Synthesis and structures of NCN pincer-type ruthenium and iridium complexes bearing protic pyrazole arms

Article abstract of DOI:10.1021/acs.organomet.7b00065

The reaction of the 1,3-bis(pyrazol-3-yl)-benzenes 1 with [RuCl(OAc)(PPh₃)₃] resulted in selective C-H cleavage at the 2-position of 1 to give the protic NCN pincer-type ruthenium(II) complexes [RuCl(R-NCN-LH₂)-(PPh₃)₂] (2; R-NCN-LH₂ = 4-R-2,6-bis(5-tert-butyl-1H-pyrazol-3-yl)phenyl). Similar cyclometalation with iridium trichloride followed by addition of triphenylphosphine led to the formation of the iridium(III) analogue [IrCl(Bu^t-NCN-LH₂)(PPh₃)]Cl (5). Treatment of the ruthenium complexes 2 with carbon monoxide afforded the carbonyl complexes [Ru(CO)(R-NCN-LH₂)(PPh₃)₂]Cl (4). On the other hand, the pyridine analogue of 1, 2,6-bis(5-tert-butyl-1H-pyrazol-3-yl)pyridine (NNN-LH₂), reacted with iridium trichloride to yield the protic NNN pincer-type complex [IrCl₃(NNN-LH₃)] (7)., 2,6-bis(5-tert-butyl-1H-pyrazol-3-yl)pyridine (NNN-LH₂), reacted with iridium trichloride to yield the protic NNN pincer-type complex [IrCl₃(NNN-LH₃)] (7).. The stronger ω donation of the NCN pincer-type ligand in comparison with the analogous NNN ligand was suggested by the CO stretching frequencies of the ruthenium carbonyl complexes 4 as well as the M-Cl distances. The catalytic activity of the ruthenium complexes 2a,b toward transfer hydrogenation of a ketone was also evaluated.

Full text of DOI:10.1021/acs.organomet.7b00065

Article
pubs.acs.org/Organometallics
Synthesis and Structures of NCN Pincer-Type Ruthenium and Iridium
Complexes Bearing Protic Pyrazole Arms
Tatsuro Toda,† Koki Saitoh,† Akihiro Yoshinari,† Takao Ikariya,† and Shigeki Kuwata*,†,‡
†School of Materials and Chemical Technology, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552,
Japan
‡PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
S
* Supporting Information
ABSTRACT: The reaction of the 1,3-bis(pyrazol-3-yl)-
benzenes 1 with [RuCl(OAc)(PPh3)3] resulted in selective
C−H cleavage at the 2-position of 1 to give the protic NCN
pincer-type ruthenium(II) complexes [RuCl(R-NCN-LH2)-
(PPh3)2] (2; R-NCN-LH2 = 4-R-2,6-bis(5-tert-butyl-1H-pyr-
azol-3-yl)phenyl). Similar cyclometalation with iridium tri-
chloride followed by addition of triphenylphosphine led to
the formation of the iridium(III) analogue [IrCl(But-NCN-
LH2)(PPh3)]Cl (5). Treatment of the ruthenium complexes 2
with carbon monoxide afforded the carbonyl complexes
[Ru(CO)(R-NCN-LH2)(PPh3)2]Cl (4). On the other hand,
the pyridine analogue of 1, 2,6-bis(5-tert-butyl-1H-pyrazol-3-
yl)pyridine (NNN-LH2), reacted with iridium trichloride to
yield the protic NNN pincer-type complex [IrCl3(NNN-LH3)] (7). The stronger σ donation of the NCN pincer-type ligand in
comparison with the analogous NNN ligand was suggested by the CO stretching frequencies of the ruthenium carbonyl
complexes 4 as well as the M−Cl distances. The catalytic activity of the ruthenium complexes 2a,b toward transfer hydrogenation
of a ketone was also evaluated.
into dinitrogen and ammonia catalyzed by a protic pincer-type
iron complex, wherein electron transfer between the complex
and the substrate molecule is associated with the proton
transfer.7 To promote electron transfer from the metal to the
substrate, we embarked on a study of the synthesis of the
corresponding NCN pincer-type complexes bearing a central
aryl group instead of pyridine, in which the electron density of
the metal center would be increased by the strong electron
donation of this monoanionic group. Unlike the NNN pincer-
type analogues, however, this class of complexes has been
unexplored so far, possibly due to apparent synthetic
incompatibility between the Brønsted basic central carbon
atoms and protic arms. In this paper, we describe the efficient
synthesis of NCN pincer-type complexes of ruthenium and
iridium through cyclometalation of an appropriately designed
protic pyrazole. Comparison with related NNN complexes is
also described.
