Methanol as a reaction medium and reagent in substrate reactions of rhodium porphyrins

Article abstract of DOI:10.1021/ic900900k

Methanol solutions of rhodium(III) tetra(ρp-sulfonatophenyl) porphyrin [(TSPP)Rh^III] have a hydrogen ion dependent equilibrium between bis-methanol, monomethoxy monomethanol, and bis-methoxy complexes. Reactions of dihydrogen (D₂) wi

Full text of DOI:10.1021/ic900900k

8550 Inorg. Chem. 2009, 48, 8550–8558
DOI: 10.1021/ic900900k
Methanol as a Reaction Medium and Reagent in Substrate Reactions of Rhodium
Porphyrins
Shan Li,† Sounak Sarkar,† and Bradford B. Wayland*,‡
†Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, and ‡Department of
Chemistry, Temple University, Philadelphia, Pennsylvania 19122
Received May 8, 2009
Methanol solutions of rhodium(III) tetra(p-sulfonatophenyl) porphyrin [(TSPP)RhIII] have a hydrogen ion dependent
equilibrium between bis-methanol, monomethoxy monomethanol, and bis-methoxy complexes. Reactions of
dihydrogen (D2) with solutions of [(TSPP)RhIII] complexes in methanol produce equilibrium distributions of a rhodium
hydride [(TSPP)RhIII-D(CD3OD)]-4 and rhodium(I) complex [(TSPP)RhI(CD3OD)]-5. The rhodium hydride complex
in methanol functions as a weak acid with an acid dissociation constant of 1.1(0.1)ꢀ10-9 at 298 K. Patterns of
rhodium hydride substrate reactions in methanol are illustrated by addition with ethene, acetaldehyde, and carbon
monoxide to form rhodium alkyl, R-hydroxyethyl, and formyl complexes, respectively. The free energy change for
the addition reaction of [(TSPP)RhIII-D(CD3OD)]-4 with CO in methanol to produce a formyl complex (ΔG°(298K)
=
-4.7(0.1) kcal mol-1) is remarkably close to ΔG°(298K) values for analogous reactions in water and benzene. Addition
reactions of the rhodium hydride ([(TSPP)RhIII-D(CD3OD)]-4) with vinyl olefins invariably yield the anti-Markovnikov
product which places the rhodium porphyrin on the less hindered terminal primary carbon center. Addition of the
rhodium-methoxide unit in [(TSPP)RhIII-OCD3(CD3OD)]-4 with olefins to form β-methoxyalkyl complexes places
rhodium on the terminal carbon for alkene hydrocarbons and vinyl acetate, but vinyl olefins that have π-electron
withdrawing substituents have a thermodynamic preference for placing rhodium on the interior carbon where negative
charge is better accommodated. Equilibrium thermodynamic values for addition of the Rh-OCD3 unit to olefins in
methanol are evaluated and compared with values for Rh-OH addition to olefins in water.
Introduction
thermodynamics for organometallic processes.1-13 Com-
parative studies of organometallic reactions of rhodium
porphyrins in benzene and water have revealed that analo-
gous reactions often occur with similar thermodynamics
because of compensating solvation terms but that different
types of solvent media can often direct reactions to occur
by substantially different mechanisms.14-19 Substrate reac-
tions in water generally occur much faster and utilize
ionic pathways that are not available in benzene. Rhodium
porphyrin substrate reactions that are distinctly different
Selection of a reaction medium is an important parameter
that can be varied to influence both the mechanism and the
*To whom correspondence should be addressed. E-mail:bwayland@
temple.edu.
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Organometallics 2006, 25, 410–415. (c) Lucey, D. W.; Helfer, D. S.; Atwood, J. D.
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pubs.acs.org/IC
Published on Web 07/30/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 17, 2009 8551
Figure 1. Tetra( p-sulfonatophenyl) rhodium(III) porphyrin ((TSPP)Rh(III)) and tetrakis(3,5-disulfonatomesityl)rhodium(III) porphyrin ((TMPS)-
Rh(III)) are representative methanol soluble porphyrins.
in water compared to benzene are observed to occur only
when water functions as a reactant. Dirhodium ketone (Rh-
C(O)-Rh) and dirhodium diketone (Rh-C(O)-C(O)-Rh)
complexes14,15 which are prominent stable species in ben-
zene20-23 are observed to react by water gas shift type
processes in water.14,15 Formation of β-hydroxyalkyl com-
plexes from reactions of olefins and rhodium(III) porphyrin
species in aqueous media also illustrate water functioning
as a reagent.17
This article reports on the reactivity patterns for rhodium
porphyrin species that occur in methanol solvent media and
on the evaluation of equilibrium thermodynamics for rho-
dium hydride and rhodium methoxide reactions with sub-
Figure 2. 1H NMR resonance for porphyrin pyrrole hydrogens at a
series of deuterium ion concentrations.
strates. Regioselectivity for reactions of olefins that produce
rhodium alkyl and β-methoxyalkyl derivatives is compared
with related reactions in water.14,15
resonances for the equilibrium distribution of 1, 2, and 3
in methanol at 298 K. Changes in the 1H NMR chemical
shift for the porphyrin pyrrole hydrogens for an equilibrium
distribution of 1, 2, and 3 as a function of the deuterium
ion concentration are illustrated in Figure 2. The mole
fraction averaged pyrrole hydrogen resonances as a func-
tion of -log[Dþ] were used in determining the acid dissocia-
Results and Discussion
Methanol and Methoxide Complexes of (TSPP)Rh(III)
in Methanol. Sulfonated tetraphenyl porphyrin deriva-
tives provide a convenient set of rhodium porphyrin
complexes for reactivity studies in methanol (CD3OD)
solutions (Figure 1).
