Metal- Versus Ligand-Centered Oxidations in Phenolato-Vanadium and -Cobalt Complexes: Characterization of Phenoxyl-Cobalt(III) Species

Article abstract of DOI:10.1021/ic970256e

The coordination chemistry of the pendent-arm macrocycles 1,4,7-tris(3,5-dimethyl-2-hydroxybenzy1)-1,4,7-triazacyclononane, L^MeH₃, 1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, L^BuH₃, 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane, L^BuH₃, and Tolman's ligand 1,4-diisopropyl-7-(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, L^PrH, with vanadium and cobalt(III) has been studied. The following complexes containing a fac-N₃O₃ donor set have been synthesized: [L^MeV^III] (1), [L^MeV^IV]PF₆ (2), [(L^MeH)V^V(O)]PF₆ (3), [L^BuV^IV]PF₆ (4), [L^OCH3V^IV]PF₆ (5), [L^MeCo^III] (6), [L^BuCo^III] (7), [L^OCH3Co^III] (8). In addition, two complexes containing the L^PrCo^III fragment have been prepared: [L^PtCo^III(acac)](ClO₄) (9) and [L^PrCo^III(Cl₄cat)]·CH₃CN (10), where acac^- represents the ligand pentane-2,4-dionate and Cl₄cat^2- is tetrachlorocatecholate. Complexes 9 and 10 have been characterized by single-crystal X-ray crystallography: 9 crystallizes in the triclinic space group P1? with a = 9.493(1) A?, b = 9.760(1) A?, c = 18.979(2) A?, α = 88.57(1)°, β= 78.60(1)°, γ = 79.24(1)°, V = 1693.3(3) A?³, and Z = 2; 10 crystallizes in the monochnic space group P2₁/n with a = 10.184(2) A?, b = 24.860(5) A?, c = 14.872(3) A?, β= 97.95(3)°, V = 3729(1) A?³, and Z = 4. Electrochemically, complexes 2,4, and 5 can be reversibly oxidized by one electron, yielding vanadium(V), and one-electron-reduced, affording vanadium(III) species; 3 can be reduced to [L^MeHV^IV(O)]. These redox processes are shown to be metal-centered. In contrast, the cyclic voltammograms of 7 and 8 display three reversible one-electron oxidations. For the monocations [7]^·+ and [8]^·+, EPR and UV-vis spectroscopies reveal that these are phenoxyl-cobalt(III) species. Thus, the redox processes are ligand-centered. Similarly, [9]⁺ is reversibly oxidized to the phenoxyl-cobalt(III) complex [9]^·2+ (S = 1/2). For 10, two reversible one-electron oxidation steps have been identified generating [10]^·+ (S = 1/2) and [10]^2·2+ (S = 1). It is unambiguously shown by EPR and resonance Raman spectroscopies that [10]^·+ contains a (phenolato)(semiquinonato)cobalt(III) unit whereas in [10]^2·2+ a phenoxyl(semiquinonato)cobalt(III) unit prevails.

Full text of DOI:10.1021/ic970256e

3702
Inorg. Chem. 1997, 36, 3702-3710
Metal- Wersus Ligand-Centered Oxidations in Phenolato-Vanadium and -Cobalt
Complexes: Characterization of Phenoxyl-Cobalt(III) Species
Achim Sokolowski, Britta Adam, Thomas Weyhermu1ller, Akihiro Kikuchi,
Knut Hildenbrand, Robert Schnepf, Peter Hildebrandt, Eckhard Bill, and Karl Wieghardt*
Max-Planck-Institut fu¨r Strahlenchemie, D-45470 Mu¨lheim an der Ruhr, Germany
ReceiVed March 4, 1997X
The coordination chemistry of the pendent-arm macrocycles 1,4,7-tris(3,5-dimethyl-2-hydroxybenzyl)-1,4,7-
triazacyclononane, LMeH3, 1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, LBuH3, 1,4,7-
tris(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane, LOCH H3, and Tolman’s ligand 1,4-
3
diisopropyl-7-(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, LPrH, with vanadium and cobalt(III)
has been studied. The following complexes containing a fac-N3O3 donor set have been synthesized: [LMeVIII]
IV
(1), [LMeVIV]PF6 (2), [(LMeH)VV(O)]PF6 (3), [LBuVIV]PF6 (4), [LOCH V ]PF6 (5), [LMeCoIII] (6), [LBuCoIII] (7),
3
III
[LOCH Co ] (8). In addition, two complexes containing the LPrCoIII fragment have been prepared: [LPr-
3
CoIII(acac)](ClO4) (9) and [LPrCoIII(Cl4cat)]‚CH3CN (10), where acac- represents the ligand pentane-2,4-dionate
and Cl4cat2- is tetrachlorocatecholate. Complexes 9 and 10 have been characterized by single-crystal X-ray
crystallography: 9 crystallizes in the triclinic space group P1h with a ) 9.493(1) Å, b ) 9.760(1) Å, c ) 18.979-
(2) Å, R ) 88.57(1)°, â ) 78.60(1)°, γ ) 79.24(1)°, V ) 1693.3(3) Å3, and Z ) 2; 10 crystallizes in the monoclinic
space group P21/n with a ) 10.184(2) Å, b ) 24.860(5) Å, c ) 14.872(3) Å, â ) 97.95(3)°, V ) 3729(1) Å3,
and Z ) 4. Electrochemically, complexes 2, 4, and 5 can be reversibly oxidized by one electron, yielding vanadium-
(V), and one-electron-reduced, affording vanadium(III) species; 3 can be reduced to [LMeHVIV(O)]. These redox
processes are shown to be metal-centered. In contrast, the cyclic voltammograms of 7 and 8 display three reversible
one-electron oxidations. For the monocations [7]•+ and [8]•+, EPR and UV-vis spectroscopies reveal that these
are phenoxyl-cobalt(III) species. Thus, the redox processes are ligand-centered. Similarly, [9]+ is reversibly
1
oxidized to the phenoxyl-cobalt(III) complex [9]•2+ (S ) /2). For 10, two reversible one-electron oxidation
1
steps have been identified generating [10]•+ (S ) /2) and [10]2•2+ (S ) 1). It is unambiguously shown by EPR
and resonance Raman spectroscopies that [10]•+ contains a (phenolato)(semiquinonato)cobalt(III) unit whereas in
[10]2•2+ a phenoxyl(semiquinonato)cobalt(III) unit prevails.
Introduction
In contrast, we have recently shown10 that, by protecting the
ortho and para positions of the phenolate pendent arms by bulky
substituents such as tert-butyl groups and introducing a redox-
inactive metal ion such as GaIII, ScIII, or ZnII,11 it is possible to
successively and reversibly oxidize the three phenolato ligands
in [LBuM] (M ) Ga, Sc) or [LZnII]- to give relatively stable
phenoxyl radical complexes. [LBu]3- represents the trianion of
1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacy-
clononane (see Chart 1). Thus, the monocation [LBuGa]•+, the
dication [LBuGa]2•2+, and the trication [LBuGa]3•3+ contain one,
two, and three coordinated phenoxyl ligands, respectively. In
this instance, ligand-centered redox processes prevail.
In essence, this work has shown that phenolates belong to
the growing class of noninnocent ligands. For a given transition
metal complex of this type, it is therefore not possible to a priori
assign an oxidation state to the metal ion involved with dn
electron configuration.
Pendent-arm macrocyclic ligands derived from N-function-
alized 1,4,7-triazacyclononane1-6 have been shown to form
extremely stable7,8 first-row transition metal complexes. In
recent years, the coordination chemistry of the tris(N-phenolato)-
pendent-arm derivatives has been extensively studied.1-8 For
example, we have been able to show4 that the potassium salt of
1,4,7-tris(5-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacy-
clononane, K3[L], reacts with VCl3 in acetonitrile quantitatively
to yield [LVIII]. Electrochemically, this species can be reversibly
oxidized, generating successively [LVIV]+ and [LVV]2+, and
reduced at very negative potentials to the monoanion [LVII]-.
Thus, four oxidation states of the central metal ion are available
without changing the ligand environment comprising a fac-N3O3
donor set.
X Abstract published in AdVance ACS Abstracts, August 1, 1997.
