13859-51-3Relevant articles and documents
Preparation of carbon supported cobalt by electrostatic adsorption of [Co(NH3)6]Cl3
D'Souza,Regalbuto,Miller
, p. 157 - 169 (2008)
Our previous paper [L. D'Souza, L. Jiao, J.R. Regalbuto, J.T. Miller, A.J. Kropf, J. Catal. 248 (2007) 165] presented the synthesis of cobalt catalysts on carbon (Timrex) and silica supports by strong electrostatic adsorption (SEA), using a cobalt hexaamine chloride ([Co(NH3)6]Cl3, CoHA) precursor. The CoHA undergoes reductive deammination in an uncontrolled manner in the presence of NaOH and adsorbs as Co3O4 on carbon with broad size distribution. The present paper extends these studies toward the end of synthesizing well-dispersed Co oxide particles in a narrow size range on carbon supports using NH4OH. Cobalt uptake versus pH was determined in NH4OH and NaOH basified solutions over a number of carbons with varying point of zero charge (PZC). The resulting materials were characterized by ICP, powder XRD, XAS, TPR and STEM. CoHA in the presence of NH4OH adsorbs as well dispersed as CoO, Co3O4 and Co(OH)2-4 depending upon the pH of the adsorption solution. These phases were undetectable by powder XRD and STEM Z-contrast imaging, but could be identified by XAS. Additionally, non-adsorbed CoHA complexes underwent transformation to [Co(NH3)5Cl]Cl2 at pH > 11 in solution. After calcinations of 250 °C, particle sizes of Co3O4 range from 20-50 A from NH4OH and 50-200 A from NaOH. Maximum metal uptake was approximately 3.3 and 2.7 μmol/m2 in presence of NaOH and NH4OH, respectively. The SEA method of preparation was compared with incipient wetness impregnation (IWI) of Co(NO3)2s6H2O; this method yields Co3O4 particles after 250 °C calcinations which are almost as small or in one case, smaller than the calcined SEA samples. Higher metal loadings can be achieved by the SEA method by successive adsorption steps with a little variation in particle size and distribution. However, the main advantage of SEA is in forming mono- or submonolayer of different Co oxide phases on carbon surface.
Enhancement of photocatalytic H2 production by metal complex electrostatic adsorption on TiO2 (B) nanosheets
Kong, Xiangchen,Gao, Zhonghui,Gong, Yue,Huang, Huiming,Wang, Haifeng,Liu, Porun,Yin, Huajie,Cui, Zhenduo,Li, Zhaoyang,Liang, Yanqin,Zhu, Shengli,Huang, Yunhui,Yang, Xianjin
, p. 3797 - 3804 (2019)
The assembly of cocatalysts on a semiconductor is a key way to promote the activity in solar hydrogen production. This process can be realized by anchoring metal complexes which can provide catalytically active sites. Despite the most recent advances, the performances and cycling stability of metal complex/semiconductor composite catalysts are still hindered by their poor contact or linkage. Herein, a series of metal complexes, such as [Co(NH3)5Cl]Cl2, [Ni(NH3)6]Cl2, and [Cu(NH3)4](OH)2, are anchored on TiO2 (B) nanosheets via a facile controllable electrostatic adsorption route. The resultant TiO2 (B) composites exhibit significantly enhanced activity and stability in photocatalytic hydrogen production compared to the blank TiO2 (B) counterpart. The [Co(NH3)5Cl]2+-TiO2 (B) composite achieves a high activity of 3.9 mmol g-1 after 8 h (with a turnover frequency (TOF) of 249 h-1), which is about 57% of that of the benchmark Pt-loaded TiO2 (B) catalyst. Further experimental and theoretical studies confirm that the strong contact between metal complexes with TiO2 (B) can facilitate photoexcited electron transfer and separation, which is responsible for the improved activity. Desorption of hydrogen atoms from ammonium in metal complexes is kinetically favorable and they can accept photoexcited electrons to generate H2. This study can open up a promising pathway to synthesize high-performance metal complex anchored semiconductors by electrostatic adsorption.
NMR spectroscopy of the solid-state isomerization of nitrito- and nitro-pentamminecobalt(III) chloride
Ooms, Kristopher J.,Wasylishen, Roderick E.
