Polyamine-Chelated Alkali Metal Compounds

4 Magnetic Resonance Studies of Polytertiary Amine Chelated Alkali Metal Compounds M. T. MELCHIOR, L. P. KLEMANN, and A. W. LANGER, JR. Downloaded by UNIV OF CINCINNATI on May 24, 2016 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/ba-1974-0130.ch004

Esso Research and Engineering Co., Linden, N.J. 07036

Magnetic resonance experiments have provided considerable structural information on chelated alkali metal compounds. The combination of H and Li NMR experiments on che lated lithium halides (Chel · LiX) with ESR experiments on chelated sodium naphthalenide (Chel · Na C H -) has shown that these systems are strongly-chelated, tight ion pairs which aggregate in aromatic hydrocarbon solvents. For a given chelating agent there is a strong correlation between the Na hyperfine interaction in Chel · Na C H and the Li chemical shift in Chel · LiBr. Evidence of a stereospecific collision complex between Chel · LiBr and aromatic solvents is derived from H NMR experiments. 1

7

+

10

8

23

+

10

8

7

1

T n the past 10 years n u c l e a r m a g n e t i c resonance *·* s p i n resonance

polytertiaryamine chelated alkali metal compounds. laboratories c o n c e r n e d alkyl compounds,

( N M R ) a n d electron

( E S R ) h a v e p r o v e d to b e v a l u a b l e tools f o r s t u d y i n g Ή

E a r l y w o r k i n these

N M R studies of d i a m i n e c h e l a t e d

lithium

p r i m a r i l y b u t y l l i t h i u m ( L i B u ) , i n paraffinic solvents.

It w a s s h o w n ( I , 2 ) that c h e l a t i o n results i n a m a r k e d u p f i e l d c h e m i c a l shift of t h e « - C H

2

protons of L i B u . T h e m a g n i t u d e of this shift correlates

w i t h t h e s t a b i l i t y of t h e c h e l a t e d L i B u ( C h e l · L i B u ) a n d p r e s u m a b l y reflects a p o l a r i z a t i o n a n d subsequent l o o s e n i n g of t h e L i - C b o n d .

There

is also a smaller d o w n f i e l d c h e m i c a l shift of t h e N - a l k y l g r o u p protons of the c h e l a t i n g agent c a u s e d b y a g e n e r a l transfer o f e l e c t r o n d e n s i t y f r o m c h e l a t i n g agent to l i t h i u m to a l k y l g r o u p .

T h i s d o w n f i e l d shift of t h e

c h e l a t i n g agent protons is a u s e f u l c r i t e r i o n for t h e presence o f c h e l a t e d l i t h i u m . F o r t h e p r o t o t y p e system T M E D · L i B u (see T a b l e I f o r c h e l a t i n g agents a n d a b b r e v i a t i o n s ) i n paraffinic solvent, this (normal) c h e l a t i o n shift is a b o u t 0.1 p p m d o w n f i e l d f o r N - C H

3

protons a n d essentially zero

113 Langer; Polyamine-Chelated Alkali Metal Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

114

P O L Y A M I N E - C H E L A T E D A L K A L I M E T A L COMPOUNDS

Table I.

Skeletal Structures of Polytertiaryamine Chelating Agents Structure

/

\

\

7

y

Abbreviation

/

X

Ν

/

TMED

Ν
N

/

\

Ν

/

\

w

Ν

/ ~ \

M

/

n-HMTT

Ν (

^4 \ N / for N - C H -

I protons.

