Dual-site aqua mononuclear nickel(II) complexes of non-heme tetradentate ligands: Synthesis, characterization and reactivity

Article abstract of DOI:10.1016/j.ica.2018.10.069

Mononuclear compounds [Ni(BQCNMe₂)(H₂O)₂](ClO₄)₂ 1 and [Ni(BQCNH₂)(H₂O)₂](ClO₄)₂ 2 of N,N^′-dimethyl-N,N^′-di(quinolin-8-yl)cyclohexane-1,2-diamine (BQCNMe₂) and N,N^′-di(quinolin-8-yl)cyclohexane-1,2-diamine (BQCNH₂) were synthesised and characterized by elemental analysis, IR/UV-Vis spectroscopy, cyclic voltammetry (CV)/differential pulse voltammetry (DPV) and X-ray powder pattern. [Ni(BQCNMe₂)(en)](ClO₄)₂ 3 and [Ni(BQCNMe₂)(phen)](ClO₄)₂ 4 were prepared by reacting 1 with ethylenediamine (en) and 1,10-phenanthroline (phen) respectively while [Ni(BQCNH₂)(en)](ClO₄)₂ 5 and [Ni(BQCNH₂)(phen)](ClO₄)₂ 6 were obtained from the reaction of 2. Compounds [Ni(BQENMe₂)(en)](ClO₄)₂ 7 and [Ni(BQENH₂)(en)](ClO₄)₂.CH₃CN 8 (BQENMe₂ is N,N′-dimethyl-N,N′-bis(8-quinolyl)ethane-1,2-diamine) and BQENH₂ is N,N′-bis(8-quinolyl)ethane-1,2-diamine) were synthesised similarly. Compounds 6 and 8 were characterized by single crystal X-ray diffractometry and their structural features are presented. The reactivity of 2 with H₂O₂/base was investigated. A new peak at 570 nm in the UV-Vis spectrum corresponding to 2a was obtained which on addition of 2-phenylpropionaldehyde (2-PPA) decays giving pseudo-first order rate constant of 9.2 × 10^?3 s^?1 and acetophenone as a major product. The catalytic hydroxylation of cumene and ethylbenzene by 1 and 2 in the presence of meta-chloroperbenzoic acid (m-CPBA) was investigated.

Full text of DOI:10.1016/j.ica.2018.10.069

Accepted Manuscript
Research paper
Dual-site Aqua Mononuclear Nickel(II) Complexes of Non-heme Tetradentate
Ligands: Synthesis, Characterization and Reactivity
Sarvesh S. Harmalkar, Dattaprasad D. Narulkar, Raymond J. Butcher, Mahesh
S. Deshmukh, Anant Kumar Srivastava, Mariappan Mariappan, Prem Lama,
Sunder N. Dhuri
PII:
S0020-1693(18)30787-4
https://doi.org/10.1016/j.ica.2018.10.069
ICA 18615
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
Revised Date:
Accepted Date:
21 May 2018
6 October 2018
31 October 2018
Please cite this article as: S.S. Harmalkar, D.D. Narulkar, R.J. Butcher, M.S. Deshmukh, A. Kumar Srivastava, M.
Mariappan, P. Lama, S.N. Dhuri, Dual-site Aqua Mononuclear Nickel(II) Complexes of Non-heme Tetradentate
Ligands: Synthesis, Characterization and Reactivity, Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/
j.ica.2018.10.069
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Dual-site Aqua Mononuclear Nickel(II) Complexes of Non-heme Tetradentate Ligands:
Synthesis, Characterization and Reactivity
Sarvesh S. Harmalkara, Dattaprasad D. Narulkara, Raymond J. Butcherb, Mahesh S. Deshmukhc,
Anant Kumar Srivastavac, Mariappan Mariappand, Prem Lamaa, Sunder N. Dhuria*
aDepartment of Chemistry, Goa University, 403206, Goa, India
bDepartment of Chemistry, Howard University, Washington, DC 20059, United States
cDepartment of Chemistry, IISER, Dr. Homi Bhabha Road Pune, 411008, India
dDepartment of Chemistry, SRM IST, Chennai, 603203, India
*Corresponding author e-mail address: sndhuri@unigoa.ac.in
Dedicated to Prof. Dr. Wolfgang Bensch on the occasion of his 65th birthday.
Abstract
Mononuclear compounds [Ni(BQCNMe2)(H2O)2](ClO4)2 1 and [Ni(BQCNH2)(H2O)2](ClO4)2
2 of N,N’-dimethyl-N,N’-di(quinolin-8-yl)cyclohexane-1,2-diamine (BQCNMe2) and N,N’-
di(quinolin-8-yl)cyclohexane-1,2-diamine (BQCNH2) were synthesised and characterized by
elemental analysis, IR/UV-Vis spectroscopy, cyclic voltammetry (CV) / differential pulse
voltammetry (DPV) and X-ray powder pattern. [Ni(BQCNMe2)(en)](ClO4)2 3 and
[Ni(BQCNMe2)(phen)](ClO4)2 4 were prepared by reacting 1 with ethylenediamine (en) and
1,10-phenanthroline (phen) respectively while [Ni(BQCNH2)(en)](ClO4)2
5
and
[Ni(BQCNH2)(phen)](ClO4)2 6 were obtained from the reaction of 2. Compounds
[Ni(BQENMe2)(en)](ClO4)2 7 and [Ni(BQENH2)(en)](ClO4)2.CH3CN 8 (BQENMe2 is N,N′-
dimethyl-N,N′-bis(8-quinolyl)ethane-1,2-diamine)
and
BQENH2
is
N,N′-bis(8-
quinolyl)ethane-1,2-diamine) were synthesised similarly. Compounds 6 and 8 were
characterized by single crystal X-ray diffractometry and their structural features are
presented. The reactivity of 2 with H2O2 / base was investigated. A new peak at 570 nm in the
UV-Vis spectrum corresponding to 2a was obtained which on addition of 2-
phenylpropionaldehyde (2-PPA) decays giving pseudo-first order rate constant of 9.2 x 10-3 s-
1
and acetophenone as a major product. The catalytic hydroxylation of cumene and
1

