Low-valent Transition Metal Complexes Research Articles

(Invited) Organic Transformations with Strongly Photoreducing Catalysts

The recent advances of synthetic methodologies benefit from molecular photoredoxcatalysis. Molecular photoredox catalysts offer particular advantages in steering reactions that involve key radical intermediates, as they enable large exoergicity for generating radical species under ambient conditions through heterobimolecular photoinduced electron transfer. To expand the catalytic potential, my group investigated photoinduced electron transfer of doublet molecules and electron-rich, low-valent metal complexes. We found that radical anions of organic molecules exhibited strong photoreducing power capable of generating synthetically important aryl radical intermediates from aryl halides which reacted with aryl boronate esters to produce C-C cross-coupled products. Our studies provided convincing evidence that the active catalyst species was the excited-state radical anion which could be generated through consecutive absorption of a photon, an electron, and another photon. In particular, femto- and nanosecond transient absorption spectroscopic investigations provided direct evidence for the generation and oxidative quenching of the excited-state radical anion. As an alternative strategy to utilize the strong photoreducing power of a molecular catalyst, we seek electron-rich, low-valent transition metal complexes as photoredoxcatalysts. Our exploration enabled us to identify heteroleptic, linear Au(I) complexes as potent photoreducing catalysts with an excited-state oxidation potential as cathodic as -2.23 V vs standard calomel electrode. These Au(I) complexes can catalyse heteroaryl C-C cross-coupling reactions of redox-resistant aryl chlorides. Our investigations demonstrate that the Au(I) complexes offer several benefits, including strong visible-light absorption, a long excited-state lifetime, and the capacity of a 91% yield in the production of free-radical intermediates.

Tetrelanes versus Tetrylenes as Precursors to Transition Metal Complexes Featuring Tridentate PEP Tetryl Ligands (E=Si, Ge, Sn).

Two synthetic approaches have until now been used to synthesize transition metal complexes having a tridentate (pincer or tripod) PEP tetryl (E=Si, Ge, Sn) ligand. These approaches differ in the metal-free precursor, tetrelane or tetrylene, that gives rise to the corresponding PEP tetryl ligand. Tetrelanes (PSiP silanes, PGeP germanes and PSnP stannanes and simple phosphane-free stannanes) have led to tetryl ligands by oxidatively adding an E-X bond (X=H, C or halogen in most cases) to the metal atom of a low-valent transition metal complex, whereas tetrylenes (PGeP germylenes and PSnP stannylenes) have led to tetryl ligands upon insertion of their E atom into an M-X bond (X=Cl in most cases) of the metal precursor or through a derivatization of the E atom after the tetrylene fragment is coordinated to the metal. For each synthetic approach, all the currently known types of PEP tetryl ligand frameworks that have been found in transition metal complexes are presented and discussed in this review.

Probing the strong electron-acceptor property of pyridine-appended 1,2,3-benzotriazole ligand in tris-homoleptic Ru(II) and Fe(II) complexes

In order to probe the σ-donor/π-acceptor properties of the fused triazolo-based pyridine-appended bidentate ligand, 1-(pyridin-2-yl)-1H-benzo[d][1–3]triazole (BTZ-2P), two discrete octahedral tris-homoleptic complexes, [Ru(BTZ-2P)3](PF6)2 (1) and [Fe(BTZ-2P)3](ClO4)2 (2) were synthesized and well characterized for the first time. Crystal structures of the complexes revealed the key geometrical parameters which highlighted the metal-ligand bonding characteristics in the complexes, especially the strong π-acceptor property of the ligand. The bonding with the BTZ-2P ligand resulted in simultaneous stabilization of the HOMO and destabilization of the LUMO of both the metal complexes, leading to anodic shift of the MII/III redox potential and blue shift of the MLCT band in absorption spectroscopy in comparison to their analogous Ru(II) and Fe(II) tris-bipyridine complexes. Experimental results and theoretical calculations confirmed that in addition to being a good σ-donor, the BTZ-2P ligand exhibits strong π-acceptor property as well compared to the classic 2,2′-bipyridine ligand. This disclosure can have future implications in stabilizing low-valent transition metal complexes with the BTZ-2P ligand.