INTRODUCTION
■
The pincer-type complexes have emerged over the past decades
as versatile scaffolds for chemical transformations and materials
science.1 The tunable nature of the steric and electronic
parameters for these purposes is one of the distinctive traits of
the pincer-type ligands. Protic pyrazole is an attractive pendant
group in the pincer-type ligand to modulate the coordination
sphere through hydrogen bonding and proton transfer.2,3
During our extensive study on metal−ligand cooperation in the
coordination chemistry of the protic N-heterocycles,4,5 we have
recently been interested in the NNN pincer-type 2,6-
bis(pyrazol-3-yl)pyridine (NNN-LH2; Chart 1) complexes
furnished with two protic pyrazole arms6−12 and investigated
their proton-donating ability and metal−ligand cooperation.
Particularly remarkable is the disproportionation of hydrazine
Chart 1. Protic NNN and NCN Pincer-Type Ligands
RESULTS AND DISCUSSION
■
Synthesis of NCN Pincer-Type Ruthenium Complexes.
The protic nature of the desired pyrazole ligand prevents us
using some of the typical synthetic protocols for the carbon-
centered pincer-type complexes such as transmetalation and
Received: January 26, 2017
© XXXX American Chemical Society
A
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C−H metalation with main-group metals.1 We thus adapted
cyclometalation13 of the 1,3-bis(pyrazol-3-yl)benzene deriva-
tives 1, which were synthesized according to Scheme 1.14
a
Scheme 1
a
For 1b, see ref 14.
Heating of 1 with the acetato complex [RuCl(OAc)(PPh3)3] in
toluene led to selective C−H cleavage at the 2-position of the
central benzene ring to afford the protic pincer-type ruthenium-
(II) complexes [RuCl(R-NCN-LH2)(PPh3)2] (2) in moderate
isolated yields (Scheme 2). The reaction of 1 with
Scheme 2
Figure 1. Crystal structures of (a) 2a·0.5(hexane) and (b) 2b·0.5THF.
The CH hydrogen atoms, the minor components of the disordered
tert-butyl groups, and the solvating molecule are omitted for clarity.
Asterisks denote atoms generated by a symmetry operation (1 − x, −y,
1 − z). Ellipsoids are drawn at the 30% probability level.
Table 1. Selected Interatomic Distances (Å) and Angles
(deg) in 2a,b and 4a
2b
2a
molecule A molecule B
4a
[RuCl2(PPh3)3] under acetate-free conditions was not effective
for the formation of 2. While cyclometalation of arenes
furnished with two Schiff-base pendants at the meta positions
has been extensively studied,15 application to the synthesis of
protic pincer-type complexes remains rare.16
a
Ru1−X
2.5867(13)
1.985(5)
2.3661(13)
2.3574(13)
2.117(4)
2.080(4)
3.528(4)
2.60
2.604(2)
1.964(6)
2.390(2)
2.383(2)
2.104(4)
2.106(4)
3.645(6)
2.73
2.595(2)
1.993(6)
2.377(2)
2.372(2)
2.126(4)
2.116(4)
3.678(6)
2.75
1.947(3)
2.038(3)
2.3834(10)
2.3985(10)
2.101(3)
2.105(3)
3.227(2)
2.32
Ru1−C4
Ru1−P1
Ru1−P2
Ru1−N2
Ru1−N4
The 1H, 13C, and 31P NMR spectra of 2 are congruent with a
1
C2v-symmetric structure. In the H NMR spectra, the NH
b
resonance appears at δ 9.53, comparable with that of the NNN
analogue [RuCl(NNN-LH2)(PPh3)2]Cl (3; δ 10.53).8 The X-
ray analysis confirmed that cyclometalation of 1 gave the
desired pincer-type framework of 2 (Figure 1). Selected bond
distances and angles are given in Table 1. The structures of 2a,b
are essentially identical except for the backbone substituent of
the pincer ligand. The two phosphine ligands lie in mutually
trans positions. One of the two NH groups makes a close
contact with the chlorido ligand in a neighboring molecule
(H1···Cl1*, 2.60−2.75 Å) to form a hydrogen-bonded dimeric
structure, as in the related NNN pincer-type complex 3. The
Ru−Cl distances of 2a (2.5867(13) Å) and 2b (2.600 Å, mean
of the two crystallographically independent molecules) are
notably longer than those of the NNN pincer-type bis-
(pyrazole) complex 3 (2.4438(6) Å) and the deprotonated
derivative [RuCl(NNN-LH)(PPh3)2] (2.498(2) Å),8 indicating
the stronger σ donation of the NCN pincer-type ligand. The
N−N−C angles around the uncoordinated nitrogen atoms
N···Cl
b
H···Cl
N2−N1−C1
N4−N3−C12
110.9(4)
112.7(5)
111.8(5)
111.5(5)
111.4(5)
112.5(5)
111.7(3)
110.9(3)
a
b
X = Cl1 (2a,b), C25 (4a). N1−H1···Cl1* for 2a,b, N3−H2···Cl1
for 4a.
