Triplet Ground State of 2,5-Dimethylene Five-Member Heterocycles

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, |ΔE S - T| to be 50 kcal/mol, which correlates with the S−T gap calculated for the parent hexatriene hydrocarbon. The ΔE S - 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.

Acid−Base-Driven Multilevel Switching of Electronic Coupling in 2,4-Dimethylene Five-Member Heterocycles

“…Anionic compounds 1c , 1h , and 1j had singlet (1 1 A‘) ground states. The results of similar calculations of 1a and 2b , also ground state singlets, have been previously reported . Compounds 3 are structurally similar to 2b and predictably all have singlet ground states.…”

“…The rapidly expanding field of high-spin organic molecules as components of molecular ferromagnets has evolved from its initial infancy of preliminary discoveries to the stage where the advance is driven by search for new molecular topologies and building blocks. The need for more robust, potent, and versatile structures has prompted the synthesis of structures with new strategies for high-spin coupling of radical sites, introduction of aromatic heterocycles as ferromagnetic coupling units, and the investigation of new radical units. − …”

“…The coupling properties of alternant hydrocarbon FCs can be reasonably well predicted based on their connectivity. Heteroatoms are known to modify an FC's properties such as its spin state, although rules governing the effects of such substitution remain elusive. − A potentially important mitigant of heteroatom-substituted FC efficacy is the protonation state of the heteroatom. Dougherty and co-workers have shown that the protonated state of an analogue of 2,6-dimethylenepyridine has been shown to favor the low-spin ground state in contrast to the high-spin ground state of the free base .…”

“…All heavy atoms were held in one plane by enforcing C s symmetry for 1b − n and 3 . Previously reported calculations of 1a and 2b were run with C 2v symmetry …”