Metalla-aromatics: Planar, Nonplanar, and Spiro.

ConspectusThe concept of aromaticity is one of the most fundamental principles in chemistry. It is generally accepted that planarity is a prerequisite for aromaticity, and typically the more planar the geometry of an aromatic compound is, the stronger aromatic it is. However, it is not always the case, particularly when transition metals are involved in conjugation and electron delocalization of aromatic systems, i.e., metalla-aromatics. Because of the intrinsic nature of transition-metal orbitals, besides planar geometries, the most stable molecular structures of metalla-aromatic compounds could take nonplanar and even spiro geometries. In this Account, we outline several unprecedented types of metalla-aromatics developed recently in our research group.Around seven years ago, we found that 1,4-dilithio-1,3-butadienes, dilithio reagents with π-conjugation, could function as non-innocent ligands and react with low-valent transition-metal complexes, generating monocyclic metalla-aromatic compounds. Later on, by taking advantage of the unique behavior of dilithio reagents and the intrinsic nature of different transition metals, we have synthesized a series of metalla-aromatic compounds, of which four types are discussed here, and each of them represents the first of its kind. First, nearly planar aromatic dicupra[10]annulenes, a 10 π-electron aromatic system with two bridging Cu atoms participating in the orbital conjugation and electron delocalization, are synthesized by annulating two dilithio reagents with two Cu(I) complexes.Second, four kinds of spiro metalla-aromatics, featuring planar (with Pd, Pt, or Rh as the spiro atom) geometry with a whole 10π aromatic system, octahedral (tris-spiro metalla-aromatics with V as the spiro atom) geometry with an entire 40π Craig-Möbius aromatic system, tetrahedral (with Mn as the spiro atom) geometry having two independent and perpendicular 6π planar aromatic rings, and tetrahedral (with Mn as the spiro atom) geometry with one planar and one nonplanar 6π aromatic rings, respectively, are generated. In sharp contrast to spiroaromaticity with carbon acting as the spiro atom described in Organic Chemistry, the metal spiro atom herein takes part in orbital conjugation and electron delocalization.Third, nonplanar aromatic butadienyl diiron complexes are realized. Different from planar aromatic systems featuring delocalized π-type overlap, this nonplanar metalla-aromaticity is achieved by the novel σ-type overlap between the two Fe 3dxz orbitals and the butadienyl π orbital, forming a 6π aromatic system. Fourth, dinickelaferrocene, a ferrocene analogue with two aromatic nickeloles, is synthesized from our monocyclic aromatic dilithionickelole and FeBr2. The aromaticity of dinickelaferrocene and its nickelole ligands is realized by electron back-donation from the Fe 3d orbital to the π* orbital of nickeloles, which also deepens our understanding of the origin of aromaticity.The search for unprecedented and exciting aromatic systems, particularly with transition metals being involved, will continue to drive this intriguing research field forward. Given the synthetic strategies and various types of metalla-aromatics developed and described, diversified metalla-aromatics of interesting structures and reaction chemistry, novel chemical bonding modes, and useful functions can be expected.

Bis-silylation of internal alkynes enabled by Ni(0) catalysis

1,2-Bis-silyl alkenes have exciting synthetic potential for programmable sequential synthesis via manipulation of the two vicinal silyl groups. Transition metal-catalyzed bis-silylation of alkynes with disilanes is the most straightforward strategy to access such useful building blocks. However, this process has some limitations: (1) symmetric disilanes are frequently employed in most of the reactions to assemble two identical silyl groups, which makes chemoselective differentiation for stepwise downstream transformations difficult; (2) the main catalysts are low-valent platinum group transition metal complexes, which are expensive; and (3) internal alkynes remain challenging substrates with low inherent reactivity. Thus, the development of abundant metal-catalyzed bis-silylation of internal alkynes with unsymmetrical disilanes is of significance. Herein, we solve most of the aforementioned limitations in bis-silylation of unsaturated bonds by developing a strongly coordinating disilane reagent and a Ni(0) catalytic system. Importantly, we sufficiently realize the stepwise recognition of the two silyl groups, making this synthetic protocol of wide potential utility.

The chemistry of dithietes, 1,2,5,6-tetrathiocins and higher oligomers.

