The quest for a better understanding of ethanol coordination to magnesium and zinc porphyrin: A combined experimental and theoretical study

Article abstract of DOI:10.1016/j.molstruc.2020.129646

An ethanol coordinated magnesium porphyrin [Mg(TDMPP)(C₂H₅OH)], (TDMPP = 5,10,15,20-tetrakis(3,5-dimethoxyphenyl)porphyrin), 1 was synthesized and characterized by single crystal X-ray diffraction method along with other standard spe

Full text of DOI:10.1016/j.molstruc.2020.129646

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The quest for a better understanding of ethanol coordination to
magnesium and zinc porphyrin: A combined experimental and
theoretical study
∗
Bishnu Prasad Borah, Smita Majumder, Karishma Devi Borah, Jagannath Bhuyan
Department of Chemistry, North Eastern Regional Institute of Science and Technology (NERIST), Nirjuli, Arunachal Pradesh, 791109, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An ethanol coordinated magnesium porphyrin [Mg(TDMPP)(C H OH)], (TDMPP = 5,10,15,20-tetrakis(3,5-
2
5
Received 14 September 2020
Revised 22 October 2020
Accepted 11 November 2020
Available online xxx
dimethoxyphenyl)porphyrin), 1 was synthesized and characterized by single crystal X-ray diffraction
method along with other standard spectroscopic techniques. This is the first known crystal structure
of an alcohol bound magnesium porphyrin of tetraphenylporphyrin derivatives. Crystal structure of the
compound shows a weak hydrogen bonding interaction (2.730(4) A˚ ) between OH group of axial ethanol
Keywords:
molecule and a peripheral methoxy group of an adjacent porphyrin molecule. This hydrogen bonding
results the formation of a weak one-dimensional supramolecular structure. The fluorescence and elec-
trochemical studies have also been performed. To explore the disadvantages of using zinc porphyrins to
study chlorophyll type magnesium porphyrins, a comparison of theoretical study using DFT level was per-
formed between compound [Mg(TDMPP)(C2H5OH)], 1 and its zinc counterpart [Zn(TDMPP)(C2H5OH)], 2.
Theoretical calculations include optimization of geometry, energy of frontier molecular orbitals (FMOs),
electronic transitions and global chemical indices. The global reactivity indices like η, μ and ω derived
from theoretical calculations for zinc and magnesium porphyrin supports that magnesium porphyrin is
more reactive than the zinc analogue. Furthermore, the Hirshfeld surface analysis was used to quantify
the weak intermolecular interactions present in the crystal lattice of compound 1.
Porphyrin
magnesium
density functional theory
Hirshfeld surface analysis
crystal structure
©
2020 Elsevier B.V. All rights reserved.
Introduction
limited with magnesium porphyrins and the study of zinc por-
phyrin was considered as model for chlorophyll type molecules. A
penta-coordinated magnesium porphyrin is known which shows
enhanced fluorescence intensity in the presence of cholesterol [14].
Fiedor and coworker carried out theoretical calculation at the DFT
level to understand water coordination in magnesium tetrapyrroles
The synthetic magnesium porphyrins have attracted significant
interest recently due to their application in different fields such as
sensors [1], catalyst [2,3], antibacterial properties [4], determina-
tion of absolute configuration of monoalcohols [5] etc., besides the
relevance with chlorophyll molecules in photosynthesis [6–10]. In
spite of so many importance magnesium porphyrins known in the
literature is very less compared to other similar porphyrins like
zinc because in the early time of porphyrin research (owing to the
synthetic difficulties of magnesium porphyrins) zinc porphyrins
were used to model chlorophyll type molecules. The study of
axial coordination of small molecules or anions to magnesium
porphyrins is an exciting topic as a water molecule is usually
present in the chlorophyll molecules in the green leaves. The axial
coordination of small molecules, anions, and coordination from the
peripheral groups of same or different compounds are thoroughly
studied for zinc porphyrins [11–13]. Similar studies are also very
and other chelates and found that the binding between Mg2 and
water is primarily electrostatic in nature and coordination number
is reduced from six to five as the binding of water in the sixth site
does not result any energy gain [15]. The one dimensional coor-
dination polymer of magnesium 3,4,5-trimethoxyphenylporphyrin
(due to metal-oxygen linking between meta-methoxy group of
+
II
ʹ
two adjacent porphyrins) [16], [Mg (TPBP)(4,4 -bpy) ] (TPBP
2
=(5,10,15,20-tetrakis[4-(benzoyloxy)phenyl)porphyrin),
bpy 4,4ʹ-bipyridine) [17], supramolecular chain network of
diaqua(5,10,15,20-tetraphenylporphyrinato-k N)magnesium–18-
crown-6, [MgTPP(H O) ](18-C-6) [18], and [MgT(p-OME)PP(H O)]
(4,4ʹ-
=
4
2
2
2
[19] are known. Magnesium tetranitrooctaethylporphyrin is found
to form similar type of one-dimensional polymeric chains with
different bidentate axial ligands like pyrazine or cyanopyridine
[20]. Using non-covalent interactions Coutsolelos and coworkers
∗
Corresponding author.
