A mild, efficient, and regiospecific catalytic cyclotrimerization of alkynes to form 1,2,4-substituted arenes has been discovered. From a cheap and air-stable [Fe(salen)]2-μ-oxo complex and readily available pinacol borane (HBpin), a monomeric [FeH(salen)]·Bpin species formed in situ acts as the active catalyst. This species is shown to feature a hemilabile salen ligand stabilized via interactions with the boron entity. The formation, identity, and reaction mechanism of the active species are supported by complementary kinetic, spectroscopic, and computational data. The active catalyst undergoes hydrometallation of a coordinated alkyne to form a vinyl iron species, stepwise additions of two more alkynes across the Fe–C bond to form a pendant triene, which upon ring-closure forms the arene product. The catalytic cycle is closed by substitution of the product with the alkyne substrate. With the active [FeH(salen)]·Bpin catalyst, atom-efficient, intermolecular trimerization is shown with high regioselectivity for a diverse range of substrate substitution patterns and presence of functional groups.