From a long time ago attempts have been made to isolate carbenes. A big motivation behind the search for a stable carbene was the fact, that oxidation state II is well known for the late members of group 14, germanium, tin and lead. Therefore it should be possible to produce a compound containing a carbon in oxidation state II, which is stable enough to be detected and possibly isolated and characterized. Additionally carbenes may be useful as building blocks in organic syntheses. They form complexes with a wide variety of main group elements and transition metals in both high and low oxidiation states. Many of these complexes are highly efficient homogeneous catalysts. Carbenes are defined as compounds possessing a divalent carbon in their structure. This carbon is bound to two adjacent groups by covalent bonds. It has two nonbonding electrons which may have parallel (singlet state) or antiparallel spins (triplet state). The simplest example of a carbene is methylene.
The area of carbene boron chemistry is a relatively new area of research. Like most other fields of carbene research it has been revived by the discovery of stable carbenes by Arduengo in 1991. Until then only a few neutral borane adducts with electroneutral carbon bases were known. Most carbon bases are electron deficient on the carbon and therefore electrophiles. However, a nucleophile center is needed to bind to an electron deficient acceptor like borane.
The new nucleophile imidazole-2-ylides make neutral carbon borane adducts easily accessible. In 1993 Kuhn et al. found that borane adducts of these carbenes can be produced in high yields by allowing the carbene to react with BH3·Me2S complex. Other examples of boron adducts with nucleophilic carbenes are adducts with boron trifluoride and trimethoxyborate.
Carbene boron adducts in which boron bears a single carbene substituent are easily accessible. Adducts with two or more carbene ligands on boron remain unknown. However, trialkylboranes with bulky substituents (e.g. trinorbornylborane, tricyclohexylborane etc.) are well documented. In view of the abundance of trialkylboranes, a compound like that appears to be a reasonable synthetic target.
A number of experimental approaches have been made. On the way interesting results have been achieved besides the main goal. These include the synthesis and characterization of a new ionic imidazolium borohydride
Table of Contents
1. INTRODUCTION
1.1. What are carbenes?
1.1.1. Definitions
1.1.2. A short history of carbene research
1.1.3. Characteristics affecting the stability of carbenes
2. ATTEMPTED SYNTHESIS OF A TRIS(IMIDAZOL-2-YLIDENE)-BORANE ADDUCT
2.1. INTRODUCTION
2.2. RESULTS AND DISCUSSION
2.2.1. General method to prepare imidazol-2-ylidenes from the corresponding imidazolium salts
2.2.2. Reaction of 1,3-dimethylimidazol-2-ylidene 9 with borane·thf complex at a 3:1 ratio
2.2.3. Attempt at the addition of 1,3-Dimethylimidazoliumchloride 25 to 2-borane-1,3-dimethylimidazolin 24 under elimination of hydrogen
2.2.4. Reaction of 1,3-dimethylimidazol-2-ylidene 9 with trimethyl borate
