Effect of ligand substituent on the reactivity of Ni(ii) complexes towards oxygen

Article abstract of DOI:10.1039/c3dt52072b

Two radical-containing Ni(ii) complexes having either parent salicylidene (complex 1) or 3,5-di-tert-butylsalicylidene (complex 2) in the ligand backbone were synthesized. Complex 2 underwent ligand centered C-H activation by aerial oxygen, forming the co

Full text of DOI:10.1039/c3dt52072b

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Effect of ligand substituent on the reactivity of
Ni(II) complexes towards oxygen†‡
Cite this: Dalton Trans., 2014, 43,
394
Samir Ghorai and Chandan Mukherjee*
Received 31st July 2013,
Accepted 26th September 2013
DOI: 10.1039/c3dt52072b
www.rsc.org/dalton
Two radical-containing Ni(II) complexes having either parent
salicylidene (complex 1) or 3,5-di-tert-butylsalicylidene (complex 2)
in the ligand backbone were synthesized. Complex 2 underwent
ligand centered C–H activation by aerial oxygen, forming the
corresponding amide complex (2a). The UV-Vis/NIR spectral changes
2 2
upon purging of molecular oxygen to 2 in CH Cl , along with ESI-MS
analysis indicated the generation of Ni–oxygen/dioxygen species as
the intermediate(s) for the amide formation. Interestingly, non-
participation of the ligand centered π-radical in the oxidation
process was observed.
Metal-oxo units have been proposed as active intermediates
for several enzymatic and biomimetic C–H bond activation
reactions. Since then, transition metal mediated dioxygen redox states. ISQ and IBQ stand for the iminobenzosemiquinone and
activation as well as the characterization of metal-oxo inter- iminobenzoquinone forms of the ligand, respectively.
Scheme 1 A schematic representation of the ligands and their different
1
mediates have drawn considerable attention from both che-
2
mists and biologists. Cu-, Co-, and Fe-based metalloenzymes
Mixed(H)
we have synthesized two new ligands, H
3
Sami
and
are well known for their oxygen activation, while Ni-based
Mixed(tBu)
1
a
H
3
Sami
, composed of both redox active aminophenol
a change in its oxidation state is favored in the presence of a
metal ion and oxygen) and redox inactive (a change in its
oxidation state is not favored in the presence of a metal ion
and oxygen) salicylidene compartments connected via a benzyl
linker (Scheme 1). The ligands differ from each other by the
metalloenzymes are very rare. This could be due to the
thermodynamically less favorable oxidation of Ni(II) by mole-
(
1
a
cular oxygen, due to its higher oxidation state. However,
electron rich ligands bound to the Ni(II) centre have been
found to favor the Ni center oxidation process by decreasing
the oxidation potential, and consequently Ni could be found
in its higher oxidation state. For instance, Ni peptide com-
plexes where the Ni center is bound to an amide ligand favor
presence of tert-butyl groups at the 3,5-positions of the salicyl-
Mixed(tBu)
idene unit in [(H Sami
)].
3
Mixed(tBu)
3
Herein, we report the synthesis of ligands [(H
3
Sami
)]
the oxidation of Ni(II) to Ni(III) under air.
Mixed(H)
3
and [(H Sami
)], and their corresponding Ni(II) com-
In continuation of our ongoing research in understanding
4
plexes. Interestingly, we have observed a remarkable substi-
tuent dependent reactivity of the Ni complexes with molecular
oxygen. Non-participation of the ligand centered π-radical of
the Ni–oxygen/dioxygen intermediate was also observed and
reported here.
schematic diagram for the synthesis of ligands
, and the corresponding Ni
complexes 1 and 2 is shown in Scheme 2. The reaction between
the effect of substituents on the reactivity of metal complexes,
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati,
781039 Assam, India. E-mail: cmukherjee@iitg.ernet.in; Fax: (+)91-361-258-2349;
Tel: (+)91-361-258-2327
th
†
‡
Dedicated to Dr Eckhard Bill on the occasion of his 60 birthday.
