RESUMEN
This Account describes our research related to the development of molecular catalysts for solution phase olefin polymerization. Specifically, a series of constrained geometry and nonmetallocene (imino-amido-type) complexes were developed for high temperature olefin polymerization reactions. We have discovered many highly active catalysts that are capable of operating at temperatures above 120 °C and producing copolymers with a useful range of molecular weights (from medium to ultrahigh depending on precatalyst identity and polymerization conditions) and α-olefin incorporation capability. Constrained geometry catalysts (CGCs) exhibit very high activities and are capable of producing a variety of copolymers including ethylene-propylene and ethylene-1-octene copolymers at high reactor temperatures. Importantly, CGCs have much higher reactivity toward α-olefins than classical Ziegler-Natta catalysts, thus allowing for the production of copolymers with any desired level of comonomer. In search of catalysts with improved performance, we discovered 3-amino-substituted indenyl-based CGCs that exhibit the highest activity and produce copolymers with the highest molecular weight within this family of catalysts. Phenanthrenyl-based CGCs were found to be outstanding catalysts for the effective production of high styrene content ethylene-styrene copolymers under industrially relevant conditions. In contrast to CGC ligands, imino-amido-type ligands are bidentate and monoionic, leading to the use of trialkyl group IV precatalysts. The thermal instability of imino-amido complexes was addressed by the development of imino-enamido and amidoquinoline complexes, which are not only thermally very robust, but also produce copolymers with higher molecular weights, and exhibit improved α-olefin incorporation. Imido-amido and imino-enamido catalysts undergo facile chain transfer reactions with metal alkyls, as evidenced by a sharp decrease in polymer molecular weight when the polymerization reactions were conducted in the presence of diethylzinc, an essential requirement for use in the production of olefin block copolymers via chain shuttling polymerization. Overall, the excellent characteristics of imino-amido-type catalysts, including high catalytic activities and ultrahigh molecular weight capabilities, make them good candidates for high temperature syntheses of block and random ethylene-α-olefin copolymers. Additionally, trialkyl imino-enamido complexes react quickly with various protic and unsaturated organic fragments, leading to a library of dialkyl precatalysts that, in several instances, resulted in superior catalysts. In conjunction with the development of transition metal catalysts, we also synthesized and evaluated activators for olefin polymerization. We found, for example, that, when conducted in coordinating solvents, the reaction between aluminum alkyls and tris(pentafluorophenyl)borane leads to the exclusive formation of alumenium borates, which are excellent activators for CGC complexes. Additionally, we developed a series of highly effective new activators featuring a very weakly coordinating anion composed of two Lewis acids coordinated to an imidazole fragment.
RESUMEN
Chemical reduction of Cp*M[N(i-Pr)C(Me)N(i-Pr)]Cl(3) (Cp* = eta(5)-C(5)Me(5)) (1, M = Mo) and (2, M = W) using 0.5% NaHg in THF provided excellent yields of the diamagnetic dinuclear end-on-bridged dinitrogen complexes {Cp*M[N(i-Pr)C(Me)N(i-Pr)]}(2)(mu-eta(1):eta(1)-N(2)) (6, M = Mo) and (8, M = W), respectively. Chemical reduction of Cp*Mo[N(i-Pr)C(NMe(2))N(i-Pr)]Cl(2) (4) with 3 equiv of KC(8) in THF similarly yielded diamagnetic {Cp*Mo[N(i-Pr)C(NMe(2))N(i-Pr)]}(2)(mu-eta(1):eta(1)-N(2)) (7). Single-crystal X-ray analyses of 7 and 8 confirmed the dinuclear end-on-bridged mu-eta(1):eta(1)-N(2) coordination mode and the solid-state molecular structures of these compounds provided d(NN) values of 1.267(2) and 1.277(8) A for 7 and 8, respectively. Based on a comparison of (15)N NMR spectra for (15)N(2) (99%)-labeled 6 and (15)N(2) (99%)-labeled 8, as well as similarities in chemical reactivity, a dinuclear mu-eta(1):eta(1)-N(2) structure for 6 is further proposed. For comparison with a first-row metal derivative, chemical reduction of Cp*Ti[N(i-Pr)C(Me)N(i-Pr)]Cl(2) (9) with KC(8) in THF was conducted to provide {Cp*Ti[N(i-Pr)C(Me)N(i-Pr)]}(2)(mu-eta(1):eta(1)-N(2)) (10) for which a d(NN) value of 1.270(2) A was obtained through X-ray crystallography. Compounds 6-8 were all found to be thermally robust in toluene solution up to temperatures of at least 100 degrees C, and 6 and 8 were determined to be inert toward the addition of H(2) or H(3)SiPh under a variety of conditions. Single-crystal X-ray analysis of meso-{Cp*Mo(H)[N(i-Pr)C(Me)N(i-Pr)]}(2)(mu-eta(1):eta(1)-N(2)) (meso-11), which was serendipitously isolated as a product of attempted alkylation of Cp*Mo[N(i-Pr)C(Me)N(i-Pr)]Cl(2) (3) with 2 equiv of n-butyllithium, revealed a smaller d(NN) value of 1.189(4) A that is consistent with two Mo(IV,d(2)) centers connected by a bridging diazenido, [mu-N(2)](2-), moiety. Moreover, meso-11 was found to undergo clean dehydrogenation in solution at 50 degrees C to provide 6 via a first-order process. Chemical oxidation of 8 with an excess of PbCl(2) in toluene solution at 25 degrees C provided a 1:1 mixture of rac- and meso-{Cp*W(Cl)[N(i-Pr)C(Me)N(i-Pr)]}(2)(mu-eta(1):eta(1)-N(2)) (12); both isomers of which provided solid-state structures through X-ray analyses that are consistent with an electronic configuration comprised of two W(IV,d(2)) centers linked through a bridging [N(2)](2-) group [cf. for rac-12, d(NN) = 1.206(9) A, and for meso-12, d(NN) = 1.192(3) A]. Finally, treatment of 6 and 8 with either 4 equiv of CNAr (Ar = 3,5-Me(2)C(6)H(3)) or an excess of CO in toluene provided excellent yields of Cp*M[N(i-Pr)C(Me)N(i-Pr)](CNAr)(2) (13, M = Mo and 14, M = W) and Cp*M[N(i-Pr)C(Me)N(i-Pr)](CO)(2) (15, M = Mo and 16, M = W), respectively. Single-crystal X-ray analyses of 13-16, along with observation of reduced IR vibrational nu(CN) or nu(CO) bond-stretching frequencies, provide strong support for the electron-rich character of the Cp*M[N(i-Pr)C(Me)N(i-Pr)] fragment that can engage in a high degree of back-donation with moderate to strong pi-acceptors, such as N(2), CNR, and CO. The collective results of this work are analyzed in terms of the possible steric and electronic factors that contribute to preferred mode of mu-N(2) coordination and the extent of N[triple bond]N activation, including complete N-N bond scission, within the now completed experimentally-derived ligand-centered isostructural series of {Cp*M[N(i-Pr)C(Me)N(i-Pr)]}(2)(mu-N(2)) compounds where M = Ti, Zr, Hf, Ta, Mo, and W.
