Bimetallic effects in homopolymerization of styrene and copolymerization of ethylene and styrenic comonomers: scope, kinetics, and mechanism

This contribution describes the homopolymerization of styrene and the copolymerization of ethylene and styrenic comonomers mediated by the single-site bimetallic "constrained geometry catalysts" (CGCs), (mu-CH2CH2-3,3'){(eta(5)-indenyl)[1-Me2Si(tBuN)](TiMe2)}2 [EBICGC(TiMe2)2; Ti2], (mu-CH2CH2-3,3'){(eta(5)-indenyl)[1-Me2Si(tBuN)](ZrMe2)}2 [EBICGC(ZrMe2)2; Zr2], (mu-CH2-3,3'){(eta(5)-indenyl)[1-Me2Si(tBuN)](TiMe2)}2 [MBICGC(TiMe2)2; C1-Ti2], and (mu-CH2-3,3'){(eta(5)-indenyl)[1-Me2Si(tBuN)](ZrMe2)}2 [MBICGC(ZrMe2)2; C1-Zr2], in combination with the borate activator/cocatalyst Ph3C+ B(C6F5)4- (B1). Under identical styrene homopolymerization conditions, C1-Ti2 + B1 and Ti2 + B1 exhibit approximately 65 and approximately 35 times greater polymerization activities, respectively, than does monometallic [1-Me2Si(3-ethylindenyl)(tBuN)]TiMe2 (Ti1) + B1. C1-Zr2 + B1 and Zr2 + B1 exhibit approximately 8 and approximately 4 times greater polymerization activities, respectively, than does the monometallic control [1-Me2Si(3-ethylindenyl)(tBuN)]ZrMe2 (Zr1) + B1. NMR analyses show that the bimetallic catalysts suppress the regiochemical insertion selectivity exhibited by the monometallic analogues. In ethylene copolymerization, Ti2 + B1 enchains 15.4% more styrene (B), 28.9% more 4-methylstyrene (C), 45.4% more 4-fluorostyrene (D), 41.2% more 4-chlorostyrene (E), and 31.0% more 4-bromostyrene (F) than does Ti1 + B1. This observed bimetallic chemoselectivity effect follows the same general trend as the pi-electron density on the styrenic ipso carbon (D > E > F > C > B). Kinetic studies reveal that both Ti2 + B1 and Ti1 + B1-mediated ethylene-styrene copolymerizations follow second-order Markovian statistics and tend to be alternating. Moreover, calculated reactivity ratios indicate that Ti2 + B1 favors styrene insertion more than does Ti1 + B1. All the organozirconium complexes (C1-Zr2, Zr2, and Zr1) are found to be incompetent for ethylene-styrene copolymerization, yielding only mixtures of polyethylene and polystyrene. Model compound (mu-CH2CH2-3,3'){(eta(5)-indenyl)[1-Me2Si(tBuN)][Ti(CH2Ph)2]}2 {EBICGC[Ti(CH2Ph)2]2; Ti2(CH2Ph)4} was designed, synthesized, and structurally characterized. In situ activation studies with cocatalyst B(C6F5)3 suggest an eta(1)-coordination mode for the benzyl groups, thus supporting the proposed polymerization mechanism. For ethylene-styrene copolymerization, polar solvents are found to increase copolymerization activities and coproduce atactic polystyrene impurities in addition to ethylene-co-styrene, without diminishing the comonomer incorporation selectivity. Both homopolymerization and copolymerization results argue that substantial cooperative effects between catalytic sites are operative.     

Catalyst/cocatalyst nuclearity effects in single-site polymerization. Enhanced polyethylene branching and alpha-olefin comonomer enchainment in polymerizations mediated by binuclear catalysts and cocatalysts via a new enchainment pathway

The binuclear "constrained geometry catalyst" (CGC) (mu-CH2CH2-3,3'){(eta5-indenyl )[1-Me2Si(tBuN)](ZrMe2)}2 [EBICGC(ZrMe2)2; Zr2] and the trityl bisborate dianion (Ph3C+)2[1,4-(C6F5)3BC6F4B(C6F5)3]2- (B2) have been synthesized to serve as new types of multicenter homogeneous olefin polymerization catalysts and cocatalysts, respectively. Additionally, the complex [1-Me2Si(3-ethylindenyl)(tBuN)]ZrMe2 (Zr1) was synthesized as a mononuclear control. For the bimetallic catalyst or bisborate cocatalyst, high effective local active site concentrations and catalyst center-catalyst center cooperative effects are evidenced by bringing the catalytic centers together via either covalent or electrostatic bonding. For ethylene homopolymerization at constant conversion, the branch content of the polyolefin products (primarily ethyl branches) is dramatically increased as catalyst or cocatalyst nuclearity is increased. Moreover, catalyst and cocatalyst nuclearity effects are approximately additive. Compared to the catalyst derived from monometallic Zr1 and monofunctional Ph3C+B(C6F5)4- (B1), the active catalyst derived from bimetallic Zr2 and bifunctional B2 produces approximately 11 times more ethyl branches in ethylene homopolymerization via a process which is predominantly intradimer in character. Moreover, approximately 3 times more 1-hexene incorporation in ethylene + 1-hexene copolymerization and approximately 4 times more 1-pentene incorporation in ethylene + 1-pentene copolymerization are observed for Zr2 + B2 versus Zr1 + B1.     