(1) Moore, D. A.; Fanwick, P. E.; Welch, M. J. Inorg. Chem. 1989, 28,
1504.
(2) Auerbach, U.; Eckert, U.; Wieghardt, K.; Nuber, B.; Weiss, J. Inorg.
Chem. 1990, 29, 938.
This dichotomy plays an important role in some copper-
containing metalloproteins such as galactose oxidase12 and
glyoxal oxidase,13 for which the active forms have been shown
(3) Martell, A. E.; Motekaitis, R. J.; Welch, M. J. J. Chem. Soc., Chem.
Commun. 1990, 1748.
(4) Auerbach, U.; Della Vedova, S. P. C.; Wieghardt, K.; Nuber, B.; Weiss,
J. J. Chem. Soc., Chem. Commun. 1990, 1004.
(5) Auerbach, U.; Stockheim, C.; Weyhermu¨ller, T.; Wieghardt, K.; Nuber,
B. Angew. Chem. 1993, 105, 735; Angew. Chem., Int. Ed. Engl. 1993,
32, 714.
(6) Auerbach, U.; Weyhermu¨ller, T.; Wieghardt, K.; Nuber, B.; Bill, E.;
Butzlaff, C.; Trautwein, A. X. Inorg. Chem. 1993, 32, 508.
(7) Clarke, E. T.; Martell, A. E. Inorg. Chim. Acta 1991, 186, 103.
(8) Martell, A. E.; Motekaitis, R. J.; Welch, M. J. J. Chem. Soc., Chem.
Commun. 1990, 1748.
(10) Adam, B.; Bill, E.; Bothe, E.; Goerdt, B.; Haselhorst, G.; Hildenbrand,
K.; Sokolowski, A.; Steenken, S.; Weyhermu¨ller, T.; Wieghardt, K.
Chem. Eur. J. 1997, 3, 308.
(11) Sokolowski, A.; Mu¨ller, J.; Weyhermu¨ller, T.; Schnepf, R.; Hilde-
brandt, P.; Hildenbrand, K.; Bothe, E.; Wieghardt, K. J. Am. Chem.
Soc., in press.
(12) (a) Ettinger, J. E.; Kosman, D. J. In Copper Proteins; Spiro, T. G.,
Ed.; Wiley and Sons: New York, 1981; p 221. (b) Whittaker, J. W.
In Metal Ions in Biological Systems; Sigel, H., Sigel, A., Eds.; Marcel
Dekker: New York, 1994; Vol. 30, p 315.
(9) Motekaitis, M. J.; Sun, Y.; Martell, A. E. Inorg. Chim. Acta 1992,
198-200, 421.
(13) Whittaker, M. W.; Kersten, P. J.; Nakamura, N.; Sanders-Loehr, J.;
Schweizer, E. S.; Whittaker, J. W. J. Biol. Chem. 1996, 271, 681.
S0020-1669(97)00256-5 CCC: $14.00 © 1997 American Chemical Society

Phenolato-Vanadium and -Cobalt Complexes
Chart 1. Ligands and Complexes
Inorganic Chemistry, Vol. 36, No. 17, 1997 3703
zyl)-1,4,7-triazacyclononane,1 LMeH3, 1,4,7-tris(3,5-di-tert-butyl-2-
hydroxybenzyl)-1,4,7-triazacyclononane,10 LBuH3, and 1,4,7-tris(3-tert-
butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane,10 LOCH H3,
3
were prepared according to published procedures.
Preparation of Complexes. [LMeVIII] (1). A deoxygenated solution
of VCl3 (0.16 g; 1.0 mmol) in CH3CN (20 mL) was heated to reflux
for 20 min under an argon atmosphere until a clear green solution of
[VCl3(CH3CN)3] was obtained. Addition of LMeH3 (0.54 g; 1.0 mmol)
and a few drops of triethylamine gave a deep green solution from which
a microcrystalline green solid material slowly precipitated. Yield: 0.36
g (63%). µeff(293 K): 2.89 µB. FAB MS (MNBA): m/z 579.3 {[M+]}.
Anal. Calcd for C33H42N3O3V: C, 68.3; H, 7.4; N, 7.2. Found: C,
68.1; H, 7.6; N, 6.9.
[LMeVIV]PF6 (2). To a solution of 1 (0.58 g; 1.0 mmol) in CH3OH
(30 mL) were added Na2S2O8 (0.3 g) and one drop of concentrated
HClO4 with stirring at room temperature. The deep violet solution
was filtered, and solid NaPF6 (1.0 g) was added, which initiated the
precipitation of violet microcrystals. Yield: 0.49 g (68%). µeff(293
K): 1.73 µB. FAB MS (MNBA): m/z 579.3 {[M+]}. Anal. Calcd
for C33H42N3O3VPF6: C, 54.7; H, 5.8; N, 5.8. Found: C, 54.5; H,
6.1; N, 5.5.
[(LMeH)VVO]PF6 (3). A solution of vanadyl acetylacetonate, VO-
(acac)2 (0.26 g; 1.0 mmol), and LMeH3 (0.54 g; 1.0 mmol) in acetone
(30 mL) was heated to 40 °C with stirring for 30 min. Addition of
solid NaPF6 (0.7 g) to the solution initiated the precipitation of violet
microcrystals. Yield: 0.24 g (32%). FAB MS (MNBA): m/z 596.7
{M+]}. Anal. Calcd for C33H43N3O4PF6V: C, 53.4; H, 5.8; N, 5.7.
Found: C, 53.2; H, 6.0; N, 5.7. 13C{1H} NMR (100 MHz, CDCl3):
δ 159.3, 151.2, 137.9, 133.5, 132.5, 132.1, 131.9, 130.3, 129.4, 128.3,
125.9, 124.9, 123.9, 123.6, 120.1, 118.6, 68.1, 64.8, 62.6, 62.0, 59.9,
57.0, 52.8, 50.5, 49.5, 21.4, 20.7, 20.6, 17.0, 16.8, 16.0 ppm.
[LBuVIV]PF6 (4). A deoxygenated solution of VCl3 (0.16 g; 1.0
mmol) in CH3CN (20 mL) was heated to reflux under an argon
atmosphere until a clear green solution was obtained, to which LBuH3
(0.78 g; 1.0 mmol) and a few drops of triethylamine were added. After
addition of solid NaPF6 (1.0 g) in the presence of air, black-violet
microcrystals formed. Yield: 0.66 g (61%). µeff(293 K): 1.83 µB.
FAB MS (MNBA): m/z 831.7 {[M+]}. Anal. Calcd for C51H78N3O3-
VPF6: C, 62.7; H, 8.0; N, 4.3. Found: C, 62.6; H, 8.3; N, 4.2.
IV
by spectroscopic methods (UV-vis, resonance Raman, EPR,
and EXAFS) to contain a tyrosyl-copper(II) rather than a
tyrosinate-copper(III) unit.
In this work, we describe the coordination and redox
chemistry of the pendent-arm macrocycles LMeH3,1 LBuH3,10 and
LOCH H3,10 the structures of which are depicted in Chart 1, with
3
vanadium and cobalt(III). As we will show, the redox chemistry
of the vanadium complexes 1-5 is metal-centered and that of
the cobalt(III) complexes 6-8 is ligand-centered. We have also
used the tetradentate ligand 1,4-diisopropyl-7-(3,5-tert-butyl-
2-hydroxybenzyl)-1,4,7-triazacyclononane, LPrH, described re-
cently by Tolman et al.,14 and prepared the two complexes 9
and 10 containing the LPrCoIII fragment and, in addition, the
redox-innocent didentate ligand pentane-2,4-dionate, acac-, or
the potentially redox-active tetrachlorocatecholate dianion,
[LOCH V ]PF6 (5). This complex was prepared as described above
3
for 4 by using LOCH H3 (0.71 g; 1.0 mmol). The violet crude product
3
was recrystallized from acetone. Yield: 0.32 (36%). Anal. Calcd
for C42H60F6N3O6PV: C, 56.1; H, 6.7; N, 4.7; V, 5.7. Found: C, 55.8;
H, 6.8; N, 4.8; V, 5.5.
