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(NSiMe3)2]2YR (R = CH2Ph·THF, CH(SiMe3)2) and {[PhC(NSiMe3)2]2Y(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-d12 or benzene-d6. The alkyls are also stable in ethereal solvents. However, {[PhC(NSiMe3)2]2Y(μ-H)}2 induces C−O cleavage in THF solutions. The complexes [PhC(NSiMe3)2]2YCH2Ph·THF and {[PhC(NSiMe3)2]2Y(μ-H)}2 are modestly active in ethylene polymerization but are inactive toward propylene and 1-hexene. Terminal alkynes react stoichiometrically with [PhC(NSiMe3)2]2YCH(SiMe3)2 and {[PhC(NSiMe3)2]2Y(μ-H)}2 to give μ-acetylide dimers, {[PhC(NSiMe3)2]2Y(μ-C⋮CR)}2 (1, R = H; 2, R = Me; 3, R = n-Pr; 4, R = SiMe3; 5, R = Ph; 6, R = CMe3). Treatment with THF leads to cleavage of these dimers, yielding [PhC(NSiMe3)2]2YC⋮CR·THF (7, R = H; 8, R = CMe3). [PhC(NSiMe3)2]2Y(μ-Me)2Li·TMEDA reacts with HC⋮CCMe3 to afford [PhC(NSiMe3)2]2Y(μ-C⋮CCMe3)2Li·TMEDA. [PhC(NSiMe3)2]2YCH(SiMe3)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(NSiMe3)2]2Y(μ-C⋮CR)}2 (1−3). Treatment of [PhC(NSiMe3)2]2YR with acetonitrile gives either C−H bond activation or insertion. For R = CH(SiMe3)2, C−H bond activation occurs, yielding {[PhC(NSiMe3)2]2Y(μ-(N,N ‘)-N(H)C(Me)C(H)C⋮N)}2 (10). For R = CH2Ph·THF a mixture of C−H bond activation (10, 10%) and insertion products, {[PhC(NSiMe3)2]2Y(μ-NC(Me)CH2Ph)}2 (11a, 50%) and {[PhC(NSiMe3)2]2Y(μ-N(H)C(Me)C(H)Ph)}2 (11b, 40%), was obtained. The hydride {[PhC(NSiMe3)2]2Y(μ-H)}2 exclusively gives insertion of acetonitrile, affording {[PhC(NSiMe3)2]2Y(μ-NC(H)Me)}2 (12). With pyridine, [PhC(NSiMe3)2]2YCH(SiMe3)2 gives C−H bond activation, whereas [PhC(NSiMe3)2]2YCH2Ph·THF and {[PhC(NSiMe3)2]2Y(μ-H)}2 undergo insertion yielding [PhC(NSiMe3)2]2Y(NC5H5R) (13, R = H; 14, R = CH2Ph). In contrast, with α-picoline, [PhC(NSiMe3)2]2YR (R = CH(SiMe3)2, CH2Ph·THF) and {[PhC(NSiMe3)2]2Y(μ-H)}2 afford the α-picolyl derivative, [PhC(NSiMe3)2]2Y(η2-(C,N)-2-CH2NC5H4 (15). The difference in reactivity of the bis(benzamidinate)-stabilized complexes compared to the corresponding Cp*2YR 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.