Metal Olefin Bond Research Articles

Cyclometallated Platinum(II) Complexes with a Phenylpropene‐Derived π/σ‐Chelator and N‐Heterocyclic Carbenes

A series of heteroleptic platinum(II) complexes of the general formula [PtX(iPrEug)(NHC)] (2–7) bearing the organometallic π/σ‐chelator iPrEug (isopropyl eugenoxyacetate), with varying X (halido) and NHC (N‐heterocyclic carbene) ligands derived from imidazole, benzimidazole, and triazole, were prepared and fully characterized by elemental analyses, ESI mass spectrometry, and IR and NMR spectroscopy. Complexes 3 and 5–7 were also characterized by single‐crystal X‐ray diffraction. These air‐stable complexes are rare examples of platinum(II) compounds containing three different types of carbon donors, that is, aryl, carbene, and olefin. Unsymmetrical NHCs led to rotameric pairs of 4 and 5 exhibiting different NMR spectroscopic features. Spectroscopic studies also revealed that the introduction of a NHC leads to a weakening of the metal–olefin bond and strengthening of the metal–aryl bond. On the other hand, backbone and thus electronic variations among the NHCs show little influence on the bonding of the iPrEug chelator.

Influence of Electronically and Sterically Tunable Cinnamate Ligands on the Spectroscopic, Kinetic, and Thermodynamic Properties of Bis(triphenylphosphine)palladium(0) Olefin Complexes

A detailed study of the influence of electronic and steric characteristics of cinnamic acid esters on the spectroscopic, kinetic, and thermodynamic properties of bis(triphenylphosphine)palladium(0) cinnamic acid ester complexes is presented (51 different new complexes included). These complexes show a dynamic behavior on the NMR spectroscopic time scale. Therefore, the rotational barriers of the olefin about the metal–olefin bond as well as the dissociation entropy and enthalpy of the olefin and the dissociation mechanism could be determined. These findings are interpreted together with the NMR spectroscopic, IR spectroscopic, and X-ray structural data (7 new structures included) concerning the influence of the different olefin ligands on the complex properties by means of Hammett plots. DFT calculations were performed to support the mechanistic conclusions.

ChemInform Abstract: Organo-Transition Metal Compounds Containing Perfluorinated Ligands

Publisher Summary This chapter discusses the effect of α-fluorine substituents on the structure and reactivity of organic ligands bound to transition-metal centers, with particular emphasis on recent chemistry of octafluorocycloocta-1,3,5,7-tetraene (OFCOT). The chemistry of transition-metal compounds containing hydrocarbon ligands continues to be intensively explored. The most common kinds of compounds are those with metal–carbon σ bonds and metal–olefin bonds. The physical properties and chemical reactivity of transition-metal compounds containing trifluoromethyl ligands are surveyed. Complexes of cyclic perfluoroolefins are also quite common. In contrast to their hydrocarbon analogues, many unsaturated fluorocarbons react with anionic transition-metal nucleophiles to yield products arising from the net displacement of fluoride ion. The fluorocarbon complexes are notably different in their chemical properties as compared to their hydrocarbon analogues. The regio- and stereochemistry of phosphine attack were unambiguously defined by a single-crystal X-ray diffraction study. The enhanced reactivity of the indenyl complexes of the transition metals toward ligand substitution, compared to their cyclopentadienyl analogues, is due to a slippage of the indenyl ligand from η5 to an η3 bonding mode, thus making the metal center coordinatively unsaturated. Further contrasting reactivity is observed using t-butylisocyanide (t-BuNC) as the exogenous ligand. No photochemical rearrangements or transannular ring closure chemistry were observed with the corresponding cyclopentadienyl analogues. Additional chromatography allowed for the isolation of minor amounts of a second isomeric complex that could not be totally separated from the major isomer.

Chlorobis(dimethylamido)(η5-2,5-dimethylpyrrolyl)titanium(IV), [Ti(NMe2)2(DMP)Cl

The structure of the title compound, [Ti(C2H6N)2(C6H8N)Cl], displays a eta5-coordination mode for the pyrrolyl ring, confirmed by the values of the slip parameter [0.073 (9) A] and the fold angle [4.6 (6) degrees ]. This coordination is confirmed by NMR data, which point to the involvement of the complex in a fluxional process in solution above 285 K, passing through an intermediate involving simultaneously a metal-azaallyl and a metal-olefin bond.

