Synthesis and structural investigations of Ni(II)- and Pd(II)-coordinated α-diimines with chlorinated backbones

Article abstract of DOI:10.1016/j.ica.2009.11.009

Novel square planar Pd(II) α-diimines [PdX₂{ArN{double bond, long}C(Cl)}₂], where Ar = C₆H₅, (2,6-Me₂C₆H₃), (2,6-ⁱPr₂C₆H₃) and X = Cl or Br, and the octahedral Ni(II) complex [NiBr₂{(C₆H₅)N{double bond, long}C(Cl)}₂(THF)₂] have been prepared and characterised by spectroscopic methods. For two of the Pd(II) complexes and the Ni(II) complex the crystal structures were determined by X-ray crystallography. A further insight into the geometry and electronic structure of [PdBr₂{(2,6-Me₂C₆H₃)N{double bond, long}C(Cl)}₂] was gained using density functional theoretical calculations (DFT). This compound resembles structurally and electronically typical olefin polymerisation pre-catalysts supported by α-diimines incorporating methyl- and 1,8-naphtalenyl substituents at the ligand backbone. The chlorine-substituted backbone of the free ligand [2,6-Me₂C₆H₃N{double bond, long}C(Cl)]₂ can be employed in further alkylation reactions to generate new multifunctional ligand prototypes with potential uses as ansa-metallocene/diimines building blocks for catalytic applications of heterobimetallic complexes.

Full text of DOI:10.1016/j.ica.2009.11.009

Inorganica Chimica Acta 363 (2010) 1157–1172
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis and structural investigations of Ni(II)- and Pd(II)-coordinated
a-diimines with chlorinated backbones
b
b,
b,
*
b
Sofia I. Pascu a, , Gabor Balazs , Jennifer C. Green , Malcolm L.H. Green , Ino C. Vei ,
*
*
John E. Warren a,c, Caroline Windsor b
a Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK
b Department of Chemistry, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, UK
c Synchrotron Radiation Source, Daresbury Laboratory, Warrington WA4 4AD, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 June 2009
Received in revised form 30 October 2009
Accepted 8 November 2009
Available online 14 November 2009
Novel square planar Pd(II)
a-diimines [PdX2{ArN@C(Cl)}2], where Ar = C6H5, (2,6-Me2C6H3),
(2,6-iPr2C6H3) and X = Cl or Br, and the octahedral Ni(II) complex [NiBr2{(C6H5)N@C(Cl)}2(THF)2] have
been prepared and characterised by spectroscopic methods. For two of the Pd(II) complexes and the Ni(II)
complex the crystal structures were determined by X-ray crystallography. A further insight into the
geometry and electronic structure of [PdBr2{(2,6-Me2C6H3)N@C(Cl)}2] was gained using density func-
tional theoretical calculations (DFT). This compound resembles structurally and electronically typical ole-
This paper is dedicated to Professor Jon
Dilworth on the occasion of his 65th
birthday.
fin polymerisation pre-catalysts supported by
a-diimines incorporating methyl- and 1,8-naphtalenyl
substituents at the ligand backbone. The chlorine-substituted backbone of the free ligand [2,6-
Me2C6H3N@C(Cl)]2 can be employed in further alkylation reactions to generate new multifunctional
ligand prototypes with potential uses as ansa-metallocene/diimines building blocks for catalytic applica-
tions of heterobimetallic complexes.
Keywords:
Pd(II) complexes
Halide-substituted a-diimines
Ethylene polymerisation catalysts
Heterobimetallic early–late transition metal
complexes
Ó 2009 Elsevier B.V. All rights reserved.
Density functional theory
1. Introduction
the metal coordination abilities and the reactivity of the resulting
complexes. Many examples of
a-diimines with a rigid backbone,
The interest in the chemistry of 1,4-disubstituted-1,4-diaza-1,3-
butadienes (R-DAB) and in their metal complexes was boosted by
the discovery by Brookhart and co-workers of highly active Ni(II)
where rotation around the C–C backbone is restricted to give the
s-cis orientation, have also been reported and their metal com-
plexes have been tested as olefin polymerisation and other organic
reaction catalysis [2–4,7].
and Pd(II)
a-olefin polymerisation catalysts containing bulky dii-
mine ligands of this family [1–6]. Ligands with R2, R3 = H, alkyl,
aryl, 1,8-naphthalenyl (Fig. 1) have been explored in this context,
showing versatile coordination behaviour and a wide variety of
reactivity patterns, which was assigned to their interesting elec-
tronic properties.
Despite a large number of reports concerning the organic chem-
istry of oxazolyl chlorides, which are known since 1907 [8], there
are only a handful of reports on the ability of (N-coordinated) tran-
sition metals to mediate their reactivity in organic transformations
involving the C(Cl)–C(Cl) backbone [9–15]. The chemistry of ha-
lide-substituted diimines (e.g. R2, R3 = Cl) is virtually unexplored
in the context of their metal coordination chemistry. Their catalytic
activity or use as potential building blocks towards the synthesis of
more complex potentially heterobimetallic systems with catalytic
applications towards organic reactions has not been explored.
The reactivity of metal-coordinated oxazolyl chlorides has not
been investigated thus far. To the best of our knowledge, there
are only very few structural investigations concerning the metal
complexes of this class of chlorine-substituted diimines, such as,
for example, the complexes [NiBr2({(p-Tol)(C6H4)N@C(Cl)}2)
(THF)2], [NiBr2({(2-(C6F5)C6H4)N@C(Cl)}2)(OH2)2], and [CoBr2({(2-
(C6F5)C6H4)N@C(Cl)}2)] [13,14].
Various synthetic routes for R-DAB ligands enabled the prepara-
tion of an array of ligands with different R1–R3 substituents, which
allowed both the steric and electronic properties to be varied.
These ligands were shown to have a versatile chemistry, acting
as N
r-donors or as g p-donors. The torsional isomers of
2-C@N
R-DAB type ligands, s-cis and s-trans, adopt conformations shown
in Fig. 1. The steric properties of R1, R2 and R3 substituents influ-
ence the stability of the N@C–C@N conformation and therefore
* Corresponding authors. Tel.: +44 1225386627 (S.I. Pascu).
E-mail address: s.pascu@bath.ac.uk (S.I. Pascu).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.11.009

