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Pure &Appl . Chem.,Vol. 64 , No. 9, pp. 1311-1317,1992.Printed in Great Britain.@ 1992 IUPAC

Photochemistry and photophysics of coordinationcompounds of the main group metalsA. Vogler' and H. NikolInstitut fiir Anorganische Chemie, Universitat Regensburg, UniversitiitsstraBe 3 1,D-8400Regensburg

Abstract - The photoproperties of main group metal complexes with the electronconfigurations s2 (e.g. Tl+, b3+, Te 4+ )and so (e.g. T13+, b4+ )were studied on thebasis of a general concept which relates characteristic excited states to typicalphotophysical and photochemical processes. The photochemistry is dominated by metal-centered sp (s2) and ligand to metal charge transfer (so) excited states which are capableof inducing inter- and intramolecular redox reactions.I N T R O D U C T I O N

The photophysics and photochemistry of transition metal complexes has become an importantbranch of inorganic chemistry as well as photochemistry.l The excited state properties of thesecompounds are now fairly well understood. On the contrary, very little is known about thephotophysics and photochemistry of coordination compounds of the main group metak2Although this lack of knowledge seems to be quite surprising there are some obvious reasonswhich hamper the study of m ain group metal complexes. Frequently, these compounds arekinetically labile and can exist with variable coordination numbers and structures in the solidstate and in solution. In addition, the structures of many complexes deviate from highlysymmetrical geometries. These properties complicate the spectroscopic identification andcharacterization of main group m etal complexes. On the other side, there are also features whichfacilitate the investigation of these compounds. Since the valence shell includes only s and porbitals the d orbitals have not to be considered for low-energy electronic transitions. Comparedto transition metals the variation of stable oxidation states of main group metals is much sm aller.This confines the number of possible products which may be generated in photoredox reactions.A few years ago we started a systematic investigation of the photophysics and photochemistry ofthe main group metal complexes.2 In this short account it is not attempted to treat this subjectcomprehensively. The current state is illustrated by a few selected examples.

S T R U C T UR E A N D B O N D I N GBefore we enter the discussion of the photoproperties it is appropriate to give a short descriptionof the structure and bond ing of main group m etal complexes. These compounds can be classifiedaccording to the electron configuration of the free main group metal ions. The so and s2configurations are most important and correspond to the highest oxidation states and those whichare smaller by two units, respectively. The other configurations such as sl, s2p2 and s2p4 areless important and will not be discussed here.

Generally, so ions form highly symmetrical complexes and their bonding pattern is illustratedby a qualitative MO scheme of an octahedral complex (Figure l).3 The n interaction is om ittedfor clarity. The ix ligands are kept to the metal by only four bonds.1311

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1312 A. VOGLER AN D H. NIKOL

7 - 7I \I

t;, \I \

M

- g- -a 1g

a l R

Figure 1 Qualitative MO-schemeof an octahedralcomplex: n orbitals of the llgands are omitted.

II

/ I /

np - -, -!?/ \n s y \\

\

M

- - t,a1

Figure 2 Qualitative MO-schemeof SKI,- inT and Cpv (disphenoidal structure) symmetry:n orbitals of the ligands are omitted,

In the case of s2 ions two valence electrons enter the antibonding alg* orbital (Figure 1) andreduce the metal-ligand bond order to three. However, in accordance with Gillespies andNyholnis VSEPR model4 most s2 complexes undergo a second order Jahn-Teller distortionwhich stabilizes the antibonding HOMO.3 For example, a highly symmetrical, tetrahedral s2complex such as SbC14- is not stab le but converted to a disphenoida l (seesaw) structure (Figure2). This distortion is associated with sp orbital mixing. As a consequence both antibonding s2electrons are not only stabilized but become also stereochemically active by the admixture of p~ h a r a c t e r . ~

E LE C TR ON I C ABSO RPT IO N S P E C TR AAlthough our knowledge of the absorption spectra of m ain group metal com plexes is ratherlimited some important results have been collected and critically reviewed by C. K. J 6 r g e n ~ e n . ~Spectral studies of solids which contain main group metal ions provide an additional s ~ u r c e . ~ , ~

Owing to the empty valence shell simple complexes of so ions display only ligand to metalcharge transfer (LMCT) bands in their absorption spectra. Typical examples are the octahedralanions SnC162- and PbC162-. The allowed lowest-energy LMCT transition involves thepromotion of an electron from the ligand orbitals to the antibonding n lg* s-orbital at the metal(Figure 1). Since the oxidation strength of so ions increases from the top to the bottom of themain groups the LMCT band of PbC162- (Figure 3) appears at longer wavelength (Arnm = 318nm ; E = 10000) than that of SnCls2- (Arnm = 22 8 nm; E = 13500).8 Recently, it has beensuggested that this shift, which causes the yellow colour of the lead complex, is caused byrelativistic effects.9

