Synthesis, structures, and one- or two-electron reduction reactivity of mononuclear lanthanide (Ho, Dy) complexes with sterically hindered o-iminobenzoquinone ligands

Abstract

Complexes of Dy and Ho (Ln) with redox-active 4,6-di-t-butyl-N-(2,6-di-isopropylphenyl)-o-iminobenzoquinone ligand (dippIQ) in different charged states have been synthesized and investigated. Besides the neutral state, the ligand is comprised as iminosemiquinolate (dippISQ–) or amidophenolate (dippAP2–). Homoligand complexes [Ln(dippIQ)2I3] (1Ln), [Ln(dippISQ)2I(thf)2] (2Ln, thf = tetrahydrofurane) and [KLn(dippAP)2(thf)4] (3Ln) have been obtained by earlier known methods that include complexation (for 1Ln) followed by reduction with 2 or 4 equivalents of KC8 (for 2Ln and 3Ln, respectively). It has been shown that upon reduction by 3 equivalents of KC8 mixed-ligand complexes [Ln(dippAP)(dippISQ)(thf)2] (4Ln) are formed. Simple synthetic methods to the high-yield syntheses of 3Ln and 4Ln have been developed, based on excessive reduction by K metal (for the former complex), followed by partial oxidation by I2 (for the latter one). In addition to the known reactivity of 3Ln towards S8 as a 2e reductant, 4Ln was shown to act as 1e reductants, according to the number of reducing dippAP2– ligands. The reduction by 3Ln results in inclusion of S52– anion in the coordination sphere, resulting in the known type [Ln(dippISQ)2(S5)]– complexes, crystallized with the [K(18-Crown-6)(thf)2]+ cation (6Ln). Unlike that, the reduction by 4Ln results in inclusion of a chain of two sulfur atoms with the formation of new type of ligand (dippAP)S22– in a complex [Ln(dippISQ){(dippAP)S2}(thf)2] (7Ln). Structures of all complexes are compared on the basis of X-ray diffraction and IR spectroscopy data. Reaction routes leading to the complexes of different types are discussed.