Metal Carbon Double Bond Research Articles

Reversible Dihydrogen Activation and Catalytic Transfer Hydrogenation with Iminophosphinoyl‐Tethered Ruthenium Carbene Complexes

AbstractIn the last decades, metal ligand cooperativity has intensively been used as tool in homogeneous catalysis for the development of potent catalysts for a variety of applications, above all hydrogenation and transfer hydrogenation reactions. Herein, we report on the synthesis of iminophosphinoyl‐tethered ruthenium carbene complexes and their use in the activation of dihydrogen and the catalytic transfer hydrogenation of p‐fluoroacetophenone. Variation of the N‐substituent to an electron‐withdrawing p‐nitrophenyl group enabled the reversible activation of elemental hydrogen via a 1,2‐addition across the metal carbon double bond both in solution and in solid state. In comparison to the thiophosphinoyl‐tethered carbene complex the presented iminophosphinoyl systems showed a considerably increased catalytic activity in transfer hydrogenation reactions, thus highlighting the tunability of the cooperative behavior of nucleophilic carbene complexes.

Benzyne addition to a metal-carbon multiple bond.

The linear μ-carbido complex [Rh2(μ-C)Cl2(dppm)2] (dppm = bis(diphenylphosphino)methane) reacts with a benzyne equivalent (Me3SiC6H4OTf-2/F-) to afford [Rh2(μ-CC6H4)(μ-Cl)(C6H5)Cl2(μ-dppm)2], in which the benzyne moiety adds across one of the two metal-carbon double bonds.

Di-ortho-beryllated Carbodiphosphorane: A Compound with a Metal–Carbon Double Bond to an Element of the s-Block

Double bonds have been realized for a wide variety of elements in the p-, d-, and f-blocks. However, no s-block metal complexes with a double bond have been identified. Here we report the synthesis...

Synthesis of Soluble Star-Shaped Polymers via In and Out Approach by Ring-Opening Metathesis Polymerization (ROMP) of Norbornene: Factors Affecting the Synthesis

The methods for one-pot synthesis of ‘soluble’ star-shaped polymers by sequential living ring-opening metathesis polymerization (ROMP) of norbornene (NBE) and cross-linking (CL) reagent using Mo(CHCMe2Ph)(N-2,6-iPr2C6H3)(OtBu)2 have been explored. The method (called the “in and out” or core-first approach) basically consists of (i) the living ROMP of NBE (formation of arm), (ii) reaction with CL (formation of core), (iii) additional living ROMP of NBE (propagating arms from the core, formation of star), (iv) end-modification via Wittig-type cleavage of metal–carbon double bonds containing polymer chain with aldehyde. Two different approaches in the core formation step (reaction with CL mixed with/without NBE) for synthesis of the high molecular weight star-shaped ROMP polymers with more branching, unimodal molecular weight distributions have been explored in detail. The method (reacting CL with NBE in the core formation step) under highly diluted conditions afforded the high molecular weight polymers with unimodal molecular weight distributions.

Formation of High-Oxidation-State Metal–Carbon Double Bonds

This tutorial explores the major pathways of forming metal–carbon double bonds in high-oxidation-state alkylidene complexes that began with the alkylidene chemistry of tantalum complexes in the 1970s and continued with the organometallic chemistry of Mo, W, and Re and the development of homogeneous catalysts for the metathesis of olefins. It also explores recent findings in surface organometallic chemistry and discusses the link between molecularly defined and heterogeneous catalysts. Recent results suggest that heterogeneous olefin metathesis catalysts that are activated toward metathesis upon exposure to olefins produce a d0 alkylidene through formation of a metallacyclopentane ring at d2 metal sites followed by “a ring contraction” to a metallacyclobutane, a reaction that was first observed in tantalum chemistry.

Cooperative P–H Bond Activation with Ruthenium and Iridium Carbene Complexes

The reactivity of nucleophilic ruthenium and iridium carbene complexes toward the P–H bond in secondary phosphines and phosphine oxides was studied. While the reactions of the free phosphines resulted in the formation of product mixtures, the oxidized congeners gave way to fast and selective P–H bond activation reactions by means of metal–ligand cooperation and net addition of the P–H bond across the metal–carbon double bond. The formed phosphoryl complexes were characterized in the solid state and in solution, indicating C–H···O hydrogen bonding as a structural motif. Computational studies to provide insights into the reaction mechanism revealed that—in contrast to other E–H bond activation reactions with carbene complexes—no concerted 1,2-addition across the M═C bond is operative. Instead, the P–H bond activation of the phosphine oxides preferably proceeds via coordination of the phosphinous acid tautomer to the metal, followed by hydrogen transfer to the carbenic carbon atom.

