Electronic effects on the rates of coupled two-electron/halide self-exchange reactions of substituted ruthenocenes

Article abstract of DOI:10.1021/ic990690b

The complexes (Me(m)Cp)(Me(n)Cp)Ru (m,n = 1,1; 0,4; 0,5; 1,5) have been synthesized along with the corresponding halometalloceniums [(Me(m)Cp)(Me(n)Cp)RuX]⁺ (X = Cl, Br, I). The two-electron/halide transfer self-exchange kinetics have been dete

Full text of DOI:10.1021/ic990690b

Inorg. Chem. 2000, 39, 1573-1577
1573
Electronic Effects on the Rates of Coupled Two-Electron/Halide Self-Exchange Reactions of
Substituted Ruthenocenes
Timothy M. Shea and T. David Westmoreland*
Department of Chemistry, Wesleyan University, Middletown, Connecticut 06459
ReceiVed June 15, 1999
The complexes (MemCp)(MenCp)Ru (m,n ) 1,1; 0,4; 0,5; 1,5) have been synthesized along with the corresponding
halometalloceniums [(MemCp)(MenCp)RuX]+ (X ) Cl, Br, I). The two-electron/halide transfer self-exchange
kinetics have been determined and compared to those of the Cp2Ru/[Cp2RuX]+ parent system. Methylation decreases
the rate of exchange monotonically, and plots of ln(kex) vs number of methyl groups are linear for constant X. It
is concluded that steric effects do not contribute significantly to the observed kinetics. Thermodynamic studies of
halide substitution equilibria support the conclusion that the electronic effects of methylation are essentially additive.
Introduction
transfer for decamethylferrocenium/decamethylferrocene relative
to the corresponding ferrocenium/ferrocene rate.7a The results
for the ferrocene-based systems have been rationalized in terms
of increased delocalization of charge with increasing methylation
and/or decreased solvent contribution to the reorganizational
energy.7 The origin of the dramatic decrease in rate for the
ruthenocenium/ruthenocene system, however, has not been
established. In particular, the relative importance of electronic
effects on the magnitude of the electronic coupling in the
transition state and the steric effects that might be expected to
inhibit the formation of a halogen-bridged transition state has
not been defined.
The characterization of multielectron/group transfer reactions,
often termed “atom-transfer reactions”,1 is of considerable recent
interest.2 The self-exchange reactions of ruthenocene and
osmocene derivatives given in eq 1 have provided a synthetically
accessible series of complexes for the study of fundamental
aspects of these reactions.3-6
[Cp2MX]+ + Cp2M f Cp2M + [Cp2MX]+
(M ) Ru, Os; X ) Cl, Br, I)
(1)
Previous studies3-6 have focused on the effects of varying the
transferred atom, the counterions, and the solvent on the
observed self-exchange kinetics and have resulted in mechanistic
proposals that invoke a halogen-bridged transition state. Ther-
modynamic and structural studies6 have also led to the conclu-
sion that electronic factors are responsible for the large halide
dependence of the rates.
Another approach to modifying the electronic properties of
the complexes is to add substituents to the cyclopentadienyl
rings. For methyl substituents, it has been shown,3 for example,
that the self-exchange rates for the [Cp*2RuX]+/Cp*2Ru couples
(Cp* ) pentamethylcyclopentadienyl) were too slow for detec-
tion by dynamic NMR techniques. This is in contrast to the
observed increase in the rate of one-electron outer-sphere
We have synthesized a series of methyl-substituted rutheno-
cenes and their haloruthenocenium derivatives and have deter-
mined their two-electron/halide transfer self-exchange rate
constants. The results lead to the conclusion that relative self-
exchange rates are governed by electronic effects, not steric
effects, for complexes with up to six methyl substituents. In
addition, the relative energetics of the reaction coordinates have
been defined from the activation parameters and the relative
reactant state free energies.
Experimental Section
Materials. Ruthenocene and decamethylruthenocene were obtained
from Strem Chemicals, Inc. and were sublimed prior to use. Cyclo-
pentadiene dimer and methylcyclopentadiene dimer were obtained from
Aldrich Chemical Co. and were cracked by distillation immediately
prior to use. Tetramethylcyclopentadiene and RuCl3‚xH2O were pur-
chased from Strem Chemicals, Inc. and were used as received. Other
materials were obtained commercially and were used as received.
