The synthesis, characterisation and photochemistry of novel organometallic complexes

Abstract

Complexation of an organic ligand by a transition metal can radically alter the ligand photochemistry. In order to investigate these effects, we synthesised a series of organometallic complexes with several different modes of ligand-metal bonding. The synthesis of various cyclopentadienyl, arene, diene and enone complexes was attempted. Novel ferrocenyl fulgides, including several methoxy-substituted analogues were synthesised, and have been shown to display E/Z-photoisomerism. In contrast with the report by McVey et al,81 we showed that methoxyferrocene would not Friedel-Crafts acetylate in the 2-
or 3-positions using aluminium chloride as the Lewis acid catalyst. Substitution only occurs at the 1'-position, which is thought to be due to deactivation of the methoxy-substituted ring by coordination of the aluminium chloride/acetyl chloride complex by the methoxy group. However, we successfully synthesised the 2- and 3-methoxyacetylferrocenes by replacing aluminium chloride by the more sterically demanding Lewis acid, diethylaluminium chloride.
All attempts at using organic fulgides as diene or enone ligands with a series of metals has shown them to be totally unsuitable for this purpose.
We succesfully synthesised a wide range of fulgide arene chromium tricarbonyl complexes. Their photochemistry was found to consist of simple E/Z-isomerisation followed by decomposition. However, their thermochemistry proved to be very interesting It was shown that a thermal electrocyclic ring closure reaction could be induced at considerably lower temperatures (i.e: -135°C ) than normal (i.e: -200°C) when a chromium tricarbonyl moiety is attached to the fulgide arene group. A crystal structure of the ring closed product from one of these fulgide chromium complexes shows that the Woodward-Hoffmann rules may have been broken by this metal-mediated ring closure. Efforts were being made to prove that this ring-closure reaction proceeded via a thermally forbidden conrotatory mechanism. Preliminary studies on the use of
these chromium tricarbonyl fulgide complexes in the synthesis of hitherto inaccessible fulgides were carried out.