Carbene Complexes Based on Dilithium Methandiides

AbstractIn the last 15 years, methandiide ligands have given access to a new class of carbene complexes with unique electronic properties. The metal–carbon interactions in these complexes cover a range of bonding situations, from highly polar interactions to metal–carbon double bonds. This flexibility has allowed the isolation of complexes with metals covering the whole periodic table, including metals that had long been reluctant to form multiple bonds with carbon atoms. Thus, recent years have seen the revelation of many unusual carbene species with interesting reactivities. In this microreview, we focus on the latest developments in the chemistry of methandiides and their use as ligands in carbene complexes. We give an overview of the geminal dianionic compounds, their properties, and molecular and electronic structures, with special focus on those compounds that are applied in transition metal chemistry. The second part of the article deals with the preparation and electronic structures of methandiide‐derived carbene complexes, and we highlight important examples that display the unique properties and efficiency of the ligands. The last section gives an overview of the reactivity and the non‐innocent behavior of these methandiide ligands.

Synthesis, structure, and reactivity of iridium perfluorocarbene complexes: regio- and stereo-specific addition of HCl across a metal carbon double bond.

Reductive activation of an α-fluorine in the perfluoroalkyl complexes Cp*(L)(i)Ir-CF2RF using Mg/graphite leads to perfluorocarbene complexes Cp*(L)Ir[double bond, length as m-dash]CFRF (L = CO, PMe3; RF = CF3, C2F5, C6F5). New complexes E-Cp*(PMe3)Ir[double bond, length as m-dash]CFC2F5 and E-Cp*(CO)Ir[double bond, length as m-dash]CFC6F5 have been characterized by single crystal X-ray diffraction studies, and a comparison of metric parameters with previously reported analogues is reported. Experimental NMR and computational DFT (B3LYP/LACV3P**++) studies agree that for Ir[double bond, length as m-dash]CFRF complexes (RF = CF3, CF2CF3) the thermodynamic preference for the E or Z isomer depends on the steric requirements of ligand L; when L = CO the Z-isomer (F cis to Cp*) is preferred and for L = PMe3 the E-isomer is preferred. When reduction of the precursors is carried out in the dark the reaction is completely selective to produce E- or Z-isomers. Exposure of solutions of these compounds to ambient light results in slow conversion to a photostationary non-equilibrium mixture of E and Z isomers. In the dark, these E/Z mixtures convert thermally to their preferred E or Z equilibrium geometries in an even slower reaction. A study of the temperature dependent kinetics of this dark transformation allows ΔG(‡)298 for rotation about the Ir[double bond, length as m-dash]CFCF3 double bond to be experimentally determined as 25 kcal mol(-1); a DFT/B3LYP/LACV3P**++ calculation of this rotation barrier is in excellent agreement (27 kcal mol(-1)) with the experimental value. Reaction of HCl with toluene solutions of Cp*(L)Ir[double bond, length as m-dash]CFRF (L = CO, PMe3) or Cp*(CO)Ir[double bond, length as m-dash]C(CF3)2 at low temperature resulted in regiospecific addition of HCl across the metal carbon double bond, ultimately yielding Cp*(L)Ir(CHFRF)Cl and Cp*(CO)Ir[CH(CF3)2]Cl. Reaction of HCl with single E or Z diastereomers of Cp*(L)Ir[double bond, length as m-dash]CFRF gives stereospecific cis-addition to give single diastereomers of Cp*Ir(L)(CHFRF)Cl; addition of HCl to several different E/Z ratios of Cp*(L)Ir[double bond, length as m-dash]CFRF affords ratios of diastereomeric products Cp*(L)Ir(CHFRF)Cl identical to the original ratio of starting material isomers. The addition of HCl is therefore demonstrated to be unambiguously regio- and stereo-specific. The observed product regiochemistry of addition of HCl to Ir[double bond, length as m-dash]CF2, Ir[double bond, length as m-dash]CFRF, and Ir[double bond, length as m-dash]C(CF3)2 ligands is the same and is not dependent on the ground state energy preference (singlet or triplet) for the free perfluorocarbene. DFT calculations on model HCl addition reactions indicate that this regiochemistry is strongly preferred thermodynamically, but predict that in H(δ+)-Cl(δ-) addition to Cp(PH3)Ir[double bond, length as m-dash]CF2, H(δ+) attack at Ir has a lower energy transition state, while for Cp(PH3)Ir[double bond, length as m-dash]CFCF3 and Cp(PH3)Ir[double bond, length as m-dash]C(CF3)2, H(δ+) attack at C is the kinetically preferred pathway. The carbene carbon atoms in Ir[double bond, length as m-dash]CFCF3 and Ir[double bond, length as m-dash]C(CF3)2 complexes are unambiguously basic towards HCl, while in the Ir[double bond, length as m-dash]CF2 analogues the carbene carbon is less basic than its Ir partner, and the eventual regiochemistry of HCl addition arises from thermodynamic control.

