Thermodynamics of rhodium hydride reactions with CO, aldehydes, and olefins in water: Organo-rhodium porphyrin bond dissociation free energies

Article abstract of DOI:10.1021/ja054548n

Tetra(p-sulfonato-phenyl) porphyrin rhodium hydride ([(TSPP)Rh-D(D ₂O)]^-4) (1) reacts in water (D₂O) with carbon monoxide, aldehydes, and olefins to produce metallo formyl, α- hydroxyalkyl, and alkyl complexes, respectively. The hydride complex (1) functions as a weak acid in D₂O and partially dissociates into a rhodium(I) complex ([(TSPP)Rh^I(D₂O)]^-5) and a proton (D⁺). Fast substrate reactions of 1 in D²O compared to reactions of rhodium porphyrin hydride ((por)Rh-H) in benzene are ascribed to aqueous media promoting formation of ions and supporting ionic reaction pathways. The regioselectivity for addition of 1 to olefins is predominantly anti-Markovnikov in acidic D₂O and exclusively anti-Markovnikov in basic D₂O. The range of accessible equilibrium thermodynamic measurements for rhodium hydride substrate reactions is substantially increased in water compared to that in organic media through exploiting the hydrogen ion dependence for the equilibrium distribution of species in aqueous media. Thermodynamic measurements are reported for reactions of a rhodium porphyrin hydride in water with each of the substrates, including CO, H₂CO, CH₃CHO, CH₂=CH₂, and sets of aldehydes and olefins. Reactions of rhodium porphyrin hydrides with CO and aldehydes have nearly equal free-energy changes in water and benzene, but alkene reactions that form hydrophobic alkyl groups are substantially less favorable in water than in benzene. Bond dissociation free energies in water are derived from thermodynamic results for (TSPP)Rh-organo complexes in aqueous solution for Rh-CDO, Rh-CH(R)OD, and Rh-CH₂CH(D)R units and are compared with related values determined in benzene.

Full text of DOI:10.1021/ja054548n

Published on Web 11/04/2005
Thermodynamics of Rhodium Hydride Reactions with CO,
Aldehydes, and Olefins in Water: Organo-Rhodium Porphyrin
Bond Dissociation Free Energies
Xuefeng Fu and Bradford B. Wayland*
Contribution from the Department of Chemistry, UniVersity of PennsylVania,
Philadelphia, PennsylVania 19104-6323
Received July 8, 2005; E-mail: wayland@sas.upenn.edu
Abstract: Tetra(p-sulfonato-phenyl) porphyrin rhodium hydride ([(TSPP)Rh-D(D2O)]-4) (1) reacts in water
(D2O) with carbon monoxide, aldehydes, and olefins to produce metallo formyl, R-hydroxyalkyl, and alkyl
complexes, respectively. The hydride complex (1) functions as a weak acid in D2O and partially dissociates
into a rhodium(I) complex ([(TSPP)RhI(D2O)]-5) and a proton (D+). Fast substrate reactions of 1 in D2O
compared to reactions of rhodium porphyrin hydride ((por)Rh-H) in benzene are ascribed to aqueous
media promoting formation of ions and supporting ionic reaction pathways. The regioselectivity for addition
of 1 to olefins is predominantly anti-Markovnikov in acidic D2O and exclusively anti-Markovnikov in basic
D2O. The range of accessible equilibrium thermodynamic measurements for rhodium hydride substrate
reactions is substantially increased in water compared to that in organic media through exploiting the
hydrogen ion dependence for the equilibrium distribution of species in aqueous media. Thermodynamic
measurements are reported for reactions of a rhodium porphyrin hydride in water with each of the substrates,
including CO, H2CO, CH3CHO, CH2dCH2, and sets of aldehydes and olefins. Reactions of rhodium porphyrin
hydrides with CO and aldehydes have nearly equal free-energy changes in water and benzene, but alkene
reactions that form hydrophobic alkyl groups are substantially less favorable in water than in benzene.
Bond dissociation free energies in water are derived from thermodynamic results for (TSPP)Rh-organo
complexes in aqueous solution for Rh-CDO, Rh-CH(R)OD, and Rh-CH2CH(D)R units and are compared
with related values determined in benzene.
Introduction
for reactions of a rhodium porphyrin hydride ((por)Rh-H) with
carbon monoxide, formaldehyde, and ethene in water for
Exploring the scope and applications of organo-metallic
transformations in water compared to those in more conventional
organic media is a major current theme of transition-metal
catalysis research.1-8 One specific objective of this study is to
provide a set of experimental equilibrium thermodynamic values
quantitative comparisons with related measurements in benzene
and also as thermodynamic bench marks for late transition-metal
hydride addition reactions. The reactivity patterns for rhodium
porphyrin hydrides ((por)Rh-H) are typical of late transition-
metal hydrides but display an exceptional breadth of observable
substrate reactions.7-13 Thermodynamic studies on a common
set of processes in organic and aqueous media provide quantita-
tive criteria for comparing substrate reactivity patterns and aid
in identifying the origins of the factors contributing to medium
effects. These classes of metal hydride reactions with unsaturated
substrates play a central role in many of the most important
catalytic processes, such as hydrogenations, hydroformylation,
and related transformations, yet thermodynamic studies for these
types of reactions remain sparse. Thermodynamic studies for
rhodium porphyrin reactions with substrates such as H2, CH4,
CO, olefins, and aldehydes provide one of the most extensive
(1) (a) Cornils, B.; Kuntz, E. G. Chapter 6. In Aqueous-Phase Organometallic
Catalysis, Concept and Application; Cornils, B., Herrmann, A. W., Eds.;
Wiley-VCH: New York, 1998. (b) Lucey, D. W.; Helfer, D. S.; Atwood,
J. D. Organometallics 2003, 22, 826. (c) Lucey, D. W.; Atwood, J. D.
Organometallics 2002, 21, 2481.
(2) (a) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed. Engl.
1998, 37, 2181. (b) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem.
Soc. 1996, 118, 5961.
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Lynn, D. M.; Mohr, B.; Grubbs, R. H.; Henling, L. M.; Day, M. W. J. Am.
Chem. Soc. 2000, 122, 6601.
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A. C.; Nadasdi, L.; Laurenczy, G. Chem.-Eur. J. 2001, 7, 193. (c) Joo,
F.; Kovacs, J.; Benyei, A. C.; Katho, A. Angew. Chem., Int. Ed. Engl. 1998,
37, 969. (d) Csabai, P.; Joo, F. Organometallics 2004, 23, 5640.
(5) (a) Kovacs, J.; Todd, T. D.; Reibenspies, J. H.; Joo, F.; Darensbourg, D. J.
Organometallics 2000, 19, 3963. (b) Darensbourg, D. J.; Joo, F.; Kannisto,
M.; Katho, A.; Reibenspies, J. H.; Daigle, D. J. Inorg. Chem. 1994, 33,
200. (c) Darensbourg, D. J.; Joo, F.; Kannisto, M.; Katho, A.; Reibenspies,
J. H. Organometallics 1992, 11, 1990. (d) Parkins, A. W. In Aqueous
Organometallic Chemistry and Catalysis; Horvath, I. T., Joo, F., Eds.;
Kluwer Academic Publisher: Dordrecht, 1997; Vol. 11.
(6) (a) Frost, B. J.; Mebi, C. A. Organometallics 2004, 23, 5317. (b)
Vancheesan, S.; Jesudurai, D. Catalysis 2002, 311. (c) Kundu, A.; Buffin,
B. P. Organometallics 2001, 20, 3635.
(7) Fu, X. F.; Basickes, L.; Wayland, B. B. Chem. Commun. 2003, 520.
(8) Fu, X. F.; Wayland, B. B. J. Am. Chem. Soc. 2004, 126, 2623.
(9) Wayland, B. B.; Duttaahmed, A.; Woods, B. A. Chem. Commun. 1983,
142.
