Vanadium Alkylidyne Initiated Cyclic Polymer Synthesis: The Importance of a Deprotiovanadacyclobutadiene Moiety.

Reported is the catalytic cyclic polymer synthesis by a 3d transition metal complex: a V(V) alkylidyne, [(dBDI)V≡CtBu(OEt2)] (1-OEt2), supported by the deprotonated ß-diketiminate dBDI2- (dBDI2- = ArNC(CH3)CHC(CH2)NAr, Ar = 2,6-iPr2C6H3). Complex 1-OEt2 is a precatalyst for the polymerization of phenylacetylene (PhCCH) to give cyclic poly(phenylacetylene) (c-PPA), whereas its precursor, complex [(BDI)V≡CtBu(OTf)] (2-OTf; BDI- = [ArNC(CH3)]2CH, Ar = 2,6-iPr2C6H3, OTf = OSO2CF3), and the zwitterion [((C6F5)3B-dBDI)V≡CtBu(OEt2)] (3-OEt2) exhibit low catalytic activity despite having a neopentylidyne ligand. Cyclic polymer topologies were verified by size-exclusion chromatography (SEC) and intrinsic viscosity studies. A component of the mechanism of the cyclic polymerization reaction was probed by isolation and full characterization of 4- and 6-membered metallacycles as model intermediates. Metallacyclobutadiene (MCBD) and deprotiometallacyclobutadiene (dMCBD) complexes (dBDI)V[C(tBu)C(H)C(tBu)] (4-tBu) and (BDI)V[C(tBu)CC(Mes)] (5-Mes), respectively, were synthesized upon reaction with bulkier alkynes, tBu- (tBuCCH) and Mes-acetylene (MesCCH), with 1-OEt2. Furthermore, the reaction of the conjugate acid of 1-OEt2, [(BDI)V≡CtBu(OTf)] (2-OTf), with the conjugated base of phenylacetylene, lithium phenylacetylide (LiCCPh), yields the doubly deprotio-metallacycle complex, [Li(THF)4]{(BDI)V[C(Ph)CC(tBu)CC(Ph)]} (6). Protonation of the doubly deprotio-metallacycle complex 6 yields 6-H+, a catalytically active species toward the polymerization of PhCCH, for which the polymers were also confirmed to be cyclic by SEC studies. Computational mechanistic studies complement the experimental observations and provide insight into the mechanism of cyclic polymer growth. The noninnocence of the supporting dBDI2- ligand and its role in proton shuttling to generate deprotiometallacyclobutadiene (dMCBD) complexes that proposedly culminate in the formation of catalytically active V(III) species are also discussed. This work demonstrates how a dMCBD moiety can react with terminal alkynes to form cyclic polyalkynes.

Tethered Alkylidenes for REMP from Carbon Disulfide Cleavage.

Reactions between tungsten alkylidyne [tBuOCO]W≡CtBu(THF)2 1 and sulfur containing small molecules are reported. Complex 1 reacts with CS2 to produce intermediate η2 bound CS2 complex [O2C(tBuC═)W(η2-(S,C)-CS2)(THF)] 8. Heating complex 8 provides a mixture of a monomeric tungsten sulfido complex 9 and a dimeric complex 10 in a 4:1 ratio, respectively. Heating the mixture does not perturb the ratio. Addition of excess THF in a solution of 9 and 10 (4:1) converts 10 to 9 (>96%) with concomitant loss of (CS)x. Both 9 and 10 can be selectively crystallized from the mixture. An alternative synthesis of exclusively monomeric 9 involves the reaction between 1 and PhNCS. Demonstrating ring expansion metathesis polymerization (REMP), tethered tungsten alkylidene 8 polymerizes norbornene to produce cis-selective syndiotactic cyclic polynorbornene (c-poly(NBE)).

Metallacyclobuta-(2,3)-diene: A Bidentate Ligand for Stream-line Synthesis of First Row Transition Metal Catalysts for Cyclic Polymerization of Phenylacetylene.

