Cationic allyl nickel complexes containing N,O-donor labile ligands: Synthesis and molecular characterisation

Article abstract of DOI:10.1016/j.molstruc.2019.01.045

Three 2-iminofuran ligands 2-(N-2,6-diisopropylphenylformimino)furan (L1), 2-(N-2,6-dimethylphenylformimino)furan (L2) and 2-(N-4-methoxyphenylformimino)furan (L3) were obtained by condensation reactions of the respective anilines with furfural. The η

Full text of DOI:10.1016/j.molstruc.2019.01.045

Accepted Manuscript
Cationic allyl nickel complexes containing N,O-donor labile ligands: Synthesis and
molecular characterisation
Marwa Belkhiria, Ali Mechria, Tiago F.C. Cruz, Clara S.B. Gomes, Moncef Msaddek,
Pedro T. Gomes
PII:
S0022-2860(19)30055-9
DOI:
https://doi.org/10.1016/j.molstruc.2019.01.045
MOLSTR 26096
Reference:
To appear in: Journal of Molecular Structure
Received Date: 6 November 2018
Revised Date: 10 January 2019
Accepted Date: 11 January 2019
Please cite this article as: M. Belkhiria, A. Mechria, T.F.C. Cruz, C.S.B. Gomes, M. Msaddek,
P.T. Gomes, Cationic allyl nickel complexes containing N,O-donor labile ligands: Synthesis and
molecular characterisation, Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/
j.molstruc.2019.01.045.
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ACCEPTED MANUSCRIPT
Graphical abstract
3
Three new cationic η -allyl Ni(II) complexes bearing 2-(N-arylformimino)furan N,O-donor
ligands have been synthesised. The complexes have been characterised by FT-IR, NMR
spectroscopy and elemental analysis. The structure of complex 2 has also been determined by
single crystal X-ray diffraction.

ACCEPTED MANUSCRIPT
Cationic allyl nickel complexes containing N,O-donor labile
ligands: Synthesis and molecular characterisation
a
a,
b
b
a
Marwa Belkhiria , Ali Mechria *, Tiago F. C. Cruz , Clara S. B. Gomes , Moncef Msaddek ,
b,
Pedro T. Gomes *
a
Laboratoire de Chimie Hétérocyclique, Produits Naturels et Réactivité: L.C.H.P.N.R, Faculté des Sciences de
Monastir, Bd de l'Environnement, 5000 Monastir, Tunisia.
b
Centro de Química Estrutural, Departamento de Engenharia Química, Instituto Superior Técnico, Universidade
de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal.
*
Corresponding authors: pedro.t.gomes@tecnico.ulisboa.pt; ali.mechria@fsm.rnu.tn
Abstract:
Three 2-iminofuran ligands 2-(N-2,6-diisopropylphenylformimino)furan (L1), 2-(N-2,6-
dimethylphenylformimino)furan (L2) and 2-(N-4-methoxyphenylformimino)furan (L3) were
obtained by condensation reactions of the respective anilines with furfural.
3
3
The η -allyl Ni(II) cationic complexes 1-3 have been synthesised by reacting [Ni(η -C H )(µ-
3
5
Br)] with the corresponding ligands L1-L3, in the presence of TlPF . All complexes have
2
6
been characterised by FT-IR, NMR spectroscopy and elemental analysis. The molecular
structure of complex 2 has been determined by single crystal X-Ray diffraction. Complexes 1
and 2 have displayed a mono bidentate chelation mode of the respective ligand, while
complex 3 showed a bis monodentate coordination framework. The respective complexes
were studied by DFT calculations, showing a tendency to form the bis monodentate species,
the formation of complex 3 being clearly the most favoured one.
3
Keywords: η -allyl complexes; cationic nickel(II) complexes; N,O-chelates; hemilabile
ligands; DFT calculations

