Molecular and electronic structures of one-electron oxidized Ni II-(dithiosalicylidenediamine) complexes: NiIII-thiolate versus NiII-thiyl radical states

Article abstract of DOI:10.1002/chem.200701108

The dithiosalicylidenediamine Ni^II complexes [Ni(L)] (R = tBu, R′ = CH₂C(CH₃)₂CH₂ 1, R′=C₆H₄ 2; R = H, R′ = CH₂C(CH ₃)₂CH₂ 3, R′ = C

Full text of DOI:10.1002/chem.200701108

DOI: 10.1002/chem.200701108
Molecular and Electronic Structures of One-Electron Oxidized
NiII–(Dithio NiIII–Thiolate versus
ACHTREUNGsalicylidenediamine) Complexes: ACHTREUGN
NiII–Thiyl Radical States
Philip A. Stenson, Ashley Board, Armando Marin-Becerra, Alexander J. Blake,
E. Stephen Davies, Claire Wilson, Jonathan McMaster,* and Martin Schrçder*[a]
Abstract: The dithiosalicylidenedia-
mine NiII complexes [Ni(L)] (R=tBu,
and 13C NMR and UV/Vis spectroscop-
ies and by single-crystal X-ray crystal-
lography. Cyclic voltammetry and cou-
lombic measurements have established
that complexes 1 and 2, incorporating
tBu functionalities on the thiopheno-
late ligands, undergo reversible one-
electron oxidation processes, whereas
the analogous redox processes for com-
plexes 3 and 4 are not reversible. The
one-electron oxidized species, 1+ and
2+, can be generated quantitatively
either electrochemically or chemically
with 70% HClO4. EPR and UV/Vis
spectroscopic studies and supporting
DFT calculations suggest that the
SOMOs of 1+ and 2+ possess thiyl rad-
R’=CH2C
ACHTREUNG
R=H, R’=CH2CACHTREUNG
C6H4 4) have been prepared by trans-
metallation of the tetrahedral com-
plexes [Zn(L)] (R=tBu, R’=CH2C-
+
E
ical character, whereas those of 1(py)2
+
A
and 2(py)2 possess formal NiIII cen-
formed by condensation of 2,4-di-R-
thiosalicylaldehyde with diamines H2N-
R’-NH2 in the presence of ZnII salts.
The diamagnetic mononuclear com-
plexes [Ni(L)] show a distorted square-
planar N2S2 coordination environment
and have been characterized by 1H-
ters. Species 2+ dimerizes at low tem-
perature, and an X-ray crystallographic
determination of the dimer [(2)2]-
Keywords: CO dehydrogenase ·
AHCTER(UGN ClO4)2·2CH2Cl2 confirms that this di-
merization involves the formation of a
hydrogenases
·
nickel
·
redox
chemistry · superoxide dismustase ·
thiolate
ꢀ
S S bond (S···S=2.202(5) Š).
Introduction
unit in these enzymes.[1c,10] NiSOD catalyses the dispropor-
tionation of the superoxide radical to hydrogen peroxide
and dioxygen via a two-step, ping-pong mechanism involving
a monomeric Ni center that cycles between the formal NiII
and NiIII oxidation states.[11,12] X-ray crystallographic studies
on the oxidized[9] and reduced[10] forms of the enzyme from
Streptomyces reveal that in the reduced NiII state the
enzyme is coordinated in a square-planar N2S2 arrangement
by two thiolate ligands derived from Cys2 and Cys6, the de-
protonated amide of the Cys2 backbone, and the N-terminal
-NH2 group of His1. On oxidation, the Ni center adopts a
square-pyramidal geometry containing the same equatorial
ligand set as for the reduced form, but with an additional
His1 imidazole ligand bound in the axial position (Figure 1).
Thus, during the catalytic cycle the metal undergoes a
change in its coordination sphere with the binding of His1
in the oxidized form of the enzyme playing a critical role in
the catalytic mechanism. Detailed spectroscopic and sup-
porting theoretical calculations on the enzyme and related
model complexes suggest that the imidazole group of His1
There is considerable support for the participation of the
NiIII oxidation state in the catalytic cycles of the [NiFe] hy-
drogenases,[1,2] acetyl coenzyme Asynthase (ACS),
CO
[3–5]
dehydrogenase,[6,7] methyl coenzyme M reductase,[8] and Ni-
containing superoxide dismustase (NiSOD).[9,10] Acommon
feature of all of these enzymes is cysteine-thiolate coordina-
tion at the Ni center in the active site. The S-ligation has
been proposed to lower the redox potential of the metal
[a] Dr. P. A. Stenson, A. Board, Dr. A. Marin-Becerra,
Prof. Dr. A. J. Blake, Dr. E. S. Davies, Dr. C. Wilson,
Prof. Dr. J. McMaster, Prof. Dr. M. Schrçder
School of Chemistry, University of Nottingham, Nottingham, NG7
2RD (UK)
Fax : (+44)115-951-3563
E-mail: j.mcmaster@nottingham.ac.uk
m.schroder@nottingham.ac.uk
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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FULL PAPER
and describe herein the synthesis and spectroscopic charac-
terization of the redox-active mononuclear complexes
[Ni(L)] 1–4, where L is derived from the condensation of
2,4-di-R-thiosalicylaldehyde with 2-dimethyl-1,3-diamino-
propane (R=tBu, 1; H, 3) or 1,2-benzenediamine (R=tBu,
2; H, 4, Scheme 1). Electrochemistry, EPR and UV/Vis
spectroscopies, and supporting DFT calculations have been
employed to probe the electronic structures of electrochemi-
cally and chemically generated [Ni(L)]+ and [Ni(L)(py)2]+
to gain insight into the role of the coordination sphere in
controlling the nature of the electronic structure of the
active site of NiSOD.
Figure 1. The structures of the active site of a) the reduced form of
NiSOD[9] and b) the oxidized form of NiSOD.[10]
needs to coordinate to the metal center following oxidation
to form a stable NiIII species at the active site of the
enzyme.[13,14] The axial coordination of the imidazole group
from His1 forces the unpaired electron into a Ni dz2-based
molecular orbital that possesses little thiolate character,
thereby suppressing thiolate-based oxidative chemistry that
could result in the formation of sulfonate groups which may
inactivate the enzyme. Previous studies of [NiII(N2S2)]n (n=
ACHTREUNG
0, 2ꢀ) compounds have demonstrated this irreversible oxi-
dation at the thiolate ligands. For example, the species [NiII-
ACHTREUNG
(N2S2)]0 containing neutral amine N-donors can react with
dioxygen to form sulfonate ligands[15] and form disulfide spe-
cies on one-electron electrochemical oxidation.[16] In con-
trast, complexes of the type [NiIIN2S2]2ꢀ containing anionic
amide N-donors exhibit reversible electrochemistry that ulti-
mately leads to the generation of NiIII centers that can be
detected as their pyridine adducts by EPR spectroscopy in
solution at low temperature.[17–19] There is one example of a
synthetic complex containing a mixed amine/amide thiolate
coordination sphere [Me4N]ACTHERUGN ACHTRE(UGN BEAAM)] (BEAAM=N-
[NiII
{2-[benzyl-(2-mercapto-2-methylpropyl)amino]ethyl}-2-mer-
capto-2-methylpropionamide), which has been shown to ex-
hibit reversible metal-based redox chemistry but with spec-
troscopic properties that are intermediate between those of
[NiACHTREUNG(N2S2)] complexes containing exclusively N-amine or N-
amide coordination.[20] It has been argued that the mixed
amine/amide coordination at the active site of NiSOD plays
a crucial role in tuning the redox chemistry and reactivity of
the active site.[13,14] Thus, filled–filled interactions between
the p-donating amide ligand and the Ni–p orbitals destabi-
lize metal-based orbitals, thereby facilitating electron trans-
II
Cꢀ
fer from Ni to O2 to generate a NiIII center, rather than S-
based radicals, in the catalytic cycle.
We report herein the chemistry of a series of Ni com-
plexes of the type [NiACHTRE(UGN N2S2)] incorporating imine N-donors
within the ligand framework to probe the effect of the incor-
poration of these N-donors on the electronic structure of a
[NiACHTREUNG(N2S2)] center. These N-donors are expected to possess
different s- and p-properties compared to amine and amide
N-donors, features that appear to be important determinants
of the nature of the SOMO within the oxidized species. We
have been interested in the study of a range of unusual
metal oxidation state and ligand–radical species based on S-
donor ligation,[21] particularly with Ni and related centers,[22]
Scheme 1. Schematic representation of the synthesis of complexes 1–4.
Chem. Eur. J. 2008, 14, 2564 – 2576
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2565

M. Schrçder, J. McMaster et al.
Results and Discussion
Table 1. Selected bond lengths [Š] and angles [8] for 7, (8)2 and 9.
7[a]
(8)2
9
Synthesis of ligands and ZnII complexes: 2,4-Di-tert-butyl-
benzenethiol (5a) was obtained by reaction of 2,4-di-tert-bu-
tylbromobenzene with Mg in Et2O followed by quenching of
the Grignard reagent with sulfur as we have reported previ-
ously for 5b.[23] Ortho-formylation of 5a to yield aldehyde
6a was achieved following modified literature proce-
dures.[24–26] The use of 1-formylpiperidine as formylating
agent in place of the previously reported N,N-dimethylform-
ꢀ
Zn(1) S(1)
2.2573(7)
2.2573(7)
2.018(2)
2.018(2)
1.278(3)
1.278(3)
2.3885(5)
2.3139(5)
2.143(2)
2.080(2)
1.295(2)
1.276(2)
2.4811(5)
2.4493(5)
2.2838(5)
2.4284(5)
2.107(2)
2.152(2)
1.280(2)
1.297(2)
108.04(2)
89.21(4)
162.75(5)
126.82(4)
89.12(5)
78.70(6)
100.85(2)
91.97(4)
128.10(4)
163.52(4)
86.25(4)
77.66(6)
79.62(2)
79.49(2)
85.60(2)
84.06(2)
2.2762(9)
2.2564(9)
2.044(2)
2.035(2)
1.265(4)
1.281(4)
ꢀ
Zn(1) S(2)
ꢀ
Zn(1) N(1)
ꢀ
Zn(1) N(2)
ꢀ
N(1) C(1)
ꢀ
N(2) C(1A)
ꢀ
Zn(1) S(2’)
ꢀ
Zn(2) S(1)
ꢀ
Zn(2) S(1’)
ꢀ
ACHTREUNGamide increased the yield from about 55% to about 80%.
