Tris(pentamethylcyclopentadienyl) Complexes of Late Lanthanides Tb, Dy, Ho, and Er: Solution and Mechanochemical Syntheses and Structural Comparisons

The tris­(pentamethylcyclopentadienyl)
complexes, (C5Me5)3Ln (Ln = Tb, Dy,
Ho, and Er), have been
synthesized to determine if structural trends observed with the larger
rare-earth metals, La–Gd, extend to the smaller metals and
to determine the level of steric crowding associated with C–H
bond activation of benzene and toluene as found for the (C5Me5)3Y complex of the smaller rare-earth metal,
yttrium. Three different synthetic methods were used. (C5Me5)3Tb was synthesized in benzene from (C5Me5)2Tb­(μ-Ph)2BPh2 and KC5Me5, which established that
it does not activate benzene. In contrast, complexes of Dy, Ho, and
Er were not isolable in benzene or toluene. (C5Me5)3Dy was synthesized in pentane from [(C5Me5)2DyH]2 and 1,2,3,4-tetramethylfulvene.
Solvent-free mechanochemistry was utilized to synthesize the Ho and
Er complexes by ball-milling (C5Me5)2Ln­(μ-Ph)2BPh2 and KC5Me5 as solids. This route also provided the Tb, Dy, and Y complexes.
Crystallographic analysis of these complexes of the smaller metals
has allowed identification of a feature in the structures of (C5Me5)3Ln that was less obvious with the
larger metals. The displacements of the methyl substituents out of
the cyclopentadienyl ring plane away from the metal do not change
substantially for complexes of the metals smaller than Gd, but the
cyclopentadienyl rings become increasingly less planar and continue
to tilt away from the metal with increasing angles.