Asymmetric cleavage of 2,2′-pyridil to a picolinic acid anion radical coordinated to ruthenium(ii): Splitting of water to hydrogen

Article abstract of DOI:10.1039/c3cc40956b

The Ru(ii)-H and water promoted asymmetric cleavage of 2,2′-pyridil to pyridine-2-carbaldehyde and unprecedented picolinic acid anion radical (PyCOOH^-) complexes, which in solution produce H₂ gas and diamagnetic picolinate complexes of ruthenium(ii) in moderate yields, is reported.

Full text of DOI:10.1039/c3cc40956b

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Asymmetric cleavage of 2,20-pyridil to a picolinic acid
anion radical coordinated to ruthenium(II): splitting of
water to hydrogen†
Cite this: Chem. Commun., 2013,
49, 4522
Received 4th February 2013,
Accepted 25th March 2013
Manas Kumar Biswas,a Sarat Chandra Patra,a Amarendra Nath Maity,b
Shyue-Chu Ke,b Thomas Weyhermullerc and Prasanta Ghosh*a
¨
DOI: 10.1039/c3cc40956b
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The Ru(II)–H and water promoted asymmetric cleavage of 2,20-pyridil
In this article, we report that the reaction of [RuII(PPh3)3(CO)(H)Cl]
to pyridine-2-carbaldehyde and unprecedented picolinic acid anion (1) with (PyCO)2 in boiling moist toluene leads to the asymmetric
radical (PyCOOHꢀꢁ) complexes, which in solution produce H2 gas and cleavage of the –C(O)–C(O)– bond producing pyridine-2-carbaldeyde
diamagnetic picolinate complexes of ruthenium(II) in moderate yields, (PyCHO) as one of the products. The redox reaction progresses
is reported.
producing ꢀhitherto unknown dark blue picolinic acid anion radical
(PyCOOHꢁ ) complexes of type trans-[RuII(PPh3)2(PyCOOHꢁꢀ)(CO)Cl]
Like aromatic o-benzoquinone, 1,2-diketone is redox non-innocent (t-2rad) as a major and cis-[RuII(PPh3)2(PyCOOHꢁꢀ)(CO)Cl] (c-2rad) as a
and its participation in electron transfer reaction is a subject of minor product. The reactions occurs via the homolytic cleavage2 of
investigation in chemical science.1 Because of the coordinating the Ru –H bond forming a 2,20-pyridil anion radical [(PyCO)2
]
II
ꢀ
ꢁ
pyridine nitrogen and keto oxygen atoms, 2,20-pyridil [(PyCO)2] complex of ruthenium(II), [Ru (PPh3)2(PyCO)2 ꢀ(CO)Cl] (Arad), as an
with a low-lying p* orbital (LUMO) can serve as a bidentate or a intermediate that is detected in a reaction with dry toluene (eqn (1)).
multidentate p-acceptor chelating agent. Transfer of electrons to Upon addition of H2O, Arad converts to t-2rad and c-2rad (eqn (2)). The
(PyCO)2 is possible via either OO- (as a 1,2-diketone acceptor) or acid anion radical complexes, t-2rad and c-2rad, are unstable in solu-
NO-chelating (as a keto–imine acceptor) routes as shown in tions and undergo electron transfer reactions evolving H2 gas, even-
Scheme S1 (ESI†). Expecting for a new multi-electron transfer tually furnishing yellow diamagnetic picolinate (PyCOOꢀ) complexes
product of (PyCO)2 incorporating two types of electron transfer of types trans-[RuII(PPh3)2(PyCOOꢀ)(CO)Cl] (t-3ac) and cis-[RuII(PPh3)2-
sites, soft transition metal reducing equivalent, ruthenium(II) (PyCOOꢀ)(CO)Cl] (c-3ac) in moderate yields (eqn (3) and (4)) as
hydride with p-acidic CO and bulkier PPh3 as coligands has been depicted in Scheme 1. This unexpected reaction that has eventually
used in a boiling aromatic hydrocarbon solvent. The result that led to the reduction of H2O to H2 by metal complexes is worth inves-
leads to the asymmetric cleavage of the –C(O)–C(O)– bond of the tigating in chemical science for sustainable and renewable sources of
coordinated (PyCO)2 in the presence of moisture, oxidizing one energy.3 In this water reduction process (PyCO)2 acts as an electron
CQO group to –COOH and transforming another CQO group to transfer agent undergoing asymmetric cleavage and affording acid
II
ꢁ
–CHO by proton reduction, is unprecedented.
