Experimental Organometallic Chemistry - ACS Publications

Chapter 4

Vacuum Line Techniques for Handling Air-Sensitive Organometallic Compounds Barbara J . Burger and John E. Bercaw

Downloaded by NANYANG TECH UNIV LIB on November 6, 2014 | http://pubs.acs.org Publication Date: November 24, 1987 | doi: 10.1021/bk-1987-0357.ch004

Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125

Vacuum line techniques for handling a i r sensitive oganometallic compounds are described. The vacuum line is discussed in six sections: (i) the pumps and main traps, (ii) the main manifold with pressure gauges, ( i i i ) work stations, (iv) the cryogenic traps and Toepler pump, (v) the inert gas purifiers and i n l e t system, and (vi) the specialty gas i n l e t system. Use of a swivel frit assembly in the synthesis of a i r sensitive compounds is described along procedures for the addition of gases, v o l a t i l e and nonvolatile liquids and solutions to reaction mixtures. High temperature and pressure reactions are discussed in terms of the use of sealed NMR tubes and heavy walled reaction vessels. The procedure for molecular weight determination using the Signer method i s also included. Our research group carries out preparative chemistry in fume hoods, on bench tops, in high pressure (Parr) reactors, in glove boxes, in Schlenk tubes, and with cannulas and septa, just as other groups concerned with the synthesis and characterization of air-sensitive organo-transition metal compounds. In this a r t i c l e we attempt to describe the less common methods, procedures and the equipment, i . e . those associated with a vacuum l i n e , which have evolved over the past fifteen years in the Bercaw group. Many of the techniques are taken from the literature and have been (hopefully) improved upon or modified for working with small amounts (50 mg - 35 g) of an organotransition metal compound. Others have evolved out of necessity, and, as far as we are aware, are not described elsewhere. 0097-6156/87/0357-0079$06.00/0 © 1987 American Chemical Society

In Experimental Organometallic Chemistry; Wayda, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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The

EXPERIMENTAL ORGANOMETALLIC CHEMISTRY Vacuum

Line

M o s t o f t h e r e a c t i o n c h e m i s t r y t h a t i s done i n o u r l a b o r a t o r y i n v o l v e s t h e u s e o f vacuum l i n e s , s u c h as t h e one shown b e l o w i n F i g u r e 1. The u s e o f a vacuum l i n e o f f e r s two m a j o r a d v a n t a g e s o v e r more s t a n d a r d S c h l e n k techniques: h o s e s and r u b b e r s e p t a , w h i c h a r e more p e r m e a b l e t o a i r and m o i s t u r e , a r e c o m p l e t e l y e x c l u d e d and q u a n t i t a t i v e m a n i p u l a t i o n o f g a s e s and v o l a t i l e l i q u i d s c a n be p e r f o r m e d c o n v e n i e n t l y . Although the c o n s t r u c t i o n o f a vacuum l i n e a s complex a s t h a t shown i n v o l v e s a r e l a t i v e l y l a r g e i n v e s t m e n t o f t i m e and money ( c a . $15,000 i n p a r t s and l a b o r ) , i t becomes a p e r m a n e n t p i e c e o f e q u i p m e n t w h i c h i s s h a r e d by two o r t h r e e g r o u p members and u s e d e x t e n s i v e l y on a r e g u l a r b a s i s . Moreover, i t i s o f t e n f a s t e r t o assemble a c o m p l i c a t e d a p p a r a t u s on t h e vacuum l i n e , r a t h e r t h a n a t a S c h l e n k l i n e , s i n c e many o f t h e r e q u i r e d components a r e permanently i n p l a c e . The vacuum l i n e c o n s i s t s o f s i x m a i n s e c t i o n s : (i) t h e pumps and m a i n t r a p s , ( i i ) t h e m a i n m a n i f o l d w i t h p r e s s u r e g a u g e s , ( i i i ) t h e work s t a t i o n s , ( i v ) t h e c r y o g e n i c t r a p s and T o e p l e r pump, (v) t h e i n e r t g a s p u r i f i e r s and i n l e t s y s t e m , and ( v i ) t h e s p e c i a l t y gas i n l e t system. E a c h vacuum l i n e i s e q u i p p e d w i t h two pumps w h i c h operate i n s e r i e s . A m e r c u r y d i f f u s i o n pump, t o g e t h e r w i t h a m e c h a n i c a l f o r e pump, p r o v i d e a r e s i d u a l p r e s s u r e o f 0, where η i s t h e number o f T o e p l e r pump cycles). An i n d i c a t i o n o f t h e amount o f r e s i d u a l g a s i s p r o v i d e d b y t h e s i z e o f t h e b u b b l e f o r m e d when t h e n



