Nickel(II) and palladium(II) complexes with acylhydrazone ligands of α-diketones: The electronic and steric factors affecting the formation of the dimeric palladium(II) complexes

Article abstract of DOI:10.1016/S0020-1693(99)00349-7

Palladium(II) acetate reacts with butane-2,3-dione bis(octanoylhydrazone) (H₂oct) giving a dimeric [Pd₂(oct)₂] complex as main product; as shown by X-ray crystal analysis, the ligands are bideprotonated and in each of them one carbonyl oxygen and the two imine nitrogens are bonded to one palladium ion, whereas one hydrazide nitrogen is bonded to the second palladium, giving rise to a N₃O square planar coordination. The core of the complex is constituted by an unusual six-membered PdN₂PdN₂ ring. The same arrangement is also found in [Pd₂(ben)₂] (H₂ben, butane-2,3-dione bis(benzoylhydrazone)) where the alkyl chains of the ligand H₂oct are substituted by phenyl rings. On the contrary, the corresponding nickel(II) complexes are monomers and the coordination is N₂O₂ square planar. Monomers are also the nickel(II) and palladium(II) complexes with butane-2,3-dione bis(thiobenzoylhydrazone) and phthalic dicarboxaldehyde bis(benzoylhydrazone).

Full text of DOI:10.1016/S0020-1693(99)00349-7

www.elsevier.nl/locate/ica
Inorganica Chimica Acta 295 (1999) 171–179
Nickel(II) and palladium(II) complexes with acylhydrazone ligands
of a-diketones: the electronic and steric factors affecting the
formation of the dimeric palladium(II) complexes
Alessia Bacchi, Mauro Carcelli *, Paolo Pelagatti, Corrado Pelizzi, Giancarlo Pelizzi,
Claudia Salati, Paolo Sgarabotto
Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Uni6ersita` di Parma, Parco Area delle Scienze 17/A,
43100 Parma, Italy
Received 12 May 1999; accepted 2 July 1999
Abstract
Palladium(II) acetate reacts with butane-2,3-dione bis(octanoylhydrazone) (H2oct) giving a dimeric [Pd2(oct)2] complex as main
product; as shown by X-ray crystal analysis, the ligands are bideprotonated and in each of them one carbonyl oxygen and the two
imine nitrogens are bonded to one palladium ion, whereas one hydrazide nitrogen is bonded to the second palladium, giving rise
to a N3O square planar coordination. The core of the complex is constituted by an unusual six-membered PdN2PdN2 ring. The
same arrangement is also found in [Pd2(ben)2] (H2ben, butane-2,3-dione bis(benzoylhydrazone)) where the alkyl chains of the
ligand H2oct are substituted by phenyl rings. On the contrary, the corresponding nickel(II) complexes are monomers and the
coordination is N2O2 square planar. Monomers are also the nickel(II) and palladium(II) complexes with butane-2,3-dione
bis(thiobenzoylhydrazone) and phthalic dicarboxaldehyde bis(benzoylhydrazone). © 1999 Elsevier Science S.A. All rights
reserved.
Keywords: Hydrazone; Nickel complexes; Palladium complexes; Dimer complexes
1(c)) even if some studies on their analytical use are
reported [8].
1. Introduction
As an extension of our work on the chelating proper-
ties of polydentate ligands derived from 1,2-diketones
The 1,4-disubstituted 1,4-diaza-1,3-butadiene (a-di-
imine) ligands (Scheme 1(a)) have been studied for their
versatile coordination behaviour and the interesting
properties of their metal complexes [1]. For example,
they can be used as metallomesogens [2] or as homoge-
neous catalysts in the copolymerization of styrene and
4-methylstyrene with carbon monoxide [3], or in the
hydrogenation of alkene or carbon–carbon cross-cou-
pling reactions [4]. These ligands are generally prepared
by condensation of a a-diketone with an amine, but
N-mono- [5] and N,N-di-substituted [6] hydrazines
(Scheme 1(b)) were also used. Less is known about the
coordination abilities of the ligands obtained from con-
densation of a-diketones and hydrazides [7] (Scheme
* Corresponding author. Fax: +39-0521-905 557.
E-mail address: carcelli@ipruniv.cce.unipr.it (M. Carcelli)
Scheme 1.
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00349-7

172
A. Bacchi et al. / Inorganica Chimica Acta 295 (1999) 171–179
[9], here we report the coordinating behaviour towards
palladium(II) and nickel(II) of some hydrazonic ligands
derived from 2,3-butadione (Scheme 1(c), R=C7H15,
C6H5) and phthalic dicarboxaldehyde (Scheme 1(d)).
The X-ray structure analysis of [Pd2(oct)2] (H2oct, bu-
tane-2,3-dione bis(octanoylhydrazone)), [Pd2(ben)2]·
EtOH (H2ben, butane-2,3-dione bis(benzoylhydra-
zone)), and Ni(tben) (H2tben, butane-2,3-dione bis-
(thiobenzoylhydrazone)) are also reported.
plates), [Pd2(ben)2]·EtOH (red flat prisms) and Ni(tben)
(dark green plates). Diffraction data for the compounds
were measured at room temperature on a Philips
,
PW1100 diffractometer, using Mo Ka (u=0.71073 A)
radiation, graphite monochromator, q/2q scan. In all
cases the intensity of one standard reflection was moni-
tored every 100 measurements, and no significant crys-
tal decay was observed throughout data collection. The
intensity data were processed with a peak-profile proce-
dure and corrected for Lorentz and polarization effects.
