Alkyne Complexes Research Articles

Tris(phosphaalkyne)tungsten Complexes

We report on the reactivity of a boryloxy-functionalized phosphaalkyne, [B]OCP ([B] = N,N′-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaboryl), toward a series of tungsten(0) reagents to afford the novel complex W(η2-PCO[B])3(CO). As with related organometallic complexes of alkynes, the C3v symmetry adopted by this complex imposes that, of the three coordinated phosphaalkyne ligands, two act as four-electron donors while the remaining species donates two electrons to the tungsten metal center. W(η2-PCO[B])3(CO) readily loses carbon monoxide in the presence of Lewis bases to afford W(η2-PCO[B])3(L) (where L = PMe3, CNtBu).

An Unprecedented Ga/P Frustrated Lewis Pair: Synthesis, Characterization, and Reactivity.

Frustrated Lewis pairs (FLPs) represent a new paradigm of main-group chemistry. The Lewis acidic centers in FLP chemistry are typically B and Al atoms in the studies reported over the past decade, and most of them are tri-coordinated with strong electron-withdrawing groups. Herein, a Ga/P system is reported which contains an unprecedented four-coordinated Lewis acidic Ga center. This Ga/P species performs classical addition reactions toward heterocumulenes, alkyne, diazomethane, and transition metal complex. Regioselective formation of the products can be rationalized by DFT calculations. The penta-coordinated gallium atom center in these products is rare in the FLP chemistry. This study enriches the diversity of FLPs and demonstrates that a four-coordinated Lewis acidic site with a donor-acceptor bond can also be FLP active.

Advantages of Group 4 Metallocene Bis(trimethylsilyl)acetylene Complexes as Metallocene Sources Towards Other Synthetically used Systems.

Active species for synthetic and catalytic applications are formed from well defined complexes or mixtures of compounds. For group 4 metallocenes, three pathways for the formation of the reactive complex fragment [Cp′2M] are known: (i) reductive mixtures and well defined complexes which are able to form the metallocene fragments either by (ii) addition or (iii) substitution reactions. In this account for each of theses systems (i)–(iii) a prominent example will be discussed in detail, (i) the Negishi reagent Cp2ZrCl2/n‐BuLi, (ii) bis(η5 : η1‐pentafulvene) complexes and (iii) metallocene bis(trimethylsilyl)acetylene complexes, to show the advantages and the disadvantages for each of these methods for synthetic applications. This account summarizes some main advantages of group 4 metallocene bis(trimethylsilyl)acetylene complexes as metallocene generating agents over other synthetically used systems. For each of the special purposes, all described systems have advantages as well as disadvantages. The aim of this overview is to help synthetic chemists in selecting the most effective system on the basis of [Cp′2M] (M=Ti, Zr) for synthetic or catalytic puposes.

Pathways to Polynuclear Complexes with α-S,N-Substituted Alkynes as Bridging Ligands.

Recently, alkyne complexes with terminal thiolate/amide substitution have been shown to act as S,N-chelate ligands in dinuclear complexes. In this study, the detailed synthesis and reactivity of W(II) alkyne complexes bearing different amino groups in the α-position are reported. The preparative scheme starts with cationic alkyne complexes [Tp'W(CO)2-η2-C,C'-C2Br(SR)]PF6 {Tp' = hydridotris(3,5-dimethylpyrazolyl)borate, R = benzyl (Bn), C2H4SiMe3}, which are obtained by applying free bromo alkynes. Subsequent nucleophilic substitution of the bromine substituent led to unsymmetrical substituted alkyne complexes [Tp'W(CO)2-η2-C,C'-C2(SR)(NHBn)]PF6 with S,N substitution in the α position of the coordinated alkyne. Depending on the base used, deprotonation of the secondary amine resulted in either neutral iminoketenyl complexes [Tp'W(CO)2-η2-C,C'-C2(SR)(NBn)] or a zwitterionic PF5 adduct. Reductive removal of the benzyl protective group was primarily observed at the imine substituent, causing a side-on/end-on rearrangement to the cyanide substituted carbene complex K[Tp'W(CO)2-η1-C(CN)(SBn)]. The reversibility of the rearrangement was proven with HBF4/Et2O, because double protonation led to [Tp'W(CO)2-η2-C,C'-C2(SR)(NH2)]BF4 exhibiting an unprecedented primary amine substitution at the coordinated alkyne. The reaction sequence starting with the thiol SC2H4SiMe3 derivative leading to the desired [Tp'W(CO)2-η2-C,C'-C2S(NHBn)] with a terminal S atom as well as the bonding situation in those complexes is discussed based on full spectroscopic characterization including X-ray structure analyses. Finally, a trinuclear complex assembled by homoleptic coordination of Pd(II) by two anionic S,N-chelates [Tp'W(CO)2-η2-C,C'-C2S(NBn)]- is presented.

