Unprecedented redox-driven ligand ejection in nickel(ii)- diiminosemiquinonate radical complexes

Article abstract of DOI:10.1039/c3cc48549h

Nickel(ii) complexes of (M:L) stoichiometries 1:1 (1) and 1:2 (2) were prepared from a polydentate ligand involving diiminosemiquinonate radicals. Both were characterized by X-ray diffraction, Vis-NIR and EPR spectroscopy as well as electrochemistry. Ligand-centered oxidation of 1 promotes ligand ejection to give 2.

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Unprecedented redox-driven ligand ejection in
nickel(II)-diiminosemiquinonate radical complexes
Cite this: DOI: 10.1039/x0xx00000x
N. Leconte,* J. Ciccione, G. Gellon, C. Philouze and F. Thomas*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
Nickel(II) complexes of (M:L) stoichiometries 1:1 (1) and 1:2
(2) were prepared from a polydentate ligand involving
diiminosemiquinonate radicals. Both were characterized by
X-ray diffraction, Vis-NIR and EPR spectroscopy as well as
electrochemistry. Ligand-centered oxidation of 1 promotes
Br
II
Pt
O
II
Pt
O
ox, +Br-, -H+
red, -Br-, +H+
P
Ph
Ph
(a)
P
Ph
Ph
OH
O
O
II
N
HO
N
N
Cu
II
ox, -Cl-, -2H+
N
Cu
Cl
ligand ejection to give 2
.
O
(b)
H
O
t-Bu
t-Bu
Molecular switches are at the heart of molecular electronic devices
and find applications in numerous fields like biosensors,[1] drug
ox
t-Bu
N
III
Ir
t-Bu
N
III
Ir
[3]
delivery,[2] nanotechnologies...
They refer to molecular systems
red
O
S
O
S
(c)
capable of undergoing reversible chemical and physical changes
[4]
triggered by external stimuli such as electric current, light or pH.
Among the platforms developed for this purpose inorganic
complexes play a central role. Redox events on the metal can indeed
trigger changes in its geometrical preference, and induce subsequent
(d)
t-Bu
t-Bu
NH HN
[5]
reorganization of the surrounding ligands.
Inorganic molecular
NH2 H2N
[6]
t-Bu
t-Bu
switches that function with ligandꢀbased redox events are rarer,
although organic molecular switches have been widely developed
Scheme 1 Redox-switchable ligands involving phenols [(a),[8] (b)
and (c)
[9]
[10]
]
during the past decade. [4] Protypical organic moieties that respond to
and molecular formula of H4L (d).
[7]
a redox stimulus by molecular motion are viologen pairs.
The
H4L (Scheme 1) was synthesized in a twoꢀstep sequence from 3,5ꢀdiꢀ
tertꢀbutylꢀ2ꢀnitrobromobenzene and 4,5ꢀdimethylꢀoꢀ
viologens do indeed not interact one with each other under their
cationic forms, but they readily assemble into donorꢀacceptor pairs
(πꢀdimers) when one of them is reduced. [7]
An alternative, yet less common, approach consists in exploiting the
redox activity of one chelating moiety of the ligand to labilize a MꢀL
phenylenediamine (see ESI for details). H4L reacted with an
equimolar amount Ni(OAc)2 .4 H2O in the presence of Et3N to give a
brown solution, from which complex 1 precipitated out as a dark
solid after exposure to air (yield 76 %). Single crystals of 1 suitable
for XꢀRay diffraction were grown by slow evaporation of an
Et2O/MeOH solution.
The structure of 1 (Fig. 1a) shows a square planar nickel(II) ion
coordinated to a dianionic tetradentate ligand. The NiꢀN1, NiꢀN2,
NiꢀN3 and NiꢀN4 bond distances (1.809(3), 1.807(2), 1.816(3) and
1.807(3) Ǻ, respectively) are consistent with a diamagnetic lowꢀspin
configuration of the metal ion. The peripherical C1ꢀN1, C7ꢀN4 bond
lengths (1.343(4) and 1.344(4) Ǻ respectively) and C2ꢀN2, C8ꢀN3
bond lengths (1.366(4) and 1.364(4) Ǻ, respectively) are
significantly shorter than the central C13ꢀN2 and C14ꢀN3 ones
(1.423(4) and 1.416(4) Ǻ, respectively), but longer than those
bond (organicꢀbased redox hemilabile ligands, oRHLs) and induce
[6]
structural changes.
Unfortunately, in these cases rearrangements
are often limited to subtle modifications of the metal coordination
sphere (Scheme 1aꢀc), making molecular motions rather small. [8ꢀ10]
We herein describe two nickel(II) complexes 1 and 2 prepared from
a polydentate oꢀphenylenediamine ligand H4L (Fig. 1) and show that
2 is capable of large redoxꢀtriggered molecular rearrangements. The
process, under the form of a reversible ligand ejection affording 1, is
initiated by electronꢀtransfer from the oneꢀelectron oxidized oꢀ
phenylenediamine group.
