Bis(3,5-dimethylpyrazol-1-yl)alkanones and their reactivity with [PdCl2(NCMe)2]: Isolation of bis(3,5-dimethylpyrazol-1-yl)propan-1-one palladium(II) chloride and C-H activation of propanone linker

Article abstract of DOI:10.1016/j.inoche.2009.10.036

Four new bis(3,5-dimethylpyrazol-1-yl)alkanones were prepared by the reaction of the respective alkanoyl chloride with 3,5-dimethylpyrazole: L1 from chloropropionyl chloride, L2 from 4-chlorobutyryl chloride, L3 from oxalyl chloride and L4 from adipoyl di

Full text of DOI:10.1016/j.inoche.2009.10.036

Inorganic Chemistry Communications 13 (2010) 123–126
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Bis(3,5-dimethylpyrazol-1-yl)alkanones and their reactivity with
[PdCl2(NCMe)2]: Isolation of bis(3,5-dimethylpyrazol-1-yl)propan-1-one
palladium(II) chloride and C–H activation of propanone linker
Nangamso Miti a, Ilia A. Guzei b, Simphiwe M. Nelana a, James Darkwa a,
*
a Department of Chemistry, University of Johannesburg, Auckland Park Kingsway Campus, Auckland Park 2006, South Africa
b Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, WI 53706, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Four new bis(3,5-dimethylpyrazol-1-yl)alkanones were prepared by the reaction of the respective alka-
noyl chloride with 3,5-dimethylpyrazole: L1 from chloropropionyl chloride, L2 from 4-chlorobutyryl
chloride, L3 from oxalyl chloride and L4 from adipoyl dichloride; all in moderate yields. Of the four com-
pounds only L1 successfully reacts with [PdCl2(NCMe)2] to yield [PdCl2(L1)] in 41% yield, which on heat-
ing in toluene leads to C–H activation to produce [PdCl(L10)], where L10 is deprotonated L1, in very low
yield.
Received 7 August 2009
Accepted 27 October 2009
Available online 30 October 2009
Keywords:
Bis(3,5-dimethylpyrazolyl)alkanone
Palladium(II) chloride complexes
C–H activation
Ó 2009 Elsevier B.V. All rights reserved.
Crystal structures
Properties of pyrazoles can be modified through functionaliza-
tion of the NH proton to form a variety of pyrazolyl compounds
that could be used as ligands. The most popular of these pyrazolyl
ligands are ‘‘scorpionates” – pyrazolylborates first developed by
Trofimenko [1]. The use of borate linkers gave rise to other linkers,
most notably alkyl and aryl linkers [2]. To date various modifica-
tions have been made to linkers that have led to different chemical
and structural behavior of metal complexes they form. Pons and
others have demonstrated that bis(pyrazolyl)alkylamine [3], ether
[4] and thioether analogues [5] can function as either bidentate
ligands that use the two nitrogen atoms of the ligands or as triden-
tate ligands that involve the heteroatom in the linker. Such linker
binding atoms have allowed the design of tridentate chromium
and nickel complexes that catalyze the polymerization or
oligomerization of ethylene [6]. Recently we have used carbonyl-
benzene [7], carbonylpyridine [8], methylbenzene [9] and methyl-
pyridine [10] linkers to prepare mainly bis(pyrazolyl)palladium
complexes some of which catalyze oligomerization or polymeriza-
tion of ethylene. For palladium complexes with pyridine-contain-
ing linkers the pyridine nitrogen binds in preference to one of
the pyrazolyl units, thus leaving this pyrazolyl unit dangling; how-
ever bis(pyrazolylmethyl)pyridine ligands act a tridentate ligand
when reacted with nickel(II) halides [11]. We have modified the
linkers in our ligands to feature alkylcarbonyl units and herein
we report how these new ligands react with [PdCl2(NCMe)2].
Reaction of alkyl acyl chlorides with two equivalents of 3,
5-dimethylpyrazole produced 1,3-bis(3,5-dimethylpyrazol-1-ly)-
alkylnones (L1 and L2) (Scheme 1) [12]. When the linker is a diacyl
chloride even more facile reactions occurred to form bis(pyrazol-
yl)alkyldiones (L3 and L4) (Scheme 2). Characterization of L2–L4
is provided as Supplementary material.
The 1H NMR spectra of L1 and L2 are characterized by two dis-
tinct peaks for the protons on the pyrazolyl rings between
5.50 ppm and 6.00 ppm, while the spectra of L3 and L4 show only
one such peak, in addition to the methyl protons on the pyrazolyl
rings and the linker protons. The presence of one or two pyrazolyl
peaks is indicative of the symmetrical or otherwise of these com-
pounds. The CH2 proton peaks differ in chemical shifts for each
compound, depending on the number of CH2 groups on the alkyl
carbonyl linker; with the CH2 protons closest to the carbonyl group
appearing more downfield due to the electron-withdrawing effect
of the carbonyl group. The presence of carbonyl groups in these
compounds was identified by their characteristic stretching fre-
quencies between 1729 and 1712 cmꢀ1 in their IR spectra. Single
crystal X-ray diffraction confirmed the structures of L1 and L4.
Crystallographic data showed that L1 and L4 have two pyrazolyl
rings with a linker consisting of two CH2 groups and a carbonyl
group next to one of the pyrazolyl rings (L1) (Fig. 1) or two pyraz-
olyl rings with a linker that consists of two CH2 groups and a car-
bonyl next to each pyrazolyl ring (L4) (Fig. 2). The two pyrazolyl
rings in L1 are close to being perpendicular to each other with a
dihedral angle of 74.23(5)° between the pz planes, whereas the
two pyrazolyl rings in L4 are much closer to being coplanar with
* Corresponding author.
E-mail address: jdarkwa@uj.ac.za (J. Darkwa).
1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2009.10.036

