Steric Effects in Lanthanide Sandwich Complexes Containing Bulky Cyclooctatetraenyl Ligands

This paper gives a full account of the complex reaction system LnCl3/(COT″)2– (COT″ = 1,4-bis­(trimethylsilyl)­cyclooctatetraenyl dianion). Four different series of organolanthanide complexes of this bulky COT ligand have been reported: (1) anionic sandwich complexes containing [Ln­(COT″)2]− anions, (2) dimeric chloro-bridged mono­(COT″) complexes, (3) cluster-centered multidecker-sandwich complexes, and (4) linear triple-decker sandwich complexes. In order to get a more complete picture of the reaction system LnCl3/(COT″)2– and to examine possible steric effects exerted by the COT″ ligand, four new rare-earth-metal COT″ complexes have been prepared: [Li­(DME)3]­[Ce­(COT″)2] (1), [Li­(THF)4]­[Nd­(COT″)2] (2), [(COT″)­Nd­(μ-Cl)­(THF)]2 (4), and (COT″)­Yb­(DIPPForm)­(THF) (5,; DIPPForm = [HC­(NAr)2]−, Ar = 2,6-diisopropylphenyl). For comparison, the COTTBS derivative [Li­(DME)3]­[Ce­(COTTBS)2] (3; COTTBS = 1,4-bis­(tert-butyldimethylsilyl)­cyclooctatetraenyl dianion) has also been synthesized. It was found that the steric effect of either COT″ or COTTBS on the molecular structures of the anionic sandwich complexes 1–3 with coplanar ring ligands is rather small. In (COT″)­Yb­(DIPPForm)­(THF) (5), the combination of COT″ with a sterically demanding formamidinate ligand in the coordination sphere of Yb3+ still leaves room for THF coordination. For example, in 4, the bulky COT″ leads to coordination of only one THF per Ln in comparison to the parent chloro-bridged dimers [(COT)­Ln­(μ-Cl)­(THF)2]2. However, in the dimeric chloro-bridged mono­(COT″) complexes the presence of COT″ reduces the number of coordinated THF molecules. Quite surprisingly, an attempt to prepare the holmium analogue of the known linear triple-decker sandwich complex (COT″)­Nd­(μ-η8:η8-COT″)­Nd­(COT″) led to formation of the isomerized triple-decker (COT″)­Ho­[μ-η8:η8-C8H6(SiMe3)2-1,5]­Ho­(COT″) (6), as a result of steric pressure-induced silyl group migration. All new complexes 1–6 have been structurally characterized by X-ray diffraction.