Iridium and ruthenium complexes of N-heterocyclic carbene- and pyridinol-derived chelates as catalysts for aqueous carbon dioxide hydrogenation and formic acid dehydrogenation: The role of the alkali metal

Article abstract of DOI:10.1021/acs.organomet.6b00806

Hydrogenation reactions can be used to store energy in chemical bonds, and if these reactions are reversible, that energy can be released on demand. Some of the most effective transition metal catalysts for CO₂ hydrogenation have featured pyridin-2-ol-based ligands (e.g., 6,6'-dihydroxybipyridine (6,6'-dhbp)) for both their proton-responsive features and for metal-ligand bifunctional catalysis. We aimed to compare bidentate pyridin-2-ol based ligands with a new scaffold featuring an N-heterocyclic carbene (NHC) bound to pyridin-2-ol. Toward this aim, we have synthesized a series of [CpIr(NHC-py^OR)Cl]OTf complexes where R = ^tBu (1), H (2), or Me (3). For comparison, we tested analogous bipyderived iridium complexes as catalysts, specifically [CpIr(6,6'-dxbp)Cl]OTf, where x = hydroxy (4_Ir) or methoxy (5_Ir); 4_Ir was reported previously, but 5_Ir is new. The analogous ruthenium complexes were also tested using [(η⁶-cymene)Ru(6,6'-dxbp)Cl]OTf, where x = hydroxy (4_Ru) or methoxy (5_Ru); 4_Ru and 5_Ru were both reported previously. All new complexes were fully characterized by spectroscopic and analytical methods and by single-crystal X-ray diffraction for 1, 2, 3, 5_Ir, and for two [Ag(NHC-py^OR)₂]OTf complexes 6 (R = ^tBu) and 7 (R = Me). The aqueous catalytic studies of both CO₂ hydrogenation and formic acid dehydrogenation were performed with catalysts 1-5. In general, NHC-py^OR complexes 1-3 were modest precatalysts for both reactions. NHC complexes 1-3 all underwent transformations under basic CO₂ hydrogenation conditions, and for 3, we trapped a product of its transformation, 3^SP, which we characterized crystallographically., we trapped a product of its transformation, 3^SP, which we characterized crystallographically.. For CO₂ hydrogenation with base and dxbp-based catalysts, we observed that x = hydroxy (4_Ir) is 5-8 times more active than x = methoxy (5_Ir). Notably, ruthenium complex 4_Ru showed 95% of the activity of 4_Ir. For formic acid dehydrogenation, the trends were quite different with catalytic activity showing 4_Ir Z> 4_Ru and 4_Ir ≈ 5_Ir Secondary coordination sphere effects are important under basic hydrogenation conditions where the OH groups of 6,6'-dhbp are deprotonated and alkali metals can bind and help to activate CO₂. Computational DFT studies have confirmed these trends and have been used to study the mechanisms of both CO₂ hydrogenation and formic acid dehydrogenation.

Full text of DOI:10.1021/acs.organomet.6b00806

Article
pubs.acs.org/Organometallics
Iridium and Ruthenium Complexes of N‑Heterocyclic Carbene- and
Pyridinol-Derived Chelates as Catalysts for Aqueous Carbon Dioxide
Hydrogenation and Formic Acid Dehydrogenation: The Role of the
Alkali Metal
†
†
†
‡
†
Sopheavy Siek, Dalton B. Burks, Deidra L. Gerlach, Guangchao Liang, Jamie M. Tesh,
†
†
†
§
Courtney R. Thompson, Fengrui Qu, Jennifer E. Shankwitz, Robert M. Vasquez,
Nicole Chambers, Gregory J. Szulczewski, Douglas B. Grotjahn,* Charles Edwin Webster,*
and Elizabeth T. Papish*
†
†
,§
,‡
,
†
†
Department of Chemistry, The University of Alabama, Box 870336, Tuscaloosa, Alabama 35487-0336, United States
Department of Chemistry, Mississippi State University, Mississippi State, Mississippi 39762, United States
‡
§
Department of Chemistry and Biochemistry, San Diego State University, 5500 Campanile Drive, San Diego, California 92182-1030,
United States
*
S Supporting Information
ABSTRACT: Hydrogenation reactions can be used to store energy in
chemical bonds, and if these reactions are reversible, that energy can be
released on demand. Some of the most effective transition metal catalysts for
CO hydrogenation have featured pyridin-2-ol-based ligands (e.g., 6,6′-
2
dihydroxybipyridine (6,6′-dhbp)) for both their proton-responsive features
and for metal−ligand bifunctional catalysis. We aimed to compare bidentate
pyridin-2-ol based ligands with a new scaffold featuring an N-heterocyclic
carbene (NHC) bound to pyridin-2-ol. Toward this aim, we have
OR
synthesized a series of [Cp*Ir(NHC-py )Cl]OTf complexes where R =
t
Bu (1), H (2), or Me (3). For comparison, we tested analogous bipy-
derived iridium complexes as catalysts, specifically [Cp*Ir(6,6′-dxbp)Cl]OTf, where x = hydroxy (4 ) or methoxy (5 ); 4 was
Ir
Ir
Ir
6
reported previously, but 5 is new. The analogous ruthenium complexes were also tested using [(η -cymene)Ru(6,6′-
Ir
dxbp)Cl]OTf, where x = hydroxy (4 ) or methoxy (5 ); 4 and 5 were both reported previously. All new complexes were
Ru
Ru
Ru
Ru
fully characterized by spectroscopic and analytical methods and by single-crystal X-ray diffraction for 1, 2, 3, 5 , and for two
Ir
OR
t
[
Ag(NHC-py ) ]OTf complexes 6 (R = Bu) and 7 (R = Me). The aqueous catalytic studies of both CO hydrogenation and
2
2
OR
formic acid dehydrogenation were performed with catalysts 1−5. In general, NHC-py complexes 1−3 were modest
precatalysts for both reactions. NHC complexes 1−3 all underwent transformations under basic CO hydrogenation conditions,
2
and for 3, we trapped a product of its transformation, 3 , which we characterized crystallographically. For CO hydrogenation
SP
2
with base and dxbp-based catalysts, we observed that x = hydroxy (4 ) is 5−8 times more active than x = methoxy (5 ). Notably,
Ir
Ir
ruthenium complex 4 showed 95% of the activity of 4 . For formic acid dehydrogenation, the trends were quite different with
Ru
Ir
catalytic activity showing 4 ≫ 4 and 4 ≈ 5 . Secondary coordination sphere effects are important under basic hydrogenation
Ir
Ru
Ir
Ir
conditions where the OH groups of 6,6′-dhbp are deprotonated and alkali metals can bind and help to activate CO .
2
Computational DFT studies have confirmed these trends and have been used to study the mechanisms of both CO
2
hydrogenation and formic acid dehydrogenation.
INTRODUCTION
of the ligands and change from L (6,6′-dhbp, neutral) to LX
2
■
(
monoanionic) and to X (dianionic) can explain the catalytic
2
Ligands that contain protic functional groups near the metal
center can accelerate proton transfer events in many different
types of reactions. Our research groups
pioneered the use of 6,6′-dihydroxybipyridine (6,6′-dhbp)
ligands (Chart 1) for the formation of metal complexes that
catalyze both oxidative and reductive reactions. The hydroxy
groups near the metal center lead to a change in ligand charge
upon deprotonation, and thereby, the electron density at the
metal is altered. The ability to instantly modulate the character
rate enhancements that are seen upon deprotonation of the
2
1
−4
5,6
metal complexes. Under oxidizing conditions, our group and
and others have
2
Ir
others have shown that both iridium (e.g., 4 and its Ir-aqua
7
,8
analogue; Chart 1) and copper complexes of 6,6′-dhbp
4
undergo decomposition reactions in solution. Furthermore, we
Received: October 22, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00806
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Chart 1. Bidentate Ligands with Proximal Protic Functional
Groups and Similar Metal Complexes in the Literature
these new metal complexes toward the catalysis of CO2
hydrogenation and the reverse dehydrogenation reaction.
RESULTS AND DISCUSSION
Synthesis and Characterization of the Ligands and
■
the Metal Complexes. The N-heterocyclic carbene-pyridinol-
OR
derived ligands (NHC-py ) are easily synthesized as the OR =
t
t
O Bu, OH, or OMe derivatives wherein O Bu serves as a
protected form for making the OH derivatives. After chelation
with the desired metal, moderate heat can drive the
deprotection of the t-butyl group to produce the hydroxy-
substituted ligand with the elimination of isobutene. OR =
OMe is a control for comparison of hydrogen bond donor
(
OH) vs hydrogen bond acceptor (OMe) properties of the
ligand.
Treating 2,6-difluoropyridine with excess sodium alkoxide
t
42−44
(
NaO Bu or NaOMe)
afforded 2-fluoro-6-alkoxypyridines
have also shown that Cu(II) complexes of 6,6′-dhbp undergo
with replacement of only one fluoride (Scheme 1). Further
ligand loss at both high (>13) and low (<4) pH in aqueous
4
solution. Therefore, we had a strong interest in affixing a
Scheme 1. Synthesis of the Imidazolium Precursors to the
OR
strong donor to the pyridinol ring to counteract the labile
nature of the 6,6′-dhbp ligand.
NHC-py Ligands (R = tBu, Me)
Carbon dioxide hydrogenation is studied here because 6,6′-
dhbp complexes and other pyridinol-based complexes of Ir(III)
are especially effective at promoting this reaction and
6
,9−11
(de)hydrogenation in general.
Other highly efficient
CO2 hydrogenation homogeneous catalysts include several
12
different iridium pincer complexes from Nozaki; Brookhart
1
3
14
and Meyer; Bernskoetter, Hazari, and Palmore; and
reaction with sodium imidazolate replaced the remaining
1
5
others. For formic acid dehydrogenation, a reusable highly
active iridium catalyst with a P,N ligand was reported by
fluoride to yield 2-alkoxy-6-(N-imidazolyl)pyridines (Im-
OR
py ). Alkylation with methyl triflate generated the imidazo-
1
6
Me
OR
Williams; other groups have performed formic acid
dehydrogenation in the course of methanol dehydrogen-
lium precursors to the NHC ligands (Im -py )OTf.
The carbene ligand was conveniently formed by deprotona-
tion of the imidazolium salt precursor. This deprotonation is
readily achieved with mild base, and the acidity of the C-2
1
7,18
ation.
Most of these (de)hydrogenation catalysts perform
+
ionic hydrogenation, which is the sequential transfer of H and
H ; other transition metal-based catalysts for ionic hydro-
genation have been reported.
−
1
proton is evident by observing H/D exchange in the H NMR
19−27
spectra in deuterated protic solvent. Silver bis(carbene)
complexes (6 and 7) were formed quantitatively through
The iridium(III) complexes based on 6,6′-dhbp and related
ligands are noteworthy for how rapidly they catalyze both CO
reaction of the imidazolium salts with Ag O and NaOH
2
45
2
6
,11
hydrogenation and the reverse reaction.
The rate accel-
(Scheme 2) (see Chart 2 for our numbering scheme). The
erations seen with 6,6′-dhbp have been attributed to electron
Scheme 2. Synthesis of NHC-pyOR Metal Complexes 1−3, 6,
and 7
donor ability, which is enhanced upon deprotonation and
+
proximal OH/O- groups facilitating the transfer of H via a
1
,28−31
metal−ligand bifunctional catalysis mechanism.
How-
ever, herein we have found that in some cases methoxy
substituents are as effective as hydroxy groups at enhancing
dehydrogenation rates. This is evident from comparing 6,6′-
dhbp and 6,6′-dimethoxybipyridine (6,6′-dmbp) complexes of
iridium and ruthenium. In addition, importantly, the role of
alkali metals in hydrogenation reactions is elucidated for the
first time with the 6,6′-dhbp scaffold.
N-Heterocyclic carbene (NHC) ligands are strong sigma
3
2−34
donors that form stable metal−carbon bonds.