Catalyst/cocatalyst nuclearity effects in single-site olefin polymerization. Significantly enhanced 1-octene and isobutene comonomer enchainment in ethylene polymerizations mediated by binuclear catalysts and cocatalysts

This Communication describes the implementation of a new binuclear homometallic organotitanium "constrained geometry catalyst" (CGC), (mu-CH2CH2-3,3'){ (eta5-indenyl )[1-Me2Si (tBuN)](TiMe2)}2[EBICGC(TiMe2)2; Ti2], together with the bifunctional activators (Ph3C+)2[1,4-(C6F5)3BC6F4B(C6F5)3]2- (B2) and new bisborane 1,4-(C6F5)2BC6F4B(C6F5)2 (BN2) in ethylene + alpha-olefin copolymerization processes. Specifically examined are the comonomers 1-octene and poorly responsive isobutene. Large increases in comonomer enchainment efficiency into the polyethylene microstructure are observed versus the corresponding mononuclear catalyst [1-Me2Si(3-ethylindenyl)(tBuN)]TiMe2 (Ti1) + Ph3C+B(C6F5)4- (B1) or B(C6F5)3 (BN) under identical polymerization conditions. In ethylene + 1-octene copolymerization, 11 times more 1-octene incorporation is observed for Ti2 + B2 vs Ti1 + B1. In ethylene + isobutene copolymerization, 5 times more isobutene incorporation is observed for Ti2 + BN2 vs Ti1 + BN.     

Half-sandwich bis(tetramethylaluminate) complexes of the rare-earth metals: synthesis, structural chemistry, and performance in isoprene polymerization

The protonolysis reaction of [Ln(AlMe(4))(3)] with various substituted cyclopentadienyl derivatives HCp(R) gives access to a series of half-sandwich complexes [Ln(AlMe(4))(2)(Cp(R))]. Whereas bis(tetramethylaluminate) complexes with [1,3-(Me(3)Si)(2)C(5)H(3)] and [C(5)Me(4)SiMe(3)] ancillary ligands form easily at ambient temperature for the entire Ln(III) cation size range (Ln=Lu, Y, Sm, Nd, La), exchange with the less reactive [1,2,4-(Me(3)C)(3)C(5)H(3)] was only obtained at elevated temperatures and for the larger metal centers Sm, Nd, and La. X-ray structure analyses of seven representative complexes of the type [Ln(AlMe(4))(2)(Cp(R))] reveal a similar distinct [AlMe(4)] coordination (one eta(2), one bent eta(2)). Treatment with Me(2)AlCl leads to [AlMe(4)] --> [Cl] exchange and, depending on the Al/Ln ratio and the Cp(R) ligand, varying amounts of partially and fully exchanged products [{Ln(AlMe(4))(mu-Cl)(Cp(R))}(2)] and [{Ln(mu-Cl)(2)(Cp(R))}(n)], respectively, have been identified. Complexes [{Y(AlMe(4))(mu-Cl)(C(5)Me(4)SiMe(3))}(2)] and [{Nd(AlMe(4))(mu-Cl){1,2,4-(Me(3)C)(3)C(5)H(2)}}(2)] have been characterized by X-ray structure analysis. All of the chlorinated half-sandwich complexes are inactive in isoprene polymerization. However, activation of the complexes [Ln(AlMe(4))(2)(Cp(R))] with boron-containing cocatalysts, such as [Ph(3)C][B(C(6)F(5))(4)], [PhNMe(2)H][B(C(6)F(5))(4)], or B(C(6)F(5))(3), produces initiators for the fabrication of trans-1,4-polyisoprene. The choice of rare-earth metal cation size, Cp(R) ancillary ligand, and type of boron cocatalyst crucially affects the polymerization performance, including activity, catalyst efficiency, living character, and polymer stereoregularity. The highest stereoselectivities were observed for the precatalyst/cocatalyst systems [La(AlMe(4))(2)(C(5)Me(4)SiMe(3))]/B(C(6)F(5))(3) (trans-1,4 content: 95.6 %, M(w)/M(n)=1.26) and [La(AlMe(4))(2)(C(5)Me(5))]/B(C(6)F(5))(3) (trans-1,4 content: 99.5 %, M(w)/M(n)=1.18).     

Diverse stereocontrol effects induced by weakly coordinating anions. Stereospecific olefin polymerization pathways at archetypal C(s)- and C(1)-symmetric metallocenium catalysts using mono- and polynuclear halo-perfluoroarylmetalates as cocatalysts

