Matrix Isolation Studies of Alkali Halide Salt Molecules with Lewis

22 Matrix Isolation Studies of Alkali Halide Salt Molecules with Lewis Acids and Bases Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 20, 2016 | http://pubs.acs.org Publication Date: March 8, 1982 | doi: 10.1021/bk-1982-0179.ch022

B R U C E S.

AULT

University of Cincinnati, Department of Chemistry, Cincinnati, OH 45221

The matrix isolation technique has been applied, in conjunction with the salt/molecule reaction tech nique, to model the high temperature gas phase reac tions of alkali halide salt molecules. The reactions with Lewis acids such as SiF , HF and CO yielded ion pair products which were quenched into inert matrices for spectroscopic study. Difficulties arising from lattice energy considerations in alkali halide salt reactions are minimized by the i n i t i a l vaporization of the salt reactant. The reaction of such salt molecules with Lewis bases, including H O and NH , yielded complexes similar in nature to transition metal coordination complexes, with binding through the alkali metal cation to the base lone pair. 4

2

2

3

I n t e r e s t i n the chemistry o f m a t e r i a l s at high temperatures has increased r a p i d l y i n the past few years. The i n t e r e s t i n the f i e l d o f high temperature chemistry i s w e l l demonstrated by the d i v e r s i t y o f techniques which have been a p p l i e d , ranging from chemiluminescent r e a c t i v e c o l l i s i o n s t o low temperature quenching i n t o i n e r t gas matrices. Of considerable importance a l s o i s the f a c t t h a t high temperature chemical r e a c t i o n s occur under s o l v e n t free c o n d i t i o n s , and chemical behavior may be a l t e r e d under these c o n d i t i o n s . The a l k a l i h a l i d e s a l t s , f o r example, have been i n v e s t i g a t e d thoroughly i n a v a r i e t y o f p o l a r s o l v e n t s , where they d i s s o l v e r e a d i l y i n t o separated ions. However, the chemistry o f the a l k a l i h a l i d e s a l t s i n the absence o f solvent i n the gas phase has not been n e a r l y as w e l l c h a r a c t e r i z e d . Recent develop ments now permit the e x p l o r a t i o n o f the chemistry o f the a l k a l i h a l i d e s a l t s , and others, under high temperature c o n d i t i o n s . One area o f i n t e r e s t i n high temperature chemistry, and c e r t a i n l y i n a l l branches o f chemistry, i s the determination o f the nature o f the i n i t i a l complex formed through the s i n g l e c o l l i s i o n r e a c t i o n o f two p a r t n e r s . For example, the i n i t i a l complex 0097-6156/82/0179-0327$05.00/0 ©

1982

A m e r i c a n Chemical Society

Gole and Stwalley,; Metal Bonding and Interactions in High Temperature Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 20, 2016 | http://pubs.acs.org Publication Date: March 8, 1982 | doi: 10.1021/bk-1982-0179.ch022

328

METAL BONDING AND INTERACTIONS

might be a h i g h l y r e a c t i v e complex, a weakly bound t r a n s i t i o n s t a t e complex, o r a r e l a t i v e l y s t a b l e ion p a i r . Knowledge o f the f i n a l products o f the system only permits i n f e r e n c e s as t o the nature o f the i n i t i a l r e a c t i o n product o r complex. The matrix i s o l a t i o n technique was s p e c i f i c a l l y developed t o a s s i s t i n the i n v e s t i g a t i o n o f i n i t i a l r e a c t i o n products and has o f t e n been a p p l i e d t o the study o f both n e u t r a l and charged chemical s p e c i e s . The i n i t i a l t h r u s t o f work i n the matrix i s o l a t i o n f i e l d was toward f r e e r a d i c a l s (_l-4) / which are f r e q u e n t l y p o s t u l a t e d i n r e a c t i o n mechanisms, but are s u f f i c i e n t l y r e a c t i v e that they o f t e n escape d e t e c t i o n and c h a r a c t e r i z a t i o n by t r a d i t i o n a l techniques. In the mid-1960's, the technology o f high temperature evaporative ovens was a p p l i e d t o matrix i s o l a t i o n , and i n c r e a s e d the range o f species a v a i l a b l e f o r study. Linevsky (_5) and others (6_-S0 demons t r a t e d the f e a s i b i l i t y o f v a p o r i z a t i o n o f i o n i c s a l t s , while s e v e r a l groups developed techniques f o r the v a p o r i z a t i o n o f metal atoms f o r use as r e a c t a n t s i n matrix i s o l a t i o n s t u d i e s (10,11,12). The l a t e 1960's and the 1970's have seen the development o f h i g h energy sources, such as vacuum u l t r a v i o l e t p h o t o l y s i s (13-16), microwave discharge (17,18) and proton beam r a d i o l y s i s (19,20) for the formation and t r a p p i n g of i o n i c s p e c i e s i n i n e r t gas matrices. In many matrix i s o l a t i o n s t u d i e s , i n c l u d i n g those which employ high temperature evaporative ovens, two r e a c t a n t s are allowed t o mix, i n a l a r g e excess o f i n e r t gas, immediately i n f r o n t of the c o l d d e p o s i t i o n s u r f a c e . The gas phase mixture i s r a p i d l y quenched and condensed onto the c o l d s u r f a c e , u s u a l l y maintained between 10 and 20 K, so that very few c o l l i s i o n s can occur between r e a c t i v e p a r t n e r s , and the i n i t i a l r e a c t i o n products of the system are i s o l a t e d . When p h o t o l y s i s i s employed i n the formation of intermediate s p e c i e s , i r r a d i a t i o n can be c a r r i e d out e i t h e r d u r i n g d e p o s i t i o n o f the r e a c t i o n mixture, o r a f t e r the i n e r t gas matrix i s r i g i d l y frozen i n p l a c e . Once d e p o s i t i o n i s complete and the i n i t i a l r e a c t i o n product i s trapped i n an i n e r t gas matrix, c h a r a c t e r i z a t i o n i s c a r r i e d out s p e c t r o s c o p i c a l l y . S e v e r a l s p e c t r o s c o p i c techniques have been used; the most common i s i n f r a r e d spectroscopy, e i t h e r d i s p e r s i v e or F o u r i e r transform. Raman s p e c t r o s c o p i c s t u d i e s have been c a r r i e d out as w e l l , but low s i g n a l l e v e l s have made t h i s approach d i f f i c u l t . When the trapped intermediate i s a f r e e r a d i c a l , e l e c t r o n spin resonance techniques are v a l u a b l e as w e l l . F i n a l l y , a number o f researchers are employing e l e c t r o n i c spectroscopy, when the species o f i n t e r e s t has an absorption i n the v i s i b l e or u l t r a v i o l e t region. The s t u d i e s which w i l l be d e s c r i b e d below were c a r r i e d out u s i n g a conventional matrix i s o l a t i o n apparatus, i n c l u d i n g a C T i model 21 c l o s e d c y c l e r e f r i g e r a t o r as a low temperature source. Such r e f r i g e r a t o r s can c o o l t o about 10 K with l i t t l e or no heat load, and t o 14-16 K with the t y p i c a l heat load generated by a high temperature oven o p e r a t i n g a t 500 C. A schematic diagram



22.

