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.