Counteranion effects on propylene polymerization rates and stereoselectivities are compared using Cs-symmetric Me2C(Cp)(Flu)ZrMe2 (1; Cp = C5H4,eta5-cyclopentadienyl; Flu = C13H8, eta5-fluorenyl) and C1-symmetric Me2Si(OHF)(CpR*)ZrMe2 (2; OHF = C13H16, eta5-octahydrofluorenyl; CpR* = eta5-3-(-)-menthylcyclopentadienyl) precatalysts activated with the mononuclear and polynuclear perfluoroarylborate, -aluminate, and -gallate cocatalysts/activators B(C6F5)3 (3), B(o-C6F5C6F4)3 (4), Al(C6F5)3 (5), Ph3C+B(C6F5)4- (6) Ph3C+FAl(o-C6F5C6F4)3- (7), Ga(C6F5)3 (8), and recently reported mono- and polymetallic trityl perfluoroarylhalometalates Ph3C+FB(C6F5)3- (9), Ph3C+FB(o-C6F5C6F4)3- (10), (Ph3C+)xFx[Al(C6F5)3]yx- (x = 1, y = 1, 11; x = 1, y = 2, 12; x = 2, y = 3, 13), Ph3C+(C6F5)3AlFAl(o-C6F5C6F4)3- (14), Ph3C+XAl(C6F5)3- (X = Cl, 15; X = Br, 16), and Ph3C+F[Ga(C6F5)3]2- (17). Temperature, propylene concentration, and solvent polarity dependence are surveyed in polymerizations catalyzed by 1 activated with cocatalysts 3-16 and with a 1:2 ratio of Ph3CCl and 5, and with a 1:2 ratio of Ph3CBr and 5, and by 2 activated with 3, 6, 7, 12, and 14. Remarkable stereocontrol with high activities is observed for 1 + 12 and 1 + 14. Polypropylene samples produced using C1-symmetric precatalyst 2 are subjected to microstructural analyses using stochastic models describing the relative contributions of enantiofacial misinsertion and backskip processes. A powerful technique is introduced for calculating interparametric correlation matrices for these nonlinear stochastic models. The collected results significantly extend what is known about ion-pairing effects in the case of Cs-symmetric precatalyst 1 and allow these findings to be applied to the case of C1-symmetric precatalyst 2 as an agent of isospecific propylene polymerization.     

Unprecedented isospecific 3,4-polymerization of isoprene by cationic rare earth metal alkyl species resulting from a binuclear precursor

The isospecific 3,4-polymerization of isoprene has been achieved for the first time by use of a combination of a binuclear rare earth metal dialkyl complex, such as [Me2Si(C5Me4)(mu-PCy)YCH2SiMe3]2 (Cy = cyclohexyl), and an equimolar amount of [Ph3C][B(C6F5)4] as a catalyst system. A DFT calculation suggested that a binuclear monocationic monoalkyl species, such as [Me2Si(C5Me4)(mu-PCy)Y(mu-CH2SiMe3)Y(mu-PCy)(C5Me4)SiMe2]+, in which the alkyl group bridges the two metal centers, could be the true catalyst species.     

Unusual reactivity of tris(pyrazolyl)borate zirconium benzyl complexes

The synthesis and reactivity of [Tp*Zr(CH2Ph)2][B(C6F5)4] (2, Tp* = HB(3,5-Me2pz)3, pz = pyrazolyl) have been explored to probe the possible role of Tp'MR2+ species in group 4 metal Tp'MCl3/MAO olefin polymerization catalysts (Tp' = generic tris(pyrazolyl)borate). The reaction of Tp*Zr(CH2Ph)3 (1) with [Ph3C][B(C6F5)4] in CD2Cl2 at -60 degrees C yields 2. 2 rearranges rapidly to [{(PhCH2)(H)B(mu-Me2pz)2}Zr(eta2-Me2pz)(CH2Ph)][B(C6F5)4] (3) at 0 degrees C. Both 2 and 3 are highly active for ethylene polymerization and alkyne insertion. Reaction of 2 with excess 2-butyne yields the double insertion product [Tp*Zr(CH2Ph)(CMe=CMeCMe=CMeCH2Ph)][B(C6F5)4] (4). Reaction of 3 with excess 2-butyne yields [{(PhCH2)(H)B(mu-Me2pz)2}Zr(Cp*)(eta2-Me2pz)][B(C6F5)4] (6, Cp* = C5Me5) via three successive 2-butyne insertions, intramolecular insertion, chain walking, and beta-Cp* elimination.     

Alkenylsilane structure effects on mononuclear and binuclear organotitanium-mediated ethylene polymerization: scope and mechanism of simultaneous polyolefin branch and functional group introduction

