Coordination and supramolecular chemistry of new bis-bidentate Schiff-base ligands

Article abstract of DOI:10.1002/ejic.200701160

The coordination chemistry of a series of bis-bidentate Schiff-base ligands with Ni^II and Cu^II has been investigated. The ligands contain two N,O-bidentate chelating salicylaldiminato units attached to polymethylene or α,α′-ortho-xylidene central spacers, through ether linkages at the 3-position of the salicyl moieties. In this paper we present a series of seven new complexes displaying several novel structural types, three of which are crystallographically characterized. Among others, the double-stranded helicates with the metal ions in the trans-square-planar geometry are of particular interest. The ortho-xylidene ligand H ₂L⁸, like the analogue H₂L⁷ previously reported by us, forms dinuclear double-stranded helicates stable both in the solid state and in solution; in the case of the ligands H ₂L^1-6, the nuclearity and the structural motif of the related complexes depend on the number (n) of methylene units in the bridge. When n = 3, 4, thermodynamically stable products from their reaction with Ni^II and Cu^II are oligomeric or polymeric species, while under kinetic control novel supramolecular architectures become accessible. The Ni^II complexes of the ligands with n = 5, 6, 8 exist as dinuclear double-stranded helicates in the solid state, while in solution they exhibit a monomer-dimer interconversion process, which has been studied using Diffusion Ordered Spectroscopy (DOSY). Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200701160

FULL PAPER
DOI: 10.1002/ejic.200701160
Coordination and Supramolecular Chemistry of New Bis-bidentate Schiff-Base
Ligands
Mauro Isola,*[a] Federica Balzano,[a] Vincenzo Liuzzo,[a] Fabio Marchetti,[a]
Andrea Raffaelli,[b] and Gloria Uccello Barretta*[a]
Keywords: Nickel / Copper / Schiff bases / Self-assembly / N,O ligands / Supramolecular chemistry
The coordination chemistry of a series of bis-bidentate Schiff-
base ligands with NiII and CuII has been investigated. The
ligands contain two N,O-bidentate chelating salicylaldimin-
ato units attached to polymethylene or α,αЈ-ortho-xylidene
central spacers, through ether linkages at the 3-position of
the salicyl moieties. In this paper we present a series of seven
new complexes displaying several novel structural types,
three of which are crystallographically characterized. Among
others, the double-stranded helicates with the metal ions in
the trans-square-planar geometry are of particular interest.
case of the ligands H2L1–6, the nuclearity and the structural
motif of the related complexes depend on the number (n) of
methylene units in the bridge. When n = 3, 4, thermodynami-
cally stable products from their reaction with NiII and CuII are
oligomeric or polymeric species, while under kinetic control
novel supramolecular architectures become accessible. The
NiII complexes of the ligands with n = 5, 6, 8 exist as dinu-
clear double-stranded helicates in the solid state, while in
solution they exhibit a monomer–dimer interconversion pro-
cess, which has been studied using Diffusion Ordered Spec-
The ortho-xylidene ligand H2L8, like the analogue H2L7 pre- troscopy (DOSY).
viously reported by us, forms dinuclear double-stranded hel-
icates stable both in the solid state and in solution; in the
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
We have previously reported a simple synthetic method
for the construction of nickel(II)-assisted supramolecular
structures of bis-N,O-bidentate Schiff bases using commer-
cially available starting materials.[3] These Schiff-base li-
gands contain two salicylaldiminato units connected by an
α,ω-dioxo-polymethylene or by the α,αЈ-dioxo-xylidene
spacer through the 3-position of salicyl moieties
(Scheme 1). In these studies, we showed that the ligand
H2L2 forms a trinuclear circular helicate, while the H2L5
and H2L7 ligands yield double-helical dinuclear complexes
based on a square-planar geometry, an unusual coordina-
tion geometry around the metal centres of these supramo-
lecular motifs. Furthermore, as a remarkable difference in
the behaviour of the dinuclear helicate of H2L5 and H2L7, a
dinuclear/mononuclear equilibrium was observed in CHCl3
solutions of the first complex, while the second one is stable
both in solid state and in solution.
Introduction
The assembly of architecturally sophisticated, high-nu-
clearity coordination complexes from the reactions of rela-
tively simple ligands with suitable metal ions has been re-
ceiving considerable attention.[1] Indeed, the development
of simple synthetic procedures for the preparation of new
systems from relatively inexpensive starting materials is one
of the major goals in the field of metallosupramolecular
chemistry. In this context, a number of attractive ligands
have been reported which contain two bidentate chelating
arms linked by a central flexible bridge. Most examples of
this type of ligand present all nitrogen donor sets, but re-
cently numerous examples of O,O and N,O mixed-donor
ligands have been described.[2]
The approach that we explore herein is to modify the
[a] Dipartimento di Chimica e Chimica Industriale, Università di length and flexibility of the bridge of this class of ligands
Pisa,
with the goal of influencing the nuclearity and the architec-
Via Risorgimento 35, 56126 Pisa, Italy
ture of their complexes. Thus, in this paper we report the
synthesis and characterization of new members of our fam-
ily of ligands (H2L1, H2L3, H2L4 and H2L8) and their NiII
complexes, the complexation studies of H2L1 and H2L8
with CuII, as well as NMR investigations on the reversible
interconversion between the mononuclear species and dinu-
clear helicates of the NiII complexes of H2L3 and H2L4.
Fax: +39-050-2219260
E-mail: misola@dcci.unipi.it
gub@dcci.unipi.it
[b] Istituto di Chimica dei Composti Organo-Metallici – Sezione
di Pisa, Dipartimento di Chimica e Chimica Industriale, Uni-
versità di Pisa,
Via Risorgimento 35, 56126 Pisa, Italy
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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Scheme 1. Synthesis of the ligands and related NiII and CuII complexes.
spectra of all the batches are very similar and display a
strong absorption characteristic of the C=N bond (at 1614
and 1616 cm–1 for the NiII and CuII derivatives, respec-
tively) shifted to lower frequency relative to that of the free
ligand (1635 cm–1), clearly indicative of metal complex-
ation. ESI-MS spectra of CHCl3–CH3CN solutions display
major peaks (with the correct isotopic distributions) corre-
sponding to [Ni2(L1)2+Na]+ (m/z 934), [Cu2(L1)2+H]+ (m/z
921) and [Cu2(L1)2+Na]+ (m/z 944), evidently indicating the
presence of a dinuclear structure in solutions for both deriv-
atives, namely [Ni2(L1)2] and [Cu2(L1)2].
Results and Discussion
The new ligands H2L1, H2L3, H2L4 and H2L8 were easily
prepared in good overall yields (50–60%) following the ge-
neral procedure previously described[3] for the other ligands
presented in Scheme 1.
The dianion of 2,3-dihydroxybenzaldehyde was gener-
ated by reaction with NaH in DMSO.[4] Subsequent nucleo-
philic displacement of the bromides from the appropriate
dibromides 1a–g yielded dialdehydes 2a–g. The ligands were
prepared by imine condensation of the dialdehydes with the
appropriate amine. Clearly, this methodology is suitable for
the preparation of a range of bis-chelating ligands with two
salicylaldimine units by varying the structural parameters
of the spacers (length and flexibility) and the substituents
on the imine nitrogen atoms. All of the ligands gave correct
elemental analyses and were further characterized by IR
The nickel derivative, in both the soluble and insoluble
forms, is diamagnetic, suggesting a square-planar geometry
1
around the NiII ions. Furthermore, its H NMR spectrum
in CDCl3 is relatively simple and shows a single set of sig-
nals that can be fully assigned, which suggests that it exists
as a single species in solution. Unfortunately, we were not
able to obtain crystals suitable for X-ray diffraction, so we
cannot infer the spatial arrangement of the NiN2O2 coordi-
1
and H NMR spectroscopy (see NMR section).
NiII and CuII Complexes of H2L1
nation planes for this dinuclear complex in the solid state.
