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Oxidation of Organometallic Compounds.

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Oxidation of Organometallic Compounds
By Jack Halpern*
Dedicated to Professor Gunther Wilke on the occasion of his 60th birthday
Stable organometallic compounds, notably of the later transition metals (groups VI-VIII),
usually are characterized by closed shell electron configurations (typically IS-electron valence shells) which are destabilized by electron addition or removal. One-electron oxidation
of such compounds results in the formation of unstable radical ions, whose characteristic
reactivity patterns include susceptibility to nucleophilic attack, disproportionation, and metal-carbon bond dissociation. Two-electron oxidation may result in dissociation or oxidation of the organic ligand. In this review studies on the chemical and electrochemical oxidations of metal carbonyls, metal-olefin complexes, and alkyl transition-metal compounds are
described. The studies encompass the following themes : (1) The kinetics and thermodynamics of the initial redox steps; (2) The characterization and reactivity patterns of the resulting
oxidation products; ( 3 ) The synthetic and catalytic applications of organometallic redox
processes.
1. Introduction
Whereas electron-transfer and simple redox reactions of
inorganic coordination compounds have been extensively
studied, and many features of their kinetics and mechanisms are well understood,"] our knowledge and understanding of the corresponding redox chemistry of organometallic compounds is much less advanced. Indeed,
only within the past decade have such redox processes received significant attention and study.
With a few notable exceptions (for example, [V(CO),]"]
and [Fe(C,H,),]@)"] stable organometallic compounds, especially of the later transition metals (groups VI to VIII),
correspond to closed shell electron configurations (often
IS-, sometimes 16-, electron valence shells) which are destabilized by electron addition or removal, generating unstable products that undergo further reactions. Equations
(1)-(3) illustrate this distinction (R=alkyl or aryl;
Hdmg = dimethylglyoximato(1 -); L = H 2 0 , py, etc.).
[ W C N)d4'
18e
5[Fe(CN)$'
(stable)
(1)
2. Oxidation of Metal Carbonyls
Our interest in this theme had its origin in our early
studies on the oxidation of CO in aqueous solution by metal ions such as Hg" and Agl [Eq. (4) and (5)].["'-1'1
-
+ 2 Hg2' + H2O CO, + Hg:' + 2 H'
CO + 2[Ag(NH;)J0 + 2 HZO * CO:' + 2Ag + 4NH:
CO
17e
[ R - C ~ ( H d r n g ) ~ L l5[R-C~(Hdrng)~Ll" (unstable)
18e
17e
dox processes, (2) the characterization of the initial (often
unstable and short-lived) organometallic oxidation products and the study of their chemical reactivities, and (3) the
roles of oxidation of organometallic compounds in catalytic oxidation and other catalytic processes.
Only insofar as they impinge upon these themes will attention be directed to certain related topics, notably chemical and electrochemical reductions of organometallic comp o u n d ~ , [ ~ . oxidative-addition
''
and reductive-elimination
and redox reactions of non-transition metal
organometallic compounds," Attention is directed in the
cited references to other reviews of some relevant aspects
of these topics.
(3)
This review is concerned particularly with the chemical
and electrochemical oxidation reactions of organo-transition metal compounds, including those containing carbonyl, olefin, and alkyl ligands. The themes to be considered
include: (1) the kinetics and mechanisms of the initial re-
(4)
(5)
These and subsequently identified oxidations of CO by
other metal ions, such as Rh"', Ni", and Cu", have been
interpreted as proceeding through mechanisms involving
hydroxycarbonyl (or metallacarboxylic acid) complexes
such as 1 [Eq. (6) and (7)].["]
Hg20
+
co
+
H,O
r.,Iedctrrmlnl"g
0
I1
[Hg-COH]@
1
+ Ha
i45,
Hg"
+ CO, + 2H'
(6)
[*] Prof. Dr. J . Halpern
Department of Chemistry, The University of Chicago
Chicago, I L 60637 (USA)
214
0 VCH Verlagrqesellrchafi mbH. 0-6940 Weinheim. 1985
0570-0833/85/0404-0274 S 02.50/0
Angew. Chem. Inr.