Alkenylsilanes of varying chain lengths are investigated as simultaneous chain-transfer agents and comonomers in organotitanium-mediated olefin polymerization processes. Ethylene polymerizations were carried out with activated CGCTiMe2 and EBICGCTi2Me4 (CGC = Me2Si(Me4C5)(NtBu); EBICGC = (mu-CH2CH2-3,3'){(eta5-indenyl)[1-Me2Si(tBuN)]}2) precatalysts in the presence of allylsilane, 3-butenylsilane, 5-hexenylsilane, and 7-octenylsilane. In the presence of these alkenylsilanes, high polymerization activities (up to 107 g of polymer/(mol of Ti.atm ethylene.h)), narrow product copolymer polydispersities, and substantial amounts of long-chain branching are observed. Regardless of Ti nuclearity, alkenylsilane incorporation levels follow the trend C8H15SiH3 < C6H11SiH3 approximately C4H7SiH3 < C3H5SiH3. Alkenylsilane comonomer incorporation levels are consistently higher for CGCTiMe2-mediated copolymerizations (up to 54%) in comparison with EBICGCTi2Me4-mediated copolymerizations (up to 32%). The long-chain branching levels as compared to the total branch content follow the trend C3H5SiH3 < C4H7SiH3 approximately C6H11SiH3 approximately C8H15SiH3, with gel permeation chromatography-multi-angle laser light scattering-derived branching ratios (gM) approaching 1.0 for C8H15SiH3. Time-dependent experiments indicate a linear increase of copolymer Mw with increasing polymerization reaction time. This process for producing long-chain branched polyolefins by coupling of an alpha-olefin with a chain-transfer agent in one comonomer is unprecedented. Under the conditions investigated, alkenylsilanes ranging from C3 to C8 are all efficient chain-transfer agents. Ti nuclearity significantly influences silanolytic chain-transfer processes, with the binuclear system exhibiting a sublinear relationship between Mn and [alkenylsilane](-1) for allylsilane and 3-butenylsilane, and a superlinear relationship between Mn and [alkenylsilane](-1) for 5-hexenylsilane and 7-octenylsilane. For the mononuclear Ti system, alkenylsilanes up to C6 exhibit a linear relationship between Mn and [alkenylsilane](-1), consistent with a simple silanolytic chain termination mechanism.     

B(C6F5)3 adducts of transition metal acyl compounds

The transition metal acyl compounds [Co(L)(CO)3(COMe)] (L = PMe3, PPhMe2, P(4-Me-C6H4)3, PPh3 and P(4-F-C6H4)3), [Mn(CO)5(COMe)] and [Mo(PPh3)(eta(5)-C5H5)(CO)2(COMe)] react with B(C6F5)3 to form the adducts [Co(L)(CO)3(C{OB(C6F5)3}Me)] (L = PMe3, 1, PPhMe2, 2, P(4-Me-C6H4)3, 3, PPh3, 4, P(4-F-C6H4)3), 5, [Mn(CO)5(C{OB(C6F5)3}Me)] 6 and [Mo(eta(5)-C5H5)(PPh3)(CO)2(C{OB(C6F5)3}Me)], 7. Addition of B(C6F5)3 to a cooled solution of [Mo(eta(5)-C5H5)(CO)3(Me)], under an atmosphere of CO gave [Mo(eta(5)-C5H5)(CO)3(C{OB(C6F5)3}Me)] 8. In the presence of adventitious water, the compound [Co{HOB(C6F5)3}2{OP(4-F-C6H4)3}2] 9, was formed from [Co(P(4-F-C6H4)3)(CO)3(C{OB(C6F5)3}Me)]. The compounds 4 and 9 have been structurally characterised. The use of B(C6F5)3 as a catalyst for the CO-induced migratory-insertion reaction in the transition metal alkyl compounds [Co(PPh3)(CO)3(Me)], [Mn(CO)5(Me)], [Mo(eta(5)-C5H5)(CO)3(Me)] and [Fe(eta(5)-C5H5)(CO)2(Me)] has been investigated.     

Cationic hafnium silyl complexes and their enhanced reactivity in sigma-bond metathesis processes with Si-H and C-H bonds

Reaction of the mixed-ring silyl methyl complex CpCp*Hf[Si(SiMe3)3]Me (4) with B(C6F5)3 in bromobenzene-d5 yielded the zwitterionic hafnium silyl complex [CpCpHfSi(SiMe3)3][MeB(C6F5)3] (7), which is stable for at least 12 h in solution. Addition of PhSiH3 to 7 rapidly produced HSi(SiMe3)3, CpCp*HfH(mu-H)B(C6F5)3, and oligomeric silane products. Reactions of CpCp*Hf(SiR3)Me (SiR3 = SitBuPh2, SiHMes2) with B(C6F5)3 rapidly produced HSiR3 in quantitative yield along with unidentified hafnium-containing species. However, reactions of Cp2Hf(SiR3)Me (SiR3 = Si(SiMe3)3 (8), SitBuPh2 (9), SiPh3 (10)) with B(C6F5)3 quantitatively produced the corresponding cationic hafnium silyl complexes 12-14. The complex Cp2Hf(SitBuPh2)(mu-Me)B(C6F5)3 (13) was isolated by crystallization from toluene at -30 degrees C and fully characterized, and its spectroscopic properties and crystal structure are compared to those of its neutral precursor 9. The sigma-bond metathesis reaction of 13 with Mes2SiH2 yielded HSitBuPh2 and the reactive species Cp2Hf(eta(2)-SiHMes2)(mu-Me)B(C6F5)3 (16, benzene-d6), which was also generated by reaction of Cp2Hf(SiMes2H)Me (11) with B(C6F5)3. Spectroscopic data provide evidence for an unusual alpha-agostic Si-H interaction in 16. At room temperature, 16 reacts with benzene to form Cp2Hf(Ph)(mu-Me)B(C6F5)3 (17), and with toluene to give isomers of Cp2Hf(C6H4Me)(mu-Me)B(C6F5)3 (18-20) and Cp2Hf(CH2Ph)(mu-Me)B(C6F5)3 (21). The reaction with benzene is first order in both 16 and benzene. Kinetic data including activation parameters (deltaH = 19(1) kcal/mol; deltaS = -17(3) eu), a large primary isotope effect (kH/kD = 6.9(7)), and the experimentally determined rate law are consistent with a mechanism involving a concerted transition state for C-H bond activation.     