Published procedures were used for syntheses of [Cp2RuX]PF6 (X )
Cl, Br, I),4,6 [Cp*RuCl2]x,8 (MeCp)2Ru,9 Cp*CpRu,8 (Me5Cp)(MeCp)-
(1) Taube, H. In Mechanistic Aspects of Inorganic Reactions; Rorabacher,
D. B., Endicott, J. F., Eds.; ACS Symposium Series 198; American
Chemical Society: Washington, DC, 1982; pp 151-179.
(2) (a) Holm, R. H. Chem. ReV. 1987, 87, 1401-1449 and references
therein. (b) Woo, L. K. Chem. ReV. 1993, 93, 1125-1136 and
references therein.
(3) Smith, T. P.; Iverson, D. J.; Droege, M. W.; Kwan, K. S.; Taube, H.
Inorg. Chem. 1987, 26, 2882-2884.
(4) (a) Kirchner, K.; Dodgen, H. W.; Wherland, S.; Hunt, J. P. Inorg.
Chem. 1989, 28, 604-605. (b) Kirchner, K.; Dodgen, H. W.;
Wherland, S.; Hunt, J. P. Inorg. Chem. 1990, 29, 2381-2385. (c)
Kirchner, K.; Han, L. F.; Dodgen, H. W.; Wherland, S.; Hunt, J. P.
Inorg. Chem. 1990, 29, 4556-4559. (d) Anderson, K. A.; Kirchner,
K.; Dodgen, H. W.; Hunt, J. P.; Wherland, S. Inorg. Chem. 1992, 31,
2605-2608.
(5) Watanabe, M.; Sano, H. Chem. Lett. 1991, 555-558.
(6) Shea, T. M.; Deraniyagala, S. P.; Studebaker, D. B.; Westmoreland,
T. D. Inorg. Chem. 1996, 35, 7699-7703.
(7) At 298 K, kex is 5.3 × 106 M-1 s-1 for [Cp2Fe]+/Cp2Fe exchange and
3.8 × 107 M-1 s-1 for [Cp*2Fe]+/Cp*2Fe exchange. (a) Yang, E. S.;
Chan, M.-S.; Wahl, A. C. J. Phys. Chem. 1980, 84, 3094-3099. (b)
Chan, M.-S.; Wahl, A. C. J. Phys. Chem. 1978, 82, 2542-2549.
(8) Gassman, P. G.; Winter, C. H. J. Am. Chem. Soc. 1988, 110, 6130-
6135.
(9) Lemay, G.; Kaliaguine, S.; Adnot, A.; Nahar, S.; Cuzak, D.; Monnier,
J. Can. J. Chem. 1986, 64, 1943-1946.
10.1021/ic990690b CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/15/2000

1574 Inorganic Chemistry, Vol. 39, No. 7, 2000
Shea and Westmoreland
Ru10 (Cp* ) pentamethylcyclopentadienyl anion; MeCp ) methyl-
invariably contaminated with ruthenocene and octamethylruthenocene.
Addition of a substoichiometric amount of [Cp2
11,12
cyclopentadienyl anion), (R4N)X2Y, and (R4N)X3
(R ) Me, Et,
RuX]+ preferentially
n-Bu; X, Y ) Cl, Br, I).
oxidizes the octamethylruthenocene present in the sample. Once the
octamethylruthenocene has been quantitatively converted to haloocta-
methylruthenocenium, additional [Cp2RuX]+ oxidizes tetramethyl-
ruthenocene only. Similarly, hexamethylruthenocene can be selectively
oxidized in the presence of pentamethylruthenocene contamination.
CD3CN for kinetic measurements was obtained from Cambridge
Isotopes in sealed glass ampules and contained <1% water as
determined by NMR. For other NMR experiments, CD3CN from
Aldrich or Cambridge Isotopes was distilled over CaH2 prior to use.
Reported chemical shifts are referenced to the solvent peak (CHD2CN
at 1.93 ppm). A Hewlett-Packard 8451A diode array UV-vis spec-
trophotometer was used for optical absorbance measurements.
Detailed preparative procedures and analytical data for all the new
complexes are available elsewhere.15 1H NMR and optical absorbance
spectral data for all oxidized metallocenes are available in the
Supporting Information.