Substitution kinetics of Cr(CO) 5(η 2- Z-cyclooctene) with tetracyanoethylene

The weakly bound Z-cyclooctene (zco) ligand in Cr(CO) 5(η 2-zco) is replaced by tetracyanoethylene (tcne) at an observable rate in the temperature range between −5 and +10 °C yielding the complex Cr(CO) 5(η 2-tcne) as the final product. The kinetics of this substitution reaction was studied in toluene solution containing 5% by volume zco by quantitative FTIR spectroscopy. The substitution reaction obeys a pseudo-first order kinetics with respect to the concentration of the starting complex. The observed rate constant, k obs, was determined at four different temperatures and five different concentrations of the entering ligand tcne in the range of 0.033–0.33 M. From the evaluation of kinetic data a possible reaction mechanism was proposed in which the rate-determining step is the cleavage of metal–olefin bond in the complex Cr(CO) 5(η 2-zco). A rate law was derived from the proposed reaction mechanism. From the dependence of k obs on the entering ligand concentration, the rate constant k 1 for the rate determining step was estimated at all temperatures. The activation enthalpy (Δ H°=100±3 kJ·mol −1) and the activation entropy (Δ S°=59±3 J·K −1·mol −1) were determined for this rate-determining step from the evaluation of k 1 values at different temperatures. The large positive value of the activation entropy is consistent with the dissociative nature of reaction. The large value of the activation enthalpy, close to the chromium–olefin bond dissociation energy, also supports this dissociative rate-determining step of the substitution reaction.

Experimental determination of the Cr-C(2)Cl(4) bond dissociation enthalpy in Cr(CO)(5)(C(2)Cl(4)): quantifying metal-olefin bonding interactions.

The bond dissociation enthalpy for the Cr-C(2)Cl(4) bond in gas-phase Cr(CO)(5)(C(2)Cl(4)) has been determined to be 12.8 +/- 1.6 kcal/mol using transient infrared spectroscopy. The results of a density functional theory-based energy decomposition analysis are used to quantify the metal-olefin bonding interactions in terms of the bonding description provided by the Dewar-Chatt-Duncanson model (sigma donation and back-bonding). The bond energy decomposition analysis reveals that metal-olefin bond strengths can be strongly influenced by the Pauli repulsion energy and by the energy necessary to deform the olefin and the metal-centered moiety from their equilibrium geometries to their geometry in the final complex. Further, a comparison between the metal-olefin bond strengths and the magnitude of the electronic interactions demonstrates that the energy associated with these deformations is the determining factor in the trends in bond enthalpies in the series of complexes Cr(CO)(5)(C(2)X(4)) (X = H, F, Cl). Though deformation of the Cr(CO)(5) moiety contributes to the overall deformation energy, the major contribution involves deformation of the olefin. This occurs as a consequence of rehybridization of the olefin as a result of metal-olefin back-bonding. The results are discussed in terms of the Dewar-Chatt-Duncanson model, which provides the accepted qualitative description of bonding in organometallic olefin complexes.

A density functional study of [M(PH3)2(η2-C2X4)] alkene complexes for the Group 10 metals Ni, Pd, Pt: the effect of electron-attracting substituents

Density functional calculations have been performed for [M(PH3)2(C2X4)] complexes with M = Ni, Pd or Pt and X = H, F or CN to study the effect of electron-attracting substituents on the metal–olefin bonding. The optimised geometries have been found to be in good agreement with experimental crystal structure data. The electronic structure has been analysed in terms of the Chatt–Dewar–Duncanson model and the contribution from π back donation was found to dominate over that from σ donation for both ethylene and substituted ethylene complexes. Reliable values have been calculated for the metal–olefin bond dissociation energies and compared with the available experimental data.

Ligand substitution kinetics in M(CO) 4 (η 2:2-1,5-cyclooctadiene) complexes (M=Cr, Mo, W) — substitution of 1,5-cyclooctadiene by bis(diphenylphosphino)alkanes

The thermal substitution kinetics of 1,5-cyclooctadiene (COD) by bis(diphenylphosphino)alkanes (PP), (C 6H 5) 2P(CH 2) n P(C 6H 5) 2 ( n = 1, 2, 3) in M(CO) 4(η 2:2-COD) complexes (M = Cr, Mo, W), were studied by quantitative FT-IR spectroscopy. The reaction rate exhibits first-order dependence on the concentration of the starting complex, and the observed rate constant depends on the concentration of the leaving COD ligand and on the concentration and the nature of the entering PP ligand. In the proposed mechanism, the rate determining step is the cleavage of one metal–olefin bond of the COD ligand. A rate-law is derived from the proposed mechanism. The evaluation of the kinetic data gives the activation parameters which support an associative mechanism in the transition states. Both the observed rate constant and the activation parameters show little variation with the chain length of the diphosphine ligand.

