11885
J. Phys. Chem. 1994, 98, 11885-11891
Electrochemical, UVNisible, and EPR Characterization of Metalloporphycenes Containing First-Row Transition Metals Francis D'Souza,tJ Pierre Boulas,t Ally M. Aukauloo,* Roger Guilard,*y$Michael Kisters? Emanuel Vogel,*,oand Karl M. Kadish'9t Department of Chemistry, University of Houston, Houston, Texas 77204-5641, LIMSAG, UMR 9953, Universiti de Bourgogne, Facult4 des Sciences "Gabriel", 6 Boulevard Gabriel, 21000 Dijon, France, and Institut f i r Organische Chemie, Universitat zu Koln, Greinstrasse 4, 50939 Koln, Germany Received: July 5, 1994; In Final Form: August 16, 1994@
Porphycenes containing d4 to dl0 first-row transition-metal ions are characterized by electrochemical, UVvisible spectroelectrochemical, and EPR measurements. The investigated compounds are represented by (0EPc)M where M = Co(II), Ni(II), Cu(II), or Zn(I1) and (0EPc)MCl where M = Mn(II1) or Fe(II1) and OEPc is the dianion of 2,3,6,7,12,13,16,17-octaethylporphycene. The metalloporphycenes exhibit electrochemical properties which are different upon reduction as compared to the corresponding metalloporphyrins, and this is attributed to the different symmetry (D2h) and smaller cavity size of the porphycene ring. The HOMO-LUMO gap in the metalloporphycenes is nearly 400 mV smaller than in related complexes of (P)Mu where P is the dianion of octaethylporphyrin (OEP) or tetraphenylporphyrin (TPP). However, like the porphyrins, the energy separation in the HOMO-LUMO gap of the porphycenes varies as a function of the size and electronegativity of the central metal ion. Electrochemical and EPR data on singly-reduced [(OEPc)M] show no evidence for the formation of a metal(1) oxidation state, and this is also the conclusion from analysis of the redox potentials. EPR spectra of electrogenerated [(OEPc)Zn]+' and [(OEPc)Ni]+*suggest the formation of an A,-type radical but with possible mixing of the closely-separated bl, orbitals. Wvisible data were used to verify several theoretical predictions, and an MO diagram is presented for the interpretation of the spectroscopic data.
Introduction Metalloporphycenes have been the subject of numerous physicochemical studies because of their close structural similarity to metalloporphyrins.1-22 In general, both sets of macrocycles exhibit quite similar electrochemical behavior upon electrooxidation but not upon electroreduction, where the metalloporphycenes are easier to reduce and less likely to exist in a low-valent metal oxidation state than the corresponding metalloporphyrins containing the same central metal ion.13,21922 Because of this easier reduction, metal complexes of porphycenes such as etioporphycene (EtioPc), tetrapropylporphycene (TPrPc), or octaethylporphycene (OEPc) have a smaller electrochemically-measuredHOMO-LUMO gap of 1.85 f 0.5 V12J3,22 as compared to 2.25 f 0.15 V generally observed for most metalloporphyrins containing the related octaethylporphyrin (OEP) or tetraphenylporphyrin (TPP) macro cycle^.^^^^^ The absorption and emission spectra of neutral metalloporphycenes containing central metal ions in their +2 or f 3 oxidation state have been well c h a r a c t e r i ~ e d , * J ~but J ~ -very ~~ little is known about the spectra of the electroreduced or electrooxidized compounds. This is investigated in the present paper, which provides a comparative W-visible spectroscopic characterization of neutral, electrooxidized, and electroreduced porphycenes containing first-row transition-metal ions. The investigated compounds are represented as (OEPc)M, 1, and (OEPc)MCl, 2, where M is a d4 to dl0 first-row transition metal and OEPc is the dianion of 2,3,6,7,12,13,16,17-octaethylport University of Houston.
* Universitk de Bourgogne. 8
Universittit zu Koln.
* Present address: Department of Chemistry, Wichita State University,
Wichita, KS 67260-0051. @Abstractpublished in Advance ACS Abstracts, October 1, 1994.
0022-365419412098-11885$04.50/0
M = Co,Ni, Cu,Zn 1
M = Mn,Fe 2
phycene. Each redox reaction of the neutral porphycene is also characterized as to the site of electron transfer by W-visible and EPR spectroscopy, and a molecular orbital diagram is presented for the interpretation of the UV-visible data. Experimental Section Chemicals. Benzonitrile (PhCN) was obtained from Aldrich Chemicals Co. and distilled over P205 under vacuum prior to use. Tetrahydrofuran (THF) was distilled over CaH2. Tetran-butylammonium perchlorate (TBAP)was purchased from Sigma Chemicals Co., recrystallized from ethyl alcohol, and dried in a vacuum oven at 40 "C for at least 1 week prior to use. Synthesis of (OEPc)Mn and (OEPc)MmCI. (OEPc)H2 and (0EPc)Zn were synthesized according to literature methods.20 A brief description for the synthesis of the other metalloporphycenes is given below. (0EPc)Ni. A mixture of 54 mg (0.1 "01) of (0EPc)Hz and 124 mg (0.5 "01) of nickel(I1) acetate tetrahydrate in 20 0 1994 American Chemical Society
D'Souza et al.
