Nickel(II) complexes of pentadentate N5 ligands as catalysts for alkane hydroxylation by using m-CPBA as oxidant: A combined experimental and computational study

Article abstract of DOI:10.1002/chem.201402391

A new family of nickel(II) complexes of the type [Ni(L)(CH ₃CN)](BPh₄)₂, where L=N-methyl-N,N′, N′-tris(pyrid-2-ylmethyl)-ethylenediamine (L1, 1), N-benzyl-N,N′, N′-tris(pyrid-2-yl-methyl)-ethylenediamine (L2, 2), N-methyl-N,N′- bis(pyrid-2-ylmethyl)-N′-(6-methyl-pyrid-2-yl-methyl)-ethylenediamine (L3, 3), N-methyl-N,N′-bis(pyrid-2-ylmethyl)-N′-(quinolin-2-ylmethyl)- ethylenediamine (L4, 4), and N-methyl-N,N′-bis(pyrid-2-ylmethyl)-N′- imidazole-2-ylmethyl)-ethylenediamine (L5, 5), has been isolated and characterized by means of elemental analysis, mass spectrometry, UV/Vis spectroscopy, and electrochemistry. The single-crystal X-ray structure of [Ni(L3)(CH₃CN)](BPh₄)₂ reveals that the nickel(II) center is located in a distorted octahedral coordination geometry constituted by all the five nitrogen atoms of the pentadentate ligand and an acetonitrile molecule. In a dichloromethane/acetonitrile solvent mixture, all the complexes show ligand field bands in the visible region characteristic of an octahedral coordination geometry. They exhibit a one-electron oxidation corresponding to the Ni^II/Ni^III redox couple the potential of which depends upon the ligand donor functionalities. The new complexes catalyze the oxidation of cyclohexane in the presence of m-CPBA as oxidant up to a turnover number of 530 with good alcohol selectivity (A/K, 7.1-10.6, A=alcohol, K=ketone). Upon replacing the pyridylmethyl arm in [Ni(L1)(CH ₃CN)](BPh₄)₂ by the strongly σ-bonding but weakly π-bonding imidazolylmethyl arm as in [Ni(L5)(CH ₃CN)](BPh₄)₂ or the sterically demanding 6-methylpyridylmethyl ([Ni(L3)(CH₃CN)](BPh₄)₂ and the quinolylmethyl arms ([Ni(L4)(CH₃CN)](BPh₄) ₂, both the catalytic activity and the selectivity decrease. DFT studies performed on cyclohexane oxidation by complexes 1 and 5 demonstrate the two spin-state reactivity for the high-spin [(N5)Ni^II-O^.] intermediate (ts1_hs, ts2_doublet), which has a low-spin state located closely in energy to the high-spin state. The lower catalytic activity of complex 5 is mainly due to the formation of thermodynamically less accessible m-CPBA-coordinated precursor of [Ni^II(L5)(OOCOC ₆H₄Cl)]⁺ (5a). Adamantane is oxidized to 1-adamantanol, 2-adamantanol, and 2-adamantanone (3°/2°, 10.6-11.5), and cumene is selectively oxidized to 2-phenyl-2-propanol. The incorporation of sterically hindering pyridylmethyl and quinolylmethyl donor ligands around the Ni^II leads to a high 3°/2° bond selectivity for adamantane oxidation, which is in contrast to the lower cyclohexane oxidation activities of the complexes. Interesting, interesting! Nickel(II) complexes with a strong π-backbonding ligand act as efficient catalysts for the oxidation of alkanes (see figure, m-CPBA=m-chloroperbenzoic acid) by stabilizing the Ni-O^. intermediate, whereas those with a better σ-donor ligand act as less efficient catalysts by destabilizing the reactive intermediate. The computed mechanism for cyclohexane hydroxylation reveals that a high-spin (S=3/2) [(L1/L5)Ni^II-O^.]⁺ species is the ground state.

Full text of DOI:10.1002/chem.201402391

DOI: 10.1002/chem.201402391
Full Paper
&
Nickel Complexes
Nickel(II) Complexes of Pentadentate N5 Ligands as Catalysts for
Alkane Hydroxylation by Using m-CPBA as Oxidant: A Combined
Experimental and Computational Study**
Muniyandi Sankaralingam,[a] Mani Balamurugan,[a] Mallayan Palaniandavar,*[a, c]
Prabha Vadivelu,*[b] and Cherumuttathu H. Suresh[b]
Chem. Eur. J. 2014, 20, 1 – 17
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Abstract: A new family of nickel(II) complexes of the type
[Ni(L)(CH3CN)](BPh4)2, where L=N-methyl-N,N’,N’-tris(pyrid-2-
ylmethyl)-ethylenediamine (L1, 1), N-benzyl-N,N’,N’-tris(pyrid-
2-yl-methyl)-ethylenediamine (L2, 2), N-methyl-N,N’-bis(pyrid-
2-ylmethyl)-N’-(6-methyl-pyrid-2-yl-methyl)-ethylenediamine
(L3, 3), N-methyl-N,N’-bis(pyrid-2-ylmethyl)-N’-(quinolin-2-yl-
methyl)-ethylenediamine (L4, 4), and N-methyl-N,N’-bis(pyr-
id-2-ylmethyl)-N’-imidazole-2-ylmethyl)-ethylenediamine (L5,
5), has been isolated and characterized by means of elemen-
tal analysis, mass spectrometry, UV/Vis spectroscopy, and
electrochemistry. The single-crystal X-ray structure of [Ni(L3)-
(CH3CN)](BPh4)2 reveals that the nickel(II) center is located in
a distorted octahedral coordination geometry constituted by
all the five nitrogen atoms of the pentadentate ligand and
an acetonitrile molecule. In a dichloromethane/acetonitrile
solvent mixture, all the complexes show ligand field bands
in the visible region characteristic of an octahedral coordina-
tion geometry. They exhibit a one-electron oxidation corre-
sponding to the NiII/NiIII redox couple the potential of which
depends upon the ligand donor functionalities. The new
complexes catalyze the oxidation of cyclohexane in the pres-
ence of m-CPBA as oxidant up to a turnover number of 530
with good alcohol selectivity (A/K, 7.1–10.6, A=alcohol, K=
ketone). Upon replacing the pyridylmethyl arm in [Ni(L1)-
(CH3CN)](BPh4)2 by the strongly s-bonding but weakly p-
bonding imidazolylmethyl arm as in [Ni(L5)(CH3CN)](BPh4)2 or
the sterically demanding 6-methylpyridylmethyl ([Ni(L3)-
(CH3CN)](BPh4)2 and the quinolylmethyl arms ([Ni(L4)-
(CH3CN)](BPh4)2, both the catalytic activity and the selectivity
decrease. DFT studies performed on cyclohexane oxidation
by complexes 1 and 5 demonstrate the two spin-state reac-
II
C
tivity for the high-spin [(N5)Ni ꢀO ] intermediate (ts1hs,
ts2doublet), which has a low-spin state located closely in
energy to the high-spin state. The lower catalytic activity of
complex 5 is mainly due to the formation of thermodynami-
cally less accessible m-CPBA-coordinated precursor of
[NiII(L5)(OOCOC6H4Cl)]+ (5a). Adamantane is oxidized to 1-
adamantanol, 2-adamantanol, and 2-adamantanone (38/28,
10.6–11.5), and cumene is selectively oxidized to 2-phenyl-2-
propanol. The incorporation of sterically hindering pyridyl-
methyl and quinolylmethyl donor ligands around the NiII
leads to a high 38/28 bond selectivity for adamantane oxida-
tion, which is in contrast to the lower cyclohexane oxidation
activities of the complexes.
from industrial processes to biological systems.[1–6] Though iron
complexes are considered to be one of the most promising
catalysts, a variety of compounds of transition metals like Mn,
Co, Cu, Ru, and Os is being investigated as catalysts for the
alkane oxidation.[7–18] The very recently developed nickel dioxy-
Introduction
The selective oxidation of saturated hydrocarbons under mild
conditions is an exciting and challenging scientific goal and
has received great attention in recent years. The conventional
hydroxylation processes usually require high temperatures and
high pressures and will give only a mixture of alcohol and al-
dehyde/ketone as the over-oxidized product. On the other
hand, oxo-iron units in many mono- and dioxygenase enzymes
catalyze these reactions very efficiently with high selectivity
under mild reaction conditions; hence the enzymatic reactions
follow methodologies different from those of conventional
processes. The mostly used considerable metal-based oxidants
are transition-metal oxo species, which are often suggested as
reactive intermediates in various oxidation reactions ranging
gen complexes, such as [NiIII (m-O)2], which exhibit hydrogen
2
abstraction from aliphatic carbon atoms in a manner similar to
the high-valent [FeIV2(m-O)2] and [CuIII (m-O)2] species, have
2
thrown light on the applicability of nickel complexes as oxidiz-
ing agents.[19–23] Also, several oxo-bridged dinuclear NiII com-
plexes have been reported to be involved in dioxygen activa-
tion chemistry.[24–34]
In addition, theoretical investigations on alkane hydroxyl-
ation catalyzed by the first-row transition-metal oxide ions
have revealed the alkane hydroxylation potential of metal–
oxygen species.[35] Itoh et al. have shown[36] that the NiII com-
plex [Ni(TPA)(OAc)(H2O)](BPh4), where TPA is tris(pyrid-2ylme-
thyl)amine and OAc is acetate, is a very efficient, alcohol-selec-
tive, and robust turnover catalyst for alkane hydroxylation with
m-chloroperbenzoic acid (m-CPBA) as oxidant. They have iso-
lated a series of NiII complexes of linear N3 and tripodal N4
and tripodal monophenolate (N3O) ligands and studied the
effect of the ligand and the counter anion on the catalytic oxi-
dation of cyclohexane by using m-CPBA. They have suggested
that the highly reactive nickel-oxo (Ni=O+) intermediate rather
than an auto-oxidation-type free radical species is involved in
the catalytic cycle.[37] Also, they have reported NiII complexes of
tripodal bis- and tris(phenolate) ligands, which are capable of
catalyzing the oxidation of cyclohexane to cyclohexanol by
using m-CPBA with up to 100% conversion based on the oxi-
dant under solvent-free conditions.[38] Hikichi et al. have crystal-
[a] Dr. M. Sankaralingam, Dr. M. Balamurugan, Prof. Dr. M. Palaniandavar
School of Chemistry, Bharathidasan University
Tiruchirappalli—620024, Tamil Nadu (India)
E-mail: palanim51@yahoo.com, palaniandavarm@gmail.com
[b] Dr. P. Vadivelu, Dr. C. H. Suresh
Chemical Sciences and Technology Division
CSIR-National Institute for Interdisciplinary Science
and Technology
Trivandrum—695019. (India)
E-mail: prabha135@gmail.com
[c] Prof. Dr. M. Palaniandavar
Department of Chemistry
Central University of Tamil Nadu
Thiruvarur—610001, Tamil Nadu, (India)
[**] m-CPBA=m-chloroperbenzoic acid.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402391.
