C-H Activation by Thermally Accesible Radical Ion Pairs

Abstract

Metal free activation of small molecules is a quickly growing field aiming for low economical and environmental footprint. However, activation of methane has proven difficult. Taking inspiration from nature, some metal-free methods involving radicals have been developed. The aim of this project is to extend this and activate methane via radical ion pairs (RIP) generated by single electron transfer (SET) in an electron donor-acceptor (EDA) complex. The SET is governed by the strength of the electron acceptor (electron affinity, EA) and donor (ionization energy, IE). These can be calculated via DFT and if they are roughly equal there will be thermal SET, eliminating the need for irradiation. The EA of several electron acceptors was calculated. DDQ-PMes3 was proposed as EDA complex since it should perform thermal SET based on the EA and IE and both reagents are commercially available. C-H activation of methane, ethane and ethylbenzene by DDQ-PMes3 was investigated computationally and found to be exergonic. However, in the lab it was found that DDQ and PMes3 irreversiably form an P-O adduct prohibiting further reactivity. Switching from PMes3 to tris-tolylamine (TTA) led to the C-H activation of TTA’s methyl group, forming a TTA-DDQ-H product. Attempts to change the non-productive C-H activation of the RIP itself to productive C-H activation of ethylbenzene were unsuccessful. The amine donor was switched to tris(para-bromo-phenyl)amine which did not lead to SET with DDQ due to a too high EA-IE energy gap. Lastly tris(diphenyl)amine was attempted, which reacted with DDQ to form either the RIP or the EDA complex of TDPA and DDQ. Future efforts should focus on obtaining a stable RIP, then testing this RIP for the C-H activation of methane. Choosing an EDA pair with a slight EA-IE gap would allow the SET to be controlled by light which might discourage non-productive reaction between the radicals.