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Metal Alkoxides D. C. BRADLEY

Department of Chemistry, Birkbeck College, University of London, London, Englan

The metal alkoxides constitute an important class of compounds characterized by the metal-oxygen-car­ bon bonding system. The strongly electronegative oxygen atom induces considerable polarity in the

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δ+δ-M-Obond,but this may be partially offset by the electrophilic nature of metals that undergo covalency expansion by coordination with donor atoms. The properties of the M-O-C system are also affected by the electronic behavior of the alkyl group. Its con­ figuration gives rise to important stereochemical effects. The metal alkoxides offer great scope for producing compounds with varied chemical and phys­ ical properties. It is surprising that only the alkox­ ides of magnesium and aluminum were of industrial importance until a few years ago.

The methods of preparing the alkoxides are varied and reflect in an interesting way the chemistry of the metals. The simplest method, involving the reaction of the metal with an alcohol (Equation 1), appears to be confined to the alkali metals, magnesium and aluminum. M + nROH -> M (OR), + n/2H

2

(1)

Even in this reaction the effect of the alkyl group is apparent, as ieri-butyl alcohol reacts much less rapidly with sodium than does methyl or ethyl alcohol. More­ over, the reactivity of an alkali metal depends on its electropositivity and thus potas­ sium combines more rapidly with the tertiary alcohol than sodium. In contrast to the alkali metals magnesium and aluminum require catalysts—e.g., iodine and mercuric chloride, respectively—to initiate the reaction between metal and alcohol. The behav­ ior of thallium merits special mention. Lamy (110) found that thallium did not react when suspended in boiling ethyl alcohol, yet a piece of the metal held in air over the surface of the alcohol readily combined to form thallous ethoxide. Apparently the metal is oxidized in air and the oxide undergoes a reversible reaction with alcohol: 4T1 + 0 -» 2T1 0 2

(2)

2

T1 0 + 2EtOH -> TlOEt + TIOH

(3)

TIOH + EtOH ^ TlOEt + H 0

(4)

2

2

According to Menzies (130) the very low solubility of thallous ethoxide in ethyl alcohol favors its formation in Equation 4. As an alternative to the direct reaction, Equation 1, the alkoxide may be obtained by combination of a metal oxide (or hydroxide) with alcohol as illustrated in Equations 10

In METAL-ORGANIC COMPOUNDS; Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

BRADLEY-METAL ALKOXIDES

11

3 a n d 4. T h u s s o d i u m e t h o x i d e m a y b e p r o d u c e d f r o m s o d i u m h y d r o x i d e a n d e t h y l a l c o h o l b y a z e o t r o p i c d e h y d r a t i o n u s i n g benzene. T h e r e m o v a l of water disturbs the e q u i l i b r i u m i n E q u a t i o n 4 a n d c o n v e r s i o n t o t h e e t h o x i d e becomes c o m p l e t e . T h e r e v e r s i b i l i t y o f t h e s o d i u m h y d r o x i d e - e t h y l a l c o h o l r e a c t i o n also i m p o s e s l i m i t a t i o n s o n t h e efficiency of s o d i u m as a reagent f o r d r y i n g aqueous e t h y l a l c o h o l . V a n a d y l a l k o x ­ ides, V O ( O R ) (142), h a v e b e e n o b t a i n e d b y t h e p r o l o n g e d t r e a t m e n t o f h y d r a t e d v a n a d i u m pent oxide w i t h alcohols. 3

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A t h i r d m e t h o d o f p r e p a r a t i o n o f t h e a l k o x i d e s d e p e n d s o n t h e m e t a l c h l o r i d e as s t a r t i n g m a t e r i a l . T h i s m e t h o d h a s b e e n w i d e l y a p p l i e d i n t h e p r e p a r a t i o n of a l k ­ oxides w h i c h c a n n o t b e o b t a i n e d b y t h e first m e t h o d . I n t h e c h l o r i d e m e t h o d t h e d e t a i l e d p r o c e d u r e d e p e n d s o n t h e r e a c t i v i t y of t h e c h l o r i d e a n d also o n t h e p r o p e r t i e s of t h e m e t a l a l k o x i d e . F o r e x a m p l e , a m o n g t h e elements i n G r o u p I V of t h e p e r i o d i c c l a s s i f i c a t i o n , s i l i c o n alone f o r m s a c h l o r i d e sufficiently r e a c t i v e t o w a r d s alcohols t o u n d e r g o c o m p l e t e r e p l a c e m e n t of c h l o r i n e b y a l k o x i d e g r o u p s . SiCl.! + 4 R O H -> S i ( O R ) + 4 H C 1

(5)

4

T h i s r e a c t i o n w a s d i s c o v e r e d b y E b e l m a n (74) i n 1846. W i t h t h e t e t r a c h l o r i d e s o f t i t a n i u m (102), z i r c o n i u m (34), a n d t h o r i u m (45), t h e degree o f s u b s t i t u t i o n decreases w i t h increase i n a t o m i c n u m b e r of t h e c e n t r a l a t o m : TiCl

+ 3 R O H -> T i C l ( O R ) , R O H + 2 H C 1

4

2

2

2 Z r C l + 5 R O H -> Z r C l ( O R ) , R O H + Z r C l ( O R ) , R O H + 3 H C 1 4

3

2

2

T h C U + 4 R O H -> T h C l , 4 R O H 4

S i m i l a r l y , t h e p e n t a c h l o r i d e s of n i o b i u m a n d t a n t a l u m (77) u n d e r g o t r i s u b s t i t u t i o n : Nb(Ta)Cl

5

+ 3 R O H -> N b ( T a ) C l ( O R ) + 3 H C 1 2

3

T h e a u t h o r a n d others h a v e r e c e n t l y f o u n d t h a t v a n a d i u m t e t r a c h l o r i d e resembles t i t a n i u m tetrachloride i n its reactions w i t h alcohols, while m o l y b d e n u m pentachloride differs f r o m n i o b i u m a n d t a n t a l u m p e n t a c h l o r i d e s b y u n d e r g o i n g d i s u b s t i t u t i o n (43): M0CI5 + 2 R O H -> M o C l ( O R ) + 2 H C 1 3

2

A n i n t e r e s t i n g f e a t u r e of these r e a c t i o n s i n v o l v i n g m e t a l c h l o r i d e s a n d a l c o h o l s i s t h a t a l t h o u g h t h e r e p l a c e m e n t of c h l o r i n e i s n o t c o m p l e t e , y e t t h e p a r t i a l r e p l a c e m e n t occurs v e r y r a p i d l y a n d exothermically. F u r t h e r m o r e , prolonged treatment of t h e r e s u l t a n t m e t a l c h l o r i d e a l k o x i d e w i t h excess b o i l i n g a l c o h o l f a i l s t o cause f u r t h e r reaction. T h i s behavior was emphasized b y the discovery that m e t a l chloride alkoxides u n d e r g o a l c o h o l i n t e r c h a n g e (32, 138). Z r C l ( O E t ) , E t O H + S P r O H -> Z r C l ( O P r ) , P r O H + 3 E t O H 2

2

i

2

Ζ ι · α ( Ο ΐ ν ) , Ρ Γ Ο Η + 3 C H O H -> Z r C l ( O C H ) 5

3

3

3

i

2

3

+ 4PrOH

I n o r d e r t o p r e p a r e t h e m e t a l a l k o x i d e i t is necessary t o e m p l o y a base w h i c h w i l l cause f u r t h e r r e p l a c e m e n t o f c h l o r i n e . I n f a c t , a t t e m p t s were m a d e as e a r l y a s 1875 (69) t o p r e p a r e t i t a n i u m t e t r a e t h o x i d e b y t h e r e a c t i o n i n v o l v i n g t i t a n i u m t e t r a c h l o ­ ride, e t h y l alcohol, a n d sodium ethoxide. However, i t remained f o r Bischoff a n d A d k i n s (11) i n 1924 t o d e m o n s t r a t e t h e effectiveness o f t h i s m e t h o d : TiCl

4

+ 3 R O H - » T i C l ( O R ) , R O H + 2HC1 2

2

T i C l ( O R ) , R O H + 2 N a O R -> T i ( O R ) + 2 N a C l + R O H 2

2

4

N e v e r t h e l e s s , M e e r w e i n a n d B e r s i n (122) f o u n d t h a t z i r c o n i u m a l k o x i d e s c o u l d n o t b e o b t a i n e d b y a s i m i l a r m e t h o d , because of t h e f o r m a t i o n of s t a b l e d o u b l e a l k o x ­ ides. T h e y c l a i m e d t h a t t h e a c i d s o d i u m z i r c o n i u m e t h o x i d e , N a H [ Z r ( O E t ) ] , w a s produced i n accordance w i t h the requirements of the following e q u a t i o n : 6

ZrCl

4

+ 5 N a O E t + E t O H -> N a H [ Z r ( O E t ) ] + 4 N a C l 6

In METAL-ORGANIC COMPOUNDS; Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

(6)

ADVANCES IN CHEMISTRY SERIES

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T h e z i r c o n i u m t e t r a e t h o x i d e was t h e n o b t a i n e d b y t r e a t m e n t of t h e a c i d s o d i u m z i r ­ c o n i u m ethoxide w i t h the equivalent a m o u n t of alcoholic hydrogen chloride. NaH[Zr(OEt) ] + HC1-> Zr(OEt) + NaCl + 2EtOH 6

