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SYNTHESIS AND TRANSIENT SPECTROSCOPY: A DUAL APPROACH TO NEW COMPOUNDS AND MATERIALS

Objective


Microsecond and nanosecond time resolved infrared (IR) spectroscopy (TRIR) has been used to investigate both the kinetics and the nature of the intermediates in the photochemical substitution reactions of cyclopentadienyliron dicarbonyl with tetrahydrofuran (THF) and organophosphites in cyclohexane and n-heptane solution at 25 C. An important feature of these experiments have been the use of both ultraviolet (UV) and visible photolysis wavelengths to distinguish between processes involving photoejection of carbonyl, which principally occurs on UV irradiation, and homolysis of the iron iron bond, which is promoted by both UV and visible light. IR spectroscopy has also been used to study the photochemical reaction of hydrogen and nitrogen molecules with cyclopentadienylmetal tetracarbonyl compounds of the group v metals, vanadium, niobium and tantalum. The reactions have been studied by Fourier transform infrared spectroscopy (FTIR) in liquid xenon solution approximately 80 C and by (TRIR) in n-heptane solution at room temperature. In nearly all cases, UV irradiation leads to substitution of only 1 carbonyl group.

Infrared spectra of long lived excited states of organometallic compounds have also been studied. Particularly successful has been the study of tungsten pentacarbonyl tetracyanopyridine, where it has been possible to defect the IR spectrum of the electronic excited state which leads to the coordinatively unsaturated intermediate.

Another area of study has been that of intramolecular exchange in norbornadieneiron tricarbonyl where an excellent agreement between predicted and observed spectra has been found.

All of the work on supercritical fluids has involved substances with critical temperatures close to room temperature. Initial studies were based on supercritical xenon which is unrivalled as a solvent for spectroscopic studies because it is totally transparent throughout the infrared (IR). The reaction chemistry has now been extended to other supercritical fluids, ethene, ethane, carbon dioxide and triflouromethane each of which has specific properties of relevance to organometallic chemistry. In general, these fluids have solvent properties similar to those of a light hydrocarbon and solubility can be varied by changing the applied pressure (and hence the density of the fluid). However, the property which has been exploited most extensively, for the synthesis of dihydrogen and dinitrogen complexes, is the complete miscibility of these fluids with dinitrogen and dihydrogen. This miscibility gives effective concentrations of these gases 5 to 10 times higher than would be achieved with conventional solvents under similar conditions. Indeed, using dihydrogen in supercritical ethane it is possible to obtain a solution in which the mole fraction of dihydrogen is greater than 0.3.

A range of compounds have been synthesized in supercritical fluids by direct photochemical substitution of carbonyl by either dinitrogen or dihydrogen. Most of these compounds were previously unknown in fluid solution at room temperature and, even for the known compounds, the synthetic route is new. The compounds include the first nonclassical dihydrogen complexes of manganese, the first complex of iron to have both a diene and an n2 dihydrogen group bound the same metal centre and compounds containing 2, or even 3, dinitrogen groups coordinated to a single metal centre. Dinitrogen and dihydrogen complexes are liable because dinitrogen and dihydrogen are good leaving groups, relatively weakly bound to the metal centre. The high concentrations of dinitrogen and dihydrogen have a purely kinetic role in stabilization of these species in the supercritical solution where an increased concentration of unbound leaving group will decrease the apparent rate of decomposition of the complex. Although this kinetic stabilization does not affect the thermodynamic stability of a complex, it is nevertheless chemically quite significant, particularly in the photochemical generation of polydinitrogen complexes. Under these conditions, loss of dinitrogen is essentially reversible while loss of carbonyl is not and the reaction is forced towards the polydinitrogen products.

Over the past few months, substantial progress has been made towards the goal of using supercritical fluids for synthetic chemistry. In particular, a relatively simple flow apparatus has been devised which allows organometallic compounds to be synthesized as a continuous rather than a batch process. The advantages of flow processes have long been known in industrial plants but this is the first application to organometallic chemistry. In a batch process, more product can only be obtained by repeating the whole process several times or by scaling the whole apparatus up in size. In a flow process, more product can be obtained by using the apparatus for a longer period of time. In addition, the reactor does not require any conventional solvents, a fact which has important implications for cleaner laboratory synthesis.

