Electronic and photoelectron spectra of vanadyl(IV) tetraphenylporphyrin

Journal

of Electron

Spectroscopy

and Related

Phenomena,

189

57 (1991) 189-197

Elsevier Science Publishers B-V., Amsterdam

Electronic and photoelectron spectra of vanadyl (IV) tetraphenylporphyrin Evelina G. Ferrer and Enrique J. Baran’ Departamento 1900 La Plats

de Quimica, (Argentina)

(First received 6 December

Facultad

de Ciencias

Exactas,

Uniuersidad

National

de La Plats,

1990; in final form 20 March 1991)

Abstract The electron absorption spectrum of vanadyl (IV) tetraphenylporphyrin was recorded and is briefly discussed. Solvent effects were also analyzed and are commented upon. The XPS spectrum was obtained using AlKa excitation and the results were compared with those obtained for related compounds.

INTRODUCTION

Vanadium is widely distributed in nature. Particularly large accumulations are often found in coal and petroleum deposits [ 1,2]. Such high levels pose problems in coal conversion, petroleum refining, and in the production of airborne vanadium as a by-product of combustion of vanadium-containing fuels. In the petroleum and coal deposits vanadium appears mainly in the form of vanadyl (IV ) porphyrinic compounds [ 3 1. As part of our current research on different aspects of the chemistry and biochemistry of vanadium, and in order to obtain a wider insight into the physicochemical properties of simple models related to natural vanadium compounds, we have now investigated the electron and photoelectron spectra of vanadyl (IV) meso-tetraphenylporphyrin (VOTPP ) . EXPERIMENTAL

Synthesis of the com.und VOTPP was prepared employing a slightly modified form of the procedure described by Bencosme et al. [4]. A solution of 100 mg of meso-tetraphenyl‘Author to whom correspondence

0368-2048/91/$03.50

should be addressed.

0 1991 EIsevier Science Publishers B.V.

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190

porphyrin (Sigma) in 100 ml of quinoline was heated to reflux in the dark, under a nitrogen atmosphere; 0.43 mg of vanadyl (IV) acetylacetonate was then added and the mixture heated and stirred for 2 h. The solvent was distilled off at reduced pressure and the crude product was extracted with CHzClz and then chromatographed using a silica gel column and a 4 : 1 mixture of Ccl, : benzene as the eluting solvent. Crystallization was achieved by slow evaporation of the solvents in a rotary evaporator. The complex was identified by UV/visible and IR spectroscopies and by TLC analysis. Measurements Electron absorption spectra were measured with a Hewlett-Packard 89500 UV/V Chemstation, using 1 cm quartz cells. Photoelectron spectra were registered with a ESCALAB-503 spectrometer. The finely ground samples were supported on adhesive tape and irradiated with the Klx line of Al (1486.6 eV at 150 W ). The binding energies were referred to the Cls value = 285.0 eV. The accuracy of the energy values is better than & 0.5 eV. RESULTS AND DISCUSSION

Electron spectrum The electron absorption spectra of porphyrins and metalloporphyrins constitute one of the most important characterization tools of these type of compounds [5]. Only two previous brief reports on the electron spectrum of VOTPP have been found in the literature but, in both cases, without analysis or discussion [4,61. Figure 1 shows the spectra of the free ligand and the investigated vanadyl complex dissolved in CH&12. Detailed band positions and molar absorptivity values are given in Table 1. Both spectra are dominated by very strong charge transfer transitions and, therefore, the typical “d-d” transitions of the V02+ cation cannot be seen. The Soret band ( ~400 nm) is only slightly modified after metallation but the four visible bands of TPP collapse into those located at 548 and 584 nm with two small side bands. Other small features around 480 and 400 nm remain also practically unmodified after the incorporation of the V02+ cation. The VOTPP has a disordered Cd, symmetry, the V-O bond length is 1.624 A and the vanadium atom lies 0.53 A above the plane of the four nitrogen atoms. The four phenyl rings are perpendicular to the N4 plane [ 71. In order to make an approximate assignment of the VOTPP spectrum, the ideal Ddhsymmetry for the metalloporphyrin molecule can be assumed and the

191 05

3

...... Tpp

1.8-

-VOTPP 0,4-

3

. ...“.

; ::

:: :i

1.4-

;: fI :i :i ; :

d -0.3c E fO.Z-

;i

* :: i':

f.E-

1.2-

Tpp

-VOlPP

; ;_ : :: i :

; : ; ; ; i : i " : :*

=I c) *0.1-

....... I

I 400

300

5oo

A

[nm)

0.0

6oo

I 500

550

600

650

700

h(nml

Fig. 1. Electron spectra of VOTPP and TPP in CH2C12. TABLE

1

Electron spectra of VOTPP Band position

and TPP in CH,Cl, E (L M-l

Assignment

cm-‘)

nm

kK

VOTPP 424 548 584 480 400

23.58 18.25 17.12 20.83 25.00

5.16 X 1.83 X 2.45 x 2.07 x 3.77 x

510 634

19.61 15.77

3.23 X lo3 1.21 x IO3

TPP 360 396 418 446 482 514 550 590 646

27.77 25.25 23.92 22.42 20.75 19.45 18.18 16.95 15.48

2.40 x 7.63 x 5.02 x 3.78 x 4.04 x 1.77 x 7.97 x 5.72 x 4.42 X

y (Soret)

lo5 lo4 lo3 IO3 10’

B a ? ?

