Synthesis and redox properties of PNP pincer complexes based on N-methyl-4,4′-bipyridinium

Article abstract of DOI:10.1002/ejic.201402349

We have designed and prepared a series of novel PNP pincer ligands based on N-methyl-4,4′-bipyridinium for the construction of stable complexes toward two steps of single electron transfer reactions, where p-quinoid-type complexes play a key role. Electrochemical studies reveal that two reduction waves of these complexes are similar to those of methylviologen and the corresponding p-quinoid type complexes form as stable two electron reduced species. The rhodium complex bearing PNP pincer ligand based on N-methyl-4,4′-bipyridinium is also found to work as a catalyst for dihydrogen production under the photoinduced electron-transfer conditions. We have designed and prepared a series of novel PNP pincer ligands based on N-methyl-4,4′-bipyridinium for the construction of stable complexes toward two steps of single electron transfer reactions, where p-quinoid-type complexes play a key role. The rhodium complex bearing PNP pincer ligand based on N-methyl-4,4′-bipyridinium is also found to work as a catalyst for dihydrogen production under the photoinduced electron-transfer conditions.

Full text of DOI:10.1002/ejic.201402349

FULL PAPER
DOI:10.1002/ejic.201402349
Synthesis and Redox Properties of PNP Pincer Complexes
Based on N-Methyl-4,4Ј-bipyridinium
Yoshihiro Miyake,*[a][‡] Kazunari Nakajima,[a] Yu Higuchi,[a] and
Yoshiaki Nishibayashi*[a]
COVER PICTURE
Keywords: Photocatalysis / Electron transfer / Dihydrogen / Ligand design / Metalloviologens
We have designed and prepared a series of novel PNP pincer
ogen and the corresponding p-quinoid type complexes form
ligands based on N-methyl-4,4Ј-bipyridinium for the con- as stable two electron reduced species. The rhodium complex
struction of stable complexes toward two steps of single elec- bearing PNP pincer ligand based on N-methyl-4,4Ј-bipyrid-
tron transfer reactions, where p-quinoid-type complexes play
a key role. Electrochemical studies reveal that two reduction
waves of these complexes are similar to those of methylviol-
inium is also found to work as a catalyst for dihydrogen pro-
duction under the photoinduced electron-transfer conditions.
chemistry.[1] For instance, methylviologen (N,NЈ-dimethyl-
4,4Ј-bipyridinium dication; abbreviated as MV2+) is known
Introduction
N,NЈ-Disubstituted-4,4Ј-bipyridinium dications, which are to work as a stable and reversible two-electron acceptor,[1]
commonly referred to as viologens, have been a quite at- and its two-electron reduced form (MV0) is recognized to
tractive class of molecules because of the intrinsic applica- have a dearomatized p-quinoid structure (Scheme 1a).[2]
A
bility in various fields. Especially, the unique properties of variety of viologen analogues have been synthesized until
high electron acceptability and stability toward reversible now,[3] and significant interests have also been focused on
redox events enable their utilization in electro- and photo- N-substituted 4,4Ј-bipyridyls bound to a transition metal,
Scheme 1. Reactivities of (a) methylviologen, (b) PNP pincer complex, and (c) metalloviologen bearing PNP pincer skeleton.
[a] Institute of Engineering Innovation, School of Engineering,
which have been called “metalloviologens”, because the in-
The University of Tokyo,
teractions between metallic center and π-conjugated 4,4Ј-
Yayoi, Bunkyo-ku, Tokyo, 113-8656, Japan
E-mail: ynishiba@sogo.t.u-tokyo.ac.jp
http://park.itc.u-tokyo.ac.jp/nishiba/
[‡] Present address: Department of Applied Chemistry, Graduate
School of Engineering, Nagoya University, Furo-cho, Chikusa-
ku, Nagoya, 464-8603, Japan
bipyridyl systems have been expected to affect optical and
magnetic properties and to promote unique reactions.[4–8]
Although various transition metal complexes having N-alk-
yl- or N-aryl-4,4Ј-bipyridinium as ligands have been synthe-
sized, the applicable skeletons for metalloviologens were
limited to monodentate 4,4Ј-bipyridyl,[4] π-extended
E-mail: miyake@apchem.nagoya-u.ac.jp
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402349.
