Coordination properties towards palladium(II) of a tridentate dianionic ligand acting as a N- or a N,O-donor

Article abstract of DOI:10.1016/j.ica.2005.06.073

The new palladium(II) complex Pd[C₅H₃N-2,6-(CONPh) ₂](η¹-NCMe) (1), prepared from N,N′-diphenyl-2, 6-pyridinedicarboxamide and Pd(OAc)₂ in acetonitrile, has been characterized via IR, ¹H NMR and single-crystal X-ray diffraction. In this compound the palladium centre is coordinated to three nitrogen donors of the anionic ligand and to the nitrogen atom of acetonitrile. Moreover, the already known Pd[C₅H₃N-2,6-(CONCH₂CH ₂Ph)₂](η¹-NCMe) (2) has been studied by ¹H NMR spectrometry and found to readily convert into the macrocyclic tetranuclear species 3, {Pd[C₅H₃N-2,6-(CONCH ₂CH₂Ph)₂]}₄ which has been isolated and characterized by IR, ¹H and ¹³C{¹H} NMR, ¹H-¹³C HETCOR and mass spectrometry, as well as by single-crystal X-ray diffraction. In 3, of S₄ symmetry, each palladium atom is coordinated to the three nitrogen atoms of the anionic ligand, while the fourth coordination position is occupied by the amidato oxygen atom of an adjacent unit. This structure is apparently maintained in CDCl₃ solution. The substitution reactions of acetonitrile in 2 with the ligands EEt₂ (E = S, Se) afford Pd[C₅H₃N-2,6- (CONCH₂CH₂Ph)₂](EEt₂) (4, E = S; 5, E = Se); these products can also be obtained by the addition of EEt₂ to 3, as shown by means of ¹H- and, in the case of E = Se, ⁷⁷Se{¹H} NMR spectroscopy in CDCl₃ solution. These results show that the Pd-O bonds of the tetranuclear species are readily broken by weakly coordinating ligands such as acetonitrile and diethylchalcogenides. Nevertheless, we are dealing with equilibrium reactions and, in some solvents, 3 can be obtained from 2, 4 or 5 being favoured by its low solubility.

Full text of DOI:10.1016/j.ica.2005.06.073

Inorganica Chimica Acta 359 (2006) 127–135
www.elsevier.com/locate/ica
Coordination properties towards palladium(II) of a tridentate
dianionic ligand acting as a N- or a N,O-donor
a,
a
a
Daniela Belli DellꢀAmico *, Fausto Calderazzo , Francesco Di Colo ,
b
a
a
Gianfranco Guglielmetti , Luca Labella , Fabio Marchetti
a
`
Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, Via Risorgimento 35, I-56126 Pisa, Italy
b
Polimeri Europa, Istituto ‘‘G. Donegani’’, Via G. Fauser 4, I-28100 Novara, Italy
Received 26 May 2005; accepted 13 June 2005
Available online 8 September 2005
Abstract
The new palladium(II) complex Pd[C5H3N-2,6-(CONPh)2](g1-NCMe) (1), prepared from N,N0-diphenyl-2,6-pyridinedicarboxa-
mide and Pd(OAc)2 in acetonitrile, has been characterized via IR, 1H NMR and single-crystal X-ray diffraction. In this compound
the palladium centre is coordinated to three nitrogen donors of the anionic ligand and to the nitrogen atom of acetonitrile.
Moreover, the already known Pd[C5H3N-2,6-(CONCH2CH2Ph)2](g1-NCMe) (2) has been studied by 1H NMR spectrometry and
found to readily convert into the macrocyclic tetranuclear species 3, {Pd[C5H3N-2,6-(CONCH2CH2Ph)2]}4 which has been isolated
and characterized by IR, 1H and 13C{1H} NMR, 1H–13C HETCOR and mass spectrometry, as well as by single-crystal X-ray diffrac-
tion. In 3, of S4 symmetry, each palladium atom is coordinated to the three nitrogen atoms of the anionic ligand, while the fourth
coordination position is occupied by the amidato oxygen atom of an adjacent unit. This structure is apparently maintained in CDCl3
solution. The substitution reactions of acetonitrile in 2 with the ligands EEt2 (E = S, Se) afford Pd[C5H3N-2,6-(CON-
CH2CH2Ph)2](EEt2) (4, E = S; 5, E = Se); these products can also be obtained by the addition of EEt2 to 3, as shown by means of
1H- and, in the case of E = Se, 77Se{1H} NMR spectroscopy in CDCl3 solution. These results show that the Pd–O bonds of the tet-
ranuclear species are readily broken by weakly coordinating ligands such as acetonitrile and diethylchalcogenides. Nevertheless, we are
dealing with equilibrium reactions and, in some solvents, 3 can be obtained from 2, 4 or 5 being favoured by its low solubility.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Palladium; Chalcogenoethers; Equilibrium study; X-ray
1. Introduction
In order to simplify the problem we wanted to possi-
bly use a metal-containing precursor whereby one coor-
dination position only could be substituted by the
selected ligand. This restricted our choice to a tridentate
di-anionic ligand. After an examination of the literature
our choice was directed towards two specific compounds
[3], namely N,N0-diphenyl-2,6-pyridinedicarboxamide
(dphpaH2) and N,N0-bis(2-phenylethyl)-2,6-pyridinedi-
carboxamide (bphepaH2), the latter, see below, contain-
ing an ethylene bridge between the NH and the Ph
groups [3a]. Both compounds are readily prepared and
they had already been established to form the corre-
sponding conjugate base upon double deprotonation.
