Mechanistic investigations of the Fe(ii) mediated synthesis of squaraines.

The scission and homologation of CO is a fundamental process in the Fischer-Tropsch reaction. However, given the heterogeneous nature of the catalyst and forcing reaction conditions, it is difficult to determine the intermediates of this reaction. Here we report detailed mechanistic insight into the scission/homologation of CO by two-coordinate iron terphenyl complexes. Mechanistic investigations, conducted using monitoring and reaction sampling techniques (IR, NMR, EPR and Mössbauer spectroscopy) and structural characterisation of isolable species, identify a number of proposed intermediates. Crystallographic and IR spectroscopic data reveal a series of migratory insertion reactions from 1 to 4. Further studies past the formation of 4 suggest that ketene complexes are formed en route to squaraine 2 and iron carboxylate 3, with a number of ketene containing structures being isolated, in addition to the formation of unbound, protonated ketene (8). The synthetic and mechanistic studies are supported by DFT calculations.