>

C.T. (L+M)

lo4 10’ lo5 lo4 lo3 lo4 lo3 lo3 lo3

analysis can be made on the basis of the well-known four-orbital model [ S-111.In this model, the LUMO is a degenerate liepair of es symmetry while the HOMOs are al, ad azu and have nearly the same energy. The HOMU-

192

LUMO energy gap is relatively small and consequently metalloporphyrins absorb light strongly in the visible region of the spectrum. Both the nearby ~1c*states, that is al,e, and azueg, have E, symmetry and are nearly degenerate. However, due to the strong configuration interaction, two separate states arise. The state with the higher energy represents the Soret band, and the low energy state appears as the weaker (a! + j?) band. The a and fi bands are considered to be vibrational levels O-O and O-l respectively of the single electronic transition. Extended Hiickel calculations of vanadyl(IV) porphyrins predict an energy of 24.04 k.K for the Soret transition and a value of 16.94 l& for the averaged (cu+B) transitions [ 121,in good agreement with the experimental data presented in Table 1 for VOTPP. The bands at 400 and 480 nm cannot be assigned with certainty, but those located at 510 and 634 nm surely originated from charge transfers of the type ligand to metal, a number of which have been theoretically predicted by extended Hiickel calculations [ 12 1. A broad feature observed around 300 nm in VOTPP (E1: 1.8~ lo*1 M-l cm-l) and in the free ligand can probably be assigned to the weakly allowed n--+n* transition named as the N band by Caughey et al. [9]. Regarding the “d-d” transitions, these are expected to lie around 17.9 k.K (b,-+e), 20.0 kK (b,-+b,) and 29.8 kK (b2+a1) as calculated with the help of X,-SW computations [ 131which, apparently, improve earlier extended Hiickel calculations [ 121. Solvent

effects

It is well known that solvent changes often dramatically affect the electron and ESR spectra of vanadyl complexes [ 14-191. Notwithstanding this, no unifled picture of the origin of the observed changes has so far been given. It is clear that a number of different effects, depending on the nature of both the complex and the solvent, may be operative. Firstly, it is well known that 7~4Z* transitions in organic molecules are often strongly affected by solvent effects [ 20 J. On the other hand, in the present complex, other, more specific, interactions should be expected: axial ligation of the solvent on the free position on the vanadium atom; hydrogen bonding, which may affect the 0 atom of the V02+ cation; electrostatic and dispersion effects; and 11x* interactions between aromatic rings and the porphyrin macrocycle. We have investigated these aspects in a qualitative way, analyzing the band shifts of the electron spectra of VOTPP in different solvents. The results of these measurements are summarized in Table 2. In some cases only the strongest absorptions could be observed due to the limited solubility of the complex in some of the employed solvents.

193 TABLE

2

Band positions (nm) of VOTPP Solvent

f?”

in different solvents Band position

n-Hexaneb Carbon tetrachloride

1.89 2.24

306

398 400

420 424

484

512

546

584

Benzene

2.28

308

400

424

482

514

548

590

Ethyl ether Chloroform Ethyl acetate Dichloromethane

4.34 4.81 6.02 8.93

310 308 -

398 -

478 480 -

510 512 508 510

546 546 546 548

582 584 582 584

Pyridineb

12.40

316

-

562

606

636

n-Butanolb

17.51

316

Isopropanolb

19.92

308

-

546

Ethanolb

24.55

320

Methanolb

32.70

DMSO

46.68

DMF

81.00

420 424 422 424 430 442 422 420 1 434 422 422 { 432 436 424 434

632 634 650 624 636 630 634

320

396 400

-

400

Principal bands (assignment)

Y

480

-

-

-

524

564

604

634

512

550

602

632

B

a!

“Dielectric constant. bVOTPP is only slightly soluble in these solvents.

The solvents are ordered according to increased dielectric constant values, i.e., reflecting in a certain form an increase in polarity [ 211. Most of the spectra are very similar to that represented in Fig. 1 for VOTPP in CH&+ In general, axial interactions in VO porphyrins are expected to be small as a consequence of the trans repulsive effect of the 0x0 group and the inaccessibility of the metal atom, raised above the plane of the porphyrin ring [ 7,221. The fact that the ligand has to enter a small cavity in order to effect coordination also causes steric restrictions, as has been demonstrated recently [ 22 1. However, some aspects of the spectroscopic behaviour of VOTPP in the different, investigated solvents strongly suggest such an interaction. In particular, the change of position of the characteristic Soret absorption may be used as a criterion to verify the possibility of an axial interaction since it is accepted [ 16,191 that a red shift of this band implies solvent coordination in this position. In n-hexane, which is the solvent with the weakest polarity, this band lies at 420 nm (Table 2). In most of the other solvents it is red-shifted by 2-4 nm, suggesting only small solvent interaction. The most important changes are

