Factors affecting imine coordination in (iminoterpyridine)MX2 (M = Fe, Co, Ni, Zn): Synthesis, structures, DFT calculations and ethylene oligomerisation studies

Article abstract of DOI:10.1039/b610562a

The bulky arylimino-terpyridine ligands, 6-{(2,6-i-Pr₂C ₆H₃)N=CR}-2,2′:6′,2″-C ₁₅H₁₀N₃ [R = H (L1), Me (L2)], have been prepared in high yield from the condensation reaction of the correspo

Full text of DOI:10.1039/b610562a

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Factors affecting imine coordination in (iminoterpyridine)MX2
(M = Fe, Co, Ni, Zn): synthesis, structures, DFT calculations
and ethylene oligomerisation studies
a
ab
Yohan D. M. Champouret, Jean-Didier Marechal,w Rajinder K. Chaggar,a
´
John Fawcett,a Kuldip Singh,a Feliu Maserasc and Gregory A. Solan*a
Received (in Montpellier, France) 25th July 2006, Accepted 22nd September 2006
First published as an Advance Article on the web 9th October 2006
DOI: 10.1039/b610562a
The bulky arylimino-terpyridine ligands, 6-{(2,6-i-Pr2C6H3)NQCR}-2,20:60,200-C15H10N3 [R = H
(L1), Me (L2)], have been prepared in high yield from the condensation reaction of the
corresponding carbonyl compound with one equivalent of 2,6-diisopropylaniline. Interaction of
an equimolar ratio of MX2 with aldimine-containing L1 in n-BuOH at 110 1C affords the
mononuclear five-coordinate complexes, [(L1)MX2] (M = Fe, X = Cl 1a; M = Co, X = Cl 1b;
M = Ni, X = Br 1c; M = Zn, X = Cl 1d), in which the metal centres occupy the terpyridine
cavities in L1 with the imino group pendant and exo to the adjacent pyridine nitrogen atom.
Similarly, a five-coordinate complex [(L2)ZnCl2] (2d) is accessible from the reaction of ketimine-
containing L2 with ZnCl2 with the non-coordinated imine group in 2d, in this case, adopting a
pseudo-exo configuration. In contrast, coordination of the imino-nitrogen atom (endo
configuration) results on reaction of (DME)NiBr2 (DME = 1,2-dimethoxyethane) with L2 to
furnish the six-coordinate complex trans-[(L2)NiBr2] (2c). With [(L2)MCl2] (M = Fe 2a; M = Co
2b) both solution and solid state IR spectroscopy suggest that the imine group prefers to adopt a
bound conformation. Quantum mechanical calculations (DFT) have been performed on
[(Lx)MX2] (Lx = L1, L2; M = Fe, Co, Ni, Zn; X = Cl, Br) and support the synthetic results
with the exo conformations energetically preferred for 1a–1d and 2d, while a distinct energetic
preference is observed for the endo conformation in 2c. For 2a and 2b, however, the closeness in
energies for endo and exo configurations suggests dynamic behaviour is likely. The iron species 1a
and 2a display low activities (on treatment with excess MAO) for ethylene oligomerisation at one
bar ethylene pressure affording highly linear a-olefins (498%); the cobalt (1b, 2b) and nickel
(1c, 2c) species are inactive. Single crystal X-ray diffraction studies have been performed on 1a,
1b, 1d, 2c and 2d.
Recently, we have been interested in the coordination
1. Introduction
chemistry and catalytic potential of divalent late transition
metal complexes containing the more extended oligopyridyl-
imine ligand, 6,600-bis(arylimino)-2,20:60,200-terpyridine (C, Fig.
1).8 By judicious choice of imino-carbon substituent (R) in C,
1 : 1 coordination compounds incorporating the metal centre
in either a tridentate terpyridyl (R = H) or a tetradentate
iminoterpyridyl (R = Me) cavity have been structurally
identified for Ni(II). By contrast, attempted isolation of a
1 : 1 complex featuring Zn(II) and the ketimine derivative of
C (R = Me) furnished only a dinuclear Zn2{bis(arylimino)-
terpyridine}-containing species. This variation in bonding
capacity has prompted us to target related ligand frames in
which bimetallic formation is unlikely.
In spite of the rich variety of transition metal complexes of
2,20:60,200:600,20 0 0 -quaterpyridine (A, Fig. 1) that have been
reported,1–5 the coordination chemistry of closely related
2,6-linked oligopyridylimine N,N,N,N-ligands such as 6,60-
bis(arylimino)-2,20-bipyridine (B, Fig. 1) is not nearly as well
developed. Nevertheless, the role of B in both supramolecular
chemistry6 and metal-mediated alkene polymerisation/oligo-
merisation catalysis7 has been documented. On the other hand
6-arylimino-2,20:6,200-terpyridine (Lx, Fig. 1), a hybrid of A
and B, has to the knowledge of the authors not been investi-
gated as a support for metal complexes.
a Department of Chemistry, University of Leicester, University Road,
Leicester, UK LE1 7RH. E-mail: gas8@leicester.ac.uk
b Department of Biochemistry, University of Leicester, University
Road, Leicester, UK LE1 7RH
Herein, the use of the shorter chain mono-imine Lx (Fig. 1)
is investigated as a support for a series of late transition
metal(II) halides. In particular, we employ a combined syn-
thetic and theoretical approach to study the factors that
influence the capacity of 6-{(2,6-i-Pr2C6H3)NQCR}-
2,20:60,200-C15H10N3 [L1 (R = Haldimine), L2 (R = Meketimine)]
to act as tri- or tetra-dentate (or both) ligands in complexes
c Institute of Chemical Research of Catalonia (ICIQ), Av. Paısos
¨
Catalans, 16, 43007 Tarragona, Spain
w Present address: Institute of Biochemistry and Biophysics Molecular
´
and Cellular Bat 430, Universite Paris Sud, XI 91405 Orsay Cedex,
France.
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Fig. 1 Quaterpyrine (A), bis(imino)bipyridine (B), terpyridylimine (Lx) and bis(imino)terpyridine (C); R = H or methyl, Ar = aryl group.
based on the 3d metals, iron, cobalt, nickel and zinc. In
addition, several of these systems are probed as precatalysts
for the oligomerisation of ethylene and their performances
related to the bonding modes possible for Lx.
Ni, X = Br 2c; M = Zn, X = Cl 2d) in high yield, respectively
(Scheme 2). All products have been characterised by FAB
mass spectrometry, IR spectroscopy and, in the cases of 1a–c
and 2a–c by magnetic measurements and diamagnetic 1d/2d
by 1H NMR spectroscopy (see Table 1 and Experimental
section). In addition, crystals of 1a, 1b, 1d, 2c and 2d have
been the subject of single crystal X-ray diffraction studies.
Crystals of 1 suitable for the X-ray determinations were
grown from acetonitrile (1a, 1b) or from a mixed chloro-
form–acetonitrile (1d) solution. The structures of 1a, 1b and
1d belong to an isomorphous series and will be discussed
together. A view of 1a is depicted in Fig. 2; selected bond
distances and angles for 1a, 1b and 1d are listed in Table 2.
