Insertion and C−H Bond Activation of Unsaturated Substrates by Bis(benzamidinato)yttrium Alkyl, [PhC(NSiMe₃)₂]₂YR (R = CH₂Ph·THF, CH(SiMe₃)₂), and Hydrido, {[PhC(NSiMe₃)₂]₂Y(μ-H)}₂, Compounds

Abstract

The reactivity of benzamidinate-stabilized yttrium complexes [PhC(NSiMe 3 ) 2 ] 2 YR (R = CH 2 Ph·THF, CH(SiMe 3 ) 2 ) and {[PhC(NSiMe 3 ) 2 ] 2 Y(μ-H)} 2 has been investigated. The complexes are thermally stable showing no sign of decomposition, ligand or solvent metalation, or H/D exchange after hours at 100 °C in cyclohexane- d 12 or benzene- d 6 . The alkyls are also stable in ethereal solvents. However, {[PhC(NSiMe 3 ) 2 ] 2 Y(μ-H)} 2 induces C−O cleavage in THF solutions. The complexes [PhC(NSiMe 3 ) 2 ] 2 YCH 2 Ph·THF and {[PhC(NSiMe 3 ) 2 ] 2 Y(μ-H)} 2 are modestly active in ethylene polymerization but are inactive toward propylene and 1-hexene. Terminal alkynes react stoichiometrically with [PhC(NSiMe 3 ) 2 ] 2 YCH(SiMe 3 ) 2 and {[PhC(NSiMe 3 ) 2 ] 2 Y(μ-H)} 2 to give μ-acetylide dimers, {[PhC(NSiMe 3 ) 2 ] 2 Y(μ-C⋮CR)} 2 ( 1, R = H; 2, R = Me; 3, R = n -Pr; 4, R = SiMe 3; 5, R = Ph; 6, R = CMe 3 ). Treatment with THF leads to cleavage of these dimers, yielding [PhC(NSiMe 3 ) 2 ] 2 YC⋮CR·THF ( 7, R = H; 8, R = CMe 3 ). [PhC(NSiMe 3 ) 2 ] 2 Y(μ-Me) 2 Li·TMEDA reacts with HC⋮CCMe 3 to afford [PhC(NSiMe 3 ) 2 ] 2 Y(μ-C⋮CCMe 3 ) 2 Li·TMEDA. [PhC(NSiMe 3 ) 2 ] 2 YCH(SiMe 3 ) 2 catalyzes the regioselective dimerization of bulky 1-alkynes. With small 1-alkynes, HC⋮CR (R = H, Me, n -Pr), no dimerization was observed and the reaction stops with the formation of the alkynyl dimers {PhC(NSiMe 3 ) 2 ] 2 Y(μ-C⋮CR)} 2 ( 1 − 3 ). Treatment of [PhC(NSiMe 3 ) 2 ] 2 YR with acetonitrile gives either C−H bond activation or insertion. For R = CH(SiMe 3 ) 2, C−H bond activation occurs, yielding {[PhC(NSiMe 3 ) 2 ] 2 Y(μ-( N, N ‘)-N(H)C(Me) C(H)C⋮N)} 2 ( 10 ). For R = CH 2 Ph·THF a mixture of C−H bond activation ( 10, 10%) and insertion products, {[PhC(NSiMe 3 ) 2 ] 2 Y(μ-N C(Me)CH 2 Ph)} 2 ( 11a, 50%) and {[PhC(NSiMe 3 ) 2 ] 2 Y(μ-N(H)C(Me) C(H)Ph)} 2 ( 11b, 40%), was obtained. The hydride {[PhC(NSiMe 3 ) 2 ] 2 Y(μ-H)} 2 exclusively gives insertion of acetonitrile, affording {[PhC(NSiMe 3 ) 2 ] 2 Y(μ-N C(H)Me)} 2 ( 12 ). With pyridine, [PhC(NSiMe 3 ) 2 ] 2 YCH(SiMe 3 ) 2 gives C−H bond activation, whereas [PhC(NSiMe 3 ) 2 ] 2 YCH 2 Ph·THF and {[PhC(NSiMe 3 ) 2 ] 2 Y(μ-H)} 2 undergo insertion yielding [PhC(NSiMe 3 ) 2 ] 2 Y(NC 5 H 5 R) ( 13, R = H; 14, R = CH 2 Ph). In contrast, with α-picoline, [PhC(NSiMe 3 ) 2 ] 2 YR (R = CH(SiMe 3 ) 2, CH 2 Ph·THF) and {[PhC(NSiMe 3 ) 2 ] 2 Y(μ-H)} 2 afford the α-picolyl derivative, [PhC(NSiMe 3 ) 2 ] 2 Y(η 2 -( C, N )-2-CH 2 NC 5 H 4 ( 15 ). The difference in reactivity of the bis(benzamidinate)-stabilized complexes compared to the corresponding Cp* 2 YR systems, e.g. the low tendency to catalyze C−C bond formation, the reduced or even the absence of C−H/C−O activation properties, and the enhanced nucleophilic and Brønsted base reactivities, is interpreted in terms of the high ionicity of the benzamidinate-stabilized yttrium complexes. The contracted yttrium orbitals are less exposed than in the dicyclopentadienyl derivatives and therefore not suited to establish the first initiating interaction with substrate molecules.