An investigation into the CO-releasing properties of
a range of iron tricarbonyl and chromium and molybdenum tetracarbonyl
complexes is presented. Iron tricarbonyl complexes containing the
2,5-bicyclo[2.2.1]heptene (norbornadiene) ligand are shown to be effective
CO-releasing molecules, in which the rate and extent of CO release
may be modulated by modification of the norbornadiene framework. Species
containing the parent norbornadiene and those with a substituent at
the 7-position of the organic ligand exhibit CO release; those containing
ester substituents at the 2- and/or 3-positions do not. A mechanism
for CO release in this species is proposed which involves initial
norbornadiene dissociation, a suggestion which is supported by the
spectroscopic data and the observation that the addition of excess
substituted norbornadiene retards the rate of CO release. CO release
from the diester-containing norbornadiene complex may be promoted
photochemically, and cell viability studies indicate that in the absence
of light this complex is nontoxic, making it an excellent candidate
for further study as a photo-CO-RM. Both the chromium and molybdenum
tetracarbonyl complexes release CO, which in the case of the molybdenum
analogue is rapid.