AULT

Alkali

Halide

Salt

Molecules

329

of the vacuum v e s s e l used i n these experiments, f o r s i n g l e or double oven experiments, i s shown i n F i g u r e 1. The ovens used i n t h i s work were s m a l l , r e s i s t i v e l y heated quartz ovens, with Chromel A h e a t i n g wire, and were g e n e r a l l y capable o f 600°C. A small s t a i n l e s s s t e e l Knudsen c e l l with 1-2 mm o r i f i c e was loaded with the s a l t of i n t e r e s t , p l a c e d i n the oven, and the system evacuated. Since the a l k a l i h a l i d e s a l t s are r e l a t i v e l y hygros c o p i c , the oven was heated to s l i g h t l y below the v a p o r i z a t i o n temperature under vacuum t o dry the s a l t . Small r e s i d u a l amounts of water were always detected, and i n t e r f e r e n c e was g e n e r a l l y minimal; however, as d i s c u s s e d below, the complex o f the a l k a l i h a l i d e s a l t with H 2 O was o f t e n weakly observed. Argon was used as the matrix m a t e r i a l i n a l l of the experiments described here, and was used without f u r t h e r p u r i f i c a t i o n . I n f r a r e d spectroscopy was the primary means o f c h a r a c t e r i z a t i o n of the r e a c t i o n products although Raman s p e c t r a were a l s o recorded f o r c e r t a i n systems. When appropriate, normal coordinate c a l c u l a t i o n s were run t o conf i r m the band assignments and to deduce f o r c e f i e l d s f o r the t r a p ped s p e c i e s . A l k a l i H a l i d e S a l t Reactions Since the adaptation of h i g h temperature oven technology to matrix i s o l a t i o n , the v a p o r i z a t i o n of h a l i d e and oxide s a l t s has become not only f e a s i b l e but r e a d i l y accomplished. Many e a r l y s t u d i e s were devoted t o i n v e s t i g a t i n g the s a l t s themselves, i n an environment f r e e from l a t t i c e i n t e r a c t i o n s and p e r t u r b a t i o n s . These l e d to matrix s t u d i e s o f the s t r u c t u r e of i o n p a i r s and t r i p l e i o n s , such as the thorough s t u d i e s by D e v l i n and coworkers on matrix i s o l a t e d a l k a l i n i t r a t e (21) , c h l o r a t e (22_) and p e r c h l o r a t e i o n p a i r s (23) . For r e l a t i v e l y simple s a l t s , such as the a l k a l i h a l i d e s , i n v e s t i g a t i o n s were conducted i n t o the s t r u c ture o f the d i m e r i c s a l t species (6_/7^8_) , which i s present i n a gas phase e q u i l i b r i u m with the monomeric s a l t species. These dimers have been found to be very s t r o n g l y bound i n a c y c l i c structure. Only o c c a s i o n a l l y have s a l t molecules been v a p o r i z e d f o r use as a reactant toward another species i n matrix i s o l a t i o n s t u d i e s . D e v l i n (24,25,26) conducted e x t e n s i v e experiments i n which s a l t molecules were v a p o r i z e d and condensed i n t o argon matrices cont a i n i n g from 1% t o 90% H 2 O or N H 3 , to study the e f f e c t s of stepwise s o l v a t i o n of the s a l t molecule, as a model f o r s o l u t i o n studies. Margrave (27_) and Snelson (28) each have used s a l t molecules as r e a c t a n t s , but most commonly toward another s a l t molecule to form a mixed s a l t dimer. The work described below, which was i n i t i a t e d at the U n i v e r s i t y of V i r g i n i a and has been continued at the U n i v e r s i t y of C i n c i n n a t i , employs a l k a l i h a l i d e s a l t molecules as r e a c t a n t s toward a v a r i e t y of s p e c i e s , i n c l u d i n g both Lewis acids and Lewis bases. The i n i t i a l i n t e n t was to r e a c t a s a l t molecule such as NaCl with HC1 i n an excess of argon t o b r i n g




330


22.

AULT

Alkali

Halide

Salt

331

Molecules

about h a l i d e anion t r a n s f e r from NaCl to HC1, forming the Na HCI2 ion p a i r . While the r e a c t i o n NaCl + HC1 -> N a + HCI2 i s endothermic by 122 Kcal/mole, energy i s regained by the e l e c t r o s t a t i c a t t r a c t i o n of the c a t i o n and anion (ion p a i r i n g energy), and the r e a c t i o n to form the i o n p a i r Na HCI2 becomes exothermic f o r charge separations l e s s than 2.72 8. For l a r g e r a l k a l i metals these r e a c t i o n s become more exothermic as a consequence of the lower i o n i z a t i o n p o t e n t i a l s of the l a r g e r a l k a l i metals. These s i m p l i s t i c thermodynamic arguments provided at l e a s t some b a s i s f o r the a n t i c i p a t i o n t h a t ion p a i r formation could occur under these c o n d i t i o n s , and the formation of new and unusual a n i o n i c species might be f e a s i b l e . These new s p e c i e s , t o the degree that they can be viewed as d i s t i n c t i o n p a i r s , are themselves s a l t s free from l a t t i c e i n t e r a c t i o n s and i n f o r m a t i o n can be gained both about the s t r u c t u r e and s t a b i l i t y of the anion, and a l s o about the nature of i o n p a i r i n g . As i n d i c a t e d above, when a l k a l i h a l i d e s a l t s are v a p o r i z e d both the monomer and dimer s p e c i e s are p r e s ent, as w e l l as higher polymers i n l e s s e r amounts. However, the dimer i s s u f f i c i e n t l y s t r o n g l y bound, by 50-80 Kcal/mole, t h a t i t does not enter i n t o the chemistry of these systems; i n s t e a d , the r e a c t i o n s of the monomer only are observed.


+

The f i r s t a p p l i c a t i o n of t h i s s a l t / m o l e c u l e technique was t o the study of the hydrogen b i h a l i d e anions, HX2 and HXY~, where X and Y represent halogens. The HF2 anion i s w e l l known and i s a very s t r o n g l y hydrogen bonded, centrosymmetriq anion i n a v a r i e t y of c r y s t a l s (29). Studies of the HCI2 anion p r o v i d e d evidence f o r two d i f f e r e n t forms of the anion, a type I asymmetric anion and a type I I centrosymmetric anion (_30) . Tetraalkylammonium c a t i o n s were employed i n t h i s o r i g i n a l work, and the c a t i o n was thought to determine whether the type I or the type I I anion was formed. While these two anions, HF2 and HClJ were w e l l i n v e s t i gated, l i t t l e was known about e i t h e r HBr2 or H I , as w e l l as the mixed halogen anions. The matrix c o d e p o s i t i o n of CsF with HF (31_,32) i n an excess of argon, up t o Ar/HF r a t i o s of 3000, y i e l d e d intense bands at 1364 and 1218 cm , w i t h i n a few wavenumbers of the HF2 band p o s i t i o n s i n i o n i c l a t t i c e s (29). The deuterium s h i f t , upon format i o n of DF2"/ was s l i g h t l y above the harmonic value of 1.41, i n d i c a t i n g t h a t the anion maintained a center o f symmetry. Similar r e s u l t s were obtained with other a l k a l i f l u o r i d e s a l t s , but the product y i e l d decreased as the r a d i u s of the a l k a l i metal c a t i o n decreased. These r e s u l t s p r o v i d e d immediate c o n f i r m a t i o n of the s a l t / m o l e c u l e technique, demonstrating t h a t f o r a known system ion p a i r formation occurred and that the spectrum o f the product anion resembled c l o s e l y t h a t of the anion i n known environments. The r e a c t i o n of Ar/HCl mixtures with a l k a l i c h l o r i d e s a l t molecules (33), and i n p a r t i c u l a r CsCl gave r i s e to a very i n tense i n f r a r e d absorption near 723 cm . Deuteration gave e v i dence f o r a center of symmetry i n the product HCI2 anion, cons i s t e n t with the formation of a type I I s p e c i e s . No evidence was 2

