Reactivity of cobalt(III) amine-thiolate complexes with terminal and bridging thiolate functions towards hydrogen peroxide

Article abstract of DOI:10.1515/znb-2000-1211

The reactivity of Co^III amine-thiolate complexes with terminal and bridging thiolate functions towards oxidizing agents has been investigated. The mononuclear Co^IIIN₃S₃ complex [Co^III(L¹)]

Full text of DOI:10.1515/znb-2000-1211

Reactivity of Cobalt(III) Amine-Thiolate Complexes with Terminal
and Bridging Thiolate Functions towards Hydrogen Peroxide
Berthold Kersting and Gabriel Siedle
Institut für Anorganische und Analytische Chemie, Universität Freiburg,
Albertstrasse 21, D-79104 Freiburg, Germany
Reprint requests to Dr. B. Kersting. E-mail: kerstber@sun2.ruf.uni-freiburg.de
Z. Naturforsch. 55 b, 1179-1187 (2000); received August 17, 2000
Cobalt Complexes, Amine-Thiolate Ligands, Sulfinate Complexes
The reactivity of Co111 amine-thiolate complexes with terminal and bridging thiolate func-
tions towards oxidizing agents has been investigated. The mononuclear Co N3S3 complex
[Coni(L1)] (2) (H3L1 represents the hexadentate ligand /V,jV',./V"-Tris(2-thio-benzyl)-1,1,1-tris-
(aminomethyl)ethane) featuring three terminal thiolate ligands, and the binuclear N3Com-
(Ai-S)3ComN3 complex [Coni2(L2)]3+ (4) (H3L2 = N,A^/V''-Tris-[2-thio-3-aminomethyl-5-terr-
butyl-benzyl]-l,l,l-tris(aminomethyl)ethane) featuring three bridging thiolates were selected.
Hydrogen peroxide was the oxidizing agent. Whereas the thiolato-bridged complex 4 is resis-
tant towards oxidation by hydrogen peroxide, 2 undergoes a ligand-based oxidation reaction
to give the amine-sulfinate complex [Co(L'')] 5, where all three terminal thiolates have been
oxidized to sulfinates without gross structural changes of the parent complex 2. In contrast to
4, the cyclic voltammogram of 2 reveals an irreversible oxidation wave at E = +0.52 V vs SCE
which can be attributed to a ligand-centered oxidation reaction. 4 and 5 have been structurally
characterized by X-ray crystallography.
Introduction
Thiolate ligands are soft ligands that can act as
(j-donor, cr-donor-7r-donor, or as cr-donor-7r-accep-
tor ligands [1]. As a consequence of the versatility
of the binding modes, metal complexes with sulfur
rich coordination spheres often exist in different
oxidation states [2-5]. Since the thiolate ligands
are also redox active species, a problem sometimes
encountered with metal thiolate complexes is the
assignment of oxidation states [6 ]. This is true in
particular for the oxidized derivatives. Gaining an
understanding of the nature of the redox products
of metal thiolate complexes is thus a considerable
and fascinating challenge. The reactivity of metal
thiolate complexes towards oxidizing agents such
as O2 or H2O2 is also of interest to the oxidative
deactivation of metallocysteinate enzymes [7 - 12].
-—
(Bu
/ /
4
fBu
Scheme 1. Structures of complexes 2 and 4 [18].
complexes in which the thiolate donors are part of
multidentate thiolate ligands [16]. By contrast, there
have been only few reports on the chemical reac-
tivity of dinuclear thiolate complexes with bridging
/i-SR groups towards oxidizing agents [17].
The oxidation of kinetically inert cobalt(III) thio-
late complexes L„Co(SR) bearing terminal thiolate
ligands (SR~) has been examined in much detail
[13-15]. Such species can be gradually oxidized to
sulfenate L„Com(S(0)R), sulfinate L„Coin(S0 2 R),
and eventually sulfonato L„Coni(OS()2R) com-
We report herein the synthesis and structures of
plexes. This reactivity has also been observed for the two Co111 amine-thiolate complexes [Co(L*)] (2)
0932-0776/00/1200-1179 $ 06.00 © 2000 Verlag der Zeitschrift für Naturforschung, Tübingen ■www.znaturforsch.com
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1180
B. Kersting and G. Siedle • Reactivity of Cobalt(III) Amine-Thiolate Complexes
1. Co"CI2, NEt3
and [Co2(L2)]3+ (4) (see Scheme 1). In both com-
plexes the cobalt centers are pseudooctahedrally co-
ordinated by three facial N and three facial S donors.
The major difference is the binding mode of the
thiolate functions. In 2, all three thiolate groups
are terminal. In 4, all thiolate groups function as
bridging groups. The chemical reactivity of the two
complexes towards oxidizing agents and the elec-
trochemical properties are presented.
