Organic and Main-Group Chemistry of the 2,4,6-Tris(trifluoromethyl

Chapter 19 Organic and M a i n - G r o u p Chemistry of the 2,4,6-Tris(trifluoromethyl)phenyl Substituent Kinetic Stabilization through Steric and Electronic Effects

Downloaded by MONASH UNIV on May 2, 2013 | http://pubs.acs.org Publication Date: April 29, 1994 | doi: 10.1021/bk-1994-0555.ch019

Frank T. Edelmann Institut für Anorganische Chemie, Universität Göttingen, Tammannstrasse 4, D—37077 Göttingen, Germany The organic and main group chemistry of the 2,4,6-tris(trifluoromethyl)phenyl substi tuent (= RF ) is reviewed. This unique ligand combines steric bulk with the possibility of electronic stabilization. In organic RF chemistry steric hindrance is an important factor guiding the reactivity of various substrates. Low coordination numbers around main group elements are effectively stabilized through the formation of short metal-fluorine interactions. Among other RF-derived substi tuents the thiolate anion RFS- promises to be a useful ligand for transition metals. During the past ten years the "Classical Double-Bond Rule" has been widely overcome through the successful preparation of stable species containing multiple bonds between heavier main group elements. The key principle leading to major achievements in this area of modern main group chemistry is kinetic stabilization ( i ) . It involves the use of sterically demanding substituents which protect the low-coordinate main group elements and thus inhibit dimerization or polymerization as well as attack of other reagents. A typical example is the diphosphene system. Simple substituted diphosphene derivatives, RP=PR (R = alkyl, aryl) are usually highly unstable. Early attempts to isolate these materials were frustrated by the formation of oligomerization products (i.e. cyclopolyphosphines, (RP) ) (2). Kinetic stabilization and hence formation of a stable monomer was achieved by introducing the very bulky 2,4,6-tri(fc-butyl)phenyl (= "supermesityl" or Mes*) substituent ( 3) , Yoshifuji's successful preparation of Mes* P=PMes* marks the beginning of vigorous research activities in the field of lowcoordinate main group compounds. Dicoordination around heavy main group elements has become a familiar sight in n

0097-6156/94/0555-0309$08.00/0 © 1994 American Chemical Society In Inorganic Fluorine Chemistry; Thrasher, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.


310

I N O R G A N I C F L U O R I N E C H E M I S T R Y : T O W A R D T H E 21ST C E N T U R Y

inorganic chemistry. Besides the diphosphene homologues R-P=As-R (4), R-As=As-R (5) and R-P=Sb-R ( 6) the i s o l a t i o n o f s t a b l e compounds c o n t a i n i n g P=C (7), Si=Si (#), Si=N (9) and Si=P (10) multiple bonds has been reported. K i n e t i c s t a b i l i z a t i o n allowed the s y n t h e s i s of numerous monomeric g e r m y l e n e s (11) and stannylenes (12). Even phosphaalkynes, R-C^p (13-15), (and to a lesser extent arsaalkynes, R-C=As (16)) have become a well i n v e s t i g a t e d c l a s s o f compounds. A s u b s t a n t i a l number o f sterically demanding ligands has been successfully e m p l o y e d . The wide r a n g e o f b u l k y s u b s t i t u e n t s includes έ-butyl, n e o p e n t y l , -CH2SiMe3, - C H ( S i M e 3 ) 2 , -C(SiMe3)3, mesityl, 2,4,6-tri(i-propyl(phenyl, 2 ,4,6-tri(t-butyl)phenyl and pentamethylcyclopentadienyl. The bis(trimethylsilyl)methyl substituent, -CH(SiMe3)2, has also been f o u n d t o be e x t r e m e l y u s e f u l f o r t h e r e a l i z a t i o n o f v e r y low c o o r d i n a t i o n numbers a r o u n d f - e l e m e n t s . T r i c o o r d i n a t e h o m o l e p t i c a l k y l s o f t h e t y p e M[CH(SiMe3 )2]3 have been reported for uranium(III) (17) and several lanthanide elements (18). Bulky amido l i g a n d s such as -N(SiMe3)2 , - N ( S i M e 2 P h ) 2 and -NMes(BMes2 ) have been u s e d in the preparation of dicoordinate d - t r a n s i t i o n metal derivatives (19). D u r i n g t h e p a s t t h r e e y e a r s t h e game has become e v e n more e x c i t i n g t h r o u g h the appearance of a new player: 2,4,6-tris(trifluoromethyl(phenyl (= RF ) . T h i s s u b s t i t u ent i s not j u s t a n o t h e r s t e r i c a l l y demanding l i g a n d . I t adds a new dimension to s t e r i c bulk: E l e c t r o n i c s t a b i l i z a t i o n . Due to the p o s s i b i l i t y of forming short metalf l u o r i n e c o n t a c t s t o t h e ortho-CFz g r o u p s o f R F , an a d d i tional stabilizing effect i s imposed on low-coordinate main g r o u p e l e m e n t s . I t i s t h e c o m b i n a t i o n o f s t e r i c and electronic stabilization which makes 2,4,6-tris(tri f l u o r o m e t h y l )phenyl truly u n i q u e among a l l o t h e r bulky s u b s t i t u e n t s r e p o r t e d so f a r . The c h e m i s t r y o f t h e 2,4,6tris(trifluoro-methyl)phenyl substituent has recently been c o m p i l e d i n a review a r t i c l e (20). In a d d i t i o n to some i m p o r t a n t earlier results this article therefore f o c u s s e s on some new d e v e l o p m e n t s i n t h i s a r e a .

