Synthesis and characterization of N3Py2 ligand-based cobalt(II), nickel(II) and copper(II) catalysts for efficient conversion of hydrocarbons to alcohols

Article abstract of DOI:10.1016/j.ica.2017.08.027

Three new complexes [Co(N3Py2)(H₂O)](ClO₄)₂ 1, [Ni(N3Py2)(H₂O)](ClO₄)₂ 2 and [Cu(N3Py2)](ClO₄)₂ 3 (N3Py2 is N,N′-dimethyl-N-(2-(methyl(pyridin-2-ylmethyl)amino)ethyl)-N′-(pyridin-2-ylmethyl)ethane-1,2-diamine) have been synthesized and characterized. Non-heme ligand N3Py2 have been prepared by Eschweiler-Clarke method and reported for the first time. Compounds 1 and 2 were characterized by single crystal X-ray structure analysis. The structure of 1 and 2 revealed that Co(II) cation in 1 and Ni(II) cation in 2 are bonded to the five nitrogen atoms of N3Py2 and a water molecule thus forming an octahedral motif [M(N3Py2)(H₂O)]²⁺. For compound 3, a square pyramidal geometry has been proposed based on the spectroscopic, elemental analysis and ESI-MS data. Compounds 1–3 were tested as catalysts in the oxidation of cumene and adamantane using m-CPBA. Comparative effect of counter anions on the product yields was observed when the perchlorates anions of 1–3 were replaced with tetraphenylborates to give compounds 1a–3a. The turnover numbers of alcohol over ketone product increased in order of catalysts, 1(1a) > 2(2a) > 3(3a).

Full text of DOI:10.1016/j.ica.2017.08.027

Accepted Manuscript
Research paper
Synthesis and Characterization of N3Py2 Ligand-Based Cobalt(II), Nickel(II)
and Copper(II) Catalysts for Efficient Conversion of Hydrocarbons to Alcohols
Dattaprasad D. Narulkar, Anant Kumar Srivastava, Raymond J. Butcher,
Kanakappan M. Ansy, Sunder N. Dhuri
PII:
S0020-1693(17)30734-X
http://dx.doi.org/10.1016/j.ica.2017.08.027
ICA 17821
DOI:
Reference:
Inorganica Chimica Acta
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8 August 2017
11 August 2017
Please cite this article as: D.D. Narulkar, A.K. Srivastava, R.J. Butcher, K.M. Ansy, S.N. Dhuri, Synthesis and
Characterization of N3Py2 Ligand-Based Cobalt(II), Nickel(II) and Copper(II) Catalysts for Efficient Conversion
of Hydrocarbons to Alcohols, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.08.027
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Synthesis and Characterization of N3Py2 Ligand-Based Cobalt(II),
Nickel(II) and Copper(II) Catalysts for Efficient Conversion of
Hydrocarbons to Alcohols
Dattaprasad D. Narulkara, Anant Kumar Srivastavab, Raymond J. Butcherc, Kanakappan M.
Ansyd, Sunder N. Dhuria*
aDepartment of Chemistry, Goa University, Taleigao Plateau, Goa, India, Tel:918326519318;
bDepartment of Chemistry, IISER, Pune, India 411008,
cDepartment of Chemistry, Howard University, Washington, DC 20059
dDepartment of Chemistry and Nanoscience, Ewha Womans University, 52, Ewhayeodae-gil, Seodaemun-gu,
Seoul 03760 Republic of Korea
E-mail:sndhuri@unigoa.ac.in
Abstract
Three new complexes [Co(N3Py2)(H2O)](ClO4)2 1, [Ni(N3Py2)(H2O)](ClO4)2 2 and
[Cu(N3Py2)](ClO4)2 3 (N3Py2 is N,N'-dimethyl-N-(2-(methyl(pyridin-2-ylmethyl)amino)-
ethyl)-N'-(pyridin-2-ylmethyl)ethane-1,2-diamine) have been synthesized and characterized.
Non-heme ligand N3Py2 have been prepared by Eschweiler-Clarke method and reported for
the first time. Compounds 1 and 2 were characterized by single crystal X-ray structure
analysis. The structure of 1 and 2 revealed that Co(II) cation in 1 and Ni(II) cation in 2 are
bonded to the five nitrogen atoms of N3Py2 and a water molecule thus forming an octahedral
motif [M(N3Py2)(H2O)]2+. For compound 3, a square pyramidal geometry has been proposed
based on the spectroscopic, elemental analysis and ESI-MS data. Compounds 1-3 were tested
as catalysts in the oxidation of cumene and adamantane using m-CPBA. Comparative effect
of counter anions on the product yields was observed when the perchlorates anions of 1-3
were replaced with tetraphenylborates to give compounds 1a-3a. The turnover numbers of
alcohol over ketone product increased in order of catalysts, 1(1a) > 2(2a) > 3(3a).
Keywords
Cobalt, nickel, copper, X-ray crystallography, cumene, adamantane, alcohol.
1

