Studies on cobalt(II), nickel(II) and copper(II) derivatives of some new meso-aryl substituted octabromoporphyrins

Article abstract of DOI:10.1016/j.poly.2005.01.008

A series of Co(II), Ni(II) and Cu(II) derivatives of a variety of porphyrins with tolyl (H₂TxTP) and naphthyl (H₂NxTP) moieties as meso-substituents are synthesised and characterised. H ₂TxTP has the tolyl functions bonded to meso-carbons at the o-, m- or p-positions of the tolyl moieties while H₂NxTP has naphthyl groups bonded through their α- or β-positions. Their octabromo derivatives (MTxOBP and MNxOBP) were also synthesised by electrophilic substitution at the β-pyrrole positions. Optical spectra showed that the Q bands remain almost unaltered for both tolyl and naphthyl derivatives of the nonbrominated porphyrins while the B bands of their naphthyl derivatives are seen to be more red-shifted than their tolyl analogues. Even though a similar trend based on the meso-substituents is observed among all MTxOBP and MNxOBP derivatives, a substantial red-shift is seen for all the bromoderivatives compared to their nonbrominated species MTxTP and MNxTP. The energy difference Δν? associated with this red-shift of the Soret band is seen to be in the range 1700-2050 cm^-1 for Co(II) and Ni(II) derivatives, the naphthyl derivatives always having higher values. Energy level reordering due to electron-withdrawing Br-substituents, distortion bringing about nonplanarity in the π-framework resulting in HOMO-LUMO level changes and the mesomeric effect due to meso-aryl moieties can be considered to be the reasons for the observed spectral changes. The EPR spectra of the Cu(II) derivatives of bromoporphyrins have lower A_∥^Cu and A_⊥^Cu hyperfine values compared to their nonbrominated analogues, indicating enhanced delocalisation of electron spin to the octabromoporphyrinato moiety. An enhanced Cu-N σ-covalency is seen in the Cu(II)-bromoporphyrins, as evident from their lower α² values.

Full text of DOI:10.1016/j.poly.2005.01.008

Polyhedron 24 (2005) 679–684
www.elsevier.com/locate/poly
Studies on cobalt(II), nickel(II) and copper(II) derivatives of
some new meso-aryl substituted octabromoporphyrins
*
Regimol G. George, M. Padmanabhan
School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills, Kottayam 686560, Kerala, India
Received 13 November 2003; accepted 24 January 2005
Abstract
A series of Co(II), Ni(II) and Cu(II) derivatives of a variety of porphyrins with tolyl (H2TxTP) and naphthyl (H2NxTP) moieties
as meso-substituents are synthesised and characterised. H2TxTP has the tolyl functions bonded to meso-carbons at the o-, m- or
p-positions of the tolyl moieties while H2NxTP has naphthyl groups bonded through their a- or b-positions. Their octabromo deriv-
atives (MTxOBP and MNxOBP) were also synthesised by electrophilic substitution at the b-pyrrole positions. Optical spectra
showed that the Q bands remain almost unaltered for both tolyl and naphthyl derivatives of the nonbrominated porphyrins while
the B bands of their naphthyl derivatives are seen to be more red-shifted than their tolyl analogues. Even though a similar trend
based on the meso-substituents is observed among all MTxOBP and MNxOBP derivatives, a substantial red-shift is seen for all
ꢀ
the bromoderivatives compared to their nonbrominated species MTxTP and MNxTP. The energy difference Dm associated with this
red-shift of the Soret band is seen to be in the range 1700–2050 cmꢀ1 for Co(II) and Ni(II) derivatives, the naphthyl derivatives
always having higher values. Energy level reordering due to electron-withdrawing Br-substituents, distortion bringing about non-
planarity in the p-framework resulting in HOMO–LUMO level changes and the mesomeric effect due to meso-aryl moieties can
be considered to be the reasons for the observed spectral changes. The EPR spectra of the Cu(II) derivatives of bromoporphyrins
have lower ACk u and AC?u hyperfine values compared to their nonbrominated analogues, indicating enhanced delocalisation of elec-
tron spin to the octabromoporphyrinato moiety. An enhanced Cu–N r-covalency is seen in the Cu(II)–bromoporphyrins, as evident
from their lower a2 values.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Keywords: Cu(II) porphyrins; Co(II) porphyrins; Ni(II) porphyrins; Octabromoporphyrins; Electronic properties; EPR studies; Tetratolylporphy-
rins; Tetranaphthylporphyrins
1. Introduction
The fact that there are twelve substitutional sites on
the periphery and a highly tunable p framework which
can act as an electron sink makes electronic and redox
modulation of the porphyrin macrocycle an interesting
and fruitful exercise leading to products of varying
chemical, optical and photophysical properties [7–12].