Zn(2) S(2’)
1H NMR spectra of crude 6a confirmed that the main con-
taminant was 5a. However, a comprehensive purification
procedure of 6a was not sought as the crude product react-
ed cleanly with 2-dimethyl-1,3-diaminopropane or 1,2-ben-
ꢀ
Zn(2) N(1’)
ꢀ
Zn(2) N(2’)
ꢀ
N(1’) C(1B)
ꢀ
N(2’) C(1C)
S(1)-Zn(1)-S(2)
S(1)-Zn(1)-N(1)
S(1)-Zn(1)-N(2)
S(2)-Zn(1)-N(1)
S(2)-Zn(1)-N(2)
N(1)-Zn(1)-N(2)
S(1’)-Zn(2)-S(2’)
S(1’)-Zn(2)-N(1’)
S(1’)-Zn(2)-N(2’)
S(2’)-Zn(2)-N(1’)
S(2’)-Zn(2)-N(2’)
N(1’)-Zn(2)-N(2’)
S(1)-Zn(1)-S(2’)
S(1)-Zn(2)-S(2’)
Zn(1)-S(1)-Zn(2)
Zn(1)-S(2’)-Zn(2)
124.78(4)
96.46(6)
123.25(8)
123.25(8)
96.46(6)
89.05(14)
117.95(4)
96.92(7)
126.70(7)
127.86(7)
98.10(7)
88.80(10)
zenediamine and ZnACHTRE(UNG OAc)2 via a template Schiff-base con-
densation reaction to afford 7–10, which crystallized readily
in yields of 80–90%.
Analytically pure yellow crystals of 7 and 9, and red crys-
tals of (8)2 were obtained by recrystallization of the crude
products. 1H NMR and 13C NMR spectra were consistent
with the formation of diamagnetic Schiff-base ZnII com-
plexes, which was confirmed by single-crystal X-ray crystal-
lographic studies of 7, (8)2, and 9. Selected bond lengths and
angles for these complexes are shown in Table 1. Complexes
7 and 9 (Figure 2) reveal monomeric ZnII centers possessing
a distorted tetrahedral N2S2 coordination sphere about the
ZnII center (the angle between the planes defined by S-Zn-S
and N-Zn-N is 86.08 and 66.78, for 7 and 9, respectively).
The solid-state structure of (8)2 reveals a thiolate-bridged
[a] Half of the complex is crystallographically independent with the ge-
ometry completed by a twofold rotation, 1ꢀx, y, ꢀz + = .
1
2
dimer with two ZnACHTREUNG(N2S2) units linked by two m2-S bridging
interactions to form a Zn2S2 diamond core (Figure 3). The
geometry at ZnII may be described as distorted trigonal bi-
pyramidal in which, for Zn(1), S(1) and N(2) occupy axial
positions and, for Zn(2), S(2’)
lution from yellow to dark brown, and traces of ZnACHTREU(GN NO3)2 in
the crude product were removed by washing with MeOH.
The overall yield using this procedure is improved greatly
and N(1’) are the axial donor
atoms. The Zn
ACHTREUNG(m-S)2Zn core
forms butterfly motif in
a
which the bridging thiolate S-
donors form the apex of a
hinge with Zn(1)···Zn(2)=
3.287(2) Š. The bond lengths
around ZnII are consistent with
those reported previously for
similar ZnII complexes with
N2S2 coordination.[27–32]
Synthesis of NiII complexes:
The NiII complexes 1–4 were
prepared from the correspond-
ing ZnII complexes 7–10, re-
spectively, via transmetallation
with
Ni
A
in
MeOH/CH2Cl2
solution
(Scheme 1). The substitution
of ZnII by NiII is accompanied
by a change in color of the so-
Figure 2. View of the molecular structure of a) 7 and b) 9 with displacement ellipsoids drawn at the 50% prob-
ability level. The H atoms are omitted for clarity.
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Redox Chemistry of Nickel (Dithiosalicylidenediamine) Complexes
FULL PAPER
II
ꢀ
N
are typical of those observed for Ni
A
bonds.[19,36,38–46] The dihedral angles between the planes de-
fined by S(1)-Ni(1)-S(2) and N(1)-Ni(1)-N(2) (15.1, 21.1, 9.0
and 15.98 for 1, 2, 3 and 4, respectively) indicate a distorted
square-planar co-ordination environment about each NiII
center. This distortion from planarity is greatest in 2 and 4
which results from the small N(1)-Ni(1)-N(2) bite angle
(87.28(7) and 86.30(5)8, respectively, c.f. 91.16(7) and
90.12(9)8 for 1 and 3, respectively) enforced by the rigid 1,2-
phenylenediimine backbone.
Electrochemistry: The cyclic voltammetry of 1 at 293 K in
CH2Cl2 containing 0.4m [NnBu4]ACTHRE[UNG BF4] as supporting electro-
lyte reveals a reversible process at E1/2 =0.16 V versus Fc+/
a
Figure 3. View of the molecular structure of (8)2 with displacement ellip-
soids drawn at the 50% probability level. The H atoms and molecules of
MeCN are omitted for clarity.
Fc (Figure 5), and a second process at Ep =0.82 V versus
Fc+/Fc. Coulometric measurements confirm the process at
E
1/2 =0.16 V versus Fc+/Fc as a one-electron oxidation, and
variable-temperature cyclic voltammetric experiments over
the range 291–226 K, reveal a temperature dependence
(Figure 5). Similar observations have been made for the
when compared to previously reported syntheses of Ni-
ACHTREUNG(tsalen)-type complexes as no thiol protection and deprotec-
tion steps are required.[33–36] Complexes 1–4 were obtained
as analytically pure brown crystals by recrystallization from
CH2Cl2. 1H NMR and 13C NMR measurements established
the formation of diamagnetic NiII centers with significant
upfield shifts in the peak positions compared to those ob-
served for 7–10.
electrochemistry of [NiACTHREUNG ACHTREUNG
(ema)]2ꢀ and [Ni(pma)]2ꢀ (H4ema=
N,N’-ethylenebis(2-mercaptoacetamide); H4pma=N,N’-1,2-
phenylenebis(2-mercaptoacetamide) which exhibit reversi-
ble oxidation processes at E1/2 =ꢀ0.34 and ꢀ0.24 V versus
SCE in DMF, respectively, that become irreversible at
ꢀ308C due to the likely formation of dimers involving the
(m-SR)2Ni bridge.[18]
formation of a NiACHTREUNG
X-ray crystal structures of 1–4: The NiII centers in 1–4
(Figure 4, see also Supporting Information SI1 and SI2) are
bound by two thiolato S-donors and two imine N donors
The cyclic voltammogram of 2 shows (Figure 5) similar
behavior and exhibits a reversible one-electron oxidation
process at E1/2 =0.10 V versus Fc+/Fc at 293 K that loses re-
versibility at temperatures below 253 K. In addition, a
ꢀ
ꢀ
with the Ni(1) S(1) and Ni(1) S(2) bond lengths (ca.
2.16 Š, Table 4) lying within the range observed for previ-
a
second, oxidation process is observed for 2 at Ep =0.93 V
ously reported NiII–S
(thiolate) complexes.[17,18,37,38] The
versus Fc+/Fc at 293 K. By contrast, 3 and 4 do not exhibit
ꢀ
ꢀ
Ni(1) N(1) and Ni(1) N(2) distances (ca. 1.89 Š, Table 2)
any reversible redox processes; instead, a series of tempera-
ture-independent
processes at Ep =0.29, 0.60
and 0.88 V versus Fc+/Fc for 3
oxidative
a
and Ep =0.41 V versus Fc+/Fc
a
for 4 were noted (see Figure
S3 in the Supporting Informa-
tion). This observation suggests
that the incorporation of tBu
groups within the ligand frame-
works in 1 and 2 stabilizes the
one-electron oxidized species,
1+ and 2+ relative to the 3+
and 4+ under ambient condi-
tions. This is consistent with
the stability conferred by tBu
substitutions in Ni complexes
containing salen-type pheno-
late ligands[47] and in [Ni-
AHCTREUNG
(buS2)2]n (n=2, 1, 0; buS2 =3,5-
ditertiarybutyl-1,2-benzenedi-
thiolate).[48]
Figure 4. Views of the molecular structures of a) 1 and b) 2 with displacement ellipsoids drawn at the 50%
probability level. H atoms are omitted for clarity.
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2567

M. Schrçder, J. McMaster et al.
Table 2. Selected bond lengths [Š] and angles [8] for 1–4 and [(2)2]2+
.
supporting electrolyte at ap-
plied potentials of 0.34 and
0.37 V versus Fc+/Fc, respec-
tively, affords green solutions
of 1+ and 2+. Samples of 1+
and 2+ in CH2Cl2 were also
prepared by chemical oxida-
tion of 1 and 2, respectively,
with 70% HClO4. At 293 K,
the X-band EPR spectra of 1+
and 2+ exhibit isotropic signals
at giso =2.073 and 2.064 with
Gaussian linewidths of 16 G
and 17 G, respectively, as de-
termined by spectral simula-
tions.[49] At 77 K, as frozen
CH2Cl2 solutions, electrochem-
ically generated 1+ and 2+,
and 2+ generated with HClO4
are EPR-silent. In contrast, 1+
generated by the reaction of 1
with HClO4, is EPR-active at
77 K as a frozen CH2Cl2 solu-
tion, and exhibits a rhombic
spectrum with g11 =2.329, g22 =
2.067 and g33 =2.027 (Figure 6).
1
2
3
4
[(2)2]2+
ꢀ
Ni(1) S(1)
Ni(1)-S(2)
2.1742(6)
2.1577(6)
1.902(2)
1.898(2)
2.1473(6)
2.1407(6)
1.882(2)
1.880(2)
2.1747(9)
2.1681(9)
1.917(2)
1.924(2)
2.1540(4)
2.1614(4)
1.8993(13)
1.9061(12)
2.138(4)
2.110(4)
1.890(10)
1.844(10)
2.153(3)
2.116(4)
1.888(10)
1.855(9)
2.202(5)
3.185(2)
ꢀ
Ni(1) N(1)
ꢀ
Ni(1) N(2)
ꢀ
Ni(2) S(3)
ꢀ
Ni(2) S(4)
ꢀ
Ni(2) N(3)
ꢀ
Ni(2) N(4)
ꢀ
S(1) S(3)
ꢀ
Ni(1) Ni(2)
ꢀ
S(1) C(3)
1.763(2)
1.761(2)
1.283(2)
1.288(2)
84.12(2)
93.09(5)
168.85(5)
169.18(5)
93.51(5)
91.16(7)
1.738(2)
1.741(2)
1.316(2)
1.318(2)
83.33(2)
96.08(5)
165.39(5)
165.79(5)
96.86(5)
87.28(7)
1.748(3)
1.744(3)
1.293(3)
1.286(4)
83.42(3)
94.09(9)
171.65(7)
174.56(7)
92.97(7)
90.12(9)
1.733(2)
1.741(2)
1.302(2)
1.298(2)
81.91(2)
97.10(4)
169.01(4)
169.24(4)
96.67(4)
86.30(5)
ꢀ
S(2) C(3A)
ꢀ
N(1) C(1)
ꢀ
N(2) C(1A)
S(1)-Ni(1)-S(2)
S(1)-Ni(1)-N(1)
S(1)-Ni(1)-N(2)
S(2)-Ni(1)-N(1)
S(2)-Ni(1)-N(2)
N(1)-Ni(1)-N(2)
S(3)-Ni(2)-S(4)
S(3)-Ni(2)-N(3)
S(3)-Ni(2)-N(4)
S(4)-Ni(2)-N(3)
S(4)-Ni(2)-N(4)
N(3)-Ni(2)-N(4)
85.61(14)
92.2(4)
164.5(3)
166.7(4)
98.5(4)
87.1(5)
85.60(14)
92.2(3)
167.8(3)
169.3(3)
98.5(4)
85.8(5)
The significant rhombic splitting of the g values and their
unknown relationship to the molecular axis system does not
permit an evaluation of the nature of the ground state in 1+
solely on the basis of the experimental EPR spectroscopic
data. Indeed the difficulties in deducing the ground states of
low-spin d7 complexes are well-documented.[18,50,51] Howev-
ꢀ
er, it seems likely that binding of ClO4 to 1+ is occurring
under these conditions to afford 1-ClO4, the flexibility of the
chelate ligand in 1+ allowing coordination of the anion
more readily compared to the more rigid, conjugated ligand
in 2+.
