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Dimolybdenum Versus Ditungsten Alkoxides RICHARD A. WALTON Purdue University, Department of Chemistry, West Lafayette, IN 47907
In the preceding paper, Malcolm Chisholm (1) has presented a cogent case for the modeling by metal alkoxides of certain aspects of the structural chemistry and reactivities of metal oxides. The focus of this work has been the dinuclear and polynuclear alkoxides of molybdenum and tungsten, an area of research which has also attracted our interest (2-4) and upon which I would now like to take this opportunity to comment. In the case of the triply bonded unbridged alkoxides M(OR), where M = Mo when R = t-Bu, i-Pr or CH-t-Bu, and M = W when R = t-Bu, (1, 5-7), the ditungsten derivative W(O-t-Bu) is much less thermally stable than Mo(OR). Furthermore, attempts to prepare other complexes of the type W(OR), where R represents a bulky alkyl group, through the reactions of W(NMe)6 with the appropriate alcohol leads to facile oxidation to tungsten(IV). This is shown, for example, in the conversion of W (NMe2)6 to W4(O-i2
6
2
2
2
6
6
2
6
2
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Pr)14H ( c o n t a i n i n g W=W bonds) (1,8), a r e a c t i o n which corresponds formally to an o x i d a t i v e a d d i t i o n . Another important example is the formation o f t e t r a n u c l e a r W4(OR)16 upon r e a c t i n g W (NMe )6 with methanol o r ethanol (1,9). In the case of the dimolybdenum Mo (OR)6 complexes, o x i d a t i v e a d d i t i o n s r e q u i r e somewhat more potent reagents, as in the conversions o f Mo (O-i-Pr)6 to doubly bonded M o ( 0 - i - P r ) by i-PrOO-i-Pr and to Mo Xi+(y-0-i -Pr) (0-iPr) i+ by the halogens X (1,10) . Q u a l i t a t i v e l y at l e a s t , the preceding observations h i n t a t an increased s t a b i l i t y of the higher v a l e n t ditungsten alkoxides over r e l a t e d dimolybdenum s p e c i e s , a trend which c o r r e l a t e s with the i n c r e a s i n g s t a b i l i t y o f higher o x i d a t i o n s t a t e s as a t r a n s i t i o n group i s descended. Our own entry i n t o t h i s area stemmed from studies we made i n t o the r e a c t i o n s between a l c o h o l - HCl(g) mixtures and the quadruply bonded complexes M2(mhp)i , where M = Mo or W and mhp i s the anion o f 2-hydroxy-6-methylpyridine. In the presence of CsCl such r e a c t i o n s give Csi Mo2Cls and CS3W2CI9 ( 2 ) , r e f l e c t i n g the greater ease of o x i d i z i n g the W2 " " core. When the t e r t i a r y phosphines P E t 3 or P-n-Pr3 are added i n place o f C s C l , each dimetal system i s o x i d i z e d one step f u r t h e r , Mo2(mhp)i 2
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0097-6156/83/0211-0269$06.00/0 © 1983 American Chemical Society Chisholm; Inorganic Chemistry: Toward the 21st Century ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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a f f o r d i n g ( R P H ) M o 2 C l H (11), a member of an already w e l l char a c t e r i z e d c l a s s of μ-hydrido bridged dimolybdenum(III) complexes (7), while W (mhp)i produces the dark green ditungsten(IV) alkox ides W Cl (y-0R)2(0R) (R0H)2(2-4) · The l a t t e r molecules possess a W=W bond represented by a σ^π " ground s t a t e e l e c t r o n i c c o n f i g u r a t i o n ( 4 ) . X-ray s t r u c t u r e determination on the methoxide and ethoxide show that these complexes possess s t r u c t u r e ^, the short 3
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2
c l
\l/°\l/
c l
W = W
V
W-W d i s t a n c e (2.48A) r e f l e c t i n g the double bond c h a r a c t e r . Oxida t i o n of 1 by 0 , H 0 , N0 or Ag(I) s a l t s gives dark red complexes ( s t r u c t u r e ^ ) , which are the ditungsten(V) analogs of ^ (W-W d i s 2
2
2
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R
ci
o Ι
o .ο. I
R
R
m
^ W ^ - ^ J J ^
R°
2
R
°R Ο tance of 2.72A)(3). In a d d i t i o n to the d i f f e r e n c e i n W-W bond orders, 1 and 2 d i f f e r i n that the former possesses two a l k o x i d e and two a l c o h o l l i g a n d s whereas i n £ there are now four t e r m i n a l a l k o x i d e s . As a r e s u l t , £ does not possess the unsymmetrical hydrogen bond that e x i s t s between the adjacent a l k o x i d e and a l c o h o l l i g a n d s i n ^L, v i z .
