2276
Reactions of Acetylenes with Noble-Metal Halides. VIII.' The Palladium Chloride Catalyzed Trimerization of 2-Butyne and 1-Phenyl-1-propyne H. Dietl, H. Reinheimer, J . Moffat, and P. M. Maitlis2 Contribution f r o m the Department of Chemistry, McMaster University, Hamilton, Ontario, Canada. Receioed October 20, 1969
Abstract: Reaction of methylphenylacetylene with bis(benzonitri1e)palladium chloride (1) in chloroform gave 1,2,4-trimethyl-3,5,6-triphenylbenzene(2, 58%), 1,3,5-trimethyl-2,4,6-triphenylbenzene(3, 3973, and 1,2,3-trimethyl-4,5,6-triphenylbenzene(4, 3%). In benzene, a complex [(PhC2Me)3PdC12]z was isolated which readily decomposed to palladium chloride, 2, and 3. 2-Butyne reacted with l in benzene to give a complex [Cl(MeC2Me)3PdC1I2 (6) and in chloroform to give [C1(MeCzMe)3PdC1PdC1~l. (7) and some complex 6 . On spectroscopic evidence, both 6 and 7 are assigned structures involving a 2-chloro-3,4,5,6-tetramethyl-2-trans,4-c~~,6-tran~-octa-2,4,6triene u-bonded at C , and r-bonded at Cz,3to palladium(I1). The ligand can take up two positions with respect
to the metal, with the coordinated double bond parallel or perpendicular to the coordination plane. Both complexes 6 and 7 decomposed readily to palladium chloride and hexamethylbenzene. A new mechanism for the trimerization of acetylenes is proposed.
I
n 1962 one of us reported3 a reinvestigation of the reaction, originally described by Malatesta, et in which diphenylacetylene was dimerized to an ethoxytetraphenylcyclobutenylpalladium chloride complex in the presence of palladium chloride in ethanol. Under slightly different conditions (aprotic solvents) from those used by Malatesta, et al., we observed that diphenylacetylene could also be catalytically trimerized to hexaphenylbenzene. In addition, a tetraphenylcyclobutadienepalladium chloride complex ([Ph4C4(PdCI2),],)was formed which eventually deactivated the cata1yst.j These results have b'een confirmed by a number of workers. 6--8 These reactions of diphenylacetylene and closely related acetylenes did not readily lend themselves to a more detailed investigation and, since the generality of the reaction was of considerable interest particularly as a very easy route to cyclobutadiene-metal complexes, we began an investigation of the reactions of some other acetylenes with palladium chloride. Our first attempts, using acetylene, propyne, and monophenylacetylene, did not lead to characterizable products. Reactions always proceeded very easily to give mixtures of metal complexes. However, the latter were polymeric and nonstoichiometric in nature and not easily handled. It is probable that the ligands in these complexes are linear polyenes; however, hydrogen-transfer and cyclization reactions are by no means excluded.$
We then turned our attention to I-phenyl-I-propyne (methylphenylacetylene, MPA) and 2-butyne (dimethylacetylene). This paper describes these l2 and gives evidence for a trimerization mechanism different from those previously considered. The following paper describes in detail some reactions of a complex isolated from the latter reactions.
Results and Discussion Reactions of Methylphenylacetylene (MPA) with Bis(benzonitri1e)palladium Chloride. In a halogenated solvent (chloroform, methylene chloride), MPA was catalytically trimerized to trimethyltriphenylbenzenes by bis(benzonitri1e)palladium chloride (1). l 3 The products included a small amount of an uncharacterized brown metal complex and the benzenoid trimers 2, 3, and 4. The trimethyltriphenylbenzene isomers 2, 3, and 4 were
(1) Part VII: S . McVey and P. M. Maitlis, J . Organometal. Chem., 19, 169 (1969). (2) Fellow of the Alfred P. Sloan Foundation and author to whom any correspondence should be addressed. (3) A. T. Blomquist and P. M. Maitlis, J . Amer. Chem. Soc., 84, 2329 (1962). (4) L. Malatesta, G. Santarella, L. M. Vallarino, and F. Zingales. A t f i Accad. Naz. Lincei, Rend., Cl. Sei. Fis. Mat. N a t . , 21, 230 (1959); Angew. Chem., 12, 34 (1960). ( 5 ) P. M. Maitlis, D. Pollock, M. L. Games, and W. J. Pryde, Can. J . Chem., 43, 470 (1965). (6) L. M. Vallarino and G. Santarella, Gazr. Chim. Ita/., 94, 252 ( 1964). (7) R. Hiittel and H . J. Neugebauer, Tetrahedron Lett., 354 (1964). (8) R. C. Cookson and D. W. Jones, J . Chem. Soc., 1881 (1965). (9) One product, isolated in only low yield from the reaction of phenylacetylene, proved to be 2,4,6-triphenylfulvene.10 (10) J. Bloodworth and P. M. Maitlis, unpublished.