Figure 5. The cyclic voltammograms of a) 1 and b) 2 (1 mm), measured at
293 K (solid line) and 233 K (dashed line), in CH2Cl2 containing 0.4m
[NnBu4]ACHTREUNG .
[BF4] as the supporting electrolyte at 100 mVsꢀ1
Figure 6. X-band EPR spectrum at 77 K of HClO4 oxidized 1 in frozen
CH2Cl2 solution. Spin Hamiltonian parameters g11 =2.329, g22 =2.067, and
EPR spectroscopy: Controlled potential electrolysis of 1
and 2 at 293 K in CH2Cl2 containing 0.4m [NnBu4][BF4] as
g
33 =2.027 and Gaussian linewidths of W11 =69 G, W22 =43 G, and W33
=
AHCTREUNG
28 G.
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Redox Chemistry of Nickel (Dithiosalicylidenediamine) Complexes
FULL PAPER
Addition of pyridine to a frozen glass of 1-ClO4, followed
by raising the temperature of the sample to about 193 K,
and a subsequent refreezing of the sample enabled the 77 K
frozen CH2Cl2 solution X-band EPR spectrum of
[1(py)2]ClO4 to be recorded (Figure 7a). The observed ani-
dine to solutions of 1 at 293 and 253 K does not change the
profile of the UV/Vis spectrum of 1, suggesting that pyridine
does not bind to 1 over this temperature range. The oxida-
tion of 1 and 2 to 1+ and 2+, respectively, was monitored
spectroelectrochemically using an optically transparent elec-
trode cell (Figure 8). In the oxidation of 1 a new asymmetri-
Figure 7. X-band EPR spectrum at 77 K of a) [1(py)2]ClO4 and b) [1-
ACHTREUNG
=
2.198, g22 =2.147, and g33 =2.039, a11 =a22 =14 G, a33 =18.5 G, Gaussian
linewidths of W11 =18 G, W22 =20 G, and W33 =16 G.
sotropy in the g tensors with g11 ꢁg22 @ g33 is typical of a
complex possessing an essentially NiIII dz2 ground-state.[18,52]
Well-defined five-line hyperfine coupling to two 14N (I=1)
nuclei in the g33 region (a33 =18.3 G) is consistent with the
co-ordination of two pyridine molecules to 1+ at axial sites
about the Ni center. Asimilar treatment of 1-ClO4 with bi-
Figure 8. a) Electronic spectra of 1 (solid line) and electrochemically gen-
erated 1+ (broken line), 3.2”10ꢀ4 moldmꢀ3 at 293 K in CH2Cl2 contain-
pyridine yields [1ACHTREUNG(bipy)]ClO4, the frozen solution X-band
EPR spectrum of which reveals five-line hyperfine splitting
in the g33 region (a33 =18.5 G) (Figure 7b) consistent with
two 14N nuclei bound to the Ni center. Fluid X-band EPR
ing 0.4m [NnBu4]
deconvolved low-energy region of the electronic spectrum of 1. b) Elec-
tronic spectra of
(solid line) and electrochemically-generated 2+
ACHTRE[UNG BF4] as the supporting electrolyte. Inset: The Gaussian-
2
(broken line) at 293 K in CH2Cl2 containing 0.4m [NnBu4]CAHTRE[UGN BF4] as the
supporting electrolyte
spectra of [1(py)2]ClO4 and [1ACTHRE(UNG bipy)]ClO4 could not be re-
corded because of the instability of these species above
195 K.
UV/Vis spectroscopy: The UV/Vis spectrum of 1 exhibits
well-defined absorptions at 406 (emax =6200mꢀ1 cmꢀ1), 306
(emax =25000mꢀ1 cmꢀ1), and 280 nm (emax =38700mꢀ1 cmꢀ1).
Shoulders are discernable at 477 (emax =1700mꢀ1 cmꢀ1) and
587 nm (emax =160mꢀ1 cmꢀ1). These lower energy features
occur at wavelengths that are about 25 nm greater than
those found in reduced NiSOD, at 450 and 563 nm, and
which have been assigned to ligand-field transitions.[14]
cally-shaped absorption appears at lmax =794 nm (emax
2900mꢀ1 cmꢀ1) together with a new shoulder at lmax
=
=
385 nm (emax =6700mꢀ1 cmꢀ1). Similar profiles have been ob-
(emb)]ꢀ (H4emb=N,N’-ethylenebis(o-mercap-
tained for [NiACHTREUNG
tobenzamide), which exhibits[17] an intense absorption fea-
ture at 714 nm (emax ca. 4000mꢀ1 cmꢀ1) that has been as-
*
signed to S/N(p)!Ni
A
AHCTREiUNG ty of the oxidation processes is confirmed by the electro-
These transitions also compare with features at 461 (emax
290mꢀ1 cmꢀ1) and 556 nm (emax =70mꢀ1 cmꢀ1), respectively,
observed for [Me4N][Ni
(BEAAM)].[20] The addition of pyri-
=
chemical reduction of 1+ and 2+, which re-generates
quantitatively the original spectra of 1 and 2, respectively.
The spectral features exhibited by electrochemically gener-
AHCTREUNG
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2569

M. Schrçder, J. McMaster et al.
ated 1+ are also observed upon chemical oxidation of 1 with
one equivalent of HClO4 in CH2Cl2 at 293 K.
The changes to the electronic spectra of electrochemically
and chemically generated 1+ in CH2Cl2 were monitored
upon cooling from 293 K to 253 K. The features at lmax =385
and 794 nm observed in UV/Vis spectra of 1+ at 293 K are
absent from the spectrum measured at 253 K (Figure 9a).
Thus, 1+ and 2+ undergo a temperature- and concentration-
dependent reversible chemical reaction or structural rear-
rangement at temperatures below 293 K that is consistent
with the variable-temperature cyclic voltammetric studies
detailed above.
X-ray crystal structure of [(2)2]ACHTER(UNG ClO4)2·2CH2Cl2: Single crys-
tals of [(2)2]AHCRTE(UNG ClO4)2·2CH2Cl2 suitable for X-ray crystallogra-
phy were obtained by layering hexane onto a solution of 2+
generated chemically by the oxidation of 2 with HClO4 in
CH2Cl2 followed by incubation at 195 K. The molecular
structure of [(2)2]ACHTRE(UNG ClO4)2·2CH2Cl2 was determined crystallo-
graphically at 120 K, and selected bond lengths and angles
are presented in Table 2. Views of the structure of the dicat-
ion [(2)2]2+ are shown in Figure 10 and confirm that in the
Figure 10. Views of the molecular structure of the dication in [(2)2]-
AHCTRE(UGN ClO4)2·2CH2Cl2 with displacement ellipsoids drawn at the 30% proba-
bility level. H atoms and tBu groups are omitted for clarity.
Figure 9. Changes in the electronic spectra of a) 1+ on going from 293,
273, and 263, to 253 K and of b) 2+ on going from 293, 272, and 253, to
233 K (3.2”10ꢀ4 moldmꢀ3 in CH2Cl2 containing 0.4m [NnBu4]
ACHTRE[UNG BF4] as the
solid-state two 2+ units undergo a dimerization involving
supporting electrolyte).
ꢀ
ꢀ
the formation of a single S S bond (S(1) S(3) 2.202(5) Š).
ꢀ
The Ni(1) Ni(2) distance (3.185(2) Š) is about 0.5 Š longer
Subsequent warming of the sample to 293 K is accompanied
by reappearance of these features and regeneration of the
original spectrum of 1+ at 293 K. The one-electron oxidation
than the sum of the empirical radius of the Ni atom
(1.35 Š),[53] which suggests that there is no formal Ni(1)
ꢀ
Ni(2) bond in [(2)2]2+. Atetragonal N S2 coordination about
2
of 2 to 2+ produces new features at lmax =665 nm (emax
=
each formal NiII center is maintained, and each Ni center
possesses a distorted square-planar geometry with dihedral
angles between the S-Ni-S and N-Ni-N planes of 20.6 and
16.28 for the planes incorporating Ni(1) and Ni(2), respec-
590mꢀ1 cmꢀ1) and lmax =460 nm (emax =1700mꢀ1 cmꢀ1) that
are similar to those of 1+ (Figure 9). Electronic spectra re-
corded between 293 K and 253 K of solutions of 2+ in
CH2Cl2 show a disappearance of the band at 665 nm and its
subsequent reappearance on the warming of the solution to
293 K (Figure 9b). This behavior is similar to that observed
for 1+. In addition, the UV/Vis spectra of electrochemically
and chemically generated 1+ and 2+ below 293 K display
significant deviations from the Beer–Lambert law between
concentrations of 1.6”10ꢀ4 moldmꢀ3 and 3.2”10ꢀ3 moldmꢀ3.
ꢀ
tively. The S(1) S(3) distance of 2.202(5) Š is about 0.15 Š
longer than the typical distances observed in Ar-S-S-Ar di-
sulfides.[38,39] The Ni(1) S(1) and Ni(2) S(3) distances
(2.138(4) and 2.153(3) Š, respectively) are slightly longer
ꢀ
ꢀ
ꢀ
ꢀ
than the Ni(1) S(2) and Ni(2) S(4) distances (2.112(4) and
ꢀ
2.116(4) Š, respectively) and are similar to the Ni(1) S(1)
ꢀ
and Ni(1) S(2) bond lengths (2.1473(6) and 2.1407(6) Š, re-
2570
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Chem. Eur. J. 2008, 14, 2564 – 2576

Redox Chemistry of Nickel (Dithiosalicylidenediamine) Complexes
FULL PAPER
ꢀ
spectively) in the parent complex 2. The Ni N distances
vary from 1.890(10) to 1.842(10) Š in [(2)2]2+ and are simi-
ꢀ
ꢀ
ꢀ
lar to the Ni N distances in 2 (Ni(1) N(1) 1.882(2); Ni(1)
ꢀ
N(2) 1.880(2) Š) These observations confirm that Ni SR
cleavage does not occur on the formation of [(2)2]2+ in con-
trast to the oxidized complexes of Fe, Ga, and Co complexes
with 1,4,7-triazacyclononane-1,4-diacetate containing a 2-
mercaptobenzyl pendant arm.[54] In addition to the forma-
ꢀ
tion of a S S bond, the stability of the dicationic dimer
[(2)2]2+ is presumably augmented by intramolecular p–p
stacking of the aromatic rings of the ligand frameworks.
