Butadiene Complex Research Articles

CS-Symmetric Pyridine(diimine) Iron Methyl Complexes for Catalytic [2+2] Cycloaddition and Hydrovinylation: Metallacycle Geometry Determines Selectivity.

A series of CS-symmetric (aryl,alkyl)-substituted pyridine(dimine) iron methyl (CyARPDI)FeCH3 complexes have been prepared, characterized, and evaluated as precatalysts for the [2+2]-cycloaddition of butadiene and ethylene. Mixtures of vinylcyclobutane and (Z)-hexa-1,4-diene were observed in each case. By comparison, C2v-symmetric, arylated (PDI) iron catalysts are exclusively selective for reversible [2+2]-cycloaddition to yield vinylcyclobutane. The alteration in the chemoselectivity of the catalytic reaction was investigated through a combination of precatalyst stability studies, identification of catalytic resting state(s), and 2H and 13C isotopic labeling experiments. While replacement of an aryl-imine substituent with an N-alkyl group decreases the stability of the formally iron(0) dinitrogen and butadiene complexes, two diamagnetic metallacycles were identified as catalyst resting states. Deuterium labeling and NOESY/EXSY NMR experiments support 1,4-hexadiene arising from catalytic hydrovinylation involving reversible oxidative cyclization leading to accessible cis-metallacycle. Cyclobutane formation proceeds by irreversible C(sp3)-C(sp3) bond-forming reductive elimination from a trans-metallacycle. These studies provide key mechanistic understanding into the high selectivity of bis(arylated) pyridine(diimine) iron catalysts for [2+2]-cycloaddition, unique, thus far, to this class of iron catalysts.

Bimolecular Reductive Elimination of Ethane from Pyridine(diimine) Iron Methyl Complexes: Mechanism, Electronic Structure, and Entry into [2+2] Cycloaddition Catalysis.

The application of bimolecular reductive elimination to the activation of iron catalysts for alkene-diene cycloaddition is described. Key to this approach was the synthesis, characterization, electronic structure determination, and ultimately solution stability of a family of pyridine(diimine) iron methyl complexes with diverse steric properties and electronic ground states. Both the aryl-substituted, (MePDI)FeCH3 and (EtPDI)FeCH3 (RPDI = 2,6-(2,6-R2-C6H3N═CMe)2C5H3N), and the alkyl-substituted examples, (CyAPDI)FeCH3 (CyAPDI = 2,6-(C6H11N═CMe)2C5H3N), have molecular structures significantly distorted from planarity and S = 3/2 ground states. The related N-arylated derivative bearing 2,6-di-isopropyl aryl substituents, (iPrPDI)FeCH3, has an idealized planar geometry and exhibits spin crossover behavior from S = 1/2 to S = 3/2 states. At 23 °C under an N2 atmosphere, both (MePDI)FeCH3 and (EtPDI)FeCH3 underwent reductive elimination of ethane to form the iron dinitrogen precatalysts, [(MePDI)Fe(N2)]2(μ-N2) and [(EtPDI)Fe(N2)]2(μ-N2), respectively, while (iPrPDI)FeCH3 proved inert to C-C bond formation. By contrast, addition of butadiene to all three iron methyl complexes induced ethane formation and generated the corresponding iron butadiene complexes, (RPDI)Fe(η4-C4H6) (R = Me, Et, iPr), known precatalysts for the [2+2] cycloaddition of olefins and dienes. Kinetic, crossover experiments, and structural studies were combined with magnetic measurements and Mössbauer spectroscopy to elucidate the electronic and steric features of the iron complexes that enable this unusual reductive elimination and precatalyst activation pathway. Transmetalation of methyl groups between iron centers was fast at ambient temperature and independent of steric environment or spin state, while the intermediate dimer underwent the sterically controlled rate-determining reaction with either N2 or butadiene to access a catalytically active iron compound.

Mechanistic Insights into Molecular Crystalline Organometallic Heterogeneous Catalysis through Parahydrogen-Based Nuclear Magnetic Resonance Studies.

