William E.
Douglas
CNRS UMR 5637, Case 007, Université de Montpellier II, 34095, Montpellier Cedex 5, France
First published on 27th January 2000
The solvent-free reaction of phenylacetylene at 115 °C in the presence of nickelocene, [(η-Cp)Ni]2(PhCCH), [(η-Cp)Ni(CO)]2, (η-Cp)Ni(NO), (η-Cp)Ni(GeBr3)(CO), (η-Cp)Ni[(P(OMe3)]Cl, (η-Cp)Ni(Ph3P)Cl, (η-Cp)Ni(Bun3P)I, or (Ph3P)2Ni(CO)2 gives rise to a mixture of cyclotrimers, linear oligomers and poly(phenylacetylene), no reaction being observed in the case of internal acetylenes. Cyclotrimer formation is favoured by the presence of (a) added phosphine (2 equiv.), or (b) (cyclopentadienyl)nickel catalysts bearing a chloro substituent at Ni. A reduction in reaction temperature results in lower conversion but favours linear oligomer and polymer formation. The extent of reaction is greatly reduced in the case of (a) nickelocene in the presence of 2 equiv. PBun3, (b) (η-Cp)Ni(GeBr3)(CO), or (c) (η-Cp)Ni(NO). The main effect of the presence of solvent, regardless of whether it is potentially coordinating (toluene) or not (n-octane), is to suppress almost completely reactions catalyzed by nickelocene.
The dimer [(η-Cp)Ni]2(PhCCH), prepared as described
4 from crude [(η-Cp)Ni(CO)]2, was isolated by chromatography followed by recrystallization from n-pentane at −78 °C to give dark-green crystals [mp 133–134 °C (blackening above 100 °C), lit.4 132–133 °C; M+ 348 (EI)].
(η-Cp)Ni(GeBr3)(CO) [M+ 464 (EI)] was prepared as described5 from pure [(η-Cp)Ni(CO)]2 in toluene. The complexes (η-Cp)Ni[P(OMe)3]Cl
6 [M+ 282 (EI)], (η-Cp)Ni[P(n-Bu)3]I
7 (mp 50–52 °C (waxy crystals formed over a period of 2 months), lit.8 red oil; M+ 452 [FAB: m-nitrobenzylalcohol]) and (η-Cp)Ni(NO)
9 [M+ 153 (EI)] were prepared as described.
Composition of final reaction mixture![]() |
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---|---|---|---|---|---|---|---|---|---|---|
Entry | Reaction temperature/time (°C/h) | Solvent | Added ligand | Time for colour change to orange-red | Aspect of final reaction mixture at RT | Polymer (%)![]() |
Linear oligomers (%)![]() |
Cyclotrimers (%)![]() |
PhC![]() ![]() |
Cyclotrimers: (polymer + linear oligomers) |
a Nickelocene (ca. 0.01 mmol) and phenylacetylene (9.1 mmol). b Determined by SEC. c Retention time 31.3–36.0 min. d Retention time 39.3–40.3 min. e Retention time 42.03 min. f Retention time 45.00 min. | ||||||||||
1 | 115/6 | none | none | 5 min | dark orange-red solid | 50 | 9 | 30 | 8 | 1∶2.0 |
2 | 65/24 | none | none | 14 h | red-brown nonviscous liquid | 10 | 15 | 4 | 74 | 1∶6.3 |
3 | 100/7 | PhMe (5 ml) | none | 1 h | orange-red solution | 2.6 | 2.8 | 2.4 | 92 | 1∶2.3 |
4 | 100/7 | n-octane (5 ml) | none | 1 h | orange-red solution | 1.5 | 2.3 | 1.3 | 95 | 1∶2.9 |
5 | 115/6 | none | PBun3 (1 equiv./(η-Cp)2Ni) | 3 h | orange-red viscous liquid | 47 | 15 | 32 | 8 | 1∶1.9 |
6 | 115/6 | none | PBun3 (2 equiv./(η-Cp)2Ni) | 6 h | orange-red liquid | 17 | 12 | 24 | 46 | 1∶1.2 |
7 | 115/6 | none | PPh3 (2 equiv./(η-Cp)2Ni) | 1 h | orange-red viscous liquid | 8 | 7 | 77 | 1.5 | 1∶0.2 |
Composition of final reaction mixture![]() |
||||||||
---|---|---|---|---|---|---|---|---|
Entry | Catalyst | Time until colour changed to orange-red | Aspect of final reaction mixture at RT | Polymer (%)![]() |
Linear oligomers (%)![]() |
Cyclotrimers (%)![]() |
PhC![]() ![]() |
Cyclotrimers: (polymer + linear oligomers) |
a Catalyst (ca. 0.01 mmol) and phenylacetylene (9.1 mmol) heated at 115 °C for 6 h. b Determined by SEC. c Retention time 31.83–32.22 min. d Retention time 39.19–40.31 min. e Retention time 42.03 min. f Retention time 45.00 min. g Reaction temperature 65 °C. | ||||||||
1 | [(η-Cp)Ni]2(PhC![]() |
