Triphosphine-Ligated Copper Hydrides for CO2 Hydrogenation: Structure, Reactivity, and Thermodynamic Studies

Article abstract of DOI:10.1021/jacs.6b05349

The copper(I) triphosphine complex LCu(MeCN)PF₆ (L = 1,1,1-tris(diphenylphosphinomethyl)ethane), which we recently demonstrated is an active catalyst precursor for hydrogenation of CO₂ to formate, reacts with H₂ in the presence of a base to form a cationic dicopper hydride, [(LCu)₂H]PF₆. [(LCu)₂H]⁺ is also an active precursor for catalytic CO₂ hydrogenation, with equivalent activity to that of LCu(MeCN)⁺, and therefore may be a relevant catalytic intermediate. The thermodynamic hydricity of [(LCu)₂H]⁺ was determined to be 41.0 kcal/mol by measuring the equilibrium constant for this reaction using three different bases. [(LCu)₂H]⁺ and the previously reported dimer (LCuH)₂ can be synthesized by the reaction of LCu(MeCN)⁺ with 0.5 and 1 equiv of KB(OⁱPr)₃H, respectively. The solid-state structure of [(LCu)₂H]⁺ shows threefold symmetry about a linear Cu-H-Cu axis and significant steric strain imposed by bringing two LCu⁺ units together around the small hydride ligand. [(LCu)₂H]⁺ reacts stoichiometrically with CO₂ to generate the formate complex LCuO₂CH and the solvento complex LCu(MeCN)⁺. The rate of the stoichiometric reaction between [(LCu)₂H]⁺ and CO₂ is dramatically increased in the presence of bases that coordinate strongly to the copper center, e.g. DBU and TMG. In the absence of CO₂, the addition of a large excess of DBU to [(LCu)₂H]⁺ results in an equilibrium that forms LCu(DBU)⁺ and also presumably the mononuclear hydride LCuH, which is not directly observed. Due to the significantly enhanced CO₂ reactivity of [(LCu)₂H]⁺ under these catalytically relevant conditions, LCuH is proposed to be the catalytically active metal hydride.

Full text of DOI:10.1021/jacs.6b05349

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Article
Triphosphine-Ligated Copper Hydrides for CO2 Hydrogenation:
Structure, Reactivity, and Thermodynamic Studies
Christopher M Zall, John C Linehan, and Aaron M. Appel
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b05349 • Publication Date (Web): 19 Jul 2016
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Triphosphine-Ligated Copper Hydrides for CO2 Hydrogenation:
Structure, Reactivity, and Thermodynamic Studies
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Christopher M. Zall, John C. Linehan, and Aaron M. Appel*
Pacific Northwest National Laboratory, P.O. Box 999, MS K2-57, Richland, Washington 99352, United States.
Supporting Information Placeholder
ABSTRACT: The copper(I) triphosphine complex LCu(MeCN)PF6 (L = 1,1,1-tris(diphenylphosphinomethyl)ethane), which
we recently demonstrated is an active catalyst precursor for hydrogenation of CO2 to formate, reacts with H2 in the presence of a
base to form a cationic dicopper hydride, [(LCu)2H]PF6. [(LCu)2H]+ is also an active precursor for catalytic CO2 hydrogena-
tion, with equivalent activity to that of LCu(MeCN)+, and therefore may be a relevant catalytic intermediate. The thermody-
namic hydricity of [(LCu)2H]+ was determined to be 41.0 kcal/mol by measuring the equilibrium constant for this reaction us-
ing three different bases. [(LCu)2H]+ and the previously reported dimer (LCuH)2 can be synthesized by the reaction of
LCu(MeCN)+ with 0.5 and 1 equivalent of KB(OiPr)3H, respectively. The solid-state structure of [(LCu)2H]+ shows three-fold
symmetry about a linear Cu-H-Cu axis and significant steric strain imposed by bringing two LCu+ units together around the
small hydride ligand. [(LCu)2H]+ reacts stoichiometrically with CO2 to generate the formate complex LCuO2CH and the sol-
vento complex LCu(MeCN)+. The rate of the stoichiometric reaction between [(LCu)2H]+ and CO2 is dramatically increased in
the presence of bases that coordinate strongly to the copper center, e.g. DBU and TMG. In the absence of CO2, the addition of a
large excess of DBU to [(LCu)2H]+ results in an equilibrium that forms LCu(DBU)+ and also presumably the mononuclear hy-
dride LCuH, which is not directly observed. Due to the significantly enhanced CO2 reactivity of [(LCu)2H]+ under these cata-
lytically relevant conditions, LCuH is proposed to be the catalytically active metal hydride.
INTRODUCTION
smaller number based on cobalt.17-20 A crucial component in
catalyst discovery and development has been the characteri-
zation of metal hydride species and their reactivity. The
thermodynamic hydride donor abilities of many first-row
metal hydrides have been studied, most extensively those of
nickel21-33 and cobalt,34-38 and more recently of several iron
complexes.14,39-42 These properties have been used to guide
catalyst development, especially for the hydrogenation of
CO2.14,19,20,33,40-42 However, no comparable data has been re-
ported for copper hydrides.
The catalytic hydrogenation of CO2 could play a central
role in the production of renewable, carbon-based fuels and
feedstocks.1-4 The identification of first-row transition metal
hydrides that can react favorably with CO2 is crucial to de-
veloping alternatives to expensive noble-metal catalysts,
thereby making this process more economically viable. In a
typical catalytic cycle, CO2 hydrogenation involves the reac-
tion of a transition metal catalyst with hydrogen to form a
metal-hydride intermediate and subsequent transfer of the
hydride to CO2 to produce formate. The favorability of each
of these steps, which can significantly affect the overall cata-
lytic activity, is a function of the thermodynamic stability of
the metal-hydride bond. Because CO2 is a relatively inert
substrate, and because first-row transition metals typically
form less reactive metal-hydride bonds than their second-
and third-row analogues, the use of first-row metal complex-
es in catalytic CO2 hydrogenation has historically been quite
limited. Before 2010, the only examples were in reports by
Inoue et al.,5,6 Evans et al.,7 and Jessop et al.8 Since that year,
and the report by Beller et al. of a tetraphosphine-ligated iron
catalyst with relatively high activity,9 many additional cata-
lysts have been reported, with most based on iron10-16 and a
Copper hydrides are widely used reagents and catalysts in
organic synthesis.44 They have been utilized most notably for
the selective reduction of carbonyl compounds,45,46 although
they catalyze numerous other reactions, such as the semihy-
drogenation47,48 or hydroalkylation49,50 of alkynes and the
hydroboration51 and hydroamination52,53 of alkenes. The
often-exceptional selectivity of these reactions is controlled
in large part by the nature of the copper hydride intermedi-
ates. Hexameric complexes are most common with mono-
dentate phosphine ligands, such as Stryker’s reagent,
[(PPh3)CuH]6.54-58 With polydentate phosphines or with
NHC ligands, lower-nuclearity structures can be obtained.58-
63
Although all of these complexes are multinuclear in the
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solid state, their speciation in solution is often unclear.64-68
Even when the isolated complex is well characterized, the
chemistry during catalysis can be more complex, with im-
portant implications for catalytic activity and selectivity.69
within several hours. This decomposition is inhibited at -35
°C or under an H2 atmosphere.
