A spacious Fe(ii)-iminopyridine self-assembled cage complex can catalyze the oxidative dimerization of alkanethiols, with air as stoichiometric oxidant. The reaction is aided by selective molecular recognition of the reactants, and the active catalyst is derived from the Fe(ii) centers that provide the structural vertices of the host. The host is even capable of size-selective oxidation and can discriminate between alkanethiols of identical reactivity, based solely on size.