4

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T h e i r f o r m u l a t i o n of the acid sodium z i r c o n i u m ethoxide was supported b y t i t r a t i o n of z i r c o n i u m t e t r a e t h o x i d e d i s s o l v e d i n b e n z e n e w i t h 2N s o d i u m m e t h o x i d e . Using thymolphthalein indicator, they obtained a n end point corresponding t o the reaction of one e q u i v a l e n t o f s o d i u m m e t h o x i d e p e r a t o m of z i r c o n i u m . R e p e a t e d e x p e r i m e n t s i n these l a b o r a t o r i e s h a v e f a i l e d t o c o n f i r m t h e d e t a i l e d r e a c t i o n s c l a i m e d b y M e e r w e i n a n d B e r s i n , a l t h o u g h t h e a u t h o r agrees t h a t a d o u b l e c o m p l e x f o r m e d i n t h e r e a c t i o n p r e v e n t s t h e i s o l a t i o n o f z i r c o n i u m t e t r a e t h o x i d e (48). H o w e v e r , i t has b e e n f o u n d t h a t b y using a n h y d r o u s a m m o n i a i n place of sodium ethoxide the tetraalkoxides of z i r c o n i u m c o u l d b e o b t a i n e d d i r e c t l y (47, 48) : ZrCl

+ 4 R O H + 4 N H -> Z r ( O R ) + 4 N H C 1

4

3

4

4

(7)

N e l l e s (188) h a d p r e v i o u s l y s h o w n t h a t t i t a n i u m a l k o x i d e s c o u l d b e o b t a i n e d f r o m the reaction i n v o l v i n g t i t a n i u m tetrachloride, alcohol, and ammonia, a n d this method h a s since b e e n c o n f i r m e d b y s e v e r a l w o r k e r s a n d is used o n a n i n d u s t r i a l scale (16, 64, 65, 109, 186, 140,141). R e c e n t l y , H e r m a n (92, 93, 164) has i n t r o d u c e d a n i n t e r e s t i n g modification of the ammonia method. T h e r e a c t i o n of t i t a n i u m t e t r a c h l o r i d e , a l c o h o l , a n d a m m o n i a i s c a r r i e d o u t i n t h e presence of a n a m i d e — e . g . , f o r m a m i d e — o r n i t r i l e , so t h a t t h e t i t a n i u m a l k o x i d e f o r m s a n u p p e r l a y e r w h i l e t h e a m m o n i u m c h l o r i d e r e ­ m a i n s i n s o l u t i o n i n t h e l o w e r l a y e r a n d n o filtration is r e q u i r e d . R e c e n t w o r k (28, 49-51) has s h o w n t h a t t h e a m m o n i a m e t h o d i s s a t i s f a c t o r y f o r t h e p r e p a r a t i o n o f t h e p e n t a a l k o x i d e s of n i o b i u m o r t a n t a l u m f r o m t h e i r p e n t a c h l o ­ rides : MCI5 + 5 R O H + 5 N H

3

M(OR) + 5NH C1 5

4

(8)

O n the other h a n d , the a m m o n i a m e t h o d failed t o produce a chloride-free product i n attempts t o prepare t h o r i u m tetraalkoxides f r o m the tetrachloride. F o r t u n a t e l y , the s o d i u m m e t h o d w a s effective (44, 46), p r e s u m a b l y because t h o r i u m does n o t f o r m s t a b l e d o u b l e a l k o x i d e s w i t h s o d i u m (cf. z i r c o n i u m ) . T h C l , 4 R 0 H + 4 N a O R -> T h ( O R ) 4

+ 4NaCl + 4ROH

4

(9)

M o r e o v e r , i t was f o u n d t h a t t h e t h o r i u m a l k o x i d e s were s t r o n g e r bases t h a n a m m o n i a i n alcoholic solution a n d this p r o b a b l y explains the failure of the a m m o n i a m e t h o d t o yield a pure thorium tetraalkoxide, Th(OR)

4

+ N H + -> T h ( O R ) 4

3

+

Th(OR) + + C l ^ T h C l ( O R ) 3

+ NH + ROH

(10)

3

(11)

3

T h e success of t h e s o d i u m m e t h o d w o u l d t h u s b e due t o t h e s t r o n g e r b a s i c i t y o f t h e alkoxide i o n relative t o the t h o r i u m alkoxide coupled w i t h the smaller solubility of sodium chloride compared w i t h a m m o n i u m chloride. I t c a n n o w b e seen h o w t h e m e t h o d o f p r e p a r a t i o n o f a m e t a l a l k o x i d e f r o m t h e m e t a l chloride is largely dictated b y the properties of the m e t a l alkoxide. T h e a l k y l group m a y p l a y a n equally i m p o r t a n t p a r t . F o r example, t h e reactions i n v o l v i n g alcohols containing a n electron-releasing a l k y l group—e.g., t e r t i a r y alcohols—lead t o the f o r m a t i o n of h y d r o l y z e d m e t a l alkoxides. A l t h o u g h the details of the h y d r o l y t i c mechanism have not yet been completely elucidated, methods of suppressing this a l ­ t e r n a t i v e r e a c t i o n h a v e been f o u n d . T h u s C u l l i n a n e a n d o t h e r s (64, 65) s h o w e d t h a t t h e presence o f a base ( p y r i d i n e ) w i t h t h e t e r t i a r y a l c o h o l w a s a n essential f a c t o r i n t h e p r e p a r a t i o n o f t i t a n i u m tetra-£eri-butoxide f r o m t h e t e t r a c h l o r i d e . T h i s m e t h o d of p r e p a r a t i o n has b e e n c o n f i r m e d a n d i t has also b e e n s h o w n t h a t d i p y r i d i n i u m h e x a chlorozirconate, ( C H N ) Z r C l , is a better starting material t h a n the tetrachloride f o r p r e p a r i n g z i r c o n i u m a l k o x i d e s . T h e s e f a c t s are consistent w i t h t h e v i e w (88) t h a t 5

6

2

6

In METAL-ORGANIC COMPOUNDS; Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

BRADLEY—METAL ALKOXIDES

13

t h e h y d r o l y s i s i s a consequence o f a s e c o n d a r y r e a c t i o n ( E q u a t i o n 13) i n v o l v i n g t h e h y d r o g e n c h l o r i d e ( f r o m t h e p r i m a r y r e a c t i o n 12) a n d t h e t e r t i a r y a l c o h o l : MCln + R O H - » MCln-i(OR) + HC1

(12)

H C 1 + R O H -> R C 1 + H 0

(13)

2

I n C u l l i n a n e ' s m e t h o d the p y r i d i n e combines w i t h the hydrogen chloride ( E q u a t i o n 12) a n d t h u s p r e v e n t s r e a c t i o n ( E q u a t i o n 1 3 ) . S i m i l a r l y n o h y d r o l y s i s occurs w h e n ( C H N ) Z r C l i s u s e d , because t h i s c o m p o u n d does n o t react w i t h alcohols a n d n o hydrogen chloride is produced. H a s l a m (88) has d e v e l o p e d a n a l t e r n a t i v e t o C u l l i ­ nane's m e t h o d b y first f o r m i n g t h e m e t a l c h l o r i d e a m m o n i a t e ( T i C l ' 8 N H ) a n d c a u s i n g t h i s t o react w i t h a n a l c o h o l i n a n i n e r t s o l v e n t . I n t h e case o f q u a d r i v a l e n t c e r i u m i t was necessary t o use t h e c o m p l e x c h l o r i d e , ( C H N ) C e C l , because t h e t e t r a c h l o r i d e does n o t exist. H o w e v e r , a r e p r e s e n t a t i v e n u m b e r o f stable c e r i u m ( I V ) a l k o x i d e s h a v e been p r e p a r e d i n c l u d i n g some v o l a t i l e m o n o m e r i c l i q u i d s (29, SO). 5

6

2

6

4

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5

6

2

3

6

(C H N) CeCl 5

6

2

6

+ 4 R 0 H + 6 N H -> C e ( O R ) 3

+ 6NH C1 + 2C H N

4

4

5

6

G i l m a n a n d others (83) h a v e r e c e n t l y i n t r o d u c e d a n ingenious m e t h o d f o r p r e p a r ­ ing insoluble alkoxides. T h e y prepared u r a n i u m tetramethoxide b y allowing l i t h i u m m e t h o x i d e t o react w i t h u r a n i u m t e t r a c h l o r i d e i n m e t h a n o l . T h e i n s o l u b l e u r a n i u m t e t r a m e t h o x i d e was i s o l a t e d b y n i t r a t i o n f r o m t h e m e t h a n o l i c s o l u t i o n o f l i t h i u m c h l o ­ ride U C 1 + 4LiOMe

U ( O M e ) + 4LiCl

4

4

T h e same a u t h o r s p r e p a r e d u r a n i u m ( I V ) a l k o x i d e s b y a n a l t e r n a t i v e m e t h o d i n v o l v i n g a l c o h o l y s i s o f the u r a n i u m t e t r a d i e t h y l a m i d e : 4LiNEt2 + U C 1 -> U ( N E t ) 4

U(NEt ) 2

2

4

+ 4LiCl

+ 4 R O H -> U ( O R ) + 4 E t N H

4

4

2

N o v e l m e t h o d s were also e m p l o y e d b y G i l m a n a n d o t h e r s (81, 82) i n p r e p a r i n g u r a n i u m ( V ) p e n t a a l k o x i d e s . I n one e x a m p l e t h e p e n t a a l k o x i d e w a s o b t a i n e d b y a i r oxidation of the u r a n i u m ( I V ) tetraalkoxide, apparently according t o the requirements of t h e f o l l o w i n g e q u a t i o n : 5U(OEt)

4

+ 0 -> 4 U ( O E t ) + U 0 2

5

2

(14)

A n alternative method involved oxidation w i t h bromine followed b y treatment w i t h sodium ethoxide: 2U(OEt) + B r 4

UBr(OEt)

4

2

2UBr(OEt)

4

+ N a O E t -> U ( O E t ) + N a B r 5

A n o t h e r u n u s u a l m e t h o d f o r t h e p r e p a r a t i o n o f a l k o x i d e s i s due t o B o y d (17), w h o has p r e p a r e d t i t a n i u m a l k o x i d e s b y c a u s i n g alcohols t o react w i t h t i t a n i u m d i s u l ­ fide. T i t a n i u m a l k y l a m i d e s m a y b e p r e p a r e d b y a n analogous m e t h o d (17). F i n a l l y , m e n t i o n m u s t be m a d e o f t h e m e t h o d o f a l c o h o l i n t e r c h a n g e ( a l c o h o l y s i s ) : M(OR)

