Axial coordination reactions with nitrogenous bases and determination of equilibrium constants for zinc tetraarylporphyrins containing four β, β ′-fused butano and benzo groups in nonaqueous media

Article abstract of DOI:10.1142/S1088424619500135

The axial coordination properties of six zinc tetraarylporphyrins with seven different nitrogenous bases were examined in CH₂Cl₂ for derivatives containing four β,β′-fused butano or benzo groups and the equilibrium constants (logK) determined using spectral titration methods. The examined compounds are represented as butano(YPh)₄PorZn and benzo(YPh)₄PorZn, where Por is the porphyrin dianion and Y is a CH₃, H or Cl substituent on the para-position of each meso-phenyl ring of the macrocycle. The initial four-coordinate butano-And benzoporphyrins will axially bind one nitrogenous base to form five-coordinate derivatives in CH₂Cl₂ and this leads to a 4-22 nm red-shift of the Soret and Q bands. The logK values range from 1.98 to 4.69 for butano(YPh)₄PorZn and from 3.42 to 5.36 for benzo(YPh)₄PorZn, with the exact value depending upon the meso and β-substituents of the porphyrin and the conjugate acid dissociation constants (pKa) of the nitrogenous base.

Full text of DOI:10.1142/S1088424619500135

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2019; 23: 1–10
DOI: 10.1142/S1088424619500135
Published at http://www.worldscinet.com/jpp/
Axial coordination reactions with nitrogenous bases
and determination of equilibrium constants for zinc
tetraarylporphyrins containing four b,b′-fused butano
and benzo groups in nonaqueous media
LinaYea,b, Yuanyuan Fang*b, Zhongping Ouc◊, Liping Wangb,c, Songlin Xueb,
Yang Lua and Karl M. Kadish*c◊
aCollege of Computer, Jilin Normal University, Siping 136000, P. R. China
bSchool of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P. R. China
cDepartment of Chemistry, University of Houston, Houston, Texas 77204-5003, USA
This paper is part of the 2019 Women in Porphyrin Science special issue.
Received 10 December 2018
Accepted 23 January 2019
ABSTRACT: The axial coordination properties of six zinc tetraarylporphyrins with seven different
nitrogenous bases were examined in CH2Cl2 for derivatives containing four b,b′-fused butano or benzo
groups and the equilibrium constants (logK) determined using spectral titration methods. The examined
compounds are represented as butano(YPh)4PorZn and benzo(YPh)4PorZn, where Por is the porphyrin
dianion and Y is a CH3, H or Cl substituent on the para-position of each meso-phenyl ring of the
macrocycle. The initial four-coordinate butano- and benzoporphyrins will axially bind one nitrogenous
base to form five-coordinate derivatives in CH2Cl2 and this leads to a 4–22 nm red-shift of the Soret and
Q bands. The logK values range from 1.98 to 4.69 for butano(YPh)4PorZn and from 3.42 to 5.36 for
benzo(YPh)4PorZn, with the exact value depending upon the meso and b-substituents of the porphyrin
and the conjugate acid dissociation constants (pKa) of the nitrogenous base.
KEYWORDS: zinc butanoporphyrins, zinc benzoporphyrins, axial coordination reactions, nitrogenous
base, equilibrium constants.
The binding of a nitrogenous base to zinc porphyrins
INTRODUCTION
has been widely used for molecular recognition as
Synthetic zinc porphyrins are one of the most studied
well as for the synthesis of new types of porphyrin
metalloporphyrin derivatives, in part because they
chelation and nanostructures [24–34]. The monomeric
have wide potential applications in the fields of solar
zinc porphyrins can bind only a single axial ligand to
cells [1–5], nanomaterials [6, 7] and supramolecular
form five-coordinate complexes in nonaqueous media
chemistry [8–13]. Among the examined compounds
[35–51] and a number of zinc porphyrins with different
are the zinc tetrabenzoporphyrins which possess an
macrocyclic structures have been examined as to their
extended conjugated system [14–19] and have attracted
coordination reactions with different nitrogenous
particular attention due to their unique UV-vis spectral
bases. Numerous equilibrium constants for these ligand
and electrochemical properties [20–23] as compared to
binding reactions were determined by electrochemical
the non-b,b′-pyrrole fused derivatives.