The formation of [(2)2]2+ is in accord with the electro-
chemical, UV/Vis and EPR spectroscopic data for these sys-
tems, which confirm the formation of the paramagnetic NiII–
thiyl radical species 1+ and 2+. These cations are in equilib-
rium in the absence of coordinating ligands/solvent with
2+
EPR-silent S S bonded dimers [(1)2] and [(2)2]2+, respec-
ꢀ
ꢀ
tively. These dimeric species incorporate direct S S bonding
and since they are highly thermally and chemically sensitive,
it was not possible to investigate these materials further.
ꢀ3
Figure 11. Kohn–Sham orbital isosurface plots at 0.05 eŠ for a) the
HOMOs of models of 1 and 2, b) the SOMOs of models of 1+ and 2+,
+
+
and c) the SOMOs of models of 1(py)2 and 2(py)2
.
DFT calculations: DFT calculations of models of 1 and 2 re-
produce the principal features of the experimental geome-
ꢀ
ꢀ
tries of 1–4. The average calculated Ni S and Ni N distan-
ces for 1 (2.181 and 1.912 Š, respectively) compare well to
the experimental distances of 3 (2.171 and 1.903 Š, respec-
tively) and are similar to the experimental distances of 1
(2.166 and 1.900 Š, respectively). Similarly, the calculated
(Figure 6) is consistent with a SOMO of p-symmetry, as re-
vealed by the DFT calculations on models of 1+ and 2+.
This symmetry is similar to that for the SOMO of [Ni-
AHCTREUNG
(emb)]ꢀ, which also exhibits a highly rhombic frozen solu-
tion EPR spectrum with g11 =2.25, g22 =2.11, and g33 =
2.04.[13,17] The DFT calculations on the models of 1+ and 2+
also suggest that the SOMOs of these species are considera-
bly more localized on the thiolate S donors than for the
ꢀ
ꢀ
Ni S and Ni N distances for 2 (2.164 and 1.908 Š, respec-
tively) are comparable to the experimental distances of 4
(2.158 and 1.903 Š, respectively) and are similar to the ex-
perimental distances of 1 (2.144 and 1.851 Š, respectively).
Moreover, the DFT calculations appear to support the sig-
(emb)]ꢀ, which involves ca. 13% amide N
SOMO of [NiACHTREUNG
character in addition to a 22% contribution from the S thio-
late donors.[13] This greater contribution to the SOMO from
the amide N-centers is consistent with the greater p-donor
ꢀ
nificantly shorter Ni S distances found in the X-ray crystal-
lographic structures of complexes 2 and 4 relative to those
of 1 and 3. Thus, the close correspondence in metrical pa-
rameters for the experimental structures of 1–4 and the
DFT-calculated structures for models of 1 and 2 suggests
that the electronic structures of these centers, and their cor-
responding oxidized forms, are well described at the qualita-
tive level by this DFT method.
capacity of these donors in [Ni
(emb)]ꢀ when compared to
ACHTREUNG
the imine N-donors in 1+ and 2+.
+
+
DFT calculations on models of 1(py)2 and 2(py)2 reveal
SOMOs with substantially different compositions relative to
those of their parent 1+ and 2+ models and which contain
significant Ni 3dz2 and N(py) character, Figure 11c. Thus, the
+
Analyses of the electronic structures of 1 and 2 show that
the redox-active orbital for these compounds is of p-symme-
try and is composed of Ni 3dxz (39%), Ni 3dx2ꢀy2 (3%), S 3pz
(30%), S 3px (5%) and N 2pz (1%) for 1 and Ni 3dxz
(42%), S 3pz (31%), and N 2pz (4%) for 2, Figure 11a. The
oxidation of models of 1 and 2 involves the removal of one
electron from each of these orbitals to generate models of
1+ and 2+ possessing SOMOs with Ni 3dxz (19%), Ni 3dx2ꢀy2
(2%), S 3pz (39%), S 3px (4%) and N 2pz (2%) and, Ni 3dxz
(21%), S 3pz (42%), and N 2pz (3%) characters, respective-
ly, Figure 11b. These orbitals are of the same symmetry as
those of their parent orbitals in the models of 1 and 2 but
with about 10% more S character. Thus, the oxidation of 1
and 2 appears to involve orbitals that possess considerable
composition of the SOMO in the model of 1(py)2 is Ni 3dz2
(29%), S 3p (25%), N(py) 2pz (11%). and in the model of
+
2(py)2 the composition is Ni 3dz2 (47%), S 3p (7%), N(py)
2pz (23%). This switch in SOMO on the binding of two pyri-
dine ligands in the axial positions of the models of 1+ and
2+ is also supported by the frozen solution EPR spectrum
+
of 1(py)2 that is typical of a NiIII center possessing a dz2
ground state with 14N hyperfine coupling in the g33 compo-
nent. This hyperfine coupling is a consequence of the 2 s
+
and 2p N(py) contributions to the SOMO in 1(py)2
.
Conclusion
Ni–S covalency to produce a [Ni
(N2S2)]+ species with thiyl-
We have synthesized and crystallographically characterized
the diamagnetic complexes [NiII
(N2S2)] containing thiolate
radical character. The rhombic EPR spectrum for 1+
AHCTREUNG
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2571

M. Schrçder, J. McMaster et al.
S- and imine N-donors and shown that 1 and 2 are capable
of supporting reversible oxidation chemistry at room tem-
perature. The analogous complexes 3 and 4 show an instabil-
ity in their oxidized states indicating that o- and p-tBu sub-
stitution of the thiolate rings limits decomposition pathways
for the oxidized products, as has been found previously for
phenolato metal complexes capable of supporting phenoxyl
radical centers.[54–58] The EPR and UV/Vis spectroscopic
measurements and supporting DFT calculations are consis-
tent with the oxidation of 1 and 2 involving a p-type orbital
with about 21% Ni and about 42–43% S character in the
oxidized forms. Thus, the oxidation of 1 and 2 is largely
ligand-based such that 1+ and 2+ possess considerable thiyl
radical character. While similar ligand-based oxidation has
The addition of pyridine to frozen solutions of 1+ fol-
lowed by warming to about 193 K and re-freezing leads to
+
the generation of 1(py)2 that possesses a SOMO with sub-
stantially less S character relative to 1+ (ca. 29% vs. 42%,
respectively) and with a dominant 3dz2 character for the Ni
contribution. For the calculated structure of 2+, the imposi-
tion of C2v symmetry prohibits combinations from S orbitals
of p-type symmetry contributing to the SOMO. For this spe-
cies the SOMO is almost exclusively derived from the Ni
3dz2 and orbital combinations from the N(py) donor atoms.
This switch in SOMO composition is reflected in the g11
>
g22 @ g33 pattern and in the resolution of 14N hyperfine cou-
+
pling in the g33 region within the EPR spectrum of 1(py)2
.
Thus, the coordination of axial N-donor ligands switches the
SOMO from one which is essentially ligand-based to one
that is more metal-based and which involves a formal NiIII
center. Our UV/Vis spectroscopic results suggest that pyri-
(emb)]2ꢀ, which also possesses phenyl
been observed for [NiACHTREUNG
thiolate units,[13,17] the incorporation of the poorer imine p-
donors in 1+ and 2+ leads to greater contributions to the
SOMO from the thiolate S-atoms in these complexes. DFT
+
dine binds only to 1+ and not 1. Therefore, 1(py)2 can only
calculations[13] on [Ni
A
form once 1 is oxidized to 1+ followed by the binding of
two pyridine donors to 1+. This observation is consistent
with the X-ray crystallographic results for NiSOD in its re-
duced and oxidized states and with recent spectroscopic and
theoretical studies on the enzyme.[9,10,14] These latter studies
donors and which might be considered to be more relevant
to the active site of NiSOD, result in an electronic structure
that is analogous to 1+, 2+ and [Ni(emb)]ꢀ. However, spec-
ACHTREUNG
troscopic and DFT studies suggest that, in MeCN/propioni-
Cꢀ
trile/butyronitrile solution, [Ni
A
a
point to a O2 reduction step in which the redox active MO
center with a dz2 ground state, most likely associated with
involves Ni/S/N(p) combinations to generate an oxidized
species that is unbound by protein ligands at the axial sites
of the Ni center. This unit subsequently binds an axial imi-
dazole ligand to ensure a NiIII SOMO with Ni 3dz2 (76%)
character and reduced S (9%) contributions. By compari-
the axial coordination of nitrile ligands.[13] Thus, 1+, 2+, [Ni-
ACHTREUNG ACHTREUNG
(ema)]ꢀ, and [Ni(emb)]ꢀ appear to exhibit similar oxidative
chemistry suggesting that the incorporation of phenyl thio-
late rather than alkyl thiolate donors has little impact on the
ligand-based or metal-based nature of the ground states in
these species.
+
son, the SOMO of the six coordinate complex 1(py)2 con-
tains significantly more S character (36%) than the SOMO
of the five-coordinate oxidized form of the enzyme. Thus,
even with the additional axial ligand in 1(py)2+, there is con-
siderable S character within the SOMO of 1(py)2+. The dif-
ferences in the compositions of the SOMOs of the oxidized
form of NiSOD, in which the axial imidazole ligand has yet
to bind and for those of the models of 1+ and 2+ are even
more significant. The SOMO in this form of the enzyme
possesses 58% Ni(p), 24% S(p), and 10% N/O(p) charac-
ter,[14] whereas for 1+ the SOMO possesses Ni (21%), S
(43%) and N (2%) character. Thus, the active site of
NiSOD appears to have evolved to minimize the S p-contri-
butions in both the imidazole bound and un-bound forms of
the oxidized enzyme whilst maintaining S-coordination to
lower the redox potential of the metal center in the enzyme.