anion radical complexes (eqn (1)–(4)). The related products were ana-
lyzed by UV-vis/NIR absorption spectra, EPR spectra of Arad, t-2rad and
a Department of Chemistry, R. K. Mission Residential College, Narendrapur,
Kolkata-103, India. E-mail: ghosh@pghosh.in; Fax: +91 33 2477 3597;
Tel: +91 33 2428 7347
ac
c-2rad and single crystal X-ray structure determination of c-3 ꢂ2toluene
1
including the DFT and TD DFT calculations on [Ru(PMe3)2-
ꢀ
((PyCO)2 ꢁ)(CO)Cl] (AMrade), trans-[Ru(PMe3)2(PyCOOHꢀꢁ)(CO)(Cl)]
b Department of Physics, National Dong Hwa University, Shou-Feng, Hualien 97401,
Taiwan
c Max-Planck Institute for Chemical Energy Conversion, Stiftstr. 34-36,
¨
45470 Mulheim an der Ruhr, Germany
† Electronic supplementary information (ESI) available: Syntheses, binding modes of
2,2-pyridil (Scheme S1), HPTLC analyses (Fig. S1), H2 detection (Fig. S2), IR spectra
(Fig. S3–S5), UV-vis/NIR absorption spectra (Fig. S6), redox potentials (Table S1),
cyclic voltammograms (Fig. S7 and S8), kinetic isotope effects (Fig. S9 and S10),
EPR spectrum (Fig. S11), optimized geometries (Fig. S12), UV-vis spectral change
(Fig. S13), photoactive orbitals (Fig. S14), crystallographic data (Table S2),
disordered ORTEP plot (Fig. S15), experimental bond parameters (Table S3),
optimized bond parameters (Table S4), comparative bond lengths (Chart S1),
electronic transition (TD DFT calculations) analyses (Table S5) and optimized
coordinates (Tables S6–S10). CCDC 833917. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c3cc40956b
Scheme 1
c
4522 Chem. Commun., 2013, 49, 4522--4524
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(t-2rMade), c-2rad and cis-[Ru(PMe3)2(PyCOOHꢀꢁ)(CO)(Cl)] (c-2Mrade) with
Table 1 UV-Vis/NIR spectral data of the complexes in CH2Cl2 at 298 K
ac
a doublet spin state and cis-[Ru(PMe3)2(PyCOOꢀ)(CO)Cl] (c-3
)
Compound
l )
max/nm (e/104 Mꢀ1 cmꢀ1
Me
with a singlet spin state.
t-2rad
c-2rad
t-3ac
762 (0.28), 687 (0.35), 631 (0.28), 577 (0.18), 528 (0.13)
770 (0.28), 697 (0.24), 632 (0.22), 578 (0.20), 536 (0.18)
762 (0.008), 684 (0.011), 634 (0.010)
1 + (PyCO)2 - Arad + PPh3 + 1/2H2
Arad + H2O - t-2rad + c-2rad + PyCHO
t-2rad - t-3ac + 1/2H2
(1)
(2)
c-3ac
767 (0.006), 691 (0.007), 629 (0.006)
(3)
(4)
and [M ꢀ Cl] at 776.10) only by a proton and is hard to affirm its
existence by mass spectra. However, the mass spectra correlate the
cleavage of the 2,20-pyridil with a picolinic acid fragment.
c-2rad - c-3ac + 1/2H2
The UV-vis/NIR absorption spectra of t-2rad, c-2rad, t-3ac and c-3ac in
CH2Cl2 are recorded at 298 K. The spectra are shown in Fig. S6 (ESI†)
and the data are summarized in Table 1. The lower energy absorption
The cleavage reactions and H2 evolution established in this
investigation are shown in Scheme 1. Reaction of 1 with (PyCO)2 in
boiling moist toluene results in unexpected dark blue solution.† In
stepwise searching, reaction of (PyCO)2 with 1 in boiling dry toluene
bands of the picolinic acid anion radical complexes, t-2rad and c-2rad
,
completely disappear in the case of acid complexes, t-3ac and c-3ac.
The radical centred excitations as the origin of these lower energy
absorptions of t-2rad and c-2rad complexes have been elucidated by TD
under argon affords dark brown solution due to formation of
II
[Ru (PPh3)2((PyCO)2 ꢀ)(CO)Cl] (Arad), an analogue of a 9,10-phenan-
ꢁ
threnesemiquinone radical complex2 as an intermediate. The geo-
metry (trans or cis) of Arad has not been successfully assigned.
