Nitrogen-Based Ligands Accelerate Ammonia Borane Dehydrogenation with the Shvo Catalyst

Article abstract of DOI:10.1021/acs.organomet.5b00409

(Chemical Equation Presented) 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.

Full text of DOI:10.1021/acs.organomet.5b00409

Article
pubs.acs.org/Organometallics
Nitrogen-Based Ligands Accelerate Ammonia Borane
Dehydrogenation with the Shvo Catalyst
Xingyue Zhang, Zhiyao Lu, Lena K. Foellmer, and Travis J. Williams*
Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, Los Angeles, California
90089-1661, United States
S
* Supporting Information
ABSTRACT: 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 deactiva-
tion. 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.
the catalyst is deactivated by hydroboration by borazine (6,
Scheme 1).14 In that study we conducted a quantitative
poisoning experiment to rule out heterogeneous nanoparticle
formation by adding 1,10-phenanthroline (phen) to the
reaction.15 Instead of the expected slowing, we saw rate
acceleration proportional to [phen]. We further observed that
when LnRu-NH3 adduct 916 was used as a catalyst precursor, it
was not deactivated by borazine (Figure 2) in the same way as
the parent system (1). With these two observations, we formed
a hypothesis that NH3 or phen can coordinate 3, which
preempts hydroboration of 3 by borazine until the complex can
associate AB by hydrogen bonding to its carbonyl group,12b,17
thus avoiding catalyst deactivation (dotted line in Scheme 1).14
Herein we challenge our hypothesis of a protective role for
NH3 or phen by analyzing the kinetics of AB dehydrogenation
with isolated pyridine- and amine-ligated homologues of the
Shvo system. In addition to refining our understanding of this
thought-provoking mechanistic issue, we report here that these
novel, air-stable, and easily prepared complexes are very
efficient catalysts for AB dehydrogenation in their own right,
releasing 2 equiv of H2 in 1 h (70 °C) at 10 mol % catalyst
loading. We show data that indicate that the success of these
new catalytic systems is based on our original hypothesis of a
protective role for their supporting pyridine ligands, and we
show reactivity data for complexes of variously substituted
pyridines that inform this proposal. We further show that
bidentate nitrogen ligands such as 2,2′-bipyridine (bipy) or
INTRODUCTION
■
The dehydrogenation of ammonia borane (AB, H3N-BH3) is a
popular approach to high-capacity hydrogen storage because of
AB’s high hydrogen density (19.6 wt %), ability to release H2
under mild conditions, and desirable physical properties.1
Accordingly, many catalytic approaches to hydrogen release
from AB have appeared, involving both aqueous2 and
anhydrous systems,3 the latter offering a more facile approach
to spent-fuel rehydrogenation.4 Among the homogeneous
systems that are known, excellent approaches based on iron,5
molybdenum,6 iridium,7 rhodium,8 nickel,9 palladium,10 and
ruthenium11,12 have been reported. Two of the first examples of
reusable ruthenium catalysts have come from our lab.
Particularly, we used Shvo’s catalyst 112 to dehydrogenate
AB, then developed catalysts 2a and 2b (Figure 1), which are
capable of releasing >2 equiv of H2 from AB and up to 4.6 wt %
of H2 in high [AB].11c,13 We have proposed a detailed
mechanism for AB dehydrogenation by Shvo’s catalyst wherein
Received: May 14, 2015
Figure 1. Ru-based AB dehydrogenation catalysts.
© XXXX American Chemical Society
A
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Scheme 1. Proposed AB Dehydrogenation Mechanism by Shvo’s Catalyst
Figure 2. Left: AB dehydrogenation catalyzed by 1 (10 mol % Ru
atom) (green diamond) and 1 with phen (0.1 equiv to Ru, blue
triangle; 0.5 equiv, red circle). Right: AB dehydrogenation catalyzed by
14 mol % Ru atom of 1 (green diamonds) and 9 (red circles).
phen can expel the tetraphenylcyclopentadienone (CPD)
ligand from the metal center, which is a previously
undocumented form of reactivity for the Shvo system.
