Ruthenocene-containing β-diketones: Synthesis, pKa′ values, keto-enol isomerization kinetics, and electrochemical aspects

Article abstract of DOI:10.1021/om700609z

Claisen condensation of acetyl ruthenocene with the appropriate methyl ester, RCOOMe, under the influence of the hindered base lithium diisopropylamide gave the β-diketones (RcCOCH₂COR) 1-ruthenocenyl-4,4,4- trifluorobutane-1,3-dione (ruthenocenoyltrifluoroacetone, 1, R = CF₃; pK_a′ = 7.36 ± 0.03), 1-ruthenocenylbutane-1,3-dione (ruthenocenoylacetone, 2, R = CH₃; pK_a′ = 10.22 ± 0.01), 1-phenyl-3-ruthenocenylpropane-1,3-dione (benzoylruthenocenoylmethane, 3, R = C₆H₅; pK _a′ = 11.31 ± 0.03), 1-ferrocenyl-3-ruthenocenylpropane- 1,3-dione (ferrocenoylruthenocenoylmethane, 4, R = Fc = (C₅H ₅)Fe(C₅H₄) = ferrocenyl; pK_a′ = ca. 12.8), and 1,3-diruthenocenylpropane-1,3-dione (diruthenocenoylmethane, 5, R = Rc = (C₅H₅)Ru(C₅H₄) = ruthenocenyl; pK_a′ = ca. 12.7). The group electronegativity of the ruthenocenyl group, Χ_Rc = 1.89 on the Gordy scale, was obtained from the linear relationship between IR carbonyl stretching frequencies of a series of methyl esters, RCOOMe, and Χ_R. A ¹H NMR kinetic study of the enol-keto interconversion resulted in accurate equilibrium constants, K_c, for this equilibrium, as well as forward and reverse rate constants of the isomerization process. Cyclic voltammetry in CH₃CN/N(ⁿBu)₄PF₆ utilizing a glassy-carbon electrode showed the ruthenocene center exhibited, in contrast to the ferrocene center, irreversible electrochemistry. The multiple peak anodic (oxidation) potentials observed are a consequence of slow isomerization kinetics and made peak assignments for the keto and enol isomers possible. The kinetics of enol to keto isomerization for 4 was also studied by cyclic and Osteryoung square wave voltammetry; obtained rate constants were mutually consistent with those obtained by the ¹H NMR technique. Kinetic rate constants, K_c, and pK_a′ and E_p,a were related to Χ_R values of the R groups in RCCOCH₂COR.

Full text of DOI:10.1021/om700609z

Organometallics 2008, 27, 353–362
353
Ruthenocene-Containing ꢀ-Diketones: Synthesis, pKa′ Values,
Keto–Enol Isomerization Kinetics, and Electrochemical Aspects
K. Christian Kemp, Eleanor Fourie, Jeanet Conradie, and Jannie C. Swarts*
Department of Chemistry, UniVersity of the Free State, P.O. Box 339, Bloemfontein 9300,
Republic of South Africa
ReceiVed June 20, 2007
Claisen condensation of acetyl ruthenocene with the appropriate methyl ester, RCOOMe, under the
influence of the hindered base lithium diisopropylamide gave the ꢀ-diketones (RcCOCH2COR)
1-ruthenocenyl-4,4,4-trifluorobutane-1,3-dione (ruthenocenoyltrifluoroacetone, 1, R ) CF3; pKa′ ) 7.36
( 0.03), 1-ruthenocenylbutane-1,3-dione (ruthenocenoylacetone, 2, R ) CH3; pK ′ ) 10.22 ( 0.01),
a
1-phenyl-3-ruthenocenylpropane-1,3-dione (benzoylruthenocenoylmethane, 3, R ) C6H5; pKa′ ) 11.31
( 0.03), 1-ferrocenyl-3-ruthenocenylpropane-1,3-dione (ferrocenoylruthenocenoylmethane, 4, R ) Fc
) (C5H5)Fe(C5H4) ) ferrocenyl; pKa′ ) ca. 12.8), and 1,3-diruthenocenylpropane-1,3-dione (dirutheno-
cenoylmethane, 5, R ) Rc ) (C5H5)Ru(C5H4) ) ruthenocenyl; pKa′ ) ca. 12.7). The group
electronegativity of the ruthenocenyl group, ꢁRc ) 1.89 on the Gordy scale, was obtained from the linear
relationship between IR carbonyl stretching frequencies of a series of methyl esters, RCOOMe, and ꢁR.
1
A H NMR kinetic study of the enol-keto interconversion resulted in accurate equilibrium constants,
Kc, for this equilibrium, as well as forward and reverse rate constants of the isomerization process. Cyclic
voltammetry in CH3CN/N(nBu)4PF6 utilizing a glassy-carbon electrode showed the ruthenocene center
exhibited, in contrast to the ferrocene center, irreversible electrochemistry. The multiple peak anodic
(oxidation) potentials observed are a consequence of slow isomerization kinetics and made peak
assignments for the keto and enol isomers possible. The kinetics of enol to keto isomerization for 4 was
also studied by cyclic and Osteryoung square wave voltammetry; obtained rate constants were mutually
1
consistent with those obtained by the H NMR technique. Kinetic rate constants, Kc, and pKa′ and Ep,a
were related to ꢁR values of the R groups in RcCOCH2COR.
to the fact that ruthenium compounds prevent, or at least
minimize, cytotoxicity induced by other chemotherapeutic
drugs,9,10 we were interested in synthesizing and characterizing
some hitherto unknown ruthenocene-containing ꢀ-diketones.
These complexes may show appreciable antineoplastic activity.
Methods used in ꢀ-diketone synthesis are wide and varied,11,12
but Cullen’s13 use of lithium diisopropylamide as a base during
the Claisen condensation of ketones with esters has been used
with considerable success for ferrocene-containing ꢀ-dike-
tones.14 ꢀ-Diketone yields were substantially better than those
obtained when condensations were performed under the influ-
ence of NaNH2, KNH2, or NaOMe.15
Introduction
ꢀ-Diketones and their metal complexes have been the subject
of many kinetic,1 catalytic,2 and medical studies.3,4 Ru-
thenocene-containing compounds have been investigated as
imaging agents,5 for photoelectrochemical uses,6 and as sur-
factants,7 as well as for other beneficial uses.8 Ruthenocene
organic chemistry parallels that of ferrocene. However, the
cyclopentadienyl (Cp ) C5H5-) ligands of ruthenocene are
much more resistant toward aromatic substitution reactions than
are those of ferrocene. Consequently, the organic chemistry of
ruthenocene has not been reported on as comprehensively. Due
Regarding the equilibrium between ꢀ-diketone keto and enol
isomers, two driving forces, the electronic driving force and
the resonance driving force, were described14,16,17 that will
determine the dominant enol isomer in solution. On the basis
* To whom correspondence should be addressed. E-mail:
swartsjc.sci@ufs.ac.za.
(1) Moon, S.; Kwan, Y. Magn. Reson. Chem. 2001, 39, 89.
(2) Cullen, W. R.; Rettig, S. J.; Wickenheimer, E. B. J. Mol. Catal.
1991, 66, 251.
(3) Bennet, I.; Broom, N. J. P.; Cassels, R.; Elder, J. S.; Masson, N. D.;
O’Hanlon, P. J. Bioorg. Med. Chem. Lett. 1999, 9, 1847.
(4) (a) Mehrotora, R. C.; Bohra, R.; Gaur, D. P. In Metal ꢀ-Diketonates
and Allied DeriVatiVes; Academic Press: London, 1978; p 268. (b) Stray,
J. In The SolVent Extraction of Metal Chelates; Macmillan: New York,
1964; pp 51–55. (c) Marcus, Y.; Keates, A. S. In Ion Exchange and SolVent
Extraction of Metal Complexes; Wiley-Interscience: New York, 1969; pp.
499–521.
(9) Tsuruo, T.; Iida, H.; Tsukagoshi, S.; Sakuari, Y. Biochem. Pharm.
1981, 30, 213.
(10) Conter, V.; Beck, W. T. Biochem. Pharm. 1983, 32, 723.
(11) March, J. In AdVanced Organic Chemistry, 4th ed.; Wiley:New
York, 1992; p 1072.
(12) Umetani, S.; Kawase, Y. H.; Le, Q. T.; Matusi, M. Dalton Trans.
2000, 2787.
(5) (a) Herken, R.; Wenzel, M. Eur. J. Nucl. Med. 1985, 10, 56. (b)
Shani, J.; Livshitz, T.; Wenzel, M. Int. J. Nucl. Med. Bio. 1985, 12, 13.
(6) Blake, A. J.; Mayers, F. R.; Osborne, A. G.; Rosseinsky, D. R.
J. Chem. Soc., Dalton Trans. 1982, 2379.
(13) Cullen, W. R.; Wickenheiser, E. B. J. Organomet. Chem. 1989,
370, 141.
(14) Du Plessis, W. C.; Vosloo, T. G.; Swarts, J. C. J. Chem. Soc., Dalton
Trans. 1998, 2507.
