ALTERNATIVES FOR CYCLOPENTADIENYL LIGANDS IN ORGANOYTTRIUM CHEMISTRY - -BIS(TERT-BUTYL)(ALKOXYDIMETHYLSILYL)AMIDO)YTTRIUM COMPOUNDS

Reaction of YCl3 . THF3.5 with 2 equiv of [Me2Si(NCMe3)(OCMe3)]Li prod uces [Me2Si(NCMe3)(OCMe3)](2)Y(mu-Cl)(2)Li . THF2 (1), which easily lo ses LiCl to give [Me2Si(NCMe3)(OMe3)](2)-YCl . THF (2). Salt metathesi s of 2 with LiBH4, LiOAr (OAr = O-2,6-(CMe3)(2)C6H3), NaN(SiMe3)(2), a nd LiCH(SiMe3)(2) gives the corresponding yttrium bis((alkoxysilyl)ami do) derivatives, [Me2Si(NCMe3)(OCMe3)](2)YR (R = BH4 . THF (3), OAr (4 ), N(SiMe3)(2) (5), CH(SiMe3)(2) (6)). The alkyl compound 6 reacts wit h H-2 in THF to give an unstable hydride {[Me2Si(NCMe3)(OCMe3)](2)Y(mu -H)}(2) (7), which was identified by H-1 NMR as a symmetric dimer in s olution. Isolation of the hydride 7 appeared not to be possible; the d isproportionation product, [Me2Si(NCMe3)(OCMe3)](3)Y (8), was obtained instead. With HC=CR, 6 undergoes protolysis of both the alkyl and the (alkoxysilyl)amido ligands to yield {Y(mu-C=CR)(3)}(n) for R = SiMe3 (9) and CMe3 (10). In contrast, polymerization to polyphenylacetylene was observed for R = Ph. Compound 6 reacts with N=CMe with metalation of the methyl group under proton transfer to the alkyl ligand to give CH2(SiMe3)(2). Insertion of another N=CMe into the new Y-C bond and 1, 3-H shift produces 2Si(NCMe3)(OCMe3)](2)Y(mu(N,N')-NH-CMe=CH-C=N)}(2) (11). The molecular structures of 6 and 11 show that the bis(N,O-bis(t ert-butyl)(alkoxydimethylsilyl)amido) ligand system is slightly more b ulky than the bis(pentamethylcyclopentadienyl) ligand set in compounds Cp2YR. A ROHF INDO/1 semiempirical molecular orbital study on a stri pped and symmetrized model of 6, [H2Si(NH)(OH)](2)YCH3, shows that the electronic properties of the bis((alkoxysilyl)amido) ligand system ar e quite different from those of [C5H5](2)YCH3 but compare well with th ose of the bis(benzamidinato) analogue [HC(NH)(2)](2)YCH3. The (alkoxy silyl)amido ligand binds dominantly through a strong, ionic Y-N bond, while the ether function coordinates only weakly. Like in the bis(benz amidinato)yttrium system, the (alkoxysilyl)amido and the alkyl ligands accumulate negative charge, resulting in essentially ionic compounds. This high ionicity makes the compounds have little tendency to engage in sigma-bond metathesis reactions and (catalytic) insertion chemistr y. Because of the absence of charge delocalization within the (alkoxys ilyl)amido ligands, these behave as strong Bronsted bases and compete successfully with the Y-C bond in C-H bond activation reactions.