ACCEPTED MANUSCRIPT
1
. Introduction:
Most of the known bidentate complexes contain rather rigid ligand frameworks, which are
intended to yield increased stabilities. On the other hand, a more flexible ligand backbone
would allow a subtler interplay between lability and stability, which may be beneficial for
some types of catalytic applications. Compounds containing N,O-donors can act as potentially
bidentate type ligands, in general displaying high versatility in their design by the introduction
of several kinds of steric and electronic features, each of the donor moieties exhibiting
specific binding characteristics. N-donor ligands coordinate well to smaller transition metal
ions such as V, Cr, Mn, Fe, Co, Ni, Cu and Zn. On the other hand, O-donor ligands tend to
coordinate to metal ions such as the larger alkali, alkaline earth metal ions and large post
transition metal ions [1-4].
Schiff base complexes derived from heterocyclic compounds have found an increased
interest in the context of bioinorganic chemistry, where complexes having simultaneously
nitrogen and oxygen donor atoms have been reported by several research groups [5–8], in
addition to their applications in the fields of luminescence [9,10] and catalysis [11, 12].
Nickel(II) complexes bearing S,S, S,O, and O,O chelating ligands have also been applied in
catalysis, demonstrating high catalytic activity for olefin oligomerisation. However, the
product distribution was difficult to control and commercialisation could not be achieved. In
contrast, the introduction of a nitrogen group into the bidentate O containing chelate Ni
systems allowed the steric properties to be varied and resulted in almost 100% selectivity for
linear hexenes with a high proportion (60-80%) being α-olefins [13]. The furan ring, as an
important part of the ligand, has been studied and attracted great attention for a long time,
significant progress being made in understanding the structure of its complexes [14–16].
Complexes of some transition metals with Schiff bases derived from furan-2-carboxaldehyde
and o-phenylenediamine [17–19], hydrazine hydrate [20], 3,3’-diaminobenzidene [21], 3-
aminodibenzofuran [22] and o-toluidine [23] were studied. Furfural Schiff bases were found
to show selectivity towards anions, allowing them to act as anion-sensitive membrane
electrodes [24], and also display nitrification inhibitory properties [25].
The common methodology for the preparation of allyl-nickel complexes consists in
3
opening the dimeric structure of [Ni(η -C H )(µ-Br)] through the addition of the appropriate
3
5
2
ligands, which leads to neutral complexes. The halide is then abstracted by thallium salts,
3
giving rise to cationic η -allyl nickel(II) complexes having a non-coordinating anion.
Recently, we employed this methodology to attempt the synthesis of Ni(II) complexes bearing
3
chelating N,S-thienylimine ligands. The reactions of [Ni(η -C H )(µ-Br)] with either one or
3
5
2
2

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two equivalents of the appropriate ligand L, in the presence of TlPF , in dichloromethane,
6
3
afforded always the corresponding complexes [Ni(η -allyl)L ][PF ], containing two
2
6
monodentate thienylimine ligands coordinated to Ni exclusively through the imine nitrogen
atoms [26], a result explained by the soft character of the sulphur donor towards the hard
nature of the nickel ion.
3
Following these results, we decided to apply this methodology to the preparation of η -
allyl Ni(II) cationic complexes containing N,O-donor ligands. Herein, we report the synthesis
and characterisation of new cationic allyl nickel(II) complexes bearing one chelating or two
monodentate 2-(N-arylformimino)furan N,O-donor ligands and the molecular structure of one
of the nickel(II) complexes containing a chelating ligand, as determined by X-ray diffraction.
2
. Experimental
2
.1. General Considerations
All experiments dealing with air and moisture-sensitive materials were carried out under an
inert atmosphere using a dual vacuum/nitrogen line and standard Schlenk techniques.
Nitrogen gas was supplied in cylinders by Air Liquide and purified by passage through 4 Å
molecular sieves. Unless otherwise stated, all reagents were purchased from commercial
suppliers (e.g., Acrös, Aldrich,) and used without further purification. All solvents to be used
under inert atmosphere were thoroughly dehydrated over a suitable drying agent (sodium for
n-hexane, and CaH for dichloromethane) and refluxed under nitrogen before use. Solvents
2
and solutions were transferred using a positive pressure of nitrogen through stainless steel
cannulae.
The deuterated solvent CD Cl used was dried by storage over 4 Å molecular sieves and
2
2
degassed by the freeze-pump-thaw method, and stored under nitrogen. NMR spectra were
1
13
recorded on a Bruker Avance III 300 spectrometer ( H, 300.130 MHz; C, 75.468 MHz),
1
1
being referenced internally to the residual H solvent resonances ( H) and the solvent
1
3
19
31
resonance ( C) relative to tetramethylsilane (δ=0); F, and P were referenced externally
using CFCl (δ=0) and H PO 85% (δ=0), respectively. Infrared spectra were recorded using
3
3
4
KBr pellets on a Jasco FT/IR 430. Elemental analyses were obtained from the Laboratório de
Análises do IST (Fisons Instrument 1108).
2
.2. General procedure for the synthesis of ligands L1-L3
The synthesis of the ligands was adapted from a previously reported method [27].
Stoichiometric amounts of furan-2-carboxaldehyde, the respective aniline (52.07 mmol) and a
3

