Bimetallic iridium-carbene complexes with mesoionic triazolylidene ligands for water oxidation catalysis

Article abstract of DOI:10.1002/ejic.201300843

Two new diiridium-triazolylidene complexes were prepared as bimetallic analogues of established mononuclear water oxidation catalysts. Both complexes are efficient catalyst precursors in the presence of cerium ammonium nitrate (CAN) as sacrificial oxidant

Full text of DOI:10.1002/ejic.201300843

FULL PAPER
DOI:10.1002/ejic.201300843
Bimetallic Iridium–Carbene Complexes with Mesoionic
Triazolylidene Ligands for Water Oxidation Catalysis
CLUSTER
ISSUE
Ana Petronilho,[a] James A. Woods,[b] Stefan Bernhard,*[b] and
Martin Albrecht*[a]
Dedicated to Professor Ernesto Carmona on the occasion of his 65th birthday[‡]
Keywords: Water chemistry / Oxidation / Iridium / Carbenes / Bimetallic complexes
Two new diiridium–triazolylidene complexes were prepared
as bimetallic analogues of established mononuclear water
oxidation catalysts. Both complexes are efficient catalyst pre-
cursors in the presence of cerium ammonium nitrate (CAN)
as sacrificial oxidant. Up to 20000:1 ratios of CAN/complex,
the turnover limitation is the availability of CAN and not the
catalyst stability. The water oxidation activity of the bimetal-
lic complexes is not better than the monometallic species at
(0.03 mM), the bimetallic complexes double their activity,
whereas the monometallic complexes show an opposite trend
and display markedly reduced rates, thereby suggesting a
benefit of the close proximity of two metal centers in this
low concentration regime. The high dependence of catalyst
activity on reaction conditions indicates that caution is re-
quired when catalysts are compared by their turnover fre-
quencies only.
0.6 m
M catalyst concentration. Under dilute conditions
active site as well as the opportunity to use the structure–
activity relationship for tailoring catalytic activity. Indeed,
the relevance of the ligand framework has been demon-
strated in various studies.[6] Appreciable performance was
noted with some first-row transition-metal complexes,[7] al-
though the most active molecular water oxidation catalysts
have emerged thus far from ruthenium and iridium com-
plexes.[8] When coordinated to a suitable ligand set, turn-
over frequencies as high as those of the natural oxygen
evolving complex (OEC) have been observed,[9] as well as
catalyst longevity that enables turnovers in the 104 range.[10]
Within this context, triazolylidene ligands,[11] a specific
class of mesoionic carbenes,[12] have recently been shown to
impart attractive and tunable properties for water oxidation
catalysis.[10,13]
The key step in water oxidation catalysis is generally ac-
cepted to be the rate-limiting O–O bond formation from
the crucial oxidized M=O species.[14] Mechanistically, two
distinct pathways have been proposed for this step, includ-
ing either the interaction of two high-valent metal–oxo spe-
cies (M=O···O=M interaction), or the nucleophilic attach-
ment of a water molecule to a metal–oxo site (M=O···OH2
interaction).[15] These different mechanisms have crucial
consequences on the ligand and catalyst design, and also
on potential catalyst immobilization strategies. A bimetallic
mechanism will clearly benefit from a close proximity of
the two metal centers (e.g., through intramolecular linkage),
whereas the nucleophilic attack will require only one metal
Introduction
Artificial photosynthesis is considered a viable and sus-
tainable process for reducing the global dependence on fi-
nite fossil-fuel resources.[1] A conceivable photosynthetic
pathway hence consists of a water reduction cycle (WRC;
2H+ + 2e– ǞH2) and a concomitant water oxidation cycle
(WOC; 2H2OǞO2 + 4H+ + 4e–), which results in an over-
all splitting of water into O2 and H2 as carbon-neutral
fuel.[2]
Although stunning performance in catalyst development
for the WRC have been achieved,[3] WOC is comparably
difficult because of the complexity of O2 formation and the
uphill thermodynamics. Oxygen evolution activity has been
demonstrated with a variety of heterogeneous and hetero-
genized catalysts.[4] Recent efforts have furthermore concen-
