Deprotonation/protonation-driven change of the σ-donor ability of a sulfur atom in iron(ii) complexes with a thioamide SNS pincer type ligand

Article abstract of DOI:10.1039/c4dt00524d

A new iron complex with a thioamide SNS pincer type ligand, [FeBr ₂(κ³-H₂L^DPM)] (κ³-H₂L^DPM = 2,6-bis(N-2,6- bis(diphenylmethyl)-4-isopropylphenylthioamide)pyridine), was synthesized. This complex reacts with NaH in THF to yield a unique Fe(ii) complex with two THF molecules, [Fe(THF)₂(κ³-L^DPM)] (κ³-L^DPM = 2,6-bis(N-2,6-bis(diphenylmethyl)-4- isopropylphenyliminothiolate)pyridine). The THF molecules of [Fe(THF) ₂(κ³-L^DPM)] can be substituted with CO and CN-xylyl to give [Fe(CO)₃(κ³-L^DPM)] and [Fe(CN-xylyl)₃(κ³-L^DPM)], respectively. The complex [Fe(CN-xylyl)₃(κ³-L ^DPM)] reacts with HBF₄ to produce [Fe(CN-xylyl) ₃(κ³-H₂L^DPM)]²⁺ with protonated thioamide units. The differences of the IR spectra before and after protonation indicate that the major binding mode of CN-xylyl to iron(ii) changes from π-back donation from metal to isocyanide to σ-donation from isocyanide to iron(ii). This indicates that the σ-donor ability of the thioamide sulfur atom is tuned by deprotonation/protonation of thioamide.

Full text of DOI:10.1039/c4dt00524d

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Deprotonation/protonation-driven change of the
σ-donor ability of a sulfur atom in iron(II)
complexes with a thioamide SNS pincer type
ligand†
Cite this: Dalton Trans., 2014, 43,
9732
Tatsuya Suzuki,a Yuji Kajitab and Hideki Masuda*a
A new iron complex with a thioamide SNS pincer type ligand, [FeBr2(κ3-H2LDPM)] (κ3-H2LDPM = 2,6-bis(N-
2,6-bis(diphenylmethyl)-4-isopropylphenylthioamide)pyridine), was synthesized. This complex reacts with
NaH in THF to yield a unique Fe(II) complex with two THF molecules, [Fe(THF)2(κ3-LDPM)] (κ3-LDPM = 2,6-
bis(N-2,6-bis(diphenylmethyl)-4-isopropylphenyliminothiolate)pyridine). The THF molecules of [Fe-
(THF)2(κ3-LDPM)] can be substituted with CO and CN-xylyl to give [Fe(CO)3(κ3-LDPM)] and [Fe(CN-
xylyl)3(κ3-LDPM)], respectively. The complex [Fe(CN-xylyl)3(κ3-LDPM)] reacts with HBF4 to produce [Fe(CN-
xylyl)3(κ3-H2LDPM)]2+ with protonated thioamide units. The differences of the IR spectra before and after
protonation indicate that the major binding mode of CN-xylyl to iron(II) changes from π-back donation
from metal to isocyanide to σ-donation from isocyanide to iron(II). This indicates that the σ-donor ability
of the thioamide sulfur atom is tuned by deprotonation/protonation of thioamide.
Received 19th February 2014,
Accepted 23rd April 2014
DOI: 10.1039/c4dt00524d
www.rsc.org/dalton
Introduction
In recent years, the coordination chemistry of “pincer-type” tri-
dentate ligands with metal ions has attracted significant atten-
tion from investigators in the fields of catalytic chemistry and
materials science.1 In particular, the iron and cobalt com-
plexes with bis-iminopyridine ligands (PDI) have been shown
to have unique properties including elevated levels of catalytic
activity for reactions such as olefin polymerization and dinitro-
gen activation, among others.2 The thioamide pincer ligand
has also been investigated. For example, both Bowman-James
and co-workers and Kanbara et al. reported that Pd and Pt
complexes with thioamide-based pincer ligands, which form a
square planar structure, respectively, exhibit catalytic activities
and photoluminescent properties.3 The thioamide group
undergoes tautomerization between its amino-thione and
imino-thiol forms, as shown in Scheme 1, and exhibits a
stronger Brønsted acidity than the corresponding amide
group. Consequently, when secondary thioamides are used as
Scheme 1 Equilibrium of the secondary thioamide group.
ligands, they are deprotonated (in the thionate form). This pro-
vides the ability to tune the electron density of the metal
center by deprotonation/protonation. Kanbara et al. also
reported that Ru(II) complexes with a thioamide pincer ligand
can regulate the electronic properties of the metal center by
deprotonation/protonation reactions of the NH groups on the
secondary thioamide units.4 However, there have been no
reports of an iron complex with a thioamide pincer ligand. In
the case of other metal complexes, the Co complex with the
thioamide pincer ligand formed a bischelate compound with
an octahedral structure.5 In addition, the iron complexes with
thiourea tridentate ligands and an amide pincer ligand, which
are similar to the thioamide pincer ligand, formed bischelate
compounds.6,7 These bischelate compounds are not expected
to have catalytic activity, because they have a tight structure
and are difficult to exchange with other substituent ligands.