1158
S.I. Pascu et al. / Inorganica Chimica Acta 363 (2010) 1157–1172
R1
N
R3
R1
N
N
R1
filtration and, after washing with Et2O and light petroleum ether
(b.p. 40–60 °C), the compounds RN(H)C(@O)–C(@O)(H)NR, where
i
R = Ph, Me, Pr, were obtained in good yield. After treatment with
PCl5 and work-up, the desired ligands 1–3 were isolated as
bright-yellow microcrystalline powders in yields comparable with
the reported methods (60–65%) [13,26].
R2
R3
R2
N
R1
The metal complexes 4–8 were synthesised according to
Scheme 1. When a suspension of [NiBr2(DME)] in THF was added
to PhN@C(Cl)–C(Cl)@NPh, a dark orange reaction mixture was ob-
tained. After filtration, all volatiles were removed under reduced
pressure and the solid was washed with pentane. The crude
product was recrystallised from THF/pentane to afford [NiBr2-
{PhN@C(Cl)}2(THF)2] (4) and this was supported by X-ray
crystallography.1
s-cis
s-trans
R1 =Alkyl,Aryl;
R2,R 3=H,Alkyl,Aryl,Cl
Fig. 1. Geometric isomers of typical a-diimines.
A solution of [PhN@C(Cl)–C(Cl)@NPh] (1) in CH2Cl2 was added
to a solution of [PdCl2(PhCN)2] in CH2Cl2 at room temperature.
The resulting orange precipitate was isolated, washed with pen-
tane and recrystallised from CH2Cl2/pentane. The complex
[PdCl2{PhN@C(Cl)}2] 5 was isolated as an orange crystalline solid
in 40% yield. Treatment of [PdCl2(PhCN)2] with [(2,6-
Me2C6H3)N@C(Cl)–C(Cl)@N(2,6-Me2C6H3)] (2) in CH2Cl2 gave an
orange solution. After 12 hours of stirring under N2, the volatiles
were removed under reduced pressure and the resulting solid
was washed with pentane. After recrystallisation of this residue
from CH2Cl2/pentane the complex [PdCl2{(2,6-Me2C6H3)N@C(Cl)}2]
(6) was obtained as an orange solid in moderate yield (40%).
A THF solution of [ArN@C(Cl)]2 (where Ar = 2,6-Me2C6H3 (2) or
2,6-iPr2C6H3 (3)) was added to a slurry of [PdBr2] in THF at ꢀ78 °C.
Reactions were carried out using 1:1 ratios of the reactants, to give
[PdBr2{(2,6-Me2C6H3)N@C(Cl)}2] (7) and [PdBr2{(2,6-iPr2C6H3)N@
C(Cl)}2] (8) respectively, both as orange solids. A significantly lower
yield was obtained in the case of the 8 with respect to 7 (ca. 3%
compared with 31%). This is presumably due to the steric hin-
drance of the iPr substituents in [(2,6-iPr2C6H3)N@C(Cl)–C(Cl)@
N(2,6-iPr2C6H3)] (8).
The compounds [PdX2{(Ar)N@C(Cl)}2)], where X = Cl, Ar = C6H5
(5) and 2,6-Me2C6H3 (6), and X = Br, Ar = 2,6-Me2C6H3 (7) and
2,6-iPr2C6H3 (8) were all characterised by elemental analysis, IR
and NMR spectroscopies. The 1H NMR spectrum of 5 in d6-DMSO
consists of three multiplets at d 7.54, 7.35 and 7.15 ppm, which
are assigned to the m-, p- and o-protons of the phenyl ring respec-
tively. The 13C{1H} NMR spectrum in the same solvent shows four
peaks at d 137.41, 128.64, 124.51 and 120.24 ppm. The use of
1H–13C HMBC and 1H–13C HMQC experiments revealed that these
signals correspond to the ipso-, p-, o- and m-carbons of the phenyl
rings, respectively. Peaks corresponding to the quaternary carbon
atoms of the diazabutadiene backbone were not observed. The
1H NMR spectrum of compound 6 in d6-DMSO shows a singlet at
d 2.06 ppm, a triplet at d 7.10 ppm and a doublet at d 7.20 ppm,
which correspond to the methyl groups, the p- and the m-protons
of the phenyl rings, respectively. The 13C{1H} NMR (d6-DMSO)
spectrum exhibits the expected peaks for the phenyl rings. For
compounds 5 and 6 no signals were observed for the quaternary
imine carbon atoms. In both cases, after 24 h in d6-DMSO solvent,
some substitution of the ligand by the solvent was observed in the
1H NMR spectra, forming [PdCl2(DMSO)2] and free ligand
[PhN@C(Cl)–C(Cl)@NPh] (1) or [(2,6-Me2C6H3)N@C(Cl)–C(Cl)@
N(2,6-Me2C6H3)] (2). The IR spectra of 5 and 6 exhibit signals for
mC=N at 1554 cmꢀ1 and at 1550 cmꢀ1 respectively. The stretching
Here we report the synthesis of novel Group 10 metal com-
plexes in the family of
-diimines (R2 = R3 = Cl) and compare their
reactivity, catalytic behaviour towards olefin polymerisation and
a
electronic properties and with those of (typical Brookhart-type)
a
-diimine pre-catalysts with R2, R3 = Me or 1,8-naphthalenyl with
related geometries. We are also interested in designing ligands that
can render active one or both of the metal centres in a potentially
heterobimetallic catalyst for organic reactions. When the two
metal centres are held in close proximity by the ligand, they can
co-operate in the transition state of the reaction, thus having the
potential for catalysing reactions more efficiently, or with different
chemo-, regio- or stereo-selectivity than found for mononuclear
species [16–19] and there has been an increase in the study of
the reactivities of complexes containing two different metal cen-
tres [18,20–22]. It is well established that both ansa-metallocenes
and a-diimine complexes are good catalysts for ethylene polymer-
isation the first giving linear and the latter giving branched poly-
ethylene [1–4,23,24]. Combining two types of catalytic centres:
early transition metal systems (such as metallocenes) and late
transition metal centres (i.e. supported by chelating diimines)
[25] could result in polymeric materials with new and useful char-
acteristics thus far unexplored. To date, promising ligands suitable
to tackle this problem have not been discovered. Only a limited
number of early–late heterobimetallic systems have been isolated
and ligand frameworks suitable to act as precursors for ‘two-in-
one’ ansa-metallocene/diimine systems are currently unavailable.
The halide-substituted diimines (also called oxazolyl chlorides or
bis-imodoylchlorides) are already of interest as building blocks to
generate such ligands, since there are a number of reports on their
further organic chemistry derivatisation via substitution reactions
at their C(Cl)–C(Cl) backbone [9–15].
We report here an efficient route to a new class of organic ligand
precursors combining both
a-diimines and fluorenyl functional
groups in the same molecule. This system could potentially lead
to new heterobimetallic complexes, with properties thus far unex-
plored, incorporating an ansa-metallocene site (known to effec-
tively bind early transition metals) and an
a-diimine (known to
coordinate efficiently to late transition metal complexes).
2. Results and discussions
2.1. Synthesis and spectroscopic characterisation
The a-diimine ligands PhN@C(Cl)AC(Cl)@NPh (1) (2,6-Me2C6H3)
N@C(Cl)C(Cl)@N(2,6-Me2C6H3) (2) and (2,6-iPr2C6H3)N@C(Cl)-
C(Cl)@N(2,6-iPr2C6H3) (3) were prepared following modifications
of the literature procedures [13,26]. The crucial step in the synthe-
sis of these bis-imodoylchlorides is the isolation of the relevant
oxalamides RN(H)C(@O)–C(@O)(H)NR. These were prepared by
reaction of the corresponding amine R–NH2 with ClC(@O)–C(@O)Cl
in Et2O. In each case, the resulting white precipitate was isolated by
frequencies mC=N were shifted to lower wavenumbers with respect
to those found for the corresponding free ligands [PhN@C(Cl)–
1
The octahedral Ni(II) compound 4 was tested as a catalyst towards the ethylene
polymerisation under mild conditions (as described above) but no significant catalytic
activity was observed and further investigations of this system were not carried out.

S.I. Pascu et al. / Inorganica Chimica Acta 363 (2010) 1157–1172
1159
Cl
Cl
Br
Ni
Br
N
N
4
Ar
Ar = C6H5
Ar
+
[NiBr2(DME)]
THF
Fl
Fl
THF
Ar
Cl
THF
N
N
N
Ar
Ar
Pd
Cl
Cl
Cl
X
X
+
[PdCl2(PhCN)2] or PdBr2
excess FlLi / THF or Et2O
OR
N
CH2Cl2 or THF
N
X
N
Cl
Cl
Ar
Ar
Ar
5-8
Pd
Fl =
X
N
N
Ar
Ar
1
Ar =Ph (
)
Pd
X = Br
Ar = 2,6-Me2C6H3 (7)
X = Cl
Ar = C6H5, ( ),
2,6-Me2C6H3 (2),
2,6-iPr2C6H3
5
Fl
3
Fl
( )
8
2,6-iPr2C6H3
( )
2,6-Me2C6H3 (6)
Ar
Ar
N
N
FlLi
Ar
Cl
Cl
H
THF
N
nBuLi / THF or Et2O
N
N
Ar
Ar
Cl
not isolated
9
Ar
N
2FlLi
THF
N
H
Ar
2
H
10
Ar = 2,6-Me2C6H3
N
Ar
Scheme 1. Reactivity of ligands 1–3 towards Group 10 metal complex precursors and alkylating agents.
C(Cl)@NPh] (1) (1664 cmꢀ1
)
and [(2,6-Me2C6H3)N@C(Cl)–
pared with mC=N 1664 cmꢀ1 in the free ligand 3, Table 1) Although
the choice of solvent and reaction conditions varied with the solu-
bility requirements of each system, mild synthetic routes led to the
facile formation of novel complexes of the type [PdX2{(2,6-
R2C6H3)N@C(Cl)}2] (where R = H, Me, iPr, X = Cl or Br). The presence
C(Cl)@N(2,6-Me2C6H3)] (2) (1660 cmꢀ1), confirming the metal
coordination in both cases. Formation of the 7 was confirmed by
1H NMR spectroscopy, mass spectroscopy and elemental analysis.
A
1H–1H COSY NMR experiment showed the coupling between
the methyl protons at d 7.00 and the aromatic protons at d 2.10
of chlorine atoms at the a-diimine backbone did not seem to hin-
der the Pd(II) coordination of the ligand.
and FAB+ spectrum confirmed the presence of the fragment corre-
sponding to [complex
–
2Br]+. The 1H NMR spectrum of
The reaction of the free ligand [2,6-Me2C6H3)N@C(Cl)}2] (2)
with one equivalent of nBuLi (in THF, ꢀ78 °C to room temperature),
followed by the in situ reaction with one equivalent of fluorenyl
lithium (from ꢀ78 °C to room temperature) led to the clean and
complete substitution of the chlorinated ligand backbone. The for-
mation of the new, potentially multidentate, neutral ligand [(2,6-
Me2C6H3)N@C(nBu)–C(Fl)@N(2,6-Me2C6H3)] 9 in ca 15% yield was
observed. Similarly, the backbone substitution of the free ligand
[2,6-Me2C6H3)N@C(Cl)}2] 2 using two equivalents of fluorenyl lith-
ium (ꢀ78 °C to room temperature) led to the conversion to the new
neutral, potentially multidentate ligand precursor [(2,6-
Me2C6H3)N@C(Fl)–C(Fl)@N(2,6-Me2C6H3)] 10 in ca. 65% yield. The
compounds 9 and 10 were characterised by FAB+ mass spectrome-
try, IR (KBr), 1H NMR, 13C{1H} NMR and X-ray diffraction. We have
thus obtained two unusual ligands having the potential to support
[PdBr2{(2,6-iPr2C6H3)N@C(Cl)}2)] (8) showed a downfield shift of
the diimine ligand resonances. For example, the CHMe2 resonances
were found at d 2.94 compared with d 2.80 in the 1H NMR spectra
of the free diimine. The 1H–1H COSY experiment shows the cou-
pling of the methyl protons at d 1.275 and the aromatic protons
at d 7.34 and d 7.22. Elemental analysis confirmed the formation
of the desired product and the FAB+ mass spectrum confirmed
the presence of the molecular ion in the correct isotopic pattern.
Further evidence is provided by the IR spectrum of 7, where the
mC=N was found shifted to a lower wavenumber (1552 cmꢀ1 com-
Table 1
Selected IR data (KBr) corresponding to metal-coordinated (where M = Ni(II) for
compound 4 and M = Pd(II) for compounds 5–8) as well as free oxazolyl chloride
ligands (compounds 1–3).
new heterobimetallic a-diimine/metallocene pre-catalyst systems.
These complexes were characterised in the solid state by X-ray
crystallography. They could potentially combine the 4-electron do-
Ligands
mC=N (cmꢀ1
)
Complexes
mC=N (cmꢀ1
)
nor abilities of
a-diimines with up to 6-electron donor (when
R = Ph
R = Ph
R = 2,6-Me2C6H3
R = 2,6-Me2C6H3
R = 2,6-iPr2C6H3
4
5
6
7
8
1590
1554
1550
1552
1552
R = 2,6-Me2C6H3
R = 2,6-Me2C6H3
R = 2,6-iPr2C6H3
1
2
3
1664
1660
1660
deprotonated) of a fluorenyl unit. These ligand precursors open
up new exciting metal coordination possibilities since they could
have the potential to coordinate in a variety of modes to both late
and/or early transition metals. Studies are in progress to further

