Cycloolefin polymerization initiated by transition metal–π-allylic complexes

A new class of catalysts for polymerization of cycloalkenes was developed, based on the interaction of these hydrocarbons with π-allylic transition metal complexes. The reaction route (decyclization or double-bond opening) and the microstructure of polyalkenamers obtained is determined primarily by the nature of the transition metal and also by the nature and the number of ligands bound to the metal. π-Allylic complexes of zirconium and chromium catalyze the polymerization of cycloalkenes in both directions. For complexes of the group VI metals the substitution of molybdenum or tungsten for chromium results in complete suppression of the double-bond opening. In the presence of Group VIII metal compounds the polymerization of cycloolefins occurs exclusively at the double bond. The introduction of acidic ligands into the internal sphere of the π-allylic compound does not affect the reaction route but results in a significant rise of the yield of polyalkenamers and enriches the content of trans-1,4-units. Lewis acids act as activators for complexes of all the metals, the halides of molybdenum and tungsten affecting favorably the decyclization. It is assumed that the ring-opening polymerization of cycloolefins proceeds via π-allylic complex formation.     

Selective Activation of Aromatic Aldehydes Promoted by Dispersion Interactions: Steric and Electronic Factors of a π-Pocket within Cage-Shaped Borates for Molecular Recognition

Selective bond formations are one of the most important reactions in organic synthesis. In the Lewis acid mediated electrophile reactions of carbonyls, the selective formation of a carbonyl–acid complex plays a critical role in determining selectivity, which is based on the difference in the coordinative interaction between the carbonyl and Lewis acid center. Although this strategy has attained progress in selective bond formations, the discrimination between similarly sized aromatic and aliphatic carbonyls that have no functional anchors to strongly interact with the metal center still remains a challenging issue. Herein, this work focuses on molecular recognition driven by dispersion interactions within some aromatic moieties. A Lewis acid catalyst with a π-space cavity, which is referred to as a π-pocket, as the recognition site for aromatic carbonyls is designed. Cage-shaped borates 1 B with various π-pockets demonstrated significant chemoselectivity for aromatic aldehydes 3 b – f over that of aliphatic 3 a in competitive hetero-Diels–Alder reactions. The effectiveness of our catalysts was also evidenced by intramolecular recognition of the aromatic carbonyl within a dicarbonyl substrate. Mechanistic and theoretical studies demonstrated that the selective activation of aromatic substrates was driven by the preorganization step with a larger dispersion interaction, rather than the rate-determining step of the C−C bond formation, and this was likely to contribute to the preferred activation of aromatic substrates over that of aliphatic ones.     

Design of Stereoelectronically Promoted Super Lewis Acids and Unprecedented Chemistry of Their Complexes

A new family of stereoelectronically promoted aluminum and scandium super Lewis acids is introduced on the basis of state‐of‐the‐art computations. Structures of these molecules are designed to minimize resonance electron donation to central metal atoms in the Lewis acids. Acidity of these species is evaluated on the basis of their fluoride‐ion affinities relative to the antimony pentafluoride reference system. It is demonstrated that introduced changes in the stereochemistry of the designed ligands increase acidity considerably relative to Al and Sc complexes with analogous monodentate ligands. The high stability of fluoride complexes of these species makes them ideal candidates to be used as weakly coordinating anions in combination with highly reactive cations instead of conventional Lewis acid–fluoride complexes. Further, the interaction of all designed molecules with methane is investigated. All studied acids form stable pentavalent‐carbon complexes with methane. In addition, interactions of the strongest acid of this family with very weak bases, namely, H₂, N₂, carbon oxides, and noble gases were investigated; it is demonstrated that this compound can form considerably stable complexes with the aforementioned molecules. To the best of our knowledge, carbonyl and nitrogen complexes of this species are the first hypothetical four‐coordinated carbonyl and nitrogen complexes of aluminum. The nature of bonding in these systems is studied in detail by various bonding analysis approaches.     