, p. 300 - 308 (2006)
Cobalt-59 and nitrogen-15 NMR spectra of the nitritopentamminecobalt(III) chloride, [(NH3)5Co-ONO]Cl2, and nitropentamminecobalt(III) chloride, [(NH3)5Co-NO 2]Cl2, isomers in the solid state have been obtained at several applied magnetic field strengths. The 59CQ NMR line shapes indicate that both the cobalt nuclear quadrupolar coupling constant (CQ) and the span of the chemical shift tensor (Ω) decrease when the complex isomerizes from [(NH3)5Co-ONO] 2+ to [(NH3)5Co-NO2]2+; CQ decreases from 23 to 10.3 MHz and Ω changes from 1650 to 260 ppm. The 15N NMR line shapes also show a significant change in the nitrogen magnetic shielding tensor upon isomerization, with Ω decreasing from 710 to 547 ppm; also, an indirect spin-spin coupling, 1J( 59Co,15N) = 63 Hz, is observed in the 15N NMR spectra of the nitro isomer. The NMR parameters are rationalized based on differences in the molecular structure of the two isomers. NMR spectra have also been recorded as the isomerization progresses with time and demonstrate the practicality of the technique for the study of solid-state isomerizations.
A recyclable cobalt(iii)-ammonia complex catalyst for catalytic epoxidation of olefins with air as the oxidant
Wang, Chenlong,Zhan, Hongju,Lu, Xinhuan,Jing, Run,Zhang, Haifu,Yang, Lu,Li, Xixi,Yue, Fanfan,Zhou, Dan,Xia, Qinghua
, p. 2147 - 2156 (2021/02/06)
[Co(NH3)6]Cl3and other ammonia complexes with different external anions or metal ions were synthesized to catalyze the epoxidation of α-pinene. The synthesized complexes were characterized using XRD, SEM, TGA, FTIR and UV spectra. With air as the oxidant, [Co(NH3)6]Cl3exhibited excellent catalytic activity for the epoxidation of α-pinene among the prepared complexes. The conversion of α-pinene reached 97.4%, with 98.3% selectivity of epoxide when using a small amount of cumene hydroperoxide (CHP) as the initiator. The results revealed that a single Co(iii) system can also catalyze the epoxidation process in the absence of Co(ii), even showing better catalytic performance than single Co(ii). Recycling experiments showed that there was no significant drop in activity after 10 cycles, demonstrating that it is a stable and efficient heterogeneous catalyst for the epoxidation of α-pinene. The excellent recycling performance may be attributed to the stability of the coordination complex itself.
Influence of the metal centers on the pKa of the pyrrole hydrogen of imidazole complexes of (NH3)5M3+, M(III) = Co(III), Rh(III), Ir(III), Ru(III)
Fazlul Hoq,Shepherd, Rex E.
, p. 1851 - 1858 (2008/10/08)
The pKa's at 298 K, μ = 0.10 (NaCl), and the temperature dependence (273-343 K) for the deprotonation of the pyrrole NH of several imidazoles coordinated to (NH3)5M3+ moieties (M = CoIII, RhIII, IrIII, RuIII) are reported. A greater importance of dn configuration over ion size is found. Data summarized for various systems are as follows (ligand, M (pK298, ΔHa° in kcal/mol, ΔSa° in eu)): imidazole = imH, CoIII (9.99, 14.0 ± 0.5, 1.3 ± 1.6), RhIII (9.97, 13.6 ± 0.3, 0.1 ± 1.3), IrIII (10.05, 13.4 ± 0.3, 1.2 ± 1.0), RuIII (8.9, 10.0 ± 0.8, 3.7 ± 1.2); 2-methylimidazole = 2-MeimH, CoIII (10.67, 17.8 ± 0.7, 11.2 ± 2.4); 2,4(5)-dimethylimidazole = 2,5-Me2imH, CoIII (11.04, 13.4 ± 0.5, 5.3 ± 1.6), RuIII (10.20, 13.2 ± 0.6, -2.1 ± 1.6). 1H NMR spectra of low-spin d6 complexes of imidazoles and ring-methylated imidazoles are discussed for CoIII, RhIII, IrIII, and RuIII. C-2 and remote ring, C-5, substituents are shifted downfield relative to the free imidazole ligand in the order H+ > CoIII > RhIII > IrIII. The C-4 position is influenced competitively by σ-withdrawal ring substituents and TIP effects for CoIII. Assignments of the remote isomer for (NH3)5M(2,5-Me2imH)3+ (M = CoIII, RuIII) are made from the 1H NMR spectra of the CoIII and RuII complexes. The RuIII complexes of 2,5-Me2imH and the imidazolate form (2,5-Me2im-) both exhibit LMCT spectra. The imidazolato form has three bands at 655, 377, and 272 nm, proposed for II1 → IId, II2 → IId, and n → IId transitions, where II1, II2, and n are the highest HOMO's of the imidazolato ring.
