Carbene transfer reactions between transition-metal ions
Shiuh-Tzung Liu and K. Rajender Reddy
Department of Chemistry, National Taiwan University, Taipei, Taiwan 106, Republic of China Received 4th March 1999
Carbene transfer processes are often involved in catalytic or metal-mediated reactions, however, a limited number of works concerning such transfer between transition metal complexes appear in the literature. This article presents an overview of the progress of this chemistry with an emphasis on examples with well-defined structures. A few studies, which are discussed in some detail, provide an insight of what has been achieved as well as an indication of the future work.
1 Introduction
Since the first discovery of stable transition metal carbene
complexes by Fischer and Maasb¨ol,1 metal carbenes have
become an important area of organometallic chemistry. A number of synthetic approaches for the preparation of carbene complexes have been reported in the last three decades, conversion of carbonyls or isocyanide groups into carbene ligands and cleavage of electron rich alkenes to give metal carbenes amongst them.2,3However, there are a few instances
where carbene complexes are prepared from other metal carbene species by a simple transfer reaction of the carbene ligand itself.4–21
The use of carbenes in various synthetic methodologies, particularly metal mediated reactions, is frequently employed in their transformation into useful organic products.22, 23
Forma-tion of metal carbene intermediates has been postulated and observed in many catalytic processes, such as olefin metathesis and the Fischer–Tropsch reactions. In fact, the
cyclopropana-tion of alkenes is one of the most widely studied reaccyclopropana-tions and metal carbene species are known to be key intermediates in transferring the carbene moiety, but the source of the carbene is generally restricted to the organic carbenoid.24Simple transfer
of a carbene ligand from one metal to another metal centre is rare. The difficulty in transferring is presumably due to the stabilization of carbene ligands via the metal centre in these pre-synthesized carbene complexes. In spite of that there are some examples demonstrating carbene transfer between metal ions, in particular with diamino-substituted carbene complexes.14–16
This article seeks to summarize such transfer processes between metal ions and illustrates the interesting feature of this type of reaction in possible further applications.
2 Carbene transfer between simple metal ions
The first example of a carbene ligand transfer between metal ions was reported by Fischer and his coworkers.4Reaction of
[Mo(Cp)(CO){C(OMe)Ph}(NO)] (1a) with [Fe(CO)5] in
ben-zene solution under photochemical conditions affords the iron carbene complex [Fe(CO)4{C(OMe)Ph}] (2a). In this study, the
authors suggested that the carbene transfers might proceed via two possible pathways: one involving a free carbene as an intermediate which then transfers to iron, and the other by the
coordination of molecule 1a with the [Fe(CO)4] fragment
followed by migration of carbene from molybdenum to iron. Later studies demonstrated that arylalkoxy- and aryldialkyl-amino-carbenes (1b and 1c) also can be transferred from the
corresponding molybdenum carbene complexes [eqn. (1)].5
Shiuh-Tzung Liu was born in Taiwan in 1954. He studied chemistry at the National Taiwan University, where he received his BS degree in 1977. In the autumn of 1980 he came to the United States and entered the graduate school of the University of Texas at Austin. He worked for his PhD with Professor Evan P. Kyba on phosphorus macrocycles (1980–1985). Af-ter spending one year as a postdoctoral fellow at Texas, he became an Associated Profes-sor at the National Taiwan University and is now a Pro-fessor of organic chemistry. His current research interests focus on the chemistry of metal car-benes and the development of new ligands for catalytic reac-tions particularly in polymeri-zation.
K. Rajender Reddy obtained his MSc (1988) and PhD (1994) from the University of Hyderabad (India) under the supervision of Dr M. V. Rajasekharan, working on higher valent manganese complexes. After a one year stay at the Nuclear Technology Institute (ITN) (Portugal) as a postdoctoral fellow, he moved to National Taiwan Uni-versity, where he is currently carrying out post-doctoral research with Professor Liu.
