A Vogler

AG Lee

G Sutton

JK Kochi

R Faggiani

SR Sagi

W Beck

English

Paukner, A.

Kunkely, Horst

Vogler, Arnd

University of Regensburg Publication Server

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 interesting but largely neglected part of inorganic photochemistry. Within a general approach to t h i s subject we studied recently photoredox reactions of [Sn(Ng)g] 2- 1) and [ pb(N 3)g] These ions are examples of complexes containing a metal with an empty valence she l l (s° electron configuration). Such compounds are characterized by low-energy ligand to metal charge transfer (LMCT) bands in t h e i r absorption spec-t r a . Reductive elimination i n i t i a t e d by LMCT excit a t i o n seems to be the t y p i c a l photo-reaction of s° complexes. We extended now these investigations to T l ( I I I ) complexes in order to test 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 electronic absorption bands were reported for a few tetrahalogeno 4 5) complexes of T l ( I I I ) * . In the present work we studied the photochemistry of [ T l ( N 3 ) 2 B r 2 r , [ T l ( b i p y ) 2 I 2 ]+ (bipy = 2,2'-bipyridyl), and [Tl (acetate) 3 ] in some d e t a i l . Spectral assignments of absorption bands were supported by the photo-chemical behavior of these complexes. Results and Discussion The compounds [ A s ( C 6 H 5 ) 4 ] [ T l ( N 3 ) 2 B r 2 J 6 ) and [Tl ( b i p y ) 2 I 2 ] I 7 \ were prepared by l i t e r a t u r e procedures. T1(CH 3C0 2) 3 x 1.5 H20 was purchased from A l d r i c h . The absorption spectrum of [ T l ( N 3 ) 2 B r 2 l " in a c e t o n i t r i l e displays a long-wavelength band at x m a v = 293 nm (e = 10200 L mol"1 cm" 1). This band i s assigned to max 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 to distinguish between N3" - T l ( I I I ) and Br" - T l ( I I I ) CT tra n -s i t i o n s . However, the photoreaction supports the former assignment. At shorter wave-length the t y p i c a l absorption features of the counterion [As(CgHg)^] + appear in the spectrum ( x m a x = 271, 265, 259 nm) 6^. Upon i r r a d i a t i o n of the LMCT band ( x i r r = 313 nm) of [Tl ( N ^ B ^ ] " in CH^CN the evolution of nitrogen was observed. Simultaneously, the inte 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 photolysis went to completion. In the la t e r stages of the photolysis the solution became cloudy due to the formation of insoluble TIBr. These observations are consistent with a reductive elimination: [ T l I H ( N 3 ) 2 B r 2 ] " T ir B r + Br" + 3 N 2 The disappearance of [ T l ( N 3 ) 2 B r 2 3 " was determined by the decrease of the extinction at x m a x = 293 nm. Upon i r r a d i a t i o n at 313 nm the reductive elimination occured with a quantum y i e l d of * = 0.3. abs Fig. 1. Spectral changes during the photolysis of 5.33-10"5 M [ T l ( b i p y ) 2 I 2 3+ in CH3CN at (a) 0 and (d) 200 s i r r a d i a t i o n time, with x . r r > 200 nm and a 1-cm c e l l . The absorption spectrum of CTl(bipy>2 I2^ + i n C H 3 C N (Fig* 1 ) consists of 3 bands at x m a v = 374 nm ( e = 4700), 302 nm (23210), and 244 nm (28200). The longest max wavelength band at 374 nm may be due to a J" - T l ( I I I ) LMCT t r a n s i t i o n since 4 5) [ T l l ^ ] shows such an absorption at x m a x = 397 nm ' . However, the photo-chemical behavior of [ T l ( b i p y ) 2 I 2 ]+ ( s e e below) i s not consistent with t h i s assignment. As a reasonable alternative the band at 374 nm may be assigned to a J" -bipy ligand to ligand (LL) CT t r a n s i t i o n . The complex [BefbipyJI^ shows such a LLCT band at 