J. Phys. Chem. 1996, 100, 4097-4103
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Radiolytic Reduction of Tetrapropylporphycene and Its Iron, Cobalt, Nickel, Copper, and Tin Complexes Dirk M. Guldi,*,† P. Neta,*,‡ and Emanuel Vogel§ Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556, Chemical Kinetics and Thermodynamics DiVision, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, and Institut fu¨ r Organische Chemie, UniVersita¨ t zu Ko¨ ln, Greinstrasse 4, 50939 Ko¨ ln, Germany ReceiVed: October 11, 1995; In Final Form: NoVember 28, 1995X
One-electron and multielectron reductions of 2,7,12,17-tetrapropylporphycene (H2TPrPc) and its Fe, Co, Ni, Cu, and Sn complexes in 2-PrOH solutions have been studied by radiolytic techniques. Formation and decay of intermediates formed upon one-electron reduction have been followed by kinetic spectrophotometric pulse radiolysis, and the absorption spectra of stable reduction products have been recorded following γ-radiolysis. H2TPrPc and SnIVTPrPc are reduced to the π-radical anions and then to the dianions, which are stable in alkaline 2-PrOH. In neutral or acidic solutions, the π-radical anions undergo proton-enhanced disproportionation and the dianions undergo protonation. With the transition metal complexes, redox reactions of FeIII/FeII and CoIII/CoII have been observed. The porphycenes of FeII, CoII, NiII, and CuII are reduced in most cases to form the π-radical anions, MIIPc•-. The exception is the case of the CoIIPc, which forms CoIIPc•in alkaline 2-PrOH but yields a short-lived CoIPc in neutral 2-PrOH. CuIIPc•- has some CuI character, resulting in rapid demetallation in acidic solutions. The radical anions MIIPc•- also undergo proton-enhanced disproportionation to yield MIIPcH2.
Introduction Porphycenes (Pc), the first structural isomers of porphyrins,1 have attracted increasing attention aimed at exploring their photophysical and photochemical properties2 by comparison to those of porphyrins. Many of these properties were found to be significantly different for these two types of molecules, in part because of the lower symmetry of porphycenes (D2h) as compared to that (D4h) of porphyrins. The redox behavior of porphycenes3-5 also shows certain differences from that of porphyrins, partly because of differences in symmetry and energy levels of the macrocycle, and in the case of their metal complexes, also because of the smaller size of the porphycene cavity compared to that of the porphyrin (the four nitrogens of the porphycene form a rectangle with N-N distances of 2.83 and 2.63 Å, whereas in the porphyrin they form a square with N-N distances of 2.89 Å).1a The latter factor results in porphycenes being less accommodating to reduced metal ions of larger radii than are porphyrins and thus leads to preferential reduction at the porphycene ligand where a similar porphyrin may be reduced at the metal. This is exemplified with the Ni and Co complexes, where the π-radical anions of CoII- and NiII-tetrapropylporphycene, CoIITPrPc•- and NiIITPrPc•-, are produced upon reduction, whereas with porphyrins the metal ion is generally reduced in such cases. The π-radical anions of free base and metalloporphycenes (MPc) also have been characterized by electron spin resonance (ESR) and other methods.3,6 To further study the above differences, we utilize the pulse radiolysis and γ-radiolysis techniques to carry out controlled one-electron and multielectron reduction of metalloporphycenes and to characterize both the stable and any unstable intermediate reduction products. We examined the free base 2,7,12,17-tetrapropylporphycene and its complexes with Fe, Co, Ni, Cu, and Sn. † University of Notre Dame and National Institute of Standards and Technology. ‡ National Institute of Standards and Technology. § Universita ¨ t zu Ko¨ln. X Abstract published in AdVance ACS Abstracts, February 1, 1996.
0022-3654/96/20100-4097$12.00/0
Experimental Section The free base porphycene, 2,7,12,17-tetrapropylporphycene (H2TPrPc), and its metal complexes were synthesized as described before.1 Five metalloporphycenes were studied, those of FeIII, CoII, NiII, CuII, and SnIV. The solvents used were analytical grade reagents from Mallinckrodt.7 Pyridine was from the same source and was vacuum distilled prior to use. Solutions containing 5 × 10-6 to 1 × 10-4 mol L-1 porphycene or metalloporphycene in the desired medium were freshly prepared before use and were irradiated after purging with Ar. Steady state irradiations were done in a Gammacell 220 60Co source with a dose rate of 1 Gy s-1. Irradiation times were up to several minutes. Optical absorption spectra were recorded within several minutes before and after irradiation. Pulse radiolysis experiments were performed with the NIST apparatus,8 which utilizes 50 ns pulses of 2 MeV electrons from a Febetron Model 705 pulser. The dose per pulse, determined by KSCN dosimetry, was generally 10-40 Gy. Kinetic traces were recorded at various wavelengths, showing bleaching of the starting porphycene or formation of the porphycene product. The differential spectra were measured after the completion of the kinetic process observed and were not corrected for the bleaching of the parent compound to obtain absolute spectra of the products. All experiments were carried out at room temperature, 21 ( 2 °C. Other details on the pulse radiolysis apparatus and the data acquisition and processing were as described previously.8 Results and Discussion The porphycenes were reduced by irradiation in deoxygenated 2-PrOH solutions. The radiolysis of 2-PrOH yields two reducing species, esol- and (CH3)2C˙ OH, as well as some methyl radicals and additional products.9
(CH3)2CHOH f esol-, (CH3)2C˙ OH, •CH3, H2, other products (1) © 1996 American Chemical Society
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Solvated electrons reduce porphyrins and metalloporphyrins with diffusion-controlled rate constants (k2 ∼ 1010 L mol-1 s-1),10 but the (CH3)2C˙ OH radicals react more slowly (k3 ∼ 107-108 L mol-1 s-1).11 Similar reactivities may be expected for the porphycenes and metalloporphycenes.