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4,4Ј:2Ј,2ЈЈ:4ЈЈ,4ЈЈЈ-quaterpyridyl,[5] 4Ј-(4-pyridyl)-2,2Ј:6Ј,2ЈЈ- pyridine ring, we have applied iridium-catalyzed selective
terpyridyl derivatives[6,7] and 2-oxazolinyl-4,4Ј-bipyridyls.[8] C–H borylation of the pyridine ring[15] as a key step be-
Pyridine-based PNP pincer ligands [2,6-bis(phosphino- cause of the versatile utility of boryl-carbon nucleophiles
mehyl)pyridines] have been known to work as rigid triden- for biaryl formation by using Suzuki–Miyaura coupling re-
tate ligands and to be introduced in various transition metal action. Reactions of 2a and 2b with bis(pinacolato)diboron
complexes, which have been reported to show unique stoi- [(Bpin)2] in the presence of catalytic amounts of [Ir(cod)-
chiometric and catalytic reactivities.[9–11] Recently, our OMe]2 (cod = 1,5-cyclooctadiene) and 4,4Ј-di-tert-butyl-
group has also reported the synthesis of dinitrogen-bridged 2,2Ј-bipyridyl (dtbpy) proceeded smoothly to give the corre-
dimolybdenum complexes supported by PNP pincer ligands sponding borylated products (3a and 3b) in 88% and 80%
and their application toward catalytic nitrogen-fixation.[10] yields, respectively. Subsequently, Suzuki–Miyaura coupling
One of the most important features of these ligands is that reactions of 3 with 4-bromopyridine afforded the corre-
the rigid tridentate backbone allows dearomatization of the sponding 4,4Ј-bipyridyls (4a and 4b) in 50% and 53% yield,
central pyridine ring induced by the deprotonation of meth- respectively. Methylation of the nitrogen atom and desulfur-
ylene moiety on the PNP pincer ligand (Scheme 1b). In ization of the phosphorus atoms in 4 were carried out by
fact, Milstein’s and other groups have reported that a series treatment with 3 equivalents of methyl trifluoromethane-
of dearomatized o-quinoid-type complexes with PNP and sulfonate (MeOTf) and successive 2 equiv. of hexamethyl-
related pincer ligands work as key intermediates in their phosphorus triamide.[16] The PNP ligand [1a][OTf] was ob-
catalytic reactions.[9c,11–13] Here, we have designed PNP tained as a viscous oil, but was rapidly decomposed into
pincer ligands based on N-methyl-4,4Ј-bipyridinium for the unidentified compounds. After an anion exchange reaction
construction of stable complexes toward two steps of single of [1a][OTf] with NaBF4, [1a][BF4] was obtained in 59%
electron transfer reactions, where p-quinoid-type complexes yield as a stable yellow solid. In contrast, [1b][OTf] was iso-
play a key role (Scheme 1c). In this paper, we have described lated as a stable solid in 51% yield. This procedure combin-
the synthesis of novel PNP pincer complexes based on N- ing C–H borylation with Suzuki–Miyaura coupling reac-
methyl-4,4Ј-bipyridinium as “metalloviologens”, and inves- tion is considered to be applicable for the synthesis of struc-
tigation of their electro- and photo-chemical reactivities in- turally diverse 4-functionalized PNP pincer ligands.
cluding catalytic dihydrogen production under the visible-
light induced electron transfer conditions.
Next, we prepared platinum and palladium complexes
bearing the pincer ligands 1 (Scheme 3). Reactions of
PtCl2(PhCN)2 with [1a][BF4] or [1b][OTf] proceeded
smoothly to give the corresponding dicationic platinum
complexes [6a][BF4]2 and [6b][OTf]2 in 82% and 35% yield,
Results and Discussion
At first, we synthesized novel PNP pincer ligands (1a and respectively (Scheme 3a). The 31P{1H} NMR spectrum of
1b) (Scheme 2). Four methyl groups at the benzylic posi- [6a][BF4]2 [δ 48.9 (s with satellite JPt,P = 2389 Hz)] indicated
tions in 1b are introduced to prevent the formation of o- the similar geometry to those of the reported platinum com-
quinoid-type complexes under basic conditions. Although plexes bearing pyridine-based PNP pincer ligands.[12c,17] Di-
various PNP pincer complexes have been prepared until cationic palladium complex [7a][BF4]2 was also prepared in
now,[9] examples of PNP pincer ligands having a substituent a similar manner from the reaction of PdCl2(PhCN)2
at the 4-position on the pyridine ring are still rare.[14] In (Scheme 3b). The unambiguous structure of [6b]2+ was de-
order to introduce a substituent at the 4-position of the termined by X-ray crystallographic analysis, and an OR-
Scheme 2. Synthesis of 1.
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TEP drawing is shown in Figure 1. The coordination geom- of the crude products exhibited the formation of a mixture
etry at the Pt atom of [6b]2+ is essentially square planar. The of [8a][BF4]2 and [9a][BF4]2 in ca. 1:1 ratio. The formation
bond lengths and angles around the coordination sphere of of RhIII complex [8a][BF4]2 indicates that AgCl generated
the Pt atom are all comparable to those of the reported in situ works as an oxidant toward RhI complex under the
platinum complex bearing pyridine-based PNP pincer li- present reaction conditions. Hence, when the reaction of
gands.[12c,17] The small dihedral angle [16.0(14)°] between [RhCl(coe)2]2 with [1a][BF4] was carried out in the presence
N-methylpyridinium ring and the pyridine ring on the PNP of both AgBF4 and AgCl, the corresponding RhIII complex
pincer core suggests an effective conjugation in metalloviol- [8a][BF4]2 was isolated in 54% yield (Scheme 4a). On the
ogen [6b]2+
.
other hand, treatment of [Rh(cod)(MeCN)2][BF4] with
[1a][BF4] afforded the corresponding RhI complex [9a]-
[BF4]2 in 83% yield (Scheme 4b). The 31P{1H} NMR spec-
trum of [9a][BF4]2 {δ = 54.7 (d, JRh-P = 134 Hz)} indicated
the similar geometry to that of the reported rhodium com-
plex bearing pyridine-based PNP pincer ligand.[12b] The un-
ambiguous structure of [8a]2+ was determined by X-ray
crystallographic analysis, and an ORTEP drawing is shown
in Figure 2. The configuration around the Rh atom of
[8a]2+ adopts a typical octahedral coordination geometry.