1160
S.I. Pascu et al. / Inorganica Chimica Acta 363 (2010) 1157–1172
develop this chemistry for tandem homogeneous catalysis
applications.
mine ligands (e.g. C@N distances of 1.2520(16) Å for 1, 1.2749(18)
and 1.2776(18) Å for 9 and 1.273(3) Å for 10; and backbone C–C
distances of 1.497(2), 1.5070(18) and 1.526(4) Å for 1, 9 and 10,
respectively. Single crystals suitable for X-ray diffraction of
[NiBr2({PhN@C(Cl)}2)(THF)2] 4 were obtained by vapour diffusion
of pentane into a THF solution of compound 4 at room tempera-
ture. This complex adopts a distorted octahedral coordination
geometry with the bromine atoms located trans to each other
and the THF molecules cis to each other. The Ni–Br bond lengths
averaging 2.534 Å are similar to those for the octahedral complexes
[NiBr2({(p-Tol)N@C(Cl)}2)(THF)2] (2.535(3) and 2.517(3) Å) [14]
and [NiBr2({(2-(C6F5)C6H4)N@C(Cl)}2)(OH2)2] (2.439(1) and
2.612(8) Å) [15]. The Ni–O (2.089(3) and 2.081(3) Å) and Ni–N
2.2. X-ray diffraction studies
The molecular structures of free ligands 1, 9 and 10 and metal
complexes 4, 5 and 7 were determined by single crystal X-ray dif-
fraction. Selected bond distances and angles are shown in Table 1
and crystallographic details in Table 2. ORTEP diagrams describing
these structures are represented in Figs. 2–7. Compounds 1, 9
and 10 were crystallised from deuterated toluene and their X-ray
diffraction structures determined. They all show s-trans geometries
and bond lengths and angles within the expected range for free dii-
Table 2
Selected bond lengths (Å) and angles (o) for the free ligands 1, 9 and 10 and for complexes 4, 5 and 7.
Comp.
Distances (Å)
Angles (°)
1
Cl(1)–C(1)
N(1)–C(2)
N(1)–C(1)
C(1)–C(1’)
1.7601(12)
1.4191(14)
1.2520(16)
1.497(2)
C(1)’–C(1)–Cl(1)
C(1)’–C(1)–N(1)
112.61(11)
122.00(13)
4
Ni(1)–Br(1)
Ni(1)–N(1)
Ni(1)–O(1)
Cl(1)–C(1)
N(1)–C(1)
N(1)–C(3)
C(1)–C(2)
Ni(1)–Br(2)
Ni(1)–N(2)
Ni(1)–O(2)
Cl(2)–C(2)
N(2)–C(2)
N(2)–C(9)
2.5470(6)
2.094(4)
2.089(3)
1.734(4)
1.261(6)
1.437(5)
1.516(5)
2.5209(6)
2.104(3)
2.081(3)
1.734(4)
1.268(6)
1.433(5)
Br(1)–Ni(1)–Br(2)
O(1)–Ni(1)–O(2)
Br(2)–Ni(1)–O(1)
Br(2)–Ni(1)–O(2)
Br(1)–Ni(1)–N(2)
Br(2)–Ni(1)–N(2)
O(2)–Ni(1)–N(2)
Cl(2)–C(2)–C(1)
C(1)–C(2)–N(2)
N(1)–Ni(1)–N(2)
Br(1)–Ni(1)–O(1)
Br(1)–Ni(1)–O(2)
Br(1)–Ni(1)–N(1)
Br(2)–Ni(1)–N(1)
O(1)–Ni(1)–N(1)
Cl(1)–C(1)–C(2)
C(2)–C(1)–N(1)
173.29(3)
88.20(15)
91.02(9)
92.92(9)
87.14(9)
90.28(9)
94.32(14)
118.2(3)
116.7(3)
78.31(13)
91.28(9)
93.45(9)
87.00(9)
86.42(9)
99.19(14)
118.5(3)
116.5(4)
5
Pd(1)–Cl(3)
Pd(1)–N(1)
Cl(1)–C(1)
N(1)–C(1)
N(1)–C(3)
C(1)–C(2)
Pd(1)–Cl(4)
Pd(1)–N(2)
Cl(2)–C(2)
N(2)–C(2)
N(2)–C(9)
2.2548(9)
2.043(3)
1.704(3)
1.277(4)
1.432(4)
1.478(5)
2.2848(9)
2.060(3)
1.698(3)
1.281(5)
1.438(4)
N(1)–Pd(1)–N(2)
Cl(3)–Pd(1)–N(1)
Cl(1)–C(1)–C(2)
Pd(1)–N(2)–C(2)
Pd(1)–N(2)–C(9)
Cl(3)–Pd(1)–Cl(4)
Cl(4)–Pd(1)–N(2)
Cl(2)–C(2)–C(1)
Pd(1)–N(1)–C(1)
Pd(1)–N(1)–C(3)
80.15(11)
92.80(7)
119.0(3)
112.4(2)
127.1(2)
91.15(3)
95.85(8)
118.8(3)
113.2(2)
125.0(2)
7
Pd(1)–N(1)
Pd(1)–N(2)
Pd(1)–Br(1)
Pd(1)–Br(2)
N(1)–C(1)
N(1)–C(3)
C(1)–Cl(1)
C(1)–C(2)
2.045(3)
2.054(3)
2.3747(7)
2.3966(4)
1.279(4)
1.449(4)
1.697(3)
1.503(4)
1.277(5)
1.444(4)
1.695(3)
N1–Pd1–N2
Br1–Pd1–Br2
Br1–Pd1–N1
Br2–Pd1–N1
Br1–Pd1–N2
Br2–Pd1–N2
Pd1–N1–C1
Pd1–N1–C3
Pd1–N2–C2
Pd1–N2–C1)
79.43(11)
90.656(19)
93.88(8)
174.81(8)
170.31(9)
96.36(8)
114.1(2)
124.6(2)
113.8(2)
125.0(2)
N(2)–C(2)
N(2)–C(11)
C(2)–Cl(2)
9
C(1)–C(2)
C(1)–C(7)
C(1)–N(2)
C(2)–C(3)
C(2)–N(1)
C(3)–C(4)
1.5070(18)
1.5266(18)
1.2749(18)
1.5141(18)
1.2776(18)
1.524(2)
C(2)–C(1)–C(7)
C(2)–C(1)–N(2)
C(7)–C(1)–N(2)
C(1)–C(2)–C(3)
C(1)–C(2)–N(1)
C(3)–C(2)–N(1)
C(2)–C(3)–C(4)
119.67(12)
114.98(12)
125.35(12)
117.19(12)
117.12(12)
125.68(12)
112.71(12)
10
N(1)–C(2)
N(1)–C(29)
N(2)–C(3)
N(2)–C(37)
C(1)–C(2)
1.273(3)
1.422(3)
1.272(3)
1.425(3)
1.526(4)
C(2)–N(1)–C(29)
C(3)–N(2)–C(37)
C(2)–C(1)–C(25)
C(2)–C(1)–C(28)
123.9(2)
123.3(2)
113.6(2)
116.9(2)