1
The reaction of equimolar amounts of [Ni(OAc)2]·4H2O In solution, the H NMR spectroscopic data do not sup-
or [Cu(OAc)2]·4H2O with H2L1 in ethanol at room tem- port a helical array (see NMR studies on the double-
perature rapidly afforded light-green or light-brown solids, stranded helicates); in fact, in comparison with the free li-
respectively, partly soluble in chloroform or dichlorometh- gand, the complex has only modest up-field shifts for most
ane. The chloroform solutions of these solids upon addition protons, the largest being ∆δ ≈ 0.3 ppm. On the contrary,
of CH3CN gave materials practically insoluble in the same in the case of the double-stranded helicate [Ni2(L7)2] meth-
solvents where they were initially soluble. Elemental analy- ylene protons are diastereotopic and the resonances of the
ses of the soluble and insoluble batches supported the for- imine and N-methyl groups undergo remarkable changes
mation of metal/ligand 1:1 complexes. Furthermore, the IR (∆δ ≈ 1.5 and 1.2 ppm, respectively) upon coordination, as
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New Bis-bidentate Schiff-Base Ligands
the helical arrangement constrains these protons in the meric or polymeric species – where the ligand structure and
shielding region generated by the aromatic rings.[3b] In the the salicylaldiminato nature of the complexes remain un-
present case, where the short trimethylene spacer is ex- modified, as suggested by the IR data – thermodynamically
pected to induce steric disposal not very dissimilar to that favoured by the absence of steric constraints.
generated by the ortho-xylidene bridging unit of [Ni2(L7)2],
1
the H NMR spectroscopic data seem consistent with the
NiII Complexes of H2L2
adoption of a more symmetrical nonhelical face-to-face
structure, which can accommodate the short bridge more
In a previous communication[3a] we reported that the re-
easily than the helical one. Figure 1 (a) shows the schematic action of H2L2 with nickel(II) acetate resulted in the forma-
representation of this presumed structure of [Ni2(L1)2], tion of a 1:1 oligomeric or polymeric material, insoluble in
where the nickel atoms are linked in the trans-square-planar common organic solvents. However, this material was solu-
geometry (the cis geometry is prevented from steric crowd- ble in pyridine and from the pyridine solution the paramag-
ing of the N-substituents), the phenyl rings and the chelat- netic complex of stoichiometry [Ni3(L2)3(py)6] was precipi-
ing arms from one ligand are superimposed, and the imine tated upon addition of Et2O. The X-ray crystal structure of
protons as well as those of the N-substituents are protrud- this compound revealed the complex to be a trimer and
ing outside the coordination planes, far from the shielding possess a trinuclear circular helical architecture, a relatively
cone of the aromatic rings.
rare class of compounds.[1c,5] This is recalled in Figure S1
(see Supporting Information), which illustrates the octahe-
dral coordination geometry of each nickel(II) ion (two pairs
of O,N coordination sites of two different ligands and two
pyridine nitrogen atoms) and its helical nature.
It is now fairly well established that assembly processes
involving polynucleating and flexible ligands are expected
to be dependent on the reaction conditions, especially rea-
gent concentrations.[5a,5e,6] Consequently, our next step was
to investigate the effect of conducting the reaction at higher
dilution to reduce intermolecular complexation that favours
the polymer formation. In fact, slow addition of a nickel
acetate solution in methanol (9.7 m) to a 1.6 m solution
of H2L2 in methanol, at room temperature, yielded a micro-
crystalline green solid, soluble in CHCl3, CH2Cl2, MeOH
and CH3CN. ESI-MS data indicated a 3:2 metal:ligand ra-
tio and the presence of two acetate ions with the peak at
highest m/z being 1133, which corresponds to [Ni3(L2)2·
(OAc)2+Na]+. Slow evaporation of a solution of the com-
plex in CH3CN afforded X-ray-quality crystals: the struc-
ture of the compound is shown in Figure 2. The complex
has the formulation [Ni3(L2)2·(OAc)2]·CH3CN and is a di-
Figure 1. Schematic representations of dinuclear face-to-face (a) mer that possesses a wrinkled metallomacrocyclic structure
and double-stranded helical (b) structure of the NiII and CuII com-
with a molecule of nickel acetate coordinated in its cavity.
plexes; the metal ions exhibit a square-planar geometry. The curves
The two bridging ligands adopt significantly different con-
represent polymethylene or o-xylidene bridges. The coordination
formations and denticities: one ligand (yellow in part B of
planes can rotate around the M···M axes depending on the length
of the bridges.
Figure 2) coordinates, as usual, each N,O-bidentate arm to
a different metal centre, while the other, (L2)2–, acts as a
For the dinuclear complex [Cu2(L1)2], the lack of experi- hexadentate chelate ligand capable of coordinating both
mental (NMR and magnetic) data and the coordination ethereal oxygen atoms in addition to the N,O-bidentate
plasticity of the CuII ion, which may provide access to a moieties. Each metal ion has a distorted octahedral geome-
variety of coordination geometries, do not allow any rea- try and the distortion amount may be inferred from the
sonable hypothesis on its structure. Nevertheless, the anal- bond lengths listed in Table 1 and from the remark that the
ogy found between [Ni2(L7)2] and [Cu2(L8)2] (vide infra) L–Ni–L angles, where L means O or N, range between 75.6°
suggests the square-planar coordination geometry and the and 173.6° for Ni(1), between 75.7° and 176.4° for Ni(2)
face-to face structure as a possible spatial disposition also and between 75.4° and 175.1° for Ni(3). The nickel atoms
for this complex.
which are part of the macrocyclic skeleton are linked by
Concerning the observed conversion of the initially two N,O-coordination sites of different ligands and one
formed soluble derivatives of both NiII and CuII to insolu- oxygen atom from each acetate ion, while the encapsulated
ble and intractable materials, it is very likely that it is a NiII reaches the hexacoordination by bonding four oxygen
process of rearrangement of kinetic products, namely the atoms (two ethereal and two phenolic) from the hexaden-
dinuclear complexes [Ni2(L1)2] and [Cu2(L1)2], to oligo- tate ligand and two oxygen atoms from the acetate ions,
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M. Isola, G. Uccello Barretta et al.
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which act as bidentate ligands. The phenyl rings, with the NiII Complexes of H2L3 and H2L4
Interestingly, we found[3a] that the ligand with the octa-
methylene spacer, H2L5, affords the double-helical dinuclear
complex [Ni2(L5)2]·THF by a self-assembly process, where
each nickel atom is linked in a trans-square-planar geome-
try, the first example of helicate based on this coordination
geometry. However, this complex in CHCl3 solution un-
dergoes a disassembly process which results in an equilib-
rium mixture of mono- and dinuclear species. These results
prompted us to study the coordination chemistry of ligands
that have a number of methylene spacers intermediate be-
tween four and eight and the properties of their NiII com-
plexes. Thus, the ligands H2L3 and H2L4 were easily pre-
pared by using the same general method as used for the
other ligands, and their coordination to nickel(II) was
achieved by stirring 1 equiv. of nickel(II) acetate with 1
equiv. of the ligand in ethanol. In both cases green precipi-
tates were formed, which were isolated by filtration. The
ESI mass spectra show peaks corresponding to [Ni2(L3)
2+H+] (m/z 965), [NiL3+H+] (m/z 483), [Ni2(L4)2+H+] (m/z
993) and [Ni(L4)+H+] (m/z 497). These data are consistent
with the coexistence in solution of a dinuclear and mononu-
adjacent chelating groups, lie two below (N and P in part
B of Figure 2; centroid–centroid 8.89 Å) and two above (M
and Q in Figure 2, B; centroid–centroid 8.96 Å) the plane
of the metal centres.
1
clear species in both cases, as has been confirmed by H
NMR studies (vide infra).
Green crystals of [Ni2(L3)2]·n-heptane were obtained by
slow evaporation of saturated chloroform/n-heptane solu-
tions of the corresponding complexes (Table 2). The X-ray
structural analyses clearly show (Figure 3; a stick-and-ball
representation is presented in Figure S2 in the Supporting
Information) that the compound has a dinuclear helical
structure with the nickel(II) centres in a trans-square-planar
geometry, linked by N,O coordination sites from two dif-
ferent ligands, the same as in [Ni2(L5)2]·THF (and in
[Ni2(L7)2]·CHCl3 and [Cu2(L8)2], vide infra).
Table 2. Bond lengths [Å] and angles [°] around the Ni atom in
[Ni2(L3)2].