2

V I

/ /

iso-HMTT

I

\

HMTP

T h i s is i n d i s t i n c t contrast to t h e b e h a v i o r

of

T M E D · L i B u i n b e n z e n e w h e r e a l a r g e u p f i e l d shift of a b o u t 0.5 p p m is e x p e r i e n c e d b y the N - C H - protons. 2

( T h i s statement takes i n t o a c c o u n t

a s m a l l c o r r e c t i o n c a u s e d b y p a r t i a l r e a c t i o n of T M E D · L i B u

with

solvent to f o r m T M E D · Li.) T h i s u p f i e l d m e t h y l e n e shift is e v i d e n c e for a stereospecific s o l u t e - b e n z e n e c o l l i s i o n c o m p l e x ( 3 ) a n d p r o v i d e s a n excellent p r o b e of chelate structure. A s i m i l a r u p f i e l d m e t h y l e n e shift has b e e n n o t e d for d i m e t h o x y e t h a n e L i A l M e

i n benzene

4

(4).

T h e h i g h r e a c t i v i t y of c h e l a t e d l i t h i u m a l k y l c o m p o u n d s limits

s t r u c t u r a l s t u d y of

pure

compounds,

severely

particularly i n

aromatic

solvents. M o s t of o u r m o r e recent w o r k o n c h e l a t e d l i t h i u m a l k y l systems u s e d Ή a n d L i N M R to observe v a r i o u s m e t a l a t i o n reactions l i k e the self7

m e t a l a t i o n or a g i n g r e a c t i o n of T M E D · L i B u i n h e p t a n e ( J , 2).

Much

of

alkali

o u r c u r r e n t i n s i g h t i n t o the s t r u c t u r a l features of

chelated

m e t a l systems comes f r o m c a r e f u l q u a n t i t a t i v e s t u d y of systems r e l a t i v e l y stable anions l i k e resonance t h e systems d e s c r i b e d i n this p a p e r . experiments o n t w o systems:

(a)

stabilized carbanions We

discuss m a g n e t i c

(5)

with and

resonance

chelated l i t h i u m halides C h e l · L i X ,

examples of t h e r e c e n t l y d i s c o v e r e d i n o r g a n i c salt chelates ( 6 ) , a n d

Langer; Polyamine-Chelated Alkali Metal Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

(b)

4.

MELCHIOR E T A L .

115

Magnetic Resonance Studies

c h e l a t e d s o d i u m salts of n a p h t h a l e n e r a d i c a l a n i o n , C h e l · N a C i H " . +

0

8

A

large a m o u n t of d a t a has b e e n c o l l e c t e d o n these systems w h i c h p r o v i d e stable m o d e l s v a l u a b l e for d e t a i l e d s t r u c t u r a l s t u d y of c h e l a t e d a l k a l i m e t a l c o m p o u n d s i n g e n e r a l . O u r d i s c u s s i o n is c o n c e r n e d w i t h the b r o a d s t r u c t u r a l i m p l i c a t i o n s of the w i d e r a n g e of e x p e r i m e n t a l results a v a i l a b l e . E v i d e n c e is p r e s e n t e d w h i c h shows that a c h e l a t e d c a t i o n is a d i s t i n c t , l o n g - l i v e d c h e m i c a l species a n d that different c h e l a t e d cations m a y coexist i n s o l u t i o n as discrete o b s e r v a b l e species.

I n v e s t i g a t i o n of

a n i o n - c a t i o n i n t e r a c t i o n shows that c h e l a t e d salts i n b e n z e n e

the

exist as

t i g h t i o n p a i r s d o w n to the l i m i t of spectrometer s e n s i t i v i t y . T h e effect Downloaded by UNIV OF CINCINNATI on May 24, 2016 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/ba-1974-0130.ch004

of c h e l a t i n g agent o n i o n p a i r s e p a r a t i o n is c o n s i d e r e d .