S.I. Pascu et al. / Inorganica Chimica Acta 363 (2010) 1157–1172
1161
(2.094(4) and 2.104(3) Å) distances are consistent with those in
[NiBr2({(p-Tol)N@C(Cl)}2)(THF)2] (2.217(13) and 2.095(12) Å for
Ni–O, 2.088(14) and 2.111(14) Å for Ni–N). The C(1)–N(1) and
C(2)–N(2) linkages (1.261(6) and 1.268(6) Å, respectively) are of
comparable length to those of [NiBr2({(p-Tol)N@C(Cl)}2)(THF)2]
(averaging 1.252 Å) and longer than those of the related free li-
gand, (2,6-iPr2C6H4)N@C(Cl)–C(Cl)@N(2,6-iPr2C6H4) (1.245 (av) Å).
The C(1)–C(2) bond length of 1.516(5) Å is similar to that for
[NiBr2({(p-Tol)N@C(Cl)}2)(THF)2] (1.516(25) Å). The C–Cl distances
(1.734 (av) Å) are slightly shorter than the corresponding C–Cl dis-
tances in [NiBr2({TolN@C(Cl)}2)(THF)2] (1.769 (av) Å). The chelate
N–Ni–N angle of 78.31(13)° is consistent with the 77.0(5)° found
in [NiBr2({(p-Tol)N@C(Cl)}2)(THF)2] [14]. The slightly distorted
octahedral coordination sphere around the nickel atom can also
be seen from the Br(1)–Ni(1)–Br(2) trans angle of 173.29(3)°. A
similar distorted geometry around the metal centre has been re-
ported earlier for [NiBr2({(p-Tol)N@C(Cl)}2)(THF)2] [14]. X-ray
quality-single crystals of [PdCl2({PhN@C(Cl)}2)] 5 were obtained
by vapour diffusion of pentane into a CH2Cl2 solution of the com-
plex at room temperature. The asymmetric unit cell comprises
one molecule of complex 5 showing a slightly distorted square–
planar geometry. Distortions from this geometry are indicated by
the deviations from a least-squares plane through the following
atoms; Pd(1) 0.034, Cl(1) 0.010, Cl(2) ꢀ0.026, N(1) ꢀ0.031 and
N(2) 0.013 Å. A closer examination of the structure reveals an
intermolecular interaction between Pd(1)ꢁꢁꢁCl(2A) of 3.217 Å which
seems to be predominantly an electrostatic interaction, between
the Pd centre with positive formal charge and the Cl atom with a
negative formal charge. The Pd–Cl linkages of 2.270 (av) Å are
slightly shorter than those of the related Pd(II) complexes
[PdCl2({(2,6-iPr2C6H4)N@C(H)}2)] (2.2799(5)–2.2834(5) Å) [27]
and [PdCl2(Ph–BIP)] (2.281(4) and 2.282(5)Å) [28]. However, the
Pd–N distances of compound 7 (2.052 (av) Å) are slightly longer
when compared with the Pd(II)–dihalide complexes mentioned
above (2.014(2)–2.025(2) Å), indicating weaker bonding between
Pd metal centre and the N atoms of oxalodiimidoyl dichlorides
compared with an
a-diimine ligand. In 5, the ligand PhN@C(Cl)–
C(Cl)@NPh (1) adopts the s-cis configuration due to chelation to
the metal. The phenyl rings are placed approximately perpendicu-
lar to the plane formed by the metal and coordinated nitrogen
atoms. In contrast, the related free ligand (2,6-iPr2C6H4)N@C(Cl)–
C(Cl)@N(2,6-iPr2C6H4) has s-trans geometry to minimise steric
interactions [15]. The N@C (1.277(4) and 1.281(5) Å) and C(1)–
C(2) bond lengths (1.478(5) Å) are similar to those found in the
earlier reported Pd(II) complex above [27] (i.e. 1.269(4) and
1.305(3) for N@C and 1.460(5) and 1.493(4) Å for CAC bond
respectively). The longer N@C bond lengths compared with com-
pound 4 (1.261(6) and 1.268(6) Å), suggests a more effective
p-
back donation from the Pd(II) with respect to Ni(II) probably due
to a better overlap of metal and ligand orbitals. The C(1)–C(2)
and C–Cl distances of compound 5 are shorter than those of com-
pound 4, indicating a small degree of electron delocalisation within
the N@C(Cl)–C(Cl)@N fragment. X-ray quality single crystals of
[PdBr2({2,6-Me2C6H3N@C(Cl)}2)] (7) were also grown by slow
diffusion of pentane into a concentrated solution of the red-orange
Fig. 2. ORTEP representation of ligand 1 (50% thermal ellipsoids, H atoms are omitted
for clarity). Asymmetric unit contains only half molecule, the other half was
generated by symmetry.
Fig. 3. ORTEP representation of compound 4 (50% thermal ellipsoids, H atoms are omitted for clarity). Each THF molecule was refined as disordered over two positions, with 0.5
occupancy each.

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S.I. Pascu et al. / Inorganica Chimica Acta 363 (2010) 1157–1172
Fig. 4. ORTEP representation of compound 5 (50% thermal ellipsoids, H atoms are omitted for clarity).
Fig. 5. ORTEP representation of compound 7 (50% thermal ellipsoids, H atoms are omitted for clarity, one of the Br ions was refined as disordered over two positions, with 0.5
occupancy each).
solid dissolved in CH2Cl2. The square planar geometry of the Pd(II)
centre in 7 is shown in Fig. 5, with the bromine atoms located
slightly above and below the plane of the molecule. All bonds
and angles are entirely consistent to those discussed above for
the structure of 5. The bond lengths for the C(1)–C(2) ligand back-
bone are within the expected range for Pd(II)-coordinated dii-
mines, 1.503(4), and close to those of C–C single bonds (typically
1.54 Å), whilst those for N(1)–C(1) (of 1.279(4) Å) and N(2)–C(2)
and (of 1.277(5) Å) indicate metal-coordinated C@N, as they are
longer than those found in free ligand 1 (of 1.2520(16) Å).
i
2.3. Reactivity of [PdBr2{(2,6-R2C6H3)N@C(Cl)}2] (R = Me, Pr) and
electronic structures by DFT calculations
To test the reactivity of the Pd(II) complexes formed, and ex-
plore possible routes towards isolation of new species based on
the further derivatisation of the backbone chlorine ligands, we
treated the Pd(II) compounds 7 and 8 with the alkylating reagents
lithium fluorenyl, phenyl lithium and CpTMSLi in THF or Et2O be-
tween 0 °C and room temperature (Scheme 1). These tests were
carried out in stoichiometric ratios as well as using vast excess of
alkylating reagents. Thus far, the substitution reactions between
Fig. 6. ORTEP representation of compound 9 (50% thermal ellipsoids).
fluorenyl lithium and complex 7 (where ligand 2 is PdBr2-coordi-

S.I. Pascu et al. / Inorganica Chimica Acta 363 (2010) 1157–1172
1163
of typical a-diimine pre-catalysts with chlorine substituents influ-
ences their activity and/or the properties of any polymer obtained.
Electronic factors, as a result of electron density withdrawal by the
chlorine atoms, could affect the activity of the Pd(II) catalysis un-
der the mild conditions. Catalysis tests were thus carried out, un-
der mild conditions, for [PdBr2{2,6-R2C6H3N@C(Cl)}2] (where
R = Me (7) and iPr (8)), using MAO as a co-catalyst (molar
ratio Al:Pd ca 310), 2–4 bar ethylene in toluene and 900
minutes reaction time at 25 °C. Pre-catalysts known to act as
typical Pd(II) Brookhart-type systems for activation of ethylene
(i.e. [PdBr2{(2,6-Me2C6H3)N@C(Me)}2] (11) and [PdBr2{(2,6-
Me2C6H3)N@C}2Nap] (Nap = 1,8-naphthalenyl) (12) were also pre-
pared via standard methods [7,29] and tested under the same con-
ditions as described above for 7 and 8. The behaviour of all Pd(II)
pre-catalysts investigated was similar: they all showed very low
activities, with only a few mg of PE isolated and activities lower
than 0.0002 ꢂ 106 g PE/mol ꢂ h ꢂ bar, and no significant amount
of oligomers was identified in the liquid phase by GC–MS for 7,
8, 11 and 12. This behaviour was entirely consistent with that re-
ported earlier for Pd(II) diimines under mild conditions [30].
Although these tests were expected to help us rationalise whether
changes in the nature of the backbone substituents affect catalytic
properties in the series, the activities of these Pd(II) precursors the
olefin polymerisation were all extremely low under the mild con-
ditions employed, regardless of the nature of the backbone and de-
spite the use of significantly longer polymerisation times, higher
catalyst loading and higher ethylene pressures than those typically
used for Ni(II) diimines [15]. Although the ethylene monomer acti-
vation by the Pd(II) centre does seem to take place in all cases, sig-
nificantly higher temperatures and pressures of monomer may be
needed to generate the polymers using these Pd(II) complexes. It
would be difficult to assign the precise reasons for the low activity
of 7 and 8 by simply comparing the catalytic behaviour towards
ethylene polymerisation with those of typical Brookhart-type pal-
ladium diimines, as the activity of all Pd diimines tested under
these mild conditions was low throughout.
Fig. 7. ORTEP representation of compound 10 (50% thermal ellipsoids).
nated) did not succeed in alkylating the metal centre or in yielding
new metal complexes with alkylated ligand backbones (Scheme 1).
Instead, only the unreacted starting materials together with small
amounts of untractable decomposition products were isolated un-
der the mild conditions applied. The direct conversion into the cor-
responding alkylated metal complexes could not be demonstrated:
it appears that the N-coordinated Pd(II) centre deactivates the
chlorine-substituted backbone of these oxazolyl chlorides but fur-
ther studies are needed to fully understand these reactions.
Group 10 metal complexes of oxazolyl chloride ligands can be
regarded structurally as closely related to the Brookhart-type
pre-catalysts for olefin polymerisation [2–4]. We are interested
in determining whether the replacement of the carbon backbone
Two initial hypotheses for the low activities of 7 and 8 in the
formation of polyethylene may be formulated: (a) electronic
factors (essentially as a result of electron density withdrawal by
Cl
Cl
Me
Me
N
N
N
N
N
N
Ar
Ar
Ar
Ar
Ar
Ar
Pd
Pd
Pd
Br
Br
Br
Br
Br
Br
C
A
B
Cl
Cl
Me
Me
N
N
N
N
N
N
Ar
Ar
Ar
Ar
Ar
Ar
Pd
Pd
Pd
Me
Me
Me
D
F
E
Fig. 8. Representation of DFT-level optimised neutral species A–C and cations D–F. (Ar = 2,6-Me2C6H3).