MPc + esol- f MPc•-
(2)
MPc + (CH3)2C˙ OH f MPc•- + (CH3)2CO + H+ (3) In alkaline solutions, the (CH3)2C˙ OH radical dissociates into the radical anion, (CH3)2C˙ O-, which is a stronger reductant that reacts with porphyrins with rate constants of 108-109 L mol-1 s-1.
(CH3)2C˙ OH + OH- a (CH3)2C˙ O- + H2O
(4)
MPc + (CH3)2C˙ O- f MPc•- + (CH3)2CO
(5)
Methyl radicals, also produced in the radiolysis of 2-PrOH, are not expected to react rapidly with the porphycene macrocycle and will disappear via reaction with the solvent or via recombination. On the other hand, methyl radicals may react rapidly with certain transition metal centers to form methylmetalloporphycenes.12 This reaction has to be considered in addition to the reduction process in the study of transition metal porphycenes. The above reactions have been studied by pulse radiolysis, where the differential spectra and reaction kinetics of transient intermediates can be determined, and by γ-radiolysis, where stable products of one-electron and multielectron reduction can be characterized. The results for each porphycene are discussed below. Free Base Porphycene. Pulse radiolysis of H2TPrPc in deoxygenated 2-PrOH solutions showed bleaching of the porphycene sharp peaks at 560, 600, and 635 nm and formation of a broad absorption at 700-820 nm (Figure 1a). This spectral change is similar to that observed upon electrochemical reduction4b and is similarly ascribed to reduction of the porphycene to its π-radical anion. Reduction by esol- (reaction 2) was diffusion-controlled, and reduction by the solvent radical (reaction 3) was found to have a rate constant of (6.6 ( 0.8) × 108 L mol-1 s-1 (determined from the dependence of the firstorder formation rate constant on the porphycene concentration). The π-radical anion is unstable under these conditions. γ-Radiolysis of a similar solution resulted in bleaching of the porphycene peaks with no apparent formation of the 700-820 nm absorption. The product was colorless with absorption peaks at 312 and 329 nm. This product is probably a protonated species formed upon disproportionation of the π-radical anion.
2 H2TPrPc•- + 2 H+ f H2TPrPc + H2TPrPcH2
(6)
In contrast to the results in neutral solutions, gradual radiolytic reduction of H2TPrPc in alkaline 2-PrOH solutions, by reaction with esol- and (CH3)2C˙ O-, led to formation of a stable π-radical anion and subsequently a dianion (Figure 1b).
H2TPrPc + e- f H2TPrPc•-
(7)
H2TPrPc•- + e- f H2TPrPc2-
(8)
These spectra are in excellent agreement with those obtained upon electrochemical reduction in aprotic solvents.4b The stability of the π-radical anion in alkaline 2-PrOH indicates that its decay in neutral solutions involves reaction with protons
Figure 1. Radiolytic reduction of H2TPrPc in 2-PrOH solutions: (a) differential spectrum monitored by pulse radiolysis of a deoxygenated neutral solution, recorded 0.2 ms after the pulse, after completion of the reduction reaction, showing bleaching of the three peaks of the starting compound and formation of the broad absorption of the π-radical anion; (b) spectra of the porphycene (solid line) and its γ-radiolytic reduction products, H2TPrPc•- (dotted line) and H2TPrPc2(dashed line), in deoxygenated alkaline (0.01 mol L-1 KOH) 2-PrOH solutions, recorded after irradiation with various doses.