A
Mixed(H)
Mixed(tBu)
Electronic supplementary information (ESI) available: Experimental proce-
H Sami
, H Sami
3
3
Mixed(R)
dures, characterization data of H
3
Sami
, 1, 2, and 2a, including IR, mass
spectra, ORTEP plots, and X-band EPR spectra. Crystallographic structural para-
Mixed(H)
1 : 1 2-aminobenzonitrile and 3,5-di-tert-butylcatechol in the
meters, and crystallographic information file of H
3
Sami
, 1, 2, and 2a.
CN
presence of Et N provided H Sami
in 72% yield. Further
CCDC 951779–951782. For ESI and crystallographic data in CIF or other
3
2
electronic format, see DOI: 10.1039/c3dt52072b
reduction of the bidentate ligand using LiAlH gave A in 70%
4
394 | Dalton Trans., 2014, 43, 394–397
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correspond to the π-radical form of the organic moiety. In
Mixed(H)
ligand H Sami
the C–C bond distances of the 3,5-
3
di-tert-butyl-containing aminophenol unit were in range of
.380(5)–1.400(5) Å (Table 2, S14‡). The C1–N1 = 1.435(5) and
1
C2–O1 = 1.375(5) Å bond distances along with the C–C bond
distances indicated the fully reduced form of the aryl ring. The
C14–N2 = 1.275(4) Å bond distance indicated its double bond
character and the C20–O2 = 1.355(3) Å bond distance was in
6
accord with the phenolic C–O bond distance. The C–C bond
6
distances of the other two C rings were found to be in the
range of 1.366(4)–1.402(4) Å with no quinoid-type distortion.
Hence, the ligand was in its fully reduced form with the benzyl
group bridging between an aminophenol and a salicylidene
unit.
All of the complexes were neutral. Complex 1 and complex
2
crystallized in the monoclinic space groups P2
respectively, while complex 2a crystallized in the orthorhombic
space group P2 . The molecular structures are shown in
1 1
/a and C2 /c1,
1 1 1
2 2
Fig. 1. Selected bond distances and bond angles are given in
Table 2 (S14–S15‡).
All of the C–C bond distances of the tert-butyl-containing C
aryl rings (amidophenolate unit) in the complexes were not within
.39 ± 01 Å range, rather, a quinoid type distortion was observed
Table 2, S14–S15‡). Furthermore, the C1–N1 = 1.364(6) [1],
6
1
(
Scheme 2 A schematic representation of the ligands and the formation
pathways of the corresponding Ni complexes.
1.371(4) [2], 1.380(5) [2a] Å and C2–O1 = 1.293(8) [1], 1.310(3) [2],
.305(5) [2a] Å (parentheses correspond to the respective
1
complex) bond distances were in between those of their single
bond and double bond values. There was shortening in the
yield. The condensation of equimolar amounts of salicylalde- C20–O2 and C14–C15 bonds, and elongation in the C14–N2
Mixed(H)
hyde and A in ethanol provided ligand H
3
Sami
in 70% bond, on going from free ligand to 1 or 2 (Table 2, S14–S15‡).
Mixed(tBu)
yield. The ligand H
3
Sami
was synthesized in situ by a These bond distance changes were mainly due to delocaliza-
: 1 condensation of A with 3,5-di-tert-butylsalicylaldehyde tion of the phenolate charge over the O2–O20–C15–C14–N2
1
−
1
in CH
3
CN under reflux. The addition of NiCl
2
·6H
2
O to unit. The shortening and elongation of these bond distances
Mixed(H)
Mixed(tBu)
3
H Sami
or to the in situ generated ligand H
3
Sami
in 2a were more pronounced due to the increase of delocaliza-
provided the corresponding complexes 1 and 2 in 51% and tion of the charge over the molecule via the newly formed
0% yield, respectively. Both complexes were stable under air amide unit. This type of shortening and elongation is
in the solid state. However, complex 2 changed gradually to common in salen complexes where the salicylidene unit is in
4
6
complex 2a when in solution.
