Paul C. Vosejpka

Intra- and intermolecular NMR studies on the activation of arylcyclometallated hafnium pyridyl-amido olefin polymerization precatalysts.

Pyridyl-amido catalysts have emerged recently with great promise for olefin polymerization. Insights into the activation chemistry are presented in an initial attempt to understand the polymerization mechanisms of these important catalysts. The activation of C1-symmetric arylcyclometallated hafnium pyridyl-amido precatalysts, denoted Me2Hf{N(-),N,C(-)} (1, aryl = naphthyl; 2, aryl = phenyl), with both Lewis (B(C6F5)3 and [CPh3][B(C6F5)4]) and Brønsted ([HNR3][B(C6F5)4]) acids is investigated. Reactions of 1 with B(C6F5)3 lead to abstraction of a methyl group and formation of a single inner-sphere diastereoisomeric ion pair [MeHf{N(-),N,C(-)}][MeB(C6F5)3] (3). A 1:1 mixture of the two possible outer-sphere diastereoisomeric ion pairs [MeHf{N(-),N,C(-)}][B(C6F5)4] (4) is obtained when [CPh3][B(C6F5)4] is used. [HNR3][B(C6F5)4] selectively protonates the aryl arm of the tridentate ligand in both precatalysts 1 and 2. A remarkably stable [Me2Hf{N(-),N,C2}][B(C6F5)4] (5) outer-sphere ion pair is formed when the naphthyl substituent is present. The stability is attributed to a hafnium/eta(2)-naphthyl interaction and the release of an eclipsing H-H interaction between naphthyl and pyridine moieties, as evidenced through extensive NMR studies, X-ray single crystal investigation and DFT calculations. When the aryl substituent is phenyl, [Me2Hf{N(-),N,C2}][B(C6F5)4] (10) is originally obtained from protonation of 2, but this species rapidly undergoes remetalation, methane evolution, and amine coordination, giving a diastereomeric mixture of [MeHf{N(-),N,C(-)}NR3][B(C6F5)4] (11). This species transforms over time into the trianionic-ligated [Hf{N(-),C(-),N,C(-)}NR3][B(C6F5)4] (12) through activation of a C-H bond of an amido-isopropyl group. In contrast, ion pair 5 does not spontaneously undergo remetalation of the naphthyl moiety; it reacts with NMe2Ph leading to [MeHf{N(-),N}NMe2C6H4][B(C6F5)4] (7) through ortho-metalation of the aniline. Ion pair 7 successively undergoes a complex transformation ultimately leading to [Hf{N(-),C(-),N,C(-)}NMe2Ph][B(C6F5)4] (8), strictly analogous to 12. The reaction of 5 with aliphatic amines leads to the formation of a single diastereomeric ion pair [MeHf{N(-),N,C(-)}NR3][B(C6F5)4] (9). These differences in activation chemistry are manifested in the polymerization characteristics of these different precatalyst/cocatalyst combinations. Relatively long induction times are observed for propene polymerizations with the naphthyl precatalyst 1 activated with [HNMe3Ph][B(C6F5)4]. However, no induction time is present when 1 is activated with Lewis acids. Similarly, precatalyst 2 shows no induction period with either Lewis or Brønsted acids. Correlation of the solution behavior of these ion pairs and the polymerization characteristics of these various species provides a basis for an initial picture of the polymerization mechanism of these important catalyst systems.

Reactions of nucleophiles with cationic bridging alkylidyne complexes

Abstract The reaction of the μ-methylidyne complex {[Cp(CO)Fe]2(μ-CH)}+ PF6− (1) with C6H5(CH3)2SiLi and CuI produced [Cp(CO)Fe]2(μ-CO)[μ-CHSi(CH3)2C6H5] (4). Reaction of p-tolyl carbyne complex {[Cp(CO)Fe]2(μ-CO)(μ-CC6H4-p-CH3)}+ BF4− (13) with nucleophiles NEt4+HFe(CO)4− and Li(CH3CuCN) gave μ-alkylidene complexes [Cp(CO)Fe]2(μ-CO)(μ-CHC6H4-p-CH3) (14) and [Cp(CO)Fe]2-(μ-CO)[μ-C(CH3)C6H4-p-CH3] (15), respectively. Treatment of ethylidyne complex {[Cp(CO)Fe]2(μ-CO)(μ-CCH3)}+ BF4− (7) with Li(CH3CuCN)·BF3 produced μ-alkylidene complex [Cp(CO)Fe]2(μ-CO)[μ-C(CH3)2] (17). Hydride abstraction from 17 by Ph3C+ PF6− produced the μ-alkenyl complex {[Cp(CO)Fe]2(μ-CO)[μ-η1,η2-C-(CH3)ue5fbCH2]}+ PF6− (18).

CCDC 857362: Experimental Crystal Structure Determination

Related Article: K.A.Frazier, R.D.Froese, Yiyong He, J.Klosin, C.N.Theriault, P.C.Vosejpka, Zhe Zhou, K.A.Abboud|2011|Organometallics|30|3318|doi:10.1021/om200167h

Rearrangement of Rhenium Allyl Vinyl Ketone Complexes

The isomerization of parallel−perpendicular allyl vinyl ketone complex C5H5(CO)Re(η2,η2-H2CCHCH2COCHCHCH2CMe3) (2) to the diastereomeric perpendicular−parallel complex C5H5(CO)Re(η2,η2-H2CCHCH2COCHCHCH2CMe3) (6) occurred without formation of an unsaturated intermediate trappable by either PMe3 or 13CO. The rearrangement of exo-C5H5(CO)Re[η2,η2-CH2CHCH(CH3)COCHCHCH2CMe3] (8-exo) occurred with retention of stereochemistry at rhenium and with enantioface inversion of both complexed alkenes. The kinetically formed parallel−perpendicular isomer 8-exo rearranged by sequential enantioface inversion of the vinyl π-bond to give parallel−parallel intermediate exo-C5H5(CO)Re[η2,η2-CH2CHCH(CH3)COCHCHCH2CMe3] (12-exo) and then enantioface inversion of the allyl π-bond to form the perpendicular−parallel isomer exo-C5H5(CO)Re[η2,η2-CH2CHCH(CH3)COCHCHCH2CMe3] (9-exo). Enolization of allyl vinyl ketone complexes was observed, but is not required for the isomerization. C−H insertion mechanisms involving a net hydrogen migr...