E-mail address: jnb@nerist.ac.in (J. Bhuyan).
https://doi.org/10.1016/j.molstruc.2020.129646
0
022-2860/© 2020 Elsevier B.V. All rights reserved.
Please cite this article as: B.P. Borah, S. Majumder, K.D. Borah et al., The quest for a better understanding of ethanol coordination to
magnesium and zinc porphyrin: A combined experimental and theoretical study, Journal of Molecular Structure, https://doi.org/10.1016/
j.molstruc.2020.129646

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designed a supramolecular donor –acceptor conjugates to model
of photosynthetic systems [21]. We and others have studied the
ethanol coordination to zinc porphyrins [22–24]. Though it is
known that alcohols can bind to magnesium porphyrins very eas-
ily, to the best of our knowledge crystal structure of magnesium
porphyrin with a bound alcohol type molecule is not known in the
literature. Therefore, here we have reported the synthesis, struc-
tural characterization of an axially ethanol coordinated magnesium
Theoretical studies
The theoretical calculations like optimization of structures,
Frontier Molecular Orbitals (FMO) energy calculation, simulation of
UV-VIS spectrum of compound were performed at DFT level using
Gaussian16 [27] software using the basis set LANL2DZ [28] along
with functional B3LYP [29,30]. The UV-VIS spectrum of compound
was simulated using TD-DFT method and compared with the ex-
perimental result. The initial geometries were taken from crystal
structure of compounds. For compound 2, the initial geometry was
taken from the crystal structure reported by Bhyappa and cowork-
ers [31]. Hirshfeld surface analysis and fingerprint plots were per-
formed using crystal explorer software [32].
porphyrin, [Mg(TDMPP)(C H OH)] and to understand profoundly
2
5
the difference between magnesium and zinc porphyrins, a theoret-
ical study of magnesium porphyrin [Mg(TDMPP)(C H OH)], 1 and
2
5
its zinc analogue, [Zn(TDMPP)(C H OH)], 2 have been undertaken.
2
5
The Hirshfeld surface analysis [25] and 2D fingerprint plots have
been studied to understand the different types of interactions
present in the crystal lattice.
Synthesis of ligand 5,10,15,20-tetrakis(3,5-dimethoxyphenyl)porphyrin
(H TDMPP)
2
Experimental
The ligand 5,10,15,20-tetrakis(3,5-dimethoxyphenyl)porphyrin
(
H TDMPP) was synthesized according to the method of Adler and
2
General considerations
Longo [33]. A sample of 8.75 mL (0.144 mmol) of 3,5 dimethoxy-
benzaldehyde was refluxed for one hour with 5 mL (0.144 mmol)
pyrrole in 250 mL propionic acid. The solution was then allowed
to cool and kept for further 24 hours. Then the dark solid residue
was collected by filtration and was washed with water and ethanol
and allowed for air dry. The yield of the compound was 1.68 g
All the reagents used in the experiments were of analytical
grade and used without any further purification. Dichloromethane,
hexane, chloroform, magnesium acetate and N,N dimethylfor-
mamide were obtained from Merck Life Science Private Limited.
