A Cu(II)-mediated C-H oxygenation of sterically hindered tripyridine ligands to form triangular Cu(II)3 complexes

Article abstract of DOI:10.1021/ic990331g

Two sterically hindered tris-pyridyl methane ligands, tris(6-methyl-2-pyridyl)methane (L1) and bis(6-methyl-2-pyridyl)pyridylmethane (L2), are newly synthesized. Under aerobic conditions, Ln (n = 1 or 2) reacts with CuX₂ (X = Cl or Br), oxygenated at the methine position to LnOH or LnOMe. The former alcoholate ligand creates trinuclear Cu(II) complexes [Cu₃(X)(LnO)₃](PF₆)₂ {(X, n) = (Br, 1) 1, (Cl, 1) 2, (Br, 2) 3, or (Cl, 2) 4} in which the alkoxide oxygen atoms bridge copper centers. The crystal structures of 1-4 are presented along with their magnetic susceptibility data. The weak antiferromagnetic coupling between the Cu(II) centers in this trinuclear arrangement is due to weak interaction of the magnetic orbitals (dz²) which are oriented along three alternate sides in a hexagon of the Cu₃O₃ core in 1-4. Under anaerobic conditions, L1 reacts with CuBr₂ to form a square pyramidal complex [CuL1Br₂] (9) with the ligand facially capping. [Cu(Br)₂(L1OMe)] (10) was obtained after the suspension of 9 in MeOH was stirred under air for 48 h. In the presence of cyclohexene, 9 is converted to [Cu(Br)(L1)]m (m = 1 or 2) 5 quantitatively to give trans-1,2-dibromocyclohexane, indicating that Br₂ is generated during the reaction. The FAB MS spectrum of [¹⁸O]-1 prepared by the reaction of L1 with CuBr₂ under ¹⁸O₂ shows that the ligand of [¹⁸O]-1 is L1¹⁸O^-. L1 ¹⁸OH, L1OCD₃, and bis(6-methyl-2-pyridyl) ketone were obtained from reaction of L1 with CuBr₂ in CD₃OD under ¹⁸O₂. These results indicate that the origins of the O atom in L1OH and L1OMe are O₂ and MeOH, respectively. On the basis of these results, a mechanism of the oxygenation of L1 in the present system will be proposed.

Full text of DOI:10.1021/ic990331g

226
Inorg. Chem. 2000, 39, 226-234
A Cu(II)-Mediated C-H Oxygenation of Sterically Hindered Tripyridine Ligands To Form
Triangular Cu(II)3 Complexes
Masahito Kodera,*,† Yoshimitsu Tachi,† Toshio Kita,† Hiromu Kobushi,† Yoshinori Sumi,†
Koji Kano,† Motoo Shiro,‡ Masayuki Koikawa,§ Tadashi Tokii,§ Masaaki Ohba,| and
Hisashi Okawa|
Department of Molecular Science and Technology, Doshisha University, Kyotanabe,
Kyoto 610-0321, Japan, X-ray Laboratory, Rigaku-Denki, Akishima, Tokyo 196, Japan,
Department of Chemistry, Saga University, Saga 840, Japan, and Department of Chemistry,
Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812, Japan
ReceiVed March 24, 1999
Two sterically hindered tris-pyridyl methane ligands, tris(6-methyl-2-pyridyl)methane (L1) and bis(6-methyl-2-
pyridyl)pyridylmethane (L2), are newly synthesized. Under aerobic conditions, Ln (n ) 1 or 2) reacts with CuX2
(X ) Cl or Br), oxygenated at the methine position to LnOH or LnOMe. The former alcoholate ligand creates
trinuclear Cu(II) complexes [Cu3(X)(LnO)3](PF6)2 {(X, n) ) (Br, 1) 1, (Cl, 1) 2, (Br, 2) 3, or (Cl, 2) 4} in which
the alkoxide oxygen atoms bridge copper centers. The crystal structures of 1-4 are presented along with their
magnetic susceptibility data. The weak antiferromagnetic coupling between the Cu(II) centers in this trinuclear
arrangement is due to weak interaction of the magnetic orbitals (dz2) which are oriented along three alternate
sides in a hexagon of the Cu3O3 core in 1-4. Under anaerobic conditions, L1 reacts with CuBr2 to form a square
pyramidal complex [CuL1Br2] (9) with the ligand facially capping. [Cu(Br)2(L1OMe)] (10) was obtained after
the suspension of 9 in MeOH was stirred under air for 48 h. In the presence of cyclohexene, 9 is converted to
[Cu(Br)(L1)]m (m ) 1 or 2) 5 quantitatively to give trans-1,2-dibromocyclohexane, indicating that Br2 is generated
during the reaction. The FAB MS spectrum of [18O]-1 prepared by the reaction of L1 with CuBr2 under 18O2
shows that the ligand of [18O]-1 is L118O-. L118OH, L1OCD3, and bis(6-methyl-2-pyridyl) ketone were obtained
from reaction of L1 with CuBr2 in CD3OD under 18O2. These results indicate that the origins of the O atom in
L1OH and L1OMe are O2 and MeOH, respectively. On the basis of these results, a mechanism of the oxygenation
of L1 in the present system will be proposed.
Introduction
phosphine ligands.2 As a similar ligand, trispyridylmethane is
known to stabilize low valence state of various metal ions8-13
and is more easily prepared than the pyrazolyl and imidazol
derivatives.14-17 However, few examples of sterically hindered
derivatives of the trispyridylmethane ligand have been known.18,19
Here, we prepared the sterically hindered trispyridylmethane
ligands tris(6-methyl-2-pyridyl)methane (L1) and bis(6-methyl-
2-pyridyl)pyridylmethane (L2). On the contrary, to the expecta-
tion that L1 and L2 would form di-µ-hydroxodicopper(II)
complexes upon reaction with CuX2 (X ) Br and Cl) and
Sterically hindered tridentate end-capping ligands, such as
derivatives of hydrotrispyrazolylborate,1 trisimidazolylphos-
phine,2 and triazacyclononane,3 are known to stabilize biologi-
cally relevant metal complexes and have led to great progress
in understanding the structures and functions of non-heme
metalloproteins.4 µ-η2:η2-Peroxodicopper(II) complexes5,6 of
sterically hindered hydrotrispyrazolylborate ligands were used
to exactly predict the Cu2O2 structure of oxyhemocyanine before
the structure of oxyhemocyanine was determined by X-ray
analysis.7 It is reported that the µ-η2:η2-peroxodicopper(II)
complex is also stabilized by sterically hindered trisimidazolyl-
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G. Inorg. Chem. 1981, 20, 2414.
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* To whom correspondence should be addressed. Telephone number:
+81-774-65-6652. Fax number: +81-774-65-6848. E-mail address: mkodera
@mail.doshisha.ac.jp.
† Doshisha University.
‡ Rigaku-Denki.
§ Saga University.
| Kyushu University.
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10.1021/ic990331g CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/23/1999

Cu(II)-Mediated C-H Oxygenation
Inorganic Chemistry, Vol. 39, No. 2, 2000 227
NaOH, equilateral triangular Cu(II)3 complexes [Cu3(X)(LnO)3]-
(PF6)2 {(X, n) ) (Br, 1) 1; (Cl, 1) 2; (Br, 2) 3, and (Cl, 2) 4}
were obtained. In the reaction, Ln is oxygenated at the methine
position to LnO-.
the aliphatic one. A ligand ketonization is known as a different
type of ligand oxygenation, in which methylene groups substi-
tuted by two imines or aromatic amines are ketonized under
air in the presence of metal ions, such as Cu(I), Cu(II), Fe(II),
Fe(III), Co(II), and Co(III).47-53 The Cu(II)-mediated keton-
ization of diiminomethane,50 dibenzoimidazolylmethane,48 di-
pyridylmethane,53 and their derivatives has been reported. The
mechanism of the ketonization seems to be a kind of autoxi-
dation via an alkylhydroperoxide intermediate.50,51 As a similar
oxidation of ligands, imidation of R-aminomalonate catalyzed
by a Co(III) ion is known.54,55 Although the ketonization is not
biologically relevant, the reactions are synthetically promising
because most of them are catalytic and quantitative. This type
of reaction, however, has never been successfully applied to
hydroxylation of a C-H group of a ligand.
Many examples of equilateral triangular Cu3 complexes have
been reported.20-32 Most of them have one or two µ3-hydroxo
(or µ3-oxo) bridges and show strong antiferromagnetic interac-
tion between the three Cu(II) ions.20-32 The structures and
magnetic properties of 1-4 are quite unique because the µ3-
bridges in 1-4 are a Br or a Cl ion, which is easily removed,
and a very weak antiferromagnetic interaction operates among
the three Cu(II) ions in 1-4. Therefore, it might be important
to clarify the reason for the weak antiferromagnetic interaction
in 1-4 on the basis of their crystal structures.