RESULTS AND DISCUSSION
Synthesis, Characterization, and Reactivity of Pyr-
■
Figure 3. Catalytic AB dehydrogenation Left: AB consumption (11B
NMR) catalyzed by 1, 11, 12, and 13 (10 mol % Ru atom, 70 °C, 1:2
C6D6/diglyme). Right: H2 release by eudiometry with 13 (10 mol %
Ru atom, 70 °C, 1:2 C6D6/diglyme with 2% EtOH).
idine Complexes. Pyridine-ligated ruthenium complexes 11−
13 were prepared by dropwise addition of the appropriate
substituted pyridine to ruthenium dimer 10 in benzene solvent
at room temperature (Scheme 2). Treatment of the reaction
mixture with hexanes yielded the corresponding products as
pale gray powders. Structural data collected on the products
were as expected; the molecular structure of 11 is shown in
Figure 3. Therein, ruthenium adopts the predicted piano stool
geometry, which has analogy to the known molecular structures
of 916 and [(2,5-Ph2-3,4-Tol2-CPD)Ru(CO)(PPh3)(pyr)].18
The complexes are bench-stable with stability similar to 1.
Compounds 11−13 affect significantly faster AB dehydro-
genation than 1 (Figure 3, left). For example, a reaction
catalyzed by 13 and 2% EtOH has a rate constant of H2
evolution of 1.40(4) × 10−3 s−1, which is 4-fold faster than an
analogous reaction of 1, with a rate constant of 3.7(1) × 10−4
s−1 (Figure 3, right).12 Moreover, 13 gives a slightly higher
extent of H2 release, 2.1 equiv. Ethanol here is a cocatalyst:
although it has known reactivity with AB, particularly when
present as solvent and activated by base,19 we have shown
Scheme 2. Synthesis of Pyridine Complexes 11−13
B
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previously that a catalytic portion is a first-order cocatalyst that
functions primarily as a proton shuttle in the Shvo system.12a
Comparable results are obtained when free pyridines are
added to reaction solutions of 1; these reactions differ from
those of isolated pyridine adducts by only a brief initiation
delay. If free nitrogen ligands are added to a reaction mixture,
they show a similar protective behavior. For example, if 1 is
treated with a 4-DMAP, Et3N, or even tetramethylethylenedi-
amine (TMEDA) in situ, saturation catalysis (the linear region
in the kinetic profile) is extended from ca. 30% conversion to
ca. 80% conversion.12 See the Supporting Information for time
course plots.
Impact of Pyridine Substitution on Reactivity. In the
dehydrogenation studies shown in Figure 3, we observe
increasing reaction rates (12 < 11 < 13) as the pyridine
ligands become less electron rich. For example, these reactions
have AB consumption rate constants of 2.04(4), 4.3(1), and
5.6(2) × 10−4 M s−1, respectively, for precatalysts 12, 11, and
13 in their saturation catalysis periods. We account for this by
considering binding affinity of the pyridine ligand to
ruthenium: the electron-rich 4-dimethylaminopyridine (4-
DMAP) ligand in 12 should bind an electrophilic metal more
tightly than an electron-neutral pyridine. The complement is
applicable to the least electron-rich 4-trifluoromethylpyridine
(4-TFMP). Accordingly, when 13 is treated with 1 equiv of 4-
DMAP, we see 12 formed in solution at room temperature
immediately with approximately 90% of 13 converted to 12 in
90 min at 70 °C. We rationalize from our observations that,
while these pyridines can increase the rate for AB dehydrogen-
ation, a less tightly binding pyridine enables a faster reaction by
facilitating ligand/substrate exchange on the ruthenium center.
This is consistent with a proposal that pyridine binding to
ruthenium is protective of the catalyst, but pyridine dissociation
is necessary for catalytic turnover.
Figure 4. Left: Metal-catalyzed dehydrogenation of 14. Right:
Dehydrogenation of AB (blue squares) and 14 (red circles) by
Shvo’s catalyst 1 (10 mol % Ru atom, 70 °C, 1:2 C6D6/diglyme).
ligation is not the major cause of reaction acceleration by added
pyridine.
Based on these observations, monodentate pyridine ligands can
play a protective role in the dehydrogenation of AB, probably by
preempting borazine hydroboration. We see that the catalysis
rate is increased and catalyst deactivation is avoided in the
presence of pyridines, whether these ligands are precomplexed
to the catalyst or added separately to the reaction. We believe
that pyridine−ruthenium ligation is important to this behavior.
