Rapid atmospheric carbon dioxide fixation by nickel(ii) complexes: meridionally coordinated diazepane-based 3N ligands facilitate fixation

Article abstract of DOI:10.1039/d1dt00299f

Octahedral complexes of the type [Ni(L)(H₂O)₃](ClO₄)₂(1and2), where L is the tridentate 3N ligand 4-methyl-1-(pyrid-2-ylmethyl)-1,4-diazacycloheptane (L1,1), or 4-methyl-1-(N-methylimidazolyl)-1,4-diazacycloheptane (L2,2), have been isolated and characterized using elemental analysis, ESI-MS and electronic absorption spectroscopy. The DFT optimized structures of1and2reveal that the tridentate 3N ligands are coordinated meridionally constituting a distorted octahedral coordination geometry around nickel(ii). In methanol solution, the complexes, upon treatment with triethylamine, generate the reactive red colored low-spin square planar Ni-OH intermediate [Ni(L1/L2)(OH)]⁺(1aand2a), as characterized by ESI-MS and electronic absorption spectroscopy, and energy minimized structures. The latter when exposed to the atmosphere rapidly absorbs atmospheric CO₂to produce the carbonate bridged dinickel(ii) complexes [Ni₂(L1/L2)₂(μ-CO₃)(H₂O)₂](ClO₄)₂(3and4), as characterized by elemental analysis and the IR spectral feature (～1608 cm^?1) characteristic of bridging carbonate. The single crystal X-ray structure of3reveals the presence of a dinickel(ii) core bridged by a carbonate anion in a symmetric mode. Both the Ni(ii) centers are identical to each other with each Ni(ii) possessing a distorted octahedral coordination geometry constituted by a meridionally coordinated 3N ligand, a carbonate ion and a water molecule. The decay kinetics of the red intermediates generated by1(k_obs, 7.7 ± 0.1 × 10^?5s^?1) and2(k_obs, 5.8 ± 0.3 × 10^?4s^?1) in basic methanol solution with atmospheric CO₂has been determined by absorption spectroscopy. DFT studies illustrate that meridional coordination of the 3N ligand and the electron-releasing imidazole ring as in2facilitate fixation of CO₂. The carbonate complex3efficiently catalyzes the conversion of styrene oxide into cyclic carbonate by absorbing atmospheric and pure CO₂with excellent selectivity.

Full text of DOI:10.1039/d1dt00299f

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DOI: 10.1039/D1DT00299F
ARTICLE
Rapid Atmospheric Carbon Dioxide Fixation by Nickel(II)
Complexes: Meridionally Coordinated Diazepane-based 3N
Ligands Facilitate Fixation
Received 00th January 20xx,
Accepted 00th January 20xx
Tamilarasan Ajaykamal,a Mitu Sharma,b Nasreen S. Islamb and Mallayan Palaniandavara*
DOI: 10.1039/x0xx00000x
Octahedral complexes of the type [Ni(L)(H2O)3](ClO4)2 1, 2, where L is the tridentate 3N ligand 4-methyl-1-(pyrid-2-
ylmethyl)-1,4-diazacycloheptane (L1, 1), or 4-methyl-1-(N-methylimidazolyl)-1,4-diazacycloheptane (L2, 2), have been
isolated and characterized using elemental analysis, ESI-MS and electronic absorption spectroscopy. The DFT optimized
structures of 1 and 2 reveal that the tridentate 3N ligands are coordinated meridionally constituting a distorted octahedral
coordination geometry around nickel(II). In methanol solution, the complexes, upon treatment with triethylamine,
generate the reactive red colored low-spin square planar Ni–OH intermediate [Ni(L1/L2)(OH)]+ (1a, 2a), as characterized by
ESI-MS and electronic absorption spectroscopy, and energy minimized structures. The latter when exposed to atmosphere
rapidly absorbs atmospheric CO2 to produce the carbonate bridged dinickel(II) complexes [Ni2(L1/L2)2(μ-CO3)(H2O)2](ClO4)2
(3, 4), as characterized by elemental analysis and IR spectral feature (~1608 cm-1) characteristic of bridging carbonate.
Single crystal X-ray structure of 3 reveals the presence of a dinickel(II) core bridged by a carbonate anion in a symmetric
mode. Both the Ni(II) centers are identical to each other with each Ni(II) possessing a distorted octahedral coordination
geometry constituted by the meridionally coordinated 3N ligand, a carbonate ion and a water molecule. The kinetics decay
of the red intermediates generated by 1 (kobs, 7.7 ± 0.1 × 10-5 s-1) and 2 (kobs, 5.8 ± 0.3 × 10-4 s-1) in basic methanol solution
with atmospheric CO2 has been determined by absorption spectroscopy. DFT studies illustrate that meridional
coordination of the 3N ligand and the electron-releasing imidazole ring as in 2 facilitate fixation of CO2. The carbonate
complex 3 efficiently catalyzes the conversion of styrene oxide into cyclic carbonate by absorbing atmospheric and pure
CO2 with excellent selectivity.
carbonic anhydrase2-3,8-9 activates and converts CO2 to
carbonic acid and the nickel-containing carbon monoxide
dehydrogenase10-11 (CODH) catalyzes the oxidation of CO to
CO2, followed by reduction of CO2 to CO. Also, dinuclear
metalloenzymes like the nickel containing urease12 are known
to selectively bind CO2 and catalyze the hydrolysis of urea as
well as redox reactions at ambient conditions.
Introduction
The concentration of the greenhouse gas carbon dioxide in our
atmosphere is ever-increasing due to continuous combustion
processes and this contributes enormously to global warming
and climate change. Currently, numerous investigations
around the world are aimed at overcoming the serious
environmental problems caused by CO2. Investigations on the
use of enzymatic,1a,2,3 chemical,1b,c photochemical,1d and
electrochemical4a methods have been undertaken to lower the
concentration of CO2 in the atmosphere by capturing and
fixing it. The development of effective chemical methods by
activating and fixing atmospheric CO2 into useful organic
compounds has gained a lot of significant attention currently.4-
In spite of its importance, only a limited number of
chemical methods of fixing CO2 and converting it into valuable
added products has been investigated. Coordination
complexes of many metal ions like iron,13-14 copper,15-16
nickel25,26 etc are known to activate atmospheric CO2 and fix it
as useful organic compounds like cyclic carbonates. Among
them, nickel complexes have attracted a lot of attention
currently.17-26 Very recently, certain dinickel azacryptand
complexes17 have been reported to selectively and reversibly
bind CO2. Also, complexes comprising Ni–OH moiety are
known to facilitate CO2 fixation.18-24 However, only a few Ni–
OH systems exist that selectively bind CO2 between two nickel
5
Apart from fixing it, its conversion to carbonate is very
important as many minerals exist as their carbonates1,6-7 on
the earth’s crust. In nature, the zinc-containing enzyme
centers.