(7) Jacob, C.; Safronov, A. Y.; Wilson, S.; Hill, H. A. O.; Booth, T. F.
J. Electroanal. Chem. 1997, 427, 161.
(15) (a) Cain, C. E.; Mashburn, T. A., Jr.; Hauser, C. R J. Org. Chem.
1961, 26, 1030. (b) Weinmayr, V. Naturwissenchaften 1958, 45, 311.
(16) Du Plessis, W. C.; Erasmus, J. J. C.; Lamprecht, G. J.; Conradie,
J.; Cameron, T. S.; Aquino, M. A. S. Swarts, J. C. Can. J. Chem. 1999, 77,
378.
(8) (a) Beagley, P.; Blackie, M. A. L.; Chibale, K.; Clarkson, C.; Moss,
J. R.; Smith, P. J. Dalton Trans. 2002, 4426. (b) Miura, H.; Taguchi, H.;
Sugiyama, K.; Matsuda, T.; Gonzalez, R. D. J. Catal. 1990, 124, 194.
10.1021/om700609z CCC: $40.75
2008 American Chemical Society
Publication on Web 01/17/2008

354 Organometallics, Vol. 27, No. 3, 2008
Kemp et al.
Scheme 1. Synthesis of ꢀ-Diketones 1–5a
a
Only the expected dominant enol isomer is shown. Fc ) ferrocenyl, Rc ) ruthenocenyl, LiDPA ) lithium diisopropylamide.
2
1
of the electronic driving force, if ꢁR > ꢁR (ꢁR ) group
electronegativity of R groups), the enol isomer R1C(OH)d
CHCOR2 in the equilibrium below should dominate because
the C atom adjacent to R1 will be more negative in character
than the C atom next to R2. On the basis of the resonance driving
force, if R1 is an aromatic substituent, such as ferrocenyl, phenyl,
or ruthenocenyl, and R2 is not, then R1COCHdC(OH)R2 should
be the dominant enol isomer even though R1 may be more
electron-donating than R2. Resonance stabilization may be
visualized in terms of the canonical structural fragments
R1(CdO)- T (R1,+)d(CO-)- that may be written for the
keto portion of the enol isomer R1COCHdC(OH)R2.
The rate of conversion between enol isomers is normally very
fast, with half-lives in the microsecond range18 not uncommon.
In contrast, conversion between enol and keto isomers may take
place over days.17
Figure 1. Plot of IR carbonyl stretching frequencies versus group
electronegativities, ꢁR, on the Gordy scale for a range of methyl
esters, RCOOMe. The R groups are indicated on the graph.
found to form at slow but different rates. Once the dynamic
equilibrium position has been reached in 1–4 days, no further
1H NMR spectral changes could be observed. In analogy to
analogous ferrocene ꢀ-diketones,16,21 we assume that the solid-
state enol isomer in the present compound series will be
RcCOCHdC(OH)R as a result of the resonance driving force
involving molecular fragments such as Rc+d(CO-)-. In
solution, the same isomer is expected to dominate.
Group Electronegativities, ꢁR. We have already reported
ꢁR values on the Gordy scale22 for each R group except the
ruthenocenyl group of the present study.14,16 Gordy scale ꢁR
values are empirical numbers that express the combined
tendency of not only one atom but also a group of atoms, such
as CF3 in the present ꢀ-diketones, to attract electrons (including
those in a covalent bond) as a function of the number of valence
electrons, n, and the covalent radius, r (in Å), of groups as
discussed elsewhere.22 They are empirically adjusted to be
aligned with Pauling atomic electronegativities. To determine
the apparent ꢁRc value, the IR carbonyl stretching frequency of
RcCOOMe was inserted in the linear plot of carbonyl stretching
frequencies of a series of other methyl esters, RCOOMe, and
ꢁR (Figure 1). ꢁRc was then calculated from the manipulated
Traditionally the view has been that electrochemical oxidation
of ruthenocene proceeds by a 2e irreversible process19,20 in
acetonitrile/tetrabutylammonium perchlorate. It was later shown
that a 1e reversible electrochemical process occurs when the
electrochemistry is performed utilizing a weakly nucleophilic
electrolyte such as tetrabutylammonium tetrakis[3,5-bis(trifluo-
romethyl)phenyl]borate in the poorly coordinating solvent
dichloromethane.20 Under these conditions, a formal reduction
potential of 1.03 V versus AgCl/Ag (1.0 M KCl) (0.57 V versus
Fc/Fc+) was obtained.
Here we report the syntheses and characterization of some
new ruthenocene-containing ꢀ-diketones. Keto–enol equilibrium
constants, the kinetics of isomerization, and the electrochemical
properties of each ligand are also presented.
Results and Discussion
Syntheses. The ꢀ-diketones RcCOCH2COR with R ) CF3
(1), CH3 (2), C6H5 (3), Fc (ferrocenyl, FeCp(C5H4); 4), Rc
(ruthenocenyl, RuCp(C5H4); 5) were synthesized according to
Scheme 1. Compound 1 was obtained in the highest yield. Due
to increased acid and base instability in moving from 1 to 5,
yields systematically became lower for the other complexes. In
the solid state, all ꢀ-diketones were found to convert quantita-
tively to the enol isomer upon aging for 3 months or longer.
However, once they were dissolved in CDCl3, keto isomers were
equation of the graph, ꢁR ) 0.0133[ν(CdO)] – 20.78 as ꢁRc
)
1.92 on the Gordy scale. Electrochemical measurements (see
below) gave ꢁRc ) 1.86. These two values are mutually
consistent and average to 1.89. Literature values of 2.27 and
2.08 were reported for the ruthenocenyl and ferrocenyl groups,
respectively, but it is uncertain what scale and/or conditions
were used.23
(21) Bell, W.; Crayston, J. A.; Glidewell, C.; Mazid, M. A.; Hursthouse,
M. B. J. Organomet. Chem. 1992, 434, 115.
(17) Du Plessis, W. C.; Davis, W. L.; Cronje, S. J.; Swarts, J. C. Inorg.
Chim. Acta 2001, 314, 97.
(22) (a) Wells, P. R. In Progress in Physical Organic Chemistry; Wiley:
New York, 1968; Vol. 6, pp 111–145. (b) Kagarise, R. E. J. Am. Chem.
Soc. 1955, 77, 1377.
(18) Iglisias, E. J. Org. Chem. 2003, 68, 2680.
(19) Kuwana, T.; Bublitz, D. E.; Hoh, G. J. Am. Chem. Soc. 1960, 82,
5811.
(23) Bennett, M. A.; Bruce, M. I. In ComprehensiVe Organometallic
Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E., Eds.; Pergamon Press:
Oxford, U.K., 1982; Vol. 4, pp 759–773.
(20) Hill, M. G.; Lamanna, W. M.; Mann, K. R. Inorg. Chem. 1991,
30, 4687.

Ruthenocene-Containing ꢀ-Diketones
Organometallics, Vol. 27, No. 3, 2008 355
Table 1. Group Electronegativities, Equilibrium Constants, Gibbs Energies (kJ mol-1), Kinetic Rate Constants (s-1), and Half-Lives (s) for
Equilibrium (2) at 293 K
a
b
c
compd (solvent, technique used)
ꢁR
Kc
∆G°
% enol
t1/2
105kobsd
105k1
106k-1
1, R ) CF3 (CDCl3, 1H NMR)
3.01
2.34
2.21d
1.87
1.87
1.87
1.87
1.87
1.87
1.87
1.87
1.89
12.3
2.2
5.7
0.9
-6.1
-1.9
-4.2
0.3
92.5
68.5
85.0
47.0
50.1
50.2
42.5
54.0
58
31900
700
11000
650
470
540
7600
6930
220
240
220
2.17(5)
98(2)
6.3(5)
2.01
68.3
5.36
50.7
73.6
64.3
3.9
5.4
180
174
162
1.6
297
9.4
563
734
637
52
46
1300
1160
1480
1880
2, R ) CH3 (CDCl3, 1H NMR)
3, R ) Ph (CDCl3, 1H NMR, ꢁPh ) 2.43d)
4, R ) Fc (CDCl3, 1H NMR)
107(3)
147(9)
128(8)
9.1(5)
10.0(6)
310(5)
290(3)
310(2)
320(10)
4, R ) Fc (CD2Cl2, 1H NMR)
1.00
0.0
4, R ) Fc (CD2Cl2 + electrolyte,e 1H NMR)
4, R ) Fc (CD3CN, 1H NMR)
1.01
0.74
1.17
1.38
1.50
1.09
0.70
-0.02
-0.7
-0.4
-0.8
-1.0
-0.2
0.9
4, R ) Fc (CH3CN + electrolyte,eCV-Rcwaves
)
)
)
)
4, R ) Fc (CH2Cl2 + electrolyte,e CV-Rcwaves
4, R ) Fc (CH2Cl2 + electrolyte,e CV-Fcwaves
4, R ) Fc (CH2Cl2 + electrolyte,e SW-Fcwaves
5, R ) Rc (CDCl3, 1H NMR)
60.0
52.2
41.2
220
132
a To determine Kc, the following NMR signals (ppm) were averaged for use in eq 5: for 1, those of COCHCO at 6.03 and COCH2CO at 3.00, and
those of C5H5 at 4.63 and 4.70; for 2, those of COCHCO at 5.64 and COCH2CO at 3.73, those of C5H5 at 4.59 and 4.61, and those of CH3 at 2.31 and
2.05; for 3, those of COCHCO at 6.28 and COCH2CO at 4.25, and those of C5H5 at 4.59 and 4.48; for 4, those of COCHCO at 5.91 and COCH2CO at
2.28, and those of C5H2 (from the substituted Cp ring) at 5.19 and 5.24; for 5, those of COCHCO at 5.83 and COCH2CO at 3.88, and those of C5H2
(from the substituted Cp ring) at 5.17 and 5.23. b Calculated from t1/2 ) ln 2/kobs c Values in parentheses indicate errors in the last digit(s). Not all
.