H3L13HCI --------------
-
[Co(L1)]
(1)
2 02
1. K3[ Co(CN)6]
[Co 2(1 )12[Co(CN)6]I3 (2)
[Co2(L )][CI04]3
2. Nal
4'
analysis, 'H and l3C NMR spectroscopy, electronic
absorption spectroscopy and by a single-crystal
structure determination of its oxidation product
[CoI1I(L1')] (5) (vide infra). The preparation of the
dinuclear complex [CoH12(L2)](C104)3 (3) has been
described previously [18]. In this study the structure
of the complex [CoUI2(L2)]3+ (hereafter denoted as
4) was determined by crystal structure determina-
tion of [Com2(L2)]2 [Co(CN)6] I3 (4'). This salt was
obtained as dark red crystals upon addition of an
aqueous solution of K3 [Co(CN)6] to a solution of
3 and Nal in methanol (eq. (2)).
Results and Discussion
Synthesis o f ligands and complexes
The synthesis of the nonadentate ligand H3L2 has
been described previously [18]. The hexadentate
ligand H3L1 was prepared in an analogous man-
ner (see Scheme 2). In the first step, three equiv-
alents of rm-butyl-(2 -formylphenyl)sulfide were
condensed with one equivalent of l,l,l-tris(amino-
methyl)ethane, followed by sodium borohydride re-
duction of the intermediate imine-thioether com-
pound to give the amine-thioether 1. Subsequent
cleavage of the arylS-fBu bonds was accomplished
by reduction with an excess of sodium in liquid am-
monia at -70 °C. The new ligand was isolated as the
trihydrochloride salt H3L1 3HC1 in an overall yield
of 83%. Its 'H NMR spectrum in D20 shows sin-
gle resonances at 6 4.67, 3.20 and 1.26 assigned to
the ArCH2, the NCH2 and the CH3 protons, respec-
tively. The aromatic protons appear as overlapping
multiplets in the region Ö 7.20 to 7.43. In the in-
frared spectrum of H3L1 3HC1 a medium band at
2410 cm- 1 can be attributed to the S-H stretching
vibration.
Characterization o f complexes
[Co(L])J (2). 2 is virtually insoluble in all com-
mon organic solvents except DMSO or DMF. Fig. 1
shows the 'H'H COSY spectrum of diamagnetic 2
in DMSO solution. The 'H NMR data are consis-
tent with the proposed structure shown in Fig. 2.
Similar to the spectrum of the free ligand the com-
plex shows a singlet for the methyl protons (CHh3)
at 6 0.52 (see Experimental Section). The aromatic
protons Hb, Hc and Hdappear as overlapping multi-
plets in the narrow region 6 7.03 to 6.98, whereas Ha
shows a well-resolved multiplet centered at 6 7.52.
While the free ligand displays two singlet reso-
nances for the methylene protons Hee and Hf,f,
2 gives rise to two AB spin systems. The sin-
gle set of resonances and the splitting of the res-
onances for the methylene protons are indicative
of local C3 symmetry and a fa c-N3 S3 configura-
tion about the cobalt atom. The assignment of the
individual resonances is based on the respective
cross peaks observed in the off-diagonal area of the
'H'H COSY map. The AB spin system centered at
b 3.69 is assigned to the benzylic methylene pro-
tons Hee . The other AB spin system centered at b
2.30 corresponds to the aliphatic methylene protons
Hf,f . In contrast to the benzylic methylene pro-
tons, the aliphatic methylene protons couple with
the adjacent secondary NH proton (broad singlet
at b 3.77). The magnitude of the vicinal coupling
constants |37(HfHg)| and |V(Hf Hg)| is 12 Hz (by
Treatment of H3L1 with one equivalent of
CoCl2 6H20 and three equivalents of a base in
methanolic solution affords adark-green suspension
containing the Co11 complex [Co^L1)]- , which
rapidly oxidizes in the presence of air to give the
neutral Co111 complex [Co111^ 1)] (2) as a dark-blue
solid (eq. (1 )). 2 was characterized by elemental
SR
2. NaBH4
(R = fBu)
1. Na/NH3f
1
2
I—
HH3,L1(R = H)
Scheme 2. Synthesis of H^L'SHCl.
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1181
B. Kersting and G. Siedle • Reactivity of Cobalt(III) Amine-Thiolate Complexes
Fig. 2. Proposed structure for the neutral complex
[Co(L')] (2) with atom labeling.
nances for aromatic and 4 resonances for aliphatic
carbon atoms).
The electronic absorption spectrum of a solu-
tion of 2 in DMF reveals two bands at 525 and
625 nm. The bands are readily assigned to d-d tran-
sitions (!Aig —> 1T2g (yi), 1 Aig —> *Tig (v \) in
Oh symmetry) of an octahedral ComN3S3 complex.
The related CoIHN3S3 complex [Co(tbm-tacn)]
(H3tbm-tacn = l,4,7-tris(4-rm-butyl-2-mercapto-
benzyl)-l,4-7-triazacyclononane) exhibits two ab-
sorptions at similar positions at 523 (i/2) and 652 nm
(u\) [19]. Relative to [Co(tbm-tacn)] the v x transi-
tion of 2 is shifted by 27 nm to lower wavelength.