[(TPA)Ni(OH)2Ni(TPA)](ClO4)2,
Thus,
the
dinuclear
where
Ni(II)
complex20
TPA is
a. School of Chemistry, Bharathidasan University, Tiruchirappalli 620024, Tamil
Nadu, India. E-mail: palanim51@yahoo.com
b. Department of Chemical Sciences, Tezpur University, Tezpur, Assam, India
†Electronic Supplementary Information (ESI) available: CCDC 1956983 (3).
Characterization of complexes, crystallographic, computational and kinetics data
are available as supplementary information. See DOI: 10.1039/x0xx00000x
tris(pyriylmethyl)amine, reacts with atmospheric CO2 to form
the carbonato complex [(TPA)Ni(CO3)Ni(TPA)](ClO4)2. Huang et
al. found that Ni(II)-hydroxide complexes of 2,6-
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Synthesis of ligands
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30
DOI: 10.1039/D1DT00299F
4-Methyl-(1-pyrid-2-ylmethyl)-1,4-diazacycloheptane (L1)
This ligand was reported by one of us earlier.30 In the present
work, it was prepared by adopting the procedure used by one
of us elsewhere for preparing 4-methyl-(1-pyrid-2-ylmethyl)-
1,4-diaza-cycloheptane.31
To
a
solution
of
N-
(a)
(b)
methylhomopiperazine (1.14 g, 10 mmol) and pyridine-2-
carboxaldehyde (1.07 g, 10 mmol) in CH2Cl2 (100 mL) was
added sodium triacetoxyborohydride with stirring (4.24 g, 20
mmol). The mixture obtained was stirred for 12 h, a saturated
solution of NaHCO3 was added, and the stirring continued for a
further 15 min. Then the solution was extracted with ethyl
acetate, and the organic layer was dried over Na2SO4, filtered
and rotaevaporated to dryness. The product was dissolved in
THF (50 mL) and treated with NaH (0.72 g, 30 mmol) to
remove traces of pyridyl-2-carbinol. After stirring the mixture
for 90 min, the solvent was removed, and the residue
extracted with several portions of hexane. The hexane extracts
were combined, and the hexane removed to obtain L1 as
brown oil which was used for preparing the complex 1. Yield:
Figure 1 a) Molecular structures of the tridentate 3N ligands L1 and L2 employed in
this study and b) the octahedral coordination geometries of [Ni(L1)(H2O)3](ClO4)2 1
and [Ni(L2)(H2O)3](ClO4)2·2H2O 2.
pyridinedicarboxamidate dianion fix CO2 very efficiently.22,23
The rapid fixation of atmospheric CO2 by hydroxo metal
complexes is of special interest, because the fundamental
understanding and application of the reaction may lead to
some practical methods of not only reducing the atmospheric
CO2 but converting it into useful compounds, in addition to
illustrating the molecular mechanism of metalloenzyme
activities and biomineralization. Also, the carbonate complexes
have been shown25 to catalyze the conversion of atmospheric
CO2 into cyclic carbonate, and the reaction is important as it
involves resource utilization, apart from having 100% atom
economy. The cyclic carbonates find wide use as chemical
intermediates,27 sustainable aprotic solvents28 and electrolytes
in Li-ion batteries.29
Herein we describe our serendipitous discovery of
spontaneous absorption and fixation of atmospheric CO2 by
the in situ generated mononuclear Ni–OH complexes of the
diazepane-based tridentate 3N pincer ligands L1 and L2 (Fig. 1)
during our studies on bis(μ-hydroxido)dinickel(II) complexes as
catalysts for alkane hydroxylation. The products of the
reaction are dinuclear Ni(II) complexes in which two nickel(II)
atoms are bridged by a carbonate anion. Interestingly, the CO2
fixing capability of the Ni-hydroxo complex with a strongly
electron-releasing N-Me-imidazole donor (L2) is higher than
that of the complex with a pyridyl donor. DFT studies on the
complexes and the red intermediates illustrate the difference
in reactivity. The μ-carbonato complex of L1 has been found to
catalyze the conversion of styrene oxide into phenylethylene
by absorption of atmospheric CO2.
1
1.03 g (46%). H NMR (400 MHz, CDCl3): δ 7.49 (m, 1H), 7.22
(m, 1H), 6.97 (m, 2H), 4.65 (s, 2H), 3.70 (s, 4H), 2.68 (m, 4H),
2.46 (d, 3H), 1.75 (m, 2H). Anal. Calcd. for C12H19N3: C, 70.20;
H, 9.33; N, 20.47. Found: C, 70.15; H, 9.48; N, 20.37. HR-MS
+
(MeOH) displays a peak at m/z 206.1651 [C12H20N3 ] (cal.
206.1652) (Fig. S18).
4-Methyl-1-(N-methylimidazol-2-ylmethyl-1,4-
diazacycloheptane (L2)
This ligand was synthesized by employing the procedure31
used for preparing L1. To
a
solution of N-
methylhomopiperazine (1.14 g, 10 mmol) and 1-
methylimidazole-2-carboxaldehyde (1.10 g, 10 mmol) in CH2Cl2
(100 mL) was added sodium triacetoxyborohydride (4.24 g, 20
mmol). The resulting mixture was stirred for 12 h, a saturated
solution of NaHCO3 added, and the stirring continued for a
further 15 min. The solution was then extracted with ethyl
acetate, and the organic layer dried over Na2SO4, filtered and
rotaevaporated to dryness. The oily product was dissolved in
THF (50 mL) and treated with NaH (0.72 g, 30 mmol) to
remove traces of imidazole carbinol. After stirring the mixture
for 90 min, the solvent was rotaevaporated, and the residue
extracted with several portions of hexane. The hexane extracts
were combined, and the hexane removed to obtain L2 as
brown oil which was used for preparing the complex 2. Yield:
Experimental
Materials
1
1.1 g (49%). H NMR (400 MHz, CDCl3): δ 6.93 (s, 1H), 6.87 (s,
1H), 4.90 (s, 5H), 4.13 (d, 4H), 2.68 (m, 5H), 2.38 (s, 2H), 1.85
(m, 2H). Anal. Calcd. for C11H20N4: C, 63.43; H, 9.68; N, 26.90.
Found: C, 63.50; H, 9.75; N, 26.77. HR-MS (MeOH) displays a
Pyridine-2-carboxaldehyde,
carboxaldehyde,
1-methylimidazole-2-
nickel(II)
N-methyl-homopiperazine,
perchlorate hexahydrate (Aldrich), sodium triacetoxy
borohydride, sodium hydride (Alfa Aesar), ethyl acetate,
hexane, THF, triethylamine, phosphorus pentoxide, sodium
hydroxide, sodium bicarbonate and sodium sulphate (Sisco
Research Laboratory, India) were used as received.
Dichloromethane, acetonitrile (Merck, India) and methanol
(Sisco Research Laboratory, Mumbai) were distilled before use.
+
peak at m/z 209.1770 [C11H20N4 ] (cal. 209.1761) (Fig. S19).
