Diastereoselectivity of the formation of planar chiral ruthenocenol in reactions of chiral tertiary phosphines with the (η4-cyclopentadienone)(η5-cyclopentadienyl)ruthenium cation

The reactions of chiral benzyl- and ethyl(phenyl)ferrocenylphosphines with (acetonitrile)(⁴-cyclopentadienone)(⁵-cyclopentadienyl)ruthenium trifluoromethylsulfonate proceed diastereoselectively through the attack on the cyclopentadienone ring to form planar chiral 2-phosphonioruthenocenols as mixtures of two diastereomers in ratios of 1.7 : 1 and 2 : 1, respectively.     

Nitroxide-mediated photo-living radical polymerization of methyl methacrylate in the presence of (η6-benzene)(η5-cyclopentadienyl)FeII hexafluorophosphate

The photoradical polymerization of methyl methacrylate (MMA) was performed at room temperature using (2RS,2’RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile) as the initiator and 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) as the mediator in the presence of (η⁶-benzene)(η⁵-cyclopentadienyl)Fe^II hexafluorophosphate (BzCpFe^II). The bulk polymerization provided narrower molecular weight distributions (Mw/Mn = 1.4 − 1.5) than the solution polymerization in acetonitrile, although BzCpFe^II was insoluble in MMA. The polymerization rate was retarded by an increase in the amount of BzCpFe^II. BzCpFe^II, which had no ability to control the molecular weight by itself, could control it through the interaction with MTEMPO. The interaction of BzCpFe^II and MTEMPO was attributed to the electron transfer involving the MTEMPO–aminoxy anion redox system and the iron redox system. The polymerization was confirmed to occur in accordance with a living mechanism because linear correlations were obtained for both the plots of the first order time–conversion and the conversion–molecular weight.     

Kinetics of η6→η5 isomerization of (η6-fluorenyl)(η5-cyclopentadienyl)iron

The kinetics of the reversible isomerization of the zwitterionic complex [(⁶-C₁₃H₉)Fe(⁵-C₅H₅)] (1) into dibenzoferrocene (2) was studied by electronic spectroscopy in the temperature range from 70 to 103 °C. The activation parameters of the reaction 1 2 were determined, E _a = 22.5 kcal mol^–1.     

η5-cyclopentadienyl-η2-styrenedicarbonylmanganese for initiation of free-radical polymerization of vinyl monomers

The polymerization of styrene, methyl methacrylate, and vinyl chloride catalyzed by η⁵-cyclopentadienyl-η²-styrenedicarbonylmanganese is studied. It is shown that the cyclopentadienyl complex of manganese containing the monomer ligand (styrene) in the coordination sphere can initiate the radical polymerization of vinyl monomers in a mild temperature range. On the basis of the experimental data and the quantum-chemical simulation of the initial stages of the process, schemes describing the initiation of polymerization under the action of the complex under study and the binary initiating system containing carbon tetrachloride are advanced. In the latter case, additional acceleration of the reaction is related to the interaction of carbon tetrachloride with the triplet form of the manganese complex that yields trichloromethyl radicals initiating polymerization.     

Theoretical exploration of structure-reactivity relationships in organometallic chemistry: butadiene insertion into the organyltransition-metal bond and conversion of the allyltransition-metal fragment in the [Ni(η-Cp)(η-phenyl)(η-butadiene)] complex

We have theoretically examined the reaction course of the butadiene insertion into the arylNi^II bond in the [Ni^II(η⁵-Cp)(η¹-phenyl)(η²-butadiene)] complex (1), by employing a gradient-corrected DFT method. Critical elementary processes have been scrutinized, viz. monomer insertion, rotational allylic isomerization and allylic η¹-σ→η³-π rearrangement. The first mechanism suggested by Lehmkuhl et al. was refined and supplemented with important details. The critical factors that determine the generation of anti-η³- and syn-η³-allyl isomers of the [Ni^II(η⁵-Cp)(1-benzyl-allyl)] product have been elucidated. This let us to rationalize the experimentally observed, almost exclusive formation of the anti-η³-allyl isomer. Butadiene preferably inserts in η²-mode into the η¹-phenylNi^II bond, initially giving rise to the η¹(C³)-allyl product species, 3σ. The direct formation of the η³-allyl product species, 3π, along the alternative path for η⁴-butadiene insertion, however, is found to be almost entirely disabled kinetically. The thermodynamically favorable η²-trans form of 1 is also shown to be more reactive in accomplishing CC bond formation. Species 3σ is indicated to be a metastable intermediate, occurring in an appreciable stationary concentration. Its respective anti and syn isomeric forms are likely to be in equilibrium, due to the facile rotational isomerization. The subsequent allylic rearrangement into the thermodynamically strongly favorable η³-allylNi^II coordination mode is shown to be the crucial elementary step that discriminates which of the isomeric η³-allyl forms is preferably generated. The higher reactivity of the anti isomer in this process decisively determines the almost exclusive formation of the anti-η³-allyl product species under kinetic control. The requirement of elevated temperatures for the anti-η³-allyl→syn-η³-allyl isomerization to occur, as revealed from experiment, is attributed to the pronounced thermodynamic stability of the η³-allylNi^II coordination.     

