Competitive O-H and C-H oxidative addition of CH3OH to rhodium(II) porphyrins

Article abstract of DOI:10.1039/b708032h

Rhodium(II) porphyrins react with CH₃OH in benzene by alternate mechanisms that give H-CH₂OH and H-OCH₃ bond activation in different methanol concentration regimes which is a rare example of transition metal reactivity with methanol. The Royal Society of Chemistry.

Full text of DOI:10.1039/b708032h

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Competitive O–H and C–H oxidative addition of CH3OH to
rhodium(II) porphyrins
Shan Li, Weihong Cui and Bradford B. Wayland*
Received (in Berkeley, CA, USA) 5th June 2007, Accepted 17th August 2007
First published as an Advance Article on the web 29th August 2007
DOI: 10.1039/b708032h
Rhodium(II) porphyrins react with CH3OH in benzene by
alternate mechanisms that give H–CH2OH and H–OCH3 bond
activation in different methanol concentration regimes which is
a rare example of transition metal reactivity with methanol.
ð1Þ
ð2Þ
Oxidative addition of the O–H units in water and alcohols to late
transition metal centers is attracting new interest because of the
potential role in catalytic substrate transformations1,2 such as
olefin hydration3,4 and photodissociation of water.5–7 Oxidative
addition of –O–H fragments is relatively unusual with only a few
examples of late 2nd and 3rd transition metal complexes that give
this type of reactivity with alcohols1,8–12 and water.1,13–17 We have
Reaction of rhodium(II) tetramesitylporphyrin ((TMP)RhII ) (6)
?
recently reported that water oxidatively adds to a rhodium(II)
complex of persulfonated tetramesitylporphyrin ((TMPS)RhII ) in
?
?
?
with methanol qualitatively parallels that of Rh(m-xylyl)Rh . At
low concentrations of methanol ([CH3OH] y 0.01 M) C–H
activation occurs slowly to give (TMP)Rh–H and (TMP)Rh–
CH2OH as the exclusive kinetic and thermodynamic product
water to form equal quantities of hydride ((TMPS)Rh–H) and
hydroxide ((TMPS)Rh–OH) complexes.18 This article reports on
reactions of rhodium(II) porphyrins with relatively high concen-
trations of methanol in benzene that produce rhodium methoxide
(Rh–OCH3) complexes as the kinetic products that subsequently
react on to hydroxymethyl complexes (Rh–CH2OH) as the
thermodynamically preferred products.19
(eq 3). Reaction of (TMP)RhII (eq 3) is much slower than reaction
?
?
?
of Rh(m-xylyl)Rh (eq 1) because of the loss of preorganization of
the transition state and the change from a bimolecular process to a
termolecular process.19 At higher concentrations of methanol
([CH3OH] ¢ 0.1 M), (TMP)RhII reacts with methanol by a fast
H–OCH3 bond activation that produces the methoxide complex
((TMP)Rh–OCH3, (7)) (eq 4) which then subsequently reacts
slowly on to produce the hydroxymethyl complex ((TMP)Rh–
?
Reactions of m-xylyl tethered rhodium bis(phenyltrimesityl-
porphyrin) bimetallo-radical complex Rh(m-xylyl)Rh (1)19–21
with CH3OH at relatively low concentrations ([CH3OH] = 0.01–
0.1 M) produce the hydroxymethyl–hydride complex (H–Rh(m-
xylyl)Rh–CH2OH (2)) as the exclusive kinetic and thermodynamic
product (eq 1) (K1(296 K) = 1.5(0.5) 6 103; DG1u(296 K) =
24.3(0.2) kcal mol21).19 At higher concentrations of methanol
?
?
1
CH2OH, (8)) (eq 3) (Fig. 1). Reaction 4 occurs to a H NMR
observable equilibrium, but reaction 3 proceeds effectively to
completion at these conditions. Evaluation of the equilibrium
thermodynamics for reaction 4 ([CH3OH] = 0.1 M) gives
?
?