(10) Wayland, B. B.; Woods, B. A.; Minda, V. M. Chem. Commun. 1982, 634.
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302.
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4039.
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6063.
9
16460
J. AM. CHEM. SOC. 2005, 127, 16460-16467
10.1021/ja054548n CCC: $30.25 © 2005 American Chemical Society

Organo-Rhodium Porphyrin Bond Dissociation Energies
A R T I C L E S
Scheme 1. Reactivity Patterns for a Rhodium Porphyrin Hydride
in D2O
sets of organo-transition-metal bond energies in organic
media.13-19 Sulfonation of the phenyl groups of tetraphenyl
porphyrin derivatives provides water-soluble rhodium porphyrin
complexes for thermodynamic studies in aqueous media that
are closely related to the benzene-soluble analogues.
Each of the (por)Rh-H reactions with unsaturated substrates,
including carbon monoxide, that were first observed in organic
media has an analogous process in water.7 Prior equilibrium
thermodynamic studies for reactions of (por)Rh-H complexes
with unsaturated substrates in organic media have been limited
to carbon monoxide12,19 and four-carbon or larger aldehydes12
because smaller aldehydes and vinyl olefins have equilibrium
constants that are too large for convenient direct determination
1
by H NMR. This restriction is largely overcome in aqueous
respectively, produce formyl (Rh-CDO), R-hydroxyalkyl
(Rh-CH(OD)R), and alkyl (Rh-CH2CH(D)X) complexes
(Scheme 1).
media where the equilibrium distribution of species in solution
can be tuned by the pH dependence. Evaluation of equilibrium
constants for reactions of dihydrogen with rhodium(III) por-
phyrin complexes in water has established a solid foundation
for equilibrium thermodynamic studies for a wide range of
additional substrate reactions in aqueous media.8 This article
reports on the reactivity and equilibrium thermodynamic studies
in water for CO, H2CO, CH2dCH2, and sets of aldehydes and
olefins with tetra(p-sulfonato-phenyl) porphyrin rhodium hydride
([(TSPP)Rh-D(D2O)]-4) (1) and the rhodium(I) derivative
([(TSPP)RhI(D2O)]-5) (2).
Aqueous solutions of the rhodium hydride (1) in D2O react
with CO (PCO ) 0.2-0.7 atm) to produce equilibrium distribu-
tions with a formyl complex [(TSPP)Rh-CDO(D2O)]-4 (4)
(Table 1, eq 4). The formyl complex is clearly identified by
13C NMR of 4 prepared by reaction of 1 with 13CO. The 13C
NMR for the formyl group of [(TSPP)Rh-13CDO(D2O)]-4 in
13
D2O (δ
) 218.7 ppm) appears as an approximate 1:2:2:1
CHO
103
quartet that results from the near equal coupling of D and
-
13
1
13C-D
103Rh-13
C
Rh with C (2J
+ J
) 78 Hz). When the H
derivative of 4 ([TSPP)Rh-13CHO]-4) is formed in H2O, the
Results and Discussion
13C NMR of the Rh-13CHO unit manifests a 13C-H coupling
The primary focus of this study was to evaluate equilibrium
thermodynamic values for reactions of CO, H2CO, CH2dCH2,
and related substrates with a rhodium porphyrin hydride in
aqueous solution. The hydride complex ([(TSPP)Rh-D(D2O)]-4)
(1) is formed in D2O by reaction of dihydrogen (H2/D2) with
[(TSPP)RhIII(D2O)2]-3 (3) (eq 1).8 The hydride (1) is a weak
acid in D2O (eq 2) (K2 (298 K) ) 8.0(0.5) × 10-8).8
13
constant of 166 Hz which gives a calculated value for J C-D of
25.5 Hz ((J C-H/J C-D) ) 6.51 Hz), and thus, a value of 27
Hz is derived for J
constant from 200 Hz for the (por)Rh-13CHO unit in ben-
zene9,11,12 to 166 Hz in water is an indication that the carbonyl
carbon has become more like an sp3 hybridized center. Water
interacting with the formyl group is clearly producing a
significant perturbation on the carbonyl unit.
13
13
C. Reduction of the 13C-H coupling
103Rh-13
1) [(TSPP)RhIII(D2O)2]-3 + D2 a
[(TSPP)Rh-D(D2O)]-4 + D+ + D2O
Aldehydes (RCHO) invariably react with 1 and 2 in D2O with
regioselectivity to form R-hydroxyalkyl complexes ([(TSPP)-
Rh-CH(OD)R(D2O)]-4) exclusively in preference to alkoxides
([(TSPP)Rh-OCH2R(D2O)]-4). Olefin substrates (CH2dCH-
(X)) usually react with 1 in acidic D2O to give anti-Markovnikov
addition to form [(TSPP)Rh-CH2CH(D)X(D2O)]-4 as the only
1H NMR observed product, but reaction of the hydride 1 in
acidic D2O with CH2dCHC6H4SO3Na is exceptional in produc-
ing comparable quantities of two isomers associated with both
anti-Markovnikov ([(TSPP)Rh-CH2CH(D)X(D2O)]-4) and Mark-
ovnikov ([(TSPP)Rh-CH(CH2D)X(D2O)]-4) addition products.
The olefins in contact with 1 in acidic D2O are sequentially
deuterated at the secondary and then the primary alkene carbon
positions. Solutions of 1 in basic D2O predominantly contain
the rhodium(I) complex (2) and react rapidly with electrophili-
cally activated olefins (CH2dCHCO2CH3 and CH2dCHC6H4-
SO3Na) to form, exclusively, the anti-Markovnikov addition
products (Rh-CH2CH(D)X). These basic D2O solutions of 2
produce selective deuteration of the secondary olefin carbon to
form CH2dCD(X).
2) [(TSPP)Rh-D(D2O)]-4
a
[(TSPP)RhI(D2O)]-5 + D+
Acid dissociation equilibria for 1 and D2O20 provide a conve-
nient means for tuning the equilibrium distribution of rhodium
porphyrin species in D2O through varying the hydrogen ion
concentration (Table 1, eqs 2 and 3). This strategy is used to
evaluate equilibrium constants for substrate reactions of the
hydride (1) and the rhodium(I) complex (2) that fall outside
the range accessible for direct measurement by 1H NMR
integration.
Reactions of the rhodium hydride ([(TSPP)Rh-D(D2O)]-4
)
(1) with carbon monoxide, aldehydes, and vinyl olefins in D2O,
(14) Cui, W. H.; Wayland, B. B. J. Am. Chem. Soc. 2004, 126, 8266.
(15) Cui, W. H.; Zhang, X. P.; Wayland, B. B. J. Am. Chem. Soc. 2003, 125,
4994.
(16) (a) Wayland, B. B. NATO ASI Ser., Ser. C 1992, 367, 69. (b) Wayland, B.
B.; Coffin, V. L.; Sherry, A. E.; Brennen, W. R. ACS Symposium Series
428; American Chemical Society: Washington, DC, 1990; 148.
(17) Wayland, B. B. Polyhedron 1988, 7, 1545.
(18) Sherry, A. E.; Wayland, B. B. J. Am. Chem. Soc. 1989, 111, 5010.
(19) Farnos, M. D.; Woods, B. A.; Wayland, B. B. J. Am. Chem. Soc. 1986,
108, 3659.
(20) Values obtained from: J. Phys. Chem. Ref. Data, 1982, 11 (Suppl. 2).
Standard state used: 298.15 K, 0.1 MPa.