Quantitative evaluation of Lewis acidity of metal ions with different ligands and counterions in relation to the promoting effects of Lewis acids on electron transfer reduction of oxygen

The g(zz) values of ESR spectra of superoxide (O(2)(.-) complexes of metal ion salts acting as Lewis acids with different ligands and counterions were determined in acetonitrile at 143 K. The binding energies (DeltaE) of (O(2)(.-)/Lewis acid complexes have been evaluated from deviation of the g(zz) values from the free spin value. The DeltaE value is quite sensitive to the difference in the counterions and ligands of metal ion salts acting as Lewis acids. On the other hand, the fluorescence maxima of the singlet excited states of 10-methylacridone/Lewis acid complexes are red-shifted as compared with that of 10-methylacridone, and the relative emission energies (Deltahnu(f)) vary significantly depending on the Lewis acidity of metal ion salts with different counterions and ligands. The promoting effects of Lewis acids were also examined on electron transfer from cobalt(II) tetraphenylporphyrin to oxygen in acetonitrile at 298 K, which does not occur in the absence of Lewis acids under otherwise the same experimental conditions. Both DeltaE and Deltahnu(f) values are well correlated with the promoting effects of Lewis acids on the electron transfer reduction of oxygen. Such correlations indicate that DeltaE and Deltahnu(f) values can be used as quantitative measures of Lewis acidity of metal ion salts with different ligands and counterions. The Lewis acidity thus determined can also be applied to predict the promoting effects of Lewis acids on organic synthesis.     

Assessment of the electronic properties of P ligands stemming from secondary phosphine oxides

We report the study of the net donating ability of monodentate and bidentate P ligands stemming from secondary phosphine oxides (SPOs). We experimentally measured and/or calculated the frequencies of CO stretching modes of various metal carbonyl complexes. The inferred electronic properties of the ligands span an unprecedented range, going from π-accepting phosphite-like compounds, to extremely electron-donating ligands outclassing N-heterocyclic carbenes.     

Lewis Acid Binding and Transfer as a Versatile Experimental Gauge of the Lewis Basicity of Fe0, Ru0, and Pt0 Complexes

A number of zerovalent ruthenium tri‐ and tetracarbonyl complexes of the form [Ru(CO)_5?nL_n] (n=1, 2) with neutral phosphine or N‐heterocyclic carbene donor ligands have been treated with the Lewis acids GaCl₃ and Ag⁺ to form a range of metal‐only Lewis pairs (MOLPs). The spectroscopic and structural parameters of the adducts are compared to each other and to related iron carbonyl based MOLPs. The Lewis basicity of the original Ru⁰ complexes is gauged by transfer experiments, as well as through the degree of pyramidization of the bound GaCl₃ units and the Ru?M bond lengths. The work shows the benefits of the MOLP concept as one of the few direct experimental gauges of metal basicity, and one that can allow comparisons between metal complexes with different metal centers and ligand sets.     

Discotic salicylaldimato metal complexes exhibiting columnar mesophases and their mixtures with rod-like salicylaldimato complexes

The synthesis and mesomorphism of a series of discotic metal complexes of salicylaldimine ligands is reported. None of the ligands was mesomorphic, but the longer chain length complexes showed a Col_h phase as evidenced by optical microscopy. Phase diagrams were constructed using rod- and disk-like salicylaldimato complexes, where one was designed to have Lewis acid characteristics and the other Lewis base properties. It was hoped that an associative Lewis acid/base interaction would prevent phase separation of the two components, which may point a way to the realization of a mixture with a biaxial nematic phase. The form of the phase diagrams is discussed.     

