1974 395Electronic Spectra of Metal Corrole AnionsBy Noel S. Hush, John M. Dyke,*t Martin L. Williams, and Ian S. Woolsey, Department of InorganicChemistry, The University, Bristol BS8 I T SThe electronic spectra of copper(i1) and nickel(ii) corrole anions are described and compared with the correspondingmetal porphyrin spectra. The spectra are interpreted with the aid of self-consistent field Pariser-Parr-Poplex-electron calculations, and comparison with similar calculations for the porphin ring allows the origin of thedifferences between the two types of spectrum to be traced.THE electronic spectra of porphyrins and their metalderivatives have been studied extensively (refs. 1-3 andreferences therein), and in general S.C.F. x-electroncalculations have been fairly successful in explainingthem.However, less attention has been given toconjugated tetrapyrrole complexes having lower sym-metry than porphyrins and with partial saturation of thex-system of the ring. Some effort has been made tointerpret experimental corrin spectra, but with lesssuccess than for p ~ r p h y r i n s . ~ . ~ Recently a variety ofmacrocyclic conjugated tetrapyrrole complexes havingrather lowcr symmetry than porphyrins has been syii-thesised by Jolinson and his co-workers,6-8 and these forman interesting series in which to study the effects ofchange in symmetry and degree of conjugation on theelectronic spectra of tetrapyrrole complexes. We nowdescribe the electronic spectra of fully conjugated metalcorrole anions and compare them with those of analogousporphyrins.The x-electron framework of these com-plexes is shown in Figure 1, and has Czo symmetry if thering system is planar.As yet, no crystallographic data are available for metalcorrole anions, but it is to be expected that the ring systemis essential131 planar, as in the case of metal porphyrins.Southampton SO9 BNH.Spectvoscopy, 1965, 16, 415.Spectvoscopy, 1970, 35, 90.J . M o l . Spectvoscopy, 1971, 36, 16.7 Pvesent address: Department of Chemistry, The University,C. VC'eiss, 13. Kobayashi, and 11. Gouterman, J . iMol.L. Edwarcls, D. H. Dolphin, and 31. Gouterman, J . ILiToZ.1,. Edwards, D. H. Dolphin, 31. Gouterman, and ,4. D. Adler,The complexes differ from metal porphyrins in having adirect link between two of the pyrrole rings rather than afourth methine bridge. As a result, the neutral free-base corrole has three of its four pyrrole nitrogens proton-ated, and bivalent metal corroles have their conjugationN a+( A 1 ( 6 )(B) Metal(I1) porphinFIGURE 1 (-4) Fully conjugated rnetal(r1) corrole anion.interrupted by the presence of an extra hydrogen in thering system.The fully conjugated metal(I1) corrole anionsconsidered here may be formed by treatment of theneutral metal corrole with base.6 In subsequent paperswe will consider the spectra of free-base corroles, neutral-metal corroles, and various corrole derivatives.P. Day, Theouet. Chim. A c t a , 1967, 7, 328.P. O'D. Offenhartz, B. H. Offenhartz, and AI.>I. Fung,-4. W. Johnson and I. T. Kay, J . Chem. SOG., 1965, 1620.D. Dolphin, R. L. N. Harris, J, L. Huppatz, A. TIr. Johnson,8 D. A. Clarke, R. Grigg, R. L. N. Harris, A. W. Johnson,J . Anzer. Chem. Soc., 1970, 92, 2966.and I. T. Kay, J . Chem. SOG. ( C ) , 1966, 30.I. T. Kay, and K. W. Shelton, J . Cheni. SOG. ( C ) , 1967, 1645J.C.S. DaltonEXPERIMENTALThe two samples used were the 8,12-diethyl-2,3,7,13,17,18-hexamethylcopper(I1) corrole, and the 2,1$-diethyl-3,7,8,12,13,17-hexamethylnickel(11) corrole, both kindlysupplied by Professor A. W. Johnson (University of Sussex).Their preparation and purification have been describedand the samples were used without further purification.The corresponding fully conjugated metal(I1) corroleanions were prepared by treatment with tetra-n-butyl-ammonium hydroxide in dimethylformamide and bysodium-film reduction in tetrahydrofuran under highvacuum.The spectra recorded for the anions prepared bythe two different methods were almost identical, and forconvenience therefore we shall only consider those measuredin tetrahydrofuran. The solvent purification and reductiontechnique have been de~cribed.