Ed. Engl. 24 (1985) 274-282
Pathways analogous to that depicted above for the oxidation of free CO extend also to the oxidation of coordinated CO. Thus, certain metal carbonyl complexes are decomposed by reaction with H,O (or OH') with the formation of CO, (or CO:") and a reduced metal carbonyl or a
metal hydride [Eq.(8) and (9)]."3.'41
in high oxidation states (for example, potential oxidants
such as Co"'). It appeared to us that a possible approach
to get around this limitation and thereby to extend the
scope for possible metal-ion oxidations of CO (and related
unsaturated molecules) would be to start with a stable metal-carbonyl complex in a low (Le., non-oxidizing) oxida-
v
-
rrurzs- fPt"C1(PEt3)z(CO)]Q
Hz0
-Ha
[ H F e o ( C 0 ) 4 ] 0 + OH'
truns- [Pt"CI(PEt,),(COOH)]
--+
Only recently have several examples of stable hydroxycarbonyl complexes, including trans-[F'tCI(PEt3)2(C00H)],
actually been intercepted and characteri~ed.['~-'~'
Such
complexes tend to be unstable and to decompose upon
warming or upon reaction with water, with loss of CO, to
form the corresponding hydrides. An unresolved mechanistic question is whether such decomposition, especially in
the presence of water, involves the direct transfer of hydrogen to the metal @-elimination of CO,) according to
Eq. (IOa) or deprotonation to yield an intermediate CO,
complex (or metallacarboxylate) according to Eq. (lob).
Both processes are inherently plausible and it is possible
that both occur in appropriate circumstances.
It is likely that the [Hg(COOH)]@intermediate 1 in reaction ( 6 ) is formed by external nucleophilic attack of water
on the transient mercury(r1)-carbonyl complex 2 [Eq.(1 I)],
analogous to the mercury(I1)-olefin complexes (or mercurinium ions) 3 that have been postulated as intermediates
in the closely related olefin oxymercuration reactions [Eq.
(12)1."~1
HgZe
+
CO e [ H g ( C O ) ] 2 e
2
HgZa
+
HzO
[Hg(COOH)]@ + HQ (11)
C H p C H 2 # [Hg(CHz'CH2)12a
HzO
--+
3
rrans- [HP?'Cl(PEt,),]
+
CO,
(9)
tion state and to oxidize the pre-formed complex to a
higher oxidation state of the metal ion which would not
normally coordinate CO.
This approach was applied successfully to effect the oxidation of C O in the mixed ligand Co' complex
[Co'(CN),(PEt,),(CO)]'
(abbreviated [L,,Co'(CO)]"), using
[Fe(CN)J3' (abbreviated X3') as oxidant.[2o1[LnCo'(CO)]o
was found to undergo two successive one-electron oxidations in aqueous solution to form the CN-bridged binuclear
Co"' complex [(NC)SFe"(CN)Co"'(CN),(PEt3)2(CO)]'o
(abbreviated [XL,CO"'(CO)]'~). The fate of the latter is
determined by the pH of the solution [Eq. (13)]; the paths
(13a), (13b), and (13c) predominate in the pH ranges <4.5,
4.5-1 1.5, and > 11.5, respectively. The rate constants k ( , , , ,
and kc13c,
at 25" are 1 . 0 lo-'
~
and 3.4 s - ' , respectively.
In the presence of excess [Fe(CN)J3' the L,Co' produced in reaction (13b) or (13c) is irreversibly oxidized to a
Co"' complex [Eq.(14)], resulting in the overall stoichiometry of Eq. (15). Under these conditions the effective reducing power of the coordinated CO, as well as of the Co'
ion, is utilized to supply electrons for the reduction of
[Fe(CN),]" so that [LnCo'(C0)]@acts as a 4-electron reductant.
[€fg(CH2CH,0H)]Q + HQ ( 1 2 )
[XL,Co'I5'
To the extent that such prior coordination of C O is an
important component of the CO oxidation pathway, the
scope of such oxidations would appear to be constrained
by the limited tendency of CO to coordinate to metal ions
Angew Chrm In,. Ed Enql. 24 ilV85) 274-282
+ 2X" + H 2 0
+
[L,CO'(CO)]~ 4X3'
--*
+ 20H'
[XL,CO"'(H,O)]~'
+ 2X"
--*
+
[XL,CO"'(H,O)]~~ 3X4'
C02
+ 20H'
e CO:'
(14)
+ CO,
(15)
+ H20
275
The apparent reversibility of the sequence of steps corresponding to Eq. (16), and the relatively slow decomposition of the intermediate CO, complex, [XL,CO'(CO,)]~~
(k, = 3.4 s '), identifies a pathway for the reduction of
coordinated CO, to CO (i.e., by Co'). Since C 0 2 has been
demonstrated to coordinate to other Co'
this
suggests a possible approach to the homogeneous catalytic
reduction of COz, an objective of considerable current interest. The recently reported catalytic reduction of CO, to
CO at carbon electrodes modified with cobalt phthalocyanine probably involves related chemistry.[**'
~
Catalysis of the oxidation of CO also has been observed
in the case of certain other metal-ion oxidants (e.g., Cu''
and Rh"') whose reduction products (Cu' and Rh') form
stable CO complexes.["] In such cases reduction may be
autocatalytic as in the reduction of RhCI3 by CO [Eq. (22)]
where both uncatalyzed and autocatalytic pathways are
observed [Eq. (23) and (24)], corresponding to the rate laws
k'[Rh"'][CO] and k"[Rh"'][Rh'(CO),], respectively.
