Reduction and Alkylation of Rhodium Porphyrins in Alcohol Solutions

Apr 25, 1996 - Neta, P.; Huie, R. E. J. Phys. Chem. 1985, 89, 1783. [ACS Full Text ACS Full Text ], [CAS]. (11) . One-electron redox reactions involvi...
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7066

J. Phys. Chem. 1996, 100, 7066-7071

Reduction and Alkylation of Rhodium Porphyrins in Alcohol Solutions. Radiation Chemical and Photochemical Studies J. Grodkowski† and P. Neta* Chemical Kinetics and Thermodynamics DiVision, National Institute of Standards and Technology, Gaithersburg, Maryland 20899

Y. Abdallah and P. Hambright Department of Chemistry, Howard UniVersity, Washington, D.C. 20059 ReceiVed: October 13, 1995; In Final Form: February 6, 1996X

Radiolytic reduction of chlororhodium(III) tetramesitylporphyrin (ClRhIIIP) in alcohol solutions forms a transient RhIIP, which reacts to yield different products under different conditions. In alkaline 2-propanol the product is RhIP-, in weakly acidic conditions HRhIIIP is formed, and under strongly acidic conditions the main products of radiolysis are the alkylated rhodium complexes, R-RhIIIP. The latter products are formed by reaction of RhIIP with alkyl radicals (R•) that are produced in the irradiated solvent (R• ) •CH3 and (CH3)2C•OH in 2-propanol). UV photolysis of ClRhIIIP in acetone/2-propanol solutions led to formation of HO(CH3)2CRhIIIP. One-electron reduction of CH3-RhIIIP occurs at the porphyrin ligand to produce a transient π-radical anion, CH3-RhIIIP•-. In alkaline solution, this transient eliminates •CH3 to form the stable RhIP-, but in neutral or acidic solutions, it undergoes disproportionation, promoted by protonation on the macrocycle, to form CH3-RhIII-chlorin and then CH3-RhIII-isobacteriochlorin upon further reduction. In the presence of CO2, the initial radiolytic reduction yields are increased. After extensive irradiation, however, the yields of reduction are decreased and a very low yield of CO was found. No reaction was detected between RhIP- or HRhIIIP and CO2, even under visible light illumination. Although this system is found to catalyze homogeneous photochemical formation of H2, no catalytic activity for CO2 reduction was found under the current experimental conditions.

Introduction

Experimental Section10

Rhodium porphyrins (RhP) have been prepared in their most stable form, RhIIIP+, the unstable RhIIP, with a radical-like behavior, and RhIP-, which is stable in the absence of oxidants or acids.1-6 The latter species can accept a proton to form the hydride, HRhIIIP. Alkyl-RhIIIP also have been prepared. These various derivatives of rhodium porphyrins have been synthesized and their redox and organometallic chemistry have been studied, as models for B12 coenzyme and to understand their role in catalysis.1-6 For example, ClRhIIITPP (tetraphenylporphyrin) has been shown to catalyze the photochemical production of H2 from alcohols7,8 and reduction of CO2 to CO under certain conditions.8 Although the solutions initially contained ClRhIIITPP, the mechanism was suggested to involve the photoexcited state of the hydride, HRhIIITPP*, reacting with another hydride molecule or with CO2, to yield the observed products. In a recent study,9 radiolytic techniques were utilized to effect controlled reduction of rhodium porphyrins, initially at the metal center and subsequently at the ligand. It was found that radiolytic reduction of ClRhIIIP to RhIP- in alkaline 2-propanol solutions involved a chain reaction, whereby an intermediate RhIIP species reacts with the alcohol and the base to produce RhIP- and an additional reducing radical from the alcohol. In the present study we examine the reactions of ClRhIIIP in irradiated 2-propanol solutions in neutral and acidic media, where different products and mechanisms are found, and we explore the possibility of reduction of CO2 by using radiation chemical and photochemical techniques.

Most experiments were carried out with chlororhodium(III) 5,10,15,20-tetramesitylporphyrin (mesityl ) 2,4,6-trimethylphenyl) to prevent dimerization of the RhIIP state.6 Several results were confirmed with tetraphenylporphyrin and its 4-methoxy and 4-fluoro derivatives. The porphyrins were obtained from Mid-Century Chemicals (Posen, IL) in the form of chloride salts, ClRhIIIP. Triethylamine (TEA), dimethyl sulfide (DMS), dimethyl sulfoxide (DMSO), and pyridine were from Aldrich; 2-propanol (2-PrOH), methanol (MeOH), acetic acid (AcOH), benzene, and the inorganic compounds were analytical grade reagents from Mallinckrodt. TEA was purified by distillation over sodium, pyridine was vaccum distilled, benzene was dried over sodium sulfate, and water was purified with a Millipore Super-Q system. Fresh solutions containing typically ∼3 × 10-5 mol L-1 porphyrin were prepared before each experiment and were deoxygenated by bubbling with Ar. Alternatively, the solutions were bubbled with CO2 or CH3Cl when these were required as additional reactants. The solutions were generally neutral, but they became slightly acidic upon irradiation (see below). For comparison, certain experiments were carried out with solutions that were initially acidified with HClO4 (5 × 10-3 mol L-1). Radiolysis was performed in a Gammacell 220 60Co source with a dose rate of 1.4 or 4.2 Gy s-1. The stable products of radiolysis were analyzed by various techniques. Changes in porphyrin structure were studied mainly by spectrophotometric measurements, where the solutions typically were sealed in a cell of 2 mm optical path length and their spectra were measured before and after irradiation with various doses. Some NMR measurements of the irradiation products were carried out in a

† On leave from the Institute of Nuclear Chemistry and Technology, Warsaw, Poland. X Abstract published in AdVance ACS Abstracts, April 1, 1996.

