Extendable nickel complex tapes that reach NIR absorptions

Article abstract of DOI:10.1039/c4cc07353c

Stepwise synthesis of linear nickel complex oligomer tapes with no need for solid-phase support has been achieved. The control of the length in flat arrays allows a fine-tuning of the absorption properties from the UV to the NIR region.

Full text of DOI:10.1039/c4cc07353c

Volume 50 Number 96 14 December 2014 Pages 15111–15272
ChemComm
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COMMUNICATION
Denis Jacquemin, Olivier Siri et al.
Extendable nickel complex tapes that reach NIR absorptions

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Extendable nickel complex tapes that reach NIR
absorptions†
Cite this: Chem. Commun., 2014,
50, 15140
Hassib Audi,a Zhongrui Chen,a Azzam Charaf-Eddin,b Anthony D’Aleo,a
´
Gabriel Canard,a Denis Jacquemin*b and Olivier Siri*a
Received 17th September 2014,
Accepted 10th October 2014
DOI: 10.1039/c4cc07353c
www.rsc.org/chemcomm
Stepwise synthesis of linear nickel complex oligomer tapes with p*-orbitals of quinoid ligands can mix extensively with the
no need for solid-phase support has been achieved. The control of valence d-orbitals of a metal center, giving complexes in which
the length in flat arrays allows a fine-tuning of the absorption the p-electrons could be fully delocalized over both metal
properties from the UV to the NIR region.
and ligand.13,14
Although oxocarbon of type 1 is probably the most used quinoid
Discrete oligomers with a very long p-conjugated path are of major ligand (more than 200 complexes based on mono-, dinuclear
interest because of both fundamental aspects and applications in compounds, and polymers),15 its stepwise metallation leading to
a large panel of areas ranging from energy and bio-imaging to complex oligomer tapes has not been described. Surprisingly, the
sensing and advanced optoelectronics.1–3 Numerous attempts corresponding nitrogenated ligand 2 has been much less investi-
have been made to achieve a full control of the p-electronic length gated since only an article of Lever et al. reported its use as ligand
and beside the random and self-organization preparations, the (prepared in situ).16 The metallation of 2 with a ruthenium pre-
stepwise synthesis of oligomers appears to be a unique method cursor did not afford the corresponding (and expected) dinuclear
for a full control over the chain length. However, if this elegant complex but instead an oxidized species 3 in which the carbonyl
method could be successfully applied in organic chemistry to the functions impede the communication between the two metal
synthesis of organic oligomers (carbon–carbon bond formation),4–8 centers. This shortcoming can be explained by the strongly limited
the access to oligonuclear complexes by coordination chemistry access to 2 owing to its in situ preparation in solution,16,17 or
(metal–ligand assembly) encountered dramatic limitations espe- deposition by sublimation on a CsI plate cooled by closed-cycle
cially for higher oligomers (e.g., synthetic inaccessibility, chemical refrigeration at 15 K.18 In contrast, N-substituted analogues of
instability, difficult purification steps and/or poor solubility). type 4 (R = alkyl or aryl) have attracted much more interest as
As such, these latter could be prepared in a stepwise manner only their easy access paved the way to their use as ligands for the
by solid-assisted syntheses involving electrode, metal surface or preparation of dinuclear complexes 5 with a high degree of
insoluble matrix.9–12 Among them, the use of resins appears to be electronic delocalization.19–21
the only method which allows reactions of coordination and the
release of the final products.9
An efficient strategy for maximizing the conjugation is to hold
the p-systems coplanar by assembling repeated metal–ligand
units as a tape-like array. In this approach, the ligand design
is a crucial factor and previous studies reported that low-lying
a
´
CINaM, UMR 7325 CNRS, Aix-Marseille Universite, Campus Luminy, case 913,
13288 Marseille Cedex 09, France. E-mail: olivier.siri@univ-amu.fr
b
´
`
CEISAM UMR CNRS 6230, Universite de Nantes, 2, rue de la Houssiniere – BP
92208 – 4322 NANTES Cedex 3 (F) and Institut Universitaire de France, 103 blvd
St Michel, 75005 Paris Cedex 05, France. E-mail: Denis.Jacquemin@univ-nantes.fr
Importantly, no metal complex oligomers based on 4 could
be attained presumably due to the steric hindrance of the
N-substituents. Consequently, we decided to reinvestigate the
isolation of 2 to study in detail its coordination chemistry
(metal–ligand assembly) since the absence of N-substituents
† Electronic supplementary information (ESI) available: Experimental details and
characterization data of the molecules are described with the spectrophysical
studies and quantum mechanical calculations. CCDC 1011676. For ESI and
