On the electronic structure of a dianion, a radical anion, and a neutral biradical (HB)11CCCC(BH)11 carborane dimer

The electronic structure of a neutral, a radical anion, and a dianion carborane dimer connected via an acetylenic bridge unit (HB)11CCCC(BH)11 is analyzed by quantum chemical methods. Geometries, relative stabilities, and singlet-triplet gaps are determined in the neutral and dianion species for the lowest-lying singlet and triplet states and for the doublet ground state in the radical anion. As for the recently studied biradical compounds derived from o-carborane, m-carborane and p-carborane [J. Chem. Theory Comput. 4 (2008) 1338] via double hydrogen abstraction, the neutral dimeric compound displays a biradical ground-state structure in which both singlet and triplet state are practically degenerate, with the singlet state lying slightly lower in energy (⩽0.005 eV) at both DFT broken-symmetry and ab initio CASPT2 levels of theory. The singlet-triplet splitting is therefore close to kB·T at room temperature, approaching the microwave region of the electromagnetic spectrum. The neutral dimer biradical becomes then a strong candidate to behave as a molecular magnet in molecular architectures based on carborane units. It is also shown that the system is a powerful electron acceptor with increasing stability from the neutral to the radical anion and dianion systems.