Equilibrium Thermodynamic Studies for Reactions of
[(TSPP)Rh-D(D2O)]-4 and [(TSPP)RhI(D2O)]-5 with CO,
Aldehydes, and Olefins in Water. Effective equilibrium
constants for reactions of [(TSPP)Rh-D(D2O)]-4 (1) and
[(TSPP)RhI(D2O)]-5 (2) with CO, aldehydes, and olefins in D2O
9
J. AM. CHEM. SOC. VOL. 127, NO. 47, 2005 16461

A R T I C L E S
Fu and Wayland
Table 1. Equilibrium Constants (Kn) and ∆G°n (kcal mol-1) Measured for Reactions of [(TSPP)Rh-D(D2O)]-4 (1) with CO and Aldehydes in
D2O (298 K)
[(TSPP)Rh
−
D(D2O)]-4 (1) reactions
Kn
∆G°
n
2) [Rh-D]-4 (1) a [RhI]-5 (2) + D+
K2 ) 8.0 (0.5) × 10-8
K3 ) 2.44 × 10-17
K4 ) 3.0 (0.3) × 103
K5 ) 4.6 (0.4) × 105
K6 ) 1.3 (0.3) × 103
K7 ) 4.9 (0.9) × 102
+9.7 (0.1)
+22.6
-4.7 (0.1)
-7.7 (0.1)
-4.3 (0.1)
-3.7 (0.1)
3) D2O a OD- + D+ (3)
4) [Rh-D]-4 + CO a [Rh-CDO]-4 (4)
5) [Rh-D]-4 + H2CO a [Rh-CH2(OD)]-4 (5)
6) [Rh-D]-4 + HC(O)CH3 a [Rh-CH(OD)CH3]-4 (6)
7) [Rh-D]-4 + HC(O)CH2CH2CH2CH3 a [Rh-CH(OD)CH2CH2CH3]-4 (7)
Table 2. Equilibrium Constants (Kn) and ∆G°n (kcal mol-1) Derived for Reactions of [(TSPP)RhI(D2O)]-5 (2) with CO and Aldehydes in D2O
(298 K)
[(TSPP)RhI(D2O)]-5 (2) reactions
Kn
∆G°
n
8) [RhI]-5 + CO + D2O a [Rh-CDO]-4 (4) + OD-
K8 ) K4K3/K2 ) 9.1 (0.3) × 10-7
K9 ) K5K3/K2 ) 1.4 (0.4) × 10-4
K9 ) K6K3/K2 ) 4.0 (0.3) × 10-7
K10 ) K7K3/K2 ) 1.5 (0.9) × 10-7
+8.2 (0.1)
+5.3 (0.1)
+8.7 (0.1)
+9.3 (0.1)
9) [RhI]-5 + H2CO + D2O a [Rh-CH2(OD)]-4 (5) + OD-
10) [RhI]-5 + HC(O)CH3 + D2O a [Rh-CH(OD)CH3]-4 (6) + OD-
11) [RhI]-5 + HC(O)CH2CH2CH3 + D2O a [Rh-CH(OD)(CH2)2CH3]-4 (7) + OD-
Table 3. Equilibrium Constants (Kn) and ∆G°n (kcal mol-1) Measured for Reactions of [(TSPP)RhI(D2O)]-5 (2) with Olefins in D2O (298 K)
[(TSPP)RhI(D2O)]-5 (2) reactions
Kn
∆G°
n
12) [RhI]-5 + CH2dCH2 + D2O a [Rh-CH2CH2(D)]-4 (8) + OD-
K12 ) 1.1 (0.7) × 10-3
K13 ) 3.3 (0.5) × 10-5
K14 ) 8.6 (0.5) × 10-2
K15 ) 2.7 (0.3) × 10-4
+4.0 (0.1)
+6.1 (0.1)
+1.5 (0.1)
+4.9 (0.1)
13) [RhI]-5 + CH2dCH(CH2)3CH3 + D2O a [Rh-CH2CH(D)(CH2)3CH3]-4 (9) + OD-
14) [RhI]-5 + CH2dCHCO2CH3 + D2O a [Rh-CH2CH(D)CO2CH3]-4 (10) + OD-
15) [RhI]-5 + CH2dCHC6H4SO3Na + D2O a [Rh-CH2CH(D)C6H4SO3Na]-4 (11) + OD-
Table 4. Equilibrium Constants (Kn) and ∆G°n (kcal mol-1) Derived for Reactions of [(TSPP)Rh-D(D2O)]-4 (1) with Olefins in D2O (298 K)
[TSPP)Rh
−
D(D2O)]-4 (1) with olefins
Kn
∆G°
n
16) [Rh-D]-4 + CH2dCH2 a [Rh-CH2CH2(D)]-4 (8)
K16 ) K2K12/K3 ) 3.6 (0.7) × 106
K17 ) K2K13/K3 ) 1.1 (0.5) × 105
K18 ) K2K14/K3 ) 2.8 (0.8) × 108
K19 ) K2K15/K3 ) 8.8 (0.3) × 105
-8.9 (0.1)
-6.9 (0.1)
-11.5 (0.1)
-8.1 (0.1)
17) [Rh-D]-4 + CH2dCH(CH2)3CH3 a [Rh-CH2CH(D)(CH2)3CH3]-4 (9)
18) [Rh-D]-4 + CH2dCHCO2CH3 a [Rh-CH2CH(D)CO2CH3]-4 (10)
19) [Rh-D]-4 + CH2dCHC6H4SO3Na a [Rh-CH2CH(D)C6H4SO3Na]-4 (11)
Table 5. Derived Proton Donor Abilities for [(TSPP)Rh-R(D2O)]-4 in D2O (298 K)
proton donor process
Kn
∆G°
n
2) [Rh-D]-4 a [RhI]-5 + D+
K2 ) 8.0 × 10-8
K21 ) K2/K5 ) 1.7 × 10-13
K22 ) K2/K6 ) 6.2 × 10-11
K23 ) K2/K16 ) 2.2 × 10-14
+9.7 (0.1)
+14.4 (0.1)
+17.4 (0.1)
+13.9 (0.1)
+18.6 (0.1)
20) [Rh-CDO]-4 a [RhI]-5 + CO + D+
K20 ) K2/K4 ) 2.7 × 10-11
21) [Rh-CH2(OD)]-4 a [RhI]-5 + H2CO + D+
22) [Rh-CH(OD)CH3]-4 a [RhI]-5 + HC(O)CH3 + D+
23) [Rh-CH2CH2(D)]-4 a [RhI]-5 + CH2dCH2 + D+
a R ) D, CDO, CH2OD, CH(OD)CH3, CH2CH2(D).
are given in Tables 1-4 (eqs 4-19). Thermodynamic param-
eters relevant to the hydride 1 and organo-rhodium complexes
functioning as a proton (H+), a hydrogen atom (H•), and a
hydride (H-) source in water are derived from the data in Tables
1-4 and are listed in Tables 5 and 6, respectively.
The equilibrium constant for reaction 4 ([(TSPP)Rh-
D(D2O)]-4 + CO a [(TSPP)Rh-CDO(D2O)]-4) was evaluated
in D2O by the integration of the porphyrin pyrrole 1H NMR for
1 and 4 and by the assumption that the solubility of CO in D2O
is the same as that in H2O.22 The hydride 1 and the rhodium(I)
complex 2 are in dynamic equilibrium in aqueous solutions
where the concentration of D+ is less than 10-5 M. The mole
fraction averaged pyrrole chemical shifts for aqueous solutions
of 1 and 2 were used to determine the concentration of the
hydride (1) for evaluation of K4 (K4 (298 K) ) 3.0 (0.3) × 103,
∆G4° (298 K) ) -4.7 kcal mol-1) (Table 1).Equilibrium
constants for reactions of [(TSPP)RhI(D2O)]-5 (2) with CO and
aldehydes are too small to be directly measured by the 1H NMR
integration method but can be derived from thermodynamic
cycles using the data in Table 1. The equilibrium constant for
the reaction of [(TSPP)RhI(D2O)]-5 (2) with CO in D2O (eq
8), K8 (298 K) ) K4K3/K2 ) 9.1 × 10-7, is derived from the
Equilibrium constants at 298 K in D2O were directly
measured for reactions of the rhodium hydride [(TSPP)Rh-
D(D2O)]-4 (1) with CO and aldehydes (eq 4-7, Table 1) and
for reactions of olefins with the rhodium(I) complex
([(TSPP)RhI(D2O)]-5) (2) (eq 8-11, Table 2). Evaluation of
the relatively large equilibrium constant for the reaction of
formaldehyde with 1 (K5 (298 K) ) 4.6 (0.4) × 105) in D2O is
aided by the hydrolysis of formaldehyde (H2CO + H2O a CH2-
(OH)2; K (298 K) ) 36.2)21 which results in only small
equilibrium concentrations of H2CO. Tuning the distribution of
1 and 2 by changing the hydrogen ion concentration provides
an effective approach to expand the range of equilibrium
1
constant values that can be determined by integration of H
NMR spectra.
