Structure sensitive bands in the vibrational spectra of metal complexes of tetraphenylporphine

Spectrochimica Acta, Vol. Printed in Great Bntam.

40A.

No.

9, pp. 863-870,

1984

0584-8539184 $3.00 + 0.00 Q 1984 Pergamon Press Ltd

Structure sensitive bands in the vibrational spectra of metal complexes of tetraphenylporphine HIROKI OSHIO,* TOMOHARUAMA,* TAKESHI WATANABE,*JAMESKINcAIDt and

KAZUO NAKAMOTO* *Department tDepartment

of Chemistry, Marquette University, Milwaukee, Wisconsin 53233, U.S.A. and of Chemistry, University of Kentucky, Lexington, Kentucky 40506, U.S.A. (Received

13 June 1983)

Abstract-The i.r. and RR spectra of twenty Fe(TPP)LL’ type complexes have been measured to locate structure-sensitive bands. In i.r. spectra, band I (135&1330cm-i) and band III (469432cm-‘) are spinstate sensitive whereas band II (806790cm-‘) is oxidation-state sensitive and slightly spin-state sensitive in the Fe(I1) state. To examine the nature of these bands, the i.r. spectra of Co(TPP), (Fe(TPP)),O and their d8 proposed. In RR spectra, band C and d2,, analogs have been measured, and empirical assignments (154551498cm-i, ap) and band D (1565%1540cm-‘, p) are spin-state sensitive whereas band E (391-376cn-‘, p) is sensitive to both spin and oxidation states. These results on RR spectra are in good

agreement with those of previous workers.

continue to be intensively investigated because of their relevance to biologically important heme proteins and the current interest in Among metalloporphyrin-based catalysts [l-3]. common metalloporphyrins, complexes of tetraphenylporphine (TPP) have been most extensively studied owing to their relatively high-yield synthesis and convenient purification [4-51. For these reasons, many of the biomimetic compounds such as “picket-fence” and “capped” porphyrins have been derived from the basic TPP structure [ 11. In addition, most of the high oxidation state metalloporphyrins of interest as potential oxidation catalysts or as models of the oxidative enzymes, peroxidase and cytochrome P-450, also employ TPP as the chelating ligand [69]. Since it is likely that workers will continue to utilize these TPPbased models, it is important to develop effective and readily accessible probes of TPP structure. Thus far, resonance Raman (RR) spectroscopy has been used almost exclusively as a structure probe of iron porphyrins [l&11]. In the case of TPP complexes, BURKE et al. [12] made empirical assignments of the RR spectra of (Fe(TPP)),O and its d8 and dzo analogs. They also noted three structure-sensitive bandsat N 1560(p), - 1360(p)and _ 390(p)cm-i ina series of the Fe(TPP)LL’ type complexes; the first band is mainly spin-state sensitive while the other two bands are oxidation-state sensitive [ 131. STONG et al. [14] found linear relationships between the core size and the vibrational frequencies of three bands at N 1570(p), N 1540(ap) and N 1460cm-r (p) in a series of M(TPP) type complexes. Finally, CHOTTARD et al. [15] found four structure-sensitive bands in the complexes: Fe(TPP)LL band A(P), type 1370_1345cm-‘; band B(dp), 1379-1365cm-‘; band C(ap), 1545-15OOcm -‘. , band D(p), 1572-1542cm-‘. They concluded that bands A, C and D are essentially spin-state sensitive and that band B is characteristic of pentacoordination. Metalloporphyrins

863

The infrared (i.r.) spectra of TPP and its metal complexes have previously been studied by THOMAS and MARTELL[ 16a] and ALBENet al. [16b] who noted several metal-sensitive vibrations in the M(TPP) and related series. KINCAIDand NAKAMOTO[ 171 assigned the metal-nitrogen vibrations of TPP complexes (33&180cm- ‘) using metal isotope techniques. In order to systematically investigate structure-sensitive bands in the i.r. spectra, we have searched for bands which are oxidation or/and spin state sensitive in the Fe(TPP)LL’ type series and are metal-sensitive in the M(TPP) type complexes. We have also compared the i.r. spectra of Co(TPP) and (Fe(TPP)),O and their d8 and dzo analogs to identify the pyrrole hydrogen (C,H) [ 191 and phenyl hydrogen vibrations, respectively. Finally, RR bands which are sensitive either to spin or oxidation state or both have been identified in the Fe(TPP)LL’ series, and the results compared with those of previous workers.