0022-3654/96/20100-7066$12.00/0

© 1996 American Chemical Society

Rhodium Porphyrins in Alcohol Solutions

J. Phys. Chem., Vol. 100, No. 17, 1996 7067

GE QE-300 spectrometer after the solvent had been evaporated at room temperature and the solids redissolved in a small volume of a deuterated solvent. Total gas evolved in the irradiated solution was determined volumetrically, the ratio between H2, CH4, and CO was determined by gas chromatography, and the yield of formic acid was determined by ion chromatography (Dionex DX-500). To observe short-lived intermediates, pulse radiolysis was carried out with the apparatus described before,11 which utilizes 50 ns pulses of 2 MeV electrons from a Febetron Model 705 accelerator. UV photolysis was carried out with a mercury lamp and visible light photolysis with a 50 W halogen lamp. All experiments were performed at room temperature, 20 ( 2 °C. Results and Discussion Radiolytic Reactions in Alcohols. Irradiation of dilute porphyrin solutions with ionizing radiation (60Co gamma used for continuous irradiation or 2 MeV electrons used for pulse radiolysis) results in the ionization of the solvent and eventual formation of various radicals and stable products. Most of the present experiments were carried out with 2-PrOH as solvent. Radiolysis of this solvent results in the formation of several reactive species: solvated electrons (esol-), hydrogen atoms, (CH3)2C•OH radicals, and •CH3 radicals.12 The stable products include H+, H2, acetone, methane, and other minor products.12

(CH3)2CHOH Df esol-, H•, (CH3)2C•OH, •CH3, H+, molecular products (1) Hydrogen atoms react with the solvent (k ) 7.4 × 107 L mol-1 s-1 in water)13 to form additional (CH3)2C•OH radicals.

H• + (CH3)2CHOH f H2 + (CH3)2C•OH

(2)

Methyl radicals may undergo a similar reaction with the solvent, but with a much lower rate constant (k on the order of 103 L mol-1 s-1).14 The overall yield, in µmol J-1, is ∼0.12 esol-, ∼0.40 (CH3)2C•OH radical, and ∼0.2 •CH3 radicals.12 Similarly, radiolysis of methanol leads to formation of esol-, •CH2OH, and •CH radicals, although with slightly different yields.12 3 Solvated electrons reduce metalloporphyrins with diffusioncontrolled rate constants (k ∼ 1010 L mol-1 s-1)15 either at the ligand or at the metal; ClRhIIIP is reduced at the metal.

ClRhIIIP + esol- f Cl- + RhIIP

(3)

The (CH3)2C•OH radical reduces metalloporphyrins more slowly (k ∼ 107-108 L mol-1 s-1),16

ClRhIIIP + (CH3)2C•OH f Cl- + RhIIP + (CH3)2CO + H+ (4) although its ionic form, (CH3)2C•O-, present in alkaline solutions, reduces more rapidly (k ∼ 108-109 L mol-1 s-1).16 Methyl radicals do not reduce porphyrins but may react with the intermediate RhIIP by binding to the metal center.

RhIIP + •CH3 f CH3-RhIIIP

(5)

Other alkyl radicals that may reduce RhIIIP, e.g. (CH3)2C•OH, also may react by addition to RhIIP.

RhIIP + (CH3)2C•OH f HO(CH3)2C-RhIIIP

(6)

The H+ formed in reactions 1 and 4 (total yield of ∼0.5 µmol J-1) may accumulate in sufficiently high concentrations to have

Figure 1. Radiolysis of RhIIIP in acidic 2-PrOH solutions. Solutions containing 3.2 × 10-5 mol L-1 porphyrin and 5 × 10-3 mol L-1 HClO4 were deoxygenated by bubbling with Ar and irradiated in a Gammacell with a dose rate of 200 Gy/min. The spectral changes were monitored after irradiation with doses of (a) 1, 2, 3, and 4 kGy and (b) 4, 7, 10, 16, 22, 28, 34, and 40 kGy.

an effect on the nature and stability of products and on the radical reactions. Mainly, H+ reacts with esol- in a diffusioncontrolled reaction to form H•,

esol- + H+ f H•

(7)

so that reaction 3 may be replaced by the sequence of reactions 2 and 4. Furthermore, H+ will affect the fate of RhIIP. This unstable species is likely to disproportionate to more stable products, to yield RhIIIP+ and either RhIP- (in basic solution) or HRhIIIP (in mildly acidic solution).

