g zz ¼ 2b
2
3 À 2b
2
2 À 2b
2
1 þ 4Kb 1 b 2
g xx ¼ 2b
2
1 À 2b
2
2 þ 2b
2
3 À 4
ffiffi ffi
3
p
Kb 1 b 3
g yy ¼ 2b
2
3 þ 2b
2
2 À 2b
2
1 À 4
ffiffi ffi
3
p
Kb 2 b 3
K ¼ 1
The unpaired electron results to be located in the d
2
z orbital and the contribution
of this orbital increases with the base coordination, while those of d xz and d yz
decrease. The g zz value increases as the axial coordination becomes stronger. This
electronic asset is responsible for stabilizing [CoPc(4Mepy) O 2 ]
− where the
unpaired electron is located on O 2 . The experimental proof comes from the ESR
spectrum, reported in Fig. 5.8 which shows g xx = g yy = 2.01 and g zz = 2.00, very
different from the magnetic tensor values of the CoPc adducts with base molecules.
The values are diagnostic of an unpaired electron strongly located onto the
superoxide radical anion, and the ESR investigation strongly answers the doubt
expressed by Malatesta.
Fig. 5.8 ESR spectrum of
[CoPc(4Mepy)O 2 ]
− in the
center of the field range. The
resonances are centered at
about 3000 G. At lower field
the resonances of [CoPc
(4Mepy)]
5.8 Discussion of the Case
95