Aqueous organometallic reactions of rhodium porphyrins: Equilibrium thermodynamics

Article abstract of DOI:10.1039/b212027e

The reactivity and equilibrium thermodynamic studies of tetra-p-sulfonatophenyl porphyrin rhodium hydride ([(TSPP)Rh-D]^-4) with CO, aldehydes and olefins that produce formyl, α-hydroxyalkyl and alkyl complexes have been explored in water and co

Full text of DOI:10.1039/b212027e

Aqueous organometallic reactions of rhodium porphyrins: equilibrium
thermodynamics†
Xuefeng Fu, Leah Basickes and Bradford B. Wayland*
Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323, USA.
E-mail: wayland@sas.upenn.edu; Fax: 215573-6743; Tel: 215898 6366
Received (in Purdue, IN, USA) 5th December 2002, Accepted 6th January 2003
First published as an Advance Article on the web 17th January 2003
The reactivity and equilibrium thermodynamic studies of
tetra-p-sulfonatophenyl porphyrin rhodium hydride
([(TSPP)Rh–D]24) with CO, aldehydes and olefins that
produce formyl, a-hydroxyalkyl and alkyl complexes have
been explored in water and compared with the related
reactions in non-aqueous media.
shifts for 1, 2, and 3 as a function of the concentration of D+ in
D2O (Fig. 1A). At D+ concentrations greater than 1025 M the
bis aquo complex 1 is effectively the only (TSPP)RhIII species
present in D2O while at D+ less than 10212 M the bis hydroxo
complex 3 predominates (Fig. 1A).
Reaction of the bis aquo species [(TSPP)RhIII(D2O)2]23 (1)
with H2/D2 (0.5–0.8 atm) results in formation of a hydride
Current emphasis on developing organometallic processes in
aqueous solution1–4 stimulated this investigation of rhodium
tetra-p-sulfonatophenyl porphyrin ((TSPP)Rh) reactivity in
water. The (TSPP)Rh species have a remarkable range of
organometallic reactions in water and this system provides an
opportunity to obtain one of the most complete descriptions of
equilibrium thermodynamics for organometallic transforma-
tions in water. Water soluble (TSPP)Rh(III) complexes react
with hydrogen to form a rhodium hydride that is a weak acid in
D2O. Reactions of the rhodium hydride in D2O with CO,
aldehydes and olefins (Scheme 1) that produce formyl, a-
hydroxyalkyl and alkyl complexes have comparable thermody-
namics and larger reaction rates in water than analogous
reactions in benzene.5–7 These substrate reactions in water are
complex [(TSPP)Rh–D]24 (4) in equilibrium with a rhodium(
I
)
species [(TSPP)RhI]25 (5) (eqns. (3) and (4)). A rhodium(II
)
metal–metal bonded dimer [(TSPP)RhII]2 (6) also forms in
the process from reaction of 1 with 5 (eqn. (5)), but at
equilibrium the reaction of 6 with hydrogen effectively removes
rhodium(II) species from the reaction system (eqn. (6)).
Equilibrium constants‡ have been determined for reactions
(3)–(6) which describe the distribution of the precursors for
organometallic reactions (T = 298 K, K3 = 0.3, K4 = 8.0 0.5
