Centered Cluster Halides for Group-Three and Group-Four Transition

19 Centered Cluster Halides for Group-Three and Group-Four

Downloaded by UNIV OF LEEDS on June 18, 2016 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0226.ch019

Transition Metals A Versatile Solid-State and Solution Chemistry Friedhelm Rogel, Jie Zhang, Martin W. Payne, and John D . Corbett Department of Chemistry, Iowa State University, Ames, IA 50011

A large number and variety of M X -type cluster halides constructed from a rare-earth metal or zirconium (X is Cl, Br, or I) are stable only when centered by an interstitial element Z. At least 22 elements are known to function as Z, and over a dozen structure types are represented. New results are presented in three different areas: the encapsulation of 3d transition elements in zirconium chlorides to give Li (Zr Cl Z)Cl phases in the Nb F structure with two independent, interpenetrating cluster networks or analogues of the K Zr Cl H structure; bonding of heavy transition metals (Ru, Rh, Pd, Re, Os, Ir, or Pt) within rare-earth-metal (R) clusters with R I Z or R I Z compositions (R is Pr, Gd, or Y); the preparation of new cluster phases from solid-state cluster compounds via room-temperature solution chemistry in CH CN. The products include the optimal 14-electron cluster ((C H ) N ) Zr Cl C , the 18-electron analogue ((C H ) N ) [(Zr Cl Fe)Cl 4-], and three examples containing the new 12-electron cluster Zr Cl Be , in which cluster-bonding electrons have clearly been removed. 6

x

2

7

6

12

12

6/2

6

15

18

7

6

12

10

3

2

2

5

4

+

4

6

12

5

+

4

4

6

4-

18

6

6

12

V J H E M I C A L CLUSTERS O F T H E M X 6

4-

1

2

n

+

ANDM

6

' X

8

m

+

TYPES have

been

k n o w n for m a n y years for t h e m o r e - c e n t r a l t r a n s i t i o n e l e m e n t s ( M is N b o r T a ; M ' is M o ; X is C l , B r , I , o r , i n o n e case, F ) . F o r t h e m o r e - r e l e v a n t M X 6

1 2

t y p e , v a r i e d b i n a r y a n d ternary n i o b i u m a n d t a n t a l u m c o m p o u n d s 0065-2393/90/0226-0369$06.25/0 © 1990 American Chemical Society

Johnson et al.; Electron Transfer in Biology and the Solid State Advances in Chemistry; American Chemical Society: Washington, DC, 1989.

370

E L E C T R O N T R A N S F E R IN B I O L O G Y A N D T H E S O L I D S T A T E

have b e e n i d e n t i f i e d that contain ( N b , T a ) 6 X

1 2

2 +

'

clusters along w i t h a d

3 +

d i t i o n a l h a l i d e anions (J). T h e electronic r e q u i r e m e n t s of these clusters s e e m to b e r e l a t i v e l y r e s t r i c t i v e , w i t h o n l y 16 or 15 electrons left i n m e t a l - m e t a l b o n d i n g orbitals i n the two charge types, respectively, after the l o w e r - l y i n g halogen valence orbitals have b e e n

filled.

The ( N b , T a ) X 6

1 2

cluster

2 + , 3 +

phases are generally synthesized at h i g h t e m p e r a t u r e a n d therefore r e p r e s e n t e q u i l i b r i u m species. O n the o t h e r h a n d , the r e v e r s i b l e oxidation of these clusters to (Nb,Ta) P d F structural r e l a t i o n s h i p (25), w h i l e t h r e e C s ions a n d the n e w Z r C l " ( D ) p r o p it o p e n . A [110] section o f this r e s u l t is s h o w n i n F i g u r e 3. 6

