Stabilization and destabilization of the Ru-CO bond during the 2,2′-bipyridin-6-onato (bpyO)-localized redox reaction of [Ru(terpy)(bpyO)(CO)](PF6)

Article abstract of DOI:10.1002/ejic.200400522

Two stereoisomers of [Ru(terpy)(bpyO)(CO)](PF₆)([1]⁺ and [2]⁺; terpy = 2,2′:6′,2″-terpyridine, bpyO = 2,2′-bipyridin-6-onato) were prepared. The pyridonato moiety in the bpyO ligand of [1]⁺ and [2]⁺ is located trans and cis, respectively, to CO. Treatment of [1]⁺ and [2]⁺ with HPF₆ produced [1H]²⁺ and [2H]²⁺, both of which contain bpyOH (bpyOH = 6-hydroxy-2,2′-bipyridine). The difference in the pK_a values of [1H]²⁺ (3.5) and [2H]²⁺ (3.9) reflects the stronger electronic interaction between CO and the pyridonato moiety in the bpyO ligand in the trans position compared with that in the cis position. The molecular structures of [1](PF₆), [2](PF ₆)·H₂O and [2H](PF₆)₂· 2H₂O were determined by X-ray structure analyses. [1]⁺ and [2]⁺ undergo one, reversible reduction at E_1/2 = -1.65 V and -1.51 V, respectively, and one irreversible reduction at E_p,c = -2.07 and E_p,c = -2.13 V, respectively. Both reductions are assigned to redox reactions localized at the terpy and bpyO ligands. Irreversible reduction of [1]⁰ results from reductive cleavage of the Ru-CO bond of [1]^-. On the other hand, a two-electron oxidation of [2] ^- almost regenerates [2]⁺ because of the depression of the reductive Ru-CO bond cleavage of [2]^- due to cyclometalation formed by an attack of oxygen of bpyO to the carbon of the Ru-CO bond. An unusually large shift of the ν(C≡O) band on going from [2]⁰ (1950 cm^-1) to [2]^- (1587 cm^-1) also supports a reversible cyclometalation driven by the bpyO-localized redox reaction. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.

Full text of DOI:10.1002/ejic.200400522

FULL PAPER
Stabilization and Destabilization of the Ru؊CO Bond During the
2,2Ј-Bipyridin-6-onato (bpyO)-Localized Redox Reaction of
[Ru(terpy)(bpyO)(CO)](PF6)
Takashi Tomon,[a] Take-aki Koizumi,[a] and Koji Tanaka*[a]
Keywords: Carbonyl complex / Electrochemistry / IR spectroscopy / Ruthenium
Two stereoisomers of [Ru(terpy)(bpyO)(CO)](PF6) ([1]+ and
[2]+; terpy = 2,2Ј:6Ј,2ЈЈ-terpyridine, bpyO = 2,2Ј-bipyridin-6-
onato) were prepared. The pyridonato moiety in the bpyO
ligand of [1]+ and [2]+ is located trans and cis, respectively,
to CO. Treatment of [1]+ and [2]+ with HPF6 produced [1H]2+
and [2H]2+, both of which contain bpyOH (bpyOH = 6-hy-
droxy-2,2Ј-bipyridine). The difference in the pKa values of
irreversible reduction at Ep,c = −2.07 and Ep,c = −2.13 V,
respectively. Both reductions are assigned to redox reactions
localized at the terpy and bpyO ligands. Irreversible reduc-
tion of [1]0 results from reductive cleavage of the Ru−CO
bond of [1]−. On the other hand, a two-electron oxidation of
[2]− almost regenerates [2]+ because of the depression of the
reductive Ru−CO bond cleavage of [2]− due to cyclometala-
[1H]2+ (3.5) and [2H]2+ (3.9) reflects the stronger electronic tion formed by an attack of oxygen of bpyO to the carbon of
interaction between CO and the pyridonato moiety in the
bpyO ligand in the trans position compared with that in the
cis position. The molecular structures of [1](PF6),
[2](PF6)·H2O and [2H](PF6)2·2H2O were determined by X-ray
the Ru−CO bond. An unusually large shift of the ν(CϵO)
band on going from [2]0 (1950 cm−1) to [2]− (1587 cm−1) also
supports a reversible cyclometalation driven by the bpyO-
localized redox reaction.
structure analyses. [1]+ and [2]+ undergo one, reversible re- (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
duction at E1/2 = −1.65 V and −1.51 V, respectively, and one
Germany, 2005)
Introduction
Polypyridylruthenium complexes exhibit a unique elec-
trochemical behavior[1] because of the low energy level of
the π*-orbitals of the ligands. For example, redox reactions
of polypyridyl ligands can be utilized as electron pools in
photo-[2] and electrochemical[3] reduction of CO2 catalyzed
by polypyridylruthenium complexes. Modification of polyp-
yridyl ligands may create new functionalities in addition to
the role of the redox centers of ruthenium complexes. For
example, a keto-enol tautomerism between 6-hydroxy-2,2Ј-
bipyridine (bpyOH) and 2,2Ј-bipyridin-6(1H)-one (bpyO)
[Equation (1)] would induce the acid-base equilibrium of
bpyOH ligated to Ru [Equation (2)], followed by resonance
between (2,2Ј-bipyridin-6-onato)- and (2,2Ј-bipyridin-6-ola-
to)ruthenium complexes [Equation (3)].
(2)
(3)
A few metal complexes with bpyO have been reported to
date,[4] but neither the equilibrium of Equation (2) nor the
resonance of Equation (3) has been discussed. The acid-
base equilibrium of Ru(bpyOH) complexes [Equation (2)]
and the subsequent resonance of Ru(bpyO) ones [Equa-
tion (3)] may allow these complexes to undergo proton-cou-
pled electron transfer. The ligand-localized redox reaction
of metalϪbpyO(H) complexes would influence the equilib-
rium of Equation (2) and the resonance of Equation (3). In
connection with this, a drastic change of basicity of the free
nitrogen of the 1,8-naphthyridine ligand of [Ru(bpy)2-
(napy-κN)(CO)]2ϩ (napy ϭ 1,8-naphthyridine) has been re-
ported. One-electron reduction of napy resulted in an at-
tack of the free nitrogen at the carbon atom of the RuϪCO
bond to form a metallacyclic ring, which smoothly opened
upon oxidation of the napy ligand.[5] Such reversible cyclo-
metalation driven by ligand-localized redox reactions falls
(1)
[a]
Institute for Molecular Science and CREST, Japan Science and
Technology Agency (JST),
5-1 Higashiyama, Myodaiji, Okazaki, Aichi 444-8787, Japan
Fax: ϩ 81-564-59-5582
E-mail: ktanaka@ims.ac.jp
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
Eur. J. Inorg. Chem. 2005, 285Ϫ293
DOI: 10.1002/ejic.200400522
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
285

T. Tomon, T. Koizumi, K. Tanaka
FULL PAPER
into a new category in a number of metallacyclic complexes [Ru(terpy)(bpyOH)]ϩ that would be produced in the reac-
bearing five-[6] and six- membered[7] rings and containing tion of [Ru(terpy)(bpyOH)Cl]ϩ with Agϩ.
