The concept of "frustrated Lewis pairs" involves donor and acceptor sites in which steric congestion precludes Lewis acid-base adduct formation. In the case of sterically demanding phosphines and boranes, this lack of self-quenching prompts nucleophilic attack at a carbon para to B followed by fluoride transfer affording zwitterionic phosphonium borates [R(3)P(C(6)F(4))BF(C(6)F(5))(2)] and [R(2)PH(C(6)F(4))BF(C(6)F(5))(2)]. These can be easily transformed into the cationic phosphonium-boranes [R(3)P(C(6)F(4))B(C(6)F(5))(2)](+) and [R(2)PH(C(6)F(4))B(C(6)F(5))(2)](+) or into the neutral phosphino-boranes R(2)P(C(6)F(4))B(C(6)F(5))(2). This new reactivity provides a modular route to a family of boranes in which the steric features about the Lewis acidic center remains constant and yet the variation in substitution provides a facile avenue for the tuning of the Lewis acidity. Employing the Gutmann-Beckett and Childs methods for determining Lewis acid strength, it is demonstrated that the cationic boranes are much more Lewis acidic than B(C(6)F(5))(3), while the acidity of the phosphine-boranes is diminished.

A strategy for polymerization catalyst design has been developed based on the steric and electronic analogy of bulky phosphinimides to cyclopentadienyl ligands. To this end, the family of complexes of the form (Cp†)TiCl2(NPR3) has been prepared and characterized. Alkyl and aryl derivatives of these species have also been synthesized, and a number have been evaluated for use as catalyst precursors in olefin polymerization. The polymerization of ethylene has been examined employing several types of cocatalyst activators. Trends and patterns in the structure−activity relationship are discussed, and the implications for catalyst design are evaluated.

Zirconium phosphinimide complexes of the form CpZr(NP-t-Bu 3 )Cl 2 (1) and Cp*Zr(NPR 3 )-Cl 2 (R ) i-Pr (2), t-Bu (3)) were readily prepared under ambient conditions via the reaction of [CpZrCl 3 ] n or Cp*ZrCl 3 with the appropriate trialkylphosphinimide lithium salt (R 3 PNLi). A series of derivatives were readily obtained via alkylation or arylation of the above dihalide precursors. These included CpZr(NP-t-Bu 3 )Me 2 (4), Cp*Zr(NPR 3 )Me 2 (R ) i-Pr (5), t-Bu (6)),. Reaction of 17 with the borane B(C 6 F 5 ) 3 yielded the zwitterionic and cationic complexes Cp*Zr(NP-t-Bu 3 )(CH 2 C(CH 3 )C(CH 3 )CH 2 B(C 6 F 5 ) 3 ) (18) and Cp*Zr(NP-t-Bu 3 )(THF)-(CH 2 C(CH 3 )C(CH 3 )CH 2 B(C 6 F 5 ) 3 ) (19). A number of the above compounds were screened for their potential as catalyst precursors in ethylene polymerization. In general, upon activation with methylaluminoxane, the resulting catalysts exhibit low activity. Efforts to understand the deactivation pathway for these zirconium catalysts involved investigating the interactions of catalyst precursors with activators. For example, reaction of 4 with the borane B(C 6 F 5 ) 3 leads to aryl group transfer and formation of catalytically inactive CpZr(NP-t-Bu 3 )(C 6 F 5 ) 2 (20). Interactions with MAO were modeled via reaction with AlMe 3 . The Zr clusters (Cp*Zr) 4 -(µ-Cl) 5 (Cl)(µ-CH) 2 ( 21) and (Cp*Zr) 5 (µ-Cl) 6 (µ-CH) 3 ( 22) were two of the products that were characterized from these reactions. The isolation of 21 and 22 infers that aryl for methyl exchange, ligand abstraction, and C-H bond activation may be catalyst deactivation pathways.

ABSTRACT:A comprehensive and chronological account of dendrimers based on [1,3,5]-triazines is provided. Synthetic strategies to install the triazine through cycloaddition, cyclotrimerization, and nucleophilic aromatic substitution of cyanuric chloride are discussed.Motivations and applications of these architectures are surveyed, including the preparation of supramolecular assemblies in the solution and solid states and their use in medicines, advanced materials, and separations when anchored to solid supports. V