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catalytic amount of formic acid were combined in ethanol and the mixture was reacted for 4 h,
at room temperature. The yellow-orange precipitates obtained were filtered, washed with cold
2
-propanol and dried under vacuum. Recrystallisation of the respective solids from cold 2-
propanol afforded the title ligands.
2
.2.1. Synthesis of ligand L1
Ligand L1 was prepared as described in the general procedure above using furan-2-
carboxaldehyde (5 g, 52.07 mmol) and 2,6-dimethylaniline (6.30 g, 52.07 mmol).
-1
1
Yield: (6.42 g, 62%). IR [ν(cm )]; 1634 (C=N). H NMR (CDCl ): δ 2.17 (s, 6H, CH ), 6.57-
3
3
1
3
1
6
1
.95 (m, 3H, Harom), 7.07-7.63 (m, 3H, Hfuran), 8.01 (s, 1H, CH=N). C{ H} NMR (CDCl ): δ
3
7.71, 111.47, 114.81, 123.33, 127.53, 145.00, 150.37, 151.28.
2
.2.2. Synthesis of ligand L2
Ligand L2 was prepared as described in the general procedure above using furan-2-
carboxaldehyde (5 g, 52.07 mmol) and 2,6-diisopropylaniline (9.22 g, 52.07 mmol).
-1
1
Yield: (10.89 g, 82%). IR [ν(cm )]; 1627 (C=N). H NMR (CDCl ): δ 1.19 (s, 12H, iPr-CH ),
3
3
1
3
1
3
.02 (sept, 2H, H ), 6.58-6.99 (m, 6H, Harom+furan), 7.99 (s, 1H, CH=N). C{ H} NMR
iPr
(
CDCl ): δ 23.07, 27.37, 111.50, 122.52, 124.03, 137.39, 145.06, 150.01, 151.25.
3
2
.2.3. Synthesis of ligand L3
Ligand L3 was prepared as described in the general procedure above using furan-2-
carboxaldehyde (5 g, 52.07 mmol) and 4-methoxylaniline (6.40 g, 52.07 mmol).
-1
1
Yield: (5.96 g, 57%). IR [ν(cm )]; 1622 (C=N). H NMR (CDCl ): δ 3.81 (s, 3H, O-CH ),
3
3
6
.54 (s, 1H, H ), 6.91 (s, 2H, Harom), 6.93 (s, 1H, Hfuran), 7.24 (d, 2H, Harom), 7.59 (s, 1H,
3
1
3
1
Hfuran), 8.30 (s, 1H, CH=N). C{ H} NMR (CDCl ): δ 54.96, 114.94, 111.57, 113.93, 121.77,
3
1
43.75, 144.79, 145.24, 151.86, 157.99.
2
.3. General procedure for the synthesis of complexes 1-3
3
A Schlenk tube was charged with [Ni(η -C H )(µ-Br)] and the respective ligand and the
3
5
2
mixture was dissolved in CH Cl (ca. 20 mL) and cooled to 0 °C. TlPF was added to the
2
2
6
mixture, which was stirred at room temperature for 60 min. After the reaction, the solution
®
was filtered through Celite and the solvent removed under vacuum. The orange-red residue
was washed with n-hexane (3×20 mL) and further recrystallised from a CH Cl /n-hexane (1:3
2
2
4

