High-Energy Processes in Organometallic Chemistry - ACS Publications

Chapter 3

Downloaded by TUFTS UNIV on December 10, 2015 | http://pubs.acs.org Publication Date: February 26, 1987 | doi: 10.1021/bk-1987-0333.ch003

Gas-Phase Transition Metal Cluster Chemistry 1

D. J. Trevor and A. Kaldor Corporate Research Laboratory, Exxon Research and Engineering, Annandale, NJ 08801

This chapter reviews reactions of Fe, Nb, V, Co, Ni , and Pt clusters with H , Co and Nb clusters with N , a variety of transition metals clusters with CO, Fe and Pt with O and Pt with a variety of C hydrocarbons carried out with laser vaporization sources and fast flow chemical reactors. A trend i s observed in which only the dissociative reactions appear to depend strongly upon cluster size. This suggests an explanation for certain size dependent behavior. The charge transfer model for bond activation i s further developed. The issue of metal cluster structure and its influence on reactivity i s discussed as well as the possibility of elucidating this structure through chemical reactivity studies. 2

2

2

6

Gas phase t r a n s i t i o n metal c l u s t e r c h e m i s t r y l i e s along c r i t i c a l c o n n e c t i n g paths between d i f f e r e n t f i e l d s o f c h e m i s t r y and p h y s i c s . For example, from t h e p h y s i c i s t ' s p o i n t of v i e w , s t u d i e s o f c l u s t e r s as t h e y grow i n t o metals w i l l present new t e s t s o f t h e t h e o r y o f metals. Questions l i k e : How i t i n e r a n t a r e t h e bonding e l e c t r o n s i n t h e s e systems? and Is t h e r e a metal t o non-metal phase t r a n s i t i o n as a f u n c t i o n o f s i z e ? a r e f r e q u e n t l y a d d r e s s e d . On t h e o t h e r hand from a chemist p o i n t o f view very s i m i l a r q u e s t i o n s a r e asked but u s i n g d i f f e r e n t t e r m i n o l o g y : How l o c a l i z e d i s t h e s u r f a c e chemical bond? and What i s t h e d i f f e r e n c e between s u r f a c e c h e m i s t r y and small c l u s t e r c h e m i s t r y ? C l u s t e r s c i e n c e i s f i l l i n g t h e v o i d between t h e s e d i f f e r e n t p e r s p e c t i v e s w i t h a new s e t o f m a t e r i a l s and measurements of p h y s i c a l and chemical p r o p e r t i e s . 1

Current address: AT&T Bell Laboratories, Murray Hill, NJ 07974

0097-6156/87/0333-0043$07.75/0 © 1987 American Chemical Society

In High-Energy Processes in Organometallic Chemistry; Suslick, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

H I G H - E N E R G Y P R O C E S S E S IN O R G A N O M E T A L L I C


44

CHEMISTRY

C l u s t e r c h e m i s t r y has l o n g been an a c t i v e area of i n o r g a n i c chemistry. T h i s f i e l d has developed a l a r g e data base and a good u n d e r s t a n d i n g of the c h e m i s t r y of the s t a b l e or p a r t i a l l y u n s a t u r a t e d complexes. The newer area of gas phase t r a n s i t i o n metal c l u s t e r c h e m i s t r y s t a r t s w i t h t h e f u l l y coordi n a t i v e l y u n s a t u r a t e d metal cluster. T h i s s p e c i e s i s h i g h l y r e a c t i v e y e t , demonstrates some interesting selectivity. The o r i g i n of t h i s s e l e c t i v i t y i s k i n e t i c and s e t s t h e p e r s p e c t i v e of t h i s r e v i e w . Three l a b o r a t o r i e s have been t h e primary c o n t r i b u t o r s i n t h e area of gas phase t r a n s i t i o n metal c l u s t e r c h e m i s t r y and t h e i r work forms t h e c e n t e r p i e c e of t h i s m i n i - r e v i e w . These are t h e Rice(_l) group, t h e Argonne(_2) group, and t h e Exxon(_3) g r o u p . Other g r o u p s , however are r a p d i l y e n t e r i n g t h i s a c t i v e f i e l d . The m a j o r i t y of the references are s i m i l a r l y grouped t o g e t h e r . P h o t o - and t h e r m a l c h e m i s t r y of s t a b l e gas phase c l u s t e r complexes are not covered i n t h i s paper. Attempts are made t o make c o n n e c t i o n s and use of t h i s data whenever p o s s i b l e . In a d d i t i o n , c l u s t e r i o n c h e m i s t r y i s not covered. These reactions have j u s t recently been extended to c l u s t e r s of a v a r i e t y of s i z e s and a l r e a d y shows some s u r p r i s i n g s i m i l a r i t y t o n e u t r a l chemi s t ry( 4 - 7 ) . T h i s o v e r v i e w i s o r g a n i z e d i n t o s e v e r a l major s e c t i o n s . The f i r s t i s a d e s c r i p t i o n of t h e c l u s t e r s o u r c e , r e a c t o r , and the general mechanisms used t o d e s c r i b e t h e r e a c t i o n k i n e t i c s t h a t w i l l be s t u d i e d . The next two s e c t i o n s d e s c r i b e the r e l a t i v e l y s i m p l e r e a c t i o n s of hydrogen, n i t r o g e n , methane, carbon monoxide, and oxygen reactions with a v a r i e t y of metal c l u s t e r s , f o l l o w e d by t h e more c o m p l i c a t e d dehydrogenation r e a c t i o n s of hydrocarbons w i t h p l a t i n u m clusters. The l a s t s e c t i o n develops a model t o r a t i o n a l i z e the observed chemical b e h a v i o r and d e s c r i b e s s e v e r a l p r e d i c t i o n s t h a t can be made from t h e m o d e l . Cluster

Source

P r e v i o u s l y , i n t e n s e beams of metal c l u s t e r s c o u l d o n l y be produced f o r t h e most v o l a t i l e m e t a l s . The l i m i t a t i o n arose from s i g n i f i c a n t m a t e r i a l s problems i n v o l v e d i n t h e c o n s t r u c t i o n of high temperature ovens. The development of a source t h a t u t i l i z e s l a s e r v a p o r i z a t i o n and subsequent c o n d e n s a t i o n i n a r a p i d l y f l o w i n g gas e l i m i n a t e d t h e m a t e r i a l s problem and has enabled j u s t about any m a t e r i a l t o be studied(la,8). Method. The laser vaporization source eliminates the material constraints inherent in conventional oven sources. This is accomplished by l o c a l i z i n g t h e h e a t i n g t o a very small area at t h e s u r f a c e of t h e sample and by e n t r a i n i n g t h e vapor produced i n a r a p i d flow of high p r e s s u r e g a s . F i g u r e 1 i s a schematic of t h e l a s e r v a p o r i z a t i o n s o u r c e . This diagram d e p i c t s a p u l s e d v a l v e on t h e l e f t which s u p p l i e s high p r e s s u r e h e l i u m flow d i r e c t l y towards t h e r i g h t . Several workers have a l s o chosen t o use c o n t i n u o u s h e l i u m f l o w s ( 2 , 6 , 9 ) . In general these sources are m o d i f i c a t i o n s of c o n v e n t i o n a l s u p e r s o n i c beam sources. A v a r i e t y of l a s e r s have been s u c c e s s f u l l y employed i s t h i s t y p e of s o u r c e . The o n l y i m p o r t a n t c r i t e r i o n i s t h a t t h e l a s e r must have s u f f i c i e n t i n t e n s i t y t o heat t h e s u r f a c e f o r v a p o r i z a t i o n . A goal at


3.