Rhodium(III) species ([(TSPP)RhIII]) in methanol
(CD3OD) occur as an equilibrium mixture of a bis-metha-
nol [(TSPP)RhIII(CD3OD)2]-3 (1), monomethanol mono-
methoxide [(TSPP)RhIII(OCD3)(CD3OD)]-4 (2), and bis-
methoxide [(TSPP)RhIII(OCD3)2]-5 (3) complexes with
the equilibrium distribution of 1, 2, and 3 dependent on
the deuterium ion concentration (eqs 1 and 2).
tion constants for [(TSPP)RhIII(CD3OD)2]-3 (1) (K1(298K)
=
6.9(0.2) ꢀ10-8) and 2 (K2(298K)=5.8(0.3)ꢀ10-13) (Figure 3)
by nonlinear least-squares fitting to the expression [δ1,2,3
=
(K1K2δ3 þ K1[Dþ]δ2 þ [Dþ]2δ1)/(K1K2þK1[Dþ] þ [Dþ]2)]15
(Figure 3). The distribution of [(TSPP)RhIII] species 1, 2, and
3in a sample with a total rhodium porphyrin concentration of
1.0 ꢀ 10-3 M in CD3OD is displayed at a series of Dþ
concentrations in Figure 4. This plot provides a convenient
means for establishing the hydrogen ion concentration con-
ditions needed to obtain the desired distribution of 1, 2,
and 3. For example, the monomethoxide [(TSPP)RhIII-
(OCD3)(CD3OD)]-4 is found to be the dominant species
present in methanol when the pD is in the range of 9-11
(Figure 4).
½ðTSPPÞRhIIIðCD3ODÞ ꢁ-3 h
½ðTSPPÞRhIIIðOCD3ÞðCD3ODÞꢁ þ Dþ ð1Þ
2
-4
½ðTSPPÞRhIIIðOCD3ÞðCD3ODÞꢁ-4 h
-5
½ðTSPPÞRhIIIðOCD3Þ2ꢁ þ Dþ
ð2Þ
The first acid dissociation constant for the bis-methanol
complex [(TSPP)RhIII(CD3OD)2]-3 (1) inCD3OD (K1(298K)
=6.9(0.2) ꢀ 10-8; ΔG°1(298K)=9.8 (0.1) kcal mol-1) is five
times larger than the dissociation constant for the bis-aquo
complex [(TSPP)RhIII(D2O)2]-3 in D2O (K = 1.4(0.2) ꢀ
10-8 ;ΔG°(298K) = 10.7 (0.1) kcal mol-1).15 This is a reversal
from the order of acid dissociation constants for pure liquid
Rapid proton exchange processes result in a single
set of mole fraction averaged porphyrin 1H NMR
(20) Cui, W.; Wayland, B. B. J. Am. Chem. Soc. 2004, 126, 8266–8274.
(21) Cui, W.; Li, S.; Wayland, B. B. J. Organomet. Chem. 2007, 692, 3198–
3206.
(22) Cui, W.; Zhang, X. P.; Wayland, B. B. J. Am. Chem. Soc. 2003, 125,
CD3OD and D2O where pure CD3OD(l) (eq 3) (K3(298K) =
1.0(0.3) ꢀ 10-18; ΔG°3(298K)=24.5 (0.1) kcal mol-1) is sub-
4994–4995.
(23) Coffin, V. L.; Brennen, W.; Wayland, B. B. J. Am. Chem. Soc. 1988,
110, 6063–6069.
stantially less acidic than pure D2O(l) (K(298K) =2.44ꢀ10-17
;

8552 Inorganic Chemistry, Vol. 48, No. 17, 2009
Li et al.
Scheme 1. Thermodynamic Comparison (at 298 K) for the Substitution of Methoxide for Methanol in CD3OD and Hydroxide for Water in D2O15 for
(TSPP)RhIII Complexes
Figure 4. Equilibrium distributions of [(TSPP)RhIII(CD3OD)2]-3 (1),
[(TSPP)RhIII(OCD3)(CD3OD)]-4 (2), and [(TSPP)RhIII(OCD3)2]-5 (3)
Figure 3. Observed limiting fast-exchange mole fraction-averaged pyr-
for a CD3OD solution with a total rhodium porphyrin concentration
role 1H NMR chemical shifts for 1, 2, and 3 in CD3OD as a function
of-log[Dþ]. The solid lineisthe nonlinearleast-squaresbest-fitlinegiving
of 1.0ꢀ10-3 M as a function of the equilibrium deuterium ion concentra-
K1(298K) = 6.9(0.2) ꢀ 10-8 and K2(298K) = 5.8(0.3) ꢀ 10-13. δ1(pyr) =
tion ([Dþ]).
9.14 ppm, δ2(pyr) =8.93 ppm, δ3(pyr) =8.74 ppm.
(Scheme 2), and the formaldehyde product oxidizes
further to formate. The methanol oxidation to formalde-
hyde is currently viewed as occurring by an intermole-
cular β C-H deprotonation of coordinated methoxide in
the relatively high dielectric and highly basic medium.
The fast quantitative air oxidation of rhodium(I) to
rhodium(III) is an important feature of the rhodium
porphyrin system that permits rhodium(III) porphyrins
to function as oxidation catalysts by directly using O2
without the need for cocatalysts.19
Scheme 2. Proposed Pathway for Stoichiometric and Catalytic
Oxidation of Methanol by Rhodium(III) Porphyrin
ΔG°(298K) =22.6 (0.1) kcal mol-1). Combining reaction 1
and the reverse of reaction 3 gives an expression
Reaction of [TSPP)RhIII(CD3OD)2]-3 with Hydrogen
in Methanol. The bis-methanol complex [(TSPP)-
RhIII(CD3OD)2]-3 (1) reacts slowly with H2/D2 (PH
2
∼ 0.5 atm) in acidic CD3OD media ([Dþ]>10-5 M) to
form the hydride complex [(TSPP)Rh-D (CD3OD)]-4 (4)
(eq 4), which occurs in equilibrium with the rhodium(I)
derivative [(TSPP)RhI(CD3OD)]-5 (5) (eq 5).
CD3OD h CD3O- þ Dþ
ð3Þ
for the substitution of methoxide for methanol (Scheme 1).