This indicates that the secondary amine donors in
H3L1 exert a slightly stronger ligand field than
the tertiary amine donors in H3tbm-tacn. Consis-
tent with this are observations that the ligand field
steadily decreases as the number of N-alkyl groups
is increased [2 0 ].
[Co1112(L2)]2[Co(CN)6] l 3 (4'). The solution-
state structure of the dinuclear Co111 complex
[Co2(L2)]3+ (4) has been described previously [18].
In order to determine unambiguously the struc-
ture of the complex a single-crystal structure de-
termination was undertaken. Suitable crystals of
4M IMeOH I.5 H2O were grown from an aqueous
methanolic solution containing [Co2(L2)](C10 4 )3,
K3 [Co(CN)ö] and Nal. The crystal structure deter-
mination revealed the salt to consist of dinuclear
[Co2(L2)]3+ trications, hexacyanocobaltate anions,
iodide anions, and methanol and water molecules
F1 / ppm
Fig. I. Homonuclear 2D shift correlation (COSY) spec-
trum of [Co(L')] in DMSO-^6 solution (6 = 4.8 -1.8). The
diagonal runs from the lower left to the upper right, re-
producing the one-dimensional spectrum. Lines between
off-diagonal peaks and peaks on the diagonal indicate the
respective homonuclear spin couplings. Resonances are
assigned according to the structure shown in Fig. 2.
graphical evaluation), whereas the coupling con-
stants |3/(H eHg)| and |37(He'Hg)| are estimated to
be smaller than 3 Hz. The Karplus equation predicts
values of 0 = 60° ± 15° (or 0 = 180° ± 15°) for
the dihedral angles 0(CHeNHg) and c/>(CHeNHg),
implying that the benzylic CH2 protons and the NH
proton adopt a staggered conformation. The dihe-
dral angles 0(CHfNHg) and 0(CHf NHg), on the
other hand, can be estimated to be close to 0 = 1 2 0 °
± 15° (or 0 = 0° ± 15°) indicating that the aliphatic
methylene protons and the NH proton adopt an
eclipsed conformation. This is also supported by
the solid-state structure of 5 (vide infra). In this
case the dihedral angles are <^)(CHeNHg) = 178°,
0(CHeNHg) = 64° (staggered conformation) and
0(CHfNHg) 118° and <£(CHf NHg) = 6 ° (eclipsed
conformation). Also consistent with the proposed
structure are the 13C NMR data. As expected for a
C3-symmetric species, the spectrum displays 10 res-
onances for 10 inequivalent carbon atoms (6 reso-
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1182
B. Kersting and G. Siedle ■Reactivity of Cobalt(III) Amine-Thiolate Complexes
Table 1. Selected bond lengths (A) and angles (°) with
e.s.d.s for 4’ 11M eOH1,5H20.
tahedral geometry (idealized C3 symmetry). It
is isostructural with the heterodinuclear complex
[NinFeHI(L2)]2+ [21]. Each cobalt atom exhibits a
distorted octahedral coordination by three nitrogen
atoms and three bridging thiolate sulfur atoms. The
average Co-N and Co-S bond lengths of the cobalt
atom bonded to the three secondary amine nitro-
gen atoms of the 1 ,1 , 1 -tris(aminomethyl)ethane cap
are at 2.016(7) and 2.274(3) A, respectively. The
corresponding values for the adjacent Co atom are
slightly shorter at 1.989(7) and 2.259(3) A, respec-
tively. The values are unexceptional and compare
well with those of other octahedral ComN3S3 com-
plexes [22], The intramolecular Co111-- Co111 dis-
tance at 3.025(1) A is also typical for tris(^-thio-
phenolato) bridged Co111 complexes [18]. On the
basis of the relatively wide Co-S-Co angles (mean
83.7(1)°), a direct attraction between the metal
atoms is precluded [23]. It should be noted that
the three secondary nitrogen atoms N(l), N(3) and
N(5) become chiral upon coordination to Co(l). For
the enantiomer shown in Fig. 2 the configurations
are S-N(l), 5-N(3) and S-N(5). This set of nitro-
gen configurations induces a A configuration about
Co(l) and this in turn results in a Z\ configuration
about Co(2).
