Phenylthiyl radical complexes of gallium(III), iron(III), and cobalt(III) and comparison with their phenoxyl analogues

Article abstract of DOI:10.1021/ja004305p

Three hexadentate, asymmetric pendent arm macrocycles containing a 1,4,7-triazacyclononane-1,4-diacetate backbone and a third, N-bound phenolate or thiophenolate arm have been synthesized. In [L¹]^3- the third arm is 3,5-di-tert-butyl

Full text of DOI:10.1021/ja004305p

J. Am. Chem. Soc. 2001, 123, 6025-6039
6025
Phenylthiyl Radical Complexes of Gallium(III), Iron(III), and
Cobalt(III) and Comparison with Their Phenoxyl Analogues
Shuji Kimura, Eckhard Bill, Eberhard Bothe, Thomas Weyhermu1ller, and
Karl Wieghardt*
Contribution from the Max-Planck-Institut fu¨r Strahlenchemie, D-45470 Mu¨lheim an der Ruhr, Germany
ReceiVed December 20, 2000
Abstract: Three hexadentate, asymmetric pendent arm macrocycles containing a 1,4,7-triazacyclononane-1,4-
diacetate backbone and a third, N-bound phenolate or thiophenolate arm have been synthesized. In [L1]3- the
third arm is 3,5-di-tert-butyl-2-hydroxybenzyl, in [L2]3- it is 2-mercaptobenzyl, and in [L3]3- it is 3,5-di-tert-
butyl-2-mercaptobenzyl. With trivalent metal ions these ligands form very stable neutral mononuclear complexes
[MIIIL1] (M ) Ga, Fe, Co), [MIIIL2] (M ) Ga, Fe, Co), and [MIIIL3] (M ) Ga, Co) where the gallium and
5
cobalt complexes possess an S ) 0 and the iron complexes an S ) /2 ground state. Complexes [CoL1]‚CH3-
OH‚1.5H2O, [CoL3]‚1.17H2O, [FeL1]‚H2O, and [FeL2] have been characterized by X-ray crystallography. Cyclic
voltammetry shows that all three [MIIIL1] complexes undergo a reversible, ligand-based, one-electron oxidation
generating the monocations [MIIIL1•]+ which contain a coordinated phenoxyl radical as was unambiguously
established by their electronic absorption, EPR, and Mo¨ssbauer spectra. In contrast, [MIIIL2] complexes in
CH3CN solution undergo an irreversible one-electron oxidation where the putative thiyl radical monocationic
intermediates dimerize with S-S bond formation yielding dinuclear disulfide species [MIIIL2-L2MIII]2+. [GaL3]
behaves similarly despite the steric bulk of two tertiary butyl groups at the 3,5-positions of the thiophenolate,
but [CoIIIL3] in CH2Cl2 at -20 to -61 °C displays a reversible one-electron oxidation yielding a relatively
stable monocation [CoIIIL3•]+. Its electronic spectrum displays intense transitions in the visible at 509 nm (ꢀ
) 2.6 × 103 M-1 cm-1) and 670sh, 784 (1.03 × 103) typical of a phenylthiyl radical. The EPR spectrum of
this species at 90 K proves the thiyl radical to be coordinated to a diamagnetic cobalt(III) ion (giso ) 2.0226;
Aiso(59Co) ) 10.7 G).
Introduction
radical or a resonance hybrid of these forms. The spin state of
the iron ion is also a matter of discussion. Lacking spectroscopic
data a theoretical DFT study has recently been published.6 The
chemistry and spectroscopy of the phenylthiyl radical has been
investigated in detail. The electronic spectrum,7 the EPR8 and
resonance Raman spectrum7 have been recorded and its structure
has been deduced from ab initio calculations.7 These data show
that the C-S bond is essentially a single bond with the unpaired
electron localized on the sulfur atom (Chart 1). This is in distinct
contrast to the corresponding phenoxyl and anilino radical both
of which show quinoide character with substantial delocalization
of the unpaired electron over the respective phenyl ring.9
Recently Stubbe et al.1 have reported the observation of a
kinetically competent thiyl radical intermediate in the ribo-
nucleoside triphosphate reductase (RTPR) from Lactobacillus
leichmannii by using rapid freeze quench techniques and EPR
spectroscopy. From similarities in mechanism between RTPR
and Escherichia coli ribonucleotide reductase (RNR) they
proposed that the latter enzyme also requires a thiyl radical for
catalysis. This radical is suggested to be a cysteinyl radical
generated from C408 in RTPR and C439 in RNR.1,2 In these
enzymes the cysteinyl radical is not S-coordinated to a transition
metal ion.
In contrast, in cytochrome P4503 a high-valent iron-oxene
species has been proposed as an intermediate in the catalytic
cycle the electronic structure of which is currently discussed
controversially. It may involve a porphyrin cation radical as is
established for compound I in horseradish peroxidase4 and some
synthetic models,5 or, alternatively an S-coordinated cysteinyl
The unsubstituted phenylthiyl is extremely reactive; it decays
via second-order kinetics with head-to-head dimerization (2k
) 9.6 × 109 M-1 s-1), generating the disulfide Ph-S-S-Ph.7
Even the sterically protected 2,4,6-tert-butylphenylthiyl decays
within minutes,10 whereas the corresponding phenoxyl and
anilino radicals11 are stable in CCl4 solution for days.
While the coordination chemistry of (phenoxyl)metal com-
plexes and their (anilino)metal analogues has in recent years
(1) Licht, S.; Gerfen, G. J.; Stubbe, J. Science 1996, 271, 477.
(2) Booker, S.; Licht, S.; Broderick, J.; Stubbe, J. Biochemistry 1994,
33, 12676.
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anisms and Biochemistry, 2nd ed.; Ortiz-de Montellano, P. R., Ed.;
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184, 40. (c) Sono, M.; Roach, M. P.; Coulter, E. D.; Dawson, J. H. Chem.
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Chem. 2000, 112, 4009; Angew. Chem., Int. Ed. 2000, 39, 3851. (b) Harris,
N.; Cohen, S.; Filatov, M.; Ogliaro, F.; Shaik, S. Angew. Chem. 2000, 112,
2070; Angew. Chem., Int. Ed. 2000, 39, 2003.
(7) Tripathi, G. N. R.; Sun, Q.; Armstrong, D. A.; Chipman, D. M.;
Schuler, R. H. J. Phys. Chem. 1992, 96, 5344.
(8) (a) Zandstra, P. J.; Michaelsen, J. D. J. Chem. Phys. 1963, 39, 933.
(b) Schmidt, V. Angew. Chem., Int. Ed. Engl. 1964, 3, 602.
(9) Adamo, C.; Subra, R.; Di Matteo, A.; Barone, V. J. Chem. Phys.
1998, 109, 10244.
(4) Schulz, C. E.; Rutter, R.; Sage, J. T.; Debrunner, P. G.; Hager, L. P.
Biochemistry 1984, 23, 4743.
(5) Bill, E.; Ding, X.-Q.; Trautwein, A. X.; Winkler, H.; Mandon, D.;
Weiss, R.; Gold, A.; Jayaraj, K.; Hatfield, W. E.; Kirk, M. L. Eur. J.
Biochem. 1990, 188, 665.
10.1021/ja004305p CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/02/2001

6026 J. Am. Chem. Soc., Vol. 123, No. 25, 2001
Kimura et al.
Chart 1
Scheme 1
been well established,12 this appears not to be the case for (thiyl)-
metal complexes. The report by Darensbourg, Krusic, Amatore,
et al.13 describes the one-electron oxidation chemistry of
RS[Cr(CO)5]2- sa diamagnetic 18e speciessto the correspond-
ing neutral radical species RS˙ [Cr(CO)5]2. From EPR spectros-
copy and extended Hu¨ckel MO calculations the authors were
able to show that, in fact, a µ2-bridging thiyl radical is formed
where the unpaired electron resides “to a large extent in a p-type
orbital on the trigonal sulfur, which is π-conjugated to d orbitals
of the neighboring Cr atoms”. Mononuclear [(CO)5CrSR]o
species have a half-life of ∼50 µs and even the above dinuclear
complex has only limited thermal stability.
In contrast, the stable paramagnetic [CpMn(CO)2(SR)] species
(S ) 1/2) described by Huttner et al.14 are probably best described
as low-spin (thiolato)manganese(II) species rather than (thiyl)-
manganese(I) since the large 55Mn hyperfine coupling constants
of 50-56 G are not compatible with a MnI-S˙ R formulation;
they resemble closely those reported for other low-spin Mn(II)
complexes of this type.15
To generate, if possible, (thiyl)metal complexes with a
biologically more relevant coordination sphere comprising
redox-innocent, O,N-spectator ligands, we have synthesized a
series of pendent arm macrocycles [L1]3-, [L2]3-, [L3]3-, the
structures of which are shown in Scheme 1. These ligands
contain a redox-stable 1,4,7-triazacyclononane-1,4-diacetate
backbone and a third, potentially redox-active phenolate or
thiophenolate pendent arm. As these trianionic ligands are
hexadentate, they are expected to form extremely stable mono-
nuclear neutral complexes, [MIIIL], with trivalent metal ions such
as gallium(III), iron(III), and cobalt(III). Electrochemical or
chemical one-electron oxidations should yield phenoxyl and thiyl
species [MIIIL•]+ which can be studied in solution by spectros-
copy (UV-vis, EPR, Mo¨ssbauer).
Experimental Section
The solvents diethyl ether, toluene, and n-pentane were dried over
Na-benzophenone and methanol, ethanol, dichloromethane, chloroform,
and tetrachloromethane over CaH2 and distilled under argon before use.
The chemicals R,R′-azoisobutyronitrile (AIBN), bromoacetic acid ethyl
ester, 2-tert-butoxycarbonyloxyimino-2-phenylacetonitrile (BOC-ON),
elemental bromine, 1.6 M n-butyllithium in n-hexane, trifluoroacetic
acid, citric acid, iron powder (φ 150 µm), hexamethylenetetramine,
mercury diacetate, potassium hydroxide, 1-chloromethyl-4-methoxy-
benzene, (4-methoxyphenyl)methanethiol, sodium metal, NaBH4, NaOH,
N-bromosuccinimide (NBS), triethylamine, phosphorustribromide, po-
tassium toluene-4-thiosulfonate, acetone, 1,4-dioxane, n-hexane, 2-pro-
panol and dimethylformamide (DMF) were obtained from commercial
sources and used without further purification. [1,4,7]Triazonane (1,4,7-
triazacyclononane, TACN) and 2-bromomethyl-4,6-di-tert-butylphenol
were synthesized as described in the literature.16,17
[1,4,7]Triazonane-1,4-dicarboxylic acid di-tert-butyl Ester (1). To
a solution of [1,4,7]triazonane (TACN) (10.0 g, 0.08 mol) in CHCl3
(200 mL) was added triethylamine (32 mL dried over Na2SO4) at room
temperature. BOC-ON (38.1 g, 0.16 mol) dissolved in CHCl3 (200 mL)
was added with stirring within 30 min. The solution was stirred at room
temperature for additional 2 h and was then concentrated under reduced
pressure. To the obtained residue was added EtOAc (300 mL). The
EtOAc solution was successively washed with 4% NaHCO3(aq) (300
mL), NaCl(aq) (300 mL), and 10% citric acid(aq) (3 × 300 mL),
whereupon the desired product moved to the aqueous phase. To the
aqueous phase was carefully added NaOH under ice-cooling until the
pH was adjusted to 10. The turbid solution was extracted with CHCl3
(3 × 200 mL). The organic phase was dried over Na2SO4 and
concentrated under reduced pressure to give a yellow oil. Upon storage
(10) Rundel, W. Chem. Ber. 1969, 102, 359.
(11) (a) Mu¨ller, E.; Ley, K. Chem. Ber. 1954, 87, 922. (b) Cook, C. D.;
Woodworth, R. C. J. Am. Chem. Soc. 1953, 75, 6242. (c) Land, E. J.; Porter,
G. J. Chem. Soc. 1961, 3540. (d) Griller, D.; Barclay, L. R. C.; Ingold, K.
U. J. Am. Chem. Soc. 1975, 97, 6151.
(12) (a) Chaudhuri, P.; Wieghardt, K. Progr. Inorg. Chem. 2001. In press.
(b) Jazdzewski, B. A.; Tolman, W. B. Coord. Chem. ReV. 2000, 200-202,
633. (c) Goldberg, D. P.; Lippard, S. J. In Mechanistic Bioinorganic
Chemistry; Thorp, H. H., Pecoraro, V. L., Eds.; Advances in Chemistry
Series 246; American Chemical Society: Washington, DC, 1995; p 59. (d)
Fontecave, M.; Pierre, J.-L. Coord. Chem. ReV. 1998, 170, 125.
(13) Springs, J.; Janzen, C. P.; Darensbourg, M. Y.; Calabrese, J. C.;
Krusic, P. J.; Verpeaux, J.-N.; Amatore, C. J. Am. Chem. Soc. 1990, 112,
5789.
1
at -30 °C white crystals precipitated. Yield: 18.8 g (74%). H NMR
(400 MHz, chloroform-d1): δ 1.42 (s, 18H, CH3(t-Bu)), 2.87 (m, 4H,
TACN ring), 3.19 (m, 4H, TACN ring), 3.40 (m, 4H, TACN ring). 13
C
NMR (100 MHz, chloroform-d1): δ 28.5 (t-Bu), 47.3, 47.7, 48.1, 48.2,
49.5, 49.8, 50.4, 51.6, 52.3, 52.5, 53.0 (TACN ring), 79.6, 79.7 (t-Bu),
155.7, 155.9 (CdO). These spectra are very similar to those reported
in ref 26b. Especially the 13C NMR spectrum indicates that two
conformers of 1 exist in solution. EI mass spectrometry: m/z ) 329
(14) Lau, P.; Braunwarth, H.; Huttner, G.; Gu¨nauer, D.; Evertz, K.; Imhof,
W.; Emmerich, C.; Zsolnai, L. Organometallics 1991, 10, 3861.
(15) (a) Gross, R.; Kaim, W. Angew. Chem. 1985, 97, 869; Angew.
Chem., Int. Ed. Engl. 1985, 24, 856. (b) Gross, R.; Kaim, W. Inorg. Chem.
1987, 26, 3596.
(16) Wieghardt, K.; Schmidt, W.; Nuber, B.; Weiss, J. Chem. Ber. 1979,
112, 2220.
(17) Sokolowski, A.; Mu¨ller, J.; Weyhermu¨ller, T.; Schnepf, R.; Hilde-
brandt, P.; Hildenbrand, K.; Bothe, E.; Wieghardt, K. J. Am. Chem. Soc.
1997, 119, 8889.

