Effect of the type of molecular nonplanar structure on the chemical reactivity of NH bonds in the coordination center of porphyrin molecule

Article abstract of DOI:10.1134/S1070363209040276

Effect of out-of-plane screwing of the porphyrin macrocycle (H ₂P) by means of tetra-meso-substitution and intracyclic N-substitution on the state of NH bonds in the molecule is explored. By applying ¹H NMR, kinetic and quantum-chemi

Full text of DOI:10.1134/S1070363209040276

ISSN 1070-3632, Russian Journal of General Chemistry, 2009, Vol. 79, No. 4, pp. 845–851. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © D.B. Berezin, O.V. Mal’tseva, 2009, published in Zhurnal Obshchei Khimii, 2009, Vol. 79, No. 4, pp. 682–688.
Effect of the Type of Molecular Nonplanar Structure
on the Chemical Reactivity of NH Bonds
in the Coordination Center of Porphyrin Molecule
D. B. Berezina and O. V. Mal’tsevab
a Ivanovo State Chemical Technological University,
pr. Engel’sa 7, Ivanovo, 153000 Russia
e-mail: berezin@isuct.ru
b Institute of the Chemistry of Solvents, Russian Academy of Sciences, Ivanovo, Russia
Received October 14, 2008
Abstract―Effect of out-of-plane screwing of the porphyrin macrocycle (Н2P) by means of tetra-meso-
substitution and intracyclic N-substitution on the state of NH bonds in the molecule is explored. By applying
1Н NMR, kinetic and quantum-chemical criteria of evaluation of the macrocycle NH-reactivity is found that
such type of Н2P screwing does not increase reactivity of the NH bonds. It is shown that increased chemical
NH activity is observed only in the porphyrins undergoing at a significant conformational screwing possess the
polarization of the molecular π-system.
DOI: 10.1134/S1070363209040276
A large number of publications reports on the study
of the effect of the out-of-plane screwing of macroring
on the chemical properties and biological activity of
porphyrins (compounds I–XIII) [1–4]. The porphyrins
being derivatives of planar aromatic porphin (I) can
become nonplanar owing to certain structural
modifications [4, 5], including the following:
(e.g., compounds XI and XIII); introduction of bulky
functional substituents to three or all four meso-
positions [isopropyl (VI), t-butyl-, adamantoyl and
others]; formation of overlapped monomeric or
dimeric structures H4P2(R)n with short spacers R [6].
(2) Modification of the coordination center of Н2P,
namely: substitution of Н atoms of intracyclic NH
groups (NR-substitution) as, e.g., in compounds III,
IV and X [7]; protonation of tertiary nitrogen atoms
with formation of Н2P mono- and dicationic forms
Н3P+ and Н4P2+ respectively [5, 8]; complexing with
(1) Modification of Н2P molecule periphery by
means of multiple (nona-, deca-, undeca- or dodeca-)
substitution of exocyclic hydrogen atoms in the
macrocycle β- and meso-positions simultaneously
R1
Ph
R
R1
R1
Ph
Ph
R1
R1
R1
R1 Ph
Ph
Ph
Ph
Ph
NR
N
N
NR
N
N
NR
N
N
NR
N
N
R1
R
R
HN
HN
HN
HN
R1
Ph
R1
Ph
R1
Ph
R1
I_VIII
Ph
XI
R
XII, XIII
IX, X
R = R1 = H (I); R = H, R1 = Ph (II); R = Me, R1 = Ph (III); R = R1 = Ph (IV); R = H, R1 = p-Pr (V); R = H, R1 = iso-Pr (VI); R = H,
R1 = cyclo-Hex (VII); R = H, R1 = tert-Bu (VIII); R = H, R1 = Et (IX); R = Me, R1 = Et (X); R = H (XII); R = Ph (XIII).