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lized the nickel(II) alkylperoxo complex [NiII(Tpipr)(OOtBu)],
where Tpipr is hydrotris(3,5-di-2-propylpyrazolyl)borate and tBu
is tert-butyl, and studied its catalytic oxidation activity towards
alkanes.[39a] The same group has found[39b,c] that nickel(II) acyl-
peroxo species are relevant to the catalytic alkane hydroxyl-
ation with m-CPBA. Very recently, we have isolated[40a] a few
nickel(II) complexes of tripodal N4 ligands and also a few
mixed ligand NiII complexes[40b] as catalysts for alkane hydroxyl-
ation reaction by using m-CPBA as the oxidant and studied the
effect of stereoelectronic factors of the ligand of the catalysts
upon the catalytic efficiency as well as the alcohol product se-
of alkanes like cyclohexane and adamantane efficiently with
good alcohol/ketone selectivity, in the presence of m-CPBA as
oxidant within two hours. Also, all the complexes catalyze the
selective oxidation of cumene to 2-phenyl-2-propanol. To illus-
trate the mechanism of the hydroxylation reactions and to
throw light on the reactive nickel-oxo intermediates a DFT
study has been undertaken for the first time, as the present oc-
tahedral complexes with N5 ligand are convenient for studying
intermediate Ni-oxo species.
lectivity. Interestingly, we obtained a linear correlation between Results and Discussion
the metal–ligand covalency parameter (b) and the total turn-
Synthesis and characterization of ligands and their nickel(II)
complexes
over number (TON). K. Ray et al.[41] trapped NiIII=O/NiIIIꢀOH in-
termediate species in the reaction of a nickel(II) salt with m-
CPBA and characterized them by using EPR and UV/Vis spectral
techniques. However, the factors determining the formation
and stabilization of the reactive intermediate species remain
still unclear.
The new pentadentate N5 ligands were synthesized according
to known procedures involving Schiff base condensation and
reductive amination with suitable modifications.[42,43] The li-
gands L1 and L2 were synthesized by reductive amination of
N-methylethylenediamine (L1)/N-benzylethylenediamine (L2)
with three equivalents of pyridine-2-carboxaldehyde by using
sodium triacetoxyborohydride. The ligands L3–L5 were synthe-
sized by Schiff base condensation of N-methylethylenediamine
with 6-methylpyridine-2-carboxaldehyde (L3)/quinoline-2-car-
boxaldehyde (L4)/1-methylimidazole-2-carboxaldehyde (L5) fol-
lowed by reduction with NaBH4 and then reductive amination
of two equivalents of pyridine-2-carboxaldehyde by using
sodium triacetoxyborohydride. All the ligands were character-
ized by 1H NMR spectroscopy and mass spectrometry. The nick-
el(II) complexes 1–5 were prepared by adding one equivalent
of ligand to a solution of [Ni(ClO4)2]·6H2O in methanol, fol-
lowed by the addition of sodium tetraphenylborate. They are
formulated as [Ni(L)(CH3CN)](BPh4)2 on the basis of elemental
analysis, UV/Vis spectroscopy, ESI-MS and X-ray crystal struc-
ture analysis of complex 3.
All the above-described observations reveal that nickel-oxo
species are potential transition-metal catalysts for an efficient
alkane hydroxylation reaction. So, we are prompted to contin-
ue to isolate nickel(II) complexes of N5 ligands (Scheme 1),
probe the effect of stereoelectronic factors of the ligand on
Description of the structure of [Ni(L3)(CH3CN)](BPh4)2 (3)
Scheme 1. Structures of the pentadentate N5 ligands employed in the study.
The molecular structure of [Ni(L3)(CH3CN)](BPh4)2 (3) is shown
in Figure 1 together with the atom numbering scheme and
the selected bond lengths and bond angles are collected in
Table 1. The complex molecule contains a NiN6 coordination
sphere with a distorted octahedral coordination geometry con-
stituted by three pyridine and two tertiary amine nitrogen
atoms of the linear pentadentate ligand and an acetonitrile
molecule. The NiꢀNpy (2.0451–2.1692 ꢁ) and NiꢀNamine (1.926-
2.196 ꢁ) bond lengths are in the ranges observed for previous-
the formation and stability of the oxo-nickel species, and study
their catalytic efficiency and alcohol selectivity in alkane hy-
droxylation. Here we report a few nickel(II) complexes of sys-
tematically varied pentadentate N5 ligands the pyridyl (py), the
imidazolyl (im), and the sterically hindering 6-methylpyridyl/
quinolyl moieties of which are expected to influence the elec-
tronic properties and the catalytic activities of the NiII com-
plexes towards alkane hydroxylation. We intend to examine
the effect of increasing the denticity from four as in the above-
mentioned NiII complexes of N4 ligands[40a] to five as in the
present complexes on the catalytic activity towards alkane hy-
droxylation. It is expected[37,40b] that an increase in denticity
from four to five and the consequent increase in the ligand
field strength would enhance the catalytic efficiency and selec-
tivity. Also, we wish to collect evidences for the involvement of
reactive intermediates in alkane hydroxylation reaction. We
have found that all the complexes catalyze the hydroxylation
ly reported nickel(II) complexes (NiꢀNpy, 2.098–2.143, NiꢀNamine
,
2.099–2.138 ꢁ).[36–38,40] The NiꢀN4py bond (2.098(2) ꢁ) is shorter
than the NiꢀNamine bonds (2.099(2), 2.138(2) ꢁ) due to sp2 and
sp3 hybridizations of the pyridyl and amine nitrogen donor
atoms respectively. However, the NiꢀN1py bond (2.141(2) ꢁ) is
longer than the NiꢀNamine bond due to the trans effect exerted
by the pyridyl N1 nitrogen atom. The NiꢀN5py bond is longer
than the NiꢀN4py and NiꢀN1py bonds (2.098(2), 2.141(2) ꢁ) due
to the sterically hindering methyl substituent adjacent to the
pyridine nitrogen atom. The NiꢀN6ACN bond is shorter than all
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provided in Figures S4, S6, and S7 in the Supporting Informa-
tion. Initially, as a bench mark calculation, the crystal structure
of complex 3 was optimized. The computed geometry is in
good agreement with the experimentally determined crystal
structure, except slight elongations in the bond lengths rang-
ing from 0.04 to 0.13 ꢁ (Figure S6 in the Supporting Informa-
tion). Hence, the same computational methodology has been
followed for optimizing the geometries of the complexes
1 and 5. As for complex 3, the complexes 1 and 5 (see the
Supporting Information for the optimized structures Figures S4
and S7) also exhibit distorted octahedral coordination
geometry.
In complex 1, the NiꢀNpy bond lengths (NiꢀN1 2.13, NiꢀN5
2.14 ꢁ) are shorter than the NiꢀNamine bond lengths (NiꢀN2
2.20, NiꢀN3 2.16 ꢁ), as expected.[36–38,40] However, the NiꢀNpy
bond (NiꢀN4 2.20 ꢁ) is equal in length to the NiꢀN2 bond but
shorter than the NiꢀN3 bond. Upon replacing one of the pyrid-
yl nitrogen atoms in complex 1 by a 6-methylpyridyl nitrogen
atom to give complex 3, the NiꢀNpy (NiꢀN5 2.21 ꢁ) bond is
weakened due to steric hindrance of the 6-methyl group but
no appreciable change in the other bond lengths is observed.
Upon replacing one of the pyridyl nitrogen atoms in complex
1 by a imidazolyl nitrogen atom to give complex 5, the NiꢀNim
(NiꢀN5 2.12 ꢁ) bond becomes stronger than the NiꢀNpy bonds
with increase in spin density of the Ni, as expected (pKa(BH+):
ImH+ 6.0, pyH+, 5.2), and no appreciable changes are ob-
served in other bonds.
Figure 1. ORTEP diagram of [Ni(L3)(CH3CN)](BPh4)2 (3) showing 50% proba-
bility thermal ellipsoids and the labeling scheme for selected atoms. All the
hydrogen atoms are omitted for clarity.
Table 1. Selected bond lengths and bond angles for complex 3.
Bond lengths [ꢁ]
Ni1ꢀN1
Ni1ꢀN2
Ni1ꢀN3
Ni1ꢀN4
Ni1ꢀN5
Ni1ꢀN6
2.141(2)
2.138(2)
2.099(2)
2.098(2)
2.143(2)
2.064(2)
Bond angles [8]
N1-Ni1-N2
N1-Ni1-N3
N1-Ni1-N4
N1-Ni1-N5
N1-Ni1-N6
N2-Ni1-N3
N2-Ni1-N4
N2-Ni1-N5
N2-Ni1-N6
N3-Ni1-N4
N3-Ni1-N5
N3-Ni1-N6
N4-Ni1-N5
N4-Ni1-N6
N5-Ni1-N6
78.11(9)
101.37(9)
173.84(9)
88.48(9)
86.98(9)
83.88(9)
96.09(9)
154.17(9)
94.12(10)
79.86(9)
77.19(9)
170.77(9)
97.68(9)
91.43(9)
107.25(10)
Electronic spectral properties
The electronic spectral data of the nickel(II) complexes are
summarized in Table 2 and the typical electronic absorption
spectrum of complex 1 is shown in Figure 2. In a dichlorome-
Table 2. UV/Vis spectral (lmax in [nm], e in [mꢀ1 cmꢀ1] in parenthesis) and
electrochemical data[a] of NiII complexes 1–5 in a dichloromethane/aceto-
nitrile solvent mixture (3:1 v/v) at 25.08C.
3
3
3
3
3
3
1
3A2g! T1g(P) A2g! T1g(F) A2g! T2g(F) A2g! E1g(D) B’
b[b]
(n3) calcd[a]
(n2)
(n1)
[%]
1
2
3
4
5
335
340
339
342
341
525 (16)
533 (15)
530 (15)
534 (18)
532 (22)
807 (16)
817 (16)
808 (11)
814 (11)
800 (19)
877 (20)
921 (20)
890 (13)
920 (16)
882 (19)
781 28
762 30
743 31
742 31
703 35
other NiꢀN bonds on account of sp hybridization of the aceto-
nitrile nitrogen atom (see above). Among the two tertiary
amine nitrogen donor atoms the N3amine nitrogen atom is coor-
dinated more strongly than the N2amine nitrogen atom, as ex-
pected. The N-Ni-N (77.19(9)–107.25(10)8) and N-Ni-N
(154.17(9)–173.84(9)8) bond angles deviate from the ideal octa-
hedral angles of 90 and 1808, respectively, revealing the pres-
ence of significant distortion in the NiII coordination geometry.