The absorption spectra of simple s2 complexes can be analyzed in terms of the s + ptransitions of the free s2 ion6j7 (Figure 4). The IS0 + 3Po transition is strongly forbidden and

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Photochemistryof coordination com pounds of main group m etals 1313

AaA

250 300 350 400 nmFigure 3 Spectral changes during the photolysis of Figure 4 Energy level scheme of1.55 10-4.M [NBu4],PbCI, in W,CN at (a) 0, 20,60, 150 s, 6, 12, 25 and 35 min (h) irradiation time,with Airr = 313 nm; l-cm cell.

a free s2 ion.

not observed, The spin-forbidden transitions to the 3P1 and 3P2 levels give rise to the so-calledA and B bands which are of moderate intensity. The C band is more intense and belongs to thespin-allowed lS0 + lP1 transition. The energy and intensity of these bands depend on the centralmetal, the ligands and the structure of the complexes. In the case of SbC14- (Figure 2) thesebands appear as shoulders (Figure 5) at A = 283 nm (A), 255 nm (B) and 235 nm (C).l0

In addition to the metal-centered sp bands LMCT absorptions may appear at shorterwavelength if the metal ion is oxidizing. The corresponding transitions terminate at the empty porbitals. For example, TeC&j2- displays the longest-wavelength LM CT band ( t lu (ligand) + lu *@), Figure 1) at A = 255 nm.11j12

Frequently s2 ions are not oxidizing but reducing. Complexes are then expected to show long-wavelength metal to ligand charge transfer (M LCT) bands if the ligands provide empty orbitals atlow energies. Accordingly, compounds such as M111(2,2'-bipyridine)X3with M = Sb, Bi and X =C1, Br, I are coloured owing to the presence of such MLC T bands in the visible region.13 So fara detailed study of these interesting compounds has been hampered by their instability insolution.P HO TO P HY S ICS

LMCT excited states are generally not luminescent. In accord with this rule so complexes haveno t been observed to emit. On the contrary, s2 complexes are frequently emissive even underambient conditiom2 In 1940 Pringsheim and Vogels reported their pioneering discovery of thephotoluminescence of Tl', Sn2+,and Pb 2+ in aqueous s01u tion.l~ n the mean time the emissionof various s2 ions in solid matrices has been studied in detail71l5 while the solution luminescenceof s2 complexes has been largely ignored. Since s2 complexes are often dissociated and thus notwell defined in aqueous solution we studied these compounds in non-aqueous solution withsolvents of low coordinating ability. The results are not only quite interesting in their own rightbut also rathe r valuable with regard to o ther aspects of the chemistry of m ain group m etals.As discussed above the absorption bands of s2 complexes can be assigned to the electronictransitions of the free s2 ions. The photoluminescence of s2 complexes which originates from the3P sp excited state shows, however, a large Stokes shift, Consequently, the emitting species is nota largely unperturbed s2 ion but a molecule with a structure which is quite different from that ofthe ground state.

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1314 A. VOGLER AN D H. NIKOL

The electronic spectra of SbC14- (Figure 5)1 may be used to illustrate this emission behavior.We suggest that the ground state distortion of SbC14- is eliminated in the excited state, Thestabilization of the seesaw structure (C,,,, Figure 2) is essentially lost in the sp excited state whichrearranges toward a symmetrical tetrahedral geometry. Such a structural change is now consistentwith the huge Stokes shift of SbC14-. The 3P1--31S0 phosphorescence which is associated with arestoration of the distortion occurs with a rate k = 8.3 * lo5 s - l .

Recently, we extended these luminescence studies to the complex ions M111C13- (M = Ge, Sn,Pb) which are trigonal pyramidal (C3,,) in the ground state and rearrange toward a highlysymmetrical trigonal planar (D3h) structure in the sp excited state.16 An analogous approach hasbeen used before in solid state chemistry. Blasse et al. have shown that s2 ions doped into hostcrystals may also undergo emissions with large Stokes shifts if the ions occupy off-center positionsin large interstices since these s2 ions can move toward the center in the sp excited state.I5 Inconclusion, it is remarkable that the elimination of the distortion in the excited state of s2complexes is quite opposite to the behavior of most transition metal complexes which are rathersymmetrical in the ground state but distorted in their excited states.