Oxygenation of C-H groups catalyzed by metal ions has
attracted the interest of chemists because of the importance in
synthetic chemistry33 and in model studies of non-heme-
monooxygenases.33-38 The oxygenation of aromatic39-42 and
aliphatic43-45 C-H groups of ligands via Cu(I)-mediated O2-
activation as models of copper-containing monooxygenases has
been reported, where O2 is bound to the dicopper(I) center and
activated to a highly electrophilic species42 to oxygenate the
aromatic C-H and to a di-µ-oxodicopper(III) complex43-46 for
In this study, we describe the synthesis of Ln (n ) 1 and 2)
and the crystal structures and magnetic properties of 1-4 and
propose a mechanism of the hydroxylation of L1 in the reaction
with CuBr2 in MeOH under air.
Experimental Section
All ordinary reagents and solvents were purchased and used as
received unless otherwise noted. Tetrahydrofuran (THF) was dried over
sodium metal and distilled. Absolute MeOH was obtained by distillation
over Mg. MeCN and CH2Cl2 were dried over P2O5 and distilled.
Measurements. Elemental analyses (C, H, and N) were carried out
at the Elemental Analysis Service Center of Kyoto University. The
amounts of copper were analyzed on a Shimazu AA-610 atomic
absorption/flame emission spectrophotometer. UV-vis absorption
spectra were recorded on a Hitachi U-3210 spectrophotometer and on
a Shimadzu UV-3100 spectrophotometer in MeCN. Infrared (IR) spectra
were taken on a Shimadzu IR-400 spectrometer with KBr disks. Fast
atom bombardment (FAB) mass spectra were obtained on a JEOL JMS-
DX 300 spectrometer using m-nitrobenzyl alcohol (NBA) as a matrix.
1H NMR spectra in CDCl3 were recorded on a JEOL JMN-A 400
spectrometer by using Me4Si as an internal standard. EPR spectra were
recorded on a JEOL JES-TE 200 spectrometer in MeCN at 4.2-77 K.
Magnetic susceptibilities were measured by the use of a HOXAN
HSM-D SQUID susceptometer in the temperature range of 4.2-100
K (applied magnetic field of 100 G) and by the use of a Faraday balance
in the temperature range 80-290 K (applied magnetic field of 3000
G). Calibrations were made with Mn(NH4)2(SO4)2‚6H2O for the SQUID
susceptometer and with [Ni(en)3][S2O3] for the Faraday balance.
Diamagnetic corrections were made with Pascal’s constants.56 Effective
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,
where øM is the molar magnetic susceptibility corrected for diamag-
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hexane was carried out by using a Shimadzu GC-8A gas chromatograph
using a thermon-1000 + H3PO4 column (3 mm diameter × 0.8 m, at
gradient temperature 80-240 °C for 20 min, He carrier 0.5 kg/cm2).
Preparations. Bis(6-methyl-2-pyridyl)methane. In dry THF (100
mL) was dissolved 2,6-lutidine (10.7 g, 0.1 mol) and the solution was
degassed by evacuation and refilling with Ar. To the solution was added
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228 Inorganic Chemistry, Vol. 39, No. 2, 2000
Kodera et al.
n-butyllithium [(1.69 M in hexane), 125 mL, 0.21 mol] at -78 °C.
The solution was warmed to room temperature and stirred for 3 h under
Ar. The solution was cooled to -78 °C and a suspension of anhydrous
ZnCl2 (30 g, 0.22 mol) in dry THF (100 mL) was added. The mixture
was warmed to room temperature and stirred for 1 h under Ar. To the
mixture were added Pd(dppf)Cl2 (0.73 g, 0.001 mol) and 2-bromo-6-
picoline (18.9 g, 0.11 mol) at -78 °C. The mixture was warmed to
room temperature and heated to reflux with vigorous stirring for 15 h
under Ar. After the resultant mixture was cooled to room temperature,
THF was removed by distillation. The residue was suspended in CHCl3
(100 mL). To the suspension were added an aqueous solution (10 mL)
of NaOH (4.4 g, 0.11 mol) and a saturated aqueous solution of Na2S‚
9H2O (26.4 g, 0.11 mol), and the mixture stirred for 1 h to give a
white precipitate. The precipitate was removed by filtration and washed
with CHCl3 (200 mL). The filtrate and washings were transferred to a
separatory funnel, and the CHCl3 layer was separated. The aqueous
layer was extracted with CHCl3 (80 mL × 5). The CHCl3 layers were
combined and dried over anhydrous Na2SO4 and concentrated to
dryness. The oily residue was purified by vacuum distillation (0.1 Torr,
80-120 °C). Bis(6-methyl-2-pyridyl)methane (13.3 g, yield 67%) was
obtained. IR data [ν/cm-1]: 3050 (pyridine C-H), 2900 (aliphatic
C-H), 1590, 1570 (pyridine ring), 1450 (C-H bending), 830, 790,
To the filtrate was added NH4PF6 (200 mg, 1.23 mmol), and the
tricopper(II) bromide complex of L1O (1) precipitated as green solid,
which was collected by filtration (182 mg, yield 37%). The powder of
1 was recrystallized from MeCN/CH2Cl2/C6H6 to give crystals (1‚C6H6)
suitable for X-ray structure analysis. 1‚C6H6: Anal. Calcd for
C63H60N9O3P2F12Cu3Br: C, 48.11; H, 3.90; N, 8.12; Cu, 12.29.
Found: C, 49.17; H, 3.98; N, 8.39; Cu, 11.90. UV-vis absorption data
(in MeCN) [λmax/nm (ꢀmax/dm3 mol-1 cm-1)]: 270 (52 600) and 770
(530). IR data [ν/cm-1] on KBr disk: 3050 (aromatic C-H), 2900
(aliphatic C-H), 1600, 1580, 1565 (pyridine ring), 1450 (C-H
bending), 1085 (C-O), 840 (PF6), and 785 (pyridine C-H bending).
FAB-MS data: m/z 1325 [M]+, 1246 [M - Br]+, 1180 [M - PF6]+,
and 1101 [M - Br - PF6]+. 5: Anal. Calcd for C19H19N3CuBr: C,
52.90; H, 4.44; N, 9.75. Found: C, 52.23; H, 4.39; N, 9.48. UV-vis
absorption data (in MeCN) [λmax/nm (ꢀmax/dm3 mol-1 cm-1)]: 254
(15 000), 268 (15 000), and 333 (2500). IR data [ν/cm-1] on KBr
disk: 3050 (aromatic C-H), 2950, 2900 (aliphatic C-H), 1590, 1570,
1560 (pyridine ring), 1450 (C-H bending), and 780 (pyridine C-H
bending). FAB-MS data: m/z 433 [M]+, 352 [M - Br]+.
Catalytic Reaction. A mixture of CuBr2 (22.3 mg, 0.1 mmol) and
L1 (289 mg, 1 mmol) in MeOH (15 mL) was stirred for 24 h to produce
a green solution. To the solution were added CuBr2 (200.7 mg, 0.9
mmol) and NaOH (50 mg, 1.25 mmol). Stirring was further continued
for 18 h. NH4PF6 (200 mg, 1.23 mmol) was added to the reaction
mixture, and 1 (312 mg, yield 64%) was obtained as green powder.
The Cu(I) complex 5 was not obtained in the catalytic reaction. The
yield of 1 was increased to 67% when CuBr2 and NaOH were added
to the resultant mixture after the reaction of L1 with the catalytic amount
of CuBr2 was continued for 2 days.
[Cu3(Cl)(L1O)3](PF6)2‚MeCN‚CH2Cl2‚C6H6 (2‚MeCN‚CH2Cl2‚
C6H6). The tricopper(II) chloride complex of L1O (2) (yield 35%) was
obtained as a bluish green powder by the reaction of L1 with CuCl2
under the stoichiometric conditions shown in the synthesis of 1. The
Cu(I) chloride complex of L1 [Cu(Cl)(L1)]m (m ) 1 or 2) (6) (yield
15%) was isolated as yellow powder. The powder of 2 was recrystal-
lized from MeCN/CH2Cl2/C6H6 to give crystals (2‚MeCN‚CH2Cl2‚C6H6)
suitable for X-ray structure analysis. 2‚MeCN‚CH2Cl2‚C6H6: Anal.
