J . Am. Chem. SOC.1982, 104, 6005-6015 ligands are 14- and 19-membered rings. Trans monomers are formed in solution even with the dphp ligand (lomembered chelate ring), albeit in small amounts. The percentage of trans monomer increases from dphp to dpdod (15-membered chelate ring) in agreement with literature reports for the trans-bonded cobalt(II1) complexes with long-chain diammines. Ring contributions, AR, for the trans monomers decrease with increasing ring size and reach zero a t a ring size of 15 members-thus we conclude that AR in trans monomeric complexes is a measure of ring strain for these large-ring chelates. For ligands with short chelate backbones such as cis-[Pt(dpe)Cl,] the anomalous coordination chemical shift observed for this and other five-membered ring systems cannot be due to “ring strain’’, as the complex trans-[Pt(dpe)Cl,] has been isolated, which also possesses a n anomalous coordination chemical shift but which cannot be chelated.
6005
Finally, trans-bonded monomeric complexes appear to be unstable for 19-membered chelate rings and above. Registry No. cis-Pt(dpe)CI2,14647-25-7;cis-Pt(dph)C12,83095-87-8; cis-Pt(dphp)CI,, 83095-88-9; ci~-[Pt(dpo)Cl~]~, 83095-89-0; cis-Pt(dpn)CI2, 83095-90-3; cis-Pt(dpd)CI2, 83095-91-4; cis-Pt(dpu)CI,, 83095-92-5; cis-[Pt(dpdod)CI2l2,83 15 1-17- 1; cis-Pt(dphd)CI2,8309593-6; cis-[Pt(dpe)C12j2, 83095-94-7; cis-[Pt(dpn)CI2j2, 83095-95-8; cis-[Pt(dpd)CI2l2,83095-96-9; cis-[Pt(dpu)CI,],, 83095-97-0; cis-[Pt(dphd)CI2l2,83095-98-1; trans-[Pt(dpe)CI,],, 83095-84-5; trans-[Pt(dph)CI2],, 83095-86-7; trans-Pt(dphp)CI,, 83 149-21-7;trans-Pt(dp)Clz, 83095-99-2; trans-Pt(dpn)C12, 83149-22-8; trans-Pt(dpd)CI,, 83 149-23-9; tr~n~-Pt(dpu)C12, 83 149-24-0; trans-Pt(dpdod)C12, 8309600-8; trans-[Pt(dphd)C12]2, 83149-25-1; trans- [ Pt(dphp)C12]2, 8309601-9; trans-[Pt(dpo)C1212,83 149-26-2;trans-[Pt(dpn)C1212,83 149-27-3; trans- [Pt (dpd)C12]2, 83 149-28-4; trans- [Pt(dp~)C12]2, 83 149-29-5; trans-[Pt(dpdod)C12]2 , 83096-02-0; K2[ PtCI,], 10025-99-7; K [Pt(C2HJCI,], 12012-50-9.