2RhIIP f RhIIIP+ + RhIP-

(8)

2RhIIP + H+ f RhIIIP+ + HRhIIIP

(9)

Products of Radiolysis of ClRhIIIP in 2-PrOH. Radiolysis of ClRhIIIP in deoxygenated alkaline 2-PrOH was found9 to produce RhIP-. Since the radiolytic yield of RhIP- was higher than the total yield of reducing radicals, the mechanism was suggested to involve not only reactions 3, 4, and 8 but also a chain process, propagated by reaction of RhIIP with 2-PrOH and the base.9 In neutral or acidic 2-PrOH, however, RhIPwas not produced. The spectral changes observed are much smaller than those in alkaline solutions;9 the 424 and 535 nm peaks shift to 413 and 525 nm and decrease slightly (Figure 1a). These spectral changes are compatible1-6 with formation of the hydride, HRhIIIP, or an alkyl-rhodium derivative, R-RhIIIP. The hydride may be formed by the disproportionation and protonation reaction 9. Formation of the hydride, however, is ruled out since the addition of base after irradiation, to convert the hydride into RhIP-, showed the absence of this product. Possibly, the hydride, if formed, decomposes by reacting with a proton to yield H2 and the oxidized porphyrin.

HRhIIIP + H+ f H2 + RhIIIP+

(10)

7068 J. Phys. Chem., Vol. 100, No. 17, 1996

Grodkowski et al.

Therefore, the main products of radiolysis are alkyl-rhodium porphyrins produced from the intermediate RhIIP by reaction with the alkyl radicals, •CH3 or (CH3)2C•OH (reactions 5 and 6). CH3-RhIIIP has been prepared and characterized before.6 The NMR spectrum of the radiolysis product showed that the main component is CH3-RhIIIP.17 The other expected product, HO(CH3)2C-RhIIIP, may be present at lower concentrations and may be less stable. In fact, although R-hydroxyalkyl-RhIIIP has been prepared by reaction of HRhIIIP with aldehydes, e.g.,4

HRhIIIP + CH2O f HOCH2RhIIIP

(11)

such reactions with ketones were not effective due to steric hindrance by the two alkyl groups on the carbonyl.5 Thus, the product of reaction 6 has not been obtained before. Although steric crowding by the methyl groups in HO(CH3)2C-RhIIIP is expected to decrease the Rh-C bond energy as compared with that in HOCH2-RhIIIP, it does not preclude the effective progress of reaction 6. Reaction 6 is much more exothermic than the reaction of HRhIIIP with acetone, and furthermore it is less critically affected by the steric crowding (due to the different geometries of the ketyl and ketone reactants). To demonstrate the formation of HO(CH3)2C-RhIIIP by reaction 6, it is necessary to prevent the parallel reaction 5. Since methyl radicals are produced in the radiolysis of many alcohols and ethers, we resorted to UV photolysis and utilized the photoreduction of acetone by 2-PrOH for the production of (CH3)2C•OH as the sole radical intermediate.

(CH3)2CO* + (CH3)2CHOH f 2(CH3)2C•OH

(12)

This radical is expected to react partly with ClRhIIIP to form RhIIP (reaction 4) and partly with RhIIP (reaction 6). Indeed, photolysis (254 nm) of ClRhIIIP in 2-PrOH solutions containing 1-5% acetone was found to lead mainly to formation of HO(CH3)2C-RhIIIP. The spectral changes observed during photolysis were similar to those during radiolysis; the peaks of the final product were at 416 and 528 nm, i.e. 2-3 nm red shifted from those observed in the radiolysis or with an authentic sample of CH3-RhIIIP. After evaporating the solvent and dissolving the product in C6D6, 1H-NMR analysis showed no trace of CH3RhIIIP. Instead, two products were tentatively identified from their negative (relative to TMS) chemical shifts and splitting patterns. One product is probably HO(CH3)2C-RhIIIP, with two CH3 groups at -2.82, and the other is reasonably (CH3)2CHORhIIIP, with two methyl groups at -0.831 (d, J ) 6.3 Hz) and the CH at -1.503 (unresolved multiplet). While the protons on the pyrrole and mesityl rings were all observed in the proper regions,17 no attempt was made to assign given resonances to specific protons in the two main products of the mixture. The second product, (CH3)2CHO-RhIIIP, is formed from the starting material, ClRhIIIP by exchanging the Cl- with a solvent anion. Straightforward exchange does not occur; it requires that the Cl- be removed first by reduction. Possibly, the second product is formed following partial decomposition of the less stable HO(CH3)2C-RhIIIP during the evaporation process. In the photochemical experiment the solvent is initially neutral and it becomes slightly acidic due to reaction 4. In the radiolytic experiment, however, the solution becomes more acidic during irradiation, due to reactions 1 and 4. This difference in acidity may be the cause for the lower stability of HO(CH3)2C-RhIIIP under the radiolysis conditions. This is confirmed by carrying out the photochemical process in acidic (5 × 10-3 mol L-1 HClO4) 2-PrOH and finding practically no HO(CH3)2C-RhIIIP as a product. Since the acidity does not prevent either the

photochemical formation of the radicals or the expected reaction 6, we conclude that the product is decomposed by the acid.