(22) Fog, P. G. T.; Gerrard, W. Solubility of Gases In Liquids: A Critical
EValuation of Gas/Liquid Systems in Theory and Practice; Wiley: Chich-
ester, NY, 1991.
(21) Bell, R. P. AdV. Phys. Org. Chem. 1966, 4, 1.
9
16462 J. AM. CHEM. SOC. VOL. 127, NO. 47, 2005

Organo-Rhodium Porphyrin Bond Dissociation Energies
A R T I C L E S
Table 6. Derived Hydride and Hydrogen Atom Donor Abilities for [(TSPP)Rh-R(D2O)]-4 Complexes in D2O (298 K)a
hydride donor process
Kn
∆G°
n
1) [RhIII(D2O)2]-3 + D2 a [Rh-D(D2O)]-4 + D+ + D2O
24) D2 a D+ + D-
K1 ) 18.2 ( 0.5
-1.7( 0.1
+53.2
K24 ) 9.6 × 10-40
25) [Rh-D(D2O)]-4 + D2O a [RhIII(D2O)2]-3 + D-
K25 ) K24/K1 ) 5.3 × 10-41
K26 ) K25/K4 ) 1.8 × 10-44
K27 ) K25/K5 ) 1.2 × 10-46
K28 ) K25/K6 ) 4.1 × 10-44
K29 ) K25/K16 ) 1.5 × 10-47
K30 = 9.8 × 10-45
+54.9 (0.1)
+59.6 (0.1)
+62.6 (0.1)
+59.2 (0.1)
+63.8 (0.1)
=60
26) [Rh-CDO(D2O)]-4 + D2O a [RhIII(D2O)2]-3 + CO + D-
27) [Rh-CH2(OD)(D2O)]-4 + D2O a [RhIII(D2O)2]-3 + H2CO + D-
28) [Rh-CH(OD)CH3(D2O)]-4 + D2O a [RhIII(D2O)2]-3 + HC(O)CH3 + D-
29) [Rh-CH2CH2(D)(D2O)]-4 + D2O a [RhIII(D2O)2]-3 + CH2dCH2 + D-
30) [Rh-D(D2O)]-4 a [RhII(D2O)]-4• + D•
31) [Rh-CDO(D2O)]-4 a [RhII(D2O)]-4• + CO + D•
32) [Rh-CH2CH2(D)(D2O)]-4 a [RhII(D2O)]-4• + CH2dCH2 + D•
K31 ) K30/K4 = 3.3 × 10-48
K32 ) K30/K16 = 2.7 × 10-51
=65
=69
a R ) D, CDO, CH2OD, CH(OD)CH3, CH2CH2(D).
following cycle by adding the reverse eq 2 (eq -2) with eqs 3
and 4.
unsaturated substrates including CO, aldehydes, and olefins
occur very slowly because all of the coordination sites adjacent
to the hydride position are occupied by the relatively rigid
porphyrin pyrrole nitrogen donors. The conventional organo-
metallic mechanism which involves substrate addition cis to the
hydride position followed by hydrogen migration to the substrate
is blocked in metallo-porphyrin complexes. Addition reactions
of porphyrin rhodium hydrides with olefins and CO in organic
media are catalyzed by (por)RhII metallo radicals which provide
a pathway for substrate binding and activation at the reactive
metal center.23 Reaction of the substrate with a porphyrin
rhodium(II) metallo radical followed by hydrogen atom transfer
from a rhodium hydride gives a radical chain process (eqs 33
and 34).23-26 Sequential addition of the radical components of
the Rh-H unit circumvents the restrictions for concerted
processes.
-2) [(TSPP)RhI(D2O)]-5 + D+ a [(TSPP)Rh-D(D2O)]-4
3) D2O a D+ + OD-
4) [(TSPP)Rh-D(D2O)]-4 + CO a
[(TSPP)Rh-CDO(D2O)]-4
8) [(TSPP)RhI(D2O)]-5 + CO + D2O a
[(TSPP)Rh-CDO(D2O)]-4 + OD-
Equilibrium constants derived for reactions of [(TSPP)RhI(D2O)]-5
with CO, formaldehyde, acetaldehyde, and butyraldehyde in
D2O are listed in Table 2.
Equilibrium constants for reactions of the hydride (1)
([(TSPP)Rh-D(D2O)]-4) with olefins are too large for direct
evaluation but can be derived from measurement of the
equilibrium constants for reactions of the rhodium(I) complex
([(TSPP)RhI(D2O)]-5) with olefins combined with acid dis-
sociation constants for D2O20 and the hydride 1, as illustrated
in the cycle below (K16 ) K2K12/K3).
33) (por)RhII• + S a [(por)RhS]•
34) [(por)RhS]• + H-Rh(por) a (por)Rh-SH + (por)RhII•
Substrate reactions of 1 in D2O are typically much faster than
uncatalyzed reactions of (por)Rh-H in benzene. This is ascribed
to a greater variety of reaction pathways in water and, in
particular, to the ability of water to support ionic species and
heterolytic processes. In aqueous solution, the rhodium hydride
(1) partially dissociates into rhodium(I) and proton fragments
(RhI:-, D+). The rhodium(I) complex (2) is an effective
nucleophile even in donor media such as water because the
monohydrate [(TSPP)RhI(D2O)]-5 is a 5-coordinate 18-electron
complex which retains a site that provides facile access to the
metal center for substrate molecules. Heterolytic dissociation
of the Rh-D unit in 1 into RhI:- and D+ has a much smaller
free-energy change (∆G° ) 9.7 kcal mol-1) than either
formation of hydride (Rh+, D-) (∆G° ) 54.9 kcal mol-1) or
homolysis to D• and RhII• (∆G° = 60 kcal mol-1) and provides
a facile pathway to generate an open reactive metal center for
substrate reactions in water (Table 6, eqs 25 and 30).
Addition of the Rh-D unit in [(TSPP)Rh-D(D2O)]-4 to
unsaturated substrates in D2O is envisioned to occur by
mechanisms that are related to acid- and base-catalyzed addition
reactions of aldehydes and olefins. General pathways for
reactions of 1 with unsaturated substrates (CH2dCHX, RCHO,
and CO) are proposed to parallel the established mechanisms
for reactions of HX with aldehydes in water (Scheme 2).
12) [(TSPP)RhI(D2O)]-5 + CH2dCH2 + D2O a
[(TSPP)Rh-CH2CH2(D)(D2O)]-4 + OD-
2) [(TSPP)Rh-D(D2O)]-4
a
[(TSPP)RhI(D2O)]-5 + D+
-3) D+ + OD- a D2O
16) [(TSPP)Rh-D(D2O)]-4 + CH2dCH2 a
[(TSPP)Rh-CH2CH2(D)(D2O)]-4
The range of equilibrium constants determined by varying
the hydrogen ion concentration in water (3.3 × 10-5 - 2.8 ×
108; ∆(∆G°) ) 17.6 kcal mol-1) far exceeds the accessible range
for K and ∆G° values that could be measured in organic media.
Directly measured equilibrium constants for substrate reactions
of the hydride 1 are found in Table 1, and the measured values
for reactions of the rhodium(I) complex 2 are found in Table 3.