ACCEPTED MANUSCRIPT
v/v) solution cooled to -20 ºC. The complexes were obtained as orange-red powders or
crystals.
2
.3.1. Synthesis of complex 1
Complex 1 was prepared using the method described above using [Ni(η -C H )(µ-Br)]
2
3
3
5
(
0.088 g, 0.25 mmol), ligand L1 (0.099 g, 0.5 mmol) and TlPF (0.174 g, 0.5 mmol).
6
Yield: (0.37 g, 83%). Found (Calc.) for (C H F NNiOP) .CH Cl : C, 41.12 (41.54); H, 3.51
1
6
18
6
3
2
2
-1
1
(
3.98); N, 2.54 (2.97). IR [ν(cm )]; 1635 (C=N), 866 (PF ). H NMR (CD Cl ): δ 2.32 (br,
6 2 2
3
3
6
H, CH ), 2.48 (br, 2H, η -allylCH , overlapping with CH ), 3.06 (br, 2H, η -allyl CH ), 5.77
3 2 3 2
3
(
br, 1H, η -allyl CH), 6.41 (br, 1H, Hfuran), 7.21 (br, 3H, Haromatic+furan), 7.60 (br, 1H, Hfuran),
1
9
1
-
31
1
8
.2 (br, 1H, N=CH). F{ H} NMR (CD Cl ): δ -71.02 (d, PF ). P{ H} NMR (CD Cl ): δ -
2 2 6 2 2
-
1
45.01 (h, PF ).
6
2
.3.2. Synthesis of complex 2
3
Complex 2 was prepared using the method described above using [Ni(η -C H )(µ-Br)]
2
3
5
(
0.088 g, 0.25 mmol), ligand L2 (0.099 g, 0.5 mmol) and TlPF (0.174 g, 0.5 mmol). Single
6
crystals suitable for X-ray analysis were obtained from a CH Cl solution double-layered with
2
2
n-hexane (1:3 v/v), at -20 ºC.
Yield: (0.44 g, 89%). Found (Calc.) (C H F N NiO P) .CH Cl : C, 41.46 (41.36); H, 4.80
2
0
26
6
2
4
3
2
2
-1
1
(
4.55); N, 4.78 (4.74). IR [ν(cm )]; 1636 (C=N), 800 (PF ). H NMR (CD Cl ): δ 1.30 (br,
6
2
2
3
3
1
3
2H, iPr CH ), 1.44 (br, 2H, η -allyl CH ), 2.63 (br, 2H, iPr CH), 3.06 (m, 2H, η -allyl CH ),
3 2 2
3
.27 (br, 2H, iPr CH), 6.05 (m, 1H, η -allyl CH), 6.94 (br, 1H, Hfuran), 7.25-7.40 (m, 6H,
1
9
1
Haromatic), 7.57 (br, 1H, Hfuran), 7.86 (br, 1H, Hfuran), 8.12 (br, 1H, N=CH). F{ H} NMR
-
31
1
-
(
CD Cl ): δ -71.34 (d, PF ). P{ H} NMR (CD Cl ): δ -144.68 (h, PF ).
2 2 6 2 2 6
2
.3.3. Synthesis of complex 3
3
Complex 3 was prepared using the method described above using [Ni(η -C H )(µ-Br)]
2
3
5
(
0.088 g, 0.25 mmol), ligand L3 (0.092 g, 0.5 mmol) and TlPF (0.174 g, 0.5 mmol).Yield:
6
(
0.27 g, 42%). Found (Calc.) for (C H F N NiO P) .CH Cl : C 48.79 (48.38); H, 3.83
2
7
30
6
2
4
3
2
2
-1
1
(
4.56); N, 4.01 (4.13). IR [ν(cm )]; 1622 (C=N), 837 (PF ). H NMR (CD Cl ): δ 2.68 (m,
6 2 2
3
3
3
2
6
H, η -allyl CH ), 3.48 (br, 2H, η -allyl CH ), 3.74 (s, 6H, OCH ), 6.08 (m, 1H, η -allyl CH),
2 2 3
.63 (br, 2H, Hfuran), 6.75-6.79 (d, 4H, Haromatic), 7.00-7.10 (m, 6H, Haromatic+furan) 7.84 (m, 4H,
1
9
1
-
31
1
Hfuran + N=CH). F{ H} NMR (CD Cl ): δ -72.31 (d, PF ). P{ H} NMR (CD Cl ): δ -
2
2
6
2
2
-
1
44.17 (h, PF ).
6
5

