Dr, Jack Haipern, Department of Chemistry, University of Chicago, Chicago, III.
Coordination chemistry C*£) homogeneous \S catalysis mong the most significant developments in the field of catalysis in recent years have been the discovery and elucidation of a variety of new, and often unusual, catalytic reactions of transition metal ions and coordination compounds. Examples of such reactions are: • The hydrogénation of olefins catalyzed by complexes of ruthenium, rhodium, cobalt, platinum, and other metals. • The hydroformylation of olefins catalyzed by complexes of cobalt or rhodium ( Oxo process ). • The dimerization of ethylene and polymerization of dienes catalyzed by complexes of rhodium. • Double-bond migration in olefins catalyzed by complexes of rhodium, palladium, cobalt, platinum, and other metals. • The oxidation of olefins to aldehydes, ketones, and vinyl esters, catalyzed by palladium chloride (Wacker process ). • The hydration of acetylenes catalyzed by ruthenium chloride. This article deals with the mechanisms of some of these reactions and with some of the general principles that underlie this relatively new and rapidly developing field of chemistry. This subject has attracted much interest in
A
recent years both because of the novelty of much of the chemistry revealed by it and because of its potential practical applications, exemplified by the Oxo and Wacker processes which have already achieved considerable industrial importance. The possible relevance of many of the catalytic reactions in this field as model systems for related heterogeneous and enzymic processes also lends interest to the subject, although attempts to exploit this theme have thus far met with only limited success. Transition metal
complexes
The majority of catalytic reactions discussed in this article involve as catalysts coordination compounds of the metals near the end of each transition series, notably the platinum groups, comprising Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, and Pt. The electron configurations of the metals in these groups are generally in the range dQ to d10, with the configuration d8 especially widely represented. Furthermore, the catalytic complexes in these groups are generally (although not invariably) of the spin-paired or low-spin type, that is, complexes in which the ligand field splittings are sufficiently large so that the d electrons first fill up (with
Some homogeneous reactions of olefins catalyzed by transition metal compSexes Reaction
Hydrogénation Hydroformylation
T y p i c a l c a t a l y s t s ( L = PPh 3 ) de
tf
Ru"
Co" 4
Active form of catalyst uncertain.
68
C&EN OCT. 31, 1966
3
RuCle " Co(CN) 5 "RuCl.oU
Double-bond migration Dimerization Oxidation a
d* Fe»
Fe(CO)5 FeH(CO)r
Coi
Rh1
Iri
CoH(CO), CoH(CO)4 CoH(CO)4
RhCIU RhCI(CO)U RhCI 3 (olefin) 2
lrl(CO)U
Pdn
ptH
"Pt(SnCI 3 )5 3 PdCU2-
RhCI 2 (C 2 H 4 ) 2 PdCL 2 "
a
Pt(SnCI 3 )5 3 -
C&EN feature
pairing, if necessary) the most stable orbitals available to them before occupying those of higher energy. First let us consider some aspects of the electronic structures and chemical reactivities of this general class of complexes, be cause of their relevance to an understanding of the catalytic properties in which we are interested. The stable coordination numbers of low-spin com plexes of transition metals range from eight to two, and exhibit a systematic inverse dependence on the number of d electrons of the metal atom. This dependence is shown in the following table: Coordination number
8 6 5
4 (Square planar) 4 (Tetrahedral) 2
Electron configuration
Examples
Mo(CN)83-, Mo(CN)84d\ d2 B M(CN)o -(M = Cr, Mn, d\ d\ Fe, Co) d7, ds Co(CN)r, Ni(CN)53~ d" Ni(CN)r Cu(CN)r, Ni (CO)* d10 io Ag(CN)o-, Au(CN) 2 -
d5, dG
Ox)
x-y*
JZL
•*>
MZEÎL/ co \
cv
\
(V")
Energy-level diagrams depict orbital splitting in complexes of various symmetries. Only the orbitals involved in accommodating the d electrons of the metal are depicted. Note that the octahedral configuration with only three stable (t2g) orbitals is most favorable for accommodation of six d electrons, whereas the two configurations of lower coordination number, each with four stable orbitals, are more favorable for the accommodation of seven or eight d electrons. The orbital symmetries are described by the symbols, xz, yz, x2-y2 . . ., as well as by the group theoretical designations a, e, t,g . . . .
