Composite materials, especially carbon fiber-reinforced polymers (CFRPs), are high-performance class of structural materials now commonly used in aircraft, marine, and other applications, with emerging large-scale use in the automotive and civil engineering applications. The difficulty of recycling these materials is a key obstacle preventing their further application in larger markets. For decades, the engineering community has pursued physical methods to recover value from end-of-life composite waste. This work has generated scalable methods to recover modest value from CFRP waste, but because of their low value recovery, these are applied to a small fraction of CFRP waste. By contrast, relatively few methods to recycle CFRPs have been based on strategic approaches systematically to deconstruct the thermoset polymers that hold them together. In this Focus Article, we will show the emergence of these structure-focused approaches to CFRP recycling and illustrate the path of this research toward the ultimate realization of methods to recover both the reinforcing fibers and the thermoset materials that comprise modern CFRPs. Navarro, C. A.,; Giffin, C. R.; Zhang, B.,; Yu, Z.,; Nutt, S. R.; Williams, T. J.; Mater. Horiz. 2021, 7, 2479 DOI:10.1039/DoMH01085E
The proliferation of increasingly useful reactions for hydrogen transfer in organic synthesis has included the introduction of many new homogeneous catalysts into the organic synthesis lexicon. Unlike the proliferation of palladium-based cross-coupling reactions in which the mechanism is generally conserved, we are learning that these emerging hydrogen transfer catalysts have a rich diversity of mechanisms for catalyst activation, speciation, C–H bond cleavage and formation, and ultimately deactivation. We find that an underappreciated commonality in the catalytic activation for some of these systems is the generation of a (carbonyl)metal group, which dominates the downstream speciation of the catalyst system. In this mini-review we highlight a few well-documented cases of this phenomenon as food for thought for those who are designing new catalytic systems to introduce into this dynamic and impactful area. Alfonso, N.; Do, V. K.; Chavez, A. J.; Chen, Y.; Williams, T. J.; Cat. Sci. Tech. 2021, DOI:10.1039/D1CY00322D
Three complexes based on an Ir–M (M = FeII, CoII, and NiII) heterobimetallic core and 2-(diphenylphosphino)pyridine (Ph2PPy) ligand were synthesized via the reaction of trans-[IrCl(CO)(Ph2PPy)2] and the corresponding metal chloride. Their structures were established by single-crystal X-ray diffraction as [Ir(CO)(μ-Cl)(μ-Ph2PPy)2FeCl2]·2CH2Cl2 (2), [IrCl(CO)(μ-Ph2PPy)2CoCl2]·2CH2Cl2 (3), and [Ir(CO)(μ-Cl)(μ-Ph2PPy)2NiCl2]·2CH2Cl2 (4). Time-dependent DFT computations suggest a donor–acceptor interaction between a filled 5dz2 orbital on iridium and an empty orbital on the first-row metal atom, which is supported by UV-vis studies. Magnetic moment measurements show that the first-row metals are in their high-spin electronic configurations. Cyclic voltammetry data show that all the complexes undergo irreversible decomposition upon either reduction or oxidation. Reduction of 4 proceeds through an ECE mechanism. While these complexes are not stable to electrocatalysis conditions, the data presented here refine our understanding of the bonding synergies of the first-row and third-row metals. Cherepakhin, V.; Williams, T. J.; Synthesis 2021, DOI:10.1055/s-0040-1706102
We report a pyridyl-phosphine ruthenium(II) catalyzed tandem alcohol amination/Pictet–Spengler reaction sequence to synthesize tetrahydro-β-carbolines from an alcohol and tryptamine. Our conditions use a Lewis acid cocatalyst, In(OTf)3, that is compatible with typically base catalyzed amination and an acid catalyzed Pictet–Spengler cyclization. This method proceeds well with benzylic alcohols, heterocyclic carbinols, and aliphatic alcohols. We also show how combining this reaction with a subsequent cycloamination enables a direct synthesis of tetracyclic alkaloids like harmicine. Nalikezhathu, A.; Cherepakhin, V.; Williams, T. J.; Org. Lett. 2020, 22, 4979-4984 DOI:10.1021/acsorglett.oco1485
Chemical recycling of thermoset composites has been focused largely on recovering high-value carbon fibers with property retention, while recovery and reuse of decomposed polymer matrix residues is generally overlooked, despite the fact that matrix recycling constitutes an essential component of a sustainable approach to the overall problem. Our previous study demonstrated that oxidative acid digestion can be deployed effectively to recover near-virgin quality carbon fibers from amine-cured epoxy composites. In the present study, we investigate the viability of recovery and reuse of the decomposed amine/epoxy residue after acid digestion of the matrix, effectively closing the recycling loop. We find that polymer matrix residues recovered from acid digestion solutions via neutralization and precipitation contain molecular components of the epoxies in which aromatic regions are preserved. The recovered matrix residues are blended into virgin resin formulations and two approaches are evaluate for potential reuse. Approach I utilizes the matrix residue as an accelerator for a virgin anhydride/epoxy formulation that contains no accelerator and thus cannot be self-catalyzed. We discover that adding matrix residue produces catalytic effects on the curing reaction. In general, anhydride/epoxy samples blended and cured with recovered matrix residues are homogenous and exhibit thermal and mechanical properties comparable to specimens cured with a commercial accelerator. Approach II deployed the matrix residue as a filler for virgin anhydride-based epoxies with a commercial accelerator to produce blended formulations. In such cases, blended formulations yielded acceptable retention of thermal and mechanical properties, provided the fraction of matrix residue added did not exceed 10 wt%. Ma, Y.; Navarro, C. A.; Williams, T. J.; Nutt, S. R.; Poly. Degrad. Stab. 2020, 175, 108125 DOI:10.1016/j.polymddegradstab.2020.109125
