Electronic structure of the porphyrin ring in an electrostatically bound

J. A. Shelnutt and D. E. Trudell. 1990,265-271 ... John A. Shelnutt , Kenneth Alston , Jui Yuan Ho , Nai Teng Yu , Tomoko Yamamoto , and Joseph M. Rif...
0 downloads 0 Views 938KB Size
Download PDF
J . Phys. Chem. 1984,88, 6121-6127

6121

Electronic Structure of the Porphyrin Ring in an Electrostatically Bound T-T Complex. Methylviologen-Metallouroporphyrin Complexes J. A. Shelnutt Solid State Materials Division 1 1 54, Sandia National Laboratories, Albuquerque, New Mexico 87185 (Received: March 13, 1984)

Methylviologen-metallouroporphyrin complexes in aqueous alkaline solution are investigated with resonance Raman difference spectroscopy and UV-visible absorption spectroscopy. The complex is found to be a 2:l a-a complex with a substantial contribution to the binding energy from electrostatic attraction between the anionic porphyrin and cationic viologen molecules. Strong axial ligation disrupts normal T-T interaction, resulting in a 1:1 complex for 5-coordinate metallouroporphyrins and total disruption of the T-a complex for 6-coordinate metals. An analysis of the absorption spectra in terms of the four-orbital model of porphyrin excited states determines the complex-related shifts in relative frontier *-orbital energies. The Raman shifts provide additional information necessary to determine absolute shifts in orbital levels. Complex-induced shifts in the al,(a) and e,(**) energy levels, but not the az,(a) level, are indicated. A 500-cm-' change in the electron-electron repulsion interaction between the excited-state configurations also accompanies methylviologen complex formation.

Introduction Uroporphyrins are naturally occurring water-soluble porphyrins. The synthetic metal derivatives are of interest as catalysts and photosensitizers in a variety of water-based chemical and photochemical reactions. A reaction of particular significance from the standpoint of metalloporphyrin-based artificial photosynthesisl-lo is that between the metallouroporphyrins and the electron acceptor methylviologen (4,4'-Wmethylpyridinium dichloride). Because of the arrangement of negative charges of the carboxylate peripheral substituents of the uroporphyrin octaanion and the positive charges of the methylviologen dication (MV2+), a strong (log K = 6) complex forms between the two molecules in alkaline aqueous solution.11-16 Consideration of molecular models shows that a reasonable structure for the complex, in view of the distance between charges at the ends of the methylviologen molecule and between charges on opposite edges of the porphyrin ring, is that with methylviologen's pyridinium rings flat against and lying across the porphyrin ring. This configuration optimizes the ionic attraction between the positive charges of the pyridinium rings and carboxylates at opposite edges of the porphyrin. Further, hydrophobic forces favor minimization of the area of ring aelectron systems exposed to water and, therefore, association of the a-systems of methylviologen and porphyrin. The tight association between methylviologen and metallouroporphyrins leads to a photochemically inactive complex for most metals; Le., no photosensitized reduction of MV2+ occurs upon irradiati011.I~ However, for metals that have strongly bound

Kalyanasundarum, K.; Gratzel, M. Helv.Chim. Acta 1980, 63, 478. (2) McLendon, G.; Miller, D. S. J . Chem. SOC.,Chem. Commun. 1980, 533. (3) Okura, I.; Thuan, N. K. J. Chem. SOC.,Faraday Trans. 1 1980, 78, 2209. (4) Harriman, A.; Richoux, M.-C. J . Photochem. 1981, 15, 335. (5) Rougee, M.; Ebbesen, T.; Ghetti, F.; Bensasson, R.V. J . Phys. Chem. 1982, 86,4404. (6) P i h i , M.-P. Chem. Phys. Lett. 1980, 75, 540. (7) Whitten, D. G. Acc. Chem. Res. 1980, 13, 83. (8) Mercer-Smith, J. A.; Mauzerail, D. Photochem. Photobiol. 1981, 34, 407. (9) Kriiger, W.; Fuhrhop, J.-H. Angew. Chem. 1982, 94, 132. (10) Fuhrhop, J.-H.; Kriiger, W.; David, H. H. Liebigs Ann. Chem. 1983, (1)

204.

(11) (12) (13) (14) (15) (16)

Mauzerall, D. Biochemistry 1965, 4, 1801. Shelnutt, J. A. J . Am. Chem. SOC.1981, 203, 4275. Shelnutt, J. A. J . A m . Chem. Soc. 1983, 105, 774. Shelnutt, J. A. J . Phys. Chem. 1983, 87, 605. Shelnutt, J. A. Inorg. Chem. 1983, 22, 2535. Shelnutt, J. A.; Dobry, M. M. J . Phys. Chem. 1983,87, 3012.

