Ligands In Transition Metal Chemistry Research Articles

Mono(Lewis Base)-Stabilized Gallium Iodide: An Unexplored Class of Promising Ligands.

Quantum-chemical (DFT) calculations on hitherto unknown base(carbene)-stabilized gallium monoiodides (LB→GaI) suggest that these systems feature one lone pair of electrons and a formally vacant p-orbital - both centered at the central gallium atom - and exhibit metallomimetic behavior. The calculated reaction free energies as well as bond dissociation energies suggest that these LB→GaI systems are capable of forming stable donor-acceptor complexes with group 13 trichlorides. Examination of the ligand exchange reactions with iron and nickel complexes indicates their potential use as ligands in transition metal chemistry. In addition, it is found that the title compounds are also able to activate various enthalpically robust bonds. Further, a detailed mechanistic investigation of these small molecule activation processes reveals the non-innocent behavior of the carbene (base) moiety attached to the GaI fragment, thereby indicating the cooperative nature of these bond activation processes. The energy decomposition analysis (EDA) and activation strain model (ASM) of reactivity were also employed to quantitatively understand and rationalize the different activation processes.

Low Valent N‐Coordinated Cations and Dications of Heavier Group 14 Elements: Lewis Acids or Bases?

AbstractLow‐valent N‐coordinated cations and dications of heavier group 14 elements are of great interest in recent years. Their unique electronic structure gives them an ambiphilic character, as they contain both a lone electron pair and an empty p‐orbital on the central metal atom. Thanks to their nucleophilic character, these compounds can act as ligands in transition metal chemistry, and conversely, their electrophilic character allows them to interact with a wide range of organic substrates and thus replace catalysts based on transition metal complexes in many chemical transformations. The aim of this article is to summarize the synthesis of N‐coordinated ionic compounds of heavier group 14 elements with their subsequent reactivity towards various nucleophiles and electrophiles.

Secondary Coordination Sphere Alkylation Promotes Cyclometalation.

Diphosphines have taken on a dominant role as supporting ligands in transition-metal chemistry. Here, we describe complexes of the type [Cp*Fe(diphosphine)(X)] (X = Cl, H) where for diphosphine = 1,2-bis(di-allylphosphino)ethane (tape), a Lewis-acidic secondary coordination sphere (SCS) was installed via allyl group hydroboration using dicyclohexylborane (HBCy2). The resulting chloride complex, [Cp*Fe(P2BCy4)(Cl)] (P2BCy4 = 1,2-bis(di(3-cyclohexylboranyl)propylphosphino)ethane), was treated with n-butyllithium (1-10 equiv), resulting incyclometalation at iron. This reactivity is contrasted with [Cp*Fe(dnppe)(Cl)] (dnppe = 1,2-bis(di-n-propylphosphino)ethane), whereby addition of n-butyllithium provides a mixture of products. Overall, cyclometalation is a common elementary transformation in organometallic chemistry; here we describe how this outcome is accessed in response to Lewis acid SCS incorporation.

Synthesis of a Triphenylphosphinimide-Substituted Silirane as a "Masked" Acyclic Silylene.

Phosphinimides are long known as useful ligands in transition metal chemistry, but examples of these in low-valent silicon chemistry are rather rare. Hence, in this work, we report on the implementation of a triphenylphosphinimide moiety as a ligand of a novel silylene that is trapped as a silirane with cyclohexene. By performing activation reactions with B(p-Tol)3, HSiEt3, N2O, and NH3, we demonstrate that the silirane exhibits a silylene-like behavior, making it a "masked" silylene. Furthermore, we treated the silirane with ethylene, propylene, and trans-butene, which led to an olefin exchange. In the case of ethylene and propylene, an additional insertion of the olefin into the silicon-silicon bonds of the respective siliranes could be achieved. As the insertion of trans-butene was not feasible, we surmise that the scope of this reactivity is restricted by the steric demand of the olefin.

N2/CO Exchange at a Vinylidene Carbon Center: Stable Alkylidene Ketenes and Alkylidene Thioketenes from 1,2,3-Triazole Derived Diazoalkenes.

