Oxidation of alcohols by [Cp*Rh(ppy)(OH)]+

Article abstract of DOI:10.1007/s007060070011

Rh(III) polypyridine complexes ([Cp*Rh(ppy)(H₂O)]²⁺; ppy = 2,2′-bipyridine, 2,2′-bipyridine-4,4′-dicarboxylate, o-phenanthroline, tetrahydro-4,4′-dialkyl-bis-oxazole) oxidize in organic or aqueous alkaline solution primary and secondary alcohols to aldehydes or ketones and are thereby reduced to the Rh(I) complexes Cp*Rh(ppy). The Rh(III) form can be regenerated by oxidants like pyruvate or oxygen, making the reaction quasi-catalytic. The reaction follows an autocatalytic pathway; hydrogen transfer from the a-CH₂ group of an alcoholate complex [Cp*Rh(ppy)(OR)]⁺ to Cp*Rh(I)(ppy) is suggested to yield the Rh(II) intermediate Cp*Rh(ppy)H as the key and rate determining step. The knowledge of Rh(III)/Rh(I) redox potentials allows to estimate the thermodynamic driving force of the reaction which is not more than about 300mV.

Full text of DOI:10.1007/s007060070011

Monatshefte fuÈr Chemie 131, 1321±1326 (2000)
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Oxidation of Alcohols by [Cp Rh( ppy)(OH)]
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È È
Ulrich Kolle and Holger Franzl
Institute of Inorganic Chemistry, Technical University of Aachen, D-52074 Aachen, Germany
Summary. Rh(III) polypyridine complexes ([Cp Rh(ppy)(H2O)]2‡; ppy ˆ 2,20-bipyridine, 2,20-
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bipyridine-4,40-dicarboxylate, o-phenanthroline, tetrahydro-4,40-dialkyl-bis-oxazole) oxidize in
organic or aqueous alkaline solution primary and secondary alcohols to aldehydes or ketones and
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by oxidants like pyruvate or oxygen, making the reaction quasi-catalytic. The reaction follows
are thereby reduced to the Rh(I) complexes Cp Rh(ppy). The Rh(III) form can be regenerated
an autocatalytic pathway; hydrogen transfer from the ꢀ-CH2 group of an alcoholate complex
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‡
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[Cp Rh(ppy)(OR)] to Cp Rh(I)(ppy) is suggested to yield the Rh(II) intermediate Cp Rh(ppy)H as
the key and rate determining step. The knowledge of Rh(III)/Rh(I) redox potentials allows to estimate
the thermodynamic driving force of the reaction which is not more than about 300 mV.
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Keywords. Alcohol oxidation; Cp Rhꢀppy† complex; Kinetics.
Introduction
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polypyridine derivatives Cp Rh(ppy)Cl2 (1 [1, 2]). Polypyridines different from
Cleavage of [Cp RhCl2]2 with various polypyridines easily leads to mononuclear
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bipy (2,20-bipyridine) and o-phenanthroline derivatives have been applied in the
same reaction by Kaim an coworkers [3]. Complexes 1 hydrolyze in water to aqua-
ions [Cp Rh(ppy)H2O]2‡ (2). The water soluble complexes 2 are stable towards
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hydrolytic loss of Cp and ppy ligands over a wide pH range. The pKs value of the
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water molecule in 2a (ppy ˆ 2; 20-bipyridine) has been determined as 8.2 Æ 0.15 [4]
which means that the species present in water at pH > 10 are hydroxo complexes
[Cp Rh(bipy)(OH)] (3). Compounds 1±3 are reduced in cyclic voltammetry and
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by suitable reducing agents (cobaltocene) to intense blue or violet Rh(I) complexes
‡
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Cp Rh(bipy) (4, ꢁmax ˆ 512 nm), where reduction potentials for Rh(III)/Rh(I) have
been found to be around 0.6 ± 0.95 V vs. SCE depending on ppy and pH [2].
The system 2/3 has previously been utilized as a catalyst for the reduction of
protons to hydrogen, a reaction that could be effected with visible light using
illuminated TiO2 colloid as the electron donor [1]. Other groups have immobilized
the system for the same purpose on an electrode surface [5] or applied it for e.g.
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the regeneration of NADH from NAD [6]. Analogous Ir complexes have been
prepared and are effective photocatalysts for the water gas shift reaction [7]. In the
present communication we report on the oxidation of alcohols by the system 2/3.
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Corresponding author

