Synthesis, characterization and electrochemistry of dipyrrinato nickel(II) complexes with different aromatic rings to meso-position

Article abstract of DOI:10.1016/j.poly.2017.02.022

Three kinds of dipyrrinato nickel(II) complexes with different aromatic substituted groups on meso-carbon position have been synthesized while their properties have been studied. The aromatic substituents of investigated compounds are phenyl (2a), 2-napht

Full text of DOI:10.1016/j.poly.2017.02.022

Accepted Manuscript
Synthesis, characterization and electrochemistry of dipyrrinato nickel (II) com-
plexes with different aromatic rings to meso-position
Zhaoli Xue, Yuan Dong, Jurui Ma, Yemei wang, Weihua Zhu
PII:
S0277-5387(17)30142-0
DOI:
http://dx.doi.org/10.1016/j.poly.2017.02.022
POLY 12489
Reference:
Polyhedron
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14 February 2017
Please cite this article as: Z. Xue, Y. Dong, J. Ma, Y. wang, W. Zhu, Synthesis, characterization and electrochemistry
of dipyrrinato nickel (II) complexes with different aromatic rings to meso-position, Polyhedron (2017), doi: http://
dx.doi.org/10.1016/j.poly.2017.02.022
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Synthesis, characterization and electrochemistry of dipyrrinato
nickel (II) complexes with different aromatic rings to meso-position
Zhaoli Xue*, Yuan Dong, Jurui Ma, Yemei wang and Weihua Zhu
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, People’s
Republic of China
Abstract: Three kinds of dipyrrinato nickel(II) complexes with different aromatic substituted
groups on meso-carbon position have been synthesized while their properties have been studied.
The aromatic substituents of investigated compounds are phenyl (2a), 2-naphthyl (2b) and 4-
1
carbazolylphenyl (2c), respectively. All compounds were characterized by H and 13C NMR
spectra, MS and UV-vis absorption spectra. These compounds were also investigated by
electrochemistry in dichloromethane containing 0.1 M tetra-n-butylammonium perchlorate
(TBAP). Each compound undergoes two reversible reductions along with one irreversible
oxidation and 2c shows another two quasi-reversible oxidations. The redox potentials of 2a-c are
consistent with the optical spectroscopy data and the relatively narrow HOMO-LUMO gaps that
are predicted in DFT calculation.The X-ray crystal structure of 5-(4-carbazolylphenyl) nickel(II)
dipyrrinato complex (2c) was also reported.
1 Introduction
Dipyrromethanes or dipyrrins are the essential building blocks in organic synthesis, which
play an important role in the synthesis of metal complex and numerous functional porphyrin
analogues [1-3]. Dipyrrin consists of two pyrrole rings linked with a sp2 carbon. Such compounds
are conveniently synthesized and the molecular structure is easily modificated like α- or β-position
substituted [4-7] of the pyrrole rings or meso-substituted [4,7-10] of carbon atom to yield different
performance dipyrromethene ligands.
Borondifluoride dipyrromethene known as BODIPY dye is a typical kind of dipyrromethane
derivatives. It was first synthesized and reported by Treibs and Kreuzer in 1968 [11]. BODIPYs
are easily to be modified with different functional groups [12-17] such as halogens, alkyl group,
aryl groups at meso- position or pyrrolic ring. These dyes are widely studied by the researchers
due to the remarkable properties including long absorption and emission wavelength [18-19],
good light stability [19], high fluorescence quantum yield [20-21], high molar extinction
coefficient [22-23].
The nitrogen atoms of dipyrromethane compounds belonging to the mild alkali have good
coordination ability for most metal ions [24-27]. Moreover, the transition metal ion can form a
stable complex under near-neutral conditions due to their smaller radius, higher charge and
ionization potential [28]. Dipyrrins can coordinate with transition metal to yield a variety of metal
complexes which have been allready applied in various areas such as transition metal catalysis,
photometric metal detection, or biomedical purposes etc. [29-39]. Therefore it aroused widespread
interest on the study of dipyrrinato metal complexes.
Recently, we have prepared and characterized a series of nickel(II) dipyrrinato complexes to

