In the oxidation of n-octane, the complex with a flexible ligand framework showed greater activity. Complexes of type 1 and 2 used in this study as catalysts in styrene oxidation.
Alkane oxidation
Factors affecting alkane oxidation
A contributing factor to the lack of reactivity of alkanes stems from their high bond energy. In addition to the inertness of alkanes, other factors contribute to the lack of reactivity of paraffins.
Oxidation of alkanes by group nine transition metals
- Cobalt
- Rhodium
- Iridium
Using the oxidant tert-butyl hydroperoxide in the oxidation of cyclohexane, a very high conversion of 100% was achieved with a good yield to cyclohexanol (39%).72 Chavez et al. Co+2, Mn+2 and Fe+3 were also used in the oxidation of cyclohexane and cyclooctane with H2O2 as the oxidant.
Styrene epoxidation
Mechanism of olefin oxidation
Formation of the ketone or aldehyde occurs via metal-induced nucleophilic attack (c) or via a metal peroxo (hydro) species (d). The first (c), involves typical Wacker-type catalysis with intermolecular (c1) or intramolecular (c2) nucleophilic attack of the hydroxide on the coordinated alkene.
Oxidation of styrene by group nine transition metals
- Co, Rh and Ir
One of the earliest studies that was performed was with cobalt salen complexes in the oxidation of styrene by Zombeck et al. When the same salt was used as a catalyst in the oxidation of styrene, a 30% conversion in high yield to benzoic acid was obtained.137.
Conclusion
Aim of study
These ligands are complexed with the transition metals Co, Rh, Ir and Ru and were used as catalysts in the oxidation of n-octane and styrene. The oxidation of both n-octane and styrene using iridium and rhodium complexes has not yet been thoroughly investigated.
Two types of cobaltaminodiphosphine complexes were synthesized and characterized by IR spectroscopy, elemental analyses, and single crystal X-ray diffraction. All complexes showed good activity as catalysts for the oxidation of n-octane using tert-butyl hydroperoxide (TBHP) as the oxidant.
Introduction
In this work, a novel approach was taken in the use of cobaltaminodiphosphine complexes in the C–H activation of n-octane. However, the valorization of medium-chain n-alkanes is of particular importance, because they form the building blocks in the chemical industry and provide a cheaper alternative raw material53-55.
Experimental
Synthesis and characterization of the compounds
Crystal structure analysis
The crystals of the complexes were each selected and individually glued to the end of glass fibers. Crystal data and structure refinement information for all the complexes are summarized in Table 2.1 (Appendix A).
Oxidation of n-octane
Data reduction was performed using the program SAINT+.58 The structure was solved by direct methods using SHELXS 59 and refined by SHELXL58. Non-H atoms were first refined isotropically and then by anisotropic refinement with full-matrix least-squares calculations based on F2 using SHELXS.60 All H atoms were geometrically positioned and allowed to ride on their respective parent atoms.
Results and Discussion
Synthesis and characterization of the compounds
The reflections were successfully indexed by an automated indexing routine built into the APEXII software package.58 The data collection method involved a 0.5° wide ω scan. A shift of the νP-N band is also noticeable in the IR spectra of complexes 1 and their corresponding ligands, which are shown in Table 2.2.
Description of the X-ray crystal structures
Oxidation of n-octane
Selectivity of the blank reaction (no catalyst) to oxidation products in a ratio of 1:5 between substrate and oxidant at 80 °C in acetonitrile. For catalyst 1, a change in selectivity to the alcohol and ketone was observed before and after addition of PPh3, but for catalyst 2, no observable change was observed.
Conclusion
These complexes showed good activity in the oxidation of styrene using TBHP as oxidant and DCE,1,2-dichloroethane as solvent. The complex with the pentyl substituent (lc) gave the highest yield of benzaldehyde and styrene oxide.
Introduction
The ligand then places a high demand on the stereochemistry of the complex, which enables the reactions of the metal ions to be quite selective.20,21. The substituent on the nitrogen atom was varied by using three different types of functional groups, a ring (cyclohexyl), a straight chain (n-pentyl) and a branched (isopropyl) substituent, with the intention of observing whether these groups have an effect on the catalytic activity and selectivity towards the oxidation products.
Experimental
Oxidation of styrene
The flask was fitted with a reflux condenser, stirred and the solution heated to the required temperature.
Results and discussion
The yield of styrene oxide decreases slightly after 2 hours for catalyst 2a due to the formation of phenylacetaldehyde (3% after 3 hours). Conversion of styrene in time and yield to benzaldehyde and styrene oxide via catalysts 1a and 2a.
Conclusion
Co(II) can bind to the oxygen of TBHP (t-BuOOH) and form Co(III) super oxo species (A).25,30,35 Oxygen-bound cobalt (Co(III)_(O2) -)) it reacts with styrene to form an active oxygen intermediate (B), which is responsible for the epoxidation reaction.30,35 The intermediate B is then rearranged into C and a cobalt catalyst is generated.
Introduction
In this study, a new approach was taken in the use of iridium and rhodium aminodiphosphine complexes in the oxidation of styrene. We report herein the synthesis and characterization of some new iridium (1) and rhodium (2) aminodiphosphine complexes and their application in the catalytic oxidation of styrene.