Triazenido Complexes of Titanocene(III)

Triazenido metal complexes have been extensively investigated for more than a century, especially transition metal complexes. However, in each of the known examples for titanium, the oxidation state of the metal is +4. Therefore, we sought to prepare a complex containing titanium(III). To achieve this goal, triazenes RNNN(H)R′ (1R, with R = R′ = Ph; Dipp; Ter; R = Mes*, R′ = mTol) and triazenido salts Li[RNNNR′] (R = R′ = Ter; R = Ter, R′ = mTol) were prepared. These were treated with suitable titanium precursors either from reaction with the alkyne complex Cp2Ti(η2‐Me3SiC2SiMe3) or in a salt metathesis with [Cp2TiCl]2 to give chelate complexes of the type Cp2Ti[RNNNR′] (2R, with R =R′ = Ph; Dipp; R = Ter; R′ = mTol). The synthesis, reactivity, and bonding of the titanocene triazenides were studied.

Activation and Photoinduced Release of Alkynes on a Biomimetic Tungsten Center: The Photochemical Behavior of the W-S-Phoz System.

The synthesis and structural determination of four tungsten alkyne complexes coordinated by the bio‐inspired S,N‐donor ligand 2‐(4′,4′‐dimethyloxazoline‐2′‐yl)thiophenolate (S‐Phoz) is presented. A previously established protocol that involved the reaction of the respective alkyne with the bis‐carbonyl precursor [W(CO)2(S‐Phoz)2] was used for the complexes [W(CO)(C2R2)(S‐Phoz)2] (R=H, 1 a; Me, 1 b; Ph, 1 c). Oxidation with pyridine‐N‐oxide gave the corresponding W‐oxo species [WO(C2R2)(S‐Phoz)2] (R=H, 2 a; Me, 2 b; Ph, 2 c). All W‐oxo‐alkyne complexes (2 a, b, c) were found to be capable of alkyne release upon light irradiation to afford five‐coordinate [WO(S‐Phoz)2] (3). The photoinduced release of the alkyne ligand was studied in detail by in situ 1H NMR measurements, which revealed correlation of the photodissociation rate constant (2 b>2 a>2 c) with the elongation of the alkyne C≡C bond in the molecular structures. Oxidation of [WO(S‐Phoz)2] (3) with pyridine‐N‐oxide yielded [WO2(S‐Phoz)2] (4), which shows highly fluxional behavior in solution. Variable‐temperature 1H NMR spectroscopy revealed three isomeric forms with respect to the ligand arrangement versus each other. Furthermore, compound 4 rearranges to tetranuclear oxo compound [W4O4(μ‐O)6(S‐Phoz)4] (5) and dinuclear [{WO(μ‐O)(S‐Phoz)}2] (6) over time. The latter two were identified by single‐crystal X‐ray diffraction analyses.

Synthesis, Spectroscopy, and Redox Studies of Ferrocene-Functionalized Coinage Metal Alkyne Complexes.

Ethynylferrocene (FcC≡CH) was utilized as a redox-active metalloligand for the synthesis of polynuclear coinage metal complexes. The reaction of [FcC≡CLi] with tri- tert-butylphosphine metal chlorides [tBu3P-MCl] (M = Au, Ag, Cu) yielded different heteronuclear ferrocene-funtionalized alkyne complexes featuring metallophilic interactions. Furthermore, the redox properties of the ferrocenyl-functionalized tetracopper complex were investigated by cyclic voltammetry and UV-vis-near-IR spectroelectrochemistry. They indicate the compounds' redox-rich nature and a weak electronic coupling between the redox-active ferrocenyl units over a large distance.

Use of Single-Metal Fragments for Cluster Building: Synthesis, Structure, and Bonding of Heterometallaboranes.