TREVOR A N D K A L D O R

Gas-Phase Transition Metal Cluster Complexes

45


one t i m e i n t h i s area was t o maximize t h e amount o f m a t e r i a l removed by t h e l a s e r . P r e v i o u s s t u d i e s i n l a s e r w e l d i n g and d r i l l i n g c l e a r l y d e s c r i b e d how t h i s can be a c h i e v e d . Use of l o n g e r p u l s e s avoids t h e c r e a t i o n of a plasma t h a t i s opaque t o t h e l a s e r , t h u s s h i e l d i n g t h e target surface. C u r r e n t l y , however, t h e l i m i t a t i o n appears t o be t h e amount o f m a t e r i a l t h a t can be c o n v e r t e d i n t o c l u s t e r s and not t h e amount v a p o r i z e d . Doubled Q-switched YAG(1,3) e x c i m e r ( 2 , 9 ) , and more r e c e n t l y copper vapor l a s e r s ( j 6 ) are t h e most p o p u l a r . Sample. T h i s source p l a c e s no r e s t r i c t i o n s on t a r g e t material. C l u s t e r s of m e t a l s , produced. For example, p o l y e t h y l e n e and alumina have been s t u d i e d as w e l l as r e f r a c t o r y metals l i k e t u n g s t e n and niobium. M o l e c u l a r s o l i d s , l i q u i d s , and s o l u t i o n s c o u l d a l s o be used. However t h e c o m p l e x i t y of t h e v a p o r i z a t i o n process and plasma c h e m i s t r y makes f o r even more complex m i x t u r e s i n t h e gas p h a s e . To date t h e t r a n s i t i o n m e t a l s ( 1 - 3 ) and e a r l y members of group 13 ( I I I A ) and 14 ( I V A ) ( 1 1 - 1 6 ) have been t h e most a c t i v e l y s t u d i e d . Clusteri ng. A f t e r a small p i e c e of t h e metal sample i s v a p o r i z e d by t h e l a s e r , t h e next step i s t o c o n t r o l i t s c o n d e n s a t i o n i n t o c l u s t e r s without significant loss to the w a l l s . This i s a c h i e v e d by v a p o r i z i n g t l i e metal i n t o a h i g h p r e s s u r e (10-3000 t o r r ) gas t h a t i s f l o w i n g ( 1 0 - 1 0 ^ cm/sec) down t h e extender tube shown i n F i g . 1. The m a j o r i t y o f t h e s m a l l (3 t o - 6 atoms) and a l l of t h e l a r g e c l u s t e r s a r e produced by c o n d e n s a t i o n i n t h i s t u b e . The h i g h p r e s s u r e not o n l y a l l o w s f o r r a p i d c o o l i n g but a l s o l i m i t s t h e d i f f u s i o n o f t h e metal atoms keeping t h e high d e n s i t y necessary f o r rapid c l u s t e r formation. Helium i s t h e best gas t o use because of i t s high thermal c o n d u c t i v i t y , chemical i n e r t n e s s and i t s s u p e r i o r p r o p e r t i e s f o r p r o d u c i n g s u p e r s o n i c beams. C l u s t e r growth appears t o be dominated by atom o r small c l u s t e r a d d i t i o n onto a few seed " c l u s t e r s " as f o l l o w s : M

n-1

+

M

< ~ " >

k" He ~ ^ ~ >

t

=

k

dnm

=

a) e x p ( - E ( n , m ) / k T ) d

B

American Chemical Society, Library 1155 in16th St., N.W„Chemistry; Suslick, K.; In High-Energy Processes Organometallic ACS Symposium Series; American Chemical Washington, D.C. Society: 20036 Washington, DC, 1987.

48


CHEMISTRY

a minimum bond s t r e n g t h of 16 kcal/mole i s set f o r detectable products. Any r e a c t i o n p r o d u c t s bond l e s s s t r o n g l y t h a n 16 kcal/mole will l i k e l y desorb before e x i t i n g t h e r e a c t o r and are t h u s not detected.


D e t e c t i on Photoioni z a t i o n t i m e - o f - f l i ght mass spectrometry is almost e x c l u s i v e l y t h e method used i n chemical r e a c t i o n s t u d i e s . The mass s p e c t r o m e t e r s , d e t e c t o r s and e l e c t r o n i c s are almost i d e n t i c a l . A major distinction is the choice of ionizing frequency and intensity. For many s t a b l e molecules m u l t i p h o t o n i o n i z a t i o n a l l o w e d for almost unit detection efficiency with controllable fragmentati on ( 2 0 ) . For c l u s t e r systems t h i s has been more d i f f i c u l t because h i g h HTser i n t e n s i t i e s g e n e r a l l y cause e x t e n s i v e d i s s o c i a t i o n of n e u t r a l s and i o n s ( 2 1 ) . T h i s has f o r c e d t h e use of s i n g l e photon ionization. T h i s works very w e l l f o r low i o n i z a t i o n p o t e n t i a l metals ( 25 atoms) c l u s t e r s are showing s i z e s e l e c t i v i t y . The Exxon group has also reported reactivities of vanadi um(3e). These are shown i n F i g . 3 . Again t h e p a t t e r n i s s i z e selective but not i d e n t i c a l t o niobium c l u s t e r s . Vanadi urn has s p e c i f i c i n e r t c l u s t e r s l i k e V , which i s s i m i l a r t o n i o b i u m ' s 8 and 10 but t h e p a t t e r n i s b e t t e r d e s c r i b e d by an even/odd a l t e r n a t i o n . This suggests p o s s i b l e a n a l o g i e s w i t h t h e one e l e c t r o n metals l i k e 6

p

+

+

6


52


CHEMISTRY

t h e a l k a l i s or c o p p e r . The r e a c t i v i t y of ^ l e v e l s o f f around n = 15, which i s e a r l i e r i n s i z e than f o r F e . The l a r g e r r a t e c o n s t a n t s f o r vanadium and niobium suggest s m a l l e r a c t i v a t i o n e n e r g i e s than found f o r i r o n . Both vanadi urn and niobium metals form di h y d r i d e s o n l y at high p r e s s u r e s ( 2 7 ) , and numerous phases w i t h hydrogen c o m p o s i t i o n s less than o n e ( 2 8 ) . Experiments were performed to saturate vanadium c l u s t e r s with deuterium. F i g u r e 4 i s a p l o t of t h e number of deuterium molecules found i n t h e p r o d u c t s . The s o l i d s t r a i g h t l i n e s are f o r D:V r a t i o s of 1 and 2 . The c o r r e s p o n d i n g curved dashed l i n e s i n c l u d e c o r r e c t i o n s f o r some bulk a t o m s ( 2 c ) . The best f i t t o t h e data i n c l u d i n g o n l y s u r f a c e atoms i n d i c a t e a s t o i chi ometry of 1.5. It i s l i k e l y t h a t t h i s high s u r f a c e s t o i chi ometry i s an i n d i c a t i o n of bulk i n c o r p o r a t i o n of d e u t e r i u m . Cobalt shows a d r a m a t i c s i z e d e p e n d e n c e ( l b , c ) t h a t resembles t h e b e h a v i o r of i r o n more so than t h a t of vanadium or n i o b i u m . The s m a l l e s t c l u s t e r t o react i s t h e t r i m e r and t h e 5-9 atom c l u s t e r s are s i g n i f i c a n t l y more i n e r t than any of t h e l a r g e r c l u s t e r s . Cobalt a l s o has a s i g n i f i c a n t d i p i n r e a c t i v i t y between 19 and 2 2 . A t h e o r e t i c a l c a l c u l a t i o n r a t i o n a l i z e d t h e onset i n r e a c t i v i t y at t h e trimer to be associated with energetic stability of the products(29). Ni c k e T r e a c t i v i t i es are r e l a t i v e l y f l a t w i t h c l u s t e r s i z e ( l c ) . For c l u s t e r s s m a l l e r than the decamer a weak even odd p a t t e r n e x i s t s down t o t h e t r i m e r , t h e s m a l l e s t c l u s t e r r e p o r t e d t o r e a c t . This p a t t e r n might be s u g g e s t i v e of a o n e - e l e c t r o n bonding scheme as used i n very e a r l y c a l c u l a t i o n s on Ni clusters(30). The Exxon group has attempted s t u d i e s "o? deuterium chemi s o r p t i on on p l a t i n u m c l u s t e r s ( 3 k ) . The high mass, l a r g e number of n a t u r a l l y o c c u r r i ng i sotopes and hi gh i oni z a t i on p o t e n t i a l s make r e a c t i vi t y experiments i m p o s s i b l e f o r p l a t i n u m . However an e s t i m a t e of t h e extent of the r e a c t i o n near s a t u r a t i o n can be made by l o o k i n g at peak broadening w i t h l e a s t - s q u a r e s - f i t t i ng p r o c e d u r e s . The average number of deuterium atoms chemi sorbed per p l a t i n u m atom i s found t o be l e s s than one f o r t h e c l u s t e r s i n t h i s s t u d y . No r e a c t i o n i s observed on t h e atom through t h e t e t r a m e r . Hydrogen chemi s o r p t i on on p l a t i n u m s u r f a c e s i s g e n e r a l l y weak and even on a stepped surface(31) H2 i s desorbed by 450 K. The expected s t o i chi ometry f o r p l a t i n u m i s one hydrogen per s u r f a c e a t o m ( 3 2 ) . T h i s i s a commonly used number i n s a t u r a t i o n chemi s o r p t i on measurements of the d i s p e r s i o n of supported metal c a t a l y s t s . It i s l i k e l y t h a t t h e temperature of the c l u s t e r s i n t h i s study i s s u f f i c i e n t t o cause s i g n i f i c a n t d e s o r p t i o n thus e x p l a i n i n g t h e i r low hydrogen a f f i n i t y . Copper c l u s t e r s , as r e p o r t e d by the Rice g r o u p ( l c ) , do not react with hydrogen. Hydrogen chemi s o r p t i on on copper s u r f a c e s i s a l s o an activated process. Surface beam s c a t t e r i n g experiments p l a c e t h i s b a r r i e r between 4-7 k c a l / m o l e ( 3 3 ) . This l a r g e value i s c o n s i s t e n t with the activated nature cT~ hydrogen chemi s o r p t i on on metal c l u s t e r s , and t h e t r e n d toward bulk b e h a v i o r f o r r e l a t i v e l y small c l u s t e r s (>25 atoms i n s i z e ) .


p

n

Nitrogen. The Rice group r e p o r t e d t h a t t h e r e a c t i o n s of di n i t r o g e n wi t h ni obi urn and c o b a l t c l u s t e r s (_lc_) e x h i b i t r e a c t i v i t y p a t t e r n s very s i m i l a r t o di h y d r o g e n . Iron has not yet been observed t o r e a c t i n the gas phase w i t h di n i t r o g e n . The Rice group d i d r e p o r t some


3.


