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

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).

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.