Comparative studies of preferential binding of group nine metalloporphyrins (M = Co, Rh, Ir) with methoxide/methanol in competition with hydroxide/water in aqueous solution

Article abstract of DOI:10.1021/ic100773y

Aqueous solutions of iridium(lll) tetra-(p-sulfonatophenyl)porphyrin [(TSPP)Ir^III] form a hydrogen ion dependent equilibrium distribution of blsaquo ([(TSPP)Ir^III(OD2)2]^3-), monoaquo/monohydroxo ([(TSPP)Ir^III(OD₂)(OD)]^4-) and blshydroxo ([(TSPP)Ir^IIICOD)₂]^5- ) complexes. Comparison of acid dissociation constants of group nine ([(TSPP)M^III(OD ₂)₂]^3-) (M = Co, Rh, Ir) complexes show that the extent of proton dissociation in water increases regularly on moving down the group from cobalt to Iridium consistent with increasing metal ligand bond strength. Addition of small quantities of methanol to aqueous solutions of [(TSPP)Ir^III] results in the formation of methanol and methoxide complexes in equilibria with aquo and hydroxo complexes that are observed by ¹H NMR. Direct quantitative evaluation of competitive equilibria of [(TSPP)Ir^III] complexes reveals a remarkable thermodynamic preference for methanol binding over that of water (ΔG° (298 K) = -5.2 kcal mol^-1) and methoxide binding over that of hydroxide (ΔG° (298 K) = -6.1 kcal mol^-1) in aqueous media. A comparison of equilibrium thermodynamic values for displacement of hydroxide by methoxide for group nine (TSPP)M^III (M = Co, Rh, Ir) complexes In aqueous media are also reported.

Full text of DOI:10.1021/ic100773y

6734 Inorg. Chem. 2010, 49, 6734–6739
DOI: 10.1021/ic100773y
Comparative Studies of Preferential Binding of Group Nine Metalloporphyrins
(M = Co, Rh, Ir) with Methoxide/Methanol in Competition with Hydroxide/Water in
Aqueous Solution
Salome Bhagan, Sounak Sarkar, and Bradford B. Wayland*
Department of Chemistry, Temple University, 130 Beury Hall, 1901N. 13th Street, Philadelphia,
Pennsylvania 12122
Received April 21, 2010
Aqueous solutions of iridium(III) tetra-(p-sulfonatophenyl)porphyrin [(TSPP)IrIII] form a hydrogen ion dependent
equilibrium distribution of bisaquo ([(TSPP)IrIII(OD2)2]3-), monoaquo/monohydroxo ([(TSPP)IrIII(OD2)(OD)]4-) and
bishydroxo ([(TSPP)IrIII(OD)2]5-) complexes. Comparison of acid dissociation constants of group nine ([(TSPP)MIII-
(OD2)2]3-) (M = Co, Rh, Ir) complexes show that the extent of proton dissociation in water increases regularly on moving
down the group from cobalt to iridium consistent with increasing metal ligand bond strength. Addition of small quantities of
methanol to aqueous solutions of [(TSPP)IrIII] results in the formation of methanol and methoxide complexes in equilibria
with aquo and hydroxo complexes that are observed by 1H NMR. Direct quantitative evaluation of competitive equilibria of
[(TSPP)IrIII] complexes reveals a remarkable thermodynamic preference for methanol binding over that of water (ΔG°
(298 K) = -5.2 kcal mol-1) and methoxide binding over that of hydroxide (ΔG° (298 K) = -6.1 kcal mol-1) in aqueous
media. A comparison of equilibrium thermodynamic values for displacement of hydroxide by methoxide for group nine
(TSPP)MIII (M = Co, Rh, Ir) complexes in aqueous media are also reported.
Introduction
toward developing water-soluble metalloporphyrins as plat-
forms for diverse organometallic transformations in aqueous
media.17-21 Aqueous solutions of rhodium porphyrin com-
plexes manifest an exceptional range of substrate reactions
that are being incorporated into catalytic processes such
as olefin oxidation reactions.15,16 Complexes of tetra(p-sulfo-
natophenyl) porphyrin rhodium [(TSPP)Rh] react with
H2/D2, D2O, CO, aldehydes, and olefins to form hydride,
hydroxide, formyl, R- and β-hydroxyalkyl, and alkyl com-
plexes in water.11,12,22 Equilibrium thermodynamic studies
for this wide range of [(TSPP)Rh] substrate reactions in water
provide one of the most comprehensive sets of thermody-
namic measurements for organometallic reactions.12,13 The
reaction chemistries of the group nine cobalt and iridium
metalloporphyrin derivatives in water are much less devel-
oped than the rhodium porphyrin systems, but recent struc-
tural characterization of ([(TSPP)IrIII(OH2)2]3-) is an
important advance.23
Environmental and energy issues have stimulated renewed
interest in utilizing water as both a reagent and a reac-
tion medium.1-21 Our effort in this area has been directed
*To whom correspondence should be addressed. E-mail: bwayland@
temple.edu.
