Organometallics 1995, 14, 3717-3723
3717
Mechanisms of Pyrolysis of Tricarbonylcyclopentadienylmanganeseand Tricarbonyl(methylcyclopentadieny1)manganese Douglas K. Russell,*s' Iain M. T. Davidson, Andrew M. Ellis, Graham P. Mills, Mark Pennington,t Ian M. Povey, J. Barrie Raynor, Sinan Saydam, and Andrew D. Workman Department of Chemistry, University of Leicester, University Road, Leicester LE1 7RH, U.K. Received December 15, 1994@ The mechanisms of the gas-phase pyrolysis of tricarbonylcyclopentadienylmanganese(1) and tricarbonyl(methylcyclopentadieny1)manganese (2) have been investigated using a combination of IR laser-powered homogeneous pyrolysis, stirred flow reactor kinetic measurements, and matrix isolation ESR spectroscopy. The observations are consistent with initial stepwise loss of CO, followed ultimately by release of 'C5H5 (for 1) or 'C5H4CH3 (for 2); the hydrocarbon radicals then abstract hydrogen from unreacted starting material. For 1, the resultant *C5H4Mnunit undergoes intramolecular rearrangement, yielding ethyne and a second species tentatively identified as a manganese carbide. For 2, the more rapid abstraction from the ring CH3 group results in formation of the fulvene CHzCsH4 followed by isomerization to benzene. These processes lead to heavy carbon contamination of deposited Mn films unless an alternative source of H atoms for abstraction by cyclopentadienyl radicals is present.
Introduction Tricarbonylcyclopentadienylmanganese(cymantrene, (q5-C5H5)Mn(C0)3,1) and its methyl derivative (methylcymantrene, (q5-CH3C5H4)Mn(C0)3,2) serve as archetypal $-bonded compounds with 1 being among the earliest known and best characterized ?-bonded systems.l They are both thermally stable, relatively volatile, low-melting solids or liquids. The physical properties and chemistry of 1 and its derivatives have been extensively investigated,2with major emphasis on the bonding and reactions of the cyclopentadienyl ring system. More recently, however, there has been some interest in the pyrolytic behavior of these compounds; 2 has found some application as an antiknock agent in g a ~ o l i n e , and ~ , ~ both 1 and 2 have been proposed as sources for magnetic dopants in processes such as metal organic chemical vapor deposition (MOCVD), metal organic vapor phase epitaxy (MOVPE),and molecular beam epitaxy (MBE).4 Little is known about the mechanistic details of the pyrolysis of 1 and 2. A recent quadrupole mass spectrometry study of the MOVPE of 2 has suggested that decomposition proceeds via successive loss of CO, followed by loss of 'C5H4CH3, and an activation energy of 236 k J mol-l for the process was
determined.5 It is well-known from our own work on both main-group6and transition-metal complexes7that the investigation of the pyrolysis of organometallic compounds is fraught with difficulties, arising largely from the competition between homogeneous and surface reaction, and that observations must be interpreted with considerable caution. On the other hand, we have also shown that the judicious use of a range of techniques, particularly that of IR laser-powered homogeneous pyrolysis (IR LPHP), can disentangle these competing effects and provide reliable results.8 In the present work, we report the application of these techniques t o the pyrolysis of 1 and 2, together with a mechanism consistent with the observations; a preliminary account of these investigations, together with those of related compounds, has appeared e l ~ e w h e r e . ~
Experimental Section The experimental methods employed in the present study have been described in detail elsewhere, and hence only brief summaries of the more significant aspects are provided here. Except where noted below, analytical investigations (FTIR, NMR, GC-MS, and ESR) were conducted using commercial instrumentation in conjunction with comparison with authentic samples.
Stirred Flow Reactor (SFR) Kinetic Measurements. The SFR consists of a spherical quartz vessel of volume 10
' Present address: Department of Chemistry, University of Auckland, Private Bag 92019, Auckland, New Zealand. Present address: School of Environmental Sciences, Nene College, Moulton Park, Northampton NN2 7AL, U.K. @Abstractpublished in Aduance ACS Abstracts, June 15, 1995. (1)Piper, T. S.; Cotton, F. A.; Wilkinson, G. J . Inorg. Nucl. Chem. 1966, 1, 165. (2) Treichel, P. M. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, U.K., 1982; Chapter 29. (3)Dictionary of Organometallic Componds; Chapman and Hall: London, 1984; Volumel. (4) Pain, G. N.; Christiansz, G. I.; Dickson, R. S.; Deacon, G. B.; West, B. 0.; McGregor, K.; Rowe, R. S. Polyhedron 1990, 9, 921.