discuss about how the para-subtituents on the meso- phenyl ring affect the electrochemistry and
the chemical peaks of the aromatic protons [40]. In this paper, we wish to reported another three
kinds of nickel(II) dipyrrinato complexes to which at the meso-position we introduces not only
phenyl ring (2a) but also naphthalene ring (2b) and carbazole ring (2c) in order to analysis how
changes the aromatic system at meso-position will affect the electrochemical properties. The X-
ray crystal structure of 2c containing meso-carbazolyl phenyl group was reported. The impact of
the substituents at meso-position in 1H NMR spectroscopy properties was also dicussed.
Scheme 1. Synthesis routes of nickel(II) dipyrrinato complexes 2a-c. Condition: i) 1a: HCl/water
(0.12 mmol/L), r.t.; 1b and 1c: ii) TFA, DCM, r.t.; iii) DDQ, Et3N; iv) Ni(OAc)2, (CH2Cl)2.
2 Experimental
2.1. Measurements
1H NMR spectra were recorded in CDCl3 on a JEOL JNM−AL 400 spectrometer. Chemical
shifts are reported in units of ppm relative to the solvent residue peaks (CDCl3, δ = 7.26 ppm for
1H and 77.16 for 13C). Cyclic voltammetry was carried out at 298 K using an EG&G Princeton
Applied Research (PAR) 173 potentiostat/galvanostat. Elemental analysis was performed on a
Perkin–Elmer 240 C elemental analyzer. A homemade three-electrode cell was used for cyclic
voltammetric measurements and consisted of a platinum button or glassy carbon working
electrode, a platinum counter electrode and a homemade saturated calomel reference electrode
(SCE). The SCE was separated from the bulk of the solution by a fritted glass bridge of low
porosity which contained the solvent/supporting electrolyte mixture. X-ray crystallographic
analyses were carried out on a Bruker Smart Apex CCD diffractometer using monochromatic Mo-
K radiation (λ = 0.71073 Å) at 153 K using the -2 scan mode. The data were corrected for
Lorenz and polarization effects. The structures were solved by direct methods and refined on F2
using the full-matrix least-squares technique of the SHELXTL-2000 program package [41].
2.2.Theoretical Calculations
DFT calculations were performed with the Gaussian09 software package by using the
B3LYP functional theory. The LANL2DZ basis set was used for Nickel and 6-31G basis sets for
carbon, hydrogen, nitrogen and oxygen [43, 44].
2.3.Chemicals

