Transition Metallophthalocyanines as Structures for Materials Design

Chapter 35

Transition Metallophthalocyanines as Structures for Materials Design

Downloaded by NORTH CAROLINA STATE UNIV on October 29, 2012 | http://pubs.acs.org Publication Date: November 18, 1994 | doi: 10.1021/bk-1994-0572.ch035

Michael Hanack Institut für Organische Chemie, Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany

Bridged macrocyclic transition metal complexes [MacM(L)] , Mac = phthalocyanine (Pc) and naphthalocyanine (Nc), M = Fe, Ru, Co and L = e.g. pyz, dib, tz, SCN , C N were synthesized and their electrical properties studied in detail. Regardless of the bridging ligand, stable semiconducting compounds are formed after doping with iodine. In general, these complexes [MacM(L)] are insoluble in organic solvents, however, soluble oligomers [R PcM(L)] were prepared using metallomacrocycles R P c M and R PcM, R = t-Bu, Et, OR' (R' = C H -C H ), M = Fe, Ru, which are substituted in the peripheral positions. We report here on the systematic investigation of the influence of the bridging ligand on the conductivity of the bridged phthalocyaninato and 2,3-naphthalocyaninato transition metal complexes. Due to the low oxidation potentials of tz and me tz and their low lying LUMO the corresponding bridged systems [MacM(L)] , L = e.g. tz, me tz, and others (see Table 1) exhibit intrinsic con ductivities. The phthalocyanines were characterized by IR, UV, H-NMR and Mößbauer (Fe) spectroscopy. n

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n

x

4

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8

11

12

25

2

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2

1

Metallophthalocyanines and -naphthalocyanines have been used recendy for the preparation of materials, which exhibit interesting semiconducting and nonlinear optical properties (l).ln this report we concentrate on the electrical properties of phthalocyaninato and naphthalocyaninato transition metal com pounds. A necessary condition for achieving good electrical conductivities in

0097-6156/94/0572-0472S08.00/0 © 1994 American Chemical Society In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.

35. HANACK

Transition Metallophthalocyanines

phthalocyaninatometal compounds is a special geometric arrangement, namely either planar or stacked. Metallophthalocyanines and naphthalocyanines are less soluble in common organic solvents. However, soluble phthalocyanines R 4 P C M (R = tBu, Et, C H O C H 5 , C H - C H ; M = Fe, Ru) and R PcM (R = C 5 H - C H 5 , O C 5 H H - O C H 5 ; M = Fe, Ru) have been prepared re cently. Quite often the solubility of tetra-substituted metallophthalocyanines is higher than the octa-substituted ones. This is due to the larger dipole moment arising from unsymmetrical structures. The synthesis of tetra-sub stituted phthalocyaninatometal compounds always leads to four structural isomers (2,3) and recently, the separation (and characterization by ^ - N M R spectroscopy) of all four isomers was successful in the case of tetrakis(2ethylhexyloxy)phthalocyaninatonickel(II) [(2-Et-C H 0) PcNi] (4). Phthalocyanines substituted in peripheral positions by long chain substituents form liquid crystalline phases (5,6), which are either discotic (from the disc like shape of the molecules) or columnar meso phases (7). In this paper we will concentrate on stacked phthalocyaninato (PcM) and naphthalocyaninato (NcM) transition metal complexes. If it is possible to arrange phthalocyaninato transition metal compounds in a stacked fashion and produce charge carriers by oxidation or reduction, good semiconducting properties can be obtained. In addition to their high thermal stability a further advantage in using these metal complexes is their accessibility. Except for the macrocyclic modification of phthalocyaninatolead(II) (8) in its monoclinic arrangement, the stacked arrangement has never been ob served. Metallophthalocyanines in general crystallize in the a- or β-modification which is not favourable for τ-orbital overlap and thus for the formation of a conduction band. However, if certain transition metal phthalocyanines, e.g. phthalocyaninatonickel(H) are doped with iodine, stacked structures are obtained which exhibit high conductivities (9). A few years ago we developed alternative route to obtain a stacked arrangement with macrocyclic transition metal compounds. This leads, as will be pointed out later, to the possibility of achieving intrinsic conductivi ties in these systems by the so-called "shish kebab" arrangement of macrocyclic transition metal compounds. Here the stacking of the metal macrocycles is achieved by bisaxially connecting the central transition metal atoms with bidentate bridging ligands (L). Such bridged macrocyclic metal com pounds [MacM(L)] with transition metals, e.g., Fe, Ru, Os, Co, Rh, in various oxidation states have been synthesized and investigated by us in detail (1,10,11). A schematic structure of these type of compounds is shown in Figure 1. 2