+ n R O H ^± M ( O R ' ) » + n R O H

n

(15)

I t a p p e a r s t h a t the m e t a l a l k o x i d e s u n d e r g o s p o n t a n e o u s a l c o h o l i n t e r c h a n g e i n s o l u ­ t i o n i n striking contrast t o t e t r a a l k y l orthosilicates, w h i c h (when pure) require the presence o f e i t h e r a c i d i c o r basic c a t a l y s t s . B y s u i t a b l e choice o f g r o u p R i n E q u a t i o n 15 t h e n e w a l k o x i d e , M ( O R ' ) m a y b e o b t a i n e d q u a n t i t a t i v e l y . I t i s u s u a l t o s t a r t w i t h a l o w e r a l k o x i d e , M ( O R ) , so t h a t the l o w e r a l c o h o l , R O H , i s easily s e p a r a t e d f r o m R O H b y d i s t i l l a t i o n . T h e m e t h o d is p a r t i c u l a r l y u s e f u l w h e r e t h e a l c o h o l R O H is r e a c t i v e — e . g . , u n s a t u r a t e d alcohols o r h i g h e r t e r t i a r y alcohols. M o r e o v e r , b y c o n ­ ducting the reaction i n a n inert solvent, quantitative yields m a y be obtained using the m

n

In METAL-ORGANIC COMPOUNDS; Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

ADVANCES IN CHEMISTRY SERIES

14

t h e o r e t i c a l q u a n t i t i e s of r e a c t a n t s . T h e s y s t e m d e s c r i b e d b y E q u a t i o n 15 i s r e v e r s i b l e a n d i t i s possible t o p r o d u c e a l o w e r a l k o x i d e f r o m a h i g h e r a l k o x i d e b y e m p l o y i n g a large excess of t h e l o w e r a l c o h o l . I n some cases—e.g., m e t h o x i d e s — t h e l o w e r a l k o x i d e s are i n s o l u b l e a n d t h e i n t e r c h a n g e i s t h u s f a c i l i t a t e d . M e h r o t r a {125) has s h o w n t h a t a l k o x i d e s c a n b e o b t a i n e d b y a n a n a l o g o u s t r a n s ­ este r i f i c a t i o n process. M(OR)

4

+ 4R"C0 R' - » M(OR') + 4 R " C 0 R 2

(16)

2

T h i s m e t h o d w a s p a r t i c u l a r l y u s e f u l f o r t h e p r e p a r a t i o n of z i r c o n i u m o r h a f n i u m t e r ­ t i a r y b u t o x i d e s w h i c h c a n n o t b e r e a d i l y o b t a i n e d b y a n y of the foregoing m e t h o d s . T h i s a c c o u n t of p r e p a r a t i v e m e t h o d s w o u l d n o t be c o m p l e t e w i t h o u t some m e n t i o n of m i x e d a l k o x i d e s Ήί(ΟΚ) (OW) _ . I n g e n e r a l these c o m p o u n d s a p p e a r t o b e u n ­ s t a b l e , because of d i s p r o p o r t i o n a t i o n i n t o t h e single a l k o x i d e s , b u t i n c e r t a i n cases stable m i x e d a l k o x i d e s h a v e been o b t a i n e d . T h u s i n a t t e m p t i n g t o p r e p a r e z i r c o n i u m tetra-£er£-butoxide f r o m e i t h e r t h e m e t h o x i d e o r e t h o x i d e b y a l c o h o l i n t e r c h a n g e i t was f o u n d t h a t t h e r e a c t i o n s ceased w i t h t h e f o r m a t i o n o f m i x e d a l k o x i d e s (40). M e h r o t r a {129) i n v e s t i g a t e d these systems f u r t h e r a n d i s o l a t e d t h e m i x e d a l k o x i d e s according t o the following reactions: x

Downloaded by CORNELL UNIV on November 21, 2012 | http://pubs.acs.org Publication Date: January 1, 1959 | doi: 10.1021/ba-1959-0023.ch002

4

Zr(OMe)

n

4

x

+ 3 B u K ) H -> ΖΓ(ΟΜβ)(ΟΒη*) + 3 M e O H 3

Ζ ( Ο Β η * ) + # O H -> Ζ ( Ο Β ) ( Ο Β η * ) + B u * O H Γ

4

Γ

3

Zr(OBu*)4 + 2 R O H -> Z r ( O R ) ( O B u ) t

2

2

+ 2Βη*ΟΗ

T h e same a u t h o r has also p r e p a r e d some m i x e d a l k o x i d e s of a l u m i n u m b y s i m i l a r m e t h o d s {126, 128). M i x e d a l k o x i d e s of t i t a n i u m were o b t a i n e d b y N e s m e y a n o v a n d N o g i n a {139), w h o u t i l i z e d t h e t i t a n i u m c h l o r i c i e - a l k o x i d e s : TiCl(OR)

3

+ R O H + N H —> T i ( O R ' ) ( O R ) 3

+ NH C1

3

4

T i t a n i u m m i x e d a l k o x i d e s c o n t a i n i n g t h r e e different a l k o x i d e g r o u p s were p r e p a r e d b y G h o s h et al. {79, 80) b y t h e f o l l o w i n g sequence of r e a c t i o n s : TiCl (OR) 2

2

+ R O H + C5H5N -> T i C l ( O R ' ) ( O R ) + C H N , H C 1

TiCl(OR')(OR)

2

5

+ R " O N a - > Ti(OR")(OR')(OR)

2

2

5

+ NaCl

Chemical Properties T h e d o m i n a t i n g p r o p e r t y of the m e t a l alkoxides is the characteristic f a c i l i t y t o h y d r o l y z e t h r o u g h t o t h e m e t a l oxide. I n f a c t , m e t a l a l k o x i d e s w i l l react w i t h p r a c ­ tically any compound w h i c h contains h y d r o x y l groups. T h u s , a ready alcohol inter­ change or a n i n t e r c h a n g e i n v o l v i n g a p h e n o l is f o u n d . F u r t h e r m o r e , c o m p o u n d s w h i c h c a n b e t r a n s f o r m e d t o h y d r o x y d e r i v a t i v e s b y e n o l i z a t i o n w i l l also b e r e a c t i v e , a n d z i r c o n i u m alkoxides combine vigorously a n d exothermically w i t h β-diketones t o f o r m t h e t e t r a k i s c h e l a t e d e r i v a t i v e s i n w h i c h the m e t a l e x h i b i t s the c o o r d i n a t i o n n u m b e r 8. I n contrast, t i t a n i u m alkoxides undergo disubstitution to f o r m the bischelate dialkoxide i n w h i c h t i t a n i u m e x h i b i t s i t s m a x i m u m c o v a l e n c y o f 6. I t m i g h t b e a r g u e d t h a t t h e β - d i k e t o n e s c o n s t i t u t e a s p e c i a l case because o f the s t a b i l i z a t i o n c o n f e r r e d b y c h e l a ­ t i o n ; h o w e v e r , H a s l a m (91) has r e c e n t l y c l a i m e d the f o r m a t i o n o f a l k e n y l oxides of t i t a n i u m b y t r e a t i n g the t i t a n i u m a l k o x i d e w i t h a l d e h y d e s : T i ( O P r 0 4 + 2 C H C H O ~> T i ( O C H = C H ) ( O P r ) + 2 P r O H 3

2

2

i

2

i

I t seems reasonable t o s u p p o s e t h a t t h i s r e a c t i o n proceeds b y a l c o h o l i n t e r c h a n g e i n v o l v i n g t h e v i n y l a l c o h o l f o r m e d b y e n o l i z a t i o n of the a l d e h y d e . T h e a u t h o r was p a r t i c u l a r l y i n t e r e s t e d t o find t h a t t i t a n i u m o r z i r c o n i u m a l k o x i d e s w o u l d not combine w i t h mercaptans t o f o r m m e t a l mercaptides although the thiol h y ­ drogen a t o m is more acidic t h a n t h e alcoholic h y d r o x y l hydrogen a t o m . T h u s i t a p p e a r s t h a t the e l e c t r o n d o n o r p o w e r o f t h e a t o m a d j a c e n t t o the a c t i v e h y d r o g e n

In METAL-ORGANIC COMPOUNDS; Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

BRADLEY—METAL ALKOXIDES

15

a t o m i s i m p o r t a n t i n these exchange ordination mechanism:

reactions w h i c h m a y w e l l b e i n i t i a t e d b y a c o ­

R ' O H + M ( O R ) „ -> I"" '—^ Μ ( 0 1 1 ) „ _ ι Ί -> R ' ( ) M ( O R ) _ i + R O H 11

H

Downloaded by CORNELL UNIV on November 21, 2012 | http://pubs.acs.org Publication Date: January 1, 1959 | doi: 10.1021/ba-1959-0023.ch002

R T h e f a i l u r e of m e r c a p t a n s t o i n t e r c h a n g e w i t h m e t a l a l k o x i d e s w o u l d t h e n b e a s c r i b e d t o ineffective c o o r d i n a t i o n b e t w e e n s u l f u r a n d t h e m e t a l . E x c h a n g e r e a c t i o n s r e a d i l y o c c u r b e t w e e n the h a l o g e n acids a n d m e t a l a l k o x i d e s , a n d M e h r o t r a (127) has s h o w n t h a t the p r o d u c t s of t h e r e a c t i o n s i n v o l v i n g h y d r o g e n c h l o r i d e a n d t h e a l k o x i d e s of s i l i c o n , t i t a n i u m , a n d z i r c o n i u m are t h e same as t h e p r o d u c t s o f t h e r e a c t i o n s of the t e t r a c h l o r i d e s w i t h alcohols. M e t a l a l k o x i d e s also c o m b i n e w i t h a c e t y l c h l o r i d e a n d i n s e v e r a l cases t h e c h l o r i d e a l k o x i d e f o r m e d i n t h e r e a c t i o n a d d s o n a m o l e c u l e of a l k y l acetate (31, 37, 102) : Ti(OEt)

