Inorganic Compounds with Unusual Properties

6 Synthesis and Selected Properties of Polymeric Sulfur Nitride, (Polythiazyl), (SN)x

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A. G. MacDIARMID, C. M. MIKULSKI, M. S. SARAN, P. J. RUSSO, M. J. COHEN, A. A. BRIGHT, A. F. GARITO and A. J. HEEGER Departments of Chemistry and Physics, University of Pennsylvania, Philadelphia, Penna. 19174

Analytically pure single crystals of polymeric sulfur nitride (polythiazyl), (SN) , suitable for solid state studies were prepared by the spontaneous room temperature polymerization of single crystals of pure S N . The polymer is relatively inert to oxygen and water. A single-crystal x-ray study revealed that an (SN) polymer molecule consists of an almost planar chain of alternating sulfur and nitrogen atoms in which the sulfur-nitrogen bond lengths are all very similar. The optical reflectance of crystalline films of (SN) from the near uv through the visible region is metal-like in character for light polarized parallel to the polymer chain axis. The covalent polymer has the optical and electrical properties of a low-dimensional metal. x

2

2

x

x

T i T e t a l s , b y d e f i n i t i o n , are those elements w h i c h possess c e r t a i n c h a r acteristic c h e m i c a l p r o p e r t i e s s u c h as ease i n f o r m i n g p o s i t i v e ions b y c h e m i c a l processes a n d c e r t a i n c h a r a c t e r i s t i c s o l i d state p h y s i c a l p r o p erties s u c h as h i g h e l e c t r i c a l c o n d u c t i v i t y ( w h i c h increases as the t e m p e r a t u r e is l o w e r e d ), h i g h reflectivity of l i g h t , g o o d t h e r m a l c o n d u c t i v i t y , d u c t i l i t y , a n d m a l l e a b i l i t y . U n t i l v e r y r e c e n t l y , i t w a s b e l i e v e d t h a t these c o l l e c t i v e properties w e r e u n i q u e to m e t a l l i c elements, b u t i t n o w appears t h a t m a n y of these properties m a y be f o u n d i n a s i m p l e i n o r g a n i c p o l y m e r that contains no m e t a l a t o m . Studies of the c o m p o u n d , p o l y m e r i c s u l f u r n i t r i d e ( p o l y t h i a z y l ), ( S N ) ^ , r e v e a l t h a t i t possesses m a n y of t h e a b o v e properties a n d m a y therefore b e t e r m e d a m e t a l even t h o u g h its m e t a l l i c characteristics are s t r o n g l y a n i s o t r o p i c .

I t is b e l i e v e d that this p o l y m e r

m a y be the f o r e r u n n e r of a w h o l e n e w class of p o l y m e r i c metals.

63 In Inorganic Compounds with Unusual Properties; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

64

INORGANIC

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S p a c e does not p e r m i t a c o m p l e t e d i s c u s s i o n of the b a c k g r o u n d i n f o r m a t i o n a v a i l a b l e o n ( S N ) * , b u t k e y l a n d m a r k s i n its s t u d y are s u m m a r i z e d v e r y briefly. P o l y m e r i c s u l f u r n i t r i d e w a s first p r e p a r e d i n 1910 b y B u r t ( 1 ) b y passing S N 4

4

v a p o r over s i l v e r gauze or q u a r t z w o o l at

1 0 0 - 3 0 0 ° C . I t w a s d e p o s i t e d outside the hot z o n e as v e r y s m a l l crystals or as a

film—when

t h i n , the film was b l u e b y t r a n s m i t t e d l i g h t , b u t , as i t

b e c a m e t h i c k e r , it b e c a m e o p a q u e a n d t o o k o n a b r o n z e m e t a l l i c luster b y reflected l i g h t . I n 1971 B o u d e u l l e et al. (2, 3, 4,5,6) f o r m e d as a p r i m a r y p r o d u c t w h e n S N 4

r e p o r t e d that S N , w h i c h w a s 2

4

2

v a p o r w a s passed o v e r h e a t e d

s i l v e r w o o l , w o u l d p o l y m e r i z e at l o w temperatures to g i v e p s e u d o single


crystals c o m p o s e d of layers of p a r a l l e l fiberlike crystals of ( S N ) * .

The

crystals w e r e too s m a l l for s i n g l e - c r y s t a l x - r a y studies, b u t t h e y w e r e u s e d i n a p r e l i m i n a r y electron diffraction investigation.