Photochemistry of Metal Carbonyl Alkyls. Study of Thermal ,&Hydrogen Transfer in Photogenerated, 16-Valence-Electron Alkyldicarbonylcyclopentadienylmolybdenum and -tungsten Complexes Romas J. Kazlauskas and Mark S . Wrighton* Contribution from the Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139. Received March 30, 1982
Abstract: Near-UV irradiation of ($-C5R’5)M(C0)3R (R’ = H, Me; M = Mo, W; R = CH3, C2H5, n-pentyl) results in efficient > 0.1) dissociative loss of CO. If the photoreaction is carried out at 77 K, the 16-valence-electronspecies resulting from C O loss can be accumulated and characterized by infrared spectroscopy. Relative intensities of the two carbonyl stretching absorptions suggest that the structure of these species is similar to that of the 18-valence-electron (7$-C5H5)Fe(CO)2R,that is, a relaxed structure. Upon warming, ($-C5R’5)M(C0)2R reacts with CO or added PPh3 to regenerate starting material or form (q5-C5R’,)M(C0),(PPh3)R, respectively. In the case where R contains @-hydrogens,warming in the absence of ligand results in 8-hydrogen transfer to form tran~-(q~-C,R’~)M(CO)~(alkene)(H). For the R groups having 8-hydrogens, optical and infrared spectroscopy give evidence for a second ($-CsR’s)M(C0)2R species that is not completely mrdinatively unsaturated and is proposed to be the immediate precursor to 8-hydrogen transfer. This species can be formed by warming or irradiating samples of the photogenerated 16-valence-electron complex and is thought to have a weak M-(@-H) bond. This species does not react rapidly with C O or PPh, at 195 K as do the (q5-C5R’,)M(CO),CH3 complexes. The activation energy AG* for the conversion of all such 16-valence-electron species to an alkene-hydride is 10 f 2 kcal/mol, and the C2H5 and C2D5species react at the same rate. Irradiation of ($-CsH,)W(C0)3CD2CH3 gives a mixture of ~~~~S-($-C,H~)W(CO)~(C~H~D)(D) and ~~U~S-(~~-C,H~)W(CO)~(C~H~D,)(H) at a temperature where both are inert. The 8-hydrogen transfer is proposed to involve a preequilibrium between the 16-valence-electronspecies and a cis-alkene-hydride complex that isomerizes to the trans alkene-hydride in the rate-determining step. ($-C5R’5)M(C0)2(alkene)(H) reacts with PPh, at 298 K to form ($C,R’,)M(CO)2(PPh3)(aIkyl). For (~S-CsH5)W(CO)2(C5Hl,)(H) the reaction rate is proportional to PPh, concentration up to -0.1 M, after which little further increase in rate is observed. The kinetics for this reaction are treated as a preequilibrium between the 16-valence-electron dicarbonyl-alkyl and an 18-valence-electron alkene-hydride with the PPh, reacting with the dicarbonyl-alkyl. The rate constant for the conversion of the pentene-hydride to the dicarbonyl-pentyl is determined to be 1.7 X s-’ at 298 K. In the case of M = Mo, R = C2H5 the equilibrium constant for this interconversion is -2 and both species are observed in alkane solution at temperatures as high as 250 K.
Introduction Photochemistry can serve as a useful tool for the generation and characterization of reactive organometallic intermediates. Among the organometallic intermediates that can be formed photochemically are 17-valence-electron metal-centered radicals via metal-metal bond cleavage and 16-valence-electron coordinatively unsaturated metal carbonyls via ligand dissociation or reductive elimination.’ Generally, a n intermediate is a transient (1) Geoffroy, G. L.; Wrighton, M. S. ’Organometallic Photochemistry”; Academic Press: New York, 1979.
molecular entity formed in the course of a reaction and is not accumulated in appreciable amounts, since the activation energy for the formation is larger than the activation energy for the decomposition or further reaction. Photochemistry offers an alternative pathway for generation of certain thermally reactive intermediates having the advantage that at sufficiently low temperatures a n intermediate can be accumulated.2 This study (2) (a) Turner, J. J.; Burdett, J. K.; Perutz, R. N.; Poliakoff, M. Pure Appl. Chem. 1977, 49, 271. (b) Perutz, R. N.; Turner, J. J. J . Am. Chem. SOC.
1975,97,4791.