[a,b] Calculated by solving the quadratic equation and by using B as
1080 cmꢀ1
.
thane/acetonitrile (3:1 v/v) solvent mixture, all the complexes
exhibit absorption bands in the range l=520–535 nm and
also a broad split band (l=877–921, 800–817 nm). The highest
Structures of nickel(II) complexes: density functional theory
(DFT) calculations
3
3
energy band is assigned to a A2g! T1g(F) (n2) transition,
whereas the band in the range l=800–817 nm is assigned to
3
3
A DFT study (by using the Gaussian 09 program[56]) has been
performed to investigate the geometrical parameters of com-
plexes 1, 3, and 5 and the optimized geometries of them are
a A2g! T2g(F) (n1) transition in a nickel(II) ion located in an oc-
tahedral environment.[40,44] The lowest energy band with an in-
tensity almost equal to that of the n1 band is assigned to the
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Figure 2. Electronic absorption spectrum of [Ni(L3)(CH3CN)](BPh4)2
(1.0ꢂ10ꢀ2 m) in a dichloromethane/acetonitrile solvent mixture (3:1 v/v) at
25.08C.
Figure 3. Cyclic voltammogram and differential pulse voltammogram of
1 mm complex [Ni(L3)(ACN)]2+ in a dichloromethane/acetonitrile mixture at
25.0 8C. Supporting electrolyte: 0.1m tetrabutylammonium perchlorate
(TBAP), scan rate: 50 and 5 mVsꢀ1 for CV for DPV, respectively.
1
3A2g! E1g(D) spin-forbidden transition. The n1 band for the
present [Ni(N5)(CH3CN)]2+ complexes is observed at energies
higher than those for the [Ni(N4)(CH3CN)2]2+ complexes[40a] of
related tetradentate ligands, which is expected due to the
higher ligand field strength of N5 ligands with higher denticity.
Similarly, the incorporation of a pyridylmethyl arm on the ter-
minal tertiary amine nitrogen atom in [Ni(iso-BPMEN)(CH3CN)2]-
(BPh4)2,[40a] where iso-BPMEN is N,N-dimethyl-N’,N’-bis(pyrid-2-
ylmethyl)ethane-1,2-diamine, to obtain complex 1 the ligand
field band is shifted from lower energy (l=842 nm) to higher
energy (l=807 nm). The n1 band position depends on the
donor atom type; thus, when the pyridylmethyl arm in com-
plex 1 is replaced an imidazolylmethyl arm to obtain complex
5, the n1 band is shifted to slightly higher energy because of
the strongly s-bonding imidazole donor atom (see above). On
the other hand, the energy of the n1 band decreases when the
pyridylmethyl arm in complex 1 is replaced by a 6-methylpyr-
idyl arm to get complex 3, and by a quinolylmethyl arm to get
complex 4, which is expected due to the weaker coordination
of the sterically hindering 6-methylpyridyl (see above) and the
quinolyl nitrogen donor atoms. Also, upon replacing the N-
methyl group in complex 1 by a benzyl group to give complex
2 the band is shifted to lower energy (l=817 nm) on account
of the steric effect of the benzyl group. The observed n1 and n2
band positions were fitted into the quadratic equation[44] relat-
ing the energy of the n1 and n3 bands and the metal–ligand
covalency parameter (b) calculated. The b values for the
[Ni(N5)(CH3CN)]2+ complexes are higher, and the B’ values cal-
culated (n˜ =703–781 cmꢀ1) are lower, than those (n˜ =773–
1038 cmꢀ1) for the [Ni(N4)(CH3CN)2]2+ complexes,[40] which is
expected.
Table 3. Electrochemical data[a] of nickel(II) complexes in dichlorome-
thane/acetonitrile solution at 25.08C.
Complex
Ep,c (CV) [V]
E1/2 (DPV) [V]
[Ni(L1)(CH3CN)]2+
[Ni(L2)(CH3CN)]2+
[Ni(L3)(CH3CN)]2+
[Ni(L4)(CH3CN)]2+
[Ni(L5)(CH3CN)]2+
1.622
1.628
1.646
1.690
1.640
1.548
1.554
1.564
1.584
1.592
[a] Potential measured versus Ag/AgNO3 (0.001m, 0.1m TBAP) in dichloro-
methane/acetonitrile solution; add 0.544 V to convert to standard hydro-
gen electrode (NHE).
the imidazolyl nitrogen donor to give complex 5, the redox
potential (E1/2 =1.592 V) is shifted (44 mV) to a more positive
value revealing that the strongly s-bonding imidazolyl nitro-
gen atom (see above) renders the oxidation of NiII to NiIII facile.
Upon replacing the pyridylmethyl arm in complex 1 by the
sterically hindering 6-methylpyridyl arm to give complex 3 or
by the quinolylmethyl arm to give complex 4, a positive shift
in the redox potential (E1/2 =20 and 36 mV for complexes 3
and 4, respectively) is observed suggesting that the oxidation
of NiII is rendered facile. The sterically hindered 6-methylpyridyl
(see above) and quinolylmethyl nitrogen donors are expected
to increase the positive charge on the NiII center making the
NiII to NiIII oxidation difficult. However, the redox potential is
shifted to more positive values revealing that the increase in
the positive charge on nickel(II) is compensated by stronger
coordination of the other nitrogen donors making the oxida-
tion of NiII to NiIII facile. A similar positive shift (6 mV) in E1/2 is
observed upon replacing the N-methyl group in complex 1 by
the sterically hindering N-benzyl group to give complex 2.
Thus, changes in the NiII/NiIII redox potential are observed
upon varying the ligand donor functionalities.
Electrochemical properties
The electrochemical properties of the nickel(II) complexes were
investigated in a dichloromethane/acetonitrile solvent mixture
by employing cyclic (CV) and differential pulse voltammetry
(DPV) on a stationary platinum electrode. All of the complexes
show an anodic oxidation wave (1.622–1.690 V) but not any
coupled reduction wave (Figure 3) corresponding to the NiII/
NiIII redox couple. The E1/2 values of the redox couple (Table 3)
fall in the range 1.548–1.592 V observed for a similar type of
nickel(II) complexes[45] and follows the trend 1<2<3<4<5.
Upon replacing one of the pyridylmethyl arms in complex 1 by
Functionalization of alkanes
The catalytic ability of the complexes 1–5 towards oxidation of
alkanes like cyclohexane, adamantane, and cumene was inves-
tigated by using m-CPBA, H2O2, and tBuOOH as oxidants in a di-
Chem. Eur. J. 2014, 20, 1 – 17
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as the major product (TON=272–487) and cyclohexanone (K;
TON=6–12), and e-caprolactone (TON=18–62) as the minor
products the alcohol/ketone selectivity (A/K=7.1–10.6) being
appreciably good. The product e-caprolactone is obtained by
the over oxidation of cyclohexanone in the presence of an
excess or unreacted m-CPBA. As m-CPBA is a reagent for the
quantitative conversion of cyclic ketones to the corresponding
lactones (Bayer–Villiger oxidation) in the absence of a metal
catalyst, obviously e-caprolactone is obtained not by metal-cat-
alyzed oxidation. Also, the complexes catalyze the selective ox-
idation of adamantane to give a total TON of 600 with good
to high 38/28 regioselectivity of 11.5. Further, they selectively
oxidize cumene to 2-phenyl-2-propanol with a total TON of
475.
Table 4. Products of the oxidation of cyclohexane catalyzed[a] by the dif-
ferent NiII complexes.
TON
Cyclohex- Cyclohex- e-capro- TON[b] benzene
Total Chloro- A/K[c] Yield[d]
[%]
anol
anone
lactone
blank
3.0
1.0
–
–
8
6
–
46
62
52
35
18
4
533
515
484
445
302
1
2
3
4
5
487
453
424
404
272
455
415
361
380
234
10.6 53.3
7.3 51.5
7.1 48.4
9.9 44.5
9.0 30.2
12
[a] Reaction conditions: catalyst (0.35ꢂ10ꢀ3 mmoldmꢀ3), substrate
(2.45 moldmꢀ3), oxidant (0.35 moldmꢀ3) in a dichloromethane/acetonitrile
solvent mixture (3:1 v/v). [b] Total TON=number of mmol of product/
number of mmol of catalyst. The TON is the average of three determina-
tions. [c] A/K=TON of cyclohexanol/(TON of cyclohexanone+TON of e-
caprolactone). [d] Yield based on the oxidant.
Complex 1 catalyzes the oxidation of cyclohexane to give
a total TON of 533 (TON(A)=487, TON(K)=12, A/K=10.6) cor-
responding to 53% conversion of the oxidant to organic prod-
ucts. The total TON observed for complex 1 with an N5 ligand
is lower than those for [Ni(TPA)(OAc)(OH2)]BPh4,[36] and [Ni(iso-
BPMEN)(CH3CN)2](BPh4)2,[40a] where TPA and iso-BPMEN are tet-
radentate N4 ligands. There is only one easily replaceable mon-
odentate ligand in complex 1, whereas there are two such li-
gands in the complexes of the N4 ligands, leading to the en-
hancement in the catalytic activity. So, it is proposed that m-
CPBA binds to the nickel(II) center in [Ni(L1)(CH3CN)]2+ by re-
placing the labile acetonitrile molecule to give the intermedi-
ate adduct species [NiII(L1)(OOCOC6H4Cl)]+ (Scheme 2), which
undergoes either 1) homolysis leading to the formation of
chloromethane/acetonitrile solvent mixture (3:1 v/v) at room
temperature under an atmosphere of nitrogen and the results
are summarized in Tables 4–6. When the catalytic activity of
the NiII complexes was studied under air, the total TON for the
cyclohexane oxidation is lower than that under a nitrogen at-
II
C
mosphere. It is probable that the [(L)Ni ꢀO ] radical intermedi-
ate generated during the catalytic cycle (see below) is
quenched by dioxygen/moisture in air to give hydroxonickel
species after some turnovers thereby decreasing the yield. The
conversion of alkanes into hydroxylated products was quanti-
fied based on gas chromatographic analysis by using authentic
samples and an internal standard. When H2O2/tBuOOH was
used as the oxidant only trace amounts of the oxidized prod-
ucts were discerned (results are not shown here) revealing that
the nickel(II) complexes are not effective as catalysts for these
oxidants. It has been reported[46] that m-CPBA is a strong oxi-
dizing agent for the oxidation of
II
+
III
+
C
[(L1)Ni ꢀO ] /[(L1)Ni =O] intermediate species (see below)
and the m-chlorobenzoic acid radical or 2) heterolysis leading
to the formation of the intermediate [(L1)NiIV=O]2+ species and
chlorobenzoic acid. The observation of chlorobenzene (75–
85%) as product corresponding to the total TON supports the
II
+
C
formation of the intermediate species [(L1)Ni ꢀO ] /
[(L1)NiIII=O]+ and reveals that the m-chlorobenzoate radical is
cyclohexane and adamantane to
the corresponding alcohols and
ketones in the absence of
a metal catalyst but only under
vigorous reaction conditions like
very high concentration of m-
CPBA, a long reaction time, and
a high temperature. Thus, con-
trol reactions performed in the
absence of complexes with m-
CPBA as oxidant yielded only
very small amounts of the oxi-
dized products (TON(cyclohex-
ane)=4, TON(adamantane)=9,
TON(cumene)=5) revealing that
all the complexes act as catalysts
towards the oxidation of alkanes.
In the presence of complexes
oxidation of cyclohexane takes
place efficiently with 300–535
total TON with cyclohexanol (A) Scheme 2. Proposed mechanism of the alkane hydroxylation.
