Architectural Editing of Polyesters and Polyurethanes via Palladium(II)-Catalyzed [3,3]-Sigmatropic Oxo-Rearrangements.

Architecture underlies the thermomechanical properties of polymers. Yet, few strategies are available to tune a polymer's architecture after it is prepared without altering its chemical composition. The ability to edit the architecture of a polymer would dramatically expand the accessible architecture-property space of polymeric materials. Herein, we disclose a backbone rearrangement approach to tune the short-chain branching of polymers. Specifically, we demonstrate that palladium(II)-catalyzed [3,3]-sigmatropic oxo-rearrangements can transform branched polyesters and polyurethanes to their linear counterparts. While the effects on materials properties are generally subtle in the case of polyesters, more dramatic changes are observed in the case of polyurethanes: two polyurethanes undergo a soluble-to-insoluble transition, and one exhibits a dramatic increase in both strain at break and toughness after rearrangement. Additionally, the incorporation of alkenes in the polymer backbone through the rearrangement enables facile deconstruction via ethenolysis. In all, we disclose a powerful and broad-scope strategy to edit the architecture of polymer backbones and thereby tune their physical and chemical properties.

Mechanistic study of pyrazole synthesis via oxidation-induced N-N coupling of diazatitanacycles.

Metal-mediated inner-sphere N-N coupling is an uncommon route to N-N bond formation. Herein, we report a mechanistic study of pyrazole formation via oxidation-induced N-N coupling of diazatitanacycles. In TEMPO oxidation reactions, the first of two oxidations is rate limiting and TEMPO coordination to Ti is critical for reactivity. In oxidations with Fc+ salts, coordinating counteranions such (eg. Cl-) aid an "inner-sphere-like" oxidation.

Bonding and Reactivity of d0 Transition Metal Imido Complexes Encoded in Their 15N NMR Signatures.

Terminal imido complexes containing metal-nitrogen multiple bonds have been widely used in organometallic chemistry and homogeneous catalysis. The role of terminal imido ligands spans from reactive sites to spectator motifs, largely depending on the nature of the metal center and its specific coordination sphere. Aiming at identifying reactivity descriptors for M-N multiple bonds, we herein explore solid-state 15N NMR spectroscopy (ssNMR) on early transition metal terminal imido complexes augmented by computational studies and show that the asymmetry parameter, κ (skew, 1 ≥ κ ≥ -1), readily available from experiments or calculations, is diagnostic for the reactivity of M-N multiple bonds in imido complexes. While inert imido ligands exhibit skew values (κ) close to 1, highly reactive imido moieties display significantly lower skew values (κ ≪ 1) as found in metallocene or bis-imido complexes. Natural chemical shielding analysis shows that skew values away from 1 are associated with an asymmetric development of π-orbitals around the M-N multiple bond of the imido moiety, with a larger double-bond character for reactive imido. Notably, this descriptor does not directly relate to the M-N-C bond angle, illustrating the shortcoming of evaluating bonding and hybridization from geometrical parameters alone. Overall, this descriptor enables to obtain direct experimental evidence for the π-loading effect seen in bis(imido) and related complexes, thus explaining their bonding/reactivity.

Evaluation of Tungsten Catalysis among Early Transition Metals for N-Aryl-2,3,4,5-tetraarylpyrrole Synthesis: Modular Access to N-Doped π-Conjugated Material Precursors.

Low-valent tungsten species generated from WCl6 and N,N'-bis(trimethylsilyl)-2,5-dimethyldihydropyrazine (Si-Me2-DHP) promotes the catalytic formation of N-phenyl-2,3,4,5-tetraarylpyrroles 3aa-ka from diarylacetylenes 1a-k and azobenzene (2a). An initial catalyst activation process is a three-electron reduction of WCl6 with Si-Me2-DHP to afford transient 'WCl3' species. Catalytically active bis(imido)tungsten(VI) species via successive one-electron reduction and N═N bond cleavage of 2a was revealed by isolating W(═NPh)2Cl2(PMe2Ph)2 from imidotungsten(V) trichloride and 2a in the presence of PMe2Ph. The superior catalytic activity of the tungsten catalyst was clarified by a density functional theory study: activation energies for the key three steps, [2 + 2]-cycloaddition of W═NPh and diarylacetylene to form (iminoalkylidene)tungsten species, enyne metathesis with second diarylacetylene, and C-N bond formation, are reasonable values for the catalytic reaction at 180 °C. In addition, this tungsten catalyst overcame two distinct deactivation processes: α-enediamido formation and aggregation of the low-valent species, both of which were observed for previously developed vanadium and titanium catalysts. We also demonstrated the synthetic utility of pentaarylpyrroles 3aa and 3ba as well as N-(2-bromophenyl)-2,3,4,5-tetraarylpyrrole 3ab by derivatizing their π-conjugated compounds 9aa, 10ba, and 11ab.