In c o n t r a s t to the remarkable thermal and a i r s t a b i l i t y of ^ and the dimolybdenum(V) analogues of Mo Xi (y-0-i^-Pr) ( 0 - i Pr) 1+ (X = CI or Br) are thermally unstable and very moisture sen s i t i v e (10). I n t e r e s t i n g l y , molybdenum analogues of ^ are unknown although the c l o s e l y r e l a t e d M o X ( 0 - i - P r ) g may be formed as a very unstable intermediate i n the o x i d a t i o n of M o ( 0 - i - P r ) 5 by halogens ( 1 0 ) , and the dimolybdenum(IV) complex Mo (0-i_-Pr) q i s a very w e l l c h a r a c t e r i z e d species ( 1 , 1 0 ) . Whereas Μ ο Χ ^ ( u - O - i - P r ) 2
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Chisholm; Inorganic Chemistry: Toward the 21st Century ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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(O-i-Pr)4 decompose even at room temperature (to evolve i s o p r o p y l h a l i d e s ) , high temperatures are r e q u i r e d to decompose ditungsten complexes of types ^ and 2, and p r e l i m i n a r y experiments i n d i c a t e that they may decompose by pathways d i f f e r e n t from the isopropoxide complexes of dimolybdenum(V). For example, the ditungsten isopropoxides of types ^ and 2 decompose at 160° i n vacuum to give propene but l i t t l e p r o p y l c h l o r i d e (12). In the case of the n-propoxide of d i t u n g s t e n ( I V ) , W C l i ( y - 0 - n - P r ) ( 0 - n - P r ) ( n - P r O H ) , d i - n - p r o p y l e t h e r i s the p r i n c i p a l v o l a t i l e (12). The doubly bonded complexes V ^ C l i ^ p - O R ) ( 0 R ) ( R 0 H ) are d i f f erent from the f u l l y s u b s t i t u t e d tungsten(IV) a l k o x i d e s W^iOR)^ (1,9), i n that the l a t t e r 8 - e l e c t r o n c l u s t e r e x h i b i t s a c o n s i d e r able degree of delocâlized W-W bonding, the s h o r t e s t W-W d i s t a n c e (2.65Â) being much longer than the W=W bond (2.48A) i n W C l i (y-0R) (0R) (R0H) (4). The p o s s i b i l i t y of c o n v e r t i n g W Cli (y-0R) (0R) (R0H) to t e t r a n u c l e a r Wi^OR)^ by s u b s t i t u t i n g OR" f o r C I " i s being pursued (12). In the course of t h i s work we have studied the a l c o h o l exchange r e a c t i o n s o f the ethoxide W C l i ( p - 0 E t ) ( 0 E t ) ( E t O H ) . Comp l e t e exchange occurs i n r e a c t i o n s with primary a l c o h o l s ROH (R = Me, n-Pr, η-Bu, n-Pent, η-Oct or i-Bu) to give W C l 4 ( y - O R ) ( 0 R ) (R0H) (12,13). On the other hand, the more s t e r i c a l l y demanding secondary a l c o h o l s R'OH (R' = i - P r , s-Bu or £-Pent) a f f o r d the mixed a l k o x i d e s W C l i ( y - 0 E t ) ( 0 R ) ( R O H ) (12). X-ray s t r u c t u r e determinations on the d e r i v a t i v e s where R' = i - P r or s-Pent (14) show that these complexes, while s t i l l possessing the same general type of s t r u c t u r e as depicted i n ^, now contain a symmetrical hydrogen-bond, v i z . 2