We report iridium catalysts IrCl(η5-Cp⁎)(κ2-(2-pyridyl)CH2NSO2C6H4X) (1-Me, X = CH3 and 1-F, X = F) for transfer hydrogenation of ketones with 2-propanol that operate by a previously unseen metal-ligand cooperative mechanism. Under the reaction conditions, complexes 1 (1-Me and 1-F) derivatize to a series of catalytic intermediates: Ir(η5-Cp⁎)(κ2-(C5H4N)CHNSO2Ar) (2), IrH(η5-Cp⁎)(κ2-(2-pyridyl)CH2NSO2Ar) (3), and Ir(η5-Cp⁎)(κ3-(2-pyridyl)CH2NSO2Ar) (4). The structures of 1-Me and 4-Me were established by single-crystal X-ray diffraction. A rate-determining, concerted hydrogen transfer step (2 + R2CHOH ⇄ 3 + R2CO) is suggested by kinetic isotope effects, Eyring parameters (ΔH≠ = 29.1(8) kcal mol−1 and ΔS≠ = −17(19) eu), proton-hydride fidelity, and DFT calculations. According to DFT, a nine-membered cyclic transition state is stabilized by an alcohol molecule that serves as a proton shuttle. Demiantes, I.; Cherepakhin, V.; Maertens, A.; Lauridsen, P. J.; Mallikarjun Sharada, S; Williams, T. J.; Polyhedron 2020, 182, 114508 DOI:10.1016/j.poly.2020.114508
We describe the mechanism, scope, and catalyst evolution for our ruthenium-based coupling of amines and alcohols, which proceeds from a [(η6-cymene)RuCl(PyCH2PtBu2)]OTf (1) precatalyst. The method selectively produces secondary amines through a hydrogen borrowing mechanism and is successfully applied to several heterocyclic carbinol substrates. Under the reaction conditions, precatalyst 1 evolves through a series of catalytic intermediates: [(η6-cymene)RuH(PyCH2PtBu2)]OTf (3), [Ru3H2Cl2(CO)(PyCH2PtBu2)2{μ-(C5H3N)CH2PtBu2}]OTf (4), and a diastereomeric pair of [Ru2HCl(CO)2(PyCH2PtBu2)2(μ-O2CnPr)]X (trans-5, X = Cl; cis-6, X = OTf). The structures of 4 and 6 were established by single-crystal X-ray diffraction. A study of catalytic activity shows that 4 is a dormant (but alive) form of the catalyst, whereas 5 and 6 are the ultimate dead forms. Electrochemical studies show that 4 is redox active and undergoes electrochemically reversible one-electron oxidation at E1/2 = 0.442 V (vs Fc+/Fc) in CH2Cl2 solution. We discuss the factors that govern the formation of 3–6 and the role of selective ruthenium carbonylation, which is essential for enabling generation of the active catalyst. We also connect these discoveries to the identification of conditions for amination of aliphatic alcohols, which eluded us until we understood the catalyst’s complex speciation behavior. Cherepakhin, V.; Williams, T. J.; ACS Catal. 2020, 10, 56-65 DOI:10.1021/acscatal.9b03679
As surface ligands play a critical role in the colloidal stability and optoelectronic properties of semiconductor nanocrystals, we used solution NMR experiments to investigate the surface coordination chemistry of Ge nanocrystals synthesized by a microwave-assisted reduction of GeI2 in oleylamine. The as-synthesized Ge nanocrystals are coordinated to a fraction of strongly bound oleylamide ligands (with covalent X-type Ge–NHR bonds) and a fraction of more weakly bound (or physisorbed) oleylamine, which readily exchanges with free oleylamine in solution. The fraction of strongly bound oleylamide ligands increases with increasing synthesis temperature, which also correlates with better colloidal stability. Thiol and carboxylic acid ligands bind to the Ge nanocrystal surface only upon heating, suggesting a high kinetic barrier to surface binding. These incoming ligands do not displace native oleylamide ligands but instead appear to coordinate to open surface sites, confirming that the as-prepared nanocrystals are not fully passivated. These findings will allow for a better understanding of the surface chemistry of main group nanocrystals and the conditions necessary for ligand exchange to ultimately maximize their functionality. Smock, S. R.; Tabatabaei, K.; Williams, T. J.; Kauzlarich, S. M.; Brutchey, R. L.; Nanoscale 2020, 12, 2764-2772 DOI:10.1039/C9NRo9233A
There are few ways to switch a catalyst’s reactivity on or off, or change its selectivity, with external radiation; many of these involve photochemical activation of a catalyst. In the case of homogeneous late-transition-metal catalysts, the metal complex itself is frequently the chromophore involved in such reactivity switching. We show here a base-pendant ligand–metal bifunctional scaffold wherein a photobase, a compound that becomes more basic in the excited state (pKa < pKa*), is used to switch the proton acceptor ability on an active site of the complex. The system differs from those with metal-centered chromophores, because the photobase operates independently of the metal. While excellent progress has been made in photoacid chemistry, neither a photoacid nor a photobase has been designed into the structure of a transition-metal catalyst where the metal is not part of the chromophore. We find that quinoline is an efficient photobase that preserves its unique properties in the close proximity of an iridium center: the efficacy of the photobase (9.3 < pKa* < 12.4) in the iridium complex is unhindered relative to the free quinoline. We apply this notion to successful photodriven deprotonation of an aliphatic alcohol, thus showing the first case of metal-orthogonal optical pKa control in a transition-metal complex. Demianets, I.; Hunt, J. R.; Dawlaty, J. M.; Williams, T. J.; Organometallics. 2020, 38, 200-204 DOI:10.1020/organomet.8boo778