We present a new class of room-temperature stable diazoalkenes featuring a 1,2,3-triazole backbone. Dinitrogen of the diazoalkene moiety can be thermally displaced by an isocyanide and carbon monoxide. The latter alkylidene ketenes are typically considered as highly reactive compounds, traditionally only accessible by flash vacuum pyrolysis. We present a new and mild synthetic approach to the first structurally characterized alkylidene ketenes by a substitution reaction. Density functional theory calculations suggest the substitution with isocyanides to take place via a stepwise addition/elimination mechanism. In the case of carbon monoxide, the reaction proceeds through an unusual concerted exchange at a vinylidene carbon center. The vinylidene ketenes react with carbon disulfide via a four-membered thiete intermediate to give vinylidene thioketenes under release of COS. We present spectroscopic as well as structural data for the complete isoelectronic series (R2C═C═X; X = N2, CO, CNR, CS) including 1J(13C-13C) data. As N2, CO, and isocyanides belong to the archetypical ligands in transition-metal chemistry, this process can be interpreted in analogy to coordination chemistry as a ligand exchange reaction at a vinylidene carbon center.

Cyclometallation of Heavier Tetrylenes: Reported Complexes and Applications in Catalysis

AbstractHeavier tetrylenes (HTs), which are the heavier analogues of carbenes, have been used as ligands in transition metal chemistry for more than 50 years. Having in mind that cyclometallation is a valuable tool to modify ligand scaffolds, with important implications in inorganic and organic chemistry, this microreview discusses in detail the reactions that have afforded complexes with cyclometallated HTs. To date, eight HT ligands, covering only silylenes and germylenes, have been reported to undergo cyclometallation, all of them on late transition metals. Structural and spectroscopic data of the isolated complexes as well as mechanistic information related to the cyclometallation processes are collected and discussed. Additionally, this minireview also surveys the catalytic applications found so far for cyclometallated HT complexes.

P atom as ligand in transition metal chemistry: Structural aspects

• Structural features of phosphide complexes have been revised and analysed. • Bonding patterns of the different coordination modes have been rationalised. • 31 P{ 1 H} NMR spectral data of these phosphide complexes have been examined. The chemistry of transition metal compounds that contain a phosphorus atom acting as ligand, sometimes called “naked” phosphorus, started at the end of the last century. However, it is still an area of permanent interest and expansion, in which new examples of complexes with novel structural features are appearing in the literature. In this review, the phosphide complexes are classified based on the different coordination modes found in structurally characterised complexes for coordination numbers from 1 to 4 at the phosphorous atom. Structural aspects are discussed and, in some cases, the bonding patterns rationalised. Additionally to the structural analysis, data from 31 P NMR spectroscopy, as a potent characterization tool, are also revised and briefly analysed.

(2-Pyridyloxy)arsines as ligands in transition metal chemistry: a stepwise As(iii) → As(ii) → As(i) reduction.

Neutral inherently tri- and tetradentate ligands of the type Ph3-xAs(PyO)x (x = 2 (1), 3 (2)) have been synthesized and characterized. Reaction of 1 with [RuCl2(PPh3)3] affords complex [PhAs(μ-PyO)2RuCl2(PPh3)] (3), whereas 2 and [RuCl2(PPh3)3] react with formation of [As(μ-PyO)2RuCl(PPh3)2] (5) and [Ph3P(PyO)]Cl (6). Treatment of complex 5 with [AuCl(tht)] (tht = tetrahydrothiophene) results in liberation of tht and formation of [AuCl(As(PyO)2)RuCl(PPh3)2] (7), featuring an (Au-As-Ru) core. For compounds 3, 5, and 7 the As-Ru and As-Au bond situation has been investigated using NBO, AIM and ELF analysis, allowing the assignment of pronounced canonical forms of σ-(AsIII→RuII) to 3, σ-(AsII-RuI) to 5 and σ-(AuI←AsI→RuII) to 7.

Reactions of an anionic chelate phosphane/borata-alkene ligand with [Rh(nbd)Cl]2, [Rh(CO)2Cl]2 and [Ir(cod)Cl]2.