4

+ 4CH C0C1

TiCl ,CH C0 Et + 3CH C0 Et

Ti(OEt)

4

+ 3CH C0C1

TiCl (OEt),CH C0 Et + 2CH C0 Et

Zr(OEt)

4

+ 4 C H C 0 C 1 -> Z r C l , C H C 0 E t + 3 C H C 0 E t

3

4

3

3

2

3

3

3

3

4

ΖΓ(ΟΡΓ04 + 4 C H C 0 C 1

3

2

3

2

3

ZrCl ,2CH C0 Pr

3

4

3

2

2

i

2

2

+ 2CH C0 Pr 3

2

i

Z r ( O P r ) , P r O H + 2 C H C 0 C 1 -> Z r C l ( O P r ) , P r O H + 2 C H C 0 P r i

i

4

3

i

2

i

2

3

2

i

Z i r c o n i u m t e t r a c h l o r i d e c o m b i n e s w i t h e i t h e r one m o l e c u l e of e t h y l acetate o r t w o molecules of i s o p r o p y l acetate, w h i l e the d i c h l o r i d e d i i s o p r o p o x i d e o f z i r c o n i u m p r e f e r s to c o o r d i n a t e w i t h i s o p r o p y l a l c o h o l r a t h e r t h a n w i t h i s o p r o p y l acetate. The mecha­ n i s m s o f these r e a c t i o n s h a v e n o t y e t been e l u c i d a t e d b u t i t is s i g n i f i c a n t t h a t r e a c t i o n s i n v o l v i n g t e r t i a r y alkoxides of t i t a n i u m o r z i r c o n i u m w i t h acetyl chloride show cer­ t a i n p e c u l i a r i t i e s (31, 37). F o r e x a m p l e , w i t h z i r c o n i u m tert-nmyl oxide a n d a c e t y l c h l o r i d e t h e first s u b s t i t u t i o n ( E q u a t i o n 17) i s r a p i d , b u t f u r t h e r s u b s t i t u t i o n t a k e s p l a c e v e r y s l o w l y e v e n i n t h e presence o f a n excess o f a c e t y l c h l o r i d e . Zr(OCMe Et) 2

4

+ C H C O C l -> Z r C l ( O C M e E t ) 3

2

3

+ CH C0 CMe Et 3

2

2

(17)

I n t h e case o f t i t a n i u m i e r i - a m y l oxide t h e effect i s s t i l l m o r e m a r k e d (37) a n d i t is c l e a r t h a t s t e r e o c h e m i c a l effects are o p e r a t i v e i n these r e a c t i o n s . A n o t h e r c h a r a c t e r i s t i c p r o p e r t y of m e t a l a l k o x i d e s i s e v i d e n t i n t h e facile r e a c ­ t i o n s b e t w e e n the a l k o x i d e s a n d t h e t e t r a c h l o r i d e s o r c h l o r i d e a l k o x i d e s . T h u s q u a n ­ t i t a t i v e y i e l d s of t h e h i t h e r t o inaccessible t i t a n i u m t r i c h l o r i d e m o n o a l k o x i d e s (37) were o b t a i n e d b y c a u s i n g t h e t e t r a a l k o x i d e t o react w i t h t h e t e t r a c h l o r i d e ( i n excess) : 3TiCl

4

+ T i ( O R ) -> 4 T i C l ( O R ) 4

3

[see also (184)1. Z i r c o n i u m a l k o x i d e s b e h a v e d s i m i l a r l y (31): ZrCl (OPr ) ,Pr OH + Zr(OPr ) ,Pr OH - » 2ZrCl(OPr ) ,Pr OH 2

i

2

i

i

4

i

i

3

i

A l t h o u g h r e a c t i o n s i n v o l v i n g s i l i c o n esters a n d s i l i c o n t e t r a c h l o r i d e o c c u r s l o w l y a t e l e v a t e d t e m p e r a t u r e s (78), y e t s i l i c o n esters a n d t i t a n i u m t e t r a c h l o r i d e u n d e r g o v i g ­ orous r e a c t i o n s (94). T h e s e r e a c t i o n s are n o w b e i n g s t u d i e d i n t h e a u t h o r ' s l a b o r a ­ tories. N e s m e y a n o v a n d others (137) h a v e i n v e s t i g a t e d t h e a c t i o n of halogens o n t i t a n i u m a l k o x i d e s . W h e n e i t h e r c h l o r i n e o r b r o m i n e was u s e d , the p r o d u c t was the t i t a n i u m dihalide dialkoxide alcoholate T i X ( O R ) , R O H . M e t a l a l k o x i d e s h a v e l o n g been of i m p o r t a n c e i n o r g a n i c c h e m i s t r y because o f their behavior with carbonyl compounds. T h u s t h e basic a l k o x i d e s p r o m o t e e n o l i z a ­ t i o n a n d c o n d e n s a t i o n — e . g . , a l d o l i z a t i o n of a l d e h y d e s (91). I n 1906 T i s c h c h e n k o d i s c o v e r e d t h a t a l u m i n u m a l k o x i d e s cause the c o n v e r s i o n o f a l d e h y d e s i n t o esters (163). 2

2

In METAL-ORGANIC COMPOUNDS; Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

ADVANCES IN CHEMISTRY SERIES

16 2 R C H 0 -> R C 0 C H R 2

2

R e c e n t w o r k b y V i l l a n i a n d N o r d (168) has r e v e a l e d h o w t h e c o n d e n s a t i o n o f a l d e ­ h y d e s i s affected b y t h e a c i d i c o r b a s i c n a t u r e o f t h e m e t a l a l k o x i d e c a t a l y s t . Alu­ m i n u m e t h o x i d e , w h i c h b e h a v e s as a " L e w i s a c i d " because o f t h e e l e c t r o p h i l i c n a t u r e of t h e a l u m i n u m , p r o d u c e s s i m p l e esters b y t h e T i s c h c h e n k o r e a c t i o n . O n t h e o t h e r h a n d , t h e m i l d l y basic d o u b l e a l k o x i d e s s u c h as M g [ A l ( O E t ) ] , C a [ A l ( O E t ) ] , N a [ M g ( O E t ) ] , M g ( O E t ) , o r C a ( O E t ) , cause t h e f o r m a t i o n o f t r i m e r i c g l y c o l esters : 4

2

4

2

2

4

2

2

3 R C H · C H O -> R C H C H · C H R . C H 0 C · R I OH

Downloaded by CORNELL UNIV on November 21, 2012 | http://pubs.acs.org Publication Date: January 1, 1959 | doi: 10.1021/ba-1959-0023.ch002

2

2

2

2

T h e s t r o n g l y basic s o d i u m e t h o x i d e caused o n l y t h e a l d o l - t y p e c o n d e n s a t i o n . L i n and D a y (112) h a v e s t u d i e d t h e " m i x e d " T i s c h c h e n k o r e a c t i o n i n w h i c h p r o p i o n a l d e h y d e w a s a l l o w e d t o condense w i t h o t h e r a l d e h y d e s i n t h e presence o f a c a t a l y t i c q u a n t i t y of a l u m i n u m i s o p r o p o x i d e i n c a r b o n t e t r a c h l o r i d e s o l u t i o n . T h e y f o u n d t h a t t h e m i x e d esters p r o d u c e d i n highest y i e l d were those d e r i v e d f r o m t h e a c i d o f t h e a l d e ­ h y d e w h i c h undergoes t h e s i m p l e T i s c h c h e n k o r e a c t i o n m o r e r e a d i l y . A n o t h e r i m p o r t a n t reaction i n v o l v i n g m e t a l alkoxides a n d carbonyl compounds is t h e M e e r w e i n - P o n n d o r f - V e r l e y - O p p e n a u e r o x i d a t i o n - r e d u c t i o n s y s t e m . A l t h o u g h a l k o x i d e s o f s o d i u m o r m a g n e s i u m were o r i g i n a l l y u s e d t o c a t a l y z e t h e r e a c t i o n , i t w a s l a t e r s h o w n b y M e e r w e i n a n d others (12%) t h a t a l k o x i d e s o f a l u m i n u m , z i r c o n i u m , t i n ( I V ) , t i t a n i u m , a n t i m o n y , o r i r o n were also effective. The readily available alu­ m i n u m a l k o x i d e s a r e m o s t u s e d because t h e y a r e g e n e r a l l y less i n c l i n e d t o cause t h e a l t e r n a t i v e c o n d e n s a t i o n r e a c t i o n s . M e e r w e i n (121) suggested t h a t t h e r e a c t i o n m e c h ­ anism involves the p r e l i m i n a r y coordination of the carbonyl oxygen t o the electrophilic aluminum to form a n intermediate complex compound:

R'

/

(OCR )

3 2

X=0 + AlV

OCHR

R*

/

(OCR )

3 2

Al

C=0

R' (OCR )

3 2

)CH

ν

2

\ CHR

f

2

0=CR I n v i e w o f recent w o r k (100, 117, 11φ, 171, 175), t h e r e seems l i t t l e d o u b t t h a t t h e reaction proceeds v i a a n i n t r a m o l e c u l a r transfer o f a n i n c i p i e n t h y d r i d e i o n f r o m c a r b i n o l t o c a r b o n y l c a r b o n a t o m s a n d t h a t a c y c l i c t r a n s i t i o n state i s i n v o l v e d :

R' , \ R-C

R / H-C-R

νy /

(OCHRj),

R R \ -H. / R'-V "C-R

OCHR, OCHR 2

R' ,\ R'— CH \

R / C—R

/

(OCHR )

