UO2 2+-mediated ring contraction of pyrihexaphyrin: synthesis of a contracted expanded porphyrin-uranyl complex

Abstract

A new mixed hexaphyrin, pyrihexaphyrin (0.1.0.0.1.0) (1), was prepared via an acid catalyzed cyclization between 5,5'-(pyridine-2,6-diyl)bis(pyrrole-2-carbaldehyde) (2) and terpyrrole (3). This expanded porphyrin undergoes a ring contraction upon metallation with uranyl silylamide [UO₂[N(SiMe₃)₂]₂] under anaerobic conditions followed by purification over basic aluminum oxide exposed to air. The uranyl-contracted pyrihexaphyrin (0.0.0.0.1.0) complex (4) produced as a result contains a unique structural architecture and possesses a formally 22 π-electron globally aromatic periphery, as inferred from NMR spectroscopy, single crystal X-ray diffraction, and computational analyses. Support for the proposed contraction mechanism came from experimental data and DFT calculations. Proton NMR and mass spectroscopic analysis provided the first insight into expanded porphyrin-mediated activation of the uranyl dication (UO₂ ²⁺).

Fig. 1. The core scaffold of porphyrin, corrole, and amethyrin with key structural components highlighted [dipyrromethane (blue), bipyrrole (red), terpyrrole (purple)].

Scheme 1. Ring contraction sequence in the anaerobic biosynthesis of cobalamin.

Scheme 2. Synthesis of pyrihexaphyrin (1).

Fig. 2. Stacked (a) UV-vis spectra in CHCl₃ and (b) CV of 1 and 4 ([c] = 1 mM) in dry acetonitrile containing 0.1 M [(n-Bu)₄N][ClO₄] as the electrolyte. All potentials are reported relative to the Fc/Fc⁺ couple.

Scheme 3. Synthesis of expanded corrole–uranyl complex (4).

Fig. 3. ORTEP plot (thermal ellipsoids set at 50% probability) of the structures obtained from single crystal X-ray diffraction analyses of (a) 1 and (b) 4 viewed from top and side (upper and lower views, respectively). Hydrogen atoms are removed for clarity.

Scheme 4. Proposed base-mediated mechanism for the observed ring contraction that yields pyrihexaphyrin (0.0.0.0.1.0)-uranyl complex 4.