Richard F. Jordan

Chemistry of Cationic Dicyclopentadienyl Group 4 Metal-Alky I Complexes

Publisher Summary This chapter discusses the chemistry of cationic dicyclopentadienyl group 4 metal–alkyl complexes. Neutral, d 0 group 4 metal alkyl complexes of general form Cp 2 M(R) 2 (R = H, hydrocarbyl) or Cp 2 M(R)(X) (X = anionic, two–electron donor) comprise the most extensively studied class of group 4 organometallics (I). For many years, it has been suspected that cationic d 0 metal–alkyl species Cp 2 M(R) + are involved in metallocene-based Ziegler–Natta olefin polymerization catalyst systems of the general type Cp 2 MX 2 /AIR n X 3 – n . Several key advances in the early 1980s generated renewed interest in the proposal that Cp 2 M(R) + ions are active species in the soluble catalyst systems. A simple but key point in the development of this chemistry was the realization that noncoordinating, nonreactive counterions, such as BPh 4 – are required for the isolation of stable salts. In 1976, Kaminsky reported the isolation of [(C 5 H 5 ) 2 Zr(CH 2 C(H)– (AIEt 2 ) 2 )][C 5 H 5 ] from the reaction of (C 5 H 5 ) 4 Zr and AIEt 3 . Cationic Cp 2 M(R)(L) + complexes exhibit a variety of interesting structural features, which result from the high Lewis acidity of the cationic d 0 metal centers. The tetrahydrofuran (THF) ligands in these complexes are labile and generally undergo rapid (nuclear magnetic resonance (NMR) time scale) exchange with free THF by dissociative or associative mechanisms at ambient temperature. Cationic, d 0 Cp 2 M(R)(L) + and base-free Cp 2 M(R) + complexes are easily prepared from readily available Cp 2 M(R) 2 compounds. There is strong evidence that Cp 2 M(R) + ions are the active species in Cp 2 MX 2 -based Ziegler–Natta olefin polymerization catalysts.

Ortho -Phosphinobenzenesulfonate: A Superb Ligand for Palladium-Catalyzed Coordination–Insertion Copolymerization of Polar Vinyl Monomers

Ligands, Lewis bases that coordinate to the metal center in a complex, can completely change the catalytic behavior of the metal center. In this Account, we summarize new reactions enabled by a single class of ligands, phosphine-sulfonates (ortho-phosphinobenzenesulfonates). Using their palladium complexes, we have developed four unusual reactions, and three of these have produced novel types of polymers. In one case, we have produced linear high-molecular weight polyethylene, a type of polymer that group 10 metal catalysts do not typically produce. Secondly, complexes using these ligands catalyzed the formation of linear poly(ethylene-co-polar vinyl monomers). Before the use of phosphine-sulfonate catalysts, researchers could only produce ethylene/polar monomer copolymers that have different branched structures rather than linear ones, depending on whether the polymers were produced by a radical polymerization or a group 10 metal catalyzed coordination polymerization. Thirdly, these phosphine-sulfonate catalysts produced nonalternating linear poly(ethylene-co-carbon monoxide). Radical polymerization gives ethylene-rich branched ethylene/CO copolymers copolymers. Prior to the use of phosphine-sulfonates, all of the metal catalyzed processes gave completely alternating ethylene/carbon monoxide copolymers. Finally, we produced poly(polar vinyl monomer-alt-carbon monoxide), a copolymerization of common polar monomers with carbon monoxide that had not been previously reported. Although researchers have often used symmetrical bidentate ligands such as diimines for the polymerization catalysis, phosphine-sulfonates are unsymmetrical, containing two nonequivalent donor units, a neutral phosphine, and an anionic sulfonate. We discuss the features that make this ligand unique. In order to understand all of the new reactions facilitated by this special ligand, we discuss both the steric effect of the bulky phosphines and electronic effects. We provide a unified interpretation of the unique reactivity by considering of the net charge and the enhanced back donation in the phosphine-sulfonate complexes.

