Anilino radical complexes of cobalt(III) and manganese(IV) and comparison with their phenoxyl analogues

Article abstract of DOI:10.1021/ja001637l

The pendent arm macrocyclic aniline ligands 1-(2-amino-3,5-di-tert-butylbenzyl)-4,7-dimethyl-1,4,7-triazacyclononane, H[L¹], and 1,4,7-tris(2-amino-3,5-di-tert-butylbenzyl)-1,4,7-triazacyclononane, H₃[L²], have been synthe

Full text of DOI:10.1021/ja001637l

J. Am. Chem. Soc. 2000, 122, 9663-9673
9663
Anilino Radical Complexes of Cobalt(III) and Manganese(IV) and
Comparison with Their Phenoxyl Analogues
Frank N. Penkert, Thomas Weyhermu1ller, Eckhard Bill, Peter Hildebrandt,
Sophie Lecomte, and Karl Wieghardt*
Contribution from the Max-Planck-Institut fu¨r Strahlenchemie, D-45413 Mu¨lheim an der Ruhr, Germany
ReceiVed May 12, 2000
Abstract: The pendent arm macrocyclic aniline ligands 1-(2-amino-3,5-di-tert-butylbenzyl)-4,7-dimethyl-1,4,7-
triazacyclononane, H[L1], and 1,4,7-tris(2-amino-3,5-di-tert-butylbenzyl)-1,4,7-triazacyclononane, H3[L2], have
been synthesized. The reaction of these ligands in methanol (1:1) and/or ethanol with CoII(ClO4)2‚6H2O and
potassium di-tert-butylacetylacetonate (1:1) or MnII(acetate)2‚4H2O in the presence of air produced the anilido
complexes [CoIII(L1)(Bu2acac)](ClO4) (1) and [MnIV(L2)]X (X ) ClO4 (2), BPh4 (2′)), respectively. The reaction
of H3[L2] and CuCl2‚2H2O in ethanol produced upon addition of NaClO4 blue crystals of [CuII(H3L2)Cl]-
(ClO4) (3) containing two coordinated and one uncoordinated aniline pendent arm. Complexes 1, 2, and 3
have been structurally characterized by X-ray crystallography. Electrochemically, 1 is reversibly oxidized with
1
formation of the paramagnetic anilino radical species [CoIII(L1•)(Bu2acac)]2+ (S ) /2) whereas the cyclic
voltammogram of 2 displays three ligand-centered one-electron-transfer oxidation waves where complexes
[MnIV(L2•)]2+, [MnIV(L2••)]3+, and [MnIV(L2•••)]4+ containing one, two, and three coordinated anilino radicals
are formed successively. These anilino radical species have been characterized by UV-vis, X-band EPR, and
resonance Raman spectroscopy. The characteristic spectroscopic features of coordinated anilino radicals have
been elucidated. In the MnIV radical species strong intramolecular exchange coupling between the metal ion
3
(t2g configuration, S ) 3/2) and one, two, or three anilino radicals (Srad ) 1/2) produces the ground states St
) 1, St ) 1/2, and St ) 0 in the di-, tri-, and tetracation of the parent complex 2, respectively.
Introduction
compounds, namely, the anilino radical, Ph-NH, and its
protonated radical cation, [Ph-N˙ H2]+, have been generated by
pulse radiolysis and studied by UV/vis,6 EPR,7 and RR
spectroscopy8 in aqueous solution as well as by computational
methods,9 their coordination chemistry with transition metal ions
has been plagued by the inherent ambiguity of assigning the
correct oxidation level to both the ligand (aminyl vs amide)
and the metal ion. (The nomenclature used in the literature is
slightly confusing. In coordination compounds we designate a
coordinated Ph-NH2 moiety as aniline (amine) and its monoan-
ion Ph-NH- as anilido whereas the coordinated neutral radical
Ph-NH is designated as anilino following organic nomencla-
ture.) The question arises whether the structures A and B
Tyrosyl radicals coordinated or uncoordinated to a transition
metal ion have been identified in a steadily increasing number
of metalloproteins as essential cofactors.1 Consequently, inor-
ganic chemists have studied in depth the coordination chemistry
of (phenoxyl)metal complexes and identified spectroscopic
markers for this class of radical compounds.2 The most effective
spectroscopic tools in this respect proved to be resonance
Raman3 and EPR spectroscopy4 in conjunction with X-ray
absorption spectroscopy (XAS).5
The coordination chemistry of the isoelectronic anilino radical
has in the past received much less attention. While the parent
(1) (a) Chaudhuri, P.; Wieghardt, K. Progr. Inorg. Chem., in press. (b)
Stubbe, J. A. Biochemistry 1988, 27, 3893. (c) Stubbe, J. A. Annu. ReV.
Biochem. 1989, 58, 257. (d) Metal Ions in Biological Systems; Sigel, H.,
Sigel, A., Eds.; Marcel Dekker, Inc.: New York, 1994; Vol. 30.
(2) (a) 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. (b) Mu¨ller, J.; Kikuchi, A.; Bill, E.;
Weyhermu¨ller, T.; Hildebrandt, P.; Ould-Moussa, L.; Wieghardt, K. Inorg.
Chim. Acta 2000, 297, 265. (c) Sokolowski, A.; Mu¨ller, J.; Weyhermu¨ller,
T.; Schnepf, R.; Hildebrandt, P.; Hildenbrand, K.; Bothe, E.; Wieghardt,
K. J. Am. Chem. Soc. 1997, 119, 8889. (d) Sokolowski, A.; Leutbecher,
H.; Weyhermu¨ller, T.; Schnepf, R.; Bothe, E.; Bill, E.; Hildebrandt, P.;
Wieghardt, K. J. Biol. Inorg. Chem. 1997, 2, 444. (e) Sokolowski, A.; Adam,
B.; Weyhermu¨ller, T.; Kikuchi, A.; Hildenbrand, K.; Schnepf, R.; Hilde-
brandt, P.; Bill, E.; Wieghardt, K. Inorg. Chem. 1997, 36, 3702.
(3) Schnepf, R.; Sokolowski, A.; Mu¨ller, J.; Bachler, V.; Wieghardt, K.;
Hildebrandt, P. J. Am. Chem. Soc. 1998, 120, 2352.
represent two resonance structures of a single electronic ground
state or, alternatively, are they two different species (redox
(6) (a) Qin, L.; Tripathi, G. N. R.; Schuler, R. H. Z. Naturforsch. 1985,
40a, 1026. (b) Wigger, A.; Gru¨nbein, A.; Henglein, A.; Land, E. J. Z.
Naturforsch. 1969, 24b, 1262. (c) Christensen, H. Int. J. Radiat. Phys. Chem.
1972, 4, 311.
(4) Babcock, G. T.; El-Deeb, M. K.; Sandusky, P. O.; Whittaker, M.
M.; Whittaker, J. W. J. Am. Chem. Soc. 1992, 114, 3727.
(5) (a) Clark, K.; Penner-Hahn, J. E.; Whittaker, M. M.; Whittaker, J.
W. J. Am. Chem. Soc. 1990, 112, 6433. (b) Clark, K.; Penner-Hahn, J. E.;
Whittaker, M. M.; Whittaker, J. W. Biochemistry 1994, 33, 12553. (c)
Knowles, P. F.; Brown, R. D., III; Koenig, S. H.; Wang, S.; Scott, R. A.;
McGuirl, M. A.; Brown, D. E.; Dooley, D. M. Inorg. Chem. 1995, 34,
3895. (d) Wang, Y.; DuBois, J. L.; Hedman, B.; Hodgson, K. O.; Stack, T.
D. P. Science 1998, 279, 537.
(7) (a) Neta, P.; Fessenden, R. W. J. Chem. Phys. 1974, 78, 523. (b)
Land, E. J.; Porter, G. J. Chem. Soc. 1961, 3540. (c) Atherton, N. M.; Land,
E. J.; Porter, G. Trans. Faraday Soc. 1963, 59, 818.
(8) (a) Tripathi, G. N. R.; Schuler, R. H. J. Chem. Phys. 1987, 86, 3795.
(b) Tripathi, G. N. R.; Schuler, R. H. Chem. Phys. Lett. 1984, 110, 542.
(9) Adamo, C.; Subra, R.; DiMatteo, A.; Barone, V. J. Chem. Phys. 1998,
109, 10244.
10.1021/ja001637l CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/22/2000

9664 J. Am. Chem. Soc., Vol. 122, No. 40, 2000
Scheme 1. Complexes from Reference 10
Penkert et al.
Scheme 2. Ligands and Complexes
isomers) with two distinctly different ground states. For example,
in 1982 Sellmann et al. reported a series of organometallic
compounds which they described as (aminyl)manganese(I)
complexes (Scheme 1).10 Kaim et al.11 in 1985 showed that the
spectroscopic data (EPR, UV/vis) of these compounds are
probably more in agreement with the formulation as (amido)-
manganese(II) species, where the manganese(II) has a low-spin
configuration (S ) 1/2).