Displacement of a neutral methanol ligand by the methox-
ide anion is highly thermodynamically favorable (ΔG°(298K)
=-14.8 (0.1) kcal mol-1). The corresponding free energy
change for substitution of hydroxide for coordinated water
in aqueous solution is ΔG°(298K)=-11.9 (0.1) kcal mol-1
(Scheme 1).15
-3
½ðTSPPÞRhIIIðCD3ODÞ2ꢁ þD2 h
-4
½ðTSPPÞRhIII -DðCD3ODÞꢁ þ Dþ þ CD3OD
ð4Þ
Stoichiometric and Catalytic Oxidation of Methanol
by Rhodium(III) Porphyrins. Rhodium(III) porphyrin
species in CD3OD at strongly basic conditions (pD>11)
are reduced to rhodium(I) porphyrin, and methanol
is oxidized to formaldehyde (Scheme 2). One distinct
difference between methanol and water as solvent media
is that the alcohol at strongly basic condition functions
as a reducing agent for rhodium(III) and thus rhodium-
(III) porphyrins are not viable reagents for substrate
reactions in strongly basic methanol. Oxidation of
methanol becomes catalytic through rapid quantitative
oxidation of (TSPP)RhI to (TSPP)RhIII by dioxygen
½ðTSPPÞRhIII -DðCD3ODÞꢁ-4 h
-5
½ðTSPPÞRhIðCD3ODÞꢁ þ Dþ
ð5Þ
The equilibrium distribution of 4 and 5 depends on the
Dþ concentration in solution. The deuterium ion ex-
change between 4 and 5 in methanol is sufficiently slow
to permit direct observation of individual sets of porphyr-
in hydrogen NMR resonances when pD is between 8 and
10 (Figure 5). This contrasts with aqueous media where
proton exchange between water and the rhodium hydride

Article
Inorganic Chemistry, Vol. 48, No. 17, 2009 8553
Figure 5. 1H NMR resonances for the pyrrole and phenyl hydrogens of an equilibrium distribution of [(TSPP)Rh-D(CD3OD)2]-4 (4) and
[(TSPP)RhI(CD3OD)]-4 (5) in CD3OD at pD = 9.21 (T = 298 K), (K5(298K) = 1.1 (0.1) ꢀ 10-9).
Table 1. Equilibrium Constants (Kn) and ΔG°(298K) (kcal mol-1) for (TSPP)Rh reactions in CD3ODa
Kn(298K)
ΔG°(298K) (kcal mol-1
)
(1) [RhIII(CD3OD)2]-3 h [RhIII(OCD3)(CD3OD)]-4 þ Dþ
(2) [RhIII(OCD3)(CD3OD)]-4 h [RhIII(OCD3)2]-5 þ Dþ
K1 = 6.9(0.2) ꢀ 10-8
K2 = 5.8(0.3) ꢀ 10-13
K3 = 1.0(0.3) ꢀ 10-18
K4 = 1.0(0.1) ꢀ 103
K5 = 1.1(0.1) ꢀ 10-9
K6 = 3.0(0.1) ꢀ 103
þ9.8(0.1)
þ16.7(0.1)
þ24.5(0.1)b
- 4.1(0.1)
þ12.2(0.1)
- 4.7(0.1)
(3) CD3OD h OCD3 þ Dþ
-
(4) [RhIII(CD3OD)2]-3 þ D2 h [Rh-D(CD3OD)]-4 þ Dþ þ CD3OD
(5) [Rh-D(CD3OD)]-4 h [RhI(CD3OD)]-5 þ Dþ
(6) [Rh-D(CD3OD)]-4 þ CO h [Rh-CDO(CD3OD)]-4
a The reported K values correspond to equilibrium constant expressions that contain all constituents given in the chemical equation including
methanol. b The pKa (298 K) of pure CH3OH (Ka (298K) = 2.5 ꢀ 10-17) was used in deriving the Ka for CD3OD (K3(298K) = 1.0 (0.3) ꢀ 10-18).
is fast on the NMR time scale.15 The equilibrium constant
for reaction 4 was evaluated in methanol from 1H NMR
shift positions and intensities in conjunction with the Dþ
concentration (Table 1). The free energy change at 298 K
for reaction of the bis-methanol complex [(TSPP)-
Rh(CD3OD)2]-3 with hydrogen (D2) in methanol (eq 4)
(ΔG°4(298K) =-4.1 kcal mol-1) is about 2.4 kcal mol-1
more favorable than the analogous reaction of [(TSPP)-
Scheme 3. Simultaneous Equilibria that Occur in the (TSPP)Rh(III)/
D2 System in CD3OD
Rh(D2O)2]-3 with D2 in water ([(TSPP)Rh(D2O)2]-3
þ
D2 h [(TSPP)Rh-D(D2O)]-4 þ Dþ þ D2O; ΔG°(298K)
=
-1.7 (0.1) kcal mol-1).15
Evaluation of the Acid Dissociation Constant for
[(TSPP)RhIII-D(CD3OD)]-4 in Methanol. The acid dis-
sociation constant for the hydride complex [(TSPP)RhIII
-
D(CD3OD)]-4 4 in CD3OD is substantially smaller
(K5(298K) = 1.1(0.1) ꢀ 10-9; ΔG°5(298K) = 12.2 (0.1) kcal
and methanol, respectively).24 The smaller extent of
heterolytic dissociation and thus acidity of the rhodium
hydride in methanol compared to water is an important
difference between water and methanol media.
mol-1
)
than the corresponding aquo complex
[(TSPP)RhIII-D(D2O)]-4 in D2O (K(298K) = 8.0 ꢀ 10-8
;
ΔG°(298K) = 9.7 (0.1) kcal mol-1). The heterolytic dis-
sociation of the Rh-D unit in water is 2.5 kcal mol-1
(Δ(ΔG°)(298K)=-2.5 kcal mol-1) more favorable than in
methanol. The difference is effectively same as the
difference in proton solvation energies in water and
Reactivity Patterns for [(TSPP)RhIII-D(CD3OD)]-4 in
Methanol with CO, Aldehydes, and Olefins. Carbon mon-
oxide, acetaldehyde, and ethene react with the hydride
methanol (Δ(ΔG°)(298K) =ΔG°Hþ(H O)- ΔG°Hþ(CH OH)
=
2
3
(24) (a) Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2007,
111, 408–422. (b) Pliego, J. R. Jr.; Riveros, J. M. Chem. Phys. Lett. 2000, 332,
597–602.