CO as a cycle component.
Treatment of [1]ϩ and [2]ϩ with one equivalent of HPF6
In this work, we have prepared two geometrical isomers produced [1H]2ϩ and [2H]2ϩ, respectively, in CH3CN. The
of [Ru(terpy)(bpyO)(CO)]ϩ with regard to the orientation addition of one equivalent of Et3N to [1H]2ϩ and [2H]2ϩ
of bpyO ([1]ϩ and [2]ϩ) and determined the pKa values of completely regenerated [1]ϩ and [2]ϩ, respectively, in
their protonated complexes ([1H]2ϩ and [2H]2ϩ). Two-elec- CH3CN [Equation (4)].
tron reduction of [1]ϩ, which has the pyridonato moiety of
the bpyO ligand trans to CO, caused reductive cleavage of
the RuϪCO bond, while two-electron reduction of [2]ϩ,
which has the pyridonato moiety of the bpyO ligand cis to
CO, formed a metallacyclic ring due to an attack of the
(4)
oxygen of bpyO at the carbon of the RuϪCO bond.
1
The H NMR spectra of [1]ϩ and [2]ϩ clearly confirm
the protonation of the oxygen atom of the bpyO ligand,
Results and Discussion
since treatment of [1]ϩ and [2]ϩ with HPF6 in CD3CN shifts
the pyridonate ring protons to lower magnetic field by
0.65Ϫ1.3 ppm, while the terpy and pyridine protons of
Syntheses and Structures of [1]؉, [1H]2؉, [2]؉, and [2H]2؉
There are two possible orientations of the pyridonato bpyO shift by only 0.1Ϫ0.25 ppm upon protonation of [1]ϩ
ring of bpyO in [Ru(terpy)(bpyO)(CO)]ϩ (vide infra): one and [2]ϩ. The pKa values of hydroxypyridine in [1H](PF6)2
is in a position trans to CO ([1]ϩ) and the other is in a and [2H](PF6)2 were determined to be 3.5 and 3.9, respec-
position cis to CO ([2]ϩ). The preparative pathways of [1]ϩ tively, by pH titrations in H2O at 25 °C. The difference in
and [2]ϩ are depicted in Scheme 1. Treatment of the pKa values of [1H]2ϩ and [2H]2ϩ can reasonably be as-
[RuCl(terpy)(bpyOH)]ϩ with AgPF6 under CO (15 atm) in cribed to the stronger electronic interaction between CO
2-methoxyethanol
produced
[Ru(terpy)(bpyO)(CO)]ϩ and the pyridonato moiety in the bpyO ligand in the trans
position compared with that in the cis position.
([1]ϩ).
The 1H NMR spectra of the PF6 salt of [1]ϩ clearly show
The crystal structures of the cations of complexes [Ru-
the selective formation of one of the two isomers under (terpy)(bpyO)(OH2)](PF6) and [Ru(terpy)(bpyOH)Cl]-
these reaction conditions. X-ray analysis of [1](PF6) demon- (PF6)[8] are depicted in Figure 1 and the Supporting Infor-
strated coordination of CO to Ru in the position trans to mation, respectively. The bond lengths and angles of
the pyridonato ring of bpyO (vide infra). On the other [Ru(terpy)(bpyO)(OH2)]ϩ are listed in Table 1. The pyri-
hand, the substitution reaction of an aqua ligand of donato ring of [Ru(terpy)(bpyO)(OH2)]ϩ and the hydroxy-
[Ru(terpy)(bpyO)(OH2)]ϩ by CO (20 atm) in 2-methoxy- pyridine ring of [Ru(terpy)(bpyOH)Cl]ϩ are situated cis to
ethanol selectively produced the other isomer of the H2O and Cl ligands, respectively. The C1ϪO1 and CϪC
[Ru(terpy)(bpyO)(CO)]ϩ with CO in the position cis to the bonds lengths of the pyridonato ring of bpyO in
1
pyridonato ring of bpyO ([2]ϩ). The H NMR spectra of [Ru(terpy)(bpyO)(OH2)]ϩ are 1.288(5) A and 1.368(7)Ϫ
˚
˚
the crude product of [2](PF6) did not show any contami- 1.417(7) A, and are similar to those in the 2-pyridonato
nation by [1](PF6). It is worthwhile to note that the orien- ligands of [Ru(terpy)(NC5H4O-κN)2(OH2)][9] and mer-
tation of bpyOH in [Ru(terpy)(bpyOH)Cl]ϩ is inverted in [Pt(NH3)2(NC5H4O-κN)Cl3][10] (NC5H4O-κN ϭ 2-pyridon-
[1]ϩ, whereas that in [Ru(terpy)(bpyO)(OH2)] is main- ato). The Ru1ϪO2 bond length is 2.152(3) A, which is quite
˚
tained in [2]ϩ. Such a difference may result from an in- close to that in [Ru(terpy)(NC5H4O-κN)2(OH2)] (2.160
[9]
˚
˚
volvement of an attack of CO to pentacoordinate A). The interatomic O1···O2 distance [2.533(5) A] be-
Scheme 1. Synthetic routes to the carbonyl complexes [1]ϩ and [2]ϩ
286
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Stabilization and Destabilization of the RuϪCO Bond During a Ligand-Localized Redox Reaction
FULL PAPER
Figure 1. Molecular structure of [Ru(terpy)(bpyO)(OH2)]ϩ with
atom labeling; hydrogen atoms are omitted for simplicity
˚
Table 1. Selected bond lengths [A] and angles [°] for [Ru(terpy)-
(bpyO)(OH2)](PF6)
Ru1ϪN1
Ru1ϪN3
Ru1ϪN5
N1ϪC1
C1ϪO1
C2ϪC3
C4ϪC5
N1ϪRu1ϪN2
N1ϪRu1ϪO2
N4ϪRu1ϪN5
O1ϪC1ϪN1
N1ϪC1ϪC2
2.089(4)
2.062(4)
2.081(4)
1.370(6)
1.288(5)
1.368(7)
1.375(6)
78.7(1)
94.5(1)
79.6(2)
120.1(4)
119.1(4)
Ru1ϪN2
Ru1ϪN4
Ru1ϪO2
N1ϪC5
C1ϪC2
C3ϪC4
2.036(4)
1.957(4)
2.152(3)
1.358(6)
1.417(7)
1.399(7)
N1ϪRu1ϪN4
N3ϪRu1ϪN4
175.8(1)
80.0(2)
O1ϪC1ϪC2
120.8(4)
tween the pyridonato and the aqua ligands suggests hydro-
gen-bond formation between them.