Preparation of

the

Starting Materials:

RFH

and

RFLÎ

The e a r l i e s t r e p o r t s on t h e p a r e n t fluorocarbon, 1,3,5t r i s ( t r i f l u o r o m e t h y l ( b e n z e n e (= R F H ) , d a t e b a c k t o 1947, when McBee and Leech described i t s synthesis v i a fluorination of 1, 3,5-(CC13 (3CeH3 (21). 1, 3 , 5 - T r i s ( t r i f l u o r o m e t h y l ( b e n z e n e was " r e d i s c o v e r e d " by Chambers e t a l . , who r e p o r t e d the f i r s t simple s y n t h e s i s o f R F H (22). It was obtained i n 33% yield by fluorination of commercially available benzene-1,3,5-tricarboxylic acid with SF4 at elevated temperatures. Reinforcement of the original reaction conditions subsequently led to a significant improvement o f t h e p r o d u c t y i e l d . A 90-95% i s o l a t e d y i e l d of 1,3,5-tris(trifluoromethyl(benzene is reproducibly

In Inorganic Fluorine Chemistry; Thrasher, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

19. EDELMANN

2,4,6-Tris(trifluoromethyl)phenyl Substituent

311

obtained when benzene-1,3,5-tricarboxylic acid is heated with excess SF4 at 170°C for 48 h (23) (Equation 1 ) :


CeH3(COOH)3

6 SF4 -— 170°C, 48h

>

(CFabCeHs

(1)

The physical and spectroscopic properties of 1,3,5(CF3 ) 3 C e H 3 have been determined (21-24), Since various fluoro-carbons are known to be rather toxic (25), R F H should be handled with great care u n t i l the physiological properties have been studied more thoroughly. Derivatization of 1,3,5-tris(trifluoromethyl)benzene generally starts with ring metalation using nbutyllithium as described by Chambers et a l . in 1987 ( 22) (Equation 2 ) : n-BuLi (CF3 )3CeH3

—>

(CF3)3CeH2Li (= R F L I )

(2)

The formation of 2,4,6-tris(trifluoromethyl)phenyllithium (= R F L I ) occurs readily in diethylether at reflux temperature. Quenching of the reaction mixture with CH3OD after 1 h resulted in the formation of the monodeuterated derivative in 90% y i e l d . However, isolated yields of main group compounds derived from the RF Li intermediate are often considerably lower. The metalation reaction works equally well on a mmol-scale or with 0.1 0.2 mol of 1,3,5-tris(trifluoromethyl)benzene. I t has been found very convenient to prepare R F L I in situ and use the resulting solutions in diethylether/hexane without any further purification for subsequent reactions. I t i s , however, possible to isolate a pure crystalline diethylether adduct, [RF L i · E t 2 Ο ] 2 (26). Evaporation of the 1,3,5- (CF3 )3 Ce H3 //2-BuLi solutions to dryness and recrystallization of the o i l y residue from hexane yields colorless crystals of [RFLi-Et20]2 in moderate y i e l d . Curiously, very pure [RFLi-Et20]2 has been obtained during attempts to prepare (RF)2Ca and (RF)2Ba (27). Treatment of in situ prepared R F L I with either anhydrous C a C l 2 or B a l 2 resulted in the formation of yellow solutions, from which [RFLi-Et20]2 was isolated as the only product by c r y s t a l l i z a t i o n from hexane. Caution: Solid [RFLi-Et20]2 decomposes violently upon contact with protic solvents like ethanol or acetone and even occasional explosions have been reported. The crystalline compound should only be prepared in small quantities and always be handled with great care! The dimeric nature of [RFLi-Et20]2 was revealed by a lowtemperature X-ray structure determination (26) (Figure 1). This structure i s already a striking example o f e l e c t r o n i c s t a b i l i z a t i o n i n RF c h e m i s t r y : The d i m e r s a r e


INORGANIC FLUORINE CHEMISTRY: TOWARD T H E 21ST CENTURY


312


19.