0002-7863/82/1504-6005$01.25/00 1982 American Chemical Society
6006 J. Am. Chem. SOC.,Vol. 104, No. 22, 1982
Kazlauskas and Wrighton
octane was further purified by stirring over concentrated H2S04/HN03 overnight, washing with H,O and NaOH solution, and then repeating the acid wash two more times. Isooctane was then stirred over 2% KMnO, in 10% H2S04overnight, washed, predried over MgSO,, and distilled from CaH, under N2. Finally, the distilled isooctane was passed down grade 1 alumina (neutral, Woelm). PPh, (Aldrich) was recrystallized L,M(alkyl) L,M-H alkene (1) three times from absolute EtOH. P(OPh), was used as received from reverse reaction, often termed olefin insertion, is a step in trancommercial sources. sition-metal-catalyzed reactions of olefins such as hydrogenation Literature procedures were used to synthesize ($-C,H,)Mand hydroformylation. In fact, @ elimination exists as either a (C0),CH3,6" (~5-CSHS)M(CO)3C2HS,6b (~s-C,H,)M(CO),(H),6cand [($-CSMe5) W (CO),] z.6d ($-C,H,) M(CO),(n-CSH was synthesized desirable or an undesirable reaction step in virtually all catalyzed by a method analogous to that for the methyl and ethyl derivatives. reactions involving metal alkyls. Not surprisingly, therefore, a ($-CsHs)M(C0)3Na in THF was treated with an excess of n-pentyl large amount of effort, both experimental4 and t h e ~ r e t i c a l has ,~ iodide and stirred overnight. THF was removed by vacuum and the gone into elucidating factors controlling this reaction. In general, yellow-brown residue extracted with hexane. The extract was filtered, @-eliminationoccurs on a single metal center and requires an open concentrated, and cooled to crystallize (~5-C,H5)M(CO)3(n-C5Hll), coordination site on the metal (preferably cis to the alkyl) and which was recrystallized an additional time from hexane. (q5-CsH,)Wa small M-C-C-H dihedral angle allowing access to the metal (CO),(n-C,H,,) is a yellow solid (mp 65 "C). The elemental analysis by the hydrogen to be transferred. Experimental work has genis satisfactory (Alfred Bernhart, West Germany). Anal. Calcd: C, 38.64; H, 3.99. Found: C, 38.50; H , 3.91. 'H NMR (CDCI,): 6 5.32 erally not allowed the direct observation of intermediates in the (s, 5 H, C5HS),1.0-1.5 (m, 11 H, n-C5Hll). (T$C~M~~)W(CO)~C,H, @-eliminationreaction because thermal extrusion of a ligand, eq was synthesized by Na/Hg reduction of [(T~-C,M~,) W(CO),], under 2, generally requires higher temperature than does the subsequent CO in T H F followed by addition of C2H,Br and worked up as for the L,M(alkyl) LW1M(alkyl) L (2) n-pentyl complex. (~5-CSMeS)W(CO),CH2CH3 is a yellow crystalline solid (mp 131-133 "c). 'H NMR (e&): 6 1.88 (t, 3 H, CHJ, 1.54 Lw1M(a1kyl) LPl(alkene)M-H (3) (s, 15 H , ring CH,), 0.98 6 (q, 2 H, CH,). I3C NMR (C6D6, 'H decoupled): 6 -9.9 6 (ring CH,), -6.0 (CH,), 20.5 (CH,), 102.5 (ring C), step, @-hydrogen transfer, eq 3, to produce an alkene hydride. 223.3, 232.8 (CO). (q5-C,Me5)W(CO),(H) was synthesized by addition Further, the low thermal activation energy associated with the of acetic acid to a THF solution of (qS-CSMeS)W(C0),Naprepared as final step, eq 4, often even precludes observation of the alkene above. (~5-C,Me,)W(CO),(H) was purified by recrystallization from hexanes (yellow crystals, mp 110-1 12 OC, some darkening) 'H NMR L L,,(alkene)M-H L,M-H alkene (4) (C6D6): 6 1.75 (s, 15 H, ring CH,), -6.52 (s, 1 H, hydride; JH-iuW= 19.4 Hz). (q5-C5HS)W(CO)2(PPh3)CH2CH3 was synthesized by reaction hydride. Our earlier communication3 has established that both ~ , ) N by ~ ~the ~ reduction of C2HSBrwith ( T J ~ - C ~ H , ) W ( C O ) ~ ( P Pformed coordinatively unsaturated alkyls and alkene hydrides can be of (~5-CSHs)W(C0)2(PPh3)C1 with Na/Hg in THF followed by the observed by near-UV irradiation of appropriate metal carbonyl workup above. The compound forms yellow crystals (mp 180 OC, dealkyl complexes at low temperature. Photoinduced C O dissociation composes with effervescence) after crystallization from CH2CI2. 