Micro-eta3:eta4-Lithiocene and eta3:eta3-zincocene incorporating 1,2-diaza-3,5-diborolyl, a cyclopentadienyl analog

A heterocyclic cyclopentadienyl analog containing only one carbon atom in the ring was prepared and a polymeric lithiocene and a monomeric zincocene containing this novel ligand have been isolated and crystallographically characterized.     

An unprecedented, reversible coordination mode rearrangement: eta3(B-H) vs.eta1(Sn) stanna-closo-dodecaborate coordination

The synthesis of the ruthenium stanna-closo-dodecaborate complex [Bu(3)MeN](2)[Ru(dppb)(MeCN)(2)(SnB(11)H(11))(2)] by an unprecedented, reversible eta(3)(B-H) to eta(1)(Sn) rearrangement of [Bu(3)MeN](2)[Ru(dppb)(SnB(11)H(11))(2)] is described and the product is characterized by multinuclear NMR spectroscopy and single-crystal X-ray diffraction.     

Coordination of Lewis acid to eta(2)-enonepalladium(0) leading to continuous structure variation from eta(2)-olefin type to eta(3)-allyl type.

The reaction of alpha,beta-unsaturated carbonyl compounds, a palladium(0) complex, and Lewis acids led to the formation of a new class of complexes showing a wide variety of structures with eta(2)-type and eta(3)-type coordination of the carbonyl compounds. The reaction of Pd(PhCH=CHCOCH(3))(PPh(3))(2) with BF(3).OEt(2) or B(C(6)F(5))(3) quantitatively gave palladium complexes 1a,b having BX(3)-coordinated eta(2)-enonepalladium structure, as revealed by X-ray structure analysis of the B(C(6)F(5))(3) adduct 1b. On the other hand, the reaction of Pd(PhCH=CHCHO)(PPh(3))(2) with BF(3).OEt(2) or B(C(6)F(5))(3) gave distorted zwitterionic eta(3)-allylpalladium complexes 3a,b, where the Pd-carbonyl carbon distance in 3a (2.413(4) A) is much shorter than that (2.96(1) A) in 1b. The values of the P-P coupling constant and (13)C chemical shift for carbonyl carbon are useful criteria for predicting how the eta(3)-coordination mode contributes to the structure of the enone-palladium-Lewis acid system. Molecular orbital calculations on the series of model complexes suggest that orbital overlap in the highest occupied molecular orbital between the palladium and carbonyl carbon is enlarged by coordination of the Lewis acid to the carbonyl group. Palladium-catalyzed conjugate addition of R-M (R-M = AlMe(3), AlEt(3), ZnEt(2)) and its plausible reaction path are also reported.     

Polynuclear magnesium and magnesium-titanium species. Syntheses and crystal structures of [Mg4(mu3,eta2-ddbfo)2(mu,eta2-ddbfo)2(mu,eta1-ddbfo)29eta1-ddbfo2],

Tetranuclear magnesium complexes with chelating alkoxo ligands have been synthesized with the aim of investigating coordinatively unsaturated magnesium sites able to bind TiX4 (X = Cl, OR), of the type necessary for the formation of the active centers in polymerization catalysts. The magnesium compound [Mg4(mu3,eta2-ddbfo)2(mu,eta2-ddbfo)2(mu,eta1-ddbfo)2(eta1-ddbfo)2] x 2CH2Cl2 (1) (ddbfo = 2,3-dihydro-2,2-dimethyl-7-benzofuranoxide) was prepared by the reaction of MgBu2 with ddbfoH in dichloromethane. Complex 1 exists as a centrosymmetric tetranuclear species with two different types of magnesium centers corresponding to octahedral MgO6 and trigonal bipyramidal MgO5 geometry. Compound 1 is monoclinic, space group P2(1/c), with a = 12.053(2) A, b = 13.323(3) A, c = 17.069(3) A, beta = 98.50(3) degrees , and Z = 4. The reaction of 1 with methanol in tetrahydrofuran (THF) gave compound [Mg4(mu3-OMe)2(mu,eta2-ddbfo)2(mu,eta1-ddbfo)2(eta1-ddbfo)2(CH3OH)5] x CH3OH x THF (2). During this reaction one of the two five-coordinate MgO5 centers in 1 is completed by a methanol molecule and becomes octahedral in 2. Species 2 belongs to the P2(1/n) monoclinic space group, with a = 13.323(3) A, b = 20.768(4) A, c = 27.584(6) A, beta = 104.26(3) degrees , and Z = 4. Compound [Mg4(mu3,eta2-thffo)2(mu,zeta2-thffo)2(mu,eta1-thffo)2[mu-OTi(DIPP)3]2] x 2CH2Cl2 (3) is formed as a result of substitution of two thffo (thffo = 2-tetrahydrofurfuroxide) ligands bonded to the five-coordinate magnesium atom in [Mg4(thffo)8] by bulky OTi(DIPP)3 (DIPP = diisopropylphenolate) groups. Crystals of 3 are monoclinic, space group P2(1/n), with a = 17.069(3) A, b = 18.421(4) A, 17.815(4) A, beta = 90.77(3) degrees , and Z = 4. The X-ray crystal structures of complexes 1-3 are discussed in terms of explaining the role of the coordinatively unsaturated magnesium site in chiral catalyst active center formation.     