A variety of donor-stabilized cationic complexes of the form [Cp(NPt-Bu 3 )TiMe(L)][MeB-(C 6 F 5 ) 3 ] and [Cp(NPt-Bu 3 )TiMe(L)][B(C 6 F 5 ) 4 ] (L ) Py, 4-EtPy, 4-t-BuPy, NC 5 H 4 NMe 2 , PMe 3 , Pn-Bu 3 , PPh 3 , P(p-MeC 6 H 4 ) 3 ) have been prepared and characterized. Prolonged storage in CH 2 Cl 2 solution resulted in chloride for methyl exchange to afford species of the formwas obtained from reactions employing the sterically demanding donor phosphine P(o-MeC 6 H 4 ) 3 , suggesting transient generation of the cation [CpTi(NPt-Bu 3 )Cl(CH 2 Cl 2 )] + . Analogous reactivity was not seen in C 6 H 5 Cl, although the species [{CpTi(NPt-Bu 3 )Me} 2 (µ-Me)][B(C 6 F 5 ) 4 ] could be formed in this solvent. The isolated zwitterionic complex TiCp(NPt-Bu 3 )Me(µ-MeB(C 6 F 5 ) 3 ) readily performs insertion chemistry into the Ti-methyl bonds with diisopropylcarbodiimide and diphenylacetylene substrates to afford the cationic species [CpTi(NPt-Bu 3 )((Ni-Pr) 2 CMe)][MeB(C 6 F 5 ) 3 ], [TiCp(NPt-Bu 3 )(PhCCPh(Me))][MeB(C 6 F 5 ) 3 ], and [TiCp(NPt-Bu 3 )(PhCCPh(Me))(PMe 3 )][MeB-(C 6 F 5 ) 3 ]. The implications of this chemistry are considered.

DFT calculations of the mechanism of polymerization for the series of catalyst models derived from CpTiMe 2 (NPR 3 ) (R ) Me, NH 2 , H, Cl, F) demonstrate the critical role of ion pairing in determining the overall barrier to polymerization and suggest that electrondonating substituents reduce this barrier. Based on these results, a family of precatalysts of general formula Cp′TiX 2 (NP(NR 2 ) 3 ) (X ) Cl, Me) were developed. This approach using computational methods to guide the synthetic efforts has afforded a new, readily accessible, and easily varied family of highly active ethylene polymerization catalysts based on titanium tris(amino)phosphinimide complexes.

Synthetic routes to the species CpZr(NPt-Bu 3 ) 2 Cl, 7, Cp 2 Zr(NPt-Bu 3 )Cl, 8, CpZr(NPt-Bu 3 ) 2 -Me, 9, Cp 2 Zr(NPt-Bu 3 )Me, 10, and CpZr(NPt-Bu 3 ) 2 Bn, 11, were developed in a manner similar to that previously reported for zirconium phosphinimide complexes. Rather than employing metathesis routes, transamination was considered to synthesize bis-phosphinimide zirconium complexes. At ambient temperature, Zr(NPt-Bu 3 ) 3 (NMe 2 ), 15, was isolated in less than 5% yield, but could be obtained cleanly via reaction of Zr(NPt-Bu 3 ) 3 Cl, 14, with LiNMe 2 . However, thermolysis of Zr(NEt 2 ) 4 with HNPt-Bu 3 afforded Zr(NPt-Bu 3 ) 2 (NEt 2 ) 2 , 12, which was subsequently converted to Zr(NPt-Bu 3 ) 2 Cl 2 , 13, upon reaction with trimethylsilyl chloride. Cationic products were generated from the reaction of Lewis acids in the presence of a donor to provide the salts [CpZr(NPt-Bu 3 )Me( THF)][MeB(C 6 F 5 ) 3 ], 16, [Cp*Zr(NPt-Bu 3 )((i-PrN) 2 -CMe)][MeB(C 6 F 5 ) 3 ], 17, and [CpZr(NPt-Bu 3 )((i-PrN) 2 CMe)][MeB(C 6 F 5 ) 3 ], 18. Similarly, reaction of [HNMe 2 Ph][B(C 6 F 5 ) 4 ] with 4 generated the salt [CpZr(NPt-Bu 3 )Me(NMe 2 Ph)]-[B(C 6 F 5 ) 4 ], 19, while reaction of 11 with B(C 6 F 5 ) 3 gave the base-free product [CpZr(NPt-Bu 3) 2 ][BnB(C 6 F 5 ) 3 ], 20. Structural considerations and preliminary MO calculations support the reactivity studies that augur well for olefin polymerization activity. Experimentally, previously reported screening using MAO as a solvent scrubber/activator with 1-4 showed only moderate polymerization activities. However, use of 20 equiv of Al(i-Bu) 3 as scavenger and 2 equiv of B(C 6 F 5 ) 3 as cocatalyst resulted in a significant increase in activity relative to that observed upon activation with MAO. Use of [Ph 3 C][B(C 6 F 5 ) 4 ] as the cocatalyst led to even higher ethylene polymerization activities.

This manuscript describes the successful synthesis and characterization of five generations of dendrimers based on melamine. Early generations of these materials appear to be single chemical entities: upon purification, no detectable impurities are observed using NMR spectroscopy, mass spectrometry, and HPLC and GPC analysis. The analysis of larger generation materials precludes an unambiguous statement of purity. The synthetic route to these targets is divergent, relying on dichlorotriazine monomers that react with a polyamine dendrimer core of generation n to produce a poly(monochlorotriazine) dendrimer of generation n + 1. Subsequently, the poly (monochlorotriazine) is derivatized through nucleophilic aromatic substitution before additional nucleophilic amines are unmasked and the process iterated.