Boudeulle

reported

r e c e n t l y t h a t the S - N b o n d lengths a l t e r n a t e d b e t w e e n short (1.58

A)

a n d l o n g (1.72 A ) b o n d s w h i c h are close to d o u b l e a n d single S - N b o n d lengths, r e s p e c t i v e l y . A l s o b o n d angles of 113.5° for S - N - S a n d 111.5° for N - S - N were reported. B y using more reliable single-crystal x-ray diffract i o n m e t h o d s , w e r e c e n t l y d e m o n s t r a t e d ( 7 ) t h a t these d a t a are n o t c o r rect (the S N 2

2

a n d ( S N ) * structures w e r e d e t e r m i n e d b y the M o l e c u l a r

S t r u c t u r e C o r p . , C o l l e g e Stations, T e x a s ) . I n 1973 L a b e s et al. (8,9)

s y n t h e s i z e d c r y s t a l l i n e b u n d l e s of i m p u r e

( S N ) a . fibers. A l t h o u g h the S : N a t o m i c r a t i o was 1:1, the m a t e r i a l c o n tained 5.48%

impurity (4.93%

0, 0 . 4 2 %

H , and 0.13%

C).

However,

m e t a l l i c - l i k e c o n d u c t i v i t y w a s o b s e r v e d i n d i r e c t i o n s p a r a l l e l to the ( S N ) ^ fibers, a n d this i n c r e a s e d s h a r p l y w i t h decrease i n t e m p e r a t u r e . S i x different samples h a d c o n d u c t i v i t i e s at r o o m t e m p e r a t u r e of 10, 89, 230, 640, 1470, a n d 1730 o h m " c m " . S i n c e the e l e c t r i c a l c o n d u c t i v i t y of a n a n i s o 1

1

t r o p i c substance c a n b e affected e n o r m o u s l y b y e v e n traces of i m p u r i t i e s , w e d e c i d e d t h a t i t w a s most i m p o r t a n t to a t t e m p t to synthesize a n a l y t i c a l l y p u r e crystals of ( S N ) * a n d to e x a m i n e the p h y s i c a l a n d c h e m i c a l p r o p e r t i e s of the m a t e r i a l . O n l y i n this w a y w o u l d i t b e possible to d e t e r m i n e w h e t h e r the m e t a l l i c - l i k e p r o p e r t i e s r e p o r t e d for ( S N ) * (8,9)

were

c h a r a c t e r i s t i c of the p u r e m a t e r i a l . T h e L a b e s ( 8 , 9 ) m e t h o d of p r e p a r i n g ( S N ) ^ u s u a l l y i n v o l v e d passing S N 4

4

v a p o r over h e a t e d s i l v e r w o o l a n d t h e n i m m e d i a t e l y c o n d e n s i n g

the i s s u i n g v a p o r s o n a c o l d finger at 0 - 8 ° C . H o w e v e r , w e f o u n d t h a t , i n o r d e r to synthesize r e p r o d u c i b l y large single crystals of a n a l y t i c a l l y p u r e i t is first necessary to p r e p a r e a b s o l u t e l y p u r e S N

(SN)

X

2

of S N ) b y p u m p i n g S N 4

2

4

silver w o o l at 2 2 0 ° C . c o l d finger, the S N 2

2

4

vapor formed from solid S N 4

4

2

(free of traces

at 8 5 ° C t h r o u g h

After being condensed on a l i q u i d nitrogen-cooled is s l o w l y s u b l i m e d f r o m the c o l d finger ( b y w a r m i n g

i t to r o o m temperature) i n t o a t r a p w i t h r e c t a n g u l a r w a l l s t h a t is i m m e r s e d

In Inorganic Compounds with Unusual Properties; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

6.

M A C D i A R M i D

i n a 0 ° C bath. W h e n S N 2

2

65

Sulfur Nitnde

Polymeric

ET AL.

crystals of the correct shape a n d d i m e n s i o n s

h a v e f o r m e d , the r e c t a n g u l a r t r a p is r a i s e d to r o o m t e m p e r a t u r e . i n i t i a l l y colorless, t a b u l a r , m o n o c l i n i c crystals of S N 2