845

846
BEREZIN, MAL’TSEVA
0
0
0
0
0
0
0
0
Saddle
Ruffle
Dome
Wave
Fig. 1. Basic types of nonplanar conformations of Н2P and its complexes with metals. Signs “+” and “–” assume position of
atoms over and below, respectively, of initial plane of macrocycle [4, 12].
metals with ionic radii much less or, on the contrary,
much larger than the size of coordination cavity (r =
2.01 Å) as in the cases of Ni(II) or Zn(II), respectively;
coordination center. If such connection really exists
and hence simple change in the conformation of Н2P
molecule can critically change its properties governing
the state of NH bond in the macrocycle then it is
prsumable that the mechanism with participation of
these processes in vivo may be the same. In particular,
the activation of chemical bonds in the reaction center
of the porphyrin molecule leads to a spasmodic growth
of their NH-acidity (by 10 and more orders of
magnitude of рKa value in DMSO [14]), to formation
of stable Н-associates of Н2P molecules and electron-
donor solvents with incomplete proton transfer [15]
and as a consequence to catalytic action of these
solvents on the processes of capture of the atoms of p-
and d-metals by porphyrin macrocycle [16].
(3) Changes in Н2P molecule leading to the loss of
aromaticity [5], for example: formation of radical
forms of porphyrins, of π-cation-radicals in parti-
cularicular; decrease in the size of π-chromophore by
means of reduction of Сβ–Сβ bonds of pyrrole rings
(chlorins, bacterio- and isobacteriochlorins, and other
compounds).
(4) Combination of various factors (dodeca-sub-
stitution and protonation, etc.) [4].
The distortion of the planar structure of the por-
phyrin macrocycle commonly leads to an appearance
of typical nonplanar conformations or combinations of
such conformations that in a certain manner change the
properties of the molecule. By extensive X-ray struc-
tural [4], spectral [1, 2, 9, 10], quantum-chemical [11],
and other studies the molecular and ionic forms of H2P
and MP with nonplanar structure were classified
according to the type of screwing as saddle, ruffle,
dome, wave, stepped, twisted, gable etc. (Fig. 1) [12].
Among these conformation the most often in Н2P and
their complexes with d-metals appear the saddle and
ruffle conformations and their combinations.
Besides the dodeca-substitution, the most pro-
nounced types of screwing of the planar structure of
Н2P molecules can be induced by substitution at meso
positions by bulky, most often alkyl, groups that leads
to a ruffle conformation, and also the substitution of a
hydrogen atom at one or more nitrogen atoms in the
coordination center with the formation of N-substituted
porphyrin analogs [7]. In the last case the saddle
conformation of asymmetric type is typical for the
molecules (Fig. 1). The present study is focused on the
study of the relations between the structure and
properties of meso-substituted Н2P with bulky groups
(ruffle, compounds VI–VIII), as well as N-substituted
(compounds III, IV, X) Н2P analogs (saddle).1
As a measure of non-planarity of porphyrin
molecules can serve not only the data of X-ray
structural analysis [4, 12] reflecting structure of the
macrocycles in solid phase but also the fluorescence
data [2, 5], results of Raman spectra [10, 13] and in
some cases NMR spectral characteristics [9] reflecting
conformational state of compounds in solution.
By their spectral characteristics, physicochemical
properties, and reactivity both meso-substituted por-
1
The ruffle and saddle nonplanar conformations are characterized
The principal problem is whether a connection
exists between nonplanar structure of the porphyrin
macrocycle and chemical reactivity of NH bonds in its
by the value (in Å) of average deviation of meso (ΔСmesо) and β
(ΔСβ) carbon atoms from the parent macrocycle plane that can
achieve 1.5 Å.
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EFFECT OF THE TYPE OF MOLECULAR NONPLANAR STRUCTURE
847
Table 1. Electron absorption spectra of meso- and N-substituted porphyrins in organic solvents
Comp.
Solvent
λСоре, nm (log ε)
λIV, nm (log ε)
λ III, nm (log ε)
λII, nm (log ε)
λI, nm (log ε)
Δλ1, nm
no.