2.2.6. Attempt to exchange dimethylamine against the 1,3-dimethylimidazolium ion 25 at tris(dimethylamino)borane
3. SYNTHESIS AND CHARACTERIZATION OF IMIDAZOLIUM BOROHYDRIDES
3.1. INTRODUCTION
3.2. RESULTS AND DISCUSSION
3.2.1. Preparation of 1,3-dimethylimidazolium borohydride 35
3.2.2. X-ray crystal structure analysis of 35
3.2.3. Preparation of 1,3,4,5-tetramethylimidazolium borohydride 37
3.3. COMPARISON OF 1H-NMR SHIFTS OF 1,3-DIMETHYLIMIDAZOLIUM SALTS AND ADDUCTS OF 1,3-DIMETHYLIMIDAZOL-2-YLIDENES WITH BORON COMPOUNDS
4. REACTIONS OF 1,3-DIALKYL- AND 1,3-DIARYLIMIDAZOLINIUM CHLORIDES WITH BORANE AND SODIUM BOROHYDRIDE
4.1. INTRODUCTION
4.2. RESULTS AND DISCUSSION
4.2.2. Attempt to the preparation of 1,3-dialkyl- and 1,3-diarylimidazolinium borohydrides
4.2.3. Reaction of 1,3-bis-(tert-butyl)imidazolinium chloride 38c with sodium hydride
4.2.4. Reaction of 1,3-dimesitylimidazolinium chloride 38a with sodium hydride, followed by borane·thf complex
5. EXPERIMENTS TOWARDS THE HYDROGENATION OF IMIDAZOLIUM-BORON ADDUCTS
5.1. INTRODUCTION
5.1.2. Apparatus
5.2. RESULTS AND DISCUSSION
5.2.1. Preparation of 1,3,4,5-tetramethylimidazol-2-ylidene borane adduct 19
5.2.2. Preparation of 2-borane-1,3-dimethyl-4,5-dichloro-imidazoin adduct 42
5.2.3. Reactions of imidazol-2-ylidene boron adducts with H2 at 900/1500 psi
5.2.4. Heating experiments with 35 and 37 to the reversibility of eq. 37
6. CONCLUSIONS AND OUTLOOK
7. EXPERIMENTAL PART
7.1. GENERAL
7.1.1. NMR spectroscopy
7.1.2. Melting Points
7.1.3. Single-crystal X-Ray structure analysis
7.1.4. Starting materials
7.2. DESCRIPTION OF THE EXPERIMENTS
7.2.1. Synthesis of 1,3-dimethylimidazol-2-ylidene 9, reaction of 1,3-dimethylimidazolium chloride 25 with sodium hydride
7.2.2. Attempt at the synthesis of a tris-(1,3-dimethylimidazol-2-ylidene)borane adduct 23a, reaction of 1,3-dimethylimidazol-2-ylidene 9 with borane·thf complex at a 3:1 ratio
7.2.3. Synthesis of 2-borane-1,3-dimethylimidazolin 24, reaction of 1,3-dimethylimidazol-2-ylidene 9 with borane·thf complex at a 1:1 ratio
7.2.4. Attempt at the addition of 1,3-dimethylimidazoliumchloride 25 to 2-borane-1,3-dimethylimidazolin adduct 24 under elimination of hydrogen
7.2.5. Attempt at the synthesis of a tris-(1,3-dimethylimidazol-2-ylidene)methylborate adduct 23c, reaction of 1,3-dimethylimidazolium chloride 25 with trimethyl borate at a 3:1 ratio in the presence of potassium tert-butoxide
7.2.6. Synthesis of 1,3-dimethylimidazol-2-ylidene trimethylborat adduct 22, reaction of 1,3-dimethylimidazol-2-ylidene 9 with trimethyl borate at a 1:1 ratio
7.2.7. Towards a tris(1,3-dimethylimidazol-2-ylidene)boronmonochloride adduct 23d, reaction of 1,3-dimethylimidazol-2-ylidene 9 with boron trichloride at a 3:1 ratio
7.2.8. Synthesis of bis-(1,3-dimethylimidazol-2-ylidene)silver(I)chloride complex 27, reaction 1,3-dimethylimidazolium chloride 25 with silver(I)oxide
7.2.9. Synthesis of the bis-(1,3-dimethylimidazol-2-ylidene)silver(I)nitrate complex 28, reaction bis-(1,3-dimethylimidazol-2-ylidene)silver(I)chloride complex 27 with silver nitrate.