The synergic property of the CO ligand, in general, can stabilize metal complexes at lower oxidation states. Utilizing this feature of the CO ligand, we have recently isolated and structurally characterized a highly fluxional molybdenum complex [{Cp*Mo(CO)2}2{μ-η2:η2-B2H4}] (2; Cp* = η5-C5Me5) comprising the diborane(4) ligand. Compound 2 represents a rare class of bimetallic diborane(4) complex corresponding to a singly bridged C s structure. In an attempt to isolate the tungsten analogue of 2, [{Cp*W(CO)2}2{μ-η2:η2-B2H4}], we have isolated a rare vertex-fused cluster, [(Cp*W)3WB9H18] (5). Having a structural likeness with the dimolybdenum alkyne complex [{CpMo(CO)2}2C2H2], we have further explored the chemistry of 2 with CO gas that yielded a homoleptic trimolybdenum complex, [(Cp*Mo)3(μ-H)2(μ3-H)(μ-CO)2B4H4] (4). In an attempt to replace the 16-electron {Cp*MoH(CO)2} moiety in 4 with isolobal fragment {W(CO)5}, we treated the intermediate, obtained from the reaction of Cp*MoCl4 and LiBH4, with the monometal carbonyl fragment {W(CO)5·THF}. The reaction indeed yielded two bimetallic clusters, [(Cp*Mo)2B4H8W(CO)4] (7) and [(Cp*Mo)2B4H6W(CO)5] (8), that seem to have been generated by the replacement of one {BH} or {BH3} vertex from [(Cp*Mo)2B5H9], respectively. All of the compounds have been characterized by various spectroscopic analyses and single-crystal X-ray diffraction studies. Electron-counting rules and molecular orbital analyses provided further insight into the electronic structure of all of these molecules.

Recent Synthetic and Catalytic Applications of Group 4 Metallocene Bis(trimethylsilyl)acetylene Complexes

With the 30 years old first example of a stable group 4 metallocene bis(trimethylsilyl)acetylene (btmsa) complex Cp2Ti(η2‐btmsa) a series of similar complexes like Cp′2M(η2‐btmsa) (M = Ti, Zr; Cp′ = Cp, Cp* as η5‐pentamethylcyclopentadienyl and Cp′2 = rac‐(ebthi) as rac‐1,2‐ethylene‐1,1′‐bis(η5‐tetrahydroindenyl) ) represent excellent sources for the generation of the very reactive coordinatively and electronically unsaturated complex fragments [Cp'2M], which were used in many synthetic and catalytic reactions. Examples for this were oresented in several papers and summarized in some reviews. In this update new reactions of group 4 metallocene bis(trimethylsilyl)acetylene complexes towards symmetrically and unsymmetrically α‐dihetero‐substituted alkynes, reactions with other alkynes, di‐ and polyynes to metallacyclopentadienes as well as the conversions of the formed products are summarized. Additionally, novel synthetic applications of the formed metallacyclopentadienes on the basis of the zirconocene bis(trimethylsilyl)acetylene complex Cp2Zr(py)(η2‐btmsa) are highlighted. Reactions with imines, neutral N‐donor ligands, E‐H compounds (E = N, O, P), molecular hydrogen, water, phosphino‐ and aminoboranes, E‐Cl compounds (E = C, P), disilylated germylenes and aryloxy ketones were described. Different self‐assembly reactions to multinuclear complexes by using group 4 metallocene bis(trimethylsilyl)acetylene complexes in C‐C coupling reactions with or without C‐H bond activation to tri‐ or tetranuclear complexes are considered, too. These examples are discussed following the general L‐M‐S principle for stoichiometric and catalytic reactions, whereby different ligands L (L = Cp′ = Cp, Cp*, rac‐ebthi), metals M (M = Ti, Zr, Hf) and substrate substituents S influence the reactivity and the obtained products.

Recombinant Expression and Purification of N-Acetylated Alpha-Synuclein.

The majority of proteins in eukaryotic cells are subject to amino-terminal (Nt) acetylation. Recombinant protein expressed using prokaryotic expression systems such as E. coli would not normally be Nt-acetylated as these cells lack the appropriate N-α-terminal acetylation complex. Here we describe a simple protocol that allows the recombinant expression and purification of Nt-acetylated alpha-synuclein (aS) from E. coli.

Acetylene and terminal alkyne complexes of copper(i) supported by fluorinated pyrazolates: syntheses, structures, and transformations.

