Three-coordinate nickel(II) and nickel(I) thiolate complexes based on the β-diketiminate ligand system

Article abstract of DOI:10.1021/ic500698v

Mononuclear nickel(II) thiolate complexes [L^tBuNi(SEt)] (1) and [L^tBuNi(aet)] (2, aet = ^-S(CH₂) ₂NH₂) (L^tBu = [HC(C(^tBu)NC ₆H₃(ⁱPr)₂)₂] ^-), supported by a bulky nacnac ligand, were synthesized by treatment of the nickel(II) bromide precursor [L^tBuNi(Br)] (I) with the potassium salts of ethanethiol and cysteamine, respectively. The nickel atom in 1 features a planar T-shaped environment, while the Ni ion within 2 shows a distorted square planar coordination geometry, as the aminoethanethiolate (aet) is coordinated as a chelating ligand. In 2 the β-diketiminate ligand binds in a rarely observed κ²C,N coordination mode. Reduction of complex 1 or its benzenethiolate analogue [L^tBuNi(SPh)] (II) by KC₈ resulted in the formation of dinuclear Ni^I thiolates (K·OEt₂)(K)[L^tBuNi(SEt)]₂ (3) and (K·OEt₂)₂[L^tBuNi(SPh)]₂ (4), respectively. In these compounds [L^tBuNi(SR)]^- units are held together by potassium cations produced in the reduction process. All compounds mentioned were structurally characterized by single-crystal X-ray crystallography.

Full text of DOI:10.1021/ic500698v

Article
pubs.acs.org/IC
Three-Coordinate Nickel(II) and Nickel(I) Thiolate Complexes Based
on the β‑Diketiminate Ligand System
Bettina Horn, Christian Limberg,* Christian Herwig, and Beatrice Braun
Humboldt-Universitat zu Berlin, Institut fur Chemie, Brook-Taylor-Str. 2, 12489 Berlin, Germany
̈
̈
S
* Supporting Information
ABSTRACT: Mononuclear nickel(II) thiolate complexes [LtBuNi(SEt)] (1)
and [LtBuNi(aet)] (2, aet = −S(CH2)2NH2) (LtBu = [HC(C(tBu)-
NC6H3(iPr)2)2]−), supported by a bulky nacnac ligand, were synthesized
by treatment of the nickel(II) bromide precursor [LtBuNi(Br)] (I) with the
potassium salts of ethanethiol and cysteamine, respectively. The nickel atom in
1 features a planar T-shaped environment, while the Ni ion within 2 shows a
distorted square planar coordination geometry, as the aminoethanethiolate
(aet) is coordinated as a chelating ligand. In 2 the β-diketiminate ligand
binds in a rarely observed κ2C,N coordination mode. Reduction of complex 1
or its benzenethiolate analogue [LtBuNi(SPh)] (II) by KC8 resulted in the
formation of dinuclear NiI thiolates (K·OEt2)(K)[LtBuNi(SEt)]2 (3) and
(K·OEt2)2[LtBuNi(SPh)]2 (4), respectively. In these compounds [LtBuNi-
(SR)]− units are held together by potassium cations produced in the reduction
process. All compounds mentioned were structurally characterized by single-
crystal X-ray crystallography.
INTRODUCTION
■
The functions of enzymes, such as the [NiFe] carbon monoxide
dehydrogenases (CODHs) that catalyze the interconversion
of CO and CO2 or the acetyl coenzyme A synthase (ACS),
mediating the conversion of CO and coenzyme A to acetyl
coenzyme A, are based on nickel centers ligated by cysteinate
ligands.1 As the ACS needs CO as the substrate, it occurs
combined with a CODH subunit, which is called C-cluster,
while the active site of the ACS is denoted as the A-cluster. Both
are depicted in Figure 1. The C-cluster of well-characterized
Moorella thermoacetica CODH/ACSMt features a NiII ion, which
is bound by three S ligands in a planar T-shaped coordination
mode, and a “unique iron atom” (Feu) in close proximity;2 in
the first coordination sphere Feu binds a histidine, a cysteine,
and a μ3-sulfide ligand plus a fourth light atom, supposed to
be a H2O/OH− ligand, which is also near to the Ni ion.3−7
The active state of the ACS contains a cysteine-ligated nickel
ion (proximal to a Fe4S4 cluster), which according to the “para-
magnetic proposal” is in the oxidation state +I prior to substrate
binding.8
The coordination environments of the Ni centers within
the C-cluster (CODH) and the ACS active site (A-cluster) have
stimulated attempts to prepare synthetic structural model com-
pounds for these metalloproteins, employing thiolates to mimic
cysteinates,7,9 and it is of particular interest to synthesize and
investigate low-coordinate model compounds with reduced
nickel centers exhibiting sulfur-based donors in its coordination
spheres. In this work, we present the syntheses of three-coordinate
nickel(II) complexes featuring a bidentate β-diketiminate ligand
Figure 1. Ball-and-stick drawing of the C-cluster (top, CODH/
ACSMt, PDB code: 3I01) and the A-cluster (bottom, ACSCh, PDB code:
1RU3).
(“nacnac”) system and one thiolate ligand and the investigation of
their reduction.
Received: March 25, 2014
© XXXX American Chemical Society
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[96.24(9)°], N1−Ni1−S1 115.87(7)° [116.09(7)°], N2−
Ni1−S1 148.03(7)° [147.65(7)°]; sum of angles 359.96(14)°
[359.98(13)°]). The Ni−S distance of 2.1506(8) Å [2.1512(8) Å]
is comparable to the value reported for the benzenethiolate
analogue II (Table 1).