(1) Mifsud, M.; Parkhomenko, K. V.; Arends, I. W. C. E.; Sheldon, R. A.
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4899.
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J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2008, 130, 2418.
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39, 477.
This article lays the foundation for comparative thermo-
dynamic studies of group nine porphyrins [(TSPP)MIII] (M =
Co, Rh, Ir) by evaluating the acid dissociation constants for
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r
pubs.acs.org/IC
Published on Web 06/14/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 14, 2010 6735
Figure 1. Water and hydroxide complexes of (TSPP) IrIII in water.
Figure 3. Equilibrium distributions of [(TSPP)IrIII(OD2)2]3- (1),
[(TSPP)IrIII(OD)(OD2)]4- (2), and [(TSPP)IrIII(OD)2]5- (3) as a func-
tion of the hydrogen ion concentration in D2O at 298 K calculated for
K1(298 K) = 4.6 ꢁ 10-8 and K2(298 K) = 2.6 ꢁ 10-11 and a total
[(TSPP)IrIII] = 1.0 ꢁ 10-3 M.
and hydroxide ligands for 1, 2, and 3 in water rapidly
interchange protons with the bulk water (298 K) (Figure 2)
which results in a single mole fraction averaged pyrrole 1H
NMR resonances for 1, 2, and 3 (Figure 2). The 1H NMR of
the bis-aquo 1 and bis-hydroxo 3 complexes can be directly
observed at limiting low and high pD respectively. The mole
fraction averaged pyrrole 1H NMR resonances for equilib-
rium distribution of 1, 2, and 3 as a function of [Dþ] were
used in determining the acid dissociation constants by non-
Figure 2. Mole fraction averaged 1H HMR (500 MHz) porphyrin pyrrole
singlet and phenyl AB pattern for 1, 2, and 3 in D2O (T = 293 K) pD of
(A) 5, (B) 8.5, and (C) 11.5.
linear least-squares curve fitting to the expression δ2,3,4(obs)
-
(pyr)=(K1K2δ3(pyr)þK1[Dþ]δ2(pyr)þ[Dþ]2 δ1(pyr))/(K1K2
water (H2O, D2O) complexes of [(TSPP)MIII] (M = Co, Rh,
Ir) in water. Equilibrium constants for competitive binding in
water-methanol mixtures show thermodynamic preference
for methanol and methoxide compared to water and hydroxide
respectively. Observations of large thermodynamic preferences
for (TSPP)IrIII to bind methanol compared to water (ΔG°
(298 K) = -5.2 kcal mol-1) and methoxide versus hydroxide
(ΔG°(298 K) = -6.1 kcal mol-1) are particularly remarkable
results that establish the capability of iridium complexes to
activate low concentrations of alcohols in water.24-26
þK1[Dþ]þ[D ] )
þ 2 13,27 giving K1(298 K) = 4.8 ꢁ 10-8, K2-
(298K)=2.6ꢁ10-11 andδ2(pyr)=8.75ppm;1[(TSPP)IrIII-
(OD2)2]3- is δ1(pyr) = 8.93 ppm, and 3[(TSPP)IrIII(OD)2]5-
is δ3(pyr)=8.57 ppm.28 The equilibrium constants agree well
with the recent report of Kanemitsu.23 Equilibrium distribu-
tion of 1, 2, and 3 in D2O as a function of the hydrogen ion
concentration is illustrated in Figure 3.
Parallel studies of Na3[(TSPP)Co(OD2)2] 18D2O in
3
D2O similarly result in a pD dependent equilibrium distri-
bution of hydroxo and aquo complexes of [(TSPP)-
CoIII].28 The acid dissociation constants and chemical
shifts were determined as K1(298 K) = 8.75 ꢁ 10-9, K2-
Results and Discussion
Aquo and Hydroxo Complexes of (TSPP)IrIII in D2O.
Dissolution of iridium(III) tetra-p-sulfanatophenyl por-
(298 K)=7.12 ꢁ 10-13 and ([(TSPP)CoIII(OD2)(OD)]4-
)
δ2(pyr) = 9.22 ppm; ([(TSPP)CoIII(OD2)2]3-) δ1(pyr) =
9.38 ppm; ([(TSPP)CoIII(OD)2]5-) δ3(pyr) = 9.10 ppm.
Aquo and Hydroxo Complexes of (TSPP)IrIII in H2O.
Equations 3 and 4 describe the equilibria between the bis-
aquo complex ([(TSPP)IrIII(OH2)2]3-) (4) and the mono
phyrin (Na3[(TSPP)IrIII(OD2)2] 18D2O) in D2O results in
3
a pH dependent equilibrium distribution of the bis-aquo
[(TSPP)IrIII(OD2)2]3- (1) with mono and bis-hydroxo com-
plexes [(TSPP)IrIII(OD)(OD2)]4- (2), [(TSPP)IrIII(OD)2]5-
(3) (eq 1,2) (Figure1). The axially coordinated water
and bis-hydroxo complexes ([(TSPP)IrIII(OH)(OH2)]4-
)
(5) and ([(TSPP)IrIII(OH)2]5-) (6) in H2O.
