The slowest water exchange at a homoleptic mononuclear metal center: Variable-temperature and variable-pressure 17O NMR study on [Ir(H2O)6]3+

Article abstract of DOI:10.1021/ja954071n

The rate constants and activation parameters for water exchange on hexaaqua and monohydroxy pentaaqua iridium(III) have been determined by ¹⁷O NMR spectroscopy as a function of temperature (358-406 K) and pressure (0.1-210 MPa) at several acidities (0.5-5.0 m). Noncoordinating trifluoromethanesulfonate (CF₃SO₃^-) was used as the counterion. The observed rate constant was of the form k = k₁ + k₂/[H⁺], where the subscripts 1 and 2 refer to the exchange pathways on [Ir(H₂O)₆]³⁺ and [Ir(H₂O)₅(OH)]²⁺, respectively. The kinetic parameters obtained are summarized as follows: k₁²⁹⁸ = (1.1 ± 0.1) × 10^-10 s^-1, ΔH₁^? = 130.5 ± 0.6 kJ mol^-1, ΔS₁? = +2.1 ± 1.7 J K^-1 mol^-1, and ΔV₁^? = -5.7 ± 0.5 cm³ mol^-1; k₂²⁹⁸ = (1.4 ± 0.6) × 10^-11 m s^-1, ΔH₂^? = 138.5 ± 4.5 kJ mol^-1, ΔS₂^? = +11.5 ± 11.6 J K^-1 mol^-1, and ΔV₂^? = -0.2 ± 0.8 cm³ mol^-1. The value obtained for k₁²⁹⁸ corresponds to a residence time of ca. 300 years. The pK_a²⁹⁸ and the volume change ΔV_a⁰ associated with the first hydrolysis of [Ir(H₂O)₆]³⁺ were determined by potentiometric and high-pressure spectrophotometric methods to be 4.45 ± 0.03 and -1.5 ± 0.3 cm³ mol^-1, respectively. Utilizing the relation k₂ = k_OHK_al, values for the first-order rate constant and the corresponding activation volume for [Ir(H₂O)₅(OH)]²⁺ were estimated to be k_OH²⁹⁸ = 5.6 × 10^-7 s^-1 and ΔV_OH^? = +1.3 cm³ mol^-1, respectively. These data are supportive of an associative interchange (I_a) mechanism for water exchange on [Ir(H₂O)₆]³⁺, but of an interchange (I) mechanism on the deprotonated species [Ir(H₂O)₅(OH)]²⁺. These mechanistic results have also been compared to those reported for other trivalent metal ions.

Full text of DOI:10.1021/ja954071n

J. Am. Chem. Soc. 1996, 118, 5265-5271
5265
The Slowest Water Exchange at a Homoleptic Mononuclear
Metal Center: Variable-Temperature and Variable-Pressure
17O NMR Study on [Ir(H2O)6]3+
Antonio Cusanelli, Urban Frey, David T. Richens,# and Andre´ E. Merbach*
Contribution from the Institut de Chimie Mine´rale et Analytique, UniVersite´ de
Lausanne-Baˆtiment de Chimie (BCH), CH-1015 Lausanne-Dorigny, Switzerland
ReceiVed December 4, 1995X
Abstract: The rate constants and activation parameters for water exchange on hexaaqua and monohydroxy pentaaqua
iridium(III) have been determined by 17O NMR spectroscopy as a function of temperature (358-406 K) and pressure
(0.1-210 MPa) at several acidities (0.5-5.0 m). Noncoordinating trifluoromethanesulfonate (CF3SO3-) was used
as the counterion. The observed rate constant was of the form k ) k1 + k2/[H+], where the subscripts 1 and 2 refer
to the exchange pathways on [Ir(H2O)6]3+ and [Ir(H2O)5(OH)]2+, respectively. The kinetic parameters obtained are
summarized as follows: k1298 ) (1.1 ( 0.1) × 10-10 s-1, ∆H1q ) 130.5 ( 0.6 kJ mol-1, ∆S1q ) +2.1 ( 1.7 J K-1
q
mol-1, and ∆V1q ) -5.7 ( 0.5 cm3 mol-1; k2298 ) (1.4 ( 0.6) × 10-11 m s-1, ∆H2q ) 138.5 ( 4.5 kJ mol-1, ∆S2
q
298
) +11.5 ( 11.6 J K-1 mol-1, and ∆V2 ) -0.2 ( 0.8 cm3 mol-1. The value obtained for k1 corresponds to a
298
0
residence time of ca. 300 years. The pKa and the volume change ∆Va associated with the first hydrolysis of
[Ir(H2O)6]3+ were determined by potentiometric and high-pressure spectrophotometric methods to be 4.45 ( 0.03
and -1.5 ( 0.3 cm3 mol-1, respectively. Utilizing the relation k2 ) kOHKa1, values for the first-order rate constant
298
and the corresponding activation volume for [Ir(H2O)5(OH)]2+ were estimated to be kOH ) 5.6 × 10-7 s-1 and
∆VOHq ) +1.3 cm3 mol-1, respectively. These data are supportive of an associative interchange (Ia) mechanism for
water exchange on [Ir(H2O)6]3+, but of an interchange (I) mechanism on the deprotonated species [Ir(H2O)5(OH)]2+
These mechanistic results have also been compared to those reported for other trivalent metal ions.
.
Introduction
Ever since the pioneering work of Eigen and Wilkins,3-5
which showed that, for aqueous solutions, the rates and
activation parameters for ligand substitution were closely similar
to those of solvent exchange, the study of the latter process has
assumed fundamental importance. Accordingly, interest in the
water exchange process has blossomed over the years resulting
in one of the largest compilations of information about ligand
substitution to date, encompassing an 18 order variation in the
magnitude of the water exchange rate constant (Figure 1).
Among the earliest reported examples of solvent exchange
kinetics was the work carried out by Hunt and Taube6,7 on the
hexaaqua chromium(III) ion [Cr(H2O)6]3+. Their efforts es-
tablished that this ion is capable of exchanging its bound water
with solvent, although the rate of this exchange was extremely
slow. Subsequently, the hexaaqua ion of rhodium was shown
to be even more inert, and kinetic parameters and solvent
exchange mechanisms for both acidic chromium(III) and
rhodium(III) have now been proposed.8-11 Interestingly, despite
much effort little is currently known about the kinetics of water
Figure 1. Mean lifetimes of a particular water molecule in the first
coordination sphere of a given metal ion, τH O, and the corresponding
2
water exchange rate constants, kH O. The tall bars indicate directly
2
determined values whereas the short bars represent values deduced from
complex formation studies.
exchange for the third-row ion, namely the hexaaqua complex
of iridium(III). This lack of kinetic information for [Ir(H2O)6]3+
has been ascribed to its high degree of inertness toward ligand
substitution, necessitating the need for extreme temperatures to
promote solvent exchange, leading in turn to potential engineer-
ing or chemical problems (i.e., [Ir(H2O)6]3+ has been shown to
be susceptible to oxidation and possibly oligomerization at high
temperature).12-14 Using specifically designed high-temperature
and high-pressure equipment, developed in-house, we have been
* Author to whom correspondence should be addressed.