Co(l)-N(l)
Co(l)-N(3)
Co(l)-N(5)
Co(l)-S(l)
Co(l)-S(2)
Co(l)-S(3)
2.016(7)
2.011(7)
2.020(8)
2.274(3)
2.270(3)
2.278(3)
Co(2)-N(2)
Co(2)-N(4)
Co(2)-N(6)
Co(2)-S(l)
Co(2)-S(2)
Co(2)-S(3)
1.987(8)
1.994(7)
1.986(8)
2.260(3)
2.251(3)
2.265(3)
N(l)-Co(l)-N(3)
N(l)-Co(l)-N(5)
N(3)-Co(l)-N(5)
S(l)-Co(l)-S(2)
S(l)-Co(l)-S(3)
S(2)-Co(l)-S(3)
N(l)-Co(l)-S(l)
92.0(3)
90.7(3)
90.2(3)
N(2)-Co(2)-N(4)
N(2)-Co(2)-N(6)
N(4)-Co(2)-N(6)
87.6(3)
89.5(3)
90.0(3)
80.79(11)
80.41(10)
80.77(10)
95.6(2)
175.9(2)
96.6(2)
95.1(2)
94.8(2)
174.1(2)
172.9(2)
93.9(3)
80.07(10) S(l)-Co(2)-S(2)
79.83(10)
80.08(10)
96.1(2)
S(l)-Co(2)-S(3)
S(2)-Co(2)-S(3)
N(2)-Co(2)-S(l)
N(2)-Co(2)-S(2)
N(2)-Co(2)-S(3)
N(4)-Co(2)-S(l)
N(4)-Co(2)-S(2)
N(4)-Co(2)-S(3)
N(6)-Co(2)-S(l)
N(6)-Co(2)-S(2)
N(6)-Co(2)-S(3)
N(l)-Co(l)-S(2) 172.0(2)
N(l)-Co(l)-S(3)
N(3)-Co(l)-S(l)
N(3)-Co(l)-S(2)
N(3)-Co(l)-S(3) 172.6(2)
N(5)-Co(l)-S(l) 172.0(2)
92.4(2)
93.8(2)
95.3(2)
N(5)-Co(l)-S(2)
N(5)-Co(l)-S(3)
92.7(2)
95.7(2)
94.2(2)
In essence the most significant structural dif-
ference between the two cobalt(III) amine-thiolate
complexes 2 and 4 concerns the coordination mode
of the thiolate sulfur atoms. In 2 all three sulfur
atoms act as terminal donors to leave an exposed
Co(RS)3 face. In 4, on the other hand, all thiolate
sulfur atoms act as bridging ligands between the two
cobalt(III) centers. The different binding modes of
the thiophenolate ligands result in different chemi-
cal reactivities of 2 and 4 towards oxidizing agents
(vide infra).
Fig. 3. Thermal ellipsoid plot of the [Co2(L2)]3+cation in Electrochemical studies
4'. terf-Butyl groups and hydrogen atoms are omitted for
clarity.
2
was further characterized by cyclic voltam-
metry. Cyclic voltammograms have been recorded
in DMF solution with tetra-/?-butylammonium hex-
afluorophosphate as the supporting electrolyte. The
CV of 2 is shown in Fig. 4. That of 4 has been in-
cluded for comparative purposes. All potentials are
referenced to the SCE.
of crystallization. The structure of the [Co(CN)6]3-
anion is unexceptional and is not further consid-
ered. Fig. 3 displays the structure of the dicobalt
complex. Table 1 lists important bond lengths and
angles.
The [Co2(L2)]3+ trication has a central N^Co(/./-
S)3CoN3 core structure (N' denotes secondary
amine nitrogen atoms) with a confacial bioc-
The CV of 2 in DMF reveals a quasi-reversible
one-electron reduction wave at E 2\/2 = -1.20 V vs.
SCE (_\EP = 124 mV). Since the amine-thiolate
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B. Kersting and G. Siedle • Reactivity of Cobalt(III) Amine-Thiolate Complexes
1183
30% H20 2
[Co(L1)]
[C od1)]
(3)
5
2
HNr-NH NH
■W
t
0"'s
Oxidation o f cobalt complexes
E/ mV vs SCE
The two cobalt complexes were treated with
H2O2 in order to probe their chemical reactivity
towards oxidizing agents. The oxidation of the
mononuclear complex with aqueous H2O2 (35%)
in DMF proceeded smoothly and resulted in the
quantitative formation of the sulfinate complex 5,
where all three terminal thiolates have been oxi-
dized to sulfinates without gross structural changes
of the parent complex 2 (eq. (3)). However, it has
not been possible to chemically oxidize 4, even by
using more drastic reaction conditions.
Once precipitated, the red sulfinate complex 5
is insoluble in all common organic solvents. The
UV/Vis spectrum (taken directly after sample prepa-
ration) is quite distinct from the spectrum observed
for 2. It features an intense absorption maximum at
450 nm. This transition is characteristic for Co111 sul-
finate complexes and can be attributed to a ligand-
to-metal charge-transfer transition arising from the
sulfinate group [13]. The IR spectrum of 5 reveals
strong bands at 1024, 1012, and 909 cm- 1 which
were not present in the spectrum of 2. They can be
attributed to the stretching vibrations of an S-bound
sulfinato ligand [24].
Fig. 4. Cyclic voltammograms of 2 and 4 [18] in DMF
solution at 295 K. Experimental conditions: 0.1 M
[Bu"4N][PF6], ca. 1 x 10-3 M sample concentration,
Pt disk working electrode, Ag wire reference electrode,
scan rate 100 mV-s"1.
ligand (L1)3- in 2 is not reduced in the potential
range of 0.0 to -1.5 V, the former process is as-
signed to a metal-centered reduction with forma-
tion of a Co11 complex. This is also supported by
the electrochemical properties of the closely related
complex [Com(tbm-tacn)], which has been reported
to undergo an irreversible one-electron reduction at
-1.35 V (vs SCE) [19].