HO(CH3)2C-RhIIIP + H+ f (CH3)2CHOH + RhIIIP+ (13) Thus, the combination of reactions 6 and 13 results in the net oxidation of RhIIP to RhIIIP+ by a typically reducing radical. Similar oxidations, that occur via addition and protonation in acidic solutions, have been reported for various organic radicals (including (CH3)2C•OH) reacting with aqueous TiIII and FeII ions.18 Reaction 13 explains the failure to observe HO(CH3)2C-RhIIIP in the radiolysis experiment and partly accounts for the much lower yield (measured in terms of decrease in ClRhIIIP concentration) observed in neutral (∼0.04 µmol J-1) and acidic (∼0.01 µmol J-1) solutions as compared with alkaline solutions (1.3 µmol J-1),9 where the main product was RhIP-. The other main factors causing the low yields are the absence of the chain reaction, which takes place only in alkaline solutions,9 and the fact that the stable products in neutral and acid solutions are formed by reaction of the intermediate RhIIP with unstable radicals, i.e. by processes that are not highly efficient due to competing radical-radical reactions. The various rhodium porphyrin species discussed in the above reactions were formulated in an incomplete fashion; that is, axial ligation of the alcohol (solvent) molecules has been ignored. When the solution contains additional compounds that are stronger ligands than alcohol, these may bind to the Rh center and may have a strong effect on the reduction and alkylation reactions. Pyridine (Py), DMS, and DMSO can bind to the rhodium porphyrin as axial ligands even when present at the 1% level in the alcoholic solutions.

Cl(ROH)RhIIIP + Py f Cl(Py)RhIIIP + ROH

(14)

Axial ligation of ClRhIIIP with these ligands caused only minimal shifts (1-2 nm) in the absorption maxima but often prevented the formation of R-RhIIIP. Addition of DMS (1%) or DMSO (1-50% by volume) to MeOH solutions of the porphyrin prevented the radiolytic formation of HOCH2-RhIIIP (or decomposed it after it was formed). CH3-RhIIIP was rapidly precipitated from MeOH by the addition of DMS or DMSO, but when it was redissolved in benzene, it was decomposed by DMS or DMSO. Addition of 1% pyridine also caused the decomposition of CH3-RhIIIP in alcohol solutions. Radiolysis in the presence of pyridine prevented formation of R-RhIIIP and directed the reduction to the porphyrin ligand; in fact, the radical anion Py2RhIIIP•- (rather than RhIIP) has been observed by pulse radiolysis as the first product of one-electron reduction.9 Triethylamine also binds to ClRhIIIP as an axial ligand, but it does not decompose CH3-RhIIIP. TEA (1-5% by volume) makes the solution basic and promotes formation of RhIP- as the main product of radiolytic reduction. When both TEA (5%) and acetic acid (0.5-5%) were added to the 2-PrOH solution of RhIIIP+ (424 and 535 nm), radiolytic reduction led to the formation of a mixture of RhIP- (400 and 497 nm) and HRhIIIP (∼412 and 525 nm).

[RhIP]- + CH3COOH f HRhIIIP + CH3COO- (15) The ratio between these two products depended on the relative concentrations of the acid and base. The hydride has been prepared before by addition of acetic acid to RhIP-.1,6 Radiolytic reduction of RhIIIP+ in the presence of acetic acid did not form the hydride efficiently because the radiolysis results in the production of a stronger acid (HCl) which decomposes

Rhodium Porphyrins in Alcohol Solutions

J. Phys. Chem., Vol. 100, No. 17, 1996 7069

the hydride. Therefore, the presence of TEA was required for neutralizing the strong acid, leaving enough of the acetic acid to promote hydride formation. Radiolytic reduction of ClRhIIIP in neutral 2-PrOH saturated with CO2 progressed more effectively than in Ar-bubbled solutions. The initial yield of reduction increased from ∼0.04 under Ar to 0.2 µmol J-1 under CO2, and the absorptions of the product were nearly the same (the peaks were at 412 and 525 nm under Ar vs 414 and 527 nm under CO2). Since CO2 is known to react very rapidly with esol- (k ) 7.7 × 109 L mol-1 s-1 in water)19 to produce the reducing radical •CO2- (E ) -1.9 V vs NHE20), it may be expected that the presence of CO2 in the solution (∼0.09 mol L-1 in 2-PrOH) will result firstly in conversion of the short-lived esol- into the longer lived •CO2-. In the absence of CO2, some of the esol- may be lost to reactions with impurities or with H+ produced by the radiolysis, but in the presence of CO2 these are efficiently converted to the reducing radicals, •CO2-, which leads to more efficient reduction of ClRhIIIP.

esol- + CO2 f •CO2-

(16)

Figure 2. Differential absorption spectrum monitored upon pulse radiolysis of CH3RhIIIP in deoxygenated 2-PrOH solutions containing 0.1 mol L-1 KOH, 100 µs after the pulse (no decay was observed over 1 ms).