194

observed in DMSO, pyridine and DMF and, in the last two mentioned solvents, a clear splitting of this band can also be seen, These observations suggest that these three solvents interact more strongly through the axial position. Also, most of the other spectral bands show the most important shifts in these three solvents, whereas they were affected to a lesser extent in all other cases. It is also interesting to comment that the solvent ordering proposed by Selbin [ 23 ] according to their tendency to occupy the axial position in VO (acac ) 2 (acac = acetylacetonate) and based on the empirical parameter &_I (energy difference between the first and second d-d bands), shows similar trends to those found in the present case. For example, methanol, ethanol, pyridine and DMSO interact more strongly than chloroform, benzene or carbon tetrachloride. It is also known that in the above-mentioned complex a slight weakening of the V=O bond occurs as the donor ability of the solvent increases [ 23,241. This has been clearly demonstrated by means of IR measurements of VO ( acac)z in different solvents [ 23,25,26]. However, it should be emphasized that as the greater effects are observed for the solvent with the greatest dielectric constant, electrostatic interactions may also be operative. Nevertheless, it is very difficult to separate both effects or to obtain an insight into their relative contributions. Axial perturbations generated by coordination of a sixth ligand to the metallic centre should affect mainly the ep(x*) orbital [ 17,18] and this therefore explains the observed shifts in some of the x+x* transitions because, as stated above, this level is involved in the transitions which are responsible for the appearance of the Soret and ( CI+ j3) bands. Finally, it is interesting to comment that in the case of benzene, although band positions are not much affected important changes are observed in their relative intensities, suggesting some type of xx* interaction of this solvent with the porphyrinic ring.

Photoelectron spectrum In order to obtain a wider insight into the electronic characteristics of the investigated complex, we have also recorded its photoelectron spectrum. The obtained data are summarized in Table 3 and Fig. 2 shows the characteristic region of the vanadium and oxygen peaks. TABLE 3 Energy values (eV> from the XPS spectrum of VOTPP referred to the C 1svalue ( = 285.00 eV 1 Nls

399.1

01s 532.9

VP,,, 516.5

VP1/2 524.3

195

515

525

535

BE

leV)

Fig. 2. @3/z, VP~,~and 01s XPS peaks in solid VOTPP.

Two types of nitrogen are found in porphyrin-free bases, the aza type whose Nls binding energy is near 398 eV and the pyrrole type of 400 eV. After metallation, the four N atoms become equivalent, with an Nls energy around 399 eV [27]. As can be seen, in the present case this level shows clearly the expected energy and the value also coincides with that previously obtained for vanadyl (IV) phtalocyanine [ 281 and is practically identical with those found in some other metalloporphyrins [ 28-30 ]. As no explicit references have been found on energy values related to vanadium and oxygen electrons of the VO moiety in porphyrins, it seems interesting to make a brief comparison of these with those found in other simple vanadyl (IV) compounds. As can be seen from Fig. 2, in our spectra the signals of VpSj2and VP~,~could be clearly differentiated. In Table 4 we compare the Vp 3j2 and 01s values of VOTPP with those obtained for some other vanadyl (IV) compounds. It is interesting to note that in VOTPP, VOPc (Pc =phthalocyanine) and VONpr (Npr = nitropruaside) , in which the coordination spheres around V02+ are dominated by N atoms, the highest energy values are found for both signals, whereas the lowest are found in VO (acac) 2,with a pure oxygen environment around the metal. VO (oxine)z, with two N and two 0 ligands, occupies a somewhat intermediate position.

196 TABLE

4

Comparison of VP~,~ and 01s energy values in different V02+ compounds Compound

VP312

01s

Ref.

VOTPP VOPC VO (acac)e VO (oxinate ) 2 VONpr

516.5 516.1 515.0 515.5 517.0

532.9 531.7 529.4 529.8 531.5

This work 31 31 31 32

CONCLUSIONS

The most prominent features of the electron spectrum of VOTPP could be assigned on the basis of the simple four-orbital model and by comparison with known data from previously investigated metalloporphyrins and theoretical models and calculations. The effect of different solvents on the electronic transitions was also investigated. Although it is difficult to establish a clear and unified model to explain the observed changes, it becomes apparent that axial ligation of the solvent in the free position on the vanadium atom plays an important role. However, other effects, such as electrostatic interactions or hydrogen bonding, may also be operative. The photoelectron spectrum of VOTPP was also discussed in detail. It shows the characteristic feature of metalloporphyrin spectra. The comparison of the VP,,, and 01s energy values with those of other vanadyl (IV) complexes shows some interesting trends. ACKNOWLEDGEMENTS

This work was supported by CONICET, E.G.F. is a research fellow from the CIC-PBA. The authors are indebted to Dr. L. Bruno Blanch for generous help during the synthesis and purification of the compound.

REFERENCES 1 2 3 4 5

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