Iron and ruthenium complexes having a pincer-type ligand with two protic amidepyrazole arms: Structures and catalytic application

Article abstract of DOI:10.1016/j.poly.2017.08.044

This paper describes the iron and ruthenium complexes ligated by 2,6-di(5-pivalamido-1H-pyrazol-3-yl)pyridine (amide-LH₂) bearing electron-withdrawing NHCOBu^t groups on the two protic pyrazole arms. Treatment of FeCl₂·4H₂O with an equimolar amount of amide-LH₂ followed by addition of two trimethylphosphine and an excess of sodium triflate gave the pincer-type iron complex [Fe(MeCN)(amide-LH₂)(PMe₃)₂](OTf)₂ (1b; OTf = OSO₂CF₃). Impact of the amido groups in 1b on the reactions with hydrazines was evaluated. Complex 1b catalyzed disproportionation of hydrazine into ammonia and dinitrogen, although the catalytic activity was lower than that of the Bu^t-LH₂ analogue 1a. X-ray analysis of 1b as well as the ruthenium complex [{RuCl₂(PPh₃)₂}₂(μ₂-amide-LH₂)₂] (2b) revealed that the pendant carboxamide groups along with the pyrazole NH units are engaged in hydrogen bonds in the second coordination sphere.

Full text of DOI:10.1016/j.poly.2017.08.044

Polyhedron xxx (2017) xxx–xxx
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Iron and ruthenium complexes having a pincer-type ligand with two
protic amidepyrazole arms: Structures and catalytic application
Yoshiko Nakahara a, Tatsuro Toda a, Shigeki Kuwata a,b,
⇑
a Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 2-12-1 E4-1 O-okayama, Meguro-ku,
Tokyo 152-8552, Japan
b PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
This paper describes the iron and ruthenium complexes ligated by 2,6-di(5-pivalamido-1H-pyrazol-3-yl)
pyridine (amide-LH2) bearing electron-withdrawing NHCOBut groups on the two protic pyrazole arms.
Treatment of FeCl2ꢀ4H2O with an equimolar amount of amide-LH2 followed by addition of two
trimethylphosphine and an excess of sodium triflate gave the pincer-type iron complex [Fe(MeCN)
(amide-LH2)(PMe3)2](OTf)2 (1b; OTf = OSO2CF3). Impact of the amido groups in 1b on the reactions with
hydrazines was evaluated. Complex 1b catalyzed disproportionation of hydrazine into ammonia and
dinitrogen, although the catalytic activity was lower than that of the But-LH2 analogue 1a. X-ray analysis
of 1b as well as the ruthenium complex [{RuCl2(PPh3)2}2(l2-amide-LH2)2] (2b) revealed that the pendant
carboxamide groups along with the pyrazole NH units are engaged in hydrogen bonds in the second coor-
dination sphere.
Received 12 July 2017
Accepted 30 August 2017
Available online xxxx
Keywords:
Pincer ligand
Pyrazole
Iron
Ruthenium
Hydrogen bond
Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction
cule is operative. Given the mechanism, the Brønsted acidity of
the pyrazole arms would greatly affect the catalytic performance.
Multiproton-responsive chelate ligands have attracted much
attention owing to their versatile roles in mimicking the active
sites of metalloenzymes and developing functional materials [1–
4]. Among such scaffolds, 2,6-di(pyrazol-3-yl)pyridines (R-LH2;
Chart 1) with a variety of substituents R in the two protic pyrazole
arms are easily accessed by conventional condensation reactions
[5]. These ligands thus have been widely used as templates for cat-
alysts [4,6–14], supramolecular architecture [15–18], and spin-
crossover compounds [19] as well as photo-emitting and sensitiz-
ing complexes [20–22]. Despite of the structural diversity, how-
ever, the substituent effect on the reactivity of this class of
compounds have scarcely been investigated [8].