3 1028, K5 = 4.2 0.5 3 103, K6 = 9.8 0.8 3 102).
28
[(TSPP)RhIII
]
23 + D2 " [(TSPP)Rh–D]24 + D+
(3)
aq
[(TSPP)Rh–D]24 " [(TSPP)RhI]25 + D+
[(TSPP)RhI]25 + [(TSPP)RhIII
(4)
(5)
(6)
23
28
]
" [(TSPP)RhII]2
aq
[(TSPP)RhII]228 + D2 " 2 [(TSPP)Rh–D]24
thought to occur by ionic pathways that involve the rhodium( )
I
metal-centered nucleophile. An advantage of using aqueous
solutions is the capability of tuning the equilibrium distribution
of species by varying the hydrogen ion concentration. This
approach is used to expand the range of rhodium hydride
substrate equilibrium thermodynamic studies in water to olefins
which have equilibrium constants too large for convenient
direct measurement in both aqueous and hydrocarbon media.
Measurement of aqueous solution equilibria involving reac-
tions of rhodium(III) porphyrins requires a knowledge of the
equilibrium distributions of bis aquo [(TSPP)RhIII(D2O)2]23
(1), mono hydroxo [(TSPP)RhIII(D2O)(OD)]24 (2) and bis
hydroxo [(TSPP)RhIII(OD)2]25 (3) complexes (eqns. (1) and
(2)).8
Aqueous solutions of the rhodium hydride complex 4 react
with CO (PCO ~ 0.2–0.9 atm; [D+] < 1025) within the time
needed to record the 1H NMR to produce an equilibrium
distribution with a formyl complex (eqn. (7)).
[(TSPP)Rh–D]24 + CO " [(TSPP)Rh–CDO]24
(7)
The formyl complex is most convincingly identified by 13C
NMR of 7 prepared by using 13CO in reaction (7). The 13C
NMR for the formyl group of [(TSPP)Rh–13CDO]24 in D2O
(d13
= 218.7 ppm) appears as an approximate 1+2+2+1
CHO
quartet that results from the near equal coupling of D and 103Rh
with 13C (2J C–D + J
= 78 Hz) (Fig. 2A).
13
103Rh–13
C
When the 1H derivative of 7 ([(TSPP)Rh–13CHO]24) is
formed in H2O, the 13C NMR of the Rh–13CHO unit manifests
a 13C–H coupling 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) and a value
of 27 Hz is derived for J
[(TSPP)RhIII(D2O)2]23 " [(TSPP)RhIII(D2O)(OD)]24 + D+
(1)
13
13
13
[(TSPP)RhIII(D2O)(OD)]24 " [(TSPP)RhIII(OD)2]25 + D+
103Rh–13
.
C
(2)
The equilibrium constant‡ for reaction (7) (K7(298 K) = 3.0
0.3 3 103) was determined in D2O by integration of the
porphyrin pyrrole 1H NMR for 4 and 7 and the solubility of CO
in water.9
Equilibrium constants‡ for reactions (1) and (2) ((T = 298 K);
K1 = 1.4
0.2 3 1028; K2 = 2.8
0.3 3 10212) were
determined from the mole fraction averaged pyrrole chemical
Scheme 1 Formation and representative small molecule reactions of
(TSPP)Rh–D in D2O.
Fig. 1 Determination of acid dissociation constants at 298 K. (A)
[(TSPP)RhIII(D2O)2]23
,
K1
(D2O)(OD)]24, K2 = 2.8 0.3 3 10212. (B) [(TSPP)Rh-D]24, K4 = 8.0
0.5 3 1028
=
1.4
0.2 3
1028 [(TSPP)RhIII-
† Electronic supplementary information (ESI) available: experimental
details. See http://www.rsc.org/suppdata/cc/b2/b212027e/
.
520
CHEM. COMMUN., 2003, 520–521
This journal is © The Royal Society of Chemistry 2003