Ni–O(1)
Ni–O(4Ј)
O(1)–Ni–N(1) 93.1(4)
O(4Ј)–Ni–N(2Ј) 93.4(4)
O(1)–Ni–O(4Ј) 175.4(3)
1.817(7)
1.829(7)
Ni–N(1)
Ni–N(2Ј)
O(1)–Ni–N(2Ј)
O(4Ј)–Ni–N(1)
N(1)–Ni–N(2Ј)
1.876(10)
1.905(10)
86.7(4)
87.2(4)
175.7(3)
Figure 2. Molecular structure of the complex [Ni3(L2)2·(OAc)2]. (A)
View illustrating the distorted octahedral geometry of the metal.
(B) Stick representation; the Schiff ligands are distinguished by
colour, emphasizing the cavity with the encapsulated NiII and ace-
tate ions (lines).
Table 1. Bond lengths [Å] around the Ni atoms in [Ni3(L2)2·(OAc)2].
Ni(1)–O(1)
Ni(1)–O(5)
Ni(1)–N(1)
Ni(1)–N(3)
Ni(1)–O(9)
Ni(1)–O(11)
Ni(2)–O(5)
Ni(2)–O(6)
Ni(2)–O(7)
1.954(3)
2.023(3)
2.051(3)
2.059(3)
2.098(3)
2.348(3)
1.990(2)
2.149(3)
2.226(3)
Ni(2)–O(8)
Ni(2)–O(10)
Ni(2)–O(11)
Ni(3)–O(4)
Ni(3)–O(8)
Ni(3)–N(2)
Ni(3)–N(4)
Ni(3)–O(10)
Ni(3)–O(12)
1.986(2)
1.997(3)
2.003(3)
1.974(3)
2.035(2)
2.052(3)
2.068(3)
2.296(3)
2.091(3)
It is noteworthy that this supramolecular structure is also
very stable in solution, as demonstrated by the presence of
the molecular peak in the ESI-MS spectrum and the obser-
vation that it can be recovered unmodified from solutions
in polar and protic solvents.
Figure 3. Molecular structure of the complex [Ni2(L3)2] projected
in a direction near to the twofold axis. Thermal ellipsoids of Ni, N
and O atoms are at 30% probability; Ј denotes y, x, –z.
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New Bis-bidentate Schiff-Base Ligands
The molecule shows a C2 symmetry; the twofold axis symmetry similar to that of [Ni2(L7)2]·CHCl3 (pseudo-C2)
passes between the Ni atoms in a direction approximately and [Ni2(L3)2], but different from them in that the twofold
perpendicular to the projection plane of Figure 3. Although axis is parallel to the coordination planes; in [Cu2(L8)2] the
the coordination around the metals is almost perfectly twofold axis crosses the copper atoms. The geometry of the
planar (max. deviations 0.07 Å), the limited length of the bonding around Cu is described in Table 3. The Cu-salicyl-
ligand causes the salicylaldiminate moieties to bend, mak- aldiminato moieties make surfaces with a more pronounced
ing moderately bowed surfaces towards the interior of the curvature with respect to the nickel derivatives, as evidenced
molecule with a phenyl(centroid)–Ni–phenyl(centroid) an- by the closer angles between the centroids of the salicyl
gle of 173.9° and a Ni···NiЈ distance of 3.617 Å. If we con- rings and the metal centres {153.7° and 164.4° vs. 173.9° of
sider fully eclipsed (rotation angle 0°) two metal coordina- [Ni2(L3)2] and 175.0° of [Ni2(L7)2]·CHCl3} and by the
tion squares where the M–N and M–O bonds (M = Ni, Cu) longer Cu···Cu distance (3.784 Å) compared to the Ni···Ni
of a coordination plane eclipse, respectively, the M–O and distances in [Ni2(L7)2]·CHCl3 and [Ni2(L3)2] (3.554 Å and
M–N bonds of the other plane, as in Figure 1 (b), the coor- 3.617 Å, respectively). Each phenylene ring of the bridge is
dination squares in Ni2(L3)2 are staggered by a rotation of approximately perpendicular only to one of the adjacent
about 29.5°. In comparison to the complex [Ni2(L5)2]·THF, salicyl rings (angle of 89.3°), while with the other one it
containing the long octamethylene bridge, the coordination makes a smaller angle (67.1°); these differences probably
environments around the metal centres of the complex reflect the large distortion from the trans-planar coordina-
[Ni2(L3)2] show a similar deviation from the planarity (max. tion around the copper centres, owing to their coordination
deviations 0.31 Å vs. 0.22–0.31 Å) but a significantly higher plasticity. The coordination squares are rotated with respect
rotation value around the Ni···Ni axis (angle of about 29.5° to each other around the Cu···Cu axis with an angle of ca.
vs. 3.5°), evidently as a consequence of the constraints in- 42°, similar to that found in [Ni2(L7)2]·CHCl3 around the
troduced by the shorter pentamethylene bridge.
Ni···Ni axis (43.7°).
For the complex [Ni2(L4)2] we were not able to obtain
crystals suitable for X-ray diffraction analysis. However,
owing to the bridge length of H2L4 intermediate between
those of H2L3 and H2L5, the presence of a helical arrange-
1
ment is both unambiguous and consistent with H NMR
studies in solution (see NMR section).
CuII and NiII Complexes of H2L8
The ligand H2L8, which possesses a skeleton identical to
that of ligand H2L7 but with the N-n-propyl instead of the
N-methyl group, was introduced to prepare more soluble
complexes and to confirm the ability of the rigid ortho-
xylidene bridge to favour the self-assembly of stable double
helicates.
Figure 4. Molecular structure of the helical complex [Cu2(L8)2].
Thermal ellipsoids of Cu, N and O atoms are at 30% probability;
Ј denotes –x, y, 1/2 – z.
The reaction of NiII and CuII acetates with ligand H2L8,
performed under the usual conditions, led to the assembly
of the dinuclear complexes [Ni2(L8)2] and [Cu2(L8)2] as
green and brown microcrystalline solids, respectively. Ele-
mental analyses show the correct composition for the com-
pounds, and characteristic peaks are observed in the ESI
mass spectra for the dinuclear complexes. Furthermore, ac-
cording to NMR measurements performed on [Ni2(L7)2]·
CHCl3 and [Ni2(L8)2] (see NMR section and ref.[3b]), MS/
MS experiments have suggested that the dinuclear com-
plexes are the unique molecular species in solution; in fact,
Table 3. Bond lengths [Å] and angles [°] around Cu atoms in
[Cu2(L8)2].
Cu(1)–O(1)
Cu(2)–O(4)
O(1)–Cu(1)–N(1) 92.7(5)
O(1)–Cu(1)–O(1Ј) 173.6(6)
O(4)–Cu(2)–N(2) 91.5(3)
O(4)–Cu(2)–O(4Ј) 169.7(4)
1.917(12)
1.886(5)
Cu(1)–N(1)
Cu(2)–N(2)
O(1)–Cu(1)–N(1Ј) 87.5(5)
N(1)–Cu(1)–N(1Ј) 176.7(7)
O(4)–Cu(2)–N(2Ј) 89.4(3)
N(2)–Cu(2)–N(2Ј) 170.3(5)
1.989(11)
1.978(7)
Unfortunately the crystalline solid of [Ni2(L8)2] was not
these experiments showed that the important ions observed suitable for X-ray diffraction, so the double helical nature
in the spectra of [Ni2(L8)2] and [Cu2(L8)2] arise from the of this complex can only be demonstrated in solution (see
fragmentation of the main peaks at m/z = 1033 and 1046, next paragraph), but there is no doubt that, considering the
respectively.
essentially identical structures of the ligands H2L7 and
Single brown crystals of [Cu2(L8)2] were obtained by H2L8, very close analogies must exist in the structural fea-
slow crystallization from chloroform/n-heptane solutions. tures of the solid state of their nickel complexes. On the
The molecular structure (Figure 4; a stick-and-ball repre- basis of these considerations, unambiguous proof of the
sentation is presented in Figure S3 in the Supporting Infor- previously[3b] proposed helicate structure of [Cu2(L7)2] came
mation) shows the complex to be a double helicate with C2 from the crystal structure of [Cu2(L8)2].