We previously reported that iridium complex 1a enables the first homogeneous catalytic dehydrogenation of neat formic acid and enjoys unusual stability through millions of turnovers. Binuclear iridium hydride species 5a, which features a provocative C2-symmetric geometry, was isolated from the reaction as a catalyst resting state. By synthesizing and carefully examining the catalytic initiation of a series of analogues to 1a, we establish here a strong correlation between the formation of C2-twisted iridium dimers analogous to 5a and the reactivity of formic acid dehydrogenation: an efficient C2 twist appears unique to 1a and essential to catalytic reactivity. Lauridsen, P. J.; Lu, Z.; Celaje, J. J.; Kedzie, E. A.; Williams, T. J.; Dalton Trans 2018, 47, 13559-13564 DOI:10.1039/C8DT03268H
Cesium lead halide perovskites are an emerging class of quantum dots (QDs) that have shown promise in a variety of applications; however, their properties are highly dependent on their surface chemistry. To this point, the thermodynamics of ligand binding remain unstudied. Herein, 1H NMR methods were used to quantify the thermodynamics of ligand exchange on CsPbBr3 QDs. Both oleic acid and oleylamine native ligands dynamically interact with the CsPbBr3 QD surface, having individual surface densities of 1.2–1.7 nm−2. 10‐Undecenoic acid undergoes an exergonic exchange equilibrium with bound oleate (Keq=1.97) at 25 °C while 10‐undecenylphosphonic acid undergoes irreversible ligand exchange. Undec‐10‐en‐1‐amine exergonically exchanges with oleylamine (Keq=2.52) at 25 °C. Exchange occurs with carboxylic acids, phosphonic acids, and amines on CsPbBr3 QDs without etching of the nanocrystal surface; increases in the steady‐state PL intensities correlate with more strongly bound conjugate base ligands. Smock, S. R.; Williams, T. J.; Brutchey, R. L.; Agnew. Chem. Int. Ed. 2018, 57, 11711-11715 DOI:10.1002/anie.201806
Selective hydrogen transfer remains a central research focus in catalysis: hydrogenation and dehydrogenation have central roles, both historical and contemporary, in all aspects of fuel, agricultural, pharmaceutical, and fine chemical synthesis. Our lab has been involved in this area by designing homogeneous catalysts for dehydrogenation and hydrogen transfer that fill needs ranging from on-demand hydrogen storage to fine chemical synthesis. A keen eye toward mechanism has enabled us to develop systems with excellent selectivity and longevity and demonstrate these in a diversity of high-value applications. Here we describe recent work from our lab in these areas that are linked by a central mechanistic trichotomy of catalyst initiation pathways that lead highly analogous precursors to a diversity of useful applications. Lu, Z.; Cherepakhin, V.; Demianets, I.; Lauridsen, P. J.; Williams, T. J.; Chem. Commun. 2018, 54, 7711-7724 DOI:10.1039/C8CCo3412E
Carbon fiber-reinforced polymers (CFRPs) are structural composites used in the aerospace and sporting goods industries. Their chief appeal lies in their high specific properties, which generally outperform metallic counterparts. There is a contemporary need for viable methods for recycling CRFPs at the end of their lifecycles and for utilizing the considerable production waste (ca. 30%) of CFRP part manufacturing. The cost associated with these waste streams is a principal economic driver inhibiting the penetration of CRFPs into larger-scale manufacturing, particularly in the automotive industry. Reported techniques for CRFP degradation involve pyrolysis or mechanical grinding of the CFRP, processes which are outlawed in some jurisdictions and can reduce the thermomechanical properties of the recycled products. In this study, we report a conceptually different approach to degrading a commercial blended benzoxazine/epoxy resin under mild, oxidative conditions. The thermosetting resin is polymerized, characterized, and then catalytically depolymerized via hydride abstraction with a ruthenium catalyst. These results demonstrate a concept for sustainable recycling of CFRP composites. Lo, J.; Nutt, S. R.; Williams T. J.; ACS Sustain. Chem. Eng. 2018, 6, 7227-7231 DOI:10.1021/acssuschemeng.8b01790
Use of benzoxazine resins in composites is limited by volatile-induced porosity, which degrades the thermomechanical properties of the product. In the present study, we demonstrate how to eliminate cure-induced volatilization and volatile-induced defects in benzoxazine composite laminates, using a chemistry-based approach. Like most resins formulated for high-temperature service, benzoxazine and benzoxazine–epoxy blends generally include solvent additives. Consequently, composite parts produced with such resins exhibit higher levels of cure-induced volatile release, often leading to porosity in the final manufactured part. Here, we develop a method to eliminate porosity by analyzing volatile release and the effects of residual solvent in a pre-commercial benzoxazine–epoxy system designed for liquid molding by resin transfer molding. Utilizing thermogravimetric analysis, nuclear magnetic resonance spectroscopy, and dynamic mechanical analysis, we correlate the concentration of residual solvent remaining within the final manufactured part with the Tg, degradation temperature, and dynamic modulus. Lastly, a resin synthesis method is demonstrated that eliminates residual solvent in order to produce composite parts with optimal surface finish and thermomechanical properties. The report outlines a methodology for optimizing blended resin chemistry for production of high-quality composite parts. Lo, J.; Zhang, X.; Williams T. J.; Nutt, S. R.; J. Composite Materials. 2018, 1481-1493 DOI:10.1177/0021998317727048
