Abstract
2,5-Dimethylene five-member heterocycles have been thought to possess exclusively singlet ground states. On the basis of the analysis of charge redistribution upon switching the state multiplicity of π-CASSCF optimized geometries, we calculate that the 2,5-dimethyleneborole dianion should exist in a triplet ground state. Additionally, the 2,5-dimethyleneborole dianion and the analogous 2,5-dimethylenealanacyclopentadiene dianion, 2,5-dimethyleneberyllacyclopentadiene trianion, and 2,5-dimethylenemagnesacyclopentadiene trianion have triplet ground states at the MCQPDT2 level in the CASSCF geometry. Furthermore, we find the upper limit of the S−T gap, |ΔES-T| to be 50 kcal/mol, which correlates with the S−T gap calculated for the parent hexatriene hydrocarbon. The ΔES-T trend approximately follows the group electronegativity scale of Boyd and Boyd derived from the bond critical point model for the heteroatom and protons attached to it. The 2,5-dimethyleneborole dianion and its analogues are members of a new class of ferromagnetic coupling units (FCs) for use in the construction of molecular organic ferromagnets.
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