d
This inverse dependence has its origin in the fact that, in general, the higher the coordination number, the fewer the d electrons that can be accommodated in stable (bond ing or nearly nonbonding) orbitals of the complex. In the case of an octahedral complex, for example, the three stable t2g orbitals (ηοη-σ-bonding or possibly slightly ττ-bonding in the case of π-acceptor ligands such as CO or CN~) can accommodate up to six d electrons. Any additional elec trons are forced to occupy the e* oribtals, which are
.[CoCCN)*4"]. d7
•Co(cN)53~ + Chr cC7
be understood in these terms. This process is analogous to that which accompanies the addition of an electron to a saturated carbon compound CX 4 , for example, CC1 4 , represented by the following simplified molecular orbital diagram :
/OOChr\ Ί—O^.' \
h
The loss of a CN~ ligand when an electron is added to the very stable de complex, Co(CN) G 3 ~, to give the pentacoordinated low-spin cobalt (II) complex, C o ( C N ) 5 3 - , can
\\
-O-c
*
^-Θ®®CH 3 1-^ Λ Ν ) 5 Ο Η / - +
»T£,
c^CciOgi*
IrrlCco)(PPh3")2 + CH 3 I-^Ir : m : lr (CH3Xco)(PPh3)2. In principle it should also be possible to accomplish the reductive cleavage of C—H and C—C bonds by similar OCT. 31, 1966 C&EN
71
(Halpern, Har rod, and James, 1966) in terms of a mecha11 nism in which the heterolytic splitting of H 2 by an Ru olefin complex is the rate-determining step as shown in the scheme at the bottom of the page. This mechanism has many features in common with that proposed by Burwell (C&EN, Aug. 22, page 66) for the heterogeneous catalytic hydrogénation on a chromia gel catalyst, shown in the following scheme
mechanisms. However, this has not yet been realized except in a few special cases such as the tautomeric equilibrium described by Chart and Davidson (1965). Rv?(c 10 H 8 )(PP) 2 ^rR^HC2-C, 0 H 7 )(PP) 2 [PP= (CH3)2 PCH2CH2PCCH3)J The reactive nature of the hydride complexes formed in many of the reactions already discussed permits these complexes to function as intermediates in homogeneous catalytic hydrogénation reactions. Thus, the reversible reaction, Ru£L 3- + H,
:RuHCl53~
H WA
Cr· Cr·
+
+ H + C1"
Q + C2H,.
The homogeneous catalytic hydrogénation of an olefin by a mechanism involving the homolytic splitting of hydrogen can be illustrated by the Co(CN) 5 8 --catalyzed hydrogénation of butadiene in aqueous solution. Detailed studies by Kwiatek and coworkers suggest that this reaction proceeds by the following mechanism:
provides a mechanism for the homogeneous exchange of H 2 with D 2 0 and for the homogeneous oxidation of H 2 by FeCl 3 , which oxidizes RuHCl 5 3 rapidly, according to RuHCl53"> 2feC]3-^RuCI63""4-2FeCI2+ H++ 0Γ
2Co(CN)c3> H2
The reactivity of the hydride intermediate in such reac tions is generally high so that its formation is the ratedetermining step in the overall catalytic reaction.
HC*
(CN)53">CH2 = CHCH
:ZHCO(CN)53"
= CH2-^CH3CH=CHCHZCo (CH)5Z~
CH3CM=CH&H2Co(ChJ)53- , H C ° C a ^L CH3CHXCH = CHX+ lCo(Oi)^ o f ! Λ + CM"
Catalytic hydrogénation
of olefins
,CH X
/!
In favorable cases, homogeneous catalytic hydrogénation of organic substrates such as olefins may also be achieved by transfer of hydrogen from the hydrido-transition metal complex. The following examples, all discovered within the past few years, illustrate how this hydrogénation can be realized for each of the three mechanisms of splitting of hydrogen (heterolytic, homolytic, and insertion) previously described. The homogeneous catalysis of the hydrogénation of fumaric acid in aqueous solution has been interpreted
A
\'
I A -β
B
C
Η"
I
\
The CN-dependent equilibrium between the σ- and π allyl intermediates accounts for the observation that hy drogénation at high CN- concentrations yields predominantly 1-butene and at low CN- concentrations predominantly frans-2-butene. The homogeneous catalysis of the hydrogénation of
'
^;c
.