Described here is a direct entry to two examples of 3d transition metal catalysts that are active for the cyclic polymerization of phenylacetylene, namely, [(BDI)M{κ2 -C,C-(Me3 SiC3 SiMe3 )}] (2-M) (BDI=[ArNC(CH3 )]2 CH- , Ar=2,6-i Pr2 C6 H3 ; M=Ti, V). Catalysts are prepared in one step by the treatment of [(BDI)MCl2 ] (1-M, M=Ti, V) with 1,3-dilithioallene [Li2 (Me3 SiC3 SiMe3 )]. Complexes 2-M have been spectroscopically and structurally characterized and the polymers that are catalytically formed from phenylacetylene were verified to have a cyclic topology based on a combination of size-exclusion chromatography (SEC) and intrinsic viscosity studies. Two-electron oxidation of 2-V with nitrous oxide (N2 O) cleanly yields a [VV ] alkylidene-alkynyl oxo complex [(BDI)V(=O){κ1 -C-(=C(SiMe3 )CC(SiMe3 ))}] (3), which lends support for how this scaffold in 2-M might be operating in the polymerization of the terminal alkyne. This work demonstrates how alkylidynes can be circumvented using 1,3-dianionic allene as a segue into M-C multiple bonds.

Ring Expansion Alkyne Metathesis Polymerization.

The synthesis, characterization, and preliminary activity of an unprecedented tethered alkylidyne tungsten complex for ring expansion alkyne metathesis polymerization (REAMP) are reported. The tethered alkylidyne 7 is generated rapidly by combining alkylidyne W(CtBu)(CH2tBu)(O-2,6-i-Pr2C6H3)2 (6) with 1 equiv of an yne-ol proligand (5). Characterized by NMR studies and nuclear Overhauser effect spectroscopy, complex 7 is a dimer. Each metal center contains a tungsten-carbon triple bond tethered to the metal center via an alkoxide ligand. The polymerization of the strained cycloalkyne 3,8-didodecyloxy-5,6-dihydro-11,12-didehydrodibenzo[a,e]-[8]annulene, 8, to generate cyclic polymers was demonstrated. Size exclusion chromatography (SEC) and intrinsic viscosity (η) measurements confirm the polymer's cyclic topology.

Isolation of an Elusive Phosphametallacyclobutadiene and Its Role in Reversible Carbon-Carbon Bond Cleavage.

The reactivity of phosphaalkynes, the isolobal and isoelectronic congeners to alkynes, with metal alkylidyne complexes is explored in this work. Treating the tungsten alkylidyne [t BuOCO]W≡Ct Bu(THF)2 (1) with phosphaalkyne (10) results in the formation of [O2 C(t BuC=)W{η2 -(P,C)-P≡C-Ad}(THF)] (13-t BuTHF ) and [O2 C(AdC=)W{η2 -(P,C)-P≡C-t Bu}(THF)] (13-AdTHF ); derived from the formal reductive migratory insertion of the alkylidyne moiety into a W-Carene bond. Analogous to alkyne metathesis, a stable phosphametallacyclobutadiene complex [t BuOCO]W[κ2 -C(t Bu)PC(Ad)] (14) forms upon loss of THF from the coordination sphere of either 13-t BuTHF or 13-AdTHF . Remarkably, the C-C bonds reversibly form/cleave with the addition or removal of THF from the coordination sphere of the formal tungsten(VI) metal center, permitting unprecedented control over the transformation of a tetraanionic pincer to a trianionic pincer and back. Computational analysis offers thermodynamic and electronic reasoning for the reversible equilibrium between 13-t Bu/AdTHF and 14.

A High-Throughput Approach to Repurposing Olefin Polymerization Catalysts for Polymer Upcycling.

Efficient and economical plastic waste upcycling relies on the development of catalysts capable of polymer degradation. A systematic high-throughput screening of twenty-eight polymerization catalyst precursors, belonging to the catalyst families of metallocenes, ansa-metallocenes, and hemi- and post-metallocenes, in cis-1,4-polybutadiene (PB) degradation reveals, for the first time, important structure-activity correlations. The upcycling conditions involve activation of the catalysts (at 0.18 % catalyst loading) with tri-iso-butyl aluminum at 50 °C in toluene. The data indicate the ability to degrade PB is a general reactivity profile of neutral group 4 metal hydrides. A simple quantitative-structure activity relationship (QSAR) model utilizing two descriptors for the distribution of steric bulk in the active pocket and one measuring the metal ion electrophilicity reveals the degradation ability improves with increased but not overbearing steric congestion and lower electrophilicity.

Double Tethered Metallacyclobutane Catalyst for Cyclic Polymer Synthesis.