Table 1 reports a summary of relevant parameters
concerning data collection and structure refinement for
the compounds. For [Pd2(ben)2]·EtOH data were cor-
rected for absorption effects by using the „-scan
method. The phase problem was solved by direct meth-
ods, using SHELXS86 [11], which allowed the retrieval of
the molecular core; the structures were completed by a
combination of least-squares refinement cycles and
analysis of the Fourier difference maps. Neutral atomic
scattering factors were employed, those for non-hydro-
gen atoms being corrected for anomalous dispersion.
Structures were refined by full-matrix least-squares on
F2 with SHELXL97 [12] for [Pd2(oct)2] and by least-
2. Experimental
Reagents of commercial quality were used without
further purification. Physical measurements were made
as described elsewhere [10]. All new compounds gave
satisfactory elemental analyses.
2.1. Crystal structure determination of [Pd2(oct)2],
[Pd2(ben)2]·EtOH and Ni(tben)
Well shaped single crystals suitable for X-ray diffrac-
tometric analysis were obtained for [Pd2(oct)2] (orange
Table 1
Crystal data and structure determination parameters for compounds [Pd2(oct)2], [Pd2(ben)2]·EtOH and Ni(tben)
Compound
[Pd2(oct)2]
[Pd2(ben)2]·EtOH
Ni(tben)
Chemical formula
Molecular weight
Crystal system
Space group
Unit cell dimensions
C40H72N8O4Pd2
941.86
monoclinic
P21/c
C38H40N8O5Pd2
901.6
triclinic
C18H16N4S2Ni
411.2
triclinic
(
(
P1
P1
,
a (A)
20.617(6)
16.856(5)
14.982(4)
10.525(6)
22.872(14)
8.700(4)
93.1(1)
110.8(1)
103.2(1)
1885(2)
2
1.588
912
10.1
1.000/0.707
9.525(2)
13.853(5)
7.051(3)
91.83(4)
96.30(3)
97.85(2)
915.0(5)
2
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
V (A )
103.49(2)
3
,
5063(2)
4
1.236
1968
7.51
Z
Dcalc (g cm−3
F(000)
)
1.492
424
13.0
v (Mo Ka) (cm−1
)
Absorption Tmax/Tmin
q Range (°)
3–27
3–27
3–28
Index range
−265h526, 05k521,
−135h513, −275k527,
−125h512, −175k517,
05l517
10 782
05l510
8205
05l58
4421
Measured reflections
Independent reflections
10 373
8205
4421
Observed unique reflections [I]2|(I)] 2036
4115
1855
Parameters/restraints
R1 (observed data)
wR2 (observed data), a, b
Goodness of fit
495/464
630/0
290/0
0.0309 a
0.0400 c
0.0351 c
0.083, 0.15, 0.00 b
0.683
0.0415, 0.6885, 0.00225 d
0.699
0.0346, 0.3402, 0.00093 d
0.427
2
2
2
a R1=SꢀꢀFoꢀ −ꢀFcꢀ ꢀ/SꢀFoꢀ .
b wR2={S[w(Fo2−Fc2)2]/S[w(Fo2)2]}1/2}, w=1/[|2(Fo2)+(aP)2+bP], P=[2Fc2+max(Fo2, 0)]/3.
c R1=SꢀꢀFoꢀ−ꢀFcꢀꢀ/SꢀFoꢀ.
d Rw=SꢀwꢀFoꢀ−ꢀFcꢀꢀ/SwꢀFoꢀ, w=a/(|2(F)+bF2).

A. Bacchi et al. / Inorganica Chimica Acta 295 (1999) 171–179
173
squares on F with SHELX76 [13] for [Pd2(ben)2]·EtOH
2.2.4. Phthalic dicarboxaldehyde bis(benzoylhydrazone)
(H2phben)
(block-diagonal matrix) and Ni(tben) (full matrix). An
I\2|(I) cut off was applied on the reflections used
in the refinement. In the last stages of the refinement
anisotropic displacement parameters were used for all
non-hydrogen atoms. For [Pd2(oct)2] rigid-bond and
similarity restraints were applied to displacement
parameters of the alkyl chains, whose CꢀC bond
distances were also restrained to be similar in all
chains. Hydrogen atoms were introduced at calculated
positions, riding on their carrier atoms, for [Pd2(oct)2],
and were localized on the Fourier difference maps
and refined isotropically for [Pd2(ben)2]·EtOH and
Ni(tben). The final geometry was analyzed by the pro-
gram PARST95 [14] and the drawings were made with
ZORTEP [15]. All calculations were performed on
an Encore91 computer at the Centro di Studio per
la Strutturistica Diffrattometrica del CNR in Parma.
Besides referring to original literature, data for
comparison with other compounds were retrieved and
analyzed by the software packages of the Cam-
bridge Structural Database [16] (release October
1998).
A total of 0.5 g (3.7 mmol) of phthalic dicarboxalde-
hyde and 1.0 g of benzoylhydrazine (7.4 mmol) were
dissolved in 50 ml of ethanol and refluxed for 1 h.
During the refluxing a white powder started to sepa-
rate; on cooling an abundant white precipitate was
obtained. Yield\90%. M.p. 170–178°C. Mass spec-
trum (CI, m/e) 370 (MH+, 100%). 1H NMR (d6-
DMSO): l 12.05 (s, D2O exchangeable, 2H, NH), 8.96
(s, 2H, HCꢁN), 7.94–7.54 (m, 14H). Selected IR bands
(cm−1, KBr): 3263m,br w(NH), 3065m w(CHar), 1657vs
w(CO), 1562m w(CN).