(110.9(4)−112.7(5)°) fall in the range of typical protic
pyrazole ligands.2
Derivatization to Carbonyl Complexes. To further
gauge the electron-donating ability of the NCN pincer-type
ligands, we next introduced a carbonyl ligand to the chlorido
complexes 2. When a solution of 2 was stirred under carbon
monoxide at room temperature, the cationic carbonyl
complexes [Ru(CO)(R-NCN-LH2)(PPh3)2]Cl (4) were ob-
tained, as shown in Scheme 2. The facile substitution of the
chlorido ligand substantiates the strong trans effect of the
B
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monoanionic NCN ligand. In fact, the corresponding reaction
of the NNN pincer-type complex 3 requires addition of a Lewis
acid and heating at 100 °C.5 The electron donation of the NCN
ligand in the cationic complexes 4 was also inferred by the CO
stretching bands at 1960 (4a) and 1963 cm−1 (4b), which are
much lower than that of the NNN pincer-type dicationic
product from 1a was rather complicated, the ESI-MS spectrum
exhibited peaks assignable to the cations [IrCl(But-NCN-
LH2)]+ and [Ir2Cl3(But-NCN-LH2)2]+. We thus considered
that chlorido-bridged oligomer(s) [IrCl2(But-NCN-LH2)]n
would be formed in this stage.19−21 As expected, addition of
an excess of triphenylphosphine to the product led to the clean
formation of the mononuclear phosphine complex [IrCl(But-
NCN-LH2)(PPh3)2]Cl (5) in moderate isolated yield. It should
be noted that similar treatment of the proligand 1b without the
tert-butyl group on the central benzene ring afforded a
complicated mixture. Unlike the reactions of the ruthenium
complex, this bulky substituent may be crucial in suppressing
C−H cleavage at the undesired 4- and 6-positions.19,22
complex [Ru(CO)(NNN-LH2)(PPh3)2](BArF )2 (1989 cm−1;
4
ArF = C6H3(CF3)2-3,5) and comparable with the wavenumber
of the uncharged bis(pyrazolato) complex [Ru(CO)(NNN-
L)(PPh3)2] (1958 cm−1).5
The crystal structure of the But-NCN complex 4a is depicted
in Figure 2. The two NH groups are engaged in hydrogen
The NCN pincer-type iridium(III) complex 5 was
1
characterized by H, 13C, and 31P NMR spectra, which are in
full agreement with the C2v-symmetric structure shown in
Scheme 3. The mononuclear structure has been established by
an X-ray analysis of the tetraphenylborate salt 6, which was
obtained by anion exchange of 5 (Figure 3 and Table 2). The
Figure 2. Crystal structure of 4a·2EtOH. The CH hydrogen atoms,
the minor components of the disordered tert-butyl groups, and one of
the solvating ethanol molecules are omitted for clarity. Ellipsoids are
drawn at the 30% probability level.
Figure 3. Structure of the cationic part of 6·2Et2O. The CH hydrogen
atoms and the minor components of the disordered tert-butyl groups
are omitted for clarity. Asterisks denote atoms generated by a
symmetry operation (2 − x, −y, − z). Ellipsoids are drawn at the 30%
probability level.
bonding with the chloride counteranion and one of the two
solvating ethanol molecules, suggesting the Brønsted acidic
nature. The carbonyl ligand lies in the pincer plane. The Ru1−
C4 bond trans to the carbonyl ligand elongated (2.038(3) Å)
relative to the chloride complex 2a (1.985(5) Å).