Calcd for C66H65N10O3P2F12Cu3Cl3: C, 48.54; H, 4.01; N, 8.39; Cu,
11.90. Found: C, 48.52; H, 4.01; N, 8.52; Cu, 11.90. UV-vis
absorption data (in MeCN) [λmax/nm (ꢀmax/dm3 mol-1 cm-1)]: 270
(63 000) and 771 (515). IR data [ν/cm-1] on KBr disk: 3050 (aromatic
C-H), 2900 (aliphatic C-H), 1600, 1580, 1565 (pyridine ring), 1450
(C-H bending), 1090 (C-O), 840 (PF6), and 785 (pyridine C-H
bending). FAB-MS data: m/z 1281 [M]+, 1246 [M - Cl]+, 1136 [M
- PF6]+, and 1101 [M - Cl - PF6]+. 6: Anal. Calcd for C19H19N3-
CuCl: C, 58.91; H, 4.95; N, 10.85. Found: C, 58.15; H, 4.76; N, 10.59.
UV-vis absorption data (in MeCN) [λmax/nm (ꢀmax/dm3 mol-1 cm-1)]:
254 (15 000), 268 (15 000), and 333 (2500). IR data [ν/cm-1] on KBr
disk: 3050 (aromatic C-H), 2950, 2900 (aliphatic C-H), 1590, 1570,
1560 (pyridine ring), 1450 (C-H bending), and 780 (pyridine C-H
bending). FAB-MS data: m/z 387 [M]+, 352 [M - Cl]+.
[Cu3(Br)(L2O)3](PF6)2 (3). The tricopper(II) bromide complex of
L2O (3) (yield 44%) was obtained as green powder by the reaction of
L2 with CuBr2 under the stoichiometric conditions shown above. The
Cu(I) bromide complex of L2 [Cu(Br)(L2)]m (m ) 1 or 2), 7, did not
precipitate during the reaction. 7 (170 mg, yield 41%) was obtained as
yellow powder by concentration of the filtrate. The powder of 3 was
recrystallied from MeCN/CH2Cl2/C6H6 to give crystals (3) suitable for
X-ray structure analysis. 3: Anal. Calcd for C54H48N9O3P2F12Cu3Br:
C, 45.31; H, 3.38; N, 8.81; Cu, 13.32. Found: C, 44.89; H, 3.43; N,
1
and 770 (pyridine C-H bending). H NMR data (δ/ppm vs Me4Si in
CDCl3): 7.47 (t, 2H, py-4), 6.99 (d, 4H, py-3, 5), 4.29 (s, 2H, -CH2-),
and 2.55 (s, 6H, CH3). FAB-MS data: m/z 198 [M]+, 106 [M -
Mepy]+.
Tris(6-methyl-2-pyridyl)methane (L1). In dry THF (100 mL) was
dissolved bis(6-methyl-2-pyridyl)methane (9.91 g, 0.05 mol) and the
solution was degassed by evacuation and refilling with Ar. To the
solution was added n-BuLi [(1.64 M in hexane), 33.5 mL, 0.055 mol]
at -78 °C. The solution was warmed to room temperature and stirred
for 3 h under Ar. To the solution were added Pd(PPh3)4 (0.58 g, 0.0005
mol) and 2-bromo-6-picoline (9.46 g, 0.055 mol) at -78 °C. The
mixture was warmed to room temperature and heated to reflux with
vigorous stirring for 15 h under Ar. The reaction mixture was cooled
to room temperature and concentrated to dryness. To the residue were
added H2O (100 mL) and CHCl3 (200 mL). The CHCl3 layer was
separated by a separatory funnel and the aqueous layer was extracted
with CHCl3 (150 mL × 4). The CHCl3 layers were combined and dried
with anhydrous Na2SO4 and concentrated to dryness. Upon addition
of Et2O to the residue, a white solid precipitated. The solid was collected
by filtration and washed with Et2O several times. L1 (8.22 g, yield
57%) was obtained. Anal. Calcd for C19H19N3: C, 78.86; H, 6.62; N,
14.52. Found: C, 78.57; H, 6.52; N, 14.22. IR data [ν/cm-1] on KBr
disk: 3050 (pyridine C-H), 2950, 2900 (aliphatic C-H), 1600, 1580,
1565 (pyridine ring), 1450 (C-H bending), 840, and 775 (pyridine
1
C-H bending). H NMR data (δ/ppm vs Me4Si in CDCl3): 7.47 (t,
3H, py-4), 7.04 (d, 3H, py-3), 6.99 (t, 3H, py-5), 5.90 (s, 1H, methine),
and 2.50 (s, 9H, methyl). FAB-MS data: m/z 289 [M]+ 197 [M -
Mepy]+.
Bis(6-methyl-2-pyridyl)(2-pyridyl)methane (L2). L2 (yield 68%)
was synthesized by the same method as L1 using 2-bromopyridine in
place of 2-bromo-6-picoline. Anal. Calcd for C19H19N3: C, 78.52; H,
6.22; N, 15.26. Found: C, 78.22; H, 6.28; N, 14.96. IR data [ν/cm-1
]
on KBr disk: 3050 (pyridine C-H), 2900 (aliphatic C-H), 1585, 1565
(pyridine ring), 1470, 1450, 1425 (C-H bending), 820, 775, and 750
1
(pyridine C-H bending). H NMR data (δ/ppm vs Me4Si in CDCl3):
8.75 (dq, 1H, py-6), 7.60, (dt, 1H, py-4), 7.49 (t, 2H, Mepy-4), 7.25 (t,
1H, py-3), 7.12 (t, 1H, py-5), 7.09 (dd, 2H, Mepy-3), 6.99 (dd, 2H,
py-5), 5.93 (s, 1H, methine), and 2.51 (s, 6H, methyl). FAB-MS data:
m/z 276 [M]+, 197 [M - py]+, 183 [M - Mepy]+.
8.84; Cu, 12.93. UV-vis absorption data (in MeCN) [λmax/nm (ꢀmax
/
[Cu3(Br)(L1O)3](PF6)2‚C6H6 (1‚C6H6). Stoichiometric Reaction.
To a solution of CuBr2 (223 mg, 1.0 mmol) in MeOH (10 mL) was
added a solution of L1 (289 mg, 1 mmol) in MeOH (5 mL) with stirring,
then a brown solid precipitated immediately. Stirring was continued
for 24 h under air. To the resultant brown suspension was added a
solution of NaOH (50 mg, 1.25 mmol) in MeOH (5 mL). The mixture
was stirred at room temperature for 18 h, and the brown suspension
turned green with a yellow precipitate. The yellow precipitate was
collected by filtration and was characterized to be the Cu(I) bromide
complex of L1 [Cu(Br)(L1)]m (m ) 1 or 2), 5 (207 mg, yield 48%).
dm3 mol-1 cm-1)]: 266 (51 700) and 757 (507). IR data [ν/cm-1] on
KBr disk: 3050 (aromatic C-H), 2900 (aliphatic C-H), 1600, 1580,
1565 (pyridine ring), 1460, 1445 (C-H bending), 1090 (C-O), 840
(PF6), and 800, 775 (pyridine C-H bending). FAB-MS data: m/z 1283
[M]+, 1204 [M - Br]+, 1138 [M - PF6]+, and 1059 [M - Br - PF6]+.
7: Anal. Calcd for C18H17N3CuBr: C, 51.80; H, 4.11; N, 10.07.
Found: C, 51.19; H, 4.09; N, 9.89. UV-vis absorption data (in MeCN)
[λmax/nm (ꢀmax/dm3 mol-1 cm-1)]: 256 (12 900), 264 (13 400), and 323
(1800). IR data [ν/cm-1] on KBr disk: 3050 (aromatic C-H), 2950,
2900 (aliphatic C-H), and 1590, 1570, 1560 (pyridine ring), 1460,

Cu(II)-Mediated C-H Oxygenation
Inorganic Chemistry, Vol. 39, No. 2, 2000 229
1450, 1430 (C-H bending), and 770 (pyridine C-H bending). FAB-
MS data: m/z 419 [M]+, 338 [M - Br]+.
by silica gel column chromatography. The developing solvent is a
mixture of CHCl3 and AcOEt. L1OH: Anal. Calcd for C19H19N3O: C,
74.29; H, 6.27; N, 13.76. Found: C, 74.31; H, 6.31; N, 13.49. IR data
[ν/cm-1] on KBr disk: 3200 (O-H), 3050 (aromatic C-H), 2950, 2900
(aliphatic C-H), 1585, 1568 (pyridine ring) 1445 (pyridine C-H
bending) 1385 (O-H bending), 1070 (C-O), and 780 (pyridine C-H
[Cu3(Cl)(L2O)3](PF6)2‚C6H6 (4‚C6H6). The tricopper(II) chloride
complex of L2O (4) (yield 39%) was obtained as bluish green powder
by the reaction of L2 with CuCl2 under the stoichiometric conditions
shown above. The Cu(I) chloride complex [Cu(Cl)L2]m (m ) 1 or 2),
8, was not isolated but detected by a IR spectrum of a crude residue
obtained by concentration of the filtrate after 4 was isolated by filtration.
