Phenoxyl radical complexes of chromium(III), manganese(III), cobalt(III), and nickel(II)

The tetradentate monoanionic macrocycles 1,4-dimethyl-7-(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane, H(L1), and 1,4-di-iso-propyl-7-(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, H(L2), form in the presence of a bidentate coligand 1,3-diphenyl-1,3-propanedionate, Ph2acac, 1,3-dimethyl-1,3-propanedionate, acac, or 1,3-di-tert-butyl-1,3-dionate, Bu2acac−, very stable octahedral complexes with di- or trivalent metal ions: [MIII(L1)(Ph2acac)](ClO4) M=CoIII (1), CrIII (3), MnIII (7), GaIII (11); [MII(L1)(Ph2acac)] M=NiII (9); [MIII(L2)(Bu2acac)](ClO4) M=CoIII (2), CrIII (4), MnIII (8), GaIII (12), [MII(L1)(Bu2acac)] M=NiII (10); [CrIII(L2)(acac)](ClO4) (5), [CrIII(L2)(C2O4)] (6). All of these monophenolatometal complexes can be electrochemically, reversibly one-electron oxidized yielding stable phenoxyl radical complexes in solution. Electronic, resonance Raman and EPR spectra prove that the phenoxyl ligand is coordinated to the corresponding metal ion. The electronic ground states of complexes containing a paramagnetic transition metal ion with dn electron configuration and a bound phenoxyl radical (S=1/2) are attained via an intramolecular anti- or ferromagnetic exchange coupling mechanism the nature of which depends on the actual dn configuration of the central metal ion: a half-filled t2g subshell allows antiferromagnetic coupling, for example [3]radical dot2+, [4]radical dot2+, [5]radical dot2+ and [6]radical dot+ have an St=1 ground state. In contrast, unpaired electrons in an eg subshell enforce a ferromagnetic coupling: [9]radical dot2+, [10]radical dot2+ have an St=3/2 ground state. The crystal structures of 1, 5, and 9 are reported.