Organometallics 1982, 1, 1629-1634
1629
A Kinetic and Mechanistic Study of the Thermolysis of Bis(pentamethylcyclopentadienyl) dimethyltitanium( I V) *$ Christine McDade,' Jennifer C. Green,' and John E. B e r ~ a w ' ~ Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, Californla 9 1 125 Received August 21, 1982
The thermal decomposition of (q5-C5Me)2Ti(CH3)2in toluene solution follows clean first-order kinetics concurrent with the evolution of and produces a single titanium product (q2-C5Me5)(C5Me4CHz)Ti(CH3), 1 equiv of methane. Labeling studies using (q5-C5Me5)2Ti(CD3)2 and (q5-C5Me5-d15)2Ti(CH3)2 show the decomposition to be intramolecular and the methane to be produced by coupling of a methyl group with a hydrogen from the other TiCH3 group Activation parameters, pH* = 27.6(3) kcal-mol-' and AS*= -2.9 (7) eu, for the decomposition of (q -C5Me5)2Ti(CH3)2 were measured. The decomposition of (q5C5Me5),Ti(CH& proceeds 2.9 times faster than (q5-C&4e5)2Ti(CD3)2, at 98 "C, whereas no significant kinetic The alternative decomposition pathways deuterium isotope effect is noted for (q5-C5Me5-d15)2Ti(CH3)2. of a abstraction and a elimination, both leading to the titanium-methylidene intermediate [ (q5C5Me5)2Ti=CH2],are discussed.
Introduction The organometallic chemistry of titanium(1V) provides many examples of isolable, yet thermally and photolytically unstable compounds containing Ti-C IJ Moderately stable compounds of the types TiR, (R = CH2C6H5, C6H5, CH2C(CH3),, CH2SiR3), TiRX3, TiRX3.L, and TiRX3.L2 (R = alkyl, alkenyl, alkynyl, aryl; X = halide, alkoxide, amide; L = oxygen, sulfur, nitrogen, or phosphorus base) have been isolated. Evolution of RH usually accompanies decomposition, although in no case has the mechanism been fully investigated. Alkyl and aryl derivatives of dicyclopentadienyltitanium(1V)are generally more stable species, and their decomposition pathways have been more extensively studied.6 Dvorak and coworkers have shown that the thermal decomposition of (q5-C5H5)2Ti(C6H,)2 probably proceeds via an o-phenylene intermediate [(q5-C5Hs)zTi(C6H4)] generated from orthohydrogen abstraction by the other phenyl group.' The related compound, (v5-C6H5),Ti(CH3),(I), decomposes even at room temperature, rapidly in light and more slowly in the dark.8pg The mechanismb) of the decomposition of 1 have proven difficult to establish, however. Van Leeuwen et al. have shown by chemically induced dynamic nuclear polarization (CIDNP) studies of photochemically initiated reactions of (q5-CJ-14Me),Ti(CHJ2that homolysis of the Ti-CH, bond does occur.l0 Similar studies of the pathway for the thermal decomposition of 1 are not as conclusive. The favored mechanisms involve loss of the methane via a-H abstraction by the other methyl group or via a-H elimination and subsequent reductive elimination of CH4.11-13 The thermal decomposition of 1 is complicated by the apparent two-stage nature of the reaction in solution: a-H abstraction yielding CHI and a dark solution, followed by solid-catalyzed a-H abstraction coupled with ring-hydrogen abstraction. Furthermore, there is evidence for yet another minor pathway which results in ethane production in both solution- and solid-state decompositions.llb Due to these complicating features, a kinetic study of the decomposition of 1 has not been feasible. In view of the increasing interest in the nature of these proposed a-H abstraction and a-H elimination processes, for example as regards their possible participation in Ziegler-Natttvpolymerization of olefins14 and in the synthesis of "Tebbe's reagent" ( q 5 Dedicated to Rowland Pettit, respected scientist and friend. *Contribution No. 6691. f
