| 1. |
|
|
| 2. |
- Mitra, Mainak, et al.
(författare)
-
An investigation of steric influence on the reactivity of FeV(O)(OH) tautomers in stereospecific C-H hydroxylation
- 2023
-
Ingår i: Dalton Transactions. - : Royal Society of Chemistry (RSC). - 1477-9226 .- 1477-9234. ; 52:12, s. 3596-3609
-
Tidskriftsartikel (refereegranskat)abstract
- Two new tetradentate N4 ligands (LN4), LN4 = Me2,Me2PyzTACN (1-(2-(3,5-dimethyl-1H-pyrazol-1-yl)ethyl)-4,7-dimethyl-1,4,7-triazacyclononane) and Me2,MeImTACN (1-((1-methyl-1H-imidazol-1-yl)methyl)-4,7-dimethyl-1,4,7-triazacyclononane) have been synthesized and their corresponding Fe(ii) complexes [FeII(Me2,Me2PyzTACN)(CF3SO3)2], 1Pz, and [FeII(Me2,MeImTACN)(CF3SO3)2], 1Im, have been prepared and characterized. Complexes 1Pz and 1Im catalyse the hydroxylation of C-H bonds of alkanes with excellent efficiencies, using hydrogen peroxide as oxidant. The high H/D kinetic isotope effect values for C-H hydroxylation, large normalized tertiary/secondary C-H (C3/C2) bond selectivities in adamantane oxidation, and high degrees of stereoretention in the oxidation of cis-1,2-dimethylcyclohexane are indicative of metal-based oxidation processes. The complexes also catalyse the oxidation of cyclooctene to form its corresponding epoxide and syn-diol. For 1Pz the epoxide is the main product, while for the analogous complex 1Im the syn-diol predominates. The active oxidant is proposed to be an [(LN4)FeV(O)(OH)]2+ species (2Pz, LN4 = Me2,Me2PyzTACN and 2Im, LN4 = Me2,MeImTACN) which may exist in two tautomeric forms related by a proton shift between the oxo and hydroxo ligands. Isotope labelling experiments show that the oxygen atom in the hydroxylated products originates from both water and hydrogen peroxide, and labelling experiments involving oxygen atom transfer to sterically bulky substrates provide indirect information on the steric influence exerted by the two ligands in the relative reactivities of the two hypervalent iron tautomers. Based on these labelling studies, the steric influence exerted by each of the ligands towards the relative reactivity of the oxo ligands of the corresponding pair of Fe(v)(O)(OH) tautomers can be derived. Furthermore, this steric influence can be gauged relative to related complexes/ligands.
|
|
| 3. |
- Mitra, Mainak, et al.
(författare)
-
Catalytic C-H oxidations by nonheme mononuclear Fe(II) complexes of two pentadentate ligands: Evidence for an Fe(IV) oxo intermediate
- 2017
-
Ingår i: Journal of Molecular Catalysis A: Chemical. - : Elsevier BV. - 1381-1169. ; 426, s. 350-356
-
Tidskriftsartikel (refereegranskat)abstract
- The oxidation reactions of alkanes with hydrogen peroxide and peracids (peracetic acid (PAA) and m-chloroperoxybenzoic acid (mCPBA)) catalysed by two Fe(II) complexes of pentadentate {N5}-donor ligands have been investigated. Kinetic isotope effect experiments and the use of other mechanistic probes have also been performed. While the total yields of oxidized products are similar regardless of oxidant (e.g. 30–39% for oxidation of cyclohexane), the observed alcohol/ketone ratios and kinetic isotope effects differ significantly with different oxidants. Catalytic reactions in H2O2 medium are consistent with the involvement of hydroxyl radicals in the Csingle bondH bond cleavage step, and resultant low kinetic isotope effect values. On the other hand, catalytic reactions performed using peracid media indicate the involvement of an oxidant different from the hydroxyl radical. For these reactions, the kinetic isotope effect values are relatively high (within a range of 4.2–5.1) and the C3/C2 selectivity parameters in adamantane oxidation are greater than 11, thereby excluding the presence of hydroxyl radicals in the Csingle bondH bond cleavage step. A low spin Fe(III)-OOH species has been detected in the H2O2-based catalytic system by UV/Vis, mass spectrometry and EPR spectroscopy, while an Fe(IV)-oxo species is postulated to be the active oxidant in the peracid-based catalytic systems. Computational studies on the Csingle bondH oxidation mechanism reveal that while the hydroxyl radical is mainly responsible for the H-atom abstraction in the H2O2-based catalytic system, it is the Fe(IV)-oxo species that abstracts the H-atom from the substrate in the peracid-based catalytic systems, in agreement with the experimental observations.
|
|
| 4. |
|
|
| 5. |
- Mitra, Mainak, et al.