0022-3654/84/2088-6121$01.50/0

fifth and sixth axial ligands, this tight complex cannot form. For example, tin(1V) uroporphyrin I dihydroxide (Sn(OH)z(UroP))k7 and iridium(II1) carbonyl uroporphyrin I hydroxide (Ir(C0)(OH)(UroP)) do not form the tight complex described above. In the case of Sn(OH)2(UroP)we will see that a weak (log K = 2) complex forms instead and it may contribute to the high activityk7 of this porphyrin as a photosensitizer of methylviologen reduction. The weak tin uroporphyrin complex is spectroscopically distinct from the a-a MVz+ complexes described in this work. In spite of the inactivity of the tight methylviologen complex in artificial photosynthetic systems the complex provides a valuable model system for other complexes of interest in which both ionic and a-a interactions come into play. One example is the association between the antimalarial drug chloroquine and its putative receptor hemin.I8 Chloroquine, like methylviologen, has an aromatic ring moiety with a positively charged group attached. Thus, a-a association of the quinoline ring with the porphyrin ring and ionic interaction of the protonated amino group of the side chain with the carboxylates of ferriprotoporphyrin IX are possible. Another reason for interest in such complexes is as a model of protein-porphyrin interactions in biological systems. The porphyrin environment in hemoproteins and chlorophyll-containing proteins may put aromatic acid residues in contact with the porphyrin ring in orientations that would not be favored by the free molecules in solution where the constraints provided by protein folding are absent. The MV2+-metallouroporphyrin complex models this situation to some degree. As will be seen, the ionic interactions between centers of charge on the two molecules dominate the formation of the MV2+ complex. By comparison, neutral aromatics, such as phenanthroline, have binding energies of about 5 kcal/m01.'~J~This is greater than 3 kcal/mol less than for the MV2+complex. The additional free energy stabilizing the methylviologen complex is a result of the ionic interaction. For the neutral phenanthroline-metallouroporphyrin complexes the charge-transfer component of the stabilization energy ranges from 0 to 3 kcal/mol.14 Thus, for the MV2+complex a less (or more) favorable orientation, as far as charge transfer is concerned, may result from constraints provided by the strong electrostatic attraction that is optimized in the complex. The present work describes UV-visible absorption and resonance Raman difference spectroscopy of the tight MVz+metallouroporphyrin complex for a series of metals. The spectroscopic results allow a complete assessment of the substantial (17) Shelnutt, J. A. J . Am. Chem. SOC.1983, 105, 7179. (18) Chou, A. C.; Rekha, C.; Fitch, C. D. Biochemistry 1980, 19,

0 1984 American Chemical Society

1543.

6122

The Journal of Physical Chemistry, Vol. 88, No. 25, 1984

electronic structure changes that occur upon complex formation. The molecular orbital parameters of Gouterman’s four-orbital model of porphyrin excited statesIg are determined for the complexes from the absorption spectral data. These parameters describe the relative energy splittings of the frontier r orbitals of the porphyrin ring and the intrinsic transition dipoles. The Raman data provide the additional information needed to determine the absolute changes in the frontier r-orbital energies upon complexation. Further, measurements of the equilibrium association constants for the series of metalloporphyrins permit an assessment of the energetics of the viologen-metallouroporphyrin interaction. The effects of axial ligation at the metal upon methylviologen binding are also determined.

Materials and Methods The metal derivatives of uroporphyrin I were obtained from Porphyrin Products and in some cases further purified by column chromatography on Sephadex G-50-40 as before.14 Raman difference spectra2s23 were taken with a difference spectrometer previously described.I4 Samples of the metalloM) in 200 p L of 0.1 M NaOH were uroporphyrin ( - 5 X added to sample and reference chambers of a Raman difference cell. Solid methylviologen (Sigma) was then added to the sample side of the partitioned cell. Methylviologen concentration was M. about 1 X As the split cell rotates at 100 Hz, the methylviologen complex and the uncomplexed metallouroporphyrin (M(UroP)) samples are alternately probed by the laser radiation. The scattered light from the complexed and uncomplexed metallouroporphyrin is separated electronically and analyzed and detected by the spectrometer. In this way the Raman spectra of the complexed and uncomplexed metallouroporphyrin are obtained simultaneously as far as the grating position is concerned. The two Raman spectra are stored in computer memory and are analyzed by subtracting the spectrum of the uncomplexed metallouroporphyrin from that of the complex. The shifts in the Raman lines of the porphyrin that result from complex formation are calculated from the peak-to-valley intensity of the deflection in the difference spectrum a t each shifted line.20-23 Only Raman lines of the metallouroporphyrin are observed, in spite of its much lower concentration than MV2+, because of resonance enhancement of the porphyrin’s scattering. Resonance enhancement occurs when the laser excitation frequency coincides with an electronic transition of the molecule. Visible excitation frequencies used in the present investigation are resonant with ?r r * transitions of the metalloporphyrin but not with transitions of MV2+. Absorption spectra of the metallouroporphyrins were obtained on a Perkin-Elmer (Model 330) UV-visible-near-IR spectrophotometer. The reported ratios qq2/qe2in Table I1 are averages of values obtained by two methods of integrating absorption-band intensitiesf. For the first method f is proportional to AAv, where A is the maximum absorbance and Au is the width of the band at half-maximum. Cutting and weighing provided a second measure of band intensities. Difference absorption spectra were used to follow the course of MV2+ binding. The spectroscopic equilibrium binding data are analyzed with a nonlinear least-squares program for 1:l or 2:l binding of MV2+ to the M(UroP)’s. The program provides a fit of the exact, coupled equations for changes in absorbance for the successive equilibria (P A + PA ( K , ) and PA + A * PA2 ( K 2 ) )as a function of the initial MV2+concentration ([Ala!. The parameters varied to obtain the fit are the apparent equilibrium constants K 1 and K2 and the extinction coefficients c1 and

-

+

(19) Gouterman, M. J. Chem. Phys. 1959, 30, 1139. (20) Kiefer, W. In “Advances in Infrared and Raman Spectroscopy“; Clark, R. J. H., Hester, R. E., Eds.; Heyden: London, 1977; Vol. 3, p 1. (21) Shelnutt, J. A,; Rousseau, D. L.; Dethmers, J. K.; Margoliash, E. Proc. Natl. Acad. Sci. U.S.A. 1979, 76, 3865. (22) Shelnutt, J. A.; Rousscau, D. L.; Dethmers, J. K.; Margoliash, E. Biochemistry 1981, 20, 6485. (23) Rousseau, D. L. J. Raman Spectrosc. 1981, 10, 94.