Conversion of vegetable-derived triglycerides to fatty acid methyl esters (FAMEs) is a popular approach to the generation of biodiesel fuels and the basis of a growing industry. Drawbacks of the strategy are that (a) the glycerol backbone of the triglyceride is discarded as waste, and (2) most available natural triglycerides in the U.S. are multiunsaturated or fully saturated, giving inferior fuel performance and causing engine problems. Here we show that catalysis by iridium complex 1 can address both of these problems through selective reduction of triglycerides high in polyunsaturation. This is realized using hydrogen from methanol or those imbedded in the triglyceride backbone, concurrently generating lactate as a value-added C3 product. Additional methanol or glycerol as a hydrogen source enables reduction of corn and soybean oils to >80% oleate. The cost of the iridium catalyst is mitigated by its recovery through aqueous extraction. The process can be further driven with a supporting iron-based catalyst for the complete saturation of all olefins. Preparative procedures are established for synthesis and separation of methyl esters of the hydrogenated fatty acids, enabling instant access to upgraded biofuels. Lu, Z.; Cherepakhin, V.; Kapenstein, T.; Williams, T. J.; ACS Sustain. Chem. Eng. 2018, 6, 5749-5753 DOI:10.1021/assuschemeng.8boo653
We introduce iridium-based conditions for the conversion of primary alcohols to potassium carboxylates (or carboxylic acids) in the presence of potassium hydroxide and either [Ir(2-PyCH2(C4H5N2))(COD)]OTf (1) or [Ir(2-PyCH2PBu2t)(COD)]OTf (2). The method provides both aliphatic and benzylic carboxylates in high yield and with outstanding functional group tolerance. We illustrate the application of this method to a diverse variety of primary alcohols, including those involving heterocycles and even free amines. Complex 2 reacts with alcohols to form the crystallographically characterized catalytic intermediates [IrH(η1,η3-C8H12)(2-PyCH2PtBu2)] (2a) and [Ir2H3(CO)(2-PyCH2PtBu2){μ-(C5H3N)CH2PtBu2}] (2c). The unexpected similarities in reactivities of 1 and 2 in this reaction, along with synthetic studies on several of our iridium intermediates, enable us to form a general proposal of the mechanisms of catalyst activation that govern the disparate reactivities of 1 and 2, respectively, in glycerol and formic acid dehydrogenation. Moreover, careful analysis of the organic intermediates in the oxidation sequence enable new insights into the role of Tishchenko and Cannizzaro reactions in the overall oxidation. Cherepakhin, V.; Williams, T. J.; ACS Catal. 2018, 8, 3754-3763 DOI:10.1021/acscatal.8boo105
Carbon fiber-reinforced polymer (CFRP) materials are widely used in aerospace and recreational equipment, but there is no efficient procedure for their end-of-life recycling. Ongoing work in the chemistry and engineering communities emphasizes recovering carbon fibers from such waste streams by dissolving or destroying the polymer binding. By contrast, our goal is to depolymerize amine-cured epoxy CFRP composites catalytically, thus enabling not only isolation of high-value carbon fibers, but simultaneously opening an approach to recovery of small molecule monomers that can be used to regenerate precursors to new composite resin. To do so will require understanding of the molecular mechanism(s) of such degradation sequences. Prior work has shown the utility of hydrogen peroxide as a reagent to affect epoxy matrix decomposition. Herein we describe the chemical transformations involved in that sequence: the reaction proceeds by oxygen atom transfer to the polymer’s linking aniline group, forming an N-oxide intermediate. The polymer is then cleaved by an elimination and hydrolysis sequence. We find that elimination is the slower step. Scandium trichloride is an efficient catalyst for this step, reducing reaction time in homogeneous model systems and neat cured matrix blocks. The conditions can be applied to composed composite materials, from which pristine carbon fibers can be recovered. Navarro, C. A.; Kedzie, E. A.; Ma, Y.; Micheal K. H.; Nutt, S. R.; Williams, T. J.; Top. Catal. 2018, 61, 704-709 DOI:10.1007/s1124-018-0917-2
One of the greatest challenges in using H2 as a fuel source is finding a safe, efficient, and inexpensive method for its storage. Ammonia borane (AB) is a solid hydrogen storage material that has garnered attention for its high hydrogen weight density (19.6 wt %) and ease of handling and transport. Hydrogen release from ammonia borane is mediated by either hydrolysis, thus giving borate products that are difficult to rereduce, or direct dehydrogenation. Catalytic AB dehydrogenation has thus been a popular topic in recent years, motivated both by applications in hydrogen storage and main group synthetic chemistry. This Account is a complete description of work from our laboratory in ruthenium-catalyzed ammonia borane dehydrogenation over the last 6 years, beginning with the Shvo catalyst and resulting ultimately in the development of optimized, leading catalysts for efficient hydrogen release. We have studied AB dehydrogenation with Shvo’s catalyst extensively and generated a detailed understanding of the role that borazine, a dehydrogenation product, plays in the reaction: it is a poison for both Shvo’s catalyst and PEM fuel cells. Through independent syntheses of Shvo derivatives, we found a protective mechanism wherein catalyst deactivation by borazine is prevented by coordination of a ligand that might otherwise be a catalytic poison. These studies showed how a bidentate N–N ligand can transform the Shvo into a more reactive species for AB dehydrogenation that minimizes accumulation of borazine. Simultaneously, we designed novel ruthenium catalysts that contain a Lewis acidic boron to replace the Shvo -OH proton, thus offering more flexibility to optimize hydrogen release and take on more general problems in hydride abstraction. Our scorpionate-ligated ruthenium species (12) is a best-of-class catalyst for homogeneous dehydrogenation of ammonia borane in terms of its extent of hydrogen release (4.6 wt %), air tolerance, and reusability. Moreover, a synthetically simplified ruthenium complex supported by the inexpensive bis(pyrazolyl)borate ligand is a comparably good catalyst for AB dehydrogenation, among other reactions. In this Account, we present a detailed, concise description of how our work with the Shvo system progressed to the development of our very reactive and flexible dual-site boron-ruthenium catalysts. Zhang, X.; Kam, L.; Trerise, R.; Williams, T. J.; A.. Chem. Res. 2017, 86-95 DOI:10.1021/acs.accounts.6boo482