trated on molecular catalyst precursors, stimulated by the
pioneering work of Meyer et al.[5] Conceptually, molecular
catalysts offer intriguing benefits such as a well-defined
[a] School of Chemistry and Chemical Biology, University College
Dublin,
Belfield, Dublin 4, Ireland,
E-mail: martin.albrecht@ucd.ie
www.ucd.ie/chem/staff/profmartinalbrecht/
[b] Department of Chemistry, Carnegie Mellon University,
4400 Fifth Avenue, Pittsburgh, PA 15213, USA
E-mail: bern@cmu.edu
http://www.chem.cmu.edu/groups/bernlab/
[‡] In admiration of his ground-breaking contributions to bond
activation and organometallic chemistry in general
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site and might become deactivated by the close proximity
In both ligands the trimethylene linker is identified by a
of another metal center.[16] Since direct mechanistic probing characteristic triplet for the NCH2 protons around δH
of the reaction pathway is extremely challenging due to the 5 ppm and a quintet for the central CH2 group at δH
=
=
multielectron, multiproton transfer involved in water oxi- 2.75 ppm (3) or δH = 2.95 ppm (4). The acidic triazolium
dation,[17] we set out to synthesize a bimetallic system that proton appears at low field (δH = 9.23 and 9.25 ppm in 3
might improve the kinetics of water oxidation if an inter- and 4, respectively).
change of two M=O units is involved in (rate-limiting) O–O
bond formation.[18] The known and prolific water oxidation
catalyst precursors 1 and 2 provided a valuable starting
point for these investigations (Figure 1).[10,13b]
These ligands were metalated according to protocols
closely related to those described for the corresponding mo-
nometallic complexes 1 and 2.[10,13b] Thus, reaction of
di(triazolium) salt 3 with Ag2O (4 equiv.) for prolonged
time at room temperature followed by transmetalation with
stoichiometric quantities of [{IrCp*Cl2}2] afforded the de-
sired complex 5 (Scheme 1). The bimetallic chelating com-
plex 6 was synthesized by means of a similar trans-
metalation protocol, although in this case, Ag2O and the
iridium precursor salt were added simultaneously and the
metalation was performed at 80 °C. Both complexes are
orange, air-stable compounds that dissolve well in MeCN,
water, and MeOH. In contrast to the ionic complex 6, the
neutral diiridium complex 5 is also very soluble in chlorin-
ated solvents.
Figure 1. Triazolylidene–iridium complexes 1 and 2 as water oxi-
dation catalyst precursors.
Complexes 5 and 6 share common spectroscopic charac-
teristics. Specifically, the Cp*/ligand ratio indicates the for-
mation of a bimetallic system rather than a chelating di(tri-
azolylidene) complex. Moreover, the NCH2 protons are
shifted to higher field. This shift difference is more pro-
Results and Discussion
Ditopic ligands that comprise the ligand scaffold of 1 nounced in 5 (ΔδH = 1.47 ppm) than in 6 (ΔδH = 0.24 ppm).
and 2, respectively, were readily accessible due to the versa- Chelation in 6 is unambiguously deduced from the reso-
tility of copper-catalyzed cycloaddition protocols.[19] Thus, nance pattern of the pyridylidene ring, which features two
reaction of diazidopropane with either phenyl or pyridyl doublets (δH = 8.71 and 8.29 ppm) and a triplet (δH
=
acetylene followed by reaction with a methylating agent af- 7.60 ppm), in agreement with the activation of one Cpy–H
forded the desired ligands 3 and 4 in moderate to high bond. In 6, most resonances are broad, and the coupling
yields (Scheme 1). Quaternization of the pyridyl-function- in the linker is poorly resolved, presumably because of the
alized di(triazole) intermediate was accomplished both with presence of two diastereoisomers due to the chirality at irid-
[Me3O]BF4 and with MeOTf, whereas MeI is not reactive ium.[20] The presence of two resonances for the Cp–CH3
enough to install four methyl groups, and methylation oc- groups in equal ratio (δH = 1.85 and 1.84 ppm) lends fur-
curs only at the pyridine. In contrast, MeI is sufficiently ther support to such a model. The iridium-bound carbon
reactive to yield the di(triazolium) salt 3 in high yields resonates at δC = 149.7 ppm in the 13C NMR spectrum of
(80%).