RESULTS AND DISCUSSION
■
Mononuclear Three-Coordinate Nickel(II) Thiolate
Complexes. As the NiII ion in [LtBuNi(Br)] (I) is already
three-coordinated,10,11 I appeared an ideal starting material for
the synthesis of a variety of low-coordinate Ni complexes (LtBu
=
[HC(C(tBu)NC6H3(iPr)2)2]−). Recently, we communicated the
synthesis of a nickel thiolate [LtBuNi(SPh)] (II) with a NiII ion
ligated by the N atoms of a bidentate β-diketiminate (“nacnac”)
ligand system and one S atom of a benzenethiolate anion in
a T-shape coordination mode by a salt metathesis reaction
involving I and 1 equiv of KSPh in tetrahydrofuran (THF).12,13
Following the same procedure, but utilizing KSEt as the thiolate
source, [LtBuNi(SEt)] (1, Scheme 1) could be accessed. Complex
1 was isolated as a deep purple solid in 88% yield.
Table 1. Selected Bond Lengths and Angles for the Nickel(II)
Thiolate Complexes II and 1
a
bond (Å)/angle (deg)
II12
1
Ni1−N1
Ni1−N2
Ni1−S1
N2−Ni1−N1
N1−Ni1−S1
N2−Ni1−S1
1.9288(18)
1.8658(19)
2.1728(7)
96.37(7)
1.921(2) [1.921(2)]
1.866(2) [1.876(2)]
2.1506(8) [2.1512(8)]
96.06(10) [96.24(9)]
115.87(7) [116.09(7)]
148.03(7) [147.65(7)]
110.39(6)
153.08(6)
a
The values in square brackets correspond to the second independent
molecule of 1.
Scheme 1. Synthesis of the Nickel(II) Thiolate Compounds
II,12 1, and 2
Solutions of complex 1 were found to be paramagnetic,
exhibiting a solution magnetic moment of μeff = 2.67 μB at room
temperature (r.t.) (Evans’ method,14 C6D6), which is consistent
with the expectations for a high-spin NiII ion (S = 1, μs.o.
=
2.83 μB). Accordingly, the 1H NMR spectrum of 1 dissolved in
C6D6 shows paramagnetically shifted signals corresponding
to the protons of the nacnac ligand over a range from 50 to
−200 ppm.15 The electronic spectra of diethyl ether solutions
of II and 1 are dominated by intense absorptions between 250
and 400 nm (II: εmax = 13 mM−1 cm−1, 1: εmax = 14 mM−1 cm−1).
Additionally, II and 1 exhibit two broad absorptions between
500 and 900 nm (εmax = 2 mM−1 cm−1), respectively. Density
functional calculations (B3LYP Def2-SVP/TZVPD, see
Supporting Information) confirm a triplet ground state with
two unpaired electrons at the Ni atom (with T-shaped planar
coordination) for both II and 1.
Cyclic Voltammetry. The redox properties of the nickel(II)
thiolates II and 1 were probed by cyclic voltammetry (CV) in
THF with 0.1 M NBu4PF6 as electrolyte. The voltammograms of
II and 1 (Figure 3) exhibit a reversible reduction wave at a low
Single crystals suitable for X-ray diffraction analysis were
obtained by cooling a hexane solution of 1 to −30 °C. They
contain two independent molecules of [LtBuNi(SEt)] (1) in the
asymmetric unit, which differ in the conformation of the thiolate
ligand. The molecular structure of one of these molecules is
displayed in Figure 2. In the solid-state structure of 1 the
coordination sphere of the three-coordinated NiII center can be
described best as T-shaped planar (N2−Ni1−N1 96.06(10)°
Figure 3. Cyclic voltammograms recorded for THF solutions
containing II or 1 (1 mM) and NBu4PF6 (0.1 M). Full scan of II
(····) and 1 () at 50 mV/s.
potential of −1.40 V (ΔEp = 76 mV, scan rate 50 mV/s) and
−1.60 V (ΔEp = 81 mV, scan rate 50 mV/s) versus Fc/Fc+,
respectively, attributable to a NiII/NiI redox couple. Additionally,
an irreversible oxidation can be observed at a higher potential of
Figure 2. Molecular structure of one of two independent molecules of
[LtBuNi(SEt)] (1) in the asymmetric unit. Thermal ellipsoids are shown
at 50% probability. All hydrogen atoms are omitted for clarity. Selected
bond lengths and angles are summarized in Table 1.
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0.24 V for 1, which becomes almost quasi-reversible at increasing
scan rates (ΔEp = 126 mV, scan rate 3158 mV/s, Figure S1, see
Supporting Information). By contrast II shows quasi-reversible
oxidation at a potential of 0.24 V (ΔEp = 86 mV, scan rate
50 mV/s) even at low scan rates. The events observed at positive
potential can be attributed to either a NiII/NiIII couple or a
ligand-based one-electron oxidation of the sulfur ligand leading
to a thiyl radical. In the case of II the positive charge might get
stabilized by the aromatic residue of the SPh ligand.
Reaction of Nickel(II) Thiolate Complexes II and 1 with
CO. Although nickel(II) carbonyl units in sulfur-rich environ-
ments have been identified in several nickel-containing enzymes,
such as CO dehydrogenases, synthetic model compounds are
rather rare.16 Therefore, hexane solutions of the thiolate
complexes II and 1 were exposed to an atmosphere of CO and
stirred for 6 h at r.t. However, after removal of the solvent mainly
the corresponding thiolate compounds II and 1, respectively,
were isolated unchanged as evidenced by 1H NMR spectroscopy.