The molecular structures of the ruthenium carbonyl com-
plexes bearing bpyO ([1]ϩ and [2]ϩ) and bpyOH ([2H]2ϩ
were determined by X-ray crystal structure analysis. The
crystal structures of the cations of these complexes are
shown in Figure 2; selected bond lengths and angles are
)
Figure 2. Molecular structures of [1]ϩ (a), [2]ϩ (b) and [2H]2ϩ (c)
with atom labeling; hydrogen atoms are omitted for simplicity
˚
listed in Table 2. The pyridonato group of the bpyO ligand more, elongation of the Ru1ϪN1 bond length [2.112(4) A],
in [1]ϩ and [2]ϩ is situated trans and cis, respectively, to the as well as the C1ϪO1 bond length of [2H]2ϩ, compared
CO ligand (Figure 2a and 2b). The orientation of bpyOH with that of [2]ϩ [2.087(9) A] demonstrates the decrease of
˚
in [2H]2ϩ is the same as that in [2]ϩ (Figure 2c). The the covalency of the RuϪN1 bond due to the protonation
Ru1ϪN bond lengths of terpy in the carbonyl complexes of the oxygen atom of bpyO in [2]ϩ [Equation (2)]. There
are 1.989(3)Ϫ2.100(3) for [1]ϩ, 2.006(9)Ϫ2.09(1) for [2]ϩ, seems to be a substantial strain in the coordination of bpyO
and 1.983(4)Ϫ2.098(4) for [2H]2ϩ. These bond lengths are to Ru in [1]ϩ because the C1ϪN1ϪRu1ϪN4 dihedral angle
close to those of similar ruthenium carbonyl complexes [15.6(3)°] in [1]ϩ is much larger than the corresponding an-
such as [Ru(terpy)(bpy)(CO)]2ϩ (1.989Ϫ2.086 A)
and gle (C1ϪN1ϪRu1ϪC26) in [2]ϩ [7(1)°]. This may be caused
[3e]
˚
[Ru(terpy)(bpy)(py)]2ϩ (1.963Ϫ2.078 A; py ϭ pyridine).
by repulsion between the lone-pair electrons of the bpyO
[11]
˚
The C1ϪO1 bond lengths of bpyO in [1]ϩ [1.250(4) A] and oxygen atom and the π-electrons of the central pyridine ring
˚
[2]ϩ [1.26(1) A] are in the range of a normal CϭO double of terpy. The shift of the oxygen atom of bpyO near the
˚
bond. It is worthwhile to note that the C1ϪO1 bond length pyridyl ring of terpy in [1]ϩ to the position cis to the CO
in [2H]2ϩ [1.326(7) A] is much longer than those in [1]ϩ and ligand of [2]ϩ will release the strain in the coordination of
˚
[2]ϩ, and is close to that of a CϪO single bond. Further- bpyO to Ru. A relatively short interatomic distance between
Eur. J. Inorg. Chem. 2005, 285Ϫ293
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T. Tomon, T. Koizumi, K. Tanaka
FULL PAPER
˚
Table 2. Selected bond lengths [A] and angles [°] for [1](PF6),
[2](PF6)·H2O, and [2H](PF6)2·2H2O
[1]ϩ
Ru1ϪN1
Ru1ϪN3
Ru1ϪN5
N1ϪC1
C1ϪO1
C2ϪC3
C4ϪC5
N1ϪRu1ϪN2
N2ϪRu1ϪC26
N4ϪRu1ϪN5
O1ϪC1ϪN1
N1ϪC1ϪC2
2.118(3)
2.085(3)
2.100(3)
1.382(4)
1.250(4)
1.355(5)
1.376(5)
77.6(1)
97.2(1)
79.3(1)
120.4(3)
117.2(3)
R1ϪN2
Ru1ϪN4
Ru1ϪC26
N1ϪC5
C1ϪC2
C3ϪC4
C26ϪO2
N1ϪRu1ϪN4
N3ϪRu1ϪN4
Ru1ϪC26ϪO2
O1ϪC1ϪC2
2.086(3)
1.989(3)
1.868(3)
1.373(4)
1.441(5)
1.404(5)
1.137(4)
98.6(1)
79.7(1)
176.0(3)
122.3(3)
[2]ϩ
Ru1ϪN1
Ru1ϪN3
Ru1ϪN5
N1ϪC1
C1ϪO1
C2ϪC3
C4ϪC5
N1ϪRu1ϪN2
N2ϪRu1ϪN4
N4ϪRu1ϪN5
O1ϪC1ϪN1
N1ϪC1ϪC2
2.087(9)
2.08(1)
2.09(1)
1.33(1)
1.26(1)
1.39(2)
1.35(2)
78.3(4)
93.6(4)
78.4(4)
120(1)
R1ϪN2
Ru1ϪN4
Ru1ϪC26
N1ϪC5
C1ϪC2
C3ϪC4
C26ϪO2
N1ϪRu1ϪC26
N3ϪRu1ϪN4
Ru1ϪC26ϪO2
O1ϪC1ϪC2
2.13(1)
2.006(9)
1.90(1)
1.40(1)
1.40(2)
1.40(2)
1.12(1)
96.2(4)
79.1(4)
173(1)
120(1)
Figure 3. Cyclic voltammograms of [1](PF6) (a) and [2](PF6) (b) in
CH3CN under N2; dE/dt ϭ 100 mV·sϪ1
120(1)
[2H]2ϩ
[Ru(terpy)2]2ϩ [13] The irreversible reduction of [1]0 prob-
.
