Cyclo[4]pyrrole with α- β direct linkages

Abstract

A cyclo[4]pyrrole bearing pyrrole C(α)-C(β) direct linkages, a contracted porphyrin analogue with no meso-carbon bridge, was synthesized from an oligoketone-related precursor. X-ray crystallography and StrainViz analysis revealed a non-planar structure with a total strain of 20.8 kcal mol^-1. The cyclo[4]pyrrole emits fluorescence in the visible region with a quantum yield of 0.026. The NICS calculations indicated a local 6π-aromatic character for each pyrrole unit, and the global π-electronic communication among the four pyrrole units was shown by the frontier orbitals in the neutral form. The cyclo[4]pyrrole underwent reversible stepwise electrochemical two-electron oxidation and the spin density of the radical cation intermediate localized on the 3,2':5',3''-terpyrrole moiety. However, spectroelectrochemical measurements and theoretical calculations indicated the contribution of a triplet diradical dication form with the spin density delocalized across the four pyrrole units and smaller dihedral angles between neighboring aromatic rings. The structural and electrochemical behavior of cyclo[4]pyrrole demonstrated the effects of ring contraction and recombination of the pyrrole-pyrrole linkages on the cyclo[n]pyrrole system.

Fig. 1. Cyclo[n]pyrroles and analogous macrocycles.

Scheme 1. Synthesis of cyclo[4]pyrrole 3.

Fig. 2. X-ray crystal structures of (a) 3 and (b) 10. (Left: top view; right: side view; thermal ellipsoids are set at the 50% probability; N-butyl groups are omitted for clarity in the side views). (c) Deformation angles α and β for the colored pyrrole rings in 3, 4 and 10. α is the averaged deviation angle between the mean planes of N–C(3)–C(4) and N–C(2)–C(3), and β is the averaged deviation angle between the mean plane N–C(2)–C(3) and a C(2)–C(external) bond.

Fig. 3. Visualization of the total strain of (a) 3, (b) 4 and (c) 10 using StrainViz analysis at the M06-2X/6-311+G(2d,p) level. (d) Calculated total, bond, angle and dihedral strain energies (in kcal mol⁻¹) of 3, 4 and 10.

Fig. 4. (a) UV-vis absorption (black) and fluorescence (red) spectra of 3 in acetonitrile. (b) Cyclic voltammogram of 3 in CH₂Cl₂.

Fig. 5. Frontier orbitals of 4 (left), 3 (middle) and 10 (right) calculated at the B3LYP/6-311G(d,p) level.

Fig. 6. (a) UV-vis absorption spectra during the electrochemical oxidation of 3 in CH₂Cl₂. (b) Spin density distribution plots for 3˙⁺ (left) and the triplet state of 3²⁺ (right) with an isovalue of 0.001.