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In Inorganic and Organometallic Polymers II; Wisian-Neilson, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.


474

INORGANIC AND ORGANOMETALLIC POLYMERS II

M = Fe . Fe *. Co , Co *. Ru *. Mn . Mn *. Cr * 2+

/Γ^

L = Ν

3

2+

3

2+

3

3

/r\

/Γ**.

Ν , Ν

2

Ν , CN Λ '

y NC , CN"

Pc ", R Pc ", R Pc " 1,2-Nc ", 2,3-Nc -, TBP " 2

4

2

2

8

2

t

2

2

Figure 1. Schematic structure of bridged macrocyclic transition metal com plexes

Within this new class of materials a large spectrum of compounds was synthesized and studied systematically with respect to their physical proper ties. The ^-electron containing bridging ligands (L) are linear organic molecules e.g. pyrazine (pyz), /7-diisocyanobenzene (dib) and substituted pdiisocyanobenzenes, tetrazine (tz) and substituted tetrazines (me2tz). If the oxidation state of the central metal atom is +3 ( C o , Fe +) charged bridging ligands such as cyanide (CN"), thiocyanate (SCN") and others can also be used. In addition to phthalocyanines and tetrabenzoporphyrins other macrocycles used are 1,2- and 2,3-naphthalocyanines and tetranaphthoporphyrins. In general, the complexes [MacM(L)] are practically insoluble in organic solvents. However, soluble oligomers [R4PcM(L)] and [R PcM(L)] have been prepared using substituted metallomacrocycles R PcM, in which R = ί-Bu, Et, OR (R* = C H - C H 5) and RgPcM (R = C H - C H 2 5 , O C H - OC H25) and M = Fe, Ru. These types of oligomers are completely soluble in most common organic solvents, thereby allowing the determination of the chain lengths by !H-NMR spectro scopy. The soluble oligomers [R4PcFe(dib)] form Langmuir-Blodgett films (12). 3+

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35.

HANACK


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The powder conductivities of most of the bridged phthalocyaninato transition metal complexes [PcM(L)] for M = Fe, Ru, Os, Co, Rh; L = pyz, dib, etc. are low in the range of 10" - 10" S/cm. However, many of these compounds can be doped either chemically (with iodine) or electrochemically (13,14) leading to good semiconducting properties ( a = 10 10" S/cm) with thermal stabilities up to 120°C. The doping does not destroy the bridged structure of the coordination polymers, because the oxidation process takes place at the macrocycle forming radical cations. The long term stability of these compounds is high, since even after two years of exposure to air and moisture, the conductivities did not change. The doping process has been studied very carefully by us using not only different macrocycles and central metal atoms, but also using different bridging ligands with a comparatively low oxidation potential. The question addressed was whether the doping process occurs only at the phthalocyanine macrocycle leading to a radical cation or also at the bridging ligand. Fig. 2 shows a phthalocyaninatoiron oligomer in which the bridging ligand is 9,10diisocyanoanthracene (9,10-dia), a ligand which has a comparatively low oxidation potential. It could be shown that doping with iodine indeed leads to oxidation of the bridging ligand, thereby increasing the powder conductivity from 3 χ 10" to 8 χ 10" S/cm (15). n

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Figure 2. Schematic structure of [PcFe(9,10-dia)]