2

The

r a p i d l y t u r n intense

b l u e - b l a c k a n d b e c o m e p a r a m a g n e t i c ( g = 2.005). A f t e r s e v e r a l h o u r s , these crystals c h a n g e spontaneously to the b r i g h t , lustrous g o l d e n c o l o r of ( S N ) * ; h o w e v e r , c h e m i c a l a n d x - r a y studies ( 7 , 1 0 ) r e v e a l t h a t t h e y s t i l l c o n t a i n l a r g e a m o u n t s of u n p o l y m e r i z e d S N . H e n c e , p o l y m e r i z a t i o n m a y 2

p o s s i b l y start at the surface of a n S N 2

2

crystal and proceed inward. T h e

2

crystals are left at r o o m t e m p e r a t u r e for a p p r o x i m a t e l y t w o a d d i t i o n a l days, a n d t h e y are

finally

unpolymerized S N . 2

2

p u m p e d o n at 75 ° C to r e m o v e a l l traces of

T h e m a t e r i a l does n o t u n d e r g o a n y c h a n g e i n a p


p e a r a n c e d u r i n g this t w o - d a y p e r i o d at r o o m t e m p e r a t u r e a n d the h e a t i n g t o 7 5 ° C . T h e (SN)

X

of S N 4

and S N

4

2

(JO, 11).

2

final

so o b t a i n e d gives n o E P R s i g n a l ; the absence

is d e m o n s t r a t e d b y a n x - r a y p o w d e r d i f f r a c t i o n s t u d y

T w o t y p i c a l analyses for the o b t a i n e d ( S N ) ^ are p r e s e n t e d i n

T a b l e I. Table I. Analysis Calcd. Found

(SN)* Crystals

0

S, %

N, %

C, %

H, %

0/ %

69.59 69.29° 70.18

30.41 30.56 30.20

0.00 0.00

0.00 0.00

0 0

e

Total,

%

100.00 99.85 100.38

d

Crystals were from two different preparations. Semiquantitative emission spec trograph ic analyses did not detect the presence of any metal impurities at the partsper-million level. Traces of oxygen are difficult to determine accurately experimentally. First set of data obtained by Galbraith Laboratories, Inc., Knoxville, Tenn. Reported as "none or trace." Second set of data obtained by Schwarzkopf Microanalytical Laboratory, Woodside, Ν. Y . Analyses were for sulfur and nitrogen only. α

6

c

d

6

T h e ( S N ) ; , . crystals f o r m e d i n this s t u d y h a v e a n e x t r e m e l y h i g h lustrous g o l d e n m e t a l l i c a p p e a r a n c e o n a l l faces, b u t the ends are d u l l , d a r k b l u e - b l a c k . T h i s is e x p e c t e d ( see b e l o w ). T h e lustrous surfaces of less w e l l f o r m e d crystals are h i g h l y s t r i a t e d ; this causes

considerable

scattering of l i g h t f r o m these surfaces at c e r t a i n i n c i d e n t angles a n d c o n s e q u e n t l y t h e y a p p e a r d a r k b l u e - b l a c k u n d e r these p a r t i c u l a r l i g h t i n g c o n ditions.

E l e c t r o n m i c r o g r a p h studies

(magnification

s p e c i a l l y chosen, i m p e r f e c t , single c r y s t a l of

100-1300)

of

a

( S N ) ^ r e v e a l e d t h a t the

crystals are c o m p o s e d of layers of fibers s t a c k e d p a r a l l e l to e a c h o t h e r a l o n g t h e l o n g axis (b axis) of the c r y s t a l . A t places w h e r e the c r y s t a l w a s s e p a r a t e d m e c h a n i c a l l y , l o n g fibrous strands of ( S N ) ^ are a p p a r e n t . T h e crystals are h i g h l y a n i s o t r o p i c , a n d t h e y m a y b e m e c h a n i c a l l y c l e a v e d v e r y e a s i l y a l o n g a p l a n e p a r a l l e l to the (SN),,. fibers. T h e crystals are also soft a n d m a l l e a b l e ; t h e y c a n be

flattened

r e a d i l y b y m i l d pressure

a p p l i e d p e r p e n d i c u l a r to the fibers to g i v e t h i n , lustrous, g o l d e n sheets.


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T h e c r y s t a l l i n e m a t e r i a l also exhibits v e r y h i g h o p t i c a l a n i s o t r o p y i n t h e v i s i b l e p o r t i o n of the s p e c t r u m : o n l y t h a t c o m p o n e n t of i n c i d e n t l i g h t w h i c h is p o l a r i z e d p a r a l l e l to the ( S N ) * fibers is reflected ( R | ) w i t h h i g h (

m e t a l l i c reflectance. T h e c o m p o n e n t of i n c i d e n t a l l i g h t w h i c h is p o l a r i z e d p e r p e n d i c u l a r to the fibers is reflected ( R