fi
I
*C
'
β
Α
^ CH3CH=CHCH3 + 20>(αι)53~
/
>B
Α
^
NN
ι "V*
I Α
3. riCbCcN) W # 5
—Co(CNV" ^
Η s
β
-
,c'
ι
Α
Α β
Β
I u
A Ν
I
72 C&EN OCT. 31, 1966
A
S
B
μ
I
u
A
ethylene and other olefins by RhCl(PPh 3 ) 3 , recently dis covered by Wilkinson and coworkers, probably involves a dihydride intermediate. A plausible mechanism for this reaction, involving steps of the type already described, is shown in the following scheme:
U—β / R H
1
^
y-|— 7 CH 1 CH 3
H++L
+iCHfCH2
CH^C^CH, Cl 7CH2CH3
Lz
Ah1/
+ c2H6
'cl
U κ *PPh3 «Λ, ^(^Λ^Μ^Γ
Isomerization
of olefins
Complexes of many transition metals, including cobalt, rhodium, iridium, iron, nickel, palladium, and platinum, catalyze double-bond migration in terminal olefins. Evi dence for a mechanism of the following type, which is probably also applicable to some of the other catalysts, has been obtained by Cramer for the rhodium chloridecatalvzed reaction: CH22R
L
/—7 L
ι
CH2R
CH
CH
v—7II
i
•
A—
cri,
Γ L
+ Ur~L \ L~C\~
/CH.R
(Λ,ΑχΑ»**&
Rhodium chloride also catalyzes many other olefin-toolefin addition reactions including the addition of ethylene to dienes, the dimerization of 1,3-dienes to linear trienes, and the highly stereospecific polymerization of dienes (Rinehart and coworkers, 1961; Canale and coworkers, 1962). The mechanisms of these reactions remain to be elucidated. Hydroformylation
of olefins
The addition of H 2 and CO to olefins to form aldehydes, that is "RCH^CHj, + \\t + CO -=*- RCH2CHZCriO
in the presence of H C o ( C O ) 4 as catalyst, constitutes a reaction of very great scientific and practical interest. The results of many investigations of this reaction, includ ing the observation of inhibition by CO, are plausibly in terpreted in terms of the following mechanism suggested by Heck and Breslow: HC*1 (oo) 4 - ^ ^ H C o 1 ^ + CO
ι
CHR
v—7 LZ
L/L
L
L RCH=CHCH3
v—711 LZ
CHR C
y-*—HI
/£. i H
^
L/
i
RCH = CH2
HcJfco),
* Ώ
• /L c
-RcM^H^CccOj
Μ-Οοτ(οο\
CH3
An alternative mechanism of olefin isomerization, in volving rearrangement through an intermediate ττ-allyl hy dride, has also been proposed and may operate in the case of some of the other catalysts, as shown in the following scheme:
RCHZ-CH=CHZ:
RCH
γ
CH2
"RcH^CH-CH*
•ι
M H
Dimerization
and polymerization
of olefins
Another olefin reaction catalyzed by rhodium chloride is the dimerization of ethylene to 1-butene (Alderson, Jenner, and Lindsay, 1965). A detailed study of this re action (Cramer, 1965) suggests the following mechanism which is closely related to the earlier mechanism of olefin isomerization:
RCH^Co
1
^ — ^ . £ C M 2 C H 2 (J
CC0) 4 -^RCH 2 CH 2 CCb X (co) 3 + CO J l - C O R
+CHjCHO
N
Co' «
IS CI,
ci/
- ^ • P d ° + H + + 3Cr
,0!
/*
^
7CI C*Ci2
c\£
/ci
In the presence of oxygen and copper (II) chloride the reduction of palladium (II) is prevented. Thus, a homo geneous catalytic cycle is established in which PdCl 4 2 ~ and CuClo serve as catalysts for the following reaction: CH1=CH1+Xo2 —=2=» CH3CH0 The corresponding oxidations of substituted olefins yield ketones, whereas oxidation in acetic acid medium yields vinyl acetates. General
observations
Many of the areas of chemistry touched on in this article are of comparatively recent origin. Indeed, nearly all of the reactions described have been discovered and eluci dated within the past decade. The selection of examples has been conditioned to some degree by my own interests as well as by emphasis on those reactions whose mecha nisms are best, although not always fully, understood. 74 C&EN OCT. 31, 1966
discussed in
• Xucleophilic catalysis of decarboxylation and hydroly sis reactions resulting from the activation of the (co ordinated) reactant by the positive charge of the metal ion. • The extensive array of cyclooligomerization, poly merization, and carbonylation reactions of acetylenes and olefins catalyzed by nickel carbonyl and its derivatives (Reppe chemistry). • The cyclotrimerization of butadiene and related reac tions catalyzed by ττ-complexes of nickel, recently dis covered by Wilke. • The polymerization of olefins by soluble catalysts of the Ziegler-Natta type. • The catalytic hydration of acetylenes. The mechanisms of many of these processes are still incom pletely understood. Several factors emerge as contributing to the great catalytic versatility of transition metal complexes in these and related reactions. Among these are:
CH,
CU
are illustra coordination its scope or