8 was synthesized by a reaction of L2 with CuCl to confirm the
formation of 8 on the reaction of L2 with CuCl2. The powder of 4 was
recrystallized from MeCN/CH2Cl2/C6H6 to give crystals (4‚C6H6)
suitable for X-ray structure analysis. 4‚C6H6: Anal. Calcd for
C60H54N9O3P2F12Cu3Cl: C, 49.19; H, 3.71; N, 8.60; Cu, 13.01.
Found: C, 48.97; H, 3.83; N, 8.57; Cu, 12.93. UV-vis absorption data
(in MeCN) [λmax/nm (ꢀmax/dm3 mol-1 cm-1)]: 266 (53 700) and 752
(456). IR data [ν/cm-1] on KBr disk: 3050 (aromatic C-H), 2950
(aliphatic C-H), 1600, 1585 (pyridine ring), 1460, 1450, (C-H
bending), 1090 (C-O), 840 (PF6), and 880, 775 (pyridine C-H
bending). FAB-MS data: m/z 1239 [M]+, 1204 [M - Cl]+, 1094 [M
- PF6]+, and 1059 [M - Cl-PF6]+. 8: Anal. Calcd for C18H17N3-
CuCl: C, 57.90; H, 4.59; N, 11.26. Found: C, 56.33; H, 4.39; N, 10.54.
UV-vis absorption data (in MeCN) [λmax/nm (ꢀmax/dm3 mol-1 cm-1)]:
254 (14 000), 263 (14 000), and 324 (2100). IR data [ν/cm-1] on KBr
disk: 3050 (aromatic C-H), 2950, 2900 (aliphatic C-H), 1590, 1570
1560 (pyridine ring), 1460, 1450, 1430 (C-H bending), and 765 (pyri-
dine C-H bending). FAB-MS data: m/z 373 [M]+, 338 [M - Cl]+.
[Cu(Br)2(L1)] (9). To a solution of CuBr2 (223 mg, 1.0 mmol) in
MeOH (5 mL) was added a solution of L1 (289 mg, 1.0 mmol) in
MeOH (5 mL) with stirring, then 9 (yield 90%) precipitated immediately
as brown solid. The brown solid of 9 was recrystallized from CH2Cl2/
C6H6/Et2O under Ar to give crystals suitable for X-ray structure analysis.
9: Anal. Calcd for C19H19N3CuBr2: C, 44.51; H, 3.74; N, 8.20; Cu,
1
bending). H NMR data (δ/ppm vs Me4Si in CDCl3): 7.49, (d, 3H,
py-3), 7.27 (t, 3H, py-4), 7.00 (d, 3H, py-5), and 2.49 (s, 9H, methyl).
FAB-MS data: m/z 306 [M]+, 288 [M - OH]+, and 213 [M - Mepy]+.
L1OMe: Anal. Calcd for C20H21N3O: C, 75.21; H, 6.63; N, 13.16.
Found: C, 74.63; H, 6.83; N, 13.05. IR data [ν/cm-1] on KBr disk:
3050 (aromatic C-H), 2950, 2900 (aliphatic C-H), 1585, 1568
(pyridine ring), 1445 (pyridine C-H bending), 1070 (C-O), and 780
1
(pyridine C-H bending). H NMR data (δ/ppm vs Me4Si in CDCl3):
7.49, (m, 6H, py-3, 5), 7.01 (t, 3H, py-4), 3.29 (s, 3H, methoxy), and
2.48 (s, 9H, methyl). FAB-MS data: m/z 320 [M]+ and 288 [M -
OMe]+. bpk: Anal. Calcd for C13H12N2O: C, 73.57; H, 5.70; N, 13.20.
Found: C, 73.53; H, 5.60; N, 13.09. IR data [ν/cm-1] on KBr disk:
3050 (aromatic C-H), 2950, 2900 (aliphatic C-H), 1680 (carbonyl),
1
1585 (pyridine ring), 1450 (C-H bending), and 1330, 990, 765. H
NMR data (δ/ppm vs Me4Si in CDCl3) 7.89, (d, 2H, py-3), 7.74 (t,
2H, py-4), 7.34 (d, 2H, py-5), and 2.63 (s, 6H, methyl). FAB-MS
data: m/z 213 [M]+.
Reaction of L1 with CuBr2 under Catalytic Conditions. The
reaction of L1 (289 mg, 1 mmol) with CuBr2 (22.3 mg, 0.1 mmol)
was carried out in MeOH (15 mL) under air. The oxygenation of L1
was monitored by 1H NMR spectra of the crude products by taking an
aliquot from the reaction mixture during the course of the reaction.
The yields of L1OH, L1OMe, and bpk were determined as described
above. Finally, the whole reaction mixture was worked up, and L1OH,
L1OMe, and bpk were isolated and purified by column chromatography
as described above.
Reaction of L1 with the Other Cu(II) Salts. To a solution of L1
(28.9 mg, 0.1 mmol) in MeOH (2 mL) was added a catalytic amount
of Cu(NO3)2, CuSO4, or Cu(ClO4)2 (L1:Cu(II) salt ) 1:0.1, mol/mol)
and the mixture was stirred under air. The reaction was followed as
described above by 1H NMR spectra. L1OH and bpk were detected as
the oxygenated products of L1, but L1OMe was not produced. For
Cu(NO3)2, the yields of L1OH and bpk after 1 day are 39% and 17%,
and after 2 days, 71% and 29%, respectively. For Cu(ClO4)2, the yields
of L1OH and bpk after 1 day are 30% and 16%, and after 2 days, 69%
and 26%, respectively. For CuSO4, the yields of L1OH and bpk after
1 day are 15% and 9%, and after 2 days, 44% and 20%, respectively.
Reaction of L1 with Cu(NO3)2 in the Presence of Br2. To a solution
of L1 (28.9 mg, 0.1 mmol) in MeOH (2 mL) were added a catalytic
amount of Cu(NO3)2 (2.0 mg, 0.01 mmol) and a drop of Br2. The
mixture was stirred under air for 1 day. The reaction mixture was
worked up and demetalated by treating with concentrated NH3/CH2-
Cl2 as described above. The oxygenated metal-free ligand was only
L1OMe. The yield of L1OMe was 54%.
12.39. Found: C, 44.23; H, 3.76; N, 8.12; Cu, 12.23. IR data [ν/cm-1
]
on KBr disk: 3025 (aromatic C-H), 2900 (aliphatic C-H), 1590, 1560
(pyridine ring), 1450 (C-H bending), and 770 (pyridine C-H bending).
FAB-MS data: m/z 443 [M - Br]+, 354 [M - 2Br]+, and 290 [M -
Cu - 2Br]+.
[Cu(Br)2(L1OMe)] (10). By mixing L1 (289 mg, 1.0 mmol) with
CuBr2 (223 mg, 1.0 mmol) in MeOH (15 mL), 9 was generated as
brown precipitate, which was stirred under air for 48 h to give a brown
suspension with yellow precipitate. The yellow precipitate was removed
by filtration. The filtrate was concentrated to dryness and the residue
was recrystallized from CH2Cl2/C6H6 to give brown crystals of 10
suitable for X-ray structure analysis. 10: IR data [ν/cm-1] on KBr
disk: 3050 (aromatic C-H), 2900 (aliphatic C-H), 1600, 1560, 1510,
(pyridine ring), 1450 (C-H bending), 1060, 1040 (ether C-O-C) and
775 (pyridine C-H bending). FAB-MS data: m/z 464 [M - Br]+, 382
[M - 2Br]+, and 320 [M - Cu - 2Br]+.
Reaction of 9 with Cyclohexene. To a solution of cyclohexene (1
mL) in CH2Cl2 (5 mL) were added 5.35 mg 9 and 3.0 µL of nitroben-
zene as an internal standard for GLC analysis. The mixture was stirred
under air for 1 day, then the brown solid 9 disappeared and yellow
solid 5 was generated. After filtration, 4.3 mg (yield 95%) of 5 was
obtained. As the sole detectable product trans-1,2-dibromocyclohexane
was generated from cyclohexene. The yield of the product was
determined by GLC analysis to be 98% based on the amount of 9 used.