ethylbenzene by 1 and 2 in the presence of meta-chloroperbenzoic acid (m-CPBA) was
investigated.
Keywords: nickel(II) compounds, non-heme ligands, spectroscopy, crystal structure,
hydroxylation.
1. Introduction
The chemistry of first row transition metal compounds containing non-heme N-donor
ligands is an area of research with growing attention [1]. In metalloenzymes, a metal active
site plays a vital role in biochemical reactions [2]. The elements of the first transition metal
series are abundant and available for specific functions as they exhibit a variety of
coordination numbers and the flexible geometries over sp, sp2 and sp3 hybridisations of
carbon [3]. The late transition metals viz. cobalt and nickel have been less investigated in
biomimetic studies due to their limited scope in biology. In recent years, a large amount of
work has been carried on understanding the roles of high valent cobalt and nickel-oxygen
intermediates in a variety of biomimetic oxidations [4-13]. The roles of nickel(II) compounds
in hydrogen gas generation and oxygen gas evolution is also documented [14-18]. The
nickel(II) compounds have been also used as the models for several metalloenzymes [19-21].
They display DNA binding-cleavage activities [22-29] and antimicrobial properties [30-32]
in biological systems and have been used in various reactions like cross-coupling reaction
[33], electrolytic water oxidation [34] and alkane oxidation [35-39] as catalysts.
In our recent work, we have reported hydroxylation of alkanes by nickel(II)
compounds of non-heme N,N′-bis(8-quinolyl)ethane-1,2-diamine (BQENH2) and N,N′-
dimethyl-N,N′-bis(8-quinolyl)ethane-1,2-diamine (BQENMe2) ligands [40]. Nam et al. have
reported iron(II) and manganese(II) compounds of BQENMe2 as the models in biomimetic
reactions [41,42]. The same group also reported [Mn(BQCNMe2)]2+ compound in water
2

oxidation catalysis and various organic oxidations [43,44]. In recent years, the structural and
spectroscopic elucidation of nickel(II)-superoxide and nickel(III)-peroxide of tetramethylated
cyclams has attracted the attention of bioinorganic chemists in the modelling chemistry
[20,21]. Although a large amount research on non-heme nickel(II) compounds has been
carried out, our understanding in this area suggests that there is scope in designing the new
topological ligands which stabilize nickel(II) ion. In the present work, we report synthesis,
characterization and reactivity of two dual site aqua mononuclear nickel(II) compounds
stabilized by N,N’-dimethyl-N,N’-di(quinolin-8-yl)cyclohexane-1,2-diamine (BQCNMe2) and
N,N’-di(quinolin-8-yl)cyclohexane-1,2-diamine (BQCNH2). The two water molecules in 1
and 2 are replaced by auxiliary bidentate ethylenediamine (en) and 1, 10-phenanthroline
(phen) affording four nickel(II) compounds (3-6). The two other compounds, 7 and 8
containing BQENMe2 and BQENH2 have been also investigated.
2. Experimental details
2.1 Materials and methods
All the chemicals were used as obtained without further purification. BQCNH2,
BQENH2, BQENMe2, [Ni(BQENH2)(H2O)2](ClO4)2] and [Ni(BQENMe2)(H2O)2](ClO4)2
1
were prepared using reported procedure [40,45]. The ligands were characterized by IR, H
and 13C NMR spectroscopy (Fig. S1-S4). IR spectra in the region 4000-400 cm-1 were
recorded on a Shimadzu (IR Prestige-21) FT-IR spectrometer. The NMR spectra were
recorded on a Bruker Avance III 400 MHz NMR spectrometer. UV-Vis spectra were
recorded on an Agilent UV-Vis spectrophotometer 8453. Elemental analysis (C,H,N) was
performed using Elementar Variomicro Cube CHNS Analyser. The redox potentials were
obtained from cyclic voltammograms (CV) and differential pulse voltammograms (DPV) on
3

Electrochemical Workstation-CH Instrument, Inc. CHI6107. In CV / DPV measurements, a
glass vessel containing sample solution was equipped with a Pt disc (working electrode), Pt
wire (counter electrode), reference electrode, Ag/AgNO3 (0.01 M) and tetrabutylammonium
hexafluorophosphate (TBAPF6) (0.1 M) as supporting electrolyte. The sample solutions were
purged with N2 gas for ~ 30 min before each measurement. The powder X-ray Diffraction
patterns were obtained using a Panalytical Xpert3 Powder X-ray Diffractometer using Cu Kα
radiation. Crystal structures of 6 and 8 were obtained using Bruker Smart Apex Duo
diffractometer at 100 K. The structural refinement was done by full-matrix least-squares
against F2 using all data (SHELXL) [46]. The non-hydrogen atoms were refined with the
anisotropic displacement parameters while the hydrogen atoms were located at the calculated
positions. Even though the structure was collected at 100 K, the quinoline moiety was found
to be disordered over two positions in 6. The site occupancy of the disordered atoms was
refined first using a variable command to get an approximate contribution from each fraction
and then fixed at that value to get the best model. The first part contains ¾ occupancy
whereas the second part contains ¼ of quinoline unit. Apart from that, the nitrogen atom
bonded to the quinoline group is also disordered over two positions (N5A = 3/4 occupancy
and N5B = 1/4 occupancy). AFIX 116 command was used to get the idealized quinoline
group and in addition, SIMU and ISOR command were also applied to get the best model for
the disordered fraction. In catalytic oxidation reactions, the organic products were analysed
using the Shimadzu GC 2014 equipped with HP capillary column (30 m x 0.25 mm x 2.5
μM) and an FID detector. The retention time and peak areas of the products were compared
with authentic samples using decane as an internal standard.
2.2 Synthesis of BQCNMe2
4

