Structure and luminescence of a nitrate-bridged heterotrinuclear Cu 2-Pr complex with compartmental Schiff base ligand

Article abstract of DOI:10.1080/00958972.2010.520315

A unique heterotrinuclear nitrate-bridged complex of hexanitrate praseodymium(III) and dicopper(II) compartmental species has been synthesized and characterized by X-ray single crystal structure analysis. The structure determination indicates that the dinuclear copper moiety undergoes a tilted deformation (with respect to the dicopper complex) upon connection to the lanthanide species via a rare nitrate bridge. The trinuclear species is highly fluorescent owing to the presence of praseodymium.

Full text of DOI:10.1080/00958972.2010.520315

Journal of Coordination Chemistry
Vol. 63, No. 21, 10 November 2010, 3714–3723
Structure and luminescence of a nitrate-bridged heterotrinuclear
Cu2-Pr complex with compartmental Schiff base ligand
KAZI SABNAM BANUy, TOTAN GHOSHy, AVERI GUHAy,
TANMAY CHATTOPADHYAYy, DEBASIS DAS*y
and ENNIO ZANGRANDOz
yDepartment of Chemistry, University of Calcutta, 92, A.P.C. Road,
Kolkata – 700 009, India
zDipartimento di Scienze Chimiche, University of Trieste, via L. Giorgieri 1,
34127 Trieste, Italy
(Received 5 March 2010; in final form 21 July 2010)
A unique heterotrinuclear nitrate-bridged complex of hexanitrate praseodymium(III) and
dicopper(II) compartmental species has been synthesized and characterized by X-ray single
crystal structure analysis. The structure determination indicates that the dinuclear copper
moiety undergoes a tilted deformation (with respect to the dicopper complex) upon connection
to the lanthanide species via a rare nitrate bridge. The trinuclear species is highly fluorescent
owing to the presence of praseodymium.
Keywords: Copper-lanthanide complex; Praseodymium(III) hexanitrate; Crystal structure,
N-(3-Aminopropyl) morpholine
1. Introduction
Heterometallic complexes of transition metals, lanthanides, and mixed transition
metal–lanthanide ions bridged by a connecting ligand continue to attract intense
interest because of their importance in solid state technology [1–3], as models for
magnetic studies [4–7], as stains for fluorescence imaging [8, 9], and their use as
precursor complexes of magnetic materials in metal organic chemical vapor deposition
(MOCVD). A search in the literature reveals a plethora of transition metals [10–12] and
heterometallic transition metal–lanthanide (e.g., CuII–LnIII) complexes with polyden-
tate ligands coordinating the metal ions. On the other hand, structural studies showing
these metals bridged by nitrate are scarce, although nitrate represents a very
common bridging ligand acting through different coordination modes, as exemplified
in scheme 1 [13]. Specifically, polynuclear Cu4Ln2 and Cu2Ln salicylidene and Cu4Ln4
*Corresponding author. Email: dasdebasis2001@yahoo.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.520315

Heterotrinuclear Cu2-Pr complex
(b) (c)
3715
(d)
(a)
(e)
M
O
M
O
M
M
O
N
O
O
O
O
N
O
O
N
M
M
N
O
M
O
O
(f)
(g)
O
O
O
O
N
O
O
M
N
O
M
N
O
M
M
O
M
(h)
(i)
(j)
M
O
O
M
O
N
O
O
M
N
O
O
N
O
M
M
O
M
M
M
M
Scheme 1. Different possible coordination modes of nitrate ligand.
derivatives [14–20] and more recently a Cu2Gd2 complex built by a substituted bis
bidentate methylenedianiline ligand were reported [21]. All the cited examples contain a
bridging nitrate between copper and the lanthanide. Jianmin et al. [22] described a
unique compound where [La(NO3)6]3ꢀ connecting either a single or two Cu(bipy)2 units
in the same compound. However, examples of [La(NO3)6]3ꢀ interacting with a dicopper
core of an ‘‘end-off’’ compartmental ligand complex via nitrate bridge are not yet
known. This study deals with the formation of a new heterotrinuclear complex, where
two copper(II) ions in close proximity are linked to the lanthanide through bridging
nitrates, corresponding to coordination mode ‘‘(e)’’ of scheme 1.
2. Experimental
2.1. Methods
Elemental analyses (carbon, hydrogen, and nitrogen) were performed using a Perkin-
Elmer 240C elemental analyzer. Copper was estimated gravimetrically with ꢀ-benzoin
oxime. Infrared spectra (4000–400 cmꢀ1) were recorded at 27ꢁC using a Shimadzu
FTIR-8400S where KBr was used as medium. Electronic spectra (800–200 nm) were
obtained at 27ꢁC using a Shimadzu UV-3101PC where dry acetonitrile/dry methanol/
nujol was used as a medium as well as a reference. Thermal analyses (TG–DTA;