In this regard, the amide-LH2 ligand recently developed by Hal-
crow and co-workers [27] is attractive since the electron-with-
drawing nature of the amide group would facilitate proton
transfer from the pincer ligand to the substrate molecule. We
describe here the synthesis and catalysis of the iron complexes
[Fe(MeCN)(amide-LH2)(PMe3)2](OTf)2 (1b). The crystal structure
of a related ruthenium complex bearing the amide-LH2 ligand,
[{RuCl2(PPh3)2}2(l2-amide-LH2)2] (2b), is also reported.
2. Experimental
Our continuing study on metal–ligand bifunctional complexes
[4,7,23–26] led to the finding that the protic pincer-type iron com-
plex [Fe(MeCN)(But-LH2)(PMe3)2](OTf)2 (1a) catalyzes dispropor-
tionation of hydrazine into dinitrogen and ammonia [6]. The
reactions using N-methylated analogous catalysts as well as sub-
stituted hydrazines suggest that proton–electron shuttling
between the Fe(But-LH2) fragment and substrate hydrazine mole-
2.1. General
All manipulations were performed under an atmosphere of
argon using standard Schlenk techniques unless otherwise speci-
fied. Solvents were dried by refluxing over sodium benzophenone
ketyl (THF, diethyl ether, and hexane), P2O5 (dichloromethane
and acetonitrile), and Mg(OMe)2 (methanol), and distilled before
use. 2,6-Di(5-pivalamido-1H-pyrazol-3-yl)pyridine (amide-LH2)
[27] and complex 2b [8] as well as [RuCl2(PPh3)3] [28] were pre-
pared according to the literature. 1H (399.8 MHz) and 31P
(161.8 MHz) NMR spectra were obtained on a JEOL JNM-ECX-400
spectrometer. 1H NMR shifts are relative to the residual CD2HCN
⇑
Corresponding author at: Department of Chemical Science and Engineering,
School of Materials and Chemical Technology, Tokyo Institute of Technology, 2-12-1
E4-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan.
E-mail address: skuwata@apc.titech.ac.jp (S. Kuwata).
http://dx.doi.org/10.1016/j.poly.2017.08.044
0277-5387/Ó 2017 Elsevier Ltd. All rights reserved.
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2
Y. Nakahara et al. / Polyhedron xxx (2017) xxx–xxx
(1 mL) of the gas phase was subjected to GLC analysis. The NMR
analysis with using 1,3,5-trimethoxybenzene as an internal stan-
dard suggested formation of a hydrazinophosphonium triflate,
(Me3PNHNH2)OTf, in 23% yield. 1H NMR (CDCl3): d 1.97 (d, 9H,
2JPH = 13.1, PMe3). 31P{1H} NMR (CDCl3): d 50.3 (s). This phospho-
nium salt was not detected in the reaction of 1a. In a separate
run, all volatile material inside was distilled into a 1 N H2SO4 aque-
ous solution after the reaction, and the residue was further
extracted with an additional amount of distilled water. The col-
lected ammonia and unreacted hydrazine were quantified by
indophenol [29] and p-(dimethylamino)benzaldehyde method
[30], respectively.
Chart 1.
(d 1.93) and CHCl3 (d 7.26), while 31P shifts are referenced to phos-
phoric acid (d 0.0), respectively. J values are given in Hz. Infrared
spectra were recorded on
a JASCO FT/IR-6100 spectrometer.
ESI-MS spectra was obtained on a JEOL JMS-T100LC (methanol
was used as a solvent). UV–Vis spectra were recorded on a JASCO
V-630 spectrometer. Nitrogen gas evolved in disproportionation
of hydrazine was determined by GLC analysis using Shimadzu
GC-2010 Plus gas chromatograph equipped with a Molecular Sieve
5A column. Elemental analyses were performed on a Perkin–Elmer
2400II CHN analyzer.
