Model studies of methyl CoM reductase: Methane formation via CH 3-S bond cleavage of Ni(I) tetraazacyclic complexes having intramolecular methyl sulfide pendants

Article abstract of DOI:10.1021/ic300017k

The Ni(I) tetraazacycles [Ni(dmmtc)]⁺ and [Ni(mtc)]⁺, which have methylthioethyl pendants, were synthesized as models of the reduced state of the active site of methyl coenzyme M reductase (MCR), and their structures and redox properties were elucidated (dmmtc, 1,8-dimethyl-4,11- bis{(2-methylthio)ethyl}-1,4,8,11-tetraaza-1,4,8,11-cyclotetradecane; mtc, 1,8-{bis(2-methylthio)ethyl}-1,4,8,11-tetraaza-1,4,8,11-cyclotetradecane). The intramolecular CH₃-S bond of the thioether pendant of [Ni ^I(dmmtc)](OTf) was cleaved in THF at 75 °C in the presence of the bulky thiol DmpSH, which acts as a proton source, and methane was formed in 31% yield and a Ni(II) thiolate complex was concomitantly obtained (Dmp = 2,6-dimesityphenyl). The CH₃-S bond cleavage of [Ni ^I(mtc)]⁺ also proceeded similarly, but under milder conditions probably due to the lower potential of the [Ni^I(mtc)] ⁺ complex. These results indicate that the robust CH₃-S bond can be homolytically cleaved by the Ni(I) center when they are properly arranged, which highlights the significance of the F430 Ni environment in the active site of the MCR protein.

Full text of DOI:10.1021/ic300017k

Article
pubs.acs.org/IC
Model Studies of Methyl CoM Reductase: Methane Formation via
CH3−S Bond Cleavage of Ni(I) Tetraazacyclic Complexes Having
Intramolecular Methyl Sulfide Pendants
Jun-ichi Nishigaki, Tsuyoshi Matsumoto,* and Kazuyuki Tatsumi*
Research Center for Materials Science, and Department of Chemistry, Graduate School of Science, Nagoya University, Furo-cho,
Chikusa-ku, Nagoya 464-8602, Japan
S
* Supporting Information
ABSTRACT: The Ni(I) tetraazacycles [Ni(dmmtc)]+ and [Ni(mtc)]+,
which have methylthioethyl pendants, were synthesized as models of the
reduced state of the active site of methyl coenzyme M reductase (MCR), and
their structures and redox properties were elucidated (dmmtc, 1,8-dimethyl-
4,11-bis{(2-methylthio)ethyl}-1,4,8,11-tetraaza-1,4,8,11-cyclotetradecane;
mtc, 1,8-{bis(2-methylthio)ethyl}-1,4,8,11-tetraaza-1,4,8,11-cyclotetrade-
cane). The intramolecular CH3−S bond of the thioether pendant of
[NiI(dmmtc)](OTf) was cleaved in THF at 75 °C in the presence of the
bulky thiol DmpSH, which acts as a proton source, and methane was formed
in 31% yield and a Ni(II) thiolate complex was concomitantly obtained
(Dmp = 2,6-dimesityphenyl). The CH3−S bond cleavage of [NiI(mtc)]+ also
proceeded similarly, but under milder conditions probably due to the lower
potential of the [NiI(mtc)]+ complex. These results indicate that the robust
CH3−S bond can be homolytically cleaved by the Ni(I) center when they are properly arranged, which highlights the significance
of the F430 Ni environment in the active site of the MCR protein.
INTRODUCTION
■
Methyl coenzyme M reductase (MCR) is a nickel enzyme
found in methanogenic archaea, which catalyzes the formation
of the heterodisulfide (CoBS−SCoM) and methane from
methyl CoM (MeSCoM) and N-7-mercaptoheptanoyl threo-
nine phosphate (HSCoB) in the final step of methanogenesis
(eq 1).1 The active site of MCR contains a nickel corphinoid
cofactor F430 (Figure 1),2,3 and the active reduced state MCRred1
is suggested to contain a Ni(I) center in the F430 according to
the EPR/ENDOR studies.4 However, the mechanism of the
MCR catalytic CH3−S bond cleavage of MeSCoM has not been
established.
MeSCoM + HSCoB → CH + CoBS−SCoM
Figure 1. Cofactor F430 in MCR.
4
0′
−1
ΔG = 30kJ mol
On the other hand, Siegbahn et al. have proposed a mec-
hanism based on theoretical studies, in which the Ni(I)-F430
state is proposed to activate the CH3−S bond of MeSCoM by
interacting with tyrosine hydroxides located around the active
site.6 However, CH3−S bond activation by Ni(I) complexes that
model the active MCRred1 state have so far failed.4 As reported
by Ram et al., the Ni(I) tetramethylcyclam complex [Ni(tmc)]+,
a model for Ni(I)-F430, does not activate C−S bonds of
thioethers.5c Jaun et al. also reported that the MCR active site
became inert when it was extracted as Ni(I)-F430 pentamethyl
(1)
Recent model studies have provided some clues to the MCR
reaction mechanism.5 Jaun and co-workers reported that photo
irradiation of a Ni(II) complex of a thioether-thiolate ligand,
bis(1-{2-(methylthio)ethyl}cyclohexanethiolate)nickel, in the
presence of the corresponding thioether-thiol compound,
1-{2-(methylthio)ethyl}cyclohexanethiol, resulted in CH3−S bond
cleavage and gave methane and the disulfide 1,2-dithiaspiro[4,5]-
decane.5b In this case, a key process would be generation
of the Ni(I)/thiyl-radical pair via photolytic homolysis of the Ni−
S(thiolate) bond, and while the results suggest an MCR catalytic
pathway, the question remains how the Ni(I)/thiyl-radical pair
could be generated in vivo.
Received: January 4, 2012
Published: March 22, 2012
© 2012 American Chemical Society
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ester, although it still cleaved sulfonium C−S bonds.7 These
results suggest that the active site environment of the protein
plays a significant role. These Ni(I) complexes coordinated by
four N atoms with a formal 17-electron configuration disfavor
further thioether coordination,8,9 and thus in the MCR active
site, the proper positioning of MeSCoM to promote interaction
with the Ni(I) center could be important. The SCoM location
in the MCRox1‑silent state as shown in Figure 2 shows that
Scheme 1
Synthesis of Ni(II) dmmtc Complexes. The dmmtc
ligand dissolved in ethanol was added to an aqueous solution of
NiCl2·6H2O, which afforded [Ni(dmmtc)(Cl)](Cl) (3) in 67%
yield as yellow-green crystals (Scheme 2). As shown by X-ray
Scheme 2
Figure 2. The MCR active site structure in the MCRox1‑silent state.
hydrogen bonds from protein residues hold the MeSCoM in a
position favorable for interaction with the Ni(I) center, which
could lead to CH3−S bond cleavage.
In the course of our MCR model studies,10 we have
synthesized cyclam derivatives having methylthioether pend-
ants, 1,8-dimethyl-4,11-bis{(2-methylthio)ethyl}-1,4,8,11-tet-
raaza-1,4,8,11-cyclotetradecane (dmmtc), and 1,8-{bis(2-
methylthio)ethyl}-1,4,8,11-tetraaza-1,4,8,11-cyclotetradecane
(mtc), as shown in Chart 1, to facilitate interaction between the
analysis, the structure features a penta-coordinate geometry
around the nickel defined by the four N atoms in a basal plane
and one Cl atom occupying the axial position.13 The four alkyl
groups on the nitrogen atoms of the dmmtc ligand are all on
the same side of the NiN4 plane similar to the reported
tetramethylcyclam analogue RSRS-[Ni(tmc)(Cl)]+.10a,14
Chart 1
Both of the chlorides of 3 were smoothly exchanged for triflate
by treatment with NaOTf in H2O, affording [Ni(dmmtc)]-
(OTf)2 (4) in 76% yield as bluish green crystals. The molecular
structure of 4 was also analyzed by X-ray crystallography
(Figure 3), and selected bond distances and angles are listed in
Table 1. The nickel assumes penta-coordination as observed for
3, although it is a thioether sulfur of the dmmtc ligand that caps
the nickel in place of a chloride. As indicated by the substantially
bent N2−Ni−N4 bond (148.75(7)°) and linear N1−Ni−N3
bond (175.01(7)°), the coordination geometry of the nickel can
be described either as a distorted square pyramid with the sulfur
occupying the apical position or as a distorted trigonal bipyramid
where N1 and N3 are at the axial positions. The Ni−S distance
(2.4005(6) Å) resembles those of previously reported penta-
coordinate nickel(II) thioether complexes.7b,15
thioether sulfur and the Ni center.11 Herein, we report the
synthesis and structures of the Ni(II) and Ni(I) complexes of the
dmmtc and mtc ligands. We also report that activation of the
CH3−S bond by the Ni(I) complexes models the MCR reaction.