A (pyridyl)phosphine-ligated ruthenium(II) catalyst is reported for the chemoselective benzylic N-alkylation of amines, via a hydrogen-borrowing mechanism. The catalyst operates under mild conditions, neat, and without a base or other additive. These conditions offer remarkable functional group compatibility for applications in organic synthesis, including reactions involving phenols and anilines, which are very difficult to achieve. Mechanistic studies suggest that, unlike other catalysts for this reaction, the redox steps are fast and reversible while imine formation is slow. We perceive that this is the origin of the selectivity realized with these reaction conditions. Celaje, J. J.; Zhang, X.; Zhang, F.; Kam, L.; Herron, J. R.; Williams, T. J.; ACS Catal. 2017, 7, 1136-1142 DOI:10.1021/acscatal.6bo30888
Di(carbene)-supported nickel species 1 and 2 are efficient catalysts for the room-temperature reduction of CO2 to methanol in the presence of sodium borohydride. The catalysts feature unusual stability, particularly for a base metal catalyst, enabling >1.1 million turnovers of CO2. Moreover, while other systems involve more expensive reducing reagents, sodium borohydride is inexpensive and easily handled. Furthermore, effecting reduction in the presence of water enables direct access to methanol. Preliminary mechanistic data collected are most consistent with a mononuclear nickel active species that mediates rate-determining reduction of a boron formate. Lu, Z.; Williams, T. J.; ACS Catal. 2016, 6, 6670-6673 DOI:10.1021/acscatal.6bo2101
Formic acid is a promising energy carrier for on-demand hydrogen generation. Because the reverse reaction is also feasible, formic acid is a form of stored hydrogen. Here we present a robust, reusable iridium catalyst that enables hydrogen gas release from neat formic acid. This catalysis works under mild conditions in the presence of air, is highly selective and affords millions of turnovers. While many catalysts exist for both formic acid dehydrogenation and carbon dioxide reduction, solutions to date on hydrogen gas release rely on volatile components that reduce the weight content of stored hydrogen and/or introduce fuel cell poisons. These are avoided here. The catalyst utilizes an interesting chemical mechanism, which is described on the basis of kinetic and synthetic experiments. Celaje, J. A.; Lu, Z.; Kedzie, E. A.; Terrile, N. J.; Lo, J. N.; Williams, T. J. Nat. Commun. 2016, 7, 11308 DOI: 10.1038/ncomms11308
Ammonia borane (AB) has high hydrogen density (19.6 wt%), and can, in principle, release up to 3 equivalents of H2 under mild catalytic conditions. A limited number of catalysts are capable of non-hydrolytic dehydrogenation of AB beyond 2 equivalents of H2 under mild conditions, but none of these is shown directly to derivatise borazine, the product formed after 2 equivalents of H2 are released. We present here a high productivity ruthenium-based catalyst for non-hydrolytic AB dehydrogenation that is capable of borazine dehydrogenation, and thus exhibits among the highest H2 productivity reported to date for anhydrous AB dehydrogenation. At 1 mol% loading, (phen)Ru(OAc)2(CO)2 (1) effects AB dehydrogenation through 2.7 equivalents of H2 at 70 °C, is robust through multiple charges of AB, and is water and air stable. We further demonstrate that catalyst 1 has the ability both to dehydrogenate borazine in isolation and dehydrogenate AB itself. This is important, both because borazine derivatisation is productivity-limiting in AB dehydrogenation and because borazine is a fuel cell poison that is commonly released in H2 production from this medium. Zhang, X.; Kam, L.; Williams, T. J. Dalton Trans. 2016, 45, 7672-7677 DOI: 10.1039/c6dt00604c
We report the synthesis and reactivity of a very robust iridium catalyst for glycerol to lactate conversion. The high reactivity and selectivity of this catalyst enable a sequence for the conversion of biodiesel waste stream to lactide monomers for the preparation of poly(lactic acid). Furthermore, experimental data collected with this system provide a general understanding of its reactive mechanism. Lu, Z.; Demianets, I.; Hamze, R.; Terrile, N. J.; Williams, T. J. ACS Catal. 2016, 6, 2014-2017 DOI: 10.1021/acscatal.5b02732
We previously reported that quantitative poisoning, a test for homogeneous catalysis, behaves oddly in the dehydrogenation of ammonia borane (AB) by Shvo’s catalyst, whereas the “poison” 1,10-phenanthroline (phen) accelerates catalysis and apparently prevents catalyst deactivation. Thus, we proposed a protective role for phen in the catalysis. Herein we account for the mechanistic origin of this accelerated AB dehydrogenation in the presence of phen and define the relevance boundaries of our prior proposal. In so doing, we present syntheses for novel amine- and pyridine-ligated homologues of the Shvo catalyst and show their catalytic efficacy in AB dehydrogenation. These catalysts are synthetically easy to access, air stable, and rapidly release over 2 equiv of H2. The mechanisms of these reactions are also discussed. Zhang, X.; Lu, Z.; Lena, K. F.; Williams, T. J. Organometallics 2015, 34, 3732-3738 DOI: 10.1021/acs.organomet.5b00409