3716
K.S. Banu et al.
thermal gravimetry–differential thermal analysis) were carried out on a Shimadzu
DT-30 thermal analyzer in flowing dinitrogen (flow rate: 30 cm3 minꢀ1).
2.2. Materials
All materials were obtained from commercial sources and used as purchased.
Solvents were dried according to standard procedure and distilled prior to use.
2,6-Diformyl-4-methylphenol was prepared according to the literature method [23].
N-3-(aminopropyl) morpholine, Pr(NO3)3 ꢂ 6H2O and Cu(NO3)2 ꢂ 3H2O were purchased
from Aldrich Chemical Company Inc. and used as received. All other chemicals were of
AR grade.
2.3. Synthesis of L
2.3.1. 2,6-Bis(N-propylmorpholine-iminomethyl)-4-methylphenolato. An
ethanolic
solution (5 mL) of 1-(2-aminopropyl)morpholine (0.288 g, 2 mmol) was added dropwise
to a heated ethanolic solution (10 mL) of 2,6-diformyl-4-methylphenol (0.164 g, 1 mmol)
and the resulting deep yellow solution was stirred for 24 h. The solvent was removed
under reduced pressure giving crude ligand (yellow liquid). Purification of L was
done by column chromatography. Yield: 0.31 gm (87%). Anal. Calcd for L: C, 66.34;
H, 8.65; N, 13.46. Found: C, 66.33; H, 8.66; N, 13.44. UV-Vis (CH3OH)/nm:
452(sh, " ¼ 17,065), 343(" ¼ 3057). 1H-NMR 2.29(s, 3H) 1.87(m, 4H), 2.1–2.4(m, 15H),
3.4(m, 4H), 3.71(m, 8H), 7.68(s, 2H, Ar–H), 8.54(s, CH¼N), 10.51 (s, Ph–OH).
13C-NMR (ꢁ, CDCl3): 20.1, 30.7, 50.4, 56.8, 76.5, 127.0, 130.3, 132.1, 159.2, 165.00.
2.4. Synthesis of the complexes
2.4.1. [Cu(II)(L0)(H2O)(NO3)]2 (1). The dinuclear copper(II) complex [Cu(II)
(L0)(H2O)(NO3)]2 was synthesized by adopting similar procedure as reported
earlier [24].
2.4.2. [Cu2L02Pr(NO3)6] [L0 ^ 2-formyl-4-methyl-6-propylmorpholine-imonomethyl
phenolato] (2). The heterotrinuclear lanthanide copper complex was prepared by
three methods as follows.
2.4.2.1. Method (a). 1 mmol of Pr(NO3)3 ꢂ 6H2O in 15 mL of acetonitrile was added
to a heated suspension of 1 mmol of [Cu(L0)(H2O)(NO3)]2 in 20 mL acetonitrile and
stirred for 3 h at room temperature. Green microcrystals suitable for X-ray single
diffraction analysis were obtained after a few days. Yield: 60%.
2.4.2.2. Method (b). To methanolic solution of 1 mmol of L (1 mmol), 1 mmol of
10 mL of methanolic solution of Pr(NO3)3 ꢂ 6H2O (0.434 g) and 1 mmol of 10 mL copper
nitrate trihydrate (0.241 g) were added and refluxed for 2 h. From the green filtrate,
microcrystals suitable for X-ray single diffraction analysis were obtained. Yield: 70%.