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Addition of excess KB(OiPr)3H to LCu(MeCN)+ or of
slightly more than 0.5 equivalents of KB(OiPr)3H to
[(LCu)2H]+ in THF or MeCN gave (LCuH)2, (Scheme 1,
top right), which was previously reported by Goeden et al. as
the product of hydrogenolysis of (CuOtBu)4 in the presence
of L.59 (LCuH)2 is a neutral, dimeric species, in which two
hydrides bridge the two copper centers. Goeden et al. noted
the thermal instability of (LCuH)2 in solution and character-
ized it at -40 °C. We similarly find that (LCuH)2 is signifi-
cantly less thermally stable than [(LCu)2H]+, decomposing
at room temperature in THF to give a black precipitate and
free L. Addition of a stoichiometric amount of CPh3BF4 to a
MeCN solution of [(LCu)2H]+ or suspension of (LCuH)2
resulted in clean conversion back to LCu(MeCN)+.
Several copper hydrides have been shown to react stoichi-
ometrically with CO2 to produce copper formates.59,62-63,70-73
Some species are highly active catalysts for CO2 reduction
when used in conjunction with stoichiometric borane74-76 or
silane77-79 reagents. Replacement of these high-energy re-
ductants with hydrogen would be significantly more practical
for applications in energy storage and fuel production. Hy-
drogenation of CO2 using molecular copper catalysts has
been demonstrated only very recently. Watari, Ikariya, and
coworkers reported that a number of simple copper salts
catalyze the hydrogenation of CO2 to formate in the presence
of strong organic bases.80 The catalytic activity was found to
vary considerably with the copper precursor and the base,
but the active catalytic species were not identified. In an in-
dependent study, we showed that the well-defined, triphos-
phine-ligated complex LCu(MeCN)+ is a more active cata-
lyst precursor for CO2 hydrogenation.81 Spectroscopic moni-
toring of the reaction revealed mechanistic information in-
cluding the catalyst resting state and two important roles for
the base in promoting catalysis. However, no hydride species
was observed throughout the reaction, and, therefore, the
identity of the active species that reacts with CO2 was not
determined. Herein, we present studies of a series of triphos-
phine-ligated copper hydrides that are plausible catalytic
intermediates, including a novel cationic dicopper hydride,
[(LCu)2H]+, a previously-reported dimer, (LCuH)2, and a
transient mononuclear hydride, LCuH. In addition, we re-
port the measurement of the thermodynamic hydride donor
ability, or hydricity, of [(LCu)2H]+. These studies represent
the first such measurement for a copper hydride and demon-
strate the thermodynamic favorability of using copper hy-
drides for CO2 hydrogenation. Finally, we report the stoichi-
ometric and catalytic reactivity of [(LCu)2H]+ with CO2 and
discuss the implications for the catalytic mechanism, includ-
ing an unusual role for the base in promoting these reactions.
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Spectroscopic and Structural Characterization of
[(LCu)2H]+ and (LCuH)2. Only a single set of ligand reso-
nances is observed in the 1H and 31P NMR spectra of
[(LCu)2H]+, indicating that the triphosphine ligands of the
copper centers are magnetically equivalent, with κ3 binding
–
of each ligand. The PF6 anion also appears as a septet in the
31P NMR spectrum and integrates to one-sixth of the signal
for the triphosphine ligands, as expected for a dicopper cati-
on. The hydride and phosphine resonances in [(LCu)2H]+
are both broadened at room temperature, likely due to the
adjacent I = 3/2 Cu nuclei. The observed broadening ob-
scures P-H coupling, even at low temperature.
Scheme 1. Synthesis of Copper Hydrides
Ph2
P
Ph2
P
Ph2
P
H-
2
Cu NCMe
Cu
H
Cu
+
P
CPh3
P
Ph2
Ph2
P
2 Ph2
P
Ph2
P
P
Ph2
Ph
LCu(MeCN)+
[(LCu)2H]+
+
2 H-
2 CPh3
H-
Ph2
P
Ph2
P
Ph2P
H
H
Cu
Cu
P
Ph2
P
Ph2
PPh2
H- = B(OiPr)3H-
(LCuH)2
RESULTS
Syntheses of Copper Hydrides [(LCu)2H]+ and
82
(LCuH)2. The reaction of LCu(MeCN)PF6 (L = 1,1,1-
tris(diphenylphosphinomethyl)ethane) with 0.5 equivalents
of KB(OiPr)3H in MeCN formed a colorless, cationic dicop-
per hydride, [(LCu)2H]+, which was isolated cleanly in 74%
yield after workup. [(LCu)2H]+ is somewhat thermally un-
stable under a nitrogen atmosphere in the solid state and
even more so in solution, forming small but noticeable
+
amounts of Cu0, H2, and the bis-ligated complex Cu(L)2
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P2’
P1’
P1
P3
H1’
Cu1’
P1
P1’
C1
C2
C1’
C2’
Cu1
Cu1 Cu1’
H1
H1
C3
P3’
C3’
P2
Figure 2. X-ray structure of (LCuH)2(MeCN). Thermal ellip-
soids are drawn at the 30% probability level. For clarity, only the
ipso carbon of each phenyl substituent is shown, and the solvent
molecules and hydrogen atoms are omitted, other than the hy-
drides. Only the major disordered component is shown. Select-
ed bond lengths (Å) and angles°: Cu1-H1, 1.63(3); Cu1-P1,
2.228(1); Cu1-P2, 2.234(4); Cu1-Cu1’, 2.3687(7); P1-Cu1-
P2, 100.8(1); P1-Cu1-H1, 117(1); P2-Cu1-H1, 108(1).