Displacement kinetics of 1,5-cyclooctadiene from M(CO)4(η2:2-1,5-cyclooctadiene) (M = Cr, Mo, W) by bis(diphenylphosphino)alkane

Abstract

Combination of ionic and metal-olefin bonds in crystalline copper(I) π-complexes

The ionic field effect on metal-olefin π-binding is considered for 24 Cu(I) compounds based on X-ray diffraction data. Structural features due to Cu…(C=C)−H…An and N−H…An bridges in cationic and zwitterionic π-complexes, respectively, are discussed. Parity of metal-olefin and hydrogen bonds in formation of these structures is shown.

Dichloro(1-trimethylsilyl-1,5-cyclooctadiene)palladium(II)

There are two independent molecules of [PdCl 2 (C 11 H 20 Si)] in the asymmetric unit. The coordination geometry about Pd II is square planar. The structure exhibits positional disorder. The silyl substitution at the vinylic position is found to cause a lengthening of the metal-olefin bond

Strength of the metal-olefin bond in titanium complexes related to Ziegler-Natta catalysis. A theoretical model study of a square-pyramidal active center postulated to be found in titanium halide-based catalysts

Strength of the metal-olefin bond in titanium complexes related to Ziegler-Natta catalysis. A theoretical model study of a square-pyramidal active center postulated to be found in titanium halide-based catalysts

Cyclopentadienyl-, Indenyl- und Trifluormethylcyclopentadienyl-Cobalt-Komplexe mit Akzeptor-substituierten Olefinen als Liganden / Cyclopentadienyl, Indenyl and Trifluoromethylcyclopentadienyl Cobalt Complexes with Acceptor-Substituted Olefins as Ligands

The complexes C5H5Co(PMe3)2 (1), C9H7Co(PMe3)2 (2) and (C5H4CF3)Co(PMe3)2 (3) react with mono- and disubstituted ethenes such as styrene, acrylic acid methylester, methylvinylketone, acroleine, acrylic acid amide, acrylonitrile, maleic anhydride, maleic and fumaric acid dimethylester, and fumaric dinitrile to give the olefin(phosphane)cobalt compounds (ring)Co(PMe3)(olefin) (4-32) in good to excellent yields. With acrylonitrile, two different rotamers are obtained which indicates that there is a considerable energy barrier for rotation around the metal-olefin bond. The reactions of 1 and 3 with maleic acid dimethylester (MDE) lead not only to the formation of the expected products (ring)Co(PMe3)(MDE) (22, 23) but also to the thermodynamically favored isomers (ring)Co(PMe3)(FDE) (24, 26) containing fumaric acid dimethylester (FDE) as ligand. A similar isomerization has been observed upon treatment of 2 and 3 with fumaric dinitrile (FDN) which gives besides the complexes (ring)Co(PMe3)(FDN) (29, 31) also the maleic dinitrile (MDN) derivatives (ring)Co(PMe3)(MDN) (30, 32). The olefin(trimethylphosphane)cobalt compounds are surprisingly inert and neither on photolysis nor on heating give the isomeric hydrido(vinyl)metal complexes.

Raman spectra of copper(I) and silver(I) complexes with 1,5-cyclooctadiene and the nature of metal-olefin bonds. Possibility of a copper(I)-olefin bond in cytochrome oxidase

The Raman spectra of copper(I) and silver(I) complexes with 1,5-cyclooctadiene (cod), [Cu(cod) 2]ClO 4 and [Ag(cod) 2]ClO 4, have been examined to determined the nature of the d 10 metal ion-olefin bonding. Judging from the slight lowering of ν(CC) stretching frequencies upon coordination of olefin to metal ions, the nature of the metal-olefin bonds in [Cu(cod) 2]ClO 4 and [Ag(cod) 2]ClO 4 seems to be predominantly by σ-bonding from the olefin π-orbital to the metal dσ orbital. The two strong vibrations that appear in the range from 420 to 350 cm −1 are assigned to metal-olefin bond stretching and their lower wavenumbers compared with those in the isoelectronic series of d 8 and d 10 metal complexes with cod also support the σ-bonding nature of the metal-cod bond. Similar lowering of ν(CC) stretching region (1650–1600 cm −1) upon reduction of oxidized cytochrome oxidase probably indicates that the peripheral 1-hydroxy-5,9,13-trimethyl-4,8,12- trans,trans-tetradecatrienyl substituent (trienyl group) on the heme a probably form part of the copper(I) ligation environment in the reduced copper-heme binuclear center.