Octahedral Chiral‐at‐Metal Iridium Catalysts: Versatile Chiral Lewis Acids for Asymmetric Conjugate Additions

Octahedral iridium(III) complexes containing two bidentate cyclometalating 5‐tert‐butyl‐2‐phenylbenzoxazole ( IrO ) or 5‐tert‐butyl‐2‐phenylbenzothiazole ( IrS ) ligands in addition to two labile acetonitrile ligands are demonstrated to constitute a highly versatile class of asymmetric Lewis acid catalysts. These complexes feature the metal center as the exclusive source of chirality and serve as effective asymmetric catalysts (0.5–5.0 mol % catalyst loading) for a variety of reactions with α,β‐unsaturated carbonyl compounds, namely Friedel–Crafts alkylations (94–99 % ee), Michael additions with CH‐acidic compounds (81–97 % ee), and a variety of cycloadditions (92–99 % ee with high d.r.). Mechanistic investigations and crystal structures of an iridium‐coordinated substrates and iridium‐coordinated products are consistent with a mechanistic picture in which the α,β‐unsaturated carbonyl compounds are activated by two‐point binding (bidentate coordination) to the chiral Lewis acid.     

Change in spin state and enhancement of redox reactivity of photoexcited states of aromatic carbonyl compounds by complexation with metal ion salts acting as Lewis acids. Lewis acid-catalyzed photoaddition of benzyltrimethylsilane and tetramethyltin via photoinduced electron transfer

The lowest excited state of aromatic carbonyl compounds (naphthaldehydes, acetonaphthones, and 10-methylacridone) is changed from the n,pi triplet to the pi,pi singlet which becomes lower in energy than the n,pi triplet by the complexation with metal ions such as Mg(ClO(4))(2) and Sc(OTf)(3) (OTf = triflate), which act as Lewis acids. Remarkable positive shifts of the one-electron reduction potentials of the singlet excited states of the Lewis acid-carbonyl complexes (e.g., 1.3 V for the 1-naphthaldehyde-Sc(OTf)(3) complex) as compared to those of the triplet excited states of uncomplexed carbonyl compounds result in a significant increase in the redox reactivity of the Lewis acid complexes vs uncomplexed carbonyl compounds in the photoinduced electron-transfer reactions. Such enhancement of the redox reactivity of the Lewis acid complexes leads to the efficient C-C bond formation between benzyltrimethylsilane and aromatic carbonyl compounds via the Lewis-acid-promoted photoinduced electron transfer. The quantum yield determinations, the fluorescence quenching, and direct detection of the reaction intermediates by means of laser flash photolysis experiments indicate that the Lewis acid-catalyzed photoaddition reactions proceed via photoinduced electron transfer from benzyltrimethylsilane to the singlet excited states of Lewis acid-carbonyl complexes.     

Effect on the aromaticity of heterocyclic ligands by coordination with ruthenium electron-withdrawing metal centers

Density functional theory calculations of polypyridyl ruthenium complexes with polyaromatic ligands have been performed to understand the metal fragment effect on the modulation of their electronic properties and the influence on the aromatic character. The change of positions of the nitrogen atoms in the ligand structure, as well as the metal moiety, seems to influence the electronic behavior of the π-extended structure and the aromatic character of the complexes at both the ground and excited states. In this framework, structural, electronic, and magnetic-based aromaticity indices were used to understand the aromaticity of the free and coordinated ligands. The aromaticity character of the ligands is highly influenced by the metal fragment, and the aromaticity/antiaromaticity is achieved according to both the electron-withdrawing capability of the ligand and the metal fragment. The electronic distribution observed on the aromatic ligand determines their π-stacking ability; thus, it is proposed that the control of the π-stacking ability is modulated according to the electronic nature of the ruthenium moiety.     