errors are known, because the error in Kc is unknown. d The value of ꢁPh ) 2.21 does not appear to fit the kinetic trends of this study well. It looks as if
ꢁPh should be larger than ꢁCH3 ) 2.34. A literature value of ꢁPh ) 2.43 was reported utilizing IR data for OdPR3,14,22 but as all other ꢁR values
originate from RCOOMe esters, we are content with the value of 2.21. e Abbreviations and conditions: electrolyte ) [N(nBu4)][B(C6F5)4] ) 5[4] ) 8.75
× 10-2 M for 1H NMR, 0.1 M in CH2Cl2 for CV or SW techniques, or “electrolyte” ) [N(nBu4)][PF6] ) 0.1 M in CH3CN for the CV technique,; CV
) cyclic voltammetric technique, SW ) Osteryoung square wave technique.
Equilibrium Constants, Kc. To determine Kc, the equilibrium
percentage of keto isomers present in CDCl3 solutions for each
ꢀ-diketone was first determined by comparing the relative
intensities of appropriate enol/keto 1H NMR signal pairs. Once
the percentage of the keto isomer was known, the equilibrium
constant
k1
[enol isomer(s)]
[keto isomer]
Kc )
)
(1)
k-1
for the equilibrium (2) in CDCl3 solutions could be evaluated.
RcCOCH2COR yk1z RcCOCHdC(OH)R
(2)
k-1
The results are summarized in Table 1. A general trend that
larger ꢁR values lead to a slight increase in Kc values was noted,
but no clear linear relationship exists between these two
quantities.
Gibbs energies for the isomerization reaction (2) was obtained
by application of eq 3.17
∆G° ) -RT ln Kc
(3)
It is obvious from Table 1 that more negative Gibbs energies
lead to a larger enol content at equilibrium. This is thermody-
namically sound because larger negative Gibbs energies imply
the thermodynamic driving force to the right of equilibrium (2)
increases.
1
Isomerization Kinetics Utilizing H NMR Spectroscopy.
Since it was found that, at equilibrium, 2-5 have measurable
quantities (>15%) of the keto isomer in CDCl3 solutions, a first-
order treatment of time-based 1H NMR data showing the
formation of keto isomers from aged samples was possible. 1H
NMR spectra of 3 in the enol form and 3 at equilibrium with
ca. 15% in the keto form are shown in Figure 2. Kinetic data
were obtained by monitoring the growth of the keto peaks,
labeled as “k”. Compound 1, however, contained at equilibrium
only 7.5% keto isomer (Table 1). This was too little to follow
time-based enol to keto conversions with accuracy by 1H NMR.
Consequently, a keto-enriched sample was studied by following
the forward reaction (i.e., keto isomer disappearance) in
equilibrium (2). This is shown in Figure 3. The insert confirms
1
Figure 2. H NMR spectra of 3 in the enol form (above) and at
equilibrium (below) at 293 K in CDCl3. The equilibrium keto
content is 14–15%, and keto signals are designated by “k”. Peak
assignments can be found in the Experimental Section.
the first-order behavior of this isomerization process, and the
slope of this graph corresponds to kobsd ) k1 + k-1
.

356 Organometallics, Vol. 27, No. 3, 2008
Kemp et al.
Figure 4. UV/vis spectra of 2 (- - -, at pH 3.22) and 2- (s, at pH
12.67) at 25 °C. I ) 0.100 mol dm-3 NaClO4 in water containing
10% CH3CN.
Figure 3. 1H NMR determined time trace showing the conversion
from keto to enol isomer for 1 at 293 K in CDCl3. Insert: a kinetic
first-order treatment of data for this process utilizing eq 8. C donates
the concentration, expressed as percent keto isomer.
Since the equilibrium constant for eq 2 is known, it was
possible to separate rate constants for the forward k1 step and
reverse k-1 step. Rate constants are summarized in Table 1.
The isomerization kinetics activity series in CDCl3 of the present
ꢀ-diketones is as follows (compare half-lives, Table 1): (fastest)
5 (R ) Rc) > 4 (R ) Fc) > 2 (R ) CH3) >> 3 (R ) Ph) >
1 (R ) CF3) (slowest).
Group electronegativities played an important role in isomer-
ization kinetics. Both k1 and k-1 increase with decreasing ꢁR
(Table 1). The reverse reaction rate in equilibrium (2), however,
increases 5 times (for 2) to 18 times (for 5) faster than the
forward reaction rate, which also explains from a kinetic point
of view why Kc ) k1/k-1 decreases with decreasing ꢁR.
Although a detailed 1H NMR solvent study for all compounds
was not performed, the effect of CDCl3 (dielectric constant 4.8),
CD2Cl2 (dielectric constant 8.9), and CD3CN (dielectric constant
37.5) on the equilibrium position and isomerization kinetics of
4 were investigated. The effect was very small. The general
trend that the CD3CN result suggests is that an increase in
dielectric constant slightly favors the keto form of
RcCOCH2COFc (i.e., leads to smaller Kc values, Table 1).
Addition of a noncoordinating electrolyte such as tetrabutylam-
monium tetrakis(pentafluorophenyl)borate had virtually no influ-
ence on the equilibrium position of RcCOCH2COFc. This result
is important for the electrochemistry described below, since it
shows that the presence of this electrolyte will not interfere with
the isomerization process when it is studied electrochemically.
In terms of how fast the equilibrium sets in, reaction half-life
values (t1/2 ) ln 2/kobsd) in Table 1 indicate the overall
isomerization process is the slowest in CD3CN (t1/2 ) 7600 s),
but the difference in t1/2 for CDCl3 (650 s) and CD2Cl2 (470 s)
is not regarded as significant.
Figure 5. Relative absorbance dependence on the pH of 1 (labeled
CF3; experimental solution concentration and wavelength were 0.11
mmol dm-3 and 315 nm, respectively), 2 (CH3; 0.15 mmol dm-3
,
319 nm) and 3 (phenyl; 0.13 mmol dm-3, 365 nm) at 25 °C. I )
0.100 mol dm-3 NaClO4 in water containing 10% CH3CN.
Table 2. pKa′ Values Determined at λexp/nm in Water Containing
10% CH3CN, as Well as Molar Extinction Coefficients, E, in Units of
dm3mol-1cm-1 at λmax/nm at 25 °Ca
b
c
compd
pKa′
λexp
ꢀ (λmax
)
λmax
1, ꢁ ) 3.01
7.36(3)
10.22(1)
11.31(4)
12.9d
315
319
365
327
365
5960 (260)
16890 (258)
13610 (333)
5840 (259)
5500 (430)
315
318
347
331
339
CF3
2, ꢁCH ) 2.34
3
3, ꢁPh ) 2.21
4, ꢁFc ) 1.87
5, ꢁRc ) 1.89
12.7e
a I ) 0.100 mol dm-3 NaClO4. b Indicated values are for the free
c
ꢀ-diketone. λmax of the ꢀ-diketonato anion. d An average estimated
value utilizing group electronegativities and cyclic voltammetric data
because 4 and 5 were too acid- and base-sensitive to determine pKa′
spectrophotometrically. e An estimate based on group electronegativities.
Apparent Acid Dissociation Constants. The apparent pKa′
values for ꢀ-diketones 1-3 were obtained from a spectroscopi-
cally monitored acid–base titration. The term “apparent” is used
in this study because no attempt was made to partition the
experimentally obtained pK a′ values between the separate pKa
values of the enol and keto isomers. A solvent system of water
containing 10% acetonitrile was used because, although the
ꢀ-diketonates 1--5- do dissolve in water at high pH, the
ꢀ-diketones 1–5 precipitate as they form upon titration with acid.
Acetonitrile was chosen as cosolvent because it has a small
influence on pKa′ determinations,14 as demonstrated by pKa′
values of a model compound, acetylacetone, in water (pKa′ )
8.83) and water containing 10% acetonitrile (pKa′ ) 8.93).
