Stable Subporphyrin meso -Aminyl Radicals without Resonance Stabilization by a Neighboring Heteroatom

A Robust Porphyrin-Stabilized Triplet Carbon Diradical

The synthesis of robust high-spin carbon radicals is an important topic in organic chemistry. Toward this end, several porphyrin-stabilized radicals have been systematically explored. A singly naphthalene-fused porphyrin radical was synthesized by a reaction sequence consisting of a Suzuki-Miyaura coupling of β-borylated porphyrin with 2-bromobenzaldehyde, addition of mesityl Grignard reagent, intramolecular Friedel-Crafts alkylation, and final oxidation with DDQ or tBuOK/O₂ . This strategy was also used to synthesize doubly naphthalene-fused porphyrins and syn- and anti-fused-anthracene-bridged porphyrin dimers. While singly naphthalene-fused porphyrin radical has been shown to be a stable monoradical, doubly naphthalene-fused porphyrins and anti-fused-anthracene-bridged porphyrin dimers have been shown to be closed-shell molecules. Finally, the syn-dimer was characterized as a surprisingly stable radical (t_1/2 =28 days under ambient air and at 80 °C) that is storable for more than several months, despite its high-spin triplet ground-state carbon diradical.

An Air-Stable, Neutral Phenothiazinyl Radical with Substantial Radical Stabilization Energy

The vital effect of radical states on the pharmacological activity of phenothiazine-based drugs has long been speculated. Whereas cationic radicals of N-substituted phenothiazines show high stability, the respective neutral radicals of N-unsubstituted phenothiazines have never been isolated. Herein, the 1,9-diamino-3,7-di-tert-butyl-N1 ,N9 -bis(2,6-diisopropylphenyl)-10H-phenothiazin-10-yl radical (SQH2 . ) is described as the first air-stable, neutral phenothiazinyl free radical. The crystalline dark-blue species is characterized by means of EPR and UV/Vis/near-IR spectroscopy, as well as cyclic voltammetry, spectro-electrochemical analysis, single-crystal XRD, and computational studies. The SQH2 . radical stands out from other aminyl radicals by an impressive radical stabilization energy and its parent amine has one of the weakest N-H bond dissociation energies ever determined. In addition to serving as open-shell reference in medicinal chemistry, its tridentate binding pocket or hydrogen-bond-donor ability might enable manifold uses as a redox-active ligand or proton-coupled electron-transfer reagent.

Acetylene and trans-Ethylene Bridged BIII -Subporphyrin Dimers

Acetylene and trans-ethylene bridged B^III -subporphyrin dimers were synthesized by cross-coupling reactions of meso-bromo B^III subporphyrin. These dimers display perturbed and red-shifted absorption spectra reaching around 750 nm and fluorescence reaching at around 850 nm with high quantum yields of 0.39 and 0.47, respectively. DFT calculations have revealed that the HOMOs and the LUMOs of both dimers are spread over the two subporphyrin units as an indication of effective conjugation between the two subporphyrin units. The large Stokes shifts and characteristic pico-second time-resolved transient absorption spectra indicated that the S₁ -states of the dimers relax with structural changes, which are larger for the trans-ethylene bridged dimer.

Stable Diporphyrinylaminyl Radical and Nitrenium Ion

Nitrenium ions, isoelectronic nitrogen counterparts of carbenes, are important intermediates in various biological and chemical processes. Herein we describe the first synthesis and characterization of a stable nitrenium ion without resonance stabilization by adjoining amino groups. Namely, a stable salt of a diporphyrinylnitrenium ion was synthesized by stepwise oxidation of the corresponding diporphyrinylamine through a stable aminyl radical. The nitrenium ion exhibits characteristic features such as a singlet ground state, enhanced double-bond character of the central C-N bonds, no reactivity toward water and methanol, and negative solvatochromic behavior.

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

Trimethylenemethane (TMM) diradical is the simplest non-Kekulé non-disjoint molecule with the triplet ground state (ΔE_ST =+16.1 kcal mol^-1 ) and is extremely reactive. It is a challenge to design and synthesize a stable TMM diradical with key properties, such as actual aliphatic TMM diradical centers and the triplet ground state with a large positive ΔE_ST value, since such species provide detailed information on the electronic structure of TMM diradical. Herein we report a TMM derivative, in which the TMM segment is fused with three Ni^II meso-triarylporphyrins, that satisfies the above criteria. The diradical shows delocalized spin density on the propeller-like porphyrin π-network and the triplet ground state owing to the strong ferromagnetic interaction. Despite the apparent TMM structure, the diradical can be handled under ambient conditions and can be stored for months in the solid state, thus allowing its X-ray diffraction structural analysis.

Stable Nitrogen-Centered Bis(imino)rylene Diradicaloids

The synthesis of stable open-shell singlet diradicaloids is critical for their practical material application. So far, most reported examples are based on carbon-centered radicals, which are intrinsically reactive, and there are very few examples of stable nitrogen-centered diradicaloids. In this full paper, a series of soluble and stable bis(imino)rylenes up to octarylene were synthesized on the basis of newly developed dibromorylene intermediates. It was found that from hexarylene onward, these quinoidal rylenes showed open-shell singlet ground states and could be thermally populated to paramagnetic triplet aminyl diradicals. They are stable due to efficient spin delocalization onto the rylene backbone as well as kinetic blocking of the aminyl sites by the bulky and electron-deficient 2,4,6-trichlorophenyl groups. They exhibited very different electronic structures, diradical character, excited-state dynamics, one-photon absorption, two-photon absorption, and electrochemical properties from their respective aromatic rylene counterparts. These bis(imino)rylenes represent a rare class of stable, neutral, nitrogen-centered aminyl diradicaloids.

A Benzene-1,3,5-Triaminyl Radical Fused with ZnII -Porphyrins: Remarkable Stability and a High-Spin Quartet Ground State

A benzene-1,3,5-triaminyl radical fused with three Zn^II -porphyrins was synthesized through a three-fold oxidative fusion reaction of 1,3,5-tris(Zn^II -porphyrinylamino)benzene followed by oxidation with PbO₂ as key steps. This triaminyl radical has been shown to possess a quartet ground state with a doublet-quartet energy gap of 3.1 kJ mol^-1 by superconducting quantum interference device (SQUID) studies. Despite its high-spin nature, this triradical is remarkably stable, which allows its separation and recrystallization under ambient conditions. Moreover, this triradical can be stored as a solid for more than one year without serious deterioration. The high stability of the triradical is attributed to effective spin delocalization over the porphyrin segments and steric protection at the nitrogen centers and the porphyrin meso positions.