The synthesis and reactivity patterns of the strained dithiete ring are compared with their dimeric tetrathiocin counterparts and higher oligomers, highlighting: (i) their cycloaddition chemistry with organic dienophiles as a route to sulfur-containing heterocycles; (ii) their oxidative addition chemistry to low valent transition metal complexes to generate transition metal dithiolate complexes and; (iii) the base-catalysed isomerizations between different dithiete oligomers.

Development of Catalytic Reduction of Renewable Carbon Resources Using Well-Elaborated Organometallic Complexes with PNNP Tetradentate Ligands

The recent depletion of fossil fuel resources has impelled industrial and academic researchers to search for alternative carbon sources. Developing sustainable resources/energy is one of the most urgent missions for human beings since the increasing resources/energy demand is in drastic conflict with the limited global fossil fuel storage. In another perspective, massive production of CO2 is inevitable and the fatal risk as far as fossil resources are continuously used as the only sources of carbon. Considerable effort has been invested in biotechnology and sustainable/green technologies to develop a chemical industry in which renewable energy resources complement dwindling fossil fuel sources for the new millennium. A round-table discussion of the US Department of Energy identified the top 30 value-added chemicals derived from biomass, which included various (poly)carboxylic acids and polyols. Non-biodegradable microplastics already plague our oceans and are destroying the world. It is really no secret. These chemicals exist in high oxidation and/or highly oxygenated/nitrogenated states, and thus, current state-of-the-art oxidation catalysts must be substantially modified so that new millennium catalysts could effectively promote the reduction and dehydration of such renewable resources. We have developed new molecular technologies based on well-elaborated metal complexes with PNNP tetradentate ligands for dehydration and reduction of unactivated amides, polycarboxylic acids, and CO2 using thermal or light energy. Those catalytic systems represent potentially sustainable methods for the production of alternative energy carriers/platform chemicals on a large scale. Herein, we offer a brief account on the development of molecular insights into mononuclear low-valent transition metal complexes for reduction of a range of renewable carbon resources.

Low-valent iron and cobalt complexes supported by a rigid xanthene-based disilylamido ligand

The dianionic ligand TIPSDAX (2,7-di-tert-butyl-9,9-dimethyl-N4,N5-bis(triisopropylsilyl)-4,5-diamido-9H-xanthene) was synthesized and employed to support low-valent first-row transition metal complexes featuring iron(II) and cobalt(II). These neutral complexes were studied structurally and electrochemically. Cyclic voltammetry revealed a variety of oxidation features in the ligand and both metal complexes, while only the Co(II)(TIPSDAX)(THF) complex exhibited a reduction event. The cobalt(II) complex was chemically reduced to afford an isolable anionic Co(I) complex whose solid-state structure revealed a considerably more linear geometry at the metal center.

Synthesis and Characterization of the {Si(NHCH2CH2NH)3[Mo(CO)3]2}2- Complex Comprising the Si(NHCH2CH2NH)32- Octahedron.

Reactions of K12Si17 with the low-valent transition-metal complex Mo(CO)3(C7H8) in ethylenediamine/toluene solutions in the presence of 2,2,2-cryptand yield the {Si(NHCH2CH2NH)3[Mo(CO)3]2}2- dianion, which contains an octahedral Si(NHCH2CH2NH)32- subunit. The SiN6 core comprises a rare example of a doubly deprotonated ethylenediame ligand in a coordination complex and is also the first structurally characterized example of a homoleptic Si(N∩N)3 trischelate. Its structure and spectroscopic properties are described.

Hafnocene-based Bicyclo[2.1.1]hexene Germylenes - Formation, Reactivity, and Structural Flexibility.