С6Н6
395 (5.30)
497 (4.19)
530 (4.07)
567 (3.92)
596 (3.18)
565 (3.86)
592 (3.05)
586 (4.02)
615 (3.80)
585 (3.71)
618 (3.22)
591 (3.81)
590 (3.73)
614 (3.78)
621 (3.81)
–2
IX
DMF
С6Н6
DMF
387 (5.31)
412 (5.19)
409 (5.11)
495 (4.16)
506 (4.35)
504 (4.18)
528 (4.03)
536 (4.13)
533 (3.85)
619 (3.74)
643 (3.94)
641 (3.61)
–2
X
С6Н6
DMF
С6Н6
418 (5.58)
416 (5.62)
426 (5.30)
514 (4.33)
513 (4.28)
499 (3.84) sh
521 (4.13)
498 (3.95) sh
532 (4.18)
sh
548 (3.96)
548 (3.91)
572 (4.26)
647 (3.68)
646 (3.68)
677 (3.77)
–1
–1
II
III
DMF
425 (5.24)
564 (4.32)
614 (3.90)
676 (3.87)
С6Н6
DMF
С6Н6
DMF
С6Н6
DMF
443
592
sh
705
–1
–4
–4
IV
444
sh
595
sh
704
417 (5.18)
415 (5.19)
420 (5.05)
420 (5.03)
521 (4.00)
518 (4.06)
526 (3.66)
524 (3.60)
559 (3.73)
533 (3.78)
561 (3.47)
560 (3.37)
603 (3.48)
599(3.53)
606 (3.13)
601 (3.00)
661 (3.51)
657 (3.54)
663 (3.31)
659 (3.17)
VI
VII
phyrins and N-substituted analogs of Н2P behave like
compounds with a nonplanar structure. Thus, all the
bands in the visible region of their electronic spectra
suffer a red shift relatively to those in the spectra of
predominantly planar Н2P (Table 1) that, however, is
not a reliable criterion of nonplanarity of the
molecules. The more convincing proofs are the
photophysical characteristics describing properties of
their excited states. As characteristics of nonplanar
porphyrins significant Stock’s shifts of neighboring
bands in fluorescence spectra compared to the
absorption bands (ΔνI), low fluorescence quantum
yields (φfl), and transitions to the triplet state (φP) [2, 4,
5] are suggested. Besides, in vibration spectra of
nonplanar porphyrins may occur typical frequency
properties [1]. The stability of the ligands and their
complexes toward elevated temperature and the action
of chemical reagents decreases [5, 18]. For example,
the temperature of the start of the thermooxidative
descomposition (tstart) of strongly nonplanar dode-
caporphyrin (XI) and N-methyltetraphenylporphin
(III) is lowered compared to almost planar compound
(II) from 407 to 244°C and 260°С, respectively.
Therewith it is established that aromaticity of strongly
nonplanar porphyrins in its canonical understanding
diminishes compared to planar molecules only by 3–
5% [9].
Obviously partial localization of the π-electron
density in the pyrrole rings of the macrocycles of
types III, IV, VI–VIII, and X leads to an increase in
basicity of the tertiary nitrogen atoms and in their
ability to bind proton even in acids of a moderate
strength [8]. Also, the ability of the studied nonplanar
Н2P to complex formation in the medium of solvents
with moderate donor–acceptor characteristics, e.g., in
acetonitrile increases [19].
1
modes [10], and in H NMR spectra are observed
downfield shifts of NH proton signals at applying
universal solvating solvents caused by deshielding of
coordination center at the screwing of planar structure
of the molecule.
Due to the activation of the out-of-plane vibrations,
the porphyrins of these structural groups show
enhanced solubility in organic solvents [17] and
unusual for the predominantly planar Н2P redox
The problem whether the nonplanar meso- (VI–
VIII) and N-substituted macrocycles (III–IV, X)
belong to the group of the porphyrins with chemically
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 4 2009

848
BEREZIN, MAL’TSEVA
Table 2. Effect of solvent nature on the position of signals
of NH protons (ΔδNH, ppm) in the Н NMR spectra of Н2P
possessing chemically active or inert NH bonds
of electron absorption spectra are not adequate for the
conclusion about the state of NH bonds in these
compounds [5].