Figure 4 shows the UV/vis spectra of 2 and 2-. From such
spectra, a suitable wavelength was identified to follow each
titration. The absorbance dependence on pH for compounds 1-3
can be seen in Figure 5. The new pKa′ values obtained from
these S curves are summarized in Table 2. For 1 it was noted
(Figure 5) that, at around the midpoint of the titration, there
was an unidentified process occurring.
However, this did not noticeably affect the pKa′ value as the
S curve retained its shape over the 2 unit pH range wherein a
pKa′ value is determined. It is possible that this unknown process
could be reversible hydroxylation, which has been previously

Ruthenocene-Containing ꢀ-Diketones
Organometallics, Vol. 27, No. 3, 2008 357
Figure 6. CVs of 2.0 mmol dm-3 solutions of RcCOCH2COR in
CH3CN/0.1 mol dm-3 N(nBu)4PF6 utilizing a glassy-carbon elec-
trode at a scan rate of 250 mV s-1: (a) 1, R ) CF3; (b) 2, R )
CH3; (c) 3, R ) C6H5; (d) 4, R ) Fc; (e) 5, R ) Rc.
Figure 7. (Top) CVs of a 2.0 mmol dm-3 enol solution and of a
near-equilibrium solution (bold line) of 4 in CH3CN/0.1 mol dm-3
N(nBu)4PF6 at 20 °C utilizing a glassy-carbon electrode and scan
rate of 100 mV s-1 recorded at the indicated times after dissolving
the sample. (Bottom) CVs of an equilibrium CH3CN solution of 4
at scan rates of 50 (smallest current), 100, 150, 200, and 250 mV
noted in other trifluoro-containing ꢀ-diketones.24,14 ꢀ-Diketone
stability in basic media are as follows: (most stable) 1 (R )
CF3) >> 2 (R ) CH3) > 3 (R ) Ph) >> 4 (R ) Fc) ≈ 5 (R
) Rc) (least stable).
No spectroscopically determined pKa′ values could be found
for 4 and 5, because they decomposed too quickly at high pH
for meaningful spectroscopic measurements. In general, though,
ꢀ-diketone acid strength decreases as ꢁR decreases. From ref
16, a linear dependence between pKa′ and ꢁR values described
by the equation pKa′ ) -6.4542ꢁR + 25 for the analogous
ferrocene series FcCOCH2COR, was observed. From this
equation, for 4 where R ) Rc, ꢁRc ) 1.89 equates with a pKa′
value of 12.8.
s-1
.
Table 3. Peak Anodic Potentials vs Fc/Fc+ and Peak Anodic
Currents for ꢀ-Diketones 1–5 in CH3CN/0.1 mol dm-3 N(nBu)4PF6,
as Well as Data for 4 in CH2Cl2
compd (% enola)
Ep,a(1)/V
ip,a(1)/µA
Ep,a(2)/V
ip,a(2)/µA
1, ꢁCF ) 3.01 (92.5) 0.609
51.3
23.2
20.3
c
20.0
c
c
9.0
c
3
2, ꢁCH ) 2.34 (68.5) 0.473
0.584
0.605
27.0
9.2
c
25.8
c
3
3, ꢁPh ) 2.21 (85.0) 0.451
4, ꢁFc ) 1.87 (42.5) 0.250b (Fc1)
0.281b (Fc2)
0.636d (Rc4)
0.314e (Fc2)
0.883f (Rc4)
0.403h (Rc2)
0.480 (Rc4)
0.484d (Rc3)
0.265e (Fc1)
+
ꢁFc ) 2.82
in CH2Cl2,e (50.1)
dimer: Ep,c ) 0.515g 0.802f (Rc3)
To estimate the pKa′ value for 5, another approach utilizing
the sum of ꢁR values for pendant R groups on ꢀ-diketones of
the type R1COCH2COR2 was used. Utilizing once again the
analogous ferrocene series FcCOCH2COR of ref 16 as well as
5, ꢁRc ) 1.89
0.213 (Rc1)
0.403h (Rc3)
c
20.8
a By 1H NMR in CDCl3 at equilibrium. b Due to poor peak
resolution, Ep,a(1) could not be measured; Ep,c ) 0.176 V. Upon
assuming ∆Ep ) 74 mV, as was found for free ferrocene under our
conditions, Ep,a could be estimated, and formal reduction potential
estimates are E°′1 ≈ 0.213 and E°′2 ≈ 0.244 V vs Fc/Fc+. c Poor peak
resolution disallowed peak current measurement. d Ep,a(3), the
ruthenocenyl oxidation potential of the enol form, and Ep,a(4), that of the
keto form, of 4 after the ferrocenyl group has already been oxidized to
ferrocenium. e In CH2Cl2/0.1 M [N(nBu)4][B(C6F5)4]. E°′1(enol) and
E°′2(keto) are estimated at 0.210 and 0.258 V respectively (see the
Supporting Information). ∆E values in CH2Cl2 are slightly larger than in
2
4, the relationship between pKa and (ꢁR1 + ꢁR ) was determined
1
2
as pKa′ ) -6.4954(ꢁR + ꢁR ) + 37.26. Since ꢁRc ) 1.89, an
estimate of the pKa′ value of 5 is 12.7. This value is exactly the
same as that obtained by inserting ꢁRc ) 1.89 into the linear
trend line pKa′ ) -4.8629ꢁR + 21.91, describing the relation-
ship between pKa′ and ꢁR values for the present ruthenocene
series, RcCOCH2COR. This result gives ample proof that, for
the present class of ꢀ-diketones, it is justified to estimate pKa′
CH3CN. f Ruthenocenyl oxidation potentials in CH2Cl2/0.1
M
1
2
values utilizing ꢁR + ꢁR values if there are no common pendant
ꢀ-diketone R substituents or ꢁR relationships if there is a
common R substituent on the ꢀ-diketone core, R1COCH2COR2.
Electrochemistry. Irreversible electrochemistry was found
for the ruthenocenyl centers of 1–5 in acetonitrile at a glassy-
carbon working electrode in the presence of 0.1 mol dm-3
N(nBu)4PF6 supporting electrolyte. Only peak anodic potentials,
Ep,a, vs Fc/Fc+ in the potential range 0.38 V < Ep,a < 0.67 V
were observed in their cyclic voltammograms (CVs) (Figures
6 and 7). In contrast, the ferrocenyl center of 4 showed reversible
electrochemistry with estimated enol and keto formal reduction
potentials E°′ ) ½(Ep,a + Ep,c) ) 0.213 and 0.244 V vs Fc/
Fc+, respectively, upon assuming ∆Ep ) Ep,a – Ep,c ) 74 mV,
[N(nBu)4][B(C6F5)4] of the enol and keto forms of 4. g In CH2Cl2, a
weak reduction potential was observed which is consistent with
dimerization of a RuIII species, in analogy with what was found for free
ruthenocene. h The provided Ep value is the observed peak potential for
the overlaid (combined) peaks 2 and 3 of 5.
the same as that for free ferrocene, at a scan rate of 50 mV s-1
(Table 3, Figures 6 and 7). In comparison, we found that for
ferrocene itself ∆Ep ) 74 mV, ip,c/ip,a ) 0.98, and E°′ ) 0.087
V vs Ag/Ag+ under similar conditions. Relevant CV parameters
are summarized in Table 3.
While 1 showed only one observable anodic peak, multiple
ruthenocenyl oxidation peaks are observed for 2–5 (Table 3 and
Figures 6 and 7). This is easily explained as follows: for 1, the
equilibrium keto content is only 7.5% (Table 1). This small
amount, which forms at a kinetic rate having a half-life of more
(24) (a) Aygen, S.; van Eldik, R. Chem. Ber. 1989, 122, 315. (b) Ellinger,
M.; Duschner, H.; Starke, K J. Inorg. Nucl. Chem. 1978, 40, 1063.

358 Organometallics, Vol. 27, No. 3, 2008
Kemp et al.
than 8 h from freshly prepared enol-enriched solutions, will be
very difficult to detect. The large ip,a(1) value reported in Table
3 for 1, compared with those of 2–5, is therefore indicative of
a large enol content at the time of measurement. In contrast,
the peak pairs for 2-5 are consistent with the existence of keto
and enol isomers in solution at the time of measurement. The
equilibrium keto content of 2-5 varies between 15 and 53%
(Table 1). From the CV of 3 (Figure 6) it was concluded that
the smaller CV peak, peak 2, belongs to the keto isomer because
the keto content at equilibrium for 3 is much less than the enol
content. By analogy, peak 2 for 2 and peaks 2 and 4 for 4 and
5 are also assigned to the keto isomers. To prove the keto and
enol peak assignments as correct, CVs were recorded of aged
samples of each compound. These samples are enol-enriched,
and the keto peaks were found to be either absent or much less
pronounced, depending on how quickly after the sample
dissolved the CVs were recorded. Figure 7 demonstrates this
for 4.
