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Angewandte
Chemie
(Table 3). The TON increased from 19 after 8 h over 52 after
24 h to 65 after 72 h, demonstrating a significant catalytic
activity even after prolonged reaction times (Table 3,
entry 5,6). Doubling the amount of ethanol in the reaction
mixture at constant total volume did not lead to an increased
methanol formation, indicating that ethyl formate formation
is not rate-limiting (Table 3, entry 7). Reduced TONs were
observed both at lower and higher ratios of MSA (Table 3,
entries 1–3), showing that only a slight excess of acid
(1.5 equiv) is required for catalyst activation.[13b] Replacing
MSA with p-toluenesulfonic acid resulted in a decreased
TON of 43, suggesting that the counterion introduced through
the acid may significantly affect the catalyst performance
(Table 3, entry 4). Variation of the ligand-to-metal ratio in the
in situ system 1 gave the best result using 2 equiv of Triphos
per [Ru(acac)3] (see the Supporting Information). Moreover,
raising the carbon dioxide pressure to 20 bar and the hydro-
gen pressure to 60 bar increased the TON to 135 (Table 3,
entry 8).
Using the isolated ruthenium complex 2 in combination
with one equivalent of the acid bis(trifluoromethane)sulfoni-
mide (HNTf2) resulted in an improved TON of 86 in
comparison to the experiment with MSA (Table 3, entry 9).
This result further corroborates the assumption of cationic
ruthenium complex 3 as catalytic active species and indicates
the enhancing effect of weakly coordinating anions. The
addition of 1.5 or 3 equivalents HNTf2 slightly reduced the
catalytic activity and a decreased TON of 77 and 65 could be
obtained (Table 3, entry 10 and 11). Most significantly, raising
the carbon dioxide pressure to 20 bar and the hydrogen
pressure to 60 bar in the reaction with 2 and 1 equiv HNTf2
resulted in a greatly increased TON of 221 (Table 3, entry 12),
which is now well in the range of even the most active
catalytic systems with reducing agents other than hydrogen.
In summary, the results of this study clearly demonstrate
the possibility to hydrogenate carbon dioxide to methanol
using a single homogeneous transition-metal catalyst under
relatively mild reaction conditions. Studies on the active
catalyst species and the detailed reaction mechanism are
currently underway in our laboratories and will provide
valuable information for rational tuning of catalyst activity.
Received: March 23, 2012
Revised: May 4, 2012
Published online: && &&, &&&&
Keywords: carbon dioxide · homogeneous catalysis ·
.
hydrogenation · methanol · ruthenium
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Experimental Section
General procedure for the homogeneous hydrogenation of carbon
dioxide to methanol: Under an argon atmosphere, catalyst
2
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(0.025 mmol), bis(trifluoromethane)sulfonimide (0.025 mmol), and
ethanol (10 mmol) were weighed into a Schlenk tube along with THF
(1.5 mL). The yellow solution was transferred via cannula to
a stainless steel autoclave under argon atmosphere. The autoclave
was pressurized with carbon dioxide to 20 bar and then hydrogen was
added up to a total pressure of 80 bar at room temperature. The
reaction mixture was stirred and heated to 1408C in an oil bath. After
24 h, the autoclave was cooled to 08C and then carefully vented. The
resulting clear yellow solution was analyzed by 1H NMR and gas
chromatography. Turnover numbers (TONs) were found to be
reproducible within DTON =Æ 5 in two independent runs for selected
experiments.
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
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