Two chloride ligands on the Rh atom are located trans to
each other. The structure of the 4,4Ј-bipyridyl core in
[8a]2+ is similar to that in [6b]2+, and N-methylpyridinium
ring is slightly twisted by 12.7(9)° with regard to the pyr-
idine ring on the PNP pincer core.
Scheme 3. Synthesis of platinum and palladium complexes bearing
1.
Scheme 4. Synthesis of rhodium complexes bearing 1a.
Figure 1. ORTEP drawing of the cationic part of [6b][Cl][OTf]. One
structure of two crystallographycally independent molecules is
shown. The structure of these two independent molecules is almost
the same. Thermal ellipsoids are shown at the 50% probability
level. Hydrogen atoms are omitted for clarity. Selected bond lengths
[Å], angles [°] and torsion angle [°]: Pt1–Cl1, 2.296(4); Pt1–N1,
2.034(9); Pt1–P1, 2.261(3); Pt1–P2, 2.273(3); N2–C29, 1.465(15);
C21–C26, 1.476(16); P1–Pt1–P2, 167.11(9); Cl1–Pt1–N1, 177.0(3);
C20–C21–C26–C25, 16.0(14).
Figure 2. ORTEP drawing of the cationic part of [8a][BF4]2. Ther-
mal ellipsoids are shown at the 50% probability level. Hydrogen
atoms are omitted for clarity. Selected bond lengths [Å], angles [°]
and torsion angle [°]: Rh1–Cl1, 2.3446(19); Rh1–Cl2, 2.3422(17);
Rh1–N1, 2.048(6); Rh1–P1, 2.3541(19); Rh1–P2, 2.3719(17); N2–
C25, 1.472(11), C17–C22, 1.490(9); Cl1–Rh1–Cl2, 179.05(6); P1–
Rh1–P2, 166.22(7); N1–Rh1–N3, 179.3(3); C18–C17–C22–C23,
Synthesis of rhodium complexes bearing 1a was also in-
vestigated (Scheme 4). At first, we examined the reaction of
[RhCl(coe)2]2 (coe = cyclooctene) with 2 equiv. of AgBF4
(1 equiv. per Rh atom) and sequential treatment with
1
[1a][BF4] in MeCN. H NMR and 31P NMR measurement 12.7(9).
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The cyclic voltammetric analysis of “metalloviologens” vealed that “metalloviologens” are easily reduced by
was performed in MeCN with 0.1 m nBu4NBF4 as an elec- [Ru(bpy)3]2+ (bpy = 2,2Ј-bipyridyl) in the presence of reduc-
trolyte (Table 1). The pincer ligands 1a and 1b have two tants under photoirradiation.[18] From these results, we con-
reversible reduction waves assigned to the formation of the sider that “metalloviologens” are applicable for photocata-
one electron reduced form (10) and the two electron reduced lytic reduction process and we examined the photocatalytic
form (1–). As expected, the corresponding two steps of re- dihydrogen production[1d,7,19–22] in the presence of [8a]-
ductions in metalloviologens such as [6a][BF4]2, [6b]- [BF4]2 (Scheme 5). When a solution of tetrafluoroboric acid
[OTf]2, [8a][BF4]2 and [9a][BF4]2 occur at much more posi- (HBF4) and triethanolamine (TEOA) in the presence of
tive potential than those of 1a and 1b, while [7a][BF4]2 has catalytic amounts of [8a][BF4]2 and [Ru(bpy)3][BF4]2 in
only one reversible reduction wave at –0.69 V and one acetonitrile/water (1:1) was irradiated by visible light (λ Ͼ
irreversible reduction wave at –1.10 V. Because the second 400 nm) for 18 h under nitrogen atmosphere, the pro-
reduction event corresponding to 1–/0 of 7a might be close duction of dihydrogen was observed with the turnover
to the reduction event corresponding to PdI/II, both reduci- number (TON) of 8.2.[23]
ton events are thought to be observed as one irreversible
reduction wave. In sharp contrast, the PNP pincer com-
plexes such as 10, 11, and 12 show only reduction waves
corresponded to the redox events on the metal center. These
results indicate that the 4,4Ј-bipyridinium moiety in
“metalloviologens” works as an efficient electron acceptor
unit, and the pincer ligands such as 1a and 1b can easily
impart an electron accepting ability to the corresponding
complexes by coordination to a variety of transition metals.
It is also noteworthy that two reduction waves of metallovi-
ologens are observed in similar to those of MV2+[2a] and
the corresponding p-quinoid type complexes are expected
to form as stable two electron reduced species (Scheme 1c).
In addition, reduction potentials of “metalloviologens” re-
Scheme 5. Photocatalytic dihydrogen production using 8a[BF4]2.
Conclusions
In summary, we have designed and prepared novel PNP
pincer complexes based on N-methyl-4,4Ј-bipyridinium as
“metalloviologens” for the construction of stable complexes
toward two steps of single electron transfer reactions. Elec-
trochemical studies revealed that two reduction waves of
metalloviologens are similar to those of MV2+ and the cor-
responding p-quinoid type complexes form as stable two
electron reduced species. The metalloviologen [8a]2+ was
also revealed to work as a catalyst for dihydrogen pro-
duction under the photoinduced electron transfer condi-
tions. Further work is currently in progress to apply
“metalloviologens” to other photocatalytic reductions.