ClRhIIIP + •CO2- f RhIIP + Cl- + CO2

(17)

CH3-RhIIIP2- + 2H+ f CH3-RhIIIPH2

The higher yield of RhIIP under CO2 increases the likelihood of its subsequent reaction with alkyl radicals (reactions 5 and 6) and increases the yield of this process. Reactions 16 and 17 should be effective also in acidic solutions (5 × 10-3 mol L-1 HClO4). But under these conditions the radiolytic yields were very low (∼0.01 µmol J-1 under Ar) due to the decomposition reaction 13, and thus addition of CO2 had only a minimal effect on the yield (∼0.02 µmol J-1). When CO2 was bubbled into solutions containing both TEA and acetic acid, formation of a mixture of RhIP and HRhIIIP was observed, as discussed above. Both of these products were unreactive toward CO2 for several hours. Products of Radiolysis of CH3-RhIIIP. CH3-RhIIIP is resistant toward reduction by NaBH4 but was found to be reduced rapidly by esol- and more slowly by (CH3)2C•O- (but not by (CH3)2C•OH). Radiolytic reduction in alkaline (0.01 mol L-1 KOH) 2-PrOH was found to result in dealkylation and formation of RhIP-. Pulse radiolysis experiments indicated that the initial one-electron reduction product is most likely a π-radical anion, on the basis of its broad absorption at 600640 nm (Figure 2).21

CH3-RhIIIP + esol- f CH3-RhIIIP•-

(18)

This radical anion may undergo two parallel reactions. In alkaline solutions it eliminates a methyl radical to form the stable RhIP-.

CH3-RhIIIP•- a •CH3 + RhIP-

(19)

In neutral and acidic solutions, however, different products were observed. The 413 and 526 nm peaks are replaced with weaker 410 and 597 nm absorptions and, upon further radiolysis, with an even weaker 410 nm peak and an increasing absorption at 584 nm (Figure 1b). After exposure to O2, the 584 nm peak disappears slowly, within several hours, but the 597 nm peak increases. The 597 nm peak is very close, but not identical, to that of the chlorin, RhIIIPH2+, and is ascribed to CH3-RhIIIPH2, formed by disproportionation of the radical anion, promoted by protonation.

2CH3-RhIIIP•- a CH3-RhIIIP + CH3-RhIIIP2- (20)

(21)

This assignment was confirmed by the finding that authentic Rh(III) tetramesitylchlorin (λmax 419 and 603 nm) can be converted to the product absorbing at 410 and 597 nm by irradiation under similar conditions (or by reaction with CH3Cl and NaBH4).22

ClRhIIIPH2 + esol- f Cl- + RhIIPH2

(22)

RhIIPH2 + •CH3 f CH3-RhIIIPH2

(23)

Further irradiation of CH3-RhIIIPH2 under the above conditions results in its conversion into the species with the 584 nm peak. This species also has broad absorptions at 544 and 475 nm and a Soret peak at 410 nm. The mechanism of formation involves an initial one-electron reduction to the radical anion,

CH3-RhIIIPH2 + e- f CH3-RhIIIPH2•-

(24)

which under neutral or acidic conditions does not dealkylate (as in reaction 19) but undergoes further reduction and protonation (as in reactions 20 and 21). On the basis of the observation that Zn tetraphenylchlorin is reduced by diimide predominantly to the isobacteriochlorin23 (whereas the free base is reduced to the bacteriochlorin) and comparison of the absorption spectra of the Zn chlorin, bacteriochlorin, and isobacteriochlorin,24 we ascribe the product observed in the present experiments (584 nm peak) to the isobacteriochlorin with a methylated rhodium center, CH3-RhIIIPH4. The radiolytic yields of the above reductions are very low. With 3 × 10-5 mol L-1 porphyrin solutions, 4 kGy was required to reduce and alkylate RhIIIP into R-RhIIIP and 40 kGy to reduce the latter products by four electrons on the ligand (to R-RhIIIPH4). A dose of 40 kGy produces a total of ∼2 × 10-2 mol L-1 of reducing equivalents (esol- and (CH3)2C•OH) in irradiated 2-PrOH solutions, i.e. ∼600 times the initial porphyrin concentration. This indicates that either the great majority of the radicals do not reduce R-RhIIIP or the reduced species are reoxidized by H+ to yield H2. To distinguish between these two possibilities, we measured the total gas evolved upon radiolysis and the yields of H2 and CH4 by gas chromatography. We find that the radiolytic yields of these gases in acidic 2-PrOH