Synthesis of NCN Pincer-Type Iridium Complexes. The
synthesis of cyclometalated octahedral iridium(III) complexes
has been studied extensively to develop the organic light-
emitting devices.17 One of the standard and reliable methods
therein involves thermal reactions of iridium trichloride and the
ligand precursor in alcohols at high temperatures.17,18 We
applied this protocol to the 1,3-bis(pyrazol-3-yl)arenes 1, as
illustrated in Scheme 3. Although the 1H NMR spectrum of the
structure of the cationic part of 6 is quite similar to the
isoelectronic NNN pincer-type ruthenium complex 3 with a
hydrogen-bonded dimeric structure. The Ir−C(central) dis-
tance of 1.976(4) Å is comparable with those in the related
aprotic complexes bearing pyrazol-1-yl23 or N-heterocylic
Table 2. Selected Interatomic Lengths (Å) and Angles (deg)
in 6 and 7
6
7
Ir1−Cl1
2.5320(10)
1.976(4)
2.3672(10)
2.3751(11)
2.079(3)
2.083(3)
3.659(3)
2.73
2.366(2)
1.980(5)
2.357(2)
2.364(3)
2.050(4)
2.058(4)
a
Ir1−X
Ir1−Y1
Ir1−Y2
b
Scheme 3
b
Ir1−N2
Ir1−N4
N1···Cl1*
H1···Cl1*
N2−N1−C1
111.5(3)
112.0(4)
110.5(5)
110.6(5)
c
N4−N3−Z
a
b
c
X = C4 (6), N5 (7). [Y1, Y2] = [P1, P2] (6), [Cl2, Cl3] (7). Z =
C12 (6), C11 (7).
C
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carbene21,24 pendants in the benzene-centered pincer ligand
(1.94(1)−1.987(9) Å).
Table 3. Transfer Hydrogenation of Acetophenone
Catalyzed by Protic Pincer-Type Ruthenium Complexes
a
Synthesis of NNN Pincer-Type Iridium Complexes.
The isolation and structural characterization of the protic NCN
pincer-type iridium complexes 5 and 6 prompted us to
synthesize the corresponding NNN pincer-type complex for
comparison. The protic NNN-LH2 ligand, like the aprotic
terpyridine,25 reacted with iridium trichloride to yield the
iridium(III) complex [IrCl3(NNN-LH2)] (7), which represents
the first example of an iridium complex bearing both NNN-LH2
and labile monodentate coligands (Scheme 4). As related
b
entry
catalyst
yield, %
1
2
3
2a
2b
81
49
81
[RuCl(NNN-LH)(PPh3)2]
a
b
Conditions: [acetophenone] = 0.1 M. Determined by 1H NMR
Scheme 4
analyses.
examined promoted the reduction of acetophenone to give 1-
phenylethanol. The catalytic activity of the But-NCN pincer-
type complex 2a was found to be higher than that of 2b without
an alkyl substituent on the center of the pincer ligand (entries 1
and 2). It is to be noted that the reaction yield with 2a remains
comparable with those with the NNN pincer-type complex
[RuCl(NNN-LH)(PPh3)2] (entry 3), which shows that the
central donor of the protic pincer ligand on the transfer
hydrogenation catalysis was not prominent.
compounds, a rhodium analogue of 7, [RhCl3(NNN-LH2)],11
as well as an iridium complex bearing a doubly deprotonated
NNN-L derivative and a tridentate chelate ligand26 has been
reported quite recently.
Figure 4 depicts the crystal structure of 7. The short contacts
between the uncoordinated nitrogen atom and the oxygen
CONCLUSION
■
We have synthesized protic NCN pincer-type complexes of
ruthenium and iridium complexes through cyclometalation of
1,3-bis(pyrazol-3-yl)benzenes in an efficient manner. For the
synthesis of the iridium complex, a bulky group at the 5-
position of the benzene ring was required to disfavor the C−H
metalation at the undesired 4- and 6-positions. Comparison of
the CO stretching frequencies of the carbonyl complexes 4 and
the NNN analogues demonstrated the strong σ-donating ability
of the NCN pincer-type ligand endowed by the monoanionic
central aryl group. The M−Cl distances are also diagnostic of
the σ donation of the pincer-type ligands. On the other hand,
the NH groups in the pendant pyrazole maintain the Brønsted
acidity, as indicated by the hydrogen bonds observed in the
crystal structures. Although the effect of the increased σ
donation of the NCN ligand to the catalysis is currently not
evident, these insights will be beneficial for future development
of the catalytic application of the protic pincer-type complexes.
Figure 4. Crystal structure of 7·2DMF. The hydrogen atoms except
for the NH and formyl hydrogens are omitted for clarity. Ellipsoids are
drawn at the 30% probability level.
EXPERIMENTAL SECTION
■
General Procedures. All manipulations were performed under an
atmosphere of argon using standard Schlenk techniques unless
otherwise specified. Solvents were dried by refluxing over sodium
benzophenone ketyl (THF, toluene, diethyl ether, 1,4-dioxane, n-
pentane, and n-hexane), P2O5 (dichloromethane), and Mg(OMe)2
(methanol) and distilled before use. DMF and ethanol (dehydrated)
were purchased from Kanto Chemical and used as received. 2-
Methoxyethanol was degassed by argon bubbling before use.
Dichloromethane-d2 and methanol-d4 were dried over P2O5 and
CaH2, respectively, and subjected to trap-to-trap distillation and three
subsequent freeze−pump−thaw cycles. Dimethyl sulfoxide-d6 was
degassed by three freeze−pump−thaw cycles and dried over MS4A.