Responsive magnetic resonance imaging (MRI) contrast agents, those that change their relaxivity according to environmental stimuli, have promise as next generation imaging probes in medicine. While several of these are known based on covalent modification of the contrast agents, fewer are known based on controlling non-covalent interactions. We demonstrate here accentuated relaxivity of a T1-shortening contrast agent, Gd-DOTP5- based on non-covalent, hydrogen bonding of Gd-DOTP5- with a novel fluorous amphiphile. By contrast to the phosphonate-containing Gd-DOTP5- system, the relaxivity of the analogous clinically approved contrast agent, Gd-DOTA- is unaffected by the same fluorous amphiphile under similar conditions. Mechanistic studies show that placing the fluorous amphiphile in proximity of the gadolinium center in Gd-DOTP5- caused an increase in τm (bound-water residence lifetime or the inverse of water exchange rate, τm = 1/kex) and an increase in τR (rotational correlation time), with τR being the factor driving enhanced relaxivity. Further, these effects were not observed when Gd-DOTA- was treated with the same fluorous amphiphile. Thus, Gd-DOTP5- and Gd-DOTA- respond to the fluorous amphiphile differently, presumably because the former binds to the amphiphile with higher affinity. (DOTP = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraphosphonic acid; DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). Wu, X.; Dawsey, A. C.; Siriwardena-Mahanama, B. N.; Allen, M. J.; Williams, T. J. J. Fluor. Chem. 2014, 168, 177-183 DOI: 10.1016/j.jfluchem.2014.09.018
A series of three phosphorescent mononuclear (NHC)–copper(I) complexes were prepared and characterized. Photophysical properties were found to be largely controlled by the NHC ligand chromophore. Variation of the NHC ligand leads to emission colour tuning over 200 nm range from blue to red, and emission efficiencies of 0.16–0.80 in the solid state. Krylova, V. A.; Djurovich, P. I.; Conley, B. L.; Haiges, R.; Whited, M. T.; Williams, T. J.; Thompson, M. E. Chem. Commun. 2014, 50, 7176-7179 DOI: 10.1039/C4CC02037E
Rhodium(I) and Iridium(I) borate complexes of the structure [Me2B(2-py)2]ML2 (L2 = (tBuNC)2, (CO)2, (C2H4)2, cod, dppe) were prepared and structurally characterized (cod = 1,5-cyclooctadiene; dppe = 1,2-diphenylphosphinoethane). Each contains a boat-configured chelate ring that participates in a boat-to-boat ring flip. Computational evidence shows that the ring flip proceeds through a transition state that is near planarity about the chelate ring. We observe an empirical, quantitative correlation between the barrier of this ring flip and the π acceptor ability of the ancillary ligand groups on the metal. The ring flip barrier correlates weakly to the Tolman and Lever ligand parameterization schemes, apparently because these combine both σ and π effects while we propose that the ring flip barrier is dominated by π bonding. This observation is consistent with metal-ligand π interactions becoming temporarily available only in the near-planar transition state of the chelate ring flip and not the boat-configured ground state. Thus, this is a first-of-class observation of metal-ligand π bonding governing conformational dynamics. Pennington-Boggio, M. K.; Conley, B. L.; Richmond, M. G.; Williams, T. J. Polyhedron 2014, 84 24-31 DOI: 10.1016/j.poly.2014.05.042
The syntheses of novel dimethylbis(2-pyridyl)borate nickel(II) complexes 4 and 6 are reported. These complexes were unambiguously characterized by X-ray analysis. In dichloromethane solvent, complex 4 undergoes a unique square-planar to square-planar rotation around the nickel(II) center, for which activation parameters of ΔH = 12.2(1) kcal mol-1 and ΔS = 0.8(5) eu were measured via NMR inversion recovery experiments. Complex 4 was also observed to isomerize via a relatively slow ring flip: ΔH = 15.0(2) kcal mol-1; and ΔS = −4.2(7) eu. DFT studies support the experimentally measured rotation activation energy (cf. calculated ΔH = 11.1 kcal mol-1) as well as the presence of a high-energy triplet intermediate (ΔH = 8.8 kcal mol-1). Celaje, J. A.; Pennington-Boggio, M. K.; Flaig, R. W.; Richmond, M. G.; Williams, T. J. Organometallics 2014, 33, 2019-2026 DOI: 10.1021/om500173j
The relaxivity of a magnetically responsive Gd complex can be controlled by non-covalent molecular recognition with a water-soluble deep cavitand. Lowered relaxivity is conferred by a self-assembled micellar “off state”, and the contrast can be regenerated by addition of a superior guest. Li, V.; Ghang, Y-J.; Hooley, R. J.; Williams, T. J. Chem. Commun. 2014, 50, 1375-1377 DOI: 10.1039/C3CC48389D
We report a novel ruthenium bis(pyrazolyl)borate scaffold that enables cooperative reduction reactivity in which boron and ruthenium centers work in concert to effect selective nitrile reduction. The pre-catalyst compound [κ3-(1-pz)2HB(N = CHCH3)] Ru(cymene)+ TfO- (pz = pyrazolyl) was synthesized using readily-available materials through a straightforward route, thus making it an appealing catalyst for a number of reactions. Lu, Z.; Williams, T. J. Chem. Commun. 2014, 50, 5391-5393 DOI:10.1039/C3CC47384H
Gadolinium-containing phosphonate-coated gold nanoparticles were prepared and then non-covalently coated with an amphiphilic fluorous monomer. The monomer spontaneously self-assembles into a non-covalent monolayer shell around the particle. The binding of the shell utilizes a guanidinium–phosphonate interaction analogous to the one exploited by the Wender molecular transporter system. Particle–shell binding was characterized by a 27% decrease in 19F T1 of the fluorous shell upon exposure to the paramagnetic gadolinium in the particle and a corresponding increase in hydrodynamic diameter from 3 nm to 4 nm. Interestingly, a much smaller modulation of 19F T1 is observed when the shell monomer is treated with a phosphonate-free particle. By contrast, the phosphonate-free particle is a much more relaxive 1H T1 agent for water. Together, these observations show that the fluoroalkylguanidinium shell binds selectively to the phosphonate-covered particle. The system's relaxivity and selectivity give it potential for use in 19F based nanotheranostic agents. Li, V.; Chang, A. Y.; Williams, T. J. Tetrahedron 2013, 69, 7741-7745 DOI:10.1016/j.tet.2013.05.092