NHCs have
been used in chelates before with pyridine rings (e.g., A; Chart
3
5−37
crystal structures of [Ag(NHC-pyOtBu)2]OTf (6) and [Ag-
1),
including in Re, Mn, and Ni catalysts for electro-
3
8−41
OMe
chemical CO reduction
but never with pyridinol as a
(NHC-py )2]OTf (7) (Figure 1) show that the two NHC-
2
OR
protic ligand. We reasoned that a bidentate ligand containing an
NHC and a pyridinol ring could offer a protic group on the
metal center in the presence of a strong donor ligand (Chart 1,
py ligands are monodentate with the pyridinol nitrogen
atoms not coordinated. The silver ions are two-coordinate with
two carbene ligands arranged with approximately linear C−
Ag−C angles (172−174°). Interestingly, in the crystal phase,
the orientation of the unbound pyridines of the two ligands
differs in the two structures. For 6, the pyridine rings orient to
OH
NHC-py ). These ligands could be tuned by deprotonation
(
producing OH/O- variants) while also containing strong metal
OH
carbon bonds. Herein, we report the NHC-py ligand and
related ethers, the Ir(III) complexes thereof, and the use of
t
one side of the Ag, and the bulky Bu groups are both above the
B
DOI: 10.1021/acs.organomet.6b00806
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
a
respectively. The hydroxyl derivative (NHC-pyOH in 2) was
synthesized by deprotection of complex 1 via superheating a
CH Cl solution or reflux of a MeCN solution. Similar to the
Chart 2. Numbering Scheme for the Compounds Herein
2
2
synthesis of 1 and 3, the complex [Cp*IrCl(6,6′-dmbp)]OTf
(
5 ) was obtained by adding two equivalents of free 6,6′-dmbp
Ir
ligand and AgOTf to one equivalent of [Cp*IrCl ] . All of
2
2
these complexes are yellow and readily recrystallize by
evaporation of acetonitrile or by diffusion with diethyl ether.
The crystal structures of these complexes are shown in Figure 2
for 1 and 2 and Figure 3 for 3 and 5Ir.
a
Note that ′ designates the corresponding metal-aqua complex; e.g., 3′
OMe 2+
=
[Cp*Ir(OH )(NHC-py )] as formed in solution by adding
2
AgOTf to 3.
Figure 2. ORTEP diagrams of the cations of 1 (top) and 2 (bottom)
with hydrogen atoms and counteranions omitted for clarity. These
structures are oriented such that the chloride is forward from the plane
OR
of the NHC-py ligand. Structural parameters are included in the
Supporting Information.
Complexes 2, 3, and 5 identically pack in the P2 /c space
Ir
1
group such that the chloride of one cation orients above the
Cp* ring of an adjacent cation forming chains along the a axis
with similar distances of 3.438, 3.678, and 3.646 Å, respectively,
from Cl1 to the centroid of Cp*. With the bulky tert-butyl
group, complex 1 packs in the P-1 space group. The 6,6′-dmbp
Figure 1. ORTEP diagrams of cations of 6 (top) and 7 (bottom) with
hydrogen atoms and counteranions omitted for clarity. Structural
parameters are included in the Supporting Information.
general plane of the NHC and Py rings. The packing of these
complexes is dominated by the accommodation of the bulky
complex, [(p-cym)RuCl(6,6′-dmbp)]Cl (5 ), was synthesized
Ru
1
previously by us, but the single-crystal structure was obtained
t
Bu groups. For 7, the methoxy-substituted pyridinol rings are
for this paper and belongs to space group P2 /n. Selected
1
oriented away from each other with out-of-plane rotation of the
bond between the NHC and Py rings observed such that one
pyridine is pointed in toward the Ag and one is pointed away
from the Ag. With little steric hindrance, the crystal packing of
these complexes is predominantly due to π-stacking between
NHC moieties.
dimensions are included in Table 1 for 1−3, 5 , and 5 and
Ir
Ru
previously reported complexes [Cp*IrCl(6,6′-dhbp)]Cl (4 )
Ir
1
,2
and [(p-cym)RuCl(6,6′-dhbp)]Cl (4 ). These complexes all
display similar coordination geometries, bond lengths, and
angles.
Ru
Complex 2 displays a strong, linear hydrogen bond between
the OH group and the O- of the triflate counterion (O1 to O4
= 2.638(3) Å, O1−H1−O4 = 173(4)°). Similarly, hydrogen
bonding interactions that range from strong to weak have been
Transmetalation of the silver bis(carbene) complexes 6 and 7
with the iridium dimer [Cp*IrCl ] and one equivalent of
2
2
Me
AgOTf afforded the chelate complexes [Cp*IrCl(NHC -
py )]OTf complexes 1 and 3 with R = Bu and Me,
OR
t
4,46
1
observed in 6,6′-dhbp complexes of Cu(II),
Ru(II), and
C
DOI: 10.1021/acs.organomet.6b00806
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
chelate rings, and the coordination environments around Ir(III)
are similar. Contrary to what is expected from the van der Waal
radii of the donor atoms (N < C), the Ir−C distances are
slightly shorter than the Ir−N distances (Ir−N = 2.10(1) Å
avg
and Ir−C = 2.04(1) Å for 1−3). This can be rationalized in
avg
terms of the strong σ bond donor character of the carbene, and
also, Ir to C back bonding is frequently present in NHC
32
complexes. One would expect the Ir−N distance to increase
as the bulk of the OR group increases, and this is true for OMe
t
and O Bu species 3 and 1 [Ir−N 2.07(1) and 2.144(1),
respectively]. Note that, to compensate, the Ir−C distance
adjusts so that the sum of the Ir−N and Ir−C distances remains
47
constant. OH species 2 shows Ir−N and Ir−C distances
between the other two, perhaps because of hydrogen bonding
of triflate to the OH, yet the sum of the Ir−N and Ir−C
distances remains equal to those of 1 and 3.
1
Characterization of complexes 1−3 by H NMR spectrosco-
py displays the shielding effect on the protons of the pyridine
ring with removal of the protecting group from the ligand
OR
NHC-py to form the hydroxy group (Figure S45). All of the
protons of the pyridine ring are shifted upfield by 0.1−0.2 ppm
for complex 2 compared to those of complex 1 with the
t
hydroxy protected by Bu and by 0.05−0.15 ppm comparing
complex 2 to 3 (hydroxy vs methoxy).
Complex 2 is protic and has a pK value of 4.9(1), which is
a
similar to the values of 4.6 and 5 previously measured for 4Ir
and 4Ru with the diprotic 6,6′-dhbp ligand bound to Ir(III) and
1
,2
Ru(II), respectively (Table 2).
As is typically seen,
Table 2. Thermodynamic Acidity (pK ) Values for the Protic
a
Metal Complexes (2, 4 , 4 ) Studied Herein and the Free
Ligand 6,6′-dhbp
Ru
Ir
Figure 3. ORTEP diagrams of the cations 3 (top) and 5 (bottom)
Ir
with hydrogen atoms and counteranions omitted for clarity. These
structures are oriented such that the chloride is forward from the plane
of the bidentate ligand. Structural parameters are included in the
Supporting Information.
compound
pKa
ref
2
4.9(1)
4.6(1)
5
this work
4Ir
2
1
4Ru
2
Ir(III). Inspecting the C−N and C−O distances near the OH/
6
,6′-dhbp
8.5
48
OH
OR group shows that these values are similar in the NHC-py
and NHC-py
OtBu
ligands of 1 and 2 as well as in 6,6′-dhbp and
6
,6′-dmbp complexes (4 and 5 and others in the literature
complexation to the metal lowers the pK value by several
Ir
Ir
a
48
that are not deprotonated).
units (6,6′-dhbp has a pK value of ∼8.5). The yellow Ir(III)
a
The bite angle of the two bidentate ligand types, NHC-pyOR
and dmbp, is nearly identical where the bite angles are within
the range of 75−76° for the N−Ir−C or N−Ir−N angles,
respectively. It is not surprising that the bite angle is retained
for the two types of ligands because both form five-membered
complexes 2 and 4 have similar absorption features in the
Ir
UV−visible spectrum, as seen in Figure 4, with the majority of
absorption occurring in the UV region by π to π* transitions of
the aromatic groups of the ligands and weaker charge transfer
(CT) and d-d transitions between 300 and 400 nm. The
OR
+
Table 1. Selected Bond Lengths and Angles of the Complexes [Cp*IrCl(NHC-py )]OTf (1−3), [Cp*IrCl(6,6′-dxbp)] (4 ,
Ir
+
5
Ir
), and [(p-cym)RuCl(dxbp)] (4 , 5 )
Ru Ru
bond lengths (Å)
a
compound
bite angle (deg)
Ir1−Cl1
2.416(4)
Ir1−N3
2.144(1)
Ir1−C4
1.997(2)
Ir1−N1 or N2
1
2
76.00(5)
75.82(9)
2.4143(7)
2.389(7) 2.43(1)
2.415(7)
2.116(2)
2.014(2)
b
3
76.1(6) 75(1)
75.92(8)
2.07(1) 2.07(2)
2.06(2) 2.09(3)
c
4Ir
2.10 (2)
5Ir
75.71(6)
2.399(5)
2.117(2) 2.108(1)
Ru1−N1 or N2
2.125(1) 2.116(1)
2.123(2)2.119(2)
Ru1−Cl1
2.3899(4)
Ru1−N3
Ru1−C4
c
4Ru
76.52(5)
76.22(9)
5Ru
2.3925(9)
a
For the bidentate ligand: C, N or N, N. Positional disorder in the bidentate ligand (NHCMe-pyOMe) for 3 leads to two sets of metrical parameters.
b
c
1,2
The second set is shown in italics. These crystal structures (for 4 and 4 ) were previously reported.
Ir
Ru
D
DOI: 10.1021/acs.organomet.6b00806
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Catalytic CO Hydrogenation: Enhancing Activity. The
2
OR
new NHC-py complexes along with bipy complexes were
tested for the catalysis of the hydrogenation of CO to formate
2
(
Table 3). Here, we only report the TON value after 18 h for
a
Table 3. Catalytic Hydrogenation of CO with 1−5
2
catalyst used as is
Cl− removed in situ
TOF
(h
10
TOF
−1
−1
entry
catalyst
TON
)
TON
(h
)
1
2
3
4
5
6
7
8
[Cp*IrCl2]2
180 (20)
1090 (20)
910 (60)
1
60.6
51
740 (30)
860 (100)
2090 (60)
2400 (70)
290 (60)
41
2
48
116
130
16
3
2020 (90)
2270 (90)
112
Figure 4. Aqueous UV/visible absorption spectra of the yellow
complexes 1−3, 4 , and 5 showing absorption primarily in the UV
4Ir
5Ir
4Ru
5Ru
126
29
Ir
Ir
b
410 (220)
region.
1070 (20)
59.4
49
2270 (100)
1220 (30)
126
b
890 (330)
67.8
electronic states of complexes 1 and 3 are shown to be nearly
identical by UV−vis, whereas deprotection of the O Bu to give
OH in complex 2 introduces a second CT absorption feature at
a
t
Conditions: The reactions were performed in 25 mL of an aqueous
solution of 0.3 mM catalyst and 1 M NaHCO at 115 °C and 300 psig
of H /CO (1:1). All TON are calculated after 18 h and are an average
of at least 3 experiments (estimated standard deviations are in
parentheses). TOF values are for the full 18 h period (TOF = TON/
1
the measurements. Entries 3, 5, and 7 are performed with OH-
containing catalysts (2 with NHC-py and 4Ir and 4Ru with 6,6′-
dhbp). The other entries used alkoxy-substituted catalysts (1 with
3
2
2
∼
355 nm comparable to the CT absorption features observed
for complex 4 . There is little change in the absorption features
Ir
of the halide bound complex 3 vs the aqua bound complex 3′
8 h). Data are reported to at most three significant figures based on
OR
(
Figure S50), suggesting that the Ir(NHC-py ) complexes
OH
have similar electronic properties regardless of whether a halide
or aqua ligand fills the coordination sphere.