Trinuclear {μ-[3,5-(CF3)2Pz]Cu}3 reacts with acetylene to produce the 2 : 1 copper(i) acetylene complex, Cu4(μ-[3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2. Related Cu4(μ-[4-Br-3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 and Cu4(μ-[4-Cl-3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 have also been isolated using the corresponding copper(i) pyrazolate and acetylene. The 1 : 1 adducts Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CH)2 and Cu2(μ-[4-Br-3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CH)2 are significantly less stable to the acetylene loss and can be observed in solution at low temperatures under excess acetylene. The X-ray crystal structures of 2 : 1 and 1 : 1 complexes, Cu4(μ-[3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 and Cu2(μ-[4-Br-3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CH)2 are reported. Raman data show a reduction in [small nu, Greek, macron]C[triple bond, length as m-dash]C stretching frequency by about ∼340 and ∼163 cm-1 in the 2 : 1 and 1 : 1 Cu(i)/acetylene complexes, respectively, from that of the free acetylene. Copper(i) pyrazolate complexes of the terminal alkynes, phenylacetylene, 1,8-nonadiyne, and 1,7-octadiyne are also reported. They form adducts involving one copper atom on each alkyne moiety. The {μ-[3,5-(CF3)2Pz]Cu}3 is also a very versatile and competent catalyst for alkyne transformations as evident from its ability to catalyze the alkyne C(sp)-H bond carboxylation chemistry with CO2, azide-alkyne cycloadditions leading to 1,2,3-triazoles including the use of acetylene itself as a substrate, and thiol addition to phenylacetylene affording vinyl sulfides.

Fluorination as tool to improve bioanalytical sensitivity and COX-2-selective antitumor activity of cobalt alkyne complexes.

The cobalt alkyne complex [(prop-2-ynyl)-2-acetoxybenzoate]dicobalthexacarbonyl (Co-ASS) is an auspicious lead, which exhibits its anticancer activity mainly by inhibition of both cyclooxygenases (COX-1 and COX-2). Since COX-2 participates in carcinogenesis, a selective inhibition of that isoenzyme is aimed. To study if fluorination increases the COX-2/COX-1 inhibition ratio of the lead, substitution was respectively performed in the positions 3, 4, 5, and 6 of the aromatic moiety. The complexes 3/4/5F-Co-ASS and to a much lower extent also 6F-Co-ASS showed cytotoxic, antimetabolic, and apoptotic effects in COX-1/2-positive HT-29 and MDA-MB-231 cells and remarkably less activity in the COX-1/2-negative MCF-7 cell line. The metabolic activity in MCF-7 cells was even unaffected up to a concentration of 40 μM. With exception of 6F-Co-ASS, the complexes strongly reduced the PGE2 synthesis in HT-29 cells and all complexes inhibited COX-2 more effectively than COX-1 in an assay at isolated enzymes. These findings point to an interference in the COX cascade as part of the mode of antitumor action. The limited cellular effects of 6F-Co-ASS are related to its poor uptake as determined by HR CS AAS/MAS. Moreover, the cellular uptake studies confirm fluorination as beneficial tool for bioanalytical labeling. The higher quantification of fluorine by HR CS MAS makes this method about 5-fold more sensitive than HR CS AAS measuring cobalt. As a further positive result, 3/4/5/6-Co-ASS demonstrated high selectivity to tumor cells due to lack of antimetabolic activity against the non-tumorigenic bone marrow stromal cell line HS-5.

A general benzylic C-H activation and C-C coupling reaction of zirconocenes mediated by C-N bond cleavage in tert-butylisocyanide - unusual formation of iminoacyl complexes.

Reactions of the zirconocene alkyne complex [rac-(ebthi)Zr(η2-Me3SiC2SiMe3)] (rac-(ebthi) = rac-1,2-ethylene-1,1'-bis(η5-tetrahydroindenyl)) with tert-butylisocyanide and methylbenzenes were investigated. Depending on the stoichiometry, the solvent and the reaction temperature different products were obtained. Starting with the end-on coordination of the isocyanide to the zirconium centre (2), elevated reaction temperatures and an excess of tert-butylisocyanide resulted after the elimination of the alkyne in the formation of zirconocene η2-iminoacyl cyanide complexes 3a-d. These complexes are formed by coupling with a benzyl fragment through C-H bond activation of a methyl group of methylbenzene. The dimeric cyanide bridged zirconocene complex 4 is formed as a side-product of this process. The unexpected dimer and the heterometallacycles were fully characterised, including X-ray crystallography.