Oxygenated Products of L1. Reaction of L1 with CuBr2 under
Stoichiometric Conditions. In the course of the reaction of L1 (289
mg, 1 mmol) with CuBr2 (223 mg, 1.0 mmol) under the stoichiometric
conditions in MeOH (15 mL) under air, an aliquot was taken from the
reaction mixture and concentrated to dryness. To the residue were added
CH2Cl2 and concentrated NH3, and the mixture was stirred vigorously
for 1 h. The aqueous layer turned blue. The CH2Cl2 layer was separated
from the aqueous layer, washed with distilled H2O (10 mL × 3), and
concentrated to dryness. To the residue was added toluene as an internal
standard. The yields of the oxygenated ligands tris(6-methyl-2-pyridyl)-
methanol (L1OH), tris(6-methyl-2-pyridyl)methoxymethane (L1OMe),
and bis(6-methyl-2-pyridyl) ketone (bpk) were determined by the
integral values of the methyl groups of the oxygenated ligands and
toluene in the 1H NMR spectrum. Finally, the whole reaction mixture
was worked up, and L1OH, L1OMe, and bpk were isolated and purified
Reaction of 5 in the Presence of Br2. To a suspension of 5 (43.3
mg, 0.1 mmol) in MeOH (2 mL) was added a few drops of Br2, and
the mixture was stirred under air. The yellow solid 5 disappeared in 1
day and a green solution appeared, then the reaction mixture was worked
up and demetalated by treating with concentrated NH3/CH2Cl2 as
described above. The oxygenated metal-free ligand was only L1OMe.
The yield of L1OMe was 62%.
2
Isotope (18O and H) Labeling Experiments. [18O]-1. A mixture
of CuBr2 (223 mg, 1.0 mmol) and L1 (289 mg, 1.0 mmol) in MeOH
(15 mL) was stirred under 18O2 for 24 h. To the reaction mixture was
added a solution of NaOH (50 mg, 1.25 mmol) in MeOH (5 mL). The
stirring under 18O2 was continued for 18 h. The 18O-labeled Cu(II)3
complex [18O]-1 (157 mg, 32%) was isolated as described in the
synthesis of 1. [18O]-1: FAB-MS data: m/z 1340-1325 [M]+, 1195-
1180 [M - PF6]+, and 1115-1100 [M - Br - PF6]+. IR data [ν/cm-1
]
on KBr disk: 3050 (aromatic C-H), 2900 (aliphatic C-H), 1600, 1580,
1565, 1450 (pyridine ring), 1075 (C-18O), 840 (PF6), and 785 (pyridine
C-H bending). The metal-free ligand of [18O]-1 was obtained by
treating [18O]-1 with concentrated NH3/CH2Cl2. FAB-MS data of the
metal-free ligand of [18O]-1: m/z 308 and 306 [M]+, 288 [M - OH]+,
and 215 and 213 [M - Mepy]+.

230 Inorganic Chemistry, Vol. 39, No. 2, 2000
Table 1. Crystallographic Data for 1‚C6H6, 9, and 10
Kodera et al.
1‚C6H6
9
10
formula
fw
cryst system
space group
a/Å
b/Å
c/Å
C63H60N9O3P2F12Cu3Br
1551.70
cubic
I23 (No. 197)
24.413(3)
C19H19N3CuBr2
512.73
triclinic
P1h (No. 2)
8.515(3)
14.598(5)
8.253(3)
C20H21N3OCuBr2
1551.70
triclinic
P1 (No. 2)
11.215(4)
11.381(3)
9.538(3)
â/deg
V/Å3
14450(4)
962.5(6)
1030.0(6)
Z
T/°C
8
23
1417
Cu KR 1.54178
27.56
0.068, 0.098
1.27
2
20
1.769
Mo KR 0.71069
53.05
0.042, 0.049
1.06
2
20
1.756
Mo KR 0.71069
49.70
0.042, 0.047
0.85
Dc/g cm-3
radiation(λ)/Å
µ/cm-1
a
R, Rw
GOF index
a R ) Σ ||Fo| - |Fc||/Σ|Fo|. Rw ) [Σw(|Fo| - |Fc|)2/Σw(Fo)2]1/2, where w ) 1/σ2(Fo).
L118OH and L1OCD3. A mixture of CuBr2 (22.3 mg, 0.1 mmol)
and L1 (289 mg, 1.0 mmol) in CD3OD (15 mL) was stirred in the
absence of NaOH under 18O2 for 2 days. The reaction mixture was
concentrated to dryness and the residue was treated with concentrated
NH3/CH2Cl2 to give a crude mixture of the oxygenated ligands L118OH,
L1OCD3, and bpk. L118OH (43 mg, 14%), L1OCD3 (97 mg, 30%),
and bpk (21 mg, 10%) were isolated and purified by a silica gel column
chromatography as described above. L118OH: FAB-MS data: m/z 308
[M18O]+, 288 [M18O - 18OH]+, and 215 [M18O - Mepy]+. IR data
[ν/cm-1] on KBr disk: 3200 (O-H), 3050 (aromatic C-H), 2950, 2900
(aliphatic C-H), 1585, 1568 (pyridine ring), 1445 (C-H bending),
1385 (O-H bending), 1060 (C-18O), and 780 (aromatic ring).
L1OCD3: 1H NMR data (δ/ppm vs Me4Si in CDCl3): 7.49, (m, 6H,
py-3, 5), 7.01 (t, 3H, py-4), and 2.48 (s, 9H, methyl). FAB-MS data:
m/z 323 [M]+and 288 [M - OCD3]+. Spectral data of bpk isolated
from the 18O-labeling reaction are exactly the same as those of bpk
obtained upon the nonlabeling reaction shown above.
Scheme 1
2-bromopyridine) as shown in Scheme 1. The stepwise method
gives better yields of L1 and L2 than the one-pot reaction pre-
viously adopted to prepare a similar tripyridylmethane ligand,
2-(bis(2-pyridyl)methyl)-6-methylpyridine.18 For the first reac-
tion of the stepwise reactions, Pd(dppf)Cl2 is a better catalyst
than Pd(PPh3)4, and the yield of the cross-coupling product is
increased in the presence of ZnCl2.63 On the other hand, for the
second reactions, better results are obtained by using Pd(PPh3)4
instead of Pd(dppf)Cl2 in the absence of ZnCl2.
Synthesis of the Cu(II)3 Complexes 1-4. At the beginning
of this study, we attempted to synthesize the di-µ-hydroxodi-
copper(II) complex63 of Ln. The reaction of L1 with CuBr2
under the stoichiometric conditions (L1:CuBr2 ) 1:1, mol/mol)
was carried out in the presence of NaOH, but the di-µ-
hydroxodicopper(II) complex was not obtained from the reaction
and a green suspension with a yellow solid was obtained. The
yellow solid is characterized as a Cu(I) complex [Cu(L1)Br]m
(m ) 1 or 2), 5, which is stable against oxidation under air. L1
may stabilize the Cu(I) state of 5 because tripyridylmethane is
known to stabilize low valence states of various metal ions,8-13
and furthermore, the methyl groups introduced at six positions
of the pyridyl groups in L1 as a steric hindrance may enhance
the stability of the Cu(I) state of 5.65 After addition of NH4PF6
to the filtrate, 1 was obtained as a green solid. The FAB MS
spectrum of 1 shows major peaks at m/z 1325, 1246, 1180, and
1040, assignable to {[Cu3(Br)(L1O)3](PF6)2}+, {[Cu3(L1O)3]-
(PF6)2}+, {[Cu3(Br)(L1O)3](PF6)}+, and {[Cu3(L1O)3](PF6)}+,
Structure Determination of Single Crystals. The structure of 1‚
C6H6 was determined on a Rigaku AFC7R diffractometer with graphite
monochromated Cu-KR radiation and a 12 kW rotating anode
generator at 296 ( 1 K. The structures of 2‚MeCN‚CH2Cl2‚C6H6, 3,
4‚C6H6, 9, and 10 were determined on a Rigaku AFC7R diffractometer
with graphite-monochromated Mo-KR radiation and a 12 kW rotating
anode generator at 293 ( 1 K. Total numbers of reflections, 1671,
4636, 2763, 4511, 7839, and 5027 were collected for 1‚C6H6,
2‚MeCN‚CH2Cl2‚C6H6, 3, 4‚C6H6, 9, and 10, respectively. The intensi-
ties of three representative reflections were measured after every 150
reflections. All six structures were solved by the direct methods
(SHELEXS 86)57 and expanded using Fourier techniques. The function
minimized was Σw(|Fo| - |Fc|)2 with w ) 1/σ2(Fo). The neutral atom
scattering factors were taken from Cromer and Waver.58 Anomalous
dispersion effects were included in Fc,59 the values for ∆f ′ and ∆f′′
being taken from the ref 60 and those for the mass-attenuation
coefficients from ref 61. All calculations were performed using the
teXan crystallographic software package.62 Key facets of the structure
determinations for 1‚C6H6, 9, and 10 are given in Table 1.
