Comparison of Rh-OCH3 and Rh-CH2OH bond dissociation energetics from methanol C-H and O-H bond reactions with rhodium(II) porphyrins

Article abstract of DOI:10.1021/ja1035489

Reaction of methanol in toluene with tetramesityl rhodium(II) porphyrin ((TMP)Rh^II·) produces a ¹H NMR-observable equilibrium with rhodium methoxide ((TMP)Rh-OCH₃(CH₃OH)) and rhodium hydride ((TMP)Rh-H) complexes. Equilibrium concentrations for each of these species, obtained from integration of ¹H NMR spectra, were used in determining the equilibrium constant, K(298K) = [Rh-OCH ₃(CH₃OH)][Rh-H]/[Rh^II·] ²[CH₃OH]² = 3.0(0.3), and free energy change, δG⁰(298K) = -0.65(0.5) kcal mol^-1, for the reaction. Equilibrium thermodynamic measurements in CD₂Cl₂ give δG⁰(298K) = -5.5(0.2) kcal mol^-1 for association of methanol with (TMP)Rh-OCH₃ to form the six-coordinate 18-electron complex (TMP)Rh-OCH₃(CH₃OH). Equilibrium measurements in conjunction with (TMP)Rh-H and CH₃O-H bond energetics are used to evaluate the (TMP)Rh-OCH₃ bond dissociation free energy (Rh-OCH ₃ BDFE(298K) = 38 (1.3) kcal mol^-1), which is 15 kcal mol^-1smaller than the Rh-H BDFE and approximately equal to the Rh-CH₂OH BDFE.