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EXPERIMENTAL ORGANOMETALLIC CHEMISTRY

mercury approaches the upper v a l v e . The p r e s s u r e i n t h e c a l i b r a t e d v o l u m e s h o u l d b e m e a s u r e d o c c a s i o n a l l y (by a l l o w i n g t h e Hg l e v e l t o r i s e t o t h e mark above t h e valve). When no d i f f e r e n c e i s d e t e c t e d f r o m r e a d i n g t o r e a d i n g , g a s c o l l e c t i o n may b e s t o p p e d . To s e p a r a t e ( h o t CuO) c o m b u s t i b l e g a s e s ( 2 3 ) from n o n c o m b u s t i b l e s , t h e sample i s f i r s t c o l l e c t e d a n d t h e t o t a l amount o f g a s i s m e a s u r e d . The s t o p c o c k l e a d i n g back t o the c r y o s c o p i c t r a p s i s c l o s e d , t h e stopcocks o p e n i n g t h e pathway a r o u n d t h e r e c i r c u l a t i n g m a n i f o l d s e c t i o n a r e opened. The t e f l o n n e e d l e v a l v e l e a d i n g back t o t h e l a s t t r a p i s opened o n l y s l i g h t l y s o t h a t e x c e s s i v e p r e s s u r e s o f g a s a r e p r e v e n t e d f r o m b u i l d i n g up a t t h e i n l e t o f t h e T o e p l e r pump. T h e g a s m i x t u r e i s now c i r c u l a t e d b y t h e T o e p l e r pump t h r o u g h t h e h e a t e d CuO s e c t i o n ( 3 2 0 ° C ) , t h e t r a p c o o l e d t o -196°C a n d b a c k i n t o t h e c a l i b r a t e d s e c t i o n , e t c . u n t i l a l l H2 i s o x i d i z e d t o H2O a n d CO i s o x i d i z e d t o CO2/ b o t h o f w h i c h a r e t r a p p e d out a t -196°C. By c l o s i n g t h e s t o p c o c k i m m e d i a t e l y above t h e l a s t s e c t i o n o f t h e c a l i b r a t e d volume, t h e r e s i d u a l , n o n c o m b u s t i b l e g a s (N2 a n d / o r CH4) may b e c o l l e c t e d a n d i t s pressure determined. By p e r i o d i c a l l y r e p e a t i n g t h e s e p r o c e d u r e s i t c a n b e d e t e r m i n e d i f more c y c l e s t h r o u g h t h e CuO t u b e a r e r e q u i r e d . T h e r e s i d u a l g a s i s m e a s u r e d and a n a l y z e d b y i n f r a r e d s p e c t r o s c o p y , g a s c h r o m a t o g r a p h y o r mass s p e c t r o m e t r y b y w i t h d r a w i n g a sample i n t o a g a s i r c e l l o r g a s sample b u l b a t t a c h e d t o t h e p o r t a t t h e t o p o f t h e T o e p l e r pump s e c t i o n o f t h e vacuum l i n e . T h e r e s i d u a l N2/CH4 i s pumped o u t o f t h e c a l i b r a t e d v o l u m e s . The t r a p a t -196°C i s warmed t o -78°C b y r e p l a c i n g t h e l i q u i d n i t r o g e n w i t h a d r y i c e / e t h a n o l i n t h e dewar s u r r o u n d i n g i t , a n d t h e CO2 i s now c o l l e c t e d b y t h e T o e p l e r pump. T h e amount o f CO2 i s , o f c o u r s e , t h e same a s t h e amount o f CO i n t h e o r i g i n a l s a m p l e . T h e d i f f e r e n c e i s t h e amount o f H2, now a s H2O i n t h e -78°C t r a p , i n t h e o r i g i n a l sample. Thus, by a p p l y i n g D a l t o n s Law o f P a r t i a l P r e s s u r e s , one c a n e a s i l y q u a n t i t a t i v e l y d e t e r m i n e t h e c o m p o s i t i o n o f H2/CH4, H2/N2, CO/CH4, CO/N2, CO/H2, CO/H2/CH4, a n d CO/H2/N2 b i n a r y a n d t e r t i a r y m i x t u r e s , a n d i f c o u p l e d w i t h mass s p e c t r o m e t r y t o q u a n t i f y t h e p r o p o r t i o n s o f methane i n N2, CO/H2/CH4/N2 quaternary gas mixtures. f