Aluminum compounds containing eta1- and/or eta5-bidentate dianionic pyrrolyl-methylamide ligands

A series of dialuminum compounds have been synthesized and their reactivity and application for lactide polymerization have been studied. The reaction of AlH3 x NMe3 with [C4H3NH(2-CH2NHtBu)] in diethyl ether generated a dimeric aluminum hydride compound, [[[C4H3N(2-CH2NtBu)]AlH]2] (1). The structure of 1 was confirmed by spectroscopy of a deuterated analogue of 1 with an Al--D function. Direct treatment of [C4H3NH(2-CH2NHtBu)] with LiAlH4 in diethyl ether resulted in colorless crystals of [[Li[micro-eta1:eta5-C4H3N(2-CH2NtBu)]2Al]2] (2) in 80 % yield after recrystallization from a toluene solution. The micro-eta1:eta5-pyrrolyl protons exhibit high-field shifts at delta=5.73, 6.15, and 6.72 comparable to a similar eta5-bonding mode in the literature. Treatment of 1 with 1 equiv acetone oxime or acetone in dichloromethane gave [[[C4H3N(2-CH2NtBu)]Al[varkappaO,varkappaN-(ON==CMe2)]]2] (3) and [[[C4H3N(2-CH2NtBu)]Al(O--CHMe2)]2] (4) in 67 % and 60 % yield, respectively. Compounds 1-4 have been characterized by X-ray diffractometry and were used as catalysts for epsilon-caprolactone polymerization.     

Unprecedented coordination modes and demetalation pathways for unbridged polyenyl ligands. Ruthenium eta1,eta4-cycloheptadienyl complexes from allyl/alkyne cycloaddition

Cationic (eta6-hexamethylbenzene)ruthenium(II) mediates the [3 + 2 + 2] cycloaddition of allyl and alkyne ligands, leading to the unexpected isolation of eta1,eta4-cycloheptadienyl complexes, an unprecedented coordination mode for transition metal complexes of simple organic rings. The nonconjugated, eta1,eta4-coordinated complex is obtained as the kinetic reaction product from treatment of the unsubstituted allyl complex with excess ethyne; this complex rearranges slowly at 80 degrees C to the thermodynamically more stable conjugated eta5-cycloheptadienyl isomer. The eta1,eta4-coordinated isomer is fluxional at room temperature, undergoing rapid and reversible equilibration with a cycloheptatriene hydride intermediate via facile beta-hydride elimination/reinsertion. The reinsertion process is remarkably regioselective, returning the nonconjugated eta1,eta4-cycloheptadienyl isomer exclusively at room temperature. For reactions incorporating dimethylacetylene dicarboxylate (DMAD) as one or both of the alkyne components, eta1,eta4-coordination appears to be both kinetically and thermodynamically favored, despite undergoing equilibration among all possible eta1,eta4-cycloheptadienyl and cycloheptatriene hydride isomers prior to arriving at one observed eta1,eta4-isomer. For this series, no isomerization to eta5-coordination is observed even upon prolonged heating. In contrast, the cyclization incorporating both DMAD and phenylacetylene proceeds directly to the eta5-cycloheptadienyl isomer at or below room temperature, indicating that eta5-coordination remains energetically accessible to this system. The DMAD-based cyclization reactions produce structurally diverse minor byproducts, including both eta1,eta4-methanocyclohexadiene and acyclic eta3,eta2-heptadienyl isomers, which have been isolated and rigorously characterized. The unusual eta1,eta4-coordination of the seven-membered ring leads to unique new organic products upon oxidative demetalation by iodinolysis. Thus, reactions with excess iodine afford bridged tricyclic cyclopropane-containing lactones or substituted cycloheptatrienes in good but sometimes variable yields, depending on the substrate and specific reaction conditions. The ruthenium in these reactions is returned in high yield as the interesting cationic mu-triiodo pseudodimer of (eta6-hexamethylbenzene)ruthenium, which is obtained as a triiodide salt. This Ru(III) complex, along with several representative Ru(II) cyclization products, has been characterized in the solid state by X-ray crystallography.     