Thus the present paper represents the first report on transition
metal ion compounds that unambiguously contain coordinated
anilino radicals. The parent anilino radical is a very reactive
intermediate that undergoes rapid radical combination reactions.
The radical cation, Ph-N˙ H2+, combines by way of C-C or
C-N coupling whereas the neutral radical, Ph-N˙ H, undergoes
C-N and N-N couplings.12 To suppress these reactions and
thereby increase the lifetime of the radicals, the ortho and para
positions of the aniline moiety must be protected by bulky
substituents. The 2,4,6-tri-tert-butylanilino radical is a stable
species in solution.7b
potentials in the cyclic voltammogram. At that time we
speculated that these might be metal-centered processes involv-
ing MnV and MnVI, but it was noted that they might also be
ligand-centered processes. If the latter is the case, these
oxidations are irreversible because the generated unprotected
anilino radicals undergo rapid combination reactions as de-
scribed above for the parent anilino radical.
In this work we have synthesized the ligands 1-(2-amino-
3,5-di-tert-butylbenzyl)-4,7-dimethyl-1,4,7-triazacyclononane,
H[L1], and 1,4,7-tris(2-amino-3,5-di-tert-butylbenzyl)-1,4,7-
triazacyclononane, H3[L2], and prepared the precursor complexes
[(L1)CoIII(Bu2acac)](ClO4) (1), [(L2)MnIV](ClO4) (2), and [(H3L2)-
CuCl]ClO4 (3) as shown in Scheme 2. The corresponding
phenolato complexes [(LPr)CoIII(acac)]ClO42e and [(LBu)MnIV]-
Experimental Section
Preparation of Ligands. 3,5-Di-tert-butyltoluene16 and 3,5-di-tert-
butyl-2-nitrotoluene17 were synthesized according to published proce-
dures.
2a
PF6 have been described previously. It has been shown that
each of the coordinated phenolato groups can be converted via
a reversible one-electron oxidation to a coordinated phenoxyl
ligand. H[LPr] represents the ligand 1,4-diisopropyl-7-(3,5-di-
tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, which was
originally prepared by Tolman et al.,13 and H3[LBu] is 1,4,7-
tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclono-
nane.2a We will compare the spectroscopic features of coordi-
nated anilino ligands with those of the corresponding coordinated
phenoxyls.
The first indication that anilino radical complexes might be
stable was obtained in 1995 in our laboratory when we reported
that the tris(anilido)manganese(IV) complex [MnIV(La)]BPh4,14
where H3[La] is 1,4,7-tris(o-aminobenzyl)-1,4,7-triazacyclono-
nane,15 which is unprotected at the ortho and para positions,
displays three irreversible oxidation waves at quite positive
3,5-Di-tert-butyl-2-nitrobenzylbromide. A solution of 3,5-di-tert-
butyl-2-nitrotoluene (24.9 g; 0.10 mol) and N-bromosuccinimide (NBS)
(17.8 g; 0.1 mol) in CCl4 (250 mL) was heated to reflux, and
azobisisobutyronitrile was added as a radical starter. The solution was
stirred without heating but with irradiation (white light) until all of the
generated succinimide floated on top of the solution. To the filtered
solution was added a saturated aqueous NaHCO3 solution with stirring.
The phases were separated, and the organic phase was dried over
MgSO4. After removal of the solvent under reduced pressure the residue
was recrystallized from methanol and then from n-pentane. Yield: 23.0
g (70%). EI mass spectrum: 329 m/z {M+}, mp 77 °C. 1H NMR-
(CDCl3): δ ) 1.31 (s, 9H, t-Bu), 1.37 (s, 9H, t-Bu), 4.32 (s, 2H, CH2),
7.35 (s, 1H, aromatic proton), 7.51 (s, 1H, aromatic proton). 13C NMR-
(CDCl3): δ ) 27.4 (CH2), 30.9 (t-Bu, CH3), 31.1 (t-Bu, CH3), 35.1
(t-Bu, C4), 36.1 (t-Bu, C4), 126.1 (Ph-H), 126.3 (Ph-H), 129.0, 140.5,
153.1.
(10) (a) Sellmann, D.; Mu¨ller, J.; Hofmann, P. Angew. Chem. 1982, 94,
708; Angew. Chem., Int. Ed. Engl. 1982, 21, 691. (b) Sellmann, D.; Mu¨ller,
J. J. Organomet. Chem. 1985, 281, 249.
(11) (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.
(15) (a) Schlager, O.; Wieghardt, K.; Nuber, B. Inorg. Chem. 1995, 34,
6449. (b) Schlager, O.; Wieghardt, K.; Grondey, H.; Rufinska, A.; Nuber,
B. Inorg. Chem. 1995, 34, 6440.
(16) Risch, N.; Meyer-Roscher, B.; Langhals, M. Z. Naturforsch. 1994,
49, 141.
(12) Mere´nyi, G.; Lind, J. In N-centered Radicals; Alfassi, Z. B., Ed.;
John Wiley and Sons Ltd.: New York, 1998; p 599.
(13) Halfen, J. A.; Young, V. G.; Tolman, W. B. Angew. Chem., Int.
Ed. Engl. 1996, 35, 1687.
(17) (a) Beets, M. G. J.; Meerburg, W.; von Essen, H. Recl. TraV. Chim.
Pays-Bas 1959, 78, 570. (b) Geuze, J.; Ruinard, C.; Soeterbraenk, J.;
Verkade, P. E.; Wepster, B. M. Recl. TraV. Chim. Pays-Bas 1956, 75, 301.
(c) Myhre, P. C.; Beng, M.; James, L. L. J. Am. Chem. Soc. 1968, 90,
2105. (d) Crivello, J. V. J. Org. Chem. 1981, 46, 3056.
(14) Schlager, O.; Wieghardt, K.; Nuber, B. Inorg. Chem. 1995, 34, 6456.

Anilino Radical Complexes of Co(III) and Mn(IV)
J. Am. Chem. Soc., Vol. 122, No. 40, 2000 9665
1-(3,5-Di-tert-butyl-2-nitrobenzyl)-4,7-dimethyl-1,4,7-triazacy-
clononane. A suspension of 1,4-dimethyl-1,4,7-triazacyclononane18
(0.79 g; 5.0 mmol), 3,5-di-tert-butyl-2-nitrobenzylbromide (2.46 g; 7.5
mmol), and solid KOH (1.7 g; 30 mmol) in toluene (50 mL) was heated
to 80 °C with stirring for 16 h. The solid material was filtered off and
discarded, and the solution was dried over MgSO4. After removal of
the solvent by evaporation under reduced pressure, the residue was
dissolved in methanol (20 mL) to which concentrated HCl (20 mL)
was added. More methanol was added until a clear solution was
obtained, which was extracted with diethyl ether (5 × 20 mL). The
solvent was removed by rotary evaporation from the acidic methanol
solution, and the residue was redissolved in methanol (20 mL) to which
Na[OCH3] was added (neutralization). After addition of aqueous 5 M
KOH (50 mL), the solution was extracted with a large amount of
n-pentane. The solution was dried over MgSO4, and the solvent was
removed. An oily product was obtained. Yield: 1.90 g (94%). EI mass
spectrum: m/z ) 404 (M+). 1H NMR(CDCl3): δ ) 1.31 (s, 9H, t-Bu),
1.36 (s, 9H, t-Bu), 2.33 (s, 6H, CH3), 2.66 (m, 8H, CH2-CH2), 2.77 (s,
4H, CH2-CH2), 3.50 (s, 2H, CH2), 7.40 (d, 1H, J ) 2.0 Hz, aromatic
H), 7.53 (d, 1H, J ) 2.0 Hz, aromatic H). 13C NMR(CDCl3): δ )
31.0 (t-Bu, CH3), 31.2 (t-Bu, CH3), 35.0 (t-Bu, C4), 35.9 (t-Bu, C4),
46.6 (CH3), 56.5, 56.8, 57.0 (CH2-CH2), 58.7 (CH2), 123.7, 125.7,
131.4, 139.5, 148.2, 151.9 (phenyl ring).
and evaporation of the solvent the solid product was obtained. Yield:
6.6 g (85%). EI mass spectrum: m/z ) 780 (M+), mp 212 °C. 1H NMR-
(CDCl3): 1.23 (s, 27H, t-Bu), 1.43 (s, 27H, t-Bu), 2.58 (broad s, 12H,
CH2-CH2), 3.52 (s, 6H, CH2), 5.04 (broad s, 6H, NH2), 6.79 (d, 3H,
J ) 2.1 Hz, Ph-H), 7.20 (d, 3H, J ) 2.1 Hz, Ph-H). 13C NMR-
(CDCl3): δ ) 30.0 (t-Bu, CH3), 31.7 (t-Bu, CH3), 34.0 (t-Bu, C4),
34.4 (t-Bu, C4), 56.3 (CH2-CH2), 64.1 (CH2), 122.6, 123.7 (Ph-H),
125.8, 132.7, 139.2, 142.6 (Ph).
Preparation of Complexes. [CoIII(L1)(Bu2acac)](ClO4) (1). To a
solution of the ligand H[L1] (0.22 g; 0.6 mmol) in methanol was added
Co(ClO4)2‚6H2O (0.16 g; 0.6 mmol) with stirring. The solution was
heated to reflux under argon for 1 h after which time potassium di-
tert-butylacetylacetonate (0.13 g; 0.6 mmol) was added. Stirring in the
presence of air was continued for 2 h. After filtration a green precipitate
formed from the solution, which was collected by filtration. Yield: 52.5
1
mg (12%). EI mass spectrum: m/z ) 714 ([Co(L1)(Bu2acac)]+). H
NMR(CDCl3): δ ) 0.68 (s, 9H, t-Bu), 1.20 (s, 18H, t-Bu), 1.43 (s,
9H, t-Bu), 2.24 (s, 3H, CH3), 2.25 (s, 3H, CH3), 2.47-3.80 (m, 12H,
CH2-CH2), 2.67 (d, 1H, J ) 15.1 Hz, CH2), 4.05 (d, 1H, J ) 15.1 Hz,
CH2), 5.78 (s, 1H, CH), 6.68 (s, 1H, Ph-H), 7.00 (s, 1H, Ph-H). 13
C
NMR(CDCl3): δ ) 27.7, 28.6, 31.1, 31.5 (t-Bu, CH3), 33.9, 35.4, 40.7,
41.0 (t-Bu, C4), 47.0, 47.6 (CH3), 55.8, 56.8, 57.5, 58.9, 59.0, 60.8
(CH2-CH2), 63.3 (CH2), 91.9 (CH), 117.1, 123.0, 123.5, 135.8, 137.7,
148.3 (Ph-H), 198.5, 200.2 (CO). Anal. Calcd for C34H60ClCoN4O6:
C, 57.09; H, 8.45; N, 7.83. Found: C, 56.63; H, 8.65; N, 7.35.
1-(2-Amino-3,5-di-tert-butylbenzyl)-4,7-dimethyl-1,4,7-triazacy-
clononane (H[L1]). The above nitro compound (1.9 g; 4.7 mmol) was
dissolved in dry tetrahydrofurane (THF) (65 mL) to which solid Li-
[AlH4] (1.8 g; 47 mmol) was carefully added with stirring. The mixture
was slowly heated to 80 °C and stirred at this temperature for 16 h. A
saturated aqueous solution (100 mL) of Na2[S2O4] was added. After
20 min the solid was filtered off and washed with water and diethyl
ether. From the filtrate the product was extracted with diethyl ether,
and KOH was added to the aqueous phase (pH ∼ 14) and again
extracted with diethyl ether. The combined ether phase was dried over
MgSO4 and the solvent removed under reduced pressure. The following
workup was done as described above: the residue was redissolved in
CH3OH/HCl and washed with CHCl3 (5 × 20 mL), the solvent was
removed, the residue was dissolved in H2O and NaOH was added, and
extraction was carried out with n-pentane. The product was obtained
as a yellow oil. Yield: 1.37 g (78%). EI mass spectrum: m/z ) 374
[MnIV(L2)]X (X ) ClO4 (2), BPh4‚1toluene (2′)). A solution of
the ligand H3[L2] (0.39 g; 0.5 mmol) and MnII(acetate)2‚4H2O (0.13 g;
0.5 mmol) in ethanol (25 mL) was heated to reflux with stirring for 2
h under an argon atmosphere. To the cooled solution were added a
few drops of triethylamine, and stirring was continued for a few hours
in the presence of air whereupon a color change from green to deep
blue was observed. To this solution NaClO4 (1.15 g) dissolved in ethanol
(50 mL) or, alternatively, Na[BPh4] (0.34 g; 1 mmol) dissolved in
ethanol (50 mL) was added, which initiated the slow precipitation of
deep-blue microcrystals of 2 and 2′ in 8 and 22% yield, respectively.
Single crystals of 2′ suitable for X-ray crystallography were obtained
by slow recrystallization of 2′ from a methanol/toluene mixture (1:1).
ESI mass spectrum (pos. ion): m/z ) 832 ([Mn(L2)]+). Anal. Calcd
for 2′, C82H109BMnN6: C, 79.14; H, 8.83; N, 6.75. Found: C, 78.92;
H, 9.03; N, 6.86.
1
(M+). H NMR(CDCl3): δ ) 1.25 (s, 9H, t-Bu), 1.42 (s, 9H, t-Bu),
[CuII(H3L2)Cl](ClO4)‚CH3OH‚1.5H2O (3). A solution of the ligand
H3[L2] (0.78 g; 1.0 mmol) and CuCl2‚2H2O (0.17 g; 1.0 mmol) in
ethanol (25 mL) was stirred at ambient temperature for 15 min after
which time a solution of NaClO4 (0.70 g) in ethanol (25 mL) was added.
Water (100 mL) was added with stirring, which initiated the precipita-
tion of blue microcrystalline 3. This material was recrystallized from
an aqueous methanol solution. Yield: 0.30 g (31%). ESI mass spectrum
(pos. ion): m/z ) 880 ([Cu(H3L2)Cl]+). Anal. Calcd for C52H91Cl2-
CuN6O6.5: C, 60.13; H, 8.83; N, 8.09. Found: C, 59.76; H, 8.74; N,
8.18.
Crystal Structure Determinations. Green single crystals of 1, and
3‚MeOH‚1.5 H2O and a blue specimen of 2′‚toluene, were picked up
from the mother liquor with a glass fiber and immediately mounted on
a Siemens SMART CCD-detector diffractometer equipped with a
cryogenic nitrogen cold stream to prevent loss of solvent. Graphite
monochromated Mo KR radiation (λ ) 0.71073 Å) was used.
Crystallographic data of the compounds are listed in Table 1. Cell
constants for 1, 2′, and 3 were obtained from a least-squares fit of the
setting angles of 6909, 2856, and 7334 reflections, respectively. Intensity
data were collected at -173(2) °C by a hemisphere run taking frames
at 0.30° in ω. Data were corrected for Lorentz and polarization effects,
and a semiempirical absorption correction for 1 and 3 was carried out
using the program SADABS.19 The Siemens ShelXTL20 software
package was used for solution, refinement, and artwork of the structures.
The structures were solved and refined by direct methods and difference
Fourier techniques. Neutral atom scattering factors of the software
package were used. All non-hydrogen atoms of 1 and 3 were refined
anisotropically, except those of disordered anions and solvent molecules,
2.31 (s, 6H, CH3), 2.60-2.75 (m, 12H, CH2-CH2), 3.60 (s, 2H, CH2),
5.04 (broad s, 2H, NH2), 6.86 (d, 1H, J ) 2.3 Hz, aromatic H), 7.18
(d, 1H, J ) 2.3 Hz, aromatic H). 13C NMR(CDCl3): δ ) 30.0, 31.7
(t-Bu, CH3), 34.0, 34.4 (t-Bu, C4), 46.6 (CH3), 55.1, 57.7, 57.8
(CH2-CH2), 63.5 (CH2), 122.4 (Ph-H), 124.1, 125.7 (Ph-H), 130.0,
132.6, 139.0 (aromatic C).
1,4,7-Tris(3,5-di-tert-butyl-2-nitrobenzyl)-1,4,7-triazacyclono-
nane. A suspension of 1,4,7-triazacyclononane (2.65 g; 20 mmol), 3,5-
di-tert-butyl-2-nitrobenzylbromide (23.0 g; 70 mmol), and solid KOH
(9.0 g; 160 mmol) in toluene (300 mL) was heated at 80 °C for 16 h
with stirring. After filtration the solution was dried over Na2SO4.
Removal of the solvent by rotary evaporation produced a pale yellow
solid, which was recrystallized from methanol. Yield: 11.3 g (65%).
EI mass spectrum: m/z ) 870 (M+), mp 134 °C. 1H NMR(CDCl3): δ
) 1.30 (s, 27H, t-Bu), 1.35 (s, 27H, t-Bu), 2.70 (broad s, 12H, CH2-
CH2), 3.48 (s, 6H, CH2), 7.40 (d, 3H, J ) 2.0 Hz, Ph-H), 7.51 (d, 3H,
J ) 2.0 Hz, Ph-H). 13C NMR(CDCl3): δ ) 31.0 (t-Bu, CH3), 31.2
(t-Bu, CH3), 35.0 (t-Bu, C4), 35.9 (t-Bu, C4), 55.6 (CH2CH2), 58.6 (CH2),
123.8, 125.8 (Ph-H), 131.3, 139.5, 148.2, 152.0 (Ph).
1,4,7-Tris(2-amino-3,5-di-tert-butylbenzyl)-1,4,7-triazacyclono-
nane, H3[L2]. To the solution of the above tris(nitro) derivative (8.71
g; 10 mmol) in THF (150 mL) was added Li[AlH4] (3.8 g; 0.1 mmol)
with stirring. The mixture was slowly heated to 80 °C and stirred at
this temperature for 16 h. The workup was performed as follows: To
the reaction mixture was added a saturated aqueous solution of Na2-
[S2O4] (100 mL). After filtration the product was extracted from the
aqueous phase with diethyl ether. Removal of the solvent produced a
pale yellow residue, which was redissolved in CHCl3. After filtration
(18) Alder, R. W.; Mowlam, R. W.; Vachon, D. J.; Weisman, G. R. J.
Chem. Soc., Chem. Commun. 1992, 507.
(19) SADABS; Sheldrick, G. M., Universita¨t Go¨ttingen, 1994.
(20) ShelXTL V.5; Siemens Analytical X-ray Instruments, Inc. 1994.

9666 J. Am. Chem. Soc., Vol. 122, No. 40, 2000
Penkert et al.
Table 1. Crystallographic Data for 1,2′‚Toluene, and 3‚MeOH‚1.5 H2O
1
2′‚toluene
3‚MeOH‚1.5H2O
chem formula
fw
space group
a, Å
b, Å
c, Å
C34H60ClCoN4O6
715.24
R3h
31.994(5)
31.994(5)
18.828(3)
90
90
120
16 691(4)
18
C82H109BMnN6
1244.50
P21/c
21.101(4)
19.209(4)
18.070(4)
90
98.14(3)
90
7251(3)
4
C52H91Cl2CuN6O6.5
1038.75
P1h
13.814(3)
13.869(3)
18.171(4)
81.48(3)
69.13(3)
61.20(3)
2849.4(11)
2
R, deg
â, deg
γ, deg
V, Å3
Z
T, K
100(2)
1.281
5.81
35 110
4569/3095
444
100(2)
1.140
2.29
19 703
6354/3159
354
100(2)
1.211
5.28
22 724
8981/6612
671
50.0
0.0689
0.1566
F calcd, g cm3
µ(Mo KR), cm-1
refl. collected
unique refl./[I > 2σ(I)]
no. parameters
2θmax, deg
R1a [I > 2σ(I)]
wR2b[I > 2σ(I)]
goodness of fit (F2)
45.0
40.0
0.0731
0.1637
1.064
0.1013
0.2192
0.955
1.040
a R1 ) Σ||Fo| - |Fc||/Σ|Fo|. b wR2 ) [Σ[w(Fo - Fc2)2]/Σ[w(Fo )2]]1/2, where w ) 1/σ2(Fo ) + (aP)2 + bP, P ) (Fo + 2Fc2)/3.
2
2
2
2
which were isotropically refined. Diffraction data of 2′ allowed
anisotropic refinement of the manganese atom only, since the number
of observed reflections was low. Hydrogen atoms were placed at
calculated positions and refined as riding atoms with isotropic displace-
ment parameters. The perchlorate anion and solvent molecules in 3
were found to be disordered. A split model with occupancy factors of
0.7 and 0.3 was used to account for the disorder of the anion, but only
two of four “oxygen positions” of the minor part could be resolved.
Occupancy factors of 0.5 were assigned to two potential methanol
molecules and a water molecule of crystallization. Disorder of the
perchlorate anion in crystals of 1 was treated by a split model with
occupancy factors of 2/3 and 1/3. A residual electron density peak of
2.04 e/Å3 remained in a potential solvent area of about 130 Å3, but
refinement of a water or methanol molecule did not fit well.
Physical Measurements. Electronic spectra were recorded on a
Hewlett-Packard HP 8452A diode array spectrophotometer (range 200-
800 nm). X-band EPR spectra of frozen solutions were measured 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. X-band EPR spectra
at room temperature were measured on a modified Varian EPR
spectrometer (Varian E-9X). The data were digitized by means of the
data station Stelar DS-EPR (Stelar Inc., Male, Italy). We thank Dr. F.
Neese (Abteilung Biologie der Universita¨t Konstanz, Germany) for a
copy of his EPR simulation program (version 1.0, 1994) which we
used for simulations of S ) 1/2 spectra measured in fluid solution.
High-spin spectra measured in frozen solution were simulated by using
the ESIM program package21a S22, S22-I32, S32 fit routines. This
program is based on the IRON_HS routine.21b The ESIM routines
allow automatic optimizations of all parameters via a SIMPLEX
routine.21c NMR spectra were recorded on Bruker Instruments (ARX
250, DRX 400, and DRX 500). Chemical shifts are referenced vs TMS
by using the signals of nondeuterated solvents as internal standards.
Cyclic voltammograms, square-wave voltammograms, and coulometric
experiments were performed on EG&G equipment (potentiostat/
galvanostat model 273A) on Ar flushed CH2Cl2 solutions of samples
containing 0.10 M [N(n-Bu)4]PF6 as supporting electrolyte under an
Ar atmosphere. Potentials were referenced vs Ag/0.10 M AgNO3/CH3-
CN electrode; ferrocene was added as internal standard. Resonance
Raman spectra were recorded with a U 1000 spectrograph (1200 nm
halographic gratings) equipped with a liquid nitrogen cooled CCD
detector (Jobin Yvon, Spex 2). The 514 nm line output of an Ar ion
laser (Innova 400) served as the excitation source. Samples of
approximate optical density of 1.5 at the excitation wavelength were
placed in a rotating quartz cell. Contributions of the solvent and
supporting electrolyte to the spectra were subtracted.
Results
Syntheses. The ligands 1-(2-amino-3,5-di-tert-butylbenzyl)-
4,7-dimethyl-1,4,7-triazacyclononane, H[L1], and 1,4,7-tris(2-
amino-3,5-di-tert-butylbenzyl)-1,4,7-triazacyclononane, H3[L2],
have been prepared by the reaction of di-3,5-tert-butyl-2-nitro-
benzylbromide with either 1,4-dimethyl-1,4,7-triazacyclononane
(1:1) or 1,4,7-triazacyclononane (3:1) in toluene over KOH
(Scheme 3). The resulting nitro compounds were reduced to
the final amino derivatives with Li[AlH4] in tetrahydrofurane.
A methanol solution of H[L1] and [Co(H2O)6](ClO4)2 (1:1)
was heated to reflux under an argon atmosphere. Potassium di-
tert-butylacetylacetonate was added, and the resulting solution
was stirred in the presence of air. From this solution green
microcrystals of diamagnetic [(L1)CoIII(Bu2acac)](ClO4) (1) were
obtained in ∼15% yield.
The reaction of H3[L2] and MnII(acetate)2‚4H2O and a few
drops of NEt3 in ethanol in the presence of air produced a deep
blue solution from which the deep-blue salts [MnIV(L2)]X2 (X
) ClO4 (2), BPh4 (2′)) were obtained in 20% yield upon
addition of NaClO4 or Na[BPh4].
The reaction of H3[L2] with CuIICl2‚2H2O in ethanol yields
upon addition of an aqueous solution of NaClO4 blue crystals
of [(H3L2)CuIICl](ClO4) (3).
Crystal Structures. The crystal structures of complexes 1,
2′, and 3 have been determined by single-crystal X-ray crystal-
lography at 100(2) K. Selected bond distances and angles are
summarized in Table 2; Figure 1 displays the structures of the
monocations in crystals of 1 (top), 2 (middle), and 3 (bottom),
respectively.
In crystals of 1 a low-spin cobalt(III) ion is octahedrally
coordinated to a tetradentate anilido (L1)1- and a bidentate di-
tert-butylacetonylacetonato ligand. The C-Nanilido bond at 1.376-
(8) Å is shorter than the C-Naniline bonds in 3 (coordinated
aniline 1.446(6) Å, 1.425(6) Å and uncoordinated aniline 1.408-
(7) Å). The structure of the monocation in 1 is very similar to
(21) (a) Bill, E. Unpublished program package ESIM. (b) Gaffney, E.
J.; Silverstone, J. In Biological Magnetic Resonance; Berliner, L. J., Reuben,
J., Eds.; Plenum: New York, 1993; Vol. 13: EMR of Paramagnetic
Molecules. (c) Press, W. H.; Flannery, B. P.; Tukolsky, S. A.; Vutterly, W.
T. Numerical Recipes; Cambridge University Press: Cambridge, 1990.

Anilino Radical Complexes of Co(III) and Mn(IV)
J. Am. Chem. Soc., Vol. 122, No. 40, 2000 9667
Scheme 3. Synthesis of Ligands
Table 2. Selected Bond Distances (Å) and Angles (deg) of
Complexes
Complex 1
Co1-O1
Co1-N4
Co1-N3
N4-C9
O2-C26
C25-C26
Co1-N4-C9
1.893(4)
1.912(6)
1.989(6)
1.376(8)
1.280(7)
1.396(9)
Co1-O2
Co1-N1
Co1-N2
O1-C24
C24-C25
1.895(4)
1.962(5)
2.044(6)
1.281(7)
1.394(9)
Figure 1. Structures of the monocations in crystals of 1 (top), 2
(middle), and 3 (bottom). Small open circles represent hydrogen atoms
at the aniline nitrogen atoms. All other hydrogens are omitted.
122.4(4)
N2-Co1-N4
175.1(2)
Complex 2′
Mn-N1
2.090(7)
2.065(8)
1.875(8)
1.395(12)
1.379(11)
126.5(7)
129.5(7)
170.1(3)
Mn-N3
Mn-N4
Mn-N6
N5-C24
2.058(8)
1.869(8)
1.878(8)
1.388(11)
Mn-N2
The manganese(IV) ion of the monocation in crystals of 2′
is in a distorted octahedral environment composed of three
facially coordinated amine nitrogen donors of the 1,4,7-
triazacyclononane backbone and three anilido nitrogen donor
atoms of the three pendant arms of the ligand (L2)3-. The anilido
nitrogen atoms are three-coordinate and sp2-hybridized. The
cation possesses idealized C3 symmetry. The structure of the
cation in 2′ is very similar to that reported previously for [MnIV-
(La)]BPh4,14 where (La)3- represents the triply deprotonated
ligand 1,4,7-tris(o-aminobenzyl)-1,4,7-triazacyclononane, which
is the unsubstituted analogue of (L2)3-. The average C-Nanilido
bond distance at 1.387(10) Å is again slightly shorter than the
average C-Naniline bond in complexes of divalent metal ions
containing the coordinated aniline ligand (H3[La]),15 e.g., in [NiII-
(LaH3)](ClO4)2‚H2O at 1.450(10) Å.
Mn-N5
N4-C9
N6-C39
Mn-N4-C9
Mn-N6-C39
N5-Mn-N1
Mn-N5-C24
N6-Mn-N2
N4-Mn-N3
126.7(6)
170.8(3)
170.7(3)
Complex 3
Cu-N1
2.065(4)
Cu-N2
Cu-N4
Cu-Cl1
N5-C32
2.066(4)
Cu-N3
2.332(4)
2.641(4)
1.446(6)
1.408(7)
2.038(4)
2.305(1)
1.425(6)
Cu-N5
N4-C12
N6-C52
Cu-N4-C12
117.9(3)
Cu-N5-C32
123.3(3)
that of the analogous (phenolato)cobalt(III) complex [CoIII(LPr)-
(acac)](ClO4).2c

9668 J. Am. Chem. Soc., Vol. 122, No. 40, 2000
Penkert et al.
Table 3. Electronic Spectra of Complexes in CH2Cl2 Solution
complex
λ
max, nm (ꢀ, L mol-1 cm-1)
1
256(2.3 × 104), 330sh(4.2 × 103), 466(2.2 × 103), 672(1.5 × 103)
[CoIII(L1•)(Bu2acac)]2+ a
296(2.0 × 104), 494(3.0 × 103), 592sh(1.8 × 103)
2
260(2.9 × 104), 296(1.8 × 104), 347sh(5.7 × 103), 502sh(2.8 × 103), 721(1.2 × 104)
[MnIV(L2•)]2+ a
[MnIV(L2••)]3+ a
[MnIV(L2•••)]4+ a
3
260(2.4 × 104), 280sh(2.1 × 104), 355(6.5 × 103), 427sh(8.5 × 103), 461(9.7 × 103), 718(8.9 × 103)
270(2.1 × 104), 420sh(1.0 × 104), 458(1.2 × 104), 572(2.0 × 104)
270(2.4 × 104), 420sh(1.1 × 104), 456(1.5 × 104), 543(3.0 × 104)
276(1.1 × 104), 325sh(5.1 × 103), 400(3.2 × 103), 618(610)
a Electrochemically generated at -40 °C in CH2Cl2 (0.10 M [(n-Bu)4N]PF6).
The monocation in crystals of 3 contains a tetradentate neu-
tral ligand H3[L2] where the three amine nitrogen atoms of the
1,4,7-triazacyclononane backbone are bound to the CuII ion
and, in addition, one of the pendant aniline arms. The second
aniline arm may be very weakly coordinated to the CuII ion;
this Cu-Naniline distance is long at 2.641(4) Å. The third
(dangling) pendant arm is definitely not coordinated. Since the
CuII ion is also bound to a Cl- ligand, it possesses a (5 + 1)
coordination sphere. The three inequivalent Cu-Namine distances
at 2.065, 2.066, and 2.332 Å indicate that the former two of
these amine nitrogens occupy basal and one the apical position
of a square-base pyramidal polyhedron, which is completed by
binding to one aniline and a chloride ion the remaining two
basal positions. As discussed above, the average C-Naniline
distances are relatively long in accordance with a C-N single
bond.
Electro- and Spectroelectrochemistry. Cyclic voltammo-
Figure 2. Cyclic voltammogram of 1 in CH2Cl2 (0.10 M [(n-Bu)4N]-
PF6) at 298 K (glassy carbon electrode; scan rates: 50, 100, 200, 300,
and 400 mV s-1).
grams and coulometric experiments of 1, 2, and 3 have been
measured in CH2Cl2 solution containing 0.10 M [(n-Bu)4N]PF6
as supporting electrolyte. Potentials were measured vs the Ag/
0.01 M AgNO3/CH3CN reference electrode; ferrocene was used
as internal standard. All redox potentials are referenced vs the
ferrocenium/ferrocene (Fc+/Fc) couple (0.40 V vs NHE). Table
3 summarizes electronic spectra of electrochemically generated
species.
The CV of 1 displays a single reversible one-electron
oxidation wave at -0.07 V and an irreversible reduction peak
at approximately -1.5 V as well as an irreversible oxidation at
Epox ) +1.10 V. The latter two processes have not been further
studied.
Figure 2 displays the CV of 1 recorded in the potential range
+0.5 to -0.6 V. This process corresponds to the formation of
the (anilino)cobalt(III) radical [CoIII(L1•)(Bu2acac)]2+ as was
judged from spectral changes observed during the coulometric
+e
Figure 3. Spectral changes observed during the coulometric oxidation
of 1 to [Co(L1•)(Bu2acac)]2+ in CH2Cl2 (0.10 M [(n-Bu)4N]PF6) at 298
K ([1] ) 5 × 10-4 M).
and correspond to the formation of one, two, and three
coordinated anilino radicals, eq 2.
1 w\
-e
x [CoIII(L1•)(Bu2acac)]2+
(1)
one-electron oxidation of 1 shown in Figure 3. The electronic
spectrum of the starting material 1 displays two d-d transitions
of an octahedral low-spin cobalt(III) ion in the visible at 466
nm (ꢀ ) 2.2 × 103 L mol-1 cm-1) and 672 (1.5 × 103) which,
+e
+e
+e
3+
[MnIII(L2)]0 w+-ee
\x 2 w\
x [MnIV(L2•)]
w\
x [MnIV(L2••)]
w\x
2+
1
1
1
1
in Oh symmetry, are assigned to Ag f T2g and A1g f T1g
transitions, respectively. Upon one-electron oxidation two
new intense absorption maxima are generated at 494 nm
(ꢀ ) 3.0 × 103 L mol-1 cm-1) and 592 (1.8 × 103). In addition,
-e
-e
-e
[MnIV(L2•••)]4+ (2)
Coulometric measurements at appropriately fixed potentials
in CH2Cl2 solution at -40 °C showed that all three oxidations
of 2 are reversible on the time scale of these experiments (∼20
min). At room temperature this is not the case; they are
irreversible. It has been possible to characterize these oxidized
species by UV/vis and EPR spectroscopy at -40 °C.
Figure 5 shows the spectral changes observed during the
oxidation of 2 to [MnIV(L2•)]2+ (top), [MnIV(L2•)]2+ to [MnIV-
(L2••)]3+ (middle), and, finally, to [MnIV(L2•••)]4+ (bottom).
a new transition at 296 nm (ꢀ ) 2.0 × 104 L mol-1 cm-1
)
appears.
Figure 4 shows the CV of 2 in CH2Cl2 (0.10 M [(n-Bu)4N]-
PF6) at different scan rates. Clearly, four reversible one-electron-
transfer waves are observed at E1/2 ) -1.12, 0.23, 0.59, and
0.96 V. The first reversible reduction process is probably metal-
centered and is assigned to the MnIV/MnIII couple. The three
successive one-electron oxidation processes are ligand-centered

Anilino Radical Complexes of Co(III) and Mn(IV)
J. Am. Chem. Soc., Vol. 122, No. 40, 2000 9669
ion induces a bathochromic shift of these π-π* bands and an
increase in intensity.
Upon further one-electron oxidation to [MnIV(L2••)]3+ the
spectrum changes again significantly. The intensity of the π-π*
anilino radical transitions at 420 and 458 nm increases, and, in
addition, a very intense new absorption maximum at 572 nm
(ꢀ ) 2.0 × 104 L mol-1 cm-1) is observed. Further oxidation
to [MnIV(L2•••)]4+ slightly increases the intensity of the anilino
π-π* transitions and dramatically enhances the latter band,
which at the same time is hypsochromically shifted to 543 nm
(ꢀ ) 3.0 × 104). The latter band is very similar to those observed
in tris(semiquinonato)chromium(III) and tris(phenoxyl)chro-
mium(III) complexes.22 Gu¨del and Dei et al.23 have assigned
4
2
this transition to a spin-forbidden transition A2g f Eg (in Oh
symmetry) corresponding to a spin-flip in the ground state. This
transition is believed to gain intensity by 3 orders of magnitude
by strong exchange coupling between the CrIII ion and the
O-coordinated radicals. We propose a similar mechanism for
the band at 572 nm in [MnIV(L2••)]3+ and at 543 nm in [MnIV-
(L2•••)]4+. Thus the electronic spectra of the three oxidized forms
of 2 clearly show that one, two, and three coordinated anilino
radicals are successively formed.
Figure 4. Cyclic voltammogram of 2 in CH2Cl2 (0.10 M [(n-Bu)4N]-
PF6) at 298 K (glassy carbon electrode; scan rates: 50, 100, 200, 300,
400, and 500 mV s-1).
Complex 3 contains one N-coordinated aniline pendent arm,
a dangling uncoordinated aniline arm, and a very weakly
N-bound aniline arm. The CV of 3 displays at ambient
temperature a quasi-reversible one-electron oxidation wave at
E1/2 ) 0.66 V and an irreversible reduction at -1.08 V. One-
electron oxidation of 3 produces the aniline radical cation, which
may be coordinated to the CuII ion or not. The oxidized form is
not stable on the time scale of a coulometric measurement, and,
therefore, it has not been possible to characterize this species
spectroscopically.
X-Band EPR Spectroscopy. The X-band EPR spectrum at
295 K of electrochemically generated [CoIII(L1•)(Bu2acac)]2+
in CH2Cl2 solution (0.10 M [(n-Bu)4N]PF6) shown in Figure 6
displays a signal centered at g ) 2.0023 with well-resolved
hyperfine structure indicating the presence of a coordinated
anilino radical. The experimental spectrum has been successfully
simulated by using the parameters indicated in Figure 6. Strong
hyperfine coupling to a 59Co nucleus (I ) 7/2) in conjunction
with coupling to a proton of the diastereotopic benzyl group
(a(59Co) ) 1.215 mT; a(1H) ) 0.856 mT) prove that the anilino
radical is coordinated. The hyperfine coupling to the anilino
nitrogen and an (NH) proton prove that the anilide in 1 is one-
electron oxidized. The spectrum of the deuterated radical
complex [Co(d4-L1•)(Bu2acac)]2+ and its simulation are also
shown in Figure 6. The spectrum has been simulated using the
same set of parameters as above for the [CoIII(L1•)(Bu2acac)]2+
species with the exception of one benzylic deuteron for which
a coupling constant of a(2H) ) 0.141 mT was found. The ratio
a(1H)/a(2H) of 6.52 is in accord with this interpretation.
By using a McConnell-type relationship for the hyperfine
coupling to the benzylic protons where the magnitude of a(1H)
depends on cos2 θ1 and cos2 θ2 as defined in Scheme 4, we
calculated an a(1H) value for the second benzylic proton of 0.38
mT. The dihedral angles θ1 and θ2 were taken from the crystal
structure of 1 and are assumed to be the same in its oxidized
form. This coupling is clearly seen in the experimental spectrum
Figure 5. Spectral changes observed during the coulometric oxidation
of (a) 2 to [MnIV(L2•)]2+, (b) [MnIV(L2•)]2+ to [MnIV(L2••)]3+, and (c)
[MnIV(L2••)]3+ to [MnIV(L2•••)]4+ in CH2Cl2 (0.10 M [(n-Bu)4N]PF6) at
233 K.
The electronic spectrum of the starting material 2 displays a
very intense anilido-to-manganese(IV) charge-transfer (CT) band
at 721 nm and a shoulder at 502 nm. Note that an absorption
minimum is observed at ∼420 nm. Upon one-electron oxidation
the intensity of the CT band at 721 nm decreases whereas two
new intense maxima appear at 427sh (8.5 × 103 L mol-1 cm-1
)
and 461 nm (9.7 × 103 L mol-1 cm-1), which are assigned to
π-π* transitions of the coordinated anilino radical. These
absorptions are observed as broad maximum at 400 nm (ꢀ )
1.3 × 103) for the uncoordinated anilino radical, Ph-N˙ H, and
for the anilino radical cation [Ph-N˙ H2]+ at 423 (4.1 × 103)
and 406 nm (3.3 × 103 L mol-1 cm-1) according to Tripathi
and Schuler.6a Thus coordination of an anilino radical to a MnIV
(22) (a) Buchanan, R. M.; Kessel, S. L.; Downs, H. H.; Pierpont, C. G.;
Hendrickson, D. N. J. Am. Chem. Soc. 1978, 100, 7894. (b) Sofen, S. R.;
Ware, D. C.; Cooper, S. R.; Raymond, K. N. Inorg. Chem. 1979, 18, 234.
(c) Downs, H. H.; Buchanan, R. M.; Pierpont, C. G. Inorg. Chem. 1979,
18, 1736. (d) Buchanan, R. M.; Clafin, J.; Pierpont, C. G. Inorg. Chem.
1983, 22, 2552.
(23) Benelli, C.; Dei, A.; Gatteschi, D.; Gu¨del, H. U.; Pardi, L. Inorg.
Chem. 1989, 28, 3089.

9670 J. Am. Chem. Soc., Vol. 122, No. 40, 2000
Penkert et al.
Figure 7. Temperature-dependent magnetization measurements of 2′
at magnetic fields of 1, 4, and 7 T. Solid lines represent a best fit using
parameters given in the text.
radical7a (a(14N) ) 0.80 mT, a(1H, NH) ) 1.29 mT). In all
cases the large values for a(14N) and the a(1H, NH) hyperfine
coupling constants indicate a significant spin density at the
anilino nitrogen. It is interesting that the a(59Co) hyperfine
coupling constant of [CoIII(LPr•)(acac)]2+ at 0.6 mT2e is signifi-
cantly smaller indicating that the Co-Nanilino bond is stronger
and more covalent than the corresponding Co-Ophenoxyl bond.
The electronic structure of solid, magnetically dilute 2′ has
been characterized by field- and temperature-dependent mag-
netization measurements shown in Figure 7 by using a SQUID
magnetometer. The compound possesses the expected S ) 3/2
ground state for an octahedral MnIV ion (d3). From simulations
an isotropic g value of 2.03, and axial and rhombic zero-field
splitting (ZFS) parameters |D| ) 1.26 cm-1 and E/D ) 0.06
have been determined. Thus, the system is in the limit of strong
ZFS with respect to X-band EPR (2D . h‚ν ∼0.3 cm-1). The
Zeeman splitting is smaller than ZFS in the whole field range
of X-band spectra, and only the transitions within the ms ) (1/2
doublet are observable. For axial symmetry (E/D ) 0) signals
would be expected at effective g values of g ∼ 4 and g| ∼ 2.
Indeed, the X-band EPR spectrum of 2′ at 5 K shown in Figure
8 displays an axial signal with well-resolved 55Mn hyperfine
splitting at g|. The spectrum was readily simulated using D,
Figure 6. X-band EPR spectra of electrochemically one-electron
oxidized 1 and d4-1 at 298 K in CH2Cl2 (0.10 M [(n-Bu)4N]PF6) solution
and their simulations using hyperfine coupling constants in units of
[mT] as depicted in the formulas (at g ) 2 a hyperfine splitting of 1
mT corresponds to a coupling constant of a ) 9.35 × 10-4 cm-1).
Conditions: frequency, 9.4420 GHz for 1 and 9.4597 GHz for d4-1;
modulation, 0.02 mT; power, 1.0 mW for 1 and 10 mW for d4-1.
Simulation parameters: Gaussian lines with Wiso ) 0.104 mT for 1
and 0.109 mT for d4-1.
Scheme 4. Definition of Dihedral Angles θ1, θ2 at the
Benzyl Group of 1
E/D values from the magnetization measurements with giso
)
2.000 and a(55Mn) ) 70 × 10-4 cm-1 (7.5 mT).
The X-band EPR spectrum of the electrochemically generated
complex [MnIV(L2•)]2+ in CH2Cl2 solution at 10 K is shown in
Figure 9. The signal around g ) 2.0 is most probably due to a
small amount (<4%) of overoxidized [MnIV(L2••)]3+ (see Figure
11). At higher power weak signals at g ) 4-7 are observed
indicating that the monoradical is an integer spin system (St )
1 or St ) 2). These signals display a different saturation behavior
than the signal at g ) 2, indicating that the signals belong to
different species. We have simulated the weak signal assuming
an St ) 1 system. The ZFS parameter Dt ∼ 2.0 cm-1 and E )
0.14 cm-1 at giso ) 2.000 were obtained, if a small impurity
(, 1%) of the starting material 2 is assumed to be present. It
is not unequivocally possible to discern an St ) 1 from an St )
2 ground state for [MnIV(L2•)]2+ on the basis of this EPR
spectrum alone as shown in Figure 10, but in light of the
as asymmetric shoulders. It has not been possible to include
this coupling satisfactorily in the simulations, but note that for
[CoIII(d4-L1•)(Bu2acac)]2+ this coupling constant is only 0.06
mT and, consequently, it is not observed in the spectrum.
Hyperfine couplings to the aromatic protons in meta position
to the NH group are not resolved; they are expected to be small.
It is instructive to compare these data with those reported
for the 2,4,6-tri-tert-butylanilino radical7b,c (a(14N) ) 0.67 mT
and a(1H, N˙ H) ) 1.19 mT) and the unsubstituted anilino
observed strong antiferromagnetic coupling in [CrIII(L•)]+ 24
a
(phenoxyl)chromium(III) species we favor an St ) 1 ground
(24) Sokolowski, A.; Bothe, E.; Bill, E.; Weyhermu¨ller, T.; Wieghardt,
K. Chem. Commun. 1996, 1671.

Anilino Radical Complexes of Co(III) and Mn(IV)
J. Am. Chem. Soc., Vol. 122, No. 40, 2000 9671
Figure 8. X-band EPR spectrum of 2 in CH2Cl2 at 5 K. Conditions:
frequency, 9.6446 GHz; modulation, 1.28 mT; power, 1.6 mW.
Simulation parameters are given in the text (Gaussian lines with angular-
dependent line widths W ) -{14, 14, 5} mT).
Figure 10. Experimental EPR spectrum (10 K) and two simulations
for S ) 1 and S ) 2 of [MnIV(L2•)]2+. Frequency-constant line widths
of Wf ) 70 mT at g ) 2 and Wf ) 60 mT at g ) 2 were used for the
simulations with S ) 1 and S ) 2, respectively. Experimental
conditions: frequency, 9.4630 GHz; modulation, 1.28 mT; power, 50.4
mW. The dotted line in the top spectrum shows a simulated contribution
from unoxidized starting material [MnIV(L2)]+ (S ) 3/2) as shown in
Figure 8, which amounts to ∼2% of the nonoxidized sample.
Figure 9. X-band EPR spectra of electrochemically generated [MnIV-
(L2•)]2+ in CH2Cl2 at 10 K recorded at power 100.6 µW, 504.1 µW,
5.0 mW, and 50.4 mW. Conditions: frequency, 9.4630 GHz; modula-
tion, 1.28 mT. The inset shows the integer-spin signal at g ∼ 4 with
increasing power.
state, which is attained via antiferromagnetic coupling between
a MnIV ion (d3, SMn ) 3/2) and an anilino radical (SRad ) 1/2).
The X-band EPR spectrum of the electrochemically generated
diradical [MnIV(L2••)]3+ in CH2Cl2 solution at 10 K is shown
in Figure 11 along with its simulation. A typical S ) 1/2 signal
at g ) 1.965 with resolved 55Mn hyperfine coupling is observed
(a(55Mn) ) 70 × 10-4 cm-1; 7.5 mT).
Figure 11. X-band EPR spectrum of electrochemically generated
[MnIV(L2••)]3+ in CH2Cl2 (0.10 M [(n-Bu)4N]PF6) at 10 K and
simulation. Conditions: frequency, 9.6447 GHz; modulation, 1.28 mT;
power, 1.6 mW. Simulation parameters are given in the text. The line
widths for the simulated spectrum are determined according to σ(field)
) (Σi)x,y,zσi2li2)1/2, where li denotes the direction cosines and σi the
angular dependent line width. Here, σi2 ) Wi + C1,i‚ν + ΣmlC2,i‚ml2,
where Wi is the intrinsic residual line width, ν is the microwave
frequency, and ml ) -5/2‚‚‚+5/2 is the magnetic quantum number of
the 55Mn nucleus (l ) 5/2). For further details, see, for instance:
Pilbrow, J. R. Transition Ion Electron Paramagnetic Resonance;
Clarendon Press: Oxford, 1990. The line shape parameters used are
W ) {0, 3.5, 3.5} mT, C1x ) 4 mT/GHz, C2y ) 2 mT, C2z ) 2 mT.
Assuming intramolecular antiferromagnetic coupling between
the MnIV ions and the anilino radicals to prevail, we expect the
triradical [MnIV(L2•••)]4+ to possess a singlet ground state (St )
0). Indeed, upon one-electron oxidation of [MnIV(L2••)]3+ its S
) 1/2 signal gradually disappears. The triradical is EPR silent.
Resonance Raman Spectroscopy. To gain further informa-
tion on characteristic spectroscopic features of coordinated
anilino radicals we have measured RR spectra of electrochemi-
cally generated [CoIII(L1•)(Bu2acac)]2+ and its deuterated ana-
logue [Co(d4-L1•)(Bu2acac)]2+ in CH2Cl2 solutions containing
0.1 M [(n-Bu)4N]PF6. The spectra were recorded at 20 °C by
using the excitation line 514 nm coincident with the anilino
radical π f π* transitions. Under the same conditions, no
Raman bands could be detected for the parent compounds 1
and d4-1 so that the RR bands in the spectra shown in Figure
12 exclusively originate from the radical complexes.

9672 J. Am. Chem. Soc., Vol. 122, No. 40, 2000
Penkert et al.
Figure 13. Calculated structures of phenoxyl, anilino, and phenylthiyl
radicals from refs 9 and 25. Bond distances are given in Å.
deuterated complex indicating that the underlying modes include
much smaller contributions of the C-H in-plane bending
coordinates.
Discussion
The structures of the uncoordinated phenoxyl,9 anilino,9 and
phenylthiyl25 free radicals have been established by hybrid
Hartree/Fock density functional calculations. The results are
summarized in Figure 13. It appears that the phenoxyl and
anilino radicals both have a quinoid type structure with two
relatively short CodCm bonds between the carbon atoms in ortho
and para positions relative to the CdX (X ) O, NH) bond. In
contrast, all the C-C bonds of the phenylthiyl radical are nearly
equidistant. The calculated spin densities and EPR spectroscopic
measurements are in excellent agreement with the interpretation
that in the phenoxyl and anilino radical the unpaired electron
is delocalized but localized in the phenylthiyl. Thus, the latter
is best described as an S-centered radical. Furthermore, the CdO
and CdN bonds in the phenoxyl and anilino radicals are
considerably shorter than in their diamagnetic one-electron
reduced anions, namely, the phenolate and anilide. These
features are in general retained in O-coordinated phenoxyl
radicals as has been verified experimentally by comparison of
the structures of [CrIIIL]‚2CH3CN and its one-electron oxidized
form [CrIIIL•](ClO4)‚3CH3CN, the latter of which contains two
phenolato and a clearly discernible phenoxyl pendent arm.24 The
ligand L is 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-
1,4,7-triazacyclononane, H3L. Therefore, one can expect that
the electronic structures of an anilino and of a phenoxyl radical
each coordinated to a transition metal ion and their spectroscopic
properties are in fact quite similar.
This is indeed the case as is shown by comparison of the
phenoxyl radical complex [(LPr•)CoIII(acac)]2+2e and its anilino
radical analogue [(L1•)CoIII(Bu2acac)]2+. Both paramagnetic
species have an S ) 1/2 ground state and display very
informative X-band EPR spectra at 295 K with a well-resolved
hyperfine splitting pattern. It is important to reemphasize that
the hyperfine coupling constant of the anilino nitrogen a(14N)
) 0.856 mT is nearly the same in the unsubstituted, free anilino
radical7a at a(14N) ) 0.80 mT and 0.67 mT in the 2,4,6-tri-tert-
butylanilino radical.7b,c
The electronic spectrum of [CoIII(L1•)(Bu2acac)]2+ displays
intense absorption maxima (>103 L mol-1 cm-1) at 494 and
592 nm, which are absent in the one-electron reduced starting
material 1. These absorptions resemble closely the π f π*
transitions of the anilino radical cation at 406 and 423 nm.6a
This assignment is bolstered by the fact that upon excitation in
resonance with one of these π f π* transitions of the
coordinated anilino radical in [CoIII(L1•)(Bu2acac)]2+ the RR
modes originating from the anilino radical are detectable. Very
similar observations have been reported for the phenoxyl radical
Figure 12. Resonance Raman spectra of electrochemically generated
[CoIII(L1•)(Bu2acac)]2+ (top) and [CoIII(d4-L1•)(Bu2acac)]2+ (bottom) in
CH2Cl2 (0.10 M [(n-Bu)4N]PF6) measured at 298 K with 514 nm
excitation.
Quantum chemical calculations have predicted similar normal-
mode patterns for anilino and phenoxyl radicals.9 For both
radicals, excitation in resonance with the π f π* transition
provides enhancement of the totally symmetric modes leading
to up to four prominent bands in the spectral region between
1200 and 1700 cm-1 3,8
As documented by various examples,
.
the principle features of these RR spectra are largely independent
of the substitution pattern of the radical.2b-e,3 Consequently, the
highest-frequency band in the RR spectrum of [Co(L1•)(Bu2-
acac)]2+ at 1541 cm-1 is readily assigned to the stretching of
the Cortho-Cmeta bonds, reflecting the quinoid structure of the
anilino radical. This band reveals only a small downshift in the
deuterated radical complex, which supports the assignment to
an essentially pure C-C stretching mode, denoted as ν8a for
the sake of simplicity.3 The distinctly lower frequency of this
mode compared to that of free unsubstituted anilino radicals8,9
may result from the tert-butyl substituents in ortho and para
position. A plausible candidate for the mode that involves the
C-NH stretching is the band at 1331 cm-1 as its frequency
agrees well with the calculated value.9 This band disappears in
the RR spectrum of the deuterated radical complex. This unique
sensitivity toward ring deuteration can be understood on the
basis of the composition of the corresponding mode in phenoxyl
radicals, denoted as ν7a, for which a substantial coupling of the
C-O stretching and the C-H in-plane bending coordinates was
found.3 It is very likely that an analogous composition holds
for ν7a of anilino radicals in view of the overall similarity in
the character of the modes with phenoxyl radicals.9 Upon ring
deuteration, this coupling is removed so that the compositions
of ν7a and the adjacent modes are drastically altered leading to
pronounced changes in the spectrum between 1300 and 1400
cm-1. Specifically, the three bands at 1331, 1357, and 1381
cm-1 of [Co(L1•)(Bu2acac)]2+ are replaced by a single band at
1392 cm-1 in [Co(d4-L1•)(Bu2acac)]2+, which most likely
originates from a mode of predominant C-NH stretching
character. The remaining bands at 1401 and 1415 cm-1 of [Co-
(L1•)(Bu2acac)]2+ reveal only slight frequency upshifts in the
(25) Tripathi, G. N. R.; Sun, Q.; Armstrong, D. A.; Chipman, D. N.;
Schuler, R. H. J. Phys. Chem. 1992, 96, 5345.

Anilino Radical Complexes of Co(III) and Mn(IV)
species [CoIII(LPr•)(acac)]2+ 2e Thus, for both CoIII complexes
the RR spectra indicate a quinoid structure of the coordinated
ligand radical.
J. Am. Chem. Soc., Vol. 122, No. 40, 2000 9673
.
plexes, one can calculate the redox potential for the couple
[MnIV(LBu)]2+/+ to be 0.23 + 0.53 ) 0.76 V. H3[LBu] represents
the ligand 1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-
triazacyclononane.2a Replacing the tert-butyl group in the para
position relative to the phenol oxygen by a methoxy group gives
the ligand H3[LOCH3], 1,4,7-tris(3-tert-butyl-5-methoxy-2-hy-
droxybenzyl)-1,4,7-triazacyclononane. This substitution in the
para position lowers the redox potential for the phenoxyl/
phenolate couple by ∼250 mV as compared to its unsubstituted
parent molecule.27 Accordingly, for [MnIV(LOCH3)]+ a reversible,
ligand-centered one-electron oxidation has been observed at 0.56
V vs Fc+/Fc.2a
The redox potentials for the couples Ph-N˙ H/Ph-NH- and
Ph-N˙ H2+/Ph-NH2 have been determined by pulse radiolysis
and electrochemical methods to be -0.99 V vs Fc+/Fc (in
dimethyl sulfoxide) and +0.445 V, respectively.26 For com-
parison, the redox potential for the couple Ph-O˙ /Ph-O- is
∼0.4 V vs Fc+/Fc.27 Thus a phenolate is relatively difficult to
oxidize, but the isoelectronic anilide is a strong one-electron
reductant. Interestingly, the redox potentials for the couples Ph-
O˙ /Ph-O- and Ph-N˙ H2+/Ph-NH2 are similar.
The electronic structures of the anilino radical species [MnIV-
(L2•)]2+, [MnIV(L2••)]3+, and [MnIV(L3•••)]4+ are governed by a
strong intramolecular antiferromagnetic coupling between the
From cyclic voltammetry it has been shown that the redox
potentials of the couples [CoΙΙΙ(LPr)(acac)]2+/1+ at 0.46 V vs
Fc+/Fc2e and [CoIII(L1)(Bu2acac)]2+/1+ at -0.07 V differ by 0.53
V. This amounts to less than one-half of the difference between
the couples Ph-O˙ /Ph-O- and Ph-N˙ H/Ph-NH- (∆E ∼1.4
V). We take this as an indication that the CoIII-N bonds of the
coordinated anilide in 1 and/or of its one-electron oxidized form
are more covalent than the CoIII-O bonds in the CoIII-phenolate
and/or CoIII-phenoxyl species. Spectroscopically, this conclu-
sion is bolstered by the hyperfine coupling constants a(59Co)
observed for the radical species [CoIII(LPr•)(acac)]2+ at 0.60 mT2e
and at 1.215 mT for the analogous [CoIII(L1•)(Bu2acac)]2+
species.
3
t2g electronic configuration of the central manganese(IV) ion
and one, two, and three anilino radicals (Srad ) 1/2). Thus
electronic ground states of S ) 1, 1/2, and 0 are observed for
the di-, tri-, and tetracation, respectively. This is also in excellent
agreement with the corresponding chromium(III) cations [CrIII-
(LOCH3•)]1+, [CrIII(LOCH3••)]2+ and [CrIII(LOCH3•••)]3+, which have
an S ) 1, 1/2, and 0 ground state, respectively.24
Acknowledgment. We thank the Fonds der Chemischen
Industrie and the Deutsche Forschungsgemeinschaft (priority
program “Radicals in Biology”) for financial support. One of
us (P.H.) is grateful to the DFG for granting a Heisenberg
fellowship. We also thank Dr. K. Hildenbrand for measurements
and simulations of EPR spectra in fluid solutions.
Interestingly, [MnIV(LBu)]+, which is the phenolate analogue
of 2, does not exhibit a ligand-centered oxidation up to +0.7 V
vs Fc+/Fc. By using the above potential difference of 0.53 V
for the structurally analogous phenolate/anilido cobalt com-
(26) (a) Jansson, M.; Lind, J.; Eriksen, T. E.; Mere´nyi, G. J. Chem. Soc.,
Perkin Trans. 2 1993, 1567. (b) Lind, J.; Shen, X.; Eriksen, T. E.; Mere´nyi,
G. J. Am. Chem. Soc. 1990, 112, 479. (c) Jonsson, M.; Wayner, D. D. M.;
Lusztyk, J. J. Phys. Chem. 1996, 100, 17539. (d) Jonsson, M.; Lind, J.;
Mere´nyi, G.; Eriksen, T. E. J. Chem. Soc., Perkin Trans. 2 1995, 61. (e)
Jonsson, M.; Lind, J.; Eriksen, T. E.; Mere´nyi, G. J. Am. Chem. Soc. 1994,
116, 1423. (f) Bordwell, F. G.; Zhang, X.-M.; Cheng, J.-P. J. Org. Chem.
1993, 58, 6410. (g) Bordwell, F. G.; Cheng, J.-P. J. Am. Chem. Soc. 1991,
113, 1736.
Supporting Information Available: Tables of crystal-
lographic and structure refinement data, atom coordinates, bond
lengths and angles, anisotropic thermal parameters, and calcu-
lated positional parameters of H atoms for complexes 1, 2′‚
toluene, and 3‚MeOH‚1.5H2O (PDF). An X-ray crystallo-
graphic file, in CIF format, is also available. This material is
available free of charge via the Internet at http://pubs.acs.org.
(27) (a) Harriman, A. J. Phys. Chem. 1987, 91, 6102. (b) Surdhar, P. S.;
Armstrong, D. A. J. Phys. Chem. 1987, 91, 6532. (c) Mere´nyi, G.; Lind,
J.; Shen, X. J. Phys. Chem. 1988, 92, 134.
JA001637L