Mechanistic Aspects of Inorganic Reactions - American Chemical

17 Proton-Transfer Reactions i n Organometallic Chemistry

Downloaded by CORNELL UNIV on May 17, 2017 | http://pubs.acs.org Publication Date: September 27, 1982 | doi: 10.1021/bk-1982-0198.ch017

RICHARD F. JORDAN and JACK R. NORTON Colorado State University, Department of Chemistry, Fort Collins, CO 80521 The Brønsted acid-base behavior of transi tion metal hydride complexes and their conjugate bases has been investigated. Deprotonation of transiton metal hydrides often results in large changes of coordination geometry. This, in turn, results in large intrinsic activation barriers for deprotonation and correspondingly small rate constants for proton transfer. Degenerate proton exchange reaction rate constants between transi tion metal carbonyl hydrides and their conjugate bases are found to span a range of more than 10 , CpCr(CO)H exhibiting the largest "self-exchange" rate constant and hydrido-osmium tetracarbonyl complexes the smallest. There is a tendency for the rate of deprotonation to decrease with de creasing acidity of the hydrido complex. 6

3

Proton transfer reactions involving organic compounds have been the subject of extensive study Cl-7), and there has also been some investigation of proton transfer reactions involving coordinated organic ligands (8). Proton transfers involving 0-H bonds (e.g., from phenols to phenolate ions) are quite fast and frequently diffusion-controlled as are proton transfers involv ing N-H bonds. In comparison, proton transfers involving carbon acids, such as nitromethane, are generally quite slow in both the forward and reverse directions. +

+ +- CHN0 J H C=N^ ^ ° ". Β + CHN0 J BH 3

2

2

2

2

(1)

The contrast is explained by the extensive electronic and struc tural reorganizations that must take place when nitromethane and 0097-6156/82/0198-0403$06.25/0 © 1982 American Chemical Society Rorabacher and Endicott; Mechanistic Aspects of Inorganic Reactions ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

MECHANISTIC ASPECTS OF INORGANIC REACTIONS


404

other carbon a c i d s a r e deprotonated o r when the corresponding anions a r e protonated. Oxygen and n i t r o g e n a r e r e l a t i v e l y e l e c t r o n e g a t i v e atoms and the lone p a i r s r e s u l t i n g from depro t o n a t i o n o f 0-H and N-H bonds are correspondingly s t a b l e , where as anions r e s u l t i n g from the deprotonation o f carbon a c i d s a r e s t a b l e o n l y when there i s e x t e n s i v e d e r e a l i z a t i o n o f the lone p a i r over more e l e c t r o n e g a t i v e atoms as i l l u s t r a t e d f o r n i t r o methane anion i n r e a c t i o n 1. This d e r e a l i z a t i o n w i l l , i n g e n e r a l , be r e f l e c t e d i n extensive s t r u c t u r a l d i f f e r e n c e s be tween carbon a c i d s and the corresponding anions. I f we consider proton t r a n s f e r r e a c t i o n s i n v o l v i n g t r a n s i t i o n metal-hydrogen bonds, i t i s apparent t h a t they should have more i n common w i t h those o f carbon a c i d s than w i t h those of oxygen o r n i t r o g e n a c i d s . T r a n s i t i o n - m e t a l anions are gen e r a l l y s t a b i l i z e d by c o n s i d e r a b l e charge d e r e a l i z a t i o n onto π-acceptor l i g a n d s such as CO. The extent o f t h e e l e c t r o n i c rearrangement r e q u i r e d as t r a n s i t i o n - m e t a l hydrides such as HCo(CO)^ and HMn(CO)^ a r e deprotonated can be gauged from the f a c t t h a t t h e i r carbonyl s t r e t c h i n g frequencies t y p i c a l l y change 1

by over 100 cm as a r e s u l t , r e f l e c t i n g a s u b s t a n t i a l decrease i n C-0 bond order as π-backbonding i n c r e a s e s . T r a n s i t i o n - m e t a l hydrides a l s o , l i k e carbon a c i d s , undergo c o n s i d e r a b l e s t r u c t u r a l change upon deprotonation. Whereas a n i t r o g e n o r oxygen lone p a i r i s s t e r e o c h e m i c a l l y a c t i v e , and the molecular geometry, t h e r e f o r e , changes l i t t l e upon deprotona t i o n , t r a n s i t i o n - m e t a l "lone p a i r s " merely i n c r e a s e the formal d - e l e c t r o n c o n f i g u r a t i o n and a r e n o t s t e r e o c h e m i c a l l y a c t i v e . The deprotonation o f a t r a n s i t i o n - m e t a l h y d r i d e , t h e r e f o r e , pro duces c o n s i d e r a b l e changes i n c o o r d i n a t i o n geometry ( 9 ) . The c o n t r a s t i s i l l u s t r a t e d by the r e c e n t l y reported (10) X-ray s t r u c t u r e o f [Me^NH][Co(CO)^]. The C-N-C angles average 109°, e s s e n t i a l l y equal

t o those

o f 110° found i n Me^N i t s e l f ( 1 1 ) ,

whereas the C-Co-C angles average 106°, c o n s i d e r a b l y l a r g e r than the C ( a x i a l ) - C o - C ( e q u a t o r i a l ) angles o f 99.7° found i n the C 3 v

symmetry HCo(CO), ( 1 2 ) . The n i t r o g e n i s thus t e t r a h e d r a l l y coordinated

i n both Me^NH

and Me^N, whereas t h e c o o r d i n a t i o n

geometry o f c o b a l t changes from a d i s t o r t e d t r i g o n a l bipyramid i n HCo(CO)^ t o an approximately t e t r a h e d r a l arrangement i n [Co(C0) ]". 4

Information P e r t i n e n t t o Proton T r a n s f e r There i s very l i t t l e i n f o r m a t i o n on proton t r a n s f e r r a t e s i n v o l v i n g M-H bonds a g a i n s t which t o check the above p r e d i c t i o n . Walker, Kresge, F o r d , and Pearson (WKFP) have r e c e n t l y reported

Rorabacher and Endicott; Mechanistic Aspects of Inorganic Reactions ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


17.