Additionally in both attempts the formation of traces of a NiI CO
complex [LtBuNi(CO)]17,18 was proved by IR spectroscopy
through its characteristic absorption band (∼2015 cm−1) cor-
responding to the C−O stretching mode. Further evidence came
from electron paramagnetic resonance (EPR) spectroscopy:
solutions of the thiolates after CO treatment exhibited the
characteristic rhombic signal caused by [LtBuNi(CO)]18 (Figure S2,
see Supporting Information). The thiolate ligands probably get
oxidized in the presence of added CO, whereas the nickel(II)
centers are reduced to nickel(I) yielding [LtBuNi(CO)] through
CO/RS· ligand substitution.19 Interestingly, no reaction could be
detected after treatment of [LtBuNi(Br)] (I) with CO for 1 d.
A Nickel(II) Aminoethanethiolate Complex. Beside
the more basic thiolates used in the syntheses of II and 1, the
introduction of ligands that can mimic the electronic and
structural character of cysteinate more closely was also attempted
to further approach the situation of the Ni center in the active
site of CODH. With this aim we tried to deprotonate L-cysteine
ethyl ester hydrochloride by different bases followed by treat-
ment with 1 equiv of [LtBuNi(Br)] (I). However, these reac-
tions either yielded in decomposition products or the starting
material was isolated back. By contrast, reaction of I with the
potassium salt of cysteamine (see Scheme 1) in THF as the
solvent led to the isolation of a red solid, after workup. After
redissolution of the red residue in diethyl ether, single crystals
suitable for X-ray diffraction were grown by fast evaporation of
the solvent. The molecular structure of the reaction product
[LtBuNi(aet)] (2, aet = 2-aminoethanethiolate) is shown in
Figure 4.
Figure 4. Molecular structure of [LtBuNi(aet)] (2). Thermal ellipsoids
are shown at 50% probability. All hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Ni1−N1 1.9593(14),
Ni1−N2 1.8947(14), Ni1−C4 2.0097(16), Ni1−S1 2.1603(5), N2−C3
1.293(2), C3−C4 1.502(2), C4−C5 1.492(2), C5−N3 1.279(2), N2−
Ni1−N1 98.52(6), N2−Ni1−C4 69.86(6), N1−Ni1−S1 88.81(4),
C4−Ni1−S1 103.31(5).
ligand at a metal center as in 2 has been found so far only in
a small number of compounds including [LMePd(acac)]20 and
[LMeGeCl3]21 (L = [HC(C(Me)NC6H3(iPr)2)2]−). Although 2
is somewhat unstable in solutionan insoluble precipitate is
formed upon storage22it could be characterized by 1H NMR
spectroscopy. The spectrum (C6D6, r.t.) shows that 2 is
diamagnetic and reveals resonances at δ(H) 7.28 and 2.34 ppm,
which correspond to the two methylene groups of the thiolate
(S(CH2)2NH2), while the amine group gives rise to a signal at
δ(H) −3.09 ppm. It is noteworthy that in solution the protons
of the two imine-based parts of the nacnac ligand (C(tBu)
N−C6H3(iPr)2) seem to be equivalent, as they result in only one
set of signals. This observation suggests a fast exchange of the
coordinated and noncoordinated imine functions. A signal for
the proton bound to the central C4 atom of the nacnac backbone
could not be detected at r.t. perhaps due to masking. Upon
cooling a THF-d8 solution of 2 (in a variable-temperature NMR
experiment), a number of signals experience significant shifts
(Figure S3, see Supporting Information), and at −40 °C the
resonance for the central CH unit became visible at δ(H) 3.9 ppm,
shifting further to δ(H) 4.8 ppm at −80 °C. The shifting of signals
with temperature may be due to a population/depopulation of an
excited triplet state23 of 2 or due to a coexisting isomer with a
significantly different structure in solution.
Reaction of 2 with O2. Complex 2 exhibits a pronounced
sensitivity toward O2, which thus was inspected more closely. The
UV−vis spectrum of 2 at r.t. dissolved in diethyl ether exhibits two
intense absorption features at 299 nm (ε = 16 mM−1 cm−1) and
331 nm (ε = 12 mM−1 cm−1) and one additional broad absorption
band at 414 nm (ε = 4 mM−1 cm−1). In the course of the reaction
with O2 these bands decrease (Figure S4, see Supporting
Information). The fact that no new band evolved may indicate
the cleavage of the Ni−S bond24 or even the total decomposition
of complex 2. The oxygenated solution was further analyzed
by mass spectrometry, and the results hinted inter alia to the
formation of a monooxygenated product, which was supported
by isotope labeling using 18O2 (Figure S5, see Supporting
Information). These results suggest an oxygenation of the sulfur
donor moiety yielding in a sulfenate complex. To further test this
assumption the product was investigated by IR spectroscopy.
However, characteristic SO stretching vibration bands (in the
region of 900−1200 cm−1)25 for oxygenated species of 2
The NiII ion in 2 is coordinated in a distorted square planar
fashion by bidentate N,S-aminoethanethiolate and nacnac
ligands. The binding of the latter at the Ni center occurs not in
the usual fashion via the two N donor functions but through
interactions with the central carbon atom (C4) and one nitrogen
atom (N2). The N2−C3 and N3−C5 bonds of 1.293(2) and
1.279(2) Å are slightly shorter than those found in other
β-diketiminate nickel(II) compounds (IIØ: 1.34 Å, 1Ø: 1.33 Å),
whereas the C3−C4 and C4−C5 bond lengths of 1.502(2) and
1.492(2) Å are significantly longer than usual (IIØ: 1.40 Å, 1Ø:
1.40 Å), indicating a diimine arrangement of the ligand. Hence,
the negative charge of the ligand appears to be no longer
delocalized over the ligand backbone but localized at the C4
atom, which thus binds as a carbanion. Both the Ni1−N2 and
Ni1−S1 distances of 1.8947(14) and 2.1603(5) Å are similar to
those found in II and 1. A κ2C,N coordination mode of a nacnac
1
could not be detected, and the results of additional H NMR
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spectroscopic measurements pointed to the formation of a
product mixture.