Each structure consists of a five-coordinate metal centre
surrounded by two chloride ligands and three pyridyl groups
from L2 with the imino group non-coordinated. With the
geometric parameter (t) in 1a, 1b and 1d being close to 0.5 >
t = 0.39 (1a), 0.51 (1b), 0.46 (1d)],11 geometries based on
distorted trigonal bipyramidal or distorted square pyramidal
could both be applicable. Nevertheless, in 1a and 1d a slight
2. Results and discussion
2.1. Ligand synthesis
Ligands
6-{(2,6-i-Pr2C6H3)NQCR}-2,20:60,200-C15H10N3
[R = H (L1), Me (L2)] can be prepared by treating the
corresponding carbonyl compound, 6-{OQCR}-2,20:60,200-
C15H10N3 (R = H, Me), with 2,6-diisopropylaniline in abso-
lute ethanol in the presence of a catalytic amount of glacial
acetic acid. To achieve satisfactory yields, the synthesis of
aldimine-containing L1 required milder conditions and shorter
reaction times (50 1C, 12 hours) while for L2 higher tempera-
tures and longer reaction times (80 1C, 72 hours) were found to
be more suitable (Scheme 1). Alternatively, L2 can be prepared
by reacting 6-{OQCMe}-2,20:60,200-C15H10N3 in neat 2,6-dii-
sopropylaniline at 160 1C over a 30 minute reaction time.9 The
precursor carbonyl compounds are not commercially available
and are synthesised by using either a palladium(0)-mediated
cross-coupling of 6-bromo-2,20-bipyridine9 with 2-(n-Bu3Sn)-
6-{C(R)CH2CH2O}-C5H3N (R = H, Me) or by treating the
lithiated derivative of 6-bromo-2,20:60-200-terpyridine with
Me2NCRO (R = H, Me) using conditions established by
Tanaka et al. for the formyl derivative 6-{OQCH}-2,20:60,200-
C15H10N3 (Scheme 1).10
2.2 Synthesis of complexes
The reaction of L1 or L2 with one equivalent of MX2 [MX2 =
FeCl2, CoCl2, (DME)NiBr2, ZnCl2] in n-butanol at 110 1C
gave complexes [(L1)MX2] (M = Fe, X = Cl 1a; M = Co, X
= Cl 1b; M = Ni, X = Br 1c; M = Zn, X = Cl 1d) and
[(L2)MX2] (M = Fe, X = Cl 2a; M = Co, X = Cl 2b; M =
Scheme 2 Reagents and conditions: (i) MX2 [MX2 = FeCl2, CoCl2,
ZnCl2, (DME)NiBr2], n-butanol, 110 1C (Ar = 2,6-i-Pr2C6H3).
Scheme 1 Reagents and conditions: (i) Pd(PPh3)4 (6 mol%), toluene, 90 1C, 72 h; (ii) HCl (4 M), 60 1C, 12 h; (iii) n-BuLi, ꢁ78 1C, Me2NC(O)R
(R = H, Me), diethyl ether–hexane–tetrahydrofuran; (iv) 2,6-i-Pr2C6H3NH2, cat. H+, ethanol, 50–80 1C, 12–72 h.
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Table 1 Selected characterisation data for complexes 1 and 2
v(CQN)a/
Compound
Colour
cmꢁ1
meffb/mB
1H NMR spectrumc
FAB mass spectrum
e
e
e
1a
1b
1c
1d
Purple-blue
Blue-green
Orange
1637
1644
1639
1644
5.2
4.3
546 [M]+, 511 [M ꢁ Cl]+
514 [M ꢁ Cl]+, 479 [M ꢁ 2Cl]+
559 [M ꢁ Br]+, 478 [M ꢁ 2Br]+
519 [M ꢁ Cl]+, 484 [M ꢁ 2Cl]+
2.5
d
3
1.23 (d, 12H, JHH 7.0 Hz, CH(CH3)2), 3.12
Pale yellow
3
(sept, 2H, JHH 7.0 Hz, CH(CH3)2), 7.1–7.2
(m, 3H, ArH), 7.6 (m, 1H, PyH), 8.04 (dd, 1H,
3JHH 7.6 Hz, 4JHH 1.4 Hz, PyH), 8.16 (app. t,
3
3
2H, JHH 7.6 Hz, JHH 7.6 Hz, PyH), 8.2–8.4
(m, 4H, PyH), 8.74 (d, 1H, 3JHH 7.3 Hz, PyH),
9.02 (d, 1H, JHH 5.0 Hz, PyH), 9.53 (s, 1H,
3
HCN).
e
2a
2b
2c
2d
Dark purple
Blue
Orange
1594
1569
1575
1634
5.3
4.6
525 [M ꢁ Cl]+, 490 [M ꢁ 2Cl]+
528 [M ꢁ Cl]+, 493 [M ꢁ 2Cl]+
573 [M ꢁ Br]+, 492 [M ꢁ 2Br]+
533 [M ꢁ Cl]+, 498 [M ꢁ 2Cl]+
e
e
2.6
d
3
Pale yellow
1.07 (d, 12H, JHH 7.0 Hz, CH(CH3)2), 2.56
(s, 3H, CH3CN), 2.93 (sept, 2H, 3JHH 7.0 Hz,
CH(CH3)2), 7.4–7.5 (m, 3H, ArH), 7.6 (m,
1H, PyH), 7.8 (m, 1H, 1.4 Hz, PyH), 8.25
(ddd, 1H, 3JHH 7.9 Hz, 3JHH 7.9 Hz, 4JHH 1.5
3
Hz, PyH), 8.40 (d, 1H, JHH 7.9 Hz, PyH),
8.4–8.6 (m, 5H, PyH), 8.66 (d, 1H, JHH 7.9
Hz, PyH).
3
a
b
Recorded on a Perkin-Elmer Spectrum One FT-IR spectrometer on solid samples. Recorded on an Evans balance at room temperature.
c
d
Recorded in CDCl3 (1d) and CD3CN (2d) at ambient temperature. Sample is diamagnetic. Broad paramagnetically shifted peaks.
e
preference towards distorted square pyramidal (t o 0.5) is
observed, as is the case for other experimental and calculated
five-coordinate structures determined in this work (vide infra).
The pyridyl nitrogen–metal distances are unsymmetrical with
the central N(2)py–M(1) distance being the shortest [2.1081(19)
(1a), 2.0302(15) (1b), 2.0729(18) (1d) A] and the distance
involving the imine-substituted pyridyl [N(3)py] group the
longest [2.2214(19) (1a), 2.1937(16) (1b), 2.2574(18) (1d) A].
Between structures the nitrogen–cobalt distances in 1b are
shorter than the corresponding values in 1a and 1d, consistent
with the ionic radii for a high spin dipositive cobalt ion being
smaller than for iron and zinc. The pendant imino group in
each structure [C(16)–N(4) 1.263(3)–1.264(3) A] is exo and
almost coplanar to the adjacent coordinated pyridine groups
[tors: N(3)–C(15)–C(16)–N(4) 174.61 (1a), 174.01 (1b), 168.21
(1d)] with the N-aryl unit essentially orthogonal to these
planes.
The preference of the MX2 unit in 1 to occupy uniquely the
terpyridyl cavity within L1 has also been observed in the
related 1 : 1 bis(aldimino)terpyridyl [{(2,6-i-Pr2C6H3NQ
CH)2terpy}MX2] (M = Fe, Ni, Zn, X = halide) complexes.8
However when a sequence of only two pyridine units is present
within the oligopyridylimine chain, imine coordination can be
achieved. For example, in the bis(aldimino)bipyridyl series,
[{(2,6-i-Pr2C6H3NQCH)2bipy}MCl2] (M = Fe,7 Ni12), the
MX2 unit fills the tridentate dipyridylimine pocket leaving
the remaining aldimine unit unbound. Nevertheless, an exo
conformation for the pendant CHQNAr unit is a feature of all
the above structural types contrasting with the pseudo-endo
disposition observed by the non-coordinated pyridine nitrogen
Table 2 Selected bond distances (A) and angles (1) for 1a, 1b and 1d
1a (M = Fe)
1b (M = Co)
1d (M = Zn)
M(1)–N(1)
M(1)–N(2)
M(1)–N(3)
M(1)–Cl(1)
M(1)–Cl(2)
C(16)–N(4)
2.175(2)
2.1514(16)
2.0302(15)
2.1937(16)
2.2630(7)
2.2733(7)
1.263(2)
2.1935(19)
2.0729(18)
2.2574(18)
2.2429(6)
2.2548(7)
1.263(3)
2.1081(19)
2.2214(19)
2.2868(8)
2.3081(7)
1.264(3)
N(1)–M(1)–N(2)
N(1)–M(1)–N(3)
N(2)–M(1)–N(3)
Cl(1)–M(1)–Cl(2)
Cl(1)–M(1)–N(1)
Cl(1)–M(1)–N(2)
Cl(1)–M(1)–N(3)
Cl(2)–M(1)–N(1)
Cl(2)–M(1)–N(2)
Cl(2)–M(1)–N(3)
74.48(8)
148.97(8)
74.53(7)
114.93(3)
97.41(6)
125.48(6)
99.22(5)
94.55(6)
119.39(6)
101.84(5)
76.64(6)
153.29(6)
76.71(6)
117.76(3)
95.34(5)
122.84(5)
97.38(4)
93.95(5)
119.20(5)
100.68(5)
75.28(7)
150.47(7)
75.24(7)
117.37(3)
97.68(5)
119.87(5)
98.39(5)
95.48(5)
122.72(5)
98.88(5)
Fig. 2 Molecular structure of 1a including the atom numbering
scheme. All hydrogen atoms, apart from H16, have been omitted for
clarity.
