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Carbene ligand
The chemistry of metal carbene complexes and of related complexes was introduced by Nobel Laureate Ernst Otto Fischer in 19641. Today, carbene complexes similar to those original Group 6 carbene complexes are referred to as Fischer carbene complexes. In this study, the somewhat neglected low–valent Fischer carbene complexes of rhenium were synthesized and studied and aspects of their chemistry were investigated.
Free carbene species
A free carbene molecule is comprised of a carbon with two substituents. One of the four carbon valence electrons is involved in σ–bonding to each of the substituents. The remaining two valence electrons are available for bonding. The carbene carbon atom therefore represents a six–electron species and is very reactive (unfilled octet). The two electrons not involved in bonding may be paired in one of the remaining orbitals and such a carbene is defined as a singlet carbene. In a triplet carbene, on the other hand, the two electrons each occupy its own orbital. Reactive carbene moieties can be stabilized by coordination to transition metals and Arduengo2 and Bertrand3 pioneered research in the isolation of free carbenes (Figure 1.1).
The bent structure of the carbene ligand comes from the carbene carbon atom being sp2 hybridized.
Applications of Fischer carbene complexes
Fischer carbene complexes have current impact in chemistry in template reactions in organic chemistry8, in catalysis9 and in materials science10. NHCs, that are superior to phosphines in many respects, are found as ancillary ligands for coupling–reaction catalysts11 and metathesis (Grubbs) catalysts12.
A metal–carbene connection can be made by attachment of the free carbene species at a vacant coordination site on a metal, which is an application of free carbenes13. This is not the most common method of carbene complex synthesis, because free carbenes are difficult to work with as they tend to dimerize.
Background/Introduction
Group 7 transition metals have an uneven number of valence electrons and thus require at least one X–type ligand (see Green classification of ligands5). Synthesis of Group 7 carbene complexes, in this study, involved low–valent dirhenium decacarbonyl or rheniumpentacarbonyl bromide. The X–type ligand of each rhenium in the dimer complex (CO)5Re–Re(CO)5 is “Re(CO)5” and in [Re(CO)5Br], it is the bromo ligand. For different complexes [M(CO)4LX] (L = CO, C(OR)Rʹ; X = halide, H, M(CO)5), we have isolobal relationships for the X–type ligands.
Chapter 1: General Introduction
1.1. Carbene ligand
1.2. Group 7 metal carbene complexes
1.3 Reactions of Group 7 metal complexes with nucleophiles
1.4 Dual nature of ligands (L, X) in Group 7 metal complexes
1.5 Carbene formation through halide catalysis
1.6 Hydroxycarbene complexes
1.7 Mechanism for decomplexation of X–type ligands of Group 7 metals
1.8 Shvo catalyst
1.9. Aims of project
Chapter 2: Synthesis of carbene complexes
2.1 Introduction
2.2 Reaction of 2–lithiumthienyl with rheniumpentacarbonyl bromide
2.3 Reaction of 2–lithiumthienyl derivatives with [Re2(CO)10]
2.4 Reaction of rhenium carbene complexes with bromine
2.5 The synthesis of tetrarhenium biscarbene complexes
2.6 Bromination of complex 8
2.7 Aldehyde compounds
2.8 Synthesis of hydroxycarbene complexes
2.9 Hydroxycarbene–acyl complex
Chapter 3: Characterization of carbene complexes
3.1 Proton NMR spectroscopy data
3.2. Carbon NMR spectroscopy data
3.3 IR spectroscopy data 3.4. Mass spectrometry data
Chapter 4: Structural features of rhenium carbene complexes
4.1 Introduction
4.2 Dirhenium monocarbene complexes
4.3 Tetrarhenium biscarbene complexes
4.4 Monorhenium monocarbene complexes
4.5 Dirhenium monocarbene aldehyde complex
4.6 Dirhenium bis–carbene/acyl complex
4.7 Summary of bond parameters around the carbene carbon atoms for the complexes
Chapter 5: Experimental
5.1 General
5.2 Synthesis
Chapter 6: Concluding comments
6.1 Summary
6.2 Future work
Appendix