EXPERIMENTAL Compounds

All the Fe(TPP)LL’ type complexes listed in Tables 1 and 4 were prepared according to the literature methods cited in these tables. The ligands, DMF (dimethylformamide), DMSO (dimethylsulfoxide), TMSO (tetramethylene sulfoxide). PMS (uentamethvlene sulfide), THT (tetrahvdrothiophene), N-MeIm (N-methylimidazole), Im ‘(imidaxole), pip (piperidine), py (pyridine), 2-MeIm (2-methylimidazole) and 1,2-DiMeIm (1,2-dimethylmidazole) were purchased from Aldrich Chemicals. Co(TPP), (Fe(TPP)),O and their d, and d,, analogs were prepared using established procedures [12] by incorporation of the metal [Co(acetate),4H,O or anhydrous FeC12] in hot (110°C) DMF solution under a nitrogen atmosphere using the aonronriate TPP derivatives (TPP. TPP-d, or TPP-d,,)_-. .. . until fluorescence disappeared. After cooling to room temperature, an equal volume of water was added to precipitate the complex. The suspension was filtered and the solid was air dried and purified by chromatography on alumina (Grade IV)

HIROKI OSHIO et al.

864

Table 1. Oxidation/spin

L, L

C.N.

state sensitive bands Oxidation state

Spin state

III

h

5

III

h

CH,COO O-&H,-p-NO,

5

III III III

h h h

SC,H,

5

III

h

III

h

5 6

III III

h h h h

5

III III III III III III II II II II II

Cl Br

P-0

S&C,H,-pC1 S&H,, HSC,H,I (DMF),S (DMSO),S (TMSW! (PMS),S (THT),S (N-MeIm),$ (Im),S i? (N-MeIm), 2-MeIm 1,2-DiMeIm

*Reference for preparation. tRoom temperature spectrum SPerchlorate salt.

(five-coordinate

in i.r. spectra

Band I 1340 1333 1339 1334 1338 1337 1341 1331 1339 1334 1340 1334 1340 1340 1334 1335 1336 1344 1343 1344 1346 1348 1349 1347 1337 1336

1 I I I 1

1 1 h h

(m) (m) (s) (s) (m) (m) (sh) (s) (m) (m) (m) (m) (m) (m) (m) (m) (m) (m) (m) (m) (m) (m) (s) (s) (m) (s)

of the Fe(TPP)LL’

series (cm-‘)

Band II

Band III

Ref.*

806 (s)

432 (w)

PO1

805 (s)

433 (m)

PI

800 (s) 799 (s) 802 (s)

434 (m) 435 (m) 435 (m)

[if]

804 (s)

434 (m)

[241

804 (s)

432 (m)

[241

803 (s) 806 (s)

433 (w) 432 (m)

[251

802 804 806 803 801 801 792 193 790

432 433 467 469 461 464 464 466 466 433 432

(m) (m) (s) (s) (s) (s) (s) (s) (s)

800 (s) 795 (s)

[231

PI

(m) (w) (w) (w) (w) (w) (w) (w) (w) (w) (w)

high spin state). See text,

with methylene chloride eluent. The fluorescence-free extracts were combined, evaporated to dryness and the resulting solid recrystallized from toluene. The H2(TPP-d,,) was prepared according to standard procedures [ 191 using benzaldehyded,. The d, analog was prepared in a similar manner except that propanoic acid-d, was used and pyrrole was allowed to equilibrate to ensure complete exchange to pyrrole-d, before addition of benzaldehyde.

on the inclined surface of a cold tip cooled to - 1OOK in tiacuo by a CT1 Model 21 closed cycle helium refrigerator. Calibration of frequency reading was made by using plasma lines from the Ar-ion laser. The accuracy and reproducibility of frequency reading were k l.Ocm-‘.