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Fig. 4 Molecular structure of 2d including the atom numbering
scheme. All hydrogen atoms have been omitted for clarity.
Fig. 3; selected bond distances and angles are listed in Table 3.
The molecular structure comprises a single nickel atom bound
by both L2 and two terminal bromide ligands so as to form a
distorted octahedral geometry. The bromide ligands are dis-
posed mutually trans [Br(1)–Ni(1)–Br(2) 175.74(10)1] with all
the four nitrogen donor atoms of L2 filling the equatorial belt.
The pyridyl nitrogen–nickel distances are unequally disposed
with the internal pyridyl distances being the shortest
[Ni(1)–N(2) 1.953(12) A, Ni(1)–N(3) 1.972(10) A] and the
external pyridyl lengths the longest [Ni(1)–N(1) 2.064(12) A].
Of the four nitrogen donor moieties, the imino nitrogen forms
the longest distance to nickel [Ni(1)–N(4) 2.191(12) A] with its
N-aryl group located almost orthogonal to the terpyridylimine
plane. The tetradentate bonding mode exhibited by L2 in 2c is
in contrast to the tridentate mode adopted by L1 in 1 but
resembles that found in the 1 : 1 complexes trans-[(quaterpyr-
idine)Ni(L)2](NCA) (L = NCMe, NCA = PF6;4d L = OH2,
NCA = BF44i) and the ketimine-containing complex trans-
[{(2,6-i-Pr2C6H3NQCMe)2terpy}NiBr2].9
Crystals of 2d suitable for X-ray determination were grown
from a mixed chloroform–acetonitrile solution. A view of 2d is
shown in Fig. 4; selected bond distances and angles are listed
in Table 4. The structure of 2d resembles 1d with a five-
coordinate zinc centre surrounded by two chloride ligands
and three pyridyl groups from L2 with the CMeQNAr group
non-coordinated. The geometry at the metal centre is also
distorted square pyramidal [t = 0.35]11 with the N(2) atom
defining the apical site and N(1), N(3), Cl(1) and Cl(2) the
basal ones. However unlike 1d, the non-coordinated imine
Fig. 3 Molecular structure of 2c including the atom numbering
scheme. All hydrogen atoms have been omitted for clarity.
atoms (one per quaterpyridine) in octahedral [(quaterpyridi-
ne)2Fe](ClO4)2.3b
Complexes 1a–d all show molecular ion peaks and/or
fragmentation peaks corresponding to the loss of halide ions.
In their infrared spectra (in solution or in solid state) n(CQN)
bands are seen at ca. 1640 cmꢁ1 corresponding to a non-
coordinated imine group and in a similar region to that for the
free ligand L1.8 Complexes 1a–c are paramagnetic and exhibit
magnetic moments of 5.2 mB, 4.3 mB and 2.5 mB (Evans balance
at ambient temperature), values that are typical of high spin
configurations corresponding to four, three, and two unpaired
1
electrons, respectively. In the H NMR spectrum of diamag-
netic 1d, signals characteristic for the aryl-H and pyridyl-H
protons are seen along with a singlet at d 9.53 for the aldimine
CHQN proton which is shifted by ca. d 1.2 downfield in
comparison with the corresponding signal in free L1. The
presence of only one doublet for the CHMe2 protons suggests
that free rotation around the aryl–nitrogen bond is opera-
tional further supporting the non-coordination of the imine.
Crystals of 2c suitable for X-ray determination were grown
by prolonged standing in chloroform. A view of 2c is shown in
group adopts
a
pseudo-exo
configuration
[tors.:
N(3)–C(15)–C(16)–N(4) 146.31] with respect to the adjacent
coordinated pyridine group. The deviation from planarity is
likely due to steric hindrance that can occur between the
Table 3 Selected bond distances (A) and angles (1) for 2c
Ni(1)–N(1)
Ni(1)–N(2)
Ni(1)–N(3)
Ni(1)–N(4)
2.064(12)
1.953(12)
1.972(10)
2.191(12)
Ni(1)–Br(1)
Ni(1)–Br(2)
C(16)–N(4)
C(16)–C(17)
2.579(2)
2.514(2)
1.313(16)
1.504(19)
Table 4 Selected bond distances (A) and angles (1) for 2d
Zn(1)–N(1)
Zn(1)–N(2)
Zn(1)–N(3)
Zn(1)–Cl(1)
2.193(2)
2.069(2)
2.308(2)
2.2654(9)
Zn(1)–Cl(2)
C(16)–N(4)
C(16)–C(17)
2.2322(9)
1.281(4)
1.465(4)
N(1)–Ni(1)–N(2)
N(1)–Ni(1)–N(3)
N(1)–Ni(1)–N(4)
N(2)–Ni(1)–N(3)
N(2)–Ni(1)–N(4)
Br(1)–Ni(1)–Br(2)
N(1)–Ni(1)–Br(1)
N(1)–Ni(1)–Br(2)
79.9(4)
N(2)–Ni(1)–Br(1)
N(2)–Ni(1)–Br(2)
N(3)–Ni(1)–N(4)
N(3)–Ni(1)–Br(1)
N(3)–Ni(1)–Br(2)
N(4)–Ni(1)–Br(1)
N(4)–Ni(1)–Br(2)
88.5(3)
88.8(3)
77.3(4)
86.5(3)
96.2(3)
95.4(3)
88.4(3)
157.0(5)
154.1(4)
77.4(5)
154.1(4)
175.74(10)
89.3(3)
N(1)–Zn(1)–N(2)
N(1)–Zn(1)–N(3)
N(2)–Zn(1)–N(3)
Cl(1)–Zn(1)–Cl(2)
N(1)–Zn(1)–Cl(1)
75.18(9)
150.70(9)
75.69(9)
129.60(3)
92.49(7)
N(1)–Zn(1)–Cl(2)
N(2)–Zn(1)–Cl(1)
N(2)–Zn(1)–Cl(2)
N(3)–Zn(1)–Cl(1)
N(3)–Zn(1)–Cl(2)
95.59(7)
110.85(7)
119.32(7)
100.75(6)
95.82(6)
87.0(3)
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imino-methyl and the chloride ligands. The effect of this steric
interaction appears also to impact on the N(3)–Zn(1) distance
with a noticeably longer bond in 2d when compared with 1d
[2.308(2) (2d) vs. 2.2574(18) (1d) A].
and 2b, however, proved problematic due to the poor solubi-
lity of the complexes and extreme broadness of the signals.
Nevertheless, the pyridyl-Ha protons could be identified as
peaks at d 103.4 and d 95.2 for 2a and 2b, respectively.
In the IR spectra for 2a–d (recorded in the solid state),
n(CQN) bands are seen between 1634–1569 cmꢁ1 with the
band for 2d (1634 cmꢁ1) falling at the higher end of the range
and consistent with a non-coordinated imine group. In con-
trast, 2c shows no CQN stretch above 1575 cmꢁ1 while in 2a
and 2b only very weak bands at ca. 1630 cmꢁ1 are apparent
suggesting that the bound imine is the preferred configuration.
The FAB mass spectra of 2a–2d each show fragmentation
peaks corresponding to the loss of one and two halide ions.
The high spin nature of 2a–c is confirmed from magnetic
measurements which indicate moments of 5.3 mB, 4.6 mB and
2.6 mB (Evans balance at ambient temperature), their magni-
tude being consistent with the presence of four, three, and two
unpaired electrons, respectively.