1

l


332


obtained f o r formation of a type I HCI2 anion under these c o n d i t i o n s , suggesting t h a t type I anion formation i n s o l u t i o n r e s u l t e d from d i s t o r t i o n of the more s t a b l e type I I anion by c e r t a i n bulky cations. S i m i l a r s t u d i e s were conducted on both (34./35J the HBr^ and HI2 anions; HBr2 was c h a r a c t e r i z e d by a s i n g l e , very i n t e n s e i n f r a r e d fundamental near 727 cm , while HI2 showed a s t r o n g abs o r p t i o n near 673 cm" . Both anions showed d i s t i n c t evidence f o r a center of symmetry, and were assigned as type II anions, while no evidence was obtained f o r a type I anion. Pimentel and coworkers observed s i m i l a r , although sharper, bands when they passed a mixture of Ar/HX/X2 through a microwave discharge p r i o r to matrix condensation (36,37,38). They assigned these bands t o the HX2 r a d i c a l , assignments which were disputed s h o r t l y t h e r e a f t e r by M i l l i g a n and Jacox_ (39_'i£) . The c l o s e agreement of the band p o s i t i o n s of the M HX2 i o n p a i r s to the bands observed by these researchers_suggests t h a t these f e a t u r e s be reassigned t o the i s o l a t e d HX2 anions. 1


1

The formation of the mixed halogen anions i s o f i n t e r e s t as w e l l ; a center of symmetry i s not p o s s i b l e under these c o n d i t i o n s , and the p o t e n t i a l e x i s t s f o r both types I and I I anions. A l l of the combinations of h a l i d e s were i n v e s t i g a t e d , from C1HF~ to BrHI , with a l k a l i metal c a t i o n s . The r e s u l t s were c o n s i s t e n t throughout the e n t i r e s e r i e s , and the s p e c t r a were c o n s i d e r a b l y more complex than those o f the HX2 anions. Moreover, each anion could be formed i n e i t h e r of two ways; the BrHCl anion might be formed through e i t h e r the r e a c t i o n o f HBr with C s C l or HC1 with CsBr. I d e n t i c a l s p e c t r a were obtained i n each case, suggesting that the r e a c t i o n s proceeded through i d e n t i c a l t r a n s i t i o n s t a t e s . C o l l e c t i v e l y , the s p e c t r a i n d i c a t e d formation of both type I and type II anions simultaneously, and the y i e l d of each type was determined by the l o c a t i o n o f the c a t i o n i n the i o n p a i r . The type I/type I I r a t i o v a r i e d with the c a t i o n ; the s m a l l e r a l k a l i metal c a t i o n s favored the type I I anion, while the l a r g e r c a t i o n s favored the type I anion. Table I l i s t s the p o s i t i o n of the hydrogen s t r e t c h i n g frequency V 3 f o r each o f the hydrogen b i h a l i d e anions s t u d i e d . In a r e l a t e d study (41), the r o l e of the cyanide anion CN as a pseudohalogen was i n v e s t i g a t e d ; the r e a c t i o n of e i t h e r CsF with HCN or CsCN with HF y i e l d e d the FHCN anion, i o n p a i r e d with the C s c a t i o n . T h i s anion appeared t o have a very strong hydrogen bond, as a n t i c i p a t e d , and the hydrogen s t r e t c h i n g frequency occurred at 1800 cm , i n d i c a t i n g a type I anion, not a c e n t r o symmetric analog of the HX2 anions. +

1


22.

AULT

Alkali

Halide

Salt

Molecules

333

Table I Hydrogen S t r e t c h i n g Band P o s i t i o n s f o r the Hydrogen B i h a l i d e Anions Anion

Type

HF2


HC1

II

II II II I II I II

2

HBr HI

2

2

HC1F HC1F

HBrF HBrF

v

Ref.

3

1364 723 727 673 2500 933 2803 849

1

cm"

32 33 34 35 32 32 32 32

Anion

Type

v

I II I II I II I II

2960 763 1110 742 1560 644 920 666

HIF" HIF

HBrCl HBrCl HIC1

HICl" HIBr HIBr

Ref.

3

1

cm"

32 32 34 34 35 35 35 35

Cesium F l u o r i d e C a t a l y s i s Studies A major a p p l i c a t i o n of the f l u o r i d e i o n i n s o l u t i o n , as w e l l as s o l i d CsF, i s the c a t a l y s i s of a wide range o f r e a c t i o n s (4245). The mechanism of t h i s c a t a l y s i s i s not e n t i r e l y c l e a r ; both a d d i t i o n and a b s t r a c t i o n r e a c t i o n s have been p o s t u l a t e d as the f i r s t step. The v a p o r i z a t i o n o f i n d i v i d u a l a l k a l i f l u o r i d e s a l t molecules and r e a c t i o n with s u i t a b l e p a r t n e r s before matrix condensation might serve as an a p p r o p r i a t e model f o r t h i s s o l u t i o n phase c a t a l y s i s . One system i n which CsF has been employed as a c a t a l y s t (46) i s i n o x i d a t i v e f l u o r i n a t i o n r e a c t i o n s of F 2 , which might proceed through the t r i f l u o r i d e anion, F 3 , analogous to the known C I 3 , Br3 and I3 anions. The s a l t / m o l e c u l e r e a c t i o n was employed to study the r e a c t i o n (47_) with F 2 i n argon matrices; a s i n g l e intense i n f r a r e d a b s o r p t i o n was detected at 550 cm , while the Raman spectrum of a s i m i l a r sample showed an intense peak at 461 cm . S i m i l a r r e s u l t s were obtained with RbF, the product bands showing v i r t u a l l y no c a t i o n s h i f t , while with KF and NaF no d i s t i n c t product bands were observed. These two bands have been assigned to the F 3 anion i n the Cs F 3 i o n p a i r , and suggest t h a t o x i d a t i v e f l u o r i n a t i o n r e a c t i o n s may w e l l proceed v i a t h i s intermediate s p e c i e s . While a valence bond s t r u c t u r a l model cannot r e a d i l y account f o r the F 3 anion, simple t r i a t o m i c molecular o r b i t a l theory p r e d i c t s a spectrum s i m i l a r o

f

C

s

F

1

1

t o the

isoelectronic XeF2

and

KrF2

(22 valence e l e c t r o n s ) .

The

agreement between the spectra of X e F 2 and F 3 i s remarkably good, and supports t h i s bonding model f o r the F 3 anion. These e f f o r t s were extended to the C I 3 and B r 3 anions, as w e l l as to the mixed t r i h a l i d e anions. The r e a c t i o n of a l k a l i c h l o r i d e s a l t s with C I 2 i n argon gave r i s e t o d i s t i n c t i v e bands between 200 and 400 cm" , i n both the i n f r a r e d and Raman s p e c t r a (48). These have been assigned t o C I 3 , which showed s i m i l a r band p o s i t i o n s i n s o l u t i o n with bulky tetraalkylammonium c a t i o n s (49). A s h i f t i n band p o s i t i o n was observed with a change i n c a t i o n , 1


334


and the i n f r a r e d and Raman s p e c t r a demonstrated t h a t mutual exc l u s i o n d i d not h o l d . Since the C I 3 i s expected to be c e n t r o symmetric i n i t s i s o l a t e d form, by analogy t o I 3 , i t i s l i k e l y t h a t the a l k a l i metal c a t i o n i s causing some p e r t u r b a t i o n , and lowering the s p e c t r o s c o p i c symmetry of the C I 3 anion. It i s noteworthy t h a t the F3 was not d e t e c t a b l y perturbed by the Cs c a t i o n , and mutual e x c l u s i o n d i d h o l d w i t h i n the l i m i t s of detectability. The mixed anions have been s y n t h e s i z e d as w e l l , f o r a l l combinations of F, CI and Br (50,51). In the cases i n which two product anions were p o s s i b l e , such as the r e a c t i o n of CsF with C1F to form e i t h e r the FC1F" or C1FF*" anions, both r e a c t i o n s were observed t o occur. Recently, Andrews (52) and coworkers completed t h i s s e r i e s of s t u d i e s by i n v e s t i g a t i n g the matrix i s o l a t e d 13 anion. The v i b r a t i o n a l frequencies o f t h i s anion are too low t o be detected i n an i n f r a r e d spectrum, and only Raman s p e c t r a were recorded. 13 has an e l e c t r o n i c absorpt i o n i n the v i s i b l e p o r t i o n of the spectrum, so t h a t resonance enhancement of the Raman spectrum was observed, as w e l l as an overtone p r o g r e s s i o n .