11886 J. Phys. Chem., Vol. 98, No. 46, 1994 mL of DMF was refluxed for 1 h, after which the solution was cooled to room temperature, 100 mL of CHZC12 added, and the solution washed with water. The solvent was evaporated and the residue crystallized from CHZClz/methanol. The product was isolated in 90% yield (48.8 mg) in the form of violet cubes (mp 217-218 "C). 'H NMR (300 MHz, CDC13): 6 (ppm) 9.33 (s, 4 H, H-9, -10, -19, -20), 3.91 (q, 8 H, H-3a, -6a, -13a, -16a), 3.83 (4, 8 H, H-2a, -7a, -12a, -17a), 1.71 (t, 12 H, H-2b, -7b, -12b, -17b), 1.61 (t, 12 H, H-3b, -6b, -13b, -16b). MS: m/z 590. UV/vis (CHZC12): Amm (nm) ( E ) 370 (39 300), 390 (llO700), 565 sh (12400), 604 (36 loo), 614 (34600). Elemental Anal. Calcd for C36H44N4Ni: C, 73.11; H, 7.50; N, 9.47. Found: C, 72.86; H, 7.50; N, 9.30. (0EPc)Co. A mixture of 54 mg (0.1 "01) of (0EPc)Hz and 260 mg (1 "01) of cobalt(II) acetylacetonate in 1.5 g of phenol was heated for 15 min to 220 "C. After cooling to room temperature, the residue was dissolved in CHzClz, washed with water, and chromatographed on neutral alumina (activity 111) with CHzClfiexane (1:l). The first green fraction was collected, the solvent evaporated, and the title compound recrystallized from CHpClfiexane (3:l). Yield: 50 mg (85%) (no mp < 300 "C). MS: m/z 596. UV/vis (CHzClz): Amm (nm) ( E ) 314 (34 800), 329 (40 500), 387 (90 500), 450 (3 900), 599 (45 200). Elemental Anal. Calcd for c36H44N4co: C, 73.08; H, 7.49; N, 9.47. Found: C, 72.94; H, 7.36; N, 9.29. (0EPc)Cu. A mixture of 54 mg (0.1 "01) of (0EPc)Hz and 262 mg (0.1 "01) of copper(II) acetylacetonate in 20 mL of DMF was refluxed for 1 h. After cooling to room temperature, 100 mL of CHzClz was added and the solution washed with water. The solvent was evaporated and the residue chromatographed on silica gel with CHzC12. Crystallization of the first green fraction from CH2Clzhexane (1:l) yielded 50 mg (83%) of the title compound (mp 240-241 "C). MS: m/z 595. UV/vis (CHzC12): Am= (nm) ( E ) 264 (11 300). 312 (14 300), 370 sh (60 000), 389 (180 000), 582 (18 800), 623 (64 000). Elemental Anal. Calcd for 3c6& & .uc: c, 72.55; H, 7.38; N, 9.40. Found: C, 72.42; H, 7.21; N, 9.35. (0EPc)FeCl. A mixture of 54 mg (0.1 "01) of (OEPc)H2 and 72 mg (0.2 "01) of iron(II1) acetylacetonate in 3 g of phenol was refluxed for 15 min. The solvent was removed by vacuum distillation and the residue dissolved in CHzCl2 and then stirred for 30 min with 50 mL of 10% aqueous sodium hydroxide. The organic layer was separated and washed twice with water. To this green solution, 50 mL of 10% HCl was added and stirred for 30 min. The organic layer was neutralized with aqueous sodium bicarbonate, washed with water, and crystallized from CHzClZhexane (1:l). The product was isolated in 72% yield as violet needles (mp 288-289 "C). 'H NMR (300 MHz, CDC13): 6 (ppm) 56.42 (br s, 4 H, H-3a, -6a, -13a, -16a), 41.11 (br s, 4 H, H-2a, -7a, -12a, -17a), 35.32 (br s, 4 H, H-2a, -7a, -12a, -17a), 25.91, (br s, 4 H, H-3a, -6a, -13a, -16a), 7.03, (s, 12 H, H-3b, -6b, -13b, -16b), 6.15, (s, 12 H, H-Zb, -7b, -12b, -17b), -12.08, (br s, 4 H, H-9, -10, -19, -20). MS: m/z 625/623. UV/vis (CHzC12): A, (nm) ( E ) 368 (89 000), 422 (9 600), 622 (37 000). Elemental Anal. Calcd for C3d-I+&J?eCl: C, 69.33; H, 7.05; N, 8.99; C1, 5.68. Found: C, 69.12; H, 6.85; N, 8.99; C1, 5.72. (0EPc)MnCl. A mixture of 54 mg (0.1 "01) of (0EPc)H2 and 130 mg (0.5 "01) of manganese(I1) acetylacetonate in 3 g of phenol was refluxed for 5 min. The solvent was removed by vacuum distillation, and the residue dissolved in CHZC12 and stirred for 30 min with 50 mL of 20% HCl. The organic layer was neutralized with aqueous sodium bicarbonate, washed with water, and crystallized from toluene. The product was isolated in 80% yield as violet needles (no mp -= 300 "C).