The active site minimizes these S p-contributions in the
redox-active orbital of the oxidized form by i) employing an
amide ligand that is an effective p-donor in the imidazole
un-bound form of the enzyme and ii) employing an axial
imidazole ligand to switch the SOMO to an orbital that is
essentially metal-based. In addition, our calculations indi-
cate that the coordination geometry of the metal center and
conformation of the ligand backbones play a major role in
determining the composition of the redox-active orbitals.[60]
Electrochemical, EPR and UV/Vis spectroscopic studies
of 1+ and 2+ show that these species undergo a chemical
process at temperatures below 293 K, that can be reversed
on warming to ambient temperature, and the isolation and
X-ray
crystallographic
characterization
of
[(2)2]-
ACHTREUNG(ClO4)2·2CH2Cl2 shows that this reaction involves the for-
mation of the diamagnetic [(2)2]2+ dimer. Similar variations
in the spectroscopic and electrochemical properties of [Ni-
ACHTREUNG
(emb)]ꢀ have also been rationalized in terms of a low-tem-
perature dimerization process that may involve the forma-
tion of [Ni2A
CHTREUNG
formulation has derived from parallel resonance Raman,
UV/Vis and TD-DFT calculations.[13] For 2+, our structural
data show that [(2)2]2+ involves a S S bond rather than the
ꢀ
direct bridging of two of the S atoms across the bimetallic
unit. The differences between the dimerization chemistry of
[Ni
tributions to the SOMO in 2+ relative to those of [Ni-
(emb)]ꢀ. This higher degree of thiyl radical character is a
ACHTREUNG
(emb)]ꢀ and 2+ may result from the greater S-atom con-
ACHTREUNG
consequence of the differences in the p-donor strength of
the N-atom donors in these complexes and may lead to the
preferential formation of a disulfide in [(2)2]2+ as is com-
monly observed for the oxidation of thiolate in solution.[59]
The relative instability and thermal sensitivity of both
[(1)2]2+ and of [(2)2]2+ precluded further study of these ma-
terials.
+
This is most apparent for the calculations of 1(py)2 and
2(py)2+; the latter complex, for which C2v symmetry was im-
posed, possesses a SOMO that has no S p-character. Thus, it
2572
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Chem. Eur. J. 2008, 14, 2564 – 2576

Redox Chemistry of Nickel (Dithiosalicylidenediamine) Complexes
FULL PAPER
10 mmol). The reaction mixture was stirred at 08C for 30 min before
being allowed to warm to room temperature and left to stir for 22 h. 1-
Formylpiperidine (2.7 mL, 24 mmol) was added dropwise such that the
temperature did not exceed 708C. The reaction mixture was stirred at
room temperature for 16 h before the addition of aqueous HCl (5%,
100 mL). Et2O (40 mL) was added and the organic layer separated. The
aqueous layer was extracted with Et2O (3”20 mL) and the organic ex-
tracts combined and dried over MgSO4. Et2O was removed in vacuo to
give a viscous orange oil which was dissolved in hexane (5 mL) and
passed through a short chromatographic column containing a silica/
is likely that the conformation of the protein side chains is
also important in fine-tuning the nature of the electronic
structure of the active site of NiSOD.
Experimental Section
Unless stated otherwise, all reactions were carried out under pure argon
using standard Schlenk techniques. Where required, solvents were dis-
tilled by standard methods before use. All chemicals were purchased
from Aldrich, except 1-formylpiperidine (Lancaster) and tert-butylchlor-
ide (Acros), and were used as received. 2,4-di-tert-butylbromobenzene
was synthesized following literature preparations.[61–63] Complexes 3 and 9
and compound 6b were prepared as described previously.[23]
hexane slurry. Ahexane/CH Cl2 mixture with an increasing proportion of
2
CH2Cl2 from 100:0 to 50:50 was passed through the column. The second
fraction (ca. 75 mL) was collected, and the solvent removed in vacuo to
give a viscous orange oil (6a, 1.48 g, 5.90 mmol, 59%), which was used
without further purification.
Synthesis of 7·0.5H2O: Asolution of Zn ACHTRE(UNG OAc)2·2H2O (0.58 g, 2.7 mmol)
NMR spectra were obtained using a Bruker DPX 300 spectrometer, and
X-band EPR spectra were recorded on a Bruker EMX spectrometer with
microwave frequencies calibrated with a Bruker ER035m NMR gaussme-
ter. EPR spectra were simulated using WINEPR SimFonia Version 1.25
(Bruker). Infrared spectra were recorded with a Nicolet Avatar 360
FTIR spectrometer, and mass spectra were obtained from the EPSRC
National Mass Spectrometry Service Center at the University of Wales,
Swansea, UK. Elemental analyses were carried out by the Microanalyti-
cal Service at the University of Nottingham with an Exeter Analytical
Inc CE-440 Elemental analyzer, and electrochemical measurements were
performed with an Autolab PGSTAT20 potentiostat. Cyclic voltammo-
grams were recorded using a three-electrode system consisting of a plati-
num working electrode, a platinum secondary electrode and an Ag/AgCl
reference electrode. Experiments were carried out at 293 K, unless stated
otherwise, under Ar using 1 mm solutions of complex in CH2Cl2 contain-
in MeOH (5 mL) was added to a solution of 6a (1.20 g, 5.3 mmol) in
MeCN (20 mL). Asolution of triethylamine (0.81 g, 8.0 mmol) in MeOH
(5 mL) was added to the mixture. Asolution of 2-dimethyl-1,3-diamino-
propane (0.22 g, 2.1 mmol) in MeOH (5 mL) was added dropwise to the
orange solution. The reaction mixture was stirred for 1 h and then fil-
tered to yield the crude product which was recrystallized from CH2Cl2 to
give yellow crystals of 7 (1.51 g, 2.4 mmol, 90%). 1H NMR (300 MHz,
CDCl3, 298 K, TMS): d=8.3 (s, 2H, N=CH), 7.6 (d, 2H, ArH), 7.1 (d,
2H, ArH), 4.2 (bs, 2H, HC-H), 3.2 (bs, 2H, HC-H), 1.8 (s, 18H, C-
A
ACHTREUNG
ACHTREUNG
A
G
A
ACHTREUNG
A
ACHTREUNG
2867 m, 1637s, 1599 w, 1466m, 1386 m, 1361m, 1263m, 1200 w, 1155m,
1080 w, 1049m, 900 w, 744 w cmꢀ1; MS (ES+): m/z: 630 [7 + H+]; ele-
mental analysis calcd (%) for C35H52N2S2Zn.0.5H2O: C 65.75, H 8.36, N
4.38: found C: 65.95, H 8.12, N 4.45. Yellow crystals of 7 suitable for X-
ray crystallography were obtained by slow diffusion of Et2O onto a
CH2Cl2 solution of 7.
ing [NnBu4]
enced to the Fc+/Fc couple which was used as an internal standard.
Where necessary, the [FeCp* ]+/[FeCp* ] couple was used as the internal
ACHTREUNG[BF4] (0.4m) as supporting electrolyte. Potentials are refer-
AHCTREUNG
2
2
standard to avoid overlapping of redox couples, and the [FeCp* ]+/
2
[FeCp* ] couple was referenced to the Fc+/Fc couple by an independent
calibration. Compensation for internal resistance was not applied. Cou-
lombic measurements were performed at 273 K under Ar in CH2Cl2 con-
Synthesis of 8·0.5MeCN·H2O: Asolution of Zn CAHTER(UNG OAc)2.2H2O (0.58 g,
taining [NnBu4]ACHTREUNG[BF4] (0.4m) as supporting electrolyte. Athree-electrode
system was used consisting of a Pt/Rh gauze basket working electrode, a
Pt/Rh gauze secondary electrode and an Ag/AgCl reference electrode.
2.7 mmol) in MeOH (5 mL) was added to a solution of 6a (1.20 g,
5.3 mmol) in MeCN (20 mL). Asolution of triethylamine (0.81 g,
8.00 mmol) in MeOH (5 mL) was added to the mixture. The reaction
flask was wrapped in foil and stirred in the dark during the dropwise ad-
dition of 1,2-phenylenediamine (0.23 g, 2.1 mmol) in MeOH (5 mL). The
red mixture was stirred in the dark for 2 h by which time a precipitate
had formed, the reaction was then stirred at 08C for a further 1 h after
which time the reaction was exposed to light and the crude product sepa-
rated by filtration. The precipitate was washed with cold MeOH and re-
crystallized from CH2Cl2 to give bright red crystals of 8 (1.41 g, 2.2 mmol,
84%). 1H NMR (300 MHz, CDCl3, 298 K, TMS): d=8.62 (s, 2H, N=
CH), 7.59 (d, 2H, ArH), 7.49 (m, 4H, ArH), 7.05 (d, 2H, ArH), 1.56 (s,
UV/Vis spectroelectrochemical measurements were carried out at 273 K
using an optically transparent electrode mounted in a modified quartz
cuvette with an optical pathlength of 0.5 mm. Athree-electrode system
was used consisting of a Pt/Rh gauze working electrode, a Pt wire secon-
dary electrode in a fritted PTFE sleeve, and an Ag/AgCl reference elec-
trode chemically isolated from the sample solution via a bridge tube con-
taining electrolyte solution terminated in a porous frit. Potentials were
applied using a Sycopel Scientific Ltd. DD10m potentiostat and UV/Vis
spectra recorded on a Perkin Elmer Lambda 16 Spectrometer. Tempera-
ture control was achieved by flowing cooled N2 across the surface of the
cell.
18H, CACTHER(NGU CH3)3), 1.28 ppm (s, 18H, CACHTRE(UGN CH3)3); IR (KBr): n˜ =2961 vs,
2906s, 2869s, 1624s, 1598s, 1364s, 1361s, 1261m, 1153m, 1112m, 1048m,
1024m cmꢀ1; MS (ES+): m/z: 637 [8 + H+]; elemental analysis calcd
(%) for C36H46N2S2Zn·0.5CH3CN·H2O: C 65.86, H 7.39, N 5.19; found: C
66.13, H 7.07, N 4.83. Red crystals of (8)2·MeCN suitable for X-ray crys-
tallography were obtained by layering MeCN onto a CH2Cl2 solution of
8.
Synthesis of 2,4-di-tert-butylbenzenethiol (5a): Adry THF solution
(40 mL) of 1-bromo-2,4-di-tert-butylbenzene (15.75 g, 58.5 mmol) was
added dropwise to magnesium (4.46 g, 58.5 mmol) in THF (280 mL). The
reaction mixture was heated at reflux for 45 min then cooled before addi-
tion of sulfur (1.88 g, 58.5 mmol) at 08C. The reaction mixture was stirred
at 508C for 10 h. After the mixture was allowed to cool, the solution was
decanted and carefully hydrolyzed with excess HCl (5%). THF was re-
moved in vacuo and the crude product extracted with CHCl3, washed
with brine, and dried over MgSO4. The CHCl3 was then removed in
vacuo and the product distilled (838C) by using a short-path Kugelruhr
apparatus to give 5a (12.15 g, 54.7 mmol, 94%). 1H NMR (300 MHz,
CDCl3, 298 K, TMS): d=7.45 (d, 1H, ArH), 7.19 (d, 1H, ArH), 7.07 (dd,
Synthesis of 9·0.25H2O: Compound 9·0.25H2O was prepared by litera-
ture methods from ZnACHTRE(UNG OAc)2·2H2O, 6b, and 2-dimethyl-1,3-diaminopro-
pane.[23] 1H NMR (300 MHz, CDCl3, 298 K, TMS): d=8.30 (s, 2H, N=
CH), 7.33 (d, 2H, ArH), 7.26 (d, 2H, ArH), 7.20 (t, 2H, ArH), 7.00 (t,
2H, ArH), 3.51 (bs, 4H, CH2), 1.08 ppm (s, 6H, CH3); 13C NMR
(75 MHz, CDCl3, 298 K, TMS): d=170.6 (N=CH), 137.1 (Ar CH), 137.1
(Ar CH), 131.1 (Ar CH), 129.4 (Ar CH), 122.4 (Ar CH), 122.4 (Ar CH),
1H, ArH), 3.60 (s, 1H, SH), 1.52 (s, 9H, C
ACHTRE(UNG CH3)3), 1.35 ppm (s, 9H, C-
68.3 (CH2), 36.2 (CACTHRE(UGN CH3)2), 24.9 ppm (CACHTREU(NG CH3)2); IR (KBr): n˜ =2952 w,
ACHTREUNG
2917 w, 2863 w, 1632s, 1587m, 1540m, 1462m, 1442 w, 1409m, 1391 w,
1336 w, 1252 w, 1203 w, 1162 w, 1127m, 1065m, 1027m, 971 w, 868 w,
760m, 752m, 721 w, 686 w, 647 w cmꢀ1; MS (ES+): m/z: 405 [9 + H+];
elemental analysis calcd (%) for C19H20N2S2Zn·0.25H2O: C 55.62, H 5.04,
Synthesis of 2,4-di-tert-butylthiosalicylaldehyde (6a): nBuLi (2.35m,
9.13 mL, 21.5 mmol) was added to a solution of TMEDA(3.25 mL,
21.5 mmol) in anhydrous hexane (30 mL) at 08C followed by 5a (2.2 g,
Chem. Eur. J. 2008, 14, 2564 – 2576
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2573