ably results from a partial RuϪCO bond cleavage of [1]Ϫ
Ru1ϪN1
Ru1ϪN3
Ru1ϪN5
N1ϪC1
C1ϪO1
C2ϪC3
C4ϪC5
N1ϪRu1ϪN2
N2ϪRu1ϪN4
N4ϪRu1ϪN5
O1ϪC1ϪN1
N1ϪC1ϪC2
2.112(4)
R1ϪN2
Ru1ϪN4
Ru1ϪC26
N1ϪC5
C1ϪC2
C3ϪC4
C26ϪO2
N1ϪRu1ϪC26
N3ϪRu1ϪN4
Ru1ϪC26ϪO2
O1ϪC1ϪC2
2.117(5)
1.983(4)
(vide infra), because two-electron reduction of
2.098(4)
2.094(4)
1.345(7)
1.326(7)
1.344(9)
1.366(7)
77.3(2)
1.909(6) [Ru(terpy)(bpy)(CO)]2ϩ leads to reductive RuϪCO bond
1.366(7)
1.410(8)
1.399(9)
1.115(7)
96.6(2)
79.2(2)
177.5(5)
122.0(5)
cleavage (CO evolution), which is a key process in the elec-
trochemical reduction of CO2 catalyzed by this complex.[3e]
The CV of [2]ϩ also shows an almost reversible [2]ϩ/[2]0
redox couple at E1/2 ϭ Ϫ1.51 V (Ep,c ϭ Ϫ1.55 and Ep,a
ϭ
93.8(2)
79.1(2)
116.5(5)
121.4(5)
Ϫ1.47 V) and an irreversible cathodic wave at Ep,c ϭ Ϫ2.13
V (Figure 3b). In contrast to the reductive cleavage of the
RuϪCO bond of [1]Ϫ, the irreversible reduction of [2]0 is
due to a cyclometalation caused by an intramolecular at-
tack of the pyridonato oxygen at the carbonyl carbon atom
(vide infra). The oxidation processes of [1]ϩ and [2]ϩ were
also examined, but both complexes were not oxidized up to
ϩ1.0 V.
the oxygen atom of bpyO and the carbon atom of CO in
[2]ϩ of 2.55 A implies a donorϪacceptor interaction be-
˚
The CV of [1H]2ϩ in CH3CN displays four cathodic
waves at Ep,c ϭ Ϫ1.40, Ϫ1.69, Ϫ1.85, and Ϫ2.05 V in the
first potential sweep (Figure 4a).[14] The cathodic wave at
Ep,c ϭ Ϫ1.40 V completely disappeared in the second po-
tential sweep, while the other three cathodic waves at Ep,c ϭ
Ϫ1.69, Ϫ1.85, and Ϫ2.05 V remained at almost the same
potentials in multiscanning potential sweeps. The addition
tween them. An increase of the distance between the oxygen
atom of bpyOH and the carbon atom of CO (O1···C26 ϭ
2.62 A) in [2H] is therefore explained by the decrease in
the basicity of the oxygen atom of bpyO upon protonation.
2ϩ
˚
Cyclic Voltammetry of [1](PF6), [2](PF6), [1H](PF6)2 and
[2H](PF6)2
The cyclic voltammograms (CVs) of [1](PF6) and [2](PF6) of one equivalent of Et3N to the solution also caused the
were measured in CH3CN (Figure 3). The CV of [1]ϩ shows disappearance of the cathodic wave at Ep,c ϭ Ϫ1.40 V in
a nearly reversible redox couple at E1/2 ϭ Ϫ1.65 V (Ep,c
ϭ
the first potential sweep. However, the second cathodic
Ϫ1.70, Ep,a ϭ Ϫ1.61 V) and a subsequent irreversible ca- wave became broad. Based on the fact that Et3NHϩ un-
thodic wave at Ep,c ϭ Ϫ2.07 V (Figure 3a), both of which dergoes an irreversible reduction at Ϫ1.75 V in CH3CN on
are assigned to terpy- and bpyO-ligand-based redox reac- a glassy carbon electrode, the broad cathodic wave at Ep,c ϭ
tions[12] by analogy with the redox behavior of Ϫ1.69 V apparently results from overlapping of the terpy-
288
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Stabilization and Destabilization of the RuϪCO Bond During a Ligand-Localized Redox Reaction
FULL PAPER
the redox behaviors of [1]ϩ and [2]ϩ are similar (one revers-
ible and one irreversible reduction). However, there is a dis-
tinct difference between the IR spectra during the reduction
and oxidation cycles of the two complexes. The ν(CϵO)
band of [1]ϩ at 1979 cmϪ1 disappears completely during the
controlled potential electrolysis of the complex at Ϫ1.70 V
in CH3CN, and a new ν(CϵO) band appears at 1932 cmϪ1
(Figure 5a). The 1932 cmϪ1 band is further shifted to 1853
cmϪ1, with a lower peak intensity, when the electrolysis po-
tential is changed from Ϫ1.70 V to Ϫ2.0 V (Figure 5b). The
decrease of the peak intensity of the 1853 cmϪ1 band com-
pared with those of the 1979 and 1932 cmϪ1 bands prob-
ably results from a gradual RuϪCO bond cleavage of [1]Ϫ;
reoxidation of the solution at 0 V did not recover the peak
intensity of the ν(CϵO) band of [1]ϩ at 1979 cmϪ1. Besides
the change of the ν(CϵO) band, two bands at 1605 and
1620 cmϪ1, assigned to the stretching modes of the pyridine
rings and the CϭO group of the bpyO ligand of [1]ϩ,
respectively, moved to 1602 and 1618 cmϪ1, and then to
1595 and 1581 cmϪ1 in the electrochemical reduction at
Ϫ1.70 and Ϫ2.0 V. The red-shift of the band of the pyridon-
ato ring of bpyO from 1620 to 1581 cmϪ1 upon two-elec-
tron reduction could be associated with a shift of the reson-
ance of Equation (3) towards the pyridonato form.