+

A second c l a s s of chemical compounds whose r e a c t i o n s are c a t a l y z e d by the f l u o r i d e i o n , and by CsF are organosilanes (42). The d i v e r s i t y of chemistry e x h i b i t e d by these compounds i s cons i d e r a b l e , and with the p o t e n t i a l f o r expanded valence i n these compounds t o f i v e and s i x coordinate intermediate anions, the a p p l i c a t i o n of the s a l t / m o l e c u l e technique was suggested. The gas phase ion/molecule r e a c t i o n s of s e v e r a l s i l a n e systems have been i n v e s t i g a t e d through i o n c y c l o t r o n resonance techniques (53,54), p r o v i d i n g thermochemical i n f o r m a t i o n about the product ions i n these systems. Perhaps the simplest r e a c t i o n i s t h a t of F with SiFit; the intermediate anion S i F s has been s t a b i l i z e d under c a r e f u l l y c o n t r o l l e d circumstances (55) , while the 2:1 adduct S i F 6 i s w e l l known (56). Less i s known about the e f f e c t s of a l k y l a t i o n on the s t a b i l i t y of the product i o n s . Salt/molecule s t u d i e s (57) of the r e a c t i o n of e i t h e r CsF or KF with SiFi+ d i l u t e d i n argon gave r i s e to a s e r i e s of product bands over c o n c e n t r a t i o n ranges from 200/1 t o 1000/1. Bands were observed at 812, 855 and 932 cm" with CsF, as w e l l as weaker bands between 400 and 500 cm as can be seen i n F i g u r e 2. These have a l l been assigned t o the S i F s anion i n reasonably good agreement with the c r y s t a l l i n e spectrum o f the anion. No evidence was detected f o r the S i F 6 anion u n t i l the sample was allowed t o d i f f u s e at somewhat e l e v a t e d temperatures (45 K). Under these c o n d i t i o n s , the bands assigned t o S i F s disappeared, and i n t e n s e bands grew i n near 730 and 480 cm" , which were assigned t o the S i F ^ anion. The o b s e r v a t i o n o f three bands above 800 cm' , the S i - F s t r e t c h i n g r e g i o n , demonstrated t h a t the symmetry of the anion i n the i o n p a i r i s not s t r i c t l y the D h t r i g o n a l bipyramid s t r u c t u r e a n t i c i p a t e d from valence bond considerations. Rather, the l o c a l symmetry i s lowered by the a l k a l i metal c a t i o n , e i t h e r a c t i v a t i n g an otherwise forbidden 1

1

1

1

3



22.

AULT

940

Alkali

Halide Salt

900

Molecules

860 820 780 ENERGY (cm-1)

335

470

430

Inorganic Chemistry

Figure 2. IR spectra of reaction products of CsF and KF with samples of Ar/SiF at varying dilution, over the spectral regions of interest (57). Key: top, reaction products of CsF with a sample of Ar/SiFj, = 1000; middle, comparable reaction at a dilution of 400:1; and bottom, reaction products of KF with a sample of Ar/SiF^ = 150. h


336


1

mode of the anion, or removing the degeneracy of the E equat o r i a l f l u o r i n e s t r e t c h i n g v i b r a t i o n . The p r e f e r r e d model i s an a x i a l i n t e r a c t i o n of the a l k a l i metal c a t i o n with one a x i a l f l u o r i n e , a c t i v a t i n g the symmetric a x i a l S i - F s t r e t c h but maint a i n i n g the degeneracy of the antisymmetric e q u a t o r i a l s t r e t c h . However, the Cz and Ci+ symmetries could not be c o n c l u s i v e l y r u l e d out. The s y n t h e s i s of the pentavalent mixed halogen s i l i c a t e anion was achieved i n p a r t as w e l l . The r e a c t i o n o f CsCl w i t h SiFii y i e l d e d the SiClFi+ anion, f o r example, and the s e r i e s was extended up to and i n c l u d i n g the SiCli+F" anion. However, attempts to synthesize the S i C l s anion with e i t h e r the Cs or K c a t i o n were u n s u c c e s s f u l . This may i n d i c a t e that e i t h e r t h i s anion i s not s t a b l e , contrary to one published r e p o r t , or t h a t there i s some inherent l i m i t a t i o n to the s a l t / m o l e c u l e technique. T h i s p o i n t w i l l be discussed i n more d e t a i l below. The dependence of the s t a b i l i t y and r e a c t i v i t y of the pentacoordinate s i l i c o n anions on the number of a l k y l groups i s of considerable i n t e r e s t , as most o r g a n o s i l i c o n chemistry i s c a r r i e d out on a l k y l s i l a n e compounds, not SiFi+. Much l e s s i s known about the p o t e n t i a l product anions i n these s t u d i e s ; an ICR study (_54) d i d i n d i c a t e t h a t the f l u o r i d e ion a f f i n i t y of the parent s i l a n e decreased d r a m a t i c a l l y with an i n c r e a s i n g number of methyl groups. The r e a c t i o n (58) o f CsF with (CH )SiFit i n argon gave r i s e to four product bands, at 742, 825, 837 and 862 cm , which might be a n t i c i p a t e d f o r a C product anion. These bands were formed with r e l a t i v e ease, suggesting t h a t the product a n i o n _ i s q u i t e s t a b l e . However, e f f o r t s t o synthesize the ( C H ) 2 S i F 3 anion were marginally s u c c e s s f u l . Weak bands were observed i n the S i - F s t r e t c h i n g region, but t h e i r breadth and l a c k of i n t e n s i t y i n d i c a t e d that t h i s anion i s only weakly bound at best under these c o n d i t i o n s . Attempts to form the remaining members of the s e r i e s , (CH ) S i F 2 and (CH ) i+SiF , were completely unsuccessful, supporting the c o n c l u s i o n from the ICR data t h a t the s t a b i l i t y decreases r a p i d l y with an i n c r e a s i n g number of methyl groups. However, these r e s u l t s suggest t h a t pentacoordinate s i l i c o n anions are s t a b l e e n t i t i e s i n s o l u t i o n , although h i g h l y r e a c t i v e , and may w e l l be the a c t i v e i n t e r m e d i ates i n organosilane r e a c t i o n s c a t a l y z e d by CsF. Another example (_59) of the c a t a l y t i c r o l e o f the f l u o r i d e ion i s i n the chemistry of COF2, i n c l u d i n g the r e a c t i o n o f COF2 with F2 i n the presence o f CsF. T h i s r e a c t i o n proceeds r e a d i l y at -78 C and i s thought to i n v o l v e attack of the F " on COF to form the COF anion. tfhis anion was f i r s t detected i n 1965, but only r e c e n t l y c h a r a c t e r i z e d s p e c t r o s c o p i c a l l y (60). Even here, a very broad spectrum was obtained, as a consequence of the c r y s t a l l i n e environment. The matrix r e a c t i o n of CsF with COF2 was i n v e s t i g a t e d to determine the s o l v e n t - f r e e chemistry of t h i s system, as w e l l as to p o s s i b l y sharpen up s p e c t r a l features (61). At Ar/COF2 d i l u t i o n s as high as 3000/1, intense product bands


v

V

3

1

2 v

3

3

3

3

2

3


22.