# Permanent address: Department of Chemistry, The Purdie Building,
University of St. Andrews, Scotland KY16 9ST.
X Abstract published in AdVance ACS Abstracts, May 15, 1996.
(1) High Pressure NMR Kinetics. Part 71. For part 70 see ref 2.
(2) To`th, E.; Vauthey, S.; Pubanz, D.; Merbach, A. E. Inorg. Chem. In
press.
(3) Eigen, M. Z. Elektrochem. 1960, 64, 115.
(4) Eigen, M. Pure Appl. Chem. 1963, 6, 97.
(5) Eigen, M.; Wilkins, R. G. AdV. Chem. Ser. 1965, No. 49, 55.
(6) Hunt, J. P.; Taube, H. J. Chem. Phys. 1950, 18, 757.
(7) Hunt, J. P.; Taube, H. J. Chem. Phys. 1951, 19, 602.
(8) Stranks, D. R.; Swaddle, T. W. J. Am. Chem. Soc. 1971, 93, 2783.
(9) Xu, F.-C.; Krouse, H. R.; Swaddle, T. W. Inorg. Chem. 1985, 24,
267.
(11) Laurenczy, G.; Rapaport, I.; Zbinden, D.; Merbach, A. E. Magn.
Reson. Chem. 1991, 29, S45.
(12) Castillo-Blum, S. E.; Richens, D. T.; Sykes, A. G. J. Chem. Soc.,
Chem. Commun. 1986, 1120.
(13) Castillo-Blum, S. E.; Sykes, A. G.; Gamsja¨ger, H. Polyhedron 1987,
6, 101.
(10) Plumb, W.; Harris, G. M. Inorg. Chem. 1964, 3, 542.
S0002-7863(95)04071-6 CCC: $12.00 © 1996 American Chemical Society

5266 J. Am. Chem. Soc., Vol. 118, No. 22, 1996
Cusanelli et al.
measurements. For all the NMR kinetic measurements the ionic
strength was maintained constant at 5.1 m with NaCF3SO3. Oxygen-
17 NMR spectra were recorded on a Bruker AM 400 instrument
equipped with a wide-bore cryomagnet (9.4 T) at an operating frequency
of 54.2 MHz. 17O NMR chemical shifts are reported with respect to
an internal reference of neat H2O (solvent).
able to solve these problems, and hence we now report the first
definitive quantitative study of the water exchange process on
iridium(III). Specifically, the rate constants and activation
parameters for water exchange on hexaaqua and monohydroxy
pentaaqua iridium(III) have been determined by 17O NMR
spectroscopy as a function of temperature (358-406 K) and
pressure (0.1-210 MPa) at several acidities (0.5-5.0 m) and a
plausible mechanism for the exchange process proposed.
Notably, the rate constant obtained for water exchange on
hexaaqua iridium(III) (1.1 × 10-10 s-1; residence time ≈ 300
years) corresponds to the slowest reported water exchange at a
homoleptic mononuclear metal center to date, and thus a further
expansion of Figure 1.4
Variable-temperature (ambient pressure) kinetic measurements were
conducted at four temperatures. At 406.3 K, the kinetics were followed
in a commercial broad-band probe thermostated with a Bruker B-VT
2000 unit, and the temperature was found to be constant to within (0.2
K as measured by a substitution technique using a platinum resistor.16
At this temperature the sample was housed in a sapphire NMR tube
and a Teflon plug was inserted into the tube to ensure that the refluxing
water solution remained in the lower 3 cm of the tube, between the
receiver coils of the NMR probe. At 392.6, 372.8, and 358.2 K the
water exchange kinetics are extremely slow and thus the samples, which
were contained in sapphire NMR tubes, were stored in a thermostated
bath and transferred at measured intervals into the NMR probe. The
spectra were then recorded at ambient temperature since the water
exchange is effectively quenched at this temperature.
Experimental Section
General Methods and Syntheses. All manipulations were per-
formed under nitrogen or argon by using standard Schlenk or vacuum
line techniques unless stated otherwise. Solutions of hexaaqua iridium-
(III) were prepared by slight modification to the literature method.15
Bi-distilled water was used as the solvent for the preparative procedures,
as well as the variable-temperature and pressure NMR work. Trifluo-
romethanesulfonic acid (Ventron) was distilled at reduced pressure.
Sodium hexachloroiridate(IV) (Johnson Matthey), 70% perchloric acid
(Merck), sodium perchlorate (Aldrich), sodium trifluoromethane-
sulfonate (Aldrich), ascorbic acid (Aldrich), sodium hydroxide (Merck),
17O-enriched water (3 atom %) (Yeda, Israel), and Dowex 50W-X2
cation exchange resin (Fluka) were used as received. All parameters
Variable-pressure kinetic measurements obtained at 358.2 K were
conducted at 0.1 (ambient pressure), 70, 140, and 210 MPa. For these
very slow high-pressure water exchanges, the samples, contained in
deformable cylindrical thin-walled Teflon cells (8.5-mm outside
diameter and 33-mm length), were kept in stainless-steel pressure
vessels which were in turn immersed in a thermostated bath. For the
NMR measurements, the pressure was released at fixed intervals, the
Teflon cells placed into conventional 10-mm Pyrex NMR tubes, and
the spectra recorded. After the NMR measurements, the samples were
repressurized and returned to the bath. The time of this operation was
short compared to the half-life of the water exchange. Each kinetic
run consisted of 12-15 spectra and was monitored for ca. 3 half-lives.
The temperature of the thermostated bath, in all cases, was measured
by a platinum resistor and was found to be constant to within (0.2 K.
reported were calculated with the data points weighted as (Ymeasured
Y
-
calculated)2/(Ymeasured)2 and the reported errors are one standard deviation
(1σ) unless stated otherwise. The iridium(III) solutions were prepared
as molalities (m) for the variable-temperature and pressure NMR work
and as molarities (M) for the acid dissociation measurements.
-
-
Preparation of [Ir(H2O)6]X3 (X ) CF3SO3 (1) or ClO4 (2)).
Na2[IrCl6]‚6H2O (2.0 g, 3.6 mmol) was treated with a deoxygenated
solution of 0.2 m sodium hydroxide (1000 mL) at 313 K for ca. 5 h.