(reaction 6), as discussed previously for porphyrins.13 Both the π-radical anion and the dianion are rapidly and quantitatively oxidized to the starting material by O2. On the other hand, the product formed in neutral solutions is oxidized by O2 much more slowly. This is another indication that it is protonated. It is not clear whether one or two protons attach to the reduced product. The site of protonation is discussed below. It should be noted that no shifts were observed in the peaks of H2TPrPc upon addition of base, indicating that no deprotonation of the pyrrolic NH groups took place with up to 0.01 mol L-1 KOH. The absorption peaks of the porphycene and the metalloporphycenes (discussed below) in various media and of the reduction products obtained under different conditions are summarized in Table 1. SnIV-Porphycene. This complex (prepared in the form of Cl2SnIVTPrPc) was studied because SnIV-porphyrins are known to yield very stable π-radical anions even in protic solvents.14 Pulse radiolysis of SnIVTPrPc in deoxygenated 2-PrOH solutions resulted in the formation of SnIVTPrPc•-, as is evident from the broad absorption at 700-820 nm, similar to that obtained with the free base and characteristic of other porphycene π-radical anions.3-5 γ-Radiolysis of a similar solution also indicated formation of the same product, which was stable for minutes. In alkaline solutions, however, the π-radical anion was completely stable and was subsequently reduced to the dianion (Figure 2a). Both of these products were oxidized quantitatively to the starting material immediately upon exposure to O2. In this experiment it was also noted that the peaks of SnIVTPrPc shifted upon addition of KOH to the 2-PrOH solution. The shifts must be due to exchange of Cl- with OHor PrO- as axial ligands.
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TABLE 1: Absorption Peaks of the Porphycenes and Their Radiolytic Reduction Products mediuma
porphycene and products
ntr or alk
H2TPrPc
ntr alk alk
H2TPrPcH2 H2TPrPc•H2TPrPc2-
ntr or acd
SnIVTPrPc
alk
SnIVTPrPc
ntr or alk
SnIVTPrPc•-
alk acd
SnIVTPrPc2SnIVTPrPcH2
alk
FeIIITPrPc
FeIITPrPc alk, CH3Cl sat CH3FeIIITPrPc pyr Py2FeIIITPrPc Py2FeIITPrPc Py2FeIITPrPcH2
peaks, nm (relative intensities)b 367 (123), 380 sh, 561 (33), 600 (28), 634 (43) 312 (39), 329 (38) 345 sh, 366, 530, 730, 830 320 sh, 335 (150), 380 (67), 400 (57) 310 (15), 370 sh, 388 (145), 395 (138), 555 (8.5), 580 sh, 598 (46), 617 (80) 310 (32), 392 (157), 398 sh, 570 sh, 616 (70), 628 (96) 350 sh, 371 (115), 550 (12), 695 sh, 750 sh, 780 (19) 320 sh, 335 (88), 376 (35), 395 (32) 400 (150), 420 (202), 510 sh, 540 sh, 550 sh, 578 (40) 355 sh, 380 (66), 430-460 sh, 575 (19), 622 (49) 375-388, 598 sh, 624 sh, 640 360 (93), 560 sh, 608 (43) 380 (255), 554 (51), 611 (91) 332 (137), 347 (154), 478 (8), 560 sh, 575 sh, 604 (42), 617 (43) 360 sh, 405 (57), 418 (60), 562 sh, 586 (24)
CoIIITPrPc CoIITPrPc CoITPrPc CoIITPrPcH2 CoIITPrPc CoIITPrPc•Py2CoIIITPrPc Py2CoIITPrPc
385 (52), 570 sh, 608 (25) 322 (30), 382 (54), 550 sh, 590 (34) ∼390, ∼620 410, 558 sh, 585 322, 382, 550 sh, 590 ∼370, 555, 650, 740, 815 372 sh, 390 (81), 552 sh, 600 (48) 384 (54), 570 sh, 612 (32)
ntr or alk
NiIITPrPc
ntr alk alk
NiIITPrPcH2 NiIITPrPc•NiIITPrPc2-
370 sh, 386 (98), 560 sh, 601 (48), 610 sh 414 (44), 485 (4), 558 sh, 570 (19) ∼370, ∼550, 730-750, 815 350, 412
ntr or alk
CuIITPrPc
ntr ntr alk acd
CuIITPrPc•CuIITPrPcH2 CuIITPrPcH2 CuIITPrPc
ntr
alk pyr
H2TPrPc
364 sh, 384 (37), 565 sh, 610 (19), 620 sh ∼740, ∼820 398, 414, 560, 572 (+ H2TPrPc) 398 sh, 414, 560 sh, 572 364 sh, 383 (77), 565 sh, 610 (34), 620 sh 368, 380 sh, 561, 601, 634
a
The porphycenes were dissolved in 2-PrOH and the solutions were deoxygenated by bubbling with Ar, except were noted. Abbreviations for the medium are the following: ntr, neutral, alk., alkaline (0.01 mol L-1 KOH); acd, acidic (0.01 mol L-1 HClO4); pyr, neutral with 1% (0.12 mol L-1) pyridine. b The relative intensities of the peaks are given in parentheses only for species that were observed with minimal or no interference by absorptions of other species. Absorption “shoulders” are denoted by “sh” and not given relative intensities.