2.5. Representative procedure for the reaction of 1 with 1,1-
diphenylhydrazine
To a solution of 1b (1.6 mg, 0.0017 mmol) in CD2Cl2 (0.5 mL) in
an NMR tube were added Ph2NNH2 (1.4 lL, 0.0085 mmol) and
1,3,5-trimethoxybenzene (0.5 mg, 0.003 mmol) as an internal stan-
dard at room temperature. After 18 h, the mixture was subjected to
1H NMR analysis.
2.2. Synthesis of [Fe(MeCN)(amide-LH2)(PMe3)2](OTf)2 (1b)
A mixture of FeCl2ꢀ4H2O (19.2 mg, 0.0966 mmol) and amide-
LH2 (41.3 mg, 0.101 mmol) in methanol (4 mL) was stirred for 4 h
at room temperature. Slow addition of diethyl ether (20 mL)
afforded an orange solid, which is tentatively assigned as
[FeCl2(MeOH)(amide-LH2)] (22.0 mg, 0.0387 mmol). ESI-MS: m/z
500.0 [FeCl(amide-LH2)+]. The solid was dissolved in acetonitrile
(2 mL), and sodium trifluoromethanesulfonate (65.5 mg,
0.381 mmol) and a 1.0 M toluene solution of trimethylphosphine
(0.08 mL, 0.08 mmol) were added. After stirring for 8 h at room
temperature, the solvent was removed in vacuo. The residue was
extracted with dichloromethane (5 mL). Slow addition of hexane
(20 mL) to the concentrated extract (to ca. 1.5 mL) afforded black
crystals, which were washed with diethyl ether (5 mL ꢁ 2). Yield:
15.0 mg (0.0157 mmol, 16% in two steps). 1H NMR (CD3CN):
d 0.70 (t, 18H, JPH = 4.0, PMe3), 1.25 (s, 18H, CMe3), 6.91 (s, 2H,
2.6. Crystallography
Single crystals suitable for X-ray analyses were mounted on a
fiber loop. Diffraction experiments were performed on a Rigaku
Saturn CCD area detector with graphite monochromated Mo K
a
radiation (k = 0.710 70 Å). Intensity data were corrected for Lor-
entz–polarization effects and for absorption. Intensity data
(6° < 2h < 55°) were corrected for Lorentz–polarization effects and
for absorption. Details of crystal and data collection parameters
are summarized in Table 1. Structure solution and refinements
were carried out by using the CrystalStructure program package
[31]. The heavy-atom positions were determined by a direct meth-
ods program (SIR92 [32]) and the remaining non-hydrogen atoms
were found by subsequent Fourier syntheses and refined by full-
3
pyrazole CH), 7.96 (d, 2H, JHH = 8.3, 3- and 5-C5H3N), 8.07 (m,
1H, 4-C5H3N), 9.10 (s, 2H, amide NH), 11.79 (s, 2H, pyrazole NH).
31P{1H} NMR (CD3CN): d 17.7 (s). IR (KBr): 1699 cm–1 (C@O). Anal.
Calc. for C31H48F6FeN8O8S2P2: C, 38.92; H, 5.06; N, 11.71. Found: C,
38.83; H, 5.17; N, 11.71%.
matrix least-squares techniques against F2 using the
SHELXL-
2014/7 program [33]. One of the tert-butyl groups in 1b was placed
at two disordered positions with 65% and 35% occupancies and
2.3. Synthesis of [{RuCl2(PPh3)2}2(l-amide-LH2)2] (2b)
Table 1
Crystal data for 1b and 2bꢀ2THFꢀhexane.
