Iridium porphyrins in CD3OD: Reduction of Ir(III), CD 3-OD bond cleavage, Ir-D acid dissociation and alkene reactions

Article abstract of DOI:10.1021/ic400240b

Methanol solutions of iridium(III) tetra(p-sulfonatophenyl)porphyrin [(TSPP)Ir^III] form an equilibrium distribution of methanol and methoxide complexes ([(TSPP)Ir^III(CD₃OD) _(2-n)(OCD₃)_n]

Full text of DOI:10.1021/ic400240b

Article
pubs.acs.org/IC
Iridium Porphyrins in CD3OD: Reduction of Ir(III), CD3−OD Bond
Cleavage, Ir−D Acid Dissociation and Alkene Reactions
Salome Bhagan, Gregory H. Imler, and Bradford B. Wayland*
Temple University, Department of Chemistry, 130 Beury Hall, 1901 North 13th Street, Philadelphia, Pennsylvania 19122, United
States
S
* Supporting Information
ABSTRACT: Methanol solutions of iridium(III) tetra(p-
sulfonatophenyl)porphyrin [(TSPP)IrIII] form an equilibrium distri-
bution of methanol and methoxide complexes ([(TSPP)-
IrIII(CD3OD)(2−n)(OCD3)n](3+n)−). Reaction of [(TSPP)IrIII with
dihydrogen (D2) in methanol produces an iridium hydride [(TSPP)-
IrIII−D(CD3OD)]4− in equilibrium with an iridium(I) complex
([(TSPP)IrI(CD3OD)]5−). The acid dissociation constant of the
iridium hydride (Ir−D) in methanol at 298 K is 3.5 × 10−12. The
iridium(I) complex ([(TSPP)IrI(CD3OD)]5−) catalyzes reaction of
[(TSPP)IrIII−D(CD3OD)]4− with CD3−OD to produce an iridium
methyl complex [(TSPP)IrIII−CD3(CD3OD)]4− and D2O. Reactions
of the iridium hydride with ethene and propene produce iridium alkyl
complexes, but the Ir−D complex fails to give observable addition with
acetaldehyde and carbon monoxide in methanol. Reaction of the
iridium hydride with propene forms both the isopropyl and propyl complexes with free energy changes (ΔG° 298 K) of −1.3 and
−0.4 kcal mol−1 respectively. Equilibrium thermodynamics and reactivity studies are used in discussing relative Ir−D, Ir−OCD3
and Ir−CD2- bond energetics in methanol.
adduct[(TSPP)Ir−D(CO)]4− without observation of an
INTRODUCTION
■
iridium formyl species (Ir−CHO). Results from equilibrium
Application of water and methanol as both solvent media and
thermodynamic and reactivity studies are used in discussing
reactants is an important direction for organometallic catalysis
research.1−27 Current interest in group nine metalloporphyrin
reactivity and catalysis studies in this area has been focused on
the behavior of complexes with sulfonated porphyrin ligands in
water5,6,8,18−23,28 and methanol.16,28 Water and methanol are
high dielectric constant liquids29,30 that have both hydrogen
bonding and donor capability, which results in many analogous
features. Methanol is distinguished by having both hydroxylic
and hydrocarbon components which result in the unusual
property of miscibility in both water and organic hydrocarbon
solvents.
Rhodium porphyrins have a more highly developed reaction
chemistry in diverse media16,18−23,31−60 compared to iridium
porphyrins.18,61−74 This Article reports on reactivity and
thermodynamic studies for methanol solutions of tetra(p-
sulfonatophenyl)porphyrin iridium(III) methanol and meth-
oxide complexes ([(TSPP)IrIII(CD3OD)2−n(OCD3)n](3+n)−).
The Ir(III) complex reacts with H2(D2) to form an iridium
hydride complex ([(TSPP)IrIII−D(CD3OD)]4−) that has an
Ir−D acid dissociation constant of 3.5 × 10−12 in methanol.