and/or spectral methods [40–43, 45–51]. However, the
axial coordination reactions of zinc tetraarylporphyrins
containing four b,b′-fused butano or benzo groups with
◊ SPP full member in good standing
nitrogenous bases have not to date been reported and it
was not known how the butano and benzo groups of the
*Correspondence to: Yuanyuan Fang, email: 170173915@
qq.com; Karl M. Kadish, email: kkadish@uh.edu.
Copyright © 2019 World Scientific Publishing Company

2
L. YE ET AL.
porphyrin would affect the axial coordination reactions.
This is addressed in the present work, where six zinc
tetrabutano- and tetrabenzotetraarylporphyrins (Chart 1)
were examined as to their coordination properties with
seven different nitrogenous bases (Chart 2).The examined
porphyrins are represented as butano(YPh)4PorZn and
benzo(YPh)4PorZn, where Por is the porphyrin dianion
and Y is a CH3, H or Cl substituent on the para-position
of each meso-phenyl ring of the macrocycle. The utilized
nitrogenous bases (L) were 3-chloropyridine (3-CP),
3-bromopyridine (3-BP), 3-acetylpyridine (3-AP), pyridine
(Py), 4-picoline (4-Pic), piperidine (Pip) and 2-picoline
(2-Pic). UV-vis spectral changes were monitored during a
titration of the porphyrin in CH2Cl2 with each nitrogenous
base. The equilibrium constants (logK) of the reactions
were determined using standard equations described in
the literature [52] and the relevant complexation reactions
are shown as in Equations 1 and 2. Relationships between
equilibrium constants and the conjugate acid dissociation
constants (pKa) of the utilized nitrogenous base are
examined and the effects of butano and benzo substituents
of the porphyrin on the logK values are discussed, with
comparisons to the zinc porphyrins lacking b,b′-fused
substituents.
Chemicals
Absolute dichloromethane (CH2Cl2) and the nitro-
genous bases in Chart 2 were purchased from Aldrich or
Sigma-Aldrich and used as received. Zinc tetrabutano- and
tetrabenzotetraarylporphyrins were synthesized according
to a procedure in the literature [53].
Determination of equilibrium constants
A series of CH2Cl2 solutions containing different
concentrations of each nitrogenous base was prepared
and used as a standard titrant. Microliter quantities of
the above standard solutions were added gradually to a
specially constructed UV-vis spectral cell (path length =
1.0 cm) containing the metalloporphyrin in 6.0 ml CH2Cl2.
After each addition, the solution was thoroughly mixed
and the spectrum was recorded. The changes in spectra
were analyzed as a function of the concentration of
added nitrogenous base and the Hill equation was used to
calculate equilibrium constants (logK) as described in the
literature [52, 54]. All UV-vis spectral measurements were
carried out at room temperature (298 K).
RESULTS AND DISCUSSION
log K
ꢀꢀꢀꢀꢁ
ꢂꢀꢀꢀꢀ
butano(YPh) PorZn + L
butano(YPh) PorZn(L)
(1)
(2)
4
4
Determination of logK
Spectral titration methods were utilized to characterize
axial coordination reactions of the Zn(II) porphyrin
with each nitrogenous base. Examples of the UV-vis
spectral changes obtained during the titration are shown
in Fig. 1 for titrations of butano(ClPh)4PorZn 1c with
3-bromopyridine (3-BP) and piperidine (Pip) in CH2Cl2.