For the symmetrical ꢀ-diketone 5, peaks 2 and 3 were
overlapping (Figure 6). Different formal reduction potentials
for side groups on symmetrical complexes in which mixed-
valent intermediates are generated are well-known in systems
that allow electron delocalization, either through bridge-mediated
paths or from a direct metal-metal interaction.16,25 This then
implies that, for 5, overlapping peaks 2 and 3 represent the
oxidation of the keto isomer Rc3COCH2CORc4 and the oxida-
tion of the mixed-valent enol intermediate Rc1,oxC(OH)d
CHCORc2, respectively. By necessity, peak 1 can only involve
Rc1C(OH)dCHCORc2 oxidation and peak 4 must involve
Rc1,oxCOCH2CORc2 oxidation. Equation 4 highlights the elec-
trochemical oxidation of enol and keto oxidation as well as the
slow chemical equilibrium that exists between the different enol
and keto forms.
chemistry study in CH3CN. However, Mann20 and, more
recently, Geiger27 reported reversible Rc oxidation to two RuIII
species, one monomeric [(C5H5)2RuIII]+ and the other dimeric
[(C5H5)2RuIIIRuIII(C5H5)2]2+, in a CH2Cl2/[N(nBu)4][B{(C6H3)
(CF3)2}4] or CH2Cl2/ [N(nBu)4][B(C6F5)4] medium, respectively.
1
It was also noted in the H NMR kinetic study (Table 1) that
more electron-withdrawing R substituents shift the keto–enol
equilibrium position to the enol side. This implies the equilib-
rium positions of the E2/K2 and E3/K3 equilibria in eq 4 lies
to the left of that for the E1/K1 equilibrium. This will be so
because Rci,ox, i ) 1–4, like the ferrocenium cation, Fc+,16 is
more electron-withdrawing than the Rc group itself.
From the above it is clear that ruthenocenyl Ep,a potentials
must be dependent on ꢁR. In Table 3 it can be seen that Ep,a(1),
the anodic ruthenocenyl oxidation potential of the enol isomer
of 1-3 and 5, becomes smaller as ꢁR decreases. In contrast,
for 4, the corresponding ruthenocenyl oxidation wave is Ep,a(3),
but the potential at which this wave is found has the second
largest value in the present compound series, despite the fact
that the ferrocenyl group has the lowest ꢁR value of all R
substituents. However, for 4, the ferrocenyl group is first
oxidized to ferrocenium during CV scans. Hence, when the first
ruthenocenyl wave (the wave for the enol isomer) is observed
at Ep,a(3) ) 484 mV vs Fc/Fc+, the group electronegativity of
the ferrocenium species,16 ꢁFc ) 2.82, is relevant. This is also
+
the second largest ꢁR value in the present compound series. Thus,
4 follows the general ꢁR dependence for ruthenocenyl oxidation
as well.
No electrochemical estimate of the pKa′ value of 5 could be
made, since the ruthenocenyl center does not represent a
thermodynamic reversible redox center and no simple linear
relationship between Ep,a(Rc) and pKa′ could be found (see the
Supporting Information).
Isomerization Kinetics Utilizing Electrochemical Mea-
surements. (a) CH3CN as Solvent. The good resolution
between keto and enol peaks of Figure 7 suggests it must in
principle be possible to follow the kinetics of isomerization for
2-5 electrochemically, because they all have a large difference
in keto content at equilibrium, and because the peak current, ip,
is directly proportional to concentration, C, according to the
Randles-Sevcik equation: ip ) (2.69 × 105)n3/2AD1/2Cν1/2 28
.
Several problems are associated with CV and OYSW measure-
ments that normally would disqualify these techniques as useful
methods in kinetic studies. The most serious of these is electrode
fouling (polluting) that may occur during the extended periods
of time required for slow kinetic changes. For 4, electrode
fouling led to the following: (i)a loss of peak resolution
especially for waves 1 and 2, (ii) a ruthenocenyl-based Ep,a
potential drift of more than 120 mV to more positive values,
and (iii) a slow systematic decrease in current intensity as the
electrode surface was compromised.
These problems were overcome by replacing measured ip
values with percent keto content relative to an internal standard,
free ferrocene, as described in the Experimental Section. Thus,
time-based CV experiments on 4 led to rate constants that only
scattered in the range (8–14) × 10-5 s-1. These CV-determined
rate constants are mutually consistent with the 1H NMR
determined rate constant of 9.1 × 10-5 s-1 (Table 1). Examples
of time-based CV scans and the workup procedure to obtain
rate constants in CH3CN solutions are available as Supporting
Information; the next section describes examples for CH2Cl2
(4)
The oxidized species E2 is not considered to be
Rc1C(OH)dCHCORc2,ox, because carbonyl (CdO) substituted
metallocenes are oxidized at potentials greater (more positive)
than those for alcohol (OH) substituted metallocenes.25e Bulk
electrolyses to determine the ruthenium oxidation state in Rcox
above failed due to compound decomposition, but it is generally
accepted that RuIV species arise in CV experiments involving
ruthenocene derivatives19,20 in CH3CN/N(nBu)4PF6 media. From
previous research,26 a RuIV species such as [(C5H4R)(Cp)-
RuIVCH3CN]2+ is very likely formed in the present electro-
(25) Leading references are: (a) Creutz, C.; Taube, H. J. Am. Chem.
Soc. 1969, 91, 3988. (b) Geiger, W. E.; van Order, N.; Pierce, D. T.;
Bitterwolf, T. E.; Rheingold, A. L.; Chasteen, N. D. Organometallics 1991,
10, 2403. (c) van Order, N.; Geiger, W. E.; Bitterwolf, T. E.; Rheingold,
A. L. J. Am. Chem. Soc. 1987, 109, 5680. (d) Pierce, D. T.; Geiger, W. E.
Inorg. Chem. 1994, 33, 373. (e) Davis, W. L.; Shago, R. F.; Langner,
E. H. G.; Swarts, J. C. Polyhedron 2005, 24, 1611.
(26) (a) Watanaba, M.; Motoyama, I.; Takayama, T.; Sato, M. J.
Organomet. Chem. 1997, 549, 13. (b) Watanaba, M.; Motoyama, I.; Shimoi,
M.; Sano, H. J. Organomet. Chem. 1996, 517, 115. (c) Smith, T. P.; Iverson,
D. J.; Droege, M. W.; Kwan, K. S.; Taube, H. Inorg. Chem. 1987, 26,
2882.
(27) Trupia, S.; Nafady, A.; Geiger, W. E. Inorg. Chem. 2003, 42, 5480.
(28) Kissinger, P. T.; Heineman, W. R. In Laboratory Techniques in
Electroanalytical Chemistry; Marcel Dekker: New York, 1984; p 82.

Ruthenocene-Containing ꢀ-Diketones
Organometallics, Vol. 27, No. 3, 2008 359
potential comparisons between the CH3CN and CH2Cl2 solvent
systems become possible (see Supporting Information for a
treatment of potential data versus both the experimentally used
Ag/Ag+ reference electrode and versus Fc/Fc+ as internal
standard). The key result from this treatment is that, although
formal reduction potentials versus Fc/Fc+ for ferrocenyl-based
waves 1 and 2 of 4 are for all practical purposes the same in
both solvents, potentials associated with ruthenocenyl waves 3
and 4 of 4 were shifted to much more positive potentials in
CH2Cl2 (Table 3). Different ruthenocene oxidation products
appear to form in CH3CN and CH2Cl2. The ruthenocenyl
fragment oxidation product in CH3CN is probably the RuIV
species [(C5H4R)(Cp)RuIV · CH3CN]2+, while in CH2Cl2, the
labile RuIII species [(C5H5)RuIII(C5H4R)]+ is first formed during
oxidation. In contrast with this, in both solvents ferrocene
oxidation only gave the FeIII species [(C5H5)FeIII(C5H4R)]+.
These results highlight different substrate-solvent interactions
(CH3CN in contrast with CH2Cl2). In particular it shows that
strong CH3CN association occurs with the oxidized ruthenocenyl
fragment of 4 but that CH3CN association with the oxidized
ferrocenyl fragment of the same molecule are so weak that for
all practical purposes, it can be neglected.
The ip,c/ip,a current ratio for waves 3 and 4 deviate substan-
tially from unity in CH2Cl2. The results, therefore, are consistent
with unstable, i.e. quickly decomposing (C5H5)RuIII(C5H4R)
fragments forming during the oxidation of the ruthenocenyl
group of 4 at 20 °C. The position of the most prominent
(C5H5)RuIII(C5H4R) fragment reduction wave at E ) ca. 0.515
V vs Fc/Fc+ at 20 °C in CH2Cl2 (Figure 8) is consistent with a
two-electron RuIII dimer reduction in analogy to free ru-
thenocene,27 because it is found at a potential ca. 290 mV
lower than Ep,a(peak 3) ) 802 mV. This peak potential
difference is too large to be associated with a simple (C5H5)
RuIII(C5H4R)]+/[(C5H5)RuII(C5H4R) couple. However, weak
peak shoulders 3red and 4red are observed at Ecathodic ) ca. 671
and 777 mV, respectively (Figure 8 and, more clearly, the
Supporting Information). A repeat CV experiment in CH2Cl2
performed not at 20 °C but at 0 °C revealed a reduction peak
associated with wave 3 at 662 mV vs Fc/Fc+ but no dimer
reduction peak at ca. 515 mV vs Fc/Fc+ (see the Supporting
Information). Despite the weakness of shoulders 3red and 4red
that obviously hampers RuIII reduction peak assignments, key
information can be extracted from them. The estimated ∆Ep )
Ep,a(peak i) – Ep,c(shoulder i) values of ca. 131 and 106 mV for
peak/shoulder pairs 3 and 4 allow them to be associated with
reversible to quasi-reversible one-electron-transfer processes.