(5) Sang, W.; Durose, K.; Brinkman, A. W.; Woods, J. J. Cryst. Growth 1991, 113, 1. (6)Grady, A. S.; Mapplebeck, A. L.; Russell, D. K.; Taylorson, M. G. J. Chem. SOC.,Chem. Commun. 1990,929. Grady, A. S.; Markwell, R. D.; Russell, D. K. J. Chem. SOC.,Chem. Commun. 1991,14. Linney, R. E.; Russell, D. K. J. Mater. Chem. 1993, 3, 587. (7) Davidson, I. M. T.; Pennington, M.; Russell, D. K.; Saydam, S. Unpublished work. (8) Atiya, G. A.; Grady, A. S.; Jackson, S. A.; Parker, N.; Russell, D. K. J. Organomet. Chem. 1989, 378, 307. (9) Davidson, 1. M. T.; Ellis, A. M.; Mills, G. P.; Pennington, M.; Povey, I. M.; Raynor, J . B.; Russell, D. K.; Saydam, S.; Workman, A. D. J . Mater. Chem. 1994, 4 , 13.
0276-733319512314-3717$09.00/0 0 1995 American Chemical Society
3718 Organometallics, Vol. 14,No. 8, 1995 cm3, at the center of which is either a smaller perforated bulb or a simple jet inlet, which provides rapid mixing and thermal equilibration of the reactant. The reactor is housed in a conventional furnace capable of providing temperatures up to the softening temperature of the quartz. Rather than a continuous flow, reactant diluted in a carrier gas (N2 or He) is admitted in a pulse or batch mode; this method confers both technical and economic advantages. Reaction in the vessel competes with the sweeping out of reagents and products, so that a controllable proportion of conversion may be achieved. Unreacted starting material and products may be analyzed directly via GC-MS or accumulated in cold traps for subsequent investigation. The output of the flame ionization or thermal conductivity detector of the GC may also be fed directly into a data-capture station for kinetic or other analysis. This kind of technique has been used with considerable success in the investigation of pyrolysis mechanisms of organosilicon compounds such as MesSiSiMes.lo Infrared Laser-Powered Homogeneous Pyrolysis. The majority of qualitative and analytical investigations were carried out using the method of IR LPHP. The experimental details and advantages of this technique have been described in an extensive review by Russel1,ll and its use in the investigation of a wide range of Al and Ga organometallic systems has been reported.I2 Briefly, static pyrolysis is conducted in a cylindrical Pyrex cell (length 10 cm, diameter 3.8 cm) fitted with ZnSe windows. The correct choice of window material is central t o the success of the technique; in comparison with the cheaper alkali-metal halides, ZnSe has a higher optical transmission at the critical wavelength of 10 pm and greater mechanical strength. Most importantly, however, it is not hygroscopic, a point of crucial importance in the study of highly moisture sensitive materials. The cell is filled with up to 10 Torr of the vapor of the material under study, together with 10 Torr of SF6. For compounds of moderate volatility, such as 1 and 2, liquid or solid can be condensed into the cell; this does not alter the basic features of the IR LPHP method but may introduce volatilization as a possible rate-limiting process. The contents of the cell are exposed to the output of a free-running continuous-wave CO2 IR laser at power levels of a few watts. The laser radiation is strongly absorbed by the SFe and is rapidly converted into heat via very efficient inter- and intramolecular relaxation. The low thermal conductivity of the SFs ensures that this process results in a highly inhomogeneous temperature profile, in which the center of the cell may be heated up to 1500 K (above which the SF6 itself decomposes), but where the cell walls remain at close to room temperature. The technique has a number of advantages. The first is that pyrolysis is initiated unambiguously in the gas phase, eliminating entirely the complications frequently introduced by competing surface reaction. The second is that the primary products of pyrolysis are ejected into cool regions of the cell, where they are not subjected t o further reaction. In favorable cases, these products may be collected as less volatile liquids (or even solids) and analyzed at leisure. On the other hand, the temperature of the pyrolysis is neither well-defined nor easy t o measure, so that comparisons with conventional methods of pyrolysis must be made with care. Such indications as are available through studies of systems with wellknown kinetic parameters ( e g . CH3C02CH3) suggest that the overall cell reaction is dominated by that at the maximum temperature. In our laboratory, reaction is monitored in the first instance by in situ FTIR spectroscopy; further confirmation of the identity of products, and quantitative measurements of relative yields, can be accomplished by selative condensation into external vessels followed by NMR (usually (10)Davidson, I. M. T.; Howard, A. V. J . Chem. Soc., F a r ~ d a yTrans. 1 1975, 71, 69.