TREVOR AND KALDOR I

I

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53

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125L' . • l • • • • I • ... I , ... i .... I • • . 5 10 15 20 25 number

of

atoms

in cluster

F i g u r e 3. L o g a r i t h m o f the r e a c t i v i t i e s o f n i o b i u m and vanadium clusters. The Exxon d a t a ( c i r c l e s ) a r e s c a l e d r e l a t i v e t o the Argonne Fe^o r e a c t i v i t y , and the R i c e d a t a ( c r o s s e s ) a r e n o r m a l i z e d to Nbg.

20 -

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15

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30

number of vanadium atoms in cluster F i g u r e 4. The number o f d e u t e r i u m m o l e c u l e s found i n p r o d u c t s on V c l u s t e r s produced w h i l e a t t e m p t i n g t o s a t u r a t e the r e a c t i o n . The s o l i d l i n e s a r e p l o t s o f D2:V r a t i o s o f 1 and 0.5, i n c l u d i n g all vanadium atoms. The dashed lines a r e c o r r e c t e d , assuming g l o b u l a r shaped c l u s t e r s . The b e s t l e a s t - s q u a r e s f i t t o the d a t a , D2:V = 0.68, i s a l s o p l o t t e d as a dashed l i n e . x


54

HIGH-ENERGY PROCESSES IN ORGANOMETALLIC CHEMISTRY


d e p l e t i o n ( l c ) but at o n l y very high r e a c t a n t flows and p r o d u c t s were not observed. Even i r o n h y d r i d e c l u s t e r s d i d not r e a c t with di n i t r o g e n i n an attempt t o study low p r e s s u r e ammonia s y n t h e s i s on gas phase i r o n c l u s t e r s ( 3 4 ) . The s i m i l a r i t y o f t h e r e a c t i v i t y p a t t e r n s f o r niobium and c o b a l t and the non-reacti v i t y of iron with nitrogen suggests that dissociative chemi s o r p t i on is taking place. D i s s o c i a t i o n of m o l e c u l a r l y chemi sorbed n i t r o g e n i s an a c t i v a t e d process on a l l m e t a l s ( 3 5 ) and i s most e x o t h e r m i c f o r t h e e a r l y metals i n t h e periodic table(36). The l i m i t e d o b s e r v a t i o n s on c l u s t e r s seems t o be consistent with these t r e n d s . Methane. Both i r o n (3b) and alumi num(3g) c l u s t e r s a r e i n e r t towards methane. M o l e c u l a r a E s o r p t i o n i s l i k e T y t o o weak f o r t h e p r o d u c t s t o be d e t e c t e d , making t h i s t e c h n i q u e o n l y s e n s i t i v e t o d i s s o c i a t i v e absorption. Although t h e C-H bond i s comparable i n s t r e n g t h t o H-H, a c t i v a t i o n o f t h e former s h o u l d be f u r t h e r c o n s t r a i n e d by s t e r i c e f f e c t s (_37) • In a d d i t i o n , f o r t h e r e a c t i o n t o be p o s s i b l e t h e sum o f the bond s t r e n g t h s of M -CH3 and M - H must exceed t h a t o f CH3-H. F o r i r o n M - H i s 67 kcal/moie r e q u i r i n g t h e M - C H bond t o at l e a s t be 33 kcal/mole. C r y o g e n i c m a t r i x s t u d i e s have found e l e c t r o n i c e x c i t a t i o n of metal atoms e s s e n t i a l f o r t h e i r i n s e r t i o n i n t o m e t h a n e ( 3 8 ) . These o b s e r v a t i o n s i m p l y at l e a s t a h i g h l y a c t i v a t e d p r o c e s s , i f not one that i s endothermic. Based on t h e l i m i t e d l i t e r a t u r e v a l u e s o f M-CH3 bond e n e r g i e s Co and Ni a r e t h e o n l y 3d t r a n s i t i o n metal atoms t h a t form sufficiently strong bonds for this reaction to be exothermi c ( 3 9 ) . Recent m o l e c u l a r beam s t u d i e s a l s o f i n d large b a r r i e r s f o r methane a c t i v a t i o n on t u n g s t e n ( 4 0 ) and ni c k e l ( 4 1 ) . p

n

n

3

Carbon monoxide. T h i s chemi s o r p t i on was f i r s t s t u d i e d on niobium and c o b a l t c l u s t e r s by t h e R i c e g r o u p ( l c ) . Although n i t r o g e n and CO a r e i s o e l e c t r o n i c t h e y behave q u i t e d i f f e r e n t l y on t h e s e m e t a l s . The d i s t i n c t s i z e s e l e c t i v e r e a c t i v i t y p a t t e r n s d i s p l a y e d by niobium and c o b a l t f o r hydrogen and n i t r o g e n c h e m i s o r p t i o n were c o m p l e t e l y absent f o r CO where a l l c l u s t e r s s t u d i e d r e a c t e d w i t h comparable r a t e s . CO r e a c t i o n s were r e c e n t l y extended t o a t o t a l o f t w e l v e t r a n s i t i o n metals by t h e Exxon g r o u p ( 3 j ) . No wide v a r i a t i o n o f r e a c t i o n r a t e a c r o s s t h e p e r i o d i c t a b l e was o b s e r v e d . F i g u r e 5 i s a bar graph of these r e s u l t s . The h e i g h t o f each r e c t a n g l e r e p r e s e n t s t h e measured r e a c t i v i t y o f a s p e c i f i c s i z e c l u s t e r w i t h CO. Almost a l l t h e metal c l u s t e r s w i t h more than t h r e e atoms r e a c t i n a f a c i l e f a s h i o n . U n l i k e hydrogen t h e s e r e a c t i o n s do not appear t o be a c t i v a t e d . In a d d i t i o n t h e p r o d u c t s d i s t r i b u t i o n s observed i n d i c a t e comparable r a t e s f o r m u l t i p l e adduct f o r m a t i o n . The mass c o m p l e x i t y , r e l a t i v e l y h i g h i o n i z a t i o n p o t e n t i a l s , and t h e known p r e v a l e n t dissociative i o n i z a t i o n o f t h e f u l l y s a t u r a t e d c a r b o n y l s ( 4 2 ) has p o s s i b l y caused t h e f a i l u r e of some i n i t i a l s a t u r a t i o n e x p e r i m e n t s ( 4 3 ) . The a b i l i t y t o s y n t h e s i z e t h e s t a b l e c a r b o n y l complexes w i l l h e l p t h i s field significantly due t o t h e vast amount o f i n f o r m a t i o n available, especially their structures. The small c l u s t e r t h r e s h o l d b e h a v i o r , suggested by t h e k i n e t i c s scheme p r e s e n t e d e a r l i e r , i s apparent i n t h i s data s e t . Assuming (1) t h e RRK form f o r k _ , (2) t h e a d d i t i o n r e a c t i o n s are not a c t i v a t e d , and (3) t h e number o f p a r t i c i p a t i n g modes a r e independent o f t h e element t y p e , t h e M-CO bond s t r e n g t h s can be grouped by e n e r g y . F o r n


3.