They are consistent with simple (C5H5)RuIII(C5H4R)]+/
[(C5H5)RuII(C5H4R) couples. Utilizing the above CH2Cl2 results,
eq 4 can be amended for compound 4 to involve electrochemical
reversible one-electron oxidation during the generation of E2,
K2, E3, and K3. Then, Rc1/Rc1,ox and Rc3/Rc3,ox will be the
FeII/III fragments (C5H5)FeII/III(C5H4R), while Rc2,ox and Rc4,ox
will be [(C5H5)RuIII(C5H4R)]+. Provision must also be made
for a two-electron reduction of at least one RuIII dimer back to
the ruthenocenyl group. With the present knowledge, the dimer
Figure 8. Sample Osteryoung square wave voltammograms (top,
at 50 Hz) and cyclic voltammograms (bottom, at scan rate 200 mV
s-1) in CH2Cl2 at 20 °C, showing how the enol form (peaks 1 and
3) becomes less prominent while the keto form becomes more
prominent (peaks 2 and 4) as an enol-enriched solution of 4
approaches equilibrium. For the CV, dotted lines show anticipated
decay curves. For the OYSW voltammograms, the dotted lines show
the peaks of waves 1 and 2 independent of the influence of each
other. This allowed measurement of ip as described in the text and
Supporting Information.
solutions. Poor peak resolution between peaks 1 and 2 disal-
lowed following of this reaction utilizing the ferrocenyl wave
of 4. Attempts to follow the kinetics in CH3CN with Osteryoung
square wave voltammetry were unsuccessful.
(b) CH2Cl2 as Solvent. In an attempt to prove the existence
of a [(C5H5)RuIII(C5H4R)]+/[(C5H5)RuII(C5H4R)] couple dur-
ing the electrochemical investigation of the present ꢀ-diketone
series, the electrochemical kinetics of 4 were reinvestigated
in a noncoordinative CH2Cl2/0.1 mol dm-3 [N(nBu)4]
[B(C6F5)4] solvent/supporting electrolyte system following
the guidelines of Geiger.27 In addition, from 1H NMR
measurements, isomerization kinetics in CH2Cl2 is almost
20 times faster than in CH3CN, which gives less time for
electrode pollution. In CH2Cl2, the resolutions of ferrocenyl
waves 1 and 2 were markedly improved, especially in the
presence of free ferrocene as internal standard. However, the
two ruthenocenyl waves at peaks 3 (Ep,a ) 0.802 V) and 4
(Ep,a ) 0.883 V) became much less resolved than was the
case in CH3CN. Clear evidence of a [(C5H5)RuIII(C5H4R)]+
species was also found, since a cathodic current for the
ruthenocenyl group during the reverse sweep could clearly
be seen, (Figure 8 and the Supporting Information).
is most likely [(C5H4R)(C5H5)RuIII-RuIII(C5H5)(C5H4R)]2+
,
another form of a RuIII-RuIII dimer in analogy with ru-
thenocene27 itself, or one of the two possible dimers that was
found for the analogous osmocene30 system.
When potentials are not referenced against the experimentally
used Ag/Ag+ reference electrode, but rather versus an internal
standard, here Fc/Fc+, as first suggested by Gagny,29 direct
Table 4 summarizes sets of data utilized to obtain isomer-
ization rate constants from three different electrochemical sets
of kinetic experiments performed in CH2Cl2. Time-based plots
(29) Gagne, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980,
19, 2855.
(30) Droege, M. W.; Harman, W. D.; Taube, H. Inorg. Chem. 1987,
26, 1309.

360 Organometallics, Vol. 27, No. 3, 2008
Kemp et al.
Table 4. Cyclic Voltammetry Data of Ferrocenyl Waves 1 and 2 (labeled as Fc-CV) and Ruthenocenyl Waves 3 and 4 (Rc-CV) Collected with
Time for the Conversion of Enol to Keto Isomers of RcCOCH2COFc in CH2Cl2 at 20 °Ca
Fc CV/Rc CV (Fc SW)
measmt time/s
ip,a(enol)/µA
ip,a (keto)/µA
-c/-c (-)c
3/6.7 (14)
8.2/11 (19)
14/17 (22)
16/21 (24)
18/23.4 (27)
-c/-c (-)c
% ketob
0d/0d (0)d
6.7/12.0 (24.1)
17.7/20.4 (35.9)
29.1/29.8 (41.5)
33.3/36.8 (46.7)
38.3/40.8 (48.2)
40d/42d (47.8)d
ln((% keto)0 - (% keto)∞)/((% keto)t - (% keto)∞)
0/0 (0)
-c/-c (-)c
41.5/49 (44)
38/43 (34)
34/40 (31)
32/36 (30)
29/34 (29)
-c/-c (-)c
0d/0d (0)d
64/64 (200)
163/163 (440)
400/400 (680)
640/640 (1160)
1120/1120 (1520)
t ) ∞
0.183/0.288 (0.702)
0.584/0.616 (1.391)
1.300/1.236 (2.027)
1.787/2.089 (3.712)
3.158/3.555 (-)e
a Values in parentheses correspond to Osteryoung square wave data of ferrocenyl waves 1 and 2 (Fc-SW). b % keto ) ip,a(keto)/(ip.a(keto) +
ip,a(enol)) × 100. c Not measured due to the time required for dissolving analytes. d Values not measured but obtained from a nonlinear least-squares
MINSQ fit. e Because the measured ip value is larger than the calculated t∞ value, this data point was not calculated.
NMR spectroscopy, cyclic voltammetry, or Osteryoung square
wave voltammetry. In contrast to the case for the ferrocenyl
group, the ruthenocenyl fragment exhibited irreversible elec-
trochemistry in CH3CN/0.1 mol dm-3 N(nBu)4PF6. Only oxida-
tion waves generating most likely a RuIV oxidized product could
be detected. In CH2Cl2/0.1 mol dm-3 [N(nBu)4][B(C6F5)4],
however, clear evidence for the electrochemical reversible
generation of unstable [(C5H5)RuIII(C5H4R)]+ and a dimerized
oxidized product was found. Kinetic measurement utilizing
cyclic voltammetry and/or Osteryoung square wave techniques
is possible, provided extensive care is taken in estimating decay
currents. Excessively long kinetic experiments are more difficult
to follow electrochemically, especially because of electrode
fouling. The accuracy of the electrochemical technique in
studying solution reaction kinetics is on a par with that of
Figure 9. Time traces showing the formation of percent keto isomer
as determined by ferrocenyl-based Osteryoung square wave (9, top),
ruthenocenyl-based cyclic (b, middle) ,and ferrocenyl-based cyclic
(2, bottom) voltammetry for 4 at 25 °C in CH2Cl2. Insert: kinetic
spectroscopic techniques. Isomerization kinetics, equilibrium
constants, pKa′ values, and ruthenocenyl oxidation potentials
were shown to be dependent on the group electronegativity of
the R side group in RcCOCH2COR. These new ligands will
open up research in new multinuclear metal coordination
complexes with interesting properties.
plot of ferrocenyl-based Osteryoung square wave data from Table
4 for this isomerization. The slope of this graph gives the first-
order rate constant kobsd ) k1 + k-1 )0.0031 s-1 (C ) percent
keto isomer).
showing how the percent keto isomer increases as the equilib-
rium position is approached from the enol side for all three
electrochemical experimental approaches are shown in Figure
9. The insert highlights the validity of a first-order treatment of
ferrocenyl-based Osteryoung square wave obtained kinetic data.
Rate constants kobsd ) k1 + k-1 for all three techniques
approached 3 × 10-3 s-1 and deviated only by a factor of about
Experimental Section
General Information. Ruthenocene (Strem) and aluminum
trichloride and other solid reagents (Aldrich) were used without
further purification. Dry acetonitrile was obtained by reflux under
nitrogen over calcium hydride, distillation onto alumina for storage,
and redistillation just prior to use. THF was dried by reflux under
a nitrogen atmosphere over sodium wire and distillation directly
before use. Water was double-distilled. Acetylruthenocene (2) was
synthesized as described before, utilizing 2.2 mol equiv31 of AlCl3.
Methyl ferrocenoate was prepared by following published guide-
lines,14 and this method was directly adapted for methyl rutheno-
cenoate. Chromatography was performed on Kieselgel 60 (Merck,
grain size 0.063–0.2 mm) utilizing 1:1 ether-hexane by volume
as eluant unless otherwise indicated. 1H NMR spectra at 20 °C
were recorded on a Bruker Advance DPX 300 NMR spectrometer
at 300 MHz, with chemical shifts presented as δ values referenced
to SiMe4 at 0.00 ppm utilizing CDCl3 as solvent. The CDCl3 was
made acid-free by passing it through basic alumina immediately
before use. Elemental analysis was conducted by the Canadian
Microanalytical Service, Ltd.
