Lanthanide NMR Shift Reagents

19 Unusually Volatile and Soluble Metal

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Chelates: Lanthanide N M R Shift Reagents R. E. SIEVERS, J. J.BROOKS, J. A. CUNNINGHAM, and W. E. RHINE 1

2

3

3

Aerospace Research Laboratories, ARL/LJ, Wright-Patterson Air Force Base, Ohio 45433 4

Novel lanthanide β-diketonate complexes have been synthe sized. Their properties include thermal, hydrolytic and oxidative stabilities, volatility, Lewis acidity, and unusually high solubility in nonpolar organic solvents. Various combi nations of these properties make lanthanide complexes useful as NMR shift reagents and fuel antiknock additives and in other applications. NMR spectral studies revealed that the Pr(III), Yb(III), and Eu(III) complexes of 1,1,1,2,2,3,3,7,7,7decafluoro-4,6-heptanedione have sufficient Lewis acidity to induce appreciable shifts in the proton resonances of weak Lewis bases such as anisole, acetonitrile, nitromethane, and p-nitrotoluene. Data from single-crystal structure determi nations indicate that the NMR shift reagent-substrate com plexes are not stereochemically rigid and that effective axial symmetry may exist by virtue of rapid intramolecular re arrangements. T y j ~ e t a l β-diketonate c o m p l e x e s d i s p l a y u n u s u a l properties. dative

a v a r i e t y of i n t e r e s t i n g a n d

A m o n g these are t h e r m a l , h y d r o l y t i c , a n d o x i

stabilities, v o l a t i l i t y , L e w i s

n o n p o l a r o r g a n i c solvents.

acidity,

and unusual solubility i n

I n general, i t is a p a r t i c u l a r c o m b i n a t i o n

of

these properties r a t h e r t h a n a n y single o n e that m a k e s possible t h e use of these c o m p l e x e s i n a diverse range of a p p l i c a t i o n s .

F o r example, be-

Present address: Dept. of Chemistry, University of Colorado, Boulder, Colo. 80302. 'Present address: Monsanto Research Corp., Dayton Laboratory, Dayton, Ohio 45407. 'Present address: Air Force Materials Laboratory, Wright-Patterson Air Force Base, Ohio 45433. * On June 30, 1975, The Aerospace Research Laboratories were abolished; con sequently, all correspondence should be directed to the first author at his present address. 1

222

In Inorganic Compounds with Unusual Properties; King, R. Bruce; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

19.

siEVERS ET AL.

Lanthanide

NMR

Shift

223

Reagents

cause of the v o l a t i l i t y a n d t h e r m a l s t a b i l i t y of c e r t a i n of these

complexes,

t h e a p p l i c a t i o n of gas c h r o m a t o g r a p h y to u l t r a t r a c e m e t a l analysis b e c a m e feasible

(J).

B e c a u s e of other c o m b i n a t i o n s of p r o p e r t i e s , these

p o u n d s are u s e d as N M R shift reagents (2,3,4), ( 5 , 6 ) , h o m o g e n e o u s catalysts stereochemical phases

studies

(8),

(7),

model

a n d selective

com

fuel antiknock additives c o m p o u n d s for

gas

gas

chromatographic

phase liquid

(9).

P e r h a p s the most w i d e l y r e c o g n i z e d use of l a n t h a n i d e β-diketonates Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch019

is as N M R shift reagents.

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

the i n t r i n s i c p a r a m a g n e t i c n a t u r e of c e r t a i n of the l a n t h a n i d e ions, b u t also of t h e L e w i s a c i d i t y , h y d r o l y t i c s t a b i l i t y , a n d h i g h s o l u b i l i t y i n n o n p o l a r o r g a n i c solvents of t h e i r complexes.

T h i s p a p e r describes o u r recent

studies of the use of these u n u s u a l chelates as N M R shift reagents. V o l a t i l e , stable r a r e e a r t h complexes w e r e o r i g i n a l l y s y n t h e s i z e d w i t h the h o p e that differences i n v o l a t i l i t y w o u l d p r o v i d e a means of s e p a r a t i n g a n d p u r i f y i n g t h e r a r e earths.

E a r l y c l a i m s that the l a n t h a n i d e a c e t y l -

acetonates w e r e v o l a t i l e w e r e later s h o w n to b e incorrect. T h e tris c o m plexes g e n e r a l l y o c c u r as h y d r a t e s , a n d t h e y d o n o t e x h i b i t the necessary s t a b i l i t y for f r a c t i o n a l s u b l i m a t i o n or gas c h r o m a t o g r a p h i c The

hydrated

l a n t h a n i d e acetylacetonates

h e a t i n g , a n d the r e a c t i o n p r o d u c t s

undergo

separation.

self-hydrolysis

on

are n o longer a p p r e c i a b l y v o l a t i l e .

T h e synthesis a n d c h a r a c t e r i z a t i o n of t h e a n h y d r o u s , s t e r i c a l l y h i n d e r e d Ln(thd)

complexes r e p r e s e n t e d a m a j o r a d v a n c e m e n t i n t h e s e a r c h for

3

v o l a t i l e , stable l a n t h a n i d e c o m p l e x e s ( 1 ). T a b l e I lists the l i g a n d s s t u d i e d most extensively i n o u r l a b o r a t o r y . Table I.