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DOI: 10.1039J/Co3uCrCn4a8l5N49aHme
[12]
expected for C=N bonds. The central ring has thus a dianionic successive oxidation of the ISQ moieties
into
diamidobenzene character, whereas the peripherical rings are diiminobenzoquinone (IBQ) (Scheme 2).
[11]
diiminobenzosemiquinone (ISQ) radicals.
Noteworthy, the three
aromatic rings are coplanar: a strong antiferromagnetic coupling is
thus expected between the two radical spins, leading to the observed
diamagnetism.
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
N
N
N
N
N
N
+ 0.02 V
- 0.40 V
Ni
Ni
Ni
N
N
N
N
N
N
H
H
H
H
H
H
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
1
1+
12+
Scheme 2. Sequential oxidation of 1.
The CV curve of 2 contrast sharply with that of 1: Oxidation waves
were indeed observed at Epa,1 = ꢀ0.18, Epa,2 = ꢀ0.04 V, Epa,3 = 0.28 V
(Fig. 2b), but they were found to be surprisingly irreversible. This
prompted us to investigate the electrochemical behaviour of the free
ligand H4L (see ESI). H4L also showed irreversible oxidation waves
in this potential range (Ep = 0.02, Ep = 0.13 V, Ep = 0.36 V),
supporting the assignment of the first anodic waves of 2 to the
oxidation of the uncoordinated diaminobenzene moieties. The anodic
shift of the coordinatedꢀIBQ/ISQ redox wave when 2 is compared to
1, most likely results from the presence of Hꢀbonds between N2 and
H5 as well as N3 and H8.
Figure 1 X-Ray crystal structures of: (a) [NiL] (1) and (b) [Ni(H2L)2] (2) shown with
30% thermal ellipsoids. H atoms (
clarity.
a,1
a,2
a,3
1 and 2) and t-Bu groups (2) are omitted for
During the synthesis of 1 we made the intriguing observation that a
green solution formed instead of the brownish one in tenꢀtimes more
concentrated media. A simple filtration allowed us to isolate a green
solid 2 (yield 66 %), whose formulation clearly differs from that of 1
After exhaustive electrolysis at +0.21 V the CV curve of 2 was
dramatically different than the original one (Fig 2c); it displays a
single reversible bielectronic oxidation wave at E1/2ox,1 = 0.78 V, and
(Fig. 1b). Although we systematically worked with
a (1:1)
ligand:metal ratio 2 is constituted by a single metal coordinated to
two ligands coordinated in a bidentate fashion. The nickel(II) ion lies
in a square planar geometry, while the C1ꢀN1, C7ꢀN4, C2ꢀN2 and
C8ꢀN3 bond distances (1.343(3), 1.311(3), 1.359(3) and 1.362(3) Ǻ,
respectively) compare fairly with those obtained for 1, confirming
that 2 also contains a bis{ISQ} core. The nonꢀcoordinating rings
exhibit typical diaminobenzene metrical parameters. It is worth
noting that Hꢀbonds exist between N2 and H5 (and opposite N3 and
H8), as evidenced by the N2ꢀH5ꢀN5 and N3ꢀH8ꢀN8 bond angles of
110(3)° and 113(3)°, respectively, as well as the bond distances N2ꢀ
N5 and N3ꢀN8 of 2.780(3) and 2.782(3) Ǻ, respectively.
red,1
red,2
five reversible reduction wave at E1/2
= ꢀ0.03 V, E1/2
= ꢀ0.40
= ꢀ1.47 V.
corresponds to a bielectronic process, whereas the other
red,3
red,4
red,5
V, E1/2
= ꢀ0.62 V, E1/2
= ꢀ0.93 V and E1/2
red,3
E1/2
reduction waves are monoelectronic processes. The four
monoelectronic redox waves are reminiscent of 1 (or its oxidation
products). Owing to the electrolysis potential (+0.21 V), which is
higher than E1/2ox,2 of 1, the oxidation product is the cation 12+, a fact
confirmed by VisꢀNIR spectroscopy. Generation of 12+ implies that
free ligand is released in solution. A pure sample of H4L was thus
subjected to electrolysis under similar conditions. The CV curve
recorded after electrolysis shows two reversible twoꢀelectron redox
ox,1
red,3
waves at E1/2
= 0.79 V and E1/2
= ꢀ0.61 V (Fig. 2d). Both
match the bielectronic waves observed for 2 after oxidation at +0.21
V, confirming that oxidation of 2 is accompanied by release of free
oxidized ligand (Scheme 3).