RESULTS AND DISCUSSION
■
Synthesis of the Cyclam Derivatives, dmmtc and mtc.
Following the report of Guilard and co-workers,12 1,4,8,11-
tetraazatricyclo[9.3.1.1]hexadecane (1) was prepared from
1,4,8,11-tetraazacyclotetradecane (cyclam) and formaldehyde,
and treated with 2-iodoethyl methyl sulfide. Alkylation occurred
site-selectively on two trans nitrogen atoms, and compound 2
was obtained in 78% yield (Scheme 1). Compound 2 was
facilely converted to dmmtc and mtc by NaBH4-reduction and
base hydrolysis, respectively.
In acetonitrile, cyclic voltammetry (CV) of 4 exhibited a single
reversible couple at −1.09 V in (vs Ag/AgNO3, Figure 4), which
we assign to the NiI/NiII redox process. This potential is close to
the value of RSRS-[Ni(tmc)]2+ (−1.14 V),5c but is considerably
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Scheme 3
configuration, and two H2O molecules are bound to the axial
positions.19
Treatment of 5a or 5b with NaOTf in H2O gave the
corresponding triflate complexes 6a and 6b, and their structures
were also confirmed by X-ray analysis. While complex 6a retains
the hexa-coordinate geometry by incorporating a second
thioether sulfur instead of the removed chloride, complex 6b
assumes a tetra-coordinate square planar geometry with
dissociation of the H2O molecules.20 The different geometry of
6a and 6b was evident from the color, light purple for 6a and
orange for 6b.
Figure 3. Molecular structure of the cation of 4 with 50% thermal
ellipsoids. Hydrogen atoms are omitted for clarity.
Table 1. Selected Bond Distances (Å) and Angles (deg) for 4
4
Ni−S1
Ni−N1
2.4005(6)
2.126(2)
The triflate complexes 6a and 6b were further converted to
the borate complexes 7a and 7b, respectively, as shown in
Scheme 4. On the basis of its purple color, we assume that 7a
has an octahedral geometry like that of 6a. However, X-ray
analysis was not performed. In contrast to the orange 6b,
simple anion exchange to produce 7b leads to a purple product,
indicating a hexa-coordinate complex. X-ray structural analysis
confirms that the two thioethers have bound to the nickel
producing a pseudo octahedral complex. Because the counter-
anions do not interact with the respective nickel cations
significantly in either 6b or 7b, the geometrical difference found
for 6b and 7b must be attributable to crystal packing, indicating
that the thermodynamic stabilities of the two geometries must
be only slightly different.
Ni−N2
Ni−N3
Ni−N4
S1−Ni−N1
S1−Ni−N2
S1−Ni−N3
S1−Ni−N4
N1−Ni−N3
N2−Ni−N4
2.1340(19)
2.131(2)
2.122(2)
84.59(5)
100.15(5)
100.30(7)
110.75(5)
175.01(7)
148.75(7)
To investigate the redox properties of 6a and 7b, cyclic
voltammograms were generated. In THF, complex 6a shows a
NiII/NiI redox couple at E1/2 = −1.36 V (vs Ag/Ag+) as shown in
Figure 6b. Complex 7b also exhibits a similar reversible NiI/NiII
redox couple at −1.35 V. These E1/2 values are ca. 0.4 V more
negative as compared to that of the penta-coordinate dmmtc
complex 4.21 While the shape of the CV plot of 7b did not
noticeably change with scan rate between 0.01 and 0.2 V s−1, that
of 6a was considerably different at a slower scan rate. At a sweep
rate of 0.01 V s−1, the anodic current of the NiII/NiI couple
became significantly smaller, and an additional irreversible anodic
wave emerged at +0.17 V (Figure 6a), indicating further reaction
of the generated Ni(I) species. Because this oxidation potential
of +0.17 V compares well with the value reported for the related
N4-coordinated Ni(II) thiolate complex [L8py2Ni(STol)](BPh4)
(L8py2 = 1,5-bis(2-pyridylmethyl)-1,5-diazacyclooctane),22 we
conclude that the Ni(I) species generated by reduction of 6a
has been converted into Ni(II) thiolate complex via reductive
cleavage of the CH3−S bond.
Figure 4. Cyclic voltammogram of 0.5 mM of 4 in CH3CN at 0.2 V s−1
with 0.1 M (Bu4N)(PF6) as the supporting electrolyte. Potentials are
referenced to Ag/AgNO3.
less negative (by ca. 0.4 V) than that of F430 pentame-
thyester.16,17
Synthesis of Ni(II) mtc Complexes. Similar to the
synthesis of 3, the mtc ligand was allowed to react with
NiCl2·6H2O in ethanol/H2O. The crude product was found
to contain two complexes, [NiII(mtc)(Cl)](Cl) (5a) and
[NiII(mtc)(OH2)2](Cl)2 (5b) (Scheme 3), which were
separated by extraction with H2O. Thus, complex 5b, insoluble
in H2O, was isolated in 16% yield as a light purple powder,
while 5a was obtained in 63% yield as a light blue powder via
evacuation of the filtrate and extraction with CHCl3. X-ray cry-
stallography revealed hexa-coordinate structures for the nickels
of both 5a and 5b (Figure 5, Tables 2 and 3). In complex 5a,
a thioether sulfur and a chloride coordinate in cis-positions,
and the mtc ligand coordinates in a fac−mer fashion, whereas
the four nitrogens of mtc in 5b are coplanar with a mer−mer
Synthesis of Monovalent Nickel Complexes [Ni-
(dmmtc)](X) (8, X = OTf; 9, X = BArF ). Noting the CV
4
data for 4 and choosing an appropriate reducing agent, treatment
of 4 with 0.3% Na/Hg in THF/CH3CN smoothly generated the
corresponding nickel(I) complex [Ni(dmmtc)](OTf) (8) in
72% yield as blue plate crystals (Scheme 5). Complex 8 was
converted to the borate analogue [Ni(dmmtc)](BArF ) (9) by
4
ion exchange with NaBArF in Et2O, which was isolated in 70%
4
yield. However, the crystals were yellow, indicating different
coordination geometries of 8 and 9 in the crystals. The molecular
structures were confirmed by X-ray analysis, showing that the
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Figure 5. Molecular structures of the cations of 5a,b, 6a,b, and 7b with 50% thermal ellipsoids. Hydrogen atoms are omitted for clarity.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 5a, 6a
Table 3. Selected Bond Lengths (Å) and Angles (deg) for 5b,
6b, and 7b
5a
6a
5b
6b
7b
Ni−S1
Ni−L1
Ni−N1
Ni−N2
Ni−N3
2.4374(4)
2.5299(6)
2.1582(13)
2.0846(17)
2.1982(13)
2.1248(13)
91.494(17)
89.68(3)
2.5614(13)
2.6187(13)
2.168(4)
a
a
Ni−L2
2.135(3)
2.167(2)
2.5460(8)
2.127(2)
2.062(3)
80.71(8)
91.89(8)
99.29(8)
88.11(8)
Ni−N1
Ni−N2
L2−Ni−N1
L2−Ni−N2
L2−Ni−N3
1.9710(19)
1.9334(17)
2.069(4)
2.042(3)
a
a
a
a
2.169(4)
92.55(10)
86.81(13)
87.45(10)
93.19(13)
Ni−N4
S1−Ni−L1
2.072(4)
a
70.47(4)
S1−Ni−N1
S1−Ni−N2
S1−Ni−N3
S1−Ni−N4
L1−Ni−N1
L1−Ni−N2
L1−Ni−N3
L1−Ni−N4
83.24(10)
164.73(11)
103.37(10)
98.95(10)
104.29(10)
97.92(11)
82.82(10)
165.19(11)
171.73(14)
94.16(14)
L2−Ni−N4
a
89.43(3)
L2 = O1 (5b), S2 (7b).