EDELMANN

2,4,6-Tris(tnfluoromethyl)phenyl Substituent

313


h e l d t o g e t h e r by l i t h i u m - f l u o r i n e i n t e r a c t i o n s . The coord i n a t i o n environment around l i t h i u m i s t h a t of a d i s t o r t e d t r i g o n a l b i p y r a m i d . E a c h l i t h i u m atom i s c o o r d i n a t e d by the ipso-carbon atoms of the phenyl rings, two f l u o r i n e atoms f r o m ortho-CFs g r o u p s and an o x y g e n atom of a diethylether ligand. Obviously the four Li-F i n t e r a c t i o n s have t o be c o n s i d e r e d t h e main stabilizing f a c t o r i n the d i m e r i c [RFLi'Et20]2 m o l e c u l e . Attempts to eliminate lithium fluoride from R F L Î and to trap the r e s u l t i n g d i r a d i c a l i n t e r m e d i a t e u s i n g f u r a n have f a i l e d (22). S o l i d R F L Î showed a r e m a r k a b l e t h e r m a l stability when h e a t e d i n a s e a l e d t u b e a t 90°C. A d d i t i o n o f furan in a similar experiment did not yield an isolable trapping product. A p p a r e n t l y no a t t e m p t s have been made t o i s o l a t e RF derivatives of the heavier a l k a l i n e metals, RFM ( M = Na,K,Rb,Cs), a l t h o u g h t h e s e o r g a n o m e t a l l i c s a r e a n t i c i p a ted to e x h i b i t interesting structural properties. Other attempts to d e r i v a t i z e 1,3,5-tris(trifluoromethyl)benzene have f a i l e d so f a r , b e c a u s e t h e t h r e e CF3 g r o u p s strongly d e a c t i v a t e the p h e n y l r i n g toward e l e c t r o p h i l i c s u b s t i t u t i o n . A c c o r d i n g l y , no r e a c t i o n was observed with acetyl c h l o r i d e and a n h y d r o u s AICI3 (28).

O r g a n i c C h e m i s t r y w i t h Rr A number o f c a r b o n - c a r b o n bond f o r m a t i o n r e a c t i o n s have been s t u d i e d i n d e t a i l by Chambers ( 22) and Filler ( 28) . Some of these transformations show that significant steric hindrance is imposed by the 2 , 4 , 6 tris(trifluoromethyl(phenyl substituent. Steric effects became especially apparent in the failure of several attempts to prepare the acetophenone d e r i v a t i v e R F C ( 0 ) C H 3 and the related vinyl ketone RFC(0)CH=CH2 (28). The elusive 2,4,6-tris(trifluoromethyl)acetophenone is apparently formed when RFLÎ is treated with acetyl chloride. Initially formed RFC(0)CH3 undergoes rapid hydrogen-metal exchange with unreacted RFLÎ and the r e s u l t i n g l i t h i u m enolate reacts with another equivalent of acetyl chloride to give the 1,3-diketone R F C ( O ) CH2C(0)CH3 as the only isolable product. This 1 , 3 d i k e t o n e a p p e a r s t o be an i n t e r e s t i n g compound i n itself as a c h e l a t i n g l i g a n d f o r t r a n s i t i o n m e t a l s , but so far no e x p e r i m e n t s i n t h a t d i r e c t i o n have been r e p o r t e d . Other attempts to prepare 2,4,6-tris(trifluorom e t h y l ( a c e t o p h e n o n e have a l s o been met only with very l i m i t e d success (28): 1 ) No reaction was o b s e r v e d when 1,3,5-tris(trifluoromethyl)-benzene was treated with a c e t y l c h l o r i d e i n t h e p r e s e n c e o f a n h y d r o u s AICI3 ( vide supra). 2) The secondary alcohol RFCH(0H)CH3 can be prepared from RFLÎ and acetaldehyde. However, it was found impossible to oxidize this alcohol with Jones r e a g e n t ( C r ) . A l t h o u g h IR s p e c t r a showed some e v i d e n c e V I