(a)
(b)
t-Bu
t-Bu
NH2
N
(c)
(d)
H
t-Bu
t-Bu
N
t-Bu
H
t-Bu
+ 0.21 V
- 0.60 V
N
N
N
Ni
Ni
N
t-Bu
N
H
N
N
H
t-Bu
H2N
H
t-Bu
t-Bu
H
12+
N
t-Bu
+ oxidized ligand, 2 H+
-1.6
-0.8
0.0
0.8
t-Bu
E / V
2
Figure 2. Cyclic voltammetry curves of (a):
1
; (b):
2
; (c):
2
after electrolysis at 0.21
Scheme 3. Redox-driven structural changes in 2.
V; (d): H4L after electrolysis at 0.21 V. Concentrations are 0.5 mM (
0.25mM ( ) in CH2Cl2 solutions (+ 0.1 M TBAP). T = 298 K, scan rate = 0.1 V /s.
The potentials are given vs. the Fc+/Fc reference.
1) and
2
We additionally examined the oxidation processes by
spectroelectrochemistry (Fig. 3ꢀ4). The electronic spectrum of 1 is
characterized by an intense LLCT transition involving the ISQ
radical at 952 nm [12 050 Mꢀ1 cmꢀ1], which shifts to 899 nm [11 320
Mꢀ1 cmꢀ1] in 1+.[13] Not surprisingly the dication 12+ does not exhibit
strong ISQ radical band above 850 nm, but simply CT transitions
involving the IBQ moieties at 738 nm [7510 Mꢀ1 cmꢀ1] and 466 nm
The electrochemical behaviour of 1 and 2 has been investigated by
cyclic voltammetry (CV) (Fig. 2). 1 undergoes four reversible
monoelectronic transfers (Fig. 2a); two in oxidation at E1/2ox,1 = ꢀ0.40
V, E1/2ox,2 = 0.02 V and two in reduction at E1/2red,1 = ꢀ0.93 V, Ep
=
c,2
ꢀ1.57 V vs. Fc+/Fc. The oxidation processes are assigned to the
2 | Chem. Commun., 2013, 00, 1-3
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[26380 Mꢀ1 cmꢀ1]. Interestingly the LLCT transition in 2 is fourꢀ
times more intense than for 1 [877 nm, 55 160 Mꢀ1 cmꢀ1]. This
enhancement may reflect the involvement of the coordinated
diaminobenzene bridge in the transition for 1. During electrolysis of
2 the 877 nm band disappeared at the expense of two intense
transitions at 410 [26 100 Mꢀ1 cmꢀ1] and 494 nm [22 600 Mꢀ1 cmꢀ1],
and a broad band at ca. 700 nm (Fig. 4b). The latter two bands
correspond to 12+ (Fig. 3c), whereas the former originates from the
oxidized free ligand (see ESI). These spectroscopic results again
diiminosemiquinonate bonds to allow for redoxꢀtriggered
ligand ejection, while keeping rebinding possible. The effect of
the pH and substituents on the process are currently
investigated in our laboratory.
This research has been funded by a grant from the University
Joseph Fourier (fond d’intervention) and the Labex ARCANE.
Notes and references
support
a
redoxꢀdriven ligand ejection, consistent with
Département de Chimie Moléculaire ꢀ Chimie Inorganique Redox (CIRE)
ꢀ UMR CNRS 5250, Université Joseph Fourier, B. P. 53, 38041 Grenoble
cedex 9, France.
electrochemical data.
Email : fabrice.thomas@ujfꢀgrenoble.fr
40
30
20
†
Electronic Supplementary Information (ESI) available: CCDCꢀ
927235ꢀ6, experimental procedures, EPR, NMR, UVꢀVis spectra and
cyclic voltammetry curves. See DOI: 10.1039/c000000x/
1
J. W. Canary, S. Mortezaei, J. Liang, Coord. Chem. Rev. 2010, 254
2249; K. W. Plaxco, H. T. Soh, Trends. Biotechnol. 2011, 29, 1.
Y. L. Zhao, Z. Li, S. Kabehie, Y. Y. Botros, J. F. Stoddart, J. I. Zink,
J. Am. Chem. Soc. 2010, 132, 13016.
B. Y. Ahn, E. B. Duoss, M. J. Motala, X. Guo, S. I. Park, Y. Xiong,
J. Yoon, R. G. Nuzzo, J. A. Rogers, J. A. Lewis, Science 2009, 232, 1590.
B. L. Feringa, Molecular Switches. WileyꢀVCH: Weinheim,
Germany, 2001.
,
(a)
(b)
2
10
(c)
ε
3
0
400
600
800
1000
λ
/ nm
4
Figure 3. Vis-NIR spectra of (a)
CH2Cl2 (+ 0.1 M TBAP). T = 298 K.
1
, (b) 1+, (c) 12+. Concentrations = 0.5 mM in
5
S. Zahn, J. W. Canary, Science 2000, 288, 1404; B. Champin, P.
Mobian, J.ꢀP. Sauvage, Chem. Soc. Rev. 2007, 36, 358. G. Periyasamy, J.
P. Collin, J.ꢀP. Sauvage, F. Remacle, Chem. Eur. J. 2009, 15, 1310.
We monitored the reversibility of the system by applying to 2 the
required potential to eject one ligand (exhaustive electrolysis at
+0.21 V), and then a cathodic potential (ꢀ0.60 V). The VisꢀNIR
spectrum recorded just after electrolysis (Fig. 4c) was clearly
different than that of 2, with a maximum peak at 894 nm [18 460 Mꢀ1
cmꢀ1] and a shoulder at 950 nm. However, after 15 h the NIR band
has both shifted to 877 nm and increased in intensity (Fig. 4d),
giving spectral features reminiscent of 2. Thus, although the reaction
6
7
A. M. Allgeirer, C. A. Mirkin, Angew. Chem. Int. Ed. 1998, 37, 894.
For recent articles see: H. Li, A. C. Fahrenbach, A. Coskun, Z. Zhu,
G. Barin, Y. L. Zhao, Y. Y. Botros, J. P. Sauvage, J. F. Stoddart, Angew.
Chem. Int. Ed. 2011, 50, 6782. A. Iordache, M. Oltean, A. Milet, F.
Thomas, B. Baptiste, E. SaintꢀAman, C. Bucher, J. Am. Chem. Soc. 2012,
134, 2653; G. Barin, M. Frasconi, S. M. Dyar, J. Iehl, O. Buyukcakir, A.
occurs slowly the ejected ligand can rebind the nickel ion after A. Sarjeant, R. Carmieli, A. Coskun, M. R. Wasielewski, J. F. Stoddart, J.
reduction (Scheme 3). [14]
Am. Chem. Soc. 2013, 135, 2466.
8
S. M. Sembiring, S. B. Collbran, D. C. Craig, Inorg. Chem. 1995, 34,
761.
9
60
40
20
0
(d)
(a)
Z. He, S. B. Colbran, D. C. Craig, Chem. Eur. J. 2003, 9, 116.
10 R. Hubner, S. Weber, S. Strobel, B. Sarkar, S. Zalis, W. Kaim,
Organometallics 2011, 30, 1414.
11 D. Herebian, E. Bothe, F. Neese, T. Weyhermüller, K. Wieghardt, J.
Am. Chem. Soc. 2003, 125, 9116.
12 The electrochemically generated 1+ is paramagnetic (
(b)
(c)
ε
S
= 1/2). It
(c)
exhibits a slightly rhombic EPR signal at g1 = 1.994, g2 = 2.004, g3
=
2.021. These values are consistent with a main ligand radical character of
the singly occupied MO (M. Orio, O. Jarjayes, H. Kanso, C. Philouze, F.
Neese, F. Thomas, Angew. Chem. Int. Ed. 2010, 49, 4989).
400 600 800 1000 400 600 800 1000
λ
/ nm
Figure 4. Vis-NIR spectra of (a):
2
; (b): 2 after electrolysis at 0.21 V; (c-d): 2 after
electrolysis at 0.21 V followed by reduction at -0.60 V [(c) 5 min after reduction,
(d) after 30 min, 2 h and 15 h]. Arrows indicate spectral changes. Concentrations
= 0.25 mM in CH2Cl2 (+ 0.1 M TBAP). T = 298 K.
13 The NIR band of
1 and 2 is assigned to an IVCT transition involving
the peripherical ISQ moieties; It formally corresponds to the transfer of
an electron from one ISQ group to the other one, and could be formulated
as (ISQ)NiII(ISQ)→(phenylenediamine)NiII(IBQ); The NIR band of 1+
reflects the mixedꢀvalent character of this compound. It is ascribed to the
(ISQ)NiII(IBQ)→(IBQ)NiII(ISQ) transition. See ref. 11.
In conclusion we herein describe the first example of
electrochemically driven ejection of a redox hemilabile ligand
involving
intramolecular Hꢀbonds and the attenuated negative charge at
the terminal chelating atoms in labilize sufficiently the Mꢀ
diiminosemiquinonate
moieties.
Both
the
14 The potential of ꢀ0.60 V is not sufficient to reduce the oxidized free
ligand. The driving force for the conversion 12+
→ 2 is therefore not
2
electron transfer to the free ligand, but rather to the complex. This
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assumption is confirmed by the conversion of
H4L (see ESI).
1 into 2 upon addition of
4 | Chem. Commun., 2013, 00, 1-3
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