Cp(NO)(CO)Mo Ph R (CO)4Fe Ph R hν (1) 1a R = OMe 1b R = OEt 1c R = NMe2 + [Mo(Cp)(NO)(CO)2] 2a-c [Fe(CO)5] M(CO)5 M(CO)4 Me N N Me Me N N Me or ∆ hν (2) cis-3a M = Cr 3b M = Mo 3c M = W 2 4a-c Cr(CO)4L Me N N Me L = PPh3, C6H11NC 5
Cr(CO)5 Cr(CO)4 Cr(CO)4
O O O O O 2 cis-6 7 (CO)5W Ph X (CO)4W Ph X Cl Au Cl Cl (CO)4W Ph X Cl Au Cl Cl (CO)4W Au Cl X Ph Cl Cl 9a-e [HAuCl4] + cis-[WCl2(CO)4] – HCl, – CO 8a X = OMe 8b X = NH2 8c X = (E)-NHMe 8d X = (Z)-NHMe 8e X = NMe2 ClAu Ph X
Under irradiation conditions, pentacarbonyl(1,3-dimethyl-4-imidazolin-2-ylidene)chromium (3a) yields a biscarbene
chromium complex 4a in boiling THF [eqn. (2)].6 It is also
found that the analogous molybdenum and tungsten complexes (4b and 4c) can be prepared by thermal reaction from the corresponding carbene complexes 3b and 3c respectively.7Here
carbene transfer between the metal atoms is expected to proceed through the initial formation of a tetracarbonyl metal carbene intermediate, which subsequently reacts with 3 to generate cis-biscarbene complexes 4. Formation of ligand substituted products 5 upon treating 3a with P(C6H5)3 or C6H11NC
indicates that the unsaturated metal intermediate can be trapped in the presence of a donor ligand. Of course, generation of a free carbene as the intermediate is excluded in this investigation.
The dissociation of a carbene ligand from the metal centre is proposed to occur through a free carbene intermediate in certain cases based on the formation of alkene products. In the
thermolysis of [Cr(CO)5{C(OMe)R}] (R = Ph, Me), the
authors suggested that the formation of alkene product (MeO)RCNCR(OMe) (R = Ph, Me) was due to the dimerization of the dissociated carbene species.25However, evidence against
such an argument has been demonstrated from an authentic preparation of 2-oxacyclopentylidene, which was found to yield dihydrofuran and cyclobutanone as the major products upon
thermolysis instead of dimerization.8 The absence of the
formation of cyclobutanone in the thermolysis of the chromium carbene complex 6 rules out intervention of the free carbene in this reaction. Based on a simple kinetic investigation the reaction pathway was proposed as in Scheme 1. This shows that
the alkene product comes from the elimination of the biscarbene chromium complex 7, which is presumably formed via a carbene transfer reaction. In addition, compound 6 undergoes
simple carbene transfer upon reaction with [W(CO)6]
generat-ing the correspondgenerat-ing complex (2-oxacyclopentylidene)-W(CO)5, which supports the possibility of carbene transfer
between the metals.
In addition, Lappert showed that the electron rich alkenes themselves could be the intermediates in such types of
reaction.3 Thermolysis of [Mo(CO)
5(LMe)] giving
cis-[Mo-(CO)4(LMe)2] {LMe = 1,3-dimethylimidazolidinylidene} which
was prepared under the same conditions using the
correspond-ing electron rich alkenes (enetetraamines) and Mo(CO)6,
indicating the possibility of formation of alkenes via the heteroatom-stabilized metal carbene complexes.
Using a carbene ligand transfer reaction, gold(i) carbenes of the type [AuCl{C(R)Ph}] (9a–d) were prepared by reaction of the corresponding tungsten carbene complexes [W(CO)5
{C(R)-Ph}] (8a–d) with HAuCl4(Scheme 2).9Transfer of the carbene
in these redox reactions is found to proceed at 0 °C and the resulting complexes are stable at room temperature. However, upon heating up to 200 °C the carbene ligand dimerizes with the formation of metal chlorides. Dimerization of the carbene moiety at 200 °C after carbene transfer from tungsten to gold indicates that the stability (kinetic or thermodynamic) of the resulting carbene complex dictates the carbene cleavage process. A possible reaction pathway (Scheme 2) for the formation of gold(i) carbene complexes involves the coordina-tion of the gold centre to form a chloride-bridged dinuclear species, which allows the carbene ligand to shift from tungsten to gold and a cis-arrangement around the tungsten ought to be the thermodynamic result. It is noticed that the oxidation state of gold is reduced from iii to i during the transfer reaction.