Heterotrinuclear Cu2-Pr complex
3717
Table 1. Crystallographic data and details of refinement for 1.
Empirical formula
Formula weight, M
Crystal system
Space group
Unit cell dimensions (A)
C32H46Cu2N10O26Pr
1254.78
Tetragonal
P43212 (no. 96)
˚
a
c
12.500(3)
30.837(5)
4818.3(18), 4
1.730
1.967
2532
3
˚
Volume (A ), Z
Calculated density (g cmꢀ3
)
ꢃ(Mo-Kꢀ) (mmꢀ1
F(000)
)
Total data
Unique data
Rint
Parameters
Goodness-of-fit
15,703
3365
0.1080
320
0.778
3863
0.0554
0.1290
Reflections [I42ꢄ(I )]
R1
wR2 (I42ꢄ(I ))a
Flack parametꢀer3
ꢀ0.03(3)
˚
Residuals (e A
)
0.843 and ꢀ0.388
a1/[ꢄ2(Fo2) þ 0.0764P)2 þ 0.0000P], where P ¼ (Fo þ Fc2)/3.
2
2.4.2.3. Method (c). To a methanolic solution (10 cm3) of 2,6-diformyl-4-methylphe-
nol (1 mmol, 0.164 g), 2 mmol (0.288 g) of N-3-aminopropylmorpholine (in 5 mL
methanol) was added dropwise and refluxed for 2 h. Then 1 mmol of copper nitrate
trihydrate (0.241 g) and 1 mmol of Pr(NO3)3 ꢂ 6H2O (0.434 g) were mixed together in
5 mL methanol and the resulting mixture was added dropwise to the refluxing ligand
solution and reflux was continued for 2 h. It was then filtered and the filtrate was kept
in CaCl2 desiccator. Green micro crystals were obtained after a few days (Yield 65%).
Anal. Calcd for C32H46N10O26Cu2Pr: C, 30.60; H, 3.60; N, 11.15%. Found: C, 30.40;
H, 3.35; N, 11.20%. Selected IR data (KBr, cmꢀ1): 1633.7, 1560.3, 1457.1, 1323.7,
1384.3 ꢂ(NO3); UV-Vis (CH3OH) nm: 373, 695.
2.5. X-ray data collection and structure determination
Diffraction data for the structure were collected at room temperature on a Nonius DIP-
˚
1030H system (Mo-Ka radiation, l ¼ 0.71073 A). Cell refinement, indexing, and scaling
of the data sets were carried out using Denzo and Scalepack [24]. The structure was
solved by direct methods and subsequent Fourier analyses and refined by full-matrix
least-squares based on F 2 with all observed reflections [25]. The contribution of
hydrogens at calculated position was introduced in the final cycles of refinement.
In order to guarantee charge neutrality, a proton should be located on one of the
morpholine amino nitrogens or on a lattice water molecule (hydronium ion), but the
two-fold symmetry of the complex and the disordered solvent molecules complicated
resolution. Two residuals in the DF map were interpreted as lattice water molecules at
half occupancy (hydrogens not located). All the calculations were performed using the
WinGX System, Version 1.70.01 [26]. Crystallographic data and experimental details
are reported in table 1. Selected bond lengths and angles are shown in table 2.