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2
.4. X-ray crystallography
Crystals of complex 2 were selected under an inert atmosphere, covered with polyfluoroether
oil, and mounted on a nylon loop. Crystallographic data for compound 2 was collected using
graphite monochromated Mo-Kα radiation (λ=0.71073 Å) on a Bruker AXS-KAPPA APEX
II diffractometer equipped with an Oxford Cryosystem open-flow nitrogen cryostat at 150 K.
Cell parameters were retrieved using Bruker SMART software and refined using Bruker
SAINT on all observed reflections. Absorption corrections were applied using SADABS [28],
structure solution and refinement were performed using direct methods with the programs
SIR2014 [29], included in the package of programs WINGX-Version 2014.1 [30] and
SHELXL [31]. All hydrogen atoms were inserted in idealised positions and allowed to refine
riding on the parent carbon atoms. Data was deposited in the CCDC under deposit no.
1
585090 for compound 2.
Table 1. Crystal data and structure refinement for complex 2.
Chemical formula
Crystal size (mm )
Formula weight
Crystal system
Space group
a (Å)
C H Cl F N Ni O P
41 54 2 12 2 2 2 2
3
0.1ꢀ0.1ꢀ0.2
1085.12
Monoclinic
C2/c
27.0269(18)
10.3375(7)
17.5800(12)
90
b (Å)
c (Å)
α (º)
β (°)
105.467(3)
90
γ (º)
V (Å )
Z
3
4733.8(6)
8
T (K)
150
1.523
34954
3429
0.0501
51.316
0.71073
1.060
-
3
ρcalc (g cm )
Total reflections
Unique reflections
Rint
2
θmax (°)
λ (Å)
-
1
ꢀ
(mm )
h,k,lmax
θmax
F(000)
32,12,21
25.680
2232
Goodness-of-fit
R (reflections)
1.102
0.0682
0.1987
1.412
wR (reflections)
2
3
Dqmax, eÅ
Dqmin, eÅ
3
-0.921
6

ACCEPTED MANUSCRIPT
2
.5. DFT calculations
All calculations were performed using the Gaussian 09 software package [32] and the
PBE1PBE functional [33]. The geometry optimisations were accomplished without symmetry
constraints using a standard 6-31G** basis set [34] for all atoms except for nickel, that used a
LanL2DZ basis set [35] with a f-polarisation function [36]. Frequency calculations were
performed to confirm the nature of the stationary points, yielding one imaginary frequency for
the transition states and none for the minima. The electronic energies obtained were converted
to free energies at 298.15 K and 1 atm by using zero-point energy and thermal energy
corrections based on structural and vibration frequency data calculated at the same level.
3
. Results and discussion
The ligands were synthesized from the condensation reaction of furan-2-carboxaldehyde
and the respective anilines in the presence of a catalytic amount of formic acid, in ethanol for
h, at room temperature (Scheme 1). Variations in the amine allowed the facile design of
4
ligands with different physical and chemical properties [37,38].
Scheme 1. Synthesis of the ligands L1-L3.
3
The new cationic η -allyl Ni(II) complexes 1-3 were prepared via the metathetic exchange
3
reaction of [Ni(η -C H )(µ-Br)] with TlPF , in the presence of the respective ligands L1-L3,
3
5
2
6
in CH Cl at 0 °C, in moderate yields (Scheme 2). The new Ni(II) complexes 1-3 were
2
2
isolated as red solids, which are soluble in soluble in CH Cl .
2
2
Both the ligands and the complexes were characterised in the solid state by FT-IR
spectroscopy, their spectra being shown in Figures S1-S6 of the Supplementary Material. The
IR spectra of the complexes were assigned by comparison with the IR spectra of the ligands,
-1
the complexes exhibiting a typical ν(C=N) band between 1622 to 1634 cm [39]. The
absorption wavenumbers do not change significantly upon coordination, meaning that π-back
7