Phenylthiyl Complexes of Ga(III), Fe(III), and Co(III)
J. Am. Chem. Soc., Vol. 123, No. 25, 2001 6027
(M+). Anal. Calcd for C16H31N3O4: C, 58.33; H, 9.49; N, 12.76.
Found: C, 58.16; H, 9.42; N, 12.75.
After which time n-hexane/Et2O (1/1 v/v, 50 mL) was added and stirred
at room temperature for 1 h. A white precipitate was collected, washed
with Et2O, and dried. Yield: 2.0 g (83%). 1H NMR (500 MHz, D2O):
δ 1.14 (s, 9H, CH3(t-Bu)), 1.19 (s, 9H, CH3(t-Bu)), 2.13 (br, 4H, TACN
ring), 2.36 (br, 8H, TACN ring), 2.95 (br, 4H, NCH2CO2), 3.49 (br,
2H, NCH2Ph), 6.79 (br, 1H), 7.04 (br, 1H). 13C NMR (126 MHz,
D2O): δ 30.0, 31.9 (CH3(t-Bu)), 33.8, 34.4 (t-Bu), 52.2, 53.6, 54.0
(TACN ring), 57.4 (CH2CO2), 60.2 (NCH2Ph(O)), 122.7, 125.8, 126.5,
137.5, 139.2, 155.2 (Carom.), 180.6 (CdO). ESI mass spectrometry in
CH3OH (negative ion mode): m/z ) 468 (M - 2Li + H)-, 462 (M -
3Li + 2H)-. Anal. Calcd for C25H38N3O5Li3: C, 62.37; H, 7.96; N,
8.73. Found: C, 62.18; H, 8.05; N, 8.55.
2-(4-Methoxybenzylsulfanyl)-benzaldehyde (6). To 120 mL of
DMF was added 2-nitrobenzaldehyde (42 g, 0.28 mol) with ice cooling.
To the clear solution was added (4-methoxyphenyl)methanethiol (45
g, 0.29 mol) and then K2CO3 (50 g, 0.36 mol) at 0 °C. This mixture
was stirred at 80 °C for 19 h and then cooled to room temperature.
The mixture was added to ice-water (2 L) with stirring. A yellow
solid was collected, washed with water, and dried under reduced
pressure. This crude solid was recrystallized from CHCl3/petroleum
ether. Pale yellow needle-shaped crystals were collected and dried under
reduce pressure. Yield: 36 g (50%). 1H NMR (250 MHz, chloroform-
d1): δ 3.74 (s, 3H, OCH3), 4.06 (s, 2H, CH2), 6.79 (m, 2H), 7.16 (m,
2H), 7.28 (m, 1H), 7.42-7.46 (m, 2H), 7.78 (m, 1H), 10.2 (s, 1H,
CHO). 13C NMR (63 MHz, chloroform-d1): δ 38.2 (CH2), 55.2 (OCH3),
114.0, 125.9, 127.9, 129.8, 130.0, 131.4, 133.8, 134.5, 141.2, 158.9
(Carom), 191.3 (CHO). EI mass spectrometry: m/z ) 258 (M+). Anal.
Calcd for C15H14O2S: C, 69.76; H, 5.46; S, 12.39. Found: C, 69.71;
H, 5.48; S, 12.40.
7-(3,5-Di-tert-butyl-2-hydoxybenzyl)-[1,4,7]triazonane-1,4-dicar-
boxylic acid di-tert-butyl Ester (2). To a solution of 1 (3.8 g, 12 mmol)
in toluene (50 mL) was added KOH (1.0 g, 18 mmol) at room
temperature. To this mixture was added 2-bromomethyl-4,6-di-tert-
butylphenol (3.8 g, 13 mmol) in toluene (30 mL) within 10 min. The
mixture was stirred at 70 °C for 6 h, filtered and dried over MgSO4,
and was concentrated under reduced pressure to give a white powder
of 2. Yield: 6.0 g (95%). 1H NMR (250 MHz, chloroform-d1): δ 1.25
(9H, CH3(t-Bu)), 1.38 (9H, CH3(t-Bu)), 1.48 (18H, CH3(t-Bu)), 2.76
(br, 4H, TACN ring), 3.33 (br, 4H, TACN ring), 3.45 (br, 4H, TACN
ring), 3.84 (2H, NCH2Ph), 6.83 (1H), 7.18 (1H), 10.2 (br, 1H, OH).
13C NMR (63 MHz, chloroform-d1): δ 28.5, 28.6, 29.5, 31.6 (CH3(t-
Bu)), 34.1, 34.8 (t-Bu), 47.6, 48.8, 49.2, 50.4, 52.9 (TACN ring), 61.6
(NCH2Ph(O)), 79.9, 80.1 (t-Bu(BOC)), 121.3, 123.0, 123.7, 135.6,
140.5, 153.9 (Carom.), 155.4 (CdO). EI mass spectrometry: m/z ) 547
(M+).
[1-(3,5-Di-tert-butyl-2-hydoxybenzyl)-[1,4,7]triazonan-4,7-ium]-
bis(trifluoroacetate) (3). To a solution of 2 (6.0 g, 11 mmol) in CH2-
Cl2 (10 mL) was added 50 v/v% CF3CO2H/CH2Cl2 (60 mL) at 0 °C.
This solution was stirred at 0 °C for 30 min and then concentrated
under reduced pressure. Et2O was added to the residue obtained. The
solvent was removed by rotary evaporation in order to remove excess
CF3CO2H. A pale white powder precipitated, which was washed with
Et2O/n-pentane (1/1 v/v) and dried under reduced pressure. Yield: 5.6
1
g (86%). H NMR (500 MHz, methanol-d4): δ 1.28 (s, 9H, CH3(t-
Bu)), 1.42 (s, 9H, CH3(t-Bu)), 2.98 (m, 8H, TACN ring), 3.62 (s, 4H,
TACN ring), 3.88 (s, 2H, NCH2Ph), 7.11 (d, 1H, J ) 2.4 Hz), 7.32 (d,
1H, J ) 2.4 Hz). 13C NMR (126 MHz, methanol-d4): δ 30.3, 32.0
(CH3(t-Bu)), 35.1, 35.9 (t-Bu), 45.0, 46.6, 51.2 (TACN ring), 58.8
(NCH2Ph(O)), 119.2, 125.1, 127.6, 139.1, 144.2, 152.8 (Carom.), 116.9
(CF3), 163.3 (q, CdO, J ) 35 Hz). ESI mass spectrometry indicated
CF3CO2H free fragment. m/z ) 348 (M′ + H)+. IR: 1694, 1663 cm-1
ν(CdO). Anal. Calcd for C25H41N3O6F6: C, 50.58; H, 6.96; N, 7.08.
Found: C, 50.72; H, 6.85; N, 7.38.
[2-(4-Methoxybenzylsulfanyl)-phenyl]-methanol (7). To a solution
of 6 (10 g, 39 mmol) in 2-propanol (300 mL) was added NaBH4 (1.2
g, 32 mmol) at room temperature. The mixture was stirred at room
temperature for 2 h and concentrated under reduced pressure. To the
residue were added 100 mL of Et2O and 500 mL of saturated NaCl-
(aq). The Et2O solution was collected, and the aqueous phase was
extracted with Et2O (2 × 100 mL). The organic phases were combined
and washed with saturated NaCl(aq) (3 × 500 mL). Then the Et2O
phase was dried over MgSO4 and concentrated. A pale yellow-white
crystalline solid was collected and dried under reduced pressure.
2,4-Di-tert-butyl-6-[1,4,7]triazonan-1-yl-methylphenol (4). To a
solution of 3 (5.5 g, 9.3 mmol) in H2O/CH3OH (250 mL/50 mL) was
slowly added 1 M NaOH(aq) until the pH was adjusted to 9-10. Then
the mixture was extracted with CHCl3 (2 × 100 mL). The CHCl3
solution was washed with saturated NaCl(aq) (100 mL), dried over
MgSO4, and concentrated under reduced pressure to give a pale yellow-
1
Yield: 10 g (99%). H NMR (250 MHz, chloroform-d1): δ 2.28 (br,
1H, OH), 3.75 (s, 3H, OCH3), 4.02 (s, 2H, CH2), 4.63 (s, 2H, CH2-
OH), 6.79 (m, 2H), 7.17 (m, 2H), 7.21-7.24 (m), 7.34-7.37 (m). 13
C
1
NMR (63 MHz, chloroform-d1): δ 39.1 (CH2), 55.1 (OCH3), 63.4 (CH2-
OH), 113.8, 127.1, 128.1, 128.2, 129.2, 129.8, 131.4, 134.2, 141.5,
158.7 (Carom). EI mass spectrometry: m/z ) 260 (M+). Anal. Calcd
for C15H16O2S: C, 69.21; H, 6.20; S, 12.29. Found: C, 69.07; H, 6.22;
S, 12.31.
white powder. Yield: 3.0 g (93%). H NMR (250 MHz, chloroform-
d1): δ 1.24 (s, 9H, CH3(t-Bu)), 1.37 (s, 9H, CH3(t-Bu)), 2.79 (m, 4H,
TACN ring), 2.90 (m, 4H, TACN ring), 2.96 (s, 4H, TACN ring), 3.78
(s, 2H, NCH2Ph), 6.0 (br), 6.81 (d, 1H, J ) 2.4 Hz), 7.19 (d, 1H, J )
2.4 Hz). 13C NMR (63 MHz, chloroform-d1): δ 29.6, 31.6 (CH3(t-
Bu)), 34.1, 34.8 (t-Bu), 46.2, 46.7, 52.9 (TACN ring), 61.8 (NCH2Ph-
(O)), 121.8, 123.3, 123.8, 135.9, 141.1, 153.4 (Carom.). EI mass
spectrometry: m/z ) 347 (M+).
1-Bromomethyl-2-(4-methoxybenzylsulfanyl)-benzene (8). To a
solution of 7 (8.8 g, 34 mmol) in 100 mL of CHCl3 was added PBr3
(2.2 mL, 23 mmol) in 50 mL of CHCl3 under ice cooling within 25
min. The solution was stirred at 0 °C for 30 min. Subsequently, the
solution was washed with saturated NaCl(aq) (3 × 500 mL). The
organic phase was dried over MgSO4 and concentrated under reduced
pressure to give a pale yellow crystalline solid. The crystals were
collected, washed with n-hexane, and dried under reduced pressure.
[4-(3,5-Di-tert-butyl-2-hydroxybenzyl)-7-ethoxycarbonylmethyl-
[1,4,7]triazonan-1-yl]-acetic acid ethyl ester (5). To a solution of 4
(2.9 g, 8.3 mol) in ethanol (30 mL) was added bromoacetic acid ethyl
ester (1.9 mL, 17 mmol) at room temperature. Sodium metal (0.4 g,
17 mmol) dissolved in ethanol (40 mL) was added under ice cooling
within 10 min. The mixture was stirred at 0 °C for 1 h. A white powder
was filtered off, and the filtrate was concentrated under reduced
pressure. Toluene was added to the residue, and the mixture was stirred
at room temperature for 2 h. A white precipitate was filtered off, and
the filtrate was concentrated under reduced pressure. The residue was
extracted with n-hexane, and the extract was concentrated under reduced
1
Yield: 4.8 g (44%). H NMR (400 MHz, chloroform-d1): δ 3.77 (s,
3H, OCH3), 4.08 (s, 2H, CH2), 4.60 (s, 2H, CH2Br), 6.80 (d, 2H, J )
8.6 Hz), 7.20 (m, 4H), 7.36 (t, 2H, J ) 7.4 Hz). 13C NMR (100 MHz,
chloroform-d1): δ 32.1 (CH2), 39.0 (CH2S), 55.2 (OCH3), 113.9, 127.2,
129.0, 129.1, 130.0, 130.5, 132.0, 136.2, 138.5, 158.8 (Carom). EI mass
spectrometry: m/z ) 322 (M+). Anal. Calcd for C15H15OSBr: C, 55.74;
H, 4.68; S, 9.90; Br, 24.72. Found: C, 55.61; H, 4.77; S, 9.91; Br,
24.69.
7-[2-(4-Methoxybenzylsulfanyl)-benzyl]-[1,4,7]triazonane-1,4-di-
carboxylic Acid Di-tert-butyl Ester (9). To a solution of 1 (4.3 g, 13
mmol) in toluene (50 mL) was added 8 (4.7 g, 15 mmol) dissolved in
toluene (60 mL) under ice cooling. Finely powdered KOH (2.4 g, 43
mmol) was added to the solution. This mixture was stirred at 80 °C
for 24 h and then cooled to room temperature and dried over MgSO4
and filtered. The filtrate was concentrated under reduced pressure to
give a yellow oil. Yield: 7.0 g (94%). This oil was used for the
1
pressure to give a yellow oil of 5. Yield: 2.6 g (60%). H NMR (250
MHz, chloroform-d1): δ 1.21-1.26 (15H, CH3(t-Bu) and Et(CH3)),
1.39 (s, 9H, CH3(t-Bu)), 2.75 (s, 4H, TACN ring), 2.90 (br, 8H, TACN
ring), 3.44 (s, 4H, CH2CO2), 3.8 (br, 2H, NCH2Ph), 4.12 (q, 4H, CH2-
(Et), J ) 7 Hz), 6.83 (br, 1H), 7.18 (br, 1H). EI mass spectrometry:
m/z ) 519 (M+).
[4-Carboxymethyl-7-(3,5-di-tert-butyl-2-hydroxybenzyl)-[1,4,7]-
triazonan-1-yl]-acetic acid Li salt (Li3[L1]). To a solution of 5 (2.6
g, 5.0 mmol) in ethanol (50 mL) was added finely powdered LiOH‚
H2O (0.63 g, 15 mmol). The mixture was stirred at 70 °C for 6 h.