Synthesis, Photophysical Properties, and Computational Analysis of Di- and Tetranuclear Alkyne Complexes of Copper(I) Supported by a Highly Fluorinated Pyrazolate

This work describes the reaction of a cyclic trinuclear complex, {[3,5-(CF3)2Pz]Cu}3, with internal alkynes to construct di- and tetranuclear copper(I) complexes with short cuprophilic contacts and the study of their intriguing bonding and photophysical properties, both experimentally and theoretically. For comparison, we have also investigated the related copper(I) trifluoroacetate 3-hexyne complexes so as to deduce and isolate the role played by the supporting pyrazolate ligand on the luminescence of these and related complexes. Some adducts reported herein feature bridging (i.e., μ2–η2,η2-) alkyne coordination modes, which is rare for copper(I)–alkyne complexes documented in the literature. Raman data show red shifts in the average υC≡C stretching frequency from 2260 cm–1 in free 3-hexyne to 2050 cm–1 in the terminal η2-/2e-donor alkyne adduct Cu2(μ-[3,5-(CF3)2Pz])2(EtC≡CEt)2 (1) and, more drastically, to 1874 cm–1 in the bridged μ2–η2,η2-/4e-donor adduct Cu4(μ-[3,5-(CF3)2Pz])4(μ-EtC≡CEt)2 (2). The te...

Dative and electron-sharing bonding in transition metal compounds.

Quantum chemical calculations using density functional theory at the BP86-D3(BJ)/def2-TZVPP level of theory are reported for transition metal compounds [TM]-L in high and low oxidation states involving carbene, carbyne, alkene, and alkyne ligands L. The nature of the [TM]-L bond is analyzed with the energy decomposition analysis - natural orbitals for chemical valence (EDA-NOCV) method. The calculations reveal that transition metal compounds with ligands, that are typically classified as donor-acceptor complexes possessing dative bonds (Fischer-type carbenes and carbynes, alkene, and alkyne complexes) or as TM compounds with electron-sharing bonds (Schrock-type carbenes and carbynes, metallacyclopropanes, and metallacyclopropenes), exhibit significant differences between the orbital interactions when closed-shell or open shell fragments are used. Fischer-type carbene complexes have much lower orbital interaction (ΔEorb ) values when singlet fragments are employed compared to triplet fragments. In contrast, singlet and triplet fragments of Schrock-type carbene complexes give similar ΔEorb values. The best description for Fischer-type carbyne complexes is found for neutral fragments in their electronic doublet state, which engage in a mixture of dative bonding (σ donation and π backdonation) and one electron-sharing π bond. The EDA-NOCV calculations of Schrock-type carbynes using open-shell species in their quartet electronic state give similar ΔEorb values as neutral fragments in their electronic doublet state. Alkene and alkyne complexes, but also metallacyclic species, are best described with singlet fragments, but the difference between the ΔEorb values for dative bonding and electron-sharing bonding using triplet fragments becomes much smaller for molecules that are considered as metallacycles. © 2018 Wiley Periodicals, Inc.

Imine hydrosilylation using an iron complex catalyst: A computational study.

The reaction mechanism for imine hydrosilylation in the presence of an iron methyl complex and hydrosilane was studied using density functional theory at the M06/6-311G(d,p) level of theory. Benzylidenemethylamine (PhCH = NMe) and trimethylhydrosilane (HSiMe3 ) were employed as the model imine and hydrosilane, respectively. Hydrosilylation has been experimentally proposed to occur in two stages. In the first stage, the active catalyst (CpFe(CO)SiMe3 , 1) is formed from the reaction of pre-catalyst, CpFe(CO)2 Me, and hydrosilane through CO migratory insertion into the FeMe bond and the reaction of the resulting acetyl complex intermediate with hydrosilane. In the second stage, 1 catalyzes the reaction of imine with hydrosilane. Calculations for the first stage showed that the most favorable pathway for CO insertion involved a spin state change, that is, two-state reactivity mechanism through a triplet state intermediate, and the acetyl complex reaction with HSiMe3 follows a σ-bond metathesis pathway. The calculations also showed that, in the catalytic cycle, the imine coordinates to 1 to form an FeCN three-membered ring intermediate accompanied by silyl group migration. This intermediate then reacts with HSiMe3 to yield the hydrosilylated product through a σ-bond metathesis and regenerate 1. The rate-determining step in the catalytic cycle was the coordination of HSiMe3 to the three-membered ring intermediate, with an activation energy of 23.1 kcal/mol. Imine hydrosilylation in the absence of an iron complex through a [2 + 2] cycloaddition mechanism requires much higher activation energies. © 2018 Wiley Periodicals, Inc.

Reactions of Group 4 Metallocene Bis(trimethylsilyl)acetylene Complexes with Nitriles and Isonitriles.