OtBu
OMe
NHC-py , 3 with NHC-py , and 5Ir and 5Ru with 6,6′-dmbp).
The similarity of the electronic states of these complexes is
again found through the electrochemical investigation of
nonaqueous solutions by cyclic voltammetry (Figures S46−
S49). For all of the complexes, 1, 2, 3, 4 , and 5 , an initial
b
Gave highly variable TON; may be forming nanoparticles.
catalysts 1−5, but in the SI, we report the pressure drop as a
function of time for these catalysts (Figures S59−79). The rate
of pressure drop is approximately constant over the course of
the 18 h for all of these catalysts (Figure S80). A control
reaction was performed with [Cp*IrCl ] (0.15 mM of dimer
or 0.30 mM in iridium) tested as a catalyst, and this produced
Ir
Ir
irreversible reduction event is observed below −1 V (all values
are reported vs SCE) forming some complex that has a return
oxidation event at a more positive potential, which is only
observed if the initial complex first undergoes the irreversible
reduction event. Aprotic complexes 1 and 3 have nearly
identical irreversible reduction events at approximately −1.60
V; upon scanning to more positive potentials, new irreversible
oxidation events at −0.90 V in MeCN were seen (Figure S46).
2 2
0
.686 mmol of formate corresponding to 180 TON (entry 1,
Table 3). Hence, [Cp*IrCl ] is 11.2−12.6 times less active
than the most active catalysts tested (3 and 4 ), implicating a
significant role of the organic ligands. No other CO reduction
products besides formate were observed in these reactions.
2 2
Ir
Complex 5 with a 6,6′-dmbp ligand exhibits similar behavior
2
Ir
with initial reduction at −1.25 V and, on return to more
positive potential, a new oxidation at −0.83 V (Figure S47).
However, when the OR group of the pyridine is deprotected to
form the protic hydroxyl group, as in the case of complexes 2
OR
For the new NHC-py derivatives, the order of activity is 3
>
1 ≈ 2, with 3 being almost twice as active as either 1 or 2.
Compounds 1−3 are best considered to be precatalysts as they
all undergo significant transformations in solution (see below).
The trend is different with complexes of the dxbp-type
ligands. The hydroxy dxbp complexes (4 or 4 ) are more
active than the methoxy complexes (5Ir or 5 ). These
hydrogenation experiments were run in 1 M NaHCO , under
basic conditions (pH 8.5), where the hydroxy groups are
deprotonated to give the more electron-donating oxyanions
that are proposed to enhance catalysis. Comparing metals in
literature examples, iridium complexes are generally more active
than ruthenium complexes, and this trend holds true here for
dhbp complexes (4Ir > 4Ru) but not for the dmbp analogues
(5Ir < 5Ru). However, the results for both 5Ir and 5Ru were
inconsistent and suggest catalyst modification (further dis-
cussed in the SI).
Comparing monodentate ligands (chloride, triflate, water) at
the catalyst active site, it has been shown that the presence of
halide can poison a (de)hydrogenation catalyst, presumably by
and 4 , the return oxidation events occur at a much larger
Ir
difference in potential than for the complexes in which the
hydroxyl group is protected. Because of the lower solubility of
Ir
Ru
complex 4 , the CV was performed in DMF (Figure S48). The
Ru
Ir
initial irreversible reduction event occurred at −1.60 V in
3
MeCN and −1.55 V in DMF for complex 2 and at −1.40 V in
DMF for complex 4 , whereas the return oxidation events
Ir
49
occur at 0.36, 0.33, and 0.95 V, respectively. The corresponding
Δ between the irreversible reduction and oxidation events are
p
1
.96 V in MeCN, 1.88 V in DMF for complex 2, and 2.35 V for
complex 4 . Recently, Re complexes with aprotic NHC-py
Ir
ligands have been reported, which similarly show a large
difference in the initial irreversible reduction event followed by
a “delayed” irreversible oxidation event; the reduction was
·−
38
ascribed to pyridyl ligand forming a radical anion (Py ). In
the case of both complex 2 and 4 , the addition of base forced
Ir
reduction to occur at more negative potentials, as could be
expected from formation of an alkoxide substituent and
diminished the current of the return irreversible oxidation
event (Figure S49).
17
favorable binding to the active site. Therefore, we removed
the chloride by adding silver triflate to the catalyst solution; the
precipitated silver chloride was easily removed by filtration. The
E
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Organometallics
Article
filtrate was then used directly for catalysis without isolation (in
situ halide removal in Table 3). For precatalysts 1−3, the
removal of the chloride does not increase the catalytic activity;
silver-triflate-treated 1 and 2 even decrease slightly in activity.
However, when treated, 4Ru (with 6,6′-dhbp bound to Ru)
shows a large increase in activity, which more than doubles with
Scheme 3. (top) Formation of Cyclometalated 3″ and
Further Products under Hydrogenation Conditions;
(bottom) Product 3SP Results when 3 is Treated with
Dichloroethane and Base
the removal of the chlorides (to 95% of the activity of 4 , the Ir
Ir
analogue). Notably, ruthenium is 10 times cheaper than1
iridium, and although Ru complex 4Ru was reported in 2011,
it had not previously been used for CO hydrogenation.
2
The base used in CO hydrogenation, NaHCO , provides a
2
3
+
Lewis acid (Na ) that can potentially impact the rate of the
reaction. To test this hypothesis, we varied the base and used
KHCO , CsHCO , and (NH )HCO , which were chosen as
3
3
4
3
+
commercially available bicarbonate salts (Table 4). The K and
Table 4. Effect of the Lewis Acid on CO Hydrogenation
2
a
with 4Ir
entry
M+
pH
TON
1
2
3
4
Na+
K+
Cs+
NH4+
8.1 (1)
8.3 (1)
8.4 (1)
7.8 (1)
1430 (70)
1410 (50)
1390 (50)
500 (80)
dichloroethane. The resulting product 3 was isolated in 48%
yield, as characterized by H NMR and MS methods (Figures
S54−55), and recrystallized by slow diffusion of ether into
acetonitrile. A crystal structure is shown in Figure S91. As
shown in Scheme 3, the product 3SP appears to result from an
SP
1
a
Conditions: The reactions were performed in 25 mL of an aqueous
solution of 0.3 mM catalyst and 0.5 M MHCO (M = Na, K, Cs, or
NH ) at 115 °C and 300 psig of H /CO (1:1). All TON are
calculated after 18 h and are an average of at least 3 experiments
3
4
2
2
S 2 reaction between 3″ and dichloroethane along with
N
surprising loss of the methyl group from the methoxy
(
estimated standard deviations are in parentheses).
substituent. In 3 , the bidentate CC ligand (bite angle =
SP
7
7.4(3)°) is best described as a zwitterion with a cationic N and
+
Cs salts give similar or higher pH values for the resulting
solution (see third column of Table 4), but the pH does drop
slightly with the NH4+ salt (pH 7.8). The Na, K, and Cs
an anionic C of the C-bound pyridyl ligand. The metal is still
Ir(III), and the complex is cationic. The metrical parameters
including Ir−C(NHC) = 1.996(5) Å and Ir−C(Py) = 2.054(7)
Å are similar to those in Table 1 and analogous Ir(III)
bicarbonate salts all gave similar TON values with 4 , whereas
Ir
21
the ammonium bicarbonate salt gave a significantly lower TON.
Similarly, when we monitor the pressure over time (Figures
S81−S84), the rate of pressure drop is similar to that of the
complexes in the literature. Although 3SP is not the same as
the cyclometalated product 3″ obtained under hydrogenation
1
conditions, the pattern of signals in H NMR spectra of 3″ and
alkali metals over the entire course of the reaction; it is much
3
does confirm that the transformation of 3 involves
SP
+
4
slower with NH . In Table 4, the almost 3-fold greater TON
cyclometalation of the pyridine ring.
seen in entries 1−3 than in entry 4 is a fact whose mechanistic
implications will be discussed in the section below on
computations.
There is precedent in the literature for cyclometalation of
37
51
bidentate NHC-pyridine ligands on iridium and rhodium.
Cyclometalation has also been reported for 6,6′-dmbp
51,52
Catalytic CO Hydrogenation: Precatalyst Transfor-
complexes of gold, palladium, and platinum,
but we have
2
mations. Interestingly, UV−vis absorption spectroscopy of
not observed cyclometalation of the dxbp complexes herein.
complex 4 shows that the catalyst remained mostly unchanged
After 18 h under basic CO hydrogenation conditions, 3 is
Ir
2
after hydrogenation, yet for 5 , significant bleaching of the CT
features was observed (Figure S52). In short, the robust nature
converted to a blue solid mixture that appears to contain 3″
(see Figure S53) and new, unidentified products. The
elemental makeup of this water-soluble blue solid is described
in the SI from XPS and EDS data (Figures S56−S58); reduced
C and loss of Cl, F, and S but retention of Ir, N, and O suggest
Ir
of 4 as compared to 5 is a main factor for the higher TON
Ir
Ir
observed in Table 3. Significant color changes were observed
OR
visually for NHC-py complexes 1−3 after hydrogenation,
50
which was quantitated by measuring UV−vis absorption
that perhaps Cp* loss occurs from 3″ after cyclometalation.
(
Figure S51). The CT absorption features for the starting
Formic Acid Dehydrogenation. Dehydrogenation of
formic acid was performed with complexes 1−5. In each trial,
1.02 M formic acid with 0.29 mM catalyst was heated at 60 °C
for 3 h (Table 5).
complexes bleached, leaving significant absorption only in the
UV range.
We were able to elucidate transformations of complex 3 (full
1
All three iridium complexes of the new NHC-pyOR ligands
(1−3) were found to be active precatalysts (entries 1−3).
These precatalysts can also be compared to iridium and
ruthenium complexes of 6,6′-dhbp and 6,6′-dmbp (entries 4−
7), of which the iridium complexes of 6,6′-dxbp are by far the
most active. We also note that the estimated standard
deviations in TON values are higher for 1−3 (as compared
with 4 and 5) and may reflect variables related to the
details, SI pp. S34−43). H NMR spectral evidence after
hydrogenation (Figure S53) shows that some 3 remains, but
some has been converted to cyclometalated product 3″ as
shown in Scheme 3. Complex 3″ is formed by treating 3 with
our hydrogenation conditions or by heating 3 in an inert
solvent with base (e.g., treating 3 with Na CO in CH Cl ).
2
3
2
2
Although we were unable to recrystallize 3″, we did trap a
cyclometalated species by treating 3 with triethylamine in
F
DOI: 10.1021/acs.organomet.6b00806
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Article
a
Table 5. Dehydrogenation of Formic Acid by Catalysts 1−5
comparable activity (entry 5). Significantly, the literature on 4Ir
describes how hydroxy groups are needed for a metal−ligand
catalyst used as is
Cl− removed in situ
9
bifunctional mechanism, but the similar reactivity of 5 and 4
Ir
Ir
entry
catalyst
TON
TOF (h−1)
TON
TOF (h−1
)
suggests that other effects are at work. Our groups provided
evidence from X-ray crystallography and reactivity studies of
the dmbp complex [(terpy)Ru(6,6′-dmbp)(H O)](OTf) that
1
2
3
4
5
6
7
1
2
3
130 (30)
90 (50)
44
31
200 (40)
140 (30)
80 (20)
66
47
2
2
180 (60)
59
27
the methoxy group can accept a hydrogen bond from the
b
b
3
4Ir
>3500
1200
1200
15
>3500
1200
1200
15
coordinated aquo ligand, and thus, hydrogen bond acceptance
b
b
5Ir
>3500
>3500
by the oxygenated substituents of 5 and 4 during catalysis
Ir
Ir
4Ru
45 (12)
46 (6)
16 (2)
may play a role. Formic acid dehydrogenation is performed
under acidic conditions (pH 1.9); hence, none of the OH
groups are deprotonated (pK = 4.6 and 5 for 4 and 4 ,
5Ru
140 (10)
45
5
a
Conditions: Aqueous formic acid (1.02 M) was treated with catalysts
−5 (0.29 mM) at 60 °C for 3 h. In the right two columns, chloride
was removed by treating with silver salts in situ. See the Experimental
Section for further details. Turnover numbers (TON) and turnover
frequency (TOF) values were calculated to two significant figures at
the end of the 3 h period by measuring the gas generated (assuming
a
Ir
Ru
1
1,2
respectively) and may behave similar to methoxy groups as
hydrogen bond acceptors and electron-donating groups. Upon
replacing the iridium with ruthenium in 4Ru (with 6,6′-dhbp)
and 5Ru (with 6,6′-dmbp), we see that the resulting catalysts are
far less active (entries 6 and 7).