Dotarem and Magnevist, two clinically available magnetic resonance imaging (MRI) contrast agents, were assessed in a high school science classroom with respect to which is the better contrast agent. Magnevist, the more efficacious contrast agent, has negative side effects because its gadolinium center can escape from its ligand. However, Dotarem, though a less efficacious contrast agent, is a safer drug choice. After the experiment, students are confronted with the FDA warning on Magnevist, which enabled a discussion of drug efficacy versus safety. We describe a laboratory experiment in which NMR spin lattice relaxation rate measurements are used to quantify the relaxivities of the active ingredients of Dotarem and Magnevist. The spin lattice relaxation rate gives the average amount of time it takes the excited nucleus to relax back to the original state. Students learn by constructing molar relaxivity curves based on inversion recovery data sets that Magnevist is more relaxive than Dotarem. This experiment is suitable for any analytical chemistry laboratory with access to NMR. Dawsey, A. C.; Hathaway, K. L.; Kim, S.; Williams, T. J. J. Chem. Educ. 2012, 90, 922-925 DOI:10.1021/ed3006902
Alcohol Dehydrogenation with a Dual Site Ruthenium, Boron Catalyst Occurs at Ruthenium Zhiyao Lu, Brock Malinoski, Ana Victoria Flores, Denver Guess, Brian L. Conley, and Travis J. Williams The complex [(κ3-(N,N,O-py2B(Me)OH)Ru(NCMe)3]+ TfO- (1) is a catalyst for transfer dehydrogenation of alcohols, which was designed to function through a cooperative transition state in which reactivity was split between boron and ruthenium. We show here both stoichiometric and catalytic evidence to support that in the case of alcohol oxidation, the mechanism most likely involves reactivity only at the ruthenium center. Lu, Z.; Malinoski, B.; Flores, A. V.; Guess, D.; Conley, B. L.; Williams, T. J. Catalysts 2012, 2, 412-421 DOI:10.3390/catal2040412
We propose a mechanistic model for three-stage dehydrogenation of ammonia–borane (AB) catalyzed by Shvo’s cyclopentadienone-ligated ruthenium complex. We provide evidence for a plausible mechanism for catalyst deactivation and the transition from fast catalysis to slow catalysis and relate those findings to the invention of a second-generation catalyst that does not suffer from the same deactivation chemistry. The primary mechanism of catalyst deactivation is borazine-mediated hydroboration of the ruthenium species that is the active oxidant in the fast catalysis case. This transition is characterized by a change in the rate law for the reaction and changes in the apparent resting state of the catalyst. Also, in this slow catalysis situation, we see an additional intermediate in the sequence of boron, nitrogen species, aminodiborane. This occurs with concurrent generation of NH3, which itself does not strongly affect the rate of AB dehydrogenation. Lu, Z.; Conley, B. L.; Williams, T. J. Organometallics 2012, 31, 6705-6714 DOI:10.1021/om300562d
Ruthenium(III) chloride hydrate is a convenient catalyst for the addition of active methylene compounds to aryl alkynes. These reactions are rapid, operationally simple, and high yielding in cases. Most significantly, no precautions are required to exclude air or water from the reactions. All reagents are commercially available at reasonable prices, and the reactions can be conducted in disposable glassware with minimal solvent. Pennington-Boggio, M. K.; Conley, B. L.; Williams, T. J. J. Organometallic Chem. 2012, 716, 6-10 DOI: 10.1016/j.jorganchem.2012.05.017
This comment describes our efforts to develop dual site catalysts for hydride manipulation. We began by analyzing the mechanism of alcohol oxidation with the ruthenium-based Shvo catalyst, which utilizes a proton transfer to template a hydride transfer from carbon to ruthenium in a single transition state. In our project we are working to extend this concept of reactivity from the use of proton transfer as a templating interaction for hydride transfer to the use of a Lewis acid to coordinate and direct a substrate to a metal. Along these lines, we have found that ammonia borane, a popular and high-weight-content hydrogen storage material, has been one of our best model substrates with which to study hydride transfer mechanisms. Our ongoing studies have thus far given new insight into the reactivity of the Shvo system, particularly regarding dehydrogenation of ammonia borane, and have enabled us to design a new, prolific, air- and water-tolerant, and reusable catalyst for ammonia borane dehydrogenation. Conley, B. L.; Williams, T. J. Comments Inorg. Chem. 2012, 32, 195-218 DOI: 10.1080/02603594.2011.642087
We report herein convenient, aerobic conditions for the oxidation of thiazolines to thiazoles and data regarding the oxidation mechanism. These reactions feature operationally simple and environmentally benign conditions and proceed in good yield to afford the corresponding azoles, thus enabling the inexpensive preparation of valuable molecular building blocks. Incorporation of a novel diimine-ligated copper catalyst, [(MesDABMe)CuII(OH2)3]2+ [−OTf]2, provides increased reaction efficiency in many cases. In other cases copper-free conditions involving a stoichiometric quantity of base affords superior results. Dawsey, A. C.; Li, V.; Hamilton, K. C.; Wang, J.; Williams, T. J. Dalton Trans. 2012, 41, 7994-8002 DOI: 10.1039/C2DT00025C
We report herein convenient procedures for the use of highly fluorinated α,ω-diols (e.g. 1) as building blocks for the rapid assembly of amphiphilic materials containing a fluorous phase region. We describe expedient conversion of the parent diols to both symmetrically and asymmetrically substituted amphiphiles via the installation of an intermediate trifluoromethanesulfonyl ester. These sulfonate esters are versatile and easily manipulated intermediates, which can be readily converted to a variety of nitrogen, halogen, and carbon groups. Moreover, we show that for guanidine-terminated fluorous amphiphiles, these molecules can bind phosphonic acid groups in aqueous media. Thus, these materials offer a new strategy for decorating phosphorylated biomolecules with fluorine-rich coatings. Wu, X.; Boz, E.; Sirkis, A. M.; Chang, A. Y.; Williams, T. J. J. Fluor. Chem. 2012, 135, 292-302 DOI: 10.1016/j.jfluchem.2011.12.011