Effects of the donor properties of alkylamines on the structural and charge characteristics of (η3-allyl)· Palladium complexes

Density functional (DFT) calculations have been performed on (³-allyl)palladium complexes with amines. The effects of the donor properties of amines on the structure of the complexes and electron density distribution have been analyzed. As the number of donor methyl groups increases in the series NH_3–n (CH₃) _n , where n = 0, 1, 2, the Pd-C bond, which is in the trans position relative to the amine, becomes longer, leading to increased probability of an intraspheric attack of the amine on the appropriate terminal carbon atom of the allyl ligand; when n = 3, this tendency is violated.Original Russian Text Copyright © 2004 by O. Yu.Tkachenko and A. P. BelovTranslated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 3, pp. 442–444, May–June, 2004.     

On the structure of [η4-2,5-bis(trimethylsilyl)cyclopentadienone](η5-cyclopentadienyl)cobalt

X-ray diffraction study has been carried out on [η⁴-bis(trimethylsilyl)cyclopentadienone](η⁵-cyclopentadienyl)cobalt (I), the first example of a (η⁴-cyclopentadienone)(η⁵-cyclopentadienyl)cobalt complex with only two substituents on the cyclopentadienone ring. Complex I crystallizes in space group P2₁/c with four molecules in the unit cell and lattice parameters a 6.5259(5), b 12.2378(9), c 21.8397(15) Å and β 90.442(7)°. Extended Hückel MO calculations are discussed for a simpler model, namely the parent cyclopentadienone complex (III).     

Gas-phase reactions of the bis(η-cyclopentadienyl)methylzirconium cation with imines

The reactions of the bis(η⁵-cyclopentadienyl)methylzirconium cation (1) with four imines have been studied in the gas phase by Fourier transform ion cyclotron resonance mass spectrometry. Reactivity of the bis(η⁵-cyclopentadienyl)methylzirconium cation is governed by the availability of a labile hydrogen in the corresponding imine. The products observed differ from those that might be expected in analogy with earlier work on ketones, aldehydes and alkenes (e.g., zirconocene η³-enolate and η³-allyl complexes): azomethyne/benzylidene species are found instead of enamines; however, the general reaction mechanism resembles that proposed for ketones and alkenes. An elimination reaction leading to final products was shown to be preceded by a fast migratory insertion/deinsertion equilibrium for alkyl-substituted imines. Such an equilibrium is not possible for aryl-substituted imines due to lack of lability of the aryl groups and thus only methane elimination was observed for these imines.     

Kinetics of the reactions of methoxide ion with tricarbonyl-η-cycloheptatrienyl salts of chromium,molybdenum and tungsten

The kinetics of the reactions of methoxide ion, obtained using triethylamine/p-toluenesulphonic acid buffers in methanol, with tricarbonyl-η-cycloheptatrienylchromium, -molybdenum and -tungsten tetrafluoroborates have been investigated at 25°C using stopped-flow spectrophotometry. Two separate processes were identified. The first is a fast reversible formation of an initial product which probably arises either by direct attack at the metal or at a carbonyl ligand affording a carbomethoxy complex [(η⁷-C₇H₇M(CO)₂CO₂Me]. The second, slower, process involves addition of methoxide ion to the cycloheptatrienyl ring to give the observed product [(η⁶-MeOC₇H₇)M(CO)₃]. This latter reaction shows metal dependence; the second order rate constants are estimated to be in the proportions 50/10/1 (Cr/Mo/W).     

