Some Recent Advances in Polypyrazolylborate Chemistry

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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.

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