0276-7333/82/2301-1629$01.25/0
Table I. Summary of Rate Constants for the Decomposition Reaction of Cp*,Ti(CH,), (2) and Its Deuterated Analormes temp, "C
Cp*,Ti(CH,),
Cp*,Ti(CD,),
(Cp*-d, $1,-
(2) (4) Ti(CH,), (5) 98.3 0.378 (18)" 0.128 ( 6 ) 0.382 (18) 115.5 2.05 (101 0.683 (32) 1.93 ( 9 ) 127.2 6.07 (29j 2.17 ( i 2 ) ' 5.78 (27) k H / k D 6 = 2.92 (10);k ~ / k ~= "1.03 (4)
" Units of s-' f o r all values of k o b d . The entry in parentheses the error limit estimated to be one standard deviation based o n repetition of the experiments. The standard deviation calculated from a n analysis of residuals f o r any single experiment was always smaller. CsH5)2TiCH2-ClAl(CH,)2 from (q5-C6H5)2TiC12 and Al(CH3)?15a system more amenable to kinetic studies is clearly deslrable. The bis(pentamethylcyclopentadieny1) analogue of 1, C P * , T ~ ( C H ~(2) ) ~(Cp* v5-C5(CH3),)has been reported to be much more stable.lB Upon heating to 110 "C in toluene solution 2 decomposes to form quantitatively (1) National Science Foundation Predoctoral Eeuow (1980-1983) and Haagen-Smit/Tyler Fellow (1980-1982). (2) Inorganic Chemistry Laboratory, Oxford. (3) Camille and Henry Dreyfus Teacher-Scholar, 1977-1982. (4) Cotton, F. A. Chem. Reu. 1966,55,551-594. (5) Wailes, P. C.; Coutts, R. S. P.; Weigold, H. 'Organometallic Chemistry of Titanium, Zirconium, and Hafnium"; Academic Press: New York, 1974. (6) Waters, J. A.; Vickroy, V. V.; Mortimer, G. A. J.Organomet. Chem. 1971,33, 41-52 and references contained therein. (7) Dvorak, J.; O'Brien, R. J.; Santo, W. J. Chem. SOC.,Chem. Commun. 1970,411-412. (8) Piper, T. S.; Wilkinson, G. J. Znorg. Nucl. Chem. 19S6,3,104-124. (9) Clause, K.; Beetian, H. Liebigs Ann. Chem. 1962, 654,8-19. (10) VanLeeuwen, P. W. N. M.; Van der Heijden, H.; Roobeek, C. G.; C. F.; Frijns, J. H. G.J. Organomet. Chem. 1981,209, 169-182. (11) (a) Erskine, G . J.; Wilson, D. A.; McCowan, J. D. J. Organomet. Chem. 1976,114, 119-125. (b) Erskine, G . J.; Hartgerink, J.; Weinberg, E. L.; McCowan, J. D. Zbid. 1979, 170, 51-61. (12) Alt, H. G.; DiSanzo, F. P.; Rausch, M. D.; Uden, P. C. J. Organomet. Chem. 1976,107,257-263. (13) Chang, B.-H.; Tung, H.-S.; Brubaker, C. H. Znorg. Chem. Acta 1981,51, 143-148. (14) Ivin, K. J.; Rooney, J. J.; Stewart, C. D.; Green, M. L. H.; Mahtab, R. J. Chem. SOC.,Chem. Commun. 1978, 604-606. (15) (a) Tebbe, F. N.; Parshall, G. W.; Reddy, G. S. J. Am. Chem. SOC. 1978,100,3611-3613. (b) Tebbe, F. N.; Parshall, G. W.; Oven+, D. W. Zbid. 1979,101,5074-5075. (c) Grubbs, R. H.; Miyashita, A. Zbad. 1978, 100,7418-7420. (d) Howard, T. R.; Lee, J. B.; Grubbs, R. H. Ibid. 1980, 102,6878-6880. (e) Lee, J. B.; Gadja, G. J.; Schaefer,W. P.; Howard, T. R.; Ikariya, T.; Straus, D. A,; Grubbs, R. H. Ibid. 1981,103,7358-7361. (16) (a) Bercaw, J. E.; Marvich, R. H.; Bell, L. G.; Brintzinger, H. H. J.Am. Chem. SOC.1972,94, 1219-1238. (b) Bercaw, J. E.; Brintzinger, H. H. Zbid. 1971, 93, 2045-2046.