1 2

6 / 2

3

5


+

3

3 h

K Z r C l H Analogues. F i n a l l y , these larger Z r C l Z units m a y also b e c o m b i n e d w i t h o t h e r c o m p o u n d anions to a l l o w access to another v a r i e t y of cluster phases, those that originate w i t h K Z r C l · Z r C l ( H ) (23). T h e cubic-close-packed Z r C l Z clusters (see F i g u r e 4) are n o w separated b y a l i k e array o f Z r C l " anions. T h e c h l o r i n e atoms i n these anions are also b o n d e d exo to a l l z i r c o n i u m vertices of the clusters. T h e separation o f the structure d e s c r i p t i o n into n e u t r a l Z r C l Z clusters a n d Z r C l ~ ions is s o m e what artificial, b u t i t is consistent w i t h a 0.3-Â difference i n distances b e t w e e n C l - Z r a n d C l - Z r C l " i n the parent. S u b s t i t u t i o n o f l o w e r - f i e l d L a or Ba for the isolated z i r c o n i u m makes a d e s c r i p t i o n i n t e r m s of 6 - 1 8 - l i k e clusters, as i n (Cs ) L a [ ( Z r C l M n ) C l " ] , i n c r e a s i n g l y m o r e apt. A n o t h e r 18-electron e x a m p l e is f o u n d i n K L a Z r C l F e a n d , as 14-electron clusters, i n K L a Z r C l B a n d K B a Z r C l C . T h e isostructural K , i R ( N b C l ) phases are also k n o w n w h e n R is a r a r e - e a r t h e l e m e n t (26). 7

2

6

1 8

2

6

6

6

1 2

2

e

6

6

1 2

6

e

1 2

5

1 2

6

2

2

3 +

2 +

+

2

3 +

6

1 2

6

s

6

2

6

1 8

2

6

1 8

1 8

0

6

1 8

A n 18-electron c l u s t e r can also b e o b t a i n e d w i t h C s L a Z r C l F e i n a d i s t o r t e d v e r s i o n o f the same structure, w h e r e there is n o w r o o m for o n l y a single u n i p o s i t i v e cation. T h e 16-electron n i o b i u m p a r e n t is C s L u N b C l (27). 6

1 8

6

1 8

Rare-Earth-Metal Clusters N e i t h e r chlorides n o r iodides o f the rare-earth metals appear to afford the great v a r i e t y o f clusters c e n t e r e d b y the l i g h t e r elements that have b e e n seen w i t h z i r c o n i u m . I n a d d i t i o n to S c ( S e C l ( B , N ) ) n o t e d e a r l i e r , t h e r e are o n l y the analogues S c ( S c I ) ( B , C ) (28) a n d a d i c a r b i d e - c o n t a i n i n g S c I C (29). T h e r e d u c e d i o d i n e content o f the latter is a c h i e v e d t h r o u g h s h a r i n g o f Γ" atoms b e t w e e n pairs o f clusters, s u c h as 6

6

12

12

6

1

Se \ Se

'

/ V

Sc Sc


n

2

377

Centered Cluster Halides for Transition Metals

ROGEL ET AL.


19.

Figure 3. A [110] section of the structure of (Cs ) [(ZréCli Mn)Clm] ~ZrClf (R3c). The cluster network defines a twisted Re0 -like unit with bridging angles of 133° (vs. 180° in Figure 2). The clusters and the ZrCl ~ ions lie on vertical threefold axes (90% ellipsoids). Jr

3

2

2

3

5

T h e C i n t e r s t i t i a l e l e m e n t is already w e l l - k n o w n from w o r k e l s e w h e r e o n a variety of g a d o l i n i u m halides, for example, i n c o n d e n s e d clusters that share m e t a l edges e i t h e r i n d i m e r s , as i n G d C l ( C ) (6), or i n an infinite c h a i n v e r s i o n G d I ( C ) (7, 24). O n the other h a n d , the y t t r i u m chlorides s e e m to be q u i t e free of c e n t e r e d octahedral cluster examples of a l l k i n d s . A n 2

1 0

1 2

1 7

2

1 8

2

2

3


378

E L E C T R O N T R A N S F E R IN B I O L O G Y A N D T H E S O L I D STATE

exploration o f r e d u c e d l a n t h a n u m chlorides suggests a s i m i l a r result is p r o b able t h e r e , too ( H . - J . M e y e r , u n p u b l i s h e d ) . E x p l o r a t i o n of 3 d metals i n cluster iodides o f r a r e - e a r t h metals (R) was i n i t i a l l y p r o m p t e d b y the discovery of Z r I Z - t y p e clusters i n w h i c h Ζ is C r , M n , F e , o r C o . A s is often the case, m a n y m o r e clusters w e r e subse q u e n t l y f o u n d t h a n j u s t z i r c o n i u m analogues. I n i t i a l l y , r a r e - e a r t h - m e t a l i o d i d e systems w e r e investigated for R (Se, Y, P r , G d ) a n d Ζ ( M n - N i ) . A considerable n u m b e r of black R I Z phases w e r e d i s c o v e r e d t h e r e i n w i t h the structure o f S c ( S c C l N ) , w h i c h is the same as that o f Z r I C w h e n the first (isolated) m e t a l atom is o m i t t e d . T h e s e phases c o n t a i n c u b i c - c l o s e p a c k e d ( A B C . . . ) R I - t y p e clusters i n w h i c h a l l t e r m i n a l positions are b o n d e d to i n n e r i o d i n e i n n e i g h b o r i n g clusters. O c t a h e d r a l holes b e t w e e n clusters o f the same stacking orientation are o c c u p i e d b y the R ion. A [110] section o f the structure for S c I C o is s h o w n i n F i g u r e 4 (30). T h e e n v i r o n m e n t o f the s e v e n t h m e t a l atom b e t w e e n t w o clusters i n this s t r u c t u r e is d e t a i l e d i n F i g u r e 5. 6