This work outlines an approach to creating a catalyst for cyclic polymer synthesis using readily available materials in only one or two steps. Combining commercially available molybdenum-alkylidene 1 with two equivalents of ene-ol proligand 2 rapidly produces, in quantitative yield (1H NMR spectroscopy), the double tethered metallacyclobutane complex 3. Characterized by variable temperature NMR studies and nuclear Overhauser effect spectroscopy (NOESY) experiments, complex 3 exhibits fluxional behavior in solution. Determined by single crystal X-ray diffraction, the solid-state structure of complex 3 reveals metrical parameters indicating that the metallacyclobutane is not predicted to undergo rapid retro-cycloaddition. However, complex 3 is a precatalyst for the polymerization of norbornene to produce cyclic polynorbornene. An NMR spectrum of a test polymerization indicates that only a small fraction of the precatalyst is activated upon exposure to monomer. Quantifying the active catalyst is possible by measuring vinyl resonances that appear in the 1H NMR spectrum. The vinyl resonances are attributable to the release of one of the tethers upon norbornene addition. Confirmation of the polymer cyclic topology comes from gel permeation chromatography (GPC), dynamic light scattering (DLS), and intrinsic viscosity (η) measurements. The double tethered metallacyclobutane complex is a novel design for catalytic cyclic polymer synthesis. The synthetic approach suggests that catalyst tuning is possible by a choice of the commercial alkylidene and alteration of the ene-ol proligand.

Tethered Tungsten-Alkylidenes for the Synthesis of Cyclic Polynorbornene via Ring Expansion Metathesis: Unprecedented Stereoselectivity and Trapping of Key Catalytic Intermediates.

This report describes an approach for preparing tethered tungsten-imido alkylidene complexes featuring a tetra-anionic pincer ligand. Treating the tungsten alkylidyne [tBuOCO]W≡CtBu(THF)2 (1) with isocyanates (RNCO; R = tBu, Cy, and Ph) leads to cycloaddition occurring exclusively at the C═N bond to generate the tethered tungsten-imido alkylidenes (6-NR). Unanticipated intermediates reveal themselves, including the discovery of [(O2CtBuC═)W(η2-(N,C)-RNCO)(THF)] (11-R) and an unprecedented decarbonylation product [(tBuOCO)W(≡NR)(tBuCCO)] (14-R), on the pathway to the formation of 6-NR. Complex 11-R is kinetically stable for sterically bulky isocyanate R = tBu (11-tBu) and is isolated and characterized by single-crystal X-ray diffraction. Finally, adding to the short list of catalysts capable of ring expansion metathesis polymerization (REMP), complexes 6-NR and 11-tBu are active for the stereoselective synthesis of cyclic polynorbornene.

Excited-State Turn-On of Aurophilicity and Tunability of Relativistic Effects in a Series of Digold Triazolates Synthesized via iClick.

iClick reactions between Au(I) acetylides PPh3Au-C≡CR, where R = nitrophenyl (PhNO2), phenyl (Ph), thiophene (Th), bithiophene (biTh), and dimethyl aniline (PhNMe2), and Au(I)-azide PPh3AuN3 provide digold complexes of the general formula R-1,5-bis-triphenylphosphinegold(I) 1,2,3-triazolate (Au2-R). Within the digold triazolate complexes the Au(I) atoms are held in close proximity but beyond the distance typically observed for aurophilic bonding. Though no bond exists in the ground state, time-dependent density functional theory interrogation of the complexes reveals excited states with significant aurophilic bonding. The series of complexes allows for tuning of the excited-state "turn-on" of aurophilicity, where ligand to metal charge transfer (LMCT) induces the aurophilic bonding. Complexes containing ligand-localized excited states, however, do not exhibit aurophilicity in the excited state. As a control experiment, a monogold complex was synthesized. The computed excited state of the monogold species exhibited LMCT to the gold ion as in the dinuclear cases, but without a partnering gold ion only a distinct N-Au-P bending occurs, revealing a potential mechanism for the excited-state turn-on of aurophilic bonding. Analysis of the steady-state electronic spectra indicates that LMCT states are achievable for compounds with sufficiently strong electron-donating ligands, and in digold complexes this is associated with enhanced fluorescence, suggestive of an aurophilic interaction.

An Application Exploiting Aurophilic Bonding and iClick to Produce White Light Emitting Materials.

The paper focuses on exploiting aurophilic bonding to produce white light emitting materials. Inorganic Click (iClick) is employed to link two or four Au(I) metal ions through a triazolate bridge. Depending on the choice of phosphine ligand (PEt3 or PPh3), dinuclear Au2-FO or tetranuclear Au4-FO complexes can be controllably synthesized (FO = 2-(9,9-dioctylfluoreneyl-)). The iClick products Au2-FO and Au4-FO are characterized by combustion analysis and multinuclear NMR, TOCSY 1D, 1H-13C gHMBC, and 1H-13C gHSQC. In addition, the photophysical properties of Au2-FO and Au4-FO were examined in THF solution. Transient absorption spectroscopy was employed to elucidate the excited state features of the gold compounds. Solution processed OLEDs were fabricated and characterized, which gave white light electroluminescence with CIE coordinates (0.34, 0.36), as seen referenced to CIE standard illuminant D65 (0.31, 0.32). TDDFT computational analysis of Au2-FO and Au4-FO reveals the origin of light emission. In the case of Au4-FO, direct excitation leads to increased aurophilic bonding in the excited state, and as a result the emission profile is broadened to cover a larger region of the visible spectrum, thus giving white light emission. Designing molecules that can access or increase aurophilic bonding in the excited state provides another tool for fine-tuning the emission profiles of gold complexes.

