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.
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