±

) o n l y v e r y w e a k l y . T h u s , the

f a c e of a n ( S N ) * c r y s t a l m a y first b e e x a m i n e d w i t h n o n p o l a r i z e d l i g h t at a n i n c i d e n c e a n g l e of 90° w i t h the a n a l y z e r adjusted to g i v e m a x i m u m i n tensity of the reflected l i g h t . W h e n the a n a l y z e r is t h e n r o t a t e d t h r o u g h 9 0 ° , the i n t e n s i t y of the reflected l i g h t is r e d u c e d e n o r m o u s l y , a n d its c o l o r changes to a d a r k b l u e - b l a c k . It is therefore a p p a r e n t w h y the ends of the ( S N ) * crystals, w h i c h consist o n l y of ends of ( S N ) * fibers, a p p e a r b l u e - b l a c k — a t these surfaces a l l i n c i d e n t r a d i a t i o n is p e r p e n d i c u l a r to the Downloaded by MONASH UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch006

( S N ) * fibers; f u r t h e r m o r e , l i g h t scattering is extensive at these less s m o o t h surfaces. A l t h o u g h ( S N ) * crystals s l o w l y b e c o m e c o v e r e d w i t h a w h i t i s h - g r a y p o w d e r after t h e y s t a n d i n a i r f o r several m o n t h s , n o i m m e d i a t e r e a c t i o n w i t h a i r is a p p a r e n t ; h o w e v e r , the p o s s i b i l i t y of instantaneous f o r m a t i o n of a n o n v i s i b l e film of o x i d i z e d or h y d r o l y z e d m a t e r i a l c o a t i n g the ( S N ) * surface cannot yet be r u l e d out. D u r i n g seven days at r o o m t e m p e r a t u r e , crystalline (b)

( S N ) * is not

attacked b y

(a)

oxygen

w a t e r v a p o r at a pressure of 4.6 torr, or ( c )

water vapor.

at 1-atm

pressure,

oxygen saturated w i t h

It is not a t t a c k e d b y degassed d i s t i l l e d w a t e r w h e n

com-

p l e t e l y i m m e r s e d i n i t for 24 h r at r o o m t e m p e r a t u r e ; h o w e v e r , after six d a y s , a l t h o u g h there is no c h a n g e i n a p p e a r a n c e of the ( S N ) * , a v e r y s m a l l a m o u n t of w h i t e s o l i d m a t e r i a l m a y b e o b t a i n e d b y e v a p o r a t i n g the water.

W h e n i t is h e a t e d i n a sealed t u b e in vacuo

at a p p r o x i m a t e l y

1 4 0 ° C , ( S N ) * d e c o m p o s e s to sulfur, n i t r o g e n , a n d p o s s i b l y other as yet unidentified materials. W h e n ( S N ) * is h e a t e d w i t h constant p u m p i n g at ^

140 ° C , g o l d e n ,

lustrous, p o l y c r y s t a l l i n e cohesive films of ( S N ) * c a n b e c o n d e n s e d o n a v a r i e t y of substrates h e l d at temperatures of 0 ° - 5 0 ° C .

X-ray powder

patterns of scrapings of these s u b l i m e d films d e m o n s t r a t e that the (SN)-,. has the same c r y s t a l structure as the ( S N ) * crystals f r o m w h i c h t h e y w e r e s u b l i m e d a n d that the films are c o m p l e t e l y free of S N 4

4

and S N . Optical 2

2

reflectance a n d x - r a y studies of the films f o r m e d o n a glass surface r e v e a l e d that the ( S N ) * fibers a l w a y s l i e p a r a l l e l to the glass surface n o ( S N ) * fibers are o b s e r v e d p e r p e n d i c u l a r to the surface. e s t i n g p o s s i b i l i t i e s of e p i t a x i a l g r o w t h of

(II);

M a n y inter-

( S N ) * films therefore

appear

l i k e l y since f u l l y o r i e n t e d e p i t a x i a l films of ( S N ) * c a n b e d e p o s i t e d v a r i o u s substrates i n c l u d i n g M y l a r , T e f l o n , a n d p o l y e t h y l e n e method

by

on this

(12).

W e p r e p a r e d single crystals of a n a l y t i c a l l y p u r e ( S N ) * t h a t w e r e sufficiently large f o r s i n g l e - c r y s t a l x - r a y studies (10)

b y s o l i d state p o l y -


6.

M A CDIARMID

E TA L .

