M. Belkhiria et al. / Journal of Molecular Structure 1171 (2018) 827e833
829
y
(M-N) are considered. The FT-IR spectra of the ligands presented a
complexes [63,64]. The imine C6eN1 and C23eN2 distances of
1.298(5) and 1.306(5) Å are in the range of reported carbon-
nitrogen double bonds [65e67]. The allyl ligand is bonded asym-
metrically to the nickel center with NieC(36) ¼ 2.001(4) Å and
NieC(38) ¼ 2.027(4) Å. A relatively large N1eNieN2 angle of
101.4(1)ꢁ is observed likely because of steric repulsion between the
two bulky thienylimine ligands. The bond angles around the Ni
atom, which sum to 360.04ꢁ, are consistent with a planar geometry
[68].
strong absorption band appeared at 1614 cmꢀ1for L1, 1621 cmꢀ1 for
L2 and 1602 cmꢀ1 for L3, typical of an imine functionality [49‒52].
The relatively strong peak at 716 cmꢀ1 has been assigned to the CeS
stretching vibration [53‒55]. From the IR spectra of both free li-
gands and complexes, it’s clear that thienyl moiety is still intact and
the formation of N,S-chelate ring, as expected, did not occur. For
complexes 1, 2 and 3, The CeS stretching vibration bands still at
about 720 cmꢀ1. However, coordination of the imine moiety to the
nickel metal center is clear from the increase of
y
(C]N) stretching
The coordination plane NieN1eC36eC38eN2 in 2 is almost flat
as indicated by the torsion angles of ꢀ1.6(5)ꢁ, ꢀ1.1(7)ꢁ and ꢀ2.2(0)ꢁ
for NieN1eC36eC38, NieN2eC38eC36 and C38eC36eN1eN2,
respectively. The imines are cis to one another and the thiophene
rings lie almost perpendicular. The plane of one of the thiophene
rings C5eS1eC2 makes an angle of 70.39ꢁ with the nickel coordi-
nation plane, while the other thiophene plane C19eS2eC22 is
almost coplanar with it.
In these syntheses, we avoided using the diethyl ether in order
to try to coordinate the thiophene ring sulfur atom, by using a
mixture of CH2Cl2/n-hexane. However, L2 acted as a monodentate
ligand and we obtained two imines coordinated to the nickel center
of complex 2, despite of the use of a stoichiometric amount (1:1)
[33,69]. As referred above, recrystallization of 2 from CH2Cl2/Et2O
led to complex 2’.
The ORTEP diagram of complex 2′ is shown in Fig. 2. Suitable
prismatic orange single crystals of 2′ were obtained. The complex 2′
crystallized in the orthorhombic P22121 space group (Table 1). The
ORTEP diagram of the molecular structure of the cationic moiety of
2′ is shown in Fig. 2, and selected bond lengths and angles are listed
in Table 2.
The molecular structure confirms the identity of 2′, as a cationic
allyl nickel complex containing the 2-iminothiophene ligand
coordinating through the imine nitrogen atom to the Ni metal
center in a monodentate fashion. The soft thiophene ring sulfur is
not coordinated to the metal while the oxygen atom of the diethyl
ether is coordinated to the Nickel during the recrystallization [70].
The structure of complex 2′ consists of a loosely associated
[Ni(ɳ3-C3H5)(L2){O(C2H5)2}]þ cation and an octahedral [PFꢀ6 ]
counter-anion (Fig. 2). The 2′ cation adopts the usual slightly dis-
torted square planar arrangement as appears from the bond angles
of 98.3(3)ꢁ, 94.1(2)ꢁ, 94.9(3)ꢁ and 72.4(4)ꢁ for NeNieC19, NeNieO,
C21eNieO and C21eNieC19 bond angles, respectively. The sum of
the bond angles around nickel in C2′ is 359.79ꢁ indicating a nearly
perfect square planar coordination geometry [71].
frequencies after complexation [56‒57] and the appearance of a
peak at 596 cmꢀ1 attribuated to stretching vibration
y(M-N). All IR
spectra present the characteristic band of the PFꢀ6 counter-ion at
around 830 cmꢀ1 [58]. In addition, the presence of the non-
coordinating PFꢀ6 counter-anion is evident from 31P and 19F NMR
spectra.