As the reaction of 1c with 3-BP proceeds (Fig. 1a), the
Soret band red-shifts from 437 to 453 nm and the 565 nm
band decreases in intensity as a new band grows in at
581 nm. Similar spectral changes are seen during the
log K
ꢀꢀꢀꢀꢁ
ꢂꢀꢀꢀꢀ
benzo(YPh) PorZn + L
benzo(YPh) PorZn(L)
4
4
EXPERIMENTAL
Instrumentation
UV-vis spectra were recorded with a Hewlett-Packard
model 8453 diode array spectrophotometer.
(a)
(b)
Chart 1. Structures of examined zinc tetrabutano- and tetrabenzotetraarylporphyrins
Copyright © 2019 World Scientific Publishing Company
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AXIAL COORDINATION REACTIONS WITH NITROGENOUS BASES AND DETERMINATION OF EQUILIBRIUM CONSTANTS
3
(a)
Chart 2. Structures of utilized nitrogenous bases
reaction of 1c with Pip as shown in Fig. 1b. The measured
(b)
spectral changes were analyzed as a function of the 3-BP
and Pip concentration at each step of the titration, thus
enabling calculation of logK for the axial coordination
reaction using the Hill equation.
For example, a diagnostic analysis plot of the measu-
red absorption at 453 nm is shown in the inset of Fig. 1a
where a plot of log[(Ai–Ao)/(Af–Ai)] vs. log[3-BP] is
linear, with a slope of 1.0, indicating that a single base
molecule is added to the four coordinate porphyrin to
give the five-coordinate butano(ClPh)4PorZn(3-BP) as
the final product. An equilibrium constant of logK =
3.09 is then calculated from the zero intercept of this
plot. Similar spectral changes were obtained during the
titration of 1c with piperidine, giving logK = 4.69 for
Fig. 1. UV-vis spectral changes of butano(ClPh)4PorZn 1c during
the formation of butano(ClPh)4PorZn(Pip) in CH2Cl2
the titration reactions with (a) 3-bromopyridine and (b) piperidine
(Fig. 1b). Similar changes in the UV-vis spectra were
in CH2Cl2 (the inset Hill plot used to calculate the number of axial
observed for the other butanoporphyrins (1a and 1b)
ligand and logK)
when the seven nitrogenous bases were used as titrant
in the CH2Cl2 solution. The calculated logK values are
given in Table 1.
is given in Table 2. As seen in the table, the Soret and
Q bands of the five-coordinate butanoporphyrins are
red-shifted by 12–22 nm as compared to the porphyrins
lacking an axial ligand. A red-shift (4–16 nm) is seen for
absorption maxima of the five-coordinate porphyrins,
benzo(YPh)4PorZn(L) as compared to the starting four-
coordinate derivative, benzo(YPh)4PorZn.
Figure 2 shows examples of the spectral changes
obtained during a titration of benzo(ClPh)4PorZn 2c with
3-AP and 4-Pic. In each case, the final porphyrin products
of the titration are characterized by red-shifted Soret
and Q bands as compared to the spectrum of the initial
porphyrin in CH2Cl2. These types of spectral changes are
typical for formation of five-coordinate Zn(II) porphyrins
in nonaqueous media [41, 45–49, 54]. The slope of the
log[(Ai–Ao)/(Af–Ai)] vs. the log[3-AP] and log[4-Pic]
was 1.0 for both plots, confirming formation of the
five-coordinate porphyrins, benzo(ClPh)4PorZn(3-AP)
with logK = 4.51 and benzo(ClPh)4PorZn(4-Pic) with
logK = 5.19.