Polymeric Sulfur Nitnde

S(c) N(c)

67

S(a) N(a)

S(WN(W

S(d) NW)

1 m e r i z a t i o n of S N 2

2

crystals. T h e ( S N ) * crystals h a v e t h e same

g r o u p ( P 2 i / c ) as, a n d are p s e u d o m o r p h s of, t h e S N 2

2

space

crystals f r o m w h i c h

t h e y are d e r i v e d (10). T h e studies r e v e a l t h a t ( S N ) * consists o f a n a l m o s t Downloaded by MONASH UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch006

p l a n a r c h a i n o f a l t e r n a t i n g s u l f u r a n d n i t r o g e n atoms as i n 1. T h e r e are f o u r S N u n i t s p e r u n i t c e l l w h i c h has a = 4.153(6), b = 4.439(5), a n d c — 7.637(12) Α , β — 1 0 9 . 7 ( 1 ) ° , a n d D structure ( R =

c h a i n distances o f S ( a ) - N ( a ) S(a)-S(b)

=

c

=

=

3

1.593(5), S ( a ) - N ( b )

2.789(2), N ( a ) - N ( b )

=

T h e refined —

1.628(7),

2.576(7), a n d S ( a ) - N ( c )

2.864(5) A a n d b o n d angles o f S - N - S — 106.2(2)°.

2.30 g / c m .

0.11) has as its m a j o r feature ( S N ) * c h a i n s w i t h i n t r a —

and N - S - N —

119.9(4)°

T h e S - N b o n d lengths a r e a l l v e r y s i m i l a r , a n d t h e y c o r r e

s p o n d to a s u l f u r - n i t r o g e n b o n d o r d e r t h a t is i n t e r m e d i a t e b e t w e e n those e x p e c t e d f o r a single a n d a d o u b l e b o n d . C e r t a i n of t h e s u l f u r - s u l f u r i n t e r c h a i n distances (3.48 A ) b e t w e e n ( S N ) * c h a i n s l y i n g i n t h e same (102) p l a n e a r e less t h a n t h e s u m o f t h e v a n d e r W a a l s r a d i i o f t w o s u l f u r atoms (3.70 A ) . T h i s suggests t h e presence o f a w e a k b u t i m p o r t a n t i n t e r c h a i n i n t e r a c t i o n ( s h o w n as d o t t e d

Q

Figure I .

SULFUR

Ο

NITROGEN

Diagrammatic representation of (SN) chains in the 102 plane of an (SN) crystal X

X


68

INORGANIC

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UNUSUAL PROPERTIES

lines i n F i g u r e 1 ) w h i c h is p r o b a b l y l a r g e l y r e s p o n s i b l e f o r c a u s i n g ( S N ) * to b e , at t h e v e r y least, a n a n i s o t r o p i c t w o - d i m e n s i o n a l m e t a l r a t h e r t h a n a one-dimensional metal. T h e o p t i c a l reflectance of t h i n films o f ( S N ) * w a s m e a s u r e d f r o m t h e n e a r U V (30,000 c m " ) t o t h e f a r I R ( 5 0 0 c m ' ) regions ( F i g u r e s 2 a n d 1

1

3 ). Reflectance c h a r a c t e r i s t i c of metals is o b s e r v e d i n t h e I R t h r o u g h t h e v i s i b l e regions, w i t h a w e l l d e f i n e d p l a s m a m i n i m u m at 22,000 c m ' corresponds (JJ).

1

that

to l i g h t p o l a r i z e d p a r a l l e l ( R ) to t h e p o l y m e r c h a i n axis M

Furthermore,

a n d R±

were

recently measured

u s i n g a face of a c a r e f u l l y p o l i s h e d s i n g l e c r y s t a l (12).

separately

T h e peaks i n

the I R at 9 9 5 a n d 685 c m " ( F i g u r e 2 ) a r e p r o b a b l y c a u s e d b y S - N 1


v i b r a t i o n a l s t r e t c h i n g m o d e s . T h e e x p a n d e d p l a s m a e d g e p o r t i o n o f this c u r v e is p r e s e n t e d i n F i g u r e 3. I n b o t h c u r v e s , t h e d a s h e d l i n e is t h e least squares c o m p u t e r D r u d e fit ( J J ) to t h e m e a s u r e d reflectance.