Insertion of alkynes into the heterocycle of (η-pentaalkyl-2,3-dihydro-1,3-diborolyl)(η-pentamethylcyclopentadienyl)ruthenium: Formation and characterization of 4-borataborepine ruthenium complexes

The violet ruthenium complex [(η⁵-C₅Me₅)Ru(η⁵-C₃B₂Me₄R¹)] (2a, R¹ = Me) reacts with terminal alkynes R²CCH to give yellow 4-borataborepine compounds [(η⁵-C₅Me₅)Ru{η⁷-(MeC)₃(R¹B)₂(R²C₂H)}] (4c, R¹ = Me, R² = Ph; 4d, R¹ = Me, R² = SiMe₃; 4e, R¹ = Me, R² = H). The insertion of alkynes into the folded C₃B₂ heterocycle of 2a causes some steric hindrance, which yields with elimination of the distant boranediyl group the corresponding boratabenzene complexes 5 as byproducts. The analogous reactions with internal alkynes R²CCR² proceed slowly and afford predominantly the boratabenzene complexes [(η⁵-C₅Me₅)Ru{η⁶-(MeC)₃(MeB)(R²C)₂}] (5f, R² = Et, 5g, R² = p-tolyl), respectively. In the latter case, three byproducts are formed: methylboronic acid and 1,2,3,4-tetra-p-tolyl-1,3-butadiene (9) due to hydrolysis of the postulated 2,3,4,5-tetra-p-tolyl-1-methylborole (10) and unexpectedly, the cationic triple-decker complex [{(η⁵-C₅Me₅)Ru}₂{μ,η⁷-(MeC)₃(MeB)₂(CH)₂}]Cl (11) having two separated CH groups. The new compounds were characterized by NMR, MS, and single-crystal X-ray studies of 4c, 5f, 9 and 11.     

Quantum chemical modeling of the cis-trans isomerization of the allyl ligand in Ni(η3-C3H5)2 in the presence of norbornadiene

Quantum chemical (DFT/PBE) modeling of the cis-trans isomerization of the allyl ligand in bis(η³-allyl)nickel in the presence of norbornadiene revealed that the type of coordination of the norbornadiene ligand affects the energy parameters of its isomerization. The Gibbs energies of activation of the rate-limiting step for different isomerization pathways range from 23.7 to 27.8 kcal mol^?1.     

(η6-1,2,3-Trimethoxybenzene) tricarbonylchromium complex: regioselectivity of the deprotonation

Deprotonation of (h⁶-1,2,3-trimethoxybenzene)tricarbonyl chromium with n-BuLi occurs mainly at the C4 carbon while the use of LiTMP gives predominantly the C5-substituted product.     

Nature of low-lying electron excited states of (η5-cyclopentadienyl) (η7-cycloheptatrienyl)niobium

Electron absorption spectra of (⁵-cyclopentadienyl)(⁷-cycloheptatrienyl)niobium in solution and in the vapor phase have been measured for the first time. Possible variants of the band assignment for the spectrum of the complex in a solution have been considered. In the spectrum of the vaporous compound the bands at 29570, 31730, and 34630 cm^–1 correspond to the electron transitions from the 4d(⁺) orbital to the lowest Rydberg p- and d-levels. The values of Rydberg transition terms and symmetry of p-states have been determined.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 634–638, April, 1994.The author is grateful to I. L. Fedyushkin for the synthesis of complex 1.     

η(6)-Hexahelicene Complexes of Iridium and Ruthenium: Running along the Helix

The first η(6)-complexes of iridium and ruthenium coordinated to helicenes have been obtained. Hexahelicene (1), 2,15-dimethylhexahelicene (2), and 2,15-dibromohexahelicene (3) react with [Cp*IrCl(2)](2) and AgBF(4) in CD(3)NO(2) to afford quantitatively the complexes [Cp*Ir(η(6)-1)][BF(4)](2) (4A), [Cp*Ir(η(6)-2)][BF(4)](2) (5A), and [Cp*Ir(η(6)-3)][BF(4)](2) (6A), respectively. In all cases, the final thermodynamic products are similar, and they exhibit coordination between the 12 e(-) metal fragment [IrCp*](2+) and the terminal ring of the helicene. Monitoring the reaction by NMR shows formation of intermediates, some of which have been fully characterized in solution. These intermediates exhibit the metal fragment coordinated to the internal rings. We have also synthesized the bimetallic complex [(Cp*Ir)(2)(μ(2)-η(6):η(6)-2)][BF(4)](4) (7), achieving coordination between two units [IrCp*](2+) and the helicene 2. Following an analogous methodology, we have prepared the complex [(η(6)-cymene)Ru(η(6)-2)][BF(4)](2) (8), which has been studied by X-ray diffraction, confirming the preferential binding to the terminal aromatic ring.     