6028 J. Am. Chem. Soc., Vol. 123, No. 25, 2001
Kimura et al.
1
following synthesis. H NMR (250 MHz, chloroform-d1): δ 1.39 (s,
CO2), 3.42 (s, 2H, NCH2Ph), 3.72 (s, 3H, OCH3), 3.95 (s, 2H, SCH2),
6.74 (d, 2H, J ) 8.5 Hz), 7.05-7.24 (m, 5H), 7.47 (br, 1H). 13C NMR
(100 MHz, chloroform-d1): δ 38.5 (SCH2), 50.4 (NCH2Ph (S)), 52.4,
52.5, 53.1 (TACN ring), 55.2 (OCH3), 62.6 (NCH2CO2), 113.9, 128.1,
129.0, 129.8, 130.0, 132.2, 132.4, 136.5, 158.8 (Carom.), 179.1 (CdO).
ESI mass spectrometry in CH3OH (negative ion mode): m/z ) 492
(M - Li)-, 486 (M - 2Li + H)-.
9H, CH3(t-Bu)), 1.46 (s, 9H, CH3(t-Bu)), 2.5-3.5 (m, 12H, TACN ring),
3.70 (s, 2H, CH2N), 3.74 (s, 3H, OCH3), 3.99 (2H, SCH2), 6.78 (m),
7.0-7.4 (m). 13C NMR (63 MHz, chloroform-d1): δ 28.4, 28.5, 38.3,
49.0, 49.1, 49.4, 49.9, 50.1, 50.3, 50.8, 53.4, 53.7, 54.2, 55.2, 58, 79.3,
79.4, 113.8, 126.0, 127.2, 129.2, 129.4, 129.7, 129.9, 136, 139, 155.4,
155.6, 158.7. EI mass spectrometry: m/z ) 571 (M+).
[4-Carboxymethyl-7-(2-mercaptobenzyl)-[1,4,7]triazonan-1-yl]-
acetic Acid Li Salt (Li2[L2H]). To a solution of 13 (0.3 g, 0.6 mmol)
in methanol (3 mL) was added Hg(OAc)2 (0.4 g, 1.3 mmol) at room
temperature. The mixture was stirred at 60 °C for 6 h and concentrated
under reduced pressure. The pale yellow residue was washed with
n-pentane (2 × 10 mL) to give a white powder. To this white powder
was added 10 mL of methanol. H2S gas was bubbled through the
solution for 20 min and then argon gas for 30 min. Black HgS was
filtered off, and the filtrate was concentrated under reduced pressure
to give a white powder. Yield 0.22 g. This compound was used for
synthesis of metal complexes without further purification. ESI mass
spectrometry in CH3OH (negative ion mode): m/z ) 388 (M - 2Li +
Na)-.
{4-[2-(4-Methoxybenzylsulfanyl)-benzyl]-[1,4,7]triazonan-1,4,7-
ium} Trifluoroacetate (10). To a solution of 9 (6.9 g, 12 mmol) in
CH2Cl2 (50 mL) was added 50 mL of CF3CO2H at 0 °C. The solution
was stirred at 0 °C for 30 min and then concentrated under reduced
pressure, yielding a red-brown residue from which a pale yellow-white
powder precipitated upon addition of Et2O. This powder was collected,
washed with Et2O, and dried in air. The purity of this powder was
confirmed by GC analysis (90% purity). Yield: 5.3 g (62%). 1H NMR
(250 MHz, chloroform-d1): δ 2.90, 3.05, 3.58 (br, 12H, TACN ring),
3.71 (s, 3H, OCH3), 3.82 (br, 2H, NCH2Ph), 3.99 (br, 2H, SCH2), 6.75
(d, 2H, J ) 8.6 Hz), 7.07 (d, 2H, J ) 8.6 Hz), 7.2-7.4 (m), 8.5 (br).
13C NMR (63 MHz, chloroform-d1): δ 38.7 (SCH2), 42.8, 44.5, 49.2
(TACN ring), 55.1 (OCH3), 56.7 (NCH2Ph), 113.9, 126.8, 128.7, 128.8,
129.8, 130.7, 130.9, 136.0, 136.3, 158.9 (Carom), 118.8 (CF3), 162.1 (q,
CdO, 35 Hz). FAB mass spectrometry (negative ion mode, matrix:
dimethoxybenzyl alcohol): m/z ) 712 (M-H)-. IR: 1678 cm-1 ν-
(CdO).
2-Bromo-1,5-di-tert-butyl-3-methylbenzene (14). To a mixture of
1,3-di-tert-butyl-5-methylbenzene (20 g, 0.10 mol) and Fe powder (1.2
g, 0.021 mol) in 200 mL of CCl4 was added Br2 (5.0 mL, 0.10 mol) in
50 mL of CCl4 under ice cooling over 2 h. The mixture was stirred at
0 °C for 2 h and filtered. The filtrate was washed with 10% NaHCO3-
(aq) (3 × 300 mL) and saturated NaCl(aq) (3 × 300 mL). The organic
phase was dried over Na2SO4 and concentrated under reduced pressure
to give a pale yellow oil. The crude oil was distilled under reduced
pressure (144.5-145.5 °C/5-6 mmHg). Yield: 18 g (65%). Colorless
1-[2-(4-Methoxybenzylsulfanyl)-benzyl]-[1,4,7]triazonane (11). To
a solution of 10 (5.0 g, 7.0 mmol) in 100 mL of water was added 1 M
NaOH(aq) under ice cooling until a pH value of ca. 10-12 was
obtained. To this mixture was added 100 mL of CHCl3, and the CHCl3
layer was collected. The aqueous phase was extracted with CHCl3 (2
× 100 mL). The combined organic phases were washed with saturated
NaCl(aq) (3 × 300 mL). The CHCl3 solution was dried over MgSO4,
filtered, and concentrated under reduced pressure. The residue was
washed with Et2O and n-hexane and dried under reduced pressure to
give a pale yellow powder. The purity of this powder was checked by
1
oil. H NMR (250 MHz, chloroform-d1): δ 1.38 (s, 9H, CH3(t-Bu)),
1.62 (s, 9H, CH3(t-Bu)), 2.52 (s, 3H, CH3), 7.21 (d, 1H, J ) 2.4 Hz),
7.41 (d, 1H, J ) 2.4 Hz). 13C NMR (63 MHz, chloroform-d1): δ 25.7
(CH3), 30.2 (CH3(t-Bu)), 31.3 (CH3(t-Bu)), 34.6 (t-Bu), 37.3 (t-Bu),
122.6, 122.9, 125.9, 139.2, 147.4, 149.2 (Carom.). EI mass spectrom-
etry: m/z ) 282 (M+). Anal. Calcd for C15H23Br: C, 63.61; H, 8.19;
Br, 28.21. Found: C, 63.51; H, 8.24; Br, 28.33.
2-Bromo-3,5-di-tert-butylbenzaldehyde (15). To a solution of 14
(60 g, 212 mmol) in CCl4 (500 mL) was added NBS (42 g, 236 mmol)
and AIBN (1.4 g, 8.5 mmol). The reaction mixture was gently heated
and then refluxed at 100 °C for 1 h and filtered at -10 °C. The filtrate
was concentrated under reduced pressure to give 2-bromo-1-bromom-
ethyl-3,5-di-tert-butylbenzene. This compound was used without further
purification. 1H NMR (250 MHz, chloroform-d1): δ 1.38 (s, 9H, CH3-
(t-Bu)), 1.62 (s, 9H, CH3(t-Bu)), 4.76 (s, 2H, CH2Br), 7.41 (d, 1H, J )
2.4 Hz), 7.53 (d, 1H, J ) 2.5 Hz). 13C NMR (63 MHz, chloroform-
d1): δ 30.1, 31.1 (CH3(t-Bu)), 34.6, 37.4 (t-Bu), 36.5 (CH2Br), 122.1,
125.8, 126.4, 138.1, 148.3, 149.9.
To the solution of the above 2-bromo-1-bromomethyl-3,5-di-tert-
butylbenzene in 500 mL of CHCl3 was added hexamethylenetetramine
(84 g, 599 mmol). The solution was refluxed at 70-80 °C for 24 h
and concentrated under reduced pressure. To the pale yellow residue
was added hexamethylenetetramine (84 g, 599 mmol) and 500 mL of
50% CH3CO2H(aq). The mixture was heated at 100-110 °C for 24 h,
after which time 100 mL of concentrated HCl(aq) was added. The
mixture was heated at 100-110 °C for 15 min and then cooled to 25
°C and extracted with Et2O (3 × 200 mL). The Et2O solution was
successively washed with 2 M HCl(aq) (3 × 500 mL) and saturated
NaCl(aq) (3 × 500 mL). The organic phase was dried over Na2SO4
and concentrated under reduced pressure to give a reddish yellow oil.
The crude oil was purified by column chromatography (Silica gel 60,
eluent: n-hexane/Et2O 100/1 (v/v)). Yield: 34-42% (pale yellow oil).
1H NMR (400 MHz, chloroform-d1): δ 1.30 (s, 9H, CH3(t-Bu)), 1.55
(s, 9H, CH3(t-Bu)), 7.73 (quasi-d, 2H), 10.5 (s, 1H, CHO). 13C NMR
(100 MHz, chloroform-d1): δ 30.1, 31.0 (CH3(t-Bu)), 34.8, 37.4 (t-
Bu), 124.5, 124.9, 131.0, 135.3, 148.4, 150.3 (Carom.), 193.9 (CHO).
EI mass spectrometry: m/z ) 296 (M+). Anal. Calcd for C15H21OBr:
C, 60.61; H, 7.12; Br, 26.88. Found: C, 60.40; H, 7.26; Br, 26.80.
(2-Bromo-3,5-di-tert-butylphenyl)-methanol (16). To a solution of
15 (21 g, 71 mmol) in 2-propanol (400 mL) was added NaBH4 (2.2 g,
58 mmol) at room temperature. The mixture was stirred at room
temperature for 2 h and concentrated under reduced pressure. The
1
GC analysis (81% purity). Yield: 2.4 g (92%). H NMR (250 MHz,
chloroform-d1): δ 2.65 (m, 8H, TACN ring), 2.95 (br, 4H, TACN ring),
3.69 (s, 2H, NCH2Ph), 3.72 (s, 3H, OCH3), 4.09 (s, 2H, CH2S), 4.9
(br), 6.7-7.3 (Harom). 13C NMR (63 MHz, chloroform-d1): δ 37.6
(SCH2), 43.4, 44.8, 50.3 (TACN ring), 55.2 (OCH3), 59.7 (NCH2),
113.8, 125.7, 128.4, 128.9, 129.2, 129.9, 130.1, 137.0, 137.1, 158.7
(Carom.). EI mass spectrometry: m/z ) 371 (M+).
{4-Ethoxycarbonylmethyl-7-[2-(4-methoxybenzylsulfanyl)-ben-
zyl]-[1,4,7]triazonan-1-yl}-acetic acid ethyl ester (12). To a solution
of 11 (2.9 g, 7.8 mmol) in ethanol (30 mL) was added bromoacetic
acid ethyl ester (1.8 mL, 16 mmol) at room temperature. The solution
was cooled to 0 °C, and sodium metal (0.37 g, 16 mmol) dissolved in
ethanol (40 mL) was added and stirred at 0 °C for 30 min. A white
precipitate was filtered off, and the filtrate was concentrated under
reduced pressure. To this residue was added 50 mL of toluene, and the
mixture was stirred for 2 h. Solid NaBr was removed by filtration, and
the filtrate was concentrated under reduced pressure to give a yellow
oil, which was extracted with Et2O. The Et2O solution was concentrated,
and the obtained residue was dried under reduced pressure. Yield: 2.6
g (61%). This product was used without further purification. 1H NMR
(400 MHz, chloroform-d1): δ 1.23 (t, 6H, CH3(Et), J ) 7 Hz), 2.74
(br, 4H, TACN ring), 2.80, (br, 4H, TACN ring), 2.88 (br, 4H, TACN
ring), 3.35 (s, 4H, NCH2CO2), 3.66 (s, 2H, NCH2Ph), 3.74 (s, 3H,
OCH3), 4.01 (s, 2H, CH2S), 4.11 (q, 4H, CH2(Et), J ) 7 Hz), 6.77 (m
2H), 7.11-7.17 (m, 4H), 7.29 (m, 1H), 7.39 (br, 1H). 13C NMR (100
MHz, chloroform-d1): δ 14.3 (CH3(Et)), 38.3 (SCH2), 55.2 (OCH3),
55.0, 55.3, 55.7 (TACN ring), 58.8 (NCH2CO2), 60.1 (NCH2Ph(S) and
CH2(Et)), 113.8, 125.7, 127.1, 129.2, 129.4, 129.7, 129.9, 136.5, 139.9,
158.7 (Carom.), 172.1 (CdO). EI mass spectrometry: m/z ) 543 (M+).
{4-Carboxymethyl-7-[2-(4-methoxybenzylsulfanyl)-benzyl]-[1,4,7]-
triazonan-1-yl}-acetic Acid Li Salt (13). To a solution of 12 (2.4 g,
4.4 mmol) in ethanol (30 mL) was added finely powdered LiOH‚H2O
(0.37 g, 8.8 mmol). The mixture was stirred at 70 °C for 17 h and
filtered. The filtrate was concentrated under reduced pressure to give
a pale yellow solid. This solid was washed with Et2O and dried under
reduced pressure. Yield: 2.1 g (95%). 1H NMR (400 MHz, chloroform-
d1): δ 2.32, 2.47, 2.65 (br, 12H, TACN ring), 3.11 (br, 4H, NCH2-