Compound 1b was prepared according to the literature procedure.14
atom in the solvating dimethylformamide molecules (2.691(7)
and 2.696(7) Å) indicate the presence of the NH protons and
the hydrogen bonds (H···O 1.79 and 1.77 Å). The Ir−
Cl(equatorial) distance of 2.3659(16) Å is much shorter than
that in the NCN pincer-type complex 6 (2.5320(10) Å), in line
with the weaker trans influence of the NNN ligand.
Catalytic Hydrogenation of a Ketone with Protic
Pincer-Type Ruthenium Complexes. Pincer-type ruthe-
nium complexes bearing the protic NNN-LH2 and its derivative
are known to catalyze transfer hydrogenation of ketones with 2-
propanol.12 To assess the impact of the central aryl group in the
NCN-LH2 pincer ligand on the catalysis, we compared the
catalytic performance of the NCN pincer-type complexes 2 and
the NNN analogue. The reactions were carried out at 50 °C for
15 h in 2-propanol containing 2 mol % of the catalyst and a
base (Table 3). All of the protic pincer-type complexes we
Other reagents were used as received. H (399.78 MHz), 13C{1H}
1
(100.53 MHz), and 31P{1H} (161.83 MHz) NMR spectra were
1
obtained on a JEOL JNM-ECX-400 spectrometer. H NMR shifts are
relative to the signal of the residual CHCl3 (δ 7.26), CHDCl2 (δ 5.32),
and DMSO-d6 (δH 2.50), while 13C{1H} and 31P{1H} NMR shifts are
referenced to CDCl3 (δ 77.0) or CD2Cl2 (δ 53.5) and phosphoric acid
(δ 0.0), respectively. Infrared spectra were recorded on a JASCO FT/
D
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IR-6100 spectrometer. ESI-MS spectra were obtained on a JEOL JMS-
T100LC spectrometer with a positive ionization mode. Elemental
analyses were performed on a PerkinElmer 2400II CHN analyzer.
Synthesis of Diethyl 5-tert-Butylisophthalate (8). The title
compound was prepared according to a procedure modified from that
described in the literature.27 A mixture of 5-tert-butylisophthalic acid
(10.3 g, 46.5 mmol) and concentrated H2SO4 (2 mL) in ethanol (100
mL) was allowed to reflux for 6 h. Evaporation of the solvent under
reduced pressure, the remaining materials were dissolved in dichloro-
methane (60 mL), and neutralized by saturated sodium bicarbonate
solution. The organic layer was separated, washed with water (50 mL)
and brine (50 mL), and dried over Na2SO4. Removal of the filtrate
afforded 8 as a white solid (11.0 g, 39.5 mmol, 85%), which was used
for the subsequent reaction without further purification.
3JHH = 7.3 Hz, 2H, m-C6H), 7.06−7.18 (m, 30H, PPh3), 9.53 (br s,
2H, NH). 13C{1H} NMR (CD2Cl2): δ 30.0 (C(CH3)3), 31.0
(C(CH3)3), 96.6 (pyrazole 4-CH), 117.0, 118.8, 127.1 (t, JCP = 3.8
Hz, PPh3), 128.3 (PPh3), 133.7 (t, JCP = 5.3 Hz, PPh3), 134.7 (t, JCP
=
17.8 Hz, PPh3), 138.8, 154.1, 161.1, 203.6 (t, JCP = 8.2 Hz, RuC).
31P{1H} NMR (CD2Cl2): δ 34.0 (s, PPh3). Anal. Calcd for
C56H55ClN4P2Ru: C, 68.46; H, 5.64; N, 5.70. Found: C, 68.85; H,
5.76; N, 5.61. Single crystals suitable for X-ray analysis were obtained
by recrystallization from THF−hexane.
Synthesis of [Ru(But-NCN-LH2)(PPh3)2(CO)]Cl (4a). To a
solution of 2a·(toluene) (117.4 mg, 0.1038 mmol) in toluene (8
mL) was introduced CO (1 atm) by freeze−pump−thaw cycles (three
times). The mixture was stirred for 14 h at room temperature. The
cream yellow precipitate that formed was filtered off and dried in
vacuo. Subsequent recrystallization from dichloromethane−hexane (3
mL/18 mL) afforded 4a as cream yellow crystals (73.4 mg, 0.0688
Synthesis of 5-tert-Butyl-1,3-bis(5-tert-butylpyrazol-3-yl)-
benzene (1a). To a suspension of NaH (60 wt % in mineral oil,
1.89 g, 47.3 mmol) in THF (20 mL) was added 3,3-dimethyl-2-
butanone (5.50 mL, 44.5 mmol). The mixture was stirred for 20 min at
room temperature and then heated to reflux. To the boiling mixture
was added 8 (5.62 g, 20.2 mmol) in THF (20 mL) over the course of
25 min, and the mixture was allowed to reflux for an additional 30 min.