(11)Russell, D.K.Chem. SOC.Reu. 1990, 19,407. (12)Russell, D. K. Coord. Chem. Rev. 1992, 112, 131.
Russell et al. in toluene-ds as solvent), GC-MS, or elemental analysis, in conjunction with comparison with authentic samples. Matrix Isolation ESR Spectroscopy. In order to identify free radicals produced in pyrolysis, reagents were pumped through a conventional resistively heated hot-wall quartz tube by means of mercury diffusion and rotary pumps at pressures much less than 1Torr. Samples for study were contained in a vessel providing for variation of source temperature and flow rate or pressure (viaa needle valve). Pyrolysis products were condensed onto a cold finger cooled to 77 K by liquid nitrogen; the whole cold finger assembly was removable for the examination of ESR or other spectra. ESR spectra were stored digitally on a computer for subsequent manipulation. Radicals could be trapped in a matrix of unreacted starting material or of a suitable inert host material such as adamantane; the latter usually provided more easily interpretable isotropic ESR spectra but did add another variable to the experimental conditions. Matrices sufficient to provide well-resolved ESR spectra could be condensed in 5-30 min, depending on the chemical system, flow rates, and pressures. Many of the ESR spectra observed arose from two or more species; these could be distinguished by judicious variation of the experimental conditions (temperature or pressure), followed by computer subtraction. The technique has been used successfully in the detection of many free radicals in organometallic pyrolyses, such as alkyl radicals from R3M (R = Me, Et, 'Pr, lBu, tBu; M = Al, Ga, In).13 Chemicals. 1 was purchased from Aldrich Chemical Co. and 2 from Johnson Matthey. CD3Mn(C0)5 was synthesized by modification of the literature method for C H ~ M ~ ( C O ) E . ~ ~ All compounds were purified by repeated trap-to-trap distillation before use and their purities checked by NMR or GCMS.
Results
SFR Kinetic Measurements. While 2 is a liquid a t room temperature (boiling point 102 "C at 10 Torr), 1 is a moderately volatile solid.3 For this reason, SFR studies (which require injection of a liquid sample) were carried out for 2 only at temperatures between 250 and 450 "C. Over this range, the pyrolysis produced free CO and 1-or 2-methylcyclopentadieneas the major gasphase products detectable in the GC-MS system, with minor amounts of CH4 and C5Hs at higher temperatures. Measurable reaction rates were obtained between 284 and 361 "C. Formation of methylcyclopentadiene (measured by a flame ionization detector) and of CO (measured by a thermal conductivity meter) over this range followed first-order behavior (order 1.0 k 0.11, as shown in Figure 1 for the formation of methylcyclopentadiene. In the lower part of the temperature range (284-342 "e>, the overall Arrhenius parameters were logA = 16.1 f 1.0 and E k J mol-l = 210 & 11 for the formation of methylcyclopentadiene and log A = 15.4 f 1.3 and E k J mol-l = 195 f 14 for the formation of CO. The Arrhenius plots curved upward at higher temperature, indicating mechanistic complexity. The Arrhenius plot for the formation of methylcyclopentadiene a t lower temperature is shown in Figure 2. IR LPHP Observations. IR laser pyrolysis of 1 or 2 alone was carried out at laser powers ranging from 1.5 to 2.0 W. As described above, temperatures in the laser pyrolysis process are not well-defined; however, independent measurements of the rate of decomposition of CH3COzCH3 indicate that these powers generate ~
(13)Mills, G. P.; Raynor, J. B.; Russell, D. K.; Workman, A. D. Unpublished work. (14) King, R. B. Organomet. Chem. Synth. 1965, 1, 147.