Full text of DOI:10.1021/ja1035489

Published on Web 09/10/2010
Comparison of Rh-OCH3 and Rh-CH2OH Bond Dissociation Energetics from
Methanol C-H and O-H Bond Reactions with Rhodium(II) Porphyrins
Sounak Sarkar,‡ Shan Li,‡ and Bradford B. Wayland*,†
Departments of Chemistry, Temple UniVersity, Philadelphia, PennsylVania 19122, and UniVersity of PennsylVania,
Philadelphia, PennsylVania 19104
Received April 26, 2010; E-mail: bwayland@temple.edu
methylene chloride was evaluated from the methanol concentra-
Abstract: Reaction of methanol in toluene with tetramesityl
tion dependence of the porphyrin pyrrole resonance position
rhodium(II) porphyrin ((TMP)RhII•) produces a 1H NMR-observable
(K1(298 K) ) 11(2) × 103; ∆G0 (298 K) ) -5.5(0.2) kcal mol-1
)
1
equilibrium with rhodium methoxide ((TMP)Rh-OCH3(CH3OH))
and rhodium hydride ((TMP)Rh-H) complexes. Equilibrium
concentrations for each of these species, obtained from integra-
tion of 1H NMR spectra, were used in determining the equi-
(Figure SI 1, Supporting Information).
(TMP)Rh-OCH3 + CH3OH a (TMP)Rh-OCH3(CH3OH)
(1)
(2)
librium constant, K(298 K)
)
[Rh-OCH3(CH3OH)][Rh-H]/
(1)
[RhII•]2[CH3OH]2 ) 3.0(0.3), and free energy change, ∆G0(298 K)
) -0.65(0.5) kcal mol-1, for the reaction. Equilibrium thermody-
namic measurements in CD2Cl2 give ∆G0(298 K) ) -5.5(0.2) kcal
mol-1 for association of methanol with (TMP)Rh-OCH3 to form
the six-coordinate 18-electron complex (TMP)Rh-OCH3(CH3OH).
Equilibrium measurements in conjunction with (TMP)Rh-H
and CH3O-H bond energetics are used to evaluate the
(TMP)Rh-OCH3 bond dissociation free energy (Rh-OCH3
BDFE(298 K) ) 38 (1.3) kcal mol-1), which is 15 kcal mol-1smaller
than the Rh-H BDFE and approximately equal to the Rh-CH2OH
BDFE.
Toluene solutions of 3 (1.0 × 10-3 M) react slowly (Figure SI
2, Supporting Information) with methanol at low concentrations
([CH3OH] < 0.01 M) by H-CH2OH bond cleavage to form the
hydroxymethyl complex 5 and hydride 4 (eq 2), but at higher
methanol concentrations ([CH3OH] > 0.08 M) the net H-OCH3
bond cleavage occurs to form the methoxide complex 2 and hydride
4 (eq 3).
CH OH
<
[
]
3
2(TMP)RhII + CH3OH. {
0.01 M} .
•
C6D5CD3
(TMP)Rh-CH2OH + (TMP)Rh-H (2)
2(TMP)RhII + 2CH3OH {0.1 M CH OH
}
3
Late transition metal alkoxides (M-OR)1-4 and R-hydroxy-
alkyl (M-CH(R)OH)5-7 complexes are important precursors and
intermediates for a diverse range of organic transformations such
as alcohol8 and olefin oxidation,9 C-O coupling,10 and carbonyl
hydrogenation.11 Substantial effort has been directed toward
evaluating the differences in thermodynamic and kinetic mecha-
nistic factors that govern the formation and utilization of an
isoelectronic series of transition metal complexes that include
methoxide (M-OCH3), hydroxymethyl (M-CH2OH), and ethyl
(M-CH2CH3) derivatives.6 Computational and experimental
studies that compare ꢀ-H migration,12 CO insertion,13 and
solution equilibria14 have advanced this area, but direct com-
parisons of metal methoxide (M-OCH3) bond dissociation
energetics with those of hydroxymethyl (M-CH2OH) and ethyl
(M-CH2CH3) derivatives has eluded experimental evaluation.
This Communication reports on using equilibrium thermody-
namic measurements for the reactions of methanol both with
rhodium tetramesityl porphyrin methoxide15 ((TMP)Rh-OCH3,
1) to form (TMP)Rh-OCH3(CH3OH) (2) and with (TMP)RhII•
(3) to produce 2 and (TMP)Rh-H (4) in order to evaluate the
RhIII-OCH3 bond dissociation energetics for 1.
•
C6D5CD3
(TMP)Rh-OCH3(CH3OH) + (TMP)Rh-H (3)
Reactivity16 and EPR17 studies have shown that one of the general
reactions of rhodium(II) porphyrin complexes is donor-induced
disproportionation to Rh(I) and Rh(III) species. The rhodium-
methoxide unit (Rh-OCH3) in 1 and 2 subsequently reacts to give
the thermodynamically preferred hydroxymethyl species, Rh-CH2OH
(Figure S2). The hydroxymethyl complex is formed by a relatively
slow termolecular pathway6 but is thermodynamically favored at all
concentrations of CH3OH. The methoxide complex forms by a donor-
induced metalloradical disproportionation route that is kinetically
preferred at high methanol concentrations (Scheme 1).
Scheme 1. Proposed Pathways for (A) C-H Bond Activation by
(TMP)RhII• Metallo-radicals and (B) O-H Bond Activation through
Donor-Induced Disproportionation of (TMP)RhII•
The five-coordinate methanol-free methoxide complex 1 was
prepared by Collman’s method15 of sequential additions of
AgPF6 and NaOCH3 to a dichloromethane solution of (TMP)Rh-I.
The equilibrium constant for methanol coordination with 1 to
form the six-coordinate 18-electron complex 2 (eq 1) in
Reaction of a 0.10 M solution of methanol in toluene-d8 with 3
(1.0 × 10-3 M) gives a 1H NMR-observable equilibrium distribution
of 3 with rhodium methoxide 2 and rhodium hydride 4 (eq 3).
† Temple University.
1
‡ University of Pennsylvania.
Complexes 2, 3, and 4 are readily identified by H NMR (Figure
9
10.1021/ja1035489  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 13569–13571 13569