7

6

1 2

1 2

6


1 2

6

1 2

1 2

3 +

7

1 2

T h e 14- o r 18-electron closed-shell b e n c h m a r k s for m a i n - g r o u p or t r a n sition-metal interstitial elements seem to b e r e l a t i v e l y i m p o r t a n t w i t h z i r c o n i u m c h l o r i d e clusters, a l t h o u g h z i r c o n i u m i o d i d e clusters w i t h 3 d transition metals usually c o n t a i n 18 or 19. T h e s e g u i d e l i n e s seem less i m portant for the rare-earth e l e m e n t s , as w e n o w find examples that range from 16 to 20 (i.e., w i t h M n , F e , C o , N i , o r C u , r e s p e c t i v e l y , i n P r I Z ) . 7

1 2

D i s t i n c t i o n s f o u n d w i t h o t h e r R hosts are still p u z z l i n g . P r e s u m a b l y t h e y d e p e n d o n s u c h subtleties as cluster a n d cavity sizes a n d alternate phase stabilities. T h u s , o n l y 18- a n d 19-electron examples ( C o , N i ) h a v e b e e n o b t a i n e d w i t h s c a n d i u m , a l t h o u g h 1 6 - 1 8 - e l e c t r o n clusters ( M n , F e , C o ) are f o r m e d b y y t t r i u m . E v i d e n t l y , c o m p l e t e occupancy o f the n o m i n a l t H O M O (for 18 electrons) is not so c r i t i c a l h e r e , perhaps because the a.o.'s o n R a n d thence the t set l i e r e l a t i v e l y h i g h w i t h these m o r e - e l e c t r o positive host e l e m e n t s . (The t set is r e a l l y i m p o r t a n t o n l y for R - R b o n d i n g because the ρ orbitals o n t r a n s i t i o n - m e t a l interstitial atoms l i e too h i g h to b e effective.) l M

6

lu

lu

A greater surprise for r a r e - e a r t h - m e t a l cluster i o d i d e s c a m e w i t h the discovery that these are also stable w h e n encapsulating 4 d a n d 5 d m e t a l atoms (31, 32). F o r e x a m p l e , the f o l l o w i n g Ζ are f o u n d i n P r I Z phases, 7

Mn

Fe

Co

Ni

Re

Ru Os

Rh Ir

Pd Pt

1 2

Cu

t h e r e b y s p a n n i n g 16- to 20-electron clusters. T h e same list is f o u n d for g a d o l i n i u m ; h o w e v e r , A g gives clusters w i t h n e i t h e r host (or the right c o n ditions have not b e e n found). O n the other h a n d , y t t r i u m does not appear to y i e l d Y I Z products w i t h N i , R h , P d , O s , I r , P t , or any of the g r o u p 7

1 2



3+

6

Figure 4. A [110] projection of the structure of Sc(Sc li2Co) with ~c vertical. The six edge-bridging iodine atoms about the waist of the trigonal antiprismatic Sc^Co) clusters (crossed ellipsoids) are also bonded to exo positions in adjoining clusters. The three iodines above and below the cluster form an octahedral cavity for the isolated Sc ion. (Both this seventh Sc atom and the Co lie on sites with 3 symmetry.) (Reproduced from ref. 30. Copyright 1988 American Chemical Society.)


380

STATE


E L E C T R O N T R A N S F E R IN B I O L O G Y A N D T H E S O L I D

Figure 5. The arrangement of two ReX^Z " clusters about the isolated R atom in R Ii Z phases. R atoms are solid and the R-Z bonding is emphasized. New compounds in this structure include those where R is Pr or Gd and Ζ is Ru, Rh, Pd, Re, Os, Ir, or Pt. 3

7

2


3+

19.