AULT

Alkali

Halide

Salt

337

Molecules

were observed at 555, 808, 919, 1039 and 1514 cm , which were assigned t o the C O F 3 anion. These occur near the absorptions observed i n the c r y s t a l l i n e spectrum, but are much sharper and b e t t e r r e s o l v e d . However, the anion shows l e s s than C 3 symmetry, which was determined f o r the c r y s t a l l i n e anion, and t h i s too can be a t t r i b u t e d t o the presence of the a l k a l i metal c a t i o n . The doubly degenerate mode i s s p l i t i n t o components a t 919 and 1039 cm , from a s i n g l e E mode at 960 cm"" i n the c r y s t a l . This s p l i t t i n g , which i s t y p i c a l of i o n p a i r i n g e f f e c t s on degenerate bands, w i l l be d i s c u s s e d i n more d e t a i l below. The band at 1514 cm was assigned to the carbonyl s t r e t c h i n g v i b r a t i o n and shows character intermediate between a s i n g l e and double bond. While t h i s r e a c t i o n proceeded r a p i d l y at h i g h d i l u t i o n s t o y i e l d i n tense product bands, the r e a c t i o n s of C s C l with C O F 2 and CsF with COC1F, e i t h e r of which might have y i e l d e d the C O C I F 2 anion, d i d not occur. T h i s h i g h l i g h t s the c o n s i d e r a b l e d i f f e r e n c e i n chemistry between the c h l o r i d e and f l u o r i d e ions, not only i n s o l u t i o n but a l s o at r e l a t i v e l y high temperatures i n the gas phase. V


1

1

1

One simple anion which has been of c o n s i d e r a b l e i n t e r e s t t o i n o r g a n i c chemists i s the fluoroformate anion, C O 2 F . S e v e r a l attempts (62/63_) have been made at the s o l u t i o n or s o l i d phase s y n t h e s i s of t h i s anion, and a l l have been u n s u c c e s s f u l , even though t h i s anion i s i s o e l e c t r o n i c with the very s t a b l e carbonate anion, CO3. However, gas phase ICR (64) r e s u l t s i n d i c a t e that the f l u o r i d e i o n a f f i n i t y of CO2 t o form the C O 2 F anion i s roughly 32 Kcal/mole. T y p i c a l condensed phase s y n t h e t i c attempts i n v o l v e d s u b j e c t i n g CsF powder t o a high pressure CO2 atmosphere at e l e v a t e d temperatures. I t has been p o s t u l a t e d (62) t h a t the l a c k of r e a c t i o n , i n view of the p r e d i c t e d s t a b i l i t y of the anion, i s due to the h i g h l a t t i c e energy of CsF, which must be overcome d u r i n g the r e a c t i o n . However, the h i g h temperature s a l t / m o l e c u l e technique overcomes the l a t t i c e energy requirement by v a p o r i z a t i o n of the s a l t p r i o r t o r e a c t i o n . Hence, a p p l i c a t i o n of t h i s technique c o u l d w e l l l e a d to the s y n t h e s i s of the C 0 F anion. 2

The matrix c o d e p o s i t i o n of CsF molecules (65) with samples of A r / C 0 at d i l u t i o n s between 250/1 to 1000/1 gave r i s e to a set of sharp, r e l a t i v e l y i n t e n s e bands at 870, 1310 and 1750 cm , along with weak, broad bands near 810, 1290 and 1800 cm When such a sample was annealed to 45 K and recooled, the sharp bands diminished i n i n t e n s i t y and the broad bands grew i n . Depo s i t i o n of CsF i n t o a pure CO2 matrix gave r i s e to these broad product bands as w e l l . The c o d e p o s i t i o n of CsF with a sample o f A r / C 0 2 gave r i s e to a s h i f t e d set of sharp bands, i n d i c a t i v e of v i b r a t i o n s i n v o l v i n g the c e n t r a l carbon atom. These bands have been assigned to the C-F and the two C - 0 s t r e t c h i n g v i b r a t i o n s i n the C 0 2 F ~ anion, i o n p a i r e d with Cs . T h i s anion should have two C - 0 s t r e t c h i n g v i b r a t i o n s near the E mode of the CO3 anion, as these v i b r a t i o n s are d e r i v e d from the E mode upon r e 2

l

1 3



338

ducing the symmetry from D3^ t o C2ft. The E mode of the carbonate anion occurs at 1495 cm , n e a r l y halfway between the 1310 and the 1750 cm bands. The 870 cm" band i s i n the C-F s t r e t c h i n g region f o r the s i m i l a r C O F 3 anion, and i s e a s i l y assigned t o the C-F s t r e t c h of the CO2F anion. While c h a r a c t e r i z a t i o n of the fluoroformate anion i s s t i l l i n progress, t h i s system p r o v i d e s another example o f the u t i l i t y o f the s a l t / m o l e c u l e technique, a l l o w i n g f o r the s y n t h e s i s of an unusual a n i o n i c species without regard to the l a t t i c e energies of the r e a c t a n t s or the products. 1


-1

1

Ion P a i r i n g E f f e c t s Salt/molecule r e a c t i o n s , as used here i n c o n j u n c t i o n with the matrix i s o l a t i o n technique, have been employed to mimic h i g h temperature gas phase ion-molecule r e a c t i o n s . However, t h i s i s not e n t i r e l y c o r r e c t ; there are d i s t i n c t , observable e f f e c t s due to the presence of the a l k a l i metal c a t i o n . I t might w e l l be questioned whether or not the product species represent t r u e i o n pairs. However, the observation of only small s h i f t s i n band p o s i t i o n s with a change of c a t i o n , and the good agreement of matrix band p o s i t i o n s with c r y s t a l data when a v a i l a b l e suggest that the species are very i o n i c i f not t r u e ion p a i r s . However, the a l k a l i metal c a t i o n does manifest i t s e l f i n the s p l i t t i n g of degenerate v i b r a t i o n a l modes f o r a number o f anions, and the magnitude of s p l i t t i n g i s cation-dependent. A l s o , t h i s t e c h nique has proven u n s u c c e s s f u l at s y n t h e s i z i n g a number o f s p e c i e s which might reasonably have been expected to be s t a b l e , such as COCI3 and S i C l s . T h i s may be due to e i t h e r an inherent l a c k of s t a b i l i t y of the anions or to a fundamental l i m i t a t i o n of the technique. While the species formed are i o n p a i r s r a t h e r than the i s o l a t e d anion of i n t e r e s t , i n f o r m a t i o n can be gained about the nature o f the c a t i o n - a n i o n i n t e r a c t i o n . These species can be considered contact i o n p a i r s , i n view of the nonpolar nature o f the argon " s o l v e n t " , and analyzed i n a manner s i m i l a r t o t h a t used f o r contact i o n p a i r s i n s o l u t i o n (66). There has been an upsurge of i n t e r e s t (67) i n i o n p a i r i n g e f f e c t s i n s o l u t i o n , and the d i f f e r e n c e s i n chemical r e a c t i v i t y of ions i n d i f f e r e n t types of i o n p a i r i n g arrangements, such as contact i o n p a i r s and solvent-separated i o n p a i r s . The matrix i s o l a t e d i o n p a i r can be used as a model t o gain information about the contact i o n p a i r i n nonpolar s o l u t i o n s . D e v l i n and coworkers have e x p l o i t e d t h i s approach i n a number of s t u d i e s , i n v e s t i g a t i n g the a l k a l i n i t r a t e , c h l o r a t e and p e r c h l o r a t e i o n p a i r s i n i n e r t matrices (21,22,23) a f t e r d i r e c t v a p o r i z a t i o n of the s a l t . Generally, the a l k a l i metal c a t i o n was found t o be i n a b i d e n t a t e arrangement with two oxygens of the anion under i n v e s t i g a t i o n , causing a s p l i t t i n g of the degenerate s t r e t c h i n g mode of the anion. Normal coordinate c a l c u l a t i o n s were used t o model t h i s s p l i t t i n g by lowering of the N-0 or C l - 0 f o r c e constant of the coordinated



22.

