Redox Studies of the Scandium Metallocene (C5H2tBu3)2ScII Lead to a Terminal Side-On (N═N)2– Complex: [(C5H2tBu3)2ScIII(η2-N2)]−

Abstract

Cyclic voltammetry measurements on the scandium(II) metallocene, Cp^ttt₂Sc^II (Cp^ttt = C₅H₂^tBu₃), reveal a −1.87 V vs Fc⁺/Fc event assigned to the [Cp^ttt₂Sc]^+/0 redox couple and a −3.09 V process assigned to the [Cp^ttt₂Sc]^0/– redox couple, which are consistent with subsequent chemical studies. Chemical oxidation of Cp^ttt₂Sc^II with AgBPh₄ gives the scandocenium cation salt [Cp^ttt₂Sc^III][BPh₄], 1. The [Cp^ttt₂Sc^III]⁺ cation does not coordinate the [BPh₄]⁻ anion or THF solvent; however, one of the methyl groups of a tert-butyl substituent has a close interaction with the Sc(III) ion. Decomposition occurs in the presence of a [ⁿBu₄N][PF₆] supporting electrolyte, and chemical studies indicate that 1 reacts with the [PF₆]⁻ anion by fluoride abstraction to form Cp^ttt₂Sc^IIIF and PF₅. Chemical reduction of 1 to Cp^ttt₂Sc^II proceeds with Cp*₂Sm^II and Cp*₂Co^II (Cp* = C₅Me₅), which is consistent with the measured reduction potentials. Cp^ttt₂Sc^II was not observed to react with N₂; however, the treatment of Cp^ttt₂Sc^II with KC₈ and 18-crown-6 (crown) under N₂ forms the (N═N)^2– complex, Cp^ttt₂Sc^III(μ-η²:η²-N₂)K(crown), 2. The reaction of Cp^ttt₂Sc^II with KC₈ in the presence of 2.2.2-cryptand (crypt) generates [K(crypt)][Cp^ttt₂Sc^III(η²-N₂)], 3, which is the first isolable complex of any metal with a terminal side-on (N═N)^2– ligand. These Ln^IIA₂/M/N₂ reactions (M = alkali metal; (A)⁻ = anion) are the Ln(II)/Ln(I) analogs of the previously reported Ln(III)/Ln(II) reactions, Ln^IIIA₃/M/N₂, in which reduction of Ln(III) complexes generates Ln(II) reactivity to form (N═N)^2– complexes.