Exchange and double-exchange phenomena in linear homo- and heteronuclear nickel(II,III,IV) complexes containing six μ₂-phenolato or μ₂-thiophenolato bridging ligands

A series of homo- and heterotrinuclear complexes containing three face-sharing octahedra has been synthesized by using the pendent arm macrocyclic ligands 1,4,7-tris(3,5-dimethyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, L⁰H₃, and 1,4,7-tris(4-tert-butyl-2-mercaptobenzyl)-1,4,7-triazacyclononane, LH₃: [{L⁰Ni(II)}₂Ni(II)] (1) and [{LCo(III)}₂Co(III)](PF₆)₃ (2); [{LCo(III)}₂Ni](n+) (n = 2 (3), 3 (4), 4 (5)); [{LNi}₂Co(III)](n+) (n = 1 (6), 2 (7), 3 (8)) and its linkage isomers [{LNi}Ni{Co(III)L}](n+) (n = 1 (9), 2 (10), 3 (11)) and, finally, the complexes [{LNi}₂Ni](n+) (n = 0 (12), 1 (13), 2 (14), 3 (15)). In complex 1 three octahedral Ni(II) ions form a linear array with two terminal [L⁰Ni(II)>]^- moieties in a facial N₃O₃ donor set and a central Ni(II) ion which is connected to the terminal ions via six phenolate bridging pendent arms of L⁰. In complexes 2-15 the three metal ions are always in the same ligand matrix yielding an N₃M(μ-S)₃M(μ-S)₃MN₃ first-coordination sphere regardless of the nature of the metal ions (nickel or cobalt) or their formal oxidation states. From temperature dependent magnetic susceptibility measurements it has been determined that 1 has an S = 3 ground state whereas in 12 it is S = 1. In order to understand this difference in exchange coupling (ferromagnetic in 1 and antiferromagnetic in 12) in two apparently very similar complexes the magnetic properties of 2-15 have been investigated. Complex 3 has an S = 1 and 4 an S = 1/4 , and 5 is diamagnetic (S = 0) as is its isoelectronic counterpart 2. This indicates the availability of the oxidation states II, III, and IV of the central NiS₆ unit. In the isostructural complexes 6, 7, and 8, two terminal nickel ions are bridged by a central diamagnetic Co(III). The exchange coupling between two terminal paramagnetic nickel ions was studied as a function of their formal oxidation state. In 6 the two Ni(II) ions are ferromagnetically coupled (S = 2); the mixed-valent Ni(II)Ni(III) species 7 has an S = ³/₂ ground state and in 8 most probably two Ni(III) ions (d⁷ low spin) give rise to an S = 1 ground state. In contrast, in the series 9, 10, and 11 where two nickel ions are in a position adjacent to each other 9 has an S = 0 (antiferromagnetic coupling), but in the mixed-valent complex 10 an S = 3/2 ground state (ferromagnetic coupling) is observed. In 11 an S = 1 ground state prevails which may be achieved by ferromagnetic coupling between two Ni(III) ions. For the trinuclear nickel complexes 12-15 an S = 1 ground state has been determined for 12, an S = 3/2 for the mixed valent complex 13, and an S = 2 for 14, and 15 exhibits an S = 3/2 ground state. The Goodenough-Kanamori rules do not provide a consistent explanation for the observed ground states in all cases. The concept of double exchange, originally introduced by Zener in 1951, appears to provide a more appropriate description for the mixed-valent species 7, 10, 13, 14, and 15. This picture is corroborated by the electrochemistry and EPR spectroscopy of complexes.