Unusual coordination mode of a 2-pyridyl ketone oxime ligand in bis(4-butylphenyl 2-pyridyl ketone oximate)palladium(II)

Article abstract of DOI:10.1016/S0020-1693(98)00299-0

In the complex PdL₂ (HL = 4-butylphenyl 2-pyridyl ketone oxime), characterised by spectroscopic techniques and X-ray diffraction analysis, the deprotonated ligand displays a N,O coordination, with the pyridyl nitrogen and the oxime oxygen, which is without precedence in this class of compounds; ¹H NMR spectra, together with structural data on PdL₂, make it possible to distinguish between the N,O and N,N coordination mode in analogous compounds.

Full text of DOI:10.1016/S0020-1693(98)00299-0

Inorganica Chimica Acta 285 (1999) 138±141
Note
Unusual coordination mode of a 2-pyridyl ketone oxime ligand
in bis(4-butylphenyl 2-pyridyl ketone oximate)palladium(II)
Mauro Carcelli*, Pietro Cozzini, Romina Marroni, Paolo Pelagatti,
Corrado Pelizzi, Paolo Sgarabotto
Á
Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Universita di Parma, Viale delle Scienze, 43100 Parma, Italy
Received 26 May 1998; accepted 8 July 1998
Abstract
In the complex PdL2 (HL ˆ 4-butylphenyl 2-pyridyl ketone oxime), characterised by spectroscopic techniques and X-ray diffraction
analysis, the deprotonated ligand displays a N,O coordination, with the pyridyl nitrogen and the oxime oxygen, which is without precedence
1
in this class of compounds; H NMR spectra, together with structural data on PdL2, make it possible to distinguish between the N,O and
N,N coordination mode in analogous compounds. # 1999 Elsevier Science S.A. All rights reserved.
Keywords: Crystal structures; Palladium complexes; Oxime complexes
1. Introduction
coordination and as an extension of our previous studies of
palladium(II) complexes with bi- and terdentate ligands [5],
we turned our attention to the complexes of aryl 2-pyridyl
ketone oximes.
2-Pyridyl ketone oximes are interesting not only for
analytical purposes [1] but also in general for their versa-
tility, that allows the design of very peculiar systems. The
complexes bis(pyridine-2-carboxaldoximinato)platinum(II)
dihydrate (1) [2] and bis(pyridine-2-methylcarboxaldoximi-
nato)platinum(II) (2) [3] have a planar structure in which
two ligand molecules coordinate to the metal atom through
the pyridyl and imine nitrogens; the molecules are stacked
giving a direct overlap of the metal orbitals along the
crystallographic c-axis so that they were studied as synthetic
metals. Metallacrowns are a new class of multinuclear
clusters that are analogous to crown ethers in both structure
and function, and recently [4] bis(2-pyridyl)ketone oxime
(Hpko) was used to realise the ®rst example of a 12-
metallacrown-4 with an `anti-metallacrown' structure,
namely Zn4(pko)4(OH)2(CH3COO)2, or inv(OH)2[12-
2. Experimental
Physical measurements were made as described pre-
viously [5].
2.1. Synthesis
2.1.1. 4-Butylphenyl 2-pyridyl ketone
2.85 g of picolinoyl chloride [6], suspended under nitro-
gen in 70 ml of CH2Cl2, are reacted with twofold moles of
AlCl3; 2.5 ml of butylbenzene in 15 ml of CH2Cl2 are added
dropwise. After stirring at room temperature for 8 h, water is
added, then the organic solution is separated and puri®ed by
chromatography (SiO2, hexane/ethyl acetate 9/1 and then
SiO2 exane/ethyl acetate 9.8/0.2). Yellow liquid, yield 20%;
IR (liquid ®lm): ꢀ(C±Har) 3010 m, ꢀ(C±Halk) 2928±2856,
MCZn ‡Nꢀpko†-4] following the proposed nomenclature. Also
2
in this complex in the chelating ring there are the pyridyl and
imine nitrogens (N,N; Scheme 1(a)), even if at the beginning
a coordination through the pyridyl nitrogen and the oxime
oxygen was also taken into consideration (N,O; Scheme
1(b)). With the aim of evaluating the possibility of the N,O
‡
ꢀ(C=O) 1661 cm 1; mass spectrum (CI): 240 (MH ,
1
100%). H NMR (CDCl3, 300 MHz): 0.92 (t, 3H, CH3),
1.34 (m, 2H, CH2), 1.61 (m, 2H, CH2), 2.66 (t, 2H, CH2),
7.27 (d, 2H, CHphenyl), 7.44 (m, 1H, CHpy), 7.85 (t, 1H,
CHpy), 7.98 (d, 3H, CHphenyl ‡ CHpy), 8.70 ppm (d, 1H,
H(a)py).
*Corresponding author. Tel.: +39-521-905432; fax: +39-521-905557;
e-mail: carcelli@ipruniv.cce.unipr.it
0020-1693/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(98)00299-0