~The electronic spectra of the metal corrole anions wererecorded on a Beckman DK 2A ratio-recording spectro-photometer at ambient temperatures. They were foundto be independent of concentration over the range loT5-10-3n~. Experimental extinction coefficients given are in1 mol-l cm-l, and oscillator strengths (f) for various bandsin the experimental spectra were calculated from therelationship lo (l), where E is the extinction coefficient andv the transition energy in wavenumbers.f = 4.319 x 10-9bdv (1)x-Electron Calculations.-Since the electronic spectra ofmetal corrole anions are determined largely by z+n*transitions of the corrole ring, as in the case of porphyrins,i t is appropriate to use S.C.F.x-electron calculations as astarting point for their interpretation. The calculationsdiscussed in this paper utilized a x-electron Hamiltoniansimplified by the approximations of Pariser, Parr, and Pople(P.P.P.).l1$l2 The basic procedure has been discussed a tlength. 13-16The calculation of the spectra of the corrole systems wasperformed with allowance of configuration interactionbetween the lowest 20 singly excited configurations.Theoscillator strengths fl and f2 of the transitions were calcu-lated by use of the length and velocity forms respectivelyof the transition moment matrix element. Expressionsfor the primitive integrals of these operators over the homo-and hetero-nuclear atomic orbitals have been given by Hushand TVilliams.17In all the calculations a planar skeleton was assumed forthe corrole ring. The atomic co-ordinates employed werethose calculated by us l8 using the bond length-bond orderrelationships of Nishimoto and F0r~ter.l~. l5 The fullskeleton possesses Czz. symmetry and the values of the co-ordinates are given in Table P, together with correspondingco-ordinates for the porphin ring system.The porphinmolecular geometry is an adaptation of that of Hoardet nl. ,19 the slight non-planarity and non-square symmetrybeing ignored by forcing the molecule into a plane and9 N. S. Hush and I. S. Woolsey, J . Amer. Chem. Soc., 1972,l o 3 . N. Murrell, ' The Theory of the Electronic Spectra ofl1 R. Pariser and R. G. Parr, J . Chem. Phys., 1953, 21,466, 767.l2 J. A. Pople, Trans. FaraEay SOG., 1953, 49, 1375.l3 R. G. Parr, ' Quantum Theory of Molecular Electronicl4 K. Nishimoto and L. S. Forster, Theoret. Chinz. Acta, 1965,94, 4107.Organic Molecules,' Methuen, London, 1963.Structure,' Benjamin, New York, 1964.3, 407.averaging symmetrically related bond distances and angles.The resulting skeleton therefore possesses D4h symmetry.TABLE 1(a) Atomic co-ordinates * for the corrole ringAtom X Y Atom x Y1 0.000 0.729 7 4-630 3.4862 -1.025 1.714 S 5.509 2-4360.357 2.927 9 4.721 1.2314 1.026 2.584 10 5.429 0.0005 2.120 3.498 20 1.224 1.2516 3.352 2.820 21 3.406 1.4710(b) Atomic co-ordinates * for the porphin ringAtom 2: Y Atom x Y2 0.681 4.217 21 0.000 2.0543 1.098 2.839 22 2.054 o*ooo4 2.444 2.4445 2.839 1.0986 4.217 0.681* All values in A, numbering as in Figure 1.RESULTS AND DISCUSSIONThe electronic spectra of the copper and nickel corroleanions obtained in tetrahydrofuran by the method pre-viously described are shown in Figures 2 and 3.Acid-base titrations in dimethylformarnide 2o have confirmedthat the formation of the metal corrole anion is due t othe removal of a single proton from the neutral-metalcorrole, so that the spectra observed for these speciesarise from a fully conjugated corrole ring, having 26c I 12 'II \ I \0 10 15 20 25 30 35:1 0 - ~ Y / cm-'FIGURE 2 Electronic spectrum of copper corrole anionin tetrahydrofuranx-electrons.The spectra of the nickel and copper com-plexes are very similar to one another and also to thoseof the corresponding nickel and copper octa-alkyl-porphins.2l5 I<. Nishimoto and L. S. Forster, Theorcl. Chim. A d a , 1966,l6 I<. Nishimoto and N. Mataga, Z . phys. Clzeim. (Frankfurt),l7 N. S. Hush and M. L. Williams, Chem. Phys. Letters, 1971,N. S. Hush, J. M. Dyke, M. L. Williams, and I. S. Woolsey,J. L. Hoard, M.J. Hsmor, andT. A. Hamor, J . Avner. Chem.4, 155.1957, 12, 335.8, 179.Mol. Phys., 1969, 17, 559.Soc., 1963, 85, 2334.