2 2

T h u s e x p e r i m e n t s u s i n g t r a c e r d e u t e r i u m e i t h e r o n s o l v e n t m o l e c u l e s (71, 11φ) o r o n t h e c a r b i n o l c a r b o n a t o m (171) gave results i n a g r e e m e n t w i t h t h i s m e c h a n i s m . T h e c y c l i c t r a n s i t i o n state i s s u s c e p t i b l e t o s t e r e o c h e m i c a l influences a n d t h i s has been c o n f i r m e d b y J a c k m a n a n d others (99, 101) a n d b y D o e r i n g a n d Y o u n g (72). T h e r e s u l t s o b t a i n e d b y D o e r i n g a n d A s c h n e r (71) s h o w t h a t a free r a d i c a l m e c h a n i s m i s extremely unlikely. A s a consequence o f t h e m e c h a n i s m d e p i c t e d i n E q u a t i o n 18 i t i s c l e a r t h a t t h e p a r t i a l positive charge induced o n the c a r b o n y l carbon a t o m facilitates the hydrogen

In METAL-ORGANIC COMPOUNDS; Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

BRADLEY—METAL ALKOXIDES

17

t r a n s f e r a n d hence affects t h e r a t e of r e a c t i o n a n d the p o s i t i o n o f e q u i l i b r i u m . T h i s e l e c t r o n i c aspect o f t h e m e c h a n i s m w a s q u a n t i t a t i v e l y c o n f i r m e d b y M c G o w a n i n 1951 u s i n g d a t a c o n c e r n i n g t h e s u b s t i t u t e d a c e t o p h e n o n e s X · C H · C O · C H (117). Jackm a n a n d M a c b e t h (98) h a v e s t u d i e d t h e k i n e t i c s of r e d u c t i o n s w i t h a l u m i n u m a l k o x ­ ides. T h e y u s e d a n elegant t e c h n i q u e i n v o l v i n g t h e r a c e m i z a t i o n o f t h e a l u m i n u m derivative of a n optically active alcohol b y the corresponding ketone, thus avoiding the complications of e m p l o y i n g t w o ketones a n d f o r m i n g m i x e d alkoxides. T h e reaction rate conformed t o the following equation: 6

4

3

τ•bOg, [ao/atj r / 1 = k -jrjy^

w h e r e a a n d a are, r e s p e c t i v e l y , t h e i n i t i a l o p t i c a l r o t a t i o n a n d t h e r o t a t i o n a f t e r t seconds, [B] is t h e c o n c e n t r a t i o n o f k e t o n e , a n d [ A ] i s t h e c o n c e n t r a t i o n o f t h e a l k ­ oxide. A c t i v a t i o n energies a n d e n t r o p i e s o f a c t i v a t i o n were d e d u c e d . Interpretation of t h e r e s u l t s s h o w e d t h a t t h e y were consistent w i t h t h e c u r r e n t t h e o r y f o r t h e r e a c t i o n m e c h a n i s m e s p e c i a l l y w h e n t h e c o m p l e x n a t u r e o f a l u m i n u m a l k o x i d e s was t a k e n i n t o account. T h e s e a u t h o r s also p o i n t e d o u t t h a t t h e efficacy o f a l u m i n u m a l k o x i d e s a s r e d u c t a n t s w a s due t o t h e i r a b i l i t y t o c o o r d i n a t e w i t h t h e c a r b o n y l c o m p o u n d t o a degree sufficient t o a l l o w t h e r e a c t i o n t o p r o c e e d a t a reasonable s p e e d . S t r o n g c o ­ ordination would simply produce a complex compound devoid of catalytic properties.

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0

t

I t seems l i k e l y t h a t t h i s m o d e r a t e degree of c o o r d i n a t i o n w i t h o t h e r m o l e c u l e s i s t h e p r o p e r t y w h i c h confers o n n u m e r o u s m e t a l a l k o x i d e s t h e i r c a t a l y t i c b e h a v i o r i n other organic reactions—e.g., transesterification, alcoholysis, e t c . M c E l v a i n a n d others (114-116) d i s c o v e r e d t h a t a l u m i n u m a l k o x i d e s w o u l d cause t h e d e a l c o h o l a t i o n of c a r b o x y l i c o r t h o e s t e r s t o k e t e n e acetals a n d t h e y p r o p o s e d t h e f o l l o w i n g c y c l i c t r a n ­ s i t i o n state f o r the r e a c t i o n m e c h a n i s m .

CH RCH «C(OR) 2

H 1.2. I t i s b e l i e v e d t h a t t h i s i s due t o d e p o l y m e r i z a t i o n o f t h e oxide a l k o x i d e s i n t h e h i g h e r b o i l i n g s o l v e n t s w i t h t h e f o r m a t i o n o f s o l v a t e d l o w e r p o l y m e r s . T h u s i t w a s suggested (86) t h a t there a r e three f u n d a m e n t a l s t r u c t u r a l models f o r t h e t i t a n i u m oxide a l k o x i d e p o l y m e r s . T h e first m o d e l i s b a s e d o n t h e t r i m e r w h i l e t h e s e c o n d m o d e l is b a s e d o n t h e s o l v a t e d d i m e r T i ( O R ) , ( R O H ) : 1 2

1

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3

4

2

1

2

2

4

3

1 2

4

2

4

a

3

1 2

4

2

8

2

T h e t h i r d system contains only three structures:

a n d is b a s e d o n a m o n o m e r i c t e t r a a l k o x i d e .

T h e u n i f y i n g feature of a l l three s t r u c t u r a l

m o d e l s i s t h e a d h e r e n c e t o 6 - c o o r d i n a t e t i t a n i u m w h i c h i s r e g a r d e d as t h e k e y p o i n t i n the theory.

T h e equations for M o d e l s I I a n d I I I are: Model II

η =

Model III

η =

6 3 -

2(h) 3

3 - (Λ)

In METAL-ORGANIC COMPOUNDS; Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

28

ADVANCES IN CHEMISTRY SERIES

B y a l l o w i n g f o r s o l v a t i o n a c c o r d i n g t o these s y s t e m s t h e results for t h e n - p r o p o x i d e ( m a i n l y M o d e l I I ) a n d i s o b u t o x i d e ( m a i n l y M o d e l I I I ) were i n m u c h b e t t e r agree­ m e n t w i t h t h e t h e o r y . A s i m i l a r i m p r o v e m e n t was o b t a i n e d f o r t h e e t h o x i d e . These p o s t u l a t e d s t r u c t u r e s are i d e a l i z e d a n d a p p l y t o t h e b e h a v i o r of these c o m p o u n d s i n b o i l i n g a l c o h o l i c s o l u t i o n s a n d w i l l n o t n e c e s s a r i l y be v a l i d f o r r e a c t i o n s o c c u r r i n g u n d e r different c o n d i t i o n s . E b u l l i o m e t r i c e x p e r i m e n t s o n t h e s e c - b u t o x i d e , n - b u t o x i d e , tertb u t o x i d e , a n d i e r i - a m y l oxide g a v e i n t e r e s t i n g r e s u l t s . T h e h y d r o l y s i s o f t h e secb u t o x i d e o c c u r r e d a t a m e a s u r a b l e r a t e w h i c h was i n c r e a s e d b y traces of a c i d a n d decreased b y traces o f a l k a l i . T h i s b e h a v i o r m a y w e l l b e caused b y t h e steric effect of t h e b r a n c h e d b u t y l g r o u p s w h i c h w o u l d b e e x p e c t e d t o oppose n u c l e o p h i l i c a t t a c k b y h y d r o x y l ions o n t h e t i t a n i u m ( E q u a t i o n 23a) i n t h e a l k a l i n e s y s t e m .

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(a)

Ti(OR) + O H - - » Ti(OH)(OR) + O R " 4

(b)

3

— T i — Ο + H 0 -> — T i

Η+ H 0

+

3

2

R

Ο R ^

\ (c)

\

+

(23)

+

— T i — Ο — Η -> — T i + R O H

\ (d)

\

+

— T i + 2 H 0 -> — T i ( O H ) + H 0 2

3

/

+

/

O n t h e o t h e r h a n d , t h e a c i d - c a t a l y z e d m e c h a n i s m d e p i c t e d i n E q u a t i o n 23b, c, a n d d w i l l n o t b e a p p r e c i a b l y affected b y t h e steric f a c t o r . I n e x p e r i m e n t s w i t h t h e n - b u t o x i d e i t w a s d e m o n s t r a t e d t h a t t h e h y d r o l y s i s was i n c o m p l e t e e v e n f o r v a l u e s o f (h) as l o w as 0.5. M o r e o v e r , t h e v e r y s m a l l degree of p o l y m e r i z a t i o n f o u n d i n b o i l i n g b u t a n o l f o r (h) as h i g h as 6 suggests t h a t stable h y d r o x y l - t i t a n i u m b o n d s are p r e s e n t . A n e q u i l i b r i u m s y s t e m o f t h e f o l l o w i n g t y p e : — T i ( O H ) + R O H ;= — T i O R + H 0 2

/

/

m a y w e l l e x p l a i n t h e u n u s u a l features i n t h e h y d r o l y s i s o f t h e n - b u t o x i d e . I t i s also l i k e l y t h a t r e l a t i v e l y stable T i - O H b o n d s are p r o d u c e d d u r i n g t h e h y d r o l y s i s o f t h e t e r t i a r y a l k o x i d e s , a l t h o u g h a t t e m p t s t o isolate a h y d r o x y c o m p o u n d were u n s u c c e s s f u l . T h e results suggested t h a t t h e i n i t i a l step Ti(OR)

4

+ H 0 -> T i ( O H ) ( O R ) + R O H 2

3

a n d also t h e c o n d e n s a t i o n step T i ( O H ) ( O R ) + T i ( O R ) -> ( R O ) T i — 0 — T i ( O R ) + R O H 3