Finally

we

d e s c r i b e a series of experiments c o n d u c t e d i n m i x e d solvents, the results of w h i c h r e v e a l a stereospecific association of a r o m a t i c solvent m o l e c u l e s w i t h a c h e l a t e d l i t h i u m salt. Experimental

Procedures

T h e p r e p a r a t i o n of c h e l a t e d l i t h i u m h a l i d e s C h e l · L i X has b e e n d e s c r i b e d elsewhere ( 6 ) . S o l u t i o n s f o r L i a n d Ή N M R e x p e r i m e n t s w e r e p r e p a r e d f r o m the s o l i d complexes. P r e p a r a t i o n of C h e l · N a C i H ~ was a c h i e v e d b y s h a k i n g a s o l u t i o n of the c h e l a t i n g agent a n d C i H w i t h a s t r i p of f r e s h l y c l e a n e d s o d i u m i n a v e r y s i m p l e c e l l j o i n e d at one e n d to a n E S R s a m p l e t u b e . T h i s t e c h n i q u e is s i m p l e a n d a l l o w s the o b s e r v a t i o n of successive i n c r e m e n t s of r e d u c t i o n . Its d i s a d v a n t a g e is t h a t i t gives a n u n k n o w n c o n c e n t r a t i o n of C h e l · N a C i H i n a n excess of reactants. A s n o t e d b e l o w i t succeeds o n l y b e c a u s e of the u n u s u a l l y s l u g g i s h e l e c t r o n transfer i n the h y d r o c a r b o n solvent. A l l samples w e r e h a n d l e d i n a d r y box. E S R s p e c t r a w e r e t a k e n o n a B r u k e r Scientific 418s X - b a n d spec t r o m e t e r e q u i p p e d w i t h H a l l - p r o b e s t a b i l i z e d m a g n e t i c field sweep. T h e 100 M H z p r o t o n N M R spectra w e r e o b t a i n e d o n a s t a n d a r d V a r i a n H A - 1 0 0 spectrometer o p e r a t i n g w i t h i n t e r n a l l o c k u s i n g a solvent peak. A l l samples c o n t a i n e d a trace of t e t r a m e t h y l s i l a n e ( T M S ) as a n i n t e r n a l reference. L i N M R spectra w e r e o b t a i n e d at 23.3 M H z o n a J E O L C 6 0 H N M R spectrometer e q u i p p e d w i t h a J N M - N S - 1 0 0 N u c l e a r S i n g l e S i d e b a n d U n i t . S a m p l e s for L i N M R c o n t a i n e d a c a p i l l a r y of 5 . 0 M L i B r i n m e t h a n o l as e x t e r n a l reference. T h e d e u t e r a t e d c h e l a t i n g agent i s o - H M T T - d , N [ C H C H N ( C D ) ] 3 , was p r e p a r e d f r o m tris-( 2 - a m i n o e t h y l ) a m i n e , p a r a f o r m a l d e h y d e - d , for m i c a c i d - d a n d D 0 via the E s c h w e i l l e r - C l a r k e r e a c t i o n ( 7 ) . 7

+

0

+

0

0

8

8

8

7

7

1 8

2

2

3

2

2

Chelated Lithium

2

Halides;

Li

7

and H 1

NMR

Results

T h e u n u s u a l p r o p e r t i e s o f p o l y t e r t i a r y a m i n e c h e l a t e d l i t h i u m salts have been noted ( β ) .

T h e h i g h s o l u b i l i t y a n d c o n d u c t i v i t y of c h e l a t e d

l i t h i u m h a l i d e s i n b e n z e n e raise a n u m b e r of i m p o r t a n t a n d i n t e r e s t i n g questions c o n c e r n i n g t h e r o l e of the a r o m a t i c solvent since these c h e l a t e d


116

POLYAMINE-CHELATED

ALKALI

METAL

COMPOUNDS

salts are n e g l i g i b l y s o l u b l e i n saturated h y d r o c a r b o n s . I n i t i a l experiments f o c u s e d o n possible e v i d e n c e for specific a n i o n s o l v a t i o n b y the a r o m a t i c solvent w i t h little or no success.

W e have, however, obtained clear-cut

e v i d e n c e for a stereospecific association of solvent molecules l o c a l i z e d at a c h e l a t e d L i c a t i o n . T h i s i n t e r a c t i o n is m a n i f e s t e d i n the Ή

and L i

+

7

c h e m i c a l shifts of the c h e l a t e d salt C h e l · L i X i n benzene. T a b l e I I shows the Ή H M T T · LiBr in C H C 1 2

e n t r y i n the t a b l e , A

c h e l

2

N M R d a t a o n 0 . 1 M i s o - H M T T a n d 0 . 1 M iso-

a n d i n benzene.