Cp2Ti(II) Mediated Rearrangement of Cyclopropyl Imines.

Ti-catalyzed oxidative alkyne carboamination with alkenes and azo compounds can yield either α,ß-unsaturated imines or cyclopropyl imines through a common azatitanacyclohexene intermediate. Herein, we report the synthesis of a model azatitanacyclohexene complex (3) through the ring-opening of a cyclopropyl imine with Cp2Ti(BTMSA) (BTMSA = bis(trimethylsilyl)acetylene). 3 readily undergoes thermal or reductant-catalyzed ring contraction to an azatitanacyclopentene (4), analogous to the proposed mechanism for forming α,ß-unsaturated imines in the catalytic reaction. A cyclopropyl imine or an α,ß-unsaturated imine could be liberated via the oxidation of 3 or 4 with azobenzene, respectively, further implicating the role of these metallacycles in the Ti-catalyzed carboamination reaction.

Synthesis of Ti Complexes Supported by an ortho-terphenoxide Ligand and their Applications in Alkyne Hydroamination Catalysis.

The synthesis of a series of Ti complexes of an aryl-linked bis-phenoxide ligand, 3,3''-di-tert-butyl-5,5''-dimethyl-[1,1':2',1''-terphenyl]-2,2''-bis(olate), (TPO)H2, is reported. This ortho-linked terphenyl ligand builds on previously reported meta- and para- linked terphenyl based ligands, completing the isomeric series of terphenoxide ligands. The 4-coordinate (TPO)Ti(NMe2)2 is an active catalyst for alkyne hydroamination with a variety of arylamines, revealing good regioselectivity in reactions with unsymmetric alkynes. Terminal alkynes such as phenylacetylene undergo additional insertion reactions with the key azatitanacyclobutene intermediates, providing further evidence that Ti aryloxide complexes are susceptible to this further reactivity.

Reassessment of N2 activation by low-valent Ti-amide complexes: a remarkable side-on bridged bis-N2 adduct is actually an arene adduct.

The complex {(TMEDA)2Li}{[Ti(N(TMS)2)2]2(µ-η2:η2-N2)2} (5-Li) is the only transition metal N2 complex ever reported with two side-on N2 adducts. In this report, the similarity of 5-Li to a new inverse sandwich toluene adduct {(PhMe)K}{[Ti(N(TMS)2)2]2(µ-PhMe)} (6-K) necessitated a re-examination of the structure of 5-Li. Through a reassessment of the original disordered crystal data of 5-Li and new independent syntheses brought about through revisitation of the original reaction conditions, 5-Li has been re-assigned as an inverse sandwich toluene adduct, {(TMEDA)2Li}{[Ti(N(TMS)2)2]2(µ-PhMe)} (6-Li). The original crystal data could be fitted almost equally well to structural solutions as either 5-Li or 6-Li, and this study highlights the importance of a holistic examination of modeled data and the need for secondary/complementary analytical methods in paramagnetic inorganic syntheses, especially when presenting unique and unexpected results. In addition, further examination of reduction reactions of Ti[N(TMS)2]3 and [(TMS)2N]2TiCl(THF) in the presence of KC8 revealed rich solvent- and counterion-dependent chemistry, including several degrees of N2 activation (bridging nitride complexes, terminal bridging N2 complexes) as well as ligand C-H activation.

3-Ethyl-6-vinyltetrahydro-2H-pyran-2-one (EVP): a versatile CO2-derived lactone platform for polymer synthesis.

3-Ethyl-6-vinyltetrahydro-2H-pyran-2-one (EVP) is a CO2-derived lactone synthesized via Pd-catalyzed telomerization of butadiene. As EVP is 28.9% by weight CO2, it has received significant recent attention as an intermediary for the synthesis of high CO2-content polymers. This article provides an overview of strategies for the polymerization of EVP to a wide variety of polymer structures, ranging from radical polymerizations to ring-opening polymerizations, that each take unique advantage of the highly functionalized lactone.

A Dual Catalyst Strategy for Controlling Aluminum Nanocrystal Growth.