Pyrolysis of Mn Cyclopentadienyl Complexes 12
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__
---
Organometallics, Vol. 14, No. 8, 1995 3719 - --
_-
/'
9
,
-12.0
-12.5
13.0
13.5
14.0
log ( r e a c t a n t )
Figure 1. Order plot for the pyrolysis of q5-MeC5H4Mn(C0)3 (2) at 340 "C ( 0 )and 290 "C (v).
Figure 2. Arrhenius plot for the formation of CHsC5H5 from (r5-MeC5H4)Mn(C0)3 (2). maximum temperatures of between 250 and 350 O C . 1 5 For 1,the postpyrolysis FTIR spectrum (see Figure 3) revealed substantial quantities of ethyne (sharp Qbranch at 729 cm-l) in addition to the gas-phase products expected on the basis of the SFR results for 2, i..e., free CO (structured P- and R-branches near 2200 cm-') and cyclopentadiene (sharp &-branches at 663 and 807 cm-l). These identifications were confirmed by GC-MS and lH NMR spectroscopy; the latter additionally yielded a molar ratio of close to 2:l for C2H2 and C5H6. For 2, the results were more complex. Although free CO and C2H2 were again readily identified, assignment of other features in the FTIR spectrum proved less straightforward; in particular, there was little sign of the expected propyne. However, the lH NMR spectrum clearly confirmed the production of both 1-and 2-methylcyclopentadiene (identified from the report of Mstislavsky et a1.16) in equimolar amounts as well as much smaller amounts of C2H2, and also substantial quantities of benzene in the majority of experiments. Unfortunately, the resonances of both the methyl and alkynyl protons in CHsC=CH are coincidentally overlapped by (15)Atiya, G. A. Ph.D. Thesis, University of Leicester, 1991. (161Mstislavsky, V. I.; Korenevsky, V. A,; Sergeyev, N. M.; Solkan, V. N. Org. Mugn. Reson. 1976,8, 368.
2000
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1600
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Wa"*""mbeiiCm-'
Figure 3. FTIR spectra of a mixture of 10 Torr of SFs and the vapor pressure of q5-C5H5Mn(C0)3(1): (A) before pyrolysis; (B)after exposure to 1.5 W COz IR laser power for 5 min. Features identified arise (1)from C5&,, and (2) from C2H2; unidentified features arise from starting material or SF6. those of C2H2, and lH NMR cannot, therefore, be used to determine the proportion (if any) of propyne. In order to provide further insight into the mechanism, f was copyrolyzed with perdeuteriated pentacarbonylmethylmanganese (CD3Mn(C0)5,3). It is known from our previous work17that pyrolysis of 3 alone occurs at a temperature significantly lower (laser powers of 0.5 W) than that required for 1 or 2, that the pyrolysis is initiated by homolysis of the CD3-Mn bond, and that reaction proceeds via a free-radical process; the only significant hydrocarbon product is CD4. Laser copyrolysis of a 1:l mixture of 1 and 3 at a power of 0.6 W (i..e.,insufficient to initiate reaction of 1 alone) resulted in CD3H (sharp &-branches at 1036 and 2991 cm-l) and CzH2 as the only hydrocarbon products; significantly, neither the FTIR nor the lH NMR spectra indicated the formation of C5Hs or CD4. At greater proportions of 3, some CD4 was evident; at higher powers, variable quantities of C5Hs could also be generated. Matrix Isolation ESR Results. Matrix isolation ESR spectra were observed for pyrolysis temperatures between 250 and 400 "C. For both 1 and 2, the appearance of the spectra depended on the precise conditions of pyrolysis temperature, flow rate, and pressure (i.e., sample source temperature). Figure 4 illustrates spectra produced by pyrolysis of 1 at two different temperatures; Figure 5 illustrates spectra obtained by subtraction at different ratios of spectra similar to those of Figure 4 produced by pyrolysis of 2. It is very evident that the ESR spectra arising from pyrolysis of both 1 and 2 contain two major components, one isotropic and the other highly anisotropic. For 1, the isotropic 1:5:10:10:5:1sextet is readily identifiable as the 'C5H5 radical;ls for 2, the isotropic component arises from more than one species and is discussed in detail below. The anisotropic component is very similar in the two spectra, the only discernible difference being that the low-field feature appears as a doublet in some spectra arising from 1 (as is evident in Figure 4). (17) Davidson, I. M. T.; Mills, G. P.; Pennington, M.; Raynor, J. B.; Russell, D. K.; Saydam, S.; Workman, A. D. Unpublished work. (18)E r a , M.; Watanabe, M.; Sakurai, H. J.Am. Chem. SOC.1980, 102, 5202.
