Spectrochimica Acta, Vol. !BA, pp. 1305 to 1311. Persamon Prem 1970. Printed in Northern Ireland

The metal-ligand vibrations in the infrared spectra of vluiofls metal phthalocyanines

T~SFXI KOBAYASRI, FUMIO KUROKAWA, NATSU UYEDA and EIJI SUITO

Institute for Chemical Research, Kyoto University, Uji, Kyoto-Fu, Japan

(Received 25 April 1969)

Abstract-The infrared spectra of phthalocyanine and seven divalent metal derivatives have been recorded in the region 400-4000 cm-l. The metal-ligand vibration and other metal dependent bands were found and the anomalies of Cu2+- and Znz+-phthalocyanines were inter- preted on the basis of electronic confgurations of outer orbital5 of the metals.

INTRODUCTION

ALTEOUGIH it is well known that some of the divalent metal derivatives of phthalo- cyanine undergo dimorphic transformation when dispersed in organic suspension media, zinc phthalocyanine exhibits anomalous behsviour to form various crystal phases depending upon the organic solvents used as the dispersion media [l]. Gener- ally, strong n-donor solvents such as aliphatic and aromatic amines produce stoichio- metric molecular complexes with zinc phthalocyanine, whereas the ordinctry solvents such as alcohol, ether, acetone and others only cause polymorphic transformation, sometimes showing stepwise processes.

These phenomena are not observed with regard to Cu- and Ni-phthdocyanines. In spite of this these derivatives as well as others which include the first transition metals not only have the same molecular configuration but also assume the same crystal structure with respect to each of the dimorphs. Such behaviour of Zn- phthalocyanine must be associated with the bonding nature of the centrsl zinc ion, which is not necessarily the same as those of other metal derivatives. Since the metal ion is co-ordinated by four inner nitrogen atoms of the macrocyclic ring of phthalo- cyanine to form a square planar configuration, the spectral data which are ascribed to the metal-ligand vibrations will render influencisl information for characterizing the nature of this type of bonding. Due to the .complexity in the structure of the macrocyclic ring, theoretical assignments for the metal-ligand vibration are almost impossible and thus, very few data have been reported so far.

In view of this, we tried to characterize the vibrational spectra on the basis of a comparative study with respect to metal-free phthelocyanine as well as its various metal derivatives.

EXPERIMENTAL

The phthalocyanine compounds used in the present work are those which are capable of purification by sublimation without being decomposed. The derivatives which include metal ions such as Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Pd2+ and Pt2+ were synthesized by heating phthalonitrile with the respective metal halides according to

[l] T. KOBAYASIEI, N. UYEDA anel E. SUITO, J. Phys. Chem. 73, 24.46 (1968).

1305

1300 T. KO~AYASEI, F. KUROKAWA, N. UYEDA and E. Sm~o

the method presented by LINSTEAD [Z]. &fe&&fiee phth~locya~e was obtained from the dilithium derivative. The specimens were purified by repeated sublima$ion of the products at 650°C in a stream of nitrogen g$$ under reduced pressure.

The infrared spectra were recorded in the range ~~~000 cm-l by the use of a Perkin-Elmer Grating Spectrometer Model 621. Each specimen for spectrometry was prepared in the form of Nujol mull or KBr disk.

RESULTS _LND DISCUSSION

The infrared absorption spectra and their frequencies for phthalocyanine and its divalent metal derivatives are shown in Fig. 1 and Table 1. Apparently, these absorp- tion bands can be divided into two groups according to whether or not the wave number as well as the intensity changed as the species of metal ion are substituted from one to the other. The effect of substitution is best noticed when a close examina- tion is made of these spectra in comparison with those for metal-free phthalocyanine which has two hydrogens in place of a divalent metal ion as shown in Fig. 2(a).

Metal-f&e phthalocyanine shows two strong absorption bands at 716 and 999 cm-l. Since these two bands were not observed iu any other spectra of metal deriva- tives and since it was reported by BOU~XIE;R and KATZ [3] that the N-H deforma- tion of protoporphyrin gave rise to sharp absorption bands at 740 and 1110 cm-l, the above two peaks are considered to be associated with the N-H out-of-plane defor- mation and in-plane deform&tion respectively. The N-H stretching vibration is expected to show an absorption near 3000 cm-l, and the weak broad band observed at 3200 cm-r for the metal free phthalocyanine only was assigned to this vibrational mode. These facts agreed well with the results reported by SEURV~LL [a].

All of the divalent metal phthalocyanine molecules are usually taken to belong to Dan point group whereas H,-phthalocysuine has apparently D, symmetry as shown in Fig. 2. The E, mode of vibration, which is infrared active for molecules with DM symmetry, is known to split into two other infrared active modes, B,, and Bau, without too much change in the peak position when the molecules happen to assume .D% symmetry as a result of, for instance, minor substitutions Such an effect of splitting is clearly shown in Table 1, particularly for those in the lower frequency region. In the higher frequency region, the characterization of doublets was difiicult because of the overlapping of a greater number of peaks made the spectra in that region much more intricate.

