Global Aromaticity in Macrocyclic Polyradicaloids: Hückel’s Rule or Baird’s Rule?

Abstract

CONSPECTUS: Aromaticity is one of the most important concepts in organic chemistry to understand the electronic properties of cyclic πconjugated molecules. Over a century, different aromaticity rules have been developed and validated. For planar monocyclic conjugated polyenes (also known as [n]annulenes), they will be aromatic if they contain [4N + 2] π electrons according to Huckel's rule, or antiaromatic if they have [4N] π electrons. Topological change from a planar to a half-twisted Mobius strip will lead to [4N] ([4N + 2]) aromaticity (antiaromaticity), which is just inverse to Huckel's rule. When the molecules are excited into the first triplet excited state, the Huckel (anti)aromaticity observed in the ground state will become reversed according to Baird's rule. Strictly speaking, these basic rules are only applicable for monocyclic conjugated systems, but some polycyclic systems such as porphyrinoids may also follow these rules if there is a dominant [n]annulene-like conjugation pathway. On the other hand, all-benzenoid polycyclic aromatic hydrocarbons usually display local aromaticity with π electrons predominantly localized at certain benzene rings according to Clar's aromatic sextet rule. In recent years, some proaromatic and antiaromatic molecules with even number of paired electrons have been found to exhibit open-shell diradical character and unique optical, electronic, and magnetic activities. One of the major driving forces is their intrinsic tendency to become aromatic in the open-shell diradical/polyradical forms. A number of stable diradicaloids and linear polyradicaloids have been successfully synthesized by using thermodynamic and kinetic stabilizing strategies. Herein, our particular interest is a type of macrocyclic polyradicaloid in which multiple frontier π-electrons are antiferromagnetically coupled with each other in a cyclic mode. Formally, these free electrons may behave like normal π-electrons in the [n]annulenes, and thus, it raises questions about their possible global aromaticity and which rule they will follow. In the past 5 years, our group has synthesized a series of macrocyclic polyradicaloids and systematically investigated their global aromaticity and electronic properties. Some important findings include: (1) global (anti)aromaticity is generally observed, but there is a balance between local aromaticity and global aromaticity; (2) most of these molecules follow Huckel's rule in the singlet state and display respective (anti)aromatic characteristics; (3) in some special cases, both Huckel's rule and Baird's rule can be applicable, and a unique annulene-within-an-annulene super-ring structure was demonstrated for the first time; (4) global antiaromaticity in the transition state is also important and a slow valence tautomerization process was observed in a supercyclobutadiene tetraradicaloid. These studies demonstrate how these open-shell macrocyclic polyradicaloids adapt their geometry and spin state to reach the lowest-energy state (aromatic). In this Account, we will main...

Predicted Reversal in N-Methylazepine/N-Methyl-7-azanorcaradiene Equilibrium upon Formation of Their N-Oxides

“…Aromaticity (antiaromaticity) is a fundamental concept to account for structure and reactivity of electron delocalized cyclic compounds, intermediates and transition states. The earliest postulates applied to cyclic π-delocalized systems (Hückel Rule [ 19 , 20 , 21 , 22 ]) but more recently include cyclic delocalized s-systems as well [ 23 ]. Aromatic species are characterized by their thermodynamic properties of inherent stabilities [ 24 ], structures with hybrid bond lengths [ 25 ], specific reactions such as preferred substitution not encountered by acyclic or nonaromatic systems [ 26 ], and secondary magnetic fields generated by ring current effects detectable by magnetic resonance spectroscopy (NMR for example).…”

Aromaticity of N‐heterocyclic carbene and its analogues: Magnetically induced ring current perspective

“…ACID is a scalar function of the magnetically induced current density tensor, which is used to understand the degree of electron delocalization in a molecule. [46] It uses the anisotropic part of the current susceptibility tensor, which is a scalar function like electron density. Then, following the similar procedure of charge density distribution, the ACID function is visualized.…”

Donor-Acceptor Conjugated Macrocycles with Polyradical Character and Global Aromaticity

“…It is evident from the prior work ( Nobusue et al., 2015 ; Das et al., 2016 ; Liu et al., 2018 , 2019 ) that the ring topology facilitates the development of polyradical character due to the high symmetry of these molecular structures. In this article, we report D−A motifs comprising π-spacer in the ring topology to achieve systems with high polyradical character ( Figure 1 D).…”

“…have reported several π-conjugated linear and macrocyclic molecules with polyradical character ( Das et al., 2016 ; Lu et al., 2016 , 2017 ; Liu et al., 2019 ; Li et al., 2019b , c ). They have synthesized a fully fused quinoidal/aromatic carbazole macrocycle showing a high diradical character with a moderate tetra- and hexaradical characters ( Das et al., 2016 ) ( Figure 1 B) and an annulene-within-an-annulene (AWA) super-ring structure with decaradical character ( Figure 1 C) ( Liu et al., 2018 , 2019 ). This raises an important question if we can achieve the polyradical character above decaradical in an alternating D−A macromolecule.…”

“…This raises an important question if we can achieve the polyradical character above decaradical in an alternating D−A macromolecule. The cornerstone of polyradical character is the subtle balance between different types of antiferromagnetic (AFM) spin-spin couplings arising within the π-conjugated macrocycle backbone ( Liu et al., 2018 , 2019 ; Li et al., 2018 ). Recent synthetic efforts on D−A macromolecules attempted to tune the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy gaps (HOMO-LUMO) by different D−A motifs to analyze optical properties ( Ball and Nuckolls, 2015 ; Van Raden et al., 2016 ; Yu et al., 2019 ; Ball et al., 2016 , 2018 , 2019 ; Darzi et al., 2015 ; Kuwabara et al., 2015 ; Li et al., 2019a ).…”