1
2 from the H NMR determined rate constant of 1.47 × 10-3
s-1 (Table 1). These spectroscopic and electrochemical equiva-
lent kinetic results are in extraordinary good agreement if one
bears in mind all the current measurement difficulties associated
with electrode fouling over extended periods of time.
Finally, the percent keto isomer values at equilibrium in
CH2Cl2 as obtained by the Fc-CV, Rc-CV and Fc-OYSW
experiments are 40, 42, and 47.8%, respectively (Table 4). These
values are in fair to good agreement with the 1H NMR
1
determined value of 50.1-50.2%. The H NMR determined
equilibrium constant Kc ) 1.00–1.01 is in equally good
agreement with the values determined by Fc-CV (1.50), Rc-
CV (1.38), and Fc-OYSW (1.09) measurements (Table 1).
Conclusion
ꢀ-Diketone Syntheses. The syntheses of 1 may serve as an
example. To a solution of acetylruthenocene (0.799 g; 2.922 mmol)
in dry THF (5 mL) was added lithium diisopropylamide (1.46 cm3
of 2.0 mol dm-3 solution; 2.92 mmol), and the reaction mixture
was stirred for 20 min while it was cooled on ice. Ethyl
trifluoroacetate (0.42 g; 0.35 cm3; 2.921 mmol) was added and the
A series of ruthenocene-containing ꢀ-diketones of the form
RcCOCH2COR with R ) CF3, CH3, Ph, Fc, Rc were synthe-
sized and characterized, and pKa′ values were determined.
Although all ꢀ-diketones converted quantitatively with time in
the solid state to one enol form, in solution these ligands exist
in a dynamic equilibrium as a mixture of keto and enol isomers.
Rate constants of these interconversions, as well as the corre-
(31) Rausch, M. D.; Fischer, E. O.; Grubert, H. J. Am. Chem. Soc. 1960,
82, 76.
1
sponding equilibrium constants, were determined utilizing H

Ruthenocene-Containing ꢀ-Diketones
Organometallics, Vol. 27, No. 3, 2008 361
reaction mixture stirred for 16 h at room temperature before an
excess of ether (>30 cm3) was added. The precipitate was filtered
off, added to an ice-cold HCl solution (0.1 mol dm-3; 100 cm3),
and stirred for 5 min and the product extracted into ether. The ether
layer was washed with H2O (3 × 100 cm3), dried over MgSO4,
filtered, and evaporated to dryness under reduced pressure. The
crude product was purified using column chromatography with 4:1
ether-hexane by volume as eluent (Rf ) 0.91). Yield: 83%. Mp:
102–104 °C. 1H NMR (300 MHz, CDCl3; δH): enol isomer (92.5%
at equilibrium), 6.03 (1H; s; COCHCO), 5.20 (2H; t; C5H4), 4.93
(2H enol overlapping with keto signal; t; C5H4), 4.63 (5H; s; C5H5);
keto isomer (7.5%), 5.12 (2H; t; C5H4), 4.93 (2H, keto overlapping
with enol signal; t; C5H4), 4.70 (5H; s; C5H5), 3.00 (2H; s;
COCH2CO). Anal. Calcd for C14F3H11O2Ru: C, 45.51; H, 3.00.
Found: C, 45.50; H, 3.00.
I of keto signal
× 100
% keto isomer )
I of keto signal + I of enol signals
(5)
(b) From Electrochemical Measurements. When electrochemi-
cal experiments were conducted, integral values, I, were replaced
by peak current values, ip, in eq 5 above to calculate the percent
keto isomer.
Keto–Enol Equilibrium Constants, Kc. Once the percent keto
isomer was known, the equilibrium constant, Kc, relating to eq 2
could be determined utilizing eq 6.
% enol isomer(s)
100 - % keto isomer
Kc )
)
(6)
% keto isomer
% keto isomer
Isomerization Kinetics. For conversions from enol to keto
isomers, solid ꢀ-diketone samples (5.5–9.6 mg/cm-3 CDCl3) older
than 3 months were quickly dissolved and 1H NMR spectra recorded
at appropriate times at 293 K until no further changes in the integral
values of signals were observed. At this stage an equilibrium set
in. The percentage keto isomer at each time interval was determined
from eq 5 utilizing the same 1H NMR keto/enol signal pairs
identified before (Table 1). From the time-based keto percentage
data fitted to eq 7,32 the observed rate constant kobsd could be
calculated. The percentage keto isomer at infinite time is that of
Kc, while k1 and k-1 relate to equilibrium (2).
Characterization Data for 2–5. Compound 2. The product was
purified using column chromatography (Rf ) 0.66). Yield: 26.8%.
Mp: 111 °C. 1H NMR (300 MHz; CDCl3; δH): enol isomer (68.5%),
5.64 (1H; s; COCHCO), 5.11 (2H; t; C5H4), 4.79 (2H enol
overlapping with keto signal; t; C5H4), 4.59 (5H; s; C5H5), 2.31
(3H; d; CH3); % keto (31.5%), 5.14 (2H; t; C5H4), 4.79 (2H keto
overlapping with enol signal; t; C5H4), 4.61 (5H; s; C5H5), 3.73
(2H; s; COCH2CO), 2.05 (3H; d; CH3). Anal. Calcd for
C14H14O2Ru: C, 53.30; H, 4.47. Found: C, 53.63; H, 4.75.
Compound 3. The product was difficult to purify using column
chromatography with 2:1 ether-hexane as eluent (Rf ) 0.66);
crystallization as for 4 worked better. Yield: 41.3%. Mp: 110 °C.
1H NMR (300 MHz; CDCl3; δH): enol (85.00%), 7.85 (2H; m;
C6H5), 7.46 (3H; m; C6H5), 6.28 (1H; s; COCHCO), 5.25 (2H; t;
C5H4), 4.83 (2H enol overlapping with keto signal; t; C5H4), 4.59
(5H; s; C5H5); keto (15.0%), 8.1 (2H; m; C6H5), 7.60 (3H; m; C6H5),
5.18 (2H; t; C5H4), 4.83 (2H keto overlapping with enol signal; t;
C5H4), 4.48 (5H; s; C5H5), 4.25 (2H; s; COCH2CO). Anal. Calcd
for C19H16O2Ru: C, 60.45; H, 4.27. Found: C, 60.38; H, 4.38.
Compound 4. Purification was by crystallization from 1:1
heptane-chloroform and allowing the chloroform to slowly evapo-
(initial % keto isomer) - (% keto isomer at infinite time)
(% keto isomer at time t) - (% keto isomer at infinite time)
ln
[
]
) (k1 + k-1)t ) kobsd
t
(7)
The individual rate constants k1 and k-1 were obtained by
simultaneously solving the equations Kc ) k1/k-1 and kobsd ) k1 +
k-1. Compounds 2-5 were studied in this manner. Hrctfa (1),
however, could only be studied by following the rate of conversion
from keto to enol isomer. Keto-enriched solutions of 1 in sufficient
concentrations for a 1H NMR study were obtained by extracting a
10 cm3 1:1 acetonitrile-ether solution (50 mg ꢀ-diketone in 10
cm-3) with 5 cm3 of 0.1 mol dm-3 aqueous NaOH. The aqueous
layer was separated, washed with CHCl3, and again separated before
2.5 cm3 of CDCl3 was added. Acidification with ice-cold 0.2 mol
dm-3 HCl allowed extraction of the ꢀ-diketone with large keto
isomer content into the CDCl3 layer. The aqueous layer was
discarded and the CDCl3 layer quickly rinsed with water before
1H NMR spectra were recorded at appropriate time intervals until
the equilibrium position was reached.