Reactivity of the bis(pentafluorophenyl)boranes ClB(C6F5)2 and [HB(C6F5)2]n towards late transition metal reagents

The reactivities of the highly electrophilic boranes ClB(C(6)F(5))(2) (1) and [HB(C(6)F(5))(2)](n) (2) towards a range of organometallic reagents featuring metals from Groups 7-10 have been investigated. Salt elimination chemistry is observed 1 between and the nucleophilic anions eta(5)-C(5)R(5))Fe(CO)(2)](-)(R = H or Me) and [Mn(CO)(5)](-), leading to the generation of the novel boryl complexes (eta(5)-C(5)R(5))Fe(CO)(2)B(C(6)F(5))(2)[R = H (3) or Me (4)] and (OC)(5)MnB(C(6)F(5))(2) (5). Such systems are designed to probe the extent to which the strongly sigma-donor boryl ligand can also act as a pi-acceptor; a variety of spectroscopic, structural and computational probes imply that even with such strongly electron withdrawing boryl substituents, the pi component of the metal-boron linkage is a relatively minor one. Similar reactivity is observed towards the hydridomanganese anion [(eta(5)-C(5)H(4)Me)Mn(CO)(2)H](-), generating a thermally labile product identified spectroscopically as (eta(5)-C(5)H(4)Me)Mn(CO)(2)(H)B(C(6)F(5))(2) (6). Boranes 1 and 2 display different patterns of reactivity towards low-valent platinum and rhodium complexes than those demonstrated previously for less electrophilic reagents. Thus, reaction of 1 with (Ph(3)P)(2)Pt(H(2)C=CH(2)) ultimately generates EtB(C(6)F(5))(2) (10) as the major boron-containing product, together with cis-(Ph(3)P)(2)PtCl(2) and trans-(Ph(3)P)(2)Pt(C(6)F(5))Cl (9). The cationic platinum hydride [(Ph(3)P)(3)PtH](+) is identified as an intermediate in the reaction pathway. Reaction of with [(Ph(3)P)(2)Rh(mu-Cl)](2), in toluene on the other hand, appears to proceed via ligand abstraction with both Ph(3)P.HB(C(6)F(5))(2) (11) and the arene rhodium(I) cation [(Ph(3)P)(2)Rh(eta(6)-C(6)H(5)Me)](+) (14) ultimately being formed.     

Activation of arene C[bond]H bonds by a cationic hafnium silyl complex possessing an alpha-agostic Si[bond]H interaction

The reaction of Cp2Hf(SiMes2H)Me (1) with B(C6F5)3 produces zwitterionic Cp2Hf(eta2-SiHMes2)(mu-Me)B(C6F5)3 (2), which is stable for >8 h at -40 degrees C in toluene-d8. Spectroscopic data provide evidence for an unusual alpha-agostic Si-H interaction in 2. At room temperature, 2 reacts with the C-H bonds of aromatic hydrocarbons such as benzene and toluene to produce Cp2Hf(Ph)(mu-Me)B(C6F5)3 (3), isomers of Cp2Hf(C6H4Me)(mu-Me)B(C6F5)3 (4-6), and Cp2Hf(CH2Ph)(mu-Me)B(C6F5)3 (7), respectively. The reaction involving benzene is first-order in both 2 and benzene; rate = k[2][C6H6]. Mechanistic data including activation parameters (DeltaH = 19(1) kcal/mol; DeltaS = -17(3) eu), a large primary isotope effect of 6.9(7), and the experimentally determined rate law are consistent with a mechanism involving a concerted transition state for C-H bond activation.     

Zwitterionic and cationic bis(phosphine) platinum(II) complexes: structural,electronic, and mechanistic comparisons relevant to ligand exchange and benzene C-H activation processes