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NMR Studies on the Stability of the Double Helicates in
Solution
(1)
Interconversion processes between complexes with dif-
ferent nuclearity, involving helicates or other supramolec-
ular structures, have been reported to be operating in sev-
eral systems.[7] In certain cases these processes are achieved
by adding an external cation[7b,7j] or anion[7a] that, through
host–guest interactions, promotes the conversion of a struc-
ture to a larger one where it remains encapsulated. Alterna-
tively, as described by Lehn and co-workers in a recent
interesting paper,[7g] the addition of Ag+ ions prompts the
conversion of polyheterocyclic monostranded helical li-
gands to dinuclear double-stranded helicates; the reversibil-
ity of the process is implemented by the addition of a strong
cryptand which scavenges Ag+ and releases it by proton-
ation. In another recent paper,[7c] Lehn and co-workers
studied the reversible assembly of helical monostranded oli-
gopyridine into double-helical dimers as a result of a bal-
ance among multiple competing interactions (intra- and
intermolecular hydrogen bonding, extensive intermolecular
aromatic stacking stabilizing the double-stranded helices).
As reported before, the dinuclear helicate [Ni2(L5)2]·THF
undergoes a disassembly process on dissolution in chloro-
form, without intervention of other species.[3a] This process
appears to be the result of the predominance of the sol-
vation energy of the mononuclear complex and of the ab-
sence of strong noncovalent CH–π and π–π interactions
which might contribute to the stabilisation of the dinuclear
helicate. The flexibility of the octamethylene bridging unit
presumably confers geometrical requirements on the ligand
framework to favour the disassembling action of the solvent
and the formation of a stable mononuclear species. Accord-
ing to this interpretation, replacement of the octamethylene
with the rigid ortho-xylidene unit resulted in the formation
of the double-stranded helicate [Ni2(L7)2]·CHCl3, stable in
chloroform solution.[3b] In light of these results, we envis-
aged that helicates derived from ligands with intermediate
bridge length and flexibility, such as [Ni2(L3)2] and [Ni2-
(L4)2], should exhibit an intermediate behaviour.
Therefore diffusion coefficients represent global parameters,
which are characteristic of molecular species as a whole. In
this respect these parameters have great potentialities in the
detection of complexed species, where an increase of molec-
ular sizes, and hence a decrease of diffusion coefficients
with respect to uncomplexed forms, should be expected.
1
Scheme 2. Numbering scheme employed for the H NMR charac-
terization of nickel complexes.
The [Ni2(L3)2] complex can be usefully employed as the
basis for our discussion, as its behaviour in solution, as de-
tected by NMR spectroscopy, gathers together several com-
mon features of this kind of complex: in the freshly pre-
pared samples a pure dinuclear species was detected, which,
in time, undergoes disassembly towards species, the nature
of which will be discussed.
1
The H NMR spectrum (Figure 5, a) of a freshly dis-
solved sample in CDCl3 solution showed only the expected
set of resonances for each proton or group of equivalent
protons (Table 4). Among assigned resonances, a surpris-
ingly high anisochrony was detected for the methylene pro-
tons H-8 directly bound to the imine nitrogen, which origi-
nated two resonances at δ = 4.16 ppm and 1.32 ppm
The nickel complexes [Ni2(L3)2], [Ni2(L4)2] and [Ni2-
(L8)2] were characterized by NMR spectroscopy by analyz-
ing homonuclear and heteronuclear scalar and dipolar in-
teractions in the Double Quantum Filtered-Correlation
Spectroscopy (DQF-COSY), Rotating-Frame Overhauser
Enhancement Spectroscopy (ROESY) and gradient Hetero-
nuclear Single Quantum Coherence (gHSQC) maps.
The numbering scheme employed for the 1H NMR char-
acterization of nickel complexes is reported in Scheme 2.
Information on the molecular sizes of the complexes to
be correlated to their nuclearities were obtained by Dif-
fusion-Ordered Spectroscopy (DOSY) measurements in
solution.[8] The DOSY technique makes it possible to mea-
sure diffusion coefficients (D) in solution, which, for spheri-
cal species, can be correlated to the hydrodynamic radius
RH by means of the Stokes–Einstein equation [Equa-
tion (1)] where k is the Boltzmann constant, T is the tem-
perature and η is the viscosity of the solvent:
Figure 5. 1H NMR (600 MHz, CDCl3, 25 °C) spectral region (3.0–
7.0 ppm) of [Ni2(L3)2] freshly dissolved (a) and one week after dis-
solution (b).
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New Bis-bidentate Schiff-Base Ligands
1
Table 4. H NMR (300 MHz, CDCl3, 25 °C) chemical shift (δ in ppm referenced to TMS as external standard) data of ligands H2L3 and
H2L4, dinuclear [Ni2(L3)2] and [Ni2(L4)2] complexes, and mononuclear [NiL3] and [NiL4] complexes.
Proton
H2L3
Dinuclear [Ni2(L3)2]
m[a] (J, Hz)
Mononuclear [NiL3]
m[a] (J, Hz)
δ
δ
δ
4
5
6
7
6.90
6.75
6.85
6.51
6.33
6.41
d (J = 7.8)
dd (J = 7.8, J = 7.8)
6.61
6.37
6.73
d (J = 7.8)
dd (J = 7.8, J = 7.8)
d (J = 7.8)
br. s
d (J = 7.8)
8.30
6.49
br. s
7.92
8
9
10
11
3.55
1.70
0.97
4.06
4.16, 1.32
1.75, 1.55
0.81
3.92, 3.83
2.00–1.40
m
m
3.50
1.94
0.98
3.83
br. t (J = 6.0)
m
t (J = 7.3)
br. t (J = 5.9)
m
t (J = 7.3)
m
m
12–13
1.95, 1.70
2.00–1.40
H2L4
Dinuclear [Ni2(L4)2]
Mononuclear [NiL4]
δ
δ
m (J, Hz)
δ
m (J, Hz)
4
5
6
7
8
9
10
11
12
13
6.90
6.75
6.85
8.30
3.55
1.70
0.98
4.04
1.89
1.57
6.52
6.29
6.25
br. d (J = 8.0)
dd (J = 8.0, J = 8.0)
dd (J = 8.0, J = 2.3)
6.62
6.39
6.71
7.85
3.48
1.95
1.00
3.83
1.75
1.47
dd (J = 7.7, J = 1.3)
dd (J = 7.7, J = 7.7)
br. d (J = 7.7)
6.29
br. s
m
m
br. s
m
m
4.25, 1.56
1.69
0.86
3.82
1.77
1.56
t (J = 7.7)
t (J = 7.3)
m
m
m
m
m
m
[a] Multiplicity, m, of the NMR signal.
(Table 4), to be compared to the value of 3.55 ppm found
in the uncomplexed ligand. This kind of differentiation can
be attributed to strong anisotropic effects generated by aro-
matic moieties. In particular, the methylene proton which
gives the NMR signal at δ = 1.32 ppm should be in the
full out-plane shielding cone generated by an aromatic ring
which points at the proton, as expected for the helical ar-
rangement of the dinuclear complex.
Moreover, an interesting trend was observed for the aro-
matic resonances: the H-6 proton was low-frequency shifted
with respect to the H-4 proton (Table 4) and a remarkable
low-frequency shift (δ = 1.81 ppm) was measured for the
imine proton H-7. These data support a high shield for the
imine proton and the adjacent aromatic proton (H-6) of the
dinuclear species, according to their spatial proximity to the
Figure 6. DOSY (600 MHz, CDCl3, 25 °C) maps of [Ni2(L3)2]
freshly dissolved (A) and one week after dissolution (B).
