Aqueous rhodium(III) hydrides and mononuclear rhodium(II) complexes

In aqueous solutions, as in organic solvents, rhodium hydrides display the chemistry of one of the three limiting forms, i.e. {Rh(I)+ H+}, {Rh(II)+ H.}, and {Rh(III)+ H-}. A number of intermediates and oxidation states have been generated and explored in kinetic and mechanistic studies. Monomeric macrocyclic rhodium(II) complexes, such as L(H2O)Rh2+ (L = L1 = [14]aneN4, or L2 = meso-Me6[14]aneN4) can be generated from the hydride precursors by photochemical means or in reactions with hydrogen atom abstracting agents. These rhodium(II) complexes are oxidized rapidly with alkyl hydroperoxides to give alkylrhodium(III) complexes. Reactions of Rh(II) with organic and inorganic radicals and with molecular oxygen are fast and produce long-lived intermediates, such as alkyl, superoxo and hydroperoxo complexes, all of which display rich and complex chemistry of their own. In alkaline solutions of rhodium hydrides, the existence of Rh(I) complexes is implied by rapid hydrogen exchange between the hydride and solvent water. The acidity of the hydrides is too low, however, to allow the build-up of observable quantities of Rh(I). Deuterium kinetic isotope effects for hydride transfer to a macrocyclic Cr(v) complex are comparable to those for hydrogen atom transfer to various substrates.     

Bent-core mesogens based on semi-flexible dicyclohexylmethane spacers

New, bent-core mesogens are described in which the core of the molecule is a semiflexible, di(4-aminocyclohexyl)methane spacer. The compounds show nematic, columnar nematic and columnar phases as shown by a combination of X-ray diffraction and optical microscopy. The potential of these new mesogens as biaxial nematic candidates is considered.     

Integration of metrological principles in a proficiency-testing scheme in the field of water analysis

Measurement traceability is universally recognised as one of the basic prerequisites for comparability of results obtained in different laboratories and is a basic aspect of metrological sciences such as analytical chemistry. This requirement is underscored by the increasing adoption of standards and measurement quality systems, such as laboratory accreditation against ISO/IEC 17025. Testing laboratories ensure traceability of their measurement results by using appropriate reference standards for calibration of instruments and control of measurement processes. For routine work in the field of water analysis, these standards are usually commercial solutions or in-house solutions prepared from pure products. Therefore, laboratories should demonstrate that their use of reference standards is appropriate and sufficient, which can be done by participation in an appropriate proficiency-testing scheme. The paper reports how measurement traceability of results from field laboratories (nitrite nitrogen, nitrate nitrogen, chloride and sulphate; all in water) can be demonstrated by participation in a proficiency-testing scheme based on reference values.     

1H NMR studies of ligand and H/D exchange reactions of cis- and trans-([14]aneN4)(H2O)RhH2+ in aqueous solutions

Substitution and exchange reactions of cis- and trans-L(1)(H(2)O)RhH(2+) (L(1) = 1,4,8,11-tetraazacyclotetradecane = [14]aneN(4)) were studied in aqueous solutions by UV-vis and (1)H NMR spectroscopies. At pH 1 and 25 degrees C, the substitution of SCN(-) for the coordinated molecule of water is rapid and thermodynamically favorable. Spectrophotometric determinations yielded the equilibrium constants K = 1.49 x 10(3) M(-1) (cis) and 1.44 x 10(3) (trans). (1)H NMR studies in D(2)O revealed a rapid dynamic process, interpreted as the exchange between coordinated water and X(-) (X = Cl, Br, or I). On the other hand, no line broadening was observed for the strongly bound ligands CN(-) and SCN(-). The complex trans-L(1)(D(2)O)RhH(2+) undergoes a base-catalyzed H/D exchange of the hydride in D(2)O with a rate constant of (1.45 +/- 0.02) x 10(3) M(-1) s(-1). The exchange in the cis isomer is very slow under similar conditions. The complex cis-[L(1)ClRhH](ClO(4)) crystallizes in the centrosymmetric Ponemacr; space group, unit cell dimensions a = 8.9805(11) A, b = 9.1598(11) A, c = 10.4081(13) A, alpha = 81.091(2) degrees, beta = 81.978(2) degrees, gamma = 88.850(2) degrees. The rhodium atom resides in a slightly distorted octahedral environment consisting of the four N atoms of the cyclam, a stereochemically active hydrogen, and a chlorine atom.     

