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 d ivalent transition metal ions in the presence of air and triethylamine. Dep ending on the metal:ligand ratio (1:1, 1:2, or 1:3) and the presence (or ab sence) of the cyclic amine 1,4-dimethyl-1,4,7-triazacyclononane (dmtacn), t he 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 crystall ography at 100(2) K. For all complexes it is unambiguously established that the O,N-coordinated o-iminobenzosemiquinonato(1-) ligand is present. Compl exes 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 s pectroscopies 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 radic al. Complex 1 contains three mutually orthogonal (L-ISQ)(-.) ligands and ha s an S-t = 3/2 ground state. It is shown that the electronic structure of 4 a and 5 is adequately described as singlet diradical containing a divalent, diamagnetic d(8) configurated central metal ion and two strongly antiferro magnetically coupled (L-ISQ)(-) radical ligands. It is concluded that the s ame electronic structure prevails in the classic bis(o-diiminobenzosemiquin onato)- and bis(o-benzosemiquinonato)metal complexes of Ni-II, Pd-II, and P t-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 ha ve been characterized spectroelectrochemically. All redox processes are sho wn to be ligand-based.