24 Some Recent Advances in Polypyrazolylborate Chemistry SWIATOSLAW TROFIMENKO Plastics Dept., Experimental Station, Ε. I. du Pont de Nemours and Co., Wilmington, Del. 19898
Selected studies of polypyrazolylborate chemistry are re viewed. X-ray and spectral studies have clarified the nature of transannular interaction in bidentate chelates containing the puckered R B(pz) M ring where the pseudoaxial R group (H, ethyl) approaches the metal close enough for three -center bonding (B-H · · · Mo; B - C - H ···Mo) to occur. The puckered nature of the metallocyclic ring in bidentate sys tems R B(pz) CoR C H leads to non-interconvertible con formational isomer pairs. Some were separated by chroma tography; in others interconversion via ring flipping was relatively facile, and the isomers were detected only spectroscopically. The stabilizing effect of an RB(pz) ligand was exploited in making a stable copper carbonyl derivative HB(pz) CuCO and a variety of five-coordinate platinum compounds of type RB(pz) PtMeL. Hybrid sandwiches containing an RB(pz) ligand and a carbocyclic moiety (C H C H , C Φ4) and new heavy metal (Ta, U) complexes were made recently. 2
2
2
2
f
5
5
3
3
3
3
5
he
5,
6
6
4
polypyrazolylborates
are r e l a t i v e l y n e w ,
uninegative chelating
agents of g e n e r a l s t r u c t u r e [ R B ( p z ) . ] " w h e r e R is a n o n c o o r d i n
4
n
n a t i n g substituent, p z is a 1 - p y r a z o l y l g r o u p , a n d η m a y b e 0, 1, or 2 ( I ). T h e y b o n d to metals a n d m e t a l l o i d s t h r o u g h the t e r m i n a l nitrogens. D e s p i t e t h e i r s e e m i n g l y exotic n a t u r e , these l i g a n d s w e r e s t u d i e d e x t e n s i v e l y since 1967 because of t h e i r v e r s a t i l i t y as c h e l a t i n g agents
and
b e c a u s e of the f o l l o w i n g a t t r a c t i v e features w h i c h d i s t i n g u i s h t h e m f r o m m o s t other l i g a n d s . 289
290
INORGANIC
COMPOUNDS
WITH
UNUSUAL PROPERTIES
( a ) A l k a l i m e t a l salts of the p a r e n t ions a n d of t h e i r s u b s t i t u t e d analogs are easily s y n t h e s i z e d ( f r o m boranes a n d p y r a z o l i d e i o n ) , a n d t h e y are stable to storage. ( b ) S t a b l e free acids d e r i v e d f r o m the R „ B ( p z ) _ p r e p a r e d a n d u s e d for t h e synthesis of other salts, e.g. u n a v a i l a b l e b y the d i r e c t route. 4
n
anions m a y b e R N , that are 4
+
( c ) T h e p y r a z o l e h y d r o g e n s c a n b e effectively u s e d for p r o t o n count, a n d as a s y m m e t r y p r o b e i n d i a m a g n e t i c a n d i n some p a r a m a g n e t i c compounds. ( d ) It is possible to i n t r o d u c e as m a n y as ten substituents i n t o the p a r e n t H B ( p z ) l i g a n d , a n d thus to alter the steric a n d e l e c t r o n i c effects a r o u n d the m e t a l w i t h o u t d e s t r o y i n g the o r i g i n a l C s y m m e t r y of the ligand. 3
3v
( e ) F o r e a c h p o l y p y r a z o l y l b o r a t e l i g a n d , there exists a n isosteric a n d isoelectronic R C ( p z ) _ c o u n t e r p a r t w h i c h has the same c o o r d i n a t i v e b e h a v i o r , b u t it is n e u t r a l a n d gives complexes w i t h a charge greater b y + 1 p e r l i g a n d . T h i s is a t r u l y u n i q u e feature. w
4
n
S i n c e the earlier w o r k ( u p to 1972) i n this area was r e v i e w e d ( 2 , 3 ) , n u m e r o u s n e w studies h a v e a p p e a r e d .