[LMeCoIII] (6). To a solution of LMeH3 (0.54 g; 1.0 mmol) in acetone
(50 mL) were added Co(CH3CO2)2‚4H2O (0.25 g; 1.0 mmol) and
triethylamine (5 mL). The suspension was heated to reflux in the
presence of air until a clear, deep green solution was obtained, from
which, upon cooling, green microcrystals precipitated. Yield: 0.39 g
(67%). FAB MS (MNBA): m/z 587.2 {[M+]}. 1H NMR (400 MHz,
CDCl3): δ 6.8 (s, 3H), 6.65 (s, 3H), 4.9 (d, 3H), 2.6-2.9 (m, 15H),
2.25 (s, 9H), 2.15 (s, 9H) ppm. Anal. Calcd for C33H42N3O3Co: C,
67.5; H, 7.2; N, 7.1. Found: C, 67.8; H, 7.4; N, 6.9.
[LBuCoIII] (7). This complex was prepared as described above for
6 by using LBuH3 (0.78 g; 1.0 mmol). Green microcrystals were
obtained. Yield: 0.54 g (64%). FAB MS (MNBA): m/z 840.5 {[M+]}.
1H NMR (400 MHz, CDCl3): δ 6.95 (d, 3H), 6.6 (d, 3H), 4.55 (d,
3H), 3.60 (m, 3H), 3.20 (m, 3H), 2.80 (d, 3H), 2.7 (m, 3H), 2.25 (m,
3H), 1.20 (s, 27H), 0.85 (s, 27H) ppm. Anal. Calcd for C51H78N3O3-
Co: C, 72.9; H, 9.4; N, 5.0. Found: C, 72.8; H, 10.0; N, 4.8.
Cl4cat2-
.
The neutral complex 10 represents an interesting test case
because both the catecholate and the phenolate can, in principle,
undergo a ligand-centered one-electron oxidation, generating a
coordinated semiquinonate and a coordinated neutral phenoxyl
radical, respectively. As we show here, it is possible to discern
unambiguously by spectroscopic methods (UV-vis, resonance
Raman, and EPR) which of the two possible valence isomers
is present in the monocation [10]•+: the (phenolato)(semi-
quinonato)cobalt(III) or the phenoxyl(catecholato)cobalt(III)
species. The corresponding dication [10]2•2+ contains a phe-
noxyl and a tetrachlorosemiquinonate ligand.
[LOCH CoIII] (8). This complex was prepared as described above
3
for 6 by using LOCH H3 (0.71 g; 1.0 mmol). Green-brown microcrystals
3
Experimental Section
were obtained. Yield: 0.56 g (66%). Electrospray ionization mass
spectrum (positive ion): m/z 761 {[M + H+]}. 1H NMR (400 MHz,
CDCl3): δ 6.64 (d, J ) 3.13 Hz, 3H), 6.23 (d, J ) 3.13 Hz, 3H), 4.58
(d, J ) 14.5 Hz, 3H), 3.68 (s, 9H), 3.51 (m, 3H), 3.21 (m, 3H), 2.76
(d, J ) 14.5 Hz, 3H), 2.70 (m, 3H), 2.26 (m, 3H), 0.87 (s, 27H) ppm.
13C{1H} NMR (100 MHz, CDCl3): δ 154.6, 148.1, 118.3, 113.9, 110.0,
62.8, 57.4, 56.5, 56.3, 34.8, 29.8, 14.5 ppm. Anal. Calcd for
C42H60N3O6Co: C, 66.2; H, 7.9; N, 5.5; Co, 7.7. Found: C, 67.0; H,
8.0; N, 5.6; Co, 7.6.
Caution. Sodium perchlorate was used in the following syntheses.
Perchlorate salts are potentially explosive and should be handled in
small quantities and with appropriate precautions.
The ligands 1,4-diisopropyl-7-(3,5-di-tert-butyl-2-hydroxybenzyl)-
1,4,7-triazacyclononane,14 LPrH, 1,4,7-tris(3,5-dimethyl-2-hydroxyben-
(14) Halfen, J. A.; Young, V. G.; Tolman, W. B. Angew. Chem., Int. Ed.
Engl. 1996, 35, 1687.

3704 Inorganic Chemistry, Vol. 36, No. 17, 1997
Sokolowski et al.
Table 1. Crystallographic Data for Complexes 9 and 10
Packard HP 8452A diode array spectrophotometer (range: 220-820
nm). Cyclic voltammetry, square-wave voltammetry, and coulometric
experiments were performed with EG&G equipment ( Model 273A
potentiostat/galvanostat). X-band EPR spectra of complexes in fluid
solution at 298 K (10-3 M CH3CN/CH2Cl2 solutions containing 0.10
M [TBA]PF6 or [TBA]ClO4) were measured on a Varian E-9 X-band
spectrometer with 100 kHz modulation frequency at 298 K in a quartz
cell (d ) 0.3 mm). The data were digitized by means of a Stelar DS-
EPR data station (Stelar snc, Mede, Italy). The spectra were simulated
by iteration of the isotropic hyperfine coupling constants a and line
9
10
formula
fw
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
C32H55ClCoN3O7
688.2
triclinic
P1h
9.493(1)
9.760(1)
18.979(2)
88.57(1)
78.60(1)
79.24(1)
1693.3(3)
2
C35H51Cl4CoN4O3
776.5
monoclinic
P21/n
10.184(2)
24.860(5)
14.872(3)
90
97.95(3)
90
3729.0(13)
4
1.383
1632
0.79
widths ∆ν1/2
. We thank Dr. F. Neese (Abteilung Biologie der
Universita¨t Konstanz) for a copy of his EPR simulation program.
Temperature-dependent measurements on frozen solutions of [10]2•2+
in the range 3.5-150 K were carried out on a Bruker ESP 300E
spectrometer equipped with an Oxford Instruments ESR 910 helium-
flow cryostat with an ITC 503 temperature controller. Temperature
stability was 0.2 K, and the temperature gradient across the sample
was estimated to be less than 0.5 K. All NMR spectra were recorded
on a 400 MHz Bruker AMX series spectrometer.
F
calc, g cm-3
F(000)
1.350
736
0.635
µ, mm-1
cryst dimens, mm
λ, Å
0.73 × 0.42 × 0.14 0.28 × 0.46 × 0.70
0.710 73
100(2)
0.710 73
100(2)
T, K
Resonance Raman spectra were recorded with a U1000 spectrograph
(2400/mm holographic gratings) equipped with a liquid-nitrogen-cooled
CCD detector (Instruments SA). The output of a dye laser (stilbene 3;
Coherent 899-01) pumped by an argon ion laser (multiline UV;
Coherent Innova 400) served as the excitation source. The laser power
at the sample was about 50 mW. In order to avoid photoinduced
degradation, the sample which exhibits an optical density of ca. 1.5 at
the excitation wavelength was deposited in a rotating cell. The Raman
scattered light was detected in 90° with a scrambler placed in front of
the entrance slit of the spectrometer to account for the polarization
sensitivity of the gratings. The spectral slit width was 2.8 cm-1. The
spectra, measured with an acquisition time of 15 s, were linearized in
wavenumbers, yielding an increment of 0.24 cm-1 and a total spectral
range of ca. 200 cm-1. Thus, several spectra covering different but
overlapping ranges are combined to give the overview spectra displayed
in this work. In these spectra, the contributions of the solvent and the
supporting electrolyte are subtracted.
no. of total data collected 14 832
no. of unique obsd data
14 718
6254 (Rint ) 0.0639) 5354 (Rint ) 0.0316)
no. of parameters refined 409
435
0.046
0.68 (-0.33)
Ra
0.054
largest residual electron 0.86 (-0.60)
density, e Å-3
a R ) ∑(|Fo| -|Fc|)/∑|Fo|.