3
,5-Dimethoxybenzaldehyde and pyrrole were purchased from
(
20%). Molecular formula of the ligand is C52H46N O Molecular
4 8;
Spectrochem Private Limited. Ethanol and propionic acid were pur-
chased from SRL.
weight 854; UV/Vis (CHCl 10 5M) λmax, nm (ε, mol–1dm cm–1):
−
3
3
,
4
20 (348000), 516(22000), 547(8000), 590 (7200), 646 (5600).
Physical Measurements
Synthesis of compound [Mg(TDMPP)(C H OH)], 1
2
5
UV-Visible spectra were recorded in a Perkin-Elmer (Lamda
A sample of 1g (1.02 mmol) of H TDMPP and 2 g (4.07
2
35) spectrophotometer. A Shimadzu FT-IR spectrophotometer (IR
mmol) of magnesium-acetate, Mg(CH COO)2 was warmed in 120
3
affinity) was used for acquiring Infra-Red spectra at 400-4000
mL DMF for one hour then a small amount of (0.2g) sodium
acetate was added. The final solution was refluxed for 7 hrs.
The solvent was removed by slow evaporation by keeping the
solution mixture on water bath. The crude product was dissolved
in dichloromethane, filtered and evaporated using rotary evap-
orator. Finally, column chromatography (neutral alumina) using
solvent mixture of dichloromethane and hexane was used for
isolation and purification of the product. The compound was
cm 1 range using pressed KBr disks. Luminescence spectral mea-
surements were acquired using an Agilent Cary Eclipse Fluores-
cence Spectrophotometer. Elemental analysis for carbon, hydrogen
and nitrogen were analyzed with Leco CHNOS 948 carbon hy-
drogen nitrogen oxygen determination. An electrochemical work-
station CH instrument CHI600E was used for cyclic voltammetry
−
(CV) experiments. Three electrode systems consist of glassy car-
bon electrode as working electrode, Pt electrode as auxiliary elec-
trode and Ag/AgCl electrode as reference electrode were used in
dichloromethane medium in nitrogen atmosphere for electrochem-
ical measurements.
re-crystallized using
a chloroform-hexane mixture with trace
amount of ethanol. The yield of compound [Mg(TDMPP)(C H OH)]
2
5
was 80%. Molecular Formula: C54H50MgN O ; Molecular Weight
4
9
9
7
23.29; Elemental analysis (%) calcd. for C54H50MgN O
C
4
9:
0.25, H 5.46, N 6.07; found C 70.50, H 6.28, N 6.21. UV/Vis
CHCl ) λmax, nm(ε, mol–1dm cm–1): 424 (600000), 553 (28000),
3
(
3
X-ray structural analysis
6
2
03 (8000). IR analysis give important peaks at 3410(ϑO-H),
923(ϑaromatic C-H),
1736(ϑ
),
1597(ϑ
),
1463(ϑ
),
C=N
C=C
C=C,pyrrole
A suitable crystal of the magnesium porphyrin was used for
data collection in a Bruker APEX-II CCD diffractometer. The in-
strument was equipped with CCD area detector and data were
collected using graphite-monochromated Mo Kα radiation (λ=
1353(ϑC-N), 1161(ϑC-Hpyrrole), 1072(ϑC-O-C), 844(ϑmethane C-H
)
and
7
55(ϑN-H out-of-plane deformation) cm 1 respectively. The IR spectrum
−
of the compound 1 is shown in supplementary information as Fig
S1
˚
0.71073 A) at room temperature (293K). All empirical absorp-
tion correction was applied using the SADABS program. All data
were acquired with Bruker APEX2 and were integrated with the
BRUKER SAINT program. SIR97 was used for solving and SHELXS-
Result and discussion
97 (Sheldrick 2008) for refining the structure. The space group
Crystal structure
of the compound was determined based on the lack of system-
atic absence and intensity statistics. Full matrix least squares/ dif-
ference Fourier cycles have been performed which located the
non-hydrogen atoms. All non-hydrogen atoms were refined with
anisotropic displacement parameters. Hydrogen atom attached to
oxygen of ethanol molecule was located from difference Fourier
map and refined isotropically. The structural figures were drawn
with the Olex2 [26].