(Me4Cp)CpRu (Me4Cp ) 1,2,3,4-Tetramethylcyclopentadienyl
Anion). Tetramethylruthenocene has not been previously reported but
can be approached by modification of published routes to pentadienyl
complexes.13 RuCl3‚xH2O (5.00 g, 24 mmol) was added under N2 to
100 mL of degassed (three freeze-thaw cycles) EtOH and was stirred
until complete dissolution had occurred to yield a green-brown solution
(2-3 h). Exactly 1.59 mL of cyclopentadiene (23.7 mmol) was added
followed by 8.00 g (65.5 mmol) of tetramethylcyclopentadiene. It is
critical that a 1:1 ratio of RuCl3 to HCp is used. If an excess of
cyclopentadiene is present, then a significant increase in the yield of
ruthenocene is observed. If there is too little, octamethylruthenocene
becomes the major product of the reaction. An amount of 15.8 g of Zn
dust was added slowly, and the solution was heated at reflux for 6 h.
The solvent and other volatiles were removed under vacuum. The solid
residue was extracted with hot hexanes, and the extracts were filtered
through a Celite pad. The filtrate was then loaded onto a short, basic
Al2O3 column and eluted with hexanes. The solvent was removed from
the combined metallocene-containing fractions by rotary evaporation
to give an oil containing (Me4Cp)CpRu (46.7%), Cp2Ru (21.4%), and
(Me4Cp)2Ru (31.9%). Separation of the metallocenes proved to be
difficult, and the mixture was used for subsequent experiments requiring
NMR Kinetics. All NMR data were collected on a Varian Gemini
300 MHz spectrometer. Integrations of 1D and 2D data were performed
using routines supplied by Varian.
For exchange rates greater than about 104 M-1 s-1, standard line
broadening techniques16 based on a temperature-dependent line width
analysis in a near-fast exchange limit were employed. For slower rates,
2D exchange spectroscopy (EXSY)17 was used to measure the rates.
To obtain kinetic data, appropriate amounts of a haloruthenocenium
salt and its corresponding reduced form were weighed into an NMR
tube and CD3CN was added to give final concentrations of the two
species near 5 mM. For tetramethylruthenocene and hexamethylrutheno-
cene, however, purified samples were not available and the oxidations
were performed in situ. In these cases, the mixture of reduced
metallocenes was placed in an NMR tube with CD3CN, and [Cp2RuX]+
was added until there were approximately equal amounts of the oxidized
and reduced forms of the compound of interest. The relative concentra-
tions of the exchanging species were determined from a 1D NMR
spectrum of the solution. The absolute concentrations were determined
after the kinetic data were obtained from optical absorbance data and
the molar absorptivities of the haloruthenoceniums.
1
tetramethylruthenocene and its derivatives. H NMR (CD3CN, ppm)
of (Me4Cp)CpRu: 4.39 (s, 1H), 4.24 (s, 5H), 1.93 (s, 6H), 1.88 (s,
6H).
A standard EXSY pulse sequence was used to acquire two-
dimensional exchange data.17 For each experiment, 256 1024W free
induction decays of eight scans each over a sweep width of 1800-
2000 Hz with a recycle delay of 0.5-20.0s were collected in phase-
sensitive mode. The integrated EXSY peak intensities were used to
determine the exchange rate constants and variances as described
elsewhere.6,17 After each EXSY experiment, the 1D spectrum was
reacquired to ensure that no decomposition had occurred. A tabulation
of the 2D integrals is given in the Supporting Information.
Oxidation of Metallocenes. Two general pathways were employed
for metallocene oxidation. For chloro- or bromometalloceniums, the
metallocene may be oxidized by Fe(III) in the presence of the halide.3
In a typical preparation, a solution of ∼1.6 mmol metallocene in 30
mL of benzene was added to a solution containing an excess of Fe(III)
in 15 mL of dilute (∼4M) aqueous HX (X ) Cl, Br). After the solution
was vigorously stirred for several hours, the aqueous phase was
separated and washed with benzene and ether. An excess of NH4PF6
dissolved in a small amount of water was added, and the solution was
allowed to stand overnight. The product was collected as a powder by
filtration, washed with benzene and ether, and recrystallized from CH3-
CN/Et2O.
A second pathway for metallocene oxidation involved oxidation by
[Cp2RuX]PF6 (X ) Cl, Br, I). It has been established from potentio-
metric studies that for the complexes reported herein the oxidizing
ability of haloruthenocenium cations decreases with increasing number
of methyl groups.14 A substoichiometric amount of [Cp2RuX]PF6 was
added to a solution of the metallocene in CH2Cl2. The mixture was
stirred for 0.5 h, then the solvent was removed by rotary evaporation
and the reduced metallocenes were extracted from the powder using
hexanes. The oxidized product in the residue was then recrystallized
from CH3CN/Et2O.
