Structure and stability of the M$_{8-n}$N$_n$C$_{12}$ (M=Ti, Zr; N=Sc, Y and n=1,2,3) Met-Cars as building blocks of cluster-assembled materials

Clusters can be used as building blocks for new materials. However, in order to form a bulk material with clusters, they should be chemically stable. This stability can be characterized by a closed-shell electronic configuration having a large HOMO-LUMO gap. Met-Cars, metal-carbon species composed of early transition metals bonded to carbon, are stable but very reactive. We propose a method to lower their reactivity by metal atom substitution with lower atomic number atoms. We report DFT results on M$_{8-n}$N$_n$C$_{12}$ (M = Ti, Zr; N = Sc, Y, and n=1,2,3) Met-Cars in the neutral, cationic and anionic charge states. Our results show that the isoelectronic M$_6$N$_2$C$_{12}$, M$_5$N$_3$C$_{12}^-$ and M$_7$N$_1$C$_{12}^+$ Met-Cars have closed-shell electronic configurations and larger HOMO-LUMO gaps (1.0-1.7 eV) than that of the M$_8$C$_{12}$. The intercluster interaction between two isolated neutral M$_6$N$_2$C$_{12}$ Met-Cars is relatively weak compared to the M$_8$C$_{12}$ dimers. Due to the weak interaction of the isolated neutral Met-Cars, their unique properties would be retained during assembly.