M o l e c u l a r Weicrht D e t e r m i n a t i o n O r c r a n o m e t a l l i c Compounds

for Non-Volatile

Of a l l t h e methods t h a t a r e a v a i l a b l e f o r d e t e r m i n i n g t h e m o l e c u l a r w e i g h t o f a n o r g a n o m e t a l l i c compound, t h e most s t r a i g h t f o r w a r d i s undoubtedly t h a t developed by S i g n e r (24.) . T h i s method i s b a s e d on t h e s i m p l e i d e a t h a t the vapor p r e s s u r e o f an i d e a l s o l u t i o n i s p r o p o r t i o n a l t o t h e c o n c e n t r a t i o n o f t h e s o l u t e ( R a o u l t ' s Law). The major advantages o f t h i s t e c h n i q u e i s t h a t i t does n o t r e q u i r e a n y s o p h i s t i c a t e d e q u i p m e n t , o n l y a b o u t 0.05 g o f t h e unknown compound a r e n e e d e d a n d i t c a n b e s u i t e d t o



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95

e x t r e m e l y a i r s e n s i t i v e compounds w i t h t h e a d a p t a t i o n s d e s c r i b e d below. The l i m i t a t i o n s o f t h i s method a r e t h a t t h e s a m p l e must be s t a b l e i n s o l u t i o n a t room t e m p e r a t u r e f o r 3-7 d a y s and t h e a c c u r a c y o f t h i s method i s ± 1 0 % a t best. The m o l e c u l a r w e i g h t a p p a r a t u s c o n s i s t s o f two g r a d u a t e d c a p i l l a r y t u b e s ( a p p r o x i m a t e l y 10 ml i n volume, c o n s t r u c t e d f r o m s e c t i o n s o f a Mohr p i p e t ) e a c h s e a l e d t o a round b u l b . The two b u l b s a r e c o n n e c t e d t o a s h o r t g l a s s tube w i t h s o l v e n t s e a l connectors. In the middle of the g l a s s tube, t h e r e i s a stopcock through which the e n t i r e a s s e m b l y c a n be e v a c u a t e d . A diagram of the a p p a r a t u s i s shown i n F i g u r e 9 . The p r o c e d u r e f o r d e t e r m i n i n g t h e m o l e c u l a r w e i g h t o f an unknown a i r s e n s i t i v e compound i s o u t l i n e d b e l o w . I n t h e g l o v e box, 0.010-0.015 g o f t h e s t a n d a r d (azobenzene o r f e r r o c e n e a r e good c h o i c e s ) i s c a r e f u l l y w e i g h e d o u t (± .1 mg) and p l a c e d i n one b u l b . Similarly, an amount o f t h e unknown i s a c c u r a t e l y w e i g h e d o u t and placed i n the other bulb. A s o l v e n t which i s very i n e r t t o w a r d t h e s a m p l e (benzene, c y c l o h e x a n e , t e t r a h y d r o f u r a n ) i s a d d e d (ça. 0.5-0.8 ml) t o e a c h b u l b and t h e a p p a r a t u s i s assembled. The b u l b s a r e c o o l e d t o -78°C, and t h e a p p a r a t u s i s e v a c u a t e d on t h e vacuum l i n e . The a s s e m b l y i s t h e n a l l o w e d t o come t o room t e m p e r a t u r e and p l a c e d (with t h e s o l v e n t i n t h e b u l b s ) i n a l o c a t i o n f r e e from d r a f t s , away f r o m d i r e c t s u n l i g h t o r room l i g h t (a c a r d b o a r d box o r l a r g e p o t work n i c e l y ) . The v o l u m e c h a n g e s h o u l d be p e r i o d i c a l l y m o n i t o r e d by t i l t i n g t h e assembly so t h a t t h e s o l u t i o n s f l o w i n t o t h e v o l u m e t r i c p o r t i o n s of the apparatus. The v o l u m e o f e a c h s o l u t i o n w i l l c h a n g e by t r a n s f e r o f s o l v e n t v a p o r u n t i l t h e v a p o r p r e s s u r e s o f t h e two s o l u t i o n e q u i l b r a t e , u s u a l l y r e q u i r i n g 3-7 d a y s . The e x p e r i m e n t a l m o l e c u l a r w e i g h t i s d e t e r m i n e d by t h e f o l l o w i n g r e l a t i o n s h i p : (mg )(MW )(ml ) x