96.19(3)
172.39(4)
83.89(4)
found for the crystal of 9 would be thermodynamically less favored
due to the coordinative saturation of the d9-electron configuration of
the monovalent nickel center.8
a
a
a
a
177.30(3)
98.65(4)
Molecular Structures of 8 and 9. The structures of 8 and
9 are shown in Figure 7, and their metric parameters are listed
in Table 4. Complex 8 contains a tetra-coordinate nickel sitting
in a nearly square planar geometry with a tetrahedral distortion
with N1 and N3 above the mean plane and N2 and N4 below it.
This structural feature is also found in the reported Ni(I) complex
RSRS-[Ni(tmc)](OTf).24 Further structural details of 8 will not be
discussed because the dmmtc ligand is fully disordered around the
axis vertically penetrating the nickel.
The borate analogue 9 contains a nickel structurally different
from that of 8. The nickel center is coordinated by a thioether
sulfur in addition to the four nitrogens, forming a distorted
square pyramidal geometry. Although it appears similar to the
85.15(4)
N1−Ni−N3
N2−Ni−N4
173.52(5)
93.63(6)
a
L1 = Cl1 (5a), S2 (6a).
nickels of 8 and 9 assume tetra- and penta-coordination,
respectively (vide infra). However, THF solutions of 8 and 9 show
the same green color. Their UV−vis spectra exhibit two common
absorptions, a CT band at 333 nm (ε = 3600 cm−1 M−1) and a
d−d* band at 717 nm (ε = 20 cm−1 M−1), which compare well
with those of RSRS-[Ni(tmc)](OTf) that includes a tetra-coordinate
geometry in solution.5c,23 Probably, the penta-coordinate structure
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Scheme 4
Figure 6. Cyclic voltammogram of 0.5 mM of 6a in THF with scan rates of (a) 0.01 V s−1, (b) 0.2 V s−1, and (c) 0.5 mM of 7b in THF with a scan
rate of 0.1 V s−1. Potentials are referenced to Ag/AgNO3.
Scheme 5
Table 4. Selected Bond Lengths (Å) and Angles (deg) in 8,
9, and 10a,b
8
9
10a
10b
Ni−S1
Ni−N1
Ni−N2
Ni−N3
2.5243(9)
2.150(2)
3.1576(7)
2.1223(5)
2.0263(5)
2.1041(6)
2.0235(6)
77.502(14)
96.70(2)
2.125(7)
2.084(6)
2.123(7)
2.076(5)
2.118(4)
2.029(5)
2.108(2)
2.176(2)
(2.118(4))
(2.029(5))
Ni−N4
2.116(2)
S1−Ni−N1
S1−Ni−N2
S1−Ni−N3
S1−Ni−N4
N1−Ni−N3
N2−Ni−N4
84.14(8)
100.96(7)
99.52(7)
93.441(13)
121.017(8)
170.296(3)
155.249(7)
107.50(8)
178.32(11)
151.24(11)
174.6(3)
157.3(3)
180
180
plane for 9 as indicated by the wider N1−Ni−N3 and N2−Ni−N4
bonds (178.32(11)° and 151.24(11)°) than the corresponding
bond angles of 4, respectively.
Synthesis of [NiI(mtc)]X (X = OTf, BArF ). Following the
4
successful synthesis of the dmmtc complexes, the synthesis of mtc
complexes of Ni(I) was similarly attempted via reduction of the
corresponding Ni(II) complexes. When complex 7a was treated
with Na/Hg at room temperature in Et2O/THF (10:1), the
solution immediately became green as was also observed for the
reduction of 4. However, the green solution evolved into reddish
brown within a few minutes, indicating thermal instability of the
reduced species as was suggested by the CV data in Figure 6a
(vide supra). To trap the transient species, reduction and
successive crystallization were performed at −40 °C, which gave
the Ni(I) mtc complex 10a as yellow green crystals in 16% yield
Figure 7. Molecular structures of the cations of 8 and 9 with 50%
thermal ellipsoids. For 8, one component of the 2-fold disordered
ligand as well as hydrogen atoms are omitted for clarity.
related Ni(II) complex 4, the Ni−S bond is reasonably
elongated by 0.12 Å due to the larger coordination sphere of
the reduced nickel. The nickel also approaches to the basal N4
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Scheme 6
(Scheme 6). A similar reduction of the trans stereoisomer 7b also
gave 10a, but with a higher yield of 51%. Partial inversion of the
mtc nitrogen atoms occurs in the course of both reductions. The
reduction of 7b occasionally afforded a trace amount of 10b,
which has the same stereo configuration as 7b. Thus, reduction of
7b first affords the intermediate Ni(I) species 10b that conserves
the nitrogen stereochemistry as shown in Scheme 6. Gradual
isomerization to the thermodynamic product 10a follows.8 The
reduction of 7a should follow a similar pathway, although the
intermediate has not been observed.
We also examined the reduction of the triflate complexes 6a and
6b. Although the triflate salt [Ni(mtc)]OTf (11) has not been iso-
lated, the reduction seems to have proceeded similarly, because anion
exchange with NaBArF4 afforded 10a although the yield was low.
The molecular structures of 10a and 10b were established
by X-ray analysis (Figures 8, 9 and Table 4).25 In complex 10a,
Figure 9. Molecular structures of the cation of 10b with 50% thermal
ellipsoids. Hydrogen atoms are omitted for clarity.
of 10b is square planar as was the case for the related Ni(II)
complex 6b, although the Ni−N distances are reasonably
longer by 0.05−0.19 Å.
EPR Spectra. The EPR spectra of 8, 9, and 10a were
recorded in the solid state Figure 10. Complex 8 exhibited an
axial signal with g∥ = 2.32 and g⊥ = 2.07 as shown in Figure 10a,
which resembles that of the square planar Ni(I) complex
[Ni(tmc)]+.27 Interestingly, the spectra of 9 and 10a also show
similar axial EPR signals with g∥ = 2.35, g⊥ = 2.10 for 9 and g∥ =
2.30, g⊥ = 2.08 for 10a, and the data are distinct from the spectra
of the penta-coordinated Ni(I) cyclam complex [Ni(cyclam)-
(CO)]+ (g1 = 2.196, g2 = 2.137, g3 = 2.017).28 The results indi-
cate that in both 9 and 10a the thioether sulfur coordination
along the z-axis is weak as suggested by the X-ray results and is
too weak to effect the unpaired electron localized in the nearly
Figure 8. Molecular structure of the cation of 10a with 50% thermal
ellipsoids. Disordered atoms and hydrogen atoms are omitted for
clarity.
2
2
orthogonal dx −y orbital.
the substituents on the nitrogen atoms are all cofacial up, and
one thioether sulfur is located above the nickel. Although this
geometry appears similar to that of Ni(I) dmmtc complex 9,
the Ni−S distance of 3.1576(7) Å is considerably longer than
that of 9 (2.5243(9) Å) and is nearly as long as the sum of their
van der Waals radii (3.43 Å).26 In addition, the nickel exhibits a
tetrahedral distortion as observed for 8, which displaces N1 and
N3 from the coordination plane toward S1, while N2 and N4
are displaced toward the opposite side of the coordination
plane. We concluded that any Ni−S interaction is insignificant.
Complex 10b also contains a tetra-coordinate nickel center, and
irrespective of the reduced Ni(I) oxidation state, the geometry
Intramolecular CH3−S Bond Activation by 8. While the
Ni(I) complex 8 was stable in the solid state, it was found to
degrade slowly in solutions via C−S bond activation. When
complex 8 was warmed in THF to 75 °C for 4 days in a sealed
flask, methane (6%), ethane (23%), and ethylene (3%) were
detected by gas chromatography.29 Because the Ni(II)−thiolate
complex 12 was concomitantly obtained from the resultant
solution as yellowish green crystals in 19% yield, the CH3−S
bond in a pendant arm of 8 must have been cleaved in the course
of the reaction (Scheme 7).30 In this reaction, the oxidized
nickel(II) complex 4 and its stereoisomer 4′ were also obtained as
blue green and light blue crystals, respectively.31 Two additional
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Figure 10. X-Band solid-state EPR spectra of (a) 8 and (b) 9 recorded at 8 K, and (c) 10a recorded at 190 K. Spectrometer settings: microwave
frequency, 9.37 GHz; microwave power, 1 mW; modulation frequency, 100 kHz; modulation amplitude, 5 G.