Structurally similar but charge-differentiated platinum complexes have been prepared using the bidentate phosphine ligands [Ph(2)B(CH(2)PPh(2))(2)], ([Ph(2)BP(2)], [1]), Ph(2)Si(CH(2)PPh(2))(2), (Ph(2)SiP(2), 2), and H(2)C(CH(2)PPh(2))(2), (dppp, 3). The relative electronic impact of each ligand with respect to a coordinated metal center's electron-richness has been examined using comparative molybdenum and platinum model carbonyl and alkyl complexes. Complexes supported by anionic [1] are shown to be more electron-rich than those supported by 2 and 3. A study of the temperature and THF dependence of the rate of THF self-exchange between neutral, formally zwitterionic [Ph(2)BP(2)]Pt(Me)(THF) (13) and its cationic relative [(Ph(2)SiP(2))Pt(Me)(THF)][B(C(6)F(5))(4)] (14) demonstrates that different exchange mechanisms are operative for the two systems. Whereas cationic 14 displays THF-dependent, associative THF exchange in benzene, the mechanism of THF exchange for neutral 13 appears to be a THF independent, ligand-assisted process involving an anchimeric, eta(3)-binding mode of the [Ph(2)BP(2)] ligand. The methyl solvento species 13, 14, and [(dppp)Pt(Me)(THF)][B(C(6)F(5))(4)] (15), each undergo a C-H bond activation reaction with benzene that generates their corresponding phenyl solvento complexes [Ph(2)BP(2)]Pt(Ph)(THF) (16), [(Ph(2)SiP(2))Pt(Ph)(THF)][B(C(6)F(5))(4)] (17), and [(dppp)Pt(Ph)(THF)][B(C(6)F(5))(4)] (18). Examination of the kinetics of each C-H bond activation process shows that neutral 13 reacts faster than both of the cations 14 and 15. The magnitude of the primary kinetic isotope effect measured for the neutral versus the cationic systems also differs markedly (k(C(6)H(6))/k(C(6)D(6)): 13 = 1.26; 14 = 6.52; 15 approximately 6). THF inhibits the rate of the thermolysis reaction in all three cases. Extended thermolysis of 17 and 18 results in an aryl coupling process that produces the dicationic, biphenyl-bridged platinum dimers [[(Ph(2)SiP(2))Pt](2)(mu-eta(3):eta(3)-biphenyl)][B(C(6)F(5))(4)](2) (19) and [[(dppp)Pt](2)(mu-eta(3):eta(3)-biphenyl)][B(C(6)F(5))(4)](2) (20). Extended thermolysis of neutral [Ph(2)BP(2)]Pt(Ph)(THF) (16) results primarily in a disproportionation into the complex molecular salt [[Ph(2)BP(2)]PtPh(2)](-)[[Ph(2)BP(2)]Pt(THF)(2)](+). The bulky phosphine adducts [Ph(2)BP(2)]Pt(Me)[P(C(6)F(5))(3)] (25) and [(Ph(2)SiP(2))Pt(Me)[P(C(6)F(5))(3)]][B(C(6)F(5))(4)] (29) also undergo thermolysis in benzene to produce their respective phenyl complexes, but at a much slower rate than for 13-15. Inspection of the methane byproducts from thermolysis of 13, 14, 15, 25, and 29 in benzene-d(6) shows only CH(4) and CH(3)D. Whereas CH(3)D is the dominant byproduct for 14, 15, 25, and 29, CH(4) is the dominant byproduct for 13. Solution NMR data obtained for 13, its (13)C-labeled derivative [Ph(2)BP(2)]Pt((13)CH(3))(THF) (13-(13)()CH(3)()), and its deuterium-labeled derivative [Ph(2)B(CH(2)P(C(6)D(5))(2))(2)]Pt(Me)(THF) (13-d(20)()), establish that reversible [Ph(2)BP(2)]-metalation processes are operative in benzene solution. Comparison of the rate of first-order decay of 13 versus the decay of d(20)-labeled 13-d(20)() in benzene-d(6) affords k(13)()/k(13-d20)() approximately 3. The NMR data obtained for 13, 13-(13)()CH(3)(), and 13-d(20)() suggest that ligand metalation processes involve both the diphenylborate and the arylphosphine positions of the [Ph(2)BP(2)] auxiliary. The former type leads to a moderately stable and spectroscopically detectable platinum(IV) intermediate. All of these data provide a mechanistic outline of the benzene solution chemistries for the zwitterionic and the cationic systems that highlights their key similarities and differences.     

Structural isomers of aryl-substituted eta(3)-propargyl complexes: eta(2)-1-Metalla(methylene)cyclopropene and eta(3)-benzyl complexes

Hydride abstraction from C(5)Me(5)(CO)(2)Re(eta(2)-PhC triple bond CCH(2)Ph) (1) gave a 3:1 mixture of eta(3)-propargyl complex [C(5)Me(5)(CO)(2)Re(eta(3)-PhCH-C triple bond CPh)][BF(4)] (5) and eta(2)-1-metalla(methylene)cyclopropene complex [C(5)Me(5)(CO)(2)Re(eta(2)-PhC-C=CHPh)][BF(4)] (6). Observation of the eta(2)-isomer requires 1,3-diaryl substitution and is favored by electron-donating substituents on the C(3)-aryl ring. Interconversion of eta(3)-propargyl and eta(2)-1-metalla(methylene)cyclopropene complexes is very rapid and results in coalescence of Cp (1)H NMR resonances at about -50 degrees C. Protonation of the alkynyl carbene complex C(5)Me(5)(CO)(2)Re=C(Ph)C triple bond CPh (22) gave a third isomer, the eta(3)-benzyl complex [C(5)Me(5)(CO)(2)Re[eta(3)(alpha,1,2)-endo,syn-C(6)H(5)CH(C triple bond CC(6)H(5))]][BF(4)] (23) along with small amounts of the isomeric complexes 5 and 6. While 5 and 6 are in rapid equilibrium, there is no equilibration of the eta(3)-benzyl isomer 23 with 5 and 6.     