314

INORGANIC FLUORINE CHEMISTRY: TOWARD THE 21ST CENTURY

for the formation of carbonyl-containing species, the ketone c o u l d n o t be i s o l a t e d . 3) A g e n e r a l route to ketones i n v o l v e s treatment of lithium carboxylates with alkyl or aryl lithium reagents. The c a r b o x y l i c acid RFCOOH (vide infra) c l e a n l y reacts with methyllithium to give the corresponding lithium salt, but the subsequent r e a c t i o n w i t h e x c e s s CH3 L i d i d n o t l e a d t o t h e f o r m a t i o n of 2 , 4 , 6 - t r i s ( t r i f l u o r o m e t h y l ) a c e t o p h e n o n e . The n o n - r e a c t i v i t y o f R F C O O L Î t o w a r d s m e t h y l l i t h i u m was a t t r i b u t e d t o severe steric hindrance i n the r e q u i s i t e tetrahedral intermediate RFC(OLi)zCH3. The case o f 2 , 4 , 6 - t r i s ( t r i f l u o r o m e t h y l l a c e t o p h e n o n e has been d i s c u s s e d i n some d e t a i l i n o r d e r t o d e m o n s t r a t e t h a t s o m e t i m e s RF c h e m i s t r y c a n be q u i t e d i f f e r e n t from that of less sterically hindered aromatic compounds. S i m i l a r p r o b l e m s a s i n t h e c a s e o f R F C ( 0 ) C H 3 were e n c o u n t e r e d i n an attempted p r e p a r a t i o n o f a r e l a t e d a r y l v i n y l ketone, RFC(0)CH=CH2 (28). When R F L I was r e a c t e d with a c r y l o y l c h l o r i d e , CH2=CH-C0C1, a m i x t u r e o f two k e t o n e s was o b t a i n e d . One o f them seemed t o be R F C ( 0 ) C H = C H 2 . The other ketone, RF C(Ο)CH2 CH2 RF , r e s u l t e d from a Michael a d d i t i o n between R F C ( 0 ) C H = C H 2 and t h e s t r o n g n u c l e o p h i l e R F L I (Equation 3). RFLI

+ CH2=CHC(0)Cl

>

RF-C(Ο)-CH=CH2 +

RF-C(0)-CH2CH2-RF

(3)

In view o f t h e s e r e s u l t s i t i s n o t s u r p r i s i n g t h a t e v e n more s t e r i c a l l y h i n d e r e d c a r b o n y l d e r i v a t i v e s o f RF a r e a l s o n o t a c c e s s i b l e (22). An a t t e m p t e d s y n t h e s i s o f ( R F ) 2 C 0 f r o m R F L I and d i m e t h y l c a r b o n a t e gave t h e m e t h y l ester of 2,4,6-tris(trifluoromethyl)benzoic acid instead (25% yield). Similarly, treatment of RFLI with oxalyl d i c h l o r i d e d i d not y i e l d the desired b e n z i l d e r i v a t i v e . In t h i s r e a c t i o n a 45% y i e l d of 2,4,6-tris(trifluoro methyl ) -2-oxoethanoic acid, RFC(0)COOH, was obtained (22). A l lthese experiments c l e a r l y demonstrate t h a t i n organic RF c h e m i s t r y steric hindrance i s an important factor guiding the r e a c t i v i t y of various substrates. In c o n t r a s t t o t h e u n s u c c e s s f u l syntheses o f v a r i o u s RF-derived ketones, the preparation of 2,4,6-tris(tri fluoromethyl )benzoic a c i d i s simple and s t r a i g h t f o r w a r d (22,28,29). I t c a n be o b t a i n e d i n 80% y i e l d by r e a c t i o n o f R F L I w i t h c a r b o n d i o x i d e and s u b s e q u e n t a c i d i f i c a t i o n . P u r e c r y s t a l l i n e R F C O O H m e l t s a t 1 9 0 - 1 9 1 ° C and h a s a p K a o f 3.0, r e n d e r i n g i t more a c i d i c t h a n m e s i t o i c a c i d ( p K a 3.8). The CF3 g r o u p s i n R F C O O H a r e v e r y s t a b l e a g a i n s t hydrolysis. Prolonged heating (24h) with either c o n c e n t r a t e d NaOH o r 96% H2SO4 l e d t o a n e a r q u a n t i t a t i v e recovery of unreacted RFCOOH (28). 2,4,6-Tris(tri f l u o r o m e t h y l ) b e n z o i c a c i d d e s e r v e s some i n t e r e s t b e c a u s e t h e c a r b o x y l g r o u p i s e x p e c t e d t o be i n a p e r p e n d i c u l a r arrangement with respect to the aromatic ring. Thus



19.