Isolation of complex [Ni(L1)(H2O)3](ClO4)2 (1)
A methanol solution (5 mL) of Ni(ClO4)2·6H2O (0.365 g, 1 mmol)
was added with stirring to a methanol solution (5 mL) of L1
(0.205 g, 1 mmol) at room temperature. The color of the
solution turned green and the solution stirred for 8 hours. The
precipitate obtained was filtered off, washed with small
amounts of cold methanol and dried over P2O5 under vacuum.
The
supporting
electrolyte
tetra-N-butylammonium
perchlorate (Aldrich) was recrystallized and used.
2 | J. Name., 2012, 00, 1-3
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Yield, 0.41 g, 72%. λmax/nm, in MeOH (Ɛmax/dm3 mol-1): 941 quantities of ice-cold methanol and then dried. The complex 4
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(105), 618 (80), 361sh. Anal. Calcd. for C12H25N3Cl2NiO11: C, was insoluble in any solvent. Anal. Calcd.DfoOrI:C1023.1H04349N/D8C1Dl2TN0i022O9193F:
27.88; H, 4.87; N, 8.13. Found: C, 27.75; H, 4.60; N, 8.22. HR- C, 33.33; H, 5.35; N, 13.52. Found: C, 33.40; H, 5.33; N, 13.42.
2-
MS (MeOH) displays a peak at m/z 322.1074, [M + Na – H2O – ATR (ν, cm−1): 1615 (CO3 ).
(ClO4)2]3+ (cal. 322.1041) (Fig. S20).
Isolation of complex [Ni(L2)(H2O)3](ClO4)2·2H2O (2)
Caution! Perchlorate salts of the compounds are potentially
A methanolic solution (5 mL) of Ni(ClO4)2·6H2O (0.365 g, 1 explosive! Only small quantities of these compounds should be
mmol) was added with stirring to a methanol solution (5 mL) prepared and suitable precautions should be taken when they
of L2 (0.208 g, 1 mmol) at room temperature. The solution are handled.
turned green and was stirred for 8 hours. The precipitate
obtained was filtered off, washed with small amounts of cold General Procedure for Cyclic Carbonate Synthesis
methanol and dried over P2O5 under vacuum. Yield, 0.45 g, A catalytic amount of the catalyst 3 (0.05 mol%) was dissolved
84%. λmax/nm, in MeOH (Ɛmax/dm3 mol-1): 903 (35), 636 (30), in CH3CN (2.0 mL) in a round bottom flask, and to this was
386sh. Anal. Calcd. for C11H30N4Cl2NiO13: C, 23.76; H, 5.44; N, added the epoxide styrene oxide (10 mmol) and Et3N (6 μM)
10.08. Found: C, 23.50; H, 5.62; N, 10.15. HR-MS (MeOH) and the reaction vessel tightly closed with a rubber septum.
displays a peak at m/z 356.7788, [M – (ClO4)2]2+ (cal. 356.1570) Atmospheric air (1 bar pressure)/pure CO2 was passed through
(Fig. S21).
the reaction mixture heated to 30-80 °C for 8 h. The product
formation was monitored by TLC for every 30 mins. After
Isolation of [Ni2(L1)2(μ-CO3)(H2O)2](ClO4)2·2H2O (3)
Method I: A methanolic solution (5 mL) of Ni(ClO4)2·6H2O completion of the reaction the vessel was cooled to ambient
(0.365 g, 1 mmol) was added with stirring to a methanol temperature and the catalyst was removed by passing the
solution (5 mL) of L1 (0.205 g, 1 mmol) at room temperature. reaction mixture through a silica-gel column. The product was
1
The green solution was stirred for 30 min and then Et3N (0.2 g, identified as 4-phenyl-1,3-dioxolan-2-one by using H and 13C
2mmol) was added when the color of the solution turned red. NMR and quantified by GC-MS/GC technique.
1
The solution was stirred for two hours in open air atmosphere. 4-Phenyl-1,3-dioxolan-2-one. H NMR (400 MHz, CDCl3): δ,
The blue colored precipitate obtained was filtrated off, washed 7.35-7.43 (m, 5H), 5.54-5.60 (t, 1H), 4.62-4.75 (m, 2H) ppm. 13
C
with small quantities of ice-cold methanol and then dried. NMR (400 MHz, CDCl3): δ, 154.70, 135.63, 129.06, 128.53,
Yield, 0.25 g, 44%.
125.63, 78.22, 71.03 ppm. GC-MS (m/z): 164.10.
Method II: A methanolic solution (5 mL) of Et3N (0.2 g, 2
mmol) was added with stirring to a green methanolic solution Physical measurements
(5 mL) of complex 1 (0.322 g, 1 mmol) at room temperature. 1H NMR spectra were recorded on a Bruker 400 MHz NMR
The red solution obtained was stirred for two hours in open air spectrometer. Chemical shifts for proton and carbon
atmosphere. The blue colored precipitate obtained was resonances are reported for the major isomer in parts per
filtrated off, washed with small quantities of ice-cold methanol million (d) relative to tetramethylsilane (d 0.00) and
and then dried. Yield, 0.13 g, 58%. λmax/nm, in MeOH chloroform (d 77.23) respectively. Multiplicities are indicated
(Ɛmax/dm3 mol-1): 897 (180), 589 (125), 368sh. Anal. Calcd. for by singlet (s), doublet (d), triplet (t) and multiplet (m). UV–
C25H46N6Cl2Ni2O15: C, 34.96; H, 5.40; N, 9.78. Found: C, 34.80; Visible and kinetic measurements were performed using a 1
2-
H, 5.42; N, 9.68. ATR (ν, cm−1): 1608 (CO3 ). HR-MS (MeOH) cm cuvette on a Varian Cary 300 UV–Vis spectrophotometer.
displays a peak at m/z 661.1806 [M + H+ – (ClO4)2]2+ (cal. FT-IR spectra were obtained using solid samples with a Bruker
661.0728) (Fig. S25). Blue colored single crystals suitable for X- ATR-Alpha (Diamond) spectrophotometer. Cyclic voltammetry
ray crystallographic analysis were obtained by slow (CV) and differential pulse voltammetry (DPV) on glassy carbon
evaporation of a CH3CN/CH3OH solution of the complex.