2 min | dark orange-red solid | 53 | 7 | 30 | 8 | 1∶2.0 |
2 | [(η-Cp)Ni(CO)]2 | 30 sec | dark orange-red viscous liquid | 40 | 20 | 35 | 8 | 1∶1.7 |
3 | (η-Cp)NiNO | (red catalyst) | brown liquid | 23 (1![]() |
2 (3![]() |
11 (1![]() |
65 (95![]() |
1∶2.3 (1∶4.0![]() |
4 | (η-Cp)Ni(GeBr3)(CO) | 1 min | orange-red liquid | 26 | 0 | 15 | 59 | 1∶1.7 |
5 | (η-Cp)Ni(PBun3)I | 1 h | orange-red viscous liquid | 56 | 8 | 29 | 8 | 1∶2.2 |
6 | (η-Cp)Ni[P(OMe)3]Cl | 1 min | orange-red viscous liquid | 38 | 15 | 40 | 3 | 1∶1.3 |
7 | (η-Cp)Ni(PPh3)Cl | (red catalyst) | red viscous liquid | 52 | 1 | 38 | 7 | 1∶1.4 |
8 | (PPh3)2Ni(CO)2 | immediate | dark orange-red solid | 35 | 0 | 60 | 6 | 1∶0.6 |
![]() | ||
Scheme 1 Reaction mechanism for nickelocene-catalyzed polymerization and cyclotrimerization of phenylacetylene in the absence of solvent. |
Previously, we have reported that under solvent-free conditions nickelocene catalyzes terminal-acetylene polymerization,1 in the case of phenylacetylene giving cyclotrimers and linear polyenes but no cyclotetramers.2,3 In solution, nickelocene, and also [(η-Cp)Ni]2(RCCR′), catalyze the polymerization of acetylenes but only in the presence of an aromatic heterocyclic amine (e.g. pyridine)
26 or AlBr3
27 (nickelocene with F3CC
CCF3 in solution has been reported to give traces of the cyclotrimer
28).
![]() | ||
Fig. 1 SEC trace for product mixture from reaction of phenylacetylene in the presence of nickelocene under the standard conditions (Table 1, entry 1). |
It having been established that the components in the reaction mixture could be distinguished by SEC, various other possible products were synthesized in order to check for any coincidence of retention times.
The mixture of tetraphenylcyclooctatetraene isomers (possibility of 7 isomers29), prepared in such a way as to contain no trace of cyclotrimers,10 showed three SEC peaks with retention times 37.90, 39.37 and 40.64 min. In particular, the peak at 37.90 min allows tetraphenylcyclooctatetraene formation in the reaction mixture to be distinguished by SEC (cf. Fig. 1 for the case where the absence of cyclotetramers has been established by other means
3).
The SEC trace of pure 1,2,4-triphenylbenzene contained a single peak with a retention time of 42.03 min, identical to that for 1,3,5-triphenylbenzene, thus showing that the two cyclotrimer isomers cannot be distinguished by SEC.
The stable tail-to-tail dimer PhCC–CH
CHPh was prepared from a commercial sample of β-bromostyrene (ca. 85% trans and 15% cis) by Pd-catalyzed coupling with phenylacetylene.14 The SEC trace of the reaction product (containing mainly E-1,4-diphenylbut-1-en-3-yne with some Z-isomer) showed a single peak with a retention time of 43.64 min. An identical trace was obtained after leaving the material to stand for 4 d. SEC can therefore be used to distinguish the presence of the dimer in the reaction mixture.
Starting from α-bromostyrene, the unstable15,16 head-to-tail dimer PhC
C–CPh
CH2 was prepared in the same way, the SEC trace of the product mixture showing peaks at 43.64 min for the head-to-tail dimer and at 46.20 min for unchanged monomer. There was also a barely perceptible peak with a retention time of 40.54 min. After 5 d, the magnitude of the peak for PhC
C–CPh
CH2 had decreased and the peak for the decomposition product at 40.54 min had become the most important (ratio of areas ca. 1∶2). The retention time of the decomposition product is consistent with it being a mixture of linear oligomers of phenylacetylene. Thus, SEC allows dimer formation to be distinguished in the reaction mixture but no differentiation between the tail-to-tail and head-to-tail isomers is possible.