The Ir−D complex reacts with CD3OD in basic methanol by a
net CD3−OD bond cleavage to form an iridium methyl
complex ([(TSPP)IrIII−CD3(CD3OD)]4−) and D2O. Alkenes
react with [(TSPP)IrIII−D(CD3OD)]4− to form iridium alkyl
complexes, but CO reacts to form only a carbon monoxide
relative Ir−D, Ir−OCD3 and Ir−CD2- bond energetics in
methanol.
RESULTS AND DISCUSSION
■
Methanol and Methoxide Complexes of (TSPP)IrIII in
CD3OD. Dissolution of iridium(III) tetra(p-sulfonatophenyl)-
porphyrin (Na3[(TSPP)IrIII(CD3OD)2]·18CD3OD) in
CD3OD results in a pH dependent equilibrium distribution
of a bis-methanol complex [(TSPP)Ir(CD3OD)2]3− (1) with
mono- and bis-methoxide complexes [(TSPP)Ir(OCH3)-
(CD3OD)]4− (2) and [(TSPP)Ir(OCD3)2]5− (3) (eqs 1 and
2) (Figure 1). The axially coordinated methanol and methoxide
ligands for 1, 2 and 3 in methanol rapidly exchange protons
with the bulk methanol (298 K) (Figure 1), which results in a
single mole fraction averaged pyrrole 1H NMR resonance for 1,
2 and 3 (Figures 2 and 3).
[(TSPP)Ir(CD3OD)2]3−
⇌ [(TSPP)Ir(OCD3)(CD3OD)]4− + D+
(1)
Received: January 30, 2013
Published: March 29, 2013
© 2013 American Chemical Society
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Figure 1. Methanol and methoxide complexes of (TSPP)IrIII in
methanol.
Figure 3. Observed limiting fast proton exchange mole fraction
averaged pyrrole 1H NMR shifts for (TSPP)IrIII complexes 1, 2 and 3
in CD3OD as a function of pD. Triangular points are experimentally
observed pyrrole shift values in methanol, and the line is calculated for
[(TSPP)Ir(CD3OD)]4− ⇌ [(TSPP)Ir(OCD3)2]5− + D+
(2)
The 1H NMR of the bis-methanol complex 1 and bis-
methoxide complex 3 can be directly obtained at limiting high
and low hydrogen ion concentrations respectively. The mole
K1(298K) = 2.3 × 10−8 and K2(298K) = 2.1 × 10−11
.
1
fraction averaged pyrrole H NMR resonances for equilibrium
ppm (Figure 3). The coordinated methanol acid dissociation
constants for [(TSPP)Ir(CD3OD)2]3− at 298 K (K1 = 2.3 ×
10−8, K2 = 2.1 × 10−11) in methanol (Table 1) are similar to the
values for the coordinated water acid dissociation constants in
D2O at 298 K for [(TSPP)Ir(D2O)2]3− (K1(D2O) = 4.8 ×
10−8, K2(D2O) = 2.6 × 10−11).8
Reaction of Hydrogen (H2/D2) with [(TSPP)-
IrIII(CD3OD)2]3− in CD3OD. Methanol solutions of [(TSPP)-
IrIII(CD3OD)2]3− at pD lower than 10 react with hydrogen
(P(H2/D2) ∼ 0.4−0.9 atm) in CD3OD to form a hydride
(deuteride) complex [(TSPP)Ir−D(CD3OD)]4− (4) (eq 3,
Figure 4). Acidic solutions of the iridium hydride complex
[(TSPP)Ir−D(CD3OD)]4− in a mixture of CH3OH/CD3OD
distributions of 1, 2 and 3 as a function of [D+] were used in
determining the acid dissociation constants for coordinated
methanol by nonlinear least-squares curve fitting to the
expression
δ
2,3,4(obs)(pyr) = (K1K2δ3(pyr) + K1[D+]δ2(pyr)
+ [D+]2δ1(pyr))
/(K1K2 + K1[D+] + [D+]2)
giving K1(298K) = 2.3 × 10−8, K2(298K) = 2.1 × 10−11 and
δ2(pyr) = 8.61 ppm; 1 [(TSPP)IrIII(CD3OD)2]3− is δ1(pyr) =
8.76 ppm, and 3 [(TSPP)IrIII(OCD3)2]5− is δ3(pyr) = 8.43
Figure 2. Mole fraction averaged 1H HMR (500 MHz) porphyrin pyrrole singlet and phenyl AB pattern for complexes 1, 2 and 3 in CD3OD (T =
298 K): (A) pD = 4.0, (B) pD = 8.5 and (C) pD = 11.5.
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Proton exchange between 4 and 5 at 298 K in methanol
solution is sufficiently slow to permit observation of separate
although slightly broadened porphyrin pyrrole hydrogen
resonances for 4 and 5.
Table 1. Equilibrium Thermodynamic Values for [(TSPP)Ir]
Reactions in Methanol (T = 298 K)
a
(TSPP)Ir reactions
K
ΔG°
[(TSPP)IrIII(CD3OD)2]3− ⇌ [(TSPP)IrIII(OCD3) 2.3 × 10−8
+10.4
(CD3OD)]4− + D+
[(TSPP)Ir−D(CD3OD)]4−
[(TSPP)IrIII((OCD3)(CD3OD)]4− ⇌ [(TSPP)
2.1 × 10−11
+14.6
−0.8
−11.2
+15.6
−2.5
−1.3
−0.4
IrIII((OCD3)2]5− + D+
⇌ [(TSPP)IrI(CD3OD)]5− + D+
(5)
[(TSPP)IrIII(CD3OD)2]3− + D2 ⇌ [(TSPP)Ir−
4.5
D(CD3OD)]4− + D+ + CD3OD
1
Evaluation of the H NMR integrated intensities for the
[(TSPP)Ir(OCD3)(CD3OD)]4− + D2 ⇌ [(TSPP) 1.9 × 108
pyrrole hydrogens of 4 and 5 along with the deuterium ion
concentration were used in determining the acid dissociation
constant for the deuteride complex ([(TSPP)Ir−D-
Ir−D(CD3OD)]4− + CD3OD
[(TSPP)Ir−D(CD3OD)]4− ⇌ [(TSPP)
3.5 × 10−12
IrI(CD3OD)]5− + D+
(CD3OD)]4−) in methanol (K5(298K) = 3.5(0.2) × 10−12
;
[(TSPP)Ir−D(CD3OD)]4− + C2H4 ⇌ [(TSPP)Ir− 63
ΔG°(298K) = 15.6(0.1) kcal mol−1) (Table 1). The acid
dissociation constant for 4 in methanol (K5(298K) = 3.5 ×
CH2CH2D(CD3OD)]4−
[(TSPP)Ir−D(CD3OD)]4− + C3H6 ⇌ [(TSPP)Ir− 9.2
CH(CH2D)CH3(CD3OD)]4−
10−12) is slightly larger than the value for [(TMPS)Ir−
[(TSPP)Ir−D(CD3OD)]4− + C3H6 ⇌ [(TSPP)Ir− 2.0
72
D(D2O)]8− in water (K(298K) = 1.8 × 10−12
)
and much
CH2CHDCH3(CD3OD)]4−
smaller than the acid dissociation constants for an analogous
rhodium complex [(TSPP)Rh−D(CD3OD)]4− in methanol
(K(298K) = 1.1 × 10−9; ΔG°(298K) = 12.2 kcal mol−1)7 and
a
−
CD3OD ⇌ D+ + OCD3 ; K = 2.0 × 10−17; ΔG0 = 22.8 kcal mol−1.
[(TSPP)Rh−D(D2O)]4− in water (K(298K) = 8.0 × 10−8
ΔG°(298K) = 9.7 kcal mol−1) (Figure 5).22
;
were used to observe the high field iridium hydride resonance
at −59.1 ppm.
The iridium(I) complex ([(TSPP)IrI(CD3OD)]5− (5))
reacts as a nucleophile with methyl iodide (CH3I) in methanol
to form the methyl complex ([(TSPP)Ir−CH3(CD3OD)]4−
(6)), which is readily identified in solution by the characteristic
high field methyl 1H NMR resonance (δ = −7.15 ppm)
(Supporting Information). Addition of triphenyl phosphite to
the methanol solution of the methyl complex (6) produces the
phosphite complex, which is readily detected in methanol
solution by 31P−CH3 coupling (δ = −7.37 ppm, 15 Hz)
between the trans phosphite and methyl groups in [(TSPP)Ir−
CH3(P(OC6H5)3)]4− (Supporting Information).