7070 J. Phys. Chem., Vol. 100, No. 17, 1996 (0.30 and 0.15 µmol J-1, respectively, after irradiation with 40120 kGy) are close to those reported previously (0.37 and 0.15 µmol J-1, at lower doses, 2-20 kGy),25 but the yields of H2 and CH4 were practically the same in the presence and absence of the porphyrin. Therefore, it appears that the great majority of the solvent radicals decay without reducing R-RhIIIP. This implies that the reduction potential of R-RhIIIP is near or slightly more negative than that of (CH3)2C•OH.26 To examine the possibility that products of rhodium porphyrin reduction may catalyze the reduction of CO2, we studied the effect of CO2 on the progress of γ-radiolysis by examining both the products obtained from the porphyrin and those formed from the solvent and CO2. In neutral solutions, the effect of CO2 was found to be an increase in the yield of radiolytic reactions of the porphyrin with no significant changes in the products formed. As discussed above, this is due to reactions 16 and 17 leading to more effective scavenging of reducing species. In acidic solutions, however, the yield of reduction was practically unchanged by CO2 in the early stages of the reduction (0-5 kGy), but upon further irradiation (10-40 kGy) CO2 decreased the yield considerably. At this stage of the radiolysis, the chlorin R-RhIIIPH2 is reduced to the isobacteriochlorin R-RhIIIPH4. Replacing some of the (CH3)2C•OH radicals with •CO2- (due to the presence of CO2) can only enhance the reduction efficiency, since the latter radical is a stronger reductant. Furthermore, the reduction product was stable in the presence of CO2. Therefore, the lowering of the yield of reduction may signify the occurrence of a different pathway for reaction of •CO -. Possibly, this radical reacts partly by binding to the 2 Rh center and this short-lived complex is then attacked by another radical, resulting either in reduction of the bound CO2to yield CO or in combination to yield other products. The production of CO was confirmed by analysis of the gaseous products formed after long irradiations (doses of 40120 kGy), but the yield was very low. Irradiation of 2-PrOH solutions saturated with CO2 resulted in the formation of H2 and CH4 with yields (0.34 and 0.17 µmol J-1, respectively) that are similar to those found in the absence of CO2. In addition, a small yield of CO was detected, ∼0.015 µmol J-1. Similar yields were found in the presence and absence of 5 × 10-5 mol L-1 ClRhIIIP in neutral 2-PrOH solutions. Although this yield of CO is fairly low, it is 1 order of magnitude higher than the yield estimated from the direct effect of radiation on the dissolved CO2 and also considerably higher than the amount that may be produced by direct action of radiation on the alcohol.27 Since the radiation doses were quite high in these experiments, it is possible that the small yield of CO is accumulated by partial radical-radical reactions, such as reactions of •CO2- with H• or esol- to give either formic acid or CO. In acidic solutions (5 × 10-3 mol L-1 HClO4) the yield of CO was also 0.015 µmol J-1 in the absence of the porphyrin, but it increased to ∼0.030 µmol J-1 in the presence of ClRhIIIP. We have also measured the yield of formic acid and found it to be nearly the same in the presence and absence of the porphyrin (∼0.05 µmol J-1 in neutral solutions and ∼0.09 µmol J-1 in acidic solutions). Thus, this system may effect some radiolytic reduction of CO2 to CO, but with very low efficiency and with no catalytic activity toward CO2. Photochemical Reduction of ClRhIIIP. Since the radiolysis experiments described above failed to show significant reduction of CO2, we carried out several photochemical experiments. Photolysis of ClRhIIIP by visible light in deoxygenated 2-PrOH or 2-PrOH/benzene solutions led to little change in the spectrum, but in the presence of the reductive quencher triethylamine (0.35

Grodkowski et al. mol L-1) a substantial decrease in the ClRhIIIP peaks and formation of the RhIP- spectrum were apparent. hν

Et3N(Cl)RhIIIP 98 Et3N•+ + Cl- + RhIIP

(25)

The quantum yield decreased at lower [TEA], and mixtures of RhIP- and HRhIIIP were formed. Addition of acetic acid increased the quantum yield28 and permitted control of the ratio of the two products. With 0.35 mol L-1 TEA in deoxygenated 2-PrOH/benzene (1:1) solutions, addition of up to 0.3 mol L-1 AcOH increased the quantum yield gradually by a up to a factor of 10 and the main product was RhIP, addition of 0.5-0.6 mol L-1 AcOH gave approximately equal concentrations of RhIP and HRhIIIP, and with 0.8-0.9 mol L-1 AcOH the product was predominantly the hydride. These results are due to the competition between reactions 8 and 9, forming the two products, and the acid-base equilibrium between these two products. Photochemical reduction of RhIIIP+ to RhIP- and/or HRhIIIP in 2-PrOH/ benzene solutions was also achieved in solutions saturated with CO2. These experiments indicated that both reduction products are stable for hours and do not react with CO2. Even under visible light photolysis (under Ar or CO2), the solutions of RhIP- and HRhIIIP were relatively stable. When the photolysis was continued beyond the first stage (up to 20 times as long as that required to obtain the first reduction products), no change in the spectrum was observed in the presence of e0.5 mol L-1 AcOH and only a small change (about 10-20%) in the presence of 0.8 mol L-1 AcOH. These long irradiations gave meaningful results only in 2-PrOH/benzene mixtures since in neat 2-PrOH the photolysis produced an insoluble product. The lack of change in spectrum upon prolonged photolysis may be due to the following effects: (a) the excited states of RhIP- and HRhIIIP undergo electron transfer with TEA that is followed by rapid back electron transfer before charge separation, or (b) the excited states react to form RhIIIP+ and other products (e.g. H2 or CO) and the RhIIIP+ is efficiently reduced by photolysis to yield back the RhIP- and HRhIIIP. To examine these possibilities, we measured the yields of H2 and CO. Photolysis of ClRhIIIP in 2-PrOH/benzene solutions containing 0.35 mol L-1 TEA, under the conditions that led to formation of mainly RhIP-, for periods 10 times longer than that required to effect complete reduction to RhIP-, resulted in formation of very little H2 (less than the concentration of porphyrin). However, in the presence of 0.8 mol L-1 AcOH in addition to the TEA, when the first product of photolysis was HRhIIIP, the yield of H2 was considerably higher and reached 10 times the porphyrin concentration. During this irradiation period, 1520% of the porphyrin were converted to reduction products other than the hydride, i.e. porphyrin ring reduction products. These results suggest that excited RhIP- is not reduced by TEA (or is reduced but undergoes rapid back electron transfer), whereas excited HRhIIIP reacts to yield H2. Since most of the HRhIIIP remains as such after the photolysis, we assume that the mechanism involves formation of H2 and RhIIIP+ and then the latter product is rapidly photoreduced to HRhIIIP. The H2 may be produced by reaction of excited HRhIIIP with TEA to form an unstable reduction product (possibly HRhIIP) which reacts rapidly with the alcohol or the acetic acid to yield H2. The possibility that excited HRhIIIP reacts with another molecule of HRhIIIP, as suggested for a similar system not containing TEA,7 cannot be ruled out but may be less likely in the presence of TEA. The small amount of ring reduction products may be formed by hydrogenation or reduction of the porphyrin ring by one of the reducing intermediates in this system. This side

