Photochemical properties of 1,3-diketonate transition metal chelates

Journal of Photochemistry

and Photobiology,

A; Chemistry,

52 (1990)

1 - 25

REVIEW PHOTOCHEMICAL PROPERTIES OF 1,3-DIKETONATE TRANSITION METAL CHELATES BRONISLAWMARCINIAK+ Faculty

of

Chemistry,

A. Mickiewicz

University,

60-780

Poznan

(Poland)

GONZALO E. BUONO-CORE Instituto

de Quimica,

Universidad

Catolica de Valparuiso,

Casilla 4059,

Valparaiso

(Chile)

(ReceivedSeptember6,1989)

Summary

The latest results concerning the photochemistry and sensitized photoreduction of transition metal 1,3-diketonates are examined. The physical and chemical aspects of the interaction mechanisms between excited singlet and triplet states of organic compounds and 1,3-diketonates are discussed. Some examples of the uses of transition metal 1,3-diketonates in photocatalysis are presented.

1. Introduction The photochemical and photophysical properties of many coordination compounds have been extensively studied in recent years and the results can be found in several monographs and reviews [ 1 - 41. The 1,3-diketonate metal complexes are some of the most widely studied compounds. The coordination ability of the 1,3-diketones is well established and large numbers have been synthesized as ligands to form complexes with various metals [ 51. Since 1,3-diketones can exist in keto and enol forms, under appropriate conditions the enolic hydrogen can be replaced by a metal cation to form the essentially planar six-membered chelate ring.

+r R2

RI

R3

0

\

0

%i

+Author

to whom

lOlO-6030/90/$3.50

correspondence

should

be addressed.

0 Elsevier Sequoia/Printed

in The Netherlands

2

have been known since the nineteenth century. Metal 1,3-diketonates Since then, their synthesis and physical and chemical properties have been described in hundreds of papers and several reviews [ 5 - 91. The great interest in the study of these complexes has been stimulated by their potential application in areas such as lH and 13C nuclear magnetic resonance (NMR) spectroscopy (shift reagents), laser technology (laser chelates), gas chromatography, the polymer industry, chemical catalysis and as anti-inflammatory and antifungal agents [ 5,8,10 - 121. The expected application of 1,3-diketonate chelates in photocatalysis, as “UV stabilizers” and as laser chelates has motivated the study of their photochemical and photophysical properties. In addition, their physical and chemical properties, e.g. solubility in a wide range of solvents, high vapour pressure, specific electronic structure (various types of low-lying excited states) and the ease of preparation with various ligands and metals has led to their application as model coordination compounds for photochemical studies. The use of sensitizers has opened up a new research area for the study of the photochemistry of 1,3-diketonate chelates. In the early 196Os, the pioneer work of El-Sayed and Bhaumik [133 first suggested the occurrence of an energy transfer from the triplet state of a sensitizer to the triplet state of a ligand in metal 1,3-diketonates. Since then a large number of reports have been published describing the possible mechanisms of interaction between excited state sensitizers and 1,3-diketonate cheIates. Owing to the large number of excited states in these chelates and their redox properties, several mechanisms may be involved in this interaction. The use of different types of sensitizers together with various 1,3-diketonate chelates and the results from direct and sensitized photolyses have contributed to the elucidation of the role played by the metal cation, the ligand, the sensitizer and the solvent. The results of these photophysical and photochemical studies are summarized in this review.

2. Absorption, metal chelates

luminescence

and photophysical

properties of 1,3-diketonate

The absorption spectroscopy of metal 1,3_diketonates has been extensively studied in the last few years. Most of the investigations have been connected with research aspects, such as the effects of substituents, ligands, metal cations and solvents, and also correlations between the energy of the charge transfer bands or ligand field bands and the reduction potential of the complexes [ 5, 6, 9, 141. A comparison of the experimental absorption spectra with the results from semiempirical calculations has enabled the absorption bands to be assigned to different electronic transitions. Transition metal 1,3-diketonates are characterized by absorption spectra rich in bands in the UV-visible region which can be assigned to three basic types of electronic transition:

(1) d-d transitions (ligand field transitions) arising from electronic transitions between the metal d orbitals perturbed by the ligand field; they are localized on the central metal atom and their properties depend on the number of d electrons, type of atomic term, local symmetry around the metal atom and the nature of the ligand field; their molar absorption coefficients E are in the range lo1 - 10’ M-l cm-*; (2) intraligand transitions arising from electronic transitions between molecular orbitals localized on the ligand; if these orbitals are not significantly perturbed by complex formation (as in the case of 1,3-diketonate chelates), intraligand transitions in the absorption spectra of the complexes (e.g. x --f n*) equivalent to the transitions in the free ligand (e.g. 7r+ 7rIT* in 1,3-diketonate anion) are observed; (3) charge transfer (CT) transitions involving electronic transitions from a metal-centred orbital to a ligand-localized orbital (metal-to-ligand charge transfer (MLCT)) or from a ligand-localized orbital to a metal-centred orbital (ligand-to-metal charge transfer (LMCT)). Figures 1 and 2 present the UV-visible absorption spectra of copper(I1) 1,3-diketonates in solution as typical examples of transition metal 1,3diketonate spectra. The observed absorption bands of Cu(acac)* (acac = 2,4_pentanedionate anion) have been assigned to the various electronic transitions on the basis of correlations with semiempirical calculations [15 211. In order to simplify the discussion, the observed bands are labelled as I - V in decreasing order of transition energy (Table 1). Except for the controversy about the assignment of band I (in the vacuum UV region), the remaining bands have been assigned as follows: band II (X = 250 nm) as the LMCT transition, band III (A = 290 nm) as the singlet-singlet intraligand 7~+ 7~* transition and bands IV and V (in the visible region) as the X

200

Fig. 1. The absorption

250

spectrum

Inm)

300

of Cu(acac)*

LOO

in methanol

500

700

at room temperature.

4

I

I

1

700 k

Fig. 2. The absorption spectra benzene at room temperature.