CPD Ligand Displacement by Bidentate Nitrogen
Ligands. When AB dehydrogenation reactions involving 1 are
treated with bidentate nitrogen ligands (e.g., phen, bipy,
TMEDA), the active catalyst for AB dehydrogenation could
involve multiple [RuLn] species. This is due to the lability of
the CPD ligand in the presence of these bidentate dinitrogen
compounds at elevated temperatures. For example, CPD is
displaced in the attempted syntheses of a TMEDA-supported
complex, 15. In conditions analogous to the syntheses of 11−
13, we observe that treatment of 10 with TMEDA results in the
formation of dinuclear species 16, where one CPD ligand is
expelled (Scheme 3). CPD displacement is confirmed by GC-
Interactions of Pyridine Complexes with BN Com-
pounds in Solution. The pyridine ligand has several reactive
options in solution. Beyond ruthenium, free pyridine can ligate
BH3; we observe pyridine-BH3 by 11B NMR in the course of
these reactions. Further evidence of pyridine ligation to boron,
nitrogen species comes from the 19F NMR handle of 13: we
observed over eight 4-TFMP species at the end of the reaction,
including free ligand.
Scheme 3. Synthesis and Structure of 16 and ORTEP
Diagram of 16 with Ellipsoids Drawn at the 50% Probability
Level and Hydrogens Omitted for Clarity
In the corresponding 11B NMR spectra, we further observe
broad signals that are consistent with polymeric boron
byproducts such as polyborazylene. In contrast, AB dehydro-
genation reactions catalyzed by 1 have borazine as the sole
boron product. On the basis of these observations, we infer that
the pyridines are ligating intermediates in dehydrogenation in a
way that disfavors borazine accumulation.
We also observe in reactions involving 12, the 4-DMAP
adduct [Me2N-C5H4N-BH2NH2-BH3], 14, which was recently
characterized by Rivard (Figure 4).20 To determine whether 14
is a necessary part of the fastest mechanistic pathway for AB
dehydrogenation or merely a cul-de-sac, we dehydrogenated an
independently prepared sample of 14. Treatment of 14 (0.11
M) with 1 (10% Ru atom) was comparable in rate to 1-
catalyzed AB dehydrogenation, if not slightly slower under the
same conditions, so we find that formation of 14 is not enabling
more rapid AB dehydrogenation. Since 14 and pyridine-BH3
are the only major pyridine−boron adducts that we observe
during catalysis or in a ruthenium-free control reaction (vide
infra), and because these dehydrogenate more slowly than AB
under our catalytic conditions, we believe that pyridine−boron
C
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MS. In addition to offering a structural suggestion for a reactive,
CPD-free AB dehydrogenation catalyst monomer, [(CO)2Ru-
(TMEDA)], the dimeric structure of 16 presents a first-in-class
Ru−Ru bond (2.7735(6) Å) formed to a Shvo-like fragment,
here featuring a single CPD ligand bridging both metal centers.
As expected, free CPD is detected when 10 reacts with
bidentate ligands phen or bipy at 70 °C. In contrast to
TMEDA, we were unable to isolate a single phen-ligated
ruthenium adduct, even when treating 10 with phen at room
temperature. A control experiment with pyridine and 10 at 70
°C yields only 11: no CPD liberation is observed. We therefore
surmise that bidentate nitrogen ligands will expel CPD from the
Shvo system under our reaction conditions, while pyridines will not.
Kinetic Effects of CPD Displacement. Formation of a
[(phen)Ru(CO)2]2+ species in situ results in a very rapid
system for AB dehydrogenation, although it is unclear what the
active catalyst(s) are in this system. For example, treating
[(phen)RuCl2(CO)2] with 2 equiv of TlOTf afforded an
excellent catalyst for AB dehydrogenation, which completed in
1.5 h under our conditions. Figure 5 (left) shows the relative
On the basis of these observations, we believe that the origins
of rate acceleration in our quantitative poisoning experiments
include the formation of a reactive, homogeneous [(phen)Ru-
(CO)2] species.