In the study of these gold carbene complexes, the authors found that the substituent on the carbene ligand affected the transfer reaction. In the case of dimethylamino substituted carbene complex 8e the reaction proceeds similarly, but yields
a mixture of [AuCl{C(NMe2)Ph}] (9e) and [AuCl3
{C(N-Me2)Ph}] (10), in which compound 10 retains the oxidation
state iii unlike the previous reaction shown in Scheme 2.10
However, the reaction of [W(CO)5{C(NMe2)Ph}] with
[HAuBr4] yields only [AuBr{C(NMe2)Ph}], which upon
oxida-tion by Br2 yields the corresponding gold(iii) complex. The
above examples show that the simple carbene transfer from Group 6 metal complexes to the gold centre is accompanied by a redox reaction at tungsten. Similar transfer forming a binuclear gold(i) carbene complex [Au2Cl2{CPhNH(CH2)n -NHCPh}] (n = 2, 6) is accomplished by treatment of the tungsten complexes [(W(CO)5)2{C(Ph)NH(CH2)nNHC(Ph)}] with 2 equiv. of [HAuCl4].11Concerning the redox chemistry of
the carbene transfer in gold complexes, there is no further investigation.
The first example of a carbene transfer in which the carbene is not stabilized by a heteroatom substituent was reported in the preparation of [Mn(Cp)(CO)2{CPh2}] (11) (Cp = h5-C5H5).12
Thermolysis of [W(CO)5{CPh2}] in the presence of
[Mn(Cp)(CO)2(THF)] results in the formation of the manganese
Scheme 1 Mechanism for the formation of the alkene from the metal carbene.
Scheme 2 Reaction pathway of carbene transfer between tungsten and gold.
TaCl3L2 t-Bu H TaCl3L t-Bu H [W(O)(OBu-t)4] L(t-BuO)Cl3Ta W(O)(OBu-t)3 H t-Bu W(O)(OBu-t)2 t-Bu H (t-BuO)Cl3Ta W(O)(OBu-t)3 H t-Bu W Cl Cl O PR3 PR3 Bu-t H – L [Ta(OBu-t)2Cl3] 12a L =PR3 12b L = THF 12c L = Cl – – L [Ta(OBu-t)4Cl] 13 + if L = PR3 14 N R R N M(CO)5 N N W(CO)4 N R R N W OC CO CO PPh3 CO Et Allyl Benzyl Pent-4-enyl Et Allyl Et Allyl Benzyl 19a R = Et 19b R = benzyl 18 N R R N W OC CO Ph2P PPh2 CO 20a R = Et 20b R = benzyl 15a 15b 15c 15d 16a 16b 17a 17b 17c M = W, M = W, M = W, M = W, M = Mo, M = Mo, M = Cr, M = Cr, M = Cr, M = W, M = W, M = W, M = W, M = Mo, M = Mo, M = Cr, M = Cr, M = Cr, R = R = R = R = R = R = R = R = R = Cl Pd Cl Pd Cl Cl N Et Et N Et N N Et N R R N Pd R N N R Cl Cl N R R N Pd NR RN Cl Cl 21b R = allyl 21c R = benzyl 21d R = pent-4-enyl 15 21a R = Et N R R N Pt Cl Cl CO N R R N Pt NR RN Cl Cl N N Pt Cl Cl 15 22a R = Et 22c R = benzyl 22d R = pent-4-enyl R = allyl 3 3 23 a-d 24 ∆ [PtCl2(PhCN)2] [PtCl2(PhCN)2]
carbene complex 11, which shows diphenylcarbene transfer from tungsten to manganese. In addition, a number of tungsten oxoneopentylidene complexes were readily obtained by alkyli-dene transfer from tantalum to tungsten.13Thus, treatment of
the tantalum complexes [Ta(NCHCMe3)Cl3L2] (12a) (L =
PMe3, PEt3, PMe2Ph) with [W(O)(OCMe3)4] in pentane
resulted in the formation of the corresponding tungsten oxoneopentylidene complexes [W(O)(CHCMe3)Cl2L2] (14). A
crystallographic study of [W(O)(CHCMe3)Cl2(PMe3)2]
showed that these species are approximately octahedral with the arrangement of trans-phosphine and cis-halide ligands. Scheme 3 illustrates the possible pathway of the carbene transfer from
tantalum to tungsten. It appears that a phosphine ligand dissociates from tantalum to generate a penta-coordinated intermediate which allows a tert-butoxide ligand to bridge between tungsten and tantalum. Subsequent ligand transfer
generates a m-neopentylidiene species 13, which permits
carbene ligand transfer to tungsten. When the starting tantalum complexes have the coordinating phosphine, ligand replace-ment continues until all the tert-butoxide ligands are transferred from tungsten to tantalum exchanging neopentylidene and tert-phosphine ligands. In the absence of tert-tert-phosphines the reaction stopped after the transfer of the second tert-butoxide, yielding [Ta(OCMe3)2Cl3] and [W(O)(CHCMe3)(OCMe3)2] as
shown in Scheme 3.