All chemicals and solvents were analytical grade which purchased from commercial sourses
and used without further purification unless otherwise specified. Pyrrole need to be purified with
column chromatography on neutral aluminum oxide to afford a colourless liquid. Reactions were
monitored by TLC using silica gel plates and crude products were purified by flash
chromatography using silica gel (200 Mesh). The 4-(9H-carbazol-9-yl)benzaldehyde was
synthesized according to the published method [45].
2.4. General Procedure for the Preparation of 1a-c
5-phenyl substituted dipyrromethane (1a)
Benzaldehyde (0.51 mL, 5 mmol) and pyrrole (2.07 mL, 30 mmol) were added into a flask under
the N2 atmosphere. The reaction was treated with 0.12mol/L aqueous HCl (100 mL) and then
shielded from light. It was stirred at room temperature and the progress was monitored by TLC.
After about 30 min, the precipitated solid sticked to the flask. And then the products precipitated
was filtered off, washed with water and hexane. The residue was purified by silica gel column
chromatography and eluted with a mixture of dichloromethane/petroleum ether (1:2) to afford a
pale white solid 1a, 45%. 1H NMR (400 MHz, CDCl3) δ = 7.92 (s, 2H), 7.33 (dd, J = 7.8, 6.7 Hz,
2H), 7.22 (d, J = 7.1 Hz, 2H), 6.70 (s, 2H), 6.17 (d, J = 2.6 Hz, 2H), 5.92 (s, 2H), 5.48 (s, 1H) ppm.
13C NMR (101 MHz, CDCl3) δ = 142.22, 132.37, 128.57, 128.45, 127.02, 117.29, 108.38, 107.28,
43.99 ppm.
5-(2-naphthyl) substituted dipyrromethane (1b)
2-naphthaldehyde (0.31 g, 2 mmol) and pyrrole (0.69 mL, 10 mmol) was dissoved in 80 mL of dry
dichloromethane under nitrogen atmosphere. Then 50 μL of trifluoroacetic acid was added in
dropwise. The reaction was shielded from light and stirred for 35 min. The mixture was washed
with saturated NaHCO3, water, brines and then dried over Na2SO4. The solvent was evaporated
and the residue was purified by silica gel column chromatography using a mixture of
dichloromethane/petroleum ether (1:3) to obtain a pale white solid 1b, 54%. 1H NMR (400 MHz,
CDCl3) δ 7.99 (s, 2H), 7.79 (ddd, J = 9.6, 8.2, 5.3 Hz, 3H), 7.64 (s, 1H), 7.52 – 7.42 (m, 2H), 7.38
(dd, J = 8.5, 1.8 Hz, 1H), 6.72 (dd, J = 4.1, 2.6 Hz, 2H), 6.18 (dd, J = 5.8, 2.8 Hz, 2H), 5.97 (s,
2H), 5.65 (s, 1H) ppm. 13C NMR (101 MHz, CDCl3) δ = 139.57, 133.49, 132.56, 132.31, 128.39,
127.87, 127.65, 126.94, 126.70, 126.20, 125.88, 117.32, 108.53, 107.39, 44.14 ppm.
5-(4-carbazolylphenyl) substituted dipyrromethane (1c)
4-(9H-carbazol-9-yl)benzaldehyde (0.54 g, 2 mmol) and pyrrole (0.69 mL, 10 mmol) was
dissoved in 100 mL of dry dichloromethane under nitrogen atmosphere. Then 65 μL of
trifluoroacetic acid was added in dropwise. The reaction was shielded from light and stirred for 60
min. The mixture was washed with saturated NaHCO3, water, brines and then dried over Na2SO4.
The solvent was evaporated and the residue was purified by silica gel column chromatography
using a mixture of dichloromethane/petroleum ether (1:1) to obtain a pale white solid 1c, 13%. 1H
NMR (400 MHz, CDCl3) δ 8.15 (d, J = 7.7 Hz, 2H), 8.06 (s, 2H), 7.51 (t, J = 10.0 Hz, 2H), 7.48 –
7.37 (m, 6H), 7.35 – 7.26 (m, 2H), 6.78 (d, J = 1.5 Hz, 2H), 6.28 – 6.18 (m, 2H), 6.01 (s, 2H),
5.61 (s, 1H) ppm. 13C NMR (101 MHz, CDCl3) δ = 141.42, 140.81, 136.41, 132.17, 129.80,
127.11, 125.93, 123.37, 120.33, 119.95, 117.58, 109.80, 108.63, 107.52, 43.75 ppm.