55


M2-CO t h e f i r s t adduct bond e n e r g i e s are g r e a t e r f o r C o , R u , P d , W, I r , and Pt t h a n V, F e , N i , Nb, and Mo. For t h e t r i m e r s , i r o n has a weaker bond t h a n t h e o t h e r metals s t u d i e d . For t h e t e t r a m e r and l a r g e r c l u s t e r s t h e r e a c t i v i t y i s c o n t r o l l e d by t h e v a l u e o f k and no l o n g e r by t h e c o m p e t i t i o n between u n i m o l e c u l a r d e c o m p o s i t i o n and c o l l i s i onal s t a b i l i z a t i o n . The l a r g e c l u s t e r regime i s not covered by t h i s model used t o make t h e c o r r e l a t i o n between k i n e t i c s and energeti c s . T h i s o r d e r i n g of Mg-CO bond s t r e n g t h s i s c o n s i s t e n t w i t h t h e t r e n d o f i n c r e a s i n g M-CO average bond s t r e n g t h s i n metal c a r b o n y l complexes as one goes down a column i n t h e p e r i o d i c t a b l e ( 4 4 ) . The n o n - r e a c t i v i t y o f Ni 2 i s i n c o n s i s t e n t w i t h t h e t r e n d of t h e c a r b o n y l complexes going a c r o s s a row. However, f o r p o l y c r y s t a l l i ne f i l m s t h e heat of a d s o r p t i o n of CO g e n e r a l l y decrease going a c r o s s a r o w ( 4 5 ) . In f a c t , groups 3 (11 IB) and 4 (IVB) have t h e l a r g e s t v a l u e s . These large v a l u e s a r e a s s o c i a t e d w i t h t h e t r e n d of e a r l y t r a n s i t i o n elements t o d i s s o c i a t e CQ(46). By groups 8-10 ( V I I I 8 ) t h e s u r f a c e and o r g a n o m e t a l l i c complexes have comparable v a l u e s . The average MC0 bond d i s s o c i a t i o n energy i n Ni (CO)4 i s 35 kcal/mole(44) which l i e s near t h e t o p of t h e range of heats of a d s o r p t i o n o f CO on n i c k e l surfaces(47). T h e r e f o r e , t h e s e measurements i n d i c a t e t h a t w i t h t h e e x c e p t i o n o f n i c k e l t h e bond e n e r g i e s f o r t h e f i r s t CO bonded t o t h e metal dimers s t u d i e d f o l l o w a s i m i l a r g l o b a l t r e n d as t h e average bond d i s s o c i a t i o n e n e r g i e s of t h e metal c a r b o n y l s . Carbon monoxide e v e n t u a l l y d i s s o c i a t e s at room t e m p e r a t u r e on a l l but some of t h e group 8-10 ( V I I IB) metals ( 4 4 ) . This d i s s o c i a t i o n occurs o n l y f o r metal s u r f a c e s which form s u f f i c i e n t l y s t r o n g m e t a l carbon p l u s metal-oxygen bonds t o break t h e 257 kcal/mole CO bond. The known v a l u e s f o r gas phase metal atoms predict t h e same trend(48). The s i m i l a r i t y i n t h e b e h a v i o r of s u r f a c e s and atoms i m p l i e s f o r t h e most p a r t t h a t t h e c l u s t e r s s h o u l d behave l i k e w i s e . This a l s o i m p l i e s h i g h l y l o c a l i z e d bonding. The n o n - r e a c t i v i t y observed f o r V2, Nbo and Moof which based on t h e s e p r i o r assumptions s h o u l d d i s s o c i a t e u ) , c o u l a mean t h a t t h e dimers a r e t o o s m a l l t o form a s t r o n g bond t o both carbon and oxygen atoms.


n

Di oxygen and Carbon D i o x i d e . Remarkable i n a way, a r e t h e very few r e a c t i o n s t u d i e s r e p o r t e d w i t h di o x y g e n ( 3 b ) . For most t r a n s i t i o n metals t h e monooxides produced from surTace o x i d a t i o n o r p u r i t y problems a r e u b i q u i t o u s and d i f f i c u l t to eliminate. In e a r l y experiments t h e Argonne group r e p o r t e d r e a c t i o n s of oxygen w i t h i r o n c l u s t e r s ( 2 a ) by having t h e r e a c t a n t present d u r i n g t h e v a p o r i z a t i o n and c l u s t e r i n g p r o c e s s . T h i s a l l o w e d f o r very h i g h energy plasma processes t o p o s s i b l y d o m i n a t e , and no r e a c t i v i t y measurements were reported. In addition the ionization laser i n t e n s i t y was s u f f i c i e n t l y high t o cause s i g n i f i c a n t f r a g m e n t a t i o n . They found t h e c o m p o s i t i o n of t h e products t o be oxygen p o o r . More r e c e n t l y t h e Exxon group r e p o r t e d i r o n r e a c t i o n s w i t h di oxygen. The d a t a was p r e s e n t e d assuming e q u i l i b r i u m i n t h e flow r e a c t o r . T h i s d a t a even w i t h t h i s q u e s t i o n a b l e assumption showed no d i s t i n c t s i z e selective behavi o r . P l a t i n u m c l u s t e r s , n = 2-11 r e a c t w i t h di oxygen at a r a t e t h a t i s w i t h i n an o r d e r of magnitude of gas k i n e t i c . There i s no d i s t i n c t size selective behavior. Products of t h e s e gas phase reactions observed w i t h 7.87 eV i o n i z a t i o n l a s e r , a r e P t p O ^ where f o r m=l,


56


CHEMISTRY

n = 5 , 7 - l l and f o r m=2, n = 8 - l l . There was a very weak product f o r Pt 02 although t h e d e p l e t i o n of t h e metal was e q u i v a l e n t t o t h e other metal c l u s t e r s . An i n c r e a s e i n i o n i z a t i o n p o t e n t i a l s upon o x i d a t i o n may be r e s p o n s i b l e f o r t h i s p a t t e r n of d e t e c t e d p r o d u c t s . P r e l i m i n a r y s t u d i e s of carbon d i o x i d e r e a c t i o n s w i t h niobium and cobalt c l u s t e r s by t h e R i c e group( l e ) have found another size selective reaction. The r e a c t i o n proceeds on small c l u s t e r s (^37) v i a d i s s o c i a t i o n , producing N b _ 0 + CO. For l a r g e r c l u s t e r s Nb C0o i s f o u n d . D i s s o c i a t i o n i m p l i e s a metal-oxygen bond s t r e n g t h > 12/ k c a l / m o l e , which i s c o n s i s t e n t w i t h t h e heat of a b s o r p t i o n on p o l y c r y s t a l l i ne f i 1ms (49) and t h e d i s s o c i a t i o n energy o f Nb-0 di atomi c ( 5 0 ) . S t u d i e s such as these extended t o o t h e r r e a c t a n t s such as N 0 and NO w i l l act t o survey metal c l u s t e r - o x y e n bond s t r e n g t h s a c r o s s t h e t r a n s i t i o n metal s e r i e s . 6

3

7

n


2

Lewi s B a s e s . A v a r i e t y of o t h e r l i g a n d s have been s t u d i e d , but w i t h o n l y a few of t h e t r a n s i t i o n m e t a l s . There i s s t i l l a l o t of room f o r s c o p i n g work i n t h i s d i r e c t i o n . Other r e a c t a n t systems r e p o r t e d are ammonia(2e), methanol ( 3 h ) , and hydrogen s u l f i de(3b) w i t h i r o n , and benzene w i t h t u n g s t e n (Tf) and p l a t i num(3a). In a q u a l i t a t i v e sense a l l of these r e a c t i o n s appear t o o c c u r a t , o r near gas k i n e t i c rates w i t h o u t d i s t i n c t s i z e s e l e c t i v i t y . The ammonia chemi sorbs on each c o l l i s i o n w i t h no s i z e s e l e c t i v e b e h a v i o r . These complexes have lower i o n i z a t i o n p o t e n t i a l i n d i c a t i v e of t h e donor type l i g a n d s . S a t u r a t i o n s t u d i e s have i n d i c a t e d a v a r i e t y of a b s o r p t i o n s i t e s on a single size cluster(51). The Exxon d a t a T o r i r o n w i t h methanol a l s o does not show s i z e s e l e c t i v e behavi o r ( 5 2 ) . The Exxon group has been a b l e t o show by i n f r a r e d m u l t i p l e photon d i s s o c i a t i o n t h a t t h e 0-H bond breaks forming methoxy on these small i r o n c l u s t e r s ( 3h). T h i s i s c o n s i s t e n t with t h e behavior of methanol at room temperature on i r o n surfaces(53). This i s t h e f i r s t example i n which t h e chemi s o r p t i on i s confirmed t o be d i s s o c i a t i v e and t h e r e a c t i o n i s not s i z e selective. However t h e Lewis a c i d i t y of t h e oxygen atom lone p a i r i n methanol i s l i k e l y s u f f i c i e n t t o cause t h e i n i t i a l product t o be m o l e c u l a r l y chemi s o r b e d . Benzene reacts at gas kinetic rates with platinum c!usters(3a). The products produced have s i g n i f i c a n t l y lower I P ' s than t h e r e a c t a n t s . This change can be very s i g n i f i c a n t . For example both t h e p l a t i n u m atom and benzene have ( - I P ' s - ) g r e a t e r than 8.9 eV w h i l e t h e P t ( C D ) s p e c i e s has an IP l e s s than 7.87 e V , i n d i c a t i n g at l e a s t a drop o f one e l e c t r o n v o l t i n i o n i z a t i o n potential. Arene complexes a l l have s i m i l a r l y low I P ' s ( 5 4 ) . With 7.87 eV i o n i z a t i o n l a s e r no dehydrogenation i s observed f o r benzene r e a c t i o n s w i t h p l a t i n u m c l u s t e r s out t o P t (the l a r g e s t c l u s t e r f o r which t h e adduct mass c o u l d be s t u d i e d r e l i a b l y ) and t h e low i o n i z a t i o n p o t e n t i a l of t h e p r o d u c t s , suggest TT bonded s t r u c t u r e s versus o x i d a t i v e a d d i t i o n t o C-H bonds. F i g u r e 6 shows t h e mass spectrum o f p l a t i n u m at t h e e a r l y stages of i t s r e a c t i o n w i t h benzene. The i n i t i a l r e p o r t (3a) of p l a t i n u m c l u s t e r c h e m i s t r y w i t h Cg hydrocarbons showed mi nor dehydrogenati on of benzene s t a r t i n g beyond the t r i m e r . These new s t u d i e s suggest that the dehydrogenation was caused by f r a g m e n t a t i o n i n d u c e d by e i t h e r t o o high o f an i o n i z a t i o n l a s e r i n t e n s i t y (work was c a r r i e d out u s i n g only 100-400 y0 of 6.42 eV photons) o r high v a p o r i z a t i o n laser -

