Synthesis, Spectroscopy, and Molecular Orbital Calculations of Subazaporphyrins, Subphthalocyanines, Subnaphthalocyanines, and Compounds Derived Therefrom by Ring Expansion¹

Abstract

A subazaporphyrin (SubAP), tert-butylated and crowned subphthalocyanines (tBSubPc and SubCRPc), a μ-oxo dimer of tert-butylated SubPc {(tBSubPc)2O}, a subnaphthalocyanine (SubNc), and monosubstituted type unsymmetrical phthalocyanine (Pc) and naphthalocyanine (Nc) analogues have been synthesized. In particular, unsymmetrical Pc's and Nc's have been prepared in moderate yields by the ring expansion reaction of structurally distorted SubPc's and SubNc's with isoindolediimine derivatives in dimethyl sulfoxide−chloronaphthalene (or chlorobenzenes or aromatic hydrocarbons such as toluene and xylene) mixtures. The compounds have been characterized by electronic absorption, magnetic circular dichroism (MCD), fluorescence emission, and nuclear magnetic resonance spectroscopy. Both the Soret bands and Q-bands shift to longer wavelength and gain intensity in the order SubAP, SubPc, and SubNc. Fluorescence quantum yields and lifetimes generally decrease with decreasing molecular symmetry. Circular dichroism and NMR spectroscopy have revealed that a SubPc with three 15-crown-5 ether voids having a phenyl group as an axial ligand (SubCRPc) forms inclusion complexes with 2,6-dimethyl-β-cyclodextrin in acetonitrile or water−acetonitrile mixtures, while the electronic absorption spectroscopy suggests that it is not dimerized by the addition of cations such as K+, Rb+, Ce+, i.e., cations which are effective in dimerizing Pc's with 15-crown-5 ether voids. Molecular orbital (MO) calculations within the framework of the Pariser−Parr−Pople approximation have succeeded in reproducing the optical absorption experimental data of not only the parent subazamacrocycles but also Pc derivatives with lower symmetry obtained by the ring expansion reaction. The correspondence between MO calculations and experiments suggests strongly that, in the metal-free unsymmetrical Pc's, two pyrrole hydrogen atoms are bound to the nitrogens along the short axis. Comparison of formation, bonding, and donation energies between SubPc and typical Pc, i.e., MgPc by natural bond orbital analysis, suggests that the distortion energy is not the major reason for the ring expansion reactivity of SubPc, and that the lack of donor−acceptor stabilization in B−N(pyrrole) bonds destabilizes SubPc. Band deconvolution of the electronic absorption and MCD spectra of SubPc with the same set of bands (with the same centers and width) experimentally identified that the excited state of the Q-band of SubPc is orbitally degenerate, with three degenerate transitions located in the 250−450-nm region. In particular, the transition at 359 nm corresponds to a shoulder seen on the red side of the Soret band tail. Time-resolved EPR analysis has shown that the size of the π-system of SubPc is, indeed, smaller than that of Pc's.

Synthesis and structure of the coordinatively unsaturated boron subphthalocyanine cation, [B(SubPc)]+

“…Protonation of the bridging O atom in -oxo iron(III) porphyrin dimers is known [27,28], so the present result reflects the basicity of the aza bridge N atoms in the subPc macrocycle relative to the bridge O atom. The neutral -oxo dimer (t-BuSubPc)B-O-B(t-BuSubPc) has been previously isolated [7,29]. The structure of the cation in 3 is shown in Fig.…”

“…B(SubPc)Cl is important as an intermediate in the formation of unsymmetrical phthalocyanines by ring expansion with a diiminoisoindoline [6,7]. Upon formation of the phthalocyanine, the boron atom is no longer bound to the macrocycle.…”

“…Upon formation of the phthalocyanine, the boron atom is no longer bound to the macrocycle. It has been proposed that the B(SubPc) + cation, formed by loss of chloride from 1, precedes the ring expansion reaction [7]. We have therefore targeted the synthesis of this cation using a very weakly coordinating counterion.…”

Synthetic approaches to asymmetric phthalocyanines and their analogues

“…In 1988 and then in 1990, Ando and Mori 99,184 as well as Kobayashi and co-workers in 1990 100 found that SubPcs undergo ring expansion reaction when treated with 1,3-diaminoisoindoline or its analogues, resulting in the metal-free asymmetric A 3 B phthalocyanines (Schemes 14-15). 99,100,[184][185][186][187][188][189][190][191][192][193][194][195][196][197][198][199][200][201][202][203] …”

“…188,195 The proposed reaction mechanism requires the cleavage of the SubPc ring, extrusion of the central boron atom, and further cyclization of the resulting open phthalonitrile trimer with the available 1,3-diiminiisoindoline. One of the initial reports 100 claimed that such a ring expansion reaction is highly selective and the asymmetric A 3 B phthalocyanine is the only major product.…”