Investigations into the photochemistry of a series of arenechromium tricarbonyl complexes have continued. The reactivity of the 16-electron arenechromium dicarbonyl is effected by the number and nature of substituents on the arene ligand. The formation of the dinuclear diarenedichromium pentacarbonyl species produced by the reaction of the primary photoproduct with the parent tricarbonyl compound has also been investigated.

The structural changes which occur in methylcyclopentadienylmanganese dicarbonyl ligand complexes when the nature of the ligand is changed have been investigated by single crystal X-ray determinations. When the ligand is a good electron donor, the methyl substituent eclipses the ligand, while a carbonyl group is eclipsed when the ligand is a good pi-acceptor.

The effect of binding potential ligands to organic polymers is also under investigation. This work has shown that it is possible, using transient spectroscopic techniques, to discriminate between chromium pentacarbonyl toluene and chromium pentacarbonyl polystyrene on the basis of their ultraviolet visible spectrum. A method of molecular weight determination using flash photolysis is currently under development.

Studies have been conducted of the reactivity of arenes with transition metal complexes: the coordination of arenes in different modes and the activation of both carbon hydrogen and carbon fluorine bonds of arenes. Arenes may coordinate without bond breaking in 3 ways, through 2, 4 or 6 carbon atoms. New crystal structures have been obtained of complexes of arenes coordinated in the eta2 and eta4 modes and the major reorganization inherent in these coordination modes have been demonstrated. It has been shown that such structures resemble coordinated alkenes and are stabilized by electron withdrawing groups on the arene rather than by electron donating groups which stabilize eta6 -arene metal complexes. Resonance Raman spectroscopy has been developed as a sensitive probe of metal dieta6-arene complexes and electron spin resonance spectroscopy has been used to characterize the first eta6-arene complexes of scandium.

It is possible to control the outcome of the reaction of a metal complex with arenes to give either metal(aryl)hydride or metal (eta2-arene) complexes depending on the choice of the arene, the metal, and the ancillary ligand. Moreover, it has been shown by laser flash photolysis that formation of a metal(aryl)hydride is preceded by a metal (eta2-arene) intermediate. When the arene is benzene this intermediate has a lifetime of approximately 1 ms at room temperature.

Carbon fluoride bands are much stronger than carbon hydrogen bonds and have proved far less reactive towards transition metal centres. However, reactions in which rhodium inserts into a carbon fluorine bond of hexafluorobenzene have now been found. Of particular importance is the fact that this reaction occurs via an isolable rhodium eta2-hexafluorobenzene complex.

In addition to the work on arene complexes, a detailed study of hydrosilation of alkenes at rhodium has revealed evidence which conflicts with the accepted mechanism of this commercially important type of reaction. An altern ative mechanism has been proposed which may have wide applicability.

Laser induced fluorescence has been developed as a technique for the detection of reactive metallocenes in low temperature matrices. The resulting spectra are rich in vibrational information, very intense and probably the best resolved electronic spectra to be recorded for any organotransition metal complex.

Studies have been conducted in the following areas:

It became clear that a complete understanding of the primary photochemistry of metal carbonyls, and other compounds of interest, would necessitate ultrafast transient absorption studies. This requires not only short lived (subpicosecond) ultraviolet (UV) pulses for excitation but also efficient and reproducible generation of a continuum for monitoring purposes. After optimization, pulses of a 100 fs duration were obtained. The performance of the apparatus was confirmed by studies with transstilbene and tetraphenylhydrazine. Ultrafast electrontransfer between thionine dyes and deoxyribonucleic acid (DNA) bases has been successfully probed.

Organic carbonyl compounds:
although the transient spectroscopy of organic carbonyls has been extensively studied by UV visible monitored flash photolysis methods, transient infrared measurements are still few. Furthermore, the possibility of exploiting the properties of these compounds for photobiological purposes such as DNA probing is of considerable interest. As part of an endeavour in this area a series of new derivatives of benzophenone and cyclopentenone have been prepared and studied.