The heterogeneous solid-gas reactions of crystals of [Rh(L2)(propene)][BArF4] (1, L2 = tBu2PCH2CH2PtBu2) with H2 and propene, 1-butene, propyne, or 1-butyne are explored by gas-phase nuclear magnetic resonance (NMR) spectroscopy under batch conditions at 25 °C. The temporal evolution of the resulting parahydrogen-induced polarization (PHIP) effects measures catalytic flux and thus interrogates the efficiency of catalytic pairwise para-H2 transfer, speciation changes in the crystalline catalyst at the molecular level, and allows for high-quality single-scan 1H, 13C NMR gas-phase spectra for the products to be obtained, as well as 2D-measurements. Complex 1 reacts with H2 to form dimeric [Rh(L2)(H)(μ-H)]2[BArF4]2 (4), as probed using EXAFS; meanwhile, a single-crystal of 1 equilibrates NMR silent para-H2 with its NMR active ortho isomer, contemporaneously converting into 4, and 1 and 4 each convert para-H2 into ortho-H2 at different rates. Hydrogenation of propene using 1 and para-H2 results in very high initial polarization levels in propane (>85%). Strong PHIP was also detected in the hydrogenation products of 1-butene, propyne, and 1-butyne. With propyne, a competing cyclotrimerization deactivation process occurs to afford [Rh(tBu2PCH2CH2PtBu2)(1,3,4-Me3C6H3)][BArF4], while with 1-butyne, rapid isomerization of 1-butyne occurs to give a butadiene complex, which then reacts with H2 more slowly to form catalytically active 4. Surprisingly, the high PHIP hydrogenation efficiencies allow hyperpolarization effects to be seen when H2 is taken directly from a regular cylinder at 25 °C. Finally, changing the chelating phosphine to Cy2PCH2CH2PCy2 results in initial high polarization efficiencies for propene hydrogenation, but rapid quenching of the catalyst competes to form the zwitterion [Rh(Cy2PCH2CH2PCy2){η6-(CF3)2(C6H3)}BArF3].

Catalyst Design Principles Enabling Intermolecular Alkene-Diene [2+2] Cycloaddition and Depolymerization Reactions.

Aryl-substituted pyridine(diimine) iron complexes promote the catalytic [2 + 2] cycloadditions of alkenes and dienes to form vinylcyclobutanes as well as the oligomerization of butadiene to generate divinyl(oligocyclobutane), a microstructure of poly(butadiene) that is chemically recyclable. A systematic study on a series of iron butadiene complexes as well as their ruthenium congeners has provided insights into the essential features of the catalyst that promotes these cycloaddition reactions. Structural and computational studies on iron butadiene complexes identified that the structural rigidity of the tridentate pincer enables rare s-trans diene coordination. This geometry, in turn, promotes dissociation of one of the alkene arms of the diene, opening a coordination site for the incoming substrate to engage in oxidative cyclization. Studies on ruthenium congeners established that this step occurs without redox involvement of the pyridine(diimine) chelate. Cyclobutane formation occurs from a metallacyclic intermediate by reversible C(sp3)-C(sp3) reductive coupling. A series of labeling experiments with pyridine(diimine) iron and ruthenium complexes support the favorability of accessing the +3 oxidation state to trigger C(sp3)-C(sp3) reductive elimination, involving spin crossover from S = 0 to S = 1. The high density of states of iron and the redox-active pyridine(diimine) ligand facilitate this reactivity under thermal conditions. For the ruthenium congener, the pyridine(diimine) remains redox innocent and irradiation with blue light was required to promote the analogous reactivity. These structure-activity relationships highlight important design principles for the development of next generation catalysts for these cycloaddition reactions as well as the promotion of chemical recycling of cycloaddition polymers.

Tris(Butadiene) Compounds versus Butadiene Oligomerization in Second-Row Transition Metal Chemistry: Effects of Increased Ligand Fields.