Figure 4. Cyclic voltammograms of [1H](PF6)2 (a) and [2H](PF6)2
(b) in the absence (Ϫ) and the presence (---) of 1.0 equiv. of Et3N
under N2; dE/dt ϭ 100 mV·sϪ1
The ν(CϵO) band at 1996 cmϪ1 of [2]ϩ shifts to 1950
cmϪ1 upon one-electron reduction of the complex at Ϫ1.70
V (Figure 5c). The 1950 cmϪ1 band of [2]0 vanishes com-
pletely under electrolysis at Ϫ2.05 V. Although no new
bands assignable to ν(CϵO) were detected in the region
1800Ϫ1900 cmϪ1, a new band emerged at 1587 cmϪ1 (Fig-
ure 5d). Re-oxidation of the resultant solution at 0 V almost
recovered the IR spectrum of [2]ϩ [ν(CϵO) band at 1996
cmϪ1].[16] Thus, the conversion between [2]ϩ and [2]Ϫ in
CH3CN is a chemically reversible process, although [2]0 un-
dergoes an irreversible reduction at Ϫ2.05 V. The 1587
cmϪ1 band of [2]Ϫ is therefore assigned to the stretching
mode of the OϪCϭO moiety of the five-membered metalla-
cycle formed by the attack of the pyridonato oxygen at the
carbonyl atom of the CO ligand [Equation (6)].
based reduction and an irreversible reduction of Et3NHϩ
produced in the reaction of [1H]ϩ with Et3N. The CV of
[2H]2ϩ displays a broad cathodic wave at Ep,c ϭ Ϫ1.58 V
and a subsequent irreversible cathodic wave at Ep,c ϭ Ϫ2.14
V (Figure 4b).[15] Addition of one equivalent of Et3N to the
solution sharpened the cathodic wave at Ep,c ϭ Ϫ1.58 V
and resulted in an irreversible cathodic wave at Ϫ1.75 V
due to the reduction of Et3NHϩ. These results indicate that
the proton of the bpyOH ligand of [1H]2ϩ and [2H]2ϩ un-
dergoes irreversible reduction at Ep,c ϭ Ϫ1.40 and Ϫ1.58 V,
respectively, to form [1]ϩ and [2]ϩ [Equation (5)].
(5)
IR Spectroscopy of [1]؉ and [2]؉ under the Electrolysis
Conditions
(6)
The IR spectra of [1](PF6) show the ν(CϵO) band and
the ν(CϭO) band of bpyO at 1979 and 1620 cmϪ1, respec-
tively, in CD3CN. The appearance of those bands at 1996
and 1612 cmϪ1 in the IR spectrum of [2](PF6) is ascribed
to the stronger electron-donor ability of bpyO located trans
This reversible ring-closing and -opening [Equation (6)]
to CO compared to when it is cis to CO (the redox poten- is also supported by the IR spectra of [2*]ϩ, which contains
tials of the reversible [1]ϩ/0 and [2]ϩ/0 couples were observed a C18O ligand. The ν(Cϵ18O) band at 1946 cmϪ1 of [2*]ϩ
at Ϫ1.65 and Ϫ1.51 V, respectively, in CH3CN). In other shifts to 1907 cmϪ1 and then to 1562 cmϪ1 upon electrolysis
words, the resonance between metal pyridinolate and pyri- at Ϫ1.70 and Ϫ2.00 V, respectively (Figure 6). Thus, the
donate forms of [1]ϩ [Equation (3)] is shifted more towards 1562 cmϪ1 band of [2*]Ϫ can reasonably be associated with
the pyridonato than in [2]ϩ. It is worthwhile to note that the isotropic shift of the 1587 cmϪ1 band of [2]Ϫ. Both [2]Ϫ
Eur. J. Inorg. Chem. 2005, 285Ϫ293
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289

T. Tomon, T. Koizumi, K. Tanaka
FULL PAPER
Figure 5. IR spectra of [1](PF6) (a and b) and [2](PF6) (c and d) under controlled-potential electrolyses at Ϫ1.70 V (a and c) and
Ϫ2.00 V (b and d) in CD3CN under N2; * denotes CD3CN peaks
As described above, the hydroxyl proton of the bpyOH
ligand of [1H]2ϩ and [2H]2ϩ undergoes irreversible re-
duction at around Ϫ1.50 V to form [1]ϩ and [2]ϩ in CH3CN
[Equation (5)]. In fact, the electrolysis of [1H]2ϩ and [2H]2ϩ
in CH3CN at Ϫ1.40 V causes the disappearance of the
ν(CϵO) bands at 1992 and 2005 cmϪ1 of these complexes,
and the appearance of the ν(CϵO) bands at 1979 and 1996
cmϪ1 of [1]ϩ and [2]ϩ, respectively. Although the redox po-
tentials of [1]ϩ and [2]ϩ are too negative to evaluate whether
the pKa values of the bpyOH ligand depend on the oxi-
dation states of the complexes, the distinct difference in the
RuϪCO bond stability between [1]Ϫ and [2]Ϫ is explained
by the cyclometallation due to the intramolecular attack of
the oxygen of bpyO at CO in the latter.
Conclusion
We succeeded in selectively synthesizing two isomers ([1]ϩ
and [2]ϩ) with a different orientation of bpyO at the ru-
thenium center. The hydrogen bond between the pyridonate
oxygen and H2O in [Ru(terpy)(bpyO)(OH2)]ϩ may play an
important role in the preparation of [2]ϩ. [1]ϩ and [2]ϩ can
Figure 6. IR spectra of [2*](PF6) under controlled potential elec-
be reversibly converted into [1H]2ϩ and [2H]2ϩ by the pro-
trolysis at Ϫ1.70 V (a) and Ϫ2.00 V (b) in CD3CN under N2;
tonation of the oxygen atom of bpyO. Based on their pKa’s,
* denotes CD3CN peaks
CVs and IR spectra, the electron density at the ruthenium
and [2*]Ϫ exhibit a band at 1587 cmϪ1. This observation center of [1]ϩ is higher than that of [2]ϩ. Such a difference
indicates the overlapping of the ring vibration mode of the is associated with the stronger trans influence of CO com-
bpyO moiety of [2]Ϫ with the stretching mode of the pared with the central pyridine of terpy, because the elec-
OϪCϭ16O group.
tron-withdrawing ability of CO induces the pyridonato
290
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Stabilization and Destabilization of the RuϪCO Bond During a Ligand-Localized Redox Reaction
FULL PAPER
Preparation of 6-Hydroxy-2,2Ј-bipyridine (bpyOH): A mixture of 6-
methoxy-2,2Ј-bipyridine (700 mg, 3.76 mmol) and pyridine hydro-
chloride (10 g) was heated under N2 at 210 °C for 90 min. After
the reaction mixture had cooled to ambient temperature, the white
solid that appeared was dissolved in H2O (50 mL). The crude prod-
uct was extracted from the aqueous solution with dichloromethane.