aDepartment of Frontier Materials, Graduate School of Engineering, Nagoya Institute
of Technology, Gokiso, Showa, Nagoya 466-8555, Japan
bDepartment of Applied Chemistry, Aichi Institute of Technology, 1247 Yachigusa,
Yakusa-cho, Toyota 470-0392, Japan. E-mail: masuda.hideki@nitech.ac.jp;
Fax: +81-52-735-5209; Tel: +81-52-735-5228
†Electronic supplementary information (ESI) available. CCDC 984342 and
984346–984349. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4dt00524d
9732 | Dalton Trans., 2014, 43, 9732–9739
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We therefore designed and prepared a unique monochelate
iron(II) complex with the thioamide pincer ligand. Here, we
report the synthesis, structure, spectroscopic properties and an
investigation of the reactivity of the new iron(II) complex with
the thioamide pincer ligand. Additionally, as an example, we
examined the coordination behavior of the isocyanide mole-
cule in deprotonation/protonation reactions of the NH groups
on the thioamide units.
Results and discussion
Preparations of the H2LDPM ligand and its iron(II) complex
Fig. 2 ORTEP view of the molecular structure of [Fe(κ3-H2LDPM)Br2]
with ellipsoids at the 50% probability level. The hydrogen atoms have
been omitted for clarity except for H(2) and H(3). Selected bond lengths
(Å) and angles (°): Br(1)–Fe(1) 2.4272(12), Br(2)–Fe(1) 2.4642(12), Fe(1)–
S(1) 2.459(2), Fe(1)–S(2) 2.458(2), Fe(1)–N(1) 2.164(4), S(1)–C(1) 1.674(5),
S(2)–C(2) 1.667(5), N(2)–C(1) 1.326(7), N(3)–C(2) 1.323(7), Br(1)–Fe(1)–
Br(2) 123.19(5), Br(1)–Fe(1)–N(1) 112.16(10), Br(2)–Fe(1)–N(1) 124.61(10),
S(1)–Fe(1)–S(2) 160.93(6).
The ligand 2,6-bis(N-2,6-diphenylmethyl-4-isopropylphenyl-
thioamide)pyridine (H2LDPM) was prepared according to a pre-
viously published method with slight modifications, where
H2LDPM is an SNS pincer-type ligand with two bulky N-2,6-
diphenylmethyl-4-isopropylphenylthioamide groups at the
2- and 6-positions of the pyridine unit. This ligand is expected
to prevent the formation of a bischelate metal complex.
The ligand was recrystallized from methanol to obtain a single
crystal suitable for X-ray analysis. The molecular structure of
H2LDPM is shown in Fig. 1. The CvS bond lengths for H2LDPM
are 1.648(2) and 1.651(2) Å and the C–N bond lengths are
1.340(3) and 1.339(3) Å, indicating that this ligand has a thio-
amide type structure (HN–(CvS)–) with a proton on each nitro-
gen atom.
(τ = 0.60);8 the bond angles Br(1)–Fe–Br(2) 123.19(5), Br(1)–Fe–
N(1) 112.16(10) and Br(2)–Fe–N(1) 124.61(10) reveal that Br(1)–
Br(2)–N(1) forms a trigonal plane and the angle S(1)–Fe–S(2)
160.93(6)° indicates that the arrangement of atoms S(1)–Fe–
S(2) is essentially linear. The Fe–N(1) bond length for
[FeBr2(κ3-H2LDPM)] is 2.164(4) Å, and the Fe–S(1) and Fe–S(2)
bond lengths are 2.459(2) and 2.458(2) Å respectively. These
Fe–N and Fe–S bond lengths are typical of those of a high-spin
iron(II) complex as reported previously.9
This geometry is similar to that of the bis(iminopyridine)
iron(II) complex [FeBr2(PDI)] reported previously.10 As the
series of the bis(imino)pyridine ligand, the iron complexes
with PDI containing thioether have previously been reported,
which is coordinated with N,N,N atoms to iron ions.11
However, we could isolate only the iron complex coordinated
with S,N,S atoms. This is the first report of a monochelate
iron(II) complex with a thioamide pincer-type ligand, although
there have been many reports on syntheses of iron(II) and
cobalt(II) complexes with thiourea coordinated ligands in the
mer configuration (each of these is an octahedral bischelate
metal complex).5,7
Treatment of H2LDPM with FeBr2 in Et2O under an argon
atmosphere at room temperature produced a dark green solu-
tion after 24 h. A single crystal of [FeBr2(κ3-H2LDPM)] was
obtained by slow evaporation from dimethoxyethane solution.
The crystal structure of [FeBr2(κ3-H2LDPM)] is shown in Fig. 2.
The coordination geometry around the metal center for
[FeBr2(κ3-H2LDPM)] is a distorted trigonal bipyramidal structure
The CvS bond lengths of thioamide units for [FeBr2(κ3-
H2LDPM)] are 1.649(4) and 1.667(5) Å and the C–N bond
lengths are 1.326(7) and 1.323(7) Å, respectively. The former
bond lengths are slightly longer than those of the metal-free
ligand (1.648(2), 1.651(2) Å), and the latter bond lengths are
shorter than those of the metal-free ligand (1.340(3), 1.339(3)
Å). These findings indicate that the ligand is in the thioamide
form (HN–(CvS)–) with a proton on each nitrogen atom.