We have been interested in developing electronically
and structurally modified porphyrins, especially on solid
polymer matrices, and have demonstrated their ability
to carry out a few enzyme mimetic reactions efficiently
[13–18]. Knowing the ability of Br substitution to bring
Synthesis of varyingly substituted porphyrins and
their metalloderivatives continues to be an active area
of research owing to their wide applications as versatile
catalysts, enzyme mimics, nonlinear optical materials
and as candidates for photodynamic therapy [1–6].
*
Corresponding author. Tel.: +91 481 273 1036; fax: +91 481 273
1009.
E-mail address: padman_mgu@yahoo.co.in (M. Padmanabhan).
0277-5387/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.01.008

680
R.G. George, M. Padmanabhan / Polyhedron 24 (2005) 679–684
about interesting electronic, redox and structural modi-
fication of aryl porphyrins, we have synthesised and
studied some of the interesting properties of a series of
octabromoporphyrins and also their Fe(III) and
Mn(III) derivatives [19–21]. The studies on such perbro-
minated porphyrins are of great relevance in the context
of growing interest on nonplanar porphyrins which are
generated either by substitution of the b-pyrrole posi-
tions by bulky/electron withdrawing substituents or by
incorporating bulky groups at the meso-positions. Real-
ising the biological significance of such distorted por-
phyrins is one of the major concerns of experimental
and theoretical chemists alike with regard to tracing
the origin of their red-shifts [22–25]. In this work we re-
port the synthesis and characterisation of Co(II), Ni(II)
and Cu(II) derivatives of some meso-tolyl and meso-
naphthyl porphyrins and also their octabromo deriva-
tives and discuss salient features of their spectral
properties.
2.1.2. Cobalt(II), nickel(II) and copper(II) complexes of
H2TxTP and H2NxTP
The metallation of the nonbrominated porphyrins
H2TxTP and H2NxTP was carried out by a known pro-
cedure. The free-base tolylporphyrins H2TxTP (0.5 g,
0.74 mmol) or free-base naphthylporphyrins H2NxTP
(0.5 g, 0.61 mmol) were dissolved in a suitable solvent
mixture (CHCl3:CH3OH, 40 mL, 1:1, v/v). To this, the
metal carrier CoCl2 Æ 6H2O (1.6 g, 7 mmol), Cu(CH3-
COO)2 Æ 4H2O (1.4 g, 7 mmol) or NiCl2 Æ 6H2O (1.7 g,
7 mmol) dissolved in glacial CH3COOH (1.5 mL) was
added for the synthesis of the respective metalloderiva-
tives. NaOAc was employed as a base. On refluxing
the reaction mixture on a water bath, the metallation
gets initiated as evident from the UV–Vis spectral mea-
surements. The metallation usually gets completed with-
in 4 h for Co(II) derivatives, 6–8 h for Ni(II) derivatives
and 2 h for Cu(II) derivatives. The reaction mixture was
washed repeatedly with water to remove the unreacted
metal salts and the metalloporphyrins formed were puri-
fied by column chromatography on basic alumina using
CHCl3 as eluent and were further purified by recrystal-
lisation using hexane/CHCl3 mixture. Metallation was
confirmed by UV–Vis spectroscopy. Yields were in the
range of 80–85% for Co(II), 70–75% for Ni(II) and
85–90% for Cu(II) complexes.