1.
Introduction
The transition metal complexes play key roles in bioinorganic chemistry as they are
used as structural model complexes for metalloenzymes and as catalysts in oxidation
reactions [1,2]. The design of novel transition metal complexes with nitrogen donor ligands is
often a target of synthetic coordination chemists. So far a variety of metal complexes bearing
amine and polypyridyl ligands exhibiting diverse structural features have been synthesised
and their roles in oxygenation reactions are well understood [3-8]. The oxidation of cheaply
available hydrocarbons in the natural feedstock to a commercially valued oxidized products
under mild conditions is one of the imperative and quite difficult chemical process [9,10].
The late transition metal complexes of first-row are now known to be equally proficient
catalysts for hydrocarbon oxidations. The selective oxidation of methane to methanol by
dioxygen activation catalyzed by an enzyme, methane monooxygenase has been reported [11-
14]. This has simultaneously led to the upsurge of research relevant to alkane oxidation by
biomimetic model complexes with much attention on bio-inspired complexes of iron as
potential catalysts [15-23]. In recent years, the cobalt(II) compounds have been widely used
in biomimetic catalysis. The cobalt(II) substituted dioxygenase enzyme showed robust
activity in vitro compared to that of the native enzyme [24]. A large number of cobalt(II)
complexes in presence of alkylperoxides as oxidants proceed via formation of an alkylperoxo
complex and perform the oxidation of hydrocarbons [25]. The high valent cobalt(IV)-oxo
species unlike the iron(IV)-oxo or manganese(IV)-oxo species are very rare and only known
in recent years. Nam and Ray group have reported the spectroscopic capture of a low spin
Co(IV)-oxo intermediate in the presence of redox-inactive metal ions and investigated their
reactivity in C-H activation and oxygen atom transfer reactions [26,27]. The spectroscopic
characterization and reactivity study of Co(IV)-oxo species have been reported [28].
2

In addition to the cobalt(II) complexes in oxygenation reactions, the several nickel(II)
compounds have been used as efficient catalysts in alkane oxidations. In a recent study, it
was observed that a [Ni(TPA)]2+ (TPA is Tris(2-pyridylmethyl)amine) complex showed high
TON (turnover numbers) of products over related TPA complexes of iron(II), manganese(II),
and cobalt(II) in the presence of m-CPBA as an oxidant [29]. Later, the handful nickel(II)
compounds were isolated and used as the catalysts in alkane hydroxylations [30-36]. It is
noted that when such reactions are carried out in the presence of alkyl peroxide or acyl
peroxide and the nickel(II) catalysts, they often proceed via the formation of a nickel(II)-
alkylperoxy or nickel(II)-acylperoxo short lived intermediate species [30-34,37].
The copper(II) complexes in enzymatic, biomimetic and chemical oxiation reactions
have also been investigated to large extend. The copper(II)-dioxygen (Cu-O2) species have
been frequently invoked as the reactive intermediate in the C-H bond activation reactions
catalyzed by copper(II) complexes [38-49]. In our recent study, we have shown the efficacy
of nickel(II) complexes containing a quinoline based tetradentate non-heme ligand in alkanes
hydroxylation [50]. In continuation to this work, herein we report the synthesis,
characterization and catalytic hydroxylation of alkanes by three new compounds of cobalt(II),
nickel(II) and copper(II) stabilized by a non-heme pentadentate ligand N3Py2.
2. Experimental
2.1 Materials and methods
All reagents were purchased from commercial sources and used without
further purification. The solvents were distilled and dried under N2 atmosphere prior to their
use. 1H and 13C-NMR spectra were recorded on a Bruker Avance III, 400 MHz, NMR
spectrometer using CDCl3. The infrared (IR) spectra in the region of 4000-400 cm-1 were
recorded on Shimadzu (IR-Prestige-21) FTIR spectrometer by diluting the compounds in KBr
3

powder. The perchlorate salts of Co(II), Ni(II) and Cu(II) were synthesized carefully by
reacting their carbonates with perchloric acid (70 %, v/v) followed by slow crystallization
[51]. The percentage of C, H and N were obtained using Elementar Variomicro Cube CHNS
Analyser. The UV-Vis spectra were recorded on Agilent diode array 8453 UV-Vis
spectrophotometer in the wavelength range of 200-1100 nm in CH3CN. The electrospray
ionization mass spectra (ESI-MS) of 1-3 in CH3CN were recorded using Applied Biosystem
Matrix-assisted Laser Desorption Ionization Time-of-flight (MALDI-TOF) spectrometer. The
cyclic voltammograms (CV) and differential pulse voltammograms (DPV) were measured
using Electrochemical Workstation-CH Instrument, Inc. CHI6107. A glass vessel containing
the sample dissolved in CH3CN was equipped with a platinum disc working electrode, a
platinum wire as a counter electrode and a reference electrode, Ag/AgNO3 (0.01 M). All the
experiments were carried out using supporting electrolyte, tetrabutylammonium
hexafluorophosphate (TBAPF6) (0.1 M) and the solutions were purged with N2 gas for
around ~30 min prior to every measurement. The potentials (Vs Ag/Ag+) were converted to
the values vs the standard calomel electrode (SCE) by adding a value of 0.29 V [52]. The
single crystals of 1 and 2 suitable for X-ray crystallography were picked up using the glycerol
loop and mounted directly on the Bruker SMART APEX-II CCD diffractometer (Mo-Kα =
0.71073 Å). The CCD data were integrated and scaled using Bruker-SAINT software
package while SHEXTL V 6.12 was used for solving and refining the structures [53]. All
non-hydrogen atoms were refined anisotropically, if not stated otherwise and the hydrogen
atoms were located at the calculated positions. In the crystal structure of 1, the disordered
atom positions (in perchlorate anions) were freely refined isotropically over two positions
using similar distances and U-restraints. The products formed in the reaction mixture were
quantified using the Shimadzu GC 2014 equipped with HP capillary column (30 m x 0.25
mm x 2.5 µM) and a FID detector.
4