M. Schrçder, J. McMaster et al.
N 6.83: found: 55.92, H 4.94, N 6.65. Yellow crystals of 9 suitable for X-
ray crystallography were obtained by slow evaporation of a solution of
the complex in CH2Cl2.
previously.[23] 1H NMR (300 MHz, CDCl3, 298 K, TMS): d=7.70 (s, 2H,
N=CH), 7.26 (m, 4H, ArH), 7.19 (dd, 2H, ArH), 7.00 (dd, 2H, ArH),
3.62 (s, 4H, CH2), 1.04 ppm (s, 6H, CH3); 13C NMR (75 MHz, CDCl3,
298 K, TMS): d=165.2 (N=CH), 145.4 (Ar CH), 133.6 (Ar CH), 130.8
(Ar CH), 130.7 (Ar CH), 130.4 (Ar CH), 122.2 (Ar CH), 69.7 (CH2), 34.8
Synthesis of 10·1.5H2O: Asolution of Zn ACHTREU(GN OAc)2·2H2O (0.55 g,
2.5 mmol) in MeOH (5 mL) was added to a solution of 6b (5.00 mmol)
in MeCN (20 mL). Asolution of triethylamine (1.5 g, 15 mmol) in
MeOH (5 mL) was added to the mixture. The reaction flask was wrapped
in foil and stirred in the dark during the dropwise addition of 1,2-phenyl-
enediamine (0.27 g, 2.5 mmol) in MeOH (5 mL). The red reaction mix-
ture was stirred in the dark for 10 h during which time an insoluble pre-
cipitate formed. Filtration of the mixture gave 10 (0.95 g, 2.3 mmol,
92%) as a red powder, which was washed with cold MeOH and used
without further purification. IR (KBr): n˜ =2973 m, 1612 vs, 1602 vs, 1577
vs, 1526s, 1451s, 1399s, 1183s, 1126m, 1075m cmꢀ1; elemental analysis
calcd (%) for C20H14N2S2Zn·1.5H2O: C 54.74, H 3.90, N 6.38; found: C
54.89, H 3.37, N 6.31.
(CACTHER(NGU CH3)2), 24.4 ppm (CACHRTE(UGN CH3)2); IR (KBr): n˜ =2959s, 2924s, 1615 vs, 1586
vs, 1540s, 1446s, 1427m, 1389m, 1221s, 1128m, 1079s, 1061m, 1035m,
948m, 747 vs, 720m cmꢀ1; MS (ES+): m/z: 399 [3 + H+], 797 [(2”3) +
H+]; elemental analysis calcd (%) for C19H20N2S2Ni: C 57.17, H 5.05, N
7.02; found: C 57.19, H 5.08, N 6.94. Brown crystals of 3 suitable for X-
ray crystallography were obtained by slow diffusion of Et2O onto a
CH2Cl2 solution of the complex.
Synthesis of 4·H2O: Asolution of Ni CAHTREU(NG NO3)2·6H2O (0.058 g, 0.2 mmol) in
MeOH (5 mL) was added with stirring to a suspension of 10·1.5H2O
(0.082 g, 0.19 mmol) in CH2Cl2 (25 mL). The mixture was heated at 408C
for 2 h during which time a slow transition from a red suspension to a
dark brown solution occurred. The solution was concentrated in vacuo to
about 7 mL and cold MeOH (40 mL) was added. The precipitate was col-
lected by filtration and washed with cold MeOH followed by Et2O
before recrystallization from CH2Cl2 to give dark brown crystals of 4
Synthesis of 1·0.5H2O: NiACHTRE(UNG NO3)2·6H2O (0.058 g, 0.19 mmol) in MeOH
(3 mL) was added to a solution of 7·0.5H2O (0.126 g, 0.20 mmol) in
CH2Cl2 (15 mL) and the solution stirred for 2 h. Excess MeOH was
added to the brown solution and the precipitate collected by filtration in
air. The crude product was washed with cold MeOH and recrystallized
1
(0.071 g, 0.17 mmol, 87%). H NMR (300 MHz, CDCl3, 298 K, TMS): d=
8.95 (s, 2H, N=CH), 7.83 (m, 4H, ArH), 7.58 (dd, 2H, ArH), 7.41 (q,
2H, ArH), 7.28 (dd, 2H, ArH) , 7.08 ppm (td, 2H, ArH); 13C NMR
(75 MHz, CDCl3, 298 K, TMS): d=157.9 (N=CH), 136.8 (Ar CH), 131.7
(Ar CH), 131.0 (Ar CH), 129.4 (Ar CH), 129.0 (Ar CH), 126.0 (Ar CH),
123.1 (Ar CH), 122.4 (Ar CH), 116.7 ppm (Ar CH); IR (KBr): n˜ =
1590m, 1576s, 1525 vs, 1457 vs, 1426m, 1392m, 1234m, 1080m,
748s cmꢀ1; MS (ES+): m/z 406 [4 + H+], 810 [(2”4)+]; elemental anal-
ysis calcd (%) for C20H14N2S2Ni·H2O: C 56.76, H 3.81, N 6.62; found: C
56.91, H 3.32, N 6.59. Dark brown crystals of 4 suitable for X-ray crystal-
lography were obtained by slow evaporation of MeOH from a 50:50
CH2Cl2/MeOH solution of the complex.
from CH2Cl2 to give brown crystals of
1 (0.11 g, 0.2 mmol, 92%).
1H NMR (300 MHz, CDCl3, 298 K, TMS): d=7.65 (s, 2H, N=CH), 7.42
(d, 2H, ArH), 7.07 (d, 2H, ArH), 3.55 (s, 4H, CH2), 1.63 (s, 18H, C-
(CH3)3), 1.03 ppm (s, 6H, CH3); 13C NMR
ACHTREUNG(CH3)3), 1.28 (s, 18H, CACHTREUNG
(75 MHz, CDCl3, 298 K, TMS): d=166.6 (N=CH), 149.5 (Ar CH), 133.0
(Ar CH), 128.3 (Ar CH), 127.5 (2 Ar CH), 127.5 (Ar CH), 69.5 (CH2),
37.8 (C
A
ACHTREUNG(CH3)3), 34.2 (CACHERT(NUG CH3)3), 31.2 (CACHTRE(UNG CH3)2), 30.0 (C-
N
1463s, 1388s, 1361s, 1266s, 1230m, 1163m, 1059m cmꢀ1; MS (ES+): m/z:
624 [1 + H+]; elemental analysis calcd (%) for C35H52N2S2Ni.0.5H2O:
C 66.45, H 8.44, N 4.43; found: C 66.55, H 8.35, N 4.38. Brown crystals of
1 suitable for X-ray crystallography were obtained by layering hexane
onto a CH2Cl2 solution of the complex.
X-ray crystallography: Crystallographic data and structure refinement pa-
rameters for complexes 1–4, 7, (8)2·MeCN, 9 and (2)2·CAHTRE(UNG ClO4)2·2CH2Cl2
Synthesis of 2·MeOH: Asolution of Ni ACHTRE(UGN NO3)2·6H2O (0.058 g, 0.19 mmol)
are summarized in Table 3 and Table 4. Crystals were mounted on a
dual-stage glass fiber and diffraction measurements collected at 150(2) K
on a Bruker SMART1000 (for 3, 4, 7, 9) or a Bruker SMART APEX
CCD area diffractometer (for 1, 2, and (8)2·MeCN) using graphite-mono-
chromated MoKa radiation (l=0.71073 Š) and w scans. Both diffractom-
eters were equipped with an Oxford Cryosystems open-flow cryostat.[64]
In the case of (2)2·ACTHRE(UGN ClO4)2·2CH2Cl2, diffraction measurements were col-
lected at 120(2) K on a Bruker SMART APEX2 CCD area diffractome-
ter using silicon [111]-monochromated synchrotron radiation (l=
in MeOH (3 mL) was added to a solution of 8·0.5CH3CN·H2O (0.096 g,
0.14 mmol) in CH2Cl2 (20 mL). The dark brown solution was stirred for
1 h before concentration in vacuo to about 5 mL. Et2O (2 mL) and
hexane (30 mL) were added, and the brown solution cooled in ice for 1 h
during which time a brown precipitate formed. The crude product was
collected by filtration and washed with cold MeOH before being recrys-
tallized from CH2Cl2 to give dark brown crystals of 2 (0.11 g, 0.18 mmol,
88%). 1H NMR (300 MHz, CDCl3, 298 K, TMS): d=9.00 (s, 2H, N=
CH), 7.84 (q, 2H, ArH), 7.58 (d, 2H,
ArH), 7.40 (q, 4H, ArH), 1.69 (s,
18H, C
ACHTREUNG
ACHTREUNG(CH3)3), 1.31 ppm (s, 18H, C-
Table 3. Crystallographic data for 7·0.13MeOH, (8)2·MeCN, and 9.