'H from (~5-C5H5)W(C0)3(n-C5Hli) results in a 16-valence-electron NMR (CDCI,): 6 7.3 (m, 15 H, phenyl), 4.70 (d, J = 1 Hz, 5 H, C,H,), ($-C5H5)W(CO),(n-C5Hi1),which can be characterized by its 1.60 (s, br, 5 H, CH,CH,). spectra at low temperatures and by its reaction chemistry at higher (~5-CSHS)W(CO),C2Ds was synthesized as above with C2DSBr temperatures. Warming this species results in @-hydrogentransfer (Merck, 99 atom % D). When compared to that of ($-C5Hs)WThis paper reports forming ~~~~-(Q~-C~H~)W(CO)~(C~H~~)(H). (C0),C2HS, the IR spectrum shows changes consistent with replacement our study of the kinetics and mechanism of this and related of C2H5 with C2D5 Y C ~ H uc,,,(calcd), ~ , uc,,,(found): 2978, 2174, 2225; 2960,2161,2205; 2930,2139,2142; 2876,2099,2170, 1460, 1066, 1070; systems. 1380, 1007, 935, cm-I. Isotopic purity was established as at least 99.3 Experimental Section atom % D by 'H NMR techniques corresponding to -2% of molecules having a light hydrogen in the @ position. Instruments. UV-vis spectra were recorded with a Cary 17 spectro($-CSHS)W(CO)3CH2CD3was synthesized as above with CD,CH21 photometer; infrared spectra were recorded with a Perkin-Elmer 180 (Stohler, 99 atom % D). The 'H NMR spectrum does not straightforgrating or Nicolet 7 199 Fourier transform spectrometer; NMR spectra wardly lead to determination of isotopic purity since the a- and P-hywere recorded with a Varian T-60, a JEOL FX90Q Fourier transform, drogens are not separated even at 250 MHz. The isotopic purity of the or Bruker 250-MHz Fourier transform spectrometer. Gas-liquid chrostarting material, CD,CH21, was established as at least 98 atom % D by matographic separations were accomplished using a Varian 2440 gas 'H NMR spectroscopy corresponding to -8% of the molecules having chromatograph with flame ionization detectors interfaced with a Hewa light hydrogen in the @ position. The gated "CI'H) NMR spectrum lett-Packard Model 33808 electronic integrator. Pentane and linear establishes that H and D do not scramble between a and @ positions upon pentenes were separated with a 0.3 cm X 9.2 m column of 20% propylene attachment to the tungsten center since the "C-'H couplings are obcarbonate on Chromasorb P at 25 "C with hexane as an internal standserved only for the a-carbon, and correspondingly I3C-,H couplings are ard. Generally, all manipulations of the organometallic complexes were observed only for the @-carbon: 6 -17.5 (t, a-carbon, 'Jc+,= 130 Hz), done under N, using a Vacuum Atmospheres drybox or under Ar using = 18 -Hz). , 'H NMR (toluene-d,) 6 4.50 (s, 5 19.5 (m, @-carbon,'.Ic conventional Schlenk line techniques unless otherwise noted. However, H, CSH,), 1.50 (s, 2 H, CHZCD,). the compounds ($-CSR'5)W(C0)3R (R' = H, CH,; R = CH,, C2HS, (75-C5HS) W(C0),CD2CH3 was synthesized as above with CH3CD21 n-pentyl) are air stable in the dark and were stored and transferred in (Stohler, 99 atom % D). Proof of structure was established by the ')C air. Melting points were determined in sealed capillaries under N, on NMR spectrum as for the complex labeled on the @-carbon. a Mel-Temp melting point apparatus and are uncorrected. Concentrated solutions of (~5-C,H,)W(CO)2(C2H,)(H) for "C NMR Chemicals. Hexane, toluene, and CHzC12were reagent grade and spectroscopy were prepared by irradiation of (qs-CSHs)W(CO)3CzHS in freshly distilled from CaH2 under N2. Tetrahydrofuran (THF) was THF-d, at -78 "C. At these temperatures the @-hydrogen-transferrefreshly distilled from Na/benzophenone under Nz. Spectroquality isoaction is slow even in an alkane solvent (tlI2.is on the order of 10 s), and is formed, which could slow we believe (~5-CSHS)W(CO)2(THF)C2H5 (3) (a) Kazlauskas, R. J.; Wrighton, M. S. J . Am. Chem. SOC.1980,102, the (3-hydrogen-transfer process. Warming to room temperature results 1727. (b) A preliminary report describing the corresponding ethyl complex in the formation of (~S-C5H,)W(CO)z(C,H,)(H). 