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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
Dawsey, A. C.; Hathaway, K. L.; Kim, S.; Williams, T. J.
J. Chem. Educ.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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