EDELMANN

2,4,6-Tns(tnfluoromethyl)phenyl Substituent

315

c o n j u g a t i o n between t h e two ιτ-systems i s e x p e c t e d t o be n e g l i g i b l e . V e r i f i c a t i o n o f t h i s hypothesis through X-ray a n a l y s e s seems h i g h l y d e s i r a b l e . A g a i n , due t o s t e r i c h i n d r a n c e , R F C O O H was r e p o r t e d t o be v e r y r é s i s t e n t t o a t t a c k by t h i o n y l c h l o r i d e . O n l y a small yield (10-15%) of R F C ( 0 ) C 1 was o b t a i n e d after h e a t i n g t h e m i x t u r e a t 7 0 ° C f o r 4 8 h (28). I n o u r hands t h e y i e l d o f a c i d c h l o r i d e was 32% (29). Pure R F C ( 0 ) C 1 i s a b e a u t i f u l l y c r y s t a l l i n e , low m e l t i n g ( 3 9 ° C ) s o l i d w h i c h i s q u i t e résistent t o h y d r o l y s i s . I t i s i n t e r e s t i n g t o note that the corresponding acid f l u o r i d e , R F C ( 0 ) F , was obtained earlier by a d i f f e r e n t route (25,30). 2,4,6T r i s ( t r i f l u o r o m e t h y l ( b e n z o y l f l u o r i d e was p r e p a r e d i n 78% y i e l d by f l u o r i n a t i o n o f benzene-1,2,3,5-tetracarboxylic a c i d w i t h S F 4 . R F C ( 0 ) F s e r v e d a s a u s e f u l s t a r t i n g material i n the synthesis of 2,4,6-tris(trifluoromethyl)aniline, RFNH2 , which i s not a c c e s s i b l e by more conventional routes. The p r e p a r a t i o n o f R F N H 2 involves conversion of the acid fluoride t o t h e amide and subsequent Hoffmann conversion t o t h e amine (overall yield 56%) (25,30). The c h e m i s t r y of RFNH2 i s largely undeveloped and c e r t a i n l y m e r i t s f u r t h e r a t t e n t i o n . The m e t h y l e s t e r R F C O O C H 3 was o b t a i n e d serendipitously i n 25% y i e l d from R F L Î and d i m e t h y l carbonate

(vide

supra)

(22).

Normal

acid-catalyzed

esterification

o f R F C O O H was f o u n d t o be u n s u c c e s s f u l (28). A s i m i l a r behavior was r e p o r t e d earlier f o r mesitoic acid. Once a g a i n s t e r i c c r o w d i n g by t h e ortho s u b s t i t u e n t s was made responsible f o r t h e n o n - a b i l i t y t o form a t e t r a h e d r a l t r a n s i t i o n s t a t e r e q u i r e d f o r t h e e s t e r f o r m a t i o n . The problem c a n be c i r c u m v e n t e d by " s t e r i c a c c e l e r a t i o n " , i.e. formation of a sterically less crowded linear acylium i o n . In the case o f R F C O O H the corresponding acylium ion RFCO i s significantly destabilized by structure C o f t h e resonance hybrid. Nevertheless, generation of R F C 0 f r o m R F C O O H a n d 96% H 2 S 0 4 and s u b s e quent r e a c t i o n w i t h e t h a n o l allowed the preparation of ethyl ester R F C O O C 2 H 5 i n low y i e l d (28). A l t e r n a t i v e l y , t h e a c y l i u m i o n c a n be g e n e r a t e d v i a a m i x e d a n h y d r i d e o f RFCOOH and t r i f l u o r o a c e t i c acid, RFC(0)0C(0)CF3. Thus, t h e h i g h l y crowded p h e n o l i c e s t e r RF C ( Ο ) O C 6 H 2 ( C H 3 ) 3 - 2 , 4 , 6 was p r e p a r e d i n 44% y i e l d by h e a t i n g a m i x t u r e o f R F C O O H , 2,4,6-trimethylphenol and t r i f l u o r o a c e t i c a n h y d r i d e (28). No f u r t h e r r e a c t i o n s o f R F C O O H have been reported and no m e t a l s a l t s o r c o m p l e x e s have b e e n f u l l y c h a r a c t e r i z e d . Thus f a r t h e d e r i v a t i v e c h e m i s t r y o f R F C ( 0 ) C 1 a l s o r e m a i n s l a r g e l y u n e x p l o r e d . O r i g i n a l l y i t was a n t i c i p a t e d t h a t R F C ( 0 ) C 1 would be t h e i d e a l starting material f o r the synthesis of the hitherto unknown phosphaalkyne R F C = P . I n i t i a l e x p e r i m e n t s have shown however, t h a t t h e p r e p a r a t i o n o f t h i s m a t e r i a l i s n o t s t r a i g h t f o r w a r d and f u r t h e r s t u d i e s a r e r e q u i r e d . The o n l y d e f i n i t e r e s u l t s o far i s the formation of a lithium diacylphosphide, +

+

+

LI RFC(0)PC(0)RF-,

from

RFC(0)C1

and L i P ( S i M e 3 ) 2


(31).

