Formation of η2-coordinated dihydropyridine-ruthenium(II) complexes by hydride transfer from ruthenium(II) to pyridinium cations

Article abstract of DOI:10.1021/om400862n

Reactions between various pyridinium cations with and without a -CF ₃ substituent at the 3-position and [Ru(tpy)(bpy)H]⁺ (tpy = 2,2′:6′,2″-terpyridine and bpy = 2,2′-bipyridine) were investigated in detail. The corresponding 1,4-dihydropyridines coordinating to a Ru(II) complex in η² mode through a C=C bond were quantitatively formed at the initial stage. The only exception observed was in the case of the 1-benzylpyridinium cation, where a mixture of two adducts with 1,4-dihydropyridine and 1,2-dihydropyridine was formed in the ratio 96:4. Cleavage of the Ru-(C=C) bond proceeded at a slower rate in all reactions, giving the corresponding dihydropyridine and [Ru(tpy)(bpy)(NCCH ₃)]²⁺ when acetonitrile was used as a solvent. Kinetic activation parameters for the adduct formation indicated that the 1,4-regioselectivities were induced by formation of sterically constrained structures.

Full text of DOI:10.1021/om400862n

Communication
pubs.acs.org/Organometallics
Formation of η2‑Coordinated Dihydropyridine−Ruthenium(II)
Complexes by Hydride Transfer from Ruthenium(II) to Pyridinium
Cations
Yasuo Matsubara,†,‡ Tatsumi Kosaka,§ Kichitaro Koga,∥ Akira Nagasawa,§ Atsuo Kobayashi,§
Hideo Konno,§,⊥ Carol Creutz,† Kazuhiko Sakamoto,§ and Osamu Ishitani*,∥
†Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000, United States
‡PRESTO, Japan Science and Technology Agency (JST), Chiyoda-ku, Tokyo 102-0076, Japan
§Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Saitama 338-8570, Japan
∥Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama 2-12-1-NE-1,
Meguro-ku, Tokyo 152-8550, Japan
S
* Supporting Information
ABSTRACT: Reactions between various pyridinium cations
with and without a −CF3 substituent at the 3-position and
[Ru(tpy)(bpy)H]+ (tpy = 2,2′:6′,2″-terpyridine and bpy =
2,2′-bipyridine) were investigated in detail. The corresponding
1,4-dihydropyridines coordinating to a Ru(II) complex in η2
mode through a CC bond were quantitatively formed at the initial stage. The only exception observed was in the case of the 1-
benzylpyridinium cation, where a mixture of two adducts with 1,4-dihydropyridine and 1,2-dihydropyridine was formed in the
ratio 96:4. Cleavage of the Ru−(CC) bond proceeded at a slower rate in all reactions, giving the corresponding
dihydropyridine and [Ru(tpy)(bpy)(NCCH3)]2+ when acetonitrile was used as a solvent. Kinetic activation parameters for the
adduct formation indicated that the 1,4-regioselectivities were induced by formation of sterically constrained structures.
ydride reduction of pyridinium cation to dihydropyridine
is an important elemental step in biological redox
(1,4-BAcPyH) and 1:1 adduct [Ru(tpy)(bpy)(1,4-BAcPyH)]2+,
which was identified when RuH+ was used. Therefore, the
carbonyl group should be able to interact with the metal center
of the hydride complexes in the transition state(s). Moreover,
Fish et al. reported that pyridinium cation with a non-
coordinating group such as methyl at the 3-position could not
be reduced by RhH+.