Tables 2 and 4 contain K and ∆G° values for substrate reactions
of 1 and 2 that are derived from the measured values in Tables
1 and 3.
(23) Paonessa, R. S.; Thomas, N. C.; Halpern, J. J. Am. Chem. Soc. 1985, 107,
4333.
(24) Wayland, B. B.; Coffin, V. L.; Farnos, M. D. Inorg. Chem. 1988, 27, 2745.
(25) Wayland, B. B.; DelRossi, K. J. J. Organomet. Chem. 1984, 276, C27.
(26) Wayland, B. B.; Sherry, A. E.; Coffin, V. L. Chem. Commun. 1989, 662.
Relative Rates and Mechanistic Inferences for Porphyrin
Rhodium Hydride Substrate Reactions in Benzene and
Water. Concerted reactions of rhodium porphyrin hydrides with
9
J. AM. CHEM. SOC. VOL. 127, NO. 47, 2005 16463

A R T I C L E S
Fu and Wayland
Scheme 2. Pathways for Addition of HX to an Aldehyde in Water
(Scheme 2). The CO reaction, in contrast with the pathways
outlined in Scheme 2, most probably occurs by concerted
addition of the RhI:- and H+ units without an intermediate. The
interaction of Rh(I) with CO is proposed to assist in the
rehybridization of the carbon center in CO from sp to sp2 in
the transition state. Interaction of the rhodium(I) nucleophile
with CO does not result in a stable complex or an observable
intermediate, but this type of interaction could provide a pathway
that stabilizes the transition state ([Rh:-‚‚‚C(O)‚‚‚H+]) for a
concerted addition of RhI- and H+ fragments to CO.
Organo-Rhodium Porphyrins as Sources of Proton, Hy-
dride, and Hydrogen Atoms. Dissociation of a proton from
[(TSPP)Rh-D(D2O)]-4 in water (∆G° ) +9.7 kcal mol-1) is
greatly preferred to either Rh-D homolysis (60 kcal mol-1) or
heterolysis to form hydride (54.9 kcal mol-1) (Table 6). The
hydride donor ability of 1 (∆G° = 54.9 kcal mol-1) places
(TSPP)Rh-D near the average position on the hydricity scale
that was recently established by DuBois.27,28 Similarly, dis-
sociation of protons from the R-carbon of Rh-CDO (4), the
â-O-D of Rh-CH2OD (5), and the â-C-D/H of Rh-CH2-
CH2D (8) occurs with a much smaller free-energy increase
(13.8-18.6 kcal mol-1) than that for either heterolysis to hydride
or homolysis to hydrogen atoms which require ∆G° values from
59 to 69 kcal mol-1 (Table 6).
In acidic media ([D+] ∼ 10-3-10-7), the hydride 1 adds
rapidly with both activated and unactivated olefins as well as
aldehydes. All of the substrate reactions of 1 in acidic media
1
are too fast to be followed by H NMR at 298 K. In acidic
media where the concentration of the rhodium(I) complex (2)
is very small, the addition of the rhodium hydride with aldehydes
and olefins probably occurs by initial protonation of the substrate
followed by reaction with either the rhodium hydride (1) or the
rhodium(I) complex (2) (Scheme 2a). Protonation of the alkene
π-bond activates both alkene carbon centers so that a thermo-
dynamically controlled distribution of regioisomers is expected.
Addition of 1 to most olefins resulted in observation of only
the anti-Markovnikov product (Rh-CH2CH(D)X) by 1H NMR,
but 4-styrenesulfonic acid sodium salt produced nearly equal
quantities of regioisomers. Olefins in contact with acidic D2O
solutions of 1 are sequentially deuterated at the secondary and
then the primary alkene carbons which are consistent with the
formation of both regioisomers.
Comparison of ∆G° (298 K) Values for Porphyrin
Rhodium Hydride Substrate Reactions of CO, Aldehydes,
and Olefins in Water and Benzene. Estimates of the differ-
ences in the solvation free-energy changes in water and benzene
((∆G°hyd) - (∆G°C H )) for (TSPP)Rh species that are found in
6
6
Table 8 are derived from hydration free energies for substrates
(Table 7) in conjunction with measured free-energy changes
for substrate reactions (Tables 1 and 4). Determination of
equilibrium constants for reactions of CO, H2CO, and CH2d
CH2 with the hydride [(TSPP)Rh-D(D2O)]-4 (1) in water
provides the first opportunity to compare experimental thermo-
dynamic values for reactions of a metal hydride with this entire
series of substrates. The observed ∆G° (298 K) values change
from -4.7 kcal mol-1 for the CO reaction to -7.7 kcal mol-1
for H2CO and -8.9 kcal mol-1 for ethene (Table 1). A change
in the equilibrium constants by a factor of 103 and a free-energy
change ∆(∆G°) of 4.2 kcal mol-1 for this series of substrate
reactions are not very large considering that transition-metal
hydride reactions with olefins to form alkyl complexes are
commonly observed, but reactions of metal hydrides with CO
that produce observable quantities of a metallo-formyl species
are extremely rare.
In basic D2O ([D+] ∼ 10-9-10-12), the rhodium hydride
is deprotonated to form the rhodium(I) complex
2
([(TSPP)RhI(D2O)]-5). The rhodium(I) complex (2) in D2O
([D+] < 10-9) reacts rapidly with aldehydes and electrophilic
olefins such as methyl acrylate and 4-styrenesulfonic acid
sodium salt to produce alkyl derivatives (eqs 9, 10, 14, and 15).
These reactions probably proceed through initial attack by the
rhodium(I) nucleophile at the electrophilic carbon center fol-
lowed by protonation from water (Scheme 2b). The net addition
of 1 to the activated olefins exclusively places the RhI:
nucleophile at the activated CH2 site to form the anti-Mark-
ovnikov products (Rh-CH2CH(D)X). In basic D2O media, the
activated olefins become selectively deuterated (CH2dCDX)
through rhodium hydride mediated exchange with D2O. Alkenes
such as ethene and propene which do not have a site that is
activated for interaction with a nucleophile are observed to react
only very slowly with 2 in basic D2O.
Free-energy changes (∆G° (298 K)) for substrate reactions
of (OEP)Rh-H in benzene have been measured for the reaction
with CO (-4.8 kcal mol-1 19
and estimated for reactions of
)
H2CO (-7.0 (0.5) kcal mol-1) and CH2dCH2 (-18 (3) kcal
mol-1), respectively. Close similarity of the ∆G° values for the
CO and H2CO reactions with (por)Rh-D species in water and
benzene indicates that the aqueous solvation free-energy changes
(∆G°hyd) for reactants and products must nearly cancel for CO
and aldehyde substrate reactions in water and benzene, but the
Addition of the rhodium hydride unit in 1 to CO in both acidic
and basic D2O ([D+] ) 10-3-10-9) produces a rhodium formyl
complex ([(TSPP)Rh-CDO(D2O)]-4) (eqs 4 and 8) at a
moderate rate. Our working hypothesis is that reactions 4 and
(27) Ellis, W. W.; Ciancanelli, R.; Miller, S. M.; Raebiger, J. W.; DuBois, M.
R.; DuBois, D. L. J. Am. Chem. Soc. 2003, 125, 12230.
(28) Ellis, W. W.; Raebiger, J. W.; Curtis, C. J.; Bruno, J. W.; DuBois, D. L.
J. Am. Chem. Soc 2004, 126, 2738.
8
proceed through the same type of transition state
([Rh:-‚‚‚C(O)‚‚‚H+]) as reactions of 1 with aldehyde and olefins
9
16464 J. AM. CHEM. SOC. VOL. 127, NO. 47, 2005

Organo-Rhodium Porphyrin Bond Dissociation Energies
A R T I C L E S
ethene reaction with 1 in water is substantially less favorable
(∆(∆G16°) = +9 kcal mol-1) than the parallel type of process
in benzene.
derive the ((TSPP)Rh-CH2OD)aq BDFE in D2O using reaction
5 gives 39 kcal mol-1 ((Rh-CH2OD)aq ) 39 kcal mol-1).