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S.I. Pascu et al. / Inorganica Chimica Acta 363 (2010) 1157–1172
Table 3
Selected crystallographic data.
1
4
5
7
9
10
Empirical formula
Formula weight
Crystal system
Space group
Unit cell dimensions:
a (Å)
C14H10Cl2N2
277.15
monoclinic
P21/n
C22H26Br2Cl2N2NiO2
639.89
monoclinic
P21/n
C14H10Cl4N2Pd
454.46
orthorhombic
P212121
C18H18Br2Cl2N2Pd
599.47
monoclinic
P21/a
C35H36N2
484.68
C44H36N2
592.78
triclinic
triclinic
ꢀ
ꢀ
P1
P1
3.91020(10)
11.0788(3)
14.3526(4)
90
92.3451(12)
90
621.24(3)
2
1.482
9.6200(1)
19.0638(2)
13.6159(2)
90
91.5258(5)
90
2496.19(5)
4
1.703
7.0602(1)
22.4867(3)
9.9806(1)
90
90
90
1584.5(1)
4
1.905
1.837
886
13.8883(3)
7.2562(2)
20.0150(4)
90
97.6232(13)
90
1999.2(1)
4
1.992
9.7372(3)
10.3731(3)
14.5123(5)
109.0433(15)
98.6066(17)
93.4458(17)
1360.65(8)
2
10.7683(4)
12.4459(5)
12.9726(6)
84.822(2)
75.203(2)
72.6660(16)
1604.4(1)
2
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
Volume (Å3)
Z
Density (calculated) (Mg/m3)
Absorption coefficient (mmꢀ1
F(0 0 0)
1.183
0.068
520
1.227
0.071
628
)
0.503
284
4.214
1280
5.197
1154
h Range for data collection
Index ranges
5–27
3–27
3–28
5–28
5–25
4–27.5
ꢀ13
ꢀ5 < h < 5
ꢀ14 < k < 14
ꢀ18 < l < 18
0 < h < 12
0 < k < 24
ꢀ17 < l < 17
0 < h < 8
0 < k < 28
ꢀ12 < l < 12
ꢀ18 < h < 18
ꢀ9 < k < 9
ꢀ25 < l < 25
ꢀ12 < h < 11
ꢀ13 < k < 13
ꢀ18 < l < 18
< h < 14
ꢀ16 < k < 15
ꢀ16 < l < 16
13 421
Reflections collected
Independent reflections
Max. and min. transmission
Parameters
2613
1397
0.9, 0.9
82
5893
5714
0.78, 0.84
334
3426
3407
0.86, 0.93
191
19 920
4577
0.39, 0.73
236
10 033
6068
0.99, 0.99
334
7330
1, 1
415
Goodness-of-fit (GOF)
Final R indices
1.0296
0.0279
1.0531
0.0437
0.9956
0.0250
1.0092
0.0327
1.0836
0.0753
1.0474
0.0579
[I > 3
r
(I)]
[I>2
ꢀ1.08; 0.82
r
(I)]
[I > 3
ꢀ0.77; 1.07
r(I)]
Largest diff. peak and hole (e Åꢀ3
)
ꢀ0.26; 0.22
ꢀ0.98; 0.61
ꢀ0.16; 0.17
ꢀ0.36; 0.35
Table 4
Experimental and calculated bond lengths (Å) and angles (°) for 7 and model complexes A, B and C.
Comp.
Pd–Br
Pd–N
N–C
C(1)–C(2)
Br–Pd–Br
N–Pd–Br
7 (exp)
A (calc)
B (calc)
C (calc)
2.385 (av)
2.387
2.400
2.050(av)
2.028
2.004
1.277(av)
1.290
1.305
1.503
1.450
1.451
1.462
90.66
89.35
90.0
95.12
95.41
95.65
94.035
2.390
2.031
1.296
91.16
the chlorine atoms resulting in the reduction of electron density on
the metal by back donation into the antibonding orbitals of the
oxazolyl chloride ligand, which could in turn affect the olefin poly-
merisation mechanism. (b) MAO is a Lewis acid and it may attack
the chlorine atoms on the ligand in preference to (or as well as) the
bromine atoms bound to the metal, leading to catalyst decomposi-
tion during the initiation stage of the ethylene polymerisation
stages. Reactions of 7 with alkylating reagents were not conclusive
thus far but seemed to suggest that once coordinated the reactivity
of the oxazolyl chloride ligand is significantly reduced.
An insight into the electronic structure of this class of Pd(II)
complexes was gained using Density Functional Theoretical calcu-
lations (DFT). The gas-phase geometries of [PdBr2{(2,6-
Me2C6H3)N@C(Cl)}2] (compound 7, gas-phase modelled geometry
A), [PdBr2{(2,6-Me2C6H3)N@C(Me)}2] (compound 11, gas-phase
modelled geometry B) and [PdBr2{(2,6-Me2C6H3)N@C}2Nap] (com-
pound 12, gas-phase modelled geometry C, where Nap = 1,8-
Naphthalenyl) were optimised without symmetry constrains.
Model species A and B have similar geometries to well established
Pd(II) halide precursors in olefin polymerisation catalysis. Their
treatment with MAO would be expected to give catalytic species
which can be simplified (for the purpose of modelling) as
[PdMe{(2,6-Me2C6H3)N@C(Cl)}2]+ (D), [PdMe{(2,6-Me2C6H3)N@
C(Me)}2]+ (E) and [PdMe{(2,6-Me2C6H3)N@C}2Nap]+ (Nap = 1,8-
naphthalenyl) (F) (Fig. 8).
Table 5
Calculated bond lengths (Å) and angles (°) for modelled species D, E and F.
D
E
F
Pd–Me
Pd–N(1)
Pd–N(2)
C(1)–N(1)
C(2)–N(2)
C(1)–C(2)
Me–Pd–N(1)
Me–Pd–N(2)
N(1)–Pd–N(2)
1.994
1.999
2.121
1.280
1.293
1.464
174.19
95.69
79.86
1.998
1.982
2.073
1.301
1.292
1.469
178.94
100.31
78.90
1.991
1.993
2.125
1.285
1.295
1.480
178.45
97.10
81.45
veals a good agreement between gas-phase models and the solid
state structures (Table 3). There is only a slight underestimation
of the C–C bond length in the backbone and of the Pd–N bond
length. The geometry of the central C2N2PdBr2 unit seems only
very slightly influenced by the nature of the substituents in the
backbone. The calculated Pd–Br bond length in A is 2.387 Å, some-
what shorter than in B or C (2.400 and 2.390 Å, respectively). Also
the Pd–N, C–N and C(1)–C(2) distances in A, B and C remain un-
changed with the nature of the backbone substituent.
The cationic species [PdMe{(2,6-Me2C6H3)N@C(Cl)}2]+ (D),
[PdMe{(2,6-Me2C6H3)N@C(Me)}2]+ (E) and [PdMe{(2,6-Me2C6H3)-
N@C}2Nap]+ (Nap = 1,8-naphthalenyl) (F) were also modelled in
the gas phase and without any symmetry restrains. All optimised
geometries fit well in the observed trends for A, B and C. The
A comparison of the calculated geometrical parameters for A
and those obtained from single crystal X-ray diffraction (for 7) re-

S.I. Pascu et al. / Inorganica Chimica Acta 363 (2010) 1157–1172
1165
changes in molecular parameters are only minor (Tables 4 and 5).
The most noticeable difference is a wider Me–Pd–N(2) angle in E
and F than in D. This implies a Me–Pd–N(2) angle closer to linearity
in E and F. The C–Cl bond length in the monocationic species D
(1.694 and 1.704 Å) is only slightly shorter than that calculated
for the neutral model complex A (1.710 Å).
0.43 for A, B and C, respectively) and that of negative charge on
bromine and (to much lesser extent) on nitrogen atoms The calcu-
lated charges on nitrogens were ꢀ0.08 for all complexes, those on
bromines were ꢀ0.24, ꢀ0.27 and ꢀ0.26 (for A, B and C, respec-
tively). Unexpectedly the chlorine atoms in compounds A and D
were found slightly positively charged (calculated Hirshfeld value
of 0.07). For D, E and F, the varying the backbone substituent from
Cl to Me and Nap led to an increase of the positive charge on pal-
The analysis of the Hirshfeld charge distribution shows the
accumulation of positive charge on palladium (ca. 0.44, 0.44 and
Fig. 9. Optimised geometry of complex 7 (model species A, a) and representations of the isosurfaces corresponding to the molecular orbitals of model species A (b), B (c), C
(d). Increased label value consistent with increase in orbital energy. Model species A–C are isostructural.