1
δNH
,
δNH
,
ΔδNH,
Porphyrin Solvent
Solvent
For solving the problem of the nature of NH-
reactivity of the porphyrins with nonplanar structure
of their π-system it is reasonable to use Н NMR,
ppm
–2.76
–1.79
–1.60
–0.90
ppm
–2.91
–2.17
–1.94
+1.00
ppm
CDCl3
CDCl3
CDCl3
CDCl3
DMSO-d6
DMSO-d6
DMSO-d6
Py-d5
–0.15
–0.38
–0.34
+1.90
II
1
VI
VII
XIа
kinetic, and quantum-chemical criteria of chemical
reactivity of NH bonds in these molecules we have
suggested previously [20]. Unfortunately, in most
a Published data [9].
1
cases the signal of NH proton in Н NMR spectra of
N-substituted porphyrins is not seen at all [7]. In
contrast, the NH signals and their dependence on the
nature of solvent are clearly seen in the spectra of
nonplanar meso-substituted porphyrins (Table 2).
Analysis of the data in Table 2 leads to a conclusion
that δNH values of compounds II, VI–VII suffer an
upfield shift at the replacement of the universally
solvating chloroform by electron-donor DMSO, while
for compound XI and the other NH-active
macrocycles the typical situation is the opposite [20].
active NH bonds has never been discussed. To our
best knowledge, to this moment there is no data on
the acidity of NH bonds in the molecules of meso-
substituted porphyrins with bulky meso-substituents.
There are rather contradictory data [5,7] on the effect
of intracyclic N-substitutents in Н2P molecule on its
acid properties. Commonly, transfer of a porphyrin
with chemically active NH bond from universally
solvating medim to a polar electron-donor solvent is
accompanied by change in the structure of the bands
of its electronic spectrum [5,14,15]. From the data in
Table 1 follows that positions of the bands of N- and
meso-substituted compounds under study in benzene
and DMF remain practically the same. Moreover,
shift of the most low-frequency Qx band in the
electron absorption spectra of compounds III, IV, VI
and VII at this replacement of the solvent, that is,
solvatochromic effect of the macrocycles, under these
conditions is not higher than 4 nm. However, the data
For confirming the fact of low chemical activity of
NH bonds in nonplanar ruffle Н2P we will use two
other criteria of NH-activity, the quantum-chemical
and kinetic ones [20].
Table 3 contains for comparison the data of
quantum-chemical calculations of enthalpy of
formation of Н2P ligands, their mono- (НP–, ΔНf(–1),
kcal mol–1) and dianions (P2–, ΔНf(–2), kcal mol–1), as
well as the values of deprotonation heat of the ligands
Table 3. Enthalpy characteristics of deprotonation [ΔHf(–1) and ΔHf(–2), kcal mol–1] and formation of mono- and dianionic
forms ΔH(–1) and ΔH(–2), kcal mol–1 of the porphyrins with planar and nonplanar structures
ΔHf(0)
,
ΔHf(–1)
,
ΔHf(–2)
,
δΔHf(0–1)
,
δΔHf(0–2)
,
ΔH(–1)
,
ΔH(–2),
Comp. no.
kcal mol–1
kcal mol–1
kcal mol–1
kcal mol–1
kcal mol–1
–43.09
–39.01
–25.64
–40.46
–38.73
–35.42
–35.23
–
kcal mol–1
kcal mol–1
221.00
200.81
264.09
20.19
347.01
777.49
I
IXа
IIа
151.91
178.54
131.62
195.96
129.24
352.27
392.90
265.25
401.29
587.56
132.54
160.12
111.92
176.05
113.26
329.70
372.72
237.53
371.91
554.62
192.37
217.27
167.04
231.19
–
19.37
18.42
19.70
19.91
15.98
22.57
20.18
27.72
29.38
32.94
347.83
348.78
347.50
347.29
351.22
344.63
347.02
339.48
337.82
334.26
774.86
773.13
769.82
769.63
–
V
VI
VII
VIII
X
–
–
–
III
IV
XII
XIII
XI
–
–
–
281.80
403.63
574.27
–16.55
–2.34
13.29
750.95
736.74
721.11
a Published data [21].