After 2 h, ascorbic acid (0.2 g) was added to the stirred iridium solution
to prevent atmospheric oxidation. Adjustment of the solution to pH 6
by dropwise addition of CF3SO3H or HClO4 (2.0 m) then gave a
precipitate of iridium(III) hydroxide hydrate [Ir(H2O)3(OH)3]. The
solution was then concentrated by rotary evaporation and the precipitate
washed with bi-distilled water (2 × 100 mL). The precipitate was
dissolved in CF3SO3H or HClO4 (0.1 m) and the resulting solution was
loaded onto a Dowex 50W-X2 cation exchange column (10 cm × 1
cm). Elution with CF3SO3H or HClO4 (2.0 m) afforded 1 in 43% yield
(1.1 g, 1.5 mmol) or 2 in 34% yield (0.7 g, 1.2 mmol) as checked by
UV/visible absorption spectroscopy.12
Potentiometric and Spectrophotometric Measurements. The
potentiometric experiments were carried out using a Metrohm Titrino
DMS 716 titration instrument equipped with a Metrohm combined glass
electrode, calibrated (pH ) -log[H+]) by titration of CF3SO3H with
carbonate free NaOH (µ ) 5.0 M NaCF3SO3). For all the acid
dissociation measurements the ionic strength was maintained constant
at 5.0 M with NaCF3SO3. The reproducibility of the pH measurements
was (0.003. Titrations with an NaOH solution ([OH-] ) 1.251 ×
10-2 M; µ ) 5.0 M NaCF3SO3) were conducted in a cylindrical-shaped
vessel, thermostated at 298.0 ( 0.1 K, on a solution which was 1.51
× 10-3 M in Ir(III) and 8.007 × 10-4 M in CF3SO3H (µ ) 5.0 M
NaCF3SO3, d ) 1.39 g cm-3). The acidified solution of [Ir(H2O)6]3+
was titrated up to a mole fraction of [Ir(H2O)5(OH)]2+ not exceeding
0.6. Under these conditions no precipitation occurred.
Preparation of [Ir(H217O)6](CF3SO3)3. A solution of [Ir(H2O)6](CF3-
SO3)3 was prepared by dissolving [Ir(H2O)3(OH)3] (2.0 g) in a 1.0 m
solution of CF3SO3H in 17O-enriched water (3 atom %) and then placed
in a sapphire NMR tube (see below for a detailed description of this
equipment) under an argon atmosphere. The sapphire tube was then
immersed in a silicone oil bath and the solution was heated at 413 K
for 24 h to allow complete enrichment of the iridium first coordination
sphere. This solution was then diluted with bi-distilled H2O (ca. 30
mL) and then loaded onto a Dowex 50W-X2 cation exchange column
(10 cm × 1 cm). Elution with 2.0 m CF3SO3H yielded the enriched
complex [Ir(H217O)6](CF3SO3)3 (1-17O) as checked by UV/visible
absorption spectroscopy. Stock solutions of [Ir(H2O)6]3+ were prepared
from the strongly acid eluate by reprecipitating [Ir(H2O)3(OH)3] with
NaOH at pH 6 and subsequently dissolving it in the appropriate amount
of CF3SO3H (0.5, 1.0, 1.5, 3.0, and 5.0 m). The solutions were stored
under an argon atmosphere at 255 K to prevent oxidation to oligomeric
products.
UV/visible spectra were obtained on acidified solutions of [Ir-
(H2O)6]3+ ([Ir3+] ) 6.04 × 10-3 M; µ ) 5.0 M NaCF3SO3) using a
Perkin Elmer Lambda 19 double beam spectrophotometer. A 1-cm
thermostated cell was used for the pH dependence measurements to
maintain a temperature of 298.0 ( 0.1 K. For the variable-pressure
experiments, a “Le Noble” piston-type cell (optical path length 1.920
cm) was used.17 The cell was immersed in the pressure transmitting
fluid (water) inside a double beam pressurizable and thermostatable
pressure bomb.17 The temperature was controlled by circulation of
liquid from a thermostat bath and the temperature was measured using
a Pt resistor placed in the body of the pressure bomb (298.0 ( 0.1 K).
The pH was measured before and after the spectrophotometric
measurements to (0.003 pH units with a Metrohm combined glass
electrode calibrated in the same manner described previously. The
potentiometric and spectrophotometric data were analyzed using the
program PSEQUAD.18
NMR Measurements. All the NMR samples were 0.03 m in Ir3+
.
Sapphire NMR Tubes. Sapphire NMR tubes, initially developed
Four concentrations of CF3SO3H (1.0, 1.5, 3.0, and 5.0 m) were used
for the variable-temperature kinetic studies, whereas three concentrations
of CF3SO3H (0.5, 1.0, 5.0 m) were used for the variable-pressure
(16) Ammann, C.; Meier, P.; Merbach, A. E. J. Magn. Reson. 1982, 46,
319.
(17) Richens, D. T.; Ducommun, Y.; Merbach, A. E. J. Am. Chem. Soc.
1987, 109, 603.
(18) Ze´kany, L.; Nagypal, I. In Computational Methods for Determination
of Formation Constants; Legatt, D. J., Ed.; Plenum Press: New York, 1985;
p 291.
(14) Castillo-Blum, S. E.; Richens, D. T.; Sykes, A. G. Inorg. Chem.
1989, 28, 954.
(15) Beutler, P.; Gamsja¨ger, H. J. Chem. Soc., Chem. Commun. 1976,
554.

Slowest H2O Exchange at a Homoleptic Mononuclear Metal
J. Am. Chem. Soc., Vol. 118, No. 22, 1996 5267
up to 80 MPa at which point the tube burst. Before the sapphire tubes
are used they are always tested to ensure that they can support a liquid
pressure of 20 MPa and a gas pressure (N2) of 15 MPa. For safety
reasons, the tubes are only used with pressures up to 10 MPa and are
frequently retested using the procedure described previously.
A typical experimental protocol for the preparation of a sapphire
NMR tube sample commences with the weighing of the solid and/or
liquid sample directly into the sapphire tube and then the valve body
and valve stem are screwed onto the tube mounting flange. Before
pressurizing, the closed tube is adjusted to the correct height in a
commercial ceramic spinner and then placed into a Plexiglas safety
shield. A schematic drawing of the safety shield (8) with the tube and
spinner (9) is shown in Figure 2B. The safety shield which was used
in the burst test up to 80 MPa acquired only minor scratches, thus
demonstrating that it is capable of providing maximum protection for
the user during pressurizing and transportation of the closed sapphire
tube. However, it is of utmost importance that the lower open part of
the safety shield be directed toward the floor.
For experiments where gases are used, the gas inlet port is connected
to a gas distribution system Via a 1/16-in. swagelock. The sapphire tube
is pressurized by turning the valve stem to the open position (1 turn).
Once the desired pressure is reached, the valve stem is turned to the
closed position and the tube is subsequently disconnected from the gas
distribution system. To use the sapphire tube under autogenic pressures,
the sample is simply introduced into the tube and the tube is then closed.
As no pressure detector is incorporated into the sapphire tube, it is
imperative that the phase diagram of the solvent (mixture) used be
carefully examined prior to experimentation to ensure that the allowed
pressure domain is not surpassed.
Results
Variable-Temperature Kinetics. The 17O enrichment of the
bound water in the hexaaqua ion [Ir(H2O)6]3+ was accomplished
by refluxing the iridium salt at 413 K for 24 h in an acidified
solution of 17O-enriched water. This enrichment step was
successful when the counterion was trifluoromethanesulfonate
(CF3SO3-). However, repeating this procedure with perchlorate
(ClO4-) as the counterion led instead to the formation of a purple
solution whose 17O NMR spectrum exhibited a single resonance
at -10 ppm not assignable to the hexaaqua (-151 ppm) or
Figure 2. (A) Schematic drawing of the sapphire tube and valve
assembly illustrating the Ti-alloy valve (1), valve body (2), valve stem
(3), Viton O-ring (4), Vespel or KelF seal (5), tube mounting flange
(6), and sapphire NMR tube (7). (B) Schematic drawing of the
Plexiglass safety cylinder (8) used during the pressurization and
transportation of the sapphire tube and the commercial spinner (9).
by Roe et al.19 and subsequently improved by Horva´th et al.,20 although
widely used today for gas/liquid systems, are not particularly well-
suited for solid systems because of their small valve opening which
makes the weighing of solids and even liquids extremely difficult. To
address this concern, we have redesigned the titanium valve to create
a much larger valve opening and thus facilitate the introduction of
material into the NMR tube as well as the cleaning of the tube.
Furthermore, the centrosymmetric design and the lighter weight of our
sapphire tube/valve system helps to dramatically minimize spinning
side bands. This new generation of sapphire NMR tubes is particularly
useful for obtaining NMR measurements at temperatures above the
boiling point of the NMR solvent (i.e., autogenic pressures) and can
also accommodate high pressures up to ca. 10 MPa.21
A schematic drawing of a 10-mm sapphire tube (Saphicon Inc.,
Milford, NH 03055) illustrating the newly designed valve is given in
Figure 2A. The valve (1) consists of the valve body (2), the valve
stem (3) with the Viton O-ring (4), the Vespel or KelF valve seal (5),
and the sapphire tube mounting flange (6) with a second Viton O-ring
(4). The sapphire tube (7) was affixed onto the mounting flange with
the composite glue Araldit AV138M/hardener HV998. All metallic
parts were constructed from titan grade 5 (6% Al, 4% V; Bibus Inco
Alloys, CH-8126 Zumikon). The combined weight of the sapphire tube
(10 mm outside diameter and 8 mm inside diameter) and the valve is
46 g, ensuring that once inserted into the NMR magnet the closed tube
can be removed and spun with a slightly increased lift and spinner
pressure.
-
ClO4 (290 ppm) oxygens. The UV/visible spectrum of this
colored solution displayed a broad absorption band with a peak
maximum at 547 nm. A 17O NMR spectrum of an isotopically
equilibrated 0.03 m aqueous solution of [Ir(H2O)6](CF3SO3)3
(3% oxygen-17) recorded at ambient temperature exhibited a
single broad resonance (1/T2 ) 1894 s-1 at 298 K) due to the
ligated water at -151 ppm. This assignment was verified by
acquiring a 17O NMR spectrum of a solution of the iridium(III)
trifluoromethanesulfonate salt at natural abundance (0.037 atom
%). The low frequency chemical shift obtained for this aqua
ion is comparable to the bound water shifts of other low-spin
6
diamagnetic t2g metal ions such as [Rh(H2O)6]3+ 11 or [Ru-
(H2O)6]2+ 22 which exhibited 17O NMR resonances at -130.5
,
and -196.3 ppm, respectively. Notably, no significant tem-
perature-dependence of the chemical shift was observed for [Ir-
(H2O)6](CF3SO3)3.
The water exchange on [Ir(H2O)6]3+ (eq 1) was followed by
[Ir(H217O)6]3+ + 6H2O h [Ir(H2O)6]3+ + 6H217O (1)
monitoring the depletion of the hexaaqua ion and the enrichment
of bulk water in oxygen-17 with time. The rate law for the
isotopic exchange illustrated in eq 1 can be expressed by eq
2,23
Two 10-mm sapphire tubes were tested and found to withstand liquid
(H2O) pressures up to 55 MPa at which point the tube came apart from
the mounting flange. A similar 5 mm outside diameter, 3.4 mm inside
diameter sapphire tube was tested and found to withstand gas pressures
(19) Roe, D. C. J. Magn. Reson. 1985, 63, 388.
(22) Rapaport, I.; Helm, L.; Merbach, A. E.; Bernhard, P.; Ludi, A. Inorg.
Chem. 1988, 27, 873.
(23) Helm, L.; Elding, L. I.; Merbach, A. E. Inorg. Chem. 1985, 24,
1719.
(20) Horva´th, I. T.; Ponce, E. C. ReV. Sci. Instrum. 1991, 62, 1104.
(21) Since dangerously high gas pressures are involved, safety precautions
must be taken when handling the sapphire NMR tubes.

5268 J. Am. Chem. Soc., Vol. 118, No. 22, 1996
-dx/dt ) k(x - x∞)/(1 - x∞)
Cusanelli et al.
(2)
where k represents the rate constant for the exchange of a
particular water molecule,24 and x and x∞ represent the mole
fractions of labeled water coordinated to the metal at the time
of sampling and at the exchange equilibrium, respectively. The
time dependence of the mole fraction x of ligated oxygen-17
water (Figure 3), obtained by integration of the resonances, was
fitted to eq 3 which was arrived at by integration of eq 2 using
x ) xo at t ) 0, and x∞ ≈ 6 × 0.03/55.5 ≈ 0.003.
x ) x∞ + (xo - x∞) exp[-kt/(1 - x∞)]
(3)
The adjustable parameters were k and xo.
As previously documented for other trivalent aqua ions,11,22
the observed water exchange rate constant k was found to be
acid dependent (see Figure 4A). This dependence can be
rationalized as being composed of two contributions according
to eq 4,
Figure 3. Mole fraction x of bound oxygen-17 water, as a function of
time, as monitored by 17O NMR spectroscopy for a solution of
[Ir(H2O)6](CF3SO3)3 (3% oxygen-17) in 1.0 m CF3SO3H at 358.2 K
and ambient pressure ([Ir3+] ) 0.03 m; µ ) 5.1 m NaCF3SO3).
k ) k1 + k2/[H+]
(4)
where k1 is the rate constant for water exchange on the hexaaqua
ion [Ir(H2O)6]3+ and k2 is the product (eq 5) of kOH, the rate
k2 ) kOHKa
(5)
constant for water exchange on the monohydroxy pentaaqua
ion [Ir(H2O)5(OH)]2+, and Ka, the first acid dissociation constant
for [Ir(H2O)6]3+ (eq 6).
[Ir(H2O)6]3+ h [Ir(H2O)5(OH)]2+ + H+
(6)
The treatment of kinetic data with respect to the rate constant
k2 has provided its share of controversy in the literature. Grant
et al.25 assumed that the hydroxo ligand does not contribute to
the exchange in the case of Fe(III) and thus applied a statistical
5
factor of /6 to the rate coefficients. Contrary to this idea, it
has also been suggested that a factor of 1/6 should be applied if
the hydroxo ligand specifically activates the water molecule
trans to it,24,26 although direct evidence for such an activation
may be difficult if not impossible to obtain given that the rate
of proton exchange with the bulk water is most certainly much
faster than the rate of the corresponding oxygen exchange. Given
these contradictory approaches, we have assumed that all six
oxygen atoms in the monohydroxy species contribute equally
to the k2 value. In any case, these considerations do not affect
the calculation of either the volumes or enthalpies of activation.27
The observed variable-temperature water exchange rate
constants, obtained at four acid concentrations and four tem-
peratures (Table 1), were least-squares fitted in a simultaneous
fashion to eqs 4 and 7, and in turn, lead to the rate constants
and activation parameters presented in Table 2.
Figure 4. (A) Acid dependence of the observed water exchange rate
constants k (s-1) for 0.03 m [Ir(H2O)6]3+ solutions (µ ) 5.1 m
NaCF3SO3) at 358.2 K and various pressures: (9) 0.1, (2) 70, (b)
140, and ([) 210 MPa. (B) Effect of pressure on the water exchange
rate constant for [Ir(H2O)6]3+ (O, k1) and for [Ir(H2O)5(OH)]2+ (0, k2)
at 358.2 K ([Ir3+] ) 0.03 m; µ ) 5.1 m NaCF3SO3).
Table 1. Water Exchange Rate Constants for [Ir(H2O)6](CF3SO3)3,
106k (s-1), as a Function of Temperature and Acidity at Ambient
Pressurea
[CF3SO3H] (m)
358.2 K
372.8 K
392.6 K
406.3 K
1.0
1.5
3.0
5.0
1.10 ( 0.01 6.42 ( 0.08 59.2 ( 0.8 237 ( 3
1.02 ( 0.01 6.11 ( 0.09 54.8 ( 0.4 217 ( 1
0.96 ( 0.01 5.85 ( 0.05 51.0 ( 0.3 199 ( 1
0.92 ( 0.01 5.64 ( 0.08 48.9 ( 0.3 191 ( 2
k ) (kBT/h) exp[(∆Sq/R) - (∆Hq/RT)]
(7)
Variable-Pressure Kinetics. The observed variable-pressure
water exchange rate constants were obtained at three acidities
and four pressures at 358.2 K (Table 3).
a Concentration of [Ir(H2O)6](CF3SO3)3 was 0.03 m and its initial
17O enrichment was 3% (µ ) 5.1 m NaCF3SO3).
A simultaneous least-squares fit of the pressure data to eqs 4
and 8 (Figure 4A), then gave the rate constants and activation
parameters presented in Table 2. The adjustable parameters
were the activation volumes ∆Viq and the rate constants at zero
pressure ki,0 (i ) 1, 2). The normalized natural logarithms of
the rate constants as a function of pressure for both the hexaaqua
ion [Ir(H2O)6]3+ and the monohydroxy pentaaqua species [Ir-
(H2O)5(OH)]2+, derived from the simultaneous least-squares fit
of the pressure data to eqs 4 and 8 (Figure 4A), are shown in
Figure 4B.
ln ki ) ln ki,0 - ∆ViqP/RT
(i ) 1, 2)
(8)
(24) Lincoln, S. F.; Merbach, A. E. AdV. Inorg. Chem. 1995, 42, 1.
(25) Grant, M.; Jordan, R. B. Inorg. Chem. 1981, 20, 55.
(26) Swaddle, T. W. AdV. Inorg. Bioinorg. Mech. 1983, 2, 95.
(27) Swaddle, T. W.; Merbach, A. E. Inorg. Chem. 1981, 20, 4212.

Slowest H2O Exchange at a Homoleptic Mononuclear Metal
J. Am. Chem. Soc., Vol. 118, No. 22, 1996 5269
Table 2. Rate Constants, Activation Parameters, and
Thermodynamic Parameters for Water Exchange on Hexaaqua and
Monohydroxy Pentaaqua Iridium(III)
species
parameter
value
[Ir(H2O)6]3+
k1298 (s-1
)
(1.1 ( 0.1) × 10-10
∆H1q (kJ mol-1
)
130.5 ( 0.6
∆S1q (J K-1 mol-1
)
+2.1 ( 1.7
∆V1q (cm3 mol-1
k1,0358 (s-1
k2298 (m s-1
∆H2q (kJ mol-1
∆S2q (J K-1 mol-1)
)
-5.7 ( 0.5
)
(8.5 ( 0.2) × 10-7
(1.4 ( 0.6) × 10-11
138.5 ( 4.5
[Ir(H2O)5(OH)]2+
)
)
+11.5 ( 11.6
-0.2 ( 0.8
∆V2q (cm3 mol-1
)
k2,0358 (m s-1
k2/k1
)
(2.4 ( 0.2) × 10-7
(1.2 ( 0.6) × 10-1
4.45 ( 0.03a
5.6 × 10-7
pKa298
kOH298 (s-1
)
∆Va0 (cm3 mol-1
pKa0
)
-1.5 ( 0.3
4.45 ( 0.01
∆VOHq (cm3 mol-1
)
+1.3
a Potentiometrically determined value.
Figure 5. Experimental and calculated spectrophotometric spectra for
[Ir(H2O)6]3+ and [Ir(H2O)5(OH)]2+ illustrating the absolute difference
between the experimentally determined curves and those calculated
using the potentiometrically determined pKa298 value of 4.45.
Table 3. Water Exchange Rate Constants for [Ir(H2O)6](CF3SO3)3,
106k (s-1), as a Function of Pressure and Acidity at 358.2 Ka
[CF3SO3H] (m) 0.1 MPa
70 MPa
140 MPa
210 MPa
Table 4. Pressure Dependence of Absorbance (A) and
Corresponding pKa for [Ir(H2O)6](CF3SO3)3
0.5
1.0
5.0
1.34 ( 0.02 1.46 ( 0.02 1.64 ( 0.02 1.72 ( 0.03
1.07 ( 0.01 1.24 ( 0.03 1.38 ( 0.02 1.54 ( 0.02
0.90 ( 0.01 1.01 ( 0.02 1.22 ( 0.02 1.29 ( 0.01
a
pressure (MPa)
A
pKa
a Concentration of [Ir(H2O)6](CF3SO3)3 was 0.03 m and its initial
17O enrichment was 3% (µ ) 5.1 m NaCF3SO3).
