Rapid transfer of hydride ion from a ruthenium complex to C1 species in water

Article abstract of DOI:10.1021/ja074158w

Solvent water (Gutmann acceptor number, AN = 55) accelerates the rate of reaction of Ru(terpy)(bpy)H⁺ with CO₂ by more than 4 orders of magnitude compared to acetonitrile (AN 18.9) (as reported in Inorg. Chim. Acta 2000, 299, 155). Water also promotes the related reductions of carbon monoxide to formaldehyde and of formaldehyde to methanol by the ruthenium(II) hydride complex. Copyright

Full text of DOI:10.1021/ja074158w

Published on Web 07/28/2007
Rapid Transfer of Hydride Ion from a Ruthenium Complex to C
1
Species in
Water
Carol Creutz* and Mei H. Chou
Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973-5000
Received June 7, 2007; E-mail: ccreutz@bnl.gov
1
,2
Water is recognized as a desirable solvent for catalysis and as
Table 1. Rate Constants and Products for the Hydride Transfer to
Acceptor A
3
a promising raw material for solar generation of fuels; however,
relatively few kinetics and mechanism studies of C
1
reduction
A
4
reactions in aqueous media have been reported. The observation
parameter
CO
2
CO
2
CH O
5
6
of Konno et al. that high solvent acceptor number (AN) enhances
kA, M- s-1 a
1
8.5 × 10
2
0.7
487
∼1 × 10
6
+
+
the rate of hydride transfer from Ru(terpy)(bpy)H (RuH , terpy
λmax, nmb
490
486
c
d
)
2,2′,6′,2′′-terpyridine; bpy ) 2,2′-bipyridine, Chart 1) to carbon
m/z product
536 (100%)
0.4 × 10
351.6 (85%)
1.4 × 10
522 (30%)
8.8 × 10
-
1
-3 e
-4 e
-4 f
kaq, s
dioxide in organic solvents has led us to characterize this reaction
in water. We find that solvent water (AN ) 55) accelerates the
a
Rate constant for hydride transfer to A (Scheme 1). b Position of lowest
CO
2
reaction rate by more than 4 orders of magnitude compared
c
energy MLCT band of hydride adduct of A. Value in parenthesis is relative
intensity at its maximum (usually first trace). d The product methanol
complex manifested as an intense peak at m/z ) 523 only when the collision
energy was reduced to 25%. e Rate of aquation at pH 5.3. Rate of aquation
in water, no buffer added.
to acetonitrile (AN ) 18.9) and that water also promotes the related
reductions of C
RuH .
1
species carbon monoxide and formaldehyde by
f
+
+
The lowest energy electronic absorption of RuH , a Ru(II)-to-
terpy charge transfer at 500 nm in water, shifts to shorter wavelength
Scheme 1
upon hydride transfer to C
reactions were followed by UV-vis spectroscopy, with both CO
and CH O requiring stopped-flow methods. All exhibited second-
order rate laws, -d[RuH ]/dt ) k
the hydride acceptor, CO (see Figure 1), CO, or CH
1
. The kinetics of the hydride-transfer
2
2
+
+
-1
A
[RuH ][A] M s where A is
7
2
2
O.
Product solutions were characterized by electrospray ionization
mass spectrometry (ESI-MS), and assignments were confirmed by
7
comparison with authentic samples prepared by other methods.
102
With CO
2
reactant, product m/z ) 536 is assigned to Ru(terpy)-
bpy)[OCH(O)] . For CO, the m/z ) 351.6 peak is assigned as
+
(
102Ru(terpy)(bpy)(OCH
(OH))[PF ](H O) (z ) 2, m ) 703). With
2+
2 6 3
1
02
+
3
formaldehyde as reactant, m/z ) 522 is Ru(terpy)(bpy)(OCH ) .
Scheme 1 summarizes the reaction sequence.
2
In contrast to previous studies of hydride transfer to free or
II
8-11
metal-bound C
1
species such as Ru (bpy)
2
(CO)(C
1
),
for each
hydride-transfer reaction studied here the initial product impli-
+
cated is the O-bonded hydride adduct: formate ion RuOCHO
+
+
(
2), formaldehyde hydrate RuOCH
2
(OH) (3), or methanol RuOCH
3
(4). This assignment is consistent with the ESI MS, UV-vis
spectrum, comparison with known samples, and the relatively
2
+
12
rapid transformation to Ru-OH
are summarized in Table 1. The importance of the Lewis acidity
of the anhydride or keto form of the C acceptor to its ability to
2
a
(λmax 477, pK 10). Results
Figure 1. The pseudo-first-order rate constant for reaction of Ru(terpy)-
1
+
13,14
(
bpy)H with CO2 at pH 5.8 as a function of CO2 concentration. Inset:
accept hydride ion is striking. For CO
2
, a pH-jump experiment
Scans taken every 500 ms with 1.5% saturated CO2 (0.45 mM, first point).