2.2.1 Synthesis of N,N'-dimethyl-N-(2-(methyl(pyridin-2-ylmethyl)amino)ethyl)-N'-
(pyridin-2-ylmethyl)ethane-1,2-diamine)
N3Py2 was prepared by following three step procedures. Step-I: To the ethanolic
solution of 2-pyridine-carboxaldehyde (3.0 g, 28.0 mmol) was added 1.52 mL of
diethylenetriamine (1.44 g, 14.0 mmol). The mixture was refluxed for ~ 5 h, cooled to room
temperature and the solvent was removed to give red semisolid product. Yield of product was
3.1 g (79 %). IR data (KBr, cm-1): 3295 ν(N-H), 3200-2700 ν(C-H), 1648 ν(C=N). Step-II: To
the ice-cold methanolic solution of imine product (3.0 g, 10.7 mmol), the sodium borohydride
(0.48 g, 12.8 mmol) was added slowly and the mixture stirred for it became brownish orange
in colour (~6 h). The solvent was removed and water (20 mL) was added to the flask
containing the crude product. The yellow viscous oil was then extracted using ethyl acetate
(30 mL x 3). Yield of product was 2.8 g (92 %). IR data (KBr, cm-1): 3295 ν(N-H), 3200-
2700 ν(C-H), 1670 ν(C=N). Step-III: The product (2.6 g, 9.1 mmol) of second step was taken
in water (3.0 mL) and cooled in ice-bath. To this mixture, formaldehyde (37 %, 22.0 mL) and
formic acid (85 %, 15.0 mL) were added and refluxed for ~24 h. The mixture was then
cooled and basified (pH = 12) using 2 M NaOH solution. The crude reddish-brown oil
obtained after extraction with chloroform (20 mL x 4) was dissolved in HCl solution (pH = ~
1). The acidic mixture was then basified using NaOH solution (pH = 12) and the product was
then extracted using diethyl ether (20 mL x 6). N3Py2 was formed as yellow oil with the
yield 2.5 g (72 %). IR data (KBr, cm-1): 3200-2700 ν(C-H), 1670 ν(C=N).1H NMR (CDCl3,
ppm): δ 8.47 (d, 2H, J = 3.2 Hz, 2-PyH), δ 7.57 (t, 2H, J = 8.2 Hz, 4-PyH), δ 7.34 (d, 2H, J =
3.8 Hz, 5-PyH ), δ 7.08 (t, 2H, J = 6.16 Hz, 3-PyH), δ 3.63 (s, 4H, Ar-CH2), δ 2.57 (s, 8H, N-
CH2), δ 2.25 (s, 3H, NMe), δ 2.25 (s, 3H, NMe) δ 2.21 (s, 6H, NMe). 13C NMR (CDCl3,
ppm): δ 159.1 (C6), 148.1 (C2), 136.2 (C4), 122.9(C5), 121.7(C3), 64.04 (Ar-CH2), 55.3 (N-
CH2), 42.7 (N-CH3).
5

2.2.2 Synthesis of [Co(N3Py2)(H2O)](ClO4)2 1
N3Py2 (0.452 g, 1.37 mmol) was dissolved in CH3CN (2 mL) and added to the
stirring CH3CN solution of Co(ClO4)2.6H2O (0.5 g, 1.37 mmol) under the N2 at room
temperature. The mixture was stirred for ~12 h and filtered. To the resulting dark reddish-
pink solution, the diethyl ether (10 mL) was added and the mixture was kept undisturbed for
crystallization. The crystals formed after two days were isolated by filtration and dried in air.
The yield of 1 was 0.7 g (80 %). Mol. Formula, C19H31N5Cl2O9Co: calc. C, 37.82; H, 5.18;
N, 11.61 %. Found, C, 37.78; H, 4.98; N, 11.62 %. IR-data (KBr, cm-1): 3420 ν(OH); 3137-
-1
2752 ν(CH); 1090, 621 ν(ClO4 ). UV-Vis data, λmax, CH3CN / nm (ɛ/dm3 mol-1cm-1): 262
(81642), 494 (42), 1028 (8). ESI-MS: m/z = 193.2 (calc. 193.2) for [Co(N3Py2)]2+ and m/z =
485.9 (calc. 485.9) for [Co(N3Py2)(ClO4)]+.
2.2.3 Synthesis of [Ni(N3Py2)(H2O)](ClO4)2 2
Compound 2 was prepared in a similar way as 1 by reacting N3Py2 (0.452 g, 1.37
mmol) and Ni(ClO4)2.6H2O (0.5 g, 1.37 mmol) in CH3CN. Yield of 2 was 0.7 g (84 %). Mol.
Formula, C19H31N5Cl2O9Ni: calc. C, 37.84; H, 5.18; N, 11.61 %. Found, C, 37.78; H, 5.26;
N, 11.90 %. IR-data (KBr, cm-1): 3420 ν(OH); 3137-2752 ν(CH); 1090, 621 ν(ClO4). UV-Vis
data, λmax, CH3CN / nm (ɛ/dm3 mol-1cm-1): 262(85439), 554(38), 920(27). ESI-MS: m/z =
193.1 (calc. 193.1) for [Ni(N3Py2)]2+ and m/z = 485.5 (calc. 485.6) for [Ni(N3Py2)(ClO4)]+.
2.2.4 Synthesis of [Cu(N3Py2)](ClO4)2 3
Compound 3 was prepared using similar methodology as in 1 and 2 wherein N3Py2
(0.452 g, 1.37 mmol) was reacted with Cu(ClO4)2.6H2O (0.5 g, 1.349 mmol) in CH3CN. The
dark blue crystalline powder of 3 was obtained and the yield of 3 was 0.64 g (78 %). Mol.
Formula, C19H31N5Cl2O9Cu: calc., C, 37.54; H, 5.14; N, 11.52 %. Found, C, 37.80; H, 5.06;
6