Ammonia Synthesis through Hydrolysis of a Trianionic Pincer Ligand-Supported Molybdenum-Nitride Complex.

Reported is the hydrolysis of a homogeneous Mo-nitride complex bearing a trianionic pincer-type ligand to produce ammonia. Treating the anionic [(ONO)]Mo≡N(OtBu)]Ph3 PCH3 with two equivalents of water produces ammonia and the dioxo complex [(ONO)]MoO2 ]Ph3 PCH3 . X-Ray crystal structures of the starting nitrido complex and product dioxo complex are presented. Evidence for ammonia release comes from GC-MS and deuterium-labelling studies. The reaction is presented in the context of a two-stage solar thermochemical dinitrogen fixation process as the solid-state nitride hydrolysis step.

ß-Alkyl Elimination: Fundamental Principles and Some Applications.

This review describes organometallic compounds and materials that are capable of mediating a rarely encountered but fundamentally important reaction: ß-alkyl elimination at the metal-Cα-Cß-R moiety, in which an alkyl group attached to the Cß atom is transferred to the metal or to a coordinated substrate. The objectives of this review are to provide a cohesive fundamental understanding of ß-alkyl-elimination reactions and to highlight its applications in olefin polymerization, alkane hydrogenolysis, depolymerization of branched polymers, ring-opening polymerization of cycloalkanes, and other useful organic reactions. To provide a coherent understanding of the ß-alkyl elimination reaction, special attention is given to conditions and strategies used to facilitate ß-alkyl-elimination/transfer events in metal-catalyzed olefin polymerization, which provide the well-studied examples.

Highly Tactic Cyclic Polynorbornene: Stereoselective Ring Expansion Metathesis Polymerization of Norbornene Catalyzed by a New Tethered Tungsten-Alkylidene Catalyst.

The tungsten alkylidyne [(t)BuOCO]W≡C((t)Bu) (THF)2 (1) reacts with CO2, leading to complete cleavage of one C═O bond, followed by migratory insertion to generate the tungsten-oxo alkylidene 2. Complex 2 is the first catalyst to polymerize norbornene via ring expansion metathesis polymerization to yield highly cis-syndiotactic cyclic polynorbornene.

Introducing "Ynene" Metathesis: Ring-Expansion Metathesis Polymerization Leads to Highly Cis and Syndiotactic Cyclic Polymers of Norbornene.

Tungsten alkylidynes [CF3-ONO]W≡CC(CH3)3(THF)2 (1) and [(t)BuOCO]W≡CC(CH3)3(THF)2 (3) react with ethylene. Complex 1 reacts reversibly with ethylene to give the metallacyclobutene (2). Complex 3 reacts with ethylene to form the tethered alkylidene (4) featuring a tetraanionic pincer ligand. Complexes 1 and 3 initiate the polymerization of norbornene at room temperature. The polymerization of norbornene by 1 is not stereoselective, whereas 3 generates a highly cis and syndiotactic cyclic polynorbornene. Comparison of the intrinsic viscosity, radius of gyration, and elution time of the synthesized cyclic polynorbornene with those of linear analogues provides conclusive evidence for a cyclic topology.

Solid State Collapse of a High-Spin Square-Planar Fe(II) Complex, Solution Phase Dynamics, and Electronic Structure Characterization of an Fe(II)2 Dimer.