The geometries, energetics, and preferred spin states of the second-row transition metal tris(butadiene) complexes (C4H6)3M (M = Zr–Pd) and their isomers, including the experimentally known very stable molybdenum derivative (C4H6)3Mo, have been examined by density functional theory. Such low-energy structures are found to have low-spin singlet and doublet spin states in contrast to the corresponding derivatives of the first-row transition metals. The three butadiene ligands in the lowest-energy (C4H6)3M structures of the late second-row transition metals couple to form a C12H18 ligand that binds to the central metal atom as a hexahapto ligand for M = Pd but as an octahapto ligand for M = Rh and Ru. However, the lowest-energy (C4H6)3M structures of the early transition metals have three separate tetrahapto butadiene ligands for M = Zr, Nb, and Mo or two tetrahapto butadiene ligands and one dihapto butadiene ligand for M = Tc. The low energy of the experimentally known singlet (C4H6)3Mo structure contrasts with the very high energy of its experimentally unknown singlet chromium (C4H6)3Cr analog relative to quintet (C12H18)Cr isomers with an open-chain C12H18 ligand.

Solid-State Molecular Organometallic Catalysis in Gas/Solid Flow (Flow-SMOM) as Demonstrated by Efficient Room Temperature and Pressure 1-Butene Isomerization.

The use of solid–state molecular organometallic chemistry (SMOM–chem) to promote the efficient double bond isomerization of 1-butene to 2-butenes under flow–reactor conditions is reported. Single crystalline catalysts based upon the σ-alkane complexes [Rh(R2PCH2CH2PR2)(η2η2-NBA)][BArF4] (R = Cy, tBu; NBA = norbornane; ArF = 3,5-(CF3)2C6H3) are prepared by hydrogenation of a norbornadiene precursor. For the tBu-substituted system this results in the loss of long-range order, which can be re-established by addition of 1-butene to the material to form a mixture of [Rh(tBu2PCH2CH2PtBu2)(cis-2-butene)][BArF4] and [Rh(tBu2PCH2CH2PtBu2)(1-butene)][BArF4], in an order/disorder/order phase change. Deployment under flow-reactor conditions results in very different on-stream stabilities. With R = Cy rapid deactivation (3 h) to the butadiene complex occurs, [Rh(Cy2PCH2CH2PCy2)(butadiene)][BArF4], which can be reactivated by simple addition of H2. While the equivalent butadiene complex does not form with R = tBu at 298 K and on-stream conversion is retained up to 90 h, deactivation is suggested to occur via loss of crystallinity of the SMOM catalyst. Both systems operate under the industrially relevant conditions of an isobutene co-feed. cis:trans selectivites for 2-butene are biased in favor of cis for the tBu system and are more leveled for Cy.

A Stable Silylene with a σ2, π- Butadiene Ligand

The synthesis of a new type of silylene 1 is reported. It adopts a bicyclo[2.1.1]hexene structure in which a hafnocene group is incorporated. The silylene is stabilized by homoconjugation with the remote C═C double bond. This is indicated by its highly shielded 29Si NMR chemical shift (δ29Si = -155) and is firmly established by its experimental molecular structure from XRD analysis. The results of a detailed bonding analysis based on DFT calculations suggest for model compounds of silylene 1 and for its heavier germanium, tin, and lead homologues uniformly electronic structures of carbene analogues that are stabilized by homoconjugation. This stabilization mode is equivalent to a σ2, π-coordination of the butadiene ligand to the element atom as it is typical for zirconocene or hafnocene butadiene complexes.

Synthesis and Electronic Structure of Iron Borate Betaine Complexes as a Route to Single-Component Iron Ethylene Oligomerization and Polymerization Catalysts

A new route to single-component iron ethylene oligomerization and polymerization catalysts is described. Treatment of readily synthesized iron butadiene complexes with B(C6F5)3 generated the corresponding betaine compounds, active catalysts for the oligomerization and polymerization of ethylene. The electronic structures of a family of iron compounds bearing tridentate, α-diimine phosphine ligands have been determined, including cases where the neutral donor has dissociated from the metal. In iron-catalyzed ethylene oligomerization with these compounds, the hemilability of the chelate has been identified as a catalyst deactiviation pathway.