-1
N, 11.79 %. IR-data (KBr, cm-1): 3030-2825 ν(CH); 1093, 621 ν(ClO4 ). UV-Vis data, λmax
,
CH3CN / nm (ɛ/dm3 mol-1cm-1): 256 (10804), 592 (216). ESI-MS data: m/z = 194.6 (calc.
194.6) for [Cu(N3Py2)]2+ and m/z = 489.1 (calc. 489.1) for [Cu(N3Py2)(ClO4)]+.
2.2.5 Synthesis of compounds 1a-3a
Three other compounds, [Co(N3Py2)(CH3CN)](BPh4)2 1a,
[Ni(N3Py2)(CH3CN)](BPh4)2 2a and [Cu(N3Py2)](BPh4)2 3a were prepared by the reaction
of 1, 2 and 3 with two equivalents of Na(BPh4) (0.45 g, 1.3 mmol) in CH3CN at room
temperature. Analytical data for compounds 1a, 2a and 3a is as follows. Yield of 1a was 86
%. Mol. Formula, C69H72N6B2Co: calc., C, 77.57; H, 6.81; N, 7.88 %. Found, C, 77.90; H,
6.72; N, 7.95 %. IR-data (KBr, cm-1): 3137-2752 ν(CH); 2280 ν(NCCH3); 734, 705 (BPh4).
Yield of 2a was 85 %. Mol. Formula, C69H72N6B2Ni: calc., C, 77.77; H, 6.81; N, 7.89 %.
Found, C, 77.60; H, 6.88; N, 7.61%. IR-data (KBr, cm-1): 3137-2752 ν(CH); 2275
ν(NCCH3); 734, 705 (BPh4). Yield of 3a was 83 %. Mol. Formula, C69H72N6B2Cu: calc., C,
77.41; H, 6.78; N, 7.85 %. Found, C, 77.84; H, 6.90; N, 7.51%. IR-data (KBr, cm-1): 3137-
2752 ν(CH); 734, 705 (BPh4).
2.3 Catalytic oxidations of cumene and adamantane
Compounds 1-3 were tested in the oxidation of alkyl hydrocarbons namely cumene
and adamantane using m-CPBA as an oxidant in CH2Cl2:CH3CN (3:1) mixture at room
temperature under N2 atmosphere. In a typical catalytic reaction, the complex was dissolved
in CH3CN (80 µL, 2.5 mM) was added to the stirring solution of cumene (350 mM) or
adamantane (250 mM) in 3:1 CH2Cl2-CH3CN (4 mL) in the presence of m-CPBA (50 mM).
At every fixed interval of time, a fraction of the reaction mixture was quenched using
7

triphenylphosphine and eluted over a silica column using diethyl ether. The eluted sample
was then directly infused on GC column using n-decane internal standard.
3. Results and Discussion
3.1 Synthesis and characterization of N3Py2
In heme and non-heme bioinspired chemistry, the ligands have been known to provide the
environment to central metal ions such that the resulting structure behaves as a functional
model for the enzymatic reactions. The transition metal complexes with different denticity
ligands (amines and pyridyl based) are viable catalysts for organic oxidations which often
proceed via formation high-valent metal-oxygen intermediates [54]. The iron(IV)-oxo and
manganese(IV)-oxo of non-heme ligands N4Py (N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)-
methylamine) and BnTPEN (N-benzyl-N,N',N'-tris(2-pyridylmethyl)-1,2-diaminoethane)
(Scheme 1) have been studied earlier in the C-H activation, epoxidation and oxygen transfer
reactions [55-57]. It has been reported that the ligands with secondary –NH groups
coordinated to metal ions tend to undergo fast degradation in the presence of oxidants such as
m-CPBA, H2O2, etc., and thus decreases the efficiency of catalysts giving low yields of
products. The complex of nickel(II), [Ni(cyclam)]2+ has been reported to give low yields of
products due to the degradation of cylcam [58]. In our earlier work, we also observed low
yields of alcohol in the alkane oxidations catalyzed by [Ni(bqenH2)](ClO4)2 (bqenH2 is N,N’-
bis(8-quinolyl)ethane-1,2-diamine) complex [50]. Thus, being inspired by the biomimetic
applications of transition metal complexes having N-methylated ligands in the field of
catalysis, herein we have report synthesis of a new pentadentate N3Py2 ligand (Scheme 1)
and its three new cobalt(II), nickel(II) and copper(II) complexes. N3Py2 was prepared in
multi step method (Scheme 2). In the first step, the diethylenetriamine and pyridine-2-
carboxaldehyde were condensed to give Schiff base imine, which was then reduced by
8