Spectral measuremenls All the ir. spectra (400&250cm~r) were measured on a Beckman 4260 i.r. spcctrophotometer. The spectra were obtained as KBr pellets or as Nujol mulls on a CsI window. The i.r. spectra of the THT and PMS complexes were measured at 5 160K by cooling their Nujol mulls on a CsI window with a CT1 Model 21 closed cvcle helium refrieerator. The i.r. spectra of Co(TPP) and its & and dzO analogs were measured by evaporating respective complex from a Knudsen cell at m 430K and codepositing the vapor with argon on a CsI window which was cooled to _ 15K by the same refrigerator. Rotation-vibration bands of standard molecules and polystyrene bands were used for frequency calibration. The accuracy and reproducibility of the frequency reading were + l.Ocm-‘. The RR spectra were recorded on a Spex Model 1401 double monochromator. Excitations at 457.9 and 514.5 nm were made by a Spectra-Physics Model 164 Ar-ion laser. Detection was made by using a cooled RCA C31034 photomultiplier in conjunction with a Spex digital photometer system. The spectra were measured as KBr pellets using rotating sample techniques. To avoid thermal decomposition by local heating, the spectra of complexes containing axial sulfur ligands were measured by attaching their KBr pellets

RESULTS AND DISCUSSION Infrared

spectra

of Fe(TPP)LL’

complexes

From careful comparison of i.r. spectra of twenty Fe(TPP)LL’ type complexes, we found that the three bands listed in Table 1 are sensitive to either spin or oxidation state or both. The frequency ranges of these three bands (I-III) are shown in Fig. 1. It should be noted that L and L’ are limited to biologically important ligands (N, 0 and S donors) and that only the bands which show more than lOcm- ’ shift by changing the oxidation and spin states are chosen. Figures 2 and 3 show their locations in the i.r. spectra of Co(TPP) and (Fe(TPP)),O, respectively. Band I appears strongly or with medium intensity in the 1349-1333 cm- 1 region, and in some cases, splits into two bands. As is seen in Fig. 1, this band is spinstate sensitive; 1349-1343 cm- 1 for low-spin, and 1341-1333cm-’ for high-spin complexes. We noted

865

Vibrational spectra of metal complexes of tetraphenylporphine that this band is metal-sensitive and shifts in the following order in the M(TPP) series:

previously[l7]

Pt(I1) 1360

>

Pd(I1) 1354

>

Ni(I1) 1353

>

Co(H) 1351

>

Cu(I1) 1346

(all in units of cm-‘, KBr pellet). Band II appears strongly in the 806790cm- ’ region. As is shown in Fig. 1, this band is oxidationstate sensitive and slightly spin-state sensitive in the Fe(H) complexes; 80&795cm-’ for high-spin and 7933790cm- 1 for low-spin complexes. Recently, we found that band II splits into two bands when Mn(TPP)O, (side-on dioxygen adduct) was formed via cocondensation of Mn(TPP) with O2 diluted in argon [32]. This splitting was attributed to the lowering of the porphyrin core symmetry from approximate D,, to Czv or lower. Band III appears with medium intensity in the range from 469 to 432cm- ‘. As shown in Fig. 1, it is an marker band; sensitive spin-state extremely 469461 cm- ’ for low-spin and 435_432cm- 1 for high spin complexes. This band is also known to be very sensitive to the change in the metal in the M(TPP) series [ 171:

Pt(II) 469

>

Pd(I1) 467

x

Ni(I1) 468

o

Co(I1) 468

>

I

p

Fe(N)

p

1s

Zn(I1) 1339

Infrared spectra ofCo(TPP), (Fe(TPP)),O and dzO analogs

HS 1340

Ag(II) 440

>

>

Zn(I1) 436

observed for the pairs of Co(TPP)/Co(TPP-d,,) and (Fe(TPP)),O/(Fe(TPP-d,,)),O. The last two frequencies of the TPP complexes differ substantially from those of the simple monosubstituted benzene

1

1330

LS

Fe(lll)

_

Fe(H)

Ad

I

I

810

800

I

790

LS

Fe(lll)

)

Fe (II)

fl

I

470

LS

HS

I

and their d,

The three structure-sensitive bands discussed above can reasonably be attributed to porphyrin core vibrations since phenyl group vibrations should not be very sensitive to the oxidation/spin state of the iron atom. In order to locate phenyl group vibrations, we have prepared the dzo analogs of Co(TPP) and (Fe(TPP)),O in which all phenyl hydrogens are deuterated, and compared their i.r. spectra with the corresponding d, compounds as shown in Figs 2 and 3. Previously, phenyl group vibrations have been assigned using isotopic pairs such as C6H5Cl/C6D,Cl [33] and C6H5CN/C6D5CN 1341. In Table 2, six pairs of the &Hs/C,D, group frequencies of these compounds have been compared with those

HS

I

Ill

>

LS

1350

II

Ag(II) 1344

>

spectra of this compound at room temperature show that it is a Fe(II1) high-spin complex.

Cu(I1) 448

(all in units of cm- ‘, KBr pellet). One compound in Table 1 needs special comments. Fe(TPP)(SC6H5)(HSC,H,) is known to be in the fivecoordinate high spin state at room temperature al-

FetIII)

though it is converted into a six-coordinate low spin complex at 4.2 K [8]. As expected, both i.r. and RR

I

450

I

HS

Fig. 1. Structure-sensitive bands in the i.r. spectra of Fe(TPP)LL’ type complexes.

430

866

HIROKI OSHIO et al.

CotTPP)

I” Ar MATRIX

1350

A) TPP-de

:I TPP-d,o Ii II 1343 I

I

.,

1600

1

.I.

1400

II

I.

I.

I

1200

.,

1000

.

I.

800

I

.,

.,

.I

400

600 CM-l

Fig. 2. Infrared

spectra

of Co(TPP)

and its d8 and dzO analogs

( ~(TPP))

J.~.‘.L.‘~‘~‘.“““““‘. 1400 1600 Fig. 3. Infrared

spectra

in Ar matrices.

2o in nsr

1 1200 of (Fe(TPP)),O

1000

600

600 CM-I 400

and its de and dzO analogs

in KBr pellets.

Vibrational spectra of metal complexes of tetraphenylporphine Table 2. Comparison

C6HSC1[33] &I& 1580/1563, 1543 147711346 144511322 12920r 10261865 7401618 6181591

867

of i.r. phenyl group vibrations for C6HSC1, &H&N, Co(TPP) and (Fe(TPP)),O (cm- ’ ) C6HSCN [34] &Id, 1599, 1584/1568 1492/1378

Co(TPP) do/&o 1605/1571 1497/1343*

(WTPP))& d, ldzo 1601/1566 _ 61 1491/1328*

1448/1330

1444/1315

1444/1300

10271871 7581643 6291599

1009/851 7031540 5231487

1007/861 7031540 5231476

*Possibly hidden under band I.

