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[(mu-eta(5):eta(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-beta-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[(mu-eta(1):eta(1)-N-2)Re(eta(5)-Cp)(BDI)][(mu-eta(5):eta(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 (eta(8)-COT)Er[(mu-eta(1):eta(1)-N-2)Re(eta(5)-Cp)(BDI)](THF)(Et2O) (3-Er) and uranium-rhenium complex [Na(2.2.2-cryptand)][(eta(5)-C5H4SiMe3)(3)U(mu-eta(1):eta(1)-N-2)Re(eta(5)-Cp)(BDI)] (4-U). Finally, we expand the scope of rhenium inverse sandwich coordination by synthesizing divalent double inverse sandwich complex Yb[(mu-eta(5):eta(5)-Cp)Re(BDI)](2)(THF)(2) (5-Yb), as well as base-free, homoleptic rhenium-rare earth triple inverse sandwich complex Y[(mu-eta(5):eta(5)-Cp)Re(BDI)](3) (6-Y).