After cooling, the mixture was treated with a 1 M HCl solution until
pH 7 and extracted with diethyl ether (15 mL × 4). The combined
organic layer was washed with brine (30 mL) and dried over Na2SO4.
Evaporation of the solvent under reduced pressure afforded 5-tert-
butyl-1,3-bis(1,3-dioxo-4,4-dimethylpentyl)benzene as a yellow oil
(8.41 g). To a boiling solution of the crude diketone in ethanol (70
mL) in an open flask was added hydrazine monohydrate (6.3 mL, 130
mmol) over the course of 10 min. The mixture was allowed to reflux
for an additional 4 h. After removal of the solvent in vacuo, the residue
was dissolved in chloroform (60 mL), washed with brine (30 mL × 3),
and dried over Na2SO4. After evaporation of the solvent under
reduced pressure, the resultant yellow solid was dissolved in warm
dichloromethane (100 mL). Keeping the solution at −30 °C yielded
1
mmol, 66%). H NMR (CD2Cl2): δ 1.21 (s, 18H, But), 1.42 (s, 9H,
4
But), 5.67 (d, JHH = 1.8 Hz, 2H, pyrazole CH), 6.91 (s, 2H, C6H2),
7.03−7.24 (m, 30H, PPh3), 11.99 (br s, 2H, NH). 13C{1H} NMR
(CD2Cl2): δ 29.9 (pyrazole-C(CH3)3), 32.1 (phenyl-C(CH3)3), 31.4
(pyrazole-C(CH3)3), 34.7 (phenyl-C(CH3)3), 97.2 (pyrazole CH),
118.0 (phenyl CH), 128.0 (t, JCP = 4.8 Hz, PPh3), 129.2 (PPh3), 131.9
(t, JCP = 21.6 Hz, PPh3), 133.7 (t, JCP = 5.3 Hz, PPh3), 138.2, 146.9,
158.2, 161.7, 192.8 (t, 2JCP = 11.1 Hz, RuC), 201.8 (t, 2JCP = 12.0 Hz,
CO). 31P{1H} NMR (CD2Cl2): δ 37.3 (s, PPh3). IR (CH2Cl2): νmax
̃
(cm−1) 1960 (CO). Anal. Calcd for C61H63ClN4OP2Ru: C, 68.69;
H, 5.95; N, 5.25. Found: C, 68.34; H, 6.13; N, 5.26. Single crystals
suitable for X-ray analysis were obtained by recrystallization from
ethanol−pentane.
Synthesis of [Ru(H-NCN-LH2)(PPh3)2(CO)]Cl (4b·H2O). This
compound was synthesized from 2b by a procedure similar to that
used for 4a. Recrystallization from wet dichlomethane−methanol−
1
diethyl ether afforded 4b·H2O in 51% yield. H NMR (CD2Cl2): δ
1.18 (s, 18H, But), 5.65 (s, 2H, pyrazole CH), 6.81 (d, 3JHH = 7.3 Hz,
1
1a as a white powder (0.934 g, 2.47 mmol, 12% over two steps). H
3
2H, m-C6H), 6.98 (t, JHH = 7.5 Hz, p-C6H), 7.02−7.24 (m, 30H,
NMR (CDCl3): δ 1.39 (s, 18H, But), 1.40 (s, 9H, But), 6.45 (s, 2H,
pyrazole CH), 7.74 (br s, 2H, 4- and 6-CH), 7.87 (br s, 1H, 2-CH),
9.96 (br s, 2H, NH). 13C{1H} NMR (CDCl3, 323 K): δ 30.4
(pyrazole-C(CH3)3), 31.3 (phenyl-C(CH3)3), 34.8 (phenyl-C(CH3)3),
99.2 (pyrazole 4-CH), 120.8 (2-CH), 122.4 (4- and 6-CH), 132.9,
150.1, 152.0, 156.7. The tert-butyl quaternary carbon signal on the
pyrazole arm is overlapped with the signal at δ 31.3, which was
confirmed by HMQC and HMBC experiments. Anal. Calcd for
C24H34N4: C, 76.15; H, 9.05; N, 14.80. Found: C, 76.01; H, 9.23; N,
14.85.