Figure 1. X-ray structure of the cationic dicopper unit in
[(LCu)2H]PF6. Thermal ellipsoids are drawn at the 30% proba-
bility level. For clarity, only the ipso carbon of each phenyl sub-
stituents is shown, and hydrogen atoms, other than the hydride,
have been omitted. Selected bond lengths (Å) and angles°:
Cu1-Cu1’, 3.044(4); Cu1-H1, 1.522(2); Cu1-P1, 2.28(5); P1-
C1, 1.86(5); C1-C2, 1.556(7); P1-Cu1-P1’, 91.3(2); P1-Cu1-
H1’124.4(1); C2-C1-P1, 111(2); C1-P1-Cu1, 110(2).
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The hydride resonance in [(LCu)2H]+ appears at a shift of
-1.46 ppm in the H NMR spectrum and integrates to the
the hydride atoms were located from the Fourier difference
map equidistant between the copper centers, along the Cu-
Cu axis. The third was placed by analogy at the same site
between the copper centers of the third cation. The Cu-H
distances for all three hydrides were allowed to refine freely
in subsequent refinements. With the caveat that these hydro-
gen atom positions cannot be determined accurately by X-
ray crystallography, the hydride in each molecule appears to
be equidistant between each copper along the Cu-H-Cu axis,
with Cu-H distances of 1.522(2), 1.548(3), and 1.548(3) Å
for the three dicopper cations in the unit cell.
1
expected 0.5 H per copper. The 31P chemical shift appears at
-20.8 ppm, very similar to the shift of -21.0 ppm reported by
Goeden et al. for the copper-coordinated phosphines of
(LCuH)2 at -40 °C in toluene-d8.59 We observe a different
shift for (LCuH)2, such that the two species can be easily
distinguished by their 31P NMR resonances. At room tem-
perature, toluene-d8 solutions of (LCuH)2 show a broad sin-
glet at -18.4 ppm, and in THF-d8 the same signal is observed
at -18.7 ppm. The chemical shift of this signal is temperature-
dependent, shifting upfield to -19.4 ppm at -40 °C, but re-
mains considerably downfield of the previously reported
chemical shift of -21.0 ppm. In addition to the broad reso-
nance for the copper-coordinated phosphines, we observe
two sharp peaks at -27.0 and -27.8 ppm for the dangling
phosphine arms of (LCuH)2 in toluene-d8 at -40 °C, which
were previously reported at -28.9 and -29.7 ppm. At room
temperature, these peaks are nearly coalesced and shift
downfield to approximately -25.0 ppm, overlapping with the
signal for free ligand, which forms from thermal decomposi-
tion of (LCuH)2.
The phosphines on the two different ligands are very near-
ly eclipsed, with average P-Cu-Cu-P torsion angles of 9.6°.
The eclipsed conformation appears to be the result of a steric
effect. The phenyl substituents on each ligand are brought
into close contact by their convergence around the small
hydride ion, and the eclipsed conformation allows them to
interpenetrate to relieve this strain; they would clash further
if the triphosphine ligands were staggered. The copper cen-
ters in [(LCu)2H]+ show a distinct distortion from tetrahe-
dral geometry. The P-Cu-P angles of 93.6(1)° are com-
pressed and the P-Cu-H angles of 122.7(8)° are elongated
relative to the 109.5° for an ideal tetrahedral metal center.83
The solid-state structure of [(LCu)2H]+ as determined by
X-ray crystallography (Figure 1) is consistent with the spec-
troscopic data, showing a hydride ligand sandwiched be-
tween two equivalent copper centers with κ3-coordinated
triphosphine ligands. The unit cell contains three independ-
ent dicopper cations. Only one-sixth of each cation is unique
due to overall D3 molecular symmetry, as the three arms of
each ligand are related by crystallographically imposed three-
fold rotational symmetry about the Cu-H-Cu axis, and the
two copper centers are related by perpendicular two-fold
rotation axes. The Cu-Cu distances in the individual cations
are 3.044(4), 3.097(6), and 3.096(6) Å, giving an average
Cu-Cu distance of 3.08(3) Å. In two of the dicopper cations,
The neutral copper hydride dimer (LCuH)2 is insoluble in
MeCN; when synthesized by reaction of LCu(MeCN)+ with
KB(OiPr)3H in MeCN, (LCuH)2(MeCN) precipitates
cleanly as a yellow crystalline solid. A single crystal grown in
this way was studied by X-ray crystallography, as shown in
Figure 2. The resulting structure is similar to that previously
reported for (LCuH)2(THF),59 except that a minor disor-
dered component is observed in the newer structure. The
major component of (LCuH)2 has inversion symmetry cen-
tered around the Cu2H2 diamond core. The disordered posi-
tion represents a diastereomeric form of the dimer, in which
the positions of the copper centers, the hydrides, and the
3
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phenyl groups are largely unchanged. However, the phos-
tions of LCu(MeCN)+ with one atm H2 in the presence of
1
2
3
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5
6
7
8
phorous atoms and the carbon atoms in the
tris(phosphinomethyl)ethane backbone of one of the two
triphosphine ligands are inverted, oriented in the opposite
direction from the major isomer. Effectively, while the ma-
jority of the molecule retains its inversion symmetry, the
backbones of the two ligands adopt localized Cs mirror
symmetry. These two isomeric forms of (LCuH)2 explain the
observation of two resonances for the dangling phosphines
in the 31P NMR spectrum of (LCuH)2. The signals do not
represent inequivalent phosphines within a single dimeric
structure, but rather two distinct structures. Refinement of
the disorder in the crystal structure suggested that the major
component represents 85.9% of the structure, corresponding
to a 6:1 ratio of the two isomers, compared to the approxi-
mately 2:1 ratio observed by 31P NMR spectroscopy. The
difference is potentially due to packing effects in the crystal.
one equivalent of the strongly basic, non-coordinating phos-
t
phazenes P1 Bu(pyrr) (pKa [BH]+ 28.4, Table 1, entry 1) and
t
P1 Bu(dma) (pKa [BH]+ 27.0 , Table 1, entry 2) or the bulky
guanidine tBuTMG (pKa [BH]+ 26.5, Table 1, entry 7)
showed slow formation of [(LCu)2H]+, along with the corre-
sponding conjugate acids. No other products or byproducts
were observed in these reactions. The ratios of
[[(LCu)2H]+]/[LCu(MeCN)+]2 and [BH+]/[B] increased
in parallel before stabilizing after approximately 1.5 weeks,
indicating the reaction had reached equilibrium. The equilib-
rium constants measured for these reactions correspond to
ΔG°H- values of 41.2 0.6, 40.9 0.3 kcal/mol, and 41.0
0.5 kcal/mol. To verify reproducibility and reversibility, the
H2 heterolysis reaction was run with varied stoichiometry
(Table 1, entries 2, 4, and 5) and in reverse (entry 6) as de-
tailed in Table 1 and in the SI. These reactions gave values
for ΔG°H- that are identical within experimental uncertainty,
confirming that these values are derived from true equilibria
rather than incomplete reactions. Altogether, combining the
data from all reactions with a measurable Keq, the hydricity of
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H2 Heterolysis Reactions and Hydricity Measurements
of [(LCu)2H]+. LCu(MeCN)+ reacts with strong bases un-
der 1 atm H2 to give [(LCu)2H]+ in reversible equilibria
(Scheme 2). The equilibrium constants for these reactions
were measured in MeCN and found to depend on the
strength of the base, as expected for heterolysis of H2. These
equilibria afforded a measure of the thermodynamic hydride
[(LCu)2H]+ is calculated to be ΔG°H- = 41.0 0.5 kcal/mol,
for which the reported uncertainty is two standard devia-
tions.