Borata-alkenes can serve as anionic olefin equivalent ligands in transition metal chemistry. A chelate ligand of this type is described and used for metal coordination. Deprotonation of the Mes2P(CH2)2B(C6F5)2 frustrated Lewis pair in the α-CH[B] position gave the methylene-bridged phosphane/borata-alkene anion. It reacted with the [Rh(nbd)Cl] or [Rh(CO)2Cl] dimers to give the respective neutral chelate [P/C[double bond, length as m-dash]B][Rh] complexes. The reaction of the [P/C[double bond, length as m-dash]B]- anion with [Ir(cod)Cl]2 proceeded similarly, only that the complex underwent a subsequent oxidative addition reaction at the mesityl substituent. Both the resulting Ir(iii)hydride complex 15 and the P/borata-alkene Rh system 12 were used as hydrogenation catalysts. The [P/C[double bond, length as m-dash]B(C6F5)2]Rh(nbd) complex 12 served as a catalyst for arylacetylene polymerization.

Phosphazenyl Phosphines: The Most Electron-Rich Uncharged Phosphorus Brønsted and Lewis Bases.

It was discovered that phosphazenyl phosphines (PAPs) can be stronger P-superbases than the corresponding Schwesinger type phosphazene N-superbases. A simple synthetic access to this class of PR3 derivatives including their homologization is described. XRD structures, proton affinities (PA), and gas-phase basicities (GB) as well as calculated and experimental pK values in THF are presented. In contrast to their N-basic counterparts, PAPs are also privileged ligands in transition metal chemistry. In fact, they are currently the strongest uncharged P-donors known, exceeding classical and more recently discovered ligands such as PtBu3 and imidazolin-2-ylidenaminophosphines (IAPs) with respect to their low Tolman electronic parameters (TEPs) and large cone angles.

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry.

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

N-Functionalized Ferrocenes: Subvalent Group XIV Element Chlorides and tert-Butyllithium-Induced C-C Bond Cleavage under Mild Conditions.

The ferrocene derivative (η5 -Cp)Fe{η5 -C5 H3 -1-(ArNCH)-2-(CH2 NMe2 )} (1; Ar=2,6-iPr2 C6 H3 )) reacts diastereoselectively with LiR by carbolithiation and subsequent hydrolysis to give (η5 -Cp)Fe{η5 -C5 H3 -1-(ArHNCHR)-2-(CH2 NMe2 )} (3: R=tBu; 4: R=Ph; 5: R=Me) in high yields. For R=tBu, the organolithium derivative (η5 -Cp)Fe{η5 -C5 H3 -1-(ArLiNCHR)-2-(CH2 NMe2 )} (2) was isolated. Compound 2 reacts with GeCl2 ⋅dioxane and SnCl2 to give the metallylene amide chlorides (η5 -Cp)Fe{η5 -C5 H3 -1-(ArMNCHtBu)-2-(CH2 NMe2 )} 6 (M=GeCl) and 7 (M=SnCl), respectively, which each contain three stereogenic centers. The potential of 7 as a ligand in transition-metal chemistry is demonstrated by formation of its complex (η5 -Cp)Fe{η5 -C5 H3 -1-(ArMNCHtBu)-2-(CH2 NMe2 )} [9, M= Sn(Cl)W(CO)5 ]. Treatment of 3 with tert-butyllithium at room temperature causes an unprecedented carbon-carbon bond cleavage whereas under kinetic control, lithiation at the Cp-3 position takes place, which leads to the isolation of (η5 -Cp)Fe{η5 -C5 H3 -1-(ArHNCHtBu)-2-(CH2 NMe2 )-3-SiMe3 } (10).

Selective Reductions of N,N-Bis{chloro(aryl)-phosphino}-amines Yielding Three-, Five-, Six-, and Eight-Membered Cyclic Azaphosphanes.

To date, a plethora of λ3 -P nitrogen-containing compounds is known. A large number of them are used as ligands in coordination chemistry and homogeneous catalysis. PN-containing compounds tend to build up cyclic moieties, which have received less attention in regard to their application as ligands in transition metal chemistry. Hence, different dehalogenation reactions of N,N-bis{chloro(aryl)-phosphino}-amines have been developed to synthesize different P-N-P containing cyclic compounds. Their coordination behavior to group VI transition metal carbonyls was explored.