M. Carcelli et al. / Inorganica Chimica Acta 285 (1999) 138±141
139
the ABSORB program [8]. The structure was solved by
Patterson and Fourier techniques and re®ned anisotropically
for non-hydrogen atoms up to R ˆ 0.028 and Rw ˆ 0.029;
the weighting function was of the form w ˆ 0.2434/
[s2(Fo) ‡ 0.0073 Fo ]. All the hydrogen atoms were located
2
from Fourier-difference maps and re®ned isotropically. All
the calculations were carried out on the ENCORE91 com-
puter of the Centro di Studio per la Strutturistica Diffratto-
metrica del CNR, Parma, by using SHELX76 [9], SIR92
[10], and PARST [11] programs.
Scheme 1.
2.1.2. 4-Butylphenyl 2-pyridyl ketone oxime
0.69 g of ketone in 30 ml of ethanol are reacted with an
excess (0.4 g) of hydroxylamine hydrochloride dissolved in
the minimum volume of water and neutralised with NaOH.
After 7 h of re¯uxing, the solution is evaporated and the
solid redissolved in chloroform to eliminate the unreacted
hydroxylamine and NaCl. Finally, the chloroform is
removed and the residue recrystallised from ethanol/water.
White solid, m.p. 1108C, yield 73%. Anal. Calc. for
C16H18N2O: C, 75.56; H, 7.13; N, 11.01%. Found: C,
75.50; H, 7.20; N, 11.09%. IR (KBr, cm 1): 3138 br
ꢀ(OH), 3015 m ꢀ(C±Har), 2928±2856 s ꢀ(C±Halk),
1593 m ꢀ(C=N), 1015 s ꢀ(N±O). 1H NMR (CDCl3,
300 MHz): 0.92 (t, 3H, CH3), 1.36 (m, 2H, CH2), 1.61
(m, 2H, CH2), 2.64 (t, 2H, CH2), 7.20±7.81 (7H, CHar), 8.65
(d, 1H, H(ꢁ)py), 10.75 ppm (br, 1H, OH).
3. Results and discussion
There is N,O coordination when the pyridine is syn to the
oxime group and this is probably possible when the two
groups that are bonded to the imine carbon have similar
steric requests. Therefore, aryl 2-pyridyl ketone oximes
seemed good candidates for this study; in particular, phenyl
2-pyridyl ketone oxime (HL0) has already been used, a
highly selective and sensitive reagent for0 palladium(II)
[12] which forms the slightly soluble PdL2 complex. All
efforts to obtain good quality crystals of this compound
failed, so the phenyl ring was functionalised with a
lipophylic butyl group to improve its solubility in organic
solvent. 4-Butylphenyl 2-pyridyl ketone is obtained by a
Friedl±Craft type acylation starting from picolinoyl chloride
and butylbenzene; it is worth noting that the reaction is
completely regioselective. The ketone is reacted with hydro-
xylamine hydrochloride to obtain the proligand 4-butylphe-
nyl 2-pyridyl ketone oxime (HL) and then HL with
palladium(II) acetate obtaining a bright yellow solid that
was characterised as bis(4-butylphenyl 2-pyridyl ketone
oximate)Pd(II); recrystallisation of the orange precipitate
from ethanol/CHCl3 (1/1 vol./vol.) gives well suited crystals
of PdL2.
A perspective view of the molecular structure of com-
pound PdL2 is illustrated in Figs. 1 and 2 while selected
distances and angles are reported in Table 1. The metal atom
is on a centre of symmetry and is bonded to the pyridyl
nitrogen and the oxime oxygen of the deprotonated ligand,
with a N2O2 square planar geometry: to our knowledge this
is the ®rst 2-pyridyl oxime complex characterised by X-ray
analysis in which the pyridyl and the imine nitrogens are not
involved together in the coordination. The six-membered
2.1.3. Bis(4-butylphenyl 2-pyridyl ketone oximate)Pd(II)
Some drops of 1 M NaOH and 0.02 g of palladium(II)
acetate dissolved in the minimum volume of acetonitrile are
added to 0.046 g of HL in 15 ml of ethanol; the solution is
stirred for 1.50 h, then the precipitate is ®ltered and dried.
Bright yellow solid, m.p. 2088C. Anal. Calc. for
C32H36N4O3Pd: C, 60.90; H, 5.75; N, 8.88%. Found: C,
60.77; H, 5.54; N, 8.41%. IR (KBr, cm 1): 3075 m
ꢀ(C±Har), 2949±2861 s ꢀ(C±Halk), 1595 m ꢀ(C=N), 1251 m
1
ꢀ(N±O). H NMR (CDCl3, 300 MHz): 0.93 (t, 6H, CH3),
1.36 (m, 4H, CH2), 1.62 (m, 4H, CH2), 2.65 (t, 4H, CH2),
7.20±7.77 (14H, CHar), 9.46 ppm (d, 2H, H(ꢁ)py).
2.2. Crystallography
Crystal data: C32H34N4O2Pd, M ˆ 613.05. Triclinic,
Ê
a ˆ 8.581(2), b ˆ 10.976(5), c ˆ 8.194(4) A, ꢁ ˆ 92.63(4),
Ê 3
ꢂ ˆ 115.75(3), ꢃ ˆ 83.91(3)8, V ˆ 691.2(5) A , Z ˆ 1,
3
Ê
Dc ˆ 1.47 g cm
;
Mo Ka radiation ꢄ ˆ 0.710688 A,
1
ꢀ
ꢅ ˆ 7.1 cm . Space group, P1; crystal dimensions 0.02,
Table 1
0.29, 0.48 mm.
Ê
Selected distances (A) and angles (8) for structure PdL2
Crystals were grown by slow evaporation of an ethanol/
CHCl3 solution of bis(4-butylphenyl 2-pyridyl ketone oxi-
mate)Pd(II). X-ray measurements were performed by a
Philips PW1100 diffractometer at 293 K. 2431 independent
Pd±O(1)
O1±N(2)
C(7)±C(2)
1.987(3)
1.349(7)
1.476(9)
Pd±N(1)
2.014(6)
1.296(9)
1.350(7)
N(2)±C(7)
C(2)±N(1)
re¯ections were measured of which 955 having Ihkl>2ꢆ(Ihkl
)
N(1)±Pd±O(1)
Pd±N(1)±C(2)
N(1)±C(2)±C(7)
89.3(2)
121.9(4)
121.1(6)
Pd±O(1)±N(2)
O(1)±N(2)±C(7)
N(2)±C(7)±C(2)
120.9(3)
121.1(5)
127.6(6)
were used in the re®nement. All data were corrected for
Lorentz and polarisation effects; absorption effects were
corrected by the method of Walker and Stuart [7] using

