Carboxylate and alkyl carbonate coordination at the hydrophobic binding site of redox-active dicobalt amine thiophenolate complexes

Article abstract of DOI:10.1021/ic011004n

A series of new dicobalt complexes of the permethylated macrocyclic hexaamine dithiophenolate ligand H₂L^Me have been prepared and investigated in the context of ligand binding and oxidation state changes. The octadentate ligand is an effective dinucleating ligand that supports the formation of bioctahedral complexes with a central N₃Co(μ-SR)₂(μ-X)CoN₃ core structure, leaving a free bridging position X for the coordination of the substrates. The acetato- and cinnamato-bridged complexes [(L^Me)Co^II₂(μ-O₂CMe)] ⁺ (2) and [(L^Me)Co^II₂(μ-O₂CCH=CHPh)] ⁺ (5) were prepared by reaction of the μ-Cl complex [(L^Me)CO^II₂(μ-Cl)]⁺ (1) with the corresponding sodium carboxylates in methanol. The electrochemical properties of these and of the methyl carbonate complex [(L^Me)Co^II₂(μ-O₂COMe)] ⁺ (8) were also investigated. All complexes undergo two stepwise oxidations at ca. E¹_1/2 = +0.22 and at E²_1/2 = ca. +0.60 V vs SCE, affording the mixed-valent complexes [(L^Me)Co^IICo^III(μ-O₂CR)] ²⁺ (3, 6, 9) and the fully oxidized Co^IIICo^III forms [(L^Me)Co^III₂(μ-O₂CR)] ³⁺ (4, 7, 10), respectively. Compounds 3, 6, 9 and 4, 7, 10 refer to acetato-, cinnamato-, and methylcarbonato species, respectively. The Co^IICo^III compounds were prepared by comproportionation of the respective Co^II₂ and Co^III₂ compounds. The Co^IIICo^III species were prepared by bromine oxidation of the Co^IICo^II forms. The crystal structures of complexes 2·BPh₄·MeCN, 3·(l₃)₂, 5·BPh₄·2MeCN, 6·(ClO₄)₂·EtOH, 7· (ClO₄)₃·MeCN·(H₂O) ₃, and 9·(ClO₄)₂·(MeOH)₂·H ₂O were determined by single-crystal X-ray crystallography at 210 K. The oxidations occur without gross structural changes of the parent complexes. The Co^IICo^III complexes are composed of high-spin Co^II (d⁷) and low-spin Co^III (d⁶) ions. The Co^IIICo^III complexes are diamagnetic. The oxidation reactions affect the binding mode of the substrates. In the Co^II₂ and Co^IICo^III forms the carboxylates bridge the two Co²⁺ ions in a symmetric μ-1,3 fashion with uniform C-O bond distances, whereas asymmetric bridging modes, with one short C=O and one long C-O distance, are adopted in the fully oxidized species. This is consistent with the observed shifts in vibrational frequencies for ν_as(C-O) and ν _s(C-O) across the series.

Full text of DOI:10.1021/ic011004n

Inorg. Chem. 2002, 41, 1140−1150
Carboxylate and Alkyl Carbonate Coordination at the Hydrophobic
Binding Site of Redox-Active Dicobalt Amine Thiophenolate Complexes
Berthold Kersting* and Gunther Steinfeld
Institut fu¨r Anorganische und Analytische Chemie, UniVersita¨t Freiburg, Albertstrasse 21,
D-79104 Freiburg, Germany
Received September 25, 2001
A series of new dicobalt complexes of the permethylated macrocyclic hexaamine dithiophenolate ligand H LMe
2
have been prepared and investigated in the context of ligand binding and oxidation state changes. The octadentate
ligand is an effective dinucleating ligand that supports the formation of bioctahedral complexes with a central
N Co(µ-SR)2(µ-X)CoN core structure, leaving a free bridging position X for the coordination of the substrates. The
3
3
II
II
acetato- and cinnamato-bridged complexes [(LMe)Co 2(µ-O CMe)]+ (2) and [(LMe)Co 2(µ-O CCHdCHPh)]+ (5) were
2
2
II
prepared by reaction of the µ-Cl complex [(LMe)Co 2(µ-Cl)]+ (1) with the corresponding sodium carboxylates in
methanol. The electrochemical properties of these and of the methyl carbonate complex [(LMe)Co 2(µ-O COMe)]+
II
2
1
2
(8) were also investigated. All complexes undergo two stepwise oxidations at ca. E 1/2 ) +0.22 and at E 1/2 ) ca.
+0.60 V vs SCE, affording the mixed-valent complexes [(LMe)Co Co (µ-O CR)]2+ (3, 6, 9) and the fully oxidized
II
III
2
Co Co forms [(LMe)Co (µ-O CR)]3+ (4, 7, 10), respectively. Compounds 3, 6, 9 and 4, 7, 10 refer to acetato-,
III
III
III
2
2
II
III
cinnamato-, and methylcarbonato species, respectively. The Co Co compounds were prepared by comproportionation
of the respective Co 2 and Co compounds. The Co Co species were prepared by bromine oxidation of the
Co Co forms. The crystal structures of complexes 2‚BPh4‚MeCN, 3‚(I3)2, 5‚BPh4‚2MeCN, 6‚(ClO ) ‚EtOH, 7‚
II
III
III
III
2
II
II
4 2
(ClO ) ‚MeCN‚(H O)3, and 9‚(ClO ) ‚(MeOH)2‚H O were determined by single-crystal X-ray crystallography at 210
4 3
2
4 2
2
II
III
K. The oxidations occur without gross structural changes of the parent complexes. The Co Co complexes are
II
III
III
III
composed of high-spin Co (d7) and low-spin Co (d6) ions. The Co Co complexes are diamagnetic. The oxidation
II
II
III
reactions affect the binding mode of the substrates. In the Co 2 and Co Co forms the carboxylates bridge the two
Co2+ ions in a symmetric µ-1,3 fashion with uniform C−O bond distances, whereas asymmetric bridging modes,
with one short CdO and one long C−O distance, are adopted in the fully oxidized species. This is consistent with
the observed shifts in vibrational frequencies for νas(C−O) and νs(C−O) across the series.
Introduction
the use of binucleating ligands proved to be a powerful
strategy for the desired arrangement of the two metal ions
(M) and the free coordination sites (X) required for substrate
binding.10-21 A promising strategy to further modulate the
The coordination chemistry of binuclear complexes of
first-row transition metal ions has been the subject of
extensive investigations in recent years.1-9 In most cases,
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Soc. 1995, 117, 12865-12866.
* To whom correspondence should be addressed. E-mail:
berthold.kersting@ac.uni-freiburg.de.
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1140 Inorganic Chemistry, Vol. 41, No. 5, 2002
10.1021/ic011004n CCC: $22.00 © 2002 American Chemical Society
Published on Web 02/01/2002

Dicobalt Amine Thiophenolate Complexes
Scheme 1. Modulation of the Chemical Reactivity of Binuclear
Complexes by a Local Hydrophobic Microenvironment of the Ligand
Matrixa
Scheme 2. Structure of the Ligand H2LMe and Schematic
Representation of the Structures of the Corresponding Metal Complexes
[(LMe)M2(X)]+ a
a The cavity representation of the ligand (LMe
)
2- should not be confused
with the one used for cyclodextrins.26
chemical reactivity could be the embedding of the binding
site in a local hydrophobic environment (Scheme 1).22
Complexes of substituted hydrotris(pyrazolyl)borates, in
which the substituents serve to form a hydrophobic binding
pocket, have already been shown to exhibit enhanced
reactivity when compared with their unmodified analogues.23-25
Complexes of cyclodextrins26 and calixarenes26-28 have also
been employed for this purpose. In contrast, the study of
related binuclear complexes is very limited.29,30
We have recently been exploring the coordination chem-
istry of the macrocyclic amine thiophenolate ligand (LH)2-
and its permethylated derivative (LMe)2-. From a structural
point of view, the two macrocycles behave in a similar
fashion. Both promote the formation of bioctahedral com-
plexes of types A and B, leaving an open position (X) in
the bridging region between the two metal ions (Scheme 2).31
The chemical reactivity of the corresponding complexes, on
the other hand, is quite different. Thus, while [(LMe)NiII (µ-
2
Cl)]+ readily undergoes substitution reactions, complex [(LH)-
NiII (µ-Cl)]+ of the unmethylated ligand was found to be
2
unreactive.31 Likewise, the remarkable ability to activate and
transform small molecules such as carbon dioxide is restricted
to complexes of the permethylated macrocycle.32 The dif-
ference in chemical reactivity has been explained by
considering the different environments about the binding
sites. So far, our studies have been confined to complexes
containing divalent metal ions such as CoII, NiII, and ZnII.
To further develop the coordination chemistry of (LMe)2-,
we decided to prepare and characterize new cobalt(III)
complexes.
a X represents the binding site of the complexes.
A vast number of stable mono- and binuclear CoIII amine
thiolate complexes have been reported in the literature.33-40
Our initial characterization of the chloro-bridged dicobalt-
(II) complex 1, however, has revealed that the CoIII oxidation
state is enormously destabilized over the CoII oxidation state
in the present complexes. In fact, the CoIII2 form of complex
1 is thermally unstable, even on the time scale of a cyclic
voltammetry experiment. As will be seen, the stability of
the oxidized species can be greatly increased when the
bridging chloro function is replaced by carboxylate coligands.
It has also been possible to access CoIII complexes of
(19) Barrios, A. M.; Lippard, S. J. Inorg. Chem. 2001, 40, 1250-1255.
(20) Karlin, K. D.; Zuberbu¨hler, A. D. In Bioinorganic Catalysis, 2nd ed.
revised and expanded; Reedijk, J., Bouwman, E., Eds.; Marcel
Dekker: New York, 1999; pp 469-534.
(21) Incarvito, C.; Rheingold, A. L.; Qin, C. J.; Gavrilova, A. L.; Bosnich,
B. Inorg. Chem. 2001, 40, 1386-1390.
(22) Hecht, S.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 2001, 40, 74-91.
(23) Trofimenko, S. Chem. ReV. 1993, 93, 943-980.
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(25) Ruf, M.; Vahrenkamp, H. Inorg. Chem. 1996, 35, 6571-6578.
(26) Reetz, M. T.; Waldvogel, S. R. Angew. Chem., Int. Ed. Engl. 1997,
36, 865-867.
(33) Jicha, D. C.; Busch, D. H. Inorg. Chem. 1962, 1, 872-883.
(34) DeSimone, R. E.; Ontko, T.; Wardman, L.; Blinn, E. L. Inorg. Chem.
1975, 14, 1313-1316.
(27) Wieser-Jeunesse, C.; Matt, D.; De Cian, A. Angew. Chem., Int. Ed.
1998, 37, 2861-2864.
(35) Beissel, T.; Glaser, T.; Kesting, F.; Wieghardt, K.; Nuber, B. Inorg.
Chem. 1996, 35, 3936-3947.
(28) Blanchard, S.; Le Clainche, L.; Rager, M.-N.; Chansou, B.; Tuchagues,
J.-P.; Duprat, A. F.; Le Mest, Y.; Reinaud, O. Angew. Chem., Int. Ed.
1998, 37, 2732-2735.
(36) Kersting, B.; Siedle, G. Z. Naturforsch., B 2000, 55b, 1179-1187.
(37) Konno, T.; Aizawa, S.; Okamoto, K.; Hidaka, J. Chem. Lett. 1985,
1017-1020.
(29) Enomoto, M.; Aida, T. J. Am. Chem. Soc. 1999, 121, 874-875.
(30) Klein Gebbink, R. J. M.; Bosman, A. W.; Feiters, M. C.; Meijer, E.
W.; Nolte, R. J. M. Chem. Eur. J. 1999, 5, 65-69.
(38) Okamoto, K.; Aizawa, S.; Konno, T.; Einaga, H.; Hidaka, J. Bull.
Chem. Soc. Jpn. 1986, 59, 3859-3864.
(39) Heeg, M. J.; Blinn, E. L.; Deutsch, E. Inorg. Chem. 1985, 24, 1118-
1120.
(31) Kersting, B.; Steinfeld, G. Chem. Commun. 2001, 1376-1377.
(32) Kersting, B. Angew. Chem. 2001, 113, 4109-4112; Angew. Chem.,
Int. Ed. 2001, 40, 3987-3990.
(40) Kersting, B.; Siebert, D.; Volkmer, D.; Kolm, M. J.; Janiak, C. Inorg.
Chem. 1999, 38, 3871-3882.
Inorganic Chemistry, Vol. 41, No. 5, 2002 1141

Kersting and Steinfeld
[(LMe)CoII (µ-Ac)]BPh4 (2‚BPh4), was prepared by adding NaBPh4
Chart 1. Designation of Cobalt Complexes
2
(342 mg, 1.00 mmol) instead of LiClO4 to the above reaction
mixture. IR (KBr, cm-1): νj ) 1585 [νas(C-O)], 1431 [νs(C-O)],
732, 703 (BPh4-).
[(LMe)CoIICoIII(µ-O2CMe)](I3)2 (3‚(I3)2). A solution of I2 (127
mg, 0.05 mmol) in acetonitrile (25 mL) was added dropwise to a
solution of 2‚ClO4 (95 mg, 0.10 mmol) in acetonitrile (25 mL).
The mixture was left to stand in air for 5 days, during which time
a few brown-red crystals of 3‚(I3)2 formed on the bottom of the
flask. These crystals were used for an X-ray crystal structure
analysis. The compound was not analyzed further because of the
low yield.
[(LMe)CoIICoIII(µ-O2CMe)](ClO4)2 (3‚(ClO4)2). A solution of
2‚ClO4 (95 mg, 0.10 mmol) in acetonitrile (25 mL) was added to
a solution of 4‚(ClO4)3 (114 mg, 0.100 mmol) in acetonitrile (25
mL). The reaction mixture was concentrated under vacuum to about
5 mL. An equal amount of ethanol was then added and the brown
solution was left to stand in air for 12 h, during which time brown-
red crystals of 3‚(ClO4)2 precipitated. The crystals were isolated
by filtration, washed with a little ethanol, and dried in air. Yield:
125 mg (60%). mp >300 °C (decomp). Anal. Calcd for C40H67-
Cl2Co2N6O10S2: C, 45.98; H, 6.46; N, 8.04; S, 6.14. Found: C,
45.10; H, 6.21; N, 7.73; S, 5.86. UV/vis (CH3CN): λmax (ꢀ) ) 456
nm (6434), 710 (1277). IR (KBr, cm-1): νj ) 1575 [νas(C-O)],
1424 [νs(C-O)].
[(LMe)CoIII2(µ-O2CMe)](ClO4)3 (4‚(ClO4)3). A solution of bro-
mine (80 mg, 0.50 mmol) in acetonitrile (5 mL) was slowly added
to a solution of 2‚ClO4 (190 mg, 0.20 mmol) in acetonitrile (25
mL). During addition the temperature was kept at 0 °C. The
resulting dark brown solution was stirred for a further 2 min to
ensure complete oxidation of 2‚ClO4. The reaction mixture was
then evaporated to dryness and the brown-black residue redissolved
in acetonitrile (10 mL). This procedure was repeated twice more
to remove the excess bromine. A solution of LiClO4‚3H2O (800
mg, 5.00 mmol) in ethanol (20 mL) was then added to give a black
microcrystalline precipitate. The crystals were filtered, washed with
a little ethanol, and dried in air. Yield: 172 mg (75%). mp 300 °C
(decomp). Anal. Calcd for C40H67Cl3Co2N6O14S2‚(H2O)3: C, 40.09;
H, 6.14; N, 7.01; S, 5.35. Found: C, 39.71; H, 6.23; N, 6.88; S,
4.75. UV/vis (CH3CN): λmax (ꢀ) ) 465 nm (12037), 640 (2664).
IR (KBr, cm-1): νj ) 1525 cm-1 [νas(C-O)], 1427 [νs(C-O)]. 1H
NMR (CD3CN, resonances in the region 2.0-4.5 ppm are broad
and could not be assigned): δ ) 7.31 (s, 4H, ArH), 1.22 (s, 18H,
t-Bu), 1.00 (s, 3H, CH3COO-).
potentially hydrolyzable substrates such as monomethyl
carbonate. The new complexes and their labels are sum-
marized in Chart 1.
We have obtained single crystals of a series of CoIICoII,
CoIICoIII, and CoIIICoIII complexes suitable for X-ray structure
determinations. Using infrared spectroscopy with the car-
boxylates as spectroscopic probes, it has been possible to
study the binding modes of the coligands as a function of
the metal oxidation states. We are unaware of any other
coordinately unsaturated Co2 thiolate complex that gives
information of structural features over an essentially iso-
structural three-member series of stable complexes.
Experimental Section
Preparation of Compounds. All manipulations were performed
under an atmosphere of argon using standard Schlenk techniques.
Solvents were purified using conventional methods and were freshly
distilled under nitrogen prior to use. The compounds [(LMe)CoII -
2
(µ-Cl)]ClO4 (1‚ClO4) and [(LMe)CoII (µ-O2COMe)]ClO4 (8‚ClO4)
2
were prepared using published procedures.32 Absorption and NMR
spectra were determined in acetonitrile solutions unless otherwise
noted.
Safety Note! Perchlorate salts of transition metal complexes are
hazardous and may explode. Only small quantities should be
prepared with great care.41
[(LMe)CoII2(µ-O2CCHdCHPh)]ClO4 (5‚ClO4). A solution of
sodium hydroxide (12 mg, 0.30 mmol) in methanol (1 mL) was
added to a solution of cinnamic acid (44 mg, 0.30 mmol) in
methanol (10 mL). This solution was then added to a solution of
1‚ClO4 (160 mg, 0.174 mmol) in methanol (30 mL), and the reaction
mixture stirred for 2 h. The resulting pale red precipitate was
filtered, washed with 1 mL of cold methanol, and dried in air.
Yield: 168 mg (81%). mp 334 °C (decomp). Anal. Calcd for
C47H71ClCo2N6O6S2: C, 54.62; H, 6.92; N, 8.13; S, 6.20. Found:
C, 53.55; H, 7.07; N, 7.89; S, 5.76. 1H NMR (CD3CN): δ ) 199.0
(s br, 4H, CH2), 185.7 (s br, 1H, CHdCHPh), 145.2 (s br, 4H,
CH2), 134.4 (s, 6H, NCH3), 106.9 (s, 1H, CHdCHPh), 77.8 (s,
4H, CH2), 76.8 (s br, 4H, CH2), 39.81 (s, 2H, CHdCHArH), 37.3
(s, 12H, NCH3), 23.4 (s, 4H, ArH), 18.76 (s, 2H, CHdCHArH),
18.72 (s, 1H, CHdCHArH), 8.5 (s, 18H, CH3), -51.0 (s br, 4H,
CH2), -69.0 (s br, 4H, CH2). UV/vis (CH3CN): λmax (ꢀ) )
445 nm (487), 524 (193), 544 (sh, 138), 567 (sh, 80), 606
(sh, 36), 1259 (43). IR (KBr, cm-1): νj ) 1642 [ν(CdC)], 1576
[νas(C-O)], 1407 [νs(C-O)]. The tetraphenylborate salt, [(LMe)-
[(LMe)CoII2(µ-O2CMe)](ClO4) (2‚ClO4). A solution of sodium
acetate (12.3 mg, 0.150 mmol) in methanol (5 mL) was added to
a solution of 1‚ClO4 (92 mg, 0.10 mmol) in methanol (50 mL).
The mixture was stirred for 2 h, during which time the color of the
solution turned from brown to pale red. A solution of LiClO4‚3H2O
(800 mg, 5.00 mmol) in methanol (2 mL) was added. The resulting
pale red microcrystalline solid was isolated by filtration, washed
with methanol, and dried in air. Yield: 76 mg (80%). mp 327 °C
(decomp). Anal. Calcd for C40H67ClCo2N6O6S2: C, 50.81; H, 7.14;
1
N, 8.89; S 6.78. Found: C, 49.72; H, 7.36; N, 8.62; S, 6.20. H
NMR (CD3CN): δ ) 200.5 (s br, 4H, CH2), 178.5 (s br, 3H, CO2-
CH3), 147.4 (s br, 4H, CH2), 132.5 (s, 6H, NCH3), 77.8 (s br, 4H,
CH2), 76.9 (s, 4H, CH2), 36.1 (s, 12H, NCH3), 22.3 (s, 4H, ArH),
8.4 (s, 18H, CH3), -53.0 (s br, 4H, CH2), -66.0 (s br, 4H, CH2).
UV/vis (CH3CN): λmax (ꢀ) ) 440 nm (467), 523 (170), 542 (sh,
121), 565 (sh, 64), 608 (sh, 21), 1262 (33). IR (KBr, cm-1): νj )
1587 [νas(C-O)], 1434 [νs(C-O)]. The tetraphenylborate salt,
(41) Wolsey, W. C. J. Chem. Educ. 1973, 50, A335-A337.
1142 Inorganic Chemistry, Vol. 41, No. 5, 2002

Dicobalt Amine Thiophenolate Complexes
Table 1. Crystallographic Data for Compounds Containing Complexes 2, 3, 5, 6, 7, and 9
7‚(ClO4)3‚MeCN‚
9‚(ClO4)2‚(MeOH)2‚
2‚BPh4‚MeCN
3‚(I3)2
5‚BPh4‚2MeCN
6‚(ClO4)2‚EtOH
(H2O)3
H2O
formula
fw
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V
C66H90BCo2N7O2S2 C40H67Co2I6N6O2S2 C75H97BCo2N8O2S2 C49H77Cl2Co2N6O11S2 C49H80Cl3Co2N7O17S2 C42H77Cl2Co2N6O14S2
1206.24
P1h
1607.38
P1h
1335.40
P1h
1179.05
P1h
1327.53
Pbca
16.003(3)
23.209(5)
37.134(7)
1142.98
P21/c
16.617(3)
20.232(4)
15.717(3)
13.319(3)
15.628(3)
16.997(3)
108.38(3)
92.24(3)
107.38(3)
3169(1)
2
10.668(2)
13.913(3)
19.424(4)
84.48(3)
87.82(3)
69.94(3)
2695.5(9)
2
13.191(3)
15.164(3)
19.734(4)
74.69(3)
83.59(3)
78.59(3)
3725(1)
2
15.457(3)
18.421(4)
20.665(4)
96.27(3)
91.86(3)
105.69(3)
5619(2)
4
93.54(3)
13792(5)
8
5274(2)
4
Z
F
calcd, g/cm3
1.264
1.980
1.191
1.394
1.279
1.440
temp, K
210(2)
210(2)
4.166
210(2)
0.550
210(2)
0.821
210(2)
0.720
210(2)
0.876
µ(Mo KR), mm-1 0.638
2θ limits, deg
total no. data
no. unique data
obsd. dataa
no. params
Rb
3-57
28 252
14 915
7050
3-57
3-57
2-57
2-57
2-57
24 448
12 603
8868
33 767
17 419
11957
51 311
26 212
7800
85 015
16 862
2431
33 380
12 663
3270
702
523
766
1115
724
548
0.0453
0.0868
0.0343
0.0904
1.850/-1.259
0.0565
0.1930
1.249/-0.843
0.0726
0.1798
1.088/-0.813
0.0913
0.2421
1.092/-0.714
0.0681
0.1701
0.989/-0.572
wR2c
max, min peaks, 0.529/-0.529
e/Å3
a Observation criterion: I > 2σ(I). b R1 ) ∑||Fo| - |Fc||/∑|Fo|. c wR2 ) {∑[w(Fo - Fc )2]/∑[w(Fo )2]}1/2
.
2
2
2
CoII (µ-O2CCHdCHPh)]BPh4 (5‚BPh4), was prepared by adding
NaBPh4 (342 mg, 1.00 mmol) instead of LiClO4 to the above
reaction mixture. IR (KBr, cm-1): νj ) 1642 [ν(CdC)], 1576
[νas(C-O)], 1406 [νs(C-O)], 732, 703 (BPh4-).
[(LMe)CoIII2(µ-O2COMe)](ClO4)3 (10‚(ClO4)3). This compound
was prepared by the same method as described above for 4‚(ClO4)3,
except that the product was recrystallized from CH3CN/MeOH.
Compound 8‚ClO4 (192 mg, 0.200 mmol) was used instead of
2‚(ClO4). Yield: 124 mg (58%). mp >290 °C (decomp). Anal.
Calcd for C40H67Cl3Co2N6O15S2: C, 41.40; H, 5.82; N, 7.24; S,
5.53. Found: C, 40.55; H, 5.67; N, 7.13; S, 5.22. UV/vis (CH3-
CN): λmax (ꢀ) ) 466 nm (9000), 643 (2048). IR (KBr): νj ) 1556
cm-1 [νas(C-O)], 1353 [νs(C-O)]. CV (CH3CN, 295 K, 0.1 M
2
[(LMe)CoIICoIII(µ-O2CCHdCHPh)](ClO4)2 (6‚(ClO4)2). This
complex was prepared by a method analogous to that described
above for 3‚(ClO4)2. The compounds 5‚ClO4 (103 mg, 0.100 mmol)
and 7‚(ClO4)3 (123 mg, 0.100 mmol) were used instead of 2‚ClO4
and 4‚(ClO4)3, respectively. Yield: 125 mg (55%). mp >300 °C
(decomp). Anal. Calcd for C47H71Cl2Co2N6O10S2: C, 49.82; H, 6.32;
N, 7.42; S, 5.66. Found: C, 50.14; H, 6.24; N, 7.33; S, 5.28. UV/
vis (CH3CN): λmax (ꢀ) ) 458 nm (6604), 710 (1330). IR (KBr,
cm-1): νj ) 1635 [ν(CdC)], 1558 [νas(C-O)], 1391 [νs(C-O)].
[(LMe)CoIII2(µ-O2CCHdCHPh)](ClO4)3 (7‚(ClO4)3). This com-
pound was prepared by the same method as described above for
4‚(ClO4)3. Compound 5‚ClO4 (207 mg, 0.200 mmol) was used
instead of 2‚ClO4. Yield: 205 mg (83%). mp >300 °C (decomp).
Anal. Calcd for C47H71Cl3Co2N6O14S2: C, 45.80; H, 5.81; N, 6.82;
S, 5.20. Found: C, 44.31; H, 5.12; N, 7.32; S, 5.11. UV/vis (CH3-
CN): λmax (ꢀ) ) 467 nm (11 598), 637 (2593). IR (KBr, cm-1):
νj ) 1631 [ν(CdC)], 1508 [νas(C-O)], 1388 [νs(C-O)]. 1H NMR
(DMSO-d6, resonances in the region 4.5-2.0 are broad and could
not be assigned): δ ) 7.38-7.30 (m, 9H), 6.63 (d, J ) 16 Hz,
1H), 5.84 (d, J ) 16 Hz, 1H), 4.11 (m), 3.88 (m), 3.52 (m), 3.47
(s br), 3.09 (s br), 3.03 (s br), 2.95 (s br), 2.88 (s br), 2.78 (s br),
2.65 (s br), 1.10 (s, 18H). 13C NMR (DMSO-d6): δ ) 181.6
(RCO2), 158.0, 153.5, 149.7, 142.9 (CH), 135.5 (CH), 133.2 (CH),
131.9 (CH), 128.9 (CH), 118.9 (CH), 66.2 (CH2), 64.9 (CH2), 62.2
(CH), 55.0, 48.5, 34.6, 30.0.
n-Bu4NPF6, 100 mV‚s-1; E (V) vs SCE): E1 ) +0.26 (∆Ep )
1/2
0.137 V); E2 ) +0.66 (0.200).
1/2
Collection and Reduction of X-ray Data. Single crystals of
2‚BPh4‚MeCN, 3‚(I3)2, and 9‚(ClO4)2‚2MeOH‚H2O were taken
directly from the reaction mixtures. Crystals of 5‚BPh4‚2MeCN and
of 6‚(ClO4)2‚EtOH were grown by slow evaporation of solutions
of the complexes in acetonitrile/ethanol (1:2) mixed-solvent systems.
Black crystals of 7‚(ClO4)3‚MeCN‚3H2O were grown by slow
evaporation of a solution of the complex in acetonitrile. The crystals
were mounted on the tips of glass fibers using perfluoropolyether
oil. Intensity data were collected at 210(2) K, using a Bruker CCD
X-ray diffractometer. Graphite monochromated Mo KR radiation
(λ ) 0.710 73 Å) was used. Crystallographic data of the compounds
are listed in Table 1. The data were processed with SAINT42 and
corrected for absorption using SADABS43 (transmission factors
1.00-0.90 (2), 1.00-0.79 (3), 1.00-0.86 (5), 1.00-0.88 (6), 1.00-
0.59 (7), 1.00-0.89 (9)). The structures were solved by using the
program SHELXS-86.44 Refinements were carried out with the
program SHELXL-93.45 PLATON was used to search for higher
symmetry.46 ORTEP-3 was used for the artwork of the structures.47
All non-hydrogen atoms were refined anisotropically, except for
those of the disordered t-Bu groups, the solvent molecules of
[(LMe)CoIICoIII(µ-O2COMe)](ClO4)2 (9‚(ClO4)2). This com-
pound was prepared by the same method described above for 3‚
(ClO4)2. Compounds 8‚ClO4 (96 mg, 0.100 mmol) and 10‚(ClO4)3
(116 mg, 0.100 mmol) were used instead of 2‚(ClO4) and 4‚(ClO4)3.
The product was recrystallized from CH3CN/MeOH. Yield: 125
mg (54%). mp >300 °C (decomp). Anal. Calcd for C40H67Cl2-
Co2N6O11S2: C, 45.29; H, 6.37; N, 7.92; S, 6.05. Found: C, 44.78;
H, 5.90; N, 7.66; S, 5.81. UV/vis (CH3CN): λmax (ꢀ) ) 450 nm
(5944), 684 (1287); IR (KBr): νj )1635 cm-1 [νasym(C-O)], 1344
[νsym(C-O)].
(42) SAINT+, V6.02; Bruker AXS; Madison, WI, 1999.
(43) Sheldrick, G. M. SADABS, Area-Detector Absorption Correction;
University of Go¨ttingen: Go¨ttingen, Germany, 1996.
(44) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, A46, 467-473.
(45) Sheldrick, G. M. SHELXL-93, Computer program for crystal structure
refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1993.
(46) Spek, A. L. PLATONsA Multipurpose Crystallographic Tool; Utrecht
University: Utrecht, The Netherlands, 2000.
(47) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
Inorganic Chemistry, Vol. 41, No. 5, 2002 1143

Kersting and Steinfeld
crystallization, and the aryl ring of the cinnamate ion from molecule
A in 6. The O atoms of all ClO4- anions in 6 and of the disordered
one in 7 were refined isotropically. Hydrogen atoms were assigned
to idealized positions and given isotropic thermal parameters 1.2
times (1.5 times for CH3 groups) the thermal parameter of the atoms
to which they were attached. Split atom models were used to
account for the disorder of tert-butyl groups in complexes 2, 6, 7,
and 9. The site occupancies of the two positions were refined as
follows: C(32a)-C(34a)/C(32b-C(34b) 0.56(1)/0.44(1) for 2, 0.55-
(3)/0.45(3) for 6, and 0.55(5)/0.45(5) for 7; and C(36a)-C(38a)/
C(36b)-C(38b) 0.55(1)/0.45(1) for 2 and 0.63(2)/0.37(2) for 9. In
the structure of 7, one ClO4- anion is disordered over two positions.
The site occupancies of the two positions were refined as follows:
O(11a)-O(14a)/O(11b)-O(14b): 0.66(2)/0.34(2).
The electrochemical properties of the dicobalt(II) com-
plexes described below implied that the oxidized CoIICoIII
and CoIIICoIII forms would be amenable to isolation using
mild oxidants. Initial attempts to prepare the mixed-valent
CoIICoIII complex 3 focused on the chemical oxidation of
the acetato-bridged derivative 2‚ClO4 with iodine. The
reaction in eq 2 was accompanied by a color change from
pale red to red-brown, indicating the formation of the desired
CoIICoIII species. However, all attempts to isolate a perchlo-
rate salt have failed. In one case, a few crystals of compo-
sition [(LMe)CoIICoIII(µ-O2CMe)](I3)2 (3‚(I3)2)sjust enough
for a single-crystal X-ray diffraction studyswere obtained.
The crystal structure determination unambiguously revealed
the presence of a mixed-valent CoIICoIII complex (see below).
Nevertheless, the yield was low and a more convenient
synthetic procedure was highly desirable.
Other Physical Measurements. Infrared spectra were recorded
over the range 4000-400 cm-1 on a Bruker VECTOR 22 FT-IR
spectrophotometer using KBr pellets. Melting points were deter-
1
mined in capillaries and are uncorrected. H NMR and 13C NMR
spectra were recorded on a Bruker AVANCE DPX-200 spectrom-
eter. Electronic absorption spectra were recorded on a Jasco V-570
UV/vis/near-IR spectrophotometer (range 400-1600 nm). Elemen-
tal analyses were performed on a Vario EL analyzer (Elementa-
ranalysensysteme GmbH). Cyclic voltammetry measurements were
carried out at 25 °C with an EG&G Princeton Applied Research
potentiostat/galvanostat Model 263 A. The cell contained a Pt
working electrode, a Pt wire auxiliary electrode, and an Ag wire
as reference electrode. Sample concentration was ca. 1.0 × 10-3
M in 0.10 M [n-Bu4N]PF6 supporting electrolyte. Cobaltocenium
hexafluorophosphate was used as internal standard (under our
The comproportionation reaction between 2‚ClO4 and the
fully oxidized CoIIICoIII species 4‚(ClO4)3 (see below) was
sought as an alternative route. As shown in eq 3, 2 reacted
smoothly with 4 in acetonitrile at 0 °C to give, after
evaporation of the solvent and recrystallization from CH3-
CN/EtOH, the isolated mixed-valent complex 3‚(ClO4)2 in
60% yield. Complex 6‚(ClO4)2 could also be readily prepared
in this manner. The complexes were isolated as air-stable,
dark brown microcrystalline powders. Our observations
indicate that this latter method is a decisively cleaner and
more general protocol than the previous one. Similarly to
their parent complexes, compounds 3‚(ClO4)2 and 6‚(ClO4)2
exhibit excellent solubility in polar-aprotic media such as
acetonitrile to give deep brown solutions, while they are
practically insoluble in ethanol or methanol. In the absence
of reducing agents, acetonitrile solutions of the CoIICoIII
complexes can be stored for several days at ambient
temperature without noticeable decomposition.
The preparation of the fully oxidized CoIIICoIII complexes
was attempted next. The addition of an excess of bromine
to solutions of the perchlorate salts of the dicobalt(II)
complexes in acetonitrile at 0 °C, followed by addition of
an excess of LiClO4 and recrystallization of the resulting
solid from EtOH/CH3CN, was found to be the method of
choice. In the case of 2‚ClO4, a black solid of composition
[(LMe)CoIII2(µ-O2CMe)](ClO4)3 (4‚(ClO4)3) could be isolated
with a 72% yield, eq 4. The Br2 oxidation of 5‚ClO4 produced
experimental conditions, the Co(Cp)2+/Co(Cp)2 couple is at E1/2
)
-1.345 V vs the Fe(Cp)2+/Fe(Cp)2 couple). All potentials were
converted to the SCE reference using tabulated values.48
Results and Discussion
Synthesis of Carboxylato-bridged Cobalt(II) Com-
plexes. The chloro-bridged complex [(LMe)CoII (µ-Cl)]+ (1)
2
was found to be an excellent starting material for the
preparation of the carboxylato-bridged dicobalt(II) complexes
2 and 5. Compound 1 reacted smoothly with 1.5 equiv of
sodium acetate or sodium cinnamate in methanol at room
temperature (eq 1) to produce a pale red solution. Upon
addition of LiClO4‚3H2O, the pale red complex [(LMe)CoII (µ-
2
O2CMe)]ClO4 (2‚ClO4) or [(LMe)CoII (µ-O2CCHdCHPh)]-
2
ClO4 (5‚ClO4), respectively, precipitated with a high yield.
Addition of NaBPh4 instead of LiClO4 gave the correspond-
ing tetraphenylborate salts 2‚BPh4 and 5‚BPh4. The new
compounds exhibit good solubility in acetonitrile, but they
are only sparingly soluble in alcohols and virtually insoluble
in halogenated hydrocarbons. Unlike the chloro-bridged
complex 1, all are stable in air, even in solution.
(48) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877-910.
1144 Inorganic Chemistry, Vol. 41, No. 5, 2002

Dicobalt Amine Thiophenolate Complexes
Table 2. Electrochemical Data, E (V), for 1-12a
E11/2 (III,II/II,II)b
E21/2 (III,III/III,II)b
∆Ec
Kc
d
no.
compound
1e
2
3
4
5
6
7
8
9
[(LMe)CoII2(µ-Cl)]+
0.47 (0.136)
0.21 (0.118)
0.22 (0.122)
0.22 (0.122)
0.20 (0.120)
0.20 (0.120)
0.20 (0.120)
0.26 (0.125)
0.26 (0.121)
0.26 (0.137)
-0.85 (0.127)
-0.84 (0.136)
0.97 (irr.)
0.60 (0.147)
0.60 (0.147)
0.60 (0.150)
0.58 (0.130)
0.57 (0.130)
0.60 (0.129)
0.66 (0.180)
0.66 (0.187)
0.66 (0.200)
-0.47 (0.132)
-0.40 (0.119)
[(LMe)CoII2(µ-O2CMe)]+
[(LMe)CoIICoIII(µ-O2CMe)]2+
[(LMe)CoIII2(µ-O2CMe)]3+
[(LMe)CoII2(µ-O2CR)]+
0.39
0.38
0.38
0.38
0.37
0.40
0.40
0.40
0.40
0.38
0.44
2.7 × 106
1.8 × 106
[(LMe)CoIICoIII(µ-O2CR)]2+
[(LMe)CoIII2(µ-O2CR)]3+
[(LMe)CoII2(µ-O2COMe)]+
[(LMe)CoIICoIII(µ-O2COMe)]2+
10
11f
12f
[(LMe)CoIII2(µ-O2COMe)]3+
n+
[(L′)3CoIII
[(L′′)CoIII
]
]
2.6 × 106
2.8 × 106
2
n+
2
a Data recorded using the perchlorate salts in CH3CN solution. All potentials are referenced to SCE. For experimental conditions see the Experimental
Section. b Ex ) (Eox + Eredp)/2 for reversible one-electron-transfer processes; values in parentheses represent peak-to-peak separations (∆Ep
)
1/2
p
|Eox - Eredp|). c ∆Ex ) |E1 - E21/2|. d Kc ) exp(38.94∆Ex1/2), at 298 K. e Reference 32. f Reference 40.
p
1/2
1/2
[(LMe)CoIII2(µ-O2CCHdCHPh)](ClO4)3 (7‚(ClO4)3) in a simi-
lar good yield (75%). The solubilities of the two fully
oxidized complexes were not as good as those of 3‚(ClO4)2
and 6‚(ClO4)2.
Table 3. Selected UV/vis and Infrared Spectroscopic Data for
Compounds 1-10a
b
c
no.
λ
max/nm (ꢀ/M-1 cm-1
)
νas, νs (CO2)
∆
1d 468 (590), 545 (162), 571 (137), 1237 (23)
2
440 (467), 523 (170), 542 (121), 565 (64),
608 (21), 1262 (33)
1587, 1434
153
Synthesis of µ-Alkyl Carbonato Complexes 9 and 10.
The bridging methyl carbonate group in complex [(LMe)-
3
4
5
456 (6434), 710 (1277)
465 (12037), 640 (2664)
445 (487), 524 (193), 544 (138), 567 (80),
606 (36), 1259 (43)
1575, 1424
1525, 1427
1576, 1407
151
98
169
CoII (µ-O2COMe)]+ (8) is, unlike the carboxylates in 2 and
2
5, a potentially hydolyzable substrate. As can be seen in eq
5, treatment of 8 with an excess of bromine, using reactions
6
7
8
458 (6604), 710 (1330)
467 (11598), 637 (2593)
441 (521), 526 (176), 543 (144), 563 (78),
606 (40), 1253 (41)
1558, 1391
1508, 1388
1630, 1336
167
120
294
9
10
450 (5944), 684 (1287)
466 (9000), 643 (2084)
1610, 1344
1556, 1353
266
203
a The UV/vis and IR spectra were taken using the perchlorate salts.
b Spectra were recorded in CH3CN solution at 295 K. Concentration of
solutions were ∼1.0 × 10-3 for CoIICoII forms and ∼1.0 × 10-5 for
CoIICoIII and CoIIICoIII complexes. c ∆ ) νas - νs. d Reference 32.
conditions similar to those described above for 4, produced
the CoIIICoIII complex 10 in good yields. Likewise, oxidation
of 8 with 10 yielded the mixed-valent CoIICoIII form 9. These
results demonstrate that all three members of this series are
also readily available.
It is noted that the oxidations are chemically reversible.
Thus, addition of suitable reducing agents such as NaBH4
to either the CoIICoIII or the CoIIICoIII species re-forms the
parent CoIICoII complexes. The new compounds gave
satisfactory elemental analyses and were characterized by
cyclic voltammetry, spectroscopic methods (UV/vis, 1H
NMR, and infrared spectroscopy), and, where possible,
single-crystal X-ray diffraction analyses.
Electrochemistry. All complexes were characterized by
cyclic voltammetry. Cyclic voltammograms have been
recorded in CH3CN solution with [Bu4N][PF6] as the
supporting electrolyte. All potentials reported are referenced
versus SCE. The electrochemical data are summarized in
Table 2.
CoIICoIII form to the fully oxidized CoIIICoIII form 4. The
CVs of the other two cobalt complexes, 5‚(ClO4) and 8‚
(ClO4), show, as expected, only very small differences from
that of 2‚(ClO4) (Table 2). Again, the CVs of their oxidized
derivatives are identical within experimental error.
The mixed-valent complexes are very stable to dispro-
portionation, as quantified by the comproportionation con-
stants Kc defined for the equilibrium shown in eq 6.
[(LMe)CoII (µ-O2CR)]+ + [(LMe)CoIII2(µ-O2CR)]3+
)
2
2[(LMe)CoIICoIII(µ-O2CR)]2+ Kc (6)
These constants are calculated by using the expression in
eq 7
∆E ) |E11/2 - E21/2| ) (RT/nF) ln(Kc)
(7)
The cyclic voltammogram (CV) of 2‚(ClO4) displays two
reversible redox waves at E11/2 ) +0.22 V (∆Ep ) 122 mV)
where ∆E is the separation of potentials for successive one-
and E2 ) +0.60 V (∆Ep ) 150 mV) vs SCE (Figure S1,
electron processes in volts.49
1/2
Supporting Information). The CVs of 3‚(ClO4)2 and 4‚(ClO4)3
using the same experimental conditions are identical within
experimental error. Thus, the reversible oxidation at E1 is
assigned to the one-electron oxidation of the CoIICoII form
to the mixed-valent CoIICoIII form 3, and the second redox
wave at E2 is assigned to the oxidation of the mixed-valent
The comproportionation constants are in the range (1.81-
2.67) × 106, showing no significant disproportionation of
the mixed-valent species 3, 6, and 9. Because of negligible
(49) See, for example: Flanagan, J. B.; Margel, S.; Bard, A. J.; Anson, F.
C. J. Am. Chem. Soc. 1978, 100, 4248-4253.
Inorganic Chemistry, Vol. 41, No. 5, 2002 1145

Kersting and Steinfeld
metal-metal interactions (see below), the large ∆E values
must be attributed to other factors such as electrostatic
effects.50
by strong thiophenolate-to-cobalt(III) charge transfer transi-
tions at ∼460 and 710 nm and at ∼465 and 640 nm,
respectively. Intense LMCT transitions are characteristic of
thiolate-bridged cobalt complexes, particularly for the more
oxidized states.32-40 The LMCT transitions of the fully
oxidized complexes are approximately twice the intensity
of the charge transfer transitions of the CoIICoIII forms,
consistent with the presence of two CoIIIN3S2O chromophores
in the former and only one such chromophore in the latter
species.
Numerous mono- and binuclear cobalt(III) amine thiolate
complexes have been prepared prior to this work,33-40 and
their electrochemical properties have been reported. There
are no previous examples in which all three members of
binuclear cobalt amine thiolate complexes have been isolated
and structurally characterized (with the same ligand). For
3+
example, the binuclear complexes [(L′)3CoIII
]
2
11 and
[(L′′)CoIII 3+ 12, where HL′ and H3L′′ represent the multi-
]
2
From the X-ray data described below one could expect a
superposition of the spectra of the constituent N3CoIIS2O and
N3CoIIIS2O ions for the mixed-valent species 3, 6, and 9.
Such a situation with distinct localized CoII and CoIII sites
would correspond to class I in Robin and Day’s classification
of mixed-valence species.54,55 In the other extreme (class IIIa
mixed-valence species featuring strong metal-metal interac-
tions) the properties of the component species are replaced
by those of a new delocalized species. Between these two
extremes lies a wide range of intermediate cases with many
gradations of metal-metal interactions (class II). However,
due to the intense ligand-to-metal charge transfer transitions
in the UV/vis spectra of the mixed-valent species (which
would obscure any of the rather weak d-d transitions of a
distinct localized CoII ion), UV/vis spectroscopy is not suited
to position the complexes in this scale. In addition, there
are no additional bands (in the 300-1600 nm region) which
could be attributable to intervalence transfer (IT) bands. It
should, however, be noted that these bands can be very weak
and situated in different areas of the spectrum, from the
visible to the mid-IR.56
dentate amine thiophenolate ligands 2,6-bis(aminomethyl)-
4-tert-butylthiophenol and N,N′,N′′-tris[2-thio-3-aminomethyl-
5-tert-butylbenzyl]-1,1,1-tris(aminomethyl)ethane, respectively,
have been isolated, but only in their CoIIICoIII forms.40 The
redox potentials of 11 and 12 are shifted by ca. 1 V to more
negative values, relative to those of the complexes described
in this study. The shifts in the redox potentials are consistent
with the change of the donor set from CoN3S2O in 2-10 to
CoN3S3 in 11 and 12. The large cathodic shifts cannot be
explained by the change of donor atoms in the bridging
region of the two metal ions alone. For example, the
replacement of one bridging thiophenolate function in 11 to
a bridging carboxylate ligand in 2 can be estimated to shift
the CoIII/II potentials by not more than ca. +0.50 V.51 Clearly,
the conversion of the terminal amine nitrogen donor atoms
from secondary in 11 and 12 to tertiary in 2-10 must also
be taken into account.
Spectroscopic Properties of 2-10. UV/vis Spectroscopy.
The electronic absorption spectra of the complexes have been
recorded in the range 300-1600 nm in acetonitrile solution
at ambient temperature. The spectra of complexes 2-4 are
representative for all other complexes; they are displayed in
Figure S2 (Supporting Information). The UV/vis spectral data
are reported in Table 3.
1H NMR Spectroscopy. The complexes 2, 4, 5, and 7
1
were further characterized by H NMR spectroscopy. The
1H NMR chemical shifts are reported in the Experimental
1
Section. The H NMR spectra obtained for the dicobalt(II)
The spectra of the pale red CoII2 complexes are very similar
and display typical weak d-d transitions of octahedral high-
spin cobalt(II) (d7) in the 300-1600 nm range.52,53 Three
spin-allowed d-d transitions are expected (4T1g(F) f 4T1g(P),
complexes 2 and 5 in CD3CN solutions at 293 K are broad
with chemical shifts ranging from +200 to -70 ppm, a range
that is consistent with the presence of paramagnetic com-
pounds.57 The spectrum of 2 shows only 11 resonances,
indicative of local C2V symmetry of the monocation in
solution. The resonances at 200.5, 147.4, 77.8, 76.9, -53.0,
and -66.0 ppm can be assigned to the six nonequivalent
methylene protons, whereas signals at 132.5 and 36.1 ppm
are due to protons of the two nonequivalent NCH3 groups.
The aromatic protons resonate at 22.3 ppm, and the t-Bu
groups give rise to a signal at 8.4 ppm. The corresponding
resonances for 5 were observed at nearly the same values.
The highly deshielded signal at 178.5 ppm in the spectrum
of 2 is assigned to the CH3 protons of the bridging acetate
group, on the basis of both its relative integration and the
absence of this signal in the spectrum of complex 5. The
4
4
4
4
4T1g(F) f A2g, and T1g(F) f T2g); however, the T1g(P)
4
and A2g levels are usually of the same energy and the
transitions to these two levels are close together.52 The
assignment of the weak band at ∼1260 nm to the 4T1g(F) f
4T2g transition is unequivocal. The more intense band at ∼450
nm most probably arises from a thiolate-to-cobalt(II) charge
transfer (LMCT) transition. All cobalt(II) complexes display
an absorption band at ∼520 nm with three shoulders at ∼540,
565, and 600 nm. These bands are attributable to components
4
of the parent octahedral ligand field transitions, T1g(F) f
4
4
4T1g(P) and T1g(F) f A2g, split by lower symmetry.
The UV/vis spectra of the mixed-valent CoIICoIII com-
plexes and the fully oxidized CoIIICoIII species are dominated
(54) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1-73.
(55) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10, 247-
422.
(56) Kaim, W.; Bruns, W.; Kohlmann, S.; Krejcik, M. Inorg. Chim. Acta
1995, 229, 143-151.
(57) Bertini, I.; Luchinat, C.; Parigi, G. Solution NMR of Paramagnetic
Molecules, Current Methods. In Inorganic Chemistry; Elsevier:
Amsterdam, 2001; Vol. 2.
(50) Sutton, J. E.; Taube, H. Inorg. Chem. 1981, 20, 3125-3134.
(51) Kru¨ger, H.-J.; Holm, R. H. J. Am. Chem. Soc. 1990, 112, 2955-2963.
(52) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th ed.; John Wiley & Sons: New York, 1999.
(53) Figgis, B. N. Introduction to Ligand Fields; Interscience: New York,
1986.
1146 Inorganic Chemistry, Vol. 41, No. 5, 2002

Dicobalt Amine Thiophenolate Complexes
Figure 1. Perspective view of the cation [(LMe)CoII2(µ-O2CMe)]+ in
crystals of 2‚BPh4. Atoms are drawn at the 50% probability level. Hydrogen
atoms are omitted for clarity.
signals for the protons of the cinnamate ion in 5 are also
deshielded. The deshielding of the protons increases (18.72,
18.76, 39.81, 106.9 (CO2CHdCH), 185.7(CO2CH)CH))
with decreasing distance from the paramagnetic cobalt(II)
ions.
Figure 2. Perspective view of the cation [(LMe)CoIII2(µ-O2CCHdCHPh)]3+
in crystals of 7‚(ClO4)3. Atoms are drawn at the 50% probability level.
Hydrogen atoms are omitted for clarity.
In contrast, the signals in the 1H NMR spectra of 4 and 7
under the same conditions are observed in the range 0-11
ppm, which is normal for CoIII compounds with low-spin d6
configuration. For 4, the aromatic and the tert-butyl protons
appear as sharp singlets at 7.31 and at 1.22 ppm, again
indicative of local C2V symmetry in solution. Unfortunately,
the signals in the 2.0-4.5 ppm region could not be assigned.
It should be noted that the signal for the methyl protons of
the coordinated acetate group is at 1.00 ppm, whereas this
signal is observed at 1.83 ppm for sodium acetate.32 The
shift changes to a higher field can be explained by the fact
that the coligand is situated in the hydrophobic binding
pocket (i.e., the shielding region of the two aryl rings of
(LMe)2-). The signals for the two olefinic CH protons of the
cinnamate ion in 7 at 6.63 and 5.84 are also shifted downfield
from their values in sodium cinnamate (which are observed
at 7.65, 6.53 ppm in CD3CN).
The 1H NMR spectrum of the mixed-valent species 3 was
less informative. The signals at 3.8 and 23.4 ppm were sharp
singlets. Further very broad signals were observed at 7.15,
12.7, and-29.2 ppm, while signals at ∼77, 65, and 42 ppm
were only just above the baseline level.58
Descriptions of the Crystal Structures. The crystal
structures of complexes 2‚BPh4‚MeCN, 3‚(I3)2, 5‚BPh4‚
2MeCN, 6‚(ClO4)2‚EtOH, 7‚(ClO4)3‚MeCN‚(H2O)3, and 9‚
(ClO4)2‚(MeOH)2‚H2O were determined by single-crystal
X-ray crystallography at 210 K. Suitable crystals of 4‚(ClO4)3
and of 10‚(ClO4)3 were not obtained. The structures of the
complexes are similar to one another. The structures of 2
and 7 are shown in Figures 1 and 2. The structures of the
other complexes are shown in Figures S3-S6 of the
Supporting Information. Table 4 lists the individual metal-
ligand bond lengths and gives information about selected
C-O and C-C distances of the coligands. All other bond
distances and angles are available in the Supporting Informa-
tion. The atomic numbering scheme for the central N3Co-
(µ-S)2(µ-O2CR)CoN3 core in 2‚BPh4 is depicted in Table 4.
This was used for all compounds.
The crystal structure of 2‚BPh4‚MeCN is composed of
discrete [(LMe)CoII(µ-O2CMe)]+ cations, tetraphenylborate
anions, and acetonitrile molecules of solvent of crystalliza-
tion. The structure of the cation is that of type B. The position
X is occupied by an acetate group. It bridges the two CoII
centers in a symmetrical µ-1,3 fashion, at a distance of
3.448(1) Å. The idealized symmetry is Cs with a mirror plane
passing through the two S atoms and the carboxyl carbon
atom of the acetate group. Both cobalt atoms are six-
coordinate with considerably distorted pseudooctahedral
geometry. The corresponding bond lengths at Co(1) and
Co(2) differ only slightly, but the individual Co-N and
Co-S bond lengths vary widely. The Co(1)-N(2) and the
Co(2)-N(5) bonds at 2.188(3) and 2.195(3) Å, respectively,
are the shortest Co-N bonds; both are in a trans position
relative to the coligand. The Co(1)-N(1) and Co(2)-N(6)
bonds at 2.345(3) and 2.335(3) Å, respectively, are the
longest Co-N bonds; both are in a trans position relative to
the two short Co-S bonds at 2.475(1) and 2.472(1) Å. The
remaining two Co-N bonds at 2.277(3) and 2.285(3) Å are
in a trans position to the long Co-S bonds. Remarkably,
the same pattern is found in the corresponding acetato-
bridged complexes of MnII,59 FeII,59 NiII, and ZnII,32 which
are all isostructural with 2. This is also observed for 3, 5, 6,
7, 8,32 and 9. The observed differences in the individual
Co-N and Co-S distances can therefore be attributed to
steric constraints of the macrocyclic ligand, rather than to a
specific metal dn electronic configuration. Finally, the C-O
distances at 1.258(4) and 1.264(4) Å are identical within
(58) Preliminary EPR measurements confirm that the CoII and CoIICoIII
2
complexes 2 and 3 are paramagnetic species. The CoIIICoIII complex
is EPR silent. The EPR spectrum of 3 (Figure S7, Supporting
Information) reveals a strong signal centered at gav > ∼4.0 indicative
of an S ) 3/2 spin ground state. We anticipate a more detailed study
of the electronic structures of compounds 2 and 3 by temperature-
dependent magnetic susceptibility measurements.
(59) Kersting, B.; Steinfeld, G. Unpublished results.
Inorganic Chemistry, Vol. 41, No. 5, 2002 1147

Kersting and Steinfeld
Table 4. Selected Bond Lengths of Cobalt Complexes 2, 3, 5, 6, 7, and 9
a There are two crystallographically independent molecules A and B in the unit cell. Values in square brackets refer to the corresponding bond lengths
of molecule B. b R ) CHPh.
Scheme 3. Schematic Representation of the Binding Mode of the
Cinnamate Ion in 5-7
for cobalt(II) complexes with this macrocyclic amine thio-
phenolate ligand.32 The cis and trans L-Co-L bond angles
deviate by as much as 20° (for O(1)-Co-N(2), O(2)-
Co(2)-N(5)) from their ideal values. As in 2, the distortions
can be traced back to the inflexible ligand geometry.
The crystal structure of 3‚(I3)2 consists of discrete [(LMe)-
CoIICoIII(µ-O2CMe)]2+ cations (Figure S3, Supporting In-
formation) and I3- anions. When the different oxidation states
of the cobalt ions in the dication 3 are neglected, the complex
can be regarded as isostructural with its parent dicobalt(II)
complex. Thus, oxidation of 2 does not affect its overall
bowl-shaped structure and occurs without loss of the bridging
coligand. The different metal-ligand bond lengths for the
two cobalt centers reflect the mixed-valent nature of the
complex and the metal-centered nature of the oxidation of
2. The dication is assigned a N3CoII(µ-SR)2(µ-O2CMe)CoIIIN3
experimental error; they are typical for bridging acetate
core structure with high-spin CoII (Co(1)) and low-spin CoIII
(Co(2)) ions, because the Co(1)-ligand distances are com-
parable with those of 2, a complex which comprises of two
high-spin CoII ions. The average Co(2)-N and Co(2)-S
bond lengths, at 2.145 and 2.334 Å, and the Co(2)-O(2)
distance of 1.904(3) Å are all short, confirming this assign-
ment. These distances are too short for an octahedral CoII
complex, but are in excellent accord with those of related
low-spin CoIIIN6-xSx64,65 and CoIIIN3S2O66 complexes. The
individual bond lengths and angles of the two thiophenolate
head units in (LMe)2- show no unusual features and are almost
identical with those in 2. This rules out the possibility that
the ligand is coordinated as a thiyl radical.67
groups.15,60,61
The crystal structure of 5‚BPh4‚2MeCN consists of discrete
[(LMe)CoII (µ-O2CCHdCHPh)]+ cations, tetraphenylborate
2
anions, and acetonitrile molecules of solvent of crystalliza-
tion. Again, the structure of the cation is of the form B
(Figure S4, Supporting Information). The cinnamate ion
bridges the two six-coordinate CoII ions in a symmetrical
µ-1,3 fashion. The substituents at the carbon-carbon double
bond are in a trans position. Note that the cinnamate ion is
not planar. The phenyl ring is twisted by 35° relative to the
alkene plane (Scheme 3). Table 4 shows that the respective
metal-ligand bond lengths are essentially identical with
those found for 2. The average Co-N bond length of 2.278-
(3) Å is significantly longer than in [CoII(NH3)6]Cl2,62 at
2.170(2) Å, and [CoII(tacn)2]I2,63 at 2.155(15) Å (tacn )
1,4,7-triazacyclononane), but comparable to literature values
(63) Ku¨ppers, H.-J.; Neves, A.; Pomp, C.; Ventur, D.; Wieghardt, K.; Nuber,
B.; Weiss, J. Inorg. Chem. 1986, 25, 2400-2408.
(64) Tyler, L. A.; Noveron, J. C.; Olmstead, M. M.; Mascharak, P. K. Inorg.
Chem. 2000, 39, 357-362.
(65) Higgs, T. C.; Ji, D.; Czernuszewicz, R. S.; Matzanke, B. F.;
Schu¨nemann, V.; Trautwein, A. X.; Helliwell, M.; Ramirez, W.;
Carrano, C. J. Inorg. Chem. 1998, 37, 2383-2392.
(66) Klingele, M. H.; Steinfeld, G.; Kersting, B. Z. Naturforsch., B 2001,
56b, 901-907.
(60) Turowski, P. N.; Armstrong, W. H.; Liu, S.; Brown, S. N.; Lippard,
S. J. Inorg. Chem. 1994, 33, 636-645.
(61) Aquino, M. A. S. Coord. Chem. ReV. 1998, 170, 141-142.
(62) Newman, J. M.; Binns, M.; Hambley, T. W.; Freeman, H. C. Inorg.
Chem. 1991, 30, 3499-3502.
1148 Inorganic Chemistry, Vol. 41, No. 5, 2002

Dicobalt Amine Thiophenolate Complexes
The crystal structure of 6‚(ClO4)2 reveals the presence of
discrete [(LMe)CoIICoIII(µ-O2CCHdCHPh)]2+ dications (Fig-
ure S5, Supporting Information). There are two crystallo-
graphically independent molecules in the asymmetric unit.
Since the two structures are essentially identical, the descrip-
tion will focus on molecule A. Unlike 5, the cinnamate ion
bridges the two Co atoms in an asymmetric manner, because
of short Co(III)-O and long Co(II)-O distances. The C-O
bond lengths are, however, uniform, indicative of a sp2-
hybridized carboxylate function, as is observed in the parent
CoIICoII form 5. Likewise, the phenyl ring of the cinnamate
ion is twisted out of the alkene plane, albeit to a lesser degree
(31°). The cation has the same average Co-N and Co-S
distances as observed in the mixed-valent CoIICoIII species
3. Thus Co(1) can be assigned an oxidation state +II (d7,
high spin) and Co(2) an oxidation state +III (d6, low spin).
The X-ray structure analysis of 7‚(ClO4)3 shows the
presence of discrete [(LMe)CoIII2(µ-O2CCHdCHPh)]3+ tri-
cations of the structure type B. This result confirms
unambiguously that the overall structure of the parent
complexes is retained in all three oxidation states (Figure
2). However, there are several differences that are worth
considering. First, the short average Co-N and Co-S
distances (for Co(1) 2.115(3) and 2.283(4) Å, for Co(2)
2.102(3) and 2.290(3) Å) together with the short Co-O bond
lengths at 1.851(9) and 1.876(9) Å are now only compatible
with a low-spin d6 configuration for both Co atoms. Second,
the cinnamate ligand shows a short C-O distance at 1.24-
(2) Å (C(39)-O(1)) and a long C(39)-O(2) bond at 1.35-
(2) Å, indicating partial double-bond character for the
C(39)-O(1) bond. Third, the cinnamate ion is almost planar
(the maximum deviations from the best least-squares plane
are at 0.02 Å). This is a striking contrast to the twisted
conformations found in the two reduced forms. Clearly, the
binding mode (the electronic structure) of the carboxylate
ligands is a function of the oxidation states of the two metal
ions. The observed trends are fully consistent with the
observed shifts in vibrational frequencies across the series
(see below).
decreasing dielectric constant of the medium.70 Clearly, more
data are needed to further investigate these phenomena.
Crystals of 9‚(ClO4)2 consist of [(LMe)CoIICoIII(µ-O2-
COMe)]2+ cations (Figure S6, Supporting Information) and
well-separated ClO4- anions. The metal-ligand bond lengths
compare well with those observed for the CoIICoIII complexes
3 and 6. Further details of the structure may be found in the
Supporting Information.
Infrared Spectroscopy. The infrared spectra of 2-10
display the bands expected for the macrocyclic amine ligand
-
(LMe)2-, counterions (ClO4 ), and carboxylate and alkyl
-
carbonate coligands. The ClO4 ions give rise to a broad
band at 1100 cm-1, which is normal for this counterion. Each
spectrum displays two further strong absorptions in the
1650-1300 cm-1 region, which are associated with the
asymmetric (νas(C-O)) and symmetric stretching modes
(νs(C-O)) of the carboxylate and alkyl carbonate coligands,
respectively.71 The observed values for νas and νs for the
dicobalt(II) complexes 2 and 5 are typical for symmetrical
carboxylate bridges. The separations (∆ ) νas - νs) between
the two C-O stretching frequencies at 153 and 169 cm-1
are also normal.71,72 As can be seen in Table 3, the
corresponding frequencies in the mixed-valent species 3 and
6 are all shifted slightly to lower frequencies (by maximal
18 cm-1) with the ∆ values remaining essentially unchanged.
Upon conversion to the CoIIICoIII forms, the C-O stretching
frequencies are much more affected; in particular, the νas
values are shifted by 50 cm-1 to lower wavenumbers when
compared with the corresponding values of the reduced
members of the respective series. Note that the same trend
is observed in the methyl carbonato complexes 8, 9, and 10.
The decreases in C-O stretching frequencies upon conver-
sion from the CoII2 to the CoIII2 forms are indicative of weaker
C-O bonds in the oxidized CoIIICoIII complexes. This is
consistent with the results from the X-ray crystal structure
determinations described above. The conclusion that emerges
is that nucleophilic substitutions at the carboxyl carbon atoms
of the substrates, in particular for those of the alkyl
carbonates, should become more feasible in the fully oxidized
forms.73 Kinetic studies are underway to explore the chemical
reactivity of the complexes as a function of the metal
oxidation states.
-
It should be noted that the presence of asymmetric RCO2
bridges is unusual for carboxylato-bridged dicobalt(III)
complexes. For example, the carboxylate substituents in
[{(tacn)Co}2(µ-OH)2(µ-O2CMe)]3+ and [{(NH3)3Co}2(µ-
OH)2(µ-O2CR)]3+ assume uniform C-O bond lengths which
are only compatible with sp2-hybridized carboxylate func-
Conclusion
The carboxylato and methyl carbonato bridged dicobalt
complexes 2-10 are redox-active species that undergo two
stepwise one-electron-oxidation processes. All three members
of each series are cleanly obtained by chemical oxidation of
the parent CoIICoII forms by using mild oxidants. The
monooxidized complexes are mixed-valent CoIICoIII com-
pounds with localized high-spin CoII (d7) and low-spin CoIII
(d6) ions. The fully oxidized forms represent diamagnetic
tions.68,69 In the present CoIII complexes, the bonding
2
situation of the carboxylates allows for assuming a coordi-
nated carbonyl group (C(39)dO(1)) and an sp3-hybridized
oxygen atom (C(39)-O(2)) as in carboxylic acids (RCOOH).
The differences may be attributed to ligand constraints or to
the more hydrophobic microenvironment about the positively
charged CoIII centers and the negatively charged carboxylate
ligands. It is well-known that Coulombic forces increase with
(70) Zubay, G. L. Biochemistry, 4th ed.; McGraw-Hill: New York, 1998.
(71) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds, 5th ed.; Wiley: New York, 1997.
(72) Deacon, G. B.; Phillips, R. J. Coord. Chem. ReV. 1980, 33, 227-250.
(73) Wahnon, D.; Lebuis, A.-M.; Chin, J. Angew. Chem., Int. Ed. Engl.
1995, 34, 2412-2416.
(67) Kimura, S.; Bothe, E.; Bill, E.; Weyhermu¨ller, T.; Wieghardt, K. J.
Inorg. Biochem. 2001, 86, 294.
(68) Wieghardt, K.; Schmidt, W.; Nuber, B.; Weiss, J. Chem. Ber. 1979,
112, 2220-2230.
(69) Maas, G. Z. Anorg. Allg. Chem. 1977, 432, 203-210.
Inorganic Chemistry, Vol. 41, No. 5, 2002 1149

Kersting and Steinfeld
CoIII species with low-spin d6 configuration for both CoIII
for his generous and continuous support. The authors thank
Gabriel Siedle for recording NMR spectra and Emma Craven
for reading the manuscript.
2
ions. The metal-centered redox reactions do not affect the
overall bowl-shaped structure of the parent complexes, the
coligands remain in a bridging position, and the solid-state
structures are retained in the solution state. Infrared spectral
changes that occur upon oxidation of the complexes suggest
that the substrates are more susceptible to nucleophilic attack
by the solvent or other reagents.
Supporting Information Available: Six X-ray crystallographic
files for 2, 3, 5, 6, 7, and 9 in CIF format; Figure S1 of the cyclic
voltammogram of compound 2; Figure S2 of the electronic
absorption spectra of complexes 2-4; Figures S3-S6 of the
molecular structures of complexes 3, 5, 6, and 9; Figure S7 of the
EPR spectra of complexes 2 and 3. This material is available free
of charge via the Internet at http://pubs.acs.org.
Acknowledgment. This work was supported by the
Deutsche Forschungsgemeinschaft (Project No. KE 585/3-
1). We are particulary grateful to Prof. Dr. H. Vahrenkamp
IC011004N
1150 Inorganic Chemistry, Vol. 41, No. 5, 2002