The photochemistry of M(CO) 4(η 4-norbornadiene) complexes of group 6 transition metals (M = Cr, Mo, W) in low-temperature matrices

Photolysis of M(CO) 4(η 4-norbornadiene) group 6 metal complexes ( 1) in low-temperature matrices involves both loss of CO and cleavage of metal—olefin bonds to an extent depending on the metal and on the excitation wavelength. In inert matrices mer-M(CO) 3(η 4-NBD) ( 2, M = Cr), fac-M(CO) 3(η 4-NBD) ( 3, M = Cr, Mo, W), and trans-vacant M(CO) 4(η 2-NBD) ( 7, M = Mo) fragments are observed as primary photoproducts and characterized by means of IR and UV-visible spectroscopy. Secondary wavelength-dependent photoreactions of these fragments include partial regeneration of the starting material and reversible 2 ⇄ 3 photoisomerization (M = Cr). Experiments in 13CO-doped matrices and with 13CO-labelled starting material complement the characterization of 1, 2, and 3 (M = Cr). Remarkably, the cis-vacant M(CO) 4(η 2-NBD) fragment 9 could not be detected. For all three metals photolysis of 1 in the presence of excess carbon monoxide results in stepwise displacement of the NBD ligand, presumably via the initial formation of 7, yielding M(CO) 5(η 2-NBD) ( 4) and M(CO) 6. The fragments 2, 3, and 7 can take up N 2 from the matrix environment to form mer-M(CO) 3(η 4-NBD)(N 2) ( 5, M = Cr), fac-M(CO) 3(η 4-NBD)(N 2) ( 6, M = Cr, Mo, W), and trans-M(CO) 4(η 2-NBD)(N 2) ( 8, M = Mo) as the major products. The relevance of these results with regard to the mechanism of the photocatalytic hydrogenation of norbornadiene is discussed.

Gehinderte Ligandbewegungen in Übergangsmetallkomplexen, XXXV [1]: Synthese und dynamisches Verhalten von Dicarbonyl[η5:1-(2-cyclopentadiendiyl)ethyl](η2-olefin)molybdän und -wolfram / Hindered Ligand Motions in Transition Metal Complexes, XXXV [1]: Synthesis and Dynamic Behaviour of Dicarbonyl[η5:1-(2-cyclopentadiendiyl)ethyl](η2-olefin)molybdenum and -tungsten

In order to investigate the coordination ability of quasi square pyramidal complexes, tricarbonyl[η5:1-(2-cyclopentadiendiyl)ethyl]molybdenum (1) and -tungsten (2) were reacted photochemically with ethylene (a), propene (b), 1-butene (c), Z-2-butene (d), E-2-butene (e), 2-methyl-propene (f), 2-methyl-2-butene (g), 2,3-dimethyl-2-butene (h), styrene (i), 2,2-dimethylcyclopropene (j), allene (k) and acetylene (I). 1 forms [Μο(η5:1-C5Η4C2H4)(CO)2(η2-olefin)] complexes with a—g, i, and k, 2 the corresponding tungsten complexes with a, g, h, and j as ligands. There is no reaction between 1 or 2 and acetylene. 1g and 2h represent the limits for complexes with sterically demanding olefin ligands and were detected only by IR spectroscopy. Prolongated UV irradiation of 2 and ethylene yields in addition to 2a [W(η5:1-C5H 4C2H4)(CO)(η2-C2H4)2] (2a′). Hindered olefin rotations around the metal-olefin bonds were detected for 1a—1c, 1e, 1f, 1i, and 2a with barrieres of activation between 50 to 75 kJ · mol-1.

Complexes cationiques η 3-1′,2,3-[(methylene-2γ-butyrolactone)-yl-3]-(η 5-cyclopentadienyl)carbonylnitrosyl molybdene: Synthese et etudes des additions nucleophiles stereoselectives

Compound (η 3-CH 2 C(O)OCH 2C H)(η 5-C 5H 5)(CO)(NO)Mo is obtained when a carbonyl ligand of the molybdenum complex (η 3-CH 2 C(O)OCH 2C H)(η 5-C 5H 5(CO) 2Mo is replaced by the isoelectronic nitrosyl cation. In a polar solvent containing the exo-endo conformers of η 3-1′,2,3-(2γ-methylenebutyrolacton-3-yl)(η 5-cyclopentadienyl)dicarbonyl molybdenum (I), the reaction is stereoselective and proceeds with conformational interconversion. The relative exo-endo ratios of I and II have been evaluated by NMR spectroscopy at low temperature. The kinetic conformer II( endo), predominant at low temperature, is the only one that reacts towards nucleophiles. No reaction is observed with the thermodynamic compound II( exo). In all the cases studied, the nucleophilic attack occurs on the methylene carbon atom of the η 2-lactone ligand and yields η 2-olefinic nitrosyl complexes. Isolation of only one diastereoisomer confirms the stereospecificity of this addition step. For the neutral η 2-olefinic complex, a conformational equilibrium has been studied by NMR spectroscopy at low temperature: the ratios of the rotamers about the metal-olefin bond varies with solvent polarity. Oxidation by cerium ions of the η 2-olefinic molybdenum complexes yielding the free organic ligand has been achieved.

