79 Phosphaalkenes, R C=PR', and Phosphaalkynes, 2
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H . W. ΚROTO and J . F. N I X O N School of Molecular Sciences, University of Sussex, Brighton, Sussex, England
The chemistry of trivalent phosphorus compounds in which phosphorus is one or two coordinate is rapidly developing. These systems contain sp or sp hybridized phosphorus and multiple π-bonds between Ρ and other elements; and until 1964 it was believed that bond formation involving ρπ-ρπ overlap was unfavourable. Subsequently certain -P=C systems resulted from the use of charged (1, 2, 3) and or delocalized systems (4) but it is only in the past few years that successful syntheses of phospha-alkenes, RC=PR', and phospha-alkynes, RC≡p, have been reported. These novel compounds are the subject of this paper. 2
2
Pyrolysis techniques developed by Kroto et al. to produce sulphur and selenium doubly bonded species in a low pressure flow system where the lifetimes are of the order of seconds, enabled species such as HCS (5), FCS (6), CHCHS (7), CHCHSe (8), CHCCS (9) and HBS (10) to be identified by microwave and/or P.E. spectroscopy. 2
2
3
3
2
An extension of this approach led to the study of some simple molecules in which trivalent phosphorus is bonded to carbon by a double or a triple bond. These spectroscopic experiments have not only yielded conclusive identification but a wealth of molecular information such as geometric and electronic structural data. More importantly, the >C=P- and -C≡P moieties could be considered as viable functional groups with interesting and well defined chemical properties. In 1961 Gier (11) reported the s y n t h e s i s o f HCEp v i a a carbon arc discharge o f P H 3 . The compound which polymerised above -120 was long regarded as a chemical c u r i o s i t y b u t i n 1968 Kroto, Nixon e t a l . (12) reported the f a c i l e synthesis o f FC=P by dehydrofluorination of C F 3 P H 2 . CF3PH2
• CF =PH 2
• FC=P
NMR, microwave and He(I) p h o t o - e l e c t r o n s p e c t r o s c o p i c s t u d i e s on "this compound have been reported (12, 1_3) . 0097-615 6/81/0171-03 8 3$05.00/ 0 © 1981 American Chemical Society
Quin and Verkade; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
384
PHOSPHORUS
A whole new c l a s s o f compounds o f the type RC=P now some o f which are l i s t e d below (14-22). HCEP
FCEP
NCCEP e.g.
PhCEP
RCH PC1 R CF PH 2
2
•
CNN
HCP
â
3
R
f
3
2
• RCEP c
E
exists,
CH CEP
CH =CHCEP
2
2
f
CF3CΞP
CHEMISTRY
CHECCEP
p
_
* NCC=P
Very r e c e n t l y (February 1981) a s i g n i f i c a n t advance has been made with the syntheses o f Me3SiC=p (^ l i f e 50 min) (Appel e t a l . ) (22) and the s t a b l e l i q u i d t-BuCEp (Becker e t a l . ) (23) by the novel route shown below. • R3S1—Ρ
(R Si) P 3
3
y
OS1R3
¥ tBuCΞp
Our r e c e n t syntheses o f CH =CH-C=p and HC^c-C=p u s i n g the HX e l i m i n a t i o n approach has i n d i c a t e d t h a t many more RC=P compounds w i l l be soon a v a i l a b l e . Our s t r u c t u r a l data i n d i c a t e t h a t the C=p bond l e n g t h 1.545 8 i s r e l a t i v e l y i n s e n s i t i v e t o the nature of the R s u b s t i t u e n t , whereas other p r o p e r t i e s , e.g. NMR parameters and I.P. data, are s i g n i f i c a n t l y a f f e c t e d by R. 2