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not the reactive intermediate as it readily undergoes decarbox-
ylation rather than hydrogen abstraction from the alkane. This
is consistent with the formation of remaining (15–25%) alkane
hydroxylated products, which reveals that the gate for the for-
methyl and the quinolylmethyl arms to be involved in p-back-
bonding. All the above-described observations reveal that a p-
accepting ligand donor like a pyridine/quinoline nitrogen atom
stabilizes the high-valent nickel-oxo intermediate species lead-
ing to an enhanced catalytic activity towards oxidation of alka-
nes, whereas a strongly s-bonding but weakly p-backbonding
ligand donor like imidazole destabilizes it, leading to a de-
creased catalytic activity. Also, interestingly, the total TON for
the cyclohexane oxidation reaction correlates linearly with the
metal–ligand covalency parameter b (Figure 4) and also the
NiII/NiIII redox couple (Figure 5). A similar linear correlation be-
tween the total TON and b has been observed[40a] by us for
mation
of
another
intermediate
species,
possibly
[(N5)NiIV=O]2+, is unlocked. DFT studies (see below) also sup-
II
+
C
port the formation of the intermediate [(L1)Ni ꢀO ] species.
Itoh et al. have proposed[37] the formation of adducts with m-
CPBA, similar to [NiII(L1)(OOCOC6H4Cl)]+, and reactive inter-
mediates, to illustrate the catalytic activity of complexes of the
type [Ni(L)(X)(X’)], where L is a tetradentate ligand and X as
well as X’ are OAcꢀ, NO3ꢀ, m-chlorobenzoic acid (m-CBA), or
phenolate. We have also successfully illustrated the trends in
the catalytic activity of nickel(II) complexes of a series of N4 li-
gands[40a] towards alkane hydroxylation by invoking the forma-
tion of such an intermediate. The proposal of
[NiII(L1)(OOCOC6H4Cl)]+ as the intermediate adduct species is
supported by the successful isolation and X-ray structure de-
termination of the nickel(II) alkylperoxo complex [NiII(Tpipr)-
(OOtBu)], which has been employed[39a] by Hikichi et al. for the
oxidation of substituted alcohols to benzaldehydes. Also, Ray
et al.[41] have used EPR and ESI-MS techniques to characterize
the intermediate species [(TMG3tren)NiꢀO(H)]n+
,
where
TMG3tren is tris(2-(N-tetramethylguanidyl)ethyl)amine and n=
1 for oxo, n=2 for hydroxo, and proposed the involvement of
[(TMG3tren)NiIII=O(H)] intermediates in alkane hydroxylation re-
actions. Further, the formation of [NiIV(O)(TMG3tren)(OTf)]+
(OTf=triflate) in the reaction of [Ni(TMG3tren)(OTf)]OTf with m-
CPBA[41] has been detected by using ESI-MS.
Figure 4. Correlation between the oxidation of cyclohexane and the
b values at 25.08C. R2 =0.93.
Upon replacing one of the pyridylmethyl arms in complex
1 by an imidazolylmethyl arm to give complex 5, cyclohexane
is catalytically oxidized with a total TON of 302 and a decrease
in the alcohol selectivity (A/K=9.0). The strongly s-bonding
imidazolyl nitrogen donor favors the formation of the reactive
intermediate with a higher oxidation state (see above) so as to
act as efficient catalysts for alkane hydroxylation. However, the
lower p-bonding ability of the imidazolyl nitrogen atom does
not facilitate stabilization of the reactive intermediate render-
ing complex 5 as a less efficient catalyst. Upon replacing the
methyl group in complex 1 by a benzyl group to give complex
2, both the total TON (515) and the A/K ratio (7.3) decrease.
The introduction of the bulky N-benzyl moiety renders the co-
ordination of the tertiary amine nitrogen atom to the NiII
center weaker, which is revealed by the slightly higher NiII/NiIII
redox potential of complex 2 (see above) leading to stronger
coordination of the other nitrogen atoms and a decrease in
the stabilization of high-valent nickel-oxo species, and hence
the lower catalytic activity of complex 2. Molecular model
building studies of complex 2 reveal that the unfavorable
steric interactions occurring between the N-benzyl group and
the pyridine ring renders the binding of m-CPBA difficult, con-
tributing to the lower yield. This is supported by the observa-
tion that complex 3 (total TON=484, A/K=7.1) and complex 4
(total TON=445, A/K=9.9) act as less efficient catalysts to-
wards cyclohexane oxidation; the sterically hindering 6-methyl
group in complex 3 and the bulky quinolyl group in complex
4 decrease the tendency of the nitrogen atom of the pyridyl-
Figure 5. Correlation between the total TON for the cyclohexane oxidation
and the NiII/NiIII redox potential. R2 =0.82.
Ni(N4) complexes; an increase in the TON with an increase in
b means that the total TON increases with an increase of the
ability of the N4 ligands to get involved in p-backbonding and
hence stabilize the reactive intermediate. However, for the
present complexes the total TON decreases with an increase in
the value of b and thus complex 5 with the imidazolyl nitrogen
donor shows the highest value for b but the lowest TON. The
catalytic ability of Ni(N5) complexes show lower catalytic ability
than Ni(N4) complexes as the former complexes make avail-
able only a single coordination site for binding with the oxi-
dant, whereas the latter complexes offer two binding sites.
Also, as only a single coordination site is made available for
the oxo group in the NiII complexes of N5 ligands, the p-back-
bonding of the oxo group with nickel is not facilitated.
Chem. Eur. J. 2014, 20, 1 – 17
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Table 5. Products of the oxidation of adamantane catalyzed[a] by the dif-
ferent NiII complexes.
Table 6. Oxidation product of the oxidation of cumene catalyzed[a] by
the different NiII complexes.
TON
1-Adamant 2-Adamant 2-Adamant TON[b]
anol anol anone
Total
38/28[c] Yield[d]
Complex
2-phenyl-2-propanol
Total TON[b]
Yield[c] [%]
[%]
blank
5
475
460
418
441
327
5
475
460
418
441
327
0.5
47.5
46.0
41.8
44.1
32.7
1
2
3
4
5
blank
6.2 2.4 0.4
9.0
595
575
527
578
512
1
2
3
4
5
464
451
413
458
402
123
114
101
109
102
8
10.6
10.9
10.9
11.5
11.0
59.5
57.5
52.7
57.8
51.2
10
13
11
8
[a] Reaction conditions: catalyst (0.35ꢂ10ꢀ3 mmoldmꢀ3), substrate
(2.45 moldmꢀ3), oxidant (0.35 moldmꢀ3) in a dichloromethane/acetonitrile
solvent mixture (3:1 v/v). [b] Total TON=number of mmol of product/
number of mmol of catalyst. The TON is the average of three determina-
tions. [c] Yield based on the oxidant.
[a] Reaction conditions: catalyst (0.2ꢂ10ꢀ3 mmoldmꢀ3), substrate
(0.4 moldmꢀ3), oxidant (0.2 moldmꢀ3) in a dichloromethane/acetonitrile
solvent mixture (3:1 v/v). [b] TON=number of mmol of product/number
of mmol of catalyst. The TON is the average of three determinations.
[c] 38/28=(TON of 1-adamantanolꢂ3)/(TON of 2-adamantanol+TON of 2-
adamantanone). [d] Yield based on the oxidant.
II
+
C
Proposed mechanism for [(L1/L5)Ni ꢀO ] -catalyzed
cyclohexane oxidation
As shown in Scheme 2, the NiII-catalyzed alkane oxidation in-
volves: 1) m-CPBA binding to NiII by replacing a CH3CN mole-
cule, 2) homolytic or heterolytic OꢀO bond cleavage of m-
Adamantane and cumene oxidation
The catalytic activity of complexes 1–5 towards the oxidation
of adamantane has been also explored and the results are
summarized in Table 5. Adamantane is efficiently oxidized to
give 1-adamantanol and 2-adamantanol as the major products
along with a small amount of 2-adamantanone as the minor
product. Complex 1 catalyzes the oxidation of adamantane to
give a total TON 595 with a high 38/28 bond selectivity (10.6).
The replacement of the methyl substituent in complex 1 by
the N-benzyl substituent to give complex 2 leads to a decrease
in the total TON (575) but with a slight increase in the 38/28
bond selectivity (10.9). Upon replacing the pyridylmethyl arm
in complex 1 by the 6-methylpyridylmethyl arm as in complex
3 (TON=527, 38/28=10.9) or the quinolylmethyl arm as in
complex 4 (TON=578, 38/28=11.5) or the imidazolylmethyl
arm as in complex 5 (TON=512, 38/28=11.0) the total TON de-
creases with the 38/28 bond selectivity remaining almost the
same. This trend in the catalytic activity is similar to that ob-
served for the cyclohexane oxidation revealing the involve-
ment of the same type of high-valent nickel-oxo intermediate
species in the adamantane oxidation. A good p-backbonding
pyridine donor facilitates the formation of the high-valent
nickel-oxo species, whereas a strongly s-donating imidazole ni-
trogen atom destabilizes it but slightly increases the 38/28
bond selectivity. However, the total TON does not correlate
well with the E1/2 or b values (Figures S1 and S2 in the Support-
ing Information). The catalytic activity of the NiII complexes to-
wards the oxidation of cumene has also been investigated and
the results are summarized in Table 6. Interestingly, all the
complexes catalyze the oxidation of cumene selectively to
form 2-phenyl-2-propanol without any side product, and
a linear correlation between the TON and the b values has
been observed (Figure S3 in the Supporting Information), illus-
trating the importance of the p-backbonding of the heterocy-
clic nitrogen donor atom.