derivatives. This may suggest the presence of strong coupling between some porphyrin core and phenyl groupvibrations. It should be noted that the i.r. spectra of Co(TPP) and its dzO analog are similar to that of (Fe(TPP)),O and its dzO analog, respectively, except for the antisymmetric Fee0 stretching band which appears near 9W870 cm - ’ in the ~-0x0 complex. In addition to the six bands listed in Table 2, the monosubstituted benzenes exhibit at least five more vibrations which show marked shifts by deuteration. No such bands were identified, however, for the TPP complexes. All the remaining bands of the Co(U) and Fe(M) TPP complexes shown in Figs 2 and 3 can be assigned to porphyrin core vibrations. In order to identify the vibrations involving the pyrrole hydrogen motions, we have compared the i.r. spectra of Co(TPP) and (Fe(TPP)),O with those of the corresponding TPP-d, complexes (Figs 2 and 3). Previously, GLADKOV ez al. [35] reported the i.r. spectra of Cu(porphin) and its d4, d8 and d,2 derivatives, and made complete band assignments based on normal coordinate calculations. They noted that the seven bands listed in Table 3 show marked shifts by the d, (pyrrole hydrogen) substitution, and assigned them to porphyrin core vibrations coupled with pyrrole hydrogen bending modes, except for the last band near 7OOcm-’ which was assigned as an out-of-plane C,-H bending mode. In Table 3, we have chosen the TPP bands which show frequencies similar to those of Cu(porphin) vibrations. The agreement between our frequencies and those OfGLADKOV et al. [35] is excellent except for Table 3. Comparison of porphyrin core vibrations involving pyrrol-hydrogen motions (cm- ’ ) Cu(porphin) [37] d, Id,

Co(TPP) do/&

(Fe(TPP))@ &Id,

1530/1510 1445/1405 1308/1259 1244/l 138 1150/1092

155211530 146311427 1317/1238 1209/l 123 1157/1080

1529/1506 1455/1401 1302/1235 1205/1115 1158/1075

iE/79l

1079/938

10741932

;:I564

715/581

720/58 1

According one band at 79 1 cm- ’ of Cu(porphin-d,). to GLADKOV et al., the 1060cm-’ of Cu(porphin) by the d, substitution (1060/791 shifts to 791cm-’ = 1.34) indicating that this vibration is almost pure C,-H bending. However, we did not observe such a band near 790cm- ‘. Instead;a new band appeared near 935cm-’ in both TPP complexes by the d, substitution. This discrepancy may indicate that the nature of vibrational coupling in this particular mode is different between the porphine and TPP complexes or that GLADKOV et al. [35] assigned it erroneously. Some insight into the sensitivity of bands I-III may be obtained by consideration of the internal coordinate composition of the normal modes. Although the vibrations may be expected to be highly mixed, deuterium shifts give some indication of the major internal coordinate contributions. Band I, which serves as a spin-state marker, exhibits only a small shift upon deuteration of the /l pyrrole hydrogens and slight sensitivity to phenyl deuteration. This is consistent with a porphyrin core mode corresponding to v(C,-C,) mixed with some v(C,-phenyl). Band II, an oxidation state marker, exhibits a substantial shift upon deuteration of the /I pyrrole hydrogens but almost no shift upon phenyl deuteration. This band may therefore be ascribed to a vibration involving deformation of the pyrrole ring including 6(C,-H) (6: in-plane bending). Band III is a very effective spin state marker which shows a moderate d, shift and a small shift with phenyl deuteration. Evidently, this band corresponds to a mode involving motion of both the pyrrole and phenyl rings and may thus be described as a low energy porphyrin core deformation mode. Resonance Raman spectra of Fe(TPP)LL’

complexes

As stated previously, CHOTTARD et al. [ 151 made the most thorough study of structure-sensitive RR bands of TPP complexes. We have, therefore, employed their band notation (except Band E) in the following discussion. Table 4 lists the RR frequencies of four structure-sensitive bands of the twenty Fe(TPP)LL complexes we studied. Figure 4 shows the frequencystructure relationship for each of these four bands. Band A( 137&l 340 cm- i, p) was originally assigned to v(C,-N)+6(C,-H) and found to be sensitive to

868

HIROKIOSHIO et al. Table 4. Oxidation/spin state sensitive bands in RR spectra of Fe(TPP)LL’ series (cm- ‘ ) Oxi. L,L’

C.N.