2.2.5. Reaction of H2oct with Pd(CH3COO)2
A total of 0.15 g (0.41 mmol) of the ligand in 30 ml
methanol/chloroform and 0.092 g (0.41 mmol) of
Pd(CH3COO)2 in methanol were refluxed for 24 h; the
TLC (SiO2, ethyl acetate) showed the presence of two
products: a dark-green (Rf=0) and an orange one
(Rf=0.7). They were separated by column chromato-
graphy using as eluents ethyl acetate (orange product)
and then chloroform (green product). Well shaped
crystals of the orange product, which turned out to
have the formula [Pd2(oct)2], were obtained by recrys-
tallization from methanol/chloroform. Yield 45%. M.p.
115–116°C. Mass spectrum (CI, m/e): 940 (M+,
2.2. Synthesis and characterization
H2ben and Ni(ben) [17], thiobenzoylhydrazine [18],
butane-2,3-dione bis(thiobenzoylhydrazone) (H2tben)
and Ni(tben) [19] were obtained following literature
findings.
1
100%), 470, 365. H NMR (CDCl3): l 0.88 (m, 12H,
(CH2)6CH3), 1.28 (m, 24H, (CH2)3CH3), 1.58 and 1.69
(m, 4H, CH2(CH2)4CH3), 1.94 and 2.16 (s, 6H,
CH3CꢁN), 2.20–2.59 (m, 8H, CH2(CH2)5CH3). Se-
lected IR bands (cm−1, KBr): 2954–2854s w(CH),
1627vs,br w(CO). Green product. Yield 20%. M.p.
\300°C. 1H NMR (CDCl3): l 8.24, 8.65 (s, D2O
exchangeable, NH), 2.09 (m, CH3CꢁN), 1.26 (m,
(CH2)6CH3), 0.88 (m, (CH2)6CH3). Selected IR bands
(cm−1, KBr): 3100w,br w(NH), 2924–2900s w(CH),
1750–1612m w(CO).
2.2.1. Butane-2,3-dione bis(octanoylhydrazone) (H2oct)
A total of 1.04 g (5.25 mmol) of octanoic hydrazide
and 0.23 ml (2.62 mmol) of 2,3-butadione were
refluxed for 24 h in 50 ml of ethanol with a few drops
of glacial acetic acid as catalyst. The end of the
reaction was confirmed by TLC (SiO2; 2:8 hexane–
ethyl acetate). Yield 95%; pale-yellow solid. M.p.
1
240°C. Mass spectrum (CI, m/e) 367 (MH+, 60%). H
NMR (CDCl3): l 0.87 (t, 6H, (CH2)6CH3), 1.24–1.33
(m, 16H, (CH2)4CH3), 1.69 (m, 4H, CH2(CH2)4-
CH3), 2.07 (s, 6H, CH3CꢁN), 2.69 (t, 4H, CH2-
(CH2)5CH3), 8.60 (s, 2H, NH). Selected IR bands
(cm−1, KBr): 3188m w(NH), 2954–2851m w(CH), 1668
vs w(CO).
2.3. [Pd2(ben)2]
A total of 0.061 g (0.27 mmol) of Pd(CH3COO)2
dissolved in 10 ml of acetonitrile were added to 0.05 g
(0.26 mmol) of the ligand suspended in 10 ml of absolute
ethanol. The brown solution was stirred for 24 h at room
temperature, filtered on Celite to remove some traces of
black palladium and left to evaporate. The solid was
recrystallized from chloroform thus obtaining red crys-
tals useful for X-ray diffraction analysis. The same
product was obtained starting from the ligand and
Li2PdCl4; then the pH of the solution was adjusted to
approximately pH 9 by adding NaOH. Yield 72%. M.p.
120°C. 1H NMR (CDCl3): l 2.19 and 2.27 (s, 6H,
CH3CꢁN), 7.34–7.51 (m, 12H, Har), 7.94 (d, 4H, Har),
8.02 (d, 4H, Har). Selected IR bands (cm−1, KBr): 3020w
w(CH), 1650m, 1613s w(CO), 1598 w(CN).
2.2.2. Butane-2,3-dione bis(benzoylhydrazone) (H2ben)
1H NMR (d6-DMSO): l 2.03 (s, 6H, CH3), 7.48–
7.59 (m, 6H, CHar), 7.85 (d, 4H, CHar), 10.78 (m, 2H,
NH).
2.2.3. Butane-2,3-dione bis(thiobenzoylhydrazone)
(H2tben)
1H NMR (CDCl3): l 1.90 (s, 6H, CH3), 6.40 (s, 2H,
NH), 7.35–7.38 (m, 6H, CHar), 7.58–7.61 (m, 4H,
CHar).

174
A. Bacchi et al. / Inorganica Chimica Acta 295 (1999) 171–179
2.3.1. Ni(ben)
bonds. In solid state the situation seems to be the same,
in fact only one w(CꢁO) band is observable. The E,E
configuration for the two imine bonds and the anti
conformation around the central CꢀC bond were found
in all the butane-2,3-dione dihydrazones which have
been characterized by X-ray diffraction [20].
1H NMR (CDCl3): l 1.87 (s, 6H, CH3CꢁN), 7.18–
7.34 (m, 6H, Har), 7.87 (d, 4H, Har).
2.3.2. Pd(tben)
A total of 0.107 g (0.3 mmol) of the ligand in 20 ml
of absolute ethanol and 0.67
g (0.3 mmol) of
After the work-up of the reaction between palladi-
um(II) acetate and H2oct (see Section 2), two different
products are obtained, one orange and the other green.