Apparent Acid Dissociation Constants. pKa′ values were
determined by measuring the absorbance on a Varian Cary 50 UV/
vis dual beam spectrophotometer at different pH values during an
acid–base titration in water or acetonitrile-water mixtures, 1:9 by
volume, I ) 0.100 mol dm-3 (NaClO4) at 298 K at wavelengths
and ꢀ-diketone concentrations as indicated in Figure 5. The
wavelengths at which the absorbance dependence on pH was
monitored was established by recording a UV/vis spectra for each
compound at low and high pH. The chosen wavelength for the pH
titration was where these two spectra exhibited meaningful different
molar extinction coefficients. These spectra are shown for 2 in
Figure 4. Titrations were performed with HClO4 from high to low
pH and then back again. ꢀ-Diketone solutions were prepared by
first dissolving the compound in acetonitrile followed by dilution
with the correct volume of aqueous NaOH at a convenient initial
1
rate. Yield: 34%. Mp: 178 °C. H NMR (300 MHz; CDCl3; δH):
enol to the Fc side (47.0%), 5.9 (1H; s; COCHCO), 5.19 (2H; t;
C5H4 Rc), 4.81 (2H + 2H Rc enol overlapping with Fc enol signal;
t; C5H4), 4.60 (5H; s; C5H5 Rc), 4.51 (2H; t; C5H4 Fc), 4.20 (5H;
s; C5H5 Fc); enol to Rc side (14.0%), 6.60 (1H; s; COCHCO), 5.14
(2H; t; C5H4 Rc), 4.81 (2H + 2H Rc enol overlapping with Fc
enol signal; t; C5H4), 4.72 (2H; t; C5H4 Fc), 4.56 (5H; s; C5H5 Rc),
3.99 (5H; s; C5H5 Fc); keto (39.0%), 5.24 (2H; t; C5H4 Rc), 5.10
(2H; t; C5H4 Fc), 4.94 (2H + 2H Rc keto overlapping with Fc
keto signal; t; C5H4 Rc), 4.61 (5H; s; C5H5 Rc), 4.23 (5H; s; C5H5
Fc), 2.28 (2H; s; COCH2CO). Anal. Calcd for C23FeH20O2Ru: C,
56.77; H, 4.14. Found: C, 56.80; H, 4.16.
Compound 5. The product was purified using column chroma-
tography (Rf ) 0.66). Yield: 28%. Mp: 182 °C. 1H NMR (300
MHz; CDCl3; δH): enol (41.2%), 5.83 (1H; s; COCHCO), 5.17 (2H;
t; C5H4), 4.84 (5H; s; C5H5), 4.78 (2H; t; C5H4); keto (58.8%),
5.23 (2H; t; C5H4), 4.83 (2H; t; C5H4), 4.81 (5H; s; C5H5), 3.88
(2H; s; COCH2CO), Anal. Calcd for C23H20O2Ru2: C, 51.96; H,
3.79. Found: C, 53.29; H, 4.08.
1
Percent Keto Isomer. (a) From H NMR Spectroscopy. The
percentage keto isomer at equilibrium was determined from eq 5
utilizing integral values, I, of the 1H NMR keto/enol signal pair(s)
identified in Table 1. Where more than one nonoverlapping keto/
enol signal pair could be identified, the average percent keto isomer
was determined. Integration of the spectra was performed such that
the methine proton, RcCOCHdC(OH)R, at 5.5-6.5 ppm was
usually assigned an integral value of 1.
(32) House, J. E. In Principles of Chemical Kinetics; W. C. Brown:
Dubuque, IA, 1997; pp 46–51.
(33) Wikins, R. G. In The Study of Kinetics and Mechanism of Reactions
of Transition Metal Complexes; Allyn and Bacon: Boston, MA, 1974; pp
44–45.

362 Organometallics, Vol. 27, No. 3, 2008
Kemp et al.
pH. pKa′ values were obtained from a least-squares fit of UV
absorbance/pH data to eq 814,33 utilizing the fitting program
MINSQ.34
developed software and analyzed with Hyperplot (JHM Interna-
tional, Inc.). For the BAS 100 instrument, data were collected with
standard BAS 100 software. All measurements were conducted
under a blanket of argon in a Faraday cage. Successive experiments
under the same experimental conditions showed that all formal
reduction and oxidation potentials were reproducible within 5 mV.
All potentials in this study were experimentally referenced against
the Ag/Ag+ couple but were then manipulated on Excel to be
referenced against Fc/Fc+, as recommended by IUPAC.38 Under
our experimental conditions, the Fc/Fc+ couple exhibited E°′ )
0.087 V vs Ag/Ag+, ip,c/ip,a ) 0.98, and ∆Ep ) 74 mV in
CH3CN,and E°′ ) 0.263 V vs Ag/Ag+, ip,c/ip,a ) 0.97, and ∆Ep )
81 mV in CH2Cl2.
Electrochemical Kinetics. Time-based CVs and OYSW volta-
mmograms of 2.0 mmol dm-3 enol-enriched solutions of 4 in
CH3CN as well as in CH2Cl2 were recorded at a scan rate of 200
mV s-1 and frequency of 50 Hz. To overcome electrode fouling
problems, manual adjustment of the amplitude of the internal
standard, free ferrocene or the ferrocenyl wave of 4, to identical
peak current levels for each time-based run and conversion of the
amended time-based peak currents to percent keto isomer allowed
the collection of good interpretable kinetic data. CV decay currents
in the free ferrocene internal marker and the ferrocenyl group peak
number 1 of Figure 8 (and all other kinetic runs) were estimated
by duplicating the decay current of the reduction half-wave of the
ferrocene marker peak and overlapping it with the free ferrocenyl
oxidation wave. Some broadening of the free ferrocenyl peak as
well as amplitude manipulation was required for realistic overlaps
and decay currents in the oxidation peaks. A realistic decay current
was considered achieved once no obvious discontinuity was
observable in each peak CV trace; i.e., no sharp corners or kinks
are obvious when the decay current is superimposed on the CV
trace. The decay current of peak 3, the enol ruthenocenyl half-
wave, was estimated by duplicating and overlapping the ferrocenyl
wavenumber 2 decay current with wave 3 (see Figure 8). The
complete procedure for estimating decay currents is described in
the Supporting Information using CH3CN results as examples. For
the Osteryoung square wave (OYSW) experiments, only peaks 1
and 2 were used to collect kinetic data, as demonstrated in Figure
8. Here, to measure peak currents, peak 1 from t ) 126 s was copied
and overlapped with all the other peak 1 and peak 2 peak positions
at different times. Currents were then measured between peaks and
neighboring peak profiles.
′
AHA10-pH + AA10-pKa
AT )
(8)
-pKa′
10-pH + 10
In eq 8, AT ) total absorbance, AHA ) the absorbance of the
ꢀ-diketone, and AA ) the absorbance of the ꢀ-diketonate. A linear
response by the pH meter (Orion Model SA 720), fitted with a
glass electrode, was ensured by calibration with IUPAC standard
buffers35 at pH ) -log RH ) 4.01, 7.00, and 12.00, respectively;
RH ) activity of H+. A test pKa′ determination was performed by
titrating the well-characterized compound acetylacetone in water.
The obtained pKa′ value of 8.83 ( 0.01 was, within experimental
error, the same as the best available published pKa′ value for
acetylacetone in water (8.878 ( 0.005 when I ) 1 mol dm-3 and
8.98 when I ) 0.0172 mol dm-3).36 It was concluded that the
electrode was calibrated to measure hydrogen ion concentration
under the conditions used.
Electrochemistry. Cyclic voltammetry and Osteryoung square
wave voltammetry were conducted using a computer-controlled
BAS Model CV-27 or BAS Model 100 B/G voltammograph. The
temperature was controlled using a water bath at 20.0 ( 0.1 °C.
Experiments were performed under argon on 2 mmol dm-3
solutions in dried distilled acetonitrile in the presence of 0.1 mol
dm-3 [N(nBu)4][PF6] as supporting electrolyte or in CH2Cl2 in the
presence of 0.1 mol dm-3 [N(nBu)4][B(C6F5)4].37 The three-
electrode cell that was used consisted of a glassy-carbon working
electrode, with surface area 7.07 mm2 pretreated by polishing on a
Buehler microcloth first with 1 µm and then ¼ µm diamond paste,
a Pt-wire counter electrode, and a reference electrode constructed
from a silver wire inserted into a solution of 0.01 mol dm-3 AgNO3
and 0.1 mol dm-3 tetra-n-butylammonium hexafluorophosphate in
acetonitrile, in a Luggin capillary with a Vycor tip. For measure-
ments in CH2Cl2, this Luggin capillary was inserted into a second
Luggin capillary with Vycor tip filled with a 0.1 mol dm-3
[N(nBu)4][B(C6F5)4] solution in CH3CN. For kinetic experiments,
a glassy-carbon working electrode with surface area 3.1 mm2 was
also used. For the BAS C-27 instrument, data uncorrected for
junction potentials were collected with an Adalab-PC and Adapt
data acquisition kit (Interactive Microwave, Inc.) with locally
(34) MINSQ, Non-Linear Parameter Estimation, V3.12; MicroMath
Scientific Software; Salt Lake City, UT, 1990.
(35) Jeffery, G. H.; Basset, J.; Mendham, J.; Denney, R. C. In Vogel’s
Textbook of QuantitatiVe Chemical Analysis, 5th ed.; Longman Scientific
and Technical: Essex, U.K., 1991; p 830.
Acknowledgment. We acknowledge the NRF under Grant
No. 2054243 and the UFS for financial support.
(36) Martell, A. E. In Stability Constants of Metal-Ion Complexes, 3rd
ed.; The Chemical Society: London, 1971; Special Publication No. 25, Part
II, p 365.
(37) LeSuer, R. J.; Buttolph, C.; Geiger, W. E. Anal. Chem. 2004, 76,
6395.
(38) Gritzner, G.; Kuta, J. Pure Appl. Chem. 1984, 56, 461.
Supporting Information Available: Text and figures giving
further explanations of electrochemisry experiments. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM700609Z