6

6

2

7



3.


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F i g u r e 5 . A bar graph of metal c l u s t e r r e a c t i v i t i e s w i t h CO on a linear scale. C l u s t e r s i z e i n c r e a s e s going i n t o t h e page and metal t y p e s a c r o s s . Once beyond a few atoms i n s i z e most a l l c l u s t e r s r e a c t at r a t e s w i t h i n an o r d e r of magnitude of each other.

500

1500

1000 mass

(amu)

Figure 6. Ti m e - o f - f l i ght mass spectrum o f CgD r e a c t i n g w i t h platinum c l u s t e r s . The peak l a b e l s r e p r e s e n t m i n P t ( C D ) . There i s a s c a l e change between n=2, m=2 and m=3. 6

n

6


6

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58

H I G H - E N E R G Y P R O C E S S E S IN O R G A N O M E T A L L I C C H E M I S T R Y

intensity. The l a t t e r may y i e l d c l u s t e r s of high enough temperature t o cause t h e r m a l l y induced d e h y d r o g e n a t i o n . In summary, r e a c t i o n s w i t h l i g a n d s t h a t r e q u i r e a bond t o be broken before chemi s o r p t i on is completed show size selective behavior. This applies to hydrogen and n i t r o g e n and l i k e l y methane. Ligands that can form bonds with these highly coordi n a t i v e l y u n s a t u r a t e d metal c l u s t e r s do so w i t h f a s t r a t e s and w i t h l i t t l e s i z e d i s c r i m i n a t i o n . T h i s a p p l i e s even t o systems t h a t e v e n t u a l l y do d i s s o c i a t e l i k e methanol and most l i k e l y the CO adducts on t h e e a r l y t r a n s i t i o n m e t a l s . Once t h e r e i s a s i g n i f i c a n t a t t r a c t i v e i n t e r a c t i o n any b a r r i e r i n t h e e n t r a n c e channel i s a b a t e d , thus e l i m i n a t i n g the size selective behavior. The c l u s t e r r e a c t i v i t i e s are n e i t h e r i d e n t i c a l t o s u r f a c e c h e m i s t r y nor c l u s t e r i n o r g a n i c c h e m i s t r y , but smoothly i n t e r p o l a t e between t h e s e fields w i t h t h e n o t a b l e e x c e p t i o n of t h e d r a m a t i c s i z e s e l e c t i v e c h e m i s t r y a s s o c i a t e d w i t h bond a c t i v a t i o n on t h e small metal c l u s t e r s . These very s i m p l e but very i n f o r m a t i v e measurements are s e n s i t i v e t o j u s t t h e i n i t i a l chemi s o r p t i on s t e p and not subsequent rearrangements and i someri z a t i o n s . Techniques t o measure of t h e f o l l o w i n g s t e p s i n r e a c t i o n s on gas phase c l u s t e r s are j u s t now being deve!oped( 3 h ) . P l a t i n u m dehydrogenation

reactions

P l a t i n u m f i l m s , c l u s t e r s and s u r f a c e s under t h e c o n d i t i o n s i n t h e flow reactor are expected to rapidly dehydrogenate most al kanes (55_). The r a t e l i m i t i n g step i s t h e i n i t i a l chemi s o r p t i on of t h e a l k a n e t o t h e metal ( 5 6 ) . These s t u d i e s are q u i t e s e n s i t i v e t o the f i r s t step and t h u s are expected t o be i n f o r m a t i v e . U n l i k e most of t h e r e a c t i o n s a l r e a d y r e p o r t e d t h e s e dehydrogenation reactions i n v o l v e d e s o r p t i o n of hydrogen from t h e metal c l u s t e r a d d u c t . This g i v e s t h e c o l l i s i o n complex a way t o remove i t s excess energy and become s t a b i l i z e d . However i f t h i s d e s o r p t i o n process becomes c o m p e t i t i v e w i t h chemi s o r p t i on then the r e a c t i v i t y observed w i l l be harder t o i n t e r p r e t . For p l a t i n u m t h e d i f f i c u l t y i n making hydrogen s t i c k s t r o n g l y suggest t h a t t h i s w i l l not be a p r o b l e m . Cyclohexane. In q u a l i t a t i v e terms small p l a t i n u m c l u s t e r s r a p i d l y and n o n - s e l e c t i v e l y chemi sorbs c y c l o h e x a n e . The p r o d u c t s are h i g h l y dehydrogenated w i t h C:H r a t i o s r a p i d l y approaching o n e . Figure 7 shows t h e g a i n i n mass upon r e a c t i o n f o r the f i r s t adduct and t h e second minus t h e f i r s t . The p l a t i n u m atom w i t h i n t h e errors dehydrogenates C^D-^ t o C^Dg. The second adduct on t h e p l a t i n u m atom does not dehydrogenate s i g n i f i c a n t l y . However by Ptg t h e p l a t i n u m c l u s t e r s appear t o be a b l e t o convert two cyclohexane molecules i n t o chemi sorbed s p e c i e s w i t h a C:D r a t i o near one. Thi s f aci 1 e dehydrogenati on i s consi s t e n t wi t h t h e ki n e t i c models derived from catalytic conversion of cyclohexane to benzene(57). These models p r e d i c t an ensemble s i z e f o r t h e a c t i v e s i t e of o n l y one atom. On t h e o t h e r hand f a c i l e dehydrogenation of cyclohexane on metal s u r f a c e under UHV c o n d i t i o n s are d e s c r i b e d u s i n g a model t h a t extends over s e v e r a l metal atoms and suggests a s p e c i f i c t y p e of i n t e r a c t i o n i s necessary f o r an e f f i c i e n t r e a c t i on ( 5 8 ) . This is obviously not t h e case for t h e very s m a l l p l a t i n u m metal clusters.


3.


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Other hydrocarbons. The dehydrogenation of normal hexane and 2 , 3 di methyl butane a l s o proceeds but not as v o r a c i o u s l y on small platinum c l u s t e r s . F i g u r e 8 i s a p l o t o f t h e hydrogen content i n t h e first adduct as a f u n c t i o n o f t h e s i z e o f t h e p l a t i n u m metal cluster. The metal atom r e a c t s v i a di hydrogen e l i m i n a t i o n t o produce 6 12 P p l a t i n u m t r i m e r i s now t h e s m a l l e s t c l u s t e r t h a t w i l l produce a C:H near o n e . The s i m i l a r i t y of s i z e dependent dehydrogenation o f t h e normal hexane and t h e branched molecule suggest t h a t t h e s e systems may not r e a d i l y a r o m a t i z e t h e s e a l k a n e s . F u r t h e r s t r u c t u r a l s t u d i e s a r e needed t o i d e n t i f y the reaction products. Chemistry s t u d i e s of a l k a n e s on p l a t i n u m s u r f a c e s under UHV conditions are l i m i t e d by t h e very weakly bound molecularly chemisorbed s t a t e ( 5 9 ) . The low s u r f a c e temperatures required to adsorb t h e molecule are i n s u f f i c i e n t t o a c t i v a t e C-H bonds. However at higher pressures at room temperature and above, extensive dehydrogenation i s e x p e c t e d . T h i s process i s t h e r m a l l y a c t i v a t e d on s u r f a c e s but i n t h e gas phase on small p l a t i n u m c l u s t e r s o c c u r s at a p p r o x i m a t e l y 1-10% gas k i n e t i c . T h i s r a p i d r e a c t i o n suggests f o r very small p l a t i n u m c l u s t e r s t h a t t h e a c t i v a t i o n b a r r i e r f o r a l k a n e s i n t h e gas phase i s at most j u s t a few k i l o c a l o r i e s per mole, and does not i n v o l v e a high degree of s t e r i c h i n d r a n c e . P t C