+
established that reaction of RuH with CO
2
is >50 times greater
-
than with HCO
3
. For CO, reaction with its hydrate, formate ion,
s-1, at least one-million times slower than the
-
5
-1
is <10
M
Chart 1
reaction with CO. Similarly for formaldehyde, the minor species
15,16
5
2
H CO
was at least 10 times more reactive than its dominant
hydrate form.
We bracket the hydricity17 of this Ru(II) hydride using our kinetic
-
1
2
data. The intercept of the plot of kobs versus [CO ] is 0.1 s , and
2
-1 -1
the slope is 8.5 × 10 M s . Then the rate constant for the reverse
10108
9
J. AM. CHEM. SOC. 2007, 129, 10108-10109
10.1021/ja074158w CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Acknowledgment. This research was carried out at Brookhaven
National Laboratory under contract DE-AC02-98CH10884 with the
U.S. Department of Energy and supported by its Division of
Chemical Sciences, Office of Basic Energy Sciences. We thank
M. Bullock, D. DuBois, E. Fujita, and J. Muckerman for extended
discussions that led to this work, S. Lymar and N. Sutin for helpful
comments on its interpretation, and N. Shaikh, K.-W. Huang, and
D. Polyansky for help with the experiments.
Supporting Information Available: Experimental details, kinetics
and products for reaction with CO , CO, and CH O, the hydricity of
2 2
formate ion. This material is available free of charge via the Internet
at http://pubs.acs.org.
References
0
-
19
Figure 2. Free energies of hydride-ion formation (∆G (H )) (circles),
(1) Jo o´ , F. Acc. Chem. Res. 2002, 35, 738.
q
+
(2) Fu, X. F.; Li, S.; Wayland, B. B. Inorg. Chem. 2006, 45, 9884.
(3) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103,
free energy of activation ∆G for hydride transfer from RuH to CO2
triangles), and (∆G (CO2)soln), the free-energy of solution of CO2
5
0
5
(
(
1
5729.
squares), as a function of solvent acceptor number.6
(
4) (a) Ogo, S.; Kabe, R.; Hayashi, H.; Harada, R.; Fukuzumi, S. Dalton Trans.
2
006, 4657. (b) Hayashi, H.; Ogo, S.; Fukuzumi, S. Chem. Commun. 2004,
reaction is e0.1 s-1, and KA,12 for the hydride transfer (from the
ratio of forward and reverse constants) is g10 M and ∆G e
5 kcal/mol. Since the hydricity of formate in water is 23 kcal/
2714. (c) Hayashi, H.; Ogo, S.; Abura, T.; Fukuzumi, S. J. Am. Chem.
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4
-1
0
(5) Konno, H.; Kobayashi, A.; Sakamoto, K.; Fagalde, F.; Katz, N. E.; Saitoh,
H.; Ishitani, O. Inorg. Chim. Acta 2000, 299, 155.
-
(
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mol18 (based on pK
(H
2
) ) 22 rather than the commonly used,
19
5
1
a
(7) The hydride complex [1][PF
6 H D
(DMSO-d δ ) -14.7; IR (KBr pellet) ν 1292 cm ;
1827 cm- , ν
3 -1 -1
6
] was prepared as described : H NMR
1
-1
+
earlier value 31), the hydricity of Ru(terpy)(bpy)H in water is e18
UV-vis 500 nm (8.5 × 10 M cm ); ESI MS in acetonitrile m/z 492.2
kcal/mol.
[Ru(terpy)(bpy)H+ 102Ru, relative intensity 100% in m/z range 200-1000].
As discussed previously5,20 for reaction of CO
+
The hydride complex undergoes very rapid exchange with the deuterons
2
with RuH and
H, these reactions involve hydride transfer via transi-
of D
O, CD OD, and C D OD, probably via a dihydrogen complex, and
2 3 2 5
Re(bpy)(CO)
3
reacts with acid (k < 10
3
M
-1
-1) to yield dihydrogen (yield g 60%);
s
the kinetics and mechanism of these reactions are currently under study.
Manipulations were carried out under dim light. As noted by Konno et
2
tion states (Scheme 2) for CO .