Table 1. Redox potentials of the compounds bearing N-methyl-
4,4Ј-bipyridinium moiety.[a]
Experimental Section
General Methods: 1H NMR (270 MHz), 31P NMR (109 MHz), and
13C NMR (67.8 MHz) spectra were recorded on a JEOL Excallibur
270 spectrometer in suitable solvents. Cyclic voltamograms were
recorded on an ALS/Chi model 610C electrochemical analyzer with
platinum working electrode in acetonitrile containing 0.1 m
nBu4NBF4 as a supporting electrolyte. All potentials were mea-
sured against an Ag/Ag+ reference electrode and converted into the
values vs. SCE. Photoirradiation was carried out by an Ushio high
pressure mercury lamp USH-250SC (250 W) with an UV cut-off
filter [SCHOTT GG400 (Ͼ 400 nm)]. Evolved dihydrogen was
quantified by a gas chromatography using a Shimadzu GC-8A with
a TCD detector and a SHINCARBON ST (6 mϫ3 mm). All reac-
tions were carried out under dry nitrogen atmosphere. Solvents
were dried by the general methods, and degassed before use. 2a was
prepared according to the literature procedure.[24]
Preparation of 2b: To a flame-dried 500 mL flask were placed 2a
(21.9 g, 54.2 mmol) and THF (108 mL). nBuLi (65.7 mL,
108 mmol, 1.65 m in hexane) was added dropwise at 0 °C, and the
resulting mixture was stirred at 0 °C for 1 h. Then, MeI (6.75 mL,
108 mmol) was added dropwise at 0 °C. After stirring at room tem-
perature for 12 h, the solvent was removed in vacuo. Saturated
[a] Potential in volts vs. SCE with cyclic voltammetry in MeCN
containing nBu4NBF4 (0.1 m) as a supporting electrolyte. Stick Pt,
Pt wire, and Ag/Ag+ were used as working, counter, and reference
electrodes, respectively. [b] Not observed. [c] Ref.[12b]
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aqueous NaHCO3 (100 mL) was added into the residue and the
resulting mixture was extracted with CH2Cl2 (50 mL ϫ3). The
combined organic layer was dried with anhydrous MgSO4 and con-
centrated in vacuo. After purification by short column chromatog-
raphy on Florisil (eluent: CH2Cl2), 2,6-bis[1-(diisopropylphosphi-
nothioyl)ethyl]pyridine was obtained and the crude product was
used for the next reaction without further purification.
from EtOH to give 4a (8.61 g, 17.9 mmol, 50% yield) as colorless
crystals (m.p. 121.7–123.8 °C). 1H NMR (CDCl3): δ = 8.74–8.71
(m, 2 H), 7.74–7.73 (m, 2 H), 7.63–7.61 (m, 2 H), 3.46 (d, J =
12.7 Hz, 4 H), 2.31–2.15 (m, 4 H), 1.24 (dd, J = 17.0 and 7.0 Hz,
12 H), 1.22 (dd, J = 17.0 and 7.0 Hz, 12 H) ppm. 31P{1H} NMR
(CDCl3): δ = 65.7 (s) ppm. C24H38N2P2S2 (480.65): calcd. C 59.97,
H 7.97, N 5.83; found C 60.14, H 8.12, N 5.58.
To a flame-dried 300 mL flask were placed 2,6-bis[1-(diisoprop-
ylphosphinothioyl)ethyl]pyridine (14.0 g, 32.5 mmol) and THF
(65 mL). nBuLi (38.9 mL, 65.0 mmol, 1.67 m in hexane) was added
dropwise at 0 °C, and the resulting mixture was stirred at 0 °C for
1.5 h. Then, MeI (4.05 mL, 65.0 mmol) was added dropwise at
0 °C. After stirring at room temperature for 12 h, the solvent was
removed in vacuo. Saturated aqueous NaHCO3 (75 mL) was added
into the residue and the resulting mixture was extracted with
CH2Cl2 (100 mL ϫ3). The combined organic layer was dried with
anhydrous MgSO4 and concentrated in vacuo. After recrystalli-
zation from EtOH, 2b (9.33 g, 20.3 mmol, 62% yield) was obtained
Preparation of 4b: To a 300 mL flask were placed 3b (9.16 g,
15.6 mmol), 4-bromopyridine hydrochloride (3.34 g, 17.2 mmol),
K3PO4 (6.64 g, 31.3 mmol), and Pd(PPh3)4 (0.547 g, 0.469 mmol).
1,4-Dioxane (78 mL) and H2O (7.8 mL) were added, and the mix-
ture was heated to 90 °C and stirred at 90 °C for 12 h. Saturated
aqueous NaHCO3 (100 mL) was added and the mixture was ex-
tracted with CH2Cl2 (100 mL ϫ3). The combined organic layer
was dried with anhydrous MgSO4 and concentrated in vacuo. The
residue was purified by column chromatography on SiO2 (eluent:
CH2Cl2/EtOAc = 4:1) and subsequent recrystallization from EtOH
to give 4b (4.44 g, 8.28 mmol, 53% yield) as a colorless solid (m.p.