The preliminary studies showed the feasibility of using SEC to follow the effects of catalysts on product distribution. Indeed, the absence of SEC peaks with retention times of ca. 37.9 min or ca. 43.6 min rules out any significant cyclotetramer or dimer formation. In all cases only the four products previously isolated and characterized for nickelocene are formed, their distribution being dependent on the nature of the nickel catalyst and reaction conditions.
No reaction of internal acetylenes (Me3SiCCSiMe3, PhC
CSiMe3, PhC
CPh) occurred under solvent-free conditions in the presence of nickelocene or the other (cyclopentadienyl)nickel catalysts. This suggests that, as with (Ph3P)2Ni(CO)2, the reaction mechanism includes an acetylene hydrogen transfer step rather than involving nickelole or other such intermediates.30,31 Probably the mechanism operating in this case is that of Meriwether (Scheme 1), the catalytic species A being formed by loss of a cyclopentadienyl ring, a reaction which is known to occur readily.32,33 Indeed, a variety of 19-electron (η-Cp)NiL2 complexes, including (η-Cp)NiPPh3(HC
CPh) and (η-Cp)Ni(HC
CPh)2, have been studied in benzene
solution and are surprisingly stable.34 The following step in the catalytic cycle is the oxidative addition of phenylacetylene giving B, such intermediate ethynyl hydride complexes of Ni having been previously isolated.25 The next step is the insertion of phenylacetylene into the Ni–C bond, a reaction which has been previously observed.35–37 This step is repeated (n − 1)-fold giving D which re-forms A by reductive elimination of linear polymer E. The cyclotrimers F can be formed by reductive elimination from D when n = 2.
In order to investigate the effect of solvents, the nickelocene-catalyzed reaction was carried out at 100 °C for 7 h in the presence of 5 ml of toluene or n-octane, the concentration of the phenylacetylene solution being ca. 17% v/v. In each case, the mixture became yellow almost immediately and after 1 h orange-red. Evaporation of the reaction mixtures gave traces of reddish-brown oils, the weights obtained corresponding to 8% and 5% overall reaction (Table 1, entries 3 and 4), respectively. The main effect of the presence of solvent, regardless of whether it is potentially coordinating (toluene) or not (n-octane), is to suppress almost completely reactions catalyzed by nickelocene. In the case of nickelocene, coordination of a solvent molecule occurs at the expense of an acetylene molecule in the 19-electron catalytic species A (Scheme 1) thus inhibiting the reaction. This is in contrast to (Ph3P)2Ni(CO)2 where the corresponding catalytic species is 18-electron and can coordinate a molecule of solvent in addition to the acetylene molecules.
The presence of 1 equiv. of PBun3 (high electron-donating ability and moderate cone angle38) per (η-Cp)2Ni (Table 1, entry 5) had little effect. In contrast, with 2 equiv. PBun3 per (η-Cp)2Ni the extent of conversion fell to 54% (Table 1, entry 6). This was accompanied by an increase in the proportion of cyclotrimers suggesting that the phosphine is present in the coordination sphere of the catalytic species. Previously it has been found that the electronic and steric factors of the phosphorus ligand play an important rôle in product distribution in the solution oligomerization of phenylacetylene catalyzed by rhodium(I) complexes,39,40 the presence of excess electron-donating phosphites
inhibiting the reaction.40 Indeed, the sterically-hindered complex (tBu2PC2H4PtBu2)Ni(C2H2) does not catalyze cyclooligomerization or polymerization of acetylenes.41
The addition of 2 equiv. PPh3 (moderate electron-donating ability and large cone angle38) per (η-Cp)2Ni had a very different result, the extent of reaction being almost complete (98.5%) with selective formation of cyclotrimers in 77% yield (Table 1, entry 7). Presumably, triphenylphosphine is present in the coordination sphere of Ni replacing one acetylene group, thus favouring reductive elimination of the cyclotrimers. Nickelocene reacts with PPh3 in cyclohexane to give Ni(PPh3)4 quantitatively,42 but (η-Cp)Ni(PPh3)Cl and CpCl are formed in the presence of CCl4.32,33 Analogous reactions occur in the case of phosphites.43 The intermediate species (η-Cp)Ni(PPh3)(σ-Cp) may be formed by a π to σ rearrangement