TABLE

900

800

II

10

(nm)

of Cu(acac)z

(full

line)

and Cu(hfac)g

(broken

line)

in

1

Transition copper(I1) Compound

Cu(acac)z

I

I

600

500

LOO

energies and 1,3-diketonates

molar

Solvent

Band

Methanol

Benzene Cu(hfac)*

Benzene

Na(acac)

Methanol

aAfter resolving ref. 22.

into

absorption

two

coefficients

-

(nm)

I II III

= 50000 41500 34100

= 200 241 293

IV V

15800

635

IV V IV V III

18500 15000 17200 14700 34100

542a 665= 580a 680a 293

components

the

absorption

h

(Vcl-6)

gaussian

in

according

cm-r)

spectra

of

Type of transition LMCT n-+7r* (intraligand)

=9400 14500 25600 49

d-d

35 40 22 26 20300

d-d d-d d-d d-d rr+lr* (intraligand)

to

described

procedure

in

5

d-d transitions. A comparison of Cu(acac), and Na(acac) absorption spectra (Table 1) provides an additional argument for the assignment of band III as the intraligand w + 7~*transition. The absorption spectra of several transition metal 1,3-diketonates have been analysed in a similar way to the copper(I1) complexes [ 5, 9, 14, 15, 18, 19, 233. Depending on the symmetry of the complex, the type of metal, its oxidation state and ligand type, absorption spectra with more or less bands are observed. In the absorption spectra of the 1,3-diketonates of the lanthanides, the following types of electronic transition are expected: intraligand and CT transitions, 4f” + 4f” -l d transitions and f-f transitions. Since the 4f orbitals are effectively shielded from the environment by the 5s and 5p orbitals, the f-f absorption bands are very sharp [ 24, 251. In fact, they are similar to those of free atoms and quite different from the broad absorption d-d bands of the transition metal complexes. The 1,3-diketonate chelates of transition metals do not emit in solution at room temperature or 77 K, with the exception of chromium(II1) complexes [ 91. In the particular case of Cr(acac)g, the observed phosphorescence (at 785 nm) in rigid solutions at 77 K has been assigned to a transition between the lowest dd excited state and the ground state of the complex (2E, + 4A2g). These luminescence properties enable more intense spectroscopic and photophysical studies to be carried out for chromium(II1) 1,3-diketonates 19, 26 - 313. In contrast with transition metal complexes, the 1,3-diketonates of lanthanides exhibit luminescence in solution, even at room temperature. The emission can occur from the excited ff level of the metal or from the excited state of the ligand if that is the lowest excited state of the complex. In the first case, sharp bands are observed due to f-f transitions centred on the metal ion, similar to those in the absorption spectra [24, 253. However, in the second case, e.g. for Gd(acac)s, the characteristic phosphorescence from the ligand triplet state is observed in rigid solution at 77 K 132, 331. This emission is similar to that shown by group 1, 2 and 3 metal acetylacetonates, e.g. Na(acac), Mg(acac)z and Al(acac)3 19, 34 - 361. Since different metals in the 1,3diketonate complex do not significantly change the energy of the intraligand 7r+ 7c* transition [9], and the phosphorescence spectra of the complexes under identical experimental conditions show a similar structure, it is reasonable to assume that the energy of the ligand triplet state is approximately constant regardless of the metal ion. However, this assumption is not fulfilled for other parameters of the triplet state. For example, the lifetime of this state changes significantly depending on the metal in the complex [34, 35, 371. The transition opposite to phosphorescence, i.e. the spin-forbidden transition to the intraligand triplet state, is usually masked by intense CT or 7r+-77* (singlet-singlet) bands in the absorption spectra. However, it has recently been proved by Ohno et al. [38] that the absorption band at h x 400 nm for chromium(II1) acetylacetonate can be assigned to an intraligand

6

singlet-triplet transition. This is the first direct observation of a singlettriplet absorption within the ligand which is well separated from other absorption bands in the spectra of transition metal 1,3_diketonates. The lack of luminescence in transition metal 1,3-diketonates is one of the reasons why so little is known about the photophysical properties of these complexes. Although there are exceptions, e.g. chromium(II1) 1,3diketonates and their derivatives, complete quantitative data are not available even in this case and are related mainly to the emitting 2E, excited state or photochemically reactive 4T,, state [9,29]. To our knowledge, all attempts at direct experimental detection of the excited states of transition metal 1,3-diketonates (with the exception of chromium(II1)) have been unsuccessful. For example, in the nanosecond flash photolysis studies of Cu(acac), in benzene, acetonitrile and methanol, no excited states or intermediates were detected on direct excitation or by sensitization with aromatic hydrocarbons and ketones [ 391. The use of laser flash photolysis techniques with picosecond time resolution and more sensitive detection systems may allow the observation of the excited states of these complexes and throw more light on their photophysical properties. For the reasons discussed above, the photophysical properties of lanthanide 1,3_diketonates are much better known and well documented [24, 40 - 431. In summary, it should be stressed that transition metal 1,3-diketonates are characterized by various types of electronically excited states and by inefficient radiation processes. The excited state of a complex populated by direct excitation or by a sensitization process can be deactivated by an efficient radiationless process or by photochemical reactions. A typical example of an energy state diagram for these compounds is presented in Fig. 3.

LO LMCT t ‘L(n,Tm

30 ck 2

3L(n,n*)

t

Id.d)

o-

ground state

Fig. 3. Diagram of the low-lying excited states of Cu(acac)z.

7

3. Photochemical reactions (direct photolysis) Quantitative photochemical studies of transition metal 1,3-diketonates began in the late 1960s and were concerned with metals such as chromium(III), manganese(III), iron(III), cobalt(III), nickel(II), copper(I1) and rhodium( III) [ 9, 44 - 611. In general, irradiation of solutions of the complexes with UV light leads to the photoreduction of the complex (manganese, iron, cobalt, copper, nickel) or to stereochemical rearrangements (chromium, rhodium). Since the direct photochemistry of metal 1,3-diketonates (ML,) has been extensively reviewed by Lintvedt [ 91 up to 1974, only basic and general considerations are presented in this paper including some recent results. The copper( II) and nickel( II) 1,3-diketonates in deoxygenated ethanolic solutions are photoreduced on 254 nm irradiation (excitation to LMCT state) and the overall reaction is presented by ML2 w

MO + 2HL

The main photoproducts are metallic copper (or nickel) and the 1,3-diketone (HL). However, the reaction pattern is more complex and can be explained by a photochemical reduction of ML2 to an M’ complex followed by a thermal reaction to MO (eqn. (5)) or a disproportionation reaction (eqn. (6))

191 ML2 w

M&* -

kz

k--2 MLL* + SH MLL- -

k4

ML-MO+ 2ML -

k3

ML+LH+S.