Results and Discussion
Preparation of L1 and L2. The sterically hindered trispyri-
dylmethane ligands L1 and L2 are prepared by stepwise cross-
coupling reactions from 2,6-lutidine and 2-bromo-6-picoline (or
(57) Sheldrick, G. M. Acta Crystallogr. A 1990, 46, 467.
(58) Cromer, D. T.; Waber, J. T. International Tables for X-ray Crystal-
lography; Kynoch Press: Birmingham:, 1974; Vol. 4.
(59) Ibers, J. A.; Hamilton, W. C. Acta Crystllogr. 1964, 17, 781.
(60) Creagh, D. C.; McAuley, W. J. Internatiuonal Tables for X-ray
Crystallography; Kluwer: Boston: 1992.
(61) Creagh, D. C.; Hubbell, H. H. Internatiuonal Tables for X-ray
Crystallography; Kluwer: Boston: 1992.
(63) Hayashi, T.; Konishi, M.; Kobori, Y.; Kumada, M.; Higuchi, T.;
Hirotsu, K. J. Am. Chem. Soc. 1984, 106, 158.
(64) Kodera, M.; Simakoshi, H.; Tachi, Y.; Katayama, K.; Kano, K. Chem.
Lett. 1998, 441.
(62) Single-crystal structure analysis software, version 1.6, Molecular
Structure Corp., The Woodlands, TX 77381, 1993.
(65) Hayashi, Y.; Suzuki, M.; Uehara, A.; Mizutani, Y.; Kitagawa, T. Chem.
Lett. 1992, 91.

Cu(II)-Mediated C-H Oxygenation
Inorganic Chemistry, Vol. 39, No. 2, 2000 231
and Cl) bonds of 1, 2, 3, and 4 are 2.836(2), 2.683(9), 2.804-
(3), and 2.675(3) Å, respectively. These distances are much
larger than those of ordinary Cu(II)-X bonds, suggesting that
the Cu-X bonds in 1-4 are weak. Actually, 1 is easily
converted to a Br-free complex [Cu3(L1O)3](PF6)2(BF4) when
treated with AgBF4. The easy exchange of the µ3-X bridges is
one of the prominent properties of 1-4. The difference of the
Cu-X bond distances in 1-4 mainly depends on the size of
the halide ions. The Cu-Cu distances of 1, 2, 3, and 4, are
3.302(3), 3.276(5), 3.276(3), and 3.248(2) Å, respectively. The
Cu-Cu distances of 1 and 2 are larger by ca. 0.027 Å than
those of 3 and 4, respectively. Since L1O in 1 and 2 is sterically
more hindered than L2O in 3 and 4, this difference may come
from the steric repulsion between the LnO ligands. The Cu-
Cu distances in 1 and 3, having a µ3-Br bridge, are larger by
ca. 0.027 Å than those in 2 and 4, having a µ3-Cl bridge,
respectively. Thus, the Cu-Cu distances in 1-4 depend on both
the steric hindrance of LnO and the size of the halide ions to a
similar extent.
The bond angles of the O(1)-Cu(1)-N(3) and the O(1*)-
Cu(1)-N(1) are the largest and the second largest in bond angles
around the Cu atom in 1 and are 170.7(4)° and 136.4(4)°,
respectively. The τ value67 calculated from these angles for 1
is 0.57. The τ values calculated for 2, 3, and 4 by the same
way are 0.60, 0.52, and 0.63, respectively. The τ values vary
from 0, for an idealized square pyramid, to 1, for an idealized
trigonal bipyramid.65 The copper coordination geometry in 1-4
can be described as a rather distorted trigonal bipyramid.
Many examples have been reported for triangular Cu3
complexes.20-32 Most of them have an equilateral triangle of
three Cu(II) ions bridged by a mono-µ3-OH or a mono-µ3-O
with a Cu-Cu distance (3.17-3.38 Å), and their Cu(II)3 cores
are supported by three lateral µ-N-O bridges of oximato
ligands20-25 (or µ-N-N bridge of pyrrazolato ligand)25 and, in
a few examples, by three lateral µ-carbonyl oxygen bridges.28,29
There is an example of a doubly µ3-OH-bridged Cu(II)3 complex
with a Cu-Cu distance of 2.808(3) Å.31 The double µ3-OH
bridges shorten the Cu-Cu distance. Stack et al. determined
the crystal structure of a doubly µ3-O-bridged Cu3(II,II,III)
complex which is formed from O2-oxidation of a Cu(I) complex
of N,N,N,N-tetramethyl-1,2-diaminocyclohexane.32 The double
µ3-O bridges stabilize the Cu(III) state. The Cu3-based triangle
is isosceles due to the Cu(III) ion to give two Cu-Cu distances,
2.641(3) and 2.705(2) Å, much shorter than the distances in
the µ3-OH-bridged Cu(II)3 complexes. The doubly µ3-OH- or
µ3-O-bridged Cu3 complexes do not have the lateral bridge. The
µ3-OH and µ3-O bridges stabilize the Cu3 core structure and
control the valence state of the Cu ion. The present Cu(II)3
complexes 1-4, however, do not have either µ3-OH or µ3-O
bridges. Instead, the µ3-position of 1-4 is occupied by a Br or
Cl ion, which weakly binds to the Cu atoms. The Cu(II)3 core
structures in 1-4 are mainly stabilized by the three lateral
µ-alkoxo bridges and only weakly supported by the µ3-X bridge.
Therefore, 1-4 are structurally quite unique in equilateral
triangular Cu(II)3 complexes reported so far.
Figure 1. ORTEP view of cationic portion of [Cu3(Br)(L1O)3](PF6)2‚
C6H6 with the atom numbering scheme (50% probability ellipsoids).
Selected bond distances (Å): Cu1-Cu1*, 3.302(3); Cu1-Br1, 2.836-
(2); Cu1-O1, 1.883(9); Cu1-O1*, 2.001(8); Cu1-N1, 2.059(10);
Cu1-N3, 1.97(1). Selected bond angles (deg): Cu1-O1-Cu1*, 116.4-
(4); O1-Cu1-O1*, 92.9(5); O1-Cu1-N1, 81.9(4); O1-Cu1-N3,
170.7(4); O1*-Cu1-N1, 136.4(4); O1*-Cu1-N3, 82.5(4); N1-Cu-
N3, 107.1(4).
respectively, suggesting that 1 has a Cu(II)3 unit, Cu3(L1O)3.
Eventually, the structure of [Cu3(Br)(L1O)3](PF6)2 1 was
revealed by the X-ray structure analysis described below. The
yield of 1 is low when CuBr2 and NaOH were added
simultaneously to a solution of L1 in MeOH and improved to
37% when NaOH was added to the reaction mixture after the
reaction of L1 and CuBr2 for 24 h under the stoichiometric
conditions (L1:CuBr2 ) 1:1, mol/mol). Furthermore, the yield
of 1 is improved to 64% when a catalytic amount of CuBr2
was used (L1:CuBr2 ) 1:0.1, mol/mol). The Cu(II)3 complexes
2-4 and the Cu(I) complexes 6-8 were obtained from the
reaction of Ln (n ) 1 and 2) with CuX2 (X ) Br and Cl) under
similar conditions.
Crystal Structures of 1-4. The ORTEP view of the cation,
[Cu3(Br)(L1O)3]2+, of 1 is shown in Figure 1 together with the
numbering scheme. The selected bond distances and bond angles
of 1 are shown in the caption of Figure 1. The ORTEP views
of 2-4 are available as Supporting Information.
The cation of 1 lies on a crystallographic 3-fold axis and,
accordingly, has exact 3-fold symmetry. The three Cu atoms
of 1 are located at the apexes of an equilateral triangle, and
bridged by three µ-alkoxo O atoms of L1O to form a Cu3O3
core. The Cu atoms are each coordinated by two pyridyl N
atoms, two bridging alkoxo O atoms, and Br atom and assumes
a distorted trigonal bipyramidal geometry. Two of three pyridyl
groups of L1O bind to different Cu atoms, and the other one is
free from coordination. The Br atom of 1 lies on the 3-fold
axis, bridges three Cu atoms to form a µ3-Br bridge, and is
surrounded by hydrophobic walls of three coordinated pyridyl
groups. The structure of 2 is similar to that of 1, but the µ3-
bridge of 2 is a chloride ion. The structures of 3 and 4 are similar
to those of 1 and 2, respectively, but the sterically less hindered
ligand (L2O) binds to the Cu atom in 3 and 4. One of two
6-methyl-2-pyridyl groups of L2O is free from coordination in
3 and 4.