Multiple Insertion of a Silyl Vinyl Ether by (α-Diimine)PdMe+ Species

This paper reports that (alpha-diimine)PdMe+ species (1) (alpha-diimine = (2,6-iPr2 C6H3)N CMeCMe N(2,6-iPr2 C6H3)) undergo multiple insertions of CH2 CHOSiPh3 (2), ultimately forming Pd allyl products. The reaction of (alpha-diimine)PdMeCl, [Li(Et2O)2.8][B(C6F5)4] (1 equiv), and 2 (8 equiv) in CH2Cl2 yields [(alpha-diimine)Pd{eta3-CH2CHCHCH(OSiPh3)CH2CH(OSiPh3)Me}][B(C6F5)4] (7-B(C6F5)4) in 83% NMR yield. 7 is formed as a 95/5 mixture of isomers (7a/b), which converts to an equilibrium 40/60 mixture at 23 degrees C. X-ray diffraction analysis established that the configuration of the kinetically favored isomer 7a is S,S,S (ent-R,R,R), where the descriptors refer to the configurations of the substituted allyl carbon and the side chain methine carbons, respectively. 7b has an R,S,S (ent-S,R,R) configuration and differs from 7a in that the Pd is coordinated to the opposite allyl enantioface. The reaction of (alpha-diimine)PdMeCl, Ag[SbF6] (1 equiv), and 2 (8 equiv) under the same conditions yields [(alpha-diimine)Pd{(eta3-CH2CHCHCH(OSiPh3)Me)}][SbF6] (8-SbF6) in 90-100% NMR yield. 8 is formed as a 90/10 mixture of isomers (8a/b) that differ in the configuration of the substituted allyl carbon and evolve to a 70/30 equilibrium mixture at 23 degrees C. These results are consistent with a mechanism involving generation of 1 and insertion of 2 to give (alpha-diimine)PdCH2CH(OSiPh3)Me+ (4). In the absence of 2, 4 undergoes beta-OSiPh3 elimination and allylic C-H activation to give (alpha-diimine)Pd(eta3-CH2CHCH2)+ (6) and Ph3SiOH. In the presence of excess 2, 4 undergoes a second insertion of 2 to form (alpha-diimine)Pd{CH2CH(OSiPh3)CH2CH(OSiPh3)Me+ (9), which can undergo beta-OSiPh3 elimination/allylic C-H activation to form 8, or insert a third equivalent of 2 ultimately leading to 7. Further chain growth is probably disfavored by steric crowding.

Palladium-Catalyzed Dimerization of Vinyl Ethers to Acetals

(Alpha-diimine)PdCl(+) species catalytically dimerize alkyl and silyl vinyl ethers to beta,gamma-unsaturated CH(2)=CHCH(2)CH(OR)(2) acetals, and they cyclize divinyl ethers to analogous cyclic acetals. A plausible mechanism comprises in situ generation of an active PdOR alkoxide species, double vinyl ether insertion to generate Pd{CH(2)CH(OR)CH(2)CH(OR)(2)} species, and beta-OR elimination to generate the acetal product. In the presence of vinyl ethers, (alpha-diimine)PdCl(+) species can be used to initiate ethylene polymerization.

Synthesis, Molecular Structure, and EPR Analysis of the Three-Coordinate Ni(I) Complex [Ni(PPh3)3][BF4]

The compound [Ni(PPh(3))(3)][BF(4)] x BF(3) x OEt(2) was isolated in crystalline form from the olefin oligomerization catalyst system Ni(PPh(3))(4)/BF(3) x OEt(2) and structurally characterized by X-ray diffraction. The influence of vibronic coupling on the EPR parameters of three-coordinate metal complexes with a 3d(9) electronic configuration was investigated within the framework of ligand field theory. Analytical expressions for g-tensor components and isotropic hyperfine coupling constants with ligand nuclei were obtained using first-order perturbation theory. It has been shown that the account of the vibronic interaction in the excited state predicts the existence of three-axial anisotropy of the g-tensor even at the level of first-order perturbation theory; two axes of the g-tensor located in a plane of three-coordinate structure can rotate about the main z axis when a compound is distorted by motion of ligands. It has been shown that in three points of the potential energy surface minimum, for which linear and quadric constants of the vibronic interactions have an identical signs, the HFS isotropic constant from one ligand is larger than HFS constants from the other two; for different vibronic constant signs the ratio between HFS constants varies on opposite. This theoretical researches are in the quality consent with experimental data for a three-coordinate Ni(I) and Cu(II) flat complexes.

Olefin Insertion into a Pd–F Bond: Catalyst Reactivation Following β-F Elimination in Ethylene/Vinyl Fluoride Copolymerization

The discrete (phosphinoarenesulfonate)Pd fluoride complex (POBp,OMe )PdF(lutidine), where POBp,OMe =(2-MeOC6 H4 )(2-{2,6-(MeO)2 C6 H3 }C6 H4 )(2-SO3 -5-MeC6 H3 )P, inserts vinyl fluoride (VF) to form (POBp,OMe )PdCH2 CHF2 (lutidine) and inserts multiple ethylene (E) units to generate polyethylene that contains -CH2 F chain ends. These results provide strong evidence that the -CHF2 and -CH2 F chain ends in E/VF copolymer generated by (phosphinoarenesulfonate)PdR catalysts form by β-F elimination of Pd(β-F-alkyl) species, VF or E insertion of the resulting (PO)PdF species, and subsequent chain growth. These results also imply that β-F elimination is not an important catalyst deactivation reaction in this system.

