5964
/
Journal of the American Chemical Society
100:18
/
August 30, 1978
hydrogen atom abstraction is easier3band that the final product (in essentially quantitative yield) is a bisneopentylidene complex, la. It is not necessarily true that the immediate precursor is also a bisneopentylidene complex (see later), although this is what we have assumed for the present. The evolution of neopentane from Nb(CH2CMe3)3(CHCMe3)3b is also accelerated dramatically on adding L and the analogous products, lb, can be isolated in -75% yield. All complexes are orange to red, crystalline, sensitive to air and moisture, and moderately soluble in pentane. A molecular weight determination for T~(CHCM~~)~(CH~CM~~)(PM~~)Z in cyclohexane (cryoscopically)showed it to be a monomer (mol wt found, 540 f 40; two runs at 0.006 and 0.01 1 M). The presence of two S . J. McLain,lSa C. D. Wood neopentylidene ligands is suggested by the instantaneous reL. W. Messerle,lSbR. R. S ~ h r o c k * ~ action ~ ~ of 1 with acetone to give 2 mol of di~obutylene.~ Department of Chemistry The IH and 13C N M R spectra of 1 show that (i) it has Massachusetts Institute of Technology a plane of symmetry and (ii) the neopentylidene ligands Cambridge, Massachusetts 02139 are nonequivalent on the N M R time scales (60-270 MHz) at 30 OC. For example, the IH N M R spectrum of TaFrederick J. Hollander, Wiley J. Youngs (CHCMe3)2(CH&Me3)(PMe3)2 at 270 MHz shows three Melvyn Rowen Churchill* CMe3 singlets, a PMe3 triplet ( J H P= 2.4 Hz), a neopentyl H, Department of Chemistry triplet at 7 9.39 ( J H P= 19 Hz), and two neopentylidene H, State University of New York at Buffalo resonances at 7 7.92 and 3.07 ( J H PS 2). On broad-bandBuffalo, New York 14214 decoupling 31Pthe neopentyl H, triplet resonance becomes a Received January 20, 1978 singlet and the broadened neopentylidene H, resonances sharpen. In the gated decoupled 13C spectrum we find two neopentylidene C, doublet resonances at 7 274 and 246 with low C H coupling constants ( J C H= 95 and 85 Hz, respectively, J c p S 5 Hz) characteristic of neopentylidene ligands in Multiple Metal-Carbon Bonds. 11.’ crowded environment^.^ When L = PMe2Ph (and M = Ta) Bisneopentylidene Complexes of Niobium and Tantalirm the gated decoupled 13CN M R spectrum establishes that the Sir: phosphine’s methyl groups are diastereotopic; two P-Me quartets are found (at 6 22.5 and 19.2; J C H = 128 Hz) but only In a recent communication’ we showed that alkylidyne one type of phenyl ring. In the four most likely structures complexes can be prepared from alkyl/alkylidene complexes by “accelerating” (with PMe3) the nondegererate a-hydrogen atom transfer from the alkylidene a-carbon atom to the alkyl a-carbon atom. An interesting question is what happens if a-hydrogen abstraction occurs in a di- or trialkyl-alkylidene complex? Either an alkyl/alkylidyne or a bisalkylidene complex in principle could be formed. Dimeric alkyl-“alkylidyne” A B C7 D complexes are known.Is2 We now want to show that bisneopentylidene complexes (of N b and Ta) can also be the ultimate (A-D) the neopentylidene ligands must not freely rotate about products of accelerated a-hydrogen abstraction reactions in the M=C axis. We favor A, based only on the relatively large di- or trineopentyl-neopentylidene complexes. They can also -CH2-phosphorus coupling constant (1 9 Hz) vs. the relatively be prepared from alkylidyne-chloride complexes by substismall =CH-phosphorus coupling constant (55Hz). tuting the chloride with the alkyl.3a Thermally unstable Ta(CH*CMe3)&1 can be prepared by Ta(CH2CMe3)3(CHCMe3)3b$c is relatively stable thermally adding HCl to Ta(CHlCMe3)3(CHCMe3) at -78 “C in as a solid or in pentane or benzene (for weeks at 25 “C) probpentane.3bWe added PMe3 in an attempt to trap the proposed ably because (i) intermolecular decomposition pathways are decomposition product, M(CH2CMe3)2(CHCMej)Cl;* the slow and (ii) the first observed change is a degenerate intrahigh yield product is, instead, Ta(CHCMe3)2(CI)(PMe3)2 (2a, molecular scrambling of a-hydrogen atoms (AC* * 28 kcal eq 2). Analogous procedures give thermally stable products mol - I ; no neopentane evolves during the scrambling process3b). In the presence of at least 2 mol of L (L = PMe3 or PMe2Ph), M(CH2CMe3),Cl however, neopentane (1 mol) does evolve, comparatively rapidly when L = PMe3 (2 h at 25 “C) but more slowly (2 weeks at 25 “C) when L = PMe2Ph. We therefore propose that a five-coordinate intermediate is formed (eq 1) in which a-