2,5-Disilylsubstituted germole dianions 1 react with hafnocene dichloride to give hafnocene-based bicyclo[2.1.1]hexene germylenes 3. Their formation proceeds via hafnocene-germylene complexes 2 that were identified by NMR and UV spectroscopy. Germylenes 3 are stabilized by homoconjugation between the empty 4p(Ge) orbital and the π-bond of the innercyclic C2═C3 double bond. This interaction can be understood as σ2, π-coordination of the butadiene part to the dicoordinated germanium atom that leaves the 16e- hafnocene moiety electronically unsaturated. We demonstrate that this new class of germylenes might serve as ligand to a variety of low-valent transition-metal complexes. The structure of the germylene ligand in complexes with Fe(0), Ni(0), and Au(I) and in reaction products with N-heterocyclic carbenes showed an intriguing structural flexibility that allows to accommodate different electronic situations at the ligating germanium atom. The origin of this structural adaptability is the interplay between the topological flexible unsaturated germanium ring and the hafnocene group.

Catalysis of CO2 Electrochemical Reduction by Protonated Pyridine and Similar Molecules. Useful Lessons from a Methodological Misadventure

Electrochemical reduction of carbon dioxide is a particularly important issue in energy and environment contemporary challenges. It mobilized for several decades many researchers in the fields of catalysis by low-valent transition metal complexes. In this context, the report that a molecule as simple as protonated pyridine could catalyze the 6e– + 6H+ conversion of CO2 into methanol and triggered quite a lot of interest and amazement, even if the Faradaic yield was relatively modest (20%) and a precious metal (Pt) had to be used as an electrode material. Successive investigations of the same system or of systems involving molecules similar to protonated pyridine produced divergent results. The amount of data available at present from several research groups is sufficiently large to try to take stock of the situation. This concerns first and foremost a critical examination of the nature and Faradaic yields of electrolysis products. It also deals with the extensive use in this research of a nondestructive e...

Addition of dihydrogen to a borylborenium center.

The activation of a H-H bond, the simplest covalent bond, is a fundamentally important process. Addition of H2 to an elemental center typically occurs on low valent transition metal or main group complexes through oxidative addition to afford metal dihydride complexes. In contrast, activation of H2 on a high valent center generally results in heterolytic cleavage of the H-H bond to a proton and hydride. Here, we report experimental and computational evidence for the addition of H2 to a borenium center in an N-heterocyclic carbene (NHC) coordinated borylborenium cation, which leads to the formation of a dihydroborenium complex accompanied by the elimination of two σ-bonded substituents, namely mesityl (Mes) and pinacolboryl (Bpin) groups, as mesitylboronic ester.

Catalysis and Inhibition in the Electrochemical Reduction of CO2 on Platinum in the Presence of Protonated Pyridine. New Insights into Mechanisms and Products.

In the framework of modern energy challenges, the reduction of CO2 into fuels calls for electrogenerated low-valent transition metal complexes catalysts designed with considerable ingenuity and sophistication. For this reason, the report that a molecule as simple as protonated pyridine (PyH+) could catalyze the formation of methanol from the reduction of CO2 on a platinum electrode triggered great interest and excitement. Further investigations revealed that no methanol is produced. It appears that CO2 is not really reduced but rather participates, on the basis of its aquation into carbonic acid, in hydrogen evolution. Actually, the situation is not that straightforward, as revealed by scrutinizing what happens at the platinum electrode surface. The present study confirms the lack of methanol formation upon bulk electrolysis of PyH+ solutions at Pt and provides a detailed account of the Faradaic yield for H2 production as a function of the electrode potential, but the main finding is that CO2 reduction is accompanied by a strong inhibition of the electrode process taking place when it is carried out in the presence of acids such as PyH+ and AcOH. Cyclic voltammetry and in situ infrared spectroscopy were closely combined to investigate and understand the nature and consequences of the inhibition process. Constant comparison between the two acids was required to decipher the course of the reaction owing to the fact that the IR responses are perturbed by PyH+ adsorption. It finally appears that inhibition is caused by the reduction of CO2 into CO, whose high affinity with platinum triggers the formation of a Pt-CO film that prevents the reaction process. Thus, a paradoxical situation develops in which the high affinity of Pt for CO helps to decrease the overpotential for the reduction of CO2 and therefore blocks the electrode, preventing the reaction process.

Diverse Reactivity of ECp* (E = Al, Ga) toward Low-Coordinate Transition Metal Amides [TM(N(SiMe3)2)2] (TM = Fe, Co, Zn): Insertion, Cp* Transfer, and Orthometalation.