Drude

reflectance is c o n c e r n e d o n l y w i t h t h e m e t a l l i c - l i k e reflectance of l i g h t , i.e. t h a t c o m p o n e n t of l i g h t t h a t is p a r a l l e l to t h e ( S N ) * fibers ( J J ) . I t w a s d e m o n s t r a t e d t h a t t h e difference b e t w e e n t h e c u r v e f o r m e a s u r e d reflectance a n d t h a t c a l c u l a t e d b y t h e D r u d e e q u a t i o n is a t t r i b u t a b l e to s u b s t a n t i a l i n t e r c h a i n c o u p l i n g ( J 2 ) . T h e m a x i m u m m e t a l l i c reflectance o b s e r v e d is a b o u t 4 5 % . T h i s v a l u e is a c t u a l l y v e r y m u c h greater t h a n i t first appears. T h u s , i t m a y b e r e c a l l e d t h a t t h e m a x i m u m m e t a l l i c reflec tance o f u n p o l a r i z e d l i g h t possible i n this system is 5 0 % since e a c h c r y s t a l d o m a i n i n t h e film w i l l reflect i n a m e t a l l i c f a s h i o n o n l y h a l f t h e i n c i d e n t l i g h t w h i c h falls u p o n i t , i.e. o n l y that c o m p o n e n t p o l a r i z e d p a r a l l e l to t h e (SN)*

fibers.

T h e r e a l m e t a l l i c reflectance is therefore o n t h e o r d e r of

9 0 % w h i c h is c h a r a c t e r i s t i c o f a m e t a l . 70

60

50

£

4

0

oc 30

20

10 04

06 08 10

20

3040 v

60 80100 (

°

m

*

200 300 χ Ι Ο ' Physical Review Letters

Figure 2. Reflectance spectrum (500-30,000 cm' ) of a thin film of (SN) . Dashed line: Drude fit to the measured reflectance (11). 1

X


6.

M A C D i A R M i D

Polymeric

ET AL.

Sulfur

69

Nitride


Physical Review Letters

Figure portion Figure dashed

3. Expanded view of the plasma edge of the reflectance spectrum of (SN) in 2. Solid line: experimental spectrum; line: Drude fit to the expenmental spectrum ( 1 1 ) . X

F r o m the p l a s m a m i n i m u m ( p l a s m a f r e q u e n c y , ω ) , one c a n r e a d i l y ρ

c a l c u l a t e τ the e l e c t r o n i c r e l a x a t i o n t i m e b y t h e c l a s s i c a l D r u d e e q u a t i o n w h i c h is w e l l e s t a b l i s h e d for m e t a l films (11). 10"

15

T h i s gives a v a l u e of 1.9

X

sec f o r ( S N ) * . F r o m τ a n d ω , one c a n c a l c u l a t e the d c c o n d u c t i v i t y ρ

of a m e t a l b y the w e l l - k n o w n e q u a t i o n

(11):

dc c o n d u c t i v i t y =

ω τ/4π ρ

2

U s i n g this r e l a t i o n s h i p , a d c c o n d u c t i v i t y of 3 Χ

10 ohm" 3

1

cm"

1

is

c a l c u l a t e d for ( S N ) * a l o n g t h e fiber axis. T h i s v a l u e agrees w e l l w i t h t h e p r e l i m i n a r y d c c o n d u c t i v i t y ( f o u r - p r o b e m e t h o d ) of a single c r y s t a l of ( S N ) * a l o n g the d i r e c t i o n of the fiber axis. W e o b t a i n e d values of 3700 o h m " c m 1

- 1

1200-

at r o o m t e m p e r a t u r e ; the c o n d u c t i v i t y increases m a r k e d l y

w h e n the t e m p e r a t u r e is l o w e r e d to 4.2 K , i n c r e a s i n g a p p r o x i m a t e l y 50-205-fold.

T h u s , ( S N ) * has a c o n d u c t i v i t y at r o o m t e m p e r a t u r e o n the

same o r d e r of m a g n i t u d e as t h a t of a m e t a l s u c h as m e r c u r y (see I I ) . I t w a s r e c e n t l y r e p o r t e d (13)

t h a t ( S N ) * becomes s u p e r c o n d u c t i n g

at 0.26K. Table II. Substance (SN)* Bi Nichrome Hg Sb Fe Cu

Table

Electrical Conductivity Conductivity

(16)

at 20°C, 3.7 8.33 10.0 10.4 23.9 1.0 5.80

Χ Χ Χ χ Χ Χ Χ

10 10 10 10 10 10 10

ohm'