PPh3), 11.69 (br s, 2H, NH). 13C{1H} NMR (CD2Cl2−CD3OD = 1:1
v/v): δ 29.8 (C(CH3)3), 31.3 (C(CH3)3), 98.3 (pyrazole CH), 121.2,
123.8, 127.9 (t, JCP = 3.8 Hz, PPh3), 129.7 (PPh3), 131.6 (t, JCP = 21.6
Hz, PPh3), 133.7 (t, JCP = 5.3 Hz, PPh3), 139.6, 158.4, 161.9, 195.4 (t,
2
2JCP = 11.1 Hz, RuC), 202.4 (t, JCP = 11.5 Hz, CO). 31P{1H} NMR
(CD2Cl2): δ 37.2 (s, PPh3). IR (CH2Cl2): νmax
Anal. Calcd for C57H57ClN4O2P2Ru: C, 66.56; H, 5.59; N, 5.45.
Found: C, 66.82 H, 5.89; N, 5.55.
̃
(cm−1) 1963 (CO).
Synthesis of [IrCl(But-NCN-LH2)(PPh3)2]Cl (5). A mixture of
IrCl3·xH2O (Ir; 52.12%, 179.5 mg, 0.487 mmol) and 1a (187.4 mg,
0.495 mmol) in degassed 2-methoxyethanol (20 mL) was allowed to
reflux for 6 h. After evaporation of the solvent under reduced pressure,
triphenylphosphine (1.029 g, 3.92 mmol) and 1,4-dioxane (20 mL)
were added to the remaining solid, and the reaction mixture was
allowed to reflux for 13 h. After removal of the solvent in vacuo,
subsequent recrystallization from dichloromethane−diethyl ether (30
mL/50 mL) afforded 5 as pale yellow crystals (305.9 mg, 0.2625
Synthesis of [RuCl(But-NCN-LH2)(PPh3)2]·(toluene) (2a·(tol-
uene)). A mixture of [RuCl(OAc)(PPh3)3] (199.0 mg, 0.203 mmol)28
and 1a (78.4 mg, 0.207 mol) in toluene (10 mL) was stirred for 15 h
at 100 °C. The mixture was filtered, and the filtrate was evaporated to
dryness. The residue was washed with diethyl ether (10 mL × 3) and
dried in vacuo. The remaining solid was extracted with toluene (10
mL). Slow addition of hexane (40 mL) to the extract afforded 2a·
(toluene) as yellowish green crystals (69.9 mg, 0.0618 mmol, 31%).
The presence of the solvated toluene was confirmed by 1H and
13C{1H} NMR. 1H NMR (CD2Cl2): δ 1.11 (s, 18H, But), 1.41 (s, 9H,
1
mmol, 54%). H NMR (CD2Cl2): δ 1.20 (s, 18H, But), 1.44 (s, 9H,
4
But), 5.79 (d, JHH = 2.2 Hz, 2H, pyrazole CH), 6.91 (s, 2H, C6H2),
7.16−7.28 (m, 30H, PPh3), 12.14 (br s, 2H, NH). 13C{1H} NMR
(CD2Cl2): δ 29.8, 31.7, 32.0, 34.8, 98.2, 118.9, 128.2 (t, JCP = 4.8 Hz,
PPh3), 128.3 (t, JCP = 26.8 Hz, PPh32), 129.9 (PPh3), 134.2 (t, JCP = 5.3
Hz, PPh3), 134.7, 146.8, 156.6 (t, JCP = 5.8 Hz, IrC), 157.9, 161.1.
31P{1H} NMR (CD2Cl2): δ −10.0 (s, PPh3). Anal. Calcd for
C60H63Cl2N4P2Ir: C, 61.84; H, 5.45; N, 4.81. Found: C, 61.85; H,
5.54; N, 4.73. Single crystals of the tetraphenylborate salt 6 for X-ray
analysis were obtained by anion exchange of 5 with excess NaBPh4 and
recrystallization from dichloromethane−diethyl ether. Data for 6 are as
4
But), 6.00 (d, 2H, JHH = 2.2 Hz, pyrazole CH), 6.97 (s, 2H, C6H2),
7.02−7.20 (m, 30H, PPh3), 9.53 (br s, 2H, NH). 13C{1H} NMR
(CD2Cl2): δ 30.0, 31.0, 32.3, 34.4, 96.4, 116.4, 127.0 (t, JCP = 3.8 Hz,
PPh3), 128.2 (PPh3), 133.7 (t, JCP = 5.3 Hz, PPh3), 134.9 (t, JCP = 17.8
Hz, PPh3), 140.8, 154.0, 161.4, 198.5 (br, RuC). The two carbon
signals on the phenyl ligand are overlapped at 116.4 ppm, which was
confirmed by HMQC and HMBC experiments. 31P{1H} NMR
(CD2Cl2): δ 33.9 (s, PPh3). Anal. Calcd for C67H71ClN4P2Ru: C,
71.16; H, 6.33; N, 4.95. Found: C, 70.86; H, 6.60; N, 4.96. Single
crystals suitable for X-ray analysis were obtained by recrystallization
from THF−hexane.