Halide Substitution Equilibria. Equilibrium constants for the halide
substitution of haloruthenoceniums were obtained using procedures
described previously.6 CD3CN solutions of the haloruthenoceniums were
mixed with substoichiometric amounts (∼0.1 molar ratio) of (R4N)-
XY2 or (R4N)X3 (R ) Me, Et, n-Bu; X,Y ) Cl, Br, I) at 298 K. The
NMR spectrum and optical absorption of the mixture were monitored
periodically until no further change was detected (6-12 h, depending
on X and Y). The UV-vis absorption spectrum of the equilbrium
mixture was fit to a sum of reference spectra of the absorbing species
using a nonlinear least-squares fitting program18 to obtain the equilib-
rium concentrations of each species. Errors in calculated concentrations
range from 1% to 5% for reactions involving iodo and bromo species
and from 5% to 10% for reactions involving chloro species.19 The UV-
vis fits and equilibrium concentrations are provided in the Supporting
Information.
The atom-transfer oxidation method has advantages over the more
direct approaches when selective oxidation of a mixture of metallocenes
is required. For example, samples of tetramethylruthenocene were
(15) Shea, T. M. Ph.D. Dissertation, Wesleyan University, 1999.
(16) Martin, M. L.; Delpuech, J.-J.; Martin, G. J. Practical NMR Spec-
troscopy; Heyden and Son: Philadelphia, 1980; pp 293-310.
(10) See Supporting Information for the following. Bretschneider-Hurley,
A.; Winter, C. H. J. Am. Chem. Soc. 1994, 116, 6468-6469.
(11) Chattaway, F. D.; Hoyle, G. J. Chem. Soc. 1923, 123, 654-662.
(12) (a) Popov, A. I.; Geske, D. H. J. Am. Chem. Soc. 1958, 80, 1340-
1352. (b) Popov, A. I.; Geske, D. H. J. Am. Chem. Soc. 1958, 80,
5346-5349. (c) Iwamoto, R. T.; Nelson, I. V. J. Electroanal. Chem.
1964, 7, 218-221.
(17) (a) Jeener, J.; Meier, B. H.; Bachman, P.; Ernst, R. R. J. Chem. Phys.
1979, 71, 4546-4553. (b) Macura, S.; Ernst, R. R. Mol. Phys. 1980,
41, 95-117. (c) Mendz, G. L.; Robinson, G.; Kuchel, P. W. J. Am.
Chem. Soc. 1986, 108, 169-173. (d) Abel, E. W.; Coston, T. P. J.;
Orrell, K. G.; Sik, V.; Stephenson, D. J. Magn. Reson. 1986, 70, 34-
53. (e) Perrin, C. L.; Dwyer, T. J. Chem. ReV. 1990, 90, 935-967.
(18) Press, W. H.; Teukolsky, S. A.; Vetterling, W. T.; Flannery, B. P.
Program MRQMIN. In Numerical Recipes in Fortran, 2nd ed.;
Cambridge University Press: Cambridge, U.K., 1992; pp 678-683.
(19) The increased error for the fits involving the chloro species is due to
the difficulty of obtaining the spectrum of an analytically pure sample
of (R4N)Cl3 with no evidence of decomposition products.
(13) (a) Gleiter, R.; Hyla-Kryspin, I.; Ziegler, M. L.; Sergeson, G.; Green,
J. C.; Stahl, L.; Ernst, R. D. Organometallics 1989, 8, 298-306. (b)
Bosch, H. W.; Hund, H.-U.; Nietlispach, D.; Salzer, A. Organome-
tallics 1992, 11, 2087-2098.
(14) Xu, A. N.; Westmoreland, T. D. Manuscript in preparation.