The dichloromethane layer was further treated with a saturated
aqueous NaHCO3 solution. The organic layer was dried with anhy-
drous sodium sulfate, and the solvents were evaporated to dryness
under reduced pressure. Recrystallization from CH2Cl2/hexane
structure of the resonance between metal-pyridinolate and -
pyridonate forms [Equation (3)]. The CVs of [1]ϩ and [2]ϩ
show one reversible and one irreversible redox wave due to
terpy- and bpyO-based redox reactions. The two-electron
reduction of [1]ϩ is followed by a partial RuϪCO bond
cleavage. On the other hand, the bpyO-based reduction of
[2]ϩ causes cyclometalation by an attack of the pyridonato
oxygen at the carbonyl carbon; re-oxidation of the resultant
[2]Ϫ almost recovers [2]ϩ. Thus, cyclometallation caused by
a bpyO-based reduction effectively supresses the reductive (1:1, v/v) gave a pale-yellow solid. Yield 520 mg (80.4%). The
identification of the compound was performed according to the
cleavage of a RuϪCO bond.
literature (1H NMR).[19]
Preparation of [Ru(terpy)(bpyOH)Cl]Cl·H2O: BpyOH (40 mg,
0.23 mmol) and Et3N (120 mg, 1.18 mmol) were added to a suspen-
sion of [Ru(terpy)Cl3] (100 mg, 0.22 mmol) in 2-methoxyethanol.
The mixture was stirred under N2 at 85 °C for 3 h, during which
time the suspension gradually changed to a deep purple solution.
After an insoluble solid was filtered off, the filtrate was concen-
trated to about 5 mL under reduced pressure. The resultant precipi-
tate was separated by filtration and recrystallized from methanol.
Yield 65 mg (49.6%). C25H21Cl2N5O2Ru: calcd. C 50.42, H 3.55, N
11.76; found C 49.98, H 3.47, N 11.60. ESI-MS: m/z ϭ 542
[Ru(terpy)(bpyOH)Cl]ϩ.
Experimental Section
Reagents: 2-Bromo-6-methoxypyridine, 2,2Ј:6Ј,2ЈЈ-terpyridine and
AgPF6 were purchased from Aldrich Chemical Co., Inc. 2-(Tri-n-
butylstannyl)pyridine was purchased from Lancaster Synthesis,
pyridine hydrochloride from Kanto Chemical Co., Inc., NH4PF6
from Wako Pure Chemical Industries, Ltd., and Me4NBF4 from
Nacalai Tesque. CD3CN was purchased from Merck (for IR meas-
urements) and Isotec (for NMR measurements). All solvents used
for preparations were reagent grade and used without further puri-
fication. [Ru(terpy)Cl3] was prepared according to the literature
method.[17]
Preparation of [Ru(terpy)(bpyO)(CO)](PF6) {[1](PF6)}: A mixture
of [Ru(terpy)(bpyOH)Cl]Cl·H2O (90 mg, 0.15 mmol) and AgPF6
(81 mg, 0.32 mmol) in 2-methoxyethanol was stirred under CO
pressure (15 atm) at 85 °C for 24 h. After the solution had cooled
to room temperature, the solution was filtered through a Celite
layer to remove the AgCl precipitate and then concentrated to
about 5 mL. Addition of an excess amount of an aqueous NH4PF6
solution gave a yellow precipitate, which was separated by fil-
tration. The resultant product was recrystallized from acetone/etha-
Physical Methods: 1H and 1H-1H-COSY NMR experiments were
performed with a JEOL LA-500 spectrometer. IR spectra were re-
corded with a Shimadzu FTIR-8300 spectrophotometer. ESI-MS
spectra were measured with a Shimadzu LCMS-2010 liquid chrom-
atograph mass spectrometer. pH titrations were performed with a
Hiranuma automatic titrator COM-980 Win. Aqueous solutions
(3 mL) of [1H](PF6)2 (1.0Ϫ1.5 mm) or [2H](PF6)2 (1.0Ϫ1.5 mm)
were titrated with an aqueous NaOH solution (0.1 n, f ϭ 1.04) at
25 °C. Cyclic voltammograms were obtained in a one-compartment
cell consisting of a glassy carbon working electrode, a platinum
counterelectrode and an Ag/AgNO3 (10 mm) reference electrode in
CH3CN containing Me4NBF4 (50 mm) and the complex (1 mm).
The cyclic voltammetric data were collected with an ALS/Chi
model 1660A electrochemical analyzer. IR spectra under con-
trolled-potential electrolysis conditions were obtained using a thin-
layer cell sandwiched between two KBr crystals with an Au mesh
working electrode, a platinum wire counterelectrode, and an Ag/
AgNO3 (10 mm) reference electrode.[3d] The measurements were
conducted in CD3CN or CH3CN containing the sample complex
(10 mm) and Me4NBF4 (50 mm) as an electrolyte.
1
nol (2:1, v/v) containing Et3N (30 µL). Yield 100 mg (53.6%). H
NMR (500 MHz, CD3CN): δ ϭ 5.65 (d, 1 H, HA3), 6.98 (d, 1 H,
HA5), 7.08 (dd, 1 H, HA4), 7.39 (ddd, 2 H, HC4), 7.61 (ddd, 1 H,
HB5), 7.69 (d, 2 H, HC3), 8.05 (td, 2 H, HC5), 8.09 (td, 1 H, HB4),
8.21 (t, 1 H, HD4), 8.26 (d, 1 H, HB3), 8.31 (m, 4 H, HC6 and HD3),
9.35 (d, 1 H, HB6) ppm. C26H18F6N5O2PRu: calcd. C 46.03, H 2.67,
N 10.32; found C 45.86, H 2.79, N 10.33. ESI-MS: m/z ϭ 534
[Ru(terpy)(bpyO)(CO)]ϩ.