crystallographic data in CIF or other electronic format see see DOI: 10.1039/
c4cc07353c
15140 | Chem. Commun., 2014, 50, 15140--15143
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should favour the formation of monodisperse nickel complex
tapes with an extension of the p-delocalization.
Herein, we report the isolation and full characterization of 2 and
its use as a ligand in the stepwise synthesis of di-, tri-, tetra- and
pentanuclear complexes 7–10. The proposed synthetic procedure
allows the purification of the oligomers by simple filtration with no
need for solid-phase support. Spectro-photophysical and theoretical
studies revealed electronic absorption bands in specific regions
from UV to NIR depending on the size of the oligomers.
The synthesis of 2 was based on the venerable Nietzki’s work
(1887)17 who mentioned spontaneous air oxidation of the electron-
rich tetraaminobenzene in water 6 yielding the 2,5-diamino-1,4-
benzoquinonediimine 2 (Scheme 1). When the same reaction was
carried out in methanol, 2 was directly precipitated as a brown solid
in 79% yield, preventing its reaction with 6 and/or other oxidized
species. The 1H NMR spectrum of 2 showed only one quinoidal
C–H signal at 5.44 ppm because of a fast intramolecular
concerted double proton transfer involving two degenerate
tautomers which exhibit in solution an average structure of
higher symmetry (see the ESI†).22 The metallation of 2 with
[Ni(acac)2] (2 equiv.) in toluene at room temperature (Scheme 2)
afforded the dinickel complex 7 as a green crystalline solid
(54% yield). When the same reaction was carried out in THF,
the trinuclear compound 8 was precipitated and isolated by
simple filtration as a violet solid (77% yield). Note that 8 could
be also directly obtained from 7 in THF without any additional
reagent. This observation clearly proved that two molecules of 7
evolve in solution into one molecule of 8 and one molecule of
[Ni(acac)2]. When 8 was dissolved in DMSO, an unprecedented
pentanuclear complex 9 could be isolated by precipitation (89%
yield), its formation occurring via a similar process (two mole-
cules of 8 give one of 9 and one of [Ni(acac)2]). The formation of
9 from 2 could be also obtained in DMSO confirming the key
role of the solvent (i.e. polarity, solubility). Thereby, the oligo-
merization completely stops when the solubility limit is
attained, leading to precipitation of the longest nickel complex
from a mixture of oligomers (see ESI†). It is noteworthy that a
tetranuclear oligomer 10 could be similarly observed (but not
isolated) from 7 and 8 (1 : 1 ratio) in DMSO. Thus, a fine-tuning
of the length becomes possible depending on the nature of the
solvent but also of the building blocks.
Scheme 2 Synthesis of oligomers 7–10.
that can be explained by a highly delocalized p-system (see Fig. 1a
and calculations below).19 X-ray diffraction study on single crystals of
7 confirmed its centrosymmetry (Fig. 2). The dianion derived from 2
acts as a tetradentate bridging ligand in a bis(chelating) fashion and
the geometry around the metals is square planar with the nickel
being quasi in the molecular plane of 7 (mean plane deviation of
0.059 and 0.002 Å for Ni(2) and Ni(1), respectively). The NiÁÁÁNi
separation of 7.65 Å is in the line with that found in a similar
dinuclear nickel complex containing a ligand of type 2.19–21,23
Examination of the bond distances within the N(1)–C(1)–C(2)–
C(3)–N(3) and N(2)–C(6)–C(5)–C(4)–N(4) moieties is consistent
with an extensive p-electronic delocalization, but with no
significant conjugation between the two 6p-subsystems since
the C(1)–C(6) and C(3)–C(4) distances are typical of single
bonds (1.498(3) and 1.494(3) Å, respectively).9
The 1H NMR spectrum of 7 showed one quinoidal C–H
signal at 4.49 ppm consistent with a centrosymmetric molecule
Molecule 8 was experimentally characterized using NMR
spectroscopy and high-resolution mass spectrometry (HRMS)
(see the ESI†).
The HRMS spectrum of 8 shows a peak at 641.0213 mass/
charge ratio (m/z) [(M + H)+] as expected for C22H22N8O4Ni3.
Scheme 1 Synthesis of aminobenzoquinonediimine 2.
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Fig. 3 UV-vis-NIR absorption spectrum of 2 and its oligomers. Colour
code: 2 in CH2Cl2 (orange), 7 in CH2Cl2 (blue), 8 in THF (red), 10 in DMSO
(green)* and 9 in DMSO (violet)*. (*) derived from a mixture in solution.
spectra of 7 and 8 exhibit broad bands peaking at 576 and
627 nm, respectively (Fig. 3).
Fig. 1 1H NMR spectrum of: (a) 7 (n = 1) in CDCl3, (b) 8 (n = 2) and (c) 10
(n = 3) in DMSO-d6. The range 4 o d o 0 ppm has been omitted for clarity.
Colour code: red (acac C–H proton), blue (quinoidal C–H proton).
The pronounced red shift of the absorption of 8 with respect
to that of 7 results from an extension of the delocalization of