Dinuclear Nickel(I) Thiolate Complexes. Structurally
characterized synthetic nickel(I) thiolate complexes are ex-
tremely scarce. We recently reported an example where an
expanded β-diketiminate ligand system with two binding pockets
was utilized as a ligand for a NiI−(μ-SR) −NiI unit with strongly
antiferromagnetically coupled NiI ions.26 Tatsumi et al. described
the syntheses of [(PPh3)Ni(SDmp)] (Dmp = 2,6-dimesitylphenyl)
and [(PPh3)Ni(μ-SR)]2 (R = 2,4,6-triisopropylphenyl or
1-adamantyl) via reactions of a NiI amide precursor with the
corresponding thiols.16c A similar dinuclear NiI complex
[(PiPr3)Ni(μ-SPh)]2 with bridging thiolate ligands could be
isolated by Johnson and co-workers after slow dimerization
of [(PiPr3)2Ni(SPh)(H)] accompanied by loss of H2 and two
phosphine ligands; an alternative synthetic route utilized a salt
metathesis reaction of PhSLi with [(PiPr3)2Ni(Cl)].27 Recently,
Duboc et al. described the synthesis of a mononuclear NiI diamine
aliphatic dithiolate complex through bulk electrolyses, which was
claimed as the first example of a representative with a purely
aliphatic thiolate N2S2 environment.28
This background and the fact that the CV measurements on
complexes II and 1 indicated that they can be reduced under
electrochemical conditions and remain stable after reduction
(→ reversibility) stimulated attempts to achieve reduction
chemically. Indeed, [LtBuNi(SEt)] (1) reacts with 1 equiv of
potassium graphite yielding a nickel(I) product.29 The pro-
duct was crystallized from diethyl ether at −30 °C, and a single-
crystal X-ray analysis identified it as the dinuclear compound
(K·OEt2)(K)[LtBuNi(SEt)]2 (3, Scheme 2).
Figure 5. Molecular structure of (K·OEt2)(K)[LtBuNi(SEt)]2 (3).
Thermal ellipsoids are shown at 50% probability. All hydrogen atoms are
omitted for clarity. Selected bond lengths and angles are summarized in
Table 2.
Table 2. Selected Bond Lengths and Angles for the Nickel(I)
Thiolate Complexes 3 and 4
bond (Å)/angle (deg)
3
4
Ni1−N1
Ni1−N2
Ni1−S1
K1···S1
K1···S1′
K1···O1
N1−Ni1−N2
N1−Ni1−S1
N2−Ni1−S1
1.9156(15)
1.9109(16)
2.1986(6)
1.9048(9)
1.9243(9)
2.2471(3)
3.3428(4)
3.1676(4)
2.7696(9)
98.04(4)
b
b
3.0683(7) [3.1023(7)]
3.0684(7) [3.1023(7)]
2.613(2)
97.47(6)
129.56(5)
144.49(3)
117.24(3)
132.85(8)
b
The values in square brackets correspond to the K2···S1 and K2···S1′
distances, respectively.
Scheme 2. Synthesis of 3 and 4 by Reaction of [LtBuNi(SR)]
(Path A: R = Et (1), Path B: R = Ph (II)) with KC8 in Hexane,
Followed by Workup with Diethyl Ether
second potassium ion (K1) is associated with one solvent
molecule (O1···K1 2.613(2) Å). The trigonal planar coordina-
tion around the nickel center found for the mononuclear
precursor 1 is maintained in 3 (N−Ni−N 97.47(6)°, N−Ni−S
129.56(5)° and 132.85(8)°, sum of angles 359.88(11)°).
However, in contrast to the distorted T-shape ligand environ-
ment of the NiII ion in 1, the solid-state structure of 3 contains
the Ni ions in Y-shaped coordination geometries, which is
reflected by two similar N−Ni−S angles per Ni center. The Ni1−
S1 bond length of 2.1986(6) Å is marginally longer (1: 2.1506(8)
[2.1512(8)] Å), whereas the Ni−N distances of 1.9156(15) and
1.9109(16) Å are comparable to the corresponding values found
in the nickel(II) thiolate complex 1 (Table 1).
To determine whether the Y-geometry arises from electronic
or steric effects, we investigated the structure of one of the
[LtBuNi(SEt)]− moieties found in 3 using density functional
theory (DFT) (B3LYP Def2-SVP/TZVPD). Geometry opti-
mization of a [LtBuNiI(SEt)]− fragment cut out of the structure
of 3 as determined by X-ray analysis gives a doublet ground
state with Y coordination. The optimized geometry shows fair
agreement with the metric data of the [LtBuNi(SEt)]− fragments
in 3, and these results therefore suggest that electronic reasons
are responsible for the preference of a Y-shaped geometry in the
NiI complex.
In 3 two crystallographically equivalent [LtBuNiI(SEt)]−
moieties are held together (Ni1···Ni1′ 7.8692(6) Å) by two K+
ions (Figure 5) produced in the reduction process, and these
cations interact with the S atoms of the thiolate ligand (S···K
3.0683(7)−3.1023(7) Å). In addition, K2 interacts with the aryl
rings of the diketiminate (Ar···K2 3.0033(4) Å), whereas the
Reduction of [LtBuNi(SEt)] (1) by one electron suggests an
oxidation state of +I for the Ni ions in the dimeric product 3, and
the spin-only value for two uncoupled NiI ions (S1 = S2 = 1/2)
amounts to 2.45 μB. Magnetic measurements at r.t. showed a
slightly higher value of μeff ≈ 3.0 μB in solution as well as in the
solid state for 3. Additional evidence for the existence of NiI ions
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4 follows the same trend as observed for the mononuclear
nickel(II) compounds 1 and II.
