PHOTOCHEMISTRY OF COORDINATION COMPOUNDS OF MAIN GROUP PETALS.
REDUCTIVE ELIMINATION OF THALLIUM(III) COMPLEXES
A.Paukner, H.Kunkely, and A.Vogler
I n s t i t u t für Anorganische Chemie der Universität Regensburg, Universitä'tsstr. 31,
8400 Regensburg, FRG
Introduction
The photochemistry of coordination compounds of the main group metals i s a rather
i n t e r e s t i n g but l a r g e l y neglected part of inorganic photochemistry. Within a general
approach t o t h i s subject we studied r e c e n t l y photoredox reactions of [Sn(Ng)g]
2- 1)
and [ b ( N ) g ]
p
3
These ions are examples of complexes containing a metal with
an empty valence s h e l l (s° e l e c t r o n c o n f i g u r a t i o n ) . Such compounds are characterized
by low-energy ligand t o metal charge t r a n s f e r (LMCT) bands i n t h e i r absorption spect r a . Reductive e l i m i n a t i o n i n i t i a t e d by LMCT e x c i t a t i o n seems t o be the t y p i c a l photor e a c t i o n of s° complexes. We extended now these i n v e s t i g a t i o n s t o T l ( I I I ) complexes i n
order t o t e s t the general v a l i d i t y of t h i s assumption.
2 3)
There i s not much known about the photochemistry of T l ( I I I ) complexes
'
. Spec-
t r a l assignments of e l e c t r o n i c absorption bands were reported f o r a few tetrahalogeno
4 5)
complexes of T l ( I I I )
*
. In the present work we studied the photochemistry of
[Tl(N ) Br r, [Tl(bipy) I ]
3
2
2
2
2
+
(bipy = 2 , 2 ' - b i p y r i d y l ) , and [Tl ( a c e t a t e ) ]
3
in some d e t a i l . Spectral assignments of absorption bands were supported by the photochemical behavior of these complexes.
Results and Discussion
The compounds [ A s ( C H ) ] [ T l ( N ) B r J
6
5
4
3
2
6 )
2
and [ T l ( b i p y ) I ] I
2
were prepared by l i t e r a t u r e procedures. T 1 ( C H C 0 )
3
2
2
7
\
x 1.5 H 0 was purchased
3
2
from A l d r i c h .
The absorption spectrum of [ T l ( N ) B r l " i n a c e t o n i t r i l e d i s p l a y s a longwavelength band at x
= 293 nm (e = 10200 L mol" cm" ). This band i s assigned t o
max
3
2
2
1
1
m a v
a LMCT t r a n s i t i o n . Since azide and bromide have comparable o p t i c a l e l e c t r o n e g a t i v i t i e s
Photochemistry and Photophysics
of Coordination Compounds, Ed. by H. Yersin/A. Vogler
© Springer-Verlag Berlin • Heidelberg 1987
i t i s d i f f i c u l t t o d i s t i n g u i s h between N " - T l ( I I I ) and Br" - T l ( I I I ) CT t r a n 3
s i t i o n s . However, the photoreaction supports the former assignment. At shorter wavelength the t y p i c a l absorption features of the counterion [As(CgHg)^]
in the spectrum ( x
+
appear
= 271, 265, 259 nm) ^ .
6
m a x
Upon i r r a d i a t i o n of the LMCT band ( x
= 313 nm) of [Tl ( N ^ B ^ ] " i n
i r r
CH^CN the e v o l u t i o n of nitrogen was observed. Simultaneously, the i n t e n s i t y of the
LMCT band decreased.
F i n a l l y , t h i s absorption disappeared when the p h o t o l y s i s went to
completion. In the l a t e r stages of the p h o t o l y s i s the s o l u t i o n became cloudy due t o
the formation of i n s o l u b l e TIBr. These observations are c o n s i s t e n t with a reductive
elimination:
[Tl
The disappearance
e x t i n c t i o n at x
m a x
I H
(N ) Br ]"
3
2
2
T i B r + Br" + 3 N
r
2
of [ T l ( N ) B r 3 " was determined by the decrease of the
3
2
2
= 293 nm. Upon i r r a d i a t i o n at 313 nm the reductive e l i m i n a t i o n
occured with a quantum y i e l d of * = 0.3.
abs
Fig. 1.
Spectral changes during the p h o t o l y s i s of 5.33-10" M [ T l ( b i p y ) I 3
5
2
at (a) 0 and (d) 200 s i r r a d i a t i o n time, with x .
r r
2
+
i n CH CN
3
> 200 nm and a 1-cm c e l l .
The absorption spectrum of CTl(bipy>2 2^
I
bands at x
max
m a v
+
i n
C H
3
(Fig* ) c o n s i s t s of 3
C N
1
= 374 nm ( = 4700), 302 nm (23210), and 244 nm (28200). The longest
e
wavelength band at 374 nm may be due t o a J " - T l ( I I I ) LMCT t r a n s i t i o n since
4 5)
[Tll^]
shows such an absorption at x
chemical behavior of [ T l ( b i p y ) I ]
2
(
+
2
= 397 nm
m a x
s
e
e
'
. However, the photo-
below) i s not c o n s i s t e n t with t h i s
assignment. As a reasonable a l t e r n a t i v e the band at 374 nm may be assigned t o a J " bipy ligand t o ligand (LL) CT t r a n s i t i o n . The complex [ B e f b i p y J I ^ shows such a LLCT
band at 368 nm ( = 7000)
The second band of [ T l ( b i p y ) I ]
e
2
+
2
at 302 nm should
be assigned t o the ™ * i n t r a l i g a n d (IL) t r a n s i t i o n of the bipy ligand which absorbs i n
t h i s region. An I " - T l ( I I I ) LMCT t r a n s i t i o n could also c o n t r i b u t e t o t h i s band
since low-energy absorptions' of t h i s type are expected t o appear near t h i s wavelength ' ^ . The t h i r d absorption at 244 nm i s c e r t a i n l y a f - T l ( I I I ) LMCT band
4
5
in agreement with the photochemical
Irradiation
(
x
m a x
behavior of [ T l ( b i p y ) I l .