3720 Organometallics, Vol. 14, No. 8, 1995
Russell et al.
For comparison with the IR LPHP work, a copyrolysis of 1 and 3 was conducted a t 220 "C, a t which temperature pyrolysis of neither 1 nor 3 alone produced observable ESR spectra. This copyrolysis resulted in an ESR spectrum identical with the anisotropic component of Figure 4, with no trace of the isotropic contribution of the 'C5H5 radical.
Discussion and Conclusions It is apparent that the overall mechanisms of pyrolysis of both 1 and 2 are complex; while the relatively high Arrhenius parameters for 2 indicate that the reactions are mainly homogeneous in the SFR system, there is little doubt that there will always be some contribution from surface reactions in pyrolyses of this type. However, all the observations for both l and 2 can be accommodated by the homogeneous reaction sequences shown in Schemes 1 and 2, where observed species are highlighted.
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HIG
Scheme 1 C,H,Mn(CO),
+ CO (1.1) - C,H,MnCO + CO (1.2) + CO (1.3) - 'C,H, + Mn (1.4) -C5H6+ +x c o (1.5) - 2C2H2+ MnC (1.6) - 'CD, + *MII(CO)~ (1.7) C,H,Mn(CO),
C,H,Mn(CO),
C,H,MnCO C,H,Mn
'C,H,
+ C,H,Mn(CO), 'C,H,Mn CD,Mn(CO),
'CD,
C,H,Mn
-
+ C,H,Mn(CO), 'C,H,
'C,H,Mn
CD,H
+ 'C,H,Mn + x C 0
(1.8)
+ RH - C,H, + 'R
(1.9)
Scheme 2 CH,C,H,Mn(CO), CH,C,H,Mn(CO),
-
-
CH,C,H,MnCO
CH,C,H,MnCO CH,C,H,Mn 'CH,C,H4
+ CO
CH,C,H,Mn(CO),
-
+ CO
CH3C,H,Mn
'CH,C,H,
+ CO
+Mn
(2.1) (2.2) (2.3) (2.4)
+ CH,C,H,Mn(CO), CH,C,H, + CH,C,H,Mn + x C 0 (2.5) 'CH,C,H,Mn - c6& + Mn (2.6)
In contrast with the LPHP observations, in the SFR experiments on 2 no C2H2 or C6H6 is observed;this could
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Figure 4. ESR spectra of the matrix-isolated products of pyrolysis of (v5-CsHdMn(C0)3(1): (A, top) at 310 "C;(B, bottom) at 365 "C. have been a result of a lower effective temperature in the SFR system, but it was more likely because the experiments were carried out at much lower pressures of 2, thus reducing the competitiveness of the sequence (2.5)and (2.6)relative to (2.4). Furthermore, the extent of decomposition is sufficiently small in the SFR experiments that CO reassociation (the reverse of reactions 2.1-2.3) is likely to be insignificant. Consequently, the kinetic experiments for 2 could be described by a simplified mechanism consisting of (2.1H2.41,followed by any hydrogen abstraction process converting the CH3C5H4 radical to the observed CH3C5H5. This simplified scheme is broadly consistent with that proposed in the MOVF'E study,, with initial loss of CO in reactions 2.1-2.3 being followed by slower dissociation of the Mn-C5H&Hs bond (reaction 2.41, but we can take the kinetic analysis further. In the work under MOVPE conditions, approximate kinetic measurements were made mass spectrometrically by following the decay of the (MnC&CH3)+ ion. This measurement was complicated by fragmentation within the ionization chamber of the mass spectrometer, since it is wellknown that the dominant species in the mass spectrum
Pyrolysis of M n Cyclopentadienyl Complexes
Organometallics, Vol. 14, No. 8, 1995 3721
Table 1. Estimated Arrhenius Parameters for Scheme 2 reacn
log An
2.1
15.5 15.5 15.5 14.5 8.0
2.2 2.3 2.4 2.5
E k J mol-' 205b 150 150 150 40
A factors in for first-order reactions and dm3 mol-' ssl for second-order reactions. Uncertainty in E is f10 kJ mol-' for (2.1) and (2.4) and larger for (2.2) a n d (2.3) (these ranges reproduce observed rate constants to within a factor of 2).