MS: m/z 624/622. UV/vis (CHZClz): A(nm) ( E ) 348 (58000), 411 (22300), 433 (31000), 567 (22000), 633 (28 OOO). Elemental Anal. Calcd for C ~ ~ 4 M n CC,l :69.39; H, 7.12; N, 8.99; C1,5.69. Found: C, 69.24; H, 7.15; N, 8.79; C1, 5.76. Methods. Cyclic voltammetry was carried out by using an EG&G Model 173 potentiostat coupled with an EG&G Model 175 universal programmer or a BAS 100 electrochemical analyzer. Current-voltage curves were recorded on a Houston Instruments Model 2000 X-Y recorder or a Houston Instruments DMP-40 plotter. A three-electrode system was used and consisted of either a platinum button or glassy carbon disk working electrode, a platinum wire counter electrode, and a saturated calomel electrode (SCE) as reference. The reference electrode was separated from the bulk of the solution by a frittedglass bridge filled with the solvent and supporting electrolyte. Solutions containing the metalloporphycenewere deoxygenated by a stream of nitrogen prior to running the experiment and were protected from air by a nitrogen blanket during the experiment. Thin-layer spectroelectrochemical measurements were carried out with a Tracor Northern 6500 multichannel analyzer/controller coupled with an EG&G Model 173 universal programmer using an optically-transparent platinum thin-layer working electrode.25 Metalloporphycenen-anion and n-cation radicals were generated for EPR measurements by bulk electrolysis in THF under an oxygen-free atmosphere. Their EPR spectra were recorded on a Bruker Model lOOD spectrometer. DPPH was used as the g marker. Results and Discussion Thin-layer UV-visible and IR spectroelectrochemistry are powerful techniques for in situ measuring the optical absorption spectra of electroreduced or electrooxidized compounds, and these data have often been used to elucidate the site of electron transfer in metallomacrocycles.24~26-29For example, metalloporphyrin n-cation and n-anion radicals usually display characteristic UV-visible and near-IR absorption bands which are quite different from those for an electrooxidized or electroreduced metalloporphyrin where the oxidation or reduction has occurred at the central metal ion or axial ligand. The data obtained from UV-vis or IR spectroelectrochemistry can be used to f i s t suggest the site of electron transfer, after which additional evidence for the assignment (metal, macrocycle, or axial ligand centered redox reaction) can be obtained using data from other measurements such as those involving EPR or NMR spectroscopy. In the present investigation, the UV-vis spectra of octaethylporphycenes containing six different fist-row transition-metal ions were measured by thin-layer spectroelectrochemistry . The compounds were also analyzed after bulk oxidation or reduction by EPR spectroscopy, and these data were then used in conjunction with the UV-visible data to assign the site of electron transfer. Each investigated porphycene was also electrochemically characterized as to the reversibility and number of electrode reactions in PhCN containing 0.1 M TBAP, and these results are briefly summarized below. Electrochemistry. Four of the six investigated metalloporphycenes contain a metal(I1) ion in their initial form, while two, (0EPc)MnCl and (OEPc)FeCI, contain a metal(II1) ion but are easily reduced to the metal(I1) form of the porphycene, after which two additional redox processes are observed (see Table 1). The half-wave potentials for these two reductions can then be compared with Eln values of the four stable M(II) derivatives, all of which undergo well-defined electroreductions between - 1.05 and - 1.47 V vs SCE. The absolute potential separation
*J. Phys. Chem., Vol. 98, No. 46, 1994 11887
Metalloporphycenes Containing Transition Metals TABLE 1: Potentials (V vs SCE) for the Oxidation and Reduction of Investigated Porphycenes in PhCN, 0.1 M TBAP ring oxidation
compd
2nd
1st
(0EPc)MnCl (0EPc)FeCl (0EPc)Co (OEPc)Nid (0EPc)Cu (0EPc)Zn
1.41 1.23 1.26 1.11 1.10 0.86
1.23b 1.06 1.13 0.82 0.83 0.66
M(II)/M(III)
Epc -0.58‘ -0.51b 0.08‘ 1.70‘
ring reduction 1st
2nd
-1.23 -0.05‘ -1.06 0.47c -1.07 1.78‘ -1.11 -1.05 -1.12
-1.49 -1.42 -1.43 -1.47 -1.39 -1.41
Epa
-0.44‘
(OEPcFeCI
e *
(0EPc)Co
G (0EPc)NI
All potentials represent reversible E112 values unless otherwise indicated. Quasireversible. Peak potentials at 0.1 V/s. Third oxidation occurs at Eli2 = 1.73 V (See Figure 2 and ref 21). a
AE1/2 260 mV
c (OEPdZn
1 I
2.00
360 mV
I
1.50
1.00
0.50
I
*
0.00
-0.50
V vs. SCE
Figure 2. Cylic voltammograms illustrating the electrooxidation of (OEPc)FeCl, (OEPc)Co, (OEPc)Ni, and (0EPc)Zn in PhCN, 0.1 M TBAP. The M(II)/M(III) reaction is indicated by a star.
360 mV
360 mV
290 mV
L
I
1
- 1.oo
-1.50 V
VI.