The difference in the free energies of hydration for the
substrate complexes (Rh-CDO and Rh-CH2OD) and the
rhodium hydride (1) (Rh-D) are nearly equal but opposite in
sign to the hydration free energy for the substrates. For these
cases, the ∆G° values for the substrate reactions in benzene
and water are comparable within less than 1 kcal mol-1. The
large difference in the hydration free energy (∼11 kcal mol-1
)
between Rh-CH2CH2D (8) and Rh-D (1) is identified as the
reason the reaction to produce the ethyl complex ([(TSPP)Rh-
CH2CH2D(D2O)]-4) (eq 11) in water is ∼9 kcal mol-1 less
favorable than the analogous substrate reaction in benzene.
(TSPP)Rh-CH2CH2(D) BDFE. The free-energy change
(∆G°) for the process of rhodium ethyl bond homolysis is
expressed by the following thermodynamic cycle which yields
Effective Organo-Rh(TSPP) Bond Dissociation Free
Energies (BDFEs) in D2O. Bond dissociation enthalpy (BDE)
is the thermodynamic parameter most often used by chemists
to compare the energetics associated with homolysis of a two-
centered bond unit. BDE values are particularly useful for
comparison of the gas-phase processes where the species present
are well defined. In aqueous media where each type of solution
species is comprised of several hydrated subspecies, the
enthalpy/entropy compensation29-31 makes the bond dissociation
free energy (BDFE) a better thermodynamic parameter for
comparison of bond homolysis energetics.
a Rh-CH2CH2D BDFE of 36 kcal mol-1
.
Free-energy changes (∆G° (298 K)) observed for the addition
reactions of the rhodium hydride 1 with CO and sets of aldehyde
and olefin substrates in aqueous media in conjunction with free-
energy changes in hydration (∆G°hyd) (Tables 1-7) are used
in deriving effective BDFEs in D2O at 298 K. Representative
thermodynamic cycles given below are used to illustrate the
derivation of the Rh-CDO, Rh-CH2OD, and Rh-CH2CH2D
BDFE values compiled in Table 9.
Effective Rh-X BDFEs in water can vary substantially from
the values in benzene depending on the component solvation
free energies. The effective Rh-D BDFE is 6 kcal mol-1 larger,
the Rh-CH2OD BDFE is equal, and the Rh-CH2CH2D BDFE
is 8 kcal mol-1 smaller in water than the corresponding BDFE
in benzene. The Rh-D BDFE is larger in water because of the
positive hydration free energy (∆G°hyd) for H• (∆G°hyd(H•) )
+4.5 kcal mol-1), and the ∆G°hyd of Rh-D (1) is more negative
than that of RhII•. The Rh-OD BDFE is larger in water even
though the ∆G°hyd of •OH is negative because the ∆G°hyd(Rh-
OD) is substantially more negative (= -7 kcal mol-1) than that
for RhII•. The largest difference in Rh-X BDFE values occurs
for the alkyl complexes where the Rh-CH2CH2D BDFE is =8
kcal mol-1 less favorable in water than in benzene. The ∆G°hyd
of Rh-CH2CH2D is =10-11 kcal mol-1 less negative than that
of RhII• as a consequence of the hydrophobicity of the alkyl
hydrocarbon chain which substantially reduces the effective
BDFE of Rh-CH2CH2D in water.
(TSPP)Rh-CDO BDFE. Production of the rhodium formyl
complex (4) by reaction of the hydride (1) with CO (eq 4) occurs
with a ∆G°4 (298 K) of -4.7 kcal mol-1. A thermodynamic
cycle gives 44 kcal mol-1 for the Rh-CDO BDFE in (TSPP)-
Rh-CDO.
Conclusions
Reactions of rhodium porphyrin hydrides with unsaturated
substrates previously reported in hydrocarbon media are ob-
served to have analogous types of substrate transformations in
aqueous media. Tetra(p-sulfonato-phenyl) porphyrin rhodium
hydride ([(TSPP)Rh-D(D2O)]-4 (1)) is a weak acid in D2O
(Ka (298 K) ) 8.0 × 10-8) and reacts with carbon monoxide,
aldehydes, and olefins to produce metallo formyl (Rh-CDO),
R-hydroxyalkyl (Rh-CH(OD)R), and alkyl complexes (Rh-
CH2CHDX), respectively. All of the substrate reactions of the
hydride (1) in acidic D2O are orders of magnitude faster than
the corresponding processes in benzene which is ascribed to
water promoting and supporting ionic pathways. Aldehydes react
rapidly and regioselectively with the rhodium hydride (1) in
both acidic and basic D2O to form R-hydroxyalkyl complexes
(Rh-CH(OD)R) rather than alkoxides (Rh-OCH(D)R). Elec-
(TSPP)Rh-CH2OD BDFE. Estimation of the (TSPP)Rh-
CH(R)OD BDFE in water can be obtained from standard free-
energy changes for reactions 5-7 in conjunction with the
(TSPP)Rh-Daq BDFE and the O-D BDFE for hydroxyalkyl
radicals (•CH(R)O-D)aq in water. A thermodynamic cycle to
(29) Liu, L.; Guo, Q. X. Chem. ReV. 2001, 101, 673.
(30) (a) Grunwald, E.; Steel, C. Pure Appl. Chem. 1993, 65, 2543. (b) Grunwald,
E.; Steel, C. J. Phys. Chem. 1993, 97, 13326. (c) Grunwald, E.; Steel, C.
J. Phys. Org. Chem. 1994, 7, 734.
(31) (a) Linert, W.; Han, L. F.; Lukovits, I. Chem. Phys. 1989, 139, 441. (b)
Mallardi, A.; Giustini, M.; Palazzo, G. J. Phys. Chem. B 1998, 102, 9168.
(c) Fang, Y.; Fan, J. M.; Liu, L.; Li, X. S.; Guo, Q. X. Chem. Lett. 2002,
116.
9
J. AM. CHEM. SOC. VOL. 127, NO. 47, 2005 16465

A R T I C L E S
Fu and Wayland
Table 7. ∆H°f, ∆G°f, and ∆G°hyd (298 K, kcal mol-1) for
dence for the distribution of solution species in water has
permitted a substantial increase in the scope of thermodynamic
studies compared to those accessible in benzene. Direct
measurement of the equilibrium constants for reactions of the
rhodium(I) complex (2) with olefins (CH2dCH(X)) in D2O to
form alkyl complexes (Rh-CH2CH(D)X) along with the acid
dissociation constant of 1 permit evaluation of equilibrium
constants as large as 2.8 × 108 for reactions of the hydride 1
with olefins and equilibrium constants as small as 2.5 × 10-7
for substrate reactions of 2.
Reactions of rhodium hydrides with CO and aldehydes have
a cancellation of relatively large solvation energies to produce
nearly equal free-energy changes in water and benzene, but the
formation of hydrophobic alkyl groups from alkene reactions
with 1 is substantially less favorable in water compared with
that in benzene. Compensation of enthalpy and entropy is an
important characteristic of aqueous media that makes the free-
energy change the best thermodynamic parameter for compara-
tive studies. Measured free-energy changes for the substrate
reactions of the hydride (1) (eqs 4-7, Table 1) in conjunction
with aqueous solvation free energies (Table 7) are used in
deriving bond dissociation free-energy (BDFE) values for
organo-rhodium porphyrin complexes in water (Table 9). BDFE
values for the metal complexes and substrate processes provide
a systematic method to anticipate new reactivity and to identify
the origins of medium-dependent behavior.