È È
U. Kolle and H. Franzl
1322
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1: Cp Rh(ppy)Cl2
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2: [Cp Rh(ppy)H2O]2‡
3: [Cp Rh(ppy)(OH)]
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4: Cp Rh(ppy)
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ppy ˆ 2,20-bipyridine (a), 2,20-bipyridine-4,40-dicarboxylate (b), o-phenanthroline (c), tetrahydro-
4,40-dialkyl-bis-oxazole (d)
Results and Discussion
When a deaerated aqueous solution of 3b at pH > 10 is treated with a primary or
secondary alcohol at ambient or slightly elevated temperature, the development of
an intense characteristic blue color within some minutes indicates the formation of
the Rh(I) complex 4b. The formation of an aldehyde or ketone can be shown by
precipitation of the corresponding 2,4-dinitrophenylhydrazone. For other polypyr-
idines such as 2,20-bipyridine or o-phenanthroline, the Rh(I) complex is insoluble in
water and will precipitate from an aqueous solution. A homogeneous reaction is
achieved in tetrahydrofurane, dimethylformamide, acetonitrile, or the alcohol as the
solvent. Since the Rh(I) complexes are air sensitive, their formation can only be
observed under anaerbic conditions. It was veri®ed that the same mixture does not
give any oxidation products in the presence of oxygen. The reaction has been
applied to a variety of different alcohols with and without additional functional
groups (Scheme 1).
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carbonyl compounds marked with (a) were identi®ed as 2,4-dinitrophenylhydrazones
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Scheme 1. Alcohols that effect the reduction of [Cp Rh(III)(ppy)(OH)] to Cp Rh(I)(ppy);

