Full Paper
changed from dark-blue to yellow. The reaction mixture was stirred
for 10 min, and then the solvent was evaporated. The complex was
crystallized from THF containing a few drops of hexane at room
temperature to give yellow crystals of 2 in 49% yield (100 mg,
0.120 mmol). M.p. 2108C (decomp.); 1H NMR (400 MHz, CD3CN,
308C): d=31.88, 18.52, 11.43, 7.49, 4.37 (s, 18H; tBu), À0.49, À4.20
(s, 18H; tBu), À4.48, À14.75 ppm; IR (KBr): n˜ =3249 (m), 2955 (s),
1623 (s), 1537 (s), 1412 (s), 1354 (m), 1308 (s), 1201 (m), 1167 (s),
1141 (w), 1068 (m), 1024 (s), 952 (w), 912 (w), 872 (m), 835 (s), 810
(m), 787 (m), 744 (s), 703 (w), 640 (w), 521 (m), 439 cmÀ1 (m); UV/
Vis (THF): lmax (e)=367 nm (1.12104 dm3 molÀ1 cmÀ1); elemental
analysis calcd (%) for C38H59CeN4O6 (808.03): C 56.49, H 7.36, N
6.93; found: C 56.75, H 7.49, N 6.62.
Experimental Section
General: All manipulations involving air- and moisture-sensitive or-
ganometallic compounds were performed under argon using stan-
dard Schlenk techniques or an argon-filled glovebox. Ligand L1
and 2,5-dimethyl-1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene
were prepared according to the literature.[16] (NH4)2Ce(NO3)6 was
purchased from Sigma-Aldrich and used as received. Anhydrous
hexane, toluene, THF, and dichloromethane were purchased from
Kanto Chemical, and were further purified by passage through acti-
vated alumina under positive argon pressure as described by
Grubbs et al.[17] MS4A was purchased from Nacalai Tesque, dried at
3008C for 2 days, and then stored inside a glovebox. CD3CN was
distilled over CaH2 and degassed before use. 1H NMR (400 MHz)
and 13C NMR (100 MHz) spectra were measured on Bruker Avance
III-400 spectrometers. Electrochemical measurements were carried
out in a glovebox at room temperature, using a 0.1m solution of
[nBu4N][PF6] in CH3CN as the supporting electrolyte, an Ag wire as
the reference electrode, and a Pt wire as the counter electrode, at
a scan rate of 100 mVsÀ1. Infrared spectra were recorded on
a JASCO FT/IR-230 spectrometer from samples in KBr pellets. Typi-
cally, 32 scans were accumulated for each spectrum (resolution
4 cmÀ1). UV/Vis spectra were measured using an Agilent 8453 UV/
Vis spectroscopy system. All melting points were measured for
samples in sealed tubes under argon atmosphere on a Büchi
M-565 melting point apparatus (18CminÀ1). Elemental analyses
were determined on a Perkin-Elmer 2400 analyzer at the Faculty of
Engineering Science, Osaka University.
Preparation of complex 3: Complex 2 (100 mg, 1.24 mmol) was
placed in a pre-dried Schlenk flask inside a glovebox and dry
CH3CN (7.0 mL) was added to prepare a 17.7 mm solution. The
Schlenk flask was removed from the glovebox and the argon
above the solution was replaced by dry oxygen. The color of the
solution immediately changed from yellow to dark-violet. It was
left to stand at room temperature for 8 h, giving nice violet
needle-shaped crystals. The solvent was decanted off to give dark-
violet crystals of 3 in 73% yield. IR (KBr): n˜ =3254 (w), 2956 (s),
2866 (m), 1627 (s), 1553 (m), 1507 (s), 1435 (s), 1412 (s), 1390 (m),
1360 (m), 1331 (w), 1255 (s), 1201 (m), 1172 (m), 1141 (w), 1068 (w),
1022 (w), 980 (w), 928 (w), 912 (w), 873 (w), 834 (m), 809 (m), 778
(m), 745 (m), 642 (w), 576 (m), 524 cmÀ1 (m); UV/Vis (THF): lmax
(e)=339 nm (1.14104), 529 nm (3.37103 dm3 molÀ1 cmÀ1); ele-
mental analysis calcd (%) for C68H102Ce2N8O12 (1502.57): C 54.31, H
6.84, N 7.45; found: C 54.33, H 6.68, N 7.37.