MW

S

s

=

X

(mg )(ml ) s

where mg mg MW MW ml ml

x

s

X

S

x

s

= = = = = =

weight of weight of molecular molecular volume o f volume o f

x

unknown i n mg s t a n d a r d i n mg w e i g h t o f unknown weight of standard standard s o l u t i o n unknown s o l u t i o n

Acknowledgments The a u t h o r s w i s h t o t h a n k members o f t h e Bercaw g r o u p , p a s t and p r e s e n t , who h a v e c o n t r i b u t e d t o t h e d e v e l o p m e n t o f t h e p r o c e d u r e s and e q u i p m e n t described i n t h i s chapter. JEB wishes t o thank P r o f e s s o r Hans H. B r i n t z i n g e r ( p r e s e n t l y a t t h e U n i v e r s i t y o f K o n s t a n z , West Germany) f o r i n s p i r i n g many o f t h e o r i g i n a l c o n c e p t s when t h e two o f u s were a t t h e U n i v e r s i t y of Michigan.




Figure 8.

Thick walled reaction vessel.

F i g u r e 9.

Apparatus f o r molecular determination.

weight


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REFERENCES AND FOOTNOTES 1.

2.


3.

4.

5.

6.

Because it operates with a Hg column, the McLeod gauge does not read the p a r t i a l pressure of mercury (ca. 10-3 torr) which continually fills those portions exposed to l i q u i d Hg. The accurate measurement of pressure requires use of a manometer. The bubbler i s less accurate since the level of the pool of mercury at the base varies s l i g h t l y as the column height varies. Our group has invested heavily in Fischer Porter brand 4 mm needle valves. These provide fine control when they are opened, and are thus preferable for those sections of the system where control i s needed, for example, for solvent removal and at the points of entry to the cryogenic trap and Toepler section. In recent years less expensive, but equally effective teflon needle valves have become available. The base of the Toepler pump and the bases of the mercury bubblers should be placed in p l a s t i c cups and secured with plaster of paris to a depth of 3 or 4 inches. Thus, mercury spillage is avoided is the glass base i s broken. The pathway of this recirculating section should have as little volume as possible out of the c i r c u i t , since gases must flow freely to prevent them from "hiding in corners" from the reactive CuO. A simplified ideal gas formula i s used to calculate the mmols of gas in the volumes: n (mmols) = p (torr) ·V (ml)/1.85 Χ 10 at 2 3 . 5 ° C . The c i r c u i t diagram i s available on request. Brown, T. L.; Dickerhoof, D. W.; Bafus, D. A.; Morgan, G. L. Rev. S c i . Instruments 1962, 33., 491492. Care should be taken not to allow the aluminum foil to contact the e l e c t r i c a l leads! Since Ar has only ca. 100 torr of vapor pressure at l i q u i d nitrogen temperature, a dangerous buildup of l i q u i d Ar w i l l occur if the pressure of Ar exceeds this value while a trap, flask, NMR tube, etc. is being cooled with l i q u i d nitrogen. Similarly care must be taken to not allow the pressure to exceed one atmosphere when cooling a part of the l i n e with N2 as the inert gas. The purity of commercially available carbon monoxide (e.g. Matheson) is normally sufficient for our experiments. Marvich, R. H . ; Brintzinger, H. H. J. Amer. Chem. Soc., 1971, 93, 2046-2048. For solvent pots of greater than 500 ml in volume, it is recommended that they be taped or otherwise treated to prevent flying glass should the flask implode. This advisory note applies whenever large glassware i s evacuated. 4