P H O T O C H E M I S T R YLigand substitutions and redox processes are the most important photoreactions of transitionmetal comp1exes.I Both reactions may also occur simultaneously as coupled processes. Reductiveeliminations and oxidative additions belong to this category.

Photosubstitutions of main group metal complexes could also occur but are not expected toyield stable products. Owing to the kinetic lability a ligand substitution or dissociation would befollowed by a facile regeneration of the starting complex. Consequently, main group metalcomplexes will mainly undergo photoredox reactions. Frequently, so and s2 complexes areinterconverted. In many cases these two-electron redox processes are accompanied by a change ofthe coordination number. Reductive eliminations and oxidative additions are thus commonphotoreactions of main group metal complexes.

Complexes of so metals are characterized by low-energy LMCT states. In accordance with thenature of this excited state the metal undergoes a reduction and the ligand an oxidation. Thereductive elimination of the octahedral anion PbC162- is a typical example of the photoreactivityof so complexes. LMCT excitation is followed by the elimination of C12. Since PbC142- is notvery stable (PbC13- + C1- + PbC142-; k = 8.5 * lo-* lrnol-l)'6 the product formation (9 =0.21 at Xirr = 313 nm ) occurs according to the stoichiometry :As indicated by the spectral variations (Figure 3) this photolysis is a clean reaction which can bedriven to completion. Although no attempt was made to detect intermediates the primaryphotochemical step may yield Pb?

Pb'VC1G2- + Pb"'C152- + * C1This assumption is based on the observation that the flash photolysis of PbC14 generates PbC13and * C1 as primary products.17 The formation of Pb2+ takes place only in secondary reactions.

The tetrahedral anion TlCl4- undergoes the photochemical elimination of C12 in deaeratedaqueous solution.I2 In order to prevent an aquation of this complex the photolysis was carriedout in solutions saturated with KCl. Light absorption by the longest-wavelength LMCT band(Amm = 24 7 nrn, E = 15850) of TlCl4- led to the formation of T1+ with a quantum yield 9 =4.5 * 10-3 at Xirr = 254 nm.

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Photochemistry of coordination compounds of main group metals 13151.6A

0.8

0.0

A821.0

0.5

0.0200 300 400 650 900 nm 200 300 400 500 runFigure 5 Electronic absorption (left side) and Figure 6 Left side: Absorption spectrum ofemission (right side) spectra of [NEt4]SbC14 2.17. M TIC1 in saturated aqueousin CH,CN at room temperature, l-cm cell. KCI solution (4.0 M). Right side: Emission

Absorption: 2.54 M; spectrum of 4.61 - M TIC1 in satu-Emission: 5.0 rated aqueous KCI solution (4.0 M) atroom temperature, l-cm cell,hex,=254 nm.M, A = 290 nm

Metal ions with a s2 configuration may be photoreduced or photooxidized. Oxidizing s2complexes such as TeC162- have LMCT states at accessible energies.ll>l2 LMCT excitation ofthis complex in CH3CN led to a reductive elimination (4 = at hirr = 254 nm) accordingto the equation:

Te'VC162- + Te11C142- + c12Chlorine was detected and determined spectrophotometricallyas C13- (A = 231 nm).18

The majority of s2 ions are reducing and less oxidizing. These s2 ions such as Tl+, n2+,andSb3' are photooxidized by 0 2 to TI3+ 14119,Sn4+ 14~20,and Sb5+ 21,22 according to theequation: M n t + O2 j M(n+2)++ 022-These photoreactions which have been known for 50 years are not only interesting in their ownright but are also of considerable importance for industrial applications such as the electrolessmetal deposition.20 Surprisingly, the nature of the reactive excited state and the mechanism ofphotooxidation has not been studied until recently.22 A detailed investigation was carried outwith SbC1b3- in CHC13. Analogous results were obtained with Tl+ n aqueous chloride.12 Firstobservations on the photooxidation of Tl+were reported by Kirkbright et al.19

Upon addition of TlCl to concentrated aqueous solutions of KC1 a chloro complex of unknowncomposition is formed. This complex is characterized by an absorption (A = 240 nm, E =4516) and an emission (Figure 6) at A = 464 nm (4 = 0.88, T = 0.62. s in theabsence of The absorption leads directly to the population of the emitting sp excited state(3P1 of the free TI+ ion). Upon exposure to air the phosphorescence is quenched to 4 = 0.1 atan oxygen concentration of M. Simultaneously, these solutions become light sensitive. Thephotolysis proceeds according to the stoichiometry (4 = 3.3 . at Xirr = 254 nm):

Tl+ + 4 C1- + 2 H + + O2+ T1111C14- + H 2 0 2The spectral changes which are associated with this photoreaction appear as the reversal of thosewhich were observed during the photolysis of TlC14- (see above).

