Graphdiynes are emerging two-dimensional sp-sp² carbon materials with electronic structures complementing those of graphene, yet their on-surface synthesis is limited by the persistence of metalated intermediates or yields disordered covalent networks. Here, we report an iron-assisted route to covalent hydrogenated graphdiyne monolayers on Au(111) from 1,3,5-tris(bromoethynyl)benzene. Low-temperature scanning tunnelling microscopy, X-ray photoelectron spectroscopy and density functional theory show that Fe scavenges chemisorbed Br byproducts forming FeBr₂, in turn promoting the removal of Au adatoms from the organometallic network, thus enabling its metalated-to-covalent conversion under mild thermal treatment. Subsequent annealing removes FeBr₂ and yields covalent, ordered domains weakly coupled to the substrate. Scanning tunnelling spectroscopy, combined with density functional theory, reveals a semiconducting gap of about 1.6 eV associated with carbon p_z frontier orbitals. This Fe-mediated on-surface synthesis strategy provides a route to atomically precise, weakly substrate-coupled graphdiyne networks and offers a design principle for two-dimensional carbon semiconductors.