7.2.10. Synthesis of the bis-(1,3-dimethylimidazol-2-ylidene)silver(I-tetrafluoroborate complex 29, reaction of bis-(1,3-dimethylimidazol-2-ylidene)silver(I)chloride complex 27 with silver tetrafluoroborate
7.2.13. Towards a bis-(1,3-dimethylimidazolium)-dichloroboron chloride complex 32a, reaction of bis-(1,3-dimethylimidazol-2-ylidene)silver(I)nitrate complex 28 with boron trichloride
7.2.14. Towards a bis-(1,3-dimethylimidazolium)-dichloroboron tetrafluoroborate complex 32b, reaction of bis-(1,3-dimethylimidazol-2-ylidene)silver(I)-tetrafluoroborate complex 29 with boron trichloride
7.2.15. Reaction of tris(dimethylamino)borane with 1,3-dimethylimidazolium chloride 25
7.2.16. Preparation of 1,3-dimethylimidazolium borohydride 35 from 1,3-dimethylimidazolium chloride 25 and sodium borohydride
7.2.17. Preparation of Preparation of 1,3,4,5-tetramethylimidazolium borohydride 37 from 1,3,4,5-tetramethylimidazolium chloride 36 and sodium borohydride
7.2.18. Attempt to the preparation of 1,3-bis-(p-tolyl)imidazolinium borohydride, reaction of 1,3-bis-(p-tolyl)imidazolinium chloride 38b with sodium borohydride
7.2.19. Attempt at the preparation of 1,3-bis-(tert-butylimidazolin)-2-ylidene 40c, reaction of 1,3-bis-(tert-butylimidazolinium) chloride 38c with sodium hydride
7.2.20. Synthesis of 1,3-dimesitylimidazolin-2-ylidene 40a, Reaction of 1,3-dimesitylimidazolinium chloride 38a with sodium hydride
7.2.21. Attempt at the synthesis of a 1,3-dimesitylimidazolin-2-ylidene borane adduct, reaction of 1,3-dimesitylimidazolin-2-ylidene 40a with borane·thf complex at a 1:1 ratio
7.2.22. Synthesis of 1,3,4,5-tetramethylimidazol-2-ylidene 18, reaction of 1,3,4,5-tetramethylimidazolium chloride 36 with sodium hydride
7.2.23. Preparation of 2-borane-1,3,4,5-tetramethylimidazolin 19, reaction of 1,3,4,5-tetramethylimidazol 2-ylidene 18 with borane·thf complex
7.2.24. Preparation of 1,3-dimethyl-4,5-dichloroimidazolium tetrafluoroborate 44, reaction of N-methyl 4,5-dichloroimidazole 43 with trimethyloxonium tetrafluoroborate
7.2.25. Preparation of 1,3-dimethyl-4,5-dichloroimidazol-2-ylidene 45, reaction of 1,3-dimethyl-4,5-dichloroimidazolium tetrafluoroborate 44 with sodium hydride
7.2.26. Preparation of 2-borane-1,3-dimethyl-4,5-dichloroimidazolin 42, reaction of 1,3-dimethyl-4,5-dichloroimidazol-2-ylidene 45 with borane·thf complex
7.2.27. Reaction of 1,3-dimethylimidazol-2-ylidene borane adduct 24 with dihydrogen at 1500 psi in DMSO-d6 for 0.5 hours
7.2.28. Reaction of 2-borane-1,3,4,5-tetramethylimidazolin 19 with dihydrogen at 900 psi in DMSO-d6 for 16 hours
7.2.29. Attempt at the reaction of 2-borane-1,3-dimethyl-4,5-dichloroimidazolin 42 with dihydrogen at 1500 psi in DMSO-d6 for 48 h
7.2.30. Attempt at the reaction of 1,3-dimethylimidazol-2-ylidene trimethylborat adduct 22 with dihydrogen at 1500 psi in DMSO-d6 for 30 h
7.3. Handling of chemicals and waste disposal
Research Objectives and Core Themes
The primary research objective of this work is the exploration of carbene-boron chemistry, specifically the synthetic accessibility of novel di- or trimeric boron-carbene adducts using 1,3-dimethylimidazol-2-ylidene as a stabilizing pedant base. The study investigates various reaction approaches to form such complexes and explores the characterization of new imidazolium borohydride salts, while also evaluating the potential reversibility of the hydrogenation of specific imidazolium-boron adducts under pressurized conditions.
- Synthesis and investigation of trimeric boron-carbene adducts.
- Characterization of novel imidazolium borohydride compounds.
- Reactivity studies of 1,3-dialkyl- and 1,3-diarylimidazolinium chlorides.
- Catalytic or stoichiometric hydrogenation experiments using high-pressure setups.
Excerpt from the Book
1.1.3. Characteristics affecting the stability of carbenes
The most important feature affecting the stability of a carbene are the adjacent substituents at the divalent carbon which promote the singlet configuration of a carbene. These substituents have to be able to donate electron density to the carbene carbon, and therefore have to possess a lone pair in their valence shells. Elements from groups 15 to 17 are suitable substituents. Experimental result showed that nitrogen seems to be suited best for this. In fact all stable (bottleable) carbenes isolated so far possess at least one nitrogen bound to the carbene. Examples with the second substituent replaced by sulphur and oxygen have been reported. Nature’s thiazole carbene 8 from vitamine B1 gives an example for this.
