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. For lead +II is even the most stable oxidation state. 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 preferred state depends on the relative energies of both states. If both orbitals are degenerate, the triplet state is favorable. Otherwise both electrons will occupy the orbital lower in energy with antiparallel spins. 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, especially because boron is not able to provide any pi-backdonation like transition metal carbene complexes, as it lacks free electron pairs.
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.
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 boranethf 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 boranethf 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 boranethf 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 boranethf 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 boranethf 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 boranethf 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 boranethf 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 Themes
The primary research objective of this thesis is the exploration and synthesis of novel boron-containing adducts derived from stable nucleophilic carbenes, specifically investigating the potential for di- or trimeric species and evaluating the reactivity of these complexes towards hydrogenation.
- Carbene-boron chemistry and the stability of imidazol-2-ylidene adducts.
- Attempted synthesis of tris(imidazol-2-ylidene)-borane adducts.
- Synthesis and characterization of imidazolium borohydrides.
- Reactions of imidazolinium chlorides with borane and sodium borohydride.
- Experimental investigation into the hydrogenation of imidazolium-boron adducts.
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 an overview of carbene chemistry, definitions, historical context, and the factors influencing their stability.
2. ATTEMPTED SYNTHESIS OF A TRIS(IMIDAZOL-2-YLIDENE)-BORANE ADDUCT: Details the experimental attempts to create higher-order boron adducts and the resulting isolation of specific mono-adducts.
3. SYNTHESIS AND CHARACTERIZATION OF IMIDAZOLIUM BOROHYDRIDES: Documents the discovery, synthesis, and characterization (including X-ray analysis) of new ionic imidazolium borohydride salts.
4. REACTIONS OF 1,3-DIALKYL- AND 1,3-DIARYLIMIDAZOLINIUM CHLORIDES WITH BORANE AND SODIUM BOROHYDRIDE: Investigates the reactivity of saturated imidazolinium derivatives and their reduction to imidazolidines.
5. EXPERIMENTS TOWARDS THE HYDROGENATION OF IMIDAZOLIUM-BORON ADDUCTS: Explores the potential reversibility of hydrogenation reactions and the influence of experimental conditions on borane-adduct stability.
6. CONCLUSIONS AND OUTLOOK: Summarizes the key findings regarding the stability and reactivity of carbene-boron adducts and suggests future avenues for research.
7. EXPERIMENTAL PART: Describes the specific methods, materials, and procedures used for the synthesis and analysis of all compounds studied in the thesis.
Keywords
Carbene chemistry, Imidazol-2-ylidenes, Borane adducts, Imidazolium borohydrides, Boron trichloride, Organometallic synthesis, NMR spectroscopy, X-ray crystallography, Hydrogenation, Imidazolidines, Carbene stability, Lewis acid-base adducts, Coordination chemistry, Transition metal-free catalysis, Chemical reactivity.
Frequently Asked Questions
What is the core subject of this thesis?
The thesis focuses on the field of carbene-boron chemistry, specifically the synthesis, structural characterization, and reactivity of various adducts formed between stable carbenes and boron compounds.
What are the primary research themes?
The study centers on the synthesis of monomeric and attempted higher-order boron-carbene adducts, the identification of novel imidazolium borohydride salts, and the assessment of the hydrogenation behavior of these systems.
What is the central research question?
The research seeks to determine whether stable carbenes can form higher-order adducts (such as tris-adducts) with boron centers and whether these adducts exhibit reversible hydrogenation behavior suitable for hydrogen storage applications.
Which scientific methods are primarily employed?
The work utilizes synthetic organic and inorganic chemistry techniques, primarily working under inert atmospheres (dry box/Schlenk techniques), alongside characterization methods like multinuclear NMR spectroscopy (1H, 11B, 13C) and single-crystal X-ray diffraction.
What is covered in the main section of the work?
The main sections document detailed experimental approaches to reacting carbenes with various boron sources (borane, trimethyl borate, boron trichloride), the isolation of imidazolium borohydrides, and systematic high-pressure hydrogenation studies.
Which keywords define this work?
Key terms include Carbene chemistry, Imidazol-2-ylidenes, Borane adducts, Imidazolium borohydrides, Organometallic synthesis, and NMR spectroscopy.
What role does DMSO play in the hydrogenation experiments?
The author observes that DMSO appears to act as a crucial catalyst for the small degree of conversion observed during hydrogenation experiments, possibly by reducing the dihydrogen species via a transition state.
Why did the attempted synthesis of the tris(imidazol-2-ylidene)borane adduct fail?
The results indicate that the boron center is electronically saturated after the formation of the mono-adduct, and the reagents and conditions tested were insufficient to force the addition of subsequent carbene ligands.
- Citation du texte
- Dipl.-Chem. Oliver Steinhof (Auteur), 2003, Investigations in the field of carbene-boron chemistry, Munich, GRIN Verlag, https://www.grin.com/document/186376