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S.I. Pascu et al. / Inorganica Chimica Acta 363 (2010) 1157–1172
ladium centres and a decrease of the negative charge on nitrogens.
The magnitude of the charge accumulation on Pd, and charge
depletion on N is almost equal for D, E and F. The Hirshfeld, Mul-
liken and Voronoi charge distributions show similar trends. Our
calculations indicate that charge distribution does not seem to be
significantly affected by the nature of backbone substituents.
For A–C the occupied MOs of highest energy are all centred on
the bromine atoms (Fig. 9). The HOMOs of A and B are mainly
formed from the p-orbitals of bromine centres, without significant
contribution from palladium. However, HOMOꢀ1 and HOMOꢀ2
show contributions from palladium d orbitals. The shapes and
energies of the MO’s of C were found qualitatively identical with
Fig. 9 (continued)

S.I. Pascu et al. / Inorganica Chimica Acta 363 (2010) 1157–1172
1167
those of A and B. The only difference was that the relative energies
of HOMO and HOMOꢀ1 are inverted, although the HOMO/
HOMOꢀ1 energy gap is very narrow (0.051 eV). The LUMOs of
A–C are centred on the delocalised backbone with relatively small
contribution from palladium. Complexes A–C are all characterised
by a relatively high HOMO/LUMO gap.
ergy than those of the bromine substituents. These calculations
suggest that the Lewis acid attack at the C–Cl backbone substitu-
ents is unfavourable with respect to that at the Pd–bromine
centres. This is consistent with the observation that halogen-con-
taining bis(imino)pyridine metal catalysts were found to be stable
to the MAO attack [31]. For D–F the HOMOs (shown in Fig. 10)
have strong contributions from the methyl group, Pd d-orbitals
In [PdBr2{(2,6-Me2C6H3)N@C(Cl)}2] (A), the lone pairs of the
chlorine substituents at the ligand backbone are much lower in en-
and from the
p
system of one aryl ligand. The HOMOꢀ1 and
Fig. 10. Optimised geometry of D (a) and representations of the isosurfaces corresponding to the molecular orbitals of D (b), E (c), F (d). Increased label value consistent with
increase in orbital energy. Model species D–F are isostructural.

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HOMOꢀ2 in A is formed exclusively from
p
system of the aryl
icant contributions from the palladium d-orbitals. The LUMO in D–
F is qualitatively identical with the LUMO in A–C. The LUMO+1
groups, whereas the corresponding orbitals of E and F show signif-
Fig. 10 (continued)

S.I. Pascu et al. / Inorganica Chimica Acta 363 (2010) 1157–1172
1169
orbitals have a relatively high palladium contribution. It is ex-
pected that this orbital would play an important role in the binding
terated solvents were degassed using three consecutive freeze–
pump–thaw cycles and kept over molecular sieves (4 Å) (d6-DMSO)
or vacuum distilled from potassium (d8-toluene) or CaH2 (CD2Cl2,
CDCl3). Solution NMR spectra were recorded using either a Varian
Mercury 300 (1H: 300 MHz, 13C: 75.5 MHz) or a Varian UNITYplus
(1H: 500 MHz, 13C: 125.7 MHz) spectrometer, and at room temper-
ature unless otherwise stated. The spectra were referenced inter-
nally relative to the residual protio-solvent (1H) and solvent (13C)
resonances relative to tetramethylsilane (1H, 13C, d = 0). Chemical
shifts (d) are expressed in ppm and coupling constants (J) in Hz.
Elemental analyses were performed by the Microanalytical Depart-
ment of the Inorganic Chemistry Laboratory, Oxford. Infra-red
spectra were recorded on a 6020 Galaxy Series FT-IR or a Perkin–
of
the
ethylene
to
cations
such
as
[PdMe{(2,6-
Me2C6H3)N@C(Cl)}2]+ (D) during the initiation step of the polymer-
isation reaction. The HOMO/LUMOꢀ1 gaps (158.8, 169.4 and
170.7 kJ/mol for D, E and F, respectively) are within the same range
for all three species. The same is true for the HOMO/LUMO gap (of
144.3, 152.5 and 156.3 kJ/mol for D, E and F, respectively). The
binding energy of ethylene to D (ꢀ120.36 kJ/mol) is very close to
that of binding to E (ꢀ121.07 kJ/mol) and F (ꢀ117.19 kJ/mol).
The inspection of these molecular orbitals suggests that there
are no significant thermodynamic differences between the elec-
tronic structure of A and those of typical Brookhart-type pre-cata-
lyst models
polymerisation with typical
extensively [32–39]. The estimated bonding energies of ethylene
to Pd-cations proved that the resulting -complexes should have
B
and C. The catalytic cycle of the ethylene
Elmer 1600 Series FT-IR instrument in the range 4000–400 cmꢀ1
.
a-diimine type catalysts was studied
Samples were prepared in the glovebox as pellets in KBr, and data
are quoted in wavenumbers (cmꢀ1). All mass spectrometry was
carried out at the EPSRC Mass spectrometry service, Swansea.
p
similar thermodynamic stabilities regardless of the presence of
electron withdrawing substituents at the C–C bridge for 7. The
inactivity of 7 (and, similarly that of 8) towards the ethylene poly-
merisation reaction in presence of MAO is likely to be of a kinetic
nature: stronger reaction conditions are likely to render this class
of complexes as catalytic towards olefin polymerisation, as their
electronic configurations, in addition to structural similarities,
4.1. Computational details
DFT calculations were carried out using the Amsterdam Density
Functional program suite ADF 2004.01 or ADF 2005.01 [40–43].
Scalar relativistic corrections were included in all calculations
using the ZORA method [44–48]. The generalised gradient approx-
imation was employed, using the local density approximation of
Vosko, Wilk and Nusair [49] together with the nonlocal exchange
correlation of Becke [50,51] and the nonlocal correlation correction
of Perdew [52]. The TZP basis sets were used with triple-f accuracy
of Slater type orbitals and two polarisation functions added. The
core electrons of the atoms were frozen up to 1s for C and N, 2p
for Cl, 3p for Br and 4p for Pd. All quoted electronic structure data
from optimised structures use an integration grid of 4.0 and have a
gradient correction applied after the SCF cycles.
seem to mimic closely those of typical Brookhart-type
with well understood catalytic behaviour.
a-diimines
3. Conclusions
New Ni(II) and Pd(II) a-diimines of the type [MX2{ArN@C(Cl)}2]
have been prepared and characterised. These complexes are inac-
tive toward ethylene polymerisation catalysis under mild condi-
tions in the presence of MAO. The presence of chlorides as the
ligand backbone substituent does not seem to affect the electronic
structure in the pre-catalyst series: [PdBr2{(2,6-Me2C6H3)N@
C(R}2], where R = Cl, Me and Nap = 1,8-naphthalenyl as shown by
DFT calculations, and the lack of activity in the ethylene oligomer-
isation reaction needs to be assigned to kinetic factors. The behav-
iour of Pd(II)–oxazolyl chloride complexes as catalysts in other
typical organic coupling reactions (e.g. Heck coupling reactions)
needs to be tested to explore their catalytic potential more widely.
4.2. X-ray diffraction studies
For single crystal X-ray diffraction all data collections were per-
formed using an Enraf-Nonius KappaCCD diffractometer with
graphite-monochromated Mo Ka radiation, (k = 0.71073 Å). Inten-
sity data were processed using the DENZO-SMN package [53]. For
10, crystals were extremely small and weakly diffracting and a lab-
oratory source (KappaCCD) as well as synchrotron radiation at SRS
Daresbury, Station 9.8 synchrotron source were used for data col-
lection (k = 0.69390). Data from the crystal measured on the labo-
ratory source showed better completion and is reported here.
The crystal structures were solved using the direct-methods
program SIR92 [54], which located all non-hydrogen atoms. Subse-
quent full-matrix least-squares refinement was carried out using
the CRYSTALS program suite [55]. Coordinates and anisotropic
thermal parameters of all non-hydrogen atoms were refined. The
H atoms were positioned geometrically after each cycle of refine-
ment. A Chebychev weighting scheme was applied in addition to
an empirical absorption correction [56].
4. Experimental details
All manipulations of air- and/or moisture-sensitive materials
were carried out under an inert atmosphere of dinitrogen or argon
using standard Schlenk line techniques, or in an inert atmosphere
dry box containing dinitrogen. Dinitrogen was purified by passage
through columns filled with molecular sieves (4 Å) and either man-
ganese (II) oxide suspended on vermiculite for the vacuum line, or
BASF catalyst for the dry box. Argon was purchased from BOC and
used as received. Filtrations were performed using modified stain-
less steel cannulae, which had been fitted with glass fibre filter
discs. Solvents and solutions were transferred through stainless
steel or Teflon cannulae, using a positive pressure of inert gas. All
glassware and cannulae were thoroughly dried at 150 °C before
use.
MAO (10% w/w in toluene) was purchased from Witco and used
as supplied. All solvents used in the preparation of air-sensitive
compounds were pre-dried over activated molecular sieves (4 Å)
and then distilled under dinitrogen over the appropriate drying
agent: toluene, benzene, THF and light petroleum (bp 40–60 °C)
were distilled over sodium metal; Et2O and pentane were distilled
over sodium–potassium alloy; CH2Cl2 was dried over CaH2; EtOH
and MeOH were dried over magnesium turnings and iodine. Deu-
4.3. Reagents and starting materials
PCl5, (2,6-Me2C6H3)NH2, (2,6-iPr2C6H3)NH2, acetic acid, p-tosyl-
sulphonic acid and ClC(@O)–C(@O)Cl (Aldrich) were used as sup-
plied. PhNH2 (Aldrich) was dried over CaH2 and distilled before
use. PhN@C(Cl)C(Cl)@NPh (1), (2,6-Me2C6H3)N@C(Cl)C(Cl)@N(2,6-
Me2C6H3) (2) and (2,6-iPr2C6H3)N@C(Cl)–C(Cl)@N(2,6-iPr2C6H3)
(3) were prepared following literature methods [13,26]. The known
ligands [2,6-Me2C6H3)N@C(Me)}2] and [{(2,6-Me2C6H3)N@C}2Nap]
were also prepared according to the literature methods [7,29].
[NiBr2(DME)], PdBr2 and [PdCl2(PhCN)2] (Aldrich) were used as
supplied.

