The tetrapyrrole ring acts as a tetradentate dianionic ligand
and can form complexes with a wide range of transition metal
ions. In many cases, additional ligands may occupy the axial
positions. Tetrapyrroles closely approach planarity, and D4h
symmetry, when coordinated with metals. The egx and egy
orbitals are degenerate and only one Q band is observed in the
absorption spectra.
Orbital diagrams of metalloporphyrins may be complicated by
the presence of metal orbitals with energies close of the HOMO
and LUMO of the prophyrin. Figure 4 shows orbital energy
TABLE I
REDOXPOTENTIALSVERSUSSCEANDOPTICALCHARACTERISTICS OFPORPHYRIN
DERIVATIVESCompound Half-wave potential (V) Q(0,0) absorption
band
E1/2ox
a
E1/2red
a
DE
a
ES1(eV) lmax(nm)H 2 TPPb 0.95 1.08 2.03 1.92 647
H 2 TPCb 0.88 1.12 2.00 1.91 650
H 2 TPBb 0.40 1.10 1.50 1.68 740
H 2 TPP-b(CF 3 ) 4 c 0.89 0.33 1.42 835
H 2 TPP-bF 8 d 1.26 0.67 2.13 637
ZnTPPe 0.77 1.35 2.02 2.12 585
ZnTPCe 0.60 1.33 1.93 2.03 610
ZnTPBe 0.18 1.28f 1.46 1.64 755
CuTPPe 0.99 1.41 2.40 578
NiTPPe 1.101.00 1.28 2.38
CoTPPe 1.190.52 1.290.92 2.48 –
FeTPPe 1.180.32 1.01*g 581
PdTPPh 1.02 1.03 2.02 2.25 553 i
aThe redox potentials taken from difference sources were offset by the difference with
respect to the redox potentials of H 2 TPP or ZnTPP reported inRef. ( 10 ), but the
differences in redox potentials (DE¼E1/2oxE1/2red) were calculated from the original
sources whenever possible.
bOxidation potential in CH 2 Cl 2 and reduction potential in dimethylformamide or
butyronitrile, spectroscopic data in CH 2 Cl 2 , benzene, or toluene, according toRef. ( 10 );
Ref. ( 11 ) gives the same oxidation potential for H 2 TPB and reportsE1/2ox¼0.40 V
versus SCE for MgBChl.
cFrom Ref. ( 12 ) but offset by the difference of redox potential of H 2 TPP.
dFrom Refs. ( 13 ) and ( 14 ) but offset by the difference of redox potentials of H 2 TPP.
eOxidation potentials in benzonitrile fromRef. ( 15 ) and reduction potentials in CH 2 Cl 2
from Ref. ( 16 ); an asterisk indicates either the metal oxidation (2þ! 3 þ) or the metal
reduction (2þ! 1 þ).
fThe macrocycle reduction potential of ZnTPP is1.52 V in the conditions ofRef. ( 16 ).
gReduction potentials for the metal reduction (2þ! 1 þ) in CH 3 ClCH 3 Cl fromRef. ( 17 ).
hFrom Ref. ( 18 ).
iFrom Ref. ( 19 ).194 LUIS G. ARNAUT