Shelnutt TABLE I: Absorption Maxima of the Soret and a Bands of Metallouroporphyrins and Shifts in the Bands and log K’s for Methvlvioloeen Comolex Formation

AB,

metal

cu Ni Pd Pt Ai3 Fe(0H) Sn(OH)* (PH 9) H2

vo

nm

A~B, nm

407 398 391 391 379 406 394 403

8 7 7 8 7 7 6 6 f 2 1

582 574 562 551 546 534 557 593 574

397

8

423

-2 1

610 538 584

XQ, nm

AAQ,

-3 -1 -1 2 1 2 0 -3 1

(ex)

log K

nm

(ey)

12 -6 -10

5.9 (2.3)

5.5 5.9 5.9 5.6 5.9 5.6 2.2 (1.8) 5.0 4.3

4

a

-7

-6

-5

-4

-3

-2

-1

LOG [MV2’]

Figure 1. Equilibrium binding data for formation of the methylviologen complex with platinum ( O ) , zinc (0),free base (a), vanadium (-), and dihydroxotin (pH 14 (A) and pH 9 (A)) uroporphyrins. e2

of the complexes PA and PA2, respectively.

Results The absorption maxima for the Soret (AB) and a (AQ) bands are listed in Table I along with the shifts (AXB and A h ) that are observed upon formation of the methylviologen-metallouroporphyrin complex. A relatively constant red shift in the Soret band of about 7 nm is observed except for the tin(1V) and vanadium(1V) uroporphyrins. The shifts in the a band are less than half as large and, depending on the metal, are both to the red and to the blue. The free base is exceptional, showing a large red shift in the x-polarized component of the Q(0-0) band and a blue shift in the y-polarized component of Q(0-0). In the last column of Table I log 6 s for the binding of MV2+ to the M(UroP)’s are given. In most cases the equilibrium constant K was obtained from a nonlinear least-squares fit to absorption difference data obtained from titration with MV2+ by assuming 1:l binding. Usually a least-squares analysis based on two MV2+ molecules binding to one M(UroP) molecule gave better fits. However, except in the case of Zn(UroP), the two equilibrium constants are nearly equal and so the log Cs for 1:l binding are reported. The binding data for Pt-, Zn-, VO-, H2-, and Sn(OH)2(UroP) are shown in Figure 1. The curve for Pt(UroP) is similar to that for Cu-, Ni-, Pd-, and Ag(UroP)-methylviologen complexes. The H2(UroP) binding curve is shifted to higher MV2+concentration (lower affinity) and is best fitted with a 2:l binding analysis (shown in Figure 1). log K 1 and log K2 are only slightly different (5.1 and 4.7), however. In contrast the VO(UroP) data are adequately fitted with 1: 1 binding and show considerably lower affinity. Fe(OH)(UroP) also binds MV2+ in a 1:l fashion, but the log K of 5.6 is typical of the 4-coordinate metallouroporphyrins. The

The Journal of Physical Chemistry, Vol. 88, No. 25, 1984 6123

Methylviologen-Metallouroporphyrin Complexes

TABLE II: Spectral Parameters for Metallouroporphyrin-Methylviologen Complexes XBt XQl EB- EQ, metal nm nm qQ2/qB2 cm-' 6854 MR 414.5 579 0.0360 6733 zn414 574 0.0499 6927 cu 404 561 0.0777 6980 Ni 399 553 0.0979 6844 Pd 398 547 0.1168 7250 Pt 386 537 0.1293

+ cm-l

2Alf

1/2(EB

cm-

0, deg 0.9 2.8 5.8 7.6 8.9

20 699 20 788 21 289 21 573 21 704 22 282

210 660 1390 1830 2170 2460

10.0

8

TABLE III: Raman Shifts' Resulting;from Formation of

8ooo

Methylviologen Complexes with h4etallouroporphyrilis in 0.1 M NaOH shifts. cm-'

metal

A V ~ ~4 v 4 -2.4 (tris pH 7.8) -2.2 (HV") -0.9 -1.7 (BV2+) -1.1 -1.9 Pdc -1.8 -3 h 1 Fe(0H) Ptd -2.4 f 0.5 Ni' -2.0 f 0.5

cu

6500

1

1I

I 6000 0.00

I

I 0.04

1

I 0.08

I

I

I

0.12

q; /q'B

Figure 2. Relationship between the Soret-a band separation and the ratio of the dipole strengths. The curve is generated by a theoretical expression given in ref 31. Tick marks along the curve mark 1O increments of 0. (See text.)

Zn(UroP) curve is anomalous in its shape and requires 2:l binding to fit the data. For Zn(UroP), log KI and log K2 differ drastically (Table I). Also shown in Figure 1 are the binding data for Sn(OH)2(UroP) in 0.1 M NaOH and A similar solution titrated with HC1 to pH 8.9. The binding affinity is clearly greatly reduced (log K = 2) for the dihydroxotin uroporphyrin and the low affinity is attributed" to prevention of normal T--a complex formation by the obligate bis axial ligands of the tin(1V) ion. The spectral data in Table I support this conclusion because the shift in the Soret band is not typical of the 7~ complex. We conclude that a weak complex that is spectroscopically distinct from the T-T complex is formed in the Sn(OH)2(UroP) case.17 Vanadium uroporphyrin also shows an anomalous shift upon complex formation. However, in contrast with the Sn(OH)2(UroP) complex, this results from a complex-induced change from the 6-coordinate, in-plane dihydroxovanadium(1V) porphyrin to the 5-coordinate, out-of-plane vanadyl porphyrin as described in previous work.16 The free energy required to replace an OH group may explain the approximately 2 kcal/mol lower affinity of the vanadium porphyrin compared to 4-coordinate M(UroP)'s. In analogy with the Sn(OH)2(UroP), the remaining strongly bound oxo ligand of the metal prevents more than one MV2+molecule from forming the usual T-T complex. This is in agreement with the fact that a 1:l binding analysis gives an excellent fit to the VO(UroP) data in Figure 1. Table I1 lists additional spectroscopic parameters for some of the M(UroP)-MV2+ complexes and values for the orbital parameters of the four-orbital model19 that are obtained from the analysis of the data which is described below. In Table 11, AB and X, are the maxima of the Soret and a bands of the complex, E B - EQis the energy separation between the Soret and a bands, and '12(EB4- E ) is the average energy of the two transitions. The quantity qQP/qB2 is the ratio of the transition dipoles squared of the a and Soret. The transition dipoles are related to the integrated intensities of the bands or oscillator strengthsfbyf Eq2, where E is the energy of the respective band. Figure 2 shows a plot of EB- E vs. qQ2/qB2for the data for the complexes given in Table 11. T i e theoretical curve through the data points is obtained by fitting by eye a curve generated by the expression relating the two quantities in the four-orbital model. The relevant resonance Raman data are tabulated in Table 111. The frequency shifts of several Raman marker lines in the