Photochemistry and photophysics in polymeric matrices:
For transient spectroscopy purposes it is often useful to incorporate the compound in polymers. On the other hand photoactive compounds may also be used as probes for polymer properties. Perfluorosulphonated polymers, such as Nafion, are of considerable interest as matrices as their polarity can be controlled by swelling in water or other solvents and they have excellent chemical stability and good transmission in the ultraviolet, visible and sections of the infrared. The uranyl ion ultraviolet (UV) has been studied as a photoprobe for hydration of Nafion.
Photocatalysts for the water gas shift reaction have been studied in Nafion, where it is possible to study the photochemical and thermally induced transformations of the var ious complexes at various pHs and extents of hydration within the film. Preliminary transient infrared studies indicated that the photoinduced decarbonylation reactions proceeded in less than 1 us.

Most of the work has concentrated on 2 topics: the photoinduced catalysis of diene hydrogenation and hydrosilylation with group 6 metal carbonyls and the observation of coalescence in the carbonyl stretching vibrational infrared (IR) bands of metal tricarbonyl ligand complexes.

In an endeavour to identify the active species in photoinduced catalysis of diene hydrogenation and to achieve a comprehensive understanding of the catalytic system, both spectroscopic studies and preparative scale experiments were included. This work was started with a thorough investigation into the photolytic behaviour of the group 6 metal tetracarbonyl eta4-norbornadiene complexes.
The catalytic hydrosilylation of norbornadiene (NBD), not previously studied, also leads to exo and endo 1,2-addition of trialkylsilane together with homo 1,4-addition from the endo side, followed by secondary exo 1,2-addition. Here, too, the active catalyst can be generated either photolytically from chromium tetracarbonyl eta4-norbornadiene or in the dark at ambient temperature from chromium tricarbonyl eta4-norbornadiene eta2-ethene. At low silane concentration both the exo 1,2-addition to NBD and the secondary exo 1,2-addition to the norbornenyl silane become negligible.

The previously observed temperature dependant coalescence of the 2 low frequency carbonyl stretching vibrational infrared (IR) bands of various iron tricatbonyl eta4-diene complexes was interpreted in terms of an ultrafast carbonyl site exchange process with a rare constant of the order of 1E12 s{-1}. This process would create psuedo C3v symmetry, thus rendering the above 2 vibrations degenerate. Investigations were extended to the study of carbon-13 carbonyl enriched samples, whereby advantage was taken from the superior solvent properties of liquid and supercritical noble gases. It was observed that the 3 band patterns of the 2 monolabelled and the 2 bislabelled isotopomers show coalescence upon increasing the temperature from appr oximately -150 C to ambient temperature and above, as could be expected as a result of ultrafast site exchange.
This project is intended to exploit the interplay betwwen synthetic chemistry and transient spectroscopy with two major objectives (i) to prepare new compounds and materials, particularly those based on arene and olefin complexes, and to study their properties and reactions (ii) to develop novel applications of transient spectroscopy to provide a better understanding of organometallic reactions in aqueous solution, of reactions in superficial fluids, and of the spectroscopy of molecules in electronic excited states. This programme will produce results of both academic and industrial significiance. The collaboration will create a very effective group involving twelve senior scientists in three EEC countries, with far more equipment and facilities than would be available within a single Institution.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

University of Nottingham
Address
University Park
NG7 2RD Nottingham
United Kingdom

Participants (4)

MAX-PLANCK-GESELLSCHAFT ZUR FOERDERUNG DER WISSENSCHAFTEN E.V.
Germany
Address
34-36,Stiftstrasse 34-36
45413 Muelheim An Der Ruhr
National Institute for Higher Education-Dublin
Ireland
Address
Glasnevin
9 Dublin
THE PROVOST, FELLOWS AND SCHOLARS OF THE COLLEGE OF THE HOLY AND UNDIVIDED TRINITY OF QUEEN ELIZABETH NEAR DUBLIN HEREINAFTER TRINITY COLLEGE DUBLIN
Ireland
Address
Trinity College
2 Dublin
University of York
United Kingdom
Address
Heslington -
YO1 5DD York