The dinuclear Co111 complex 4 is also redox ac-
tive [18]. The two one-electron reduction waves at
potentials of E 1 i/2 = -0.40 V and at E 21/2 = -0.84 V
correspond to the formation of the mixed-valent
ComCon and the fully-reduced ConCon forms, re-
spectively. The value of the formal potential Emidfor
the dinuclear complex 4 (£mid= (E 1 1/2 + £ 2 1/2)/2 ) =
-0.66 V) is shifted by ca. 0.51 V to more anodic po-
tentials, indicative of a stabilization of the Co11 oxi-
dation state relative to the Co111 state upon changing
the three terminal to three bridging thiolate ligands.
Description o f the crystal structure o f 5
In addition to the one-electron reduction of Co111
to Co11 at -1.20 V the CV of 2 reveals an irreversible
oxidation at E 2 = +0.52 V vs SCE. This is in marked
contrast to the dinuclear complex 4, which cannot be
oxidized in the potential window of 0.0 V to +1.5 V.
It is therefore unlikely that the irreversible oxidation
of 2 corresponds to an oxidation of Co111 to CoIY.
Hence the oxidation must be ligand-centered. It is
assumed that a coordinated thiyl radical is formed
which is not very stable on the time scale of a cyclic
voltammetry experiment. The same conclusions can
be drawn from the chemical reactivity studies pre-
sented below.
The crystal structure determination of 5 con-
firmed the formulation of the oxidation product of 2
as an amine-sulfinate complex. The complex crys-
tallizes in the space group P i with two formula units
per unit cell. Fig. 5 shows the molecular structure
of the neutral Co111 complex with the atomic num-
bering scheme. Table 2 lists selected bond lengths
and angles.
The Co111 complex possesses an idealized C3
symmetry with the C3 axis passing through the
methyl carbon atom of the l,l,l-rm-(amino-
methyl)ethane cap and the Co atom. The three facial
amine nitrogen atoms and the three facial sulfinate
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1184
B. Kersting and G. Siedle • Reactivity of Cobalt(III) Amine-Thiolate Complexes
C(40)
Fig. 5. Perspective view of the structure of
5 with atomic numbering scheme. Hydrogen
atoms bonded to carbon atoms are omitted
for clarity. H (l)-0(5) 1.97 Ä, H (2 )-0 (l)
1.95 A, H(3) -0(3)2.01 A.
Table 2. Selected bond lengths (A) and angles (°) with Coin-N bond lengths usually lie in the range of 1.96
e.s.d.s for 5.
to 2.02 A [26]. The differences in the Co-N distances
are indicative of a trans effect of the S-bonded sulfi-
2.067(4)
2.065(4)
2.082(4)
2.2336(14)
2.232(2)
2.237(2)
1.481(4)
1.434(4)
1.486(3)
1.421(4)
1.483(4)
1.423(4)
Co-N(l)
Co-N(2)
Co-N(3)
Co-S(l)
Co-S(2)
Co-S(3)
S(l)-0(1)
S(l)-0(2)
S(2)-0(3)
S(2)-0(4)
S(3)-0(5)
S(3)-0(6)
nate groups [13]. A similar effect has been observed
in the complex [Co(en)2(S(0 )2CH2CH2NH2)]3+
where the trans Co-N bond length (N trans to
the S-bound sulfinate group) is significantly longer
(by 0.049(5) A) than the average cis Co-N bond
length [27].
There are two sets of S=0 bond lengths with av-
erage bond lengths of 1.483 A (for 0(1), 0(3), 0(5))
and 1.426 A (for 0(2), 0(4), 0(6)), respectively (see
Table 2). The differences in the S=0 bond lengths
can be ascribed to the fact that the oxygen atoms
0(1), 0(3) and 0(5) are involved in intramolecular
hydrogen bonding interactions with the amine hy-
drogen atoms (mean distance for SO - HN at 1.98 A)
to cause a weakening of the respective sulfur oxygen
double bonds.
N(l)-Co-N(2)
N(l)-Co-N(3)
N(2)-Co-N(3)
S(l)-Co-S(2)
S(l)-Co-S(3)
S(2)-Co-S(3)
N(l)-Co-S(l)
N(l)-Co-S(2)
91.4(2)
87.7(2)
89.2(2)
90.38(6)
92.00(6)
88.62(6)
92.39(12)
175.58(13)
N(l)-Co-S(3)
N(2)-Co-S(l)
N(2)-Co-S(2)
N(2)-Co-S(3)
N(3)-Co-S(l)
N(3)-Co-S(2)
N(3)-Co-S(3)
87.84(13)
87.23(11)
92.19(11)
178.88(11)
176.38(13)
89.80(13)
91.62(13)
sulfur atoms (S-bonded sulfinate groups) form a
trigonally distorted octahedral coordination around
the cobalt atom. Similar to [Co2(L2)]3+ and other
Co111 complexes that are facially coodinated by a
l,l,l-rrä(aminomethyl)ethane cap, the S-NO), S-
N(3), and S-N(5) set of nitrogen configurations
induces a A configuration about the cobalt atom
(the R configuration of the nitrogen atoms in the
enantiomer requires a A configuration for the Co
atom) [25].
