High-spin carboxylate polymers [M(OOCCMe3)2]n of group VIII 3d metals

A coordination polymer of the general formula [Co(OOCCMe₃)₂]_n (2) was prepared by mild thermolysis of the coordination polymer of variable composition [(HOOCCMe₃)_xCo(OH)_n(OOCCMe₃)_2−n ]_m, the dinuclear cobalt complex Co₂(μ-H₂O)(OOCCMe₃)₄(HOOCCMe₃)₄, the tetranuclear cobalt cluster Co₄(μ₃-OH)₂(OOCCMe₃)₆(HOEt)₆, and the hexanuclear cluster [Co₆(μ₄-O)₂(μ_n-OOCCMe₃)₁₀(C₄H₈O)₃(H₂O)]·1.5(C₄H₈O) (7) in organic solvents. In the crystal, the polymer has a chain structure. Unlike thermolysis of cobalt pivalates, thermolysis of the dinuclear complex Ni₂(μ-H₂O)(OOCCMe₃)₄(HOOCCMe₃)₄ gave rise to the hexanuclear complex Ni₆(μ₂-OOCCMe₃)₆(μ₃-OOCCMe₃)₆ (3). The magnetic properties of compound 2 are substantially different from those of 3. Compound 2 undergoes the magnetic phase transition into the ordered state at T _c = 3.4 K (H = 1 Oe), whereas compound 3 exhibits antiferromagnetic properties. Solid-state decomposition of polymeric cobalt carboxylate 2 (below 350 °C) afforded the octanuclear cluster Co₈(μ₄-O)₂(μ₂-OOCCMe₃)₆(μ₃-OOCCMe₃)₆ (9) as the major product, which sublimes without decomposition. Decomposition of 3 gave nickel oxide as the final product. Pivalates 2 and 3 reacted with 2,3-lutidine in acetonitrile at 80 °C to form the isostructural dinuclear complexes (2,3-Me₂C₅H₃N)₂M₂(μ-OOCCMe₃)₄ (M = Co or Ni). The structures of compounds 3 and 7 were established by X-ray diffraction. The structure of polymer 2 was determined by powder X-ray diffraction analysis. Dedicated to Academician O. M. Nefedov on the occasion of his 75th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1841–1850, November, 2006.     

High-spin Ni(II) clusters: triangles and planar tetranuclear complexes

The exploration of the NiX(2)/py(2)CO/Et(3)N (X = F, Cl, Br, I; py(2)CO = di-2-pyridyl ketone; Et(3)N = triethylamine) reaction system led to the tetranuclear [Ni(4)Cl(2){py(2)C(OH)O}(2){py(2)C(OMe)O}(2)(MeOH)(2)]Cl(2)·2Et(2)O (1·2Et(2)O) and [Ni(4)Br(2){py(2)C(OH)O}(2){py(2)C(OMe)O}(2)(MeOH)(2)]Br(2)·2Et(2)O (2·2Et(2)O) and the trinuclear [Ni(3){py(2)C(OMe)O}(4)]I(2)·2.5MeOH (3·2.6MeOH), [Ni(3){py(2)C(OMe)O}(4)](NO(3))(0.65)I(1.35)·2MeOH (4·2MeOH) and [Ni(3){py(2)C(OMe)O}(4)](SiF(6))(0.8)F(0.4)·3.5MeOH (5·3.5MeOH) aggregates. The presence of the intermediate size Cl(-) and Br(-) anions resulted in planar tetranuclear complexes with a dense hexagonal packing of cations and donor atoms (tetramolybdate topology) where the X(-) anions participate in the core acting as bridging ligands. The F(-) and I(-) anions do not favour the above arrangement resulting in triangular complexes with an isosceles topology. The magnetic properties of 1-3 have been studied by variable-temperature dc, variable-temperature and variable-field ac magnetic susceptibility techniques and magnetization measurements. All complexes are high-spin with ground states S = 4 for 1 and 2 and S = 3 for 3.     