368 nm ( e = 7000) The second band of [ T l ( b i p y ) 2 I 2 ]+ at 302 nm should be assigned to the ™ * intraligand (IL) t r a n s i t i o n of the bipy ligand which absorbs in 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 contribute to t h i s band since low-energy absorptions' of t h i s type are expected to appear near t h i s wave-length 4' 5^. The t h i r d absorption at 244 nm is c e r t a i n l y a f - T l ( I I I ) LMCT band in agreement with the photochemical behavior of [ T l ( b i p y ) 2 I 2 l+ . I r r a d i a t i o n ( x m a x > 200 nm) of [ T l ( b i p y ) 2 I 2 ]+ in CH^CN was accompanied by spectral changes (Fig. 1) which are consistent with a reductive elimination according to: [ T i n l ( b i p y ) 2 I 2 ]+ - T l + + 2 bipy + I £ In the photolyzed solution the absorption maximum at 360 nm i s caused by I2» The new band at 286 nm i s apparently a superimposition of absorption maxima of I 2 (xm a x = 289 nm) and free bipy ( * v = 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 ) 2 I 2 f the quantum yiel d s of the reductive elimination (* = 0.54 at x. r r = 254 nm, 0.03 at 302 nm, and 2.9x10 at 366 nm) decreased with increasing wavelength of i r r a d i a t i o n . The photochemistry of some thai 1ium(111) carboxylates of the type T1(RC00) 3 where 2) R i s a larger a l i p h a t i c group was studied by Kochi and Bethea . The photolysis of these compounds in benzene solution led to the formation of T l ( I ) and oxidation of carboxylate (RC00" - e" -R - + C 0 2 ) # In the present work we investigated the photolysis of T1(CH 3C00) 3 which does not dissolve in benzene. In the s o l i d state T l ( I I I ) i s e s s e n t i a l l y hexacoordinated by three chelating acetate ligands . In aqueous solution the acetate ligands are p a r t i a l l y substituted Since in a c e t o n i t r i l e such ligand substitutions are generally less e f f i c i e n t i t is'assumed that T1(CH 3C00) 3 dissolves in CH3CN with-out d i s s o c i a t i o n . The absorption spectrum of T1(CH 3C00) 3 in a c e t o n i t r i l e (Fig. 2) abs 0.7 0.0 200 300 400 500 nm Fig, 2 Absorption spectra of 9.9x10~5 M [Tl(CH 3COO) 3] (a) and i t s photolysis product (b) in CH3CN, x i r r > 200 nm, 1-cm c e l l i s rather featureless. The complex starts to absorb in the v i s i b l e region. The absorp-t 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 to acetate - T l ( I I I ) LMCT t r a n s i t i o n s . The spectral changes which accompanied the i r r a d i a t i o n (white l i g h t from a high-pressure mercury arc) of T1(CH 3C00) 3 in CH3CN indicated the formation of T l ( I ) acetate (Fig. 2) which shows absorption bands a t x ^ = 256 nm and 214 nm. Although the oxidation products were not i d e n t i f i e d i t i s assumed that the reductive elimina-t i o n takes place according to the equation: T l I n ( C H 3 C 0 0 ) 3 -^* Tl^CH^OO) + 2 C0 2 + 2 CH^ The methyl radicals may undergo dimerization or other secondary reactions. At x. 254 nm the quantum y i e l d for the disappearance of T1(CH 3C00) 3 was * = 0.14. It dropped to * = 0.01 at * i r r = 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 cited therein. 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) SuttonTS. J. Austr. J. Sei. 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. 

Photochemistry of Coordination Compounds of Main Group Metals. Reductive Elimination of Thallium(III) Complexes

Photochemistry of Coordination Compounds of Main Group Metals. Reductive Elimination of Thallium(III) Complexes

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