donor ability, or hydricity (ΔG°H-), of [(LCu)2H]+, which
can be calculated from the equilibrium constant for the H2
heterolysis reaction and the pKa of the protonated base (see
the discussion). The equilibrium constants were determined
Stoichiometric Reactions of [(LCu)2H]+ with CO2. We
investigated the reaction of [(LCu)2H]+ with CO2, as sum-
marized in Scheme 2, bottom. With 1 atm CO2, this reaction
is quite slow, taking approximately 4 days to reach comple-
tion. The only product observed in solution is
LCu(MeCN)+. The formate product precipitates as the
mononuclear complex LCuO2CH, which is largely insoluble
in MeCN at ambient temperatures. X-ray quality single crys-
tals grown directly from the reaction solution yielded a struc-
ture identical to that originally reported for LCuO2CH.84
The formation of this product from CO2 even at low temper-
ature and pressure is consistent with the favorability predict-
ed by the hydricity value (see the discussion). However, the
rate of CO2 reactivity was significantly slower than expected
1
by H and 31P NMR spectroscopy for CD3CN solutions of
LCu(MeCN)+ (5-10 mM) and equimolar amounts of base.
Equilibrium constants for these reactions and hydricity val-
ues calculated from these values are summarized in Table 1.
No hydride formation was observed when using NEtiPr2
or NEt3 (pKa [BH]+ 18.8), TMG (1,1,3,3-
tetramethylguanidine, pKa [BH]+ 23.4), or DBU (1,8-
diazabicycloundec-7-ene, pKa [BH]+ 24.3),85 which are
therefore absent from Table 1. The lack of an observed reac-
tion under these conditions suggests Keq << 1 M-1 atm-1, and
for
a
catalytically
active
intermediate.
therefore, using the pKa of DBU, ΔG°H- < 43 kcal/mol. Reac-
Scheme 2. Formation and CO2 Reactivity of [(LCu)2H]+
+
+
Ph2
P
Ph2
P
Ph2
P
Keq
2
Cu
Cu NCMe + Base + H2 (1 atm)
H
Cu
+ H[Base]+
P
Ph2
Ph2
P
P
Ph2
Ph2
P
CD3CN, 22°C
P
P
Ph2
Ph2
+
+
Ph2
P
Ph2
P
O
O
Ph2
P
Ph2
P
H
3-4 days
+ CO2 (1 atm)
+
Cu
Cu NCMe
Cu
H
Cu
CD3CN, 22 °C
P
Ph2
Ph2
P
P
Ph2
Ph2
P
P
P
P
P
Ph2
Ph2
Ph2
Ph2
4
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Table 1. Equilibrium Constants for H2 Heterolysis Reactions and Hydricity Values Calculated for [(LCu)2H]+ in CD3CN.
1
2
3
4
5
6
7
8
9
Entry
M+/M2H+
M+
P
H2 (atm)
1.0
1.7
1.0
1.7
1.0
1.0
1.0
Base/H[Base]+
pKa(H[Base]+)
28.4d
K
eq (M-1 atm-1)
450 230
595
32 30
17
37
52
7.3 5.3
ΔG°H- (kcal/mol)
40.9 0.3
41.0
t
1
2
3
4
5
6
7
P1 Bu(pyrr)c
t
M+
P1 Bu(pyrr)c
28.4d
M+
P1 Bu(dma)b
27.0d
41.2 0.6
40.9
41.3
41.5
41.0 0.5
t
M+
P1 Bu(dma)b
27.0d
t
M+
P1 Bu(dma)b (0.5 equiv)
27.0d
t
M2H+
M+
H[P1 Bu(dma)]+b
27.0d
t
tBuTMGa
26.5e
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
All data are for reactions at 22 3 ˚C using 5-10 mM Cu+ in CD3CN. a tBuTMG = 2-tert-Butyl-1,1,3,3-tetramethylguanidine. Keq and
ΔG°H- values with reported uncertainties are the averages of three experiments each, and uncertainties are reported as two standard devia-
t
t
tions. b P1 Bu(dma) = tert-Butylimino-tris(dimethylamino)phosphorane. c P1 Bu(pyrr) = tert-Butylimino-tris(pyrrolidino)phosphorane.
dpKa values taken from ref 85. epKa value taken from ref 81.
Use of [(LCu)2H]+ as a Catalyst Precursor for CO2
Hydrogenation. To assess whether [(LCu)2H]+ was a via-
ble catalytic intermediate, it was tested as a catalyst precursor
(Scheme 3). A CD3CN solution of [(LCu)2H]+ (1.25 mM)
and DBU (50 mM) was degassed and charged with 40 atm
H2/CO2, then heated to 83°C. The reaction progress was
The addition of 10 equivalents of DBU (5 equivalents per
Cu) led to a dramatic increase in the rate of the reaction of
[(LCu)2H]+ with CO2. The reaction was complete within 15
minutes, and LCu(DBU)+ was the only copper species ob-
1
served by H and 31P NMR spectroscopy, indicating that the
formate was an outer-sphere anion (Scheme 4, top). The
formate peak was observed at 8.6 ppm, also consistent with
an outer-sphere anion, yet it was broadened, indicating ex-
change between LCuO2CH and LCu(DBU)+ near the NMR
timescale, as observed during catalysis. A similar rate en-
hancement for the reaction of [(LCu)2H]+ with CO2 was
observed when using a 20-fold excess of TMG. However,
much slower CO2 reactivity was observed when using
tBuTMG: the hydride was not consumed until after 5 hours
when using this base.