CONCLUSIONS
■
We show here that amine and pyridine ligands can accelerate
the 1-catalyzed dehydrogenation of AB. Monodentate pyridines
enable accelerated dehydrogenation of AB without CPD
displacement. We propose that the reason for this is that
pyridines can protect the catalyst against hydroboration by
borazine. Bidentate (N−N) ligands appear to accelerate
catalysis through an additional mechanism where a relatively
more reactive ruthenium species is formed upon loss of CPD.
Further experiments on improved H2 production using these
catalysts and strategies are under way.
EXPERIMENTAL SECTION
■
I. General Procedures. All air- and water-sensitive procedures
were carried out either in a Vacuum Atmospheres glovebox under
nitrogen (0.5−10 ppm of O2 for all manipulations) or using standard
Schlenk techniques under nitrogen. Ethanol was distilled from sodium
ethoxide and stored over molecular sieves. Deuterated NMR solvents
were purchased from Cambridge Isotopes Laboratories. Benzene,
benzene-d6, diethylene glycol dimethyl ether (diglyme, Alfa Aesar),
tetraethylene glycol dimethyl ether (tetraglyme, Alfa Aesar),
tetramethylethylenediamine (Alfa Aesar), and triethylamine (Et3N,
Alfa Aesar) were dried over sodium benzophenone ketyl and distilled
prior to use. Shvo’s catalyst was purchased from Strem Chemicals and
used without further purifications. Ammonia borane (NH3BH3) was
purchased from Sigma-Aldrich and used under a N2 atmosphere
without further purifications. The integrity of this material was
1
checked regularly by H and 11B NMR. Pyridine, 4-DMAP, 4-TFMP,
and 1,10-phenanthroline were purchased from Alfa Aesar and used
without further purification. 2,2-Bipyridine was purchased from
Lancaster Synthesis Inc. and used without further purification.
Dimer 10 was synthesized according to Mays, Morris, and Shvo.21
Ammonia adduct 9,14,16 pyridine adduct 14,20 and Ru(Phen)-
22
Cl2(CO)2 were prepared according to literature procedures.
Figure 5. Left: Dehydrogenation of AB by 1 (green circles),
(phen)RuCl2(CO)2 with 2 equiv of TlOTf (blue diamonds), and 11
(red squares). Right: Dehydrogenation of AB by 1 (green circles),
(phen)RuCl2(CO)2 with 2 equiv of TlOTf (blue diamonds), and 11
(red squares). Conditions are 10 mol % Ru atom, 70 °C, 1:2 C6D6/
diglyme.
Sonication procedures were done in a VWR desktop sonic cleaner
bath. GC-MS spectra were acquired with a Thermo Focus GC
equipped with a DSQII mass-selective detector. H and 11B NMR
1
spectra were obtained on Varian 600, 500, and 400 MHz
spectrometers (600 MHz in 1H, 192 MHz in 11B) with chemical
shifts reported in units of ppm. All 1H chemical shifts were referenced
to the residual H solvent (relative to TMS). All 11B chemical shifts
1
rates of AB consumption: these conditions are significantly
more reactive than 1, but less reactive than 11. As expected, no
mechanism change is obvious in this time course plot.
Presumably, part of the reason that phen did not poison the
Shvo system as expected, but rather accelerated it, was the
generation of a portion of a relatively reactive [(phen)Ru-
(CO)2] species in situ. In line with this proposal, the rate of AB
consumption with phen-treated 1 lies nicely between the rates
of dehydrogenation with 1 and with our [(phen)Ru(CO)2]
species, which is consistent with a mechanism that involves a
[(phen)Ru(CO)2] catalyst that is attenuated by interaction
with a CPD-ligated moiety.
We surmise that CPD was not involved in this accelerated
reaction. Ruthenium must have been involved, however,
because while we observe that phen, TMEDA, or 4-DMAP
will release H2 from AB without ruthenium, these are slow and
have different product selectivities: 11B NMR shows these to
give a branched cyclotetraborazane1j and only traces of borazine
after 4 h at 70 °C.
were referenced to BF3-OEt2 in diglyme in a coaxial external standard
(0 ppm). All 19F chemical shifts were referenced to a CFCl3 external
standard. NMR spectra were taken in 8-inch J-Young tubes (Wilmad)
with Teflon valve plugs. All spectra were processed using MesRe Nova
(v. 9.0.0-12821). FTIR were taken on KBr salt plates on a Bruker
Vertex 80 FTIR. Accurate mass spectrometry was conducted by the
University of California, Riverside, High Resolution Mass Spectrom-
etry Facility. Elemental analysis was conducted by the University of
Illinois Urbana−Champaign Microanalysis Laboratory.