Kinetics of the reactions Br + NO2 + M and I + NO2+ M

The reactions Br + NO₂ + M → BrNO₂ + M (1) and I + NO₂ + M → INO₂ + M (2) have been studied at low pressure (0.6-2.2 torr) at room temperature and with helium as the third body by the discharge-flow technique with EPR and mass spectrometric analysis of the species. The following third order rate constants were found k₁₍₀₎ = (3.7 ± 0.7) × 10^?31 and k₂₍₀₎ = (0.95 ± 0.35) × 10^?31 (units are cm⁶ molecule^?2 s^?1). The secondary reactions X + XNO₂ → X₂ + NO₂ (X = Br, I) have been studied by mass spectrometry and their rate constants have been estimated from product analysis and computer modeling.     

Collisional energy transfer in the reactions I + NO + M → INO + M and I + NO2 + M → INO2 + M

The low-pressure recombination rate constants of the reactions I + NO + M → INO + M (with 14 different M) and I + NO₂ + M → INO₂ + M (with 26 different M) have been measured at 330°K by laser flash photolysis. The collision efficiencies β_c are analyzed and compared with other thermal activation systems. Whereas β_c increases in one reaction with an increasing number of atoms in M, practically no such effect is found when, for the same M, different reactions with varying complexities of the reacting molecules are considered.     

Robust hydrogen-bonded self-assemblies from biimidazole complexes. Synthesis and structural characterization of [M(biimidazole)2(OH2)2]2+ (M = Co2+, Ni2+) complexes and carboxylate modules

A robust heteromeric hydrogen-bonded synthon [R2(2) (9)-Id] is exploited to drive the modular self-assembly of four coordination complexes [M(H2biim)2(OH2)2]2+ (M = Co2+, Ni2+) and carboxylate counterions. This strategy allowed us to build molecular architectures of 0-, 1-, and 2-dimensions. A hydrogen-bonded 2D-network with cavities has been designed, which maintains its striking integrity after reversible water desorption-resorption processes.     

Rate Determination of the CO2* Chemiluminescence Reaction CO + O + M CO2* + M

Electronically excited carbon dioxide (CO₂*) is known for its broadband emission, and its detection can lead to valuable information; however, owing to its broadband characteristics, CO₂* is difficult to isolate experimentally, and its chemical kinetics are not well known. Although numerous works have monitored CO₂* chemiluminescence, a full kinetic scheme for the excited species has yet to be developed. To this end, a series of shock‐tube experiments was performed in H₂‐N₂O‐CO mixtures highly diluted in argon at conditions where emission from CO₂* could be isolated and monitored. These results were used to evaluate the kinetics of CO₂*, in particular the main CO₂* formation reaction CO + O + M CO₂* + M (R1). Based on collision theory, the quenching chemistry of CO₂* was estimated for 11 collision partners. The final mechanism developed for CO₂* consists of 14 reactions and 13 species. The rate for (R1) was determined to within about ±60% using low‐pressure experiments performed in five different (H₂‐)N₂O‐CO‐Ar mixtures, as follows: where R is the universal gas constant in cal/mol‐K and T is the temperature in K. Final mechanism predictions were compared with experiments at low and high pressures, with good agreement at both conditions for the temperature dependence of the peak CO₂* and the CO₂* species time histories. Comparisons were also made with previous experiments in methane–oxygen mixtures, where there was slight overprediction of CO₂* experimental trends, but with the results otherwise showing a dramatic improvement over an earlier mechanism. Experimental results and model predictions were also compared with past literature rates for CO₂*, with good agreement for peak CO₂* trends and slight discrepancies in CO₂* species time histories. Overall, the ability of the CO₂* mechanism developed in this work to reproduce a range of experimental trends represents an important improvement over the existing knowledge base on chemiluminescence chemistry.     