1
monitored in operando by H NMR spectroscopy in a setup
identical to our previously reported reactions using
LCu(MeCN)+ as a catalyst.81 The reaction progress is shown
in Figure 3. Clean and essentially complete conversion to
H[DBU]O2CH was observed within 5 hours, indicating that
[(LCu)2H]+ is an active catalytic precursor. In fact, the cata-
lytic performance, including the rate of formate production
and the species observed in solution, was essentially indistin-
guishable from reactions using LCu(MeCN)+ as a catalyst
precursor.81 The initial catalyst resting state was the base
adduct, LCu(DBU)+, and converted over time to LCuO2CH
as the reaction progressed, with a concomitant decrease in
catalytic activity. Because the calculation of the TOF and
TON values depend on the catalyst concentration, in this
case they are reported relative to the LCu(DBU)+ concentra-
tion of 2.5 mM, rather than the starting concentration of 1.25
mM for [(LCu)2H]+. This gives an initial TOF of 7.5 h-1,
equivalent to the TOF measured using LCu(MeCN)+ as a
catalyst precursor.
Stoichiometric Reactivity of [(LCu)2H]+ in the Pres-
ence of DBU and Other Bases. In the catalytic reaction,
complete conversion from [(LCu)2H]+ to LCu(DBU)+ was
evident in the first spectrum following gas addition. This
result suggested that [(LCu)2H]+ reacts with CO2 much
faster under catalytic conditions than in the stoichiometric
studies described above. The dramatically increased rate
suggested that either the presence of H2 or the presence of
the base altered the rate of CO2 insertion. Accordingly, we
investigated the effect of added base on the stoichiometric
reaction of [(LCu)2H]+ with CO2.
Scheme 3. CO2 Hydrogenation using [(LCu)2H]+
[(LCu)2H]+
(1.25 mM)
20 H(DBU)+ O2CH-
+
H2/CO2
(40 atm)
20 DBU
(50 mM)
CD3CN
83°C, 5 h
20
15
10
5
0
0
1
2
3
4
5
Time (h)
Figure 3. Progress for CO2 hydrogenation using [(LCu)2H]+ as
a catalyst precursor. Reaction conditions are indicated in
Scheme 3. TON defined as mol [HCO2 ]/mol [LCu(DBU)]+
–
as determined by NMR spectroscopy. The initial linear region
(dashed line) gives an initial TOF (TON/h) of 7.5 h-1.
5
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Scheme 4. Reactions of [(LCu)2H]+ in the presence of excess DBU
Page 6 of 13
1
2
3
4
5
6
+
+
Ph2
P
Ph2
P
Ph2
P
+ 10 equiv. DBU
< 15 min
O
O
+ CO2 (1 atm)
Cu
2
+
H
Cu
Cu DBU
H
P
Ph2
Ph2
P
P
P
Ph2
Ph2
P
P
Ph2
Ph2
LCu(DBU)+
[(LCu)2H]+
7
8
9
+
+
N2
+
Ph2 Ph2
Ph2
P
Ph2
P
Ph2
P
Ph2
P
P
P
Ph2P
+ 10-60 DBU
+
Cu DBU
Cu H
Cu
H
Cu
Cu
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
< 15 min
15 min - 20 h
P
Ph2
P
Ph2
P
Ph2
P
P
Ph2
P
Ph2
P
P
P
P
PPh2
Ph2
Ph
Ph2
Ph2
Keq = 5 x10-3
2Ph2
[(LCu)2H]+
[(L)2Cu]+
+ Cu0 + 1/2 H2
LCu(DBU)+
LCuH,
not observed
In the absence of CO2, addition of excess DBU to
[(LCu)2H]+ led to a color change from colorless to pale
yellow and the immediate appearance of LCu(DBU)+ in the
1H and 31P NMR spectra, followed by slow formation of the
bis-ligated copper(I) complex [Cu(L)2]+. The latter species
appeared in trace amounts after 10 min and steadily grew in
over the next 20 h. A small amount of hydrogen became visi-
DISCUSSION
Hydricity of [(LCu)2H]+ and Implications for CO2
Hydrogenation. For efficient catalysis, the energies of the
catalytic intermediates need to be balanced. In the context of
CO2 hydrogenation, a metal hydride must be sufficiently
reactive to transfer a hydride to CO2 but stable enough to be
regenerated efficiently using hydrogen and a base. This
thermodynamic balance can be conveniently assessed by
1
ble after 1 hr in the H NMR spectrum. A black precipitate
was also formed on the same timescale as [Cu(L)2]+. The
amount of LCu(DBU)+ that was formed and the rate of de-
composition to [Cu(L)2]+ both increased with a greater ex-
cess of added DBU, consistent with the formation of
LCu(DBU)+ and a new (unobserved) hydride species in a
rapid equilibrium, followed by slow decomposition to give
Cu0, [Cu(L)2]+, and H2. Under the assumption that the un-
observed hydride is a mononuclear species that forms in a
1:1 stoichiometry with LCu(DBU)+ (Scheme 4, bottom)
and with NMR features that overlap with those of
[(LCu)2H]+, Keq for the equilibrium in Scheme 4 was calcu-
lated to be (4.8 3.3)x10-3 from the initial 31P spectra of re-
actions using approximately 10-60 equiv DBU relative to
[(LCu)2H]+. This equilibrium constant corresponds to a
quantifying the hydricity value, or ΔG°H-, of the metal hy-
dride.23 The hydricity is defined as the free energy required
to cleave the metal-hydride bond heterolytically, forming
free hydride and a metal cation (eq 1 in Scheme 5). A lower
hydricity (corresponding to a more positive value of ΔG°H-)
indicates that the M-H species is a poorer hydride donor
and, equivalently, that the free metal ion is a better hydride
acceptor. The thermodynamic favorability of hydride trans-
fer from a metal hydride to CO2 can be evaluated by compar-
ing the hydricity of the metal species to that of formate,
which has been estimated to be 44 kcal/mol in MeCN.21-23,86
Because the transfer of a hydride to CO2 (eq 2) is the reverse
of the hydricity half-reaction for formate, the overall reac-
tion, hydride transfer from a metal to CO2 (eq 3), will be
favorable only if the hydricity of the M-H species is less than
44 kcal/mol.
ΔG° value of 3.2
0.2 kcal/mol for the formation of
LCu(DBU)+ and LCuH from [(LCu)2H]+ and DBU, as
shown in Table 2.
Scheme 5. Favorability of H– transfer from M+ to CO2
Table 2. Equilibrium Data for DBU-promoted Decomposi-
Reaction
ΔG° (kcal/mol)
tion of [(LCu)2H]+ into LCu(DBU)+ and LCuH in CD3CN.