Pwton-Transjer

JORDAN AND NORTON

Reactions

405

(13) a stopped-flow study o f the deprotonation o f s e v e r a l hy d r i d e s by sodium methoxide i n methanol i n which they conclude t h a t the r a t e s " a r e remarkably s m a l l f o r a base as s t r o n g as methoxide i o n " and "are q u i t e comparable t o r a t e s o f r e a c t i o n of n i t r o p a r a f f i n s w i t h hydroxide i o n " . However, most o f the hydrides s t u d i e d were p o l y n u c l e a r . We b e l i e v e t h a t systematic i n f o r m a t i o n on t h e common mono n u c l e a r hydrides i s needed and have begun such an i n v e s t i g a t i o n . We have chosen a c e t o n i t r i l e as a s o l v e n t f o r s e v e r a l reasons: 1) i t s h i g h d i e l e c t r i c constant and s t r o n g s o l v a t i o n o f a l k a l i metal c a t i o n s make i t a good s o l v e n t f o r o r g a n o m e t a l l i c anions and minimize c o m p l i c a t i o n s due t o i o n p a i r i n g i n s o l u t i o n ; 2) i t d i s s o l v e s most t r a n s i t i o n - m e t a l hydrides without r e a c t i n g w i t h them; 3) i t s low s e l f - i o n i z a t i o n constant (14) permits the use of a wide range o f a c i d and base s t r e n g t h s ; 4) the e x i s t e n c e o f accurate pK data f o r a wide range o f n i t r o g e n bases (15, 16) a allows s t r a i g h t f o r w a r d determination o f pK values f o r organo m e t a l l i c hydrides by IR spectroscopy. We have used a c e t o n i t r i l e d i s t i l l e d from ?2^5 * n i t r o g e n and handled by standard fl

u n
0 s ( C 0 ) H 4

4

3

4

[Os(CO) CH ]' + CH X •* 0 s ( C 0 ) ( C H ) 4

3

3

(2)

3

4

3

2

+ [Os(CO) CH ]" (3) 4

2

3

(4)

With methyl t o s y l a t e as CH X, the r a t e s o f r e a c t i o n s 2 and 3 3

are approximately equal, whereas w i t h methyl f l u o r o s u l f o n a t e as Ch* X, r e a c t i o n 2 i s much f a s t e r (17). Thermodynamic data on the a c i d i t y o f o r g a n o m e t a l l i c hy d r i d e s should help i d e n t i f y s i t u a t i o n s where apparent r e a c t i o n s of a c i d i c t r a n s i t i o n - m e t a l hydrides a c t u a l l y r e s u l t from t h e i r conjugate bases. A case i n which both species can r e a c t but g i v e d i f f e r e n t products (as was p o i n t e d out by P r o f . Espenson three years ago (18)) i s the a d d i t i o n o f hydridocobaloximes, HCo(dmgH) B, t o o l e f i n s w i t h electron-withdrawing s u b s t i t u e n t s 3

2

CH =CHX, r e a c t i o n s 5 and 6.



406

HCo(dmgH) B + CH =CHX -> CH CHXCo(dmgH) B 2

2

3

(5)

2

Co(dmgH) B~ + CH =CHX -» 5 XCH CH Co(dmgH) B 2

2

2

2

2

(6)

The d i r e c t i o n o f a d d i t i o n depends upon whether the hydridocobaloxime o r i t s conjugate anion i s t h e r e a c t i v e species (19, 20). The f a c t t h a t these r e a c t i o n s can both be observed sug gests (but does n o t prove) t h a t proton exchange between HCo(dmgH) B and Co(dmgH) B~ i s slow. Downloaded by CORNELL UNIV on May 17, 2017 | http://pubs.acs.org Publication Date: September 27, 1982 | doi: 10.1021/bk-1982-0198.ch017

2

2

Another way i n which a t r a n s i t i o n - m e t a l hydride may con c e i v a b l y i n t e r a c t w i t h a s u b s t r a t e i s by rate-determining pro ton t r a n s f e r t o the s u b s t r a t e i t s e l f , r e a c t i o n 7. M-H + S •* M" + S H

+

(7)

An understanding o f k i n e t i c a c i d i t y i s necessary i n order t o d i s t i n g u i s h such mechanisms from other ways i n which hydrogen may become attached t o a s u b s t r a t e , e.g., hydrogen atom t r a n s f e r , r e a c t i o n 8, and hydride t r a n s f e r , r e a c t i o n 9. M-H + S + ·Μ + *SH M-H + S -» M

+

+

(Β)

SH"

(9)

I t should be both p o s s i b l e and p r o f i t a b l e t o d i v i d e a l l reac t i o n s o f t r a n s i t i o n - m e t a l hydrides i n t o these three c l a s s e s . Even concerted r e a c t i o n s , i n which the metal i n t e r a c t s w i t h the s u b s t r a t e a t the same time as i t s hydrogen l i g a n d does, can u s e f u l l y be d i v i d e d according t o which o f t h e above hydrogen t r a n s f e r r e a c t i o n s they most resemble. Proton T r a n s f e r i n Osmium-Tetracarbonyl Complexes A l l o f our measurements o f proton t r a n s f e r r a t e s i n aceto n i t r i l e have been done by v a r i o u s NMR techniques. One o f t h e s i m p l e s t has been used t o measure t h e r a t e s o f t r a n s f e r from c i s - 0 s ( C 0 ) H t o Et^N and back again (21). Whereas a mixture o f 4

cis-0s(C0) H 4

2

2

and K[0s(C0) H] shows two sharp resonances i n t h e 4

hydride r e g i o n o f the *H NMR ( F i g u r e 1 ) , the a d d i t i o n o f Et^N t o 0s(C0) h* 4

2

gives

resonances

a t t h e same two p o s i t i o n s which,

however, are q u i t e broad a t some temperatures ( F i g u r e 2 ) . The resonances correspond t o unreacted 0s(C0) h* and t o t h e 4

0s(C0) H 4

2

produced by i t s p a r t i a l deprotonation by Et^N.