II. Preparative and Spectroscopic Details. 11: Ru-pyridine
adduct 11 was prepared under air by dissolving 10 (50 mg, 0.046
mmol, 1 equiv) in benzene (5 mL). Pyridine (15 μL, 0.18 mmol, 4
equiv) was dissolved separately in benzene (0.1 mL) and added
dropwise to the 10/benzene solution. The reaction mixture color
lightened from an orange to a lighter yellow-orange. The reaction was
stirred at room temperature overnight to ensure completion. The
reaction was then passed through a pipet filter filled with cotton and
Celite, and benzene was removed by rotary evaporation. The residue
was dried under reduced pressure overnight. Hexanes (5 mL) was
added to the residue, and the mixture was immersed in a sonication
bath for 15 min. A pale yellow-white precipitate was filtered out and
D
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washed with hexanes. The solid was then lyophilized from a benzene
solution to give a pale yellow-white powder in 84% yield (46 mg).
1H NMR (600 MHz, benzene-d6): δ 8.19 (d, J = 7.7 Hz, 4H, Ph),
8.10 (d, J = 4.8 Hz, 2H, C2,6 of Pyr), 7.31 (d, J = 7.0 Hz, 4H, Ph), 7.06
(t, J = 7.6 Hz, 4H, Ph), 6.94 (t, J = 7.4 Hz, 2H, Ph), 6.86 (t, J = 7.5 Hz,
4H, Ph), 6.80 (t, J = 7.4 Hz, 2H, Ph), 6.44 (t, J = 7.7 Hz, 1H, C4 of
Pyr), 6.05 (t, J = 6.7 Hz, 2H, C3,5 of Pyr). 13C{1H} NMR (150 MHz,
benzene-d6): δ 201.46 (CO), 170.05 (C1 of Cp), 155.68 (C2,6 of Pyr),
137.07 (C4 of Pyr), 134.69 (Ph), 132.79 (Ph), 132.57 (Ph), 130.34
(Ph), 128.35 (Ph), 128.18 (Ph), 127.95 (Ph), 126.43 (Ph), 125.51
(C3,5 of Pyr), 104.71 (C2,5 of Cp), 80.81 (C3,4 of Cp). FTIR (ν, cm−1):
2008.2, 1956.4 (M − CO’s), 1615.8 (CO). ESI-MS for [M − H]+:
calcd 622.0951 g/mol, found 622.0950 g/mol. Mp: 181−187 °C, dec,
black crust.
12: Ru-4-DMAP adduct 12 was prepared under air by dissolving 10
(50.0 mg, 0.046 mmol, 1 equiv) in benzene (5 mL). 4-
Dimethylaminopyridine (22.6 mg, 0.18 mmol, 4 equiv) was separately
dissolved in benzene (0.1 mL), immersed in a sonication bath briefly
until homogeneous, and added dropwise to the 10/benzene solution.
The reaction mixture color lightened from an orange to a lighter
yellow-orange. The reaction was stirred at room temperature overnight
to ensure completion. The reaction was then passed through a pipet
filter filled with cotton and Celite, and benzene was removed by rotary
evaporation. The residue was dried under reduced pressure overnight.
Diethyl ether (5 mL) was added to the residue, and the resulting
suspension was immersed in a sonication bath for 15 min. A pale
yellow-white precipitate was filtered out and washed with hexanes. The
solid was then lyophilized from a benzene solution to give a pale
yellow-white powder in 60% yield (37 mg).
Tetramethylethylenediamine (14 μL, 0.18 mmol, 2 equiv) was
separately dissolved in hexanes (20 mL). The 10/benzene solution
was added dropwise into the hexanes solution. The reaction was
stirred vigorously at room temperature overnight. The color of the
heterogeneous reaction mixture lightened from orange to pale yellow-
white upon completion. Solvent was then removed by rotary
evaporation. The residue was dried under reduced pressure overnight.
Hexanes (5 mL) was added to the residue, and the mixture was
immersed in a sonication bath for 15 min. A pale yellow-white
precipitate was filtered out and washed with hexanes. The solid was
then lyophilized from a benzene solution to give a dull white-yellow
powder in 89% yield (49 mg).