However, the formation of it in solution was detected by mass and
EPR (vide infra) spectra. Upon addition of one drop of water, the
brown solution turned dark blue and the reaction mixture was
refluxed for further 10 min and cooled at 298 K. A dark blue solid
of the picolinic acid anion radical complex, trans-[RuII(PPh3)2-
(PyCOOHꢁꢀ)(CO)Cl] (t-2rad), separated out, which was filtered. The
filtrate was evaporated under vacuum, and a bluish-green mass of cis-
[RuII(PPh3)2(PyCOOHꢁꢀ)(CO)Cl] (c-2rad) was collected. Mass, UV-vis/
NIR and EPR spectra of t-2rad and c-2rad were recorded (vide infra).
In solution, t-2rad and c-2rad radical complexes liberate H2 molecules
and were themselves transformed to yellow diamagnetic picolinate
(PyCOOꢀ) complexes of types trans-[RuII(PPh3)2(PyCOOꢀ)(CO)Cl]
(t-3ac) and cis-[RuII(PPh3)2(PyCOOꢀ)(CO)Cl] (c-3ac) in moderate yields
as given in Scheme 1. Since, t-2rad and c-2rad liberate H2 because of
intra-electron transfer, crystallization of these reactive radical com-
plexes even in argon did not succeed. The transformation of c-2rad to
c-3ac is faster than that of the trans-analogue. Formation of the
picolinate complexes was confirmed by single crystal X-ray structure
DFT calculations on trans-[Ru(PMe3)2(PyCOOHꢀꢁ)(CO)(Cl)] (t-2Mrade
)
and cis-[Ru(PMe3)2(PyCOOHꢀꢁ)(CO)(Cl)] (c-2Mrade) (vide infra) complexes.
The redox potential values of the trans and cis radical isomers
are different (Table S1, ESI†). Both the complexes display two
ꢀ
quasi-reversible redox waves due to PyCOOH/PyCOOHꢁ (t-2rad
,
,
ꢀ0.85 V; c-2rad, ꢀ1.00 V) and RuIII/RuII (t-2rad, +0.20 V; c-2rad
+0.42 V with respect to Fc+/Fc, couple) redox couples as illustrated
in Fig. S7 and S8 (ESI†). However, the PyCOOH/PyCOOHꢁꢀ couplꢀe
is not stable. The redox wave due to the PyCOOH/PyCOOHꢁ
couple of c-2rad diminishes faster than that of ꢀt-2rad. Both the
redox waves of t-2rad due to PyCOOH/PyCOOHꢁ and RuIII/RuII
couples appear to be more reversible at 253 K (Fig. S8, ESI†).
The isotopic effect on the cleavage reaction was investigated using
D2O. It was observed that the reaction was facile with moist-toluene
than in D2O–toluene solvent mixtures. The cleavage reaction needs
more time with D2O. It predicts that the O–H/O–D bond cleavage
leads to the C–C bond cleavage. However, the rates of transformation
of c-2rad to c-3ac and deuterated c-2Drad to c-3Dac were compared using
UV-vis/NIR spectra. It was observed that kH/kD E 1.04, i.e., evolution of
H2 displays a secondary kinetic isotopic effect (kH and kD are the rates
of transformation of c-2rad to c-3ac and c-2Drad to c-2Dac). The details of
these kinetic studies are given in the ESI† (page 6, Fig. S9 and S10).
ac
ac
1
determination of c-3 ꢂ2toluene. However, a suitable CIF for t-3
could not be made. t-3ac was isolated by diffusion of n-hexane to
the dichloromethane solution of t-2rad. c-3 ꢂ2toluene has been iso-
lated as single crystals from the toluene solution of c-2rad. The
formation of pyridine-2-carbaldeyde (PyCHO) as another important
cleaved product was confirmed by HPTLC experiments with the
filtrate of the reaction (ESI,† Fig. S1) and the EI mass spectrum.
The evolution of H2 gas was confirmed using GC. The gas evolved was
injected into the GC instrument. H2 was identified by an exact match
with the retention time of an authentic sample (Fig. S2, ESI†).