The 1H-NMR spectrum of [FeBr2(κ3-H2LDPM)] in THF-d8
exhibited broadened peaks, indicating that the iron(II) complex
is in the high-spin form. The [FeBr2(κ3-H2LDPM)] complex
decomposed in MeOH, resulting in a yellow precipitate of
Fig. 1 ORTEP view of the molecular structure of H2LDPM with ellipsoids
at the 50% probability level. The hydrogen atoms and aromatic substitu-
ents on the N-phenyl unit have been omitted for clarity except for H(2)
and H(3). Selected bond lengths (Å) and angles (°): C(1)–N(2) 1.340(3),
C(2)–N(3) 1.339(3), C(1)–S(1) 1.648(2), C(2)–S(2) 1.651(2), S(1)–C(1)–N(2)
123.93(16), S(1)–C(1)–C(3) 121.41(14), N(2)–C(1)–C(3) 114.67(17), S(2)–
C(2)–N(3) 124.74(14), S(2)–C(2)–C(7) 121.85(14), N(3)–C(2)–C(7)
113.41(16).
H2LDPM
.
The solid-state magnetic moment of [FeBr2(κ3-
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Scheme 2 Synthesis of compounds [FeBr2(κ3-H2LDPM)] and [Fe(THF)2(κ3-LDPM)].
H2LDPM)] was determined by the Evans method (using a mag- [FeBr2(κ3-H2LDPM)] complex described above. These obser-
netic susceptibility balance) to be 5.4μB at 23 °C, which is con- vations are explained in terms of a structural change involving
sistent with the presence of four unpaired electrons. This deprotonation of the NH groups on the secondary thioamide
finding has also been reported for the bis(imino)pyridine unit to form imino-thiolate. The deprotonation of NH groups
iron(II) complex, [FeBr2(PDI)].12
was confirmed in a comparison of IR spectra of [FeBr2(κ3-
H2LDPM)] and [Fe(THF)2(κ3-LDPM)]. The NH stretching vibration
bands of [FeBr2(κ3-H2LDPM)] were found at 3327 cm−1 and at
3157 cm−1, while the analogous bands in [Fe(THF)2(κ3-LDPM)]
disappeared upon reaction with NaH. The formation of the
imino-thiolate is indicated by the metal-to-ligand bond
lengths. The lengths of Fe–N(1) (2.1593(19)), Fe–S(1) (2.3600(8)),
and Fe–S(2) (2.3621(8)) bonds for [Fe(THF)2(κ3-LDPM)] are
each shortened upon deprotonation. This indicates that the
σ-donation of the imino-thiolate ligand is strengthened relative
to the bond strength of the protonated thioamide form. This
Preparation of [Fe(THF)2(κ3-LDPM)]
Stirring of [FeBr2(κ3-H2LDPM)] with excess NaH in THF solution
under a dinitrogen atmosphere produces the bis-THF iron(II)
compound [Fe(THF)2(κ3-LDPM)] (Scheme 2), which is also air-
and moisture-sensitive. A single crystal of [Fe(THF)2(κ3-LDPM)]
suitable for X-ray analysis was obtained from THF solution.
The crystal structure of [Fe(THF)2(κ3-LDPM)] is shown in Fig. 3.
The coordination geometry of [Fe(THF)2(κ3-LDPM)] is square
pyramidal (τ = 0.16) with one pyridine nitrogen atom, N(1), two
sulfur atoms, S(1) and S(2), and one THF oxygen atom, O(1), in
the equatorial plane and one THF oxygen atom, O(2), in the
axial position. This Fe(II) complex with THF molecules is avail-
able for substitution reaction.13
1
complex is paramagnetic, as determined by its H-NMR spec-
trum in THF-d8. The solid-state magnetic moment of
[Fe(THF)2(κ3-LDPM)] was determined by magnetic susceptibility
measurements to be 5.2μB at 23 °C, which is consistent with
that of an iron(II) complex with four unpaired electrons.
Such a combination does not typically form a stable
complex.14 The C–S bond lengths of [Fe(THF)2(κ3-LDPM)] are
1.744(3) and 1.758(3) Å and the C–N bond lengths are 1.286(4)
and 1.284(3) Å. The C–S bond lengths are longer and the CvN
bond lengths are shorter in comparison with those of the
Reaction of [Fe(THF)2(κ3-LDPM)] with CO
The red precipitate of [Fe(THF)2(κ3-LDPM)] reacted with CO in
toluene, and the resulting orange solution was stored at
−30 °C (Scheme 3). The resulting complex [Fe(CO)3(κ3-LDPM)]
was in the form of a yellow crystalline solid (precipitated from
toluene solution), and was characterized by 1H-NMR and
13C-NMR spectrometry, infrared spectroscopy and elemental
analyses. The complex [Fe(CO)3(κ3-LDPM)] is diamagnetic, and
1
its H-NMR spectrum displays resonances in the diamagnetic
range. In 13C-NMR, the carbon atoms of the coordinated CO
ligands were clearly observed at 206.9 and 200.3 ppm with an
integration ratio of 1 : 2 and the thioamide carbon atoms were
observed at 169.5 ppm. Fortunately, we obtained a single
crystal of the complex [Fe(CO)3(κ3-LDPM)], and the X-ray analy-
sis revealed that the iron ion is engaged by the thioamide
pincer-type ligand and three carbonyl ligands (Fig. 4). The
coordination geometry is distorted octahedral with the SNS
Fig. 3 ORTEP view of the molecular structure of [Fe(THF)2(κ3-LDPM)] pincer type ligand (Fe–N(1) 1.980(2), Fe–S(1) 2.2676(11), Fe–
with ellipsoids at the 50% probability level. The hydrogen atoms have
been omitted for clarity. Selected bond lengths (Å) and angles (°): Fe(1)–
S(1) 2.3600(8), Fe(1)–S(2) 2.3621(8), Fe(1)–O(1) 2.0639(19), Fe(1)–O(2)
2.1418(18), Fe(1)–N(1) 2.1593(19), S(1)–C(1) 1.744(3), S(2)–C(2) 1.758(3),
S(2) 2.2638(11)), in which one coordinated CO molecule (Fe–
C(4) 1.791(3)) is positioned within the pincer plane, and two
CO molecules (Fe–C(3) 1.861(3) and Fe–C(5) 1.825(3)) are
located in trans positions to each other. The Fe–S(1) and Fe–
S(2) bonds are shorter than those of [Fe(CN-xylyl)3(κ3-LDPM)] as
N(2)–C(1) 1.286(4), N(3)–C(2) 1.284(3), S(1)–Fe(1)–S(2) 151.30(3), O(2)–
Fe(1)–N(1) 162.05(8).
