Communication pubs.acs.org/IC
Naphthylbipyrrole-Containing Amethyrin Analogue: A New Ligand for the Uranyl (UO22+) Cation Gonzalo Anguera,† James T. Brewster, II,† Matthew D. Moore, Juhoon Lee, Gabriela I. Vargas-Zúñiga, Hadiqa Zafar, Vincent M. Lynch, and Jonathan L. Sessler* Department of Chemistry, The University of Texas at Austin, 105 East 24th Street, Stop A5300, Austin, Texas 78712-1224, United States S Supporting Information *
whether putative cation coordination would give rise to equally dramatic color changes. As detailed below, the new naphthoincorporated expanded porphyrin (3) allows the formation of a bench-stable uranyl complex (7) concurrent with a formal twoelectron oxidation of the ligand. The net results are easy-todiscern changes in the spectroscopic features (and color) of the system.
ABSTRACT: Using naphthobipyrrole as a functional building block, a new expanded porphyrin, naphthoisoamethyrin, was prepared in 85% yield under acid-catalyzed [4 + 2] MacDonald coupling conditions. Treatment of naphthoisoamethyrin with the nonaqueous uranyl silylamide salt [UO2[N(SiMe3)2]2·2THF] yielded the corresponding uranyl complex. Upon metalation, naphthoisoamethyrin undergoes a two-electron oxidation to yield a formal 22 π-electron aromatic species, as inferred from 1H NMR and UV−vis spectroscopy, as well as cyclic voltammetry.
N
The synthesis of 3 is summarized in Scheme 1. Briefly, quaterpyrrole (5)21 was condensed with naphthobipyrrole
ew expanded porphyrins are of interest in terms of understanding the electrochemical features of oligopyrroles and the determinants of aromaticity. They can also act as ligands to stabilize metal complexes whose structures can differ from those of the better-studied porphyrins and, separately, have been explored for use in photodynamic and radiation therapy, among other application areas.1−5 Common methods employed to prepare expanded porphyrins include the condensation of an aldehyde with pyrrole or a pyrrolic precursor (e.g., MacDonald and Rothemund cyclization),6,7 oxidative cyclization,8−11 and metal-catalyzed or -mediated cyclization (e.g., nickel-mediated, Suzuki cyclization, or McMurry coupling).12,13 Extensive efforts have also been made to modify various expanded porphyrin cores. Naphthobipyrrole has proven to be useful in this regard; it gives rise to more rigid systems that are typically characterized by red-shifted UV−vis absorption spectral features.14−17 Here, we report the use of a [4 + 2] MacDonald coupling protocol to generate a new naphthobipyrrole-based expanded porphyrin, namely, naphthoisoamethyrin (3). We also report its conversion to a bench-stable uranyl complex (7). Previous work on related hexaphyrin systems (e.g., amethyrin (1) and isoamethyrin (2)) has established the versatility of these frameworks in coordinating disparate metal cations in a number of distinct coordination modes.7,10,18,19 In particular, these and other 1-based macrocycles have been shown to coordinate the high-valent uranyl (UO22+) dication. In many cases, significant changes in the absorption spectral features are seen across the UV and visible ranges as a result of a change in the ligand oxidation state.8,20 However, the systems studied to date have all been relatively flexible. We thus sought to explore whether analogues bearing a rigidifying naphthobipyrrole element could serve as a similar scaffold for actinyl cation complexation and © 2017 American Chemical Society
Scheme 1. Synthesis of 3 and the Uranylisoamethyrin Complex 7 from 5 and 6
dialdehyde (6)16 under acid-catalyzed MacDonald conditions to give 3 in 85% yield after flash column chromatography purification.22 Compound 3 was characterized via standard spectroscopic means (cf. electrospray ionization). The UV−vis spectrum of 3 is characterized by a broad absorption feature extending up to 610 nm. This stands in contrast to what is seen in the case of the previously reported 2, which is characterized by absorbance bands extending only to ca. 580 nm (Figure 1).8 However, the absorption coefficient for the macrocycle 3 is lower (418 nm; ε = 4.5 × 104 M−1·cm−1) than those of the previously reported isoamethyrins.8,22 These spectral differences are ascribed to the presence of the rigidifying naphthyl moiety. Single crystals suitable for X-ray diffraction analysis were grown via the slow evaporation of CHCl3/hexanes (1:1, v/v). As can be seen from an inspection of Figure 2, the metal-free ligand 3 Received: July 3, 2017 Published: August 7, 2017 9409
DOI: 10.1021/acs.inorgchem.7b01668 Inorg. Chem. 2017, 56, 9409−9412
Communication
Inorganic Chemistry
normal laboratory conditions, the complex was isolated in a 75% yield. When uranyl acetate dihydrate [UO2(OAc)2·2H2O] was used, complex 7 was obtained in ca. 18% yield. The addition of exogenous water to the reaction with uranyl acetate (H2O/ CH3OH/DCM, 1:1:1, v/v/v) resulted in diminished yields (