A freshly dissolved sample of [Ni2(L8)2] in CDCl3 solu-
salicyl moiety of the second ligand, as expected on the basis tion, prepared starting from crystals of the dinuclear spe-
of their helicate structure.
cies, showed both a unique set of resonances in the 1H
Confirmation of the nuclearity of the complex was ob- NMR spectrum and a single diffusion coefficient in the
tained by measuring the diffusion coefficient of the freshly DOSY map (D = 5.65ϫ10–10 m2 s–1), comparable to that
dissolved complex and comparing it with those of the free of the [Ni2(L3)2] complex.
ligand and of a freshly dissolved sample of [Ni2(L8)2] in
It is noteworthy that no changes were detected either in
CDCl3 solution, which exists in a well defined dinuclear the 1H NMR spectrum or in the value of the diffusion coef-
form.
ficient of [Ni2(L8)2] at different times. The diffusion coeffi-
The DOSY map of the [Ni2(L3)2] complex (Figure 6, A) cient of 5.65ϫ10–10 m2 s–1 can therefore be considered as
showed a single diffusion coefficient of 6.70ϫ10–10 m2 s–1, evidence of a dinuclear species, apart from some deviations
lower than that of the H2L3 ligand (D = 8.29ϫ10–10 m2 s–1), determined by the differences in the size of the ligands.
indicating an increase in the molecular sizes determined by
the metal complexation, assigned to the dinuclear species behaviour quite different with respect to the [Ni2(L8)2] one
on the basis of the X-ray (see previous paragraph). as it originated an interconversion equilibrium mixture
However, in solution the [Ni2(L3)2] complex showed a
Eur. J. Inorg. Chem. 2008, 1363–1375
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1369

M. Isola, G. Uccello Barretta et al.
FULL PAPER
which, after one week, showed the presence of detectable
Further evidence of the self-association of monomeric
amounts of the dinuclear complex (53%) and another spe- species came from ROE data. In fact, methyl protons (H-
cies (47%), attributable to the mononuclear complex, as al- 10) of the propyl chain produced ROEs on the bridging
ready evidenced in the case of the [Ni2(L5)2]·THF com- methylene chain (H-11, H-12 and H-13), which could not
plex.[3a]
be attributed to intramolecular interactions and are in ac-
The DOSY analysis of such an equilibrium mixture, cordance with a stacking of monomeric units.
which was fully characterized (Table 4), pointed out the It is noteworthy that in this case a third species, charac-
presence of the dinuclear species (D = 6.70ϫ10–10 m2 s–1), terized by an intermediate diffusion coefficient (D =
already discussed, and for the mononuclear species an un- 4.90ϫ10–10 m2 s–1) (Figure 7, B), which is unaffected by di-
expectedly low diffusion coefficient (D = 2.60ϫ10–10 m2 s–1) lution, was detected. This value was quite similar to that
was measured (Figure 6, B). As a matter of fact, the above observed for the dinuclear species. This last species could
value was too low not only for a mononuclear complex, but be a different conformation of the dinuclear complex, which
also for the dinuclear one and, hence, seemed to belong to is likely to be the face-to-face structure already discussed
a larger species.
for [Ni2(L1)2]. Accordingly, the methylene protons H-8 are
With the aim of gaining a better insight into this aspect, isochronous as in the mononuclear species (Tables 5 and 6)
we analysed, by DOSY technique, a solution of [NiL6], and its H-7 protons are remarkably higher frequency
which exists as mononuclear complex.[3a] The measured dif- shifted with respect to the same protons in the dinuclear
fusion coefficient for this species was 2.89ϫ10–10 m2 s–1, helicate. As a matter of fact, in the double-stranded helical
very similar to that measured in the equilibrium mixture structure the H-7 protons and low-frequency shifted H-8
obtained from the dissolution of the [Ni2(L3)2] complex.
protons (δ = 1.56 ppm) are faced to the aromatic rings,
Progressive dilution experiments of [NiL6] led to an ap- whereas for the face-to-face arrangement the H-7 and H-8
proximately twofold concomitant increase in the diffusion protons protrude from the coordination plane, far away
coefficient (D = 5.22ϫ10–10 m2 s–1 for a 1:20 dilution), sup- from the aromatic rings and, hence, are unaffected by
porting the self-association of the mononuclear species in shielding anisotropic effects.
solution. This behaviour is in accord with the well known
Table 5. 1H NMR (600 MHz, CDCl3, 25 °C) chemical shift (δ in
ppm referenced to TMS as external standard) data of the third
phenomenon observed with NiII and CuII complexes of bi-
dentate Schiff bases, which lead to the formation of dimers
species observed in the equilibrium mixture originated by the
[Ni2(L4)2] complex.
Proton
or polymers through intermolecular metal–oxygen interac-
tions.[9]
δ
m[a] (J, Hz)
The [Ni2(L4)2] complex behaved likewise to the already
4
5
6
7
8
9
10
11
12
13
6.58
6.45
6.71
6.98
3.08
1.82
0.97
3.83
1.74
1.46
d (J = 7.8)
dd (J = 7.8, J = 7.8)
discussed [Ni2(L3)2] complex: in a freshly dissolved sample
only one species with
a
diffusion coefficient of
d (J = 7.8)
6.50ϫ10–10 m2 s–1 (Figure 7, A) was present, corresponding
to a dinuclear species (see Table 4 for characterization). At
equilibrium another species, with a diffusion coefficient of
1.70ϫ10–10 m2 s–1, was formed at the expense of the dinu-
clear species, attributable to the self-aggregated mononu-
clear complex (Figure 7, B). Accordingly, dilution measure-
ments left the diffusion coefficient of the dinuclear complex
unchanged, while bringing about a significant increment in
the diffusion coefficient (D = 4.00ϫ10–10 m2 s–1 for the 1:10
s
t (J = 6.5)
m
t (J = 7.4)
m
m
m
[a] Multiplicity, m, of the NMR signal.
It should be noted that the conversion of the helicate to
diluted solution) of the mononuclear complex formed by a a face-to-face structure would be a low-energy process, as
disassembling process in solution.
could occur through an intramolecular rearrangement in-
volving an untwisting of the helicate structure without li-
gand dissociation. Thus, it is likely that the hexamethylene
bridging unit provides optimal geometric requirements to
gain access, through this pathway, to the face-to-face struc-
ture. On the other hand, similar topologies, not found in all
the other cases studied, are not accessible for more rigid
helicates. However, for helicates that contain longer and
more flexible bridging units, such as octamethylene and
dodecamethylene, similar face-to-face topologies may be ac-
cessible, although they are kinetically less stable.
Finally, for the [Ni2(L1)2] complex, for which ESI-MS
analysis showed evidence of its nuclearity, DOSY analysis
confirmed the presence of a species with a diffusion coeffi-
cient of 5.40ϫ10–10 m2 s–1, according to a dinuclear coordi-
nation complex.
Figure 7. DOSY (600 MHz, CDCl3, 25 °C) maps of [Ni2(L4)2]
freshly dissolved (A) and one week after dissolution (B).
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New Bis-bidentate Schiff-Base Ligands
Table 6. 1H NMR (300 MHz, CDCl3, 25 °C) chemical shift (δ, ppm referenced to TMS as external standard) of mononuclear ([NiL5]
and [NiL6]) and dinuclear ([Ni2(L7)2]·CHCl3 and [Ni2(L8)2]) complexes.
X
R
Ligand
Complex
δ
H-4
H-5
H-6
H-7
H-8
(CH2)8
(CH2)12
o-Xylidene
o-Xylidene
nPr
nPr
Me
nPr
H2L5
H2L6
H2L7
H2L8
[NiL5]
[NiL6]
6.63
6.62
6.78
6.72
6.38
6.38
6.36
6.34
6.71
6.71
6.59
6.57
7.96
7.59
6.79
6.63
3.53
3.51
[Ni2(L7)2]·CHCl3
[Ni2(L8)2]
3.81, 0.95
It is interesting to observe that dinuclear complexes have ene group directly bound to the nitrogen atom, in which
some common NMR spectroscopical features: they are all one proton is cisoid to the imine proton and the other pro-
characterized by a remarkable anisochrony of the methylene ton, transoid to it, faces the ortho-xylidene bridge.
protons of the propyl chains directly bound to the nitrogen
atom (Tables 4 and 6); a reliable trend in the relative posi-
tions of the aromatic resonances is always found as the
highest-frequency signal is originated from the H-4 protons
(Tables 4 and 6); and, finally, the H-7 imine proton is low-
frequency shifted with respect to the corresponding mono-
nuclear species (Tables 4 and 6).
The contrary is true for the mononuclear complexes as
they all have the H-6 protons resonating at higher frequency
than the H-4 protons (Tables 4 and 6), with isochronous
methylene protons H-8 giving rise to a signal at about
3.5 ppm, as observed in the free ligands.