Substituted Ruthenocenes
Inorganic Chemistry, Vol. 39, No. 7, 2000 1575
Table 1: Self-Exchange Rates and Free Energies of Activation for
(MemCp)(MenCp)Ru/[(MemCp)(MenCp)RuX]+ Couples in CD3CN at
298 K
constant for eq 2 must be obtained from a thermodynamic cycle
as given in the scheme below.
a
m,n
X
kex (M-1 s-1
16.5 ( 0.1
1950 ( 87
)
∆Gq (kJ mol-1
)
0,0
0,0
0,0
1,1
1,1
1,1
0,4
0,4
0,5
0,5
0,5
1,5
1,5
1,5
Cl
Br
I
Cl
Br
I
Cl
Br
Cl
Br
I
Cl
Br
I
66.1 ( 0.2
54.2 ( 0.9
36.9 ( 0.2 d
75.1 ( 0.1
60.3 ( 0.1
(2.1 ( 0.2) × 106 b,c
0.42 ( 0.02
166.1 ( 3.6
(3.3 ( 0.7) × 104 b
<0.01f
47.1 ( 0.3 d,e
10 ( 4
67.2 ( 0.8
<0.01f
As described previously,6 from the spectrophotometrically
estimated equilibrium constants for reaction 3b and the known
trihalide reduction potentials,12,20 the equilibrium constants for
reaction 2 can be determined. The results are given in Table 2.
6.55 ( 0.32
841 ( 18
68.3 ( 0.1
56.3 ( 0.1
<0.01f
4.37 ( 0.28
550 ( 50
69.3 ( 0.2
57.3 ( 0.2
Discussion
a Estimated using the Eyring equation except as noted. b Determined
by NMR line broadening. c From ref 4b. d Estimated from the enthalpy
and entropy of activation. e ∆Hq ) 30.5 ( 0.5 kJ mol-1; ∆Sq ) -48
For all the ruthenocenes, the self-exchange rates exhibit a
strong dependence on the nature of the transferred group. As
shown in Table 1, for fixed (m + n), the rate constants decrease
in the order I > Br > Cl. This ordering is well established for
the parent ruthenocene and osmocene systems3-6 where it has
been interpreted as reflecting the influence of both the differ-
ences in M-X bond strengths and the magnitude of halide-
mediated electronic coupling between the metals in the transition
state.6
( 7 kJ mol-1 K-1 f Too slow for EXSY.
.
Results
Self-Exchange Rates. Self-exchange rates determined either
by line broadening or EXSY techniques for the series of
compounds are presented in Table 1. In the EXSY spectra, cross-
peaks between the resonances of the reduced and oxidized forms
provided evidence for the exchange reaction. For species
containing multiple proton resonance frequencies (e.g., hexa-
methylruthenocene), identical rate constants were obtained
regardless of which pairs of exchanging resonances were
analyzed.
From Table 1 it is also clear that the self-exchange rates for
constant X decrease monotonically with increasing (m + n).
The trend is due to the electronic effects of methylation on the
reaction coordinate energetics and/or the steric effects of
methylation on the formation of the bridged transition-state
structure. The steric effects may, in turn, arise in two distinct
ways. Large substituents on the Cp rings might inhibit formation
of the halide-bridged intermediate. Such intermolecular steric
effects would raise the activation barrier and lead to a slower
rate of exchange, as is observed. At lower levels of methylation,
it would be possible to minimize these effects by arranging the
substitutents away from the intermolecular space as the transition
state forms. For high levels of methylation (e.g., decamethyl-
ruthenocene) no alternative conformations are possible and direct
steric effects might become important. Alternatively, steric
interactions between the two Cp rings in the bent oxidized
metallocenium may influence the exchange rate. At higher
degrees of methylation, steric crowding may inhibit the reor-
ganization of the metallocene to the bent metallocenium
structure. This additional intramolecular steric contribution to
the activation energy would decrease the exchange rate.
Figure 1 presents a plot of ln(kex) vs (m + n) for the data in
Table 1. The plots are essentially linear over the limited range
of data available. If steric effects were dominating the differ-
ences in exchange rates, it would be expected that the plots
would show marked deviations from linearity and that the rates
would decrease more quickly than linearly, particularly at higher
values of (m + n). The data in Figure 1 suggest, however, that
steric effects are not important contributors to the differences
in exchange kinetics, at least up to six methyl groups.
Although analytically pure samples of tetramethylruthenocene
and hexamethylruthenocene were not obtained, the self-exchange
rate constants could still be determined via the EXSY technique.