Experimental
- Synthesis and characterization of compounds
- Crystal structure analysis
- Oxidation of styrene
Compound 1d was synthesized according to the procedure described for 1a, except that [Ph2PN(Ph)PPh2] (0.25 mmol, 0.12 g) was used. Data were scaled and reduced using DENZO-SMN software.40 Absorption correction was performed using SADABS.38 The program Olex2 was used to create molecular graphics images.41 All non-hydrogen atoms were refined anisotropically.
Results and Discussion
- Synthesis and characterization of the compounds
- Description of crystal structures
- Oxidation of styrene
The yield of benzaldehyde is comparable to the first experiment, however, the yield of styrene oxide increases from cycle 1 (2%) to cycle 3 (6%). The yield of benzaldehyde is comparable to the first experiment, however the yield of styrene oxide decreased (from 15% to 5.
Conclusion
Six PNP or aminodiphosphine ligands were synthesized and complexed to the transition metals iridium and rhodium to give [(η5-C5Me5)MCl{η2-P,Pʹ′-(PPh2)2NR}]PF6, where M= Ir (1) and Rh (2) and R= cyclohexyl (a), iso-propyl (b), pentyl (c), phenyl (d), chlorophenyl (e) and methoxyphenyl (f). These complexes showed good activity in the oxidation of n-octane using tert-butyl hydroperoxide (TBHP) as the oxidant.
Introduction
In general, phosphine-based ligands are limited in their use in oxidation reactions due to loss and degradation of the ligand. 70 Ruthenium-based phosphine complexes have been studied in the oxidation of n-octane, but low conversions and overoxidation prevail. 71, 72 Activation of alkane C–H bonds by complexes rhodium and iridium has not been thoroughly investigated. With the worldwide increase in the production of gas to liquid plants, the production of medium- to long-chain paraffins is increasing, increasing the need to functionalize these hydrocarbons.74-76 Therefore, in this study, we report the use of iridium(1) and rhodium(2) aminodiphosphine "PNP".
Experimental
- Crystal structure analysis
- Oxidation of n-octane
Compounds 1 and 2 were synthesized and characterized according to the procedure described in Chapter 4, Sections 4.3 and 4.4. The flask was fitted with a reflux condenser, stirred, heated to the required temperature and kept at this temperature for 48 hours in an oil bath.
Results and discussion
- Description of crystal structures
- Oxidation of n-octane
Examination of iridium (1) and rhodium (2) catalysts with different substituents on the nitrogen atom (a-f). The selectivity to oxidation products by Ir catalysts is shown in Fig.
Conclusion
The oxygen-centered radicals generated in eqn 1 and 4 attack the hydrocarbon with the formation of alkyl radicals (eqn. Increased ketone selectivity occurs when ROO attacks t-BuOO through mixed molecular Russel termination (eqn 11). 102 Oxidation of the most deeply of alcohols that form ketones is caused by the M(I)L complex (eqn.
The rhodium catalyst carrying the ligand with the phenyl substituent on the nitrogen atom is the most active catalyst in the series. All complexes showed good activity as catalysts in the oxidation of n-octane and styrene using tert-butyl hydroperoxide (TBHP) as oxidant.
Introduction
One of the key steps in ruthenium-catalyzed oxidation reactions is the formation of an intermediate ruthenium-oxo species through the mediation of a suitable oxidant, 4,10-14 and ruthenium catalysts have been extensively explored in epoxidation reactions using oxidants of different and ligand systems. 1-6,8-25 The products of these reactions, such as carbonyls and epoxides, are widely used in perfumes, dyes, and in pharmaceutical and fine chemical synthesis.26-36. The oxidation of saturated hydrocarbons by ruthenium complexes to more valuable products has also gained the interest of scientists.23,37-48 A selective, efficient, and catalytic system must be established to functionalize unactivated paraffin C-H bonds.37,49- 55 Many challenges, such as the chemical inertness of alkanes, the preferential activation of substrates containing sp2 hybridized C‒H bonds over sp3 hybridized C‒H bonds, and cases where the intermediates are more reactive than the alkane and thus can react more easily with the metal center, are widespread in the activation of alkanes.56,57-59 One of the ways to overcome these problems is to use a suitable ligand system, which can increase the catalytic activity and selectivity through modifications of ligand and associated electronic backbone changes.3,44,60 Such a ligand system may include the aminodiphosphine ligand system or PNP.
Experimental
- Synthesis and characterization of the compounds
- Crystal structure analysis
- Oxidation studies
For the analysis of the products of the oxidation of styrene, a PerkinElmer Auto System gas chromatograph equipped with a flame ionization detector (FID) set at 290 °C was used. A two-necked pear-shaped flask was charged with 5 mg of the respective catalyst, benzophenone (as an internal standard), styrene, the respective oxidant and 10 ml of the solvent.
Results and discussion
- Synthesis and characterization of the compounds
- Description of crystal structures
- Oxidation of n-octane
- Oxidation of styrene
Selectivity for the oxidation products of the blank reaction (no catalyst) at a substrate:oxidant molar ratio of 1:5. This could be due to the decomposition of TBHP to form tert-butanol, which competes for the same coordination site on the metal.9 The blank reaction, without catalyst, in the chamber.
Conclusion
The difference in activity of the complexes is attributed to the basicity of the substituents on the nitrogen atom.
The activities of the different catalysts, which carry the different substituents on the ligand backbone, are comparable. The difference in the activity of the catalysts bearing the different substituents on the nitrogen atom of the ligand backbone can be attributed to the basicity of the ligand backbone.
