Dissociative Photochemistry: H-H, C-H, Si-H, B-H, C-F activation

Ultraviolet irradiation may lead to photodissociation of ligands from organometallic carbonyl, dihydride or ethene complexes. The resulting ‘fragment’ is often amazingly reactive, living for a few microseconds before it succumbs and coordinates a new ligand or undergoes oxidative addition.1,2 Even methane or xenon may attack these highly reactive fragments such as (C5H5)Rh(CO). Nevertheless, we can determine the structure of such fragments and their rates of their reactions. We aim to discover new photo-active organometallics in order to make unusual molecules and to activate C–H, C–F, B-H and Si–H bonds. Sometimes a hydrogen-containing substrate may coordinate via a sigma bond without breaking a bond to hydrogen. Such sigma-ligands include dihydrogen, methane and silanes.3

A methane activation reaction

Homogeneous catalysis finds industrial use in reactions such as hydrosilation and stereoselective polymerisations. Synthetic chemists use catalysis in metal-catalysed C-C coupling procedures. Underlying these processes are the same types of metal fragments as those that are generated photochemically: that means that photochemical mechanisms can tell us about fundamental steps in catalysis, such as oxidative addition of H-H or Si-H bonds.

In recent projects, we have measured the first rates of oxidative addition of B-H bonds.4 We have also used photochemistry to generate silyl hydride complexes with well-defined dynamic behaviour, in which we can prove that η2-silane complexes are intermediates in dynamic rearrangement.5 We have used competition reactions to show that B-B, B-H, C-H and Si-H oxidative addition reactions occur at rhodium cyclopentadienyl centres with negligible selectivity.6 We have also linked theory and experiment (with Eisenstein in Montpellier, France) in C-H activation reactions: these studies correlate the metal-carbon bond energies of the product with the carbon-hydrogen bond energies of the organic precursor.7,8 We now aim to observe highly reactive sigma-complexes by nuclear magnetic resonance and time-resolved spectroscopy (with Duckett in York and George in Nottingham).

A B-H oxidative addition reaction

Selected Publications

  1. Organometallic Intermediates: Ultimate Reagents, Chem Soc Rev, 1993, 361.
  2. Photochemistry of M(PP3)H2 (M = Ru, Os), J Am Chem Soc, 1997, 119, 8459.
  3. Transition metal alkane complexes, Chem Rev, 1996, 96, 3125.
  4. Photochemical reactions yielding Rh(η2-silane) intermediates, Dalton Trans, 2004, 3331
  5. Photochemical oxidative addition of B-H bonds at Ru and Rh, Chem Commun, 2004, 242
  6. Photo-induced oxidative addition of B-H, B-B and Si-H bonds at Rh, Organometallics 2006, 25, 5093
  7. Bond energy M-C/C-H correlations: theory and experiment, Chem Commun, 2003, 490
  8. Validation of the M-C/H-C bond enthalpy relationship by DFT, J Am Chem Soc, 2006, 128, 8350.