Scheme 7
Ni(II) complexes, the Ni(II)−CH3 complex [Ni(dmmtc)-
(CH3)]+ (13) and the Ni(II) amido complex 14 that has lost
one thioether pendant, were observed in the ESI-TOF-MS.
Although there have been several reports of C−S bond cleavage
mediated by transient Ni(I) species that were generated in situ
and have not been isolated or fully characterized,5a,b,32 this
reaction would serve as a rare example of C−S bond cleavage
mediated by an isolated well-characterized Ni(I) complex.
Ni(II) complexes 4 and 4′, the methyl complex 13, and the
amido complex 14. In addition to these products, DmpSCH3
was obtained in 12% yield.35
The molecular structure of 12 was confirmed by X-ray analysis
as shown in Figure 11. The nickel assumes a penta-coordination,
As a coenzyme B model, we also examined the similar
reaction of 8 in the presence of added thiol. Sterically less
demanding thiols such as p-toluenethiol were directly reduced
by the Ni(I) center to afford H2 and the nickel(II) thiolate.33
However, the very bulky arenethiol DmpSH (Dmp = 2,6-
dimesitylphenyl)34 did not react at room temperature, and thus
in the presence of 5 equiv of DmpSH, complex 8 was allowed
to warm to 75 °C in THF (Table 5). The thiol addition did not
Table 5. Hydrocarbon Yields from the Ni(I) Complex 8
additives
CH4 (%)
C2H6 (%)
C2H4 (%)
6
23
3
3
Figure 11. Molecular structure of the cation of 12 with 50% thermal
ellipsoids. Hydrogen atoms are omitted for clarity.
5 equiv of DmpSH
31
trace
cause significant acceleration of the reaction, but GC analysis
indicated that the methane yield was increased up to 31% while
ethane formation was almost completely suppressed. The nickel
products were almost identical to those from the run in the
absence of the thiol; the thiolate complex 12 was isolated in
21% yield, and ESI-TOF-MS showed the formation of the
resembling that of 4. However, the Ni−S1 bond is 0.1 Å shorter
(Table 6).36
Mechanism: Reaction of 8. To corroborate the source
of the methyl fragment in the products, we prepared the bis
S-trideuteriomethyl analogue of the dmmtc ligand dmmtc-d6
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Table 6. Selected Bond Lengths (Å) and Angles (deg) for 12
Scheme 9
12
Ni−S1
Ni−N1
2.2954(13)
2.133(4)
Ni−N2
2.153(3)
Ni−N3
2.161(3)
Ni−N4
2.132(3)
CH3−S Bond Activation of [NiI(mtc)]X (X = OTf, BArF ).
4
S1−Ni−N1
S1−Ni−N2
S1−Ni−N3
S1−Ni−N4
N1−Ni−N3
N2−Ni−N4
87.81(11)
106.37(10)
98.27(10)
106.15(10)
173.93(14)
147.32(14)
The negatively shifted redox potential of 10a as compared to
the dmmtc complex 8 suggests a higher driving force and thus
a faster reaction (Scheme 10). Indeed, the reduction of 10a
occurs at a lower temperature and in hours rather than days.
When 10a was stirred for 12 h at room temperature in THF,
formation of methane (18%), ethane (trace), and ethylene (2%)
was observed by GC analysis. The ESI-TOF-MS spectra showed
a signal at m/z = 391.1 attributable to the Ni(II) thiolate
complex 15. The mass spectra also exhibited another strong
signal at m/z = 405.1, which is 1 mass-unit smaller than that of
10a. Probably the NH proton of the mtc ligand is consumed as a
hydrogen source for methane formation, and the Ni(II) amide
complex 16 would also form. Although the X-ray crystallographic
analysis of 15 and 16 has not been achieved, their structures
were confirmed for their triflate analogues (vide infra).
and synthesized the Ni(I) complex [Ni(dmmtc-d6)](OTf)
(8-d6). When the reaction of 8-d6 was similarly examined in
the presence of DmpSH, formation of CD3H and C2D6 was
observed by GC−MS, indicating that the methane and the
ethane resulted from the methylthio portions of the dmmtc
pendants. In addition, the CD3-coordinated Ni(II) complex
[Ni(dmmtc-d6)(CD3)]+ was observed by ESI-TOF-MS analysis,
and the deuterated methylthioether DmpSCD3 was obtained.
On the other hand, the ethylene was not deuterated. The forma-
tion of the amido complex 14 requires the loss of a
CH2CH2SCH3 group, which must be the source of the ethylene.
On the basis of these results, a proposed reaction mechanism
is depicted in Scheme 8. Activation of the CH3−S bond,
It is notable that the methane yield was considerably higher
when the Ni(I) species were prepared in situ from 7a at
−40 °C in THF by treatment with Na/Hg and successive
removal of the reductant. When the solution was warmed to
room temperature and stirred for 6 h, the methane yield was
increased to 34%, while the yields of ethane and ethylene did
not vary significantly. Probably the reduction of 7a first
afforded the Ni(I) complex with retention of the ligand
conformation as shown in Scheme 6, in which the CH3−S bond
could be cleaved more facilely than the case of 10a, although
the reason for the higher reactivity is unclear.
Scheme 8
We have also carried out the same reaction with the triflate
analogue 6a (Scheme 11). Because the corresponding Ni(I)
complex has not been isolated as described above, complex 6a
was similarly reduced in situ over Na/Hg in THF, and the
solution was stirred for 6 h at room temperature. The GC
analysis showed the formation of methane in 30% yield and
ethylene in 2% yield, while ethane was not detected. In the MS
spectra, the Ni(II)−thiolate complex 17 and the Ni(II)−amide
complex 18, the triflate analogues of 15 and 16, respectively,
were observed. Recrystallization of the crude mixture afforded
17 in 9% yield as green crystals and 18 in 5% yield as deep
blue crystals,30 and their structures were confirmed by X-ray
crystallography.
The molecular structures of 17 and 18 are shown in
Figures 12 and 13, and selected bond distances and angles are
summarized in Table 7. In the thiolate complex 17, the
nitrogen stereo configuration of the mtc ligand matches that of
the starting complex 6a. However, the thioether sulfur does not
coordinate to the nickel in 17 probably due to the strong
coordination of the thiolate, and thus the nickel of 17 becomes
trigonal bipyramidal. The axial Ni−N distances (2.1513(17)
and 2.2013(18) Å) are elongated from those in equatorial
positions of 2.0683(17) and 2.0780(16) Å.
achieved by the intramolecular interaction of the thioether sulfur
and the Ni(I) center, results in homolytic cleavage of the CH3−S
bond.37 The “CH3” radical would give methane via hydrogen
abstraction and ethane by self-coupling. It may also add to
another nickel(I) complex 8 to give the nickel(II) methyl
complex 13, although it would release “CH3” radical as suggested
in the thermal reaction of the related Ni(II) methyl complex
[Ni(tmc)CH3](OTf).38
In the presence of the thiol DmpSH, the formation of methane
was enhanced, while that of ethane was largely suppressed. The
result indicates that the thiol serves as a hydrogen donor.
On the other hand, ethylene, which derives from the
methylthioethyl pendants, can be formed via activation of
the other C−S bond of the thioether, the CH2−SCH3 bond.
As shown in Scheme 9, homolysis of the C−S bond would
generate a nickel(II) thiolate and a radical NiII(SMe)−N−
CH2−CH2·, which would be further converted to ethylene,
Ni(II) amide 14, and a CH3S· fragment.