In the case of [Ni2(L8)2] (see Table 7 for characteriza-
tion), where an ortho-xylidene bridging unit is present, the
detection of intramolecular dipolar interactions originated
by the methylene protons directly bound to the nitrogen
atom allowed us to impose stereochemical constraints in
solution which are in agreement with the double-stranded
helicate structure determined by the X-ray analysis for the
analogous [Ni2(L7)2]·CHCl3 complex.[3b]
Table 7. 1H NMR (300 MHz, CDCl3, 25 °C) chemical shift (δ in
Figure 8. Traces of the ROESY map (300 MHz, CDCl3, 25 °C, mix
ppm referenced to TMS as external standard) data of the dinuclear
0.4 s) of Ni2(L8)2 corresponding to the high-frequency (a) and low-
Ni2(L8)2 complex.
frequency (b) shifted resonances of methylene group H-8.
Proton
δ
m[a] (J, Hz)
According to the X-ray structure,[3b] the proton of the
methylene group bound to the nitrogen atom, in spatial
proximity to the imine proton, is located in the shielding
region of the salicyl moiety of the second ligand, whereas
the proton of the same group, transoid to the imine proton,
is far away from any aromatic shielding region and its
chemical shift is similar to that measured for the corre-
sponding isochronous methylene protons of the mononu-
clear species (Tables 4 and 7).
4
5
6
7
8
9
10
11
12
13
6.72
6.34
6.57
d (J = 7.7)
dd (J = 7.7, J = 7.7)
d (J = 7.7)
6.63
s
m
3.81, 0.95
1.82, 1.50
0.77
5.12, 5.02
7.52
m
t (J = 7.3)
d (J = 13.4)
m
7.29
m
The length and flexibility of the bridging chains in the
other complexes, together with the partial superimposition
of some resonances, prevented us from obtaining stereo-
chemical information in solution from NOE data.
[a] Multiplicity, m, of the NMR signal.
Indeed, the high-frequency shifted proton H-8 (δ =
3.81 ppm) generated an intense NOE on the methylene pro-
tons H-11 of the ortho-xylidene bridge (Figure 8, a),
whereas the low-frequency shifted methylene proton H-8 (δ
= 0.95 ppm) produced an intense NOE on the imine proton
Conclusions
In this paper we have described some new members of a
H-7 (Figure 8, b) indicating an arrangement of the methyl- class of ligands derived from two salicylaldiminato moieties,
Eur. J. Inorg. Chem. 2008, 1363–1375
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1371

M. Isola, G. Uccello Barretta et al.
FULL PAPER
connected at the 3-position by a α,ω-dioxo-polymethylene
or α,αЈ-dioxo-ortho-xylidene bridge, and their coordination
chemistry with NiII and CuII.
Taking into account the results of this work and those
previously reported, it is possible to conclude that all these
ligands are capable of generating, by self-assembly, interest-
ing new compounds of varying nuclearity and unusual
structural features.
Experimental Section
The ligand H2L2 was prepared as described previously.[3] IR spectra
were collected on a Perkin–Elmer Paragon 500 FTIR spectropho-
tometer. ESI mass spectra were recorded with a PE Sciex API III
plus triple quadrupole mass spectrometer equipped with an atmo-
spheric pressure ionization source and an articulated ionspray
interface. The sample was dissolved in chloroform (1 mgmL–1) and
diluted to 1:100 with acetonitrile. Experimental conditions in-
cluded: IS voltage, 5.5 kV; OR voltage, 90 V. NMR measurements
were performed on a Varian VXR-300 spectrometer operating at
300 MHz for 1H and on a Varian INOVA 600 operating at
600 MHz for 1H using a 5-mm broadband inverse probe with z-
axis gradient. The sample temperature was maintained at 25 °C.
The NMR spectra were recorded in CDCl3 and the 1H NMR
chemical shifts were referenced to TMS as external standard. The
2D NMR spectra were obtained by using standard sequences. The
double-quantum-filtered (DQF) COSY experiments were recorded
with the minimum spectral width required; 256 increments of 8
scans and 2 K data points were acquired. The relaxation delay was
5 s. The data were zero-filled to 2 Kϫ1 K and a Gaussian function
was applied for processing in both dimensions. The phase-sensitive
ROESY spectra were acquired with the minimum spectral width
required in 2 K data points using 8 scans for each of the 256 t1
increments; the mixing time ranged from 200 to 600 ms. A
Gaussian function was applied for processing in both dimensions.
The gradient gHSQC spectrum was obtained in 32 transients per
increment into a 2048ϫ256-point data matrix and zero filled to
2048ϫ1024 points. DOSY experiments were carried out by using
The ligands H2L3–5,7,8, upon complexation to nickel(II)
and – in the cases of H2L7 and H2L8 – to copper(II) ions,
afford double-helical dinuclear complexes exhibiting as a
special feature a trans-square-planar geometry around the
metal ions.
With the ligands containing polymethylene spacers, the
nuclearity of the complexes depends on the number of
methylene units: by packing an increasing number of meth-
ylene groups in the linker chain, low nuclearity species are
favoured. In fact, in the case of ligands with the tri- and
tetramethylene spacers, oligomeric or polymeric materials
are the final stable products of the reaction with the NiII
and CuII ions. Accordingly, the dinuclear derivatives
[Ni2(L1)2] and [Cu2(L1)2], to which face-to-face structures
were attributed, are kinetic products which likely undergo
a process of rearrangement to afford insoluble, thermody-
namically stable species with higher nuclearities. On the
other hand, the trend to higher nuclearities for the shorter
ligands is not contradictory to the formation of the trinu-
clear circular helicate [Ni3(L2)3(py)6] and of [Ni3(L2)2-
(OAc)2], where the macrocyclic dinuclear structure encapsu-
lates a molecule of nickel acetate. In fact, the first is the
product of the competition of the strongly coordinating
pyridine with the ligand (L2)2–, while in the case of the sec-
ond one, which can only be achieved under high-dilution
conditions, the stability of the cyclic dinuclear core can
likely be ascribed to the encapsulation of the molecule of
nickel acetate. When the number of linker chain atoms be-
comes 5, and up to 8, the ligands become binucleating and
afford double-stranded nickel(II) helicates. However, while
these complexes are stable in the solid state, a monomer–
dimer interconversion process is observed in chloroform
solution by NMR spectroscopy. In the equilibrium mixture
the monomer to dimer ratio varies from the value 47/53,
observed for the derivative with the pentamethylene spacer,
to the value 95/5 of the derivative with the octamethylene
spacer. These results indicate an increasing tendency of
these ligands to become mononucleating as the length and
the flexibility of the bridge increase. In accordance with this
a
stimulated echo sequence with self-compensating gradient
schemes, a spectral width of 8000 Hz and 32 K data points. Typi-
cally, a value ranging from 120 to 250 ms was used for ∆, 1.0 ms
for δ, and g was varied in 30 steps (8 transients each) to obtain an
approximately 90–95% decrease in the resonance intensity at the
largest gradient amplitudes. The baselines of all arrayed spectra
were corrected prior to processing the data. After data acquisition,
each FID was apodized with 1.0-Hz line broadening and Fourier
transformed. The data were processed with the DOSY macro (in-
volving the determination of the resonance heights of all the signals
above a pre-established threshold and the fitting of the decay curve
for each resonance to a Gaussian function) to obtain pseudo-two-
dimensional spectra with NMR chemical shifts along one axis and
calculated diffusion coefficients along the other.
Preparation of Precursor Dialdehydes: These compounds were pre-
pared from 2,3-dihydroxybenzaldehyde and relative dibromide ac-
cording to our previously reported procedure.[3]
1,3-Bis(3-formyl-2-hydroxyphenoxy)propane: White crystals from
cyclohexane, m.p. 112–120 °C. IR (Nujol): ν = 1658 (C=O) cm–1.