ML,

MLL-

(2)

(3) (4)

-L

(5)

MO + ML1

(6)

where SH is a hydrogen-atom-donating solvent. In the case of trivalent transition metal 1,3-diketonates (MLs) such as manganese(III), iron(II1) or cobalt(III), the photolysis at the LMCT band in hydrogen-atom-donating solvents leads to the formation of an Mn complex and the 1,3-diketone ML3 $+

ML,(SH),

+ HL

In some cases, e.g. Cr(tfac)3 and Rh(tfac)j (tfac = l,l,l-trifluoro-2,4-pentanedionate anion), the ML3 complex can undergo isomerization during the irradiation (eqn. (9)). The general reaction scheme can then be described as

(8) (9) The quantum yields depend on the excitation wavelength and the solvent. The following mechanism is postulated hV

ML3 (trans) -ML ML2L* +

3

ML3 (cis) or ML3 (trans)

SH

ML2L. ----+ML,+++HL ML2 -

SH

*---+ML2L~

ML,(SH)

2

(10) (1‘1) (12) (13)

where ML2L* represents the species which formally contains the MI’ cation bound to a 1,3_diketone radical (L- ). Reactions (2) - (6) and (10) - (13) can explain most of the experimental results for ML, photolysis and are generally accepted [9, 57]However, as has recently been shown for Co(tfac)s using flash photolysis, the ML2L. complex (eqn. (10)) can also undergo intramolecular rearrangement and additional reaction with the solvent [58]. To illustrate more clearly the direct photochemistry of transition metal 1,3-diketonates, the results from photochemical studies of copper( II) complexes are discussed in more detail. The irradiation with 254 nm light (excitation of the LMCT band) of CuL2 in deoxygenated alcoholic solutions leads to the formation of copper(I) complexes as black precipitates (CuL), the free ligand (HL) and products derived from solvent oxidation (e.g. formaldehyde in the case of methanol) [9, 39, 471. During prolonged irradiation metallic copper is formed (eqn. (1)). The organic products can be identified by gas chromatography-mass spectrometry (GC-MS), and the CuL complex by reaction with triphenylphosphine to form CuL(PPh,), (a stable complex) [ 391, similar to the method described by Chow and coworkers [ 62,631. The reaction proceeds only in hydrogen-atom-donating solvents and in carefully deoxygenated solutions, since the presence of oxygen leads to the reverse reaction, i.e. oxidation of CuL to CuL2. Quantitative photochemical studies of copper acetylacetonates have been carried out by Lintvedt [9] and Gafney and Lintvedt [47] and complemented by one of us [39] _ The quantum yields of photoreduction of three copper chelates in various solvents and at different excitation wavelengths are shown in Table 2. The following conclusions can be inferred from the results presented: (1) the quantum yields depend on the excitation wavelength; the photoreduction occurs only during direct excitation to the LMCT state (254 nm);

9 TABLE

2

Quantum

yields of photoreduction

of CuLz chelates

Compound

Solvent

Lx

Cu(acac)2 Cu(tfac);? Cu(hfac)z Cu(acac)2 Cu( acac), Cu(tfac)z Cu(hfac)z Cu( acac)z Cu( acac):! Cu( acac)* Cu(acac)z Cu(tfac)z Cu(hfac)2 Cu(acac)a

Methanol Methanol Methanol Ethanol Ethanol Ethanol Ethanol 2-Propanol Methanol Methanol Methanol Methanol Methanol Acetonitrile

254 254 254 254 254 254 254 254 313 366 647 647 647 254

(nm)

in solution

at room

temperature

@c

Reference

0.015 zk0.003 0.023 f 0.005 0.026 + 0.005 0.017 + 0.004 0.018 f 0.005 0.027 + 0.007 0.036 f 0.006 0.025 + 0.006 <2x10* <7x10+ <7x10-+ <1x10* <2x10”
39 39 39 39 47 47 47 39 39 39 39 39 39 39

(2) the quantum yields increase with the number of CF, groups in the ligands; (3) the quantum yields are solvent dependent; they are shown to increase as the hydrogen-atom-donating ability of the solvent increases, i.e. methanol < ethanol < 2-propanol. The relationship between @, and X,,, indicates that the reaction observed is LMCT photoreduction. However, as suggested by’ Yokoi 1213 when re-examining the spectroscopic properties of CuL2 complexes, the dd states in these complexes are not “pure” ligand field states; they are partially mixed with the LMCT state. The lack of photochemical reduction during 647 nm irradiation (Table 2) does not support such a hypothesis. However, the @‘c values for 647 nm excitation represent the upper limit that could be measured under the experimental conditions used. A small admixture of the LMCT state with a dd state could not be detected in the experiment performed. The introduction of CF, groups in the complex increases the a, values on excitation at 254 nm. If the reduction is caused by electron migration from the ligand to the metal, then a relationship between the polarographic reduction potentials of the chelates and the photoreduction quantum yields is expected. The half-wave polarographic reduction potentials Et,z of the CuL2 complexes determined under identical experimental conditions are -0.502 V, -0.169 V and +0.038 V for Cu(acac)2, Cu(tfac)* and Cu(hfac)* (hfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate anion) respectively [64], and correlate well with the @‘c values (Table 2). A linear relationship of @‘c vs. El/z for five copper( II) 1,3-diketonate chelates can be found in ref. 9. This relationship shows that as the ease of reduction increases, the quantum yield also increases, and this can be taken as an indirect argument for the assignment of the photoreactive state as the LMCT state.