SYNTHESIS AND STEREOCHEMISTRY OF PENTACARBONYL(PHOSPHOLE)METAL(0)- AND TETRACARBONYLBIS(PHOSPHOLE)METAL(0) COMPLEXES OF 6B ELEMENTS

Abstract

Pentacarbonylphospholemetal(0) and cis-tetracarbonylbis(phosphole)metal(0) complexes were synthesized from the thermal reaction of M(CO)₃(THF) and M(CO)₄(COD) (M: Cr, Mo, W) with corresponding phosphole (1-phenyl-3,4-dimethylphosphole, 1-phenyl-3-methylphosphole, and 1-phenylphosphole). These complexes were isolated as orange crystals by column chromatography on silicagel at 253 K and crystallization from n-hexane at 223 K and characterized by means of IR and NMR (¹H, ¹³C, and ³¹P). Spectroscopic data shows that the phosphole is coordinated to the transition metal through its phosphorus atom rather than through the conjugated diene unit in the both types of complexes. The tetracarbonylbis(phosphole)metal(0) complexes were found to have cis-arrangement of two phosphole ligands. Comparing ¹³C-NMR chemical shifts of the complexes with the free ligands, one can deduce that the involvement of the phosphorus atom in the ring π-electron delocalization is drastically reduced upon coordination. This is attributed to the stronger [sgrave]-donation but weaker π-accepting ability of the phosphorus atom in the phosphole ligands compared to the carbonyl groups.     

2-alkyl-1,3-butadiene polymerization under the influence of π-allylic complexes of transition metals

Stereoregulation in the polymerization of 2-alkyl-1,3-butadienes with transition metal π-allylic complexes has been studied. The direction of isoprene polymerization is shown to be a function of the nature of the metal and ligands in the allylic compound. The presence of acidic ligands in π-allylic complexes of Zr, Cr, Mo, and Co contributes to 1,4-addition and increases the selectivity of π-allylic nickel complexes, favoring cis-1,4-structure formation. Investigation of the model reaction of 2-alkyl-1,3-butadienes with bis(π-perdeuterocrotyl nickel iodide) revealed that active sites have an π-allylic type structure. The mechanism of formation of π-allylic adducts and the main factors which determine the dependence of direction and rate of polymerization on the nature of a monomer in the diene series: 2-methyl-1,3-butadiene(isoprene), 2-ethyl-1,3-butadiene, 2-isopropyl-1,3-butadiene, and 2-tert-butyl-1,3-butadiene, are discussed.     

Anionic N-heterocyclic carbenes with N,N'-bis(fluoroaryl) and N,N'-bis(perfluoroaryl) substituents

A series of rhodium complexes, [Rh(cod)(NHC-F(x))(OH(2))] (cod = 1,5-cyclooctadiene; NHC = N-heterocyclic carbene), incorporating anionic N-heterocyclic carbenes with 2-tert-butylmalonyl backbones and 2,6-dimethylphenyl (x = 0), 2,6-difluorophenyl (x = 4), 2,4,6-trifluorophenyl (x = 6), and pentafluorophenyl (x = 10) N,N'-substituents, respectively, has been prepared by deprotonation of the corresponding zwitterionic precursors with potassium hexamethyldisilazide, followed by immediate reaction of the resulting potassium salts with [{RhCl(cod)}(2)]. These complexes could be converted to the related carbonyl derivatives [Rh(CO)(2)(NHC-F(x))(OH(2))] by displacement of the COD ligand with CO. IR and NMR spectroscopy demonstrated that the degree of fluorination of the N-aryl substituents has a considerable influence on the σ-donating and π-accepting properties of the carbene ligands and could be effectively used to tune the electronic properties of the metal center. The carbonyl groups on the carbene ligand backbone provided a particularly sensitive probe for the assessment of the metal-to-ligand π donation. The ortho-fluorine substituents on the N-aryl groups in the carbene ligands interacted with the other ligands on rhodium, determining the conformation of the complexes and creating a pocket suitable for the coordination of water to the metal center. Computational studies were used to explain the influence of the fluorinated N-substituents on the electronic properties of the ligand and evaluate the relative contribution of the σ- and π-interactions to the ligand-metal interaction.     