For the last few years, research concerning diaminocarbene ligands has received much attention.26This type of carbene is a
strong s-donor towards various metal ions and is found to be
readily transferred between metal centres.14–16In addition, it is
observed that the ligands surrounding the metal and N-substitution in complexes 15–20 affect the carbene transfer dramatically in both the reaction rate and structural change.15
As shown in Scheme 4, complex 15a reacts with [PdCl2(PhCN)2] to yield a chloro-bridged dipalladium carbene
complex 21a, whereas complexes 15b–d give the biscarbene
palladium species 21b–d in high yields. 13C NMR clearly
indicates the formation of Pd–carbene compounds, shifts of carbene carbon appeared around 198–199 ppm, which are shifted upfield compared to the analogous Group 6 metal complexes (in the range 207–209 ppm). The trans-biscarbene palladium complexes then isomerize to the thermodynamically more stable cis-form.
Unlike palladium complexes, both carbene and carbonyl ligands are transferred from tungsten to platinum in several
cases (Scheme 5). Reactions of 15a, 15c and 15d with [PtCl2(PhCN)2] in dichloromethane solution result in the
formation of 22a, 22c and 22d respectively. The infrared
carbonyl absorptions around 2100–2110 cm21are essentially
identical with those reported in cis-[PtCl2(CO)(R3P)].27Such
spectral data imply the donating properties of diaminocarbene are similar to that of trialkylphosphines. Treatment of these
carbonyl–carbene complexes 22a, 22c and 22d with Me3NO
provides the corresponding biscarbene complexes 23a, 23c and
23d, respectively. Apparently, the formation of biscarbene
complexes proceeds via a ligand re-distribution reaction, i.e., carbene transfer takes place between platinum metals to provide the thermodynamically more stable biscarbene complexes. The Scheme 3 A possible mechanism for neopentylidene ligand transfer.
Scheme 4 Formation of palladium carbene complexes via the reaction of 15a–d with [PdCl2(PhCN)2].
Scheme 5 Preparation and reactions of platinum carbonyl–carbene complexes.
N R R N Rh R N N R Cl N R R N Au R N N R [Rh(CO)2Cl]2 [AuCl(Me2S)] 15 N N Rh N N Cl – Cl – 25a R = allyl 25c R = benzyl 25d R = 4-pentenyl 26 27a R = Et 27b R = allyl 27c R = benzyl 27d R = pent-4-enyl [Rh(CO)2Cl]2 R = allyl CO
N-allyl substituted biscarbene complex 23b was obtained directly from the reaction of 15b with [PtCl2(PhCN)2] in
refluxing chloroform solution. It is evident that the p-bond (h2
-bond) via the N-allyl substituent assists the carbene transfer, but inhibits the transfer of the carbonyl ligand. However, the chain length of the N-substituent influences the nature of the reaction. Complex 15d undergoes both carbonyl and carbene transfer to form 22d followed by thermally induced intramolecular substitution to yield an alkene substituted complex 24. The chemical shifts of the carbene carbon (166–174 ppm) for mono and biscarbene platinum complexes appear to be much more upfield than the corresponding palladium–carbene complexes, which is a trend in these metal carbene complexes. The 13C
chemical shifts of the carbene carbon have become a convenient tool to investigate these metal carbene species.
Similar to Pt(ii) and Pd(ii) complexes, carbene ligand transfer reactions are also observed with rhodium(i) and gold(i) metal
ions. Reactions of 15a, 15c, 15d with [RhCl(CO)2]2 gave
biscarbene rhodium complexes 25a, 25c and 25d respectively (Scheme 6). In the case of the N-allyl substituted carbene 15b, the p-coordinated product 26 was obtained directly under the
same conditions. The 13C NMR chemical shifts for the carbene
carbon appeared around 200–215 ppm with a rhodium–carbon coupling JRh-C ~ 35 Hz. As for the gold biscarbene complexes
27a–d, which are similar to those prepared by the ligand
substitution reactions of LAuCl (L = PPh3, Me2S) with
C-imidazolyllithium,28was formed by the reaction of 15a–15d
with [AuCl(Me2S)] in dichloromethane at room temperature.
13C NMR resonances for the carbene carbon appear in the range
203–205 ppm which is comparable to other related gold carbene complexes.