1
I
-2.00
SCE
Figure 1. Cyclic voltammograms illustrating the electroreduction of (0EPc)M in PhCN, 0.1 M TBAP.
between these two reductions varies between 260 and 360 mV (see Figure 1 and Table 1) and these values can be compared with the 280-410 mV values seen for the two successive oneelectron reductions of related tetrapropylporphycene derivatives containing the same central metal ions.13 The M(III)/M(II) electrode reactions of (0EPc)FeCl and (0EPc)MnCl are both irreversible in that each is coupled to a chemical reaction following electron transfer. The chemical reaction involves an axial ligand dissociation which leads to an electroactive species whose reoxidation is observed at more
positive potentials than for the original reduction. A similar electrochemical behavior has been reported for a number of Fe(III), Cr(III), and Mn(II1) porphyrins containing anionic axial ligands and was interpreted in terms of a classic “square scheme” where the axial anionic ligand (such as C1-) dissociates after electroreduction but reassociates after reoxidation of the unligated M(II) p r o d u ~ t , ~thus ~ , ~giving ~ , ~ ~two separate redox couples, each with its own characteristic redox potentials. (0EPc)FeCl and (0EPc)MnCl also undergo two one-electron oxidations in PhCN, 0.1 M TBAP. The Mn(II1) complex is more difficult to oxidize than the Fe(1II) derivative (see Table l), and this is also the case for analogous iron and manganese porphyrins where the electrooxidations lead to a stepwise formation of metalloporphyrin n-cation radicals and dications.23,2431 (0EPc)Co and (0EPc)Ni can also be electrochemically converted to porphycene x-cation radicals and dications in addition to an M(II)/M(III) reaction (see Figure 2 and Table 1). The two ring-centered oxidations of (0EPc)Co occur ufrer the formation of the Co(II1) complex, and this contrasts with the Ni porphycene where the x-mono and dications are generated prior to the formation of the Ni(II1) species.21 The electrochemical behavior of (0EPc)Co in PhCN is almost identical to that reported for (TPrPc)Co,13 ( T P P ) C O , ~and ~,~~ ( O E P ) C O ~under ~ , ~ ~the same solution conditions, and in all these cases, the first one-electron oxidation involves the formation of a Co(II1) complex followed by generation of the Co(II1) porphycene n-cation radical and dication. The three oneelectron oxidations of (0EPc)Ni are all reversible in PhCN, and the overall reaction involves the ultimate generation of a Ni(II1) porphycene dication.21 A correlation between E112 for each oxidation of the investigated porphycenes and the thud ionization potential of the metal is shown in Figure 3. The E112 for the M(II)/M(III) reactions of (OEPc)Ni, (OEPc)Co, and (0EPc)Fe (indicated by a star in Figure 2) all correlate fairly well with the third ionization potential of the metal, especially when considering the different positive charges on the compounds under conditions
D'Souza et al.
11888 J. Pkys. Ckem., Vol. 98, No. 46, 1994
2'o
I
4
Co
'Fe "
26
30
'
34
Ni
Cu
t
Zn
#Li3' a A
'
I
42
3rd ionization Potential
Figure 3. Correlation of oxidation potentials vs the third ionization potential of the porphycene metal ion. The solid circles (0)represent the M(II)/M(III)reaction, while the open symbols represent the first (0)and second (0)ligand-centered oxidations.
where the metal is oxidized.35 On the other hand, the two ringcentered oxidations occur at potentials which are shifted only to a small extent with changes in the third ionization potential of the central metal. These data are self-consistent and clearly support the assignment of Ni(II1) formation after electrogeneration of a Ni(I1) porphycene dication. Site of Electron Transfer. Figure 4a shows the UV-visible spectral changes which are obtained during the first electroreduction of (0EPc)Co in PhCN. Comparative spectra for three of the neutral and singly-reduced (OEPc)Mm derivatives are shown in Figure 5, and data for all six investigated compounds are summarized in Tables 2 and 3. The six singly-reduced metal(I1) derivatives exhibit an almost identical spectral pattern, independent of the central metal; Le., each compound has a broad UV band around 370 nm, a single visible band around 1 I I 550 nm, and either two or three near-IR bands. The [(OEPc)M]-' 350 550 750 950 Wavelength (nm) complexes also have similar UV-vis spectra (see Table 2), and these may be compared to several related [(TPrPc)M]-' derivaFigure 4. Thin-layer spectral changes obtained for (0EPc)Co (a) during tives which have earlier been characterized as metallothe fnst one-electron reduction at -1.2 V, (b) during the fnst oneelectron oxidation at +0.6 V, and (c) during the second one-electron porphycene n-anion radical^.'^^'^ Thus, the similar electrooxidation at +1.2 V in PhCN, 0.1 M TBAP. reduction behavior and the similarity in optical absorption spectra for the six electroreduced [(OEPc)M] species suggests (OEPc)FelllC1under the same solution conditions. This is also formation of a porphycene n-anion radical in all six cases. consistent with the formation of a porphycene n-cation radical. Spectral changes obtained during the first and second oneHowever, the electrogenerated product adsorbs on the electrode electron oxidations of (0EPc)Co are also shown in Figure 4. surface, and spectral data on these latter compounds are not The first electron abstraction results in a small red shift of the listed in the present study. absorption bands, and the final UV-vis spectrum (Figure 4b) EPR Studies. Figure 6 illustrates EPR spectra for two of is similar to that of a metal(II1) porphycene such as the singly-oxidized and singly-reduced compounds, (0EPc)Ni (OEPc)FeC1.22 The spectral changes in Figure 4b are also and (0EPc)Zn. The four signals are isotropic and centered comparable to those seen upon the first one-electron oxidation around g = 2.00, consistent with ligand-centered reductions and of (TPrPc)Co, (OEP)Co, or (TPP)Co in a bonding, nonaqueous oxidations. The n-anion radicals (Figure 6a, b) exhibit hyperfine solvent such as PhCN, THF, MeZSO, or structures, and this contrasts with the n-cation radicals (Figure The second one-electron oxidation of (0EPc)Co occurs at 6c, d) which show only a single broad signal devoid of hyperfine E1/2 = 1.13 V and leads to a Co(II1) porphycene n-cation radical. structures. The isotropic signal observed for the nickel n-cation As this reaction proceeds, the Soret and visible bands of the and n-anion spectra is invariant over a temperature range of singly-oxidized compound decrease in intensity and new bands 250-300 K, thus indicating an absence of either Ni(1) or appear at 393, 515, 753Sh,and 813 nm (see Figure 4c). The final UV-vis spectrum of electrogenerated [ ( O E P C ) C O ~ ~is~ ] ~ + Ni(II1) products in the (0EPc)Ni electrode reactions. comparable to the spectrum of electrogenerated [(OEPc)ZnU]+', Simulation of the [(OEPc)Ni]-' EPR spectrum shows that the hyperfine coupling originates from the four nitrogens (UN despite a difference in the oxidation state of the Zn(I1) and Co(II1) central metal ions and a different overall charge on the = 0.65 G) and four hydrogens of the macrocycle which are directly attached to the carbons of the porphycene ring system complexes. The UV-vis spectra of these two compounds are (UH = 1.45 G). The protons of the eight ethyl groups at the also similar to spectra obtained after the first one-electron P-pyrrole positions of the OEPc macrocycle show nc~appreciable oxidation of (OEPc)Ni", (OEPc)Curr, (OEPc)MnIr1C1, or
J. Phys. Chem., Vol. 98, No. 46, 1994 11889
Metalloporphycenes Containing Transition Metals
A W
V
Figure 6. EPR spectra of electrogenerated (a) [(OEPc)Ni]-', (b) [(OEPc)Zn]-', (c) [(OEPc)Ni]+', and (d) [(OEPc)Zn]+' at 140 K in THF, 0.1 M TBAP.
l
350
550
750 Wavelength (nm)
'
950 350
i
'
550
I
8
750
l
950
Wavelength (nm) Figure 5. W-visible spectra of (a) (OEPc)Mn and (b) [(OEPc)Mn]-' in PhCN, 0.1 M TBAP where M = Mn, Co, or Zn.
TABLE 2: W-Visible Data of [(OEPc)M]-' in PhCN, 0.2 M TBAP Mn(I1) Fe(II) Co(I1) Ni(I1) Cu(I1) Zn(I1) a
386 (0.38) 384 (0.36) 373 (0.3) 379 (0.63) 382 (0.48) 377 (0.47)
559 (0.09) 562 (0.14) 557 (0.10) 554 (0.27) 557 (0.11) 554 (0.15)
66Yh (0.10) 736Sh(0.07) 746 (0.10) 668Sh(0.06) 735 (0.1 1) 740 (0.27) 699Sh(0.05) 755 (0.12) 720Sh(0.11) 776sh(0.16)
903 (0.09) 851 (0.09) 844 (0.09) 838 (0.19) 858 (0.14) 874 (0.18)
Shoulder, sh.
TABLE 3: UV-Visible Data of (0EPc)M in PhCN, 0.2 M TBAP
A,,
M Mn(I1)" Fe(I1)" Co(I1) Ni(II) Cu(I1) Zn(II)
3885h(0.39) 366sh(0.38) 370Sh(0.28) 370Sh(0.58) 372sh(0.15) 376Sh(0.45)
404 (0.50) 378 (0.50) 389 (0.44) 395 (1.38) 395 (0.28) 397 (0.75)
(E
x 10-5)
62Ph (0.18) 569Sh(0.17) 557sh(0.09) 568Sh(0.26) 583Sh(0.05) 597Sh(0.19)
665 (0.40) 623 (0.31) 602 (0.26) 606 (0.58) 626 (0.19) 643 (0.62)
a Additional low-intensity bands observed and ascribed to higher vibrational transitions.
coupling with the unpaired electron on [(OEPc)Ni]-'. A similar simulation is obtained for [(OEPc)Zn]-' with UN = 0.64 G and UH = 1.36 G. Once again, there is no appreciable hyperfine coupling originating from the protons of the eight ethyl groups on the OEPc macrocycle. Renner et ~ 1 . have l ~ reported that [(TPrPc)Ni]-' has significant n-electron localization on the methylene protons of the propyl entities, but the simulated EPR spectra of [(OEPc)Ni]-' and [(OEPc)Zn]-' show no hyperfine interactions which might originate from protons of the ethyl substituents. Thus, the difference between [(TPrPc)Ni]-' and [(OEPc)Ni]-' might be related to the fact that the eight ethyl groups of OEPc are out of the macrocyclic plane in (0EPc)Ni due to ring ruffling effects,20as compared to (TPrPc)Ni, which is planar in solution, both in its neutral and singly-electron-reduced Two types of x-cation radicals are known in the case of
metalloporphyrins. One is for complexes where the electron has been abstracted from the al, orbital of the HOMO level, while, in the other, the a2, orbital of the metalloporphyrin loses an e l e ~ t r o n . ~The ~ *EPR ~ ~ signals for each of these two radicals are distinctly different. A nine-line EPR spectrum is seen for the A2, Jt-cation radicals, while a broad signal devoid of hyperfine splitting is observed for the AI, n-cation radicals. The splitting in the Az, metalloporphyrin radical arises from the presence of a high-spin density at both the four equivalent nitrogens and the four meso carbon atoms of the c ~ m p l e x . ~ ' . ~ ~ The observed nine-line spectrum is primarily due to an interaction of the unpaired electron with the four imine nitrogens of the compound since the meso positions are linked to four phenyl rings which are perpendicular to the porphyrin plane. This contrasts with the AI, radical, where the spin density is located only at the a and?!, carbon atoms of the metalloporphyrin pyrrole units and not at the nitrogen atoms. Hence, no hyperfine splitting is observed. As seen in Figure 6, the electrochemicallygenerated n-cation radicals of (0EPc)Ni and (0EPc)Zn show no hyperfine couplings over a temperature range of 250-300 K in THF. The signals of [(OEPc)Ni]+' and [(OEPc)Zn]+' are both centered at g = 2.00 and have spectral line widths, AH, of 10.5 and 10.0 G, respectively. As will be discussed in a following section, the MO diagram for the metalloporphycenes shows that the a, orbital of these compounds is similar to the al, orbital of those metalloporphyrins which have no spin density located at the four nitrogen atoms. Thus, the broad signal of these n-cation radicals can be assigned to an Au-type radical, but as discussed below, a Bl,-type radical is also possible. Interpretation of Optical Spectra. Optical absorption data for the investigated metal(I1) porphycenes are listed in Table 3. Four of the investigated complexes show four major absorption bands, while two, (OEPc)Mnn and (OEPc)Fen,exhibit additional low-intensity bands which are ascribed to higher vibrational transitions. The near-UV (Soret) absorption band of (OEPc)Mnis blue-shifted by 20-25 nm, while the visible bands are red-shifted by 30-40 nm as compared to the related (0EP)M" derivative^.^^^^^ The red shift in the lowest energy band, which corresponds to the HOMO-LUMO energy gap (AEl,?),indicates a smaller energy gap for metalloporphycenes as compared to the corresponding metallooctaethylporphyrin (OEP) derivatives. As discussed above, a decreased HOMOLUMO gap is also seen by us and others in the redox potential measurements. The optical absorption spectra of metalloporphycenes were
11890 J. Phys. Chem., Vol. 98, No. 46, 1994 D4h
D'Souza et al. D2h
>
14 0.67 (0EP)M
Figure 7. Schematic orbital energy level diagrams for (0EP)M and
0.79
0.75
(g)
0.83
17 r
(0EPc)M.
initially rationalized by Oertling et al. l7 using the four orbital model developed by Gouterman for interpretation of metalloporphyrin absorption and emission s p e ~ t r a . Later, ~ ~ . ~Waluk ~ et al. lo carried out extensive molecular orbital calculations using a classical perimeter model as well as CNDO/S, INDO/S and PPP calculations. From these studies, it was concluded that the effect of perturbation on the metalloporphycenes is similar to that of the metalloporphyrins and remains nearly degenerate for the HOMO orbitals. On the other hand, the metalloporphycene LUMO orbitals are split as a result of reduced symmetry of the macrocycle. Thus, the overall HOMO and LUMO molecular orbital splitting patterns of the metalloporphycenes will differ substantially from those of the metalloporphyrins. Figure 7 shows a comparative molecular orbital energy level diagram for (0EPc)M and (0EP)M. The labelling of the MO levels was anived at by symmetry considerations and by analysis of data from previous s t u d i e ~ . ~ As ~ , ' seen ~ in this figure, the HOMO orbitals of (OEPc)M, (au and bl,) are closely separated so that their relative positions might be altered upon changing the nature of the substituents on the porphycene periphery and/ or the central metal ion. The small separation between the two HOMO levels of (0EP)M (a2, and al,) (AHOMO) and an almost zero separation between the two LUMO levels (ALUMO) of these compounds can be compared to a small AHOMO (bl, and a,) and a large ALUMO (b3g and bzg) for (0EPc)M. A calculation has earlier been performed on a Be(I1) porphycene, and the estimated energy separations were given as AHOMO = 0.1 eV and ALUMO = 1.4 eV.l0 The CNDO/S calculations and perimeter model predicted a polarization sequence of yj,x,y for the two visible and two Soret bands of the metalloporphycene, and evidence for this polarization sequence was obtained from polarized UV-visible absorption, emission, and circular dichroism measurements. The fact that the low-energy transition is higher in intensity than the preceding visible band (see Table 3) also supports the assigned y,x,x,y polarization pattern. The fact that the two HOMO levels are very close and might be altered depending upon the substituents on the ring periphery suggest that a different polarization sequence can also be anticipated for the visible and Soret bands. It was earlier suggested17that the forbidden Q transitions of metalloporphyrins are allowed for metalloporphycenes, and this would lead to relatively high-intensity visible bands for these compounds as compared to the low-intensity visible bands of the related metalloporphyrins. This is indeed the case for the investigated OEPc derivatives, all of which show visible bands with high molar absorptivities as compared to the related metalloporphyrins. The ratio of molar absorptivities between
0.71
Ionic Radius
(0EPc)M
0
'0 r
16
'E
I 0
zn*
/
0
I
/
0
I
k
14 1.5
1.6 Metal
1.7
1.8
1.9
2
Electronegativity