2. Experimental
Pyrrole (Fluka) was distilled over KOH pellets, under
reduced pressure before use. The a-naphthaldehyde and
b-naphthaldehyde (E. Merck) were used as such. All the
solvents employed in the present study were of spectral
grade and were distilled before use. Liquid bromine pro-
cured from Ranbaxy and basic alumina obtained from
Acmes were used as received. The metal salts (E. Merck)
were also used without further purification. The UV–Vis
spectral measurements were carried out with a Shima-
dzu 160A UV–Vis spectrophotometer. Elemental analy-
ses were done using a Perkin–Elmer CHN analyzer. The
EPR Spectra were measured on a Varian E-109 X-band
spectrometer with the micropower set at 5 mW and with
a modulation frequency of 100 K. The solution of cop-
per(II) porphyrins was made in a toluene–methanol
mixture with a concentration ca. 10ꢀ4 M. The ꢀgꢁ marker
employed was TCNE radical.
2.1.3. Cobalt(II), nickel(II) and copper(II) complexes of
H2TxOBP and H2NxOBP
The metallation of all the bromoporphyrins could be
carried out at room temperature. To a solution of the
metal carrier of CoCl2 Æ 6H2O (2 g, 8.75 mmol), Ni-
Cl2 Æ 6H2O (2.12 g, 8.75 mmol) or Cu(CH3COO)2 Æ
4H2O (1.75 g, 8.75 mmol) in CHCl3:MeOH mixture
(60 mL, 2:1, v/v) was added H2TxOBP (0.60 g,
0.46 mmol) or H2NxOBP (0.66 g, 0.46 mmol), 2 mL of
glacial MeCO2H and 1 g of powdered NaOAc. The
reaction mixture was stirred either at room temperature
or on a water bath for about 1–2 h. The completion of
the metallation reaction was monitored by electronic
spectroscopy. The mixture was then taken in a separat-
ing funnel, washed repeatedly with water after which the
organic phase was separated and dried over anhydrous
Na2SO4. The compound was purified by column chro-
matography (basic Al2O3, CHCl3 eluent) to obtain the
metalloderivative and then by recrystallisation using
hexane/CHCl3 mixture (yield 95%).
2.1. Preparation
2.1.1. Free-base porphyrins H2TxTP, H2NxTP,
H2TxOBP and H2NxOBP
The free-bases of tetratolyl (H2TxTP) and tetranaph-
thyl porphyrins (H2NxTP) including those reported ear-
lier [19] were synthesised by Adlerꢁs method involving
the condensation between pyrrole and the respective
aldehydes in propionic acid medium [26]. Their bromi-
nated free-bases, tolyl (H2TxOBP) and naphthyl por-
phyrins (H2NxOBP), were prepared by brominating
the Cu(II) derivatives of the respective porphyrins
H2TxTP and H2NxTP, based on the reported procedure
[19,27]. The yields of the bromoderivatives were in the
range 75–80%.
3. Results and discussion
The free-base porphyrins, which are purified by care-
ful column chromatography, are characterised by chem-
1
ical analysis, electronic and H NMR studies [19–21].
1
All the free-base porphyrins gave reasonably good H
NMR signals as reported by us earlier [19]. The effect

R.G. George, M. Padmanabhan / Polyhedron 24 (2005) 679–684
681
due to various aryl groups at the meso-positions and
also due to the Br substitution at the eight b-pyrrole
positions on the optical absorption, fluorescence emis-
sion and redox potential of the free-base porphyrins
has been reported by us recently [19,20].
As mentioned in the preparatory section all the
metalloporphyrins were purified by column chromatog-
raphy. The compounds were characterised by electronic
and EPR studies. The various Co(II), Ni(II) and Cu(II)
derivatives of both the nonbrominated and brominated
porphyrins prepared are summarised in Table 1, wherein
the substituents indicated in (1) are specified.