BQCNMe2 was prepared by a complete modification of reported procedure [45]. To a
stirred THF solution (40 mL) of BQCNH2 (4.0 g, 10.86 mmol), 21.0 mL of aqueous
formaldehyde (37 %) (6.48 g, 217 mmol) was added. The solution slowly turned dark red
after ~ 5 min. To this red mixture, the sodium cyanoborohydride (1.38 g, 22.0 mmol) was
added slowly till an yellow colour solution is obtained. The solution was stirred for ~ 24 h
and THF was removed which afforded yellow crude powder. This crude product on
recrystallization in hot ethanol resulted in a crystalline solid. Yield was 3.0 g. Anal. Calc. for
C26H28N4 (%): C, 78.75; H 7.12; N 14.13; Found: C, 78.35; H, 7.12; N, 14.06. Selected IR
1
bands (KBr, cm-1): 1562 ν(C=N); 3134-2746 ν(CH). H-NMR (400 MHz, CDCl3): δ (ppm)
8.73 (d, 2H, J = 1.2 Hz, 2-QnH), 7.99 (d, 2H, J = 8 Hz, 4-QnH), 7.26 (m, 4H, 3-QnH & d 5-
QnH), 7.14 (d, 2H, J = 7.6 Hz, 6-QnH), 6.60 (d, 2H, J = 7.2 Hz, 7-QnH), 4.70 (d, 2H, J = 8
Hz, NCH), 2.43 (s, 6H, NMe), 2.29 (d, 2H, J = 12.4 Hz, CH), 1.75 (d, 2H, J = 7.2 Hz, CH),
1.60 (d, 2H, J = 8 Hz, CH), 1.28 (m, 2H, CH). 13C NMR (CDCl3): δ (ppm) 149.4, 146.4,
142.3, 136.2, 129.7, 126.7, 120.5, 117.7, 115.5, 63.2 (NCH), 33.6 (NMe), 30.3 (NCHCH2),
26.0 (NCHCH2CH2).
2.3 Synthesis of compounds 1-2
Ni(ClO4)2 • 6H2O (2.2 g, 6.0 mmol) was dissolved in CH3CN (5 mL) and treated with
BQCNMe2 (2.38 g, 6.0 mmol) in CH2Cl2 (5 mL) resulting in red coloured solution. The
solution was filtered after 2 h and violet colour crystalline powder was obtained by slow
diffusion of diethylether. Yield of 1 was 3.2 g. Anal. Calc. for 1 C26H32N4Cl2O10Ni (%): C,
45.25; H, 4.67; N, 8.12. Found: C, 44.91; H, 4.56; N, 8.01. Selected IR bands (KBr, cm-1):
-1
3433 ν(OH); 3020-2814 ν(CH); 1101 (ν3), 624 (ν4) (ClO4 ). λ(nm/dm3mol-1cm-1): 228
(12092), 293 (2498), 542 (9), 890 (10).
Compound 2 was prepared using BQCNH2 (2.2 g, 6.0 mmol) by employing a similar method
5

as above. Yield of 2 was 2.9 g. Anal. Calc. for 2 C24H28N4Cl2O10Ni (%): C, 43.54; H, 4.26; N,
8.86. Found: C, 43.42; H, 4.26; N, 8.63. Selected IR bands (KBr, cm-1): 3435 ν(OH); 3246
-1
ν(NH); 3030-2825 ν(CH); 1087 (ν3), 624 (ν4) (ClO4 ). λ (nm/dm3mol-1cm-1): 230 (17796),
302 (3923), 526 (12), 905 (12).
2.4 Reactivity of 1 and 2 with en and phen to obtain compounds 3-6
The ethylenediamine, en (0.09 g, 1.5 mmol) was added to the violet coloured CH3CN
solution of 1 (0.1 g, 1.5 mmol). After ~ 1 h the solution was filtered in a glass vial (10 mL)
and left open in a dessicator containing diethylether for slow diffusion. A pale-red crystalline
compound was obtained. Yield of 3 was 0.12 g. Anal. Calc. for 3 C28H36N6Cl2O8Ni (%): C,
47.09; H, 5.08; N 11.77. Found: C, 47.48; H, 5.28; N, 11.64. Selected IR bands (KBr, cm-1):
-1
3296 ν(NH); 3024-2816 ν(CH); 1109 (ν3), 624 (ν4) (ClO4 ). λ (nm/dm3mol-1cm-1): 230
(15657), 291 (3569), 525 (13), 866 (11).
Compound 4 was prepared by taking phen instead of en. Yield of 4 was 0.13 g. Anal.
Calc. for 4 C38H36N6Cl2O8Ni (%): C, 54.70; H, 4.35; N, 10.07. Found: C, 54.64; H, 4.30; N,
-1
9.33. Selected IR bands (KBr, cm-1): 3028-2814 ν(CH); 1095 (ν3), 623 (ν4) (ClO4 ). λ
(nm/dm3mol-1cm-1): 226 (32632), 268 (16113), 525 (15), 866 (13).
Compounds 5 and 6 were prepared by reaction of en and phen with
[Ni(BQCNH2)(H2O)2](ClO4)2 (2) in CH3CN. The single crystals of 6 were grown by slow
diffusion of diethylether into its CH3CN solution. Anal. Calc. for 5 C26H32N6Cl2O8Ni (%): C,
45.51; H, 4.70; N, 12.25. Found: C, 45.43; H, 4.51; N, 12.15. Selected IR bands (KBr, cm-1):
-1
3346 ν(NH); 3020-2827 ν(CH); 1089 ν3, 624 ν4 ν(ClO4 ). λ (nm/dm3mol-1cm-1): 230 (20666),
303 (3986), 535 (13), 886 (8). Anal. Calc. for 6 C36H32N6Cl2O8Ni (%): C, 53.63; H, 4.00; N,
10.42. Found: C, 53.68; H, 3.95; N, 10.24. Selected IR bands (KBr, cm-1): 3255 ν(NH); 2983-
-1
2808 ν(CH); 1093 (ν3), 623 (ν4) (ClO4 ). λ (nm/dm3mol-1cm-1): 225 (25559), 270 (12184),
526 (15), 875 (10).
6

2.5 Synthesis of compounds 7 and 8
Compounds 7 and 8 were synthesized by reacting [Ni(BQENMe2)(H2O)2](ClO4)2 and
[Ni(BQENH2)(H2O)2](ClO4)2 with en as above. The single crystals of 8 were obtained by
slow diffusion of diethylether. Yields of 7 and 8 were 0.12 and 0.13 g respectively. Anal.
Calc. for 7 C24H30N6Cl2O8Ni (%): C, 43.67; H, 4.58; N, 12.73. Found: C, 43.39; H, 4.36; N,
-
12.86. Selected IR bands (KBr, cm-1): 3275 ν(NH); 3030-2825 ν(CH); 1093 ν3, 621 ν4 (ClO4
1). λ (nm/dm3mol-1cm-1): 230 (15657), 291 (3569), 538 (13), 894 (11). Anal. Calc. for 8
C22H26N6Cl2O8Ni (%): C, 41.80; H, 4.15; N, 13.30. Found: C, 41.31; H. 3.87; N, 12.96.
-1
Selected IR bands (KBr, cm-1): 3300 ν(NH); 3030-2825 ν(CH); 1093 (ν3), 621(ν4) (ClO4 ). λ
(nm/dm3mol-1cm-1): 227 (57263), 297 (11759), 315 (10169), 530 (12), 862 (9).
2.6 Reactivity of 1 and 2 with H2O2 in presence of TBAH
On addition of 10 eq. of H2O2 (30 %) and 5 eq. tetrabutylammonium hydroxide
(TBAH) to the CH3CN solution of 2 (0.25 mM in 2ml) gave an intense band at 570 nm in the
UV-Vis spectrum. This species (2a) formed was kinetically quite stable. To the intermediate
solution, 40 eq. of 2-PPA was added which resulted in the decay of the peak at 570 nm. The
solution after the end of the reaction was analysed by GC and the products formed were
quantified based on the catalyst used. Acetophenone was obtained as the major product in the
reaction with 2-PPA (vide infra). Interestingly under identical reaction conditions, 1 showed
no reactivity with H2O2 and TBAH.
2.7 Catalytic reactions by 1-8.
The efficacy of compounds 1-8 as a catalyst was tested in the catalytic oxidation of
hydrocarbons such as cumene and ethylbenzene in presence of meta-chloroperbenzoic acid
7

(m-CPBA) oxidant in CH2Cl2 / CH3CN (3:1) under N2 atmosphere at room temperature. The
products formed in the reaction were analysed and quantified by GC using internal standard
n-decane.
3. Results and discussion
3.1 Synthesis of BQCNMe2 and 1 and 2
Several non-heme metal compounds which carry out selective oxidation of C-H and
C=C bonds are known in natural systems [47-48]. The topology of ligands has very a high
influence on the stability and reactivity patterns of high valent metal-oxygen intermediates
[45]. The chemical structures of few non-heme N-donor ligands which have been extensively
employed to stabilize high valent iron(IV/V) and manganese(IV/V)-oxo species are shown in
Scheme 1. The reactivity of these metal-oxo species in biomimetic oxidations (O-transfer, C-
H activation, aromatic hydroxylation and epoxidation) has been well understood [49]. The
non-heme metal compounds containing ligands with N-H groups when used in organic
substrate oxidations often results in degradation of ligand itself in presence of oxidants like
m-CPBA and thus result in low yields of organic products [50]. The lower yields of
hydroxylated products were also obtained by us when we used the nickel(II) compounds
containing BQENH2 which has two –NH groups [40]. In the present investigation, we have
chosen a tetradentate N-methylated ligand BQCNMe2 for stabilizing nickel(II) compounds
(Scheme 2). The BQCNMe2 was earlier prepared by alkylating BQCNH2 with a strong base
n-BuLi and CH3I. One of the issue with the method is that it works only at a very low
temperature (-78˚C) and under inert atmosphere due to the pyrophoric nature of n-BuLi. To
overcome this cumbersome procedure, we adopted a simple methodology for the synthesis of
BQCNMe2 wherein we have carried out the reductive methylation of BQCNH2 using aqueous
formaldehyde and sodium cyanoborohydride at room temperature [40].
8