S t r u c t u r e s a n d A b b r e v i a t i o n s of / ? - D i k e t o n e s Θ Ο

Ο

R—C—CH—C—R R

CH C(CH ) CF CF CF CF CF CF 2

2

3

2

2

acac thd fod dfhd

3

3

3

Abbreviation

- C H -C(CH ) -C(CH ) -CF

3

3

2

R*

1

3

3

3

3

3

A n extension of this r e s e a r c h l e d to t h e p r e p a r a t i o n of

fluorinated

β-diketones w h i c h f o r m stable a n d even m o r e v o l a t i l e r a r e e a r t h c o m plexes.

D e t a i l e d studies

Ln(dfhd)

3

(13,

14)

of

the

Ln(thd)

3

(J),

Ln(fod)

3

(12),

and

c o m p l e x e s r e v e a l e d t h a t the v o l a t i l i t y of the tris

c o m p l e x is d i r e c t l y p r o p o r t i o n a l to the degree of fluorine s u b s t i t u t i o n a n d


224

INORGANIC

COMPOUNDS

WITH

i n v e r s e l y p r o p o r t i o n a l to the i o n i c r a d i u s ( 1 5 )

UNUSUAL

PROPERTEES

of the m e t a l a t o m for a

g i v e n series of chelate complexes. I n 1969 (16) of E u ( t h d ) terol.

3

i t was r e p o r t e d that a d d i t i o n of the b i s - p y r i d i n e a d d u c t

i n d u c e d l a r g e shifts i n t h e p r o t o n N M R s p e c t r u m of choles

Subsequently, it was reported that the unsolvated E u ( t h d )

e v e n m o r e effective

as a n N M R shift reagent

was

3

S i n c e these

(17).

first

reports a p p e a r e d , m o r e t h a n 400 papers h a v e b e e n p u b l i s h e d (2, 3,

4)

i n this field o n subjects r a n g i n g f r o m s p e c t r a l c l a r i f i c a t i o n to the selection Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch019

of the best shift reagent for a g i v e n a p p l i c a t i o n . T h e reagents m o s t w i d e l y u s e d i n the e a r l y studies w e r e the t r i s - t h d complexes of E u ( I I I ) , P r ( I I I ) , a n d Y b ( I I I ) . I n general, E u ( t h d ) whereas P r ( t h d ) Yb(thd)

3

3

3

and Y b ( t h d )

3

i n d u c e d o w n f i e l d shifts

i n d u c e s u p f i e l d shifts. A l t h o u g h the shifts i n d u c e d b y

are u s u a l l y greater t h a n those i n d u c e d b y the P r a n d E u a n a

logs, m u c h of t h e fine s t r u c t u r e i n the N M R spectra is often lost because of s i g n a l b r o a d e n i n g . D e s p i t e the successes a c h i e v e d w i t h the L n ( t h d )

complexes

3

used

as shift reagents, they h a v e l i m i t e d s o l u b i l i t y i n the u s u a l N M R solvents ( 1 8 , 1 9 ) s u c h as c h l o r o f o r m a n d c a r b o n t e t r a c h l o r i d e , a n d t h e y are n e a r l y i n s o l u b l e i n solvents s u c h as a c e t o n i t r i l e , n i t r o m e t h a n e , a n d I n a d d i t i o n , the i n t e r a c t i o n b e t w e e n

the L n ( t h d )

3

p-dioxane.

chelates a n d

weak

n u c l e o p h i l e s is often not s t r o n g e n o u g h to result i n complexes

which

e x h i b i t l a r g e i n d u c e d shifts. D u r i n g the course of o u r research o n v o l a t i l e r a r e e a r t h complexes, w e f o u n d t h a t the f o d chelates w e r e m o r e soluble i n a w i d e r a n g e of solvents t h a n either the acetylacetonates or the t h d complexes.

W e postu

l a t e d that the presence of the electronegative fluorine atoms increases the L e w i s a c i d i t y of the m e t a l w h i c h results i n a stronger i n t e r a c t i o n w i t h v a r i o u s n u c l e o p h i l e s (20). t i v e to E u ( t h d )

3

T h e greater L e w i s a c i d i t y of E u ( f o d )

3

rela

was d e m o n s t r a t e d b y the r e s o l u t i o n of resonances i n a

m i x t u r e of i s o m e r i c azoxybenzenes

(21).