H
systems, i.e., conversion of coenzyme NAD(P)+ to NAD(P)H,1
and in the preparation of various synthons used in heterocyclic
synthesis.2 Factors that control the positions of the ring at
which a hydride attacks have attracted particular interest.3
Transition-metal hydride and formyl complexes of Ru(II),4
Re(I),4c,5 Rh(III),6 and Ir(III)7 have been established as
hydride donors for the reduction of pyridinium-type cations. In
some cases, a carbamoyl group, which is a substituent at the 3-
position of the ring in the NAD(P)+ models, has been thought
to play an important role as a directing group to induce 1,4-
regioselectivity.4a,c,6b For instance, we found that a Ru complex
coordinated by the carbamoyl group of 1-benzyl-1,4-dihydro-
nicotinamide (BNAH), i.e., [Ru(tpy)(bpy)(BNAH)]2+ (RuB-
NAH2+; tpy = 2,2′:6′,2″-terpyridine and bpy = 2,2′-bipyridine)
was produced as an important intermediate of 1,4-regioselective
hydride reduction of 1-benzyl-3-carbamoylpyridinium cation
(BNA+) by [Ru(tpy)(bpy)H]+ (RuH+).4c This strongly
suggests that the interaction between the carbamoyl group of
BNA+ and the central metal of the Ru complex is an important
factor in the 1,4-regioselectivity. Fish et al. also have suggested
the importance of a similar interaction in the hydride reduction
of NAD(P)+ models with [Cp*Rh(bpy)H]+ (RhH+; Cp* =
pentamethylcyclopentadienyl).6b,c Reduction of 1-benzyl-3-
acetylpyridinium cation (BAcPy+), bearing an acetyl group
Herein, we report reactions of RuH+ with pyridinium cations
without a substituent or with a noncoordinating group (−CF3)
at the 3-position of the pyridinium ring (Scheme 1). These
cations could react with RuH+ to give the corresponding
dihydro form(s), and we found that new types of Ru(II)-bound
η2-1,4-dihydropyridine adducts are formed during these
reactions (Scheme 1).
Scheme 1. Formation of the Reaction Intermediates in the
Reactions of Pyridinium Cations with RuH+
4c
instead of the carbamoyl group of BNA+, by RuH+ or
Received: August 30, 2013
Published: October 29, 2013
6b,c
RhH+
selectively gave the corresponding 1,4-dihydro form
© 2013 American Chemical Society
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Organometallics
Communication
species (Ru1dH(1,4)2+) with the formation of 1dH(1,4) and
an equivalent amount of RuAN2+ during the initial stage of the
reaction, confirmed by an 1H NMR spectrum immediately after
−
In a typical run, PF6 salts of 1-benzyl-3-trifluoromethylpyr-
idinium cation (1a+; 15 μmol) and RuH+ (15 μmol) were
dissolved in 0.7 mL of acetonitrile-d3 at room temperature.
Quantitative formation of the corresponding 1,4-dihydro form
of 1a+ (1aH(1,4)) and a solvento complex, [Ru(tpy)(bpy)-
(NCCH3)]2+ (RuAN2+), was observed within 5 min (eq 1).
PF6 salts of both 1d+ (9 μmol) and RuH+ (8 μmol) were
−
dissolved in 0.7 mL of acetonitrile-d3 at room temperature
(Figure S5a). A trace amount of another intermediate species
(Ru1dH(1,2)2+, described below) was also detected (the inset
of the same figure).
The structure of Ru1dH(1,4)2+ could be assigned to a 1:1
adduct of [Ru(tpy)(bpy)]2+ and the 1,4-dihydro form of 1d+
(1dH(1,4)), in which the signals from the protons at 2- and 3-
positions were observed at magnetic fields different from those
at the 5- and 6-positions in the 1H−1H gradient-COSY
spectrum (Figure S6). In an NOE experiment for Ru1dH-
(1,4)2+, two NOEs of the protons at the 5- and 6-positions of
the 1dH(1,4) unit with the proton at the 6-position of bpy in
the Ru complex unit were observed with similar magnitude
(Figure S7). In the following stage of the reaction, a decrease in
Ru1dH(1,4)2+ and an increase in 1dH(1,4) and RuAN2+ were
mainly observed, while a small amount of another adduct
(Ru1dH(1,2)2+) with the 1,2-dihydro form of 1d+ (1dH(1,2))
instead of the 1dH(1,4) unit was produced (Figure S5b and the
inset of Figure S5a). Finally, both of the adducts were
dissociated to give RuAN2+ and the corresponding dihydro
form, i.e., 1dH(1,4) and 1dH(1,2), in 94% and 3% yields,
respectively (Figure S5c).