The

l i n e width i s produced by the r a p i d o p e r a t i o n o f r e a c t i o n 10 i n both d i r e c t i o n s .



17.

JORDAN AND NORTON

t 1.93

Proton-Transfer

Reactions

• 0.05

407

t • -9.03 -10.4

Figure 1. The 100-MHz Ή NMR spectrum of cis-Os(COhH (0.05 M), K[Os(CO) H](0.08 M), and hexamethyldisiloxane (0.01 M) (used as an internal line width standard) in CD CN. Chemical shifts are illustrated in B. The signal at M .93 is due to residual solvent protons. 2

k

S

-40

ι

10.4δ

Figure 2. The 100-MHz *H NMR spectra (hydride region only) of a CD CN solution of 0.3 M cis-OsfCO^Hi and 1.5 M Et N as a function of temperature. Chemical shifts are illustrated in 8. S

s


408


cis-0s(C0) H 4

+ Et N

2

J k

3

1

0s(C0) H

+ E t NH

4

(10)

r D i v i s i o n o f the excess l i n e w i d t h , A ( 0 s ( C 0 ) H ) 4

crease i n the 0 s ( C 0 ) H 4

( i . e . , the i n

2

l i n e w i d t h due t o r e a c t i o n 10), by the

2

c o n c e n t r a t i o n o f Et^N g i v e s the forward r a t e constant k^, and a s i m i l a r procedure g i v e s k . 7lA(Os(CO) H ) 4


C

f

=

2

(11)

ïlt^NÎ 7lA(0s(C0) H ) ^ — [Et NH ] A

k

=

r

(12)

3

The 0 s ( C 0 ) H 4

peak i s both s m a l l e r and, a t a g i v e n temperature,

broader, as e q u i l i b r i u m 10 l i e s f a r t o the l e f t . The v a l u e o f the e q u i l i b r i u m constant f o r r e a c t i o n 10 can be c a l c u l a t e d from k^/k^ tensities

from the r a t i o o f the i n t e g r a t e d i n

o f the 0 s ( C 0 ) H 4

and 0 s ( C 0 ) H

2

peaks, and from spec

4

t r o p h o t o m e t r y (carbonyl r e g i o n IR) t i t r a t i o n of 0 s ( C 0 ) H 4

2

with

E t N . F o r t u n a t e l y , a l l t h r e e v a l u e s agree q u i t e w e l l . The Κ thus determined and the known pK of E t N H i n a c e t o n i t r i l e (15, a D 16) g i v e the v a l u e f o r the pK o f 0s(C0),H i n a c e t o n i t r i l e as l i s t e d i n Table I . * In p r i n c i p l e , the NMR l i n e widths a l s o r e f l e c t 0 s ( C 0 ) H / [0s(C0) H] p r o t o n exchange. ( E t N / E t N H p r o t o n exchange i s 3

+

Q

9

a

Z

4

2

+

4

3

3

extremely r a p i d and t h e i r NMR s p e c t r a a r e completely averaged under a l l c o n d i t i o n s . ) However, t h e sharp l i n e s seen f o r the 0 s ( C 0 ) H / K [ 0 s ( C 0 ) H ] mixture show t h a t such exchange i s ex tremely slow. (IR s t u d i e s (21) c o n f i r m t h a t there i s l i t t l e contact i o n p a i r f o r m a t i o n w i t h o r g a n o m e t a l l i c ions i n a c e t o n i t r i l e and suggest t h a t a r e a c t i o n which i s slow f o r K[0s(C0) H] 4

2

4

4

will

also

be slow

for

[Et NH] [0s(C0) H].) 3

4

I t i s , however,

p o s s i b l e t o measure the r a t e o f t h i s p r o t o n exchange by another technique, s a t u r a t i o n t r a n s f e r (22).

As shown i n F i g u r e 3, i r

r a d i a t i o n of the 0s(C0) H

causes a decrease i n the

resonance

4

intensity

o f the 0 s ( C 0 ) H 4

2

resonance.

From M , the q

intensity



4

4

HOs(C0) 0s(CO) H

4

Os(CO) (CH )H

2

3

Et N

Os(CO) H

3

3

Et N

tetramethy1guanidine

3

morpholine

CpW(CO) H

4

pyridine

CpMo(CO) H

3

pyridine

3

CpCr(CO) H

r

20.4

3

Et N

4

3

0s(C0) CH "

3

0s(C0),H" Et N

3

CpW(C0) " morpholme

3

CpMo(C0) "

3

CpCr(C0) "

HOs(CO),Os(CO) " E t /

23.0(0C)

20.8

16.1

13.9(0C)

13.3

Base Employed for Kinetic Acidity Measurement

3

negligible 12

not measured, but presumably v e r y slow 34

0.075 480

640 1.0 χ 10^

2.4 χ 1 0

4

1

7.4 χ 1 0

25C,M" s"

1

Second-order Rate Constant f o r H+ T r a n s f e r

9.1

8.5

6.5

8.1 11.3

6.8

8.8

ΔΗ*, kcal/mol

-23

-23

-24

-19 2

-20

- 7

AS*, cal/deg/mol

Thermodynamic and K i n e t i c A c i d i t i e s o f Metal Hydrides i n A c e t o n i t r i l e

Base Employed f o r Thermo pK o f Metal dynamic A c i d i t y a Metal Hydride Measurement Hydride (25C)

Table I .


