Synthesis and crystal structure of dinuclear cyclopalladated 1,2- and 1,3-bridged squarato complexes

Article abstract of DOI:10.1016/S0020-1693(00)00091-8

Two new dinuclear Pd(II) cyclometalated complexes of formula [L₂Pd₂(μ-l,2-sq)(C₅H₁₁N)₂] (1) and [L₂Pd₂(μ-1,3-sq)(C₅H₁₁N)₂]·2[(C₅H₁₂N)BF₄] (1a) [HL= 2,6-dimethylazobenzene], have been synthetized from a reaction of the mononuclear [LPd(acetone)₂][BF₄] with the piperidinium salt of squaric acid, [C₅H₁₂N]₂[sq]. The single crystal X-ray analysis of complexes 1 and 1a shows that the sq ligand is found to bridge in both μ-1,2- and μ-1,3-coordination modes. These are the first examples of squarato Pd(II) complexes characterized structurally up to now. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(00)00091-8

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Inorganica Chimica Acta 304 (2000) 219–223
Synthesis and crystal structure of dinuclear cyclopalladated
1,2- and 1,3-bridged squarato complexes
Alessandra Crispini *, Daniela Pucci, Iolinda Aiello, Mauro Ghedini
Dipartimento di Chimica, Uni6ersita` della Calabria, I-87030 Arca6acata (CS), Italy
Received 28 October 1999; accepted 28 January 2000
Abstract
Two new dinuclear Pd(II) cyclometalated complexes of formula [L2Pd2(m-1,2-sq)(C5H11N)2] (1) and [L2Pd2(m-1,3-
sq)(C5H11N)2]·2[(C5H12N)BF4] (1a) [HL=2,6-dimethylazobenzene], have been synthetized from a reaction of the mononuclear
[LPd(acetone)2][BF4] with the piperidinium salt of squaric acid, [C5H12N]2[sq]. The single crystal X-ray analysis of complexes 1
and 1a shows that the sq ligand is found to bridge in both m-1,2- and m-1,3-coordination modes. These are the first examples of
squarato Pd(II) complexes characterized structurally up to now. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Crystal structures; Palladium complexes; Cyclometalated complexes; Squarato complexes
ands [8], we have investigated the formation of
complexes between sq and the cyclopalladated 2,6-
dimethylazobenzene molecular fragment LPd(II). We
report here the synthesis, characterization and crystal
structures of two new binuclear Pd(II) cyclometalated
complexes containing the m-1,2- and m-1,3-bridged
squarato ligands, respectively. At the time of writing, as
far as we know, these are the first examples of squarato
1. Introduction
The coordination chemistry of the squarato anion
(sq=C4O42−, the dianion of 3,4-dihydroxycyclobut-3-
ene-1,2-dione) has attracted the interest of researchers
in many different areas because of the various coordi-
nation modes it can display. Several transition metal
complexes, in which sq behaves as a bridge connecting
two or more metal atoms, have been described.
In the major part of the structurally characterized
complexes of metals such as Fe(II), Fe(III), Ni(II) and
Cu(II), sq is coordinated in a m-1,3 fashion giving
binuclear [1] and chain structures [2], whereas the m-1,2
coordination mode has been reported for binuclear and
chain complexes of Cu(II), Ni(II) [3,4] and Pt(II) [5].
Moreover, with Cu(II) and Ni(II), sq has been found to
act as a tetramonodentate ligand and polynuclear com-
pounds have been obtained [6]. In all cases, the metal
squarate complexes have been found interesting in
terms of structural relationship between their respective
solid state architectures [7].
palladium(II)
structurally.
complexes
to
be
characterized
2. Experimental
Elemental analyses were performed with a Perkin–
Elmer 2400 analyzer. The infrared spectra were ob-
tained on a Perkin–Elmer F.T.2000, as KBr pellets. All
the commercially available chemicals were used as re-
ceived. The complex [LPd(m-Cl)]2 was prepared as de-
scribed previously [9].
2.1. Synthesis of 2,6-dimethylazobenzene squarato
bridged cyclopalladated complex
[L2Pd2(v-sq)(C5H11N)2] (1)
As part of our solid state studies on dinuclear cyclo-
palladated complexes containing different bridging lig-
* Corresponding author. Tel.: +39-984-492 047; fax: +39-984-
492 044.
E-mail address: a.crispini@unical.it (A. Crispini)
In a typical preparation, a sample of 0.1 g of [LPd(m-
Cl)]2 (0.14 mmol) was added to a solution of AgBF4
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(00)00091-8