Rhodium Porphyrins in Alcohol Solutions reaction is estimated to result in destruction of most of the porphyrin before a turnover number of 100 is reached, making these conditions unsuitable for sustained hydrogen production. The photochemical yield of H2 in the above solutions was similar when the solutions were saturated with either Ar or CO2. For the latter case, we also analyzed for CO as a product and found it to be below the detection limit of our system, i.e. at least 500 times less than the yield of H2. This is in contrast with the catalytic activity reported for ClRhIIITPP in the photoreduction of CO2 to CO in alcohol solutions,8 which was observed in 2-PrOH solutions (without TEA) at high temperatures and pressures. The design of those experiments suggests that catalysis of CO2 reduction may require the presence of local high concentrations or aggregates of the porphyrin. Cyclic voltammetry experiments also indicated no catalyzed reduction of CO2 by rhodium porphyrins.29 In summary, although rhodium porphyrins catalyze the photochemical formation of H2, they appear to be ineffective as truly homogeneous catalysts for reduction of CO2 under ambient conditions. Acknowledgment. This research was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy. We thank Professor J.-M. Save´ant for helpful discussions. References and Notes (1) Ogoshi, H.; Setsune, J.; Yoshida, Z. J. Am. Chem. Soc. 1977, 99, 3869. (2) Wayland, B. B.; Woods, B. A. J. Chem. Soc., Chem. Commun. 1981, 700. Wayland, B. B.; Woods, B. A.; Minda, V. M. J. Chem. Soc., Chem. Commun. 1982, 634. Wayland, B. B.; Woods, B. A.; Pierce, R. J. Am. Chem. Soc. 1982, 104, 302. Wayland, B. B.; Del Rossi, K. J. J. Organomet. Chem. 1984, 276, C27. Del Rossi, K. J.; Wayland, B. B. J. Am. Chem. Soc. 1985, 107, 7941. Bosch, H. W.; Wayland, B. B. J. Chem. Soc., Chem. Commun. 1986, 900. Coffin, V. L.; Brennen, W.; Wayland, B. B. J. Am. Chem. Soc. 1988, 110, 6063. Sherry, A. E.; Wayland, B. B. J. Am. Chem. Soc. 1990, 112, 1259. (3) Paonessa, R. S.; Thomas, N. C.; Halpern, J. J. Am. Chem. Soc. 1985, 107, 4333. (4) Van Voorhees, S. L.; Wayland, B. B. Organometallics 1985, 4, 1887. Wayland, B. B.; Van Voorhees, S. L.; Wilker, C. Inorg. Chem. 1986, 25, 4039. (5) Wayland, B. B. Polyhedron 1988, 7, 1545. (6) Wayland, B. B.; Ba, S.; Sherry, A. E. J. Am. Chem. Soc. 1991, 113, 5305. Wayland, B. B.; Ba, S.; Sherry, A. E. Inorg. Chem. 1992, 31, 148. (7) Irie, R.; Li, X.; Saito, Y. J. Mol. Catal. 1983, 18, 263; 1984, 23, 17, 23. (8) Saito, Y.; Li, X.; Kurahashi, K.; Shinoda, S. Sci. Papers Inst. Phys. Chem. Res. (Jpn.) 1984, 78, 150. (9) Grodkowski, J.; Neta, P.; Hambright, P. J. Phys. Chem. 1995, 99, 6019. (10) The mention of commercial equipment or material does not imply recognition or endorsement by the National Institute of Standards and