Kinetics of the reaction Cl + NO2 + M

The termolecular rate constant for the reaction Cl + NO₂ + M has been measured over the temperature range 264 to 417 K and at pressure 1 to 7 torr in a discharge flow system using atomic chlorine resonance fluorescence at 140 nm to monitor the decay of Cl in an excess of NO₂. The results are\documentclass{article}\pagestyle{empty}\begin{document}$k_1^{{\rm He}} = 9.4{\rm } \times {\rm }10^{ - 31} \left({\frac{T}{{300}}} \right)^{ - 2.0 \pm 0.05} {\rm cm}^6 {\rm s}^{ - {\rm 1}}$\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$k_1^{{\rm N}2} = (14.8{\rm } \pm {\rm }1.4){\rm } \times {\rm 10}^{ - 31} {\rm cm}^6 {\rm s}^{ - 1}$\end{document} at 296 K where error limits represent one standard deviation. The systematic error of k₁ measurements is estimated to be about 15%. Using a static photolysis system coupled with the FTIR spectrophotometer the branching ratio for the formation of the two possible isomers was found to be ClONO(?75%) and CINO₂(?25%) in good agreement with previous measurements.     

A study of the reaction NO3 + NO2 + M → N2O5 + M(M = N2, O2)

The gas-phase reaction of the NO₃ radical with NO₂ was investigated, using a flash photolysis-visible absorption technique, over the total pressure range 25–400 Torr of nitrogen or oxygen diluent at 298 ± 2 K. The absolute rate constants determined (in units of 10^?13 cm³ molecule^?1 s^?1) at 25, 100, and 400 Torr total pressure were, respectively, (4.0 ± 0.5), (7.0 ± 0.7), and (10 ± 2) for M = N₂ and (4.5 ± 0.5), (8.0 ± 0.4), and (8.8 ± 2.0) for M = O₂. These data show that the third-body efficiencies of N₂ and O₂ are identical, within the error limits, and that previous evaluations for M = N₂ are applicable to the atmosphere. In addition, upper limits were determined for the rate constants of the reactions of the NO₃ radical with methanol, ethanol, and propan-2-ol of ?6 × 10^?16, ?9 × 10^?16, and ?2.3 × 10^?15 cm³ molecule^?1 s^?1, respectively, at 298 ± 2 K.     

Analysis of the unimolecular reaction N2O5 + M ⇌ NO2 + NO3 + M

Recent experimental results on the thermal decomposition of N₂O₅ in N₂ are evaluated in terms of unimolecular rate theory. A theoretically consistent set of fall-off curves is constructed which allows to identify experimental errors or misinterpretations. Limiting rate constants k₀ = [N₂] 2.2 × 10^?3 (T/300)^?4.4 exp(?11,080/T) cm³/molec·s over the range of 220–300 K, k_∞ = 9.7 × 10¹⁴ (T/300)^+0.1 exp(?11,080/T) s^?1 over the range of 220–300 K, and broadening factors of the fall-off curve F_cent = exp(-T/250) + exp(?1050/T) over the range of 220–520 K have been derived. NO₂ + NO₃ recombination rate constants over the range of 200–300 K are k_rec,0 = [N₂] 3.7 × 10^?30 (T/300)^?4.1 cm⁶/molec²·s and k_rec,∞ = 1.6 × 10^?12 (T/300)^+0.2 cm³/molec·s.     

Kinetic study of the recombination reaction OD + NO2 + M → DNO3 + M

Rate constants for the recombination reaction OD + NO₂ + M → DNO₃ + M have been determined in the falloff region (5–500 torr) and at 297 ± 2 K, in the presence of He, N₂, and SF₆ as third bodies, by using a pulsed laser photolysis-resonance absorption technique. Values of k₀, k_x and the falloff parameter F_c have been estimated. Our rate constants were, within the experimental uncertainty, the same as those reported for the reaction of OH radicals with NO₂.     