The formation of positional isomers, i.e., 1,2- and 1,6-dihydro
isomers, and one-electron-reduced dimers (1a2) was not
1
observed in the H NMR spectrum of the sample prepared at
room temperature (Figure 1b). We could detect the
1
The H NMR data of Ru1aH(1,4)2+ and Ru1dH(1,4)2+,
along with the adduct of BNAH with RuH+ (RuBNAH2+), are
summarized in Table S1.
The chemical shifts observed in the range from 0.7 to 6.4
ppm clearly indicate that the pyridinium rings were already
reduced in the process of forming the adducts. Moreover, the
pyridinium rings exhibited the 1,4-dihydro structure (except for
Ru1dH(1,2)2+), which was established by the two doublets and
two triplets that were observed for the protons on the
1
●
,
Figure 1. H NMR spectra measured (a) at low temperature (red
2+
●
Ru1aH(1,4) ; black , 1aH(1,4)) and (b) at room temperature after
mixing RuH+ (15 μmol) and 1a+ (15 μmol) in acetonitrile-d3 (0.7
mL). The peaks marked with *, †, ‡, and § are attributed to excess 1a+,
water, residual solvent protons, and contaminating toluene and ether,
respectively.
1
dihydropyridine ring of the adducts. The H NMR spectra of
the adducts showed the following characteristics (Ru1dH-
(1,4)2+ is discussed as a typical example): (1) there were none
of the magnetically equivalent protons of the dihydropyridine
ring and those of the tpy ligand in the Ru complex, indicating
that the adduct has C1 symmetry; (2) geminal coupling of the
protons at the 4-position was observed (2J ≈ 20 Hz), and both
proton signals were observed at higher magnetic fields, where
one was more strongly shielded (−2.02 ppm) than the other
(−0.51 ppm); (3) geminal coupling of the methylene protons
of the benzyl substituent at the 1-position was also observed,
and shielding effects on the methylene protons were observed
to different extents as well; (4) shielding effects were observed
at the 2- and 5-positions (2-position, −0.87 ppm, 5-position,
−0.60 and −0.54 ppm), whereas the proton at 6-position was
deshielded (+0.64 ppm).
These results strongly indicate that the 1,4-dihydropyridine
moiety coordinates to the Ru center with one of the CC
bonds in an η2 mode but not by the N lone pair in η1 mode
because of the following reasons: (1) the W(0) complex having
1-acetyl-1,2-dihydropyridine as a ligand coordinated in an η2
mode to the W(0) center via the CC bond between the 3-
and 4-positions has been reported,8 and in the 1H NMR
spectrum of this complex, a similar geminal coupling of the
methylene groups and similar magnetic shielding effects of the
dihydropyridine protons were observed;9 (2) the magnitude of
shielding effects of the protons at the 4-position was obviously
larger than that of RuBNAH2+, in which the carbamoyl group at
intermediate by measuring the 1H NMR spectrum immediately
after defrosting the solution that was prepared and frozen in
liquid nitrogen (Figure 1a). The intermediate quickly
disappeared as the solution warmed, and simultaneously,
equivalent amounts of 1aH(1,4) and RuAN2+ appeared.
When [Ru(tpy)(bpy)D]+ was used, the deuterium was
exclusively incorporated at the 4-positions in the intermediate
and then in the product without H/D exchange with protons in
the reaction solution (Figure S1 in the Supporting
Information). The intermediate was determined to be a 1:1
adduct of [Ru(tpy)(bpy)]2+ and 1aH(1,4) (Ru1aH(1,4)2+)
1
based on kinetic studies of the reaction (vide infra) and H
NMR spectrum. In the IR spectrum of Ru1aH(1,4)2+ (Figure
S2), the absorptions attributed to CC stretching modes were
observed at lower frequencies (1604 and 1685 cm−1) compared
with that of 1aH(1,4) (1628 and 1695 cm−1).8
Similar adduct formation was observed for the reaction of
RuH+ with 1-methyl-3-trifluoromethylpyridinium (1b+) and 1-
isopropyl-3-trifluoromethylpyridinium (1c+). The adducts
Ru1bH(1,4)2+ and Ru1cH(1,4)2+ were converted to 1bH(1,4)
and 1cH(1,4), respectively, with formation of RuAN2+ in
quantitative yield (Figures S3 and S4).