Obviously, due to both the more bulky thiolate ligands and the
altered interactions of the K+ ions the Ni···Ni distance is much
larger in the solid-state structure of 4 (9.0041(4) Å) than it is in 3.
Notably, the Ni ions in 4 are in a coordination environment
that can be described best as T-shaped (N−Ni−N 98.04(4)°,
N−Ni−S 144.49(3)° and 117.24(3)°), which is in conflict with
the Y-shaped geometry as found in the molecular structure of 3
and theoretically for a [LtBuNi(SEt)]− model (see above). In fact,
DFT predicts a Y-shape also for [LtBuNi(SPh)]−, so that the
T-shape found will have its origin in restrictions caused by
aggregation via the K+ ions.
Interestingly, the solubilities found for 3 and 4 are
complementary: while 3 is only sparingly soluble in hexane but
dissolves well in Et2O, 4 behaves the other way round. Magnetic
measurements at r.t. for 4 found μeff = 2.79 μB (Evans’ method,14
C6D6, μs.o. = 2.45 μB). The EPR spectrum recorded for a diethyl
ether solution of 4 at 77 K (Figure S6, see Supporting
Information) exhibited a rhombic signal (gx = 2.463, gy =
2.108, gz = 2.068)31 that is similar to the one recorded for the
Y-coordinated NiI complex 3, which suggests a similar Y-shaped
structure in solution.
Figure 6. EPR spectrum of 3 dissolved in diethyl ether at 77 K. Black
line: experimental spectrum with MgO/Cr3+ (*) as an internal standard,
red line: simulation. The g values were determined to be gx = 2.461, gy =
2.110, and gz = 2.068 (spectrometer frequency: 9.22 GHz).
in 3 was provided by an EPR spectrum recorded for a diethyl
ether solution at 77 K (Figure 6), which exhibited a rhombic
signal (gx = 2.461, gy = 2.110, gz = 2.068) that is typical for nacnac
NiI complexes (S = 1/2).11,30
The results of analogous reduction studies with II showed that
the steric demand of the thiolate ligand does not affect the
formation of a NiI dimer: treatment of II with 1 equiv of KC8 in
hexane and storage of a diethyl ether solution of the product
at r.t. afforded red crystals of (K·OEt2)2[LtBuNi(SPh)]2 (4)
(Scheme 2), as confirmed by single-crystal X-ray diffraction
analysis (Figure 7).
Complexes 3 and 4 represent novel examples of the still small
family of structurally characterized nickel(I) thiolate complexes:
to our knowledge these are the first dinuclear representatives,
where the thiolates do not function as bridging ligands.
Treatment of 3 and 4 with CO. Exposure of a hexane
suspension of 3 to an atmosphere of CO at r.t. immediately pro-
duced the mononuclear NiI carbonyl complex [LtBuNi(CO)]17 as
1
the major product (as evidenced by the H NMR and IR
spectrum of the crude product), accompanied by the formation
of KSEt. The stability of the latter allows for the exchange of
the thiolate ligands by CO. Upon treatment of a hexane solution
of the benzenethiolate derivative 4 it reacts with CO in an
analogous manner to give [LtBuNi(CO)] and KSPh, as proved by
IR and NMR spectroscopy.
CONCLUSION
■
Inspired by the 3-fold S-ligated nickel centers found in
metalloenzymes such as the CODH or ACS, the introduction
of thiolates into the coordination spheres of [LtBuNi]+ moieties
was investigated. Complex [LtBuNi(SEt)] (1) could be accessed
displaying a three-coordinate Ni center with a T-shaped
environment. In an attempt to approach the electronic situation
provided by a cysteinate ligand more closely aminoethanethio-
late was employed as a ligand, too, which led to a nickel(II)
complex with a distorted square planar ligand coordination
geometry composed of a chelating NH2(CH2)2S− unit and
a diketiminate ligand in an unusual κ2C,N coordination mode.
The thiolate compounds did not exhibit a pronounced reactivity
toward CO even though [LtBuNi(CO)] was found in traces.
Reduction of 1 and its benzenethiolate analogue by KC8 led to
rare examples of structurally characterized nickel(I) thiolate
complexes, which may serve as models for intermediates in the
catalytic cycle of the ACS.
Figure 7. Molecular structure of (K·OEt2)2[LtBuNi(SPh)]2 (4).
Thermal ellipsoids are shown at 50% probability. All hydrogen atoms
are omitted for clarity. Selected bond lengths and angles are summarized
in Table 2.
As with 3, complex 4 is composed of two crystallographically
equivalent [LtBuNi(SPh)]− moieties held together by K+ ions.
However, while in 3 linkage by the K+ cations occurs through
electrostatic interactions involving nacnac aryl rings and the
thiolate S atoms, in 4 the phenyl residues of the thiolate
ligands rather than their S atoms (showing comparatively large
distances to the K+ ions, see Table 2), are involved: each K+ ion
interacts with one aryl ring of the nacnac ligand system (Arnacnac···
K 3.0777(3) Å) and one phenyl ring of the benzenethiolate
(ArSPh···K 2.9825(3) Å) as well as with one Et2O molecule
(O1···K1 2.7696(9) Å). The Ni1−S1 bond length of 2.2471(3)
Å is longer compared to the corresponding bonds found in the
nickel(II) thiolate complexes (1 and II; 2.15−2.17 Å; Table 1).
The increase of the Ni−S distance by ∼0.05 Å on going from
ethanethiolate in nickel(I) complex 3 to benzenethiolate in
EXPERIMENTAL SECTION
■
General Procedures. All manipulations were carried out in a
glovebox or else by means of Schlenk-type techniques involving the use
of a dry argon atmosphere. The 1H and 13C NMR spectra were recorded
on a Bruker DPX 300 NMR spectrometer at r.t. (1H: 300.1 MHz, 13C:
75.5 MHz). 1H and 13C chemical shifts are reported in ppm and
E
dx.doi.org/10.1021/ic500698v | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
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were calibrated internally to the solvent signals (C6D6: δH 7.16 and δC
128.06 ppm, deuterated dimethyl sulfoxide (DMSO-d6): δH 2.50 ppm).