˜
1H NMR (CDCl3): δ = 11.05 (s, 2 H, OH), 9.87 (s, 2 H, CHO),
7.18–6.87 (m, 6 H, C6H3), 4.28 (t, 4 H, OCH2), 2.37 (t, 2 H, CH2)
ppm. C17H16O6 (316.09): calcd. C 64.55, H 5.10; found C 64.81, H
finding, we found[3a] the ligand H2L6, with the dodecameth- 5.32.
ylene bridge, to act only as a tetradentate chelating unit in
1,5-Bis(3-formyl-2-hydroxyphenoxy)pentane: White crystals from
the mononuclear complex [NiL6].
chloroform/n-hexane, m.p. 102–106 °C. IR (Nujol): ν = 1652
˜
1
(C=O) cm–1. H NMR (CDCl3): δ = 11.05 (s, 2 H, OH), 9.93 (s, 2
Furthermore, following these concepts, the polymethyl-
ene bridge was replaced by the short, rigid and helically
H, CHO), 7.22–6.92 (m, 6 H, C6H3), 4.10 (t, 4 H, ArOCH2), 2–1.7
preorganized ortho-xylidene unit to form the ligands H2L7 [m, 6 H, (CH2)3] ppm. C19H20O6 (344.12): calcd. C 66.27, H 5.85;
and H2L8. As expected, the dinuclearity and the helical
found C 66.57, H 6.08.
structure of the related NiII and CuII complexes, found in
1,6-Bis(3-formyl-2-hydroxyphenoxy)hexane: White crystals from
the solid state, are maintained in chloroform solution, as
determined on the basis of NMR and ESI-MS measure-
ments.
chloroform/n-hexane, m.p. 138–142 °C. IR (Nujol): ν = 1652
˜
1
(C=O) cm–1. H NMR (CDCl3): δ = 11.07 (s, 2 H, OH), 9.93 (s, 2
H, CHO), 7.21–6.92 (m, 6 H, C6H3), 4.08 (t, 4 H, ArOCH2), 1.92–
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New Bis-bidentate Schiff-Base Ligands
1.6 [m, 8 H, (CH2)4] ppm. C20H22O6 (358.14): calcd. C 67.03, H
6.19; found C 66.92, H 6.28.
(5) [Ni(L1)2 + 3 H]+, 965 (traces) [Ni2(L1)2 + H]+. 1H NMR, see
text. C52H68N4Ni2O8 (992.37): calcd. C 62.80, H 6.89, N 5.63;
found C 62.97, H 6.94, N 5.82.
Preparation of the Ligands: The ligands were synthesized following
standard procedures.[3]
[(Ni)2(L8)2]: Green microcrystalline powder from chloroform/n-
heptane, m.p. 194–199 °C. IR (Nujol): ν = 1616 (C=N) cm–1. ESI-
H2L1: Yellow crystals from chloroform/n-hexane, m.p. 72–75 °C.
˜
MS: m/z (%) = 461 (70) [H2L7 + H]+, 517 (38) [NiL7 + H]+, 977
1
IR (Nujol): ν = 1635 (C=N) cm–1. H NMR (CDCl ): δ = 14.21
˜
3
1
(5) [Ni(L7)2 + 3 H]+, 1033 (traces) [Ni2(L7)2 + H]+. H NMR, see
(s, OH), 8.29 (t, J = 1.2 Hz, 7-H), 6.97 (dd, J = 7.9, J = 1.6 Hz, 4-
H), 6.85 (dd, J = 7.9, J = 1.6 Hz, 6-H), 6.74 (dd, J = 7.9, J =
7.9 Hz, 5-H), 4.29 (t, J = 6.1 Hz, 11-H), 3.55 (dt, J = 6.7, J =
1.2 Hz, 8-H), 2.41 (m, 12-H), 1.71 (m, 9-H), 0.98 (t, J = 7.5 Hz,
10-H) ppm. C23H30N2O4 (398.22): calcd. C 69.32, H 7.59, N 7.03;
found C 69.54, H 7.22, N 6.86.
text. C56H60N4Ni2O8 (1032.31): calcd. C 65.02, H 5.85, N 5.42;
found C 65.18, H 5.91, N 5.36.
[(Cu)2(L8)2]: Brown crystals from chloroform/n-heptane, m.p. 168–
172 °C. IR (Nujol): ν = 1620 (C=N) cm–1. ESI-MS: m/z (%) = 461
˜
(100) [H2L8 + H]+, 1046 (10) [Cu2(L8)2 + H]+, 1067 (25) [Cu2(L8)2
+ Na]+. C56H60Cu2N4O8 (1044.22): calcd. C 64.41, H 5.79, N 5.37;
found C 64.67, H 5.95, N 5.58.
H2L3: Yellow crystals from chloroform/n-hexane, m.p. 40–43 °C.
1
IR (Nujol): ν = 1632 (C=N) cm–1. H NMR (CDCl ): δ = 14.19
˜
3
Preparation of [Ni3(L2)2·(OAc)2]·CH3CN: A solution of Ni(OAc)2·
4H2O (0.24 g, 0.97 mmol) in MeOH (100 mL) was added dropwise
to a solution of H2L2 (0.4 g, 0.97 mmol) in MeOH (600 mL). The
solution obtained was concentrated (10 mL), added to H2O
(50 mL) and extracted with chloroform (3ϫ50 mL). The combined
organic layers were dried with Na2SO4, concentrated and added to
Et2O (20 mL). The solid precipitated was collected by filtration,
washed with diethyl ether and dried in vacuo (0.28 g). Recrystalli-
zation from acetonitrile led to green crystals, m.p. Ͼ 240 °C. IR
(s, OH), 8.30 (t, J = 1.2 Hz, 7-H), 6.90 (dd, J = 8.0, J = 1.5 Hz, 4-
H), 6.85 (dd, J = 7.8, J = 1.5 Hz, 6-H), 6.75 (dd, J = 8.0, J =
7.8 Hz, 5-H), 4.06 (t, J = 6.6 Hz, 11-H), 3.55 (dt, J = 6.6, J =
1.2 Hz, 8-H), 1.95 (m, 12-H), 1.70 (m, 9-H and 13-H), 0.97 (t, J =
7.5 Hz, 10-H) ppm. C25H34N2O4 (426.25): calcd. C 70.39, H 8.03,
N 6.57; found C 70.52, H 8.15, N 6.29.
H2L4: Yellow crystals from chloroform/n-hexane, m.p. 100–103 °C.
1
IR (Nujol): ν = 1632 (C=N) cm–1. H NMR (CDCl ): δ = 14.19
˜
3
(s, OH), 8.30 (t, J = 1.2 Hz, 7-H), 6.90 (dd, J = 8.0, J = 1.6 Hz, 4-
H), 6.85 (dd, J = 7.8, J = 1.6 Hz, 6-H), 6.75 (dd, J = 8.0, J =
7.8 Hz, 5-H), 4.04 (t, J = 6.6 Hz, 11-H), 3.55 (dt, J = 6.8, J =
1.2 Hz, 8-H), 1.89 (m, 12-H), 1.70 (m, 9-H), 1.57 (m, 13-H), 0.98
(t, J = 7.4 Hz, 10-H) ppm. C26H36N2O4 (440.26): calcd. C 70.88,
H 8.24, N 6.36; found C 70.66, H 8.01, N 6.18.
(Nujol): ν = 1626 (COO–), 1614 (C=N) cm–1. ESI-MS: m/z (%) =
˜
414 (20) [H2L2 + H]+, 469 (50) [NiL2 + H]+, 1044 (5) [Ni2(L2)2·OAc
+ Na2]+, 1133 (10) [Ni3(L2)2·(OAc)2 + Na]+. C52H66N4Ni3O12·
CH3CN (1153.30): calcd. C 56.09, H 6.02, N 6.06; found: C 56.19,
H 6.26, N 6.16.
H2L8: Yellow crystals from chloroform/n-hexane, m.p. 74–79 °C.
X-ray Diffraction Studies: The X-ray diffraction experiments were
carried out at room temperature (T = 293 K) by means of a Bruker
P4 four-circle diffractometer. In all experiments graphite-mono-
chromated Mo-Kα radiation was used. The samples were glued at
the top of glass fibres and handled in air. The intensities were cor-
rected for Lorentz and polarization effects and for absorption by
the ψ-scan method[10] for [Ni3(L2)2(OAc)2]·acetonitrile and
[Cu2(L8)2] and by an integration method based on the crystal
habit[11] for [Ni2(L3)2]·n-heptane. The structure solutions were ob-
tained by means of the automatic direct methods contained in the
SHELXS97 program[12] for [Ni3(L2)2(OAc)2]·acetonitrile and
[Ni2(L3)2]·n-heptane and in the SIR-92 program[13] for [Cu2(L8)2].