Subtle reactivity patterns of non-heteroatom-substituted manganese alkynyl carbene complexes in the presence of phosphorus probes

The non-heteroatom-substituted manganese alkynyl carbene complexes (eta5-MeC5H4)(CO)2Mn=C(R)C[triple bond]CR'(3; 3a: R = R'= Ph, 3b: R = Ph, R'= Tol, 3c: R = Tol, R'= Ph) have been synthesised in high yields upon treatment of the corresponding carbyne complexes [eta5-MeC5H4)(CO)2Mn[triple bond]CR][BPh4]([2][BPh4]) with the appropriate alkynyllithium reagents LiC[triple bond]CR' (R'= Ph, Tol). The use of tetraphenylborate as counter anion associated with the cationic carbyne complexes has been decisive. The X-ray structures of (eta5-MeC5H4)(CO)2Mn=C(Tol)C[triple bond]CPh (3c), and its precursor [(eta5-MeC5H4)(CO)2Mn=CTol][BPh4]([2b](BPh4]) are reported. The reactivity of complexes toward phosphines has been investigated. In the presence of PPh3, complexes act as a Michael acceptor to afford the zwitterionic sigma-allenylphosphonium complexes (eta5-MeC5H4)(CO)2MnC(R)=C=C(PPh3)R' (5) resulting from nucleophilic attack by the phosphine on the remote alkynyl carbon atom. Complexes 5 exhibit a dynamic process in solution, which has been rationalized in terms of a fast [NMR time-scale] rotation of the allene substituents around the allene axis; metrical features within the X-ray structure of (eta5-MeC5H4)(CO)2MnC(Ph)=C=C(PPh3)Tol (5b) support the proposal. In the presence of PMe3, complexes undergo a nucleophilic attack on the carbene carbon atom to give zwitterionic sigma-propargylphosphonium complexes (eta5-MeC5H4)(CO)2MnC(R)(PMe3)C[triple bond]CR' (6). Complexes 6 readily isomerise in solution to give the sigma-allenylphosphonium complexes (eta5-MeC5H4)(CO)2MnC(R')=C=C(PMe3)R (7) through a 1,3 shift of the [(eta5-MeC5H4)(CO)2Mn] fragment. The nucleophilic attack of PPh2Me on 3 is not selective and leads to a mixture of the sigma-propargylphosphonium complexes (eta5-MeC5H4)(CO)2MnC(R)(PPh(2)Me)C[triple bond]CR' (9) and the sigma-allenylphosphonium complexes (eta5-MeC5H4)(CO)2MnC(R)=C=C(PPh(2)Me)R' (10). Like complexes 6, complexes 9 readily isomerize to give the sigma-allenylphosphonium complexes (eta5-MeC5H4)(CO)2MnC(R')=C=C(PPh2Me)R'). Upon gentle heating, complexes 7, and mixtures of 10 and 10' cyclise to give the sigma-dihydrophospholium complexes (eta5-MeC5H4)(CO)2MnC=C(R')PMe2CH2CH(R)(8), and mixtures of complexes (eta5-MeC5H4)(CO)2MnC=C(Ph)PPh2CH2CH(Tol)(11) and (eta5-MeC5H4)(CO)2MnC=C(Tol)PMe2CH2CH(Ph)(11'), respectively. The reactions of complexes 3 with secondary phosphines HPR(1)(2)(R1= Ph, Cy) give a mixture of the eta2-allene complexes (eta5-MeC5H4)(CO)2Mn[eta2-{R(1)(2)PC(R)=C=C(R')H}](12), and the regioisomeric eta4-vinylketene complexes [eta5-MeC5H4)(CO)Mn[eta4-{R(1)(2)PC(R)=CHC(R')=C=O}](13) and (eta5-MeC5H4)(CO)Mn[eta4-{R(1)(2)PC(R')=CHC(R)=C=O}](13'). The solid-state structure of (eta5-MeC5H4)(CO)2Mn[eta2-{Ph2PC(Ph)=C=C(Tol)H}](12b) and (eta5-MeC5H4)(CO)Mn[eta4-{Cy2PC(Ph)=CHC(Ph)=C=O}](13d) are reported. Finally, a mechanism that may account for the formation of the species 12, 13, and 13' is proposed.     

Zwitterionic ring-borylated ansa-chromocene complexes

Ring borylation of [Me4C2(eta5-C5H4)2CrCO] by B(C6F5)3 affords the zwitterionic complex {Me4(eta5-C5H4)(eta5-C4H3B(C6F5)3)}CrH(CO) (1), the first structurally characterized bent-metallocene complex of Cr(4+). This species decomposes thermally to the zwitterionic species {Me4(eta5-C5H4)(eta5-C4H3B(C6F5)3)}Cr (2) and the ionic species [Me4C2(eta5-C5H4)2CrCO][HB(C6F5)3] (3). The molecular structure of 2 is also described.     