its fully reduced form. Hence, in the complexes, the amido-
Single crystal X-ray diffraction measurements for ligand phenolate part of the ligand was found to be in its one-
Mixed(H)
H
3
Sami
, complex 1, complex 2, and complex 2a were electron oxidized ISQ form and no oxidation in the salen unit
Mixed(H)
performed at 296(2) K. Ligand H Sami
crystallized in was observed.
the triclinic space group P ˉ1 . The asymmetric ligand was com-
prised of three different kinds of C aryl ring. The C–C bond
3
6
distances of the C aryl rings can be taken into consideration
6
when assigning the oxidation state of the ligand. In the fully
6
reduced form, the C–C bond distances in the C aryl ring are
5
,6
all 1.39 ± 0.01 Å. In the case of the one-electron oxidized
iminobenzosemiquinone (ISQ) or two-electron oxidized imino-
benzoquinone (IBQ) forms of the ligand (Scheme 1), discrete
alternative short and long C–C bond distances (i.e. a quinoid-
5
type distortion) in the C
6
aryl ring are expected. Furthermore,
in the ISQ form, the CPh–NPh and CPh–OPh (CPh, NPh, and OPh
stand for the phenyl carbon atom, N, and O atom attached to
the phenyl ring, respectively) bond distances are 1.35 ± 01 Å
5
and 1.30 ± 0.01 Å, respectively. These bond distances are
Mixed(H)
Fig. 1 Ball and stick representation of the (A) H
3
Sami
, (B) 1, (C) 2,
in between single bond and double bond character, and and (D) 2a molecular structures. Hydrogen atoms are omitted for clarity.
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In the complexes, the central Ni1 atom was coordinated by
two N and two O atoms. The Ni1–O1, Ni1–N1 and Ni1–N2
bond distances were almost equal for both complexes, and
found to be within the 1.86 ± 0.01 Å range. These bond dis-
tances were in accord with the +II oxidation state of the Ni
5
b,c,6c,d
center.
Interestingly, the Ni1–O2 bond distance was
∼0.02 Å shorter in 2 compared to in 1, indicating the higher
covalency of the bond caused by the presence of the electron
donating tert-butyl substituent at the 3,5-positions of the
parent salicylidene unit. This difference in covalency seemed
to be responsible for the observed difference in reactivity
under air between 1 and 2. A higher electron density at the
−
Ni(II) center via PhO in 2 might facilitate the required stabili-
1
b,7
zation of Ni to its higher oxidation state,
oxygenation reaction.
and hence the
A detailed solid-state structural comparison was performed
and the parameters are given in Table 1 (S13‡). The Ni atom in
1
was observed to be almost planar with respect to both the
P and R planes, while that in 2 was 0.30 Å below and 0.34 Å
above the P and R planes, respectively. Furthermore, it was
found that the position of the central Ni atom relative to the
plane passing through the salicylidene unit differed remark-
ably (0.01 Å towards the benzyl carbon atom for 1 and 0.3 Å
away from the benzyl carbon atom for 2). These positional
differences of the Ni(II) center seemed to facilitate formation
of the oxygen–metal center adduct.
The X-band EPR spectra of 1, 2, and 2a (Fig. 9, S8‡) dis-
played isotopic resonance signals at g = 2.003 (1), g = 2.005 (2)
and g = 2.002 (2a) (parentheses indicate the complex). Thus,
the neutral complexes can be best described as Ni(II)-mono
Scheme 3 A mechanistic proposal for the aerial oxidation of 2 to 2a.
the increase of the product marking band at ∼485 nm.
This increase was related to the increase of the EPR signal
(
Fig. 13, S11‡).