1
as a white solid (m.p. 177.0–178.4 °C). 1H NMR (CDCl3): δ = 7.68– Ͼ 200 °C). H NMR (CDCl3): δ = 8.77–8.74 (m, 2 H), 8.04–8.03
7.59 (m, 3 H), 2.37–2.23 (m, 4 H), 1.78 (d, J = 14.9 Hz, 12 H), 1.20 (m, 2 H), 7.67–7.65 (m, 2 H), 2.40–2.26 (m, 4 H), 1.85 (d, J =
(dd, J = 16.1 and 7.2 Hz, 12 H), 0.89 (dd, J = 15.7 and 7.0 Hz, 12
H) ppm. 31P{1H} NMR (CDCl3): δ = 79.9 (s) ppm. C23H43NP2S2
(459.67): calcd. C 60.10, H 9.43, N 3.05; found C 59.91, H 9.45, N
3.05.
14.6 Hz, 12 H), 1.23 (dd, J = 16.2 and 7.3 Hz, 12 H), 0.94 (dd, J
= 15.7 and 7.0 Hz, 12 H) ppm. 31P{1H} NMR (CDCl3): δ = 80.0
(s) ppm. C28H46N2P2S2 (536.75): calcd. C 62.66, H 8.64, N 5.22;
found C 62.41, H 8.85, N 5.22.
Preparation of 3a: To a 500 mL sealed vessel were placed [Ir(cod)-
OMe]2 (815 mg, 1.23 mmol), 4,4Ј-di-tert-butyl-2,2Ј-bipyridyl
(662.8, 2.47 mmol), and THF (205 mL). Then, 1a (16.6 g,
41.1 mmol) and bis(pinacolato)diboron (7.82 g, 30.8 mmol) were
added, and the resulting mixture was heated to 90 °C and stirred
at 90 °C for 12 h. The solvent was removed in vacuo, and the resi-
due was purified by column chromatography on Florisil (eluent:
Preparation of [1a][BF4]: To a 50 mL Schlenk flask were placed 4a
(481 mg, 1.00 mmol) and CH2Cl2 (10 mL). Methyl trifluoro-
methanesulfonate (330 μL, 3.00 mmol) was added, and the re-
sulting solution was stirred at room temperature for 14 h. Tris(di-
methylamino)phosphine (365 μL, 2.00 mmol) was added dropwise,
and the resulting mixture was stirred at room temperature for 1 h.
The solvent was removed in vacuo, and the residue was extracted
Et2O) to give 3a (19.1 g, 36.1 mmol, 88% yield) as a pale yellow with Et2O (ca. 20 mL). The organic layer was poured into H2O
1
solid (m.p. 118.6–118.9 °C). H NMR (CDCl3): δ = 7.56 (s, 2 H),
3.41 (d, J = 13.2 Hz, 4 H), 2.36–2.23 (m, 4 H), 1.34 (s, 12 H), 1.23
(dd, J = 17.0 and 6.8 Hz, 12 H), 1.20 (dd, J = 17.3 and 6.8 Hz, 12
(10 mL) containing NaBF4 (333 mg, 3.03 mmol). After vigorous
stirring, white precipitate was collected by filtration and washed
with H2O and Et2O to give [1a][BF4] (306 mg, 0.591 mmol, 59%
yield) as a white solid. Analytically pure material was obtained by
recrystallization from MeOH/Et2O. 1H NMR ([D6]acetone): δ =
9.17, (d, J = 6.5 Hz, 2 H), 8.57, (d, J = 6.5 Hz, 2 H), 7.69, (s, 2,
H), 4.63, (s, 3 H), 3.08, (d, J = 1.9 Hz, 4 H), 2.04–1.77, (m, 4 H),
1.11, (dd, J = 11.6, and 7.0 Hz, 12 H), 1.06, (dd, J = 13.2, and
7.0 Hz, 12 H) ppm. 31P{1H} NMR ([D6]acetone): δ = 13.4, (s) ppm.
C25H41BF4N2P2 (518.36): calcd. C 57.93, H 7.97, N 5.40; found C
57.65, H 8.04, N 5.20.
H) ppm. 331P{1H} NMR (CDCl3):
C25H46BNO2P2S2 (529.52): calcd. C 56.71, H 8.76, N 2.65; found
C 56.88, H 8.91, N 2.46.
δ = 65.8 (s) ppm.
Preparation of 3b: To a 300 mL sealed vessel were placed [Ir(cod)-
OMe]2 (431 mg, 0.650 mmol), 4,4Ј-di-tert-butyl-2,2Ј-bipyridyl
(350 mg, 1.30 mmol), and THF (218 mL). Then, 2b (9.98 g,
21.7 mmol) and bis(pinacolato)diboron (4.14 g, 16.3 mmol) were
added, and the resulting mixture was heated to 90 °C for 12 h. The
solvent was removed in vacuo, and the residue was purified by col-
umn chromatography on Florisil (eluent: CH2Cl2) to give 3b
(10.2 g, 17.4 mmol, 80% yield) as a pale yellow solid (m.p. 193.1–
194.7 °C). 1H NMR (CDCl3): δ = 7.84–7.83 (m, 2 H), 2.42–2.26
(m, 4 H), 1.79 (d, J = 14.6 Hz, 12 H), 1.35 (s, 12 H), 1.20 (dd, J =
16.1 and 6.9 Hz, 12 H), 0.88 (dd, J = 15.7 and 7.0 Hz, 12 H) ppm.