3718
K.S. Banu et al.
ꢁ
˚
Table 2. Selected coordination bond lengths (A) and angles ( ) for the copper in 1.
1
Cu–N(1)
Cu–O(1)
1.913(11)
1.956(10)
1.970(11)
1.979(11)
2.493(8)
2.655(8)
92.5(5)
166.3(5)
80.4(5)
99.4(5)
O(1)–Cu–O(20)
O(10)–Cu–O(20)
N(1)–Cu–O(4)
O(1)–Cu–O(4)
O(10)–Cu–O(4)
O(20)–Cu–O(4)
Cu–O(1)–Cu0
166.6(5)
89.1(5)
92.7(4)
80.5(4)
97.7(4)
92.7(4)
99.6(5)
124.1(11)
71.5(4)
Cu–O(10)
Cu–O(20)
Cu–O(4)
Pr–O(4)
N(1)–Cu–O(1)
N(1)–Cu–O(10)
O(1)–Cu–O(10)
N(1)–Cu–O(20)
C(9)–O(2)–Cu0
O(4)–Pr–O(40)
Symmetry code: primed atoms at x, ꢀy þ 1/2, z.
3. Results and discussion
3.1. Synthesis and characterization
The ‘‘end-off’’ compartmental ligand L was synthesized by conventional Schiff-base
condensation maintaining aldehyde–amine ratio as 1 : 2 in ethanol. The ligand was
1
characterized by spectroscopic techniques. The H-NMR and 13C-NMR spectral data
of the ligand (Supplementary material), with their corresponding assignments, are
reported in section 2 and the data are consistent with the proposed structure.
The heterotrinuclear lanthanide–copper complex was prepared by sequential reaction as
well as by applying template synthesis. The composition and the structure of the
complex are determined by routine physico-chemical techniques, such as elemental
analysis, IR and UV-Vis methods, and X-ray single crystal structure analysis. During
complexation L was converted to L0.
The IR stretching frequencies of nitrates of this heterotrinuclear complex differ
significantly from those of the corresponding dicopper and Pr(NO3)6, species
(Supplementary material). In Pr(NO3)6 a highly intense broad band with maxima at
1447 and 1330 cmꢀ1 and in dicopper complex highly intense band with only one band
maxima at 1378 cmꢀ1 corresponding to nitrate are observed. The heterotrinuclear
copper–lanthanide complex exhibits highly intense band split into three, well-resolved
sharp bands at 1457, 1384, and 1324.
IR and UV-Vis stretching frequencies, attributed to azomethine ꢂ(C¼N), skeletated
vibrations, and nitrates ꢂ(NO3ꢀ) were observed at 1634, 1560, 1324, and 1384 cmꢀ1 for
the heterotrinuclear lanthanide–copper complex, respectively. Electronic spectra of
ligand and heterotrinuclear lanthanide–copper complexes have been studied in the solid
state (dispersed in Nujol mull) as well as in solution using CH3OH as solvent.
The absorption bands for the ligand are observed from 343 to 452 nm. Bands at
370–380 nm for the complex are due to both phenoxido–Cu(II) and hydroxide–Cu(II)
2
2
LMCT as a single band. For a d9 system, the Eg ! T2g transition is expected to take
place at 800 nm for octahedral geometry and this band will undergo a significant blue
shift on distortion of octahedral coordination to square-pyramidal and square-planar
structure [27]. In 1, the band at ꢃ690 nm due to the d–d transition is in agreement with
a square pyramidal geometry around copper. Bands at ꢃ1400 nm for the mixed
lanthanide–copper complex are due to f–f transitions. Since no significant shifting in

Heterotrinuclear Cu2-Pr complex
3719
Figure 1. ORTEP drawing of the copper moiety of 1 with atom labeling of the crystallographic independent
part (primed atoms at y, x, ꢀz þ 1).
band positions is observed in solid state electronic spectra compared to the
corresponding spectra recorded in solution, it may be assumed that solution and
solid state structures are very similar.
3.2. Structure description of 1 and 2
X-ray structural analysis confirms the formation of the trinuclear complex. An ORTEP
view of the phenoxido-bridged copper dimer of 1 with atom labeling scheme of the
independent part is shown in figure 1, while the entire complex is depicted in figure 2.
The compound can be described as an adduct formed by a dinuclear copper cationic
part bound to [Pr(NO3)6]3ꢀ. The lanthanide resides on a crystallographic two-fold axis.
The copper ions, chelated by unsymmetrical tridentate ligands, possess a distorted
square pyramidal coordination sphere. Two phenoxido-bridged oxygens, an imine
nitrogen, and a carbonyl oxygen are located in the basal plane. The Cu–N bond
˚
distance [1.913(11) A] appears slightly shorter than the Cu–O bond lengths which range
˚
from 1.956(10) to 1.979(11) A and the apical distance Cu–ONO2 is, as expected,
0
ꢁ
˚
significantly elongated (2.493(8) A). The bond angle Cu–O1–Cu of 99.6(5) leads to an
˚
intermetallic separation of 2.998(4) A. The praseodymium is chelated by six nitrates
˚
with Pr-O distances between 2.585(9) and 2.655(8) A, the longest value is that pertaining
to O4 bound to copper. In fact, O4 bridges the metals with a Cu–O4–Pr angle of
138.7(4). Conversely, making allowance for the esd.’s, the nitrate N–O4 bond length
˚
(1.308(14) A) is the longest among the nitrates. The intermetallic Pr–Cu distance is
˚
4.817(2) A. The morpholine moieties assume the usual chair conformation and
are pendant from the copper–phenolate dimer surface, opposite to [Pr(NO3)6].