Further studies verify the diaminocarbene transfer from tungsten to copper and silver complexes. Reactions of 15 with [Cu(MeCN)4][BF4] or [AgBF4] in dichloromethane or
chloro-form yield the corresponding copper(i) and silver(i) complexes (28 and 29). However, the resulting complexes are sensitive to oxygen and moisture and readily decompose to yield the N,N-dialkylimidazolidin-2-ylidinium salts, i.e., cleavage of the metal–carbene bond followed by protonation. Nevertheless, these copper and silver carbene complexes generated in situ in
CDCl3 have been characterized by 1H NMR, 13C NMR and
mass spectroscopy under anhydrous conditions. 13C NMR
shows signals around 197–199 ppm for the corresponding Cu– C(carbene) carbon and 202–204 ppm for the Ag–C(carbene) with the isotope coupling values ranging from 168–170 Hz for
107Ag–C and 192–196 Hz for 109Ag–C.
In fact, reactions of Group 6 diaminocarbene complexes with [Cu(MeCN)4][BF4] or [AgBF4] in the presence of water provide
the corresponding imidazolidin-2-ylidinium salt directly. Scheme 7 shows the possible reaction pathway for the cleavage of the metal–carbene bond from the tungsten carbene com-plexes 15. The diaminocarbene ligand is initially transferred to the copper or silver metal and then dissociates and behaves as a
strong base, which is protonated to yield the iminium salt. It has been demonstrated by theoretical calculations that the simple diaminocarbenes are strong bases, even stronger than the N,N-disubstituted imidazolylidenes (an aromatic heterocyclic car-bene).29Formation of C2 deuterated product in acetonitrile-d
3
or a mixture of THF and D2O indicates that the proton comes
from the solvents. Furthermore, this cleavage reaction is accelerated under acidic conditions. Reaction of 15a with Cu(i) is complete within a few minutes in 10% HBF4solution. Even
a catalytic amount of copper ions (10% mol) can accomplish the same reaction in acid media. Such a cleavage process is also observed in the reactions of the biscarbene palladium and
platinum complexes 21 and 23 with [AgBF4] or
[Cu-(MeCN)4][BF4] in the presence of water. These results show the
possibility of diaminocarbene transfer among the late transition metal ions.
The metal ions, molybdenum- and chromium-diamino-carbene complexes (19 and 20) follow the same general trend as tungsten. In terms of substituent effect, the p-bond functionality
on the nitrogen atom accelerates the carbene transfer and influences the structure of the products as shown in Schemes 4–6. In the case of palladium, the rate of formation of the biscarbenes 21b–d is faster than that of the chloro-bridged carbene complex 21a. Ligands on the Group 6 carbene complexes also influence the nature of carbene transfer. The
p-coordinated tungsten carbene complex 18 does not undergo carbene transfer with Pd and Pt, whereas the triphenylphosphine substituted tungsten carbenes 19a–b readily undergo carbene transfer with [PtCl2(PhCN)2] and result in the formation of
platinum carbene phosphine complex [eqn. (3)]. Bisphosphine ligand transfer was observed upon the reaction of 20a–b with [MCl2(PhCN)2] (M = Pd or Pt) as shown in eqn. (4). Here, the
carbene moiety is believed to transfer to the platinum initially, however in the presence of a bidentate phosphine ligand carbene–metal bond cleavage occurs. These reactions suggest that diaminocarbenes are as good s-donors as phosphines. Such
phosphine ligand transfer from tungsten to palladium was also observed in the tungsten–neopentylidine complexes. Reaction
of [W(O)(NCHMe3)Cl2(PMe3)2] with 0.5 or 1 equiv. of
[PdCl2(PhCN)2] generates [PdCl2(PhCN)(PMe3)] and
[PdCl2(PMe3)2] demonstrating phosphine transfer from W to
Pd. However, no carbene transfer was observed in the latter example.13
Ligands around the metal centre are also important for the carbene transfer reactions. It is found that [PdCl2(CH3CN)2],
[PdCl2(Ph3P)2], [PdCl2(COD)] or [PdCl(Me)(COD)] do not
react with the tungsten diaminocarbene complexes 15. On the other hand chromium and tungsten complexes of 1,3-thiazolin-2-ylidene (30) are found to undergo carbene transfer upon
treating with [PdCl2(COD)] to give the trans-biscarbene
palladium complexes 31 [eqn. (5)],17which indicates that the
coordinating ability of heteroatom-substituted carbene com-plexes affects the reactions.