Magnetic Properties of 1-4. The magnetic susceptibility
measurements of 1-4 were made on the crystals in the
temperature range 4.2-290 K. The temperature dependence of
the magnetic susceptibility (øM) and effective magnetic moment
(µeff) per molecule of 1 is shown in Figure 2. The temperature
(66) Kodera, M.; Terasako, N.; Kita, T.; Tachi, Y.; Kano, K.; Yamazaki,
M.; Koikawa, M.; Tokii, T. Inorg. Chem. 1997, 36, 3861.
(67) Addison, A. W.; Rao, T. N.; Reedijk, J.; Rijin, J. V.; Verschoor, G.
C. J. Chem. Soc., Dalton Trans. 1984, 1349.
The Cu-N and Cu-O bond distances in 1-4 are typical of
other Cu(II) complexes. The distances of the Cu-X (X ) Br

232 Inorganic Chemistry, Vol. 39, No. 2, 2000
Kodera et al.
cm-1 20-25
)
or pyrrazolato (-200 cm-1 27
)
ligands and slightly
)
lower than the values (-12 and -15 cm-1 28,29
reported for
the mono-µ3-OH-bridged Cu(II)3 complexes with the µ-carbonyl
oxygen bridges. The three Cu(II) ions in the µ3-OH(or O)-
bridged complexes are magnetically coupled to each other by a
superexchange interaction through the µ3-OH (or O) bridge. The
strong magnetic interaction for the oximato- and pyrrazolato-
bridged complexes is owing to the effective overlap of the
2
2
magnetic orbitals (dx -y ) because of the coplanarity of the
coordination planes of the three subunits.25,27 On the other hand,
the relatively weak magnetic interaction for the Cu(II)3 com-
plexes bridged by µ-carbonyl oxygen is due to orthogonality
2
2
of the three magnetic orbitals (dx -y ) which are directed to the
µ3-OH bridge.29 The coordination geometry about each copper
atom in 1-4 is nearly trigonal bipyramidal, which leads to a
dz2 ground state. The three major lobes of the dz2 orbitals
(magnetic orbitals) are oriented along three alternate sides in a
hexagon of the Cu3O3 core in 1-4 and cannot overlap each
other. This leads to the very weak magnetic interaction between
the Cu(II) ions in 1-4.
Figure 2. Temperature dependence of øM (O) and µeff (b) of 1‚C6H6.
Solid curves are based on eq 1, using J ) -19.5 cm-1, g ) 2.02, and
NR ) 60.
The 4.2 K EPR spectrum of 1, however, shows a feature of
a hyperfine structure due to the parallel component with gparallel
) 2.25, which is larger than gperpendicular. This is indicative of a
Table 2. Magnetic Susceptibility Parameters
compound
J/cm-1
g
NR
2
2
dx -y electronic ground state, showing that the coordination
geometry of the Cu atom in 1-4 in solution is square pyramidal.
The hyperfine splitting of the parallel component is a compli-
cated multiplet and broadened due to a spin delocalization
between the three Cu atoms. The EPR spectra of 2-4, at 4.2
K, are similar to that of 1, but the hyperfine splittings of the
parallel components of 2-4 are much more complicated. This
may be explained by contribution of the quartet state of the
Cu(II)3 unit due to the weaker antiferromagnetic interaction of
2-4 compared with 1. The EPR spectra of 1-4 are available
as Supporting Information.
1‚C6H6
-19.5
-3.70
-4.80
-0.30
2.02
2.01
2.00
2.01
60
60
60
60
2‚MeCN‚CH2Cl2‚C6H6
3
4‚C6H6
dependence of øM and µeff in the temperature range from 4.2 to
290 K for 2-4 are available as Supporting Information.
As the temperature is lowered, the µeff per molecule of 1
decreases from 2.95 at 290 K to 1.75 at 4.2 K, indicating that
an antiferromagnetic interaction operates among the three Cu-
1
(II) ions. When three paramagnetic metal ions of S ) /2 are
equivalent and form an equilateral triangle, the magnetic
exchange interaction may be described by the single parameter
J. The temperature dependence of øM for 1-4 is simulated by
the equation shown below:25
Formation of 9 and 10. [Cu(Br)2(L1)] (9) is formed by the
reaction of L1 with CuBr2 under anaerobic conditions. Just after
mixing L1 with CuBr2 in MeOH, 9 precipitated as a brown solid.
The yield of 9 is quantitative. [Cu(Br)2(L1OMe)] (10) is formed
by stirring 9 in MeOH under air. The structures of 9 and 10
were determined by X-ray analyses. The ORTEP view of 9 is
shown in Figure 3. The Cu atom in 9 is coordinated to three
pyridyl N atoms of L1 and two Br atoms to create a distorted
square pyramidal geometry. The structure of 9 shows that L1
is unmodified in the first step of the reaction of L1 with CuBr2.
The oxygenation of L1 must proceed via 9 by considering the
quantitative yield of 9. The ORTEP view of 10 is shown in
Figure 4. The structure of 10 reveals that the methine carbon
of L1 is substituted by a methoxy group to form tris(6-methyl-
2-pyridyl)methoxymethane (L1OMe). The Cu atom in 10 is
coordinated to two pyridyl N atoms and an O atom of L1OMe
and two Br atoms to create a distorted square pyramidal
geometry. These structures of 9 and 10 clearly demonstrate that
L1 is converted to L1OMe in the reaction of L1 with CuBr2 in
MeOH under air.
øM ) (Ng2â2/4kT)[5 + exp(-3J/kT)] ×
[1 + exp(-3J/kT)]-1 + NR (1)
An excellent fit of the øM-T data to the simulation was
obtained when J ) -19.5 cm-1, g ) 2.02, and NR ) 60 were
assumed for 1. The best fitting parameters for 1-4 are
summarized in Table 2.
The isotropic exchange interaction among three ions of S )
1/2 placed at the apexes of an equilateral triangle splits the three
degenerate energy levels into one quartet state with a total spin
3
1
ST ) /2 and two degenerate doublet states with ST ) /2. The
3
1
separation energy between the ST ) /2 and ST ) /2 states is
58.5 cm-1 (-3J) for 1. The µeff value, 1.75 µB, of 1 per molecule
at 4.2 K indicates that the ground doublet states are occupied
predominantly and the spin of the tricopper unit is close to 1/2.
On the other hand, the µeff values at 4.2 K for 2, 3, and 4 are
1.95, 1.93, and 2.80 µB, respectively, larger than 1.73 µB of the
When 9 was converted to 5 in the presence of cyclohexene,
trans-1,2-dibromocyclohexane was obtained quantitatively but
not cis-1,2-dibromocyclohexane, indicating that cyclohexene is
brominated by Br2 generated in the reaction system. The driving
force to generate Br2 may be owing to the high stability of 5.
It seems that the Br2 generated plays an important role in the
oxygenation of L1.
1
spin only value for S ) /2. In 4, the µeff values are almost
constant at temperatures between 4.2 and 290 K, indicating that
the magnetic interaction between the Cu(II) ions is very weak
and the excited quartet state is considerably populated even at
4.2 K.
The J values, -0.30 to -4.8 cm-1, found for 2-4 are much
lower than the values reported for the mono-µ3-OH(or O)-
bridged Cu(II)3 complexes with oximato (-122 to -1000
Oxygenated Products of L1. During the reaction of L1 with
CuBr2 in MeOH under air, an aliquot was taken from the
reaction mixture, concentrated, and treated with concentrated

Cu(II)-Mediated C-H Oxygenation
Inorganic Chemistry, Vol. 39, No. 2, 2000 233
Scheme 2
the catalytic conditions, the yields of L1OH, bpk, and L1OMe
for 1 day of reaction are 24, 17, and 52%, respectively, and the
reaction is complete in 2 days. Both in the stoichiometric and
catalytic reactions, the major product is L1OMe. The catalytic
reaction is more efficient than the stoichiometric one. This is
also found in the Cu-mediated ligand ketonization.53 In the
catalytic reaction, the metal-free L1 is oxygenated, but in the
stoichiometric reaction, L1 of 5 is oxygenated. The conforma-
tional freedom of the pyridyl groups in L1 of 5 is restricted by
the coordinated Cu(I) ion. If the oxygenation of L1 proceeds
via a radical intermediate at the methine carbon, the Cu(I) ion
prevents the methine carbon in L1 of 5 from taking the planar
configuration required for the radical intermediate. This may
be one reason the stoichiometric reaction is less efficient than
the catalytic reaction.