The synthesis of Al nanocrystals (Al NCs) is a rapidly expanding field, but there are few strategies for size and morphology control. Here we introduce a dual catalyst approach for the synthesis of Al NCs to control both NC size and shape. By using one catalyst that nucleates growth more rapidly than a second catalyst whose ligands affect NC morphology during growth, one can obtain both size and shape control of the resulting Al NCs. The combination of the two catalysts (1) titanium isopropoxide (TIP), for rapid nucleation, and (2) Tebbe's reagent, for specific facet-promoting growth, yields {100}-faceted Al NCs with tunable diameters between 35 and 65 nm. This dual-catalyst strategy could dramatically expand the possible outcomes for Al NC growth, opening the door to new controlled morphologies and a deeper understanding of earth-abundant plasmonic nanocrystal synthesis.

Tunable and recyclable polyesters from CO2 and butadiene.

Carbon dioxide is inexpensive and abundant, and its prevalence as waste makes it attractive as a sustainable chemical feedstock. Although there are examples of copolymerizations of CO2 with high-energy monomers, the direct copolymerization of CO2 with olefins has not been reported. Here an alternative route to functionalizable, recyclable polyesters derived from CO2, butadiene and hydrogen via an intermediary lactone, 3-ethyl-6-vinyltetrahydro-2H-pyran-2-one, is described. Catalytic ring-opening polymerization of the lactone by 1,5,7-triazabicyclo[4.4.0]dec-5-ene yields polyesters with molar masses up to 13.6 kg mol-1 and pendent vinyl side chains that can undergo post-polymerization functionalization. The polymer has a low ceiling temperature of 138 °C, allowing for facile chemical recycling, and is inherently biodegradable under aerobic aqueous conditions (OECD-301B protocol). These results show that a well-defined polyester can be derived from CO2, olefins and hydrogen, expanding access to new polymer feedstocks that were once considered unfeasible.

Ruthenium hydrides encapsulated in sol-gel glasses exhibit new ultrafast vibrational dynamics.

Vibrational dynamics were measured by IR pump-probe spectroscopy and two-dimensional IR spectroscopy for triruthenium dodecacarbonyl and the undecacarbonyl hydride that forms when it is encapsulated in an alumina sol-gel glass. For comparison, a triruthenium undecacarbonyl hydride salt was also synthesized and studied in neat solution to identify the potential influence of the confined solvent environment on the dynamics experienced by carbon monoxide ligands. The vibrational lifetime was found to be significantly decreased for both hydride species relative to the dodecacarbonyl compound. Conversely, spectral diffusion of the CO vibrations was measured to be faster for the parent compound. The most significant dynamic changes occurred upon transformation from the starting compound to the hydride, while only minor differences were observed between the dynamics of the freely dissolved and sol-gel encapsulated hydrides. The results suggest that the structural change to the hydride has the largest impact on the dynamics and that its improved catalytic properties likely do not originate from confined solvent effects.

α-Diimine synthesis via titanium-mediated multicomponent diimination of alkynes with C-nitrosos.

α-Diimines are commonly used as supporting ligands for a variety of transition metal-catalyzed processes, most notably in α-olefin polymerization. They are also precursors to valuable synthetic targets, such as chiral 1,2-diamines. Their synthesis is usually performed through acid-catalyzed condensation of amines with α-diketones. Despite the simplicity of this approach, accessing unsymmetrical α-diimines is challenging. Herein, we report the Ti-mediated intermolecular diimination of alkynes to afford a variety of symmetrical and unsymmetrical α-diimines through the reaction of diazatitanacyclohexadiene intermediates with C-nitrosos. These diazatitanacycles can be readily accessed in situ via the multicomponent coupling of Ti[triple bond, length as m-dash]NR imidos with alkynes and nitriles. The formation of α-diimines is achieved through formal [4 + 2]-cycloaddition of the C-nitroso to the Ti and γ-carbon of the diazatitanacyclohexadiene followed by two subsequent cycloreversion steps to eliminate nitrile and afford the α-diimine and a Ti oxo.

Electronic Structure Analysis and Reactivity of the Bimetallic Bis-Titanocene Vinylcarboxylate Complex, [(Cp2Ti)2(O2C3TMS2)]†.