(författare)
-
Highly enantioselective epoxidation of olefins by H 2 O 2 catalyzed by a non-heme Fe(ii) catalyst of a chiral tetradentate ligand
- 2019
-
Ingår i: Dalton Transactions. - : Royal Society of Chemistry (RSC). - 1477-9226 .- 1477-9234. ; 48:18, s. 6123-6131
-
Tidskriftsartikel (refereegranskat)abstract
- The chiral tetradentate N4-donor ligand, 1-methyl-2-({(S)-2-[(S)-1-(1-methylbenzimidazol-2-yl methyl)pyrrolidin-2-yl]pyrrolidin-1-yl}methyl) benzimidazole (S,S- PDBz L), based on a chiral dipyrrolidine backbone, has been synthesized and its corresponding Fe(ii) complex has been prepared and characterized. The X-ray structure of the complex reveals that the Fe(ii) ion is in a distorted octahedral coordination environment with two cis-oriented coordination sites occupied by (labile) triflate anions. The ability of the iron complex to catalyze asymmetric epoxidation reactions of olefins with H 2 O 2 was investigated, using 2-cyclohexen-1-one, 2-cyclopenten-1-one, cis-β-methylstyrene, isophorone, chalcones and tetralones as substrates. Different carboxylic acids were used as additives to enhance yields and enantioselectivities, and 2-ethylhexanoic acid was found to give the best results. The catalysis results indicate that the Fe(ii) complex is capable of effecting comparatively high enantioselectivities (>80%) in the epoxidation reactions.
|
|
| 6. |
- Mitra, Mainak, et al.
(författare)
-
Nonheme Fe(IV) Oxo Complexes of Two New Pentadentate Ligands and Their Hydrogen-Atom and Oxygen-Atom Transfer Reactions
- 2015
-
Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 1520-510X .- 0020-1669. ; 54:15, s. 7152-7164
-
Tidskriftsartikel (refereegranskat)abstract
- Two new pentadentate {N5} donor ligands based on the N4Py (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) framework have been synthesized, viz. [N-(1-methyl-2-benzimidazolyl)methyl-N-(2-pyridyl)methyl-N-(bis-2-pyridyl methyl)amine] (L1) and [N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)amine] (L2), where one or two pyridyl arms of N4Py have been replaced by corresponding (N-methyl)benzimidazolyl-containing arms. The complexes [FeII(CH3CN)(L)]2+ (L = L1 (1); L2 (2)) were synthesized, and reaction of these ferrous complexes with iodosylbenzene led to the formation of the ferryl complexes [FeIV(O)(L)]2+ (L = L1 (3); L2 (4)), which were characterized by UV–vis spectroscopy, high resolution mass spectrometry, and Mössbauer spectroscopy. Complexes 3 and 4 are relatively stable with half-lives at room temperature of 40 h (L = L1) and 2.5 h (L = L2). The redox potentials of 1 and 2, as well as the visible spectra of 3 and 4, indicate that the ligand field weakens as ligand pyridyl substituents are progressively substituted by (N-methyl)benzimidazolyl moieties. The reactivities of 3 and 4 in hydrogen-atom transfer (HAT) and oxygen-atom transfer (OAT) reactions show that both complexes exhibit enhanced reactivities when compared to the analogous N4Py complex ([FeIV(O)(N4Py)]2+), and that the normalized HAT rates increase by approximately 1 order of magnitude for each replacement of a pyridyl moiety; i.e., [FeIV(O)(L2)]2+ exhibits the highest rates. The second-order HAT rate constants can be directly related to the substrate C–H bond dissociation energies. Computational modeling of the HAT reactions indicates that the reaction proceeds via a high spin transition state.
|
|
| 7. |
- Olivé-Busom, Júlia, et al.
(författare)
-
Evidence of otitis media and mastoiditis in a Medieval Islamic skeleton from Spain and possible implications for ancient surgical treatment of the condition
- 2021
-
Ingår i: International Journal of Paleopathology. - : Elsevier BV. - 1879-9817 .- 1879-9825. ; 32, s. 17-22
-
Tidskriftsartikel (refereegranskat)abstract
- Objective: To evaluate lesions on a cranium from the Iberian Peninsula and assess its medico-historical and paleopathological significance.Materials: The skeletal remains of a juvenile individual found in a Medieval Islamic grave (10th -16th century) in Eastern Spain.Methods: Macroscopic examination of the left and right temporal bones, binocular microscopy, X-ray, and Scanning Electron Microscopy - Energy Dispersive X-ray Spectroscopy (SEM-EDS) were performed.Results: A sub-oval perforation superior to the right mastoid process and pathological changes on the right temporal bone were identified. SEM-EDS confirmed the presence of copper in the surrounding area of the perforation.Conclusions: The observed pathological changes are most likely compatible with otitis media and subsequent mastoiditis. The sub-oval perforation could be interpreted either as an abscess or as evidence of a surgical procedure (mastoidectomy) or a combination of both; and the Cu traces may be the result of an associated object or, possibly, the application of a plaster with copper acetate used as medical treatment.Significance: This case contributes to the paleopathological record and the interpretation of similar cases, and also helps in the understanding of medical care and treatment in Medieval Islam.Limitations: The lack of similar pre-modern cases of surgical intervention limits comparability to clinical cases. Suggestion for further research: Exploration into indicators of health care in past populations.
|
|