We describe an efficient homogeneous ruthenium catalyst for the dehydrogenation of ammonia borane (AB). This catalyst liberates more than 2 equiv of H2 and up to 4.6 system wt % H2 from concentrated AB suspensions under air. Importantly, this catalyst is robust, delivering several cycles of dehydrogenation at high [AB] without loss of catalytic activity, even with exposure to air and water. Conley, B. L.; Williams, T. J. J. Am. Chem. Soc. 2011, 133, 14212-14215 DOI: 10.1021/ja2058154
A convenient laboratory experiment is described in which NMR magnetization transfer by inversion recovery is used to measure the kinetics and thermochemistry of amide bond rotation. The experiment utilizes Varian spectrometers with the VNMRJ 2.3 software, but can be easily adapted to any NMR platform. The procedures and sample data sets in this article will enable instructors to use inversion recovery as a laboratory activity in applied NMR classes and provide research students with a convenient template with which to acquire inversion recovery data on research samples. Williams, T. J.; Kershaw, A. D.; Li, V.; Wu, X. J. Chem. Ed. 2011, 88, 665-669 DOI: 10.102/ed1006822
Shvo's cyclopentadienone-ligated ruthenium complex is an efficient catalyst for the liberation of exactly two molar equivalents of hydrogen from ammonia-borane, a prospective hydrogen storage medium. The mechanism for the dehydrogenation features a ruthenium hydride resting state from which dihydrogen loss is the rate-determining step. Conley, B. L.; Williams, T. J. Chem. Commun. 2010, 46, 4815-4817 DOI:10.1039/C003157G
A boron-pendant ruthenium species forms a unique agostic methyl bridge between the boron and ruthenium atoms in the presence of a ligating solvent, acetonitrile. NMR inversion−recovery experiments enabled the activation and equilibrium thermochemistry for formation of the agostic bridge to be measured. The mechanism for bridge formation involves displacement of an acetonitrile ligand; thus, this is a rare example of a case where an agostic C−H ligand competitively displaces another tightly binding ligand from a coordinatively saturated complex. Characterization of this complex gives unique insights into the development of C−H activation catalysis based on this ligand−metal bifunctional motif. Conley, B. L.; Williams, T. J. J. Am. Chem. Soc. 2010, 132, 1764-1765 DOI: 10.1021/ja909858a
Chem. Rev. 2010, 110 (4), 2294-2312 DOI:10.1021/cr9003133
Cyclopentadienone-ligated ruthenium complexes, such as Shvo's catalyst, are known to oxidize reversibly alcohols to the corresponding carbonyl compounds. The mechanism of this reaction has been the subject of some controversy, but it is generally believed to proceed through concerted transfer of proton and hydride, respectively, to the cyclopentadienone ligand and the ruthenium center. In this paper we further study the hydride transfer process as an example of a coordinatively directed hydride abstraction by adding quantitative understanding to some features of this mechanism that are not well understood. We find that an oxidant as weak as acetone can be used to re-oxidize the intermediate ruthenium hydride without catalyst re-oxidation becoming rate-limiting. Furthermore, C–H cleavage is a significantly electrophilic event, as demonstrated by a Hammett reaction parameter of ρ = –0.89. We then describe how the application of our mechanistic insights obtained from the study have enabled us to extend the ligand-directed hydride abstraction strategy to include a rare example of an iron(0) oxidation catalyst. European Journal of Inorganic Chemistry 2009, (2), 295-302 DOI:10.1002/ejic.200800975
Air- and water-tolerant C−H activation is observed in reactions of [(diimine)Pt(μ2-OH)]22+ dimers with allylic and benzylic C−H groups. The reactions proceed in good yields under mild conditions. Mechanistic studies indicate that the active species is the monomeric [(diimine)Pt(OH2)]2+ dication. The related palladium species, [(diimine)Pd(μ2-OH)]22+, exhibit similar stoichiometric activations and also effect catalytic oxidation of cyclohexene to benzene with molecular oxygen as the terminal oxidant. Williams, T. J., Caffyn, A. J.; Hazari, N.; Oblad, P. F.; Labinger, J. A.; Bercaw, J. E.; J. Am. Chem. Soc. 2008, 130, 2418-2419 DOI:10.1021/ja076740q
The Rh(I)-catalyzed [3 + 2] cycloaddition of cyclopropenones and alkynes is found to provide a highly efficient and regiocontrolled route to cyclopentadienones (CPDs), building blocks of widespread use in the synthesis of natural and non-natural products, therapeutic leads, polymers, dendrimers, devices, and antigen presenting scaffolds. The versatility of the method is explored with 23 examples representing a wide range of alkyne variations (arylalkyl-, dialkyl-, heteroarylalkyl-) and diaryl- as well as arylalkylcyclopropenones. The reactions often proceed in high yield using minimal catalyst loadings and in all cases examined proceed with high or complete regioselectivity. The reaction is readily scalable to produce gram quantities of cycloadduct and provides a unique and versatile route to CPDs that would be otherwise difficult to obtain. Wender, P. A.; Paxton, T. J.; Williams, T. J.; J. Am. Chem. Soc. 2006, 128, 14814-148159 DOI:10.1021/ja065868p
Drei‐Komponenten‐[2+2+1]‐Cycloadditionen gelingen mit Dienen statt Alkenen in einer intermolekularen, RhI‐katalysierten Variante der Pauson‐Khand‐Reaktion (siehe Schema). Die höhere Reaktivität der Diene ermöglicht einen effizienten Zugang zu Alkenylcyclopentenonen ausgehend von leicht erhältlichen Ausgangsstoffen. R1, R2=Alkyl, Silyl, Carbonyl; R3=Me, Bn. Wender, P. A.; Deschamps, N. M.; Williams, T. J.; Agnew. Chem. Int. Ed. 2004, 43, 3076-3079 DOI:10.1002/ange200454117