Stoichiometric and Catalytic Solid–Gas Reactivity of Rhodium Bis-phosphine Complexes

The complexes [Rh(iBu2PCH2CH2PiBu2)L2][BArF4] [L2 = C4H6, (C2H4)2, (CO)2, (NH3)2; ArF = 3,5-C6H3(CF3)2] have been synthesized by solid−gas reactivity via ligand exchange reactions with, in some cases, crystallinity retained through single-crystal to single-crystal transformations. The solid-state structures of these complexes have been determined, but in only one case (L2 = (NH3)2) is the cation ordered sufficiently to enable its structural metrics to be determined by single crystal X-ray diffraction. The onward solid-state reactivity of some of these complexes has been probed. The bis-ammonia complex [Rh(iBu2PCH2CH2PiBu2)(NH3)2][BArF4] undergoes H/D exchange at bound NH3 when exposed to D2. The bis-ethene complex [Rh(iBu2PCH2CH2PiBu2)(C2H4)2][BArF4] undergoes a slow dehydrogenative coupling reaction to produce a material containing a 1:1 mixture of the butadiene complex and a postulated mono-ethene complex. The mechanisms of these processes have been probed by DFT calculations on the isolated Rh cations....

Ethylene Reactions of a [ReH(η2-BH4)(NO)(PPh3)2] Complex: Reductive Elimination of Ethane and Oxidative Coupling to Butadiene

The borohydride complex [Re+IH(η2-BH4)(NO)(PPh3)2] (1ph) reacts with ethylene to yield [Re+IH2(η2-C2H4)(NO)(PPh3)2] (2ph) and triethylborane formed by ethylene hydroboration. Subsequent ethylene insertion into the Re–H bond of 2ph and uptake of another 1 equiv of ethylene led to the kinetically stable cis-hydrido–ethyl complex [Re+IH(Et)(η2-C2H4)(NO)(PPh3)2] (3ph). 3ph was found to slowly reductively eliminate ethane. The rate of this process was determined by quantitative NMR spectroscopy in the temperature range from 293 to 338 K, enabling calculation of the activation parameters (ΔH⧧ = 68.7 kJ mol–1, ΔS⧧ = −94 J mol–1 K–1; half-life time 1.8 h at 303 K). The reaction was found to follow first-order kinetics in c(3ph) and is zeroth order in c(C2H4) and c(PPh3), ruling out preceding ligand dissociation. The presumptive intermediate [Re–I(η2-C2H4)(NO)(PPh3)2] could not be traced, since it rapidly reacted further with ethylene, furnishing the stable butadiene complex [Re–I(η2-C2H4)(η4-C4H6)(NO)(PPh3)] (4ph...

Vinyl Oxidative Coupling as a Synthetic Route to Catalytically Active Monovalent Chromium

Reaction of the deprotonated form of cis-{(t-Bu)N(H)P[μ-N(t-Bu)](2)PN(H)(t-Bu)} with CrCl(3)(THF)(3) afforded the trivalent cis-{(t-Bu)NP[μ-N(t-Bu)](2)PN(t-Bu)}[Li (THF)])CrCl(2) (1). Subsequent reaction with 2 equiv of vinyl Grignard (CH(2)=CH)Mg Cl gave the butadiene derivative (cis-{(t-Bu)NP[μ-N(t-Bu)](2)PN(t-Bu)}[Li(THF)])Cr(cis-η(4)-butadiene) (3) formally containing the metal in its monovalent state. The presence of the monovalent state was thereafter confirmed by DFT calculations. The coordination of the butadiene unit appears to be rather robust since reaction with Me(3)P afforded cleavage of the dimeric ligand core but not its displacement. The reaction formed the new butadiene complex [(t-Bu)N-P-N(t-Bu)]Cr(cis-η(4)-butadiene)PMe(3) (4) containing a regular NPN monoanion. In agreement with the presence of monovalent chromium, complexes 3 and 4 act as single-component self-activating catalysts for selective ethylene trimerization and dimerization, respectively.