5, and at δC = 154.5 and 153.9 ppm in 6.
Scheme 1. Synthesis of complexes 5 and 6.
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The electrochemical properties of complexes 5 and 6 are (0.29 versus 0.17 s–1), which corresponds to some 25% ac-
unsurprising and do not differ significantly from the mono- tivity of the iridium centers in 5 relative to 1. The trend is
metallic model compounds and from related species.[21] For opposite when using the chelating pyridylidene–triazolyl-
example, complex 6 does not show any distinct redox fea- idene species 2 and 6 as precursors. The TOFmax measured
ture below +1 V (aqueous solution, 1 m HNO3, pH = 0) from the bimetallic precatalyst is indeed twice as high as
and reveals solvent discharge (i.e., electrocatalytic water oxi- that of the monometallic species (0.14 versus 0.07 s–1).
dation) above this potential.
Maximum rates for 6 were achieved after about 3 min {i.e.,
Both complexes were evaluated as catalysts for the oxi- at a stage of the reaction when CAN was still in large excess
dation of water in the presence of sacrificial [Ce(NO3)6]- (Ͻ10% CAN conversion, namely, CAN/[Ir] ratio
(NH4)2 (CAN) as terminal oxidant. An initial set of experi- Ͼ1000:1)}. After about 40 min of reaction, the turnover fre-
ments was carried out at approximately 0.6 mm concentra- quencies of 1, 2, 5, and 6 are essentially identical and grad-
tion of catalyst in a 0.67 m aqueous solution of CAN. Un- ually decrease as CAN is exhausted. This may support a
der these conditions, essentially all CAN was consumed mechanistic model that involves multiple active species with
within 90 min, and oxygen production was close to the differing levels of catalytic activity as postulated by Beller
theoretical limit (Figure 2; Table 1, entries 1–4). Inspection and co-workers.[17e] Accordingly, heterogeneous (and kinet-
of the early stages of the reaction indicates only very minor ically less competent) species become increasingly prevalent
differences in conversion between the monometallic species as the reaction proceeds, whereas at initial stages, homoge-
1 and the bimetallic complexes 5 and 6 (inset Figure 2). neously operating catalysts are predominant.
This similarity is remarkable, because complexes 5 and 6
possess two active metal centers, and hence a higher activity
would be expected. Monitoring the rate of oxygen evolution
reveals a delicate influence of the ligand scaffold. Thus, the
bimetallic complex 5 is significantly slower than the mono-
metallic analogue within the first 1000 s of the reaction
(Figure 3, Table 1). The maximum turnover frequency
(TOFmax) of the catalytically active species derived from 1
is about twice as high as that of the bimetallic precursor
Figure 3. Changes in turnover frequency in water oxidation cataly-
sis with a CAN/complex ratio of approximately 1000:1 (0.6 mm
complex) for complexes 1, 2, 5, and 6 over the first 1000 s of O2
evolution (TOFs measured by manometry with a custom-built
transducer setup; see the Experimental Section). Solid lines repre-
sent the average of triplicate measurements, faint areas indicates
error bands (95% confidence).
A second set of experiments was subsequently performed
under more dilute conditions. Lower concentrations are ex-
pected to increase the relevance of mononuclear processes
(i.e., water nucleophilic attack) in monometallic systems;
however, in bimetallic complexes O–O bond formation
Figure 2. Oxygen evolution with a CAN/complex ratio of approxi-
mately 1000:1 (0.6 mm complex) for complexes 1, 2, 5, and 6. Inset:
Oxygen evolution at initial 15 min of the reaction.
might still occur through a M=O···O=M interaction due
to the locally high concentration of metal centers. Water
Table 1. Catalytic performance of complexes 1, 5, and 6.[a]
oxidation with the four iridium complexes 1, 2, 5, and 6
was thus carried out at approximately 0.035 mm catalyst
concentration with a CAN/complex ratio of approximately
20000:1 (Table 1, entries 5–8; Figure 4). Oxygen evolution
is complete after around 24 h (Figure 4). At this stage, the
turnover numbers reached values slightly higher than 5000,
thus indicating essentially complete consumption of CAN
and efficient conversion of the redox equivalents for O2 pro-
duction with all three catalytic systems. The fact that the
sacrificial oxidant is turnover-limiting rather than catalyst
deactivation suggests that turnover numbers might be
markedly increased when using a flow reactor instead of the
batch system applied here.