-2.4 kcal mol-1, where ΔG°Hþ(H O) and ΔG°Hþ(CH OH)
are the free energies for solvation of the proton in water
2
3

8554 Inorganic Chemistry, Vol. 48, No. 17, 2009
Li et al.
Table 2. Equilibrium Constants and ΔG° (kcal mol-1) for (TSPP)Rh Reactions in Methanol (298 K) Derived from Data in Table 1
a
Kn
ΔG°
(12) [RhIII(OCD3)(CD3OD)]-4 þ D2 h [Rh-D(CD3OD)]-4 þ CD3OD
(13) [RhIII(OCD3)2]-5 þ D2 h [RhI(CD3OD)]-5 þ CD3OD
K12 = K4/K1 = 1.4 ꢀ 1010
K13 = K4K5/(K1K2) = 2.7 ꢀ 1013
K14 = K1/K3 = 6.9 ꢀ 1010
K15 = K4K20 = 5.2 ꢀ 1040
K16 = K4K6 /K1 = 4.3 ꢀ 1013
K17 = K3K6 /K5 = 2.7 ꢀ 10-6
K18 = K1 /K5 = 63
-13.8(0.1)
-18.3(0.1)
-14.8(0.1)
-55.5(0.1)
-18.6(0.1)
7.6(0.1)
-2.5(0.1)
-16.3(0.1)
-53.2b
(14) [RhIII(CD3OD)2]-3 þ OCD3- h [Rh(OCD3)(CD3OD)]-4 þ CD3OD
(15) [RhIII(CD3OD)2]-3 þ D- h [Rh-D(CD3OD)]-4 þ CD3OD
(16) [RhIII(OCD3)(CD3OD)]-4 þ D2 þ CO h [Rh-CDO(CD3OD)]-4 þ CD3OD
(17) [RhI(CD3OD)]-5 þ CD3OD þ CO h [Rh-CDO(CD3OD)]-4 þ OCD3
-
(18) [RhIII(CD3OD)2]-3 þ [RhI(CD3OD)]-5 h[Rh(OCD3)(CD3OD)]-4 þ [Rh-D(CD3OD)]-4
(19) [RhIII(CD3OD)2]-3 þ [RhI(CD3OD)]-5 þD2 h 2[Rh-D(CD3OD)]-4 þ CD3OD
(20) Dþ þ D- h D2
K19 = K4 /K5 = 9.1 ꢀ 1011
K20 = 5.2 ꢀ 1037
a The reported K values correspond to equilibrium constant expressions that contain all constituents given in the chemical equation including water
(T = 298 K). b E°(2Hþ þ 2e- h H2) = 0.00 V and E°(2H- h H2 þ 2e-) = þ2.23 V were used to evaluate ΔG°(Hþ þ H- h H2) = -51.4 kcal mol-1 and
ΔG°(Dþ þ D- h D2) = -53.2 kcal mol-1. CRC Handbook of Chemistry and Physics, 71st ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1990-1991;
pp 8-38, 6-11, and 8-17.
1
Figure 6. High-field H NMR resonances for (A) (TSPP)Rh-CH2-CH2-OCD3 and (B) [(TSPP)Rh-CH2-CH(OCD3)-CH3. (Inset: Observation of the
β-OCH3 group in (TSPP)Rh-CH(CO2CH3)-CH2-OCH3 and very slow exchange of the -OCD3 group in CD3OD. (a) t = 0, (b) t = 1 month, (c) t =
2 months, and (d) t = 3 months.)
complex [(TSPP)Rh-D(CD3OD)]-4 (4) (pD=6 - 8, 298
K) in methanol to produce rhodium formyl [(TSPPRhIII
-
Scheme 4. Reactions of (TSPP)Rh(III) with Styrene or Methyl Acry-
late in Methanol
CDO(CD3OD)]-4 (6) (eq 6), R-hydroxyethyl [(TSPP)-
RhIII-CH(OD)CH3(CD3OD)]-4 (7) (eq 7), and ethyl
[(TSPP)RhIII-CH2CH2D(CD3OD)]-4 (8) (eq 8) com-
plexes respectively.
-4
½ðTSPPÞRh-DðCD3ODÞꢁ þ CO h
-4
½ðTSPPÞRh-CDOðCD3ODÞꢁ
ð6Þ
-4
½ðTSPPÞRh-DðCD3ODÞꢁ þ CH3CHO h
-4
½ðTSPPÞRh-CHðODÞCH3ðCD3ODÞꢁ
ð7Þ
ð8Þ
-4
½ðTSPPÞRh-DðCD3ODÞꢁ þ CH2 ¼ CH2 h
give organo-rhodium complexes corresponding to anti-
Markovnikov regioselectivity which places rhodium on
the terminal primary carbon (eq 9). The regioselectivities
observed for addition of rhodium hydride (Rh-D) with
olefins in methanol are identical to those previously
observed for the analogous processes in water.16
-4
½ðTSPPÞRh-CH2CH2DðCD3ODÞꢁ
The reaction of CO with 4 to produce the formyl complex
(eq 6) achieves a 1H NMR measurable equilibrium
(K6(298K)=3.0 ꢀ 103; ΔG°6(298K)=-4.7 (0.1) kcal mol-1
)
(Table 1) which is the same ΔG° value as that measured for
the addition of rhodium hydride [(TSPP)Rh-D] with CO to
give a formyl complex in water.15,16
-4
½ðTSPPÞRh-DðCD3ODÞꢁ þ CH2 ¼ CHY h
-4
½ðTSPPÞRhIII-CH2CH2YðCD3ODÞꢁ
ð9Þ
Addition reactions of the rhodium hydride 4 with
both unactivated alkene hydrocarbons and activated
vinyl olefins like styrene and methyl acrylate exclusively
Y ¼ H, alkyl, C6H5, CðOÞOCH3

Article
Inorganic Chemistry, Vol. 48, No. 17, 2009 8555
Table 3. Directly Measured Equilibrium constants (Kn) and ΔG° (kcal mol-1) Values for the Reactions of [(TSPP)RhIII(CD3OD)2]-3 with olefins in CD3OD (T = 298 K)
-5
Reaction of [(TSPP)RhI(CD3OD)]
with CH3I and
by the set of characteristic widely spread porphyrin
[(TSPP)RhIII(CD3OD)] -3 in Methanol. The rhodium(I)
complex [(TSPP)RhI(CD3OD)]-5 (5) retains the proper-
ties of a nucleophile in methanol in spite of the fact that
methanol is both a donor and a hydrogen bonding
medium. This is illustrated by the immediate reaction
of 5 with CH3I to form a rhodium methyl complex
[(TSPP)RhIII-CH3(CD3OD)]-4 (9) (eq 10). At acidic con-
ditions ([Dþ] > 10-6 M) where the rhodium hydride is the
predominant species, the reaction with CH3I to form 9
occurs very slowly because of the low level of heterolytic
Rh-H dissociation in methanol.
