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 ser...

Chemical reduction of (η5-C5Me4R)M[N(R‘)C(X)N(R‘)]Cl2 (M = Zr or Hf, X = NMe2 or Me, R = H or Me, R‘ = i-Pr, Et) with 3 equiv of potassium graphite (KC8) in tetrahydrofuran (THF) provides modest isolated yields of the corresponding “side-on-bridged” dinitrogen complexes, {(η5-C5Me4R)M[N(R‘)C(X)N(R‘)]}2(μ-η2:η2-N2) (1−6). Single-crystal X-ray analyses of these compounds provide d(N−N) bond length values of 1.518(2), 1.581(4), 1.600(6), 1.611(4), 1.630(4), and 1.635(5) Å for 1−6, respectively, that correlate with an increasing fold angle of the M2N2 core for an increase in d(N−N). Compounds 1−6 all undergo hydrosilylation and hydrogenation with PhSiH3 and H2 to provide the corresponding N-functionalized products, 7 and 8, resulting from single addition of these reagents at 25 °C. Compounds 5 and 6 reacted with ethylbromide at 25 °C to provide the N-alkylated products 9a and 9b, respectively, while reaction of 5 with 1 equiv of Br2 provided a near quantitative yield of the diazene dibromide 10 which was obtained as a minor co-product in the N-alkylation of 5.

Chemical reduction of Cp*Ta[N( i Pr)C(Me)N( i Pr)](Cl)3 (Cp* = η5-C5Me5) (1) with 2.5 equiv of potassium graphite (KC8) in tetrahydrofuran (THF) provides a 70% yield of {Cp*Ta[N( i Pr)C(Me)N( i Pr)]Cl}2(μη:η-ηN2) (2). With 4 equiv of KC8, reduction of 1 provides a 34% isolated yield of {Cp*Ta[N(iPr)C(Me)N( i Pr)]}2(μη:η-N2) (3). Single-crystal X-ray analyses of 2 and 3 provide N−N bond lengths of 1.288(10) and 1.313(4) Å, respectively, which is indicative of substantial hydrazido [μ-N2]4- character for these compounds. While 2 is thermally robust and chemically inert to hydrogenation and hydrosilylation in solution, above 0 °C, 3 spontaneously and quantitatively converts to {Cp*Ta[N( i Pr)C(Me)N( i Pr)N]}2 (4) in a 7:1 ratio of cis and trans isomers. Crystallographic analyses of both cis- and trans-4 reveal an absence of N−N bonding within the four-membered ring. Hydrosilylation of 3 at 25 °C using PhSiH3 occurs with 1,4-addition across the TaN−NTa framework to provide a quantitative yield of the hydrosilylated product, 5. Finally, hydrogenation of 3 in pentane at 0 °C, provides an 80% yield of the stereospecific 1,4-addition dihydride product 6. Single-crystal X-ray analyses of 5 and 6 provide N−N bond lengths of 1.284(4) and 1.307(6) Å, respectively. Finally, cis-4 was found to readily react with PhSiH3 at 25 °C to provide 7 as the product of stereospecific ring-opening via σ-bond metathesis of a Ta−N single bond. Crystallographic analysis of 7 confirms the absence of bridging hydrides and the noncyclic nature of this compound.

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 ...

Planar chiral arene-tethered ruthenium complexes were applied to the Diels-Alder reaction of methacrolein and cyclopentadiene, with enantiomeric excesses up to 70%. The influence of a chiral phosphoramidite ligand on the catalytic selectivity was examined, along with counterion effects. The potential of asymmetric activation using the mixture of diastereomers formed from a racemic tethered complex and an enantiopure phosphine directly in the catalysis was investigated.