Inorganica Chimica Acta 321 (2001) 1–4

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Kinetics of the photoreduction ofbis(2,4-pentanedionato)copper(II) in chloroform

Wilson Ng, Patrick E. Hoggard *Department of Chemistry, Santa Clara Uni�ersity, 500 El Camino Real, Santa Clara, CA 95053-0270, USA

Received 27 December 2000; accepted 16 March 2001

Abstract

Under 254 nm irradiation in chloroform, bis(2,4-pentanedionato)copper(II) is converted to CuCl·2HCl, from which CuClprecipitates at concentrations of 10−3 M or higher. The photoreaction appears to be completely solvent-initiated, that is, the ratedepends on the fraction of light absorbed by the solvent. Both �CCl3 radicals and HCl, produced photolytically, are able to initiatethe reaction. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Bis(2,4-pentanedionato)copper(II); Acetylacetonato complexes; Photoreduction; Chloroform; Solvent-initiated

1. Introduction

Gafney and Lintvedt studied the photochemistry ofCu(acac)2, Hacac=2,4-pentanedione, in ethanol, char-acterized by reduction to metallic copper and 2,4-pen-tanedione [1]. The suggested mechanism involved adetachment of one arm of the diketone in the excitedstate, followed by ligand loss to form solvatedCu(acac), then thermal loss of the second ligand. Hy-drogen abstraction by �acac radicals yielded Hacac [1].

Gafney and Lintvedt found the quantum yield of thisprocess to be 0.018 under 254 nm irradiation [1]. Werepeated these experiments in chloroform, because un-der 254 nm irradiation there exists the possibility of asolvent-initiated process, caused by the reaction ofmetal complex with the products of the photolysis ofCHCl3 [2], which could be compared with the reactionof the excited state metal complex taking place at thesame time. Preliminary experiments revealed that noprecipitate formed when the initial concentration ofCu(acac)2 was 10−4 M or below, so that the progress ofthe reaction could be followed spectrophotometrically.

Solvent- and metal-initiated photoreactions in chlori-nated solvents often lead to the same products [3–5],and have to be distinguished kinetically. Metal-initiatedreaction rates depend on the light absorbed by thereactant metal complex, fRI0, where I0 is the lightintensity incident on the sample and fR is the fraction oflight absorbed by the metal complex, which, for areaction R�P may be written [2]:

fR={1−10− (�R[R]+�P[P]+AS)}�P[R]

�R[R]+�P[P]+AS

(1)

In Eq. (1), �R and �P are the extinction coefficients ofthe reactant and product, and AS the absorbance of thesolvent, all at the irradiation wavelength. If the reactionis solvent-initiated, the rate depends on fSI0, where fS,the fraction of light absorbed by the solvent, can bewritten as [2]:

fS={1−10− (�R[R]+�P[P]+AS}AS

�R[R]+�P[P]+AS

(2)

The kinetic rate law for the reaction, that is, thedependence of the rate on concentrations, may revealthe dependence on fR, fS, or both. There are some ratelaws that can be interpreted as either metal- or solvent-initiated. For example, fR is functionally the same asfS[R]. To distinguish them would require a comparisonof rate laws at different wavelengths.

* Corresponding author. Tel.: +1-408-554-7810; fax: +1-408-554-7811.

E-mail address: phoggard@scu.edu (P.E. Hoggard).

0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 0 2 0 -1693 (01 )00502 -3

W. Ng, P.E. Hoggard / Inorganica Chimica Acta 321 (2001) 1–42

2. Experimental

Cu(acac)2, anhydrous CuCl, 2,4-pentanedione, andCHCl3 were obtained from Aldrich and used as re-ceived. Chloroform was HPLC grade, stabilized withethanol.

Photolyses were carried out with an Osram HBO 100W/2 lamp in an Oriel Q housing. Light was passedthrough a 22.5-cm monochromator onto a stirred 1.0-cm cuvette containing 3.0 ml of the sample solution.Incident light intensities were measured in triplicate byferrioxalate actinometry [6,7]. The progress of the reac-tion was followed spectrophotometrically on aHewlett–Packard 8453 diode array spectrometer.

