Reduced 2,2’‐Bipyridine Lanthanide Metallocenes Provide Access to Mono‐C5Me5 and Polyazide Complexes

Exploration of the multielectron reduction chemistry of the lanthanide complexes Cp*₂LnCl(bipy), Cp*₂Ln(bipy), and [Cp*₂Ln(bipy)]¹⁻ highlights the unusual reactivity available from this redox-active platform. A variety of unusual polysulfide, polyazide, and mixed-valence complexes are accessible, many of which are formed through the loss of a Cp* ligand and the generation of mono-Cp* complexes.

Abstract

Exploration of the reduction chemistry of the 2,2’-bipyridine (bipy) lanthanide metallocene complexes Cp*₂LnCl(bipy) and Cp*₂Ln(bipy) (Cp* = C₅Me₅) resulted in the isolation of a series of complexes with unusual composition and structure including complexes with a single Cp* ligand, multiple azide ligands, and bipy ligands with close parallel orientations. These results not only reveal new structural types, but they also show the diverse chemistry displayed by this redox-active platform. Treatment of Cp*₂NdCl(bipy) with excess KC₈ resulted in the formation of the mono-Cp* Nd(III) complex, [K(crypt)]₂[Cp*Nd(bipy)₂], 1, as well as [K(crypt)][Cp*₂NdCl₂], 2, and the previously reported [K(crypt)][Cp*₂Nd(bipy)]. A mono-Cp* Lu(III) complex, Cp*Lu(bipy)₂, 3, was also found in an attempt to make Cp*₂Lu(bipy) from LuCl₃, 2 equiv. of KCp*, bipy, and K/KI. Surprisingly, the (bipy)¹⁻ ligands in neighboring molecules in the structure of 3 are oriented in a parallel fashion with intermolecular C⋅⋅⋅C distances of 3.289(4) Å, which are shorter than the sum of van der Waals radii of two carbon atoms, 3.4 Å. Another product with one Cp* ligand per lanthanide was isolated from the reaction of [K(crypt)][Cp*₂Eu(bipy)] with azobenzene, which afforded the dimeric Eu(II) complex, [K(crypt)]₂[Cp*Eu(THF)(PhNNPh)]₂, 4. Attempts to make 4 from the reaction between Cp*₂Eu(THF)₂ and a reduced azobenzene anion generated instead the mixed-valent Eu(III)/Eu(II) complex, [K(crypt)][Cp*Eu(THF)(PhNNPh)]₂, 5, which allows direct comparison with the bimetallic Eu(II) complex 4. Mono-Cp* complexes of Yb(III) are obtained from reactions of the Yb(II) complex, [K(crypt)][Cp*₂Yb(bipy)], with trimethylsilylazide, which afforded the tetra-azido [K(crypt)]₂[Cp*Yb(N₃)₄], 6, or the di-azido complex [K(crypt)]₂[Cp*Yb(N₃)₂(bipy)], 7 a, depending on the reaction stoichiometry. A mono-Cp* Yb(III) complex is also isolated from reaction of [K(crypt)][Cp*₂Yb(bipy)] with elemental sulfur which forms the mixed polysulfido Yb(III) complex [K(crypt)]₂[Cp*Yb(S₄)(S₅)], 8 a. In contrast to these reactions that form mono-Cp* products, reduction of Cp*₂Yb(bipy) with 1 equiv. of KC₈ in the presence of 18-crown-6 resulted in the complete loss of Cp* ligands and the formation of [K(18-c-6)(THF)][Yb(bipy)₄], 9. The (bipy)¹⁻ ligands of 9 are arranged in a parallel orientation, as observed in the structure of 3, except in this case this interaction is intramolecular and involves pairs of ligands bound to the same Yb atom. Attempts to reduce further the Sm(II) (bipy)¹⁻ complex, Cp*₂Sm(bipy) with 2 equiv. of KC₈ in the presence of excess 18-crown-6 led to the isolation of a Sm(III) salt of (bipy)²⁻ with an inverse sandwich Cp* counter-cation and a co-crystallized K(18-c-6)Cp* unit, [K₂(18-c-6)₂Cp*]₂[Cp*₂Sm(bipy)]₂ ⋅ [K(18-c-6)Cp*], 10.

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