220
A. Crispini et al. / Inorganica Chimica Acta 304 (2000) 219–223
2.3. Single-crystal analyses
(0.05 g, 0.28 mmol) in acetone (5 ml); the mixture was
stirred overnight and filtered to remove the AgCl. The
filtrate was concentrated and added to a suspension of
squaric acid (0.016 g, 0.14 mmol) and piperidine (0.029
ml, 0.28 mmol) in methanol (12 ml). The mixture was
shaken for 24 h and the orange solid formed, complex
1, was filtered off, recrystallized from chloroform–
methanol and dried under reduced pressure. Yield:
0.054 g (42%). Anal. Calc. for C42H48N6O4Pd2: C,
55.21; H, 5.30; N, 9.20. Found: C, 53.77; H, 5.21; N,
9.53%.
A summary of selected crystallographic data for 1
and 1a are given in Table 1. A poor quality orange
crystal of 1 (0.24×0.30×0.08 mm) was obtained from
a solution of CHCl3–CH3OH while a red–orange crys-
tal of 1a of dimensions 0.40×0.22×0.35 mm was
isolated from the acetone–methanol mother solution of
1. Diffraction measurements were carried out on a
Siemens R3m/V automated four-circle diffractometer
equipped with a graphite-monochromated Mo Ka radi-
,
ation (u=0.71073 A). The data were corrected for
Lorentz polarization effects. For complex 1, an empiri-
cal absorption correction was applied using a method
based upon azimuthal (c) scan data [10] (min/max
coefficient transmission of 0.462/0.757). Both structures
were solved using the SHELXTL PLUS package [11] and
refined by full-matrix least-squares refinement on F.
While the Pd, N, O, the squarato C atoms were an-
isotropically refined for complex 1, all non-H atoms
were assigned anisotropic thermal parameters for com-
2.2. Synthesis of 2,6-dimethylazobenzene squarato
bridged cyclopalladated complex,
[L2Pd2(v-sq)(C5H11N)2]·2[(C5H12N)BF4] (1a)
The mother solution obtained from the separation of
1 gave rise to red–orange crystals of 1a by slow evapo-
ration in air. Several attempts to synthesize 1a sepa-
rately have been made unsuccessfully. Anal. Calc. for
C52H72N8O4B2F8Pd2: C, 49.59; H, 5.76; N, 8.90. Found:
C, 49.15; H, 5.94; N, 9.49%.
plex 1a. Hydrogen atoms were placed in calculated
2
,
positions with fixed (U=0.08 A ) contributions. The
weighting scheme used in the last refinement cycle was
2
w
−1=|2ꢀFoꢀ+qꢀFcꢀ , with q=0.0049 for 1 and q=
Table 1
Crystal data and structure refinement for complexes 1 and 1a
0.0094 for 1a. A co-crystallized molecule of CH3OH
was found for complex 1 with a site-occupation factor
of 0.5. Only the isotropic thermal parameters of the
solvent molecule atoms were refined and the hydrogen
atoms were not included in the refinement.
Complex
1
1a
Empirical formula
C42H48N6O4Pd2
·0.5CH3OH
929.7
C42H48N6O4Pd2
·2[C5H12N]BF4
1259.6
Formula weight
Temperature (K)
298
298
,
Wavelength (A)
Crystal system
0.71073
monoclinic
0.71073
triclinic
3. Results and discussion
Unit cell dimensions
Reaction of the dinuclear chloro-bridged complex
[LPd(m-Cl)]2 with two equivalents of silver tetrafluorob-
orate salt, followed by treatment of the mononuclear
solvato adduct, [LPd(acetone)2][BF4], with a methanolic
suspension of the piperidinium salt of squaric acid,
[C5H12N]2[sq], yielded two different crystalline materi-
als, 1 and 1a. The former is the main reaction product
and was obtained as an orange microcrystalline solid
which precipitate from the acetone–methanol solution,
while the latter is a by-product which was isolated as
red–orange crystals after slow evaporation of the
mother solution.
Complexes 1 and 1a were characterized by elemental
analysis, IR spectroscopy and single crystal X-ray
diffraction.
Elemental analysis data for both 1 and 1a show the
presence of two coordinated piperidine ligands. In addi-
tion, in the case of complex 1a, the data accounted for
1 plus two co-crystallized molecules of piperidinium
tetrafluoroborate salt. The IR spectra of 1 (1473 and
1515 cm−1) and 1a (1530 cm−1) show strong bands
which can be assigned to a combination of CꢀO and
,
a (A)
13.756(6)
16.684(6)
18.950(9)
11.466(11)
11.875(5)
13.062(7)
113.11(4)
108.24(6)
101.97(6)
1438(2)
1
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
93.13(4)
3
,
V (A )
4343(3)
4
1.422
0.875
Z
Dcalc (g cm−3
)
1.452
0.700
Absorption coefficient
(mm−1
F(000)
)
1900
1.5–24
646
1.5–22.5
q Range for data
collection (°)
Index ranges
05h515,
05k519,
−215l521
6851/2107
05h512,
−125k512,
−145l513
3769/2910
Reflections
unique/observed
[I\2|(I)]
Parameters
305
343
Final R indices R a, R% b 0.0837, 0.0850
0.0455, 0.0490
0.60
Goodness-of-fit
1.05
a R=ꢁꢀꢀFoꢀ−ꢀFcꢀꢀ/ꢁꢀFoꢀ.
b R%=[ꢁ w(ꢀFoꢀ−ꢀFcꢀ)2/ꢁ wꢀFoꢀ ]
.
2 1/2