A simple and effective magnesium metalation to porphyrin
type molecules was developed by Lindsey and co-workers [34].
An axially coordinated water molecules is usually seen in the
crystal structure of chlorophyll type molecules [35,36] which re-
sults a hydrogen bonding interaction between axial water molecule
and peripheral methyl ester carbonyl oxygen atom in chlorophyll
molecules [37,38]. The hydrogen bonding may have some role in
2

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Fig. 1. Perspective view of compound [Mg(TDMPP)(C2H5OH)].
Table 1
Crystallographic dataa for complex [Mg(TDMPP)(C2H5OH)].
the energy dissipation process of photosynthesis. The suitable crys-
tal for single crystal X-ray diffraction studies of compound 1 was
obtained from DCM-ethanol-hexane mixture. The compound crys-
Complex
[Mg(TDMPP)(C2H5OH)]
C54H50MgN O
tallized in a monoclinic crystal system with P2 /C space group
1
Chemical Formula
Formula weight
Crystal system
Space group
4
9
and the perspective view of the compound is given in the Fig. 1.
923.29
Monoclinic
P21/C
In the complex 1, the metal magnesium is positioned at a dis-
˚
tance of 0.322A above the porphyrin plane with Mg-N(py) bond
Temperature (K)
293
˚
length are between 2.075 to 2.080A [Mg1-N1=2.080(2), Mg1-
a, b, c ( A˚ )
24.437(5), 8.792(5), 22.912(5)
90.000(5)˚,109.384(5)˚,90.000(5)˚
33, 12, 31
N2=2.075(2), Mg1-N3=2.076(2), Mg1-N4=2.079(2)], which agrees
α, β, γ
h, k, lmax
well with known Mg-N(py) bond length of magnesium porphyrins
A˚ 3
[
16,39]. The axial Mg-O (ethanol) bond distance is found as
˚
V
4644(3)
Z
4
2
.079(2)A which is in the range of all Mg-O bond distance of re-
Radiation Type
Wavelength (Ǻ)
Completeness
Mo Kα
0.71069
0.997
ported magnesium porphyrins [15,40]. A hydrogen bonding inter-
action between OH group of the axial ethanol molecule and a pe-
ripheral methoxy groups of a neighbouring porphyrin molecule,
and another similar hydrogen bonding interaction between a pe-
ripheral methoxy groups and axial ethanol molecule of another
neighbouring porphyrin makes the compound 1 as a weak one-
dimensional hydrogen bonded supramolecular structure (Fig. 2).
The hydrogen bonding distance between the methoxy oxygen atom
−
−
1
Absorption coefficient
dcalcd
0.102 mm
1.321 gcm
3
Data collection diffractometer
Absorption correction
No. of reflections collected/ unique
Rint
Bruker APEX-II CCD diffractometer
SADABS (Bruker, 2002)
90289/12369
7.12%
b
c
R indices
R1 = 9.15 %, wR2= 22.25 %
˚
S (Goodness of fit)
Deposition number
1.16
and axially linked ethanol molecule is 2.730(4) A. Although, it is
CCDC 1984143
very much challenging to locate hydrogen atoms (as only one elec-
tron is present) using X-ray diffraction [41–42] we could locate the
hydrogen atom attached to oxygen of ethanol molecule from dif-
ference Fourier map and refined isotropically. All the bond lengths
and bond angles of bound ethanol are within the normal ranges
a
b
c
Mo Kα radiation.
R1 = ꢀ||F0| - |Fc||/ꢀ|F0|.
wR2 = {ꢀ[w(F0 - Fc ) ]/ꢀ[w(F02)2]}1/2
2
2 2
.
[Z- no. of formulae units per unit cell; a,b,c– length of the cell edges; V- volume
of the unit cell; dcalad- density of unit cell calculated].