Preparation of [Ru(terpy)(bpyOH)(CO)](PF6)2 {[1H](PF6)2}: Yellow
crystals of [1H](PF6)2 were obtained by crystallization of [1](PF6)
from CH3CN/H2O (1:1, v/v) containing 1.5 equiv. of an aqueous
1
HPF6 solution (1 m). H NMR (500 MHz, CD3CN): δ ϭ 6.97 (d,
1 H, HA3), 7.41 (ddd, 2 H, HC4), 7.68 (d, 2 H, HC3), 7.76 (dd, 1 H,
HA4), 7.79 (ddd, 1 H, HB5), 7.90 (d, 1 H, HA5), 8.11 (td, 2 H, HC5),
8.26 (td, 1 H, HB4), 8.38 (t, 1 H, HD4), 8.43 (d, 2 H, HC6), 8.47 (d,
2 H, HD3), 8.51 (d, 1 H, HB3), 9.45 (d, 1 H, HB6) ppm. ESI-MS:
Preparation of 6-Methoxy-2,2Ј-bipyridine:
A toluene solution
(35 mL) containing 2-bromo-6-methoxypyridine (1.0 g, 5.32
mmol), 2-(tri-n-butylstannyl)pyridine (3.0 g), LiCl (2.2 g, 53 mmol),
and [PdCl2(PPh3)2] (290 mg, 0.42 mmol) was deaerated by bubbling
N2 throug it. The mixture was then refluxed under N2 for 18 h, and
an aqueous solution (10 mL) of NaF (2.2 g, 53 mmol) was added
to the solution at room temperature. The resultant solution was
further stirred at ambient temperature for 30 min. An insoluble
solid was filtered off, and the filtrate was treated with a 5% Na2CO3
aqueous solution in a separating funnel. The organic layer was
dried with anhydrous sodium sulfate, and the solvents were evapo-
rated to dryness under reduced pressure. The obtained oil was dis-
solved in CHCl3/EtOAc (95:5, v/v), and purified on a silica gel
column. Yield: 830 mg (83.7%). The identification of the com-
pound was performed according to the literature (1H NMR).[18]
m/z ϭ 267.5 [Ru(terpy)(bpyOH)(CO)]2ϩ
.
Preparation of [Ru(terpy)(bpyO)(OH2)](PF6): Na2CO3 (91 mg,
0.86 mmol) and Et3N (0.4 mL) were added to a suspension of [Ru-
(terpy)Cl3] (250 mg, 0.57 mmol) in MeOH/H2O (1:2 v/v, 80 mL),
and then the suspension was refluxed under N2 for 150 min. The
resultant solution was cooled to room temperature, filtered, and
the filtrate was concentrated to dryness under reduced pressure.
The resulting solid was dissolved in ethanol and any insoluble salt
was removed by filtration. Then, the solution was concentrated to
dryness, and the solid was purified by column chromatography on
a Cosmosil column, with MeOH as eluent. An aqueous solution
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291

T. Tomon, T. Koizumi, K. Tanaka
(5 mL) of [Ru(terpy)(CO3)(OH2)] (60 mg) and bpyOH (30 mg, C26H23F12N5O4P2Ru: calcd. C 36.29, H 2.69, N 8.14; found C
FULL PAPER
0.17 mmol) was stirred at ambient temperature for 2Ϫ3 d, and the
solution was then filtered. Addition of an excess amount of
NH4PF6 to the filtrate gave [Ru(terpy)(bpyO)(OH2)](PF6) as a red-
purple powder which was isolated by filtration, and recrystallized
from acetone/H2O (4:1, v/v). Yield 45 mg (11.8%). 1H NMR
(500 MHz, [D6]acetone): δ ϭ 6.71 (d, 1 H, HA3), 6.89 (ddd, 1 H,
HB4), 7.30 (d, 1 H, HB3), 7.56 (ddd, 2 H, HC4), 7.62 (td, 1 H, HB5),
7.67 (d, 1 H, HA5), 7.79 (t, 1 H, HA4), 8.05 (td, 2 H, HC5), 8.11 (d,
2 H, HC3), 8.20Ϫ8.25 (m, 2 H, HB6 and HD4), 8.64 (d, 2 H, HC6),
8.76 (d, 2 H, HD3) ppm. C25H20F6N5O2PRu: calcd. C 44.92, H
3.01, N 10.48; found C 45.16, H 3.20, N 10.44. ESI-MS: m/z ϭ 524
[Ru(terpy)(bpyO)(OH2)]ϩ.
36.26, H 2.80, N 8.13. ESI-MS: m/z ϭ 267.5 [Ru(terpy)(bpyOH)-
(CO)]2ϩ
.
Preparation of [Ru(terpy)(bpyO)(C18O)](PF6) {[2*](PF6)}: Et3N (0.1
equiv.) was added to a CH3CN/H218O (4:1, v/v, 1 mL) solution of
[2](PF6) (40 mg) and the mixture was heated at 50 °C for 2 d. After
the solution had cooled to room temperature, the solvent was re-
moved completely. The orange solid thus obtained was recrys-
tallized from CH3CN/EtOH (1:4, v/v). The purity of the complex
was checked by comparison of the parent peaks of m/z ϭ 534 ([2]ϩ)
and 536 ([2*]ϩ) in the ESI-MS, and of the ν(C16O) and ν(C18O)
bands at 1996 and 1946 cmϪ1 in the IR spectra. Based on these
results, the sample used in the experiments contained about 80%
[2*](PF6) and 20% [2](PF6). Yield 18 mg. ESI-MS: m/z ϭ 536
[Ru(terpy)(bpyO)(C18O)]ϩ.