*O N. S. Hush and R. 1;. McMeeking, unpublished results1974An intense absorption is observed in the near-u.v. re-gion (24 000-26 000 cm-l), and a weaker system of bandsis seen in the visible region (16 000-20 000 cm-l), anal-ogous to the Soret (or B ) and visible (Q) bands in porphinspectra.. In discussing corrole spectra, therefore, we shallrefer to these as the Soret and visible bands. Assuming61 57 1 , -EV0E; 3- 2L.)U I0!0FIGURE 3 Electronic spectrum of nickel corrole anionin tetrahydrofuranDBh symmetry for metal porphins, we predict the Q and Bbands to be doubly degenerate, and to arise largely fromtransitions between the two highest occupied molecularorbitals (3azu and lal,) and the lowest vacant molecularorbitals, the e, pair. The appearance of several peaksin the visible region of porphin spectra has thereforebeen interpreted as vibrational fine structure of theelectronic transitions.21,22 Typically two peaks areobserved in the visible region with a separation of ca.1250 cm-l.The lower-energy peak is in general themore intense, and has been assigned to the 0-0 vibra-tional component of the first singlet x+z* transi-tion, and the second to the 0-1 vibrational componentof the same electronic transition.2 In contrast, theSoret band docs not normally exhibit any fine structure.The observed corrole spectra are not markedly difierentfrom those of a typical porphin.However, the lowersymmetry of corroles requires that some modificationof the assignment used for porphin spectra is necessaryin discussing corrole spectra; in particular we no longerexpect degenerate transitions.Table 2 shows the calculated n-molecular orbitalenergies and symmetries for the fully conjugated corroleand porphin rings. Even with the lower symmetry ofthe corrole ring, the relationship between the orbitals isclear. The calculated 7a2 and 8b, corrole orbitals areseparated by only 0.46 el7, whilst the next higher orbital,9b,, is almoit 3 eV higher in energy. The 7a, and 8A1lcvek corresporitl to the two components of the lowestvacant orbitals in porphins, the 4e, pair. Similarly the6a2 and 7 4 corrole orbitals correspond to the highestfilled porphin orbitals lalu and 3azu respectively, thecharge distributions within corresponding orbitals beingfairly similar.The correlation between other orbitalsis less obvious, but since the visible and Soret bands inporphins arise mainly from transitions between theTABLE 2Calculated x-molecular orbital energies/eV and symmetriesfor the fully conjugated corrole and porphin ringsCorrole Porphinr- c 3 AOrbital Orbitalnumber and number and20 l l b , 10.981 22, 23 6 ~ g 8.09719 9a, 10-156 21 361, 7.27318 lob, 9-788 20 3bzt1 7.09317 8% 9.423 18, 19 5eg 6.49816 9bl 9.320 17 %tS 6.31014 7a, 5.919 14, 15 4eg 2.71213 7b1 2.347 13 3a2, -1.362- 3.967symmetry EnergyIeV symmetry Energy/eV16 Sb, 6.375 16 2blU 3.77912 6a2 1.346 1211 6bI1 ~ 1 , -2.1392bzu -4.195-0.315 10, 11 3eg9 10 5~3, -0.4089 5b1 -0.733 8 2a2, -4.3598 4a, -0.856 6, 7 2e, - 4-5757 3a, - 1.318Both the corrole and porphin rings have 26 x-electrons.The orbitals reduce as follows on going from D4h (porphin)Their net charges are 3 - and 2 - respectively.t o Czv (corrole) symmetry with the present axis system.pg ---+ b, + a2a2u - bl4, a,bz, + a2blu + b,lalu, 3azu, and 4e, pair, it is not surprising that the visibleand Soret bands of the corrole anions closely resemblethose of the corresponding porphins.Table 3 gives the calculated x-electronic spectra of thefully conjugated 26 n-electron corrole and porphin rings.Two transitions are predicted to appear in the visibleregion at 12 460 cm-l and 15210 cm-l in the case ofcorroles compared with the doubly degenerate transitionpredicted at 14090 cm-l for porphins.The two bandscorresponding to the degenerate component transitionsof the porphin Soret band are predicted at 21 680 cm-land 23 130 cm-l for corroles, while the porpliin transitionsare calculated to be at 25 090 cm-l.The lowest n+n* transition in the corrole ring ispredicted to be quite intense and 2700 cm-l lower inenergy than the second x+x* transition. I t seemsreasonable therefore to assign the bands observed a t16 960 cm-l and 17 510 cm-l for the nickel and coppercorrole anions respectively, as the 0-0 vibronic compo-nent of the lowest singlet x+n* transition, and those at17 950 cm-l and 18 720 