1H NMR (600 MHz, methylene chloride-d2): δ 7.59 (dd, J = 8.3, 1.3
Hz, 8H, Ph), 7.12 (t, J = 7.5 Hz, 8H, Ph), 7.10−7.02 (m, 24H, Ph),
2.51 (s, 4H, ethylene), 2.05 (s, 12H, tetramethyls). 13C NMR (150
MHz, benzene-d6): δ 201.79 (CO), 167.62 (C1 of Cp), 134.34 (Ph),
132.24 (Ph), 132.18 (Ph), 130.46 (Ph), 128.14 (Ph), 127.98 (Ph),
127.81 (Ph), 126.39 (Ph), 103.68 (C2,5 of Cp), 82.46 (C3,4 of Cp),
57.02 (ethylene), 53.27 (tetramethyls). FTIR (ν, cm−1): 2012.3,
1950.6 (M − CO’s), 1648.4 (CO). Anal. Calcd: C, 68.39; H, 5.08;
N, 2.28. Found: C, 68.1; H, 4.72; N, 2.15. Mp: 182−191 °C, dec, black
crust.
Unsymmetrical Dimer 16. 16 is not readily separated from 15 and
17 (see Supporting Information). Two separate synthetic routes are
described.
1. Preparation of 15, 16, and 17 from 10 and TMEDA was
conducted by dissolving 10 (50 mg, 0.046 mmol, 1 equiv) in benzene
(5 mL) under air. Tetramethylethylenediamine (7 μL, 0.09 mmol, 1
equiv) was added to the solution while stirring vigorously. The
reaction was then stirred in a 35 °C oil bath overnight. The resulting
dark red-purple solution was then passed through a filter filled with
cotton and Celite, the solvent was removed by rotary evaporation, and
the residue was dried under reduced pressure. Hexanes (5 mL) was
added to the residue, and the resulting suspension was immersed in a
sonication bath for 5 min to produce a dark red powder. The powder
was isolated via filtration and washed with hexanes. Benzene was
added to the powder, and the resulting solution was lyophilized to
dryness. NMR analysis of the powder showed 17 as the major product,
with 16 and 15 as minor products. Minimum fresh benzene was added
to dissolve a portion of the powder (ca. 15 mg), and vapor diffusion
was conducted with hexanes selectively to yield crystals of 16 for X-ray
analysis.
1H NMR (400 MHz, benzene-d6): δ 8.33−8.30 (m, 4H, Ph), 7.80−
7.77 (m, 2H, C2,6 of DMAP), 7.39−7.36 (m, 4H, Ph), 7.12 (t, J = 7.8
Hz, 4H, Ph), 6.97 (t, J = 7.4 Hz, 2H, Ph), 6.88 (t, J = 7.4 Hz, 4H, Ph),
6.84−6.79 (m, 2H, Ph), 5.34 (d, J = 7.3 Hz, 2H, C3,5 of DMAP), 1.77
(s, 6H, methyls of DMAP). 13C NMR (101 MHz, CDCl3): δ 200.94
(CO), 169.46 (C1 of Cp), 154.51 (C2,6 of DMAP), 154.26 (C4 of
DMAP), 134.04 (Ph), 132.54 (Ph), 132.26 (Ph), 130.05 (Ph), 127.69
(Ph), 127.61 (Ph), 127.46 (Ph), 125.80 (Ph), 108.63 (C3,5 of DMAP),
103.18 (C2,5 of Cp), 80.42 (C3,4 of Cp), 39.11 (methyls of DMAP).
FTIR (ν, cm−1): 2009.0, 1939.3 (M − CO’s), 1627.1 (CO). ESI-
MS for [M − H]+: calcd 665.1373 g/mol, found 665.1376 g/mol. Mp:
199−209 °C, dec, black crust.
13: Ru-4-TFMP adduct 13 was prepared by dissolving 10 (50 mg,
0.046 mmol, 1 equiv) in benzene (5 mL) under air. 4-
Trifluoromethylpyridine (21 μL, 0.18 mmol, 4 equiv) was dissolved
in benzene (0.1 mL) and added dropwise to the 10/benzene solution.