Differing Reactions of Functionalized Hydrocarbons with Cp*M(NO)(alkyl)(η3-allyl) Complexes of Molybdenum and Tungsten

Cp*W(NO)(CH2CMe3)(η3-CH2CHCHMe) (1) is known to initiate facile and selective aliphatic C−H bond activations of hydrocarbons at ambient temperatures. Its ability to effect C−H activations of unfunctionalized hydrocarbon portions of more complex molecules containing various functional groups has now been investigated in some detail. In addition, molybdenum analogues of 1 have also been examined in order to see how the C−H activation chemistry is affected when the central metal is changed. Thermolyses of 1 in neat 1-chloropropane, 1-chlorobutane, and 1-bromobutane at room temperature result in activation of the terminal C−H bonds at the end opposite the carbon−halogen linkage and the clean formation of the alkyl-allyl complexes Cp*W(NO)(CH2CH2CH2Cl)(η3-CH2CHCHMe) (2), Cp*W(NO)(CH2(CH2)2CH2Cl)(η3-CH2CHCHMe) (3), and Cp*W(NO)(CH2(CH2)2CH2Br)(η3-CH2CHCHMe) (4), respectively. No reaction occurs with the C−Cl or C−Br bonds in the haloalkanes even though they are weaker than the C−H bonds that are activated. Similarly, treatment of 1 with n-Bu2O yields exclusively the terminal sp3 C−H activated product, Cp*W(NO)((CH2)4O(CH2)3CH3)(η3-CH2CHCHMe) (5), whereas the reaction with THF results in the single activation of a secondary sp3 C−H bond α to the oxygen atom in THF and the formation of Cp*W(NO)(C4H7O)(η3-CH2CHCHMe) (6). Consistently, reaction of 1 with ethylcyclohexane results in preferential activation of one of the primary sp3 C−H linkages of the ethyl group and the formation of Cp*W(NO)(CH2CH2C6H11)(η3-CH2CHCHMe) (7). The Cp*Mo(NO)(alkyl)(η3-allyl) complexes analogous to 1 are generally thermally unstable and react at or slightly above room temperature. The first member of this family of complexes to be studied was Cp*Mo(NO)(CH2CMe3)(η3-C3H5) (8), whose thermolysis in the presence of pyridine at 35 °C over 3 days leads to the formation of Cp*Mo(NO)(C5H5N)(η2-CH2═CHCH2CH2-t-Bu) (9), an η2-olefin complex in which the allyl and neopentyl ligands have coupled. The related Cp*Mo(NO)(CH2SiMe3)(η3-CH2CHCHMe) complex (11) exists as a 2:1 mixture of isomers distinguishable by the orientation of the endo, syn allyl ligand. In the less sterically congested major isomer, the methyl group on the allyl ligand is adjacent to the NO ligand, but in the minor isomer the methyl group is adjacent to the more sterically demanding CH2SiMe3 ligand. In general, the thermal reaction of 11 is similar to that of 1. Spectroscopic monitoring indicates that the loss of TMS from 11 at room temperature results in the formation of a 16e η2-diene intermediate complex that can be trapped with PMe3 as an 18e adduct, Cp*Mo(NO)(η2-CH2═CHCH═CH2)(PMe3) (12). However, reactions of 11 with various substrates (e.g., pentane, Et2O, and mesitylene) all lead to a single product, Cp*Mo(NO)(η4-trans-butadiene) (13). Evidently, the formation of the 18e butadiene complex is favored over the activation of a relatively electron-poor C−H bond by this molybdenum system. The results of DFT calculations on the model reaction of CpW(NO)(η2-CH2═CHCH═CH2) with propane confirm that the rate-determining step is the cleavage of a propane C−H bond and that the activation barrier for terminal activation is 8.6 kJ/mol lower in energy than that for internal activation. All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of complexes 3, 5, 6, 8, 9, and 13 have been established by X-ray crystallographic analyses.

Reactions of Ruthenium Cp Phosphine Complex with 4,4-Disubstituted-1,6-Enynes: Effect of Methyl Substituents in the Olefinic Fraction