II
IV
C
CPBA to generate the Ni ꢀO or Ni =O intermediate, and 3) oxi-
dation of the alkane by the intermediate. For the computation-
al investigation of the mechanism of catalysis, NiII complexes
of L1 and L5 were considered as they show a large difference
in the yield of the product (Table 4). It should be noted that so
far no detailed study on the mechanism of a nickel(II)-mediat-
ed alkane oxidation has been attempted and many DFT meth-
ods have been used in the present study to locate the ground-
state configuration of the reactive intermediate. Previously,
Comba et al. investigated the FeIV(O)- or FeV(O)-catalyzed cyclo-
hexane oxidation by using similar DFT methods.[47] The ener-
IV
II
C
getics computed for Ni =O and Ni ꢀO species in their different
spin levels (B3LYP/B2 method) show that the NiIV=O intermedi-
ate is significantly destabilized by more than 900 kJmolꢀ1 com-
II
C
pared to the corresponding Ni ꢀO species (see Table S1 in the
Supporting Information). These results strongly support the
homolytic OꢀO bond cleavage pathway and involvement of
III
II
C
Ni =O/Ni ꢀO species in the reaction proposed above
(Scheme 2), ruling out the formation of the NiIV=O intermedi-
ate. Also, all attempts to optimize [(L1)NiIII=O]+ and [(L5)NiIII=
O]+ in the high-spin state (S=3/2) only led to the formation of
II
C
the high-spin Ni ꢀO species 1bhs (1(Ni) =1.56, 1(O1) =1.16) and
5bhs (1(Ni) =1.57, 1(O1) =1.16), respectively (Figures 6 and 7). The
1(Ni) and 1(O1) values of 1bhs and 5bhs indicate the presence of
two unpaired electrons on the Ni and one unpaired electron
on the O1, which are ferromagnetically coupled. The comput-
ed singly occupied molecular orbitals (SOMOs) of 1bhs and
5bhs (Figure 8) indicate that one of the two unpaired electrons
of the Ni resides in a non-bonded dx2ꢀy2 orbital, whereas the
other one resides in a s*dz2–pz orbital (with respect to the Ni–
O1 interaction), and the unpaired electron on the O1 is located
in a p*dyz–py orbital. The low-spin (S=1/2) intermediate [(L1/
II
+
C
L5)Ni ꢀO ] species 1bls (1(Ni) =1.56, 1(O1) =ꢀ0.83) and 5bls
(1(Ni) =1.58, 1(O1) =ꢀ0.82) have also been modeled and they are
found to be less stable than 1bhs and 5bhs by +25.1 and
+16.2 kJmolꢀ1, respectively (Figures 6 and 7). In addition, the
&
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Figure 8. Singly occupied molecular orbitals for 1bhs and 5bhs. Molecular or-
bitals are named based on the Ni–O1 interactions. The x, y, and z axes point
along the NiꢀN5, NiꢀN4, and NiꢀN3 directions, respectively (atom number-
ing is similar to the optimized geometries in Figures 6 and 7).
II
+
C
Figure 6. Optimized geometries of [(L1)Ni ꢀO ] (1b) in their high-spin
(S=3/2) and low-spin (S=1/2) states along with the low-spin [(L1)NiIII=O]+
species. Energies in [kJmolꢀ1], bond lengths in [ꢁ], and angles in [8].
II
C
methods. It involves initial hydrogen abstraction by the Ni ꢀO
species (ts1) leading to the formation of NiIIꢀOH and the cyclo-
hexyl radical (radint), which reacts over through the CꢀO re-
bound step (ts2) to form the cyclohexanol product coordinat-
ed to the NiI species.
Relative stabilities of high-spin and low-spin intermediate
II
+
C
[(L1/L5)Ni -O ] species
Initially, the reliability of the B3LYP/B1 method was tested by
optimizing the coordination geometry of [Ni(L3)(CH3CN)]2+ (3)
(Figure S4 in the Supporting Information), which reproduces
the X-ray crystal structure well, except slight elongations in the
NiꢀN bond lengths (0.05 to 0.10 ꢁ). The spin density on the Ni
center (1(Ni)) is 1.58 with the remaining spins delocalized on the
nitrogen atom, which confirms the presence of two unpaired
electrons on the NiII in complex 3. As the spin densities (1(Ni)
)
of 1bhs and 5bhs are almost equal to that of NiII in complex 3,
II
+
C
Figure 7. Optimized geometries of [(L5)Ni ꢀO ] (5b) in their high-spin
(S=3/2) and low-spin (S=1/2) states along with the low-spin [(L5)NiIII=O]+
species. Energies in [kJmolꢀ1], bond lengths in [ꢁ], and angles in [8].
it is clear that both 1bhs and 5bhs are high-spin [(L1/L5)NiIIꢀ
+
C
O ] species. As mentioned above, the high-spin states 1bhs
and 5bhs are more stable than the corresponding low-spin spe-
cies. In order to verify that this energy preference of the high-
spin state is not an artifact of the level of theory used, a bench-
mark study was conducted by using six other DFT methods,
namely, B3LYP-D, wB97XD, B97D, M06-2X, B3LYP*, and B3LYP**
along with single-point calculations by using the TPSSh and
OLYP methods (Table 7).[48,59] All the methods consistently
show the high-spin state as the ground states for both 1b and
5b. In the case of 1b, the low-spin states 1bls and 1b0ls are de-
stabilized by +15.0 and +75.0 kJmolꢀ1, respectively, relative to
low-spin (S=1/2) intermediate [(L1/L5)NiIII=O]+ species 1bl0s
(1(Ni) =0.16, 1(O1) =0.76), and 5b0ls (1(Ni) =0.08, 1(O1) =1.11) with
one unpaired electron on the nickel have been computed and
they are less stable than 1bhs and 5bhs by +75.0 and
+56.5 kJmolꢀ1 respectively, and so they are discarded from
mechanistic investigation.
For the mechanistic investigation on the alkane oxidation
II
+
II
+
C
C
step, both high- and low-spin [(L1)Ni ꢀO ] and [(L5)Ni ꢀO ]
species have been considered as
they are the species most likely
formed in the homolytic path-
way (Scheme 2). We now pro-
pose a mechanism for the NiIIꢀ
C
O -catalyzed cyclohexane oxida-
tion (Scheme 3), which is similar
to the one proposed by Comba
et al.[47] for the FeIV=O/FeV=O-cat-
II
+
C
alyzed mechanism based on DFT Scheme 3. Proposed mechanism for the [(L1/L5)Ni ꢀO ] -catalyzed cyclohexane oxidation/hydroxylation.
Chem. Eur. J. 2014, 20, 1 – 17 www.chemeurj.org ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9
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II
+
Table 7. Relative free energies in [kJmolꢀ1] of [(L1/L5)Ni ꢀO ] species
with different DFT methods in their high-spin (S=3/2) and low-spin (S=
1/2) states. The columns 1b0ls and 5bl0s show the [(L1/L5)NiIII=O]+ with
low-spin (S=1/2) state.
C
II
+
II
+
Methods[a]
[(L1)Ni -O ]
[(L5)Ni -O ]
C
C
1bhs
1bls
1b’ls
5bhs
5bls
5b’ls
(S=3/2) (S=3/2) (S=3/2) (S=3/2) (S=3/2) (S=3/2)
B3LYP
+0.0
+0.0
+0.0
+0.0
+0.0
+25.8
+17.4
+15.8
+14.9
+16.3
+36.5
+18.9
+74.8
+74.6
+72.9
+53.1
+63.1
+61.6
+77.4
+0.0
+0.0
+0.0
+0.0
+0.0
+0.0
+0.0
+18.1
+16.9
+14.8
+13.6
+17.8
+34.3
+19.4
+50.9
+57.8
–
+60.2
+39.5
+38.3
+58.4
B3LYP-D
wB97XD
M06–2X
B3LYP*
B3LYP-D//OLYP +0.0
B3LYP-D//TPSSh +0.0
[b]
Figure 9. Computed free energy profile (B3LYP/B2//B3LYP/B1 level) for the
II
+
[(L1)Ni ꢀO ] (1b)-catalyzed cyclohexane oxidation. Energies in [kJmolꢀ1].
C
[a] For all methods the B1 basis sets were employed, optimization with
other DFT methods such as BP86, B3LYP** were failed to produce all
three intermediates. [b] Reproduces the 5bls spin state.
the high-spin state 1bhs. Similarly, the low-spin states 5bls and
5b0ls are +14.0 and +54.0 kJmolꢀ1, respectively, less stable
than the high-spin state 5bhs. For both 1b and 5b, the spin
density distribution at the Ni and O1 atoms are very similar to
those described at the B3LYP/B1 level. Overall, all the six meth-
II
+
C
ods show that the high-spin [(L1/L5)Ni ꢀO ] species is the
ground state. Due to the consistently lower energy gaps ob-
served for 1bhs–1bls and 5bhs–5bls, both the 1bhs and 5bhs
species are selected for mechanistic studies.
In order to further verify the reliability of the B3LYP/B1
method to compute the geometries and reactivities of the NiII
complexes, we have optimized the intermediate hydroxo [NiIII-
(TMG3tren)(OH)]2+ as well as the oxo [NiIII(TMG3tren)(O)]+ spe-
cies, which were spectroscopically characterized by Ray et al.[41]
in both the high-spin (S=3/2) and the low-spin (S=1/2) state.
We found that the low-spin state of [NiIII(TMG3tren)(OH)]2+ is
more stable than the corresponding high-spin state by
+19.8 kJmolꢀ1 (Figure S5 in the Supporting Information),
which is in agreement with the experimental findings. On the
other hand, only the high-spin state of [NiIII(TMG3tren)(O)]+ is
located as the minimum and all attempts to optimize the low-
spin state of [NiIII(TMG3tren)(O)]+ were fruitless. Although
a direct comparison of the present N5-ligand-coordinated [(L1/
Figure 10. Computed geometries of the transition states along with the key
II
+
C
spin densities for [(L1)Ni ꢀO ] (1bhs)-catalyzed cyclohexane hydroxylation.
Bond lengths in [ꢁ].
which leads to the formation of [(L1)NiII(OH)]+ and a cyclohexyl
radical (radint). This step is associated with the transition state
ts1hs (Figure 10, E=+58.6 kJmolꢀ1), with CꢀH and O1ꢀH bond
lengths of 1.24 and 1.28 ꢁ, respectively, and the NiꢀO1 bond
being elongated by 0.04 ꢁ (vs. 1bhs). The spin density of the
Ni, O1, and C centers of ts1hs (1(Ni) =1.59, 1(O1) =0.78, 1(C) =0.42)
indicate the formation of a terminal OꢀH bond and the cyclo-
hexyl radical. Subsequently, ts1hs decays into radquartet (E=
ꢀ15.1 kJmolꢀ1, 1(Ni) =1.6, 1(O1) =0.15, 1(C) =0.98), in which the
unpaired electron of the cyclohexyl carbon atom couples ferro-
magnetically with the unpaired electrons of NiII in 1bhs (see
Figure S6 in the Supporting Information for the optimized
structures). As for ts1hs, the ts1ls (E=+67.0 kJmolꢀ1, 1(Ni) =1.59,
1(O1) =ꢀ0.53, 1(C) =ꢀ0.37) has been also computed and is
found to be slightly uphill (by +8.3 kJmolꢀ1) compared to
ts1hs. It then leads to the generation of raddoublet (E=
ꢀ9.0 kJmolꢀ1, 1(Ni) =1.60, 1(O1) =0.13, 1(C) =ꢀ0.98) in which the
II
+
C
L5)Ni ꢀO ] species with the intermediate species of Ray et al.
is difficult, it is interesting to see that only a terminal NiIIIꢀOH
bond is present in the low-spin ground state of [NiIII-
(TMG3tren)(OH)]2+, whereas only a terminal NiIIIꢀO bond is
II
+
C
present in the high-spin ground state of [(L1/L5)Ni ꢀO ] .