The first compound containing BQCNMe2, [FeII(BQCNMe2)(CF3SO3)2] was reported
by Britovsek et al. which showed high reactivity in the oxidation of cyclohexane [45]. Nam
et al. then prepared [MnII(BQCNMe2)(CF3SO3)2] which worked out to be an excellent
catalyst in water oxidation [43,44]. In the present work, we have prepared and characterized
two new compounds [Ni(BQCNMe2)(H2O)2](ClO4)2 (1) and [Ni(BQCNH2)(H2O)2](ClO4)2
(2) (Scheme 2).
3.2 Substitution of cis-waters by en or phen in 1 and 2
The dual site cis-water compounds 1 and 2 were reacted with symmetrical bidentate
auxiliary ligands en and phen in a 1:1 stoichiometric ratio to afford four new compounds viz.
[Ni(BQCNMe2)(en)](ClO4)2 3, [Ni(BQCNMe2)(phen)](ClO4)2 4, [Ni(BQCNH2)(en)](ClO4)2
5 and [Ni(BQCNH2)(phen)](ClO4)2 6 in nearly quantitative yields (Scheme 2). We have
earlier investigated the substitution of two cis-H2O molecules of [Ni(BQENH2)(H2O)2]2+ and
[Ni(BQENMe2)(H2O)2]2+ by bpy and phen [40]. In addition to 3-6, we also report synthesis
and characterization of [Ni(BQENMe2)(en)](ClO4)2] 7 and [Ni(BQENH2)(en)](ClO4)2 8.
3.3. Infrared spectra of ligands and compounds 1-8
Infrared spectra of ligands and solid samples of 1-8 were recorded in the region of
4000-400 cm-1 by diluting samples with KBr powder. IR spectra of BQCNMe2, 1, 3 and 4 are
shown in Fig. 1 while the IR spectra of BQCNH2, 2, 5 and 6 are depicted in Fig. S5 (SI). The
IR spectra of 1 and 2 exhibited broad bands at ~ 3547 and ~ 3405 cm-1 respectively
corresponding to O-H vibrations of water. IR spectrum of 3 exhibits N-H stretching
vibrations due to the incorporation of en while similar peaks were not seen in 1 and 4 as
amines are methylated. On the other hand, 2, 5 and 6 showed peaks corresponding to the N-H
vibrations in the region 3350-3430 cm-1. The bands due O-H vibrations are absent in the IR
spectra of 3-6 indicating that the two waters are replaced by en and phen. IR spectra of 7 and
9

8 also suggested replacement of waters by en (Fig. S6 in SI). The additional band due to C-N
vibration was observed in the IR spectrum of 8 due to CH3CN. All complexes have
incorporated perchlorate anions as evidenced from well resolved absorption bands at 1093 (s)
and 621 (m) cm-1 [51,52].
3.4. UV-Vis spectra of 1-8
UV-Visible spectra of 1-8 in CH3CN were recorded to obtain the information on
electronic transitions of nickel(II) ion and the ligands. UV-Vis spectra of 1, 3 and 4 and those
of 2, 5 and 6 are depicted in Fig. S7 (SI). The weak d-d bands are observed for all the
3
compounds in the visible region. Out of expected three d-d bands (3A2g → T2g(F), 3A2g →
3T1g(F), 3A2g → 3T1g(P)) for complexed nickel(II) ion, only two bands are seen in the UV-Vis
spectrum. The two d-d bands in 1 and 3 occur nearly at the same wavelengths (540 and 894
nm), while the bands in 4 shifted to slightly lower wavelengths (526 and 852 nm). This
3
3
observation reveals that phen has incorporated in 4. The d–d band assigned to A2g → T2g
transition is centered at 850-900 nm while the second peak due to 3A2g → 3T1g(F) transition is
centered at 450-550 nm [53]. The third high energy transition 3A2g → 3T1g(P) transition is not
observed as it is tailing towards high energy CT region [54]. The bands in the higher energy
region are assigned to intra-ligand charge transfer transitions the n-π* (270-320 nm) and π-π*
(220-230 nm). The UV-Vis spectra of compounds 7 and 8 can be explained on similar lines
(Fig. S8 in SI).
3.5. Electrochemical properties of 1-8
Compounds 1-8 were investigated using cyclic voltammetry (CV) and differential
pulse voltammetry (DPV) to understand their electrochemical behavior that correlates with
structural and reactivity properties. The quasi-reversible voltammograms corresponding to
10

the Ni(II) / Ni(I) and Ni(I) / Ni(II) couples were observed in all the cases. CV / DPV plots of
1 are shown in Fig. 2. The E1/2 value of these compounds lies between -1.19 to -1.38 V (Fig
S9 in SI). BQCNH2 and BQCNMe2 ligands showed no redox signals under identical
conditions. The CV / DPV plots of 7 and 8 are shown in Fig S10 in SI.
3.6. Crystal structure description of 6 and 8 and PXRD description of 1-8
The single crystals of 6 and 8 were obtained by slow diffusion of diethylether into
their CH3CN solutions. The remaining compounds did not give crystals suitable for single
crystal analysis under identical conditions after several attempts, hence they were studied by
Powder X-ray pattern. Compounds 1 and 2 exhibit the sharp Bragg lines suggesting that both
compounds are highly crystalline. However, the powder patterns of 1 and 2 do not match on
overlaying on one another indicating that they have different structures (Fig. 3). This is
obvious because 1 contains BQCNMe2 while 2 contain BQCNH2.
The technical details of data collection and selected refinement parameters of 6 and 8 are
given in Table 1. Both 6 and 8 crystallize in the centrosymmetric triclinic space group Pī with
all the atoms located in general positions. The crystal structure of 6 contains a unique Ni(II)
cation, one bidentate ligand phen, BQCNH2 functioning as a tridentate ligand and two unique
perchlorate anions (Fig. 4). In contrast the crystal structure of 8 contain an unique Ni(II)
cation, one bidentate en, tetradentate BQENH2, two perchlorate anions and an acetonitrile
solvate (Fig. 4). In both compounds the two crystallographically independent perchlorate
anions lie outside the coordination sphere and act as counter anions. One more additional
feature in the structure of 8 is that it has crystal lattice CH3CN molecule which contributes to
the stability of the structure. In compound 6, the Ni(II) ion is coordinated by five N atoms. A
notable fact being one of the quinoline rings has a two-fold disorder (Fig S11 in SI). The
distances of Ni from the phen nitrogens N1 and N2 are 2.001(3) and 2.156(3) Å respectively
11

