A Stable Trimethylenemethane Triplet Diradical Based on a Trimeric Porphyrin Fused π‐System

Abstract

Trimethylenemethane (TMM) diradical is the simplest non-KekulØ non-disjoint molecule with the triplet ground state (DE ST =+16.1 kcal mol À1 )a nd is extremely reactive.I tis ac hallenge to design and synthesize as table TMM diradical with key properties,s uch as actual aliphatic TMM diradical centers and the triplet ground state with al arge positive DE ST value,s ince such species provide detailed information on the electronic structure of TMM diradical. Herein we report aT MM derivative,i nw hicht he TMM segment is fused with three Ni II meso-triarylporphyrins,t hat satisfies the above criteria. The diradical shows delocalized spin density on the propeller-like porphyrin p-network and the triplet ground state owingt ot he strong ferromagnetic interaction. Despite the apparent TMM structure,t he diradical can be handled under ambient conditions and can be stored for months in the solid state,t hus allowing its X-raydiffraction structural analysis.Trimethylenemethane (TMM, 1 in Figure 1) is arepresentative non-KekulØ and non-disjoint diradical possessing intrinsic opposed spin-pairing tendency. [1] Its abnormal electronic structure and reactivity have provided fundamental understanding in theoretical chemistry [2] and have promoted various useful synthetic methods. [3] TMM usually behaves as an extremely reactive species that can survive only at cryogenic temperatures and displays ad istinct ah igh-spin ground state with alarge singlet-triplet energy gap of DE ST = + 16.1(1) kcal mol À1 ,a so ne of the largest values for organic diradicals. [4] Stabilization of these TMM-related high-spin species will allow detailed understanding of the relationship of the electronic properties with regard to their thermal stability,which is important for the development of moleculebased magnetic materials and spintronics. [5] Thus,i ti s ac hallenge to realize as table TMM diradical that holds key properties such as actual aliphatic TMM radical centers and atriplet ground state with alarge positive DE ST value.Along this line,s everal TMM-based stable high-spin molecules have been developed to date. [6,7] As as uccessful example,t he Yang diradical 2 [6a-d] showed both strong ferromagnetic coupling and stability similar to other typical phenoxy radicals.However,its methylene ends of TMM were located as parts of aromatic rings,h ence significantly contributing to improvement of the chemical stability.O nt he other hand, TMM derivatives without such structural features were highly reactive. [7] In fact, 3 was generated in situ by reduction of the corresponding dication and directly subjected to characterization by ESR spectroscopy under inert conditions. [7b] In recent years,w eh ave explored stable porphyrinoid radicals,i nw hich as pin is delocalized over the large pelectronic networks of porphyrinoids. [8,9] Reported examples include meso-oxy radicals, [8b-d] meso-aminyl radicals, [8e,f] and meso-ammoniumyl cation radicals [8g] that can be handled and stored under ambient conditions.Amethyl radical was also stabilized at...

Hydrogen-Bonding-Assisted Substituent Engineering for Modulating Magnetic Spin Couplings and Switching in m-Phenylene Nitroxide Diradicals

“…In recent years, organic magnetic molecular switches have emerged with their promising application prospects in the field of magnetic materials − due to their structural diversity, ease with which they can be synthesized experimentally, and comprehensive magnetic, mechanical, optical, and electronic properties, − which stimulate extensive theoretical and experimental explorations of various novel functional materials. − Chen et al rationally designed organic magnets with switchable two-way couplers for magnetic modulation or switching through the lactam–lactim tautomerization. Rajca et al synthesized three conformationally constrained m -phenylene-bridged nitroxide (NO) diradicals with good environmental stability and strong ferromagnetic (FM) coupling, as further confirmed by theoretical calculations.…”