Phenylthiyl Complexes of Ga(III), Fe(III), and Co(III)
J. Am. Chem. Soc., Vol. 123, No. 25, 2001 6029
residue was extracted with Et2O (3 × 200 mL). The collected extracts
were washed with saturated NaCl(aq) (3 × 500 mL), dried over Na2-
SO4, and concentrated under reduced pressure yielding pure white
crystals. Yield: 18.4 g (87%). 1H NMR (250 MHz, chloroform-d1): δ
1.32 (s, 9H, CH3(t-Bu)), 1.54 (s, 9H, CH3(t-Bu)), 2.37 (s, 1H, OH),
4.76 (br, 2H, CH2), 7.36 (d, 1H, J ) 2.5 Hz), 7.44 (d, 1H, J ) 2.5 Hz).
13C NMR (63 MHz, chloroform-d1): δ 30.1, 31.3 (CH3(t-Bu)), 34.8,
37.3 (t-Bu), 66.9 (CH2), 120.4, 124.3, 124.7, 140.8, 147.5, 149.9 (Carom.).
EI mass spectrometry: m/z ) 298 (M+). Anal. Calcd for C15H23OBr:
C, 60.20; H, 7.75; Br, 26.70. Found: C, 60.38; H, 7.82; Br, 26.53.
Alternatively, 16 was prepared as follows. To the described 2-bromo-
1-bromomethyl-3,5-di-tert-butylbenzene (66.0 g, 0.18 mol) in 1,4-
dioxane (530 mL) was added NaOH (13.2 g, 0.33 mol) in water (550
mL). This mixture was refluxed at 115 °C for 9 h, and Et2O (500 mL)
was added. The Et2O solution was washed with 2 M HCl(aq) (3 ×
500 mL) and saturated NaCl(aq) (3 × 500 mL). The Et2O phase was
dried over MgSO4 and concentrated under reduced pressure to give a
pale yellow oil. This crude oil was purified by column chromatography
(Merck Silica gel 60, eluent: n-hexane/Et2O 10/3 (v/v)). White crystals.
Yield: 32.1 g (59%).
chloroform-d1): δ 1.33 (s, 9H, CH3(t-Bu)), 1.59 (s, 9H, CH3(t-Bu)),
3.80 (s, 3H, OCH3), 3.98 (s, 2H, SCH2), 5.03 (s, 2H, CH2Br), 6.85 (m,
2H), 7.24 (m, 2H), 7.47 (m, 2H). 13C NMR (63 MHz, chloroform-d1):
δ 31.2, 31.6 (CH3(t-Bu)), 34.6 (CH2Br), 34.9, 37.9 (t-Bu), 42.5 (SCH2),
55.3 (OCH3), 114.0, 124.8, 126.9, 129.1, 130.2, 130.8, 143.9, 151.7,
153.9, 158.9 (Carom.). EI mass spectrometry: m/z ) 434 (M+). Anal.
Calcd for C23H31OBrS: C, 63.45; H, 7.18; Br, 18.35; S, 7.35. Found:
C, 64.44; H, 7.31; Br, 18.03; S, 7.05.
7-[3,5-Di-tert-butyl-2-(4-methoxybenzylsulfanyl)-benzyl]-[1,4,7]-
triazonane-1,4-dicarboxylic Acid Di-tert-butyl Ester (20). To a
solution of 1 (2.5 g, 7.6 mmol) in toluene (30 mL) was added 19 (3.7
g, 8.5 mmol) in toluene (35 mL) under ice cooling over 10 min. Finely
powdered KOH (1.4 g, 25 mmol) was added to the solution. The
mixture was stirred at 80 °C for 24 h and cooled to 25 °C, dried over
MgSO4, and filtered. The filtrate was concentrated under reduced
pressure to give a yellow solid. Yield: 5.2 g (100%). This solid was
used without further purification. EI mass spectrometry: m/z ) 683
(M+). Anal. Calcd for C39H61N3O5S: C, 68.49; H, 8.99; N, 6.14; S,
4.68. Found: C, 68.52; H, 8.93; N, 6.03; S, 4.79.
{4-[3,5-Di-tert-butyl-2-(4-methoxybenzylsulfanyl)-benzyl]-[1,4,7]-
triazonan-1,4,7-ium} Trifluoroacetate (21). To a CF3CO2H/CH2Cl2
(50 v/v%) solution (60 mL) was added 20 (4.5 g, 6.6 mmol) at 0 °C.
The solution was stirred at 0 °C for 30 min and concentrated under
reduced pressure. To the red-brown residue was added an n-hexane/
Et2O mixture and the solvent was evaporated under reduced pressure.
This procedure was repeated to remove excess CF3CO2H. To the residue
was added an n-hexane/Et2O mixture. After stirring at room temperature
Toluene-4-thiosulfonic Acid S-(4-Methoxybenzyl) Ester (17). To
a solution of 1-chloromethyl-4-methoxybenzene (15.0 g, 0.10 mol) in
acetone (75 mL) was added potassium toluene-4-thiosulfonate (22.5
g, 0.10 mol) dissolved in acetone (200 mL). The mixture was refluxed
at 70 °C for 8 h, filtered, and concentrated under reduced pressure.
The residue obtained was extracted with Et2O (3 × 100 mL), and a
colorless precipitate was filtered off. The Et2O solution was washed
with saturated NaCl(aq) (3 × 300 mL) and dried over Na2SO4. Removal
of the solvent yielded a crude residue, which was recrystallized from
1
a yellow-white powder was obtained. Yield: 3.8 g (70%). H NMR
(250 MHz, chloroform-d1): δ 1.28 (s, 9H, CH3(t-Bu)), 1.52 (s, 9H,
CH3(t-Bu)), 2.93 (br, 4H, TACN ring), 3.10 (br, 4H, TACN ring), 3.58
(br, 4H, TACN ring), 3.66 (s, 2H, NCH2Ph), 3.73 (s, 3H, OCH3), 4.15
(s, 2H, SCH2), 6.76 (d, 2H, J ) 8.5 Hz), 7.02 (d, 2H, J ) 8.5 Hz),
7.21 (br, 1H), 7.47 (br, 1H), 8.6 (br). 13C NMR (63 MHz, chloroform-
d1): δ 31.1, 31.5 (CH3(t-Bu)), 34.7, 37.7 (t-Bu), 42.6 (SCH2), 43.9,
44.2, 48.3 (TACN ring), 55.2 (OCH3), 56.7 (NCH2), 114.0, 124.4, 126.3,
128.6, 130.0, 131.2, 140.5, 151.8, 154.3, 159.0 (Carom.), 118.4 (CF3),
161.7 (q, CdO, J ) 37 Hz). ESI mass spectrometry (acetone): m/z
) 484 (M′ + H)+. IR: 1679 cm-1 ν(CdO). Anal. Calcd for
C35H48N3O7F9S: C, 50.91; H, 5.86; N, 5.09; F, 20.71; S, 3.88. Found:
C, 51.12; H, 5.95; N, 5.10; F, 20.92; S, 3.78.
1-[3,5-Di-tert-butyl-2-(4-methoxybenzylsulfanyl)-benzyl]-[1,4,7]-
triazonane (22). To a solution of 21 (3.6 g, 4.4 mmol) in CHCl3 (100
mL) were added 3.0 mL of NEt3 with ice cooling. After stirring at 0
°C for 30 min, the organic phase was washed with NaCl(aq) (3 × 300
mL), dried over MgSO4 and concentrated under reduced pressure to
give a pale yellow-white powder. The purity of this powder was checked
by GC analysis (83% purity). Yield: 1.9 g (90%). 1H NMR (250 MHz,
chloroform-d1): δ 1.29 (s, 9H, CH3(t-Bu)), 1.53 (s, 9H, CH3(t-Bu)),
2.65 (br, 8H, TACN ring), 2.84 (br, 4H, TACN ring), 3.68 (s, 2H,
NCH2Ph), 3.76 (s, 3H, OCH3), 4.03 (s, 2H, SCH2), 6.79 (m, 2H), 7.11
(m, 2H,), 7.37 (quasi-d, 1H), 7.42 (quasi-d, 1H). 13C NMR (63 MHz,
chloroform-d1): δ 31.3, 31.6 (CH3(t-Bu)), 34.8, 37.6 (t-Bu), 42.9
(SCH2), 45.5, 45.8, 52.0 (TACN ring), 55.2 (OCH3), 59.4 (NCH2),
113.9, 123.1, 125.4, 129.4, 130.0, 130.7, 144.6, 150.9, 153.4, 158.8
(Carom.). EI mass spectrometry: 483 (M+).
{4-Carboxymethyl-7-[3,5-di-tert-butyl-2-(4-methoxybenzylsulfa-
nyl)-benzyl]-[1,4,7]triazonan-1-yl}-acetic Acid Li Salt (23). To a
solution of 22 (1.9 g, 3.9 mmol) in ethanol (30 mL) was added
bromoacetatic acid ethyl ester (0.9 mL, 8.1 mmol) at room temperature.
The solution was cooled at 0 °C. Sodium metal (0.19 g, 8.3 mmol)
dissolved in ethanol (20 mL) was added at 0 °C over 5 min. After
stirring at 0 °C for 30 min, the solution was concentrated under reduced
pressure to give a pale yellow-white residue. To this residue was added
toluene (50 mL), and the mixture was stirred for 90 min and filtered.
The filtrate was concentrated under reduced pressure to give a yellow
oil to which was added n-hexane, and a white powder was removed
by filtration. The filtrate was concentrated under reduced pressure to
give a crude compound. From NMR and mass spectrometry, it was
established that the crude oil contained ca. 65 wt % of product and ca.
35 wt % di-substituted compound. The crude oil reacted with
bromoacetatic acid ethyl ester (0.15 mL) and sodium metal (30 mg).
1
CH3OH, affording colorless crystals. Yield: 22.8 g (77%). H NMR
(250 MHz, chloroform-d1): δ 2.41 (s, 3H, CH3Ph), 3.73 (s, 3H, OCH3),
4.18 (s, 2H, CH2), 6.73 (m, 2H), 7.07 (m, 2H), 7.25 (m, 2H), 7.71 (m,
2H). 13C NMR (63 MHz, chloroform-d1): δ 21.5 (CH3), 39.8 (CH2),
55.2 (OCH3), 114.1, 125.2, 126.9, 129.7, 130.3, 142.0, 144.5, 159.3
(Carom.). EI mass spectrometry: m/z ) 308 (M+). Anal. Calcd for
C15H16O3S2: C, 58.44; H, 5.23; S, 20.76. Found: C, 58.56; H, 5.28; S,
20.72.
[3,5-Di-tert-butyl-2-(4-methoxybenzylsulfanyl)-phenyl]-metha-
nol (18). To a solution of 16 (17.6 g, 59 mmol) in 100 mL of Et2O
was added 1.6 M n-BuLi in n-hexane (79.2 mL, 127 mmol) under an
argon atmosphere with ice cooling. The solution was stirred at 0 °C
for 10 min and then at room temperature for 3 h. To the pale yellow
solution was added 17 (18.5 g, 60 mmol) in 200 mL of Et2O at -70
°C and 30 mL of Et2O was added. The solution was stirred at -70 °C
for 1 h and then at ca. -30 °C for 4 h. To the mixture was added 2 M
HCl(aq) (500 mL), and the Et2O layer was collected. The aqueous phase
was extracted with Et2O (2 × 200 mL). The organic phases were
combined and washed with 2 M HCl(aq) (2 × 500 mL) and saturated
NaCl(aq) (3 × 500 mL). The solution was dried over Na2SO4 and
concentrated under reduced pressure to give a crude oil. This oil was
purified by column chromatography (Merck Silica gel 60, eluent:
n-hexane/Et2O 10/5 (v/v) or toluene/Et2O 10/1 (v/v)). Yield: 8.6 g
1
(39%). H NMR (250 MHz, chloroform-d1): δ 1.32 (s, 9H, CH3(t-
Bu)), 1.58 (s, 9H, CH3(t-Bu)), 2.68 (t, 1H, OH, J ) 6.5 Hz), 3.79 (s,
3H, OCH3), 3.86 (s, 2H, CH2), 4.97 (d, 2H, CH2OH, J ) 6.4 Hz), 6.84
(m, 2H), 7.21 (m, 2H), 7.37 (d, 1H, J ) 2.2 Hz), 7.46 (d, 1H, J ) 2.3
Hz). 13C NMR (63 MHz, chloroform-d1): δ 31.2, 31.6 (CH3(t-Bu)),
34.9, 37.7 (t-Bu), 42.8 (SCH2), 55.3 (OCH3), 65.5 (CH2OH), 114.1,
123.9, 124.5, 129.0, 129.2, 130.1, 146.8, 151.7, 153.8, 158.9 (Carom.).
EI mass spectrometry: m/z ) 372 (M+). Anal. Calcd for C23H32O2S:
C, 74.16; H, 8.66; S, 8.59. Found: C, 73.99; H, 8.68; S, 8.75.
1-Bromomethyl-3,5-di-tert-butyl-2-(4-methoxybenzylsulfanyl)-
benzene (19). To a solution of 18 (2.5 g, 6.7 mmol) in CHCl3 (30 mL)
was added PBr3 (0.83 mL, 8.7 mmol) in CHCl3 (10 mL) over 15 min,
and 10 mL of CHCl3 was added. The clear pale yellow solution was
stirred at 0 °C for 90 min and washed with NaBr(aq) (4 × 300 mL).
The organic phase was dried over Na2SO4 and concentrated under
reduced pressure to give a pale yellow oil. Yield: 3.1 g. The compound
was used without further purification. For analytical measurements the
crude product was purified by column chromatography (Merck Silica
gel 60, eluent: n-hexane/Et2O 100/2(v/v)). 1H NMR (250 MHz,