Summary
In the present study the reactivities of Co(III)
amine-thiolate complexes with terminal and bridg-
ing thiolate groups towards oxidizing agents have
been investigated. Complex 2 with three terminal
The average Co-N bond distance at 2.071(4) A ligands can be oxidized by H2O2 to give the sulfi-
in the sulfinate complex 5 is unusually long in nate complex 5, where all three terminal thiolates
comparison with the corresponding values in the have been oxidized to S-bound sulfinate groups. In
contrast, the binuclear complex 4, in which all three
thiolates act as bridging groups, is inert towards ox-
amine-thiolate complex 4 (vide supra) and other
CoinN3S3 complexes. In the latter complexes the
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B. Kersting and G. Siedle • Reactivity of Cobalt(III) Amine-Thiolate Complexes
1185
idizing agents. These results also nicely corroborate
with the electrochemical properties of the two com-
plexes. Whereas 2 undergoes an irreversible oxida-
tion at +0.52 V vs. SCE, 4 cannot be electrochemi-
cally oxidized up to potentials of +1.50 V vs. SCE.
It has recently been demonstrated that metal thio-
late complexes can be oxidized to yield transient
complexes with coordinated thiyl radicals [28]. It is
likely that the irreversible oxidation of 2 also cor-
responds to the formation of such a species. The
reason behind the different reactivities of the two
complexes can be explained by the non-availability
of lone pairs of electrons of thiolates in bridging
ß-SR groups as opposed to terminal thiolates.
stirred for 10 min at 60 °C and for further 12h at r. t. to give
a viscous yellow oil. To the suspension was added a solu-
tion of NaBH4 (4.54 g, 120 mmol) in ethanol (75 ml) and
the resulting homogeneous reaction mixture was stirred
for further 2 h. Water was added (50 ml), the pH of the
solution was adjusted to 1 by the addition of hydrochloric
acid, and stirring was continued for an additional 30 min.
The solution was made alkaline by the addition of 5 M
KOH solution (final pH ~ 12 - 13). The product was ex-
tracted with dichloromethane (3 x 50 ml). The combined
extracts were dried with K2CO3, filtered, and the filtrate
evaporated to dryness under reduced pressure. The result-
ing viscous yellow oil was not purified further (yield of
crude product: 12.4 g, 95%). - *H NMR (200.13 MHz,
CDCI3): 6 = 7.46 - 7.35 (m, 6 H, ArH), 7.20 - 7.10 (m, 6
H, ArH), 3.93 (s, 6 H, ArCH2), 2.46 (s, 6 H, CH2), 1.20
(s, 9 H, C(CH3)3), 0.80 (s, 3H, CCH3). - 13C{ 1H} NMR
(50 MHz, CDCI3): 6 = 146.1, 139.1, 132.3, 129.7, 129.4,
126.9, 57.2, 53.6, 47.5, 38.8, 31.6, 22.4.
Experimental
Materials and methods
The compounds l,l,l-tris(aminomethyl)ethane [29],
terr-butyl(2-formylphenyl)sulfide [30], and [Co2(L2)]-
(0 0 4 )3 (3) [18] were prepared according to published
procedures. All other chemicals were of reagent grade and
used without further purification. Solvents were predried
over molecular sieves and freshly distilled from appropri-
ate drying agents. Unless otherwise noted, all operations
were carried out under a pure argon atmosphere.1H NMR
and 13C{1H} NMR spectra were recorded on a Bruker
AVANCE DPX 200 or a Varian 300 unity spectrometer.
All chemical shifts are quoted on the 6 scale with TMS
or the solvent as internal standard. Coupling constants
are expressed in Hz. Melting points were determined in
capillaries and are uncorrected. Elemental analyses were
determined with a Perkin Elmer Elemental Analyzer 240.
IR spectra were recorded on a Bruker VECTOR 22 FT-
IR-spectrophotometer as KBr pellets. Absorption spectra
were recorded on a Jasco V-570 UV/VIS/NIR spectrom-
eter. Cyclic voltammetry measurements were carried out
at 25 °C with an EG&G Princeton Applied Research Po-
tentiostat/Galvanostat Model 263 A. The cell contained
a Pt working electrode, a Pt wire auxiliary electrode, and
an Ag wire as reference electrode. Concentration of so-
lutions were 0.1 M in supporting electrolyte (Bu^NPFö)
and ca. 1.0 x 10~3 M in sample. Ferrocenium/ferrocene
was used as internal standard. All potentials were con-
verted to the SCE reference [31].
Synthesis of N,N',N"-tris(2-thio-benzyl)-1,1,1-tris-
(aminomethyl)ethane (H3L1)
To a solution of sodium (1.06 g, 46.0 mmol) in liquid
ammonia (130 ml) was added a solution of compound 1
(5.02 g, 7.70 mmol) in THF (20 ml) at -70 °C. After stir-
ring for an additional 1 h, excess sodium was destroyed
by careful addition of solid ammonium chloride to give a
yellow solution. After the ammonia was allowed to evap-
orate at atmospheric pressure, the remaining THF was
evaporated at reduced pressure. The resulting solid was
hydrolyzed with water (50 ml) and acidified with 6 M
hydrochloric acid (final pH ca. 1) to give a viscous yel-
low oil. The supernatant solution was decanted and the
oily residue was washed with water (20 ml), and this
process was repeated one more time. The oily residue
solidified upon addition of ethanol (30 ml) to give 4.03 g
(88%) of the product H3L' -3HC1 as a pale-yellow solid.
The crude material was sufficiently pure for metal com-
plex synthesis. M. p. 164 - 168 °C (decomp.). - IR(KBr):
v 2966m (CH), 2781m, 2729m, 2632m, 2576m (NH2+),
2410m cm“ 1 (SH). - ’H NMR (200.13 MHz, D20): 6 =
7.43 - 7.20 (m, 12 H, ArH), 4.67 (s, 6 H, ArCH2), 3.20
(s, 6 H, CH2), 1.26 (s, 3H, CCH3).
Synthesis of [Co(L])] (2)
To a solution of H^L' ^HCl (296 mg, 500 //mol) in
methanol (7 ml) was added a solution of C0CI2 6 H2O
(119 mg, 500 //mol) in methanol (3 ml) and the resulting
Synthesis of N,N',N"-tris(2-tert-butylsulfanylbenzyl)-
1 ,1 , 1 -tris(aminomethyl)ethane 1)
To a solution of teAt-butyl(2-formylphenyl)sulfide green solution was stirred for 5 min. A solution of NEt3
(11.8 g, 61.0 mol) in absolute ethanol (25 ml) was added
a solution of l,l,l-tris(aminomethyl)ethane (2.34 g,
(202 mg, 2.00 mmol) in methanol (1 ml) was added to
give a dark-green suspension. The flask was exposed to
20.0 mmol) in ethanol (5 ml). The reaction mixture was air and stirred for further 2 h. The resulting dark-blue
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B. Kersting and G. Siedle • Reactivity of Cobalt(III) Amine-Thiolate Complexes
X-ray crystallography
solid was isolated by filtration, washed with methanol
( 3 x 5 ml), and dried in air. Yield: 189 mg (70%). M.p.
326 °C (decomp.). - UV/Vis (DMF): Amax(£) = 422 sh, 525
(141), 612 nm (170 M-1cm_1). - IR(KBr): u = 3244m
(NH), 3046m, 2916w, 2872w cm-1 (CH). - 'H NMR
(200.13 MHz, DMSO-d6): 6 = 1H, <37.52 (dd, V = 6.1 Hz.
4/= 1.6 Hz, 3 H, ArH), 7.01 (m, 9 H, ArH),4.33 (dd, 2J =
11.0 Hz, V < 3.0 Hz, 3 H, ArCH), 3.77 (s br., 3 H, NH),
3.04 (d, 2J = 11.0 Hz, 3 H, ArCH), 2.32 (dd, 2J= 12.0 Hz,
V = 12.0 Hz, 3 H, NCH2), 2.18 (dd, 2J = 12.0 Hz, 3J =
5.0 Hz, 3 H, NCH2), 0.52 (s, 3 H, CH3). - l3C{1H} NMR
(50.32 MHz, DMSO-dö): b = 146.1, 134.9, 132.7, 131.3,
127.9, 123.0, 53.1, 52.5, 40.1, 22.0. - C26H3oCoN3S3
(539.66): calcd. C 57.86, H 5.60, N 7.79; found C 57.64,
H 5.42, N 7.25.
Single crystals of 4' • 11 MeOH • 1.5 H20 were
grown from an aqueous methanolic solution containing
[Co2(L2)](C104)3, Nal, and Na3[Co(CN)6]. Single crys-
tals of complex 5 were taken directly from the reaction
mixture. The crystals were mounted on glass fibers us-
ing perfiuoro-polyether oil. Crystal data were collected at
180(2) K using a Bruker SMART diffractometer. Graphite
monochromated Mo-KQradiation (0.71073 A) was used
throughout. The data were processed with SAINT [32]
and corrected for absorption using SADABS [33] (trans-
mission factors: 0.74 - l.00 for 4', 0.78 - 1.00 for 5).
The crystal structures were solved by direct methods in
SHELXS-86 [34], and refinements were carried out on
F 2 values using the program SHELXL-93 [35].
Crystal data and refinement details for 4' • 1IMeOH
• 1.5H20: (C99H173Co5l3N,80i2.5S6, Mr = 2683.26), red-
brown crystal, crystal size 0.43 x 0.38 x 0.15 mm3,
monoclinic, space group C2/c (No. 15), a = 36.767(7),
b = 18.331(4), c = 25.080(5) Ä, ß = 128.25(3)°, V =
13274(5) Ä3, Z = 4, n(Mo-Ka) = 1.460 mm"1, pc =
1.343 g em-3, F(000) = 5532, 41933 reflections mea-
sured, 15567 unique (Rmt = 0.1776), 4765 observed re-
flections [/> 2cr(/)]. All non-hydrogen atoms were refined
anisotropically except for the carbon and oxygen atoms of
one disordered tert-butyl group and the MeOH and H20
solvent molecules of crystallization. These were refined
isotropically. Hydrogen atoms were placed at calculated
positions and positional parameters were refined using
a riding model. No hydrogen atoms were calculated for
the H20 solvates. An isotropic split-atom model was ap-
plied for the disordered tert-butyl methyl carbon atoms.
The multiplicities of the respective orientations were re-
fined as follows: C(21a)-C(23a): 0.62(2), C(21b)-C(23b):
0.38(2). Final residuals R\ = 0.0638, wR2 = 0.1475 [/ >
2<j(/)], R\ = 0.2467, wR2 = 0.2252 (all data), largest dif-
ference peak, A pmax = 1.194 eA-3, and hole _ lp min =
-0.777 eA-3.
Crystal data and refinement details for 5 (C26H3oCo-
N3O6S3, Mr = 635.64): red block, crystal size 0.25 x 0.22
x 0.10 mm3, triclinic, space group P l,a= 10.493(2), b =
10.753(2), c = 12.749(3) A, a = 81.90(3), ß = 74.46(3),
7 = 67.36(3)°, V = 1278.0(5) Ä3, Z = 2, //(Mo-Kq) =
0.967 mm-1, pc = 1.652 g em"3, F(000) = 660, 11435
reflections measured, 5798 unique (Rint = 0.0774), 3220
observed reflections [/> 2a(/)]. Positional and anisotropic
atomic displacement parameters were refined for all non-
hydrogen atoms. Hydrogen atoms were placed geometri-
cally and positional parameters were refined using a riding
model. Isotropic atomic displacement parameters for hy-
drogen atoms were constrained to be 1.2 (1.5 for methyl
groups). Final residuals R 1 = 0.0512, wR2 = 0.1312 [/
> 2a(D], Rl = 0.1111, wR2 = 0.1759 (all data), largest
Synthesis of [Co2(L2 )]2[Co(CN)6]h (4')
To a solution of [Co2(L2)](C104)3 (3) (15 mg, 13 /umol)
and Nal (40 mg, 0.27 mmol) in methanol (15 ml) was
added dropwise with stirring a solution of K3[Co(CN)6]
(4.3 mg, 13 //mol) in water (0.2 ml). The solution was
filtered and kept at room temperature for 12 h, during
which time red crystals of 4'1 lM e0H1.5H20 formed.
The crystals quickly lose the solvent molecules of crys-
tallization upon storage in air at 298 K. This complex
was identified by an X-ray structure determination (see
below).
Synthesis of [Co(L])] (5)
To a solution of complex 2 (50 mg, 93 //mol) in DMF
(10 ml) was added 1ml of a 35% aqueous H20 2 solution.
The color of the solution gradually changed from dark-
blue via green to red. The reaction mixture was allowed
to stand for 12 h. The red crystalline product was isolated
by filtration, washed with methanol and diethyl ether,
and dried in air. Yield: 41 mg (72%). The compound
is insoluble in all common organic solvents. - IR(KBr):
v = 3142m (NH), 3058m, 2928w, 2869w (CH), 1024 m,
1012m (SO), 980w, 909s c m '1(SO). - UV/VIS (DMF):
A(e) = 450sh nm (690 M ^cm ’ 1). - Q 6H30C0N3S3O6
(635.65): calcd. C 49.13, H 4.76, N 6.61; found C 48.87,
H 5.41, N 6.21.
Reaction of 3 with H2O2
To a solution of 3 (115 mg, 0.10 mmol) in DMF (10 ml)
was added 2 ml of a 35% aqueous H20 2 solution. The re-
action mixture was allowed to stand for 12 h. The solution
was evaporated to dryness, water (10 ml) was added, and
the resulting brown precipitate collected by filtration. The
analytical data of this material are identical to that of 3.
Complex 3 was recovered in > 80% yield.
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1187
B. Kersting and G. Siedle • Reactivity of Cobalt(III) Amine-Thiolate Complexes
difference peak, A pmax = 0.678 eA 3, and hole A pm\n =
-0.552 eA“3.
CCDC reference numbers 146943 (4'), 146942 (5).
E-mail: deposit@ccdc.cam.ac.uk
Acknowledgements
The authors thank Prof. Dr. H. Vahrenkamp for his
generous support. Financial support by the Deutsche For-
schungsgemeinschaft and the Wissenschaftliche Gesell-
schaft Freiburg is also gratefully acknowledged.
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