A Photolabile Fe Catalyst for Light-Triggered Alkyd Paint Curing

In search for cobalt replacements for alkyd paint curing we show that the photo-active complex [(Cp)Fe(C₆H₆)]+ (Cp = cyclopentadienyl) acts as a latent catalytic drier that allows for photochemical control over the onset of curing, without the need for anti-skinning agents such as the volatile MEKO normally used to prevent curing during paint storage. The highly soluble neutral complex [(Cp)Fe(Ch)] (Ch = cyclohexadienyl) readily converts to the photo-active complex [(Cp)Fe(C₆H₆)]⁺ upon oxidation in alkyd, allowing the latter to be dosed in a wide range of concentrations. Infrared and Raman studies show similar spectral changes of the alkyd paint matrix as have been observed in alkyd curing mediated by the known commercial cobalt- and manganese-based driers Durham NUODEX® Cobalt 10 Neo and NUODEX® DryCoat. The new [(Cp)Fe(Ch)] / [(Cp)Fe(C₆H₆)]⁺ system performs equally well as both commercial paint driers in terms of drying time, and outperforms NUODEX® DryCoat by showing a hardness development (increase) similar to the cobalt-based drier. Based on an observed light-dark on/off effect and EPR studies we propose that photolysis of [(Cp)Fe(C₆H₆)]+ generates short-lived active Fe^II species, explaining the excellent latency. The novel alkyd curing system [(Cp)Fe(Ch)] / [(Cp)Fe(C₆H₆)]⁺ presented herein is the first example of an intrinsically latent paint curing catalyst that is: (1) based on an abundant and harmless transition metal (Fe), (2) doesn’t require any anti-skinning agents, and (3) shows excellent performance in terms of drying times and hardness development.