-

4v3

h 9 b

-2.2 -2.4 -1.9 -2.3 -3.0 O f 1 -4.6 f 0.5 -5.2 f 1

440

-4.3 -4.0 -3.2 -4.8

-1.3 -1.8 -1.7 -2.1 -1.6 f 1 -3.8 -2f1 -2f1 -4.2 f 0.5 -5.4 f 0.5 -8.6 f 1 -1.4 h 0.5

"Possible errors are f0.2 cm-l unless noted. *Errors may be larger as a result of overlapping Raman lines. CSomePd(UroP) photodecomposition; MVZ+complex stable. dShifts relative to dimer: Pt(UroP) photodecomposes. CSpin-statemixture.

L 1280

1360

1440

1520

FRE0 UENCY

1600

1680

( c M- ')

Figure 3. Resonance Raman spectra and Raman difference spectra (X4) comparing the heptylviologen-copper uroporphyrin I complex (-) with

the uncomplexed copper uroporphyrin in 0.1 M NaOH solution. Shifts resulting from complex formation are listed in Table 111. (..a)

high-frequency region between 1280 and 1680 cm-I that result from formation of the viologen complex are listed. The relatively small shifts are measured by Raman difference spectroscopy and the errors are about f0.2 cm-' in most cases. Representative Raman difference data are shown in Figure 3, where the spectrum of the heptylviologen-Cu(UroP) complex is compared with the uncomplexed Cu(UroP) spectrum in the Raman marker line region. The Raman difference spectra are also shown expanded by a factor of 4. From the deflection in the difference spectra the shifts in the Raman lines resulting from complex formation are calculated from Au = 0.381'Zd/Zo, where r is the line width, Id is the peak-to-valley intensity in the difference spectrum, and 1, is the intensity of the Raman The Raman lines for which shifts are given have all been shown to be sensitive to the oxidation state of iron in hemes and hem o p r o t e i n ~ , ring ~ ~ . ~reduction,26 ~ and charge in antibonding T (24) Spiro, T. G.; Strekas, T. C. J . Am. Chem. SOC.1974, 96, 338. (25) Spiro, T. G.; Burke, J. M. J . Am. Chem. SOC.1976, 98, 5482. (26) Ksenofontova, N. M.; Maslov, V. G.; Sidorov, A. N.; Bobovich, Ya. S.Opt. Spectrosc. (Engl. Trans/.) 1976, 40, 462.

6124

The Journal of Physical Chemistry, Vol. 88, No. 25, 1984 FOUR-ORBITAL MODEL (tan 28-A1,/A';,)

I

(a) MOLECULAR ORBITALS

'

egx-egy-

lbl MOLECULAR STATES

Figure 4. Definition of the energy parameters of the four-orbitalmodel. (a)Metailo.

orbitals of the ring.25,27 The vibration v4, in particular, is well characterized as a a-density and oxidation-state marker line. The vibrations v3, ~19,and vl0 are sensitive to core size27,28(centerto-nitrogen(pyrro1e) distance) in addition to a-charge density. The v2' vibration is sensitive to peripheral substitution in addition to oxidation ~ t a t e . ~ ~ . ~ " The shifts due to complex formation generally are small (0-9 cm-', and are all to lower frequency relative to the uncomplexed metallouroporphyrin frequencies.. In fact, for the Cu(UroP)-MVZ+ complex all lines above 1000 cm-l have lower frequencies than the uncomplexed porphyrin. Except for ~19,for which overlapping Raman lines make accurate determination of the shifts difficult, the shifts in all of the lines are of about the same magnitude, roughly -3 f 2 cm-I. This pattern of Raman line shifts, roughly equal and negative shifts in all of the marker lines, is reminiscent of the shifts in low-spin hemes as the a basicity of a series of bis ligands increases.25 The shifts are attributed to progressive reduction of the a-charge density in the lowest empty e,(a*) orbital of the porphyrin ring.25,27

Discussion Analysis of the UV-Visible Absorption Spectra of Metallouroporphyrins. In previous work3' Gouterman's four-orbital modelIg of porphyrin a a * states was used to derive an expression relating the ratio (qQ2/qB2)qf dipoles squared for the B (Soret band) and Q (aband) transitions to the energy separation of the bands EB - Eq. The two quantities, qQ2/qB2and EB- EQ, depend upon only three of the four molecular orbital parameters of the four-orbital model. These are the following: i9,31 (1) A,,, the splitting of the two nearly degenerate highest filled a orbitals, a,, and a2u;(2) r / R = ( R , - R , ) / ( R , R 2 ) ,where R I and R , are the dipoles for the transitions a,, eg and a,, e,, respectively; and (3) A I / , the two-electron configuration interaction matrix element coupling the excited-state configurations (ai,ai,e:) and ( a ~ u a ~ u eThe ~ ) .three energy parameters are defined schematically in Figure 4. It was shown3' that if A , / and r / R are held constant and A,, is varied continuously, a curve is generated in the E , - EQ vs. q Q 2 / q B 2 plane. When the experimental values of EB - E for a series of uroporphyrin metal derivatives are plotted vs. qQ ?/qB2, the data fall along the theoretical curve generated by reasonable values of the constants A I / and r / R . Thus, it was shown that the metal influences primarily the orbital parameter Ai,. With arguments based on the symmetry properties of the a,, and a2, orbitals, it is clear that the a2, orbital is the one affected by the metal-porphyrin interaction. This is reasonable because the a', orbital has nodes at the pyrrole nitrogens and cannot interact