A
mixture of [RuCl2(PPh3)3] (166.8 mg, 0.1740 mmol) and
1b
2bꢀ2THFꢀhexane
amide-LH2 (68.8 mg, 0.168 mmol) in dichloromethane (10 mL)
was stirred for 16 h at room temperature. After removal of the sol-
vent, recrystallization from THF–hexane (5 mL/15 mL) afforded
2bꢀ2THFꢀhexane as red crystals. The thoroughly dried sample was
found to lose the solvating molecules on the basis of 1H NMR spec-
troscopy and combustion analysis. Yield: 156.8 mg (0.07088 mmol,
84%). 1H NMR (CDCl3): d 1.18 (s, 36H, CMe3), 6.91 (t, 24H, J = 7.5,
Formula
M
Cryst. system
Space group
a (Å)
C
31H48F6FeN8O8P2S2
C128H144Cl4N14O6P4Ru2
2442.48
monoclinic
P21/n
26.295(7)
14.963(4)
30.494(8)
90
99.282(3)
90
11841(5)
93
956.67
orthorhombic
Pbcn
41.556(14)
11.028(4)
19.136(6)
90
90
90
8770(5)
93
8
b (Å)
c (Å)
a
(°)
4
PPh3), 7.04 (d, 4H, JHH = 2.2, pyrazole CH), 7.13 (t, 12H, JHH = 7.0,
b (°)
3
PPh3), 7.21 (d, 4H, JHH = 7.7, 3- and 5-C5H3N), 7.30–7.34 (m, 24H,
c
(°)
3
PPh3), 7.59 (t, 2H, JHH = 8.0, 4-C5H3N), 10.33 (br s, 4H, NH), 12.41
V (Å3)
T (K)
Z
4
(br d, 4H, JHH = 2.4, NH). 31P{1H} NMR (CDCl3): d 35.3 (s). IR
4
(KBr): 1687 cm–1 (C@O). Anal. Calc. for C114H114Cl4N14O4P4Ru2: C,
l
(Mo K
Dcalc (g cm–3
Limiting indices
a
) (mm–1
)
)
0.591
1.449
0.461
1.370
–34 ꢂ h ꢂ 33
–19 ꢂ k ꢂ 19
–39 ꢂ l ꢂ 39
27062
61.90; H, 5.19; N, 8.86. Found: C, 62.18; H, 5.44; N, 8.51%.
–53 ꢂ h ꢂ 53
–14 ꢂ k ꢂ 14
–24 ꢂ l ꢂ 24
9407
0.0547
551
2.4. Representative procedure for catalytic disproportionation of
hydrazine wtih 1
No. of unique reflection
Rint
No. variables
0.1125
1396
To a dichloromethane–methanol (1 mL/1 mL) solution of 1b
(9.2 mg, 0.0096 mmol) in a Schlenk tube (inside volume: ca.
R1 [I > 2
r
(I)]
0.0800
0.2263
1.140
0.0919
0.1841
1.180
wR2 (all data)
33 mL) was added anhydrous hydrazine (6.1
lL, 0.19 mmol), and
Goodness of fit (GOF) on F2
the mixture was stirred for 18 h at room temperature. An aliquot
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Y. Nakahara et al. / Polyhedron xxx (2017) xxx–xxx
3
3. Results and discussion
3.1. Synthesis and structure of iron complex 1b
The amide-LH2 iron complex 1b furnished with electron-with-
drawing groups on the two pyrazole arms was obtained according
to the synthetic protocol for the But-LH2 analogue 1a as shown in
Scheme 1 [6,34,35]. Treatment of iron(II) chloride with an equimo-
lar amount of amide-LH2 in methanol afforded a paramagnetic
orange solid, which we tentatively assigned as [FeCl2(MeOH)
(amide-LH2)] by ESI-MS spectroscopy. The But-LH2 analogue has
been isolated and structurally characterized [6]. Subsequent addi-
tion of two equivalents of trimethylphosphine and an excess of
sodium trifluoromethanesulfonate led to the formation of 1b. The
1H NMR spectrum exhibits two low-field singlets at d 11.79 and
9.10 (2 H each) ascribed to the NH protons. The 31P{1H} NMR spec-
trum showing only one singlet at d 17.7 is also consistent with a C2v
symmetric structure of 1b as well at the diamagnetic nature.
Fig. 1 depicts the crystal structure of 1b. The bond distances and
angles in the iron–pincer framework are almost identical with
those of the But-LH2 analogue 1a [6] as summarized in Table 2.