As a further contribution to our studies on the stabil-
ity of the metal–chalcogen bond [1], it appeared worth-
while to further explore the coordination properties of
chalcogenoethers, by using palladium(II) as the metal
acceptor. As a matter of fact, the coordination of sele-
noethers and telluroethers to transition metal complexes
is a subject of renovated considerable interest [2].
*
Corresponding author. Tel.: +39 050 2219 206; fax: +39 050 2219
246.
E-mail address: belli@dcci.unipi.it (D. Belli DellꢀAmico).
0020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.06.073

128
D. Belli Dell’Amico et al. / Inorganica Chimica Acta 359 (2006) 127–135
This was an important premise, as bivalent cations were
expected, and had been established, to provide neutral
complexes, presumably amenable to studies in organic
solvent solution. Prior art in this field concerns the prep-
aration of the iron(II) and cobalt(II) [3a] derivatives of
both ligands and the crystal and molecular structure of
the palladium(II) complex derived from bphepaH2. Fi-
nally, palladium(II) was selected for this study in view
of its well-established kinetic lability [4].
2. Experimental
2.1. General
All manipulations were performed under an atmo-
sphere of dry dinitrogen. Solvents were dried and
distilled following standard procedures. Diethylsulfide
(Merck, 98%) and diethylselenide (Aldrich) were used
as received. Palladium(II) acetate [6], N,N0-diphenyl-
2,6-pyridinedicarboxamide [3] and N,N0-bis(2-phenyl-
ethyl)-2,6-pyridinedicarboxamide [3] were prepared
according to published procedures. Elemental micro-
analyses (C, H, N) were performed at Dipartimento
(CH2)nPh
NH
CO
N
`
di Scienze Farmaceutiche, Universita di Pisa. IR
CO
NH
spectra were recorded with a Perkin–Elmer Paragon
500 FTIR spectrophotometer. The NMR mono-
and bidimensional spectra were recorded using a
Varian-Gemini 200BB instrument, with the exception
of the spectrum of 3, which was measured with a
Varian Inova 600 spectrometer. Chemical shifts d
(CH2)nPh
This paper reports the preparation and the crystal
and molecular structure of the new compound (N,N0-di-
phenyl-2,6-pyridinedicarboxamidato) (g1-acetonitrile)-
palladium(II), 1. The preparation of the more soluble
already known [3b] [N,N0-bis(2-phenylethyl)-2,6-
pyridinedicarboxamidato](g1-acetonitrile)palladium(II),
Pd(bphepa)(NCMe), 2, evidenced that it easily loses
MeCN and is converted to the MeCN-free tetranuclear
derivative [Pd(bphepa)]4, 3, which has been studied both
spectroscopically and crystallographically.
are reported in ppm from TMS for H and 13C nu-
1
clei and from SeMe2 for 77Se. UV/Vis spectra were
obtained with a Perkin–Elmer Lambda 9 spectropho-
tometer. DCI/MS spectra (positive and negative ions)
were recorded with a magnetic double focalization
reverse geometry Finnigan MAT 8400 spectrometer.
One drop of a diluted CHCl3 or CH2Cl2 solution
of the sample was used to load the DCI wire. The
DCI heating current was raised linearly to 1 A at a
rate of 20 mA sꢀ1. Isobutane was employed as a re-
agent gas at a pressure of 0.2–0.3 mbar. In a typical
experiment, the source was heated only by the
0.2 mA emission current of the filament (80–
100 ꢁC). The magnetic field was scanned in the range
m/z 60–2000 at full (3 keV) accelerating voltage (m/
Dm resolution, 2000; 10% valley). An upward qua-
dratic scan of the magnetic field (1.3 s scan time;
1.5 s total cycle time) was adopted. Operating condi-
tions of the ESI source positive ions/negative ions
were as follows: spray voltage, 5–4 kV; capillary volt-
age 22 to ꢀ13 V; tube lens offset voltage ꢀ5 to
ꢀ35 V; sheath gas (N2), 50 units (roughly
1.25 L minꢀ1); capillary temperature, 200 ꢁC. The
sample, dissolved in CH2Cl2, or MeOH, or MeCN,
was injected with a syringe pump at a flow of
5 lL minꢀ1. The instrument was operated from m/z
150 to 4000. ESI/MS spectra (positive and negative
ions) were recorded with an ion-trap Finnigan LCQ
spectrometer equipped with an ESI source and a syr-
inge pump. Details of the experimental setup have
been reported earlier [7a]. Mass-spectral data on
compound 2 showed a main peak associated with a
palladium-containing fragment at m/z 478 [corres-
ponding to (M ꢀ MeCN + H)+, both in CHCl3 (DCI/
MS, positive ions) and in MeCN (ESI/MS, positive
ions)].
CH2CH2Ph
Ph
N
CO
N
N
Pd
N
CO
N
NCMe
NCMe
Pd
N
CO
CO
Ph
CH2CH2Ph
2
1
Thus, the carboxamidato group expands its coordina-
tion ability to the oxygen atom, thus leading to a
remarkable molecular rearrangement. While this work
was in progress the crystallographic data of another
crystal modification of 3 have appeared [5].
The treatment of 2 or 3 with EEt2 (E = S, Se) affords
Pd(bphepa)(EEt2) (4, E = S; 5, E = Se).