Isolation of [Ni2(L2)2(μ-CO3)(H2O)2](ClO4)2 (4)
electrode were performed in MeOH at 25 °C. The
voltammograms were generated using CH instruments 620C
Method I: A methanol solution (5 mL) of Ni(ClO4)2·6H2O (0.365 electrochemical analyzer. A three electrode system used to
g, 1 mmol) was added with stirring to a methanol solution (5 study the electrochemical behavior of complexes (0.001 M)
mL) of L2 (0.208 g, 1 mmol) at room temperature. The solution consisted of a glassy carbon working electrode (A, 0.0707 cm2),
turned green and was stirred for an additional 30 min with the a platinum wire auxiliary electrode and a saturated calomel
addition of Et3N (0.2 g, 2mmol). The color of the solution reference electrode and TBAP (0.1 M) was used as the
turned red and the solution further stirred for 30 min in open supporting electrolyte. Solutions were deoxygenated by
air atmosphere. The resulting green colored precipitate was purging with nitrogen gas for 15 min prior to the
filtrated off, washed with small quantities of ice-cold methanol measurement. Microanalyses (C, H and N) were carried out
and then dried.
using PR 2400 Series II PerkinElmer equipment. High resolution
Method II: A methanol solution (5 mL) of complex 2 (0.356 g, 1 mass analyses were performed using electron spray ionization
mmol) was added with stirring to a methanol solution (5 mL) (ESI) technique on a Thermo Scientific exactive plus EMR
of Et3N (0.2 g, 2mmol) at room temperature. The green instrument. GC-MS and GC analysis were performed on Agilent
solution was turned into red. The red color solution further 5977E GCMSD using HP-5 MS ultrainert (30 m × 250 μm × 0.25
stirred for 30 min in open air atmosphere. The resulting green μm) capillary column.
colored precipitate was filtrated off, washed with small
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
Crystallographic data collection, refinement and structure [Ni(L1)(H2O)3](ClO4)2 1 and [Ni(L2)(H2O)3](ClO4)2·2H2O 2 on the
View Article Online
DOI: 10.1039/D1DT00299F
basis of elemental analysis, High Resolution Mass
solution
The molecular structures of the complex 3 was unambiguously Spectrometry, conductivity measurements and UV-visible
determined by measuring the X-ray intensity data on a Bruker spectroscopy and the formulae supported by the energy
SMART APEX II single crystal X-ray CCD diffractometer having minimized octahedral structures of cations of 1 and 2 (cf.
graphite-monochromatized Mo-Kα (λ = 0.71073 Å) radiation at below, Fig. 3). ESI-MS reveal that both 1 (m/z 322.1074, [M +
ambient temperature. The structure was solved using direct Na – H2O – (ClO4)2]3+) and 2 (m/z 356.7788, [M – (ClO4)2]2+)
methods with SHELXS-97 and refined using SHELXL-97, remain as mononuclear complex species in solution. Such
SHELXTL and SHELXL-2014, SHELXTL.41 The graphics interface complexes are interesting but no single crystals of them could
package used was PLATON, and the figures were generated be grown to determine their molecular structures using X-ray
using the ORTEP 3.07 generation package.42 The positions on diffraction. The dinickel(II) complexes 3 and 4 were prepared
all the atoms were obtained by direct methods. Metal atoms in by adding one equivalent of the ligand L1 or L2 to a methanol
each complex were located from the E-maps and non- solution of Ni(ClO4)2·6H2O to generate the complexes 1 and 2
hydrogen atoms were refined anisotropically. The hydrogen in situ, followed by adding excess of Et3N in methanol, and
atoms bound to the carbon were placed in geometrically exposing the red solution obtained to atmospheric CO2. They
constrained positions and refined with isotropic temperature were also prepared by treating a methanolic solution of their
factors, generally 1.2 Ueq of their parent atoms. parent complexes 1 and 2 by with Et3N and then exposing it to
Crystallographic data are listed in Table 1 and Table S1.
atmospheric CO2. They are formulated as [Ni2(L1/L2)2(μ-
CO3)(H2O)2](ClO4)2·nH2O (3: n=2; 4: n=0) on the basis of
elemental analysis, High Resolution Mass Spectrometry (m/z
Computational studies
DFT studies have been carried out to understand the effect of 661.1806 [M + H+ – (ClO4)2]2+), and UV-visible spectroscopy,
replacing pyridyl nitrogen by imidazolyl nitrogen on the and the formula confirmed by the X-ray crystal structure of 3
capability of the tridentate ligands to involve in meridional (cf. below, Fig. 2). They can be synthesized also by bubbling
coordination to Ni(II). The coordination geometries of Ni(II) CO2 or by adding a carbonate salt to the red solution. The
complexes 1 and 2 with triplet spin in the ground state were molar conductivity of 1, 2 and 3 in methanol solution (ΛM/Ω−1
fully optimized by using Density Functional Theory function cm2 mol−1: 237–260) fall in the range observed for 1:2
(DFT) at U-B3LYP level of theory by employing Gaussian 09 electrolytes.
program package.43 The calculations were carried out with a
mixed basis set of LanL2DZ for the nickel centre, which has a Description
of
the
structure
of
[Ni2(L1)2(μ-
relativistic effective core potential with a valence basis set, and CO3)(H2O)2](ClO4)2·4H2O 3
6-31G for the remaining atoms.44-46 The normal mode analyses The molecular structure of the complex cation of [Ni2(L1)2(μ-
were performed to check the minimal energy nature of the CO3)(H2O)2](ClO4)2·4H2O 3 is shown in Fig. 2 and the packing
geometry. The complexes 1 and 2 were considered as diagram shown in Fig. S2. It consists of a dinickel(II) core
dipositive cations while nickel metal was considered in +2 formed by bridging of two Ni(II) centers through a carbonate
oxidation state. The solvation of the complexes was carried anion. Both the Ni(II) centers are identical to each other with
out by employing CPCM method using methanol as the each Ni(II) possessing a distorted octahedral coordination
solvent. The coordination geometries of Ni(II) intermediate geometry constituted by the three nitrogen atoms of
complexes 1a and 2a, derived from 1 and 2 respectively, with meridionally coordinated L1, two oxygen atoms of the bridging
singlet spin in the ground state were fully optimized by using CO32−, and the oxygen atom of a water molecule. The
Density Functional Theory function (DFT) at U-B3LYP level of meridional coordination of L1 is similar to that observed in the
theory by employing Gaussian 09 program package,39 as copper(II) complex [Cu(L1)Cl2].32 The diazepane backbone of
described above for 1 and 2. The complexes 1a and 2a were the tridentate 3N ligand accounts for its meridional
considered as monopositive cations while nickel metal was coordination to Ni(II), which is in contrast to the facial
considered in +2 oxidation state. The solvation of the coordination of the tridentate ligand bis(pyrid-2-
complexes was carried out by employing CPCM method using ylmethyl)(tert-butyl)amine (tBu-DPA) to Ni(II).21 This is
methanol as the solvent.
interesting as 3N ligands with two pyridyl donors like bis(pyrid-
2-ylmethyl)amine (DPA)33-35 and tBuDPA21 are involved in the
more commonly adopted cis-facial34,35 rather than meridional
coordination. The Ni–Namine (2.082(4), 2.107(4) Å) and Ni–Npy
(2.068(4) Å) bond lengths (Table 1) fall in the ranges observed
already for similar Ni(II) complexes.20,37 The carbonate ion is
bound to both the Ni(II) centers in a μ2-κ2:κ2 mode,20,26 with
the Ni–O(CO32−) bond lengths being 2.088(3) and 2.073(4) Å.