4

3

3

were b o t h slower t h a n t h e subsequent h y d r o l y s i s o f T i ( O H ) ( O R ) , T i ( O H ) ( O R ) , a n d T i ( O H ) ( O R ) w h e r e R is a t e r t i a r y a l k y l g r o u p . T h u s i n t h e e b u l l i o m e t r i c h y d r o l y s i s of t h e éeré-butoxide t h e first a d d i t i o n o f w a t e r (h) < 0.1 caused t h e d e p o s i t i o n o f a n i n ­ s o l u b l e c o m p o u n d a n d t h e e x p e r i m e n t was d i s c o n t i n u e d . W i t h t h e éeri-amyl oxide the t i t a n i u m compound remained monomeric throughout the experiment [final value of (h) = 3 . 0 ] . O t h e r recent w o r k o n t h e h y d r o l y s i s o f t i t a n i u m a l k o x i d e s i n c l u d e s t h a t o f D ' A d a m o a n d K i e n l e (68), w h o f o u n d t h a t n e o p e n t y l a l c o h o l was r e c o v e r e d b y the hydrolysis of t i t a n i u m neopentyl oxide, thus showing t h a t the t i t a n i u m - o x y g e n b o n d i s d i s r u p t e d a n d n o t t h e c a r b o n - o x y g e n b o n d . N e s m e y a n o v a n d others (135) have isolated the following compounds: ( E t O ) T i 0 a n d ( R O ) T i O T i ( O R ) where R = E t , P r , or B u . 3

2

2

3

8

n

3

2

3

n

In METAL-ORGANIC COMPOUNDS; Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

3

29

BRADLEY—METAL ALKOXIDES The paper

b y Bistan and Gomory

(12) c o n f i r m e d

t h e conclusions

of previous

w o r k e r s t h a t t h e h y d r o l y s i s o f t h e b u t o x i d e i s i n c o m p l e t e i n t h e l a t e r etages a n d t h e y showed

t h a t t h i s effect is e v e n

more

m a r k e d i n the hydrolysis of other t i t a n i u m

alkoxides a n d aryloxides—e.g., h e p t y l , cetyl, benzyl, phenyl, a n d m-cresyl derivatives. Some of t h e i r results are quoted i n T a b l e I I I .

Table III.

Results of Bistan a n d Gomory (h)

R in Ti(OR)4 Added 0.5 1.0 2.0

Recovered

— — 0.47

Consumed 0.5 1.0 1.53

n-Heptyl

0.5 1.0 2.0

0.04 0.07 0.52

0.46 0.93 1.48

Cetyl

0.5 1.0 2.0

0.07 0.08 0.55

0.43 0.92 1.45

Benzyl

0.5 1.0 2.0

0.04 0.16 0.69

0.46 0.84 1.31

Phenyl

0.5 1.0 2.0

0.12 0.14 0.83

0.38 0.86 1.17

0.5 1.0 2.0

0.09 0.17 0.82

0.41 0.83 1.18

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n-Butyl

m-Cresyl

P r e s e n t k n o w l e d g e of t h e h y d r o l y t i c r e a c t i o n s o f t i t a n i u m a l k o x i d e s m a y t h e n be s u m m a r i z e d as f o l l o w s . T h e r a p i d i t y a n d completeness of t h e h y d r o l y s i s depend o n t h e size o f t h e a l k y l g r o u p ; h y d r o l y s i s i s s l o w e r a n d less c o m p l e t e t h e h i g h e r t h e alkyl group. I t is p a r t i c u l a r l y slow f o r t h e higher homologs after t h e r e m o v a l of t h r e e q u a r t e r s of t h e a v a i l a b l e a l k o x i d e g r o u p s a n d i t i s possible t h a t r e l a t i v e l y s t a b l e T i - O H g r o u p s a r e p r e s e n t i n a l c o h o l i c s o l u t i o n a n d t h a t a n e q u i l i b r i u m is e s t a b l i s h e d b e t w e e n h y d r o l y s i s a n d e s t e r i f i c a t i o n . I n a l l cases t h e degree o f p o l y m e r i z a t i o n i n t h e e a r l y stages of h y d r o l y s i s i s v e r y s m a l l a n d f o r t h e l o w e r a l k o x i d e s t h i s m a y b e a consequence o f t h e t e n d e n c y o f t i t a n i u m t o e x h i b i t t h e c o o r d i n a t i o n n u m b e r 6 i n c o m p a c t s t r u c t u r e s . I n n o case does t h e p o l y m e r i z a t i o n r e s e m b l e t h e c h a r a c t e r i s t i c b e h a v i o r o f s i l i c o n i n t h e f o r m a t i o n o f silicones. A n o t h e r g e n e r a l f e a t u r e o f t h e partial hydrolysis products of t i t a n i u m alkoxides is their tendency t o disproportionate when heated a n d to produce the volatile tetraalkoxide a n d a more highly polymerized n o n v o l a t i l e r e s i d u e . T h e s e p r o p e r t i e s a l l suggest t h a t t h e T i - 0 b o n d s i n t h e a l k o x i d e s are r e l a t i v e l y w e a k a n d t h e u l t i m a t e p r o d u c t o f h y d r o l y s i s i s t i t a n i u m d i o x i d e . I t is c l e a r t h a t o n l y a s m a l l b e g i n n i n g h a s so f a r b e e n m a d e i n t h e s t u d i e s o f t h e h y d r o l y s i s of m e t a l a l k o x i d e s a n d i n t e r e s t i n g d e v e l o p m e n t s

are anticipated i n t h e near

future.

Industrial Applications of Metal Alkoxides A s u r v e y o f t h e recent l i t e r a t u r e r e v e a l s a r a p i d increase i n t h e i n d u s t r i a l i m ­ p o r t a n c e of m e t a l a l k o x i d e s . I n p a r t i c u l a r , t i t a n i u m a l k o x i d e s a r e b e i n g p r o d u c e d o n a n e v e r - i n c r e a s i n g scale. F o r e x a m p l e , t h e m a n u f a c t u r e a n d sale of t i t a n i u m b u t o x i d e has b e e n d o u b l e d i n B r i t a i n d u r i n g t h e l a s t y e a r , w h i l e i n t h e U n i t e d S t a t e s t h e o u t p u t o f t i t a n i u m a l k o x i d e s w a s q u a d r u p l e d d u r i n g 1955. T h e o u t p u t o f t h e products c a n n o w be reckoned i n tons p e r a n n u m . T h e o r i g i n of this r e m a r k a b l e development i n t i t a n i u m c h e m i s t r y m a y be traced t o 1947 w h e n K r a i t z e r , M c T a g g a r t , a n d W i n t e r (109) r e p o r t e d t h a t t i t a n i u m b u t o x i d e c o u l d b e used i n m a k i n g h e a t - r e s i s t a n t p a i n t s . A n o t h e r i m p o r t a n t i m p e t u s o c c u r r e d i n 1950 w h e n S p e e r a n d C a r m o d y (161) a n n o u n c e d t h a t t i t a n i u m a l k o x i d e s c o n f e r r e d

In METAL-ORGANIC COMPOUNDS; Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

30

ADVANCES IN CHEMISTRY SERIES

water-repellent properties o n textile fabrics, leather, wood, a n d other materials. Since t h e n the alkoxides of t i t a n i u m a n d other metals have been used increasingly i n these a n d o t h e r r e l a t e d a p p l i c a t i o n s .