O
R
N
3
5
2
6
4
Pd
N
N
O
R
O
N
N
N
Pd
R
O
R
R
O
R
R
N
N
Pd
N
N
N
O
Pd
N
O
R
O
3
R = CH2CH2Ph

D. Belli Dell’Amico et al. / Inorganica Chimica Acta 359 (2006) 127–135
129
2.2. (N,N0-Diphenyl-2,6-pyridinedicarboxamidato)(g1-
acetonitrile)palladium(II), Pd(dphpa)(NCMe) (1)
nances], 48.3, 47.4, 36.8 and 36.7 (4 different CH2);
1H-coupled-13C NMR (200 MHz, CDCl3) d (ppm):
169.4 (CO), 154.2 and 152.4 [2 different C(py)–CO],
140.7 [C(py)–H, JC–H = 168 Hz], 140.2 and 137.9 [2 dif-
ferent C(U)–CH2], 132–123 [m, 2 different C(py)–
H + C(U)–H resonances], 48.2 (t, JC–H = 137 Hz,
N,N0-Diphenyl-2,6-pyridinedicarboxamide,dphpaH2,
(5 g, 15.8 mmol) was suspended in a solution of
Pd(OAc)2 (3.55 g, 15.8 mmol) in 1 L of acetonitrile.
The resulting mixture was stirred for about 36 h, and
the solvent was then removed in vacuo giving crude 1
(6.63 g, 91% yield) as a yellow-green solid, recrystallized
as orange needles from acetonitrile. Elemental Anal.
Calc. for C21H16N4O2Pd: C, 54.5; H, 3.5; N, 12.1.
Found: C, 53.3; H, 3.6; N, 12.1%. IR (KBr, Nujol):
1616 cmꢀ1(mCO); (CD3CN, CaF2 cell): 1624 cmꢀ1 (mCO).
1H NMR (DMF-d7) d (ppm): 8.5–8.3 (m, py), 8.1–7.8
(m, py), 7.5–6.9 (m, 10H, arom.), 2.15 (s, 3H, MeCN).
The DCI mass spectral data (CHCl3 as solvent, positive
ions) showed the main peak attributable to a palladium-
containig fragment at m/z 422 [(M ꢀ MeCN + H)+].
The ESI-MS data (MeCN, MeOH, positive ions)
showed the main peak attributable to a palladium-con-
taining fragment at m/z 462 (M)+, 461 (M ꢀ H)+, 485
(MNa)+. The presence of MeCN was confirmed by the
direct inlet probe Electron Ionization analysis (DIP/
EI) at 70 eV.
CH2), 47.4 (t, JC–H = 139 Hz, CH2), 36.7 (t, JC–H
=
129 Hz, CH2).
1
The H–13C HETCOR (200 MHz, CDCl3) spectra,
see Fig. 1, were obtained with the following acquisition
parameters: delay 1 s, single acquisition time 0.079 s,
overall acquisition time ꢁ60 h).
Mass spectral data: main peak associated with a pal-
ladium-containing fragment at m/z 478 [corresponding
to (1/4M + H)+], both in CH2Cl2 (DCI or ESI/MS, po-
sitive ions) and in MeCN (ESI/MS, positive ions). Peaks
of low intensity corresponding to [(1/4M + Na)
+ (1/4M)n]+ with n = 1–7 are present (ESI/MS, positive
ions, MeCN as solvent).
2.4. Crystallographic study of compounds 1 and 3
The X-ray diffraction experiments were carried out by
means of a Bruker P4 diffractometer operating with
graphite-monochromated Mo-Ka radiation. The intensi-
ties were measured with the x/2h scan mode, collecting a
redundant set of data. Three standard reflections were
measured every 97 measurements to check sample de-
cay. The intensities were corrected for Lorentz and
polarisation effects and for absorption by means of a
w-scan method [8]. The absorption correction was not
applied to the intensities of compound 3. The structure
solutions were obtained by means of the automatic di-
rect methods contained in SHELXS97 [9] for compounds
1 and SIR-92 [10] for compound 3. The refinement, based
on full-matrix least-squares on F2, was done by means of
the SHELXL97 [9] programme. Some other utilities con-
tained in the WINGX suite [11] were also used.
A single crystal of 1 Æ MeCN was grown from aceto-
nitrile and sealed in a Lindemann capillary under a
dinitrogen atmosphere saturated with acetonitrile
vapours. The structure solution of 1 was found in the
non-centrosymmetric Pna21 space group. Besides the
complex molecule, maxima corresponding to the heavy
atoms of one acetonitrile lattice molecule were also
found. The hydrogen atoms were in part located in
the difference Fourier map and in part introduced in
calculated positions. The final refinement cycle, per-
formed with anisotropic thermal parameters for all
the heavy atoms and constraining the hydrogen coordi-
nates to ꢁrideꢀ on the connected carbon atoms, gave the
reliability factors listed in Table 1, along with other
details concerning crystallographic data collection and
structure determination.
2.3. Tetrakis{l-[N,N0-bis(2-phenylethyl)-2,6-
pyridinedicarboxamidato]}tetrapalladium(II),
[Pd(bphepa)]4 (3)
Compound 2 (0.419 g, 0.808 mmol) was refluxed in
toluene (ꢁ200 mL) for ꢁ4 h, followed by removal of
the solvent in vacuo, addition of heptane (50 mL), and
filtration of the resulting solid, which was dried in vacuo
(0.174 g, 45% yield). The yellow solid is soluble in chlo-
roform or dichloromethane, sparingly soluble in cold
toluene and almost insoluble in heptane or hexane. Ele-
mental Anal. Calc. for C92H84N12O8Pd4: C, 57.8; H, 4.4;
N, 8.8. Found: C, 56.9; H, 4.8; N, 8.4%. IR (KBr,
Nujol): 1592 and 1551 cmꢀ1 (mCO); (CH2Cl2, CaF2 cell):
1598 and 1558 cmꢀ1 (mCO). UV/Vis (1,2-dichloroethane):
k
max = 275 nm
(e = 8900 Mꢀ1cmꢀ1).