9734 | Dalton Trans., 2014, 43, 9732–9739
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tive to the two carbonyl carbon atoms at the trans positions,
although it was not significantly affected by their C–O bond
lengths. In the IR spectrum of [Fe(CO)3(κ3-LDPM)], two
vibrations were detected as the coordinated CO stretching
bands. From the above findings, the 2044 cm−1 band was ten-
tatively assigned as that of the two carbonyls located at the
trans positions and the 1993 cm−1 as a carbonyl in the pincer
plane.
Reaction of [Fe(THF)2(κ3-LDPM)] with 2,6-dimethylbenzene
isocyanide (CN-xylyl)
The reaction of [Fe(THF)2(κ3-LDPM)] with 3 equiv. of CN-xylyl in
toluene afforded [Fe(CN-xylyl)3(κ3-LDPM)] as
a red crystal
1
(Scheme 3) which was characterized by H-NMR and 13C-NMR
spectrometry, infrared spectroscopy and elemental analyses.
1
Scheme 3 Synthesis of compounds [Fe(CO)3(κ3-LDPM)] and [Fe(CN-xylyl)3-
(κ3-LDPM)].
The H-NMR spectrum of the complex displays resonances in
the normal magnetic field region, indicating that the complex
is in the low-spin state. The methyl protons of the xylyl group
were clearly observed at 2.268 and 1.878 ppm with a 1 : 2 inte-
gral ratio, although certain peaks assignable to aromatic
protons observed in the 6.7–7.2 ppm region are less informa-
tive due to overlapping of peaks. The compound [Fe(CN-
xylyl)3(κ3-LDPM)] was stable under air and in the presence of
moisture. Fortunately, we obtained a single crystal suitable for
X-ray analysis. The crystal structure analysis, as shown in
Fig. 5, clearly revealed that the complex [Fe(CN-xylyl)3-
(κ3-LDPM)] has a distorted octahedral structure with an SNS pincer-
type ligand, one isocyanide carbon atom in the pincer plane
and two isocyanide carbon atoms located at trans positions to
Fig. 4 ORTEP view of the molecular structure of [Fe(CO)3(κ3-LDPM)]
with ellipsoids at the 50% probability level. The hydrogen atoms have
been omitted for clarity. Selected bond lengths (Å): Fe(1)–S(1) 2.2676(11),
Fe(1)–S(2) 2.2638(11), Fe(1)–N(1) 1.980(2), Fe(1)–C(3) 1.861(3), Fe(1)–C(4)
1.791(3), Fe(1)–C(5) 1.825(3), S(1)–C(1) 1.743(3), S(2)–C(2) 1.745(3), N(2)–
C(1) 1.291(3), N(3)–C(2) 1.285(4), S(1)–Fe(1)–S(2) 173.39(3), S(1)–Fe(1)–
N(1) 86.73(7), S(1)–Fe(1)–C(3) 90.51(11), S(1)–Fe(1)–C(4) 93.98(11), S(1)–
Fe(1)–C(5) 89.49(12), S(2)–Fe(1)–N(1) 87.05(8), S(2)–Fe(1)–C(3) 87.04(11),
S(2)–Fe(1)–C(4) 92.25(11), S(2)–Fe(1)–C(5) 92.41(12), N(1)–Fe(1)–C(3)
87.88(10), N(1)–Fe(1)–C(5) 86.99(10), C(3)–Fe(1)–C(4) 92.30(12), C(4)–
Fe(1)–C(5) 92.82(12).
