs(18C6)]⁺ in N-butyl-4-methyl-pyridinium tetrafluoroborate ([BMPy][BF₄], I), in 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄], II) and in 1-butyl-3-methylimidazolium dicyanamide ([BMIM][N(CN)₂], III) were measured with NMR ¹³³Cs technique at 23–50 °C. The stability of cesium complex in RTILs is estimated to be in the range between water and DMFA. Stability constants for [Cs(18C6)]⁺ are found to decrease as temperature is increasing. The following values for lgK(Cs+L) and ΔH(Cs+L) at 23 °C are determined: 2.6 (0.3), ?47(1) kJ/mol (RTIL I); 2.8(0.3), ?80(3) kJ/mol (RTIL II) and 3.03 (0.08), ?47(2) kJ/mol (RTIL III). It is demonstrated that enthalpy change promotes complex formation while the corresponding change of entropy is negative and provides decomposition of [Cs(18C6)]⁺.     

Stability constant of the sodium complex with 18-crown-6 in the mixed water-ionic liquid solvent

The stability constant of the Na⁺ complex with dibenzo-18-crown-6, [Na(DB18C6)]⁺, was measured by the ²³Na NMR method in the mixed water-ionic liquid solvent at 23°C. N-Butyl-4-methylpyridinium tetrafluoroborate (BMPTB) was used as room temperature ionic liquid. It was found that for the solvent containing 0.8 mol fraction of water and 0.2 mol fraction of BMPTB, the value log K = 0.77 ± 0.05. The addition of BMPTB to water was found to increase the solubility of the crown ether but to decrease the sodium nitrate solubility.     

Phase separation and crystallinity in proton conducting membranes of styrene grafted and sulfonated poly(vinylidene fluoride)

A series of proton exchange membranes have been prepared by the preirradiation grafting method. Styrene was grafted onto a matrix of poly(vinylidene fluoride) (PVDF) after electron beam irradiation. Part of the samples was crosslinked with divinylbenzene (DVB) or bis(vinylphenyl)ethane (BVPE). Subsequent sulfonation gave membranes grafted with poly(styrene sulfonic acid) and marked PVDF‐g‐PSSA. It was found that the intrinsic crystallinity of the matrix decreased in both the grafting and the sulfonation reaction in all the membranes. The graft penetration and the ion conductivity are influenced strongly by the crosslinker. The ion conductivity is considerably lower in crosslinked membranes than in noncrosslinked ones. Generally, the mechanical strength decreases with crosslinking. The membranes show a regular phase separated structure in which the sulfonated grafts are incorporated in the amorphous parts of the matrix polymer. The phase separated domains are small, of the order of magnitude of 100–250 nm. These were resolved on transmission electron micrographs and on atomic force images but could not be resolved with microprobe Raman spectroscopy. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1741–1753, 1999