124
N. Miti et al. / Inorganic Chemistry Communications 13 (2010) 123–126
O
O
N
N
N
n
Et3N
+
Cl
Cl
2
N
N
N
n
- Et3NHCl
H
n = 1 (L1); n = 2 (L2)
Scheme 1.
O
O
O
O
N
N
N
n
Et3N
+
Cl
Cl
2
N
N
N
n
- Et3NHCl
H
n= 0 (L3); n=4 (L4)
Scheme 2.
Fig. 3. ORTEP diagram of 2 with 50% thermal probability ellipsoids.
Fig. 1. ORTEP diagram of L1 with 50% thermal probability ellipsoids.
a palladium complex [13] suggested that the carbonyl linker pro-
motes such hydrolysis. In contrast L1 produces [PdCl2L1] (1) but
in low yield (41%) and is accompanied by the formation of
[PdCl2(3,5-Me2pzH)2] which indicates that there is partial hydroly-
sis [14]. The 1H NMR spectrum of complex 1 shows a spectrum pat-
tern similar to that of L1, but differs in chemical shifts. There are
four methyl protons that have shifted downfield compared to L1,
and three less intense methyl peaks. One of the latter methyl peaks
is more intense than the others, suggesting that a fourth weak
methyl peak overlaps with this peak. The two sets of methyl pro-
tons imply that there exist two conformers of 1, boat and chair
conformers that are in equilibrium in solution. Two sets of dou-
blets of doublets were observed in the CH2 region at 4.23 ppm
and 5.27 ppm; each attributable to CH2 protons in the ligand back-
bone. The NMR data therefore suggest that while the two conform-
ers interconvert through the flipping of the ligand alkyl backbone,
the room temperature 1H NMR spectrum is only an average of the
four CH2 groups of the conformers. Pons and co-workers have
shown over the years that alkyl linker pyrazolyl complexes readily
interchange forms in solution, so this observation is not uncom-
mon [4a,15].
Fig. 2. ORTEP diagram of L4 with 50% thermal probability ellipsoids.
a dihedral angle of 16.28(8)° between them. The N–C(@O) bond
length in L1 is shorter (1.4020(14) Å) than the N–C(@O) bond
length of the nitrogen bound to the methylene group (N(3)–C(8)
(1.4601 Å) due to conjugation. Other distances agree within exper-
imental error and fall in the usual ranges.
Reaction of L2–L4 with [PdCl2(NCMe)2] rapidly led to the for-
mation of [PdCl2(3,5-Me2pzH)2]. We recently reported that the
partial hydrolysis of (pyrazol-1-ylcarbonyl)benzene to pyrazole in
Upon heating 1 transforms into a tridentate [Pd(N^C^N)Cl]
complex (2), via a C–H activation of the CH2 next to the carbonyl

N. Miti et al. / Inorganic Chemistry Communications 13 (2010) 123–126
125
O
O
N
N
N
N
N
[PdCl2(NCMe)2]
N
N
N
Pd
Cl
Cl
1
- HCl
O
N
N
N
N
Pd
Cl
2
Scheme 3.
group thereby eliminating HCl (Scheme 3); albeit it in very low
yield and accompanied by decomposition to intractable products.
1H NMR spectrum of 2 clearly shows three doublets of doublets
signals between 3.60 ppm and 4.46 ppm that confirm that there
are in fact three inequivalent protons in the linker, two from the
CH2 and one from the CH a proton left after the C–H activation
and elimination of HCl. The structure of 2 was confirmed by single
crystal X-ray crystallography as tridentately bound palladium
complex in which a deprotonated L1 binds the palladium through
two N and a C in a distorted square planar geometry (Fig. 3). In this
structure the ligand acts a tridentate chelate, coordinating to the
metal in a ‘‘butterfly” fashion by forming two five-membered
metallocycle rings. Ring Pd1–N1–N2–C6–C7 possesses a ‘‘twisted”
conformation in which atoms C7 and Pd reside above and below
the plane of the other three atoms. Ring Pd–N4–N3–C8–C7 is best
described as an envelope with C7 as the flap atom. The Pd(II) atom
is displaced from the coordination plane defined by atoms Cl, N1,
C7, and N4 by 0.0457(7) Å, therefore these five atoms are
essentially coplanar. The bond angles around Pd in 2 are: C(7)–
Acknowledgement
Financial support from the National Research Foundation
(South Africa) for this project is gratefully acknowledged.
Appendix A. Supplementary material
CCDC 740261, 740262 and 740263 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.inoche.2009.10.036.
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