The carboxamide groups are nearly coplanar with the pincer
ligand. These NH groups, facing the same direction as the pyrazole
NH groups, are engaged in hydrogen bonding with the triflate
counteranions. The N(pyrazole)ꢀ ꢀ ꢀOTf distances (2.815 Å, mean)
in the amide-LH2 complex 1b are slightly shorter than those in
the But-LH2 analogue 1a (2.873 Å, mean), possibly owing to the
two-point interaction between the pincer ligand and counteranion.
Meanwhile, the carbonyl oxygen atoms in the carboxamide groups
are free from hydrogen bonding interactions in contrast to the
related bis(pincer)-type complex [Fe(amide-LH2)2](BF4)2 reported
by Halcrow and co-workers [27].
Scheme 1. Synthesis of 1b.
Fig. 1. Crystal structure of 1b. The CH hydrogen atoms as well as the minor
component of the disordered tert-butyl group are omitted for clarity. Thermal
ellipsoids are drawn at the 30% level.
3.2. Catalytic disproportionation of hydrazine with iron complexes 1
refined with restraint geometries. For 2bꢀ2THFꢀhexane, one of the
amide carbonyl groups was placed at two disordered positions
with 50% occupancies and included in the refinements with
restraint geometries. The solvated hexane molecule was also disor-
dered; the carbon atoms therein were included in the refinements
with fixed isotropic thermal parameters and restraint geometries.
The hydrogen atoms except for those in the disordered moieties
were included in the refinements with a riding model.
The electron-withdrawing amide pendants in 1b would
increase the Brønsted acidity of the protic pincer ligand. To evalu-
ate the substituent effect on the catalysis, we tested catalytic dis-
proportionation of hydrazine with 1b. The reaction with
a
substrate/catalyst ratio of 20 took place at room temperature to
afford ammonia and dinitrogen catalytically, although the yield
was significantly lower in comparison with the But-LH2 complex
1a (Table 3). According to the proposed proton–electron shuttling
Table 2
Selected interatomic distances (Å) and angles (°) for 1.
1b
2.2824(12)/2.2829(12)
1a
Fe(1)–P(1)/P(2)
2.2834(8)/2.2798(8)
Fe(1)–N(2)/N(4)
Fe(1)–N(5)
1.953(3)/1.964(3)
1.904(3)
1.976(2)/1.981(2)
1.913(2)
Fe(1)–NCMe
1.909(3)
1.918(2)
N(1)–N(2)/N(3)–N(4)
N(2)–C(3)/N(4)–C(9)
N(6)–C(12)/N(7)–C(17)
O(1)–C(12)/O(2)–C(17)
N(1)ꢀ ꢀ ꢀO(3)/N(3)ꢀ ꢀ ꢀO(6)a
H(1)ꢀ ꢀ ꢀO(3)/H(2)ꢀ ꢀ ꢀO(6)a
N(6)ꢀ ꢀ ꢀO(4)/N(7)ꢀ ꢀ ꢀO(7)
H(3)ꢀ ꢀ ꢀO(4)/H(4)ꢀ ꢀ ꢀO(7)
1.345(4)/1.360(4)
1.352(5)/1.342(5)
1.379(5)/1.374(4)
1.224(5)/1.231(5)
2.825(4)/2.804(4)
1.98/1.95
1.358(3)/1.353(2)
1.349(3)/1.349(3)
—
—
2.898(3)/2.848(3)
1.98/1.96
—
—
2.943(5)/3.007(5)
2.07/2.15
P(1)–Fe(1)–P(2)
172.40(5)
176.34(3)
N(2)–Fe(1)–N(4)
159.11(13)
158.86(7)
N(2)–Fe(1)–N(5)/N(4)–Fe(1)–N(5)
N(2)–N(1)–C(1)/N(4)–N(3)–N(11)
N(1)–N(2)–C(3)/N(3)–N(4)–N(9)
Ref.