B e f o r e these m o r e recent d e v e l
opments are discussed, some b a c k g r o u n d i n f o r m a t i o n o n the c h a r a c t e r i s t i c features of the p o l y p y r a z o l y l b o r a t e l i g a n d s a n d t h e i r chelates is p r e s e n t e d . T h e b o n d i n g of p o l y p y r a z o l y l b o r a t e s to m e t a l is d e t e r m i n e d p r i m a r i l y b y the n u m b e r of p y r a z o l y l groups a t t a c h e d to b o r o n .
Dipyrazolylborates
are necessarily b i d e n t a t e , a n d t h e y f o r m w i t h d i v a l e n t t r a n s i t i o n m e t a l ions complexes,
1, that are s i m i l a r to β-diketonates b u t t h a t are a l w a y s
m o n o m e r i c for steric reasons. T h e major difference b e t w e e n d i p y r a z o l y l borates a n d β-diketonates is t h a t the R B ( p z ) M r i n g is not p l a n a r b u t 2
2
is p u c k e r e d i n the b o a t f o r m as i n S t r u c t u r e 2.
N - N
/
x
R B 2
2
\
/
M
N - N
\
BR
T h i s results i n a s y m -
2
m e t r y a n d n o n - i d e n t i t y of the R g r o u p s , one of w h i c h ( t h e p s e u d o a x i a l ) is close to a n d sometimes interacts w i t h the m e t a l , e v e n w h e n R is H or a n a l k y l g r o u p . I f one of the R groups i n R B ( p z ) 2
i t c a n also b o n d to the m e t a l .
2
is another p y r a z o l y l r i n g , t h e n
W i t h d i v a l e n t t r a n s i t i o n metals,
com
p o u n d s w i t h S t r u c t u r e 3 are f o r m e d w h e r e a s w i t h t r i v a l e n t metals the
24.
TROFiMEΝκο
Polypyrazolylborate
291
Chemistry
analogous cations are f o r m e d . T h e l i g a n d R B ( p z )
3
(R ^
p z ) is t h e first
k n o w n e x a m p l e of a u n i n e g a t i v e t r i d e n t a t e l i g a n d w i t h C
3 v
symmetry,
a n d i t is analogous to a c y c l o p e n t a d i e n i d e i o n i n b e i n g u n i n e g a t i v e , i n s u p p l y i n g six electrons, a n d i n o c c u p y i n g three c o o r d i n a t i o n sites. I n t h i s context, the o c t a h e d r a l c o m p o u n d s (3 ), w h i c h are of D
s y m m e t r y , are
Sd
analogs of metallocenes.
T h e r i g i d c e n t r a l cage s t r u c t u r e persists i n a l l
t h e i r s u b s t i t u t e d d e r i v a t i v e s . A c c o r d i n g l y , t h e i r p r o p e r t i e s c h a n g e less d r a m a t i c a l l y w i t h s u b s t i t u t i o n t h a n do those of the b i d e n t a t e chelates, 1.
W h e n the f o u r t h b o r o n substituent is also a p y r a z o l y l g r o u p , t h e resulting B ( p z )
4
l i g a n d is s t i l l b a s i c a l l y t r i d e n t a t e of l o c a l C
m e t r y , a n d i t forms chelates ( S t r u c t u r e 3 ) w i t h R = compounds,
3 v
sym
pz. I n most such
the three c o o r d i n a t e d a n d one u n c o o r d i n a t e d p z
groups
m a i n t a i n t h e i r separate i d e n t i t y , as w a s d e m o n s t r a t e d b y N M R spectros c o p y of the C o ( I I ) d e r i v a t i v e ( 4 ) . a n a l o g , a l l f o u r p z groups exchange r a p i d l y ( 5 ) .
B y contrast, i n the i s o m o r p h o u s z i n c
are s p e c t r o s c o p i c a l l y
The B ( p z )
4
e q u i v a l e n t a n d thus
l i g a n d m a y also act i n tetradentate
( b i s - b i d e n t a t e ) f a s h i o n , f o r m i n g spiro cations of the 4 t y p e (6)
which
m a y c o n t a i n i d e n t i c a l or d i s s i m i l a r b r i d g i n g units. Polypyrazolylborates have been w i d e l y used i n organometallic chem istry.