2.3 Density functional theory calculations
Given our recent findings on the flexibility of coordination
behaviour exhibited by the more extended oligopyridylimine C
(see Fig. 1),8 the capacity of Lx to adopt both tri- (with imino
group exo) or tetra-dentate (with imino group endo) bonding
modes on coordination to an MX2 unit was not entirely
surprising. In this previous study the nature of the imino-
carbon substituent (Me vs. H) was found to be highly influen-
tial on the bonding mode displayed by the ligand while the role
of the 3d metal centre (or possibly the halide employed) was
not clear. To complement the synthetic work undertaken
herein and to compare with our previous studies, DFT calcu-
lations have been performed on [(Lx)MX2] (M = Fe, Co, Ni,
Zn; X = Cl, Br) for both L1 and L2 with the intent of
understanding more fully the factors that influence the binding
mode of Lx.
1
In the H NMR spectrum of diamagnetic 2d, sharp signals
are seen for the aryl-H and pyridyl-H protons along with a
singlet at d 2.56 corresponding to the CMeQN group; shifted
by ca. 0.3 ppm downfield in comparison with free L2. The
isopropyl methyl protons are seen as a single doublet at d 1.07
suggesting that as with 1d the N-aryl group in 2d can freely
rotate, indicating that the imine is not bound. By contrast the
1H NMR spectrum of 2c shows broad paramagnetically
shifted peaks between d ꢁ4.1 and 130.9 (see Experimental
section). Some degree of assignment can be made on compar-
ison of the chemical shifts and relaxation times with previously
reported nickel(II)–pyridyl systems.13 For example, the single
pyridyl-Ha proton can be identified as the most downfield
In order to evaluate the validity of our DFT approach,
geometry optimisations have initially been performed on
selected complexes that have been the subject of X-ray deter-
minations in this work. Specifically, optimised structures of
trans-[(L2)NiBr2] (opt-2cendo), [(L1)ZnCl2] (opt-1dexo) and
[(L2)ZnCl2] (opt-2dexo) were compared with the X-ray data
for 2c, 1d and 2d, respectively. The calculated structures are in
good agreement with their respective crystallographic counter-
parts with their overall structural features well reproduced:
octahedral for opt-2cendo, distorted square pyramidal for opt-
1dexo and distorted square pyramidal for opt-2dexo (Table 5).
With regard to opt-2dexo, the structural arrangement of the
aryl and imine moieties is maintained when compared to 2d,
though more displaced towards idealised square pyramidal
0
signal (d 130.9), the six pyridyl-Hb and pyridyl-Hb protons to
the signals at d 92.9, 82.5, 72.3, 66.8, 59.5, 35.0 while the
pyridyl-Hg protons are assigned to the more upfield signals at
d 18.0, 16.9, 13.3. A related assignment of the signals for 2a
Table 5 Selected calculated bond distances (A) and angles (1) and other structural parameters for [(L1)MX2] (opt-1) and [(L2)MX2] (opt-2) in
both exo- and endo-conformations
M = Fe, X = Cl
M = Co, X = Cl
M = Ni, X = Br
M = Zn, X = Cl
M = Ni, X = Cl
1aexo 2aexo 1aendo 2aendo 1bexo 2bexo 1bendo 2bendo 1cexo 2cexo 1cendo 2cendo 1dexo 2dexo 1dendo 2dendo 1c0exo 2ce0 xo 1c0endo 2ce0 ndo
Bond lengths
M(1)–N(1)
M(1)–N(2)
M(1)–N(3)
M(1)–N(4)
M(1)–X(1)
M(1)–X(2)
C(16)–N(4)
2.25 2.25 2.23 2.22 2.19 2.2
2.13 2.12 2.19 2.2 2.1 2.07 2.14 2.12 2.02 1.99 2.04 2.03 2.19 2.16 2.23 2.22 2.02 1.99 2.04 2.04
2.28 2.35 2.18 2.18 2.2 2.26 2.12 2.1 2.16 2.17 2.03 2.02 2.29 2.41 2.23 2.21 2.15 2.16 2.04 2.03
4.66 4.95 2.29 2.32 4.63 4.89 2.24 2.26 4.65 4.87 2.29 2.25 4.65 4.99 2.55 2.47 4.63 4.85 2.29 2.24
2.32 2.34 2.46 2.47 2.33 2.35 2.48 2.46 2.48 2.51 2.6 2.63 2.32 2.31 2.38 2.42 2.34 2.38 2.45 2.48
2.34 2.36 2.54 2.51 2.33 2.36 2.53 2.51 2.47 2.5 2.64 2.65 2.32 2.33 2.4 2.4 2.35 2.38 2.48 2.48
1.29 1.29 1.29 1.29 1.28 1.29 1.29 1.29 1.29 1.29 1.29 1.29 1.28 1.29 1.29 1.29 1.28 1.29 1.29 1.29
2.15 2.17 2.13 2.13 2.12 2.11 2.25 2.27 2.32 2.31 2.13 2.13 2.13 2.13
Bond angles
N(1)–M(1)–N(2) 75.3 75.5 74.4 73.8 75.5 76.0 75.8 76.3 78.6 79.2 78.5 78.8 73.4 73.5 72.0 72.5 78.6 79.2 78.5 78.6
N(1)–M(1)–N(3) 150.6 151.6 146.8 146.7 152.7 153.4 146.8 152.3 157.3 159.7 156.4 157.4 146.9 146.8 144.6 146.0 157.3 159.8 156.4 157.0
N(2)–M(1)–N(3) 75.4 76.6 72.4 73.3 77.2 78.4 73.9 76.1 78.7 80.5 78.0 78.6 73.7 74.5 72.6 73.5 78.7 80.6 77.9 78.4
X(1)–M(1)–X(2) 135.9 146.2 173.5 171.1 129.0 143.5 174.8 171.4 147.0 161.1 174.8 168.8 127.6 133.9 162.6 168.6 145.3 161.0 175.6 171.3
X(1)–M(1)–N(1) 94.6 91.9 85.4 85.2 91.5 89.2 86.1 89.7 91.7 89.4 88.9 87.8 96.0 94.1 85.2 85.2 92.1 89.4 87.9 89.0
X(1)–M(1)–N(2) 116.2 114.6 95.0 89.8 119.7 115.2 92.3 86.8 110.7 100.4 84.9 89.7 120.0 123.8 101.5 101.5 113.3 99.7 90.3 85.6
X(1)–M(1)–N(3) 97.5 95.1 98.1 99.4 100.4 97.5 95.6 90.9 96.0 94.1 89.4 92.6 97.9 96.3 102.2 102.2 96.2 93.9 92.3 89.2
X(2)–M(1)–N(1) 94.1 90.5 88.7 86.4 91.1 87.7 89.1 86.8 92.8 89.3 88.8 89.9 95.7 91.5 88.7 88.7 93.0 89.5 89.9 88.9
X(2)–M(1)–N(2) 107.8 98.6 86.0 85.0 110.2 99.3 84.7 84.8 102.2 97.8 83.8 85.2 112.3 101.5 92.0 92.0 101.3 98.7 85.5 85.5
X(2)–M(1)–N(3) 95.6 98.56 88.2 86.0 100.0 101.1 87.5 88.5 92.2 93.7 88.5 87.6 99.2 103.2 92.3 92.3 92.2 93.7 88.1 89.2
ta
0.25 0.09 —
—
0.40 0.17 —
—
0.17 0.03 —
—
0.32 0.22 —
—
0.20 0.02 —
—
a
t = structural index parameter = (b ꢁ a)/60 where b = the largest L–M–L angles and a = second largest (see ref. 11).