Figure 3. ORTEP view of [CuBr2(L1)] (9) with the atom numbering
scheme (50% probability ellipsoids). Selected bond distances (Å): Cu-
Br1, 2.398(2); Cu1-Br2, 2.455(3); Cu1-N1, 2.10(1); Cu1-N2, 2.02-
(1); Cu1-N3, 2.33(1). Selected bond angles (deg): Br1-Cu1-Br2,
93.06(9); Br1-Cu1-N1, 93.3(3); Br1-Cu1-N2, 163.4(3); Br1-Cu1-
N3, 101.3(3); Br2-Cu1-N1, 172.6(3); Br2-Cu1-N2, 91.3(3); Br2-
Cu1-N3, 92.1(3); N1-Cu1-N2, 81.5(4); N1-Cu1-N3, 90.2(4); N2-
Cu1-N3, 94.6(4).
The oxygenation of L1 is also catalyzed by Cu(NO3)2, Cu-
(ClO4)2, and CuSO4 (L1:Cu salt ) 1:0.1, mol/mol) in MeOH
under air to give L1OH and bpk, but not L1OMe. The order of
efficiency in the oxygenation of L1 is CuBr2 . Cu(NO3)2 >
Cu(ClO4)2 > CuSO4. Since L1OH and bpk were obtained by
using any Cu(II) salt, an autoxidation of L1 catalyzed by a Cu-
(II) ion under air may be proposed as a reaction pathway for
the formation of L1OH and bpk. On the other hand, since
L1OMe was obtained only by using the Cu(II) halide salts,
another reaction pathway must exist for the formation of
L1OMe. When the reaction of L1 with a catalytic amount of
Cu(NO3)2 in MeOH under air was carried out in the presence
of a drop of Br2, L1OMe was detected as a major product. When
a few drops of Br2 were added to a suspension of 5 in MeOH,
5 was converted to 10. L1OMe was also obtained from reaction
of L1 with Br2 without any Cu salt in MeOH under air. These
results indicate that Br2 is essential for the formation of L1OMe.
Figure 4. ORTEP view of [Cu(Br)2(L1OMe)] 10 with the atom
numbering scheme (50% probability ellipsoids). Selected bond distances
(Å): Cu1-Br1, 2.378(2); Cu1-Br2, 2.405(2); Cu1-O1, 2.382(7);
Cu1-N1, 2.049(9); Cu1-N3, 2.037(9). Selected bond angles (deg):
Br1-Cu1-Br2, 93.61(7); Br1-Cu1-O1, 102.5(2); Br1-Cu1-N1,
174.7(2); Br1-Cu1-N3, 94.3(2); Br2-Cu1-O1, 98.8(2); Br2-Cu1-
N1, 91.5(2); Br2-Cu1-N3, 171.7(3); O1-Cu1-N1, 78.1(3); O1-
Cu1-N3, 77.0(3); N1-Cu1-N3, 80.6(3).
Isotope-Labeling of the Oxygenated Ligands. The FAB MS
spectrum of the 18O-labeled Cu(II)3 complex [18O]-1 prepared
under 18O2 shows parent peaks between m/z 1325 and 1345 due
to {[Cu3(Br)(L1O)3](PF6)2}+, indicating that the origin of O
atom in L1OH is O2. To know the origin of the oxygen atom
of L1OH and L1OMe, the reaction of L1 with CuBr2 was
carried out in CD3OD under 18O2. The oxygenated products of
L1 isolated from this reaction were L118OH, bpk, and L1OCD3
on the basis of all spectral data including FAB MS spectra.
L118OH and L1OCD3 clearly demonstrate that 18O2 is incor-
porated into L118OH and CD3OD into L1OCD3. The two
different origins of O atoms for L1OH and L1OMe clearly show
that two different reaction pathways exist for the oxygenation
of L1.
NH3/CH2Cl2. A crude mixture of the oxygenated metal-free
ligands L1OH, bpk, and L1OMe was obtained from the CH2-
Cl2 extracts. When CuCl2 was used instead of CuBr2, the same
products were obtained. The transformation of L1 in the reaction
with CuBr2 in MeOH under air and in the treatment with NaOH
is shown in Scheme 2.
1
The reaction is followed by H NMR spectra of the crude
mixture of the oxygenated metal-free ligands. The yields of
L1OH, bpk, and L1OMe for 1 day of reaction under the
stoichiometric conditions are 10, 4, and 27%, respectively. After
the reaction is continued for 2 days, these are increased to 18,
5, and 34%, respectively. In the stoichiometric reaction, it takes
more than 1 week to complete the oxygenation of L1. Under

234 Inorganic Chemistry, Vol. 39, No. 2, 2000
Kodera et al.
Scheme 3
carbon of L1 and the methylene carbon substituted by two
aromatic amines, such as pyridyl, benzimidazolyl, and imid-
azolyl derivatives, may be easily oxidized to the carbon radicals
because they must be stabilized by π-conjugation with the
aromatic groups. However, for the formation of L1OMe, the
reaction proceeds via the bromination of the methine carbon
by Br2 and the methanolysis of L1Br. Thus, the formation of
L1OMe is totally different from the ketonization.
Conclusion
Newly designed sterically hindered trispyridylmethane ligands
L1 and L2 are synthesized. The reaction of Ln (n ) 1 and 2)
with CuX2 (X ) Br and Cl) formed unique equilateral triangular
Cu(II)3 complexes 1-4, on the contrary to the expectation that
Ln would form di-µ-hydroxodicopper(II) complexes. Com-
pounds 1-4 are quite unique in that they do not have neither
µ3-hydroxo or µ3-oxo bridges and very weak antiferromagnetic
interactions operate between the three Cu(II) ions. In the course
of the reaction of L1 with CuBr2 in MeOH under air, the
oxygenated metal-free ligands L1OH, bpk, and L1OMe were
detected. Since this reaction is quite unique, various studies
including the isotope labeling with 18O2 and CD3OD were
carried out to clarify the mechanism. The oxygenation of L1
with CuBr2 in MeOH under air proceeds through the two
different reaction pathways. In one of them, L1OH and bpk are
produced via a kind of autoxidation similar to the ketonization.
Another one is the formation of L1OMe, and the mechanism
might be explained by the methanolysis of L1Br generated from
bromination of L1 with Br2.
Proposed Mechanism of the Oxygenation of L1. A
proposed mechanism for the oxygenation of L1 to L1OH, bpk,
and L1OMe is shown in Scheme 3. Two different reaction
pathways exist in the oxygenation of L1 with CuBr2 in MeOH
under air. One is for the formation of L1OH and bpk and another
is for the formation of L1OMe. The formation of L1OH and
bpk may be explained by an autoxidation catalyzed by a Cu(II)
ion on the basis of the following two pieces of evidences: (1)
L1OH and bpk are produced by using any Cu(II) salt under air
and (2) the origin of O atom in L1OH is O2. The mechanism
of the autoxidation of L1 is proposed as follows. Initially, L1
is oxidized to a radical (L1•) at the methine carbon in the
presence of a Cu(II) ion under air. L1• reacts with O2 to form
•
L1O2 , which abstracts H• from L1 to give L1O2H and L1•.
L1O2H decomposes to L1OH and bpk. The facts that (1) Br2 is
essential for the formation of L1OMe and (2) the origin of O
atom of L1OMe is MeOH suggest the following reaction
pathway for the formation of L1OMe. L1 is brominated at the
methine carbon by Br2, generated when 9 is converted to 5, to
form L1Br, methanolysis of which affords L1OMe.
The ketonization of ligand has been well-studied and is a
kind of autoxidation.50,51 The first step of the ketonization may
be one electron (1-e) oxidation of ligand by a metal ion under
air. This reaction is accelerated in the presence of a base (OH-
and amines), indicating that the 1-e oxidation is coupled with a
proton trap by the base. The second step is oxygenation of the
resultant carbon radical (L•) by O2, which forms alkylhydro-
peroxide intermediate. The oxygenation of L1 to L1OH and
bpk may be similar to the ketonization because both these
reactions are catalyzed by various Cu(II) salts and proceed via
formation of alkylhydroperoxide intermediate. The methine
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research on Priority Areas (No. 10149101
“Metal-assembled Complexes”) and a Grant to RCAST at
Doshisha University from the Ministry of Education, Science,
Sports, and Culture, Japan.
Supporting Information Available: X-ray crystallographic files
in CIF format for 1‚C6H6, 2‚MeCN‚CH2Cl2‚C6H6, 3, 4‚C6H6, 9, and
10; ORTEP diagrams of 2-4, figures for temperature dependence of
øM and µeff in the temperature range from 4.2 to 290 K for 2-4, and
ESR spectra at 4.2 K for 1-4. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC990331G