'H NMR (THF-d,): has appeared: Alt, H. F.; Eichner, M. E. Angew. Chem., In?.Ed. Engl. 1982, 6 4.42 (s, 5 H, C,H,), -5.5 (s, 1 H, -H) (bound ethylene obscured by 21, 78. alkyl resonance of starting material at 6 1.54). I3C NMR (THF-d,, -5 (4) (a) Whitesides, G. M.; Gaasch, J. F.; Stedronsky, E. R. J . Am. Chem. "C): 6 217.5 (CO), 90.9 (C,H,), 21.0 (ethylene). The spectrum is Soc. 1972, 94, 5258. McDermott, J. X.; White, J. F.; Whitesides, G. M. Ibid. unchanged upon cooling to -50 OC. More dilute solutions of ($1976, 98,6521. (b) Reger, D. L.; Culbertson, E. C. Ibid. 1976,98, 2789. (c) Evans, J.; Schwartz, J.; Urquhart, P. W. J. Orgummet. Chem. 1974,82, C37. (d) Ikariya, T.; Yamamoto, A. Ibid.1976,120, 257. A recent report on the photoinduced j3-hydrogen-transfer reaction of Th and U alkyls has appeared: (6) (a) Piper, T. S.; Wilkinson, G. J . Inorg. Nucl. Chem. 1956,3, 104. (b) Bruno, J. W.; Kalina, D. G.; Mintz, E. A.; Marks, T. J. J . Am. Chem. SOC. Davison, A.; McCleverty, J. A.; Wilkinson, G. J . Chem. SOC.1963, 1133. (c) 1982, 104, 1860. Fischer, E. 0.Inorg. Synth. 1963, 7, 136. (d) King, R. B.; Iqbal, M. 2.;King, (5) Thorn, D. L.; Hoffmann, R. J . Am. Chem. SOC.1978, 100, 2079. A. D., In J . Organomef.Chem. 1979, 171, 53. (e) George, T. A,; Turnipseed, C. D. Inorg. Chem. 1973, 12, 384. Lauher, J. W.; Hoffmann, R. Ibid. 1976, 98, 1729. represents an application of this and other techniques to the characterization of intermediates in the reaction of metal carbonyl alkyls. A preliminary account of this work has a ~ p e a r e d . ~ @ elimination of an alkene, eq 1, from a metal alkyl or the
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+
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+
+
-+
+
Photochemistry of Metal Carbonyl Alkyls C,H,)W(CO),(C,H,)(H) were generated by flash photolysis (Xenon flash tubes, 5000 V, 2000 J) of ($-CSHS)W(C0)3C2HSin isooctane or toluene. Irradiations. Two General Electric Blacklight bulbs (355 f 20 nm, -2 X lod einstein/min) were used for irradiation unless otherwise noted. Quantum yields at 366 nm were measured in a merry-go-r~und~~ using lo-, M (q5-C,R’5)M(C0)3Rwith the appropriate ligand concentration. Three-milliliter samples in 13 X 100 mm test tubes were freeze-pumpthaw degassed ( species thermally react to form tran~-($-C~R’~)M(CO)~(alkll kene)(H). Such reaction can be detected a t temperatures less (75-C5R’5)M ( CO) ,(alkene) ( H ) (45-C5R’5)M ( CO)2R ki structure I1 than 195 K, but the extent of conversion depends on whether M = Mo or W: reaction goes to completion for M = W but only (12) to -65% for M = Mo. k-IO The fact that the alkene-hydride complexes are the trans (t15-CsR’s)M(C0)2R (oS-CsR’s)M(C0)2R (1 3) isomers raises the question of whether the yellow species that we structure I1 structure I have assigned as the dicarbonyl-alkyl of structure I1 is in fact k14 the cis alkene hydride. Both the UV-vis and the I R spectra of ($-C5R’s)M(CO),R L (qS-CSR’5)M(C0)2LR (14) structure I the yellow species are inconsistent with it being the cis-alkenehydride complex. The IR spectra of cis- and t r a n ~ - ( $ - C ~ R ’ ~ ) M - k10k7/k-10the rate of formation of (oS-C5R’5)M(C0)2LRgives (CO),LR complexes in the C O stretching region are very simikI2as the intercept of the plot of (initial rate)-’ vs. [LI-I, if the lar,10J1-2’whereas structure I1 