Since the π-radical anion and the dianion were stable in neutral and alkaline 2-PrOH, we attempted to produce the protonated reduction product by radiolytic reduction of SnIVTPrPc in acidic 2-PrOH solutions (0.01 mol L-1 HClO4). In this case, indeed, we observed formation of a different product (Figure 2b), which was not oxidizable by O2. On the basis of the behavior of metalloporphyins in similar experiments,14,15 we suggest that this product is protonated on a pyrrole ring, similar to a chlorin,16 and we formulate it as SnIVTPrPcH2. Similar products are formed by reduction of other metalloporphycenes, which have similar absorption spectra and similar resistance to O2 oxidation (see below). The protonated reduced species formed from the free base, however, had a different spectrum and was readily oxidized by O2. Again, by comparison to the behavior of porphyrins, we ascribe this product to a species protonated at a bridge position and a nitrogen, i.e., equivalent to the phlorin16 produced from free base porphyrins. Unlike in the case of porphyrins, protonation of the reduced free base porphycene forms a product that absorbs only in the
Figure 2. Radiolytic reduction of SnIVTPrPc in 2-PrOH solutions: (a) in neutral solution, showing the spectrum of the starting material (solid line) and the reduction products, SnIVTPrPc•- (dashed line) and SnIVTPrPc2- (dotted line); (b) in acidic (0.01 mol L-1 HClO4) 2-PrOH, showing the spectrum of the starting material (solid line) and of the protonated 2e-reduction product (dotted line).
UV region, indicating that the extended conjugation of the macrocycle is broken. FeIII-Porphycene. The differential absorption spectrum recorded by pulse radiolysis of a deoxygenated solution of FeIIITPrPc in alkaline 2-PrOH shows bleaching of the porphycene peaks and formation of sharp peaks at 400 and 650 nm (Figure 3a). This spectral change is ascribed to reduction of FeIIITPrPc to the FeII state. The spectral changes observed by γ-radiolysis (Table 1) are in agreement with this differential spectrum and with spectra obtained by electrochemical reduction.4b In contrast to the cases of the free base and the SnIV porphycene discussed above, we have to consider the potential reaction of methyl radicals with FeIITPrPc. To assess the contribution of this reaction, we studied this system under CH3Cl instead of Ar. Methyl chloride, present in large excess over the porphycene (>0.1 mol L-1), will react with all the esol- to form methyl radicals.
CH3Cl + esol- f •CH3 + Cl-
(9)
Thus, FeIIITPrPc will be reduced to FeIITPrPc only via reaction 10,
FeIIITPrPc + (CH3)2·C˙ OH f FeIITPrPc + (CH3)2CO + H+ (10) and the reduced product may react with methyl radical by
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Figure 3. Radiolytic reduction (and methylation) of FeIIITPrPc in 2-PrOH solutions: (a) differential absorption spectrum monitored by pulse radiolysis of a deoxygenated alkaline (0.01 mol L-1 KOH) 2-PrOH solution, recorded 0.2 ms after the pulse, showing reduction to FeIITPrPc; (b) γ-radiolysis of FeIIITPrPc in alkaline 2-PrOH solution saturated with CH3Cl (the arrows show the decay of the starting material and the formation of CH3FeIIITPrPc); (c) differential spectrum monitored by pulse radiolysis of a deoxygenated 2-PrOH solution containing 1% pyridine, recorded 0.2 ms after the pulse, showing reduction of Py2FeIIITPrPc to the FeII complex; (d) γ-radiolysis of a solution similar to that in part c (the arrows show the direction of absorption changes during irradiation).
addition.