+
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2
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.--H»„
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Thus the hydrogen-bonding i s apparently dependent, amongst other f a c t o r s , upon the nature of the a l k y l s u b s t i t u e n t s . Perhaps the more h i g h l y branched secondary a l k y l s u b s t i t u e n t s increase the hydrophobic environment about the hydrogen bond, thereby f a v o r i n g a symmetrical i n t e r a c t i o n . In any event, i t i s apparent that t h i s i n t e r a c t i o n i s best described by a very broad, shallow p o t e n t i a l function. The hydrogen-bonding i n t e r a c t i o n s w i t h i n the complexes W Clit( y - 0 R ) ( 0 R ) ( R 0 H ) and W Cli (μ-OR) ( 0 R ) ( R OH) may provide the molecular analogues with which to model the s t r u c t u r e and r e a c t i v i t i e s of t r a n s i t i o n metal oxide c a t a l y s t s that possess surface hydroxyl groups. The thermal treatment which i s o f t e n c a r r i e d out i n the pretreatment of metal oxides ( l e a d i n g to the l o s s of -OH 2
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groups and the e v o l u t i o n of H2O) may have i n t e r e s t i n g i n the thermolysis of the ditungsten a l k o x i d e s .
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analogies
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Literature Cited
1. Chisholm, M. H. preceding paper in this symposium. 2. DeMarco, D.; Nimry, T.; Walton, R. A. Inorg. Chem., 1980, 19, 575. 3. Cotton, F. Α.; DeMarco, D.; Kolthammer, B.W.S.; Walton, R. A. Inorg. Chem., 1981, 20, 3048. 4. Anderson, L. B.; Cotton, F. Α.; DeMarco, D.; Fang, Α.; Ilsley, W. H.; Kolthammer, B.W.S.; Walton, R. A. J. Am. Chem. Soc., 1981, 103, 5078. 5. Chisholm, M. H.; Cotton, F. Α.; Murillo, C. Α.; Reichert, W. W. Inorg. Chem., 1977, 16, 1801. 6. Akiyama, Α.; Chisholm, M. H.; Cotton, F. Α.; Extine, M. W.; Haitko, D. Α.; Little, D.; Fanwick, P. E. Inorg. Chem., 1979, 18, 2266. 7. Cotton, F. Α.; Walton, R. A. "Multiple Bonds Between Metal Atoms", J. Wiley and Sons, 1982, and references therein. 8. Akiyama, M.; Chisholm, M. H.; Cotton, F. Α.; Extine, M. W.; Haitko, D. Α.; Leonelli, J.; Little, D. J. Am. Chem. Soc., 1981, 103, 779. 9. Chisholm, M. H.; Huffman, J. C.; Kirkpatrick, C.C.;Leonelli, J.; Folting, K. J. Am. Chem. Soc., 1981, 103, 6093. 10. Chisholm, M. H.; Kirkpatrick, C. C.; Huffman, J. C. Inorg. Chem., 1981, 20, 871. 11. Pierce, J. L.; DeMarco, D.; Walton, R. Α., submitted for publication. 12. DeMarco, D.; Walton, R. A. unpublished results. 13. Clark, P. W.; Wentworth, R.A.D. Inorg. Chem., 1969, 8, 1223. 14. Cotton, F. Α.; DeMarco, D.; Walton, R. A. unpublished results. RECEIVED October 8, 1982
Chisholm; Inorganic Chemistry: Toward the 21st Century ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
