Reactivity of (Pyridine-Diimine)Fe Alkyl Complexes with Carbon Dioxide

The reaction of CO₂ with (PDI)­FeMe (<b>1</b>), (PDI)­Fe­(Me)­PMe₃ (<b>1-PMe</b>_<b>3</b>) and [(PDI)­FeMe]­[BPh₄] (<b>2</b>, PDI = 2,6-(2,6-^<i>i</i>Pr₂–C₆H₃–NCMe)₂–C₅H₃N) generates (PDI)­Fe­OAc (<b>3</b>), (PDI)­Fe­(OAc)­PMe₃ (<b>3-PMe</b>_<b>3</b>), and [(PDI)­FeOAc]­[BPh₄] (<b>4</b>), respectively. Kinetic data and solvent effects provide evidence that these reactions occur by precoordination of CO₂ to the Fe center regardless of the charge state and thus favor an insertion mechanism for carboxylation. Carboxylation of <b>1-PMe</b>_<b>3</b> requires initial dissociation of PMe₃ to generate <b>1</b>, which reacts with CO₂; <b>1-PMe</b>_<b>3</b> itself does not react directly with CO₂. CO₂ reacts 5 times faster with neutral <b>1</b> than with cationic <b>2</b> (at 0 °C), which is ascribed to the higher nucleophilicity of the Fe–Me group in <b>1