Cl Br p-0 CH,COO =,H,-00, SC3,H, SC3,H,-~I SC,HS, HSC,HS t (DMF),S

5

5 5 5 5 5 5 5 6 6 6 6 6 6 6 6 6 6 5 5

IRE:;2$$ (PM%: (THT),S (N-MeIm),$ (WA (Pip), (PY), (N-MeIm), 2-MeIm 1,2-DiMeIm

III III III III III III III III III III III III III III III 11 II II II II

Spin state

Band A(p) Band C(ap) Band D(p) Band E(p)

h h h h h h h h h h h

1362 1362 1359 1360 1361 1361 1361 1361 1360 1360 1358 1367 1367 1369 1368 1355 1358 1353 1343 1344

1 I 1 I I 1 1 h h

Ref.*

1514 1513

1553 1551

390 389

1513 1515 1516

1551 1552 1552

390 389 389

[::I [*31

1514 1516 1516 1509

1552 1552 1553

390 388 389

I;]

1507 1501 1545

1550 1548 1553

391 388 389

[::I’

1548 1540

1554 1564

390

;::I’

1536 1539 1531

1563 1560 1553

391 382 384

[:J

1534 1498 1499

1557 1541 1539

380 376 373

;:z;

[81

[251

WI

VI

[311

*Reference for preparation. tRoom temperature spectrum (five-coordinate high spin state). See text. SPerchlorate salt.

A (PI

Fdlll) HS Fe(")

LS

I 1370

1340

LS

Fe(lll) u Fe(M)

LS

H

HS

I

I

D (PI

LS

p

I

HS

Fetlll)

HS

HS

FeW I 395

HS

I

1565

E (PI

I

1505

LS

Feflll) _ Fe(H)

I

1525

1545

1535

LS I

HS 365

Fig. 4. Structure-sensitive bands in the RR spectra of Fe(TPP)LL’ type complexes. both oxidation and spin states by BURKE et al. [13]. Later, CHOTTARD et al.[15], through an extensive study including Fe(I1) low-spin complexes of strongly a-acidic axial ligands, concluded that this band should be regarded as a spin-state marker band since its

frequency for low-spin Fe(II) complexes ranges from 1369 to 1355cm-’ which covers all the Fe(II1) complexes they studied (136&1370cm-I). It should be noted, however, that this band is sensitive to both oxidation and spin states as is seen in Fig. 4 if we

Vibrational

spectra

869

of metal complexes of tetraphenylporphine

consider only low-spin Fe(I1) complexes having nitrogen-base ligands. Band C (1545-1498 cm- ‘, ap) was originally assigned by BURKE et al. [13] to v(C,C,). Its frequency decreases linearly as the porphyrin center to pyrrole nitrogen (C,-N,) distance increases [14]. CHOTTARD et al. [15] found that this band is spin-state sensitive; 1540cm-‘for1ow-spinand1516-1500cm-’forhighspin complexes regardless of their oxidation states. Our results shown in Fig. 4 are in good agreement with previous workers [13-l 51. Band D (1565-1540cm-i, p) wasoriginallyassigned

Sensitivity of RR bands to spin-state has been attributed to expansion or out-of-plane deformation of the porphyrin core [ 10, 111, either of which would produce a weakening of the bonds at the methine positions and a decrease of the associated frequencies. As previously pointed out [ 171, these same bands are also expected to be sensitive to the metal ion due to variable occupation of the u-antibonding d.,>-I’2 orbital. In fact, the spin-state sensitive i.r. bands we observed were also metal-sensitive, and their frequenties decrease as the metal-porphyrinato nitrogen (M-N,) distance increases in the M(TPP) series [38].