This Review summarizes reactions of Group 4 metallocene bis(trimethylsilyl)acetylene complexes with nitriles and isonitriles. Selected examples of reactions of Group 4 metallocene alkyne complexes of the type Cp'2 M(η2 -Me3 SiC2 SiMe3 ) with different nitriles and isonitriles are described following the general L-M-S principle, in which stoichiometric and catalytic reactions are influenced by ligands L, metals M, and substrate substituents S. The influence of all these parameters is highlighted. The basic reactions with nitriles and in some cases with isonitriles were investigated in detail and are summarized. Selected further reactions of the products again depend on L, M, and S. Most interesting feature for this is the formation of mono- and polynuclear 1-metalla-2,5-diazacyclopenta-2,4-dienes, which show great potential for the synthesis of complex heterocycles.

Thermally Stable Gold(III) Alkene and Alkyne Complexes: Synthesis, Structures, and Assessment of the trans‐Influence on Gold–Ligand Bond Enthalpies

The reaction of [C^C)Au(OEt2 )2 ]+ with 1,5-cyclooctadiene or norbornadiene affords the corresponding olefin complexes [(C^C)Au(COD)]SbF6 and [(C^C)Au(NBD)]SbF6 , which are thermally stable in solution and the solid state (C^C=4,4'-di-tert-butylbiphenyl-2,2'-diyl). The crystal structures of these complexes have been determined. By contrast, dienones such as dibenzylideneacetone are O- rather than C=C-bonded. The reactions of (C^C)Au(OAcF )(L) (L=PMe3 or CNxyl) with B(C6 F5 )3 in the presence of bis(1-adamantyl)acetylene give the mixed-ligand alkyne complexes [(C^C)Au(AdC≡CAd)(L)]+ , the first complexes of their type in gold chemistry. In the presence of an excess of acetylene these compounds are thermally stable in solution and as solids. The bonding of n- and π-donor ligands to AuIII fragments and the effect of the trans influence exerted by N- and C-donors was explored with the aid of DFT calculations. Results show that the Au-L bond enthalpies trans to anionic C are 35-60 % of the enthalpies trans to N, with strong π-acceptors being particularly affected. In comparison with [Me2 Au]+ , the [(C^C)Au]+ fragment is more polar and in bond enthalpy terms resembles Me2 Pt.

Synthesis, characterization, and reactivity of the first uranium metallocene 1,2-bis(diphenylphosphino)acetylene complexes

Two new uranium metallacyclopropenes, (C5Me4R)2U(η2-Ph2PCCPPh2) (R = Me, Et) were prepared by reducing the corresponding (C5Me4R)2UCl2 complexes with KC8 in the presence of 1,2-bis(diphenylphosphino)acetylene (Ph2PCCPPh2). Both compounds were fully characterized by a combination of elemental analysis and multinuclear NMR, UV–visible–NIR, and IR spectroscopies. Differences in the electronic spectra of these novel compounds and the known (C5Me5)2U(η2-Me3SiCCSiMe3) are discussed. Also presented is the solid-state structure of (C5Me4Et)2U(η2-Ph2PCCPPh2), which reveals significant distortions of the coordinated 1,2-bis(diphenylphosphino)acetylene (Ph2PCCPPh2) ligand.

Synthesis, Structure, Bonding, and Reactivity of Metal Complexes Comprising Diborane(4) and Diborene(2): [{Cp*Mo(CO)2}2{μ‐η2:η2‐B2H4}] and [{Cp*M(CO)2}2B2H2M(CO)4], M=Mo,W

AbstractThe reaction of [(Cp*Mo)2(μ‐Cl)2B2H6] (1) with CO at room temperature led to the formation of the highly fluxional species [{Cp*Mo(CO)2}2{μ‐η2:η2‐B2H4}] (2). Compound 2, to the best of our knowledge, is the first example of a bimetallic diborane(4) conforming to a singly bridged Cs structure. Theoretical studies show that 2 mimics the Cotton dimolybdenum–alkyne complex [{CpMo(CO)2}2C2H2]. In an attempt to replace two hydrogen atoms of diborane(4) in 2 with a 2e [W(CO)4] fragment, [{Cp*Mo(CO)2}2 B2H2W(CO)4] (3) was isolated upon treatment with [W(CO)5⋅thf]. Compound 3 shows the intriguing presence of [B2H2] with a short B−B length of 1.624(4) Å. We isolated the tungsten analogues of 3, [{Cp*W(CO)2}2B2H2W(CO)4] (4) and [{Cp*W(CO)2}2B2H2Mo(CO)4] (5), which provided direct proof of the existence of the tungsten analogue of 2.