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 4 2009

EFFECT OF THE TYPE OF MOLECULAR NONPLANAR STRUCTURE
849
δΔHf, kcal mol–1
(ΔН–i) into mono- and dianions in a vacuum for
basically flat not NH-active porphyrins I–II, V, IX,
and of nonplanar meso-substituted porphyrins VI–
VII and N-substituted H2P analogs III–IV, X. The
accepted as estimates δΔНf(0–2) values (Table 3) show
that unlike the dodeca-substitution of Н2P (compound
XI), the meso- or N- substitution in the macrocylcle
does not lead to the chemical activation of the NH
bonds (Fig. 2). The δΔНf(0–2) values only slightly tend
to shift to the positive side (by 5–8 units only), while
even in the compounds with weak NH-activity like
Н2ТBP (XII) and H2TPTBP (XIII) this shift attains
26.5 and 23.3 units respectively (Table 3). Inasmuch
as the N-substituted porphyrins are monohydric acids,
analysis of their quantum-chemical data can be based
on the δΔНf(0–1) values. It follows from the data in
Table 3 and in Fig. 2 that this value also, compared to
the NH-active compounds XII and XIII, practically
does not grow relatively to unsubstituted porphin (I).
Thus, quantum-chemical calculations confirm the
conclusion on the absence of a noticeable chemical
activity of NH bonds in nonplanar meso-substituted
Н2P and N-substituted porphyrin analogs.
δΔHf(0–1)
VIII
VII
I
V
VI
X
20
III
IV
VII
0
–20
δΔHf(0–2)
VI
II
V
IX
–40
I
VII
Fig. 2. Plots of characteristics of relative stability of the
Н2P mono- and dianions (δΔНf(0–1) and δΔНf(0–2), kcal mol–1)
with chemically inactive NH bond against molecular
structure (data of [21]).
activity, but at attaining certain concentration of the
product an equilibrium is established [23].
Н(NR)Р + МХ2(Solv)n–2
–2 Solv
#
[Н(NR)P···MX2(Solv)n–4
]
→ (X)M(NR)P + HX + (n – 4)Solv.
(1)
An additional argument in favor of the absence of
NH–activity in N-substituted porphyrins is the
negative activation entropy in the reactions of their
coordination with d-metal salts. On the contrary,
positive ΔS# = S# – Sinit in NH–reactive Н2P we
attribute to a substantial solvation of the initial state
on the reaction coordinate (Table 4) [5].
Now let us consider the data that form the basis of
the kinetic criterion of NH-reactivity. From the
experimental values obtained for the reaction of
complex formation (1) of the ligands III and X
follows (Table 4) that the rates of complex formation
of N-substituted compounds fall with the increase in
the electron-donor property of solvent: propanol-1 >
DMF > DMSO > Pу [22], that corresponds to the
behavior of porphyrins whose NH bond is localized,
i.e., chemically inert. Moreover, in pyridine that
activates NH bonds the rate of reaction of nonplanar
Н(N–Me)TPP (III) with zinc acetate not only grows
relatively to the media with lower coordinating
Data on the kinetics of complex formation of
nonplanar porphyrins in a ruffle conformation are
practically absent. Our study on an example of H2T(i-
Pr)P (VI) and H2T(cyclo-Hex)P (VII) (ΔCmeso for
their complexes with Ni(II) are respectively 0.74 and
0.77 Å [4]) showed that rates of this reactions with
copper, zinc and cadmium acetate in electron-donor
Table 4. Kinetic parameters of reaction (1) of N-substituted porphyrins with Zn(OAc)2 in organic solvents
Comp. no. Csalt ×103 mol l–1
Solvent
kv298, l mol–1 s–1
Еа, kJ mol–1
ΔS≠, J mol–1 K–1
2.6
0.014
5.20
fast
77.0±1.4
–30±2
Py
X
2.0
2.0
0.2
0.2
52.8±2.8
–56±3
DMSO
DMF
–
–
131.4
fast
7.1±0.1
–189±15
–
–
PrOH-1
Py
2.6
slow
6.14
fast
–
–
–124±7
–
III
2.0
2.0
0.2
0.2
34.1±2.8
DMSO
DMF
–
32.8±2.0
–
18.8
fast
–119±9
–
PrOH-1
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 79 No. 4 2009

850
BEREZIN, MAL’TSEVA
solvents (DMF, DMSO) for which is expected
formation of Н-associates with active NH bonds are
very low and the respective metallocomplexes do not
form practically in the temperature range 298–338 K.