M+ + H−
ΔGH
−
M-H
(1)
(2)
Entry
Equiv. DBU
Keq
(x10-3)
4.9
3.3
7.5
3.6
4.6
4.8 3.3
ΔG°
(kcal/mol)
3.2
−
H− + CO2
- 44
ΔG°H - 44
HCO2
M+ + HCO2
−
−
+
M-H CO2
(3)
1
2
3
4
5
10
10
20
36
54
3.4
2.9
3.4
3.2
The hydricity value also determines the favorability of re-
generating a hydride from the free metal ion using H2 and a
base, shown for the case of [(LCu)2H]+ in Scheme 6. The
reverse of this H2 heterolysis equilibrium (eq 4) can be com-
bined with the hydride-donor ability of H2 (eq 5), and the
strength of the base (eq 6) in a thermodynamic cycle yield-
ing the hydricity half-reaction for the metal hydride (eq 7).23
Average
3.2 0.2
All data are for reactions at ambient temperatures using 1.5-2.5 mM
[(LCu)2H]+ in CD3CN. See SI for details of the calculations.
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The free energy of eq 5 has been estimated to be 76 kcal/mol short distances are simply a result of the geometric con-
1
2
3
4
5
6
7
8
in MeCN.23 The equilibrium constant (Keq) for regenerating
a metal hydride in MeCN can therefore be calculated if its
hydricity and the pKa of the protonated base are known. Al-
ternatively, if the hydricity is not known, the thermodynamic
cycle in Scheme 6 can be used to calculate it from equilibra-
tion of the H2 heterolysis reaction in MeCN with a base of
known strength. The equation for the hydricity calculation in
that case is given in eq 8.87-89
straints imposed by the ligand bridging the metal centers.90-93
Whether due to these steric or electronic effects, short Cu–
Cu distances are ubiquitous in copper hydrides, making the
long Cu–Cu distance of >3 Å in [(LCu)2H]+ unique. All
structurally-characterized copper hydrides, except for
[(LCu)2H]+, contain at least one Cu–H–Cu unit with a Cu–
Cu distance of 2.89 Å or shorter,94 with most having a Cu–
Cu distance in the range of 2.4-2.6 Å.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 6. Thermodynamic cycle relating hydricity and
H2 heterolysis equilibria.
Considering that truly linear M–H–M groups are quite ra-
re,95-97 and no other examples exist among copper hydrides, it
is possible that the apparent position of the hydride in
[(LCu)2H]+ is actually the average of several disordered
structures with bent geometries. There is precedent for this
phenomenon in the structures of other dinuclear hydrides.98-
Reaction
ΔG° (kcal/mol)
[(LCu)2H]+ +
H[Base]+ + 2 MeCN
2 LCu(MeCN)+
+ Base + H2
1.364logKeq
(4)
H+ + H−
H[Base]+
H2
(5)
(6)
76
101
However, the long Cu–Cu distance in this complex prob-
Base + H+
-1.364pKa(BH+)
ably precludes the attractive interactions between the metal
centers that would lead to a bent geometry. In any case, X-
ray diffraction techniques cannot resolve this issue.
[(LCu)2H]+
+ 2 MeCN
2 LCu(MeCN)+ + H−
ΔG°H
−
(7)
(8)
ΔG°H− = 1.364logKeq - 1.364pKa(BH+) + 76
Comparison of Cationic and Neutral Copper Hy-
drides. The cationic nature of [(LCu)2H]+ is unusual among
copper hydrides and probably attenuates its hydricity. Most
known copper hydrides are neutral species and are likely
even stronger hydride donors. Three other cationic dicopper
hydrides have been reported: two are N-heterocyclic carbene
(NHC) complexes, [(IPrCu)2H]+ and the very similar
[(SIPrCu)2H]+.50,62 The third, [(dpmppm-Cu)2H]+, is a
tetraphosphine complex reported by Tanase and coworkers,
in which each copper center is coordinated to three phos-
We determined the hydricity of the dicopper hydride cati-
on [(LCu)2H]+ by measuring the equilibrium constants for
its formation from LCu(MeCN)+ via H2 heterolysis. These
reactions formed [(LCu)2H]+ only in the presence of strong
bases (pKa [BH]+ > 26). The equilibrium constants gave a
consistent value of ΔG°H- = 41.0
0.5 kcal/mol for
[(LCu)2H]+. This value is in a relatively advantageous range
for CO2 hydrogenation. It is 3 kcal/mol less positive than
that of formate, which provides sufficient but not excessive
driving force for the reaction with CO2. A greater hydricity
phines and the bridging hydride (dpmppm
= meso-
bis[(diphenylphosphinomethyl)phenylphosphino]methane
(lower ΔG°H-) would provide even more of a driving force,
which might lead to a faster reaction at room temperature.
This change might be achieved by increasing the donor
strength of the phosphines on the triphosphine ligand. How-
ever, very low values of ΔG°H- correspond to more reactive
species that can be difficult to regenerate in a catalytic cycle.
Structural Characterization of [(LCu)2H]+. Although
the chemistry of multinuclear copper hydrides is extensive,
[(LCu)2H]+ is one of only two structurally-characterized
cationic dicopper hydrides containing phosphine ligands.72
The most notable structural features in [(LCu)2H]+ are its
apparently linear Cu–H–Cu unit, the long Cu–Cu distance
within this unit, and the eclipsed arrangement of the phos-
phines around this axis. All of these features appear to be
related to the steric pressure imposed by bringing two cop-
per-triphosphine units together around one small hydride
ligand. Many multicopper(I) clusters show very short inter-
metallic distances that have been attributed to a direct attrac-
tive (cuprophilic) interaction between the copper centers or
to three-center, two-electron bonding; in other cases, the
).72 This complex is thus quite analogous to [(LCu)2H]+.
1
The Cu–H H NMR chemical shifts for all of the known cat-
ionic CuH species are upfield relative to neutral copper hy-
drides, which have positive chemical shifts.102 The average
Cu–H distances of 1.54 Å in [(LCu)2H]+ are also shorter
than the distances in (LCuH)2 (1.66 and 1.81 Å),59 con-
sistent with a more electron-deficient hydride ligand. The
Cu–H distances are intermediate between those in the other
cationic species [(dpmppm-Cu)2H]+ (1.62 Å) and
[(IPr)Cu]2H+ (1.45 Å). However, its Cu–Cu distance of
3.08 Å is significantly longer than those of [(dpmppm-
Cu)2H]+ (2.7948(7) Å) and [(IPr)Cu]2H+ (2.533(2) Å),
which have distinctly bent Cu–H–Cu angles of 120˚ and
122˚, respectively.