The elemental analysis of the orange compound sug-
gests the formula Pd(oct); in the mass spectrum the
molecular peak is twice the value of Pd(oct). In the IR
spectrum a strong band at 1627 cm−1 attributable to
an uncoordinated CꢁO group is still present, whereas in
the 1H NMR spectrum the peaks of the hydrazone
hydrogens are absent but there are two sets of signals
for the two arms of the ligand thus suggesting an
asymmetric coordination mode.
Pd(CH3COO)2 in ethanol/acetonitrile were mixed and
stirred for 1 h at room temperature; immediately the
formation of a brown precipitate was observed. The
solid was collected and dried. Yield 68%. M.p.\300°C.
1H NMR (CDCl3): l 2.17 (s, 6H, CH3CꢁN), 7.28 (t,
4H, Har), 7.42 (t, 2H, Har), 8.01 (d, 4H, Har).
2.3.3. Ni(tben)
1H NMR (CDCl3): l 2.05 (s, 6H, CH3CꢁN), 7.31 (t,
4H, Har), 7.45 (t, 2H, Har), 8.01 (d, 4H, Har).
2.3.4. Pd(phben)
The single-crystal X-ray determination made clear
the structure of the complex: a perspective view of
[Pd2(oct)2] is shown in Fig. 1, along with the labelling
scheme. Table 2 reports the most relevant molecular
geometric parameters. The complex is a dinuclear
dimer, where two bideprotonated oct2− molecules act
as asymmetric tridentate ligands towards one metal and
as monodentate ligands towards the other metal. Each
metal is coordinated in a square planar fashion by one
carbonyl and both iminic nitrogen atoms belonging to
one ligand (O1, N2 and N3 for Pd1, and O3, N6 and
N7 for Pd2), and by the remaining amidic nitrogen
belonging to the other ligand (N8 for Pd1 and N4 for
Pd2). Both the coordination arrangements do not devi-
ate significantly from planarity. Thus, five chelation
rings are present, two five-membered pairs formed by
the tridentate arrangement of each ligand around the
metals, and one six-membered dipalladacycle defined by
the two NꢀN bridging units between the two metals
(N3ꢀN4 and N7ꢀN8). The amidic nitrogen atoms adja-
cent to the coordinated carbonyls (N1 and N5) are not
involved in coordination, as well as the carbonyl oxy-
gens adjacent to the coordinated amidic nitrogens (O2
and O4). The four five-membered chelation rings are all
A total of 0.05 g (0.13 mmol) of the ligand were
dissolved in boiling absolute ethanol, then 0.03 g (0.13
mmol) of Pd(CH3COO)2 in acetonitrile was added.
During the refluxing (2 h) the clear solution became
dark-green and some precipitate formed. The solvent
was removed and the powder redissolved in 40 ml of
chloroform; yellow microcrystals were obtained from
the chloroform solution by adding 10 ml of absolute
ethanol and leaving at −20°C for 1 week. Yield 75%.
1
M.p. 164–170°C (dec.). H NMR (d6-DMSO): l 9.41
(s, 2H, HCꢁN), 8.34 (dd, 2H, Har a(CꢁN)), 8.10 (d, 4H,
Har a(CꢁO)), 7.95 (dd, 2H, Har b(CꢁN)), 7.46–7.59 (m,
6H, Har). Selected IR bands (cm−1, KBr): 3050m,br
w(CHar), 2990w,br w(CH), 1592m w(CN).
2.3.5. Ni(phben)
A total of 0.1 g of the ligand (0.26 mmol) and a slight
excess of Ni(CH3COO)2·4H2O in hot absolute ethanol
were refluxed for 1 h. On standing an orange powder
formed that was dried and characterized. Yield 70%.
M.p. 261–262°C. 1H NMR (CDCl3): l 9.07 (s, 2H,
HCꢁN), 8.11 (d, 4H, Har a(CꢁO)), 7.79 (d, br 4H, Har),
7.45–7.35 (m, 6H, Har). Selected IR bands (cm−1
,
KBr): 3050m,br w(CHar), 2993w,br w(CH), 1595m
w(CN).
3. Results and discussion
The potentially quadridentate ligands H2oct (Scheme
1(c), R=C7H15) and H2ben (Scheme 1(c), R=C6H5)
are obtained by condensation of 2,3-butadione and the
opportune hydrazide in 1:2 molar ratio. In solution
1
they are symmetric (in the H NMR spectrum there is
only one peak for the two methyl groups and the two
hydrazone hydrogens), probably with the two arms in
the less hindered E,E configuration around the imine
Fig. 1. ORTEP view of [Pd2(oct)2], with thermal ellipsoids drawn at
30% probability level. For clarity, hydrogen atoms are omitted and
labelling of alkyl chains is not reported.