O f p r i m a r y interest is the last

, the c h e l a t i o n shift for i s o - H M T T · L i B r r e l a t i v e

to free i s o - H M T T i n e a c h of the t w o solvents. CH C1 Downloaded by UNIV OF CINCINNATI on May 24, 2016 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/ba-1974-0130.ch004

2

2

T h e values of A

c h e

i in

are analogous to w h a t is o b s e r v e d for l i t h i u m a l k y l systems s u c h

as T M E D · L i B u i n paraffinic solvents. T h e s m a l l ( ^ 0 . 1 field shift of t h e N - C H the N - C H 2

protons

3

ppm)

down-

protons a c c o m p a n i e d b y essentially n o shift of

is t y p i c a l for l i t h i u m chelates

i n non-aromatic

solvents a n d is w h a t w e h a v e r e f e r r e d to as a n o r m a l c h e l a t i o n shift. O b s e r v a t i o n of this n o r m a l A

c h e

i is e v i d e n c e t h a t i s o - H M T T · L i B r does

exist as the chelate i n C H C 1 . 2

Table II.

2

Proton N M R Chemical Shifts for i s o - H M T T , Free and Complexed with L i B r CH2CI2

CQHG

CHs

CH «

CH

CH *

i s o - H M T T (0.1M) i s o - H M T T - L i B r (0.1M)

-2.183 -2.319

-2.442 -2.448

-2.139 -2.240

-2.541 -1.875

Achei, (ppm)

-0.136

-0.006

-0.101

+0.666

a

2

Z

2

Average shift for A B pattern, all shifts in ppm downfield from T M S . 2

2

T a b l e I I shows t h a t t h e c h e l a t i o n shift A e i is q u i t e different i n ch

b e n z e n e s o l u t i o n . H e r e a g a i n the b e h a v i o r of i s o - H M T T · L i B r is a n a l o gous to T M E D · L i B u , w i t h a large u p f i e l d shift of the N - C H - protons. 2

T h i s l a r g e u p f i e l d m e t h y l e n e shift ( l a r g e p o s i t i v e A

c h e

i)

i n b e n z e n e is

g e n e r a l for c h e l a t e d l i t h i u m c o m p o u n d s a n d is seen as a m a n i f e s t a t i o n of a stereospecific c o l l i s i o n c o m p l e x b e t w e e n b e n z e n e a n d the p o s i t i v e e n d of the m o l e c u l a r d i p o l e m o m e n t of C h e l · L i X . T o s t u d y this u n u s u a l i n t e r a c t i o n w e h a v e c a r r i e d out extensive experiments i n m i x e d C H C 1 — 2

b e n z e n e solvents, some results of w h i c h are discussed b e l o w .

2

In addition

to s t u d y i n g the o r i g i n of this u p f i e l d m e t h y l e n e c h e l a t i o n shift i n b e n z e n e , w e h a v e t a k e n a d v a n t a g e o f its existence i n a n u m b e r of w a y s d e s c r i b e d below. T h e L i N M R spectra of C h e l · L i X h a v e p r o v e d sensitive to v a r i o u s 7

s t r u c t u r a l features. F i g u r e 1 shows the

7

L i c h e m i c a l shifts for P M D T ·


4.

117


MELCHIOR E T AL.

-1.00

Downloaded by UNIV OF CINCINNATI on May 24, 2016 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/ba-1974-0130.ch004

χ = CI

.50

M

Figure 1.

2.00

1.50

1.00 LiX

Lithium NMR chemical shifts for PMDT · LiX, X Cl, Br, I; room temperature in toluene

7

L i X ( X == C I , B r , or I ) at r o o m t e m p e r a t u r e i n toluene.