[
43]. Moreover, the position and bond length of H attached to
oxygen (ethanol) of the geometry optimized structure O-H (0.977
˚
A) corroborates the position of H of the single crystal data [O-H
˚
[Mg(TDMPP)(C2H5OH)] is presented in supporting information
Figure S3. The ethanol bound zinc porphyrin showed a red shift
of Soret and Q bands from ethanol free parent porphyrin due to
the axial ligation of the ethanol molecule which brings the metal
out of the porphyrin plane and alters the symmetry and energetic
distribution of frontier molecular orbitals [22,34]. The UV-visible
spectroscopic data of some of known similar compounds are given
in Table 2.
(
0.92(5) A].
The packing diagram of the compound 1 is given in the supple-
mentary information Figure S2. The crystallographic data for the
complex [Mg(TDMPP)(C H OH)] is given in Table 1.
2
5
UV-visible and Luminescence studies
The
UV-visible
spectrum
of
the
compound
[
Mg(TDMPP)(C H OH)] shows
a
typical Soret band (at 425
There are reports of formation of isoporphyrins from zinc por-
phyrin upon addition of oxidizing agents such as ceric ammonium
nitrate in nucleophilic solvents [22,45–48]. To understand the gen-
eration of similar type of compounds from magnesium porphyrin
2
5
nm) and two Q bands (in the range 550–600 nm), arises due to
∗
porphyrin π–π electronic transition [44]. The electronic spec-
tra of free base porphyrin H TDMPP and magnesium porphyrin
2
3

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Fig. 2. Intermolecular H-bonding interacton present in compound [Mg(TDMPP)(C2H5OH)].
Table 2
UV/Vis data of some Mg porphyrins.
Compound
Solvent
Soret band λ [nm]
Q bands λ [nm]
Reference
[
[
[
[
[
Mg(TDMPP)(C2H5OH)]
Mg(TPP)(THF)2]
Mg(TPP)(H2O)]
Chloroform
Toluene
424
426
423
405
428
553,603
This paper
524,564, 604
563,602
40
35
9
Chloroform
Benzene
MgTPP(C60Im)]
568, 608
563,603
Mg(TMPP)(H2O)]
Dichloromethane
14
very less (Fig. 4) compared to dilute solution (10 6M). This lower
intensity at higher concentration may be due to the effect of con-
centration quenching observed for zinc porphyrins [46,50]. Mg-
porphyrins show higher quantum yield than corresponding Zn-
porphyrins and free base porphyrins due to its longer excited state
lifetime [51–53]. Recently, we have determined the quantum yield
of a zinc isoporphyrin and found that isoporphyrin have lower
value of quantum yield than parent zinc porphyrin due to the loss
of macrocyclic aromaticity [22].
−
we have added ceric ammonium nitrate to a solution of the mag-
nesium porphyrin 1. Upon addition of ceric ammonium nitrate and
other oxidizing agents, the electronic spectrum changes immedi-
ately with the appearance of two peaks in the lower energy region
of electronic spectrum (700-900 nm) and simultaneous decrease of
the inteensity of Soret band. But, the electronic spectrum within
a couple of minutes changes to spectrum of porphyrin acid dica-
tion (Supporting Information Figure S4) may be due to dematal-
lation. The isolation and full characterization of the intermediate
isopophyrin type species was not sucessful even when used in a
basic solution to stop the demetallation. It is known that magne-
sium porphyrin undergoes dematallation even in the trace amount
of acidic water [34]. The electronic spectra of magnesium por-
phyrin and the intermediate with strong absorbance similar to iso-
porphyrin type species is shown in Fig. 3. It displayed the Soret
peak at 430 nm whereas the two Q bands appeared at 765 and
Electrochemistry
Electrochemical measurements were carried out using cyclic
voltammetery three electrode system to measure the redox po-
tential of compound [Mg(TDMPP)(C H OH)] in dichloromethane
2
5
medium. Similar to zinc porphyrins, magnesium porphyrins un-
dergo only porphyrin ring (ligand) centered redox reactions
[22,54].
8
58 nm.