[LPrCoIII(acac)](ClO4) (9). To a solution of the ligand LPrH (0.086
g; 0.2 mmol) in acetonitrile (10 mL) was added CoII(acac)2 (0.052 g;
0.2 mmol) in the presence of air. The solution was stirred at 20 °C for
1 h. Solid NaClO4 (0.5 g) and H2O (5 mL) were then added. The
solution was allowed to stand at 0 °C for 12 h, during which red-
brown microcrystals precipitated. Yield: 0.09 g (65%). ESI MS
(positive ion; CH3CN): m/z 588 {[LPrCo(acac)]+}. 1H NMR (400 MHz;
CDCl3): δ 7.02 (d, J ) 2.4 Hz, 1H), 6.72 (d, J ) 2.35 Hz, 1H), 5.56
(s, 1H), 4.48 (d, J ) 14.2 Hz, 1H), 3.7-2.5 (m, 14H), 2.67 (d, J )
14.9 Hz, 1H), 2.23 (s, 3H), 1.73 (s, 3H), 1.60 (d, J ) 6.4 Hz, 3H),
1.31 (s, 9H), 1.22 (s, 9H), 1.19 (d, J ) 6.4 Hz, 3H), 1.14 (d, J ) 6.4
Hz, 3H), 1.10 (d, J ) 6.5 Hz, 3H) ppm. Anal. Calcd for C32H55N3O7-
CoCl: C, 55.8; H, 8.1; N, 6.1. Found: C, 55.7; H, 7.9; N, 6.0.
[LPrCoIII(Cl4cat)]‚CH3CN (10). To a solution of LPrH (0.043 g,
0.1 mmol) and N(Et)3 (14 µL) in acetonitrile (5 mL) was added CoCl2
(13 mg; 0.1 mmol). The solution was stirred at ambient temperature
in the presence of air for 30 min. Tetrachlorocatechol (24 mg; 0.1
mmol) was added, and the solution was stirred for 1 h at 20 °C.
Addition of water (5 mL) and cooling to 0 °C initiated the precipitation
of aubergine microcrystals. Yield: 0.040 g (54%). FAB MS (dimethoxy-
benzyl alcohol matrix): m/z 734 {[LPrCo(Cl4cat)]+}. 1H NMR (400
MHz; CDCl3): δ 6.92 (d, J ) 2.5 Hz, 1H), 6.67 (d, J ) 2.4 Hz, 1H),
5.03 (d, J ) 13.04 Hz, 1H), 3.85-2.2 (m, 14H), 2.93 (d, J ) 13.18
Hz, 1H), 1.51 (s, 3H), 1.21 (s, 9H), 1.16 (d, J ) 6.40 Hz, 3H), 1.10 (d,
J ) 6.38 Hz, 3H), 1.07 (s, 9H), 1.05 (d, J ) 6.53 Hz, 3H) ppm. Anal.
Calcd for C33H48N3O3CoCl4‚CH3CN: C, 54.1; H, 6.6; N, 7.2. Found:
C, 54.1; H, 6.4; N, 7.1.
X-ray Crystallography. Crystal data and data collection and
refinement details are summarized in Table 1 (and corresponding tables
in the Supporting Information). Graphite-monochromated Mo KR
X-radiation was used throughout. Intensities of a red-brown crystal
of 9 and an aubergine crystal of 10 were collected at 100(2) K,
respectively, by using the Siemens SMART system; they were corrected
for Lorentz and polarization effects, but no corrections for absorption
were carried out. The structures were solved by conventional Patterson
and difference Fourier methods by using the Siemens SHELXTL-PLUS
program package.15 The function minimized during full-matrix least-
squares refinement was ∑w(|Fo| - |Fc|)2. All non-hydrogen atoms were
refined anisotropically. The hydrogen atoms were placed at calculated
positions and refined with isotropic thermal parameters.
Results
Synthesis of Complexes. Green microcrystals of [LMeVIII]
(1) precipitated slowly from an anaerobic reaction mixture of
the ligand LMeH3 and VCl3 in acetonitrile to which a few drops
of triethylamine for deprotonation of the ligand had been added.
Complex 1 was oxidized in methanol with Na2S2O8. Upon
addition of NaPF6, violet crystals of [LMeVIV]PF6 (2) were
obtained. Both 1 and 2 are paramagnetic and display effective
magnetic moments at 293 K of 2.89 µB (1) and 1.73 µB (2)
indicative of octahedral vanadium(III) (d2) and vanadium(IV)
(d1), respectively.
When VIVO(acac)2, where acac represents the monoanion
pentane-2,4-dionate, was treated with 1 equiv of LMeH3 in
acetone in the presence of air and NaPF6 was added, violet
microcrystals of [(LMeH)VVO]PF6 (3) were obtained. Complex
3 is diamagnetic. In the IR spectrum, a sharp band at 947 cm-1
indicates the presence of a terminal oxo group (ν(VdO)). In
1
the H NMR spectrum, six well-resolved singlets in the range
δ 2-2.5 are observed which are assigned to the protons of six
chemically nonequivalent methyl groups. The 13C NMR
spectrum of 3 displays six methyl carbon signals at δ ∼20; in
the range 50-70 ppm, nine resonances of the methylene carbon
atom (ethyl and benzyl groups) are clearly detected, and of the
expected 18 resonances of the aromatic carbon atoms, 16 are
observed in the range 120-160 ppm. These data clearly
indicate that all three pendent arms of the macrocyclic ligand
[LMeH]2- are different. We propose the structure for the
monocation in 3, shown in Chart 2, with two coordinated
phenolates and one dangling phenol. In complexes 1 and 2,
Physical Measurements. Electronic spectra were recorded on a
Perkin-Elmer Lambda 19 (range: 220-1400 nm) or on a Hewlett
the trianion [LMe 3-
is coordinated in an octahedral fashion to
]
a V(III) and a V(IV) central ion, respectively. Here the three
coordinated phenolate pendent arms are equivalent. We previ-
(15) Sheldrick, G. M. Full-matrix least-squares refinement package SHELX-
TL-PLUS. Universita¨t Go¨ttingen, 1991.

Phenolato-Vanadium and -Cobalt Complexes
Inorganic Chemistry, Vol. 36, No. 17, 1997 3705
Chart 2. Proposed Structures for 3 (left) and 4 (right)
Table 2. Selected Bond Distances (Å) and Angles (deg) in 9
and 10
Complex 9
Co-O1
1.880(2)
1.889(2)
2.040(2)
1.275(4)
1.398(4)
1.393(4)
Co-O2
Co-N1
Co-N2
O2-C29
O1-C9
1.887(2)
1.948(2)
2.040(3)
1.281(4)
1.345(3)
Co-O3
Co-N3
O3-C31
C30-C31
C29-C30
Figure 1. Perspective view of the monocation in crystals of 9.
O1-Co-O2
O2-Co-O3
O2-Co-N1
O1-Co-N3
O3-Co-N3
O1-Co-N2
O3-Co-N2
N3-Co-N2
92.83(9)
92.07(9)
87.94(9)
176.40(9)
92.37(9)
89.59(9)
92.67(9)
87.54(9)
O1-Co-O3
O1-Co-N1
O3-Co-N1
O2-Co-N3
N1-Co-N3
O2-Co-N2
N1-Co-N2
89.93(8)
88.96(9)
178.89(9)
89.85(9)
88.73(10)
174.68(9)
87.36(10)
Complex 10
Co-O1
Co-O3
Co-N2
O2-C28
O1-C9
1.892(2)
1.898(2)
2.033(2)
1.326(3)
1.329(3)
Co-O2
Co-N1
Co-N3
O3-C29
C28-C29
1.897(2)
1.949(2)
2.059(2)
1.316(3)
1.422(4)
O1-Co-O2
O2-Co-O3
O2-Co-N1
O1-Co-N2
O3-Co-N2
O1-Co-N3
O3-Co-N3
N2-Co-N3
88.61(8)
86.67(8)
89.89(9)
89.24(8)
95.32(9)
175.54(8)
92.34(8)
86.90(9)
O1-Co-O3
O1-Co-N1
O3-Co-N1
O2-Co-N2
N1-Co-N2
O2-Co-N3
N1-Co-N3
85.79(8)
95.67(8)
176.23(9)
176.96(8)
88.18(9)
95.33(8)
86.44(9)
Figure 2. Perspective view of the neutral complex in crystals of 10.
diamagnetic species. Therefore, we propose that the cobalt-
(III) (d6 low spin) ions are in an octahedral environment
comprising three facially bound amine nitrogen and three
phenolate oxygen donors.
The ligand LPrH is tetradentate and contains three tertiary
amine nitrogen donors and one pendent phenol arm. Thus, the
reaction of LPrH with CoII(acac)2 in the presence of air in
acetonitrile solution affords, upon addition of NaClO4, red-
brown microcrystals of [LPrCoIII(acac)](ClO4) (9). From a
similar reaction of CoCl2, LPrH, and tetrachlorocatechol (Cl4-
catH2) in acetonitrile, aubergine microcrystals of the neutral
diamagnetic complex [LPrCoIII(Cl4cat)] (10) were obtained.