Multimetallic redox cooperativity features heavily in both bioinorganic and synthetic reactions. Here, the electronic structure of the bimetallic Ti/Ti complex 11, [(Cp2Ti)2(O2C3TMS2)] has been revisited with EPR, confirming a predominantly TiIII/TiIII electronic structure. Reactions of 11 with 2,6-dimethylphenyl isocyanide (CNXyl), TMSCl, MeI, and BnCl were explored, revealing differential redox chemistry of the bimetallic core. In reactions with CNXyl and TMSCl, the metallacyclic TiIII center remained unperturbed, with reactions taking place at the pendent κ2(O,O)-titanocene fragment, while reaction with MeI resulted in remote oxidation of the metallacyclic Ti center, indicative of a cooperative redox process. All structures were studied via X-ray diffraction and EPR spectroscopic analysis, and their electronic structures are discussed in the context of the covalent bond classification (CBC) electron counting method.

Multicomponent syntheses of 5- and 6-membered aromatic heterocycles using group 4-8 transition metal catalysts.

In this Perspective, we discuss recent syntheses of 5- and 6-membered aromatic heterocycles via multicomponent reactions (MCRs) that are catalyzed by group 4-8 transition metals. These MCRs can be categorized based on the substrate components used to generate the cyclized product, as well as on common mechanistic features between the catalyst systems. These particular groupings are intended to highlight mechanistic and strategic similarities between otherwise disparate transition metals and to encourage future work exploring related systems with otherwise-overlooked elements. Importantly, in many cases these early- to mid-transition metal catalysts have been shown to be as effective for heterocycle syntheses as the later (and more commonly implemented) group 9-11 metals.

Ti-Catalyzed and -Mediated Oxidative Amination Reactions.

Titanium is an attractive metal for catalytic reaction development: it is earth-abundant, inexpensive, and generally nontoxic. However-like most early transition metals-catalytic redox reactions with Ti are difficult because of the stability of the high-valent TiIV state. Understanding the fundamental mechanisms behind Ti redox processes is key for making progress toward potential catalytic applications. This Account details recent progress in Ti-catalyzed (and -mediated) oxidative amination reactions that proceed through formally TiII/TiIV catalytic cycles.This class of reactions is built on our initial discovery of Ti-catalyzed [2 + 2 + 1] pyrrole synthesis from alkynes and azobenzene, where detailed mechanistic studies have revealed important factors that allow for catalytic turnover despite the inherent difficulty of Ti redox. Two important conclusions from mechanistic studies are that (1) low-valent Ti intermediates in catalysis can be stabilized through coordination of π-acceptor substrates or products, where they can act as "redox-noninnocent" ligands through metal-to-ligand π back-donation, and (2) reductive elimination processes with Ti proceed through π-type electrocyclic (or pericyclic) reaction mechanisms rather than direct σ-bond coupling.The key reactive species in Ti-catalyzed oxidative amination reactions are Ti imidos (Ti≡NR), which can be generated from either aryl diazenes (RN═NR) or organic azides (RN3). These Ti imidos can then undergo [2 + 2] cycloadditions with alkynes, resulting in intermediates that can be coupled to an array of other unsaturated functional groups, including alkynes, alkenes, nitriles, and nitrosos. This basic reactivity pattern has been extended into a broad range of catalytic and stoichiometric oxidative multicomponent coupling reactions of alkynes and other reactive small molecules, leading to multicomponent syntheses of various heterocycles and aminated building blocks.For example, catalytic oxidative coupling of Ti imidos with two different alkynes leads to pyrroles, while stoichiometric oxidative coupling with alkynes and nitriles leads to pyrazoles. These heterocycle syntheses often yield substitution patterns that are complementary to those of classical condensation routes and provide access to new electron-rich, highly substituted heteroaromatic scaffolds. Furthermore, catalytic oxidative alkyne carboamination reactions can be accomplished via reaction of Ti imidos with alkynes and alkenes, yielding α,ß-unsaturated imine or cyclopropylimine building blocks. New catalytic and stoichiometric oxidative amination methods such as alkyne α-diimination, isocyanide imination, and ring-opening oxidative amination of strained alkenes are continuously emerging as a result of better mechanistic understanding of Ti redox catalysis.Ultimately, these Ti-catalyzed and -mediated oxidative amination methods demonstrate the importance of examining often-overlooked elements like the early transition metals through the lens of modern catalysis: rather than a lack of utility, these elements frequently have undiscovered potential for new transformations with orthogonal or complementary selectivity to their late transition metal counterparts.

Reactivity of terminal imido complexes of group 4-6 metals: stoichiometric and catalytic reactions involving cycloaddition with unsaturated organic molecules.

Imido complexes of early transition metals are key intermediates in the synthesis of many nitrogen-containing organic compounds. The metal-nitrogen double bond of the imido moiety undergoes [2+2] cycloaddition reactions with various unsaturated organic molecules to form new nitrogen-carbon and nitrogen-heteroatom bonds. This review article focuses on reactivity of the terminal imido complexes of Group 4-6 metals, summarizing their stoichiometric reactions and catalytic applications for a variety of reactions including alkyne hydroamination, alkyne carboamination, pyrrole formation, imine metathesis, and condensation reactions of carbonyl compounds with isocyanates.

Spectroscopic Study of Sol-Gel Entrapped Triruthenium Dodecacarbonyl Catalyst Reveals Hydride Formation.

Triruthenium dodecacarbonyl exhibits increased catalytic activity toward hydrogenation reactions when encapsulated in alumina sol-gels. In this study, we demonstrate structural and electronic changes induced by the encapsulation process. Fourier transform infrared (FTIR) spectroscopy reveals that the carbonyl vibrational modes dramatically red shift during aging in the sol-gel glass. These shifts are attributed to the formation of the metal hydride: [HRu3(CO)11]-. A comparison to the FTIR spectrum of synthesized [NEt4][HRu3(CO)11] confirms this assignment. XPS studies show that the Ru 3d5/2 peak of [HRu3(CO)11]- also shifts to lower binding energy, consistent with an increased electron density on the Ru nuclei compared to Ru3(CO)12 and confirmed by density functional calculations. This study should open the door to further investigations into the hydride's role in the previously observed catalytic activity. To the best of our knowledge, this is the first study to identify the presence of [HRu3(CO)11]- in the alumina sol-gel.

Synthesis and Characterization of Tantalum-Based Early-Late Heterobimetallic Complexes Supported by 2-(diphenylphosphino)pyrrolide Ligands.

The synthesis of the metalloligand Ta(κ2-NP)3Cl2 (NP = 2-diphenylphosphinopyrrolide) and its coordination chemistry with group 9 and 10 metals is reported. Treatment of Ta(κ2-NP)3Cl2 with group 9 and 10 metals resulted in clean formation of the heterobimetallic complexes Cl2Ta(µ2-NP)3M (M = Ni (2), Pd (3)) or Cl2Ta(µ2-NP)3MCl (M = Rh (4), Ir (5)). Each pair of complexes is isostructural and contains three phosphinopyrrolide ligands that bridge the metal centers. The d10 or d8 complexes are all diamagnetic and X-ray crystallographic analysis reveals similarly short metal-metal distances, ranging from 2.2979(5) Å to 2.4366(2) Å. Despite the similar bonding metrics in 2-5, treatment with an L type donor (2,6-dimethylphenylisocyanide (CNXylyl)) reveals 3 different coordination geometries in TaNi(CNXylyl) (6), TaPd(CNXylyl) (7), and TaIr(CNXylyl) (8). While complexes 6, 7, and 8 all bind the isocyanide at the late metal, ligand rearrangements are observed in the first row complex 6. Complex 7 binds the isocyanide in the axial position while equatorial binding is observed in 8. All isocyanide adducts maintain close metal-metal contacts in the solid state.

Cp2Ti(κ2-tBuNCNtBu): A Complex with an Unusual κ2 Coordination Mode of a Heterocumulene Featuring a Free Carbene.

Although there are myriad binding modes of heterocumulenes to metal centers, the monometallic κ2-ECE (E = O, S, NR) coordination mode has not been reported. Herein, the synthesis, isolation, and physical characterization of Cp2Ti(κ2-tBuNCNtBu) (3) (Cp = cyclopentadienyl, tBu = tert-butyl), a strained 4-membered metallacycle bearing a free carbene, is described. Computational (DFT, CASSCF, QT-AIM, ELF) and solid-state CP-MAS 13C NMR spectroscopic analysis indicate that 3 is best described as a free carbene with partial Ti-Cß bonding that results from Ti-N π-bonding mixing with N-C-N σ-bonding of the bent N-C-N framework. Reactivity studies of 3 corroborate its carbene-like nature: protonation with [LutH]I results in the formation of a Ti-formamidinate (4), while oxidation with S8 yields a Ti-thioureate (5). Additionally, a related bridged dititanamidocarbene, (Cp2Ti)2(µ-η1,η1-CyNCNCy) (10) (Cy = cyclohexyl) is reported. Taken together, this work suggests that the 2-electron reduction of heterocumulene moieties can allow access to unusual free carbene coordination geometries given the proper stabilizing coordination environment from the reducing transition metal fragment.