1

3 3 3 3 3 5

5


70

INORGANIC

COMPOUNDS

WITH

UNUSUAL

PROPERTIES

S i n c e the l e n g t h of a s u l f u r - n i t r o g e n single b o n d is e x p e c t e d to b e a p p r o x i m a t e l y 1.74 A a n d that of a s u l f u r - n i t r o g e n d o u b l e b o n d ~ 1.54 A (14, 15),

for a s u l f u r - n i t r o g e n b o n d o r d e r of 1.5, a n e x p e r i m e n t a l l e n g t h

of a p p r o x i m a t e l y 1.64 A w o u l d be e x p e c t e d . F r o m s i m p l e v a l e n c e concepts, the b o n d i n g b e t w e e n s u l f u r a n d n i t r o g e n atoms i n a n

bond (SN)*

c h a i n m a y therefore be c o n s i d e r e d as d e r i v e d , to a first a p p r o x i m a t i o n , f r o m the t w o extreme resonance forms

M

S-N Downloaded by MONASH UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch006

S=N

/

c

S

( 2 a n d 3 ) ( i n w h i c h a l l atoms

=N

X

S=N

s-N

S - N

e x h i b i t n o r m a l o x i d a t i o n states a n d valences ) to g i v e a resonance h y b r i d species ( 4 )

i n w h i c h a l l b o n d s are i n t e r m e d i a t e b e t w e e n d o u b l e a n d

single b o n d s . It s h o u l d be stressed that this d e s c r i p t i o n is o b v i o u s l y a n o v e r - s i m p l i f i c a t i o n since, a l t h o u g h a l l S - N b o n d s are i n t e r m e d i a t e

in

l e n g t h b e t w e e n single a n d d o u b l e b o n d s , t h e y are n o t a l l exactly e q u a l experimentally. A l t e r n a t i v e l y , since the N S m o n o m e r is the s u l f u r a n a l o g of

NO

( w h i c h has one u n p a i r e d electron i n a π* o r b i t a l ) , ( S N ) * m a y be r e g a r d e d as m a d e u p f r o m the p o l y m e r i z a t i o n of

•m o n o m e r u n i t s , w h i c h l i k e N O , m i g h t b e e x p e c t e d to h a v e a n i t r o g e n s u l f u r b o n d o r d e r of 2.5. P o l y m e r i z a t i o n t h e n gives 5; e a c h n i t r o g e n a t o m


6.

MACDIARMID

ET

Polymeric

AL.

Sulfur

71

Nitride

\ Ρ \

f

6 is d e p i c t e d as b e i n g associated w i t h e i g h t electrons, e a c h s u l f u r w i t h n i n e electrons. T h e n i n t h e l e c t r o n c a n b e r e g a r d e d as present i n one of t h e π* orbitals of e a c h - S ~ N - u n i t .

T h e s e π* o r b i t a l s o n adjacent - S ~ N -


units c a n t h e n o v e r l a p to g i v e some m u l t i p l e - b o n d c h a r a c t e r to the other S - N l i n k a g e s , viz.

6.

It s h o u l d b e n o t e d t h a t this q u a l i t a t i v e t r e a t m e n t

differs f r o m the p r e v i o u s resonance h y b r i d a r g u m e n t — i t does not suggest t h a t the a m o u n t of m u l t i p l e b o n d c h a r a c t e r i n a l l S - N l i n k a g e s is neces sarily exactly the same.

T h e h a l f - f i l l e d o v e r l a p p i n g ττ* orbitals of

each

N S u n i t are e x p e c t e d to p r o d u c e a h a l f - f i l l e d c o n d u c t i o n b a n d i n t h e p o l y m e r m o l e c u l e i n a m a n n e r t h a t is s o m e w h a t s i m i l a r , f o r e x a m p l e , to the p r o d u c t i o n of a h a l f - f i l l e d c o n d u c t i o n b a n d i n m e t a l l i c L i * " p o l y m e r " b y the o v e r l a p p i n g of h a l f - f i l l e d 2s a t o m i c o r b i t a l s . F r o m these observations, i t appears t h a t ( S N ) * e x h i b i t s s t r o n g l y a l l the m a j o r c o l l e c t i v e properties c h a r a c t e r i s t i c of a m e t a l i n a d i r e c t i o n p a r a l l e l to the ( S N ) * class

of

metals.

covalent

fibers.

