The photochemistry of group eight metal dihydrides of the type M(PP)2H2 (M =
Ru, Os, Fe; PP = dmpe, depe, dppe) complexes has been extensively explored by Perutz.62,64-68 Their reactions in the presence of substrates such as silanes and H2 led to the oxidative addition products to form the M(PP2)H(SiRmHn-m) and
to regenerate the starting material respectively. The photochemical activity of those complexes towards C-H activation was found to be poor due to the rapid recombination with H2. An improvement in the reactivity for aromatic and
activation has been achieved mainly for reactions of Ru(PP)2H2 in the presence
of HBpin.3,70 η2 - Coordination species for this kind of complexes have not been observed. A more detailed explanation will follow in chapters three and six.
1.5 Time resolved methods and detection of transients
Time resolved methods are among the most used for experimental characterization of reaction intermediates. The rapid development in laser technology, digital electronics and fast detection systems gave access to methods of monitoring very short-lived reaction intermediates allowing their detection down to the nano, pico, femto second time domain. The spectral evolution followed at very short time intervals can be achieved by laser flash photolysis71 (LFP). In this way, a very short pulse of high intensity radiation (laser or flash lamp) initiates the reaction which is then followed by monitoring the changes in absorption or emission. The intermediate generated by the rapid photoexcitation is formed in acceptable yield to be detected by UV/vis or IR spectroscopy.
Spectral and kinetic information are obtained by the use of time resolved methods. A point-by-point spectrum can be built by looking at different frequencies at one moment in time while a transient decay can be obtained at one wavelength. Laser flash photolysis has been used in these studies to obtain information about reactive intermediates formed after flash photolysis of complexes cis-[Ru((R,R)-Me-BPE)2(H)2] and cis-[Ru((R,R)-Me-DuPHOS)2(H)2].3
1.6 Outline of the thesis
An outline of the thesis is provided below. A more detailed introduction is provided in each chapter.
Chapter two. The photochemical reactions of complex CpRh(PMe3)(C2H4) in
the presence of pentafluoropyridine, 2,3,5,6 tetrafluoropyridine and 4- substituted analogues are explored. The mechanism for reactions with the 4- functionalised tetrafluoropyridines assessed by NMR spectroscopy, KIE and DFT calculations. Reaction products are fully characterised by multinuclear NMR spectroscopy, mass spectrometry and X-ray crystallography.
Chapter three. The photochemical reaction of the bidentate-phosphine
ruthenium complex Ru(dppe)2H2 in the presence of borolanes HBpin and HBcat
is investigated by NMR spectroscopy. The nature of the products is discussed by comparison with previously isolated ruthenium-boryl complexes. On the same lines, the photoreactivity of CpRh(C2H4)2 in the presence of HBpin and
B2pin2 is tested in hexane as solvent in order to detect reaction intermediates.
The catalytic ability towards borylation of heptane for the system is also determined by GC-MS method.
Chapter four. The photo-reactivity of CpRh(PR3)(C2H4) complexes (R = PMe3,
PPh3, PMePh2 ) towards the aminoborane H2BN(iPr)2 is investigated by NMR
spectroscopy and X-ray crystallography. The photochemical reaction of [CpRh(C2H4)2] in the presence of a silazane is also studied by NMR
spectroscopy.
Chapter five. The photochemistry of Tp`Rh(PMe3)H2 and
Tp`Rh(CNneopentyl)(
η
2-PhN=C=N-neopentyl) is investigated in the presence of C5F5N, HBpin, primary, secondary and tertiary silanes. The reactions areLFP is also performed on a ruthenium complex with a bidentate non chiral- phosphine. The results are discussed in comparison to data previously observed.
Chapter seven. Crystallographic analyses of different ruthenium and rhenium
complexes are given.
1.7 Aim of the project
The project was designed to probe comparisons between different substrates in oxidative addition reaction and especially to discover more about B-H and B-B oxidative addition. The main strands of the project were:
• Competition between C-H and C-F activation.
• Target the selectivity for C-H bonds relative to B-B, B-H or Si-H bonds comparing different metals and ancillary ligands.
• Measure the rates of B-B and B-H oxidative addition reactions by laser flash photolysis.
• Search for reaction intermediates.
• Determine the rates and activation energies for their conversion to products.
• Investigate the reaction via theoretical calculations in collaboration with Prof. Eisenstein.
2. Photochemical cyclometallation via HF elimination:
synthesis and mechanism
2.1. Introduction
The activation of strong carbon-fluorine bonds is a great challenge in organometallic chemistry. Many reviews have been published looking at different aspects of the topic.1-3 Nevertheless, not many examples of cyclometallation via C-F activation have been reported.
Albrecht reviewed cyclometallation reactions using d-block transition metals showing metallacycles have been successfully applied in organic transformations, catalysis and in various other domains of materials science.4 Since then, many other papers have been published presenting characterisations of new metallacycles,5,6 applications in hydrodefluorination catalysis,7 oxygen sensing8 and transfer hydrogenation.9
The reactivity of rhodacycles and iridacycles, for instance, has been described by the work of Jones and Li, where a wide range of metallacycles is involved in insertion reactions with different unsaturated ligands.10
Palladacycles have been investigated in reactions with phosphines to form new Pd complexes,11 and cycloplatinated complexes have showed great
photophysical properties opening themselves to a wide set of applications as light emitting devices or luminescent molecular sensors.12-14 A search into
the literature highlighted few papers with metallacycles formed by C-F activation. Most of them proceed by intramolecular C-F oxidative addition favoured by the presence of a chelating ligand. The ligands coordinate firstly through the hetero atom (N or S) to the metal centre and the C-F bond is then oxidatively added. A metal fluoride is detected in all the following examples, either as final product or as intermediate in the catalytic cycle. No formation of HF has been detected or mentioned as side product in any of these publications.
activation were in fact observed using π-coordinated cobalt(0) compounds. Following their previous work, Li and coworkers looked at the reaction also from the catalytic side; they reported later that new organic fluorides are formed by carbonylation reactions of cobaltacycles (Scheme 2).16