381

Centered Cluster Halides for Transition Metals

ROGEL ET AL.

six elements. H o w e v e r , i n the cases of O s , R h , I r , a n d P t , this result is apparently because an alternate cluster phase, Y I i Z , i n t r u d e s . B o t h s t r u c ture types o c c u r o n l y w i t h y t t r i u m clusters c o n t a i n i n g cobalt or r u t h e n i u m . Z i r c o n i u m seems to show v e r y little of an analogous c h e m i s t r y w i t h most of the 4 d a n d 5 d elements. 6

0

T h e versatility of N a t u r e is nicely d e m o n s t r a t e d b y this n e w structure t y p e , Y I R u (31), w h i c h occurs for y t t r i u m w h e n Ζ is C o , N i , R u , R h , O s , Ir, or P t , b u t so far only for P r I O s a n d not (yet) for g a d o l i n i u m . T h e r e d u c e d i o d i n e content is a c h i e v e d , as w i t h S e I C , t h r o u g h s h a r i n g of i n n e r i o d i n e atoms. I n this case, the shared atoms f o r m infinite chains o f b r i d g e d clusters rather than pairs. A p o r t i o n of this c o n s t r u c t i o n i n Y I O s is s h o w n i n F i g u r e 6. Pairs of 14 atoms b r i d g e octahedral edges i n a d j o i n i n g clusters, whereas p a r a l l e l bridges i n v o l v i n g 15 are of the m o r e c o m m o n 1 ^ type. 6

1 0

6

1 0

6

n

2


6

1 0

T h e i d e a l octahedral cluster w o u l d e x h i b i t a t H O M O , w h i c h m a y b e the reason for a significant compression of the isolated octahedra i n Y I R u along what is close to a fourfold axis r o u g h l y n o r m a l to the c h a i n d i r e c t i o n . T h i s distortion is i n the d i r e c t i o n to p r o d u c e an e g r o u n d state a n d possibly occurs because the ρ orbitals o n Ζ (which n o r m a l l y w o u l d y i e l d the opposite distortion) l i e so h i g h as to be u n i m p o r t a n t . Instead, the distortion is d e t e r m i n e d m o r e b y Y - Y interactions (30). A l t h o u g h the Y - R u b o n d s i n Y I R u differ b y 0.21 Â, the difference is o n l y 0.09 Â i n Y I O s . T h e smaller difference may m e a n the b e h a v i o r is m o r e c o m p l e x a n d less obvious t h a n anticipated. T h e r e seems to be no significant compression i n Y I i o I r (17 electrons) (32). T h e average Y - O s distance i n Y I O s is 0.17 Â less t h a n that i n the i n t e r m e t a l l i c Y O s . W e have o b s e r v e d this general effect w i t h m a n y n o v e l M Z clusters (19, 21, 30, 31) i n w h i c h it m i g h t be i m a g i n e d that w e have s i m p l y c a p t u r e d the smallest e l e m e n t of an i n t e r m e t a l l i c i n t e r a c t i o n . 4

lu

6

1 0

6

1 0

4

u

6

1 0

6

6

1 0

3

6

Zirconium Cluster Chlorides in Nonaqueous Solutions A large family of c e n t e r e d z i r c o n i u m cluster chlorides M ( Z r C l Z ) C l can be obtained i n about 12 structural arrays i n w h i c h the clusters are h e l d together b y c h l o r i n e bridges w i t h Cl "° or C l functionalities. T h e stabilities of these clusters peak sharply at 14 cluster electrons. T h e q u e s t i o n n a t u r a l l y arises as to w h e t h e r a greater range of electronic stabilities m i g h t exist i n metastable clusters p r e p a r e d near r o o m t e m p e r a t u r e , p a r a l l e l i n g the b e havior w i t h N b 6 X units w h e r e the 15- a n d 16-electron units o b t a i n e d i n h i g h - t e m p e r a t u r e phases can be o x i d i z e d to 14-electron examples i n aqueous solution. 2 C

fl

1 2

I

6

1 2

n

f l _ i

n +

T h e solution process e n v i s i o n e d requires not just solvation of the p r o d u c t ions, b u t a suitable n u c l e o p h i l e to o p e n u p the Z r - C l - Z r linkages. E x tensive explorations of this c h e m i s t r y have r e v e a l e d a sizable list of reactions 6

6


Johnson et al.; Electron Transfer in Biology and the Solid State Advances in Chemistry; American Chemical Society: Washington, DC, 1989. 6

Figure 6. A portion of the infinite cluster chain in Y IwOs. Pairs of 14 atoms bridge yttrium edges in both clusters. 15 atoms have a more normal I™ function, bridging one edge and bonding to metal vertex in an adjoining cluster. Inversion centers at the osmium atoms generate infinite chains of such bridged clusters.