and the trans-alkene-hydride I1 + I interconversion is assumed to be fast. Figure 4 shows the complexes have very different spectra in this region. Also, the effect of varying PPh, concentration on the initial ratio of ($-400-nm absorption for I1 is too low in energy to be associated C5H5)W(C0)2(PPh3)C2H5 and (?5-C5H5)W(Co>2(C2H4>(H) with a cis alkene hydride, since the trans alkene hydride (Figure formed from irradiation of ($-CsH5) W ( C 0 ) 3 C 2 H 5in isooctane 2b) absorbs well above this energy. Finally, irradiation of the solution at 298 K. These data accord well with the data for the Mo-C2HS complex a t 77 K followed by warmup to 223 K only thermal reaction of ( Q ~ - C ~ H ~ ) W ( C O1-pentene)(H) ),( with PPh3 yields one hydride resonance, 6 -5.2, for the trans alkene hydride via eq 12-14 in that the PPh,, at a given concentration, suppresses despite the presence of species of structure 11. the 8-hydrogen-transfer products by an amount near that expected To summarize the thermal chemistry of ($-C5R’s)M(C0)2R on the basis of the intermediacy of a common 16-valence-electron so far, we note that (i) back-reaction with C O is apparently facile species I in the thermal reaction of the alkene hydride with L and for dicarbonyl alkyls of structure I, (ii) for the C2H5 and n-C5HII in the photochemical substitution of the tricarbonyl. species conversion of structure I to I1 occurs in competition with The ability to photochemically prepare the t r a n ~ - ( q ~ - C ~ R ’ ~ ) M the back-reaction with CO, and (iii) the structure I1 species appear (CO)2(alkene)(H) complexes illustrates another example of a to thermally react to give trans-(~S-C5R’5)M(CO)2R(alkene)(H). thermally inaccessible species. Thermal chemistry of (a5To further substantiate the reactivity differences associated with C5R’5)M(C0)3Rrequires temperatures well above 298 K to obtain structures I and 11, we have carried out photosubstitution of 0.7 a significant rate of reaction, and the product from the metal m M (q5-C5R’5)M(C0)3Rin 195 K isooctane solutions containing carbonyl is mainly ($-C5R’5)zM2(C0)6, not an alkene hydride. 1 m M PPh3. The procedure is to irradiate the sample in an The temperatures necessary to promote dissociative loss of C O infrared cell with an intense UV source for a short period and from ($-C5R’5)M(C0)3R are apparently sufficiently great to lead then to rapidly analyze to determine the thermal rate of formation to good rates for further reaction of the alkene-hydride or to other of (~5-C5R’s)M(CO)2(PPh3)R and (~5-C5R’S)M(CO)2(alkene)(H) thermal processes of ($-C,R’,)M(CO),R. tran~-(q~-C~R’~)(where possible) from reaction of the photogenerated ( q S Mo(CO)2(alkene)(H) is obviously quite thermally labile (deC5R’5)M(C0)2R. For R = C H , we find that (q5-C5R’5)Mcomposes above -250 K), and the W complexes are quite labile (C0)2(PPh3)CH3is formed immediately ( < l o s). For R = C2H5 above 350 K. The thermal chemistry at high temperatures requires we find that a certain amount of (I~-C~R’~)M(CO)~(PP~~)C~H~ detailed study in order to comment further on the photochemistry is formed instantly, but this is accompanied by the presence of a t low temperature vs. thermal chemistry. (q5-C5R’5)M(C0)2C2H5that slowly gives ($-C5R’s)M(CO)2Mechanisms of Formation of trans -(q5-C5R’5)M(CO),(a1(C2H4)(H)but no additional PPh3 substitution product. The slow kene)(H). Formation of the alkene-hydride complexes from
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co
+
(19) (a) Mills 111, W. C.; Wrighton, M. S. J . Am. Chem. SOC.1979, 101, 5832. (b) Kazlauskas, R. J.; Robbins, J. L.; Wrighton, M. S., to be submitted for publication. (20) Kinney, R. J.; Jones, W. D.; Bergman, R. G. J. Am. Chem. Soc. 1978, 100, 635, 7902 and references therein.