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Oxidation of Alcohols by [Cp Rh(ppy)(OH)]
1323
In some cases, the oxidation product of the alcohol was identi®ed as the
aldehyde or ketone by conversion to the respective 2,4-dinitrophenylhydrazone. GC
analysis of the reaction mixture did not show any carboxylic acid or other oxidation
products after several cycles. The overall reaction is thus as formulated in Eq. (1). In
the presence of an excess of alcohol, the conversion of Rh(III) to Rh(I) was
quantitative.
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‰Cp Rhꢀppy†ꢀOH†Š ‡ RCH2OH ‡ OH ! Cp Rhꢀppy† ‡ RCHO ‡ 2H2O
ꢀ1†
Though the reduction of Rh(III) to Rh(I) with concomitant aldehyde/ketone
formation due to the involvement of Rh(I) in the reaction cycle (see below)
only proceeds under anaerobic conditions, the whole reaction can still be made
quasi-catalytic if oxygen is admitted after one cycle to reoxidize Rh(I) to Rh(III).
In this way, many cycles (more than 10, depending on ppy and pH) have been
effected.
Reaction scheme and kinetics
As mentioned above, the reaction only proceeds in deaerated solutions, and there is
always an induction period before the development of the blue color commences.
The length of this induction period varies between seconds and several minutes
depending on the reaction medium and the deaeration procedure; it can be shortened
by adding a small amount of Rh(I) complex to the deaerated solution. These
observations strongly suggest an autocatalytic pathway with the participation of the
Rh(I) complex. Since the product-forming step of an alcohol to aldehyde conversion
is the abstraction of the ꢀ-hydrogen we suggest that Rh(I) is effective in this step. A
Rh(II) hydride formed this way would be rapidly converted to Rh(I) and a proton as
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outlined below (RH ˆ Cp Rhꢀppy†).
‡
‡
!
RHꢀIII†ꢀOH† ‡ RCH2OH
RHꢀIII†OCH2R ‡ H2O
ꢀ2†
ꢀ3†
ꢀ4†
ꢀ5†
‡
‡
RHꢀIII†OCH2R ‡ RHꢀI† ! RHꢀII† ‡ RHꢀII†H ‡ RCHO
‡
‡
RHꢀII† ‡ RHꢀII†H ! RHꢀI† ‡ RHꢀI†H
‡
RHꢀI†H ‡ OH ! RHꢀI† ‡ H2O
The 1st step in the sequence, i.e. complexation of the alcohol to Rh(III) either as an
alcoholate as given in Eq (2) or as an alcohol complex, is inferred from the
observation that the reaction is strongly inhibited by stronger, ꢂ-donors such as
amines which obviously block the residual coordination site at Rh(III) for alcohol
complexation. It further accounts for the fact that the reaction requires alkaline
conditions, since formation of a Rh(III) alcoholate is feasible only at high pH.
Formation of an alcoholate by the hydrogen donating alcohol is invoked in transfer
hydrogenations, particularly those catalyzed by Rh(ppy) complexes [8]. Deprotona-
tion of protonated Rh(I) (Eq. (5)) would proceed completely at pH ꢁ 9 where the
alcohol oxidation is still very slow. A pKa around 8 was evaluated from spectro-
electrochemistry for Cp Rh(I)(bipy)H (bipy ˆ 4,40-dicarboxylato-2,20-bipyridine).
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In these experiments, the reduction of Cp Rh(bipy)(H2O)2‡ gives the colored Rh(I)
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U. Kolle and H. Franzl
1324
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complex Cp Rh(I)(bipy) only at pH ꢁ 8. Below this pH, the reduction product is
nd
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the much less colored protonated complex Cp Rh(I)(bipy)H . In the 2 step (Eq.
(3)), a hydrogen atom is abstracted from the ꢀ-CH2 group of the alcohol by Rh(I)
generating formally two Rh(II) complexes. Though transfer of hydrogen, e.g. in
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alcohol dehydrogenase to NAD to give NADH, may be viewed as hydride transfer
as well as the corresponding step in transfer hydrogenations (1st half of Eq. (6)), in a
recent system detected by Wieghardt and coworkers [9], where alcohols have been
oxidized aerobically by a Cu bisphenolamine complex, ꢀ-hydrogen transfer is
considered a radical transfer step.
Electron transfer between protonated and unprotonated Rh(II) species (Eq. (4))
forms protonated and unprotonated Rh(I) complexes, where the former will be
deprotonated at the pH given. Note that the seeming contradiction in oxidation
states in Eq. (4) is due to the fact that the Rh bound hydrogen is counted (by
convention) as a hydride at the left hand side but as a proton at the right hand side.
Equations (2)±(5) not only take care of the fact that the reaction requires
alkaline conditions and accounts for the overall autocatalytic pathway, but also
explains why it does not proceed under aerobic conditions: rapid reoxidation of any
Rh(I) formed quenches step two and thus the whole cycle. It also rules out the 2nd
mechanistic alternative, i.e. deprotonation of the Rh(III)alcoholate complex at the
ꢀ-CH2 group, or the hydride derived thereof by ꢃ-insertion (Eq. (6)). It has, however,
been observed that under strong alkaline conditions, i.e. at pH ꢁ 13, the reaction
mechanism changes. The induction period is shortened or even absent, and the time
pro®le of Rh(I) after about one half life resembles more that of an ordinary 1st order
reaction. Operation of Eq. (6) would readily account for this behavior.
OH
‡
RHꢀIII†OCH2R ! RHꢀI†H ‡ RCHO ! RHꢀI† ‡ H2O
ꢀ6†
A kinetic run at 23ꢂC of a solution containing 1.12 Á 10 3 M 1c (ppy ˆ
o-phenanthroline) and 0.01 M KOH in 2 cm3 i-propanol was simulated according
to the reaction scheme with the CKS kinetic simulator [8]. A starting concentration
of 5 Á 10 5 M Rh(I) was given to initiate the reaction, and the initial concentrations
of alcohol and water were set to 0.1 M for practical reasons (to keep the required
number of particles and thus the time for simulation within a reasonable limit).
The equilibrium of Eq. (2) was ®xed arbitrarily to K2 ˆ 10 with rate constants
1
1
k2 ˆ 5 Á 105, k ˆ 5 Á 104, and k4 ˆ k5 ˆ 5 Á 104 M Á min to ensure the pre-
2
equilibrium and the consecutive ractions to be fast in comparison to the rate
determining step (Eq. (3)). The simulated curve (Fig. 1) was obtained with
k3 ˆ 700 M Á min . Though the simulated curve is somewhat more curved than
the experimental one, where also the beginning of the reaction is lacking, the order
of the slow reaction, i.e. Eq. (3), is well reproduced. Note that the rate constant k3
has no real signi®cance; only the product K2 Á k3 ˆ 7 Á 104 M Á min can be
1
1
1
1
deduced from the observed kinetics.
A 1st order dependence of the rate on [OH ] and thus a strong support for the
participation of Eq. (2) in the reaction scheme was revealed when a kinetic run at
1/10 of the base concentration was simulated with the same rate constant and gave
very good agreement between experimental and simulated slopes in the nearly
linear part of the c vs. t curve.