7. 8. 9. 10.

11. 12. 13.


98



14.

Heating the solvent pot above room temperature w i l l only s l i g h t l y increase the rate of transfer, and has the undesirable effect of causing solvent condensation along the path of the transfer. 15. Thicker Apiezon H grease is used to compensate for the poorer fit of standard taper joints and to reduce the amount of solvent wash out. The joints are greased only along the top half of the ground glass section to minimize contamination from grease; however, we have never noted deleterious effects of Apiezon greases on our compounds. 16. If the high vacuum working station is used the bubbler/manometer may be accurately used as a manometer since the pools of Hg are interconnected (see Figure 1). 17. If a very reactive, noxious or poisonous reagent is used, the residual reagent should be trapped out into one of the removable traps, and the reagent should be carefully transferred to the fume hood where it is allowed to warm slowly. 18. A section of a Mohr pipet or graduated centrifuge tube sealed to a 14/20 standard taper outer joint have proven to be quite satisfactory. 19. Caution: the reaction flask may be cooled only to -78°C; cooling to l i q u i d nitrogen temperature w i l l lead to dangerous buildup of l i q u i d Ar. If the reaction flask must be cooled to l i q u i d nitrogen temperature, N2 should be used as the inert gas and great care must be exercised to avoid condensation of l i q u i d nitrogen. 20. Care should be taken to avoid cooling the teflon valves. The much different coefficients of thermal expansion for teflon and glass results in leakage when the valve is cooled much below room temperature. 21. We have found that Wilmad (507 pp) NMR tubes provide adequate resolution for routine high f i e l d NMR spectra. 22. A noncondensable gas i s operationally defined as one that has more than ca. 0.01 torr pressure at l i q u i d nitrogen temperature. 23. Care should be taken to avoid O2, even in small amounts, as a component of these gas mixtures. An explosive mixture with H2, CO or CH4 could easily be produced and detonated by sparks at the electrodes of the relay or on contact with the hot CuO. 24. E . P . Clark, Ind. Eng. Chem., Anal. Ed. 1941, 13, 820.. RECEIVED July 31, 1987


Chapter 4: Application 1

Assay of Ligand-Derived Gases Following Outer-Sphere Oxidation Eric G. Lundquist and Kenneth G. Caulton


Department of Chemistry, Indiana University, Bloomington, IN 47405

A method for the quantitative evolution and estimation of a ligand-derived gas is described. Dissociation of the gas (CO, H2, CO2, N2, etc.) i s promoted by outersphere oxidation of the compound of interest using [Fe(bipy)3] (PF6)3 in acetonitrile solvent. Examples described include the analysis of CO content i n Fe (CO)12 and Co (CO) . 3