Multidentate silyl ligands in transition metal chemistry.

Silyl ligands are of great interest in transition metal coordination chemistry since they exhibit a strong trans-influence and create electron rich metals centres, which are in turn eligible to activate reagents. This review presents and discusses the use of multidentate Si-based ligand systems in transition metal coordination chemistry. These include tri- and tetradentate pincer and scorpionate-type ligands, which have been developed in recent years in order to tailor both steric and electronic properties for achieving a defined control over the reactivity of a transition metal complex. A discussion of synthetic and structural aspects of several subgroups of ligand architectures is presented. Some compounds allow for a detailed insight into the activation process of silanes or are capable of activating small molecules.

The Sterically Demanding Thiosilyl Group SSi(SiMe3)(3) as a Ligand in Transition Metal Chemistry

The increasing number of metalloid clusters, synthesized in the last couple of years, gives new insights into the formation process of metals or semi-metals. Metalloid clusters of the transition metals, especially the coinage metals are thereby of particular interest and the number of structurally characterized metalloid gold clusters has increased over the last decade. Most of these clusters are stabilized via aryl thiolate ligands. Herein, the synthesis and crystal structures of the lithium and potassium salts of the bulky SSi(SiMe3)3 ligand are presented together with the synthesis of suitable MI precursors (M = Cu, Ag, Au) for further build-up reactions to metalloid clusters.

Post-Functionalization: A Useful Method for the Synthesis of Donor-Func­tionalized N-Heterocyclic Carbene–Transition-Metal Catalysts

Functionalized N-heterocyclic carbene ligands are particularly useful and widely applied ligands in transition-metal chemistry and catalysis. In this article we introduce an emerging synthetic method for functionalized N-heterocyclic carbene ligands developed from the reactivity study on cyclometalated N-hetero­cyclic carbene compounds, namely postfunctionalization. The status quo of this method for the preparation of novel donor-functionalized N-heterocyclic carbene complexes and their application in catalysis are discussed.

Carbonylation of Cyclotrisilenes

Carbon monoxide (CO) is a common ligand in transitionmetal chemistry. In organic chemistry, it is frequently used to introduce a C1 unit, although carbonylations, such as hydroformylation or the Pauson–Khand reaction, generally require a transition-metal catalyst, even for strained (and thus highly reactive) cyclic compounds. Only a few reports on the reactivity of CO towards compounds of main-group elements other than carbon are available. Although known since the pioneering work by Schlesinger and Burg in the 1930s, borane–CO adducts are usually unstable at room temperature. Only recently, Piers et al. reported a stable adduct between the very Lewis acidic perfluorinated pentaphenylborole and CO. The hydroborating Lewis acid (F5C6)2BH incorporates CO assisted by a alkenyl-functionalized phosphine in a frustrated Lewis pair (FLP) mechanism. A B(C6F5)3-based FLP is capable of stoichiometric CO reduction with dihydrogen. A few coordinatively unsaturated and thus Lewis acidic organic species, such as transient triplet carbenes, and Bertrand s cyclic alkyl amino carbenes react with CO in the absence of transition metals, forming ketenes. Despite growing appreciation of the capability of main group species to activate small molecules, such as H2 and NH3, [10] reactions of stable low-valent compounds of the heavier elements with CO have not, to our knowledge, been reported. Transient silylene–CO adducts, however, have been observed in the gas phase and in cold matrices. We have recently shown that reactions of cyclotrisilenes with N-heterocyclic carbenes (NHCs) reversibly afford cyclotrisilene–NHC adducts, which in the case of 1a can undergo reversible ring-opening to form a highly unsaturated NHCstabilized disilenylsilylene. Isonitriles also react with cyclotrisilenes, and, mindful of the isoelectronic relationship to CO, we became interested in the reactivity of the latter. Herein, we report the facile reaction of carbon monoxide with cyclotrisilenes in the absence of a catalyst to form highly functionalized cyclic silenes. Exposure of a benzene solution of cyclotrisilene 1a to 1 atm of CO at 25 8C results in precipitation of a yellow powder within a few hours, which owing to its insolubility cannot be characterized spectroscopically. A reaction for several days without stirring under otherwise identical conditions afforded bright yellow single crystals. Elemental analysis confirmed the incorporation of one equivalent of CO per molecule of 1a. X-ray crystallography revealed the product to be the tricyclic Brook-type bis(silene) 2a, formally arising from two molecules each of 1a and CO (Scheme 1, Figure 1). With a p-donating oxygen atom at carbon, 2a is reminiscent of donor-stabilized silenes with inverse polarization,