10

The role of the solvent in the photoreduction of copper(I1) 1,3-diketonates can be explained by the abstraction of a hydrogen atom from the solvent by the MLL. complex formed in the primary photochemical process (eqn. (3)). On the basis of reactions (2) - (4), the quantum yield of photoreaction aC can be expressed by (14) where aILILL.= (kZ/(k2 + k_*)) is the quantum yield of MLL- complex formation. The role of the solvent is expressed by the last term, which illustrates the efficiency of decay of the MLL- complex in the hydrogen abstraction reaction (eqn. (3)). If k4 3- k,[SH], the increase in the hydrogen-atomdonating ability of the solvent also increases the second term in eqn. (14) and therefore increases the value of aC. The influence of the solvent on “non-chemical” processes, i.e. kz, k-, and k4 (eqns. (2) - (4)), should be less effective than in the chemical reaction (eqn. (3)) [9, 47, 531. Not many studies on the photochemical properties of 1,3-diketonate lanthanide complexes can be found in the literature. Brittain [65] has reported that irradiation with 311 nm light (excitation to intraligand state) of a terbium(II1) complex leads to the photodecomposition of the chelate. The primary photochemical step is the photodissociation of a 1,3-diketone ligand with very low quantum yields (0.001 - 0.002) which is very dependent on the solvent. Excitation to an ff excited state (centred on the metal) can also lead to photoreactions of the complex. In this case, the irradiation must be carried out in coordinating solvents such as pyridine, ethanol or acetone and in the presence of oxygen [66]. The mechanism has been postulated to be photosubstitution of a ligand by a solvent molecule followed by a dark reaction with oxygen to form a 3,4,5-triketone [66]. The mechanism of the photodecomposition of lanthanide f,3-diketonates is still unclear and future detailed studies are required in this field. 4. Quenching of excited states of organic compounds by 1,3-diketonate metal chelates (physical aspects) The crucial step in the sensitized photoreduction of 1,3-diketonate transition metal chelates is the interaction between the excited state of the sensitizer (organic compound) and the metal complex. The possible mechanisms for this interaction are discussed in this section. 4.1. Quenching of triplet states The quenching of the triplet states of organic compounds by ML,, complexes has been the subject of many investigations which started with the pioneer work of Bell and Linschitz [67] and Hammond and coworkers [68,69]. Since then the development of new methods and experimental

11 TABLE

3

Quenching of benzophenone tion at room temperature

Quencher

Cu(acac)z Cu(tfac)z Cu(hfac)z Ni( acac), Co(acac)z Zn( acac)z Mg(acac)z acacH hfacH Ln(acac)3

aTaking rr = 5.1 bTaking 7~ = 31 =In methanoi (7 *Average value dysprosium( III)

phosphorescence

by 1,3-diketonate

metal

chelates

in solu-

In acetonitrile

In benzene K x 10-3 (M-l)

kqx10-9a (ha-1 s-l>

K x 10-S (M-l)

k,x

10’+b

(M-’

s-r)

16.8 17.5 16.7 5.0 8.0 2.6 G 9oc 0.69 13.1 1.7*

3.3 3.4 3.3 1.0 1.6 0.51 G 0.4c 0.14 2.6 0.3

145 150 135 105 81 40 13 6.5 90 17*

4.7 4.8 4.4 3.4 2.6 1.3 0.42 0.21 2.9 0.6

w [78]. w [78]. = 0.26 ,% 1781). for samarium(III), europium(III), acetylacetonates [ 7 9 1.

gadoIinium(III),

terbium(II1)

and

techniques has allowed the physical aspects of this process to be studied in more detail 170 - 771. The two most prevalent mechanisms of quenching by ML, complexes have been shown to be energy and/or electron transfer; these mechanisms are first discussed for the quenching of benzophenone triplets. The results obtained from quenching of benzophenone phosphorescence in solution at room temperature are summarized in Table 3 [ 78, 793; the data for lanthanide 1,3-diketonates, Mg(acac)z, acacH and hfacH are included for comparison. It has recently been shown that the quenching of triplet benzophenone by Ln(acac), occurs by an energy transfer mechanism via the triplet state of the ligand. This does not seem to be the case for transition metal acetylacetonates in which the quenching process is much more complex and energy transfer (to ligand-localized and ligand field states) and/or electron transfer mechanisms may be involved. Since transition metal 1,3diketonates do not emit in solution at room temperature and transient absorption spectra of their excited states have not been detected (Section 2), a direct experimental observation for the participation of energy transfer has not been obtained. However, no intermediates suggesting an electron transfer quenching have been detected [ 781. Furthermore, no correlation between the quenching rate constants and the reduction or oxidation potentials of expected donors or acceptors, typical of electron transfer quenching, has been found [ 791. A comparison of the results for Ln(acac)3, Mg(acac)2 and ML n complexes (Table 3) suggests

12

that energy transfer to a ligand-localized triplet state is not the main mechanism of quenching by transition metal 1,3_diketonates. Therefore, it can be inferred from these observations that the quenching of triplet benzophenone by transition metal 1,3_diketonates cannot be described by a single mechanism common to all ML, complexes (Table 3). The involvement of energy and/or electron transfer is strongly dependent on the type of quencher (complex) used. A systematic investigation of the quenching of a series of appropriately chosen organic compounds by a single 1,3-diketonate chelate may throw more light on the contribution of energy and electron transfer (vide infra). The lack of direct methods for studying the participation of energy and electron transfer processes in the quenching of excited states by ML, complexes has been overcome by using indirect methods which are now widely applied by most researchers in this field. One of the most fruitful approaches is to study the quenching of a series of donors by a single ML,, complex and to look for a correlation between the experimental rate constants k, and the triplet state energy of the donor ET and the free energy change for electron transfer AGe1 (obtained from spectroscopic and electrochemical data). Such a treatment was first suggested by Balzani and coworkers [ 80,811, and has recently been reviewed by Wilkinson st al. [82]. The latter group, using the laser flash photolysis technique, have studied the quenching of triplet states of a set of organic molecules by several chromium(III) and iron(II1) 1,3_diketonates [ 72 - 771. The observed correlation between log k, and ET for Cr(acac)3, Cr(dpmJ3 (dpm = 2,2,6,6-tetramethyl-3,5-heptandionate anion) and Fe(acac), is taken as evidence of energy transfer quenching. They have also suggested that the stepwise increase in log k, is indicative of an energy transfer to the upper excited states of the quenchers. However, the results observed for Cr(hfac)3 have been interpreted in terms of competitive energy and electron transfer and the quenching by Fe(hfac)3 and Fe(tfac), occurs predominantly by a reversible electron transfer [76, 77, SZ]. Since the sensitized photoreduction of copper(I1) 1,3-diketonates is discussed as an example in Section 5, the physical aspects of the quenching of the excited states of organic compounds by copper(I1) complexes are discussed in detail in this section. The quenching of triplet state aromatic hydrocarbons by CuLZ complexes can be interpreted according to a general scheme including a combination of energy and electron transfer h Z