A. Bacchi et al. / Inorganica Chimica Acta 295 (1999) 171–179
175
Table 2
in five complexes is the bridging performed by acyclic
NꢀN systems, like those present in [Pd2(oct)2]. In gen-
eral, it is observed that the ligands of the former class
induce a boat conformation for the dimetallacycle, with
metals at the flaps, or in some cases a planar ring is
found. On the contrary, ligands similar to the present
one induce the butterfly folding described above. Of
particular interest is the comparison between [Pd2(oct)2]
and bis(m2-acetophenone N-phenylhydrazone-C,N,N%)-
bis(trimethylphosphite)-dipalladium [21], and bis(tri-
methoxyphosphine - (m2 - 2 - (1 - (4 - nitrophenylhydra-
zono)ethyl) phenyl-C,N,N%)-palladium) [22], the only
other palladium complexes containing acyclic NꢀN
bridges in the six-membered dimetallacycle. In both
cases the span of the butterfly wings (118 and 117°) is
narrower than in [Pd2(oct)2], whereas the metals are
,
Selected bond distances (A) and angles (°) with s.u.s in parentheses
for [Pd2(oct)2]
Bond lengths
Pd1ꢀO1
Pd1ꢀN2
Pd1ꢀN3
Pd1ꢀN8
Pd2ꢀO3
Pd2ꢀN4
Pd2ꢀN6
Pd2ꢀN7
O1ꢀC1
O2ꢀC13
O3ꢀC21
O4ꢀC33
N1ꢀN2
N1ꢀC1
2.05(1)
2.04(1)
2.02(1)
2.14(1)
2.040(9)
2.08(1)
1.97(1)
2.01(1)
1.36(3)
1.24(2)
1.34(2)
1.26(3)
1.40(3)
1.36(2)
1.31(2)
N3ꢀN4
N3ꢀC3
1.41(2)
1.38(2)
1.38(2)
1.37(2)
1.32(2)
1.31(2)
1.42(2)
1.33(2)
1.43(2)
1.51(3)
1.50(3)
1.54(2)
1.50(2)
1.49(2)
1.53(3)
N4ꢀC13
N5ꢀN6
N5ꢀC21
N6ꢀC22
N7ꢀN8
N7ꢀC23
N8ꢀC33
C1ꢀC4
C2ꢀC3
C13ꢀC14
C21ꢀC24
C22ꢀC23
C33ꢀC34
N2ꢀC2
,
slightly farther apart (Pd···Pd=3.788 and 3.803 A).
Bond angles
O1ꢀPd1ꢀN2
O1ꢀPd1ꢀN3
O1ꢀPd1ꢀN8
N2ꢀPd1ꢀN3
N2ꢀPd1ꢀN8
N3ꢀPd1ꢀN8
O3ꢀPd2ꢀN4
O3ꢀPd2ꢀN6
O3ꢀPd2ꢀN7
N4ꢀPd2ꢀN6
N4ꢀPd2ꢀN7
N6ꢀPd2ꢀN7
Pd1ꢀO1ꢀC1
Pd2ꢀO3ꢀC21
N2ꢀN1ꢀC1
Pd1ꢀN2ꢀN1
Pd1ꢀN2ꢀC2
The presence of the five condensed chelation rings in
the dinuclear unit induces major angular distortion in
the square coordination around the metals, mostly by
reducing to about 80° the angles between cis donors
belonging to the five-membered rings, and by widening
to about 100° the angles belonging to the six-membered
ring. The analysis of bond distances of the hydrazonic
skeleton of [Pd2(oct)2] shows variations difficult to ra-
tionalize completely on the basis of electronic delocal-
ization and steric strains, and probably mostly due to
statistical fluctuations. The most remarkable feature is
that there are no relevant differences between the bond
79.0(6)
159.0(5)
99.7(5)
79.9(6)
177.8(5)
101.3(5)
99.2(4)
80.7(5)
159.7(4)
178.7(4)
101.0(5)
79.1(6)
108(1)
Pd1ꢀN3ꢀN4
Pd1ꢀN3ꢀC3
Pd2ꢀN4ꢀN3
N6ꢀN5ꢀC21
Pd2ꢀN6ꢀN5
Pd2ꢀN6ꢀC22
Pd2ꢀN7ꢀN8
Pd2ꢀN7ꢀC23
Pd1ꢀN8ꢀN7
Pd1ꢀN8ꢀC33
O1ꢀC1ꢀN1
N2ꢀC2ꢀC3
N3ꢀC3ꢀC2
O3ꢀC21ꢀN5
N6ꢀC22ꢀC23
N7ꢀC23ꢀC22
123.2(9)
112(1)
116.0(8)
107(1)
117.4(9)
118(1)
124.9(9)
116(1)
113.5(8)
130(1)
129(2)
110(2)
119(1)
130(2)
114(2)
113(1)
105(1)
105(2)
119(1)
119(1)
,
length of the bridging (1.36(4) A) and non-bridging
,
(1.34(4) A) NꢀN systems, probably due to the possibil-
ity of charge delocalization on the central molecular
core — in the other two palladium complexes [21,22],
the bridging NꢀN distances are slightly longer than
,
planar within 0.04 A, whereas the six-membered dipal-
ladacycle has a butterfly shape with the hinge along the
N4ꢀN8 bond; consequently, the two square planar co-
ordination systems make a dihedral angle of 133°, and
,
here, 1.38 and 1.40 A, respectively, due to the presence
of aromatic substituents instead of the C(O)-alkyl
groups. The whole complex has a rough two-fold sym-
metry around the central dipalladacycle Pd1ꢀN3ꢀN4ꢀ
Pd2ꢀN7ꢀN8. The alkyl chains involved in the carbonyl
coordination (C4ꢀC10 and C24ꢀC30) are extended and
roughly contained in the planes of the butterfly wings,
whereas the free carbonyl protrude almost orthogonally
out of the hinge, as shown by the torsion angles
C3ꢀN3ꢀN4ꢀC13=71(2)° and C23ꢀN7ꢀN8ꢀC33=
69(2)°. The chains C33ꢀC40 and C13ꢀC20 extend in
different directions, perturbing the approximate overall
two-fold molecular symmetry (Fig. 1). The crystal
,
the Pd1···Pd2 distance is 3.724(2) A (Fig. 2). A great
number of six-membered dimetallacycles MꢀNꢀNꢀ
MꢀNꢀN are structurally known, but in almost all of
them the bridging groups belong to aromatic heterocy-
cles such as pyridazine, pyrazole or triazole; some
others contain bridging ethylendiaminic ligands. Only
,
packing shows no relevant interaction below 3.40 A,
with the exception of the contacts C31···O4(i )=3.35(2)
,
A, C31ꢀH···O4(i )=136(1)° and C32···O4(i )=3.35(2)
,
A, C32ꢀH···O4(i )=129(1)° (i= −x, 1−y, 2−z); the
extended alkyl chains of adjacent molecules face each
other.