316


Y e l l o w c r y s t a l l i n e L i RF C ( Ο ) P C ( Ο ) RF " was obtained i n 24% yield and fully characterized by spectroscopic methods. A l t h o u g h t h i s d i a c y l p h o s p h i d e a n i o n seems t o be an attractive chelating ligand f o r t r a n s i t i o n s metals, the r e a c t i v i t y of L i R F C ( 0 ) P C ( 0 ) R F " towards metal h a l i d e s has n o t y e t been i n v e s t i g a t e d . The only i n i t i a l experi ment showed t h a t L i RF C ( 0 ) PC ( Ο ) RF ~ did not react with ( C s M e s ) T i C l 3 , but f u r t h e r work i s r e q u i r e d t o i n v e s t i g a t e the l i g a n d p r o p e r t i e s o f the R F C ( 0 ) P C ( 0 ) R F ~ anion. Another p o t e n t i a l l y i n t e r e s t i n g l i g a n d f o r transi t i o n metals i s the recently reported dithiocarboxylate anion R F C S S " (32). Treatment of in situ prepared RFLI w i t h excess carbon d i s u l f i d e r e s u l t s i n the f o r m a t i o n of a r e d - b r o w n s o l u t i o n , f r o m w h i c h R F C S S H c a n be i s o l a t e d after acidification (55% yield). Pure 2,4,6-tris( t r i f l u o r o m e t h y l ) d i t h i o b e n z o i c a c i d forms a d a r k r e d oily liquid. Orange crystalline [Cs Hi ο NH2]* RF C S S " has been p r e p a r e d by n e u t r a l i z i n g t h e f r e e a c i d w i t h p i p e r i d i n e i n p e n t a n e s o l u t i o n (32). The c o o r d i n a t i o n chemistry of the R F C S S " anion s t i l l awaits f u r t h e r i n v e s t i g a t i o n . Yet another anion formally derived from 2 , 4 , 6 t r i s ( t r i f l u o r o m e t h y l ) b e n z o i c a c i d has a l r e a d y been p r o v e n to be a useful chelating ligand. The very highly s u b s t i t u t e d b e n z a m i d i n a t e a n i o n [ R F C ( N S i M e 3 ) 2 ] " has been synthesized in one step by adding RFLI to bis(trimethylsilyl)carbodiimide (33). L i [RF C(NSiMe3 )z ] i s o b t a i n e d in 8 0 % yield in the form of large, colorless, transparent crystals after r e c r y s t a l l i z a t i o n f r o m h e x a n e . The p r e s e n c e o f two bulky t r i m e t h y l s i l y l substituents i n a d d i t i o n t o t h e CF3 groups is responsible f o r the high s o l u b i l i t y of the lithium benzamidinate derivative in unpolar solvents such as pentane or hexane. Li[RFC(NSiMe3)2] is also quite v o l a t i l e and slowly s u b l i m e s a t room t e m p e r a t u r e under m o d e r a t e vacuum. The [RFC(NSiMe3)2]" a n i o n r e p r e s e n t s the l i m i t of s t e r i c s a t u r a t i o n . A c c o r d i n g l y Li[RFC(NSiMe3)2] does not r e a c t w i t h M e 3 S i C l to g i v e the f u l l y silylated benzamidine derivative RF C(=NSiMe3 )[N(SiMe3 )z ]. The 2,4,6-tris(trifluoromethyl)-N,N'-bis(trimethylsilyl)benz amidinate anion e x h i b i t s i n t e r e s t i n g l i g a n d properties i n coordination compounds with lanthanide and actinide e l e m e n t s (33). As p a r t o f a g e n e r a l s t u d y on f-element complexes c o n t a i n i n g bulky chelating ligands, we have demonstrated that silylated benzamidinate anions of the type [RC6H4C(NSiMe3)2J" can be regarded as "steric cyclopentadienyl equivalents" (33). The cone a n g l e o f the silylated benzamidinate is comparable with that of cyclopentadienyl and v i r t u a l l y a l l d i f f e r e n t types of f element c y c l o p e n t a d i e n y l c o m p l e x e s have a n a l o g u e s i n t h e benzamidinate chemistry of these elements. Typical examples are compounds like [RCeH4C(NSiMe3 ) 2 ] 3 L n (34), [ R C e H 4 C ( N S i M e 3 )2 J3UC1 (33), [ RCe H4 C (NSiMes )z ] 3 UMe (35) or [RCeH4C(NSiMe3)2]2Ln(THF)2 (36), which correspond to the well-known f-element cyclopentadienyl complexes Cp3Ln, +