Figure 8. Plot of I,, vs (a) metal ionic radius and (b) metal electronegativity of the investigated metalloporphycenes. the visible and the Soret bands in solutions of porphycenes in PhCN containing 0.2 M TBAP ranges from 0.42 for (0EPc)Ni to 0.82 for (0EPc)Zn and depends upon the specific central metal ion and its charge (see Table 3). Much smaller values are seen for metalloporphyrins where the ratio ranges between 0.1 and 0.2.24,31,m341 The intensity ratio of the two bands depends upon central metal ion and varies in the order Ni > Co > Fe > Cu > Mn > Zn in the (0EPc)MII series. The highest intensity ratio is seen for (0EPc)Zn and might indicate a minimal mixing of the dl0 metal orbitals with those of the porphycene macrocycle. Spectral Correlations. A number of correlations have been reported between the metalloporphyrin UV-visible spectra and properties of the macrocycle substituents, the central metal ion, and the coordinated axial ligands.29s39-43 Differences in optical properties among a series of metalloporphyrins have been related to interactions between the metal orbitals and those of the porphyrin ring system, which, in turn, will influence the one-electron energy of the a2, orbital. Generally, substituent effects on the UV-visible spectra of metalloporphyrins are additive, but this is not the case for metalloporphycenes where an observed nonadditivity has been ascribed to a low symmetry of the porphycene ring system as well as to associated steric factors related to the peripheral substitution.13317 The position of the lowest energy transition in the (OEPc)Mn complexes varies with the ionic radius of the central metal ion as shown in Figure 8a. It was expected that the energy of the HOMO orbitals would also vary with the size of the central metal ion, and as a result, the optical absorption bands are expected to have different maxima for each compound. This is the case and is due to the fact that the porphycene core widens in size with increasing size of the central metal ion.17 As a result, the relative energy of the HOMO levels, especially that of the bl, orbital which directly interacts with the metal orbitals,
Metalloporphycenes Containing Transition Metals
J. Phys. Chem., Vol. 98, No. 46, I994 11891
(1 1) Vogel, E. Pure Appl. Chem. 1990, 62, 557. is decreased, which leads to an overall red shift in the absorption (12) Renner, M. W.; Forman, A.; Wu, W.; Chang, C. K.; Fajer, J. J. spectrum. Similar effects have been reported for metalloAm. Chem. SOC.1989, 111, 8618. porphyrin~.*~ Interestingly, the electronegativity of the central (13) (a) Gisselbrecht, J. P.; Gross, M.; Kijcher, M.; Lausmann, M.; metal ion also correlates well with the lowest energy optical Vogel, E. J. Am. Chem. SOC. 1990,112,8618, (b)Bemard, C.; Gisselbrecht, J. P.; Gross, M.; Vogel, E.; Lausmann, M. Znorg. Chem. 1994, 33, 2393. transitions of the metalloporphycene (see Figure 8b). This type (14) Aramendia, P. F.; Redmond, R. W.; Nonell, S.; Schuster, W.; of behavior has also been reported for metalloporphyrin~,2~~~~ Braslavsky, S. E.; Schaffner, K.; Vogel, E. Photochem. Photobiol. 1986, and two effects are envisioned. One is that the electron should 44, 555. (15) Levanon, H.; Toporowicz, M.; Ofir,H.; Fessenden, R. W.; Das, P. shift from the metal toward the ligand as the central metal ion K.; Vogel, E.; Kocher, M.; Pramod, K. J. Phys. Chem. 1988, 92, 2429. becomes less electronegative. The second is that the melectron (16) Berman, A.; Michaeli, A,; Feitelson, J.; Bowman, M. K.; Noms, density located on the metal pn orbital should move back onto J. R.; Levanon, H.; Vogel, E.; Koch, P. J. Phys. Chem. 1992, 96, 3041. the ligand, thus raising the energy of the a, orbital through a (17) Oertling, W. A.; Wu, W.; L6pez-Garriga, J. J.; Kim, Y.; Chang, C. K. J. Am. Chem. SOC. 1991, 113, 127. diminishing of conjugation effects. The near linear correlation (18) Martire, D. 0.;Jux, N.; Aramendia, P. F.; Negri, R. M.; Lex, J.; between 2, and electronegativity in Figure 8b is in agreement Braslavsky, S. E.; Schaffner, K.; Vogel, E. J. Am. Chem. SOC.1992, 114, with the occurrence of both effects. It is likely that the less 9969. (19) Toporowicz, M.; Ofr, H.; Levanon, H.; Vogel, E.; Kocher, M.; electronegative central metal ions donate n-electron density into Pramod, K.; Fessenden, R. W. Photochem. Photobiol. 1989, 50, 37. the pn orbital, thus raising the energy of the bl, orbital. (20) Vogel, E.; Koch, P.; Hou, X.-L.; Lex, J.; Lausmann, M.; Esters, Finally, one must look at the Ni(I1) and Zn(I1) n-cation M.; Aukauloo, M. A.; Richard, P; Guilard, R. Angew. Chem., Int. Ed. Engl. radicals which lack hyperfine splittings and can therefore be 1993, 32, 1600. (21) Kadish, K. M.; Van Caemelbecke, E.; Boulas, P.; D’Souza, F.; assigned as an A,-type radical. However, the possibility of a Vogel, E.; Esters, M.; Medforth, C. J.; Smith, K. M. Znorg. Chem. 1993, Bl,-type n-cation radical cannot be ruled out since the energy 32, 4177. difference between these two orbitals is small. The fact that (22) Kadish, K. M.; Boulas, P.; D’Souza, F.; Aukaulw, M. A.; Guilard, R.; Lausmann, M.; Vogel, E. Inorg. Chem. 1994, 33, 47 1. the spectral line width, AH, for the n-cation radicals is larger (23) Kadish, K. M. Prog. Znorg. Chem. 1986, 34, 435. than for the related nanion radicals suggests that there could (24) Felton, R. H. In The Porphyrins; Dolphin, D., Ed.; Academic: New be a mixing of these two HOMO levels which would result in York, 1978; Vol. V, Chapter 3. a broad spectrum. (25) Lin, X. A,; Kadish, K. M. Anal. Chem. 1985, 57, 1498.