0 1982 American Chemical Society
McDade, Green, and Bercaw
1630 Organometallics, Vol. 1, No. 12, 1982
Scheme 1. Statistical Product Distribution for Pyrolysis of 4 (97.0%Deuteration of Ti-CH, Groups; (Cp* q5C5(CH3)S; Fv* E C5(CH3)4CH,)
0.60p\
83.3%
Cp*Fv*Ti-CD2H
Cp*, Ti /CD3 C'D,
+
83.3%
CD4
6 Cp*Fv*Ti-CD2H
+
CD3H 10.3%
Cp*Fv*Ti-CDH,
+
CD4
N
2 g
5.2%
I 010-
L-
Scheme 11. Statistical Product Distribution for Pyrolysis of 5
(96.9%Deuteration of q5-C5(CH,), Groups; Cp* qS-C,(CH,),; Fv* :C,(CH,),CH,) 38.9% (CP*Z-~,~)T~(CH& A (Cp*FvX-dz9)TiCHzD + CH, 38.9% 004b
I
I
1
600
1200
I800
L
2400 l i m e , sec
I
I
3000
3600
9
4200
(Cp*Fv*-d2dTiCH3
Figure 1. Representative plot of data from a kinetic run: the (2) at 127.2 O C . thermolysis of Cp*2Ti(CH3)2
37.3%
3
Cp*,Ti(CH3), Cp*(C5Me4CH2)Ti(CH3)+ CHI (1) 2 3 Preliminary experiments indicated that cleanly firstorder kinetic results could be expected from this thermal decomposition. A kinetic and mechanistic study of the thermal decomposition of 2 involving rate measurements taken over a range of temperatures and isotopic ,H substitutions are reported herein.
A
The thermal decompositions of Cp*,Ti(CH3), (2) and its deuterated analogs Cp*,Ti(CD3), (4) and (Cp*d,&,Ti(CH,), (5) in toluene solution were followed by 'H and 2H NMR a t 98.3,115.5, and 127.2 OC. The reaction kinetics, as measured by loss of starting dimethyl compound relative to an internal ferrocene reference with time, consistently proved to be cleanly first order for greater than 3 half-lives. A representative plot is presented in Figure 1. The observed rate constants are summarized in Table I. Attempts to determine the rate by measuring the increase in integrated intensity of the product Ti-CY3 (Y = H or D) resonance, even when not complicated by the presence of more than one isotopic substitution pattern (vide infra), gave non-first-order plots, Rather, the plots are indicative of a subsequent, slower first-order reaction" in which the turquoise product 3 apparently decomposes slowly a t the elevated temperatures used (eq 2), although no new signals were observed in the NMR.
A
CHI
,--
..
4
-
Cp*(C5Me4CH2)TiCH3
A
?
(2)
Verification of the intramolecular nature of the decomposition of 2 is provided by crossover experiments. A toluene solution of Cp*(C5Me4CH2)TiCH3 (3) and Cp*,Ti(CD,), (4) (1:l)was heated in a sealed NMR tube a t 110 "C for 2 h, the time scale of most of the kinetics experiments. No evidence for Ti-CH, exchange was found. In fact, the ZH NMR spectrum (observed at 76.8 IvIHz) for (17) Frost, A. A.; Pearson, R. G. 'Kinetics and Mechanism A Study of Homogeneom Chemical Reactiona",3rd ed.;Wiley New York, 1981; pp 166-172.
1.2%
CH,
36.1%
9
(c~F~*-~,,)TEH,
+
CH,
1.201~
10
L ( C ~ F v * - d z 7 ) T i C +~
CH,
16.1%
9