I t is, therefore, the first m e m b e r of a n e w

polymeric

materials—lower dimensional,

polymeric

I t is h i g h l y l i k e l y t h a t a w h o l e b r o a d n e w field of

polymeric

metals b a s e d o n d i a t o m i c n e u t r a l , c a t i o n i c , o r a n i o n i c m o n o m e r i c u n i t s t h a t c o n t a i n one u n p a i r e d e l e c t r o n c o u l d i n d e e d exist. A p r i m e r e q u i s i t e appears to b e t h a t the r e a l or h y p o t h e t i c a l m o n o m e r u n i t h a v e o n e u n p a i r e d electron. N e u t r a l species of this t y p e m i g h t b e sought f r o m b i n a r y c o m p o u n d s that c o n t a i n one element f r o m a n o d d - n u m b e r e d g r o u p i n the P e r i o d i c T a b l e a n d one f r o m a n e v e n - n u m b e r e d g r o u p , e.g. A s S . C a t i o n i c or a n i o n i c u n i t s c o u l d , i n p r i n c i p l e , b e o b t a i n e d b y r e m o v i n g o r a d d i n g a n e l e c t r o n , r e s p e c t i v e l y , to b i n a r y c o m p o u n d s i n w h i c h b o t h atoms w e r e from either even-numbered T a b l e , e.g.

or odd-numbered

(CS)~ and ( P N ) . +

groups

i n the

Periodic

I n this respect, i t m a y b e n o t e d t h a t a

species s u c h as ( C S ) * ' * is i s o e l e c t r o n i c w i t h ( N S ) . Species i s o e l e c t r o n i c X

w i t h ( N S ) * are, of course, n o t l i m i t e d to ions. isoelectronic w i t h N S a n d consequently

F o r e x a m p l e , H C S is

( H C S ) * — a n d its d e r i v a t i v e s ,

( R C S ) * , i f t h e y c o u l d b e s y n t h e s i z e d — m i g h t b e e x p e c t e d to b e metals. I t appears t h a t there is m u c h n e w c h a l l e n g i n g s y n t h e t i c c h e m i s t r y b o t h i n o r g a n i c a n d o r g a n i c - t o b e c a r r i e d o u t i n o r d e r to a s c e r t a i n t h e extent a n d i m p o r t a n c e of this n e w area of c h e m i s t r y t h a t is

concerned

w i t h c o v a l e n t p o l y m e r s w h i c h are metals.


72

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PROPERTIES

Acknowledgment T h e authors w i s h to t h a n k J . M . T r o u p a n d B . A . F r e n z o f t h e M o l e c u l a r S t r u c t u r e C o r p . f o r m a n y h e l p f u l discussions c o n c e r n i n g t h e s i n g l e c r y s t a l x-ray studies.


Literature Cited 1. Burt, F. P., J. Chem. Soc. (1910) 1171. 2. Boudeulle,M.,Douillard, Α., Michel, P., Vallet, G., C. R. Acad. Sci Ser. C (1971) 272, 2137. 3. Boudeulle,M.,Michel, P., Acta Crystallogr. A (1972) 28, S199. 4. Boudeulle, M., Douillard, Α., J. Microsc. Paris (1971) 11, 3. 5. Boudeulle, M., Ph.D. thesis, University of Lyon, 1974. 6. Boudeulle, M., Cryst. Struct. Commun. (1975). 4, 9. 7. MacDiarmid, A. G., Mikulski, C. M., Russo, P. J., Saran, M. S., Garito, A. F., Heeger, A. J., Chem. Commun. (1975) 476. 8. Walatka, Jr., V. V., Labes, M. M., Perlstein, J.H.,Phys. Rev. Lett. (1973) 31, 1139. 9. Hsu, C.H.,Labes, M. M.,J.Chem. Phys. (1974) 61, 4640. 10. MacDiarmid, A. G., Mikulski, C. M., Russo, P. J., Saran, M. S., Garito, A. F., Heeger, A. J., J. Am. Chem. Soc. (1975) 97, 6358. 11. Bright Α. Α., Cohen, M. J., Garito, A. F., Heeger, A. J., Mikulski, C. M., Russo, P. J., MacDiarmid, A. G., Phys. Rev. Lett. (1975) 34, 206. 12. Bright, Α. Α., Cohen, M. J., Garito, A. F., Heeger, A. J., Mikulski, C. M., MacDiarmid, A. G., Appl. Phys. Lett. (1975) 26, 612. 13. Greene, R. L., Street, G. B., Suter, L. J., Phys. Rev. Lett. (1975) 34, 577. 14. Lu,C.,Donohue, J., J. Am. Chem. Soc. (1944) 66, 818. 15. Sharma, B., Donohue, J., Acta Crystallogr. (1963) 16, 891. 16. "Handbook of Chemistry and Physics," 52nd Ed., p. E-72, The Chemical Rubber Co., Cleveland (1971-1972). RECEIVED January 24, 1975. Work supported in part by the National Science Foundation through the Laboratory for Research on the Structure of Matter and grants GH-39303 and GP-41766X, and by the Advanced Research Projects Agency through grant DAHC 15-72-C-0174.


Inorganic Compounds with Unusual Properties

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