Acknowledgment. The authors are thankful to Melvin Zandler for helpful discussions. The support of the Robert A. Welch Foundation (Grant E-680), the National Institutes of Health (Grant GM 25 172), and the National Science Foundation (Grant CHE-8822881) is gratefully acknowledged. M. K. and E. V. are indebted to the Deutsche Forschungs-Gemeinschaft for financial support. References and Notes (1) Vogel, E.; Kocher, M.; Schmickler, H.; Lex, J. Angew. Chem., Znt. Ed. Engl. 1986, 25, 257. (2) Ofir,H.; Regev, A,; Levanon, H.; Vogel, E.; Kocher, M.; Balci, M. J. Phys. Chem. 1987, 9Z. 2686. (3) Schliipmann, J.; Huber, M.; Toporowicz, M.; Kocher, M.; Vogel, E.; Levanon, H.; Mobius, K. J. Am. Chem. SOC. 1988, 110, 8566. (4) Vogel, E.; Grigat, I.; Kocher, M.; Lex, J. Angew. Chem., Int. Ed. Engl. 1989, 28, 1655. ( 5 ) Vogel, E.; Kocher, M.; Lex, J.; Ermer, 0. Isr. J. Chem. 1989, 29, 257. _ .. . (6) Jux, N.; Koch, P.; Schmickler, H.; Lex, J.; Vogel, E. Angew. Chem., Int. Ed. Engl. 1990, 29, 1385. (7) Will, S.; Rahbar, A.; Schmickler, H.; Lex, J.; Vogel, E. Angew. Chem., Int. Ed. Engl. 1990, 29, 1390. (8) Nonell, S.; Aramendia, P. F.; Heihoff, K.; Negri, R. M.; Braslavsky, S. E. J. Phys. Chem. 1990, 94, 5879. (9) Schliipmann, J.; Huber, M.; Toporowicz, M.; Plato, M.; Kocher, M.; Vogel, E.; Levanon, H.; Mobius, K. J. Am. Chem. SOC. 1990, 112, 6463. (10) Waluk, J.; Muller, M.; Swiderek, P.; Kbcher, M.; Vogel, E.; Hohlneicher, G.; Michl, J. J. Am. Chem. SOC. 1991, 113, 5511.
(26) Electrochemical and Spectroelectrochemical Studies of Biological Redox Components; Kadish, K. M., Ed.; Advances in Chemistry Series 201; American Chemical Society: Washington, DC, 1982 and references therein. (27) Spectroelectrochemistry, Theory and Practice; Gale, R. J., Ed.; Plenum: New York, 1988. (28) Mu, X. H.; Kadish, K. M. Electroanalysis 1990, 2, 15. (29) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic: New York, 1978; Vol. III, Chapter 1. (30) (a) Kadish, K. M. In Iron Porphyrins; Lever, A. B. P., Gray, H. B., Eds.; Addison Wesley: Reading, MA, 1983; Part 2, p 161. (b) Bottomley, L. A.; Kadish, K. M. Inorg. Chem. 1981, 20, 1348. (31) Fuhrhop, J.-H.; Kadish, K. M.; Davis, D. G. J. Am. Chem. SOC. 1973, 95, 5140. (32) D’Souza; F.; Villard, A.; Van Caemelbecke, E.; Franzen, M.; Boschi, T.; Tagliatesta, P.; Kadish, K. M. Inorg. Chem. 1993, 32, 4042 (33) Mu, X. H.; Kadish, K. M. Znorg. Chem. 1989, 28, 3743. (34) Hu, Y.; Han, B. C.; Bao, L. Y.; Mu, X. H.; Kadish, K. M. Znorg. Chem. 1991, 30, 2444. (35) Electrochemical data for the (0EPc)MnCl derivative do not fit the straight line plot. The lack of fit to similar plots is also seen for manganese po’phyrins when compared to other transition-metal porphyrins (see ref 42). (36) Furenlid, L. R.; Renner, M. W.; Smith. K. M.; Fajer, J. J. Am. Chem. SOC. 1990, 112, 1634. (37) Fajer, 3.; Davis, M. S. Electron Spin Resonance of Porphyrin x-Cations and Anions. In The Porphyrins; Dolphin, D., Ed.; Academic: New York, 1978; Vol. III. (38) Gross, Z.; Barzilay, C. Angew. Chem., Znt. Ed. Engl. 1992,31,1615. (39) Gouterman, M. J. Chem. Phys. 1959, 30, 1139. (40) Nappa, M.; Valentine, J. S . J. Am. Chem. SOC. 1978, 100, 5075. (41) Wang, R.; M.-Y.; Hoffman, B. M. J. Am. Chem. SOC. 1984, 106, 4235. (42) Wolberg, A.; Manassen, J. J. Am. Chem. SOC.1970, 92, 2982. (43) Convin, A. H.; Chivvis, A. B.; Poor, R. W.; Whitten, D. G.; Baker, E. W. J. Am. Chem. SOC. 1968, 90, 6577.