We studied chemical reactions of Cp(PPh(3))(2)RuCl with nine 1,6-enyne compounds (1-4, 8, 12, 19, 21, and 22) in which the triple bond is associated with propargylic alcohol and the olefinic group has various substituted methyl groups. For the enyne compounds 1-3 with no substituted methyl group, the reaction takes place at the propargylic alcohol first giving the allenylidene complex 6 which could undergo a skeletal rearrangement to yield the disubstituted vinylidene complex 7. By changing the propargylic alcohol to propargylic ether, the reaction gives the carbene complex 10 as the major product and the butadiene complex 9 by a cyclization reaction as the minor product. For enyne 12 with two methyl groups at the terminal carbon of the olefinic part, formation of either of the carbene complexes 15 and 16 with a substituted cyclopentenyl ring at Calpha or the vinylidene complex 17 is controlled by the use of solvent. For the formation of 15 and 16, a C-C bond-forming cyclization reaction is proposed to occur at Cbeta in an intermediate where the triple bond is pi-coordinated. However, for the vinylidene intermediate, the reaction may proceed by the formation of the allenylidene, which undergoes a retro-ene reaction to bring about cleavage of the dimethyl substituted allyl group giving 17. For two enynes 21 and 22 where each olefinic portion is internally substituted with one methyl group, two vinylidene complexes 23 and 24 each with a five-membered ring bonded at Cbeta are isolated. The reaction proceeds via formation of an allenylidene intermediate followed by a cyclization at Cgamma. Stabilization of the cationic charge by the presence of methyl subsituents clearly controls the reaction pathway to give different products. These chemical reactions and their mechanisms are corroborated by structure determinations of five ruthenium complexes using single crystal X-ray diffraction analysis.

On the syntheses, NMR spectroscopic and structural characterisations of [CuL(C2H4)]·PF6 and [Cu2L2(μ-C4H6)]·2PF6: L = (±)-N,N′-bis(2,4,6-trimethylbenzylidene)-1,2-diaminocyclohexane

Copper(i) (+/-)-N,N'-bis(2,4,6-trimethylbenzylidene)-1,2-diaminocyclohexane reacts with either ethene or butadiene to give the corresponding alkene complexes. In each case crystallographic studies show that the alkene coordinates in an eta(2) fashion. In the butadiene complex a dimer is formed where the butadiene bridges two copper complexes. Variable temperature NMR spectroscopy of the ethene adduct demonstrates fluxional behaviour, which is attributed to the rotation of the coordinated ethene relative to the complex, DeltaG(double dagger)(255) = 50 +/- 2 kJ mol(-1).

Crystal structure of η5-1-(tris(pentafluorophenyl)boranylmethyl)-2,3,4,5- tetramethylcyclopentadienyl-η5,η1-3,6-di-tert-butyl-9,10,11-trimethylbicyclo( 6.3.0)undeca-3-en-8,10-dienyl)titanium(IV), Ti(C50H48BF15)

C50H48BF15Ti, monoclinic, P121/c1 (no. 14), a = 12.007(2) A, b = 19.511(4)A, c = 20.010(4)A, 3 = 98.90(3)°, V = 4631.3A, Z = 4, Rgt(F) = 0.052, wRref(F) = 0.120, T = 200 K. Source of material An amount of B(C6F5)3 (321 mg, 0.63 mmol) was dissolved in 10 ml of warm n-hexane under Ar. The obtained solution was filtered and added to a yellow-greenish filtrate of pentamethylcyclopentadienyl-trimethylcyclopentadienyl-cyclooctyne titanium(II) [1] (289 mg, 0.60 mmol) in 15 ml of warm n-hexane. The resulting mixture was allowed to stand in argon atmosphere at room temperature. In 3 days in the yellow solution the red-brown fine crystals were formed on the bottom and the walls of the vessel. The crystals were separated from the mother liquor by decanting, washed with n-hexane, and dried in vacuum to give 373 mg of the title complex. After repeated recrystallization from 20 ml of a mixture THF/n-hexane (1:1, v/v) a small amount of crystals well suited for X-ray structure analysis was obtained (m.p. 257260 °C, dec. under argon). Experimental details The hydrogen atoms (except those attached toC13, C14 andC18) were included at calculated positions and refined by using the riding model. The remaining H atoms were refined isotropically. Discussion Ion pairs [Cp)2ZrR][RB(C6F5)3] which are formed by the abstraction of methyl anions from group 4 dimethyl complexes Cp)2MMe2 (M = Ti, Zr; Cp) = substituted or unsubstituted 1cyclopentadienyl) with the strong Lewis acid B(C6F5)3 are highly active catalysts for the polymerization of 4-olefins [2]. Metallocene olefin, alkyne and butadiene complexes undergo also a similar activation for the catalytic polymerization [3]. In the title crystal structure, the titanium(IV) is coordinated by the [Me4C5-CH2-B(C6F5)3] ligand and shows the typical environment [4]. The most interesting feature is the formed trimethylcyclopentadienylcyclooctenyl ligand interacting unsymmetrically with the metal (d(Ti1—C13) = 2.222(3) A, d(Ti1—C14) = 2.410(3) A). From the Ti—C and the corresponding C—C bond distances (d(C13—C14) = 1.439(5) A and d(C14—C15) = 1.379(5) A) one can describe the interaction of this ligand by a #-Ti—C13 bond. The type of the structure is unprecedented. During the reaction the hydrogen is shifted to the trimethylcyclopentadienylcyclooctyne ligand giving a trimethylcyclopentadienylcyclooctenyl ligand. The remaining double bond does not coordinate to the metal. 64 Z. Kristallogr. NCS 223 (2008) 64-66 / DOI 10.1524/ncrs.2008.0029 © by Oldenbourg Wissenschaftsverlag, Munchen