The 1-benzylpyridinium cation (1d+) has no substituent at
the 3-position, but it reacted with RuH+; however, the reaction
was comparatively slow and was complete in about 12 h even at
room temperature. It afforded a more stable intermediate
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Communication
the 3-position coordinates to the metal center and the protons
at the 4-position are relatively close to the tpy ligand; (3) it
seems reasonable that the π bonding encumbered free rotation
of the dihydropyridine on the metal center, resulting in the C1-
symmetrical appearance of the dihydropyridine and the tpy
protons of Ru1dH(1,4)2+ in the NMR spectrum, as described
above; (4) the shifts of the CC stretching bands of
Ru1aH(1,4)2+ to lower frequencies (−24 and −10 cm−1)
indicate weak interactions in comparison with the reported
shifts (−100 to −200 cm−1) of η2-form structures of
[Ru(NH3)5(alkene)]2+.10 The binding position of the dihy-
dropyridine rings to the Ru(II) center could be determined on
the basis of NMR experiments. The NOE data of Ru1dH-
(1,4)2+ indicate that the carbon atoms at both the 5- and 6-
positions were located at similar distances from the metal
center.
equilibrium reaction followed by an irreversible reaction to
yield a product; the first reaction corresponds to formation of
Ru1dH(1,4)2+, and the irreversible reaction corresponds to the
cleavage of Ru1dH(1,4)2+. This kinetic model is supported by
an analysis of a steady-state approximation (see section F in the
Supporting Information for details).
Meanwhile, formation of Ru1dH(1,2)2+ was observed as a
very minor reaction without an induction period, while the
formation of 1dH(1,2) had an induction period (about 1 h).
Since the formation of 1dH(1,2) was observed after the
formation of Ru1dH(1,2)2+ and stopped after the disappear-
ance of Ru1dH(1,2)2+, 1dH(1,2) is likely to have been formed
via cleavage of Ru1dH(1,2)2+ and not via isomerization of
1dH(1,4).11
There are two possible mechanisms to give Ru1dH(1,2)2+
(Scheme 2): (A) direct formation from 1d+ and RuH+ and/or
In Ru1aH(1,4)2+−Ru1cH(1,4)2+, the magnetic shielding
effects of the protons at the 5- and 6-positions by binding to the
[Ru(tpy)(bpy)]2+ moiety (from −0.40 to −0.52 ppm and from
+0.20 to +0.50 ppm, respectively) are similar to those for
Ru1dH(1,4)2+ (−0.54 and +0.64 ppm, respectively).
Scheme 2. Possible Reaction Pathways
This similarity of the shielding effect is also applicable to all
other protons. Thus, the 1,4-dihydropyridine moieties of the
adducts Ru1aH(1,4)2+−Ru1dH(1,4)2+ exhibit the η2-C(5)
C(6) binding mode as shown in Scheme 1.
Is the formation of the η2 adducts the key to achieving
regioselective hydride transfer to pyridinium cations? The
reaction of RuH+ with 1d+ offers a test case, since two isomers
(Ru1dH(1,4)2+ as a main product and Ru1dH(1,2)2+ as a very
minor product) were observed in the NMR experiment. Figure
2 shows the time courses of concentrations of RuH+,
(B) rearrangement of the 1,4-dihydropyridine moiety in
Ru1dH(1,4)2+ to the 1,2-isomer on the [Ru(tpy)(bpy)]2+
moiety. Path A seems reasonable as the major pathway,
because no induction period was readily observed in Figure 2;
on the other hand, an antarafacial [1,3]-sigmatropic shift such
as path B would be sterically hindered. To clarify this, we tested
the reaction using higher concentrations of RuH+ (44 mM) and
1d+ (175 mM) and found that Ru1dH(1,2)2+ was clearly
formed without any induction period and, in the initial stage of
the reaction, the ratio of Ru1dH(1,4)2+ to Ru1dH(1,2)2+ was
96:4 (Figure S9). This ratio, comparable to the product yields
of 1dH(1,4) and 1dH(1,2) described above (94% and 3%,
respectively), indicates that path A was operating. Therefore,
the formation of the 1:1 adducts is the key stage to determine
the regioselectivity of the free dihydropyridine products. Note
that the reaction of 1d+ with RuH+ involving both paths A and
B was slower than the reactions of 1a+−1c+.