ACCEPTED MANUSCRIPT
bonding from the metal to the ligands is not significant possibly due to low chelating strength.
-1
A strong absorption band is exhibited at approximately 800-866 cm , for all three
Scheme 2. Synthesis of the complexes 1-3.
complexes, corresponding to the characteristic fingerprint stretching vibration of the non-
-
-1
coordinated PF counter-ion [40]. For complex 2, weak bands are observed at 596 cm (ν(Ni-
6
-1
N)) [41] and at 414 cm (ν(Ni-O)) [42].
The three complexes were characterised in solution by NMR spectroscopy, the respective
1
spectra being consistent with their formulation. In general, the H NMR spectra of the
complexes (Figures S13-S15 of the Supplementary Material) are somewhat broad. Although
the compounds were analytically pure, they showed a relatively thermally unstable behaviour
in solution, leading to a slight degree of decomposition, possibly giving rise to paramagnetic
Ni impurities, which may a reason for the slightly broad features of the resonances. The
thermally instability is likely justified by the overall low donor ability and hemilability of the
potentially bidentate ligands. Contrarily to the observed with the analogous N,S thienylimine
ligands [26], which behave exclusively as monodentate ligands coordinated through the N-
atom to nickel, complexes 1 and 2 exhibit the expected Ni:ligand ratio of 1:1 for a species
containing a chelating N,O 2-iminofuran ligand. Conversely, a 1:2 ratio was found for
1
complex 3. The H NMR of 1 and 2 reveal changes in the chemical shifts of CH=N, which
were only slightly downfield shifted in relation to the free ligands by 0.1 ppm [43], whereas
the CH=N of 3 is upfield shifted from 8.30 to 7.84 ppm after complexation to nickel. In all
8