Fig. 5 ORTEP view of the molecular structure of [Fe(CN-xylyl)3-
(κ3-LDPM)] with ellipsoids at the 50% probability level. The hydrogen atoms
have been omitted for clarity. Selected bond lengths (Å) and angles (°):
Fe(1)–S(1) 2.2807(10), Fe(1)–S(2) 2.2744(10), Fe(1)–N(1) 1.9844(17), Fe(1)–
C(78) 1.871(4), Fe(1)–C(79) 1.809(3), Fe(1)–C(80) 1.874(4), S(1)–C(1) 1.743(3),
S(2)–C(2) 1.739(2), N(2)–C(1) 1.288(4), N(3)–C(2) 1.286(3), S(1)–Fe(1)–
S(2) 172.33(3), S(1)–Fe(1)–N(1) 86.49(8), S(1)–Fe(1)–C(78) 90.39(10), S(1)–
Fe(1)–C(79) 95.04(11), S(1)–Fe(1)–C(80) 90.24(10), S(2)–Fe(1)–N(1)
86.00(7), S(2)–Fe(1)–C(78) 91.13(10), S(2)–Fe(1)–C(79) 92.45(11), S(2)–
Fe(1)–C(80) 87.90 (10), N(1)–Fe(1)–C(78) 89.49(10), N(1)–Fe(1)–C(80)
87.90(10), C(78)–Fe(1)–C(79) 91.17(13), C(79)–Fe(1)–C(80) 91.41(13).
described below and the Fe–CO bond in the pincer plane is
significantly shorter than the other two Fe–CO bonds, indicat-
ing that the Fe–CO bond in the pincer plane is shortened by
the trans influence of the thioamide ligand. On the other
hand, the Fe–CO bonds of the other two carbonyls are
elongated by trans influence through the π-back donation from
metal to CO ligand. Further support for trans influence is pro-
vided by 13C-NMR data; the carbonyl carbon atom in the
pincer plane is observed in a lower magnetic field region rela-
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each other. The lengths of Fe–N(1), Fe–S(1), Fe–S(2), and Fe– of the [Fe(THF)2(κ3-LDPM)] complex were easily substituted with
C(79) bonds in the pincer plane are 1.9844(17), 2.2807(10), CO and CN-xylyl molecules to give [Fe(CO)3(κ3-LDPM)] and
2.2744(10), and 1.809(3) Å, respectively, and the Fe–C(79) bond [Fe(CN-xylyl)3(κ3-LDPM)], respectively. These complexes each
length is shorter than the Fe–C(78) (1.871(4) Å) and Fe–C(80) had a distorted octahedral structure in a low spin state. The
(1.874(4) Å) bonds. The lengths of the two C–S bonds are 1.743(3) imino-thiolate units of the [Fe(CN-xylyl)3(κ3-LDPM)] complex
and 1.739(2) Å, and those of two C–N bonds are 1.288(4) can be protonated to thioamide units to give [Fe(CN-xylyl)3-
and 1.286(3) Å, respectively. These bond lengths are almost the (κ3-H2LDPM)]·2(BF4). The band representing the CN stretching
same as those of [Fe(CO)3(κ3-LDPM)] and [Fe(THF)2(κ3-LDPM)] vibration of the coordinated CN group of [Fe(CN-xylyl)3-
with the imino-thiolate ligand in spite of the difference in (κ3-LDPM)] was shifted by 34 cm−1 to a higher energy region upon
their spin states (the former is low-spin and the latter is high- protonation. The above findings may be explained in terms of
spin). However, the C–S and C–N bonds for [Fe(CN-xylyl)3- the σ and π bonding characters between metal and ligands as
(κ3-LDPM)] are clearly elongated and shortened, respectively, in follows: the σ-donation from the sulfur atoms to the iron ion is
comparison with those of [FeBr2(κ3-H2LDPM)] with the thio- weakened by conversion from imino-thiolate to thioamide,
amide ligand described above.
which reduced the π-back donation from the metal ion to iso-
Next, [Fe(CN-xylyl)3(κ3-LDPM)] was mixed with 2 equiv. of cyanide and increased the σ-donation from the isocyanide
HBF4 in toluene.
A brown precipitate of [Fe(CN-xylyl)3- carbon to the metal ion. These suggest that they are controlled
(κ3-H2LDPM)]·2(BF4) was obtained in which the two imino-thiolate by the process of deprotonation/protonation.
nitrogen atoms were protonated. The 1H-NMR spectrum of the
precipitate in CDCl3 solution has an NH proton peak at
10.51 ppm, indicating that the imino-thiolate nitrogen of the
Experimental
Materials
compound is protonated to afford [Fe(CN-xylyl)3(κ3-H2LDPM)]2+.
Such a phenomenon was not observed when a weaker acid
All of the reagents and solvents employed are commercially
such as trimethylammonium chloride and 2,6-lutidinium
tetrafluoroborate was added. Further evidence of protonation
was provided by a comparison of IR spectra of [Fe(CN-
xylyl)3(κ3-LDPM)] and [Fe(CN-xylyl)3(κ3-H2LDPM)]·2(BF4); the NH
stretching vibration band for [Fe(CN-xylyl)3(κ3-H2LDPM)]·2(BF4)
appeared at 3266 cm−1 upon addition of HBF4 to [Fe(CN-
xylyl)3(κ3-LDPM)], indicating that the imino-thiolate groups are
converted to thioamides. Furthermore, the band representing
the stretching vibration of the coordinated isocyanide units
ν(CuN) was shifted from 2110 cm−1 to 2144 cm−1 upon proto-
nation of the imino-thiolate units. This stretching vibration is
significantly higher than that of the metal free CN-xylyl mole-
cule (2121 cm−1), which indicates that the σ-donation from the
sulfur atoms to the metal ion is weakened with the conversion
from imino-thiolate to thioamide groups. We explain this
finding as follows. The π-back donation from the metal ion to
the π* orbitals of isocyanide ligands is reduced and the
σ-donation from the σ* orbital of the isocyanide carbon to the
metal ion increases.