[(TSPP)IrIII(CD3OD)2]3− + D2
⇌ [(TSPP)Ir−D(CD3OD)]4− + CD3OD + D+
(3)
Reaction 3 was permitted to equilibrate for a period of two
weeks at 298 K before determination of the equilibrium
1
constant by integration of the H NMR for 2 and 4 and
measurement of the solution pH (K3(298K) = 4.5(0.5); ΔG°3
= −0.8(0.1) kcal mol−1) (Table 1). The equilibrium constant
for the reaction of dihydrogen with [(TSPP)Ir(OCD3)-
(CD3OD)]4− (2) (eq 4) is too large for direct evaluation by
1H NMR, but the free energy change for reaction 4 can be
derived from the measured ΔG° values for reactions 1 and 3
(Scheme 1, ΔG4° = ΔG3° − ΔG1°; ΔG4°(298K) = −11.2 kcal
mol−1).
Reduction of Ir(III) to Ir(I) by Methanol. Methanol
solutions of [(TSPP)IrIII(OCD3)2]5− (3) (pD > 11) are
kinetically stable for days at 298 K, but upon warming to 343 K
the (TSPP)IrIII is quickly reduced to [(TSPP)IrI(CD3OD)]5−
(5) and methanol is oxidized to formaldehyde (Scheme 2).
Iridium(III) porphyrins are thus only metastable reagents for
substrate reactions in strongly basic methanol. Oxidation of
methanol becomes catalytic through quantitative dioxygen
oxidation of (TSPP)IrI to (TSPP)IrIII (Scheme 2), and
formaldehyde oxidizes further under these conditions to
formate. The methanol oxidation to formaldehyde is currently
viewed as occurring by an intermolecular β C−H deprotona-
tion of methoxide coordinated to Ir(III) in the basic high
[(TSPP)Ir(OCD3)(CD3OD)]4− + D2
⇌ [(TSPP)Ir−D(CD3OD)]4− + CD3OD
(4)
Acid Dissociation Constant of [(TSPP)Ir−D(CD3OD)]4−
in Methanol. The iridium hydride complex [(TSPP)Ir−
D(CD3OD)]4− (4) in methanol occurs in a 1H NMR
observable equilibrium with the iridium(I) derivative [(TSPP)-
IrI(CD3OD)]5− (5) (eq 5) in the pD range of 10.5−11.5.
1
Figure 4. H NMR (500 MHz, CD3OD,pD = 4.8). Equilibrium among [(TSPP)IrIII(CD3OD)2]3−, H2 and [(TSPP)Ir−D(CD3OD)]4−.
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Scheme 1. Thermodynamic Cycle for D2 Reaction with [(TSPP)Ir(OCD3)(CD3OD)]4−
1
IrI(CD3OD)]5− (5) occur at H NMR observable concen-
trations. The net process corresponds to reaction of the hydride
(4) with CD3OD to form the methyl complex (6) and D2O (eq
6) where [(TSPP)IrI(CD3OD)]5− (5) functions as a catalyst.
The proposed reaction pathway is illustrated in Scheme 3 . This
process is an unusual example of CD3−OD bond cleavage
which is driven thermodynamically by formation of water.
Scheme 3. Reaction of [(TSPP)Ir−D(CD3OD)]4− and
CD3OD To Form [(TSPP)Ir−CD3(CD3OD)]4− and Water
Catalyzed by [(TSPP)IrI(CD3OD)]5− in Methanol
Figure 5. Acid dissociation constant for [(TSPP)Ir−D(CD3OD)]4− in
CD3OD from [(TSPP)IrI(CD3OD)]5− in equilibrium with the
[(TSPP)Ir−D(CD3OD)]4− as a function of pD from 1H NMR.
Nonlinear least-squares fit gives K5(298K) = 3.5 × 10−12
.
dielectric medium. Fast quantitative air oxidation of iridium(I)
to iridium(III) is an important feature of the iridium porphyrin
system that permits iridium(III) porphyrins to function as
oxidation catalysts by directly using O2 without the need for a
cocatalyst.
Reaction of [(TSPP)Ir−D(CD3OD)]4− with CD3OD To
Form [(TSPP)Ir−CH3(CD3OD)]4− and Water Catalyzed by
[(TSPP)IrI(CD3OD)]5− in Methanol. Methanol solutions that
contain only the hydride ([(TSPP)Ir−D(CD3OD)]4− (4))
(pD < 10.5) or the iridium(I) complex ([(TSPP)-
IrI(CD3OD)]5− (5)) (pD > 12.0) are kinetically stable over
periods of months. When the pD is in the range of 10.8−11.8
the iridium hydride complex ([(TSPP)Ir−D(CD3OD)]4− (4))
is observed to react with methanol to form the methyl complex
[(TSPP)Ir−CD3(CD3OD)]5− (6) and water (eq 6). Formation
of an iridium−methyl complex from reaction with methanol
was first reported by Chan.18 Reaction 6 in CH3−OH results in
formation of [(TSPP)Ir−CH3(CH3OH)]4−, which is easily
observed in solution by the characteristic high field methyl
Reaction of [(TSPP)Ir−D (CD3OD)]4− with Alkenes in
CD3OD. Methanol (CD3OD) solutions of the iridium hydride
complex 4 ([(TSPP)Ir−D(CD3OD)]4−) (1 × 10−3 M) in
vacuum adapted NMR tubes were pressurized with ethene,
propene, 1-pentene and 1-hexene gases each at 100 Torr
1
pressure, and changes in the H NMR spectra were followed
1
over a period of days. The H NMR spectra for CD3OD
solutions of 4 with ethene and propene showed the appearance
of the characteristic high field porphyrin ring current shifted
resonances for metalloporphyrin alkyl complexes ((por)M−R),
but the longer chain alkenes did not produce 1H NMR
observable concentrations of organoiridium porphyrin species.
Reaction of [(TSPP)Ir−D(CD3OD)]4− with ethene produ-
ces the iridium ethyl complex [(TSPP)Ir−CH2CH2D-
(CD3OD)]4− (7), which is identified in methanol solution by
1
resonance in the H NMR spectrum (δIr−CH3 = −7.15).
[(TSPP)Ir−D(CD3OD)]4− + CD3OD
⇌ [(TSPP)Ir−CD3(CD3OD)]4− + D2O
(6)
Formation of the [(TSPP)Ir−CD3(CD3OD)]4− complex
only occurs at a finite rate in the pD range (10.8−11.8) where
both [(TSPP)Ir−D(CD3OD)]4− (4) and [(TSPP)-
1
the high field region of the H NMR (−1.85 (br, 2H, −CH2)
and −5.68 (br, 2H, −CH2D) (eq 7). Equilibrium concen-
Scheme 2. Proposed Pathway for Methanol Reduction of Iridium(III) Porphyrins at High pD
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trations of the iridium ethyl complex [(TSPP)Ir−CH2CH2D-
(CD3OD)]4−, [(TSPP)Ir−D(CD3OD)]4− and ethene deter-
interactions favor placing the metalloporphyrin on a primary
carbon, but M−C covalent bonding is maximized by placing the
metal center on a secondary carbon which better stabilizes the
negative charge.75 The increased importance of covalent
bonding compared to rhodium and the smaller steric
requirements of TSPP compared to TMPS may combine in
the (TPPS)Ir−D system to alter the regioselective addition
with propene. The small free energy change in the addition
reaction of propene with 4 to produce [(TSPP)Ir−
CH2CHDCH3(CD3OD)]4− (ΔG9°= −0.4 kcal mol−1))
compared to related Rh−D reaction in water (∼−5 kcal
mol−1) results in part from the previously recognized increase
in the difference between the Ir−D and Ir−CH2 BDFEs
compared to the Rh−D and Rh−CH2 BDFE values.