A l a r g e n u m b e r of c o m p o u n d s c o n t a i n i n g the b i - or t r i d e n t a t e
l i g a n d b o n d e d to a m e t a l a l r e a d y c o n t a i n i n g v a r i o u s o r g a n i c were synthesized and studied.
T h e tridentate R B ( p z )
3
moieties
l i g a n d forms
o r g a n o m e t a l l i c c o m p o u n d s analogous to the h a l f - s a n d w i c h e s b a s e d C 5 H 5 (e.g.
on
5) w h i c h , as a r u l e , are m o r e stable t h a n t h e i r c y c l o p e n t a
d i e n y l counterparts, a n d sometimes these h a l f - s a n d w i c h e s c a n exist o n l y i n the p o l y p y r a z o l y l b o r a t e series. reviews (2, 3 ) .
T h e s e w e r e d i s c u s s e d i n the e a r l i e r
A g e n e r a l i d e a of the w a y a n R B ( p z )
3
group functions
i n o r g a n o m e t a l l i c c h e m i s t r y m a y be o b t a i n e d f r o m Schemes I a n d I I .
292
INORGANIC
RB(pz) - +
M(CO)
3
(M =
H
-*·
^
> RB(pz)3M(C0) -
+
3
100 L»
3C0
8a
3
RB(pz) M(CO) R' 3
3
+ 7
50°C
*-RB(pz) M(CO) C H 3
allyl halide DMF,
I
3
R'X
DMF,
Scheme
PROPERTIES
iRB(pz) M(CO) H
+
C H
UNUSUAL
Cr, Mo, W)
OH"
7
e
COMPOUNDS WITH
2
7
7
RB(pz) M(CO) -n-allyl 3
50°C
2
ArNV D M F , 25°C *
R
B
(P )3M(CO) -N=N-Ar z
2
C1NO, - 2 5 ° c /
\ ^ X
RB(pz) M(NO) (N Ar)Cl 3
[RB(pz) M(N Ar)X]
2
° ( R O N O , C1NO)
3
R B (pz) M ( C O ) N O — t νγν«~χ~-/ C H 2
2
m i n e d b y C o t t o n a n d co-workers
2
2
as d e t e r
2
i n d i c a t e s t h a t s u c h i n t e r a c t i o n does
i n d e e d o c c u r b u t that o n l y o n e o f t h e p s e u d o a x i a l m e t h y l e n e
hydrogens
f o r m s a C - H - M o three-center b o n d w i t h a Η - M o d i s t a n c e o f 2.15 A (26).
N M R studies o f this a n d t h e r e l a t e d ττ-allyl
about
compound
r e v e a l e d that t h e s t r u c t u r e f o u n d i n t h e c r y s t a l also exists i n s o l u t i o n at l o w t e m p e r a t u r e as t h e 12.4 τ p e a k splits i n t o t w o : at 14.3 τ a n d at 10.8 τ ( n o n b r i d g i n g ).
O n e type of
fluxionality
(bridging)
o b s e r v e d i n this
m o l e c u l e is a n o s c i l l a t o r y e x c h a n g e o f m e t h y l e n e h y d r o g e n s b o n d e d to Mo.
T h i s process has a n a c t i v a t i o n energy o f a b o u t 14 k c a l / m o l e . A t
h i g h e r t e m p e r a t u r e s , a r i n g i n v e r s i o n occurs w i t h a n a c t i v a t i o n e n e r g y o f 17-20 k c a l / m o l e . T h i s a p p r o x i m a t e s t h e s t r e n g t h o f t h e C - H - M o i n t e r action
(27).
I n t h e 18-e p y r a z o l e a d d u c t o f this c o m p o u n d ,
t h e base a t t a c k e d
trans to t h e C - H - M o b o n d ; t h e flip-back o f t h e B - E t g r o u p r e s u l t e d i n a nonbonded
B - M o distance o f 3.806 A , t h e longest f o u n d i n a n y c o m
p o u n d o f this t y p e , a n d t h e B ( p z ) M o r i n g a s s u m e d a d i s t o r t e d c h a i r 2
conformation
(28).
T h e f o r m a t i o n o f a three-center C - H - M o b o n d c o m p e t e s
success
f u l l y w i t h olefinic c o o r d i n a t i o n i n t h e c o m p o u n d E t B ( p z ) M o ( C O ) C H 2
(29).