Direct Observation of Metal Ketenes Formed by Photoexcitation of a Fischer Carbene using Ultrafast Infrared Spectroscopy

Fischer carbenes are commonly used as reagents in the synthesis of new carbon–carbon bonds, a reaction made possible by the unique chemistry of the formal metal–carbon double bond. Nevertheless, the photoinduced reactions of these complexes are relatively poorly understood. For instance, it has been postulated but not confirmed that visible irradiation leads to photocarbonylation, in which a CO ligand inserts into the metal–carbon bond to form a metal ketene intermediate. Here, we report the first direct observation of this intermediate following 400 nm photoexcitation of the model group 6 Fischer carbene Cr(CO)5[CCH3(OCH3)]. Using ultrafast time-resolved infrared spectroscopy (TRIR), we observe the formation of three distinct metal ketene structures, which we assign as a singlet and two isoenergetic triplet excited-state structures. The singlet relaxes to the ground state on a time scale of ∼35 ps, whereas the two triplets are long-lived (>2 ns). TRIR of the tungsten analogue yields no evidence for a metal ketene structure, consistent with the limited reactivity of this complex. The results directly elucidate the fundamental role of triplet metal ketenes in the photoreactivity of Fischer carbene complexes.

Methandiide as a non-innocent ligand in carbene complexes: from the electronic structure to bond activation reactions and cooperative catalysis.

The synthesis of a ruthenium carbene complex based on a sulfonyl-substituted methandiide and its application in bond activation reactions and cooperative catalysis is reported. In the complex, the metal-carbon interaction can be tuned between a Ru-C single bond with additional electrostatic interactions and a Ru=C double bond, thus allowing the control of the stability and reactivity of the complex. Hence, activation of polar and non-polar bonds (O-H, H-H) as well as dehydrogenation reactions become possible. In these reactions the carbene acts as a non-innocent ligand supporting the bond activation as nucleophilic center in the 1,2-addition across the metal-carbon double bond. This metal-ligand cooperativity can be applied in the catalytic transfer hydrogenation for the reduction of ketones. This concept opens new ways for the application of carbene complexes in catalysis.

Synthesis of 1‐Aza‐2‐borabutatriene Rhodium Complexes by Thermal Borylene Transfer from [(OC)5MoBN(SiMe3)2

Borylene on me: An unprecedented borylene transfer to metal-carbon double bonds afforded the title compounds. Preliminary studies revealed a thermally induced change of coordination mode from BC to CC and subsequent highly stereoselective CH activation by the BC bond. [(η5-C5H5)Rh(PCy3){(B,C-η2)-(SiMe3)2N=B=CCH2)}] and its rearrangement product were both characterized by X-ray crystallography (see picture).

Infrared study on the nature of active centers in anionic polymerization

The active species structure in alkali metal-initiated polymerization in different media has been investigated, living polydienes of low molecular weight being used as the main objects. An investigation of the IR spectra of living polydienes in the regions from 350 to 650 cm−1 and from 1500 cm−1 to 1700 cm−1 made it possible to find out the influence of the synthesis conditions on the metal-carbon and carbon-carbon double bond stretching frequencies in the active center. In alkali metal-initiated polymerization of hydrocarbon monomers the character of the polymerization process is greatly dependent on the nature of alkali metal and the solvent. By comparison of the spectral data with the microstructure of polymers formed under the same conditions the possible structures of different types of active centers are discussed.