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Changing the macrocycles in the coordination polymers shown in Fig. 1 from phthalocyanine to one with an extended τ-electron system, e.g. 2,3-naphthalocyanine leads to an interesting effect concerning the semicon ducting properties of the corresponding bridged systems. A systematic investigation of the oxidation potentials of the metallomacrocycles used for


476


the synthesis of the corresponding bridged systems by cyclic voltammetry in different solvents shows that the first and second oxidation potentials of the metallomacrocycles are very much dependent upon the structure: the phthalocyaninatoiron (PcFe) used here has almost the same oxidation potential as the unsymmetrical 1,2-naphthalocyaninatoiron (1,2-NcFe). 2,3-Naphthalocyaninatoiron, however, exhibits a much lower oxidation potential as compared to TBPFe and 2,3-TNPFe. The lower the oxidation potential of the metallomacrocycle, the higher the conductivity of the corresponding [MacM(L)] system, e.g. the bridged compound [2,3-NcFe(dib)] shows a powder con ductivity of 4 χ 10" S/cm, which is due to oxygen doping of the macrocycle. PcFe, PcRu, PcOs and 2,3-NcFe react with tetrazine (tz) and dimethyltetrazine (me2tz) depending on the conditions with formation of the corre sponding monomers MacM(L) (L = tz, me^tz) and the bridged systems [MacM(L)] (L = tz, me^tz) (16). The tetrazine bridged systems, in contrast to other bridged compounds [MacM(L)] with M = Fe, Ru or Os and L e.g pyz or dib, show good semiconducting properties without external oxidative doping ( σ = 0.05 - 0.3 S/cm) (16). The powder conductivities of bridged transition metallomacrocycles in the non-doped state are listed in Table I. A l l the bridged complexes [MacM(L)] (L = pyz, tz, dabco), contain cofacially arranged macrocycles, which are separated by approximately the same distance (about 600 pm). While the monomelic complexes PcM(L) (L = pyz, dabco, tz; M = Fe, Ru, Os) show insulating behaviour, it can be seen (Table I) that the ligand L has a significant effect on the conductivity of the bridged complexes [MacM(L)] . The following conclusion can be drawn from Table I: because dabco is a ligand containing no τ-orbitals that are capable of interaction with the metallomacrocycle (see below), the complex [PcFe(dabco)] is an insulator. A clear increase in conductivity is observed for the pyrazine-bridged com pounds [MacM(pyz)] , which exhibit conductivities in the low semiconduct ing region. However, by changing the bridging ligand from pyrazine to stetrazine the conductivity is increased by 3 to 5 orders of magnitude without external oxidative doping (see Table I). One of the factors responsible for the electrical conductivities in bridged macrocyclic transition metal complexes [MacM(L)] is the band gap, which is determined by the energy difference between the LUMO of the bridging ligand and the HOMO of the transition metallomacrocycle. There fore to achieve semiconducting properties the metallomacrocycle should con tain a high lying HOMO, while a bridging ligand with a low lying LUMO should be used. A systematic change of the bridging ligand in this respect has been investigated. It was found that not only tetrazine (tz) and dimethyltetrazine (me^tz), but also other bridging ligands, e.g., p-diaminotetrazine n


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HANACK

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can be used to prepare the corresponding bridged macrocyclic transition metal compounds. /?-Diaminotetrazine (NH^tz is coordinated to the ruthe nium atom in [PcRuiNH^tz],, via the nitrogen atoms of the amino groups: its powder conductivity is about 100 times higher than that of [PcRu(p(NH^CgUi)],!, in which the bridging ligand /nphenylenediamine contains no hetero atom in the aromatic ring (Table I). On the other hand, electron withdrawing substituents in the peripheral positions of the macrocycle show the expected effect: e.g. [(CN)4PcFe(tz)] has a conductivity which is at least 3 orders of magnitude less than the conductivities of other tetrazine-bridged compounds investigated so far (Table I) (17).


n

Table I. DC-Powder Electrical Conductivities of Monomelic and Bridged Macrocyclic Transition Metal Complexes. 1

