  The rules that govern spin exchange interaction in pristine graphene
nanostructures are constrained by the bipartite character of the lattice, so
that the sign of the exchange is determined by whether magnetic moments are on
the same sublattice or else. The synthesis of graphene ribbons with perfect
zigzag edges and a fluoranthene group with a pentagon ring, a defect that
breaks the bipartite nature of the honeycomb lattice, has been recently
demonstrated. Here we address how the electronic and spin properties of these
structures are modified by such defects, both for indirect exchange
interactions as well as the emergent edge magnetism, studied both with DFT and
mean field Hubbard model calculations. In all instances we find that the local
breakdown of the bipartite nature at the defect reverts the sign of the
otherwise ferromagnetic correlations along the edge, introducing a locally
antiferromagnetic intra-edge coupling and, for narrow ribbons, also revert the
antiferromagnetic inter-edge interactions that are normally found in pristine
ribbons. Our findings show that these pentagon defects are a resource that
permits to engineer the spin exchange interactions in graphene based
nano-structures.