sodium borohydride in step II [59-61]. In the final step, the amine was N-methylated
following Eschweiler-Clarke reaction (Scheme 2). N3Py2 was characterized using IR, 1H and
13C NMR spectroscopic techniques (Fig. 1 and SI, Fig. S1-S3).
3.2 Synthesis and characterization of 1-3
The reaction of N3Py2 with metal salts, Co(ClO4)2.6H2O or Ni(ClO4)2.6H2O or
Cu(ClO4)2.6H2O in CH3CN in equal molar concentration afforded us three new compounds 1,
2 and 3 (Scheme 3). The characteristic spectroscopic features of N3Py2 in all compounds
were traced from their IR and UV-Vis spectra. The cyclic and differential pulse voltammetric
techniques (CV/DPV) were used to obtain redox potentials of 1-3. Based on the IR and UV-
Vis spectroscopy, C,H,N analysis, ESI-MS and CV/DPV data, the compounds 1-3 were
unambiguously formulated as [Co(N3Py2)(H2O)](ClO4)2, [Ni(N3Py2)(H2O)](ClO4)2 and
[Cu(N3Py2)](ClO4)2. Compounds 1 and 2 were then characterized by single-crystal X-ray
structure analysis. Compounds 1-3 were tested as catalysts in the oxidation of two alkyl
hydrocarbons, cumene and adamantane using m-CPBA as an oxidant [35,55-57]. We also
investigated the counter anion effect on the product yields formed in the oxidation of cumene
and adamantane. The perchlorates of 1-3 were replaced by tetraphenylborates in acetonitrile
to afford us compounds [Co(N3Py2)(CH3CN)](BPh4)2 1a, [Ni(N3Py2)(CH3CN)](BPh4)2 2a
and [Cu(N3Py2)](BPh4)2 3a.
3.2.1. Infrared spectra of N3Py2, 1-3 and 1a-3a
Infrared spectroscopy is an affordable technique used by many inorganic chemists to
extract the information on the ligation behaviour of ligands in the metal complexes. When
ligand coordinates, the original bands due ligand often shift slightly to the lower
wavenumbers [62]. We have used IR spectroscopy for the characterization of ligand and the
9

complexes 1-3 and 1a-3a. IR spectra of products obtained in step I-II showed stretching
vibrations due to N-H bonds, while the IR spectrum of N3Py2 showed no N-H signals
indicating N3Py2 has formed by methylation [SI, Fig. S1]. The overlaid IR spectra of N3Py2
and compounds 1-3 are shown in Fig. 2a. IR spectra of 1 and 2 shows a broad band centered
at ~3420 cm-1 which has been assigned to the O-H vibration of water molecule. Interestingly,
no O-H vibration was observed for compound 3 which suggest the absence of water molecule
and thus square pyramidal geometry was proposed as in [Cu(bpmen)(ClO4)]+ (bpmen is
tetradentateN,N′-dimethyl-N,N′-bis(2-pyridylmethyl)ethane-1,2-diamine) [63]. The lack of
water or solvent molecule in nickel(II) and copper(II) complexes has been reported [63,64].
Further, the IR spectra of 1-3 shows strong absorption signals at ~1090 cm-1 and ~621 cm-1
due uncoordinated perchlorates [62]. When we replaced two perchlorates by two
tetraphenylborate anions in 1-3, the IR spectra of resulting compounds 1a-3a showed the
bands at ~734 cm-1 and ~705 cm-1 corresponding to tetraphenylborates (SI, Fig. S4) [25]. In
addition to tetraphenylborate bands, we also observed a new band at 2275 cm-1 for 1a and
2280 cm-1 for 2a due to incorporation of CH3CN molecule [62,65]. Thus, from IR spectra, we
infer that H2O and CH3CN molecules are essential components to stabilize the structures of
1-2 and 1a-2a (vide infra). IR spectrum of 3a showed no vibration attributed to CH3CN
indicating that complex 3a adopt same geometry as in 3 [66,67].
3.2.2 Electronic spectra of 1-3
UV-Vis spectra of compounds 1-3 were recorded in CH3CN showed an intense band
at ~262 nm in all compounds and this band has been assigned to the intra-ligand charge
transfer transition. The weak bands due to spin allowed d-d transitions were observed in the
visible region (Fig. 2b). In the UV-Vis spectrum of 1, the bands at ~494 and ~1028 nm were
4
4
4
4
→
assigned to T1g(F)→ A2g(F) and T1g(F) T2g(F) transitions respectively [68]. For
10