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Oxidation of Alcohols by [Cp Rh(ppy)(OH)]
1325
Fig. 1. Experimental (ÐÐÐ) and simulated (Á Á Á Á Á Á) concentration time pro®le for the reaction
a 1.2 mM solution of 2c (L ˆ o-phenanthroline) with i-propanol in 0.01 M KOH according to Eqs.
(2)±(5)
Thermodynamics
Alcohol oxidation catalyzed by metal complexes has utilized strongly oxidizing
metals in high oxidation states such as Mn(III), Co(III), and Ni(III) in most cases. The
formation of oxidation products beyond the aldehyde/ketone stage is a frequently
encountered problem, in particular with oxidation of primary alcohols. In contrast, the
redox couple 2b/4b for example is characterized by potentials of 0.56 (Epc) and
0.50 (Epa) V vs. SCE. The corresponding values for 2a/4a are 0.66 and 0.54 V
[2]. On the other hand, the driving force for Eq. (7) can be estimated as 0.8 V vs.
SCE at pH 13.
C2H5OH ‡ 2OH ! CH3CHO ‡ 2H2O ‡ 2e
ꢀ7†
This leaves an overall driving force for Eq. (1) of only 150 to 250 mV
(ÁGꢂ ˆ 0:3 ± 0.5 kJ/mol), much less than is typical for alcohol oxidation by
high oxidation state transition metal complexes. The low driving force may be
responsible for the clean oxidation in particular of primary alcohols to aldehydes. It
may ®nd application in cases where substrates feature oxidation sensitive
functionalities such as conjugated multiple bonds. The small driving force, on the
other hand, can lead to a reversal of the reaction for activated, i.e. stronger oxidizing
carbonyl compounds. It was found that e.g. pyruvate readily oxidizes Rh(I) back to
Rh(III), presumably with formation of lactate, under the same conditions where
ordinary alcohols are reduced.
‡
CH3CꢀO†COO ‡ RhꢀI† ‡ 2H2O ! CH3CHꢀOH†COO ‡ RhꢀIII†OH ‡ OH
ꢀ8†
Experimental
Rh(III)ppy complexes 2/3 were prepared as previously described [1, 2]. For alcohol oxidation
experiments they may be generated in situ by adding an equivalent of the respective polypyridine to a
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slurry of [Cp RhCl2]2 in methanol.

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U. Kolle and H. Franzl: Oxidation of Alcohols by [Cp Rh(ppy)(OH)]
1326
Pentamethylcyclopentadienyl-(4S,40S)-4,40,5,50-tetrahydro-4,40-methyl-bisoxazole-chlororhodium
tetra¯uoroborate (2d)
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2d was prepared analogously to the polypyridine complexes [2] after dehalogenation of [Cp RhCl2]2
with one equivalent of AgBF4 in methanol or acetone. 1H NMR (300 MHz, ꢄ, acetone-d6): 1.63, 1.91
(d, J ˆ 6:3 Hz, Me), 1.95 (s, 15H, Cp ), 3.58 (m, 2H, H4), 4.8 (m, 2H, H40), 5.11 (m, 2H, H5) ppm;
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cyclic voltammogram (CH2Cl2:acetonitrile ˆ 1:1, Bu4NPF6, v ˆ 200 mV/s): Epc ˆ 0:94 (irrev),
Epa ˆ 0:43 (irrev).
Pentamethylcyclopentadienyl-(4S,40S)-4,40,5,50-tetrahydro-4,40-i-propyl-bisoxazole-chlororhodium
tetra¯uoroborate (2e)
1H NMR (300 MHz, ꢄ, CD2Cl2): 1.00, 1.07 (d, J ˆ 6:5 Hz, 2H each, Me), 1.5 (m, 3H, CH(Me)2 and
CH2), 1.87 (s, 15 H, Cp ), 4.3 (dd, J ˆ 7:3, 8.6 Hz, 2H, H4), 4.51 (d, J ˆ 8:6 Hz, 2H, H40), 4.77 (m,
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2H, H5) ppm.
Oxidation of alcohols was conducted in 5 cm3 septum sealed vessels charged with a 10
10 4 M
3
solution of the Rh(III) complex in H2O (ppy ˆ 2,20-bipyridine-4,40-dicarboxylate), THF, DMF, or the
alcohol as solvent, to which 0.1 M aqueous KOH and the alcohol were added. Experiments with
alcohols collected in Scheme 1 were performed with ꢃ10 3 M 3a, 10 2 M KOH, and 10 2 M
alcohol in THF:H2O ˆ 1:1 to ensure the solubility of 4a. After deaerating the vessels with Ar by
means of a syringe needle, the reaction starts by developing a blue to violet color. Repeated reduction
and reoxidation of the blue solution was effected by injecting air slightly less than required for
stoichiometric reoxidation of Rh(I). In one experiment, a 20 cm3 septum sealed vessel was charged in
the same way, and after the reduction had come to completion once, oxygen was continuously
injected by means of an automatic pipette (Metrohm 702 Titrino) at a rate ensuring the presence of
residual Rh(I) throughout. Polypyridine complexes generally allowed many reduction-reoxidation
cycles. Bisoxazole complexes which in cyclic voltammetry showed similar reduction-oxidation
behavior and developed dark red solutions in the presence of alcohol and base gave only one cycle.
Kinetic measurements were performed in a 5 mm optical glass cell ®tted with a rubber seal.
The absorption of the Rh(I) phenanthroline complex at 700 nm was followed with a J&M Tidas diode
array spectrophotometer at 1 min intervals. The reaction scheme was simulated with the CKS, Vers. 1,
kinetic simulator program [10] and compared with the experimental curves using Origin 6.01.
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È È
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È
È
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È È
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Received July 27, 2000. Accepted September 18, 2000