phenyl H NMR resonances that have been previously
1
reported for 1H NMR spectra of [(por)RhII]2 com-
plexes17,25 and other metal-metal bonded face to face
dimers.26
Equilibrium Thermodynamics for Reactions of [(TSPP)-
RhIII] Complexes with Hydrogen in Methanol. Methanol
solutions of rhodium(III) tetra( p-sulfonatophenyl) por-
phyrin [(TSPP)RhIII] complexes with methanol and
methoxide axial ligands react with dihydrogen H2/D2 to
form an equilibrium distribution of six rhodium species
including
a rhodium hydride, rhodium(I) anion,
rhodium(II) dimer, and three rhodium(III) methanol/
methoxide complexes (Scheme 3). By adjusting the Dþ
and dihydrogen concentrations it is possible to produce
1H NMR measurable equilibria of [(TSPP)Rh] species in
methanol. A series of equilibrium constants for most of
the processes in Scheme 3 were measured directly by 1H
NMR and are summarized in Table 1. The measured
thermodynamic values found in Table 1 were used to
derive additional thermodynamic values for reactions
12-19, which are listed in Table 2.
-5
½ðTSPPÞRhIðCD3ODÞꢁ þCH3I h
-4
½ðTSPPÞRhIII-CH3ðCD3ODÞꢁ þ I-
[(TSPP)RhI(CD3OD)]-5 also reacts with methanol so-
lutions of [(TSPP)RhIII(CD3OD)2]-3 (1) to produce an
equilibrium distribution with a RhII-RhII bonded dimer
[(TSPP)RhII(CD3OD)]2-8 (10) (eq 11).
ð10Þ
-5
-3
½ðTSPPÞRhIðCD3ODÞꢁ þ ½ðTSPPÞRhIIIðCD3ODÞ2ꢁ
Reactions of (TSPP)RhIII Species with Olefins in
Methanol. Rhodium methanol/methoxide complexes
(1, 2, and 3) in methanol react with olefins to produce
-8
h½ðTSPPÞRhIIðCD3ODÞꢁ2 þ CD3OD
The rhodium(II) dimer(10) never becomes the majo-
ð11Þ
1
rity species but achieves a H NMR observable con-
(25) Wayland, B. B.; Van Voorhees, S. L.; Wilker, C. Inorg. Chem. 1986,
25, 4039–4042.
(26) Collman, J. P.; Barnes, C. E.; Collins, T. J.; Brothers, P. J. J. Am.
Chem. Soc. 1981, 103, 7030–7032.
centration in a narrow range of pD (pD 7.8-8.2). The
RhII-RhII bonded dimer 10 is recognized in solution

8556 Inorganic Chemistry, Vol. 48, No. 17, 2009
Li et al.
Table 4. Equilibrium Constants (Kn) and ΔG° (kcal mol-1) Values for Reactions of [(TSPP)RhIII-OCD3(CD3OD)]-4 with Olefins in CD3OD Derived from Table 3
(T = 298 K)
-3
½ðTSPPÞRhIIIðCD3ODÞ2ꢁ þ CH2dCHC3H7 h ½ðTSPPÞ
rhodium β-methoxyalkyl complexes. Formation of the β-
alkoxyalkyl rhodium complexes ([(TSPP)RhIII-CH 2CH-
-4
RhIII-CH2CHC3H7ðOCD3ÞðCD3ODÞꢁ þ D þ ð23Þ
(R)(OCD3)(CD3OD)]-4) in CD3OD is followed by the
appearance of characteristic high field H NMR reso-
nances that result from porphyrin ring current effects on
1
convert to an equilibrium distribution with the thermo-
the hydrogens in groups bonded to the rhodium center
dynamically preferred products where rhodium is at-
(Figure 6). Formation of the β-alkoxyalkyl species in
CH3OH and then dissolution in CD3OD permits obser-
tached to the internal carbon and methoxide is on the
terminal primary carbon center (eqs 24-26; Scheme 4).