In the case of metal phthalocyauiues, the absorption bands apparently corre- sponding to those which appeared as the doublets for ~~-phthalocya~ewere detected as singlets which exhibited considerable shifts to higher frequencies as shown in Table 2. The corresponding absorption peaks are always shifted in the same order

Ni > Co > Fe > Cu > Zn > H, and Pd > Pt,

strongly indicating that the absorption bands which are affected by the species of substituted metal ion are metal dependent ones.

[Z] F. H. MOSER and A. L. THOMAS, Phthalocyanine compounds, AC% Monograph 157, pp. 104- 138. Reinhold (1963).

[3] L. J. BOUCKER and J. J. K&IX, J. Am. Chem, Sot. 89, 1340 (1966). [4] H. I?. SEURVLL and L. PINZUTI, Can. J. Chern. 44, 126 (1966).

The metal-ligand vibrations in the in&bred spectra of various metal phthalocyanines 1307

Ni-

Pd-

P?-

n I

1600 cm_, 1200 800 1 30

Fig. 1. Infrared absorption spectra of Ha-phthalocyanine and its metal derivatives. * Mark 7a indicates the absorption bands by Nujol. t No distinct absorption bands were observed in the higher frequencies region

than 1650 cm-l.

The medium intensity band which appeared as a singlet peak in the range 888 and 919 cm-l for each specimen is characteristic only of metal derivatives and is strongly dependent on the species of the metal ion as shown in Fig. 3. Since no corresponding absorption peak was observed for H,-phthalocyanine, the band of this type can be taken as being associated with the metal-ligand vibrations. These find- ings, that the metal-ligand absorption bands are observed in such a high frequency region, indicate the extraordinary stability of the metal-pythalocyanines due to the strong bonding between the metal ion and the four surrounding nitrogen atoms in the pyrrol rings. In fact, all of the metal derivatives used here are known to be so stable that they recrystallize without decomposition even at 660°C if the sublimation is carried out in an inert atmosphere.

1308 T. KOBAYASHI, F. KUROKAWA, N. UYEDA and E. Surxo

Table 1. Infrared absorption spectrum of phthalocyanine and its metal derivatives*

H*- Fe- CO- Ni- cu- Zll- Pd- Pt.

420 vwt 436 m 489 w 498 m 662 sh 669 w 617 m 626 sh 676 sh 683 m 716 “8 726 “B 730 “8 746 “s 760 sh 766 m 776 8

870 a 876 sh

942 sh 960 m 999 “8

1036 w 1066 sh 1089 8 1llOsh 1116 “a

1160x11 1176 m 1182 sh 1240 m 1272 s 1296 s 1316 B 1338 sh 1400 VW 1433 8 1466 w 1476 w 1496 s 1630 w 1600 m

1610 m

3260 w

420 VW 418 “xv 437 m 436 m

618 m 619 m

674 m 642 m

674 m 642m

676 VW 683 w

673 VW 686 m

726 sh 734 “8

723 ah 730 “8

766 8 770 m 781 s 800 VW

876 m 909 m

964 w

766 “8 7139 m 780 s 800 VW 866 sh 874 m 911 s 946 m 962 m

1000 VW 1069 sh 1084 “8 1097 sh 1120 “B

996 VW 1072 s 1086 “8 1100 sh 1120 “8

1163 8 1171 sh 1196”w

1162 8 1172 m 1196 w

1288 B 1332 “8

1288 s 1333 “8

1420 B 1426 8

1606 8 1610 B

1606 B 1620 s

1690 sh 1606 m

1690 sh 1609 m

420 VW 438 m

621 m

678 m 646 m

670 VW 680 w

726 sh 731”s

766 s 770 m 780 8 800 “w 866 ah 876 m 916 m 946 m 963 m

996 VW 1076 ah 1086 “S 1098 ah 1120 “8

1164 s 1170 sh 1196 w

1288 B 1333 “8

1426 B

1603 B 1620 8 1627 sh 1696 sh 1606 m

420 VW 436 m

609 m

676 m 640 m

676 VW 687 w

728 “s

7633 B 770m 779 8 796 VW 866 ah 876 m 898 8 946 w 963 m

1000 w 1064 a 1087 “B 1099 8 1118 “8

1162 s 1170 sh 1190”~

1286 8 1330 “8

1416 s

1600 8 1606 sh

1686 w 1603 m

420 VW 438 m

602 m

676 m 637 m

680 m 696 m

728 “8

762 “s 773 8 780 “8 797 “w

878 8 888 s 946 m 966 In

1002 m 1060 “8 1090 “8 11028 1117 “B 1122 sh 1162 s 11721~1

1284 vs 1332 “s

1408 s

1480 “s

1680 w 1604 m

420 VW 438 m

6141”

678 m 648 VW

723 “s 730 sh

766 s 767 B 778 m 806 w 860 w 880 w 911 m 940 sh 949 m

1000 VW 1070 s

1107 s 1120 “S 1126 sh

1167 s 1188 w

1286 8 1330 s 1360 m

1418 s

1480 sh 1603 8

1683 w 1606 m

420 VW 439 m

616 w

681 m 646 VW

726 “s 740 sh

760 s 770 8 778 m 816 w 866 m 880 w 919 m 943 m 960 In

1000 VW 1076 s

1111 s 1124 “s 1130 sh

1170s 1190~

1290 s 13318 1363 m

1418 ah 1428 8

1480 sh 1606 sh 1610 8 1688 w 1610 m

* Frequencies in on+. t “, very; 8, strong; m, medium; w, weak; sh, shoulder.