7·0.13MeOH
35.13H52.5N2O0.13S2Zn
(CH3)3); 13C NMR (75 MHz, CDCl3,
(8)2·MeCN
9
298 K, TMS): d=159.2 (N=CH),
149.1 (Ar CH), 145.1 (Ar CH), 143.6
(Ar CH), 131.9 (Ar CH), 131.5 (Ar
CH), 128.8 (Ar CH), 128.1 (Ar CH),
116.1 (Ar CH), 38.0 (CACHTREUNG
(CACHTREUNG(CH3)3), 31.2 (CACHTREUNG
empirical formula
C
C74H95N5S4Zn2
1313.53
C19H20N2S2Zn
405.86
Mr
634.28
crystal system
space group
a [Š]
b [Š]
c [Š]
monoclinic
C2/c
32.560(2)
11.0318(9)
10.3704(7)
90
triclinic
P1
monoclinic
P21
¯
12.5404(8)
17.7470(11)
18.1994(12)
67.259(2)
89.896(2)
72.798(2)
3539.2(4)
2
9.4649(11)
9.7759(11)
9.8843(11)
90
95.914(2)
90
(CACHTREUNG
2906m, 2867m, 1599m, 1570m,
1507m, 1462m, 1384 vs, 1362m,
1161m, 735m cmꢀ1; MS (ES+): m/z:
629 [2+]; elemental analysis calcd
a [8]
b [8]
g [8]
104.921(2)
90
3599.4(7)
4
1.170
0.823
volume [Š3]
Z
909.7(2)
2
(%) for C36H46N2S2Ni.CH3OH:
C
67.17, H 7.62, N 4.23; found: C 67.39,
H 7.20, N 3.81. Dark brown crystals
of 2 suitable for X-ray crystallography
were obtained by slow evaporation of
MeOH from a 50:50 CH2Cl2/MeCN
solution of the complex.
1calcd [gcmꢀ3
]
1.233
1.482
m [mmꢀ1
]
0.84
1.582
crystal size [mm]
reflections collected
unique reflections (Rint
R [F>4s(F)]
0.31”0.18”0.16
11279
4419 (0.020)
0.044
0.51”0.40”0.12
31481
0.25”0.22”0.10
5366
)
15994 (0.020)
0.035
3334 (0.039)
0.031
Synthesis of 3: Compound 3 was pre-
pared from Ni
9·0.25H2O by our method reported
RwF (all F2)
0.134
0.095
0.068
ACHTREUNG(NO3)2·6H2O and
GOF
1.046
1.03
0.995
2574
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 2564 – 2576

Redox Chemistry of Nickel (Dithiosalicylidenediamine) Complexes
FULL PAPER
Table 4. Crystallographic data for 1–4 and [(2)2]ACTHERU(NG ClO4)2·2CH2Cl2.
1
2
3
4
[(2)2]AHCTER(UNG ClO4)2·2CH2Cl2
empirical formula
Mr
crystal system
space group
a [Š]
b [Š]
c [Š]
a [8]
b [8]
C35H52N2NiS2
623.62
monoclinic
P21/c
12.763(2)
14.940(2)
18.229(3)
90
93.030(3)
90
C36H46N2NiS2
629.58
monoclinic
P21/c
8.1604(7)
25.658(2)
16.4775(13)
90
100.590(1)
90
C19H20N2NiS2
399.2
C20H14N2NiS2
405.16
monoclinic
P21/c
6.9449(5)
17.7885(13)
13.5370(10)
90
100.582(2)
90
C80H110Cl6N4Ni2O8S4
1714.08
monoclinic
P21/c
24.296(5)
18.854(5)
18.330(5)
90
111.067(5)
90
7835(3)
4
triclinic
¯
P1
8.5697(18)
9.0569(19)
11.767(3)
76.164(3)
87.506(3)
89.673(3)
886.0(6)
2
g [8]
volume [Š3]
Z
3471.0(2)
4
1.193
3391.2(8)
4
1.233
1643.9(4)
4
1.637
1calcd [gcmꢀ3
]
1.496
1.333
1.453
m [mmꢀ1
]
0.704
0.721
1.438
0.851[a]
crystal size [mm]
reflections collected
unique reflections (Rint
R [F>4s(F)]
0.33”0.19”0.15
21583
8188 (0.024)
0.043
0.28”0.14”0.04
23990
8084 (0.040)
0.039
0.25”0.23”0.04
7692
3955 (0.033)
0.040
0.49”0.44”0.22
14974
3895 (0.017)
0.023
0.02”0.02”0.02
37375
8188 (0.1895)
0.088
)
RwF (all F2)
0.102
0.088
0.096
0.061
0.224
GOF
1.11
0.96
1.036
1.05
0.953
[a] l=0.6868 Š.
0.6868 Š) and w scans from Daresbury SRS Station 9.8.[65,66] Empirical
absorption corrections were applied using SADABS.[67] Structures were
solved by direct methods using SHELXS-97[68] and, in the case of (2)2·
(ClO4)2·2CH2Cl2, SIR-92[69] and refined with SHELXL-97.[70] Hydrogen
atoms were geometrically placed in calculated positions and constrained
[1] a) Y. Higuchi, T. Yagi, N. Yasuoka, Structure 1997, 5, 1671–1680;
b) Y. Higuchi, H. Ogata, K. Miki, N. Yasuoka, T. Yagi, Structure
1999, 7, 549–556; c) D. J. E. Spencer, A. C. Marr, M. Schrçder,
Coord. Chem. Rev. 2001, 219–221, 1055–1074.
[2] A. Volbeda, M. H. Charon, C. Piras, E. C. Hatchikian, M. Frey, J. C.
Fontecilla-Camps, Nature 1995, 373, 580–587.
to ride on their parent atom with Uiso(H)=xUeqACTHRE(UNG parent), where x=1.2 or
1.5.
[3] V. Svetlitchnyi, H. Dobbek, W. Meyer-Klaucke, T. Meins, B. Thiele,
P. Romer, R. Huber, O. Meyer, Proc. Natl. Acad. Sci. USA 2004,
101, 446–451.
[4] C. Darnault, A. Volbeda, E. J. Kim, P. Legrand, X. Vernede, P. A.
Lindahl, J. C. Fontecilla-Camps, Nat. Struct. Biol. 2003, 10, 271–279.
[5] T. I. Doukov, T. M. Iverson, J. Seravalli, S. W. Ragsdale, C. L.
Drennan, Science 2002, 298, 567–572.
[6] C. L. Drennan, J. Y. Heo, M. D. Sintchak, E. Schreiter, P. W.
Ludden, Proc. Natl. Acad. Sci. USA 2001, 98, 11973–11978.
[7] H. Dobbek, V. Svetlitchnyi, L. Gremer, R. Huber, O. Meyer, Science
2001, 293, 1281–1285.
[8] U. Ermler, W. Grabarse, S. Shima, M. Goubeaud, R. K. Thauer, Sci-
ence 1997, 278, 1457–1462.
DFT calculations: Calculations were performed by using the Amsterdam
Density Functional (ADF) suite version 2005.01.[71] The open-shell re-
stricted scalar relativistic DFT calculations employed a Slater type orbital
(STO) triple-z-plus one polarization function all-electron basis sets from
the ZORA/TZP database of the ADF suite for all atoms. The local den-
sity approximation (LDA) with the correlation potential due to Vosko
et al.[72] was used in all of the DFT calculations. Gradient corrections
were performed using the functionals of Becke[73] and Perdew (BP).[74]
+
Models of 1, 1+, and 1(py)2 were optimized without any symmetry con-
+
straints, whereas those of 2, 2+, and 2(py)2 were optimized in idealized
C2v symmetry. The z axis is perpendicular to the NiS2N2 plane and the y
axis bisects the N(1)-Ni-N(2) angle in the starting geometries in the coor-
dinate frames employed in the calculations. Within each model, the tBu
groups of each thiolate ring were substituted by an H atom to decrease
the computational effort required for the calculations.
[9] D. P. Barondeau, C. J. Kassmann, C. K. Bruns, J. A. Tainer, E. D.
Getzoff, Biochemistry 2004, 43, 8038–8047.
[10] J. Wuerges, J-W. Lee, Y. I. Yim, H-S. Yim, S-O. Kang, K. Djinovic
Carugo, Proc. Natl. Acad. Sci. USA 2004, 101, 8569–8574.
[11] A. F. Miller, Curr. Opin. Chem. Biol. 2004, 8, 162–168.
[12] J. S. Valentine, D. L. Wertz, T. J. Lyons, L. Liou, J. J. Goto, E. B.
Gralla, Curr. Opin. Chem. Biol. 1998, 2, 253–262.
CCDC-278858, CCDC-654478, CCDC-654479, CCDC-654480, CCDC-
654481, CCDC-654482, CCDC-654483, CCDC-654484, CCDC-654485
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallograph-
ic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[13] A. T. Fiedler, T. C. Brunold, Inorg. Chem. 2007, 46, 8511–8523.
[14] A. T. Fiedler, P. A. Bryngelson, M. J. Maroney, T. C. Brunold, J. Am.
Chem. Soc. 2005, 127, 5449–5462.
[15] C. A. Grapperhaus, M. Y. Darensbourg, Acc. Chem. Res. 1998, 31,
451–459.
[16] G. J. Colpas, M. Kumar, R. O. Day, M. J. Maroney, Inorg. Chem.
1990, 29, 4779–4788.
Acknowledgements
[17] H. Krüger, R. H. Holm, Inorg. Chem. 1987, 26, 3645–3647.
[18] H. Krüger, G. Peng, R. H. Holm, Inorg. Chem. 1991, 30, 734–742.
[19] H. J. Krüger, R. H. Holm, J. Am. Chem. Soc. 1990, 112, 2955–2963.
[20] J. Shearer, N. Zhao, Inorg. Chem. 2006, 45, 9637–9639.
[21] a) A. J. Blake, R. O. Gould, T. I. Hyde, M. Schrçder, J. Chem. Soc.
Chem. Commun. 1987, 431–433; b) A. J. Blake, R. O. Gould, T. I.
Hyde, M. Schrçder, J. Chem. Soc. Chem. Commun. 1987, 1730–
1732; c) G. Reid, A. J. Blake, T. I. Hyde, M. Schrçder, J. Chem. Soc.
Chem. Commun. 1988, 1397–1399; d) A. J. Blake, A. J. Holder, T. I.
We thank the EPSRC for financial support and CONACyT and
DGAPA-UNAM for support to A. M.-B. We also thank the EPSRC Na-
tional Service for Mass Spectrometry at the University of Wales, Swansea
for mass spectra and the EPSRC X-ray Crystallography Service at the
University of Southampton and the STFC Daresbury Laboratory for
data collection from single crystals of [(2)2]ACHTRE(UGN ClO4)2·2CH2Cl2. M.S. grate-
fully acknowledges receipt of a Royal Society Wolfson Merit Award and
of a Royal Society Leverhulme Trust Senior Research Fellowship.
Chem. Eur. J. 2008, 14, 2564 – 2576
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2575

M. Schrçder, J. McMaster et al.
Hyde, M. Schrçder, J. Chem. Soc. Chem. Commun. 1987, 987–988;
e) A. J. Blake, L. M. Gordon, A. J. Holder, T. I. Hyde, G. Reid, M.
Schrçder, J. Chem. Soc. Chem. Commun. 1988, 1452–1454; f) A. J.
Blake, A. J. Holder, T. I. Hyde, M. Schrçder, J. Chem. Soc. Chem.
Commun. 1989, 1433–1434; g) A. J. Blake, A. J. Holder, T. I. Hyde,
H-J. Küppers, M. Schrçder, S. Stçtzel, K. Wieghardt, J. Chem. Soc.
Chem. Commun. 1989, 1600–1602; h) A. J. Blake, G. Reid, M.