2.5. Preparation of compound 2a-c
5-phenyl substituted nickel(II) dipyrrinato complex (2a).
1a (0.11 g, 0.5 mmol) was dissoved in 15 mL of dry 1,2-dichloroethane and DDQ (0.12 g, 0.5
mmol) was added into the solution for stirring 30 minutes, then 2 mL of triethylamine (TEA) was
added and stirred for 10 mins. The reaction solution was treated with Ni(OAc)2 (0.09 g, 0.5 mmol)
and reluxed for another 2 hours. After that, the reaction solution was cooled to room temperature
and solvents were removed by evaporation under reduced pressure. The residue was washed by
saturated NaHCO3, water, brine and dried over anhydrous Na2SO4. The solvent was evaporated
again and the crude product was purified by silica gel column chromatography using a mixed
eluent of dichoromethane/petroleum ether (1:1) to afford the pure nickel(II) dipyrrinato complexes
1
2a in 40% yield, green solid. 2a: green solid, Yield: 40%; H NMR (400 MHz, CDCl3, 25˚C,
TMS): δ = 9.38 (s, 4H, α-dipyrrin), 7.46 (dd, J = 10.7, 4.3 Hz, 6H, Ar-H), 7.44-7.37 (m, 8H, Ar-H
and β-dipyrrin), 6.74 (d, J = 4.1 Hz, 4H, β-dipyrrin) ppm. 13C NMR (101 MHz, CDCl3, 25˚C,
TMS) δ = 163.9, 140.4, 138.0, 134.0, 130.8, 129.9, 129.1, 128.8, 127.4, 126.9 ppm. UV-vis
(CH2Cl2): max () = 328 (16989), 472 (32146) nm; MS (MALDI-TOF): Calcd for C30H23N4Ni
[M+H]+: 497.127, Found: 497.125; Elemental Anal. Calc. for C30H22N4Ni: C, 72.47; H, 4.46; N,
11.27; Found: C, 72.83; H, 4.36; N, 11.76.
5-(2-naphthyl) substituted nickel(II) dipyrrinato complex (2b)
The synthesis process is the same as 2a. 1b (0.14 g, 0.5 mmol) was reacted with DDQ (0.12 g, 0.5
mmol), 2 mL of TEA and Ni(OAc)2 (0.09 g, 0.5 mmol) in 15 mL of dry 1,2-dichloroethane to get
2b. Green solid,Yield: 41%; 1H NMR (400 MHz, CDCl3, 25˚C, TMS): δ = 9.39 (s, 4H, α-dipyrrin),
7.98-7.84 (m, 8H, naphthyl-H), 7.63-7.52 (m, 6H, naphthyl-H), 7.48 (d, J = 3.9 Hz, 4H, β-
dipyrrin), 6.77 (d, J = 4.1 Hz, 4H, β-dipyrrin) ppm. 13C NMR (101 MHz, CDCl3, 25˚C, TMS) δ
(ppm) = 139.3, 134.4, 133.5, 132.3, 130.4, 128.5, 128.4, 127.7, 126.9, 126.6 ppm. UV-vis
(CH2Cl2): max () = 324 (21158), 475 (46931) nm; MS (MALDI-TOF): Calcd for C38H26N4Ni
[M+H]+: 597.159, Found: 597.157; Elemental Anal. Calc. for C38H26N4Ni: C, 76.41; H, 4.39; N,
9.38; Found: C, 76.85; H, 4.78; N, 9.52.
5-(4-carbazolylphenyl) substituted nickel(II) dipyrrinato complex (2c)
1c (0.19 g, 0.5 mmol) was dissoved in dry 1,2-dichloroethane and DDQ (0.12 g, 0.5 mmol) was
added into the solution for stirring 30 minutes, then the triethylamine (TEA) was added and stirred
for 10 mins. The reaction solution was treated with Ni(OAc)2 (0.09 g, 0.5 mmol) and reluxed for
another 2 hours. After that, the reaction solution was cooled to room temperature and solvents
were removed by evaporation under reduced pressure. The residue was washed by saturated
NaHCO3, water, brine and dried over anhydrous Na2SO4. The solvent was evaporated again and
the crude product was purified by silica gel column chromatography using a mixed eluent of
dichoromethane/petroleum ether (1:1) to afford the pure nickel(II) dipyrrinato complexes 2c in 43%
yield, green solid. Green solid, Yield: 43%; 1H NMR (400 MHz, CDCl3, 25˚C, TMS): δ = 9.51 (s,
4H, α-dipyrrin), 8.18 (d, J = 7.7 Hz, 4H, carbazole ring C-H), 7.71-7.65 (m, 8H, Ar-H), 7.60 (s,
4H, carbazole ring C-H), 7.55 (d, J = 8.2 Hz, 4H, carbazole ring C-H), 7.49-7.44 (m, 4H, β-
dipyrrin), 7.36-7.31 (m, 4H, carbazole ring C-H), 6.93 (s, 4H, β-dipyrrin) ppm. 13C NMR (101
MHz, CDCl3, 25˚C, TMS) δ (ppm) = 147.3, 141.4, 140.6, 140.4, 139.4, 139.1, 132.3, 132.2, 126.3,