cm-l as the 0-1 vibronic com-ponents of the same transition. With this assignmentthe vibrational separations are 990 cm-l for the nickelcorrole anion and 1210 cm-l for the copper corrole anion,compared with 1230 and 1260 cm-l in the case of nickeland copper octaethylporphins.2For both corroles and porphins, we predict the lowestn+x* transition to be cn.4000 cm-l lower in energy thanis observed experimentally. This deficiency of the21 M. Gouterman, J . Chem. Phys., 1959, 30, 1139.22 R. Gale, A. J. McCafferty, and &I. D. Rowe, J.C.S. DlzltoTi,1972, 596J.C.S. Daltoncalculations has also been encountered in other P.P.P. would therefore correspond to the second Soret bandx-electron calculations on porphins.l, The second transition.x+n* transition in the corrole anions is not resolved, but The situation in the case of the nickel corroleseems to contribute to the overall absorption in the anion Soret band is less clear owing to the rathei-region between 19 000 and 22000 cm-l, which is broader profile of the band.? However, it is evidentgreater than in the case of porphins.The absorption that it is composed of several components as predicted.in this region is thus considered to arise from a combina- The calculated and experimental transition energiestion of the second x+x* transition and further vibronic are in reasonable agreement for the Soret bands, althoughcomponents of the first n+n* transition. the calculated values are ca.2000 cm-I too low in energy.TABLE 3Calculated x-electronic spectra for the fully conjugated corrole and porphin ringsTransition energylO-%~/cm-~and polarisation12.46 (y)15.21 ( x )21-68 ( x )23-13 ( y )28.22 (y)68-33 (x)31.23 (y)32-83 (x)33.02 (3')34.83 ( x )34.95 ( y )35.99 (x)37.65 ( x )38.11 ( y )39-76 ( x )Transiticn energy10-3v/cm-'14-09 ((5)25.09 (B)33.26 (N)37-23 (I.)41.1843-9450.0163.01CorroleOscillator strength0-285 0.0100.019 0.0031.632 0.3090.658 0.1160*000 0-0010.294 0.0890.306 0.1180.101 04460.336 0.1070.16s 0.0560.000 o*ooo0.007 0.0060.445 0.1060.012 0-0040-089 0.025f l f 2PorphinOscillator strengthf i j-20.052 0.0183-938 0.6741-106 0-4100.762 0.2601.223 0-3740.040 0-0020.075 0~0000.324 0.050Main components of transition13+ 14 (0.88), 12 + 15 (0.09)13 -> 15 (0.71), 12 + 14 (0.28)13 + 15 (0*26), 12 + 14 (0.68)12 -> 15 (0*84), 13 + 14 (0.08)11 ___t 14 (0.83), 8 + 15 (0.09) lo-+ 14 (0*85), 11 __t 15 (0.04)9 + 14 (0*64), 10 __t 15 (0.28)11 -+ 1 5 (0.59), 8 __+ 14 (0.32)13 + 17 (0*91), 11 + 14 (0.03)8 + 14 (0*58), 11 + 15 (0.29)10 + 15 (0-39), 8 __t 15 (0.31)13 -> 16 (0.88), 13 + 18 (0.05)13 + 18 (0*86), 13 + 16 (0.03)8 + 15 (0.57), 10 + 15 (0.31)9 + 15 (0.88), 10 + 14 (0.03)Main components of transition *13 -> 15 (0.67), 12 __t 4(0*30)12 ___t 14 (0*66), 13 -+ 15 (0.26)9- 14 (0-69), 10+ 16 (0.13)8 __t 14 (0.77), 9 I_t 14 (0.11)13 __t 18 (0.42), 13 ___t 19 (0.44)11 __t 16 (0-71), 9 + 15 (0.08)12- 18 (0.97)11 ___t 17 (0.98)Polarisations of corrole transitions arc parallel ( x ) and perpendicular (y) t o thc C , axis.Figures in parentheses after the maincomponents of transitions are the squares of the C.T. vector coefficients. The configuration interaction in the case of porphininvolved the following 16 allowed transitions: 13+ 14, 15, 18, 19; 1 2 , - ~ 14, 15, 18, 19; 11 + 16, 18; 10 .< 16,17; 9-> 14, 15, and 8+ 14, 15.* All transitions are doubly degenerate. The component transitions are for one of the degenerate pair, the ,fl and fi values are thetotal oscillator strengths for the degenerate transitions.The integrated absorption intensities of the observedvisible bands of the corrole anions (Table 4) are morethan twice those of the corresponding porpliins.This isqualitatively predicted by the& values, but not by thef,values, the former being in reasonable agreement with theabsolute experimental values.The Soret bands observed for nickel and copper corroleanions differ somewhat from those of the analogousporphins, being composed of several components, whereasmetal porphin Soret bands exhibit no structure.,The Soret band of the copper complex appears to showvibrational structure (peaks were observed at 23 840and 24 200 cm-1) and it seems reasonable to assign theseas vibrational components of the