A. Crispini et al. / Inorganica Chimica Acta 304 (2000) 219–223
221
piperidine nitrogen atom complete the coordination
sphere of each metal atom. The internal geometry of
the 2,6-dimethylazobenzene ligand is normal, with bond
distances and angles in good agreement with those
observed for previous described Pd(II) azobenzene
complexes [13]. The orthometalated phenyl rings are
essentially coplanar with the five-membered cyclopalla-
dated rings, while the rotationally free phenyl rings are
significantly twisted with tilt angles of 77 and 97°.
These values are very similar to those found in other
structurally characterized Pd(II) complexes containing
2,6-dimethylazobenzene ligand [8]. The piperidine lig-
ands are found to be in a chair conformation and the
angles in the rings are close to tetrahedral (109–116°).
The least-square planes through the rings form angles
of 71 and 69° with the respective coordination planes.
The mean plane through the squarato ligand
Fig. 1. Crystal structure of complex 1. H atoms and CH3OH
molecules are omitted for clarity. Thermal ellipsoids are shown at the
40% level.
Table 2
Selected structural parameters for complex 1
,
[−0.113(2) A maximum deviation from planarity at
,
O(3) and 0.102(2) A at O(4)] is found to be nearly
Pd(1)ꢀN(1)
Pd(1)ꢀO(1)
Pd(2)ꢀN(4)
Pd(2)ꢀO(2)
O(1)ꢀC(1)
O(3)ꢀC(3)
C(1)ꢀC(2)
C(2)ꢀC(3)
2.03(2)
2.12(2)
1.99(2)
2.13(2)
1.28(3)
1.21(3)
1.46(4)
1.48(4)
Pd(1)ꢀN(3)
Pd(1)ꢀC(5)
Pd(2)ꢀN(6)
Pd(2)ꢀC(24)
O(2)ꢀC(2)
O(4)ꢀC(4)
C(1)ꢀC(4)
C(3)ꢀC(4)
2.11(2)
1.98(3)
2.10(2)
1.97(3)
1.25(4)
1.25(4)
1.45(4)
1.49(5)
orthogonal to the two coordination mean planes (83
and 79°, respectively). The PdꢀO bond lengths of
,
2.12(2) and 2.13(2) A are found to be similar to those
observed for a palladium(II) complex with m-oxalate
,
bridge (2.09(1) and 2.14(1) A) [14]. The two CꢀO
coordinated bond distances, as well as the ketonic
C(3)ꢀO(3) and C(4)ꢀO(4) ones (Table 2), are in the
range of values found for the free squaric acid and its
salts (1.21–1.32 for single CꢀO bond, 1.21–1.28 for
ketonic CꢁO bond) [15,16].
N(1)ꢀPd(1)ꢀN(3) 171.3(9)
N(1)ꢀPd(1)ꢀO(1)
N(1)ꢀPd(1)ꢀC(5)
O(1)ꢀPd(1)ꢀC(5)
N(4)ꢀPd(2)ꢀO(2)
N(4)ꢀPd(2)ꢀC(24)
93.2(9)
77.8(10)
170.4(9)
92.6(9)
N(3)ꢀPd(1)ꢀO(1)
N(3)ꢀPd(1)ꢀC(5)
94.1(8)
94.7(10)
N(4)ꢀPd(2)ꢀN(6) 172.5(9)
N(6)ꢀPd(2)ꢀO(2)
N(6)ꢀPd(2)ꢀC(24)
93.2(7)
94.9(10)
79.1(11)
Fig. 2(a) shows a perspective view of complex 1a.
The structure is built of discrete dinuclear molecules,
where a squarato anion bridges in a m-1,3-bis(monoden-
tate) fashion two centrosymmetrically palladium atoms
O(2)ꢀPd(2)ꢀC(24) 171.2(10)
CꢀC bond stretching vibrations. As far as the sq ligand
is concerned, it has been pointed out previously that the
IR spectroscopy is not a useful tool to distinguish
between its different bonding modes [12]. Thus, the
above data can only demonstrate that in 1 and 1a the
sq ligand experiences two different electronic environ-
ments. Moreover, with respect to complex 1a, the pres-
ence of the piperidinium tetrafluoroborate salt is
supported by the observation of the characteristic te-
trafluoroborate broad band between 1107 and 1037
,
(intramolecular Pd···Pd distance of 7.904(1) A). The
two molecules of piperidinium tetrafluoroborate salt are
such that the piperidinium cations are linked by ionic
and hydrogen-bonding interactions (Fig. 2(a)). Main
bond distances and angles are given in Table 3. No
main differences with complex 1 are observed with
reference to the geometrical parameters of the azoben-
zene ligand, the metallocycle moieties or the piperidine
ligand.
The squarato anion is planar and located in a plane
nearly orthogonal to the coordination mean plane (di-
hedral angle 91.2(5)°). The uncoordinated oxygen atom
of the sq ligand, O(2), is subject only to hydrogen bond
interaction with the nitrogen atom of the piperidinium
cm−1
.
The actual squarato coordination modes were deter-
mined by single crystal X-ray analysis. A perspective
view of complex 1 with the atomic-labeling scheme is
presented in Fig. 1. Selected bond lengths and angles
are given in Table 2. The structure consists of binuclear
,
cation (N···O(2) distance of 2.72(1) A). The CꢀC bond
,
lengths (mean 1.456(10) A) are in the same range as
those reported for other O1O3 bis(monodentate)
units
built
by
two
palladium(II)
moieties,
LPd(C5H11N), bridged by a squarato ligand coordi-
,
squarates (means range from 1.445 to 1.481 A). The
nated to the Pd(II) ions in a m-1,2 mode and with an
CꢀCꢀC angles are close to 90° and the OꢀCꢀC angles
vary between 132.8 and 136.3° as in all reported struc-
tures in which the squarato is not chelating.
,
intramolecular Pd···Pd distance at 7.354(4) A. Each
palladium(II) ion displays a distorted square-planar
coordination with
azobenzene ligand. One squarato oxygen atom and the
a
cyclometalated 2,6-dimethyl
It is worth noting that the squarato anion is sand-
wiched between two rotationally free phenyl rings along

