Issue 12, 2017

Effective stabilization of a planar phosphorus(iii) center embedded in a porphyrin-based fused aromatic skeleton

Abstract

Organophosphorus(III) compounds usually take on stable pyramidal structures with a large inversion barrier of 30–35 kcal mol−1. In contrast, diphenylphosphine-fused Ni(II) porphyrin, where the phosphorus atom is directly attached at the meso-position and embedded in a rigid skeleton, exhibits a considerably planarized phosphorus center. Here we report the synthesis of a mesityl-substituted Ni(II) porphyrin analogue, 6, which allowed an evaluation of the inversion barrier (ΔG203) by variable temperature 1H NMR spectroscopy which showed it to be exceptionally small, at 14.0 kcal mol−1. The observed small inversion barrier has been attributed to conformational constraint imposed by the fused structure. In addition, it was thought that the planar transition state is stabilized by the Ni(II) porphyrin network that allows the contribution of a 22π-aromatic circuit involving phosphorus lone-pair electrons. Along this postulate, we attempted to engineer diarylphosphine-fused porphyrins with smaller inversion barriers by replacing the fused benzene rings with five-membered heterocyclic rings such as thiophene, benzothiophene, benzofuran, indole, benzothiophene 1,1-dioxide, and thiophene 1,1-dioxide. In that order, the aromatic character of the heterocycle decreases, which leads to increasing contribution of the 22π-aromatic circuit. Actually, the inversion barrier of the phosphorus center becomes smaller in this order and reaches 8.7 kcal mol−1 for thiophene 1,1-dioxide-fused Ni(II) porphyrin 15, supporting the postulate.

Graphical abstract: Effective stabilization of a planar phosphorus(iii) center embedded in a porphyrin-based fused aromatic skeleton

Supplementary files

Article information

Article type
Edge Article
Submitted
05 Sept. 2017
Accepted
05 Okt. 2017
First published
06 Okt. 2017
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY-NC license

Chem. Sci., 2017,8, 8231-8239

Effective stabilization of a planar phosphorus(III) center embedded in a porphyrin-based fused aromatic skeleton

K. Fujimoto and A. Osuka, Chem. Sci., 2017, 8, 8231 DOI: 10.1039/C7SC03882H

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