It was expected that the final UV-vis spectrum of each
five-coordinate porphyrin should vary with the porphyrin
structure but be independent of the nitrogenous base
used in the titration. This is the case as shown by spectra
in the dashed lines of Fig. 3 for butano(CH3Ph)4PorZn
and Fig. 4 for benzo(CH3Ph)4PorZn. A summary of the
spectral absorption maxima of the five-coordinate deriva-
tives, butano(YPh)4PorZn(L) and benzo(YPh)4PorZn(L),
Effect of the pKa on logK
It has long been known that the magnitude of equili-
brium constants for the axial coordination reactions of
zinc porphyrins with nitrogenous bases depends upon
their pKa values. Generally, for a given porphyrin, the
order of logK is consistent with that of pKa values,
i.e. the larger the pKa value of the base, the larger the
logK for the ligand binding reaction [49, 50]. Linear
correlations are seen between the logK and pKa values
of the bases used in the current study as shown in Figs 5
and 6 for the currently investigated butanoporphyrins
and benzoporphyrins. Similar relationships have been
reported for zinc tetraphenylporphyrin (Ph)4PorZn [50],
Copyright © 2019 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2019; 23: 3–10

4
L. YE ET AL.
Table 1. Equilibrium constants (logK) for the axial coordination reactions between zinc porphyrins and nitrogenous
bases in CH2Cl2
Nitrogenous base
pKaa
(Ph)4PorZn
logKa
3.11
logK (butano(YPh)4PorZn)
logK (benzo(YPh)4PorZn)
cpd-1a
cpd-1b
2.85
2.93
3.17
3.41
3.73
4.58
2.10
cpd-1c
cpd-2a
cpd-2b
3.87
3.90
4.42
4.87
5.00
5.24
3.55
cpd-2c
3-CP (1)
3-BP (2)
3-A (3)
Py (4)
2.81
2.84
3.18
5.29
5.98
2.46
2.57
3.04
3.28
3.54
4.43
1.98
2.99
3.09
3.22
3.68
3.92
4.69
2.29
3.81
3.84
4.37
4.52
4.76
5.13
3.42
3.94
3.96
4.51
5.09
5.19
5.36
3.74
3.14
3.31
3.82
4-Pic (5)
Pip (6)
3.96
11.1
5.96
5.09
2-Pic (7)
2.36
aData taken from Ref. 50.
(a)
(b)
Fig. 2. UV-vis spectral changes of benzo(ClPh)4PorZn 2c obtained during the titration reactions with (a) 3-acetylpyridine and
(b) 4-picoline in CH2Cl2 (the inset the Hill plot used to calculate the number of axial ligands and logK)
(OPP)Zn and (DPP)Zn [49] (where OPP and DPP are the
dianions of 2,3,5,10,12,13,15,20-octaphenylporphyrin
2,3,5,7,8,10,12,13,15,17,18,20-dodecaphenylporphyrin,
respectively). The slope of the logK vs. pKa plots range
from 0.199 to 0.219 for the butanoporphyrins (see Fig. 5)
and from 0.146 to 0.163 for the benzoporphyrins (see
Fig. 6). Both sets of slopes are smaller than the slopes for
TPP (0.236), OPP (0.314) and DPP (0.268) derivatives
[49] under similar solution conditions.
It should be pointed out that the logK values for
reaction of the butano- and benzoporphyrins with
2-picoline (7) are much smaller than that for the
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J. Porphyrins Phthalocyanines 2019; 23: 4–10

AXIAL COORDINATION REACTIONS WITH NITROGENOUS BASES AND DETERMINATION OF EQUILIBRIUM CONSTANTS
5
Fig. 3. UV-vis spectra of butano(CH3Ph)4PorZn 1a obtained before (solid line) and after (dashed line) addition of nitrogenous bases
in CH2Cl2
Fig. 4. UV-vis spectra of benzo(CH3Ph)4PorZn 2a obtained before (solid line) and after (dashed line) addition of nitrogenous bases
in CH2Cl2
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J. Porphyrins Phthalocyanines 2019; 23: 5–10