[XL,CO"'(COOH)]~~
%
[XL,CO"'(CO)]~'
Hi0
(16)
-
[XL"CO'(CO2)]~Q
Because of the effectiveness of CO, as a leaving group,
redox pathways such as (17) may result in the formation of
"coordinatively unsaturated" species, e.g., [XL,CO']~"(or
[L,Co']',
which presumably is formed readily from
[ X L , C O ' ] ~by~ loss of X4'), with high affinities for a variety of ligands, notably those (e.g., olefins) that are effective
in stabilizing low oxidation states. This opens up the possibility of synthetic approaches involving the "oxidative
substitution" of CO by other ligands to generate complexes that are not readily synthesized by other methods,
such as 4 and 5 [Eq. (17)-(19)].L201
- H20
CH>=CHCN
[ L,C ~ ( C H Z ' C H C N )I'
(18)
4
[L"C
[Rhi"]
+
[Rhl(CO)z]
-
2 CO
[Rd]
[Rgl
+
[Rd(CO),]
[Rhl"(CO),I
H,O,
- H"
Other catalytic applications of the metal-carbonyl chemistry described above include catalysis of the water-gas
shift reaction [Eq. (25)]['2."-251and the use of combinations
of CO and H,O as a source of hydrogen or of electrons to
effect hydrogenation, hydroformylation [Eq. (26)], or reduction [Eq. (27)].[23.26,271
M echanistic schemes for reactions ( 2 5 ) and (26) are depicted in Schemes I and 2, respectively.
21"
CO
CH2=CHCH2OH
+ HzO
H2 + CO, (-
OH
"
HCOY)
(25)
[ L,C2(CH,=CHCHzOH)10
i
- OHa
0
RCH=CH,
RNOz
+ 2CO + HzO
+ 3 CO + H2O
Ibc(co)T1
II
(26)
' RCHzCH2CH + COz
RNHz + 3 CO1
When the oxidation of [L,CO'(CO)]~according to Eq.
(13b) and (13c) is conducted in the presence of an excess
of CO, the coordinatively unsaturated [L,Co']' intermediate is readily trapped by CO to reconstitute the initial
[L,Co'(CO)I0 complex. This results in the establishment of
a catalytic cycle for the oxidation of CO [Eq. (20)]. When
the oxidant is [Fe(CN),]3e the overall catalytic reaction
corresponds to Eq. (21).[201
-2 $
[C2(CO)]
rcd"cco,1
HzO
Scheme I . Mechanism of the Fe(CO),-catalyzed water-gas shift reaction [Eq
W1.
276
Angrw. Chem. Inf. Ed. Engl. 24 il985) 274-282
0
which involve both TI- and a-bonded organometallic intermediates, an example being the widely accepted mechanism of the Wacker reaction [Eq. (32)].
/OH@
[PdC13(CHZCH20H)I2'
Pd"
+ CH3CHO + H' + 3C1°
(32)
The evidence upon which this mechanism rests is not en)Lo.
tirely conclusive, particularly with respect to the interme-
L
[HFeo(CO)4]e
' 0
[RCH,CH,&Feo(CO),]'
Scheme 2. Mechanism of the Fe(CO),-catalyzed hydroformylation of alkenes
IEq. (2611.