N R R N M R N N R BF4–
28a M = Cu, R = ethyl 28b M = Cu, R = allyl 28c M = Cu, R = benzyl 29a M = Ag, R = ethyl 29b M = Ag, R = allyl 29c M = Ag, R = benzyl N R R N M(CO)5 N R R N M+ N R R N N R R N N R R N H N R R N MCl2 Cu(I) or Ag(I) dissociation 2 " H+ " M = Cr, Mo, W M = Pd, Pt Cu(I) or Ag(I) 2 N R R N W OC CO COPPh3 CO N R R N Pt PPh3 Cl Cl 19a R = Et 19b R = benzyl [PtCl2(PhCN)2] (3) N R R N W OC CO Ph2P PPh2 CO P Ph2 M Ph2 P Cl Cl N R R N H 20a R = Et 20b R = benzyl M = Pd, Pt + (4) [MCl2(PhCN)2] S H N EtOOC MeS M(CO)5 S H N EtOOC MeS PdCl2 M = Cr, W [PdCl2(COD)] 2 trans- (5) 31 30 N Et Et N H N Et Et N Ag N Et Et N N Et Et N Pd N Et Et N Cl Cl N Et Et N Au Br N Et Et N Ag N Et Et N Au [AgBr2] – X – Ag2O X = Br [PdCl2(MeCN)2] X = PF6 [AuCl(Me2S)] 32 33 35 34 36 2 2 [AuCl(Me2S)] [PF6] – [PF6] – Ag2O
All the above examples illustrate the ability of the diamino-carbene ligand to transfer among various metal complexes. Formation of diaminocarbene complexes can be represented as the adduct of a strong Lewis-base carbene and a Lewis-acid organometallic fragment. The diaminocarbene transfer between transition metals in these reactions is likely to be the thermodynamic product. The pathway of such transfer proc-esses may occur through the initial coordination of the nitrogen atom of the carbene moiety to the unsaturated metal centre followed by the carbene shift to form the product. However, the intermediates have not been isolated to verify the mechanism of these diaminocarbene transfer reactions. Qualitative studies by NMR spectroscopy show that the relative rate of the cleavage of the diaminocarbene ligand by the copper(i) ion was found to decrease in the order 15a > 21 ~ 22a > 23a. This may be due to the steric bulk of the phosphine which hinders the access of copper to the carbene ligand, indicating the possibility of weakening the coordination of the heteroatom toward copper to assist the transfer.
The diaminocarbene complexes of Pd(ii), Pt(ii), Rh(i), Au(i), Cu(i) and Ag(i) complexes resulting from transfer from Group 6 metal complexes do not undergo the reverse transfer reaction with [M(CO)6] (M = Cr, Mo, W), i.e. no carbene transfer from
Pd(ii), Pt(ii), Rh(i), Au(i), Cu(i) and Ag(i) to W(0) is observed. In contrast to this trend, a carbene transfer from silver to gold and palladium metal ions is reported with 1,3-dimethylbenzimi-dazol-2-ylidene (Et2-Bimy) ligand, a heterocyclic aromatic
carbene.16Treatment of silver complexes 32 and 33 with
[AuCl(SMe2)] in a 1+1 molar ratio results in the formation of
the mono and biscarbene gold complexes [AuBr(Et2-Bimy)]
(34) and [Au(Et2-Bimy)2][PF6] (35) respectively (Scheme 8).
However, compounds 34 and 35 could not be obtained directly from the carbene precursor and the metal complex under basic phase transfer catalytic conditions. trans-[PdCl2(Et2-Bimy)2]
(36) was also obtained in high yield by the reaction of
[PdCl2(MeCN)2] with 32 in 1+1 molar ratio in
dichloro-methane.
Formation of carbene transfer intermediates is also proposed in certain catalytic reactions. Recently Sierra and co-workers found that palladium acetate catalyzes the carbene ligands of chromium(0) complex to undergo self-dimerization and C–H insertions under mild conditions.31In this work, they found that
addition of catalytic amounts of [Pd(OAc)2] to a THF solution
of chromium carbene complexes 37 and 38 and triethylamine resulted in the formation of the alkene products 40–41 as shown Scheme 7 Reaction pathway of MNC cleavage by Cu(i) and Ag(i).