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irradiation of the tricarbonyl precursors involves first the loss of C O to form the coordinatively unsaturated intermediate. The absorption of light produces an electronically excited state that undergoes facile loss of C O at temperatures as low as 77 K, whereas, significant thermal reaction of the tricarbonyl species
I
I
I
I
. .
/---
J
/
/
/
--
---
, / '
12 0-
. .-------1
/
-3000
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II
O
1
I
I
1
I / [PPh3]
Z O L 100 200 300 400 500 T I M E , SEC
'
, M-'
Figure 3. Kinetic analysis for the reaction of PPh, with (q5-CsHs)W(CO)2(pentene)(H).The inset shows the natural logarithm of absorbance changes
as a function of time in toluene containing 0.42 M PPh,. The slope corresponds to a for initial formation of (q5-CSHS)W(C0)2(PPh,)(n-CsH,l) pseudo-first-order rate constant of 1.4 X ~ O - ' S - ~ . The dashed curve represents a plot of initial rate vs. PPh, concentration (use top and left scale), showing a limiting rate of 1.7 X lo-) M. The open symbols represent data obtained in isooctane, while the darkened symbols represent data obtained with toluene as a solvent. The smooth curve represents a plot of (initial rate)-' vs. [PPh,]-' (bottom and right scales). Scheme I. Possible Mechanisms for Formation of Alkene Hydride from Dicarbonyl Alkyla MECHANISM
I
D I R E C T FORMATION OF ALKENE HYDRIDE
MECHANISM 2
P - H Y D R O G E N TRANSFER IS RATE DETERMINING
MECHANISM 3 .
c / s - m n s I S O M E R I Z A T I O N I S RATE D E T E R M I N I N G
P h 3 l
Figure 4. Relative rates of P-hydrogen transfer and capture by PPh,.
The ratio (&lS%) of (qs-CsHs)W(CO)2(PPh,)R and (qs-CsHs)W(C0)2(alkene)(H) (or D) is plotted as a function of PPh, concentration from 0 to 16 mM. Measurements were done in isooctane solution at 300 K with 1 mM starting complex irradiated with a single flash from a GTE Sylvania flashcube. Spectra were obtained within 60 s of reaction to minimize any contribution from secondary thermal reaction of alkene hydride with PPh3.
