Synthesis and characterization of the dihydrosalpren ligand (H2L1) and of its trinuclear Ni(II) complex

Article abstract of DOI:10.1016/j.inoche.2009.05.003

The synthesis of the new unsymmetrical tetradendate Schiff ligand N-(2-hydroxybenzyl)-N′-(2-hydroxybenzylidene)-1,3-diaminopropane (H₂L¹) is reported. The ligand comprises two different coordination moieties: a rigid salicylaldimmine

Full text of DOI:10.1016/j.inoche.2009.05.003

Inorganic Chemistry Communications 12 (2009) 608–610
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis and characterization of the dihydrosalpren ligand (H2L1)
and of its trinuclear Ni(II) complex
*
Mauro Isola, Vincenzo Liuzzo , Fabio Marchetti
Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via Risorgimento 35, I-56126 Pisa, Italy
a r t i c l e i n f o
a b s t r a c t
The synthesis of the new unsymmetrical tetradendate Schiff ligand N-(2-hydroxybenzyl)-N0-(2-hydrox-
ybenzylidene)-1,3-diaminopropane (H2L1) is reported. The ligand comprises two different coordination
moieties: a rigid salicylaldimmine unit and a more flexible (2-hydroxybenzyl)-amino (hydrogenated
salicylaldimmine) unit. The reaction of H2L1 with Ni(OAc)2ꢀ4H2O (1:1 molar ratio) leads to the spontane-
ous formation of a trinuclear complex with composition {[Ni(L1)OH2]2(OAc)2Ni}ꢀ2H2O, characterized by
X-ray crystallography, where two [Ni(L1)] units act as O,O-bidentate chelate to a Ni(II) ion.
Ó 2009 Elsevier B.V. All rights reserved.
Article history:
Received 3 April 2009
Accepted 4 May 2009
Available online 15 May 2009
Keywords:
Tetradentate
Schiff base
Unsymmetrical ligand
Nickel
Dihydrosalpren
It is known that the mononuclear metal complexes of tetraden-
tate Schiff base ligands H2L show the interesting feature of acting
as ‘‘complex ligands” [1]. This property has been used to prepare
and study a wide variety of adducts between transition metal com-
plexes [ML] and alkaline [2], alkaline-earth [3], transition [1,3,4]
and f-element [5] metal ions. Actually, important research efforts
are devoted to the synthesis and characterization of homo- and
hetero-trimetallic complexes of H2L and relative reduced ligands,
due to their applications in various fields [6–9].
natively, it is possible to improve the ligand’s flexibility by hydro-
genation of the imine bonds [10]. This method drastically modifies
the electronic nature of the ligand and its donor strength, as the
donor ability of the amine nitrogen is slightly stronger than those
of the imine nitrogen atom.
In this paper we report the synthesis of the new unsymmetrical
tetradentate ligand N-(2-hydroxybenzyl)-N’-(2-hydroxybenzilid-
ene)-1,3-diaminopropane (dihydrosalpren, H2L1) and a study on
its coordination ability.
OH
NH
HO
OH HO
(CH2)2
(CH2)3
salen
N
N
N
salprn
(CH2)3
H2L
(CH2)4
salbu
H2L1
Several studies have established that the hosting ability of [ML]
complexes is dependent on a fine balance between flexibility and
rigidity of the ligand [3]. Conformationally active ligands favour
the geometrical disposition of the complex in relation to the guest
metal dimensions. On the other hand, the rigidity appears to be an
important requirement for the formation of an efficient cage struc-
ture. A simple method to favour the flexibility consists of increas-
ing the length of the polymethylene spacer between the nitrogen
atoms, leaving the electronic nature of the ligand unaltered. Alter-
To the best of our knowledge, no studies are reported in litera-
ture on the preparation of the ligand H2L1 and related metal
complexes, while some reports describe the preparation of the
half-hydrogenated salen ligand (H2salen) [11–13]. In these studies,
the key step in the synthesis of the ligand is the mono-condensa-
tion reaction of the salicylaldehyde with an excess of ethylene
diamine, followed by the reduction with NaBH4 or NaBH3CN and
subsequent imine condensation of the resulted diamine with the
salicylaldehyde. Following this suggestion, we carried out the same
sequence of reactions using the salicylaldehyde and an excess of
1,3-propandiamine (from 1:4 to 1:10 molar ratio) in high dilution
* Corresponding author. Tel.: +39 050 2219443; fax: +39 050 2219260.
E-mail address: vinlz@dcci.unipi.it (V. Liuzzo).
