Electronic structure of bis(o-iminobenzosemiquinonato)metal complexes (Cu, Ni, Pd).: The art of establishing physical oxidation states in transition-metal complexes containing radical ligands

The ligand 2-anilino-4,6-di-tert-butylphenol and its 2-(3,5-dichloroanilino)-4,6-di-tert-butylphenol analogue react in CH3CN or CH3OH solutions with divalent transition metal ions in the presence of air and triethylamine. Depending on the metal:ligand ratio (1:1, 1:2, or 1:3) and the presence (or absence) of the cyclic amine 1,4-dimethyl-1,4,7-triazacyclononane (dmtacn), the following complexes have been isolated as crystalline solids: [Co-III(L-ISQ)(3)] (1); [Cu-II(dmtacn)(L-ISQ)]PF6 (2); [Cu-II(L-ISQ)(2)] (3); [Ni-II(L-ISQ)(2)] (4a); [Ni-II(L-Cl(ISQ))(2)] (4b); [Pd-II(L-ISQ)(2)] (5). (L-ISQ)(-) represents the monoanionic o-iminobenzosemiquinonate radical (S-rad = 1/2). Compounds 1-5 have been characterized by single-crystal X-ray crystallography at 100(2) K. For all complexes it is unambiguously established that the O,N-coordinated o-iminobenzosemiquinonato(1-) ligand is present. Complexes 3, 4b, and 5 are square planar molecules which possess an S-t = 1/2, 0, and 0 ground state, respectively, as was established by H-1 NMR and EPR spectroscopies and variable-temperature magnetic susceptibility measurements. Complex 2 possesses an S-t = 1 ground state which is attained via strong intramolecular ferromagnetic coupling (J = +195 cm(-1)) between the d(x2-y2) magnetic orbital of the Cu-II ion and the pi -orbital of the ligand radical. Complex 1 contains three mutually orthogonal (L-ISQ)(-.) ligands and has an S-t = 3/2 ground state. It is shown that the electronic structure of 4a and 5 is adequately described as singlet diradical containing a divalent, diamagnetic d(8) configurated central metal ion and two strongly antiferromagnetically coupled (L-ISQ)(-) radical ligands. It is concluded that the same electronic structure prevails in the classic bis(o-diiminobenzosemiquinonato)- and bis(o-benzosemiquinonato)metal complexes of Ni-II, Pd-II, and Pt-II. The electrochemistry of all complexes has been investigated in detail. For 3, la, and 5 a series of reversible one-electron-transfer waves leads to the formation of the anions and cations [M(L)(2)](2-,1-,1+,2+) which have been characterized spectroelectrochemically. All redox processes are shown to be ligand-based.