Preparation of [Ru(terpy)(bpyO)(CO)](PF6)·H2O {[2](PF6)·H2O}: A
2-methoxyethanol solution of [Ru(terpy)(bpyO)(OH2)](PF6)
(45 mg, 0.07 mmol) was stirred under CO pressure (20 atm) at 90
°C for 12 h. The reaction mixture was then concentrated to about
5 mL under reduced pressure. Addition of an excess amount of an
aqueous NH4PF6 solution to the solution gave an orange precipi-
tate, which was separated by filtration. Orange crystals were ob-
tained by vapor diffusion of diethyl ether into an EtOH solution
containing Et3N (15 µL). Yield 22 mg (44.9%). 1H NMR
(500 MHz, CD3CN): δ ϭ 6.60 (d, 1 H, HA3), 7.00 (d, 1 H, HB3),
7.05 (ddd, 1 H, HB4), 7.24 (d, 1 H, HA5), 7.45 (ddd, 2 H, HC4),
7.54 (dd, 1 H, HA4), 7.81 (td, 1 H, HB5), 7.90 (d, 2 H, HC3), 8.07
(td, 2 H, HC5), 8.12 (d, 1 H, HB6), 8.39 (d, 2 H, HC6), 8.42 (t, 1 H,
HD4), 8.54 (d, 2 H, HD3) ppm. C26H20F6N5O3PRu: calcd. C 44.83,
H 2.89, N 10.05; found C 44.71, H 2.93, N 10.09. ESI-MS: m/z ϭ
534 [Ru(terpy)(bpyO)(CO)]ϩ.
Crystallographic Study of [Ru(terpy)(bpyO)(OH2)](PF6), [1](PF6),
[2](PF6)·H2O, and [2H](PF6)2·2H2O: Crystals for X-ray analyses of
the complexes were obtained as described above. A suitable single
crystal for the measurement was mounted on a glass fiber. Data
were collected at Ϫ100 °C on a Rigaku/MSC Mercury CCD dif-
fractometer with graphite-monochromated Mo-Kα radiation (λ ϭ
˚
0.71070 A). All data were collected and processed using the Crystal
Clear program (Rigaku). All the calculations were carried out with
the teXsan software package.[20] All structures were solved by direct
methods and expanded using Fourier techniques. Refinements were
performed anisotropically for all non-hydrogen atoms by the full-
matrix least-squares method. Hydrogen atoms were placed at the
calculated positions and were included in the structure calculation
without further refinement of the parameters. Crystal data and pro-
Preparation of [Ru(terpy)(bpyOH)(CO)](PF6)2·2H2O {[2H](PF6)2· cessing parameters are summarized in Table 3. CCDC-240927 for
2H2O}: Orange crystals of [2H](PF6)2·2H2O were obtained by crys-
tallization of [2](PF6) from CH3CN/H2O (1:1, v/v) containing 1.5
[Ru(terpy)(bpyO)(OH2)](PF6), -217817 for [1](PF6), -217818 for
[2](PF6)·H2O and -217819 for [2H](PF6)2·2H2O contain the sup-
equiv. of an aqueous HPF6 solution (1 m). 1H NMR (500 MHz, plementary crystallographic data for this paper. These data can be
CD3CN): δ ϭ 7.14 (d, 1 H, HB3), 7.27 (ddd, 1 H, HB4), 7.42Ϫ7.45
obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html
[or from the Cambridge Crystallographic Data Centre, 12 Union
(m, 3 H, HA3 and HC4), 7.80 (d, 2 H, HC3), 8.01 (td, 1 H, HB5),
8.10Ϫ8.20 (m, 4 H, HA4, HA5 and HC5), 8.38 (d, 1 H, HB6), 8.43 Road, Cambridge CB2 1EZ, UK; Fax: ϩ 44-1223-336-033; E-mail:
(d, 2 H, HC6), 8.51 (t, 1 H, HD4), 8.58 (d, 2 H, HD3) ppm. deposit@ccdc.cam.ac.uk].
Table 3. Crystallographic data for [Ru(terpy)(bpyO)(OH2)](PF6), [1](PF6), [2](PF6)·H2O and [2H](PF6)2·2H2O
[Ru(terpy)(bpyO)(OH2)](PF6)
[1](PF6)
[2](PF6)·H2O
[2H](PF6)2·2H2O
Empirical formula
Formula mass
Crystal system
Space group
Color
C25H20F6N5O2PRu
668.50
monoclinic
P21/n
red purple
10.413(1)
14.505(1)
16.601(1)
C26H18F6N5O2PRu
678.49
monoclinic
C2/c
C26H20F6N5O3PRu
696.51
orthorhombic
Pca21
orange
15.457(1)
C26H23F12N5O4P2Ru
860.50
triclinic
¯
P1
orange
orange
˚
a [A]
41.034(3)
8.5910(5)
14.349(1)
8.6983(4)
14.0966(1)
14.8504(2)
62.952(9)
78.28(1)
84.67(1)
1587.9(2)
2
˚
b [A]
16.176(1)
11.3332(8)
˚
c [A]
α [°]
β [°]
γ [°]
98.593(4)
95.404(3)
3
˚
V [A ]
2479.3(4)
4
1.791
0.30 ϫ 0.15 ϫ 0.10
5036.0(6)
8
1.790
0.20 ϫ 0.20 ϫ 0.10
2833.7(6)
4
1.623
0.25 ϫ 0.13 ϫ 0.06
Z
Dcalcd [g·cmϪ3
]
1.80
Crystal size [mm]
0.40 ϫ 0.20 ϫ 0.10
173
0.71070
0.708
0.079
0.201
T [K]
173
173
173
˚
λ [A]
0.71070
0.778
0.067
0.153
0.71070
0.768
0.048
0.108
0.71070
0.687
0.070
0.176
µ [mmϪ1
]
[a]
R1
[b]
Rw
[a]
[b]
2
2 2
2 2 1/2
R1 ϭ Σ||Fo| Ϫ |Fc||/Σ|Fo| for I Ͼ 2.0σ(I) data. Rw ϭ Σ[w(Fo Ϫ Fc ) /Σw(Fo ) ]
.
292
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Stabilization and Destabilization of the RuϪCO Bond During a Ligand-Localized Redox Reaction
FULL PAPER
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[9]
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(bpyOH)Cl]ϩ was determined unequivocally by the X-ray crys-
tal structure analysis but the quality of the crystals of the com-
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and lengths (R1 ϭ 11.8%, Rw ϭ 20.5%).
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[1g]
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J.-P. Sauvage, J.-P. Collin, J.-C. Chambron, S. Guil-
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A. Bignozzi, T. A. Heimer, F. N. Castellano, G. J. Meyer, Inorg.
[1i]
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[10]
[11]
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