Stable organic radicals have received considerable attention recently, not only for purely scientific interest in their structures and properties, but also in applications, such as organic magnets, electronic devices, organic batteries, and optical materials. [1] To design stable organic radicals, kinetic stabilization by steric hindrance and delocalization of an unpaired electron by resonance through effective conjugation is essential to attain high stability. In most cases, the unpaired electron is delocalized on planar p-electronic systems, such as cyclopentadienyl and phenalenyl. Isolation of a stable organic radical on nonplanar p systems is quite rare. [2] However, such nonplanar stable organic radicals would have intriguing properties owing to unique features of their curved pelectronic systems.
Expanded porphyrins, which are porphyrin analogues with more than five pyrrolic subunits, have attracted much attention because of their fascinating optical, electrochemical, and coordination properties. [3] In particular, hexaphyrin(1.1.1.1.1.1) exhibits multi-redox behavior in which two stable oxidation states, namely [26]hexaphyrin 1 and [28]hexaphyrin 2, are easily interconvertible through two-electron oxidation and reduction (Scheme 1). [4] [26]Hexaphyrin 1 exhibits distinct aromaticity in the rigid and flat structure; in contrast, [28]hexaphyrin 2 exists as a dynamic mixture of predominantly Möbius aromatic conformations along with a minor Hückel-antiaromatic planar structure.
Although the metalation of 1 has been extensively examined, [5] free-base 2 remains unexplored in coordination chemistry. [6] Palladium metalation of 1 provides a Möbiusaromatic [28]hexaphyrin monopalladium complex 3. [5c] Consequently, we anticipated the formation of the same Möbiusaromatic complex 3 from 2 with palladium salts; however, this was not to be the case. Treatment of 2 with PdCl 2 in CH 2 Cl 2 / MeOH under a nitrogen atmosphere led to complex 4 in 40 % yield as a major product, along with 3 in 4 % yield (Scheme 2). [7] The new complex 4 was air-and moisturestable, both in solid and solution states. High-resolution electrospray-ionization time-of-flight (HR-ESI-TOF) mass measurement of 4 has the parent-ion peak at m/z = 1706.8417 ([M] À ; calcd for C 66 H 12 N 6 F 30 Pd 2 Cl À : 1706.8421), thus revealing the product is a bis(palladium) complex. However, the complex 4 did not exhibit any clear 1 H NMR spectroscopy Scheme 1. [26]Hexaphyrin and [28]hexaphyrin. Ar = C 6 F 5 .