Metal-centred planar [15]annulenes

The discovery of ferrocene¹ heralded the advent of modern organometallic chemistry. Characterized by the π-coordination of a metal by one or two planar annulene anions, ferrocenes and their analogues^2-4 exemplify the archetype of out-of-plane annulene metal complexes. By contrast, the integration of metal within the annulene core to form in-plane annulene metal complexes featuring metal-carbon σ bonds has been obstructed not only by the synthetic difficulty and the non-planarity of annulenes with appropriate internal dimensions, but also by the difficulty of embedding the metal. These challenges have prevented the isolation of such in-plane annulene metal complexes. Here we report the preparation of three metal-centred planar [15]annulene frameworks. The most symmetrical fragment has D_5h symmetry, with the metal centre shared by five identical five-membered rings. Density functional theory calculations demonstrate that metal d orbitals participate in conjugation with these five-membered rings, rendering all of them aromatic. The overall framework bears a loose structural and spectroscopic analogy to metallo-expanded porphyrins with multiple aza donors⁵, which thus provides a nexus between annulene chemistry and classic heteroatom-based coordination chemistry. The present systems display high stability and are easily functionalized. We thus suggest that metal-centred planar annulenes could emerge as promising building blocks for materials science.