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Heat-Resistant Paint F o l l o w i n g t h e i r o r i g i n a l w o r k i n 1947, W i n t e r a n d o t h e r s (14, 62, 68, 109, 173, 174) h a v e p u b l i s h e d a series o f a r t i c l e s o n t h e uses o f t i t a n i u m a l k o x i d e s i n h e a t resistant paints. T h e y have shown t h a t the p a r t i a l l y h y d r o l y z e d butoxide m a y be used i n place of the tetrabutoxide a n d t h a t the chloride content of the t i t a n i u m c o m ­ p o u n d should b e k e p t below 0 . 0 0 5 % ; otherwise corrosion w i l l occur due t o the f o r m a ­ t i o n o f h y d r o c h l o r i c a c i d . C o n t r i b u t i o n s t o t h i s field h a v e also c o m e f r o m o t h e r a u t h o r s , n o t a b l y R o z a n (153), H a n c o c k a n d S i d l o w (87), T h o m a s (162), a n d S i d l o w (158, 159). T h e h e a t - r e s i s t a n t t i t a n i u m p a i n t i s u s u a l l y c o m p o s e d o f t i t a n i u m t e t r a ­ butoxide ( o r a partial hydrolysis p r o d u c t ) , a l u m i n u m powder pigment, a n d a solvent such as white spirit ( i n d u s t r i a l solvent, p e t r o l e u m f r a c t i o n ) . A c c o r d i n g t o S i d l o w (158, 159) t h e r e l a t i v e p r o p o r t i o n s o f t i t a n i u m b u t o x i d e a n d a l u m i n u m a r e c r i t i c a l f o r t h e f o r m a t i o n o f a c o a t i n g w i t h o p t i m u m heat a n d c o r r o s i o n resistance. I t a p p e a r s t h a t the f u n c t i o n of the t i t a n i u m alkoxide is t o produce t i t a n i u m dioxide u n d e r c o n d i ­ tions i n w h i c h i t w i l l b o n d together the protected surface—e.g., steel—and the a l u m i ­ n u m p i g m e n t . S u c h coatings a r e t h e n s t a b l e f o r p r o l o n g e d p e r i o d s a t 6 0 0 ° C . w h i c h i s near t o the melting point of a l u m i n u m . T h e s e c o a t i n g s a r e m o s t efficient u n d e r c o n d i t i o n s o f c o n t i n u o u s h i g h t e m p e r a t u r e — i.e., l o w h u m i d i t y — w h i l e u n d e r m o r e h u m i d c o n d i t i o n s c o r r o s i o n m a y o c c u r because o f t h e p o r o u s n a t u r e o f t h e c o a t i n g . C o x a n d W i n t e r (62) h a v e i m p r o v e d t h e c o r r o s i o n resistance b y i n c o r p o r a t i n g z i n c m e t a l p i g m e n t i n p l a c e o f t h e a l u m i n u m , b u t t h e g a i n i n c o r r o s i o n resistance i s offset b y a loss i n h e a t resistance a n d m a x i m u m o p e r a t i n g temperature for the zinc-titanium paint appears t o be about 400°C. (melting point of z i n c , 4 1 8 ° C ) . T h e same a u t h o r s a r e i n v e s t i g a t i n g t h e p o s s i b i l i t y o f u s i n g t i t a n i u m alkoxides i n antifouling paints for m a r i n e use. T h e s u i t a b i l i t y o f t i t a n i u m b u t o x i d e as a p a i n t m e d i u m is closely l i n k e d t o i t s h y d r o lytic properties. A l t h o u g h a l l t i t a n i u m alkoxides are u l t i m a t e l y h y d r o l y z e d t o t h e dioxide a n d could i n principle be used i n heat-resistant a l u m i n u m - t i t a n i u m paints, t h e l o w e r a l k o x i d e s a r e so r a p i d l y h y d r o l y z e d b y e v e n traces o f w a t e r t h a t t h e y w o u l d n o t p r o d u c e s t a b l e p a i n t m e d i a . O n t h e o t h e r h a n d , t h e less r e a c t i v e h i g h e r a l k o x i d e s hydrolyze more slowly a n d the d r y i n g of the paint would then be impeded. With t i t a n i u m b u t o x i d e t h e i n i t i a l r a t e o f h y d r o l y s i s is r a p i d b u t t h e p r o d u c t s a r e s o l u b l e , so t h a t a stable p a i n t m e d i u m i s p o s s i b l e . F u r t h e r h y d r o l y s i s t o i n s o l u b l e p r o d u c t s i s s l o w e r b u t i t s t i l l o c c u r s a t a reasonable r a t e w h e n a film o f t h e p a i n t i s e x p o s e d t o the atmosphere. T h e ultimate formation a n d bonding of the t i t a n i u m dioxide p r o b ­ a b l y o c c u r w h e n t h e c o a t i n g is h e a t e d i n a i r . A n o t h e r use f o r t i t a n i u m b u t o x i d e w a s f o r e s h a d o w e d b y C o x a n d W i n t e r (62). T h e y suggested t h a t a m e d i u m c o n t a i n i n g m i c a a n d t h e b u t o x i d e w o u l d g i v e a c o a t i n g w i t h good h i g h temperature insulating properties. Considerable quantities of t i t a n i u m butoxide are n o w being manufactured f o r the production of heat-resistant p a i n t a n d i t w i l l b e i n t e r e s t i n g t o see w h e t h e r t h e a l k o x i d e s o f o t h e r m e t a l s s u c h a s z i r c o n i u m , t h o r i u m , n i o b i u m , o r t a n t a l u m w i l l find s i m i l a r a p p l i c a t i o n s . A n o t h e r p o s s i b i l i t y i s t h e i n c o r p o r a t i n g o f m e t a l s o t h e r t h a n a l u m i n u m as p i g m e n t s i n o r d e r t o raise t h e m a x i m u m operating temperature above 600°C.

Water Repellency T h e use o f m e t a l a l k o x i d e s as w a t e r - r e p e l l e n t agents h a s d e v e l o p e d r a p i d l y since t h e p a p e r b y S p e e r a n d C a r m o d y (161). T h e problem facing the chemist is that of p r o d u c i n g agents w h i c h w i l l c o n f e r p e r m a n e n t w a t e r - r e p e l l e n t p r o p e r t i e s o n v a r i o u s

In METAL-ORGANIC COMPOUNDS; Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

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materials. T h u s the water repelleney of a treated textile fabric m u s t n o t be i m p a i r e d b y repeated l a u n d e r i n g , a n d water-repellent leather footwear m u s t be capable of w i t h ­ s t a n d i n g t h e effects o f w e a t h e r i n g . S i d l o w (158) suggests t h a t t i t a n i u m a l k o x i d e s o n their o w n are more suitable for the waterproofing of leather rather t h a n textiles. T h e literature contains m a n y claims f o r t h e waterproofing properties of m a n y t i t a n i u m a l k o x i d e s , oxide a l k o x i d e s , o r a c y l a t e s (4, 5, 18, 20, 84, 111). Z i r c o n i u m a l k o x i d e s h a v e also b e e n t e s t e d a n d M a i l a n d e r (118) c l a i m e d t h a t s o l u t i o n s c o n t a i n i n g 0.1 t o 0 . 3 % of z i r c o n i u m e t h o x i d e a n d 1.0 t o 1 0 % o f p e t r o l e u m w a x , p e t r o l a t u m , o r p o l y m e r i z e d octadecyl v i n y l ether i n aromatic o r halogenated a l i p h a t i c solvents w o u l d i m p a r t water r e p e l l e n e y t o t e x t i l e s . O t h e r a l k o x i d e s of z i r c o n i u m a n d o f a l u m i n u m were also effec­ tive i n similar preparations (118). T h e w a t e r - r e p e l l e n t p r o p e r t i e s of t h e silicones a r e w e l l k n o w n a n d i t w a s n o t s u r ­ p r i s i n g t h a t m i x t u r e s o f silicones a n d m e t a l a l k o x i d e s s h o u l d b e t e s t e d . T s u k a d a a n d T s u j i (166) f o u n d t h a t a s o l u t i o n c o n t a i n i n g 2 . 5 % o f silicone a n d 0 . 2 % o f a l u m i n u m e t h o x i d e i n benzene r e n d e r e d t e x t i l e fibers w a t e r - r e p e l l e n t . S i m i l a r l y , i t h a s been c l a i m e d t h a t m i x t u r e s o f silicones a n d t i t a n i u m a l k o x i d e s i n p e t r o l e u m s o l u t i o n a r e s u i t a b l e f o r w a t e r p r o o f i n g l e a t h e r (#7) a n d t e x t i l e s (132). Silicone-zirconium alkoxide p r e p a r a t i o n s h a v e also been u s e d f o r t h e same p u r p o s e s (87). B o y d (15, 22) p r e p a r e d l i q u i d c o p o l y m e r s b y t h e p a r t i a l h y d r o l y s i s i n benzene s o l u t i o n o f a l k y l a l k o x y l silanes i n t h e presence o f t i t a n i u m a l k o x i d e s , a c y l a t e s , o r a l k y l a m i d e s . T h e s e c o p o l y m e r s were u s e d f o r w a t e r p r o o f i n g t e x t i l e s . A m o s t i n t e r e s t i n g f e a t u r e o f t h e s i l i c o n e - m e t a l a l k o x i d e w a t e r - r e p e l l e n t s is t h a t t h e y a r e m o r e effective t h a n e i t h e r t h e silicone o r t h e m e t a l a l k o x i d e a l o n e . I t is k n o w n t h a t m e t a l a l k o x i d e s cause t h e final " c u r i n g " o f silicones t o o c c u r a t 5 0 ° t o 8 0 ° C . i n s t e a d o f t h e n o r m a l t e m p e r a t u r e of 2 0 0 ° t o 2 5 0 ° C . r e q u i r e d b y t h e s i l i c o n e . I n t h i s c o n n e c t i o n t h e m e t a l a l k o x i d e p r o b a b l y hastens t h e p o l y m e r i z a t i o n process b y r e a c t i o n s i n v o l v i n g t h e a l k o x i d e a n d t h e S i - O H g r o u p s w h i c h cause cross l i n k i n g t o occur. H o w e v e r , the enhancement of water-repellent properties m a y b e due t o a d d i ­ t i o n a l f a c t o r s besides t h i s s i l i c o n e - c u r i n g effect. T h u s t h e silicones w i t h t h e best w a t e r ­ p r o o f i n g p r o p e r t i e s a r e those c o n t a i n i n g some S i - Η g r o u p s (151) a n d i t m a y be t h a t t h e m e t a l a l k o x i d e s i n t e r a c t w i t h these S i - Η g r o u p s . M o r e o v e r , m e t a l a l k o x i d e s a r e w a t e r ­ p r o o f i n g agents i n t h e i r o w n r i g h t , p r e s u m a b l y because o f i n t e r a c t i o n w i t h fibers. T h e r e f o r e i t seems possible t h a t i n a d d i t i o n t o t h e c u r i n g effect, t h e m e t a l a l k o x i d e s m a y i n t e r a c t w i t h b o t h fiber a n d silicone a n d t h u s a c t as a b o n d i n g a g e n t . Other organic compounds of metals—e.g., 2-ethyl hexoate—enhance the water-repellent p r o p ­ erties o f silicones (24), so t h a t t h i s effect is n o t specific t o m e t a l a l k o x i d e s .