Crystal Structures. The structures of 9 and 10 have been
determined at 100(2) K by single-crystal X-ray crystallography.
Figure 1 shows the structure of the monocation in crystals of
9, and Figure 2, that of the neutral complex 10. Selected bond
distances and angles are summarized in Table 2.
In both mononuclear complexes, the cobalt ion is surrounded
by a facial N3O3 donor set. The monoanionic ligand [LPr]- is
tetradentate, and its pendent arm is coordinated as a phenolate.
The average Co-Ophen bond at 1.885 Å is very short in
comparison with those of other known phenolate cobalt(III)
species.17 Two further coordination sites are occupied in 9 by
a chelating pentane-2,4-dionate ligand and in 10 by a tetrachlo-
rocatecholate. The average C-Ophenolate bond at 1.337 Å in 9
and 10 and the average C-Ocat bond at 1.321 Å in 10 indicate
that the phenolate and tetrachlorocatecholate ligands prevail (and
ously reported4 the crystal structure of an analogous complex
[LVIV]BPh4, where L represents 1,4,7-tris(5-tert-butyl-2-hy-
droxybenzyl)-1,4,7-triazacyclononane.
By using the corresponding ligands LBuH3 and LOCH H3 and
3
similar reaction conditions as described above for the synthesis
of 1, we obtained, in the presence or absence of air, violet
crystals of the vanadium(IV) complexes [LBuVIV]PF6 (4) and
IV
[LOCH V ]PF6 (5) as the only isolable products when NaPF6
3
was added to the solution. Both compounds are paramagnetic
with µeff at ∼1.7 µB. Figure S1 (Supporting Information)
displays the X-band EPR spectrum of 5 in acetonitrile solution
at 298 K. The observed 51V hyperfine coupling constant of
1.07 mT (geff ) 1.99) is very similar to those of other octahedral
vanadium(IV) complexes16 and proves that 5 is a genuine V(IV)
species with a 3d1 electronic configuration. Note that 2, 4, and
5 do not exhibit a ν(VdO) stretching mode in the infrared in
the range 900-1000 cm-1
.
Reaction of the pendent-arm macrocycles LMeH3, LBuH3, and
LOCH H3 with Co (CH3CO2)2‚4H2O in acetone in the presence
II
3
of air affords, upon addition of triethylamine, green micro-
crystals of the neutral complexes [LMeCoIII] (6), [LBuCoIII] (7),
and [LOCH Co ] (8), respectively. The H NMR spectra prove
III
1
3
that the three pendent arms are equivalent in each of these
(17) Riley, P. E.; Pecoraro, V. L.; Carrano, C. J.; Raymond, K. N. Inorg.
Chem. 1983, 22, 3096.
(18) Adams, D. M.; Dei, A.; Rheingold, A. L.; Hendrickson, D. N. J. Am.
Chem. Soc. 1993, 115, 8221.
(16) Mabbs, F. E.; Collison, D. Studies in Inorganic Chemistry: Electron
Paramagnetic Resonance of d Transition Metal Compounds; Elsevi-
er: Amsterdam, 1992; Vol. 16.

3706 Inorganic Chemistry, Vol. 36, No. 17, 1997
Sokolowski et al.
a
Table 3. Redox Potentials of the Complexes
The CV (cyclic voltammogram) of the oxovanadium(V)
species 3 shows one reversible one-electron-transfer wave at
-0.39 V which is assigned to the VV/VIV couple. The enormous
stabilization of the +V oxidation state by the presence of a
terminal oxo group is well established in vanadyl coordination
chemistry.20
The square-wave voltammogram of 7 and the CV of 8 both
display three fully reversible one-electron-transfer waves. This
is shown in Figure 4. Similar observations have been reported
for the corresponding tris(phenolato) complexes of GaIII, ScIII,
CrIII, and FeIII.10 These oxidation processes are ligand-centered
as in eq 2 and indicate that one, two, and three coordinated
complex solvent
E11/2,V
E21/2,V E31/2,V E41/2,V E5
1/2
1
2
3
4
CH3CN
CH3CN
CH3CN
CH3CN
0.33 -0.55
0.33 -0.55
1.06 (irr) -0.39
0.37 -0.49
5
CH3CN 1.38 (irr) 1.07 (irr)
0.13 -0.71
0.38 -0.50 -2.41
0.01
-0.10
[L′VIII]b
7
8
9
10
CH2Cl2 0.75
CH3CN 0.43
CH3CN 0.46
CH3CN 0.62
0.41
0.18
0.16
a Redox potentials, Ex1/2 ) (Ep + Epred)/2, are referenced Vs the
ferrocenium/ferrocene (Fc+/Fc) couple; for irreversible (irr) processes,
peak potentials, Epox, are given. Conditions: solvent containing 0.10
M [TBA]PF6 or [TBA]ClO4 supporting electrolyte; glassy carbon
working electrode; Ag/AgCl (saturated LiCl/C2H5OH) reference elec-
ox
+e
-e
[LCo ]
y
z [LCo ]
y
+ez
III 3+
III 2+
-e
E1
E2
1/2
1/2
[LCoIII]+ y+-eez [LCoIII] (2)
trode; scan rate 200 mV s-1 b L′H3 ) 1,4,7-tris(5-tert-butyl-2-hydroxy-
.
benzyl)-1,4,7-triazacyclononane; ref 4.
E3
1/2
not their phenoxyl or semiquinonate forms).18 All Co-O
distances in 9 and 10 are smaller than 1.90 Å. We take this as
clear structural evidence that, at 100 K in the solid state, the
cobalt ions are in the oxidation state +III and no valence isomers
such as CoII(semiquinonate) species are present.19
Electrochemistry. The electrochemistry of the complexes
was investigated by cyclic and square-wave voltammetry and
controlled-potential electrolysis (coulometry) of acetonitrile or
dichloromethane solutions containing 0.10 M tetra-n-butylam-
monium hexafluorophosphate ([TBA]PF6) as supporting elec-
trolyte. All redox potentials are referenced to the ferrocenium/
ferrocene couple (Fc+/Fc); they are summarized in Table 3.
Table 4 gives details of the electronic spectra of the complexes.
The cyclic voltammograms of the tris(phenolato)vanadium
complexes 1, 2, 4, and 5 are very similar; they display two
reversible one-electron-transfer waves in the potential range
-1.5 to +0.9 V Vs Fc+/Fc. The square-wave voltammogram
of 5 shown in Figure S2 (Supporting Information) shows two
additional, irreversible oxidation peaks at +1.07 and 1.38 V,
which probably correspond to ligand-centered processes. The
reversible electron transfer processes are assigned to metal-
centered processes VIII/VIV and VIV/VV as in eq 1. Stepwise
phenolates are successively oxidized to the corresponding
phenoxyl radical. Thus, the trication [LCoIII]3+ contains three
coordinated phenoxyl ligands, whereas the dication has two
phenoxyls and one phenolate and the monocation contains one
phenoxyl and two phenolates. On the time scale of a coulo-
metric experiment at -30 °C, only the monocations of 7 and 8
are stable enough to allow their spectroscopic characterization.
In both cases, two new absorption maxima at ∼400 and >600
nm are observed for the monocations (Table 4 and Figure S3
(Supporting Information)). As we showed previously10 for
+
OCH3
[LOCH Ga] , [L
Sc]+ and [LBuGa]+, [LBuSc]+, as well as for
3
+
Bu
+
the complexes [LOCH Fe] and [L Fe] , these bands are typical
3
of coordinated phenoxyl radicals.