H

r o d u c t s

T

n

e

Models of s i z e s e l e c t i v e

reactivity

The i n i t i a l goal of a successful model i s t o r a t i o n a l i z e t h e s i g n i f i c a n t and s u r p r i s i n g o b s e r v a t i o n s . The f i r s t o b j e c t i v e w i l l be t o e x p l a i n t h e s i z e dependence found i n t h i s s i m p l e c h e m i s t r y i . e . why i n going from F e i y t o F e 2 does t h e r a t e c o n s t a n t i n c r e a s e by a f a c t o r of a thousand/ The second goal i s t o r a t i o n a l i z e t h e s t r i k i n g c o r r e l a t i o n , f i r s t made by t h e Exxon g r o u p , between t h e v a r i a t i o n i n i o n i z a t i o n p o t e n t i a l s and t h e l o g a r i t h m o f t h e r a t e c o n s t a n t f o r chemi s o r p t i on o f di hydrogen ( 3 c ) . F i n a l l y s i n c e many o f t h e r e a c t i o n s are f a c i l e t h e minimum c r i t e r i a i n l i g a n d e l e c t r o n i c s t r u c t u r e w i l l be sought t h a t w i l l a s s u r e n o n - a c t i v a t e d chemi s o r p t i o n . The above o b s e r v a t i o n suggests t h a t t h e e l e c t r o n i c s t r u c t u r e must play a significant role in determining s i z e - s e l e c t i ve chemistry. An i o n i z a t i o n potential i n a simple one-electon a p p r o x i m a t i o n measures only the o r b i t a l energy o f t h e h i g h e s t o c c u p i e d m o l e c u l a r o r b i t a l (HOMO). T h i s e n e r g y , e s p e c i a l l y as t h e system gets l a r g e , i s o n l y one o f t h e numerous o r b i t a l s t h a t contribute to the c l u s t e r ' s t o t a l s t a b i l i t y . Both t h e success of frontier fragment-orbital analysis i n organometallic reactivity s t u d i e s ( 6 0 ) and t h e r e l a t i v e l y small changes i n a c t i v a t i o n e n e r g i e s n e c e s s a r y t o e x p l a i n t h e observed variations i n rate constants, suggest e l e c t r o n i c s t r u c t u r e p l a y s t h e p i v o t a l r o l e . In a d d i t i o n t h e recent successes i n e x p l a i n i n g a l k a l i metal c ! u s t e r ( 6 1 ) d i s t r i b u t i o n s and IPs based on a j e l l i u m model suggest e l e c t r o n i c e f f e c t s are d i r e c t l y apparent and do not n e c e s s a r i l y d e p i c t themselves i n terms of s p e c i f i c geometric s t r u c t u r e s . An a l t e r n a t i v e approach i s t o r e l a t e v a r i a t i o n s i n r e a c t i v i t y w i t h metal c l u s t e r s ' s t r u c t u r e s . U n f o r t u n a t e l y even l e s s i s known about t h e i r geometric arrangement than t h e i r e l e c t r o n i c s t r u c t u r e . Suggestions made i n t h e l i t e r a t u r e w i l l be summarized and s i m p l e i d e a s of t h e degree of coordi n a t i ve u n s a t u r a t i o n w i l l be p r e s e n t e d . 2


60


88 86

X \

84

X

•

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H^ • • X N K a _ B B E X X X X •

•

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—B

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76


(second-first) adduct

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-e 80

74

first adduct

\X

82

78

• X

J

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2

4

_J_ 6

_l 8

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J_

12

14

number of platinum atoms

F i g u r e 7. A p l o t o f t h e adduct masses (product - bare m e t a l ) produced i n r e a c t i n g p l a t i n u m c l u s t e r s w i t h c-CgD-jo. The l i n e s f o r t h e f i r s t adduct ( s o l i d ) and s e c o n d - f i r s t (dashed) adduct masses a r e drawn t o h e l p guide t h e eye through t h e s c a t t e r i n t h e data.

n

1

1

1

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Pt +m((CH ) CHCH(CH ) )— > n

1

2

3

4

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Number of Platinum Atoms

6

J 1

3

2

3

I

I

L_

2

3

4

2

5

6

Number of Platinum Atoms

Figure 8 . P l o t s o f t h e number o f hydrogens r e t a i n e d i n t h e p r o d u c t s formed i n r e a c t i n g p l a t i n u m c l u s t e r s w i t h n - C g H i * and (CH ) CHCH(CH ) . 3

2

3

2


3.



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Empirical f i t and Charge T r a n s f e r . F i g u r e 9 i s a p l o t of t h e logarithm of the bi m o l e c u l a r rate constants for deuterium chemi s o r p t i on as a f u n c t i o n of s i z e f o r i r o n , niobium and vanadi urn clusters. The dashed l i n e i s a p l o t of t h e i o n i z a t i o n p o t e n t i a l s of t h e s e same c l u s t e r s s c a l e d by an e m p i r i c a l charge t r a n s f e r m o d e l . The s u r p r i s i n g o b s e r v a t i o n of a s i g n i f i c a n t a n t i c o r r e l a t i on between a metal c l u s t e r s ' i o n i z a t i o n p o t e n t i a l and t h e i r r a t e of r e a c t i o n w i t h deuterium/hydrogen s t r o n g l y suggested charge t r a n s f e r from t h e metal cluster to was e s s e n t i a l i n a c t i v a t i n g t h e bond. The IPs are scaled according to


k

n

= A exp(-e(IP(n)-E )/k T) 0

B

where E Q and e are determined from a l e a s t - s q u a r e s f i t t o t h e r a t e c o n s t a n t s , assuming A i s independent of c l u s t e r s i z e . The A f a c t o r used i s o b t a i n e d by assuming t h a t t h e l a r g e c l u s t e r p l a t e a u i n t h e i r o n and vanadi urn c l u s t e r r e a c t i v i t i e s corresponds t o zero a c t i v a t i o n energy. The same value i s then used f o r niobium as f o r vanadi urn clusters. With t h i s assumption any c l u s t e r h a v i n g an i o n i z a t i o n p o t e n t i a l l e s s than E Q w i l l have zero a c t i v a t i o n e n e r g y . The e s c a l e f a c t o r can be i n t e r p r e t e d i n terms of t h e amount of charge t r a n s f e r or as w i l l be shown l a t e r a c l u s t e r e l e c t r o n e g a t i v t y d i f f e r e n c e . The i n c l u s i o n o f t h e kgT f a c t o r i n t h e exponent was chosen t o draw a c l o s e analogy w i t h t h e A r r h e n i us e x p r e s s i o n s i n c e t h e Argonne group has observed thermal a c t i v a t i o n f o r t h e i r o n s y s t e m . The c o r r e l a t i o n i n F i g . 9 i s best f o r i r o n , e s p e c i a l l y f o r d u s t e r w i t h more than e i g h t atoms. The o p p o s i t e behavior i s observed for the smaller clusters of niobium and i r o n . This departure along with (1) o b s e r v a t i o n s by t h e Rice group that p o s i t i v e l y charged i o n s of niobium have s i m i l a r r e a c t i v i t y p a t t e r n s as t h e n e u t r a l c l u s t e r s ( l e , g ) and (2) t h e report by t h e Argonne group of size selective behavi o r on ammoni ated clusters that have s i g n i f i c a n t l y lower i o n i z a t i o n p o t e n t i a l s than t h e bare c l u s t e r s ( 6 2 ) , n e c e s s i t a t e s t h e i n t e r p r e t a t i o n of e i n terms more general t h a n T h e f r a c t i o n a l charge t r a n s f e r . E l e c t r o n i c s t r u c t u r a l model. The s i z e s e l e c t i v e r e a c t i v i t y of t h e s e metal c l u s t e r s T~s s u r p r i s i n g . C e r t a i n l y t h e metal c l u s t e r are coordi n a t i v e l y unsaturated. It appears that coordi n a t i v e u n s a t u r a t i o n i s e s s e n t i a l t o s a t i s f y t h e e n e r g e t i c c r i t e r i a but more s p e c i f i c aspect of t h e e l e c t r o n i c s t r u c t u r e must play a r o l e i n c o n t r o l l i n g t h e a c t i v a t i o n energy of t h e p r o c e s s . A series of papers by Shustorovi ch(63) and/or Baetzo1d(64) summarized i n a recent a r t i c l e ( 6 5 ) have addressed t h e problem of chemi s o r p t i on on metal s u r f a c e s i n terms of e l e c t r o n a c c e p t i n g and donating interactions. Saillard and Hoffmann (66) developed q u a l i t a t i v e l y i d e n t i c a l p i c t u r e s of t h e s e i n t e r a c t i o n s but s t a r t i n g from fragment o r b i t a l t y p e a n a l y s i s . These papers are o n l y a few o f the theoretical discussions that consider hydrogen activation, however we w i l l use t h e i r approach because i t address t h e problem i n a f a s h i o n t h a t can i n t e r p o l a t e between t h e o r g a n o m e t a l l i c c l u s t e r and the b u l k . S t a r t i n g from t h e p o i n t of view of d i s c r e t e molecules t h e r e are four f r o n t i e r orbital and two primary i n t e r a c t i o n s . The f i r s t