The P-C d i s t a n c e s i n simple phosphines, phospha-alkenes and phospha-alkynes are summarised i n F i g u r e 1 and the r e s u l t s c o n t r a s t e d with C-C, C=C and C=C bond lengths when a simple r e l a t i o n s h i p i s seen to e x i s t . A d d i t i o n a l l y the data f o r the phosphabenzene systems shows e x c e l l e n t agreement with e x p e c t a t i o n . Compounds o f the type R C=PX were f i r s t p o s t u l a t e d by Haszeldine and coworkers (24-27) as r e a c t i o n intermediates i n the r e a c t i o n o f c e r t a i n perfluoroaLkyl phosphines with bases. These s p e c i e s were f i r s t i s o l a t e d and c h a r a c t e r i s e d by s p e c t r o s c o p i c techniques by Kroto and Nixon and coworkers ( 2 8 ) . Microwave and/ or NMR data on CH =PH, CH =PC1 and CF =PH were obtained on samples generated by the p y r o l y s i s o f s u i t a b l e p r e c u r s o r s , but subsequently a v a r i e t y o f a l t e r n a t i v e s y n t h e t i c r o u t e s have been developed to o b t a i n the f o l l o w i n g phospha-alkene molecules (28-34): 2
2
2
2
CH =PH, CH =PC1, CH =PBr, CH =PF, 2
2
2
2
CF =PH, CF =PCF , PhRC=PCl, e t c . 2
2
3
Quin and Verkade; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
KROTO A N D NIXON
Phosphaalkenes
and Phosphaalkynes
385
— ι — 1.5
1.6 r C-P
Figure L
- (°A)
vs. r _ . Gives covalent radius of Ρ; Sp , 1.07 °A; Sp , 1.00; Sp, 0.94. 3
Plot of r _ c
1.8
1.7
P
c
c
Quin and Verkade; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
2
PHOSPHORUS CHEMISTRY
386 ,
,,
R SiCR R PX 3
CF PH 3
• R S i X + CR'R'^PX
2
3
* CF =PH
2
2
Thermal s t a b i l i z a t i o n o f phospha-alkenes can be i n f l u e n c e d by the attachment o f a heteroatom t o the P=C bond as evidenced by the elegant work o f Becker ( 3 5 - 4 0 , 4 3 ) , Appel ( 4 1 ) and Issleib ( 4 2 ) .
e.g.
RP(SiMe ) 3
^OSiMe • R-P=C^ tBu
+ tBuCOCl
2
^Cl PhP(SiMe ) 3
1
2
+ R N=C'
R ί
X
R
3
R
1
^ Me Si
N-C=P ^Ph
3
In 1978 B i c k e l h a u p t e t al_. ( 4 4 ) r e p o r t e d the f i r s t s t a b l e phospha-alkene with carbon s u b s t i t u e n t s o n l y . u s i n g the f o l l o w i n g s y n t h e t i c approach. RPC1
2
— • RPClCHPh
• RP=CPh
2
(R - m e s i t y l )
2
(L) Kroto, Nixon e t a l . have r e p o r t e d s e v e r a l examples o f complexes o f the phospha-alkene, L, RP=CPh (R = m e s i t y l ) ( 4 5 ) . In p r i n c i p l e phospha-alkenes can coordinate t o t r a n s i t i o n metals i n any o f the three modes shown but so f a r evidence only f o r type (a) has been obtained. 2
C — R
C— R
/ /
C— R
/ A R*— Ρ s v
R*— Ρ \ M
f
R — Ρ 4 M
*M
(a)
(b)
A ν M
(c)
Complexes obtained a r e : cis-M(CO) i*L (M = Cr,Mo,W) ? M(C0) 5 L (M = W); R h C l ( P P h ) L ; RhCl(CO)L ; R h ( C H ) L ; P t X L (X = Cl,I,Me)j P t C l ( P E t ) L . The l a t t e r i s the s u b j e c t o f an X-ray s t r u c t u r a l a n a l y s i s ( 4 6 ) . These r e s u l t s a r e important i n r e l a t i o n t o other r e c e n t developments i n t h e c o o r d i n a t i o n 2
3
2
2
2
9
7
2
2
2
3
Quin and Verkade; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
79.
KROTO A N D NIXON
Figure 2.
Phosphaalkenes
Structure
and
Phosphaalkynes
of cis-PtC 1 (Ρ Et 3)(P(mes)=CPh2). 2
Quin and Verkade; Phosphorus Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
387
388
PHOSPHORUS
CHEMISTRY
chemistry o f RPNR' and RPO (47) where phosphorus i s the coordinating s i t e . Of p a r t i c u l a r i n t e r e s t i s the r e c e n t l y determined s t r u c t u r e o f the complex c i s - P t C l 2 ( P E t ) (P(mes) = CPÏ12) shown i n F i g u r e 2. The p r e v i o u s l y p o s t u l a t e d (46) mode o f c o o r d i n a t i o n o f the phospha-alkene t o the metal atom u s i n g the lone p a i r on phosphorus has now been confirmed. The phosphorus-carbon double bond i s 1.668 8 i n e x c e l l e n t agreement with our p r e v i o u s l y p u b l i s h e d data from microwave spectroscopy on simpler uncoordinated phospha-alkenes ( i t should be noted t h a t the s t r u c t u r e o f f r e e Ρ(mes)=CPh i s not known). The angles subtended a t phosphorus namely