79.34(13)/79.77(13)
110.8(3)/110.5(3)
105.7(3)/106.3(3)
This work
79.47(7)/79.40(7)
111.3(2)/111.7(2)
105.21(14)/105.7(2)
[6]
a
For 1a, the O(3) and O(6) atoms are labeled as O(1) and O(4) in the original paper [6].
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Y. Nakahara et al. / Polyhedron xxx (2017) xxx–xxx
Table 3
Catalytic disproportionation of hydrazine with iron complexes 1.
2OTf
PMe3
N
Fe
N
PMe3
R
Fe cat =
4 NH3 + N2
Fe cat
N
(S/C = 20)
N
H
3 N2H4
CH2Cl2–MeOH
r.t., 18 h
N
N
R
H
C
Me
Yieldsb
Conv. of N2H4 (%)a
NH3
N2
a
c
Catalyst
R
1b
1a
ButCONH
But
57
87
12.8
21.3
2.4
4.2
a
b
c
Determined by colorimetry.
In moles per mole of Fe complex.
Determined by GC.
Scheme 2. Proposed mechanism for catalytic disproportionation of hydrazine [6]. Fe = Fe(PMe3)22+. The intermediates A–E were not observed and isolated. The total charge of
the iron complexes (2+) and the substituents on the pyrazole rings are omitted.
mechanism shown in Scheme 2 [6], the result may be rationalized
as follows. The increased acidity of 1b would accelerate the initial
proton transfer from the pyrazole units to the coordinated hydra-
zine (A to C). However, the deprotonated pyrazolato units in C
should be less basic for 1b, and hence the subsequent proton shift
from the second molecule of hydrazine to the pincer ligand (C to E)
is retarded, which would lead to catalyst deactivation through
reductive elimination of the phosphonium salt 3. In fact, the
NMR spectra of the reaction mixture revealed the formation of a
Scheme 3. Synthesis of 2b.
phosphonium salt, tentatively characterized as the decomposition
product 3a, only in the reaction using 1b.
The reactions of 1 with 1,1-diphenylhydrazine also support the
acceleration of the initial proton transfer with electron-withdraw-
ing amide pendants. Since there is no distal hydrazine hydrogen
atom in the intermediate D in this case, the reaction affords the
decomposition product 3b exclusively without the second
proton–electron transfer from hydrazine to the iron(IV) pincer
unit. In the reactions, both the conversion of diphenylhydrazine
and yield of 3b were higher for the amide-LH2 complex 1b (69%
conv., 36% yield) than the But-LH2 complex 1a (24% conv., 10%
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Y. Nakahara et al. / Polyhedron xxx (2017) xxx–xxx
5
Fig. 2. Crystal structures of 2bꢀ2THFꢀhexane. The CH hydrogen atoms, phenyl groups except for the ipso carbon atoms, and the minor component of the disordered group as
well as the solvating molecules are omitted for clarity. Thermal ellipsoids are drawn at the 30% level.
been characterized by 1H and 31P{1H} NMR spectroscopy [8]. In
Table 4
addition, the But-LH2 [36], Bun-LH2 [13], and Me-LH2 [8] ligands
Selected interatomic distances (Å) and angles (°) for 2b and 2c.
are known to form the mononuclear pincer-type complexes [RuCl
(R-LH2)(PPh3)2]Cl (e.g. 2a), while the substituent-free H-LH2 ligand
affords the dinuclear complex 2c similar to 2b (Chart 2) [36]; both
types of core structures have been confirmed by X-ray crystallog-
raphy. The 31P{1H} NMR appears diagnostic of the product nuclear-
ity. The resonances for the mononuclear complexes 2a and 2a0 are
observed at d 24.8 and 24.1, while those of the dinuclear complexes
appear at a lower field (d 35.3 for 2b, d 42.1 for 2c). On the other
hand, Halcrow proposed that the 31P chemical shift depends on
the electronic properties of the pendant group on the pyrazole ring
[8].