This fact evidences significantly in favor of localized
NH bond in the meso–substituted porphyrin molecules.
conformation (Fig. 1). As seen, in the saddle
conformation the protons are deviated from the initial
macrocycle plane while in the ruffle conformation the
planar structure of N4H2 coordination center is not
distorted.
EXPERIMENTAL
It can be noted also that meso-substituted Н2P with
bulky substituents as a rule do not enter in the reaction
of complex formation with d-metal salts in acetic acid
and other solvents with expressed proton-donor
function. Distortion of the macrocycle planar structure
leads to a partial localization of π-electron density in
pyrrole rings and to increased basicity of the molecule.
Since the porphyrin protonated forms (Н3P+ and Н4P2+)
forming in proton-donor media are inactive in the
reaction (1), this rule is fulfilled also for other
nonplanar Н2P such as N- (III–IV, X) or dodeca-
substituted (XI, XIII) molecules. Some of the dodeca-
substituted compounds still enter in the reaction (1) in
AсOН but with a very low rate [24]. The best medium
for the synthesis of MP with nonplanar meso-
substituted porphyrins is a polar solvent with moderate
donor–acceptor properties, e.g., acetonitrile. The
reaction (1) with more screwed Н2Т(cyclo-Hex)P
(VII) proceeds in this solvent very rapidly, while with
Н2Т(iso-Pr)P (VI) with kv ~ 10 l mol–1 s–1 at 298 К.
Porphyrins II–IV, VI, VII, IX, and X were
prepared, purified and identified along the described
procedures [7,19,27,28]. Solvents were dried and
purified by commonly used methods described in [29].
Electron adsorption spectra of the explored
compounds were registered on a Hitachi U 2020
1
spectrophotometer, the Н NMR spectra on a Bruker
spectrometer with operating frequency 200 MHz at
303 K. Progress of reaction (1) was followed
spectrophotometrically according to the procedure
described in [19,23]. The change in enthalpy at the
formation of neutral and deprotonated forms of the
porphyrins was estimated using quantum-chemical
calculations by semiempirical AM1 method (Hyper-
chem 5.0 program package) with the formulas (2, 3)
taking enthalpy of proton formation in gas phase
ΔHf(H+) = 367.2 kcal mol–1.
ΔH(–i) = iΔHf(H+) + ΔHf(–i) – ΔHf(0)
ΔHf(0–i) = ΔHf(0) – ΔHf(–i).
,
(2)
(3)
Thus, all three quantitative criteria confirm the
absence of chemical activity of NH bonds in both
meso-substituted porphyrins with bulky functional
groups leading to formation of ruffle conformation
[4] and with N-substituted compounds with low
symmetrical moderately saddle-like conformation.
Thus, according to quantum-chemical calculations
of geometry, the ΔСβ values for compounds X and
III are approximately equal to 0.1 and 0.5 Å,
respectively [25]. The data obtained allow to
conclude that the chemical activity of NH bonds in
Н2P molecules does not appear at any screwing
planar structure of the molecule but specifically
depends on the type of conformation of the screwed
macrocycle. Only the pronounced symmetrically
screwed saddle conformation of Н2P possessing high
orthogonal dipole moment [26] leads to the
appearance of NH-activity in properly nonplanar
Н2P. The nonplanar Н2P with ruffle conformation in
crystalline state and in solutions [4] are low polar
and do not display chemical activity of NH bonds.
Another reason for the absence of activity of the NH
protons of the coordination center in ruffle
porphyrins follows from the structure of the ruffle
ACKNOWLEDGMENTS
The authors are grateful to professor A.S.
Semeikin, Chair of organic chemistry, Ivanovo State
Chemical Technological University, for the kindly
provided objects of investigation and to Russian
Foundation for Basic Research (grant no. 06-03-
96327) and Foundation for Promotion of National
Science for the financial support.
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