Organo-Rare-Earth Complexes Supported by Chelating Diamide Ligands

The synthesis of a series of heteroleptic ate-complex-free rare-earth(III) diamide complexes
by alkane and amine elimination reactions, starting from Ln(CH2SiMe3)3(THF)2 and Ln(NiPr2)3(THF)x (x = 1, Ln = Sc, Lu; x = 2, Ln = Y), respectively, is described. 2,6-Bis(((2,6-diisopropylphenyl)amino)methyl)pyridine (H2BDPPpyr) formed monomeric complexes of the
types (BDPPpyr)Ln(CH2SiMe3)(THF)x (x = 1, Ln = Sc, Lu; x = 2, Ln = Y) and (BDPPpyr)Ln(NiPr2)(THF) (Ln = Sc, Lu), which display enhanced stability for the smaller metal center
scandium, for diisopropylamide coordination, and in donor solvents such as THF. Conversion
of the silylalkyl complexes into their amide derivatives via secondary alkane elimination,
i.e., reaction with HNEt2 and HN(SiHMe2)2, increased the complex stability. The mono-THF adduct complexes (BDPPpyr)Sc(L)(THF) show a nonfluxional structure in solution,
which contrasts the dynamic behavior of the corresponding bis-THF adduct complexes of
the larger elements lutetium and yttrium. Sterically less encumbered 2,6-bis((mesitylamino)methyl)pyridine (H2BMespyr) gave the less stable complexes (BMespyr)Ln(CH2SiMe3)(THF)x
(x = 1, Ln = Sc; x = 2, Ln = Lu; the Y derivative could not be isolated). Again, subsequent
silylalkyl/silylamide ligand exchange gave the complexes (BMespyr)Ln[N(SiHMe2)2](THF),
exhibiting considerably increased stability. The complexes (BDPPoxyl)Ln(CH2SiMe3)(THF)
(Ln = Sc, Lu, Y), derived from a nonfunctionalized diamide ligand (H2BDPPoxyl = 1,2-bis(((2,6-diisopropylphenyl)amino)methyl)benzene), were isolated in high yields. The 4- and
5-coordinate complexes (BDPPoxyl)Sc(CH2SiMe3)(THF) and (BDPPpyr)Sc(CH2SiMe3)(THF),
respectively, were also characterized by X-ray diffraction structure determinations. All of
the 5-coordinate scandium complexes derived from the BDPPpyr ligand effectively polymerize
methyl methacrylate in a living manner (Mw/Mn < 1.5), affording mainly atactic polymer at
ambient temperature.