Preparation of [1b][OTf]: To a 50 mL Schlenk flask were placed 4b
(1.41 g, 2.63 mmol) and CH2Cl2 (26 mL). Methyl trifluorometh-
anesulfonate (0.865 mL, 7.89 mmol) was added, and the resulting
solution was stirred at room temperature for 14 h. Tris(dimethyl-
amino)phosphine (0.960 mL, 5.26 mmol) was added dropwise, and
the resulting mixture was stirred at room temperature for 1 h. After
31P{1H} NMR (CDCl3): δ = 80.4 (s) ppm. C29H54BNO2P2S2 removal of solvent in vacuo, the resulting solid was washed with
(585.63): calcd. C 59.48, H 9.29, N 2.39; found C 59.37, H 9.31, N
2.45.
H2O and Et2O to give [1b][OTf] (1.42 g, 1.34 mmol, 51% yield) as
white solid. Analytically pure material was obtained by
a
recrystallization from MeOH/Et2O. 1H NMR ([D6]acetone): δ =
9.20 (d, J = 6.9 Hz, 2 H), 8.65 (d, J = 6.9 Hz, 2 H), 7.84 (s, 2 H),
4.65 (s, 3 H), 1.94–1.82 (m, 4 H), 1.66 (d, J = 10.3 Hz, 12 H), 1.07
(dd, J = 14.3 and 7.3 Hz, 12 H), 0.87 (dd, J = 10.1 and 6.9 Hz, 12
Preparation of 4a: To a 500 mL flask were placed 3a (19.1 g,
36.1 mmol), 4-bromopyridine hydrochloride (7.86 g, 40.4 mmol),
K3PO4 (15.4 g, 72.5 mmol), and Pd(PPh3)4 (1.25 g, 1.08 mmol).
1,4-Dioxane (180 mL) and H2O (18 mL) were added, and the mix-
ture was heated to 90 °C and stirred at 90 °C for 12 h. Saturated
aqueous NaHCO3 (200 mL) was added and the resulting mixture
was extracted with CH2Cl2 (200 mL ϫ3). The combined organic
layer was dried with anhydrous MgSO4 and concentrated in vacuo.
The residue was purified by column chromatography on SiO2 (elu-
ent: hexane/EtOAc = 8:2 to 2:8) and subsequent recrystallization
H) ppm. 31P{1H} NMR ([D6]acetone):
δ = 44.6 (s) ppm.
C30H49F3N2O3P2S (636.74): calcd. C 56.59, H 7.76, N 4.40; found
C 56.34, H 7.84, N 4.20.
Preparation of [6a][BF4]2: To a 100 mL Schlenk flask were placed
PtCl2(PhCN)2 (771 mg, 1.63 mmol), AgBF4 (320 mg, 1.64 mmol)
and MeCN (25 mL), and the mixture was heated to 70 °C and
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stirred at 70 °C for 12 h. After cooling to room temperature, a solu-
tion of [4a][BF4] (846 mg, 1.63 mmol) dissolved in MeCN (17 mL)
was added dropwise and the resulting mixture was stirred for 1.5 h.
After the filtration through Celite, volatiles were removed in vacuo.
The residue was washed with MeOH and Et2O to give [6a][BF4]2
(1.11 g, 1.33 mmol, 82% yield) as a white solid. Analytically pure
material was obtained by recrystallization from MeCN/Et2O. 1H
NMR (CD3CN): δ = 8.78 (d, J = 6.7 Hz, 2 H), 8.30 (d, J = 6.7 Hz,
and MeCN (16 mL), and the mixture was heated to 70 °C and
stirred at 70 °C for 12 h. After cooling to room temperature, a solu-
tion of 2,6-bis(diisopropylphosphinomethyl)pyridine (359 mg,
1.06 mmol) dissolved in MeCN (10 mL) was added dropwise and
the resulting mixture was stirred at room temperature for 1 h. After
the filtration through Celite, volatiles were removed in vacuo. The
residue was washed with MeOH and Et2O to give 11 (265 mg,
0.466 mmol, 44% yield) as a pale yellow solid. 1H NMR (CD3CN):
2 H), 7.99–7.96 (m, 2 H), 4.34 (s, 3 H), 3.84–3.81 (m, 4 H), 2.73– δ = 7.98–7.92 (m, 1 H), 7.55 (d, J = 8.0 Hz, 2 H), 3.83–3.80 (m, 4
2.61 (m, 4 H), 1.43–1.34 (m, 12 H), 1.27–1.17 (m, 12 H) ppm. H), 2.54–2.39 (m, 4 H), 1.42–1.32 (m, 12 H), 1.24–1.16 (m, 12 H)
31P{1H} NMR (CD3CN): δ = 48.9 (s with satellite JPt,P = 2389 Hz)
ppm. 31P{1H} NMR (CD3CN):
δ
=
55.0 (s) ppm.
ppm. C25H41B2ClF8N2P2Pt (835.71): calcd. C 35.93, H 4.95, N C19H35BClF4NP2Pd (568.12): calcd. C 40.17, H 6.21, N 2.47; found
3.35; found C 36.11, H 5.12, N 3.39.
C 40.35, H 6.06, N 2.59.