1H
NMR
(600 MHz, CDCl3): d (ppm): 8.05 (t, J = 7.8 Hz, 4H,
py), 7.85 (dd, J = 7.8 and 1.5 Hz, 4H, py), 7.43 (dd,
J = 7.8 and 1.0 Hz, 4H, py), 7.25–7.10 (m, arom.),
6.93–6.92 (m, 12H, arom.), 6.83 (dd, J = 7.5 and
1.5 Hz, 8H, arom.), 5.92 (dt, J = 7.5 and 12.5 Hz, 4H,
CH2), 3.68 (ddd, J = 12.5, 10.0 and 5.5 Hz, 4H, CH2),
2.99 (ddd, J = 12.5, 10.0 and 6.0 Hz, 4H, CH2), 2.85
(ddd, J = 13.0, 10.0 and 5.5 Hz, 4H, CH2), 2.66–2.49
(3 incompletely resolved ddd, 12H, CH2), 2.36–2.30
(incompletely resolved ddd, 4H, CH2). 13C{1H} NMR
(200 MHz, CDCl3, satd. soln.) d (ppm): 169.4 and
169.1 (2 different CO), 154.2 and 152.4 [2 different
C(py)–CO], 140.8 [C(py)–H], 140.3 and 138.0 [2 different
C(U)–CH2], 128.8, 128.5, 128.2, 128.1, 126.7, 125.9,
125.6 and 124.9 [2 different C(py)–H + C(U)–H reso-
Well -formed single crystals of 3 separated out from a
CDCl3 solution of the compound (about 0.1 M)

130
D. Belli Dell’Amico et al. / Inorganica Chimica Acta 359 (2006) 127–135
Fig. 1. 1H–13C HETCOR spectrum of 3 in CDCl3 solution.
ꢀ
Table 1
in the non-centrosymmetric I4 space group. The asym-
Crystal data and details of the structure refinement for compounds 1
and 3
metric unit corresponds to one fourth of the molecule,
the whole frame being produced by the action of the 4
ꢀ
1 Æ MeCN
23H19N5O2Pd
503.83
298(2)
3
operator. Since the first stages of refinement, the abnor-
mally high values of the thermal parameters of the ben-
zyl moieties suggested the presence of disorder in these
groups. Taking into account the direction of the longer
axis of its prolated ellipsoids, the disorder was handled
as they were distributed on two limiting positions. Each
benzyl moiety was then refined as composed by two
units with imposed hexagonal geometry and isotropic
thermal parameters and fixing to 1 the total occupancy
of the site. The hydrogen atoms were placed in calcu-
lated positions and refined with the above-mentioned
riding condition. The reliability factors resulting from
the final refinement cycle are listed in Table 1, along
with other details concerning crystallographic data col-
lection and structure determination.
Formula
Formula weight
Temperature (K)
C
C92H84N12O8Pd4
1911.31
293(2)
0.71073
tetragonal
˚
Radiation k (A)
0.71073
Crystal system
Space group
orthorhombic
Pna21 (no. 33)
7.7043(8)
15.072(2)
18.268(2)
2121.3(4)
4
1.578
0.905
2051 [0.0362]
282
1.106
ꢀ
I4 (no. 82)
˚
a (A)
13.615(5)
13.615(5)
23.224(9)
4305(3)
2
1.474
0.885
2771 [0.0631]
173
1.062
˚
b (A)
˚
c (A)
3
˚
Volume (A )
Z
qcalc (Mg/m3)
l (mmꢀ1
)
Independent reflections [Rint
Refined parameters
Goodness-of-fita on F2
R1, wR2 (I > 2r(I))a
R1, wR2 (all data)a
a
]
0.0384, 0.1117
0.0515, 0.1190
0.0588, 0.1515
0.0736, 0.1679
2.5. [N,N0-bis(2-phenylethyl)-2,6-pyridinedicarboxa-
midato](diethylsulfide)palladium(II),
Pd(bphepa)(SEt2) (4)
2
2 1=2
R(Fo) =RiFojꢀjFci/RjFoj; RwðF 2oÞ ¼ ½R½wðF o2 ꢀF 2c Þ ꢂ=R½wðF o2Þ ꢂꢂ
;
2
w ¼ 1=½r2ðF o2ÞþðAQÞ þBQꢂ; where Q ¼ ½MaxðF o2;0Þþ2F c2ꢂ=3; GOF ¼
½R½wðF 2o ꢀ F 2c Þ ꢂ=ðN ꢀ PÞꢂ1=2, where N, P are the numbers of observa-
2
tions and parameters, respectively.
Crystals of 4 were obtained by cooling to 4 ꢁC a solu-
tion of 3 in SEt2. The single crystal X-ray diffraction study
showed that we were dealing with the title species.
Although the crystallographic data are not of a high qual-
ity, they allowed the connectivity and the geometry of the
containing an excess of SeEt2 (5:1), by vapour diffusion
of pentane in the course of an attempt to obtain crystals
of Pd(bphepa)(SeEt2), 5. Structure solution was found

D. Belli Dell’Amico et al. / Inorganica Chimica Acta 359 (2006) 127–135
131
metal coordination sphere to be established. Structure
ylselenide (13.9 lL, 0.12 mmol) was added and a 1H
NMR spectrum was recorded after ꢁ3 h. d (ppm): 8.1
(t, J = 7.7 Hz, 1H, py), 7.8 (d, J = 7.7 Hz, 2H, py),
7.4–7.1 (m, 10H, arom.), 3.5 (2 overlapping t, 4H,
CH2–N), 2.9 (2 overlapping t, 4H, CH2–U), 2.75 (br,
CH2–Se–Pd), 1.5 (br t, CH3 of SeEt2). A 77Se{1H}
NMR spectrum was also recorded. d (ppm): 244, 227.
ꢀ
solution was found in the P1 space group. Cell parame-
˚
ters: a = 13.334(5), b = 13.662(5), c = 16.312(5) A; a =
3
˚
88.07(5), b = 89.25(5), c = 65.14(5)ꢁ; V = 2694.7(16) A .
Attempts to set up a rational and high yield prepara-
tion of 4 produce samples containing 3 as an impurity.
2.6. [N,N0-bis(2-phenylethyl)-2,6-pyridinedicarboxa-
midato](diethylselenide)palladium(II), Pd(bphepa)
(SeEt2) (5)
3. Results and discussion
Compound 2 (0.842 g, 1.62 mmol) was dissolved
in 10 mL of dichloromethane and SeEt2 (0.5 mL,
4.50 mmol) was added. The mixture turned orange
immediately after mixing. After stirring overnight, hep-
tane (50 mL) was added, and the resulting suspension
was filtered. The yellow solid was washed with heptane
and dried in vacuo (0.803 g, 81% yield). Elemental Anal.
Calc. for C27H31N3O2PdSe: C, 52.7; H, 5.1; N, 6.8.
Found: C, 52.6; H, 5.1; N, 7.0%. IR (KBr, Nujol):
1595 cmꢀ1 (mCO); 1H NMR (CDCl3) d (ppm): 8.1 (t,
J ꢁ 7.5 Hz, 1H, py), 7.8 (d, J ꢁ 7.5 Hz, 2H, py), 7.5–
6.7 (m, arom.), 3.5 (br t, 4H, CH2–N), 3.0–2.5 (m, 8H,
CH2–U + CH2–Se), 1.5 (t, J ꢁ 7.5 Hz, 6H, CH3 of
SeEt2). All resonances are rather broad. 13C{1H}
NMR (CDCl3) d (ppm): 171.7 (CO), 151.2 (py), 141.0
(py), 140.1 (arom.), 128.7 (arom.), 128.3 (arom.), 126.0
(arom.), 124.4 (py), 49.8 (CH2 of chelated ligand), 36.4
(CH2 of chelated ligand), 23.8 (CH2–Se), 14.6 (CH3);
77Se{1H} NMR (CDCl3): 244 ppm (versus SeMe2). A
3.1. Compound 1
When an acetonitrile solution of Pd(OAc)2 was
treated with an equimolar amount of N,N0-diphenyl-
2,6-pyridinedicarboxamide, dphpaH2, palladium(II) co-
ordinates to the corresponding diamidato dianion,
affording complex 1 as for Eq. (1).
PdðOAcÞ2 þ dphpaH2 þ MeCN
! PdðdphpaÞðNCMeÞ þ2HOAc
ð1Þ
1
By removing the volatiles in vacuo, the crude product
was obtained, which was recrystallized from acetonitrile
as yellow-orange needles, sparingly soluble in almost all
organic solvents except N,N-dimethylformamide and
acetonitrile. The solid-state IR spectrum of the product
shows a C=O stretching band at 1616 cmꢀ1, which is
ꢁ40 cmꢀ1 lower than that of dphpaH2. The H NMR
1
spectrum of 1 (recorded in DMF-d7 solution), contains
two sets of resonances between 8.5 and 7.8 ppm and be-
tween 7.5 and 6.9 ppm, due to the pyridine protons and
to the phenyl protons, respectively. A singlet at
2.15 ppm is ascribed to the methyl group of free acetoni-
trile, suggesting that DMF displaces acetonitrile from
the coordination sphere of palladium.
1H NMR spectrum was measured on a ꢁ1.5 · 10ꢀ3
M
solution of 5 in CDCl3. d (ppm): 8.1 [m, pyridine (py)
H of 5 and 3], 7.9 (dd, J = 7.8 and 1.2 Hz, py of 3),
7.8 (d, J = 7.5 Hz, py of 5), 7.45 (dd, J = 7.8 and
1.2 Hz, py of 3), 7.4–7.1 (m, arom. H of 5 and 3), 6.95
(m, arom. H of 3), 6.85 (m, arom. H of 3), 5.95 (m,
CH2 of 3), 3.7 (br, CH2 of 3), 3.5 (2 overlapping t,
CH2–N of 5), 2.9–2.8 (m, CH2–U + CH2–Se of 5), 2.6
(q, J = 7.5 Hz, CH2 of free SeEt2), 2.35 (br, CH2 of 3),
1.6 (t, J = 7.5 Hz, CH3 of coordinated SeEt2), 1.4 (t,
J = 7.5 Hz, CH3 of free SeEt2).
A schematic drawing of the solid-state structure of 1,
derived from X-ray diffraction data on a single crystal of
1 Æ MeCN, is reported in Fig. 2, some relevant bond dis-
tances and angles are listed in Table 2.
The palladium coordination environment is approx-
imately square-planar. The N(2) atom exhibits the
In a control experiment, compound 2 (50 mg,
0.096 mmol) was dissolved in CDCl3 (ꢁ0.5 mL, about
0.19 M)), SeEt2 (ꢁ20 lL, 0.179 mmol) was added and
˚
maximum deviation (0.0542 A) from the best plane.
The metal centre is 0.0224 A away from this plane on
˚
a
1H NMR spectrum was acquired after ꢁ1 h. d
the opposite side with respect to N(2). The Pd–N(3)
and Pd–N(4) distances range between 1.96 and
(ppm): 8.1 (t, J = 7.7 Hz, 1H, py), 7.8 (d, J = 7.7 Hz,
2H, py), 7.4–7.1 (m, 10H, arom.), 3.5 (2 overlapping t,
4H, CH2–N), 2.9 (2 overlapping t, 4H, CH2–U), 2.7
(br, 4H, CH2–Se of both free and coordinated SeEt2),
2.0 (s, 3H, MeCN), 1.5 (br t, J = 7.3 Hz, 6H, CH3 of
both free and coordinated SeEt2). A 77Se{1H} NMR
spectrum was also acquired. d (ppm): 244 (free SeEt2),
227 (coordinated SeEt2).
˚
2.04 A, which are expected values for square-planar
palladium(II) to this type of ligands [12]. The N(2)
atom is 1.953(8) A away from the metal centre: compara-
tively short Pd–N(py) bonds have been reported [12,13]
for palladium(II) square-planar complexes having a sim-
ilar structure, e.g., [Pd(MeCN)(pybox)](BF4)2 [13],
(pybox = 2,6-bis[40-(S)-phenyloxazoline-20-yl]pyridine)
˚
˚
The interaction of the tetranuclear compound 3 with
where the Pd–N(py) distance is 1.927(7) A. The N(1)–Pd–
SeEt2 was also studied. Compound
0.083 mmol Pd) was dissolved in CDCl3 (0.5 mL), dieth-
3 (39.7 mg,
N(3) angle of 1 is 159.5(3)ꢁ, while the N(2)–Pd–N(4) angle
is 176.0(3)ꢁ. Analogous departures from the ideal

132
D. Belli Dell’Amico et al. / Inorganica Chimica Acta 359 (2006) 127–135
4PdðbphepaÞðNCMeÞ ꢀ ½PdðbphepaÞꢂ4 þ 4MeCN ð2Þ
2
3
In the course of this work we found that solutions of
other adducts of the type Pd(bphepa)L (L = SEt2, SeEt2)
separate 3 with release of free L, as we discuss later on.
Actually, the most beautiful single crystals of 3 were
grown, in an attempt to obtain crystals of Pd(bphepa)
(SeEt2), from a ꢁ0.1 M solution of 3 in CDCl3, in the
presence of excess SeEt2 (molar ratio Se/Pd ꢁ 5), by slow
diffusion of pentane vapors. The crystals of the tetranu-
clear complex 3 separated out, presumably due to its low
solubility.
Two different projections of the molecular structure
of 3, which crystallizes in the tetragonal system, are
shown in Fig. 3. The more relevant bond lengths and an-
gles are listed in Table 3.
Fig. 2. View of the molecular structure of 1. Thermal ellipsoids are at
30% probability.
Table 2
˚
Selected bond distances (A) and angles (ꢁ) for compound 1
Pd–N(1)
Pd–N(2)
Pd–N(3)
Pd–N(4)
2.076(8)
1.953(8)
2.040(8)
2.017(9)
N(1)–Pd–N(2)
N(2)–Pd–N(3)
N(4)–Pd–N(3)
N(4)–Pd–N(1)
79.4(3)
80.2(3)
100.2(3)
100.3(3)
square-planar geometry have been reported [14] for simi-
lar complexes.
Finally, it should be noticed that the phenyl groups
form a dihedral angle of ꢁ50ꢁ with the average coordi-
nation plane of palladium, thus presumably minimizing
the steric hindrance. A similar phenomenon occurs in
[Pd(MeCN)(pybox)](BF4)2, where the corresponding
dihedral angles are 79ꢁ and 76ꢁ [13]. In complex 1,
Pd–N distances and N–Pd–N angles are analogous to
the corresponding metrical parameters of complex 2
[15].
3.2. Compound 3
The already known compound 2 was also studied be-
cause it was considered that the ethylene chain inter-
posed between the nitrogen atom and the phenyl
group on passing from 1 to 2, could modify the proper-
ties, for instance the solubility, of the palladium(II) pre-
cursor. The new findings largely met these expectations:
2 shows a high solubility for instance in aromatic hydro-
carbons or chloroalkanes. Nevertheless we have found
that, in solution, 2 is in equilibrium with the tetranuclear
species 3 according to Eq. (2).
Fig. 3. Two projections of the molecular structure of 3: A, perpen-
ꢀ
dicular, and B, parallel to the 4 axis. Only the most populated positions
Compound 3 was prepared by refluxing 2 in toluene,
followed by removal of the volatiles, acetonitrile in-
cluded, under reduced pressure.
of the disordered benzyl groups are shown. Thermal ellipsoids are at
30% probability. Dotted line shows the short Pdꢃ ꢃ ꢃH(16A0) distance,
0
00
˚
2.539 A. = 1 ꢀ y, x, 2 ꢀ z, = y, 1 ꢀ x, 2 ꢀ z.

D. Belli Dell’Amico et al. / Inorganica Chimica Acta 359 (2006) 127–135
133
Table 3
be equivalent to those in the periphery of the molecule.
In addition, as free rotation about the CH2ꢀU bond is
reasonably assumed, the ortho-protons will give rise to
a single peak, and the same applies to the meta-protons.
Moreover, the 8.05 ppm triplet is assigned to the pyr-
idine proton in 4-position (cfr. 8.1 ppm for 2); the two
double doublets at 7.85 and 7.43 ppm, respectively, cor-
respond to the other two pyridine protons (cfr. the dou-
blet at 7.5 ppm for 2). Further, there are a multiplet at
7.25–7.10 ppm, and a couple of signals at 6.92 and
6.83 ppm, respectively. Both the multiplet and the latter
two resonances integrate for five protons (by assuming
the integral of the 8.1 ppm triplet as the unit), and arise
from the two unequivalent aromatic nuclei. For compar-
ison, in complex 2 the protons of the two aromatic nu-
clei (therein equivalent to one another) originate one
resonance around 7.2 ppm. Finally, there are eight
structured signals between 5.92 and 2.30 ppm (each of
them is a doubled double doublet, and some of them
are not well-resolved), each of which integrates for one
proton. Each of these resonances can be ascribed to
one of the eight non-equivalent methylene protons on
the pendant phenylethyl groupings. Each resonance is
structured because each methylene proton couples with
the geminal one, as well as with each proton on the adja-
cent methylene group. The spectrum computed (LEQUOR
program [16]) by using the spectral parameters reported
in the experimental part gives an excellent fit to the
experimental one.