Paint Driers and Modifiers M e t a l a l k o x i d e s a r e n o w b e i n g u s e d t o accelerate t h e d r y i n g o f p a i n t s (9, 28, 60, 61, 154). I n g e n e r a l , l a r g e r p r o p o r t i o n s o f m e t a l a l k o x i d e a r e i n c o r p o r a t e d c o m p a r e d with the usual proportions of metal naphthenates but the alkoxide improves the quality of t h e c o a t i n g i n a d d i t i o n t o a c c e l e r a t i n g t h e d r y i n g process. I t is believed that the m e t a l a l k o x i d e reacts w i t h t h e h y d r o x y l g r o u p s i n t h e p a i n t m e d i u m , t h u s c a u s i n g cross l i n k i n g a n d p o l y m e r i z a t i o n . T h i s p r o p e r t y has been utilized i n t h e modification of p a i n t s a n d l a c q u e r s (104, 14?, HP, 165, 172). I n p a r t i c u l a r , t h e cellulose ester films are c o n s i d e r a b l y i m p r o v e d i n h e a t a n d s o l v e n t resistance a n d also i n m e c h a n i c a l p r o p ­ erties (8). T h e l o w e r a l k o x i d e s o f t i t a n i u m cause i m m e d i a t e g e l a t i o n a n d t h i s effect i s obviated either b y using a solution of the alkoxide i n its o w n alcohol o r b y incorporat­ i n g a less r e a c t i v e c o m p l e x a l k o x i d e (75, 155). T h e h y d r o l y t i c p r o p e r t i e s o f t h e t i t a n i u m a l k o x i d e s a r e a g a i n seen t o b e t h e d o m i n a t i n g f a c t o r i n t h e c o n t r o l o f a v a l u a b l e process. T h e less r e a c t i v e c o m p l e x a l k o x i d e s a r e d e r i v e d f r o m c h e l a t i n g h y d r o x y c o m p o u n d s s u c h as 1 : 3 d i o l s — e . g . , o c t y l e n e g l y c o l — k e t o a l c o h o l s — e . g . , d i a c e tone a l c o h o l — o r e a s i l y e n o l i z a b l e dérivâtes—e.g., / ? - d i k e t o n e s o r β - k e t o e s t e r s . I t i s p r o b a b l e t h a t i n some o f these m o n o m e r i c c o m p o u n d s t h e t i t a n i u m is e x e r t i n g i t s

In METAL-ORGANIC COMPOUNDS; Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

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ADVANCES IN CHEMISTRY SERIES

m a x i m u m c o v a l e n c y a n d t h i s f a c t o r c o u p l e d w i t h t h e s t a b i l i t y o f chelate r i n g s r e n d e r s t h e t i t a n i u m m u c h less v u l n e r a b l e t o n u c l e o p h i l i c a t t a c k b y t h e o x y g e n o f h y d r o x y l groups a n d thus accounts for the lower reactivity.

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Other Applications T h e s i l i e o n e - m e t a l a l k o x i d e c o p o l y m e r s a r e also u s e d as f i l m - f o r m i n g m e d i a (1, 6, 73, 108). G u l l e d g e (86) c l a i m s t h a t t h e " t i t a n a t e d o r g a n o s i l o x a n e " p o l y m e r s p r o ­ d u c e h a r d coatings w h i c h are o p a q u e t o u l t r a v i o l e t l i g h t a n d s u i t a b l e f o r i n c o r p o r a t i o n i n refrigerator enamels. A n o t h e r use f o r t h e less r e a c t i v e t i t a n i u m a l k o x i d e s i s as a d h e s i o n p r o m o t e r s (13, 70). A t h i n a d h e r e n t f i l m o f t i t a n i u m d i o x i d e i s f o r m e d b y t h e e v a p o r a t i o n a n d slow h y d r o l y s i s of d i l u t e s o l u t i o n s o f t h e t i t a n i u m a l k o x i d e s . T h i s l a y e r o f t i t a n i u m d i o x i d e c a n b e p r o d u c e d on surfaces o f a w i d e v a r i e t y o f m a t e r i a l s a n d i t p r o m o t e s t h e a d h e s i o n of p a i n t s a n d l a c q u e r s w h i c h w o u l d n o t o t h e r w i s e s t i c k . B o y d (19) c l a i m s t h a t g o o d a d h e s i v e c o m p o u n d s a r e o b t a i n e d b y m o d i f y i n g a m i n o p l a s t resins t h r o u g h t h e i n c o r ­ p o r a t i o n of p o l y m e r i z e d t i t a n i u m alkoxides, acylates, o r amides. A n i n t e r e s t i n g a p p l i c a t i o n o f t i t a n i u m o r z i r c o n i u m a l k o x i d e s is i n t h e r a p i d d r y i n g of p r i n t i n g i n k s (88). A d d i t i o n o f t h e m e t a l a l k o x i d e t o t h e w e t p r i n t causes r a p i d d r y i n g d u e t o i n t e r a c t i o n of t h e a l k o x i d e w i t h h y d r o x y c o m p o u n d s i n t h e i n k . I t is clear t h a t the characteristic properties of m e t a l alkoxides lead t o consider­ a b l e v e r s a t i l i t y i n t h e i r a p p l i c a t i o n s a n d i t is c o m m o n t o f i n d t h a t a g i v e n m e t a l a l k o x ­ i d e p r e p a r a t i o n i s c l a i m e d t o b e u s e f u l i n s e v e r a l different a p p l i c a t i o n s . I n a d d i t i o n t o t h e e x a m p l e s a l r e a d y c i t e d , m e t a l a l k o x i d e s a r e also f i n d i n g o t h e r uses s u c h a s : p o l y m e r i z a t i o n c a t a l y s t s (176), m o r d a n t s (17), s i z i n g agents (18), d i s ­ p e r s i n g a n d a n t i s l u d g i n g c o m p o u n d s (4, 13, 111), d i e l e c t r i c s (165), c a t a l y s t s i n p o l y ­ ester f o r m a t i o n (53-55), c a t a l y s t s i n t r a n s e s t e r i f i c a t i o n (143, 144), d components i n e n a m e l s (152). T h e m e t a l a l k o x i d e s a r e also s u i t a b l e f o r p r e p a r i n g p u r e m e t a l oxides o r h i g h l y " a c t i v e " oxides (85, 95, 96, 106). A c c o r d i n g t o K e a r b y (105) t h e a l u m i n u m a l k o x i d e s m a y b e u s e d t o reduce t h e a l d e h y d e s p r o d u c e d i n t h e O x o process a n d , a f t e r h y d r o l y s i s a n d s e p a r a t i o n o f t h e a l c o h o l s , a n " a c t i v e " a l u m i n a i s also o b t a i n e d . A n o v e l use f o r t i t a n i u m a l k o x i d e s i s i n t h e p r e p a r a t i o n o f b a r i u m t i t a n a t e (76). a

n

T h e r e is t h e f u t u r e p o s s i b i l i t y t h a t m e t a l a l k o x i d e s m a y b e u t i l i z e d i n t h e s e p a r a ­ t i o n a n d p u r i f i c a t i o n o f m e t a l s . I n t h i s case t h e p h y s i c a l p r o p e r t i e s o f t h e a l k o x i d e s w i l l be brought into p l a y , for i t has been demonstrated t h a t the v o l a t i l i t y of a m e t a l a l k o x i d e i s c o n t r o l l e d b y t h e size a n d shape o f t h e a l k y l g r o u p . W o r k i s a l r e a d y i n progress i n these l a b o r a t o r i e s o n t h e s e p a r a t i o n o f z i r c o n i u m a n d h a f n i u m b y t h e f r a c ­ t i o n a l d i s t i l l a t i o n o f t h e i r a l k o x i d e s . I n t h i s p a r t i c u l a r case, one is p r o b a b l y d e a l i n g w i t h t h e m o s t d i f f i c u l t e x a m p l e t o choose f o r a m e t a l a l k o x i d e s e p a r a t i o n o w i n g t o t h e p r o x i m i t y o f b o i l i n g p o i n t s o f t h e c o m p o n e n t s ( b o i l i n g p o i n t difference o f a b o u t 1° t o 2°C). H o w e v e r , p r e l i m i n a r y r e s u l t s (170) s h o w t h a t t h e m e t h o d w o r k s a n d h o l d s p r o m i s e f o r l a r g e scale o p e r a t i o n s . T h i s could be i m p o r t a n t i n t h e atomic energy p r o g r a m , w h i c h requires hafnium-free z i r c o n i u m . A m o n g other systems being studied is t h e s e p a r a t i o n o f n i o b i u m a n d t a n t a l u m b y f r a c t i o n a l d i s t i l l a t i o n of t h e i r a l k o x i d e s a n d t h i s s h o u l d b e c o n s i d e r a b l y easier t h a n t h e z i r c o n i u m - h a f n i u m s e p a r a t i o n . T h e d i s t i l l a t i o n of u r a n i u m p e n t a e t h o x i d e h a s b e e n p a t e n t e d as a m e a n s o f e n r i c h i n g t h e u r a n i u m - 2 3 5 i s o t o p e (52). A n a d v a n t a g e o u s f e a t u r e o f t h e a l k o x i d e s e p a r a t i o n is t h e readiness w i t h w h i c h t h e m e t a l a l k o x i d e c a n b e h y d r o l y z e d t o g i v e t h e p u r e oxide a n d t h e a l c o h o l . T h i s s h o u l d b e o f s p e c i a l i m p o r t a n c e w h e r e t h e p u r e m e t a l is p r o d u c e d b y r e d u c t i o n of t h e oxide a n d i t s h o u l d also b e possible t o r e c o v e r a n d recycle the alcohol. T h e m e t a l a l k o x i d e s a r e b e i n g a p p l i e d i n d u s t r i a l l y o n a n e v e r - i n c r e a s i n g scale a n d i t seems l i k e l y t h a t t h i s e x p a n s i o n w i l l c o n t i n u e i n t h e f u t u r e as t h e w i d e p o ­ t e n t i a l i t i e s of these c o m p o u n d s a r e f u r t h e r d e v e l o p e d .

In METAL-ORGANIC COMPOUNDS; Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

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Acknowledgment The author is most indebted to William Wardlaw, for introducing him to this fascinating branch of chemistry in 1947 and for his continued support and encourage­ ment. The author is especially grateful to the numerous collaborators who have been engaged in alkoxide chemistry in these laboratories and with whom it has been a pleasure and privilege to work. Thanks are also due to J . H . Harwood and R. Sidlow of Peter Spence and Sons, Ltd., and to J . G. Evans and G. Landells of the Bradford Dyers' Association, Ltd., for their information concerning the industrial application of metal alkoxides.

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RECEIVED f o r review A p r i l 10, 1957.

In METAL-ORGANIC COMPOUNDS; Advances in Chemistry; American Chemical Society: Washington, DC, 1959.