FeIITPrPc + •CH3 f CH3FeIIITPrPc
(11)
Steady state radiolysis of FeIIITPrPc in CH3Cl-saturated 2-PrOH solution results in spectral changes (Figure 3b) that are significantly different from those detected in the absence of CH3Cl, i.e., formation of bands at 360 and 608 nm. These peaks are ascribed to CH3FeIIITPrPc and are somewhat comparable to the peaks reported for C6H5FeIIIetioPc.12 These peaks are not apparent in the spectrum obtained without methyl chloride. Pulse radiolysis experiments also indicate significant difference between the absorption time profiles in the absence and presence of CH3Cl. Whereas reduction under Ar leads to formation of a stable product (FeIITPrPc monitored at 650 nm), the traces observed under CH3Cl show that the initial formation step is followed by a slow decay, supporting the occurrence of reactions 10 and 11. The spectrum recorded ∼100 µs after the pulse, i.e., after the initial formation is complete, resembles that found upon reduction. The subsequent decay due to alkylation was too slow to be complete during the time scale used in the pulse experiments. However, the differential spectrum monitored after 2 ms is in agreement with the findings of the γ-radiolysis experiments. In the above discussion we have neglected the axial ligation of the FeIIITPrPc. The compound contains a Cl- ion, and in 2-PrOH solutions, it is likely to bind a solvent molecule at the other axial position. In the presence of KOH, the 2-PrOH axial ligand may dissociate into 2-PrO-, and the Cl- ligand also may be exchanged with 2-PrOH or 2-PrO-. These axial ligands apparently do not prevent the attachment of the methyl radical on the metal center (reaction 11). It is possible, however, to replace the 2-PrOH axial ligand with a stronger coordinating ligand, such as pyridine, and this may result in the inhibition of reaction 11. Addition of 1% pyridine (0.12 mol L-1) to the 2-PrOH solution of FeIIITPrPc causes a shift of the Q bands from 575 and 622 nm to 554 and 611 nm. These changes are
due to formation of Py2FeIIITPrPc (the monopyridine complex was obtained at lower pyridine concentrations). The observed blue shifts are in accord with the increased ligand strength of pyridine and reflect a lowering of the electron density at the metal center. γ-Radiolysis of this solution leads to formation of FeIITPrPc with peaks at 332, 347, 478, 560, 575, 604, and 617 nm. These changes resemble those found in an earlier spectroelectrochemical study upon reduction of ClFeIIITPrPc in CH2Cl2.4b The differential spectrum recorded by pulse radiolysis under identical conditions (Figure 3c) is in excellent agreement with the spectra found previously4b and following γ-radiolytic reduction (Figure 3d). FeIITPrPc, formed in the above experiments, is oxidized by O2 rapidly and quantitatively to FeIIITPrPc. Further radiolytic reduction of FeIITPrPc resulted in formation of a product with absorption peaks at 405, 418, and 586 nm. This product was oxidized by O2 very slowly (over several hours), suggesting that it is a protonated 2e-reduction product. The one-electron-reduction product from FeIITPrPc was not observed in this case. An attempt to observe it as a short-lived transient in the pulse radiolysis experiments failed because of the sensitivity of the FeIITPrPc to traces of O2. CoII-Porphycene. CoIITPrPc was found to be stable in noncoordinating solvents but was slowly oxidized by air in strongly coordinating solvents, similar to the behavior of some CoII-porphyrins.17 Therefore, solutions of CoIITPrPc were prepared in carefully deoxygenated solvents. Upon exposure of a 2-PrOH solution to O2, the spectrum of CoIITPrPc, with peaks at 322, 382, and 590 nm, changes slowly to a spectrum with peaks at 385 and 608 nm, indicating oxidation to CoIIITPrPc. The same spectral changes were observed upon radiolytic oxidation of CoIITPrPc in dichloroethane solutions.18 Addition of KOH (0.01 mol L-1) and particularly pyridine (0.12 mol L-1) to the 2-PrOH solution of CoIITPrPc accelerated the oxidation of this compound by O2. These additives cause changes in the axial ligation of the CoIITPrPc, presumably from (PrOH)2CoIITPrPc to (PrO-)2CoIITPrPc by addition of base and to Py2CoIITPrPc by addition of pyridine (at lower pyridine
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concentration, (PrOH)PyCoIITPrPc was obtained). The electrondonor properties of the axial ligands affect the electron density at the metal center. The ionic form (PrO-) and the strongly coordinating pyridine increase the electron density at the cobalt center, compared to the neutral form (PrOH), and thus decrease the redox potential for the CoIII/CoII couple and facilitate oxidation by O2. Storing an O2-saturated 2-PrOH solution containing 0.12 mol L-1 pyridine and CoIITPrPc for 1 h results in complete oxidation of the CoII to CoIII. Steady state radiolysis of this system, after purging with N2, resulted in the complete reduction of the CoIII to CoII (Figure 4c). Pulse radiolysis of CoIITPrPc in deoxygenated 2-PrOH solutions, containing either 0.01 mol L-1 KOH or 0.12 mol L-1 pyridine, resulted in the reduction of this compound to its π-radical anion CoIITPrPc•-:
L2CoIITPrPc + e- f L2CoIITPrPc•-
(12)
where L represents the axial ligands PrO- or pyridine and erepresents all reducing species. This is evident from the differential spectrum (Figure 4a) showing a broad absorption at 700-800 nm, which is in agreement with the spectral changes discussed above. On the other hand, the differential spectrum recorded with a neutral 2-PrOH solution (Figure 4b) is completely different. It shows the bleaching of the CoIITPrPc absorption at 320 and 590 nm, but no broad absorptions formed above 700 nm. Instead, sharp absorptions are formed at 390 and 620 nm, indicating reduction to the CoI state:
CoIITPrPc + e- f CoITPrPc
(13)
The reaction of •CH3 radicals, also produced in the radiolysis of 2-PrOH,
CoIITPrPc + •CH3 f CH3CoIIITPrPc
(14)
does not appear to have a significant contribution in the above experiment (as indicated by the finding that γ-radiolysis under CH3Cl yields a product absorbing at 385 and 582 nm, which is oxidized by O2). The decay of CoITPrPc leads to formation of a product, observed after γ-radiolysis with main peaks at 410 and 585 nm, that is stable under oxygen. By comparison to the results obtained with other porphycenes, this product is ascribed to a protonated ligand-reduced species, produced by disproportionation of CoITPrPc:
2CoITPrPc + 2H+ f CoIITPrPc + CoIITPrPcH2 (15) A similar mechanism has been suggested previously for the disproportionation of NiI-porphyrins19 and involves a transfer of an electron from the metal to the ligand, driven by protonation at the ligand. NiII-Porphycene. The differential spectrum recorded by pulse radiolysis of a deoxygenated NiIITPrPc solution in 2-PrOH (Figure 5a) shows bleaching of the porphycene peaks at 386 and 600 nm and formation of a broad absorption in the 700840 nm range with peaks at 740 and 820 nm. This spectrum resembles that obtained upon controlled electrolysis of NiIITPrPc in tetrahydrofuran3 and is similarly ascribed to the π-radical anion NiIITPrPc•- formed by one-electron reduction (by reactions equivalent to 2 and 3). The absorption spectra recorded after γ-radiolysis of an identical solution with increasing doses exhibit a gradual decrease of the porphycene peaks, but instead of formation of the 740 and 820 nm peaks, three new peaks at 414, 558, and 570 nm are found. This finding indicates that the initial reduction product observed in the pulse radiolysis,
Figure 4. Radiolytic reduction of cobalt porphycenes in 2-PrOH solutions: (a) differential absorption spectrum monitored by pulse radiolysis of a deoxygenated alkaline (0.01 mol L-1 KOH) 2-PrOH solution of CoIITPrPc, recorded 0.2 ms after the pulse, showing bleaching of the starting material and formation of its π-radical anion; (b) differential absorption spectrum obtained with a neutral 2-PrOH solution of CoIITPrPc, recorded 0.2 ms after the pulse, showing reduction of the metal center from CoII to CoI; (c) γ-radiolytic reduction of Py2CoIIITPrPc to the CoII complex (the arrows show the direction of absorption changes during irradiation).
although it appeared to be stable over several milliseconds, decays at longer times to a different product. This stable product found after γ-radiolysis was not sensitive to O2 and is most likely a protonated 2e-reduction product, NiIITPrPcH2, as discussed above. To suppress the protonation and enhance the lifetime of NiIITPrPc•-, we added 0.01 mol L-1 KOH to the NiIITPrPc solution in 2-PrOH. The spectrum recorded upon steady state radiolysis indicates formation of new absorptions with peaks at 370, 555, 665, 730, 750, and 815 nm (Figure 5b). These peaks are in line with those observed in the pulse radiolysis experiments in neutral 2-PrOH (Figure 5a) and with earlier spectra found upon electrochemical reduction of NiIITPrPc3 and indicate formation of a stable NiIITPrPc•- in alkaline 2-PrOH. The high stability of the π-radical anion, which is in contrast to the behavior of NiII-porphyrins under similar conditions,19 enables us to follow the second reduction product formed in
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Figure 6. γ-Radiolytic reduction of CuIITPrPc in acidic (0.01 mol L-1 HClO4) 2-PrOH solutions, indicating formation of the free base porphycene. Insert shows spectra on an enlarged (×5) scale.
which results in acid-catalyzed demetallation of this species.
CuIITPrPc•- (T CuITPrPc) + 2H+ f CuI + H2TPrPc (16)
Figure 5. Radiolytic reduction of NiIITPrPc in 2-PrOH solutions: (a) differential absorption spectrum monitored by pulse radiolysis of a deoxygenated alkaline (0.01 mol L-1 KOH) 2-PrOH solution, recorded 0.2 ms after the pulse, showing bleaching of the starting material and formation of the π-radical anion; (b) γ-radiolysis of the same solution, the arrows showing disappearance of the starting material and formation of a stable π-radical anion.