˚
Selected bond distances (A) and angles (ꢁ) for compound 3
Pd–N(1)
Pd–N(2)
Pd–N(3)
Pd–O(20)
2.008(9)
1.908(8)
2.042(9)
2.017(9)
N(1)–Pd–N(2)
N(2)–Pd–N(3)
O(20)–Pd–N(1)
O(20)–Pd–N(3)
82.0(4)
80.2(4)
97.6(4)
100.3(3)
The apex has the same meaning as in Fig. 3.
While this work was in progress, a paper appeared [5]
describing another crystal modification of compound 3.
In our modification the four palladium atoms in the
molecule are equivalent by 4 (S4) symmetry. The already
known phase of 3 crystallizes in the triclinic P1 space
group without symmetry elements relating parts of the
molecule.
The molecular structure of the triclinic modification
(3tricl) is essentially analogous to that of the tetragonal
one (3tetr), as for the Pd–N distances [Pd–N(py) =
ꢀ
ꢀ
˚
˚
1.913(9) A for 3tricl, 1.908(8) A for 3tetr; ÆPd–NCOæ =
˚
˚
2.03(1) A for 3tricl, 2.025(6) A for 3tetr] and N–Pd–N
angles [OCN–Pd–NCO = 161.3(4)ꢁ for 3tricl, 162.1(4)ꢁ
for 3tetr] within each mononuclear unit. The Pd–N
˚
lengths fall within the range of 1.91–2.06 A, which is
normal for palladium(II) square-planar complexes [12].
Significant differences are observed between the two
crystalline phases. The difference in the Pd–O distances,
The resonance centred at 5.92 ppm shows a remark-
able downfield shift with respect to the corresponding
resonances of 2, located at 3.35 and 2.75 ppm. Such a
shift may be explained taking the solid-state structure
of 3 into consideration: it is seen (Fig. 3) that the
˚
˚
2.064(8) A for 3tricl and 2.017(9) A for 3tetr, is more than
five times the standard deviation. Being the Pd–O bond
the hinge which joins the four {Pd[C5H3N-2,6-
(CONPh)2]} moieties, we suppose that the tetragonal
arrangement allows a better disposition of the whole
molecule, producing a slightly stronger Pd–O bond.
Both crystal structures exhibit a nearly square-planar
coordination geometry for each palladium atom.
However, the major difference between the triclinic
and the tetragonal phases is the orientation of the termi-
nal benzyl groups, which are easily visualized by super-
imposing the two molecular structures. A detailed
analysis of this subject should include the disorder of
the benzyl groups in the tetragonal phase, which is be-
yond the scope of this report.
0
˚
H(16A ) proton is 2.57 A away from the palladium atom
(Pd in the figure) belonging to the contiguous unit, the
Pd–H(16A0) vector being almost orthogonal to the coor-
dination plane. Based on literature [17,18], the H(16A0)
proton is shifted downfield due to paramagnetic anisot-
ropy arising from Pd. This hypothesis is consistent with
the van Vleckꢀs theory, which states that a weak
paramagnetism can be induced by external magnetic
fields on atoms whose d orbitals are not completely
filled, and whose fundamental-state total spin quantum
number equals 0, such as palladium(II) (d8). The
13C{1H} NMR spectrum of 3 can be interpreted simi-
larly to the 1H NMR spectrum. A bidimensional
1H–13C HETCOR spectrum of complex 3, see Fig. 1,
confirmed the proton couplings.
Compound 3 was studied via NMR in chloroform
solution. The 600 MHz H NMR spectrum in CDCl3
1
(see Fig. 1 and Supplementary Material) is consistent
with the solid-state structure of Fig. 3, taking into consid-
eration that the S4 symmetry axis passing through the
centre of the macrocycle makes the palladium-containing
units equivalent to one another, and that each methylene
group bears diastereotopic protons, consistent with the
absence of symmetry planes.
To conclude, NMR data are consistent with the so-
lid-state structural data of tetragonal modification of
3. It should be noticed that the triclinic crystal modifica-
tion reported in the literature [5], when in solution,
would be expected to give NMR spectra more compli-
cated than those previously discussed. As a matter of
fact, the palladium-containing units of the triclinic
Moreover, the phenylethyl protons located in the in-
ner part of the (ꢃ ꢃ ꢃO@C–N–Pdꢃ ꢃ ꢃ)4 macrocycle, will not

134
D. Belli Dell’Amico et al. / Inorganica Chimica Acta 359 (2006) 127–135
modification are not equivalent, since the S4 symmetry
element is absent.
to note that attempts to separate pure solid 4 from its
solutions (in CnH2n + 2 ꢀ xClx) by addition of a hydrocar-
bon as pentane or by evaporation of the solvent met
with some difficulties because of the competing precipi-
tation of 3, observed even in the presence of an excess
of SEt2.
In the ESI/MS [7] mass spectra of 3, recorded by
using a CH2Cl2 solution, the most intense peak is due
to the palladium-containing mononuclear unit (m/z
476 for negative ions and 478 for positive ions), as the
result of the tetranuclear structure undergoing fragmen-
tation. This kind of cleavage, involving Pd–O@C bonds,
is consistent with the well-established [19] low stability
of this type of bonds.
It should be noted that the ESI/MS spectra of 2 show
the most intense fragment at m/z 477 (negative ions) and
478 (positive ions). The agreement with the mass spectra
of 3 can be accounted for by postulating that the Pd–
NCMe bond of 2 breaks up on ionization, in keeping
with the rather low stability of Pd–NCMe bonds re-
ported in the literature [20].
As far as the reaction of SeEt2 with 2 is concerned,
when a molar ratio of about 2 between the reagents
1
was employed: the H NMR spectrum of the reaction
mixture showed the resonance at 2.0 ppm due to uncom-
plexed acetonitrile, but no signal attributable to coordi-
nate MeCN. Therefore, in these conditions the
equilibrium involving 2 and SeEt2 (see Eq. (5)) appears
to be almost totally shifted to the right. 77Se{1H}
NMR spectrum of the mixture exhibited two signals at
228 and 244 ppm, respectively, the former due to unco-
ordinated SeEt2, the latter being attributed to ligated
SeEt2. The downfield shift of 16 ppm of coordinated
SeEt2 is rather modest; however it is in the range of
those reported in the literature [21,22]. It is interesting
3.3. Reactivity of compounds 2 and 3 with EEt2
(E = S, Se)
1
to point out that in the H NMR spectrum only one
We have observed that products 2 and 3 interact with
the diethylchalcogenides EEt2 (E = S, Se) to afford the
complexes 4 and 5 according to equilibria (3)–(6).
set of rather broad signals due to SeEt2 was observed
[at 2.7 (q) and 1.5 (t) ppm], suggesting that free and
coordinate ligands undergo rapid exchange on the
NMR time scale. The apparent contrast between the
1
77Se- and H NMR data can be settled by considering
PdðbphepaÞðNCMeÞ þSEt2
2
that the greater difference in chemical shift of selenium
signals, with respect to hydrogen, will correspond to
an enhancement of the coalescence temperature. The
presence of well separate signals due to the free and
ꢀ PdðbphepaÞðSEt2Þ þNCMe
ð3Þ
ð4Þ
4
1=4½PdðbphepaÞꢂ4 þSEt2 ꢀ PdðbphepaÞðSEt2Þ
1
4
3
coordinate thio-ether in the H NMR spectrum of solu-
tions containing 4 and SEt2 could be explained with a
slower ligand exchange rate, which well agrees with
the different kinetic constants measured for substitution
reactions in platinum(II) complexes, SeR2 being better
nucleophiles than SR2 [23].
The diethylselenide adduct 5, as prepared from 2
and SeEt2 in dichloromethane solution, can be ob-
tained pure in the solid state by addition of heptane
in the presence of an excess of SeEt2, to avoid the
concomitant precipitation of the tetranuclear deriva-
tive 3. The formation of 3 is clearly bound to the
set up of equilibrium (6) in solution, the slight solubil-
ity of this tetranuclear species causing its prompt
precipitation.
PdðbphepaÞðNCMeÞ þSeEt2
2
ꢀ PdðbphepaÞðSeEt2Þ þNCMe
ð5Þ
ð6Þ
5
1=4½PdðbphepaÞꢂ4 þSeEt2 ꢀ PdðbphepaÞðSeEt2Þ
5
3
1
The H NMR spectrum of the reaction mixture be-
tween 2 and SEt2 in the molar ratio 1:1, showed a quar-
tet and a triplet at 2.7 and 1.5 ppm, respectively,
assigned to the methylene and the methyl protons of
coordinate SEt2. These resonances were accompanied
by weak signals, assigned to uncomplexed diethylsulfide
at 2.6 and 1.3 ppm. Resonances at 2.15 and at 2.0 ppm
were assigned to coordinated and uncomplexed acetoni-
trile, respectively. These findings agree with the presence
in solution of equilibrium (3) involving the formation of
the diethylsulfide adduct, 4. From the ratio between the
integrals of the resonances at 2.0 and 2.15 ppm, an equi-
librium constant K of about 15 was estimated. Thus, the
Pd–SEt2 bond appears to be slightly more strong than
the Pd–NCMe bond [20]. Product 4 can be obtained also
by reacting 3 with an excess of SEt2. Crystals of 4 sepa-
rated out by a solution obtained by adding 3 to neat
SEt2 when it was stored at about 4 ꢁC. It is interesting
4. Conclusions
The complex Pd(bphepa)(NCMe), 2, used in this
work as precursor to study the relative stability of the
derivatives Pd(bphepa)(ER2) (E = S, Se), releases aceto-
nitrile in solution, being in equilibrium with [Pd(bphe-
pa)]4, 3, which shows low solubility in hydrocarbons.
Both the complexes, 2 and 3, react with EEt2 (E = S,
Se) with formation of Pd(bphepa)(EEt2): in all cases

D. Belli Dell’Amico et al. / Inorganica Chimica Acta 359 (2006) 127–135
135
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`
The authors thank the Ministero dellꢀ Universita e
della Ricerca (MIUR) for financial support (Programmi
di Interesse Nazionale 2004–5). We also thank Chimet
SpA, Badia al Pino, Arezzo, Italy, for a loan of palla-
dium as a solution of tetrachloropalladate(II). Thanks
are due to Dr. Donata Catalano for the 1H NMR
simulation.
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Appendix A. Supplementary data
´
´
[12] (a) D.J. Cardenas, A.M. Echavarren, M.C. Ramırez de Arellano,
Crystallographic data for compounds 1 and 3 have
been deposited in the form of CIF files with the Cam-
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numbers 271557 and 271558, respectively. Copies of
the data may be obtained free of charge, on application
to The Director, CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK, fax: +44 1223 336 033, email: deposit
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ccdc.cam.ac.uk. The experimental and the LEQUOR com-
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