The frequency of this kind of absorption increases in the same order as that for other metal dependent bands described above. According to IRVINCIWILLIAMS [5] the order of the stability constant for divalent metal complexes is

Zu < Cu > Ni > Co > Fe > M.n

and is in good agreement with the order of the second ionization potential of the

[6] H. IRVING and R.J.P. WILLUMS, J.Chem.Soc. 3192 (1963).

The metal-ligand vibrations in the infrared spectra of various metal phthalocyanines 1309

9-xN6) N

N "N &?I N

M ... F,t’Co~Ni:‘C~,Tinf’Pd:‘ptZf

(a) (b)

Fig. 2. Molecule+ of II,-phthalocyanine and metal derivatives.

Table 2. Absorption frequencies showing the splitting behaviour of Ha-phthalocyanine

D2.h J% Fe- co- Ni-

Da Cu- Zn- Pd- Ptl-

489 498

518 519 521 509 512 514 516

552 559

574 574 578 575 575 578 581

617 625

642 642 645 640 637 648 645

Fe -

co-

Ni-

Pd-

Pf -

Fig. 3. I&wed absorption speotra in the region 850-950 om‘-l.

1310 T. KOBAYASEI, F. KUROKAWA, N. UYEDA and E. SUITO

metals. It was reported by BOUCHER and KATZ [3] that the ligand deformation bands of several metal derivatives of porphyrin shifted toward higher frequencies in the order as follows,

Ni > Co > Cu > Zn > Mg.

Since this result is in good agreement with the present one, it is likely that the anomaly of the &r-derivative concerning the order of the frequency shift is a general tendency for such a complex molecule which assumes a square-planar configuration with a copper ion in it.

I I I I I I I I I I I

I I I I I I

II I I I I

lIElIul

lIinIcl

Fig. 4. Diagrammatic presentation of the electronic configurations of the outer orbit& of divalent metals. The black points show the electrons donated by

ligand nitrogens.

Most of the divalent metals except the zinc ion can form an inner orbital complex of square-planar structure as a result of &@ orbital hybridization. The electronic configuration in the outer orbitals of a metal in phthalocyanine may be schematically expressed as shown in Fig. 4. Thus, a set of CL@ orbitals of the metal, which are associated with a square-planar arrangement of covalent bonding, must be filled with four electron pairs donated by the ligand nitrogen.

For a divalent copper ion, the electronic configuration of the outer orbitals is either 3d4s4p2 or 4e494d according to whether the unpaired electron is elevated to the 4~ orbital or stays in the 3d orbitals as shown in Fig. 4(b). Both configurations are known to be energetically equivalent and capable of forming stable square-planar bonding. However, the fact that the magnetic moment of Cu-phthalocyanine is 1.73 B.M. as reported by MELLOR and CRAIG [6] suggests that the Cu-phthalocyanine is an inner orbital complex, that is, the electronic conf?guration is (a) in Fig. 4. Thence, the unpaired electron stays in the outermost 499 orbital, and produces a repulsive force against the bonding of the copper ion with the four surrounding nitrogen atoms, resulting in the anomaly of copper phthalocyanine concerning the order of frequency shift, which differs from that predicted by IRVING-WILLIES theory.

For a divalent zinc ion, the 3d orbital wholly filled with 10 electrons cannot give any contribution to form covalent bonding. Therefore, most of the four-co-ordinated

[S] D. P. MJZLLOR and D. P. CRAIG, J. Proo. Roy. Sot. N.S. Wales 75, 27 (1941).

The metal-ligand vibrations in the infrared spectra of various metal phthalocyanines 1311

complexes of zinc ion have a tendency to assume a tetrahedral configuration using 4&ps hybrid.

In the case of zinc-phthalocyanine, however, the metal ion is forced to fit into a rigid macrocyclic ring which a priori requires a square-planar configuration.

In order to satisfy this requirement, the outer 44 orbital must be used for &+t%d hybridization, resulting in the formation of an outer-orbital complex. Since the stability of this type of complex is generally less than that of an inner complex, it is not unreasonable that the frequency of the metal dependent band for zinc-phthalo- cyanine is always the minimum as far as the phthalocyanines of divalent first transi- tion metals are concerned.

Further information about the metal ligand vibrations in metal-phthalocyanines is expected to be obtained in the far infrared region.