In a previous paper,10 we reported a linear correlation between
III +
OCH3 III
the redox potentials, E31/2, of the [LOCH M ] /[L
M ] couple
3
(M ) Sc, Fe, Cr) and the average O-M-O bond angle R. This
angle is a measure for the extent of M-Ophenolate π-bonding
effects. A pronounced MdO double-bond character of the three
phenolate-to-metal bonds (note: phenolates are good π-donors)
induces an opening of this angle (R > 90°). Although we have
not been able to determine R for 8, it is reasonable to assume
a value of 89-90°. This would be in line with the structure
determinations of 9 and 10 and the fact that Co-Ophenolate bonds
are single bonds since the t2g orbitals are filled and therefore
they cannot accept oxygen-to-cobalt(III) π bonding. It is
gratifying that the E31/2 of 8 fits the above linear correlation at
R ∼ 90° as is shown in Figure 5. Thus, the extent of M-O
π-bonding tunes the redox potentials of the ligand-centered
redox process. A potential range of ∼400 mV is observed for
[LV ]
y
+ez [LVIV]+ y+-eez [LVIII]
(1)
V 2+
-e
E3
E4
1/2
1/2
3-
L ) [LMe]3-, [LBu]3-, [LOCH
]
3
controlled-potential electrolysis of 5 allows the spectroelectro-
III +
OCH3 III
the [LOCH M ] /[L
M ] couple on going from CoIII (no
3
IV +
chemical characterization of [LOCH VV]2+, [LOCH V ] , and
3
3
π-bonding) to ScIII (strong π-bonding).10 We cannot offer a
theoretical rationale for the observed linearity of this correlation
at this point.
[LOCH V ]. The UV-vis spectra of these species in CH3CN
III
3
solution are shown in Figure 3. In the visible region, the VV
and VIV species exhibit intense phenolate-to-vanadium charge
transfer bands which are shifted bathochromically with increas-
ing oxidation state and increase in intensity at the same time.
Note that the three spectra display an absorption minimum at
380-400 nm. Coordinated phenoxyl radical complexes display
an intense maximum (ꢀ > 4 × 103 L mol-1 cm-1) in this
region.10,11,14 The absence of such maxima in the spectra of
[LVV]2+ and [LVIV]+ supports therefore the assumption that the
two successive one-electron oxidations of [LVIII] are metal-
rather than ligand-centered.
Complex 9 contains only one oxidizable coordinated pheno-
late group, and consequently, the CV of 9 (Figure S4 (Sup-
porting Information)) displays only one reversible one-electron-
transfer wave at 0.46 V in the potential range +0.9 to -0.20
V. At more negative potentials, an irreversible reduction at Ep,red
) -0.80 V is observed which we assign to a metal-centered
reduction of the cobalt(III) to a labile cobalt(II) ion. Figure 6
(top) shows the spectral changes observed upon electrochemical
one-electron oxidation of red-brown 9 yielding the paramagnetic
violet dication [LPrCoIII(acac)]•2+. The intense new absorption
maxima at 405 and 545 nm indicate that a phenoxyl radical
has been generated.
(19) (a) Buchanan, R. M.; Pierpont, C. G. J. Am. Chem. Soc. 1980, 102,
4951. (b) Adams, D. M.; Dei, A.; Rheingold, A. L.; Hendrickson, D.
N. Angew. Chem. 1993, 105, 954; Angew. Chem., Int. Ed. Engl. 1993,
32, 880.
(20) Bonadies, J. A.; Carrano, C. J. J. Am. Chem. Soc. 1986, 108, 4088.

Phenolato-Vanadium and -Cobalt Complexes
Table 4. Electronic Spectral Data for the Complexes
Inorganic Chemistry, Vol. 36, No. 17, 1997 3707
complex
solvent
λ )
max, nm (ꢀ, L mol-1 cm-1
1
2
3
4
5
6
7
8
9
10
CH3OH
CH3OH
CH3OH
CH3OH
CH3CN
CH3OH
CH2Cl2
CH3CN
CH3CN
CH3CN
576 (590), 380 sh, 271 (1.6 × 104), 216 (1.7 × 104)
570 (1.4 × 104), 420 sh, 270 (1.9 × 104)
575 (9.0 × 103), 420 (5.0 × 103), 274 (1.3 × 104), 217 (2.7 × 104)
580 (7.5 × 103), 430 sh, 320 sh, 273 (1.5 × 104)
634 (1.0 × 104), 459 (7.7 × 103), 324 sh, 282 (2.5 × 104), 252 (5.0 × 104)
602 (1.0 × 103), 404 (1.4 × 103), 262 (2.9 × 104), 214 (2.9 × 104)
578 (480), 480 sh, 410 (3.1 × 103), 368 (2.9 × 103), 317 sh, 272 (2.7 × 104), 258 (2.6 × 104)
592 (650), 450 sh, 380 (3.8 × 103), 330 sh, 320 sh, 278 (2.5 × 104), 254 sh
699 (220), 490 (1.1 × 103), 289 (1.9 × 104), 241 (1.9 × 104)
502 (800), 307 sh (1.7 × 104), 242 (3.9 × 104)
Electrochemically Generated Complexes
[LOCH V ]
CH3CN
CH3CN
CH2Cl2
CH3CN
CH3CN
CH3CN
CH3CN
609 (1.5 × 103), 392 sh, 287 (3.0 × 104), 252 (3.6 × 104)
III
3
[LOCH V ]
870 (3.2 × 104), 604 sh, 324 sh, 282 (2.6 × 104), 252 (5.2 × 104)
V
2+
3
[LBuCoIII]•+ b
760 (1.4 × 103), 398 (4.8 × 103), 382 sh, 286 (2.4 × 104), 256 (2.1 × 104)
678 (1.6 × 103), 418 sh, 402 (4.9 × 103), 288 (2.1 × 104), 246 (2.1 × 104)
545 (1.5 × 103), 405 (4.7 × 103), 304 (2.5 × 104), 270 sh (2.1 × 104), 240 sh (2.0 × 104)
641 (2.5 × 103), 426 (4.0 × 103), 406 (3.1 × 103), 327 (1.6 × 104), 244 (3.5 × 104)
524 (4.3 × 103), 432 sh (4.5 × 103), 412 (7.2 × 103), 392 sh (5.2 × 103), 314 (2.8 × 104)
[LOCH Co ]
III •+ a
3
[9]•2+
[10]•+
[10]2•2+ a
a Generated by controlled-potential electrolysis at -30 °C in CH3CN (0.10 M [TBA]PF6) solution or in CH2Cl2.
b
Figure 3. Electronic spectra of 5 and its electrochemically reduced
III
OCH
V
[LOCH V ] and oxidized [L
V ]2+ forms in CH3CN (0.10 M [TBA]-
3
3
PF6).
Replacement of the redox-innocent acetylacetonate ligand in
9 by the dianionic tetrachlorocatecholate affording 10 has an
interesting effect on the corresponding CV shown in Figure S4
(bottom). In the potential range +1.1 to -0.50 V, two reversible
one-electron-transfer waves are observed, and at very negative
potentials, an irreversible process is detected with Ep,red ) -1.3
V and Ep,ox ) -0.70 V, which is assigned to the CoIII/CoII
couple. Coulometric measurements established unambiguously
that both reversible one-electron processes correspond to
oxidations of 10. Figure 6 (bottom) shows the electronic spectra
of the parent complex 10, as well as its one- and two-electron-
oxidized forms, respectively. The two oxidized species were
generated at controlled potentials of 0.35 and 0.8 V Vs Fc+/Fc,
respectively, and their spectra were recorded in CH3CN solution
containing 0.10 M [TBA]PF6. Oxidation of aubergine 10 to
its monocation [10]•+ is accompanied by a color change to blue-
green. In the electrochemically generated paramagnetic mono-
cation [10]•+, either the catecholate is oxidized, to the corre-
sponding semiquinonate or the phenolate is oxidized yielding
a phenoxyl ligand. The electronic spectrum in conjunction with
the EPR and resonance Raman spectra (see below) of [10]•+
suggests that the catecholate ligand is oxidized to give the
monocation [LPrCoIII(Cl4-semiquinonate)]•+. In the electronic
spectrum of [10]•+, three intense new absorption maxima at 406,
426, and 641 nm are observed which do not resemble the
features in the spectrum of [9]•2+ containing a phenoxyl ligand.
Further one-electron oxidation of [10]•+ affording [10]2•2+
induces a dramatic change in the electronic spectrum, and a
color change to violet is observed. A very intense absorption
Figure 4. Square-wave voltammogram (top) in CH2Cl2 and cyclic
voltammograms (bottom) in CH3CN (0.10 M [TBA]PF6) of 7 and 8,
respectively, at a glassy carbon working electrode.
maximum at 412 nm and one at ∼520 nm indicate that the
phenolate has now been converted to a phenoxyl ligand. Thus,
the dication [10]2•2+ contains a semiquinonate and a phenoxyl
radical coordinated to a cobalt(III) ion.