The kinetically preferred initial product (eq 24) places the
1
vation of the -OCH protons by H NMR (δ = 1.47
3
ppm) which very slowly decline in intensity by exchange
bulky rhodium porphyrin at the least sterically encum-
of methoxide groups with CD3OD (Figure 6, inset).
bered terminal carbon, but the thermodynamically pre-
Reactions of alkenes including ethene, propene,and
ferred isomer places rhodium at the internal carbon
pentene with [(TSPP)RhIII- OCD 3(CD3OD)]-4 produce
center (eq 25).
regiospecific products with rhodium attached exclusively
to the terminal primary carbon [(TSPP)RhIII-CH - CH
-3
½ðTSPPÞRhIIIðCD3ODÞ2ꢁ þ CH2dCHC6H5 h ½ðTSPPÞ
2
2(OCD3)(CD3OD)]-4 (11), [(TSPP)RhIII-CH2CH(OC-
D3)CH3(CD3OD)]-4 (12), and [(TSPP)RhIII-CH 2 -CH-
-4
RhIII-CH2CHðOCD3ÞC6H5ðCD3ODÞꢁ þ D þ ð24Þ
(OCD 3)C3H7(CD3OD)]-4 (13), (eqs 21-23). Vinyl
acetate which has a π-donor methoxide substituent also
-3
½ðTSPPÞRhIIIðCD3ODÞ2ꢁ þ CH2dCHC6H5 h ½ðTSPPÞ
reacts with 1 to give the regioselectivity observed for
alkenes [(TSPP)Rh-CH CH(OCD3)(OC(O)CH3)(CD3-
2
OD)] (15). Reactions of 1 with olefins activated by
π-electron withdrawing groups such as styrene and meth-
yl acrylate initially produce a kinetic product where
rhodium adds to the terminal carbon, but then slowly
-4
RhIII-CHðC6H5ÞCH2ðOCD3ÞðCD3ODÞꢁ þ D þ
ð25Þ
-3
½ðTSPPÞRhIIIðCD3ODÞ2ꢁ þ CH2dCHCO2CH3
-3
½ðTSPPÞRhIIIðCD3ODÞ2ꢁ þCH2dCH2 h
h ½ðTSPPÞRhIII-CHðCO2CH3Þ-CH2ðOCD3Þ
-4
½ðTSPPÞRhIII-CH2CH2ðOCD3ÞðCD3ODÞꢁ þ Dþ ð21Þ
-4
ðCD3ODÞꢁ þ D þ
ð26Þ
-3
½ðTSPPÞRhIIIðCD3ODÞ2ꢁ þ CH2dCHCH3 h ½ðTSPPÞ
Thermodynamic preference for rhodium bonding to
the internal carbon results from better stabilization of
-4
RhIII-CH2CHCH3ðOCD3ÞðCD3ODÞꢁ þ D þ ð22Þ

Article
Inorganic Chemistry, Vol. 48, No. 17, 2009 8557
negative charge, and this electronic effect more than
compensates for the larger destabilizing steric effect for
this isomer.27 The kinetic and thermodynamic regioselec-
tivities for addition of the Rh-OH unit in water17 and
Rh-OCD3 unit in methanol are the same for all olefins
studied.
The larger acid dissociation constant for (TSPP)Rh-D in water
(Ka(298K) = 8.0 ꢀ 10-8) compared to methanol can be
explained in terms of the slightly larger solvation energy for
protons in water compared to methanol (Δ(ΔG°)=-2.4 kcal
mol-1).24 The rhodium hydride [(TSPP)Rh-D(CD3OD)]-4 in
methanol reacts with CO, acetaldehyde, and olefins to produce
formyl, R-hydroxyethyl, and alkyl complexes respectively
which are also prominent substrate reactions in water16 and
benzene.28 Compensating solvation energies for reactants
and products yields equal thermodynamics to produce a
rhodium formyl complex from reactions of the rhodium
hydride with CO in methanol and water (K(298K) = 3.0 ꢀ
103, ΔG°(298K) = -4.7 kcal mol-1) and very nearly equal
thermodynamics for the analogous reaction in benzene.28
Addition of rhodium hydride [(TSPP)RhIII-D(CD3OD)]-4
to olefins invariably forms the anti-Markovnikov product
which places the rhodium porphyrin unit on the less hindered
terminal primary carbon center. The rhodium methoxide unit
in [(TSPP)RhIII-OCD3(CD3OD)]-4 reacts with alkene hydro-
carbons and vinyl acetate to give β-methoxyalkyl complexes
where rhodium is attached to the terminal primary carbon, but
reaction with olefins that are activated by π-acceptor groups
results in placing the rhodium prophyrin at the more sterically
hindered interior carbon center (Tables 3 and 4). The regios-
electivity and equilibrium thermodynamics for addition of the
rhodium methoxide unit in [(TSPP)RhIII-OCD3(CD3OD)]-4
to olefins in methanol complexes parallel the reported results
for olefin reactions of the analogous hydroxide complex
[(TSPP)RhIII-OD(D2O)]-4 in water.17 Continuing thermody-
namic studies of cobalt, rhodium, and iridium porphyrin
substrate reactions are directed toward achieving direct com-
parisons of bond dissociation free energies (BDFE) for group
9 metalloporphyrins in diverse solvent media.
Equilibrium Thermodynamics for Rhodium(III) Reac-
tions with Olefins in CD3OD. Equilibrium constants
for reactions of [(TSPP)RhIII(CD3OD)2]-3 (1) with
1
olefins in CD3OD were directly evaluated by H NMR
along with measurement of the Dþ concentration (pD =
4-6, 298 K). The equilibrium constants and free energy
changes for reactions of 1 with olefins (reactions 21-26)
are summarized in Table 3, and values derived for reac-
tions of [(TSPP)RhIII(ODC3)(CD3OD)]-4 are in Table 4.
