The Solvent-Initiated Photochemistry ofTetrachloropalladate(II) in Chloroform

Anh L. Le and Patrick E. Hoggard*

Department of Chemistry, Santa Clara University, Santa Clara, CA

Received 26 June 2007, accepted 27 July 2007, DOI: 10.1111/j.1751-1097.2007.00202.x

ABSTRACT

Under 254 nm irradiation in chloroform, [PdCl4]2) is protonated

to H2PdCl4 by HCl, generated through absorption of light by

CHCl3. This stands in contrast to the behavior of [PtCl4]2) and

[Pt(bpy)Cl2], which undergo photooxidation under the same

conditions.

INTRODUCTION

Both [PtCl6]2) and [PdCl6]

2) can be reduced to the tetrachlo-rometallate(II) ions by exposure to near-UV light in thepresence of an oxidizable substrate, each passing through a

detectable M(III) intermediate (1–6). On the other hand, wereported that [PtCl4]

2) in CHCl3 is oxidized to [PtCl6]2) upon

exposure to 254 nm radiation (7). The photoxidation occurs asa result of light absorbed by the chloroform, leading to C–Cl

bond homolysis and the eventual formation of CCl3OOH,which acts as the oxidizing agent (7). A similar solvent-initiated photooxidation from Pt(II) to Pt(IV) was observed to

take place with [Pt(bpy)Cl2] (bpy = 2,2¢-bipyridine) in chlo-roform (8). Palladium(IV) is less stable relative to reduction tothe +2 state than platinum(IV), and it is therefore an

interesting question whether [PdCl4]2) behaves in a similar

fashion when exposed to 254 nm radiation.

MATERIALS AND METHODS

Chloroform was HPLC grade, stabilized with ethanol. (Bu4N)2[PdCl4]was synthesized by mixing aqueous solutions of Na2[PdCl4] andBu4NBr (both Aldrich Chemical Co.). The resulting precipitate wasfiltered, washed with ethanol and air-dried.

Photolyses were carried out on solutions in 1 cm quartz cuvettes.Some samples were deoxygenated by bubbling argon for 5–10 min.Light from a 100 W mercury lamp (Oriel) was passed through a12.5 cm monochromator in order to irradiate samples at 254 nm orother wavelengths.

Initial concentrations of (Bu4N)2[PdCl4] were determined from theextinction coefficient at the 275 nm peak, 1.40 · 104 MM

)1 cm)1 from aBeer’s law plot.

Photolysates were analyzed by gas chromatography with mass-sensitive detection, using a Shimadzu QP-5000 GC-MS with a RestekXTI-5 column. The oven start temperature was 40�C and a lineartemperature gradient of 20� per minute was applied. The instrumentwas operated in splitless injection mode. Chlorine-containing productswere identified from their mass spectra.

RESULTS

Figure 1 shows a sequence of spectra taken during the 254 nmphotolysis of (Bu4N)2[PdCl4] in CHCl3. It is characterized by agradual blueshift in the dominant UV peak, initially at

275 nm, followed by a redshift, accompanied by a significantdecrease in extinction. Exhaustive photolysis yielded a spec-trum with a maximum absorbance in the near-UV at approx-

imately 288 nm. The development of the spectra in Fig. 1suggested that a three component system was involved, inwhich two products were formed sequentially.

Because HCl is generated in the photolysis of chloroform,several milliliters of anhydrous HCl were bubbled into asample of (Bu4N)2[PdCl4] in CHCl3 to test for a directreaction. The result was a spectrum, shown in Fig. 2, dupli-

cating the spectrum achieved through exhaustive photolysis.We previously observed that when [PtCl4]

2) in CHCl3 wasphotooxidized by irradiation at 254 nm and then photore-

*Corresponding author email: phoggard@scu.edu (Patrick E. Hoggard)� 2007TheAuthors. JournalCompilation.TheAmericanSociety ofPhotobiology 0031-8655/08

Figure 1. Sequential photolysis at 254 nm of a 7.8 · 10)5MM solution

of (Bu4 N)2PdCl4 in CHCl3.

Photochemistry and Photobiology, 2008, 84: 86–89

86

duced at a higher wavelength, the spectrum of the resultingPt(II) complex was shifted (7). This was attributed to theformation of the protonated species H2PtCl4 (7), analogous to

protonated complexes formed in CHCl3 by other complexes inthe presence of HCl (9–11). We therefore assign the spectrumof the product from the reaction of [PdCl4]

2) with hydrogen

chloride to H2PdCl4. The protons are assumed to be associatedwith the chlorides, but the nature of that association hasalways been unclear (12). The intermediate species can then be

assigned to [HPdCl4]).

The spectrum of the presumed [HPdCl4]) was determined

by optimization to fit the sequence of spectra shown in Fig. 1

and similar data sets, using mass balance as a constraint(13,14). With this spectrum, shown in Fig. 2, satisfactory fits toall the photolysate spectra were achieved. The higher extinc-tion coefficient of the intermediate is consistent with an

unsymmetrical structure, while the smaller extinction coeffi-cient of the final product suggests a symmetrical structure.Possible structures are shown below.

PdCl

Cl Cl

ClH Pd

Cl

Cl Cl

ClH H

The photolysis was repeated with a solution through whichargon was bubbled for 10 min. The reaction proceeded towardH2PdCl4 at approximately the same rate. Bubbling O2 into a

solution also had no effect on the rate of reaction.When a [PdCl4]

2) solution was irradiated at 313 nm for30 min, no detectable reaction took place.