3-
4-
½ðTSPPÞIrIIIðOD2Þ2ꢀ h½ðTSPPÞIrIIIðODÞðOD2Þꢀ þ Dþ
3-
4-
½ðTSPPÞIrIIIðOH2Þ2ꢀ h½ðTSPPÞIrIIIðOHÞðOH2Þꢀ þ Hþ
ð1aÞ
ð1Þ
4-
5-
½ðTSPPÞIrIIIðODÞðOD2Þꢀ h½ðTSPPÞIrIIIðODÞ2ꢀ þ Dþ
ð2Þ
4-
5-
½ðTSPPÞIrIIIðOHÞðOH2Þꢀ h½ðTSPPÞIrIIIðOHÞ2ꢀ þ Hþ
ð2aÞ
(24) Ugrinova, V.; Ellis, G. A.; Brown, S. N. Chem. Commun. 2004, 468.
(25) Saidi, O.; Blacker, A. J.; Farah, M. M.; Marsden, S. P.; Williams,
J. M. J. Chem. Commun. 2010, 46, 1541.
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Chan, K. S. Organometallics 2010, 29, 1343.
(27) Sutter, T. P. G.; Rahimi, R.; Hambright, P.; Bommer, J. C.; Kumar,
M.; Neta, P. J. Chem. Soc., Faraday Trans. 1993, 89, 495.
(28) See Supporting Information.

6736 Inorganic Chemistry, Vol. 49, No. 14, 2010
Bhagan et al.
Non-linear least-squares curves fitting of the pH depen-
dence of the pyrrole 1H NMR resonances for equilibrium
distribution of 4, 5, and 6 were used to evaluate the acid
dissociation constants13,29 for compounds 4 and 5, (K1a=
1.1 ꢁ 10-7) (K2b= 5.3 ꢁ 10-11). Plots of the observed
pyrrole 1H NMR shifts as a function of the hydrogen ion
concentration ([Dþ]; [Hþ]) for the H2O and D2O com-
plexes of [(TSPP)IrIII] and the calculated best fit lines are
shown in Figure 4. The H2O complexes are more acidic
those of D2O as a result of the smaller zero-point energy
of D2O30 (Figure 4).
Results from determining acid dissociation constants
for aquo and hydroxo complexes of [(TSPP)MIII] (M =
Co, Rh, Ir) in D2O by application of the same 1H NMR
method are given in Table 1.13,31-33
The relative acidities of the [(TSPP)MIII(OH2)2]3-
(M = Co, Rh, Ir) complexes increase regularly in pro-
ceeding down group nine from cobalt to iridium which
reflects the trend of increasing metal-ligand bond
strengths.34-36 The free energy changes for displacement
of water by hydroxide derived for [(TSPP)MIII(OH2)2]3-
(M = Co, Rh, Ir) complexes (eqs 3, 4; ΔG3° = ΔG1° þ
ΔG5°, ΔG4° = ΔG2° þ ΔG5°) (Table1). The binding of
hydroxide is substantially more favorable than water
in all cases and the difference increases when moving
down group nine from cobalt to iridium. Preference for
the hydroxide anion over water decreases substantially as
the effective metal site positive charge is attenuated in go-
ing from the diaquo complex ([(TSPP)MIII(OD2)2]3-) to
Figure 4. Observed limiting fast exchange mole fraction averaged
pyrrole 1H NMR chemical shifts for compounds 1, 2, and 3 in D2O and
4, 5, and 6 in H2O. The points are experimentally determined values, and
the lines are calculated for K1a (298 K) = 1.1 ꢁ 10-7 and K2b (298 K) =
5.3 ꢁ 10-11
.
1
of [(TSPP)IrIII in pure CD3OD and observing the H
NMR allowed the assignment of the pyrrole resonance at
8.74 ppm to [(TSPP)IrIII(CH3OH)2]3- (8) which also
identifies the CH3 resonance for the coordinated metha-
nol at -2.34 ppm (Figure 5). The remaining pyrrole reso-
nance is assigned to [(TSPP)IrIII(OD2)(CH3OD)]3- (7).
Equilibria corresponding to sequential substitution of
water by methanol are described by eqs 6 and 7.
Characteristic 1H NMR resonances for compounds 1-3
and 7-10 are found in Table 2.
the mono hydroxo monoaquo complexes ([(TSPP)MIII
(OD2)(OD)]4-) (Table 1).