Substrates and Radicals
compound
∆H°
∆G
°
∆G°
hyd
f
f
CO
-26.4a
0a
-32.8a
0a
4.1a
H2
+4.2k
-2.1l
-4.5i
-3.5j
-3.18j
1.27j
H2O
H2CO
CH3CHO
-57.8a
-25.9a
-39.7a
-49.0b
12.5a
-54.6a
-26.3-24.5a
-30.8a
-27.4b
16.3a
CH3CH2CH2CHO
CH2dCH2
CH2dCH(CH2)3CH3
H•
-9.96b
52.1a
20.9b
1.68j
48.6a
+4.5k
-2.4l
-4.1j
-5.11j
-5.02j
1.83j
•OH
+9.3a
10.3a
+8.2a
6.7a
•CHO
•CH2(OH)
•CH(OH)CH3
•CH2CH3
-4.0c/-4.25d
-14.4e/-15.2f
28.8g
0.0d
•CH2(CH2)4CH3
6.0h
2.49j
a J. Phys. Chem. Ref. Data 1982, 11 (Suppl. 2). Standard state used:
298.15 K, 0.1 MPa. b Chemical thermodynamics of organic compounds;
Stull, D. R., Westrum, E. F., Jr., Sinke, G. C., Eds.; John Wiley & Sons,
Inc.: New York, 1969. c Ruscic, B.; Berkowitz, J. J. Phys. Chem. 1993,
97, 11451-11455. d Hohnsom, R. D., III; Hudgens, J. W. J. Phys. Chem.
1996, 100, 19874-19890. e Holmes, J. L.; Lossing, F. P.; Mayer, P. M. J.
Am. Chem. Soc. 1991, 113, 9723-9728. f Ruscic, B.; Berkowitz, J. Chem.
Phys. 1994, 101, 10936-10946. g Lee, J.; Chen, C.-J.; Bozzelli, J. W. J.
Phys. Chem. A 2002, 106, 7155-7170. h Lossing, F. P.; Maccoll, A. Can.
J. Chem. 1976, 54, 990-992. i Bell, R. P. AdV. Phys. Org. Chem. 1966, 4,
1. j Sandberg, L.; Casemyr, R.; Edholm, O. J. Phys. Chem. B 2002, 106,
7889-7897. k Han, P.; Bartels, D. M. J. Phys. Chem. 1990, 94. l Abraham,
M. H.; Whiting, G. S.; Fuchs, R.; Chambers, E. J. J. Chem. Soc., Perkin
Trans. 1990, 2, 291.
Table 8. Estimated Difference in the Free Energy of Solvation
∆(∆G°) (kcal mol-1, 298 K) for (TSPP)Rh Species
Experimental Section
a
b
General Procedures. All manipulations were performed on a high-
vacuum line equipped with a Welch Duo-Seal vacuum pump or in an
inert atmosphere box unless otherwise noted. Substrates were degassed
by freeze-pump-thaw cycles just before use. Tetra(p-sulfonato-phenyl)
porphyrin sodium salt was purchased from Mid-Century Chemicals.
Research grade hydrogen and carbon monoxide were purchased from
Matheson Gas Products and used without further purification. Carbon-
13 labeled carbon monoxide was purchased from Cambridge Isotope
Laboratories, Inc.
(TSPP)Rh species
∆
(
∆
G
°
)
∆(∆G° )
sol
hyd
(Rh-D)-(RhII•
)
-2.4
-4.2
+5.2
+5.6
-4.0
-3.2
+3.7
+3.9
(Rh-D)-(Rh-CDO)
(Rh-D)-(Rh-CH2OD)
(Rh-D)-(Rh-CH(OD)CH2CH2CH3)
(Rh-D)-(Rh-CH2CH3)
-10
-11
a ∆(∆G°hyd) is defined as the difference in the hydration free energies
for the (TSPP)Rh species. b ∆(∆G°sol) is defined as the difference in the
solvation free energies (∆G°hyd) in water and benzene (∆G°C6H6).
Proton NMR was obtained on a Bruker AC-360 interfaced to an
Aspect 300 computer at ambient temperature. Chemical shifts were
referenced to 3-trimethylsilyl-1-propanesulfonic acid sodium salt.
Carbon-13 NMR spectra were obtained on an AMX-500 instrument.
Synthesis of [(TSPP)RhIII(D2O)2]-3 (3), [(TSPP)Rh-D(D2O)]-4 (1),
and [(TSPP)RhI(D2O)]-5 (2) were reported in the previously published
paper.7,8 Substrate reactions of [(TSPP)Rh-D(D2O)]-4 (1) and
[(TSPP)RhI(D2O)]-5 (2) with CO, aldehydes, and olefins in water were
carried out by vacuum transfer of the substrate to vacuum-line adapted
NMR tubes containing preformed samples of 1 and 2 in D2O at a
defined hydrogen ion ([D+]) concentration, and then dihydrogen was
repressurized into the sample to suppress the formation of (TSPP)RhIII
species. The total concentration of rhodium complexes is maintained
in the range 1 × 10-3-5 × 10-4 M to ensure that each species is
monomeric.
Table 9. Rh-X Bond Dissociation Free Energies for (por)Rh-Xa
b,c
Complexes in D2O and C6D6 (kcal mol-1
)
Rh
−X
BDFE(D2O)
BDFE(C6D6)
Rh-D
60
59
62
44
39
37
36
34
(41)
54
53
(<57)
49
39
37
44
(42)
49
Rh-H
Rh-OH
Rh-CDO
Rh-CH2OD
Rh-CH(OD)CH3
Rh-CH2CH2D
Rh-CH2CHD(CH2)3CH3
Rh-CH3
a The porphyrin (por) is TSPP in water and a TPP derivative in benzene
(ref 14, 18, and 12). b BDFE of Rh-X is defined as the change in free
energy (∆G° (298 K)) for homolytic dissociation of Rh-X into RhII• and
X• units in the designated medium. c Values in parentheses are estimated.
Proton NMR was used to identify solution species and to determine
the distribution of each species at equilibrium. Equilibrium constants
1
were evaluated from the intensity integrations of H NMR for each
trophilic olefins such as methyl acrylate react quickly with
rhodium(I) ([(TSPP)RhI(D2O)]-5 (2)) in D2O to form the anti-
Markovnikov addition products (Rh-CH2CH(D)X), but unac-
tivated alkenes such as ethene react very slowly. Anti-
Markovnikov addition also occurs in acidic media for 1 with
most of the olefins studied, with the exception of 4-styrene-
sulfonic acid sodium salt, which gives a mixture of isomers.
Equilibrium constants (298 K) were evaluated for each of
the substrate reactions in water (Tables 1-4). The pH depen-
species in combination with D+ concentration measurement and the
solubility of the small organic substrate in water.22 Equilibrium
concentrations of [(TSPP)Rh-D(D2O)]-4 for the substrate reactions
were determined from the averaged 1H NMR chemical shifts in
combination with the D+ ion concentration which determines the
equilibrium distribution of [(TSPP)RhI(D2O)]-5 and [(TSPP)Rh-
D(D2O)]-4
.
Reaction of [(TSPP)Rh-D(D2O)]-4 with CO. A solution of 1 (10-3
M) was pressurized with a mixture of H2/CO containing 70% CO (0.6
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16466 J. AM. CHEM. SOC. VOL. 127, NO. 47, 2005

Organo-Rhodium Porphyrin Bond Dissociation Energies
A R T I C L E S
atm) to form [(TSPP)Rh-CDO(D2O)]-4 (4). The equilibrium constant
was evaluated from the 1H NMR integrations of rhodium hydride and
rhodium formyl porphyrin pyrrole hydrogens at 8.78 and 8.89 ppm,
respectively.
hydride with all the substrate olefins go to completion within the time
1
needed to run an H NMR, but the reactions of [(TSPP)RhI(D2O)]-5
with unactivated olefins such as ethene and hexene are much slower
and take months to achieve an equilibrium distribution. The thermo-
dynamic equilibrium constants were evaluated from the integration of
the 1H NMR for the rhodium alkyl and rhodium(I) complexes in
combination with the proton concentration which determines the
equilibrium concentration of [(TSPP)Rh-D(D2O)]-4. The substrate
olefin hydrogens slowly reduce in 1H NMR intensity compared to the
internal reference, which indicates that the olefins are partially
deuterated. 4-Styrenesulfonic acid sodium salt becomes almost fully
deuterated (CD2dCDX) indicating that both regioisomers are being
formed in acidic D2O. In basic D2O media, the activated olefins become
selectively deuterated (CH2dCDX) through rhodium hydride mediated
exchange with D2O.