ꢀc
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[t = 0.35 (2d) vs. 0.22 (opt-2dexo)]. This discrepancy is likely
due to the gas phase conditions employed for the calculations
which are exempt of crystal packing interactions that are
evident in 2d. While agreement between experimental and
theoretical angles for opt-1dexo, opt-2cendo and opt-2dexo are
good with discrepancies less than 61, bond lengths are slightly
more divergent with average differences around 0.1 A (Table
5). Nevertheless, the expected variations in the metal ligand
distances (e.g., M–N distances: Fehigh spin E Zn 4 Cohigh spin
4 Ni) between the different 3d metal complexes employed for
all experimentally characterised structures are particularly well
reproduced in these theoretical calculations. Due to the size of
the system, it was viewed that our approach leads to good
agreement with the experimental data and demonstrates that
using the B3LYP functional with this basis set constitutes a
viable method for studying systems described in this work. To
extend the investigation, we have applied this theoretical
approach to the two other X-ray characterised complexes 1a
and 1b along with 1c, 2a, 2b and the chloride analogues of 1c
and 2c, [(Lx)NiCl2] [Lx = L1 (1c0), L2 (2c0)].
Table 6 Potential (DE) energies (kcal molꢁ1) between endo and exo
conformations for opt-1 and opt-2 using DE = Eendo ꢁ Eexo
DE
DE
opt-1a
opt-1b
opt-1c
opt-1d
opt-1c0
6.7
9.7
2.1
7.5
1.6
opt-2a
opt-2b
opt-2c
opt-2d
opt-2c0
0.2
ꢁ0.5
ꢁ2.5
3.3
ꢁ3.1
tural types: (i) with the imine co-planar to the adjacent
pyridine and the metal centre adopting a distorted square
pyramidal geometry (range for t: 0.17–0.40) (Fig. 5b), (ii) with
the imine unit out-of-the plane formed by the adjacent pyr-
idine and the metal centre exhibiting a distorted square based
pyramidal geometry (range for t: 0.09–0.22) (Fig. 5c), (iii) with
the imine co-planar to the adjacent pyridine, the halides
disposed in a linear fashion and the metal centre displaying
a more idealised square pyramidal geometry (t = 0.02–0.03)
(Fig. 5d). The particular type of exo conformation exhibited
by a complex (Fig. 5b–d) is found to depend not only on Lx
but also on the metal centre. The nature of the halide (Cl vs.
Br) ligand appears not to be influential in these systems. For
all the L1-containing complexes (opt-1exo), the structures
adopted are based on the conformation shown in Fig. 5b with
the two nickel versions displaying the more idealised square
pyramidal geometries [t = 0.17 (opt-1cexo), 0.20 (opt-1c0exo)].
On the other hand, complexes bound by the ketimine ligand
L2 can adopt two of the exo structures, the precise type being
determined by the metal centre with the iron, cobalt and zinc
Optimised structures for 1a, 1b, 1c, 1d, 1c0, 2a, 2b, 2c, 2d and
2c0 have been determined based on the imine nitrogen atom
being bound (opt-1aendo, opt-1bendo, opt-1cendo, opt-1dendo, opt-
1c0endo, opt-2aendo, opt-2bendo, opt-2cendo, opt-2dendo, opt-2c0endo
)
and unbound (opt-1aexo, opt-1bexo, opt-1cexo, opt-1dexo, opt-
1c0exo, opt-2aexo, opt-2bexo, opt-2cexo, opt-2dexo, opt-2c0exo);
selected bond lengths and angles are listed in Table 5. By
inspection of the computed structures it is evident that the
endo conformations adopt a common octahedral structure,
trans-[(Lx)MX2], regardless of Lx, metal or halide (Fig. 5a). In
contrast, the exo conformations fall into three distinct struc-
complexes (opt-2aexo
, opt-2bexo, opt-2dexo) preferring the
Fig. 5 Common optimised structural types for 1 and 2: (a) opt-2endo (also applicable to opt-1endo) (FeCl2, CoCl2, NiBr2, NiCl2, ZnCl2); (b) opt-1exo
(FeCl2, CoCl2, NiBr2, NiCl2, ZnCl2); (c) opt-2exo (FeCl2, CoCl2, ZnCl2); (d) opt-2exo (NiBr2, NiCl2).
ꢀc
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structure in Fig. 5c while the nickel systems (opt-2cexo, opt-
2c0exo) prefer that in Fig. 5d.
sideration of the dn configuration is useful. For the complexes
displaying five-coordinate geometries only in the case of the
nickel(II) species are they adequately described as square
pyramidal, while the zinc(II) species along with the high spin
iron(II) and cobalt(II) systems as described as distorted square
pyramidal.15 This deviation of nickel (d8) can be rationalised
by consideration of the energy gain that would occur when the
three non-bonding d-orbitals are fully occupied in the square
pyramidal case (opt-1cexo, opt-1c0exo, opt-2cexo, opt-2c0exo). For
Fe (d6), Co (d7) and Zn (d10) the nature of the dn configura-
tions has no significant impact on the resting distorted square
pyramidal geometry. Moreover, it appears that the stability of
a high spin d8-square pyramidal geometry relative to the high
spin d8-octahedral geometry is delicately balanced with the
presence of the ketimine moiety in 2c being sufficient to drive
the endo formation. A similar but less dramatic effect is also
apparent with the iron and cobalt derivatives. In the case of
the zinc complexes, endo configurations do not form which is
likely due to electron–electron repulsions between the fully
occupied d-shell and the incoming imine-nitrogen donor.
To investigate the relative stability of endo versus exo
structural types for a particular Lx–metal ion combination
and to probe the potential for conformation conversions, the
difference in energy between endo and exo structures for
[(L1)MX2] (1) and [(L2)MX2] (2) was determined (Table 6).
The agreement between computed energy differences and
the available experimental data is satisfactory. For species opt-
1a, opt-1b, opt-1c, opt-1d and opt-2d, the experimentally ob-
served exo-form is favoured by a value between 2.1 and
9.7 kcal molꢁ1. For opt-2c, the experimentally observed endo
form is computed to be more stable by 2.5 kcal molꢁ1
.
Interestingly, for opt-2a and opt-2b, where experimental data
supports the presence of the endo form, a small energy
difference of 0.5 kcal molꢁ1 at most between the computed
structures would suggest the coexistence between two species
in equilibrium. Furthermore the fact that the calculations
allow the least stable isomers to be computed, additional
useful information can be gleaned. As highlighted above the
calculations show that the behaviour depends little on the
nature of the halide (Cl or Br). With regard to energy
differences two major trends are apparent. The first trend is
that the L1-containing systems (opt-1) always favour the exo
form with respect to the corresponding L2-containing systems
(opt-2), with the difference in preference oscillating between 4
and 10 kcal molꢁ1. The second trend is that the nickel
complexes (opt-1c, opt-1c0, opt-2c, opt-2c0) have a larger pre-
ference towards the endo form than the rest of the species,
which can be quantified at Z 5 kcal molꢁ1. It is also worth
mentioning that both trends are additive. For example, opt-2c
and opt-2c0 are the only cases where the endo form is clearly
favoured because of the joint presence of the ketimine L2
ligand frame and the nickel metal centre.
2.4 Screening of complexes as catalysts for ethylene
oligomerisation/polymerisation
Late transition metal complexes of the first row (e.g., Fe, Co,
Ni) have, in recent years, been widely employed as catalysts for
alkene polymerisation and/or oligomerisation.16 In particular,
complexes containing multidentate nitrogen donor ligands
incorporating bulky aryliminepyridine moieties have been
central to many of the key developments, one such example
being the [{(2,6-isopropylphenylimino)bipyridine)}FeCl2]/
MAO system reported by Gibson et al. that has been shown
to be highly active for alkene oligomerisation.7 With the intent
of probing the effect of an additional pyridine group within the
ligand frame in the latter system, we have screened the two
iron complexes (1a and 2a), along with the cobalt (1b, 2b) and
nickel (1d, 2d) species, as precatalysts for the oligomerisation/
polymerisation of ethylene.