126.1, 125.8, 125.6, 125.5, 123.7, 123.6, 120.4, 120.3, 118.1 109.8 ppm. UV-vis (CH2Cl2): max ()
= 325 (26823), 340(26129), 470 (61600) nm; MS (MALDI-TOF): Calcd for C54H37N6Ni [M+H]+:
827.243, Found: 827.241; Elemental Anal. Calc. for C54H36N6Ni: C, 78.37; H, 4.38; N, 10.15;
Found: C,78.57; H, 4.31; N, 10.26.
3 Results and discussion
3.1. X-ray diffraction analysis
Fig. 1. The crystal structure of 2c (a) top view and (b) side view with thermal ellipsoids set at 50%
probability.
The single crystal of compound 2c was obtained by slow evaporation of methanol to the
dichloromethane solution over several weeks. The X-ray structure of 2c was shown in Figure 1
and relevant details were summarized in Fig. 1 and Table S1, S2 and Fig. S1 of the Supporting
Information. As expect the Ni(II) ion was as coordinated to four pyrrolic nitrogen atoms with the
average bond lengths 1.885 Å for Ni-N in a distorted tetrahedron conformation. The dihedral
angle of the two dipyrrin ligands is 53.75o which is much more smaller as compare to the reported
Ni(II) dipyrrin complexes [37-40]. To one of the dipyrromethane ligand, the carbazole ring and
the ring that contains two pyrrole rings are almost at same plane while the central phenyl ring
takes orthogonal comformation to that plane. The dihedral angle of the two carbazole rings
(carbazole 1 and 2) is 55.17o. The distance between the two meso-carbon atoms C5-C5 is 6.724 Å
while the distance between the two nitrogen atoms of the carbazole rings is 18.141 Å.
3.2. 1H NMR spectroscopy

Fig. 2. 1H NMR spectra of 2a-c in CDCl3. The asterisk denotes a solvent peak.
1
The H NMR spectra of 2a-c recorded in CDCl3 are shown in Fig. 2 and Fig. S2-S4 of the
Supporting Information. The singlet peaks that located at 9.38, 9.39 and 9.51 ppm was indentified
as α-H of pyrrole ring for 2a, 2b and 2c, respectively. While the signals at 6.74, 6.77 and 6.93
ppm were assigned to β-H of pyrrole ring for 2a, 2b and 2c, respectively. From the change of the
chemical shift of α-H and β-H, it is easily to found that the introduction of three different aromatic
ring have much lower effect to the chemical shift of pyrrolic protons as compare to the Ni(II)
dipyrrin complexes when only introduces para-substituents on the meso-phenyl rings has great
affect the chemical peak [40].
3.3. UV-Vis spectroscopy
Fig. 3. UV-Vis absorption spectra of compound 2a (black), 2b (blue) and 2c (red) recoreded in
CH2Cl2 at room temperature.

The UV-Vis absorption specctra of 2a-c in CH2Cl2 is given in Fig. 3. All spectra show a
dominant absorption peak near 470 nm, weaker absorptions up to 350 nm. The shapes of
absorption spectra present tiny change among 2a-c. The maximum absorption wavelengths of the
complexes varied very little (2-5 nm) in the CH2Cl2, corresponding to 2a (472 nm), 2b (475 nm)
and 2c (470 nm), respectively. The extinction coefficient of 2c was quite higher than 2a and 2b,
indicating that the energy absorption capacity of 2c is stronger due to the introduction of carbazole
unit to the para-position of meso-phenyl ring.
3.4 Electrochemical Properties
Table 1 Half-wave potential (V vs. SCE) of 2a-c in CH2Cl2 containing 0.1 M TBAP
Oxidation
2nd
-
-
Reduction
2nd
-1.57
-1.46
-1.44
Compound
H-L(V)d
b
c
3rd
1st
ΔEO
-
-
1st
-1.19
-1.16
-1.14
ΔER
-
-
1.07a
1.04a
1.03
0.38
0.30
0.30
2.22
2.20
2.17
2a
2b
2c
1.59a
1.29
0.26
a Irreversible peak potential at 0.10 V/s scan speed.
b Potential difference between the second and first oxidations.
c Potential difference between the second and first reductions.
d The potential difference between the first oxidation and first reduction.
Fig.4. Cyclic voltammograms of 2a-c in CH2Cl2 containing 0.1 M TBAP recorded at 0.10 v/s.
The redox properties of 2a-c were examined in CH2Cl2 containing 0.1 M tetra-
nbutylammonium perchlorate (TBAP) as supporting electrolyte at room temperature. Cyclic
voltamograms of these compounds are shown in Fig. 4 and the half-wave potentials and peak