The luminescence properties were studied in dichloromethane
solution at two different concentrations. The compound was found
to show two well-defined emission peaks at 608 and 661 nm
The cyclic voltammetric experiments were carried out using
tetrabutylammonium perchlorate (TBAP) as supporting electrolyte
in inert atmosphere. The compound shows two reversible oxida-
tion peaks. The first peak at E1/2= 0.69 V (vs. Ag/AgCl) is due to the
one electron oxidation of porphyrin to form porphyrin π-cation
(
Fig. 4) when excited at Soret band for (Q(0,0)) and (Q(0,1)) re-
spectively which is good agreement with emission peaks of other
Mg-porphyrins found in the literature [15,49]. The intensity of lu-
−4
+·
minescence spectrum at higher concentration (10 M) was found
radical [Mg(TDMPP)] and the second oxidation peak at 0.91 V vs
4

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Fig. 3. Electronic spectrum of [Mg(TDMPP)(C2H5OH)], (red trace) and on addition
of CAN, (black trace).
Fig. 5. Cyclic voltammogram of compound
[Mg(TDMPP)(C2H5OH)]
in
dichlormethane.
−
6
Fig. 4. Luminescence spectra of the compound 1 at concentration 10
trace) and 10 4 M (black trace) in dichloromethane (λ ex = 420 nm).
M (red
−
++
Ag/AgCl is due to formation of porphyrin dication [Mg(TDMPP)]
(
Fig. 5). The lower oxidation potential supports the oxidation of
++
compound to its dication [Mg(TDMPP)]
by chemical oxidants
like ceric ammonium nitrate (CAN) and subsequent conversion to
magnesium isopophyrin.
Theoretical calculations
The geometry of compound [Mg(TDMPP)(C H OH)] and
2
5
[
Zn(TDMPP)(C H OH)] were optimized by DFT method using basis
2 5
set LANL2DZ [28] along with functional B3LYP [29,30]. Using
the geometrically optimized structure electronic energy for the
compound was calculated to investigate the electronic property in-
tensively and to understand the contributions to chemical bonding
of metal center as well as surrounding ligand part by the TD-
SCF (Time Depending- Self Consistent Field) method. The partial
molecular energy level diagram has been derived from the elec-
tronic energy calculation using the contours of selective frontier
orbitals (HOMOs-LUMOs) and is shown in Fig. 6. From the energy
level diagram, it is found that metal center and axially bound
ethanol moiety does not have significant contribution towards the
Fig. 6. Frontier molecular orbital energy level diagram of compound
[Mg(TDMPP)(C2H5OH)]
5

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Table 3
Comparison of experimental and theoretical bond parameters of [Mg(TDMPP)(C2H5OH)], 1.
Bond
Experimental( A˚ )
Theoretical( A˚ )
Angle
Experimental(°)
Theoretical(°)
Mg-N1
2.080(2)
2.075(2)
2.076(2)
2.079(2)
2.079(2)
2.087
2.086
2.083
2.085
2.091
N1-Mg-N2
N1-Mg-N4
N3-Mg-N4
N3-Mg-N2
N1-Mg-N3
Mg1-O9-C53
N1-Mg-O9
88.77(8)
88.38(8)
88.77(8)
88.56(8)
162.683
128.2(2)
98.760
89.364
88.694
89.614
88.623
165.066
128.912
97.056
Mg-N2
Mg-N3
Mg-N4
Mg-O9(ethanol)
Table 4
frontier molecular orbitals (FMOs) as the metal center magnesium
has no d-orbital. The energy difference between HOMO and LUMO
for compound 1 is found to be as 2.66 eV which is responsible
for the lowest energy transition at 604 nm. The HOMO orbital
is mainly contributed by the four nitrogen Pz orbitals of pyrrole
whereas the LUMO orbitals are predominantly constructed by the
π- bond character of pyrrolic carbons.
The global reactivity indices for compound 1 and 2, (η- Chemical
hardness, s- chemical softness, μ-chemical potential, ω-global
electrophilicity index).