Olefin Complexes of Low‐Valent Rhenium

AbstractReplacement of the MeCN group in the acetonitrile‐cis‐dibromo(nitrosyl)‐trans‐bis(phosphane)rhenium compounds 1a,b (R = iPr a, R = Cy b) with ethylene afforded the olefin derivatives [Re(η2‐H2C=CH2)(NO)(PR3)2Br2] (2a,b) (R = iPr a, R = Cy b). Compound 1a could be converted into the dimethyl species [Re(MeCN)(NO)(PiPr3)2Me2] (3a) applying MeLi in toluene; the related methylation of 1b, however, failed. Abstraction of a Br– ion from 1a,b with [Na][BAr′4] in acetontrile yields the air‐stable salts [Re(MeCN)2(NO)(PR3)2Br]+[BAr′4]– (4a,b) (R = iPr a, R = Cy b) and under 1 bar of H2 complexes 1a,b were converted into the known dihydrogen species [Re(η2‐H2)(NO)(PR3)2(Br)2] (5a,b) (R = iPr a, R = Cy b). Reduction of [Re(MeCN)(NO)(PR3)2Br2] (1a,b) with Na/Hg under 1 bar of C2H4 afforded the butadiene complex [Re(η4‐C4H6)(η2‐H2C=CH2)(NO)(PR3)2] (6a,b) (R = iPr a, R = Cy b) via oxidative coupling of two coordinated ethylene groups followed by double β‐H shift and subsequent reductive H2 elimination from the formed dihydride complex. Reduction of the complexes [Re(CO)(NO)(PR3)2Cl2] (7a,b) (R = iPr a, R = Cy b) with Na/Hg yields the pentacoordinate species [Re(CO)(NO)(PR3)2(η2‐H2C=CH2)] (8a,b) (R = iPr a, R = Cy b) under the same conditions as for 6a,b. Reaction of 8a with 1 equiv. of B(C6F5)3 leads to the [Re(CO)2{NOB(C6F5)3}(PiPr3)2] compound (9a) and to the carbonyl nitrosyl complexes [Re(CO)2(NO)(PiPr3)2] (10a) with evolution of ethylene. The same reaction of 8a and 8b, but applying 1 bar of CO, leads to exclusive formation of 9a,b. Complexes 4a, 4b, 6b, 8a, 9a, and 9b were characterized by X‐ray diffraction studies.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Metallacycloheptatrienes of Iridium(III): Synthesis and Reactivity

The Ir(I)−butadiene complex TpMe2Ir(η4-CH2C(Me)C(Me)CH2) (1) (TpMe2 = hydrotris(3,5-dimethylpyrazolyl)borate) reacts with ≥3 equiv of DMAD (RC⋮CR, R = CO2Me) in CH2Cl2 at 60 °C, in the presence of ...