The susceptibility of coordinated CO to nucleophilic attack by water and the instability of the resulting hydroxycarbonyl complexes usually precludes the direct detection
and characterization of oxidized metal-carbonyl complexes in aqueous solution. In less nucleophilic solvents
such as acetonitrile, oxidation may simply result in the liberation of coordinated CO and other ligands [Eq. (28) and
(29), C4H6= b~tadiene].[~']
[Fe(CO),]
3 [Fe(CO)5]e (?) 3 Fe"
[Fe(CO),(C,H,)]
-
Fe"
+ 5CO
(28)
+ 3CO + C4H6
(29)
In some cases the lifetimes of the oxidized species are
sufficiently long that the electrochemical oxidations of metal carbonyls or mixed ligand carbonyl complexes to the
corresponding 17-electron radical cations yield reversible
cyclic voltammograms as in the example of Eq. (30) and
the corresponding oxidations of [Cr(CO),(PPh3)],'2'1
[CpMn(CO)(PPh,),],f301[Fe(CO),(PR3),],[3'1[Cr(CO),I]Q,[321
[Ta(CO)2(dmpe)2C1]0,[331 [ C ~ C O ( P P ~ , ) , ] , [ ~ ~and
~
[Cr(CO),]f3s1(Cp = cyclopentadienyl; dmpe = bis( 1,2-dimethy1phosphino)ethane). Several such species have been
characterized by electron absorption and EPR spectroscopy and at least one, [Cr(CO),(PPh,),]@', has recently been
isolated as a stable salt.[361One decomposition pathway of
such 17-electron radical cations in solution is through disproportionation [Eq. (31)].f371
2[Cr(CO),lo
[Cr(CO),]
+ [Cr(C0),l2'
(-
Cr"
+ 6CO)
(31)
unstable
3. Oxidation of Metal-Olefin Complexes
Metal ions such as Pd"[38,391
and T1"'[40.4'1
are useful oxidants for olefins, particularly in view of the distinctive selectivities which they characteristically exhibit. Mechanistic pathways for such reactions have been postulated
Angew. Chrm. Int. Ed. En.91 24 11985) 274-282
diacy of the o-bonded organometallic species
[PdCl3(CH2CH20H)]'' for which there is only indirect evid e n ~ e ' ~ ' .(although
~~'
analogous [Tl>C-CcOH]z" intermediates have been identified in the oxidations of certain
To the extent that initial n-coordinaolefins by T111').[40a1
tion of the olefins to the metal ion is an important component of the oxidation pathway, such oxidations (like the
corresponding oxidations of CO discussed above)'"]
would appear to be constrained by the limited tendencies
of olefins to coordinate to metal ions in high oxidation
states. Accordingly, one strategy for extending the scope of
metal-ion oxidations (paralleling that described above for
CO oxidation) would be to start with a stable olefin complex of a metal ion in a low oxidation state and to oxidize
the pre-formed complex to a higher oxidation state with an
external oxidant.
An illustration of the successful application of this approach is provided by the oxidation of Zeise's anion,
[Pt"C13(CH2=CH,)]', in aqueous HCI solution by C12.1421
The addition of a stoichiometric amount of Clz results in
the oxidation of the coordinated ethylene to chlorohydrin
[Eq. (33)]. Excess C1, further oxidizes [Pt"Cl,]'e to
[Pt'"Cl6]Z0.
[R"CII(CH~=CH,)]'
+ CI2 + H 2 0
--t
(33)
CI
I
[PtCI,]"
OH
I
+ CH2-CH2 + H"
A detailed study of this reaction has revealed that it proceeds according to the stepwise mechanism in Scheme 3.14']
The first step of this sequence apparently involves the
rapid oxidative addition of C1, to [Pt"Cl,(CH,=CH,)]o to
form [Pt'"CIs(CH2=CH2)]', followed by nucleophilic attack of C1' on the coordinated ethylene to form the 0bonded complex [Pt'VC15(CHzCH2CI)]2e.In the second
step, a reversible solvolytic nucleophilic displacement of
the chloride, apparently reflecting the activating influence
of the p-Pt substituent, yields the p-hydroxy o-complex
[PtlvCI,(CH2CH,0H)]'e, the analogue of the postulated
[Pd"C13(CH2CH20H)]2eintermediate in the Wacker reaction [Eq. (32)]. In the third step, reductive elimination of
chlorohydrin takes place. The latter step exhibits a first-order kinetic dependence on the C1' concentration, suggesting that it actually proceeds via nucleophilic displacement
at the Pt-bonded carbon atom by an external C1' ion [Eq.
(34) and (35)].This is consistent with our earlier suggestion
that the mechanism of the reverse reaction, i.e., of "oxidative addition" of organic halides to low-valent dX metals
such as iridium(]), also involves such a nucleophilic displa~ement.[~.~']
277
resulting complex has proved to be a useful method of liberating the organic product [Eq. (36)1281and (37)1441].