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1316 A. VOGLER AND H. NIKOL

The photooxidation of Tl+by oxygen in the presence of C1- is assumed to proceed according tothe following simplified reaction scheme:Tl+ + h v + T l + *l l + * + T l + + hvTl+ + Tl+ + heatTl+* O2 + 4 C1- + 2 H++ 1111C14- + H202

light absorption ('So+ 3P,)phosphorescence (3P1+ 'So)radiationless deactivation and othernonreactive quenching processesphotooxidation/oxygen quenching

The quantum yield of photooxidation is calculated to be #J = 0.9 if the emission data areapplied to this kinetic scheme. However, since the experimental photooxidation quantum yieldwas only #J = 0.03 oxygen quenching leads only partially to the photooxidation or thephotooxidation is partially reversed by an efficient back electron transfer.

In the presence of an excess of oxygen and chloride a photostationary state is established sinceTI+ s photooxidized and Tl3+ photoreduced.12 The overall reaction0 2 + 2 HC1 + H202 + C12

proceeds then as a photocatalysis.

O U T L O O KThe photochemistry of main group metal complexes seems to become an important part ofinorganic photochemistry. While considerable progress has been achieved with simplecoordination compounds the photochemistry of main group organometallics has not yet beenstudied systematically although sporadic observations have been reported. With regard topotential applications the photocatalysis by s2/so metal ion redox pairs is anticipated to play animportant role in the future. The dehydrodimerization of organic compounds by atomic mercuryin the gas phase24 as well as in solutiond5 may be a first example of such a photocatalysis.Although the mechanism of this photolysis is largely unknown the redox pair Hg(s2)/Hg2' (so)could act as photocatalyst. The conversion and chemical storage of solar energy might be anotherattractive application of the photocatalysis by s2/so systems. Finally, it should be kept in mindthat most main group metal compounds absorb only UV light. It is desirable to photosensitizereactions of such compounds for visible light. That this is possible has been shown in a few cases.For example, the photocatalytic oxidation of alkenes was achieved with visible light by usingantimony porphyrins as sensitizers and catalysts.26 However, more studies are needed to fullyunderstand and optimize such useful photoreactions.

AcknowledgementsSupport of this research by BMFT (Grant 0329075A) and the Deutsche Forschungsgemeinschaft(Grant Vo 211/8-1) is gratefully acknowledged. We thank P. Hub er for emission lifetimemeasurements.

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(b) Concepts of Inorganic Photochemistry; Adamson, A. W., Fleischauer, P. D., Eds.; Wiley-(c) Ferraudi, G. J. Elements of Inorganic Photochemistry; Wiley-Interscience: New York,

New York, 1970.Interscience: New York, 1975.1988.(2) Vogler, A.; Paukner, A.; Kunkely, H. Coord. Chem. Rev. 1990, 97, 285.

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(3 ) Albright, T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital Interactions in Chemistry;Wiley:(4 ) (a) Gillespie, R. J.; Nyholm, R. S . Q. ev. Chem. SOC.1957, 1, 339.New York, 1985.(b) Gillespie, R. J. Molecular Geometry; Van Nostrand Reinhold: London, 1972.(c) Gillespie, R. J.; Hargittai, I. The VSEPR Model of Molecular Geometry; Allyn and(5 ) (a) Jfirgensen, C. K. Absorption Spectra and Chemical Bonding in Complexes; PergamonBacon: Boston, 199 1 .Press: Oxford, 1964.(b) Jfirgensen, C. K. Oxidation Numbers and Oxidation States; Springer: B erlin, 1969.(6) Fukuda, A.; Inohara, K.; Onaka, R. J. Phys. SOC.Japan 1964,19, 274.(7) Ranfagni, A.; Mugnai, D.; Bacci, M.; Viliani, G.; Fontana, M. P. Adv . Phys. 1983,32, 23 .(8 ) Becht, A. diploma thesis; University of Regensburg, 1990.(9 ) (a) Pyykko, P. Chem. Rev. 1988, 8,563.(10) Nikol, H.; Vogler, A. J. Am. Chem. SOC. 1991, 13, 988.(11) (a) Couch, D. A.; Wilkins, C. J.; Rossman, G. R.; Gray, H. B. J. Am. Chem. SOC.

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