Cr(CO)5 MeO Ar Ar OMe Ar MeO Et3N, RT (6) [Pd(OAc)2] 37 40 41 Cr(CO)5 ArCH2O Me [Pd(OAc)2] ArCH2O (7) Et3N, RT 38 Cr(CO)5 Me2N Ar Y O Ar 42 [Pd(OAc)2] Et3N, ∆ H2O (8) 39 Y Y = COOMe, CN [Pd] X R 43 O O Cr(CO)5 Cr(CO)5 O O (OC)5Cr [Pd(OAc)2] Et3N, RT (9) 44
in eqns. (6) and (7). For aminocarbenes 39 the transfer reaction to palladium is accelerated in the presence of acrylate or acrylonitrile and the carbene moiety is then incorporated into these unsaturated substrates to yield the addition product 42 [eqn. (8)]. In all these reactions, the palladium carbene complex
43 via the transfer process from tungsten is believed to be the
key intermediate for the catalytic cycles. A synthetic approach leading to 44 is achieved by adopting this technique [eqn. (9)].
3 Carbene transfer in di- and poly-nuclear
complexes
3.1 Carbene transfer resulted in the formation of di- and poly-nuclear complexes
Although numerous di- and poly-nuclear carbene complexes appear in the literature, there are only a few instances where they are synthesized by a carbene transfer reaction from the mononuclear carbene complex. A trinuclear carbene species prepared by such an approach was achieved by Fischer and co-workers in the photochemical reaction of [MoCp(CO){C(O-Me)Ph}(NO)] with tetracarbonylnickel(0) in benzene solution, resulting an unstable trinuclear nickel carbene complex [Ni3(CO)3{m-C(OMe)Ph}3].4 It is clear that the cluster is
formed via a carbene transfer process from the molybdenum to the nickel followed by the construction of metal–metal bonds. Stone and co-workers discovered various carbene-bridged di-and tri-nuclear complexes which were formed by the reaction of [M(CO)5{C(OMe)Ph}] [M = Cr, or W] with [Pt(C2H4)2(PR3)]
{R = PBut
2Me or P(c-C6H11)} involving the carbene transfer
process.18Scheme 9 summarizes the study of these reactions.
Reaction of [Pt(C2H4)2(PR3)] with tungsten carbene 45
pro-vides a triangular WPt2 cluster 50 as the exclusive product,
whereas the chromium carbene 46 produces a CrPt2-trinuclear
complex (51) and a mixture of platinum di- and tri-nuclear species (52 and 53) under similar conditions. Rationalization of the formation of these metal–metal bonded species is shown in Scheme 9 and the bridging carbene species 47 generated via the coordination of the carbene to ‘Pt(C2H4)(PR3)’ ought to be the
key intermediate. Such species undergo either dissocation of
Ru Ru Ru O Ru Ru Ru Ru Ru Ru O = CO
‘M(CO)5 fragment’ to yield 49 (a carbene transfer product,
route a in Scheme 9) which leads to the formation of homonuclear platinum carbene complexes, or the rearrange-ment of the ligand to construct the heteronuclear species 48 (route b) which reacts with ‘Pt(C2H4)(PR3)’ to yield 50. From
the product distribution, it appears that the ease of carbene transfer depends on the metal, i.e. the chronium carbene is able to proceed through route a, but not the tungsten carbene.
Reaction of [Mn(Cp)(CO)2{C(OMe)Ph}] with
[Pt(C2H4)2(PR3)] produces a mixture of homonuclear platinum
carbene complexes [Pt3{m-C(OMe)Ph}3(PMe3)3] (54) and
[Pt3{m-C(OMe)Ph}2(m-CO)(PR3)3] (55), without the formation
of any compound containing bridging carbene across Pt–Mn bonds, indicating that the transfer of a carbene group from the manganese complex to platinum is easier than those from chromium and tungsten. On the other hand, the reaction of the
2-oxacyclopentylidene–manganese complex 56 with
[Pt(C2H4)2(PR3)] (R3 = But2Me) affords a mixture of
[MnPt(C4H6O)(CO)4(PR3)I] (57) and [{Pt(m-CO)(PPR3)}3]
(58). The carbene moiety in 57 is exclusively bonded to the platinum centre [eqn. (10)],19 which illustrates the expected
stronger interaction of the Pt–C bond than the Mn–C. The recent work on the Pauson–Khand type of reaction with the substrate of trimethylsilylalkynyl tungsten carbene 59 reported by Moreto and co-workers was the only example involving carbene transfer reaction.20 Treatment of tungsten
complex 59 or the p-coordinated form 60 with Co2(CO)8
provides a carbene transfer product 61 (Scheme 10).