The reaction mixture color lightened from orange to a lighter yellow-
orange. The reaction was stirred at room temperature overnight to
ensure completion. The reaction was then passed through a pipet filter
filled with cotton and Celite, and benzene was removed by rotary
evaporation. The residue was dried under reduced pressure overnight.
Hexanes (5 mL) was added to the residue, and the mixture was
immersed in a sonication bath for 15 min. A pale yellow-white
precipitate was filtered out and washed with hexanes. The solid was
then lyophilized from a benzene solution to give a dull yellow-orange
powder in 82% yield (52 mg).
2. In a separate synthesis of 16, 10 (7.62 mg, 0.014 mmol of Ru
atom, 1 equiv of Ru atom) and Ru3(CO)12 (3.0 mg, 0.014 mmol of Ru
atom, 1 equiv of Ru atom) were dissolved in benzene-d6 (0.5 mL) in a
J-Young NMR tube in a nitrogen-filled glovebox. Tetramethylethyle-
nediamine (2.1 μL, 0.014 mmol, 1 equiv) was added, and the tube was
swirled to allow solvation of all reactants. The tube was put in a 45 °C
oil bath, and after 10 min, the orange solution had darkened to a
darker orange; then the tube was left to react for 36 h before NMR
analysis. Although the major product (16) could not be separated from
1
other Ru-containing compounds in the reaction, it matched H NMR
shifts of the minor product observed above (see Supporting
Information for further details).
1H NMR (400 MHz, benzene-d6): δ 7.91, 6.99, 6.94, 6.86, 6.73, 2.18
(s, 6H), 1.65−1.68 (m, 8H, ethylene and methyl), 1.47 (m, 2H).
III. Mechanistic Studies Using 11B NMR. In a typical reaction,
7.7 mg of AB was combined with catalyst (10 mol %) in a J-Young
NMR tube while in a glovebox under nitrogen. The AB concentration
and catalyst concentrations may be varied. Diglyme or tetraglyme (0.4
mL) and benzene-d6 (0.2 mL) were added to the tube. The sample
tube was immediately inserted into a preheated, preshimmed, and
prelocked NMR (70 °C) tube, and the kinetic monitoring
commenced. Disappearance of AB in the solution was monitored by
the relative integration of its characteristic peak in the 11B spectrum
(−22 ppm) and the BF3-OEt2 standard. The acquisition involved a
1.84 s pulse sequence in which 16 384 complex points were recorded,
followed by 1 s relaxation delay. To eliminate B−O peaks from the
borosilicate NMR tube and probe, the 11B FIDs were processed with
backward linear prediction. Safety note: caution should be used when
1H NMR (400 MHz, benzene-d6): δ 8.18−8.14 (m, 4H, Ph), 8.08
(d, J = 5.9 Hz, 2H, C2,6 of TFMP), 7.31−7.28 (m, 4H, Ph), 7.06 (t, J =
7.7 Hz, 4H, Ph), 6.96−6.91 (m, 2H, Ph), 6.87 (dd, J = 8.2, 6.4 Hz, 4H,
Ph), 6.84−6.78 (m, 2H, Ph), 6.08 (d, J = 5.8 Hz, 2H, C3,5 of TFMP).
13C NMR (101 MHz, CDCl3): δ 199.98 (CO), 168.46 (C1 of Cp),
156.78 (C2,6 of TFMP), 139.72 (q, J = 35.4 Hz, C4 of TFMP), 133.04
(Ph), 132.09 (Ph), 131.74 (Ph), 130.06 (Ph), 128.02 (Ph), 127.87
(Ph), 127.79 (Ph), 126.49, (Ph), 122.09 (q, J = 273.7 Hz, CF3), 121.69
(q, J = 3.5 Hz, C3,5 of TFMP), 103.89 (C2,5 of Cp), 81.42(C3,4 of Cp).
19F NMR (564 MHz, benzene-d6): δ −65.69. FTIR (ν, cm−1): 2015.8,
1960.1 (M − CO’s), 1606.1 (CO). ESI-MS for [M − H]+: calcd
690.0825 g/mol, found 690.0831 g/mol. Mp: 200−207 °C, dec, black
liquid.
Dimer 15: Ru-TMEDA dimer 15 was prepared under air by
dissolving 10 (50 mg, 0.046 mmol, 1 equiv) in benzene (5 mL).