AULT

Alkali

Halide

Salt

339

Molecules

oxygens r e l a t i v e to the noncoordinated oxygens. These workers a l s o included the p o s s i b i l i t y of a nonzero force constant between the metal c a t i o n and the coordinated oxygens, and were able to reproduce the observed spectra q u i t e w e l l f o r a v a r i e t y of cations. The d i f f e r e n c e , AK, between the force constants of the coordinated and f r e e oxygens was found to be s u b s t a n t i a l , on the order of 3 mdyne/X, suggesting that i o n p a i r i n g e f f e c t s are very important i n these matrix i s o l a t e d systems. The i n t e r a c t i o n of the c a t i o n with the anion was described i n terms of p o l a r i z a t i o n of the anion by the c a t i o n , and force constant trends a n a l yzed r e l a t i v e to the p o l a r i z i n g power of the c a t i o n s . This provided an adequate but not p e r f e c t d e s c r i p t i o n , so that a covalent i n t e r a c t i o n term was introduced as w e l l , f o l l o w i n g the_ lead of e a r l i e r workers (68). A s i m i l a r study (69) of the M BF^ ion p a i r i n t h i s laboratory l e d to comparable r e s u l t s , i n c l u d i n g a s p l i t t i n g of the F? s t r e t c h i n g mode of the anion, and a AK value of 4.17 mdyne/A. Figure 3 shows a p l o t of the B-F s t r e t c h ing v i b r a t i o n s as a function of AK i n a monodentate, C 3 , arrangement. An a l t e r n a t i v e view of the i n t e r a c t i o n of an a l k a l i metal c a t i o n with a f l u o r i d e - c o n t a i n i n g anion i s one of Lewis acid/base competition. The reactions discussed i n the preceding s e c t i o n involved the r e a c t i o n of an a l k a l i f l u o r i d e s a l t with a Lewis a c i d with subsequent f l u o r i d e ion t r a n s f e r to the Lewis a c i d . However, the a l k a l i metal c a t i o n i s a Lewis a c i d as w e l l , and the degree of p e r t u r b a t i o n of the anion by the c a t i o n may be dependent on the d i f f e r e n c e s i n f l u o r i d e i o n a f f i n i t y of the Lewis a c i d and the a l k a l i metal c a t i o n . The f l u o r i d e ion a f f i n i t i e s for a v a r i e t y of Lewis acids are w e l l known from ICR (53,54,64) s t u d i e s , while the f l u o r i d e ion a f f i n i t i e s f o r a l k a l i metal cations are the h e t e r o l y t i c bond d i s s o c i a t i o n energies of the gas phase a l k a l i f l u o r i d e molecules V

MF

M

+

+

F

These can be evaluated through a thermodynamic c y c l e (70), and the r e s u l t s are given i n Table II f o r both Lewis acids and a l k a l i metal c a t i o n s . C l e a r l y , the a l k a l i metal cations are stronger Lewis acids than the n e u t r a l species l i s t e d i n Table I I ; Table II Lewis A c i d HF SiFi, BF 3

C0F C0 2

PF

5

2

F

Affinity

50 Kcal/mole 68 71 34 32 >71

M

+

F

Affinity

182 153 139 133 130 162

Kcal/mole



340


Figure 3. A plot of the B-F stretching frequencies of the BFf anion ion paired in a monodentate arrangement with an alkali metal cation, as a function of AK, the difference in force constant between coordinated and noncoordinated fluorines (69).


22.

AULT

Alkali

Halide

Salt

341

Molecules

however, energy i s gained back through the e l e c t r o s t a t i c a t t r a c t i o n i n an i o n p a i r , and i o n p a i r formation i s f e a s i b l e . Nonet h e l e s s , the d i f f e r e n c e i n f l u o r i d e i o n a f f i n i t y between the Lewis a c i d and metal c a t i o n may determine the degree o f p e r t u r b a t i o n and anion d i s t o r t i o n . C e r t a i n l y , Table I I demonstrates why C s i s g e n e r a l l y the c a t i o n of choice f o r both the b e s t y i e l d of product i o n , and the l e a s t p e r t u r b a t i o n of the anion. These values a l s o suggest t h a t when the metal c a t i o n has a s u f f i c i e n t l y high f l u o r i d e i o n a f f i n i t y r e l a t i v e to a given Lewis a c i d , _ p r o d uct formation w i l l not occur. T h i s was observed f o r the F 3 s y s tem; r e a c t i o n of CsF or RbF with F 2 gave l a r g e product y i e l d s , while the r e a c t i o n o f KF or NaF with F 2 d i d not y i e l d any d i s t i n c t product s p e c i e s . These r e s u l t s i n d i c a t e t h a t there i s a l i m i t to the e f f e c t i v e n e s s of the s a l t / m o l e c u l e technique, and the l a c k of formation of such s p e c i e s as C O C I 3 may be due to t h i s l i m i t r a t h e r than the inherent s t a b i l i t y of the anion.


+

C l e a r l y , the e f f e c t s o f i o n p a i r formation i n matrix i s o l a t i o n s p e c t r a are apparent, p a r t i c u l a r l y i n the removal of degeneracy of v i b r a t i o n a l modes. However, these can be f i t q u i t e w e l l with normal coordinate c a l c u l a t i o n s which show t h a t i o n p a i r i n g e f f e c t s cause s i g n i f i c a n t f o r c e constant changes. Moreover, i t i s apparent t h a t the e n e r g e t i c s of i o n p a i r formation do provide a l i m i t as to the s p e c i e s which can be s t u d i e d i n t h i s f a s h i o n . However, f o r the species which can be s t a b i l i z e d i n t h i s manner, the technique i s s t i l l e f f e c t i v e and p r o v i d e s a model f o r both ion-molecule r e a c t i o n s and CsF c a t a l y s i s . A l k a l i H a l i d e S a l t Reactions with Lewis Bases The a p p l i c a t i o n of the s a l t / m o l e c u l e r e a c t i o n technique to the study of r e a c t i o n s with Lewis bases such as H 2 O and N H 3 p r e sents the p o s s i b i l i t y f o r a d i f f e r e n t type o f i n t e r a c t i o n which may f i n d some analogy i n t r a n s i t i o n metal c o o r d i n a t i o n chemistry. The s t r u c t u r e of s m a l l complexes such as MX*H 0 are of c o n s i d e r able i n t e r e s t both e x p e r i m e n t a l l y and t h e o r e t i c a l l y . These s t u d i e s were i n i t i a t e d as a r e s u l t of the o b s e r v a t i o n of s e v e r a l bands i n the spectrum of a l k a l i h a l i d e s a l t s i n argon which c o u l d not r e a d i l y be assigned to the i s o l a t e d s a l t s p e c i e s . Rather, i t was shown t h a t these bands were due to r e a c t i o n o f the s a l t with impurity H 2 O , which was always present i n these experiments t o some degree. A study was then i n i t i a t e d to i n v e s t i g a t e these bands, and the nature of the r e a c t i o n complex. Numerous experiments (71) were conducted u s i n g a wide range of a l k a l i h a l i d e s a l t s , i n c l u d i n g a l l of the halogens, and a l l a l k a l i metals except L i . In a d d i t i o n , s e v e r a l cyanide s a l t s were a l s o employed. The r e s u l t i n g band p o s i t i o n s were r e l a t i v e l y cons i s t e n t , except when CsF was employed, where very d i f f e r e n t spect r a were obtained. T h i s was r a t i o n a l i z e d i n terms of the unique r e a c t i v i t y of the f l u o r i d e i o n , and the focus of the study was d i r e c t e d toward the remaining s a l t s . In most cases, four product 2