Determination of the rate coefficients of the SO2 + O + M → SO3 + M reaction

Rate coefficients of the title reaction R₃₁ (SO₂ + O + M → SO₃ + M) and R₅₆ (SO₂ + HO₂→ SO₃ + OH), important in the conversion of S(IV) to S(VI), were obtained at T = 970–1150 K and ρ_ave = 16.2 μmol cm^?3 behind reflected shock waves by a perturbation method. Shock‐heated H₂/O₂/Ar mixtures were perturbed by adding small amounts of SO₂ (1%, 2%, and 3%) and the OH temporal profiles were then measured using laser absorption spectroscopy. Reaction rate coefficients were elucidated by matching the characteristic reaction times acquired from the individual experimental absorption profiles via simultaneous optimization of k₃₁ and k₅₆ values in the reaction modeling (for satisfactory matches to the observed characteristic times, it was necessary to take into account R₅₆). In the experimental conditions of this study, R₃₁ is in the low‐pressure limit. The rate coefficient expressions fitted using the combined data of this study and the previous experimental results are k_31,0/[Ar] = 2.9 × 10³⁵ T^?6.0 exp(?4780 K/T) + 6.1 × 10²⁴ T^?3.0 exp(?1980 K/T) cm⁶ mol^?2 s^?1 at T = 300–2500 K; k₅₆ = 1.36 × 10¹¹ exp(?3420 K/T) cm³ mol^?1 s^?1 at T = 970–1150 K. Computer simulations of typical aircraft engine environments, using the reaction mechanism with the above k_31,0 and k₅₆ expressions, gave the maximum S(IV) to S(VI) conversion yield of ca. 3.5% and 2.5% for the constant density and constant pressure flow condition, respectively. Moreover, maximum conversions occur at rather higher temperatures (～1200 K) than that where the maximum k_31,0 value is located (～800 K). This is because the conversion yield is dependent upon not only the k_31,0 and k₅₆ values (production flux) but also the availability of H, O, and HO₂ in the system (consumption flux). © 2010 Wiley Periodicals, Inc. *

1 This article is a U.S. Government work and, as such, is in the public domain of the United States of America.

Int J Chem Kinet 42: 168–180, 2010     

Gas-phase H/D exchange reactions of polyamine complexes: (M + H)+, (M + alkali metal+), and (M + 2H)2+

Gas-phase hydrogen/deuterium exchange reactions between noncovalent polyamine complexes and D2O, CH3OD, or ND3 are undertaken in a quadrupole ion trap mass spectrometer. Structural features of the protonated polyamines can be differentiated by the rates and overall extent of exchange, specifically the presence of propylene units and/or a cyclic structure noticeably decreases exchange compared to the exchange observed for acyclic polyamines with only ethylene bridges between amino groups. Significant differences are observed for singly protonated vs. doubly protonated complexes, where the doubly protonated complexes undergo more efficient exchange at a higher rate than the analogous singly protonated complexes. Molecular modeling calculations suggest that more diffuse conformations may exist for the higher charge states, thus facilitating H/D exchange. In addition, H/D exchange reactions between the alkali metal cationized complexes and ND3 are nearly quenched, compared to the significant exchange seen for singly protonated complexes. A conformational change or the loss of a low energy reaction pathway may explain the limited exchange reactions seen when a bulky cation replaces a proton in the complex.     

Temperature and third-body dependence of the rate constant for the reaction O + O2 + M → O3 + M

The flash photolysis resonance fluorescence technique was used to measure the rate constants of the reaction O + O₂ + M → O₃ + M (M = N₂, O₂, Ar, and He) as a function of temperature. The results for the rate constants are given by The activation energies with N₂, O₂, and Ar as third bodies are equal within the experimental error, (?1370 → 340 cal/mol), and the relative third-body efficiencies at 298 K for N₂, O₂, Ar, and He are 1.00, 0.99, 0.69, and 0.60, respectively.     

A C2-symmetric, basic Fe(III) carboxylate complex derived from a novel triptycene-based chelating carboxylate ligand

A triptycene-based bis(benzoxazole) diacid ligand H(2)L2(Ph4) bearing sterically encumbering groups was synthesized. Treatment of H(2)L2(Ph4) with Fe(OTf)(3) afforded a C(2)-symmetric trinuclear iron(III) complex, [NaFe(3)(L2(Ph4))(2)(μ(3)-O)(μ-O(2)CCPh(3))(2)(H(2)O)(3)](OTf)(2) (8). The triiron core of this complex adopts the well known "basic iron acetate" structure where the heteroleptic carboxylates, comprising two Ph(3)CCO(2)(-) and two (L2(Ph4))(2-) ligands, donate the six carboxylate bridges. The (L2(Ph4))(2-) ligand undergoes only minor conformational changes upon formation of the complex.     