E
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carrying out these reactions, as the release of hydrogen can lead to sudden
pressurization of reaction vessels.
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IV. Hydrogen Quantification. In a typical reaction, 7.7 mg of AB
(0.25 mmol) was combined with catalyst (10 mol %) in a 2 mL
Schlenk bomb equipped with a Teflon stir bar while in a glovebox
under nitrogen. Benzene (0.2 mL) and diglyme containing 2 mol %
ethanol (0.4 mL) were added to the flask (made from a stock solution
of 3.7 μL of EtOH in 5 mL of diglyme). A eudiometer was constructed
as follows: the side arm of the valve of the Schlenk flask was connected
to a piece of Tygon tubing, which was adapted to 20 gauge (0.03 inch)
Teflon tubing with a needle. The tubing was threaded through the
open end of a buret that was sealed with a Teflon stopcock on the
other end. The buret was filled with water. The entire apparatus was
then inverted into a 500 mL Erlenmeyer filled with water and clamped
onto a metal ring stand. The reactor’s valve was opened to release gas
from the reactor headspace while heating in a regulated oil bath. The
volume of liberated gas was recorded periodically until gas evolution
ceased. Liberated hydrogen was quantified by recording its volume
displacement in the eudiometer.
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V. GC-MS Detection of Free CPD. In a typical reaction, 10 and a
bidentate nitrogen ligand (1,10-phenanthroline, TMEDA, or 2,2′-
bipyridine) were added into a vial at room temperature. The reaction
was placed in a 70 °C oil bath for 12−16 h. The solvent was then
removed under reduced pressure, and the residue was immersed in a
sonication bath in hexanes for 1 min. The supernatant was then passed
through a Celite filter and analyzed by GC-MS. CPD elutes at ca. 16.8
min with this method, with the major mass peaks being 384 m/z
(CPD) and 178 m/z (portion of acetylene). See the Supporting
Information for spectra.
CDCC 1062142 (11) and 1062141 (16) contain supplementary
crystallographic data for this article.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00409.
Graphical NMR data and detailed kinetic plots (PDF)
X-ray crystallographic data for compound 11 (CIF)
X-ray crystallographic data for compound 16 (CIF)
Compounds 11 and 16 (MOL)
(6) Buss, J. A.; Edouard, G. A.; Cheng, C.; Shi, J.; Agapie, T. J. Am.
Chem. Soc. 2014, 136, 11272−11275.
(7) Denny, M. C.; Pons, V.; Hebden, T. J.; Heinekey, M.; Goldberg,
K. I. J. Am. Chem. Soc. 2006, 128, 12048−12049.
AUTHOR INFORMATION
Corresponding Author
*E-mail (T. J. Williams): travisw@usc.edu.
■
(8) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. J. Am. Chem.
Soc. 2003, 125, 9424−9434. (b) Jaska, C. A.; Manners, I. J. Am. Chem.
Soc. 2004, 126, 1334−1335. (c) Shrestha, R. P.; Diyabalanage, H. V.
K.; Semelsberger, T. A.; Ott, K. C.; Burrell, A. K. Int. J. Hydrogen
Energy 2009, 34, 2616−2621. (d) Douglas, T. M.; Chaplin, A. B.;
Weller, A. S. J. Am. Chem. Soc. 2008, 130, 14432−14433. (e) Alcaraz,
G.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2010, 49, 7170−7179.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was sponsored by the National Science Foundation
(CHE-1054910) and the Hydrocarbon Research Foundation.
We thank the NSF (DBI-0821671, CHE-0840366, CHE-
1048807) and NIH (1 S10 RR25432) for sponsorship of
research instrumentation. Fellowship assistance from The
Sonosky Foundation of the USC Wrigley Institute (X.Z.,
Z.L.) and the Johns Hopkins Center for Talented Youth
(L.K.F.) is gratefully acknowledged. We acknowledge Dr. Brian
Conley and Jeff Celaje, and Nicky Terrile for insightful
discussions. We thank Dr. Ralf Haiges for help with X-ray
crystallography.
(10) (a) Kim, S.-K.; Han, W.-S.; Kim, T.-J.; Kim, T.-Y.; Nam, S. W.;
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