0.1
40
70
100
140
170
200
0.521
0.523
0.523
0.524
0.525
0.526
0.530
4.45
4.44
4.44
4.43
4.42
4.42
4.39
Thermodynamics for the Acid Dissociation of [Ir-
(H2O)6]3+. To calculate the kinetically relevant values for water
exchange on the monohydroxy species, the thermodynamic
parameters associated with the acid dissociation of the hexaaqua
ion were first determined. The equilibrium constant (Ka) for
the acid dissociation reaction of [Ir(H2O)6]3+, represented in
eq 6, was determined potentiometrically as well as spectropho-
tometrically.
a Obtained from
[Ir(H2O)6](CF3SO3)3 at 266 nm and at 298.0 K (µ ) 5.0 M NaCF3SO3).
a
6.04
×
10-3
M
aqueous solution of
298.0 K were conducted on a 6.04 × 10-3 M Ir(III) solution of
pH 4.174. The spectra were taken at 100 and 200 MPa with
increasing pressure and at 170, 140, 70, and 40 MPa with
decreasing pressure. The pH of the solution, measured before
and after the variable-pressure experiments, was unchanged. The
absorbance changes with pressure, followed at 266 nm, were
then converted to pKa values (Table 4) using eq 9 where b is
the optical path length (1.920 cm) and ꢀIr3+ and ꢀIrOH2+ are the
The titration data obtained from three independent potentio-
metric trials was utilized simultaneously to give a pKa298 value
of 4.45 ( 0.03 (Table 2) which is in good agreement with the
value of 4.37 measured by Gamsja¨ger et al. in 1.05 m
NaClO4.28,29 The spectrophotometric data, which were acquired
298
over a pH range of 0.979-4.368 (Figure 5), lead to a pKa
value of 4.29 ( 0.05. This estimate is in agreement, within
error (assuming 2σ for the errors), to the potentiometrically
determined value and thus further corroborates the validity of
our results. This agreement is substantiated in Figure 5 which
illustrates schematically the experimental spectrophotometric
spectra and the absolute difference between these measurements
and the calculated curves derived from the potentiometric pKa298
value of 4.45. Included in this figure are the calculated limiting
spectra for [Ir(H2O)6]3+ (peak maxima λ/nm (ꢀ/M-1 cm-1) at
262 (36.6) and 311 (30.8)) and for [Ir(H2O)5(OH)]2+ (peak
maxima λ/nm (ꢀ/M-1 cm-1) at 266 (60.7) and 314 (43.5)). The
experimentally determined rate constant k2, when combined with
the first hydrolysis constant Ka (eq 5), yielded an estimate of
5.6 × 10-7 s-1 for kOH, the rate constant for water exchange on
the monohydroxy pentaaqua ion [Ir(H2O)5(OH)]2+ (Table 2).
The volume change ∆Va0 accompanying the acid dissociation
reaction in eq 6 was determined by observing the effect of
A ) b[Ir(III)][(ꢀIr3+) + (ꢀIrOH2+)(10pH-pK )]/(1 + 10
)
pH-pKa
a
(9)
molar absorptivities (M-1 cm-1) of the two species in equilib-
rium (i.e., [Ir(H2O)6]3+ and [Ir(H2O)5(OH)]2+, respectively). The
pKa values obtained in this manner were then fitted to eq 10,
0
0
leading to a ∆Va value of -1.5 ( 0.3 cm3 mol-1 and a pKa
value of 4.45 ( 0.01 (Table 2).
pKa ) pKa0 + ∆Va0P/2.303RT
(10)
The difference between the experimental volume of activation
∆V2 and the volume change ∆Va associated with the first
hydrolysis of [Ir(H2O)6]3+ (eq 11) gave an estimate of +1.3
q
0
∆VOHq ) ∆V2q - ∆Va0
(11)
cm3 mol-1 for ∆VOHq, the volume of activation for water
exchange on the monohydroxy pentaaqua species [Ir(H2O)5-
(OH)]2+ (Table 2).
pressure on the optical absorbance A due to [Ir(H2O)5(OH)]2+
.
A series of variable-pressure experiments up to 200 MPa at
(28) Gamsja¨ger, H.; Beutler, P. J. Chem. Soc., Dalton Trans. 1979, 1415.
(29) When only the first dissociation constant for [Ir(H2O)6]3+ was
considered, a good fit with the acquired potentiometric data was obtained.
However, we were unable to fit the potentiometric data to a second
dissociation constant for the hexaaqua ion.
Discussion
Synthesis and Characterization of [Ir(H2O)6](CF3SO3)3.
The perchlorate salt of hexaaqua iridium(III) was first character-

5270 J. Am. Chem. Soc., Vol. 118, No. 22, 1996
Cusanelli et al.
Table 5. Rate Constants and Activation Parameters for Water Exchange on Hexaaqua and Monohydroxy Pentaaqua Trivalent Metal Ions
(Including Thermodynamic Parameters for the Acid Dissociation of the Hexaaqua Ions)
species
[M(H2O)6]3+
parameter
Ga(III)a
Ti(III)b
Fe(III)c
Cr(III)d
Ru(III)e
68
Rh(III)f
66.5
Ir(III)g
ionic radius (pm)h
62
67
64
61
68
k1298 (s-1
)
4.0 × 102 1.8 × 105 1.6 × 102
2.4 × 10-6 3.5 × 10-6 2.2 × 10-9 (1.1 ( 0.1) × 10-10
∆H1q (kJ mol-1
)
67.1
43.4
64.6
108.6
+11.6
-9.6
Ia
89.8
-48.2
-8.3
Ia
131.2
+29.3
-4.1
Ia
130.5 ( 0.6
+2.1 ( 1.7
-5.7 ( 0.5
Ia
∆S1q (J K-1 mol-1
)
+30.1
+5.0
Id
+1.2
-12.1
A, Ia
+12.1
-5.4
Ia
∆V1q (cm3 mol-1
)
mechanism
[M(H2O)5(OH)]2+ k2298 (m s-1
)
1.4 × 101
+7.8
3.9
11.4 × 104
+7.8
2.9
1.1 × 10-6 1.5 × 10-8 (1.4 ( 0.6) × 10-11
∆V2q (cm3 mol-1
)
-1.1
4.1
-2.1
2.7
+1.2
3.5
-0.2 ( 0.8
4.45 ( 0.03
pKa298
kOH298 (s-1
kOH/k1
)
1.1 × 105
1.2 × 105
1.8 × 10-4 5.9 × 10-4 4.2 × 10-5 5.6 × 10-7
275
750
75
170
-3.0
+0.9
I
19100
-0.2
+1.5
I
5100
∆Va0 (cm3 mol-1
)
+1.5
+6.2
Id
+0.8
+7.0
Id
-3.8
+2.7
I
-1.5 ( 0.3
∆VOHq (cm3 mol-1
)
+1.3
I
mechanism
a Reference 43. b Reference 42. c References 25 and 27. d Reference 9. e Reference 22. f Reference 11. g This work. h Reference 44.