compound 2 the bands at 554 and 920 nm were attributed to 3A2g(F)→3T1g(F) and
3A2g(F)→3T2g(F) transitions respectively [50,69,70]. The third band due to 4T1g(F) → 4T1g(P)
in 1 and 3A2g(F)→3T1g(P) in 2 were tailed in UV region due to overlapping with charge
transfer band of N3Py2 and hence not seen [70,71]. Unlike low intensity bands in the UV-Vis
spectra of 1 and 2, the compound 3 exhibited a high intensity band at ~592 nm. Such bands
are often seen in the spectrum when a molecule is non-centrosymmetric [72]. The high
absorbance band was also observed in the UV-Vis spectrum of a square pyramidal non-
centrosymmetric [CuIIDIEN-pyr)]2+ complex [66,67].
3.2.3 ESI-Mass spectrometry
The ESI-MS spectra of compounds 1 and 2 are shown in Fig. 3a and 3b. The ESI-MS
spectrum of 1 in CH3CN showed two prominent mass peaks at m/z = 193.2 and 485.9, which
were assigned to [Co(N3Py2)]2+ and [Co(N3Py2)(ClO4)]+ species respectively (Fig. 3a).
Compound 2 showed two peaks at m/z = 193.1and 485.5 corresponding to [Ni(N3Py2)]2+ and
[Ni(N3Py2)(ClO4)]+ species respectively (Fig. 3b). On measuring the ESI-MS spectrum of 3,
the two peaks at m/z = 194.6 and 489.1 corresponding to [Cu(N3Py2)]2+ and
[Cu(N3Py2)(ClO4)]+ ions respectively were observed (data no shown).
3.2.4 Crystal structures of 1 and 2
Single crystals of 1 and 2 suitable for X-ray crystallography were obtained by the
slow diffusion of diethyl ether into their CH3CN solutions. The technical details of data
acquisition and selected refinement results for 1 and 2 are listed in Table 1. The selected bond
lengths and bond angles for 1 and 2 are shown in Table 2. It has been reported that the metal
complexes with the pentadentate N5 ligands can exist in four isomeric forms [73] (SI,
Scheme S1). Based on this report and our X-ray structural characterization, we propose the
11

isomeric structure II for compounds 1 and 2 (Fig. 4 and Fig. 5). Compounds 1 and 2 are
isostructural and crystallize in a centrosymmetric space group P21/c. The crystal structures of
both consist of a centrally located metal(II) ion (Co(II) in 1 and Ni(II) in 2), a pentadentate
N3Py2 and a H2O molecule besides two crystallographically independent perchlorates (Fig. 4
and Fig. 5). The octahedral [MN5O]2+ unit (M = Co(II), Ni(II)) is slightly distorted due to the
presence of two types of nitrogen-donor atoms (two of pyridyl part and three of tertiary
amine backbone) and a water molecule. The octahedral distortion is clearly evident from the
(N-Co-N and N-Ni-N) cis angles which range from 76.30(2)-108.71(3) Å in 1 and from
76.92(14)-109.00(14) Å in 2 and similarly from the trans angles which range between
154.63(2)-173.45(2) Å in 1 and 160.04(15)-172.88(14) in 2 (Table 3). In both the structures,
the pyridyl nitrogen atoms are disposed syn to one another and the triamine part occupies the
facial positions of the octahedron. The similar dispositions of donor atoms were also
observed in compounds with pyrrole based pentadentate ligands [74]. The examples of
triamine nitrogens occupying meridional positions of octahedron are also reported [75].
The perchlorates in 1 and 2 only behave as the counter anions for [Co(N3Py2)]2+ and
[Ni(N3Py2)]2+ cations and do not participate in bonding with cobalt(II) or nickel(II) ions
(Fig. 4 and Fig. 5). However, the perchlorate anions play important role in building the
extended network through its oxygen atoms (O12 and O14 in 1 and O6 and O8 in 2) which
are involved in weak hydrogen bonding with a water molecule (Table 3). The resulting
supramolecular three-dimensional metal organic framework structures formed by Cl-O···H
contacts, [MN5O]2+ motifs, and perchlorates anions are shown in Fig. 6 and Fig. S5 in SI. The
two O-Cl-O and two H-O-H linked through hydrogen bonds forms a twelve membered ring
structure in 1 and 2 and these rings resemble like a chair form of cyclohexane when viewed
along the ‘ab’ plane (Fig. 6a, SI, Fig. S5a). The hydrogen bond distances in 1 and 2 are quite
shorter than the sum of their Van der Waals radii of the atoms involved in the hydrogen
12

bonding. The hydrogen bond distances range from 2.02 (4) to 1.988 (7) Å in 1 and from
2.037 (21) to 2.070 (21) Å in 2 (Table 3). Hydrogen bond distances in 1 are relatively shorter
than in 2 with an average difference of 0.0495 Å. It is evident from the structural data that the
hydrogen bonding basically originates due the presence of a water molecule and perchlorate
ions in 1 and 2. In spite of using acetonitrile as the solvent, the incorporation of water in the
crystal structure of 1 and 2 could be justified based on the use of metal perchlorate
hexahydrates in the synthesis. In other related nickel(II) compounds, the incorporation of
water molecule was also observed although the reactions were carried out in methanol
[29,30,32]. When the perchlorates ions were replaced by tetraphenyborates in acetonitrile, the
new compounds 1a and 2a indeed showed the presence of acetonitrile. Our attempts to grow
single crystals of [Cu(N3Py2)](ClO4)2 3 were not fruitful and hence the compound 3 along
with 1 and 2 were also characterized by powder X-ray diffraction (PXRD) technique. The
PXRD patterns of 1 and 2 were identical and support the notion that they have same space
groups as obtained from their single crystal X-ray structures [Fig. 7]. On the contrary, the
PXRD pattern of 3 was quite different and did not match with the powder patterns of 1 and 2
[Fig. 7]. The single crystal X-ray data and PXRD patterns thus confirm that 1 and 2 are
isostructural while 3 crystallizes in a different space group.
3.2.5 Electrochemical properties of 1-3
Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques were applied
to understand the redox behavior of the compounds 1-3. CV of compound 1 in CH3CN
showed the anodic and cathodic waves corresponding to the Co(II)/Co(III) couple with a E =
½
0.56 V v/s Ag/AgNO3 (0.86 V v/s SCE) (Fig. 8a). The E value was further confirmed from
½
the DPV technique and was in good agreement with those of known cobalt(II) complexes
with multidentate nitrogen donor ligands [76-78]. When we measured CV of 2 under
13