1
mined by integration of the H NMR were used in evaluating
the equilibrium constant for reaction 7 at 298 K (K7 = 63 and
ΔG7° = −2.5 kcal mol−1) (Table 1). Addition of the
(TSPP)Ir−D to ethene in methanol (ΔG7° = −2.5 kcalmol−1)
is similar to that for the reaction of ethene with (TMPS)Ir−D
in water(ΔG° = −3.7 kcal mol−1).72
[(TSPP)Ir−D(CD3OD)]4− + C2H4
⇌ [(TSPP)Ir−CH2CH2D(CD3OD)]4−
(7)
The reaction of [(TSPP)Ir−D(CD3OD)]4− with propene
results in observation of an equilibrium distribution of two
isomers. The major isomer corresponds to the iridium
isopropyl complex [(TSPP)Ir−CH(CH2D)CH3(CD3OD)]4−
(8) which is identified in solution by distinct doublets at
−2.20 ppm (−CH(CH2D)CH3) and a broad peak at −6.33
ppm (−CH(CH2D)CH3)) (eq 8). The minor isomer is the
i r i d i u m n - p r o p y l d e r i v a t i v e [ ( T S P P ) I r −
CH2CHDCH3(CD3OD)]4− (9), which is identified by the
distinctive high field resonances for the iridium propyl complex
at −1.70 ppm (−CH2 CHDCH3 ), −3.01 ppm
(−CH2CHDCH3) and −5.82 ppm (−CH2CHDCH3) (eq 9).
Equilibrium constants and ΔG° values for reactions 8 and 9
Reaction of [(TSPP)Ir−D (CD3OD)]4− with CO in CD3OD
in Methanol. The hydride complex [(TSPP)Ir−D-
(CD3OD)]4− (4) does not react with CO (PCO = 700 Torr)
in methanol to form an iridium formyl complex ([(TSPP)Ir−
CDO(CD3OD)]4−), but simply produces a trans hydrido
carbonyl adduct ([(TSPP)Ir−D(CO)]4− (10)) (eq 10). The
inability to obtain observable quantities of an iridium formyl
complex contrasts with rhodium porphyrin hydrides where
reaction with CO (PCO < 1 atm) to produce metallo−formyl
complexes is a prominent characteristic reaction in ben-
zene,36,76 water27 and methanol.17
1
were evaluated at 298 K by H NMR (K8 = 9.2(0.2), ΔG8° =
−1.3 kcal mol−1; K9 = 2.0(0.5), ΔG9° = −0.4 kcal mol−1)
[(TSPP)Ir−D(CD3OD)]4− + CO
(Table 1).
⇌ [(TSPP)Ir−D(CO)]4− + CD3OD
(10)
[(TSPP)Ir−D(CD3OD)]4− + CH2CHCH3
⇌ [(TSPP)Ir−CH(CH2D)CH3(CD3OD)]4−
An increase in the Ir−H relative to the Ir−C is consistent
with the inability of the iridium hydride complex (4) to
produce observable quantities of the α-hydroxyethyl complex
([(TSPP)Ir−CH(OD)CH3(CD3OD)]4−), which is a thermo-
dynamically favorable reaction of the corresponding rhodium
porphyrin system.42
Difference between the Bond Dissociation Free
Energy of Ir−OCD3 and Ir−D for [(TSPP)Ir−X(CD3OD)]4−
Complexes in CD3OD. The difference between the [(TSPP)-
Ir−OCH3] and [(TSPP)Ir−D] units can be estimated as
shown from analysis of the bonds breaking and forming as
derived from eq 4 (Scheme 5).
