Infrared spectra of some alkyl platinum compounds. Part I. Comparison with the spectra of chemisorbed hydrocarbons

Transcription

1 Infrared spectra of some alkyl platinum compounds. Part I. Comparison with the spectra of chemisorbed hydrocarbons B. A. MORROW Department of Chemistry, University of Ottawa, Ottawa 2, Canada Received March 24, 1970 A series of triphenylphosphine stabilized platinum dialkyl compounds of the type (Ph2P),PtR2 where R = methyl, ethyl, ethyl-d,, n-propyl, n-butyl, and n-hexyl have been synthesized and their infrared spectra in the CH stretching region have been obtained. These spectra are discussed in terms of the infrared spectra of hydrocarbons which are chemisorbed on transition metal catalysts. Canadian Journal of Chemistry, 48, 2192 (1970) Infrared spectroscopy has been widely used to study the species produced during the chemisorption of hydrocarbons on silica-supported transition metal catalysts (1-5). The interpretation of such spectra has been based mainly on examination of the bands in the C-H stretching region (vch) between 3300 and 2700 cm-' because the silica support absorbs strongly between 2000 and 1300 cm-' and absorbs totally below 1300 cm-l. Implicit in the interpretation of such spectra has been the assumption that the infrared spectrum of a hvdrocarbon attached to a surface transition metal atom will resemble the spectrum of a pure hydrocarbon (1-3). On this basis, comparisons of the spectra of chemisorbed hydrocarbons with those of pure hydrocarbons have been used to deduce the nature of the surface species present. However, such an interpretive procedure is subject to the criticism that the functional group of a hydrocarbon which is attached to a surface metal atom might not absorb at the same frequency as its pure hydrocarbon counterpart (2, 3). A major difficulty in interpreting the infrared spectra of chemisorbed hydrocarbons has been the lack of suitable transition metal alkyl compounds with known infrared spectra which can be used as model compounds for purposes of comparison. Chatt and Shaw (6, 7) have synthesized some bis(trimethy1phosphine)dialkylplatinum compounds and Adams (8) has published a spectrum of the methyl derivative in the vch region and has reported some tentative frequencies for the ethyl and n-propyl derivatives. However, trimethylphosphine also absorbs in the same spectral region and obscures part of the metal-alkyl spectrum. In the present work a series of cis-platinum dialkyl compounds have been synthesized of the type (Ph,P),PtR, (Ph = phenyl) where R = methyl, ethyl, ethyl-d,, n-propyl, n-butyl, and n-hexyl. All of these are new compounds except for the dimethyl derivative which has been reported by Chatt and Shaw (6). The infrared spectra in the vch region have been obtained and are discussed in terms of the infrared spectra of chemisorbed hydrocarbons. In these compounds, the absorptions due to the phenyl portion of the molecule lie above 3000 cm-i and do not obscure the spectrum of the aliphatic groups whose absorptions lie below 2960 cm-l. Experimental and Results The platinous dialkyl compounds were prepared by the reaction of cis-dichlorobis(triphenylphosphine)platinum (11), (Ph3P),PtCI2, with an ether ioiutioi of the corresponding alkyllithium. The method was essentially the same as that used by Chatt and Shaw (6, 7) for the synthesis of the corresponding trimethylphosphine and triethylphosphine stabilized dialkyl derivatives. cis- (Ph3P)2PtC12 was prepared by the reaction of K2PtCI, with Ph3P as described by Jensen (9). The diethyl derivative was also prepared by the reaction of ci~-(ph~p)~ptcl with CH3CH2MgBr with slightly lower yield and the di(ethy1-d3) derivative was prepared by this method using CD3CH2Br as starting material. This contained 98% deuterium in the terminal position and was supplied by Merck, Sharp and Dohme Ltd. of Montreal. Products were crystallized from light petroleum ether (30-60 "C) except for the dimethyl derivative which was crystallized from benzene. Yields, melting points, and analytical data are shown in Table 1. Nuclear magnetic resonance spectra were run on a Varian Associates T60 instrument and the ratio of the integrated intensities of the phenyl to alkyl signals agreed to within a few percent of the theoretical. The spectrum of the dimethyl derivative was identical to the published spectrum (10). Infrared spectra were run as dilute solutions in CCI4 (about 0.01 M, path length of 0.5 mm) at low resolution on a Beckmann IR-20 instrument, and at high resolution in the vch region on a Perkin-Elmer model 13G infrared spectrometer using a spectral slit width of about 2 cm-'. The high resolution spectra of