Solution magnetic susceptibilities were determined by the Evans’
method14 at r.t. with a Bruker DPX 300 (300.1 MHz) or AV 400
(400.1 MHz) NMR spectrometer. The samples were measured in C6D6
with 1% tetramethylsilane (TMS), together with a capillary tube that
contained C6D6 with 1% TMS as an internal standard. The magnetic
moment of the solid sample was determined by using a magnetic
balance (Alfa) at r.t. Ultraviolet−visible (UV−vis) spectra were
obtained on an Agilent 8453 UV−visible spectrophotometer using
quartz cuvettes. Mass spectra (electrospray ionization) were recorded
on an Agilent Technologies 6210 Time-of-Flight LC-MS instrument.
Infrared (IR) spectra were recorded using solid samples prepared as
KBr pellets with a Shimadzu FTIR-8400S-spectrometer. Microanalyses
were performed on a HEKAtech Euro EA 3000 analyzer. EPR
spectra were recorded at the X-band spectrometer ERS 300 (ZWG/
Magnettech, Berlin/Adlershof) equipped with a fused quartz Dewar
for measurements at liquid nitrogen temperature. The g factors
were calculated with respect to a Cr3+/MgO reference (g = 1.9796).
CV was performed with a GAMRY Reference 600 potentiostat. The
3-electrode setup consisted of a 3 mm glassy carbon disc working,
Radiometer M241Pt counter, and Pt wire pseudoreference elec-
trodes. Electrochemical measurements were carried out at r.t. and
under an argon atmosphere. At the end of each experiment a small
amount of ferrocene was added to the respective solution, and all data
were then referenced against the Fc/Fc+ redox couple as an internal
standard. Cyclic voltammograms were collected at scan rates of
50−3158 mV/s.
spectrometry (ESI-MS) (250 V, pos): m/z = 636.3879 ([LtBuNi-
(aet)+H]+, calcd: 636.3861), 658.3695 ([LtBuNi(aet)+Na]+, calcd:
̃
658.3681); IR (KBr): υ = 3312 (m), 3265 (w), 3249 (w), 3056 (m),
3050 (m), 2962 (vs), 2928 (s), 2903 (s), 2868 (s), 2835 (m), 1612 (s),
1603 (s), 1586 (s), 1512 (w), 1477 (s), 1461 (s), 1441 (m), 1429 (m),
1408 (s), 1396 (s), 1383 (m), 1364 (m), 1324 (s), 1252 (m), 1214 (m),
1189 (m), 1178 (m), 1160 (w), 1122 (m), 1091 (m), 1080 (m), 1061
(s), 1039 (m), 1028 (m), 999 (s), 960 (m), 936 (m), 889 (w), 877 (m),
865 (w), 850 (m), 820 (w), 800 (s), 770 (s), 760 (s), 689 (w) cm−1;
elemental analysis (%) calcd for C37H59N3NiS (636.64 g mol−1):
C 69.80, H 9.34, N 6.60; found: C 69.48, H 9.41, N 6.10.
Synthesis of (K·OEt2)(K)[LtBuNi(SEt)]2 (3). A dark blue hexane
solution of [LtBuNi(SEt)] (1) (100 mg, 0.16 mmol) was treated with
KC8 (23 mg, 0.17 mmol, 1.1 equiv) and stirred for 12 h. All volatiles of
the resulting red suspension were removed in vacuum. The residue was
extracted with diethyl ether (10 mL). Removal of the solvent yielded 3
as a red solid, which was dried in vacuum (73 mg, 0.05 mmol, 65%).
1H NMR (C6D6, 300.1 MHz): δ = 3.5 (br), −1.1 (br) ppm; μeff
(solution, C6D6, 300.1 MHz) = 2.90 μB; μeff (solid, r.t.) = 3.08 μB, UV−
vis (Et2O): λmax (ε in mM−1 cm−1) = 285 (sh, 26), 315 (33), 387 (11),
̃
450 (sh, 6), 550 (sh, 2), 700 (1) nm; IR (KBr): υ = 3052 (m), 3011 (m),
2958 (vs), 2905 (s), 2868 (s), 1904 (w), 1849 (w), 1618 (w), 1581 (w),
1532 (m), 1503 (s), 1463 (m), 1444 (m), 1425 (s), 1404 (vs), 1365 (s),
1324 (s), 1253 (m), 1219 (m), 1195 (m), 1184 (m), 1155 (m), 1096
(m), 1057 (w), 1040 (w), 1028 (m), 964 (w), 935 (m), 889 (m),
878 (m), 843 (w), 803 (m), 776 (m), 757 (m), 707 (w), 680 (w), 648
(w) cm−1; elemental analysis (%) calcd for C78H126K2N4Ni2OS2
(1395.57 g mol−1): C 67.13, H 9.10, N 4.01; found: C 66.92, H 9.00,
N 4.09.
Materials. Solvents were dried employing an MBraun Solvent
Purification System. [LtBuNi(Br)]32 (I) and [LtBuNi(SPh)]12 (II) were
prepared according to literature procedures. To synthesize the potas-
sium salts KSR (R = Et, −(CH2)2NH2) the appropriate thiol was treated
with a slight excess of KOMe (1.1 equiv) suspended in THF and stirred
for 12 h. The resulting suspension was filtered, and the residue was dried
in vacuum.