Entry Complex [mm]
CAN/complex TON
TOFmax [s–1]
1
2
3
4
5
6
7
8
1 (0.65)
2 (0.63)
5 (0.65)
6 (0.69)
1 (0.034)
2 (0.035)
5 (0.033)
6 (0.034)
1100
1100
1100
1100
22000
22000
22000
22000
260
250
240
230
5300
5400
5500
5100
0.29 (Ϯ0.01)
0.07 (Ϯ0.01)
0.17 (Ϯ0.03)
0.14 (Ϯ0.01)
0.22 (Ϯ0.05)
0.21 (Ϯ0.03)
0.47 (Ϯ0.06)
0.29 (Ϯ0.02)
[a] Reactions with 0.6 mm complex loading typically performed in
6 mL H2O; reactions with 0.03 mm complex in 10 mL H2O. Turn-
over numbers (TONs) calculated at 10000 s and at 48 h, respec-
tively.
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With both mono- and bimetallic complexes that contain
the bidentate chelating carbene ligand, the maximum turn-
over rate doubles upon dilution of the catalyst concentra-
tion (e.g., from 0.14 s–1 to 0.29 s–1 for 6). Yet, complex 6 is
considerably less active than the precatalyst derived from
bimetallic 5 with the monodentate bis(carbene) ligand. The
lower activity of complex 6 might be attributed to the en-
tropic challenges to reach the appropriate bis(Ir=O) config-
uration because of the chelating ligand, or indeed to other
aspects such as the different partial charge due to the ab-
sence or presence of anionic ligands. In any case, the oppo-
site concentration dependence of the rates observed here
with mono- and bimetallic systems is remarkable and seems
to disfavor a mere complex degradation and formation of
colloidal species as the catalytically active species.[17e,23] In
agreement with this conclusion, headspace analysis of the
reaction at different stages by MS did not reveal any CO2
above the detection threshold (Ͻ1 ppm).
Figure 4. Oxygen evolution with a CAN/complex ratio of approxi-
mately 20000:1 (0.03 mm complex) for complexes 1, 2, 5, and 6.
Inset: oxygen evolution at initial 1.5 h of the reaction.
In contrast to the experiments performed at higher cata-
lyst loading, inspection of early stage conversions with low
iridium concentrations revealed a significantly higher pro-
ductivity of the bimetallic complex 5 than the monometallic
analogue 1 (inset Figure 4). The different initial reaction
rates are illustrated in Figure 5, which indicates a maximum
turnover frequency of complex 5 that is now twice as high
as that of 1 (0.47 s–1 versus 0.22 s–1).[22] This behavior is
in sharp contrast to the measurements at higher catalyst
concentration, whereby the bimetallic species is less efficient
(Table 1). These observations might be directly correlated
to a potential change in mechanism for these complexes
under dilute conditions. Tentatively, the decrease of rate for
the monometallic species might be attributed either to a
rapid heterogenization (formation of catalytically active col-
loids) or to an increasing relevance of water nucleophilic
attack relative to the interaction of two iridium–oxo spe-
cies.[8d] The opposite effect, namely, the rate increase of the
bimetallic complexes under dilute conditions, might specu-
latively arise from the suppression of undesired intermo-
lecular processes[16] that might occur at higher catalyst con-
centrations, and the preponderance of a mechanism that
involves a bimetallic Ir=O···O=Ir interaction in the key
step.