II
+
C
Computed mechanism of the [(L1/L5)Ni ꢀO ] -catalyzed
cyclohexane hydroxylation
The computed free energy profile at the B3LYP/B2//B3LYP/B1
level of the two reaction steps, passing through the transition
states ts1 and ts2 (Scheme 3) relative to the infinitely separat-
II
+
C
ed [(L1)Ni ꢀO ] (1bhs) and cyclohexane is shown in Figure 9. In
the first step, one of the equatorial hydrogen atoms of cyclo-
hexane is abstracted by the terminal oxygen atom of 1bhs,
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cyclohexyl radical is antiferromagnetically coupled with the un-
paired electrons of the NiII. Interestingly, the computed inter-
mediates radquartet and raddoublet are very close in energy (ꢀ15.1
and ꢀ9.0 kJmolꢀ1, respectively), which was verified by optimiz-
ing the intermediates with the B3LYP*/B1 method and radquartet
is more stable than raddoublet by only +3.9 kJmolꢀ1
.
In the next step, the radical carbon atom approaches the
NiIIꢀOH species, which is modeled by varying the CꢀO bond
length. The transition states for this step are found at quartet
and doublet surfaces as ts2quartet (E= +89.1 kJmolꢀ1, 1(Ni) =1.65,
1(O1) =0.22, 1(C) =0.44) and ts2doublet (E= +37.7 kJmolꢀ1, 1(Ni)
=
1.36, 1(O1) =0.12, 1(C) =ꢀ0.65), respectively. The transition state
ts2quartet is associated with an activation energy of
+89.1 kJmolꢀ1, which is +30.5 kJmolꢀ1 higher than ts1hs for
hydrogen abstraction. In contrast, the ts2doublet is associated
with an activation energy, which is +20.9 kJmolꢀ1 lower than
ts1hs and +29.2 kJmolꢀ1 lower than ts1ls. Thus, it is evident
that, as the radquartet and raddoublet states are very close in
energy and are thermodynamically very stable, they will easily
interchange with one another[47,48] through spin flipping and
the reaction pathway follows the low-energy barrier route in-
volving ts2doublet rather than the one involving the higher
energy transition state ts2quartet. The spin-state analysis of the
transition states reveal that both will end up to give the same
rebound product in which NiII is reduced to NiI, which is con-
firmed by modeling the rebound product (1c, E=
ꢀ111.2 kJmolꢀ1, 1(Ni) =0.94).
Figure 12. Computed geometries of the transition states along with the key
II
+
C
spin densities for [(L5)Ni ꢀO ] (5bhs)-catalyzed cyclohexane hydroxylation.
Bond lengths in [ꢁ].
II
+
radquartet (E=ꢀ12.5 kJmolꢀ1, 1(Ni) =1.62, 1(O1) =0.16, 1(C) =0.98)
C
The computed free-energy profile for [(L5)Ni ꢀO ] (5b)-cat-
alyzed cyclohexane hydroxylation is shown in Figure 11 and
the energetics are quoted relative to the free intermediate
5bhs species plus the cyclohexane molecule. The optimized in-
and raddoublet (E=ꢀ11.0 kJmolꢀ1, 1(Ni) =1.61, 1(O1) =0.23, 1(C)
=
ꢀ0.98) intermediates, which indicates the occurrence of facile
spin flipping between the radquartet and raddoublet species. As for
1b, the CꢀO rebound step for 5b also pursues a trend similar
to 1b in such a way that the activation energy barrier of
ts2quartet (E= +82.8 kJmolꢀ1) is higher than that of ts2doublet (E=
+33.8 kJmolꢀ1). Both the ts2 states will lead to the formation
of NiI-rebound product with overall reaction energy of
ꢀ122.2 kJmolꢀ1.
Overall, the computed energetics given in Figures 9 and 11
lead to the following mechanistic details: 1) the ground-state
configuration of the active species is in a high-spin state (1bhs
or 5bhs) and will pass through a hydrogen-abstraction transi-
tion state (ts1hs) to yield a more stable radical intermediate
radquartet, which is in equilibrium with raddoublet by readily under-
go spin flipping; 2) in the CꢀO rebound step, raddoublet follows
the low-energy pathway involving the ts2doublet to yield the
product. Thus, it is interpreted that the reaction preferably
goes through high-spin ts1hs for hydrogen abstraction and
low-spin ts2doublet for a CꢀO rebound step, that is, a two spin
state reactivity; 3) for both 1b and 5b, a minor pathway
through the low-spin transition state (ts1ls) is also possibly op-
erating along with the above-described major pathway occur-
ring through the high-spin transition state ts1hs, as both 1bhs
and 5bhs may coexist due to a small energy gap for the spin
cross over; 4) overall a two spin-state reactivity is predicted for
Figure 11. Computed free energy profile (B3LYP/B2//B3LYP/B1 level) for the
II
+
[(L5)Ni ꢀO ] (5b)-catalyzed cyclohexane oxidation. Energies in [kJmolꢀ1].
C
termediates associated with this pathway are shown in Fig-
ure S7 in the Supporting Information and the transition-state
geometries are shown in Figure 12. In the case of 5b, the hy-
drogen-abstraction step proceeds through the transition states
ts1hs (E= +52.0 kJmolꢀ1, 1(Ni) =1.59, 1(O1) =0.79, 1(C) =0.42) and
ts1ls (E= +60.8 kJmolꢀ1, 1(Ni) =1.60, 1(O1) =ꢀ0.53, 1(C) =ꢀ0.36).
As found for 1b, the ts1ls for 5b is found to be slightly higher
in energy than ts1hs by +8.9 kJmolꢀ1. Here again, the hydro-
gen abstraction leads to the formation of the more degenerate
II
+
C
the [(L1/L5)Ni ꢀO ] -catalyzed cyclohexane hydroxylation reac-
tion wherein the rate-determining step is the hydrogen ab-
straction.
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Comparison of the reaction pathways modeled for
atom acts as an efficient catalyst towards oxidation of alkanes
by stabilizing the high-valent reactive nickel-oxo intermediate
species, whereas that with a good s-donating imidazole nitro-
gen atom acts as a less efficient catalyst by destabilizing the
reactive intermediate. The computed results show that the
II
+
II
+
C
C
[(L1)Ni ꢀO ] (1b) and [(L5)Ni ꢀO ] (5b)
The reaction pathways modeled for the 1b- and 5b-catalyzed
cyclohexane hydroxylation are essentially the same. However,
a comparison of the energetics of ts1 in the high-spin (S=3/2)
and low-spin (S=1/2) states for 1b and 5b reveals that the
ts1hs and ts1ls of 1b have activation energies slightly higher (ꢁ
+6.0 kJmolꢀ1) than those of 5b, which means that the catalyt-
ic efficiency of 1b is less than that of 5b. However, on the
basis of the computational method used in this study and the
data obtained, the expected error limit is ꢂ(10.0–
II
+
C
high-spin (S=3/2) [(5N)Ni ꢀO ] species is found to be the
ground state with the low-spin (S=1/2) excited state located
close to it. The cyclohexane hydroxylation occurs through
II
+
C
a major pathway involving the high-spin [(5N)Ni ꢀO ] species
and two transition states, ts1hs and ts2doublet. However, a minor
pathway for the same species but with a low-spin state and
two transition states, ts1ls and ts2doublet may also operate be-
15.0) kJmolꢀ1 [49]
.
So, it is evident that both 1b and 5b can be
cause both the high-spin and low-spin [(5N)Ni ꢀO ] species
II
+
C
considered to have almost the same activation energy barriers
and hence would compete with each other to be equally effi-
cient towards cyclohexane hydroxylation. However, it should
be noted that 1b produces cyclohexanol (53.3%) in higher
amount than 5b (30.2%). To throw light on the higher catalytic
activity of 1b and to get further mechanic insights, the homo-
lytic OꢀO bond cleavage of [NiII(L1/L5)(OOCOC6H4Cl)]+ (1a/5a)
coexist with a small energy gap for the spin cross over. Overall,
II
+
C
a two spin-state reactivity is predicted for the [(5N)Ni ꢀO ]
-catalyzed cyclohexane hydroxylation reactions wherein the
rate-determining step is the hydrogen abstraction. Also, the
DFT studies illustrate that if the homolytic OꢀO bond cleavage
of the peroxo species [NiII(L)(OOCOC6H4Cl)]+ to form the reac-
II
+
C
tive [(L)Ni ꢀO ] intermediate is thermodynamically less fa-
vored then the catalytic activity would be decreased. The cata-
lytic ability of the present Ni(N5) complexes are lower than
those of Ni(N4) complexes suggesting that at least two coordi-
nation sites are needed for conferring a better catalytic effi-
II
+
C
to obtain [(L1/L5)Ni ꢀO ] (1b/5b) along with the m-CPBA rad-
ical (Scheme 2), has been modeled. It was found that the reac-
tion energy (DER.E) for the formation of 1bhs is +99.3 kJmolꢀ1
,
which is +8.4 kJmolꢀ1 lower than that for 5bhs (DER.E = +
107.7 kJmolꢀ1), and this means that the 1bhs species [(L1)NiIIꢀ ciency on the complexes. The present study also suggests that
+
O ] is thermodynamically more easily accessible than the 5bhs
a robust NiII catalyst for alkane oxidation may be designed by
incorporating strongly p-backbonding heterocyclic nitrogen
donor atoms on a tetradentate ligand.
C
II
+
C
species [(L5)Ni ꢀO ] from 1a and 5a, respectively. It is evident
that p-backbonding by the pyridine nitrogen atom stabilizes
II
+
C
the high-valent intermediate [(L1)Ni ꢀO ] (1bhs) species,
whereas s-donation by the imidazole nitrogen atom destabiliz-
II
+
Experimental Section
C
es the [(L5)Ni ꢀO ] (5bhs) species. The thermodynamically
more stable 1bhs is expected to stabilize the associated ts1hs
species, and hence with the inclusion of formation energy of
1bhs, ts1hs has an activation energy barrier slightly lower (ꢁ +
2.0 kJmolꢀ1) than that of 5bhs. This trend is consistent with the
experimentally observed higher catalytic activity of [Ni(L1)-
(CH3CN)]2+ (1) than [Ni(L1)(CH3CN)]2+ (5). Thus, the homolytic
OꢀO bond cleavage step plays a key role in determining the
catalytic activity of 1 and 5 and so the catalytic activity mainly
depends upon the thermodynamic stability of 1bhs and 5bhs.
However, it should be noted that the DER.E value for the forma-
tion of 1bls (DER.E = +124.4 kJmolꢀ1) is almost the same as
that for 5bls (DER.E = +123.9 kJmolꢀ1).
Materials
Pyridine-2-carboxaldehyde, N-methylethylenediamine, N-benzyle-
thylenediamine, nickel(II) perchlorate hexahydrate, adamantane,
cumene, m-chloroperbenzoic acid, sodium tetraphenylborate,
sodium borohydride, 1-methylimidazole-2-carboxldehyde, tetra-N-
butylammonium bromide (Aldrich), 6-methylpyridine-2-carboxlde-
hyde, quinoline-2-carboxaldehyde, sodium triacetoxyborohydride
(Alfa Aesar), acetic acid glacial, ethyl acetate, triethylamine, sodium
sulphate (Merck, India), and cyclohexane (Ranbaxy) were used as
received. Dichloromethane, diethyl ether, tetrahydrofuran, acetoni-
trile (Merck, India), and methanol (Sisco Research Laboratory,
Mumbai) were distilled before use. The supporting electrolyte
tetra-N-butylammonium perchlorate (TBAP) was prepared in water
and re-crystallized twice from aqueous ethanol.