6030 J. Am. Chem. Soc., Vol. 123, No. 25, 2001
Kimura et al.
After the reaction was completed, a pale yellow oil (1.2 g) was obtained
and used for the next reaction without further purification. To the oil
(1.2 g) in ethanol (30 mL) was added finely powdered LiOH‚H2O (0.16
g, 3.8 mmol) at room temperature. The mixture was stirred at 70 °C
for 6 h and filtered. The filtrate was concentrated under reduced pressure
to give a yellow oil. To this oil was added an n-hexane/Et2O solution,
and the solvent was evaporated to remove ethanol. During this
procedure a white powder precipitated. The powder was washed with
n-pentane, collected, and dried under reduced pressure. Yield: 0.92 g
(38%). ESI mass spectrometry in methanol (negative ion mode): m/z
) 604 (M - Li)-, 598 (M - 2Li + H)-. IR: 1609, 1594 cm-1 ν(Cd
O).
[4-Carboxymethyl-7-(3,5-di-tert-butyl-2-mercaptobenzyl)-[1,4,7]-
triazonan-1-yl]-acetic Acid Li Salt (HLi2L3). The synthesis of this
compound was performed under an atmosphere of argon. To 23 (0.20
g, 0.33 mmol) and Hg(CH3CO2)2 (0.22 g, 0.69 mmol) was added
methanol (10 mL). The solution was stirred at 60 °C for 6 h and then
concentrated under reduced pressure. The residue was washed with
n-pentane (2 × 10 mL) and dried under reduced pressure to give a
white powder. To the powder was added methanol (10 mL). Through
this suspension was bubbled H2S gas for 20 min and then argon for 30
min. Black HgS was filtered off, and the colorless filtrate was
concentrated under reduced pressure to give a white powder. Yield:
0.20 g. This powder was used for synthesis of metal complexes without
further purification. ESI mass spectrometry in methanol (negative ion
mode): m/z ) 478 (M - 2Li + H)-.
as described above. Yield 34 mg (52%). ESI mass spectrometry in CH3-
OH (positive ion mode): m/z ) 540 (M0 + Na)+, 1057 (2M0 + Na)+.
[GaL2]. To HLi2L2 (46 mg, 0.13 mmol) in CH3CN (2 mL) was added
a solution of GaCl3 (23 mg, 0.13 mmol) in CH3CN (4 mL). The mixture
was refluxed at 90 °C for 1 h. After addition of 50 µL of NEt3 the
mixture was refluxed at 90 °C for additional 3 h and concentrated under
reduced pressure to give a white powder. The crude powder was
recrystallized from hot CH3OH. A white precipitate was collected,
washed with cold CH3CN and Et2O, and dried under reduced pressure.
Yield: 11 mg (20%). ESI mass spectrometry in CH3OH (positive ion
mode): m/z ) 456 (M0 + Na)+, 891 (2M0 + Na)+. Anal. Calcd for
C17H22N3O4SGa: C, 47.03; H, 5.11; N, 9.68. Found: C, 46.85; H, 5.02;
N, 9.73.
[CoL2]. A solution of HLi2L2 (48 mg, 0.13 mmol) and CoCl2‚6H2O
(31 mg, 0.13 mmol) in CH3CN (6 mL) was refluxed at 90 °C for 1 h.
50 µL of NEt3 were added, and the solution was refluxed at 90 °C
under an atmosphere of argon for 1 h and then in the presence of air
for 2 h. The solution was concentrated under reduced pressure to give
a green solid, which was recrystallized from hot CH3OH. Red crystals
were collected, washed with cold CH3OH and Et2O, and dried under
reduced pressure. Yield: 15 mg (27%). ESI mass spectrometry in CH3-
OH (positive ion mode): m/z ) 446 (M0 + Na)+, 869 (2M0 + Na)+.
Anal. Calcd for C17H22N3O4SCo: C, 48.23; H, 5.24; N, 9.93. Found:
C, 48.16; H, 5.28; N, 10.03.
[FeL2]. To HLi2L2 (45 mg, 0.12 mmol) in CH3CN (2 mL) was added
FeCl3 (20 mg, 0.12 mmol) in CH3CN (4 mL). The solution was refluxed
at 90 °C for 2 h. To this solution was added 50 µL of NEt3, and the
solution was refluxed at 90 °C for additional 2 h and concentrated under
reduced pressure to give a greenish blue solid. The crude solid was
recrystallized from hot CH3CN. Black crystals were collected, washed
with cold CH3CN, and dried under reduced pressure. Yield: 8.3 mg
(16%). Crystals suitable for X-ray crystallography were grown from
methanol. ESI mass spectrometry in CH3CN (positive ion mode): m/z
) 443 (M0 + Na)+, 863 (2M0 + Na)+. Anal. Calcd for C17H22N3O4-
SFe: C, 48.59; H, 5.28; N, 10.00. Found: C, 48.46; H, 5.22; N, 9.88.
[GaL3]. To HLi2L3 (59 mg, 0.12 mmol) in CH3OH (6 mL) was added
GaCl3 (21 mg, 0.12 mmol) in CH3OH (4 mL). The mixture was refluxed
at 90 °C for 1 h followed by the addition of 50 µL of NEt3, and the
mixture was refluxed at 90 °C for additional 3 h. Subsequently the
mixture was filtered under argon, and the filtrate was concentrated to
about half the volume, and the solution was stored at -20 °C. A white
precipitate was collected, washed with cold CH3CN, and dried under
reduced pressure. Yield: 6 mg (9%). ESI mass spectrometry in CH3-
OH (positive ion mode): m/z ) 568 (M0 + Na)+, 584 (M0 + K)+, 619
(M0 + K + 2H2O)+, 647 (M0 + K + 2CH3OH)+, 1115 (2M0 + Na)+.
Anal. Calcd for C25H38N3O4SGa‚ 1 1/6H2O: C, 52.92; H, 7.17; N, 7.41.
Found: C, 52.86; H, 7.30; N, 7.29.
[CoL3]. A solution of HLi2L3 (50 mg, 0.1 mmol) and CoCl2‚6H2O
(25 mg, 0.1 mmol) in CH3CN (4 mL) was refluxed at 90 °C for 1 h.
NEt3 (50 µL) was added, and the mixture was stirred at 90 °C for an
additional hour and then at 90 °C in the presence of air for 2 h. The
mixture was concentrated under reduced pressure to give a dark green
solid. To the solid was added 5.5 mL of CH3OH, and the color of the
solution turned brown. After 2 days at 25 °C, brown, needle-shaped
crystals precipitated which were collected, washed with cold CH3CN,
and dried under reduced pressure. Yield: 9 mg (17%). Crystals of
[CoL3]‚1 1/6H2O were suitable for X-ray crystallography. ESI mass
spectrometry in CH3OH (positive ion mode): m/z ) 558 (M0 + Na)+,
1093 (2M0 + Na)+. Anal. Calcd for C25H38N3O4SCo: C, 56.07; H,
7.15; N, 7.85. Found: C, 55.94; H, 7.25; N, 7.76.
Preparation of Complexes. [GaL1]. To a solution of the ligand
Li3L1 (118 mg, 0.25 mmol) in CH3CN (2 mL) was added GaCl3 (41
mg, 0.23 mmol) in CH3CN (4 mL). The mixture was refluxed at 90 °C
for 4 h and concentrated under reduced pressure to give a colorless
crude polycrystalline precipitate. The crude crystals were recrystallized
from CH3OH/CH3CN. Colorless crystals were collected, washed with
cold CH3CN, and dried under reduced pressure. Yield: 88 mg (71%).
1H NMR (500 MHz, methanol-d4): δ 1.26 (s, 9H, CH3(t-Bu)), 1.35 (s,
9H, CH3(t-Bu)), 2.29 (m, 1H), 2.96 (m, 4H), 3.12 (m, 4H), 3.24 (m,
1H), 3.42 (m, 2H), 3.53 (s, 2H), 3.62 (s, 2H), 3.83 (d, 1H, J ) 12 Hz),
4.29 (d, 1H, J ) 12 Hz), 6.93 (d, 1H, J ) 2.5 Hz), 7.27 (d, 1H, J )
2.6 Hz). ESI mass spectrometry in CH3OH (positive ion mode): m/z
) 552 (M0 + Na)+ 1083 (2M0 + Na)+. Anal. Calcd for C25H38N3O5-
Ga: C, 56.62; H, 7.22; N, 7.92. Found: C, 56.70; H, 7.29; N, 7.76.
[CoL1]. A solution of Li3L1 (51 mg, 0.11 mmol) and CoCl2‚6H2O
(25 mg, 0.11 mmol) in CH3OH (6 mL) was refluxed at 90 °C under an
argon atmosphere for 2 h. Then the solution was refluxed at 90 °C in
the presence of air for 2 h. The red-wine colored solution was filtered,
and the filtrate was concentrated under reduced pressure. The residue
was recrystallized from CH3OH/acetone. Brown crystals were collected,
washed with cold CH3CN, water, and acetone, and dried under reduced
pressure. Yield: 33 mg (60%). Crystals of [CoL1]‚CH3OH‚1.5H2O
suitable for X-ray crystallography were grown from CH3OH/CH3CN
1
by slow evaporation of the solvent. H NMR (400 MHz, methanol-
d4): δ 1.25 (s, 9H, CH3(t-Bu)), 1.40 (s, 9H, CH3(t-Bu)), 2.48 (m, 1H),
2.66 (m, 1H), 2.96 (m, 3H), 3.2∼3.4 (m), 3.4∼3.8 (m, 4H), 3.9∼4.1
(m, 3H), 6.90 (d, 1H, J ) 2.5 Hz), 7.18 (d, 1H, J ) 2.6 Hz). ESI mass
spectrometry in CH3OH (positive ion mode): m/z ) 542 (M0 + Na)+,
1061 (2M0 + Na)+. Anal. Calcd for C26H45N3O7.5Co: C, 53.97; H,
7.84; N, 7.26. Found: C, 54.49; H, 7.73; N, 7.61.
[FeL1]. To a solution of Li3L1 (53 mg, 0.11 mmol) in CH3CN (2
mL) was added FeCl3 (21 mg, 0.13 mmol) in CH3CN (4 mL). The
reaction mixture was refluxed at 90 °C for 4 h and concentrated under
reduced pressure to give a purple crude solid, which was recrystallized
from hot CH3OH. Purple crystals were collected, washed with cold
CH3OH, and dried under reduced pressure. Yield: 28 mg (49%).
Crystals of [FeL1]‚H2O suitable for X-ray crystallography were obtained
from CH3OH/CH3CN by slow evaporation of the solvent. ESI mass
spectrometry in CH3OH (positive ion mode): m/z ) 539 (M0 + Na)+,
1055 (2M0 + Na)+. Anal. Calcd for C25H38N3O5Fe: C, 58.14; H, 7.42;
N, 8.14. Found: C, 58.21; H, 7.49; N, 8.05.
Physical Measurements. EI, ESI, or FAB mass spectra were
recorded on a Finnigan MAT 8200, a Finnigan MAT 95 or a HP 5989
mass spectrometer. 1H and 13C NMR spectra were measured on Bruker
ARX 250, DRX 400, or DRX 500 spectrometers using the solvent as
internal standard. Electronic absorption spectra were measured in the
range 200-1000 nm on a Perkin-Elmer Lambda 19 spectrophotometer.
Cyclic voltammograms were measured by use of an EG&G potentiostat/
galvanostat 273A on solutions containing 0.1 M TBA(PF6) as supporting
electrolyte in a conventional electrochemical cell. Glassy carbon and
Pt wire were used as working and counter electrode, respectively.
Potentials were referenced to the Ag/0.01 M AgNO3 CH3CN or
[57FeL1]. To 57Fe (7.20 mg, 0.13 mmol) was added 30 mL of
concentrated HCl(aq), and the mixture was refluxed at 120 °C for 24
h. Then the yellow solution was concentrated under reduced pressure
to give yellow 57FeCl3. Preparation of the iron complex was performed

Phenylthiyl Complexes of Ga(III), Fe(III), and Co(III)
J. Am. Chem. Soc., Vol. 123, No. 25, 2001 6031
Table 1. Crystallographic Data for [FeL1]‚H2O, [CoL1]‚CH3OH‚1.5 H2O, [FeL2], [CoL3]‚1.17 H2O
[FeL1]‚H2O
[CoL1]‚CH3OH‚1.5 H2O
[FeL2]
[CoL3]‚1.17 H2O
chem formula
fw
C25H40FeN3O6
534.45
C26H45CoN3O7.5
578.58
C17H22FeN3O4S
420.29
C25H40.33CoN3O5.17
556.59
S
space group
a, Å
b, Å
c, Å
P1h, No. 2
12.468(2)
17.667(3)
25.111(4)
79.81(2)
79.28(2)
89.99(2)
5346(2)
8
C2/c, No. 15
49.743(7)
8.8465(12)
13.293(2)
90
98.90(2)
90
P21/n, No. 14
7.3937(6)
13.8182(12)
17.329(2)
90
91.293(12)
90
P3hc1, No. 165
27.694(5)
27.694(5)
13.188(3)
90
90
120
R, deg
â, deg
γ, deg
V, Å
5779.2(14)
8
1770.0(3)
4
8760(3)
12
Z
T, K
100(2)
1.328
27104/45.0
13738/8280
1285/15
6.07
100(2)
1.330
16812/55.0
6556/4237
356/3
6.42
0.0544
0.1292
100(2)
1.577
30347/60.0
5161/4600
235/0
9.99
0.0250
0.0671
100(2)
1.266
13187/35
1850/924
187/0
6.96
0.0726
0.1652
F calcd, g cm-3
refl. collected/2θmax
unique refl/I > 2σ(I)
no. of params/restr
µ(Mo KR), cm-1
R1a
0.0646
0.1576
wR2b(I > 2σ(I))
a Observation criterion: I > 2σ(I). R1 ) Σ||Fo| - |Fc||/Σ|Fo|. b wR2 ) [Σ[w(Fo2 - Fc2)2]/Σ[w(Fo )2]1/2 where w ) 1/σ2(Fo ) + (aP)2 + bP, P )
2
2
2
(Fo + 2Fc2)/3.
propionitrile solution. Ferrocene was used as internal standard, and
potentials were referenced versus the Fc+/Fc couple. Spectroelectro-
chemical measurements were performed by using an optically transpar-
ent thin-layer electrochemical cell (OTTLE cell). During oxidation/
reductions in the OTTLE cell spectral changes were recorded on a
Hewlett-Packard HP 8453 diode array spectrophotometer in the range
190-1100 nm. X-band EPR spectra were measured on a Bruker ESP
300 Spectrometer equipped with an Oxford Instruments ESR 910
helium-flow cryostat with an ITC 503 temperature controller. One-
electron oxidations of [57FeL1], [CoL1], and [GaL1] were achieved
electrochemically in a conventional electrochemical cell by using the
following conditions: [complex] ) 0.5 mM, T ) -20 °C. For the
electrochemical oxidation of [CoL3] at -60 °C under an argon
atmosphere, a 0.2 mM solution (25 mL) was employed. All samples
were immediately frozen in liquid N2 after the one-electron oxidation
and transferred to the respective spectrometer. 57Fe Mo¨ssbauer spectra
were measured on an Oxford Instruments Mo¨ssbauer spectrometer.
57Co/Rh was used as the radiation source. The minimum experimental
line widths were 0.24 mms-1. The temperature of the samples was
controlled by an Oxford Instruments Variox Cryostat. Isomer shifts
were determined relative to R-iron at 300 K.
and refined as riding atoms with isotropic displacement parameters.
Hydrogen atoms of solvate water molecules in [FeL1]‚H2O and
[CoL1]‚CH3OH‚1.5 H2O were located from the difference map, and
O-H distances were restrained to be equal within errors using the SADI
instruction in ShelXTL.
Results
Preparation of Ligands and Complexes. The hexadentate
macrocycles containing a 1,4,7-triazacyclononane backbone and
three identical, N-bound pendent arms such as acetates, [La]3- 20
,
phenolates, [Lb]3- 21 or thiophenolates, [Lc]3- 22
, and their
,
extremely stable,23 neutral complexes of GaIII, FeIII, and CoIII
have been thoroughly studied. They serve as benchmarks for
the present study where we have synthesized the corresponding
mixed pendent arm macrocycles [L1]3-, [L2]3-, and [L3]3- and
their octahedral complexes of GaIII, FeIII, and CoIII. The
structures of the ligands are depicted in Scheme 1.
These mixed pendent arm macrocycles were designed to
contain a redox-inert 1,4,7-triazacyclononane-1,4-diacetate24 unit
in addition to one potentially redox-active phenolate or thiophe-
X-ray Crystallographic Data Collection and Refinement of the
Structures. A violet single crystal of [FeL1]‚H2O, and dark brown
crystals of [CoL1]‚CH3OH‚1.5 H2O and [FeL2], and an orange red
specimen of [CoL3]‚1.17 H2O were fixed with perfluoropolyether on
glass fibers and mounted on a Nonius Kappa-CCD diffractometer
equipped with a cryogenic nitrogen cold stream. Graphite monochro-
mated Mo KR radiation (λ ) 0.71073 Å) was used throughout.
Crystallographic data of the compounds are listed in Table 1. Cell
constants were obtained from a least-squares fit of the setting angles
of several thousand strong reflections. Intensity data were corrected
for Lorentz and polarization effects. Intensity data collected from a
thin plate-shaped crystal of [FeL1]‚H2O were corrected for absorption
using the program MulScanAbs embedded in the PLATON99 program
suite18 yielding max and min transmission coefficient of 0.92 and 0.77,
respectively. The Siemens ShelXTL19 software package was used for
solution, refinement, and artwork of the structures, and neutral atom
scattering factors of the program were used. All structures were solved
and refined by direct methods and difference Fourier techniques. Non-
hydrogen atoms were refined anisotropically except carbon atoms and
solvate in compound [CoL3]‚1.17 H2O since the number of observed
reflections in the data set was not sufficient to refine all atoms
anisotropically. Hydrogen atoms were placed at calculated positions
(20) (a) Wieghardt, K.; Bossek, U.; Chaudhuri, P.; Herrmann, W.; Menke,
B. C.; Weiss, J. Inorg. Chem. 1982, 21, 4308. (b) Takahashi, M.; Takamoto,
S. Bull. Chem. Soc. Jpn. 1977, 50, 3413. (c) Arishima, T.; Hamada, K.;
Takamoto, S. Nippon Kagaku Kaishi 1973, 1119. (d) Bossek, U.; Hanke,
D.; Wieghardt, K.; Nuber, B. Polyhedron 1993, 12, 1. (e) Craig, A. S.;
Parker, D.; Adams, H.; Bailey, N. A. J. Chem. Soc., Chem. Commun. 1989,
1793. (f) Moore, D. A.; Fanwick, P. E.; Welch, M. J. Inorg. Chem. 1990,
29, 672. (g) Matthews, R. C.; Parker, D.; Ferguson, G.; Kaitner, B.; Harrison,
A.; Royle, L. Polyhedron 1991, 10, 1951. (h) Craig, A. S.; Helps, I. M.;
Parker, D.; Adams, H.; Bailey, N. A.; Williams, M. G.; Smith, J. M. A.;
Ferguson, G. Polyhedron 1989, 8, 2481.
(21) (a) Moore, D. A.; Fanwick, P. E.; Welch, M. J. Inorg. Chem. 1989,
28, 1504. (b) Auerbach, U.; Eckert, U.; Wieghardt, K.; Nuber, B.; Weiss,
J. Inorg. Chem. 1990, 29, 938. (c) Martell, A. E.; Motekaitis, R. J.; Welch,
M. J. J. Chem. Soc., Chem. Commun. 1990, 1748. (d) Auerbach, U.; Della
Vedova, B. S. P. C.; Wieghardt, K.; Nuber, B.; Weiss, J. J. Chem. Soc.,
Chem. Commun. 1990, 1004. (e) Auerbach, U.; Stockheim, C.; Weyher-
mu¨ller, T.; Wieghardt, K.; Nuber, B. Angew. Chem. 1993, 105, 735; Angew.
Chem., Int. Ed. Engl. 1993, 32, 714. (f) Auerbach, U.; Weyhermu¨ller, T.;
Wieghardt, K.; Nuber, B.; Bill, E.; Butzlaff, C.; Trautwein, A. X. Inorg.
Chem. 1993, 32, 508. (g) Clarke, E. T.; Martell, A. E. Inorg. Chim. Acta
1991, 186, 103.
(22) (a) Beissel, T.; Bu¨rger, K. S.; Voigt, G.; Wieghardt, K.; Butzlaff,
C.; Trautwein, A. X. Inorg. Chem. 1993, 32, 124. (b) Beissel, T.; Glaser,
T.; Kesting, F.; Wieghardt, K.; Nuber, B. Inorg. Chem. 1996, 35, 3936.
(23) (a) Clarke, E. T.; Martell, A. E. Inorg. Chim. Acta 1991, 186, 103.
(b) Motekaitis, R. J.; Sun, Y.; Martell, A. E. Inorg. Chim. Acta 1992, 198-
200, 421.
(18) Spek, A. L. University of Utrecht, The Netherlands, 1999.
(19) (19)ShelXTL, V.5; Siemens Industrial Automation Inc.; 1994.
(24) Flassbeck, C.; Wieghardt, K. Z. Anorg. Allg. Chem. 1992, 608, 60.