The C–O bond lengths (C–O2, 1.281(4), C–O2a, 1.281(4), C–O3,
1.304(6) Å) suggest that the bound carbonate ion is strongly
delocalized. Also, the two pyridyl moieties are oriented in
Results and discussion
Synthesis and characterization of ligands and complexes
The
ligands
4-methyl-1-(pyrid-2-ylmethyl)-1,4-
diazacycloheptane30 (L1) and 4-methyl-1-(N-methylimidazolyl)-
1,4-diaza-cycloheptane (L2) were prepared according to a
known reductive amination procedure.31 The nickel(II)
complexes 1 and 2 were isolated in good yields as green solids
by adding one equivalent of L1 or L2 to a methanol solution of
one equivalent of Ni(ClO4)2·6H2O. They are formulated as
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directions opposite to each other and the two coordinated and reactivities but unfortunately, we failed in ourVieawttAertmiclepOtsnlitnoe
DOI: 10.1039/D1DT00299F
water molecules are also
obtain single crystals of 4.
DFT studies: Structures of nickel(II) complexes 1, 2 and
intermediates 1a, 2a
The optimized geometries of cations of the complexes 1 and 2
have been computed using density functional theory (DFT).
The optimized structures of the complexes are shown in Figs 3
and 4, and the geometrical parameters viz. bond lengths,
optimized energies, frontier MOs, and HOMO–LUMO energy
gaps have been calculated (Table 2, S2, S3 and S4; Fig. 5, 6, 7
and S15, 16†) at B3LYP hybrid exchange-correlation function
with 6-31G/LANL2DZ combined basis sets using Gaussian 09
program package. The distorted octahedral coordination
geometries of Ni(II) cations are constituted by all the three
nitrogen atoms of meridionally coordinated L1 and L2, and the
oxygen atoms of three water molecules. The Ni–Namine (1:
2.140, 2.115; 2: 2.132, 2.149 Å), Ni–Npy (2.077 Å) and Ni–Nim
(2.059 Å) bond lengths (Table 2) fall in the ranges observed
already for similar Ni(II) complexes,37,38 suggesting that the
optimized structures are reliable and can be used to
understand the structural variations and reactivity. Of the
three Ni–O bonds, the one (Ni–O1, 2.088 Å) trans to Ni–N2 is
much shorter than the other two (Ni–O2, 2.152; Ni–O3, 2.136
Å). To understand the difference in reactivities of 1 and 2
towards CO2, the optimized geometries of the red
intermediates [Ni(L1)(OH)]+ 1a and [Ni(L2)(OH)]+ 2a, obtained
(cf. below) by treating respectively 1 and 2 in methanol
solution with triethylamine, have been also computed (Fig. 4).
Figure 2 ORTEP diagram of complex cation of 3 showing 50% probability thermal
ellipsoids and the labeling scheme for selected atoms. All the hydrogen atoms,
solvent molecules and the perchlorate counter ions are omitted for clarity.
Table 1 Selected bond lengths [Å] and bond angles [°] for [Ni2(L1)2(μ-CO3)(H2O)2](ClO4)2
·4H2O 3
Bond lengths [Å]
Bond Angle [°]
Ni1–N1
Ni1–N2
Ni1–N3
Ni1–O1
Ni1–O2
Ni1–O3
2.068(4)
O1–Ni1–O2 60.98(11)
O1–Ni1–O3 159.04(13)
O2–Ni1–O3 98.24(12)
N1–Ni1–N2 82.83(15)
N2–Ni1–N3 77.53(15)
N1–Ni1–N3 160.09(15)
2.082(4)
2.107(4)
2.2641(11)
2.088(3)
2.073(4)
Figure 3 The computed coordination structures of cations of [Ni(L1)(H2O)3](ClO4)2 1 and
[Ni(L2)(H2O)3](ClO4)2·2H2O 2.
N3–Ni1–O3 91.82(16)
N1–Ni1–O1 86.53(11)
oriented similarly, which has implications for the proposed
mechanism of CO2 fixation (cf. below). Such a compact
coordination geometry of 3 with the diazepane backbone
accounts for the spontaneous and rapid fixation of
atmospheric CO2 by 1 and 2. A comparison of the structure of
3 with that of 4 would have thrown light on their formation
Figure 4 The energy minimized structures computed for the cationic intermediate species
[Ni(L1)(OH)]+ 1a and [Ni(L2)(OH)]+ 2a.
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The intermediates possess low-spin square planar
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coordination geometry formed by the thDreOeI:n10it.1r0o3g9e/nD1aDtTo0m02s99oFf
meridionally coordinated L1 and L2, and an oxygen atom of
hydroxyl group. The Ni–Namine (1: 1.967, 1.960; 2: 1.984, 1.996
Å), Ni–Npy (1.915 Å) and Ni–Nim (1.890 Å) bond lengths (Table
2) fall in the ranges observed for related square planar low-
spin Ni(II) complexes in their X-ray crystal structures,25,38
suggesting that the optimized structures of 1a and 2a are
reliable and can be used to understand the reactivity and
structural variations (Fig. 4, see in ESI†). Thus, upon formation
of 1a and 2a from respectively 1 and 2, all the Ni–N and Ni–O1
bond lengths in them decrease, illustrating that the nitrogen
donor atoms come closer to Ni(II), facilitated by the diazepane
Table 2 Computed structural parameters and values of the HOMO–LUMO energy gap
in complexes 1, 2 and the intermediates 1a and 2a.
Parameters
1
2
1a
2a
Bond
lengths [Å]
Ni–N(1)
Ni–N(2)
Ni–N(3)
Ni–O(1)
Ni–O(2)
Ni–O(3)
2.140
2.115
2.077
2.088
2.152
2.136
2.132
2.149
2.059
2.086
2.149
2.132
1.967
1.960
1.915
1.844
-
1.984
1.996
1.890
1.842
-
back bone, to effect a higher ligand field strength and pair the
2
electron spins in Ni(II) to give the low-spin (dxz,dyz)4(dxy)2(dz
)
2
configuration. It is expected that the [Ni(L1/L2)(OH)]+ species
with the latter configuration would act as an electron-rich
super nucleophile, like the reactive [(4N)CoI] species with
-
-
(dxz,dyz)4(dxy)2(dz
2
)
configuration, obtained by two-electron
2
reduction of alkylcobalamin, towards electrophilic carbon
atom of CO2 (cf. below). The Ni–O1(H2O) bond length also
decreases upon deprotonation from 2.088 (1) to 1.844 (1a)
and 2.086 (2) to 1.842 (2a) Å, illustrating the stronger
coordination of oxygen atom of hydroxyl group. Computations
reveal that the electron-releasing imidazole donor in the
former destabilizes39 the HOMO orbital in 2a (1a: -6.3604; 2a: -
6.2700 eV) relative to the pyridyl donor in 1a (cf. above).
Similarly, the LUMO in 2a is more destabilized than that in 1a
(1a: -1.8444; 2a: -1.6393 eV) with the HOMO – LUMO energy
gap being lower in 1a than in 2a (1a: 4.5160; 2a: 4.6307 eV).