Compound

ORJ [S/cnr ]* 1 0

PcFe(dabco) [PcRu(dabco) χ 1.4 CHCl ] PcFe(pyz) [PcFe(pyz)] PcFe(tz) [PcFe(tz)] PcRu(tz) [pcRu(tz)] PcOs(tz) [PcFe^tz)],! tPcRu(NH2) tz] rPcRu^(NH2)C H ] [PcRuCl (tz)] [PcOs(pyz)] [PcOs(tz)] [2,3-NcFe(pyz)] [2,3-NcFe(tz) χ 0 . 2 C H C l ] [(me) PcFe(pyz)] [(me) PcFe(tz)] [PcRu(me2tz)] [(CN) PcFe(pyz)] [(CN) PcFe(tz)] 2

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The low band gaps of all the tetrazine-bridged coordination polymers with group VIII transition metals [MacM(L)] [Mac = Pc; M = Fe, Ru, Os; Mac = 2,3-Nc; M = Fe; L = e.g. tz, me^tz, (NH^tz] can be demonstrated by physical properties that are not shown by the corresponding systems [MacM(L)] in which L = e.g. pyz or dib: all the tetrazine-bridged coordination polymers with group VIII transition metals [MacM(tz)] show broad bands in the UV/Vis/NIR spectra between 1250 and 2500 nm with different maxima, e.g., [PcFe(tz)] at 1515 nm (0.75 eV) and [PcRu(tz)] at 1053 nm (1.18 eV). The corresponding pyrazine-bridged systems [PcM(pyz)] (M = Fe, Ru, Os) exhibit normal UV/Vis spectra with Soretand Q-bands between 245 and 700 nm, respectively, but show no absorption between 1200 and 2000 nm. The absorption bands in the near infrared correlate well with the electrochemically estimated energy gap between the HOMOs of the different metallomacrocycles and the LUMO of s-tetrazine in all the tetrazine-bridged systems described here. The broad band observed in the absorption spectra of these complexes can be assigned to charge transfer process from the metallomacrocycle to the 7r*-orbital of j-tetrazine. n

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Table Π. Mofibauer Data of s-Tetrazine Bridged Complexes. Complex

h fmm/sj

B-PcFe PcFe(py) PcFe(pdz)2 PcFe(tz) [PcFe(tz)] [PcFe(bpytz)] PcFe(me2tz) x 0.5 m^tz 2

c

2

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(me) PcFe (me) PcFe(py) [(me) PcFe(tz] 8

8

2

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(meO) PcFe (meO) PcFe(py) [(meO) PcFe(tz)] c 8

8

2

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a

b

c

d

n

d

AgQ fmm/sj

0.38 0.26 0.21 0.15 0.13 0.24 0.36

2.58 2.02 1.82 1.79 2.23 1.90 2.67

0.38 0.26 0.14

2.55 1.97 2.11

0.36 0.25 0.15

2.50 1.95 2.19

Measured at room temperature, unless otherwise indicated. Relative to metallic iron. Additional doublet, due to PcFe present. Measured at 77° K.


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479

The MoBbauer data of j-tetrazine-coordinated iron complexes are summarized in Table II along with the data for the iron macrocycles and the corresponding pyridine complexes (18). For the tetrazine-bridged complexes [MacFe(tz)] the isomer shift δ is similar to the monomelic PcFe(tz) whereas a clear increase of A Q in comparison to [PcFe(pyz)] is measured. This is a further indication for a low band gap, because this effect can be ex plained by the thermal activation of electrons of the highest occupied band. As the contribution of such delocalized electrons to the occupation of metal centred d-orbitals is diminished, the increase in A Q is the result of the low band gap of the quasi one-dimensional chain structure of the tetrazine-bridged complexes. Other examples of macrocyclic transition metal complex exhibiting in trinsic semiconducting properties are the cyano-bridged systems [MacM(CN)] (Mac = Pc, TBP, 2,3-Nc; M = Fe, Co). The cyano-bridged phthalocyanine complex [PcCo(CN)] can be easily synthesized, e.g., from PcCoCl with an excess of NaCN to form the soluble NatPcCoiCN)^ (19). The crystal structure of this monomer shows the bisaxial coordination of the CN-groups. HCN can be cleaved from the monomer using different methods, e.g. treatment with H 0 at 100°C, to form [PcCo(CN)] in quantitative yield (19). It was shown by physical methods that these compounds possess the cyano-bridged structure (19,20). All of the cyano-bridged macrocyclic metal complexes show comparatively high powder conductivities in the range of 10-2 - 10-1 S/cm. The bridged transition metal complexes described here are of interest for technical applications due to their comparatively high thermal stability and their good semiconducting properties without external oxidative doping. n