Preparation of CeIV complex 1: A solution of the disodium salt of
ligand L1, {NH(CH2CH2N=CHC6H2-3,5-tBu2-2-ONa)2} (250 mg, 4.31
10À1 mmol), in THF (20 mL) was added to
a solution of
Preparation of hydroxo-bridged CeIV complex 4: TEMPO-H
(9.8 mg, 3.0510À2 mmol) was added to a solution of complex 2
(50.0 mg, 3.0910À2 mmol) in CH3CN (12.5 mL) under argon. Dioxy-
gen was then bubbled into the solution and the reaction mixture
was left to stand for 3 h. The color changed from yellow to red-
brown as complex 4 was quantitatively formed (see Figures S7 and
S8). It was isolated in 82% yield. M.p. 2498C (decomp.); 1H NMR
(400 MHz, CD3CN, 308C): d=8.68 (s, 2H; N=CH), 7.50 (s, 2H; Ar),
7.29 (s, 2H; Ar), 4.16–4.08 (brm, 1H; N-H), 4.00–3.91 (brm, 4H;
CH2), 3.88–3.80 (brm, 4H; CH2), 1.37 (s, 18H; tBu), 1.31 ppm (s,
18H; tBu); IR (KBr): n˜ =3369 (w), 3280 (w), 3092 (m), 2960 (w), 2604
(w), 2259 (vs), 2115 (w), 1943 (w), 1943 (w), 1883 (w), 1391 (w),
1192 (m), 1101 (m), 1038 (s), 889 (m), 837 (s), 824 (s), 779 (s), 749
(s), 740 (s), 728 (s), 715 (s), 705 (s), 672 (s), 653 cmÀ1 (s); UV/Vis
(THF): lmax (e)=329 (1.70104), 476 nm (5.63103 dm3 molÀ1 cmÀ1);
ESI-MS (negative mode) [(C34H51N3O2)2(NO3)2Ce2(OH)2(CH3CN)(OH)]À:
m/z 1562. Despite several attempts, we were unable to obtain
reproducible elemental analysis data for this complex. Because of
the low solubility of complex 4, no 13C NMR spectral data could be
obtained.
(NH4)2Ce(NO3)6 (236 mg, 4.3010À1 mmol) in THF (30 mL) via a can-
nula at room temperature under Ar atmosphere. After stirring for
2 h, the reaction mixture had turned blue and a white precipitate
had separated. All volatiles were removed in vacuo, and then the
blue product was extracted with toluene (215 mL). The solvent
was evaporated, and the product was isolated as a blue powder in
76% yield (254 mg). Single crystals suitable for X-ray diffraction
study were obtained by cooling a saturated solution in toluene/
hexane. The cyclic voltammogram of complex 1 is shown in Fig-
ure S5, which features a reversible CeIV/CeIII wave. M.p. 2158C
1
(decomp.); H NMR (400 MHz, CDCl3, 308C): d=8.61 (s, 2H; N=CH),
7.59 (s, 2H; Ar), 7.13 (s, 2H; Ar), 4.17 (brt, 2H; CHH), 3.98 (brd, 2H;
CHH), 3.42–3.52 (brm, 2H; CHH), 3.19–3.33 (brm, 2H; CHH), 3.05–
3.16 (brm, 1H; N-H), 1.50 (s, 18H; tBu), 1.30 ppm (s, 18H; tBu);
13C{1H} NMR (100 MHz, CDCl3, 308C): d=167.7 (N=CH), 166.3 (C-O
of Ar), 143.2 (C-C(CH3)3 of Ar), 135.2 (C-C(CH3)3 of Ar), 129.8 (C-H of
Ar), 129.4 (C-H of Ar), 127.3 (C-CH=N of Ar), 62.1 (CH2-N=CH), 50.6
(CH2-CH2-NH), 35.2 (C(CH3)3), 33.9 (C(CH3)3), 31.9 (C(CH3)3), 30.8 ppm
(C(CH3)3); IR (KBr): n˜ =3230 (w), 2960 (s), 2868 (m), 1620 (s), 1556
(m), 1512 (vs), 1467 (s), 1436 (s), 1412 (m), 1392 (m), 1362 (w), 1332
(w), 1262 (vs), 1201 (w), 1175 (w), 1100 (m), 1020 (s), 875 (w), 836
(s), 810 (s), 775 (w), 747 (w), 734 (m), 528 (m), 450 cmÀ1 (w); UV/Vis
(THF): lmax (e)=340 (1.19104), 594 nm (3.48103 dm3 molÀ1 cmÀ1);
ESI-MS (negative mode) [(C34H51N3O2)(NO3)2Ce]: m/z 797. Despite
several attempts, we were unable to obtain reproducible elemental
analysis data for this complex. The 1H and 13C NMR spectra are
shown in Figure S3.
Catalytic alcohol oxidation: The typical procedure for alcohol oxi-
dation was as follows. The requisite alcohol (2.0410À1 mmol),
complex 1 (0.1010À1 mmol, 5 mol%), MS4A (40 mg), and TEMPO
(0.1910À1 mmol, 10 mol%) were added to a J. Young Schlenk
tube (30 mL) under argon. CH3CN (1 mL) was added by means of
a syringe, and then the argon inside of the tube was replaced by
O2 at atmospheric pressure. After heating the closed reaction
vessel at 858C for 28 h, it was allowed to cool to room tempera-
ture and a portion of the reaction mixture was diluted with CD3CN
Preparation of CeIII complex 2: A solution of 2,5-dimethyl-1,4-
bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene (32.0 mg, 1.20
1
10À1 mmol) in toluene (5 mL) was added to
a
solution of
for H NMR spectroscopic analysis. The yield was determined from
1 (200 mg, 2.5010À1 mmol) in toluene (5 mL) at room tempera-
the relative intensities of the signals of the product and the start-
1
ture inside a glovebox. The color of the solution immediately
ing compound. A typical H NMR spectrum is shown in Figure S1.
Chem. Eur. J. 2016, 22, 4008 – 4014
4013
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim