The molecular and electronic structures of monomeric cobalt complexes containing redox noninnocent o-aminobenzenethiolate ligands

Article abstract of DOI:10.1016/j.ica.2010.03.042

Dark blue [PPh₄][Co^III(²L)] (2), where (²L)^2- represents the closed-shell dianion of 4,6-di-tert-butyl-2-[(pentafluorophenyl)amino]benzenethiol, has been synthesized from the reaction of H₂

Full text of DOI:10.1016/j.ica.2010.03.042

Inorganica Chimica Acta 363 (2010) 2702–2714
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Inorganica Chimica Acta
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The molecular and electronic structures of monomeric cobalt complexes
containing redox noninnocent o-aminobenzenethiolate ligands
*
Stephen Sproules, Ruta R. Kapre, Nabarun Roy, Thomas Weyhermüller, Karl Wieghardt
Max-Planck-Institut für Bioanorganische Chemie, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany
a r t i c l e i n f o
a b s t r a c t
Dark blue [PPh4][CoIII(2L)] (2), where (2L)2ꢀ represents the closed-shell dianion of 4,6-di-tert-butyl-2-
[(pentafluorophenyl)amino]benzenethiol, has been synthesized from the reaction of H2(2L) and CoCl2
(2:1) in acetonitrile with excess NEt3, brief exposure of the solution to air, and addition of [PPh4]Br.
The oxidation of 2 with one equivalent of iodine produces the neutral species [CoIII(2Lꢀ)2I]0 (3), where
Article history:
Received 16 February 2010
Accepted 12 March 2010
Available online 19 March 2010
(2Lꢀ)1ꢀ represents the one-electron oxidized
(4Lꢀ)3ꢀ is the
p
radical anion of (2L)2ꢀ. Crystalline [CoIII(4Lꢀ)] (4), where
Dedicated to Animesh Chakravorty
p
radical monoanion of bis-2,20-(1,2-diphenylethylenediimine)-benzenethiolate, was precip-
itated from a toluene reflux of [CoII(3L)2], where (3L)2ꢀ is the closed-shell monoanion of 2-(phenylmeth-
ylamino)benzenethiol. The reduction of 4 with CoCp2 under anaerobic conditions yielded dark violet
crystals of [CoCp2][CoIII(4L)] (5). The reaction of Zn(CH3CO2)2 with 2-phenylbenzothiazoline in methanol
resulted in the formation of [ZnII(3L)2]0 (6). The two monoanions 2, and 5, along with [N(n-Bu)4][Co(abt)2]
(1) (abt2ꢀ = o-aminobenzenethiolate), and neutral 4 have all been shown by X-ray crystallography to be
square planar. A tetrahedral geometry was adopted by 6. From temperature dependent (3–300 K) mag-
netic susceptibility measurements, it was established the monoanions have a triplet ground state char-
acterized by a large zero field splitting. EPR measurements of 4, and electrochemically oxidized 1 and
2 reveal distinctly different spin Hamiltonian parameters that are interpreted with the aid of density
function theoretical (DFT) calculations. It is shown that oxidation states describing a d6 Co(III) or d7 Co(II)
cannot be unambiguously assigned for these neutral and monoanionic species.
Keywords:
Cobalt
Aminobenzenethiolate
Noninnocent ligands
Electronic structure
DFT
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
techniques [3–5], that facilitate a physical oxidation state assign-
ment to the metal (and ligand).
The evaluation of the correct valence electron count for a given
transition metal coordination compound is an important task for
the understanding and prediction of its spectroscopic properties
and or its reactivity. Perhaps the most effective means of account-
ing for electrons is the concept of an oxidation state. A formal oxi-
dation state of any given metal ion is a nonmeasurable integer
which is commonly defined as ‘‘the charge left on the metal after
all the ligands have been removed in their closed-shell form” [1].
The distinction is made with a physical or spectroscopic oxidation
state, a value derived from a known dn configuration that is mea-
surable quantity determined by various spectroscopic and theoret-
ical techniques [2]. In many cases, the formal and physical
oxidation states are identical. However, they differ if one of the li-
gands in the complex is an open-shell organic radical [3]. These
radical ligand metal ion complexes, M–Lꢀ, constitute a rapidly
growing class of coordination compounds. Identification of such a
bonding situation experimentally has been improved by involving
a combination of spectroscopic, crystallographic and theoretical
While there is no impediment to assigning a formal oxidation
state, there are limits to successfully defining a physical oxidation
state, and in the vast array of characterized coordination com-
pounds of the first row transition metals, cobalt complexes are per-
haps the most demanding [6–9]. Several recent studies have
shown that the bonding arrangement in four-coordinate com-
plexes of cobalt with two noninnocent ligands cannot be simply
described using integer oxidation levels for the metal and ligands
because the energy of the cobalt 3d orbitals is comparable to the
ligand
p orbitals [5,6,9].
Herein, we present the coordination chemistry of cobalt com-
plexes chelated by aminobenzenethiolate ligands (Scheme 1). They
can exist at four distinctly different protonation and oxidation lev-
els [10,11]; the structural parameters associated with these states
are shown in Scheme 2. Birker et al. reported the isolation of a
monomeric complex with two unsubstituted o-aminobenzenethio-
late(2ꢀ) ligands, (abt)2ꢀ, bound to cobalt, [N(n-Bu)4][Co(abt)2] (1)
[12]. It was shown to have an S = 1 ground state, however, this
compound had not been structurally characterized. Therefore, we
report its crystal structure along with the structure of [PPh4]-
[CoIII(2L)2] (2), where (2L)2ꢀ represents the sterically encumbered
* Corresponding author.
E-mail address: wieghardt@mpi-muelheim.mpg.de (K. Wieghardt).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.03.042

S. Sproules et al. / Inorganica Chimica Acta 363 (2010) 2702–2714
2703
Scheme 2.
[ZnII(3L)2] (6) as confirmed by a X-ray crystallographic analysis.
Throughout this work, we will describe these compounds based
on the most favored electronic structure as preferred by the exper-
imental data, despite the ambiguity in assigning an oxidation state
for the central cobalt ion.
2. Experimental section
2.1. Synthesis of complexes
All manipulations were carried out with standard Schlenk line
and glove box techniques. Synthesis of the ligand 4,6-di-tert-2-
[(pentafluorophenyl)amino]benzenethiol, H2(2L) has been de-
scribed in Ref. [11]. The compounds [N(n-Bu)4][CoIII(abt)2] (1)
[12], [CoII(3L)2] [13], and [CoIII(4L)] (4) [13] have all be prepared
according to published procedures.
2.2. [PPh4][CoIII(2L)2] (2)
The ligand H2(2L) (0.60 g; 1.50 mmol) was dissolved in dry ace-
tonitrile (15 cm3) under anaerobic conditions. To this yellow solu-
tion was added CoCl2 (96 mg; 0.75 mmol) and triethylamine
(0.40 cm3; 3.0 mmol) and the reaction mixture was stirred under
Ar for 15 min. Upon limited exposure to air, the green solution be-
came dark blue, after which the acetonitrile was removed under
vacuum and replaced by dichloromethane (20 cm3). Tetraphenyl-
phosphonium bromide, [PPh4]Br (0.30 g; 0.75 mmol) was added
under anaerobic conditions (glovebox) and the mixture stirred
for another 15 min. before filtering through Celite. The volume of
the filtrate was reduced to 5 cm3 and n-hexane (25 cm3) was added
to afford a blue precipitate of 1. The powder was separated by fil-
tration and stored under an inert atmosphere. Single crystals suit-
able for X-ray diffraction study were grown by slow diffusion of
diethyl ether into a dichloromethane solution of the product yield-
ing 1ꢁEt2O. Yield: 0.60 g (79%).
Scheme 1. Ligands and complexes.
4,6-di-tert-butyl-2-[(2-pentafluorophenyl)-amino]benzenethiolate(2ꢀ)
ligand [11]. We have also prepared the complex [CoIII(2Lꢀ)2I]
(3) from the oxidation of the parent monoanion with molecular
iodine.
Tetradentate
bis-2,20-(1,2-diphenylethylenediimine)benzen-
ethiolato(4-), (4L)4ꢀ, is formed via a carbon-carbon bond formation
from two 2-(phenylmethyleneamino)benzenethiolate(1ꢀ) units,
(3L)1ꢀ, while coordinated to either a nickel or cobalt transition me-
tal ion (Scheme 3) [13]. The presence of bulky aromatic substitu-
ents ensured the isolation of
a monomeric charge neutral
[CoIII(4Lꢀ)] (4). High-quality X-ray crystallography at cryogenic
temperatures for this and its one-electron reduced monoanion,
[CoCp2][CoIII(4L)] (5) provide a means of determining whether this
redox event is metal- or ligand-centered. Finally, Kochin et al. re-
ported formation of [ZnII(4Lꢀꢀ)] from the reaction of 2-phenylbenzo-
thiazoline with zinc acetate [14]. However, when the reduction of
the alkane bridge with sodium borohydride failed – a technique
partially successful for nickel analogue [15] – the formulation of
this species was doubtful. We show here the compound is actually
Anal. Calc. for C64H60N2F10PS2Co: C, 63.99; H, 5.03; N, 2.33; Co,
4.61. Found: C, 64.06; H, 5.12; N, 2.46; Co, 4.96%. Electrospray ion-
ization (ESI) mass spectrum (CH2Cl2, neg. ion mode): m/z = 860.2
[CoL2]ꢀ.
2.3. [CoIII(2Lꢀ)2I] (3)
The complex [PPh4][Co(2L)2] (2) (0.08 g; 0.07 mmol) was dis-
solved in dry dichloromethane (15 cm3) under anaerobic condi-

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S. Sproules et al. / Inorganica Chimica Acta 363 (2010) 2702–2714
2.7. X-ray crystallographic data collection and refinement of the
structures
Single crystals of compounds 1, 2ꢁEt2O, 4, 5ꢁ0.5MeCN, and 6
were coated with perfluoropolyether, picked up with nylon loops
and were immediately mounted in the nitrogen cold stream of a
Bruker–Nonius KappaCCD diffractometer equipped with a Mo-tar-
get rotating-anode X-ray source. Graphite monochromated Mo K
a
radiation (k = 0.71073 Å) was used throughout. Final cell constants
were obtained from least squares fits of all measured reflections.
Intensity data were corrected for absorption using intensities of
redundant reflections using SADABS [16]. The structures were readily
solved by Patterson methods and subsequent difference Fourier
techniques. The Siemens ShelXTL [17] software package was used
for solution and artwork of the structures, SHELXL97 [18] was used
for the refinement. All non-hydrogen atoms were anisotropically
refined and hydrogen atoms were placed at calculated positions
and refined as riding atoms with isotropic displacement parame-
ters, except for some methyl-hydrogen atoms in disordered aceto-
nitrile molecules of crystallization, which could not be reliably
located. Crystallographic data of the compounds are listed in
Table 1.
Scheme 3. Synthesis of [M(3L)2] and [M(4L)].
tions. To this dark blue solution was added I2 (0.02 g; 0.18 mmol)
dissolved in n-hexane (15 cm3) and stirred for 20 min. The solvent
was then removed in vacuo, the residue reconstituted in diethyl
ether and filtered through Celite. Evaporation of the solvent under
reduced pressure yielded the product as a green black microcrys-
talline solid. Yield: 0.05 g (56%).
Anal. Calc. for C40H40N2F10S2CoI: C, 48.59; H, 4.07; N, 2.84; Co,
5.96; I, 12.83. Found: C, 48.41; H, 3.88; N, 2.86; Co, 5.66; I,
13.06%. ESI mass spectrum (CH2Cl2, neg. ion mode): m/z = 860.9
[M–I]ꢀ.
A phenyl ring in the [PPh4]+ cation (C81–C86), and a diethyl-
ether molecule of crystallization (O100–C104) in 2 were found to
be disordered. A split atom model was refined giving an occupation
ratio of about 0.53:0.47. SAME, EADP and SADI restraints of SHEL-
XL97 were used for the refinement.
An ill defined acetonitrile molecule was detected to be disor-
dered next to a crystallographic inversion center in 5. A split atom
model with constrained bond distances using the DFIX instruction
of ShelXL was refined. Hydrogen atom positions could not be lo-
cated for this molecule.
2.4. [CoIII(4Lꢀ)] (4)
A suspension of [CoII(3L)2] [13] (0.22 g; 0.45 mmol) in toluene
(15 cm3) was heated for 30 min. The dark teal solution was filtered
and the solvent removed by rotary evaporation. The dark blue res-
idue was reconstituted in dichloromethane, filtered, then eluted on
a silica gel (230–400 mesh) chromatography column using dichlo-
romethane as eluent. The first dark blue band was collected and re-
moval of the solvent under reduced pressure yielded 4 as a metallic
aubergine microcrystalline solid. Yield: 0.11 g (50%).
2.8. Physical measurements
Electronic absorption spectra of complexes from the spectro-
electrochemical measurements were recorded on a HP 8452A
diode array spectrophotometer (200–1100 nm). Cyclic voltam-
mograms were recorded with an EG&G potentiostat/galvanostat.
Variable temperature (3–300 K) magnetization data were re-
corded in a 1 T magnetic field on a SQUID magnetometer (MPMS
Quantum Design). Multiple-field variable temperature measure-
ments were done at three different fields (1, 4, and 7 T) for
which the magnetization was equidistantly sampled on a 1/T
temperature scale. The experimental magnetic susceptibility data
were corrected for underlying diamagnetism using tabulated
Pascal’s constants. X-band EPR spectra were recorded on a Bru-
ker ELEXSYS E500 spectrometer and simulated with XSophe dis-
tributed by Bruker Biospin GmbH [19]. Elemental analyses were
performed by H. Kolbe at the Mikroanalytischen Labor in Mül-
heim an der Ruhr, Germany.
Anal. Calc. for C26H20N2S2Co: C, 64.58; H, 4.17; N, 5.80; Co,
12.25. Found: C, 64.4; H, 4.1; N, 5.6; Co, 12.2%.
2.5. [CoCp2][CoIII(4L)] (5)
To a deep blue solution of 4 (0.24 g; 0.50 mmol) in dichloro-
methane (25 cm3) under an Ar atmosphere was added cobaltocene
(95 mg; 0.50 mmol). After the solution was stirred at ambient tem-
perature for 1 h, the violet precipitate that emerged was isolated
by filtration. Single crystals suitable for diffraction analysis were
grown by slow evaporation of the compound dissolved in a 1:1
mixture of acetonitrile/methanol. Yield: 0.26 g (80%).
Anal. Calc. for C36H30N2S2Co2: C, 64.29; H, 4.46; N, 4.17. Found:
C, 64.2; H, 4.4; N, 4.2%.
2.9. Calculations
All DFT calculations were performed with the ORCA program
[20]. The complexes were geometry optimized using the BP86
functional [21]. Scalar relativistic effects have been included
using the ZORA method [22]. Since these species are monoa-
nions, or neutral complexes generated in solution, the conduc-
tor-like screening model (COSMO) was applied using CH2Cl2 as
the solvent [23]. The all-electron basis sets were those reported
by the Ahlrichs group [24,25]. Triple-n-quality basis sets with
one set of polarization functions (TZVP) were used for the cobalt
and the coordinating oxygen, nitrogen and sulfur atoms. The
remaining atoms were described by slightly smaller polarized
split-valence SV(P) basis sets that are double-n-quality in the va-
2.6. [ZnII(3L)2] (6)
Zinc acetate (1.1 g; 5.0 mmol) was added to refluxing solution
of 2-phenylbenzothiazoline (2.1 g; 10.0 mmol) in methanol
(50 cm3). After stirring for 30 min, an orange precipitate was col-
lected by filtration and washed with methanol, then diethyl ether.
X-ray quality single crystals of 6 were obtained by slow evapora-
tion of a dichloromethane/methanol solution of the complex.
Yield: 1.7 g (70%).
Anal. Calc. for C26H20N2S2Zn: C, 63.68; H, 4.08; N, 5.71. Found: C,
63.7; H, 4.2; N, 5.8%.

S. Sproules et al. / Inorganica Chimica Acta 363 (2010) 2702–2714
2705
Table 1
Crystallographic data for 1, 2ꢁEt2O, 4, 5ꢁ0.5MeCN, and 6.
1
2ꢁEt2O
4
5ꢁ0.5MeCN
6
Chemical formula
Formula weight (Fw
Space group
A (Å)
B (Å)
C (Å)
C
28H46N3S2Co
C68H70F10N2OPS2Co
1275.28
P1, No. 2
C26H20N2S2Co
483.49
C37H31.5N2.5S2Co
693.13
C26H20N2S2Zn
489.93
)
547.73
ꢀ
P21/n, No. 14
13.2840(10)
16.1915(15)
13.6370(12)
90
98.972(4)
90.00
2897.3(4)
4
P21/n, No. 14
8.4252(9)
24.270(3)
10.7163(12)
90
101.483(3)
90
2147.4(4)
4
P21/n, No. 14
10.1342(10)
17.186(2)
17.838(2)
90
91.672(3)
90
3105.5(5)
4
P21/n, No. 14
26.557(2)
7.0143(8)
26.686(2)
90
118.652(3)
90
4362.3(7)
8
13.3109(12)
14.5142(12)
17.405(6)
101.217(3)
101.769(3)
98.852(3)
3162.7(5)
2
a
(°)
b (°)
(°)
c
V, Å
Z
T (K)
100(2)
1.256
100(2)
1.339
100(2)
1.495
100(2)
1.483
100(2)
1.492
qcalc. (g cmꢀ3
)
Reflections collected (2Hmax
Unique reflections (I > 2 (I))
Number of parameters (restraints)
)
65433/60.00
8447/6851
222/1
0.71073/7.57
0.0410/1.060
0.0816
62261/62.00
20132/15830
815/31
0.71073/4.36
0.0489/1.020
0.1074
34574/61.92
6805/5278
288/0
0.71073/10.10
0.0486/1.074
0.0883
82019/62.00
9798/8658
398/6
0.71073/12.34
0.0841/1.194
0.2363
125768/63.12
14546/12335
559/0
0.71073/13.34
0.0365/1.063
0.0714
r
k (Å)/
l
(K
a
) (cmꢀ1
)
R1a/goodness of fitb
c
wR2 (I > 2
r
(I))
Residual density (e Åꢀ3
)
+0.46/ꢀ0.26
+1.24/ꢀ1.14
+0.49/ꢀ0.45
+2.200/ꢀ0.784
+0.49/ꢀ0.36
a
b
c
Observation criterion: I > 2
r
(I). R1
=
R
||Fo| ꢀ |Fc||/
R
|Fo|.
GOF = [
wR2 = [
R
[w(Fo2 ꢀ Fc2)2]/(n ꢀ p)]1/2
.
R
[w(F2o ꢀ F2c )2]/
R r
[w(Fo2)2]]1/2 where w = 1/ 2(Fo2) + (aP)2 + bP, P = (Fo2 + 2F2c )/3.
lence region and contain a polarizing set of d functions on the
non-hydrogen atoms [24]. Auxiliary basis sets used to expand
the electron density in the calculations were chosen to match
the orbital basis. The self-consistent field calculations were
tightly converged (1 ꢂ 10-8 Eh in energy, 1 ꢂ 10ꢀ7 Eh in the den-
sity charge, and 1 ꢂ 10ꢀ7 in the maximum element of the DIIS
[26] error vector). The geometry search for all complexes was
carried out in redundant internal coordinates without imposing
geometry constraints. Electronic energies and properties were
calculated using the B3LYP functional [27]. In this case, with
the same basis sets as for the optimizations. Natural population
analysis [28] was done through an interface of ORCA to the gen-
nbo program version 5.0. Corresponding [29] and canonical orbi-
tals and density plots were obtained using Molekel [30]. We
describe our computational results for 3 using the broken-sym-
metry (BS) formalism [11,31].
als [13,33]. The neutral compound [ZnII(3L)2]0 (6) was prepared by
the combination of zinc acetate and 2-phenylbenzothiazoline in
methanol. In contrast to the cobalt compound, refluxing 6 in tolu-
ene did not result in any formation of [ZnII(4Lꢀꢀ)]0 [14].
3.1.1. Crystal structure determination
The crystal structures of complexes 1, 2, 4, 5, and 6 have been
determined by single crystal X-ray crystallography at 100(2) K by
using Mo Ka radiation. Fig. 1 displays the structures of the monoa-
nions in 1 and 2, whereas the structures of the monoanion in 5 and
neutral 6 are shown in Fig. 2. The structure of the neutral molecule
in 4 is very similar to that on the monoanion in 5 and is shown in
the Fig. S1. Selected bond distances and angles are presented in Ta-
bles 2 and 3.
Both monoanions in 1 and 2 are strikingly square planar with a
trans-CoN2S2 moiety, where the cobalt ion occupies a crystallo-
graphic inversion center (a = 0.0°). For 2, the bulky pentafluor-
3. Results and discussion
ophenyl substituents are parallel with respect to each other and
make a dihedral angle of 79.1° to the CoN2S2 plane. The average
C–S, C–N, and aromatic C–C distances are indicative of the pres-
ence of two identical o-iminobenzenethiolate(2ꢀ) ligands (type
C, Scheme 2) rendering the oxidation state of the central cobalt
ion +III. Thus the structural formulae [CoIII(abt)2]1ꢀ and
[CoIII(2L)2]1ꢀ represent the correct oxidation level of the ligands
(type C, Scheme 2) and of the central cobalt ion.
3.1. Synthesis and characterization
Two equivalents of the ligand 4,6-di-tert-butyl-2-[(2-pentaflu-
orophenyl)-amino]benzenethiolate H2(2L), prepared according to
Ref. [11], were combined with one equivalent of CoCl2 and four
equivalents of triethylamine in acetonitrile under anaerobic condi-
tions. A brief exposure to the atmosphere followed by addition of
tetraphenylphosphonium bromide lead to the isolation of dark
blue [PPh4][CoIII(2L)2] (2) in high yields. This compound is readily
oxidized by iodine (Eq. (1)) to the neutral species [CoIII(2Lꢁ)2I]0
(3), which was isolated as black microcrystalline powder.
The structures of the neutral complex in 4 and of the mono-
anion in 5 are also similar; both contain a nearly perfect square
planar cis-CoN2S2 unit. The dihedral angle between the two CoNS
planes,
a, is 8.8° and 2.2° for 4 and 5, respectively. The phenyl
substituents of the alkane bridge are trans with respect to the
CoN2S2 plane, making dihedral angles of 89.5° and 82.4°, and
89.3° and 84.4° for 4 and 5, respectively, and are also nearly par-
allel to each other (4: 11.4°; 5: 22.6°). In the monoanion the
average C–S bond at 1.76(1) Å and the average C–N bond at
1.38(1) Å describe a closed-shell tetraanion (4L)4- (each amino-
benzenethiolate arm type C, Scheme 2) which again renders
the oxidation state of the central cobalt ion +III: [CoIII(4L)]1ꢀ. In
the corresponding neutral complex, the C–S distance in shorter
at 1.737 0.006 Å as is the average C–N bond at 1.367 0.01 Å.
These distances are intermediate between those of a dianion
h
i
h
i
0
½PPh4ꢃ Co ð LÞ þ I2 ! Co ð L Þ2I þ ½PPh4ꢃþ þ Iꢀ
ð1Þ
III
III
ꢀ
2
2
2
The oxidation of 2 to 3 is predominately ligand-based, as de-
tailed below. Similar complexes of this type have been described
recently [32].
Dark aubergine [CoIII(4Lꢀ)]0 (4) was simply prepared via a tolu-
ene reflux of the dark brown [CoII(3L)2]0 precursor in a synthesis
devised by Kawamoto et al. [13] depicted in Scheme 3. This meth-
od has been used to prepare analogous complexes with other met-

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S. Sproules et al. / Inorganica Chimica Acta 363 (2010) 2702–2714
ð3Þ
The CV of 4 recorded in the range ꢀ2.0 to 0.5 V also displays
three transfer waves (Fig. 3). The one-electron reduction and oxi-
dation processes are fully reversible, whereas the second one-elec-
tron reduction to the dianionic species is only quasi-reversible as
determined by controlled potential coulometry; the CV after coulo-
metric reduction exhibiting new peaks. Despite the diminished
intensity for the reversible oxidation, all three waves represent
one-electron processes and the electron transfer series is summa-
rized in Eq. (4).
h
i
h
i
h
i
h
i
2ꢀ
1ꢀ
0
1þ
ꢀe
ꢁ
þe
ꢀe
ꢁ
þe
ꢀe
ꢁ
þe
II
III
III
ꢀ
III
ꢀꢀ
4
4
4
4
Co ð LÞ
Co ð LÞ
Co ð L Þ
Co ð L Þ
ð4Þ
S¼1=2
S¼1
S¼1=2
S¼0
The electronic spectra of 1, 1ox, 2, and 3 are overlaid in Fig. 4.
Monoanionic 1, 2 and 5 possess a similar profile, that is, two in-
tense bands (
e
ꢄ 104 Mꢀ1 cmꢀ1) in the far visible region between
500 and 700 nm (Table 5). The more prominent band at higher en-
ergy has been described as a 1b1u ? 2b2g transition composed of a
mixture of ligand-to-ligand (LLCT) and ligand-to-metal (LMCT)
charge transfer character owing to the considerable ligand content
of the acceptor orbital (see DFT calculations) [6,9,32]. Importantly,
no monoanionic species has any transitions in the near IR region
that is a fingerprint for a ligand mixed valency giving rise to an
intervalence charge transfer (IVCT) band. Similar spectra have been
recorded for square planar monoanionic cobalt complexes with
two dithiolene, o-iminophenolate, and diimine ligands
[6,7,9,32,35–38]. In all cases, the absence of near IR bands supports
an oxidation state of +III for the central cobalt ion.
Fig. 1. Perspective views of the monoanions in crystals of 1 (top) and 2 (bottom).
The electronic spectrum of 4 has one very intense band at
658 nm (
e
= 3.9 ꢂ 104 Mꢀ1 cmꢀ1) and one moderately intense band
type C and a
p radical monoanion of type D (Scheme 2). Thus,
at 805 nm (7800 Mꢀ1 cmꢀ1) that are described as LMCT transitions
(Fig. 5). Most importantly, there is a noticeable IVCT band at
1382 nm (3240 Mꢀ1 cmꢀ1) that has been observed previously in
the spectra of neutral cobalt complexes with similar ligands
[6,7,35–38]. Treatment of 1 with one equivalent to ferrocenium
hexafluorophosphate yielded [Co(abt)2]0, whose electronic spec-
trum shown in Fig. 4, has a relatively weak band at 1173 nm
(1500 Mꢀ1 cmꢀ1). This IVCT band indicates ligand mixed valency
in these neutral species whose electronic structure is defined as
[CoIII(L)(Lꢀ)].
the neutral form contains a one (delocalized)
p radical ligand,
(4Lꢀ)3ꢀ, and a Co(III) central ion: [CoIII(4Lꢀ)]0.
Finally, the neutral complex in 6 contains a tetrahedral ZnN2S2
polyhedron
(a = 81.4°) with two N,S-coordinated, closed-shell
monoanions (3L)1ꢀ. The average C–S distance is long at 1.76 Å
and the average C–N double bond length is observed at 1.29 Å
characteristic for type B ligands.
3.1.2. Electro- and spectroelectrochemistry
Cyclic voltammograms of complexes have been recorded at
25 °C in CH2Cl2 solutions containing 0.1 M [N(n-Bu)4]PF6 support-
ing electrolyte at a glassy carbon working electrode at scan rates
of 25–800 mV sꢀ1. All potentials listed in Table 4 and referenced
versus the ferrocenium/ferrocene couple (Fc+/Fc).
Oxidation of 4 yields a species without any features in the near
IR region (Fig. 5). This monocationic species is described as
[CoIII(4Lꢀꢀ)]1+ where both arms of the tetradentate ligand have been
one-electron oxidized to
p radical oxidation level (type D). Its spec-
trum is similar in band position and intensity to the monoanionic
complexes 1, 2, and 5, and that for five-coordinate 3, which has the
formula [CoIII(2Lꢀ)2I]0 (Fig. 4). Most interestingly, the latter has a
reasonably intense band at 1339 nm (6800 Mꢀ1 cmꢀ1). When com-
pared to the electronic spectra of the corresponding [FeIII(2Lꢀ)I]
[11], the Co analog has its most prominent transition 130 nm lower
in energy than the Fe complex due to the larger effective nuclear
charge of the cobalt ion that stabilizes the d orbital manifold. How-
ever, the Fe compound only has a weak shoulder at 822 nm
(ꢅ3000 Mꢀ1 cmꢀ1), so a shift of ꢅ500 nm is too large to be attrib-
uted to the effective nuclear charge of cobalt alone. The exact nat-
ure of this transition is not known at this time, and highlights the
complexity of the intrinsic electronic structures of these com-
pounds where two redox active ligands are bound to a central Co
ion, in that the highly covalent orbitals give rise to low-energy
LLCT and/or LMCT bands.
The CV of 1 recorded in the range of ꢀ0.2 to 2.0 V displays a
reversible one-electron oxidation at E21=2 ¼ ꢀ0:61 V and a quasi-
reversible reduction at E21=2 ¼ ꢀ1:73 V that are assigned as in Eq.
(2). The one-electron electrochemical or chemical oxidation affords
the neutral species that rapidly dimerizes to [CoIII(abt)(abtꢀ)]2
[32,34]. The presence of the pentafluorophenyl substituents on
(2L)2ꢀ, and phenyl groups on the bridge in (4L)4ꢀ, prevent similar
processes occurring in 2 and 4.
h
i
h
i
h
i
2ꢀ
1ꢀ
0
ꢀe
ꢁ
þe
ꢀe
ꢁ
þe
CoIIðabtÞ2
CoIIIðabtÞ2
CoIIIðabtÞðabtꢀÞ
ð2Þ
S¼1=2
S¼1
S¼1=2
The CV of 2 recorded in the range ꢀ2.0 to 1.0 V is shown in
Fig. S3. Three one-electron transfer waves have been observed:
E21=2 ¼ ꢀ1:54 V; E12=2 ¼ ꢀ0:37 V; E12=2 ¼ ꢀ7:1 V that may be assigned
to the processes described by Eq. (3).

S. Sproules et al. / Inorganica Chimica Acta 363 (2010) 2702–2714
2707
Ms =
1 levels. The experimental data were simulated with
|D| = 47 cmꢀ1 and g = 2.15, consistent with the values reported
for the square planar complexes [Co(tbbdt)2]1- (tbbdt = 3,5-di-
tert-butylbenzene-1,2-dithiolate) [9] and [Co(tdt)2]1ꢀ (tdt = tolu-
ene-3,4-dithiolate) [39]. The sign of D was determined from vari-
able temperature variable field (VTVH) measurements shown in
the inset of Fig. 6: D = +41.2 cmꢀ1; g = 2.003. Very similar results
have been obtained for the monoanionic complexes 2 and 5. Again
an S = 1 ground state has been established for both, with the data
successfully modeled using |D| = 29.0 cmꢀ1 and g = 2.32, for 2
(Fig. S5), and D = +31.0 cmꢀ1, E/D = ꢀ0.385 and g = 2.10 [40], for 5
(Fig. S6).
3.1.4. EPR spectroscopy
X-band EPR spectra have been recorded on electrochemically
oxidized 1 and 2. These two S = 1/2 neutral species have very sim-
ilar EPR parameters, namely a rhombic g-tensor with very little
anisotropy and a modest 59Co (I = 7/2, 100% natural abundance)
magnetic hyperfine coupling. Fig. 7 shows the spectrum of 1ox
;
the spectrum of 2ox is found in Fig. S7. Table 7 summarized the spin
Hamiltonian parameters obtained from simulations of the spectra.
In contrast, the EPR spectrum of 4 has a large anisotropy
(Dg = 0.686), with the magnetic hyperfine equivalent to the elec-
trochemically generated species described above (Fig. 8). Clearly
a different parentage of the unpaired spin is seen in 4 compared
to 1ox and 2ox. Other chemically isolated neutral species with
two noninnocent bidentate ligands exhibit similar g anisotropy
and hyperfine couplings (Table 7) that are typical of cobalt-based
paramagnets irrespective of how the spin system is defined [6–
8,35,37,41,42]. The EPR spectra of the latter closely resemble that
reported for octahedral Co(III) (d6, SCo = 0) complex coordinated
by
p
radical ligands, such as [(tren)CoIII(tmbdtꢀ)]2+ (tren = tris(2-
aminoehtyl)amine;
tmbdt = 3,6-bis(triemthylsilyl)benzene-1,2-
dithiolate) [43].
In order to rationalize the different spin Hamiltonian parame-
ters, it has been proposed that electrochemically generated 1ox
and 2ox attract two adventitious water molecules that stabilize a
diamagnetic, low-spin Co(III) central ion that is ligated by a
p rad-
ical monoanion (Lꢀ)1ꢀ (SL = 1/2). This is the analogous electronic
structure to [(tren)CoIII(tmbdtꢀ)]2+. As detailed below, DFT calcula-
tions support such a structure, though crucially only the cis-aquo
complex gives the desired spin ground state; the trans-aquo spe-
cies is calculated as having a metal-centered singly occupied
molecular orbital.
Fig. 2. Perspective views of the monoanion in crystals of 5 (top) and the neutral
molecule in crystals of 6 (bottom).
Charge neutral 6 has been diagnosed as [ZnII(3L)2]0 with two
monoanionic closed-shell ligands. This tetrahedral complex exhib-
its one weak CT band at 447 nm (2000 Mꢀ1 cmꢀ1, Fig. S4). The most
reduced member of this electron transfer series, dianionic
[CoII(4L)]2ꢀ, has no prominent LMCT or LLCT bands (Fig. 5), which
is consistent with a low-spin Co(II) central ion and one tetraanionic
ligand. The spectral features between 500 and 800 nm are rem-
nants of the monoanionic compound that underscore the quasi-
reversible nature of this redox process.
3.2. DFT calculations
3.2.1. Calculated geometries
All complexes were geometry optimized using ORCA at the
BP86 level of theory. For monoanionic 1 and 2, the computed struc-
tures are in very good agreement with the experimental data
(Table 8). The Co–N and Co–S bond lengths are overestimated
maximally by 0.01 Å. The most revealing bonds are the C–N, C–S,
and C–C distances of the aromatic ring. In each case, the long
C–N (ꢅ1.39 Å) and C–S (ꢅ1.78 Å) bonds, and essentially equivalent
C–C distances (avg. 1.41 Å) support the presence of two o-amino-
benzenethiolate(2ꢀ) ligands (type C, Scheme 2) in 1 and 2. The pen-
3.1.3. Magnetochemistry
The temperature dependence of the molar magnetic suscepti-
bilities of solid samples of complexes have been measured in the
temperature range 3–300 K by using a SQUID magnetometer at
1.0 T external magnetic field. The results are summarized in Ta-
ble 6. Fig. 6 shows the temperature dependence of the magnetic
moment of 1. Above 50 K, the complex shows a temperature inde-
pendent magnetic moment of 2.90 lB indicating the presence of
two unpaired electrons per cobalt ion (S = 1 ground state). The de-
crease of leff at <50 K has been successfully modeled by a large
zero field splitting of the S = 1 ground state into Ms = 0 and
tafluorophenyl substituents in
orthogonal (87°) to the CoN2S2 as in the crystal structure.
2
are similarly positioned
In the absence of a crystal structure, we have geometry opti-
mized the neutral species [Co(abt)2]0 (1ox). This square planar mol-
ecule exhibits similar Co–N and Co–S bond lengths to those in the
monoanion 1, however, there are noticeable differences in the
intraligand C–N, C–S and aromatic C–C distances. Somewhat short-
er C–N (1.369 Å) and C–S (1.753 Å) bond lengths and a slight
inequivalence of the C–C bonds in the aromatic ring are consistent

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S. Sproules et al. / Inorganica Chimica Acta 363 (2010) 2702–2714
Table 2
Selected bond distances (Å) and angles (deg) in 1, 2ꢁEt2O, and 6.
Complex 1
Co(1)–S(1)
N(1)–C(2)
C(2)–C(3)
C(5)–C(6)
2.1872(4)
1.371(2)
1.406(2)
1.390(3)
Co(1)–N(1)
C(1)–C(2)
C(3)–C(4)
1.840(1)
1.413(2)
1.385(2)
S(1)–C(1)
C(1)–C(6)
C(4)–C(5)
1.753(2)
1.393(2)
1.392(3)
S(1)–Co(1)–N(1)
87.46(5)
a
0.0
Complex 2
Co(1)–S(1)
C(2)–C(7)
C(4)–C(5)
C(7)–N(8)
Co(2)–S(31)
C(32)–C(37)
C(34)–C(35)
C(37)–N(38)
2.1739(4)
1.414(2)
1.401(2)
1.402(2)
2.1687(4)
1.418(2)
1.398(2)
1.399(2)
Co(1)–N(8)
C(2)–C(3)
C(5)–C(6)
1.869(1)
1.412(2)
1.389(2)
1.409(2)
1.867(1)
1.413(2)
1.397(2)
1.415(2)
S(1)–C(2)
C(3)–C(4)
C(6)–C(7)
1.766(2)
1.401(2)
1.404(2)
N(8)–C(9)
Co(2)–N(38)
C(32)–C(33)
C(35)–C(36)
N(38)–C(39)
S(31)–C(32)
C(33)–C(34)
C(36)–C(37)
1.762(2)
1.402(2)
1.399(2)
S(1)–Co(1)–N(8)
86.41(4)
S(31)–Co(2)–N(38)
86.71(4)
a
0.0
Complex 6
Zn(1)–S(1)
Zn(1)–N(27)
C(1)–C(2)
C(4)–C(5)
N(7)–C(8)
C(21)–C(22)
C(24)–C(25)
N(27)–C(28)
2.2728(5)
2.113(1)
1.402(2)
1.389(2)
1.293(2)
1.403(2)
1.386(2)
1.291(2)
Zn(1)–N(7)
S(1)–C(1)
C(2)–C(3)
2.093(1)
1.762(2)
1.384(2)
1.400(2)
1.758(2)
1.381(2)
1.395(2)
Zn(1)–S(21)
C(1)–C(6)
C(3)–C(4)
2.2646(2)
1.407(2)
1.391(3)
1.429(2)
1.405(2)
1.386(3)
1.435(2)
C(5)–C(6)
C(6)–N(7)
S(21)–C(21)
C(22)–C(23)
C(25)–C(26)
C(21)–C(26)
C(23)–C(24)
C(26)–N(27)
S(1)–Zn(1)–N(7)
88.29(4)
S(21)–Zn(1)–N(27)
88.57(4)
a
81.4
Table 3
Selected bond distances (Å) and Angles (deg) in 4 and 5ꢁ0.5 MeCN (in brackets).
Co(1)–S(1)
Co(1)–S(12)
Co(1)–N(7)
Co(1)–N(10)
S(1)–C(1)
C(1)–C(6)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(6)–N(7)
2.1384(6)
2.1388(6)
1.816(2)
1.815(2)
1.738(2)
1.417(3)
1.398(3)
1.379(3)
1.407(3)
1.372(3)
1.411(3)
1.365(3)
[2.173(3)]
[2.168(3)]
N(7)–C(8)
C(8)–C(9)
1.470(3)
1.556(3)
1.465(3)
1.371(3)
1.411(3)
1.390(3)
1.395(3)
1.377(3)
1.397(3)
1.413(3)
1.735(2)
[1.469(14)]
[1.399(15)]
[1.466(15)]
[1.380(14)]
[1.404(17)]
[1.398(18)]
[1.38(2)]
[1.826(10)]
[1.825(10)]
[1.760(12)]
[1.425(16)]
[1.399(15)]
[1.393(18)]
[1.400(18)]
[1.391(17)]
[1.410(16)]
[1.367(14)]
C(9)–N(10)
N(10)–C(11)
C(11)–C(16)
C(16)–C(15)
C(15)–C(14)
C(14)–C(13)
C(13)–C(12)
C(12)–C(11)
C(12)–S(12)
[1.39(2)]
[1.389(17)]
[1.407(18)]
[1.765(13)]
S(1)–Co(1)–N(7)
a
89.02(6)
8.8
[89.0(3)]
[2.2]
N(10)–Co(1)–S(12)
89.41(6)
[88.8(3)]
Table 4
Redox potentials (V) of complexes versus Fc+/Fc.
Complexa
E11=2
E21=2
E31=2
b
c
d
1
2
4
ꢀ1.73 qr
ꢀ1.54 r
ꢀ0.61 r
ꢀ0.37 r
ꢀ0.61 r
+0.71 qr
ꢀ0.04 r
ꢀ1.81 qr
r = Reversible; qr = Quasi-reversible.
a
Recorded in CH2Cl2 solutions containing 0.1 M [N(n-Bu)4]PF6 at 20 °C using a
scan rate of 100 mV sꢀ1
.
b
1ꢀ/2ꢀ Couple.
c
0/1ꢀ Couple.
d
1+/0 Couple.
with a Co(III) ion bound by one o-aminobenzenethiolate(2ꢀ) and
radical (Table 8). The
one o-iminothiobenzosemiquinone(1ꢀ)
p
Fig. 3. Cyclic voltammogram of 4 in a CH2Cl2 solution (0.10 M [N(n-Bu)4]PF6) at
ꢀ5 °C with a scan rate of 100 mV sꢀ1 (glassy carbon working electrode). Potentials
are references versus the Fc+/Fc couple.
optimized structure suggests the unpaired ligand spin is delocal-
ized, though this may not be the case in the compound itself. In
the interest of the measured EPR spectrum for this S = 1/2 species,
we have added two water molecules to the system and generated
a six-coordinate [CoIII(abt)(abtꢀ)(OH2)2]. This complex has been
examined by theoretical calculations, where both the cis- and
trans-[CoIII(abt)(abtꢀ)(OH2)2] forms have been geometry optimized.
The trans isomer was a meager 2.6 kcal molꢀ1 more stable than the
cis conformation, however the latter had Co–OH2 bonds at 2.116 Å

S. Sproules et al. / Inorganica Chimica Acta 363 (2010) 2702–2714
2709
Fig. 5. Electronic spectra of [4]1+ (purple), [4] (black), [4]1ꢀ (red), and [4]2ꢀ (blue) in
CH2Cl2 solution (0.10 M [N(n-Bu)4]PF6) at ꢀ5 °C.
Fig. 4. Electronic spectra of 1 (black), 1ox (red), 2 (green), and 3 (purple) in CH2Cl2
solutions at 20 °C.
3.2.2. Bonding schemes
Table 5
The bonding schemes in monoanionic and neutral [Co(L)2]1ꢀ
have been examined previously using both ab initio and DFT meth-
Electronic spectra of complexes in CH2Cl2 solutions.
Complex kmax (nm) (e )
, 104 Mꢀ1 cmꢀ1
1ox
1
ods. The outcome of these studies, with either two
a-diimine
1173 (0.15), 799 (0.40), 656 (1.39)
[6,36], -iminopyridine [37,38], o-aminophenolate [6,41], or
a
695 (sh, 1.31), 664 (1.37), 566 (1.95), 512 (sh, 0.79), 385 (sh, 0.97)
743 (1.40), 617 (2.33), 552 (sh, 0.84), 365 (2.43)
1339 (0.68), 881 (1.18), 736 (2.77), 560 (sh, 0.51)
756 (0.66), 592 (1.51), 500 (sh, 0.54), 346 (sh, 1.10)
1386 (0.32), 805 (0.78), 658 (3.91), 557 (sh, 0.50), 342 (sh, 1.01)
867 (0.14), 634 (0.61), 570 (1.59), 538 (sh, 0.57), 504 (0.42), 426
(sh, 0.38), 354 (sh, 0.90)
dithiolene ligands [9,35], was that a complicated electronic struc-
ture arose since the energy of the cobalt d orbitals is comparable
to the energy of the ligand
2
3
41+
4
p orbitals. Despite these systems con-
taining two asymmetric aminobenzenethiolate ligands, they pos-
sess approximate D2h point symmetry due to their square planar-
based geometries.
a
41ꢀ
42ꢀ
6
413 (sh, 0.59), 344 (sh, 1.98)
447 (0.20)
The calculated electronic ground state is 3B1g with the following
electronic configuration: (1ag)2(2ag)2(1b3g)2(1au)2(1b2g)2(1bu)2
a
Complex 5.
0
(2b2g)1(2b3g)1(1b1g
)
(Fig. 9). However, despite the MO manifold
showing two doubly occupied d orbitals, two singly occupied d
orbitals and one empty dxy orbital that is considered the hallmark
for an intermediate-spin Co(III) ion, assigning an unambiguous
spectroscopic oxidation state for the metal (and oxidation level
to the ligands) is rendered impossible due to the considerable li-
gand character in the 2b2g orbital (Table 9). The actual electronic
structure of monoanionic complexes at this time is best repre-
and 2.154 Å similar to reported cobalt-aquo compounds [44],
whereas the trans structure was discounted by the very long bonds
at 2.43 Å. Both the Co–L and intraligand distances are similar to the
four-coordinate species and suggest a Co(III) ion with one coordi-
nated ligand radical. The O–Co–O angle is at 79.5° while the two
CoNS planes meet with a dihedral angle of 84.6°.
sented by the following resonance forms [CoIII(L2ꢀ
(L2ꢀ)]1ꢀ M [CoII(L2ꢀ)(L1ꢀꢀ)]1ꢀ M [CoII(L1ꢀꢀ)(L2ꢀ)]1ꢀ
The Mulliken
spin population analysis finds 1.61 -spins (spin-up) at the cobalt
)
.
The geometry optimized structure of [CoIII(2Lꢀ)2I]0 (3) exhibits
even shorter C–N and C–S bond lengths than observed in 1ox. Fur-
thermore, there is a pronounced quinoidal distortion of the aro-
matic ring. This distances are reminiscent of the intraligand bond
lengths in crystallographically characterized [PtII(2Lꢀ)2]0 and geom-
etry optimized iron analogue [FeIII(2Lꢀ)2I]0 [11], such that we can
described this five-coordinate complex as a Co(III) ion bound by
one apical iodo at 2.468 Å that is typical of similar complexes
a
ion that indicates a slightly greater weight to the Co(III) form
(Fig. 10; Table 10), and we have written the formula as such
throughout this article.
It has been argued that the description of neutral [Co(L)2]0 in
terms of a single configuration is slightly oversimplified, but for
all practical purposes adequate [6]. One-electron oxidation of
monoanion 1 occurs with the removal of an electron from the sin-
gly occupied 2b2g orbital such that it is now the LUMO of [Co(2L)2]0
(Fig. 9). The highest occupied orbital is the Co-based 2b3g SOMO,
[6,32], and two o-iminothiobenzosemiquinone(1ꢀ)
p radial
ligands.
such that the calculated electronic configuration for this S = 1/2 spe-
The geometry optimized structures of neutral 4 and monoan-
ionic 5 have intraligand bond distances in reasonable agreement
with their crystal structures. The slight shortening of the C–N
and C–S bond lengths in 4 a consequence of some ligand radical
character, similar to that calculated for 1ox and 1oxꢁ2H2O. More-
over, the pendant phenyl groups are nearly orthogonal to the
CoN2S2 plane in each case (ꢅ87°) and the small twist of the geom-
etry about the Co ion toward a tetrahedron is mapped at 5.0° and
4.2° for 4 and 5, respectively, and in excellent agreement with the
crystallographic data.
0
cies is then (1ag)2(2ag)2(1b3g)2(1au)2(1b2g)2(1bu)2(2b3g)1(2b2g
)
(1b1g)0. Unlike that observed for the analogous bis(diimine) com-
plexes [6], the oxidation does not result in any substantial changes
to the compositions of the 2b2g and 2b3g orbitals, however there is
a marked increase in the Co dxz character in the 1b1g orbital (from
32.5% in 1 to 62.3% in 1ox, shown in blue in Fig. 9). Again, like the
monoanionic species, the pervasive covalency in this neutral spe-
cies renders a physical oxidation assignment of the metal impossi-
ble, such that we can only represent the electronic structure by the

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S. Sproules et al. / Inorganica Chimica Acta 363 (2010) 2702–2714
Table 6
Magnetochemical data of complexes 1, 2, 4, and 5.
Complex
Sa
leff
Db
g
1
1
1
/
1
1
1
1
1
1
2.90 lB (100–300 K)
3.22 lB (80–300 K)
1.93 lB (20–300 K)
3.00 lB (50–300 K)
+41.2
+29.0
2.003
2.32
2.28
2.10
2.19
2.09
2.17
2.094
2.26
2c
1
4
5
2
+31.0
+37.4
+39.4
+32.0
+35.0
+57.0
[N(n-Bu)4][CoIII(bdt)2]d
[N(n-Bu)4][CoIII(tdt)2]d
[N(n-Bu)4][CoIII(tbbdt)2]e
[N(n-Bu)4][CoIII(tmbdt)2]f
[CoCp2][CoIII(LN,O)2]g
2.80 lB (50–300 K)
2.84 lB (50–300 K)
3.10 lB (50–300 K)
a
Total spin ground state.
b
In cmꢀ1. The sign of D was determined by VTVH magnetization measurements
at 1.0, 4.0, 7.0 T.
c
No VTVH data obtained, however, D is assumed to be positive.
d
(bdt)2ꢀ = Benzene-1,2-dithiolate. Ref. [39].
e
Ref. [9].
f
(tmbdt)2ꢀ = 3,6-Bis(trimethylsilyl)benzene-1,2-dithiolate. Ref. [35].
g
(LN,O)
2ꢀ = 2-(2-Trifluoromethyl)aniline-4,6-di-tert-butylphenol. Ref. [6].
Fig. 7. X-band EPR spectra of 1ox at 10 K in CH2Cl2 solution containing 0.1 M [N(n-
Bu)4]PF6 (conditions: frequency 9.63 GHz; power 0.1 mW; modulation 0.75 mT).
Experiment (solid line) and simulation (dashed line).
following resonance structures: [CoIII(L2ꢀ)(L1ꢀꢀ)]0 M [CoIII(L1ꢀꢀ
(L2ꢀ)]0 M [CoII(L1ꢀꢀ)(L1ꢀꢀ)]1ꢀ. The spin density plot of 1ox exhibits
1.48 -spins (spin-up) on Co and 0.3 b-spins (spin-down) on the li-
)
a
gands that lends more weight to the Co(III) structure (Fig. 10).
Throughout this manuscript we documented such an oxidation
state in the formulae for these neutral species. A similar distribu-
tion is found in the spin density plot of 4 (Fig. S9), where in concert
with the crystallographic parameters, a Co(III) central ion is
favored.
As shown in the electron configuration of 1ox, the unpaired elec-
tron located in the dyz orbital should lead to a significantly large g
anisotropy that is clearly not evident in the EPR spectrum of this
coulometrically generated neutral species. Interestingly, the EPR
spectrum of 4 is highly anisotropic, such that while it is also ex-
Fig. 6. Temperature-dependence of the magnetic moment, leff, lB, of a powdered
sample of 1 recorded in a 1.0 T external magnetic field. The filled squares represent
the experimental data, whereas the solid line represents the simulation (g = 2.153;
|D| = 47.1 cmꢀ1; TIP = ꢀ417 ꢂ 10ꢀ6 cm3 molꢀ1). Inset shows the variable field and
variable temperature dependence of the magnetization of 1. The solid lines
represent best fits obtained using g = 2.003, D = +41.2 cmꢀ1, and S = 1.
Table 7
EPR Spin Hamiltonian g-tensor and 59Co magnetic hyperfine tensor (A ꢂ 10ꢀ4 cmꢀ1) derived from simulation of frozen solution spectra.
Complex
gx
gy
gz
<g>a
D
gb
Axx
Ayy
Azz
<A>c
19.7
Reference
1ox
2.004
2.046
2.016
2.000
2.005
2.32
1.97
1.991
2.05
2.004
2.030
2.046
2.018
2.032
2.81
2.30
2.051
1.91
2.008
2.028
2.702
2.036
2.050
3.45
2.71
2.810
3.53
2.005
2.035
2.255
2.018
2.029
2.86
2.327
2.284
2.497
2.502
0.004
0.018
0.686
0.036
0.045
1.13
0.74
0.819
1.48
43
19
18
4.7
-1.3
17
37
25.6
25
0
0
0
30
54.5
34
80
0
16
69
28.4
4.7
7.3
190
49
44.8
140
240
This work
This work
This work
[43]
[35]
[42]
[42]
[6]
[37]
[41]
2ox
29.3
15.5
13.1
20.2
80.3
55.3
23.5
65
4
[CoIII(tren)(tmbdt)]2+
[CoIII(tmbdt)(tmbdtꢀ)(OH2)x]0
[CoII(LO,Oꢀ)2(Et2O)]0
d
[CoII(LO,O)2]2ꢀ
e
[Co(LN,N)2]0
f
[Co(LNpy)2]0
30
52.2
g
[Co(LN,O)2]0
1.949
1.936
3.620
1.671
50
114.1
h
a
b
c
<g> = (gx + gy + gz)/3.
D
g = gz ꢀ gx.
<A> = (Axx + Ayy + Azz)/3.
(LO,O
(LO,O
d
e
f
ꢀ
)
1ꢀ = bis(2,6-isopropylphenyl)glyoxyl
p
radical.
)
2ꢀ = bis(2,6-isopropylphenyl)glyoxylate.
LN,N = N-phenyl-o-phenylenediimine.
g
h
LNpy
LN,O
=
=
a
-Iminopyridine.
a-Iminoketone.

S. Sproules et al. / Inorganica Chimica Acta 363 (2010) 2702–2714
2711
geometry that stabilizes a low-spin Co(III) ion with one unpaired
electron residing in a predominantly (87%) ligand-centered orbital
(Scheme S2). Three doubly occupied and two empty Co d orbitals
describe a low-spin Co(III) SCo = 0 central ion. This arrangement is
directly analogous to [(tren)CoIII(tmbdtꢀ)]2+ that possesses similar
spin Hamiltonian parameters (Table 7) [43]. Interestingly, the trans
structure gave rise to an intermediate-spin Co(III) SCo = 1 antiferro-
magnetically coupled to one ligand
p
radical (SL = 1/2). This resulted
in a metal-centered dz2 (2ag orbital) paramagnet which although
consistent with the low g anisotropy of the EPR spectrum, a consid-
erable Co 4s contribution would be expected to generate a sizeable
magnetic hyperfine interaction.
The electronic structure of 3 was derived from a B3LYP BS(2,2)
MS = 1 single point calculation on the BP86 optimized coordinates.
The MO manifold shown in Fig. 11 presents a d orbital occupancy
consistent with an intermediate-spin Co(III) SCo = 1. The binding of
the iodo ligand lifts the Co ion out of the N2S2 plane, reducing the
ligand character in the 2b2g orbital that is now effectively a Co-
based SOMO (78% metal character). The composition of the 2b3g
and 1b1g orbitals are the same for the monoanionic and neutral
species described above. The two ligand-based SOMOs (79% and
80% ligand character, respectively) found in the spin-down mani-
fold are antiferromagnetically coupled to the SCo = 1 center that
gives a St = 0 spin ground state. The orbital overlap integrals of
S = 0.37 and S = 0.70 underscore the strength of the coupling in this
system, similar to iron analogue [11]. Fig. 10 shows a spin density
plot (Mulliken analysis) with a-spin density in red and b-spin den-
sity in yellow for 3.
Fig. 8. X-band EPR spectra of 4 in a CH2Cl2/toluene solution at 10 K (conditions:
frequency 9.44 GHz; power 0.1 mW; modulation 1.0 mT). Experiment (solid line)
and simulation (dashed line).
4. Conclusions
plained by the aforementioned resonance forms, we can further-
more add that the total doublet ground state results from either
a high-spin Co(II) SCo = 3/2 central ion antiferromagnetically cou-
Complexes of cobalt containing two bidentate or one tetraden-
tate aminobenzenethiolate-based ligand have been prepared and
their electronic structures characterized by spectroscopic and the-
oretical techniques. The consequence of the energetic equivalence
pled to two ligand
ate-spin Co(III) SCo = 1 antiferromagnetically coupled to one ligand
radical monoanion (SL = 1/2). The net result in both instances is a
p radical monoanions (SL = 1), or an intermedi-
of the Co 3d manifold and the ligand
p orbitals delivers consider-
p
able ligand character to the frontier orbitals that precludes a defin-
itive a physical oxidation state assignment to both the cobalt ion
and the ligands. This is not a new concept, since several studies
of various cobalt systems with different nitrogen-, oxygen-, and
sulfur-donor noninnocent ligands have arrived at the same conclu-
sion [6,9,42]. A comparison of the electron and spin distribution
from a natural population analysis of various monoanionic cobalt
metal-centered doublet ground state.
The stark difference in the g anisotropy between 1ox and 2ox on
the one hand, and 4 on the other has been ascribed to inclusion of
two adventitious water molecules in the former to give a six-coor-
dinate species [CoIII(L)(Lꢀ)(OH2)2]. Only the cis conformation
yielded the correct electronic structure, that as an octahedral
Table 8
Calculated and experimental (in parentheses) bond lengths (Å) in 1, 2, 1ox, 1oxꢁ2H2O, 3, 4, and 5.
1
2
1ox
1oxꢁ2H2O
3
4
5
Co–N
Co–S
C–N
1.846 (1.840)
2.196 (2.187)
1.378 (1.371)
1.781 (1.753)
1.431 (1.413)
1.421 (1.393)
1.405 (1.406)
1.411 (1.385)
1.410 (1.392)
1.406 (1.390)
1.880 (1.869)
2.183 (2.179)
1.403 (1.402)
1.784 (1.766)
1.428 (1.414)
1.413 (1.404)
1.424 (1.412)
1.419 (1.401)
1.410 (1.401)
1.408 (1.389)
1.819
2.174
1.369
1.753
1.434
1.422
1.410
1.401
1.418
1.396
1.888
2.176
1.361
1.766
1.442
1.428
1.411
1.404
1.419
1.396
1.842
2.261
1.375
1.763
1.436
1.424
1.411
1.404
1.416
1.398
1.878
2.176
1.374
1.728
1.441
1.426
1.441
1.397
1.433
1.388
1.821 (1.816)
2.154 (2.139)
1.377 (1.368)
1.757 (1.737)
1.436 (1.414)
1.423 (1.401)
1.410 (1.406)
1.400 (1.388)
1.416 (1.392)
1.397 (1.384)
1.838 (1.826)
2.187 (2.171)
1.385 (1.374)
1.785 (1.763)
1.433 (1.415)
1.421 (1.404)
1.404 (1.403)
1.411 (1.391)
1.408 (1.395)
1.407 (1.386)
C–S
C1–C2
C1–C6
C2–C3
C3–C4
C4–C5
C5–C6

2712
S. Sproules et al. / Inorganica Chimica Acta 363 (2010) 2702–2714
Fig. 9. Qualitative MO schemes for [Co(abt)2]1ꢀ (1) and [Co(abt)2]0 (1ox). Kohn–Sham canonical orbitals are depicted for 1 (left) and 1ox (right).
species (Table 11) shows a d population between 7.57 and 7.82 and
the spin density ranging 1.47–1.78. The charge excess over the for-
mal d6 or d7 electron configuration arises from the covalent popu-
mines) complexes [6], the same electronic structure is not so
apparent, however, the charge and spin population analysis favor
a [CoII(LN,Nꢀ)2] with a low-spin Co(II) (SCo = 1/2) antiferromagneti-
lation of the otherwise unpopulated Co dxy orbital due to
r
cally coupled to one of two ligand p radicals (SL = 1). The square
donation from the ligand. These values show the central ion to
be more reduced than typical Co(III) and more oxidized than typical
Co(II) complexes. Thus, in [Co(L)2]1ꢀ the cobalt ion is somewhere
between Co(II) d7 and Co(III) d6, and the electronic structure can
planar [CoII(LN,Oꢀ)2]0 with two aminophenolate ligands has similar
metric and spectroscopic parameters [6]. The tetrahedral ana-
logues have been diagnosed as having a quartet ground state
(S = 3/2) that arises from either: a) antiferromagnetic coupling be-
only be represented by the resonance forms [CoIII(L2ꢀ
)
tween a high-spin Co(III) ion (SCo = 2) and one ligand p radical
(L2ꢀ)]1ꢀ M [CoII(L2ꢀ)(L1ꢀꢀ)]1ꢀ M [CoII(L1ꢀꢀ)(L2ꢀ)]1ꢀ
.
Despite the
crystallographic analysis strongly favoring a Co(III) ion with two
closed-shell ligands, there are several spectroscopic indicators for
some partial Co(II)-ligand radical character in related systems
[6,7,9,35,42,45]. A notable departure from this ambiguity is pack-
Table 9
Percentage composition of the selected orbitals of complexes as obtained from DFT
(COSMO) calculation at the scalar relativistic ZORA-B3LYP Level with CH2Cl2 as the
solvent.
aged in the form [CoII(LNpy)(LNpyꢀ)]1ꢀ (LNpy
= a-iminopyridine)
Complex
MO
Co(dyz
)
Co(dyz
57.0
)
N(2pz)
S(3pz)
C(2pz)a
[38], with its diamagnetic ground state derived from antiferromag-
netic coupling between a low-spin Co(II) (SCo = 1/2) and ligand
p
1
2b3g
2b2g
2b3g
2b2g
2b3g
2b2g
2b3g
2b2g
2b3g
2b2g
79.9
1.2
14.6
8.2
5.0
3.6
12.8
3.4
13.6
3.8
5.2
9.2
0.2
16.2
2.4
17.2
3.1
16.3
2.5
6.2
3.4
6.6
1.2
7.2
3.4
4.7
2.5
4.9
1.4
radical (SL = 1/2).
2
73.3
77.1
75.0
74.1
66.1
53.2
50.1
57.6
For the neutral compounds, a similar degree of covalency is
apparent, however there are contrasting spin state systems that
give rise to the doublet ground state that are dependent on the
noninnocent bidentate ligand involved. For [CoII(LO,Oꢀ)2(OEt2)]0
[42], the spectroscopic, crystallographic and computational studies
clearly indicate an electronic structure with a high-spin Co(II) ion
(SCo = 3/2) antiferromagnetically coupled to two diketyl radicals
1ox
4
5
12.0
11.3
a
The aromatic carbons bonded to the heteroatoms N and S.
(SL = 1). For square planar (or tetrahedrally distorted) bis(a-dii-

S. Sproules et al. / Inorganica Chimica Acta 363 (2010) 2702–2714
2713
Fig. 10. Mulliken Spin Density plots for (a) [CoIII(abt)(abtꢀ)]1ꢀ (1), (b) [CoIII(abt)(abtꢀꢀ)]0 (1ox), (c) [CoIII(abt)(abtꢀ)(OH2)2]0 (1oxꢁ2H2O), and (d) [CoIII(2Lꢀ)2I]0 (3).
Table 10
Mulliken spin populations analyses as obtained from DFT (COSMO) calculation at the
scalar relativistic ZORA-B3LYP level with CH2Cl2 as the solvent.
a
Complex
qCo
qL
1
+1.61
+1.66
+1.48
+0.06
+1.55
+1.44
+1.58
+0.39
+0.34
ꢀ0.48
+0.94
ꢀ1.48
ꢀ0.44
+0.42
2
1ox
1oxꢁ2H2O
3
4
5
a
Spin density distributed on the aminobenzenethiolate ligands only.
monoanion (SL = 1/2); or b) a high-spin Co(II) ion (SCo = 3/2) bound
by two closed-shell ligands [36]. This is contrasted here by
[CoIII(LN,S)(LN,Sꢀ)]0, where a central cobalt ion in the +III oxidation
state is the preferred structure, though reality from the perspective
of a population analysis is again between a Co(II) and Co(III)
resonance forms (Table 11). It can be seen that the aminobenzen-
ethiolate ligand is more difficult to oxidized that its
a-diimine,
a-iminopyridine, diketonate, and aminophenolate analogues. This
trend is realized through the diverse electronic structures arising
from the various neutral compounds compared with their corre-
sponding monoanionic species.
The electronic structure description for the five-coordinate
[CoIII(Lꢀ)2I]0, with either two aminophenolate or aminobenzenethi-
olate ligands, is more definitive, with both the spectroscopic and
theoretical data supporting a intermediate-spin Co(III) central ion
(SCo = 1) antiferromagnetically coupled to two ligand
(SL = 1).
p radicals
Fig. 11. Qualitative MO scheme for neutral 3 (S = 0) as derived from a BS(2,2) DFT
calculation (B3LYP/ZORA).
Among four-coordinate complexes of the first row transition
metals with two noninnocent bidentate ligands, it is apparent that
those with a cobalt central ion are the most perplexing. Cobalt
occupies a unique position in the periodic table where the compa-
rable energies of the Co 3d and ligand orbitals generate highly
covalent systems that defy physical oxidation level (state) assign-
ment to either the cobalt or the ligand.

2714
S. Sproules et al. / Inorganica Chimica Acta 363 (2010) 2702–2714
[15] F. Lalor, M.F. Hawthorne, A.H. Maki, K. Darlington, A. Davison, H.B. Gray, Z.
Table 11
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Corrections, Version 2006/1, Universität Göttingen, Göttingen, Germany, 2006.
[17] SHELXTL 6.14, Bruker AXS Inc., Madison, WI, 2003.
[18] G.M. Sheldrick, SHELXL97, Universität Göttingen, Göttingen, Germany, 1997.
[19] G.R. Hanson, K.E. Gates, C.J. Noble, M. Griffin, A. Mitchell, S. Benson, J. Inorg.
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[20] F. Neese, Orca, an Ab Initio, Density Functional and Semiempirical Electronic
Structure Program Package, Version 2.7, Revision 1523; Universität Bonn,
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Comparison of the charge and spin populations at the Co ion resulting from a natural
population analysis of the one-electron density of the ground state resulting from
scalar relativistic ZORA-B3LYP DFT calculations in CH2Cl2 solution by using COSMO
model.
Complex
Electrons-3d
Electrons-4s
Spin-d
Reference
1
7.57
7.61
7.60
7.64
7.82
7.58
0.29
0.30
0.53
0.51
0.51
0.47
1.58
1.43
1.47
1.14
1.59
1.78
This work
This work
[6]
[6]
[9]
1ox
[CoIII(pda)2]1ꢀ
a
[CoII(pda)2]0
a
[21] (a) A.D. Becke, J. Chem. Phys. 84 (1988) 4524;
[CoIII(bdt)2]1ꢀ
b
(b) J.P. Perdew, Phys. Rev. B 33 (1986) 8822.
[CoIII(tmbdt)2]1ꢀ
[35]
c
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a
b
c
pda = O-phenylenediamine.
bdt = Benzene-1,2-dithiolate.
tmbdt = 3,6-Bis(trimethylsilyl)benzene-1,2-dithiolate.
(b) P. Pulay, J. Comput. Chem. 3 (1992) 556.
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We thank Dr. Eckhard Bill for useful discussions. R.R.K. and N.R.
are grateful to the Max Planck Society for a doctoral stipend. S.S. is
similarly grateful for a postdoctoral fellowship. We thank the
Fonds der Chemischen Industrie for financial support.
Appendix A. Supplementary material
CCDC 720621, 720622, 720623, 720624 and 757525 contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplemen-
tary data associated with this article can be found, in the online
version, at doi:10.1016/j.ica.2010.03.042.
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