Kinetics and Mechanism of the Reaction of Chloroplatinum(II) Complexes with Alkyl Radicals

The reaction between certain platinum(II) complexes and alky radicals produces an unstable organoplatinum(III) intermediate, {Pt^III -R}. The kinetics of this step were evaluated by laser flash photolysis with ABTS² (2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate) ion) and TMPD (tetramethylphenylenediamine) as kinetic probes. The rate constants for PtCl₄^2? are: k_Pt/10⁸ L mol^?1 s^?1 = 5.2, 2.8 and 0.27 for CH_3, C₂H₅, and CH₂Cl in aqueous solution at pH 1. Those with cis-Pt(NH₃)₂Cl₂ are somewhat smaller, and those for Pt(NH₃)₄²⁺ too small to measure will) this technique. The product analysis indicates that the decomposition of organoplatinum takes place by hydrolysis and (for R = C₂H₅ only) by β-elimination, The kinetic isotope effect on die β-elimination of DCH₂CH₂PtC₄,^2? is k_H/k_D = 1.2. The β-elimination step produces a Pt^III-hydride that releases hydrogen gas and forms {Pt_III-OH}. The short-lived Pt(III) intermediate may disproportionate or oxidize the Co^II complex that is produced in the radical-generating step.     

Nitrous acid as a source of NO and NO(2) in the reaction with a macrocyclic superoxorhodium(III) complex

The title reaction takes place according to the stoichiometry 2L(2)RhOO(2+) + 3HNO(2) + H(2)O --> 2L(2)Rh(OH(2))(3+) + 3NO(3)(-) + H(+) (L(2) = meso-Me(6)-[14]ane-N(4)). The kinetics are second order in HNO(2) and independent of the concentration of L(2)RhOO(2+), rate = (k(1) + k(2)[H(+)])[HNO(2)](2), where k(1) = 10.9 M(-1) s(-1) and k(2) = 175 M(-2) s(-1) at 25 degrees C and 0.10 M ionic strength. The reaction produces two observable intermediates, the nitrato (L(2)RhONO(2)(2+)) and hydroperoxo (L(2)RhOOH(2+)) complexes. The product analysis and kinetics are indicative of the initial rate-controlling formation of NO and NO(2), both of which react rapidly with L(2)RhOO(2+) in subsequent steps. The reaction with NO produces mainly L(2)RhONO(2)(2+), which hydrolyzes to L(2)Rh(OH(2))(3+) and NO(3)(-). Another minor pathway generates the hydroperoxo complex, which was detected by its known reaction with Fe(aq)(2+). The reaction of NO(2) with L(2)RhOO(2+) requires an additional equivalent of HNO(2) and produces L(2)Rh(OH(2))(3+) and NO(3)(-) via a proposed peroxynitrato complex L(2)RhOONO(2)(2+). This work provides strong evidence for the long-debated reaction between HNO(2) and H(2)NO(2)(+) to generate N(2)O(3).     

Generation of a peroxynitrato metal complex from nitrogen dioxide and coordinated superoxide

The reaction between photogenerated NO(2) radicals and a superoxochromium(III) complex, Cr(aq)OO(2+), occurs with rate constants k(Cr)(20) = (2.8 +/- 0.2) x 10(8) M(-)(1) s(-)(1) (20 vol % acetonitrile in water) and k(Cr)(40) = (2.6 +/- 0.5) x 10(8) M(-)(1) s(-)(1) (40 vol % acetonitrile) in aerated acidic solutions and ambient temperature. The product was deduced to be a peroxynitrato complex, Cr(aq)OONO(2)(2+), which undergoes homolytic cleavage of an N-O bond to return to the starting materials, the rate constants in the two solvent mixtures being k(H)(20) = 172 +/- 4 s(-)(1) and k(H)(40) = 197 +/- 7 s(-)(1). NO(2) reacts rapidly with 10-methyl-9,10-dihydroacridine, k(A)(20) = 2.2 x 10(7) M(-)(1) s(-)(1), k(A)(40) = (9.4 +/- 0.2) x 10(6) M(-)(1) s(-)(1), and with N,N,N',N'-tetramethylphenylenediamine, k(T)(40) = (1.84 +/- 0.03) x 10(8) M(-)(1) s(-)(1).     