The green product of the reaction between H2oct and
palladium(II) acetate has not been unambiguously
characterized. The spectroscopic data are not clear
Fig. 2. Butterfly geometry for five fused chelation rings in [Pd2(oct)2].
A similar arrangement is found in [Pd2(ben)2].

176
A. Bacchi et al. / Inorganica Chimica Acta 295 (1999) 171–179
The five-membered chelated rings are planar within
,
0.02 A, and the six-membered dipalladacycle has a
butterfly conformation with a dihedral angle of 121°
around the hinge at N4ꢀN8 (133° in [Pd2(oct)2]). The
smaller width of the butterfly span determines a shorter
,
Pd1···Pd2 distance for [Pd2(ben)2] (3.669(3) A) than for
,
[Pd2(oct)2] (3.724(2) A). The angular distorsions in-
duced by the chelation strains on the coordination
geometry are comparable with those observed above,
whereas the metal–donor distances, more precisely de-
termined than for [Pd2(oct)2], reveal that the bonds to
N2 and N6 are significantly shorter than the others,
due to the fact that the formal negative charge resonat-
ing along the NꢀN and CO systems is donated to only
one metal, whereas for N3, N4, N7 and N8 it is shared
by two metals. The analysis of bond distances along
the hydrazonic core shows that the bridging NꢀN
bonds are longer than the non-bridging ones (1.413(7)
Fig. 3. ORTEP view of [Pd2(ben)2], with thermal ellipsoids drawn at
50% probability level. For clarity, hydrogen atoms are omitted.
enough to rule out the presence of the acetate as
counterion. The band at 1750 cm−1 in the IR region
and the D2O exchangeable signals in the NMR spec-
trum imply that at least one arm of the ligand is not
deprotonated and its carbonyl group is not coordi-
nated to the metal. The complexing ability of the
ligand in the neutral form is in fact very poor: in the
reaction between NiCl2·4H2O and H2oct in methanol,
free ligand is clearly visible in the TLC (SiO2, 1:1
hexane–ethyl acetate) after 4 h at refluxing but if the
pH is risen above 9 by adding aqueous NaOH, the
complex forms immediately.
It is known [23] that the coordinating ability of the
carbonyl group in the acylhydrazones is deeply influ-
enced by the R substituents (Scheme 1(c)) and in par-
ticular it becomes greater as the conjugating ability of
R increases. With the aim to force the N2O2 coordina-
tion around the palladium ion by increasing the donor
ability of the CꢁO groups, the alkyl chains of H2oct
are substituted by phenyl rings thus obtaining the
H2ben ligand. It reacts with palladium(II) acetate giv-
ing a dimeric complex, [Pd2(ben)2], with the same spec-
troscopic peculiarities of [Pd2(oct)2]. Contrary to the
above reaction with H2oct, now only the dimeric
product formed: H2ben is easily deprotonated because
the negative charge can be delocalized in the phenyl
rings. The reaction was also made in high dilution
conditions (10−6 M), but the dimer was, again, the
only product.
,
and 1.37(1) A, respectively), in contrast with the find-
ings for [Pd2(oct)2].
Table 3
,
Selected bond distances (A) and angles (°) with s.u.s in parentheses
for [Pd2(ben)2]·EtOH
Bond lengths
Pd1ꢀO1
Pd1ꢀN2
Pd1ꢀN3
N8ꢀPd1
Pd2ꢀO3
Pd2ꢀN4
Pd2ꢀN6
Pd2ꢀN7
O1ꢀC1
O2ꢀC13
O3ꢀC21
O4ꢀC33
N1ꢀN2
N1ꢀC1
2.051(5)
1.904(6)
1.995(5)
2.032(5)
2.055(5)
2.057(6)
1.920(6)
2.017(5)
1.277(8)
1.22(1)
1.286(9)
1.237(8)
1.380(7)
1.327(8)
1.303(7)
N3ꢀN4
N3ꢀC3
1.414(7)
1.295(8)
1.35(1)
N4ꢀC13
N5ꢀN6
N5ꢀC21
N6ꢀC22
N7ꢀN8
N7ꢀC23
N8ꢀC33
C1ꢀC4
1.363(7)
1.334(8)
1.295(7)
1.412(7)
1.301(9)
1.320(8)
1.481(9)
1.47(1)
C2ꢀC3
C13ꢀC14
C21ꢀC24
C22ꢀC23
C33ꢀC34
1.51(1)
1.480(9)
1.46(1)
N2ꢀC2
1.52(1)
Bond angles
O1ꢀPd1ꢀN2
O1ꢀPd1ꢀN3
N8ꢀPd1ꢀO1
N2ꢀPd1ꢀN3
N8ꢀPd1ꢀN2
N8ꢀPd1ꢀN3
O3ꢀPd2ꢀN4
O3ꢀPd2ꢀN6
O3ꢀPd2ꢀN7
N6ꢀPd2ꢀN4
N7ꢀPd2ꢀN4
N6ꢀPd2ꢀN7
Pd1ꢀO1ꢀC1
Pd2ꢀO3ꢀC21
N2ꢀN1ꢀC1
Pd1ꢀN2ꢀN1
Pd1ꢀN2ꢀC2
79.5(2)
159.7(2)
100.2(2)
80.3(2)
176.1(2)
100.1(2)
100.6(2)
79.1(2)
Pd1ꢀN3ꢀN4
Pd1ꢀN3ꢀC3
Pd2ꢀN4ꢀN3
N6ꢀN5ꢀC21
Pd2ꢀN6ꢀN5
Pd2ꢀN6ꢀC22
Pd2ꢀN7ꢀN8
Pd2ꢀN7ꢀC23
N7ꢀN8ꢀPd1
C33ꢀN8ꢀPd1
O1ꢀC1ꢀN1
N2ꢀC2ꢀC3
N3ꢀC3ꢀC2
O3ꢀC21ꢀN5
N6ꢀC22ꢀC23
N7ꢀC23ꢀC22
125.0(4)
113.7(5)
108.9(4)
108.7(5)
118.5(4)
117.6(5)
124.1(4)
113.4(5)
112.1(4)
130.2(4)
127.0(7)
111.8(6)
115.6(6)
125.8(7)
113.9(6)
114.8(6)
A perspective view of the [Pd2(ben)2] molecule is
shown in Fig. 3, while relevant geometric parameters
are listed in Table 3. [Pd2(ben)2] is a dimeric dinuclear