Conclusion
Straightforward synthetic access to bimetallic triazolylid-
ene–iridium complexes has been disclosed. The complexes
are active catalyst precursors for CAN-mediated water oxi-
dation. When compared to the monometallic analogues, the
activity is improved when water oxidation is performed un-
der dilute catalyst concentrations. This result might point
to a change in the mechanism that is related to the concen-
tration of iridium complexes and emphasizes the difficulties
associated with using turnover frequencies as the key pa-
rameter for the evaluation of catalyst precursors. These re-
sults thus strongly underline the relevance of reaction con-
ditions, especially when comparing different catalyst sys-
tems. Our results tentatively support a bimetallic O–O
bond-forming mechanism at low iridium concentration, al-
though we cannot confidently rule out an oxidative cleavage
of the linker, which curtails any benefits from a dimetallic
catalyst precursor, especially at higher iridium concentra-
tions, and might lead to a heterogeneous catalyst. Synthetic
efforts directed towards installing linkers with different ri-
gidity between the two triazolylidene iridium active sites are
currently in progress.
Experimental Section
General Comments: The syntheses of complexes 1[13b] and 2[10] are
reported elsewhere. The 1,3-diazidopropane was prepared as de-
scribed previously[24] and was used without isolation as a THF
solution (ca. 3 m). All other starting materials and reagents were
obtained from commercial sources and were used as received. Mi-
crowave reactions were carried out with a Biotage Initiator 2.5 op-
erating at 100 W irradiation power. NMR spectra were recorded
with Varian spectrometers operating at 300–600 MHz. Chemical
shifts δ are reported in ppm (J in Hz) relative to Me4Si or residual
protio solvents. Signals were assigned with the aid of two-dimen-
sional cross-coupling experiments. Elemental analysis was per-
Figure 5. Changes in turnover frequency in water oxidation cataly-
sis with a CAN/complex ratio of approximately 20000:1 (0.03 mm
complex) for complexes 1, 2, 5, and 6 over the first 10000 s of O2
evolution (TOFs measured by manometry). Solid lines represent
average of triplicate measurements; faint areas indicate error bands
(95% confidence).
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formed with an Exeter Analytical CE440 elemental analyzer. High-
resolution mass spectrometry was carried out with a Micromass/
Waters Corp. USA liquid chromatography time-of-flight spectrom-
eter equipped with an electrospray source.
[{IrCp*Cl2}2] (129 mg, 0.16 mmol) was added, and the mixture was
stirred for another 24 h. The mixture was filtered through Celite,
and the filtrate was evaporated to dryness. The solid residue was
dissolved in CH2Cl2, filtered through Celite, and triturated with
Et2O to give an orange precipitate, which was isolated and dried
under vacuum, yield 70 mg (41%). 1H NMR (500 MHz, CDCl3):
Synthesis of 3: Diazidopropane (3 mL, approx 3 m in THF, 9 mmol)
and phenylacetylene (1.5 mL, 16 mmol) were dissolved in a mixture
of THF/water (1:1, 15 mL). CuSO4 (110 mg, 0.70 mmol) and so-
dium ascorbate (1.4 g, 7 mmol) were added and the mixture was
reacted under microwave irradiation at 100 °C for 1 h. The solvents
were removed under reduced pressure, and the residue was sus-
pended in CH2Cl2 and washed consecutively with aqueous NH3
(10%), water, and brine. After drying over MgSO4 and solvent