Transformation of Metal–Organic Framework Secondary Building Units into Hexanuclear Zr-Alkyl Catalysts for Ethylene Polymerization

We report the stepwise and quantitative transformation of the Zr6(μ3-O)4(μ3-OH)4(HCO2)6 nodes in Zr-BTC (MOF-808) to the [Zr6(μ3-O)4(μ3-OH)4Cl12]6- nodes in ZrCl2-BTC, and then to the organometallic [Zr6(μ3-O)4(μ3-OLi)4R12]6- nodes in ZrR2-BTC (R = CH2SiMe3 or Me). Activation of ZrCl2-BTC with MMAO-12 generates ZrMe-BTC, which is an efficient catalyst for ethylene polymerization. ZrMe-BTC displays unusual electronic and steric properties compared to homogeneous Zr catalysts, possesses multimetallic active sites, and produces high-molecular-weight linear polyethylene. Metal-organic framework nodes can thus be directly transformed into novel single-site solid organometallic catalysts without homogeneous analogs for polymerization reactions.

Synthesis and Reactivity of NHC-Supported Ni2(μ2-η2,η2-S2)-Bridging Disulfide and Ni2(μ-S)2-Bridging Sulfide Complexes

The (IPr)Ni scaffold stabilizes low-coordinate, mononuclear and dinuclear complexes with a diverse range of sulfur ligands, including μ(2)-η(2),η(2)-S2, η(2)-S2, μ-S, and μ-SH motifs. The reaction of {(IPr)Ni}2(μ-Cl)2 (1, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with S8 yields the bridging disulfide species {(IPr)ClNi}2(μ(2)-η(2),η(2)-S2) (2). Complex 2 reacts with 2 equiv of AdNC (Ad = adamantyl) to yield a 1:1 mixture of the terminal disulfide compound (IPr)(AdNC)Ni(η(2)-S2) (3a) and trans-(IPr)(AdNC)NiCl2 (4a). 2 also reacts with KC8 to produce the Ni-Ni-bonded bridging sulfide complex {(IPr)Ni}2(μ-S)2 (6). Complex 6 reacts with H2 to yield the bridging hydrosulfide compound {(IPr)Ni}2(μ-SH)2 (7), which retains a Ni-Ni bond. 7 is converted back to 6 by hydrogen atom abstraction by 2,4,6-(t)Bu3-phenoxy radical. The 2,6-diisopropylphenyl groups of the IPr ligand provide lateral steric protection of the (IPr)Ni unit but allow for the formation of Ni-Ni-bonded dinuclear species and electronically preferred rather than sterically preferred structures.

Olefin polymerisation promoted by monodicarbollide complexes of group 4 metals

The behavior of (C2B9H11)M(NEt2)2(NHEt2) [M=Ti (1), Zr (2)] mono-dicarbollide complexes in olefin and styrene polymerisation has been investigated. Compounds 1 and 2, when activated by MAO (Al/M=100 to 1000), polymerise ethylene with good activity. Surprisingly, 2 can also be activated with small amounts of triisobutylaluminum (TIBA, Al/Zr=3 to 40). The ethylene polymerisation activity of the 2/TIBA catalyst (50°C, 5 atm, toluene–chlorobenzene) is greater than that reported for the CpZrX3/MAO catalysts but lower by about one or two orders of magnitude than zirconocene–MAO systems (e.g. Cp2ZrCl2/MAO, rac-(ethylene-bis-1-idenyl)ZrCl2/MAO). The GPC (gel permeation chromatography) curve of the polyethylene sample obtained with 2/TIBA catalyst (50°C, 5 atm, toluene–chlorobenzene) is bimodal indicating that at least two species are active in this system and one is more active than the other. The 2/MAO system (50°C, 5 atm, toluene–chlorobenzene) slowly polymerises propylene to atactic polymer. Under the same experimental conditions 1-pentene does not polymerise at all. Compounds 1 and 2, activated with MAO, produce syndiotactic polystyrene (sPS) with very low activity. Attempts to copolymerise styrene with ethylene by the 1/MAO and 2/MAO systems afforded a mixture of the two homopolymers.

Heterolytic H–H and H–B Bond Cleavage Reactions of {(IPr)Ni(μ-S)}2

Kinetic and DFT computational studies reveal that the reaction of {(IPr)Ni(μ-S)}2 (1, IPr = 1,3-bis(2,6-diisopropyl-phenyl)imidazolin-2-ylidene) with dihydrogen to produce {(IPr)Ni(μ-SH)}2 (2) proceeds by rate-limiting heterolytic addition of H2 across a Ni-S bond of intact dinuclear 1, followed by cis/trans isomerization at Ni and subsequent H migration from Ni to S, to produce the bis-hydrosulfide product 2. Complex 1 reacts in a similar manner with pinacolborane to produce {(IPr)Ni}2(μ-SH)(μ-SBPin) (3), showing that heterolytic activation by this nickel μ-sulfide complex can be generalized to other H-E bonds.