Divalent Lanthanide Metallocene Complexes with a Linear Coordination Geometry and Pronounced 6s–5d Orbital Mixing

Abstract

A small but growing number of molecular compounds have been isolated featuring divalent lanthanides with 4f n 5d z 2 1 electron configurations. While the majority of these possess trigonal coordination geometries, we previously reported the first examples of linear divalent metallocenes Ln(Cp iPr5 ) 2 (Ln = Tb, Dy; Cp iPr5 = pentaisopropylcyclopentadienyl). Here, we report the synthesis and characterization of the remainder of the Ln(Cp iPr5 ) 2 (1-Ln) series (including Y and excluding Pm). The compounds can be synthesized through salt metathesis of LnI 3 and NaCp iPr5 followed by potassium graphite reduction for Ln = Y, La, Ce, Pr, Nd, Gd, Ho, and Er, by in situ reduction during salt metathesis of LnI 3 and NaCp iPr5 for Ln = Tm and Lu, or through salt metathesis from LnI 2 and NaCp iPr5 for Ln = Sm, Eu, and Yb. Single crystal X-ray diffraction analyses of 1-Ln confirm a linear coordination geometry with pseudo-D 5d symmetry for the entire series. Structural and ultraviolet−visible spectroscopy data support a 4f n+1 electron configuration for Ln 2+ = Sm, Eu, Tm, and Yb and a 4f n 5d z 2 1 configuration for the other lanthanides ([Kr]4d z2 1 for Y 2+ ). Characterization of 1-Ln (Ln = Y, La) using electron paramagnetic resonance spectroscopy reveals significant s−d orbital mixing in the highest occupied molecular orbital and hyperfine coupling constants that are the largest reported to date for divalent compounds of yttrium and lanthanum. Evaluation of the room temperature magnetic susceptibilities of 1-Ln and comparison with values previously reported for trigonal Ln 2+ compounds suggests that the more pronounced 6s−5d mixing may be associated with weaker 4f−5d spin coupling.

Synthesis and Crystallographic Characterization of a Reduced Bimetallic Yttrium ansa-Metallocene Hydride Complex, [K(crypt)][(μ-CpAn)Y(μ-H)]2 (CpAn = Me2Si[C5H3(SiMe3)-3]2), with a 3.4 Å Yttrium–Yttrium Distance

“…Complex 5 exhibited a room temperature χ M T value of 30.04 emu K mol –1 under an applied dc magnetic field of 0.1 T, which is only slightly lower than the predicted value of 31.18 emu K mol –1 for a dinuclear complex containing one Dy 3+ ion (4f 9 ) and one Dy 2+ ion with an L – S coupled 4f 9 (5d z 2 ) 1 electron configuration, Figure . The predicted room temperature χ M T value for an L – S coupled Dy 2+ ion was calculated using L = 5 and S = S 5d z 2 + S 4f = 3 . In contrast, the value for [(C 5 i Pr 5 )­Dy] 2 (μ-I) 3 was 51.77 emu K mol –1 , which is consistent with the parallel alignment of the electrons on both Dy centers as a result of a single-electron Dy–Dy bond.…”

“…The predicted room temperature χ M T value for an L−S coupled Dy 2+ ion was calculated using L = 5 and S = S 5dz2 + S 4f = 3. 48 In contrast, the value for [(C 5 i Pr 5 )Dy] 2 (μ-I) 3 was 51.77 emu K mol −1 , 47 which is consistent with the parallel alignment of the electrons on both Dy centers as a result of a singleelectron Dy−Dy bond. At low temperatures, a decline in the χ M T product of 5 is observed due to thermal depopulation of the M J manifolds of the Dy ions.…”

“…While this work was in progress, the use of 4d 1 and 4f n 5d 1 electron configurations to form Ln−Ln bonds has been reported in a mono-cyclopentadienyl iodide bridged bimetallic system, [(C 5 i Pr 5 )Ln] 2 (μ-I) 3 (Ln = Y, Gd, Tb, Dy). 47,48 ■ RESULTS…”

“…UV–visible and EPR spectroscopy as well as magnetic characterization support the presence of Ln­(II) species. While this work was in progress, the use of 4d 1 and 4f n 5d 1 electron configurations to form Ln–Ln bonds has been reported in a mono-cyclopentadienyl iodide bridged bimetallic system, [(C 5 i Pr 5 )­Ln] 2 (μ-I) 3 (Ln = Y, Gd, Tb, Dy). , …”

Divalent metallocenes of the lanthanides – a guideline to properties and reactivity

“…Although divalent metallocenes of Tm, 7 Dy, 8 and Nd 9 were established some years ago, metallocenes of most of the other rare earth metallocenes were published only very recently. 10,11 These newly accessible divalent lanthanides are mostly considered as the ''non-classical'' divalent compounds. This review focuses mainly on the ''classical'' divalent lanthanide metallocenes of Sm, Eu and Yb ligated by the permethylated cyclopentadienyl ligand C 5 Me 5 .…”