4*2(D.

k-en

k--d

..&*I--+-ID+2Q*

UW

skd

3D + 2Q s 4*2(D*. _ .Q) k-d k4 G

k-e1

4, 2(,+.

.

.Q-) __f kbt lD +

2Q

(15b)

13 TABLE

4

Rate constants k, of quenching of the triplet states of organic donors by Cu(acac)z and Cu(hfac)T in benzene at room temperature and the energies of the triplet states of the donors [ 83 ]

Number

Compound

ET

(cm-l)

1

2 3 4 5 6 7 8 9 IO 11

Phenanthrene Naphthalene Chrysene Pyrene Anthracene Perylene Tetracene 5,12_Diphenyltetracene Rubrene Pentacene p-carotene

21600 21300 19800 16800 14700 12400 10300 10000 9300 8000 6300

Cu(ucac)~

Cu(hfac)z

lo-+

k,

lo-k,

(M-r

s-l)

(M-

2.5 2.2 2.2 1.9 1.4 0.70 0.025 0.0066 G 0.0013 0.0079 s 0.05

1 s-1

)

3.6 3.1 3.0 2.0 1.5 1.5 0.90 0.43 0.13 0.60 0.41

where s is the spin statistical factor, k d is the diffusion-controlled rate constant and k-d and kbt are the dissociation rate constants for the encounter complex and charge transfer complex respectively. It has recently been shown that the quenching of triplet aromatic hydrocarbons by Cu(acac), in benzene is mainly due to energy transfer (eqn. Wa)) 1331, with the exception of donors with low-lying triplets (ET < 11000 cm-l) and pentacene in particular, in which electron transfer plays a significant role (Table 4). The rate constants for quenching of the triplet states of 11 aromatic hydrocarbons by copper(I1) complexes in solution have been measured by laser flash photolysis (Table 4). A dependence of log k, on ET, typical of energy transfer quenching, is observed for Cu( acac), , i.e. a linear increase in k, in the endoergonic region followed by an extended plateau of rate constants for exoergonic quenching (Fig. 4). A quantitative treatment, similar to that of Balzani et al. [ 801, has been used to determine the transmission coefficient K,, and the intrinsic barrier parameters AG,f,(O) for energy transfer quenching by Cu(acac)z: K,, = (4.3 _+1.3) X 10m3; AGefn(O)= 800 _+200 cm -l. The transmission coefficient derived is in agreement with results reported for chromium(II1) and iron(II1) acetylacetonates [76, 771. However, the intrinsic barrier indicates a significant distortion of the Cu(acac)2 excited state formed by the energy transfer. The mixing of a dd state with the LMCT state in Cu(acac);!, suggested by Yokoi [ 211 (Section 3), may be a reason for this effect. In contrast, experimental data have shown that the quenching of triplet aromatic hydrocarbons by Cu(hfac)2 occurs mainly by electron transfer (eqn. (15b)). Figure 5 shows the dependence of ks on the free energy of electron transfer AG,l

14

10

10

9

11

l

l

8 r Y tz

10

7

I

8 I

xx*

Cu(acacI2

*

Cu(hfacI2

i l

l

6

5

5

10

15 ET

25

20

x10 -3

I

km’)

Fig. 4. Dependence of quenching rate constants k, on the energy ET of the triplet states of aromatic hydrocarbons (1 - 11, Table 4) and benzophenone (12) quenched in benzene solution at room temperature. The observed electronic transitions to the lowest excited states of the copper(D) complexes are shown as arrows. (The curve is the best fit to the Cu(acac)a points, with the exception of pentacene and benzophenone; see ref. 83.)

10

-

5l-

m Y

0” 8Ix s = Cu (acacj2 l

l

l

Cu(hfacJ2

7I_

9 Ei-

5 0

0.5 -AGeI

1.0

IeV)

Fig. 5. Dependence of log k, on the free energy change AGel of electron transfer from the donor triplet state of the aromatic hydrocarbons (1 - 11, Table 4) and benzophenone (12) to the copper(D) complexes. (The curve is the best fit to the Cu(hfac)* points, with the exception of benzophenone; see ref. 83.)

15 AGel

=

F(EoX

-Rred)

-ET

+ Aw

(16)

where F is the Faraday constant, Eox and Ered are the oxidation and reduction potentials of the donors and quenchers respectively and Aw represents the coulombic interaction energy and changes in solvation free energy of separate ions and the encounter complex (Aw is small when donor and quencher are uncharged species: approximately 0.1 eV [SZ]). The transmission coefficient and intrinsic barrier parameters obtained for electron transfer quenching by Cu(hfac)* (K,~ = 0.014 f 0.003; AG,f,(O) = 2100 f 400 cm-‘) are similar to those for Cr(hfac), and Fe(hfac)3 [76, 771, and are consistent with the values predicted by the outer-sphere electron transfer theory. The quenching of triplet aromatic ketones by copper( II) 1,3diketonates in solution has been studied for benzophenone derivatives possessing electron-donating or electron-withdrawing groups [84, 851. In contrast with aromatic hydrocarbons, the correlation between the quenching rate constants and the energy term (-ET -Ered) (eqn. (16)) suggests an electron transfer from CuL2 to benzophenone as the quenching mechanism (see Section 5). As shown in Fig. 5, the opposite direction for the electron transfer can be excluded for benzophenone (12). However, the relatively good agreement of the rate constant k, of quenching of triplet benzophenone (12) by Cu(acac), and Cu(hfac)* using an “energy transfer plot” (Fig. 4) means that we cannot completely disregard the participation of an energy transfer mechanism. A more detailed discussion on the quenching of the triplet states of aromatic hydrocarbons and ketones by copper(I1) 1,3-diketonates can be found in refs. 83 and 84.