3720
K.S. Banu et al.
Figure 2. Molecular structure of 1, showing the disordered lattice water molecules O1w and O2w interacting
with the copper center and through H-bond to nitrates and morpholine nitrogens.
Two residuals in the electron density map were attributed to disordered lattice water
(occupancy 0.5). One bridges at H-bond distance the morpholine nitrogen
˚
a nitrate oxygen of a symmetry related complex
(Ow2 ꢂ ꢂ ꢂ N2 ¼ 2.86 A) and
˚
˚
(Ow2 ꢂ ꢂ ꢂ O12 ¼ 2.87 A). The other (O1w) is located at 3.259 A from the copper–ion to
indicate possible six-coordination. The molecular structure of the dinuclear copper
derivative [Cu(L0)(H2O)(NO3)]2 [28] showed octahedral Cu(II) coordinated at axial
˚
positions by an aquo and a nitrate (axial distances of 2.450(7) and 2.484(10) A,
respectively). However, it is worth noting that the centrosymmetric copper–phenolato
dimer has practically coplanar atoms, contrary to the propeller-like twisted conforma-
tion assumed here by the phenolato units that seem to wrap the [Pr(NO3)6] moiety.
˚
˚
Nevertheless, the intermetallic copper distances (2.998(4) A in 1 and 3.015(1) A in the
dicopper precursor) appear comparable within 3ꢄ. A selection of bond lengths and
angles is given in table 2.
3.3. Absorption and fluorescence properties
The complex exhibits strong fluorescence in methanol (figure 3). The dinuclear
copper(II) complex is nearly non-fluorescent, as is evidenced from figure 3 (excitation
was done at lmax 386 nm, since the dinuclear copper(II) complex exhibits absorption
lmax at 386 nm). However, when the dinuclear copper complex is combined with the
praseodymium species, an intense fluorescence band, nearly twice as intense as the
fluorescence exhibited by pure Pr(NO3)3 ꢂ 6H2O (figure 4), was observed upon
excitation at lmax 371 nm in methanol (the complex shows a UV-Vis band at
lmax ¼ 371 nm) with quantum yield (’ ¼ 0.0155). The above-mentioned data suggest
that 1 exhibits luminescence owing to the presence of Pr, whereas dinuclear copper(II)
complex is non-fluorescent due to deactivation of excited state via non-radiative
channel owing to the presence of paramagnetic copper(II).

Heterotrinuclear Cu2-Pr complex
3721
50
40
30
20
10
0
a
b
400
450
500
550
600
Wavelength (nm)
Figure 3. Luminescence spectra (a) of 1 and (b) of [Cu(L0)(H2O)(NO3)]2 in methanol.
40
20
350
400
450
500
550
Wavelength (nm)
Figure 4. Luminescence of pure Pr(NO3)3 ꢂ 6H2O in methanol.
4. Conclusions
A heterotrinuclear copper(II)-praseodymium complex was synthesized, where the
praseodymium connected with both dinuclear copper(II) centers simultaneously
through a rare nitrate bridge. The fluorescence spectra of the dinuclear copper complex
and heterotrinuclear copper–lanthanide complex were investigated in methanol, where

3722
K.S. Banu et al.
the former remains non fluorescent and the latter exhibits good fluorescence due to the
presence of Pr(III).
Supplementary material
CCDC reference number for 1 is 730014. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif or from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge, CB2 lEZ, UK (Fax: þ44-1223/336-033; Email: deposit@ccdc.cam.ac.uk).
Acknowledgment
This study is supported by the UGC Major Research Project (F. no. 34-308/2008 (SR)
Dated: 31 December 2008).
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