Conclusion
Synthesis of the ligands
A new family of nickel(II) complexes of pentadentate N5 li-
gands has been isolated, characterized, and their ability to
carry out alkane hydroxylation by using m-CPBA as oxidant has
been studied. The X-ray crystal structure of one of these com-
plexes possesses a distorted octahedral nickel(II) coordination
geometry constituted by the N5 ligand and one solvent mole-
cule. The UV/Vis spectra reveal an octahedral coordination ge-
ometry for all of the complexes in solution. All the complexes
catalyze the oxidation of cyclohexane, adamantane, and
cumene with high TONs and selectivity. Interestingly, the nick-
el(II) complex with a strongly p-backbonding pyridine nitrogen
N-Methyl-N,N’,N’-tris(pyrid-2-ylmethyl)ethylenediamine (L1): The
ligand was prepared as reported[42] elsewhere. Yield: 1.39 g (80%);
1H NMR (400 MHz, CDCl3): d=2.28 (s, 3H; NCH3), 2.54–2.77 (m, 4H;
NCH2CH2N), 3.70 (s, 2H; NCH2 of pyridyl), 3.90 (s, 4H; NCH2 of pyr-
idyl), 7.00–8.56 ppm (m, 12H; pyridyl); 13C NMR (400 MHz, CDCl3):
d=159.21, 148.63, 148.58, 135.81, 122.52, 122.37, 121.43, 121.36,
63.33, 60.26, 55.03, 51.55, 42.19 ppm; MS (EI): m/z calcd for
C21H25Nþ5 : 347.21; found: 347.3.
N-Benzyl-N,N’,N’-tris(pyrid-2-ylmethyl)ethylenediamine (L2): The
ligand was prepared as reported[43] elsewhere. Yield: 1.25 g (59%);
1H NMR (400 MHz, CDCl3): d=2.60–2.70 (m, 4H; 2ꢂCH2), 3.53 (s,
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Synthesis of the NiII complexes
2H; CH2), 3.68 (s, 2H; CH2), 3.73 (s, 4H; 2ꢂCH2), 7.05–7.10 (m, 3H),
7.19–7.22 (m, 5H), 7.39–7.41 (m, 3H), 7.56–7.66 (m, 3H), 8.42–
8.51 ppm (m, 3H; Py, Ar); 13C NMR (400 MHz, CDCl3): d=160.09,
159.65, 148.85, 148.70, 139.11, 136.22, 128.64, 128.09, 126.79,
122.64, 122.59, 121.75, 121.69, 60.71, 60.65, 60.51, 58.91, 52.15,
51.83 ppm; MS (EI): m/z calcd for C27H29Nþ5 : 423.24; found: 423.3.
[Ni(L1)(CH3CN)](BPh4)2 (1): A solution of [Ni(ClO4)2]·6H2O (0.365 g,
1 mmol) in methanol (5 mL) was added to a solution of L1
(0.347 g, 1 mmol) in methanol (5 mL) with stirring at room temper-
ature when the color of the solution turned to indigo. After stirring
the solution for 30 min, acetonitrile (5 mL) was added. The solution
was stirred for additional 30 min when the color of the solution
turned blue. Upon metathesis of the complex by adding NaBPh4
(0.684 g, 2 mmol) in methanol (3 mL), a pink-colored precipitate
was obtained, which was filtered off, washed with small quantities
of ice-cold methanol and then dried. Yield, 0.91 g (84%); MS (ESI):
m/z: 202.67 [MꢀCH3CNꢀ2BPh4]2+; elemental analysis calcd (%) for
C71H68N6B2Ni: C 78.55, H 6.31, N 7.74; found: C 78.63, H 6.42, N
7.79.
N-Methyl-N,N’-bis(pyrid-2-ylmethyl)-N’-(6-methylpyrid-2-ylme-
thyl)ethylenediamine (L3): Step I: N-Methyl-N’-(6-methylpyrid-2-yl-
methyl)ethylenediamine was synthesized by adding 6-methylpyri-
dine-2-carboxaldehyde (0.619 g, 5 mmol) in methanol (20 mL) to N-
methyl-ethylenediamine (0.780 g, 10 mmol) in methanol (20 mL)
and then stirring the solution for twelve hours. Then the solution
was cooled to 08C and sodium borohydride (7.5 mmol) was added
and the stirring continued for twelve hours. The solvent was re-
moved under vacuum, the residue was extracted by using ethyl
acetate and the water was removed by using sodium sulphate.
The solvent was removed under vacuum to give a pale yellow oil,
which was used for the preparation of L2 without further purifica-
tion.
[Ni(L2)(CH3CN)](BPh4)2 (2): A procedure analogous to that used to
prepare complex 1 was followed by using L2 (0.423 g, 1 mmol) in-
stead of L1. The pure product was isolated as a purple precipitate.
Yield: 0.94 g (81%); MS (ESI): m/z: 240.67 [MꢀCH3CNꢀ2BPh4)2+; el-
emental analysis calcd (%) for C77H72N6B2Ni: C 79.61, H 6.25, N 7.23;
found: C 79.56, H 6.33, N 7.31.
Step II: To a mixture of N-methyl-N’-(6-methylpyrid-2-ylmethyl)ethy-
lenediamine (0.895 g, 5 mmol), pyridine-2-carboxaldehyde (1.08 g,
10 mmol), and acetic acid glacial (570 mL, 10 mmol) in tetrahydro-
furan (50 mL) was added sodium triacetoxyborohydride (3.75 g,
17.7 mmol) and the solution was stirred for 48 h. The reaction was
quenched with saturated sodium hydrogencarbonate solution and
extracted with ethyl acetate. The organic fractions were combined,
dried (Na2SO4), and the solvent was removed under reduced pres-
sure to obtain the product. The yellow oil obtained was purified by
using flash chromatography and then used for complex prepara-
[Ni(L3)(CH3CN)](BPh4)2 (3): This complex was prepared by using
a procedure analogous to that used to prepare complex 1 and L3
(0.361 g, 1 mmol) was used instead of L1. The pure product was
isolated as a purple precipitate. Yield 0.86 g (78%); MS (ESI): m/z:
209.67 [MꢀCH3CNꢀ2BPh4]2+
;
elemental analysis calcd for
C72H70N6B2Ni: C 78.64, H 6.42, N 7.64; found: C 78.56, H 6.57, N
7.72. Single crystals suitable for X-ray crystallographic analysis were
obtained by slow evaporation of a solution of the complex in a di-
chloromethane/acetonitrile solvent mixture.
1
tion. Yield: 1.60 g (88%); H NMR (400 MHz, CDCl3): d=2.29 (s, 3H;
[Ni(L4)(CH3CN)](BPh4)2 (4): A procedure analogous to that used to
prepare complex 1 was used and L4 (0.397 g, 1 mmol) was used in-
stead of L1. The pure product was isolated as a purple precipitate.
Yield 0.81 g (71%); MS (ESI): m/z: 234.48 [MꢀCH3CNꢀ2BPh4]2+; ele-
mental analysis calcd for C75H70N6B2Ni: C 79.32, H 6.21, N 7.40;
found: C 79.28, H 6.28, N 7.20.
NCH3), 2.76–2.80 (m, 4H; NCH2CH2N), 2.59 (s, 3H; CH3), 3.71 (s, 2H;
NCH2 of pyridyl), 3.92 (s, 4H; NCH2 of pyridyl), 7.00–8.60 ppm (m,
11H; pyridyl); 13C NMR (400 MHz, CDCl3): d=159.26, 148.69,
148.68, 135.61, 122.32, 122.27, 121.33, 121.32, 63.23, 60.22, 55.13,
51.45, 42.12, 28.51 ppm; MS (EI): m/z calcd for C22H27Nþ5 : 361.23:
found: 361.3.
[Ni(L5)(CH3CN)](BPh4)2 (5): This complex was isolated by employ-
ing the procedure analogous to that used to prepare complex
1 and by using L5 (0.350 g, 1 mmol) instead of L1. The pure prod-
uct was isolated as a purple precipitate. Yield 0.87 g (80%); MS
(ESI): m/z: 224.07 [MꢀCH3CNꢀ2BPh4]2+; elemental analysis calcd
for C70H69N7B2Ni: C 77.23, H 6.39 N 9.01; found: C 77.35, H 6.31, N
9.11.
N-Methyl-N,N’-bis(pyrid-2-ylmethyl)-N’-(quinolin-2-ylmethyl)ethy-
lenediamine (L4): The procedure employed for the preparation of
L3 was used for obtaining L4 and quinoline-2-carboxaldehyde was
used instead of pyridine-2-carboxaldehyde. The brown oil was puri-
fied by using flash chromatography and the obtained product was
used for complex preparation. Yield: 1.41 g (71%); 1H NMR
(400 MHz, CDCl3): d=2.29 (s, 3H; NCH3), 2.76–2.82 (m, 4H;
NCH2CH2N,), 3.73 (s, 2H; NCH2 of quinolyl), 3.94 (s, 4H;NCH2 of pyr-
idyl), 7.00–8.56 ppm (m, 14H; aromatic); 13C NMR (400 MHz,
CDCl3): d=159.23, 148.62, 148.38, 135.51, 129.86, 128.61, 127. 32,
125. 80, 122.31, 122.29, 121.34, 121.39, 63.13, 60.22, 60.56, 55.15,
51.45, 42.11 ppm; MS (EI): m/z calcd for C25H27Nþ 397.23; found:
Caution!
Perchlorate salts of the compounds are potentially explosive. Only
small quantities of these compounds should be prepared and suit-
able precautions should be taken when they are handled.
:
5
397.3.
N-Methyl-N,N’-bis(pyrid-2-ylmethyl)-N’-(imidazole-2-ylmethyl)e-
thylenediamine (L5): The procedure employed for the preparation
of L1 was used for L5 and imidazole-2-carboxaldehyde was used
instead of pyridine-2-carboxaldehyde. The colorless oil was purified
by using flash chromatography and the obtained product was
used for complex preparation. Yield: 1.36 g (78%);1H NMR
(400 MHz, CDCl3): d=2.29 (s, 3H; NCH3), 2.76–2.81 (m, 4H;
NCH2CH2N), 3.59 (s, 3H; NCH3), 3.71 (s, 2H; NCH2 of imidazolyl),
3.91 (s, 4H; NCH2 of pyridyl), 6.70–8.56 ppm (m, 10H; pyridyl);
13C NMR (400 MHz, CDCl3): d=156.21, 152.14, 148.67, 135.63,
127.35, 124.23, 120.34, 120.37, 62.21, 60.27, 53.11, 51.44, 48.92,
42.14, 28.56 ppm; MS (EI): m/z calcd for C20H26Nþ6 : 350.22; found:
350.3.