Quenching of singlet states The singlet states of organic compounds are also effectively quenched by 1,3-diketonate metal chelates. The fluorescence of benzene (Es = 37 500 cm-l) can be quenched efficiently by transition metal and lanthanide 1,3diketonate complexes and this effect can be interpreted as an energy transfer to a ligand-localized and LMCT state of the complexes [78, 84, 861. The observed benzene-sensitized emission of terbium( III) and europium( III) 1,3-diketonates and the similar sensitization and quenching rate constants of all Ln(acac)s complexes used can be taken as direct evidence for an energy transfer quenching via ligand-localized excited states. However, in the case of Cu(acac)*, the energetically-allowed energy transfer to an LMCT state can also take place [ 871. The latter process can then be responsible for the sensitized photoreduction of Cu( acac), , as in the case of the direct photolysis (see Sections 3 and 5). The quenching of low-lying excited singlet states of aromatic compounds (i.e. tetracene (Es = 21 100 cm-l) and trimethylated pyrichrominium ion (Es = 23 600 cm-‘) [88]) by ML, complexes occurs with an almost dif4.2.

16

fusion-controlled rate. An energy transfer to the ligand field states has been suggested as the main mechanism of quenching [ 78, 841.

5. Sensitized photoreduction (chemical aspects)

of 1,3-diketonate

transition metal chelates

Although direct photolysis has been carried out for various metal 1,3-diketonates (Section 3), the sensitized photoreduction has been observed [94, 9.51 and cobalt(II) only for copper [ 84, 87, 89 - 931, nickel(R) [96] chelates. The first study reporting on the sensitized photoreduction of copper(I1) 1,3-diketonates presented only qualitative results 1893. Further work by Chow and coworkers 162, 90, 911 has provided more quantitative data on the mechanism for the benzophenone-sensitized photoreduction of Cu(acac)* in alcohols. The general reaction pattern is Cu(acac)*

+ RR’CHOH

-

PhzCO

Cu( acac) + acacH + RR’CO

(17)

(A > 3”0”0nm) The main photoproducts are a copper(I) complex (Cu(acac)), acetylacetone and products derived from oxidation of the solvent (formaldehyde in the case of methanol). Metallic copper is formed after prolonged irradiation, as in the case of direct photolysis (Section 3). The same reaction pattern has been observed for fluorinated derivatives of Cu( acac), , i.e. Cu(tfac)2 and Cu(hfac)2 [92]. The mechanism describing the sensitized photoreduction of copper(I1) 1,3-diketonates by aromatic ketones ( 3K) in hydrogen-donating solvents (SH) is shown below 3K + CuL2 -

k,

[K’_.

- - cuL,‘+]

(18)

C C+SH-

kl9

K+CuL+LH+S.

(19)

bo

C-K+CuL, An electron transfer from the transition metal complex to the triplet state of the ketone to form an exciplex C has been suggested as the key step in the mechanism of photoreduction. The exciplex can undergo either product formation (by reaction with the solvent) or reversion to the ground state of the reactants (reverse electron transfer). Therefore the competition between reactions (19) and (20) affects the efficiency of the sensitized photoreduction. This effect can be better illustrated by the observed increase in the quantum yields of photoreduction with an increase in the hydrogen-donating ability of the solvents, e.g. 2-propanol > ethanol > methanol % benzene

17

(no reaction) [93]. The generation of the acetylacetonyl radical (acac. ) during the photoreaction (eqn. (21)) has been demonstrated by trapping with nitroso compounds to form stable nitroxide radicals and by addition reactions with olefins [90, 911 [I(‘-.

. .

Cu( acac) 2- ‘1 __f

+ Cu(acac) + acac*

[K’--**Cu(acac)+*-eacac-]

+

K (21)

The photoreduction of Cu(acac)* occurs only if the acac- radical can be efficiently scavenged by reaction with hydrogen-donating solvents (reaction (19)), by a radical trapping agent or by a molecule of hydrogen [ 871. Since flash photolysis techniques do not provide direct evidence of the presence of exciplexes, radical ions or other intermediates, indirect methods have been used to gain more insight into the mechanism proposed in eqns. (18) - (20). The effect of the 1,3diketone ligand on the benzophenonesensitized photoreduction of copper complexes has been investigated 1921. The rates of interaction between triplet benzophenone and copper(I1) complexes containing various ligands (i.e. acac, tfac, hfac) are similar and close to the diffusion-controlled rate. However, the limiting quantum yields of photoreduction vary significantly depending on the Iigand and the hydrogen-donating properties of the solvent. Thus a study of the effect of the 1,3-diketone ligand does not provide clear evidence of electron transfer participation in the quenching process. The primary photochemical process proposed in eqn. (18) is supported by the correlation between the k, values for the interaction of the triplet states of aromatic ketones with Cu(acac)2 and the reduction potentials of the ketones [93]. The k, values for benzophenone, xanthone, propiophenone, p-methoxyacetophenone and 2-acetonaphthone, obtained by monitoring the T-T absorption, are not significantly different and only small changes in (-ET -Ered) are found for these ketones. Nevertheless, the correlation between log k, and (-ET - Ered) is moderately good, suggesting the possibility of electron transfer quenching according to eqn. (18). Similar conclusions can be drawn from the results reported for the quenching of some aromatic ketones by Ni( acac), in methanol [ 94 1. The correlation of log k, us. (-ET - Ered) (eqn. (16)) can be a much better argument for electron transfer quenching if an appropriate series of ketones with various triplet state energies and reduction potentials is used. For example, a series of benzophenone derivatives containing electrondonating or electron-withdrawing groups have been used for quenching by Cu(acac), (Table 5) [85]. The E T values were determined from the phosphorescence spectra in alcohols at 77 K and the half-wave reduction potentials were measured in acetonitrile us. a saturated calomel electrode (SCE) 1971. The k, values were calculated by quenching of the phosphorescence of the benzophenones and by determining the quantum yields of the triplet-sensitized photoreduction of Cu(acac)2. Excellent agreement is observed between the two methods (Table 5). The good correlation obtained for log k, us. (-ET -E red) strongly suggests that the interaction between