342


bands were observed, two i n the region 3100-3250 cm , and two between 400 and 700 cm Complete i s o t o p i c s t u d i e s , u s i n g both deuterium and 0 l a b e l i n g , i n d i c a t e d that a l l four v i b r a t i o n s were hydrogenic i n nature and that the two hydrogens were equival e n t i n the complex. A number of s t r u c t u r e s f o r the complex were considered; a d e f i n i t e dependence o f the band p o s i t i o n s on M was noted, as w e l l as a s l i g h t dependence on X . In a d d i t i o n , the p o s i t i o n s o f the two upper bands, which are assigned t o 0-H s t r e t c h i n g modes, i n d i c a t e d some hydrogen bonding was o c c u r r i n g . The remaining f e a t u r e s i n the spectrum were reminiscent of coordinated aquo complexes suggesting a d e f i n i t e M interaction, which was c o n s i s t e n t with the M dependence o f the band p o s i tions. The favored s t r u c t u r e of the complex was a C symmetry t r i g o n a l pyramid with the metal c a t i o n bound d i r e c t l y t o the oxygen o f the H 0 and the anion s i t t i n g at the base of the p y r a mid, i n t e r a c t i n g e l e c t r o s t a t i c a l l y with the a l k a l i metal c a t i o n and hydrogen bonding t o the two hydrogens. Other s t r u c t u r e s might be e n v i s i o n e d f o r the complex which f i t the observed data but there was i n s u f f i c i e n t data t o reach a more d e f i n i t i v e conclusion. D e v l i n and coworkers (24,25,26) s t u d i e d the i n t e r a c t i o n between metal n i t r a t e and c h l o r a t e i o n p a i r s and water, and concluded that the i n t e r a c t i o n i s between the metal c a t i o n and the oxygen on the coordinated H2O. In a d d i t i o n , they observed some evidence f o r hydrogen bonding i n the second c o o r d i n a t i o n sphere s i m i l a r t o the r e s u l t s here. These researchers worked i n a q u i t e d i f f e r e n t c o n c e n t r a t i o n regime and monitored the complex by obs e r v i n g perturbed bands o f the anion r a t h e r than the water molec u l e , but the r e s u l t s were nonetheless c o n s i s t e n t . The analogous complex between a s a l t molecule and N H 3 was s t u d i e d as w e l l t o i n v e s t i g a t e s i m i l a r i t i e s to ammine c o o r d i n a t i o n chemistry. The d e p o s i t i o n of a v a r i e t y o f s a l t molecules with mixtures o f argon and N H 3 gave r i s e to s e v e r a l product bands which were i n d i c a t i v e of a s t r o n g l y bound complex (72). The most prominent f e a t u r e was an intense band near 1100 c m 7 which showed a d i s t i n c t metal c a t i o n dependence and very l i t t l e anion dependence. T h i s band has been assigned to the N H 3 deformation mode i n the complex which i s g e n e r a l l y the most intense i n coord i n a t e d ammine complexes (73), s h i f t s up from f r e e N H 3 near 970 cm , and i s very s e n s i t i v e to the s t r e n g t h o f i n t e r a c t i o n and hence the c a t i o n . Other modes d e t e c t e d i n c l u d e d two N-H s t r e t c h i n g modes near 3400 cm" and an NH r o c k i n g mode at low frequencies. The N-H s t r e t c h i n g modes were not s h i f t e d by a s i g n i f i c a n t amount from f r e e N H 3 , suggesting t h a t no hydrogen bonding was t a k i n g p l a c e , and the i n t e r a c t i o n was d i r e c t l y through the metal c a t i o n and the n i t r o g e n lone p a i r as i n t r a d i t i o n a l c o o r d i n a t i o n chemistry. S i m i l a r r e s u l t s were obtained by D e v l i n (24,25,26) e t a l . f o r the complexes formed between the metal n i t r a t e s a l t s and N H 3 i n argon m a t r i c e s , although the c o n c l u s i o n s were again based on the s h i f t s of the N0 bands, not N H 3 complex bands. 1 8


+

s

2

-_r

1

1

3

3


22.

AULT

Alkali

Halide

Salt

Molecules

343

These s t u d i e s were extended b r i e f l y to a number of other Lewis bases, i n c l u d i n g dimethyl ether to look f o r trends r e l a t i n g to base s t r e n g t h or proton a f f i n i t y (74). The major s p e c t r a l features o f the complexes were s h i f t e d v i b r a t i o n a l modes of the base, but these s h i f t s were much l e s s s u b s t a n t i a l than f o r H2O or NH3, and very l i t t l e i n f o r m a t i o n about the nature or s t r u c t u r e of the complex could be e x t r a c t e d .


Conclusions The r e a c t i o n s of a l k a l i h a l i d e and other s a l t molecules i n the gas phase are of c o n s i d e r a b l e i n t e r e s t to high temperature chemists; r e a c t i o n s of CsF i n e i t h e r the gas phase or condensed phases are a l s o of i n t e r e s t t o c a t a l y t i c chemists. The matrix i s o l a t i o n technique has proven i t s e l f v a l u a b l e i n the area o f high temperature chemistry; while i n e r t matrices are condensed at 15 K, the technique allows a high temperature r e a c t i o n to be i n i t i a t e d i n f r o n t of the c o l d surface and then r a p i d l y quenched to t r a p the i n i t i a l products of the high temperature r e a c t i o n . Salt/molecule r e a c t i o n s with f l u o r i d e acceptors o r Lewis acids appear, i n a l l cases s t u d i e d t o date, t o form i o n p a i r complexes as the i n i t i a l r e a c t i o n product f o r such Lewis a c i d s as HF, SiFi+, BF3 and COF2. There has been no evidence o f a b s t r a c t i o n as the i n i t i a l step, although some systems may e x i s t where t h i s step w i l l be p r e f e r r e d . Some s p e c t r o s c o p i c e f f e c t s of i o n p a i r i n g are observed i n the s p e c t r a of the product anions, but these s t i l l allow f o r the study of unusual anions i n a r e l a t i v e l y unperturbed environment. The r e a c t i o n of a s a l t molecule with Lewis bases has been e f f e c t i v e as w e l l i n forming intermediate complexes. In the systems s t u d i e d , the complexes appear bound through the metal c a t i o n of the s a l t molecule t o the lone p a i r on the Lewis base. However, i n the case of H2O, there appears t o be some hydrogen bonding i n t e r a c t i o n as w e l l . These products g e n e r a l l y mimic t r a n s i t i o n metal c o o r d i n a t i o n complexes, although the s t r e n g t h of the i n t e r a c t i o n i s c o n s i d e r a b l y weaker as might w e l l be expected. The s a l t molecule technique, i n c o n j u n c t i o n with matrix i s o l a t i o n , appears t o be a promising technique f o r the study of high temperature r e a c t i o n s , and a f u r t h e r development of t h i s approach should be b e n e f i c i a l . Extension t o a d d i t i o n a l i o n p a i r adducts would appear f e a s i b l e , a l l o w i n g f o r the study o f unusual f l u o r i d e c o n t a i n i n g anions. Extension may a l s o be p o s s i b l e t o oxide s a l t v a p o r i z a t i o n and r e a c t i o n t o form oxyanions with a -2 charge i n matrix i s o l a t e d t r i p l e anions, but t h i s avenue has only been b r i e f l y explored. Perhaps the r e s u l t s obtained t o date and des c r i b e d here as w e l l as the numerous s t u d i e s which could not be mentioned, w i l l s t i m u l a t e f u r t h e r study, both experimental and t h e o r e t i c a l , i n t h i s area of high temperature chemistry.


METAL

344

BONDING AND

INTERACTIONS

Acknowledgments


The author g r a t e f u l l y acknowledges support o f much o f the work described above by the Research Corporation under grant #8305, the N a t i o n a l Science Foundation under grant CHE78-27643, and the Henry and Camille Dreyfus Foundation. The considerable a s s i s t a n c e o f Dr. L e s t e r Andrews, p a r t i c u l a r l y i n the e a r l y phases of t h i s research, i s a l s o g r a t e f u l l y acknowledged.