The concentration of Cu(acac)2 during the reactionwas determined from the its extinction coefficient at 305nm in CHCl3, 2.14 (�0.08)×104 M−1 cm−1 from aBeer’s Law plot. At that wavelength, the extinction

coefficient of the photochemical product was 4×103

M−1 cm−1.The fraction of light absorbed by the reactant and

solvent at 254 nm was determined from slightlymodified forms of Eqs. (1) and (2)

fR={1−10−A254−AS}�R[R]

A254+AS

(3)

fS={1−10−A254−AS}AS

A254+AS

(4)

For these equations, �R, the extinction coefficient ofCu(acac)2, is 1.30 (�0.05)×104, AS, the absorbance ofchloroform at 254 nm, is 0.081, and A254 is the solutionabsorbance measured at 254 nm.

3. Results

When solutions with [Cu(acac)2] concentrationsgreater than 2×10−4 M were photolyzed in CHCl3 at254 nm, a white precipitate formed, which was iden-tified as CuCl by comparing the solubility in DMSOand UV spectrum with an authentic sample. Gafneyand Lindtvedt also found a CuCl precipitate fromphotolysis in chloroform [1]. Fig. 1 shows absorptionspectra during the irradiation of a solution of[Cu(acac)2] in CHCl3. Isosbestic points occurred at 255,286, and 351 nm, and the Cu(acac)2 peak at 296 nmwas replaced by a peak at 275 nm.

2,4-Pentanedione in chloroform has a peak at 275nm, with an extinction coefficient of 6.7 (�0.4)×103

[8]. Hacac (2 equiv.) would account for most of theproduct absorbance observed at 275 nm. CuCl is essen-tially insoluble in chloroform, but it was soluble to theextent of approximately 10−3 M when anhydrous HClwas first bubbled into the chloroform. The spectrum inthis solution exhibited a peak at 276 nm (�=7×102).We take this as evidence that the twofold adduct withHCl is formed, because of the similarity to the spectrumof [CuCl3]2− in acidic aqueous solution [9,10]. Wetherefore represent the main photolysis products asCuCl·2HCl (or H2CuCl3) and Hacac.

Initial rates of reaction under 254 nm irradiation atconstant irradiation intensity were determined over arange of concentrations of [Cu(acac)2]. The generalresult was that the smaller the initial concentration of[Cu(acac)2], the faster the reaction. This is typical be-havior for some solvent-initiated reactions, in particularwhen the rate law depends on fS, but not directly on theconcentration of the reactant [11]. In fact, a plot ofinitial rate as a function of fS, shown in Fig. 2, shows alinear dependence, from which we conclude that path-ways other than through light absorption by the solventcontribute little to the overall rate.

Initial rates of reaction were determined for solutionswith a constant [Cu(acac)2] concentration, irradiated

Fig. 1. Sequential spectra from the 254 nm irradiation of a 1.0×10−4 M solution of [Cu(acac)2] in CHCl3.

Fig. 2. Initial rate of reaction of different concentrations of[Cu(acac)2] in CHCl3 under 254 nm irradiation (I0=3×10−10 ein-stein s−1) as a function of the fraction of light absorbed by thesolvent. Error bars are the standard errors for the slope of theconcentration vs. time curves; R2=0.51.

W. Ng, P.E. Hoggard / Inorganica Chimica Acta 321 (2001) 1–4 3

Fig. 3. Initial rate of reaction of a 7×10−5 M solution of [Cu(acac)2]in CHCl3 under 254 nm irradiation as a function of incident lightintensity. The slope, 34�1 M einstein−1, corresponds to a quantumyield of 0.10, based on total light absorbed; R2=0.96.

decelerate, but by so little that the reactions wouldappear to be zero order.

When anhydrous HCl was introduced into CHCl3solutions of [Cu(acac)2], an immediate reaction oc-curred, with products spectrophotometrically indistin-guishable from those produced by photolysis.

To test whether Cl� or �CCl3 radicals can initiate thereaction, the 254 nm photolysis was carried out in CCl4.Again a spectrophotometrically identical productresulted.

No photochemical reaction was observed when chlo-roform solutions of [Cu(acac)2] were irradiated at 313nm, nor was a reaction observed when acetonitrilesolutions were irradiated at either 254 or 313 nm. Thissupports the solvent-initiated nature of the photoreac-tion in CHCl3. In fact, it is somewhat puzzling that noreaction was observed under these conditions, sinceGafney and Lindvedt observed decomposition to cop-per metal with a quantum yield of 0.02 under 254 nmirradiation in ethanol [1].