(CH2CMe3)C13is treated with 2 mol of LiCH2CMe3 in pentane at -78 ‘C: 6(CHCMe3) 232 (Jcn = 76 f 2 Hz), T (CKNe3) 6.25, T (CHAH~CMe3) 9.08, T (CHA&CMe3) 9.25 = 14 Hz). (b) Ta(s5-CsMe5)(CHPh)(CH2Ph)CI is formed when Ta(CH2Ph)3C12 is treated with LiC5Me5in THF for 1 day: 8 (CHPh) 222 (Jcn = 85 f 2 Hz), T (CWh) 6.71, T (CHAHBPh) 7.66, T (CH,&Ph) 7.82 (&Ab = 11 Hz). Ta(05-C5Me4Et)(CHPh)(CH2Ph)CI forms similarly and has been analyzed. Calcd for TaC25H3&I: C, 54.91; H, 5.52; CI, 6.48. Found: C. 54.30; H, 5.40; CI, 6.37. (13) (a) W. Mowat and G. Wilkiqson, J. Chem. SOC.,Dalton Trans., 1120 (1973); (b) F. Huq, W. Mowat, A. C. Skapski, and G. Wilkinson, Chem. Commun., 1477 (1971). (14) (a) M. R. Churchill, F. J. Hollander, and R. R. Schrock, J. Am. Chem. SOC., 100,647 (1978); (b) M. R. Churchill and F. J. Hollander. Inorg. Chem., 17, 1957 (1978). (15)(a) National Science Foundation Predoctoral Fellow, 1975-1978; (b) Dow Chemical Central Research Fellow, 1976-1977; (c) Alfred P. Sloan Fellow, 1976-1978.
( Me3CCH2),M=C
/H
‘CMe,
L
“(Me,CCH,),M(CHCMe,)L”
,f,,,L CH, -M’
L = PMe, or PMepPh
/
(1)
firM 2a,
= Ta
>I’L
b, M = Nb L = PMe, or
3
PMepPh (2)
“(MeJCCH,)M(CHCMeJ2L” 4 ,M(CHCMe3)2(CH,CMe,)L2 la, M =
Ta b,M=Nb
0002-7863/78/1500-5964$01 .OO/O
where L = PMe2Ph or M = Nb. Apparently even the larger, less basic PMe2Ph can coordinate to incipient M(CH2CMe3)2(CHCMe3)Cl to give (most likely) thermally 0 1978 American Chemical Society
Communications to the Editor
5965
unstable M(CH2CMe,)2(CHCMe,)(Cl)(L)which eliminates neopentane and adds another L to give 2. These species are much more soluble in pentane than 1 and also more volatile; Ta(CHCMes)z(Cl)(PMe& sublimes without decomposition at 80 "C (1 k ) . A cryoscopic molecular weight determination showed Ta(CHCMe3)2(Cl)(PMe3)2 to be a monomer in cyclohexane (mol wt found, 550). The reaction of 2a with LiCH2CMe3 gives 1 and with LiCH3 gives 3. On reaction with LiCD2CMe3, Ta(CHCMe3)2(Cl)(PMe3)2 gives Ta(CHCMe3)2(CD2CMe3)(PMe3)2 specifically; the signal for the neopentyl H, protons is absent in the 'H NMR spectrum and solely diisobutylene-do is formed on reaction with acetone. Therefore, H, scrambling in la, as in Ta(CH2CMe3)3(CDCMe3),3b is slow. The -30 to -50 "C IH and I3C N M R spectra of 2 and 3 are entirely analogous (including chemical shifts and coupling constants) to those of 1. We therefore propose the structures of 2 and 3 are analogous to that of 1. However, as the temperature is raised, the two neopentylidene groups equilibrate. For example, in Ta(CHCMe3)2(Cl)(PMe3)2, the two C, signals at 6 273 (JCH = 98 f 3 Hz; J c p = 5 f 2 Hz) and 241 ( J C H = 86 f 3 Hz; Jcp = 5 f 2 Hz) coalesce (at 300 K, 22.63 MHz) to give a doublet ( J C H= 91 f 3 Hz) at 257 (50 OC at 15.1 MHz); therefore, AG*300 13.2 kcal mol-'. For Ta(CHCMe&(CH3)(PMe3)2 AG*353 16.3 kcal mol-' and for Ta(CHCMe3)2(CH2CMe3)(PMe3)2AG*373 = 17.2 kcal mol-' by both I3C and IH N M R . Added PMe3 does not exchange with coordinated PMe3 during the equilibrationg and AG* is independent of complex concentration. We therefore propose that the equilibration process is intramolecular but do not yet know whether the TBP skeleton rearranges or whether the neopentylidene ligands only rotate (synchronously or separately) about the axial M=C bonds.6 The reaction of 2a (L = PMe3) with NaCp yields 1 mol of PMe3 and monomeric 4 (mol wt found, 482, in cyclohexane) in which the two neopentylidene ligands (eq 3) are nonequiv-
-
-
$-C5R5
I
+ LiCH2CMe3
C1-Ta-PMe,
4%
Me2
C
\
CMe,
CMe3
(4)
arrangement is if R = R' # CMe3 or if R # R' in a RCH~MECR' complex remains to be seen. Also, our findings still do not tell us whether alkyl-alkylidyne or bisalkylidene complexes form first when di- or trialkyl-alkylidene complexes lose alkane.l0 We are in the process of preparing and studying the formation and reactions of other bisalkylidene complexes. Among other things we hope to determine under what conditions, if any, an olefin complex does form, and whether this olefin might be in equilibrium with the free olefin.I2 We have found so far that small olefins react with 2 in a characteristic way.I3 For example, ethylene produces 2 mol of 4,4-dimethyl- 1-pentene. Surprisingly, however, 3 mol are formed when ethylene reacts with 1.
Acknowledgments. Acknowledgment is made to the National Science Foundation (CHE76-07410) for support of this research and to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support in the form of a fellowship to G.A.R. We also thank the Francis N. Bitter National Magnet Laboratory for use of their high-field N M R facilities. References and Notes
NaCp
Ta(CHCMe3)2(Cl)(PMe3)2 --+
Me,lC 4
alent on the N M R time scales at low temperatures (I3C, (67.89 MHz) at 6 255 and 235 at -30 "C ( J C H= 97 f 3 and 90 f 3 Hz, respectively), IH, (60 MHz) at 7 1.74 and 7.97 at -60 OC, both in toluene-ds). On warming the sample they equilibrate (I3C, at 6 247 (JCH = 94 f 3 Hz at 60 "C), iH at T 4.65 at 80 OC, AG* 12.5 kcal mol-') most likely by rotation of the neopentylidene ligands about the M=C bonds. The fact that the ground state is not an olefin complex cannot be ascribed to their being mutually trans (as in the proposed ground state structures of 1 or 2). The stability of 4 (a 16e complex) relative to the trans-di-tert-butylethylenecomplex (14e) might then be ascribed to the fact that Ta is electron deficient and would prefer a 16e count and/or that the activation energy to form the hindered trans-di-tert-butylethylene is prohibitively high. It is noteworthy that 4 is also formed quantitatively on treating TaCp(CCMe3)(Cl)(PMe3)2' with '/2Mg(CH2CMe3)2(dioxane) in ether; an analogue containing q5-C5Me5 can be prepared similarly employing LiCH2CMe3 in pentane. (eq 4). This finding suggests that monomeric neopentyl-neopentylidyne complexes can rearrange to bisneopentylidene complexes. Whether they always do and how general this re-
-
S. J. McLain, C. D. Wood, L. W. Messerle, R. R. Schrock, F. J. Hollander, W. J. Youngs, and M. R. Churchill, J. Am. Chem. Soc., in press. W. Mowat and G. Wilkinson, J. Chem. Soc., Dalton Trans., 1120 (1973). (a) Satisfactory elemental analyses have been obtained for the reported complexes. (b) R. R. Schrock and J. D. Fellmann, J. Am. Chem. Soc., 100, 3359 (1978). (c) R. R. Schrock, J. Am. Chem. SOC., 96, 6796 (1974). R. R. Schrock, J. Am. Chem. SOC.,98, 5399 (1976). Why Jcn varies so greatly (from 76 Hz in Ta(q5-C5Me5)(CH2CMe3)(CHCMe3)CI' to 131 Hz in Nb(q5-C~H5)(CHMe&Ie) will be discussed elsewhere. We should note that the C, and H, signals in the 13C and 'H NMR spectra of Nb complexes are broadened significantly owing to coupiing to 93Nb(I = 9/2, 100%). The barrier to rotation of the methylene ligand in TaCp2(CH2)(CH3)is estimated to be 1 2 1 kcal mol-'. The barriers to rotation in similar =CHPh and =CHCMe3 complexes are 19.3 and 16.8 kcal mol-', respectively, most likely due to progressively greater steric crowding in the coordination sphere: R. R. Schrock, C. D. Wood, L. W. Messerle, and L. J. Guggenberger, J. Am. Chem. SOC., 100,3793 (1978). The two =CHCMe3 ligands and the CH2CMe3ligand are coplanar and arranged in a pinwheel fashion. Note that the =CHCMe3 ligand is perpendicular to the trigonal pseudoplane in E but lies in the plane in C. This product can be trapped with TiC5H5 (M = Ta) to give Ta(q5-C5H5)(CH2CMe3)2(CHCMe3).3b The triplet signal for bound PMe3 does eventually collapse to a singlet and bound PMe3 exchanges with any added PMe3.This process is most facile in more crowded Ta(CHCMe3)2(CH2CMe3)(PMe& It clearly is not related to the lower temperature intramolecular alkylidene ligand equilibration. We now believe that this may depend on the structure of both the starting material and the prduct and the types of ligands in each. In one case, which may eventually prove to be a special case, a neopentylidene L? hydrogen is removed by an external base." L. J. Guggenberger and R. R. Schrock. J. Am. Chem. Soc., 97, 5935 (1975). Electron rich olefins such as PhNCH2CH2NPhC=CNPhCH2CH2NPh will add to metals in low oxidation states to give bis "carbene" complexes (see M. F. Lapped and P. L. Pye. J. Chem. Soc., Dalton Trans., 2172 (1977), and references therein). In one case such olefins could be metathesizedslowly by a rhodium catalyst (D. J. Cardin, M. J. Doyle, and M. F. Lapped, J. Chem. SOC., Chem. Commun., 927 (1972)).However, to what extent these findings are related to the corresponding reactions of ordinary olefins remains an open question. (See R. J. Haines and C. J. Leigh, Chem. Soc. Rev., 4, 155
5966
Journal of the American Chemical Society
(1975).) (13) S. J. Mclain, C. D. Wood, andR. R. Schrock, J. Am. Chem. SOC.,99,3579 (19771.
100:18
/ August 30,1978
Table I. Isotopic Distribution of Isobutanes Obtained from la, lb, and I C
(14) Alfred P. Sloan Foundation Fellow, 1976-1978.
solvent
product
J. D. Fellmann, G . A. Rupprecht C. D. Wood, R. R. Schrock*14 Department of Chemistry Massachusetts Institute of Technology Cambridge, Massachusetts 02139 Received March 27. 1978
Reductive Elimination of Isobutane from an Isobutyl Hydride Derivative of
Bis(pentamethy1cyclopentadienyl)zirconium
la lb
Sir: The majority of transition metal complexes bearing cishydride and alkyl ligands are evidence only by the detection of alkane, presumably generated by facile reductive elimination from the unstable hydridoalkyl compounds.' In several cases such intermediates have been characterized in solution,'.2 but there appear to be very few hydridoalkyl metal complexes which are sufficiently stable for their isolation at room temp e r a t ~ r e . ~ -Consequently ' there have been few systematic investigations of this important class of compounds with regard to the preferred modes of decomposition or their reactivity toward H2, olefins, or donor ligands. The paucity of stable hydridoalkyl compounds has been recognized by Norton,6 who noted further that, within a homologous series of cis-dihydrido, -dialkyl, and -hydridoalkyl metal complexes, the last is always least stable. This same relative order of stability is displayed within the homologous series of bis(pentamethylcyclopentadienyl)zirconium(IV) compounds. Thus, while toluene solutions of the dihydrido derivative, (q5-C5Me5)2ZrH2,8 and dimethyl derivative, (~~-C5Me5)2Zr(CH3)2,~ are stable for weeks at 100 "C, the isobutyl hydride complex, (q5-CsMe5)2Zr(H)(CH2CHMe2) (1),9 decomposes during the course of several hours at 75 "C providing isobutane quantitatively (NMR). Isobutane is formed at lower temperatures during the reaction of 1 with ethylene (25 "C) or H2 (-15 "C). The relatively high thermal stability of 1, the ease with which regiospecific deuterium labeling of 1 can be and the clean, stoichiometric character of these reactions have permitted some studies, reported herein, which bear on the mechanism of isobutane formation in this system. [q5-C5(CH3)s12Zr(H)(CH2CHMe2)( l a ) , [v5-C5(CH3)512Zr(D)(CH2CDMez) (1bL and [v5-CdCD3)512 Zr(D)(CH2CDMe2) (IC) are obtained quantitatively (NMR) via treatment of benzene solutions of [q5-C5(CH3)s]2ZrH2, [q5-Cs(CH3)5]2ZrD2, and [q5-C5(CD3)5]2ZrD2, respectively, with isobutylene at 25 "C (eq 1). Compound 1 may be isolated as a pale yellow microcrystalline material, which on the basis of analytical, IR, 'H NMR,9 and molecular weight datal2 appears to have the normal monomeric, pseudotetrahedral arrangement of ligands around Zr.
Reaction with C2H4 C6D6 2a lb C7D8 2b (+2c)" Reaction with C2D4 la C7D8 2a -k 2db Not detected; large excess of C2H4 used. b See text.
'
-
[q5-Cs(CX3)5]2ZrY2 + CH2=CMe2
[a5-C5(CX3)5I2Zr(Y)(CH2CYMe2)(1) la,X = Y = H b , X = H; Y = D c,X=Y=D
When pyrolyzed in benzene or toluene solution in vacuo at 74 "C, 1 quantitatively liberates isobutane (NMR). Under these conditions the organozirconium products appear to un0002-7863/78/1500-5966$01 .OO/O
la
dergo further reaction(s) and the final products have not all been identified. l 3 Nonetheless, the rate of disappearance of 1 and the rate of appearance of isobutane, monitored by lH NMR spectrometry (benzene&), are in very good agreement and obey a first-order relationship -d[la]/dt = d[CHMe,]/dt = kle[la]
(2) s-l at 74 "C. Activation pawith kl, = (7.2 f 0.5) X rameters, calculated from rate data obtained at 105,90, and 74 "C were AH* = 21 f 1 kcal mol-' and AS* = -5 f 0.5 eu. Pyrolysis of l b at 74 "C proceeds at a nearly identical rate (klb = (7.7 f 0.5) X s-'), whereas IC decomposes roughly half as fast (k1, = (3.8 f 0.5) X s-1). The isotopic distributions for the isobutanes obtained on pyrolysis of la, lb, and IC were determined by a combination of IR and mass ~ p e c t r o m e t r y 'and ~ ~ are summarized in Table I. Surprisingly, pyrolysis of l b gives only 2-deuterio-2-methylpropane (2b). The results obtained for IC and for a 1:l mixture of l a and ICconfirm that isobutane is generated by intramolecular coupling of the isobutyl moiety with a hydrogen from a cyclopentadienyl methyl group. Simple coupling of the alkyl and hydride substituents originally cis in 1 is thus contradicted as are radical or other bimolecular processes. Compound 1 reacts rapidly with a H2 (1 atm) in toluene at - 15 "C (t 112 N 5 min) to give (q5-C5Me5)2ZrH2and isobutane (eq 3). Once again the mechanism which first comes to mind, namely reductive elimination of alkane followed by trapping of the permethyl zirconocene fragment with H2, is not observed. Rather, the labeling pattern for the isobutanes (Table I) indicates exclusive coupling of the isobutyl group with hydrogen (or deuterium) from the ambient gas.
-
(q5-C5Me5)2Zr(H ) (CH2CHMe2)
($-C5Me&ZrH2
+ H2 + CH3CH(CH3)2
(3)
The mechanism outlined in Scheme I accounts for these data for the pyrolysis of 1 and its reaction with Hz. Thus the firstorder dependence on [I] observed for the rate of pyrolytic decomposition, the small, negative entropy of activation (AS* = -5 (f0.5) eu), and the observation of a kinetic deuterium isotope effect only on deuteration of the cyclopentadienyl methyl groups14bare fully in accord with a rate-determining, unimolecular rearrangement of 3 to 4 followed by reductive elimination of isobutane with the generation of 5. The final 0 1978 American Chemical Society
