H. Hori et al. / Inorganic Chemistry Communications 6 (2003) 300–303
301
with sapphire windows was charged with an Et3N=DMF
solution (9 ml, Et3N 0.80 M) containing 1 (1.1 mg, 2.38
lmol). Then the reactor was connected to a CO2 cylin-
der through the needle valve and charged to the desired
CO2 pressure. The sample solution was then irradiated
for 0–25 h with stirring. In all runs, the temperature was
maintained at 27 °C. After irradiation, the pressure was
released, and the gas was collected in a sampling bag
and subjected to measurements.
consisting of a UV detector, an ODS column, and a
mobile phase consisting of a mixture (60:40, v/v) of
methanol and KH2PO4–NaOH buffer (0.05 M).
3. Results and discussion
The irradiation-time dependence of CO formation
using 1 in Et3N=DMF solutions under various pressures
is shown in Fig. 1. When the reaction was carried out
under normal pressure (Fig. 1a), steady CO formation
stopped at ca. 3 h, and the amount of CO after 25 h of
irradiation was 19.6 lmol, which corresponds to a
turnover number of 8.2 (Table 1). Addition of a 23-fold
excess of Et4NCl (ClÀ source) to the reaction solution
increased the duration of steady CO formation to ca. 5 h
(Fig. 1b) and the turnover number at 25 h was 11.5. The
saturated CO2 concentration in the Et3N=DMF solu-
tion under normal pressure was measured to be 0.19 M.
Increasing the CO2 pressure increased the CO2 concen-
tration in the Et3N=DMF solution; that is, 1.21 MPa of
CO2 gas raised the CO2 concentration in the liquid
phase to 2.63 M (Table 1). The increased CO2 concen-
tration in the reaction solution had a favorable effect on
the catalytic CO formation (compare Figs. 1a, c, and d).
At higher-than-normal pressures, steady CO formation
continued much longer than it did under normal pres-
sure either with or without excess ClÀ ions. The CO
formation rate, calculated from the data in the region
where the CO amount increased linearly with respect to
time, increased with increasing pressure (Table 1). The
CO formation rate under 2.45 MPa of CO2 was 6.80
lmol hÀ1, which is 1.3 times the rate obtained under
normal pressure. At 2.45 MPa of CO2, the CO amount
after 25 h of irradiation reached 99.5 lmol, which cor-
responds to a turnover number of 41.8. This value is 5.1
times that obtained under normal pressure without
excess ClÀ ions and 3.6 times that obtained with excess
ClÀ ions.
2.3. Measurements
The saturated CO2 concentrations in the Et3N=DMF
mixtures (Et3N 0.80 M) under high pressure were de-
termined from the weight of charged CO2, the pressure,
and the volumes of the gas and liquid phases. The CO2
concentrations under normal pressure were obtained by
a titration method [13]. The amount of CO was mea-
sured with a gas chromatograph equipped with a
methanizer, a molecular-sieve column, and a flame
ionization detector. The amount of 1 during irradiation
was measured with a reversed-phase HPLC system
Fig. 1. Irradiation-time dependence of CO formation catalyzed by 1
under various CO2 pressures: (a) 0.10 MPa; (b) 0.10 MPa with excess
ClÀ ions; (c) 1.21 MPa; (d) 2.45 MPa. Irradiation wavelength was 365
nm.
To determine why CO formation increased at higher
CO2 pressures, we used HPLC to measure the concen-
tration of 1 during the photocatalytic reduction of CO2.
Table 1
CO formation data catalyzed by 1 under several CO2 pressures
Entry
PðCO2Þ ðMPaÞ
[CO2]a (M)
CO formation rateb (lmol hÀ1
)
CO amountc (lmol)
Turnover numberd
1
0.10
0.10
1.21
2.45
0.19
0.19
2.63
5.44
5.14
5.11
5.95
6.80
19.6
27.3
76.2
99.5
8.2
11.5
32.0
41.8
2e
3
4
a CO2 concentration in the Et3N (0.80 M)/DMF mixture.
b Calculated from the data in the time region where the CO amount increased linearly with respect to time; 0–3 h for entries 1 and 2 (a and b in
Fig. 1) and 0–5 h for entries 3 and 4 (c and d in Fig. 1).
c CO amount after 25 h of irradiation.
d Turnover number ¼ (mol of CO at 25 h)/(mol of catalyst 1 used).
e NEt4Cl (6.07 mM) was added.