6032 J. Am. Chem. Soc., Vol. 123, No. 25, 2001
Kimura et al.
Scheme 2
Scheme 3
Scheme 4
nolate pendent arm. The preparation of such mixed ligands in
good yields and high purity is not straightforward.25
We have successfully employed a synthetic route where only
two amine nitrogens of the 1,4,7-triazacyclononane starting
material are N-protected by its reaction with 2 equiv of 2-tert-
butoxycarbonyloxyimino-2-phenylacetonitrile (BOC-ON) yield-
ing 1,4,7-triazacyclononane-1,4-dicarboxylic acid di-tert-butyl
ester (1) in 74% yield and high purity (Scheme 2).26 Compound
1 possesses only one (unprotected) secondary amine to which
the desired phenolate or thiophenolate arm can now be appended
by the reaction of 1 with either 2-bromomethyl-4,6-di-tert-
butylphenol (Scheme 2), or 1-bromomethyl-2-(4-methoxyben-
zylsulfonyl)benzene (8, Scheme 3), or 1-bromomethyl-3,5-di-
tert-butyl-2-(4-methoxybenzylsulfonyl)benzene (19, Scheme 4),
yielding compounds 2, 9, 20, respectively, which were N-
deprotected with CF3COOH in CH2Cl2 to give the ammonium
salts 3, 10, and 21. Neutralization of these species with base
(NaOH or NEt3) gave the N-monosubstituted 1,4,7-triazacy-
clononane derivatives 4, 11, and 22. The reaction of these
compounds with bromoacetic acid ethyl ester produced com-
pounds 5, 12, and the precursor of 23 (not isolated) which were
deesterified with LiOH‚H2O, yielding the lithium salt Li3[L1]
(Scheme 2), 13 (Scheme 3), and 23 (Scheme 5). The S-protected
compounds 13 and 23 react with Hg(acetate)2, yielding Li2[L2H]
and Li2[L3H] in good yields (see below).
sulfonic acid S-(4-methoxybenzyl)ester 17. Thus, in situ lithiated
16 reacts with 17 cleanly to give 18 which reacted with PBr3
to give 19. On the other hand, the precursor 8 was synthesized
as is shown in Scheme 3 where 6 was generated from
2-nitrobenzaldehyde and (4-methoxyphenyl)methanethiol in
DMF. The 4-methoxybenzyl S-protecting group was readily
cleaved off from 13 and 23 by Hg(acetate)2 in methanol and
passing of H2S through the solution to produce HgS and the
ligands Li2[L2H] and Li2[L3H], respectively.
The synthesis of the sulfenylating agents 8 and 19 for the
generation of the S-protected 1,4,7-triazacyclononane derivatives
9 and 20 is outlined in Schemes 3 and 4, respectively. We have
successfully used a route described by Shibata et al.27 who
reported the excellent sulfenylating ability of toluene-4-thio-
(25) (a) Erhardt, J. M.; Grover, E. R.; Wuest, J. D. J. Am. Chem. Soc.
1980, 102, 6365. (b) Weisman, G. R.; Vachon, D. J.; Johnson, V. B.;
Gronbeck, D. A. J. Chem. Soc., Chem. Commun. 1987, 886. (c) Blake, A.
J.; Fallis, I. A.; Gould, R. O.; Parsons, S.; Ross, S. A.; Schro¨der, M. J.
Chem. Soc., Chem. Commun. 1994, 2467. (d) Wainwright, K. P. Coord.
Chem. ReV. 1997, 166, 35.
The gallium complexes [GaIIIL1], [GaIIIL2], and [GaIIIL3] were
prepared in acetonitrile or methanol solutions of GaCl3, the
(26) (a) Itoh, M.; Hagiwara, D.; Kamiza, T. Tetrahedron Lett. 1975, 16,
4393. (b) Kovacs, Z.; Sherry, A. D. Tetrahedron Lett. 1995, 36, 9269.
(27) Shibata, N.; Baldwin, J. E.; Jacobs, A.; Wood, M. E. Synlett 1996,
519.

Phenylthiyl Complexes of Ga(III), Fe(III), and Co(III)
J. Am. Chem. Soc., Vol. 123, No. 25, 2001 6033
Scheme 5
lithium salts of the respective ligand and triethylamine as
colorless microcrystalline materials. The corresponding cobalt
complexes [CoIIIL1], [CoIIIL2], and [CoIIIL3] were synthesized
similarly from CoIICl2‚6H2O in the presence of air. Compound
[FeIIIL1] and [FeIIIL2] were obtained from acetonitrile solutions
of FeCl3 and the respective ligand (1:1). All diamagnetic
Figure 1. Structures of the neutral molecules in crystals of [CoIIIL1]‚CH3-
OH‚1.5H2O (top) and [CoIIIL3]‚1.17H2O (bottom).
0.87, 0.62, and 0.35 V which correspond to the reversible
formation of one-, two-, and three-coordinated phenoxyl radicals,
respectively.28 In contrast, the tris(thiophenolato) analogue
[GaLc] displays in chloroform only an irreversible, ligand-based
oxidation at 1.08 V, which has not been further characterized.22b
Figure 3 (top) displays the cv of [GaL1] which exhibits a
single, fully reversible one-electron transfer wave at 0.63 V,
corresponding to the generation of a coordinated phenoxyl
radical in [GaIIIL1•]+. This monocation is stable in solution at
ambient temperature. It is paramagnetic with an S ) 1/2 ground
state. The X-band EPR spectrum of the electrochemically
generated monocation exhibits a broad, unresolved, isotropic
signal at g ) 2.006 at 50 K (not shown). The electronic spectra
of [GaL1] and its one-electron oxidized form are shown in Figure
4 (top). [GaL1] is colorless, and the monocation is yellow. The
1
complexes of GaIII and CoIII were characterized by H NMR
and electro spray ionization (ESI) mass spectrometry (see
Experimental Section). From Mo¨ssbauer spectroscopy (see
below) it was established that the iron complexes [FeL1] and
[FeL2] both contain a high-spin iron(III) ion. The compound
[FeL3] has not been obtained.
The crystal structures of [CoIIIL1]‚CH3OH‚1.5H2O, [CoIIIL3],
[FeL1]‚H2O, and [FeL2] have been determined by single-crystal
X-ray crystallography at 100 K. Figure 1 shows the structures
of the neutral molecules in crystals of the former two, whereas
Figure 2 displays the structures of the latter two. Clearly, all
species contain a hexadentate macrocyclic ligand with a
common 1,4,7-triazacyclononane-1,4-dicarboxylato fragment
and an O-, or S-coordinated phenolato or thiophenolato pendent
arm.
intense absorption maxima at 417 nm (ꢀ ) 4.6 × 103 M-1 cm-1
)
and 401 nm (3.9 × 103 M-1 cm-1, sh) are indicative of the
phenoxyl radical28 in [GaL1•]+.
Table 2 summarizes bond lengths and angles. Structures of
complexes containing tris(acetato)-,20 tris(phenolato)-,21 and tris-
(thiophenolato)22 analogues [La]3-, [Lb]3-, and [Lc]3- (Scheme
1) have been described previously.
Figure 5 shows the cv’s of [GaL2] and [GaL3] which were
recorded at 25 and -20 °C, respectively (no difference
observed). Both species display an irreversible oxidation wave
at 0.90 and 0.82 V, respectively, which in the case of [GaL2] is
accompanied by a weak irreversible reduction wave at ∼1.55
V. The latter is only observed if the oxidation wave is scanned
first. This indicates that this reduction wave belongs to a species
formed after one-electron oxidation has occurred.
Electro- and Spectroelectrochemistry. Cyclic voltammo-
grams (cv) of complexes have been recorded in acetonitrile or
dichloromethane solutions containing 0.1 M [TBA]PF6 as
supporting electrolyte at 25, -20, -30, or -60 °C. Ferrocene
was used as internal standard, and all potentials are referenced
versus the Fc+/Fc couple. Table 3 summarizes the results.
(a) Gallium Complexes. Complexes [GaL1], [GaL2], and
[GaL3] contain a redox-inactive GaIII ion. Therefore, all redox
activity observed must be ligand-based. As expected, [GaLa] is
redox-inactive in the potential range 1.2 to -2.0 V,20 whereas
[GaLb] displays three reversible one-electron-transfer waves at
This irreversible one-electron oxidation of [GaL3] has been
followed spectroelectrochemically. Three isosbestic points are
observed at 279, 250, and 232 nm, and no absorption >380
(28) Adam, B.; Bill, E.; Bothe, E.; Goerdt, B.; Haselhorst, G.; Hilden-
brand, K.; Sokolowski, A.; Steenken, S.; Weyhermu¨ller, T.; Wieghardt, K.
Chem. Eur. J. 1997, 3, 308.

6034 J. Am. Chem. Soc., Vol. 123, No. 25, 2001
Kimura et al.
Table 3. Redox Potentials of Complexesa,b
complex
E, V vs Fc+/Fca
[GaL1]
[FeL1]
[CoL1]
[GaL2]
[FeL2]
[CoL2]
[GaL3]c
[CoL3]c
0.63 r.
0.73 r.
0.36 r.
0.90 irr.
0.65 irr.
0.60 irr.
0.82 irr.
0.55 r.
-1.14 r.
-1.38 irr; -0.40 irr.
-1.5 to -1.6 irr.
-0.83 r.
-1.28 irr.
-1.6 irr., -0.5 irr.
a For reversible (r.) one-electron processes E1/2 is given (E1/2
)
(Ep,ox + Ep,red)/2, whereas for irreversible (irr.) processes the peak
potentials Ep,ox and/or Ep,red are given. b Conditions: glassy carbon
working electrode, scan rate 100 mV s-1; acetonitrile 0.1 M [TBA]PF6
supporting electrolyte, 25 °C. c CH2Cl2 solution (0.1 M [TBA]PF6) at
-20 °C.
Figure 2. Structures of the neutral molecules in crystals of [FeIIIL1]‚
H2O (top) and [FeIIIL2] (bottom).
Table 2. Selected Bond Lengths (Å) of Complexes
Figure 3. Cyclic voltammograms of [ML1] complexes (M ) Ga, Fe,
Co) in CH3CN solution (0.10 M [TBA]PF6) recorded at scan rates 100,
200, 300, and 500 mV s-1 at a glassy carbon electrode at 25 °C.
of a monocationic coordinated phenoxyl radical species
[FeIIIL1•]+, whereas the reduction is metal-centered, yielding
[FeIIL1]-. The radical cation has been characterized by its
absorption spectrum (Figure 4) and its X-band EPR spectrum
which is shown in Figure 6. The electronic spectrum of [FeIIIL1]
is dominated by an intense phenolato-to-iron charge-transfer
band at 520 nm and an absorption minimum at ∼400 nm. The
monocation displays the typical intense phenoxyl radical bands
at 412, 458, and 523 nm.28,29
[FeIIIL1]‚ [CoIIIL1]‚CH3OH‚
[CoIIIL3]‚
H2Oa
1.5 H2O
[FeIIIL2]
1.17 H2O
M-X
1.875(4)
1.986(4)
1.977(4)
2.177(5)
2.228(5)
2.207(5)
1.910(2)
1.918(2)
1.937(2)
1.928(3)
1.928(2)
1.951(2)
2.3221(4) 2.273(4)
M-O1
M-O3
M-N1
M-N2
M-N3
1.978(1)
1.965(1)
2.177(1)
2.211(1)
2.205(1)
1.934(8)
1.927(8)
1.931(11)
1.939(10)
1.981(10)
The X-band EPR spectrum of [FeIIIL1] recorded in perpen-
dicular (normal) mode at 8 K displays a rhombic spectrum with
an intense signal at g ≈ 4.3 typical of an octahedral high-spin
iron(III) ion. This signal disappears upon one-electron oxidation,
and a relative intense signal at g ≈ 8.5 is observed in parallel
mode at 10 K. This is attributable to |∆MS| ) 2 transition of
an integer spin system (S ) 2). As shown previously, coordi-
a There are four crystallographically independent molecules in the
unit cell; the bond lengths of only one molecule are given here.
nm is generated upon oxidation which, as we will show below,
immediately rules out the formation of the expected thiyl radical
as a stable species. Instead, we propose that the putative
[GaL3•]+ radical dissociates rapidly at the Ga-S˙ R bond and
forms a dangling, uncoordinated thiyl radical which dimerizes
nated phenoxyl radicals (Srad
)
1/2) couple strongly in an
antiferromagnetic fashion to a high-spin iron(III) ion (SFe ) 5/2),
yielding the observed St ) 2 ground state.28,29 We have also
measured the Mo¨ssbauer spectra of [FeIIIL1] and its one-electron
oxidized form [FeIIIL1•]+ which are shown in Figure 7. From
rapidly to the disulfide dinuclear complex, [GaL3-L3Ga]2+
,
shown in Scheme 6. Exactly the same behavior is proposed for
[GaL2] upon oxidation.
(b) Iron Complexes. Figure 3 (middle) shows the cv of
[FeIIIL1] which exhibits a reversible oxidative and a reductive
one-electron transfer wave at 0.73 V and -1.14 V, respectively.
The oxidation is ligand-based and corresponds to the formation
(29) Snodin, M. D.; Ould-Moussa, L.; Wallmann, U.; Lecomte, S.;
Bachler, V.; Bill, E.; Hummel, H.; Weyhermu¨ller, T.; Hildebrandt, P.;
Wieghardt, K. Chem. Eur. J. 1999, 5, 2554.