It is seen that among the nonbrominated porphyrins,
the naphthyl derivatives undergo easier metallation than
the tolyl derivatives. This could be attributed to the pos-
sible distortional change brought about on the porphy-
rin ring in H2NxTP by the bulkier meso-naphthyl
groups as compared to the tolyl porphyrins. The metal-
lation of all the bromoporphyrins are also found to be
very facile as it could be carried out at room tempera-
ture. It is known that all perbrominated porphyrins tend
to exist in a highly distorted form because of the pres-
ence of bulky Br atoms at the eight b-pyrrole positions
[27,28]. Since the mechanism of metallation involved
has been proposed to be following a ‘‘sitting atop mech-
anism,’’ such a distortion also can be expected to facili-
tate metallation processes. Further, the electron
withdrawing effect due to the eight Br-atoms at the
b-pyrrole positions can be expected to help substantially
the deprotonation step of the –NH protons, augmenting
the metallation. Compared to cobalt(II) and nickel(II)
derivatives, the copper(II) metalloporphyrins are seen
to be easily formed.
R
X
X
X
X
N
N
N
R
R
M
N
X
X
X
X
R
(1)
Table 1
The Co(II), Ni(II) and Cu(II) derivatives of porphyrins developed in the study
R
X
M
Porphyrin
Analysis (%)a
C
H
N
H
Co
Co
Ni
CoNaTP
CoNaOBP
NiNaTP
NiNaOBP
CuNaTP
CuNaOBP
83.10(82.50)
47.20(47.90)
82.76(82.60)
48.20(47.90)
82.18(82.14)
47.77(47.75)
4.19(4.12)
1.90(1.86)
4.02(4.13)
1.74(1.86)
4.06(4.10)
1.82(1.85)
6.49(6.40)
3.40(3.70)
6.32(6.42)
3.60(3.72)
6.36(6.38)
3.78(3.70)
Br
H
Br
H
Ni
Cu
Cu
Br
H
Co
Co
Ni
CoNbTP
CoNbOBP
NiNbTP
NiNbOBP
CuNbTP
CuNbOBP
82.62(82.50)
47.45(47.90)
82.71(82.60)
47.50(47.90)
82.20(82.14)
47.92(47.75)
4.24(4.12)
1.84(1.86)
4.18(4.13)
1.99(1.86)
4.11(4.10)
1.77(1.85)
6.50(6.40)
3.68(3.70)
6.38(6.42)
3.84(3.72)
6.30(6.38)
3.74(3.70)
Br
H
Br
H
Ni
Cu
Cu
Br
H
Co
Co
Ni
CoToTP
CoToOBP
NiToTP
79.32(79.20)
41.50(42.38)
79.10(79.22)
42.48(42.39)
78.75(78.70)
42.19(42.24)
4.87(4.95)
2.12(2.06)
5.06(4.95)
2.01(2.06)
4.86(4.91)
2.10(2.05)
7.63(7.70)
4.42(4.12)
7.76(7.70)
4.24(4.12)
7.60(7.65)
4.14(4.10)
Br
H
Br
H
Ni
NiToOBP
CuToTP
CuToOBP
Cu
Cu
Br
H
Co
Co
Ni
CoTmTP
CoTmOBP
NiTmTP
79.14(79.20)
41.85(42.38)
79.26(79.22)
42.40(42.39)
78.62(78.70)
42.31(42.24)
4.92(4.95)
2.18(2.06)
4.98(4.95)
2.03(2.06)
4.96(4.91)
2.08(2.05)
7.80(7.70)
4.24(4.12)
7.74(7.70)
4.04(4.12)
7.64(7.65)
4.13(4.10)
Br
H
Br
H
Ni
NiTmOBP
CuTmTP
CuTmOBP
Cu
Cu
Br
H
Co
Co
Ni
CoTpTP
CoTpOBP
NiTpTP
79.32(79.20)
42.01(42.38)
79.07(79.22)
42.33(42.39)
78.79(78.70)
42.04(42.24)
4.87(4.95)
2.36(2.06)
5.05(4.95)
2.12(2.06)
4.76(4.91)
2.17(2.05)
7.63(7.70)
4.10(4.12)
7.78(7.70)
4.26(4.12)
7.69(7.65)
4.13(4.10)
Br
H
Br
H
Ni
NiTpOBP
CuTpTP
CuTpOBP
Cu
Cu
Br
a
Calculated values are given in parentheses.

