Tris[p-(N-oxyl-N-tert-butylamino)phenyl]amine, -methyl, and -borane Have Doublet, Triplet, and Doublet Ground States, Respectively

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Abstract

N,N,N-Tris[p-(N-oxyl-tert-butylamino)phenyl]amine (N) was obtained as dark violet plates by lithiation of tris(4-bromophenyl)amine, followed by reaction with 2-methyl-2-nitrosopropane and oxidation of the resulting tris(hydroxyamine) with Ag2O. An X-ray crystal and molecular structure analysis of the monoclinic single crystal with space group Cc showed that it has neither C3 symmetry nor zwitterionic quinonoid structure. An EPR spectrum in MTHF solution at room temperature consisted of seven lines (aN = 4.06 G (3N) at g = 2.0058). The temperature dependence of the magnetic susceptibility data on N in the range 2−300 K was analyzed in terms of a triangular coupling model for three S = 1/2 spins to give a set of best-fit parameters which placed a doublet state as the ground state with a quartet state lying 559 K (= 1.11 kcal mol-1) above it. B3LYP/6-31G*//B3LYP/6-31G computations were performed on both spin states of a model molecule (N') in which all three N-tert-butyl groups in N were replaced with N-methyl groups. The doublet state was found to be more stable than the quartet state by 0.84 kcal mol-1. Tris[p-(N-oxyl-tert-butylamino)phenyl]methyl (C) gave a monoclinic single crystal with space group Cc (no. 9) and has been shown to have a benzoquinoneimine N-oxide type diradical structure. Temperature dependence of its magnetic susceptibility showed that C has a triplet ground state with a singlet state lying 410 K above the triplet. Reactions of the corresponding organolithium compounds with BF3·OEt2 were not applicable to give tris[4-(N-oxyl-tert-butylamino)phenyl]borane (B) but its 2,2',2' ',6,6',6' '-hexamethyl derivative (B'). In the trigonal crystal with space group R3̄ (no. 148), the three benzene rings are tilted by 49.2° with respect to the valence plane of the boron atom. The magnetic susceptibility data on B' were consistent with a doublet ground state with an energy gap to a quartet state by 9.9 K (= 0.0197 kcal mol-1). All the results together with ab initio MO studies are consistent with a picture that the ground spin states of title compounds N, C, and B are doublet, triplet, and doublet, respectively. The electronic structures are explained qualitatively by p-(N-oxyl-tert-butylamino)phenyl homologues of (•H2C)3X in which X = N, C·, and B atoms, and two, one, and no π-electrons are contained, respectively.

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