The IR spectra (Fig. S3–S5, ESI†) and mass spectra confirm the
presence of (PyCO)2 in Arad. No cleavage occurs at this stage. However,
the IR spectra of t-2rad and c-2rad are superimposable to those of the
ac
1
A
rad, t-2rad and c-2rad are paramagnetic while t-3ac and
ac
1
c-3 ꢂ2toluene are diamagnetic. The fluid solution (CH2Cl2) isotropic
EPR spectra at 298 K of Arad and t-2rad are illustrated in panels (a)
and (b) of Fig. 1. The EPR spectrum of c-2rad is displayed in Fig. S11
(ESI†). Simulations of the spectra revealed the g values and hyperfine
coupling parameters. The g-values confirm the presence of organic
radicals in all three complexes: Arad, g = 1.9967, A(31P) = 12 G; t-2rad
,
g = 2.002, A(4 H) = 10.9 G; c-2rad, g = 2.002, A(4 H) = 9.89 G. The
1
1
hyperfine splitting patterns of Arad are quite different from those of
diamagnetic picolinate complexes, t-3ac and c-3 ꢂ2toluene and the IR
spectra do not support the presence of the PyCO function in t-2rad
and c-2rad complexes. It is to be noted that because of the mixing with
the acid analogue, the IR spectrum of c-2rad displays two overlapping
metal carbonyl frequencies at 1922 and 1943 cmꢀ1 while only one
ac
1
n
CRO at 1943 cmꢀ1 is observed for t-2rad. The molecular composition
of t-2rad and c-2rad (m/z of [M + Na]+ at 835.53, 833.53 and [M ꢀ Cl]
Fig. 1 (a) Hyperfine splittings of the X-band EPR spectra of (a) Arad, (b) t-2rad
at 777.60) differs from that of t-3ac and c-3ac (m/z of [M + Na]+ at 834.07 (black, experimental and red, simulated spectra).
c
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consistent with those obtained from the single crystal X-ray struc-
ac
1
ture determination of c-3 ꢂ2toluene. It is to be noted that the
experimental and calculated bond lengths of the picolinic acid
anion radical are significantly different from those of the picolinate
chelates (ESI,† Chart S1). The calculated C3–N1 (1.393 Å) distance
of the pyridine ring of the picolinic acid anion radical is longer than
that (exp., 1.339(10) Å; cal., 1.350 Å) in the picolinate chelate while
the C2–C3 (1.404 Å) length in the picolinic acid anion radical is
much shorter than that (exp., 1.487(12) Å; cal., 1.525 Å) in the
picolinate chelate. A similar trend has been established precisely by
Wieghardt et al. in one electron reduced iminopyridine complexes
of 3d elements.5 The C2–O2 length in the anion radical is also
comparatively shorter. The C2–O3 distance (1.363 Å) is remarkably
longer in the acid anion radical compared to that, 1.227(10) Å
(cal., 1.231 Å), in the acid chelate. The two alternate C–C bonds in the
picolinic acid anion radical (C3–C4 and C5–C6, 1.426 and 1.421 Å)
are longer than the average C–C length in a pyridine ring.
Fig. 2 ORTEP plot (30% probability level) of c-3acꢂ2toluene (H atoms and
1
solvents are excluded for clarity).
t-2rad and c-2rad. The spectrum of Arad is consistent with the presence
of two equivalent PPh3 ligands. The hyperfine couplings due to four
equivalent pyridine H atoms correlate excellently with the existence
of the picolinic acid anion radical in t-2rad and c-2rad complexes.
Similar spectral features of the bipyridine anion radicals have been
documented in the literature.4
The moderately strong absorption bands of t-2rad and c-2rad above
600 nm which are absent in picolinate complexes, t-3ac and c-3ac, as
shown in Fig. S5 (ESI†), are characteristics of a picolinic acid anion
radical complex. The lower energy absorption maxima of t-2rad and
c-2rad of LMCT origins disappear during proton induced electron
transfer reaction reducing H+ to H2 slowly in solution as shown in Fig.
S13 (ESI†). Origins of the lower energy absorptions of t-2rad and c-2rad
have been investigated by TD DFT calculations on t-2rMade and c-2Mrade in
CH2Cl2 solvent using the CPCM model. The responsible photoactive
orbitals are illustrated in Fig. S14 (ESI†). Calculations have predicted
three lower energy absorption bands at 744.5, 709.9 and 617.6 nm
due to LMCT i.e., SOMO (a-HOMO) to LUMO (dz2) or LUMO +
1(dx2ꢀy2) orbitals as summarized in Table S6 (ESI†). The transition
Single crystal X-ray structure determinations have confirmed
the C–C bond cleavage forming the picolinic acid from the (PyCO)2
ac
and trans and cis geometries of the t-3ac and c-3 ꢂ2toluene com-
1
plexes. However, t-3ac poorly diffracts and the crystallographic data
ac
1
%
are not reported here. c-3 ꢂ2toluene crystallizes in the P1 space
group. The molecular geometry with the atom labelling scheme is
illustrated in Fig. 2 (also Fig. S15, ESI†). The crystallographic data
and the bond parameters of c-3 ꢂ2toluene are summarized in
Tables S2 and S3 (ESI†) (CCDC 833917).