682
R.G. George, M. Padmanabhan / Polyhedron 24 (2005) 679–684
3.1. Electronic spectra
for them is seen to be in the range 1915–2130 cmꢀ1
,
the naphthyl species having larger differences than the
tolyl derivatives. As in the case of cobalt(II) derivatives,
the nickel(II) species of all the brominated porphyrins
also show red-shifted B and Q bands, which is evident
The electronic spectra of the metal complexes were
recorded in CHCl3 at 298 K. The data are presented in
Table 2. Some of the salient features associated with
the electronic spectra are given below.
ꢀ
from Table 2. The Dm for the B band for these complexes
The Co(II), Ni(II) and Cu(II) porphyrins of the non-
brominated forms of the tetratolyl and tetranaphthyl
porphyrins show characteristic absorption peaks of typ-
ical meso-aryl derivatives. We see that the B band of the
naphthyl derivatives in all their metalloderivatives have
absorptions at higher wavelength compared to their to-
lyl derivatives. However, we do not observe any percep-
tible difference in the absorption maxima for both B and
Q bands among the various tolyl derivatives and also
within the naphthyl species.
was found to be between 1770 and 1940 cmꢀ1 which is
comparatively lower than that for their Co(II)
derivatives.
In the case of Cu(II) bromoporphyrins, the expected
red-shifts are also seen for both B and Q bands, but the
significant feature, however, is the appearance of two
overlapping peaks centered ca. 445 and 470 nm for the
B band of the Cu(II) complexes of brominated porphy-
rins compared to their nonbrominated species. This phe-
nomenon is not manifested by their analogous Ni(II)
and Co(II) derivatives. In all the three metal derivatives,
The cobalt(II) complexes of the brominated porphy-
rins manifest significant changes compared to their non-
brominated species. Both B and Q bands for the
complexes are seen to be red-shifted compared to their
ꢀ
Dm is seen to be the lowest for the ortho-tolyl porphyrins
and the highest for their naphthyl derivatives. The
underlying factors involved in the above spectral fea-
tures could be summarised as below.
ꢀ
nonbrominated derivatives. The red-shift Dm was calcu-
lated as the energy difference of the Soret band in wave-
numbers between the complexes of brominated and
The fact that the naphthyl porphyrins have a greater
red-shifted Soret band as compared to the tolyl deriva-
tives in all their Co(II), Cu(II) and Ni(II) complexes
ꢀ
nonbrominated porphyrins. The energy difference Dm
Table 2
Electronic spectral data of Co(II), Ni(II) and Cu(II) derivatives of brominated and nonbrominated porphyrins in CHCl3 at 298 K
b
B band (nm) (e · 10ꢀ4 a
)
Q band (nm) (e · 10ꢀ4 a
)
Þ
ꢀ1
ꢀ
Dm ðcm
Porphyrin
CoToTP
CoTmTP
CoTpTP
CoNaTP
CoNbTP
CuToTP
CuTmTP
CuTpTP
CuNaTP
CuNbTP
NiToTP
409(19.95)
412(21.38)
528(2.69)
530(2.81)
412(20.89)
414(20.42)
530(2.81)
530(2.63)
416(20.89)
414(23.99)
531(2.75)
537(2.39)
415(24.55)
414(25.12)
538(2.51)
540(2.51)
419(23.44)
420(24.55)
541(2.39)
542(2.34)
415(16.22)
416(16.98)
527(1.99)
528(2.04)
NiTmTP
NiTpTP
417(17.38)
419(16.60)
530(2.08)
528(2.06)
NiNaTP
NiNbTP
CoToOBP
CoTmOBP
CoTpOBP
CoNaOBP
CoNbOBP
CuToOBP
CuTmOBP
CuTpOBP
CuNaOBP
CuNbOBP
NiToOBP
NiTmOBP
NiTpOBP
NiNaOBP
NiNbOBP
a
420(16.98)
445(16.60)
531(2.08)
560(1.28)
1918
1950
2050
2128
2012
448(16.98)
450(17.38)
562(1.35)
564(1.35)
454(16.98)
454(16.98)
565(1.31)
566(1.35)
446(13.18), 462(sh)