Synthesis of (K·OEt2)2[LtBuNiSPh]2 (4). A dark green hexane
solution of [LtBuNi(SPh)] (II) (100 mg, 0.15 mmol) was treated with
KC8 (22 mg, 0.16 mmol, 1.1 equiv) and stirred for 12 h. The suspen-
sion was filtered, and the solvent was evaporated under vacuum.
Crystallization from diethyl ether at −30 °C yielded red crystals of 4,
which were dried in vacuum (68 mg, 0.04 mmol, 58%). 1H NMR (C6D6,
300.1 MHz): δ = 3.6 (br), −0.9 (br) ppm; μeff (solution, C6D6, 300.1
MHz, 21 °C) = 2.79 μB; UV−vis (Et2O): λmax (ε in mM−1 cm−1) = 290
(sh, 33), 315 (41), 390 (sh, 10), 421 (sh, 9), 535 (sh, 2), 780 (1) nm; IR
Synthesis of [LtBuNi(SEt)] (1). [LtBuNi(Br)] (I) (300 mg, 0.47
mmol) was dissolved in 20 mL of THF, KSEt (52 mg, 0.52 mmol, 1.1
equiv) was added, and the reaction mixture was stirred for 12 h. After
filtration, the solvent was evaporated under vacuum, and the residue was
extracted with hexane (20 mL). Filtration yielded a dark blue solution.
The solvent was removed in vacuum to give 1 as a dark violet solid (256
̃
(KBr): υ = 3052 (w), 2958 (vs), 2927 (s), 2905 (s), 2868 (m), 1620 (w),
1575 (m), 1532 (m), 1502 (m), 1460 (m), 1444 (m), 1424 (m), 1402
(vs), 1365 (s), 1323 (s), 1253 (w), 1219 (m), 1195 (w), 1184 (w), 1155
(m), 1096 (m), 1080 (m), 1024 (m), 935 (m), 889 (w), 803 (m), 777
(m), 758 (m), 740 (m), 697 (m) cm−1; elemental analysis (%) calcd
for C90H136K2N4Ni2O2S2 (1565.78 g mol−1): C 69.04, H 8.75, N 3.58;
found: C 69.31, H 8.84, N 3.99.
1
mg, 0.41 mmol, 88%). H NMR (C6D6, 300.1 MHz): δ = 48 (4H, Ar-
mH), 31 (4H, CH(CH3)2), 14 (3H, SCH2CH3), 10 (12H, CH(CH3)2), 8
(12H, CH(CH3)2), 4 (18H, C(CH3)3), −54 (2H, Ar-pH), −201 (1H,
CHCC(CH3)3) ppm; μeff (C6D6, 400.1 MHz) = 2.67 μB; UV−vis (Et2O):
λmax (ε in mM−1 cm−1) = 290 (sh, 12.5), 304 (sh, 13), 315 (14), 340 (sh,
Reaction of (K·OEt2)(K)[LtBuNi(SEt)]2 (3) with CO. At r.t., an
orange-red hexane suspension of (K·OEt2)(K)[LtBuNi(SEt)]2 (3)
(27 mg, 19 μmol) was treated with excess CO and stirred for 15 min.
The precipitation of a white solid was observed. Filtration of the
suspension* and removal of all volatile materials afforded a red-brown
solid of [LtBuNi(CO)] (10 mg, 17 μmol, 44%) as revealed by a 1H NMR
spectrum of a benzene-d6 solution featuring two broad signals, which
are characteristic for the nickel(I) carbonyl compound. 1H NMR
̃
9), 385 (sh, 2), 567 (2), 701 (2) nm; IR (KBr): υ = 3060 (m), 3050 (m),
3016 (m), 3003 (m), 2957 (vs), 2925 (s), 2866 (s), 1918 (w), 1854 (w),
1537 (m), 1508 (s), 1463 (m), 1442 (m), 1433 (m), 1381 (m), 1374 (m),
1352 (vs), 1317 (s), 1251 (m), 1221 (m), 1209 (m), 1196 (m), 1181 (m),
1153 (w), 1136 (m), 1098 (m), 1059 (w), 1053 (w), 1032 (w), 968 (m),
933 (m), 887 (w), 819 (m), 802 (m), 798 (m), 778 (s),
754 (m), 673 (w), 648 (w) cm−1; elemental analysis (%) calcd for
C37H58N2NiS (621.63 g mol−1): C 71.49, H 9.40, N 4.51; found: C 71.62,
H 9.50, N 4.20.
̃
(300.1 MHz, C6D6): δ = 3.4 (br), 2.9 (br) ppm; IR (KBr): υ = 2009
(s, CO) cm−1. *1H NMR spectroscopy of a DMSO-d6 solution of the
remaining cruddy white residue proved the formation of KSEt. 1H NMR
(300.1 MHz, DMSO-d6): δ = 2.21 (q, 3JHH = 7.3 Hz, 2H, KSCH2CH3),
1.04 (t, 3JHH = 7.3 Hz, 3H, KSCH2CH3) ppm.
Synthesis of [LtBuNi(aet)] (2). [LtBuNi(Br)] (I) (150 mg,
0.23 mmol) was dissolved in 20 mL of THF, KS(CH2)2NH2 (30 mg,
0.26 mmol, 1.1 equiv) was added, and the reaction mixture was stirred
for 12 h. The suspension was filtered, and the solvent was evaporated
under vacuum. The resulting residue was washed with hexane (5 mL)
and dried in vacuum to give 2 as a red solid (90 mg, 0.14 mmol, 60%).