([CH3OH] . 0.5 M), Rh(m-xylyl)Rh has now been observed
to react with CH3OH by an alternate reaction pathway that gives
H–OCH3 bond addition to form methoxide and hydride com-
plexes CH3O–Rh(m-xylyl)Rh–OCH3 (3), H–Rh(m-xylyl)Rh–OCH3
(4), and H–Rh(m-xylyl)Rh–H (5) in a mole ratio of 1 : 2 : 1
(eq 2). Oxidative addition of the H–OCH3 unit to rhodium(II)
centers gives a statistical distribution of products which is distinc-
tively different from the selective intramolecular H–CH2OH
oxidative addition at low CH3OH concentrations. The rapid
H–OCH3 oxidative addition that kinetically dominates at high
concentrations of methanol is tentatively ascribed to a route
K4(296 K)
=
4.4(0.6)
6
1022
,
DG4u(296 K)
=
1.84(0.08) kcal mol21. The methoxide complex occurs as a
methanol adduct at 0.10 M CH3OH. Repeating reaction 4 using
toluene permits 1H NMR observation of the coordinated
methanol and methoxide at lower temperatures. Exchange of
involving donor induced disproportionation of rhodium(II) (2RhII
?
+ 2:B = [RhIII(B)2]+[RhI]2) which has several precedents in
rhodium porphyrin chemistry.22,23 Over a period of days at 296 K,
the Rh–OCH3 centers at 0.1 M CH3OH are converted quantita-
tively to Rh–CH2OH units as the thermodynamic products.
Fig. 1 High-field 1H NMR (C6D6) resonances for the reaction of
(TMP)RhII with CH3OH ([CH3OH] = 0.1 M). Reaction time: (a)
?
10 minutes; (b) 5 days; (c) 11 days. (Rh–OCH3: d = 22.35 ppm, d, 3H,
3J103Rh–H = 1.5 Hz; Rh–CH2OH: d = 21.53 ppm, dd, 2H, 3JH–H = 8.0 Hz,
2J103Rh–H = 3.3 Hz).
Department of Chemistry, University of Pennsylvania, Philadelphia,
PA 19104-6323, USA. E-mail: wayland@sas.upenn.edu;
Fax: 215573 6743; Tel: 215898 8633
4024 | Chem. Commun., 2007, 4024–4025
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methanol from (TMP)Rh–OCH3(CH3OH) with the bulk
broadens the NMR resonance of the coordinated CH3OH beyond
observation at 296 K. Temperature dependence of the coordinated
methanol provides activation parameters for the exchange. The
oxygen donor and hydrogen bonding capability for CH3OH
results in self association and adduct formation with the rhodium
complexes. Differential solvation of reactants and products
complicates the precise descriptions for reactions 3 and 4 and
the interpretation of the solution equilibrium studies for reaction 4.
2(TMP)RhII
+ CH3OHsol
?
sol
= (TMP)Rh–CH2OHsol + (TMP)Rh–Hsol
(3)
2(TMP)RhII
+ CH3OHsol
?
sol
= (TMP)Rh–OCH3 sol + (TMP)Rh–Hsol
(4)
The combination of equilibrium studies (eq 1, 4) indicates that
isomerisation (eq 5) of a rhodium porphyrin methoxide complex
((por)Rh–OCH3) to a hydroxymethyl species ((por)Rh–CH2OH)
at 0.1 M methanol is free energy favorable (DG5u y 26 kcal mol21).
Scheme 1 Proposed reaction pathways for C–H and O–H bond
reactions of rhodium(II) porphyrins.
This research was supported by the Department of Energy,
Division of Chemical Sciences, Office of Science through grant
DE-FG02-86ER-13615.
(TMP)Rh–OCH3 sol = (TMP)Rh–CH2OHsol
(5)
The difference in the substrate bond dissociation enthalpy values
(kcal mol21) (CH3O–H (104.6) and HOCH2–H (96.1))24 is the
dominant energy contribution that makes conversion of the
methoxy to the hydroxymethyl complex thermodynamically
favorable.
Notes and references
1 O. Blum and D. Milstein, J. Am. Chem. Soc., 2002, 124, 11456.
2 G. W. Parshall and S. D. Ittel, Homogeneous Catalysis, Wiley, New
York, 2nd edn, 1992, pp. 100–104.
3 B. Cornils and W. A. Herrmann, Aqueous-Phase Organometallic
Catalysis, Wiley, Weinheim, Germany, 2nd edn, 2004, p. 51.
4 N. D. Jones, P. Meessen, U. Losehand, B. O. Patrick and B. R. James,
Inorg. Chem., 2005, 44, 3290.
