Rare earth mixed sandwich complexes with tetraalkylphospholide and cyclooctatetraenide ligands

A series of rare earth (mono)phospholide mixed sandwich complexes of the general form [M(PC₄R₄)(COT)(THF)_n] (M = Sc, Y, La, Lu; R = Me, Et; COT = cyclooctatetraenide, {C₈H₈}^2–; n = 0 to 2) have been isolated by the treatment of iodide precursors, [M(COT)(I)(THF)_n], (1M, M = Sc, Y, Lu, n = 2; M = La, n = 3) with potassium phospholide salts, [K(PC₄R₄)]_n (R = Me, Et). The solid-state molecular structures and speciation of these sandwich complexes depends upon both the ionic radius of the rare earth metal, along with small steric and solubility differences which arise between the two per-alkylated phospholide ligands. The smaller {PC₄Me₄} ligand gave monomeric Lewis-base free [M(C₈H₈)(PC₄Me₄)] (2M, M = Sc, Lu) with smaller rare earths Sc(III) and Lu(III), but moving to larger ions Y(III) and La(III), the products were poorly soluble and could only be isolated as THF-adducts, [Y(C₈H₈)(PC₄Me₄)(THF)] (3) and [La(C₈H₈)(PC₄Me₄)(THF)₂] (4). The slightly increased steric demands of {PC₄Et₄} gave monomeric Lewis-base free complexes for Sc(III), Lu(III), and Y(III) in [M(C₈H₈)(PC₄Et₄)] (5M, M = Sc, Y, Lu), whereas for La(III) a dimeric complex was isolated, [La(C₈H₈)(µ-PC₄Et₄)]₂ (6). We also report the synthesis and molecular structures of 1Sc and 1Lu, as well as [LuI₃(THF)₃] (7) for the first time. All complexes were characterised by single crystal X-ray diffraction, multi-nuclear NMR, UV-Vis-NIR and ATR-IR spectroscopies in addition to elemental analysis.