Influence of Radical Bridges on Electron Spin Coupling

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• 作者：Torben Steenbock ; David A. Shultz ; Martin L. Kirk ; Carmen Herrmann
• 刊名：Journal of Physical Chemistry A
• 出版年：2017
• 出版时间：January 12, 2017
• 年：2017
• 卷：121
• 期：1
• 页码：216-225
• 全文大小：641K
• ISSN：1520-5215

文摘

Increasing interactions between spin centers in molecules and molecular materials is a desirable goal for applications such as single-molecule magnets for information storage or magnetic metal–organic frameworks for adsorptive separation and targeted drug delivery and release. To maximize these interactions, introducing unpaired spins on bridging ligands is a concept used in several areas where such interactions are otherwise quite weak, in particular, lanthanide-based molecular magnets and magnetic metal–organic frameworks. Here, we use Kohn–Sham density functional theory to study how much the ground spin state is stabilized relative to other low-lying spin states by creating an additional spin center on the bridge for a series of simple model compounds. The di- and triradical structures consist of nitronyl nitroxide (NNO) and semiquinone (SQ) radicals attached to a meta-phenylene(R) bridge (where R = −NH^•/–NH₂, −O^•/OH, −CH₂^•/CH₂). These model compounds are based on a fully characterized SQ–meta-phenylene–NNO diradical with moderately strong antiferromagnetic coupling. Replacing closed-shell substituents CH₃ and NH₂ with their radical counterparts CH₂^• and NH^• leads to an increase in stabilization of the ground state with respect to other low-lying spin states by a factor of 3–6, depending on the exchange–correlation functional. For OH compared with O^• substituents, no conclusions can be drawn as the spin state energetics depend strongly on the functional. This could provide a basis for constructing sensitive test systems for benchmarking theoretical methods for spin state energy splittings. Reassuringly, the stabilization found for a potentially synthesizable complex (up to a factor of 3.5) is in line with the simple model systems (where a stabilization of up to a factor of 6.2 was found). Absolute spin state energy splittings are considerably smaller for the potentially stable system than those for the model complexes, which points to a dependence on the spin delocalization from the radical substituent on the bridge.