-

-

+

+

(27) Spiro, T. G . In "Iron Porphyrins"; Lever, A. B. P., Gray, H. B., Eds.; Addison-Wesley: Reading, MA, 1982; Part 11, p 89. (28) Spaulding, L. D.; Chang, C. C.; Yu,N.-T.; Felton, R. H. J . Am. Chem. SOC.1975, 97, 2517. (29) Adar, F. Arch. Biochem. Biophys. 1975, 170, 644. (30) Adar, F. Arch. Biochem. Biophys. 1977, 181, 5 . (31) Shelnutt, J. A. J . Phys. Chem. 1984, 88, 4988.

uroporphyrin6

Figure 5. Energy levels of the frontier

?r molecular orbitals for the uncomplexed metallouroporphyrins (a) and the methylviologen complexes (b). Positions of the a2" orbital for metals other than Mg are indicated by dashed lines.

directly with the metal orbitals. On this basis the relative orbital energies for the highest filled a orbitals were determined for the series of metall~uroporphyrins.~~ The approximate energy levels of the a,, and a2, orbitals of the metallouroporphyrins are illustrated schematically in Figure 5a. The metallouroporphyrin for the same series of metals exhibit a distinctly different relationship between EB - EQ and qQ2/qB2; nevertheless, the same value of A , / and a different value of r / R generate a fit of the experimental data for the dimers.31 Although the separation of the a,, and a2, orbitals is larger for the dimers than for the monomers, the metal-dependent changes in the a2u level remain the same. Effects of Complex Formation on the Highest Occupied a Molecular Orbitals. The curve in Figure 2, which represents a fair fit to the experimental data for the methylviologen complexes, is generated by using the same value of r / R (-0.173) as for the uncomplexed metallouroporphyrin monomers. However, a smaller value for 2A1/ is required, 6750 cm-' for the complexes vs. 7250 cm-' for the uncomplexed M(UroP)'s. The tick marks along the theoretical curve in Figure 2 represent 1' increments of 8, defined by tan 20 = A l g / A ! [ , starting at 8 = OD at the left end of the curve. From the position of each metal data point along the theoretical curve, 0 can be estimated. The estimated values of 0 and 2A1,, the separation of the a,, and azu orbitals, are given in Table I1 for each metal. The values of 2AI, are about 150 cm-' larger for the methylviologen complexes than for the corresponding uncomplexed M(UroP)'s. However, the metal-dependent changes in 2Ai, for different metals are about the same as for the monomeric and dimeric M(UroP)'s within experimental error. From the analysis of the data plotted in Figure 2 we conclude (1) the splitting of the al, and a2, orbitals increases by 150 cm-' due to binding of methylviologen and (2) the configuration interaction matrix element decreases by 500 cm-' to 6750 cm-'. We have assumed that the dominant interaction with MV2+is totally the consequences of this assumption will be symmetric (D4*); considered later. Effects of Complex Formation on the Configuration Interaction. G o ~ t e r m a noriginally '~ estimated 2A1," by neglecting any 0 dependence. In this case EB = EQ + 2Al//cos 20 i= EQ + 2A!,", for small constant 8. Therefore, a plot of EBvs. EQ should give a straight line with unit slope and intercept 2 A l / . This procedure was carried out by Gouterman for metallotetraphenylporphyrins. The slope in Gouterman's data is near 1; moreover, assuming a slope of 1 the intercept 2A1/ is about 6700 cm-'. (32) Shelnutt, J. A,, Dobry, M. M.; Satterlee, J. D. J . Phys. Chem. 1984, 88, 4980. (33) Satterlee, J. D.; Shelnutt, J. A. J. Phys. Chem. 1984, 88, 5487.

The Journal of Physical Chemistry, Vol. 88, No. 25, 1984 6125

Methylviologen-Metallouroporphyrin Complexes

23,000

27,000

M(UroP)'s

- r c 22,000

26,000

I

A r

iI

i_.

7 UNCOMPLEXED

k v

m

0

w

\

w

//'MV2'

I

y

COMPLEXES

21,000

r

25,000

20,000

500

0

1000 A1

24,000 17,000

EQ (cm-1) Figure 6. Energy of the Soret band vs. energy of the

band for fourth-period divalent metals, magnesium, and platinum. Solid line is the least-squaresfit with the Pt point omitted; the dashed line indicates unit slope.

= E(e,) - E(a1,)

- E(a2,)l

+ AI,

point omitted.

CY

Treating the metallouroporphyrins in this manner leads to an estimate of 7400 cm-I for 2Alg1'.31 However, the plot of E B vs. EQfor the experimental data actually has a slope of 1.2 as a result of the neglected '6 dependence. The 6-dependent method gives 7250 cm-] for 2Alg11.31 The MV2+-M(UroP) complexes can also be treated in this fashion and a plot of EB vs. EQis shown in Figure 6 . The dashed line has a slope of 1 and the intercept gives 2A1/ = 6920 cm-I, a value somewhat higher than the value of 6750 cm-I obtained from the &dependent analysis of the data in Figure 2. However, the 6 dependence is evident for the complex in that a least-squares line has a slope of 1.3. The conclusion is that, whether the spectral data are analyzed by the approximate method that ignores 6' dependence or the more accurate 0-dependent method, 2A1," for the MV2+complexes is lower than for the uncomplexed M(UroP)'s, The approximate method also shows that the value (6750 cm-l) given by the 8dependent method is reasonable, being above the estimated value of 2Al/ for the tetraphenylporphyrins and below the value for the uncomplexed uroporphyrins. Effects of Complex Formation on the Lowest Unoccupied a Molecular Orbital. Discussion has focused so far on changes in the energies of the filled frontier a orbitals, but the spectral data can be further analyzed to determine the metal- and complexinduced changes in the T * orbitals. To accomplish this we plot for each metal the average energy of the Q and B transition, '/,(EB + EQ),vs. the value of A!, obtained from the analysis of the data plotted in Figure 2 and listed in Table IL31 The result is shown in Figure 7. The average transition energy, regardless of the values of the orbital parameters, is AI,', the fourth and last of the molecular orbital parameters of the four-orbital model. A l l is equal to the average energy of the separations between the a,, and eg orbitals and the a,, and eg orbitals, i.e. A l l = %[2E(eg) - E ( a d