ac
1
The spin density distributions of Arad, t-2rad and c-2rad were
elucidateꢀd by the unrestricted DFT calculations on [Ru(PMe3)2-
((PyCO)2 ꢁ)(CO)Cl] (AMrade), trans-[Ru(PMe3)2(PyCOOHꢀꢁ)(CO)(Cl)]
(t-2rMade) and c-2rad with a doublet spin state. The gas-phase geometry
of cis-[Ru(PMe3)2(PyCOOHꢀꢁ)(CO)(Cl)] (c-2Mrade) was also optimized.
The optimized geometries are shown in Fig. S12 (ESI†). The calcu-
lated bond parameters of all these radical complexes are summar-
features of picolinate complexes have also been elucidated by TD DFT
ac
Me
calculation on c-3 in dichloromethane using the CPCM model.
In this case, no lower energy excitations have been predicted. The
ac
Me
calculated lower energy excitations of c-3 are 329.01 and 324.58 nm
rad
Me
rad
Me
ized in Table S4 (ESI†). c-2 and t-2 have comparable energies
with oscillator strengths (f) of 0.015 and 0.042 respectively.
In conclusion, formation of a 2,20-pyridil anion radical and its
asymmetric cleavage by water affording unprecedented picolinic
acid anion radical complexes with a transition metal ion, which
produce hydrogen gas and picolinate complexes, is reported.
Financial support received from DST (SR/S1/IC/0026/2012) and
CSIR (3231/NS-EMRII), New Delhi, India, is gratefully acknowl-
edged. M. K. B. is thankful to CSIR for the fellowship (8/531(007)/
2012-EMR-I). We are thankful to Prof. J. K. Bera, IIT, Kanpur,
India, for providing us the facility to record hydrogen evolution.
and both the isomers are isolable experimentally. Mulliken spin
population analyses authenticated that the spin densities are pri-
marily localized on the 2,20-pyridil ligand in AMrade and PyCOOH
chelate of t-2Mrade and c-2rad. The spin density plots of AMrade, t-2Mrade and
c-2rad are shown in Fig. 3. In ArMade, spin density is mainly localized on
rad
the trans-CO–CO-fragment while in t-2 and c-2rad complexes it is
Me
mainly localized on the pyridine chelate forming a picolinic acid
anion radical, PyCOOHꢀꢁ. The spin density distribution features
correlate well with the fluid solution EPR spectra of Arad and pyridine
H hyperfine couplings of t-2rad and c-2rad complexes at 298 K (Fig. 1).
The gas phase geometry of cis-[Ru(PMe3)2(PyCOOꢀ)(CO)(Cl)] Notes and references
(c-3aMce) has also been optimized with the singlet spin state for
1 (a) G. H. Spikes, E. Bill, T. Weyhermu¨ller and K. Wieghardt, Angew.
ac
Me
comparison. The calculated bond parameters of c-3
are
Chem., Int. Ed., 2008, 47, 2973; (b) G. H. Spikes, S. Sproules, E. Bill,
T. Weyhermu¨ller and K. Wieghardt, Inorg. Chem., 2008, 47, 10935.
2 M. K. Biswas, S. C. Patra, A. N. Maity, S.-C. Ke, N. D. Adhikary and
P. Ghosh, Inorg. Chem., 2012, 51, 6687.
3 (a) W. T. Eckenhoff and R. Eisenberg, Dalton Trans., 2012, 41, 13004;
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5 (a) C. C. Lu, E. Bill, T. Weyhermu¨ller, E. Bothe and K. Wieghardt,
J. Am. Chem. Soc., 2008, 130, 3181; (b) A. C. Bowman, C. Milsmann,
C. C. H. Atienza, E. Lobkovsky, K. Wieghardt and P. J. Chirik, J. Am.
Chem. Soc., 2010, 132, 1676.
rad
Fig. 3 Mulliken spin population analyses of ArMade, t-2 and c-2rad
.
Me
c
4524 Chem. Commun., 2013, 49, 4522--4524
This journal is The Royal Society of Chemistry 2013