-
3-
½ðTSPPÞMIIIðOD2Þ2ꢀ þ OD- h
3-
½ðTSPPÞIrIIIðOD2Þ2ꢀ þ CH3ODh
4-
½ðTSPPÞMIIIðODÞðOD2Þꢀ þ D2O
ð3Þ
3-
½ðTSPPÞIrIIIðOD2ÞðCH3ODÞꢀ þ D2O
ð6Þ
ð7Þ
4-
½ðTSPPÞMIIIðODÞðOD2Þꢀ þ OD- h
3-
½ðTSPPÞIrIIIðOD2ÞðCH3ODÞꢀ þ CH3ODh
5-
½ðTSPPÞMIIIðODÞ2ꢀ þ D2O
ð4Þ
ð5Þ
3-
½ðTSPPÞIrIIIðCH3ODÞ2ꢀ þ D2O
Equilibrium constants for reactions 6 and 7 were eval-
uated from the integrated intensities of the porphyrin and
methanol H NMR resonances and the D2O concentra-
Dþ þ OD- hD2O
1
Free Energy Changes in the Replacement of Water in
[(TSPP)IrIII(OH2)2]3- by Methanol in Acidic D2O. Addi-
tion of a small quantity of CH3OH (1.0-0.1 ꢁ 10-3 M) to
an acidic (pH = 4) D2O solution of [(TSPP)IrIII (5 ꢁ 10-4
M) resulted in observation of three distinct pyrrole re-
sonances that are assigned to the three possible water and
methanol complexes [(TSPP)IrIII(OD2)(CH3OD)]3- (7),
[(TSPP)IrIII(CH3OH)2]3- (8), and [(TSPP)IrIII(OD2)2]3-
(1) (Figure 5). The high field resonance at -2.44 ppm that
is not present in deuterated methanol samples is assigned
to the coordinated methanol in 7 (Figure 5). Dissolution
tion (55.2 M), (K6 (298 K) = 6.3 ꢁ 103, ΔG°6 (298 K) =
-5.2 kcal mol-1 and K7 (298 K) = 2.5 ꢁ 103,ΔG°7 (298K) =
-4.6 kcal mol )
-1 28 (Table 3).
Addition of methanol to aqueous solutions of (TSPP)MIII
(M=Co, Rh) results in a single mole fraction averaged
pyrrole resonances and the absence of high field reso-
nances for coordinated methanol which are indicative of
fast interchange of methanol and water complexes which
contrasts with the limiting slow exchange observed for
(TSPP)IrIII complexes.
Free Energy Changes in the Replacement of Hydroxide
by Methoxide in Basic D2O. Basic D2O solutions (pD =
11-13) of [(TSPP)IrIII (5 ꢁ 10-4 M) with a small addition
of CH3OH (0.1-1.0 ꢁ 10-3 M) resulted in observation of
equilibrium distributions of hydroxo and methoxide
complexes [(TSPP)IrIII(OD)2]5- (3), [(TSPP)IrIII(OD)-
(OCH3-)]5- (9), and [(TSPP)IrIII(OCH3-)2]5- (10)
(29) Cookson, R. F. Chem. Rev. 1974, 74, 5.
(30) Guillot, B.; Guissani, Y. Fluid Phase Equilib. 1998, 150-151, 19.
(31) McLendon, G.; Bailey, M. Inorg. Chem. 1979, 18, 2120.
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1613.
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^ ꢀ
(35) Grossman, J. C.; Mizel, A.; Cote, M.; Cohen, M. L.; Louie, S. G.
1
(Figure 6, eqs 8, 9). Assignments of the H NMR for 3,
9, and 10 are based on the changes in the observed
Phys. Rev. B 1999, 60, 6343.
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Article
Inorganic Chemistry, Vol. 49, No. 14, 2010 6737
Table 1. First and Second Acid Dissociation Constants and Free Energy Changes (K1, K2, ΔG1°, ΔG2°) (298 K) and Free Energy Changes for Reactions That Substitute
Hydroxide for Water (ΔG3°, ΔG4°) (298 K) in [(TSPP)MIII(OD2)2]3- (M = Co, Rh, Ir) Complexes in Aqueous Solutions
group 9 (TSPP)MIII
K1
K2
ΔG1° (kcal mol-1
)
ΔG2° (kcal mol-1
)
ΔG3° (kcal mol-1
)
ΔG4° (kcal mol-1
)
Co (D2O)a
Rh (D2O)b
Ir (D2O)
8.8 ꢁ 10-9
1.4 ꢁ 10-8
4.8 ꢁ 10-8
1.1 ꢁ 10-7
7.1 ꢁ 10-13
2.8 ꢁ 10-12
2.6 ꢁ 10-11
5.3 ꢁ 10-11
þ10.9
þ10.6
þ9.90
þ9.41
þ16.4
þ15.6
þ14.3
þ13.9
-11.7
-12.0
-13.7
-14.5
-6.1
-6.8
-8.2
-10.0
Ir (H2O)
a see Supporting Information. b Reference 13.
Analogous studies of [(TSPP)RhIII] and [(TSPP)CoIII] in
highly basic D2O (pD=11-13) with addition of known
quantities of methanol result in observing sets of hydro-
xide and methoxide complexes that exhibit characteristic
1
porphyrin pyrrole and high field H NMR resonances
paralleling the observations for [(TSPP)IrIII] complexes
of hydroxide and methoxide.28
Equilibrium constants for reactions corresponding to
displacement of hydroxide by methoxide for the group
nine [(TSPP)MIII] (M = Co, Rh, Ir) complexes are summ-
arized in Table 4. All of the [(TSPP)MIII] (M = Co,
Rh, Ir) reactions where methoxide substitutes for hydro-
xide in water are thermodynamically favorable (Table 4).