Gafney and Lindtvedt also noted a qualitative andquantitative dependence of the photoreaction on oxy-gen concentration. The reaction was slower in solutionsthat were open to the atmosphere, and no observablereaction occurred when air was bubbled through thesolution during irradiation [1]. Curiously, Parkanyi etal. noted that a reaction with an unidentified copperproduct takes place under 300 nm irradiation, but onlyin the presence of oxygen [12]. In CHCl3, we found nodifference between air-saturated and nitrogen-saturatedsolutions.

4. Discussion

The photolysis of chloroform yields HCl and �CCl3radicals [13,14], both of which can evidently initiate thereaction with [Cu(acac)2].

CHCl3�h�

Cl�+ �CHCl2 (5)

Cl�+CHCl3�HCl+ �CCl3 (6)

�CHCl2+CHCl3�CH2Cl2+ �CCl3 (7)

A simple sequence of steps by which HCl could reactwould involve the protonation of the pentanedionylradical (�acac), possibly in a concerted fashion.

[Cu(acac)2]+HCl�k1

[Cu(acac)Cl]+Hacac (8)

[Cu(acac)Cl]�k2

CuCl+acac� (9)

acac�+CHCl3�k3

Hacac+ �CHCl2 (10)

If a steady state is assumed for pentanedionyl radi-cals and [Cu(acac)Cl], but not for HCl, these stepsallow a fair fit to the experimental data in Fig. 4, butshowing extensive curvature (acceleration) at the lowestconcentration. More importantly, the rate of formationof HCl would have to be two to three times greater

Fig. 4. Change in reactant concentration with irradiation time forfour solutions of [Cu(acac)2] in CHCl3. Solid lines are fit numericallyto Eqs. (13) and (14), with a=4.8×10−5, b=166, c=1.0×10−6,d=8.8×10−6; �exc=254 nm, I0=3×10−10 einstein s−1.

with different intensities of 254 nm light. Fig. 3 showsa plot of initial rate versus I0, showing a linear depen-dence of the rate on the light intensity. This implies thesame dependence on the fraction of light absorbed,either fR or fS or both. If only the data at very lowintensity are examined, however, the rate appears tohave a square root dependence on the intensity.

When photolyses were monitored throughout muchof the course of reaction, the simple relationship sug-gested by initial rate data, i.e. that the rate is propor-tional to fSI0, is blurred. Fig. 4 shows that the reactionaccelerates with time, except at the lowest concentra-tions. Because the product absorbs slightly more at 254nm than the reactant, the reaction would be expected to

W. Ng, P.E. Hoggard / Inorganica Chimica Acta 321 (2001) 1–44

than the rate of photon absorption. The hydrogenabstraction in Eq. (10) could be problematic, since theO�H bond energy in the predominant enol form ofpentanedione is estimated at 368�25 kJ mol−1 [15],while the C�H bond energy in CHCl3 is estimated at400�4 kJ mol−1 [16]. A similar problem has beennoted for pentanedionatocobalt complexes [8], forwhich pentanedione was also observed as a photolysisproduct. The feasibility of the process may be ascrib-able to solvation effects. Eq. (8) also raises questionsabout bond energies.

Much of the discrepancy between the model, Eqs. (8)and (10), and experiment may be removed if a radicalchlorination also takes place. The trichloromethyl radi-cal is generally an effective chlorinating agent for metalcomplexes [3–5,17].

�CCl3+ [Cu(acac)2]�k4

[Cu(acac)Cl]+acac�+ :CCl2(11)

2�CCl3�k5

C2Cl6 (12)

Assuming a steady state for CCl3, the following equa-tions result:

d[R]dt

= −b [HCl][R]−2�

1+ (1+c�/[R]2)1/2 (13)

d[HCl]dt

=afS−b [HCl][R] (14)

where �=dfS+b [R][HCl]. Fig. 4 presents a numericalfit to Eqs. (13) and (14), optimizing a–d. There areother mechanisms that lead to the same set of equa-tions, but they have in common the sole photochemicaldependence on the light absorbed by the solvent.

The quantum yield is thus expected to be concentra-tion-dependent. The initial rate data from Fig. 3 implya quantum yield of 0.10 based on total light absorbed,but 1.8 based on the light absorbed by the solvent at a

7×10−5 M concentration of [Cu(acac)2]. There needbe no chain process to account for this, since eachCHCl3 homolysis event yields one HCl molecule andtwo �CCl3 radicals, all of which may initiate reaction.Also, the HCl pathway must generate a radical whenCu(II) is reduced. Thus the maximum quantum yieldshould be 4.

Acknowledgements

This work was supported by the National ScienceFoundation through grant CHE-9625664.

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