However, the HOMO – LUMO+1 energy gap in 1a is higher
Optimized
-2.81 ×
-2.85 ×
104
-6.8254
-2.39 ×
104
-6.3604
-2.44 ×
104
-6.2700
energy (eV) 104
HOMO (eV) -6.8415
LUMO (eV) -1.9448
-0.5883
6.2371
-1.8444
4.5160
-1.6393
4.6307
HOMO–
LUMO
energy gap
(eV)
4.8967
Figure 6 Electron density plot of HOMO of intermediates 1a and 2a, LUMO+1 of 1a
and LUMO of 2a.
Figure 5 HOMO and LUMO band gap Energy profile diagram for 1 and 2 were
calculated in U-B3LYP level of theory with mixed basis sets LANL2DZ/6-31G and
methanol as a solvent in CPCM method.
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Ɛmax/dm3 mol-1: 897 (180), 589 (125), 368sh, Fig. 8 and Table 3)
View Article Online
DOI: 10.1039/D1DT00299F
typical of octahedral coordination geometry around Ni(II). In
CH3CN solution, the ν1 and ν2 bands are slightly shifted
(λmax/nm, Ɛmax/dm3 mol-1: 905 (165), 580 (95), 396sh, Table 3),
revealing the stronger binding of CH3CN solvent molecules to
Ni(II). While the lowest energy band is assigned to ν1
transition, the higher energy band to ν2 transition and the
intense highest energy band around 368 nm to ν3 transition in
Ni(II) located in an octahedral environment. The ν1 band
energy (897 nm) of 3 is higher than that of 1, suggesting that
the carbonate anion chelates more strongly in 3 contributing
to the higher ligand field strength.
500
450
400
350
300
250
Figure 7 HOMO and LUMO band gap Energy profile diagram for 1a and 2a were
calculated in U-B3LYP level of theory with mixed basis sets LANL2DZ/6-31G and
methanol as a solvent in CPCM method.
200
a
150
than the HOMO – LUMO energy gap in 2a (1a: 4.7103; 2a:
4.6307 eV), which is consistent with the higher energy of the
CT band observed (cf. above) for 1a (1a: 524; 2a: 540 nm). It
may be noted that they have similar electron density
distribution. It may be noted that LUMO+1 in 1a and LUMO in
2a have similar electron density distribution (Fig. 6, 7).
b
100
50
400
500
600
700
800
900 1000 1100
Wavelength (nm)
Figure 8 Electronic absorption spectra of [Ni2(L1)2(CO3)(H2O)2](ClO4)2·2H2O 3 in
(a) Methanol and (b) Acetonitrile solution at 25 °C.
Electronic absorption spectral and electrochemical properties
The electronic spectral features observed for 1 and 2 in
methanol solution (λmax/nm, Ɛmax/dm3 mol-1: 1, 941 (105), 618
(80), 361sh, 2, 903 (35), 636 (29), 386sh, Table 3, Figure S4)
reveal that the octahedral coordination geometries of the
complexes are maintained in solution. The lowest energy band
0.00000
-0.00001
-0.00002
-0.00003
3
3
is assigned to A2g → T2g(F) (ν1) transition while the higher
3
energy band to 3A2g → T1g(F) (ν2) transition in Ni(II) located in
the octahedral environment. The intense shoulder observed
3
3
around 360 (1) and 390 nm (2) is assigned to A2g → T1g(P)
(ν3) transition. The lower intensity absorption spectral feature
3
1
around 750 nm is assigned to A2g → E1g(D) spin-forbidden
transition. All these assignments are in good agreement with
those observed for similar octahedral Ni(II) complexes
reported previously.40 The ν1 band energy of 1 (941 nm) is
lower than that of 2 (903 nm), which is expected of the more
basic N-Me-imidazole nitrogen donor (pKa: pyH+, 5.2, MeImH+,
7.0), and hence its stronger coordination in 2. Also, the ν1 band
2
1
-0.00004
energy of
1
is higher than that (1096 nm) of
[Ni(tBuDPA)(H2O)3](ClO4)2,21 which reveals that the diazepane
backbone of the meridionally coordinated L1 confers a ligand
field strength higher than the cis-facially coordinating tBuDPA.
In methanol solution, the carbonato complex 3 displays well
defined electronic absorption spectral features (λmax/nm,
-1.1
-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
E (V)
Figure 9 Cyclic voltammograms of complexes 1 (1.9 × 10-3 M) and 3 (1.8 × 10-3 M) in
methanol solution at 25 °C. Conditions: supporting electrolyte, 0.1 M TBAP; Scan rate,
50 mVs-1; reference electrode, Calomel; working electrode, Glassy carbon; Counter
electrode, platinum plate.
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DOI: 10.1039/D1DT00299F
Table 3 UV-visible spectral data of nickel(II) complexes 1, 2, 3 and intermediates 1a and 2a in CH3OH solvent at 25 °C.
Complex
Solvent
Electronic spectra
Wavelength (nm)
CV
DPV
E1/2 (V)
Ɛ
Epc (V)
Epc1/2 (V)
(M-1cm-1)
[Ni(L1)(H2O)2](ClO4)2 (1)
Methanol
Methanol
941 (ν1)
618 (ν2)
361 (ν3)
105
85
sh
-0.841
-0.984
-0.960
-
-0.663
-0.781
-0.760
-
-0.745
-0.901
-0.825
-
[Ni(L2)(H2O)2](ClO4)2·2H2O (2)
903 (ν1)
636 (ν2)
386 (ν3)
35
30
sh
[Ni2(L1)2(CO3)(H2O)2](ClO4)2·2H2O (3) Methanol
[Ni2(L1)2(CO3)(H2O)2](ClO4)2·2H2O (3) Acetonitrile
897 (ν1)
589 (ν2)
376 (ν3)
180
125
sh
905 (ν1)
577 (ν2)
396 (ν3)
165
100
sh
[Ni(L1)(OH)]+ 1a
intermediate
Methanol
Methanol
474
524
3,700
3,620
-
-
-
-
-
-
[Ni(L2)(OH)]+ 2a
intermediate
463
540
1,310
sh
Conc. 1: 4.5 × 10-3, 2: 3.1 × 10-3 M; 1a, 2a: 2 × 10-4, 3: 7.6 × 10-4 M in methanol; 3, 2.7 × 10-3 M in acetonitrile solvent. Cyclic voltammograms and DPV of complexes 1
(1.9 × 10-3 M), 2 (5.2 × 10-3) and 3 (1.8 × 10-3 M) in methanol. Conditions: supporting electrolyte, 0.1 M TBAP; Scan rate, CV: 50 mVs-1, DPV: 2 mVs-1, for all complexes,
reference electrode, calomel; working electrode, glassy carbon.