This phenomenon

d e m o n s t r a t e d i n d e p e n d e n t l y b y gas c h r o m a t o g r a p h y

was

(GC)

also

(9)—fluori-

n a t e d β-diketonate complexes i n t e r a c t m o r e s t r o n g l y w i t h o r g a n i c n u c l e o p h i l e s t h a n d o n o n f l u o r i n a t e d ones. A s i m i l a r G C s t u d y that c o n c e n t r a t e d on E u ( f o d )

3

(10)

r e l a t e d the strengths of these interactions to the d o n a t

i n g abilities of the o r g a n i c n u c l e o p h i l e s a n d to steric constraints. T h e f o d complexes

are n o w the most w i d e l y u s e d class of N M R shift reagents

b e c a u s e of greater c o n v e n i e n c e i n use a n d w i d e r a p p l i c a b i l i t y to w e a k nucleophiles. F u r t h e r s u b s t i t u t i o n of

fluorine

atoms i n the β-diketone s i d e c h a i n s

has l e d to the synthesis a n d c h a r a c t e r i z a t i o n of the L n ( d f h d ) as N M R shift reagents.

A l t h o u g h the h y d r a t e d L n ( f o d )

m o r e s o l u b l e i n c h l o r o f o r m t h a n the h y d r a t e d L n ( d f h d )

3

3

3

complexes

complexes

are

complexes, t h e

l a n t h a n i d e d f h d complexes h a v e s u p e r i o r s o l u b i l i t y i n d i o x a n e a n d aceto-


19.

siEVERS E T A L .

Table II.

Lanthanide

NMR

225

Shift Reagents

S o l u b i l i t y o f Some L a n t h a n i d e β - D i k e t o n a t e S h i f t R e a g e n t s * Solvent

Complex

Acetonitrile

Ln(dfhd) -xH 0 Ln(fod) -xH 0 Ln(thd) 3

3

2

2

Dioxane

>1 0.8 insoluble

6

6

3

Chloroform

>1 0.03 0.03

0.08 0.5 0.03

° Solubility is given in g/g. Drying the shift reagent over P4O10 increases its solubility in chloroform.


6

nitrile (Table I I ) .

I t is i m p o r t a n t to h a v e shift reagents t h a t are s o l u b l e

a n d t h a t c a n f u n c t i o n w e l l i n solvents s u c h as a c e t o n i t r i l e a n d d i o x a n e b e c a u s e m a n y c o m p o u n d s of b i o l o g i c a l i m p o r t a n c e are s o l u b l e o n l y i n solvents s u c h as these. T h e d f h d c o m p l e x e s a p p e a r to b e v e r y p r o m i s i n g for s u c h a p p l i c a t i o n s . T h e h i g h d e g r e e of s o l u b i l i t y of the d f h d c o m p l e x e s i n d e u t e r a t e d a c e t o n i t r i l e m a k e s i t p o s s i b l e to m e a s u r e e x p e r i m e n t a l l y the d e g r e e o f h y d r a t i o n of these complexes.

T h e i n t e g r a t e d i n t e n s i t y of t h e H 0 p r o t o n 2

resonance c o m p a r e d w i t h t h a t of the m e t h i n e p r o t o n of the shift reagent p r o v i d e s a measure of the r e l a t i v e a m o u n t of w a t e r present. T h e c h e m i c a l shift of the w a t e r protons d e p e n d s o n the c o n c e n t r a t i o n , b u t i t is o b s e r v e d d o w n f i e l d f r o m the m e t h i n e p r o t o n resonance of the s h i f t reagent. T y p i c a l r e s i d u a l w a t e r after d r y i n g 3 days in vacuo o v e r P4O10 is 0.5

mole/mole

e u r o p i u m chelate. T h i s a m o u n t of w a t e r does n o t seriously i n t e r f e r e w i t h t h e a b i l i t y of t h e c o m p l e x to f u n c t i o n effectively as a shift reagent. T h e i n c r e a s e d L e w i s a c i d i t y of the L n ( d f h d )

complexes relative to

3

the t h d a n d the f o d c o m p o u n d s is d e m o n s t r a t e d b y a c o m p a r i s o n of t h e i n d u c e d shifts i n C D C 1

solutions of s u c h w e a k bases as a c e t o n i t r i l e ,

3

n i t r o m e t h a n e , a n d anisole ( T a b l e I I I ) . m e t h y l protons of e a c h c o m p o u n d .

T h e i n d u c e d shifts are for

Although P r ( d f h d )

3

and

the

Yb(dfhd)

3

i n d u c e the largest shifts, l i n e b r o a d e n i n g is a p p r e c i a b l e ; i n fact, i n e x p e r i T a b l e I I I . C o m p a r i s o n of S h i f t s I n d u c e d i n the S p e c t r a of W e a k L e w i s B a s e s α

Anisole Complex

Acetonitrile

0.1 R:S

0.1 R:S

0.3 R:S

0.1 R:S

0.3

-1.58 2.03 0.68 0.22 0.35

-1.50 2.28 0.77 0.40 0.38

-3.45 5.67 1.92 0.85 0.75

-1.20 1.50 0.45 0.20 0.20

-2.27 3.00 0.88 0.23 0.23

b

Pr(dfhd) Yb(dfhd) E u (dfhd), E u (fod) E u (thd) 3 3

3

3

Nitromethane R:S

Shifts are given in ppm. Data obtained at 60 M H z with 10~ mole shift reagent dissolved in 0.5 g C D C 1 . R:S—the mole ratio of shift reagent to substrate. 0

4

3

6


226

INORGANIC

COMPOUNDS

WITH

UNUSUAL PROPERTIES

ments w i t h t e t r a h y d r o f u r a n ( T H F ) , no fine structure w a s o b s e r v e d i n the T H F p r o t o n resonances.

