Methodology: Synthetic, NMR, theory, laser flash

We try to bring many techniques to bear on the same problem. NMR and X-ray crystallography serve to identify stable products. 2-D NMR reveals fluxional behaviour and solution conformations. Laser flash photolysis (time-resolved spectroscopy, timescale 10–8 – 10–1 s) is ideal for spotting short-lived intermediates and measuring their reaction rates. Ultrafast spectroscopy informs us about the earliest stages of reactions (10–12 – 10–10 s). Time and again the different methods prove complementary to one another.

NMR Methods

Nuclear magnetic resonance provides our most powerful method of studying molecular structure in solution. Often we design our complexes so that each nucleus has spin ½, or an isotope with spin ½, so ensuring that the spin-spin coupling pattern identifies the molecule uniquely. Two-dimensional NMR methods can be used to link the spectra of different nuclei (HETCOR), to find nuclei which are close in space in order to identify conformers (NOESY), and to spot nuclei undergoing chemical exchange (EXSY). For instance, we have studied (C5H5)Rh(PMe3)(η2-naphthalene) (A) by all these methods.1 We make use of the Centre for Magnetic Resonance which is equipped with state-of-the-art Bruker 600 and 700 MHz spectrometers that opened in 2006.

Structures studied by NMR

 

Setup of laser and NMR equipment

We have designed facilities for laser irradiation within the NMR probehead with S. B. Duckett. We have a continuous He-Cd laser (325 nm) and a pulsed XeCl excimer laser available (308 nm). This system works even at low temperature and a pressure of 3 atm of gas. We have used this system to observe labile reaction intermediates such as solvent complexes.2

Selected Publications

  1. Structure and dynamic exchange in rhodium η2-naphthalene and rhodium η2-phenanthrene complexes: quantitative NOESY and EXSY studies, Organometallics, 2000, 19, 672.
  2. NMR characterisation of unstable solvent and dihydride complexes generated at low temperature by in-situ UV irradiation, Chem Commun, 2002, 2836.
  3. Photochemical reactions of Rh(η5-C5H5)(t-butylacrylate)2 with silanes: dynamics of isomer interconversion via Rh(η2-silane) species, Dalton Trans, 2004, 3331.

Time-resolved Spectroscopy

Photochemical initiation of a reaction with a pulsed laser often generates a transient species, which then goes on to form products over the succeeding fractions of a second. That transient could be an excited state of the absorbing molecule or a reaction intermediate generated from it. In our experiments, we monitor the transient by UV/visible absorption or emission, or by IR absorption. There are two dimensions to such experiments, the spectrum measured at any point in time, and the kinetics usually measured at one point in the spectrum. Bimolecular reactions of transients usually occur on a timescale of nanoseconds to milliseconds. The same timescales are appropriate to forbidden transitions of excited states. The initial photochemical act and reaction with solvent, and allowed interconversions of excited states may occur much faster: from femtoseconds to picoseconds.1,2 Such processes must be studied by ultrafast spectroscopy. We have exploited these methods to study 16-electron d8 reaction intermediates, such as Ru(dmpe)2, (dmpe = Me2PCH2CH2PMe2) generated from 18-electron hydride precursors. Ultrafast methods show that it is formed within about 10–11 s of the initial laser flash, and possibly faster. Nanosecond methods show that it reacts with H2 with a rate constant greater of about 6 × 109 dm3 mol–1 s–1. We used ultrafast IR spectroscopy to demonstrate photo-induced electro transfer in a metalloporhyrin appended with a rhenium carbonyl bipyridine group.3

The time dimensions of time-resolved spectroscopy or laser flash photolysis
The time dimensions of time-resolved spectroscopy or laser flash photolysis

An example of flash photolysis data: Ru(PP3) is formed from Ru(PP3)H2 and reacts with benzene
An example of flash photolysis data: Ru(PP3) is formed from Ru(PP3)H2 and reacts with benzene

ultrafast IR spectra
Example of ultrafast IR spectra of a metalloporphyrin appended with a rhenium(bipyridine)tricarbonyl group. The spectra are measured at intervals between 1 and 200 ps after the laser flash (see right of figure).

Selected Publications

  1. Tilden Lecture Organometallic intermediates — ultimate reagents, Chem Soc Rev, 1993, 21, 361.
  2. Recent example of time-resolved spectroscopy Light-controlled ion switching: [(bpy)Re(CO)3L]+ complex, J Phys Chem A, 2004, 108, 9037.
  3. Ultrafast charge separation in a photo-reactive rhenium-appended porphyrin assembly by Picosecond Transient IR Spectroscopy, J Am Chem Soc, 2006, 128, 4253.

Laboratory and Instrumental Facilities

The York preparative laboratories were fully renovated in 1999 and are equipped with modern fume-cupboards, Schlenk lines and glove box. The laser laboratory is also purpose-built and houses a kinetic spectrometer equipped with an excimer and a YAG laser for nanosecond and microsecond spectroscopy. We make use of the Centre for Magnetic Resonance which is equipped with state-of-the-art Bruker 600 and 700 MHz spectrometers that opened in 2006. Several NMR spectrometers are in regular use with lower operating frequencies of 300, 400 and 500 MHz. X-ray diffraction measurements are carried out on a Bruker Smart diffractometer with area detector; the crystals are usually cooled to 115 K by a cold gas stream.