Interestingly, Lalic and coworkers demonstrated that the
cationic [(NHC-Cu)2H]+ complexes showed markedly low-
er reactivity towards other electrophiles than their corre-
sponding
neutral
hydrides,
[(IPr)CuH]2
and
[(SIPr)CuH]2.50 Nevertheless, all of the known cationic di-
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Page 8 of 13
copper hydrides react with CO2 to produce formate com-
substitution of DBU for the formate ligand to produce a se-
cond equivalent of LCu(DBU)+ and free formate (eq 11).
1
2
3
4
5
6
7
8
plexes. The reaction of [(dpmppm-Cu)2H]+ with CO2 was
described as slow, although it proceeded largely to comple-
tion in < 1 day, making it more rapid than the reaction of
[(LCu)2H]+ with CO2. A dicationic tricopper hydride with
diphosphine (dcpm) ligands has also been reported. Its CO2
reactivity was not specifically reported, but it is stable to pro-
tic solvents and even air.103 This stability is remarkable for a
copper hydride complex and indicates that coordination of a
third cationic copper center to a hydride ligand results in a
dramatic loss of reactivity.
[(LCu)2H]+ + DBU → LCuH + LCu(DBU)+
(9)
LCuH + CO2 → LCuO2CH
LCuO2CH + DBU → LCu(DBU)+ + –O2CH
(10)
(11)
In support of this mechanism, we observe the formation
of LCu(DBU)+ in an unfavorable equilibrium upon the addi-
tion of excess DBU to [(LCu)2H]+ in the absence of CO2.
Slow decomposition to [Cu(L)2]+ and a black precipitate is
observed on a longer timescale. We propose that the mono-
nuclear hydride that is presumably formed in this reaction is
not observed because its 31P NMR spectral features are simi-
lar to and overlap with those of [(LCu)2H]+, which remains
the major copper-containing species in solution and is struc-
turally similar to the trigonally symmetric LCuH. The 1H
NMR resonances for LCuH could be obscured by those of
[(LCu)2H]+ or of DBU, which is present in a large excess.
(LCuH)2, which could form from dimerization of LCuH, has
distinct 31P NMR resonances that are not observed. The re-
action of LCuH with LCu(DBU)+ (the reverse of eq 9) is
thus apparently faster than dimerization of LCuH. Likewise,
no precipitate or [Cu(L)2]+ is observed in the presence of
CO2, indicating that the reaction of LCuH with CO2 is faster
than its self-decomposition. Using the assumptions that
LCuH is formed in a 1:1 stoichiometry with LCu(DBU)+
and that its 31P NMR resonance is overlapped with that of
[(LCu)2H]+, we have estimated an equilibrium constant for
the DBU-promoted dissociation of [(LCu)2H]+, Kdiss ≈ (4.8
3.3) x10-3, corresponding to a ΔG° value of 3.2 0.2
kcal/mol for this reaction. These studies indicate that
LCu(DBU)+, [(LCu)2H]+, and LCuH are the copper-
containing species present at appreciable concentrations
under catalytic conditions.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The decreases in reactivity upon coordinating each addi-
tional copper center are due in part to the Lewis acidity of
the metal ion and in part to the steric shielding provided by
the additional metal-ligand fragment. The former is a ther-
modynamic effect and should be reflected in the hydricity,
while the latter is purely kinetic and would have no effect on
the hydricity. We have attempted to measure the hydricity of
(LCuH)2 for a quantitative comparison between well de-
fined and analogous cationic and neutral species, but its
thermal decomposition prevents the establishment of equi-
libria required for such thermodynamic measurements.
Stoichiometric Reactivity of [(LCu)2H]+ with CO2.
Given the hydricity of 41.0 kcal/mol measured for
[(LCu)2H]+ and the hydricity of 44 kcal/mol for formate,
the reaction of [(LCu)2H]+ with CO2 is predicted to be fa-
vorable by 3 kcal/mol. At room temperature, [(LCu)2H]+
does react with 1 atm CO2, yielding the inner-sphere formate
complex LCuO2CH. However, the reaction is quite slow,
taking ~3-4 days for complete conversion. Surprisingly, the
rate of reaction is dramatically increased in the presence of
excess base. In the presence of 10 equivalents of DBU or
TMG, [(LCu)2H]+ reacts with CO2 in less than 15 min,
yielding two equivalents of the base adduct LCu(DBU)+ or
LCu(TMG)+ and an outer-sphere formate anion. In the
Implications for the Catalytic Mechanism. We previ-
ously proposed a simple mechanism for catalysis involving
(a) heterolytic H2 cleavage by the copper-base adduct
LCu(DBU)+ to form an unobserved copper hydride and
protonated base, (b) CO2 insertion into the copper-hydride
bond to form a formate complex, and (c) displacement of the
formate by DBU to regenerate LCu(DBU)+.81 Two of the
catalytic intermediates have been directly observed through
our in operando NMR studies. Whether LCu(MeCN)+ or
[(LCu)2H]+ was used as the catalyst precursor, the base ad-
duct LCu(DBU)+ was the only copper-containing species
observed in the initial spectra. With greater reaction time and
conversion, LCuO2CH became the predominant copper
species. These two intermediates are in rapid equilibrium at
the reaction temperature of 83 °C. The rapid exchange be-
tween DBU and formate keeps the catalyst active and solu-
t
presence of the bulky base BuTMG, the reaction rate is in-
creased to a lesser degree, reaching completion after 5 h. We
previously showed that DBU and TMG bind readily to the
t
copper centers, whereas BuTMG binds only very weakly.81
Because tBuTMG is a much stronger base but a much weaker
ligand, these results indicate that the increased CO2 reactivi-
ty with DBU and TMG is related to the coordinating nature
of these bases, rather than their basicity.
A plausible explanation for the enhanced CO2 reactivity in
the presence of DBU is that the base binds to one of the cop-
per centers in [(LCu)2H]+, breaking up the dinuclear struc-
ture and forming both LCu(DBU)+ and a more active, mon-
onuclear copper hydride species, LCuH (eq 9). LCuH re-
acts with CO2 much faster than [(LCu)2H]+ does, forming
LCuO2CH as the initial product (eq 10), followed by rapid
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Journal of the American Chemical Society
ble, whereas the formate complex is insoluble in MeCN on
its own.
The remaining steps in our previously proposed catalytic
Scheme 7. Potential Catalytic Mechanisms.