GC-MS analysis of solutions following 30 to 60 min ofphotolysis yielded C2Cl6 as the major chlorine-containing peakfollowing the solvent band, with a smaller peak identifiable asC2HCl5.

DISCUSSION

Chloroform decomposes under irradiation at wavelengthsshorter than about 260 nm, the reaction proceeding by bondhomolysis to form, following hydrogen abstraction from the

solvent, trichloromethyl radicals (15,16).

CHCl3!hm �CHCl2 þ Cl� ð1Þ

Cl � þCHCl3 ! HClþ �CCl3 ð2Þ

�CHCl2 þ CHCl3 ! CH2Cl2 þ �CCl3 ð3Þ

In the absence of oxygen, C2Cl6 is formed by the dimeriza-tion of CCl3 radicals, and C2HCl5 from the combination of

dichloromethyl and trichloromethyl radicals. In the presenceof oxygen, CCl3 can additionally go on to form CCl3OOradicals (17–20) and CCl3OOH (19).

At 254 nm, with [PdCl4]2) concentrations around

1 · 10)4MM, the palladium complex and its photoproducts

absorb considerably more light than chloroform, whoseabsorbance is 0.081 at 254 nm (21). The formation of H2PdCl4could take place either through direct photolysis of chloro-form, Eq. (2), or through Pd–Cl bond homolysis. The latter isreasonable, considering that the UV spectrum of tetrachloro-

palladate is dominated by LMCT (ligand-to-metal chargetransfer) bands (22). The unsaturated PdCl3

2) could thenabstract chlorine from the solvent to return to the +2 state.

PdCl2�4 !hmPdCl2�3 þ Cl� ð4Þ

Cl � þCHCl3 ! HClþ �CCl3 ð5Þ

PdCl2�3 þ CHCl3 ! PdCl2�4 þ �CHCl2 ð6Þ

These two possibilities can be distinguished by establishing

the dependence of the reaction rate on the fraction of lightabsorbed by the solvent and by the tetrachloropalladate. Therate of a photochemical reaction may be expressed as I0fu ⁄V,where I0 is the incident light intensity, u is the quantum yield,V the sample volume and f is the fraction of light absorbed bywhichever species is responsible for the reaction. The relation-

ship between the fraction of light absorbed by the metalcomplex (reactant), fR, or the solvent, fS, and the concentra-tion of the metal complex, [R], is given by (23).

fR ¼ 1� 10�ðeR½R�þeP½P�þASÞn o eR½R�

eR½R� þ eP½P� þ ASð7Þ

fS ¼ 1� 10�ðeR ½R�þeP½P�þASÞn o AS

eR½R� þ eP½P� þ ASð8Þ

In these equations, AS is the absorbance of the solvent at theirradiation wavelength, P is an inactive photoproduct, and eRand eP are the extinction coefficients of reactant and product at

the irradiation wavelength.To avoid the complication that the photoproducts,

[HPdCl4]) and H2PdCl4, can be assumed to be photoactive if

[PdCl4]2) is, rate comparisons were done as initial rate

Figure 2. Spectra of {2Bu4N+;PdCl4

2)} (reactant), H2PdCl4 (finalphotoproduct) and {Bu4N

+;HPdCl4)} (intermediate product, derived

from numerical fit) in CHCl3.

Photochemistry and Photobiology, 2008, 84 87

experiments over a range of initial concentrations of(Bu4N)2[PdCl4]. The fractions of light absorbed by [PdCl4]

2)

(fR) and by CHCl3 (fS) were calculated from Eqs. (7) and (8),assuming no product and the measured extinction coefficient,

eR, at 254 nm, 9.4 · 103 MM)1 cm)1.

A negative correlation between initial rate and fR is shownin Fig. 3, which argues against tetrachloropalladate being the

predominant photoactive species. Figure 4, on the other hand,exhibits a positive correlation between the initial rate and fS,which, given the experimental errors in determining rates, can

be reasonably projected through the origin. Thus we infer thatlight absorbed by the chloroform is mainly responsible for theHCl that converts [PdCl4]

2) to H2PdCl4.

In chloroform, ions are present predominantly as ion pairs(24). The protonation of the tetrachloropalladate ion by HClshould thus be formulated as:

2Bu4Nþ;PdCl2�4

� �þHCl! Bu4N

þ;HPdCl�4� �

þ Bu4Nþ;Cl�f g

ð9Þ

Bu4Nþ;HPdCl�4

� �þHCl! H2PdCl4 þ Bu4N

þ;Cl�f g ð10Þ

CONCLUSION

We found no evidence of a photooxidation reaction analogous

to that of [PtCl4]2) in chloroform. The only reaction that took

place was the protonation of [PdCl4]2) by HCl produced by the

photolysis of CHCl3. The photolysis was hindered, rather than

promoted, by higher concentrations of the metal complex,

which reduced the amount of light available to the chloroform.CCl3OOH, generated during the photolysis of CHCl3 in thepresence of O2 (25), did not cause net oxidation to a

palladium(IV) species, although it is possible that oxidationdoes take place, but is followed by a rapid reduction throughanother pathway.

Acknowledgements—We thank the donors of the Petroleum Research

Fund, administered by the American Chemical Society, for support of

this research. Additional support was received from the National

Science Foundation through grant CHE-0079152.

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