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3 2194 CANADIAN JOURNAL OF CHEMISTRY. VOL. 48, 1970 TABLE 2 Observed alkyl CH stretching frequencies of (Ph3P),PtR2 I I I I I c m-i FIG. 3. Infrared spectra of (Ph3P)Pt(CH2CD3), in the vch and vcd regions. The dotted portion of A is the spectrum of (Ph3P)Pt(CH2CH3)2 as in Fig. 1B. shown that the intensity of the band near 2960 cm-i [v(as)ch,] remains approximately constant as thechain length increases but the intensity of the CH, band at 2925 cm-i [v(as)ch,] increases in almost direct proportion to the chain length. Therefore, the ratio of the optical density of these bands is a useful measure of the relative proportion of these groups present. From an examination of the infrared spectra of a number of tin and lead alkyl compounds, Sheppard and Ward conclude (3) that the Jones relationship is still approximately valid, although use of the hydrocarbon data may lead to an underestimate of the ratio n by one methylene unit. In the present work the frequencies of the major bands in the spectrum of the n-propyl, n-butyl, and n-hexyl derivatives agree quite well with the Fox and Martin (12) assignments, the agreement improving as the chain length increases (Table 2). Further, the ratio of the optical density of the band near 2925 cm-i also increases with the chain length, and using the Ward and Sheppard (3) criteria, are in quite good agreement with that which would be predicted for each particular case. It is particularly worth noting that the spec- Ratio of optical Frequency Optical density of R (cm-') density v(as)ch,/v(as)ch,* - Methyl Ethyl *as = asymmetric stretching mode using the assignmefits of Fox and Martin (12). tsh = shoulder. TABLE 3 Infrared frequency data for saturated hydrocarbons Frequency cm-' Assignment Group (11, 12) (1 I)* C-CH vch(as) 2872k 10 vch(s) C-CH2-2926k _+ 10 vch(as) vch(s) C-CH 2890k 10 vch *as = asymrne1ric.s = symmetric. trum of the alkyl portion of the n-butyl derivative is almost an exact replica of the spectra which has frequently been observed in studies of ethylene and butene hydrogenation on nickel and platinum (2,4, 5, 14) and which has always been attributed to a chemisorbed n-butyl group. This is the first

4 MORROW: INFRARED SPECTRA OF SOME ALKYL PLATINUM COMPOUNDS. PART I 2195 direct piece of experimental evidence which corroborates this assignment. In a comparison of the spectrum of trans- (Me,P),PtClMe [Me = methyl] and (Me,P),- PtClCD,, Adams (8) deduced that the vch absorptions of Pt-CH, occurred at about 2930, 2885, and 2815 cm-'. However, the two higher frequency bands only appeared as shoulders on either side of an intense P-CH, band. A similar spectrum was obtained for cis-(me,p),ptme,. The spectrum of the dimethyl platinum derivative in this work agrees very well with that "predicted" by Adams, and its general form is similar to the vch spectra of other metal-ch, compounds in having two strong high frequency bands and a weaker low frequency band (3, 15-17). Adams (8) assigned the low frequency 2815 cm-i band to the symmetric vch mode and was unable to decide which of the two high frequency bands was attributable to the asymmetric vch mode, or indeed, why there were two bands at all. In view of the intensities, it would appear more reasonable to assign the 2934 and 2874 cm-i bands to the asymmetric and symmetric CH stretching modes, respectively, and the lower frequency mode to an overtone of FCH which is brought up in intensity by Fermi resonance interaction (3, 15, 17, 18). However, any assignment can only be speculative at this time and either gas phase band contour data, or the frequencies of the partially deuterated methyl species would be needed to further confirm any assignment. There is increasing evidence in the literature that the traditional Fox and Martin assignments may not be correct (19, 22) and it is anticipated that the data for the isotopic dimethyl platinum derivative will be forthcoming in a later part of this series. The spectrum of the diethyl platinum derivative in the vch region (< 3000 cm-') is not typical of the spectrum to be expected for an ethyl group (3, 18, 20) which normally has a pair of bands near 2960 and 2870 cm-' due to the CH,, and another pair near 2925 and 2855 cm-' due to CH,. However, the observed spectrum of the Pt--ethyl group is apparently similar to that observed by Adams (8) for cis-(me,p),pt(ch,- CH3), which had a strong band at 2840 cm-' and weaker bands near 2933 and 2892 cm-' appearing as shoulders on either side of the intense P-CH, band at 2905 cm-i. In view of the similarity in the spectrum of the Pt-(n-butyl) derivative in this work and that of a chemisorbed n-butyl group (2, 4) it is worth noting the form of the Pt-ethyl spectrum since a chemisorbed ethyl species has long been postulated as an intermediate during the hydrogenation of ethylene to ethane (2). The spectrum of the ethyl radical has never been unambiguously identified by infrared spectroscopic techniques and conversely, no spectrum similar to that of the Pt-ethyl group in this work has ever been observed (4). However, it is considered that chemisorbed ethyl groups would not necessarily be on the surface under the static reaction conditions generally employed for infrared studies of this reaction (4), but it could exist as a transient under dynamic reaction conditions. In an attempt to assign the major features in the Pt-ethyl spectrum, the partially deuterated compound c~s-(p~,p),p~(ch,cd,)~ was synthesized and its spectrum is shown in Fig. 3. Clearly, the strong bands at 2937 and 2848 cm-i in the spectrum of the Pt-CH,CH, compound have disappeared from the vch region and probably appear as the two bands at 2174 and 2107 cm-' in the vcd region (vch/vcd = in each case), and are attributable to the methyl group. The spectrum in the vcd region is very similar to that reported by Adams (8) for cis- (Me,P),Pt(CD,),. However, the remaining features in the vch region are a broad strong doublet at 2909 and 2893 cm-' and a weak doublet at 2841 and 2817 cm-', which can possibly be assigned respectively as the asymmetric and symmetric stretching modes of the CH, group with the doublet splitting arising from coupling between the two methylenes. This would correspond to the traditional Fox and Martin Assignment. However, it is also possible that the stronger pair of bands near 2900 cm-i might be the asymmetric and symmetric stretching frequencies of the methylene group (19, 21, 22) and that the low frequency bands might be overtone or combination bands. Similar features have been reported by Nolin and Jones (18) who compared the spectrum of diethyl ketone CH,CH,COCH,CH,, with that of diethyl ketone-d,, CD,CH,COCH,CD,. Again, further work is necessary to confirm the assignments for an ethyl group. In the present context, it will be necessary to synthesize the other partially deuterated derivative and this work will be presented as a later part of this series.