The amide complex 18 in Figure 13 contains a square planar
nickel coordinated by four N atoms of the mtc ligand, while the
thioethers do not interact with the nickel center. The Ni−N4
distance of 1.8747(19) Å is significantly shorter than the other
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Scheme 10
Scheme 11
Table 7. Selected Bond Lengths (Å) and Angles (deg) for 17
and 18
17
18
Ni−S1
Ni−N1
Ni−N2
Ni−N3
2.2343(5)
2.1513(17)
2.0683(17)
2.2013(18)
2.0780(16)
88.45(4)
1.9685(12)
1.9680(18)
1.9642(12)
1.8747(19)
Ni−N4
S1−Ni−N1
S1−Ni−N2
S1−Ni−N3
S1−Ni−N4
N1−Ni−N3
N2−Ni−N4
134.93(4)
96.94(4)
127.13(5)
174.60(6)
97.74(6)
179.54(7)
177.71(6)
Figure 12. Molecular structure of the cation of 17 with 50% thermal
ellipsoids. Hydrogen atoms are omitted for clarity.
robust CH3−S bond was cleaved by nickel(I) when the
thioether sulfur was arranged close to the nickel center. The
protein residues around the MCR active site seem to play a
crucial role to arrange the substrates properly. As the Ni(I) mtc
complex cleaved the CH3−S bond more facilely as compared to
that of the dmmtc complex, the Ni(I)/Ni(II) potential would
be important for efficient catalysis. Another important finding is
that the CH3−S bond cleavage promoted by the nickel(I)
occurs homolytically to afford a nickel(II) thiolate and a “CH3”
radical. These result supports the suggestion that the Ni(I)
state is a plausible active state in MCR catalysis.
EXPERIMENTAL SECTION
■
General Procedures. All reactions and manipulations were
conducted under an inert atmosphere of dry nitrogen by employing
standard Schlenk techniques or a glovebox under a nitrogen
atmosphere. Pentane, hexane, ether, THF, and acetonitrile were
degassed and purified by the method of Grubbs, where the solvents
were passed over columns of activated alumina and supported copper
catalyst supplied by Hansen & Co. Ltd. Fluorobenzene was distilled
from sodium benzophenone ketyl. Methanol, ethanol, and nitro-
methane were purchased and used without further purification. C6D6
was dried over sodium and distilled prior to use. 1H NMR (600 MHz)
and 13C{1H} NMR (243 MHz) spectra were recorded on a JEOL
ECA-600. The signals were referenced to the residual proton peak
of the deuterated solvents. UV−vis spectra were recorded in 10 mm
quartz glass cells on a JASCO V560 or SHIMAZU UV3150
spectrometer. Electrospray ionization time-of-flight mass spectrometry
(ESI-TOF-MS) spectra were obtained from a Micromass LCT TOF-
MS spectrometer. Elemental analyses were recorded on a LECO-
CHNS-932 elemental analyzer where the crystalline samples were
sealed in silver capsules under nitrogen. EPR spectra were collected on
Figure 13. Molecular structure of the cation of 18 with 50% thermal
ellipsoids. Hydrogen atoms are omitted for clarity.
Ni−N distances by 0.09 Å, clearly indicating that N2 is
coordinating as an amide.
SUMMARY
■
As models of the MCR active site in the reduced state, Ni(I)
complexes coordinated by cyclam derivatives that carry
methylthioether pendants have been synthesized and inves-
tigated for their reactivity. A significant finding is that the
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a Bruker EMX-plus spectrometer at X-band frequencies with a liquid
helium cryostat. Nickel halides, nickel triflate, and the other common
reagents were purchased and used without further purification. 1,4,8,11-
Tetraazatricyclo[9.3.1.1]hexadecane,12 2-iodoethyl methyl sulfide,39 and
DmpSH34 were prepared according to literature procedures.
temperature for 3 h. After the solution was concentrated to ca. 1 mL in
vacuo, the green residue was extracted with chloroform (5 × 100 mL),
dried over MgSO4, and evaporated to dryness to give 3 as a yellow
green powder in 70% yield. Single crystals suitable for X-ray analysis
were obtained by recrystallization from CH2Cl2/Et2O. ESI-TOF-MS
(CH3CN): m/z = 469.2 ([M]+). UV−vis (CH3CN): λmax/nm (ε) =
427 (124), 722 (34). Anal. Calcd for C18H40N4S2NiCl2: C, 42.70; H,
7.96; N, 11.07; S, 12.67. Found: C, 42.56; H, 7.55; N, 10.95; S, 12.30.
[Ni(dmmtc-d6)(Cl)](Cl) (3-d6). Complex 3-d6 was prepared as
described for 3 using dmmtc-d6. ESI-TOF-MS (CH3CN): m/z =
475.1 ([M]+).
[Ni(dmmtc)](OTf)2 (4). To an aqueous solution (10 mL) of 3
(1.00 g, 1.98 mmol) was added sodium trifluoromethanesulfonate
(1.04 g, 6.04 mmol), and the mixture was stirred at room temperature
for 1 h. The blue solution was extracted with nitromethane, and the
organic phases were collected, dried over MgSO4, and evaporated to
dryness. The bluish-green solid was dissolved in acetonitrile (5 mL),
and THF (200 mL) was added. After standing for a day, bluish green
crystals of 4 were obtained in 76% yield. ESI-TOF-MS (CH3CN):
m/z (%) = 217.2 ([M]2+) (100), 583.1 ([M + OTf]+) (90). UV−vis
(CH3CN): λmax/nm (ε) = 345 (1670), 616 (57). Anal. Calcd for
C20H40N4S4F6NiO6: C, 32.75; H, 5.50; N, 7.64; S, 17.49. Found: C,
33.07; H, 5.10; N, 7.67; S, 17.43.
Materials. 1,8-Bis(2-methylthioethyl)-4,11-diazoniatricyclo-
[9.3.1.1]hexadecane Diiodide (2). Compound 2 was prepared by
a modified literature procedure.12 To an acetonitrile solution
(100 mL) of 1,4,8,11-tetraazatricyclo[9.3.1.1]hexadecane (1)
(4.00 g, 178 mmol) was added 2-iodoethyl methyl sulfide (∼80%
purity, 23.0 g, 91.1 mmol) rapidly. The solution was stirred at
room temperature for 48 h. The white precipitate was filtered,
washed with acetonitrile, and dried under vacuum to give 2
1
(8.72 g) in 78% yield. H NMR (DMSO-d6): δ 1.75 (m, 2H,
β-CH2), 2.19 (s, 6H, S−CH3), 2.36 (m, 4H), 2.64 (d, 2H, J =
14.4 Hz), 2.86−3.10 (m, 8H), 3.16 (t, 2H, J = 14.4 Hz), 3.33−
3.35 (m, 4H), 3.46−3.52 (m, 4H), 3.65−3.72 (m, 4H), 4.24 (t,
2H, N−CH2−N, J = 14.4 Hz), 5.22 (d, 2H, N−CH2−N, J = 8.9
13
Hz). C{1H} NMR (DMSO-d6): δ 15.31 (S−CH3), 19.65 (S−
CH2), 24.75 (β-CH2), 46.91 (N−CH2), 47.38 (N−CH2), 51.43
(α-CH2), 57.97 (α-CH2), 59.40 (α-CH2), 76.79 (N−CH2−N).
Anal. Calcd for C18H38N4S2I2: C, 34.40; H, 6.09; N, 8.91; S,
10.20. Found: C, 33.70; H, 5.75; N, 9.01; S, 9.82.
1,8-Bis(2-[D3]methythioethyl)-4,11-diazoniatricyclo[9.3.1.1]-
hexadecane Diiodide (2-d6). The deuterated compound 2-d6 was
prepared as described for 2, but using CD3SCH2CH2I prepared from
CD3SCH2CH2Cl accordingly.39
[Ni(dmmtc-d6)](OTf)2 (4-d6). Complex 4-d6 was prepared as
described for 4 from 3-d6. ESI-TOF-MS (CH3CN): m/z (%) = 220.2
([M]2+) (100), 589.1 ([M + OTf]+) (90).