of a cyclopentadienyl ligand.32,44 In the presence of excess phosphine, ionic species such as [(η-Cp)Ni(PR3)2]+Cl− are formed.45,46 Alkyl compounds of the type (η-Cp)Ni(PPh3)R are sufficiently stable to be isolated and characterized, an investigation of the n-alkyl derivatives suggesting reversible olefin β-elimination accompanied by nickel hydride formation.47 Nineteen-electron complexes (η-Cp)Ni(PR3)2 have been isolated and characterized, and have been shown to be most probably intermediates in the reactions of nickelocene with phosphines.48
The complex (η-Cp)Ni(NO) gives only 35% conversion at 115 °C. Decrease in the reaction temperature to 65 °C results in a reduction in both the extent of reaction (to 5%) and, as with nickelocene, the proportion of cyclotrimers (Table 2, entry 3). Loss of the NO ligand enabling formation of [(η-Cp)Ni]2(PhC CH) occurs only with difficulty. Indeed, it has been found that nitrosyl exchange with 15NO did not occur for (η-Cp)Ni(NO) in 10 d at 120 °C.53
The complex (η-Cp)Ni(GeBr3)(CO) gave very similar results (Table 2, entry 4) to those for (η-Cp)Ni(NO), suggesting that although the CO ligand is readily lost (as shown by the initially green solution turning orange-red after only 1 min at 115 °C), the presence of the electron-withdrawing GeBr3 substituent5 hinders subsequent reactions with phenylacetylene. The reaction mechanism may involve 20-electron species (Scheme 1 with (η-Cp)(GeBr3)Ni in place of (η-Cp)Ni). No reaction was observed with PhC
CPh.
In the case of (η-Cp)Ni(Bun3P)I, the product distribution and extent of reaction (Table 2, entry 5) were very similar to those for nickelocene (Table 1, entry 1), but the reaction mixture did not pass through a green stage. Here too the reaction mechanism may involve 20-electron species [Scheme 1 with (η-Cp)NiI in place of (η-Cp)Ni].
The complexes (η-Cp)Ni[(P(OMe3)]Cl and (η-Cp)Ni(PPh3)Cl (Table 2, entries 6 and 7) gave very similar results, the product mixtures being much richer in cyclotrimers than in the case of nickelocene. Since the phosphorus ligands are quite different in nature,38 the phosphine groups are probably not present in the active catalytic species. The reaction mechanism in each case may involve 20-electron species [Scheme 1 with (η-Cp)NiCl in place of (η-Cp)Ni]. Indeed, in neither case was a green stage observed corresponding to formation of [(η-Cp)Ni]2(PhCCH). This is in contrast to the case of nickelocene in the presence of 2 equiv. PPh3 (Table 1, entry 7) where cyclotrimer formation is favoured, the phosphine presumably playing a rôle in the co-ordination sphere of Ni (vide supra).
Finally, for comparison, the effect of (Ph3P)2Ni(CO)2 was investigated in the absence of solvent (Table 2, entry 8). Unlike the reaction in benzene where only cyclotrimer and linear trimer are afforded,19 the solvent-free reaction of phenylacetylene in the presence of (Ph3P)2Ni(CO)2 gives linear polymer in addition to cyclotrimer consistent with the much greater concentration of phenylacetylene favouring a larger value of n in intermediate D [Scheme 1 with (Ph3P)2 in place of (η-Cp)]. No reaction was observed with PhCCPh.
In summary, the oligomerization and cyclotrimerization of phenylacetylene with high conversion is catalyzed under solvent-free conditions by a wide variety of cyclopentadienylnickel complexes, internal acetylenes being unreactive. Cyclotrimer formation is favoured by the presence of (a) 2 equiv. of phosphine in the reaction mixture, or (b) (cyclopentadienyl)nickel catalysts bearing a chloro substituent at Ni. A reduction in reaction temperature results in lower conversion but favours linear oligomer and polymer formation. The extent of reaction is greatly reduced in the case of (a) (η-Cp)Ni(NO), (b) (η-Cp)Ni(GeBr3)(CO), or (c) nickelocene in the presence of 2 equiv. PBun3. The main effect of the presence of solvent, regardless of whether it is potentially coordinating (toluene) or not (n-octane), is to suppress almost completely reactions catalyzed by nickelocene. It can be concluded that under solvent-free conditions (cyclopentadienyl)nickel compounds in general are active catalysts for the polymerization of terminal acetylenes.
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