The cyclic voltammetry (CV) responses obtained for 1 and 2 in was stirred for two hours in an open atmosphere, a blue
methanol solution reveal that the Ni(II)/Ni(I) redox couples are colored precipitate was obtained. The product was identified
completely irreversible, as no reoxidation wave is observed and formulated as [Ni2(L1)2(μ-CO3)(H2O)2](ClO4)2·2H2O 3, which
(Fig. 9, S5). The Ni(II)/Ni(I) redox potential of 2 is more is confirmed by its X-ray crystal structure (Fig. 2). The sharp IR
negative than that of 1 (E1/2: 1, -0.745; 2, -0.901 V vs calomel, spectral feature observed around 1608 cm-1 is characteristic of
DPV, Table 3), which is expected of the more basic N-Me- carbonate anion involved in symmetric μ-mode of
imidazole nitrogen donor (cf. above) and hence its stronger coordination as found in the X-ray crystal structure.25,36 When
coordination in 2. This is in agreement with the observation of pure CO2 gas was bubbled through the red solution, the color
ν1 band for 2 at energy higher than that for 1. The Ni(II)/Ni(I) changed from red to green much faster than when exposed to
redox of 3 is irreversible (E1/2: -0.825 V, Table 3), which is more atmospheric CO2, and the blue colored product obtained was
negative than that of its monomeric Ni(II) complex 1, which is identified as 3 by using its UV-Vis spectral features. When a
consistent with the observation of the ν1 band at energy methanolic solution of NaOH was used in the place of Et3N, a
higher for 3 than that for 1.
similar red coloured solution was formed and finally a blue
precipitate of 3 was obtained. When a methanolic solution of
Carbon Dioxide Fixation: Characterization and reactivity of the complex was acidified with dil. HCl, solvated Ni2+ ions are
intermediate [Ni(L1/L2)(OH)]+ species
formed, indicating that the complex 3 is unstable. However,
When a methanolic solution of Ni(ClO4)2·6H2O was added with when sodium hydroxide was added to a methanolic solution of
stirring to a solution of an equivalent amount of L1 in 3 no change in the absorption spectrum was observed,
methanol at room temperature a green solution was obtained. revealing that the carbonato complex is stable in basic
The latter was treated with two equivalents of Et3N to give a solution. The Ni(II) complex 4 of L2 isolated by adopting the
red colored solution (pH, 10). Interestingly, when the latter procedure used for 3 was found to have the composition
8 | J. Name., 2012, 00, 1-3
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possibly to a metal to ligand charge transfer (MLCT) transition.
We have already established the ability of the related
diazepane-based 4N ligands N,N′-bis(2-pyrid-2-ylmethyl)-1,4-
diazacycloheptane and N,N′-bis((1-methyl-1H-imidazole-2-
View Article Online
DOI: 10.1039/D1DT00299F
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
524 nm
474 nm
yl)methyl)-1,4-diaza-cycloheptane to impose
a
low-spin
configuration on Ni(II).25,38 A similar mononuclear square
planar complex [(L3)NiII-OH]−, where L3 is the diacidic
tridentate 3N ligand 2,6-pyridinedicarboxamidate, has been
prepared, characterized (λmax, 411, 490 nm) and X-ray
structure determined.23,24 When N2 gas was purged
continuously for an hour through solutions of 1 and 2 in
methanol to remove any dissolved dioxygen, and then Et3N
was added, the red solutions characteristic of the intermediate
species 1a and 2a were generated. When dioxygen was
bubbled through this red solution no change was observed,
suggesting that the intermediate is stable towards dioxygen.
We were able to obtain the electronic absorption spectrum
(436, 540sh nm) of the red colored [Ni(L2)(OH)]+ species 2a
generated in situ from 2, but not its HR-MS as it is very
reactive (cf. below).
0
20
40
60
80
100
Time (min)
350
400
450
500
550
600
650
700
750
Wavelength (nm)
Figure 10 Reaction of the intermediate [Ni(L1)(OH)]+ 1a upon exposure to
atmospheric CO2 followed at 5 min interval. There are two isosbestic points at 400
and 625 nm. The intermediate was generated by adding Et3N (4 × 10-4 M) to complex
1 (2 × 10-4 M). Inset: Plot of absorbance vs time, observed at 474 nm. kobs = 7.7 ± 0.1
× 10-5 s-1 (t1/2 = 2.5 hrs).
The kinetics of decay of the [Ni(L1/L2)(OH)]+ intermediates,
generated in situ upon adding Et3N in methanol to a
methanolic solution of 1 and 2, upon exposure to atmospheric
air to give 3 and 4 was followed by monitoring the decreases in
intensity of the intense bands around 474 nm for 1 (Fig. 10)
and that around 463 nm for 2 (Fig. S12, S13). The plots of
absorbance vs time for both 1 (kobs, 7.7 ± 0.1 × 10-5 s-1; t1/2, 2.5
hrs) and 2 (kobs, 5.8 ± 0.3 × 10-4 s-1; t1/2 , 0.33 hrs) are linear,
showing that the intermediate 2a reacts with excess CO2
almost 10 times faster than 1a in methanol. It may be noted
that 4 was formed instantaneously in basic solution upon
exposure of 2 to atmospheric CO2. When a CO2-saturated
methanolic solution was used the reaction was completed
again instantaneously, and so the kinetics of the fast CO2
fixation reaction could not be followed. It is proposed that the
conversion of the high-spin complex 1 to the reactive low-spin
square planar species is initiated by the deprotonation of
water molecule coordinated to Ni(II) trans to the central N2
nitrogen atom of 3N ligand upon adding Et3N base, which is
followed by the loss of two weakly coordinated water
molecules oriented in directions opposite to each other
(Scheme 1). As the Ni(II)-bound hydroxyl group in the reactive
intermediate [Ni(3N)(OH)]+ is polarized and stabilized in basic
medium, its electron-rich nucleophilic oxygen atom, as
revealed by the HOMO of intermediates (Fig. 6), attacks the
nucleophilic carbon atom of CO2 to form the carbonate bound
to the mononuclear Ni(II) species [Ni(3N)(H2O)(OCO2)]+. The
monodentatively coordinated carbonate in the latter readily
binds to Ni(II) in another molecule of the reactive intermediate
[Ni(3N)(OH)]+ opposite to the sterically hindering diazepane
ring, which is followed by protonation of the hydroxyl group in
the latter, to form the dimeric carbonate-bound Ni(II) complex
3 as the final product. The mechanism proposed is very similar
to the generally accepted carbonic anhydrase mechanism
mediated by Zn(II) in the presence of a base. The
deprotonation of the Zn(II)-coordinated water molecule
generates a hydroxide ion bound to tetrahedral Zn(II) (d10), the
[Ni2(L2)2(μ-CO3)(H2O)2](ClO4)2, same as that of 3 (Table 3). As it
is insoluble in any solvent it was not studied further. It is
possible that the coordination of the more basic and smaller
imidazole donor in L2 causes efficient packing in the solid state
resulting in insolubility of 4. When the red colored solution
obtained upon adding two equivalents of Et3N to one
equivalent of [Ni(L1)(H2O)3]2+ 1 was subjected to ESI-MS
analysis a mass peak at m/z (pos.) = 303.2044 was observed
(Fig. S22), which agrees well with the value of m/z = 303.0858
calculated for the intermediate Ni(II)-hydroxyl species
