Binding and photodissociation of CO in iron(ii) complexes for application in positron emission tomography (PET) radiolabelling

Abstract

(R-DAB)FeI₂ complexes containing bidentate diimide ligands (R-DAB = RNCH–CHNR; R = ⁱPr, c-C₆H₁₁) have been investigated for their ability to react with carbon monoxide to form iron(II) dicarbonyl complexes, (R-DAB)FeI₂(CO)₂. Solution IR spectroscopy revealed two νCO stretches between 2000 and 2040 cm⁻¹ corresponding to a cis-arrangement of the carbonyl ligands around the iron. Photochemical decarbonylation was achieved by UV irradiation (365 nm), which occurred within 5 min as evidenced by solution IR spectroscopy. (c-C₆H₁₁-DAB)FeI₂ has been characterised by X-ray crystallography. Reactions using ¹¹C-labelled carbon monoxide were investigated and revealed that both (R-DAB)FeI₂ species were not effective as trapping complexes due to the low concentrations of [¹¹C]CO used in these experiments. A Fe(TPP)(THF)_x (TPP = tetraphenylporphyrin) complex was investigated with unlabelled CO and the monocarbonyl adduct Fe(TPP)(THF)CO was formed in situ as identified by IR spectroscopy (νCO = 1966 cm⁻¹) yet was stable to CO loss upon UV irradiation. Carbonylation reactions of in situ-generated Fe(TPP)(THF)_x using [¹¹C]CO revealed that 97% of the [¹¹C]CO stream could be trapped in one pass of the gas at room temperature and at atmospheric pressure.