62


i n t e r a c t i o n i s d o n a t i o n from t h e metal c l u s t e r t o t h e l i g a n d through t h e c l u s t e r ' s HOMO and t h e r e a c t a n t ' s lowest unoccupied m o l e c u l a r o r b i t a l (LUMO). The o t h e r i n t e r a c t i o n i s f o r t h e c l u s t e r t o a c t as t h e a c c e p t o r u s i n g i t s LUMO and t h e r e a c t a n t ' s HOMO. The e n e r g i e s o f these o r b i t a l s o b t a i n e d by EHT f o r a v a r i e t y of r e a c t a n t s and a c a n o n i c a l c l u s t e r a r e shown i n F i g . 1 0 . From t h i s diagram i t i s i m m e d i a t e l y obvious why t h e t r a n s i t i o n metals a r e e f f e c t i v e i n bond rearragements. These metals s u p p l y e l e c t r o n d e n s i t y t h a t b r i d g e s t h e H0M0-LUM0 gap which i s a s s o c i a t e d w i t h t h e h i g h s t a b i l i t y o f t h e closed shell reactants. Interactions i n v o l v i n g these electrons supply paths t h a t a v o i d high a c t i v a t i o n b a r r i e r s . The e l e c t r o n r i c h c h a r a c t e r o f metals i m p l y t h a t they should be very e f f e c t i v e d o n o r s , i n s p i t e o f t h e l a r g e energy s e p a r a t i o n between t h e metal-HOMO and t h e LUMO o f t h e r e a c t a n t . C o n s i d e r t h e hydrogen molecule approaching a metal c l u s t e r . The l o n g - r a n g e i n t e r a c t i o n w i l l be r e p u l s i v e because t h e m a j o r i t y o f t h e e l e c t r o n s i n t h e c l u s t e r a r e s p i n - p a i r e d and h o l d i n g t h e c l u s t e r together. This long-range repulsion creates a barrier to the reaction. As t h e hydrogen molecule gets c l o s e r , t h e c l u s t e r a c t s as an e " d o n o r , i n t e r a c t i n g w i t h a of H and as an a c c e p t o r w i t h t h e bonding molecular orbital of the absorbate. Both of these i n t e r a c t i o n s weaken t h e absorbate bond and s t r e n g t h e n t h e bonding t o the c l u s t e r . Thus i f t h e a t t r a c t i v e i n t e r a c t i o n overwhelms t h e repulsion, dissociative chemi s o r p t i on w i l l occur with a small a c t i v a t i o n b a r r i e r i n t h e entrance c h a n n e l . The a c t i v a t i o n energy w i l l depend upon a compromise between t h e r e p u l s i o n and t h e l o n g e s t range a t t r a c t i o n . The donor i n t e r a c t i o n i s l o n g e r - r a n g e and w i l l i n c r e a s e as o r b i t a l s e p a r a t i o n d e c r e a s e s . T h e r e f o r e t h e lower IP c l u s t e r s a r e b e t t e r l o n g - r a n g e donors than a c c e p t o r s and t h u s have smaller activation energies. T h i s i s e x a c t l y what i s needed t o r a t i o n a l i z e t h e c o r r e l a t i o n between a c l u s t e r ' s IP and i t s a b i l i t y t o a c t i vate h y d r o g e n . T h i s s i m p l e model p r e d i c t s f o r a s u f f i c i e n t l y low IP c l u s t e r t h a t bonds can be broken without a b a r r i e r and t h a t metals on t h e l e f t end o f t h e p e r i o d i c t a b l e s h o u l d be t h e most f a c i l e , both i n agreement w i t h our o b s e r v a t i o n s . I f t h e a n t i bondi ng a t y p e o r b i t a l i n t h e reactant i s s i g n i f i c a n t l y higher i n energy, t h e b a r r i e r could be s u f f i c i e n t t o prevent r e a c t i o n s from being d e t e c t e d . This i s a p p a r e n t l y t h e case f o r methane. These arguments change when extended t o ^ b o n d e d r e a c t a n t s l i k e N , CO, and CgHg where now t h e LUMO has TT character. Small populations i n t h e LUMO w i l l no l o n g e r guarantee dissociation. Instead a s t a b l e m o l e c u l a r chemi s o r p t i on bond can f o r m . Since t h e s e experiments have a much h i g h e r s e n s i t i v i t y f o r a c t i v a t i o n e n e r g i e s ( a few ki l o c a l o r i e s / m o l e ) than f o r s t a b l e bonds (16 kcal/mole) m o l e c u l a r bonding dominates t h e r e a c t i v i t y . The LUMO o f n i t r o g e n i s high enough t h a t m o l e c u l a r s t a t e s do not form (even on t h e bulk s u r f a c e t h e y need t o be s t a b i l i z e d by a l k a l i promotion) and f o l l o w the same p a t t e r n as i n h y d r o g e n . Iron c l u s t e r s appear t o be on t h e verge o f r e a c t i n g w i t h n i t r o g e n i n t h e s e e x p e r i m e n t s - l i k e l y higher temperatures or a l k a l i promotion will test t h i s simple model. F i n a l l y m o l e c u l a r oxygen w i l l look l i k e a di radi c a l t o even t h e s m a l l e s t metal c l u s t e r and s h o u l d react without s i z e s e l e c t i v i t y even i f t h e f i n a l products are d i s s o c i a t i v e . T h i s model f o r bond a c t i v a t i o n has a l r e a d y f a i l e d s e v e r a l s i m p l e 2

2





63

dorter size (number of atoms)

F i g u r e 9. P l o t s o f t h e r a t e c o n s t a n t s (X) o f i r o n , vanadi urn and niobium c l u s t e r s r e a c t i n g w i t h di hydrogen/di d e n t e r i urn, and t h e i r r e s p e c t i v e bare c l u s t e r i o n i z a t i o n p o t e n t i a l s ( s o l i d l i n e s ) s c a l e d as d e s c r i b e d i n t h e t e x t .

Figure 10. I n d i c a t e d a r e t h e HOMO and LUMO o r b i t a l e n e r g i e s o b t a i n e d form EHT c a l c u l a t i o n s f o r a v a r i e t y o f r e a c t a n t s . In t h e center are estimated orbital energies f o r a canonical metal cluster.


64


tests. First, reactions o f t h e charged c l u s t e r s show s i m i l a r p a t t e r n s , which i n d i c a t e s i m p l e charge t r a n s f e r i s i n a d e q u a t e . In a d d i t i o n t h e r e p u l s i v e i n t e r a c t i o n t h a t caused t h e b a r r i e r i n t h e first place i s l i k e l y i n s i g n i f i c a n t compared t o t h e a t t r a c t i v e charge-induced d i p o l e i n t e r a c t i o n . In t h e o t h e r d i r e c t i o n t h e model f a i l s as w e l l . The Argonne group has found s i z e - s e l e c t i v e reactivity of hydrogen on i r o n c l u s t e r s t h a t are s a t u r a t e d w i t h ammonias. These ammine-Fe c l u s t e r s have s i g n i f i c a n t l y lower i o n i z a t i o n potentials and s t i l l show s i z e s e l e c t i v i t y ; again c o n f l i c t i n g w i t h a s i m p l e charge t r a n s f e r m o d e l . The s i m p l e s t e x t e n s i o n of t h i s model i s t o i n c l u d e both donor and a c c e p t o r i n t e r a c t i o n s . The r e l e v a n t parameter i s the Mulliken electronegativity which averages t h e t h e IP and e l e c t r o n a f f i ni t y ( E A ) . U n f o r t u n a t e l y at t h i s time i n s u f f i c i e n t EA data i s a v a i l a b l e f o r m e a n i n g f u l l t e s t s . As seen i n F i g . 9 t h e c o r r e l a t i o n between IP and r e a c t i v i t y changes s i g n i f i c a n t l y f o r small c l u s t e r s . In f a c t f o r many o f t h e c l u s t e r s under e i g h t atoms i n s i z e t h e c o r r e l a t i o n i s i n t h e o p p o s i t e sense. The c o n s i d e r a t i o n s given so f a r have been based on e n e r g e t i c s and have not i n c l u d e d any d e t a i l valency or symmetry. The assumption i s t h a t t h e d e n s i t y o f s t a t e s i s s u f f i c i e n t l y high t h a t at any energy there i s an o r b i t a l that could s a t i s f y any o f t h e s e further restrictions. As t h e c l u s t e r gets s m a l l e r t h i s becomes more d i f f i c u l t and t h u s d e p a r t u r e s from t h e " b u l k " model s h o u l d o c c u r . In a d d i t i o n t h e c l u s t e r IPs a l s o i n c r e a s e s i g n i f i c a n t l y making a c c e p t o r i n t e r a c t i o n s more i m p o r t a n t .