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Figure 5. ESR spectra of the matrix-isolated products of pyrolysis of (r5-MeC5H4)Mn(C0)3 (2) at two different temperatures, computer subtracted to separate the isotropic (A, top) and anisotropic (B, bottom) contributions. of the parent compound 1 is the (MnCsHs)+ In our system, this complication is avoided by the use of GC-MS, and we had the further advantage that the SFR technique allows separate measurement of the kinetics of formation of all stable products, CO and CH3C5H5 in this case. The pyrolysis mechanism is complex, as indicated by the curvature of the Arrhenius plots observed in this work. It was undoubtedly even more complex in the MOVPE s t ~ d yespecially ,~ as that pyrolysis was taken to much higher conversion than was our work, with rate constants as high as 0.8 s-l. In these circumstances, with extensive secondary reactions, it is not appropriate to identify a single rate-determining step; the mechanism has to be considered as a whole. This we have done by analyzing the simplified scheme by numerical integration using the KIN& package.20 This analysis requires Arrhenius parameters for the individual reactions; the estimation of A factors for reactions 2.1-2.4 is straightforward using published data,21but that of activation energies is much less so. (19)Winters, R. E.; Kiser, R. W. J . Organomet. Chem. 1965,4,190. (20)Turanyi, T.; Berces, V. S.;Vadja, S. Int. J . Chem. Kinet. 1989, 21, 83. Turanyi, T. J . Math. Chem. 1990, 5,203.
Sang and co-workers5quote 97.5 and 256.6 k J mol-l, respectively, for the Mn-CO and Mn-C5H4CH3 bond strengths; they associated the activation energy of 236 k J mol-' observed by them with reaction 2.4. However, we found that numerical integration with activation energies based on these bond strengths of 97.5 and 256.6 k J mol-l was completely inconsistent with the observed kinetic behavior for the formation of CO and CH3C5H5. There is some confusion over the absolute values of bond dissociation energies in manganese carbonyls,21,22but there is good evidence for a much higher value for the first Mn-CO bond strength. For 1, it has been measuredZ3as 195 k J mol-l in solution, with a suggested gas-phase value of 230 k J mol-l; the average Mn-CO bond energy has been estimated24at 125 k J mol-'. Numerical integration with 200 kJ mol-l for E2.1, 150 kJ mol-' for E2.2 and E2.3, and 256 kJ mol-l for E 2 . 4 gave excellent agreement with our observed Arrhenius parameters for the formation of CO, but those for the formation of CH3C5H5 were reproduced extremely poorly. We conclude that E2 4 is substantially lower than 256 kJ mol-l, i.e.,that the Mn-C5&CH3 dissociation energy is much lower in the Mn-CsH4CH3 fragment than it is in the parent compound 2. This reasonable conclusion is consistent with both our observation of the C5H5 radical, and not the MnCbH5 radical, in the pyrolysis of 1 and the observed significant difference between the first and second dissociation energies in sandwich compound^.^^ Accordingly, varying E 2 . 4 by trial and error, we found that it had to be reduced to 150 kJ mo1-l in order to obtain good agreement with the experimental Arrhenius parameters for the formation of C5H5CH3. The final Arrhenius parameters are given in Table 1. Thus, our kinetic study confirms that the first Mn-CO bond dissociation energy in 2 is at least 200 k J mol-l and provides an upper limit of 150 k J mol-l for the MnC5H4CH3 dissociation energy in the Mn-CsH4CH3 fragment. The observation of free monomeric cyclopentadiene in the pyrolysis of 1 suggests that the major reaction pathway for loss of the 'C5H5 radical is hydrogen abstraction, presumably from unreacted parent material (or perhaps some intermediate) as in (1.5), rather than dimerization. The fate of the resulting 'C5H4Mncontaining species cannot be determined with certainty, (21)Smith, G.P.Polyhedron 1988, 7 , 1605 and references therein. (22) Connor, J. A,; Zafarani-Moattar, M. T.; Bickerton, J.; El Saied, N. I.; Suradi, S.; Carson, R.; A1 Takhin, G.; Skinner, H. A. Organometallics 1982, 1, 1166. (23)Klassen, J. K.;Selke, M.; Sorenson, A. A,; Yang, G. K. J.Am. Chem. SOC.1990,112, 1267. (24) Chipperfield, J.R.; Sneyd, J. C. R.; Webster, D. E. J . Organomet. Chem. 1979, 178, 177. (25) Robles, E. S.;Ellis, A. M.; Miller, T. A. J . Phys. Chem. 1992, 96,8791.
