Conclusions and acknowledgment:

Introduction:

Electrochemistry of [(TMpyP)M]n+(X-)n (X- = Cl-,I-, or BPh4- and n = 4) where M is one of

fifteen different metal ions

Eric Van Caemelbeckea,c, Agathe Derbina, Rachel Garciaa, Peter Hambrightb, and Karl Kadisha

aDept. of Chemistry, University of Houston, Houston, TX 77204 bDept. of Chemistry, Howard University, Washington, D.C. 20059 cHouston Baptist University, Houston, TX 77074

• To determine the electrochemical properties of different [(TMpyP)M]n+(X-)n complexes in non aqueous media, where n = 4.

Background-

• To examine the effect of:

• the metal ion

• porphyrin counteranion (X-)

Experimental Procedure:

Goals-

On the electrochemical properties with emphasis on the overall number of electrons added to the porphyrin and/or the reversibility of the electron transfer of each electrode process.

N

NN

NN+

N+

+N

N+

CH3

CH3

H3C

CH3

X-MX-

X-

X-

M= Metal Ion in +II or +III oxidation stateX-= Cl-, BPh4

-, or I-

X-

Metal Ion Effect-

Porphyrin Counteranion (X-) Effect-

[(TMpyP)M]n+(X-)n

• Group AGroup A compounds undergo 3 processes, II, III and IV. Process II is a reversible process in which the Epa-Epc separation is 30-40 mV, suggesting that is a 2-electron transfer. The remaining processes are irreversible.

• Group CGroup C compounds undergo a reversible M(III)/M(II) electrode process (Process I) prior to the other processes.

• Group BGroup B compounds have more than 3 processes. Process II is similar to process II of Group A and Group C. The metal ions in this category effects the remaining processes which maybe due to low electronegativity of those metal ions.

12 μA

7 μA

5.5 μA

12 μA

4 μA

7 μA

= Group A

= Group B

= Group C

Metal Ion in +II or +III oxidation

state (M)

=

[(TMpyP)Cu][(TMpyP)Cu]4+4+(Cl(Cl--))44

[(TMpyP)Cd][(TMpyP)Cd]4+4+(BPh(BPh44--))44

[(TMpyP)FeCl][(TMpyP)FeCl]4+4+(Cl(Cl--))44

0.0 -0.4 -0.8 -1.2

E (in V vs. SCE)[(TMpyP)VO][(TMpyP)VO]4+4+(Cl(Cl--))44

[(TMpyP)Zn][(TMpyP)Zn]4+4+(Cl(Cl--))44

[(TMpyP)AuCl][(TMpyP)AuCl]4+4+(Cl(Cl--))44

Cyclic Voltammetry , Normal Pulse Voltammetry, and Time-Dependant Spectral Characterization using a diode array UV-Visible Spectrophotometer. All were performed under Nitrogen atmosphere with TBAP as the supporting electrolyte.

Techniques Employed-

Data-

The formation of a peak around the 650-900 nm region when a potential more

negative than Process II is applied suggests that a phlorin anion is formed

Time-Dependant Spectral Changes after Process II

E ( in V vs. SCE)

[(TMpyP)Zn][(TMpyP)Zn]4+4+((BPhBPh44--))44

[(TMpyP)Zn][(TMpyP)Zn]4+4+((ClCl--))44

The change in counteranion effects the processes after process II for Group B compounds

Proposed Mechanism-

The first reduction (Process II) is coupled with a chemical reaction involving the

macrocycle which leads to a phlorin anion while the following processes involve

reduction of the N-methylpyridyl groups.

These compounds have received attention due to their possible applications in medicine. With specific metal ions, they can be tested as tumor and liver contrast agents in mice using magnetic resonance imaging techniques1 and shown to act against the human immunodeficiency virus2 as well as mad-cow disease3. They have been studied in both aqueous and non-aqueous media4. Non-aqueous media is ideal for studies since it can provide an insight to the behavior of the compound under particular cellular environments, thus giving us some possible explanations in-vitro.

[(TMpyP)Zn][(TMpyP)Zn]4+4+(BPh(BPh44--))44

0.2 M TBAP

0.1 M TBAP + 0.1 M TBABPh4

0.2 M TBABPh4

E ( in V vs. SCE) 0.0 -0.4 -0.8 -1.2

[(TMpyP)Zn][(TMpyP)Zn]4+4+(Cl(Cl--))44

[(TMpyP)Pb][(TMpyP)Pb]4+4+(BPh(BPh44--))44

[(TMpyP)Cd][(TMpyP)Cd]4+4+(BPh(BPh44--))44

Normal Pulse Voltammetry

0.0 -0.4 -0.8 -1.2E (in V vs. SCE)

This data combined with the cyclic voltammetry data indicates a total of

6 electrons transferred.

The current heights for each porphyrin are an approximate 1:2

ratio between the 2 steps

Under different values of the ClO4

-/BPh4- ratio, the data indicates that

there are porphyrin species in equilibrium prior to the processes involving the 1-methyl-4-pyridyl

substituents.

0.0 -0.4 -0.8 -1.2

Compounds with M(II) or electroinactive M(III) central metals can be reduced by an overall number of 6 electrons. The first 2-electron reduction is followed by a chemical reaction leading to a phlorin anion and this is followed by further reductions of the 1-methyl-4-pyridyl substituents. When the metal ion is an electroactive M(III), the porphyrin undergoes a M(III)/M(II) process prior to the electrode reactions involving the porphyrin π-ring system and the meso 1-methyl-4-pyridyl substituents. Depending on the electronegativity of the metal ion, the exact mechanism the porphyrin undergoes differs as well as the number of processes observed.

The support of the Robert A.Welch Foundation (K.M.K., Grant E-680) is gratefully acknowledged.References-

1Ferrer-Sueta,G; Batinic-Haberle.; Spasojrvic, I; Fridovich, I.; Radi, R. Chem. Res. Toxicol. 1999,12, 442.

2Dixon, D. W.; Schinazi, R.; Marzilli, L. G. Ann. N.Y. Acad. Sci. 1990, 616,511.

3Caughey, W. S.; Raymond, L. D., Horiuchi, M.; Caughey, B. Proc. Natl. Acad. Sci. U.S.A 1998, 95, 12117

4P. Hambright in “Inorganic Organometallic and Coordination Chemistry” in “The Porphyrin Handbook”, Karl. M. Kadish, Kevin M. Smith, R. Guilard; Eds.; Chapter 18, pp 129-200