The reactivity of the carbenoid group 13 metal ligands ECp* (E = Al, Ga) toward low valent transition metal complexes [TM(btsa)2] (TM = Fe, Co, Zn; btsa = bis(trimethylsilyl)amide) was investigated, revealing entirely different reaction patterns for E = Al and Ga. Treatment of [Co(btsa)2] with AlCp* yields [Cp*Co(μ-H)(Al(κ2-(CH2SiMe2)NSiMe3)(btsa))] (1) featuring an unusual heterometallic bicyclic structure that results from the insertion of AlCp* into the TM-N bond with concomitant ligand rearrangement including C-H activation at one amide ligand. For [Fe(btsa)2], complete ligand exchange gives FeCp*2, irrespective of the employed stoichiometric ratio of the reactants. In contrast, treatment of [TM(btsa)2] (TM = Fe, Co) with GaCp* forms the 1:1 and 1:2 adducts [(GaCp*)Co(btsa)2] (2) and [(GaCp*)2Fe(btsa)2] (3), respectively. The tendency of AlCp* to undergo Cp* transfer to the TM center appears to be dependent on the nature of the TM center: For [Zn(btsa)2], no Cp* transfer is observed on reaction with AlCp*; instead, the insertion product [Zn(Al(η2-Cp*)(btsa))2] (4) is formed. In the reaction of [Co(btsa)2] with the trivalent [Cp*AlH2], transfer of the amide ligands without further ligand rearrangement is observed, leading to [Co(μ-H)4(Al(η2-Cp*)(btsa))2] (5).

Synthetic Transformations through Alkynoxy-Palladium Interactions and C-H Activation.

Organic synthesis based on straightforward transformations is essential for environmentally benign manufacturing for the invention of novel pharmaceuticals, agrochemicals, and organoelectronic materials in order to ultimately realize a sustainable society. Metal-catalyzed C-H bond-cleaving functionalization has become a promising method for achieving the above goal. For site-selective C-H bond cleavage, so-called directing groups, i.e., ligands attached to substrates, are employed. Commonly utilized directing groups are carbonyls, imines, carboxyls, amides, and pyridyls, which σ-donate electron pairs to metals. On the other hand, unsaturated substrates such as alkenes and alkynes, which participate largely as reactants in organic synthesis, are prepared readily by a wide variety of synthetic transformations and are also employed as reactants in organometallic chemistry. Moreover, such unsaturated groups form complexes with some metals by ligation of their p orbitals via donation and back-donation. However, the use of unsaturated bonds as directing groups has not been studied extensively. We have been involved in the development of methods for the cleavage of C-H bonds by means of transition-metal catalysts to achieve new carbon-carbon bond-forming reactions and incidentally came to focus on the alkynoxy group (-OC≡C-), which shows a ketene-like resonance structure. We expected the alkynoxy group to interact electrophilically with a low-valent transition-metal complex in order to cleave adjacent C-H bonds. In this Account, we summarize our recent achievements on C-H activation based on interactions of palladium with the alkynoxy group in alkynyl aryl ethers. The alkynoxy group plays two roles in the transformation: as a directing group for adjacent C-H bond activation and as an acceptor for the carbon and hydrogen fragments. A typical example is palladium-catalyzed ortho-C-H bond activation in alkynoxyarenes followed by sequential insertion/annulation with internal alkynes and the alkynoxy group to produce 2-methylidene-2H-1-benzopyrans. Mechanistic studies have shown that the presence of both oxygen and alkynyl moieties is essential for selective ortho-C-H bond activation and subsequent annulation. In addition to internal alkynes, norbornene, allenes, isocyanates, and ketenes produce the corresponding oxacycles. It is worthy of note that benzoxadinones formed by the reaction with isocyanates exhibit solid-state luminescence. In addition, 2-methylphenyl alkynyl ethers and 2-alkynoxybiaryls undergo intramolecular annulation at the benzylic γ-position and aryl δ-position via C-H bond activation to give benzofurans and dibenzopyrans, respectively. The disclosed methods allow us to construct useful π-conjugated systems in a straightforward manner.

ChemInform Abstract: The Role of Low Valent Transition Metal Complexes in Homogeneous Catalysis: An EPR Investigation

AbstractReview: 57 refs.