ACCEPTED MANUSCRIPT
complexes, the iminic proton resonance was observed at 8.20 and 8.12 for complexes 1 and 2,
-
respectively, and 7.84 for complex 3. The presence of the non-coordinated PF counter-ions
6
1
9
1
31
1
in complexes 1-3 was also confirmed by the respective F{ H} and P{ H} NMR spectra.
Because of the harder nature of the O-atom of the ligands employed in this work, in
relation to the S-atom of the previously reported thienylimine ligands [26], its affinity for
nickel is higher and the bulkier N,O-ligand precursors L1 and L2 behave as bidentate towards
the metal. However, the expected higher electron density at the N-atom in ligand precursor L3
(which has a para-substituted methoxy donor group), the lower stereochemical protection of
this ligand and a possibly weak/labile chelation of its furan moiety are likely sufficient to
thermodynamically favour the coordination of two N-monodentate 2-(N-4-
methoxyphenylformimino)furan (L3) ligands, leading to the formation of complex 3. In
summary, depending on their stereochemical and electronic natures, these ligands can
coordinate with 1:1 or 1:2 metal to ligand ratios with or without oxygen-to-metal bonding
[
44].
In particular, the bidentate chelation mode of the ligands in complex 2 was unambiguously
confirmed by single crystal X-ray diffraction. Crystals of complex 2 suitable for X-ray
diffraction were grown from a CH Cl /n-hexane solution at -20 °C. The crystallographic data
2
2
for complex 2 is presented in Table 1 (see Experimental Section). Complex 2 crystallised in
the monoclinic system, within the C2/c space group. The molecular structure of complex 2 is
depicted in Figure 1, selected bond lengths and angles being shown in Table 2.
Complex 2 reveals a structure consisting of a cationic nickel centre containing iminofuran,
3
-
η -allyl moieties and an octahedral PF counter anion, without any direct interactions, as
6
depicted from the large distance between the metal and the nearest fluorine atom (Ni···F3 =
4
.431(7) Å). Although classic hydrogen bonds were not found, four non-classic hydrogen
bonds can be observed, consistent with the distances C(6)−H(6)ꢀꢀꢀF(3) of 2.48 Å [C(6)ꢀꢀꢀF(3)
.303(6) Å, C(6)−H(6)ꢀꢀꢀF(3) 144º], C(13)−H(13)ꢀꢀꢀN(1) of 2.39 Å [C(13)ꢀꢀꢀN(1) 2.881(6) Å,
3
C(13)−H(13)ꢀꢀꢀN(1) 109º], C(16)−H(16)ꢀꢀꢀN(1) of 2.40 Å [C(16)ꢀꢀꢀN(1) 2.905(6) Å,
C(16)−H(16)ꢀꢀꢀN(1) 111º], and C(19)−H(19B)ꢀꢀꢀF(5) of 2.51 Å [C(19)ꢀꢀꢀF(5) 3.231(12) Å,
C(19)−H(19B)ꢀꢀꢀF(5) 130º]. The geometry around the cationic nickel centre in complex 2 can
be described as square planar, when considering the plane formed by the atoms C19 and C21,
the allyl group and the atoms O1 and N1 of the ligand. The relatively small N1-Ni-O bite
9

ACCEPTED MANUSCRIPT
Figure 1. Molecular of the cation of complex 2, determined by X-ray diffraction, showing
-
5
0% probability ellipsoids. Hydrogen atoms, the PF anion and a CH Cl molecule were
6
2
2
omitted for clarity.
angle in complex 2 of 88.22(13)º, is a result of chelating ligand steric constraints. The bond
angles around the Ni atom sum c.a. 360°, being consistent with a square planar geometry. The
metal chelation ring (formed by the atoms Ni1-N1-C6-C2-O1) in complex 2 is almost flat.
The allyl carbons (C19, C20 and C21) give rise to an angle of 61.7(6)° with the plan of metal-
chelate ring. The Ni-O bond length in complex 2 is equal to 1.982(3) Å, being longer than the
Ni-N length, of 1.921(3) Å. This feature has been analogously reported [45,46], but still
shorter than those reported for both nickel(II)furan [41] or nickel(II) iminopyridine [47-49]
complexes. The Ni-Callyl bond lengths in complex 2 lie between 1.963(5) and 2.013(5) Å and
3
compare well with the Ni-C distances observed in other related Ni(II) η -allyl complexes
[
50,51].
3
The η -allyl, phenyl and furan planes give rise to angles of 61.7(6)°, 91.56(15)° and
1
.52(14)° with the nickel coordination plane, respectively.
10