identical conditions, we observed the peak attributed to Ni(II)/Ni(III) redox couple centered
at quite higher potential of 1.39 V v/s Ag/AgNO3 (1.68 V v/s SCE) (Fig. 8b) [35, 70, 79]. On
contrary to the positive E values of 1 and 2, the complex 3 exhibited a reversible peak at E
½
½
= -0.52 V v/s Ag/AgNO3 (-0.22 V v/s SCE) due to Cu(II)/Cu(I) redox couple (Fig. 8c)
[47,80,81]. This large deviation of E value for 3 compared to those of 1 and 2 suggest that
½
the compound 3 have different structural properties. N3Py2 showed no peaks under the
identical conditions indicating the redox peaks in the CV of 1-3 are solely due the metal ions.
The CV plots of 1 as well as 2 and 3 recorded at different scan rates were identical and
showed a proportional increase in the peak currents (SI, Fig. S6-S8). ∆Ep values at different
scan rates suggest the quasi-reversible redox phenomenon occurring in 1 and 2 while the
reversible behavior in 3 (SI, Table S1).
3.3. Catalytic oxidations of cumene and adamantane by 1-3 and 1a-3
Since compounds 1-3 were stabilized by a non-heme ligand N3Py2 and differ from
each other in terms of structural and electronic properties, we then decided to test their utility
in the catalytic hydroxylation of alkyl hydrocarbons (cumene and adamantane) using m-
CPBA as an oxidant. The catalytic oxidation of cumene by 1-3 gave 2-phenyl-2-propanol as
the major product along with acetophenone (Table 4, Scheme 4). The oxidation of alkanes
catalyzed by nickel(II) complexes in presence of m-CPBA with different counter anions such
as acetate and nitrate have been previously reported [30]. It was observed that the yields of
organic products were highly influenced by the type of counter anions present in the
nickel(II) complexes. On similar lines, we investigated the catalytic reactions of three
compounds 1a-3a which were prepared by a simple metathesis reaction of 1-3 with two
equivalents of sodium tetraphenylborate. Interestingly, the TONs of 2-phenyl-2-propanol and
acetophenone increased considerably when compounds 1a-3a were used as catalysts instead
14

of 1-3 (Table 4). This observation suggests that the product yields could be fine-tuned by
using different counter anions keeping the metal ions and N3Py2 unchanged. We also studied
the time dependent oxidation of cumene in the presence of catalysts 1-1a and 2-2a. 2-phenyl-
2-propanol increased with time and the yields were higher for 1 than those for 2 (Fig. 9). On
reacting adamantane (250 mM) with 1-3 and 1a-3a (5 x 10-5 M) in presence of m-CPBA (50
mM), we obtained 1-adamantanol in high yields and 2-admantanol with 2-adamantanone as
the minor products (Table S2, Scheme 4). The yields of products for catalysts 1-3 and 1a-3a
in the hydroxylation of cumene and adamantane increased in the order 1(1a) > 2(2a) > 3(3a)
and our results were comparable with the reported cases [82]. When the reactions were
carried out only in presence of m-CPBA and substrates or in presence of 1-3 or 1a-3a and
substrates, no organic products were detected [83]. When H2O2 and t-BuOOH were used as
oxidants instead of m-CPBA, only trace amounts of alcohol products were formed [32,35].
4. Conclusion
We have reported the synthesis and characterization of three new complexes,
[Co(N3Py2)(H2O)](ClO4)2 1, [Ni(N3Py2)(H2O)](ClO4)2 2 and [Cu(N3Py2)](ClO4)2 3
stabilized by a non-heme ligand N3Py2. Compounds 1 and 2 were structurally characterized
by single crystal X-ray diffractometry. Both are isostructural and crystalized in a
centrosymmetric space group P21/c. The structures of 1 and 2 shows cobalt(II) and nickel(II)
ions coordinating to N3Py2 and H2O molecule forming slightly distorted octahedral
geometry. PXRD patterns suggest of 1-3 suggest that the compound 3 has different structure.
Based on spectroscopic and elemental analysis, the square pyramidal geometry has been
proposed for 3. The catalytic activity of 1-3 was studied in the C-H activation of cumene and
adamantane in presence of m-CPBA. The cumene gave 2-phenyl-2-propanol as well as
acetophenone products in good yields while adamantane affored 1-adamantanol as the major
15

product. The counter anion effect on product yields by replacing perchloarates of 1-3 with
tertraphenylborates was investigated.
Acknowledgement
SND acknowledge UGC and DST, New Delhi for the support to Department of Chemistry,
Goa University through their SAP and FIST programs. DDN thanks UGC, New Delhi for
SRF Fellowship (UGC-BSR). SND thank Dr. M. Sankaralingam, Post-doctoral fellow at
Centre for Biomimetic Systems, Ewha Womans University, Seoul, Korea for valuable
discussions.
Supplementary materials
CCDC 1532118 and 1532119 contains the supplementary crystallographic data for 1 and 2.
These data can be obtained free of charge from ‘The Cambridge Crystallographic Data
Centre’ via www.ccdc.cam.ac.uk/data_request/cif. Supplementary information (SI) associated
with this article can be found in the online version.
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21

