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Macrocyclic Metal Complexes - Phthalocyanines

Phthalocyanines

Phthalocyanine is a planar 18 -electron heterocyclic aromatic system derived from porphin. The more systematic name is [therefore] tetraazatetrabenzoporphyrine. The annullation of further benzene units leads to the naphthalocyanines 1,2-NcH ₂ and 2,3-NcH ₂ , respectively (figure 1).



Abb.1 a.)Porphin, b.)PcH ₂ , c.)PcM, d.)1,2-NcH ₂ (C _4h -Isomer), e.) 2,3-NcM
A large variety of complexes of the planar macrocycle Pc ^2- and its derivatives have been described in the literature [1] . Main group metals (e.g.Si, Ge) as well as numerous transition metals like Fe, Co or Ni serve ascentral units.

Research Areas

Due to their macrocyclic nature including extended -systems, phthalocyanines and naphthalocyanines are capable of forming organic conductors. Charge transport is enabled by specific orientation, which can be enhanced by doping, e.g. with iodine. One distinguishes between planar, stacked [2] and axially bridged [3] polymer systems (figure 2).
Some years ago, we developed a method for the transformation of phthalocyaninato- and naphthalocyaninato-transition metal compounds into coordination polymers as shown in figure2. The bridging is effected by neutral ligands with sigma-donor ability such as pyrazine (pyz), tetrazine (tz), diisocyanobenzene (dib) etc. In this macrocycle system, central transition metal atom and the bridging ligand can be systematically varied to influence the physical properties such as conductivity, nonlinear optical properties and others.



= Pc, 1,2-Nc, 2,3-Nc, TBP, TPP
M = transition metal (+II, +III), e.g. Fe, Ru, Os, Co, Rh, Mn, Cr
L = pyz, tz, bpy, dib, me ₂ dib, me ₄ dib, CN ^- SCN ^-
Abb.2 Axiallly bridged Macrocyclics

These bridged macrocyclic metal complexes (shish-kebab polymers) can be obtained in high yield and purity. Due to the choice of adequate bridging ligands, e.g. tetrazine, shish-kebab polymers of the structural type depicted in figure 2 display intrinsic semiconducting properties, while on the other hand chemical and electrochemical doping are still possible without destroying the polymer structure. Bridged systems with central metal atoms in the oxidation state +3 (e.g. Fe ³⁺ , Co ³⁺ , Rh ³⁺ ) are obtained with CN ^- SCN ^- or N ^3- . The preparatively easily accessible µ-cyano-polymer [PcCo(CN)] _n , which displays good semiconducting properties with _RT = 2·10 ^-2 S/cm, without external doping should be mentioned here[3] .
A more recent issue amongst polymeric metal macrocycles are the ladder polymers. This concept is realized by the synthesis of monomeric macrocycles with D _2h -symmetry capable of Diels-Alder reactions, such as the hemiporphyrazines, and their reactions with dienes and dienophiles (Scheme 1).[4] .


Scheme 1: Synthesis of a ladder polymer on Hemiporphyrazine Basis

According to Scheme 1, syntheses of ladder polymers on phthalocyanine basis are also being investigated. To this end, investigations are under progress to synthesize phthalocyanines with D _2h -symmetry capable undergoing Diels-Alder reaction are synthesized and reacted with dienes and dienophiles to form ladder polymers. The double bonds required for conjugation are introduced into the obtained ladder polymers chemically.
Due to the fact that the D _2h phthalocyanines depicted in Scheme 1 are unsymmetrically substituted phthalocyanines, our research group is intensivly working on the separation of such substituted phthalocyanines: peripherally tetrasubstituted phthalocyanines are always obtained as a mixture of the four possible constitutional isomers (D _2h , C _2v , C _4h , C _s Their definite analyticaland preparative separation and characterization by nuclear magnetic resonance was achieved for the first time by us recently [5] .

Further phthalocyaninato-metal compounds such as PcTiO and peripherally substituted soluble derivatives are at present investigated for their photoconducting properties.

Some tetra- and octasubstituted phtahlocyanines show liquid crystalline behaviour depending on their substituents, which are being studied for their suitability as Langmuir-Blodgett films andas optical limiting compounds.

Phthalocyaninatometal compounds and comparable macrocyclic metal complexes are prepared for molecular pattern recognition and their sensory properties investigated. Phthalocyaninato-silicon complexes bridged by acetylene [PcSiC _{2 n} are synthesized for use in the construction of nanostructures.
Water-soluble phthalocyaninato Zinc complexes of different structures and with different substituent patterns are synthesized and tested systematically for their application potentials in photodynamic cancer therapy.

Certain tetra- and octasubstituted phthalocyaninatometal complexes embedded in suitable polymer films display positive effects in the gas separation of nitrogen and oxygen. The main aspect of research here is the dependence of this property on the central metal atom and the peripheral substituents.

Recently, some transition metal phthalocyanines substituted with sixteen alkoxy groups (linear, branched and fluorinated alkoxy groups) were synthesized from the corresponding tetraalkoxyphthalodinitriles. In the UV/Vis spectra of these hexadecaalkoxy substituted phthalocyanines a bathochromic shift of the Q band up to 80 nm relative to the unsubstituted phthalocyanines was observed. The hexadecaalkoxy substituted phthalocyanines show excellent solubility in organic solvents. Systems with long linear side chains exhibit unexpected low melting points [7] .

Electrochemistry plays an important role in our research group in the characterization of the soluble macrocycles. Electronic properties of the monomeric and polymeric complexes are characterized by cyclic voltammetry and spectroelectrochemistry.