Reactions of Scorpionate-Anchored Yttrium and Lutetium Dialkyls with Terminal Alkynes: From Bimetallic Complexes with Bridging Enynediyl Ligands to Monomeric Terminal Dialkynyl Complexes

The reactions of scorpionate-anchored yttrium and lutetium dialkyls
with terminal alkynes were investigated, and the nature of the complexes
was found to depend on both the ancillary scorpionate ligand and the
alkyne substituent. Reaction of (TpMe2)Ln(CH2SiMe3)2(THF) (1; Ln = Y, Lu) with
the terminal alkynes HCCR (R = Ph, SiMe3, tBu, adamantyl (Ad)) afforded dimeric complexes. The solid-state
structures, determined by single-crystal X-ray diffraction, showed
the two metal centers bridged by two asymmetric alkynyl ligands and
a coupled dialkynyl ligand, the latter exhibiting the unusual enyne
bonding motif: [{(TpMe2)Ln(μ-CCR)}2(μ-RC4R)] (Ln = Y, Lu, R = Ph (3),
SiMe3 (4), tBu (5); Ln = Y, R = Ad (6-Y)). Protonolysis of complexes 3-Y and 4-Y with 2,4,6-trimethylphenol gave a
2:1 mixture of free alkyne and (Z)-enyne; however, with 5-Y and 6-Y both enyne
and butatriene were obtained in addition to the free alkyne. Dissolution
of dimeric 3-Lu and 4-Y in THF resulted
in cleavage of the bridging units to give (TpMe2)Ln(CCR)2(THF) (Ln = Lu, R = Ph (7); Ln = Y, R = SiMe3 (8)); however, the putative monomers could not
be isolated, as they were only stable in THF solution. In the presence
of 1 equiv of 2,2′-bipyridine, 1-Lu reacted with
HCCtBu to give the monomeric bipyridine adduct
(TpMe2)Lu(CCtBu)2(bipy)
(9-Lu). Compound 9-Lu is seven-coordinate
in the solid state with the metal center coordinated to a κ3-TpMe2 ligand, two terminal alkynyl ligands, and
a κ2-bipyridine ligand. Reaction of (TptBu,Me)Ln(CH2SiMe3)2 (2; Ln = Y, Lu) with 2 equiv of HCCPh yielded
the terminal dialkynyl complexes (TptBu,Me)Ln(CCPh)2 (10), on the basis of
characteristic NMR spectroscopic data. Complexes 1-Y and 1-Lu reacted with HCCTrit* (Trit* = tris(3,5-di-tert-butylphenyl)methyl) to give the corresponding monomeric
complexes (TpMe2)Ln(CCTrit*)2(THF) (11; Ln = Y, Lu). The monomeric nature and the presence of
terminal alkynyl ligands in 11-Lu was verified by X-ray
crystallography. The complex adopts a six-coordinate distorted-octahedral
structure, with the Lu center coordinated to a κ3-TpMe2 ligand, two terminal alkynyl ligands, and a THF
molecule. NMR-tube reactions of the dimeric complexes 3-Y and 4-Y and the dialkyl complexes 1-Y and 1-Lu with excess terminal alkynes showed that the complexes
are able to dimerize terminal alkynes, albeit with low activity.