-
ized by Beutler and Gamsja¨ger in 197615 and its coordination
number in solution was also determined by these same workers
using an 18O-tracer technique.30 Subsequently, the hexaaqua
structure of this complex was verified by a crystal structure
obtained on the corresponding caesium sulfate alum CsIr(SO4)2‚-
12H2O.31 Despite the fact that almost 20 years have passed
since the initial discovery of [Ir(H2O)6]3+, to date few, if any,
kinetic investigations of successfully monitored anation reactions
have been reported for this species. Attempts at measuring the
water exchange in perchloric acid solutions by an oxygen-18
isotopic dilution method in sealed tubes at 393 K were impaired
by oxidation yielding purple solutions believed to contain
iridium(IV).13 Furthermore, anation of [Ir(H2O)6]3+ (2.3 × 10-3
M) by either Cl- or Br- ions (3.7 M) was not detected despite
the fact the reaction was monitored at 313 K for over 15 days.13
In retrospect, this result is not surprising since based on our
results the half-life for water exchange at this temperature (313
K) is ca. 15 years. Similar studies with thiocyanate were
impaired by the competing acid decomposition of thiocyanate
at the elevated temperatures and prolonged reaction times
required.13 This lack of information sponsored the belief that
hexaaqua iridium(III) was the most inert metal hexaaqua ion
yet obtained and that successful kinetic studies of this complex,
including the fundamental process of water exchange, may prove
difficult, if not impossible.13
The key to solving this problem came during our attempts to
enrich the bound water in the hexaaqua ion [Ir(H2O)6](ClO4)3.
Refluxing the iridium salt at 413 K for 24 h in a perchloric
acid solution of 17O-enriched water gave an unwanted purple
solution. The spectroscopic data obtained for this solution (17O
NMR (H2O): -10 ppm (singlet); UV/visible: peak maximum
λ (nm) 547 in 1.0 m H+) are comparable to those obtained for
a purple iridium(IV) solution which has been assigned to a
dimeric oxygen-bridged species resulting from electrochemical
oxidation of [Ir(H2O)6]3+ in perchloric acid solution.12,14
Notably, repeating our enrichment step using the trifluoro-
methanesulfonate salt, [Ir(H2O)6](CF3SO3)3, gave only the
desired 17O-enriched product, without the formation of the
colored solution. This result indicates that the purple iridium-
(IV) solution mentioned previously is most likely formed as a
result of reactions with the perchlorate counterion at these high
temperatures.32 Furthermore, a 17O NMR spectrum obtained
of a 5.0 m solution of trifluoromethanesulfonic acid in 3% 17O-
enriched water, after it was refluxed at 423 K for 147 h, showed
no measurable exchange between the CF3SO3 oxygen atoms
and the bulk water oxygen atoms. Taken together, these results
strongly suggested that [Ir(H2O)6](CF3SO3)3 was an ideal
candidate for a successful water exchange kinetic study and also
provided a qualitative estimate of the upper limit for the
-
exchange of CF3SO3 with H2O.
Water Exchange on [Ir(H2O)6]3+. The inertness of the
iridium(III) solvate prevented us from studying its kinetics by
the classical NMR line-broadening technique.33 Instead, the
isotopic labeling method was used, employing 17O NMR
spectroscopy to follow the solvent exchange. Accurate rate
constants were obtained from fits of resonance integrations to
first-order equations; thus, no assumptions about relaxation
mechanisms had to be made, as is often the case with line-
broadening techniques. Notably, the obvious advantage of NMR
spectroscopy over mass spectrometry for the study of slow
exchange reactions is that the former technique can be used to
discern different kinetic processes as well as the formation of
possible side products without the need for previous chemical
transformations.
Our directly determined value of the water exchange rate
constant for [Ir(H2O)6]3+ (Table 5) is in keeping with estimated
upper limits for the rate constant of Cl- anation (<10-9 M-1
s-1) on this same species.13 Previous substitution studies on
6
the t2g analogs [Co(H2O)6]3+ 34 and [Rh(H2O)6]3+ 35 have
indicated an increase in inertness in going from Co3+ to Rh3+
and then to Ir3+ which is in keeping with the effect of increasing
ligand-field stabilization.36 A similar overall trend has been
observed for H2O exchange of the aquapentaammine complexes
[Co(NH3)5(H2O)]3+ 37,38
(H2O)]3+ 40
,
[Rh(NH3)5(H2O)]3+ 38,39
, and [Ir(NH3)5-
.
The negative volume (-5.7 cm3 mol-1) and near zero entropy
(+2.1 cm3 mol-1) of activation obtained for [Ir(H2O)6]3+ (Table
5) is supportive of an associative pathway for the water exchange
process on this species. However, to differentiate between an
associative interchange Ia and a limiting associative A mecha-
nism, the magnitude of ∆Vq must be considered. Its value is
less negative than that calculated by Swaddle et al. for first-
row metal ions for an associative A mechanism (-13.9 cm3
(33) (a) Merbach, A. E. Pure Appl. Chem. 1982, 54, 1479. (b) Merbach,
A. E. Pure Appl. Chem. 1987, 59, 161.
(34) Conocchioli, T. J.; Nancollas, G. H.; Sutin, N. Inorg. Chem. 1965,
5, 1.
(35) Swaminathan, K.; Harris, G. M. J. Am. Chem. Soc. 1966, 88, 4411.
(36) Basolo, F.; Pearson, R. G. In Mechanisms of Inorganic Reactions;
Wiley: New York, 1967; p 145.
(37) Hunt, H. R.; Taube, H. J. Am. Chem. Soc. 1958, 80, 2642.
(38) Gonzalez, G.; Moullet, B.; Martinez, M.; Merbach, A. E. Inorg.
Chem. 1994, 33, 2330.
(30) Beutler, P.; Gamsja¨ger, H.; Baertschi, P. Chimia 1978, 5, 163.
(31) Armstrong, R. S.; Beattie, J. K.; Best, S. P.; Skelton, B. W.; White,
A. H. J. Chem. Soc., Dalton Trans. 1983, 1973.
(32) Notably, UV/visible spectroscopy may be used to obtain an estimate
of the fraction of Ir(III) oxidized by ClO4-, although such a study was not
performed in this work.
(39) Swaddle, T. W.; Stranks, D. R. J. Am. Chem. Soc. 1972, 94, 8357.
(40) Tong, S. B.; Swaddle, T. W. Inorg. Chem. 1974, 13, 1538.