These trends identified in the calculations can be qualita-
tively rationalised. With regard to the role played by the
ligands themselves, both steric and electronic effects are likely
operational. The reduced donor capability of an aldimine (L1)
nitrogen over a ketimine (L2) nitrogen supports the preference
of L1 to adopt the exo form while the improved donor
capacity of L2 allows endo configurations to be accessed.14
In terms of steric variations, the smaller steric hindrance in L1
imparted by the H imino-carbon substituent when occupying
the position closer to the metal means exo configurations are
preferential, while for L2 the bulkier methyl substituent is less
likely to adopt the exo configuration with the result that the
endo arrangement will prevail. With respect to the metal centre
and the resultant coordination geometries displayed, a con-
On activation with 400 equivalents of methylaluminoxane
(MAO) only the iron complexes 1a and 2a showed any activity
for ethylene oligomerisation; the cobalt and nickel systems
were inactive. The results for 1a and 2a employing one atmo-
sphere of ethylene are summarised in Table 7.
Several points emerge from inspection of the data. Both
complexes show only low activity for ethylene oligomerisation
with the ketimine species (2a) showing slightly higher activity
when compared with the aldimine species (1a). The oligomeric
products are typical of iron-based catalysts giving greater than
Table 7 Ethylene oligomerisation with the iron(II) catalystsa
Entry Pre-catalyst Massb/g Activity/g mmolꢁ1 hꢁ1 barꢁ1 Internal olefinc (%) External olefinc (%) Av. chain length,c Cn ad
1
2
1a
2a
0.040
0.100
4
10
1
2
99
98
16.4
16.2
0.84
0.72
a
General conditions: 1 bar ethylene Schlenk test carried out in toluene (40 cm3) at ambient temperature using 4.0 mmol MAO (Al : M = 400 : 1),
b
0.01 mmol precatalyst, over one hour. Reactions were terminated by addition of dilute HCl. Mass of oligomer isolated. Oligomerisation
product percentages and average chain length, Cn, calculated via integration of 1H NMR spectra. Determined by GC; a = (rate of propagation)/
((rate of propagation) + (rate of chain transfer)) = (moles of Cn+2)/(moles of Cn).
c
d
ꢀc
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98% linear a-olefins (C6–C26). Schulz–Flory distribution of
oligomers are observed in both cases with values ca. 0.76.17
The explanation for the low activity of these iron systems
(1a/MAO or 2a/MAO) when compared to the highly active
previously reported procedures. All other chemicals were
obtained commercially and used without further purification.
3.2 Synthesis of 6-{(2,6-i-Pr2C6H3)NQCMe}-2,20:60,200-
C15H10N3 (L2)
[{(2,6-diisopropylphenylimino)bipyridine)}FeCl2]/MAO7
is
uncertain but could be attributable to two reasons or a
combination of both. Firstly, the imino arm 1a/2a could be
coordinated in the putative cationic active catalyst and thus
block the approach of an incoming ethylene monomer. In-
deed, this explanation has been suggested as one possible
reason for the inactivity of [{bis(2,6-diisopropylphenylimino)-
bipyridine}FeCl2]/MAO towards ethylene.7 Alternatively if
the imino group does not coordinate, the low activity could
be due to the ligand support acting more like a substituted
terpyridine ligand in a manner similar to that seen in the
sterically encumbered [(6,600-diarylterpyridine)FeCl2]/MAO
systems in which only very low activities are reported; the
steric bulk of the 6-position in this case inhibiting monomer
approach.18 Nevertheless, it is likely that the nature of the
imino carbon substituent plays a key role in dictating which
pathway results in a manner akin to that seen for the pre-
catalytic species 1 and 2.
Prepared via a two-step procedure:
(a) 6-Acetyl-2,20:6,200-terpyridine. An oven-dried Schlenk
flask equipped with a magnetic stir bar was evacuated and
backfilled with nitrogen. The flask was charged with 6-bromo-
2,20:60,200-terpyridine (0.714 g, 2.29 mmol) and a mixture of
diethyl ether (22 ml), hexane (11 ml) and tetrahydrofuran (11
ml). The solution was cooled to ꢁ78 1C and n-butyllithium
(1.5 ml, 2.40 mmol, 1.6 M in hexane) added dropwise over
10 min resulting in a dark turquoise coloured solution. After
15 min of stirring, N,N-dimethylacetamide (0.43 ml, 4.58
mmol) was added and the reaction mixture stirred for a further
15 min at ꢁ78 1C. The solution was allowed to warm to room
temperature and quenched with water and then left to stir
overnight. The resulting deep red solution was extracted with
ethyl acetate (3 ꢂ 25 ml) and the organic layer washed with
saturated sodium chloride solution (3 ꢂ 25 ml) before being
dried over magnesium sulfate. Following filtration the solvent
was removed under reduced pressure to give a dark brown oil
that solidified on prolonged standing giving a pale yellow
solid. The product was crystallised from ethanol at ꢁ30 1C
and collected by filtration to afford 6-acetyl-2,20:6,200-terpyr-
idine as a pale yellow solid. Yield 80% (0.504 g, 1.83 mmol);
mp: 185–187 1C. 1H NMR (250 MHz, CDCl3): d 8.76 (dd,
3
Experimental
3.1 General
All reactions, unless otherwise stated, were carried out under
an atmosphere of dry, oxygen-free nitrogen, using standard
Schlenk techniques or in a nitrogen purged glove box. Solvents
were distilled under nitrogen from appropriate drying agents
and degassed prior to use.19 The infrared spectra were re-
corded on a Perkin-Elmer Spectrum One FT-IR spectrometer
on solid samples. The ES (electrospray) and the FAB mass
spectra were recorded using a micromass Quattra LC mass
spectrometer and a Kratos Concept spectrometer with metha-
nol or NBA as the matrix, respectively. 1H and 13C NMR
spectra were recorded on a Bruker ARX spectrometer (250 or
300 MHz) at ambient temperature; chemical shifts (ppm) are
referred to the residual protic solvent peaks and coupling
constants measured in Hertz (Hz). Oligomer products were
analysed by GC, using a Perkin-Elmer Autosystem XL chro-
matograph equipped with a flame ionisation detector and 30 m
PE-5 column (0.25 mm thickness), injector temperature 45 1C
and the following temperature programme: 45 1C/7 min,
45–195 1C/10 1C minꢁ1, 195 1C/5 min, 195–225 1C/10 1C
minꢁ1, 225 1C/5 min, 225–250 1C/10 1C minꢁ1, 250 1C/22 min.
Magnetic susceptibility studies were performed using an Evans
balance (Johnson Matthey) at room temperature. The mag-
netic moment was calculated following standard methods20
and corrections for underlying diamagnetism were applied to
the data.21 Elemental analyses were performed at the Science
Technical Support Unit, London Metropolitan University.
The reagents, 2,6-diisopropylaniline, MAO (10 wt% in
toluene), metal dichlorides and (DME)NiBr2 (DME = 1,2-
dimethoxyethane) were purchased from Aldrich Chemical Co.
and used without further purification. The compounds,
6-{(2,6-i-Pr2C6H3)NQCH}2ꢁ2,20:60,200-C15H10N3 (L1)9 and
6-bromo-2,20:60,200-terpyridine22 were prepared according to
4
3
3JHꢁH 7.7, JHꢁH 2.5, 1H, Py-H), 8.64 (d, JHꢁH 7.5, Py-H),
8.56 (d, 3JHꢁH 7.4, Py-H), 8.51 (dd, 3JHꢁH 7.5, 4JHꢁH 2,6, 1H,
3
4
Py-H), 8.38 (dd, JHꢁH 8.0, JHꢁH 2.8, 1H, Py-H), 7.98 (m,
1H, Py-H), 7.87 (m, 1H, Py-H), 7.78 (m, 1H, Py-H), 7.27 (m,
1H, Py-H), 2.79 (s, 3H, CH3CQO). 13C {1H} NMR (62.5
MHz, CDCl3): d 24.8 (CH3CQO), 120.0 (Py), 120.1 (Py),
120.4 (Py), 120.5 (Py), 122.8 (Py), 122.9 (Py), 135.9 (Py), 136.7
(Py), 137.0 (Py), 148.2 (Py), 151.9 (Py), 153.5 (Py), 154.4 (Py),
154.5 (Py), 155.9 (Py), 199.4 (CQO). IR (cmꢁ1) 1700 (C = O),
1564, 1426, 1078, 790, 748, 668. ESIMS: m/z 276 [M + H]+.