18

TABLE 5 Correlation of the rate constants of quenching of the triplet states of substituted benzophenones by Cu(acac)z with (-ET - Ered) [85] -Ered

(-ET-

Ered)

Substituted benzophenones

ETa

(kJ mol-‘)

(kJ mol-‘)

(kJ mol-‘)

4,4’-Dimethoxy 4-Methoxy Q-Methyl Unsubstituted 4-Chloro 4-Trifluoromethyl

292.5 288.7 289.5 289.5 288.3 285.4

194.5 184.1 179.5 176.2 168.6 152.3

-97.9 -104.6 -110.0 -113.4 -119.7 -133.1

b

j3’

log k, d

log k, e

0.037 0.040 0.098 0.26 = 0.40 0.84

9.08 9.32 9.38 9.56 9.80 10.1

8.95 9.26 9.42 9.66 = 9.76 10.3

aKetone triplet energies obtained from the O-O band of the phosphorescence spectra at 77 Kin CpHsOH-CHSOH (4:l) [85,97]. bHalf-wave reduction potentials determined in CH&N us. SCE [97]. CLimitingquantum yields. dVaIues obtained from phosphorescence quenching. eVaIues obtained from quantum yield determinationsof Cu(acac)z photoreduction. the triplet state of benzophenone

tron transfer from the complex

and Cu(acac)* is dominated by an electo the ketone (eqn. (18)) and that this

process is responsible for the photoreduction. However, the lack of correlation between Iog k, and ET clearly indicates that energy transfer is not the mechanism of quenching. The increase in the limiting quantum yield p with an increase in the reduction ability of the ketone (Table 5) can be taken as an additional argument in favour of the proposed mechanism for the sensitized photoreduction (eqns. (18 - (20)). Nickel(I1) 1,3_diketonates can also undergo photoreduction sensitized by aromatic ketones. The general reaction pattern and mechanism have been investigated in detail by Chow et cd. [ 94, 951 and are similar to those of Cu( acac),. Phenylalkyl ketones, which undergo Norrish type II reactions, have also been used as sensitizers for mechanistic studies of the sensitized photoreduction of Cu(acac)? [98, 991. It has been shown that Cu(acac)* can interact with the transients involved in the Norrish type II reaction in two different ways. (1) Triplet quenching leading to the photoreduction of the complex; this seems to be the predominant process in the case of butyrophenone which has a relatively long-lived triplet. (2) Biradical scavenging, which does not lead to new products, but modifies the product ratios of the Norrish type II reaction (cyclobutanol and acetophenone). The yield of cyclobutanol increases as the concentration of Cu(acac), increases. It has been proposed that the interaction of Cu(acac)2 with the triplet biradicals (3B) induces an

intersystem crossing to singlet biradicals (I&,) (eqn. (28)) which possess an average conformation different, from the singlet biradicals ( ‘Ba) produced by the intrinsic intersystem crossing (eqn. (25)). The fraction of biradicals which yields cyclobutanols is approximately doubled for those biradicals

19

which undergo intersystem crossing assisted by Cu(acac)2. It seems that Cu(acac), shows some preference for those biradicals in a cisoid conformation since they can coordinate to the copper centre more easily. The change in product ratios and the value of the rate constant for trapping of the biradical by the complex (hr = 2 X lo9 M-l s-l, in MeOH), measured by flash photolysis, are taken as a clear indication of biradical scavenging and induced intersystem crossing [ 991. Reactions (22) - (30) illustrate the mechanism proposed (K is ketone, B is biradical and the superscript indicates the multiplicity) hv

K-w

Is’

3K -

3B

3K

3K + Cu(acac)2 3B--

(22) (23) k,

reduction products

lB a

(24) (25)

‘B a -

CH&OPh + olefin

(26)

lB a -

cyclobutanol

(27)

3B + Cu(acac)a -

kT

Cu(acac), + lBb

‘Bb -

CH$OPh

‘B,, -

cyclobutanol

+ olefin

(23) (29) (30)

Although triplet aromatic hydrocarbons are efficiently quenched by copper(I1) acetylacetonates [83], they do not sensitize the photoreduction of Cu(acac),. It has been shown that the quenching occurs mainly as a result of energy transfer; electron transfer, which is responsible for the photoreduction, is not feasible for energetic reasons. The photoreduction of Cu(acac), can also be sensitized by the singlet state of benzene in the presence of hydrogen-donating species 1871. When benzene is used as sensitizer as well as solvent, the reduction of the complex takes place only if a suitable hydrogen donor is added or if the reaction is carried out in an atmosphere of hydrogen (but not nitrogen). The agreement between the quenching rate constant obtained from benzene fluorescence quenching (Section 4) and that obtained from the quantum yield of Cu(acac), photoreduction, is taken as evidence of singlet state sensitization. The process can be explained as an energy transfer which takes place from the Sr state of benzene to an LMCT state of the complex, followed by photoreduction. Aromatic hydrocarbons with S1 state energies lower than that of benzene do not sensitize the reduction of Cu(acac)z. In this case, energy transfer to a photoreactive LMCT state of the complex is energetically unfavourable.