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S.I. Pascu et al. / Inorganica Chimica Acta 363 (2010) 1157–1172
4.4. Synthesis of [NiBr2{PhN@C(Cl)}2(THF)2] (4)
Elemental analysis: Anal. Calc. C18H18Br2Cl2N2Pd.0.5 THF: C,
37.80; H, 3.49; N, 4.41. Found: C, 37.86; H, 3.34; N, 4.61%.
A suspension of [NiBr2(DME)] (0.111 g, 0.360 mmol) in THF
(30 mL) was added to PhN@C(Cl)–C(Cl)@NPh (1) (0.100 g,
0.361 mmol) at room temperature. The resulting orange-red reac-
tion mixture was stirred for 18 h, followed by filtration. The vola-
tiles were removed under reduced pressure and the residue
washed with pentane (2 ꢂ 5 mL). The crude product was recrystal-
lised from THF/pentane at ꢀ20 °C. [NiBr2({PhN@C(Cl)}2)(THF)2], 6
was obtained as an orange crystalline solid (Yield: 0.130 g, 56%).
Elemental analysis: Anal. Calc. for C22H26Br2Cl2N2NiO2: C, 41.3;
N, 4.38; H, 4.10. Found: C, 42.03; N, 5.00; H, 4.02%. IR (KBr): mC=N
IR (KBr): m
C=N 1554 cmꢀ1; 1H NMR (CD2Cl2, 500 MHz, 298 K): m-
NPh 7.00 (t, 2H, 3JHH = 7.3) p-NPh 7.30 (t, 1H, 3JHH = 7.3) Me 2.10 (s,
3H); 13C{1H}NMR (CD2Cl2, 125.7 MHz, 298 K) p-NPh 127.0 (s) o-
NPh 124.1 (s) m-NPh 120.1 (s), 2,6-Me2 16 (s); FAB+: 435.20 (37)
[Mꢀ2Br]+.
4.8. Synthesis of [PdBr2{(2,6-iPr2C6H3)N=C(Cl)}2] (8)
A yellow solution of ligand 3 (3.321 g, 7.5 mmol) was reacted
with [PdBr2] (1.985 g, 7.5 mmol) in THF to give the desired product
via an analogous route to that reported above for 7. Filtration of the
reaction mixture gave an insoluble, dark-red solid (yield 1.869 g)
and, upon removal of volatiles from the filtrate fraction, an oily
red-orange solid. Washing the latter with pentane (20 mL) fol-
lowed by Et2O (20 mL) and filtration yielded 8 as a bright orange
solid, which was dried under reduced pressure over night (yield
0.167 g, 3.1% yield). From the additional solid fractions isolated
from the reaction mixture, only the unreacted ligand 3 was isolated
and identified by 1H NMR.
1590 cmꢀ1
4.5. Synthesis of [PdCl2{PhN@C(Cl)}2] (5)
yellow solution of PhN@C(Cl)–C(Cl)@NPh (0.100 g,
.
A
0.361 mmol) in CH2Cl2 (10 mL) was added dropwise at room tem-
perature to a dark orange solution of [PdCl2(PhCN)2] (0.115 g,
0.300 mmol) in CH2Cl2 (10 mL). The reaction mixture was stirred
for a further 60 h, after which time an orange precipitate had
formed. The volatiles were removed under reduced pressure to af-
ford an orange solid, which was washed with pentane (2 ꢂ 10 mL)
and recrystallised from CH2Cl2/pentane. [PdCl2({PhN@C(Cl)}2)], 9
was obtained as an orange crystalline solid (yield: 0.082 g, 60%).
The above procedure, but using [PdCl2(COD)] (0.086 g,
0.300 mmol) in THF (10 mL) only resulted in the recovery of start-
ing materials.
Elemental analysis: Anal. Calc. C26H34Br2Cl2N2Pd THF: C, 45.97;
H, 5.40; N, 3.57. Found: C, 45.43; H, 5.58; N, 4.05%.
IR (KBr): m
C=N 1554 cmꢀ1; 1H NMR (CD2Cl2, 500 MHz, 298 K): m-
NPh 7.22 (d, 2H, 3JHH = 7.8) p-NPh 7.22 (t, 1H, 3JHH = 6.9), iPr 2.94 (q,
2
2
2H, JHH = 6.3) and 1.28 (dd, 12H, Me, JHH = 7.2); 13C{1H}NMR
(CD2Cl2, 125.7 MHz, 298 K) p-NPh 130.0 (s), o-NPh 124.1 (s), m-
NPh 120.2 (s), 30 CHMe2(s,), CHMe2 24 (broad); FAB+: 708.95
(14) [M]+.
Elemental analysis: Anal. Calc. for C14H10Cl4N2Pd: C , 37.00; N,
6.16; H, 2.22. Found: C, 37.01; N, 5.83; H, 2.71%.
IR (KBr):
m
C=N 1554 cmꢀ1. 1H NMR (d6-DMSO, 500 MHz, 298 K):
4.9. Synthesis of 9
3
3
m-NPh 7.54 (t, 2H, JHH = 7.3) p-NPh 7.35 (t, 1H, JHH = 7.3) o-NPh
7.15 (t, 2H, JHH = 7.3). 13C{1H}NMR (d6-DMSO, 125.7 MHz, 298 K)
A cold (ꢀ78) solution of nBuLi (0.155 g, 6 mmol) in Et2O (30 mL)
was added dropwise to a solution of 2 (2 g, 6 mmol) in 50 mL Et2O
(ꢀ78 °C). The reaction mixture was allowed to reach room temper-
ature with stirring giving a yellow suspension over 15 h. This was
cooled again to ꢀ78 °C and a cold (ꢀ78 °C) solution of fluorenyl
lithium in Et2O (1.033 g, 6 mmol) was added dropwise. The mix-
ture was allowed to reach room temperature under stirring, over
night. Removals of volatiles afforded a white-yellow solid. From
this, compound 9 was extracted in 30 mL toluene. Removal of sol-
vent under reduced pressure yielded 0.436 g of compound 9 as a
yellow solid (15% yield).
3
ipso-NPh 137.41 (s) p-NPh 128.64 (s) o-NPh 124.51 (s) m-NPh
120.24 (s).
4.6. Synthesis of [PdCl2{(2,6-Me2C6H3)N@C(Cl)}2] (6)
A dark orange solution of [PdCl2(PhCN)2] (0.230 g, 0.600 mmol)
in CH2Cl2 (10 mL) was treated with a yellow solution of (2,6-
Me2C6H3)N@C(Cl)–C(Cl)@N(2,6-Me2C6H3) (0.200 g, 0.600 mmol)
in CH2Cl2 (10 mL) at room temperature. The reaction was stirred
for 60 h, after which time the solvent was removed under reduced
pressure. The residue was washed with pentane (2 ꢂ 20 mL) and
recrystallised from CH2Cl2/pentane. [PdCl2({(2,6-Me2C6H3)N@
C(Cl)}2)], 10 was isolated as an orange solid. Yield: 0.165 g, 54%
based on [PdCl2(PhCN)2].
22
21
20
21'
20'
23
19
23'
Elemental analysis: Anal. Calc. for C18H18Cl4N2Pd: C, 42.34; N,
5.49; H, 3.55. Found: C, 42.37; N, 4.97; H, 3.89%.
18
N
16
14
IR (KBr) mC=N 1550 cmꢀ1
;
1H NMR (d6-DMSO, 500 MHz, 298 K)
12
17
13
3
3
H
m-NPh 7.20 (d, 4H, JHH = 7.8), p-NPh 7.10 (t, 2H, JHH = 7.8), Me
2.06 (s, 12H); 13C{1H}NMR (d6-DMSO, 125.7 MHz, 298 K) ipso-
NPh 144.03 (s), C–Cl 138.53 (s), p-NPh 125.41 (s), m-NPh 128.14
(s), o-NPh 124.76 (s).
15
6
7
11
8
10
N
13'
5'
2'
9
12'
11'
5
1
9'
2
3
3'
10'
4.7. Synthesis of [PdBr2{(2,6-Me2C6H3)N@C(Cl)}2] (7)
4
THF (100 mL) was added to [PdBr2] (2.752 g, 10.3 mmol) stirred
for 1 h at r.t. and cooled to ꢀ78 °C. To this, a cooled solution of li-
gand 2 (3.445 g, 10.3 mmol) in THF (100 mL, ꢀ78 °C)) was added
dropwise. Stirring continued for 18 h during which time the reac-
tion returned to room temperature. This was followed by filtration.
The filtrate was kept and the volatiles were removed to yield the
desired complex as a red-orange solid (yield 1.914 g, 31% yield).
Crystals suitable for analysis by X-ray diffraction were grown via
the slow diffusion (4 days) of pentane into a CH2Cl2 solution of 7.
IR (KBr): mC=N broad: 1642–1620 cmꢀ1 1H NMR (C6D6, 500 MHz,
;
298 K): H10–H13; H100–H130: 7.5–7.4 (b multiplet, 8H); H7 4.7
(b, 1H), H3, H30 7.01 (d, 2H), H4 6.90 (dd, 1H), H21, H210 6.741
(bd, 2H), H22 6.49 (dd, 1H); H15 1.22 (b, 2H), H16 1.15 (b, 2H)
H17 0.84 (b, 2H), H18 0.65 (b, 3H); H5, H50 2.02 (b, 6H), H23,
H2301.62 (b, 6H); 13C{1H}NMR (CD2Cl2, 125.7 MHz, 223 K) C14
170.4 (s), C1 and C19 148.2 and 148.5 (overlapped s), C2, C20, C20,
C200 125.2 (b), C3, C30, C20, C200 128.2 (b), C4 and C22 123.3 and