410



M

0

t î -9.03 -10.4

1.93 0.05

Figure 3. The 100-MHz NMR spectra at 40°C of a CD CN solution of cisOs(CO) H (0.85 M) and K[Os(CO) H] (0.34 M), with (MJ and without (M ) irradiation at the hydride resonance of K[Os(CO) H] (indicated by the arrow in lower spectrum). Ratio M„: M = 0.7. Chemical shifts are illustrated in δ. The signal at hi.93 is due to residual solvent protons. Hexamethyldisiloxane (020 M), resonance W.05, is an internal intensity standard. S

h

t

h

0

k

0


Proton-Transfer

JORDAN AND NORTON

17.

Reactions

411

of the Os(CO)^R^ s i g n a l without i r r a d i a t i o n , M^, after time

i t s intensity

prolonged i r r a d i a t i o n , and the s p i n - l a t t i c e r e l a x a t i o n of O s i C O ) ^ ^ , the r a t e constant f o r p r o t o n exchange

, Os-Os , _ . κ can be obtained. Os-Os

1

2

[Os(CO) H-]


4

V

M (Os(CO),H )

0s(C0

W

^(0.(00)^)

1 ]

( 1 3 )

The

experiment can a l s o be performed i n the reverse d i Os-Os r e c t i o n and k determined independently. However, as the time s c a l e of t h i s r a t e measurement technique i s determined by T,

and as the T j of 0s(C0) H~

1

4

onds a t 30C) ject

to

than t h a t of 0 s ( C 0 ) H 4

considerable

0s(C0) H

irradiation

4

i s a great d e a l longer (240 sec (34.6 seconds) and i s sub

2

u n c e r t a i n t y , the i s more

value

reliable.

obtained

From

a

single

from ex

periment, the two values agree reasonably w e l l ( f o r example, 0.058 M" s" versus 0.037 M" s" a t 30C), but the measured r a t e constant v a r i e s somewhat from experiment t o experiment. Consid e r i n g the slowness of t h i s p r o t o n t r a n s f e r , the r a t e constant v a r i a t i o n may r e f l e c t v a r y i n g degrees of a d v e n t i t i o u s c a t a l y s i s . 1

1

1

1

As l i t t l e as 10 M of an i m p u r i t y w i t h the k i n e t i c b a s i c i t y of Et^N would e x p l a i n the observed r a t e v a r i a t i o n . The a d d i t i o n of a s m a l l amount of e t h a n o l (which i s not s i g n i f i c a n t l y depro tonated by 0s(C0) H~) f a i l e d t o a f f e c t the r a t e . 4

We have a l s o looked a t c i s - 0 s ( C 0 ) ( C H ) H / E t N proton t r a n s f e r by observing the c o l l a p s e of the hydride q u a r t e t ( F i g u r e 4) (21) . The observed and c a l c u l a t e d s p e c t r a are q u i t e s i m i l a r to the 1957 ones - which have become standard textbook m a t e r i a l of Grunwald, Loewenstein, and Meiboom f o r the deprotonation of 4

CH NH 3

+ 3

3

3

(23).

Other Complexes F i n a l l y , we have measured the r a t e constants of proton t r a n s f e r r e a c t i o n s i n v o l v i n g (r) -C H )M(C0) H (M = Cr, Mo, W) 5

5

5

3

by watching the coalescence of the c y c l o p e n t a d i e n y l s i g n a l s of the anion and of the n e u t r a l hydride ( F i g u r e 5) and by d e t e r mining the excess l i n e widths due t o proton t r a n s f e r i n a mix-



Β


A

Figure 4. The hydride region 100-MHz *H NMR spectra observed for 0.31 M cis-Os(CO) (CH )H and 1.20 M Et N in CD CN as a function of temperature (5°C) where kt = 14 s' and calculated for the exchange rates (with respect to ch-Os(CO) (CH )H) shown. Key: A, observed; and B, calculated. k

s

s

S

1

h

s


Proton-Transfer

JORDAN AND NORTON

Reactions

413


-10

-36

Figure 5. The 100-MHz H NMR spectra of a CD CN solution of ( -C H )W(CO)sH (0.092 M) and [( -C H )W(CO) Y (0.058 M) as a function of T. Only the region containing the cyclopentadienyl resonances is shown. Chemical shifts are illustrated in δ. X

5

v

S

5

5

S

V

s


5

5


414

t u r e o f (n -C H )W(CO) H and morpholine 5

5

5

(21). A l l o f these r a t e

3

constants have been measured over a range o f temperatures which give r a t e s a p p r o p r i a t e f o r the v a r i o u s methods o f measurement. A c t i v a t i o n parameters and values o f the r a t e constants extrapo l a t e d t o 25C are given i n Table I , as are pK values a r i s i n g a from IR determination (checked by NMR when p o s s i b l e as above f o r Os(CO) H /Et N) o f e q u i l i b r i u m constants f o r the deprotonation 4

2

3


of the hydride complexes by v a r i o u s n i t r o g e n bases. Conclusions These r e s u l t s suggest a number o f g e n e r a l i z a t i o n s : 1) The e q u i l i b r i u m a c i d i t y decreases down a column i n the p e r i o d i c t a b l e (Cr>Mo»W). While t h i s g e n e r a l i z a t i o n has been o f f e r e d before (24, 25), there has been no previous q u a n t i t a t i v e measurement o f the a c i d i t y o f a l l three members o f such a series. 2) The k i n e t i c a c i d i t y ( r a t e constant f o r metal-to-metal proton exchange) a l s o decreases down a column (Cr>Mo»W) i n the p e r i o d i c t a b l e . This p a r a l l e l s the order o f r a t e s we have observed f o r the d i n u c l e a r e l i m i n a t i o n o f methane from (n -C H ) Zr(CH ) 5

5

5

suggests

2

3

that

5

and HM(CO) (r| -C H )

2

3

the l a t t e r

(n -C H ) Zr(CH ) 5

2

3

(M = C r , Mo, W) and

5

transfer.

+ HM(CO) (n -C H ) ->

5

5

5

r e a c t i o n i n v o l v e s proton

5

3

2

5

5

(14)

CH + (n -c H ) zr 4

*M(co) (n -c H )

5

5

5

CH

5

2

2

5

5

3

3) Replacement o f a hydride l i g a n d by a methyl s u b s t i t u ent decreases both the thermodynamic and the k i n e t i c a c i d i t y of the remaining hydrogen, w h i l e i t s replacement by an a d d i t i o n a l Os(CO) H u n i t i n c r e a s e s the thermodynamic a c i d i t y but de creases the r a t e o f deprotonation. The same a d d i t i o n a l d e r e a l i z a t i o n t h a t decreases the pK o f 0 s ( C 0 ) H r e l a t i v e t o that a 2 οζ of 0 s ( C 0 ) H presumably increases the e l e c t r o n i c rearrangement t h a t must occur upon deprotonation and, t h e r e f o r e , decreases the r a t e o f t h a t process. 4) The t r a n s f e r o f protons from metal hydrides t o metal 4

o

4

o

o

2


17.

JORDAN AND NORTON

Proton-Transfer

Reactions

415

anions i s much slower than t r a n s f e r t o n i t r o g e n bases of compar able to

strength.

F o r example,

(n -C H )W(CO) ~ (pK 5

5

5

3

transfer

from

o f conjugate a c i d

a

(iy*-C,-H,.)W(CO) H 3

16.1) i s 156 times

slower than t o morpholine (pK

of conjugate a c i d 16.6); t r a n s f e r a from Os(CO) H t o 0 s ( C 0 ) H " (pK of conjugate a c i d 20.8) i s over 6,000 times slower than t r a n s f e r t o t r i e t h y l a m i n e (pK^ o f con j u g a t e a c i d 18.5). I t appears t h a t the p r e d i c t i o n t h a t p r o t o n t r a n s f e r processes i n v o l v i n g metals would be slower than com parable processes i n v o l v i n g n i t r o g e n i s c o r r e c t . An A l t e r n a t i v e Mechanism. C o n s i d e r i n g the f a c i l i t y o f the e l e c t r o n t r a n s f e r r e a c t i o n s t o which a g r e a t d e a l o f t h i s sym posium has been devoted, we have t o worry whether our "proton t r a n s f e r " r e a c t i o n s may not r e a l l y be the r e s u l t o f e l e c t r o n t r a n s f e r i n the reverse d i r e c t i o n f o l l o w e d by hydrogen t r a n s f e r . As Bergman (26) has r e c e n t l y reported t h a t another h y d r i d e anion may a c t as a o n e - e l e c t r o n reducing agent, and as we have e v i dence i m p l i c a t i n g 0 s ( C 0 ) H as an i n t e r m e d i a t e i n a number o f


4

2

4

fl

#

4

other r e a c t i o n s , the mechanism o f r e a c t i o n s 15 and 16 has t o be considered as an a l t e r n a t i v e t o d i r e c t 0 s ( C 0 ) H / 0 s ( C 0 ) H ton exchange. 4

0s(C0) H 4

2

+ 0s(C0) H~ 4

t r a n

0 s ( C 0 ) H " + 0s(C0) H4

2

4

t r a

f

f e r

Jf

e r

2

4

pro

0 s ( C 0 ) H " + 0s(C0) H- (15) 4

2

4

0 s ( C 0 ) H " + 0 s ( C 0 ) H (16) 4

4

2

The o p e r a t i o n o f t h i s a l t e r n a t i v e mechanism may e x p l a i n why the r a t e of t h i s r e a c t i o n appeared t o v a r y from experiment t o exper iment, although a more l i k e l y e x p l a n a t i o n i s a d v e n t i t i o u s catalysis. D i r e c t i o n s f o r Future Work. The measurement o f r a t e s o f p r o t o n t r a n s f e r from a s i n g l e a c i d t o more bases d i f f e r i n g o n l y i n thermodynamic base s t r e n g t h should a l l o w the c o n s t r u c t i o n o f Bronsted p l o t s o f k i n e t i c versus thermodynamic a c i d i t y . The bases we have used a t t h i s e a r l y stage o f development o f the s u b j e c t have i n v o l v e d d i f f e r e n t p r o t o n acceptor atoms and cannot be so used (although comparison o f the Et^N t r a n s f e r r a t e s o f 0s(C0) H 4

2

and Os(CO) (CH )H suggests a Bronsted s l o p e Of o f about 4

3

0.5). The slopes o f such p l o t s should g i v e v a l u a b l e knowledge of the d e t a i l s o f p r o t o n t r a n s f e r processes i n v o l v i n g metals. We i n t e n d t o extend t h i s work t o i n c l u d e the comparison o f hydrides o f d i f f e r e n t types. I n c o n t r a s t t o o r g a n i c a c i d s ,



416

where the protons are bound t o oxygen, n i t r o g e n , o r carbon i n a few simple ways, a c i d i c i n o r g a n i c hydrides are known f o r v i r t u a l l y a l l the t r a n s i t i o n elements and d i s p l a y great s t r u c t u r a l variety. A p a r t i c u l a r l y f a s c i n a t i n g c l a s s , j u s t now becoming w i d e l y recognized, i s t h a t o f i n t e r s t i t i a l h y d r i d e s . An example


is

[Rh^^(CO)^^H^_ ] n

n

, where η ranges

from two t o f o u r , one o f

many i n t e r e s t i n g s t r u c t u r e s f o r which we a r e indebted t o t h e l a t e Paolo C h i n i (27) . The hexagonal-close-packed rhodium framework i s i l l u s t r a t e d i n F i g u r e 6. One, two, o r three pro tons occupy semi-octahedral holes (28, 29). The r a t e o f de p r o t o n a t i o n o f such systems i s undoubtedly v e r y slow and may r e f l e c t t h e r a t e a t which i n t e r s t i t i a l hydrogens migrate t o surface s i t e s .



17.

JORDAN AND NORTON

Proton-Transfer

417

Reactions

2 interstitial H Figure 6.

The hexagonal-close-packed rhodium framework of

[Rh (CO)nH . ] ~. ÎS


5 n

n


418

Acknowledgments This work was supported by NSF Grant No. CHE79-20373 and by the Colorado S t a t e u n i v e r s i t y Regional NMR Center, funded by Grant No. CHE78-18581. The authors are g r a t e f u l t o Dr. Bruno Longato and Robin E d i d i n f o r p r e l i m i n a r y work on d i n u c l e a r e l i m i n a t i o n r a t e s and t o the A l f r e d P. Sloan Foundation f o r a f e l l o w s h i p t o J . R. N.

Literature Cited Downloaded by CORNELL UNIV on May 17, 2017 | http://pubs.acs.org Publication Date: September 27, 1982 | doi: 10.1021/bk-1982-0198.ch017

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

Bell, R. P. "The Proton in Chemistry"; Cornell U. Press: Ithaca, Ν. Υ., 1973. Reutov, O. Α.; Beletskaya, I. P.; Butin, Κ. P. "CH-Acids"; Pergamon: New York, 1978. Albery, W. J. Ann. Rev. Phys. Chem. 1980, 31, 227. Hibbert, F. In "Comprehensive Chemical Kinetics"; Bamford, C. H.; Tipper, C. F. Η., Eds.; Elsevier: Amsterdam, 1977; Vol. 8, Chap. 2. Crooks, J . E. ibid., Chap. 3. Simmons, E. L. Progr. Reaction Kinetics 1977, 8, 161. Eyring, Ε. M.; Marshall, D. B.; Strohbusch, F.; Süttinger, R., This Volume, Chap. 3. Bannister, C. E.; Margerum, D. W.; Raycheba, J. M. T.; Wong, L. F. "Proton Transfer", Symposia Faraday Soc., Vol. 10; The Chemical Society: London, 1975; p 78. Frenz, Β. Α.; Ibers, J. A. "Transition Metal Hydrides", Muetterties, E. L . , Ed., Marcel Dekker: New York, 1971; Chap. 3. Calderazzo, F.; Fachinetti, G.; Marchetti, F.; Zanazzi, P. F. J. Chem. Soc., Chem. Commun. 1981, 181. Beagley, B.; Hewitt, T. G. Trans. Faraday Soc. 1968, 64, 2561. McNeil, Ε. Α.; Scholer, F. R. J. Am. Chem. Soc. 1977, 99, 6243. Walker, H. W.; Kresge, C. T.; Ford, P. C.; Pearson, R. G. J. Am. Chem. Soc. 1979, 101, 7428. Coetzee, J. F. Prog. Phys. Org. Chem. 1967, 4, 45. Kolthoff, I. M.; Chantooni, M. K.; Bhowmik, S. J. Am. Chem. Soc. 1968, 90, 23. Coetzee, J. F.; Padmanabhan, G. R. J. Am. Chem. Soc. 1965, 87, 5005. Evans, J.; Okrasinski, S. J.; Pribula, A. J.; Norton, J. R. J. Am. Chem. Soc. 1976, 98, 4000. Chao, T.-H.; Espenson, J. H. J. Am. Chem. Soc. 1978, 100, 129; and references therein. Schrauzer, G. N.; Windgassen, R. J. J. Am. Chem. Soc. 1967, 89, 1999.



17.

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Proton-Transfer

Reactions

419

20. Johnson, M. D.; Tobe, M. L . ; Wong, L. Y. J. Chem. Soc. A. 1968, 929. 21. Jordan, R. F.; Norton, J. R. J. Am. Chem. Soc. 1982, 104, 1255. 22. Faller, J. W. "Determination of Organic Structures by Physical Methods", Nachod, F. C., Zuckerman, J. J., Eds.; Academic Press: New York, 1973; Vol. 5, Chap. 2. 23. Grunwald, E.; Loewenstein, Α.; Meiboom, S. J. Chem. Phys. 1957, 27, 630. 24. Beck. W.; Hieber, W.; Braun, G. Z. Anorg. Allg. Chem. 1961, 308, 23. 25. Lokshin, Β. V.; Pasinsky, Α. Α.; Kolobova, Ν. E.; Anisimov, D. N.; Makarov, Y. V. J. Organomet. Chem. 1973, 55, 315. 26. Jones, W. D.; Huggins, J. M.; Bergman, R. G. J. Am. Chem. Soc. 1981, 103, 4415. 27. Albano, V. G.; Ceriotti, Α.; Chini, P.; Ciani, G.; Martinengo, S. J. Chem. Soc., Chem. Commun. 1975, 859. 28. Albano, V. G.; Ciani, G.; Martinengo, S.; Sironi, A. J. Chem. Soc., Dalton Trans. 1979, 978. 29. Ciani, G.; Sironi, Α.; Martinengo, S. J. Chem. Soc., Dal ton Trans. 1981, 519. RECEIVED April 27,

1982.


General Discussion—Proton-Transfer Reactions i n Organometallic Chemistry Leader: Dennis T u c k


DR. GREGORY GEOFFROY (Pennsylvania State U n i v e r s i t y ) : There was a recent paper [ V i d a l , J . L. ; Walker, W. E. Inorg. Chem. 1981, 20, 249] which i n d i c a t e d t h a t hydridorhodiumtetracarbonyl i s much more a c i d i c than h y d r i d o c o b a l t t e t r a c a r b o n y l . This goes a g a i n s t your general statement t h a t a c i d i t y decreases w i t h i n c r e a s i n g atomic number w i t h i n a group. Can you e x p l a i n that? DR. NORTON: I read t h a t paper w i t h some care. I am a f r a i d I am not convinced by t h e r e s u l t . The c o b a l t and i r i d i u m r e s u l t s a r e b e l i e v a b l e . They have been c a r e f u l l y measured and they agree w i t h the general t r e n d . The rhodium r e s u l t i s not w e l l measured, and I am i n c l i n e d , t h e r e f o r e , t o be s u s p i c i o u s o f the f a c t t h a t i t doesn't f i t the general t r e n d . DR. GEOFFROY: Do you know anything about d i h y d r i d o r u t h e niumtetracarbonyl compared t o d i h y d r i d o i r o n c a r b o n y l ? DR. NORTON: No. DR. HENRY TAUBE (Stanford U n i v e r s i t y ) : There i s i n the l i t e r a t u r e a d e s c r i p t i o n o f bis-ethylenediaminedihydride osmium(IV) prepared by M a l i n [ M a l i n , J . ; Taube, H. Inorg. Chem. 1971, 10, 2403]. The i n t e r e s t i n g t h i n g i s t h a t the compounds you d e s c r i b e , i n which you have f o u r carbon monoxides i n p l a c e of the two ethylenediamines and where the formal o x i d a t i o n s t a t e i s lower, a r e a c t u a l l y much more a c i d i c than M a l i n s d i h y d r i d e , i n which the o x i d a t i o n s t a t e o f Os i s 4+. The comparison i s n ' t exact, because M a l i n * s work was done i n aqueous s o l u t i o n , and bases a r e n o t as strong i n aqueous s o l u t i o n as i n n o n p r o t i c s o l v e n t s , but, n e v e r t h e l e s s , i t t e s t i f i e s t o the strong e l e c t r o n withdrawing power o f something l i k e carbon monoxide. Malin observed no d i m i n u t i o n i n the hydride nmr s i g n a l when h i s spe2+ cies, Osien)^^ , was d i s s o l v e d i n 2M aqueous hydroxide. 1

DR. NORTON: I t i s a l s o worth n o t i n g t h a t t h e osmium d i hydride i s a r a t h e r weak a c i d i f you compare i t t o these o t h e r s . DR. STEPHEN NEUMANN (Eastman Kodak Co.): I n your observa t i o n o f the proton t r a n s f e r r a t e s between the metal base and the amine base, do you have any f e e l i n g f o r whether the slower t r a n s f e r i s unique t o the metal bases o r whether i t has t o do more w i t h the b u l k o f t h e o v e r a l l base p a r t i c i p a t i n g i n the proton t r a n s f e r ? F o r i n s t a n c e , would a s t e r i c a l l y hindered amine show r a t e s s i m i l a r t o the metal bases. DR. NORTON: That i s a d i f f i c u l t q u e s t i o n t o answer a t bottom. But I t h i n k the general t r e n d i s c l e a r , namely t h a t a l l


17.

JORDAN AND NORTON

Proton-Transfer

Reactions

421


of the metal bases a r e slower than a l l o f the comparable n i t r o gen bases. I t i s d i f f i c u l t t o b e l i e v e t h a t t h i s phenomenon i s simply due t o s t e r i c hindrance. I suppose t h a t we should t r y p u t t i n g a s t e r i c a l l y hindered n i t r o g e n base i n t o t h e system. I t i s f a i r t o say, however, t h a t many o f these metal anions a r e not e x c e p t i o n a l l y hindered. F o r example, hydridoosmiumtetracarbonyl anion i s a t r i g o n a l b i p y r a m i d a l species w i t h minimal s t e r i c con gestion. DR. JOHN BRAUMAN ( S t a n f o r d U n i v e r s i t y ) : One o f the p o i n t s t h a t i s i n t e r e s t i n g i n these proton t r a n s f e r r e a c t i o n s i s the o b s e r v a t i o n t h a t o f t e n hydrogen-bonded intermediates don't seem to be o f much consequence as c o n t r a s t e d w i t h oxygen o r n i t r o g e n lone p a i r bases. There i s a v e r y l o n g , i n v o l v e d , and sometimes confusing literature about c o r r e l a t i o n s o f hydrogen bond strengths o r hydrogen bonding w i t h a c i d i t y , which i n v a r i a b l y break down i n p e c u l i a r s i t u a t i o n s . I s there any i n d i c a t i o n t h a t you can see hydrogen bonding i n these metal hydrides? DR. NORTON: An e x c e l l e n t attempt t o observe such hydrogen bonding was made r e c e n t l y by F a c h i n e t t i , e t a l . [Calderazzo, F.; F a c h i n e t t i , G. ; M a r c h e t t i , F.; Z a n a z z i , P. F. J . Chem. S o c , Chem. Commun. 1981, 181]. They took h y d r i d o c o b a l t t e t r a c a r b o n y l and t r i e t h y l a m i n e , and c r y s t a l l i z e d out a species which one can o n l y d e s c r i b e as the t e t r a c a r b o n y l c o b a l t a t e o f protonated t r i ethylamine. They proposed some type o f i n t e r a c t i o n between t h e hydrogen and a face o f the c o b a l t t e t r a h e d r a l complex, b u t i t was c l e a r t h a t the i n t e r a c t i o n was almost e n t i r e l y w i t h n i t r o gens. The c o n c l u s i o n I would draw i s t h a t the complex appears t o proceed d i r e c t l y t o f u l l p r o t o n a t i o n o f the amine without any observable evidence f o r a hydrogen bonded i n t e r m e d i a t e . DR. BRAUMAN: But there a r e much weaker bases, such as e t h e r s , which might show i n f r a r e d changes i n the metal hydride. DR. NORTON: The metal-hydrogen s t r e t c h e s tend t o be v e r y broad. This would hamper such o b s e r v a t i o n s . I am aware o f no r e p o r t s o f s o l v e n t dependence o f metal-hydrogen s t r e t c h e s . DR. DENNIS TUCK ( U n i v e r s i t y o f Windsor): There are some s t a b l e d i m e t a l carbonyl species i n which the hydrogen bond a c t u a l l y holds the two anions together ( i . e . , HX^ s p e c i e s ) . Are these r e l e v a n t t o the k i n e t i c s you a r e l o o k i n g a t ? DR. NORTON: We a r e expecting t o look a t such systems but haven't y e t done so. C l e a r l y , the next step i s t o begin l o o k i n g a t cases where the hydrogen i s b r i d g i n g two metal systems. DR.

JAMES

ESPENSON

(Iowa

State

University):

You have


422


t a l k e d about r e a c t i o n s i n which the metal hydride a c t s as a proton donor toward a Lewis base. Are there cases i n which the same metal complex can a c t as a hydride donor toward a Lewis acid?


DR. NORTON: I am not aware o f any cases where the same complex has been shown t o a c t i n both ways. I n g e n e r a l , one f i n d s t h a t metal hydrides tend t o be f a i r l y good a t e i t h e r one t h i n g o r the other. DR. THOMAS MEYER ( U n i v e r s i t y o f North C a r o l i n a ) : I feel t h a t you have glossed over something t h a t i s a c t u a l l y v e r y i n t e r e s t i n g , and t h a t i s the whole q u e s t i o n o f the Bronsted r e l a t i o n s h i p . You have now observed a l a r g e number o f systems, chromium, molybdenum, tungsten, which e x h i b i t t r i c a r b o n y l an i o n s . Have you looked a t r a t e constants f o r a l l those species w i t h the same hydride? You would then have three bases, a l l with identical structures. DR. NORTON: The answer i s , as y e t , no. I t h i n k t h e ex periment t o do would be t o take a weak base w i t h a l l t h r e e o f those hydrides as a c i d s and look a t the l i n e broadening. By v a r y i n g the c o n c e n t r a t i o n o f t h e weak base over an enormous range, one should be a b l e t o o b t a i n the p r o t o n t r a n s f e r r a t e constants o f a l l three o f those hydrides t o t h e same weak base. That i s the next experiment on our l i s t . DR. DALE MARGERUM (Purdue U n i v e r s i t y ) : You need t o be c a r e f u l because you may not o b t a i n a t r u e Bronsted p l o t from what you have j u s t d e s c r i b e d . You don't want t o make a p l o t where each system has a d i f f e r e n t rearrangement. You want t o be c e r t a i n t o have a common type o f a c i d o r base w i t h which you are r e a c t i n g . The l i t e r a t u r e i s f u l l o f i n v e r s e Bronsted r e l a t i o n ships i n which there has been a poor choice o f r e a c t i o n a c i d base p a i r s . Much c o n f u s i o n has been generated as a r e s u l t . DR. NORTON: U n f o r t u n a t e l y , a l l o f these systems tend t o be rather d i f f e r e n t s t e r i c a l l y . That, as I understand i t , makes the whole concept t o t a l l y i n a p p l i c a b l e . But i n the case t h a t Dr. Meyer has c i t e d , one would be v a r y i n g o n l y the c e n t r a l metal w i t h a common base. Simply by v a r y i n g the c e n t r a l m e t a l , I don't t h i n k one would be changing the geometry t h a t much. DR. MARGERUM: On the c o n t r a r y , I t h i n k you are changing i t v e r y much. You could use one o f your a c i d s and r e a c t i t w i t h a s e r i e s o f d i f f e r e n t bases, a l l o f which are s i m i l a r amine bases. DR. NORTON: That i s , i n f a c t , another s e t o f experiments on our l i s t . I would presume from what you are saying t h a t the proper approach would be t o take a constant h y d r i d e w i t h a


17.

JORDAN AND NORTON

Proton-Transfer

Reactions

s i n g l e metal and a v a r i e t y o f c l o s e l y r e l a t e d bases r a t h e r s m a l l pK^ range and measure the t r a n s f e r r a t e s .

423 over a

DR. MARGERUM: R i g h t . Otherwise, you w i l l have d i f f e r e n t r e o r g a n i z a t i o n a l terms o r d i f f e r e n t work terms, a l l o f which i s going t o confuse your Bronsted r e l a t i o n s h i p .


DR. WILLIAM TROGLER (Northwestern U n i v e r s i t y ) : You looked a t exchange between t h e monohydride anion o f osmium and t h e n e u t r a l d i h y d r i d e . Have you ever s t u d i e d the b i m o l e c u l a r s e l f exchange f o r a n e u t r a l d i h y d r i d e ? T h i s c o u l d be f o l l o w e d by mixing one sample l a b e l l e d w i t h deuterium and one w i t h hydrogen. DR. NORTON: No. Bergman has seen one such case. I t h i n k t h i s r e a c t i o n can take p l a c e o n l y when t h e r e i s some type o f base i n the medium t o c a t a l y z e t h a t e q u i l i b r a t i o n . I don't see i t as a completely non-acid-base phenomenon.


Mechanistic Aspects of Inorganic Reactions - American Chemical

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