Mononuclear metal complexes of organic carboxylic acid derivatives: synthesis, spectroscopic characterization, thermal investigation and antimicrobial activity

Two Schiff base ligands bearing organic acid moiety, vis., N-(2-thienylmethylidene)-2-amino-4-chlorobenzoic acid (HL(1)) and N-(2-hydroxybenzylidene)-2-amino-4-chlorobenzoic acid (H(2)L(2)) have been synthesized by the interaction of 2-thiophenecarboxaldehyde and 2-hydroxybenzaldehyde with 2-amino-4-chlorobenzoic acid. Co(II), Ni(II), Cu(II) and Zn(II) complexes of these ligands have been prepared. They are characterized on the basis of analytical data, molar conductance, IR, (1)H NMR, UV-vis, mass spectra, magnetic measurements, thermal analysis and X-ray powder diffraction technique. The molar conductance data reveal that these complexes are non-electrolytes. The ligands are coordinated to the metal ions in a terdentate manner with ONO/ONS donor sites of the carbonyl oxygen, azomethine nitrogen and phenolic oxygen or thiophenic sulphur. An octahedral structure is proposed for the prepared metal complexes and some ligand field parameters (D(q), B and beta) in addition to CFSE were calculated. The thermal stability of the metal complexes is evaluated. The Schiff base ligands and their metal complexes have been tested against four species of bacteria as well as four species of fungi and the results have been compared with some known antibiotics.     

Mass spectra of π-cyclopentadienyl-diene cobalt complexes

Many reports on the mass spectra of organotransition-metal complexes have appeared in recent years,¹ whilst there have only been a few reports on the mass spectra of transition metal olefin complexes, some metal carbonyl olefin complexes²³⁴ and π-cyclooctenyl-π-cyclooctadienyl cobalt.⁵ Recently fragmentation paths of π-cyclopentadienyl-cyclooctadiene rhodium were elucidated by King.⁶ The present authors found metastable ions in the mass spectra of π-cyclopentadienyl-diene cobalt complexes as well as in the mass spectra of π-cyclopentadienyl-diene rhodium complexes.⁷. In the present paper the authors wish to report the mass spectra of several π-cyclopentadienyl diene cobalt complexes.     

Substituent effects in π-(tricarbonylchromium)arenes: I. 1H NMR Spectroscopy of alkylsubstituted π-(tricarbonylchromium)benzenes

Some thirty substituted π-(tricarbonylchromium)benzenes, including some crowded alkylsubstituted complexes, e.g. (l',l'-diethylpropyl)-, (1'-t-butyl-2',2'dimethylpropyl)-, 1,2-di-t-butyl- and 1,2,4-tri-t-butyl-π-(tricarbonylchromium)benzenes, have been prepared. ¹H NMR spectra have been recorded from acetone solutions of the complexes. Using the chemical shift increments of the aromatic protons on complex formation of C₆H₅CH-t-Bu₂, as a reference for the eclipsed conformation with the substituent staggered with respect to the carbonyl ligands, the contribution of this conformer to the conformational equilibrium of the other monoalkylsubstituted complexes has been estimated, ¹H NMR spectroscopy turns out to be a useful probe in conformation analysis of π-(tricarbonylchromium)arenes.     

New entries to water-compatible Lewis acids

Lewis acid catalysis has attracted much attention in organic synthesis as it often affords access to unique reactivity and selectivity under mild conditions. Although various kinds of Lewis acids have been developed and applied in industry, these Lewis acids must be generally used under strictly anhydrous conditions, as the presence of even a small amount of water interferes with the reactions due to preferential reaction of the Lewis acids with water rather than the substrates. In contrast to this, rare earth and other metal complexes have been found to be water-compatible. Furthermore, Bi(OTf)(3)- and Ga(OTf)(3)-basic ligand complexes have also been found to be stable in water, and have been used as water-compatible Lewis acids. This application is particularly significant, as Bi(OTf)(3) and Ga(OTf)(3) themselves are unstable in the presence of water, but are stabilized by the basic ligands. This observation has led to the development of a new approach to Lewis acid catalysis in which Lewis acids that are generally unstable in the presence of water are rendered amenable to aqueous systems when combined with basic ligands. In particular, the use of chiral basic ligands leading to new types of water-compatible chiral Lewis acids may enable a wide range of asymmetric catalysis in aqueous media.     