I n s i m i l a r experiments w i t h

Eu(dfhd) , 3

there was no d i s c e r n i b l e b r o a d e n i n g . T h e greater L e w i s a c i d i t y of the L n ( d f h d )

shift reagents is also

3

d e m o n s t r a t e d b y the fact that a d d u c t s are f o r m e d a n d r e l a t i v e l y l a r g e shifts are i n d u c e d e v e n w i t h w e a k L e w i s bases s u c h as

nitrobenzene

d e r i v a t i v e s . T h e i n d u c e d shifts are p e r h a p s best i l l u s t r a t e d b y the p a r a s u b s t i t u t e d d e r i v a t i v e s s u c h as p - c h l o r o n i t r o b e n z e n e Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch019

T h e d a t a for i n d u c e d shifts w i t h E u ( d f h d ) Table I V .

3

and p-nitrotoluene.

are s u m m a r i z e d i n T a b l e I V .

N M R Shifts i n /r-Chloronitrobenzene and ^ - N i t r o t o l u e n e I n d u c e d b y A d d i t i o n of E u ( d f h d ) 3

p-Nitrotoluene

p-Chloronitrobenzene ortho-H meta-H methyl-H

a

1.9 1.2 0.33

1.5 0.5

° Shifts are given in ppm. Data obtained at 60 M H z with 10~ mole complex dissolved in C D C 1 ; mole ratio of shift reagent to substrate, 0.3. 4

3

T h e e l e c t r o n - w i t h d r a w i n g c h l o r o g r o u p reduces

the L e w i s b a s i c i t y of

the n i t r o g r o u p , a n d therefore the i n d u c e d shifts f o r

p-chloronitrobenzene

are s m a l l e r , as m i g h t b e e x p e c t e d f r o m c o n s i d e r a t i o n of t h e H a m m e t t s i g m a f u n c t i o n . P l o t s of c h e m i c a l shift vs. m o l e r a t i o of shift r e a g e n t - t o substrate for each p r o t o n are l i n e a r over the range of 0 . 0 - 0 . 5 m o l e r a t i o . It is a s s u m e d that the slopes of these lines give the m a g n i t u d e s of the i n d u c e d shifts w h i c h c o n t a i n i n f o r m a t i o n a b o u t the geometry

of

the

complex. I n order to a p p r o x i m a t e the c o n f o r m a t i o n of the c o m p l e x i n s o l u t i o n , a c a l c u l a t i o n of the t y p e d e s c r i b e d b y W i l l c o t t et al. (22)

(assuming

effective a x i a l s y m m e t r y ) was m a d e o n the p - n i t r o t o l u e n e c h e m i c a l shift d a t a (see

Ref. 2, p . 1 4 3 ) . B e c a u s e a x i a l s y m m e t r y is a s s u m e d , the s i m p l i -

fied M c C o n n e l l - R o b e r t s o n e q u a t i o n c a n b e used. I n several c a l c u l a t i o n s the p r i n c i p a l m a g n e t i c axis w a s v a r i e d so t h a t i t w a s d i r e c t e d f r o m the e u r o p i u m a t o m to various points a l o n g t h e l i n e b i s e c t i n g the

O-N-O

angle. A b r o a d m i n i m u m was o b t a i n e d ; therefore, w i t h i n the a b o v e c o n straints, t h e o r i e n t a t i o n of the p r i n c i p a l m a g n e t i c axis does n o t a p p e a r to affect seriously the c a l c u l a t e d gross g e o m e t r y of the c o m p l e x .

T h e best

fit of the N M R d a t a w a s o b t a i n e d w h e n the E u a t o m w a s p o s i t i o n e d 2.2

±

0.2 A a b o v e the p l a n e of the n i t r o g r o u p a n d c o p l a n a r i t y of the n i t r o g r o u p a n d the b e n z e n e r i n g was a s s u m e d . T h e c a l c u l a t i o n i n w h i c h t h e p r i n c i p a l m a g n e t i c axis was a s s u m e d to b e c o l i n e a r w i t h t h e E u - N v e c t o r is i l l u s t r a t e d i n F i g u r e 1. R e l a t i v e to e i t h e r the L n ( t h d ) Ln(dfhd)

3

3

or the L n ( f o d )

3

chelates, use of t h e

complexes offers a n a d d i t i o n a l a d v a n t a g e since t h e /?-diketone


19.

siEVERS E T A L .

Lanthanide

NMR

Shift

227

Reagents

Figure 1. Calculated coordination geometry of the europiump-nitrotoluene interaction has o n l y the m e t h i n e p r o t o n resonance w h i c h c a n p o s s i b l y c o i n c i d e w i t h the substrate p r o t o n resonances.