+
2
> 200 nm) of [ T l ( b i p y ) I ]
2
2
+
2
i n CH^CN was accompanied by
s p e c t r a l changes ( F i g . 1) which are c o n s i s t e n t with a reductive e l i m i n a t i o n according
to:
[Ti
n l
(bipy) I ]
2
+
2
- T l + 2 bipy + I
+
£
In the photolyzed s o l u t i o n the absorption maximum at 360 nm i s caused by I » The new
2
band at 286 nm i s apparently a superimposition of absorption maxima of I
289 nm) and f r e e bipy ( *
v
(
2
x
m a x
=
= 280 nm). The presence of released bipy i s also i n d i -
ITlaX
cated by bands at x
= 235 and 243 nm.
max
In accordance with the assignments of the absorption bands of [ T l ( b i p y ) I
2
the quantum y i e l d s of the reductive e l i m i n a t i o n (* = 0.54 at x .
302 nm, and 2.9x10
The photochemistry
r r
2
f
= 254 nm, 0.03 at
at 366 nm) decreased with i n c r e a s i n g wavelength of i r r a d i a t i o n .
of some t h a i 1ium(111) carboxylates of the type T1(RC00) where
2)
3
R i s a l a r g e r a l i p h a t i c group was studied by Kochi and Bethea
. The p h o t o l y s i s of
these compounds i n benzene s o l u t i o n led t o the formation of T l ( I ) and o x i d a t i o n of
carboxylate (RC00" - e" -R - + C 0 )
2
#
In the present work we i n v e s t i g a t e d the p h o t o l y s i s of T1(CH C00)
3
3
which does
not d i s s o l v e i n benzene. In the s o l i d s t a t e T l ( I I I ) i s e s s e n t i a l l y hexacoordinated by
three c h e l a t i n g acetate ligands
p a r t i a l l y substituted
. In aqueous s o l u t i o n the acetate ligands are
Since i n a c e t o n i t r i l e such ligand s u b s t i t u t i o n s are
g e n e r a l l y less e f f i c i e n t i t is'assumed that T1(CH C00)
3
out d i s s o c i a t i o n . The absorption spectrum of T1(CH C00)
3
d i s s o l v e s i n CH CN w i t h -
3
3
3
i n a c e t o n i t r i l e ( F i g . 2)
abs
0.7
0.0
300
200
400
500
nm
Fig, 2
Absorption spectra of 9.9x10~
in CH CN, x
3
i r r
M [Tl(CH COO) ] (a) and i t s p h o t o l y s i s product (b)
5
3
3
> 200 nm, 1-cm c e l l
i s rather f e a t u r e l e s s . The complex s t a r t s t o absorb i n the v i s i b l e region. The absorpt i o n increases towards shorter wavelength. A maximum appears at 203 nm (e =12430)
while shoulders occur at 240 nm ( e = 6440) and 395 nm ( e = 1660). These absorptions
can c e r t a i n l y be assigned t o acetate - T l ( I I I ) LMCT t r a n s i t i o n s .
The s p e c t r a l changes which accompanied the i r r a d i a t i o n (white l i g h t from a highpressure mercury arc) of T1(CH C00)
3
3
i n CH CN i n d i c a t e d the formation of T l ( I )
3
acetate ( F i g . 2) which shows absorption bands a t x ^ = 256 nm and 214 nm. Although
the o x i d a t i o n products were not i d e n t i f i e d i t i s assumed that the reductive e l i m i n a t i o n takes place according t o the equation:
Tl (CH C00)
I n
3
3
-^* T l ^ C H ^ O O ) + 2 C0 + 2 CH^
2
The methyl r a d i c a l s may undergo d i m e r i z a t i o n or other secondary r e a c t i o n s . At x.
254 nm the quantum y i e l d f o r the disappearance of T1(CH C00) was * = 0.14. I t
3
dropped t o * = 0.01 at *
i r r
3
= 395 nm.
References
1) Vogler, A.; Quett, C ; Paukner, A.; Kunkely, H. J . Am. Chem. Soc. 1986, 108,
8263.
2) Kochi, J . K.; Bethea, T. W. J . Org. Chem. 1968, 33, 75.
3) Sagi, S. R.; Prakasa Raju, 6. S.; Appa Rao, K.; Prasada Rao, M. S. Talanta 1982,
29, 413 and references c i t e d t h e r e i n .
4) Matthews, R. W.; Walton, R. A. J . Chem. Soc. A 1968, 1639.
5) Day. P.; Seal, R. H. J . Chem. Soc. Dalton 1972, 2054.
6) Beck, W.; Fehlhammer, W. P.; Pöllmann, P.; Schuierer, E.; F e l d l , K. Chem. Ber.
1967, 100, 2335.
7) S u t t o n T S . J . Austr. J . S e i . Res., A 1951, 4, 654.
8) Coates, G. E.; Green, S. I . E. J . Chem. Soc. 1962, 3340.
9) Faggiani, R.; Brown, I . D. Acta Cryst. B 1978, 34, 2845.
10) Lee, A. G. The Chemistry of Thallium, Elsevier,"Amsterdam 1971.