1H NMR (360 MHz, D2O) δ (ppm) ([(TSPP)Rh-CDO (D2O)]-4
1
1
(4)): 8.89 (s, 8H, pyrrole), 8.54 (d, 4H, o-phenyl, J H- H ) 8 Hz), 8.31
1
1
1
1
(d, 4H, o-phenyl, J H- H ) 8 Hz), 8.20 (d, 8H, m-phenyl, J H- H ) 8
Hz). 13C NMR (500 MHz, D2O) δ (ppm): 218 (q, 1C, formyl, J
) 28 Hz, J H- ) 25 Hz).
103
13
Rh-
C
2
13
C
Reactions of [(TSPP)RhI(D2O)]-5 and [(TSPP)Rh-D(D2O)]-4
with Aldehydes in Water. Aqueous solutions of paraformaldehyde
are a source of H2CO in equilibrium with CH2(OD)2 (H2CO + D2O a
CH2(OD)2) that reacts with the hydride 1 (10-3 M) to form a rhodium
hydroxy methyl complex [(TSPP)Rh-CH2(OD)]-4 (5). The equilibrium
concentration of the formaldehyde monomer (H2CO) in D2O was
1
1H NMR (360 MHz, D2O) δ (ppm) ([(TSPP)Rh-CH2CH2D]-4
(8)): 8.64 (8H, pyrrole), 8.33-8.14 (16H, phenyl), -4.6 (br, 2H,
CH2CH2D), -5.7 (br, 2H, CH2CH2D).
determined by integration of the H NMR for H2C(OD)2 relative to
3-trimethylsilyl-1-propanesulfonic acid sodium salt as a concentration
standard and the reported equilibrium constant for the formaldehyde
hydration reaction (K (298 K) ) 36.2). The equilibrium constant of
the formaldehyde reaction with Rh-D was determined by integration
of the 1H NMR for [(TSPP)Rh-D(D2O)]-4 and [(TSPP)Rh-CH2(OH)-
(D2O)]-4 and combined with the concentration of [H2CO].
1HNMR (360 MHz, D2O) δ (ppm) ([(TSPP)Rh-CH2(CH2)3CH2-
DCH3]-4 (9)): 8.71 (8H, pyrrole), 8.39-8.16 (16H, phenyl), 0.02 (3H,
CH2CHD(CH2)3CH3), -0.29 (2H), -0.86 (2H), -1.92 (2H), -4,95
(1H), -6.08 (2H).
1H NMR (360 MHz, D2O) δ (ppm) ([(TSPP)Rh-CH2(OH)(D2O)]-4
(5)): 8.8 (8H, pyrrole), 8.30-8.00 (16H, phenyl), -2.86 (d, 2H, CH2).
Solutions of [(TSPP)Rh-D(D2O)]-4 (5 × 10-4 M) react with
acetaldehyde to form R-hydroxyalkyl complexes [(TSPP)Rh-CH(OD)-
CH3(D2O)]-4 (6). The extent of conversion to the R-hydroxyalkyl
complex decreases in basic solution and becomes unobservable when
[D+] is less than 10-10 M. The equilibrium concentrations of the
1H NMR (360 MHz, D2O) δ (ppm) ([(TSPP)Rh-CH2CH(D)CO2-
CH3]-4 (10)): 8.70 (s, 8H, pyrrole), 8.29 (d, 4H, phenyl, J H- H ) 8
1
1
1
1
1
1
Hz), 8.25 (d, 4H, phenyl, J H- H ) 8 Hz), 8.14 (d, 4H, phenyl, J H- H
1
1
) 8 Hz), 8.11 (d, 4H, phenyl, J H- H ) 8 Hz), -3.26 (t, 1H,
1
1
2
1
CH2CHD(CO2CH3), J H- H ) 6.2 Hz, J H- H ) 1.0 Hz), -5.62 (d of d,
1
1
103
1
2H, CH2CHD(CO2CH3), J H- H ) 6.2 Hz, J
) 2.8 Hz).
Rh- H
1H NMR (360 MHz, D2O) δ (ppm) ([(TSPP)Rh-CH2CH(D)C6H4-
SO3Na]-4 (11)): 8.62 (8H, pyrrole), 8.41-8.0 (16H, phenyl), -3.2 (t,
1H, CH2CH(D)C6H4SO3Na), -5.5 (d of d, 2H, CH2CH(D)C6H4SO3-
Na). 1H NMR (360 MHz, D2O) δ (ppm) ([(TSPP)Rh-CH(CH2D)C6H4-
SO3Na]-4): 8.59 (8H, pyrrole), 8.41-8.0 (16H, phenyl), -4.17 (t, 1H,
CH(CH2D)C6H4SO3Na), -4.34 (d, 2H, CH(CH2D)C6H4SO3Na).
Estimation of Bond Dissociation Free Energies (∆G° (298 K))
of D-X from Values of H-X in Water. The BDFEs of (D-D)aq
and (D-OD)aq are estimated on the basis of the corresponding H-H
and H-OH values and the zero-point energy difference. The difference
between H-H and D-D is =1.7 kcal mol-1, and the difference between
H-OH and D-OD is =1.5 kcal mol-1 which results from the difference
in stretching frequencies. The derived BDFEs for D-Daq and D-ODaq
are 103.4 and 117.6 kcal mol-1, respectively.
1
aldehydes in D2O were determined by H NMR relative to a standard
3-trimethylsilyl-1-propanesulfonic acid sodium salt. Equilibrium con-
stants (298 K) were measured by integration of the 1H NMR for
[(TSPP)Rh-D(D2O)]-4 and [(TSPP)Rh-CH(OH)CH3(D2O)]-4 com-
bined with the concentration of the acetaldehydes.
1H NMR (360 MHz, D2O) δ (ppm) ([(TSPP)Rh-CH(OD)CH3-
(D2O)]-4 (6)): 8.44 (8H, pyrrole), 8.3-8.0 (16H, phenyl), -2.6 (br,
1H, CH(OD)CH3), -4.4 (br, 3H, CH(OD)CH3).
Reaction of [(TSPP)RhI(D2O)]-5 and [(TSPP)Rh-D(D2O)]-4 with
Ethene, Hexene, Methyl Acrylate, and 4-Styrenesulfonic Acid
Sodium Salt. A solution of 2 (5 × 10-4 M) reacted with ethene, hexene,
methyl acrylate, and 4-styrenesulfonic acid sodium salt to form [(TSPP)-
Rh-CH2CH2D(D2O)]-4, [(TSPP)Rh-CH2CH(D)(CH2)3CH3(D2O)]-4
,
[(TSPP)Rh-CH2CH(D)CO2CH3(D2O)]-4, and [(TSPP)Rh-CH2CH(D)-
C6H4SO3Na(D2O)]-4, respectively. Reactions of 1 (5 × 10-4 M) with
ethene, hexene, and methyl acrylate produce the same products as those
of 2, with the exception of 1 with 4-styrenesulfonic acid sodium salt
which produces both [(TSPP)Rh-CH2CH(D)C6H4SO3Na(D2O)]-4 and
[(TSPP)Rh-CH(CH2D) C6H4SO3Na(D2O)]-4. The reactions of rhodium
Acknowledgment. This research was supported by the
Department of Energy, Division of Chemical Sciences, Office
of Science, through grant DE-FG02-86ER-13615.
JA054548N
9
J. AM. CHEM. SOC. VOL. 127, NO. 47, 2005 16467