The thermodynamic preference for complexing with meth-
oxide compared to hydroxide increases slightly from CoIII
Figure 5. [TSPP]IrIII complexes in (A) D2O with a small concentration
of methanol and (B) in pure CD3OD.
Table 2. Characteristic 1H HMR (500 MHz) Resonance for the Porphyrin
Pyrrole and Methyl of Coordinated CH3OH/CH3O- for [(TSPP)IrIII] Methanol/
Water Species in D2O
(ΔG° =-1.5 kcal mol-1),toRhIII (ΔG° =-1.9 kcal mol-1
)
and becomes remarkably large for IrIII (ΔG° = -6.1 kcal
(TSPP)IrIII species
pyrole
MeOH/OMe
mol-1) (Table 4).
[(TSPP)IrIII(OD2)2]3- (1)
8.93
8.75
8.588
8.84
8.76
8.585
8.45
Trends in 1H NMR Parameters for ([(TSPP)MIII
-
[(TSPP)IrIII(OD)(OD2)]4- (2)
[(TSPP)IrIII(OD)2]5- (3)
(OCH3)(OH)]5-). The porphyrin pyrrole and coordi-
nated methoxide methyl 1H NMR resonances for
[(TSPP)MIII(OCH3)(OD)]5- (M = Co, Rh, Ir) are sum-
[(TSPP)IrIII(OD2)(CH3OD)]3- (7)
[(TSPP)IrIII(CH3OD)2]3- (8)
[(TSPP)IrIII(OCH3)(OD)]5- (9)
[(TSPP)IrIII(OCH3)2]5- (10)
-2.44a
-2.34a
-2.51b
-2.55b
1
marized in Table 5. Porphyrin ring current H NMR
reasonances have a (1 - 3 cos2 θ)/r3 dependence which
results in downfield shifts for the in-plane pyrrole hydro-
gens and upfield shifts for the M-OCH3 group near the
center of the π orbitals.39,40 The ring current shifts also
scale with the π electron population, and the regular
decrease in the downfield pyrrole hydrogen shifts in
going from the CoIII to the IrIII derivatives is consistent
with decreasing π electron population resulting from inc-
reased M-porphyrin π mixing. Decreasing high field
shifts for the M-OCH3 group for M = Co, Rh, Ir is also
consistent with this model (Table 5).
a CH3OH. b OCH3
.
-
intensities for the pyrrole and methoxide resonances as
the methanol concentration is varied.
5-
½ðTSPPÞIrIIIðODÞ2ꢀ þ OCH3 - h
5-
½ðTSPPÞIrIIIðODÞðOCH3Þꢀ þ OD-
ð8Þ
5-
½ðTSPPÞIrIIIðODÞðOCH3Þꢀ þ OCH3 - h
Comparison of the Free Energy Changes for Replacement
of D2O by OD- and OCH3 from ([(TSPP)MIII(D2O)-
-
5-
½ðTSPPÞIrIIIðOCH3Þ2ꢀ þ OD-
ð9Þ
(OD)]4-) (M = Co, Rh, Ir) in Water. Reactions that sub-
stitute methoxide and hydroxide for water in (TSPP)MIII
complexes are depicted by eqs 4 and 11 and the equilib-
rium thermodynamic values given in Table 6. Reactions 4
and 11 provide the best opportunity to compare the
interactions of methoxide and hydroxide with a set of
group nine (Co, Rh, Ir) complexes.
Reactions 8 and 9 describe processes where hydroxide is
displaced by methoxide in water which provide a direct
comparison of the hydroxide and methoxide binding with
[(TSPP)IrIII] in water. Equilibrium constants for reactions
8 and 9 were evaluated by 1H NMR integrated intensities
for the [(TSPP)IrIII] species, pD measurements, and the
-
equilibrium concentrations of OD- and OCH3 from
reaction 10 (K10 (298 K) = 0.3)37,38 in water. Equilibrium
constantsK8 and K9 evaluated at298 K, Table, (K8 (298 K)
=3.2 ꢁ 104, ΔG°8 (298 K)=-6.1 kcal mol-1 and K9(298 K)
= 8.6 ꢁ 103, ΔG°9(298 K) = -5.4 kcal mol-1).
4-
½ðTSPPÞMIIIðODÞðOD2Þꢀ þ OCH3 - h
5-
½ðTSPPÞMIIIðODÞðOCH3Þꢀ þ D2O
ð11Þ
The free energy change for displacement of D2O by OD-
CH3OH þ OH- hH2O þ CH3O-
ð10Þ
(ΔG4°(298 K) becomes steadily more favorable in passing
(39) Abraham, R. J.; Medforth, C. J. Magn. Reson. Chem. 1988, 26, 803.
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6738 Inorganic Chemistry, Vol. 49, No. 14, 2010
Bhagan et al.