HRMS (FAB): Calcd for C17H14N3O [M + H]+ 276.113 69,
found 276.113 64.
(b) L2. 6-Acetyl-2,20:6,200-terpyridine (0.100 g, 3.64 mmol)
was dissolved in the minimum quantity of absolute ethanol (10
ml) and 2,6-diisopropylaniline (0.69 ml, 3.64 mmol) along with
one drop of acetic acid was introduced. The solution was
heated to reflux and stirred for 72 h. On cooling to room
temperature the precipitate was filtered off and washed with
ethanol to give L2 as a pale yellow solid. Yield 60% (0.948 g,
2.18 mmol); mp: 242–245 1C. 1H NMR (300 MHz, CDCl3): d
3
1.10 (d, JHꢁH 6.7, 12H, CH(Me)2), 2.28 (s, 3H, CH3CQN),
2.81 (sept, 2H, CH(Me)2), 7.0–7.2 (m, 3H, Ar–H), 7.2–7.3 (m,
1H, Py-H), 7.7–7.9 (m, 1H, Py-H), 7.91 (dd, 3JHꢁH 7.9, 3JHꢁH
7.9, 2H, Py-H), 8.35 (d, 3JHꢁH 7.9, 1H, Py-H), 8.42 (d, 3JHꢁH
7.9, 1H, Py-H), 8.53 (d, 3JHꢁH 7.9, 1H, Py-H), 8.59 (d, 3JHꢁH
7.9, 1H, Py-H), 8.67 (m, 2H, Py-H). 13C {1H} NMR (75 MHz,
CDCl3): d 16.3 (CH3CQN), 21.9 (CH3), 22.2 (CH3), 27.3
(CH), 120.1 (Py), 120.2 (Py), 121.0 (Py), 122.5 (Ar), 122.8
(Ar), 134.8 (Py), 136.3 (Py), 136.8 (Py), 142.0 (Ar), 148.1 (Py),
ꢀc
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154.4 (Py), 154.5 (Py), 155.1 (Py), 155.9 (Py), 166.1 (CQN). IR
(cmꢁ1) 2952, 1630 (CQN), 1562, 1422, 1360, 1263, 1075, 990,
777, 744. ESIMS: m/z 435 [M + H]+. HRMS (FAB): Calcd
for C29H31N4 [M + H]+ 435.254 90, found 435.254 87.
and hexane added to induce precipitation of the product. The
solid was filtered off, washed with hexane and dried over-
night under reduced pressure to afford [{6-{(2,6-i-
Pr2C6H3)NQCMe}-2,20:60,200-C15H9N3}FeCl2] (2a) as
a
dark-purple solid. Yield: 72% (0.096 g, 0.171 mmol). 1H
3.3 Synthesis of [{6-iminoformyl-2,20:60,200-terpyridine-
(2,6-diisopropylanil)}MX2] (1)
NMR (CDCl3, 293 K): d 103.4 (o1.6 ms, Ha).
(b) 2b, M = Co, X = Cl. Using an analogous procedure to
that described for 2a employing anhydrous CoCl2 (0.031 g,
0.238 mmol) and L2 (0.103 g, 0.238 mmol, 1 eq.) gave [{6-
{(2,6-i-Pr2C6H3)NQCMe}-2,20:60,200-C15H9N3}CoCl2] (2b) as
a blue solid. Yield: 70% (0.094 g, 0.166 mmol). 1H NMR
(CDCl3, 293 K): d 95.2 (br, Ha).
(a) 1a, M = Fe, X = Cl. An oven-dried Schlenk flask
equipped with a magnetic stir bar was evacuated and back-
filled with nitrogen. The flask was charged with anhydrous
FeCl2 (0.030 g, 0.238 mmol) in n-BuOH (10 ml) and the
contents stirred at 110 1C until the iron salt had completely
dissolved. L1 (0.100 g, 0.238 mmol, 1 eq.) was added and the
reaction mixture stirred at 110 1C for a further 20 min. After
cooling to room temperature, the suspension was concentrated
and hexane added to induce precipitation of the product. The
solid was filtered off, washed with hexane and dried overnight
under reduced pressure to afford [{6-{(2,6-i-Pr2C6H3)
NQCH}-2,20:60,200-C15H9N3}FeCl2] (1a) as a purple-blue
powder. Yield: 68% (0.088 g, 0.162 mmol). Recrystallisation
from a hot acetonitrile solution gave 1a as purple plates. Anal.
Calcd for C28H28N4FeCl2: C, 61.43; H, 5.12; N, 10.24. Found
C, 60.69; H, 5.06; N, 10.30%.
(c) 2c, M = Ni, X = Br. Using an analogous procedure to
that described for 2a employing (DME)NiBr2 (0.073 g, 0.238
mmol) and L2 (0.103 g, 0.238 mmol, 1 eq.) gave [{6-{(2,6-i-
Pr2C6H3)NQCMe}-2,20:60,200-C15H9N3}NiBr2] (2c) as an
orange solid. Yield: 75% (0.117 g, 0.179 mmol). Recrystallisa-
tion from a chloroform solution at 0 1C gave 2c as orange
plates. 1H NMR (CDCl3, 293 K): d 130.9 (o0.1 ms, Ha), 92.9
(2.07 ms, HbHb0), 82.5 (2.25 ms, HbHb), 72.3 (2.24 ms, HbHb0),
66.8 (2.54 ms, HbHb0), 59.5 (2.59 ms, HbHb0), 35.0 (3.66 ms,
H
b0b0), 18.0 (7.37 ms, Hg), 16.9 (6.30 ms, Hg), 13.3 (8.20 ms,
Hg), 12.0–4.1 (Ar–H, CMeQN, Ar–CHMe2). Anal. Calcd for
C29H30N4NiBr2: C, 46.69; H, 4.02; N, 7.26. Found C, 47.09;
H, 3.97; N, 7.11%.
(b) 1b, M = Co, X = Cl. Using an analogous procedure to
that described for 1a employing CoCl2 (0.031 g, 0.238 mmol)
and L1 (0.100 g, 0.238 mmol, 1 eq.) gave [{6-{(2,6-i-
Pr2C6H3)NQCH}-2,20:60,200-C15H9N3}CoCl2] (1b) as a blue-
green powder. Yield: 70% (0.091 g, 0.166 mmol). Recrystalli-
sation from a hot acetonitrile solution gave 1b as green blocks.
Anal. Calcd for C28H28N4CoCl2: C, 61.09; H, 5.09; N, 10.18.
Found C, 60.76; H, 5.15; N, 10.12%.
(d) 2d, M = Zn, X = Cl. Using an analogous procedure to
that described for 2a employing anhydrous ZnCl2 (0.032 g,
0.238 mmol) and L2 (0.103 g, 0.238 mmol, 1 eq.) gave [{6-
{(2,6-i-Pr2C6H3)NQCMe}-2,20:60,200-C15H9N3}ZnCl2] (2d) as
a yellow solid. Yield: 65% (0.088 g, 0.155 mmol). Prolonged
standing at room temperature of an acetonitrile–chloroform
mixture containing the complex gave 2d as pale yellow blocks.
Anal. Calcd for C29H30N4ZnCl2: C, 61.01; H, 5.26; N, 9.82.
Found C, 61.21; H, 5.12; N, 9.97%.
(c) 1c, M = Ni, X = Br. Using an analogous procedure to
that described for 1a employing (DME)NiBr2 (0.073 g, 0.238
mmol) and L1 (0.100 g, 0.238 mmol, 1 eq.) gave [{6-{(2,6-i-
Pr2C6H3)NQCH}-2,20:60,200-C15H9N3}NiBr2] (1c) as an or-
ange solid. Yield: 80% (0.122 g, 0.190 mmol). Anal. Calcd
for C28H28N4NiBr2: C, 52.62; H, 4.39; N, 8.77. Found C,
52.89; H, 4.71; N, 8.64%.