Preparation of [6b][OTf]2: To a 50 mL Schlenk flask were placed
PtCl2(PhCN)2 (751 mg, 1.59 mmol) and MeCN (16 mL). A solu-
tion of [1b][OTf] (1.01 g, 1.59 mmol) dissolved in MeCN (16 mL)
was added dropwise and the resulting mixture was stirred at room
temperature for 12 h. After concentration in vacuo, the residue was
washed with Et2O to give a crude solid containing [6b][Cl][OTf].
The crude solid was dissolved in MeCN (16 mL) and NaOTf
(544 mg, 3.16 mmol) was added into the mixture. After concentra-
tion in vacuo, the residue was washed with H2O. Recrystallization
from MeCN/Et2O gave [6b][OTf]2 (497 mg, 0.551 mmol, 35% yield)
Preparation of [8a][BF4]2: To a 20 mL Schlenk flask were placed
[RhCl(coe)2]2 (35.9 mg, 0.0500 mmol), AgBF4 (19.6 mg,
0.100 mmol), AgCl (14.2 mg, 0.100 mmol), and MeCN (1 mL). The
mixture was stirred at room temperature for 24 h. A solution of
[1a][BF4] (51.8 mg, 0.100 mmol) dissolved in MeCN (1 mL) was
added dropwise and the resulting mixture was stirred at room tem-
perature for 16 h. After the filtration through Celite, volatiles were
removed in vacuo. Recrystallization from MeCN/Et2O gave
[8a][BF4]2 (44.0 mg, 0.0536 mmol, 54% yield) as orange crystals.
1H NMR ([D6]acetone): δ = 9.23 (d, J = 6.9 Hz, 2 H), 8.64 (d, J =
6.9 Hz, 2 H), 8.39 (s, 2 H), 4.67 (s, 3 H), 4.39–4.36 (m, 4 H), 3.23–
3.10 (m, 4 H), 2.84 (s, 3 H), 1.59–1.44 (m, 24 H) ppm. 31P{1H}
1
as an orange solid. H NMR (CD3CN): δ = 8.82 (d, J = 6.8 Hz, 2
H), 8.47 (d, J = 6.8 Hz, 2 H), 7.81–7.77 (m, 2 H), 4.37 (s, 3 H),
2.89–2.76 (m, 4 H), 1.83–1.79 (m, 12 H), 1.52–1.44 (m, 12 H), 1.24–
1.16 (m, 12 H) ppm. 31P{1H} NMR (CD3CN): δ = 72.9 (s with
satellite JPt,P = 2380 Hz) ppm. C31H49ClF6N2O6P2PtS2 (1016.34):
calcd. C 36.64, H 4.86, N 2.76; found C 36.49, H 5.00, N 2.61.
NMR ([D6]acetone):
δ = 51.2 (d, JRh-P = 78.9 Hz) ppm.
C27H44B2Cl2F8N3P2Rh·1/4Et2O (838.56): calcd. C 40.11, H 5.59,
N 5.01; found C 40.35, H 5.66, N 4.75.
Preparation of [9a][BF4]2: To a 50 mL Schlenk flask were placed
[Rh(cod)(MeCN)2][BF4] (377 mg, 0.99 mmol) and MeCN (20 mL).
A solution of [1a][BF4] (521 mg, 1.01 mmol) dissolved in MeCN
(10 mL) was added dropwise, and the resulting mixture was stirred
at room temperature for 1 h. After the concentration in vacuo, the
residue was washed with EtOH and Et2O to give [9a][BF4]2
(620 mg, 0.828 mmol, 83% yield) as a red solid. 1H NMR ([D6]-
acetone): δ = 9.13 (d, J = 7.0 Hz, 2 H), 8.59 (d, J = 7.0 Hz, 2 H),
8.08 (s, 2 H), 4.57 (s, 3 H), 3.64–3.61 (m, 4 H), 2.55 (s, 3 H), 2.44–
2.32 (m, 4 H), 1.42–1.33 (m, 12 H), 1.28–1.20 (m, 12 H) ppm.
31P{1H} NMR (CD3CN): δ = 54.7 (d, JRh-P = 134 Hz) ppm.
C27H44B2F8N3P2Rh (749.12): calcd. C 43.29, H 5.92, N 5.61; found
C 42.85, H 5.99, N 5.16.
Preparation of 10: To
a 50 mL Schlenk flask were placed
PtCl2(PhCN)2 (349 mg, 0.740 mmol), AgBF4 (144 mg, 0.740 mmol)
and MeCN (11 mL), and the mixture was heated to 70 °C and
stirred at 70 °C for 20 h. After cooling to room temperature, a solu-
tion of 2,6-bis(diisopropylphosphinomethyl)pyridine (297 mg,
0.875 mmol) dissolved in MeCN (7.4 mL) was added dropwise. The
resulting mixture was stirred at room temperature for 6 h. After
concentration in vacuo, the residue was washed with Et2O. After
the filtration through with CH2Cl2 as an eluent, recrystallization
from CH2Cl2/Et2O gave 10 (336 mg, 0.512 mmol, 69% yield) as a
1
white solid. H NMR ([D6]acetone): δ = 8.22–8.16 (m, 1 H), 7.83–
7.76 (m, 2 H), 4.09–3.06 (m, 4 H), 2.79–2.67 (m, 4 H), 1.49–1.40
(m, 12 H), 1.33–1.24 (m, 12 H) ppm. 31P{1H} NMR ([D6]acetone):
δ = 48.8 (s with satellite JPt,P = 2409 Hz) ppm. C19H35BClF4NP2Pt
(656.78): calcd. C 34.75, H 5.37, N 2.13; found C 34.49, H 5.18, N
1.99.