Journal of the American Chemical Society
92:8
/ April 22, 1970
(1 1) Preliminary communications on part of this work have appeared: (a) H. Dietl add P. M. Maitlis, Chem. Commun., 481 (1968); (b) H. Reinheimer, H. Dietl, J. Moffat, D. Wolff, and P. M. Maitlis, J . Amer. Chem. Soc., 90,5321 (1968). (12) P. M. Maitlis, H. Reinheimer, H. Dietl, and J. Moffat, Amer. Chem.. Soc., Dic. Petrol. Chem., Prepr., 14, B156 (1969). (1 3) Bis(benzonitri1e)palladium chloride, prepared by the method of Kharasch, et al.,14 was used because of its solubility and ease of preparation. We also observed that the benzonitrile ligands coordinated only very weakly to the metal. In a Nujol mull the complex showed a sharp Y C N at 2290 cm-1, but when the complex was in solution two bands, ascribed to Y C N (at 2230 and 2290 cm-l), of about equal intensity were always observed, irrespective of solvent, The latter is due to coordinated PhCN, while the position of the former band is identical with that of free benzonitrile itself. Since bands of approximately equal intensity were observed, it suggests that an equilibrium such as
IfnPhCN + (PhCNPdCln),
rz(PhCN)zPdC1z
lies well over to the right-hand side, In fact when such solutions were allowed to stand, palladium chloride slowly crystallized out, indicating that the equilibrium
(PhCNPdC12),
nPhCN
+ (PdCL),
also lies over to the right. As far as we have been able to tell, benzonitrile, in the concentrations present, plays no significant role in these reactions, and therefore the complex (PhCN)gPdCIz may for practical purposes be regarded simply as weakly solvated PdCL. A similar conclusion has been arrived at by Kitching and Moore.15 (14) M. S. Kharasch, R. C. Seyler, and F. R. Mayo, J . Amer. Chem. Soc., 60, 882 (1938). (15) W. Kitching and C. J. Moore, Inorg. Nucl. Chem. Lett., 4, 691 (1968).
2277
The solid complex 5 also decomposed, beginning at loo", to give a sublimate of white crystals (2 and 3) and a
Me
residue of palladium chloride. Unfortunately, any metal complex which gives rise to the trimers 2 and 3 (and possibly 4) is bound to be a mixture of isomers (as 2 2 + 3 + 4 indicated by its nmr spectrum). This severely limits the + [(MeCiPh),PdC1,I, structural and mechanistic information which can be ob5 tained. By analogy, however, we suggest that this complex is a mixture of isomers, each of which has the Me P h G ; same basic skeleton as that of [(MeCzMe)3PdC12]2 ( 6 )discussed below. Ph Ph Reactions of 2-Butyne with Bis(benzonitri1e)palladium 3 4 Chloride. When the reaction of 2-butyne with separated by fractional crystallization and their relative (PhCN)2PdClz was carried out under very carefully conamounts estimated by a combination of crystallization trolled conditions in benzene at 5 ",we obtained in 50 and quantitative pmr analysis as 58:39:3. Isomers 2 yield a yellow crystalline complex [(MezC~)3PdClz]z (6). and 3 have previously been prepared by trimerization of This complex appeared to behave similarly to [(PhC2MPA with Hg(Co(CO),), by Hubel and Hoogzand.16 Me)aPdClz]z(5) described above. It readily gave hexaOur spectroscopic data were in agreement with these methylbenzene (HMB) and PdClz in chloroform or structures. The isolation of the 1,2,3-trimethyl-4,5,6-triphenyl- methylene chloride solution, or on heating. An intermediate in the decomposition was shown by nmr to be benzene (4) was unexpected, and this compound had not another complex, [(MezC2)3(PdC12)2], (7). been reported previously. The identification as 4 folThe reaction of 2-butyne with (PhCN)2PdCI2 in lowed from its analysis and molecular weight (both chloroform was followed by nmr. Below -40" a commass spectroscopic and osmometric) and particularly