New metal aminocarbene-substituted cyclopentadienyliron half sandwich complexes

Some new half sandwich cyclopentadienyl aminocarbene complexes were prepared by treatment of the corresponding alkoxycarbene complexes with primary and secondary amines, including enantiopure chiral aminoalcohols and amines with acetyl, phosphine, alkene, and alkyne functionalities, all of which proved to be compatible with the substrates. In two cases the complexes were crystallographically characterized indicating that the metal–carbon double bond is – in contrast to some alkoxycarbene analogues – not in conjugation with the cyclopentadienyl π-system in the solid state. Some of the complexes were characterized by cyclic voltammetry showing similar behavior as the corresponding alkoxycarbene complexes. The complexes are the first examples of half sandwich complexes with a cyclopentadienyl-aminocarbene ligand system.

High oxidation state alkylidene and alkylidyne complexes

Electron deficient high oxidation state early transition metal complexes that contain metal-carbon double or triple bonds and bulky supporting ligands have been found to be highly reactive catalysts for the alkene and alkyne metathesis reactions, respectively.

N‐Heterocyclic Carbenes: Generation under Mild Conditions and Formation of Group 8–10 Transition Metal Complexes Relevant to Catalysis

AbstractStable mono‐ and dicarbene adducts of late transition metals are readily accessible either by reaction of imidazolium salts with metal complexes bearing basic ligands, or by the cleavage of chloro‐ and acetato‐bridged dinuclear metal complexes with the free carbenes (e.g., 1,3‐dimethylimidazoline‐2‐ylidene). A general novel method for the deprotonation of N‐substituted azolium salts in liquid ammonia–the liquid ammonia route–is described. This method yields not only the known free monimeric 1,3‐dimethylimidazoline‐2‐ylidene in quantitative yield, but also otherwise hardly accessible derivatives. For example, imidazoline‐2‐ylidenes with linear, branched, cyclic, heteroatom‐substituted and even chiral hydrocarbon residues can be obtained. The nucleophilic behaviour of 1,3‐dimethyl‐imidazoline‐2‐ylidene is reported and compared with that of other donor ligands. Novel carbene complexes of RuII, RhIPdII, OsIIand IrIare presented. Reaction of the potentially chelating ligand 1,1′‐(1,2‐ethylene)‐3,3′‐dimethyldiimidazoline‐2,2′‐diylidene with [(COD)‐RhCl]2yields a dinuclear complex, in which two rhodium centres are linked by the dicarbene bridge. Four single‐crystal X‐ray diffraction structures ofnewmetalcarbene complexes (Rh, Os) are reported. N‐Heterocyclic carbene complexes of Group 8–10 transition metals are both thermally and chemically very stable. They do not show the typical reactivity of metal–carbon double bonds. For a number of reasons, these complexes must be regarded as donor adducts of the Lewis‐basicimidazoline‐2‐ylidene ligand and the Lewis‐acidicorganometallic fragment.

Photochemistry of Metalloporphyrin Carbene Complexes

The photochemistry of metalloporphyrins and other tetrapyrrolic systems continues to attract considerable interest.1 There have been numerous studies on the photoactivity of various porphyrin complexes with single-bonded axial ligands, including halogens, oxo-anion species, molecular oxygen,2 and a variety of metals, including chromium, manganese, and iron.3 The photochemistry of metalloporphyrins with multiple-bonded axial ligands has not been explored. We report here the photochemistry of several iron porphyrin carbene and vinylidene complexes. Irradiation of these complexes with visible light cleaves the iron-carbon double bond, resulting in a four-coordinate iron(II) porphyrin and a free carbene. This chemistry is unique in the area of transition metal carbene or alkylidene compounds. Porphyrins with multiple-bonded axial ligands are especially relevant to both biological and catalytic oxidative processes.4 Iron porphyrin carbene and nitrene species are isoelectronic with a terminal iron-oxo complex, the proposed intermediate of the cytochrome P-450 monooxygenases and its synthetic analogs. Most recently, the catalytic cyclopropanation of olefins by iron porphyrins using ethyl diazoacetate was thought to involve a porphyrin carbene complex.5 We prepared Fe(TPP)CCl2, Fe(TPP)CBr2, Fe(TPP)CClF, and Fe(TPP)CC(p-C6H4Cl)2 under anaerobic conditions following Mansuy’s procedure using iron metal as the reductant (TPP ) 5,10,15,20-tetraphenylporphyrinate).6 These compounds were further purified via crystallization from benzene/pentane under argon in an inert atmosphere box and then fully characterized.7 To examine the photochemistry of these complexes, degassed benzene solutions (∼1 × 10-4 M) were irradiated with a 300 W Xe arc lamp, which was filtered to remove both infrared and ultraviolet (<360 nm) light in order to prevent sample heating and porphyrin bleaching, respectively. In addition, the photolysis cells were thermostated to an internal solution temperature of 20 °C. In all cases, irradiation of the parent compounds cleanly produced a new porphyrin identified as Fe(TPP) (λmax ) 419, 442, and 537 nm), as shown in Figure 1 for Fe(TPP)CCl2. Both the Soret and Q-bands of the parent compounds are photoactive.8 The formation of Fe(TPP) was confirmed by addition of pyridine after photolysis of the carbene or vinylidene complexes, yielding the bispyridine complex, Fe(TPP)(py)2. Also as expected, exposure of photolysed solutions to oxygen resulted in the formation of the μ-oxo dimer species, [Fe(TPP)]2O. The production of Fe(TPP) in this reaction is indicative of homolytic cleavage of the metal-carbon double bond and production of a carbene, as in the following reaction:

Transition Metal Fischer-Type Complexes. Density Functional Analysis of the Systems (CO)5CrEH2 (E = C, Si, Ge, Sn) and (CO)5MCH2 (M = Mo, W, Mn+)

The electronic and molecular structures of the title compounds have been investigated using density functional theory within the local density approximation, adding nonlocal corrections to exchange and correlation energy as a perturbation. The transition metal main group double bond was analyzed in terms of σ and π bond contributions. The main difference between chromium pentacarbonyl complexes with the carbene fragment compared to their higher homologues is a significant drop in the intrinsic π bond strength: Dπ,int(MC) = 202 kJ/mol, Dπ,int (MSi) = 82 kJ/mol, Dπ,int (MGe) = 72 kJ/mol, and Dπ,int (MSn) = 51 kJ/mol. For the carbene complexes with transition metals of the chromium triad, the intrinsic π bond strengths are very similar: Dπ,int (CrC) = 202 kJ/mol, Dπ,int (CrMo) = 204 kJ/mol, and Dπ,int (CrW) = 221 kJ/mol. Relativistic effects are responsible for the increased π bond strength in the tungsten complex. The bond strengths for the metal carbon double bond rank as BE(WC) > BE(CrC) > BE(MoC). Molecular orbital arguments are provided to explain the calculated trends. Further, the question of the rotational barriers around the ME bond is addressed, and the differences in the geometry for the eclipsed as well as for the staggered conformation of (CO)5MEH2 are analyzed.

Amphiphilie kationischer η2-phosphinocarbenkomplexe des wolframs

η 2-Phosphinocarbene complexes of tungsten containing a trimethyl-phosphine ligand show an amphiphilic behavior. With the Lewis acid SMe + an electrophilic attack occurs at the metal-carbon double bond to give 1-tungsta-2-phospha-4-thiabicyclo[1.1.0]butane complexes. On the contrary the Lewis base, PMe 3, adds at the carbene carbon atom to yield a cationic trimethylphosphoraneylidene complex. However, treatment of dicarbonyl( η 5-cyclopentadienyl)[ η 2-(diphenylphosphino)methyl-carbene]-tungsten-tetraphenylborate with the less nucleophilic triethylamine causes deprotonation at the methyl group providing dicarbonyl)( η 5-cyclopentadienyl)- η 2-diphenylphosphinovinyltungsten.

The Metal Carbon Double Bond in Fischer Carbenes: A Density Functional Study of the Importance of Nonlocal Density Corrections and Relativistic Effects

The Metal Carbon Double Bond in Fischer Carbenes: A Density Functional Study of the Importance of Nonlocal Density Corrections and Relativistic Effects

Heisenberg model for radical reactions. Part 3. Direct exchange coupling between transition metal ions and triplet methylene

Ab initio UHF calculations have been carried out for naked transition metal—methylene ions. The low-spin open-shell states are found to be the ground states of these ions, in accord with the GVB-CI results. The low-spin transition metal—methylene ions are regarded as direct exchange-coupled systems formed from high-spin transition metal ions and the triplet monocentric diradical. The direct exchange couplings between these species can be described on the basis of the Heisenberg model. The CASSCF calculations using UHF natural orbitals (UNOs) have been carried out for low-spin states, showing that UNOs and their occupation numbers are good trials. However, they provide poor binding energies for the transition metal—carbon double bonds and indicate the necessity for dynamical correlation corrections. The approximate spin-projected UHF Møller—Plesset and UHF coupled-cluster methods provide reasonable binding energies for the transition metal methylene ions, which are close to those of modified coupled-pair functional (CPF) and average CPF methods.