Sugiura S, Kubo T, Haketa Y, Hori Y, Shigeta Y, Sakai H, Hasobe T, Maeda H. Deprotonation-Induced and Ion-Pairing-Modulated Diradical Properties of Partially Conjugated Pyrrole-Quinone Conjunction.
J Am Chem Soc 2023;
145:8122-8129. [PMID:
36976916 DOI:
10.1021/jacs.3c01025]
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Abstract
Quinoidal molecules based on dipyrrolyldiketone boron complexes (QPBs), in which pyrrole units were connected by a partially conjugated system as a singlet spin coupler, were synthesized. QPB, which was stabilized by the introduction of a benzo unit at the pyrrole β-positions, formed a closed-shell tautomer conformation that showed near-infrared absorption. The deprotonated species, monoanion QPB- and dianion QPB2-, showing over 1000 nm absorption, were formed by the addition of bases, providing ion pairs in combination with countercations. Diradical properties were observed in QPB2-, whose hyperfine coupling constants were modulated by ion-pairing with π-electronic and aliphatic cations, demonstrating cation-dependent diradical properties. VT NMR and ESR along with a theoretical study revealed that the singlet diradical was more stable than the triplet diradical.
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