Catalytic oxidations
The oxidation of alkanes was carried out under a nitrogen atmos-
phere at room temperature witꢀh3in two hours. In a typical reaction,
the nickel(II) complex (0.35ꢂ10 mmoldmꢀ3) was added to a mix-
ture of alkanes (2.45 moldmꢀ3
(0.35 moldmꢀ3) in a CH2Cl2:CH3CN mixture (3:1 v/v). After 2 h the
reaction mixture was quenched with triphenylphosphine and fil-
tered over a silica column and then eluted with diethyl ether. An
internal standard (bromobenzene) was added at this point and the
solution was subjected to GC analysis. The mixture of organic
products were identified by Agilent GC-MS and quantitatively ana-
lyzed by HP 6890 series GC equipped with a HP-5 capillary column
)
and oxidant m-CPBA
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(30 mꢂ0.32 mmꢂ2.5 mm) by using a calibration curve obtained
with authentic compounds. All of the products were quantified by
using GC (FID) with the following temperature program: injector
temperature=1308C, initial temperature=608C, heating rate=
108Cminꢀ1 to 1308C, increasing the temperature to 1608C at
a rate of 28C minꢀ1, and then increasing the temperature to 2608C
at a rate of 58Cminꢀ1, FID temperature=2808C. GC-MS analysis
was performed under conditions identical to those used for GC
analysis. The averages of three measurements are reported.
SHELXTL[54] program was used for space group and structure deter-
mination, and least-squares refinements on F2. The structure was
solved by the heavy-atom method and other non-hydrogen atoms
were located in successive difference Fourier syntheses. After locat-
ing the NiII complex with the bulky anions, interlinked diffused
peaks with residual electron density ranging from 2.3–1.5 eꢁꢀ3
were observed in the difference Fourier map, which can be attrib-
uted to disordered solvent molecule (acetonitrile used for crystalli-
zation) present in the crystal lattice. Attempts were made to model
these residual electron density peaks, but were unsuccessful be-
cause the peaks obtained were diffused and there was no obvious
major site occupations for the solvent molecules. The PLATON/
SQUEEZE[55] option has been used to correct the data for the con-
tribution from of the disordered solvent. Final cycles of least-
squares refinements with the modified data set improved both the
R values and the goodness of fit significantly. Crystal data and ad-
ditional details of the data collection and refinement of the struc-
ture are presented in Table 8. The selected bond lengths and bond
angles are listed in Table 1. CCDC 898033 contains the supplemen-
tary crystallographic data for complex 3. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Physical measurements
Elemental analyses were performed on a Perkin–Elmer Series II
1
CHNS/O Analyzer 2400. H NMR spectra were recorded on a Bruker
400 MHz NMR spectrometer. All the ligands were purified on TELE-
DYNE CombiFlash Rf Flash chromatography. Electronic spectra
were recorded on a Agilent 8453 Diode Array Spectrophotometer.
Low-temperature spectra were obtained on a Agilent 8453 Diode
Array Spectrophotometer equipped with an UNISOKU USP-203
cryostat. ESI-MS analyses were recorded on a Micromass Quattro II
triple quadrupole mass spectrometer. Cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) were performed at (25ꢂ
0.2)8C by using a three-electrode cell configuration. A platinum
sphere, a platinum plate, and Ag(s)/AgNO3 were used as working,
auxiliary, and reference electrodes, respectively. The platinum
sphere electrode was sonicated for two minutes in dilute nitric
acid, dilute hydrazine hydrate, and in double distilled water to
remove the impurities. The reference electrode for non-aqueous
solution was Ag(s)/Ag+, which consists of a Ag wire immersed in
a solution of AgNO3 (0.01m) and tetra-N-butylammonium perchlo-
rate (0.1m) in acetonitrile placed in a tube fitted with a vycor plug.
The instruments utilized included an EG & G PAR 273 Potentiostat/
Galvanostat and P-IV computer along with EG & G M270 software
to carry out the experiments and to acquire the data. The tempera-
ture of the electrochemical cell was maintained by a cryo-circulator
(HAAKE D8-G). The E1/2 value observed under identical conditions
for the ferrocene/ferrocenium (Fc/Fc+) couple in acetonitrile was
0.102 V with respect to the Ag/Ag+ reference electrode. The exper-
imental solutions were deoxygenated by bubbling research-grade
nitrogen and an atmosphere of nitrogen was maintained over the
solution during measurements. The products were analyzed by
using a Hewlett Packard (HP) 6890 GC series Gas Chromatograph
equipped with a FID detector and a HP-5 capillary column (30 mꢂ
0.32 mmꢂ2.5 mm). GC-MS analysis was performed on an Agilent
GC-MS equipped with 7890A GC series (HP-5 capillary column) and
5975C inert MSD under conditions that are identical to that used
for GC analysis.
Table 8. Crystallographic data for complex 3.
empirical formula
formula weight [gmolꢀ1
crystal habit, color
crystal system
crystal size [mm3]
space group
a [ꢁ]
C72H70B2N6Ni
1099.67
pink
monoclinic
0.08ꢂ0.08ꢂ0.07
P21/c
22.0950(5)
]
b [ꢁ]
17.3654(4)
c [ꢁ]
18.4523(5)
a [8]
90
b [8]
102.842(10)
g [8]
90
6902.8(3)
V [ꢁꢀ3
]
Z
4
1calcd [gcmꢀ3
F(000)
]
1.058
2328
temperature [K]
293(2)
no. of reflns collected
no. of unique reflns
radiation (MoKa) [ꢁ]
goodness-of-fit on F2
128631
14792 (R(int)=0.0433)
0.71073
1.084
no. of refined parameters
R1/wR2[I>2s(I)][a]
R1/wR2 (all data)
726
R1 =0.0437, wR2 =0.1295
R1 =0.0608, wR2 =0.1368
Crystallographic data collection, refinement, and structure
solution
[a] R1=[S(jjFojꢀjFc j j)/SjFo j]; wR2={[S(w(F2oꢀFc2)2)/S(wFo4)]1/2}.
The diffraction experiments were carried out on a Bruker SMART
APEX diffractometer equipped with a CCD area detector. High-
quality crystals, suitable for X-ray diffraction were chosen after
careful examination under an optical microscope. Intensity data for
the crystals were collected by using MoKa (l=0.71073 ꢁ) radiation
on a Bruker SMART APEX diffractometer equipped with a CCD area
detector at 296 K. The structure was solved by direct methods by
using the SHELXS-97.[50] The refinement and all further calculations
were carried out by using SHELXL-97.[51] The SMART[52] program
was used for collecting frames of data, indexing reflection, and de-
termination of lattice parameters; the SAINT[52] program was used
for integration of the intensity of reflections and scaling; the
SADABS[53] program was used for absorption correction, and the
Computational details
In the current study, all geometries are fully optimized by the
B3LYP-DFT method by using the Gaussian 09 program.[56] The cal-
culations were carried out with a mixed basis set (B1) of LanL2DZ
for the Ni center, which has a relativistic effective core potential
with a valence basis set and 6-31G** for the remaining atoms.[57]
All the intermediates were confirmed by the frequency calculations
as minima on the potential energy surface. The transition states
were characterized by a single negative frequency and verified fur-
ther by animating the frequency that correspond to the CꢀH bond
elongation/CꢀO bond formation, as concern to this study. For se-
&
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lected transition states, intrinsic reaction coordinate (IRC) calcula-
tions were preformed to confirm that the transition state is indeed
connected to the corresponding reactant and the product in the
potential energy surface. Further, to get the reliable energetics,
single-point calculations were performed on the B3LYP/B1-opti-
mized geometries by using LanL2DZ for the Ni center and 6-311+
+G** for the remaining atoms (basis set B2). The quoted energies
are those calculated at the B3LYP/B2//B3LYP/B1 level, including the
free energy corrections obtained from the B3LYP/B1-optimized ge-
ometries. In addition, the single-point calculations at B3LYP/B1
have also been performed with all electron TZVP basis sets for all
atoms (free energy corrections obtained from the B3LYP/B1-opti-
mized geometries) for representative systems. This has been com-
pared with the B3LYP/B2//B3LYP/B1 energies (see Table S2 in the
Supporting Information). The B3LYP/TZVP//B3LYP/B1 level energet-
ics are found to be very close to the B3LYP/B2//B3LYP/B1 energet-
ics (up to the variation of 3–6 kJmolꢀ1). Hence, the B3LYP/B2//
B3LYP/B1 free energies are discussed throughout in the text unless
otherwise mentioned. It is known that there are instances that
B3LYP predicts incorrect ground states, especially for systems
having small low-spin and high-spin gaps.[58] In the case of
fractometer at the School of Chemistry. We thank Dr. Balachan-
dran Unni Nair, Central Leather Research Institute, Chennai for
providing the ESI-MS facility.
Keywords: density functional calculations · hydroxylation · N
C
ligands · nickel · NiꢀO species
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We sincerely thank the Council of Scientific and Industrial Re-
search, New Delhi, for a Senior Research Fellowship to M.S.
and the Department of Science & Technology, SERB, for the
award of Fast track Young Scientist Fellowship to P.V. (Ref. No:
CS-232/2012). This work was supported by the DST Nano Mis-
sion. Project (SR/NM/NS-110/2010 (G)) and a CSIR project
(CSIR/01(2462)/11/EMR-II). Professor M. Palaniandavar is a recipi-
ent of the DST Ramanna Fellowship [Scheme No. SR/S1/RFIC-
01/2007 and SR/S1/RFIC-01/2010]. We also thank the Director
of the National Institute for Interdisciplinary Science and Tech-
nology (NIIST)-CSIR, Trivandrum, for providing the computa-
tional facility. We thank the Department of Science and Tech-
nology (FIST program), New Delhi for the use of the X-ray dif-
Chem. Eur. J. 2014, 20, 1 – 17
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Received: February 27, 2014
Published online on && &&, 0000
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FULL PAPER
Interesting, interesting! Nickel(II) com-
plexes with a strong p-backbonding
ligand act as efficient catalysts for the
oxidation of alkanes (see figure, m-
CPBA=m-chloroperbenzoic acid) by sta-
&
Nickel Complexes
M. Sankaralingam, M. Balamurugan,
M. Palaniandavar,* P. Vadivelu,*
C. H. Suresh
C
bilizing the NiꢀO intermediate, whereas
&& – &&
those with a better s-donor ligand act
as less efficient catalysts by destabilizing
the reactive intermediate. The comput-
ed mechanism for cyclohexane hydrox-
ylation reveals that a high-spin (S=3/2)
Nickel(II) Complexes of Pentadentate
N5 Ligands as Catalysts for Alkane
Hydroxylation by Using m-CPBA as
Oxidant: A Combined Experimental
and Computational Study
II
+
C
[(L1/L5)Ni ꢀO ] species is the ground
state.
Alkane Hydroxylation
The selective oxidation of saturated hydrocarbons under
mild conditions is challenging. In their Full Paper on page
&& ff., M Palaniandavar, P. Vadivelu et al. investigated the
reactivity of a new series of nickel complexes with
pentacoordinated N ligands both experimentally and
computationally. They found that the use of strong p-
C
backbonding ligands stabilize a NiꢀO intermediate, thus
leading to high activity.
Chem. Eur. J. 2014, 20, 1 – 17
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