Alternatives for Cyclopentadienyl Ligands in Organoyttrium Chemistry: Bis(N,O-bis(tert-butyl)(alkoxydimethylsilyl)amido)yttrium Compounds

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Reaction of YCl3 . THF3.5 with 2 equiv of [Me2Si(NCMe3)(OCMe3)]Li produces [Me2Si(NCMe3)(OCMe3)](2)Y(mu-Cl)(2)Li . THF2 (1), which easily loses LiCl to give [Me2Si(NCMe3)(OMe3)](2)-YCl . THF (2). Salt metathesis of 2 with LiBH4, LiOAr (OAr = O-2,6-(CMe3)(2)C6H3), NaN(SiMe3)(2), and LiCH(SiMe3)(2) gives the corresponding yttrium bis((alkoxysilyl)amido) 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 with 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 solution. Isolation of the hydride 7 appeared not to be possible; the disproportionation 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 ([Me2Si(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(tert-butyl)(alkoxydimethylsilyl)amido) ligand system is slightly more bulky than the bis(pentamethylcyclopentadienyl) ligand set in compounds Cp*2YR. A ROHF INDO/1 semiempirical molecular orbital study on a stripped and symmetrized model of 6, [H2Si(NH)(OH)](2)YCH3, shows that the electronic properties of the bis((alkoxysilyl)amido) ligand system are quite different from those of [C5H5](2)YCH3 but compare well with those of the bis(benzamidinato) analogue [HC(NH)(2)](2)YCH3. The (alkoxysilyl)amido ligand binds dominantly through a strong, ionic Y-N bond, while the ether function coordinates only weakly. Like in the bis(benzamidinato)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 chemistry. Because of the absence of charge delocalization within the (alkoxysilyl)amido ligands, these behave as strong Bronsted bases and compete successfully with the Y-C bond in C-H bond activation reactions.