the γ-radiolysis, i.e., NiIITPrPc2-. The absorption peaks resemble those obtained in the electrochemical studies and are listed in Table 1. Both the dianion and the π-radical anion are rapidly oxidized by O2 to the starting material, but in the absence of O2, the dianion has a shorter lifetime than the π-radical anion and is converted within several minutes to the protonated product. CuII-Porphycene. The differential spectrum recorded upon pulse radiolytic reduction of CuIITPrPc in 2-PrOH shows bleaching of the porphycene absorptions at 386 and 600 nm and formation of broad absorption maxima at 740 and 820 nm. These absorptions were stable for several milliseconds. They resemble those found upon reduction of NiIITPrPc and are similarly ascribed to the π-radical anion CuIITPrPc•-. The spectral changes monitored following γ-radiolysis indicate formation of new absorption with peaks at 414, 560, and 572 nm. No recovery of CuIITPrPc was observed after addition of O2. The latter peaks also resemble those observed in the γ-radiolysis of NiIITPrPc and are similarly ascribed to a protonated reduction product formed via disproportionation of CuIITPrPc•-. Besides this product, absorption peaks at 366, 380, 562, 584, 601, and 634 nm developed gradually with time. These peaks indicate formation of the free base H2TPrPc, a behavior similar to that observed upon reduction of CuII-porphyrins.20 One-electron reduction of CuII-porphyrins produces species that exhibit absorption spectra characteristic of π-radical anions, but these species undergo demetallation, probably because of a partial CuI character of the reduction product. The large radius21 of the CuI ion enhances demetallation. The present observations of the demetallation of CuIITPrPc upon reduction indicate that the π-radical anion CuIITPrPc•- also has a partial CuI character,
In irradiated neutral 2-PrOH solutions, apparently, this demetallation takes place along with the disproportionation so that both the free base porphycene and the CuIITPrPcH2 are observed. The competition between these two processes can be strongly affected by addition of acid or base. Addition of 0.01 mol L-1 KOH inhibited the demetallation completely so that all the π-radical anions were converted to the chlorin-type product, whereas addition of 0.01 mol L-1 HClO4 enhanced the reduction-induced demetallation to such an extent (Figure 6) that the chlorin-type product was not produced at all. Summary and Conclusions Porphycenes and metalloporphycenes, like their porphyrin analogues, are reduced by solvated electrons with diffusioncontrolled rate constants (∼1010 L mol-1 s-1) and by the (CH3)2C˙ OH and (CH3)2C˙ O- radicals somewhat more slowly (∼108-109 L mol-1 s-1, depending on the radical and the metal center). Radiolytic one-electron reduction of the free base porphycene, H2TPrPc, and its SnIV, NiII, and CuII complexes in 2-PrOH solutions leads to formation of π-radical anions. The CoII complex, on the other hand, yields CoIITPrPc•- in alkaline 2-PrOH but forms an unstable CoITPrPc in neutral solution. This behavior is due to the effect of the axial ligands; the PrOligands in alkaline solutions increase the electron density at the metal center, compared to PrOH ligands, and thus inhibit reduction at the metal compared to reduction at the porphycene macrocycle. This is in contrast to the behavior of CoIIporphyrins, which are generally reduced to CoIP complexes rather than to CoIIP•- radical anions. This difference is most likely due to the larger cavity of the porphyrin, compared to that of the porphycene. The former can accommodate the larger reduced metal ion, and thus, the electronic effect of the ligand in controlling the site of reduction is less pronounced. The reduction product of CuIITPrPc has a spectrum characteristic of radical anions, but it also exhibits a behavior indicative of some CuI character, i.e., it rapidly demetallates in acidic solutions. This finding parallels the observation with CuIIporphyrins.20 The π-radical anions formed from the porphycenes have varying stability in neutral 2-PrOH solutions but were generally stable in alkaline solutions. The stability was greatest for the SnIV complex, in parallel with previous findings with porphyrins,14 and is related to the high electronegativity of this metal center. Increased electronegativity of the metal center reduces the negative charge density on the macrocycle π-system, and this results in more facile reduction (more positive reduction potential) and less facile protonation. The decay of the π-radical