1H NMR (C6D6, 300.1 MHz): δ = 7.72 (d, 3JHH = 7.6 Hz, 4H, Ar-mH),
Reaction of (K·OEt2)2[LtBuNi(SPh)]2 (4) with CO. At r.t., an
orange-red hexane solution of (K·OEt2)2[LtBuNi(SPh)]2 (4) (39 mg,
25 μmol) was treated with excess CO and stirred for 10 min. The
precipitation of a white solid was observed. All volatiles of the resulting
red-brown suspension were removed in vacuum. The residue was
extracted with hexane. Filtration* and removal of the solvent afforded a
red-brown solid of [LtBuNi(CO)] (14 mg, 24 μmol, 48%) as revealed by a
1H NMR spectrum of a benzene-d6 solution featuring two broad signals,
which are characteristic for the nickel(I) carbonyl compound. 1H NMR
3
7.28 (br, 2H, SCH2CH2NH2), 5.97 (t, JHH = 7.6 Hz, 2H, Ar-pH),
4.90 (br, 4H, CH(CH3)2), 2.34 (br, 14H, CH(CH3)2 (12H) and
SCH2CH2NH2 (2H)), 1.50 (br, 12H, CH(CH3)2), 1.09 (s, 18H,
C(CH3)3), −3.09 (br, 2H, SCH2CH2NH2) ppm; 13C NMR (C6D6,
75.5 MHz): δ = 158.8 (Ar-oC), 145.0 (Ar-iC), 132.0 (Ar-pC), 119.2
(Ar-mC), 30.8 (C(CH3)3), 30.4 (CH(CH3)2), 26.1 (CH(CH3)2), 25.1
(CH(CH3)2) ppm; UV−vis (Et2O): λmax (ε in mM−1 cm−1) = 299 (16),
331 (12), 394 (sh, 3.5), 414 (4) nm; electrospray ionization mass
̃
(300.1 MHz, C6D6): δ = 3.4 (br), 2.9 (br) ppm; IR (KBr): υ = 2011
(s, CO) cm−1. *1H NMR spectroscopy of a DMSO-d6 solution of
the remaining cruddy white residue proved the formation of KSPh.
F
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Inorganic Chemistry
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Table 3. X-ray Structure Parameters for the Nickel(II) and Nickel(I) Thiolate Complexes Discussed in This Work
1
2
3
4
formula
C37H58N2NiS
621.62
C37H59N3NiS
636.64
C78H126K2N4Ni2OS2
C90H136K2N4Ni2O2S2
1565.77
monoclinic
P21/n
fw
1395.57
cryst syst
triclinic
monoclinic
Pc
orthorhombic
space group
P1
̅
Pbcn
a /Å
9.6736(3)
9.7346(3)
41.7577(11)
95.973(2)
90.744(2)
113.835(2)
3570.81(18)
4
10.4799(4)
14.9728(5)
11.5528(4)
90
26.4765(15)
12.7088(4)
18.3156(6)
19.4283(6)
90
b /Å
17.9487(10)
c /Å
16.7544(9)
α /deg
90
β /deg
107.423(3)
90
90
102.5970(10)
90
γ /deg
V /Å3
90
1729.62(11)
2
7962.0(8)
4413.5(2)
2
Z
4
calc density/g cm−3
μ /mm−1
1.156
1.222
1.164
1.178
0.628
0.650
2.329
0.615
F(000)
1352
692
3024
1692
θ /deg
2.30−26.60
3.29−27.90
2.97−66.73
2.42−28.34
reflections collected/unique
final R indices [I > 2σ(I)]
R indices (all data)
GOF
45 193/14 924 [Rint = 0.0931]
R1 = 0.0436, wR2 = 0.0908
R1 = 0.0773, wR2 = 0.0963
0.847
19 613/7951 [Rint = 0.0363]
R1 = 0.0252, wR2 = 0.0507
R1 = 0.0294, wR2 = 0.0515
1.020
58 562/7010 [Rint = 0.0508]
R1 = 0.0355, wR2 = 0.0837
R1 = 0.0542, wR2 = 0.0925
1.010
152 169/10 977 [Rint = 0.0631]
R1 = 0.0326, wR2 = 0.1158
R1 = 0.0377, wR2 = 0.1229
1.035
3
1H NMR (300.1 MHz, DMSO-d6): δ = 6.98 (d, JHH = 6.9 Hz, 2H,
Ar-oH), 6.63 (t, 3JHH = 7.3 Hz, 2H, Ar-mH), 6.39 (t, 3JHH = 7.6 Hz, 1H,
Ar-pH) ppm.
REFERENCES
■
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Crystal Structure Determinations. The data collections (see
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ASSOCIATED CONTENT
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S
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(3), and CCDC 984796 (4) contain the supplementary
crystallographic data for this paper. These data can be obtained
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AUTHOR INFORMATION
■
Corresponding Author
(11) Holland, P. L.; Cundari, T. R.; Perez, L. L.; Eckert, N. A.;
Lachicotte, R. J. J. Am. Chem. Soc. 2002, 124, 14416−14424.
(12) Holze, P.; Horn, B.; Limberg, C.; Matlachowski, C.; Mebs, S.
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2750−2753.
*E-mail: Christian.limberg@chemie.hu-berlin.de.
Notes
The authors declare no competing financial interest.
(13) For similar three-coordinate iron thiolates supported by the same
bulky β-diketiminate ligand system, compare Chiang, K. P.; Barrett, P.
M.; Ding, F.; Smith, J. M.; Kingsley, S.; Brennessel, W. W.; Clark, M. M.;
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signal for the methylene bridge (SCH2CH3) was detected, which may be
due to its close proximity to the paramagnetic nickel center.
ACKNOWLEDGMENTS
■
We are grateful to the Cluster of Excellence “Unifying Concepts
in Catalysis” funded by the Deutsche Forschungsgemeinschaft
(DFG) as well as the Humboldt-Universitat zu Berlin for
financial support. Dr. S. Mebs is thanked for the X-ray analysis
of 1. We also thank C. Matlachowski and S. T. Li for EPR
measurements.
̈
G
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Inorganic Chemistry
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̈
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H
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