222
A. Crispini et al. / Inorganica Chimica Acta 304 (2000) 219–223
the a axis, with an interplanar distance and angle of
crystal packing, each rotationally free phenyl ring of
one molecule is found to be parallel and separated by
,
3.48(3) A and 18.6(2)°, respectively (Fig. 2(b)). In the
,
an interplanar distance of 3.18(2) A with the one from
another molecule.
Concluding, through reaction between [LPd(ace-
tone)2][BF4] and [C5H12N]2[sq] two different dimeric
complexes, 1 and 1a, both characterized by single crys-
tal X-ray diffraction analysis, have been isolated. It is
worth noting that no crystal structures of Pd(II) com-
plexes containing the squarato anion coordinated in
different modes have been described so far. Complex 1
contains the squarato ligand coordinated in a m-1,2
fashion whereas complex 1a, which stoichiometrically
differs from 1 by two molecules of [C5H12N][BF4]
weakly interacting via ionic and hydrogen bonds, ex-
hibits a m-1,3-bonded squarato anion. Thus, these re-
sults confirm further how delicate the electronic balance
is which controls the coordination chemistry of the
squarato dianion.
4. Supplementary material
Atomic coordinates and Ueq, intramolecular bond
distances and angles, hydrogen coordinates and
isotropic displacement parameters, anisotropic displace-
ment parameters for 1 and 1a (13 pages) can be ob-
tained from the authors on request.
Acknowledgements
This work was partially supported by the Italian
Ministero dell’Universita´ e della Ricerca Scientifica e
Tecnologica (MURST) and Regione Calabria (POP
94/99).
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Table 3
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Pd(1)ꢀN(1)
Pd(1)ꢀO(1)
O(1)ꢀC(1)
C(1)ꢀC(2)
C(2)ꢀC(1a)
2.004(7)
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Pd(1)ꢀN(3)
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O(2)ꢀC(2)
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2.079(7)
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O(1)ꢀPd(1)ꢀC(5)
93.7(2)
78.4(3)
171.2(3)

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