4. Oxidation of Cobalt Alkyl Complexes
I
The most extensive studies on the chemical and electrochemical oxidations of alkyl transition-metal compounds
have been performed on organobis(dimethylg1yoximato(1 -))organocobalt(m) complexes [ R c ~ ( H d m g ) ~ L(ab]
breviated [CoR]), 6,where L is an axial ligand such as water or pyridine, and on related organocobalt complexes
such as 7, which have Schiff base compounds as lig a n d ~ . ' ~ ~ - Such
~ ~ , * Iorganocobalt compounds also have
been studied extensively in other contexts because of their
relevance as coenzyme B t 2 model^.[^^-'^^
c1
Scheme 3. Mechanism of the [F'tCI,]'"-catalyzed oxidation of ethylene by C12
8
1421.
[Pt'VC15(CH2CH20H)]2G
+ CI' --t
[Pt"C1,]3e
[Pt"Clj]3Q --t [ P t " C I p
+ C1CH2CH20H
+ CIQ
(35)
The known reaction of C2H4 with [Pt"C14]2Qto regenerate [Pt"CI3(CH2=CH2)lQ completes a catalytic cycle for
the highly selective oxidation of CzH4 to CICH2CH20H
(Scheme 3). All the intermediate species and the component steps of this catalytic cycle have been directly observed and characterized. This general approach would appear to be capable of extention to encompass the catalysis
of the oxidation of olefins and related substrates by other
metal complexes with potentially distinctive selectivities.
Few oxidation reactions of other metal-olefin complexes
have been examined in any detail. Commonly, oxidation
of metal-olefin complexes, and especially of mixed olefincarbonyl complexes, results in liberation of the coordinated olefin (see, e.g., Eq. (29), C4H6=b~tadiene).''~'
In recent years, "metal assisted syntheses", involving the
construction or elaboration of organic molecules that are
coordinated to metals (e.g., to fragments such as Fe(CO),
and Cr(CO),), have achieved increasing importance in synthetic organic methodology.'28~441
The application of this
approach typically results in the production of the target
organic molecule as a coordinated ligand. Oxidation of the
OMe
278
OMe
GMe
Me
Me
6, L
(34)
=
H,@
7, L
= PY
[CoR] compounds ( R = alkyl or benzyl) have been found
to undergo reversible chemical and electrochemical oneelectron oxidations in aqueous solution to generate the
corresponding
17-electron
radical
cations,
[ R C O ( H ~ ~ ~ ) ~ ( H (abbreviated
~O)]'
[CoR]', Eq. (38)).L47.501
[CoR]
[CoR]'
(38)
The [CoR]' ions, which could also be generated by
chemical oxidation with [IrC1,]ze [Eq. (39)], are unstable at
room temperature and are decomposed by nucleophilic attack of water to form Ca" and ROH [Eq. (40)].147.5"1
In accord with this scheme, the rate law of the overall reaction
[Eq. (41)] was found to conform to Eq. (42).
[CoR] + [IrC1,]2e
[CoR]'
+ HzO
*
[CoRl + [IrCI,]*"
+ [IrCI,]"
[Co"] + ROH + H Q
h-iu
[CoR]'
(39)
-
(40)
+ H20
[Co"]
+ ROH + [IrC1J3' + H'
(41)
for the electrochemical oxidations (deterValues of
mined by cyclic voltammetry) and of the kinetic parameters, determined from the kinetic measurements, are summarized in Table
As expected, the trend of
values is in the direction of increasing ease of oxidation with
increasing electron donor power of R. At the same time
the trend of lifetimes
of the [CoR]@ ions, i.e.,
CH3, CzH5> i-C3H7 and p-NOZC6H4CH2
> C6HSCH2
p-CH30C6H4CH2, is consistent with a decomposition
mode involving nucleophilic attack on R.
At - 78 "C (in aqueous methanol or methylene chloride)
the [CoR]@ ions (generated by oxidation with Ce'" or
*
Angew. Chem. Int. Ed. Enyl. 24 119851 274-282
Table I . Thermodynamic and kinetic parameters for the oxidation of 6
CH,
CzH.
n-C,Hr-C,H,
p-CH,OC,H,CH,
p-CH,C<,H ,CH,
C,H,CHp-FC, H K H ,
p-CIC,H,CH,
p-NO-C, H,CH7
0.902
0.878
0.867
0.856
-
0.849
0.859
0.873
0.876
0.907
~
~
-
3.5.10'
3.4.105
1.4. 10'
==5.104
~ 5 . 1 0 ~
=I.IO~
51.102
4.6.10-'
1.1.10-3
1.9.10-3
2.7. lo-'
1.6.10-'
0.9. 10-'
9.3.10-3
32
3.9.10-2
2 . 8 . lo-'
2.6. lo-'
1.7.10-2
3.2.10-3
> 10'
8.3
9. I
1.7
1.7. lo-'
[a] Relative to the a t u r a t e d calomel electrode. [b] K 3 s = k i q / k _ 3 9
In related studies on the electrochemical oxidations of a
variety of [ R C ~ ( H d r n g ) ~ Land
l
[RCo(Schiff base)] complexes in acetonitrile solutions, the initially produced
[Co'"R] radical cations were found to exhibit several different reactivity patterns, depending on the ligands used
and on whether nucleophiles such as pyridine were added.[531In addition to nucleophilic displacement of R'
(either by pyridine [Eq. (46)], or by an oxygen atom of the
Schiff base ligand) homolytic dissociation of the Co-R
bond with formation of alkyl radicals was observed in
some cases [Eq. (47)J.
