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

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-η 5 :η 5 -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-η 1 :η 1 -N 2 )Re(η 5 -Cp)(BDI)][(μ-η 5 :η 5 -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 (η 8 -COT)Er[(μ-η 1 :η 1 -N 2 )Re(η 5 -Cp)(BDI)](THF)(Et 2 O) ( 3-Er ) and uranium-rhenium complex [Na(2.2.2-cryptand)][(η 5 -C 5 H 4 SiMe 3 ) 3 U(μ-η 1 :η 1 -N 2 )Re(η 5 -Cp)(BDI)] ( 4-U ). Finally, we expand the scope of rhenium inverse sandwich coordination by synthesizing divalent double inverse sandwich complex Yb[(μ-η 5 :η 5 -Cp)Re(BDI)] 2 (THF) 2 ( 5-Yb ), as well as base-free, homoleptic rhenium-rare earth triple inverse sandwich complex Y[(μ-η 5 :η 5 -Cp)Re(BDI)] 3 ( 6-Y ).