10 Chemistry of Bis(pentamethylcyclopentadienyl) -zirconium Dihydride
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JOHN E. BERCAW Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125
5
The title compound, (η -C Me ) ZrH (7), is prepared by treating {(η -C Me ) ZrN } N with dihydrogen. Structure 7 is monomeric, in contrast to polymeric {(η -C H ) ZrH } , and is very soluble in hydrocarbons and in ethers. Its H NMR spectrum exhibits a resonance attributable to the two equiva lent hydrides (singlet, δ 7.46) at an unusually low field, and the Zr-H stretching frequency (1555 cm ) is much lower than other monomeric transition metal hydrides. The chemical re activity of the Z r - H moiety is distinctly hydridic in contrast to the acidic character of group VI-VIII transition metal hy drides. In the presence of D , the hydride ligands are ex changed readily, even at —80°C, to yield H and HD, and at +70°C, all 30 methyl hydrogens exchange with a D atmo sphere. Structure 7 promotes the reduction of CO and ole fins. Several intermediates in these reactions have been iden tified, and possible mechanisms for these transformations are discussed. 5
5
2
2
5
5
5
2
2
2
2
5
5
5
2
2
x
1
-1
2
2
2
M
ost of the interest i n transition metal hydrides has focused on complexes of group V I to group VIII metals. These hydrides of the latter transition metals are understood now fairly w e l l and have been identified as i n termediates i n important catalytic reactions such as the hydrogénation, h y d r o f o r m y l a t i o n , hydrosilylation, and isomerization of olefins. O n the other h a n d , relatively little is k n o w n about the properties of group I V and V transition metal hydrides. Recent work b y Schwartz and co-workers (J ) w i t h ( ( C H ) Z r ( H ) C l ) demonstrated that z i r c o n i u m hydrides promote certain difficult transformations w i t h h i g h regioselectivity; therefore, hydrozirconation has gained recognition as a useful method i n organic synthesis. Furthermore, there are indications that 5
5
2
0-8412-0390-3/78/33-167-136/$05.00/0 © American Chemical Society Bau; Transition Metal Hydrides Advances in Chemistry; American Chemical Society: Washington, DC, 1978.
x
10.
Bis(pentamethylcyclopentadienyl)zirconium
BERCAW
137
Dihydride
early transition m e t a l hydrides can be used as catalysts for C H b o n d activation (2, 3, 4) a n d for C O reduction (5). T h e best-defined z i r c o n i u m hydrides are derivatives of bis(cyclopentadienyl)-zirconium.
T h e first were prepared i n 1966 by treating (n-C5Hs) 2
Z r ( B H ) w i t h t r i m e t h y l a m i n e (Reactions 1 a n d 2) (6, 7).
Downloaded by CORNELL UNIV on September 23, 2016 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/ba-1978-0167.ch010
4
2
C H ) ZrH } 5
5
2
2
whereas
x
The compound
(η-
can also be obtained by treating ( ( r 7 - C H ) Z r C l } 0 w i t h L i A l H , 5
treating ( 7 7 - C H ) Z r C l 5
C 5 H 5 ) Z r ( H ) C l ) U> 9). 2
x
5
5
2
2
with
5
5
2
2
4
LiAlH(0-ri-C H )3 4
yields
9
(5
Investigations of the c h e m i c a l a n d physical properties
of these compounds are severely l i m i t e d by their solubility, however.
Only
( 7 7 - C H ) Z r ( H ) ( B H ) 4 appears to be m o n o m e r i c ; both ( 7 7 - C H ) Z r ( H ) ( C l ) ) 5
5
5
5
2
5
5
2
x
and |(77 -C5H ) ZrH ) are insoluble, apparently as a result of polymeric structures 5
5
2
2
x
w i t h Z r - H - Z r b r i d g i n g groups.
M o r e recently, a related z i r c o n i u m h y d r i d e
species was prepared by hydrogénation of bis(?7 -indenyl)dimethylzirconium(IV) 5
(Reaction 3) (10).
T h i s c o m p o u n d is d i m e r i c , a n d on the basis of H N M R a n d l
ir data, its structure is probably that shown below:
O u r o w n interest i n z i r c o n i u m hydrides stems f r o m work w i t h d i n i t r o g e n and carbonyl derivatives of bis(pentamethylcyclopentadienyl)zirconium(II) 12, 13).
(11,
W e f i n d that these compounds are m u c h more amenable to study than
the parent ( r - C s H s Z r derivatives because of their greater stability, higher solubility, a n d enhanced c r y s t a l l i z a b i l i t y . D u r i n g these studies, we devised synthetic routes to the title c o m p o u n d (r; -C5Me5) ZrH and began to investigate 5
2
2
its unusual c h e m i c a l properties.
Synthesis and Characterization
of
(rfi-CMeZrH
T h e development of the chemistry of organometallic compounds containing the p e n t a m e t h y l c y c l o p e n t a d i e n y l l i g a n d has been h a m p e r e d by the lack of a simple, high yield synthesis of 1,2,3,4,5-pentamethylcyclopentadiene
(14).
We
have recently expanded the method first reported by Sorensen et al. (15) to a large scale preparation w h i c h follows Scheme 1 (16).
T h i s procedure is a p p l i c a b l e
Bau; Transition Metal Hydrides Advances in Chemistry; American Chemical Society: Washington, DC, 1978.
138
TRANSITION M E T A L HYDRIDES
Scheme 1 0 Br
Li
Li
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1 R=CH
OR'
R
3
2
R=C H
3
R=rt-C H
7
4
R=/?-C H
9
5
R=/7