Reactions of Ruthenium–Aminophosphane Complexes with Diynes: P–N Bond Activation and Formation of Novel Phosphaallyl, Azaallyl, and Aminocarbene Complexes

AbstractThe aminophosphane complexes [RuCp(PPh2NRR′2)(CH3CN)2]+ and [RuCp*(PPh2NRR′2)(CH3CN)2]+ (NRR′ = NHnPr, NEt2, NC5H5) react with 1,6‐heptadiyne and 1,7‐octadiyne to yield novel η3‐phosphaallyl–η2‐vinylamine complexes of the types [RuCp{η3‐(P,C,C)‐PPh2CH=C–(CH2)n–η2‐(C,C)–C=CHNRR′2}]+ and [RuCp*{η3‐(P,C,C)‐PPh2CH=C–(CH2)n–η2‐(C,C)‐C=CHNRR′2}]+ (n = 3, 4). These complexes are the kinetic products but eventually form the η1‐phosphaallyl–η3‐azaallyl complexes [RuCp{η1‐(P)‐PPh2CH=C–(CH2)n–η3‐(C,C,N)‐CCHNRR′2}]+ and [RuCp*{η1‐(P)‐PPh2CH=C–(CH2)n–η3‐(C,C,N)‐CCHNRR′2}]+, respectively. With n = 3 elevated temperatures are required, while with n = 4 this conversion takes places already at room temperature. The only exception is [RuCp*(PPh2NHnPr)(CH3CN)2]+ where amido butadiene complexes [RuCp*{η1‐(N)‐NnPrPPh2‐η4‐CH=C(CH2)nCH=CH2}]PF6 (n = 3, 4) are obtained instead. In the case of [RuCp{η1‐(P)‐PPh2CH=C–(CH2)3–η3‐(C,C,N)‐CCHNRR′2}]+ with NRR′ = NEt2 and NC5H5, a further rearrangement took place at elevated temperatures affording the aminocarbenes [RuCp{=C(NRR′)‐η2‐(C,C)‐C(CH2)3C–CH2–(η1‐(P)‐PPh2)}]+. Representative X‐ray structures are presented. Moreover, conceivable mechanisms for all these reaction sequences are established by means of DFT/B3LYP calculations. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Structural studies on platinum alkene complexes and precursors

A number of platinum complexes, precursors to alkene complexes (Pt2Cl4(PPh3)2 and cis-PtCl2(CH3CN)(PPh3)), alkene complexes (cis-PtCl2(C2H4)(PPh3), cis-PtCl2(C3H6)(PPh3) and cis-PtCl2(1-C6H12)(PPh3)), the diamination product of a 1,3-butadiene platinum complex and the 1,2,3,4-tetramethylcyclobutadiene complex resulting from dimerization of 2-butyne have been synthesized, characterized and the structures determined by X-ray diffraction. The ethylene complex, cis-PtCl2(C2H4)(PPh3), has been a useful reagent for preparing other alkene complexes. Reaction of a bound butadiene complex with diethylamine yielded a diamination product with anti-Markovnikov stereochemistry. An attempt at binding cis-butyne to the metal center resulted in metal-assisted formation of 1,2,3,4-tetramethylcyclobutadiene with previously unreported geometry.

The (butadiene)zirconocene route to active homogeneous olefin polymerization catalysts

(Butadiene)zirconocene is observed to exist as an equilibrium mixture of (s- cis-) and (s- trans-η 4-C 4H 6)ZrCl 2 isomers. The system adds a variety of unsaturated organic reagents to form metallacyclic (allyl)metallocene products. In some cases, a second equivalent of a reagent is taken up, which forms the basis of a variety of useful template coupling reactions of butadiene at the bent metallocene framework. The Lewis acid B(C 6F 5) 3 adds to (butadiene)zirconocene and to a great variety of butadiene complexes of ansa-metallocenes and related systems to give zwitterionic metallocene–butadiene–borate betaines. Most of these systems are active homogeneous Ziegler– Natta olefin polymerization catalysts, that do not require additional activation. Catalyst activities are often in a similar range to those observed for other catalyst activation procedures used in this chemistry. In the case of the (butadiene)metallocene/B(C 6F 5) 3 systems we can often observe the first olefin insertion step. This feature was utilized to carry out a variety of mechanistic studies of the essential carbon–carbon coupling steps that take place at such bent metallocene catalyst systems. Even reactions with the functionalized monomer methylmetacrylate could be followed in this way. Some chelate ligand late metal systems were also included in these studies. However, these systems mostly behaved differently, even in cases where some structural similarities were observed.