A flash photolysis-resonance fluorescence study of the reaction of atomic hydrogen with molecular oxygen H + O2 + M → HO2 + M

Using the technique of flash photolysis-resonance fluorescence, absolute rate constants have been measured for the reaction H + O₂ + M → HO₂+M over a temperature range of 220–360°K. Over this temperature range, the data could be fit to an Arrhenius expression of the following form: The units for k_Ar are cm⁶/mole-s. At 300°K the relative efficiencies for the third-body gases Ar:He:H₂:N₂:CH₄ were found to be 1.0:0.93:3.0:2.8:22. Wide variations in the photoflash intensity at several temperatures demonstrated that the reported rate constants were measured in the absence of other complex chemical processes.     

Determination of the rate coefficients for the reactions IO + NO2 + M (air) --> IONO2 + M and O(3P) + NO2 --> O2 + NO using laser-induced fluorescence spectroscopy

Laser-induced fluorescence spectroscopy via excitation of the A2pi(3/2) <-- X2pi(3/2) (2,0) band at 445 nm was used to monitor IO in the presence of NO2 following its generation in the reactions O(3P) + CF3I and O(3P) + I2. Both photolysis of O3 (248 nm) and NO2 (351 nm) were used to initiate the production of IO. The rate coefficients for the thermolecular reaction IO + NO2 + M --> IONO2 + M were measured in air, N2, and O2 over the range P = 18-760 Torr, covering typical tropospheric conditions, and were found to be in the falloff region. No dependence of k1 upon bath gas identity was observed, and in general, the results are in good agreement with recent determinations. Using a Troe broadening factor of F(B) = 0.4, the falloff parameters k0(1) = (9.5 +/- 1.6) x 10(-31) cm6 molecule(-2) s(-1) and k(infinity)(1) = (1.7 +/- 0.3) x 10(-11) cm3 molecule(-1) s(-1) were determined at 294 K. The temporal profile of IO at elevated temperatures was used to investigate the thermal stability of the product, IONO2, but no evidence was observed for the regeneration of IO, consistent with recent calculations for the IO-NO2 bond strength being approximately 100 kJ mol(-1). Previous modeling studies of iodine chemistry in the marine boundary layer that utilize values of k1 measured in N2 are hence validated by these results conducted in air. The rate coefficient for the reaction O(3P) + NO2 --> O2 + NO at 294 K and in 100 Torr of air was determined to be k2 = (9.3 +/- 0.9) x 10(-12) cm3 molecule(-1) s(-1), in good agreement with recommended values. All uncertainties are quoted at the 95% confidence limit.     

Thienyl carboxylate ligands bound to M2 quadruple bonds involving molybdenum and tungsten. Models for dimetallated polythiophenes

Thienyl-carboxylate and -dicarboxylate groups attached to dinuclear centers (M = Mo or W) having MM quadruple bonds show interesting electronic properties and provide insight into the probable nature of related dimetallated polythiophenes.     

A spectroscopic study of the equilibrium NO2 + NO3 + M 2 N2O5 + M and the kinetics of the O3/N2O5/NO3/NO2/ air system

The equilibrium constant, K_eq of the reaction NO₂ + NO₃ + M 2 N₂O₅ + M has been determined for a small range of temperatures around room temperature in air at 740 torr by direct spectroscopical measurements of NO₂, NO₃, and N₂O₅. At 298 K, K_eq was determined as (3.73 ± 0.61) × 10⁻¹¹ cm³ molecule⁻¹. Averaging this and 11 other independent evaluations of K_eq yields K_eq = (3.31 ± 0.82) × 10⁻¹¹ cm³ molecule⁻¹, where the uncertainty is given as one standard deviation. The kinetics of the O₃/NO₂/N₂O₅/NO₃/ air system was studied in a static chamber at room temperature and 740 torr total pressure. Evidence of a unimolecular decay reaction of NO₃, NO₃ → NO + O₂, was found and its rate coefficient was estimated as (1.6 ± 0.7) × 10⁻³ s⁻¹ at 295 ± 2 K.