Russell et al.
3722 Organometallics, Vol. 14, No. 8, 1995 but it seems very likely that they are the source of the ethyne observed, as indicated in (1.6). The precise nature of the other product(s), designated "MnC" in (1.6), is not clear from our observations alone, but it is reasonable to assign the anisotropic ESR signal of Figure 4 to this species. In this context, it is of significance to note that MBE using 1 or 2 alone does result in the deposition of Mn very heavily contaminated with C26and that amorphous MnC is reported to exhibit magnetic behavior a t or below room t e m p e r a t ~ r e . ~ ~ The results of the copyrolysis experiments are accounted for in reactions 1.7 and 1.8. CD3 radicals can be generated from 3 at temperatures much lower than those required for decomposition of 1 or 2. These radicals are good hydrogen abstractors, as indicated by the generation of CDsH, and will therefore produce the 'C5H4Mn species through a different route, as in reaction 1.8; this will then decompose as in (1.6). This is consistent *ith the production of C2H2 in the absence of C5Hs in the IR LPHP experiments and with the observation of the anisotropic ESR signal free of *C5H5 in the matrix isolation studies. Finally, if the C5H5 radicals are provided with an alternative and more available source of H atoms, they will preferentially abstract from the latter, as in (1.9), rather than parent material, as in (1.5). This will prevent formation of the *C5H4Mnsystem and in turn that of the MnC. This fact accounts for the greatly reduced C contamination observed in the MBE synthesis of MnAs from 2 and AsH3, since the H atoms in the latter are more readily abstracted than those in the former.26 The nature of the analogue of the abstraction reaction (2.5) in the pyrolysis of 2 is of considerable interest. The absence of 5-methylcyclopentadiene is not surprising, since equilibration of all three isomers via 1,5-H shifts is likely t o be rapid at the temperature of the pyrolysis. Furthermore, by analogy with the fact that abstraction of H atoms from the CH3 group of toluene is more facile than that of the ring protons, we expect to observe the products of the considerably more facile abstraction reaction from the CH3 group, rather than the ring protons, of 2. This results in a fulvene-type species, which will then rapidly rearrange t o the observed benzene, as in (2.6). The isotropic component of the ESR spectrum of the matrix-isolated products of pyrolysis of 2 consists of two components (Figure 5A). One is a well-resolved and strong 1:2:1 triplet with a hyperfine splitting of 16 G and a line width of 3.5 G; the second is a weaker and less well-resolved doublet of doublets with splittings of 40 and 25 G. The former we assign to a C-centered n-radical with the unpaired electron localized largely on the CH2 group of a Me-deprotonated methylcyclopentadiene fragment. This assignment was confirmed by the results of Hiickel molecular orbital calculations for the following radicals: A,the neutral five-electron radical *C5H4CH3(the monomethylated analogue of the 'C5H5 radical); and B, the seven-electron radical anion 'CHzCsH4- (which serves as a prototype for radicals formed by H-abstraction from the methyl group); the results of these calculations are shown in Figure 6. (26) Wright, P. J. Personal communication. (27)Hauser, J. J. Phys. Reu. B 1980,22,2554.