6
L. YE ET AL.
Table 2. UV-vis spectral data (lmax, nm) of butano(YPh)4PorZn(L) and benzo(YPh)4PorZn(L) (Y = CH3, H or Cl and
L = nitrogenous base) in CH2Cl2
Y
nitrogenous base (L)
butano(YPh)4PorZn(L)
benzo(YPh)4PorZn(L)
Soret
Q
DBa
DQb
Soret
Q1
Q2
DBa
DQ1b
DQ2b
CH3
none
437
452
452
449
452
452
453
451
566
584
582
582
584
585
586
583
457
471
471
471
471
471
472
470
605
613
614
614
612
613
614
611
650
655
655
657
657
657
656
657
3-chloropyridine (1)
3-bromopyridine (2)
3-acetylpyridine (3)
pyridine (4)
15
15
12
15
15
16
14
18
16
16
18
19
20
17
14
14
14
14
14
15
13
8
9
9
7
8
9
6
5
5
7
7
7
6
7
4-picoline (5)
piperidine (6)
2-picoline (7)
H
none
435
450
451
448
450
451
452
449
564
581
581
580
583
583
586
581
455
469
469
469
469
470
470
469
604
613
613
613
613
613
613
612
651
655
656
656
657
656
656
657
3-chloropyridine (1)
3-bromopyridine (2)
3-acetylpyridine (3)
pyridine (4)
15
16
13
15
16
17
14
17
17
16
19
19
22
17
14
14
14
14
15
15
14
9
9
9
9
9
9
8
4
5
5
6
5
5
6
4-picoline (5)
piperidine (6)
2-picoline (7)
Cl
none
437
453
453
450
453
453
454
451
565
581
581
582
582
584
587
583
457
471
472
471
472
472
472
471
604
615
616
616
615
615
616
615
654
660
660
660
659
660
660
660
3-chloropyridine (1)
3-bromopyridine (2)
3-acetylpyridine (3)
pyridine (4)
16
16
13
16
16
17
14
16
16
17
17
19
22
18
14
15
14
15
15
15
14
11
12
12
11
11
12
11
6
6
6
5
6
6
6
4-picoline (5)
piperidine (6)
2-picoline (7)
aRed-shift (nm) for Soret band of the five-coordinate porphyrin as compared to the none axial ligand compound.
bRed-shift (nm) for Q band of the five-coordinate porphyrin as compared to the none axial ligand compound.
reaction with 4-picoline (5), even though both bases
have essentially the same pKa values (see Table 1). This
behavior was previously observed for (Ph)4PorZn [50]
and can be attributed to steric hindrance by the methyl
group on the 2-position of the base. The difference
in equilibrium constants (DlogK) between 4-picoline
and 2-picoline was 1.60 for (Ph)4PorZn [50]. Similar
differences of DlogK (1.56~1.63) are seen for the
butanoporphyrins, but a smaller DlogK (1.34~1.45) is
seen for the benzoporphyrins.
and the largest was seen for the meso-ClPh derivative
(1c) of the butanoporphyrins when a specific nitrogenous
base was utilized as the titrant. It is also the case for
the benzoporphyrins. For example, the logK value for
formation of benzo(CH3Ph)4PorZn(3-CP) is 3.81 but it
is 3.94 for formation of benzo(ClPh)4PorZn(3-CP). This
result indicates that the porphyrins containing electron-
withdrawing meso-ClPh substituents favor generation
of the five-coordinate derivatives in CH2Cl2 solutions
containing nitrogenous bases.