[Ni(mtc)(Cl)](Cl) (5a) and [Ni(mtc)(OH2)2](Cl)2 (5b). To an ethanol
solution (20 mL) of mtc (1.40 g, 4.02 mmol) was added an aqueous
solution (30 mL) of NiCl2·6H2O (1.00 g, 4.21 mmol). This was stirred
at room temperature for 3 h. After the mixture was concentrated to
10 mL, a light purple precipitate was collected and washed with
ethanol to give 5b as a purple powder in 16% yield. The filtrate was
evaporated to dryness, and the residue was extracted with chloroform,
dried over MgSO4, and concentrated to give 5a as a light blue powder
in 63% yield. Single crystals of 5a and 5b suited for X-ray analysis
were obtained by cooling an CH3CN/Et2O solution for 5a and by
slow evaporation of a methanol solution for 5b. 5a, ESI-TOF-MS
(CH3CN): m/z = 441.1 ([M]+). UV−vis (CH3CN): λmax/nm (ε) =
379 (17), 608 (11). Anal. Calcd for C16H36N4S2NiCl2·C2H3N: C,
41.63; H, 7.57; N, 13.49; S, 12.35. Found: C, 41.67; H, 7.20; N, 13.39;
S, 12.80. 5b, ESI-TOF-MS (CH3OH): m/z = 441.1 ([M + Cl −
2(H2O)]+) Anal. Calcd for C16H40N4S2Cl2NiO2: C, 37.37; H, 7.84; N,
10.89; S, 12.47. Found: C, 36.91; H, 7.88; N, 10.74; S, 11.79.
RRRR-[Ni(mtc)](OTf)2 (6a). To an aqueous solution (10 mL) of
RRRR-[Ni(mtc)(Cl)](Cl) (5a) (480 mg, 1.00 mmol) was added sodium
trifluoromethanesulfonate (510 mg, 2.96 mmol), and the mixture was
stirred at room temperature for 1 h. The purple solution was extracted
with nitromethane, and the organic phases were collected, dried over
MgSO4, and concentrated to give 6a as a violet powder in 88% yield. The
crude product was purified by cooling a saturated THF/Et2O solution to
give violet crystals. ESI-TOF-MS (CH3CN): m/z (%) = 555.2 ([M +
OTf]+) (100), 203.1 ([M]2+) (30). UV−vis (CH3CN): λmax/nm (ε) =
352 (26), 554 (17). Anal. Calcd for C18H36N4S4F6NiO6: C, 30.65; H,
5.14; N, 7.94; S, 18.18. Found: C, 31.14; H, 5.27; N, 7.96; S, 17.68.
RRSS-[Ni(mtc)](OTf)2 (6b). Method A: Complex 6b was prepared as
described for 6a from 5b (250 mg, 0.486 mmol) and NaOTf (260 mg,
1.51 mmol) in 91% yield. Method B: To an ethanol solution (50 mL)
of nickel(II) triflate hexahydrate (464 mg, 1.00 mmol) was added an
ethanol solution (10 mL) of mtc (314 mg, 0.900 mmol), and the
orange solution was stirred overnight. Concentration of the solution
gave an orange crystalline solid as the first crop of 6b. The filtrate was
evaporated, and the resulting sticky solid was dissolved in THF and
after standing for several days gave an orange precipitate. After
filtration, the solid was washed with THF and EtOH/THF (1:10)
to afford 6b. The total yield of 6b was 65%. ESI-TOF-MS (CH3CN):
m/z (%) = 555.2 ([M + OTf]+) (100), 203.1 ([M]2+) (30). UV−vis
(CH3CN): λmax/nm (ε) = 285 (2100), 479 (21). Anal. Calcd for
C18H36N4S4F6NiO6: C, 30.65; H, 5.14; N, 7.94; S, 18.18. Found: C,
31.05; H, 5.28; N, 7.97; S, 18.29.
1,8-Bis(2-methylthioethyl)-4,11-dimethyl-1,4,8,11-tetraazatetra-
decane (dmmtc). To a suspension of 2 (2.00 g, 3.18 mmol) in ethanol
(50 mL) was added sodium tetrahydroborate (1.20 g, 31.7 mmol) in
small portions, and the solution was refluxed for 3 h. After the mixture
was cooled to room temperature, aqueous HCl (ca. 3 M, 30 mL) was
added slowly, and most of the ethanol was removed by evaporation.
The aqueous residue was diluted by water (100 mL) and adjusted to
pH > 12 by addition of aqueous NaOH (conc.). The mixture was
extracted with chloroform (3 × 100 mL), and the organic phases were
collected and dried over MgSO4, and evaporated to dryness to give
dmmtc as a sticky white solid in 80% yield, which is sufficiently
pure for further use. Recrystallization from hexane gave the analytically
pure product. 1H NMR (C6D6): δ 1.52 (quint, J = 6.2 Hz, 4H, β-CH2),
1.87 (s, 6H, S−CH3), 2.11 (s, 6H, N−CH3), 2.38 (t, 8H, J = 6.2 Hz,
N−CH2), 2.46−2.50 (m, 8H, N−CH2+S−CH2) 2.54 (t, J = 6.9 Hz,
4H, S−CH2), 2.62−2.63 (m, 4H, N−CH2). 13C{1H} NMR (C6D6):
δ 15.52 (S−CH3), 25.47 (β-CH2), 32.40 (S−CH2), 43.31 (N−CH3),
51.50 (N−CH2), 52.07 (α-CH2), 54.84 (α-CH2), 55.21 (α-CH2),
55.65 (α-CH2). ESI-TOF-MS (MeOH): m/z = 377.1 ([M + H]+).
Anal. Calcd for C18H40N4S2: C, 57.40; H, 10.70; N, 14.87; S, 17.03.
Found: C, 57.37; H, 10.50; N, 14.91; S, 16.56.
1,8-Bis(2-[D3]methylthioethyl)-4,11-dimethyl-1,4,8,11-tetraazate-
tradecane (dmmtc-d6). Using 2-d6, the deuterated ligand dmmtc-d6
was prepared as described for dmmtc. ESI-TOF-MS (MeOH): m/z =
383.1 ([M + H]+).
1,8-Bis(2-methylthioethyl)-1,4,8,11-tetraazatetradecane (mtc).
Compound 2 (2.00 g, 3.18 mmol) was dissolved in an aqueous
NaOH solution (3 M, 100 mL), and the solution was refluxed for 1 h.
The product was extracted with chloroform (3 × 100 mL), and the
organic phases were collected, dried over MgSO4, and evaporated to
dryness to give mtc as a sticky white solid in 83% yield, which is
sufficiently pure for further use. Recrystallization from hexane gave
1
the analytically pure product. H NMR (C6D6): δ 1.58 (quint, 4H,
β-CH2), 1.89 (s, 6H, S−CH3), 2.29 (t, 4H, S−CH2), 2.37 (m, 4H,
N−CH2), 2.51−2.56 (m, 8H, N−CH2), 2.61−2.68 (m, 8H, N−CH2).
13C{1H} NMR (C6D6): δ 15.59 (S−-CH3), 26.94 (β-CH2), 31.02
(S−CH2), 48.50 (N−CH2), 50.82 (N−CH2), 53.30 (α-CH2), 53.67
(α-CH2), 55.16 (α-CH2). ESI-TOF-MS (MeOH): m/z = 349.1
([M + H]+). Anal. Calcd for C16H36N4S2: C, 55.12; H, 10.41; N,
16.07; S, 18.40. Found: C, 55.51; H, 10.12; N, 15.76; S, 18.87.
[Ni(dmmtc)(Cl)](Cl) (3). To an ethanol solution (20 mL) of dmmtc
(1.00 g, 2.65 mmol) was added an aqueous solution (30 mL) of
NiCl2·6H2O (678 mg, 2.86 mmol), which was stirred at room
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RRRR-[Ni(mtc)](BArF ) (7a). To a suspension of 6a (210 mg, 0.298
temperature, both the solution and the headspace-gas of the Schlenk
tube were analyzed by GC, which shows the yields of methane, ethane,
and ethylene as 6%, 23%, and 3%, respectively, as a sum of those in
the solution and gas phases. The solution was filtered to give the
crystalline solids 4 and 4′. The blue green crystals of 4 and light blue
crystals of 4′ were isolated in ∼2% yield by selecting the crystals under
a microscope. The filtrate was evaporated and washed by THF/Et2O
to remove the remaining small amount of 8. The residue was dissolved
in THF and concentrated to give brownish green crystals of 12 in
19% yield. 12, ESI-TOF-MS (THF): m/z = 419.2 ([M]+). UV−vis
(THF): λmax/nm (ε) = 377 (1600), 712 (21). Anal. Calcd for
C18H37N4S3F3NiO3: C, 37.97; H, 6.55; N, 9.84; S, 16.89. Found: C,
38.13; H, 6.62; N, 9.81; S, 16.49. 4′: ESI-TOF-MS (CH3CN), m/z =
217.1 ([M]2+). Anal. Calcd for C20H40N4S4F6NiO6: C, 32.75; H, 5.50;
N, 7.64; S, 17.49. Found: C, 32.83; H, 5.60; N, 7.60; S, 17.08.