=

( A l l L i chemical 7

shifts are m e a s u r e d r e l a t i v e to a n e x t e r n a l s t a n d a r d of 5 . 0 M L i B r i n m e t h a nol. ) F i g u r e 1 shows that there is about a 1-ppm d o w n f i e l d shift i n g o i n g f r o m c h l o r i d e to i o d i d e i n P M D T · L i X . F i g u r e 2 shows t h e v a r i a t i o n w i t h c o n c e n t r a t i o n of L i c h e m i c a l shifts for C h e l · L i B r i n b e n z e n e f o r the five 7

c h e l a t i n g agents s t u d i e d . T h e l i m i t i n g d i l u t i o n c h e m i c a l shifts f o r C h e l · L i B r d e r i v e d f r o m F i g u r e 2 are t a b u l a t e d i n T a b l e I I I . I n a d d i t i o n to these variations w i t h a n i o n a n d c h e l a t i n g agent, the

7

L i c h e m i c a l shift is also

i n f l u e n c e d b y stereospecific association of C h e l · L i X w i t h a r o m a t i c s o l vents to r o u g h l y the same degree as is the Ή c h e m i c a l shift of the c h e l a t i n g agent. T h e L i c h e m i c a l shift of 0 . 1 M i s o - H M T T · L i B r i n b e n z e n e is 7

about 0.6 p p m u p f i e l d f r o m that of 0 . 1 M i s o - H M T T · L i B r i n C H C 1 . 2

2

T h i s m a y b e c o m p a r e d w i t h the Ή N M R d a t a i n T a b l e I I . T h e r e is also c o n c e n t r a t i o n d e p e n d e n c e of the less p r o n o u n c e d t h a n for Ή .

7

L i c h e m i c a l shift a l t h o u g h

somewhat

C l e a r l y the L i c h e m i c a l shift of C h e l · L i X 7

is e q u a l l y sensitive to the n a t u r e of the a n i o n , c h e l a t i n g agent, a n d solvent, as w e l l as c o n c e n t r a t i o n . T h i s discourages attempts to i n t e p r e t the m a g n i t u d e of the o b s e r v e d

7

L i c h e m i c a l shifts. I n the f o l l o w i n g w e w i l l d r a w

conclusions f r o m the fact that L i c h e m i c a l shift differences exist i n g i v e n 7

situations w i t h o u t a t t e m p t i n g to r a t i o n a l i z e the m a g n i t u d e of the shifts themselves. Chelated Sodium Naphthalenide;

ESR

Results

C h e l a t e d s o d i u m n a p h t h a l e n i d e salts C h e l · N a C i H " h a v e +

0

p r e p a r e d via s o d i u m m e t a l r e d u c t i o n of solutions of C i H 0

agent i n benzene.

8

8

been

and chelating

W h i l e N a C i H " has b e e n s t u d i e d extensively i n ether +

0

8



118

P O L Y A M I N E - C H E L A T E D A L K A L I M E T A L COMPOUNDS

JL

_L

0.5 M

Chel-LiBr

Figure 2. Lithium NMR chemical shifts for Chel · LiBr in benzene as a function of the concentration (moles/liter) of chelated salt. Chemical shifts are in ppm downfield from external 5 M LiBr in methanol. 7

solvents (8,9), solvents.

this is t h e first s t u d y of s u c h salts b y E S R i n h y d r o c a r b o n

T h e o b s e r v e d spectra h a v e a n u m b e r of u n u s u a l features.

A

t y p i c a l s p e c t r u m is s h o w n i n F i g u r e 3, that of P M D T · N a C i H " p r e +

0

8

p a r e d b y N a metal reduction of a benzene solution 0.002M i n C i H a n d 0

8

P M D T . S e v e r a l features o f this s p e c t r u m a r e w o r t h n o t i n g : ( a ) T h e s p e c t r u m i n F i g u r e 3 is c h a r a c t e r i z e d b y n a r r o w lines ( < 0.1 G ) d e s p i t e the fact t h a t the system contains a n excess of the p a r e n t h y d r o c a r b o n C i H . T h i s indicates a n u n u s u a l l y s l u g g i s h e l e c t r o n exchange between C h e l · N a C i H " a n d the parent C i H i n the hydroc a r b o n solvent. ( P r e p a r a t i o n of N a C i H " i n a n ether solvent s u c h as T H F u s i n g o u r t e c h n i q u e p r o d u c e d o n l y a single b r o a d E S R l i n e . ) T h i s o b s e r v a t i o n is discussed b e l o w . 0

8

+

0

8

0

+

0

8

8


4.

MELCHIOR E T A L .

119


Table III. Observed N a Hyperfine Interactions and Limiting Dilution L i Chemical Shifts for Chelated Ion Pairs 2 3

7

Number of Nitrogens

Chelating Agent

a";

C

iso-HMTT

b

0.36

Observed N a hf interaction for Chel · N C i H " in benzene. Observed limiting dilution L i chemical shift for Chel · L i B r in benzene, in ppm downfield from 5M L i B r in methanol (see Figure 2). See section on effect of chelating agent on ion pairing. α

2 3

a

b


ppm 2.08 1.55 1.00 0.83

1.80 1.12 0.89 0.91 0.20 0.71

2 3 4 4* 5 4

TMED PMDT n-HMTT HMTP

Li,

G"

+

0

8

7

c

( b ) T h e s p e c t r u m i n F i g u r e 3 is that of a n i o n p a i r e v i d e n c e d b y t h e s u p e r p o s i t i o n of a N a hyperfine ( h f ) i n t e r a c t i o n o n t h e 25-line p a t t e r n w h i c h w o u l d b e o b s e r v e d f o r a free C i H ~ a n i o n . F i g u r e 3 shows s p l i t t i n g of t h e c e n t r a l l i n e a n d t h e outermost free i o n l i n e i n t o quartets b y the N a h f c o u p l i n g constant a = 1.12 G . ( c ) T h e N a h f c o u p l i n g constant i n this a n d r e l a t e d systems is c o n c e n t r a t i o n i n d e p e n d e n t a n d c h a r a c t e r i s t i c of t h e p a r t i c u l a r c h e l a t i n g agent. O b s e r v e d vaues of a are l i s t e d i n T a b l e I I I . W i t h t w o of t h e c h e l a t i n g agents, T M E D a n d H M T P , t i m e d e 2 3

0

2 3

8

N a

2 3

N a

p e n d e n t E S R spectra w e r e observed. T h e o v e r a l l b e h a v i o r of t h e T M E D (0,0)

1 gauss

a a

0

N a

= 1.840 gauss (4H) = 1.115 gauss (1 Na)

Figure 3. ESR spectrum of PMDT · Na C H ~ ion pairs in benzene. Spectrum shows Na hf splitting of the 25-line pattern of the C H ~ anion. This splitting is shown for the central (0, 0) and the outermost (2,2) line of the free anion. +

10

8

23


10

8

120

POLYAMINE-CHELATED

ALKALI

METAL

COMPOUNDS

[TMED-Na C Hg~] +

10

η

1 gauss Downloaded by UNIV OF CINCINNATI on May 24, 2016 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/ba-1974-0130.ch004

a1

a

Η

4.920 gauss (4H)

Η

1.865 gauss (4H)

Na

0.485 gauss (2 Na)

Figure 4. ESR spectrum of initial product of sodium reduction of C H + TMED in benzene. Spectrum is that of an ion cluster and shows hf interaction with two equivalent Na cations. 10

8

23

system is v e r y c o m p l i c a t e d a n d is n o t discussed i n d e t a i l . T w o p r o d u c t s are o f i n t e r e s t — t h e i n i t i a l p r o d u c t , a n d t h e p r o d u c t o b s e r v e d after l o n g times i n contact w i t h s o d i u m . T h e E S R s p e c t r u m of t h e i n i t i a l p r o d u c t is s h o w n i n F i g u r e 4. T h i s s p e c t r u m shows h f i n t e r a c t i o n w i t h t w o e q u i v a lent

2 3

N a n u c l e i , i n d i c a t i n g i n i t i a l f o r m a t i o n of a t r i p l e i o n or d i m e r i c i o n

pair ( T M E D · N a C i H " ) . +

0

8

T h e l o n g contact t i m e p r o d u c t is a t y p i c a l

2

t i g h t i o n - p a i r T M E D · N a C i H " , d a t a f o r w h i c h is i n c l u d e d i n T a b l e I I I . +

0

8

T h e s e species are v e r y s i m i l a r to the t w o species o b s e r v e d w h e n N a C i H " +

0

8

is p r e p a r e d i n T M E D as a solvent (10) e v e n t h o u g h t h e order i n w h i c h the t w o species a p p e a r is i n v e r t e d . T h e second case of t i m e d e p e n d e n t spectra is t h e o b s e r v e d d e c a y of initially formed H M T P · N a C +

H - i o n pairs w i t h a

1 0

8

m i x t u r e of this species a n d another w i t h a

N a

0.05 G (0.15 M H z ) w e o b t a i n e d a conservative u p p e r l i m i t

0

for the rate constant for e l e c t r o n transfer i n E q u a t i o n 3, fci < sec" . 1

T h i s u p p e r l i m i t is s i g n i f i c a n t l y b e l o w

e l e c t r o n transfer rate constants i n ether solvents ( 1 2 ) . tions (—

+

0

6

- 1

for

A t h i g h concentra

1 0 " M ) of C h e l · N a C i H ~ w e h a v e o b s e r v e d 2

10 M

the range of values

8

concentration

d e p e n d e n t b r o a d e n i n g w h i c h c a n be a t t r i b u t e d to s p i n - s p i n exchange, E q u a t i o n 4. I n this case conservative estimates of the q u a n t i t i e s i n v o l v e d l e a d to the c o n c l u s i o n k

>

2

5 X

10 M 8

- 1

sec" . T h i s l o w e r l i m i t is c o m 1

p a r a b l e to s p i n - s p i n i n t e r a c t i o n rates ( ~ s i d e r e d to be diffusion c o n t r o l l e d (13).

10 ) 9

i n ether solvents

con

P r e s u m a b l y t h e n the l o w v a l u e

of fci i n these systems is not c a u s e d b y a n u n u s u a l l y l o w f r e q u e n c y anion-neutral molecule

encounters.

of

C o n c e r t e d e l e c t r o n transfer as i n

E q u a t i o n 3 requires a s y m m e t r i c t r a n s i t i o n state i n w h i c h the c a t i o n is shared equally b y two C i H 0

molecules (12,14).

8

W i t h a tightly chelated

c a t i o n steric considerations m a k e s u c h a t r a n s i t i o n state difficult to v i s u a l i z e . A l t e r n a t i v e p a t h w a y s i n v o l v e either d e c o m p l e x a t i o n Chel.Na+C

1 0

H - τ± C h e l + N a + C i o H r ,

(6)

8

or i o n - p a i r d i s s o c i a t i o n C h e l · N a + C i H - C h e l · N a + + C i H ~ , 0

8

0

(7)

8

e i t h e r of w h i c h l e a d to h i g h a c t i v a t i o n energies for the o v e r a l l e l e c t r o n transfer process. Effect of Chelating Agent on Ion Pairing. W e h a v e a l r e a d y n o t e d t h a t the o b s e r v a t i o n of

2 , 3

N a h f i n t e r a c t i o n i n the E S R spectra of C h e l ·

N a C i H " is d i r e c t e v i d e n c e that these chelated salts exist as t i g h t i o n +

0

8

pairs i n v e r y d i l u t e s o l u t i o n , at least o n the E S R t i m e scale. T h e a v a i l a b l e 7

L i N M R d a t a o n c h e l a t e d l i t h i u m h a l i d e s s h o w t h a t these systems are

also t i g h t i o n p a i r s d o w n to the l i m i t s of present spectrometer sensitivity. F i g u r e 1 shows t h e L i c h e m i c a l shift of P M D T · L i X ( X = 7

as a f u n c t i o n of c o n c e n t r a t i o n i n benzene.