I n the d f h d complexes there are o n l y

3 m e t h i n e protons i n the chelate s h e l l whereas there are 3 m e t h i n e a n d 27 Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch019

teri-butyl

protons i n the L n ( f o d )

b u t y l protons i n the L n ( t h d ) very unlikely i n E u ( d f h d )

3

3

complexes a n d 3 m e t h i n e a n d 54 tert-

complexes.

3

F u r t h e r m o r e , i n t e r f e r e n c e is

complexes since t h e m e t h i n e p r o t o n resonance

experiences a shift i n the opposite d i r e c t i o n f r o m t h a t of the substrate resonances; for E u ( d f h d )

3

d r i e d over P4O10, this resonance appears u p -

field f r o m T M S . C o n s e q u e n t l y , o n e c a n a c c o m p l i s h the same objective of e l i m i n a t i n g i n t e r f e r i n g p r o t o n peaks b y s u b s t i t u t i n g f l u o r i n e atoms

for

m e t h y l groups i n the f o d l i g a n d as w a s d o n e p r e v i o u s l y b y d e u t e r a t i o n . M a n y questions c o n c e r n i n g t h e i n t e r p r e t a t i o n of N M R shift reagent data remain unanswered. H o p e f u l l y , single-crystal structural determinations of shift r e a g e n t - s u b s t r a t e complexes

w i l l allow basic

structural

p r i n c i p l e s to b e d e d u c e d . T h e structures of E u ( t h d ) ( D M F ) , E u ( t h d ) 3

2

3

( D M S O ) , Eu(thd) (l,10-phenanthroline), and Y b ( t h d ) ( D M S O ) 3

3

were

r e c e n t l y d e t e r m i n e d ( 2 3 ) . T h e s e structures, i n c o n j u n c t i o n w i t h structures d e t e r m i n e d i n other laboratories (see

Ref. 2, p p . 3 6 8 - 3 6 9 ) , l e a d to s e v e r a l

generalizations c o n c e r n i n g the stereochemistry of shift reagent complexes. I n the s i n g l e - c r y s t a l s t r u c t u r e of E u ( t h d ) ( D M S O ) , t w o c o n f o r m a 3

tions of the c o m p l e x o c c u p y the same u n i t c e l l . T h e t w o c o n f o r m a t i o n s h a v e the same gross stereochemistry, b u t t h e y differ s i g n i f i c a n t l y i n d e t a i l . T h e r e f o r e , this i n d i c a t e s t h a t t h e N M R shift reagent complexes are n o t s t r u c t u r a l l y r i g i d , b u t r a t h e r t h a t the c o o r d i n a t i o n p o l y h e d r o n a n d t h e l i g a n d s c a n b e easily d i s t o r t e d b y p a c k i n g effects. example.

The

same

Eu(thd) (DMF) 3

2

phenomenon

was

also

T h i s is n o t a n i s o l a t e d

observed

i n crystalline

w h i c h also contains t w o n o n - e q u i v a l e n t m o l e c u l e s i n

the u n i t c e l l . A c o m p a r i s o n of Y b ( t h d ) ( D M S O ) a n d E u ( t h d ) ( D M S O ) reveals 3

3

that these c o m p o u n d s c r y s t a l l i z e i n different space groups a n d t h a t t h e r e are significant differences structures.

i n the stereochemistry of

t h e i r s o l i d state

T h e c o o r d i n a t i o n g e o m e t r y of t h e Y b c o m p l e x c a n best b e

d e s c r i b e d as a t r i g o n a l b a s e - t e t r a g o n a l base p o l y h e d r o n whereas t h e E u c o m p l e x c a n best b e d e s c r i b e d as a d i s t o r t e d p e n t a g o n a l b i p y r a m i d . I n t r a m o l e c u l a r a n d i n t e r l i g a n d contacts b e t w e e n

terf-butyl

groups

indicate

that there are no serious steric interactions. T o date, the p r e f e r r e d stereoc h e m i s t r y of

h i g h e r coordinate

complexes

has n o t b e e n

successfully

c o r r e l a t e d w i t h factors s u c h as c r y s t a l field s t a b i l i z a t i o n , l i g a n d - l i g a n d


228

INORGANIC

COMPOUNDS WITH

UNUSUAL PROPERTIES

interactions, a n d s o l v a t i o n energies, a n d the c r y s t a l l a t t i c e energy appears to b e the most i m p o r t a n t factor that d e t e r m i n e s the s o l i d state s t r u c t u r e (24).

W e c o n c l u d e that the o b s e r v e d differences b e t w e e n the s o l i d state

structures of Y b ( t h d ) ( D M S O ) a n d E u ( t h d ) ( D M S O ) m a y b e c a u s e d 3

3

p r e d o m i n a n t l y b y p a c k i n g considerations. T h e b u l k y tert-butyl

g r o u p s o n the t h d l i g a n d d o not a p p e a r to b e

the d o m i n a n t factor i n d e t e r m i n i n g or l i m i t i n g the c o o r d i n a t i o n g e o m e t r y to a n y one c o n f i g u r a t i o n . I n t h e o c t a c o o r d i n a t e c o m p l e x E u ( t h d ) ( D M F ) , 3

2


t h e l i g a n d s f o r m a d i s t o r t e d square a n t i p r i s m w i t h t w o of the t h d moieties f o r m i n g one s q u a r e face a n d one t h d a n d the t w o D M F l i g a n d s (cis to e a c h o t h e r ) f o r m i n g the other face. Eu(thd)

3

and E u ( a c a c )

tures (25).