• The existence of relatively stable but highly reactive complexes of transition metals which, by virtue of their electron configurations and coordination numbers, exhibit reactivities closely related to those of the reactive inter mediates of organic chemistry namely, free radicals, carbenes, and carbanions. As noted previously, the promi nence of d" and, particularly, d s complexes in homogeneous catalysis is related to this theme. • The ability of transition metals to stabilize a variety of otherwise unstable reaction intermediates through co ordination as ligands in relatively stable but reactive com plexes. Among these are σ-bonded ligands such as hydride and alkyl groups and ττ-bonded species such as allyl, cyclobutadiene, and the like. Reaction mechanisms involving such intermediates, which are prohibitively endothermic in the absence of catalysts, are thus rendered feasible. The ability of certain complexes to dissociate molecular hydro gen and other stable molecules reflects this property. • The ability of a transition metal atom to assemble or orient within the framework of its coordination shell several components of a reaction (template effect). This factor is undoubtedly important in such reactions as the cyclo oligomerization of acetylenes or dienes and in the metalion-promoted syntheses of porphyrins and, as shown by Busch, other macrocyclic ligands. • The ability of certain transition metal complexes to facilitate rearrangements within their coordination shells by virtue of the existence of two or more stable configura-
tions of the complex differing in coordination or oxidation number. An example of this effect, which is probably of widespread importance in catalytic reactions, is the ex istence of two stable configurations, corresponding to the coordination numbers 4 and 5, for d8 complexes. This effect facilitates such rearrangements as the proposed step in the hydroformylation reaction: ο
CH^c Cco)4 ^ = cHj.cc· Cco) 3 Related insertion rearrangements probably play an impor tant role in many other catalytic addition reactions. Future
trends
Homogeneous catalysis by coordination compounds con tinues to be an active and fruitful field of research. Cur rently important lines of research are directed particularly to three areas: • The search for and discovery of new catalytic reac tions. • The more detailed elucidation of the mechanisms of the many reactions which are, as yet, incompletely under stood. • The discovery and characterization of new coordina tion compounds, often containing novel ligands, which are of interest as potential catalysts or catalytic intermediates or whose study might contribute to a better understanding of related catalysts or catalytic intermediates. Recent progress in all three of these areas has been im pressive, and there is every indication that the present intensive pace of research and discovery will continue for some years. Many important problems, both of understanding and of application, remain to be solved. Among these is the attainment of such objectives as the homogeneous catalytic activation of saturated hydrocarbons and of molecular nitrogen, which still await major breakthroughs. The re cent success of Volpin and Shur in fixing nitrogen with transition metal catalysts of the Ziegler-Natta type, as well as the recent discovery of two transition metal complexes containing coordinated nitrogen, that is R u ( N H 3 ) 5 N 2 2 + (Allen and Senoff, 1965) and IrCl(N 2 ) ( P P h 3 ) 2 (Collman and Kang, 1966), reinforce the hope that attainment of at least one of these objectives may be close. There is also reason to hope and anticipate, although the complexity and difficulty of this goal should not be under estimated, that the detailed understanding that is emerging of catalytic mechanisms in these relatively simple homo geneous systems will ultimately make a meaningful con tribution to our understanding of related catalytic phe nomena in heterogeneous and enzymic systems. In spe cific instances this has already occurred; one example is the insight which has been gained into certain aspects of Vitamin B 1 2 chemistry through studies on related cobalt complexes, such as the carbonyl (Heck), cyanide (Kwiatek, Halpern), and dimethylglyoxime (Schrauzer) com plexes.
SUGGESTIONS FOR ADDITIONAL READING 1. Gray, H. B., "Electrons and Chemical Bonding," W. A. Ben jamin, Inc., New York, 1964. 2. Halpern, J., "Catalysis by Coordination Ann. Rev. Phys. Chem., 16, 103 (1965).
Compounds,"
3. Heck, R. F., "Insertion Reactions of Metal Complexes," p. 181, in "Mechanisms of Inorganic Reactions," Ad vances in Chemistry Series No. 49, American Chemical Society, Washington, D.C., 1965. 4. Jones, Mark M., "Elementary Coordination Chemistry," Prentice-Hall, Inc., Englewood Cliffs, N.J., 1964. 5. Nyholm, R. S., "Structure and Reactivity of Transition Metal Complexes," Proceedings of the Third International Congress on Catalysis, p. 25, North-Holland Publishing Co., Amsterdam, 1965. 6. "Reactions of Coordinated Ligands and Homogeneous Catalysis" (Symposium) Advances in Chemistry Series No. 37, American Chemical Society, Washington, D.C., 1963. 7. Schrauzer, G. N., "Coordination Chemistry and Catalysis: Investigation on the Synthesis of Cyclooctatetraene by the method of W. Reppe," Angew. Chem. Int. Ed., 2, 105 (1963). 8. Schrauzer, G. N., "Some Advances in the Organometallic Chemistry of Nickel," p. 2, in "Advances in Organometal lic Chemistry," Stone, F. G. A. and West, R., eds., Aca demic Press, New York, 1965.
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