(8)
[(TSPP)Ir−D(CD3OD)]4− + CH2CHCH3
⇌ [(TSPP)Ir−CH2CHDCH3(CD3OD)]4−
(9)
Thermodynamic preference for the Ir−D addition with
propene to give the Markovnikov regioisomer is opposite to
that observed for propene reactions with (TMPS)Ir−D in
water72 and with (TSPP)Rh−D and (TMPS)Rh−D complexes
in water and methanol (Scheme 4).20,22 Repulsive steric
Scheme 4. Addition Reactions of Propene with [(TSPP)Ir−
The iridium hydride [(TSPP)Ir−D(CD3OD)]4− (4) bond
dissociation free energy (BDFE) is larger than the iridium
methoxide BDFE for [(TSPP)Ir(OCD3)(CD3OD)]4− (2) in
methanol by 13.3 kcal mol−1 ([Ir−D] − [Ir−OCH3] = 13.3
kcal mol−1). The iridium hydroxide BDFE is 3.6 kcal mol−1
larger than the iridium hydride BDFE ([Ir−OD] − [Ir−D] =
3.6 kcal mol−1) for the similar (TMPS)Ir complexes in water,72
which places the Ir−OH BDFE in water at ∼17 kcal mol−1
larger than the Ir−OCH3 BDFE in methanol. The difference of
14 kcal mol−1 in H−OCH3 and H−OH bond dissociation
energies77 is dominated by the difference in the radical
stabilization energies of the methoxy and hydroxyl radicals,
D(CD3OD)]4− in Methanol
Scheme 5. BDFE of [(Ir−OCD3)- (Ir−D)] Derived from the Reaction of [Ir-OCD3(CD3OD)]4− and D2
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SUMMARY AND CONCLUSIONS
■
Solutions of iridium(III) tetrakis(p-sulfonatophenyl)porphyrin
[(TSPP)IrIII] in methanol form a hydrogen ion dependent
equilibrium distribution between three species [(TSPP)Ir-
(CD3OD)2]3−, [(TSPP)Ir(OCD3)(CD3OD)]4− and [(TSPP)-
Ir(OCD3)2]5−. The coordinated CD3OD acid dissociation
constants for [(TSPP)Ir(CD3OD)2]3− are K1 = 2.3 × 10−8
and K2 = 2.1 × 10−11, which are somewhat smaller than those of
[(TSPP)Ir(D2O)2]3− in water (K1(H2O) = 4.8 × 10−8,
K2(H2O) = 2.6 × 10−11). Reaction of [(TSPP)IrIII] species in
CD3OD with dihydrogen (H2/D2) results in formation of an
iridium hydride complex [(TSPP)Ir−D(CD3OD)]4− with an
equilibrium constant of K3 = 4.5, ΔG°(298K) = −0.8 kcal
mol−1, which is smaller than the corresponding reaction of
[(TSPP)RhIII]) in methanol (K = 1.0 × 103, ΔG°(298K) =
−4.1 kcal mol−1). The less favorable ΔG° for the (TSPP)IrIII
compared to (TSPP)RhIII system results from larger Ir−
methanol binding compared to Rh−methanol that more than
compensates for the increase in metal hydride bond energy
(([Ir−(CD3OD] − [Rh(CD3OD]) > ([Ir−D] − [Rh−D])).
The acid dissociation constant for [(TSPP)Ir−D(CD3OD)]4−
measured in methanol is 3.5 × 10−12 (ΔG°(298K) = 15.6 kcal
mol−1) and is comparable to that for [(TMPS)Ir−D(D2O)]8−
in water (K = 1.8 × 10−12, ΔG°(298K) = 16.0 kcal mol−1) and
much smaller than that for [(TSPP)Rh−D(CD3OD)]8− in
methanol (K = 1.1 × 10−9) which primarily reflects the larger
Ir−D bond energy. The Ir(I) complex ([(TSPP)Ir(I)−
(CD3OD)]5−) in the pD range of 10.8 to 11.8 is observed to
catalyze reaction of the iridium hydride ([(TSPP)Ir−D-
(CD3OD)]4−) with methanol to form an Ir−CH3 complex
[(TSPP)Ir−CD3(CD3OD)]4− and water. This thermodynami-
cally favorable CD3−OD bond cleavage occurs at a significant
rate only in the pD range where both the Ir−D and Ir(I) have
substantial equilibrium concentrations which maximize the
opportunity for a concerted process.
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ASSOCIATED CONTENT
* Supporting Information
Materials and experimental procedures and H NMR spectra.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
1
AUTHOR INFORMATION
Corresponding Author
*E-mail: bwayland@temple.edu.
Notes
■
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This research was initiated with support from the Department
of Energy, Division of Chemical Sciences and the Office of
Science through Grant DE-FG02-09ER-16000.
Chem. Soc. 1992, 114, 1673.
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Chem. Soc. 1987, 109, 6513.
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