-066*7 -0.35(3.6)
B
\._I'
0.28 (2.4)
Figure 6. Hiickel molecular orbital calculations for (A) *C5H4CH3and (B) 'CH&sH4-. Numbers are Huckel coefficients and (in parentheses) predicted hyperfine splittings using a McConnell &-factorof 29.9 G. In Figure 6, the numbers in brackets are predicted hyperfine couplings for a McConnell &-value of 29.9 G, the same as that for C ~ H S It. ~is ~clear that radical B fits the experimental observations almost exactly. Coupling to the CH2 protons matches the observed triplet splitting of 16 G very closely and is very similar to that found for the CH2 protons in the benzyl radical.29 The other predicted splittings are, not surprisingly, lost in the line width of 3.5 G. We cannot be certain whether this radical is bound to an Mn-containingunit, although the lack of anisotropy or Mn hyperfine splitting tends to suggest otherwise (the radical 02Mn(C0)5, with the unpaired electron located largely on 0 2 , exhibits anisotropic Mn hyperfine structure of 8.3 and 6.4 G30). The observations are clearly inconsistent with both the calculations for the free 'C5H4CH3 radical (hyperfine splittings of 10.7G for C2 and 4.2 G for C3) and previous observations of this specie^.^^^^^ We conclude, therefore, the lower activation energy for the reaction (2.5) in the pyrolysis of 2 results in very rapid loss of the 'C5H4CH3 radical, with the result that only the abstraction product is observed in the ESR spectrum. The fate of this abstraction product is clearly of interest. Since it is an isomer of benzene, it seems very likely that it is the origin of this product (which is never observed in the pyrolysis of 1). The large hyperfine splittings of the secondary radical of Figure 5a suggest that it is a o-radical, probably of the vinyl type 'CH=CHR, arising from fragmentation of the 'C5H4CH3 rings. Hyperfine splittings from the a- and P-protons in radicals of this type are very dependent on bond angles and p:s ratios in the orbital of the unpaired electron. Thus in the vinyl radical itself (R = HI, couplings are 15.7 G (a),68 G (PI),and 34 G whereas in the methylvinyl radical (R = Me), the It is unlikely couplings are 58 G (PI) and 38 G that it is an allylic radical, since such species are n-radicals with a maximum hyperfine splitting of 15 G.31 The production of alkynes in the LPHP system, and their absence in SFR, merits additional comment. As (P2).32133
(28) Zandstra, P. J. J . Chem. Phys. 1964,40, 612. (29)Dixon, W. T.; Norman, R. 0. C. J . Chem. SOC.1964,4857. (30)Mach, K.; Novakova, J.; Raynor, J. B. J . Organomet. Chem. 1992,439,341. (31)Barker, P. J.; Davies, A. G.; Fisher, J. D. J.Chem. Soc., Chem. Commun. 1979,587. (32) Fessenden, R. W.; Schuler, R. H. J . Chem. Phys. 1963,39,2147. (33)Adrian, F. J.; Cochran, E. L.; Bowers, V. A. Free Radicals in Organic Chemistry; American Chemical Society: Washington, DC, 1962.
Organometallics, Vol. 14, No. 8,1995 3723
Pyrolysis of M n Cyclopentadienyl Complexes described above, the very low reagent pressure in the SFR system is designed specifically to minimize surface reaction and also t o result in low reactant conversion. Together, these will reduce alkyne production in comparison with the LPHP system, where longer residence times and higher pressures will increase the competitiveness of reactions 5. However, it is evident both from the nonlinear nature of Figure 2 and from other observations7J7that there may be a contribution from heterogeneous processes in the SFR system. It is likely that the stepwise CO loss of reactions 1-3 in Schemes 1 and 2 operates both in the gas phase and on the surface; what is likely to differ is the subsequent fate of abstraction products. That some closely related processes do occur on the surface is evident from the
heavy C contamination of Mn films26and the observation of the anisotropic ESR signals; however, it is very likely that any resultant alkyne undergoes further surface-catalyzed conversion, presumably into an involatile polymeric material.
Acknowledgment. We thank the SERC for their extensive support of this work, including equipment grants (to D.K.R., J.B.R., and I.M.T.D.), postdoctoral fellowships (to M.P., I.M.P., and A.D.W.), and a postgraduate studentship (to G.P.M.). We also gratefully acknowledge support from the Government of Turkey through a studentship to S.S. OM9409579