CI, B r , or I )

T h e d o w n f i e l d c h e m i c a l shift


126

POLYAMINE-CHELATED

ALKALI

METAL

COMPOUNDS

f o r t h e series C l , B r , I p e r h a p s is c a u s e d b y i n c r e a s e d c o v a l e n c y of the L i X b o n d b u t this is c o m p l i c a t e d b y other factors. of the

7

A n adequate

theory

L i N M R c h e m i c a l shifts i n c h e l a t e d l i t h i u m salts w i l l h a v e to

c o n s i d e r the c o m b i n e d

effects of a n i o n , c h e l a t i n g agent, solvent,

c o n c e n t r a t i o n or degree of aggregation.

and

O u r results i n d i c a t e that these

f o u r factors are of c o m p a r a b l e m a g n i t u d e . T h e i m p o r t a n t feature of the d a t a i n F i g u r e 1 for t h e present d i s c u s s i o n is the absence of a n y t r e n d t o w a r d 3 c o m m o n free i o n v a l u e f o r 8u at l o w concentrations.

It seems

clear f r o m F i g u r e 1 t h a t there is no i o n - p a i r d i s s o c i a t i o n at the

concen

trations u s e d . Downloaded by UNIV OF CINCINNATI on May 24, 2016 | http://pubs.acs.org Publication Date: June 1, 1974 | doi: 10.1021/ba-1974-0130.ch004

W e n o w consider the role of the c h e l a t i n g agent i n d e t e r m i n i n g the i o n - p a i r i n t e r a c t i o n i n c h e l a t e d salts. T h e i o n - p a i r i n t e r a c t i o n is m a n i fested i n the Table

III.

2 3

N a h f interactions o b s e r v e d for C h e l · N a C i H " g i v e n i n +

It was

noted

N a C i H " were observed. +

0

8

a b o v e that t w o

0

8

i o n - p a i r species

HMTP ·

W e postulate that the t w o species

represent

the l i n e a r pentadentate H M T P

chelating N a

nitrogens g i v i n g h f interactions of a

N a

=

+

w i t h either f o u r or

0.91 G a n d a

five

< 0.2 G , respec

N a

t i v e l y . P r o v i d i n g w e m a k e this a s s u m p t i o n c o n c e r n i n g H M T P , the d a t a i n T a b l e I I I s h o w that the m a g n i t u d e of a

N a

decreases as the n u m b e r of

n i t r o g e n bases increases. T h e o r d e r i n w h i c h the m a g n i t u d e of a

N a

places

the c h e l a t i n g agents p a r a l l e l s that o b s e r v e d for d c c o n d u c t i v i t y of C h e l · N a C i H " (15) +

0

8

a n d t h e r m a l s t a b i l i t y of r e l a t e d l i t h i u m salts (6, 16).

is g e n e r a l l y r e c o g n i z e d that the

2 3

It

N a h f i n t e r a c t i o n i n N a C i H " is a v e r y +

sensitive p r o b e of the g e o m e t r y of the i o n p a i r (17):

0

8

T h i s h f interne > n

is a c o m p l i c a t e d f u n c t i o n of the precise l o c a t i o n of the N a c a t i o n over +

t h e p l a n e of the C i H " a n i o n (17). 0

tion.

We

Nevertheless i t is a reasonable

8

a p p r o x i m a t i o n t h a t a decrease i n #

Na

first

reflects a n i n c r e a s e d i o n p a i r separa

c o n c l u d e .from this that as the c h e l a t i n g agent

in Chel ·

N a C i H " becomes m o r e effective, the average i o n p a i r s e p a r a t i o n i n +

0

8

creases across the series T M E D < PMDT < HMTPiv «

n-HMTT < iso-HMTT

Polyamine-Chelated Alkali Metal Compounds

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