while amine nitrogens N4, N5A and N5B are 2.022(3), 2.041(4) and 2.154(12) Å
respectively. The Ni(II) ion is 2.012(3) Å away from quinoline nitrogen N3 (Table 2). The
axial positions of trigonal bipyramidal (tbp) geometry are occupied by N4 of amine part and
N1 of phenanthroline part, on the other hand, the equatorial positions are occupied by N5A of
amine, N2 of phen and N3 of quinoline moiety. The bond angles N2-Ni-N3; 105.30˚(11), N2-
Ni-N5A; 111.05˚(15) and N3-Ni-N5A; 143.03˚(16) sum to 359.38˚, the value which is very
close to the expected 360o of tbp geometry. The bond angle, N1-Ni-N4; 176.48˚(12) deviates
from 180˚ indicating that the structure of 6 is slightly distorted. The chlorine atoms of
perchlorate anions are all involved in the hydrogen bonding with the N’s of BQCNH2 ligand
as well as phen ligand moieties (Fig. 5, Table 3). In the crystal structure of 6, the Ni(II) ion
adopts trigonal bipyramidal geometry despite the ligand BQCNH2 being tetradentate which is
quite rare. It is often seen that the tridentate ligands along with auxiliary bidentate ligands
adopt trigonal bipyramidal geometry with metal ions [55-58].
In the crystal structure of 8 the two amine nitrogen atoms N11 and N14 occupy the
adjacent positions, while the quinolyl nitrogen atoms, N1 and N23 are located trans to each
other (Fig. 4). The remaining two cis sides of the octahedron are occupied by the two
nitrogen atoms of the en ligand. The Ni-N bond distances range from 2.0918(14) to
2.1191(13) Å (Table 2). All the Ni-N bond distances and N-Ni-N bond angles are in good
agreement with literature values [59-62]. There is a small deviation from the normal bond
angle of octahedron (90° for cis and 180° for trans) indicates a distorted octahedral geometry.
The cis angle varies from 80.17(5) to 98.35(5) and the trans angle varies from 167.47(5) to
178.51(5). The oxygen atoms and chlorine atom of perchlorate anions and the nitrogen atom
of CH3CN molecule are all involved in the hydrogen bonding with the N’s of BQENH2 as
well as en ligand moieties (Fig. 6, Table 3). The two perchlorate anions associated with each
{NiN6}2+ octahedron are connected to three other octahedra by O···H-N hydrogen bonding
12

interactions (Fig. 6a). These interactions further extend connecting other {NiN6}2+ octahedra
and [ClO4]- tetrahedra resulting in a three dimensional network structure (Fig. 6b). All
{NiN6}2+ octahedra and [ClO4]- tetrahedra are organized symmetrically when viewed through
all the three crystallographic planes (bc, ab, ac) (Fig. 7, Fig. S12 in SI). In the crystal
structure of 8, all the octaheda are aligned and far apart at a distance of 12.33 Å (Fig 7b).
3.7 Reactivity of 1 and 2 with H2O2 in presence of TBAH
On adding 10 eq. of H2O2 to a CH3CN solution of 2 (0.25 mM in 2 ml) in the
presence of 5 eq. TBAH at room temperature resulted in the formation a new species (2a) as
evidenced by the appearance of a peak at 570 nm in the UV-Vis spectrum of 2 with a colour
change from light green to purple (Fig 8a). Similar observations were earlier reported for the
Ni(III)-oxygen species by others [63,64]. Assuming that the intermediate (2a) formed in the
solution could be Ni(III)-oxygen species, we then treated this solution with 40 eq. of 2-
phenylpropionaldehyde (2-PPA) at 25˚C. The spectral changes for the decay of 570 nm peak
with time were monitored (Fig 8b). The isobestic points were observed at 365 nm and 727
nm. The peak at 570 nm slowly decayed following pseudo-first order kinetics affording us a
pseudo-first order rate constant, kobs of 9.2 x 10-3 s-1 (Fig. 8b inset). The kobs values increased
with increase in the concentration of 2-PPA and thus afforded us a second order rate constant,
k2 = 9.05 x 10-4 M-1 s-1 (Fig. S13 in SI). On subjecting final reaction solution to GC analysis,
the acetophenone was obtained as the major product, these results suggest that the
intermediate formed at 570 nm has nucleophilic character [65-68]. Efforts are underway to
elucidate the structure and formula of this intermediate. In the case of 1, no clear formation of
new species was observed in the reaction 1 and H2O2 / TBAH (Fig. S14 in SI).
3.8. Catalytic hydroxylation of alkanes by 1 and 2
13