J. Phys. Chem., Vol. 100, No. 17, 1996 7071 Technology, nor does it imply that the material or equipment identified are necessarily the best available for the purpose. (11) Neta, P.; Huie, R. E. J. Phys. Chem. 1985, 89, 1783. (12) Johnsen, R. H.; Becker, D. A. J. Phys. Chem. 1963, 67, 831. Russell, J. C.; Freeman, G. R. J. Phys. Chem. 1968, 73, 808. Ponomarev, A. V.; Pikaev, A. K. High Energy Chem. 1986, 20, 162. Pikaev, A. K. Contemporary Radiation Chemistry. Radiolysis of Gases and Liquids; Nauka: Moscow, 1986. Jha, K. N.; Freeman, G. R. J. Am. Chem. Soc. 1973, 95, 5891. Spinks, J. W. T.; Woods, R. J. An Introduction to Radiation Chemistry, 3rd ed.; Wiley: New York, 1990; pp 415-426. (13) Fessenden, R. W.; Verma, N. C. Faraday Discuss. Chem. Soc. 1977, 63, 104. Smaller, B.; Avery, E. C.; Remko, J. R. J. Chem. Phys. 1971, 55, 2414. (14) Thomas, J. K. J. Phys. Chem. 1967, 71, 1919. Kantrowitz, E. R.; Hoffman, M. Z.; Endicott, J. F. J. Phys. Chem. 1971, 75, 1914. (15) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1988, 17, 513. (16) (a) Neta, P.; Scherz, A.; Levanon, H. J. Am. Chem. Soc. 1979, 101, 3624. (b) Guldi, D. M.; Hambright, P.; Lexa, D.; Neta, P.; Save´ant, J.-M. J. Phys. Chem. 1992, 96, 4459. (17) The 1H NMR of CH3RhIIITMP (δ in C6D6) shows 8.75 (pyrrole H, s, 8 H), 7.21 (m-H, s, 4 H), 7.07 (m′-H, s, 4 H), 2.44 (p-CH3, s, 12 H), 2.26 (o-CH3, s, 12 H), 1.73 (o′-CH3, s, 12 H), and -5.25 (103Rh-CH3, d, 3 H, J ) 2.60 Hz). The positions and relative intensities are practically identical to those obtained with an authentic sample of CH3RhIIITMP synthesized by borohydride reduction of RhIIITMP in MeOH under CH3Cl and to the values reported in ref 6. (18) Behar, D.; Samuni, A.; Fessenden, R. W. J. Phys. Chem. 1973, 77, 2055. Bakac, A.; Espenson, J. H.; Lovric, J.; Orhanovic, M. Inorg. Chem. 1987, 26, 4096. (19) Gordon, S.; Hart, E. J.; Matheson, M. S.; Rabani, J.; Thomas, J. K. Discuss. Faraday Soc. 1963, 36, 193. (20) Schwarz, H. A.; Dodson, R. W. J. Phys. Chem. 1989, 93, 409. (21) A similar spectrum, with a broad peak near 660 nm, was observed upon pulse radiolysis of CH3RhIIITSPP (tetrasulfonatophenylporphyrin) in deoxygenated aqueous 2-PrOH (0.1%) solutions at pH 10. This absorption decayed to zero within 50 ms and the final product was RhITSPP. (22) Rh(III) tetramesitylchlorin was found to be methylated also by exposure to CH3Cl in alkaline 2-PrOH due to thermal reaction, as found9 for the rhodium porphyrin. (23) Whitlock, H. W., Jr.; Hanauer, R.; Oester, M. Y.; Bower, B. K. J. Am. Chem. Soc. 1969, 91, 7485. (24) Keegan, J. D.; Stolzenberg, A. M.; Lu, Y.-C.; Linder, R. E.; Barth, G.; Moscowitz, A.; Bunnenberg, E.; Djerassi, C. J. Am. Chem. Soc. 1982, 104, 4305. (25) Johnson, R. H.; Becker, D. A. J. Phys. Chem. 1963, 67, 831. However, at much lower doses the yields were significantly higher (Basson, R. A.; van der Linde, H. J. J. Chem. Soc., Faraday Trans. 1 1974, 70, 431). (26) The reduction potential of (CH3)2C•OH in water was estimated to be -1.4 V vs NHE.20 The half-wave reduction potential of CH3-RhIIITPP in THF was reported to be -1.45 vs SCE (Anderson, J. E.; Liu, Y. H.; Kadish, K. M. Inorg. Chem. 1987, 26, 4174), i.e. -1.21 V vs NHE. It is possible that in 2-PrOH the two potentials are closer in value. (27) Spinks, J. W. T.; Woods, R. J. An Introduction to Radiation Chemistry, 3rd ed.; Wiley: New York, 1990; pp 419-425. (28) The increased quantum yield in the presence of AcOH is probably due to partial reaction with TEA to form ions (Et3NH+ and CH3CO2-) which enhance charge separation. Similar experiments in acetonitrile gave similar results, and the yield was increased upon addition of acetic or 0.1 mol L-1 tetrapropylammonium perchlorate (this salt was not sufficiently soluble in 2-PrOH to test its effect directly in that solvent). (29) Save´ant, J.-M. Private communication.

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