1
:1 of CO /H ). TON values are an average of at least 3 experiments
2 2
(
estimated standard deviations are in parentheses). Entries 2, 4 and 6
The shaded cells in Table 5 show the reactivity of OH-
substituted catalysts from which it is readily apparent (entries 2
and 4) that 6,6′-dhbp-ligated catalysts are far more active than
OH
were performed with OH-containing catalysts (2 with NHC-py and
4Ir and 4Ru with 6,6′-dhbp). The other entries used alkoxy-substituted
catalysts (1 with NHC-py , 3 with NHC-py , and 5 and 5
with 6,6′-dmbp). These experiments went to 99.8−100% conversion
of formic acid to gaseous products; in addition to measuring gas
formation, we also double checked these values by H NMR
spectroscopic analysis of an aliquot of the solution phase, and these
reactions only leave 0−0.2% of formic acid unconsumed.
OtBu
OMe
OH
Ir
Ru
the complexes of the NHC-py ligand. The lesser reactivity
b
OR
may be caused by transformation(s) of the NHC-py -ligated
1
complexes.
The catalyst [Cp*IrCl(6,6′-dmbp)]OTf (5 ) was further
Ir
studied over a longer time course as a highly active species of
novel structure. Table S22 shows the longevity of 5 by adding
Ir
transformation of the NHC-pyOR complexes 1−3 (see the SI
for further details). Because complexes 4 and 5 appear to be
robust catalysts, we focus on them here.
substrate (formic acid) after each reaction cycle was complete
(every 3 h). UV−vis absorption spectra of the catalytic mixtures
before and after dehydrogenation show that the complexes are
unaltered (Figure S87). Complete conversion of formic acid to
gaseous products occurs quantitatively (∼100% yield) for five
Notably, when we replace 6,6′-dhbp with 6,6′-dmbp in
[
Cp*Ir(6,6′-dmbp)Cl]OTf (5 ), we see that the catalyst has
Ir
Figure 5. Diagram of minima with relative energies (solvent corrected energies in kcal/mol) of CO hydrogenation catalyzed by 4 and 5 with the
2
Ir
Ir
+
assistance of Na ion. The blue bars with the blue solid line represent the pathway of 4 , and the red bars with the red solid line represent the
Ir
pathway of 5 . The dashed line represents the pathway via the Ir-hydride with protonated pyridine species.
Ir
G
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Article
Figure 6. Free energy diagram (solvent corrected energies in kcal/mol) of CO insertion into the iridium hydride of 4 and 5 with and without the
2
Ir
Ir
+
assistance of a Na ion. The blue bars with the solid line represents the pathway of 4 , and red bars with the solid line represent the pathway of 5 .
The dashed line represents the pathways without the assistance of the Na ion.
Ir
Ir
+
5
3−58
cycles run over 3 d. Only after gentle heating at 60 °C for 3 d
without added substrate does catalyst deactivation occur.
Thereafter, when formic acid is added, a decrease in TOF
was observed. A maximal TON of 21,000 was observed at this
point (after 8 d), but the ultimate value would be higher
impact the rates.
We note that most of the studies on the
role of alkali metals have been in alcoholic solvents, and ours is
2
+
the first to use water. Although [Cp*Ir(OH )(6,6′-dhbp)] ,
2
4 ′, has been computationally studied for CO hydro-
Ir
2
59
genation, our mechanism is unique in describing the role of
1
because the catalyst is still active. A H NMR spectrum
the alkali metal and in contrasting dxbp complexes 4 and 5 .
Under our typical hydrogenation conditions (Table 3), [Na ] =
1 M and [H ] = 3.2 × 10 M at pH 8.5. Thus, we invoke the
use of Na rather than H (as used by Ertem et al. ) under
basic conditions.
Ir
Ir
+
acquired after concentrating the same reaction mixture shows
that the major component is still the original catalyst (Figure
S88).
+
−9
+
+
59
A lower loading of catalyst 5 (0.0028 mol % catalyst or 10
Ir
times lower relative to the conditions of Table 5) was also
The proposed mechanisms of CO hydrogenation catalyzed
2
+
investigated (Table S23). Here, the TOF is ∼2.8 times faster at
by 4 and 5 with and without the assistance of Na ion are
Ir
Ir
∼
3300 h− with only 29% conversion at 3 h. For comparison,
1
shown in Figure 5 and Figure S85, respectively. Hydrogenation
reactions were run in aqueous base (pH 8.5), and thus for 4Ir,
the 6,6′-dhbp ligand will first be deprotonated to form the
−1
the TOF values over 3 h are ∼1200 h at 0.028 mol % of 5 . A
0-fold increase in turnover frequency was anticipated with
Ir
1
−
dropping the catalyst loading, but it appears that at the low
catalyst loading the reaction occurs more slowly due to the
saturation of all the catalyst sites or that catalyst decomposition
species with the dianionic ligand (6,6′-bobp, 6,6′-bis-O -
bipyridine). The activation of dihydrogen leads to Ir−H , a σ
2
complex, as the first intermediate. The acidic dihydrogen
interferes. At 0.0028 mol % of catalyst 5 loading, the reaction
complex can transfer a proton two ways. With 5 , proton
Ir
Ir
does go to 94% conversion (TON = 33,000) after 24 h; thus, if
given enough time, the yield of product is nearly quantitative
even at very low catalyst loadings.
transfer to solvent occurs directly leading to Ir−H. With 4 , the
Ir
O of the dianionic ligand can be protonated to give the Ir−H−
OH species before loss of a proton and formation of Ir−H
5
9
Computational Study of Catalytic CO Hydrogena-
(water may assist this process). [The formation of a
protonated pyridine species (Ir−H−NH) is energetically
unfavorable for both 4 and 5 (17.7 and 18.6 kcal/mol,
2
tion. Hydrogenation reactions described herein using 6,6′-
dhbp are proposed to proceed via a metal−ligand bifunctional
mechanism. We expect that the OH/O- groups will play a role
in transferring protons. This mechanism will be illustrated using
4Ir, which was the most active catalyst for hydrogenation. We
Ir
Ir
respectively).] After the formation of Ir−H, one CO molecule
2
+
could be involved with the assistance of Na ion to form the
Ir−H−CO species. The Ir−H−CO -Na species could then be
2
2
will compare 4Ir computationally with 5 , the 6,6′-dmbp
converted to the Ir-formate species (Ir-OCHO-Na), which
Ir
−
complex, which was considerably less active. The anionic
then would generate the final product HCOO . The species
+
+
oxygen of the deprotonated 6,6′-dhbp ligand can bind Na to
thus formed would be lower-energy with the Na interacting
activate CO2 and hold substrate near the metal center.
Interactions between alkali metals and substrates have literature
precedent, and the identity of the alkali metal can greatly
with the dianionic 6,6′-bobp ligand as compared with the dmbp
ligand (for 4 and 5 : Ir−H−CO -Na at −5.7 and −3.4 kcal/
Ir
Ir
2
mol and Ir-OCHO-Na at −9.8 and −5.0 kcal/mol,
H
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Article
Figure 7. Diagram of minima with relative energies (solvent corrected energies in kcal/mol) of intramolecular proton transfer-involved formic acid
dehydrogenation catalyzed by 4 and 5 . The blue bars with the solid line represent the pathway of 4 , and red bars with the solid line represent the
Ir
Ir
Ir
pathway of 5Ir.
+
respectively). The weaker stabilization of Na ion with 5 as
we note that ammonium can still donate hydrogen bonds but it
is a weaker Lewis acid than Na .)
Ir
+
compared to 4 was also confirmed by the longer Na-OMe
Ir
distances in the Ir−H−CO -Na (2.40 vs 2.17 Å) and Ir-
The H cleavage process catalyzed by 4 (Figure S86)
2
2
Ir
OCHO-Na species (2.36 vs 2.20 Å). Thus, 4 is predicted to
demonstrates that the pathway from Ir−H σ complex to Ir-
Ir
2
have better catalytic activity than 5 in CO hydrogenation with
hydride with hydroxy group species (Ir−H−OH) was more
Ir
2
alkali metals present, as is experimentally observed. Without
favorable than the pathway from Ir−H σ complex to Ir-
2
+
Na ion, a much higher-energy intermediate is observed (Ir-
hydride with protonated pyridine species (Ir−H−NH) (11.0 vs
OCHO at 18.8 and 12.4 kcal/mol for 4 and 5 , respectively;
28.7 kcal/mol). It is worth noting that an Ir-dihydride species
Ir
Ir
Figure S85). This shows the importance of the Lewis acid
(Ir-2H) was observed for the H cleavage process catalyzed by
5Ir (Figure S86). The transition state of the conversion between
2
+
(
Na ) in CO hydrogenation.
2
The related transition states (proceeding from Ir−H−CO
the Ir−H σ complex and Ir-2H dihydride species has not yet
2
2
+
to Ir-OCHO) located in Figure 6 verify the importance of Na
been located; however, this process will be facile. The free
energy of activation for the conversion between Ir-2H
dihydride and Ir-hydride with protonated pyridine species
(Ir−H−NH) of 5 was much higher than that of 4 (33.9 vs
ion in the CO insertion step of the hydrogenation reaction.
2
The transition state free energies are much higher without the
assistance of a sodium ion (22.8 and 25.6 kcal/mol for 4 and
Ir
Ir
Ir
+
5Ir, respectively). With the Na ion present, CO insertion into
28.7 kcal/mol). Computations of the CO insertion process
2
2
+
the Ir−H of 4 was achieved by two lower-energy intermediate
with the assistance of Na ion and H cleavage process were
Ir
2
+
Na -ion stabilized species (Ir-HCOO-Na at 0.7 kcal/mol with a
consistent with the experimental observation that 4 has better
Ir
weak Ir−H interaction and Ir-HCOO-Na at −7.9 kcal/mol
without direct interaction between Ir to H; Figure 6). The
catalytic activity than that of 5 in CO hydrogenation.
In summary, our proposed mechanism invokes a role for the
Ir
2
turnover-limiting step in the CO insertion process catalyzed by
oxyanion of dhbp and explains why the dhbp complexes are
2
+
4Ir with the assistance of Na ion gave a much lower Gibbs free
more active than the dmbp complexes for CO hydrogenation
2
+
energy compared to that without the assistance of Na ion (3.2
at basic pH. Remarkably, the important role of the secondary
coordination sphere in hydrogenation may help explain why the
vs 22.8 kcal/mol). Similar results were also observed for the 5Ir
(6.1 vs 25.6 kcal/mol). These related transition states during
activity of 4Ru is 95% of that seen with 4 . In this manner, a less
Ir
the CO hydrogenation reaction catalyzed by 4 and 5 show
the important role of the Na ion in the stabilization of reaction
expensive metal (ruthenium) can work nearly as well as iridium
when paired with an appropriate ligand.
2
Ir
Ir
+
intermediates and also demonstrate that 4 is predicted to have
Computational Study of Formic Acid Dehydrogen-
ation. Recall that OH and OMe groups in dxbp complexes (4Ir
Ir
better catalytic activity than 5 in CO hydrogenation. (We
Ir
2
propose that the use of NH4+ would give activation barriers
and 5 ) produced similar rates of dehydrogenation. Under
Ir
+
slightly higher than that observed for Na based upon Table 4;
acidic conditions (pH 1.9 for dehydrogenation of aqueous
I
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Article
Figure 8. Diagram of minima with relative energies (solvent corrected energies in kcal/mol) of intermolecular proton transfer-involved formic acid
dehydrogenation catalyzed by 4 and 5 . The blue bars with the solid line represent the pathway of 4 , and the red bar with the solid line represent
Ir
Ir
Ir
the pathway of 5Ir.