For example, a solution containing roughly equal concentrations
of (Me4Cp)CpRu and [(Me4Cp)CpRuBr]+ was obtained by
titrating [Cp2RuBr]+ into a mixture of tetramethylruthenocene,
octamethylruthenocene, and ruthenocene. The EXSY spectrum
of this mixture, obtained at a mixing time of 3.0 s, showed cross-
peaks between the tetramethyl resonances only. Any peaks due
to cross-reactions with the other ruthenocene species present
were indistinguishable from the spectral noise, and the rate
constants reported in Table 1 are based on an analysis that
included only the tetramethylruthenocene/tetramethylrutheno-
cenium-based diagonal and cross-peaks. An analysis was also
performed that included diagonals for all metallocene species
present and that used the spectral noise level as an upper limit
for the unobserved cross-peak intensities. For both analyses,
the calculated self-exchange rate constants for the species of
interest were identical within the error of the experiment.
Halide Substitution Thermodynamics. As has been shown
previously for the parent [Cp2RuX]+ species,6 the relative
reaction coordinate energetics for the self-exchange reactions
can be constructed from the free energies of activation and the
equilibrium constants for the generalized halide substitution
reaction
Steric effects do, however, appear to be important for more
highly methylated species. Extrapolation of the data in the figure
predicts that the rate of the [Cp*2RuI]+/Cp*2Ru self-exchange
should be approximately 1 M-1 s-1, a rate easily accessible by
EXSY. No evidence for cross-peaks could be obtained from
[(MemCp)(MenCp)RuX]+ + Y- f
[(MemCp)(MenCp)RuY]+ + X- (2)
The halometalloceniums are relatively good oxidants, however,
and the direct reaction of halometalloceniums with halides gives
ruthenocene and mixed trihalide anions.6 Thus, the equilibrium
(20) Popov, A. I.; Swenson, R. F. J. Am. Chem. Soc. 1955, 77, 3724-
3726.

1576 Inorganic Chemistry, Vol. 39, No. 7, 2000
Shea and Westmoreland
Table 2. Equilibrium Constants for Eq 3b and Calculated Differences in Free Energies of Formation for Haloruthenoceniums at 298 K
+
-
∆G[(Me
∆∆G°(X,I)
c
Cp)(Me Cp)RuX] ,I
m
n
m,n a
X,Y
K([(Me
(kJ mol-1
)
(kJ mol-1
)
b
+
-
)
Cp)(Me Cp)RuX] /I
m
n
3
0,0
0,0
1,1
1,1
0,5
0,5
5,5
5,5
Br,I
Cl,I
Br,I
Cl,I
Br,I
Cl,I
Br,I
Cl,I
18 ( 6
8 ( 10
25 ( 5
49 ( 6
27 ( 4
55 ( 7
30 ( 5
46 ( 9
30 ( 7
54 ( 7
-63 ( 8
-103 ( 8
-65 ( 4
-109 ( 7
-68 ( 6
-100 ( 9
-68 ( 8
-108 ( 8
3.0 ( 0.4
12 ( 10
0.05 ( 0.01
925 ( 5
0.09 ( 0.02
13 ( 1
a For the formulation [(MemCp)(MenCp)RuX]+. b Calculated ∆G for eq 2. c Calculated ∆Gformation of [Cp2RuX]+ relative to that of [Cp2RuI]+.
two-electron/halide transfer via a halide-bridged transition state.
Furthermore, the negative slope of the plot suggests that the
transition states exhibit a decrease in net bond order relative to
the reactant state because as the number of methyl groups is
increased, the ruthenocene becomes more nucleophilic. If bond
formation were the dominant process, the increased nucleophi-
licity of the ruthenocene would lead to rate increases with
increasing methylation. The decrease of rate with increasing
nucleophilicity of the ruthenocene suggests that the new Ru-X
bond is formed largely after the transition state and that the net
Figure 1. Plot of ln(kex) vs total number of methyl groups (m + n)
for self-exchange reactions in Table 1: X ) Cl (9), Br (b), I (2). The
lines represent linear least-squares fits of the data to ln(kex) ) M(m +
n) + B. The best fit values of M and B are the following: (X ) Cl)
-1.46 ( 0.22, 2.53 ( 0.81; (X ) Br) -1.01 ( 0.11, 7.39 ( 0.46; (X
) I) -1.37 ( 0.18, 13.97 ( 0.72.
Ru-X bond order in the transition state has decreased. This
interpretation is supported by the thermodynamic studies
discussed below.
The thermodynamic results in Table 2 show that for constant
(m + n) the calculated relative free energy of formation of each
metallocenium, ∆∆G°(X,I), becomes more negative in the order
I > Br > Cl. Furthermore, for constant X, the ∆∆G°(X,I) values
remain essentially constant within the error of the experiment
and are thus independent of the number of methyl substituents.
