Pyrrolic macrocycles with stabilized triplet states: Metal-centered and ligand-centered separation of unpaired electrons

Abstract

Two groups of cationic macrocycles with contrasting behaviors have been investigated. The boron group has singlet states well below triplet states. This is the standard behavior for most macrocycles without transition metals. The aluminum group has triplet states stabilized through metal-centered and macrocycle-centered separation of unpaired electrons. The cationic boron subphthalocyanine (1B) has a triplet state whose unpaired electrons are delocalized in the conjugated ring. This is not surprising because a conjugated macrocycle can act as an electron buffer for multiple electrons. On the other hand, coexistence of metal-centered and ligand-centered distributions of unpaired electrons leads to the stabilized triplet state of 1Al. Results from the spin density, natural bond orbital (NBO) charges, and electron localization function (ELF) indicate distinguishable electron distributions between 1B and 1Al. The metal-centered unpaired electron in the triplet state of 1Al leads to the longer Al–N bond length and smaller N–Al–N bond angle compared with these in its singlet state. Consistent with this, the TDDFT study suggests a significant energy relaxation after the first vertical singlet-to-triplet transition. The ligand-centered unpaired electron in the triplet state of 1Al leads to the odd number of π electrons in the macrocycle. NICS values of this macrocycle are distinguishable from these of an aromatic or an antiaromatic macrocycle. The two highest singly occupied orbitals in the triplet state of 1Al are almost degenerate, spatially separated, and have little overlap. This rationalizes its stabilized triplet state. The stabilization mechanism of an unpaired electron at the tetrahedral corner above the central aluminum can also be rationalized by considering the spatial requirement in the valence shell electron pair repulsion (VSEPR) model. The metal-centered and ligand-centered separation of unpaired electrons also leads to stabilized triplet states in some other cationic macrocycles. It is hoped that this theoretical study will stimulate more synthetic attempts to access the experimentally unknown 1Al and more interest in the experimentally known 1B.