The products formed in these reactions have low thermal sta-
bility in solution, at room temperature, and show broad signals in
NMR spectra in CD2Cl2 that indicate a fluxional behavior. For
example, the 13C{1H} resonances corresponding to the allylic CH2
groups can be observed only at ꢀ20 ꢁC at43.2 and 70.0 ppm and
broaden to the baseline at room temperature. Although the thermal
instability of the nickel complexes prevented us from observing the
coalescence of these signals, this observation indicates a dynamic
behavior of the ɳ3-allyl ligand [59].
The 1H, 19F and 31P NMR spectra were obtained despite the
instability of the complexes. Analysis of the cationic nickel com-
plexes 2 and 3 by 1H NMR reveals changes in chemical shifts
compared to free ligands. For example, the sharp singlet and
doublet signals from the ortho-isopropyl and para-methyl groups of
the free ligands L2 and L3, at 1.20 and 2.38 ppm, respectively, split
into broad singlets in complexes 2 and 3. The 1H NMR spectra of
complexes 1e3 show similar trends and present, in all cases, proton
resonances assigned to the ligand eCH]N around 8.5e8.9 ppm,
which were downfield shifted in relation to the free ligand
(8.30 ppm) after complexation to nickel. The 19F and 31P NMR
spectra are consistent with the cationic allylnickel(II) complexes
formulation.
2.2. X-ray diffraction studies
Suitable prismatic orange single crystals of complex 2 were
obtained by crystallization from CH2Cl2/n-hexane, at ꢀ20 ꢁC. This
complex crystallizes in the orthorhombic Pbca space group
(Table 1). The corresponding ORTEP diagram is shown in Fig. 1
confirms the identity of complex 2, its structure consisting of an
organometallic cation and a [PFꢀ6 ] counter-anion.
Selected bond lengths and angles are listed in Table 2. The
[Ni(ɳ3-C3H5)(L2)2]þ cation adopts the usual slightly distorted
square planar arrangement, taking into account the imine nitrogen
atoms N1 and N2 of each of the thienylimine ligands and the two
terminal carbon atoms of the allyl group (C36 and C38), showing
that the thiophene moieties are not coordinated to Ni [60].
Complex2 reveals a structure consisting of a loosely associated
[allyl Ni(L2)2]þ cation and octahedral PFꢀ6 counter-anion without
direct interactions because of the large distance between the metal
and the nearest fluorine atom (Ni/F1 ¼4.922 Å). Another aspect of
the relatively weak cationeanion attraction is the presence of
CeH/F hydrogen bonding with ranging distances between 2.365
and 3.944 Å [61].
The diethyl ether ligand lie plane and perpendicular (89.64ꢁ) to
the O Ni N plane, while 59.97ꢁ is the angle between the allyl and O
Ni N planes. The 2,6-disubstituted phenyl ring is also almost
perpendicular to the chelating plane with an angle of 88.13ꢁ.
The allyl ligand appears much more symmetrically arranged
than in 2, with NieC(21) ¼ 2.008(9) Å, NieC(19) ¼ 1.984(9) Å and
with a shorter NieC(20) of 1.94(1) Å [72].
The NieO bond length is 1.986(5) Å, which is longer than NieN
bond length of 1.945(5) Å compares well with the bonds lengths of
[O,N] nickel complexes [73].
Because the coordination of the sulfur heteroatom to the nickel
center was not observed, two possible explanations for this
behavior were considered. First, the coordination ability of diethyl
ether is too great in comparison to the sulfur in thiophene. Rieb et al.
studied the reaction of bis(imino)thiophene ligands with FeCl2 and
no coordination of thiophene has been observed in this case [74],
confirming that sulfur atom is also a poor donor in these systems.
Second, an aromatic behavior of the thiophene moieties may ther-
modynamically exclude the coordination to the nickel [75].
The nitrogen-nickel bonds of 1.992(3) Å and 1.958(3) Å are
slightly longer than those reported in diamine nickel(II) systems
[40,62] and
a slightly shorter than iniminopyridine nickel