1
formic acid), the OH groups in dhbp will not be deprotonated
and will behave similarly to the OMe groups. Again, the
dehydrogenation of formic acid has been studied previously
3.5 kcal mol− as a function of bipy substituent, consistent with
operation of intermolecular proton transfer pathways in the
formic acid dehydrogenation process catalyzed by 4Ir and 5Ir
.
5
9
with 4 ′, but our study is unique in comparing the role of OH
Ir
vs OMe groups.
CONCLUSIONS
■
The possible mechanisms of formic acid dehydrogenation
In summary, we have synthesized a new bidentate ligand with
an NHC ring bound to a pyridinol ring. The resulting NHC-
complexes of iridium(III) are moderately active
precatalysts for the hydrogenation of CO . Low activity is
catalyzed by 4 and 5 with intramolecular or intermolecular
Ir
Ir
proton transfer (Figures 7 and 8) were also explored. Formic
acid can first bind to Ir via the hydroxyl group (Ir-HOCHO)
under acidic aqueous conditions. Then, a lower energy species
is formed by conversion to the species (Ir-HCOOH) with
formic acid’s OH hydrogen bonded species to the OH or OMe
of the dxbp ligand. The hydrogen bonds between OH of Ir-
HCOOH and N of the dxbp and the following intramolecular
proton transfer lead to the formation of an Ir-formate species
with protonated pyridine (Ir-HCOO-NH). Subsequent β-
OR
py
2
OR
observed when these NHC-py complexes (1−3) are used for
formic acid dehydrogenation. The NHC-pyridinol-derived
ligands appear to undergo a cyclometalation reaction under
basic conditions, and we have trapped a cyclometalated
product.
In contrast, the 6,6′-dhbp ligands and complexes thereof are
more stable and products of further transformations are not
observed. The iridium 6,6′-dhbp complexes have been used for
hydride elimination produces CO and the Ir-hydride species
2
6
,9
with protonated pyridine (Ir−H−NH). The iridium hydride
CO hydrogenation in the literature,
and similarly, we
2
2
can be protonated intramolecularly by the NH to form an η -H
2
observe that these are highly active catalysts for both
hydrogenation and dehydrogenation. However, we have
extended this work toward ruthenium and 6,6′-dmbp ligands.
Our groups and others have described the secondary
coordination sphere influence of 6,6′-dhbp and its benefits
σ complex (Ir−H ), which finally releases an H molecule. The
2
2
relatively small differences of the values of reaction energies in
the formic acid dehydrogenation process catalyzed by 4 and
Ir
5Ir with intramolecular proton transfer is consistent with the
similar catalytic activities observed. Figure 8 focuses on
intermolecular proton transfer, for example, that of HCOOH
directly to solvent as it binds to Ir to give the Ir-formate species
toward reductive catalysis, specifically, CO reduc-
2
1
,6,9,53,60,61
tion.
Not surprisingly, under basic conditions,
catalysts for hydrogenation with the diprotic ligand 6,6′-dhbp
(Ir-OCHO). Here, the dhbp analogue is stabilized relative to
are 5−8 times more active than catalysts using the aprotic
the dmbp complex by intramolecular hydrogen bonds. In
contrast, subsequent minima in Figure 8 differ by no more than
analogue 6,6′-dmbp. Remarkably, ruthenium (4 ) is nearly
Ru
(95%) as active as iridium (4 ) with 6,6′-dhbp, and this can be
Ir
J
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Article
rationalized in terms of the secondary coordination sphere
having a big impact on catalysis despite the difference in metal.
measured at 294(2) K; the detector was placed at a distance 6.000 cm
64
from the crystal. The collected frames were integrated with the Saint
software package using a narrow-frame algorithm. Data were corrected
Computationally, this assertion is supported by observing how
65
+
for absorption effects using the multiscan method in SADABS. The
binding Na near the metal (in 4 ) serves to lower the free
Ir
66
space group was assigned using XPREP of the Bruker ShelXTL
energy barriers for CO hydrogenation.
66
66
2
package, solved with ShelXT, and refined with ShelXL and the
graphical interface ShelXle. All non-hydrogen atoms were refined
However, we see the secondary coordination sphere is not as
important for dehydrogenation under acidic conditions. Here,
iridium is more active than ruthenium, and methoxy and
hydroxy perform similarly (in 5 and 4 , respectively). Thus,
67
anisotropically. H atoms attached to carbon were positioned
geometrically and constrained to ride on their parent atoms. Specific
structure determination details are included in the Supporting
Information.
Ir
Ir
the nature of x in dxbp does not matter if the pH is such that
the OH groups are not deprotonated. Electronically, OH and3
OMe groups are similar and both can accept hydrogen bonds.
Computationally, we can explain this similarity in terms of
similar energy barriers for dehydrogenation of formic acid with
4Ir and 5Ir.
In summary, we have determined how subtle ligand changes
can influence reactivity and stability of iridium catalysts for CO2
hydrogenation and formic acid dehydrogenation. We have also
determined when the secondary coordination sphere influences
the activity of the catalysts (and when it does not). With
iridium complexes of dxbp-type ligands as (de)hydrogenation
catalysts, hydroxy groups perform better than methoxy groups
when the oxyanions can form, but when hydroxy groups remain
neutral, they behave similarly to methoxy substituents.
Synthesis of 2-(tert-Butoxy)-6-(1H-imidazol-1-yl)pyridine
t
O Bu
(py -Im). A Schlenk flask with stir bar was loaded with KOtBu
(1.485 g, 13.2 mmol, 1.2 equiv) in a glovebox and sealed with a rubber
septum, and THF (20 mL) was added via cannula. Under N2(g)
atmosphere, 2,6-difluoropyridine (1.00 mL, 11.0 mmol, 1.00 equiv)
was added dropwise to the stirring solution via syringe to form 2-(tert-
butoxy)-6-fluoropyridine. This reaction was stirred for 2 h. A separate
Schlenk flask with stir bar was charged with powdered sodium hydride
(0.292 g, 12.2 mmol, 1.1 equiv) and anhydrous DMF (30 mL) and
then sealed with a rubber septum. This reaction flask was attached to
the Schlenk line and, with a positive flow of nitrogen solid imidazole
(0.8257 g, 12.1 mmol, 1.1 equiv), was slowly added to the stirring
suspension to form sodium imidazolate. The reaction was stirred 1 h
under nitrogen until hydrogen gas ceased to evolve. The THF reaction
solution prepared previously was transferred via cannula to the DMF
reaction solution with stirring. With positive nitrogen flow, a reflux
condenser was attached, and the flask was heated to 70 °C overnight
with stirring. The reaction mixture was cooled to room temperature;
the THF was removed from the mixture via a rotary evaporator, and
the remaining reaction mixture was transferred to a separatory funnel
with DI H O (300 mL). The product was extracted with CH Cl (40
EXPERIMENTAL SECTION
General Procedures. All ligand and metal complex syntheses were
performed under a nitrogen atmosphere in a glovebox or by utilizing
standard Schlenk line techniques with oven-dried glassware. H and
■
1
1
3
2
2
2
C NMR spectra were acquired at room temperature on a Bruker
mL × 3). The organic phase was washed with brine (80 mL), dried
AV360 360 MHz or AV500 500 MHz spectrometer, as designated, and
referenced to the solvent peak. Mid-IR spectra were collected on a
Bruker Alpha ATR-IR spectrometer. Mass spectrometric data were
collected on a Waters AutoSpec-Ultima NT spectrometer with
electron ionization method. Elemental analyses were performed by
NuMega Resonance Laboratories, Inc., San Diego, CA. Electronic
spectra were measured on a PerkinElmer Lambda 35 UV−visible
spectrometer. Cyclic voltammetry experiments were conducted in a
over MgSO , and filtered; the filtrate was concentrated to dryness on a
4
rotary evaporator, and the residue was dried under high vacuum to
t
O Bu
afford a honey-colored oil identified as desired product py -Im
1
(
8
7
2.287 g, 10.5 mmol, 95.5%). H NMR (360 MHz, CDCl , ppm): δ
3
3
.18 (s, 1H, Im-CH (N-CH-N)), 7.49 (t, J = 8.3 Hz, 1H, Py-CH),
.45 (t, J = 1.2 Hz, 1H, Im-CH), 7.05 (t, J = 1.2 Hz, 1H, Im-
CH), 6.74 (d, J = 7.8 Hz, 1H, Py-CH), 6.45 (d, J = 8.1 Hz, 1H,
Py-CH), 1.51 (s, 9H, OC(CH ) ) (see Figure S1). C { H}NMR
HH
3
3
HH
HH
3
3
HH
HH
1
3
1
0
.1 M solution of Bu NPF6 in acetonitrile (MeCN) or N,N′-
3 3
4
t
(
(
125.76 MHz, CDCl , ppm): δ 163.02 (C ( BuO-CN)), 146.21
dimethylformamide (DMF) with a glassy carbon working electrode,
3
Py
CPy (N-C-N)), 140.54 (C ), 134.77 (C (N-CH-N)), 130.19 (C ),
a Pt counter electrode, and a Ag/AgCl reference electrode on a
Py Im Im
+
CHI760C Potentiostat. The redox potentials are calibrated to Fc/Fc
115.99 (CIm), 110.91 (CPy), 102.84 (CPy), 80.28 (OC(CH
3
)
3
), 28.41
−1
and reported vs SCE. A Fisher Scientific accumet glass electrode
calibrated with standard buffer solutions was used to measure pH
values. Pressurized gas reactions were performed in a Parr reaction
vessel. SEM data were collected with a JEOL 7000F Field Emission
Gun (FEG) for secondary and backscattered electron images. EDS
data were collected using an Oxford system with Silicon Drift Detector
(OC(CH ) ) (see Figure S2). FT-IR (ATR, cm ): 3114 (w), 2976
3 3
(w), 2931 (w), 1673 (m), 1603 (m), 1571 (s), 1478 (m), 1445 (vs),
1382 (m), 1364 (m), 1323 (m), 1277 (m), 1247 (m), 1232 (s), 1158
(m), 1132 (m), 1103 (m), 1056 (s), 1013 (m), 930 (m), 910 (m), 841
(m), 792 (s), 730 (m), 653 (s), 609 (w), 469 (w), 406 (w) (see Figure
S4). EI-MS (EI ): m/z found (expected): 217.1 ([py -Im]+
+
OtBu
=
+
OH
+
+
(
SDD) with Aztec software. XPS data were collected using a Kratos
[C12H
15
N
3
O] , 217.12), 161.0 ([py -Im] = [C
8 7 3
H N O] , 161.06)
AXIS 165 XPS with a Mono (A1)(144W) anode, 50.0 meV step, 1000
ms dwell time, and hybrid lens mode with resolution of pass energy 20.
Materials. Dry solvents were obtained via the Glass Contour
Solvent System built by Pure Process Technology, LLC. All reagents
were used as purchased and degassed under vacuum as needed. The
(see Figure S3).