[ColVR]'
[Co'"R]'
PbOz) were found to be stable for many hours and could
be characterized by EPR.[47-491
The results convincingly
support the formulation of these ions as cobalt(rv) complexes, i.e., as [Col"R']' rather than [Co"'R0]@. Warming
such solutions ( C H 3 0 H / H 2 0 , 4 : 1) of [CoR]',
R = C6H5CH2,to ca. - 20°C resulted in decomposition according to Eq. (40). On the other hand, when R = C2H5,decomposition followed a different course, the products and
kinetics of which are consistent with Eq. (43) and (44).['01
2[CoR]'
[CoR]
+ [CoR]*'
+ py
-
---*
[Co"]
[Co"']
+ Rpy'
+ Ro
(46)
(47)
Much of the research o n the redox chemistry of organocobalt compounds such as [RCo(Hdmg),(H,O)] has been
motivated, at least in part, by the relevance of such compounds as coenzyme B,, (5'-deoxyadenosylcobalamin)
models. However, at this stage there is no evidence, and little reason to suspect, that oxidation or reduction of coenzyme B,, is involved in the cobalt-carbon bond dissociation step which triggers the coenzyme BI2-promotedrearrangement and in which the coenzyme's distinctive biochemical role is manife~ted."~-''~
(43)
5. Implications for Halogen Cleavage of MetalAlkyl Bonds
The reversible disproportionation of [CoR]' [Eq. (43)]
finds parallels in the corresponding reactions of other organometallic radical cations, e.g., [Cr(CO),]@ [Eq. (3 l)].[3'1
Failure to detect the disproportionative pathway in the
case of the benzyl and sec-alkyl cobalt complexes presumably reflects the much higher reactivities of these complexes toward nucleophilically induced decomposition
[Eq. (40)] as revealed by the values of k,, in Table 1.
Examination of the data in Table 1 reveals an unusual
pattern of relationships between the rate constant (k39)and
equilibrium constant (K39=k39/k--39)for reaction (39). For
the series of p-substituted benzylcobalt complexes examined, the rate constant k,, exhibits a significantly larger dependence on the variation of R than the corresponding
equilibrium constant K39.These data give rise to a Marcus
plot of logk,, vs logK3, with an anomalously large slope
of ca. 2-3 instead of the slope of ca. 0.5 expected for a
simple outer-sphere electron-transfer reaction (and found,
for example, in the related oxidations of Pb% compounds
by [IrCI,]'0).[81 The results suggest that reaction (39) may
actually be a stepwise process of the type depicted by Eq.
(45) in which the second step presumably involves a structural rearrangement as well as an internal electron transfer.
In such a case the measured rate constant and equilibrium
constant would not refer to the same process. The testing
of this suggestion calls for measurements on a faster time
scale than can be achieved by the electrochemical and
chemical oxidation procedures employed in these studies,
e.g., pulse radiolysis.
In contrast to the familiar halogen cleavage reactions of
organomercurials, which typically proceed with retention
of configuration at the metal-bonded a-carbon atom, and
which have been interpreted as front-side electrophilic displacement processes,[571halogen cleavage of alkyl transition-metal complexes, e.g., [RC0(Hdmg)~(H,0)1 and
[RFe(qS-C5H5)(CO),], commonly proceeds with inversion
of configuration,'" 591 The latter reactions have been alternatively interpreted as proceeding through (1) direct backside electrophilic attack at the a-carbon atom,L5X1
or (2)
one-electron oxidation of the metal alkyl followed by nucleophilic attack of halide ion on the resulting radical cation,[4h
Ce'". H',
- pyHm
A n y e w . Clirm Int. Ed. Enql. 24 (198s) 274-282
591
The demonstration that stable [ R C ~ ( H d r n g ) ~ ( H ~ ora)]@
dical cations could be generated in solution by oxidation
of [RCo(Hdmg),(H20)] permitted a direct test of interpretation (2) to be made. Employing the sequence of steps depicted by Eq. (48), it was demonstrated that CI' does indeed react directly with [RCo(Hdmg),(H,O)]@, displacing
Re with virtually quantitative inversion of configuration of
the a-carbon atom. Accordingly, such a process does ap-
HzCbMeOH
*
279
pear to be a viable step in halogen cleavage reactions of
metal-carbon bonds that occur with inversion of configuration,[5'1
6. Oxidation of Dialkyl Transition-Metal
Compounds. Oxidatively Induced Reductive
Elimination
Diverse reactivity patterns have been observed in studies
on the oxidation of dialkyl transition-metal compounds.