Compound
η (eV)
s (eV)
μ (eV)
ω (eV)
Mg(DMTPP)(EtOH)
Zn(DMTPP)(EtOH)
1.3298
1.3389
0.752
0.747
-3.841
-3.521
11.094
9.259
Some selective bond lengths and bond angles have been tab-
ulated in Table 3 with their experimental and calculated val-
ues. The values agree well with the experimental data obtained
from the crystal structure. These values are also in agree with
less deviation with earlier reported magnesium porphyrin com-
plexes [15,16,39,40]. The calculated UV-Visible spectrum shows
three bands, the intense Soret band (at about 420 nm) and two
Q-bands at around 565 nm similar to the experimental spectrum
with slight variation from experimental λmax values. The calculated
and experimental UV-Visible spectrum of the compound is shown
in supplementary information as Fig S6.
tions using HOMO-LUMO energies, and are listed in Table 4.
Chemical hardness, η of Zn(DMTPP)(EtOH) is slightly higher than
Mg(DMTPP)(EtOH), therefore Zn species is little a bit harder than
the Mg complex. Similarly, the chemical potential, μ is found to be
a bit higher negative for Mg complex. This may be due to pres-
ence of filled d orbital present in Zn metal. This values indicates
that Mg(DMTPP)(EtOH) is more reactive than Zn(DMTPP)(EtOH)
and suggests that magnesium porphyrin looses electron eas-
ily than zinc analogue. The parameter ω is smaller in case of
Mg(DMTPP)(EtOH) complex which means it is better electrophilic
in nature than the corresponding Zn(DMTPP)(EtOH) complex.
Global reactivity indices
Hirshfeld surface analysis and fingerprint plots
Employing the energy calculation data of the optimized struc-
ture of the Mg-porphyrin and zinc porphyrin the global reactivity
indices were calculated to understand the differences between zinc
and magnesium porphyrins. The chemical hardness (η) [55] and
chemical potential [56] (μ) were calculated utilizing the follow-
ing equations as mentioned below and subsequently the value of
chemical electrophilicity index [57] (ω) was calculated. And the
Hirshfeld surfaces analysis and calculation of associated finger-
print region has recently become an indispensible approach to in-
vestigate intensively different type of strong as well as weak in-
teractions present in the crystal lattice. The term Hirshfeld surface
was given in honour to F. L. Hirshfeld, who first used the ‘‘stock-
holder partitioning’’ model for defining atoms in molecules [61].
It divides the crystal space into molecular regions where aggre-
gate electron distribution of spherical atoms for a molecule (pro-
molecule) exceeds half of the total electron density of the crystal
softness (s) is 1
.
η
ELU MO − EHOMO
Chemical hardness, η =
Chemical potential, μ =
(1)
(2)
(3)
2
(procrystal) [62,63]. Weighing function w(r) can be used to explain
ELU MO + EHOMO
Hirshfeld surface, where w(r) ≥ 0.5. Here, ρa(r) is spherically av-
2
eraged electron density [62,63].
ꢀ
ꢁꢀ
μ2
w(r) =
ρ
a
(r)
ρ
a
(r) = ρ
(r)/ρprocrystal
promolecule
(4)
(
r)
Global electrophilicity index, ω =
η
a∈molecule
a∈crystal
The chemical hardness describes the reactivity and hence the
stability of the chemical species; it is a measure of resistance to
changes in its electronic configuration. Chemical potential is the
correlation between the electronic energy changes according to the
number of electrons involved during the charge transfer. The pa-
rameter global electrophilicity index is important which describes
the maximum energetic stabilization of a chemical species that
arises from accepting electronic charge. It is a positive quantity
which describes the tendency of the electrophile to get an extra
electronic charge determined by the chemical potential, μ. Since
an electrophile is capable of accommodating electrons, its energy
decreases when it accepts an electron and hence chemical po-
tential is negative, but hardness is positive. Very limited study
is known for the chemical reactivity indices of magnesium por-
phyrins [58–60].
Fingerprint plots can be drawn from Hirshfeld surface and was
introduced to remove the complication of displaying two dis-
similar but important distances mapped on a three-dimensional
molecular surface in a two-dimensional format. We have quan-
tified various kinds of major interactions for the compound
[Mg(TDMPP)(C H OH)] in crystal packing mode using this Hirsh-
2
5
feld surface analysis and correlated with 2D fingerprint plot. Be-
sides that, different quantitative measurements like volume (V ),
H
surface area (S ), globularity (G), asphericity (ꢁ) have also been
H
analyzed. The globularity [64] is observed less than 1 indicating
the more structured molecular surface not spherical.