Synthesis of 2-Methoxy-6-(1H-imidazol-1-yl)pyridine
OMe
(py -Im). A Schlenk flask with stir bar was loaded with NaH
(1.377g, 57.4 mmol, 1.1 equiv) and THF (250 mL) in a glovebox and
then sealed with a rubber septum. Under nitrogen, anhydrous MeOH
(4.23 mL, 104.4 mmol, 2.0 equiv) was added dropwise to the NaH
suspension with stirring at 0 °C for 30 min until the bubbling had
stopped. 2,6-Difluoropyridine (4.732 mL, 52.2 mmol, 1.0 equiv) was
added dropwise to the reaction mixture via syringe. The reaction was
stirred for 12 h under nitrogen at room temperature. The solvent was
removed under reduced pressure. To the Schlenk flask containing
crude 2-(methoxy)-6-fluoropyridine was added anhydrous DMF (250
mL) via cannula transfer. With a positive flow of nitrogen, solid
imidazole (4.265 g, 62.64 mmol, 1.2 equiv) was added to the DMF
solution. The reaction mixture was stirred at 0 °C for 15 min; then,
with positive flow of nitrogen, NaH (1.378 g, 57.42 mmol, 1.1 equiv)
was added to the reaction flask and stirred at 0 °C until the bubbling
ceased. The reaction flask was allowed to warm to room temperature;
62
62
compounds 6,6′-dmbp, 6,6′-dhbp, [(p-cym)RuCl(6,6′-dmbp)]Cl
1
1
(
(
5 ), [(p-cym)RuCl(6,6′-dhbp)]Cl (4 ), [Cp*IrCl(6,6′-dhbp)]Cl
Ru
Ru
2
2
4 ), and [Cp*IrCl(6,6′-dmbp)]Cl were prepared according to
Ir
previously published procedures. High purity grade (>97%) formic
acid was used as purchased from AMRESCO, Inc. The compressed
gases CO and 50:50 vol CO /H were purchased from Airgas and
2
2
2
used without further purification.
XRD Structure Determination of 1−3, 3 , 5 , 5 , 6, and 7.
SP
Ir
Ru
Single crystal samples of complexes 1−3, 3 , 5 , 5 , 6, and 7 were
SP
Ir
Ru
mounted on glass filaments on a Bruker Apex2 CCD-based X-ray
63
diffractometer equipped with an Oxford N-Helix Cryosystem at
100 °C and fine focus Mo-target X-ray tube (λ = 0.71073 Å)
operated at 2000 W power (50 kV, 40 mA). The X-ray intensities were
−
K
DOI: 10.1021/acs.organomet.6b00806
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1
a reflux condenser was attached, and the flask was heated overnight at
3.98 (s, 3H, OCH ) (see Figure S13). 13C{ H} NMR (125.76 MHz,
3
8
0 °C open to air. The reaction mixture was cooled to room
CDCl , ppm): δ 163.93 (C (MeO-CN)), 143.69 (C ), 142.06
3
Py
Py
1
temperature; the DMF was removed from the mixture via a rotary
evaporator, and DI water (400 mL) was added to the remaining
reaction mixture and then transferred to a separatory funnel. The
organic phase was extracted with DCM (80 mL × 4), dried over
MgSO , and filtered. The filtrate was concentrated to dryness on a
rotary evaporator, and the residue was dried under high vacuum to
(C ), 134.62 (C (N-CH-N)), 124.76 (C ), 120.63 (q, J = 320.1
Py Im Im CF
Hz, CF of triflate), 119.17 (C ), 112.64 (C ), 105.31 (C ), 54.33
3
Im
Py
Py
19
(OCH ), 36.93 (NCH ) (see Figure S14). F NMR (338.86 MHz,
3
3
−1
CDCl , ppm): δ −78.59 (s, CF of triflate). FT-IR (ATR, cm ): 3135
3
3
(w), 3102 (w), 1626 (m), 1575 (m), 1543 (w), 1486 (m), 1448 (m),
1431 (m), 1368 (m), 1250 (s), 1223 (s), 1152 (s), 1101 (s), 1026 (s),
988 (w), 856 (w), 804 (w), 746 (w), 636 (s), 613 (m), 572 (m), 516
4
OMe
afford a honey-colored oil identified as the desired product py -Im
1
+
(
6.003 g, 34.29 mmol, 65% yield). H NMR (500 MHz, CDCl , ppm):
(m), 484 (w) (see Figure S16). EI-MS (EI ): m/z found (expected):
3
3
Me
OMe
+
+
Me
OMe
δ 8.18 (s, 1H, Im-CH (N-CH-N)), 7.48 (t, J
CH), 7.44 (t, J = 1.3 Hz, 1H, Im-CH), 7.02 (t, J = 1.1 Hz, 1H,
Im-CH), 6.71 (d, J
= 7.9 Hz, 1H, Py-
190.1 ([Im -py ] = [C10
H
12
N
3
O] , 190.10), 175.1 ([Im -py
] = [C H N O] , 175.08) (see Figure S15).
3 9 9 3
H, H
3
3
+
+
− CH
H,H
H,H
3
3
Me
OtBu
= 7.7 Hz, 1H, Py-CH), 6.48 (d, J
= 8.2 Hz,
Synthesis of [Ag(NHC -py
bottomed Schlenk flask with stir bar was loaded with (Im
py )OTf (2.007 g, 5.25 mmol, 1 equiv) and Ag
)
2
]OTf (6). An oven-dried round-
H, H
H, H
1
3
1
Me
1
H, Py-CH), 3.79 (s, 3H, OCH ) (see Figure S5). C { H}NMR
-
3
OtBu
(
(
(
125.76 MHz, CDCl , ppm): δ 163.49 (C (MeO-CN)), 146.80
2
O (0.622 g, 2.68
3
Py
C ), 140.81 (C ), 134.77 (C (N-CH-N)), 130.27 (C ), 115.92
mmol, 0.5 equiv) in a glovebox and sealed with a rubber septum. Dry
CH Cl (45 mL) was added via cannula at the Schlenk line to make a
Py
Py
Im
Im
C ), 108.47 (C ), 103.39 (C ), 53.43 (OCH ) (see Figure S6). FT-
2
2
Im
Py
Py
3
−1
IR (ATR, cm ): 3111 (w), 2982 (w), 2951 (w), 1612 (m), 1599 (m),
1
1
black slurry. The flask was covered in foil to block light. Aqueous
NaOH (1.3 mL, 2.6 mmol, 0.5 equiv) was added dropwise via syringe
with stirring. The reaction mixture was stirred for 24 h at RT. Excess
578 (s), 1471 (m), 1445 (s), 1417 (m), 1376 (m), 1320 (m),
262(m), 1247 (m), 1233 (s), 1155 (m), 1130 (m), 1102 (m), 1056
(
s), 1027 (m), 990 (m), 902 (m), 855 (m), 829 (s), 788 (m), 727 (s),
Ag
with CH
with DI water (3×), dried over MgSO
4
2
O was removed by suction filtration over Celite, which was washed
Cl (80 mL). The organic phase of the filtrate was washed
, and filtered, and the filtrate
6
71 (m), 652 (m), 608 (m), 475 (w), 449 (w) (see Figure S8). EI-MS
2
2
+
OMe
+
+
(
EI ): m/z found (expected): 175.1 ([py -Im] = [C H N O] ,
9
9
3
1
75.07) (see Figure S7).
was concentrated to dryness. The resulting slightly colored, viscous oil
Synthesis of 1-(6-(tert-Butoxy)pyridin-2-yl)-3-methyl-1H-
was further dried under high vacuum to yield an airy white solid
identified as the desired product (1.828 g, 2.54 mmol, 96.8% yield). H
Me
OtBu
1
imidazol-3-ium triflate (Im -py )OTf. Dry DMF (30 mL)
was added via cannula to an evacuated flask containing 2-(tert-butoxy)-
6
bar. Methyl trifluoromethanesulfonate (MeOTf) (1.31 mL, 11.6
mmol, 1.1 equiv) was added dropwise with stirring at 0 °C. A white
solid gradually formed with stirring. The reaction mixture was stirred
for 12 h under nitrogen at room temperature. The white solid product
was collected by suction filtration and washed with Et O. More
product precipitated from the filtrate with the addition of Et O, which
was also collected by filtration. The white solid was combined and
dried under vacuum to yield the product (Im -py )OTf (3.0098 g,
NMR (360 MHz, CDCl , ppm): δ 7.67 (broad s, 1H, CH of the
3
3
3
-(1H-imidazol-1-yl)pyridine (2.287 g, 10.5 mmol, 1 equiv) and a stir
NHC), 7.55 (t, J = 8.1 Hz, 1H, Py-CH), 7.44 (d, J = 7.5 Hz,
H,H H,H
3
1H, Py-CH), 7.31 (d, J = 15.4 Hz, 1H, CH of the NHC), 6.69 (d,
H,H
3
JH,H = 8.0 Hz, 1H, Py-CH), 4.02 (s, 3H, NCH ), 1.57 (s, 9H,
OC(CH ) ) (see Figure S17). C { H}NMR (125.76 MHz, CDCl ,
3
13
1
3
3
3
ppm): δ 179.30 (CNHC (carbene bound to Ag)), 163.08 (CPy (MeO-
1
2
CN)), 148.44 (C ), 140.70 (C ), 123.55 (C ), 120.84 (q, J
Py
Py
NHC
CF
=
2
321.4 Hz, CF of triflate), 120.43 (C ), 113.27 (C ), 107.14 (C ),
3 NHC Py Py
80.80 (OC(CH ) ), 39.39 (NCH ), 28.53 (OC(CH ) ) (see Figure
3 3 3 3 3
Me
OtBu
−1
S18). FT-IR (ATR, cm ): 3143 (w), 3126 (w), 3084 (w), 2977 (w),
2933 (w), 1626 (w), 1607 (m), 1567 (m), 1451 (s), 1403 (m), 1403
(w), 1386 (w), 1361 (m), 1302 (w), 1257 (s), 1235 (s), 1158 (s),
1140 (s), 1029 (s), 988 (m), 927 (m), 913 (m), 844, (m), 809 (m),
794 (m), 743 (m), 636 (s), 572 (m), 516 (m), 469 (w). (see Figure
1
7
.892 mmol, 75% yield). H NMR (360 MHz, CD OD, ppm): δ 9.58
3
(
broad s, 1H but integrates low due to H/D exchange with CD OD,
3
3
3
Im-CH (N-CH-N)), 8.24 (d, J = 2.0 Hz, 1H, Im-CH), 7.90 (t, J
HH
HH
3
=
8.3 Hz, 1H, Py-CH), 7.78 (d, J = 2.0 Hz, 1H, Im-CH), 7.37 (d,
HH
3
3
+
Me
JHH = 7.9 Hz, 1H, Py-CH), 6.87 (d, J = 8.3 Hz, 1H, Py-CH), 4.07
S20). EI-MS (EI ): m/z found (expected): 569.2 ([Ag(NHC
-
HH
1
3
OtBu
+
+
Me
OtBu
(
{
s, 3H, NCH ), 1.66 (s, 9H, OC(CH ) ) (see Figure S9).
H}NMR (125.76 MHz, CD OD, ppm): δ 164.83 (C ( BuO-C
C
py
(NHC -py )] = [C22
Calcd for [C27
11.68%. Found: C, 44.84%; H, 4.75%; N, 11.41%.
)
2
] = [C26
H
34AgN
6
O
2
] , 569.18), 512.1 ([Ag(NHC -py
)
3
3 3
1
t
Me
O
+
+
H
25AgN
O
] , 512.11) (see Figure S19). Anal.
H N O F SAg] = (6): C, 45.07%; H, 4.76%; N,
34 6 5 3
3
Py
6
2
N)), 145.20 (C ), 143.11 (C ), 136.26 (C (N-CH-N)), 125.91
Py
Py
Im
1
(
C ), 121.77 (q, J = 319.5 Hz, CF of triflate), 120.49 (C ),
Im CF 3 Im
Me
OMe
1
(
(
1
15.85 (C ), 106.24 (C ), 82.56 (OC(CH ) ), 37.08 (NCH ), 28.88
Synthesis of [Ag(NHC -py
) ]OTf (7). In a glovebox, an
2
Py
Py
3
3
3
−1
OC(CH ) ) (see Figure S10). FT-IR (ATR, cm ): 3160 (w), 3141
oven-dried Schlenk round-bottomed flask with stir bar was loaded with
3 3
Me
OMe
w), 3119 (w), 2987 (w), 1627 (w), 1614 (w), 1565 (m), 1544 (m),
453 (m), 1441 (m), 1368 (m), 1343 (s), 1309 (s), 1253 (s), 1220
(Im -py )OTf (3.162 g, 9.33 mmol, 1 equiv), Ag
O (1.081 g, 4.66
2
mmol, 0.5 equiv), and dry CH Cl (15 mL) forming a black slurry.