20

6. Uses of 1,3-diketonate metal chelates in photocatalytic reactions Photochemical reactions of organic compounds catalysed by transition metal complexes have received a great deal of attention in the last 15 years and a large number of papers have been published in this field. Some excellent reviews discussing the different aspects of photocatalysis by coordination compounds can be found in the literature [ 100 1, and therefore only a few examples involving metal 1,3_diketonates are discussed here. The first,report describing the participation of a metal 1,3-diketonate in a photocatalytic reaction appeared in 1974 [loll. UV irradiation (greater than 250 nm) of 3-chloro-l-butene in the presence of CuCl,, Cu(OAc), or Cu( acac), induces the photoisomerization of the substrate to give 1-chloro-2-butene CH,-YH-CH=CH, Cl

c$

CH,-CH=CH-CH, 2

(31) A1

Although the role of copper has not been explained, it is speculated that the activation of the allylic chloride may occur by an LMCT excitation of a Cu(II)-substrate complex [lOOa] in a similar manner to the mechanism proposed for the metal-ion-catalysed photopolymerization of tetrahydrofuran (THF) [ 1021. The photochemical oxidation (by dioxygen) of tetraline (1,2,3,4tetrahydronaphthalene) is catalysed by Fe(acac)3 [lo33

(32)

The results show that the addition of Fe(acac)3 enhances the rate of the photoinitiated reaction and that this effect is more pronounced with increasing temperature. It is concluded that the rate-determining step is a thermal reaction which takes place after the absorption of light. In a related study, the catalytic effects of transition metal 1,3-diketonates on the photochemical decomposition of 1,2,3,4-tetrahydro-lnaphthyl hydroperoxide (THP) have been investigated [ 1041. The effects of iron(III), cobalt(III), cobalt(II), manganese(III), copper( chromium(III) and nickel(I1) acetylacetonates on the kinetics of THP photolysis and the formation of products have been studied. Cobalt(II), manganese(III), copper(I1) and nickel(II) are catalytically active in the thermal decomposition of THP, whereas iron(II1) and cobalt(II1) do not catalyse the decomposition without light. A sensitizing or photocatalytic effect of the complexes is proposed and, since these complexes are photoreduced on exposure to light, the catalytic species are assumed to be cobalt(I1) and iron( II) acetylacetonates.

21

The effect of cobalt(III), cobalt(II), iron(I1) and manganese(I1) 2,4pentanedionates on the photochemical oxidation of aromatic hydrocarbons has been investigated [ 1051. By comparing the rate of oxygen consumption for the photochemical oxidation of ethylbenzene it has been shown that cobalt(II1) acetylacetonate has a pronounced catalytic effect on the reaction. The cobalt(II1) complex is reduced to cobalt(I1) during the photoreaction; this decrease in concentration does not cause a decrease in the rate of photochemical reaction, but it has a marked effect on the subsequent thermal reaction: Thus it is concluded that cobalt(II1) 2,4_pentanedionate does not act as a genuine homogeneous catalyst in the photochemical oxidation of ethylbenzene, but it plays an important role in the initial stages of the reaction by acting as an initiator [ 1051. Metal 1,3-diketonate chelates have also been shown to have some catalytic effects on photochemical isomerization, rearrangement and/or hydrogenation of olefins and polyolefins. The photoisomerization of cis,cis1,5+yclooctadiene (cc-1,5-COD) to cis, trans-1,5-cyclooctadiene (c&1,5COD) has been shown to be catalysed by Cu’acac generated in the sensitized photoreduction of Cu( acac), 11061. GC monitoring of the sensitized reaction of Cu(acac)* and benzophenone in the presence of a large excess of cc-1,5-COD shows that ct-1,5-COD is formed at the expense of the cis,cis isomer and reaches a photostationary state in 7 - 8 h. The induction period at the beginning of the irradiation is taken as an indication that the intermediate Cu( acac) (cc-l ,5-COD) formed during the sensitized photoreduction is the species catalysing the light-induced COD isomerization. The mechanism is shown to be analogous to that proposed for the isomerization by other copper(I) compounds [ 1 OOb] . A similar catalytic effect is observed for Cu’acac in the photoisomerization of 1,5,9_cyclododecatrienes (1,5,9-CDT) [62a]. For example, the benzophenone-sensitized photoreduction of Cu(acac)z in benzene in a hydrogen atmosphere in the presence of ttt-1,5,9-CDT or ctt-1,5,9-CDT gives a yellow photolysate without the formation of metallic copper. GC monitoring of the reaction shows the formation of the CDT isomers and a photostationary state is reached in 6 - 8 h. It is proposed that a Cuiacacbenzene complex mediates the isomerizations, but a Cu’(CDT) complex must be involved in a mechanism analogous to the photocatalysed isomerization of the trienes by copper(I) chloride [ 1071. The role of Ni’acac complexes in the photocatalytic hydrogenation of olefins and dienes has also been studied by Chow and Li [ 1081. An extensive account of these reactions can be found in ref. 108b.

7. Final remarks In spite of the considerable amount of work that has been carried out in the last 10 years on the photochemistry of transition metal 1,3-diketonate chelates, further studies are required to clarify many aspects of the

22

mechanisms involved in the sensitized reduction of these complexes. A combination of thermodynamic and kinetic arguments as well as those obtained from mechanistic studies (presented in Sections 4 and 5) must be confirmed by direct evidence which proves the mechanism of interaction between the excited states of organic compounds and 1,3-diketonate chelates. Nanosecond and picosecond flash photolysis, electron spin resonance (ESR) and photoconductivity techniques may prove to be useful for the direct observation of the excited states of ML, complexes and other intermediates involved in the sensitized photoreduction. The few examples described in this paper on the photocatalytic properties of some metal 1,3-diketonates suggest that further studies in this area may provide interesting results concerning the possible applications of the system in the catalysis of a variety of organic reactions. Acknowledgments The authors wish to express their gratitude to Professor Y, (Department of Chemistry, Simon Fraser University, Burnaby, for his interest, stimulating discussions, guidance and inspiration out their stay in his laboratory. B. M, thanks the Polish Academy of for financial support.

L. Chow Canada) throughSciences

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24 60 61 62

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85 86 87 88 89 90 91 92 93 94 95 96 97

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