The asphericity [65] was found as 0.091 showing isotropic na-
ture of the structure. The red specified spot in the dnorm sur-
face, Fig. 7(a), reveals a strong hydrogen bonding interaction be-
tween axially coordinated O-H group of ethanol with the O-atom
of neighboring m-OCH3 group of successive Mg-porphyrin moi-
ety and the faint red or white spots denote comparatively weaker
The chemical parameters η, μ and ω for both Zn and Mg por-
phyrin complexes are obtained from the above mentioned equa-
6

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B.P. Borah, S. Majumder, K.D. Borah et al.
Journal of Molecular Structure xxx (xxxx) xxx
Fig. 7. Different types of Hirshfeld surfaces of compound [Mg(TDMPP)(C2H5OH)] mapping with (a) dnorm ranging from -0.5 A˚ (red) to 1.75 A˚ (blue), (b) shape-index, (c)
curvedness, (d) 2D fingerprint plot for compound [Mg(TDMPP)] with de vs. di (e), (f), (g) and (h) are showing de vs. di for H···H, C···H, O···H, and N···H interaction respectively
with percentage.
dispersive interactions. Shape index and curvedness surface anal-
ysis also provide information about the crystal packing modes
and short contacts. The orange concave regions in shape index,
Fig. 7(b), are due to the acceptor atoms present in crystal whereas
the blue regions for donor H-atoms. In Fig. 7(c) the curvedness is
shown, which is a function of root mean square of surface of the
crystal, indicating no flat surface. It appeals there exists no stack
type interaction in the crystal packing. Calculation of 2D finger-
print plot of di vs de represents various kinds of respective short
contacts contributions in percentage towards the Hirshfeld surface
non-redox metallo-porphyrins. Hirshfeld surface analysis was also
performed to quantify the intramolecular interactions present in
the crystal structure.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
[
Fig. 7(d)]. From the analysis it is evident that the largest contribu-
CRediT authorship contribution statement
tion is from the H···H contact which is equivalent to 55% [Fig. 7(e)].
Apart from this some other interactions like C···H, O···H, N···H are
also present in the crystal as shown in (f), (g), (h) respectively of
Fig. 7.
Bishnu Prasad Borah: Investigation, Visualization, Writing -
original draft. Smita Majumder: Investigation, Software, Visual-
ization. Karishma Devi Borah: Software, Validation. Jagannath
Bhuyan: Conceptualization, Writing - review & editing.
Conclusions
Acknowledgments
An
ethanol
bound
magnesium
porphyrin
SM
acknowledge
national
postdoctoral
fellowship
[
Mg(TDMPP)(C H OH)] have been synthesized and character-
2
5
(
NPDF,PDF/2017/000865) from SERB, DST. The work was sup-
ized using conventional spectroscopic methods along with single
crystal X-ray diffraction method. The crystal structure reveals
that the compound shows one dimensional weak H-bonding
supramolecular structure due to axial ethanol molecule and a
methoxy group of an adjacent porphyrin. The electrochemistry
showed two reversible oxidation peaks at E1/2= 0.69 V and 0.91
V vs Ag/ AgCl respectively due to porphyrin ligand oxidation to
form π-cation radical and dictation. Theoretical calculation was
done using DFT method for magnesium porphyrin and its zinc
analogue. Furthermore, the disadvantages of using zinc porphyrin
for the study of magnesium porphyrin is computationally explored
in this work comparing the global reactivity indices like η, μ and
ω derived from theoretical calculations for zinc and magnesium
porphyrins. Theoretical calculations support that magnesium por-
phyrin is more reactive that zinc porphyrins. These results of this
work might have implications in the further study of reactivity
differences between magnesium and similar biologically relevant
ported by the project (MTR/2019/000223) from SERB, DST, New
Delhi. Authors are thankful to Gauhati University, Guwahati, Assam
for single crystal X-ray diffraction data.
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2020.129646.
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