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

The reaction of CO2 with (PDI)­FeMe (1), (PDI)­Fe­(Me)­PMe3 (1-PMe3) and [(PDI)­FeMe]­[BPh4] (2, PDI = 2,6-(2,6-iPr2–C6H3–NCMe)2–C5H3N) generates (PDI)­Fe­OAc
(3), (PDI)­Fe­(OAc)­PMe3 (3-PMe3), and [(PDI)­FeOAc]­[BPh4] (4), 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 1-PMe3 requires initial dissociation
of PMe3 to generate 1, which reacts with CO2; 1-PMe3 itself does
not react directly with CO2. CO2 reacts 5 times
faster with neutral 1 than with cationic 2 (at 0 °C), which is ascribed to the higher nucleophilicity
of the Fe–Me group in 1.