List of Figures and schemes
Scheme 1. Chemical structures of non-heme pentadentate nitrogen donor ligands.
Scheme 2. Three step synthetic strategy used in the preparation of N3Py2.
22

Fig. 1. 1H-NMR spectrum of N3Py2 recorded in CDCl3.
Scheme 3. Synthetic method for preparation of 1-3.
23

Fig. 2. (a) The overlaid IR spectra of N3Py2 and 1-3 (b) UV-Vis spectra of compounds 1-3
(10 mM) in CH3CN.
Fig. 3. ESI-MS spectra of 2 mM solution of (a) 1 (b) 2 recorded in CH3CN.
24

Fig. 4. Crystal structure of [Co(N3Py2)(H2O)](ClO4)2 showing atom labelling scheme.
Displacement ellipsoids are drawn at 50% probability except for H atoms which are shown as
circles of arbitrary radius. The perchlorate anions are not shown for the clarity.
Fig. 5. Crystal structure of [Ni(N3Py2)(H2O)](ClO4)2 showing atom labelling scheme.
Displacement ellipsoids are drawn at 50% probability except for H atoms, which are shown
as circles of arbitrary radius.
25

Fig. 6. (a) Cyclic structure formed due to hydrogen bonding interactions in 2 with atom
labelling scheme of atoms involved hydrogen bonding. Hydrogen atoms attached to carbon
are omitted for clarity (b) Enlarged view of a three dimensional network in 2 showing the
symmetric organisation of [Ni(N3Py2)(H2O)]2+ cations and perchlorate anions in the
crystallographic ab plane.
26

Fig. 7. Comparative PXRD patterns of [Co(N3Py2)(H2O)](ClO4)2 1 (red line),
[Ni(N3Py2)(H2O)](ClO4)2 2 (green line) and [Cu(N3Py2)](ClO4)2 3 (blue line).
Fig. 8. CV(black line) and DPV(red line) of a) 1, b) 2 and c) 3 recorded at scan rate of 100
mVs-1 in CH3CN containing 0.1 M of TBAPF6 as supporting electrolyte against Ag/AgNO3
(0.01 M) reference electrode.
27

Fig. 9. Bar graphical diagram showing 2-phenyl-2-propanol formation with time in the
reactions of cumene and A) [Co(N3Py2)(H2O)](ClO4)2 1 vs [Co(N3Py2)(CH3CN)](BPh4)2 1a
B) [Ni(N3Py2)(H2O)](ClO4)2 2 vs [Ni(N3Py2)(CH3CN)](BPh4)2 2a in CH2Cl2/CH3CN (3:1
v/v) mixture at room temperature.
Scheme 4. The reaction scheme showing conversion of cumene and adamantane to 2-phenyl-
2-propanol and 1-adamantanol respectively by catalysts 1-3 and 1a-3a.
28

Table 1 Technical details of data acquisition and selected refinement results for 1 and 2.
1
2
Empirical formula
Formula weight
Crystal description
Crystal colour
C19H31Cl2N5CoO9
603.32
Block
C19H31Cl2N5NiO9
603.08
Block
Brick red
Blue
Crystal system
Space group
Temperature (K)
Unit cell dimensions
Monoclinic
P21/c(no. 14)
100(2)
a = 8.967 (3)Å
b = 32.954 (12)Å
c = 8.662 (3) Å
α = 90 °
Monoclinic
P21/c(no. 14)
100(2)
a = 9.027 (4)Å
b = 33.134 (15)Å
c = 8.668 (4) Å
α = 90 °
β = 105.509 (8)°
γ = 90°
2466.4(16)
4
β = 105.805 (6)°
γ = 90°
2494.6(19)
4
volume (Å3)
Z
Radiation type (Mo-Kα)/Å
Crystal size (mm)
Diffractometer
Absorption correction
0.71073
0.18 x 0.13 x 0.07
Bruker Smart APEX-II Duo Bruker Smart APEX-II Duo
Semi-empirical from
equivalents
3363
1.625
0.972
0.71073
0.400 x 0.30 x 0.10
multi scan
No. measured reflections
Calculated density (mg/m3)
Absorption coefficient (mm-1)
F(000)
6824
1.606
1.051
1259.2
1252
θ range for data collection
Limiting indices
0.618 to 25.280
-10 ≤ h ≤ 8
-39 ≤ k ≤ 39
-10 ≤ l ≤ 10
Full-matrix least-squares on
2.34 to 25.04
-9 ≤ h ≤ 10
-37 ≤ k ≤ 39
-10 ≤ l ≤ 9
Full-matrix least-squares on F2
Refinement method
F2
Data / restraints / parameter
Final R Indices[I>2σ(I)]
R indices (all data)
4455 / 23 / 352
R1 = 0.0995, wR2 = 0.2029 R1 = 0.0572, wR2 = 0.1100
R1 = 0.1222, wR2 = 0.2143 R1 = 0.0718, wR2 = 0.1160
4366 / 0 / 328
Goodness of fit on F2
1.211
1.0720
Largest diff. peak and hole(eÅ-3)
1.531 and -1.797e.Å-3
27156 / 4455 [R(int) =
0.1163]
1.0110 and -0.8095 e.Å-3
Reflections collected / unique
14244 / 4366 [R(int) = 0.0444]
29