Finally, linear relationships are also obtained between
the logK values for ligand binding and the sum of
the Hammett constants (4s) for groups on the meso-
phenyl groups of the porphyrins. An example is given
in Fig. 7 which illustrates the plots of 4s vs. logK of the
Effect of porphyrin substituents on logK
As seen in Table 1, the smallest equilibrium constant
was seen for the meso-CH3Ph substituted compound (1a)
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AXIAL COORDINATION REACTIONS WITH NITROGENOUS BASES AND DETERMINATION OF EQUILIBRIUM CONSTANTS
7
Fig. 6. Plots of logK for (a) benzo(CH3Ph)4PorZn 2a,
(b) benzo(Ph)4PorZn 2b and (c) benzo(ClPh)4PorZn 2c vs. pKa
of the nitrogenous bases (1–7) which are identified in Table 1
Fig. 5. Plots of logK for (a) butano(CH3Ph)4PorZn 1a and
(b) butano(Ph)4PorZn 1b and (c) butano(ClPh)4PorZn 1c vs. pKa
of the nitrogenous bases (1–7) which are identified in Table 1
butanoporphyrins and benzoporphyrins reacting with
piperidine, pyridine and 3-chloropyridine in CH2Cl2. The
slopes of the plots in Fig. 7 range from 0.160 to 0.321
for the butanoporphyrins and from 0.081 to 0.350 for the
benzoporphyrins. Similar plots were earlier constructed for
a series of meso-aryl substituted zinc porphyrins titrated
with pyridine and gave a slope of 0.188 in benzene [42].
From the values of logK in Table 1, one can see
that the b,b′-fused butano and benzo groups of the
porphyrins exert a significant effect on the equilibrium
constants of the porphyrins. The logK values decrease
in the order: benzo(YPh)4PorZn > (YPh)4PorZn >
butano(YPh)4PorZn, when the same nitrogenous base is
used as the titrant in CH2Cl2.
series of porphyrins (Fig. 8). As seen in the figure, much
larger slopes (~0.30) are seen in the plots involving the
butanoporphyrins than in those for the benzoporphyrins
where the slopes are ~0.12, thus, suggesting that pyridine
binding is more sensitive to the reduction and oxidation
of the butanoporphyrins.
Except for the pKa of the base and substituents of the
meso and b-pyrrole positions of the porphyrins, other
factors such as the solvent utilized for the ligand binding
reactions, the charge of the porphyrins and the method of
calculating logK may also have an effect on the measured
binding constants. This is seen by the data in Table 3
where the logK values determined by different methods
for a number of the zinc porphyrins titrated with pyridine
in different solvents are summarized. As seen in the
table, a logK value of 3.58 is determined for (Ph)4PorZn
by a calorimetric method, but logK = 3.72–3.78 values
were determined by spectroscopic methods in the same
A correlation is also seen between the logK for pyridine
binding and half-wave potentials for the first reduction
and first oxidation of the butano and benzoporphyrins
in pyridine. Linear relationships are observed for both
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8
L. YE ET AL.
Table 3. Binding constants (logK) for axial reactions of various
zinc porphyrins with pyridine in different solvents
Porphyrin
Solvent
logK
Method
Ref.
(Ph)4PorZn
benzene
benzene
benzene
CH2Cl2
CH2Cl2
CH2Cl2
CCl4
3.72
3.78
UV-vis
UV-vis
42
45
(Ph)4PorZn
(Ph)4PorZn
3.58 calorimetry 40
(Ph)4PorZn
4.04
3.84
3.82
UV-vis
UV-vis
UV-vis
49
41
50
(Ph)4PorZn
(Ph)4PorZn
(Ph)4PorZn
4.30 calorimetry 40
3.61 calorimetry 40
(Ph)4PorZn
CHCl3
(Ph)4PorZn
cyclohexane 4.40
UV-vis
43
[(Ph)4PorZn]+
CH2Cl2
CH2Cl2