5
al., the hydride complex is not oxygen sensitive, but must be rigorously
Scheme 2
protected from the carbon dioxide in air. Thus reagents were prepared
under argon with freshly drawn mill-Q water and transferred with use of
syringe techniques. Kinetics runs were carried out with 0.03-0.1 mM
+
RuH and the C
1
species usually in at least 10-fold excess at 22 °C.
Experiments with CO used 100% and 50% CO/50% Ar-saturated solutions
in water (0.94 and 0.47 mM, respectively). Commercial CO /Ar mixtures
were used to vary the [CO ], and the reactions were monitored by diode
2
2
array on an Applied Photophysics stopped-flow spectrometer. Mass spectra
were simulated using Isotope Distribution Calculator (http://www2.
sisweb.com/mstools/isotope.htm) and monitored on a Thermo Finnigan
LCQ MS. Samples for comparison with the products of the C
1
reactions
was reacted with RuH+ in methanol
2
The reactions cannot involve Ru binding of O, followed by
were synthesized as follows: CO
-
transfer of H , since substitution reactions at the Ru(II) center are
to give the O-bonded formate complex as established by X-ray crystal
5
21
structure by Konno et al. The formaldehyde adduct was prepared through
many orders of magnitude too slow (for binding of acetonitrile,
2
+
2
reaction of Ru(terpy)(bpy)(H O) with formaldehyde at pH 9 (borate
-
4
-1 -1 18
k ) 0.3 × 10
On the basis of the work presented, it is evident that water is an
excellent solvent for the hydride transfer to the free C acceptors.
M
s
)
to account for the observed rates.
buffer); this reaction is relatively rapid, evidently involving Ru-OH attack
on CH O (substitution on carbon, not ruthenium). The methanol complex
was prepared by dissolving Ru(terpy)(bpy)(H O)[PF in methanol for
2
2
6 2
]
1
an hour, followed by evaporation to dryness. The digitized absorbance-
time data from kinetics runs were least-squares-fit to an exponential
function using Origin.
It is of great interest to understand the basis of this reactivity
enhancement, which is much greater than expected from dielectric
(8) Tanaka, K.; Ooyama, D. Coord. Chem. ReV. 2002, 226, 211.
continuum considerations.5
,20
(9) Gibson, D. H.; Sleadd, B. A.; Yin, X.; Vij, A. Organometallics 1998, 17,
We compare the effect of solvent
2689.
acceptor number on thermodynamic and kinetic parameters in
Figure 2.
(
10) Toyohara, K.; Nagao, H.; Mizukawa, T.; Tanaka, K. Inorg. Chem. 1995,
34, 5399.
(
(
11) Nagao, H.; Mizulawa, T.; Tanaka, K. Inorg. Chem. 1994, 33, 3415.
12) Davies, N. R.; Mullins, T. L. Aust. J. Chem. 1967, 20, 657.
Acceptor number reflects the electrophilic properties of the
solvent; with increasing AN, the negative charge on the hydride
ligand is increasingly stabilized. The trend observed here for
(13) Lymar, S. V.; Hurst, J. K. J. Am. Chem. Soc. 1995, 117, 8867.
14) Kern, D. M. J. Chem. Ed. 1960, 37, 14.
(
(
15) Walker, J. F. Formaldehyde, 3rd ed.; R. E. Krieger: Huntington, NY,
1964.
0
-
6
∆
2
G (H ) has also been found for chloride ion. The plot in Figure
(
(
16) Greenzaid, P.; Luz, Z.; Samuel, D. Trans. Faraday Soc. 1968, 64, 2780.
17) Curtis, C. J.; Miedaner, A.; Raebiger, J. W.; DuBois, D. L. Organometallics
2004, 23, 511.
strongly suggests that the thermodynamics of formation of the
hydride ion is responsible for the exceptional solvent sensitivity of
the hydride-transfer rate to solvent acceptor number and encourages
us to explore the scope of this reactivity enhancement in future
experiments with other metal-hydride donors and both metal-bound
and free hydride acceptors.
(
(
18) Creutz, C.; Chou, M. H. unpublished work.
19) Kelly, C. A.; Rosseinsky, D. R. Phys. Chem. Chem. Phys. 2001, 3, 2086.
(20) Sullivan, B. P.; Meyer, T. J. Organometallics 1986, 5, 1500.
21) Davies, N. R.; Mullins, T. L. Aust. J. Chem. 1968, 21, 915.
(
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J. AM. CHEM. SOC.
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VOL. 129, NO. 33, 2007 10109