available. All anhydrous solvents were purchased from Wako
Ltd, and were degassed with argon. 2,6-Pyridinedicarbonyl
dichloride was synthesized according to a previously published
method.15
General methods
All manipulations were carried out under an atmosphere of
purified argon or dinitrogen gas in an mBRAUN MB 150B-G
glovebox or by standard Schlenk techniques. 1H-NMR and
13C-NMR spectra were measured on a Varian Mercury 300
spectrometer, and 1H chemical shifts were estimated relative
to TMS as an internal standard. Fourier transform infrared
(FT-IR) spectra of solid compounds were measured as KBr
pellets using a JASCO FT/IR-410 spectrophotometer. Elemental
analyses were obtained with a Perkin-Elmer CHN-900 elemen-
tal analyzer. Magnetic moment measurements (Evans method)
in the solid state were obtained using a Sherwood Scientific
Ltd MSB-MKI magnetic susceptibility balance.
X-ray crystallography
Each crystal was mounted on a glass fiber and diffraction data
were collected using a Rigaku/MSC Mercury CCD with graphite
Conclusion
In order to examine the binding behavior of the thioamide monochromated Mo-Kα radiation at −100 °C or −150 °C.
pincer type ligand toward the metal and effects relating to Crystal data and experimental details are listed in Table S1
deprotonation/protonation of its thioamide group, we syn- (ESI†). The crystal structures of all Fe(II) complexes were solved
thesized
a
new monochelate iron(II) complex, [FeBr2- by a combination of direct methods (SIR9216 or SIR200417) and
(κ3-H2LDPM)], with a thioamide SNS pincer type ligand which Fourier techniques. All non-hydrogen atoms were refined ani-
includes a bulky substituent group, κ3-H2LDPM. The complex sotropically. Hydrogen atoms were refined by the riding model
was characterized and ligand substitution reactions were inves- using the appropriate HFIX command in SHELXL97.18 The
tigated. To the best of our knowledge, this is the first report of Sheldrick weighting scheme was applied. Plots of ∑(|F0| −
a monochelate iron(II) complex with a thioamide pincer-type |Fc|)2 versus |F0|, reflection order in data collection, sin θ/λ,
ligand. The two bromide ions of the iron complex were substi- and various classes of indices showed no unusual trends.
tuted with THF molecules in THF solution. The THF ligands Neutral atomic scattering factors were taken from Inter-
9736 | Dalton Trans., 2014, 43, 9732–9739
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national Tables for X-ray Crystallography edited by Cromer and (CH3)2), 1.10 (d, 12H, CH3). ESI TOF MS: m/z 1120.5 [M + Na]+,
Waber.19 Anomalous dispersion effects were included in M = 1097.5 (C77H67N3S2). FTIR (KBr, cm−1): 3301(NH), 994
20
Fcalc
,
where the values for Δf′ and Δf″ were taken from those (CvS). Anal. Calcd for C77H67N3S2: C, 84.19; H, 6.15; N, 3.38;
of Creagh and McAuley.21 The values for the mass attenuation S, 5.84. Found: C, 83.75; H, 5.93; N, 3.83; S, 5.52.
coefficients are those of Creagh and Hubbell.22 All calculations [FeBr2(κ3-H2LDPM)]. To
diethylether (3 mL) solution
a
were performed using the crystallographic software package, of H2LDPM (100 mg, 9.10 × 10−2 mmol) was added FeBr2
CrystalStructure.
(8.28 mg, 9.10 × 10−2 mmol), followed by stirring at room
temperature. After 24 hours, the mixture was filtered and
washed with diethylether to give a green powder. The powder
Syntheses of compounds
2,6-Bis(diphenylmethyl)-4-isopropylaniline. The compound was dissolved in CH2Cl2, and the precipitate was filtered. The
was prepared according to the method described by Markó solvent was removed under vacuum to yield a green precipitate
et al.23 Diphenylmethanol (10.0 g, 0.0542 mol), HCl (36% HCl, of [FeBr2(κ3-H2LDPM)] (100.4 mg, 84%). The complex was
2 mL), ZnCl2 (2.00 g, 0.0146 mol), and 4-isopropylaniline recrystallized from DME as dark green needle-like crystals. ESI
(3.66 g, 0.0271 mol) were added to a 100 mL Schlenk tube. The TOF MS: m/z 1235.3 [M − Br]+, M = 1313.0 (C77H67N3S2FeBr2).
mixture was heated at 160 °C to melt the diphenylmethanol. Anal. Calcd for C77H67N3S2FeBr2: C, 70.37; H, 5.14; N, 3.20;
Once cooled, the solid was dissolved in CH2Cl2 and washed S, 4.88. Found: C, 70.09; H, 5.24; N, 2.98; S, 5.01.
successively with NaHCO3. After drying over Na2SO4, the
[Fe(THF)2(κ3-LDPM)]. To a THF (3 mL) solution of [FeBr2-
mixture was filtered. Evaporation of the solvent in vacuo (κ3-H2LDPM)] (100 mg, 7.61 × 10−2 mmol) was added NaH (20 mg,
yielded an off-white solid, which was washed with n-hexane to 8.33 × 10−1 mmol) with stirring at room temperature. After
give 2,6-bis(diphenylmethyl)-4-isopropylaniline as
a white 2 hours, the color of the reaction mixture changed from green
powder (9.66 g, yield: 76%). 1H-NMR (δ/ppm vs. TMS in CDCl3, to orange-red. The precipitate was filtered and the mixture was
300 MHz): 7.27 (t, 8H, CHPh), 7.21 (t, 4H, CHph), 7.08 (d, 8H, stored at −30 °C overnight, leading to formation of orange
CHph), 6.42 (s, 2H, CHph), 5.45 (s, 2H, CH(Ph)2), 3.26 (s, 2H, crystals of [Fe(THF)2(κ3-LDPM)] (38 mg, 39%).