445(sh), 468(13.49)
448(sh), 469(13.80)
458(sh), 473(13.49)
467(13.50) (broad)
448(24.55)
579(1.73) 624(sh)
581(1.77) 625(sh)
582(1.86) 628(sh)
583(1.82) 627(sh)
580(1.77) 627(sh)
560(1.90)
1620, 2510
1725, 2729
1833, 2833
2032, 2725
2340
1770
1920
451(25.70)
451(26.30)
561(1.95)
561(1.95)
1810
1940
456(25.12)
455(26.30)
563(1.99)
563(1.95)
1800
Molar extinction coefficient e in 10ꢀ4/dm3 molꢀ1 cmꢀ1
.
b
Dꢀm ¼ ꢀm2 ꢀ ꢀm1, where ꢀm1 is the energy of the B band for brominated porphyrin and ꢀm2 that for the corresponding nonbrominated porphyrin.

R.G. George, M. Padmanabhan / Polyhedron 24 (2005) 679–684
683
for both brominated and nonbrominated forms could be
attributed to the higher mesomeric participation of the
naphthyl moieties with the macrocyclic p-framework
by adjusting the porphyrin-aryl dihedral angle. Because
of a less extended p-system, the tolyl groups will have
lesser conjugative participation with the porphyrin
p-framework than their naphthyl analogues. The fact
that all the metalloderivatives of o-tolyl porphyrins in
their brominated and nonbrominated forms have their
B and Q bands absorbing at lower wavelength could
be attributed to the inability of the o-tolyl groups to tilt
in order to reduce the dihedral angle for better conjuga-
tive interaction because of higher steric strain caused by
–CH3 groups at the ortho-position of the meso-tolyl
moieties.
The pronounced red-shift observed for the Co(II),
Ni(II) and Cu(II) derivatives of all the brominated por-
phyrins compared to their nonbrominated derivatives
can be explained in terms of the electronic change caused
by the electron-withdrawing Br-groups coupled with dis-
tortion of the porphyrin framework brought about by
the bulky Br-atoms at the eight pyrrole b-positions
[19,27,28]. The trend in the red-shift of B band observed
for tolyl and naphthyl derivatives of the Br-substituted
porphyrins is similar for their Co(II), Ni(II) and Cu(II)
complexes and could be explained by considering the
various factors which influence the energies of both the
HOMO and LUMOs of the metalloporphyrins. It is evi-
dent that the eight Br substituents at the b-pyrrole posi-
tions would drain out electron density from the
porphyrin – framework making the macrocycle more
susceptible to reduction [19,27]. The concerted action
by the electronegative Br atoms would also make the sys-
tem resistant to oxidation. These would result in lower-
ing of both LUMOs and the HOMO levels. One of the
major factors for the red-shift of the porphyrins (in both
B and Q) is essentially this electronic effect. Coupled with
this electronic modulation there is also a significant effect
due to a steric factor. The bulky Br-atoms at the eight
pyrrole b-positions would bring about major structural
distortion within the porphyrin framework to ease steric
strain. Such a distortion would cause a significant pertur-
bation in conjugation, the effect of which would be signif-
icant alteration of p-energy levels including
destabilisation of the HOMO and some lowering of the
LUMOs. INDO level calculation has testified such a
reordering in the energy levels of the frontier orbitals
on distortion [29]. Because of the lowered symmetry
the degenerate eꢁg orbitals (LUMOs) can be expected to
be split, lowering the energy of one component. A sub-
stantial reduction in the HOMO–LUMO gap is, there-
fore, expected because of such a geometry variation.