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Table 2. Selected bond lengths (Å) and angles (º) of complex 2.
Bond lengths
Ni1-N1
Ni1-O1
Ni1-C19
Ni1-C20
Ni1-C21
N1-C6
1.921(3)
1.982(3)
1.963(5)
1.986(6)
2.013(5)
1.281(6)
1.447(5)
1.397(5)
1.373(5)
N1-C7
O1-C2
O1-C5
Bond angles
N1-Ni1-O1
N1-Ni1-C19
N1-Ni1-C20
N1-Ni1-C21
O1-Ni1-C20
O1-Ni1-C21
C19-Ni1-C21
Ni1-N1-C6
C6-N1-C7
82.2(1)
100.9(2)
135.7(2)
173.3(2)
137.4(2)
103.4(2)
73.4(2)
115.3(3)
119.2(3)
111.9(2)
142.1(3)
Ni1-O1-C2
Ni1-O1-C5
In order to rationalise the observations made related to the formation of the three
complexes of this study, some DFT calculations were performed. To that effect, geometry
+
optimisations on the cations of the mono chelated complexes of the type Ni Ln, on the
+
cations of the bis chelated complexes of the type Ni Ln , and on the respective ligands, Ln,
2
with n=1, 2 and 3, were carried out (see the atomic coordinates for the optimised species in
the respective xyz files of the Supplementary Material). In the case of complex 2, the one that
was characterised in the solid state by single-crystal X-ray diffraction, it can be seen that its
experimental and theoretical structures are similar (see Table S1 of the Supplementary
Material).
We also utilised the frequencies calculations obtained for the cations of complexes 1-3,
using the same functional, in order to simulate their C=N stretching vibrations. The
comparison of the experimental and calculated IR stretching vibrations is presented in Table
11

ACCEPTED MANUSCRIPT
S2 of the Supplementary Material and, as can be observed, they are well comparable albeit
theoretically overestimated, mainly owing to the overestimation of the respective force
constants [52].
The consistency in the bond parameters of complex 2 in both the X-ray diffraction, as well
as its DFT optimised structure indicates that the theoretical methods used are suitable for the
present system. Therefore, we proceeded to a more detailed thermodynamic analysis of the
system.
+
Subsequently, the free energy balances of the reactions of conversion of Ni Ln into
+
Ni Ln were determined (Scheme 3). From these results it is possible to observe that,
2
+
thermodynamically, the Ni Ln products are favoured in all of the considered structures, with
2
the reaction involving ligands L1 and L2 virtually representing a chemical equilibrium (ꢁG
-1
being equal to –4 and –1 kcal mol , respectively). However, the reaction corresponding to the
formation of complex 3, which was, in fact, the isolated product, is clearly the most favoured
one, reinforcing the rational made above concerning its formation.
-1
Scheme 3. Energy balances (kcal mol ) determined by DFT calculations for the formation of
bis(N-monodentate iminofuran) allyl nickel(II) complexes from the addition of an iminofuran
ligand to mono(N,O-chelating iminofuran) allyl nickel(II) complexes.
4
. Conclusions
We have described, in the present work, a convenient synthetic procedure for the
preparation of new cationic allylnickel complexes supported by 2-(N-arylformimino)furan
-
ligands with [PF ] as the counter anion (1-3).
6
The chemistry of N,O ligands L1-L3, used in this work, has led to different coordination
3
modes. In the three newly prepared η -allyl Ni(II) cationic complexes 1-3 it was observed that
12

ACCEPTED MANUSCRIPT
the 2-iminofuran derivatives displayed both bidentate and monodentate coordination modes,
respectively, in complexes 1-2 and 3.
DFT studies on all complexes indicated that the formation of the bis chelated monodentate
complex 3 is clearly the most favoured outcome of all the reported synthetic procedures.
5
. Acknowledgements
We thank the Fundação para a Ciência e a Tecnologia, Portugal, for financial support (Project
UID/QUI/00100/2013) and for fellowships to T.F.C.C. and C.S.B.G., (PD/BD/52372/2013-
CATSUS PhD programme, and SFRH/BPD/107834/2015, respectively).
6
. Supplementary material
CCDC 1585090 contains the crystallographic data for the structural analysis of 2. Copy of this
information can be obtained free of charge via
http://www.ccdc.cam.ac.uk/Community/Requestastructure/Pages/DataRequest.aspx or from
the Cambridge Crystallographic Data Centre, 12 Union Road, CambridgeCB2 1EZ; United
Kingdom; Fax: +44 (0)1223 336033.
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Highlights
3
•
•
•
Synthesis of η -allyl Ni(II) cationic complexes containing N,O-donor ligands
Characterisation by elemental analysis, IR and NMR
Single crystal X-ray diffraction structure of one of the complexes has been determined