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Acknowledgment The reported results are based on the very efficient experimental work of my co-workers Anton Beck, Reinhold Dieing, Michael DreBen, Carola Feucht, Ronald GroBhans, Ahmet GUI, Shigeru Hayashida, Andreas Hirsch, Sabine Kamenzin, Young-Goo Kang, Armin Lange, José Osio Barcina, Jorg Pohmer, Haiil Ryu, Gabriele Schmid, Hanna Schultz, Michael Sommerauer and Elisabeth Witke. My sincere thanks go to all of them.


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Literature (1) (2) (3)


(4) (5) (6) (7) (8) (9) (10) (11)

(12) (13)

(14) (15) (16) (17) (18)

(19) (20)

Schultz, H., Lehmann, H . , Rein, M., Hanack, M . in "Structure and Bonding 74", Springer-Verlag, Heidelberg 1991, p. 41. Hanack, M., Thies, R. Chem. Ber. 1988, 121, 1225. Hanack, M., Meng, D., Beck, Α., Sommerauer, M., Subramanian, L.R. J. Chem.Soc.,Chem. Commun. 1993, 58. Hanack, M., Schmid, G., Sommerauer, M . Angew. Chem. 1993, 105, 1540. Giroud-Godquin, A . - M . , Maitlis, P.M. Angew. Chem. 1991, 103, 370. Espinet, P., Esternelas, M.A., Oro, L.A., Serrano, J.L., Sola, E. Coord. Chem. Rev. 1992, 117, 215. Chandraskhar, S., Ranganath, G.S. Rep. Prog. Phys. 1990, 53, 570. Ukei, K. Acta Cryst. 1973, B29, 2290. Schramm, C.J., Scaringe, R.P., Stojakovic, D.R., Hoffman, B.M., Ibers, J.A., Marks, T.J. J. Am. Chem. Soc. 1980, 102, 6702. Hanack, M., Deger, S., and Lange, A . Coord. Chem. Rev. 1988, 83, 115. Hanack, M., Datz, Α., Fay, R., Fischer, K., Keppeler, U., Koch, J., Metz, J., Mezger, M., Schneider, O., and Schulze, H.-J. "Handbook of Conducting Polymers", M . Dekker Inc., New York 1986, Vol. 1. Rauschnabel, J. Diploma Thesis, Tübingen, 1992. Diel, B.N., Inabe, T., Jakki, N.K., Lyding, W., Schneider, O., Hanack, M., Kannewurf, C.R., Marks, T.J., Schwarz, L.H. J. Am. Chem.Soc.1984, 106, 3207. Keppeler, U., Schneider, O. Stöffler, W., Hanack, M . Tetrahedron Lett. 1984, 25, 3679. Hanack, M., Ryu, H . Synth. Met. 1992, 46, 113. Hanack, M., Lange, Α., Groβhans, R. Synth. Met. 1991, 45, 59. Hanack, M., Groβhans, R., Chem. Ber. 1989, 122, 1665. Hanack, M., Keppeler U., Lange, Α., Hirsch Α., and Dieing, R., in: "Phthalocyanines, Properties and Applications", V C H Publishers, Weinheim 1993, Vol. 2, p. 43. Metz, J., Hanack, M . J. Am. Chem. Soc. 1983, 105, 828. Hanack, M., Hedtmann-Rein, C., Datz, Α., Keppeler, U., Münz, X . Synth. Met. 1987, 19, 787.

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Transition Metallophthalocyanines as Structures for Materials Design

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