The major red-shift observed on bromination can be
attributed essentially to these factors.
It is known that while Co(II) and Ni(II) porphyrins
show affinity to Lewis-bases and ligating solvents and
coordinate them at the axial positions of the metal cen-
tre, while Cu(II) porphyrins exhibit no such tendency
and remain unligated. We have verified this with Le-
wis-bases like pyridine and imidazole for all the MTxTP
and MNxTP of nonbrominated porphyrins. We were,
however, interested in knowing whether Cu(II) deriva-
tives of bromoporphyrins would have any affinity to-
wards Lewis-bases because of the strong electron
accepting ability of the bromoporphyrinato framework
(compared to the nonbrominated porphyrins). While
Co(II) and Ni(II) derivatives of the bromoporphyrins
were found to show enhanced affinity towards Lewis-
bases, their Cu(II) derivatives were still resistant to
any adduct formation, as evident from practically no
change seen in the electronic spectra even in an excess
of base.
3.2. EPR spectra
The EPR spectra of all the Cu(II) porphyrins were
measured at the X-band frequency to understand both
the electronic and bonding features between the para-
magnetic Cu(II) and the porphyrinato moiety. The spec-
tra were recorded in a toluene–methanol mixture at
liquid nitrogen temperature. While two hyperfine peaks
of the parallel components of the anisotropic spectra are
clearly discernible in the low field, the other two are seen
overlapped in the perpendicular component. The spin
Table 3
EPR spin-Hamiltonian parameters of copper(II) derivatives of brominated and nonbrominated porphyrins
Porphyrin
gi
g^
ACk u ðmTÞ
ANk ðmTÞ
1.5
AC?u ðmTÞ
AN? ðmTÞ
a2
CuToTP
CuTmTP
CuTpTP
CuNaTP
CuNbTP
CuToOBP
CuTmOBP
CuTpOBP
CuNaOBP
CuNbOBP
a
2.21
2.21
2.20
2.20
2.21
2.21
2.22
2.20
2.22
2.21
2.07
2.07
2.07
2.07
2.07
2.05
2.06
2.07
2.05
2.05
20.3
20.0
19.8
20.0
20.0
19.1
19.0
19.3
19.1
19.1
3.3
3.3
3.0
3.5
3.3
3.1
3.0
3.0
3.1
3.1
1.47
1.48
1.48
1.47
1.48
1.50
1.51
1.52
1.51
1.51
0.564
0.555
0.550
0.555
0.555
0.532
0.528
0.535
0.535
0.532
1.5
1.5
1.5
1.5
n.ra
n.ra
n.ra
n.ra
n.ra
Not resolvable.

684
R.G. George, M. Padmanabhan / Polyhedron 24 (2005) 679–684
Hamiltonian parameters for the copper(II) compounds
were extracted from the spectra by a known procedure
[30], by considering the axial symmetry of the metallo-
porphyrins. The value of a2 (r-covalency factor) was
also evaluated from the EPR parameters using the
Kivelson and Neiman equation [31], a2 = (Ai/P +
(gi ꢀ 2.0023) + 3/7(g^ ꢀ 2.0023) + 0.04), where P is the
spin–orbit coupling factor. It is known that metallopor-
phyrins are sensitive to the molecular environment
around them. This has been well demonstrated by the
difference in electronic spectra observed for these com-
plexes. The EPR data also yield similar results, as exem-
plified by the variations in the spin Hamiltonian
parameters for Cu(II) in different molecular environ-
ments. The various parameters evaluated for the
Cu(II)-porphyrins are presented in Table 3.
0.528 indicating more r-covalency between copper
and nitrogen facilitated by the electron-withdrawing
Br substituents, which are in direct interaction with
the porphyrin p-system.
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