M-N,(A) Band I (cm-‘) Band III (cm-‘) to v(C&) + 6 (C-H) by BURKE et al. [ 131. Similarly to band C, its frequency decreases linearly as the core size increases. Both BURKE et al. [ 131 and CHOTTARD et al. [ 151 found this band to be spin-state sensitive. As is shown in Fig. 4, we also found this to be the case. The six-coordinate, low-spin Fe(TPP) (1,2-DiMeIm), is formed when Fe(TPP) is reacted with excess base at low temperature [36]. Recently, WALTERS [37] observed its D and A bands at 1554 and 1354cm-‘, respectively, which are in perfect agreement with those of six-coordinate, low spin complexes listed in Table 4. It should be noted that similarly to bands I and III of the i.r. spectra, bands C and D of the RR spectra which are spin-state sensitive are also known to be metalsensitive [14]. Band E (391-376cm‘, p) is due to a porphyrin core deformation mode, and serves as an excellent oxidation-state marker according to BURKE et al. [ 131. This is clearly demonstrated in Fig. 4 where all the Fe(III) complexes are in the 391-388 cm- ’ range while all the Fe(I1) complexes are in the 384376 cm-i range. Finally, CHOTTARD et al. noted that all fivecoordinate complexes exhibit band B(dp) near 1370cm-i. Although it is often difficult to identify this band due to its proximity to band A, we were able to confirm its presence in all the five-coordinate complexes we studied (137&1366cm-‘). Correlation of spectral parameters with structural features Most of the previous studies on structure-sensitive RR bands have dealt with metalloporphyrins which bear heavy atom substituents at the C, positions and hydrogens at the methine carbons [l 11. The nature of porphyrin core vibrations between these and TPP complexes are markedly different since 6(C,-H), which couples with core vibrations in TPP complexes [ 123, does not exist in the former. Nevertheless, as has been shown above, several RR as well as i.r. bands can be utilized as effective markers of spin and oxidation states.

Ni(I1) 1.928 1353 468

< x z

Co(I1) 1.949 1351 468

< > >

Cu(II) 1.981 1346 448

< > >

Zn(I1) 2.036 1339 436

Occupation of this orbital induces sigma strain into the macrocycle which is evidently relieved via distortion and weakening of the methine bridge bonds[39]. In fact, calculations by WARSHEL [40] provide convincing evidence that core expansion is accompanied by methine bridge stretching and deformation. In the case of octaalkylporphine complexes, these spin-state sensitive bands, associated with the methine bridge atoms, were shown to be sensitive to deuteration at these positions [41]. In the TPP series, spinstate sensitive bands are not so easily identified since these positions are occupied by phenyl groups. Also, extensive mixing of 6(C,-H) in all symmetry species [ 121 may render vibrations associated with v(C,-C,) sensitive to deuteration at the B-pyrrole positions (i.e. TPP-ds). Such ambiguities are evident upon consideration of the data presented here. Thus, spin-state marker bands, III (i.r.), D (RR) and A (RR) show some sensitivity to /I-pyrrole deuteration although they are likely associated with v(C,&,,) or 6 (C,C,C,). However, bands I (i.r.) and C (RR), which are clearly effective spin-state markers, exhibit deuterium sensitivity consistent with expectations; i.e. a small de shift and a moderate or small d,, shift. Sensitivity to oxidation state is most often ascribed to alternations in 7c back-donation to the porphyrin ring which, in the case of octaalkylporphines, leads to changes in a frequency associated with pyrrole breathing [ll]. In the case of TPP complexes, these frequencies should be most sensitive to d, substitution. It is satisfying, therefore, that both of the oxidation state marker bands [(i.e. II (i.r.), E (RR) and A (RR)] show relatively large shifts upon p-pyrrole deuteration but only small or zero shifts upon phenyl deuteration. It should be noted, however, that the distinction of spin- and oxidation-state sensitivity is not always clearcut. For example, band A of the RR spectra is sensitive to both spin and oxidation states. Band II of the i.r. spectra, which is basically an oxidation-state marker, is also slightly spin-state sensitive in the Fe(I1) complexes. In order to understand the structure-sensitivity

870

HIROKI OSHIO et al.

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of these bands on a quantitative basis, it is necessary to carry out theoretical calculations such as those of WARSHEL and LAPPICIRELLA [42]. In summary, although correlation of structuresensitive bands with specific atomic groupings is not as satisfactory for the TPP series as for the octaalkylporphines, several useful marker bands have been identified and the limitations of their use indicated.