19.

383

Centered Cluster Halides for Transition Metak

ROGEL ET AL.


that do not w o r k , b u t some that w i l l . T h e best q u a n t i f i e d systems are those from w h i c h single crystals have b e e n isolated a n d c h a r a c t e r i z e d s t r u c t u r a l l y . A substantial n u m b e r of other solutions exist f r o m w h i c h suitable crystals have not yet b e e n obtained. T h e clusters are naturally good r e d u c i n g agents a n d r a p i d l y react w i t h water, alcohol, a n d acetone, w i t h both oxidation a n d solvolysis. A c e t o n i t r i l e seems q u i t e suitable as a solvent u n d e r m a n y circumstances, whereas d i m e t h y l f o r m a m i d e a n d d i m e t h y l sulfoxide oxidize most substrates too r e a d i l y . S o l u t i o n of cluster species appears to r e q u i r e not o n l y l i g a t i o n b u t also complexation of the a l k a l i - m e t a l cation b e t t e r than that afforded b y the solvent alone. B o t h c r o w n ethers (18-crown-6, 15-crown-5) a n d 2,2,2-crypt have served this purpose w e l l , a l t h o u g h crystalline derivatives have b e e n isolated m o r e f r e q u e n t l y as R N or ( C H ) P salts. T h e reactions are c a r r i e d out w i t h c o n v e n t i o n a l v a c u u m techniques a n d m u l t i c h a m b e r e d glass apparatus e q u i p p e d w i t h poly(tetrafluoroethylene) (Teflon) n e e d l e valves. +

4

6

5

4

+

A c o m m o n m o d e of reaction produces a Z r C l Z a n i o n f r o m a c h l o r i d e p o o r e r (bridged) phase. R e t e n t i o n of a 14-electron u n i t is i l l u s t r a t e d b y reaction 1. 6

KZr Cl C(s) 6

qfa

1 5

g^T >
C l e

e

e _ i

) are b o u n d to

the cluster m o r e strongly, so that t h e n e w Z r C l Z ~ clusters d e s c r i b e d i n 6

1 8

n

this chapter s h o u l d represent t h e e n d p o i n t e v e n i n t h e presence o f large


a n d less-polarizing countercations. A l t h o u g h s u c h effects appear r e a l , t h e y are small (only a f e w h u n d r e d t h s o f an A n g s t r o m i n most cases), whereas the cluster e l e c t r o n count appears to b e m o r e i m p o r t a n t i n d e t e r m i n i n g distances. T a b l e I compares t h e Z r - C , Z r - Z r , a n d Z r - C l

e

distances i n t h e t w o

( ( C H 5 ) N ) 4 ( Z r C l C ) · 2 C H C N structures w i t h those i n three solid-state 2

4

6

1 8

3

cluster carbides, t h e last o f w h i c h contains 15 rather than 14 electrons. T h e first two e x t e n d e d structures have o n l y b r i d g i n g chlorines i n exo positions. T h e Z r - C l ° separations are seen to shorten b y 0.04 Â w h e n these b e c o m e

Table I. Comparison of ZreChgC " Cluster Dimensions with Those in Cluster Networks 4

[(C H ) N ] Zr Cl C 2

5

4

+

4

6

18

4

Parameter

Tricl.

Monocl.

Cluster electrons d(Zr-C) d(Zr-Zr) d(Zr-Cl)

14 2.296 3.248 2.597

14 2.295 3.245 2.586

Zr Cl C 6

14

KZr Cl C

a

14 2.286 3.232 2.630

6

d

15

b

14 2.279 3.223 2.640**

Cs£r Cl C 6

16

c

15 2.261 3.197 2.596, 2.689*

N O T E : All d values are given in angstrom units. Standard deviations for all structures listed are

Centered Cluster Halides for Group-Three and Group-Four Transition

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