Solutions of [(TSPP)Rh–D]24 (4) in D2O react with
acetaldehyde to form an a-hydroxy ethyl complex [(TSPP)Rh–
CH(OD)CH3]24 (8) (eqn. (8)) during the time required to record
the substrate to the rhodium center and sequential addition of
the (por)Rh and H fragments. Reactions of (por)Rh–H com-
plexes with CO and olefins in benzene are catalyzed by
(por)RhII· metalloradicals and provide a radical chain pathway
for stepwise addition.11 The rhodium hydride is observed to be
a weak acid in water and thus a facile source of [(TSPP)RhI]25
(5) which functions as a metal-centered nucleophile in water.
Substrate reactions of 4 in water thus have a potential alternate
pathway for reaction involving the stepwise addition of the
1
the H NMR. The equilibrium constant for reaction (8) was
obtained by integration of the 1H NMR for each of the
constituents in the equilibrium distribution of 4, 8 and CH3CHO
(K8(298 K) = 1.4 0.1 3 103). Equilibrium constants for
reactions of 4 with olefins (eqn. (9)) are too large for accurate
direct measurement.
rhodium(
I) nucleophile and proton fragments of 4 (eqns. (12)
[(TSPP)Rh–D]24 + CH3CHO
and (13)).
" [(TSPP)Rh–CH(OD)CH3)]24
(8)
[(TSPP)Rh–D]24 + CH2NCH(X)
(por)RhI:2 + H2CO " (por)Rh–CH2O2
(12)
" [(TSPP)Rh–CH2CH(D)X]24
(9)
(por)Rh–CH2O2 + H+ " (por)Rh–CH2OH
(13)
The equilibrium constant for reaction (9) was derived
indirectly from a thermodynamic cycle by measuring the
Relatively fast reactions of 4 with aldehydes and olefins
equilibrium constant for reaction of the rhodium( ) complex (5)
I
activated by electron withdrawing groups are consistent with
with olefins and D2O in basic solution (eqns. (10) and (11)) in
combination with the equilibrium constants (298 K) for
reactions (4) (K4 = 8.0 3 1028) and (11) (K11 = [D2O]/
Kw(D2O) = 4.13 1016) (K9 = K10K4K11 = 3.3 3 109K10).
the rhodium( ) species having a prominent role in the reaction of
I
4 in water. Reaction of 4 with CO in water by this unusual
pathway is plausible, but is speculative at this time.
The equilibrium thermodynamic studies reported in this
article provide a foundation for obtaining thermodynamic
measurements on a much wider scope of substrate reactions
available to (TSPP)Rh species in the +1 to +3 oxidation state in
water. Our continuing studies in this area include a focused
effort to elucidate the mechanism for CO activation in water and
the development of strategies to exploit the rapid reactions of 4
in aqueous substrate transformations.
[(TSPP)RhI]25 + CH2NCH(X) + D2O
" [(TSPP)Rh–CH2CH(D)X]24 + OD2
(10)
OD2 + D+ " D2O
(11)
The equilibrium constant‡ for reaction (10) using methyl
acrylate (CH2NCHCO2CH3) is 0.086 (K10(298 K) = 0.086)
which permits evaluation of the equilibrium constant for
addition of the Rh–H (4) to methyl acrylate (K9(methyl acrylate)
= 2.8 3 108).
The authors acknowledge support of this research from the
Department of Energy, Division of Chemical Sciences, Office
of Science through grant DE-FG02-86ER-13615.
Addition reactions of [(TSPP)Rh–D]24(4) with CO, alde-
hydes and olefins to produce formyl (7), a-hydroxyalkyl (8),
and alkyl (9) derivatives are analogous to previously observed
substrate reactions of (TPP)Rh–H in benzene.5–7 Measurement
of the equilibrium constant (298 K) to produce the formyl
complex (Rh–CDO) by reaction (7) in water gives a DG°7(298
K) of 24.7 kcal mol21 which compares with a DG°(298 K) of
23.7 kcal mol21 to produce (TPP)Rh–CHO from reaction of
(TPP)Rh–H with CO in benzene.10 Solvation of the formyl
complex in water is undoubtedly very different from that in
benzene, yet there is only a small net change in DG°(298 K) for
the process in water compared to benzene.
References
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with regioselectivity to form an a-hydroxyalkyl [(TSPP)Rh–
CH(OD)CH3]24, in preference to an alkoxide. The hydride 4
reacts with olefins that have either electron withdrawing or
releasing substituents to give anti-Markovnikov regioselectivity
which suggests that the steric demands of the porphyrin may
have a dominant influence. Anti-Markovnikov regioselectivity
is also observed for (por)Rh–H addition reactions in benzene
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to occur substantially faster than the analogous reactions in
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Fig. 2 (A) 13C NMR of [(TSPP)Rh–13CDO]24 in D2O. 2J C–D + J
13
103Rh–13
C
= 78 Hz. (B) 13C NMR of [(TSPP)Rh–13CHO]24 in DMF. J
= 166
13C–H
Hz.
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