T h e 1,10-phenanthroline a d d u c t s of

also h a v e s q u a r e a n t i p r i s m a t i c s o l i d state s t r u c

3

Differences

between

the latter t w o

a t t r i b u t e d to the presence of b u l k y

terf-butyl

structures c a n n o t

groups.

be

I n fact, i t is the

t h d c o m p l e x w h i c h has the most c o m p a c t c o o r d i n a t i o n p o l y h e d r o n , a n d the β-diketonate r i n g i n closest p r o x i m i t y to the 1,10-phenanthroline g r o u p has a s m a l l e r f o l d angle a b o u t the o x y g e n - o x y g e n

vector.

Perhaps the

greater e l e c t r o n - d o n a t i n g a b i l i t y of t h e t h d l i g a n d m a y a c c o u n t f o r t h e s h o r t e n i n g of t h e L n - O b o n d lengths i n the t h d c o m p l e x .

I n contrast to

the a b o v e p a t t e r n , the n u c l e o p h i l e s i n E u ( t h d ) ( p y r i d i n e ) 3

Ho(thd) (picoline) 3

(27)

2

2

(26)

and

are b o n d e d to opposite faces of a s q u a r e a n t i -

p r i s m a n d are as far a p a r t f r o m e a c h other as possible. A n a l y s e s of these structures i m p l y that several arrangements of t h e l i g a n d s about the c e n t r a l m e t a l a t o m are possible. I f one wishes to extract s t e r e o c h e m i c a l i n f o r m a t i o n a b o u t substrates i n s o l u t i o n f r o m the data o b t a i n e d i n the N M R experiments, t w o p h y s i c a l mathematical models

are a v a i l a b l e .

F o r shifts i n d u c e d b y a d i p o l a r

( p s e u d o c o n t a c t ) m e c h a n i s m , the M c C o n n e l l - R o b e r t s o n e q u a t i o n ( E q u a t i o n 1) (28, 29)—where

u, 0 , a n d φ are the s p h e r i c a l p o l a r coordinates {

t

of the i t h r e s o n a t i n g nucleus i n the c o o r d i n a t e system of the p r i n c i p a l m a g n e t i c axes—relates the d i r e c t i o n a n d m a g n i t u d e of the shift to t h e geometry

of

AHi/H

the substrate—chelate c o m p l e x . = Κ (3 cos Bi -

substrate-chelate

l ) / r \ + K' (sin θ cos 2 φ,·) M

2

c o m p l e x has a x i a l s y m m e t r y ( a C

I f the 2

3

{

(1)

or h i g h e r axis of r o t a t i o n ) , E q u a t i o n 1

m a y b e s i m p l i f i e d to the m o r e m a n a g e a b l e f o r m of E q u a t i o n 2 w e r e Si is the angle b e t w e e n the p r i n c i p a l m a g n e t i c axis a n d the vector f r o m t h e l a n t h a n i d e i o n to t h e i t h r e s o n a t i n g nucleus. AHi/H B r i g g s et al. (30)

= Κ (3 cos Bi 2

l)/r\

(2)

h a v e s h o w n that a n e q u a t i o n s i m i l a r i n f o r m to

E q u a t i o n 2 c a n b e d e r i v e d a n d t h a t i t s h o u l d b e v a l i d w h e n the substrate


19.

siEVERS ET AL.

l i g a n d undergoes

Lanthanide

NMR

Shift

229

Reagents

free r o t a t i o n a b o u t a n axis p a s s i n g t h r o u g h the l a n

t h a n i d e i o n as w e l l as w h e n the s u b s t r a t e - c h e l a t e c o m p l e x forms three or m o r e i n t e r c o n v e r t i n g rotamers that are e q u a l l y p o p u l a t e d . S i g n i f i c a n t l y , i n this d e r i v a t i o n no a priori m e t r y of the c o m p l e x .

assumptions are m a d e c o n c e r n i n g the s y m

I n the B r i g g s et al. e q u a t i o n , θι n o w denotes the

a n g l e b e t w e e n t h e r o t a t i o n axis a n d the vector f r o m the p a r a m a g n e t i c l a n t h a n i d e i o n to the i t h resonating n u c l e u s . A l t h o u g h almost a l l the r e p o r t e d c r y s t a l structures of l a n t h a n i d e Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch019

NMR

shift reagent

complexes

are d e v o i d

of

any symmetry

element

greater t h a n the t r i v i a l C i r o t a t i o n axis, this n e e d n o t e l i m i n a t e t h e p o s s i b i l i t y of effective a x i a l s y m m e t r y i n s o l u t i o n . T h e c o o r d i n a t i o n

geometry

e x h i b i t e d b y h i g h e r c o o r d i n a t e l a n t h a n i d e ions i n a c r y s t a l l i n e a r r a n g e m e n t a p p a r e n t l y is affected b y p a c k i n g considerations. A d i r e c t c o m p a r i son of s o l i d state a n d s o l u t i o n s t r u c t u r e as d e t e r m i n e d f r o m N M R e x p e r i ments m a y not b e v a l i d b e c a u s e of the great difference i n the t i m e scales of the t w o t e c h n i q u e s .