benzene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
benzene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
toluene
3.98 Electrochem 50
2.06 Electrochem 50
[(Ph)4PorZn]-
(p-CH3OPh)4PorZn
(p-CH3Ph)4PorZn
(m-CH3Ph)4PorZn
(p-FPh)4PorZn
(m-CH3OPh)4PorZn
(p-ClPh)4PorZn
(m-FPh)4PorZn
(m-ClPh)4PorZn
(m-BrPh)4PorZn
(p-tBuPh)4PorZn
(p-HOPh)(p-tBuPh)3PorZn
cis(p-HOPh)2(p-tBuPh)2PorZn
(p-HOPh)3(p-tBuPh)PorZn
(p-HOPh)4PorZn
butano(CH3Ph)4PorZn
butano(Ph)4PorZn
butano(ClPh)4PorZn
benzo(CH3Ph)4PorZn
benzo(Ph)4PorZn
benzo(ClPh)4PorZn
(OPP)Zn
3.69
3.72
3.80
3.91
3.84
4.04
4.09
4.14
4.19
3.73
3.71
3.69
3.67
3.65
3.28
3.41
3.68
4.52
4.87
5.09
4.15
4.66
4.84
UV-vis
UV-vis
UV-vis
UV-vis
UV-vis
UV-vis
UV-vis
UV-vis
UV-vis
UV-vis
UV-vis
UV-vis
UV-vis
UV-vis
UV-vis
UV-vis
UV-vis
UV-vis
UV-vis
UV-vis
UV-vis
UV-vis
UV-vis
42
42
42
42
42
42
42
42
42
46
46
46
46
46
tw
tw
tw
tw
tw
tw
49
49
38
Fig. 7. Plots of logK for the coordination reactions of
(a) butano(YPh)4PorZn 1a–1c and (b) benzo(YPh)4PorZn 2a–2c
with 3-chloropyridine, pyridine, piperidine vs. the Hammett
substituent constants (4s), Y = CH3, s = -0.17; Y = H, s = 0.0;
Y = Cl, s = 0.23 [55]
Fig. 8. Plots of pyridine binding constants (logK) vs. half-wave
potentials (E1/2) for the first oxidation and first reduction of
butano(YPh)4PorZn 1a–1c and benzo(YPh)4PorZn 2a–2c in
pyridine (potentials taken from Ref. 53)
(DPP)Zn
Br8(Ph)4PorZn
[50]. This is consistent with that the porphyrin having a
positive charge being easier to accept a donor, while the
negatively charged porphyrin is harder to accept a donor
to form the five coordinate complex.
benzene solvent. When CH2Cl2 was used as a solvent,
slightly larger logK values (3.82–4.04) were obtained for
(Ph)4PorZn as compared to those determined in benzene
for the same compound.
As compared to the logK of 3.82 [50] for neutral
(Ph)4PorZn, its singly oxidized form [(Ph)4PorZn]+ has
a bigger value of logK (3.98), while its singly reduced
form [(Ph)4PorZn]- has a smaller value of logK (2.06)
CONCLUSION
Zinc tetrabutano and tetrabenzotetraarylporphyrins
can axially bind one nitrogenous base to formation of
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J. Porphyrins Phthalocyanines 2019; 23: 8–10

AXIAL COORDINATION REACTIONS WITH NITROGENOUS BASES AND DETERMINATION OF EQUILIBRIUM CONSTANTS
9
five-coordinate derivatives, leading to a 4–22 nm red-shift
of the Soret and Q bands in UV-vis spectra of the porphyrin
in CH2Cl2. The equilibrium constants (logK) for the
coordination reactions were determined for each porphy-
rin.The logKvalues follow the order: benzo(YPh)4PorZn >
(YPh)4PorZn>butano(YPh)4PorZn, whiletheorderforthe
nitrogenous bases is: 3-chloropyridine < 3-bromopyridine
< 3-acetylpyridine < pyridine < 4-picoline < piperidine.
This is consistent with the order of pKa values for these
nitrogenous bases. The pKa value of 2-picoline is the same
as that of 4-picoline but its logK values are much smaller
than that of 4-picoline due to steric hindrance by methyl
group at the 2-position.
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Acknowledgments
We gratefully acknowledge support from the National
Natural Science Foundation of China (Grant 21501070,
21606099), Jiangsu University Foundation (Grant
15JDG131), China Postdoctoral Science Foundation
(Grant 2017M611707) and the Robert A. Welch Found-
ation (K.M.K. Grant E-680).
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