Reactions between [(TSPP)RhIII-OCD3(CD3OD)]-4
(2) and olefins to form rhodium β-methoxyalkyl com-
plexes are relatively fast and thermodynamically highly
favorable. The reactions proceed effectively to comple-
tion which prohibits direct evaluation of the equilibrium
constants by 1H NMR. Equilibrium thermodynamics
between 2 and olefins however can be evaluated indirectly
by using the measured thermodynamic values for reac-
tions of the rhodium bis-methanol complex 1 with olefins
and the acid dissociation constant of 1 (K1). The derived
thermodynamic values for addition of Rh-OCD3 unit to
olefins (reactions 27-32) are summarized in Table 4. The
free energy changes for adding the Rh-OCD3 unit to
alkene hydrocarbons in methanol are very similar to the
previously reported values for addition of the Rh-OD
unit to the alkenes in water.17 The average difference for
the ΔG° values favors the Rh-OCD3 addition to alkenes
by about 1.0 kcal mol-1. One specific example is the
addition of the (TSPP)Rh-OCD3 to ethene in methanol
(eq 27; ΔG°27 = -6.9(0.1) kcal mol-1) which compares to
a ΔG°(298K) of - 5.9(0.1) kcal mol-1 for the addition of
the (TSPP)Rh-OD unit to ethene in water.17
Experimental Section
Materials and Instruments. The sodium salt of free-base
tetra(p-sulfonatophenyl)porphyrin Na4[(TSPP)H2] xH2O was
purchased from Sigma Adlrich and used as received without any
3
Summary and Conclusions
further purification. The rhodium complex Na3[(TSPP)RhIII
-
(CH3OH)2] was synthesized using a literature method.29 Metha-
nol-d4 (CD3OD, 99.8 D%) was purchased from Cambridge
Isotope Laboratories, Inc., and used without further purifica-
tion. Sodium methoxide (NaOCH3) and trifluoromethanesul-
fonic acid-d (CF3SO3D) were purchased from Aldrich and
dissolved in CD3OD to make 0.01 M stock solutions. 3-Tri-
methylsilyl-1-propanesulfonic acid sodium salt (CH3)3SiCH2-
The broad conclusion from reactivity and thermodynamic
studies is that the general behavior of rhodium porphyrins is
remarkably similar in methanol and water but that differences
in ligand binding and solvation energies have a significant
impact on the thermodynamic parameters. Methanol soluble
rhodium(III) porphyrins [(TSPP)RhIII] in methanol establish
equilibria between bis-methanol (1), monomethoxide mono-
methanol (2), and bis-methoxide (3) complexes, which parallel
the analogous mono- and bis-hydroxide complexes in water.15
The methanol complexes are more acidic than the water
complexes which is a manifestation of larger free energy
change for CD3O- substituting for coordinated CD3OD
(ΔG°(298K) = -14.8 kcal mol-1) when compared to OD-
substituting for coordinated water (ΔG°(298K) = -11.7 kcal
mol-1) (Scheme 1). In strongly basic methanol (CD3OD)
(pD>11), the (TSPP)RhIII-OCH3 complexes are reduced to
a rhodium(I) complex which is a reaction that cannot occur
with water or benzene. The rhodium(III) species in methanol
(pD < 8) react with hydrogen (D2) to form a hydride complex
[(TSPP)Rh-D(CD3OD)]-4 that behaves as a weak acid
(K(298K) = 1.1 ꢀ 10-9; ΔG°(298K) = 12.2(0.1) kcal mol-1).
CH2CH2SO3Na H2O was purchased from Eastman Kodak
3
Company and used as an internal standard for the calibration
of chemical shift and intensity integration of species in CD3OD.
Reagent grade of hydrogen, carbon monoxide, ethene, and
propene were purchased from Matheson Gas Products. Pen-
tene, hexene, styrene, methyl acrylate, and vinyl acetate were
purchased from Aldrich and used as received. 1H NMR spectra
were obtained on a Bruker AC-300 or AM-500 spectrometer
interfaced to an Aspect 300 computer at ambient temperature.
pH measurements were performed on a Hanna 212 pH meter
connected with a Thermo micro-electrode for NMR cells.
General Procedures. To a 5 mm NMR tube with vacuum
adapter, 25 μL of a 5 ꢀ 10-3 M aqueous solution of 3-trimethyl-
silyl-1-propanesulfonic acid sodium salt was added followed by a
(28) Farnos, M. D.; Woods, B. A.; Wayland, B. B. J. Am. Chem. Soc.
1986, 108, 3659–3663.
(29) Krishnamurthy, M. Inorg. Chim. Acta 1977, 25, 215–218.
(27) Harvey, J. N. Organometallics 2001, 20, 4887–4895.

8558 Inorganic Chemistry, Vol. 48, No. 17, 2009
Li et al.
drop of ∼5 ꢀ 10-3 M aqueous solution of Na3[(TSPP)RhIII
-
formed right after vacuum transfer of alkyl olefins (R = H,
CH3, and CH2CH2CH3) into vacuum adapted NMR tubes
containing CD3OD solutions of [(TSPP)Rh(OCD3)(CD3-
(CH3OH)2]. Water was then removed from the NMR tube
followed by addition of 0.4 mL of CD3OD. To this methanol
solution of (TSPP)RhIII, portions of 0.01 M NaOCH3 in CD3OD
or 0.01 M CF3SO3D in CD3OD were added to adjust the acidity
of the sample. At the desired pH, substrates were introduced to
OD)]-4
.
[(TSPP)Rh-CH2CH2(OCD3)(CD3OD)]-4 (11). 1H NMR
(300 MHz, CD3OD) δ (ppm): 8.83 (s, 8H, pyrrole), 8.40 (d,
1
the NMR tube and reactions are followed by H NMR. Reso-
4H, o-phenyl, J
7.2 Hz), 8.24 (d, 4H, m-phenyl, J
m-phenyl, J1H- H = 7.2 Hz), -5.76 (br t, 2H(R)), -2.52, (t,
2H( β), J1H- H =6 Hz).