To obtain further mechanistic insights into the reactions, we
employed the stopped-flow technique to follow the formation
of the adducts of 1a+−1c+ with RuH+ and the subsequent
cleavage of Ru1aH(1,4)2+−Ru1cH(1,4)2+. Two-step spectral
changes were observed, and in each step, there were isosbestic
points (Figures S10−S12). These results are consistent with the
results of the low-temperature NMR experiments as described
above. In the first stage (<5 s), the metal-to-ligand charge-
transfer absorption band of the Ru complex was blue-shifted
(λmax from 535 to 451 (1a+), 443 (1b+), and 465 (1c+) nm);
this is attributed to the formation of the adduct. The spectrum
of the adduct subsequently changed to that of the solvento
complex [Ru(tpy)(bpy)(DMF)]2+ (λmax 485 nm12) on a time
scale of several tens to several hundreds of seconds; this is
attributed to the cleavage of the adduct. Since in the adduct
formation the observed pseudo-first-order rate constants were
proportional to the concentrations of 1a+−1c+, the second-
order rate constants (k2) for the reactions could be determined.
2+
Figure 2. Concentrations of RuH+ (black ), RuAN (black ),
●
○
2+
2+
■
▲
1dH(1,4) (red ), Ru1dH(1,4) (red ), Ru1dH(1,2) (green
+
□
△
), and 1dH(1,2) (green ) over time during the reaction of RuH
with 1d+ at 22 °C. The initial concentrations of RuH+ and 1d+ are 6.0
and 6.3 mM, respectively. Fitting curves are based on a kinetic model
using Scheme 2
Ru1dH(1,4)2+, Ru1dH(1,2)2+, 1dH(1,4), 1dH(1,2), and
RuAN2+ during the reaction of RuH+ with 1d+, where the
initial concentrations of RuH+ and 1d+ were 6.0 and 6.3 mM,
respectively. In the initial stage of the reaction, rapid formation
of Ru1dH(1,4)2+ and the corresponding decay of RuH+ were
observed, followed by comparatively slow changes in the
concentrations. This behavior can be explained by an
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ASSOCIATED CONTENT
* Supporting Information
■
S
Figures, tables, a detailed experimental section, characterization
data, activation parameters, and stopped-flow analysis data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(9) (a) Harrison, D. P.; Zottig, V. E.; Kosturko, G. W.; Welch, K. D.;
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AUTHOR INFORMATION
Corresponding Author
*E-mail for O.I.: ishitani@chem.titech.ac.jp.
■
Present Address
⊥Research Institute for Innovation in Sustainable Chemistry,
National Institute of Advanced Industrial Science and
Technology (AIST), 1-1-1 Higashi, Tsukuba 305-8565, Japan.
Notes
The authors declare no competing financial interest.
(13) (a) Cordone, R.; Harman, W. D.; Taube, H. J. Am. Chem. Soc.
1989, 111, 2896. (b) Surendranath, Y.; Harman, W. D. Dalton Trans.
2006, 3957.
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ACKNOWLEDGMENTS
■
This work was supported by a Grant-in Aid for Exploratory
Research (24655046) from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT). The research at
Brookhaven National Laboratory (BNL) was conducted under
contract DE-AC02-98CH10884 with the U.S. Department of
Energy and supported by its Division of Chemical Sciences,
Geosciences and Biosciences of the Office of Basic Energy
Sciences. We thank Dr. Etsuko Fujita (BNL) for helpful
comments.
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