A v e r a g i n g of several d i s s y m e t r i c arrays, s u c h as

those f o u n d i n t h e c r y s t a l s t r u c t u r e of E u ( t h d ) ( D M S O ) , leads to a n 3

e q u i v a l e n t a n d p o s s i b l y a n a x i a l l y s y m m e t r i c d e s c r i p t i o n for the t h a n i d e shift r e a g e n t - s u b s t r a t e c o m p l e x .

lan

L o w p o t e n t i a l energy b a r r i e r s

b e t w e e n the i d e a l i z e d h i g h e r c o o r d i n a t e p o l y h e d r a m i g h t also p e r m i t the t i m e - a v e r a g e d s o l u t i o n c o n f i g u r a t i o n to b e s i g n i f i c a n t l y different f r o m t h a t d i s p l a y e d i n the c r y s t a l . T h i s a v e r a g i n g process, w h a t e v e r the details, m u s t be r a p i d w i t h respect to the N M R t i m e scale s i n c e shift r e a g e n t studies at a m b i e n t temperatures a l w a y s r e v e a l a single N M R s p e c t r u m that is the average of free a n d c o m p l e x e d substrate a n d of a l l i n t e r m e d i a t e species i n s o l u t i o n . C r y s t a l s t r u c t u r e d e t e r m i n a t i o n s , h o w e v e r , i n d i c a t e d that, i n the seven-coordinate s u b s t r a t e - l a n t h a n i d e shift reagent

complex,

steric c r o w d i n g is not as serious as w a s once b e l i e v e d , a n d t h a t free r o t a t i o n a b o u t the L n - X b o n d is possible.

C o n s e q u e n t l y there m a y b e t w o

processes o p e r a t i n g i n s o l u t i o n that a l l o w successful a p p l i c a t i o n of t h e s i m p l i f i e d e q u a t i o n : ( a ) free r o t a t i o n of the substrate a n d ( b ) r a p i d i n t e r c o n v e r s i o n of g e o m e t r i c isomers to p r o d u c e a n effective a x i a l l y s y m m e t r i cal complex. L a n t h a n i d e complexes

w i t h o p t i c a l l y a c t i v e β-diketones h a v e

u s e d to d e t e r m i n e the p u r i t y of o p t i c a l isomers (see

been

c h a p . 4, p. 87 of

R e f . 2 for a r e v i e w ) . T h e most w i d e l y u s e d c h i r a l shift reagents are b a s e d o n 3-trifluoroacetyl-ci-camphor, the a n i o n of w h i c h is d e s i g n a t e d f a c a m . T h e c r y s t a l structure d e t e r m i n a t i o n of the D M F a d d u c t of fluoroacetyl-d-camphorato)

reagent, has b e e n c o m p l e t e d dimer,

tris(3-tri-

p r a s e o d y m i u m ( I I I ) , t h e first of a c h i r a l shift (31).

T h e a s y m m e t r i c u n i t contains the

( facam ) P r ( D M F ) P r ( facam ) , w i t h the D M F oxygen 3

3

3

f o r m i n g b r i d g e s b e t w e e n the t w o P r ( f a c a m )

3

moieties.

atoms

Therefore, each

P r ( I I I ) i o n is n i n e - c o o r d i n a t e w i t h a g e o m e t r y best d e s c r i b e d as a c a p p e d



230

INORGANIC

COMPOUNDS

WITH

UNUSUAL PROPERTIES

Figure 2. A stereoscopic drawing of the dimer (facam) Pr(DMF) Pr(facam) . Average bond distances and angles: Pr-O(facam), 2.46(3) A; Pr-O(DMF), 2.60(2) A; Pr-Pr, 4.078(9) A; and Pr-0(DMF)-Pr, 103.6(1.8) A. 3

square a n t i p r i s m (see

s

s

F i g u r e 2 ) . B y ignoring the ring backbones a n d

f o c u s i n g only o n t h e P r O i core, one c a n see t h a t there is a p s e u d o - m i r r o r 2

5

p l a n e c o n t a i n i n g t h e three b r i d g i n g D M F o x y g e n atoms. T h e presence of the a s y m m e t r i c centers i n t h e f a c a m b a c k b o n e prevents t h e e n t i r e d i m e r from having mirror symmetry.

E v e n i n this m o l e c u l e , w h i c h contains

l a r g e f a c a m l i g a n d s , there is s t i l l sufficient space i n t h e c o o r d i n a t i o n sphere of P r ( I I I ) to a c c o m m o d a t e three D M F groups. the u n s o l v a t e d complexes,

O n e m a y conclude that

p a r t i c u l a r l y f o r the e a r l y m e m b e r s

of t h e

l a n t h a n i d e series, s h o u l d b e a b l e to b i n d l a r g e n u c l e o p h i l e s w i t h l i t t l e difficulty.