One-electron oxidation of trans-[Me,Co"'(dpnH)] 8[*lor
trans-[Me,Co"'(tim)] 9[*]in acetonitrile to the corresponding Co'" complexes was found to be irreversible (by cyclic
voltammetry) and generally to result in homolysis of one
of the Co-C bonds, yielding a stable monoalkyl Co"'
compound and a free CHQ radical whose fate (dimerization, H abstraction, or oxidation by excess oxidant) depends on the reaction conditions [Eq. (49)].[601
ble to be isolated and characterized by EPR. The decomposition products (C4Hlo,C2H6,and C2H, when R=C,H,)
were interpreted as arising from cage reactions of a pair of
R' radicals. On the other hand, the two-electron oxidation
product, 11, whose formation is irreversible, apparently
undergoes a concerted reductive elimination to yield R2 directIy.['']
Other reported examples of oxidatively induced reductive elimination include that depicted by Eq. (52) (R=alkyl or aryl, L = PEt3).I6'I
[(Aryl)(R)Ni"L,] d [ ( A r y l ) ( R ) N i ' ' ' L , ]
[ArylNi"lX,LZ]
[Aryl,Ni"'XLJ
[AryINi"XL,]
2[Me2Co'"(chelate)]"'+
[MeCo"'(chelate)]'"+"O
-
-
[ArylNi"'X,L,]
+ Me'
Oxidation of cis-[R,Fe"(bpy),] ( R = Me, Et, nPr, etc.) revealed still other reactivity patterns [Eq. (5 I)].["'b1
11
4
f
I
J.
2 R@(-+R2
etc.)
[Fc?]
f
(51)
R,
The one-electron oxidation product, 10, whose formation was electrochemically reversible, was sufficiently staI*]
d p n H =2,3,Y, IO-tetramethyl-l,J,X,l I-tetraaza-1,3,8,lO-undecatetraen-I I 01-1-olate:tim = 2,3,Y,IO-tetramethyl- 1,4,8,1 I-tetraaza-I,3,8,IO-cyclotetradecatetrdene
280
(53)
+ [ArylNiiiXL2] [Aryl,Ni"'XL,] + [Ni"X2L,]
Ary12 + [Ni'XL2]
-
+ ArylX
--*
(54)
+
[Ni1'X2L,] Aryl:
(55)
(56)
Extensive studies also have been conducted on chemical
and electrochemical oxidations of non-transition metal alkyl compounds such as &Pb, R$n, and R2Hg.18] The
course of the oxidation of Me4Pb in acetic acid is typical
of that found for such systems [Eq. (57)-(6O)J.
[Me,Pb]
+ 2[IrCI,]"
%
[Me,PbOAc]
[F;]
(52)
(49)
(50a)
10
+ [Ni'L,]
7. Oxidation of Non-Transition Metal Alkyls
I)'
Oxidation of cis-[R,Pt"(PR;),]
(R=CH,,
C,H,;
PR;= PMe,Ph, PPh3) complexes by [IrC1,]20 in acetonitrile
afforded two types of products, depending on the nature
of R and PR; (X=CI',
CH3CN, etc.). cis[Me2Pt(PMe2Ph),] reacted predominantly according to Eq.