Adjacent methyl to remote methyl isomerization of (4-methylimidazole)pentaamminecobalt(III)
Hoq, M. Fazlul,Johnson, Craig R.,Paden, Susan,Shepherd, Rex E.
, p. 2693 - 2700 (2008/10/08)
Isomers of (4-methylimidazole)pentaamminecobalt(III) have been isolated as chloride salts with the methyl group of the imidazole directed away from the five NH3 ligands (remote = R) and near the NH3 ligands (adjacent = Ad). Isomers have been assigned by a separate X-ray diffraction study. Isomers have been characterized by 1H NMR spectra showing shifts (ppm) relative to the free ligand (L) as follows: 4-CH3 Ad = 2.22, R = 2.33, L = 2.27; C2-H Ad = 7.78, R = 7.88, L = 7.65; C5-H Ad = 7.08, R = 6.73 (C4-H), L = 6.77. In Tris or pyridinium buffers two paths are found for the isomerization of Ad to R. One path (k0 = (5.83 ± 2.18) × 10-8 s-1) is attributed to the isomerization of the parent 4-methylimidazole species Ad. A second path, first order in [OD-], is attributed to the imidazolato form Ad-H (kOH = 3.19 ± 0.07 M-1 s-1). The activation parameters for the kOH path are ΔH≠ = 32.4 ± 4.1 kcal/mol and ΔS≠ = 16.8 ± 11.7 eu. A mechanism is presented, suggesting a largely dissociative-like transition state. Comparisons are made to the linkage isomerization of (NH3)5Co3+ coordinated to ONO- and N1 of the 5-methyltetrazolato ligand. While the isomerization of the 5-methyltetrazole ligand is faster by 105 relative to that for 4-methylimidazole, the anion forms favor the 4-methylimidazolato isomerization by a factor of 400 vs. the 5-methyltetrazolato case. The difference is attributed to different basicity of the lone electron pairs of these ligands The pKa's of (NH3)5CoX3+ have been found at 25.0°C and μ = 0.10 and are as follows (X, pKa): imidazole, 9.99; 2-methyhmidazole, 10.67; 4-methylimidazole (Ad), 10.46; 4-methylimidazole (R), 10.70. The acid dissociation constants of R and Ad were studied as a function of temperature, yielding values of ΔH°, ΔS° as follows: (Ad) 17.7 ± 0.5 kcal/mol, 11.2 ± 16 eu; (R) 15.4 ± 0.6 kcal/mol, 2.5 ± 1.8 eu. The values of ΔH° are within a kilocalorie of imidazole's pyrrole pKa (17.6 kcal/mol) while the values of ΔS° are more favored by charge dispersal for Ad and R by about 18 to 10 eu, respectively.
Importance of ion association in the induced reactions of cobalt(III)-acido complexes. 2. Nitrosation of the (NH3)5CoN32+ ion
Buckingham, David A.,Clark, Charles R.,Webley, Wayne S.
, p. 3353 - 3360 (2008/10/08)
The nitrosation of (NH3)5CoN32+ (CoN32+) in the presence of added anions Y- = Cl, NO3, ClO4 follows the rate law -d[CoN32+]/dt = (k0 + ky[Y-])[H+][HNO2][CoN 32+] with k0 = 6.6 × 102 mol-2 dm6 s-1 and ky = 1.70 × 105 (Cl), 4.48 × 103 (NO3), 2.21 × 103 (ClO4) mol-3 dm9 s-1 at 25.0°C and I = 1.0 mol dm-3 (NaSO3CF3). The rate is little affected by the ionic strength (I > 0.2 mol dm-3) and Y- = CF3SO3- does not affect the rate. The kinetic data are interpreted in terms of NO+(aq) and YNO reacting with CoN32+ with respective second-order rate constants of 3.3 × 109 and 1.9 × 108 (Cl), 3.5 × 107 (Br) mol-1 dm3 s-1. Y- species (Cl-, NO3-) are included in the products of the reaction (as CoY2+), but not via the YNO species, and the [CoY2+]/[CoOH23+] ratios at variable and constant I are interpreted in terms of competition between Cl-, NO3-, ClO4- and CF3SO3- (NO3- > ClO4- > Cl- > CF3SO3-) from within the ion-paired Y-,CoN3ONY2+ intermediates. For Y- = Cl, NO3 this species leads to 40% CoCl2+/60% CoOH23+ and 56% CoNO32+/44% CoOH23+ entry, respectively. Comment is made on the lifetime of the Y-,CoN3ONY2+ intermediate, and comparisons are made with oxidative processes involving Cl2, H2O2, and HOCl.