Reduction of Tetrapropylporphycene and Its Complexes anions in protic solvents probably occurs via protonation of the dianion,13,14 which may be present in equilibrium with the radical anion and the parent compound. Therefore, increased electronegativity of the metal center enhances the stability of the π-radical anion by retarding protonation.14 The stability of the porphycene π-radical anions in protic solvents is much greater than that observed with the corresponding porphyrins under similar conditions,11,13-15 indicating a less facile protonation of the reduced porphycene macrocycle. This is in agreement with the finding that the reduction potentials of porphycenes are less negative than those of the corresponding porphyrins. Protonation of 2e-reduced porphycenes appears to form two different products. The free base yields a product that is readily oxidized by O2, which, by comparison to the phlorins produced from porphyrins,15,16 is assumed to be protonated at a bridge carbon and a pyrrolic nitrogen. Metalloporphycenes yield reduction products that are not readily oxidized and are assumed to be protonated at the pyrrole β-positions, in parallel with the chlorins produced from porphyrins.15,16 Further studies are necessary to fully characterize these protonated reduction products of porphycenes. Acknowledgment. This research was supported in part by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy. This is Contribution No. NDRL-3879 from the Notre Dame Radiation Laboratory. References and Notes (1) (a) Vogel E.; Ko¨cher, M.; Schmickler, H.; Lex, J. Angew. Chem., Int. Ed. Engl. 1986, 25, 257. (b) Vogel, E.; Balci, M.; Pramod, K.; Koch, P.; Lex, J.; Ermer, O. Angew. Chem., Int. Ed. Engl. 1987, 26, 928. (2) Ofir, H.; Regev, A.; Levanon, H.; Vogel, E.; Ko¨cher, M.; Balci, M. J. Phys. Chem. 1987, 91, 2686. Levanon, H.; Toporowicz, M.; Ofir, H.; Fessenden, R. W.; Das, P. K.; Vogel, E.; Ko¨cher, M.; Pramod, K. J. Phys. Chem. 1988, 92, 2429. Waluk, J.; Mu¨ller, M.; Swiderek, P.; Ko¨cher, M.; Vogel, E.; Hohlneicher, G.; Michl, J. J. Am. Chem. Soc. 1991, 113, 5511. (3) Renner, M. W.; Forman, A.; Wu, W.; Chang, C. K.; Fajer, J. J. Am. Chem. Soc. 1989, 111, 8618. (4) (a) Gisselbrecht, J. P.; Gross, M.; Ko¨cher, M.; Lausmann, M.; Vogel, E. J. Am. Chem. Soc. 1990, 112, 8618. (b) Bernard, C.; Gisselbrecht,
J. Phys. Chem., Vol. 100, No. 10, 1996 4103 J. P.; Gross, M.; Vogel, E.; Lausmann, M. Inorg. Chem. 1994, 33, 2393. (5) D’Souza, F.; Boulas, P.; Aukauloo, A. M.; Guilard, R.; Kisters, M.; Vogel, E.; Kadish, K. M. J. Phys. Chem. 1994, 98, 11885. (6) Schlu¨pmann, J.; Huber, M.; Toporowicz, M.; Ko¨cher, M.; Vogel, E.; Levanon, H.; Mo¨bius, K. J. Am. Chem. Soc. 1988, 110, 8566. Schlu¨pmann, J.; Huber, M.; Toporowicz, M.; Plato, M.; Ko¨cher, M.; Vogel, E.; Levanon, H.; Mo¨bius, K. J. Am. Chem. Soc. 1990, 113, 6463. (7) The mention of commercial equipment or material does not imply recognition or endorsement by the National Institute of Standards and Technology nor does it imply that the material or equipment identified is necessarily the best available for the purpose. (8) Neta, P.; Huie, R. E. J. Phys. Chem. 1985, 89, 1783. (9) Russell, J. C.; Freeman, G. R. J. Phys. Chem. 1968, 73, 808. Pikaev, A. K. Contemporary Radiation Chemistry. Radiolysis of Gases and Liquids; Nauka: Moscow, 1986. Spinks, J. W. T.; Woods, R. J. An Introduction to Radiation Chemistry, 3rd ed.; Wiley: New York, 1990; p 425. (10) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1988, 17, 513. (11) (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. (12) Kadish, K. M.; D’Souza, F.; Van Caemelbecke, E.; Boulas, P.; Vogel, E.; Aukauloo, A. M.; Guilard, R. Inorg. Chem. 1994, 33, 4474. (13) Baral, S.; Neta, P.; Hambright, P. Radiat. Phys. Chem. 1984, 24, 245. (14) Baral, S.; Hambright, P.; Neta, P. J. Phys. Chem. 1984, 88, 1595. Richoux, M.-C.; Neta, P.; Harriman, A.; Baral, S.; Hambright, P. J. Phys. Chem. 1986, 90, 2462. (15) Sutter, T. P. G.; Rahimi, R.; Hambright, P.; Bommer, J. C.; Kumar, M.; Neta, P. J. Chem. Soc., Faraday Trans. 1993, 89, 495, and references therein. (16) Scheer, H. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 2, Part B, Chapter 1, p 1. Scheer, H.; Inhoffen, H. H. Ibid.; Chapter 2, p 45. (17) Kadish, K. M.; Lin, X. Q.; Han, B. C. Inorg. Chem. 1987, 26, 4161. (18) Radiolysis of 1,2-dichloroethane solutions under air produces strongly oxidizing radical cations (Arai, S.; Ueda, H.; Firestone, R. F.; Dorfman, L. M. J. Chem. Phys. 1969, 50, 1072. Shank, N. E.; Dorfman, L. M. J. Chem. Phys. 1970, 52, 4441. Wang, Y.; Tria, J. J.; Dorfman, L. M. J. Phys. Chem. 1979, 83, 194.6) as well as more weakly oxidizing peroxyl radicals. (19) Nahor, G. S.; Neta, P.; Hambright, P.; Robinson, L. R.; Harriman, A. J. Phys. Chem. 1990, 94, 6659. (20) Kumar, M.; Neta, P.; Sutter, T. P. G.; Hambright, P. J. Phys. Chem. 1992, 96, 9571. (21) Lange’s Handbook of Chemistry, 13th ed.; Dean, J. A., Ed.; McGraw-Hill: New York, 1985; pp 3-124.
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