NH2), 2.55 (m, 1H, CH(CH3)2), 0.93 (d, 6H, CH3).
FTIR (KBr, cm−1): 3082, 3059, 3024, 2958, 2924, 2868 (CH3),
2,6-Bis(N-2,6-bis(diphenylmethyl)-4-isopropylphenylamide)- 1948, 1884, 1807, 1598 (CvC), 1565 (CvN). Anal. Calcd for
pyridine. 2,6-Pyridinedicarboxylic dichloride (5.00 g, 10.6 × C85H79N5S2O2Fe: C, 78.86; H, 6.15; N, 3.25; S, 4.95. Found: C,
10−3 mol) was dissolved in 100 mL of dry THF. To this solution 77.53; H, 6.23; N, 3.18; S, 4.83.
was added 2,6-bis(diphenylmethyl)-4-isopropylaniline (1.09 g,
[Fe(CO)3(κ3-LDPM)]. A toluene (3 mL) solution of [Fe-
5.34 × 10−3 mol) and triethylamine (1.50 mL, 0.0106 mol) at (THF)2(κ3-LDPM)] (100 mg, 7.71 × 10−2 mmol) was placed under
0 °C. The mixture was stirred at room temperature overnight. 1 atm of carbon monoxide at −196 °C. When the resulting
The precipitate of Et3N·HCl was filtered and the solvent was reaction mixture was warmed to ambient temperature and
removed under vacuum, dissolved in CHCl3 and purified by stirred for 48 h, the solution color changed from orange to
silica gel column chromatography. 2,6-Bis(N-2,6-bis(diphenyl- brownish yellow. After 48 hours, the solvent was removed
methyl)-4-isopropylphenylamide)pyridine was isolated as a under vacuum, forming a dark brown powder of [Fe(CO)3-
1
white powder (yield: 3.51 g, 62%). H-NMR (δ/ppm vs. TMS in (κ3-LDPM)]. The brown powder was dissolved in toluene and stored
CDCl3, 300 MHz): 7.95 (t, 1H, CHpy), 8.27 (d, 2H, CHpy), 7.98 at −50 °C for 24 hours, leading to formation of [Fe(CO)3-
1
(s, 2H, NH), 5.43 (s, 4H, CH(Ph)2), 6.87 (br, 16H, CHPh), 6.87 (κ3-LDPM)] crystals (42 mg, 44%). H-NMR (δ/ppm from TMS in
(br, 16H, CHPh), 6.87 (br, 8H, CHPh), 6.73 (s, 4H, CHPh), 2.72 benzene-d6, 300 MHz): 7.71 (t, 1H, Py(4)), 7.29–7.13 (overlapped
(m, 2H, CH(CH3)2), 1.05 (d, 12H, CH3). ESI TOF MS: m/z with solvent peaks), 5.976 (s, 4H, CH(Ph)2), 2.508 (m, 2H, CH-
1088.7 [M + Na]+, 1066.7 [M + H]+. FTIR (KBr, cm−1): 3371(NH), (Me)2), 0.970 (d, 12H, CH(CH3)2). 13C-NMR (δ/ppm from
1687 (CvO).
2,6-Bis(N-2,6-bis(diphenylmethyl)-4-isopropylphenylthioamide)- 145.6, 143.9, 143.8, 143.5, 132.3, 129.8, 129.3, 128.3, 128.1,
pyridine (H2LDPM). Lawesson’s reagent (3.79 g, 9.33
126.5, 126.2, 126.0, 125.9, 52.5 (CH(Ph2)), 33.6 (CH(CH3)2),
solvent in CDCl3): 206.9 (CO), 200.3 (CO), 169.5 (CS), 159.1,
×
10−3 mol) was added to a solution of 2,6-bis(N-2,6-bis- 24.1 (CH(CH3)2). FTIR (KBr, cm−1): 3082, 3059, 3024, 2958,
(diphenylmethyl)-4-isopropylphenylamide)pyridine (5 g, 4.69 × 2924, 2868 (CH3), 2044, 1993 (CO), 1948, 1884, 1807, 1598
10−3 mol) in toluene (30 mL), and the mixture was heated at (CvC), 1565 (C–N). Anal. Calcd for C80H68N3S2FeO3: C, 77.53;
100 °C for 6 h. After cooling, the precipitate was filtered and H, 5.53; N, 3.39, S, 5.17. Found: C, 77.85; H, 5.72 2.88; N 4.89;
the solvent was removed under vacuum. The precipitate was S, 4.82.
dissolved in CHCl3 and purified by silica gel column chrom-
[Fe(CN-xylyl)3(κ3-LDPM)]. To a toluene (3 mL) solution of
atography. 2,6-Bis(N-2,6-bis(diphenylmethyl)-4-isopropyl- phe- [Fe(THF)2(κ3-LDPM)] (100 mg, 7.71 × 10−2 mmol) was added
nylthioamide)pyridine was isolated as a yellow powder of the 2,6-xylylisocyanide (30.4 mg, 2.32 × 10−1 mmol) with stirring at
H2LDPM ligand (2.53 g, 49%). 1H-NMR (δ/ppm vs. TMS in room temperature. After 6 hours, the mixture was stored
CDCl3, 300 MHz): 8.02 (t, 1H, CHpy), 8.77 (d, 2H, CHpy), 9.42 at −50 °C overnight, leading to formation of red crystals of
(s, 2H, NH), 5.37 (s, 4H, CH(Ph)2), 6.97 (d, 8H, CHPh), 6.64 (d, [Fe(CN-xylyl)3(κ3-LDPM)] (88 mg, 74%).