Schrçder, J. Chem. Soc. Dalton Trans. 1990, 3363–3373.
[22] a) C. W. G. Ansell, J. Lewis, P. R. Raithby, J. N. Ramsden, M.
Schrçder, J. Chem. Soc. Chem. Commun. 1982, 546–547; b) J. Lewis,
M. Schrçder, J. Chem. Soc. Dalton Trans. 1982, 1085–1089; c) A. J.
Blake, R. O. Gould, M. A. Halcrow, A. J. Holder, T. I. Hyde, M.
Schrçder, J. Chem. Soc. Dalton Trans. 1992, 3427–3431;
d) N. D. M. J. Branscombe, A. J. Atkins, A. Marin-Becerra, E. J. L.
McInnes, F. E. Mabbs, J. McMaster, M. Schrçder, Chem. Commun.
2003, 1098–1099; e) Q. Wang, E. Barclay, A. J. Blake, E. S. Davies,
D. J. Evans, A. C. Marr, E. J. L. McInnes, J. McMaster, C. Wilson,
M. Schrçder, Chem. Eur. J. 2004, 10, 3384–3396; f) M-C. Chalbot,
A. M. Mills, A. Spek, G. J. Long, E. Bouwman, Eur. J. Inorg. Chem.
2003, 453–457; g) A. J. Amoroso, S. S. M. Chung, D. J. E. Spencer,
M. G. Glenny, J. P. Danks, A. J. Blake, P. A. Cooke, C. Wilson, M.
Schrçder, Chem. Commun. 2003, 2020–2021; h) W. Zhu, A. C.
Marr, Q. Wang, F. Neese, D. J. E. Spencer, A. J. Blake, P. A. Cooke,
C. Wilson, M. Schrçder, Proc. Natl. Acad. Sci. USA 2005, 102,
18280–18285; i) A. Perra, E. S. Davies, J. R. Hyde, W. Qiang, J.
McMaster, M. Schrçder, Chem. Commun. 2006, 1103–1105.
[23] P. A. Stenson, A. Marin-Becerra, A. J. Blake, C. Wilson, J. McMas-
ter, M. Schrçder, Chem. Commun. 2006, 317–319.
[24] E. Block, V. Eswarakrishnan, M. Gernon, G. Ofori-Okai, C. Saha,
K. Tang, J. Zubieta, J. Am. Chem. Soc. 1989, 111, 658–665.
[25] G. D. Figuly, C. K. Loop, J. C. Martin, J. Am. Chem. Soc. 1989, 111,
654–658.
[26] D. M. Giolando, K. Kirschbaum, Synthesis 1992, 451–452.
[27] U. Brand, H. Vahrenkamp, Chem. Ber. 1995, 128, 787–791.
[28] A. R. Cowley, J. R. Dilworth, P. S. Donnelly, E. Labisbal, A. Sousa,
J. Am. Chem. Soc. 2002, 124, 5270–5271.
[29] V. L. Goedken, G. G. Christoph, Inorg. Chem. 1973, 12, 2316–2320.
[30] D. J. E. Spencer, A. J. Blake, S. Parsons, M. Schrçder, J. Chem. Soc.
Dalton Trans. 1999, 1041–1042.
[31] V. V. Tkachev, E. A. Kordrashkina, L. O. Atovmyan, J. Struct.
Chem. 1977, 18, 833–841.
[45] D. Sellmann, R. Prakash, F. W. Heinemann, Eur. J. Inorg. Chem.
2004, 10, 1847–1858.
[46] G. Steinfeld, B. Kersting, Chem. Commun. 2000, 205–206.
[47] O. Rotthaus, F. Thomas, O. Jarjayes, C. Philouze, E. Saint-Aman,
J. L. Pierre, Chem. Eur. J. 2006, 12, 6953–6962.
[48] D. Sellmann, H. Binder, D. Häußinger, F. W. Heinemann, J. Sutter,
Inorg. Chim. Acta 2000, 300–302, 829–836.
[49] F. E. Mabbs, D. Collinson, Electron Paramagnetic Resonance of d-
Transition Metal Compounds, Elsevier, Amsterdam, 1992.
[50] B. R. McGarvey, Can. J. Chem. 1975, 53, 2498–2511.
[51] Y. Nishida, S. Kida, Bull. Chem. Soc. Jpn. 1978, 51, 143–149.
[52] N. Takvoryan, K. Farmery, V. Katovic, F. V. Lovecchio, E. S. Gore,
L. B. Anderson, D. H. Busch, J. Am. Chem. Soc. 1974, 96, 731–742.
[53] J. C. Slater, J. Chem. Phys. 1964, 41, 3199–3204.
[54] a) S. Kimura, E. Bill, E. Bothe, T. Weyhermüller, K. Wieghardt, J.
Am. Chem. Soc. 2001, 123, 6025–6039; b) T. Glaser, T. Beissel, E.
Bill, T. Weyhermüller, V. Schunemann, W. Meyer-Klauke, A. X.
Trautwein, K. Wieghardt, J. Am. Chem. Soc. 1999, 121, 2193–2208;
c) T. Glaser, T. Beissel, E. Bill, T. Weyhermüller, W. Meyer-Klauke,
K. Wieghardt, Inorg. Chem. 1999, 38, 722–732; d) G. Steinfeld, B.
Kersting, Chem. Commun. 2000, 3, 205–206.
[55] L. Benisvy, E. Bill, A. J. Blake, D. Collison, E. S. Davies, C. D.
Garner, C. I. Guindy, E. J. L. McInnes, G. McArdle, J. McMaster, C.
Wilson, J. Wolowska, Dalton Trans. 2004, 3647–3653.
[56] L. Benisvy, E. Bill, A. J. Blake, D. Collison, E. S. Davies, C. D.
Garner, G. McArdle, E. J. L. McInnes, J. McMaster, S. H. K. Ross,
C. Wilson, Dalton Trans. 2006, 258–267.
[57] L. Benisvy, A. J. Blake, D. Collison, E. S. Davies, C. D. Garner,
E. J. L. McInnes, J. McMaster, G. Whittaker, C. Wilson, Chem.
Commun. 2001, 1824–1825.
[58] L. Benisvy, A. J. Blake, D. Collison, E. S. Davies, C. D. Garner,
E. J. L. McInnes, J. McMaster, G. Whittaker, C. Wilson, Dalton
Trans. 2003, 1975–1985.
[59] J. K. Howie, J. J. Houts, D. T. Sawyer, J. Am. Chem. Soc. 1977, 99,
6323–6326.
[60] C. S. Mullins, C. A. Grapperhaus, P. M. Kozlowski, J. Biol. Inorg.
Chem. 2006, 11, 617–625.
[61] R. Edler, J. Voss, Chem. Ber. 1989, 122, 187–191.
[62] C. M. D. Komen, F. Bickelhaupt, Synth. Commun. 1996, 26, 1693–
1697.
[63] P. C. Myhre, T. Reiger, J. T. Stone, J. Org. Chem. 1966, 31, 3425–
3426.
[64] J. Cosier, A. M. Glazer, J. Appl. Crystallogr. 1986, 19, 105–107.
[65] R. J. Cernik, W. Clegg, C. R. A. Catlow, G. Bushnell-Wye, J. V. Flah-
erty, G. N. Greaves, M. Hamichi, I. Burrows, D. J. Taylor, S. J. Teat,
J. Synchrotron Radiat. 1997, 4, 279–286.
[66] W. Clegg, J. Chem. Soc. Dalton Trans. 2000, 3223–3232.
[67] G. M. Sheldrick, SADABS, Program for Correcting Area Detector
Data, Bruker AXS Inc., Madison, Wisconsin, USA, 2001.
[68] G. M. Sheldrick, SHELXS-97, Program for Crystal Structure Solu-
tion, University of Gçttingen, Gçttingen, Germany, 1997.
[69] A. Altomare, G. Cascaravo, C. Giacovazzo, A. Gualardi, J. Appl.
Crystallogr. 1993, 26, 343–350.
[32] T. Tuntulani, J. H. Reibenspies, P. J. Farmer, M. Y. Darensbourg,
Inorg. Chem. 1992, 31, 3497–3499.
[33] M. F. Corrigan, B. O. West, Aust. J. Chem. 1976, 29, 1413–1427.
[34] N. Goswami, D. M. Eichhorn, Inorg. Chem. 1999, 38, 4329–4333.
[35] N. Goswami, D. M. Eichhorn, Inorg. Chim. Acta 2000, 303, 271–276.
[36] T. Yamamura, M. Tadokoro, R. Kuroda, Chem. Lett. 1989, 1245–
1246.
[37] S. Fox, Y. Wang, A. Silver, M. J. Millar, J. Am. Chem. Soc. 1990,
112, 3218–3220.
[38] W. J. J. Smeets, A. Spek, R. K. Henderson, E. Bouwman, J. Reedijk,
Acta Crystallogr. Sect. C 1997, 53, 1564–1566.
[39] F. H. Allen, J. Acta. Crystallogr. Sect. B. 2002, 58, 380–388.
[40] A. J. Atkins, D. Black, A. J. Blake, A. Marin-Becerra, S. Parsons, L.
Ruiz-Ramirez, M. Schrçder, Chem. Commun. 1996, 457–464.
[41] A. J. Atkins, A. J. Blake, M. Schrçder, J. Chem. Soc. Chem.
Commun. 1993, 1662–1665.
[42] N. D. J. Branscombe, A. J. Blake, A. Marin-Becerra, W .S. Li, S. Par-
sons, L. Ruiz-Ramirez, M. Schrçder, Chem. Commun. 1996, 2573–
2574.
[70] G. M. Sheldrick, SHELXL-97, Program for Crystal Structure Re-
finement, University of Gçttingen, Gçttingen, Germany, 1997.
[71] SCM, Vrije Universiteit Amsterdam; http://www.scm.com, Amster-
dam, The Netherlands; C. F. Guerra, J. G. Snijders, G. te Velde, E. J.
Baerends, Theor. Chem. Acc. 1998, 99, 391–403; G. te Velde, F. M.
Bickelhaupt, S. J. A. van Gisbergen, G. C. Fonseca, E. J. Baerends,
J. G. Snijders, T. Ziegler, J. Comput. Chem. 2001, 22, 931–967.
[72] S. H. Vosko, L. Wilk, M. Nusair, Can. J. Phys. 1980, 58, 1200–1211.
[73] A. D. Becke, Phys. Rev. A 1988, 38, 3098–3100.
[43] J. Hanss, H. J. Krüger, Angew. Chem. 1998, 110, 366–369; Angew.
Chem. Int. Ed. 1998, 37, 360–363.
[74] J. P. Perdew, Phys. Rev. B 1986, 33, 8822–8824.
[44] D. Sellmann, F. Geipel, F. Lauderbach, F. W. Heinemann, Angew.
Chem. 2002, 114, 654–656; Angew. Chem. Int. Ed. 2002, 41, 632–
634; .
Received: July 18, 2007
Published online: January 21, 2008
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