Cobalt(iii) and copper(ii) hydrides at the crossroad of catalysed chain transfer and catalysed radical termination: a DFT study

Abstract

Metal complexes that mediate radical polymerisation may also lead to catalysed chain transfer (CCT) or to catalysed radical termination (CRT), both processes occurring via the same type of hydride intermediate. What leads these intermediates to prefer reacting with the monomer, leading to CCT, or with radicals, leading to CRT, was unclear. We report here a DFT investigation of the comparative reactivity of two different hydride complexes, [(TMP)Co^III(H)] (TMP = tetramesitylporphyrin) and [(TPMA)Cu^II(H)]⁺ (TPMA = tris(2-pyridylmethyl)amine), generated from [Co^II(TMP)] and [Cu^I(TPMA)]⁺, versus the monomer and radical, using the ˙CH(CH₃)(COOCH₃) and ˙C(CH₃)₂(COOCH₃) radicals as models for the growing PMA and PMMA radical chains. The unsubstituted porphyrin was used as a model for full quantum mechanical (QM) calculations, but selected calculations on the full TMP system were also carried out by the hybrid QM/MM approach, treating the mesityl substituents at the molecular mechanics (MM) level. The calculations provide a basis for rationalizing the experimentally observed strong activity of the cobalt system in catalysed chain transfer (CCT) polymerization without a reported activity so far for catalysed radical termination (CRT), whereas the copper system leads to CRT but does not promote CCT. In essence, the key factors in favour of CCT for the cobalt system are a very low barrier for H transfer to the monomer and the much greater concentration of the monomer relative to the radical, yielding v_CCT > v_CRT. For the copper system, on the other hand, the greater barrier for H transfer to the monomer makes the CCT rate much slower, while the CRT quenching pathway favourably takes place through an electronically barrierless pathway with incipient stabilization at long C⋯H distances. The different spin states of the two systems (spin quenching along the CCT pathway for the Co system and along the CRT pathway for the Cu system) rationalize the observed behavior. The new acquired understanding should help design more efficient systems.