The Role of Low Valent Transition Metal Complexes in Homogeneous Catalysis: An EPR Investigation

Electron paramagnetic resonance (EPR) spectroscopy is an extremely versatile technique for the characterisation of homogeneous catalysts involving paramagnetic centres. The paramagnetic parent pre-catalyst, the activated catalyst itself or any resulting reactive intermediates, can all be monitored in situ in order to delineate the role of the formal metal oxidation state and the influence of ligand structure on the resulting catalytic activity. In this review of our recent results, we describe how highly reactive, low valent transition metal complexes of formal oxidation states Ni(I), Fe(I) and Cr(I) coordinated by N-heterocyclic carbene (NHC) or phosphine ligands and which are active for a range of cross-coupling reactions or ethylene oligomerisation, have been investigated by EPR. Analysis of the EPR spectra revealed (i) how the influence of the NHC ring size affects the electronic properties of the Ni(I) centre, (ii) how low spin Fe(I) intermediates are catalytically competent on-cycle species in cross-coupling reactions and (iii) how intramolecular structural rearrangements involving Cr(I) centres occur following the addition of a co-catalyst to the reaction medium. These results demonstrate the utility of EPR to probe the structure–reactivity relationships in paramagnetic homogeneous catalysts, providing information not readily accessible by other techniques.

A Dimetalloxycarbene Bonding Mode and Reductive Coupling Mechanism for Oxalate Formation from CO2.

We describe the stable and isolable dimetalloxycarbene [(TiX3 )2 (μ2 -CO2 -κ(2) C,O:κO')] 5, where X=N-(tert-butyl)-3,5-dimethylanilide, which is stabilized by fluctuating μ2 -κ(2) C,O:κ(1) O' coordination of the carbene carbon to both titanium centers of the dinuclear complex 5, as shown by variable-temperature NMR studies. Quantum chemical calculations on the unmodified molecule indicated a higher energy of only +10.5 kJ mol(-1) for the μ2 -κ(1) O:κ(1) O' bonding mode of the free dimetalloxycarbene compared to the μ2 -κ(2) C,O:κ(1) O' bonding mode of the masked dimetalloxycarbene. The parent cationic bridging formate complex [(TiX3 )2 (μ2 -OCHO-κO:κO')][B(C6 F5)4], 4[B(C6 F5)4], was simply deprotonated with the strong base K(N(SiMe3 )2 ) to give 5. Complex 5 reacts smoothly with CO2 to generate the bridging oxalate complex [(TiX3 )2 (μ2 -C2 O4 -κO:κO'')], 6, in a C-C bond formation reaction commonly anticipated for oxalate formation by reductive coupling of CO2 on low-valent transition-metal complexes.

ChemInform Abstract: Asymmetric Nitrene Transfer Reactions: Sulfimidation, Aziridination and C—H Amination Using Azide Compounds as Nitrene Precursors

AbstractReview: [39 refs.

Asymmetric Nitrene Transfer Reactions: Sulfimidation, Aziridination and C–H Amination Using Azide Compounds as Nitrene Precursors

Nitrogen functional groups are found in many biologically active compounds and their stereochemistry has a profound effect on biological activity. Nitrene transfer reactions such as aziridination, C-H bond amination, and sulfimidation are useful methods for introducing nitrogen functional groups, and the enantiocontrol of the reactions has been extensively investigated. Although high enantioselectivity has been achieved, most of the reactions use (N-arylsulfonylimino)phenyliodinane, which co-produces iodobenzene, as a nitrene precursor and have a low atom economy. Azide compounds, which give nitrene species by releasing nitrogen, are ideal precursors but rather stable. Their decomposition needs UV irradiation, heating in the presence of a metal complex, or Lewis acid treatment. The examples of previous azide decomposition prompted us to examine Lewis acid and low-valent transition-metal complexes as catalysts for azide decomposition. Thus, we designed new ruthenium complexes that are composed of a low-valent Ru(II) ion, apical CO ligand, and an asymmetry-inducing salen ligand. With these ruthenium complexes and azides, we have achieved highly enantioselective nitrene transfer reactions under mild conditions. Recently, iridium-salen complexes were added to our toolbox.