Reaction of 8 with DmpSH. In a 200 mL Schlenk tube with a
Young valve were dissolved complex 8 (120 mg, 0.205 mmol) and
DmpSH (345 mg, 1.00 mmol) in THF (5 mL), and then they were
stirred at 75 °C for 4 d. After being cooled to room temperature, both
the solution and the headspace-gas of the Schlenk tube were analyzed
by GC, which shows the yields of methane and ethylene as 31% and
3%, respectively, as a sum of those in the solution and gas phases. The
solution was filtered to give a small amount of 4 and 4′ as crystalline
solids. The filtrate was evaporated, and the residue was first extracted
with ether, evaporated, and separated by HPLC to give DmpSCH3 in
12% yield. The ether insoluble residue was extracted with THF and
concentrated to give 12 as yellow green crystals in 21% yield.
Synthesis of DmpSCH3. To a THF solution (50 mL) of DmpSH
(345 mg, 1.00 mmol) was added n-butyllithium (1.6 M in hexane,
0.66 mL, 1.05 mmol) at −20 °C, and the mixture was stirred at room
temperature for 30 min. Iodomethane (0.10 mL, 1.6 mmol) was added
to the solution and stirred for 1 h. After evaporation, the residue
was dissolved in CH2Cl2. The organic layer was washed with water
(100 mL), dried over MgSO4, and evaporated to dryness to give
DmpSCH3 as a white powder in 80% yield. 1H NMR (CDCl3): δ 1.71
(s, 3H, SCH3), 2.04 (s, 12H, o-CH3 of Mes), 2.33 (s, 6H, p-CH3 of
Mes), 6.95 (s, 4H, m-CH of Mes), 7.06 (d, J = 7.6 Hz, 2H, m-CH of
Dmp), 7.36 (t, J = 7.6 Hz, 1H, p-CH of Dmp). Anal. Calcd for
C25H28S: C, 83.28; H, 7.83; S, 8.89. Found: C, 83.11; H, 7.42; S, 8.30.
Reaction of 11. [Ni(mtc)]OTf (11) was prepared by treatment of
6a (140 mg, 0.198 mmol) with 0.3% Na/Hg (10 g) in THF (3 mL)
at −40 °C followed by filtration. The THF solution of 11 was
warmed to room temperature and stirred at for 6 h. Both the solu-
tion and the headspace-gas of the Schlenk tube were analyzed by
GC, which shows the yields of methane and ethylene were 30% and
2%, respectively, as a sum of those in the solution and gas phases.
The solution was evaporated and extracted with fluorobenzene.
After slow addition of ether, the solution stood for a day to give deep
blue crystals of 18 (5% yield) with a small amount of green crystals
of 17. After the solution was evaporated to dryness, the solid
was dissolved in fluorobenzene (1 mL). Vapor diffusion of pentane
afforded 17 as green crystals in 9% yield. 17, ESI-TOF-MS (THF):
m/z = 391.0 ([M]+). UV−vis (THF): λmax/nm (ε) = 405 (160).
Anal. Calcd for C16H33N4S3F3NiO3·C3H2.5F0.: C, 38.72; H, 6.07; N,
9.51; S, 16.32. Found: C, 38.82; H, 6.53; N, 9.63; S, 15.96. 18,
ESI-TOF-MS (THF): m/z = 405.1 ([M]+). UV−vis (THF):
λmax/nm (ε) = 357 (1200). Anal. Calcd for C17H35N4S3F3NiO3·C3H2.5F0.5:
C, 39.81; H, 6.26; N, 9.28; S, 15.94. Found: C, 39.72; H, 6.03; N, 9.66;
S, 15.78.
4 2
mmol) in fluorobenzene (10 mL) was added NaBArF4 (536 mg, 0.605
mmol), and the mixture was stirred at room temperature for 1 h. After
the precipitates were filtered off, the filtrate was evaporated and
washed with Et2O/hexane (1:1) to give 7a as a light purple powder in
90% yield. UV−vis (THF): λmax/nm (ε) = 550 (22). Anal. Calcd for
C80H60N4S2B2F48Ni: C, 45.03; H, 2.83; N, 2.63; S, 3.01. Found: C,
45.07; H, 2.91; N, 2.63; S, 2.71.
RRSS-[Ni(mtc)](BArF ) (7b). To a solution of 6b (315 mg, 0.447
4 2
mmol) in MeOH/CH3CN (a 2:1 mixture, 10 mL) was added NaBArF
4
(720 mg, 0.812 mmol), and this was stirred at room temperature for
1 h. The mixture was evaporated to dryness and washed with water to
give 7b as a light purple powder in 74% yield. Single crystals suitable
for X-ray analysis were grown by cooling a saturated THF/Et2O
solution. UV−vis (THF): λmax/nm (ε) = 476 (15). Anal. Calcd for
C80H60N4S2B2F48Ni: C, 45.03; H, 2.83; N, 2.63; S, 3.01. Found: C,
45.41; H, 2.86; N, 3.01; S, 2.70.
[Ni(dmmtc)](OTf) (8). Complex 4 (1.00 g, 1.36 mmol) was dissolved
in a THF/CH3CN (a 5:1 mixture, 10 mL), and 0.3% Na/Hg (50 g)
was added. After the mixture was stirred for 20 min, ether was added to
the green solution to afford a light blue powder. The product was
dissolved into fluorobenzene, and the insoluble NaOTf was filtered off.
Slow addition of ether to the filtrate afforded 8 as light blue crystals
in 72% yield. ESI-TOF-MS (THF): m/z = 434.2 ([M]+). UV−vis
(THF): λmax/nm (ε) = 333 (3600), 717 (20). Anal. Calcd for
C19H40N4S3F3NiO3: C, 39.05; H, 6.90; N, 9.59; S, 16.46. Found: C,
39.42; H, 6.71; N, 9.65; S, 16.04. EPR spectrum was recorded at 8 K for
a finely grinded crystalline light blue powder.
[Ni(dmmtc-d6)](OTf) (8-d6). Complex 8-d6 was prepared from 4-d6
as described for 8. ESI-TOF-MS (THF): m/z = 440.2 ([M]+).
[Ni(dmmtc)](BArF ) (9). To a suspension of 8 (116 mg, 0.198 mmol)
4
in ether (10 mL) was added NaBArF4 (178 mg, 0.201 mmol), and this
was stirred at room temperature for 1 h. After filtration, vapor diffusion
of pentane into the filtrate at −40 °C resulted in the formation of
yellow crystals of 9 in 70% yield. ESI-TOF-MS (THF): m/z = 434.2
([M]+). Anal. Calcd for C50H52N4S2BF24Ni: C, 46.25; H, 4.04; N, 4.31;
S, 4.94. Found: C, 46.56; H, 4.06; N, 4.29; S, 4.45. EPR spectrum was
recorded at 8 K for a finely grinded crystalline yellow powder.
RSRS-[Ni(mtc)](BArF ) (10a). During this procedure, the temper-
4
ature was kept below −40 °C. Complex 7b (200 mg, 0.0937 mmol)
was dissolved in THF/ether (a 1:10 mixture, 2 mL), and 0.3% Na/Hg
(10 g) was added. After being stirred for 20 min, the mixture was
filtered, and pentane was added slowly to the filtrate to give 10a as a
green precipitate. Vapor diffusion of pentane into a solution of 10a in
THF/Et2O (a 1:20 mixture, 1 mL) afforded 10a as yellow green
crystals in 51% yield. 10a; UV−vis (THF): λmax/nm (ε) = 335 (3100).
Anal. Calcd for C48H48N4S2BF24Ni: C, 45.38; H, 3.81; N, 4.41; S, 5.05.