2
2
7
7
W i t h a c h o i c e o f a t t a i n i n g 18-e c o n f i g u r a t i o n t h r o u g h p e n t a h a p t o
b o n d i n g to C H 7
7
or through C - H - M o bonding, the molecule opted for
the latter. I t is n o t clear, h o w e v e r , w h e t h e r t h e p e n t a h a p t o s t r u c t u r e i s s t e r i c a l l y possible. T h e complex
(0 Η ) Β(ρζ) Μο(0Ο) -7Γ-0Η 0Μβ0Η β
5
2
2
2
t r u e 16-e c o n f i g u r a t i o n i n t h e c r y s t a l (30).
2
possesses a
2
T h e r e is n o i n t e r a c t i o n w i t h
a n o r t h o C - H as m i g h t b e e x p e c t e d b e c a u s e t h e necessary o r i e n t a t i o n of t h e p s e u d o a x i a l p h e n y l g r o u p w o u l d i m p a r t too m u c h steric s t r a i n o n t h e rest o f t h e m o l e c u l e . T h e v a l u e of t h e d i h e d r a l a n g l e b e t w e e n p z groups i n v a r i o u s b i d e n t a t e p o l y p y r a z o l y l b o r a t e c o m p o u n d s varies g r e a t l y ( T a b l e I ) . I t r e flects
the d e p t h o f t h e b o a t c o n f o r m a t i o n
i n the B ( p z ) M ring a n d 2
depends o n the nature of M , o n the nature of the pseudoaxial R group,
298
INORGANIC
Table I.
COMPOUNDS WITH
Dihedral Angles between Pyrazolyl Rings in Bidentate Complexes Dihedral
Compound EtoB(4-Brpz)oBEto RB(pz) MXYZ [H B(pz)o] Co 3
2
2
[EtoB(pz)o] Ni E t B (pz) o M o ( C O ) ο- -€ΐίοΟφΟΆ> H B (3,5-Me pz) oMo (CO) o C H 2
π
2
2
UNUSUAL PROPERTIES
2
7
7
Angle
180° 120° 121° 114° 113° 111° 104° 102°
Ref. 31 9 33 32 27 23
a n d o n the m o d e of i n t e r a c t i o n of the p s e u d o a x i a l g r o u p w i t h M .
R
g r o u p s , w h i c h are not n o r m a l l y c o n s i d e r e d p r o n e to b o n d i n g , are b r o u g h t close e n o u g h to the m e t a l for i n t e r a c t i o n / b o n d i n g to o c c u r b e c a u s e of the v e r y flexible n a t u r e of p o l y p y r a z o l y l b o r a t e l i g a n d s a n d t h e i r adjust a b l e b i t e , w h i c h p e r m i t the a d o p t i o n of a w i d e r a n g e of c o n f o r m a t i o n s . A t the one extreme, w h e r e M is B R as i n 13, the d i h e d r a l angle is almost 2
Br
Br 13 180°
T h e tridentate H B ( p z )
(31).
a n g l e of 120°.
3
l i g a n d s h o u l d a n d does h a v e a n
P r e s e n c e of a B - H - M o b o n d gives rise to the smallest
angle (102° a n d 1 0 4 ° ) .
E t B ( p z ) l i g a n d s , w i t h less s t r a i n r e q u i r e d to 2
2
b r i n g the C - H close to M , h a v e a m o r e r e l a x e d angle ( 1 1 1 ° a n d 1 1 3 ° ) e v e n t h o u g h the latter a n g l e is f o u n d i n a c o m p o u n d that is d e v o i d of C - H - M bonds, [ E t B ( p z ) ] N i (32). 2
hedral
[H B(pz) ] Co, 2
2
2
2
two
2
It is n o t e w o r t h y that i n the t e t r a
d i s t i n c t l y different c o n f o r m a t i o n s
of
the
l i g a n d s are e n c o u n t e r e d ( 3 3 ) ; one has a d i h e d r a l a n g l e of 121°, the other 114°. A g o o d i l l u s t r a t i o n of g e o m e t r i c a n d c o n f o r m a t i o n a l stereoisomerism i n b i d e n t a t e p o l y p y r a z o l y l b o r a t e complexes is p r o v i d e d b y the stable,
24.
TROFiMEΝκο
Polypyrazolylborate
299
Chemistry
r e d perfluoroalkyl C o ( C H ) derivatives prepared b y K i n g a n d B o n d 5
These compounds
(34).
(C H ). 5
5
h a v e the g e n e r a l s t r u c t u r e H B ( p z ) _ C o R n
4
n
£
T h e p o l y p y r a z o l y l b o r a t e l i g a n d is a l w a y s b i d e n t a t e , a n d t h e r e
5
is n o exchange b e t w e e n the c o o r d i n a t e d a n d u n c o o r d i n a t e d p z groups. With R B(pz) 2
2
l i g a n d s w h e r e b o t h R g r o u p s are i d e n t i c a l , t w o
con
f o r m a t i o n a l isomers are p o s s i b l e ( 1 4 a n d 1 5 ) b e c a u s e of t h e p u c k e r e d n a t u r e of the r i n g .
A l t h o u g h these isomers s h o u l d b e c a p a b l e of i n t e r -
c o n v e r s i o n t h r o u g h i n v e r s i o n of the C o ( p z ) B r i n g , this w a s n o t o b s e r v e d . 2
I n fact, these isomers c o u l d b e s e p a r a t e d i n p u r e state b y c o l u m n c h r o m a t o g r a p h y . W i t h H B ( p z ) , f o u r isomers are t h e o r e t i c a l l y p o s s i b l e ( t h e 3
t w o g e o m e t r i c isomers, 16 a n d 17, p l u s t h e i r r e s p e c t i v e c o n f o r m a t i o n a l isomers i n w h i c h C H 5
5
and R
£
are r e v e r s e d ) .
W h e n Rf was C F
3
or
n - C F , h o w e v e r , o n l y a single i s o m e r w a s o b t a i n e d ; i t p r o b a b l y has 3
7
S t r u c t u r e 16 w i t h C F a n d S t r u c t u r e 17 w i t h n - C F . O n t h e o t h e r h a n d , 3
C F 2
5
3
7
gave a n i n s e p a r a b l e m i x t u r e of isomers. I t is r e m a r k a b l e t h a t c o n
f o r m a t i o n a l isomers w e r e i s o l a t e d , b u t g e o m e t r i c isomers w e r e not.
14
15
16
17
A n o t h e r i n t e r e s t i n g recent d e v e l o p m e n t is t h e successful synthesis of h y b r i d s a n d w i c h e s t h a t c o n t a i n one R B ( p z )
3
l i g a n d a n d one c a r b o c y c l i c
ligand—e.g. [ R B ( p z ) C o C H ] , ( C H ) C C o ( p z ) B R , [ C H R u ( p z ) 3
B p z ] P F , and 6
5
5
+
6
5
4
4
3
6
6
[ C M e R h ( p z ) B H ] P F . T h e y were prepared by 5
5
3
6
3
dis-
300
INORGANIC
COMPOUNDS
WITH
UNUSUAL PROPERTIES
p l a c e m e n t reactions o n t h e a p p r o p r i a t e h a l i d e precursors ( 3 5 ) . X - r a y c r y s t a l l o g r a p h i c studies c o n f i r m e d t h e s t r u c t u r a l assignments f o r t h e last two compounds (36, 3 7 ) . Finally,
two communciations
reporting
polypyrazolylborates
of
h e a v i e r metals h a v e a p p e a r e d : W i l k i n s o n a n d c o - w o r k e r s ( 3 8 ) d e s c r i b e d the synthesis o f t h e s e v e n - c o o r d i n a t e [ H B ( p z ) ] 2 T a M e 2
2
3
while Bagnall
a n d E d w a r d s ( 3 9 ) p r e p a r e d the s u b l i m a b l e , green, air-sensitive H B ( p z ) 3
U C I 0 C 5 H 5 as w e l l as [ H B ( p z ) ] U , 2
UC1
2
(40).
T h e above
findings
2
4
[ H B ( p z ) ] U , and [ H B ( p z ) ] 3
4
3
2
bear witness to a sustained a n d r a p i d
g r o w t h of p o l y p y r a z o l y l b o r a t e c h e m i s t r y .
Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.
Trofimenko, S.,J.Am. Chem. Soc. (1966) 88, 1842. Trofimenko, S., Acc. Chem. Res. (1971) 4, 17. Trofimenko, S., Chem. Rev. (1972) 72, 497. Jesson, J. P., Trofimenko, S., Eaton, D. R.,J.Am. Chem. Soc. (1967) 89, 3148. Trofimenko, S., J. Am. Chem. Soc. (1969) 91, 3183. Trofimenko, S., J. Coord. Chem. (1972) 2, 75. Clark, H.C.,Manzer, L. E., Inorg. Chem. (1974) 13, 1291. Ibid. (1974) 13, 1996. Davies, B. W., Payne, N.C.,Inorg. Chem. (1974) 13, 1843. Clark, H. C., Manzer, L. E.,J.Chem. Soc. Chem. Commun. (1973) 870. Kadonaga, M., Yasuoka, N., Kasai, N., J. Chem. Soc. Chem. Commun. (1971) 1597. Rush, P. E., Oliver, J. D.,J.Chem. Soc. Chem. Commun. (1974) 996. Trofimenko, S.,J.Am. Chem. Soc. (1969) 91, 588. Meakin, P., Trofimenko, S., Jesson, J. P., J. Am. Chem. Soc. (1972) 94, 5677. Clark, H.C.,personal communication. Davies, B. W., Payne, N.C.,personal communication. Bruce, M. I., Ostazewski, A. P. P., J. Chem. Soc. Dalton Trans. (1973) 2433. Bruce, M. I., personal communication. Arcus, C. S., Wilkinson, J. L., Mealli, C., Marks, T. J., Ibers, J. Α.,J.Am. Chem. Soc. (1974) 96, 7564. Abu Salah, Ο. M., Bruce, M. I.,J.Organomet. Chem. (1975) 87, C 15. Trofimenko, S., Inorg. Chem. (1970) 9, 2493. Kosky, C. Α., Ganis, P., Avitabile,C.,Acta Cryst. Sect. Β (1971) 27, 1859. Cotton, F. Α., Jeremic, M., Shaver, Α., Inorg. Chem. Acta (1972) 6, 543. Calderon, J. L., Cotton, F. Α., Shaver, Α., J. Organomet. Chem. (1972) 42, 419. King, R. B., Bond, Α., J. Am. Chem. Soc. (1974) 96, 1338. Cotton, F. Α., LaCour, T., Stanislowski, A.G.,J.Am. Chem. Soc. (1974) 96, 754. Cotton, F. Α., Stanislowski, A. G.,J.Am. Chem. Soc. (1974) 96, 5074. Cotton, F. Α., Frenz, Β. Α., Stanislowski, A. G., Inorg. Chim. Acta (1973) 7, 503. Cotton, F. Α., Day, V. W., J. Chem. Soc. Chem. Commun. (1974) 415. Cotton, F. Α., personal communication Holt, Ε. M., personal communication. Echols, Η. M., Dennis, D., Acta Cryst. Sect. Β (1974) 30, 2173.
24. TROFiMENKO Polypyrazolylborate Chemistry
301
33. Guggenberger, L. J., Prewitt, C. T., Meakin, P., Trofimenko, S., Jesson, J. P., Inorg. Chem. (1973) 12, 508. 34. King, R. B., Bond, Α., J. Am. Chem. Soc. (1974) 96, 1334. 35. O'Sullivan, D. J., Lalor, F. J., J. Organomet. Chem. (1973) 57, C58. 36. Ferguson, G., Restivo, R. J., J. Chem. Soc. Chem. Commun. (1973) 847. 37. Ferguson, G., private communication. 38. Williamson, D. H., Santini-Scampucci, C., Wilkinson, G.,J.Organomet. Chem. (1974) 77, C25. 39. Bagnall, K. W., Edwards, J., J. Organomet. Chem. (1974) 80, C14. 40. Bagnall, K. W., Edwards, J., du Preez, J. G. H., Warren, R. F.,J.Chem. Soc. Dalton Trans. (1975) 140. RECEIVED February 6, 1975.
