Triple Inverse Sandwich versus End-On Diazenido: Bonding Motifs across a Series of Rhenium–Lanthanide and –Actinide Complexes

Abstract

While synthesizing a series of rhenium–lanthanide triple inverse sandwich complexes, we unexpectedly uncovered evidence for rare examples of end-on lanthanide dinitrogen coordination for certain heavy lanthanide elements as well as for uranium. We begin our report with the synthesis and characterization of a series of trirhenium triple inverse sandwich complexes with the early lanthanides, Ln­[(μ-η5:η5-Cp)­Re­(BDI)]3(THF) (1-Ln, Ln = La, Ce, Pr, Nd, Sm; Cp = cyclopentadienide, BDI = N,N′-bis­(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate). However, as we moved across the lanthanide series, we ran into an unexpected result for gadolinium in which we structurally characterized two products for gadolinium, namely, 1-Gd (analogous to 1-Ln) and a diazenido dirhenium double inverse sandwich complex Gd­[(μ-η1:η1-N2)­Re­(η5-Cp)­(BDI)]­[(μ-η5:η5-Cp)­Re­(BDI)]2(THF)2 (2-Gd). Evidence for analogues of 2-Gd was spectroscopically observed for other heavy lanthanides (2-Ln, Ln = Tb, Dy, Er), and, in the case of 2-Er, structurally authenticated. These complexes represent the first observed examples of heterobimetallic end-on lanthanide dinitrogen coordination. Density functional theory (DFT) calculations were utilized to probe relevant bonding interactions and reveal energetic differences between both the experimental and putative 1-Ln and 2-Ln complexes. We also present additional examples of novel end-on heterobimetallic lanthanide and actinide diazenido moieties in the erbium–rhenium complex (η8-COT)­Er­[(μ-η1:η1-N2)­Re­(η5-Cp)­(BDI)]­(THF)­(Et2O) (3-Er) and uranium–rhenium complex [Na­(2.2.2-cryptand)]­[(η5-C5H4SiMe3)3U­(μ-η1:η1-N2)­Re­(η5-Cp)­(BDI)] (4-U). Finally, we expand the scope of rhenium inverse sandwich coordination by synthesizing divalent double inverse sandwich complex Yb­[(μ-η5:η5-Cp)­Re­(BDI)]2(THF)2 (5-Yb), as well as base-free, homoleptic rhenium–rare earth triple inverse sandwich complex Y­[(μ-η5:η5-Cp)­Re­(BDI)]3 (6-Y).