5 2196 CANADIAN JOURNAL OF CHEMISTRY. VOL. 48, 1970 The financial support of the National Research Council of Canada is gratefully acknowledged. The author also wishes to thank Dr. A. D. Westland for many helpful discussions, and Miss M. Flett for assistance with the experimental part of this work. 1. L. H. LITTLE. Infrared spectra of adsorbed molecules. Academic Press Inc., New York R. P. EISCHENS and W. A. PLISKIN. Advan. Catal. 10, 1 (1958). 3. N. SHEPPARD, and J. W. WARD. J. Catal. 15, 50 (1969). 4. B. A. MORROW and N. SHEPPARD. Proc. Roy. Soc. Ser. A. 311, 391 (1969). 5. B. A. MORROW and N. SHEPPARD. Proc. Roy. Soc. Ser. A. 311,414 (1969). 6. J. CHATT and B. L. SHAW. J. Chem. Soc. 705 (1959). 7. J. CHATT and B. L. SHAW. J. Chem. Soc (1959). 8. D. M. ADAMS. J. Chem. Soc (1962). 9. K. A. JENSEN. Z. Anorg. Chern. 229,237 (1936). E. D. GREAVES, R. BRUCE, and P. M. MAITLIS. Chem. Commun. 860 (1967). L. J. BELLAMY. Infrared spectra of complex molecules. 2nd ed. Methuen. London J. J. Fox and A. E. MARTIN. Proc. Rov. Soc. Ser. -. A. 167, 257 (1938); 175, 208 (1940). 13. R. N. JONES. Spectrochim. Acta, 9, 235 (1957). 14. J. ERKELENS and TH. J. LIEFKENS. J. Catal (1967). 15. D. B. POWELL and N. SHEPPARD. S~ectrochim. Acta, 21, 559 (1965). 16. D. E. CLEGG and J. R. HALL. Spectrochim. Acta, 21, 357 (1965). 17. J. H. S. GREEN. Spectrochim. Acta, 24A, 863 (1968). 18. B. NOLIN and R. N. JONES. J. Amer. Chem. Soc. 75, 5626 (1953). 19. F. WINTHER and D. 0. HUMMEL. Spectrochim. Acta, 25A, 425 (1969). 20. B. NOLIN and R. N. JONES. Can. J. Chem. 34, 1392 (1956). 21. N. SHEPPARD. Private communication. 22. S. C. GRAHAM. Spectrochirn. Acta, 26A, 345 (1970).