Table 3. Measured Kn and ΔG°n Values for the Reactions of [(TSPP)IrIII] in D2O with Methanol Described by eqs 6, 7 at Acidic Conditions and eqs 8, 9 at Basic Conditions
measured [(TSPP)IrIII] reactions
Kn (298 K)
ΔG°n (kcal mol-1
-5.2(0.1)
)
3-
3-
½ðTSPPÞIrIIIðOD2Þ2ꢀ þ CH3ODh½ðTSPPÞIrIIIðOD2ÞðCH3ODÞꢀ þ D2O
6
7
8
9
6.3(0.3) ꢁ 103
3-
3-
2.5(0.2) ꢁ 103
32 (3) ꢁ 103
-4.6(0.1)
-6.1(0.2)
-5.4(0.2)
½ðTSPPÞIrIIIðOD2ÞðCH3ODÞꢀ þ CH3ODh½ðTSPPÞIrIIIðCH3ODÞ2ꢀ þ D2O
5-
5-
½ðTSPPÞIrIIIðODÞ2ꢀ þ OCH3 - h½ðTSPPÞIrIIIðODÞðOCH3Þꢀ þ OD-
5-
5-
8.6(0.3) ꢁ 103
½ðTSPPÞIrIIIðODÞðOCH3Þꢀ þ OCH3 - h½ðTSPPÞIrIIIðOCH3Þ2ꢀ þ OD-
in water (eq 12) are influenced by contributions from
the solvation free energies of both reactants and pro-
ducts along with metal-ligand interactions. Solvation
free energies for
M-LðaqÞ þ L0ðaqÞhM-L0ðaqÞ þ L
ð12Þ
ðaqÞ
CH3OH(g) (-5.1 kcal mol-1 41
)
and H2O(g) (-6.3 kcal
mol
)
-1 41 in H2O(l) favor displacement of the water ligand
Figure 6. 1H NMR (500 MHz) of observed equilibrium species in D2O
with addition of 1 ꢁ 10-4 M CH3OH at pH = 11.75. The pyrrole reso-
nances of compounds 3, 9, and 10 and corresponding upfield peaks
(magnified ꢁ 4) are shown.
by ∼1 kcal mol-1. The difference in solvation energies of
the metal complexes is anticipated to be small and par-
tially compensates for the difference in the aquation free
energies for methanol and water. Additionally, repulsive
steric interactions of CH3OH as a ligand are larger than
that for H2O. Thus, the preference of (TSPP)MIII binding
methanol over water in water must be attributed to M-L
bonding interactions.
Table 4. Equilibrium Constants (Kn) and ΔG°n (kcal mol-1) for Substitution of
Methoxide for Hydroxide on [(TSPP)MIII(OD)2]5-;(K8, ΔG°8) and [(TSPP)MIII
(OD)(OCH3)]5-; (K9, ΔG°9) (M = CoIII, RhIII, IrIII) at pH 12
-
) )
K8 (298 K) ΔG°8 (kcal mol-1 K9 (298 K) ΔG°9 (kcal mol-1
Methanol displacing water from ([(TSPP)IrIII(OD2)2] 3-
)
CoIII 12(3)
-1.5(0.2)
-1.9(0.1)
-6.1(0.2)
3.0(2)
10(3)
-0.6(0.04)
-1.4(0.1)
-5.4(0.2)
(eq 6) (K6(298 K) = 6.2(0.3) ꢁ 103, ΔG°6(298 K) =
-5.2(0.1) kcal mol-1) results in a large thermodynamic
preference of (TSPP)IrIII binding methanol compared to
water, and the dominant contribution to ΔG°6 must arise
from more favorable bonding interaction. When metha-
nol functions as a ligand to bind with metals, the methyl
group interaction with the oxygen center attenuates the
charge separation at the oxygen donor site and promotes
M-CH3OH covalent bonding relative to the M-H2O
bonding. The M-L interactions within group nine (Co,
Rh, Ir) are dominated by increasing importance of cova-
lent bonding acceptor properties from Co to Rh to Ir.
Thus, the larger covalent bonding donor affinity of
methanol/methoxide compared to water/hydroxide when
matched with the larger covalent bonding acceptor prop-
erties of (TSPP)IrIII amplifies the difference between
(TSPP)IrIII and the RhIII and CoIII derivatives.