Dynamic equilibration of eta 1-carbene and eta 2-alkyne moieties within an alkynylcarbene dimanganese complex

The coordination of an additional [Cp(CO)2Mn] fragment to the alkyne linkage of an alkynylcarbene complex of the type Cp(CO)2Mn=C(R')C identical to CR" yields a highly fluxional molecule, in which the [eta 1-carbene] and [eta 2-alkyne] moieties are seen to exchange rapidly on the NMR time scale.     

Carbon-oxygen bond cleavage with eta9,eta5-bis(indenyl)zirconium sandwich complexes

Treatment of the eta9,eta5-bis(indenyl)zirconium sandwich complex, (eta9-C9H5-1,3-(SiMe3)2)(eta5-C9H5-1,3-(SiMe3)2)Zr, with dialkyl ethers such as diethyl ether, CH3OR (R=Et, nBu, tBu), nBu2O, or iPr2O resulted in facile C-O bond scission furnishing an eta5,eta5-bis(indenyl)zirconium alkoxy hydride complex and free olefin. In cases where ethylene is formed, trapping by the zirconocene sandwich yields a rare example of a crystallographically characterized, base-free eta5,eta5-bis(indenyl)zirconium ethylene complex. Observation of normal, primary kinetic isotope effects in combination with rate studies and the stability of various model compounds support a mechanism involving rate-determining C-H activation to yield an eta5,eta5-bis(indenyl)zirconium alkyl hydride intermediate followed by rapid beta-alkoxide elimination. For isolable eta6,eta5-bis(indenyl)zirconium THF compounds, thermolysis at 85 degrees C also resulted in C-O bond cleavage to yield the corresponding zirconacycle. Both mechanistic and computational studies again support a pathway involving haptotropic rearrangement to eta5,eta5-bis(indenyl)zirconium intermediates that promote rate-determining C-H activation and ultimately C-O bond scission.     

Relative rates for the amination of eta3-allyl and eta3-benzyl complexes of palladium

Reactions of nucleophiles with metal-bound hydrocarbyl pi-ligands bound in an eta3-fashion are key steps in a variety of carbon-carbon and carbon-heteroatom bond-forming reactions. To reveal factors that control the rates of reaction of nucleophiles with this type of ligand, the rates of reactions of an aromatic and an aliphatic amine with a series of eta3-allyl, eta3-benzyl, and eta3-phenethyl palladium complexes ligated by the bisphosphine (R)-BINAP to form allylic and benzylic amines were measured. These data showed that the less common addition to an eta3-benzyl complex is faster than the more common addition to an eta3-allyl complex. The relative rates of reaction followed the trend naphthylmethyl > naphthylethyl > benzyl > 1,1-dimethylallyl > allyl. Computational studies suggest that there is a correlation between the amount of positive charge at the site of nucleophilic attack and the rate of C-N bond formation.     

Chemistry of Ru(eta6-1,3,5-cyclooctatriene)(eta2-dimethyl fumarate)2

The chemistry of a novel zerovalent Ru complex, Ru(eta6-cot)(eta2-dmfm)2 (1) (cot=1,3,5-cyclooctatriene; dmfm=dimethyl fumarate), is reviewed with a focus on its reactivity toward phosphines, amines, and H2O, as well as arenes and p-quinones. A variety of novel zerovalent Ru complexes were synthesized from Ru(eta6-cot)(eta2-dmfm)2 (1), and it was shown that the complexes preferably bear both electron-donating and -accepting ligands simultaneously to exhibit thermodynamic stability. The first isolable zerovalent Ru aqua complexes were successfully prepared, and in these complexes, the generation of a chiral center on the O atom of the coordinated H2O was disclosed. In addition, the characteristic catalytic activity of 1 in organic synthesis was considered by reviewing recently developed novel reactions: (i) dimerization of 2,5-norbornadiene to pentacyclo[6.6.0.0(2,6).0(3,13).0(10,14)]tetradeca-4,11-diene (PCTD), (ii) intramolecular hydroamination of aminoalkynes to cyclic imines, (iii) formal [4+2] cycloaddition of alkynes with dmfm to trans-4-cyclohexene-1,2-dicarboxylates, and (iv) co-dimerization of dihydrofurans with alpha,beta-unsaturated esters to 2-alkylidenetetrahydrofurans. The products obtained here are expected to be used as novel functional organic monomers.     

eta1:eta2-Alkynyl-bridged W-Si complexes: formation, structure, and reaction with acetone