Found: C, 45.26; H, 3.59; N, 4.11; S, 5.05. EPR spectrum was
recorded at 190 K for a finely grinded crystalline green powder.
A small amount of 10b was obtained as yellow crystals together with
10a when pentane was quickly added to the filtrate after the same
reduction, and the structure was characterized by X-ray analysis.
GC Analysis of the Gaseous Products. The various Ni(I)
complexes were allowed to react in a Schlenk tube with a Young valve,
and the gaseous products were analyzed with a Shimadzu GC-8A gas
chromatograph equipped with TCD detector and integrator Shimadzu
CR-6A, using a 2 m long packed column SHINCARBON-ST.
Measurement conditions: CH4; injector temperature = 120 °C,
detector temperature = 120 °C, column temperature = 60 °C, flow
50 mL/min. C2H4 and C2H6; injector temperature = 180 °C, detector
temperature = 180 °C, column temperature = 150 °C, flow 50 mL/
min. The products were identified by comparison of their retention
times with those of authentic standard compounds. Their yields were
determined from the areas of the corresponding eluted peaks using
calibration curves. The deuterated products, CD3H and C2D6, were
determined by GC−MS Shimadzu GCMS-QP2010 with a 30 m long
Rt-Qplot column under isothermal conditions at 40 °C.
X-ray Structural Determination. Crystal data and refinement param-
eters for the structurally characterized complexes are summarized in
Table 8. Single crystals were coated with oil (CryoLoop, Immersion
Oil, Type B or Paraton, Hampton Research Corp.) and mounted on
loops. Diffraction data were collected on a Rigaku AFC-8 instrument
equipped with a Mercury CCD detector (for 5a, 6a, 6b, 8, 10b, 12,
17) or with a Saturn 70 CCD detector (for 4, 18, RSRS-
[Ni(tmc)]OTf) or Rigaku AFC-10 instrument equipped with a Saturn
70 CCD detector (for 3, 4′, 5b, 7b, 9, 10a). The measurements were
made by using graphite-monochromized Mo Kα radiation (λ = 0.71070 Å)
under a cold nitrogen stream. The frame data were integrated and
Reaction of 8. In a 200 mL Schlenk tube with a Young valve was
dissolved the Ni(I) complex 8 (120 mg, 0.206 mmol) in THF (5 mL),
and then it was stirred at 75 °C for 4 d. After being cooled to room
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corrected for absorption with the Rigaku/MSC CrystalClear program
package. The structures were solved with use of direct methods (SIR-
92 or SIR-97) and standard difference map techniques and were
refined by full-matrix least-squares procedures on F2 using the Rigaku/
MSC CrystalStructure package. Anisotropic refinement was applied to
all non-hydrogen atoms except for the disordered atoms that include a
dmmtc ligand for 8, a CH3SCH2CH2 chain for 10a, a triflate anion for
697−716. (f) Pfaltz, A.; Jaun, B.; Fassler, A.; Eschenmoser, A.;
̈
Jaenchen, R.; Gilles, H. H.; Diekert, G.; Thauer, R. K. Helv. Chim. Acta
1982, 65, 828−865.
(4) (a) Rospert, S.; Bocher, R.; Albracht, S. P. J.; Thauer, R. K. FEBS
̈
Lett. 1991, 291, 371−375. (b) Goubeaud, M.; Schreiner, G.; Thauer,
R. K. Eur. J. Biochem. 1997, 243, 110−114. (c) Telser, J.; Davydov, R.;
Horng, Y.-C.; Ragsdale, S. W.; Hoffman, B. M. J. Am. Chem. Soc. 2001,
123, 5853−5860. (d) Duin, E. C.; Piskorski, R.; Mahlert, F.; Clay,
M. D.; Goenrich, M.; Thauer, R. K.; Jaun, B. J. Biol. Inorg. Chem. 2004,
9, 563−574. (e) Mahlert, F.; Bauer, C.; Jaun, B.; Thauer, R. K.; Duin,
E. C. J. Biol. Inorg. Chem. 2002, 7, 500−513.
6a, trifluoromethyl groups of BArF −, and the crystalline solvent THF
4
for 10a and fluorobenzene for 17 and 18, in which the ratios were
refined freely while the total occupancy of the components was con-
strained to unity (see the Supporting Information). All of the
hydrogen atoms were placed at calculated positions. Additional
crystallographic data are given in the Supporting Information.
(5) (a) Zilbermann, I.; Golub, G.; Cohen, H.; Meyerstein, D. Inorg.
Chim. Acta 1994, 227, 1−3. (b) Signor, L.; Knuppe, C.; Hug, R.;
Schweizer, B.; Pfaltz, A.; Jaun, B. Chem.-Eur. J. 2000, 6, 3508−3516.
(c) Ram, M. S.; Riordan, C. G.; Ostrander, R.; Rheingold, A. L. Inorg.
Chem. 1995, 34, 5884−5892.
(6) (a) Pelmenschikov, V.; Blomberg, M. R. A.; Siegbahn, Per E. M.;
Crabtree, R. H. J. Am. Chem. Soc. 2002, 124, 4039−4049.
(b) Pelmenschikov, V.; Siegbahn, Per E. M. J. Biol. Inorg. Chem.
2003, 8, 653−662.
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(b) Jaun, B.; Pfaltz, A. J. Chem. Soc., Chem. Commun. 1986, 293−296.
(8) A five-coordinated Ni(I) azamacrocyclic complex was reported,
see: Suh, M. P.; Oh, K. Y.; Lee, J. W.; Bae, Y. Y. J. Am. Chem. Soc.
1996, 118, 777−783.
(9) The Ni(I) azamacrocyclic complexes have been reported, see:
(a) Szalda, D. J.; Fujita, E.; Sanzenbacher, R.; Paulus, H.; Elias, H.
Inorg. Chem. 1994, 33, 5855−5863. (b) Suh, M. P.; Kim, H. K.; Kim,
M. J.; Oh, K. Y. Inorg. Chem. 1992, 3620−3625. (c) Furenlid, L. R.;
Renner, M. W.; Szalda, D. J.; Fujita, E. J. Am. Chem. Soc. 1991, 113,
883−892.
ASSOCIATED CONTENT
* Supporting Information
■
S
Crystallographic data in CIF format for 3, 4, 4′, 5a,b, 6a,b, 7b,
8, 9, 10a,b, 12, 17, 18, RSRS-[Ni(tmc)]OTf, synthesis of RSRS-
[Ni(tmc)](OTf) and [Ni(tmc)](BArF ), molecular structures
4
and metric parameters of 3, RSRS-[Ni(tmc)](OTf), and 4′, CV
data for 8, and EPR spectrum of [Ni(tmc)](BArF ). This
4
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: i45100a@nucc.cc.nagoya-u.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(10) (a) Nishigaki, J.; Matsumoto, T.; Tatsumi, K. Eur. J. Inorg. Chem.
2010, 5011−5017. (b) Nishigaki, J.; Matsumoto, T.; Tatsumi, K. Inorg.
Chem. 2012, 51, 3690−3697.
■
This research was financially supported by Grant-in-Aids for
Scientific Research (nos. 18GS0207 and 23000007) from the
Ministry of Education, Culture, Sports, Science, and Technol-
ogy, Japan. We thank Prof. Tatsuhisa Kato (Kyoto University)
for fruitful discussions on EPR spectra. We are grateful to Prof.
Roger E. Cramer for discussions and careful reading of the
manuscript.
(11) Previously, Kaden and co-workers synthesized the nickel(II)
complexes having similar cyclam derivatives with appended CH3−S
group and investigated their redox properties by CV, see: (a) Schmid,
C. L.; Kempf, C.; Taubert, A.; Neuburger, M.; Zehnder, M.; Kaden, T. A.
Helv. Chim. Acta 1996, 79, 1011−1020. (b) Schmid, C.; Neuburger, M.;
Zehnder, M.; Kaden, T. A. Helv. Chim. Acta 1997, 80, 241−252.
(12) Royal, G.; Dahaoui-Gindrey, V.; Dahaoui, S.; Tabart, A.;
Guilard, R.; Pullumbi, P.; Lecomte, C. Eur. J. Org. Chem. 1998, 1971−
1975.
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1
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