(50a), whereas cis-[Et,Pt(PPh,),] reacted predominantly according to Eq. (50b). For cis-[Me,Pt(PPh,),], competition
between the two pathways was observed.(6'"1
I
Aryl-R
Reductive elimination from a Ni"' intermediate according to Eq. (53)-(55) has been invoked to interpret the
cross-coupling reactions of aryl halides (ArylX) with arylnickel complexes (ArylNiXL,, L = PEti and similar ligands) to yield biaryls [Eq. (56)].["l
[Ni'XL2] + ArylX
[Me,Co"'(chelate)]"@
--t
+ MeCl + [lrC1,]30 + [IrCli]20
(60)
Differences between this behavior and that found for related oxidations of alkyl transition-metal compounds include the following: 1) Step (57) appears to be essentially
irreversible and the postulated [&Pb]' (or [R,Hg]') radical cations are too short-lived to be detected kinetically,
electrochemically, or spectroscopically. 2) In contrast to
the oxidized organo-transition metal complexes, for which
alternative decomposition modes, including nucleophilic
attack, disproportionation, or reductive elimination,
usually are observed, [&Pb]' and [R2Hg]' decompose exclusively by metal-carbon bond homolysis. The origin of
these differences may be that the highest occupied molecular orbital (HOMO) from which the electron is removed in
the oxidation of [ k P b ] presumably is a metal-carbon 0bonding orbital, whereas in the case of most transition metal complexes, such as [RnCoI''] or [R,Fe"], it is a nonbonding d - ~ r b i t a l . [ ~ ~ '
Angew. Chem. Int. Ed. Engl. 24 11985) 274-282
Finally it should be noted that not all processes in which
oxidation of an organometallic compound apparently occurs actually proceed through such a step. Thus oxidative
cleavage of the Cr-C bond in [PhCHzCr"'(HzO)s]Z@
by
oxidants such as Fe3@,CuZe,02,and HzOz in aqueous solution exhibits first-order kinetics that are independent of
the nature or concentration of the oxidant [Eq. (61)].'651
Net reaction:
[ P h C H, C(1I:
H 2 0 )5]
rate-determmmg
* PhCH,@ +
1
+ PPh,-+[L\.V(CO),(PPh,)]
+ L
1
Cr3'aq
8. Oxidatively Induced CO Insertion
There is extensive evidence that one-electron oxidation
of transition metal complexes containing both alkyl and
CO ligands promotes migratory insertion of C O into the
metal-alkyl bond [Eq. (62)]. Specific examples encompassing this phenomenon are shown in Eq. (63) and (64).'66-h8'
Analogous mechanisms [Eq. (70)-(72)] have been invoked to account for the oxidatively induced migratory insertion reaction of [(q5-C5H5)(PPh3)(CO)Fe(CH,)](abbreviated [Fp-CH3]) [Eq. (73)].[7'1
.,e
Initiation:
[Fp-CH,]
+
[F'p-CH,]'f
[Fp-ClI,J@
CO
-
[Fp-C-C€131@
+
(70)
0
I(
[Fp-C-CH3]@
P r opagation:
[Fp-CH,]
(71)
(72)
ROH,CO
((59)
C r Z Q a q (61)
;:Yon
PhCH20H
[L,W(CO),]
P
[F~-&-cH,~+ [ F ~ - c H , I ~
0
I1
RCOR'
CUCI,
It has been shown that such CO insertion reactions proceed with retention of configuration at the a-carbon
Oxidatively induced migratory insertion of C O
has been utilized in synthetic applications such as b-lactam
synthesis [Eq. (65), Cp = T ~ ' - C ~ H ~ ] . [ ~ ~ ]
9. Redox-Catalyzed Reactions of Organometallic
Cornpounds
The distinctive reactivity patterns of oxidized organometallic compounds may be incorporated into catalytic cycles
which proceed through electron-transfer-propagated chain
mechanisms.
An example of such catalysis is provided by the substitution reactions of metal carbonyls such as cis-[L2W(CO),]
( L = M e C N or py), whose rates are enhanced by chemical
or electrochemical oxidation according to the mechanism
of Eq. (66)-(69), for which the chain length has been estimated to be ca. 20.[701
Angew Chem l n t Ed Enql 24 il98S) 274-282
Net r e a c t l o n :
[Fp-CH,]
+ CO
9
[Fp-C-CH,]
(73)
These processes parallel other examples of "electrontransfer catalysis", the principle of which has been recognized for some time also in other context^.[^^-'^^
10. Concluding Remarks
The study of the oxidation reactions of organometallic
compounds constitutes a relatively new field of research
which has developed almost entirely within the past decade. The results yielded by the limited studies conducted
thus far already reveal a rich and versatile array of chemistry associated with the primary redox processes themselves, with the distinctive electronic structures and reactivity patterns of the species produced by oxidation of stable organometallic compounds, and with the synthetic and
catalytic applications of oxidatively induced organometallic reactions. At the same time many problems, both of understanding and of application, remain to be addressed.
Research activity in this field continues at a high level and
with great promise for the realization of important new
discoveries and insights.
Support of our research in this field by the National
Science Foundation is gratefully acknowledged.
Received: October 19, 1984 [A 531 IE]
German version: Angew. Chem. 97 (1985) 308
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