'

(cm- 1

Figure 7. Average energy of the Soret and CY bands vs. the estimated value of A , , for metal uroporphyrin derivatives and their complexes with methylviologen. Lines represent least-squares fits to the data with Pt

19,000

18,000

1500

(1)

Because the a,, orbital does not interact directly with the metal atom, E(al,) is considered a constant (at least, for metals from a row of the periodic table). In addition, if the e, level is not influenced by changing from one metal to another along a series (Le., by changing A,,), then E(e,) is constant and eq 1 predicts a linear dependence with unit slope in a plot such as shown in

Figure 7. Considering only the first-row transition metals and Mg, the data in Figure 7 are accurately fitted (correlation coefficient = 0.99) by a straight line with a slope of 1.14 f 0.02 for both complexed and uncomplexed M(UroP)'s. Because the slopes are close to 1, from eq 1 we conclude that the e,(**)-orbital energy is almost unaffected by metal substitution for the first-row transition metals. This holds for the MV2+complexes as well as the uncomplexed M(UroP)'s (Figure 7). Although there is not a significant metal dependence of AI; for different metals other than that expected as a result of the change in A,,, there is an overall change in A l i brought about by MV2+ binding. This change in A,; is given by the difference in intercepts in Figure 7 for the cornplexed (20820 crn-l) and uncomplexed (20510 cm-I) M(UroP)'s and is equal to 310 cm-'. This quantity represents the difference in the intrinsic separation of the frontier n and a * orbitals for the complexed and uncomplexed metallouroporphyrins. From eq 1 a shift in intercept could result from either a metal-independent decrease in the e&n*) level, a metal-independent increase in the al,(a) and a,,(") levels, or both. The platinum uroporphyrin point in Figure 7 lies above the line for the first-row transition metals. The Pt(UroP) orbital energy levels are apparently shifted with respect to the first-row transition metals as might be expected. In particular, the larger filled d, orbitals of Pt might be expected to interact more strongly with the porphyrin e,(**) orbital, thereby raising its energy. Furthermore, according to eq l a row-dependent increase in the eg level would account for the high Pt point in Figure 7. In fact, extending the results for the first-row transition metals eq 1 predicts no metal dependence of e, energy within other rows of the periodic table, but rathe: a displaced linear relationship between Al, and '/,(EB EQ) = Al;. In summary, complex formation has the following effects on the orbital parameters: (1) r/R is unchanged; (2) AI," is reduced by 500 cm-' relative to the uncomplexed metallouroporphyrins; (3) the changes in A , , as the metal is varied are the same as for the uncomplexed M(UroP)'s; (4) A',, however, increases by about 150 cm-I for all metals; and ( 5 ) the intrinsic transition energy from filled to unfilled frontier orbitals decreases by 310 cm-'. The metal-dependent change in A,, has been traced to stabilization of the a2, orbital.31 The Raman marker line shifts resultiag from complexation allow us to speculate about which orbital levels shift, causing the change in A l i a Shifts in the Raman Electronic Structure Marker Lines. Previous work has been shown that the Raman core-size marker lines are also sensitive markers of a,,-orbital e n e r g ~ . ' ~ , ' ~ In J~-~~