evaporation, compound 1 was obtained as a white powder 643 mg
(25%). The solid was dissolved in MeCN (15 mL) and MeI
(1.1 mL, 18 mmol) was added. The mixture was stirred under mi-
crowave irradiation at 100 °C for 2 h. Et2O was added, and the
formed white precipitate was filtered off and dried under vacuum
3
δ = 7.65 (d, JH,H = 6.4 Hz, 4 H; HAr), 7.41–7.37 (m, 6 H; HAr),
5.00 (br. s, 4 H; NCH2), 3.75 (s, 3 H; NCH3), 3.0–2.9 (m, 2 H; C–
CH2–C), 1.36 (s, 30 H; Cp–CH3) ppm. 13C{1H} NMR (125 MHz,
CDCl3): δ = 149.7 (Ctrz–Ir), 132.6, 129.7, 127.8 (3ϫCPhH), 127.8
(Ctrz–Ph), 127.7 (CPh–trz), 88.1 (CCp–Me), 50.6 (NCH2), 37.3
(NCH3), 31.5 (br., C–CH2–C), 9.0 (CH3–Cp) ppm. HRMS (ES+):
calcd. for [M + Na]+ 1177.2200; found 1177.2164. C41H52Cl4Ir2N6
(1155.15): calcd. C 42.63, H 4.54, N 7.28; found C 42.78, H 4.26,
N 6.45.[25]
Synthesis of 6: Compound 4a (100 mg, 0.15 mmol), [{IrCp*Cl2}2]
(115 mg, 0.15 mmol) and Ag2O (100 mg, 0.43 mmol) were sus-
pended in dry MeCN (10 mL) and stirred at 80 °C for 24 h in the
absence of light. After cooling to room temp., the reaction mixture
was filtered through Celite and Et2O was added, which induced the
formation of an orange precipitate. This precipitate was dissolved
in MeOH, filtered, and layered with Et2O, which induced slow for-
mation of an orange precipitate. Further precipitation was ac-
complished upon storing the mixture at –23 °C for 2 h. The precipi-
1
to afford 3 (965 mg, 81%). H NMR (500 MHz, [D6]DMSO): δ =
9.25 (s, 2 H; CHtrz), 7.79–7.75 (m, 4 H; CHPh), 7.71–7.66 (m, 6 H;
3
CHPh), 4.89 (t, JH,H = 6.9 Hz, 4 H; NCH2), 4.32 (s, NCH3), 2.75
3
(quintet, JH,H = 6.9 Hz, 2 H; C–CH2–C) ppm. 13C{1H} NMR
(125 MHz, [D6]DMSO): δ = 142.9 (Ctrz–Ph), 132.0, 129.9, 129.8
(3ϫCPhH), 129.7 (CtrzH), 123.1 (CPh–trz), 50.4 (NCH2), 39.4
(NCH3), 28.5 (C–CH2–C) ppm. HRMS (ES+): calcd. for
[M – 2I]2+ 180.1031; found 180.1036. C21H24I2N6·0.5H2O: calcd. C
40.47, H 4.04, N 13.48; found C 40.38, H 3.74, N 13.11.
1
tate was collected and dried under vacuum, yield 79 mg (36%). H
3
NMR (400 MHz, CD3CN): δ = 8.71 (d, JH,H = 7 Hz, 2 H; Hpy),
3
3
8.29 (d, JH,H = 7 Hz, 2 H; Hpy), 7.60 (t, JH,H = 7 Hz, 1 H; Hpy),
4.9–4.6 (m, 4 H; NCH2), 4.61, 4.58 (2ϫs, 3 H; NCH3), 2.75–2.65
(m, 2 H; C–CH2–C), 1.85, 1.84 (2ϫs, 15 H; Cp–CH3) ppm.
13C{1H} NMR (100 MHz, CD3CN): δ = 155.2 (Cpy–trz), 154.5
(Ctrz–Ir), 153.9 (Cpy–Ir), 152.5 (CpyH), 149.2 (Ctrz–py), 140.0, 125.2
(2ϫCpyH), 94.4 (CCp–Me), 50.9 (NCH2), 44.7, 44.2 (2ϫNCH3),
29.5 (C–CH2–C), 8.8 (CH3–Cp) ppm. HR-MS (ES+): calcd. for
Synthesis of 4: Diazidopropane (3 mL, approx 3 m in THF, 9 mmol)
and 2-ethynylpyridine (1.0 mL, 16 mmol) were dissolved in a THF/
water mixture (1:1, 15 mL). CuSO4 (110 mg, 0.70 mmol) and so-
dium ascorbate (1.4 g, 7 mmol) were added, and the mixture was
reacted under microwave irradiation at 100 °C for 1 h. The solvents
were removed under reduced pressure, and the residue was sus-
pended in CH2Cl2 and washed sequentially with aqueous NH3
(10%), water, and brine. After drying over MgSO4 and evaporation
of all volatiles, the di(triazole) was obtained as a white powder
(908 mg, 54%). The triazole (900 mg, 2.7 mmol) was dissolved in
a mixture of 1,2-dichloroethane (15 mL) and MeOH (3 mL), and
MeOTf (2.5 mL, 23 mmol) was added. After stirring for 48 h at
90 °C, the mixture was cooled to room temp. Addition of Et2O
yielded a oily residue, which was separated and washed with copi-
ous amounts of Et2O to give compound 4a (950 mg, 39%) as a
white solid.