Reaction of nitrogen monoxide with a macrocyclic superoxorhodium(III) complex produces an observable nitratorhodium intermediate

The rapid (k > or = 10(6) M(-1) s(-1)) reaction between NO and L(2)(H(2)O)RhOO(2+) (L(2) = meso-Me(6)-[14]ane-N(4)) generates two strongly oxidizing, scavengeable intermediates, believed to be NO(2) and L(2)(H(2)O)RhO(2+). A mechanism is proposed whereby a peroxynitrito complex L(2)(H(2)O)RhOONO(2+) is formed first. The homolysis of O-O bond produces NO(2) and L(2)(H(2)O)RhO(2+) which were trapped with ABTS(2)(-) and Ni([14]aneN(4))(2+). In the absence of scavengers, the decomposition of L(2)(H(2)O)RhOONO(2+) produces both free NO(3)(-) and a rhodium nitrato complex L(2)(H(2)O)RhONO(2)(2+), which releases NO(3)(-) in an inverse acid-dependent process. The total yield of L(2)(H(2)O)RhONO(2)(2+) is 70%. In a minor, parallel path, NO and L(2)(H(2)O)RhOO(2+) react to give nitrite and the hydroperoxo complex L(2)(H(2)O)RhOOH(2+).     

Acid-catalyzed oxidation of iodide ions by superoxo complexes of rhodium and chromium

Superoxometal complexes L(H(2)O)MOO(2+) (L = (H(2)O)(4), (NH(3))(4), or N(4)-macrocycle; M = Cr(III), Rh(III)) react with iodide ions according to the stoichiometry L(H(2)O)MOO(2+) + 3I(-) + 3H(+) --> L(H(2)O)MOH(2+) + 1.5I(2) + H(2)O. The rate law is -d[L(H(2)O)MOO(2+)]/dt = k [L(H(2)O)MOO(2+)][I(-)][H(+)], where k = 93.7 M(-2) s(-1) for Cr(aq)OO(2+), 402 for ([14]aneN(4))(H(2)O)CrOO(2+), and 888 for (NH(3))(4)(H(2)O)RhOO(2+) in acidic aqueous solutions at 25 degrees C and 0.50 M ionic strength. The Cr(aq)OO(2+)/I(-) reaction exhibits an inverse solvent kinetic isotope effect, k(H)()2(O)/k(D)2(O) = 0.5. In the proposed mechanism, the protonation of the superoxo complex precedes the reaction with iodide. The related Cr(aq)OOH(2+)/I(-) reaction has k(H)2(O)/k(D)2(O) = 0.6. The oxidation of (NH(3))(5)Rupy(2+) by Cr(aq)OO(2+) exhibits an [H(+)]-dependent pathway, rate = (7.0 x 10(4) + 1.78 x 10(5)[H(+)])[Ru(NH(3))(5)py(2+)][Cr(aq)OO(2+)]. Diiodine radical anions, I(2)(*)(-), reduce Cr(aq)OO(2+) with a rate constant k = 1.7 x 10(9) M(-1) s(-1).     

Hydrogen-atom transfer from transition metal hydroperoxides, hydrogen peroxide, and alkyl hydroperoxides to superoxo and oxo metal complexes

Superoxochromium(III) complexes L(H2O)CrOO2+ (L = (H2O)4 and 1,4,8,11-tetraazacyclotetradecane) oxidize hydroperoxo complexes of rhodium and cobalt in an apparent hydrogen-atom transfer process, i.e., L(H2O)CrOO2+ + L(H2O)RhOOH2+ --> L(H2O)CrOOH2+ + L(H2O)RhOO2+. All of the measured rate constants fall in a narrow range, 17-135 M-1 s-1. These values are about 2.5-3.0 times smaller in D2O, where the hydroperoxo hydrogen is replaced by deuterium, and coordinated molecules of water by D2O. The failure of the back reaction to take place in the available concentration range places the O-H bond dissociation energy in RhOO-H2+ at or=80 kJ/mol) in the driving force for the two types of reactions. A chromyl ion, CrIVaqO2+, oxidizes L(H2O)RhOOH2+ and the cobalt analogs to the corresponding superoxo complexes. The rate constants are approximately 102-fold larger than those for the oxidation by CraqOO2+. The oxidation of tert-BuOOH by CrIVaqO2+ has k = 160 M-1 s-1 and exhibits an isotope effect kBuOOH/kBuOOD = 12. Hydrogen atom transfer from H2O2 to CraqOO2+ is slow, k approximately 10-3 M-1 s-1.