plex, identified as the solvated .rr-acetylene complex 8, from its pmr spectrum. The latter showed, apart from was formed. In the presence of excess 2-butyne at ca. phenyl resonances, two singlets due to the two different -25", the complex 7 was formed, and at higher temtypes of methyls, in the ratio I : 2. l7 The infrared specperatures, in the presence of 2-butyne, this was transtra of all three isomers wereo very similar, and they all formed into complex 6 . These reactions are summarshowed a maximum at 2820 A in the ultraviolet. ized in Scheme I. Although the full extent of the catalytic trimerization was not investigated, we were able to trimerize at least Scheme I 63 mol of the acetylene, MPA, per mol of 1. C,H, +5' In benzene some of the trimers 2, 3, and 4 were also MeCeCMe + (PhCN)2PdCl, [(Me2CJ3PdCI2], obtained, but the major product (ca. 90 was a yellow 6 complex [(MeCzPh)3PdC12]2(5). This material was CHCI,, +a5 /CHCI,, -50 very labile. Attempts to purify it led to materials having a higher palladium and chlorine content and some trimers. This was presumably due to a reaction such as 8 7
+
z
-
z)
n[(MeCzPh)sPdCl~]~ +n(MeCtPh)l
x,
+ n[(MeC*Ph)r(PdCl~)~]
The complex was reasonably stable in the solid. In benzene solution the pmr spectrum showed a very complex pattern in the methyl region which was not analyzable. This did not change noticeably with time, despite the occurrence of the above decomposition. The decomposition of 5 to a mixture of trimers and palladium chloride was fast in chloroform or methylene chloride. Only the two isomers 2 and 3 (in the ratio of 3 :2) were detected in the product. An analogous reaction occurred when a benzene solution was treated with various ligands (L = triphenylphosphine, acetonitrile, or benzonitrile). In each case the appropriate palladium complex, L2PdC12, and the trimers 2 and 3 were obtained. 5
+ 4L
2
+ 3 + 24PdC12
(16) W. Hubel and C. Hoogzand, Chem. Ber., 93, 103 (1960). (17) Themethyl resonances observed were at 6 2.19 (1) and 2.28 (2) for 2, 6 2.18 for 3,and 6 2.30 (2) and 2.37 (1) for 4 in benzene. In CHzCh these were shifted to 1.67 ( I ) and 2.00 (2) for 2, 1.62 for 3, and 2.06 (2) and 2.33 (1) for 4. The resonances to lowest field are those of methyls most shielded by phenyls (e.g., in 3). The differences in chemical shift between the two solvents are also revealing and are largest (0.52, 0.56 ppm) for a methyl situated between two phenyls. A methyl between a methyl and a phenyl experiences a solvent shift of 0.24 (or 0.28) ppm, while one between the two methyls, i.e., the 2-methyl in the 1,2,3-trimethyl isomer, 4, is hardly moved at all (0.04 ppm). This provides useful confirmatory evidence for the structure of 4.
P H C 1 3 , +30i
I P P h , , -50"
MeC2Me
+ (Ph3P)2PdC12
HMB
+ PdC12
Complex 6 , [(Me2C2)3PdC12]z.Although this complex is crystalline, the crystals obtained were too small for X-ray analysis, and we have been forced to rely on spectroscopic and chemical evidence in order to formulate a structure for it. This evidence is summarized. (a) The complex 6 decomposed either on heating the solid to over IOO", or in halogenated solvents (CHC13, CH2C12) at 30" (Figure l), or in solution on reaction with halogens (Brz, I,) at -15", to HMB. The other product was palladium chloride in the first two cases. This implied that three (rather than two or four) acetylenes were associated with each metal atom in the complex, which was therefore probably a chlorinebridged dimer. (18) Under some conditions, for example, when 6 was allowed to stand undisturbed in chloroform for several days, a brown precipitate with composition agreeing with [(Me~C?)3(PdCl&l~ was obtained in addition to hexamethylbenzene.