Square-planar high-spin Fe(II) molecular compounds are rare, and until recently, the only four examples of non-macrocyclic or sterically driven molecular compounds of this kind shared a common FeO4 core. The trianionic pincer-type ligand [CF3-ONO]H3 (1) supports the high-spin square-planar Fe(II) complex {[CF3-ONO]FeCl}{Li(Sv)2}2 (2). In the solid state, 2 forms the dimer complex {[CF3-ONO]Fe}2{(µ-Cl)2(µ-LiTHF)4} (3) in 96% yield by simply applying a vacuum or stirring it with pentane for 2 h. A detailed high-frequency electron paramagnetic resonance and field-dependent (57)Fe Mössbauer investigation of 3 revealed a weak antiferromagnetic exchange interaction between the local iron spins which exhibit a zero-field splitting tensor characterized by negative D parameter. In solution, 2 is in equilibrium with the solvento complex {[CF3-ONO]FeCl(THF)}{Li2(Sv)4} (2·Sv) and the dimer 3. A combination of frozen solution (57)Fe Mössbauer spectroscopy and single crystal X-ray crystallography helped elucidate the solvent dependent equilibrium between these three species. The oxidation chemistry of 2·Sv was investigated. Complex 2 reacts readily with the one-electron oxidizing agent CuCl2 to give the Fe(III) complex {[CF3-ONO]FeCl2}{Li(THF)2}2 (4). Also, 2·Sv reacts with 2 equiv of TlPF6 to form the Fe(III) complex [CF3-ONO]Fe(THF)3 (5).

N-Heterocyclic Carbene-Gold(I) Complexes Conjugated to a Leukemia-Specific DNA Aptamer for Targeted Drug Delivery.

This report describes the synthesis and characterization of novel N-heterocyclic carbene (NHC)-gold(I) complexes and their bioconjugation to the CCRF-CEM-leukemia-specific aptamer sgc8c. Successful bioconjugation was confirmed by the use of fluorescent tags on both the NHC-Au(I) complex and the aptamer. Cell-viability assays indicated that the NHC-Au(I) -aptamer conjugate was more cytotoxic than the NHC-gold complex alone. A combination of flow cytometry, confocal microscopy, and cell-viability assays provided clear evidence that the NHC-Au(I) -aptamer conjugate was selective for targeted CCRF-CEM leukemia cells.

Fast "Wittig-like" reactions as a consequence of the inorganic enamine effect.

The tungsten alkylidyne [CF3-ONO]W≡CC(CH3)3(THF)2 (3) {where CF3-ONO = (MeC6H3[C(CF3)2O])2N(3-)} supported by a trianionic pincer-type ligand demonstrates enhanced nucleophilicity in unusually fast "Wittig-like" reactions. Experiments are designed to provide support for an inorganic enamine effect that is the origin of the enhanced nucleophilicity. Treating complex 3 with various carbonyl-containing substrates provides tungsten-oxo-vinyl complexes upon oxygen atom transfer. The rates of reactivity of 3 are compared with the known alkylidyne (DIPP)3W≡CC(CH3)3 (DIPP = 2,6-diisopropylphenoxide). In all cases (except acetone), complex 3 exhibits significantly faster overall rates than (DIPP)3W≡CC(CH3)3. New oxo-vinyl complexes are characterized by NMR, combustion analysis and single crystal X-ray diffraction. Treating 3 with acid chlorides provides the tungsten oxo chloride species [CF3-ONO]W(O)Cl (4) and disubstituted alkynes. In the case of acetone the oxo-vinyl complex results in two rotational isomers 10syn and 10anti. The rate of isomerization was determined for the forward and reverse directions and was complimented with DFT calculations.

Trianionic pincer and pincer-type metal complexes and catalysts.

Trianionic pincer and pincer-type ligands are the focus of this review. Metal ions from across the periodic table, from main group elements, transition metals, and the rare earths, are combined with trianionic pincer ligands to produce some of the most interesting complexes to appear in the literature over the past decade. This review provides a comprehensive examination of the synthesis, characterization, properties, and catalytic applications of trianionic pincer metal complexes. Some of the interesting applications employing trianionic pincer and pincer-type complexes include: (1) catalyzed aerobic oxidation, (2) alkene isomerization, (3) alkene and alkyne polymerization, (4) nitrene and carbene group transfer, (5) fundamental transformations such as oxygen-atom transfer, (6) nitrogen-atom transfer, (7) O2 activation, (8) C-H bond activation, (9) disulfide reduction, and (10) ligand centered storage of redox equivalents (i.e. redox active ligands). Expansion of the architecture, type of donor atoms, chelate ring size, and steric and electronic properties of trianionic pincer ligands has occurred rapidly over the past ten years. This review is structured according to the type of pincer donor atoms that bind to the metal ion. The type of donor atoms within trianionic pincer and pincer-type ligands to be discussed include: NCN(3-), OCO(3-), CCC(3-), redox active NNN(3-), NNN(3-), redox active ONO(3-), ONO(3-), and SNS(3-). Since this is the first review of trianionic pincer and pincer-type ligands, an emphasis is placed on providing the reader with in-depth discussion of synthetic methods, characterization data, and highlights of these complexes as catalysts.