Preparation of 12: To
[RhCl(coe)2]2 (84.0 mg,
a
20 mL Schlenk flask were placed
0.117 mmol), AgBF4 (46.0 mg,
0.236 mmol), AgCl (33.3 mg, 0.234 mmol), and MeCN (2.3 mL).
The mixture was stirred at room temperature for 16 h. A solution
Preparation of [7a][BF4]2: To a 50 mL Schlenk flask were placed
PdCl2(PhCN)2 (564 mg, 1.47 mmol), AgBF4 (286 mg, 1.47 mmol)
and MeCN (22 mL), and the mixture was heated to 70 °C and
stirred at 70 °C for 15 h. After cooling to room temperature, a solu-
tion of [1a][BF4] (762 mg, 1.47 mmol) dissolved in MeCN (15 mL)
was added dropwise and the resulting mixture was stirred at room
temperature for 6 h. After the filtration through Celite, volatiles
were removed in vacuo. The residue was washed with MeOH and
Et2O to give [7a][BF4]2 (821 mg, 1.10 mmol, 75% yield) as a pale
yellow solid. Analytically pure material was obtained by recrystalli-
zation from DMF/Et2O. 1H NMR (CD3CN): δ = 8.79 (d, J =
6.8 Hz, 2 H), 8.28 (d, J = 6.8 Hz, 2 H), 7.97 (s, 2 H), 4.36 (s, 3 H),
3.93–3.90 (m, 4 H), 2.59–2.44 (m, 4 H), 1.45–1.35 (m, 12 H), 1.29–
1.20 (m, 12 H) ppm. 31P{1H} NMR (CD3CN): δ = 55.3 (s) ppm.
C25H41B2ClF8N2P2Pd·2/3DMF (795.80): calcd. C 40.75, H 5.78, N
4.69; found C 40.87, H 5.99, N 5.01.
of
2,6-bis(diisopropylphosphinomethyl)pyridine
(79.4 mg,
0.234 mmol) dissolved in MeCN (2.3 mL) was added dropwise, and
the resulting mixture was stirred for 20 h. After the filtration
through Celite, volatiles were removed in vacuo, and the residue
was washed with MeOH. Recrystallization from MeCN/Et2O gave
12 (32.4 mg, 0.0505 mmol, 22% yield) as orange crystals. 1H NMR
(CD2Cl2): δ = 7.81 (t, J = 8.1 Hz, 1 H), 7.49 (d, J = 8.1 Hz, 2 H),
4.05–4.02 (m, 4 H), 3.17–3.04 (m, 4 H), 2.52 (s, 3 H), 1.56–1.40 (m,
24 H) ppm. 31P{1H} NMR (CD2Cl2): δ = 50.1 (d, JRh-P = 78.7 Hz)
ppm. C21H38BCl2F4N2P2Rh·3/4MeCN (671.90): calcd. C 40.22, H
6.04, N 5.73; found C 40.32, H 6.09, N 6.09.
Photocatalytic Production of Dihydrogen: A typical experimental
procedure for the photocatalytic dihydrogen production using
[8a][BF4]2 is described below. In a 50 mL Schlenk flask equipped
with silicon rubber septa, were placed triethanolamine (310.0 mg,
2.08 mmol), H2O (1.0 mL), and MeCN (1.0 mL) under nitrogen
Preparation of 11: To
a 50 mL Schlenk flask were placed
PdCl2(PhCN)2 (405 mg, 1.06 mmol), AgBF4 (207 mg, 1.06 mmol) atmosphere. To the mixture were added aqueous HBF4 (27 μL,
Eur. J. Inorg. Chem. 2014, 4273–4280
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0.20 mmol, 48 wt.-%), [8a][BF4]2 (8.2 mg, 10 μmol), and [Ru-
(bpy)3][BF4]2 (7.3 mg, 10 μmol). The resulting reaction mixture was
stirred and irradiated at λ Ͼ 400 nm for 18 h. The amount of pro-
duced dihydrogen was quantified by headspace analysis using gas
chromatography. 100 μL samples of headspace were injected into
the gas chromatograph.
Crystallographic Data: CCDC-998946 (for [6b][Cl][OTf]), and
-998947 (for [8a][BF4]2) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[6]
Supporting Information (see footnote on the first page of this arti-
cle): Tables for photocatalytic dihydrogen production and crystallo-
graohic data of 6b and 8a.
Acknowledgments
This work was supported by the Japanese Ministry of Education,
Culture, Sports, Science and Technology (MEXT) (through the
Funding Program for Next Generation World-Leading Research-
ers, GR025, and a Grant-in-Aid for Scientific Research on Innov-
ative Areas). The authors also thank the University of Tokyo for
support (X-ray analysis; Research Hub for Advanced Nano Char-
acterization).
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Received: April 24, 2014
Published Online: August 26, 2014
Eur. J. Inorg. Chem. 2014, 4273–4280
4280
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim