Dynamic n.m.r. studies of the exchange processes in the complexes [M(η-C_5H_5)(exo-η-RCH=CH_2)H], M = Nb, Ta, lead to the proposal of a new mechanism involving intermediates with agostic bonding

Bercaw, John E.

Burger, Barbara J.

Green, Malcolm L. H.

Santarsiero, Bernard D.

Sella, Andrea

Trimmer, Mark S.

Wong, Luet-Lok

English

Caltech Authors - Main

734 J .  CHEM. SOC., CHEM. COMMUN., 1989 
A New Mechanism for Exchange Processes observed in the Compounds 
[M(q-CSH5)2( exo-q-RCH=CH2)H], M = Nb and Ta 
John E. Bercaw,a Barbara J. Burger,* Malcolm L. H. Green,*b Bernard D. Santarsiero,a Andrea Sella,b Mark S. 
Trimmer,a and Luet-Lok Wongb 
a Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 9 1 125, 
U. S.A. 
b Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, U.K. 
Dynamic n.m.r. studies of the exchange processes in the complexes [M(q-C5H5)(exo-q-RCH=CH2)H], M = Nb, Ta, lead 
to the proposal of a new mechanism involving intermediates with agostic bonding. 
We have reported a detailed investigation of the dynamic 
processes in the exo-niobium complexes [Nb(q-CSH5)2(exo-q- 
RCH=CH2)H] (1).1 For these exo-complexes it was found that 
exchange between the Nb-H and the vinylic CHZ hydrogens 
was n ~ ~ c h  faster (by ca. 2 orders of magnitude) than the 
exchange between the chemically inequivalent c5 rings. For 
the exo-propene-hydride complex (1, R = Me), an approxi- 
mate analysisf showed the rates of exchange of the Nb-H and 
the two inequivalent methylene hydrogens [k(H - CH2)], 
'F The exchange of the ex0 methyl group with the Nb-H and the 
methylene hydrogens was treated as a two-site exchange process. 
J.  CHEM. SOC., CHEM. COMMUN., 1989 
I 
Insertion 
735 
I 
( A )  
QNbg 
c( (- 
Figure 1. Previous proposed mechanism I1 showing the pyramidal 
intermediate and the inversion process. 
and the exo-methyl group [k(Me ts H)], to be similar, and as 
for R # Me cases, faster than that for the  exchange between 
the inequivalent C5 rings [k(Cp)]. 
The mechanism originally proposed for the exchange 
processes for the ex0 isomers is shown in Figure 1. Here, the 
alkene inserts into the Nb-H bond forming a 16-electron 
a-alkyl species (A), in which the metal centre adopts a 
'pyramidal' structure. Rotation about the C,-Cp bond fol- 
lowed by p-elimination gives the observed alkene/hydride 
scrambling. In order to effect exchange between the C5 rings, 
it is necessary for the o-alkyl group to undergo a 180" rotation, 
and the metal centre to invert so as to bring the methyl group 
to the correct orientation for the 13-elimination step. The 
slower rate of the C5 ring exchange process was proposed to 
arise from a non-negligible steric barrier for rotation and/or 
inversion at the metal centre. 
In the light of the recent evidence for an in-place rotation 
mechanism for hydrogen scrambling in alkene-hydride com- 
plexes,*.3 we propose an alternative mechanism (mechanism 
Table 1. Rate constantsa of hydrogen scrambling in [M(q-C5H&(em- 
q-MeCH=CH2)H] (M = Nb, Ta). 
M = N b  M = T a  
T/K k(Me c, H) k(H t, CH2) 
293 0.12(4) 0.39(6) 337 0.14(1) 0.15(2) 
298 0.24(4) 0.81(6) 347 0.32(2) 0.27(4) 
303 0.36(4) 1.47(12) 357 0.95(13) 0.79(11) 
34gd 40.8' 125.0' 
TIK k(Me H H) k(H t, CH2) 
313 1.32(16) 4.02(39) 412b 53.5' 
a Reported in s-1, standard deviations (n 26) are in parentheses. 
The reported coalescence temperature for the resonances due to the 
q-C5H5 ligands of the tantalum compound. k(Cp) = 18.0 s-l .  
Extrapolated values. d The reported coalescence temperature for 
the resonances due to the 1]-C5H5 ligands of the niobium compound, 
k(Cp) = 10.4 s-l. 
11 In-place rotatton I t  
r 
Q 
11 
( b )  
Figure 2. Proposed new mechanism I1 for hydrogen scrambling in (1, 
R = Me). (a) The process of rotation about the M-C, bond, and 
p-elimination are assumed to be fast compared to the cleavage of the 
agostic interaction, which is proposed to be rate limiting. (b) The new 
pathway for exchange of the propene methyl group with the metal 
hydride via the species Q. 
736 J.  CHEM. SOC., CHEM. COMMUN., 1989 
11, Figure 2) to account for the exchange processes in the 
niobium compounds. In mechanism 11, the methylene 
hydrogendhydride exchange occurs via the in-place rotation 
of an agostic alkyl intermediate. Exchange of the C5 rings 
occurs via the rate-limiting dissociation of the agostic 
hydrogen in the q2-alkyl intermediate to form a symmetrical 
16-electron o-alkyl intermediate, in which the a-alkyl group 
undergoes rapid rotation about the Nb-C, bond. We propose 
a symmetrical intermediate, i.e. with the M-C, bond aligned 
with the C2 axis, since there are no obvious reasons, steric or 
electronic,4 for a 16-electron a-alkyl species such as (A) to be 
pyramidal. Indeed, such a structure is found in the crystallo- 
graphically characterised isoelectronic compound [V(q- 
C5H5)2C1] .5 If the 16-electron a-alkyl species is symmetrical, 
then the relatively slower rates of exchange between the C5 
rings in mechanism I must be due to slow rotation arising from 
steric ligand repulsion. 
In mechanism I, in order to account for the observed 
exchange rates in the propene hydride complexes, a full 
rotation of the a-isopropyl group in the intermediate followed 
by p-elimination results in exchange of the C5 rings. Rotation 
by 60" about the M-C, bond (isopropyl rock), followed by 
P-elimination, results in exchange of the M-H with the 
exo-methyl group without scrambling of the C5 rings. In 
mechanism 11, the hydride to methyl exchange proceeds via 
the intermediate Q (Figure 2b). A 16-electron a-isopropyl 
complex is not formed at any time during this formal 
substitution reaction. If the isopropyl rock or the formation of 
Q is fast on the time-scale of the B-elimination step, then 
k(H ++ CH2) = k(Me ++ H), whereas k(H ++ CH2) > 
k(Me c) H) if they are slow. It is therefore important to 
determine these two rate constants accurately. 
The rate constants of the four-site exchange system consist- 
ing of the exo-methyl group, the inequivalent methylene 
hydrogens, and the hydride ligand in the complexes [M(q- 
C5H5)2(q-MeCH=CH2)H], M = Nb, Ta,l.6 were determined 
by variable temperature multisite spin saturation transfer 
(SST)$ n.m.r. experiments.7.8 The results are shown in Table 
1. The data show that for both the niobium and tantalum 
complexes k(Me - H) > k(Cp), and for the Nb compound 
k(H t) CH2) ==3 k(Me t) H), but within experimental error 
k(H t, CH2) = k(Me t) H) for the tantalum compound. 
The observation for the tantalum compound that k(H) ++ 
CH2) = k(Me +-+ H) can be explained in both mechanisms in 
terms of propene insertion being the rate-determining step in 
$ Multisite irradiation in the SST experiments was carried by 
sequential irradiation of the desired number of sites for 100-200 ms 
with a switching time of 5 ms between sites (decoupler gated off). The 
total irradiation time was varied by recycling through the frequency 
list the necessary number of times. 
both processes. In the case of the niobium compound, the 
isopropyl rock or the formation of Q must be slow compared 
to fi-elimination, and therefore k(H - CH2) > k(Me t--, H). 
In other words there must be a significant barrier to a 60" 
'rotation' of the isopropyl group. In mechanism I this barrier is 
due to steric ligand repulsion whereas in mechanism I1 it arises 
from specific agostic interactions. The kinetic data obtained in 
this work do not definitively distinguish between these two 
possibilities. However, there are observations in the literature 
which provide circumstantial evidence which lead us to prefer 
mechanism I1 over I. Thus there is already evidenc-e for the 
in-place rotation mechanism ,233 and agostic q2-isopropyl 
compounds have been characterised.gJ0 Concerning the 
proposal of the participation of the intermediate (or transition 
state) Q in the exo-methyl to Nb-H exchange process, we note 
that there are many examples of related agostic-alkyl ++ alkyl 
interchanges in complexes such as [kn(q-C6H8-fi)(CO)3] ,1 
viz. C-H - M H-6 a.Indeed the 
q3-isopropyl system in Q clearly is similar to the q3-isopropyl 
system in the in-place rotation mechanism. 
We conclude that for (1; R = Me) we can account for all the 
kinetic data if there is some mechanism for a restricted 60" 
rotation of the isopropyl group and that the proposal of agostic 
alkyl intermediates (electronic restriction) is a more compell- 
ing explanation than that of interligand steric interactions. 
We thank NATO (research grant 287-84) for support, and 
the Gas Research Institute for partial support, of this work. 
We thank D. O'Hare, C. M. Dobson, and S. J. Heyes for 
helpful discussion. 
Received, 20th December 1988; Com. 8/0#965C 
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A new mechanism for exchange processes observed in the compounds [M(η-C_5H_5)_2(exo-η-RCH = CH_2)H], M = Nb and Ta

Dynamic NMR Spectroscopy,'

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A new mechanism for exchange processes observed in the compounds [M(η-C_5H_5)_2(exo-η-RCH = CH_2)H], M = Nb and Ta

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