140
M. Carcelli et al. / Inorganica Chimica Acta 285 (1999) 138±141
complexes where the ligands act as N,O bridging over
two Pd ions [15,16], but in all the cases the crystallographic
data are poor, so a comparison with the value found in PdL2
Ê
Ê
(1.987(3) A, 1.98±2.03 A in the literature examples) prob-
ably is not really signi®cant. There are no unusual angles or
distances in the organic moiety. The bulky butylphenyl
groups and the twist boat six-membered rings concur to
prevent stacking interactions between the palladium ions
such as those occurring between platinum ions in 1 and 2 (in
PdL the closest distance between palladium ions in parallel
complexes is one unit cell along the c axis).
From the 1H NMR spectrum it is possible to say that the
isolated complex is not a mixture of linkage isomers and
does not present ¯uxional behaviour, at least at room
temperature; moreover, the resonance attributable to
H(6), the hydrogen adjacent to the pyridyl nitrogen, results
diagnostic of the type of coordination: in fact, the atom is
less deshielded in the N,O-complex PdL2 (9.47 ppm) than in
the N,N-complexes (10.28 ppm in 1). This is because in
PdL2 the interaction with the oxime oxygen of the other
ligand molecule is weaker due to the fact that the oxygen is
bonded to the metal and seems to be farther (O(1)Á Á ÁH(6)
Ê
Ê
2.31(8) versus 2.18(5) A in 1 or 2.12(8) A in 2).
The N,O coordination is not so unfavourable, at least in
these aryl 2-pyridyl ketone oximes. From the reaction
between palladium(II) and the E isomer of HL0 [17], that
is stable in solution [18] and `preorganised' for N,N coor-
dination, a bright yellow powder is obtained: in its 1H NMR
spectrum there are two doublets at 10.08 and 9.47 ppm (1/2
ratio; 8.66 ppm in the proligand) that allow us to conclude
that the solid is a mixture of (a), in which the ligand has
retained its con®guration, and (b), where the ligand has
undergone isomerisation to assume the Z form.
Fig. 1. A perspective view of the complex PdL2.
4. Supplementary material
Crystallographic data have been deposited with the Cam-
bridge Crystallographic Data Centre, University Chemical
Laboratory, 12 Union Road, Cambridge CB2 1EZ, UK.
Fig. 2. A side view of the coordination around the metal atom;
4-butylphenyl groups are omitted for clarity.
Acknowledgements
chelation ring is severely distorted and can be described as a
twist boat using the Cremer and Pople parameters [13]
(q2 ˆ 0.476(5), q3 ˆ 0.183(4); ' ˆ 25.3(6), ꢇ ˆ 69.0(5)).
The phenyl and pyridyl rings make an angle of 42.58 and
27.48 with the plane of the oxime group (C(7)N(2)O(1)); the
N2±C7±C8±C9 and N2±C7±C2±C3 torsion angles are
This work has been supported by Ministero dell'Univer-
Á
sita e della Ricerca Scienti®ca e Tecnologica (MURST).
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