Synthesis, structure and properties of benzoic acids bearing para- or meta-imino nitroxides or ortho-nitronyl nitroxide radical centres

Abstract

2-(4-Carboxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-yloxyl ( p-IMBAH), 2-(3-carboxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-yloxyl (m-IMBAH) and 2-(2-carboxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-3-oxido-1H-imidazol-3-ium-1-yloxyl (o-NITBAH) have been prepared. The m-IMBAH and p-IMBAH radicals crystallised in monoclinic space group P2₁/n while the o-NITBAH radical occupied the monoclinic space group Cc. In p-IMBAH, a 12-membered ring is formed via strong hydrogen bonds between two water molecules and two radicals, this unit being propagated along the b axis via additional hydrogen bonds. In m-IMBAH, the radicals are arranged in dimers held together by strong hydrogen bonds between the acid and the imine functions, each dimer being connected with its neighbours via hydrogen bonds forming alternating chains along the c axis. No cyclic arrangements are evident in o-NITBAH, the radicals being organized into infinite chains. In each case, the product of paramagnetic susceptibility with T lies in the 0.362 to 0.374 emu K mol^–1 range at room temperature while the reciprocal susceptibility follows a Curie-Weiss law above 20 K with θ=+1.38, +5.24 and –0.81 K, respectively, for p-IMBAH, m-IMBAH and o-NITBAH. The isotropic ¹⁴N hyperfine coupling constants are found from solution experiments to be 9.20 (a_N1) and 4.28 (a_N2) G for the imino radicals and 7.59 G for the nitronyl nitroxide radical. The principal values of the g-tensor measured on single crystals afforded an average of 2.0066/7 while in solution g_iso=2.0060 for p/m-IMBAH and 2.0065 for o-NITBAH.

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