first Soret band transi-tion, since the calculations predict the second electronictransition in the Soret band to occur 1440 cm-l higher inenergy than the first, and to have a smaller absorptionintensity than the first.The shoulder on the high-energy side of the observed Soret band around 25 500 cm-lA similar result occurs in the porphin calculations, wherethe Soret band is predicted to lie some 500 cm-l lower inenergy than is observed. This is in contrast to theearlier results of Gouterman et aZ.lP2 who calculated theenergy of the porphin Soret band to be ca. 3000 cm-lhigher than the experimental value.Our calculated visible-Soret band separation forporphins is 11 000 cm-l compared with the experimentalvalue of ca. 8000 cm-l. This represents a slight improl-e-ment on Gouterman's work where the separation witspredicted to be 12 500 cm-l. In the case of corroles,the lower-energy visible and Soret band transitions arecalculated to have a separation of 9220 cm-l, a decreaseof 1780 cin-l compared with the porphins.The experi-The general broadening of the spectrum of the nickelcorrole anion compared with the copper corrole anion mayresult from greater buckling of the corrole ring due to stericinteractions of the 2 , l &ethyl substituents in the nickel corroleanion. I n the copper corrole anion the 2,18-substituents areless bulky methyl groups1974mental value is 6330 cm-l in the case of the copper corroleanion, a decrease of 1010 cm-l compared with the copperoctaet hy1porpliin.2 The calculated decrease in theTABLE 4( a ) Peak positions and intensities for the electronic spectrum ofcoppcr corrole anionPeak position1 0% jcm-l17.5118-7210.0723-8424-20:3 1 *6436.00Intensity3.101.210.8412.7714.442-052.181 0-4Elntegr,ttcd absorptionintensity, f(oscillator strength) Limits of integration0.283 (visible) v < 22.001.180 (Soret)0.50622.00 < v < 27.5027-50 < v < 34.00(6) Feak positions and intensities for the electronic spectrum ofnickel corrolc anionPcaii positicn16.9617.9519.0624.5025.1431.7839-221 0-"V,'cn1-'Intensity1.791.150.724.266.021.242.1410-4&Integratetl absorptionintensity, f(oscillator strength) Limits of integration0.205 (visible) v < 21.001.053 (Siorct\0.27321.00 < v < 29-0029-00 < v < 34-50lisible-Sorct band separation of corroles compared withporphins is thus in reasonable agreement with experi-ment.The decreased visible-Soret band separation in thecase of corroles arises from changes in the amount ofconfiguration interaction between the transitions re-sponsible for these bands.Also, the increased intensityof the visible bands and decrease in intensity of the Soretband of corroles, compared with porphins, is a furthermanifestation of the reduced configuration interactionbetween the transitions. This effect also occurs in thetetra-azaporphins and phthalocyanines,l and has alsobeen considered in the description of corrin ~ p e c t r a . ~The lower symmetry of corroles means that manymore transitions are allowed than in porphins, and this isreflected in the appearance of the corrole anion spectraat energies higher than the Soret band. In particular,the first two predicted transitions above the Soret bandcorrespond to forbidden transitions in porphins, andoccur a t 28220 and 28 330 cm-l, the latter being themore intense of the pair. The configurational excita-tions in corroles corresponding to the N and L bands inporphins are 4a,+7a2,8b, and 50,+7a2,8b,. The splittingof their degeneracy and the introduction of further transi-tions forbidden in porphins is presumably responsible forthe general broadening of the spectra at high energies,so that no firm assignment of bands in this region ispossible.In conclusion we add that while the x-electron calcula-tions do not give an exact description of the observedcorrole spectra, they do give reasonable semiquantitativepredictions, and their deficiencies are known. They alsoshow rather clearly how the spectra differ from those ofanalogous porphyrins, and consequently we may hope-fully expect to be able to use them to trace the changesin the x-electronic spectra of other related conjugatedtetrapyrrolic rings systems.We thank the S.R.C. for maintenance grants.[3/1524 Received, 20th July, 1973