In many stable carbenes the divalent carbon is part of a five membered ring containing a C-C double bond. This ring system possesses a delocalized π-system that may play an important role in stabilizing the carbene. Steric hindrance by the substituents at nitrogen can also be used to further enhance the stability of the carbene.
Summary of Chapters
1. INTRODUCTION: Provides a comprehensive overview of carbene definitions, their historical research context, and the fundamental electronic characteristics that influence their stability.
2. ATTEMPTED SYNTHESIS OF A TRIS(IMIDAZOL-2-YLIDENE)-BORANE ADDUCT: Details experimental approaches to synthesize trimeric boron-carbene adducts, including reaction conditions with borane, trimethyl borate, and boron trichloride.
3. SYNTHESIS AND CHARACTERIZATION OF IMIDAZOLIUM BOROHYDRIDES: Documents the serendipitous discovery, isolation, and structural characterization of new ionic imidazolium borohydride salts.
4. REACTIONS OF 1,3-DIALKYL- AND 1,3-DIARYLIMIDAZOLINIUM CHLORIDES WITH BORANE AND SODIUM BOROHYDRIDE: Explores the reactivity of fully saturated imidazolinium derivatives and their reduction pathways in the presence of borane and borohydride reagents.
5. EXPERIMENTS TOWARDS THE HYDROGENATION OF IMIDAZOLIUM-BORON ADDUCTS: Presents the investigation of metal-free hydrogenation pathways and the reversibility of the formation of boron-carbene adducts under pressure.
6. CONCLUSIONS AND OUTLOOK: Summarizes the key experimental findings and offers insights into potential future research directions for the stabilization of boron-carbene systems.
7. EXPERIMENTAL PART: Lists the detailed protocols, technical instrumentation, and analytical data for all synthetic procedures described in the thesis.
Keywords
Carbene, Boron Chemistry, Imidazol-2-ylidene, Borohydride, Imidazolium Salts, Hydrogenation, Adduct Synthesis, Catalysis, NMR Spectroscopy, X-Ray Crystallography, Donor-Acceptor Complexes, Organometallic Synthesis, Reactivity Studies, Borane-THF, Stability
Frequently Asked Questions
What is the core focus of this scientific publication?
The work primarily focuses on exploring and synthesizing carbene-boron adducts, specifically investigating the stability and synthesis of monomeric and higher-order complexes using nucleophilic imidazol-2-ylidenes.
What are the primary themes discussed?
The key themes include the synthesis of boron-carbene adducts, the structural analysis of novel imidazolium borohydride salts, reactivity studies with various boron-based reagents, and experiments examining the potential reversibility of hydrogenation reactions.
What is the main research question or goal?
The central goal was to synthesize di- or trimeric boron-carbene adducts using 1,3-dimethylimidazol-2-ylidene, and to characterize the reactivity and hydrogen-storing potential of related imidazolium-boron species.
Which scientific methods are utilized in this research?
The research relies heavily on synthetic organic and inorganic chemistry, including vacuum line techniques, Schlenk methods, NMR spectroscopy (1H, 11B, 13C), and single-crystal X-ray diffraction for structural analysis.
What is covered in the main section of the paper?
The main sections document detailed experimental procedures and discussions regarding the attempted synthesis of tris-adducts, the characterization of new borohydride salts, and the investigation of saturation/hydrogenation experiments in stainless steel pressure reactors.
What are the characterizing keywords of the research?
Key terms include Carbenes, Boron-adducts, Imidazolium borohydrides, Catalysis, Organometallic reactivity, and Structural characterization.
How is the stability of carbenes related to their substituents?
Stable carbenes generally require adjacent substituents with lone pairs, such as nitrogen, to donate electron density to the vacant valence shell of the divalent carbon, promoting a singlet configuration.
What significance do the heating experiments have in this work?
The heating experiments were conducted to test the reversibility of the hydrogen addition to boron-carbene adducts, evaluating their potential for safe hydrogen storage in fuel cell or engine applications.
- Quote paper
- Dipl-Chem. Oliver Steinhof (Author), 2003, Investigations in the field of carbene-boron chemistry, Munich, GRIN Verlag, https://www.grin.com/document/80000