Dietl, Reinheimer, Moffat, Maitlis
/ Acetylenes with Noble-Metal Halides
2278
530 (m) (vpd-c),28 665 or 723 (vs) (vc-c1),29 1512 (m) (vCmc ~oordinated~l*~6~30), and 1624 cm-’ (m) uncoordinated). In addition, in CS2 or CCL solution, bands arising from C-C and C-H vibrations and deformations were seen at 953 (w), 1025 (s), 1029 (s); 1056 (m), 1072 (m), 1322 (w), 1361 (m), 1377 (m), 1384 (sh), 1411 (sh), 1423 (sh), 1431 (m), 1442 (m), 1450 (sh), 2872 (m), 2902 (m), 2929 (m), and 2966 cm-’ (m). Of particular interest is the absence of bands in the region (>3000 cm-l) where vinylic vC-”s are usually found. a 1 This agrees with the pmr spectrum. The electronic spectrum of 6 has been recorded but does not appear to be of help in assigning a structure. 3 2 These results indicate the presence of three acetylenes linked to each other and to each metal atom, an asymmetric Pd2C12bridge linking the two halves of the dimer, and an organic ligand containing a C-C1 and a coordinated and an uncoordinated double bond. The six
2
A
i ppm.
Figure 1. Nmr spectra (60 MHz) of the decomposition of complex 6 in CDCla at 34”: (A) complex 6 ; (B) HMB and complex 7; (C) HMB and traces of complex 7.
(b) The pmr spectrum of the complex in chloroform at 20” showed six singlets with equal intensities due to six inequivalent methyl groups. l9 This indicated a considerable asymmetry in the organic ligand and ruled out simple complexes involving dimethylacetylene, tetramethylcyclobutadiene, hexamethyl(Dewar benzene), or H M B as ligands. (c) The ir spectrum of 6 was examined in several media. The most significant bands in Nujol and their proposed assignments are: 242 (m) and 273 (m) (vpd-c,, bridging),20395 (m) ( Y p d - ~ l e f i n ) , ~ ~ ’ ~ 487 ~’~’ (w) or (19) The same result was obtained in CCh and CSZsolutions; however, in benzene the lowest field peak was split into a doublet (6 2.04, 2.12). (20) Bands due to terminal Pd-CI bonds are usually higher than this, in the region 339-366 cm-1.21122 Two bands due to bridging Pd-C1 bonds in the PdzCIz unit are usually observed, in the regions 255-280 and 294-308 cm-1. However, the position of the lower of these bands is markedly influenced by the nature of the trans ligand. In complexes such as [(allyl)PdCI]~,where the bridge is symmetrical, two bands close together, in the range 244-262 cm-1, are observed.23 Powell and Shaw however, noted that when an asymmetric MC12 bridge is present, as in [(allyl)~RhCl]~, and where the M-C1 bond lengths are quite different, the two bridging P M - C I bands also differ ~onsiderably.2~ We have shown,llb and this has been confirmed by Crociani, et a1.,25 that in complexes such as
OMe
r
CI
where a very asymmetric bridge is expected, owing to the very high trans influence of the Pd-C u bond, YPdCl bands at 222 and 272 cm-l are present. Our observation of bands at 242 and 273 cm-1 for vpdci in 6 is consistent with the presence of a n asymmetric PdzCh unit, but one not as asymmetric as in the cyclooctenyl complex (above). This can be rationalized in terms of a smaller trans influence exerted by a sp* carbon than by a sp3 carbon. Our spectra also indicate the absence of terminal Pd-C1 bonds. (21) D. M. Adams, “Metal-Ligand and Related Vibrations,” Edward Arnold, London, 1967. (22) D. M. Adams and P. J. Chandler, J . Chem. Soc., A , 588 (1969). (23) M. S. Lupin, J. Powell, and B. L. Shaw, ibid., 1410 (1966). (24) J. Powell-and B. L. Shaw, ibid., 583 (1968). (25) B. Crociani, P. Uguagliati, J. Boschi, and U. Belluco, ibid.,2869 (1968). (26) M. J. Grogan and I