Electronic effect of η 5-cyclopentadienyl and η 3-allyl ligands on the nature of the metal—olefin bond: Reversal of the relative olefin affinity of palladium(II) and platinum(II) ions

The barrier to olefin rotation in [Pt(η 3-CH 2CMeCH 2)(olefin)(PPh 3)]PF 6 ( 3) (olefin = CH 2CH 2, E-MeCHCHMe) has been found to be extremely low compared to those in the other known, 4-coordinate olefin complexes of Pt II. This can be ascribed to the smaller steric congestion around the olefin in 3. The corresponding barrier in [Pt(η 5-C 5H 5)(CH 2CH 2)(PPh 3]ClO 4 ( 2), possessing likewise small steric congestion, was substantially higher than that in 3 (olefin = CH 2CH 2). The 13C and 31P NMR measurements have revealed much larger J(Pt-C(olefin)) in 2 than that in 3 (olefin = CH 2CH 2), while J(Pt-P) are comparable in these two. Stability constant data suggested that Pd II ion in the Pd(η 5-C 5H 5)(PPh 3) + moiety is a better π-donor to olefins than Pt II ion in the Pt(η 3-CH 2CMeCH 2)(PPh 3) + moiety, a reversal of the normal trend in the relative olefin affinity of these metal ions. The above spectral and stability features have been related to the electronic effect of the Cp ligand in enhancing the π back-bond interaction in one particular orientation of the CC bond.

A NUCLEAR MAGNETIC RESONANCE AND MASS SPECTRAL STUDY OF SOME ALKENE COMPLEXES OF RHODIUM(I) AND IRIDIUM(I): COORDINATED ALKENE ROTATIONAL BARRIERS, APPEARANCE POTENTIALS AND ESTIMATED HEATS OF FORMATION

Abstract The activation energies (ΔG) for olefin rotation in ethylene complexes of Rh(I) and Ir(I) have been estimated by signal coalescence from their temperature dependent 1H nmr spectra in solution. The barriers decrease with increase in the electron accepting nature of the associated ligand (η5-cyclopentadienyl, pentane-2, 4-dionate, carboxylate and substituted derivatives thereof) in accordance with the expected decrease in the importance of the π-contribution to the metal-olefin bond. Barriers are substantially higher for Ir(I) than for corresponding Rh(I) complexes. The complex temperature dependent spectra of di-μ-chlorotetrakis(4-methylpenta-1, 3-diene dirhodium(I) are interpreted in terms of both intra- and intermolecular processes. The greater ΔG values found for Ir(I) compared to Rh(I) complexes are reflected in the different mass spectral fragmentation patterns for corresponding members of the two series. Appearance potentials and heats of formation are estimated for [cpRh(C2 H4)2] and [cpRh(...

Metal π-complexes of cyclooctatetraenes

Abstract A 1:1 adduct of benzoycyclooctatetraene and silver perchlorate, C 12 H 10 ·AgClO 4 , 1 , has been isolated and characterized by X-ray crystallography. Complex 1 crystallizes in space group P 2 1 / c with Z = 4, a = 8.045(2), b = 17.412(4), c = 8.501(1) A, β 91.13(2)°, and V = 1190.6(4) A 3 . The structure has been refined to R F = 0.051 and R wF = 0.072 for 2165 observed Mo- K α data and 154 variables. The perchlorate group exhibits two unequally populated orientations about the central chlorine atom. The three moieties in compound 1 are interconnected by ionic (metaloxygen) and covalent (metalolefin and metalaromatic) bonds to form layers parallel to the bc plane. The coordination about the silver ion may be described as distorted trigonal bipyramidal, the ligand sites being filled by two non-adjacent double bonds of a benzocyclooctatetraene molecule, one aromatic CC bond of a neighbouring benzocyclooctatetraene molecule, and two oxygen atoms belonging to different perchlorate groups. This is the first reported instance of simultaneous donor π-bonding from aromatic and olefinic ligands to a silver ion.