The oxidation of cumene and ethylbenzene gave 2-phenylpropane-2-ol and 1-
phenylethanol respectively as the major products and acetophenone as the minor product
(Table 4). On comparing the oxidising ability of 1 and 2 it is observed that 1 showed slightly
higher yields of hydroxylated products over 2. In compounds 1 and 2 the two solvent
molecules coordinated to Ni(II) ion are labile thus making the geometry flexible for oxidation
of Ni(II) to Ni(III) as compared to compounds 3-8 where Ni(II) ion is coordinately saturated.
The Ni(III) species proposed here in the catalytic oxidation of cumene and ethylbenzene by 1
and 2 was further traced using CV / DPV which showed anodic peak potentials for Ni(II) /
Ni(III) couple at centred 0.689 in 1 and 0.637 V in 2 (Fig. S15 in SI). Based on the yields of
hydroxylated products obtained in the catalytic reactions and the appearance of Ni(II) / N(III)
anodic peaks in the CV / DPV we suggest that the catalytic cycle may involve H-atom
abstraction of alkanes by Ni(III)-oxygen intermediate [69]. Compounds 3-8 were also tested
in the catalytic oxidations of alkanes however no products were obtained in these cases [40].
Conclusion
We have reported synthesis and characterization of two new dual-site aqua nickel(II)
compounds 1 and 2 stabilized by non-heme BQCNMe2 and BQCNH2. BQCNMe2 was
prepared by a modified procedure. Compounds 3-6 were prepared by simple substitution of
labile solvent molecules of 1 and 2 by relatively stronger bases en and phen. Compounds 7
and 8 with the other two ligands BQENMe2 and BQENH2 have been also reported. All
compounds were characterized by C, H, N analysis, IR, UV-Vis and CV / DPV techniques to
obtain their correct formulas. We attempted to obtain single crystals of 1-8 and we were
successful in getting crystals of only 6 and 8, hence their structures are reported. Both 6 and 8
crystallizes in a centrosymmetric triclinic Pī space group and isostructural with 5. Compound
6 shows five coordinated geometry while 8 has a slightly distorted octahedral structure with
14

additional crystal lattice acetonitrile. The reaction of 2 with H2O2 / TBAH forms a reactive
species as evidenced from the UV-Vis spectrum which undergoes deformylation of 2-PPA to
acetophenone. Finally, we reported catalytic hydroxylation of cumene and ethylbenzene by 1
and 2 using m-CPBA.
Acknowledgment
SND thank UGC, New Delhi (F.No.37-576/2009 (SR) and CSIR, New Delhi (No.
01(2923)/18/EMR-II) for financial support. SH thank Goa University for the research
studentship. DDN acknowledge support from UGC (BSR fellowship).
Supplementary Information (SI)
The SI file contains additional figures (Fig. S1-S15) and checkCIF/PLATON reports of
structures of 6 and 8. The crystallographic data of compounds 6 and 8 are deposited in the
Cambridge Crystallographic Data Centre with CCDC nos. 1842331 (6) and 1486163 (8). The
data can be freely downloaded from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
15

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19

List of schemes and figures
Scheme 1. Structures of known non-heme tetradentate N-donor ligands
Scheme 2. Synthetic methods used to obtain the compounds 1-6
20

Fig. 1. IR spectra of BQCNMe2 ligand, 1, 3 and 4.
Fig. 2. CV (solid line) and DPV (dotted line) of 1 recorded in CH3CN containing 0.1 M of
TBAPF6 as supporting electrolyte against 0.01 M Ag/Ag+ reference electrode.
21

Fig. 3. X-ray powder patterns of 1 and 2.
22

Fig. 4. – The crystal structure of compound (6) showing the atom labelling scheme and the
penta coordination sphere of Ni(II) (top), intramolecular H-bonding is shown in broken line.
The crystal structure of compound (8) showing the atom labelling scheme and the hexa
coordination sphere of Ni(II) (bottom). Displacement ellipsoids are drawn at the 50%
probability level excepting for the H atoms, which are shown as circles of arbitrary radius.
For clarity the disordered atoms and the anions in 6 (See Fig. S11) are not shown.
23

Fig. 5. Hydrogen bonding interactions in 6 with atom labelling scheme of atoms involved
hydrogen bonding along bc plane. O3ʹ, O4ʹ, O8Bʹ, H13ʹ, H4Nʹ and H6BNʹ are symmetry
generated atoms. The symmetry code for all symmetry generated atoms are 1-x, 1-y, 1-z.
Fig. 6. a) H-bonding situation around the unique anions. Hydrogen atoms which are not
involved in hydrogen bonding are omitted for clarity b) Enlarged view of network of
hydrogen bonding in 8 showing symmetric organization of cations [Ni(BQENH2)(en)]2+ and
anions [ClO4]- in a crystallographic ac plane.
24

Fig. 7. a) A view along a showing the crystallographic packing of {NiN6}2+ cations (blue)
and perchlorate anions in polyhedral representation. b) alignment of {NiN6}+2 octahedra and
perchlorate anions along a axis showing separation between the two geometric units.
25

Fig. 8. a) UV-Vis spectra of 2 (black line) and new peaks formed at 570 and 384 nm (red
line) in CH3CN at 25˚C on addition of H2O2 / TBAH due to species 2a; b) UV-Vis spectral
changes occurring at 570 nm on addition of 2-PPA (10 mM). Inset shows the pseudo first
order time trace for the decay of peak 570 nm on addition of 2-PPA.
26