Literature Cited

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

Andrews, L. Ann. Rev. Phys. Chem. 1971, 22, 109. Pimentel, G. C.; "Formation and Trapping of Free Radicals" A. M. Bass and H. P. Broida, ed. Chapt. 4, Academic Press, New York, 1960. Milligan, D. E . ; Jacox, M. E. in "Physical Chemistry, An Advance Treatise, 4" Chapter 4, Academic Press, New York, 1969. Craddock, S.; Hinchcliffe, A. J . "Matrix Isolation" Cambridge University Press, New York, 1975. Linevsky, M. J . J. Chem. Phys. 1961, 34, 587. Snelson, A.; Pitzer, K. S. J . Phys. Chem. 1963, 67, 882. Schick, S.; Schnepp, O. J. Chem. Phys. 1964, 41, 463. Ismail, Z. K.; Hauge, R. H.; Margrave, J. L. J . Mol. Spectrosc. 1975, 54, 402. Mann, D. E . ; Calder, G. V.; Seshadri, K. S.; White, D.; Linevsky, M. J . J. Chem. Phys. 1967, 46, 1138. Hastie, J . W.; Hauge, R. H.; Margrave, J . L. Ann. Rev. Phys. Chem. 1970, 21, 475. Andrews, W. L. S.; Pimentel, G. C. J. Chem. Phys. 1966, 44 2361. Andrews, L. J . Chem. Phys. 1969, 50, 4288. Milligan, D. E . ; Jacox, M. J . J . Chem. Phys. 1969, 51, 2150. Brus, L. E . ; Bondybey, V. E. J . Chem. Phys. 1975, 63, 3123. Prochaska, F. T.; Andrew, L. J . Chem. Phys. 1978, 68, 5577. Wight, C. Α.; Ault, B. S.; Andrews, L. J. Chem. Phys. 1976, 65, 1244. Jacox, M. E. Chem. Phys. 1976, 12, 51. Andrews, L . ; Tevault, D.; Smardzewski, R. R. Appl. Spectrosc. 1978, 32, 157. Andrews, L . ; Grzybowski, J. M.; Allen, R. O. J . Phys. Chem. 1975, 79, 904. Prochaska, F. T.; Andrews, L. J . Chem. Phys. 1977, 67, 1091. Smith, D.; James, D. W.; Devlin, J . P. J . Chem. Phys. 1971, 54, 4437. Smyrl, N.; Devlin, J. P. J . Chem. Phys. 1974, 60, 2540. Ritzhaupt, G.; Devlin, J . P. J . Chem. Phys. 1975, 62, 1982. Ritzhaupt, G.; Devlin, J . P. J. Phys. Chem. 1975, 79, 2265. Smyrl, N.; Devlin, J . P. J . Chem. Phys. 1974, 61, 1596. Ritzhaupt, G.; Devlin, J . P. Inorg. Chem. 1977, 16, 486.


22.

AULT

Alkali

Halide

Salt

Molecules

27.

Kana'an, A. S.; Hauge, R. H.; Margrave, J . L. J . Chem. Soc. Faraday Trans. II 1976, 1991. 28. Snelson, A.; Cyvin, B. N.; Cyvin, S. J . J. Mol. Struc. 1975, 24, 165. 29. Jones, L. H.; Penneman, R. A. J . Chem. Phys. 1954, 22, 781. 30. Evans, J . C.; Lo, G. Y-S. J. Phys. Chem. 1966, 70, 11. 31. Ault, B. S. J . Phys. Chem. 1978, 82, 844. 32. Ault, B. S. J . Phys. Chem. 1979, 83, 837. 33. Ault, B. S.; Andrews, L. J . Chem. Phys. 1975, 63, 2466. 34. Ault, B. S. Andrews, L. J . Chem. Phys. 1976, 64, 1986. 35. Ellison, C. M.; Ault, B. S. J . Phys. Chem. 1979, 83, 832. 36. Noble, P. N.; Pimentel, G. C. J . Chem. Phys. 1968, 49, 3165. 37. Bondybey, V.; Pimentel, G. C.; Noble, P. N. J . Chem. Phys. 1971, 55, 540. 38. Noble, P. N. J . Chem. Phys. 1972, 56, 2088. 39. Milligan, D. E . ; Jacox, M. J. J . Chem. Phys. 1970, 53, 2034. 40. Milligan, D. E . ; Jacox, M. J . J . Chem. Phys. 1971, 55, 2550. 41. Ault, B. S. J . Phys. Chem. 1979, 83, 2634. 42. Clark, J . H. Chem. Rev. 1980, 80, 429. 43. Corey, E. J.; Venkateswarill, A. J. Amer. Chem. Soc. 1974, 94, 6190. 44. Kraihanzel, C. S.; Poist, J . E. J . Organomet. Chem. 1967, 8, 239. 45. Chan, T. H.; Mychajlowski, W. Tetrahedron Lett. 1974, 3479. 46. Hohorst, F. A.; Shreeve, J . M. J. Amer. Chem. Soc. 1967, 89, 1809. 47. Ault, B. S. Andrews, L. J . Amer. Chem. Soc. 1976, 98, 1591. 48. Ault, B. S. Andrews, L. J . Chem. Phys. 1976, 64, 4853. 49. Evans, J . C.; Lo, G. Y-S. J. Chem. Phys. 1966, 44, 3638. 50. Ault, B. S.; Andrews, L. Inorg. Chem. 1977, 16, 2024. 51. Miller, J . H.; Andrews, L. Inorg. Chem. 1979, 18, 988. 52. Andrews, L . ; Prochaska, E. S.; Loewenschuss, A. Inorg. Chem. 1980, 19, 463. 53. Haartz, J . C.; McDaniel, D. H. J. Amer. Chem. Soc. 1973, 95, 8562. 54. Murphy, M. K.; Beauchamp, J . L. J . Amer. Chem. Soc. 1977, 99, 4992. 55. Clark, H. C.; Dixon, K. R.; Nicholson, J . G. Inorg. Chem. 1969, 8, 450. 56. Begun, G. M.; Rutenberg, A. C. Inorg. Chem. 1967, 6, 2212. 57. Ault, B. S. Inorg. Chem. 1979, 18, 3339. 58. Ault, B. S.; Tandoc, U. Inorg. Chem. in press. 59. Lustig, M.; Pitochelli, A. R.; Ruff, J . K. J . Amer. Chem. Soc. 1967, 89, 2841. 60. Christie, K. O.; Curtis, E. C.; Schack, C. J . Spectrochim. Acta A 1975, 3 1 , 1035. 61. Ault, B. S. J. Phys. Chem. 1980, 84, 3448. 62. Lawlor, L.; Passmore, J . Inorg. Chem. 1979, 18, 2923. 63. Martineau, E . ; Milne, J . B. Chem. Comm. 1971, 1327. 64. McMahon, T. C.; Northcott, C. J . Can. J. Chem. 1978, 56, 1069.


;

;

;


346

65. 66. 67. 68. 69.


70. 71. 72. 73. 74. 75.


A u l t , B. S. t o be p u b l i s h e d . E d g e l l , W.; L y f o r d , J . J . Amer. Chem. Soc. 1971, 93, 6407. Hogen-Esch, T. E.; Smid, J . J . Amer. Chem. Soc. 1966, 88, 307. Hester, R. E.; S c a i f e , C. W. J . J . Chem. Phys. 1967, 47, 5253. Hunt, R. L.; A u l t , B. S. Spectrochim. Acta Part A, 1981, 37, 63. Handbook of Chemistry and Physics, Chemical Rubber Co., 50th ed. A u l t , B. S. J . Amer. Chem. Soc. 1978, 100, 2426. A u l t , B. S. J . Amer. Chem. Soc. 1978, 100, 5773. Nakamoto, K. " I n f r a r e d Spectra o f Inorganic and C o o r d i n a t i o n Compounds" 3rd ed., W i l e y - I n t e r s c i e n c e , New York, 1978. A u l t , B. S. unpublished r e s u l t s . K a l l e n d o r f , C. J . ; A u l t , B. S. J . Phys. Chem. 1981, 85, 608.

RECEIVED

August 26,

1981.


Matrix Isolation Studies of Alkali Halide Salt Molecules with Lewis

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