C42H50F6Ir2N9O6S2 [M
–
2MeOTf, CH3]2+ 670.1244; found
670.1313. C49H60F12Ir2N10O12S4 (1721.73): calcd. C 34.18, H 3.51,
N 8.14; found C 34.10, H 3.38, N 7.31.[25]
Catalytic Water Oxidation: For measurements at higher concentra-
tion, a solution of the indicated catalyst (1 mL; see Table 1 for final
catalyst concentrations) was added to a sealed vial that contained
CAN (5 mL, 0.8 m, 4.0 mmol). Experiments under dilute condi-
tions were performed by adding the indicated catalyst (0.1 mL) to
a sealed vial that contained CAN (10 mL, 0.8 m, 8.0 mmol). For
both sets of experiments, the resulting pressure increase was moni-
tored by means of manometry. End points were verified by gas
chromatography and corrected for nitrogen contamination. Head-
space MS analysis was performed at different stages of the reaction.
Further experimental details are reported in the literature.[6a]
Another batch of di(triazole) (430 mg, 1.3 mmol) was dissolved in
CH2Cl2 (10 mL), and [Me3O]BF4 (1.3 g, 10 mmol) was added. The
mixture was stirred for 36 h, treated with an excess amount of
Et2O, and the precipitate was added. The mixture was stirred for
36 h, treated with an excess amount of Et2O, and the precipitate
was isolated and recrystallized from water. The first batch of crys-
tals (79 mg, 12%) yielded analytically pure 4b. Spectroscopic data
1
for 4a and 4b were identical. H NMR (500 MHz, D2O): δ = 9.23
3
3
(s, 2 H; CHtrz), 9.18 (d, JH,H = 6.1 Hz, 2 H; Hpy), 8.78 (t, JH,H
=
Acknowledgments
8.0 Hz, 2 H; Hpy), 8.35–8.31 (m, 4 H; Hpy), 7.70–7.67 (m, 4 H;
Hpy), 5.02 (t, 3JH,H = 7.3 Hz, 4 H; NCH2), 4.29 (s; NCH3), 4.27 (s;
This work has been financially supported by the European Re-
search Council (ERC StG 208651, ERC PoC 324609), and by the
Science Foundation Ireland (Solar Research Cluster). S. B. grate-
fully acknowledges support by the National Science Foundation
(CHE-1055547). We thank Johnson Matthey for a generous loan
of iridium.
3
NCH3), 2.94 (quintet, JH,H = 7.3 Hz, 2 H; C–CH2–C) ppm.
13C{1H} NMR (125 MHz, D2O): δ = 149.7 (CpyH), 149.7 (Cpy
–
trz), 147.3 (CpyH), 133.3 (CtrzH), 132.8 (Ctrz–py), 132.5 (CpyH),
131.3 (CpyH), 51.3 (NCH2), 47.5, 39.4 (2ϫNCH3), 27.3 (C–CH2–
C) ppm. C21H26B4F6N8 (547.72): calcd. C 34.19, H 3.55, N 15.19;
found C 34.23, H 3.42, N 15.19.
Synthesis of 5: Compound 3 (100 mg, 0.16 mmol) and Ag2O
(151 mg, 0.64 mmol) were suspended in dry MeCN (10 mL) and
stirred at room temp. for 3 d in the dark. Subsequently,
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batches of complexes 5 and 6 consistently revealed correct C/
H values but N percentages that were too low. Potentially, the
bimetallic samples might form barely combustible iridium ni-
tride fractions that lower the measured fraction of N2.
Received: July 3, 2013
Turnover frequencies for all complexes were highest within the
first 10–30 min of the catalytic experiments [i.e., at a stage when
conversions were less than 10% and CAN was present in a
large excess amount (ca. 18000:1 CAN/[Ir])]. At this stage,
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Published Online: August 30, 2013
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