1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2009.05.003

M. Isola et al. / Inorganic Chemistry Communications 12 (2009) 608–610
609
NH2
(CH2)3
OH
NH
O
N
i
ii
H2L1
Ni
NH2
N
O
(CH2)3
(H2C)3
NH2
1
2
Scheme 1. (i) (1) dimethylglyoxime (2 equiv.), MeOH, 30 min, reflux. (2) NaBH4 (3 equiv.), 2 h, reflux. (ii) salicyladehyde (1 equiv.), MeOH, 2 h,reflux.
conditions. In all the attempts we were able to isolate only the tetr-
ahydrogenated product, H4salprn, in high yields (>70%).
The slow addition of a nickel acetate solution in H2O to a solu-
tion of H2L1 in methanol (1:1 molar ratio), at room temperature,
rapidly afforded a green microcrystalline powder [17]. Recrystalli-
zation of this product from CH3CN resulted in the formation of
crystals suitable for X-ray analysis. The structure of the compound
is shown in Fig. 1 [18]. The complex has the formulation {[Ni(L1)O-
H2]2(OAc)2Ni}ꢀ2H2O and adopts a structural motif common for the
tri-nickel acetate complexes of the Salpr and H4Salpr ligands [19–
21]. The structure shows a trinuclear centrosymmetric complex
consisting of three Ni2+atoms in a linear array. The Ni(2) ion is lo-
cated at the inversion centre of the complex in an octahedral envi-
ronment. This atom is coordinated by the four phenolic oxygen
atoms from the two [NiL1] moiety that form a plane and, near-per-
pendicularly to it, by two oxygen atoms from the two acetate
bridging ligands that connect the central metal to the outer metal
ions. The greatest deviation of the bond angles from those expected
for an ideal octahedral geometry is found for O(1)Ni(2)O(2) with
80.07° and O(1)Ni(2)O(2’) with 99.93°. The remaining bond angles
are close to the ideal values for the octahedral coordination [93.1°
for O(1)Ni(2)O(5’) and 86.9° for O(1)Ni(2)O(5)]. The bond distances
Ni(2)O in the equatorial plane are similar (2.044 and 2.047 Å,
respectively), while the axial Ni(2)O(5) bond distance is longer
(2.130 Å).
To overcome the difficulties we encountered in the mono-con-
densation of the 1,3-propandiamine with the salicylaldehyde, we
considered the known octahedral Ni(II) complex 1 [14] a possible
starting material. Thus, using this compound, the ligand H2L1 was
readily prepared in a one-pot procedure, as outlined in Scheme 1.
After demetallation of 1 with dimethylglyoxime and separation
from Ni(dmgly)2 [15], the solution obtained was treated with
NaBH4 to hydrogenate the imine bond. The intermediate 2 ob-
tained was not isolated, but was reacted in situ with the salicylal-
dehyde to give the ligand H2L1 in good yield (65%)[16].
The unsymmetrical nature of the ligand was confirmed by IR
and 1H NMR spectroscopy. In particular, the IR spectrum displays
a single sharp band at 3279 cm1, due to the NH stretching vibra-
tion, and another at 1629 cm1, characteristic of the azomethine
(C@N) group. The 1H NMR spectrum in CDCl3 is relatively simple
and shows a single set of signals, which suggests that a single spe-
cies exists in solution. The signals of the central methylene of the
bridge appear as two triplets around 1.9 ppm, while two multiplets
present at 2.7 and 3.7 ppm are attributed to the protons of the
methylene units linked to the amine and imine nitrogen atoms,
respectively. The signals of the benzylic protons appear as a singlet
at 4 ppm and the complex pattern around 7 ppm, attributed to aro-
matic protons, confirm the unsymmetrical nature of the ligand.
The terminal nickel ions Ni(1) and Ni(10) are in an octahedral
environment and are coordinated by two oxygen and two nitrogen
atoms of (L1)2 in the basal plane, and by two O atoms from one
bridging acetate group and one coordinated water molecule in
the axial positions. The [NiL1] unit are butterfly-shaped, with the
dihedral angles formed by the two aryl rings of 58.0°. The greatest
deviations from an ideal octahedral geometry are found in the
O(1)Ni(1)O(2) (80.78°) and in N(2)Ni(1)N(1) (95.7°) angles. The
two equatorial Ni(1)O(phenolic) bond lengths are practically iden-
tical (2.029 and 2.032 Å), while the axial Ni(1)O(4) and Ni(1)O(3)
distances are rather different (2.050 and 2.165 Å, respectively) thus
displaying the relative weakness of the bond with the uncharged
water oxygen. Furthermore, the two Ni(1)N bond lengths are only
slightly different (2.057 and 2.085 Å), but the N(1)C(benzylic) bond
length (1.328 Å) is significantly shorter than the N(2)C(benzylic)
bond length (1.451 Å), indicating the different nature of the nitro-
gen atoms [immine N(1) and amine N(2)].