C O M M U N I C A T I O N S
1). The high-field doublet from Rh-OCH coupling (J
) 1.5 Hz) centered at -2.46 ppm is associated with 2, and the
resonance centered at -40.0 ppm with a 103Rh-H coupling of 43.2
Hz identifies the five-coordinate hydride complex 4.
103
103Rh-OCH
prior estimates of the near equivalence of late transition metal M-H
and M-OCH3 bond energetics.21 The Rh-OCH3 BDFE is ∼4 kcal
mol-1 smaller than the Rh-CH2CH3 BDFE and is virtually equal to
the BDFE for the isomeric Rh-CH2OH unit.
Coordinated alkoxide groups in late transition metal complexes have
been shown by Bergman’s group to function as remarkably strong
donors in hydrogen bonding with alcohols.3 Exceptionally strong
MOR-alcohol bonds may result from synergism of hydrogen bonding
with decreasing the dπ-pπ repulsions. Binding of methanol with the
methoxide in HIr(OCH3)(PEt3)3Cl was suggested as a possible
explanation for the large entropy change (∆S0 ) -67(4) cal K-1 mol-1)
in the oxidative addition of CH3O-H with Ir(PEt3)3Cl.14 The effective
Ir-OCH3 BDE of 61-68 kcal mol-1 may thus be a composite of the
Ir-OCH3 BDE and the dissociation of a hydrogen-bonded methanol,
similar to reaction 6, where (TMP)Rh-OCH3(CH3OH) (2) involves
dissociation of a Rh-methanol complex and Rh-OCH3 bond ho-
molysis. Spectroscopic and equilibrium studies of the interaction of
methanol with solutions of (TMP)Rh-OCH3 (1) are consistent with
formation of only the 18-electron complex (TMP)Rh-OCH3(CH3OH)
(5) in the range of methanol concentrations studied ([CH3OH] )
0.001-1.0 M). Hydrogen bonding of methanol with the coordinated
methoxide in 1 and 2 is inhibited by the steric demands of the porphyrin
mesityl groups.
Figure 1. Characteristic 1H NMR (500 MHz) for an equilibrium distribution
of (TMP)Rh-OCH3(CH3OH) (2) and (TMP)Rh-H (4) from reaction of
(TMP)RhII• (3, 1 × 10-3 M) with CH3OH (0.10 M) in toluene at 298 K: (a)
pyrrole of 3, (b) m-phenyl of 3, (c) pyrrole of 2, (d) pyrrole of 4, (e) -OCH3
of 2, and (f) hydride of 4.
The concentrations of 2, 3, and 4 obtained from integration
of the 1H NMR spectrum were utilized in determining the
equilibrium constant at 298 K for reaction 3 (K3(298 K) )
3.0(0.3); ∆G0(298 K) ) -0.65(0.05) kcal mol-1).The free energy
change for the reverse of reaction 3 in combination with the
bond dissociation free energies (BDFEs) of Rh-H6 (eq 4) and
CH3O-H20 (eq 5) gives a free energy change of 43.7(1.1) kcal
mol-1 for reaction 6 (Scheme 2). The sum of the Rh-OCH3
BDFE and the free energy change to form the methanol complex
Isomerization of 1 to 5 (eq 9) in toluene is observed by 1H NMR
to proceed effectively to completion.
(TMP)Rh-OCH3 h (TMP)Rh-CH2OH
(9)
(1)
(5)
Rearrangement of the -OCH3 organic fragment to CH2OH is
2 is ∼43.7(1.1) kcal mol-1
.
20
thermodynamically favored by ∼8.5 kcal mol-1
,
and trading an
Scheme 2. Free Energy Change (∆G0(298 K), kcal mol-1) for the
Homolytic Dissociation of (TMP)Rh-OCH3(CH3OH) (2) in Toluene
(T ) 298 K; L ) TMP)
O-H for a C-H unit thus drives the isomerization.
Late transition metal alkoxides (M-OCHR2) with filled dπ
orbitals should be thermodynamically unstable with respect to
isomerization to R-hydroxyalkyl (M-C(OH)R2) complexes in the
absence of additional energy terms from interactions such as M-OR
alkoxide hydrogen bonding with alcohols. Addition of late transition
metal hydrides with aldehydes and ketones should invariably have
a thermodynamic preference to produce metal R-hydroxyalkyl
derivatives over metal alkoxides.
Acknowledgment. This research was supported by the Depart-
ment of Energy, Office of Basic Energy Science, through Grant
DE-FG02-09ER16000.
Evaluation of ∆G0 (298 K) for methanol complex formation with
7
1 permits closing the thermodynamic cycle to obtain the Rh-OCH3
BDFE for 1 (38.2(1.7) kcal mol-1, Scheme 3). Dissociative
Supporting Information Available: Details on the solution struc-
tures and equilibrium measurements. This material is available free of
charge via the Internet at http://pubs.acs.org.
6
processes like 8 typically have ∆S0 values ∼27(4) cal K-1 mol-1
,
which provides an estimate of 46(2) kcal mol-1 for the Rh-OCH3
bond dissociation enthalpy (BDE) for 1.
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