Planar carbon radical's assembly and stabilization, a way to design spin-based molecular materials

In this work, we report the first computational study on the assembly and stabilization of a novel kind of radical, i.e., the planar tetracoordinate carbon radical CAl(4)(-). Based on the 6-31+G(d)-UB3LYP, UMP2 and UCCSD(T) calculations on charged [D(CAl(4))M](q-), saturated [D(CAl(4))M(n)] and extended (CpM)(p)(CAl(4))(q) sandwich-like compounds (D = CAl(4)(-), Cp(-); M = Li, Na, K, Be, Mg, Ca), we find that for the six metals, the planar radical CAl(4)(-) can only be assembled in the "hetero-decked sandwich" scheme (e.g. [CpM(CAl(4))](q-)) rather than the traditional "homo-decked sandwich" scheme. Moreover, the low and high spin states of the designed sandwich-like species are perfectly degenerate during assembly. This can be ascribed to the good spin conservation of the CAl(4)(-) deck and the good spatial separation between two CAl(4)(-) decks. Our results show for the first time that the planar radical CAl(4)(-) can act as a new type of spin-embedded "superatom" for cluster assembly when it is assisted by a rigid partner like Cp(-). The good spin-conservation of CAl(4)(-) is very promising for the future design of novel paramagnetic and diamagnetic materials. The ionic, clustering and radical interactions between the two decks are analyzed in detail, which is quite crucial to improve the insight and understanding of the nature and origin of the interactions of the "deck-core-deck" in the metallocenes. Such information is also important in understanding the radical reactions and designing novel spin-based molecular materials. The present study should be expected to enrich the flat carbon chemistry, radical chemistry, metallocene chemistry and combinatorial chemistry.