N o w t h a t x - r a y d a t a are a v a i l a b l e , i t is a p p a r e n t that steric

c r o w d i n g is not as severe as w a s o r i g i n a l l y b e l i e v e d .

I t is also

note-

WOrthy that i n a l l structures d e t e r m i n e d , the L n - X b o n d lengths a r e i n the e x p e c t e d ranges b u t t h a t m a j o r r e o r g a n i z a t i o n s of t h e l i g a n d s a b o u t the m e t a l atom are o b s e r v e d .

Literature Cited 1. Eisentraut, K. J., Sievers, R. E., J. Am. Chem. Soc. (1965) 87, 5254. 2. Sievers, R. E., Ed., "Nuclear Magnetic Resonance Shift Reagents," Aca demic, New York, 1973. 3. Cockerill, A. F., Davies, G. L. O., Harden, R.C.,Rackham, D. M., Chem. Rev. (1973) 73, 553. 4. Reuben, J., Prog. Nucl. Magn. Reson. Spectrosc. (1973) 9, 1. 5. Tischer, R. L., Eisentraut, K. J., Scheller, K., Sievers, R. E., Bausman, R. C., Blum, P. R., "New Rare Earth Antiknock Additives that Are Potential Substitutes for Tetraethyl Lead," Aerospace Res. Lab., Wright-Patterson Air Force Base, Ohio (1974) Rep. ARL TR-74-0170 (Nat. Tech. Inf. Ser. Rep. AD/A-006, 151/5 WP). 6. Eisentraut, K. J., Tischer, R. L., Sievers, R. E., "Rare Earth β-Ketoenolate Antiknock Additives in Gasoline," U.S. Patent 3,794,473 (1974). 7. Sievers, R. E., et al., unpublished data.



19. SIEVERS ET AL. Lanthanide NMR Shift Reagents

231

8. Kutal,C.,Sievers, R. E., Inorg. Chem. (1974) 13, 897. 9. Feibush, B., Richardson, M. F., Sievers, R. E., Springer, Jr., C. S.,J.Am. Chem. Soc. (1972) 94, 6717. 10. Brooks, J. J., Sievers, R. S.,J.Chromatogr. Sci. (1973) 11, 303. 11. Sicre, J. E., Dubois, J. T., Eisentraut, K. J., Sievers, R. E., J. Am. Chem. Soc. (1969) 91, 3476. 12. Springer, C. S., Meek, D. W., Sievers, R. E., Inorg. Chem. (1967) 6, 1105. 13. Richardson, M. F., Sievers, R. E., Inorg. Chem. (1971) 10, 498. 14. Scribner, W. G., Smith, B. H., Moshier, R. W., Sievers, R. E., J. Org. Chem. (1970) 35, 1969. 15. Sievers, R. E., in "Coordination Chemistry," S. Kirschner, Ed., p. 270, Plenum, New York, 1969. 16. Hinckley, C.C.,J.Am. Chem. Soc. (1969) 91, 5160. 17. Sanders, J. K. M., Williams, D. H., Chem. Commun. (1970) 422. 18. Demarco, P. V., Elzey, T. K., Lewis, R. B., Wenkert, E., J. Am. Chem. Soc. (1970) 92, 5734. 19. Ibid. (1970) 92, 5737. 20. Rondeau, R. E., Sievers, R. E.,J.Am. Chem. Soc. (1971) 93, 1522. 21. Rondeau, R. E., Sievers, R. E., Anal. Chem. (1973) 45, 2145. 22. Willcott, M. R., Lenkinski, R. E., Davis, R. E., J. Am. Chem. Soc. (1972) 94, 1742. 23. Cunningham, J. Α., Sievers, R. E., Proc. 10th Rare Earth Res. Conf., Carefree, Ariz., April-May 1973. 24. Blight, D. G., Kepert, D. L., Theor. Chim. Acta (1968) 11, 51. 25. Watson, W. H., Williams, R. J., Stemple, N. R., J. Inorg. Nucl. Chem. (1972) 34, 501. 26. Cramer, R. E., Seff, K., Chem. Commun. (1972) 400. 27. Horrocks, Jr., W. De W., Sipe, J. P., Luber, J. R.,J.Am. Chem. Soc. (1971) 93, 5258. 28. McConnell, H. M., Robertson, R.E.,J.Chem. Phys. (1958) 29, 1361. 29. Horrocks, Jr., W. DeW.,J.Am. Chem. Soc. (1974) 96, 3022. 30. Briggs, J. M., Moss, G. P., Randall, E. W., Sales, K. P., J. Chem. Soc. Chem. Commun. (1972) 1180. 31. Cunningham, J. Α., Sievers, R. E., J. Am. Chem. Soc. (1975) 97, 1586. RECEIVED February 26, 1975.


Lanthanide NMR Shift Reagents

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