RhIII 26(5)
IrIII 32(3) ꢁ 103
8.6(0.2)x103
Table 5. Porphyrin Pyrrole Resonances and Methoxide Methyl 1H NMR
(500 MHz) Resonances of Group Nine Metal [(TSPP)MIII(OCH3)(OD)]5-
Group 9 Species
pyrrole
OMe
[(TSPP)CoIII(OCH3)(OD)]5-
[(TSPP)RhIII(OCH3)(OD)]5-
[(TSPP)IrIII(OCH3)(OD)]5-
9.01
8.92
8.59
-3.59
-2.80
-2.51
Table 6. Derived Free Energy Changes (ΔGn° (kcal mol-1) (298 K)) for Reactions
That Displace Water by X (X = OD-/OCH3-) [(TSPP)MIII] (M = Co, Rh, Ir) in
Aqueous Media: [(L)MIII(OD2)(OD)]4- þ X- h [(L)MIII(OD)(X)]5- þ D2O
[(TSPP)MIII(OD2)(OD)]4-
OD-
OCH3
-
[(TSPP)CoIII(OD2)(OD)]4-
[(TSPP)RhIII(OD2)(OD)]4-
[(TSPP)IrIII(OD2)(OD)]4-
-6.1
-6.8
-8.2
-7.7
-8.9
-14.4
Reduction of [(TSPP)IrIII] by Methoxide. The capabil-
ity for accomplishing metal complex promoted and cat-
alyzed alcohol oxidations in water is dependent on
substrate coordination and activation and favorable
thermodynamics for methanol and methoxide binding
with [(TSPP)MIII] (M = Co, Rh, Ir) in aqueous solution
is a major factor in obtaining processes involving alco-
hols such as oxidation to aldehydes42 and amine alkyla-
tion25 in water. The dramatic preferential selectivity of
[(TSPP)IrIII] for binding methanol and methoxide in
water illustrates how iridium(III) has the potential to be
an effective catalyst for alcohol reactions in water. Re-
duction [(TSPP)IrIII] (5 ꢁ 10-4 M) to [(TSPP)IrI] or
from cobalt (-6.1 kcal mol-1) to rhodium (-6.8 kcal
mol-1) to iridium (-8.2 kcal mol-1) (Table 6). The
-
(ΔG11°(298 K)) for substitution of D2O by OCH3 also
becomes more favorable in moving down group nine
(Co(-7.7), Rh(-8.9), Ir(-14.4) kcal mol-1), but there is
a substantially larger change between rhodium (-8.9 kcal
mol-1) and iridium (-14.4 kcal mol-1) which is not
observed for the OD- reactions (Rh(-6.8), Ir(-8.2) kcal
mol-1). There is thus a specific feature of the binding of
methoxide with iridium(III) which is distinguishable from
hydroxide interactions. A greater importance of covalent
bonding in M-OCH3 compared to M-OD could be a
major contribution to this observation.
Contributing Factors to ΔG° for Ligand Displacements
in Water. Thermodynamics of ligand displacement reactions
(41) Goncalves, P. F. B.; Stassen, H. Pure Appl. Chem. 2004, 76, 231.
(42) Blum, O.; Milstein, D. J. Organomet. Chem. 2000, 593-594, 479.

Article
Inorganic Chemistry, Vol. 49, No. 14, 2010 6739
buffer solutions of pH = 4.01, 7.00, and 10.01. Meso-tetra-
phenylporphyrin was synthesized by the method of Adler.44
Sulfonation of meso-tetra phenylporphyrin sodium salt
was achieved and subsequently purified by the method of
Srivastava.45
Concentration of Complexes and Ionic Strength of Aqueous
Solutions. Thermodynamic studies of (TSPP)Ir complexes in
water were carried out at concentrations less than 2 ꢁ 10-3 M to
minimize molecular and ionic association. Most equilibrium
constant measurements were performed at a low ionic strength
(μ ∼ 10-3) where the ion activity coefficients approach unity.
Synthesis of Na3[(TSPP)MIII(D2O)2] (M = Co, Rh, Ir). Na3-
[(TSPP)MIII(OD2)2] (M = Co, Rh, Ir) was synthesized follow-
ing reported methods by Ashley.32,33 Dissolution of 1 in D2O
Figure 7. 1H NMR (500 MHz, 298 K) of [(TSPP)IrI]5- with pyrrole
peak at 8.26 ppm and AA0 BB0 phenyl pattern centered at 8.15 ppm.
[(TSPP)Ir-H] by low concentrations of methanol ((1-5)
ꢁ 10-3 M) in basic D2O illustrate the effectiveness of this
system (eq 13, 14, Figure 7). The formation of Ir-OCH3,
Ir-H and IrI provide an entry point to many substrate
transformations.
results in solutions of the bis aquo complex [(TSPP)IrIII
-
(OD2)2]3- (1) in an equilibrium distribution with the mono
and bis hydroxo complexes, [(TSPP)IrIII(OD2)(OD)]4- (2),
[(TSPP)IrIII(OD)2]5- (3) . Na3[(TSPP)IrIII(OD2)2] 1H NMR
(500 MHz, D2O) δ(ppm): 8.93 (s, 8H, pyrrole), 8.44 (d, 8H,
1
1
o-phenyl, J1
= 8 Hz), 8.25 (d, 8H, m-phenyl, J1
= 8
½ðTSPPÞIrIIIðODÞðOCH3Þꢀ5- h
H- H
H- H
Hz). Na3[(TSPP)CoIII(D2O)2] 1H NMR (500 MHz, D2O)
1
H- H
δ(ppm): 9.38 (s, 8H, pyrrole), 8.41 (d, 8H, o-phenyl, J1
=
-
5-
½ðTSPPÞIr-DðODÞꢀ þ CH2O
ð13Þ
1
8 Hz), 8.23 (d, 8H, m-phenyl, J1H- H = 8 Hz). Na3[(TSPP)RhIII
(D2O)2] 1H NMR (500 MHz, D2O) δ(ppm): 9.15 (s, 8H,
= 8 Hz), 8.25 (d, 8H,
H- H
1
pyrrole), 8.44 (d, 8H, o-phenyl, J1
5-
- 6
½ðTSPPÞIr-DðODÞꢀ h½ðTSPPÞIrIðODÞꢀ þ Dþ ð14Þ
m-phenyl, J1H- H = 8 Hz). UV-vis (CH3OH) 408 nm, 516 nm.