potentials for the irreversible electrode reactions are summarized in Table 1.
In CH2Cl2, the cyclic voltamograms of 2a-c show two reversible reductions and ΔER ranged
from 0.30 to 0.38 V vs SCE. 2a and 2b show one irreversible oxidation, while 2c shows two
quasi-reversible oxidation along with one irreversible oxidation.Compare to 2a, the reduction
potential values of 2b and 2c show anodic shift, which indicate they are easier to be reduced while
the oxidation potential show cathodic shift. The potential differences between the first reduction
and fitst oxidation (H-L gap) are 2.22, 2.20 and 2.17 V vs SCE for 2a, 2b and 2c, respectively.
This gap is a little lower as compare to the value to the reported metalloporphyrins [1]. Also the
oxidation potential values of 2c are very different from 2a and 2b, suggesting that substitution by
carbazolyl phenyl group at the meso-position affected the electrochemical properties more than the
phenyl and naphthyl moieties.
3.5 Electronic structures and theoretical investigation
In order to get further insight into electronic absorption spectra of 2a-c, DFT calculations were
performed at B3LYP/6-31G(d) level. The optimized molecular structures together with the
fronitizer molecular orbital maps are shown in Fig. 5. As can be deduced from Fig. 5, for 2a-c,
both LUMO and HOMO are almost identically localised, mainly on the dipyrromethene ligands.
This indicates that the first oxidation together with the first reduction should happen to the
dipyrromethene ligands instead of the central nickel ion [40]. Moreover, by introduced carbazolyl
moieties to the para-position of meso-phenyl ring is stabilized not only the HOMO but also the
LUMO level while the HOMO-LUMO gap is much more lower as compare to 2a and 2b. This
tendency is identical to the electrochemical properties of 2a-c.
Fig.5. Partial MO digrams of 2a-c at an isodurface value of 0.05 a.u.and the relative energies of

the frontier π-MOs.
4 Conclusion
In conclusion, three kinds of nickel(II) dipyrrinato complexes with different aromatic rings at
1
meso-position have been successfully been prepared. Their H and 13C NMR spectra, UV-Vis
spectra and electrochemical studies were carried out. Also, the X-ray crystal structures of 2c was
solved, which is asymmetric and appear to distorted geometry. In the cyclic voltammograms, each
complex undergoes two reversible reduction, 2a and 2b show one irreversible oxidation while 2c
shows two quasi-reversible oxidation along with one irreversible oxidation. The introduction of
expanded aromatic system to the meso-position to nickel(II) dipyrrinato complexes seems to have
weak effect both to the chemical peaks and the redox properies.
Acknowledgments
This work was supported by grants from the Natural Science Foundation of China (No. 21301074
and 21171076), the Natural Science Foundation of Jiangsu Province (No. BK20130483), China
Postdoctoral Science Foundation (No. 2013M540415 and 2014T70474), Postdoctoral Science
Foundation of Jiangsu Provience (1302153C) and Jiangsu Provience Universities Natural Sciences
Fund (No. 13KJB150010).
Appendix A. Supplementary data
CCDC: 1513054 contains the Supplementary crystallographic data for 2c [42]. These data can
be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
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