n

Local coordi n a t i v e s a t u r a t i o n . For t h e very small c l u s t e r t h e i d e a oT drawi ng on fHe f i e 1 d o"f s t a b l e c l u s t e r complexes i s very attractive. Lauher(69) and Teo(70) have presented rules that d e s c r i b e t h e t o t a l bonding c a p a b i l i t i e s o f t h e metal c l u s t e r s . These work w e l l f o r s a t u r a t e d and p a r t i a l l y u n s a t u r a t e d metal c l u s t e r s . However t h e high degree of coordi nati ve u n s a t u r a t i o n i s s i g n i f i c a n t l e a v i n g t h e s e r u l e s w i t h l i t t l e p r e d i c t i v e power. However L . Brewer has t a k e n a s l i g h t l y d i f f e r e n t approach and e x p l a i n e d t h e s t r u c t u r e of a l l o y s and c r y s t a l h a b i t of t h e t r a n s i t i o n e l e m e n t s . T h i s work i s a b l e t o p r e d i c t t h e s t r u c t u r e o f most bulk metal s y s t e m s . This p o i n t s out t h a t even f o r metals t h a t a r e very m a l l e a b l e and have l i t t l e p r e f e r e n c e f o r d i r e c t i o n a l bonding s i m p l e r u l e s can s t i l l be developed and might be very a p p l i c a b l e t o c l u s t e r s . For niobium and c o b a l t c l u s t e r s s t r u c t u r e s have been proposed based upon t h e elements behavi o r ( 7 1 ) . Niobium's s p e c i f i c i n e r t n e s s has been a s s o c i a t e d w i t h s t r u c t u r e s t h a t a r e analogous t o c l o s e packed s u r f a c e o f W(110) which a l s o has an a c t i v a t i o n b a r r i e r f o r hydrogen chemi s o r p t i o n . Since t h e IPs a r e a l s o expected t o be h i g h e r f o r c l o s e d packed s t r u c t u r e s t h e s e two s e t s of o b s e r v a t i o n s a r e i n agreement. This model at i t s c u r r e n t stage of development requires different s t r u c t u r e s f o r each system and as y e t has not been u s e f u l i n making p r e d i c t i o n s . It i s i m p o r t a n t t o note t h a t none o f t h e s e arguments have addressed t h e f i r s t concern of why. B u t , have s h i f t e d the question t o what i n t h e e l e c t r o n i c s t r u c t u r e o f t h e s e c l u s t e r s i s c a u s i n g t h e IP, r e a c t i v i t y , and l i k e l y v a r i a t i o n s i n s t r u c t u r e w i t h s i z e .


3.



Physical


65

Characterization

Through out t h i s mi n i - r e v i ew c o r r e l a t i o n s have been made between chemical and p h y s i c a l properties o f t h e metal clusters. The simultaneous study of t h e i r c h e m i s t r y and t h e a p p l i c a t i o n of s t a n d a r d molecular-beam chemical p h y s i c s probes have c o o p e r a t i v e l y enabled t h e r a p i d growth of t h i s new a r e a . Physical properties currently measurable are i o n i z a t i o n p o t e n t i a l s ( 3 d - f , 3 i ,72) photoi oni z a t i on effi ciencies(73), magnetic moments (_l£JT photof ragmentati on of i o n s ( 7 4 , 7 5 ) and i n some cases n e u t r a l s , p o l a r i z a b i 1 i t e s ( 7 6 ) , and some strong " i n f r a r e d absorpti o n s " ( 3 h ) . The number of systems s t u d i e d and t h e measurement methods a r e r a p i d l y g r o w i n g . T h i s data base i s e s s e n t i a l i n t h e u n d e r s t a n d i n g of t h e compromises t h a t are i n v o l v e d in metal bonding, as t h e m e t a l l i c state evolves from small clusters. Theory has p a r t i c i p a t e d i n a l l aspects of this area's development. The almost hand-waving arguments used t o r a t i o n a l i z e t h e i r chemical b e h a v i o r need t e s t i n g and w i l l l i k e l y be r e p l a c e d by more e l e g a n t q u a n t i t a t i v e d i s c u s s i o n s . The t h e o r e t i c a l a s p e c t s ( 7 7 ) and most p h y s i c a l p r o p e r t y measurements(78) of small metal c l u s t e r s have been r e c e n t l y r e v i e w e d . Summary The f i e l d o f gas-phase t r a n s i t i o n metal c l u s t e r c h e m i s t r y has expanded r a p i d l y due t o t h e development of t h e l a s e r vaporization source and t h e f a s t flow chemical r e a c t o r . The work from t h e t h r e e major l a b o r a t o r i e s have been r e v i e w e d . Many a d d i t i o n a l l a b o r a t o r i e s are d e v e l o p i n g c l u s t e r c h e m i s t r y programs and w i l l soon c e r t a i n l y make s i g n i f i c a n t c o n t r i b u t i o n s . In summary a few " g e n e r a l i z a t i o n s " have been f o u n d . F i r s t , s i z e selective c h e m i s t r y i s s t r o n g l y a s s o c i a t e d w i t h chemi s o r p t i on t h a t requires bond-breaking. Second, metal c l u s t e r s r e a c t r a p i d l y w i t h ligands that m o l e c u l a r l y chemisorb even when t h e eventual products i n v o l v e d i s s o c i a t i o n of t h e l i g a n d . Dehydrogenation of C - a l k a n e s on small p l a t i n u m c l u s t e r s t a k e e x c e p t i o n t o t h i s . The charge transfer model suggested to rationalize the c o r r e l a t i o n between i o n i z a t i o n p o t e n t i a l and r e a c t i v i t i e s o f i r o n , vanadium, and niobium w i t h di hydrogen fails f o r other systems. However a model that takes into account t h e f r o n t i e r orbital i n t e r a c t i o n s , although h i g h l y s i m p l i s t i c , does account f o r a v a r i e t y of observations. T h i s model suggests extensions that include e l e c t r o n a f f i n i t i e s as w e l l as IPs and t h e p o s s i b i l i t y of d e v e l o p i n g an e l e c t r o n e g a t i v t y s c a l e f o r c l u s t e r s as a f u n c t i o n of t h e i r s i z e . Geometric s t r u c t u r e of t h e bare metal c l u s t e r s and t h e complexes formed by r e a c t i o n are unknown and present a s i g n i f i c a n t e x p e r i m e n t a l challenge. Chemical s t u d i e s are s t a r t i n g t o i m p l y something about t h e s t r u c t u r e o f t h e p r o d u c t s and w i l l be i n v a l u a b l e u n t i l more d i r e c t chemical p h y s i c s probes are a v a i l a b l e . 6

Acknowledgments We wish t o thank t h e c u r r e n t and past members of t h e c l u s t e r c h e m i s t r y groups at Rice U n i v e r s i t y , Argonne N a t i o n a l L a b o r a t o r y and Exxon Research and E n g i n e e r i n g C o . Rick S m a l l e y , Steve R i l e y , E r i c


66


CHEMISTRY

P a r k s , Ken Reichmann, E r i c R o h l f i n g , Rob Whetten, M i t c h Z a k i n , and Don Cox deserve thanks f o r communication of t h e i r data before p u b l i c a t i o n and/or a l l o w i n g i t s use f o r t h e f i r s t time i n t h i s mi ni review. The open communication among t h e s e c o l l a b o r a t o r s and c o m p e t i t o r s have made t h i s an e x t r e m e l y enjoyable, e x c i t i n g , and r a p i d l y p r o g r e s s i n g area of r e s e a r c h .

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