1
2
3
4
5
6
7
8
LCu(MeCN)+ + DBU
- MeCN + MeCN
LCu(DBU)+
(a)
cycle involve the formation and CO2 reactivity of a transient
copper hydride. Any of the three copper hydrides identified
in this study could plausibly be the intermediate that reacts
with CO2. [(LCu)2H]+ can be formed under H2 and is ther-
modynamically well suited to CO2 hydrogenation. The rate,
conversion, and species observed in solution for reactions
using [(LCu)2H]+ as a precatalyst were identical to those in
our previous studies, where LCu(MeCN)+ was used as the
catalyst precursor.81 The identical performance of the two
catalyst precursors is consistent with [(LCu)2H]+ being an
accessible intermediate but does not necessarily indicate that
it is the CO2-reactive hydride. On the contrary, the stoichi-
ometric reaction of [(LCu)2H]+ with CO2 is too slow to be
catalytically relevant, as discussed above. Instead, our stoi-
chiometric studies suggest that binding of DBU to the di-
copper complex forms an unobserved mononuclear hydride,
LCuH, that is much more reactive.
H2
-O2CH
HDBU+
DBU
9
+ LCu(DBU)+
- LCu(DBU)+
LCuHCuL+
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
LCuH
LCuO2CH
+ DBU
CO2
(b)
LCu(MeCN)+ + DBU
- MeCN + MeCN
LCu(DBU)+
-O2CH
H2
DBU
DBU
Goeden et al. previously prepared the dimeric hydride
complex (LCuH)2 and showed that it reacted with CO2,59
making it another potential intermediate. However,
(LCuH)2 presumably forms from dimerization of LCuH,
and there is no reason to suspect that dimerization assists its
reaction with CO2, or that it occurs at all under catalytic
conditions. On the contrary, mononuclear copper hydrides
are often believed to be the active intermediates in solution
even when the structurally characterized complexes are pol-
ynuclear.64-67 Notably, Stryker and coworkers studied
(LCuH)2 as a catalyst for the hydrogenation of ketones and
found a half-order dependence on [(LCuH)2], indicating
that the dimer dissociated to form a more reactive mononu-
clear hydride.65 Moreover, our equilibrium studies suggest
that recombination of LCuH with LCu(DBU)+ to make
[(LCu)2H]+ outcompetes the dimerization of LCuH. The
factors disfavoring dimerization will be especially significant
under catalytic conditions, where the concentration of
LCuH is very small and vastly exceeded by the concentration
of LCu(DBU)+. In addition, we have found that (LCuH)2 is
soluble in benzene and THF but insoluble in MeCN; in con-
trast, no catalytic CO2 hydrogenation is observed in benzene
or THF, but catalysis occurs in MeCN. Therefore, we believe
it is unlikely that (LCuH)2 is an active intermediate, or even
present in appreciable concentrations. For these reasons, we
propose that the CO2-reactive hydride is LCuH.
LCu(H2)+
LCu(DBU)+
LCuO2CH
HDBU+
CO2
LCuH
LCuHCuL+
LCu(DBU)+
DBU
Pathways for H2 Activation and Hydride Formation.
The remaining mechanistic questions center around the
pathways for forming LCuH from LCu(DBU)+ and H2. Our
previous studies suggested that the coordinated base within
LCu(DBU)+ remains bound and is intimately involved in the
H2 activation and hydride formation step(s).81 Two plausible
scenarios consistent with the available data are shown in
Scheme 7, where the key distinction is whether [(LCu)2H]+
lies on the catalytic cycle, or whether it is formed in an un-
productive, off-cycle equilibrium. Scheme 7(a) shows a sim-
ple modification to our originally proposed mechanism,
where LCuH is formed in a bimolecular reaction of H2 with
LCu(DBU)+. In this case, the formation of [(LCu)2H]+ is an
off-cycle equilibrium caused by binding of the resting state
LCu(DBU)+ to the transient hydride. In Scheme 7(b),
[(LCu)2H]+ is formed first from H2 heterolysis and subse-
quently reacts with DBU to yield LCuH and LCu(DBU)+. In
both cases, coordination of DBU to [(LCu)2H]+ is essential
to generating the active hydride and promoting catalysis. In
Scheme 7(a), [(LCu)2H]+ is off-pathway, and DBU binding
pushes the catalyst back on-cycle. In Scheme 7(b),
9
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[(LCu)2H]+ is an essential intermediate, but DBU binding is
Notes
1
2
3
4
5
6
7
8
The authors declare no competing financial interest
still necessary to pushing the cycle forward. We cannot dis-
tinguish between the two mechanistic possibilities in
Scheme 7 without a more detailed knowledge of the mecha-
nism of H2 activation and hydride formation. Kinetic studies
that address this issue are underway.
ACKNOWLEDGMENT
The authors thank Prof. James Mayer for helpful discussions.
This material is based upon work supported by the U.S. De-
partment of Energy, Office of Science, Office of Basic Energy
Sciences, Division of Chemical Sciences, Geosciences & Biosci-
ences. Pacific Northwest National Laboratory is operated by
Battelle for the US Department of Energy.
SUMMARY AND CONCLUSIONS
To identify potential intermediates in the copper-
catalyzed hydrogenation of CO2, we have synthesized and
characterized two isolable copper hydrides and uncovered
evidence for the existence of one other transient species, all
using the same triphosphine ligand. All of these complexes
are potential catalytic intermediates. We have measured the
thermodynamic hydricity of one of these hydrides,
[(LCu)2H]+, which is the first such measurement reported
for a copper hydride. The value, ΔG°H- = 41.0 kcal/mol, in-
dicates that [(LCu)2H]+ is a powerful hydride donor and
energetically well suited to CO2 hydrogenation. The cationic
character of [(LCu)2H]+ is somewhat unusual for copper
hydrides. Neutral copper hydrides are more common and are
expected to be even stronger donors. The thermodynamic
data therefore suggest that copper hydrides have been largely
overlooked as potential catalysts for CO2 hydrogenation.
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Despite the thermodynamic favorability for the reaction of
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appears to be the reactive intermediate under catalytic condi-
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ASSOCIATED CONTENT
Experimental procedures, characterization of compounds,
NMR spectra, reaction data for equilibrium studies. This mate-
rial is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*aaron.appel@pnnl.gov
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158-165.
(102) For instance, the Cu–H chemical shift of (LCuH)2 is 1.83 ppm,59
while for [(LCu)2H]+ it is -1.46 ppm. As with the Cu-H distances, the shift
for [(LCu)2H]+ is between those of [(dpmppm-Cu)2H]+ (δ 0.16 ppm)72
and [(IPr)Cu]2H+ (δ -4.13 ppm). 62 The shift of [(SIPr)Cu]2H+ (δ -4.52
ppm) is similar to that of [(IPr)Cu]2H+.50 In general, it appears that the
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+ H2
+ [Base]
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CO2
LCu O2CH
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LCu
slow
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-
+ 2 H-
+ H-
[Base]
LCu
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H
CO2
LCu
CuL
LCu H
LCu O2CH
fast
H
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