Phenylthiyl Complexes of Ga(III), Fe(III), and Co(III)
J. Am. Chem. Soc., Vol. 123, No. 25, 2001 6035
Figure 6. X-band EPR spectra of frozen CH3CN solution at [FeL1] at
8 K measured in perpendicular (normal) mode (top) and of [FeL1•]+ in
frozen CH3CN (0.10 M [TBA]PF6) measured in the parallel mode at
10 K. Conditions (top) frequency: 9.6325 GHz; power: 0.1 mW;
modulation: 19.9 G; (bottom) frequency: 9.3465 GHz; power: 5.1
mW; modulation: 19.9 G.
Scheme 6
Figure 4. Electronic spectra of [ML1] (- - -) and [ML1•]+ (s) species
in CH3CN (0.10 M [TBA]PF6); M ) Ga, Fe, Co.
Figure 5. Cyclic voltammograms of [GaL2] at 25 °C in CH3CN and
[GaL3] at -20 °C in CH2Cl2 solution (0.10 M [TBA]PF6) recorded at
scan rates 100, 200, 300, and 500 mV s-1 at a glassy carbon electrode.
simulations the following spin Hamiltonian parameters were
found: For [FeIIIL1] an isomer shift, δ, of 0.46 mm s-1 and
quadrupole splitting parameter, ∆EQ ) -0.74 mm s-1, were
established at 4.2 K from an applied field (7 T) spectrum (η )
1; Aiso/gNâN ) -211 T). These data are typical for octahedral
high-spin iron(III) complexes.28,29 For [FeIIIL1•]+ these values
do not change much (δ ) 0.44 mm s-1 and |∆EQ| ) 0.55 mm
s-1 (measured at 80 K in zero-field)). This clearly demonstrates
that one-electron oxidation of [FeIIIL1] is not metal-centered (no
formation of Fe(IV)).
reduction at -0.83 V which is assigned to the FeIII/FeII couple.
Thus, the phenolato pendent arm in [FeIIIL1] stabilizes the ferric
oxidation state by 310 mV as compared to its thiophenolato
analogue [FeIIIL2] but by 840 mV as compared to three-
coordinated carboxylates in [FeIIILa].20a The irreversible oxida-
tion wave probably involves the generation of a coordinated
thiyl radical which rapidly undergoes FeIII-S˙ R bond scission
and dimerization, yielding an [FeL2-L2Fe]2+ disulfide species
(Scheme 6). Due to this disulfide formation the cv displays upon
re-reduction a small, broad reduction peak at ∼-0.3 V which
The cv of [FeIIIL2] in acetonitrile (0.10 M [TBA]PF6) at 25
°C (Figure S1) exhibits an irreversible oxidation at Ep,ox ) 0.65
V at a scan rate of 100 mV s-1 and a reversible one-electron

6036 J. Am. Chem. Soc., Vol. 123, No. 25, 2001
Kimura et al.
Figure 8. X-band EPR spectra of electrochemically generated (-60
°C) [CoIIIL3•]+ (top) at 90 K in frozen CH2Cl2 (0.10 M [TBA]PF6) and
of [CoIIIL1•]+ (bottom) in CH3CN (0.10 M [TBA]PF6) solution at 90
K, respectively. Conditions (top and bottom): frequency: 9.6366,
9.6345 GHz; power: 4.0 and 0.1 mW; modulation: 5.7 and 5.7 G.
Figure 7. Applied field (7 T) Mo¨ssbauer spectrum at 4.2 K of a frozen
CH3CN solution of [FeIIIL1] (top) and simulation (solid line) using spin
Hamiltonian parameters given in the text. Bottom: zero-field Mo¨ssbauer
spectrum at 80 K of a frozen CH3CN (0.10 M [TBA]PF6) solution of
electrochemically generated [57FeL1•]+ (0.5 mM). The isomer shift, δ,
is referenced vs R-Fe at 298 K.
is not observed scanning the potential range +0.1 to -2.0 V
only.
(c) Cobalt Complexes. The cv of [CoIIIL1] shown in Figure
3 displays a reversible one-electron oxidation wave at 0.36 V
and a reduction peak at -1.38 V which is accompanied by a
reoxidation peak at -0.40 V. Coulometric and spectroelectro-
chemical measurements at a fixed potential of -1.5 V show
that this reduction is a one-electron process which does not lead
to decomposition of the reduced form [CoIIL1]- since subsequent
reoxidation quantitatively regenerates the spectrum of [CoIIIL1].
The irreversible appearance of this process is attributed to slow
electrode-to-complex electron-transfer kinetics frequently ob-
served for CoIII/CoII couples. Thus, the reduction of [CoIIIL1]
is metal-centered.
In contrast, the one-electron oxidation is ligand-based. A
coordinated phenoxyl radical, [CoIIIL1•]+, is generated as was
judged by its electronic spectrum shown in Figure 4.30 An
intense new maximum at 402 nm (ꢀ ) 3.8 × 103) and maxima
at 634 (2.1 × 103), 532 (1.3 × 103, sh) indicate the phenoxyl
radical formation. The X-band EPR spectrum of [CoIIIL1•]+ in
frozen acetonitrile at 90 K shown in Figure 8 displays a broad,
nearly isotopic signal which was simulated with parameters gx
) 2.0076, gy ) 2.0095, gz ) 2.0044 and hyperfine coupling of
Figure 9. Cyclic voltammograms of [CoIIIL3] in CH3CN (0.10 M
[TBA]PF6) at 25 °C at scan rates 100, 200, 300, 500, and 1000 mV
s-1 (top) and in CH2Cl2 (0.10 M [TBA]PF6) at -20 °C at scan rates
50, 100, 200, 300, 500, and 1000 mV s-1 (bottom).
which is independent of the scan rate (50-500 mV s-1) and an
irreversible reduction peak at -1.28 V which probably corre-
sponds to the metal-centered but kinetically hindered CoIII/CoII
couple. We have not investigated the electrochemistry of this
species in further detail.
the organic radical to the 59Co (I ) 7/2) nucleus of ACoxx ) ACo
The electrochemistry of [CoIIIL3] is interesting. The cv of
[CoL3] in acetonitrile at 25 °C shown in Figure 9 (top) displays
an irreversible oxidation at Ep,ox ) 0.53 V which is nearly
independent of the scan rate (100-1000 mV s-1) and an
irreversible reduction at Ep,red ) -1.36 V which is more clearly
dependent on the scan rate. The latter is again assigned to the
CoIII/CoII couple, whereas the former involves the oxidation of
the coordinated thiophenolate. Under these experimental condi-
tions the thiyl radical dissociates via Co-S˙ R bond scission and
dimerizes with formation of a disulfide [CoIIIL3-L3CoIII]2+ as
described above for [GaL3]. After scanning through the oxida-
tion wave upon re-reduction, a small wave at -0.5 V is
observed. This peak is not observed when the reduction scan is
yy
) 2.2 G, Azz ) 13.6 G (ACoiso ) 6.0 G). The hyperfine splitting
is not resolved. These parameters are very similar to those
reported for [CoIII(LPr•)(acac)]2+ where HLPr is 1,4-diisopropyl-
7-(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane
and acac- is pentane-2,4-dionate (g ) 2.0047; ACo ) 6.0 G,
iso
aHiso ) 6.55 G).30 In this case the hyperfine splitting was clearly
resolved.
In contrast, the cv of [CoIIIL2] in acetonitrile (not shown)
displays an irreversible one-electron oxidation peak at 0.60 V
(30) Sokolowski, A.; Adam, B.; Weyhermu¨ller, T.; Kikuchi, A.; Hilden-
brand, K.; Schnepf, R.; Hildebrandt, P.; Bill, E.; Wieghardt, K. Inorg. Chem.
1997, 36, 3702.

Phenylthiyl Complexes of Ga(III), Fe(III), and Co(III)
J. Am. Chem. Soc., Vol. 123, No. 25, 2001 6037
successfully simulated by using the following parameters: gx
) 2.0339, gy ) 2.0159, gz ) 2.0180, Axx(59Co) ) 2.2 G;
1
Ayy(59Co) ) 2.5 G; Azz(59Co) ) 27.3 G; Aiso(59Co) ) /3(Axx
+
Ayy + Azz) ) 10.7 G, giso ) 2.0226.
In comparison, the 2,4,6-tri-tert-butylphenylthiyl radical
generated in the solid state by photolysis shows an axial
spectrum with g ) 2.015 and g×e7×e7 ) 2.003.10,31d EPR studies
of Jeevarajan, Fessenden, and others show that the Ph-S• radical
has a high g factor of 2.0252, indicating that the unpaired spin
is highly localized on the sulfur31 (no hyperfine splitting to the
phenyl protons has been reported).
Mass Spectroscopic Evidence for Dimer Formation. To
obtain evidence for the dimer formation during the electro-
chemical oxidations of [CoL3], [GaL2], and [GaL3] we have
chemically oxidized these species in CH3CN solutions at -35
°C and recorded their electrospray mass spectra. As reagents
we have used the outer-sphere one-electron oxidants [NiIII(tacn)2]-
(ClO4)3 (for [CoL3]) (tacn ) 1,4,7-triazacyclononane) and the
thianthrene radical monocation tetrafluoroborate (for [GaL2] and
[GaL3]) in 10-20% excess.
Immediately after completion of these chemical oxidations
the ESI mass spectra were recorded. For [CoL3] an m/z ) 1169.6
peak corresponds to {[CoL3-L3Co](ClO4)}+. Similarly, for
[GaL2] the m/z ) 885.2 peak corresponds to {[GaL2-L2Ga]F}+,
and for GaL3 the peak m/z ) 1109.4 corresponds to the
monocation {[GaL3-L3Ga]F}+. In addition strong peaks m/z,
where z ) 2, have been observed at 535.2 for the dinuclear
dication of oxidized [CoL3]; at 433.2 and 434.0 two peaks
corresponding to a 69Ga:71Ga ratio of 60:40 for the dinuclear
dication [GaL2-L2Ga]2+ have been observed. The observation
of the latter m/z peaks rules out the possibility that the m/z with
z ) 1 peaks correspond to monocationic clusters consisting only
of two mononuclear cations [ML•]+ and an anion.
Figure 10. Electronic spectra of [CoL3] (- - -) and electrochemically
generated [CoIIIL3•]+ (s) at -61 °C in CH2Cl2 (0.10 M [TBA]PF6).
The latter spectrum was obtained as described in the text from solutions
of [CoL3] (1 mM) which were oxidized to 30 and 50% only.
started at 0 V. It probably corresponds to the reduction of the
putative disulfide complex.
Figure 9 (bottom) displays the cv of [CoIIIL3] recorded in
dichloromethane (0.1 M [TBA]PF6) at -20 °C at scan rates
50-1000 mV s-1. A reversible one-electron transfer wave is
now observed at 0.55 V; ∆Ep ) Ep,ox - Ep,red ) 80 mV at scan
rates >200 mV s-1. In addition, a kinetically delayed reduction
peak at -1.6 V and a corresponding oxidation peak at -0.5 V
are observed which are assigned to the CoIII/CoII couple. The
cv recorded at 25 °C shows the same behavior, but the oxidation
wave is only quasi-reversible.
We have investigated the one-electron oxidation of [CoIIIL3]
in CH2Cl2 solution, yielding [CoIIIL3•]+ by spectroelectrochem-
istry and coulometry. Figure 10 shows the electronic spectra of
the starting material [CoIIIL3] and of its one-electron oxidized
form [CoIIIL3•]+ in CH2Cl2 at -61 °C. The latter spectrum has
been calculated in the following manner. A solution of [CoIIIL3]
in CH2Cl2 (1 mM) has been oxidized at a fixed potential of
+0.7 V by controlled potential coulometry to only 30 and 50%
with respect to the starting material. The resulting electronic
spectra were recorded, and contributions from unreacted starting
material were subtracted. The resulting spectra for [CoIIIL3•]+
were identical in both cases, indicating the stability of both
species on the time scale of these experiments (1-2 min) using
the above experimental conditions at -61 °C.
Discussion
In the following section we will first focus on the thermo-
dynamic and spectroscopic features of phenylthiyl radicalss
uncoordinated and coordinated to a metal ion.
The redox potentials relative to the standard hydrogen
electrode (SHE) for the uncoordinated species [p-X-C6H4-Y•]/
[p-X-C6H4-Y]- and [p-X-C6H4-Y•H]+/[p-X-C6H4-YH] (where
X represents H or a methyl group and Y is either oxygen or
sulfur) have been measured in aqueous solution by pulse
radiolytic studies of electron-transfer equilibria.32 To compare
these values to those obtained in the present study by cyclic
voltammetry we have subtracted 0.40 V from the values
referenced versus SHE32 since the Fc+/Fc couple has a redox
potential of 0.400 V versus SHE.33 We have also assumed that
the measured peak potentials, Ep,ox, for the irreversible one-
electron oxidations of [ML] complexes in Table 3 are related
to the unknown half-wave potentials, E1/2, by the equations E1/2
) (Ep,ox + Ep,red)/2 and ∆E ) (Ep,ox - Ep,red) ) 80 mV at a
100 mV s-1 scan rate under our experimental conditions. These
The electronic spectrum of [CoIIIL3] exhibits two relatively
weak d-d transitions in the visible at 659 nm (ꢀ ) 430 M-1
cm-1) and 485 nm (870 M-1 cm-1). Upon one-electron
oxidation the spectrum changes significantly. Intense new,
asymmetric maxima appear at 509 nm (ꢀ ) 2.6 × 103 M-1
cm-1) and 784 nm (1.03 × 103 M-1 cm-1) with a shoulder at
670 nm. These spectral features resemble closely those reported
for the phenylthiyl radical7 which displays a broad, asymmetric
maximum at 460 nm (ꢀ ) 2.5 × 103 M-1 cm-1). Reports on
the spectrum of this radical in the literature encompass only an
experimental range <550 nm, and no information is available
on longer wavelength maxima. The spectra of [CoIIIL1•]+ (Figure
4) and [CoIIIL3•]+ (Figure 10) are rather similar. This points to
the formation of a moderately stable thiyl radical in [CoIIIL3•]+.
That this thiyl radical is coordinated to a cobalt(III) ion in
[CoIIIL3•]+ was unambiguously proven by X-band EPR spec-
troscopy. Figure 9 (top) shows this spectrum of a frozen CH2-
Cl2 (0.1 M [TBA]PF6) solution of [CoIIIL3•]+ at 90 K. The
electrochemical generation of the radicals was performed at -60
°C, and care was taken to ensure that the samples never warmed
up before or during the recording of this EPR spectrum.
The EPR spectrum exhibits a rhombic signal at g ≈ 2 and
well-resolved hyperfine splitting between an organic radical spin
E
1/2 values are ordered in a redox potential ladder in Figure 11.
From columns 1-4 the following conclusions can be drawn
which all reflect the fact that thiyl radicals are S-centered,
(31) (a) Cited in ref 7. (b) Jeevarajan, A. S.; Fessenden, R. W. J. Phys.
Chem. 1992, 96, 1520. (c) Schmidt, U.; Mu¨ller, A. Angew. Chem. 1963,
75, 299; Angew. Chem., Int. Ed. Engl. 1963, 2, 216. (d) Rundel, W.;
Scheffler, K. Z. Naturforsch. 1963, 18b, 984.
(32) Armstrong, D. A.; Sun, Q.; Schuler, R. H. J. Phys. Chem. 1996,
100, 9892.
(33) (a) Koepp, H. M.; Wendt, H.; Strehlow, H. Z. Elektrochem. 1960,
64, 483. (b) Gagne´, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem.
1980, 19, 2854.
1
7
S ) /2 and the 59Co nucleus (I ) /2). The spectrum was