S.I. Pascu et al. / Inorganica Chimica Acta 363 (2010) 1157–1172
1171
2
298 K): Nap ring H1 8.08 (d, 1H, JHH = 9), H2 7.46 (dd, 1H,
123.4 (overlapped s), C7 37.2 (s), C15 30.1 (s), C23, C230 C5, C50 16.2
(b), C18 1.5 (s), C16, C17 2.0 (b), C13, C130 127.2 (b), C12, C120 129.2
(b), C11, C110 124.2 (b), C10, C100 123.2 (b); FAB+: 485.5 (13) [M]+.
2JHH = 6), H3 6.57 (d, 1H, JHH = 9), m-NPh 7.24 (d, 2H, JHH = 7.8)
2
3
p-NPh 7.22 (t, 1H, JHH = 6.9), 2,6-Me2 2.40 (s, 6H); FAB+:
3
575.038(40) [MꢀBr]+.
4.10. Synthesis of 10
4.12. Ethylene polymerisation
A cold (ꢀ78 °C) solution of fluorenyl lithium (2.06 g, 12 mmol)
in Et2O (30 mL) was added dropwise to a solution of 2 (2 g,
6 mmol) in 50 mL Et2O (ꢀ78 °C). The reaction mixture was allowed
to reach room temperature under vigorous stirring, giving a yellow
suspension over 15 h. Removals of volatiles afforded a yellow solid.
From this, compound 10 was extracted in 30 mL toluene. Removal
of solvent under reduced pressure yielded compound 10 as a yel-
low solid (2.31 g, 65% yield).
MAO (8.5 ꢂ 10ꢀ3 mol Al, in 500 mL toluene) was transferred un-
der Ar to a dried 1-L stainless steel reactor equipped with a mag-
netic stirrer under 2 bar ethylene pressure at 25 °C. The solution
was allowed to stir for 5 min (300 rpm) after which time a solution
of the Pd pre-catalyst (27.5 ꢂ 10ꢀ6 mol in 50 mL toluene) was
added to the reactor (Al:Pd molar ratio was 310:1). After stirring
for 5 min, the reactor contents were degassed and the ethylene
pressure controlled between 2 and 4 bar. The reaction mixture
was stirred vigorously for 900 min, after which the pressure was
released and the reaction quenched with an excess of HCl in MeOH
(ꢃ2 M HCl). For each of the pre-catalysts tested (for 7, 8, 11 and 12)
only formation of a small amount of PE was observed (less than
300 mg), which was filtered, washed with diluted HCl (to remove
Al), water and acetone and dried under reduced pressure. These
tests gave extremely low activities (lower than 0.0002 ꢂ 106 g/
mol Pd ꢂ h ꢂ bar). GC–MS chromatographs and spectra were re-
corded using a Hewlett Packard 5890 Gas Chromatograph fitted
with a non-polar column connected to a Trio-1000 Mass Spectrom-
eter operating Electron Impact (70 eV) and Chemical Ionisation (CI)
modes (NH3). This detected only traces of PE oligomers, as posi-
tively charged species.
4
3
12
2
13
5
11
10
1
H
7
8
6
N
9
H
N
Elemental analysis: Anal. Calc. C44H36N2: C, 89.15; H, 6.12; N, 4.72.
Found: C, 85.67; H, 5.79; N, 4.77%.
IR (KBr):
m
C=N 1642 cmꢀ1; 1H NMR (CD2Cl2, 500 MHz, 223 K): H3
Acknowledgements
7.05 (d, 2H, 3JH3–H4 = 7.05) H4 6.91 (b, 1H), H5 2.01 (s, 6H), H7 3.90
5
5
3
(m, 1H, JH7–H12 1.2) H10 7.80 (m, 2H, JH10–H13 0.7, JH10–H11 = 7.5)
SIP thanks the EPSRC Mass Spectrometry Service, Swansea, Roy-
al Society, St. Anne’s College Oxford and University of Bath for
funding, STFC for a research grant to support crystallographic
work, Drs. Matthew Jones, Stuart Dubberley and Prof. Pedro Gomes
for helpful discussions, Dr. Nick Rees for assistance with NMR
experiments. GB thanks the Humboldt Foundation and Oxford
Supercomputing Centre for funding.
4
3
3
H11 7.37 (m, 2H, JH11–H13 = 1.2 JH11–H12 = 7.4, JH10–H11 7.5) H12
7.30 (m, 2H, 3JH12–H13 = 7.4, 4JH10–H12 = 1.2, 3JH11–H12 = 7.4,
5JH7–H12 = 1.2) and H13 7.56 (m, JH12–H13 = 7.4, JH11–H13 = 1.2,
5JH10–H13 = 0.7); 13C{1H}NMR (CD2Cl2, 125.7 MHz, 223 K) C1
148.7, C2 125.1, C3 128.2, C4 123.2, C5 18.3, C7 37.2, C8 143.7,
C9 141.9, C10 120.1, C11 127.05, C12 127.0, C13 125.4; FAB+:
591.37 (8) [M]+.
3
4
Appendix A. Supplementary material
4.11. General method for the preparation of Brookhart-type diimines
11 and 12
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2009.11.009.
An adaptation of reported methods was used [7,29]. Th ligand
([2,6-Me2C6H3)N@C(Me)}2] for 11, or [{(2,6-Me2C6H3)N@C}2Nap]
for 12) (1.70 mmol) was dissolved in 10 mL of THF in a Schlenk
tube under N2 atmosphere. In each case, this solution was added
via a cannula to a suspension of PdBr2 (0.431 g, 1.62 mmol) in
THF (20 mL) and the resulting red-brown mixture was stirred for
3 days, followed by separation by filtration. The filtrate was col-
lected, and the volatiles removed under reduced pressure resulting
in orange solids 11 or 12. The products were washed with
3 ꢂ 10 mL pentane, dried under reduced pressure and character-
ised by elemental analysis, FAB+ mass spectrometry and 1H NMR.
Yields: 0.52 g of 11 (57.5%) and 0.60 g of 12 (56.6%).
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Anal. Calc. for. C25H24Br2N2Pd THF: C, 45.77; H, 4.96; N, 4.45.
Found: C, 45.90; H, 4.55; N, 4.80%; 1H NMR (CDCl3, 300 MHz,
3
298 K): m-NPh 7.24 (d, 2H, JHH = 7.8) p-NPh 7.22 (t, 1H,
3JHH = 6.9), C(Me), 2,6-Me2 2.33 (s, 6H), 1.50 (s, 3H); FAB+:
482.022(35) [MꢀBr]+.
4.11.2. Characterising data 12
Anal. Calc. for C25H24Br2N2Pd THF: C, 52.95; H, 4.30; N, 3.86.
Found: C, 52.43; H, 4.24; N, 3.44%; 1H NMR (CDCl3, 300 MHz,

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