[Ni(L1)(OH)+Na]2+. The same intermediate species was
generated in basic ethanol (m/z, 303.2050, Fig. S23) and
isopropanol (m/z, 303.2046, Fig. S24) solutions but not in basic
acetonitrile and DMF solutions. When a small amount of
methanol was added to the green solution of 1 in basic (Et3N)
acetonitrile or DMF solution, the solution became red in color
characteristic of the [(L1/L2)Ni(II)-OH] species 1a and 2a. All
these observations reveal that alcoholic solvents stabilize the
Ni(II)-hydroxo intermediate by hydrogen-bonding possibly with
the hydroxyl group coordinated to the intermediate species
[Ni(L1)(OH)]+. The magnetic moments of 1 and 1a in methanol
solution were determined by using Evans’ method (Fig. S3).49
While 1 is found to be paramagnetic (µeff, 2.93 BM) 1a is
diamagnetic. The latter possesses a low-spin square planar
configuration, as diagnosed from the red color, magnetic
moment determined by Evan’s method and the presence of
intense absorption bands around 474 and 524 nm observed
selectively in alcoholic solvents and absence of bands
characteristic of octahedral ligand field. The former band is
assignable to 1A1g → A2g ligand field transition in Ni(II) with a
square planar coordination geometry, and the other one
1
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DOI: 10.1039/D1DT00299F
Scheme 1 Proposed reaction pathway for fixation of atmospheric CO2 using [Ni(L1/L2)(H2O)3]2+ in the presence of Et3N in methanol solution forming the reactive intermediates
[(L1/L2)Ni–OH]+ and [(3N)Ni(H2O)(OCO2)]+ and then carbonato-bridged dinickel(II) complex
oxygen atom of which acts as a nucleophile for the crystal structure of the tetrameric ‘dimer of dimer’ complex
electrophilic carbon atom of CO2 in the most-efficient [Ni4(tBuDPA)4(μ-CO3)2(H2O)4](ClO4)4. It is interesting to note
physiologically important enzymatic hydration of CO2 to form that the electron density in the LUMO of 2a is distributed on
bicarbonate.47 Similarly, the present square planar [(3N)Ni- the ligand orbitals as well as Ni(II) dx -y
2
2
orbital; however, in 1a
(Fig. 6) the LUMO is localized on pyridyl ring, and the LUMO+1
configuration, stabilized by the meridionally and hence only contains electron density distributed over all the ligands
strongly coordinated diazepane ligand, the Ni(II)-bound as well as on the dx -y orbital of Ni(II) similar to the LUMO of
2
OH]+ species with Ni(II) in low-spin
(dxy,dxz,dyz)6(dz
)
2
2
2
hydroxide acts as a better nucleophile for the electrophilic 2a. The HOMO – LUMO energy gap (4.6307 eV) in 2a is lower
carbon atom of CO2 in the minimal reaction [(3N)Ni-OH]+ + CO2 than the HOMO – LUMO+1 energy gap (4.7103 eV) in 1a (Fig.
→ [(3N)Ni(H2O)(OCO2)]+. Thus, the extremely high reactivity 7) by 1.83 kcal mole-1, which correlates well with the higher
of the in situ generated [Ni(3N)(OH)]+ species is ascribed to its energy needed for the conversion of low-spin 1a to an
square planar configuration with the σ-electron density octahedral complex 3 with high-spin Ni(II) configuration, which
exposed. Similar mononuclear square planar [(L3)NiII-OH]− involves de-pairing of the two electrons in dz
complexes (cf. above), have been isolated and the kinetics and promoting one of them to the dx -y orbital in 3. This illustrates
2
orbital in 1a and
2
2
mechanistic study of its fast CO2 fixation reaction in DMF has the observed lower reactivity of 1a in fixing CO2.
been studied in detail.22-24 One might indeed expect the
formation
of
transient
bicarbonate
complex Catalytic conversion of CO2 into Cyclic Carbonate
[(3N)Ni(H2O)(OC(O)OH)] via Lindskog mechanism,48 which The ability of the μ-carbonate complex 3 to catalyze the
subsequently reacts with the cationic species [(3N)Ni-OH]+ to conversion of CO2 to cyclic carbonate was investigated in the
form the bridging carbonate dimer. Thus, it is clear that a more soluble CH3CN as solvent at 30, 40, 50, 60 and 80 °C over
meridionally bound tridentate 3N ligand system suitable to a period of 8 h, by following the already reported procedure.25
confer a low-spin square planar geometry on Ni(II) in Styrene epoxide (5 mmol) was used as the substrate and
[(L1/L2)Ni(II)-OH] complexes is essential to enhance the reacted with atmospheric CO2 (1 bar) and pure CO2 at a
nucleophilicity of the Ni(II)-bound hydroxyl group that directly catalyst load of 0.05 mol% and 2 equivalents of Et3N as a base
takes part in CO2 activation. It has been observed21 that the in the absence of a co-catalyst. The product was identified by
1
tBuDPA ligand changes from facial to meridional coordination using H and 13C NMR as phenylethylene carbonate (4-phenyl-
when its Ni(II)-OH complex activates CO2 and in fact, both the 1,3-dioxolan-2-one) as the major one, further confirmed by
modes of ligand coordination have been observed in the X-ray GC-MS (m/z, 164.1) and quantified by GC-MS/GC technique.
10 | J. Name., 2012, 00, 1-3
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The yields were 9-15% with excellent selectivity (>99%) at the Mr. Aditya Borah, Department of Chemistry, IITB, Mumbai is
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catalyst load of 0.05 mol% at ambient temperature and thanked for help in solving the X-ray strDuOctI:u1r0e.1.0W39e/Dt1DhaT0n0k29D9rF.
pressure (1 bar). The blank reaction performed only in the M. Murali, National College, Tiruchirappalli, for the
presence of Et3N showed the formation of only a very trace electrochemical measurement facility.
amount of the cyclic carbonate. When pure CO2 (1 atm) was
used the product yield increased. When the temperature was
increased the yield also gradually increased: 9 (30), 25 (40), 39
Notes and references
(50), 55 (60), 72% (80 °C) (Table S5), without any change in the
selectivity under the same experimental conditions. Thus, the
reaction temperature and pressure of CO2 have a strong
influence on the catalytic CO2 fixation reaction.
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Conclusions
3
4
Thus, the octahedral nickel(II) complexes of two diazepane-
based 3N ligands [Ni(L1/L2)(H2O)3]2+ generate the intermediate
complex species [(L1/L2)Ni–OH]+ in methanolic solution upon
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Author Contributions
Almost all the work is carried out by Mr. T. Ajaykamal and Prof. M.
Palaniandavar at School of Chemistry, Bharathidasan University,
Tiruchirappalli 620024, Tamil Nadu, India. Professor Islam and Mitu
Sharma collected the X-ray structure data at Tezpur University,
Tezpur, Assam, India for complex 3.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
We thank Indian National Science Academy (INSA), New Delhi,
for the INSA Senior Scientist position to M.P. and the Science
and Engineering Research Board (SERB), New Delhi for a
research project with financial support (EMR/2015/002222) to
M. P. HR-MS facility sanctioned to School of Chemistry,
Bharathidasan University through DST-FIST is acknowledged.
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