Tri-wing graphene nano-paddle-wheel with a single-file metal joint: Formation of multi-planar tetracoordinated-carbon (ptC) strips

Using density-functional theory (DFT), we show that edge-passivated zigzag graphene nanoribbons by a metal ligand M (ZGNR-M) can form various nanostructures through a single-file metal-chain joint. We have investigated structural properties of ZGNR-M (M = Al, Sc, Ti, V, Cr, Mn, Fe, or Co) based bi-wing nanostructures and tri-wing "nano-paddle-wheels", named as bi-ZGNR<M> and tri-ZGNR<M>, respectively. In particular, we explore whether one or more wings in bi-ZGNR<M> or tri-ZGNR<M> nanostructures can entail a strip of planar tetracoordinated carbon (ptC), a concept originally proposed by Hoffman et al. (J. Am. Chem. Soc.1970, 92, 4992) for molecular species. We find that although the ptC is energetically less favorable than nonplanar tetracoordinated carbon in bi-ZGNR<M> nanostructures (M = Al, Sc, Ti, V, Cr, Mn, Fe, or Co), surprisingly, the ptC strip can be fully stabilized in two wings of tri-ZGNR<Al> and in all three wings of tri-ZGNR<Ti> "nano-paddle-wheel". We also show that tri-ZGNR<Ti> can be a structural unit for building a three-dimensional (3D) titanium-graphene framework (TiGF), the first predicted 3D porous material with segments of ptC strips.