The water-soluble iridium complexes
trans-Ir(CO)L2X (X = Cl, OH; L =
PPh2(m-C6H4SO3K) (TPPMS),
P(m-C6H4SO3Na)3
(TPPTS)) have been used to extend the range of solvent
effect for H2 activation at an iridium(I) center and
to examine the effect of pH changes on
the rate of H2 addition to an organometallic center.
Hydrogen adds to the square-planar
iridium(I) complexes in H2O and DMSO to give products
analogous to those for trans-Ir(CO)(Cl)(PPh3)2 in organic solvents.
The kinetic deuterium isotope effect,
k
H/k
D = 1.1 (L
=
TPPMS) and 1.2 (L = TPPTS), for addition of H2 to
trans-Ir(CO)(Cl)L2 in water is
the same
as that observed for
trans-Ir(CO)(Cl)(PPh3)2
in toluene and indicates a common mechanism.
More polar solvents accelerate the rate of H2 addition
to trans-Ir(CO)(Cl)L2,
indicating
polarity in the transition state. Increasing the pH causes a
decrease in the rate of H2 addition
for trans-Ir(CO)(Cl)(TPPMS)2 and
trans-Ir(CO)(Cl)(TPPTS)2 in
water. This pH effect, coupled
with a shift of νCO to higher frequency in water, is
ascribed to protonation/hydrogen bonding
to the iridium center. The hydride ligands undergo H/D exchange
with D2O in a process
that is quite dependent on the trans ligand.
Iridium complexes containing the TPPMS ligand (TPPMS =
PPh2(m-C6H4SO3K))
have
been prepared and studied in DMSO and H2O as a comparison
to the PPh3 analogues in
toluene. Measurements of the pH of the complexes show that most
are almost neutral, but
Ir(CO)(H)(TPPMS)3 is basic (pH = 10.7).
The basicity of Ir(CO)(H)(TPPMS)3 is
confirmed
by its reaction with H2O to give
Ir(CO)(H)2(TPPMS)3
+.
In aqueous solution, reaction of trans-Ir(CO)(OH)(TPPMS)2 with H2 produces
only
fac-Ir(CO)(H)3(TPPMS)2
not the usual mixture
of facial and meridional isomers. This is attributed to a
hydrogen-bonding interaction
between the two cis-TPPMS ligands in H2O.
Reaction of
trans-Ir(CO)(Cl)(TPPMS)2
with
CO in water produces
[Ir(CO)2(TPPMS)2
+]Cl-
and
[Ir(CO)3(TPPMS)2
+]Cl
sequentially in a
reaction that is pH dependent. Reaction of
trans-Ir(CO)(OH)(TPPMS)2 with CO
results in
products based on the water gas shift-reaction. These complexes of
TPPMS are spectroscopically very similar to the PPh3 analogues, but display
significantly different reactions in H2O.
A significant enhancement in the rate of oxidative addition of H2 to square-planar iridium(r1) complexes in water is observed; kinetic studies of the addition of H2 to trans-[Ir(CO)Cl(PPh3)2] in toluene and of H2 to truns-[Ir-(CO)Cl(PPh2(C6H4S03K-rn))2] in water show a factor of 45 increase in rate constant in water; this solvent effect is shown to be general for the solvents toluene, chlorobenzene, N,Ndimethylformamide, dimethyl sulfoxide and water, and has important ramifications for catalysis in water.
Over the last several decades the communications industry has developed small, high-light transmitting fiber-optic materials. Because of their inherently non-perturbing nature fiber optics have found wide spread use in many diverse areas of analytical spectroscopy. Of all the spectroscopic techniques, fiber-optic-based fluorescence has probably enjoyed the greatest popularity. Unfortunately, while successful analyses have been achieved via fiber-optic probes, only a fraction (that is, spectral information) of the total information content from the fluorescence process has been utilized. In this paper, recent results from our laboratory are presented for the successful implementation of the remaining dynamic and steady-state fluorimetric measurements in conjunction with fiber-optic sensing.
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