2
2
(
(
6
(
2
s), 1153 (s), 1097 (m), 1028 (s), 985 (m), 931 (m), 911 (m), 855
m), 838 (m), 796 (m), 772 (m), 757 (m), 726 (w), 697 (w), 634 (s),
The flask was covered in foil to block light. The flask was attached to
the Schlenk line, and under nitrogen, aqueous NaOH (3.0 mL, 4.66
mmol, 0.5 equiv) was added dropwise via syringe with stirring. The
12 (m), 572 (m), 516 (s), 485 (w), 464 (w) (see Figure S12). EI-MS
+
Me
OtBu
+
+
EI ): m/z found (expected): 232.1 ([Im -py ] = [C H N O] ,
reaction mixture was stirred for 24 h at RT. Excess Ag
by suction filtration over Celite, which was washed with CH
mL). The filtrate organic phase was washed with DI water (3 times),
dried over MgSO , and filtered, and the filtrate was concentrated to
2
O was removed
13
18
3
32.14), 176.1 ([ImMe-py ] = [C H N O] , 176.08) (see Figure
OH
+
+
Cl (30
2
9
10
3
2
S11).
Synthesis of (ImMe-pyOMe)OTf. Dry DMF (150 mL) was added
via cannula to an evacuated flask containing 2-(methoxy)-6-(1H-
imidazol-1-yl)pyridine (6.003 g, 34.28 mmol, 1 equiv) and a stir bar.
Methyl trifluoromethanesulfonate (MeOTf) (4.27 mL, 37.72 mmol,
4
dryness. The resulting slightly gray colored solid was further dried
under high vacuum and identified as the desired product (2.25 g, 3.55
1
mmol, 76.1% yield). H NMR (360 MHz, CD
CN, ppm): δ 7.77 (d,
3
3
3
1
.1 equiv) was added dropwise with stirring at 0 °C. The reaction
JHH = 1.8 Hz, 1 H, CH of the NHC), 7.74 (t, JHH = 7.9 Hz, 1H, Py-
CH), 7.37 (d, JHH = 1.8 Hz, 1H, CH of the NHC), 7.33 (d, JHH = 7.6
Hz, 1H, Py-CH), 6.80 (d, JHH = 8.0 Hz, 1H, Py-CH), 3.94 (s, 3H,
3
3
mixture was stirred for 12 h under nitrogen at room temperature.
DMF was removed via rotary evaporator to afford a honey-colored oil,
which then solidified with agitation. The honey-colored solid was
3
13
1
NCH
), 3.75 (s, 3H, OCH
) (see Figure S21). C{ H}NMR (125.76
3
3
collected by suction filtration and washed with Et O (60 mL) to
MHz, CDCl
3
, ppm): δ 180.30 (CNHC (carbene bound to Ag), 163.90
2
obtain a white solid. The solid was further dried under vacuum to yield
the product (Im -py )OTf (12.79 g, 37.72 mmol, 72% yield). H
(CPy (MeO-CN)), 149.08 (CPy), 141.44 (CPy), 123.69 (CNHC),
Me
OMe
1
1
121.14 (q, JCF = 320.5 Hz, CF
of triflate), 120.65 (CNHC), 110.75
3
NMR (500 MHz, CDCl , ppm): δ 9.83 (s, 1H, Im-CH (N-CH-N)),
(CPy), 107.68 (CPy), 54.13 (OCH
3
), 39.87 (NCH
3
) (see Figure S22).
3
3
3
19
8
.11 (t, J = 2.0 Hz, 1H, Im-CH), 7.80 (t, J = 8.0 Hz, 1H, Py-
CH), 7.58 (t, J = 2.0 Hz, 1H, Im-CH), 7.41 (d, J = 7.8 Hz, 1H,
Py-CH), 6.85 (d, J = 8.3 Hz, 1H, Py-CH), 4.12 (s, 3H, NCH3),
F NMR (338.86 MHz, CDCl
3
, ppm): δ −78.16 (s, CF
3
of triflate)
HH
HH
3
3
−1
(see Figure S23). FT-IR (ATR, cm ): 3155 (w), 3131 (w), 3107 (w),
3012 (w), 2957 (w), 1602 (3), 1583 (m), 1472 (s), 1435 (m), 1270
HH
HH
3
HH
L
DOI: 10.1021/acs.organomet.6b00806
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(
(
s), 1237 (s), 1146 (s), 1027 (s), 980 (m), 858, (m), 791 (m), 739
m), 717 (m), 634 (s), 571 (m), 516 (m), 469 (w). (see Figure S25).
Synthesis of [Cp*IrCl(NHCMe-pyOH)]OTf (2). An oven-dried
round-bottomed flask with stir bar was charged with [Cp*IrCl-
+
Me
Me
OtBu
EI-HRMS (EI ): m/z found (expected): 485.1 ([Ag(NHC
py ) ] = [C H AgN O ] , 485.09), 189.1 ([NHC -py ] =
-
(NHC -py )]OTf (0.1008 g, 0.136 mmol) and dry MeCN (20
OMe
+
+
Me
OMe +
mL). The solution was refluxed overnight under nitrogen. The
2
20 22
6
2
+
[
[
C H N O] , 189.09) (see Figure S24). Anal. Calcd for
solution was brought to room temperature and layered with dry Et
2
O
1
0
11
3
C H N O F SAg] = (7): C, 39.70%; H, 3.49%; N, 13.23%.
(100 mL). Yellow crystals were collected via suction filtration yielding
2
1
22
6
5 3
Me
OH
Found: C, 39.66%; H, 3.49%; N, 13.24%.
the desired product [Cp*IrCl(NHC -py )]OTf (0.0525 g, 0.0763
1
Synthesis of [Cp*IrCl(NHCMe-pyOtBu)]OTf (1). An oven-dried
Schlenk flask with stir bar was loaded with [Cp*IrCl ] (0.5344 g
mmol, 56.0% yield). H NMR (360 MHz, CD CN, ppm): δ 10.3
3
3
3
(broad s, 1H, OH), 7.91 (t, JH, H = 7.9 Hz, 1H, Py-CH), 7.80 (d, JHH
2
2
3
0
.670 mmol, 1 equiv), AgOTf (0.1787 g, 0.695 mmol, 1.04 equiv), and
= 2.2 Hz, 1H, CH of the NHC), 7.38 (d, JHH = 2.0 Hz, 1H, CH of the
Me
OtBu
3
3
[
Ag(NHC -py )2]OTf (0.5006 g, 0.696 mmol, 1.04 equiv) under
NHC), 7.31 (d, JHH = 7.7 Hz, 1H, Py-CH), 6.94 (d, JHH = 8.5 Hz,
1H, Py-CH), 3.96 (s, 3H, NCH ), 1.74 (s, 15H, CH of Cp*) (see
Figure S30). 13C{ H}NMR (125.76 MHz, CD
CN, ppm): δ 168.19
N2(g). Dry CH Cl (50 mL) was dispensed via cannula. An immediate
3
3
2
2
1
color change from orange to yellow was observed. The reaction
mixture was stirred protected from light for 18 h, resulting in the
accumulation of a tan precipitate (AgCl). The reaction mixture was
filtered over Celite with suction, which was washed with CH Cl (10
3
(
(
CNHC (carbene bound to Ir), 165.00 (CPy (HO-CN)), 151.66
C ), 144.71 (C ), 126.70 (C ), 122.03 (q, J = 321.0 Hz, CF
3
1
Py
Py
NHC
CF
of triflate), ∼118.31 (C
peak appears to be under the solvent
NHC
2
2
peak), 109.02 (C ), 103.34 (C ), 93.49 (ring C of Cp*), 38.41
mL). The product was crystallized by layering the filtrate with Et O
Py
Py
2
19
(
NCH ), 10.17 (CH of Cp*) (see Figure S31). F NMR (338.86
(
80 mL). The resulting yellow rod crystals were collected by suction
3 3
MHz, CD CN, ppm): δ −79.34 (s, CF of triflate). FT-IR (ATR,
filtration and washed with ether yielding the desired product
3
3
−1
Me
OtBu
cm ): 3101 (w), 2917 (w), 2799 (w),1626 (m), 1582 (m), 1485 (m),
472 (m), 1400 (w), 1323 (w), 1294 (m), 1229 (s), 1213 (m), 1176
[
Cp*IrCl(NHC -py )]OTf (0.9341 g, 1.257 mmol, 93.7%
1
yield). Pure microcrystalline product could also be collected by
quickly adding Et O to the filtrate. H NMR (360 MHz, CDCl ,
ppm): δ 8.14 (d, J = 2.4 Hz, 1H, CH of the NHC), 8.01 (t, J
1
(m), 1151 (m), 1020 (s), 884 (w), 804 (m), 745 (m), 719 (m), 693
2
3
=
3
3
(m), 633 (s), 604 (m), 514 (m), 486 (w), 436 (w) (see Figure S33).
H,H
H,H
+
Me
Me
3
EI-MS (EI ): m/z found (expected): 537.1 ([Cp*IrCl(NHC
-
-
8
.0 Hz, 1H, Py-CH), 7.77 (d, JH,H = 8.2 Hz, 1H, Py-CH), 7.34 (d,
OH
+
+
3
3
py )] = [C H ClIrN O] , 537.12), 501.1 ([Cp*Ir(NHC
JH,H = 2.2 Hz, 1H, CH of the NHC), 7.03 (d, J = 8.5 Hz, 1H, Py-
19 23
3
H,H
OH
+
+
Me
OH
py )] = [C H IrN O] , 501.15), 651.2 ([Cp*Ir(NHC -py
)
1
9
23
3
CH), 4.02 (s, 3H, NCH ), 1.73 (s, 15H, CH of Cp*), 1.58 (s, 9H,
3
3
+
+
13
1
(OTf)] = [C20
H
23
F
3
IrN O S] , 651.10) (see Figure S32). Anal. Calcd
3 4
(
CH ) C) (see Figure S26). C { H}NMR (125.76 MHz, CD CN,
3
3
3
for [C H N O F SClIr·H O] = (2·H O): C, 34.06%; H, 3.72%; N,
20
24
3
4
3
2
2
ppm): δ 167.88 (CNHC (carbene bound to Ir), 164.92 (CPy (tBuO-C
1
5.96%. Found: C, 34.42%; H, 3.79%; N, 5.91%.
N)), 152.28 (C ), 144.35 (C ), 126.64 (C ), 122.15 (q, J
=
Py
Py
NHC
CF
Synthesis of [Cp*IrCl(6,6′-dmbp)]OTf (5 ). A similar procedure
Ir
3
9
(
21.3 Hz, CF of triflate), 118.85 (CNHC), 112.84 (C ), 105.35 (C ),
3 Py Py
Me
OtBu
was followed as for the synthesis of [Cp*IrCl(NHC -py )]OTf
with the following differences. The regents and amounts used were
3.27 (ring C of Cp*), 87.45 (s, OC(CH ) ), 37.95 (NCH ), 29.28
3
3
3
OC(CH ) ), 9.93 (CH of Cp*) (see Figure S27). FT-IR (ATR,
3 3 3
1
−
[Cp*IrCl ] (0.1416 g 0.178 mmol, 1 equiv), AgOTf (0.0951 g, 0.370
2 2
cm ): 3118 (w), 2985 (w), 1619 (m), 1575 (w), 1480 (m), 1444
m), 1399 (m), 1374 (m), 1301 (m), 1258 (s), 1222 (m), 1136 (s),
028 (s), 910 (m), 823 (w), 800 (m), 763 (m), 717 (w), 694 (m), 636
s), 569 (m), 514 (m), 454 (w) (see Figure S29). ESI-MS: m/z found
mmol, 2.08 equiv), and 6,6′-dmbp (0.0801 g, 0.370 mmol, 2.08 equiv).