Structur-ally, it shows that the carbene group shifts from tungsten to cobalt leading to the dicobalt complex 61. Unlike the chemistry of diaminocarbene transfer between tungsten and platinum, the
p-coordination at the tungsten metal centre has no effect on the
transfer.
In addition to the carbene transfer between mononuclear complexes as discussed in the previous section, bridging carbenes are also able to undergo a similar reaction in binuclear systems. Bergman and Theopold discovered that a m-CH2unit
of the dicobalt complex 62 is transferred to form a new bridging dinuclear carbene complex in the reaction of 62 with [Rh(Cp)(CO)2] [eqn. (11)].21It is also found that an equimolar
mixture of 62 and its MeCp analogue 65 in benzene at 63 °C produces a statistical ratio of 62, 65 and the mixed dimer 66 [eqn. (12)], indicating the equilibration between these species.
This observation suggests a reversible transfer of the carbene ligand between complexes.
3.2 Carbene migration in di- and poly-nuclear complexes
Migration of the carbene ligand in di- and poly-nuclear complexes is a common process in carbene transfer reactions as well as in metal induced catalytic processes, and indeed the direct observation of carbene migration is reported in some examples.
Interconversion of bridge-to-terminal carbene is an example of where the carbene is found to be in a dynamic state. Typical examples of this kind of isomerization appear in the complexes of [Ru2Cp(m-CO)(m-CMe2)],32[Co2(CO)4( m-CH2)(m-dppm)]33
and [CoRh(Cp)2(CO)2(m-CH2)].21As for the cluster, Shapley
and Holmgren reported that the fluxional behavior of [Ru3(CO)10(m-CO)(m-CH2)] can be attributed to the migration
of bridging carbene and carbonyl ligands.34 Scheme 11
illustrates the mechanism which involves the conversion of bridge-to-terminal carbene species.
Another route involving carbene “migration” in the cluster was illustrated by Barnes and co-workers in the complex of Scheme 10 Carbene transfer in the Pauson–Khand reaction.
*CpRh CoCp Co Cp H2 C O O H2 C Co CoCp *CpRh O O Cp *CpRh CoCp Co Cp CH2 O O *Cp = Me5C5 67 [RhCp*(CpCo)
2(m-CO)2(m-CH2)] (67).35The crystal structure
of 67 exhibits a triangular arrangement of the metal atoms with carbene ligand bridging the Rh–Co edge and the two carbonyl ligands bridging the remaining Rh–Co and Co–Co edges.
Spectroscopic data of a CD2Cl2 solution of 67 at room
temperature are consistent with this structure. However, upon standing overnight complex 67 isomerized to a new species with the carbene ligand bridging to the Co–Co edge. They observed additional signals from the other isomer in which the carbene is capped to the three metal units, which might be the intermediate for the carbene transfer (Scheme 12). All these
experimental observations clearly indicate the labile nature and various bonding modes of the carbene ligands and these species are believed to be the intermediates for many catalytic processes.
4 Summary
In this article examples of carbene or alkylidene ligand transfer between transition metals in an inter- or intramolecular fashion have been considered. These investigations illustrate the plausibility of such transfer between various transition metals. Some of these reactions proceed efficiently to generate the new carbene species, which is not easily achieved by a traditional approach. As mentioned, the carbene transfer might be an important catalytic process and more effort in mechanistic understanding could help to design better catalysts for relevant synthetic applications. Indeed, Sierra and co-workers demon-strated that palladium catalyzes the transformation of chro-mium(0) carbenes into useful organic products.
It is also appears that the driving force directing such transfers is not fully understood. Nevertheless, the resulting carbene transfer product ought to be the thermodynamically favored one. Indeed, a crystallographic study on the related species [WPt(CO)5(m-C(OMe)Ph)(PMe3)2], clearly shows that
the bond distance of Pt–C [2.04(1) Å] is shorter than that of W– C [2.48(1) Å], i.e. the stronger bond of Pt–C, reflecting the tendency of the carbene ligand transferring from tungsten to
platinum.30 Of course, more experimental and theoretical
studies may provide greater insight into these carbene transfer reactions.
5 Acknowledgements
It is a pleasure to thank all our colleagues whose work has contributed to this Review for their skillful experimental work as well as their intellectual contribution. S.T.L. would like to
thank the National Science Council, Republic of China for supporting our research work over the past twelve years.
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