2

8

An extremely valuable development in the f i e l d of metal carbonyl chemistry was the p u b l i c a t i o n of an a n a l y t i c a l method for the determination of the number of carbonyl ligands contained in a complex (1_,.2). This became increasingly important with the discovery of metal carbonyl c l u s t e r s , since the weight percent carbon between various molecular formulae varies l i t t l e when the atom ration C/M is no longer integral. Thus, the rhodium carbonyls Rh (C0) , R h ( C 0 ) , Rh (C0) have C/Rh ratios (4, 3, and 2.67) which tend towards a l i m i t and correspondingly s i m i l a r percent carbon values. Classical elemental analysis is of essentially no use in establishing the number of carbonyl ligands in the large anionic clusters of the Longoni/Chini group (3). For example, the recent examination of an apparent s t r u c t u r a l isomer of Ru^CCO)-^, while i t gave acceptable percent carbon ( c a l c , 22.54, found, 22.04), would perhaps benefit from direct and accurate measurement of a CO content (4_). S i m i l a r l y , i d e n t i f i c a t i o n of the prodigious array of osmium carbonyls of high nuclearity (0S (C0)Q, Os-^CO)^, 0 s ( C 0 ) , 0 s ( C 0 ) , 0 s ( C 0 ) , Os (CO) , 0 s ( C 0 J ) r e l i e s l i t t l e , i f at a l l , on elemental analysis C5. In each instance, the modern phase of metal carbonyl chemistry has r e l i e d on x-ray diffraction to determine not only bond lengths and angles, but even molecular formula; modern x-ray diffraction has become a technique of q u a l i t a t i v e as w e l l as quantitative analysis. Yet, there are cases where carbonyl ligands comprise such a small fraction of the t o t a l x-ray scattering power that even the number of CO ligands cannot be reliably detected (ÇjJ)* In addition, there are compounds 2

8

4

12

6

16

2

5

16

5

19

b

18

6

20

?

21

0097-6156/87/0357-0099506.00/0 © 1987 American Chemical Society


100


i n which d i s o r d e r f r u s t r a t e s a c c u r a t e c r y s t a l l o g r a p h y , compounds which ' ' r e f u s e ' to c r y s t a l l i z e .

as w e l l

as


1

A g a i n s t t h i s background, we have sought an a n a l y t i c a l procedure w h i c h r e l i a b l y ( i . e . q u a n t i t a t i v e l y ) c o n v e r t s l i g a n d s to g a s e o u s m o l e c u l e s which are then q u a n t i t a t e d by standard PVÏ measurements u s i n g a u t i l i t y vacuum l i n e (8). S i n c e a l l i d e a l gases occupy the same m o l a r volume r e g a r d l e s s of m o l e c u l a r w e i g h t , t h i s procedure i s p a r t i c u l a r l y advantageous f o r l i g h t w e i g h t l i g a n d s , where g r a v i m e t r i c p r o c e d u r e s s u f f e r t h e i r most a c u t e i n a c c u r a c i e s . T h u s , w h i l e the method has been i n t r o d u c e d i n the a b o v e p a r a g r a p h s f o r c a r b o n y l l i g a n d s , i t s a d v a n t a g e s a r e g r e a t e s t f o r the l i g h t e s t of a l l l i g a n d s , h y d r i d e ( l e a d i n g to H ), w h i c h a l s o s u f f e r from poor v i s i b i l i t y to x - r a y s b e c a u s e of t h e i r l o w s c a t t e r i n g power ( 9 ) . E x t e n s i o n of the m e t h o d o l o g y to o t h e r l i g a n d s w h i c h m i g h t be r e l e a s e d as a gas (e.g. N , C 0 , C H^, C 0^ "", BH^ , C 0 , etc.) i s l e f t to the i m a g i n a t i o n of the reader. 2

2

2

2

2

0

2

3

Method The method of e v o l u t i o n of a l i g a n d as a gas r e l i e s on the p r i n c i p l e t h a t r e m o v a l of e l e c t r o n s from a complex w i l l weaken m e t a l - l i g a n d bonds. W h i l e t h i s i s o b v i o u s i n the c a s e where the o x i d a t i o n removes m e t a l - l i g a n d bonding e l e c t r o n s (e.g. the d complexes WMe^ o r R e H y ( P R ) ) , i t can a l s o be t r u e when the e l e c t r o n s a r e more l o c a l i z e d on the m e t a l . Thus, to the e x t e n t t h a t the metal-carbonyl bond depends on b a c k b o n d i n g , r e m o v a l o f the d - e l e c t r o n s f r o m M (C0) w i l l weaken the M/C bond. S a i d i n a n o t h e r way, carbonyl l i g a n d s become i m p r o b a b l e f o r m e t a l s i n h i g h f o r m a l o x i d a t i o n states. For h y d r i d e l i g a n d s , o x i d a t i o n of a m e t a l / h y d r i d e complex w i l l a c h i e v e a s t a t e where the r e d u c i n g power of the h y d r i d e l i g a n d i s s u f f i c i e n t to cause i n t e r n a l e l e c t r o n t r a n s f e r to the ( o x i d i z e d ) m e t a l , p e r h a p s g e n e r a t i n g a c o o r d i n a t e d H m o l e c u l e ( E q u a t i o n 1). C o o r d i n a t e d m o l e c u l a r H i s known to be k i n e t i c a l l y l a b i l e , j u s t as 3