The short bite of acetate ligand constraints the Ni(1) and Ni(2)
coordination octahedra to tilt by 19.4° around the O(1)ꢀ ꢀ ꢀO(2) axis.
The presence of the inversion centre on Ni(2) thus produces in the
trinuclear complex the stair like structure shown in the figure.
The crystal structure is stabilized by a hydrogen bond network
involving the coordinated water molecule (O(3)) and the lattice
water (O(6)). The O(3) ligand water molecule behaves as a donor
toward O(6) one (O(3)ꢀ ꢀ ꢀO(6) 2.689 Å) and alternatively as a donor
or acceptor toward the O(3) ligand of the nearest trinuclear com-
plex in the a direction (O(3)ꢀ ꢀ ꢀO(3’’) 2.880 Å, ’’ = ꢁx, 1 ꢁ y, 1 ꢁ z).
The O(6) lattice water molecule accepting a hydrogen bond from
Fig. 1. The molecular structure of {[Ni(L1)OH2]2(OAc)2Ni}ꢀ2H2O. Hydrogen interac-
tions are represented by dotted lines. Thermal ellipsoids are at 30% probability,
‘ = 1 ꢁ x, 1 ꢁ y, 1 ꢁ z. Bond distances (Å) around Ni atoms: Ni(1)O(2), 2.029(2);
Ni(1)O(1), 2.032(2); Ni(1)O(4), 2.050(3); Ni(1)N(1), 2.057(3); Ni(1)N(2), 2.085(3);
Ni(1)O(3), 2.165(3); Ni(2)O(2), 2.044(2); Ni(2)O(1), 2.047(2); Ni(2)O(5), 2.130(2).

610
M. Isola et al. / Inorganic Chemistry Communications 12 (2009) 608–610
O(3) donates a hydrogen bond to O(5) acetate oxygen (O(5)ꢀ ꢀ ꢀO(60)
2.693 Å) as shown by dotted lines in Fig. 1 .
[8] Y. Song, P. Gamez, O. Roubeau, I. Mutikainen, U. Turpeinen, J. Reedijk, Inorg.
Chim. Acta 358 (2005) 109.
[9] K. Matsumoto, B. Saito, T. Katsuki, Chem. Commun. (2007) 3619.
[10] M. Asadi, K.A. Jamshid, A.H. Kyanfar, Synth. React. Inorg. Metal-Org. Nano-Met.
Chem. 37 (2007) 77.
Attempts to isolate the mononuclear complex [NiL1] failed. In
particular, the use of stoichiometric defects of nickel ion (molar ra-
tio ligand:nickel 1:0.8) or a slow addition of a methanol solution of
nickel acetate in to methanol solution of H2L, invariably lead to the
formation of the trinuclear complex above descript. This indicates
a great tendency of [NiL1] to act as a ‘‘complex ligands” in a similar
way as the H4Salpr ligand [18], while, for the preparation of the tri-
nickel species of Salpr ligand a stoichiometric excess of nickel ion is
needed [20–22].
[11] S.K. Mandal, K. Nag, J. Chem. Soc., Dalton Trans. (1984) 2839.
[12] A. Berkessel, Bioorg. Chem. 19 (1991) 101.
[13] The synthesis of [Ni–H2salen] and [Cu–H2Salen] complexes, obtained by
oxidative dehydrogenation of relative full hydrogenated complexes, have been
described by Elias et al. (Angew. Chem. Int., Ed. Engl. 31 (1992) 623) and
Reglinsky et al. (Inorg. Chim. Acta, 359 (2006) 2455), respectively.
[14] R.C. Elder, Aust. J. Chem. 31 (1978) 35.
[15] P.J. Burke, D.R. Mcmillin, J. Chem. Soc., Dalton Trans. (1980) 1794.
[16] N-(2-hydroxybenzylidene)-N’-(2-hydroxybenzyl)-1,3-propanediamine (H2L1).
A
mixture of dimethylglyoxime (DMG) (3.3 g, 29 mmoli) and 1 (6 g, 14 mmol)
was refluxed for 30 min in methanol (50 mL). The red powder of Ni(DMG)2
was filtered off over a celite plug. NaBH4 (1.5 g, 40 mmol) was added and the
solution refluxed for 2 h. Finally, salicylaldehyde (4 g, 32 mmol), dissolved in
methanol (20 mL), was added and the resulting mixture was refluxed for
additional 2 h. After cooling, the residue was treated with H2O (50 mL) and
extracted with chloroform (3 ꢂ 50 mL). The organic phase was dried over
Na2SO4, filtered and evaporated to dryness. The crude yellow solid was
purified by crystallization from cyclohexane. Yield 65 %. M.p.: 89–92 °C. IR
In conclusion, we have developed a convenient and efficient
procedure to synthesize a new unsymmetrical tetradentate Schiff
ligand that contains a rigid salycilaldimine coordination unit and
more flexible (2-hydroxybenzyl)-amino coordination unit. The NiII
complex of this ligand shows a great tendency to act as a ‘‘complex
ligand”, forming trinuclear species. Efforts to investigate other me-
tal complexes based on this new ligand are underway in our
laboratory.