The refinement, based on full-matrix least-squares on F2, were
done by means of the SHELXL97[12] program. Some other utilities
contained in the WINGX suite[14] were also used. The more rel-
evant crystal parameters are listed in Table 8.
1
IR (Nujol): ν = 1630 (C=N) cm–1. H NMR (CDCl ): δ = 14.18
˜
3
(s, OH), 8.31 (t, J = 1.2 Hz, 7-H), 7.55 (m, 12-H), 7.31 (m, 13-H),
7.00 (dd, J = 8.0, J = 1.2 Hz, 4-H), 6.87 (dd, J = 7.8, J = 1.2 Hz,
6-H), 6.71 (dd, J = 8.0, J = 7.8 Hz, 5-H), 5.34 (s, 11-H), 3.56 (dt,
J = 6.9, J = 1.2 Hz, 8-H), 1.73 (m, 9-H), 0.99 (t, J = 7.5 Hz, 10-
H) ppm. C28H32N2O4 (460.23): calcd. C 73.02, H 7.00, N 6.08;
found C 73.15, H 6.78, N 5.90.
Preparation of the Complexes: The complexes were synthesized fol-
lowing standard procedures.[3]
[Nix(L1)x]: Green microcrystalline powder, m.p. Ͼ240 °C. IR (Nu-
jol): ν = 1614 (C=N) cm–1. ESI-MS: m/z (%) = 399 (50) [H L1 +
˜
2
H]+, 455 (30) [NiL1 + H]+, 934 (2) [Ni2(L1)2 + Na]+. 1H NMR
(CDCl3): δ = 7.97 (br. s, 7-H), 6.71 (br. d, J = 6.0 Hz, 6-H), 6.64
(br. d, J = 8.0 Hz, 4-H), 6.38 (dd, J = 6.0, J = 8.0 Hz, 5-H), 3.97
(m, 11-H), 3.51 (m, 8-H), 2.17 (m, 12-H), 1.93 (m, 9-H), 0.97 (t, J
= 6.2 Hz, 10-H) ppm. C23H28N2NiO4: calcd. C 60.69, H 6.20, N
6.15; found C 60.77, H 6.36, N 6.02.
The structure solution of [Ni3(L2)2(OAc)2]·acetonitrile was found
¯
in the P1 space group. The difference Fourier map showed the pres-
ence of a lattice acetonitrile molecule. One of the ligands presented
disorder in one of the terminal propyl groups and in the intermedi-
ate C4 chain. Both of the disordered groups were refined as distrib-
uted in two different positions. The hydrogen atoms were placed in
calculated positions and left to ride on the connected carbon atoms.
The final refinement cycles were performed with anisotropic ther-
mal parameters for heavy atoms that were not disordered and iso-
tropic for the others, giving the final reliability factors listed in
Table 8.
[Cux(L1)x]: Brown microcrystalline powder, m.p. Ͼ240 °C. IR (Nu-
jol): ν = 1616 (C=N) cm–1. ESI-MS: m/z (%) = 400 (80) [H L1 +
˜
2
H]+, 921 (10) [Cu2(L1)2 + H]+, 944 (15) [Cu2(L1)2 + Na]+.
C23H28CuN2O4 (460.03): calcd. C 60.05, H 6.13, N 6.09; found C
60.13, H 6.22, N 6.28.
[Ni2L3 ]·n-heptane: Green crystals from chloroform/n-heptane, m.p.
2
125–126 °C. IR (Nujol): ν = 1610 (C=N) cm–1. ESI-MS: m/z (%)
˜
= 427 (100) [H2L1 + H]+, 483 (83) [NiL1 + H]+, 910 (8) [Ni(L1)2 +
3 H]+, 965 (10) [Ni2(L1)2 + H]+. 1H NMR, see text. C57H80N4Ni2O8
(1064.47): calcd. C 64.18, H 7.56, N 5.25; found C 64.36, H 7.73,
N 5.27.
[Ni2(L3)2]·n-heptane crystallizes in the trigonal symmetry class and
the structure solution was found in the chiral space group P3121.
The asymmetric unit is made up of one half molecule placed near
the Wickoff position a astride a twofold axis which generates the
whole molecule. The packing of the molecules leaves a channel
along the threefold screw-helix 31, where a series of electron density
[(Ni)2(L4)2]: Green microcrystalline powder from chloroform/n-
heptane, m.p. 132–134 °C. IR (Nujol): ν = 1610 (C=N) cm–1. ESI-
˜
MS: m/z (%) = 441 (80) [H2L1 + H]+, 497 (100) [NiL1 + H]+, 938
Eur. J. Inorg. Chem. 2008, 1363–1375
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1373

M. Isola, G. Uccello Barretta et al.
[Cu2(L8)2]
FULL PAPER
Table 8. Crystal data and structure refinements.
Compound
[Ni3(L2)2(OAc)2]·acetonitrile
[Ni2(L3)2]·n-heptane
Empirical formula
Formula weight
Crystal system
Space group
a [Å]
b [Å]
c [Å]
C54H69N5Ni3O12
1156.27
C57H80N4Ni2O8
1066.67
trigonal
P3121 (no. 152)
18.964(3)
18.964(3)
13.013(4)
90
90
120
4053.0(15)
3
1.311
C56H60Cu2N4O8
1044.16
monoclinic
C2/c (no. 15)
21.516(3)
9.002(1)
27.510(3)
90
107.652(7)
90
5077.3(11)
4
triclinic
¯
P1 (no. 2)
13.313(2)
13.648(2)
16.390(3)
93.43(1)
100.64(1)
110.186(7)
2722.4(6)
2
α [°]
β [°]
γ [°]
U [Å3]
Z
Dcalcd [Mgm–3]
1.411
1.092
1.366
0.897
µ [mm–1]
0.754
No. of measured reflections
No. of unique reflections [Rint
No. of parameters
R1, wR2 [I Ͼ 2σ(I)][a]
R1, wR2 [all data][a]
Goodness of fit[a] on F2
8233
7131 [0.0171]
670
0.0386, 0.0878
0.0573, 0.0971
1.001
4589
3423 [0.0525]
296
0.0636, 0.1246
0.1493, 0.1578
1.003
3821
2986 [0.0472]
317
0.0674, 0.1174
0.1692, 0.1547
1.045
]
2 2
2
[a] R(Fo) = Σ||Fo| – |Fc||/Σ|Fo|; Rw(Fo2) = {Σ[w(Fo2 – Fc ) ]/Σ[w(Fo2)2]}1/2; w = 1/[σ2(Fo2) + (AQ)2 + BQ] where Q = [MAX(Fo2, 0) + 2Fc ]/
2
2 2
3; GOF = [Σ[w(Fo – Fc ) ]/(N – P)]1/2, where N and P are the number of observations and parameters, respectively.
maxima were attributed to a disordered lattice n-heptane molecule.
Acknowledgments
The disorder present in this site of the structure induced disorder
in two propyl moieties of the [Ni2(L3)2] molecule. The disordered
This work was supported by the Ministero dell’Università e della
propyl group was refined as distributed in two different conforma-
tions, by fixing to one the total occupancy for each atom. For the
disordered solvent molecule, it was impossible to recognize any
conformation and the maxima positions were introduced in the
model as carbon atoms and left free to move in the refinement
process. The hydrogen atoms of the [Ni2(L3)2] molecule were intro-
duced in calculated positions and left to move riding on the con-
nected carbon atoms. The hydrogen atoms of the disordered solvent
were disregarded. The final refinement cycles were performed with
anisotropic thermal parameters for all the ordered heavy atoms and
isotropic for the others. The final reliability factors are listed in
Table 8.
Ricerca Scientifica (MIUR), Programma di Ricerca Scientifica di
Notevole Interesse Nazionale 2004-05, Progetto “High perform-
ance separation systems based on chemo- and stereoselective mo-
lecular recognition” (grant 2005037725) and by the Istituto di Chi-
mica dei Composti OrganoMetallici-Consiglio Nazionale delle
Ricerche (ICCOM-CNR).
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Received: October 26, 2007
Published Online: February 5, 2008
Eur. J. Inorg. Chem. 2008, 1363–1375
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1375