Reactions of (eta5-C5Me4R)(CO)2(MeCN)WMe (R = Me, Et) with HPh2SiCCtBu gave the novel alkynyl-bridged W-Si complexes, (eta5-C5Me4R)(CO)2W(mu-eta1:eta2-CCtBu)(SiPh2) (R = Me, Et), whose alkynyl ligands bridge the tungsten and silicon atoms in an eta1:eta2-coordination mode. The structures of these complexes were fully characterized, including X-ray crystallography. Treatment of (eta5-C5Me5)(CO)2W(mu-eta1:eta2-CCtBu)(SiPh2) with acetone resulted in acetone insertion into the silicon-alkynyl linkage followed by intramolecular C-H activation of the tBu group to give the chelate-type alkyl-alkene complex, (eta5-C5Me5)(CO)2W(eta1:eta2-CH2CMe2C=CHSiPh2OCMe2).     

A tris-pyrazolylborate ligand with hemilabile O-donor groups. Examples of eta3, eta5, eta6 and bridging modes of bonding to Li+, Na+, K+, Tl+ and Ca2+ ions

A tris-pyrazolylborate ligand bearing ether appendages is shown to be a potential hemilabile ligand based on NMR studies and structural characterization of its eta3, eta5, eta6, and micro-binding modes in coordination with Li+, Na+, K+, Tl+, and Ca2+ ions.     

Novel eta1-N,eta2-C,C-binding mode between pyridinyl-functionalized cyclopentadienyl ligands and iridium

The reaction between 2-(2,3,4,5-tetramethyl-cyclopenta-1,3-dienyl)-pyridine 1 and IrCl3 was performed in an attempt to synthesize a cyclometalated system with decreased pi conjugation in the ligand. An unexpected reduction and rearrangement of bis-pyridinyl-cyclopentadienyl cyclometalated Ir(III) intermediate 2 took place yielding bis-pyridinyl-fulvene Ir(I) complex 4, which exhibits a novel bis-eta1-N,eta2-C,C-binding mode between the pyridinyl-functionalized fulvene ligand and iridium. The iridium atom in 4 is not sandwiched between two cyclopentadienyl moieties; rather, the two cyclopentadienyl groups adopt a pi-pi stacking arrangement with a centroid-centroid distance of 3.494 A. The Cl/P(wedge)O ligand-exchange reaction between 4 and 2-[(diphenylphosphanyl)-methyl]-1,1,1,3,3,3-hexafluoro-propan-2-ol 5 led to loss of one pyridinyl-functionalized fulvene ligand and produced complex 6, in which the remaining pyridinyl-functionalized fulvene ligand exhibits the mono-eta1-N,eta2-C,C-binding mode.     

Wavelength-dependent photofragmentation and photoionization of gaseous (eta4-cycloocta-1,5-diene)(eta5-cyclopentadienyl)cobalt

Gas-phase photoreactions and photoproducts of the mixed-ligand compound (eta(4)-cycloocta-1,5-diene)(eta(5)-cyclopentadienyl)cobalt are reported. Significant amounts of the monoligated complexes CoCOD and CoCp are produced, and the relative amounts are wavelength dependent. The COD ligand (with the weakest metal-ligand bonds) is always preferentially labilized as expected, but the relative amounts of the CoCOD and CoCp fragments change by 1 order of magnitude as the excitation wavelength is changed. The gas-phase photoreactions exhibit other surprising features that are uncommon in the photoreactions of organometallic compounds in the gas phase. Significant amounts of the intact cation are formed, in contrast to most reported reactions where fragmentation of the weak metal-ligand bonds precedes ionization. Most surprisingly, fragmentation of the ligands occurs while the ligands are still coordinated. The resulting metal complexes contain ligand fragments that remain coordinated to the metal. Breaking several carbon-carbon bonds with retention of weaker metal-ligand bonds is unexpected. For example, C(5)H(5) undergoes fragmentation while still coordinated to the cobalt, yielding smaller compounds such as Co(CH)(+), Co(C(2)H(2))(+), Co(C(3)H(3))(+), and Co(C(4)H(6))(+). Correspondingly, coordinated COD yields Co(C(6)H(6))(+), Co(C(5)H(5))(+), Co(C(4)H(6))(+), Co(C(3)H(3))(+), Co(C(2)H(2))(+), and Co(CH)(+). The wavelength dependence of the ligand labilization is examined by utilizing mass-selected resonance enhanced multiphoton ionization spectroscopy. Both broad bands and sharp lines are observed in the mass-selected excitation spectra. The action spectra obtained in the gas phase while monitoring the cobalt ion follow the absorption onset found in solution. The changes in fragmentation pathways are interpreted in terms of the initially accessed excited state.