8H, CHPh), 7.13 (t, 8H, CHPh), 6.56 (t, 8H, CHPh), 7.15 (t, 8H,
1H-NMR (δ/ppm from TMS in benzene-d6, 300 MHz): 7.783
CHPh), 6.41 (t, 8H, CHPh), 6.84 (s, 4H, CHPh), 2.78 (m, 2H, CH- (d, 2H, Py(3, 5)), 7.419 (d, 8H, Ph(2, 6)), 6.714 (t, 1H, Py(4)),
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6.781–7.124 (m, 14H, Ph, xylyl), 6.477 (m, 3H, xylyl), 6.377 (d,
4H, xylyl), 5.976 (s, 4H, CH(Ph)2), 2.508 (m, 2H, CH(Me)2),
2.268 (s, 6H, xylyl), 1.878 (s, 12H, xylyl), 0.970 (d, 12H, CH
(CH3)2). 13C-NMR (δ/ppm from solvent in CDCl3): 175.18
(xylyl), 175.29 (xylyl), 160.79 (CS), 147.85, 145.21, 144.12,
142.12, 138.13, 135.61, 134.68, 134.47, 132.12, 130.05, 129.43,
129.31, 128.04, 128.50, 127.75, 127.66, 126.48, 125.59, 124.22,
51.97 (Ph2CH), 33.63 (Ar-CH(CH3)2), 24.28(Ar-CH(CH3)2), 19.45
(xylyl), 18.50 (xylyl). FTIR (KBr, cm−1): 3082, 3059, 3024, 2953,
2922, 2854 (CH3), 2110, 2087 (CN), 1948, 1884, 1807, 1598
(CvC), 1538 (C–N). Anal. Calcd for C104H92N6S2Fe: C, 80.80;
H, 6.00; N, 5.44; S, 4.15. Found: C, 79.67; H, 6.04; N, 5.16;
S, 4.10.
1418; B. L. Small and M. Brookhart, Macromolecules, 1999,
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2 G. J. P. Britovsek, S. Mastroianni, G. A. Solan,
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[Fe(CN-xylyl)3(κ3-H2LDPM)](BF4)2. To a toluene (3 mL) solu-
tion of [Fe(CN-xylyl)3(κ3-LDPM)] (100 mg, 7.71 × 10−2 mmol)
was added HBF4 (20 μL, 1.47 × 10−1 mmol) with stirring at
room temperature. After 6 hours, the reaction mixture changed
from a red solution to a brown slurry. The precipitate was col-
lected on a funnel, washed with pentane and dried (79 mg,
60%).
1H-NMR (δ/ppm from TMS in benzene-d6, 300 MHz): 10.51
(s, 2H, NH), 8.59 (t, 1H, Py(4)), 8.16 (d, 2H, Py(3, 5)), 7.40–5.44
(overlapped with solvent peaks), 5.44 (s, 4H, Ar-CHPh2), 2.69
(m, 2H, CH(CH3)2), 2.52 (s, 6H, xylyl), 1.89 (s, 12H, xylyl), 0.99
(d, 12H, CH(CH)2).
5 L. E. Karagiannidis, P. A. Gale, M. E. Light, M. Massi and
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13C-NMR (δ/ppm vs. solvent in CDCl3): 194.71 (CS), 155.71
(xylyl), 150.57 (xylyl), 142.56, 142.16, 140.89, 134.88, 134.63,
131.77, 130.30, 130.17, 129.25, 129.06, 128.86, 128.73, 128.51,
128.44, 127.86, 127.58, 127.24, 126.54, 126.16, 52.03 (Ph2CH),
33.91 (Ar-CH(CH3)2), 23.69 (Ar-CH(CH3)2), 19.38 (xylyl), 18.52
(xylyl).
6 M. Ray, D. Ghosh, Z. Shirin and R. Mukherjee, Inorg.
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FTIR (KBr, cm−1): 3269 (NH), 3082, 3059, 3024, 2953, 2922,
2869 (CH3), 2175, 2144 (CuN), 1948, 1884, 1807, 1150, 1127,
1077, 1051 (BF4). Anal. Calcd for C104H94N6S2FeB2F8: C, 72.56;
H, 5.50; N, 4.88; S, 3.73. Found: C, 71.86; H, 5.54; N, 4.50;
S, 3.52.
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Acknowledgements
We thank Professor M. D. Fryzuk (The University of British
Columbia) for his helpful comments. We gratefully acknowl-
edge the support of this work by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, 11 T. M. Smit, A. K. Tomov, V. C. Gibson, A. J. P. White and
Science and by Japan Society for the Promotion of Science
“Strategic Young Researcher Overseas Visits Program for
Accelerating Brain Circulation”.
D. J. Williams, Inorg. Chem., 2004, 43, 6511–6512;
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