+

6126 The Journal of Physical Chemislry, Vol. 88, No. 25, 1984

fact, on the basis of the changes in 2A1, and the corresponding shifts in vl0 as a result of changing metals, we expect about a 4.3-cm-' increase in vl0 for every 100 cm-' decrease in the orbital energy (in the vicinity of CU).~'We would expect a similar sized increase in the other core-size marker lines v3 and v19 For v4, on the other hand, a much smaller increase is expected due to a,, stabilization, roughly 0.9 cm-l/100 cm-I stabilization of 37 a,,. According to point 4 above, A,, increases by 150 cm-' upon complexation. Therefore, if the 150-cm-' shift is all due to a decrease in a2,, then we expect greater than a 6-cm-I increase in vlo, but less than a 2-cm-' increase in v4. Examination of Table I11 shows the Raman data are inconsistent with such disparate shifts in v4 and vlo. For example, the best Raman data are for the Cu(UroP) complex and it shows almost equal shifts for v4 and vlo of about 2 cm-I. We conclude that the Raman shifts are not a result of stabilization of the a,, orbital alone and most likely result from the 150-cm-' destabilization of the al, orbital illustrated in Figure 5b and changes in the e,(.*) level. The intrinsic separation of the a and a* frontier orbitals also decreases by 310 cm-' for the complex. About 75 cm-' of this is accounted for by the a,, orbital destabilization, but the rest is the result of a 235-cm-' decrease in the e, orbital as shown in Figure 5b. Because the pattern of shifts in the Raman marker lines for the MV2+ complexes usually is associated with changes in eg(?r*)-orbital d e n ~ i t y , ~it~ is~ interesting ~ ' * ~ ~ to see if the present results are compatible with such an interpretation of the Raman shifts. Such considerations are complicated by the fact that both the a,,- and e,-orbital energies change upon complex formation. For the moment let's ignore the change in the a,, orbital. Then by the accepted interpretation of a decrease in frequency of all of the Raman marker lines, an increase in e,(**) charge density is predicted. The eg(a*) orbital could obtain a small charge initially through back-bonding with the filled d, and dyzorbitals of the metal. Charge-transfer interaction of eg(a*) with an unfilled a acceptor orbital of MV2+ (with transition energy above the Soret) would result in stabilization of e,(a*) as observed (Figure 5b) but also a loss of charge density, not the required gain. However, we would expect stronger interaction with the nearby B state than with the Q state and the shift in the Soret maximum is larger than the shift in the LY band. If, indeed, MV2+ is an acceptor and the porphyrin a donor, then it is somewhat surprising that the most likely donor orbitals of the ring, the top filled a,, and a,, orbitals, are not more affected by the interaction. On the other hand, an interaction with a low-lying filled donor orbital of MV2+would provide the required gain in eg(a*) density, but the interaction would likely raise the e, level not lower it as needed to explain the effect of MV2+binding on the absorption spectrum (Figure 5). Another interpretation of the Raman line shifts is possible and may be necessary, Upon dimerization much larger changes in the al, and eg orbitals occur than for the complex-induced changes reported here.31,32For dimerization the a,, moves up by 2500 cm-' and the e, moves up by 1500 cm-'. Nevertheless, the Raman marker line shifts are of similar magnitude to those for the MV2+ complexes, i.e., 1-3-cm-' increases in frequency for v4, v3, v19, and vl0. (The substituent-dependent Raman line vzl, however, decreases in frequency upon dimerization.) Notice that an increase in e,(a*) energy is associated with an increase in marker line frequencies upon dimerization; a decrease in e,(..*) energy is associated with a decrease in marker line frequencies upon complex formation. A discrepancy arises, however, because of the great (34) Shelnutt, J. A.; Straub, K. D.; Rentzepis, P. M.; Gouterman, M.; Davidson, E. R. Biochemistry 1984, 23, 3946: (35) Shelnutt, J. A.; Ondrias, M. R. Inorg. Chem. 1984, 23, 1175. (36) Kitagawa, T.; Ogoshi, H.; Watanabe, E.; Yoshida, 2.J. Phys. Chem. 1975, 79,2629. (37) The change from Cu to Ni results in an increase in u I 0 by 19 cm-' from data in ref 15. Cu Ni substitution lowers the energy of the a2"(r) orbital by 440 cm-' as detetmined from Table I1 of the present work. The the shift in uIo. shift in u4(Cu-.Ni) is 4 cm-' (ref 15), roughly +

Shelnutt disparity in the size of the changes in eg level, 1500 vs. 235 cm-I, in spite of the fact that the same size Raman line shifts are observed in the two cases. A reasonable way out of this dilemma obtains if one assumes that the Raman marker lines are proportional to e,(a*) energy and inversely proportional to alu(a) energy. Then, small shifts observed in the dimer Raman spectra are explained by the cancellation of an increase in frequency resulting from the raised e, level by the decrease resulting from the raised a,, level. On the other hand, for the MV2+ complexes the decrease in marker line frequencies is a sum of decreases resulting from a lower eg orbital and a higher al, orbital. This would imply that the Raman lines are almost twice (1.9 times) as sensitive to eg as to al, energy. An increase in the al, orbital lowers marker line frequencies; an increase in e, orbital energy raises marker line frequencies. This is reminiscent of the effect of removing charge from frontier orbitals of the porphyrin. Removing charge from the a', orbital (neutral a-dication) lowers v4 by 9 cm-'; removing charge from * the e, orbital (a-dianion neutral) raises ,.v by 14 ~ m - l . ~ Thus, for charge as well as energy the marker line frequency is more sensitive to e,(.*) than to a,,(a) and the "sense" of the frequency shift is the same. Effect of Lower Symmetry. In the foregoing discussion we have considered the intermolecular interaction to be totally symmetric. It almost surely is not, so we must consider the effect of lower symmetry components of the interaction between the two molecules. The in-plane a a* transitions that give the Q and B bands have E, symmetry and transform like x and y . The Q and B states themselves arise from mixing of excited-state configurations by the electron-electron repulsion terms of the molecular Hamiltonian. The primary components of the interaction Hamiltonian affecting the Q and B transitions of the uncomplexed metallouroporphyrin are those that influence the mixing of the electronic configurations, and, therefore, the symmetry of these components must be contained in the direct product E, @ Eu.19,39 The direct product contains A , , interactions, which we have already considered, and the nontotally symmetric components of A,,, B,,, and B,,. The B,, and BZgintermolecular interactions induce splitting of the x and y components of the doubly degenerate Q and B states, thereby producing a splitting of the absorption band^.'^,^^ N o splitting or enhanced asymmetry of the LY and Soret bands is noted in the spectra of the MV2+ complex. Further, no significant broadening of the bands that might indicate unresolved splitting is noted. Also, B , , and BZgcomponents of the MVZ+-M(UroP) interaction can give rise to absorption band shifts as a result of the asymmetric intensity distribution of the split components of the absorption bands.Ig The AZginteraction may introduce an interaction between the al, and a2uorbitalslg that results in mixing of the Q and B states, but not mixing within the degenerate components of these states. If needed, the theoretical expressions can be generalized to include these nontotally symmetric perturbations.

-

-

-

Conclusions The methylviologen-metallouroporphyrin complexes provide a valuable model system for drug-receptor and protein-heme interactions. Their investigation is important also because of their central role in desirable photochemistry in biomimetic photosynthesis of gaseous fuels. Because of the added strength of binding of MV2+to M(UroP)'s over a-a complexes with neutral aromatics it is clear that electrostatics play a large role in formation of the complex. Also, by comparison with the phenanthroline c ~ m p l e x e s and ' ~ by estimation of the hydrophobic force that results from reducing the surface area of ?r-electron systems exposed to water," a-r contact between the ring systems of the two molecules is favored by about 2.5 kcal/mol. This places the MVZ+molecule flat against and (38) Aleksandrov, I. V.; Yeletskii, N. P.; Sidorov, A. N . Biofizika 1980, 25, 279; Biophysics (Engl. Transl.) 1981, 25, 389. (39) Shelnutt, J. A. J . Chem. Phys. 1980, 72, 3948.