complex like [Pd2(oct)2], in which two doubly deproto-
nated ben2− ligands bridge two metals by two NꢀN
systems. Each ligand acts as a N2O tetradentate donor
towards one metal and as a N monodentate donor
towards the other metal to complete the square coordi-
159.2(2)
173.8(3)
99.9(2)
80.1(2)
107.4(4)
107.9(4)
107.6(5)
118.5(4)
118.7(5)
,
nation, which in both cases is planar within 0.10 A.

A. Bacchi et al. / Inorganica Chimica Acta 295 (1999) 171–179
177
These findings can be compared with those observed
for the cation trans-diammine[2,3-butanedione bis-
(4-methoxybenzoylhydrazonato)]cobalt(III) [24], where
both the ligand arms are deprotonated and coordinate
the metal by the hydrazonic N and the carbonyl O.
This is the only other known crystal structure contain-
ing a butane-2,3-dione hydrazonic ligand [25]1: CꢁN=
1.307(1), NꢀN=1.37(1), NꢀC(O)=1.325(6), CꢁO=
,
1.295(4), C(O)ꢀC6H4OCH3=1.47(1) A. These values
are in good agreement with those observed for the NO
donor systems in [Pd2(ben)2] and indicate that both the
PdꢀNꢀNꢀCO chelation ring and the terminal aromatic
groups are involved in a certain degree of charge delo-
calization. The situation is significantly different for the
ligand arms containing the NN bridging groups and the
free carbonyls: the coordination of the second N with-
draws electronic density from the NꢀN bond and from
the bond between the first N and Pd, which both
elongate. The NꢀC(O) length is practically unaffected
by the different coordination mode, and its partial
multiple bond character is built at the expense of the
Pd2ꢀN4 and Pd1ꢀN8 bonds, which are practically
equivalent to Pd1ꢀN3 and Pd2ꢀN7. The shortening of
the uncoordinated CꢁO bonds is accompanied by a
slight lengthening of the CꢀC6H5 bonds. Even if the
structural data for [Pd2(oct)2] are less reliable, they
seem to suggest that the lack of resonance for the
C(O)-alkyl system shifts the electronic delocalization
more towards the NꢀC(O) and NꢀN bonds, the former
becoming sensitive to the CO coordination, the latter
increasing the bond order.
As observed for [Pd2(oct)2], the free carbonyls in
[Pd2(ben)2] point away from the condensed chelation
rings, as shown by torsion angles C3ꢀN3ꢀN4ꢀC13=
80(1)° and C23ꢀN7ꢀN8ꢀC33=64(1)°. The phenyl rings
bonded to the coordinated carbonyls are almost copla-
nar with the CꢁO groups (O1ꢀC1ꢀC4ꢀC9= −12° and
O3ꢀC21ꢀC24ꢀC29=11°), whereas the phenyls bonded
to the free carbonyls C13 and C33 are rotated by 35°
and −58°, respectively.
Fig. 4. ORTEP view of Ni(tben), with thermal ellipsoids drawn at 50%
probability level. For clarity, hydrogen atoms are omitted.
The ligand H2tben is synthesized starting from bu-
tane-2,3-dione and thiobenzoylhydrazine with the aim
to substitute oxygen with sulfur, that has more affinity
for palladium(II); the reactions with Ni(II) and Pd(II)
1
acetates give only one product. Considering that the H
NMR spectra are highly symmetric, the signals of the
hydrazone hydrogens are absent and in mass spectra
there is the molecular peak of the monomer, it is
possible to conclude that both complexes are mono-
meric and square planar, with the bideprotonated lig-
and that coordinates to the metal with the two imine
nitrogens and the two sulfur atoms. This arrangement
is confirmed by the X-ray structure analysis carried on
the Ni(tben) complex. The molecular structure and
labelling scheme of Ni(tben) are viewed in Fig. 4, and
relevant geometric parameters are listed in Table 4. The
neutral complex is monomeric and the doubly deproto-
nated tben2− ligand behaves as a symmetric N2S2
donor, by coordinating the Ni atom in a square planar
Table 4
,
Selected bond distances (A) and angles (°) with s.u.s in parentheses
for Ni(tben)
Bond lengths
NiꢀS1
NiꢀN2
NiꢀN3
NiꢀS2
C1ꢀS1
C1ꢀC4
C1ꢀN1
N1ꢀN2
2.160(2)
1.865(3)
1.862(4)
2.168(1)
1.750(5)
1.481(7)
1.317(6)
1.368(6)
N2ꢀC2
C2ꢀC3
C3ꢀN3
N3ꢀN4
N4ꢀC13
C13ꢀS2
C13ꢀC14
1.311(6)
1.488(6)
1.309(6)
1.377(5)
1.296(6)
1.768(5)
1.467(6)
The crystal packing is characterized by the presence
of an ethanol molecule, which is strongly hydrogen-
,
bonded to O4 (O30···O4=2.761(9) A, O30ꢀH···O4=
166(9)°). The only other relevant interaction below 3.40
,
,
A is: C6···O2(i )=3.353(9) A, C6ꢀH···O2(i )=130(7)°
(i=1−x, 1−y, 2−z).
Bond angles
S1ꢀNiꢀN2
S1ꢀNiꢀN3
S1ꢀNiꢀS2
N2ꢀNiꢀN3
N2ꢀNiꢀS2
N3ꢀNiꢀS2
S1ꢀC1ꢀN1
NiꢀS1ꢀC1
C1ꢀN1ꢀN2
NiꢀN2ꢀN1
From the literature [17] it is known that the corre-
sponding Ni(II) complex, Ni(ben), has a N2O2 square
planar structure: in the IR spectrum there are no
w(CꢁO) bands and in H NMR the two arms of the
ligand are equivalent and give rise to only one set of
signals.
86.4(1)
169.8(1)
103.44(5)
83.4(2)
170.1(1)
86.8(1)
123.2(4)
95.3(1)
110.6(4)
124.5(3)
NiꢀN2ꢀC2
N1ꢀN2ꢀC2
N2ꢀC2ꢀC3
C2ꢀC3ꢀN3
NiꢀN3ꢀC3
NiꢀN3ꢀN4
N3ꢀN4ꢀC13
N4ꢀC13ꢀS2
NiꢀS2ꢀC13
115.7(3)
119.7(4)
112.6(4)
111.9(4)
116.3(3)
123.7(3)
111.7(4)
123.1(4)
94.8(2)
1
1 The crystal structure of [Ni(o-phen)(ethoxy)(Hben)] is reported,
but structural parameters are not deposited or published.

178
A. Bacchi et al. / Inorganica Chimica Acta 295 (1999) 171–179
palladium. The corresponding nickel(II) derivative,
Ni(phben), is diamagnetic and has the same spectro-
scopic features as Pd(phben) therefore the same coordi-
nation is proposed.
geometry. The strain of the penta-atomic chelation
rings together with the steric hindrance of sulfur atoms
induce a substantial angular deformation in the square
coordination geometry, by widening the S1ꢀNiꢀS2 an-
gle (103.44(5)°) and narrowing the remaining ones,
which range from 83.4(1) to 86.8(1)°. The molecular
core, comprising the three fused chelation rings,
methyls C11 and C12, and atoms C4 and C14, is planar
It is possible to conclude that the palladium atom
preference for nitrogen donors instead of oxygen and
the ligand constrains lead to dimerization in [Pd2(oct)2]
and [Pd2(ben)2]. On the contrary, when the carbonyl
oxygens of the ligand are substituted by the softer and
larger sulfur atoms, the high affinity between palladi-
um(II) and sulfur stabilizes the monomeric Pd(tben)
form. If the ligand is obtained from a 1,4-diketone
instead of a 1,2-diketone, the central chelation ring
is seven-membered and correspondingly the OꢀMꢀO
angle narrows: this seems to release the ligand strains
that destabilize the N2O2 square planar coordination
and, as a result, the monomer Pd(phben) is obtained
as the only product.
,
within 0.07 A, and the phenyl groups C4ꢀC9 and
C14ꢀC19 form with it dihedral angles of 16 and 2°,
respectively. The electronic delocalization along the sys-
tem CꢁNꢀNꢀC(S)ꢀC6H5 is similar to the one observed
for the non-bridging arm of the ligand in [Pd2(ben)2].
The coordination is described by the averaged bond
distances: NiꢀS=2.164(6), NiꢀN=1.863(4), CꢀS=
,
1.76(1) A. In the crystallographic literature, only one
complex containing the ligand tben — diacetyl-bis-
(thiobenzoylhydrazonato) - dioxo - (3,6 - diphenyl - 9 -
methyl - 4,5,7,8 - tetraza - 2 - thiabicyclo(4.4.0)dec - 3,6,8 -
trien-10-olato)-uranium(ii) [26] — and one closely re-
lated nickel complex — butane-2,3-dione-di(phenyl-
thioacetyl-hydrazonato) nickel(ii) [27] — are known.
The nickel complex is identical to Ni(tben), except for
the fact that the terminal ꢀCH2ꢀC6H5 substituents do
not allow the extension of the resonance beyond the
ꢀC(S)ꢀ groups. The reported average bond distances
are: NiꢀN=1.853(1), NiꢀS=2.151(1), CꢀS=1.746(6),
Acknowledgements
Thanks are due to the Centro di Studio per la
Strutturistica Diffrattometrica del CNR and Centro
Interfacolta` Misure ‘Giuseppe Casnati’ of Parma for
technical assistance.
,
CꢁN=1.301(5), NꢀN=1.39(1), NꢀC(S)=1.294(8) A.
It is seen that the larger electron-withdrawing capabili-
ties of terminal phenyls in Ni(tben) determine an elon-
gation of the metal–donor and CꢀS distances.
The crystal packing of Ni(tben) shows no relevant
features apart from van der Waals interactions.
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