(KBr, nujol): 3279 (m m
NH) cm1, 1629 ( C@N) cm1. 1H NMR (CDCl3) d (ppm):190
(dt, 2H, CH2), 2.75 (t, 2H, CH2), 3.70 (t, 2H, CH2), 3.99 (s, 2H, CH2), 6.7–7.4 (m,
8H, ArH), 8.35 (s, 1H, H–C@N).
[17] {[Ni(L1)OH2]2(OAc)2Ni}.2H2O. A water solution (5 mL) of Ni(OAc)2ꢀ4H2O (0.13 g,
0.52 mmol) was added dropwise to the ligand H2L1 (0.15 g, 0.52 mmol)
dissolved in methanol (15 mL). A green solid rapidly precipitated. This was
filtered and dried. Yield 0.16 g. Crystals suitable for X-ray analysis were
retrieved from acetonitrile solution. M.p.: >320 °C. IR (KBr, nujol): 3256, 3446
Appendix A. Supplementary material
CCDC 724555 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.inoche.2009.05.003.
(m m m m –
OH) cm1; 3282, 3260 ( NH) cm1; 1636 ( C@N) cm1; 1590, 1482 ( COO) cm1
Found: C 48.91%, H 5.55%, N 6.12%. Expected for C38H50N4O12Ni3: C 49.03%, H
5.41%, N 6.02%.
[18] X-ray Crystal structure determination of {[Ni(L1)OH2]2(OAc)2Ni}ꢀ2H2O:
ꢀ
C38H50N4Ni3O12
,
M = 930.89, triclinic, space group P1, a = 9.881(2),
b = 10.415(2), c = 10.808(1) Å,
a
= 68.82(1), b = 80.47(1),
c = 86.60(1)°,
V = 1022.8(3) Å3, calcd = 1.511 g cm3,
q
l
= 1.028 m cm1, Z = 1, Mo
Ka
radiation, k = 0.71073 Å, T = 296 K. The unit cell content consists in one
{[Ni(L1)OH2]2(OAc)2Ni} centrosymmetric molecule and two water lattice
molecules. The hydrogen atoms of the acetate ligands have been introduced
in the model as statistically disordered on two limit positions. Programmes
used: SHELX97 (G.M. Sheldrick, SHELX97, Structure Solution and Refinement
Package, Universität Göttinegen, 1997) and WINGX (L.J. Farrugia, J. Appl. Cryst.
30 (1997) 565) suite R1 = 0.0481, wR2 = 0.0901; GOF = 1.053 for 3172 unique
reflections and 250 parameters.
References
[1] E. Sinn, C.M. Harris, Coord. Chem. Rev. 4 (1969) 391. and reference therein
reported.
[2] (a) S.J. Gruber, C.M. Harris, E.J. Sinn, Chem. Phys. 49 (1968) 2183;
(b) L.G. Armstrong, H.C. Lip, L.F. Lindoy, M. McPartlin, P.A. Tasker, J. Chem. Soc.,
Dalton Trans. (1977) 1771.
[3] L. Carbonaro, M. Isola, P. La Pegna, L. Senatore, Inorg. Chem. 38 (1999) 5519.
[4] Da-Hua Shi, Zhong-Lu You, Chen Xu, Qiang Zhang, Hai-Liang Zhu, Inorg. Chem.
Commun. 10 (2007) 404.
[5] G. Condorelli, I. Fragala, S. Giuffrida, A. Cassol, Z. Anorg, Allg. Chem. 412 (1975)
251.
[19] J. Reglinski, M.K. Taylor, A.R. Kennedy, Inorg. Chem. Commun. 9 (2006) 736.
[20] D. Ulku, F. Ercan, O. Atakol, F.N. Dincer, Acta Crystallogr. Sect. C 53 (1997)
1056.
[21] A. Elmali, V. Elerman, I. Svoboda, H. Feuss, K. Griesar, W. Haase, Z. Naturforsch.
B 51 (1995) 665.
[6] J. Reglin oski, S. Morris, D.E. Stevenson, Polyhedron 21 (2002) 2167.
[7] O. Atakol, R. Boca, I. Ercan, F. Ercan, H. Fuess, W. Haase, R. Herchel, Chem. Phys.
Lett. 423 (2006) 192.
[22] Zhong Lu You, Z. Anorg. Allg. Chem. 632 (2006) 664.