This result suggests a simple additive effect of each methyl
group on the overall energetics. That is, addition of any number
of methyl groups shifts the free energies of the X ) Cl, Br,
and I metalloceniums by approximately the same amount. This
lack of change in ∆∆G°(X,I) as the number of methyl groups is
increased is consistent with the notion that methyl groups exert
a relatively straightforward electronic effect on the energetics
of the system.
the EXSY spectra of the system, however, even for mixing times
as long as 20 s. Thus, at higher levels of methylation, the rates
must decrease more quickly than extrapolation of the data in
the figure would imply. The most likely source of such a
deviation is the onset of significant steric effects inhibiting the
formation of the halide-bridged species in the decamethyl
systems.
Intramolecular steric effects are not likely to be important
for these reactions, however. If significant intramolecular Cp*-
Cp* steric interactions are present, there should be significant
differences in the energies of the eclipsed and staggered
conformations of [Cp2*RuX]+ with staggered being favored.
This would also imply a significant barrier to Cp* rotation and
the inequivalence of the methyl groups on the NMR time scale.
For all complexes at all temperatures, we have observed NMR
spectroscopic features consistent with free rotation of each Cp*
group about the Cp*-M axis and no NMR inequivalencies are
evident. Thus, even if there are energetically preferred confor-
mations, interconversion is very fast on the time scale of the
atom-transfer reaction and cannot play a significant role in the
observed kinetics. It is also of interest to note that X-ray
crystallographic data indicate that the Cp(centroid)-Ru-Cp-
(centroid) angle in the [Cp2RuBr]+ cation is 36.7°.21 The
corresponding angle in [Cp*2RuBr]+ is 30.7°, and the closest
approach of two methyl hydrogen atoms on different Cp* rings
is 2.76 Å.22 The significantly smaller angle in the decameth-
ylruthenocenium is unexpected if intramolecular steric crowding
were important.
These results may be interpreted in terms of a model in which
methylation of the cyclopentadienyl rings essentially changes
only the effective electronegativity of the ruthenium and thus
the Ru-X bond strength. As the degree of methylation increases,
the electronegativity of the metal decreases, the difference in
electronegativity, øX - øRu, increases, and the Ru-X bond
strength increases. Since the kinetic results indicate that net
Ru-X bond breaking is a feature of the transition state, this
interpretation would predict that increasing methylation would
decrease the exchange rate, which is consistent with the
experimental observations.
From the data in Tables 1 and 2, it is possible to construct
reaction coordinate diagrams for each self-exchange, as has been
previously demonstrated for the Cp2Ru/[Cp2RuX]+ and Cp2-
Os/[Cp2OsX]+ systems.6 Specific energetic parameters for the
systems for which complete data are available can be calculated
from the parameters in Tables 1 and 2 and are given in the
Supporting Information. It is noteworthy that for a constant
degree of methyl substitution, the relative energetics of the three
halide transfer reactions are remarkably constant. In all cases,
the transition states are closer in energy than the corresponding
reactant states, providing further support for the conclusion that
the transition state involves net bond breaking.
Figure 1 is essentially a “Hammett-type” plot, and the similar
slopes of the data for all three halides may be interpreted as
reflecting similar mechanisms in each case, presumably coupled
(21) Sohn, Y. S.; Schlueter, A. W.; Hendrickson, D. N.; Gray, H. B. Inorg.
Chem. 1974, 13, 301-304.
(22) Deraniyagala, S. P.; Westmoreland, T. D. Organometallics 1995, 14,
1239-1241.

Substituted Ruthenocenes
Inorganic Chemistry, Vol. 39, No. 7, 2000 1577
Supporting Information Available: Tables of NMR and optical
absorbance data for the new compounds, reaction coordinate parameters
and diagram, integrated EXSY peak intensities, and a table and figures
of the optical absorbance data for equilibrium mixtures corresponding
to reaction 3b. This material is available free of charge via the Internet
at http://pubs.acs.org.
Concluding Remarks
The thermodynamic and kinetic results given above indicate
that in both the reactant states and the transition states, the effects
of up to six methyl groups are electronic and quantitatively
conform to a Hammett relationship. Steric effects play no
significant role in the overall reaction. These results suggest
that all the self-exchanges proceed by a similar mechanism, i.e.,
through a linear Ru-X-Ru bridged transition state.
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