3.5 Ethylene oligomerisation
An oven dried 200 ml Schlenk vessel equipped with magnetic
stir bar was evacuated and backfilled with nitrogen. The vessel
was charged with the precatalyst (0.01 mmol) and dissolved or
suspended in toluene (40 ml). MAO (4.0 mmol, 400 eq.) was
introduced and the reaction mixture left to stir for 5 minutes.
The vessel was purged with ethylene and the contents magne-
tically stirred under 1 bar ethylene pressure at room tempera-
ture for the duration of the test. After 1 h, the test was
terminated by the addition of dilute aqueous hydrogen chlor-
ide (5 ml). The organic phase was separated and dried over
magnesium sulfate and filtered. GC analysis was performed by
taking an aliquot of the solution. For analysis of the oligomers
by 1H NMR spectroscopy, the solvent was removed on a
rotary evaporator and the residue dissolved in CDCl3.
(d) 1d, M = Zn, X = Cl. Using an analogous procedure to
that described for 1a employing ZnCl2 (0.032 g, 0.238 mmol)
and L1 (0.100 g, 0.238 mmol, 1 eq.) gave [{6-{(2,6-i-
Pr2C6H3)NQCH}-2,20:60,200-C15H9N3}ZnCl2] (1d) as a pale
yellow solid. Yield: 65% (0.086 g, 0.155 mmol). Recrystallisa-
tion from an acetonitrile–chloroform solution gave 1d as pale
yellow blocks. Anal. Calcd for C28H28N4ZnCl2: C, 60.39; H,
5.03; N, 10.06. Found C, 60.51; H, 4.99; N, 10.21%.
3.4 Synthesis of [{6-iminoacetyl-2,20:60,200-terpyridine-
(2,6- diisopropylanil)}MX2] (2)
(a) 2a, M = Fe, X = Cl. An oven-dried Schlenk flask
equipped with a magnetic stir bar was evacuated and back-
filled with nitrogen. The flask was charged with anhydrous
FeCl2 (0.030 g, 0.238 mmol) in n-BuOH (10 ml) and the
contents stirred at 110 1C until the iron salt had completely
dissolved. L2 (0.103 g, 0.238 mmol, 1 eq.) was added and the
reaction mixture stirred at 110 1C for a further 20 min. After
cooling to room temperature, the suspension was concentrated
3.6 Density functional calculations
Quantum mechanical calculations have been carried out using
the Gaussian 03 package of programs.23 Density functional
theory (DFT) was applied, in particular the functional Becke’s
three-parameter hybrid exchange method combined with the
LYP correlation functional (B3LYP).24 The quasi-relativistic
ꢀc
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New J. Chem., 2007, 31, 75–85 | 83

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Table 8 Crystallographic and data processing parameters for complexes 1a, 1b, 1d, 2c and 2d
Complex
Formula
1a
1b
1d
2c
2d
C28H28Cl2N4Fe ꢃ
CH3CN
588.35
C
28H28Cl2N4Co ꢃ
C28H28Cl2N4Zn ꢃ
CHCl3
C29H30Br2N4Ni ꢃ
3.5CHCl3
1070.89
C29H30Cl2N4Zn
CH3CN
591.43
M
Crystal size/mm3
T/K
Crystal system
Space group
a/A
b/A
c/A
a/1
676.18
0.20 ꢂ 0.16 ꢂ 0.11
570.84
0.18 ꢂ 0.16 ꢂ 0.13
0.22 ꢂ 0.17 ꢂ 0.05
0.17 ꢂ 0.25 ꢂ 0.35
0.35 ꢂ 0.23 ꢂ 0.05
150(2)
Monoclinic
P2(1)/c
150(2)
150(2)
Monoclinic
P2(1)/c
150(2)
Monoclinic
C2/c
150(2)
Triclinic
Monoclinic
P2(1)/c
10.089(3)
17.124(5)
16.991(5)
90
96.459(5)
90
2916.8(15)
4
1.347
1228
0.799
22391
5730
0.0285
0/407
ꢀ
P1
10.0666(12)
17.260(2)
16.987(2)
90
96.542(2)
90
2932.3(6)
4
1.333
1224
0.724
20994
5155
0.0383
0/367
R1 = 0.0408,
wR2 = 0.1039
R1 = 0.0497,
wR2 = 0.1089
1.045
10.3842(7)
16.8912(11)
17.4972(12)
90
96.1660(10)
90
3051.3(4)
4
1.472
1384
1.269
23670
5986
0.0366
0/374
R1 = 0.067,
wR2 = 0.0884
R1 = 0.0471,
wR2 = 0.0919
33.824(8)
10.759(3)
23.446(6)
90
100.613(4)
90
8386(3)
8
1.696
4264
3.068
31356
8219
0.1464
0/454
R1 = 0.1088,
wR2 = 0.2653
R1 = 0.2232,
wR2 = 0.3250
10.2739(19)
10.5553(19)
12.623(2)
94.301(3)
101.922(3)
96.325(3)
1324.4(4)
2
1.431
592
1.155
10435
5134
0.0362
0/330
R1 = 0.0440,
wR2 = 0.1004
R1 = 0.0567,
wR2 = 0.1055
b/1
g/1
U/A3
Z
Dc/Mg mꢁ3
F(000)
m(Mo-Ka)/mmꢁ1
Reflections collected
Independent reflections
Rint
Restraints/parameters
Final R indices (I 4 2s(I))
R1 = 0.0336,
wR2 = 0.0920
R1 = 0.0409,
wR2 = 0.0947
1.036
All data
Goodness of fit on F2 (all data)
0.974
0.893
1.003
P
P
P
P
a
2
Data in common: graphite-monochromated Mo-Ka radiation, l = 0.710 73 A; R1 = JFo| ꢁ |FcJ/ |Fo|, wR2 = [ w(Fo ꢁ Fc2)2/ w(Fo2)2]1/2
,
P
wꢁ1 = [s2(Fo)2 + (aP)2], P = [max(Fo2,0) + 2(Fc2)]/3, where a is a constant adjusted by the program; goodness of fit = [ (Fo ꢁ Fc2)2/
2
(n ꢁ p)]1/2 where n is the number of reflections and p the number of parameters.
effective core potential (ECP) LANL2DZ was used for all
metal atoms.25 The basis set for both atoms in the valence
double-z contraction was associated with this ECP. The
valence double-z with polarisation 6-31 G(d) basis was used
for N and Cl and the minimal basis STO-3G for C and H.26,27
halide complexes, [(Lx)MX2] (M = Fe, Co, Ni, Zn). It is
apparent that not only the imino-carbon substituent but also
the nature of the metal centre influences the bonding mode
adopted by Lx. For nickel, both tetra- and tri-dentate bonding
modes are possible while for zinc only tridentate bonding is
evident. The steric/electronic attributes of the particular imine
donor (ketimine vs. aldimine) coupled with stabilisation effects
imparted by specific dn configurations has been offered as an
explanation for these trends. Moreover it has been shown that
lability of the ketimine donor is most likely to occur for the
high spin iron and cobalt complexes although no clear evi-
dence for this has been identified in the complexes prepared.
Nevertheless, the propensity for imine coordination has been
suggested as an explanation for the very low catalytic activities
for ethylene oligomerisation displayed by the iron systems.
3.7 Crystallography
Data for 1a, 1b, 1d, 2c and 2d were collected on a Bruker
APEX 2000 CCD diffractometer. Details of data collection,
refinement and crystal data are listed in Table 8. The data were
corrected for Lorentz and polarisation effects and empirical
absorption corrections applied. Structure solution by direct
methods or Patterson (2d) and structure refinement based on
full-matrix least-squares on F2 employed SHELXTL version
6.10.28 Hydrogen atoms were included in calculated positions
(C–H = 0.96 A) riding on the bonded atom with isotropic
displacement parameters set to 1.5 Ueq (C) for methyl H atoms
and 1.2 Ueq (C) for all other H atoms. All non-H atoms were
refined with anisotropic displacement parameters. Atoms
C(25) and C(26) in 1b were disordered and modelled.
CCDC reference numbers 615609–615613.
Acknowledgements
We thank the University of Leicester for financial assistance.
References
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ꢀc
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