m

2

n

2

2

-Xe" L M (H) —* n

m

n

L M m

n + X

(H)

X + n

—» L M m

n + X

2

- (H) . (H ) n

2

X +

2

(1)

a r e the CO l i g a n d s i n an o x i d i z e d ( e s p e c i a l l y a p a r a m a g n e t i c ) c a r b o n y l complex. Thus, e v o l u t i o n of gaseous l i g a n d may be e x p e c t e d to ensue i n the presence of an abundance of incoming l i g a n d chosen to s t a b i l i z e a h i g h e r m e t a l o x i d a t i o n s t a t e . We h a v e f o u n d a c e t o n i t r i l e to be a s a t i s f a c t o r y s o l v e n t / l i g a n d , but t h i s c h o i c e might be p r o d u c t i v e l y a l t e r e d by o t h e r s as t h e i r p a r t i c u l a r system d i c t a t e s . A p p l i c a t i o n of the method t o p o l y h y d r i d e compounds has been d e s c r i b e d i n some d e t a i l , as has the d e t r i m e n t a l i n f l u e n c e of p r o t i c n u c l e o p h i l e s as s o l v e n t s or i m p u r i t i e s ( 9 ) . The c h o i c e of o x i d a n t remains. Our r e a s o n i n g has been t h a t we w i s h to a v o i d i n n e r s p h e r e e l e c t r o n t r a n s f e r r e a g e n t s s i n c e the group ( h a l i d e , o x a l a t e , etc.) w h i c h b r i d g e s d u r i n g e l e c t r o n t r a n s f e r can a l s o s t a b i l i z e the o x i d i z e d unknown a g a i n s t q u a n t i t a t i v e gas e v o l u t i o n . E x a m p l e s i n c l u d e c o n v e r s i o n of F e ( C 0 ) to F e ( C 0 ) ^ I . 5


2

4.1

LUNDQUIST AND CAULTON

Assay of Ligand-Derived Gases

Our work to date has employed FeCbipy)^ (bipy = 2 , 2 ' - b i p y r i d i n e ) , w i t h E°(MeCN) = 1.21 v. as w e l l as the w e a k e r o x i d a n t C p F e (E° « +0.55 v) b o t h v s . S.C.E. E a c h has the a d d i t i o n a l a d v a n t a g e t h a t i t undergoes a v i v i d c o l o r change on r e d u c t i o n (from deep b l u e , i n each c a s e , to deep r e d or p a l e y e l l o w , r e s p e c t i v e l y ) . H e r e a g a i n , the r e a d e r i s encouraged to seek a l t e r n a t i v e ( s u p e r i o r ) c h e m i c a l o x i d a n t s a c c o r d i n g to the d i c t a t e s o f the s y s t e m u n d e r s t u d y . A more g e n e r a l p r o c e d u r e w o u l d be t o use an e l e c t r o d e to e f f e c t " o x i d a t i v e s t r i p p i n g " o f l i g a n d as a gas, s i n c e the q u e s t i o n of o x i d a t i o n p o t e n t i a l i s more r e a d i l y u n d e r the c o n t r o l of t h e experimenter. +


2

Procedure The e x p e r i m e n t a l p r o c e d u r e c o n s i s t s of l o a d i n g a v a c u u m - t i g h t r e a c t i o n v e s s e l ( t y p i c a l dimensions, 3 X 20 cm) w i t h a magnetic s t i r b a r , 5-6 m o l e s of o x i d a n t (e.g. [Fe(bipy)

Experimental Organometallic Chemistry - ACS Publications

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