Slowest H2O Exchange at a Homoleptic Mononuclear Metal
mol-1 41
and, more importantly, significantly less negative than
J. Am. Chem. Soc., Vol. 118, No. 22, 1996 5271
)
donating capability of HO- which is responsible for strengthen-
ing the bond between it and the metal center while concomi-
tantly weakening the remaining metal-water bonds. Thus the
ligated water molecules become more labile which, in turn,
favors a dissociative activation process. This mechanistic
change is most pronounced for Fe(III) where a changeover from
Ia for [Fe(H2O)6]3+ to Id for [Fe(H2O)5(OH)]2+ was reported
(Table 5). This mechanistic change toward dissociation is less
pronounced in [Ir(H2O)5(OH)]2+ as evidenced by the near zero
value for the volume of activation for this species. Small
the most negative experimental value (Ti3+, -12.1 cm3 mol-1).42
Thus, the water exchange mechanism on the hexaaqua iridium-
(III) species is best represented as an associative interchange
mechanism Ia. A similar mechanism has been assigned to the
trivalent metal hexaaqua ions Fe3+, Cr3+, Ru3+, and Rh3+ (Table
5). In contrast, water exchange on the hexaaqua gallium(III)
ion [Ga(H2O)6]3+ has been suggested to proceed by a dissocia-
tive interchange Id process where filled orbitals, size of the metal
ion, and electrostatics are believed to determine the pattern of
behavior observed.43
q
positive values of ∆VOH were obtained for the corresponding
ruthenium and rhodium monohydroxy pentaaqua species as well
(Table 5). These results indicate that both bond making and
bond breaking contribute equally to the transition state (within
experimental error) and are therefore supportive of an inter-
It is clear from the literature covering the kinetics of
homoleptic hexaaqua ions that properties such as charge, ionic
radius, and electron distribution in the valence orbitals are
important contributors to the mechanism of water exchange on
the respective aqua species.24 Therefore, for a meaningful
discussion we will focus our comparisons of [Ir(H2O)6]3+ to
ruthenium(III) and its second-row analog rhodium(III), espe-
cially in light of the fact that other third-row trivalent hexaco-
change I mechanism for water exchange on [Ir(H2O)5(OH)]2+
.
The extrapolated water exchange rate constant at 298 K for
[Ir(H2O)6]3+ is ca. 20 times slower than that for [Rh(H2O)6]3+
(Table 5). This trend is even more pronounced for the
corresponding monohydroxy species where the rate constant for
water exchange at 298 K for [Ir(H2O)5(OH)]2+ is ca. 75 times
slower than that for [Rh(H2O)5(OH)]2+ (Table 5). Furthermore,
in reality this pathway is actually closer to 3 orders of magnitude
slower in acidic media due to a further 1 order of magnitude
difference in the respective pKa values (Table 5). The poorer
labilizing effect demonstrated by the HO- ligand in [Ir(H2O)5-
(OH)]2+ may be ascribed to the larger electronic population on
Ir3+, which may suppress effective σ-donation from the bound
OH-. The high energy of the σ-accepting eg orbitals, resulting
from a larger ligand field splitting energy for the Ir3+ ion, may
also minimize the capability of the HO- ligand to labilize the
ordinated aqua ions have yet to be characterized.
6 44
Regardless that the ionic radii for Ir3+ (68 ppm, t2g
)
and
5 44
Ru3+ (68 ppm, t2g
)
are identical, the latter species exhibits
greater associative character than the former (Table 5). This
may be attributable to the larger electronic population on Ir3+
which disfavors the approach of a seventh water molecule at
the transition state, diminishing its associative character com-
pared to that of Ru3+. Notably, Ir3+ exhibits greater associative
character than its valence isoelectronic ruthenium partner Ru2+
(73 ppm, t2g6),22 despite the larger ionic radius of the latter ion.
This result is attributed to the decrease in charge on going from
iridium to ruthenium which will disfavor nucleophilic attack
and lead to a weakening of the metal water bond. This latter
comparison suggests that the charge of a hexaaqua ion may be
a larger contributor to the mechanism of water exchange than
the ionic radii of the respective metal ion. The kinetic data
remaining bound water molecules on [Ir(H2O)5(OH)]2+
.
Conclusion
This work provides the first definitive quantitative evidence
for water exchange on iridium(III). The rate constants and
activation parameters for water exchange on hexaaqua and
monohydroxy pentaaqua iridium(III) have been determined by
17O NMR spectroscopy as a function of temperature (358-406
K) and pressure (0.1-210 MPa) at several acidities (0.5-5.0
m). It is important to note that in all cases noncoordinating
trifluoromethanesulfonate (CF3SO3-) was used as the counter-
ion. The water exchange rate constant for [Ir(H2O)6]3+ was
calculated to be 1.1 × 10-10 s-1 (residence time of ca. 300
years), corresponding to the slowest documented water exchange
at a homoleptic mononuclear metal center to date. The negative
volume and near zero entropy of activation for the water
exchange on [Ir(H2O)6]3+ are supportive of an associative
interchange Ia mechanism.
also indicates that Ir3+ behaves in a more associative manner
6
than its analog Rh3+ (66.5 pm, t2g
)
44 despite the larger overall
electronic population on Ir3+. This behavior is ascribed to the
larger ionic radius for Ir3+, which is not completely compensated
for by the lanthanide contraction, and thus facilitates the
approach of a water molecule at the transition state.
Water Exchange on [Ir(H2O)5(OH)]2+. The hydrolysis in
water of trivalent hexaaqua ions of the form [M(H2O)6]3+ is a
kinetically important process since it results in the production
of a significant amount of the conjugate base [M(H2O)5(OH)]2+
(eq 6). The presence of this species can then lead to an
alternative pathway for water exchange.
The water exchange rate constant obtained for the monohy-
droxy pentaaqua ion of iridium [Ir(H2O)5(OH)]2+ (5.6 × 10-7
s-1) is considerably larger than that measured for the corre-
sponding parent ion [Ir(H2O)6]3+ (1.1 × 10-10 s-1), and more
importantly, this increase in the rate constant is also ac-
companied by an increase in dissociative character toward a
more positive volume of activation. Similar behavior has been
reported for all the monohydroxy pentaaqua metal ions (Table
5). This drastic mechanistic difference between both exchange
pathways is most plausibly ascribed to the strong electron-
Estimations of the rate constant and activation volume for
water exchange on [Ir(H2O)5(OH)]2+ indicate that this species
is much more reactive and has more dissociative character than
the hexaaqua ion, as is the case for other pairs of trivalent
hexaaqua and monohydroxy pentaaqua ions.
Acknowledgment. We thank the Swiss National Science
Foundation for a visiting position for D. T. R. and for financial
support. The authors are also indebted to Dr. G. Laurenczy
for helpful discussions along with Mr. R. Ith for technical
assistance.
(41) Swaddle, T. W. Inorg. Chem. 1983, 22, 2663.
(42) Hugi, A. D.; Helm, L.; Merbach, A. E. Inorg. Chem. 1987, 26, 1763.
(43) Hugi-Cleary, D.; Helm, L.; Merbach, A. E. J. Am. Chem. Soc. 1987,
109, 4444.
(44) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751.
JA954071N