ja409625x_si_003.cif (32.93 kB)
Direct Arylations for Study of the Air-Stable P‑Heterocyclic Biradical: From Wide Electronic Tuning to Characterization of the Localized Radicalic Electrons
dataset
posted on 2013-11-20, 00:00 authored by Shigekazu Ito, Yasuhiro Ueta, Trang
Thi Thu Ngo, Makoto Kobayashi, Daisuke Hashizume, Jun-ichi Nishida, Yoshiro Yamashita, Koichi MikamiWe have developed methods for installing
aryl substituents directly
on the phosphino groups of the 1,3-diphosphacyclobutane-2,4-diyl system.
The aryl substituents tuned the electronic and structural characteristics
of the biradical unit both in solution and in the solid state. 1-tert-butyl-2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobuten-4-yl
anion, prepared from phosphaalkyne (Mes*CP; Mes* = 2,4,6-tBu3C6H2) and t-butyllithium, was allowed to react with an electron-deficient N-heterocyclic
reagent. The corresponding N-heteroaryl-substituted P-heterocyclic
biradicals were produced via SNAr reactions. Biradicals
bearing perfluorinated pyridyl substituents exhibited photoabsorption
properties comparable to those of previously reported derivatives
because the highest occupied and lowest unoccupied molecular orbit
levels were reduced by a similar amount. In contrast, the triazine
substituent reduced the band gap of the biradical unit, and the large
red shift in the visible absorption and high oxidation potential were
further tuned via subsequent SNAr and Negishi coupling
reactions. The amino-substituted triazine structure provided a strongly
electron-donating biradical chromophore, which produced unique p-type
semiconducting behavior even though there was no obvious π-overlap
in the crystalline state. The single-electron transfer reaction involving
Mes*CP, phenyllithium, and iodine afforded 1,3-diphenyl-2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl via
the intermediate P-heterocyclic monoradical. The tetraaryl-substituted
symmetric biradical product was used to determine the electron density
distribution from the X-ray diffraction data. The data show highly
localized radicalic electrons around the skeletal carbon atoms, moderately
polarized skeletal P–C bonds in the four-membered ring, and
no covalent transannular interaction.