1
1H- H=7.2 Hz), 8.29 (d, 4H, o-phenyl, J1H- H
1
=
nancefor methylhydrogens of 3-trimethylsilyl-1-propanesulfonic
acid sodium salt was set at 0 ppm, and other chemical shifts were
referenced to it. Concentration of each species in solution was
determined by comparison with the concentration of 3-trimethyl-
silyl-1-propanesulfonic acid sodium salt. Proton NMR spectra
were used to identify solution species and determine the distribu-
tion of species at equilibrium. When a reaction reached equilib-
rium, resonances in the 1H NMR spectrum were integrated and
the deuterium ion concentration was measured. For reaction with
gases such as carbon monoxide, solubility data in methanol at
various gas pressures were used to determine the gas concentra-
tion in CD3OD.30
1
1H- H = 7.2 Hz), 8.20 (d, 4H,
1
1
[(TSPP)Rh-CH2CH(OCD3)CH3(CD3OD)]-4 (12). 1H NMR
(300 MHz, CD3OD) δ (ppm): 8.83 (s, 8H, pyrrole), 8.40 (d, 4H,
o-phenyl, J1H- H
7.2 Hz), 8.24 (d, 4H, m-phenyl, J
m-phenyl, J
1
= 7.2 Hz), 8.29 (d, 4H, o-phenyl, J1H- H
1
=
1
1H- H = 7.2 Hz), 8.20 (d, 4H,
1
1H- H =7.2 Hz), -5.70 (m, 1HA(R)), -5.76 (m, 1HB-
(R)), -2.50 (m, 1H(β)), -1.97 (d, 3H(γ), 6 Hz).
[(TSPP)Rh-CH2CH(OCD3)C3H7(D2O)]-4 (13). 1H NMR
(300 MHz, CD3OD) δ (ppm): 8.68 (8H, pyrrole), 8.50-8.10
(16H, phenyl), -5.96 (m, 1HA(R)), -5.75 (m, 1HB(R)), -3.02
(m, 1H(β)), -2.66 (m, 1HA(γ)), -1.77 (m, 1HB(γ)), -0.72 (m,
2H(δ)), -0.31 (t, 3H(ε)).
Measurement of [Dþ] in CD3OD. The glass electrode of a pH
meter responds as efficiently to deuterium ions as it does with
hydrogen ions, but the magnitude of the response is different.31
For identical solutions, the p(RD) values differ from pH read-
ings by a constant of 0.447 (RD = activity of Dþ, p(RD) - pH =
0.447)32 when RD was measured in molal concentration. Con-
version to molar units yields the relationship between pH read-
ings and actual pD values as pD = pHreading þ 0.41 at 25 °C.
Acid Dissociation Constants for (TSPP)RhIII(CD3OD)2 and
(TSPP)RhIII(OCD3)(CD3OD) in CD3OD. At a series of [Dþ],
the chemical shifts of the porphyrin pyrrole hydrogen were
recorded. The single resonance at each pD manifests the
fast exchange among [(TSPP)RhIII(CD3OD)2]-3 (1), [(TSPP)-
RhIII(OCD3)(CD3OD)]-4 (2), and [(TSPP)RhIII(OCD3)2]-5 (3).
The chemical shift of this single peak is the mole fraction averaged
signal of (1), (2), and (3). Fitting into the δ1,2,3 = (K1K2δ3 þ
K1[Dþ]δ2 þ [Dþ]2δ1)/(K1K2 þ K1[Dþ] þ [Dþ]2) gives K1 =
Reactions of [(TSPP)Rh ]
III -3 with Styrene and Methyl Acry-
late. Mixing methanol solutions of (2) with styrene and methyl
acrylate initially produces [(TSPP)Rh-CH2CH(OCD3)C6H5-
(CD3OD)]-4(14) and [(TSPP)RhCH2CH(OCD3)CO2 CH3-
-4
(D2O)]
(16) as the kinetically preferred isomers which
subsequently rearrange to produce [(TSPP)Rh-CH(C6H5)-
CH2-OCD3(CD3OD)]-4 (17) and [(TSPP)Rh-CH(CO2CH3)-
CH2-OCD3(CD3OD)]-4 (18) as the thermodynamically pre-
ferred products.
[(TSPP)Rh-CH(CO2CH3)CH2-OCD3(CD3OD)]-4 (18). 1H
NMR (250 MHz, CD3OD) δ (ppm): 8.90 (s, 8H, pyrrole), 8.47-
8.20(m, 16H, phenyl), 1.86 (s, 1H, OCH3), -1.79 (t, 1H, CH-
(CO2CH3), 3J1H- H
4.1 Hz, 2J1H- H
Hz, 2J1H- H
=11.2 Hz).
1
= 11.2 Hz), -3.06 (d of d, 1H, CH2, 3J1H- H
=
1
1
= 11.2 Hz), -4.9 (d of t, 1H, CH2, 3J1H- H
= 4.1
1
1
6.9(0.2) ꢀ 10-8 and K2 =5.8(0.3) ꢀ 10-13
.
[(TSPP)Rh-CH2CH(OCD3)C6H5(CD3OD)]-4 (14). 1H NMR
(360 MHz, CD3OD) δ (ppm): 8.80 (s, 8H, pyrrole), 8.46-8.20(m,
16H, phenyl), -1.95 (m, 1H, CH2), -5.15 (m, 1H, CH2), -5.65
Reactions of (TSPP)RhIII with Ethene, Propene, and Pentene
in Methanol. [(TSPP)Rh-CH2CH(OCD3)R(CD3OD)]-4 was
(m, 1H, CH). [(TSPP)Rh-CH(C6H5)CH2-OCD3(CD3OD)]-4
:
1H NMR (360 MHz, CD3OD) δ (ppm): 8.80 (s, 8H, pyrrole),
8.46-8.20 (m, 16H, phenyl), -1.67 (m, 1H, CH2), -2.97 (m, 1H,
CH2), -4.30 (m, 1H, CH).
(30) Fog, R. G. T.; Gerrard, W. Solubility of Gases in Liquids: A Critical
Evaluation of Gas/Liquid Systems in Theory and Practice; Wiley: Chichester,
1991.
(31) Glasoe, P. K.; Long, F. A. J. Phys. Chem. 1960, 64, 188–190.
(32) (a) Gray, R.; Bates, R. G.; Robinson, R. A. J. Phys. Chem. 1964, 68,
1186–1190. (b) Gray, R.; Bates, R. G.; Robinson, R. A. J. Phys. Chem. 1960, 64,
3806–3809. (c) Gray, R.; Bates, R. G.; Robinson, R. A. J. Phys. Chem. 1965, 69,
2750–2753.
Acknowledgment. This research was supported by the De-
partment of Energy Office of Basic Energy Science through
Grant DE-FG02-09ER16000.