Nouveaux complexes π-allyliques de trifluoracetato-molybdene ou - tungstene

The synthesis, physico-chemical propperties and crystal structure of two new isomorphous π-allyldicarbonyl (molybdenum or tungsten) trifluoroacetates, solvated by 1,2-dimethoxyethane, are reported.Pseudo-octahedral surrounding of the metal ion is formed by the coordination of two carbonyl ligands, of a bidentate molecule of diether lying in the square plane of the metal atom, and of a symmetrical π-allyl group and a monodentate trifluoroacetate counteranion, both ligands forming the out of plane apices.     

Slow Reactant–Water Exchange and High Catalytic Performance of Water‐Tolerant Lewis Acids

³¹P nuclear magnetic resonance (NMR) spectroscopic measurement with trimethylphosphine oxide (TMPO) was applied to evaluate the Lewis acid catalysis of various metal triflates in water. The original ³¹P NMR chemical shift and line width of TMPO is changed by the direct interaction of TMPO molecules with the Lewis acid sites of metal triflates. [Sc(OTf)₃] and [In(OTf)₃] had larger changes in ³¹P chemical shift and line width by formation of the Lewis acid–TMPO complex than other metal triflates. It originates from the strong interaction between the Lewis acid and TMPO, which results in higher stability of [Sc(OTf)₃TMPO] and [In(OTf)₃TMPO] complexes than other metal triflate–TMPO complexes. The catalytic activities of [Sc(OTf)₃] and [In(OTf)₃] for Lewis acid‐catalyzed reactions with carbonyl compounds in water were far superior to the other metal triflates, which indicates that the high stability of metal triflate–carbonyl compound complexes cause high catalytic performance for these reactions. Density functional theory (DFT) calculation suggests that low LUMO levels of [Sc(OTf)₃] and [In(OTf)₃] would be responsible for the formation of stable coordination intermediate with nucleophilic reactant in water.     

The Importance of Ligand-Induced Backdonation in the Stabilization of Square Planar d10 Nickel π-Complexes

The electronic nature of Ni π-complexes is underexplored even though these complexes have been widely postulated as intermediates in organometallic chemistry. Herein, the geometric and electronic structure of a series of nickel π-complexes, Ni(dtbpe)(X) (dtbpe=1,2-bis(di-tert-butyl)phosphinoethane; X=alkene or carbonyl containing π-ligands), is probed using a combination of ³¹P NMR, Ni K-edge XAS, Ni K_β XES, and DFT calculations. These complexes are best described as square planar d¹⁰ complexes with π-backbonding acting as the dominant contributor to M−L bonding to the π-ligand. The degree of backbonding correlates with ²J_PP from NMR and the energy of the Ni 1s→4p_z pre-edge in the Ni K-edge XAS data, and is determined by the energy of the π*_ip ligand acceptor orbital. Thus, unactivated olefinic ligands tend to be poor π-acids whereas ketones, aldehydes, and esters allow for greater backbonding. However, backbonding is still significant even in cases in which metal contributions are minor. In such cases, backbonding is dominated by charge donation from the diphosphine, which allows for strong backdonation, although the metal centre retains a formal d¹⁰ electronic configuration. This ligand-induced backbonding can be formally described as a 3-centre-4-electron (3c-4e) interaction, in which the nickel centre mediates charge transfer from the phosphine σ-donors to the π*_ip ligand acceptor orbital. The implications of this bonding motif are described with respect to both structure and reactivity.