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requires a temperature in excess of 298 K. Thus, it is clear that irradiation allows the @-hydrogentransfer to occur a t low temIn order to measure the rate of reaction of the dicarbonyl-alkyl perature because C O loss is not rate limiting. The issue we now complexes, we have used infrared spectroscopy to monitor the address is whether the rate of formation of t r a n ~ - ( q ~ - C ~ R ' ~ ) M -metal carbonyl species. Figure 5 shows spectral changes following (C0)2(alkene)(H) from the dicarbonylalkyl, k7, is in fact limited a t 195 K in isothe irradiation of (q5-CSH5)W(CO),(n-C5Hll) by the rate of the @-hydrogen-transferprocesses. octane solution. The irradiation initially produces the 16-vaWe consider the three mechanistic possibilities as represented lence-electron species (structure 11) that reacts slowly in a subin Scheme I. In the first, where the @-hydrogentransfer is rate sequent thermal step to form the trans alkene hydride. The is generated directly limiting, trans-(q5-CsR's)M(C0)2(alkene)(H) rapid-scan F T I R method allows simultaneous monitoring of from the dicarbonyl-alkyl, In the second, a slow @-hydrogen product and starting material with molecular specificity. The inset transfer to form a cis-alkene-hydride complex is followed by a shows that good kinetics can be obtained. fast cis -,trans isomerization. In the third mechanism, there is Figure 6 shows a summary of rate data collected as a function a fast interconversion between the dicarbonyl-alkyl and a cis of temperature for a variety of complexes with either the FT I R alkene-hydride, and cis trans isomerization of the alkenerapid-scan or conventional single-wavelength monitoring vs. time. hydride is rate limiting. We have measured the rate of reaction In all cases the rate of reaction is directly proportional to conof the dicarbonyl alkyl for a variety of systems and conditions. centration of the dicarbonyl alkyl. It should be realized that the The data to be elaborated below are best interpreted as supporting rate data have relatively large experimental errors. Any given the third mechanism, where cis trans isomerization is the determination has an error in the temperature of 1 2 K and in rate-determining step in the formation of the trans alkene hydride. the observed rate of f20%. Table I11 summarizes the activation
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J . Am. Chem. Soc., Vol. 104, No. 22, 1982 6013
Photochemistry of Metal Carbonyl Alkyls
t
2
109 Thermal Reoction
\ 4\
Dicarbonyl -pentyl
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I
/Tr~corbonyl
2100
, ,
'Eo25>?,
-0 6 0 -Pentyl
2000
,
~-
2
j
4 6 8 TIME l s e c l
-8-
I
1900
I800
- 9-
WAVENUMBERS
Figure 5. Infrared spectral changes accompanying the thermal reaction of ($-C5HS)W(C0)2(n-C5Hll)as shown by difference infrared spectra. ($-C5H5)W(CO)3(n-C5Hll)in isooctane (- 1 mM) at approximately 195 K is irradiated briefly with a xenon arc lamp. The spectrum immediately after irradiation is the trace marked t = 0 and shows large negative peaks corresponding to the disappearance of starting material. Positive peaks are seen at 1948 and 1862 cm-I corresponding to the and also at 1973 and 1900 cm-' formation of (T$-C,H~)W(CO)~(C~H~,) The successive spectra corresponding to (q5-C5H5)W(CO)2(pentene)(H). show in real time the thermal conversion of the dicarbonyl pentyl (structure 11) to the alkene hydride. The inset shows a plot of the natural logarithm of an absorbance ratio vs. time for the appearance of the alkene hydride at 1900 cm-' ( X ) and the disappearance of the dicarbonyl alkyl at 1950 cm-' (0). Both result in the same line whose slope corresponds to the first-order rate constant for reaction.
-10k
c
-
-3
-4L
Table 111. Kinetic Parameters for Conversion of (v5-C, R', )M(CO), R to n'~rans-(77~-C, R',)M(CO), (alkene)(H) AH*, kcal/ kcav mol mol AS*,
A@,
(i
starting compd" H, )W(CO), C, H, (TI~-C~H,)W(CO),C,D~
(i
11 11 9 8 (~5-C,H,)W(CO),(~-C,Hll) 11 11 (v5-C,Me,)W(CO),C,H, 10 10 (q5-C,H,)Mo(CO),C,H, (7 ,-C,
eu
25%) 25%) (i25%) 11 11
-8 -8 -23 -10 -18
k,, s-' (195 K,
k207C) (t,,,, s)
\ \
A 5.0 -
0.06 (11) 0.1 (7)
0.02 (30) 0.04 (17) 0.004(170)
a All compounds indicated have been photogenerated by 355-nm irradiation of the parent tricarbonyl in alkane media in an infrared cell at low temperature. Under the conditions of the thermal reactions the dicarbonyl alkyls have structure 11.
parameters and k , and t 1 / 2a t 195 K for the systems studied. It is noteworthy that all of the complexes have approximately the same activation parameters. Further, the dicarbonyl-C2H5 and -C2D5 complexes react at a similar rate. The similarity in rate for the dicarbonyl-C2H5 and