5 N. S. Lewis and D. G. Nocera, Proc. Natl. Acad. Sci. U. S. A., 2006,
103, 15729.
6 R. S. Paonessa, A. L. Prignano and W. C. Trogler, Organometallics,
1985, 4, 647.
7 V. Balzani, L. Moggi, M. F. Manfrin, F. Bolletta and M. Gleria,
Science, 1975, 189, 852.
8 R. Dorta, H. Rozenberg, L. J. W. Shimon and D. Milstein, Chem.–Eur.
J., 2003, 9, 5237.
9 O. Blum and D. Milstein, Angew. Chem., Int. Ed. Engl., 1995, 34, 229.
10 M. B. Sponsler, B. H. Weiller, P. O. Stoutland and R. G. Bergman,
J. Am. Chem. Soc., 1989, 111, 6841.
11 P. K. Monaghan and R. J. Puddephatt, Inorg. Chim. Acta, 1982, 65,
L59.
The reaction of the C–H bond of methanol with rhodium(II)
(eq 1) has previously been shown to occur by a metallo-radical
pathway that involves two rhodium(II) centers and the substrate in
the transition state (Scheme 1A). The C–H bond reactions of
rhodium(II) with CH3OH (eq 1, 3) are thermodynamically more
favorable than the H–OCH3 oxidative addition even at high
concentrations of methanol ([CH3OH] y 3 M). The observed
O–H bond activation must result from a pathway that becomes
more kinetically preferred as the concentration of methanol
increases. Strong donor molecules like pyridine (.2 equiv.) are
known to produce disproportionation of rhodium(II) porphyrins
into rhodium(I) and rhodium(III) bis-donor adducts. Substantially
stronger bonding of rhodium(III) with donor molecules compared
to rhodium(II) is the thermodynamic driving force for the
12 D. Milstein, J. Am. Chem. Soc., 1986, 108, 3525.
13 R. Dorta, H. Rozenberg, L. J. W. Shimon and D. Milstein, J. Am.
Chem. Soc., 2002, 124, 188.
disproportionation. Methanol adduct formation with (por)RhII
?
at high concentrations of CH3OH is proposed to induce
disproportionation to rhodium(III) and rhodium(I) and provide a
facile route for the observed H–OCH3 bond cleavage and addition
to the rhodium centers (Scheme 1B).
14 D. Morales-Morales, D. W. Lee, Z. Wang and C. M. Jensen,
Organometallics, 2001, 20, 1144.
15 M. J. Burn, M. G. Fickes, J. F. Hartwig, F. J. Hollander and
R. G. Bergman, J. Am. Chem. Soc., 1993, 115, 5875.
16 T. Yoshida, T. Okano, K. Saito and S. Otsuka, Inorg. Chim. Acta, 1980,
44, L135.
17 M. Yoon and D. R. Tyler, Chem. Commun., 1997, 639.
18 X. Fu, S. Li and B. B. Wayland, Inorg. Chem., 2006, 45, 9884.
19 W. Cui and B. B. Wayland, J. Am. Chem. Soc., 2004, 126, 8266.
20 W. Cui, X. P. Zhang and B. B. Wayland, J. Am. Chem. Soc., 2003, 125,
4994.
21 X.-X. Zhang and B. B. Wayland, J. Am. Chem. Soc., 1994, 116,
7897.
22 L. Zhang, C. W. Fung and K. S. Chan, Organometallics, 2006, 25, 5381.
23 B. B. Wayland, K. J. Balkus, Jr. and M. D. Farnos, Organometallics,
1989, 8, 950.
The observed isomerization of the methoxide complexes
((por)Rh–OCH3) to hydroxymethyl species ((por)Rh–CH2OH) is
proposed to go through the metallo-radicals ((por)RhII ) that
?
occur in equilibrium. At very high concentrations of methanol
in benzene ([CH3OH] . 5 M) or in pure methanol, fully selective
H–OCH3 bond activation occurs (eq 4) and the methoxide product
(Rh–OCH3) is indefinitely kinetically trapped relative to conver-
sion to the thermodynamically preferred hydroxymethyl complex
(Rh–CH2OH) by the vanishingly small equilibrium concentration
of rhodium(II).
24 S. J. Blanksby and G. B. Ellison, Acc. Chem. Res., 2003, 36, 255.
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