1
follows. H NMR (CD2Cl2): δ 1.12 (s, 18H, But), 1.38 (s, 9H, But),
4
6.16 (d, JHH = 2.1 Hz, 2H, pyrazole CH), 6.85−6.89, 7.00−7.37 (m,
50H, PPh3 and BPh4), 6.98 (s, 2H, m-CH), 9.54 (br s, 2H, NH).
Synthesis of [RuCl(H-NCN-LH2)(PPh3)2] (2b). This compound
was synthesized from 1b by a procedure similar to that used for 2a.
31P{1H} NMR (CD2Cl2): δ −13.3 (s, PPh3).
1
4
Yield: 34%. H NMR (CD2Cl2): δ 1.10 (s, 18H, But), 5.96 (d, JHH
=
Synthesis of [IrCl3(NNN-LH2)]·2DMF (7·2DMF). To a boiling
solution of IrCl3·xH2O (Ir 52.12%, 218.3 mg, 0.592 mmol) in ethanol
1.8 Hz, 2H, pyrazole CH), 6.69 (t, 3JHH = 7.5 Hz, 1H, p-C6H), 6.82 (d,
E
DOI: 10.1021/acs.organomet.7b00065
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(25 mL) was added dropwise a solution of 2,6-bis(5-tert-butyl-1H-
pyrazol-3-yl)pyridine (193.4 mg, 0.598 mmol)8 in ethanol (45 mL),
and the mixture was allowed to reflux for 3 h in the dark. After the
solution was cooled to room temperature, the reaction mixture was
filtered. After evaporation of the filtrate in vacuo, the residue was
recrystallized from DMF−diethyl ether (25 mL/150 mL). The orange
microcrystalline solid that formed was filtered off, washed with diethyl
ether (10 mL × 2) and hexane (10 mL), and dried in vacuo (274.7 mg,
work was partially supported by a Grant-in-Aid for Scientific
Research on Innovative Areas “Stimuli-responsive Chemical
Species (No. 2408)” (JSPS KAKENHI Grant Number
JP15H00928). T.T. acknowledges a JSPS Research Fellowship
for Young Scientists.
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0.358 mmol, 60%). H NMR (DMSO-d6): δ 1.43 (s, 18H, But), 7.14
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Catalytic Transfer Hydrogenation of Acetophenone. A
solution of potassium tert-butoxide (0.5 M, 1.3 equiv/Ru) in THF
was added to a mixture of ruthenium complex (2 mol %) and
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1
determined by H NMR using 1,3,5-trimethoxybenzene as an internal
standard.
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
Kα radiation (λ= 0.71070 Å). Intensity data were corrected for
Lorentz−polarization effects and for absorption. Details of crystal and
data collection parameters are shown in the Supporting Information.
Structure solution and refinements were carried out by using the
CrystalStructure program package.29 The heavy-atom positions were
determined by a direct methods program (SIR9230), and the
remaining non-hydrogen atoms were found by subsequent Fourier
syntheses. The non-hydrogen atoms other than those mentioned
below were refined anisotropically by full-matrix least-squares
techniques based on F2. Some tert-butyl groups in 2a·0.5(hexane),
2b·0.5THF, 4a·2EtOH, and 6·2Et2O were placed at two disordered
positions and isotropically refined; for 2a·0.5(hexane) and 4a·2EtOH,
the isotropic thermal parameters were fixed and the structures were
refined with restraint geometries. The solvating hexane in 2a·
0.5(hexane), THF in 2b·0.5THF, and one of the two diethyl ether
molecules in 6·2Et2O were partially disordered and the non-hydrogen
atoms therein were included in the refinements with fixed isotropic
thermal parameters and restraint geometries. The hydrogen atoms in
these disordered moieties were not included in the refinements. The
OH hydrogen atoms in the solvated ethanol molecules in 4a·2EtOH
were found in the difference Fourier map and refined with a riding
model. The rest of the hydrogen atoms were placed at calculated
positions and included in the refinements with a riding model.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00065.
1H, 13C, and 31P NMR spectra (PDF)
Crystallographic data (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail for S.K.: skuwata@apc.titech.ac.jp.
■
ORCID
Shigeki Kuwata: 0000-0002-3165-9882
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are thankful for the PRESTO program on “Molecular
Technology and Creation of New Functions” from JST. This
■
F
DOI: 10.1021/acs.organomet.7b00065
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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