The Journal of Physical Chemistry, Vol. 88, No. 25, 1984 6127

Methylviologen-Metallouroporphyrin Complexes lying across the porphyrin ring and is compatible with strong electrostatic attraction between the positive charges at the ends of MV2+ and the carboxylates on opposite edges of the uroporphyrin macrocycle. A a- interaction is also indicated by the strong effect of MV2+ binding on the in-plane transitions. The disruption of the usual 2:l a-a complex by fifth and sixth axial ligands of the metal further suggests that contact of the a-systems is required. In cases of 6-coordination, e.g., Sn(OH),(UroP), a weak complex forms that is spectroscopicallydistinct from the r-r complex. The strong a-a complex has been shown to be photochemically inactive in' photosensitizing MV2+ r e d u ~ t i o n . ~Finally, ~ , ~ ~ the similarity of the changes in the Raman and absorption spectra for all of the remaining metal derivatives of uroporphyrin points to a-a rather than metal interaction with MVZ+. A relationship between the intensities and energies of the a a * absorption bands Q and B has been found and analyzed in terms of a theoretical expression31given by the four-orbital model19 of porphyrin excited states. Distinct relationships of this kind have -porphines,"O now been observed for metallouroporphyrins,31~40 -octaethylp~rphyrins,~~ - p r o t o p ~ r p h y r i n s ,and ~ ~ -tetraphenylporphyrin^.'^*^^ The relationship also holds for a-a complexes and the awith methylviologen (Figure 2) and phenanthr~line~l Fitting the theoretical expression31 to experimental data provides values for three of the four molecular orbital parameters of the four-orbital model. Further analysis of the energy relationships as described in relation to Figure 7 allows the fourth parameter Algl to be determined. The four molecular orbital parameters completely determine relative energies of the topfilled and lowest empty a orbitals and their relative dipole strengths.40 Differences between the M O parameters for the complexed and uncomplexed metalloporphyrins help to determine the effect of the intermolecular interaction on the porphyrin. For the MV2+ complexes with metallouroporphyrins the changes are (1) a 150-cm-' increase in separation of the two highest top-filled orbitals, (2) a 310-cm-' decrease in the average separation of the two highest occupied orbitals and the lowest a * orbital, and (3) a 500-cm-' decrease in the electron-electron repulsion matrix element. A decrease in electron-electron repulsion might result from a decrease in charge density in the porphyrin ring. The frequency shifts in the Raman marker lines upon MV2+ complex formation are small (- 1-3 cm-I). The shift in the

-

(40) Shelnutt, J. A,, submitted for publication in J . Am. Chem. SOC. (41) Shelnutt, J. A., unpublished results.

oxidation-state marker line (v4) is approximately the same as the shifts in the core-size marker lines (v3, v19, and vl0) and all shifts are to lower frequency relative to the uncomplexed M(UroP). The pattern of equal shifts in these marker lines was previously observed upon M(UroP) a-a dimer formation.31 The Raman shifts appear incompatible with a shift in the a2,(a) orbital energy. Metal s u b ~ t i t u t i o nand ~ ~ axial ~ ~ ~ligation , ~ ~ of a c r - d o n ~ affect r ~ ~ primarily the level through conjugation with an empty pr metal 0 r b i t a 1 . ~These ~ ~ ~ perturbations ~ of the metal-porphyrin interaction result in a pattern of marker line shifts in which v4 exhibits a shift that is small compared to shifts in the core-size lines. Phenanthroline-metallouroporphyrin a-a complexes how a similar pattern of shifts.l4JS Because this aZu-dependent pattern of shifts does not occur for the MV2+ complex, we conclude that the aZulevel is unaffected. The pattern of marker line shifts is compatible with shifts in the alu(a)- and e,(**)-orbital energies provided the marker lines are about twice as sensitive to the e, level as to the al, energy. The observed pattern of marker line shifts has been previously associated with e,(a*)-charge den~ity,~"~' but this interpretation appears to be inconsistent with detailed orbital energy shifts for the MV2+ complexes. When used in conjunction, Raman difference spectroscopic measurements and a detailed analysis of the absorption spectra of a series of non-hyperporphyrins provide changes in orbital energies resulting from complex formation, a g g r e g a t i ~ n and ,~~ peripheral substitution.@ Further structural information, possibly from N M R spectroscopy, is required to determine the mechanism of intermolecular interaction and conformation of the complex. On the basis of the distribution of charge in the al, and e, orbitals it is predicted that the a-and @-carbonpositions of the ring will be most affected by the interaction. It is the aim of such investigations to correlate detailed electronic structure changes in porphyrin complexes with reactivity. Such studies of metalloporphyrins in other environments should aid in this effort. Acknowledgment. I thank Martin Gouterman for a helpful discussion and Mary M. Dobry for technical assistance. This work was performed at Sandia National Laboratories and supported by the U S . Department of Energy contract DE-AC04-76DP00789 and the Gas Research Institute contract 5082-260-0767. Registry No. Mg(UroP), 84254-36-4; Zn(UroP), 55972-25-3; Cu(UroP), 78991-92-1; Ni(UroP), 84098-84-0; Pd(UroP), 91841-54-2; Pt(UroP), 91798-65-1; Ag(UroP), 92719-94-3; Fe(OH)(UroP), 8425429-5; Sn(OH)2(UroP), 9201 1-53-5; H,(UroP), 607-14-7; VO(UroP), 86472-06-2; MV2+, 4685-14-7.