List of Tables
Table 1- Technical details of crystal data and structural refinement parameters of 6 and 8.
Parameters
6
8
Empirical formula
Formula weight
Crystal description
Crystal colour
C36H32Cl2N6NiO8
806.28
Prismatic
pale green-purple
Triclinic
C24H29Cl2N7NiO8
673.15
Block
dark red
Triclinic
Crystal system
Space group
Pī
Pī
Temperature (K)
Unit cell dimensions
100(2)
100(2)
a = 8.6908(9) Å
b = 11.9334(12)Å
c = 18.6099(19)Å
α = 100.900(2) °
β = 101.104(2)°
γ = 107.0900(10)°
1747.1(3)
a = 10.9192(11) Å
b = 12.3327(12)Å
c = 12.6497(13)Å
α = 60.9370(10) °
β = 70.0320(2)°
γ = 75.4400(2)°
1391.9(2)
volume (Å3)
Z
2
2
Radiation type (Mo-Kα)/Å
Crystal size (mm)
X-ray Diffractometer
0.71073
0.71073
0.16 x 0.12 x 0.09
Bruker Smart APEX-II Duo
0.32 x 0.22 x 0.16
Bruker Smart APEX-II
Duo
Absorption correction
No. measured reflections
Calculated density (mg/m3)
Absorption coefficient (mm-1)
F(000)
Multi scan
26448
1.533
0.771
Multi scan
20590
1.606
0.951
696
832
θ range for data collection
Limiting indices
1.850 to 25.000
-10 ≤ h ≤ 10
-14 ≤ k ≤ 14
-22 ≤ l ≤ 22
Full-matrix least-squares
on F2
1.899 to 25.000
-12 ≤ h ≤ 11
-14 ≤ k ≤ 14
-15 ≤ l ≤ 15
Full-matrix least-squares on F2
Refinement method
Data / restraints / parameter
Final R Indices[I > 2σ(I)]
R indices (all data)
6149 / 217 / 579
R1 = 0.0535, wR2 = 0.1369 R1 = 0.0238, wR2 = 0.0629
R1 = 0.0586, wR2 = 0.1402 R1 = 0.0254, wR2 = 0.0638
1.060
1.134 and -0.890
4876 / 0 / 388
Goodness of fit on F2
Largest diff. peak and hole
(eÅ-3)
1.063
0.460 and -0.410
Reflections collected / unique
26448 / 6146
20590 / 4876
[R(int) = 0.0558]
[R(int) = 0.0236]
27

Table 2 – Selected bond lengths (Å) and angles (°) for 6 and 8.
Compound 6
Ni1-N1
Ni1-N3
Ni1-N5A
2.001(3)
2.012(3)
2.041(4)
Ni1-N2
Ni1-N4
Ni1-N5B
2.156(3)
2.022(3)
2.154(12)
N1-Ni-N2
N1-Ni-N4
N1-Ni-N5B
N2-Ni-N4
N2-Ni-N5B
N3-Ni-N5A
N4-Ni-N5A
80.42(11)
176.48(12)
88.1(3)
99.94(12)
127.5(4)
N1-Ni-N3
N1-Ni-N5A
N2-Ni-N3
N2-Ni-N5A
N3-Ni-N4
N3-Ni-N5B
N4-Ni-N5B
99.08(11)
93.43(14)
105.30(11)
111.05(15)
84.23(12)
127.1(4)
143.03(16)
83.16(14)
89.0(3)
Compound 8
Ni1-N1
Ni1-N14
Ni1-N25
2.1191(13)
2.1032(14)
2.0977(14)
Ni1-N11
Ni1-N23
Ni1-N28
2.1105(14)
2.1094(13)
2.0918(14)
N1-Ni1-N11
N1-Ni1-N23
N1-Ni1-N28
N11-Ni1-N23
N11-Ni1-N28
N14-Ni1-N25
N23-Ni1-N25
N25-Ni1-N28
80.17(5)
167.47(5)
92.65(5)
89.57(5)
97.18(5)
94.44(5)
91.92(5)
82.69(5)
N1-Ni1-N14
N1-Ni1-N25
N11-Ni1-N14
N11-Ni1-N25
N14-Ni1-N23
N14-Ni1-N28
N23-Ni1-N28
91.49(5)
98.35(5)
85.79(6)
178.51(5)
80.51(5)
175.27(5)
95.79(5)
Note: The values in the parentheses indicate estimated standard deviations.
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Table 3 – Hydrogen bonding Parameters (Å, °) for 6 and 8.
Compound 6
D-H/Å
1.00
1.00
0.84
0.84
D-H···A
H···A/Å
2.12
2.50
2.56
2.15
D···A/Å
2.7302
3.2878
3.3389
2.9352
D-H···A/°
117
135
155
155
N5A-H5AN···N(6A)
N5A-H4N···O(3)a
N4-H4N···O(5B)
N(4)-H4)N···O(8B)
a = 1-x, 1-y, 1-z
Compound 8
D-H/Å
0.83
0.85
D-H···A
H···A/Å
2.18
2.17
2.21
2.11
D···A/Å
D-H···A/°
169
176
166
164
N(11)-H(11)···O(11)a
N(14)-H(14) ···O(21)b
2.9942
3.0158
3.1038
2.9974
3.1478
3.0649
N(25)-H(25A) ···O(24)c 0.91
N(25)-H(25B) ···O(24)b 0.91
N(28)-H(28A) ···N(14)d 0.91
N(28)-H(28B) ···O(13) a 0.91
2.29
2.17
158
168
a = x,-1+y, z; b = 1-x,1-y, -z; c = -1+x, y, z; d = -x, 1-y,1-z
Table 4. The yields of oxidised products formed in the catalytic oxidation of cumene and
ethylbenzene by 1 and 2 in presence of m-CPBAa analysed by GC.
substrate
cumene
alcohol (A)
(TON)b
(A)
ketone (K)
(TON)b
(B)
A / K
2-phenylpropane-2-ol 460
165
acetophenone 60
acetophenone 42
7.6
4.0
1
2
ethylbenzene 1-phenylethanol
cumene
2-phenylpropane-2-ol 420
acetophenone 55
acetophenone 32
7.7
3.9
ethylbenzene 1-phenylethanol
130
a
Note: Reaction conditions: [Ni2+] = 0.5 mM; [m-CPBA] = 0.5 M, [substrate] = 1 M in CH3CN at 25˚C for 90
min under N2. bTurnover number [(moles of product)/(moles of catalyst)] determined by GC.
29