Diatomic PN – trapped in a cyclo-tetraphosphazene

(Me3Si)2NPCl2, which is formally capable of eliminating two equivalents of Me3SiCl, is shown to be a suitable starting material to prepare highly reactive PN species by successive elimination of Me3SiCl. Me3SiCl elimination can be triggered either thermally and/or by addition of a Lewis acid such as GaCl3, thus leading to the formation of a highly labile aminochlorophosphenium cation in [(Me3Si)2NPCl][GaCl4] and iminophosphenium salt [Me3Si–NP][GaCl4] upon warming to ambient temperatures. This work describes the synthesis and characterization of a cyclo-tetraphosphazane in [PN(dmb)]4 (5) (dmb = 2,3-dimethyl-1,3-butadiene) obtained by thermal elimination of Me3SiCl from (Me3Si)2NPCl2 at 120 °C in toluene solution. The reactive intermediate Me3SiNPCl was trapped with dmb to form the cyclic phosphazane Me3SiN(dmb)PCl, which eventually oligomerizes to give 5. In the presence of dmb or chd (chd = 1,3-cyclohexadiene) (Me3Si)2NP(OTf)2 reacts by eliminating Me3SiOTf to yield the spirocyclic phospholenium salts [Me3SiN(dmb)P(dmb)][OTf] (7) and [Me3SiN(chd)P(chd)][OTf] (8), of which the solid state structures were successfully determined. 7 decomposes when exposed to moisture to give an unprecedented cyclic ammonium phosphinoxide [P(O)H(dmb)2NH2][OTf] (9). Tetraphosphazane 5 is shown to be a versatile ligand in transition metal chemistry. It coordinates in an η3-fashion in {[PN(dmb)]4Mo(CO)3} (5·Mo) and is able to coordinate a second metal fragment, exemplified by the formation of the ditungsten complex{[PN(dmb)]4W2(CO)7} (5·W2) with a semi-bridging carbonyl ligand. All compounds were structurally characterized and the bonding situation was investigated by density functional theory and natural bond orbital analysis (NBO).

New silyl-substituted phosphaalkenes Mes*P[dbnd]C(Cl)–Si(Cl)PhR, promising building blocks in silicon and phosphorus chemistry

Three new silyl-substituted phosphaalkenes Mes*PC(Cl)–Si(Cl)PhR (R = chlorine, 1; R = 9-methylfluoren-9-yl, 2; R = 9-trimethylsilylfluoren-9-yl, 3) were obtained and completely characterized. The coordination compounds 4 and 5, resulting from the reaction between Au(SMe2)Cl and 1 and 3, respectively, are also described. All these compounds are potential precursors of phosphasilaallenes, not stabilized until now and new ligands in transition metal chemistry.

P–N/P–P Bond Metathesis for the Synthesis of Complex Polyphosphanes

A unique hexaphosphane featuring a 2,2'-bi(1,2,3-triphosphacyclopentane) moiety (19) and an ethylene-bridged bis-isotetraphosphane (27(c,m)) were both selectively prepared in efficient one-pot syntheses from easily accessible tris(3,5-dimethyl-1-pyrazolyl)phosphane (14) and 1,2-bis(phenylphosphanyl)ethane (18(c,m)). The formation of 27(c,m) is an example of a highly efficient P-P bond formation via protolysis. In contrast, the formation of 19 comprises P-N/P-P bond metathesis steps. This constitutes a novel synthetic approach toward the preparation of complex polyphosphanes. Detailed spectroscopic investigations form the basis for a mechanistic understanding of this unprecedented methodology. Furthermore, the preparation of a unique dinuclear iron-carbonyl complex which features hexaphosphane 19 as a bridging ligand illustrates the potential use of complex polyphosphanes such as 19 as ligands in transition metal chemistry.