Ni(II) or Ni(III)-superoxo, -peroxo, -hydroperoxo, -oxide, and
-
hydroxo species do not give rise to a strong absorption band
8
at ∼652 nm. Moreover, the addition of mCPBA (1 equiv.) to
the CH Cl solution of 2 gave rise to similar changes in the
2
2
UV-Vis/NIR spectral features (Fig. 14, S12‡) as air did. There-
fore, the species showing absorption at ∼652 nm were not the
corresponding -superoxo, -peroxo, or -hydroperoxo species.
(
iminobenzosemiquinone) complexes, i.e. Ni(II) coordinated to
a π-radical anion.
A CH Cl solution of 2 was continuously purged with
Herein, the band at ∼652 nm was attributed to the generation
9
of a diamagnetic species X , having a delocalizing radical at
2
2
2
the benzyl position (Scheme 3).
oxygen gas and time-dependent UV-Vis/NIR spectra were simul-
taneously recorded (Fig. 2). A gradual and regular increase in
the absorption band (dotted lines) at ∼652 nm was observed
until 70 min. Noticeably, the band at ∼835 nm, known as a
The ESI (positive mode) mass spectrum of the reaction
mixture (2 + molecular oxygen) at 75 min showed peaks at
m/z = 613.30 and m/z = 630.30, in addition to the molecular
mass peaks for 2 and 2a (Fig. 4, S8‡). The m/z = 613.27 peak
and its isotope distribution pattern correspond to a [{(2 − H) +
5
c
marker of π-radical character, showed little change in absorp-
tion (ε = 5000 M− cm ). This confirmed no reaction and/or
1
−1
+
+
2
O} + H] species, i.e. [X + H] , while the m/z = 630.26 peak
participation of the ligand centered π-radical [2, ∼835 nm
+
+
ε, 5500 M− cm )]. The generation of the band at ∼652 nm
1
−1
represents a [{2 + OO} + H] unit, i.e. [X + H] , a dioxygen–2
1
(
adduct (Fig. 15, S12‡). When the reaction was carried out with
was related to the formation of diamagnetic species (28%) as
evidenced by the decrease of the EPR signal (Fig. 13, S11‡).
After 70 min, the band at ∼652 nm gradually decreased with
1
8
2
O , the reaction solution exhibited ion peaks at m/z = 615.29
1
8
+
and 613.34. These peaks corresponded to [2 + O] and
1
8
[2a (with one O)] species with the expected isotope distri-
bution pattern (Fig. 14, S12‡). This labeling experiment con-
firmed the incorporation of one oxygen atom from molecular
oxygen into 2a, in addition to the transient intermediate
(
Fig. 15, S12‡). Considering the above experimental (UV-Vis/
NIR, X-band EPR, MS) observations, the oxidation process
seemed to proceed via the route presented in Scheme 3.
Conclusions
Mixed(H)
Fig. 2 UV-Vis/NIR spectra of
H
3
Sami
, 1, 2, and 2a (left).
To conclude, two asymmetric ligands and their corresponding
2 2
Time-dependent UV-vis/NIR spectral changes of 2 at RT in CH Cl in
the presence of molecular oxygen (right).
Ni(II) complexes (1 and 2) coordinated to a π-radical anion
396 | Dalton Trans., 2014, 43, 394–397
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were synthesized. Among them, 2 underwent ligand centered
C–H bond activation, and consequently a keto (CvO) bond for-
mation by reaction with air as the sole oxidant. It was found
from X-ray crystallography that the position of the Ni center
above the salicylidene plane, and its higher covalency were
necessary to activate Ni(II) towards reaction with air. In
contrast to 1, the tert-butyl groups present at the salicylidene
unit in 2 stabilize the generated radical-containing transient
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8
e
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Mixed(H)
CCDC 951782 [H Sami
], 951779 [1], 951780 [2], and
3
9
51781 [2a] contain the supplementary crystallographic data
for this paper.
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7
8
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Acknowledgements
This project was funded by DST (SR/FT/CS-86/2011). SG thanks
UGC (India) for doctoral fellowship. Dr T. K. Paine and Dr
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1
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Dalton Trans., 2014, 43, 394–397 | 397