1
Acid Dissociation Constant Measurement for [(TSPP)MIII
(OD2)2]3- (M = Co, Ir) in water. Samples of ([(TSPP)MIII
-
-
Summary
Acid dissociation constants for ([(TSPP)MIII(OD2)2]3-
(OD)2]5-) (M = Co, Ir) (4) were prepared by mixing standar-
dized D2O solution of NaOD with the stock solutions of complex
1 (0.5-1.0 ꢁ 10-3 M) in NMR tubes. A series of DCl and NaOD
deuterium oxide solutions were used to tune the pH values. A plot
)
(M = Co, Rh, Ir) (Table 1) in conjunction with equilibrium
constants for substitution of OCH3- and CH3OH for hydro-
xide and water are used to identify trends in ligand binding by
group nine porphyrin complexes. Equilibrium thermody-
namic studies quantitatively describe the preferenial binding
of anions (OD-, OCH3-) over that of neutral ligands (D2O,
CH3OH) for all of the group nine metalloporphyrin com-
plexes. The magnitude of the difference in binding energies
for anionic compared to neutral ligands decreases substan-
tially as the effective positive charge of the metal complex
decreases. All observations are consistent with a regular
increase in the metal ligand binding on moving down group
nine (Co, Rh, Ir) porphyrin complexes. The acid dissociation
constants and free energy changes in moving down group
nin-e (Ir > Rh > Co) demonstrate the general trend. Ex-
ceptionally large thermodynamic preferences are observed
for (TSPP)IrIII to bind methanol compared to water and
methoxide versus hydroxide and indicate the special cap-
ability of iridium(III) complexes to activate low concentra-
tions of alcohols in water.
1
of the pyrrole hydrogen H NMR chemical shifts to pD value
(pD = pH þ 0.41) and fit bynon-linear least-squares curve fitting
to the equation: δ2,3,4(obs)(pyr) = (K1K2δ3(pyr) þ K1[Dþ]δ2(pyr)
þ [Dþ]2 δ1(pyr))/(K1K2 þ K1[Dþ] þ [Dþ]2).
Equilibrium Constant Measurement for [(TSPP)MIII] þ
Methanol (M = Co, Rh, Ir) in D2O. To NMR tubes containing
basic (pD = 11) solutions of ∼10-3 M ([(TSPP)MIII(OD)2]5-
)
(M = Co, Rh, Ir) various quantities of methanol were added,
and the NMR spectra recorded. Similarly, small quantities of
methanol were made to NMR tubes containing acidic (pD = 3)
solutions of ∼10-3 M ([(TSPP)MIII(OD2)2]3-) (M = Co, Rh,
Ir), and the NMR spectra recorded. Integration of upfield
resonances against pyrrole resonances allowed the evaluation
of equilibrium constants for axially coordinated methoxide and
methanol species in equilibrium with hydroxide and water group
nine metalloporphyrins.
Synthesis of [(TSPP)Ir-D(OD2)]4-/[(TSPP)IrI(OD2)]5-. 0.4
mL [(TSPP)IrIII(OD2)2]3- D2O stock solutions (1.2-1.8 ꢁ 10-3
M, [Dþ] < 10-9 M) was added into a vacuum adapted NMR
tube. A 10 μL addition of a stock 2.4 ꢁ 10-1 M methanol/
deuterium oxide solution was added into the NMR tube and
degassed. The tube was heated for 2 days at 343 K, and the 1H
NMR (500 MHz, D2O) was obtained at 360 K, shown in
Figure 7.
Experimental Section
General Considerations. D2O was purchased from Cambridge
Isotope Laboratory Inc. and degassed by three freeze-pump-
thaw cycles before use. Proton NMR spectra were obtained on a
Bruker AvanceIII 500 MHz at 293 K. Chemical shifts were
referenced to 3-trimethyl silyl-1 propane sulfonic acid sodium
salt. Proton NMR spectra was used to identify solution species
and to determine the distribution of species at equilibrium. pH
measurements are performed on Thermo Scientific XL15 m and
Orion 9802 glass electrode43 precalibrated by Thermo Orion
Acknowledgment. This research was supported by the
Department of Energy, Division of Chemical Sciences and
Office of Science through Grant DE-FG02-09ER-16000.
Supporting Information Available: Calculations ofequilibrium
constants, tables and representative NMR spectra. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(43) Glasoe, P. K.; Long, F. A. J. Phys. Chem. 1960, 64, 188.
(44) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour,
J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476.
(45) Srivastava, T. S.; Tsutsui, M. J. Org. Chem. 1973, 38, 2103.