6038 J. Am. Chem. Soc., Vol. 123, No. 25, 2001
Kimura et al.
be established, because [FeL3] is not available, but the potential
for [GaL3] is again very much higher than that observed for
[CoL3]. This is an indication that the Ga-S bond is significantly
stronger than the Fe-S bond. Both metal ions are expected to
display more π acidity due to their empty and half-filled t2g
6
subshell, respectively, contrasting in this respect the filled t2g
subshell of a cobalt(III) ion. It is also quite remarkable that
oxidation of the coordinated phenolate to the corresponding
phenoxyl in [ML1] complexes is more easily achieved than
oxidation of the analogous thiophenolate complexes. This is,
as described above, in contrast to the electrochemistry of the
uncoordinated ligands in column 1-4. We take this as an
experimental indication that the metal-to-sulfur (thiophenolate)
bonds possess significantly more covalent character than the
metal-to-oxygen (phenolate) bonds.
The electronic spectra of [CoL1•]+, [GaL1•]+, and [FeL1•]+
(Figure 4 and Table 4) clearly show the presence of a phenoxyl
radical by their intense (ꢀ > 3 × 103 M-1 cm-1) asymmetric
transitions 380-420 nm and a less intense transition >600 nm.
This has been reported for numerous (phenoxyl)metal com-
plexes, irrespective of the nature of the metal ion.12 It is therefore
gratifying that the electronic spectrum of [CoIIIL3•]+ shows
exactly the same features: an intense, broad, and asymmetric
transition at 509 nm (ꢀ ) 2.6 × 103 M-1 cm-1) and a transition
at 784 nm (ꢀ ) 1.0 × 103 M-1 cm-1). As pointed out above,
the former transition resembles closely that reported for the free
phenylthiyl radical at 460 nm (ꢀ ) 2.5 × 103 M-1 cm-1).7,34
The X-band EPR spectrum of the uncoordinated phenylthiyl
radical shows a large g-value anisotropy which has been
interpreted in terms of spin-orbit coupling (giso ) 2.0252),31
which mixes two states of different orbital angular momentum.
In the ground state of the radical the unpaired electron is
localized in a nonbonding orbital of the sulfur,8a,b and conse-
quently, no resolved hyperfine splitting to the protons of the
phenyl ring has been detected. This is in marked contrast to
phenoxyl radicals, the EPR spectra of which display typical
organic radical signals at g ≈ 2.0045 with well-resolved
hyperfine splitting to phenyl protons due to significant delo-
calization of the unpaired electron over the phenyl ring.35
In Table S21 we have summarized EPR spectroscopic data
on cobalt(III) complexes containing coordinated phenoxyl,30,36
anilino,37 thiyl, and semiquinonato38-40 ligands. With the
exception of the present (thiyl)cobalt complex, [CoIIIL3•]+, all
complexes display EPR signals centered at g ≈ 2.00 with, in
most cases, well-resolved 59Co hyperfine splitting typical for
the presence of a coordinated organic radical. For (semiquinon-
ato)cobalt(III) complexes the A(59Co) hfcs are observed in the
rather narrow range 9.8-11 G,38-40 whereas for (phenoxyl)-
cobalt(III) species30,36 this value is significantly smaller in the
range 5-6 G. The semiquinonates are O,O-coordinated bidentate
ligands, whereas the phenoxyls are O-coordinated monodentate.
It has recently been shown37 that a monodentate anilino radical
coordinated to a cobalt(III) ion displays a rather strong 59Co
Figure 11. Redox potential ladder for phenoxyl and phenylthiyl
(coordinated and uncoordinated) radicals. Potentials are referenced vs
the Fc+/Fc couple where the potentials given in columns 1, 2, 3, and
4 have been obtained by subtracting 0.40 V from the values given in
ref 32 vs NHE. Columns L1, L2, L3 give the redox potentials for the
couples [ML•]+/[ML] from the present work where the Ep,ox values for
irreversible processes given in Table 3 were converted to E1/2 values
by subtracting 40 mV, assuming that E1/2 ) (Ep,ox + Ep,red)/2 and ∆E
) (Ep,ox - Ep,red) ) 80 mV under our experimental conditions (scan
rate 100 mV s-1). Column 1 gives values for the couples [C6H5O˙ H]+/
[C6H5OH] (top value) and [C6H5O•]/[C6H5O]- (lower value); column
2, those of the couples [C6H5S˙ H]+/[C6H5SH] (top) and [C6H5S•]/
[C6H5S]- (bottom); column 3, those of the corresponding p-methylphe-
nol(ate) derivatives; and 4, those of the corresponding p-methylthiophenol-
(ate) derivatives.
contrasting in this respect the phenoxyls which are de-
localized:32 (i) The one-electron oxidation of phenolate or
p-methylphenolate is energetically more favorable by ∼600 mV
than the corresponding oxidations of the phenol and p-
methylphenol (columns 1 and 3). This difference is reduced to
∼400 mV for the corresponding thio-derivatives in columns 2
and 4. (ii) As expected, the substitution of the parent phenol
(or thiophenol) at the para position by an electron-donating
methyl group lowers the redox potentials for the respective
phenolates and phenols by 110 and 90 mV (columns 1, 3),
respectively, which decreases to 50 and 40 mV, respectively,
for the thio derivatives (columns 2 and 4). (iii) It is always easier
to oxidize the analogous thio derivatives than their corresponding
oxygen analogues by 50-100 mV (compare columns 1, 2 with
3, 4). Clearly, protonation of a given phenolate or thiophenolate
has the most dramatic effect on the respective redox potential.
We now turn to the thermodynamic properties of coordinated
phenoxyl and thiyl radicals. In previous work on tris(phenolato)-
metal complexes, (M ) Ga, Co, Fe), we had established that
the redox potentials for the couples [MLc•]+/[MLc] vary from
0.35 V for the gallium28 to 0.38 V for the iron28 and to 0.01 V
for the cobalt complex.30 It was shown that they correlate with
the π donor bond strength of the M-Ophenolato bond which is
the least strong one in low-spin CoIII (t2g6) complexes, but
relatively strong in GaIII (d0), and the strongest one in high-
spin FeIII (d5) complexes. Exactly the same order of the redox
potentials, namely E1/2 is the lowest for [CoL1], intermediate
for [GaL1], and highest for [FeL1], has been found in the present
work. It is therefore rather unexpected that the redox potentials
for the [ML2•]+/[ML2] couples display a different order. Whereas
the potential for the cobalt(III) species, [CoL2], is still the lowest,
it is closely followed by that for [FeL2], and that for [GaL2] is
significantly higher. For the series [ML3] this order could not
(34) Bonifacic, M.; Weiss, J.; Chaudhry, S. A.; Asmus, K.-D. J. Phys.
Chem. 1985, 89, 3910.
(35) (a) Neta, P.; Fessenden, R. W. J. Phys. Chem. 1974, 78, 523. (b)
Stone, T. J.; Waters, W. A. J. Chem. Soc. 1964, 213.
(36) Mu¨ller, J.; Kikuchi, A.; Bill, E.; Weyhermu¨ller, T.; Hildebrandt,
P.; Ould-Moussa, L.; Wieghardt, K. Inorg. Chim. Acta 2000, 297, 265.
(37) Penkert, F. N.; Weyhermu¨ller, T.; Bill, E.; Hildebrandt, P.; Lecomte,
S.; Wieghardt, K. J. Am. Chem. Soc. 2000, 122, 9663.
(38) Wicklung, P. A.; Beckmann, L. S.; Brown, D. G. Inorg. Chem. 1976,
15, 1996.
(39) Buchanan, R. M.; Pierpont, C. G. J. Am. Chem. Soc. 1980, 102,
4951.
(40) Kessel, S. L.; Emberson, R. M.; Debrunner, P. G.; Hendrickson,
D. N. Inorg. Chem. 1980, 19, 1170.

Phenylthiyl Complexes of Ga(III), Fe(III), and Co(III)
J. Am. Chem. Soc., Vol. 123, No. 25, 2001 6039
Table 4. Electronic Spectra of Complexes in CH3CN Solution
complex
λ )
max, nm (ꢀ, M-1 cm-1
[GaL1]
233 (9400), 286 (3500)
[GaL1•]+ a
[FeL1]
417 (4600), 401 (3900, sh), 302 (1.1 × 104)
520 (3400), 323 (5400), 272 (9.9 × 103, sh), 236 (1.3 × 104)
523 (2100), 458 (3500, sh), 412 (1.1 × 104), 283 (1.4 × 104)
607 (300, sh), 522 (600), 390 (600), 266 (1.7 × 104), 225 (1.9 × 104, sh)
634 (2100), 532 (1300, sh), 402 (3800), 304 (1.5 × 104), 241 (1.9 × 104)
292 (1.6 × 103, sh), 262 (1.3 × 104, sh)
[FeL1•]+ a
[CoL1]
[CoL1•]+ a
[GaL2]
[FeL2]
595 (2800), 450 (900,sh), 291 (9600), 253 (1.3 × 104)
[CoL2]
629 (300), 504 (500,sh), 402 (1.1 × 103), 311 (1.7 × 104), 230 (1.9 × 104, sh)
283 (2000, sh), 252 (1.3 × 104)
[GaL3]
[CoL3]
639 (520), 489 (1.0 × 103), 322 (1.9 × 104), 281 (1.2 × 104, sh), 224 (2.5 × 104, sh)
659 (430), 485 (870), 326 (1.6 × 104), 286 (1.0 × 104), 262 (9.6 × 103)
784 (1.0 × 103), 670 (790, sh), 509 (2600), 332 (1.5 × 104), 284 (1.2 × 104), 267(1.3 × 104, sh)
[CoL3] b
[CoL3•]+ a,b
a Electrochemically generated (0.10 [TBA]PF6). b CH2Cl2 solution.
hyperfine coupling of 12.15 G (in addition to strong nitrogen
hyperfine coupling and coupling to the N-H proton). The
observed EPR spectrum of [CoL3•]+ is fully in line with the
interpretation as a cobalt(III) species containing a coordinated
thiyl radical: a high giso value of 2.0226 and the absence of
phenyl proton hyperfine coupling indicates its S-centered
character. Interestingly, the 59Co hyperfine coupling of 10.7 G
is rather large. We take this as an indication that the Co-S
bond in [CoIIIL3•]+ is strong and covalent as is the Co-N bond
in the anilino radical species [CoIII(LNH•)(Bu2acac)]2+ where
does not show hyperfine splitting to the benzyl or phenyl ring
protons.
These coordinated phenylthiyl radicals display significantly
enhanced reactivity in solution at 20 °C compared to their
phenoxyl analogues. The M-S˙ R bonds are quite weak and
dissociate in solution with dimerization yielding dinuclear
disulfide complexes [ML-LM]2+
.
3
Ligand abbreviations: H3(LOCH ) ) 1,4-dimethyl-7-(3-tert-
butyl-5-methoxy-2-hydroxy-benzyl)-1,4,7-triazacyclononane;
LPrH ) 1,4-diisopropyl-7-(3,5-di-tert-butyl-2-hydroxy-benzyl)-
1,4,7-triazacyclononane; Ph2acac- ) diphenylacetylacetonate;
acac- ) 2,4-pentanedionate; Bu2acac- ) di-tert-butylacetylac-
etonate; LNHH ) 1-(2-amino-3,5-di-tert-butylbenzyl)-4,7-dim-
ethyl-1,4,7-triazacyclononane; trien ) triethylenetetramine;
(LNH 1-
represents the monoanion of 1-(2-amino-3,5-di-tert-
)
butylbenzyl)-4,7-dimethyl-1,4,7-triazacyclononane.37 The Co-O
bond in (phenoxyl)cobalt(III) species is then considerably less
covalent.36,30
DBsq•- ) 3,5-di-tert-butyl-o-benzosemiquinonate; DCat2-
3,5-di-tert-butylcatecholate; bpy ) 2,2′-bipyridine; salen2-
)
)
Conclusions
N,N′-ethylenebis(salicylideneiminate)(2-); Cl4sq- ) tetrachlo-
We have shown that the mononuclear, neutral complexes of
GaIII, FeIII, CoIII containing a trisanionic macrocyclic ligand with
one o,p-substituted phenolate and two carboxylate pendent arms,
[ML1], are reversibly one-electron oxidized, yielding stable
(phenoxyl)metal(III) cations [ML1•]+ which were characterized
by their electronic EPR and, in case of iron, by their Mo¨ssbauer
spectra. It is unequivocally shown that in these instances ligand-
based oxidations occur rather than metal-based.
rosemiquinonate.
Acknowledgment. We thank the Fonds der Chemischen
Industrie and the Deutsche Forschungsgemeinschaft (Priority
program “Radicals in Biology”) for financial support. Mrs.
Heike Schucht is thanked for skillfull help with the X-ray
crystallography.
Supporting Information Available: Figure S1 showing the
cv of [FeIIIL2]; Table S21 summarizes EPR parameters of
(radical)cobalt(III) complexes; tables of crystallographic and
structure refinement data, atom coordinates, bond lengths and
angles, anisotropic thermal parameters, and calculated positional
parameters of H atoms (PDF). An X-ray crystallographic file,
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
Replacing the phenolate pendent arm by an o,p-di-tert-
butylthiophenolate in [L3]3- and the preparation of the complex
[CoL3] has allowed the electrochemical generation of the
monocation [CoIIIL3•]+ in CH2Cl2 solution at -61 °C. It has
been shown that this species is an S-coordinated phenylthiyl
radical which displays characteristic transitions in the visible.
The X-band EPR spectrum exhibits a 59Co hyperfine split signal
(A(59Co) ) 10.7 G) at giso ) 2.0226. In agreement with the
notion that thiyls are S-centered radicals, the above monocation
JA004305P