EPR Spectroscopy. X-band EPR spectra of the electro-
chemically generated radical complexes [9]•2+, [10]•+, and
[10]2•2+ in CH3CN solution containing 0.10 M [TBA]PF6 or
[TBA]ClO4 supporting electrolyte are shown in Figure 7.
Because of the short lifetime of electrochemically generated
[7]•+ and [8]•+ at 20 °C, the EPR spectra were recorded at 77
K. Under these conditions, a broad unresolved signal at g )
1
2.004 (CH3CN solution) indicative of the expected S ) /2
ground state was observed. These spectra are very similar to
III •+ 10
the spectrum reported for [LOCH Ga ] , although the width
3
of the signals is significantly broader than that of the latter.
This is probably due to an unresolved 59Co hyperfine splitting.

3708 Inorganic Chemistry, Vol. 36, No. 17, 1997
Sokolowski et al.
Figure 5. Correlation of the redox potentials, E31/2, of the couples
+
OCH
3
[LOCH M] /[L
M] Versus the Ophen-M-Ophen bond angle R. This
3
line is a linear regression.
Figure 7. X-band EPR spectra of electrochemically generated (a) [9]•2+
at 295 K, (b) [10]•+ at 295 K, and (c) [10]2•2+ at 5 K in CH3CN solution.
The dotted lines represent simulations with g values and hyperfine
coupling constants given in the text. Microwave power: (a, b), 5 mW;
(c) 1 mW. Modulation amplitude: (a, b), 0.3 mT; (c) 1.5 mT.
10 yields a (semiquinonato)cobalt(III) species. A number of
such species have been characterized previously by EPR
spectroscopy, and in all cases where the semiquinonate is
coordinated to cobalt(III) as a didentate ligand, cobalt hyperfine
coupling constants in the range 9-12 G have been
reported.19a,21-23
X-band spectra of the diradical [10]2•2+, which was electro-
chemically prepared at -30 °C in acetonitrile solution (0.10
[TBA]PF6), were measured in frozen solution only. The
observed absorption derivative traces in the range 3-150 K are
a superposition of two subspectra (Figure 7c): a very broad
pattern in the field range 0-400 mT and a narrow isotropic
resonance composed of eight weakly resolved hyperfine lines
(a ) 20 G) centered at g ) 2.0. The prominent g ) 2 line was
the only detectable feature at room temperature, but its intensity
was found to be sample-preparation dependent. Quantitation
of this signal revealed in “bad” preparations 0.9 spin per
molecule which decreased to 0.1 spin per molecule in the most
elaborate, low-temperature preparations of [10]2•2+. Therefore,
we discard this half-integer-spin signal and attribute it to [10]•+
or to some other decomposition product.
The appearance of the broad subspectrum with two extremely
broad lines at 270 and 400 mT in the vicinity of g ) 2 and a
narrow trough at g ) 4 is typical of a spin-triplet spectrum with
zero-field splitting on the order of the microwave quantum (D
≈ hν; 0.3 cm-1 at X-band). The feature at g ) 4.12 is then
assigned to a ∆m ) 2 transition. The triplet character of the
spectrum proves the presence of the diradical [10]2•2+, which
Figure 6. Top: Spectral changes observed during electrochemical one-
electron oxidation of 9 in CH3CN (0.10 M [TBA]ClO4) at 20 °C.
Bottom: Electronic spectra of 10 (20 °C) and electrochemically
generated [10]•+ (20 °C) and [10]2•2+ (-30 °C) in CH3CN (0.10 M
[TBA]PF6).
The spectrum of [9]•2+ displays a signal at g ) 2.0047.
Hyperfine coupling to the 59Co nucleus (I ) 7/2), aCo ) 6.0 G,
and to one benzylic proton, aH ) 6.55 G, is detected. Using
these parameters with a Gaussian line width of 2.27 G, the
resulting simulation is shown in Figure 7a (dotted line). The
spectrum clearly indicates the presence of a coordinated
phenoxyl radical where the two benzylic protons are magneti-
cally not equivalent (as in its reduced form 9, for which the 1H
NMR spectrum shows two diastereotopic protons). The hy-
perfine splitting to the second benzylic proton and to the
aromatic ring proton in a meta position is not resolved.
The spectrum of the corresponding radical [10]•+ shown in
Figure 7b shows a simple eight-line hyperfine pattern at g )
2.0044. The hyperfine splitting constant, aCo, was determined
to be 11.0 G. A simulation using this parameter and a Gaussian
line width of 4.70 G is shown in Figure 7b (dotted line).
Clearly, no hyperfine coupling to a benzylic proton is observed.
We take this as good evidence that one-electron oxidation of
(21) Wicklund, P. A.; Beckmann, L. S.; Brown, D. G. Inorg. Chem. 1976,
15, 1996.
(22) Kessel, S. L.; Emberson, R. M.; Debrunner, P.; Hendrickson, D. N.
Inorg. Chem. 1980, 19, 1170.
(23) Lange, C. W.; Conklin, B. J.; Pierpont, C. G. Inorg. Chem. 1994, 33,
1276.

Phenolato-Vanadium and -Cobalt Complexes
Inorganic Chemistry, Vol. 36, No. 17, 1997 3709
coordination to a metal ion, this mode shifts to lower frequencies
and the ν7a/ν8a intensity ratio is nearly reversed.11,25 The RR
spectrum of [9]•2+ reveals no intense band at ∼1510 cm-1, but
a weak shoulder of the 1468 cm-1 peak at 1487 cm-1 is
attributable to the ν7a mode. Its low intensity as well as its
frequency separation relative to the ν8a mode of 125 cm-1 is
consistent with previous findings on coordinated phenoxyl
radicals.11,25 In this respect, the spectrum of [9]•2+ resembles
those of related phenoxyl-metal complexes, but the relative
RR intensities of the bands at 1468, 1452, 1430, and 1401 cm-1
are significantly stronger than those observed previously. This
effect may be due to the differing resonance conditions. In the
present case, the RR spectra were excited at the low-energy
side of the phenoxyl π f π* transition whereas the other spectra
were recorded with excitation at the high-energy side.27
It is important that the RR spectrum of [10]•+ does not
resemble that of [9]•2+. The observed features at 1524, 1475,
1464 (sh), 1345, and 1222 cm-1 do not correspond to vibrations
of a coordinated phenoxyl radical. We assign these frequencies
to modes of a coordinated tetrachlorosemiquinonate, where the
band at 1524 cm-1 is due to a C-O stretching frequency, in
agreement with previous IR spectroscopic measurements on
p-semiquinonates.22,28 Tripathi et al. have also shown that this
mode dominates the RR spectrum of a m-semiquinonate
radical.29 Presumably, this intensity distribution is similar for
o-semiquinonates. Regardless of a detailed assignment of all
observed bands, it can be safely concluded that the monocation
[10]•+ does not contain a phenoxyl ligand.
Figure 8. Resonance Raman spectra of electrochemically generated
[9]•2+ (top) and [10]•+ (bottom) in CH3CN (0.10 M [TBA]ClO4 and
[TBA]PF6, respectively) at 298 K (λexc ) 427 nm).
exhibits intramolecular spin coupling. This broad spectrum was
observed from 2.8 to 150 K without significant changes in line
shape. From the variation of the intensity as a function of the
temperature, as was determined from single integrations of the
trough signal at g ) 4.12 in the range 6-120 K with 50 µW
power, the singlet-triplet splitting of the diradical was estimated
Discussion
Octahedral tris(phenolato)metal complexes containing the
to be +24 ((10) cm-1
.