+


+



19. EDELMANN

2,4,6-Tris(trifluoromethyl)phenyl Substituent 317

C p 3 U C l , Cp3UMe and C p 2 L n ( T H F ) 2 (37). The i n t r o d u c t i o n o f ortho s u b s t i t u e n t s i n the phenyl ring should signifi cantly increase the steric bulk of the benzamidinate a n i o n s . I t was t h e r e f o r e a n t i c i p a t e d t h a t the reactivity of the s t e r i c a l l y demanding [RFG(NSiMe3 )2 ]~ a n i o n would resemble t h a t o f the p e n t a m e t h y l c y c l o p e n t a d i e n y l ligand. The experimental results have revealed that this is e x a c t l y the case. Towards l a n t h a n i d e and a c t i n i d e ions the s t e r i c a l l y demanding [RFC(NSiMe3)2]" l i g a n d behaves j u s t l i k e Cp* . G i v e n u r a n i u m and t h o r i u m as an example, Marks e t a l . have shown t h a t U C I 4 and ThCl4 r e a c t with two o r more e q u i v a l e n t s o f a Cp* t r a n s f e r r e a g e n t t o g i v e exclusively Cp*2UCl2 and Cp*ThCl2 resp. as the only i s o l a b l e products (38). Trisubstitution, i.e. formation o f Cp*3UCl or Cp*3ThCl, was not observed. A striking similarity was found for the analogous reactions of L i [ RF C ( NSiMe3 )2 ] w i t h UC14 and ThCl4 . I n b o t h c a s e s the disubstituted products [RFC(NSiMe3)2]2AcCl2 (Ac = U,Th) were i s o l a t e d . T h e r e was no e v i d e n c e f o r t h e f o r m a t i o n o f trisubstituted derivatives [RFC(NSiMe3)z ] 3 A c C l . The a c t i n i d e b e n z a m i d i n a t e complexes [RFC(NSiMe3)2]2UCI2 and [ R F C ( N S i M e 3 ) 2 ] 2 T h C l 2 have been f u l l y c h a r a c t e r i z e d by Xr a x c r y s t a l l o g r a p h y (33). The l i g a n d e n v i r o n m e n t i n t h e s e formally six-coordinate actinide complexes is very s i m i l a r as i n t h e b e n t m e t a l l o c e n e s C p * 2 A c C l 2 (Ac = U,C1) (Figure 2). Further evidence f o r the steric similarity between Cp* and the bulky benzamidinate ligand [RFC(NSiMe3)2]~ came f r o m l a n t h a n i d e c h e m i s t r y . U n t i l v e r y r e c e n t l y , Cp* was the only ligand known to form complexes of the g e n e r a l type L 2 L n ( μ - C l ) 2 L i ( T H F ) 2 , i n which l a n t h a n i d e and l i t h i u m i o n s a r e b r i d g e d by two h a l i d e l i g a n d s t o g i v e a four-membered r i n g s y s t e m (39). S u b s t i t u t i o n r e a c t i o n s o f anhydrous lanthanide trichlorides with LiCp* in THF solution usually yield complexes of the composition C p * 2 L n ( u - C l ) 2 L i ( T H F ) 2 . The c o o r d i n a t i o n o f l i t h i u m h a l i d e i l l u s t r a t e s the tendency of the l a n t h a n i d e i o n s t o adopt h i g h f o r m a l c o o r d i n a t i o n numbers. A v e r y s i m i l a r complex was synthesized by using the "steric Cp* equivalent" [ R F C ( N S i M e 3 )z ]~ . Treatment of anhydrous neodymium t r i c h l o r i d e w i t h two e q u i v a l e n t s o f L i [ R F C ( N S i M e 3 ) 2 ] gave exclusively the d i s u b s t i t u t e d product [ R F C ( N S i M e 3 )i ] 2 Nd(y-Cl)2Li(THF)2 (40) ( E q u a t i o n 4 ) . NdCl3(THF)2

+

2 L i [ RFC(NSiMe3 )z ]

THF >

[ R F C(NS iMe3 )2 ] 2 N d ( μ - C l h L i ( T H F ) z

(4)

-LiCl Once again the X-ray structure determination revealed a striking analogy between the neodymium benzamidinate and the bent metallocene derivative Cp* Nd(u-Cl)2Li(THF)2 (Figure 3). 2


318



F i g u r e 3. M o l e c u l a r s t r u c t u r e Nd(u-Cl) Li(THF)2.

o f [ R F C ( N S i M e 3 )2]z-

2


19. EDELMANN

2,4,6-Tris(trifluoromethyl)phenyl Substituent

319

The most r e c e n t a d d i t i o n t o t h e f a m i l y o f o r g a n i c RF derivatives i s 2,4,6-tris(trifluoromethyl)benzyl alcohol, R F C H 2 0 H (41). This seemingly unspectacular compound was p r e p a r e d by a s t a n d a r d p r o c e d u r e f r o m R F L I and ( C H 2 0 ) n . RF CH2 OH was i s o l a t e d i n low y i e l d (15%) a s a l o w - m e l t i n g c r y s t a l l i n e s o l i d . What makes R F C H 2 OH i n t e r e s t i n g i s i t s unusual crystal structure. S i x RFCH2OH molecules are connected v i a hydrogen b r i d g e s . The r e s u l t i n g h e x a m e r i c a r r a y resembles the c h a i r c o n f o r m a t i o n of a "supercyclohexane" ( F i g u r e 4 ) . The unexpected structure of RF CH2 OH clearly d e m o n s t r a t e s t h a t more work i s needed t o e l u c i d a t e t h e m o l e c u l a r s t r u c t u r e s o f " s i m p l e " o r g a n i c RF d e r i v a t i v e s .


