Europium(II) and Ytterbium(II) Cyclic Organohydroborates with Agostic Interactions

The divalent lanthanide bis((cyclooctane-1,5-diyl)dihydroborate) complexes {K(THF)4}2{Ln{(μ-H)2BC8H14}4} (Ln
= Eu, 3; Yb, 4) were prepared by a metathesis reaction between (THF)xLnCl2 and K[H2BC8H14] in THF in a 1:4
molar ratio. Although the reaction ratios were varied between 1:3 and 1:6, complexes 3 and 4 were the only
lanthanide 9-BBN hydroborates produced. Because of disorder of THF in crystals of 3 and 4, good single-crystal
X-ray structural data could not be obtained. However, when the potassium cation was replaced by the
tetramethylammonium cation or when MeTHF (2-methyltetrahydrofuran) was employed in place of THF, good quality
crystals were obtained. Complexes [NMe4]2[Ln{(μ-H)2BC8H14}4] (Ln = Eu, 5; Yb, 6) were afforded by metathesis
reactions of NMe4Cl with 3 and 4 in situ. On the basis of the single-crystal X-ray diffraction analysis, the four
9-BBN tetrahydroborate ligands are tetrahedrally arranged around the lanthanide cation in 5 and 6. The two structures
differ in that one α-C−H bond from each of the four {(μ-H)2BC8H14}4 units exhibits an agostic interaction with
Eu(II) in 5 but, in complex 6, only two of the α-C−H bonds form agostic interactions with Yb(II). Complexes
{K(MeTHF)3}2{Ln{(μ-H)2BC8H14}4} (Ln = Eu, 7; Yb, 8) were produced by employing MeTHF in place of THF.
The structures of 7 and 8 display connectivity between the anion {Ln{(μ-H)2BC8H14}4}2- and the cation
{K(MeTHF)3}+, in which the potassium not only interacts directly with the hydrogens of the Ln−H−B bridged bonds
but is also involved in agostic interactions with α-C−H bonds. By systematically examining the structures of complexes
3−8 and taking into account the previously reported complexes (THF)4Ln{(μ-H)2BC8H14}2 (Ln = Eu, 1; Yb, 2), it
is concluded that Eu(II) appears to have a better ability to form agostic interactions than Yb(II) because of its larger
size, even though Yb(II) has a higher positive charge density.