2bꢀ2THFꢀhexane
2cꢀ2CHCl3
Ru(1)–P(1)/P(2)
Ru(2)–P(3)/P(4)
Ru(1)–Cl(1)/Ru(2)–Cl(3)
Ru(1)–Cl(2)/Ru(2)–Cl(4)
Ru(1)–N(2)/N(11)a
Ru(2)–N(4)/N(9)a
Ref.
2.3374(14)/2.3260(15)
2.3540(15)/2.3299(15)
2.4181(13)/2.4108(14)
2.4346(13)/2.4282(14)
2.207(4)/2.220(4)
2.3409(11)/2.3342(11)
2.3243(11)/2.3380(10)
2.4141(10)/2.4159(10)
2.4358(10)/2.4489(10)
2.135(3)/2.169(3)
2.156(3)/2.166(3)
[36]
2.207(4)/2.189(4)
This work
a
For 2c, the N(11) and N(9) atoms are labeled as N(9) and N(7) in the original
paper [36].
In the crystal of 2b, the four pyrazole NH groups is involved in
intramolecular hydrogen bonding with the chlorido ligands inside
the dinuclear core (Nꢀ ꢀ ꢀCl(1)/Cl(3): 2.913(4)–3.073(4) Å, NHꢀ ꢀ ꢀCl
(1)/Cl(3): 2.32–2.56 Å) as in the H-LH2 analogue 2c. The four car-
boxamide NH groups form additional hydrogen bonds outside
the core (Nꢀ ꢀ ꢀCl(2)/Cl(4): 3.098(4)–3.233(4) Å, NHꢀ ꢀ ꢀCl(2)/Cl(4):
2.28–2.45 Å). The latter interactions appear to contribute the for-
mation of the dinuclear structure of 2b with significant twist of
the amide-LH2 ligands and deviation from the ideal C2v symmetry.
These hydrogen bonds are strong enough to preserve the dinuclear
structure of 2b even in much polar solvents such as methanol and
DMF, which has been confirmed by 1H NMR spectroscopy. In con-
trast, the formation of the dinuclear complex 2c without pendant
groups appears to be driven by the reduced steric hindrance
around the pyrazole nitrogen atoms distal from the central pyri-
dine [36]. The N–N–C angles around the uncoordinated nitrogen
atoms in 2b (111.9(4)–112.8(4)°) are commonly observed for pro-
tic pyrazole ligands [37]. The inside Ru(1)–Cl(1) and Ru(2)–Cl(3)
bonds (2.414 Å, mean) are shorter than the outside Ru(1)–Cl(2)
and Ru(2)–Cl(4) bonds (2.431 Å, mean) possibly due to the repul-
sion between the two inside chlorine atoms (Cl(1)ꢀ ꢀ ꢀCl(3): 3.305
(2) Å).
yield) in agreement with the increased Brønsted acidity of the pin-
cer ligand in 1b.
3.3. Synthesis and structure of ruthenium complex 2b
To gain deeper understanding of the substituent effect in the
pyrazole-based protic pincer-type complexes, we next explored
ruthenium complexes bearing the amide-LH2 ligand. Treatment
of [RuCl2(PPh3)3] and an equimolar amount of amide-LH2 led to
the formation of the dinuclear ruthenium complex 2b, where
two amide-LH2 ligands bridge the two ruthenium atoms
(Scheme 3). X-ray analysis revealed the dinuclear structure of 2b
(Fig. 2 and Table 4), and the 1H and 31P{1H} NMR spectra are in line
with an approximately C2v symmetric structure. Halcrow’s group
independently reported that a similar reaction affords a mononu-
clear pincer-type complex [RuCl(amide-LH2)(PPh3)2]Cl, which has
4. Conclusion
We have described the crystal structures of the octahedral
mono(amido-LH2)-type iron and ruthenium complexes 1b and 2b
featuring an amide pendant at the 5-position of the protic pyrazole
Chart 2.
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6
Y. Nakahara et al. / Polyhedron xxx (2017) xxx–xxx
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We are thankful for the PRESTO program on ‘‘Molecular Tech-
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