This energy value indicates an
3-
3
]
pendent-arm macrocyclic ligands [L
]
Me 3-, [LBu]3-, and [LOCH
intramolecular ferromagnetic coupling of the semiquinonate and
the phenoxyl radical with a coupling constant J ) +12 ((5)
cm-1 using the spin-Hamiltonian H ) -2JSsemiquin‚Sphenoxyl
(Ssemiquin ) Sphenoxyl ) 1/2). It is interesting to note that in tris-
(semiquinonato)cobalt(III) complexes the semiquinonate-semi-
quinonate coupling has been found to be antiferromagnetic.23
display a rich and diverse redox chemistry even when the central
metal ion itself is redox inactive, e.g. ScIII, GaIII, or ZnII.10,11,25
It has been shown that in the latter instances the coordinated
phenolates undergo reversible one-electron oxidations yielding
coordinated phenoxyl species provided that the ortho and para
positions of the aromatic rings are protected by sterically
demanding groups such as a tert-butyl or methoxy group. Note
that two methyl groups in these positions on each ring as in
1
An S ) /2 ground state has been observed.
Resonance Raman (RR) Spectra. Figure 8 displays the RR
spectra of electrochemically generated species [9]•2+ and of
[10]•+ in acetonitrile solution (0.10 M [TBA]PF6 or [TBA]ClO4)
recorded with excitation at 427 nm in resonance with the
corresponding absorption maxima of both species shown in
Figure 6.
[LMe 3- are not sufficient to stabilize the phenoxyl radicals.30
]
For instance, the CV of [LMeScIII] in CH3CN displays two
irreVersible oxidation waves at quite anodic potentials, whereas
III
for the corresponding complexes [LBuScIII] and [LOCH Sc ] three
3
reVersible one-electron-transfer waves in the same potential
The spectrum of [9]•2+ exhibits features at 1593, 1468, 1452,
range have been reported, respectively.10
.
1430, and 1401 cm-1 The band at 1593 cm-1 is readily
The situation in the above cases is quite clear and unequivo-
assigned to the CdC stretching mode (ν8a), which in uncoor-
dinated phenoxyl radicals is found at 1585 cm-1 (2,4-dimeth-
ylphenoxyl)24 but in phenoxyl-metal complexes is found at
higher frequencies.11,25 Hence, the 7 cm-1 upshift in the
spectrum of [9]•2+ points to a coordinated phenoxyl radical.
Further support of this notion comes from the behavior of the
C-O stretching mode ν7a and its RR intensity relative to that
of the ν8a mode. For free phenoxyl radicals the ν(C-O)
cal; i.e., the UV-vis, EPR, and RR spectra of the corresponding
1
monocations [LM]•+ with an S )
/ ground state clearly
2
demonstrate the presence of one coordinated phenoxyl radi-
cal.10,11,25 If the central metal ion is a transition metal with a
dn (n is >1 but <10) electron configuration, it is not a priori
possible to discern one-, two-, and three one-electron oxidations
between ligand- and metal-centered processes and each case
has to be carefully assessed by spectroscopy (or, if possible,
by crystallography).
vibration is found above ∼1510 cm-1 26
, with an approximately
An interesting example in this respect is the complex [LOCH
-
10-fold intensity compared to that of the ν8a mode. Upon
3
CrIII], which undergoes three reversible, successive one-electron-
(24) (a) McGlashen, M. L.; Eads, D. D.; Spiro, T. G.; Whittaker, J. W. J.
Phys. Chem. 1995, 99, 4918. (b) Mukherjee, A.; McGlashen, M. L.;
Spiro, T. G. J. Phys. Chem. 1995, 99, 4912.
(25) Sokolowski, A.; Leutbecher, H.; Weyhermu¨ller, T.; Schnepf, R.; Bothe,
E.; Bill, E.; Hildebrandt, P.; Wieghardt, K. J. Biol. Inorg. Chem., in
press.
(26) (a) Chipman, D. M.; Liu, R.; Zhou, R.; Pulay, P. J. Chem. Phys. 1994,
7, 5023. (b) Johnson, C. R.; Ludwig, M.; Asher, S. A. J. Am. Chem.
Soc. 1986, 108, 905. (c) Tripathi, G. N. R.; Schuler, R. H. J. Phys.
Chem. 1988, 92, 5129.
(27) Tripathi, G. N. R.; Schuler, R. H. J. Chem. Phys. 1984, 81, 113.
(28) Matsunaga, Y. J. Chem. Phys. 1964, 41, 1609.
(29) (a) Tripathi, G. N. R.; Chipman, D. M.; Miderski, C. H.; Davis, H.
F.; Fessenden, R. W.; Schuler, R. H. J. Phys. Chem. 1986, 90, 3968.
(b) Tripathi, G. N. R. In Time ResolVed Spectroscopy; Clark, R. J. H.,
Hester, R. E., Eds.; John Wiley and Sons: New York, 1989; p 184.
(30) Altwicker, E. R. Chem. ReV. 1967, 67, 475.
(31) Sokolowski, A.; Bothe, E.; Bill, E.; Weyhermu¨ller, T.; Wieghardt, K.
J. Chem. Soc., Chem. Commun. 1996, 1671.

3710 Inorganic Chemistry, Vol. 36, No. 17, 1997
Sokolowski et al.
transfer oxidations yielding a mono-, di-, and tricationic species.
Complexes 9 and 10 represent interesting cases because the
latter contains two different oxidizable ligands, namely a
coordinated phenolate and a catecholate ligand. Species [9]•+
shows the spectroscopic features of a coordinated phenoxyl
radical whereas the monooxidized form of 10, namely [10]•+,
shows features in the EPR and RR spectrum typical of a
coordinated semiquinonate ligand. These spectral characteristics
of a bound phenoxyl and a semiquinonate differ significantly
and allow their unequivocal identification. It is also of
significance that the two ligand radicals in [10]2•2+ (each with
S ) 1/2) are intramolecularly ferromagnetically coupled, yielding
an S ) 1 ground state. This type of coupling is most probably
due to the fact that the two magnetic orbitals are orthogonal in
the dication diradical.
[LOCH Cr](ClO4) has been characterized by X-ray crystal-
3
lography.31 It was established that it contains two coordinated
phenolates, one coordinated phenoxyl radical, and a chromium-
(III) ion. The metrical details of the phenolate and phenoxyl
ligands differ significantly, and gratifyingly, the experimentally
observed C-C and C-O distances of the phenoxyl radical in
[LOCH CrIII]•+ agree nicely with those reported for phenoxyl and
3
tyrosyl from density functional calculations.32
In contrast, one-electron oxidation of [L′VIII], where L′
represents 1,4,7-tris(5-tert-butyl-2-hydroxybenzyl)-1,4,7-triaza-
cyclononane, also yields a monocation but the crystal structure
of [L′V]BPh4 shows unambiguously that the monocation
contains three equivalent coordinated phenolates and a central
VIV. In this case, clearly, a metal-centered oxidation prevails.4
The present study on the analogous complexes 1, 2, 4, and 5
underlines this point. The UV-vis spectral changes observed
on going from VIII to VIV to VV cannot be accounted for by
involving ligand-centered phenoxyl formation. All the data
available point to metal-centered redox processes.
Acknowledgment. We thank the Fonds der Chemischen
Industrie for financial support Professor Y. Sasaki for granting
a leave of absence to A.K. P.H. is grateful to the Deutsche
Forschungsgemeinschaft for a Heisenberg stipend.
The cobalt(III) complexes 7 and 8, on the other hand, display
three reversible one-electron-transfer steps yielding mono-, di-,
and trications, respectively. The monocations [7]•+ and [8]•+
are stable enough in solution at -30 °C to allow their
spectroscopic characterization. Both possess an S ) 1/2 ground
state, as was established by their EPR spectra. The UV-vis
spectra of these species demonstrate that a coordinated phenoxyl
ligand is present. Thus, the oxidation state +IV for the central
cobalt ion is not accessible in the potential range investigated.
Supporting Information Available: Figure S1, showing the EPR
spectrum of 5, Figure S2, displaying the square-wave voltammogram
of 5, Figure S3, exhibiting the spectral changes observed upon one-
electron oxidation of 7 and 8, respectively, Figure S4, showing the
cyclic voltammograms of 9 and 10, and tables of crystallographic and
structure refinement data, atom coordinates and Ueq values, bond lengths
and angles, anisotropic thermal parameters, and calculated positional
parameters of hydrogen atoms for complexes 9 and 10 (17 pages).
Ordering information is given on any current masthead page.
(32) Qin, Y.; Wheeler, R. A. J. Am. Chem. Soc. 1995, 117, 6083.
IC970256E