Nitronyl and imino nitroxides: improvement of Ullman's procedure and report on a new efficient synthetic route.

The synthesis of nitronyl and imino nitroxides has been reexamined with the aim of both increasing yields and of offering opportunities for new structures. The conditions for the formation of 2,3-bis(hydroxyamino)-2,3-dimethylbutane, the key intermediate of Ullman's route, have been carefully studied, and a new procedure is proposed, which affords the free base in a very pure form and up to 60% yield. Full characterization of this intermediate including an X-ray crystal structure is presented. An alternative synthetic route through 2,3-diamino-2,3-dimethylbutane and the corresponding imidazolidines which bypasses the delicate synthesis of the bis(hydroxyamino) compound is described. It is shown that 3-chloroperbenzoic acid is an effective oxidant for the transformation of adequately substituted imidazolidines into nitronyl nitroxides, which are obtained in high yield. An illustration of the potentialities of this new route, a new nitronyl nitroxide with two ethyl substituents in positions 4 and 5 of the imidazoline ring, is reported. The scope and limitations of the two routes are discussed.

Antiferromagnetic Coupling in a Gadolinium(III) Semiquinonato Complex.

The strongest antiferromagnetic coupling to Gd(III) so far reported was found for the complex [Gd(Hbpz(3))(2)(dtbsq)] small middle dot2 CHCl(3) (1; Hbpz(3)=hydrotris(pyrazolyl)borate; dtbsq=3,5-di-tert-butylsemiquinonato; see structure). At 245 K the magnetic susceptibility of 1 is lower than expected for two uncorrelated spins of 7/2 and 1/2, and the lowering of chiT with increasing temperature suggests that this is due to antiferromagnetic interaction between Gd(III) and the radical.

Synthesis and structure of a complex having a quartet ground state with three entirely different spin carriers: nitronyl nitroxide, o-semiquinone, and CuII.

A "spin diverse" S = 3/2 ground-state complex, NN-SQCuTpCum,Me, has been prepared. The three S = 1/2 spin carriers are nitronyl nitroxide (NN), o-semiquinone (o-SQ), and cupric ion. The solid-state structure of the ZnII derivative, NN-SQZnTpCum,Me (C56H69BN8O4Zn), was determined: monoclinic, P2(1)/c, a = 12.5781(12) A, b = 17.7408(17) A, c = 24.440(2) A, alpha = 90.00 degrees, beta = 98.240(2) degrees, gamma = 90.00 degrees, Z = 4. The results of X-ray structural characterization of the ZnII derivative suggest substantial interaction between the two spin carriers NN and o-SQ. Indeed, strong intramolecular exchange coupling has been determined by variable-temperature magnetic susceptibility studies. Intraligand ferromagnetic exchange is considerably greater than kT, such that only the triplet state is populated at 300 K, and CuII-ligand exchange is ferromagnetic, with J = +75 cm-1.