A color change of orange to yellow occurred with the addition of
solvent. The reaction mixture was concentrated to dryness after
filtering, and the desired product was collected as a yellow powder,
(
1
(
(
5
5
Me
OtBu
+
+
expected) 594.1 ([Cp*IrCl(NHC -py )] = [C H ClIrN O] ,
2
3
32
3
which was recrystallized from MeCN solution and layered with dry
Me
OH
+
+
94.19), 538.0 ([Cp*IrCl(NHC -py )] = [C H ClIrN O] ,
1
19
24
3
Et O (0.2458 g, 0.338 mmol, 94.9% yield). H NMR (360 MHz,
Me
OH
+
+
2
38.12), 502.1 ([Cp*Ir(NHC -py )] = [C H IrN O] , 502.15)
3
3
1
9
23
3
CD CN, ppm): δ 8.12 (t, J = 8.4 Hz, 2H, Py-CH), 7.95 (d, J =
.7 Hz, 2H, Py-CH), 7.26 (d, J = 8.5 Hz, 2H, Py-CH), 4.10 (s, 6H,
OCH ), 1.53 (s, 15H, CH of Cp*) (see Figure S40). C { H}NMR
3
H,H
H,H
(
see Figure S28). Anal. Calcd for [C H N O F SClIr] = (1): C,
3
24
32
3
4 3
7
H,H
3
8.75%; H, 4.34%; N, 5.65%. Found: C, 38.57%; H, 4.69%; N, 5.59%.
13
1
Synthesis of [Cp*IrCl(NHCMe-pyOMe)]OTf (3). The same
3
3
(
(
1
9
125.76 MHz, CD CN, ppm): δ 165.43 (C (MeO-CN)), 155.55
Me
3
Py
procedure was followed as for the synthesis of [Cp*IrCl(NHC
py )]OTf with the following differences. The reagents and amounts
used were [Cp*IrCl ] (0.200 g 0.261 mmol, 1 equiv), AgOTf (0.0671
-
C ), 144.02 (C ), ∼118 (peaks for CF carbon are very weak),
OtBu
Py
Py
3
17.65 (C ), 110.91 (C ), 90.01 (ring C of Cp*), 58.65 (OCH ),
Py
Py
3
2
2
19
.75 (CH3 of Cp*) (see Figure S41). F NMR (338.86 MHz,
Me
OMe
g, 0.261 mmol, 1 equiv), and [Ag(NHC -py )2]OTf (0.1652 g,
.261 mmol, 1 equiv). A color change of orange to yellow occurred
CD CN, ppm): δ −79.33 (s, CF of triflate) (see Figure S42). FT-IR
3
3
0
−1
(
7
ATR, cm ): 441 (w), 463 (w), 515 (m), 570 (m), 634 (s), 681 (w),
with the addition of solvent. The resulting crystals that grew were
yellow rods of the pure product (0.2737 g, 0.390 mmol, 75.0% yield).
11 (w), 743 (w), 797 (s), 814 (w), 1028 (s), 1074 (m), 1142 (s),
1
194 (w), 1222 (m), 1255 (s), 1276 (m), 1303 (w), 1349 (w), 1425
1
3
H NMR (500 MHz, CDCl , ppm): δ 8.08 (d, J = 2.3 Hz, 1H, CH
3
HH
(w), 1484 (m), 1572 (m), 1603 (m), 2856 (w), 3075 (w) (see Figure+
3
3
of the NHC), 8.04 (t, J = 8.2 Hz, 1H, Py-CH), 7.70 (d, J = 7.8
Hz, 1H, Py-CH), 7.33 (d, J = 2.3 Hz, 1H, CH of the NHC), 6.91
HH
HH
S44). ESI-MS: m/z found (expected): 579.1 ([Cp*IrCl(6,6′-dmbp)]
3
+
+
HH
= [C H ClIrN O ] , 579.14), 543.1 ([Cp*Ir(6,6′-dmbp)]
=
2
2
27
2
2
3
+
(
d, J = 8.2 Hz, 1H, Py-CH), 4.13 (s, 3H, OCH ), 4.03 (s, 3H,
H
H
3
[C H IrN O ] , 543.16) (see Figure S43). Anal. Calcd for
22 26 2 2
13
1
NCH ), 1.78 (s, 15H, CH of Cp*) (see Figure S35). C { H}NMR
3
3
[C H N O F SClIr] = (5 ): C, 37.91%; H, 3.74%; N, 3.85%.
23 27 2 5 3 Ir
Found: C, 37.95%; H, 3.81%; N, 3.77%.
(
125.76 MHz, CDCl , ppm): δ 166.90 (C
(carbene bound to Ir),
3
NHC
1
(
1
3
64.43 (CPy (MeO-CN)), 151.21 (C ), 145.21 (C ), 125.91
Py
Py
Procedure for Catalytic Hydrogenation of CO . Catalyst
2
1
CNHC), 121.03 (q, J = 320.8 Hz, CF of triflate), 118.38 (C ),
solution (0.3 mM, 25 mL) in 1 M NaHCO3(aq) was added to a Parr
high-pressure vessel. The vessel was purged at least 3 times and then
pressurized to 300 psig with 50:50 CO /H . The vessel was heated at
CF
3
NHC
04.75 (C ), 104.66 (C ), 92.61 (ring C of Cp*), 58.35 (OCH ),
Py Py 3
19
7.77 (NCH ), 10.02 (CH of Cp*) (see Figure S36). F NMR
3
3
2
2
(
338.86 MHz, CDCl , ppm): δ −78.13 (s, CF of triflate) (see Figure
115 °C with stirring for 18 h. After the reaction time, the vessel was
3
3
−1
S37). FT-IR (ATR, cm ): 3107 (w), 2918 (w), 1620 (m), 1580 (w),
488 (m), 1454 (m), 1385 (m), 1360 (m), 1299 (m), 1260 (s), 1223
m), 1144 (s), 1058 (s), 974 (m), 794 (m), 753 (w), 694 (m), 602 (s),
cooled to room temperature, and the pressure was released. The
1
1
(
amount of formate produced was determined by H NMR
spectroscopy in D O with isonicotinic acid as an internal standard.
2
+
5
71 (m), 516 (m), 479 (w) (see Figure S39). EI-MS (EI ): m/z found
Halide Removal in Situ for Hydrogenation. Silver triflate (1 equiv
for catalysts of formula [Cp*IrCl(L)]OTf and 2 equiv for catalysts
with formula [Cp*IrCl(L)]Cl and [(p-cym)RuCl(L)]Cl) was added
to the freshly prepared stock solution of aqueous catalyst (0.3 mM, 50
mL) and allowed to stir at room temperature in the absent of light for
at least 6 h. The reaction mixture was filtered over Celite with suction.
Me
OMe
+
+
(
5
expected): 551.1 ([Cp*IrCl(NHC -py )] = [C H ClIrN O] ,
20 25 3
Me OMe + +
51.13), 516.2 ([Cp*Ir(NHC -py )] = [C H IrN O] , 516.16)
20 25 3
(
2
5
see Figure S38). Anal. Calcd for [C H N O F SClIr·2H O] = (3·
21 26 3 4 3 2
H O): 34.21%; H, 4.10%; N, 5.70%. Found: C, 33.93%; H, 3.92%; N,
2
.68%.
M
DOI: 10.1021/acs.organomet.6b00806
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
A 25 mL portion of the filtrate was used without further
characterization according to the catalytic hydrogenation procedure
previously stated.
Procedure for Catalytic Dehydrogenation of Formic Acid. A
stock solution of catalyst (0.3 mM, 100 mL) was freshly prepared in
water. Three simultaneous trials were run by transferring 25 mL
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: dbgrotjahn@mail.sdsu.edu.
*E-mail: ewebster@chemistry.msstate.edu.
*E-mail: etpapish@ua.edu.
(
0.075 mmol, ∼ 0.003 mol % catalytic loading) of stock solution to
ORCID
three separate 100 mL Schlenk flasks each with tubing to an upturned
water filled graduated cylinder in a water basin. The reaction flasks
were heated to constant temperature of 60 °C with stirring, and high
purity formic acid (1.00 mL, 26.5 mmol) was added to each reaction
flask. The dehydrogenation reactions were monitored for 3 h for all
reactions and for longer times as indicated. The evolved gas was
measured, and the TON was calculated based on the number of moles
Fengrui Qu: 0000-0002-9975-2573
Robert M. Vasquez: 0000-0002-6921-7764
Douglas B. Grotjahn: 0000-0002-2481-7889
Charles Edwin Webster: 0000-0002-6917-2957
Elizabeth T. Papish: 0000-0002-7937-8019
Notes
of the catalyst used divided by the moles of CO produced (calculated
2
as one-half of the total volume of gas generated and assuming the ideal
gas law).
The authors declare no competing financial interest.
Halide removal in situ for dehydrogenation. Silver triflate (1
equiv for catalysts of formula [Cp*IrCl(L)]OTf and 2 equiv for
catalysts with formula [Cp*IrCl(L)]Cl and [(p-cym)RuCl(L)]Cl) was
added to the freshly prepared stock solution of aqueous catalyst (0.3
mM, 100 mL) and allowed to stir at room temperature in the absent of
light. After 12 h of stirring, the reaction mixture was filtered over Celite
with suction and used without further characterization according to the
catalytic dehydrogenation procedure previously stated.
ACKNOWLEDGMENTS
■
We thank NSF CAREER for past support (Grants CHE-
0846383 and CHE-1360802 to E.T.P. and her group), NSF
EPSCoR Track 2 Seed Grant to E.T.P. for support during 2016
(
PI N. Hammer, Grant OIA-1539035), Drexel University for
past support, and the University of Alabama for generous
financial support (including a UCRA award to N.C. and
CARSCA support). Computational work was partially
supported by the Mississippi State University Office of
Research and Economic Development and the National
Science Foundation (OIA-1539035). Computations were
performed at the Mississippi State University High Perform-
ance Computing Collaboratory and the Mississippi Center for
Supercomputing Research. D.B.B thanks a GAANN program
fellowship (Grant P200A150329). R.M.V. thanks the NIH
SDSU IMSD Program Grant GM058906-16. We also thank
Qiaoli Liang (the University of Alabama) for MS analysis.
Finally, we thank the members of the Papish group for
assistance and suggestions. We thank Nicholas J. White for
early work on this project. We thank Matthias Zeller (Purdue
University) and Allen Oliver (Notre Dame) for help with X-ray
structure determination. We thank Johnny R. Goodwin for
SEM and Rob Holler and Tabitha Sutch for XPS experiment
assistance.
Computational Methods. The mechanisms were proposed via
68
DFT computations using Gaussian 09 (revision E01). Gas phase
69,70
geometry optimizations were carried out with PBEPBE
functional
and basis set 1 (BS1). In BS1, iridium utilized the Couty and Hall
71−73
modified-LANL2DZ
basis set and the associated LANL2DZ
effective core potential, and all other atoms (C, O, N, Na, and H) used
7
4−77
the 6-31G (d′)
basis sets. Harmonic vibrational frequency
computations were performed to verify the nature of all stationary
points. For the solvation effect in aqueous conditions to be
approximated, the self-consistent reaction field (SCRF) single-point
computations with the SMD solvation model on gas-phase
optimized geometries were performed. Nondefault self-consistent
7
8
−6
field (SCF) convergence (10 ) and density fitting approximation
79,80
(
with AUTO keyword)
single-point SMD solvation computations. Spherical harmonic 5d and
f functions and a pruned integration grid containing 75 radial shells
were used in geometry optimizations and
7
and 302 angular points per shell were used for all computations. Free
energy corrections were determined at 1 atm and 298.15 K. The
experimental value of proton solvation energy in water (−265.9 kcal
−
1
81,82
83,84
mol )
and experimental Gibbs free energy of proton (−6.28 kcal
were used to calculate the relative reaction energy of the
−1
mol )
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ASSOCIATED CONTENT
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*
S
Supporting Information
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DOI: 10.1021/acs.organomet.6b00806
Organometallics XXXX, XXX, XXX−XXX

Organometallics
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