Structural characterization of four members of the electron-transfer series [PdII(L)2)2]n (L = o-iminophenolate derivative; n= 2-, 1-, 0, 1+, 2+).: Ligand mixed valency in the monocation and monoanion with S=1/2 ground states

The reaction of the ligand 2-(2-trifluoromethyl)anilino-4,6-di-tert-butylphenol, H-2(L-1(IP)), and PdCl2 (2:1) in the presence of air and excess NEt3 in CH2Cl2 produced blue-green crystals of diamagnetic [Pd-II(L-1(ISQ))(2)] (1), where (L-1(ISQ))(center dot-) represents the o-iminobenzosemiquinonate(1-) pi radical anion of the aromatic (L-1(IP))(2-) dianion. The diamagnetic complex 1 was chemically oxidized with 1 equiv of Ag(BF4), affording red-brown crystals of paramagnetic (S = (1)/(2)) [Pd-II(L-1(ISQ))(L-1(IBQ))](BF4) (2), and one-electron reduction with cobaltocene yielded paramagnetic (S = (1)/(2)) green crystals of [Cp2Co][Pd-II(L-1(ISQ))(L-1(IP))] (3); (L-1(IBQ))(0) represents the neutral, diamagnetic quinone form. Complex 1 was oxidized with 2 equiv of [NO]BF4, affording green crystals of diamagnetic [Pd-II(L-1(IBQ))(2)](3)(BF4)(4){(BF4)(2)H}(2)(.)4CH(2)Cl(2) (5). Oxidation of [Ni-II(L-1(ISQ))(2)] (S = 0) in CH2Cl2 solution with 2 equiv of Ag(ClO4) generated crystals of [Ni-II(L-1(IBQ))(2)(ClO4)(2)](.)2CH(2)Cl(2) (6) with an S = 1 ground state. Complexes 1-5 constitute a five-membered complete electron-transfer series, [Pd(L-1)(2)](n) (n = 2-, 1-, 0, 1+, 2+), where only species 4, namely, diamagnetic [Pd-II(L-1(IP))(2)](2-), has not been isolated; they are interrelated by four reversible one-electron-transfer waves in the cyclic voltammogram. Complexes 1, 2, 3, 5, and 6 have been characterized by X-ray crystallography at 100 K, which establishes that the redox processes are ligand centered. Species 2 and 3 exhibit ligand mixed valency: [Pd-II(L-1(ISQ))(L-1(IBQ))](+) has localized (L-1(IBQ))(0) and (L-1(ISQ))(center dot-) ligands in the solid state, whereas in [Pd-II(L-1(ISQ))(L-1(IP))](-) the excess electron is delocalized over both ligands in the solid-state structure of 3. Electronic and electron spin resonance spectra are reported, and the electronic structures of all members of this electron-transfer series are established.