Stereochemical control mechanisms in propylene polymerization mediated by C1-symmetric CGC titanium catalyst centers

This work analyzes stereochemical aspects of olefin polymerization processes mediated by the C1-symmetric constrained geometry catalyst H2Si(ind)(tBuN)TiCH3+ (ind = indenyl), including the role of the cocatalyst/counteranion. The energetics of catalyst activation are first analyzed and shown to compare favorably with experiment. The energetics of heterolytic ion pair separation are next scrutinized, and the effects of solvation environment are assessed. Computed thermodynamic profiles for ethylene insertion at H2Si(ind)(tBuN)TiCH3+ indicate that the kinetics of insertion processes at the H2Si(ind)(tBuN)TiR+ cation can be analyzed in terms of SCF potential energies. We next compare the energetic profile for ethylene insertion at the naked H2Si(ind)(tBuN)TiCH3+ cation with that at the related H2Si(ind)(tBuN)TiCH3+H3CB(C6F5)3- ion pair to understand counterion effects. It is seen that the counterion, although affecting overall catalytic activity, does not significantly influence enchainment stereochemistry or polymer microtacticity. Next, the second ethylene insertion at H2Si(ind)(tBuN)Ti(nC3H7)+H3CB(C6F5)3- is analyzed to evaluate counteranion influence on the propagation barrier. It is found that the ethylene uptake transition state is energetically comparable to the first insertion transition state and that solvation has negligible effects on the energetic profile. These findings justify analysis of the propylene insertion process within the less computationally demanding "naked cation" model. Thus, monomer enchainment at H2Si(ind)(tBuN)TiR+ is analyzed for H2Si(ind)(tBuN)TiCH3+ + propylene (first insertion) and for H2Si(ind)(tBuN)Ti(iC4H6)+ + propylene (second insertion). Data describing the first insertion highlight the sterically dominated regioselection properties of the system with activation energies indicating that olefin insertion regiochemistry is predominantly 1,2 (primary), while the second insertion similarly reflects the catalyst stereoinduction properties, with steric effects introduced by the growing chain (mimicked by an isobutyl group) preferentially favoring insertion pathways that afford isotactic enrichment, in agreement with experiment.     

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 Br?nsted ([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 Br?nsted 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.     

Synthesis and structural investigation of N,N',N' '-trialkylguanidinato-supported zirconium(IV) complexes

The addition of 2 equiv of N,N',N' '-triisopropylguanidine (guanH(2)) to Zr(CH(2)Ph)(4) produced the bis(guanidinato)bis(benzyl)zirconium complex [((i)PrNH)C(N(i)Pr)(2)](2)Zr(CH(2)Ph)(2) (1). The mono(guanidinato) complex [((i)PrN)(2)C(NH(i)Pr)]ZrCl(3) (2) was accessible by the reaction of 2 equiv of guanH(2) with ZrCl(4). Guanidinium hydrochloride, [C(NH(i)Pr)(3)]Cl, is a byproduct of this reaction. When crystallized from THF, complex 2 was isolated as the THF adduct [((i)PrNH)C(N(i)Pr)(2)]ZrCl(3)(THF) (2-THF). The mixed cyclopentadienyl guanidinato complex [eta(5)-1,3-(Me(3)Si)(2)C(5)H(3)][((i)PrNH)C(N(i)Pr)(2)]ZrCl(2) (3) was prepared by treatment of [1,3-(Me(3)Si)(2)C(5)H(3)]ZrCl(3) with the in situ generated lithium triisopropylguanidinate salt. The reaction of guanH(2) with [1,3-(Me(3)Si)(2)C(5)H(3)]ZrMe(3) affords the dimethyl derivative [eta(5)-1,3-(Me(3)Si)(2)C(5)H(3)][((i)PrNH)C(N(i)Pr)(2)]ZrMe(2) (4). Definitive evidence for the molecular structures of these products is provided through single-crystal X-ray characterization of 1, 2-THF, and 3, which are presented. The extent of pi delocalization within the guanidinato ligand is discussed in the context of the metrical parameters obtained from these structural studies.     

Novel Mo(V)-dithiolene compounds: characterization of nonsymmetric dithiolene complexes by electrospray ionization mass spectrometry

Three new Mo(V) dithiolene compounds have been synthesized by addition of alkynes ((Me(3)Si)(2)C(2) (TMSA), (Me(3)Si)(2)C(4), and (Ph)(2)C(4) to MoO(2)S(2)(2-) in a MeOH/NH(3) mixture: [Mo(2)(O)(2)(mu-S)(2)(eta(2)-S(2))(eta(2)-S(2)C(2)H(2))](2)(-) 1, [Mo(2)(O)(X)(mu-S)(2)(eta(2)-S(2))(eta(2)-S(2)C(2)Ph(C(2)Ph))](2-) 2 (X = O or S), and [Mo(2)(O)(2)(mu-S)(2)(eta(2)-S(2))(eta(2)-S(2)C(2)H(C(2)H))](2-) 3. The structure of 1 as determined by single-crystal X-ray diffraction study (space group Pbca, a = 13.3148(1) A, b = 15.7467(4) A, c = 28.4108(7) A, V = 5956.7(2) A(3)) is discussed. 2 and 3 have been identified by ESMS (electrospray mass spectrometry), (1)H NMR, (13)C NMR, and infrared spectroscopies. This investigation completes our previous study devoted to the addition of DPA (C(2)Ph(2)) to MoO(2)S(2)(2-) which led to [Mo(2)(O)(X)(mu-S)(2)(eta(2)-S(2))(eta(2)-S(2)C(2)Ph(2))](2-) 4 (X = O or S). A reaction scheme is proposed to explain the formation of the different species present in solution. The reactivity of the remaining nucleophilic site of these complexes (eta(2)-S(2)) toward dicarbomethoxyacetylene (DMA) is also discussed.