the conjugated p-system. The electronic absorption spectra of
the trimer 10 and of the tetramer 9 peaking at labs = 693 and
757 nm, respectively, confirmed this hypothesis. As expected,
extension of the tape length increases the bathromic shift,
allowing a fine-tuning of the absorption in the entire visible
region as well as in the NIR region.
In order to gain more insights into the relationship between
the electronic spectrum and the p-electron delocalization in
these compounds, theoretical calculations were performed on
7–10 (see the ESI† for details). These studies show that the
optimal structures are either perfectly planar (7), or very close to
planarity. For instance, the longest tape 9 belongs to the D2 point
group, but the deviation from planarity is small suggesting that
crystal packing effects might yield perfectly flat compounds
(Fig. 4). The DFT-computed HOMO–LUMO gap (HLG) for 7, 8,
10, 9 are 3.22, 2.81, 2.63 and 2.55 eV, confirming the constant
decay of the gap with increasing size (see ESI†). More impressive
is the increase of the isotropic polarisability of values 460, 807,
1179 and 1558 a.u. for the same four compounds, respectively,
highlighting the strong increase of p-delocalization. The TD-DFT
calculations on the solvated molecules reveal for all one strongly
dipole-allowed S0–S1 transition in the visible domain. The delta
density plots can be found in Fig. 5 for the four members of the
series. It is obvious that the excited state corresponds to a typical
p–p* transition with alternating regions of gain/depletion of
density, the single-bonds separating the two sub-structures
(e.g., C(4)–C(3) and C(1)–C(6) in 7) gaining electron density
(mostly in red) after absorption, as expected. One notices that
the excited-state spreads over several units, so that one needs to
reach rather long tapes to achieve convergence of the transition
Fig. 2 Structure of 7 (anisotropic displacement parameters at 50%):
top view and side view. Selected bond lengths (Å): N(4)–C(4) = 1.319(3),
C(4)–C(5) = 1.394(3), C(5)–C(6) = 1.393(3), C(6)–N(2) = 1.320(3), N(1)–C(1) =
1.318(3), C(1)–C(2) = 1.379(3), C(2)–C(3) = 1.386(3), C(3)–N(3) = 1.316(3),
C(1)–C(6) = 1.498(3), C(3)–C(4) = 1.494(3).
1
Its H NMR spectrum showed one olefinic C–H signal at 4.63 ppm
integrating 4 protons (the two acac C–H protons are used as a
standard reference since I = 2 in all cases, see Fig. 1b). The formation
of the tetranuclear complex 10 from 7 and 8 in solution could be
1
followed by the H NMR spectrum (see the ESI†) that revealed the
presence of a broad signal around 4.75 ppm integrating for 6 C–H
quinoidal protons (I = 6H). Unfortunately, its isolation as a solid could
not be achieved because of its rapid co-precipitation with 9 that is also
formed during the reaction between 7 and 8 (1 : 1 ratio) in DMSO. In
contrast, pentanuclear complex 9 could be isolated (directly from 8) by
precipitation in DMSO as a blue solid. Its characterization could be
carried out only in the solid state (elemental analysis and solid NMR
using 8 as the reference, see the ESI†) owing to its very poor solubility.
Ligand 2 shows an intense absorption band at 317 nm (Fig. 3) that
is assigned to intraquinone transitions.13 Its stepwise metallation
could be followed in DMSO by UV-Vis-NIR absorption, indicating the
successive 7 - 8 - 9 and/or 10 conversion (see ESI†). The absorption
Fig. 4 DFT optimal structure of the pentanuclear complex 9.
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OS acknowledges the Centre National de la Recherche
´
Scientifique, the Ministere de la Recherche et des Nouvelles
Technologies for financial support and PhD grant to ZC. DJ
acknowledges the European Research Council (ERC) and the
´
Region des Pays de la Loire for financial support in the frame-
work of a Starting Grant (Marches-278845) and a recrutement
´
sur poste strategique, respectively. This research used resources
of (1) the GENCI-CINES/IDRIS, (2) CCIPL (Centre de Calcul
Intensif des Pays de Loire), (3) the local Troy cluster acquired
´
thanks to Region des Pays de la Loire.
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be reached with longer tapes.
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of mono- di- tri- and tetramers 7–10 with extension of the
p-delocalization. The synthetic accessibility – based on the
unique behaviour of ligand 2 and the key role of the solvent
(oligomerization completely stops when the solubility limit is
attained) – is highly efficient because of a purification step by simple
filtration with no need for solid-phase support. Importantly, the
possibility of controlling the length of the oligomers allows a fine-
tuning of the absorption properties from the UV to the NIR region.
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This journal is ©The Royal Society of Chemistry 2014
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