Halogenation of the Hexaphosphabenzene Complex [(Cp*Mo)2(μ,η6:η6-P6)]: Snapshots on the Reaction Progress

The oxidation of [(Cp*Mo)₂(μ,η⁶:η⁶-P₆)] ( 1 ) with halogens or halogen sources was investigated. The iodination afforded the ionic complexes [(Cp*Mo)₂(μ,η³:η³-P₃)(μ,η¹:η¹:η¹:η¹-P₃I₃)][X] (X=I₃⁻, I⁻) ( 2 ) and [(Cp*Mo)₂(μ,η⁴:η⁴-P₄)(μ-PI₂)][I₃] ( 3 ), while the reaction with PBr₅ led to the complexes [(Cp*Mo)₂(μ,η³:η³-P₃)(μ-Br)₂][Cp*MoBr₄] ( 4 ) [(Cp*MoBr)₂(μ,η³:η³-P₃)(μ,η¹-P₂Br₃)] ( 5 ) and [(Cp*Mo)₂(μ-PBr₂)(μ-PHBr)(μ-Br)₂] ( 6 ). The reaction of 1 with the far stronger oxidizing agent PCl₅ was followed via time- and temperature-dependent ³¹P{¹H} NMR spectroscopy. One of the first intermediates detected at 193 K was [(Cp*Mo)₂(μ,η³:η³-P₃)(μ-PCl₂)₂][PCl₆] ( 8 ) which rearranges upon warming to [(Cp*Mo)₂(μ-PCl₂)₂(μ-Cl)₂] ( 9 ), [(Cp*MoCl)₂(μ,η³:η³-P₃)(μ-PCl₂)] ( 10 ) and [(Cp*Mo)₂(μ,η⁴:η⁴-P₄)(μ-PCl₂)][Cp*MoCl₄] ( 11 ), which could be isolated at room temperature. All complexes were characterized by single-crystal X-ray diffraction, NMR spectroscopy and their electronic structures were elucidated by DFT calculations.     

Exo-H- andendo-H-η4-pentamethylcyclopentadiene complexes of platinum

A mixture ofendo-H andexo-H isomers (1a and1b) of the (⁴-C₅Me₅H)PtCl₂ complex was prepared by the reaction of K₂PtCl₄ with C₅Me₅H in MeOH. The mixture of isomers reacts with CpTl in the presence of TiBF₄ to give a novel complex, [(⁴-C₅Me₅H)Pt(⁵-C₅H₅)]⁺BF₄ ^–, as a mixture ofendo-H- andexo-H-isomers (2a and2b). The data of¹H and¹³C NMR spectroscopy of the resulting complexes are discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 514–517, March, 1994.     

Synthesis and reactions of dicarbonyl (η2-indene)-η5-indenyl complexes of manganese and rhenium

Photochemical reactions of M(CO)₃(⁵-C₉H₇), where M=Mn (1) or Re (2), with indene have produced ²-indene complexes M(CO)₂(²-C₉H₈)(⁵-C₉H₇), where M=Mn (3) or Re (4). Deprotonation of complex3 witht-BuOK in THF at –60 °C gives the anion [Mn(CO)₂(¹-C₉H₇)(⁵-C₉H₇)^– (5), in which there occurs a rapid interchange of the Mn(CO)₂(⁵-C₉H₇) group between positions 1 and 3 in the ¹-indenyl ligand. The reaction of complex4 with Ph₃CPF₆ in CH₂Cl₂ at 0 °C leads to the complex [Re(CO)₂(³-C₉H₇)(⁵-C₉H₇)PF₆, whereas the similar reaction of complex3 gives only decomposition products even at –20 °C.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1280–1285, July, 1993.     

Vapor-phase electronic absorption spectrum of (η5-cyclopentadienyl)(η5-cycloheptatrienyl)chromium

The UV and visible absorption spectra of (Cp)(Ch)Cr (Cp = η⁵-C₅H₅, Ch = η⁷-C₇H₇) vapor show surprisingly well-resolved Rydberg structure. The first ionization potential (5.603 ± 0.007 eV) has been determined as a convergence limit of three Rydberg series. (Cp)(Ch)Cr is the first sandwich complex in which the lowest Rydberg transitions terminating at ns and np(σ) orbitals have vibrational structure.