M e t a l D e r i v a t i v e s c o n t a i n i n g RF a n d R r S L i g a n d s M a i n g r o u p m e t a l d e r i v a t i v e s c o n t a i n i n g RF , R F O and RF S l i g a n d s h a v e been t h e main t o p i c o f t h e f i r s t review article on RF c h e m i s t r y (20). Therefore o n l y a few o f t h e s e r e s u l t s w i l l be m e n t i o n e d h e r e i n a d d i t i o n t o some new developments in that area. Perhaps the most s i g n i f i c a n t r e s u l t i n RF c h e m i s t r y was t h e s y n t h e s i s o f t h e f i r s t d i a r y l p l u m b y l e n e , ( R F ) 2 P b (42). Bright yellow, crystalline (RF)2Pb c a n be p r e p a r e d by r e a c t i n g PbCl2 with two equivalents of RFLI in diethyl ether/hexane s o l u t i o n . In c o n t r a s t t o p r e v i o u s l y r e p o r t e d r e a c t i o n s o f PbCl2 with a r y l l i t h i u m or aryl Grignard reagents, the formation of (RF>2Pb is very clean and no disproportionation is observed. An X-ray crystal s t r u c t u r e a n a l y s i s r e v e a l e d t h a t ( R F ) 2 P b i s monomeric i n the s o l i d s t a t e . T h i s compound i s a n o t h e r good example for t h e e l e c t r o n i c s t a b i l i z a t i o n o f l o w - c o o r d i n a t e main group elements induced by t h e RF substituent. Four intramolecular Pb-F c o n t a c t s contribute to the unusual s t a b i l i t y of the diarylplumbylene (Figure 5). Disappointingly, but not s u r p r i s i n g l y ( i n e r t pair e f f e c t ! ) , ( R F ) 2 P b i s v e r y r e l u c t a n t t o undergo o x i d a t i v e addition reactions ( 32) . So f a r i t was n o t p o s s i b l e t o p r e p a r e any l e a d ( I V ) derivatives containing RF ligands ( e . g . ( R F ) 2 . P b C l 2 o r ( RF )2 Pb ( SPh )2 ). However, i t was f o u n d t h a t ( R F ) 2 P b c a n s e r v e as a u s e f u l s t a r t i n g m a t e r i a l i n the preparation of unsolvated lead(II) thiolates. The reaction of (RF>2Pb with two equivalents of 2 , 4 , 6 tris(trifluoromethyl)benzenethiol, RFSH, in hexane s o l u t i o n provided bright yellow crystalline (RFS)2Pb i n h i g h y i e l d (42). The o n l y b y - p r o d u c t i n t h i s r e a c t i o n i s the volatile fluorocarbon 1, 3 , 5 - t r i s ( t r i f l u o r o m e t h y l ) b e n z e n e . Thus t h e method a p p e a r s t o be an e l e g a n t way o f making lead(II) alkoxides or thiolates and deserves further elucidation. Unsolvated (RrS)2Pb has so f a r r e s i s t e d a l l attempts to o b t a i n s u i t a b l e s i n g l e c r y s t a l s for an X-ray analysis. In one occasion repeated crystallization from toluene produced large, yellow block-like crystals. An X-ray structure determination



320


Figure

4. M o l e c u l a r

Figure

structure

5. M o l e c u l a r

o f hexameric

structure

of

[RFCH2 0H]6.

(RF)2PD.

r e v e a l e d t h a t t h i s m a t e r i a l was a new compound r e s u l t i n g from c o n t a m i n a t i o n o f the o r i g i n a l l e a d ( I I ) t h i o l a t e w i t h s m a l l amounts o f oxygen. T h i s r e s u l t e d i n t h e f o r m a t i o n of the unusual oxygen-centered lead thiolate cluster PD5 0(SRF)8

(43)

(Figures

6 and

7).


19. EDELMANN

2,4,6-Tris (trifluoromethyl)phenyl Substituent

321


0-(

Figure

6.

Molecular

structure of PD5 0 ( S R F ) S .

F i g u r e 7. C o o r d i n a t i o n p o l y h e d r o n in PD5 0 ( S R F ) S showingtwo loosely coordinated toluene molecules (RF s u b s t i t u e n t s o m i t t e d f o r c l a r i t y ; d i s o r d e r a t s u l f u r atom S8).


322


Generally the chemistry of metal complexes containing RFS ligands is already fairly extensive. Starting point is 2,4,6-tris(trifluoromethyl)benzenet h i o l , R F S H , w h i c h was f i r s t d e s c r i b e d by Chambers et a l . in 1 9 8 7 (22). R F S H i s i s o l a t e d as an o i l y l i q u i d w i t h a p u n g e n t o d o r . The acidity of R F S H i s so high that it f o r m s a s t a b l e ammonium t h i o l a t e . N H 4 R F S " i s o b t a i n e d in high y i e l d as a w h i t e p r e c i p i t a t e when R F S H i s t r e a t e d w i t h HN

Organic and Main-Group Chemistry of the 2,4,6-Tris(trifluoromethyl

Recommend Documents