Structures and properties of 6-aryl substituted tris(2-pyridylmethyl)-amine transition metal complexes

Article abstract of DOI:10.1039/b207406k

A series of metal(II) complexes of [6-(2′,5′-dimethoxyphenyl)-2-pyridylmethyl]bis(2-pyridylmethyl) amine (L) have been prepared. X-Ray crystal structures have been determined for L and its metal(II) chloride complexes for Mn, Fe, Co, Ni, Cu and Zn and the results compared. The preparation and crystal structure of the unusual carbonato-bridged copper-tetramer [Cu₄(L)₄(CO₃)₂][BF₄] ₄·5.2H₂O is also described. The solution-state structures of the complexes are deduced from their physicochemical and spectroscopic properties. The Zn(II) complex, [ZnCl₂(L)], shows inversion about the ligand amine group on the NMR timescale - results from a variable temperature NMR study are presented and allow estimation of the barrier to amine inversion as 56 ± 0.5 kJ mol¹. Overall, it is found that the intramolecular steric interactions introduced by substitution of the tris(2-pyridylmethyl)amine (tpa)-skeleton by a single 6-aryl group result in significant changes to the structures and properties of the resulting metal complexes: in particular the aryl-substitution in L causes (i) a weaker ligand-field compared to tpa favouring high-spin complexes, (ii) a tendency toward lower coordination numbers, and (iii) hemilability in the Ni(II) and Cu(II) complexes - the aryl-substituted leg of the ligand (L) is coordinatively labile.

Full text of DOI:10.1039/b207406k

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Structures and properties of 6-aryl substituted tris(2-pyridylmethyl)-
amine transition metal complexes
Zhicong He, Donald C. Craig and Stephen B. Colbran*
School of Chemical Sciences, University of New South Wales, Sydney, NSW 2052, Australia
Received 29th July 2002, Accepted 30th September 2002
First published as an Advance Article on the web 22nd October 2002
A series of metal() complexes of [6-(2Ј,5Ј-dimethoxyphenyl)-2-pyridylmethyl]bis(2-pyridylmethyl)amine (L) have
been prepared. X-Ray crystal structures have been determined for L and its metal() chloride complexes for Mn,
Fe, Co, Ni, Cu and Zn and the results compared. The preparation and crystal structure of the unusual carbonato-
bridged copper-tetramer [Cu4(L)4(CO3)2][BF4]4ؒ5.2H2O is also described. The solution-state structures of the
complexes are deduced from their physicochemical and spectroscopic properties. The Zn() complex, [ZnCl2(L)],
shows inversion about the ligand amine group on the NMR timescale — results from a variable temperature NMR
study are presented and allow estimation of the barrier to amine inversion as 56 0.5 kJ molϪ1. Overall, it is found
that the intramolecular steric interactions introduced by substitution of the tris(2-pyridylmethyl)amine (tpa)-skeleton
by a single 6-aryl group result in significant changes to the structures and properties of the resulting metal complexes:
in particular the aryl-substitution in L causes (i) a weaker ligand-field compared to tpa favouring high-spin
complexes, (ii) a tendency toward lower coordination numbers, and (iii) hemilability in the Ni() and Cu()
complexes — the aryl-substituted leg of the ligand (L) is coordinatively labile.
The potentially tetradentate ligand tris(2-pyridylmethyl)amine
(tpa) was first prepared in 1967 by Anderegg and Wenk,1 and
complexes of tpa are now characterised for most metal ions.1–32
Most recently, iron and copper complexes of tpa and its simple
derivatives have risen to special prominence as mimics for bio-
logical metal centres, especially those that bind and activate
dioxygen.3–9 The study of these complexes has contributed
greatly to understanding of iron–dioxygen and copper–
dioxygen chemistries, and has led to new catalysts for oxidation
of organic substrates using the clean oxidants, oxygen or
hydrogen peroxide.
Substitution of the tpa skeleton has recently emerged as a
powerful strategy for changing and controlling the properties of
(tpa)metal complexes, including structure, magnetism and,
most importantly, stoichiometric and catalytic reactivity.3–18
To date, simple alkyl substitution of one or more of the tpa
pyridines at the 6-position has most often been employed:
intramolecular steric interactions in the complexes cause the
more highly substituted derivatives to bind metal ions more
weakly thus favouring high-spin (iron) complexes or (copper)
complexes with “dangling” non-coordinated pyridylmethyl
“legs”. For example: [Fe(tpa)(CH3CN)2][ClO4]2 is a low-spin
iron() complex whereas [Fe(Me2tpa)(CH3CN)2][ClO4]2 and
[Fe(Me3tpa)(CH3CN)2][ClO4]2 are high spin iron() species;
these are important changes since the spin state of these iron
compounds profoundly influences their reactivity, for instance,
with organic peroxides.7 Likewise, copper() complexes of alkyl-
tpa derivatives exhibit different oxygen reactivities compared
to the parent (tpa)Cu() compound,8,9 and different structures
and reactivities are found for even the simplest copper() com-
plexes — for example whereas trigonal bipyramidal (tbp)
[Cu(κ4N-tpa)Cl]ϩ is obtained from copper() chloride and
tpa,10 the crystals obtained from Me3tpa and copper() chloride
contain a solid solution of square pyramidal (sp) [Cu(κ3N-
Me3tpa)Cl2] and sp-[Cu(κ4N-Me3tpa)Cl]ϩ and its chloride
counter ion.11
characterisation of the copper() complexes, [Cu(Phntpa)]ϩ
(n = 1–3),12 the X-ray crystal structures of [Cu(Ph3tpa)][BPh4]
and [Cu(Ph3tpa)(MeCN)2](ClO4)2,13 copper complexes of tpa
derivatives with benzocrown ether substituents as a second
(s-block) metal ion binding site,14 and some cadmium and
zinc complexes of chiral ligands based on the tpa skeleton that
have phenyl substituents.15 More recently, Que and co-workers
reported that the 6-phenyl-substituent within [Fe(Phtpa)-
(MeCN)2]2ϩ is hydroxylated following treatment with tert-
butylperoxide and base,16 and Mandon et al. demonstrated that
6-(4-methoxyphenyl)-substitution of a tpa-pyridine suffices for
that “leg” to be unbound in the ferric chloride (FeCl3) com-
plex.17 In this contribution, we describe first d-series metal()
complexes of [6-(2Ј,5Ј-dimethoxyphenyl)-2-pyridylmethyl]bis-
(2-pyridylmethyl)amine (L) which demonstrate that substitu-
tion of the tpa-skeleton by a single aryl substituent is sufficient
to significantly change the structures and properties of the
resulting complexes.
Results
Syntheses
Ligand. Ligand L was synthesised by two routes. Both started
with monolithiation of 1,4-dimethoxybenzene, reaction of
the monolithio product with tri(iso-propyl)borate and then
hydrolysis with hydrochloric acid to give the arylboronic acid,
which was coupled with 6-bromo-2-pyridylcarboxaldehyde
under Suzuki conditions to afford 6-(2Ј,5Ј-dimethoxyphenyl)-2-
In contrast to alkyl-substitution of tpa, aryl-substitution has
been relatively little and only recently exploited. Canary and co-
workers have reported the preparations and electrochemical
4224
J. Chem. Soc., Dalton Trans., 2002, 4224–4235
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b207406k

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Table 1 Selected key bond length (Å) and bond angle (Њ) data for the metal() chloride complexes
a
[MnCl2(L)]
[FeCl2(L)]
[CoCl(L)][BPh4]
2.262(1)
[Ni2Cl2(L)2]Cl2
[CuCl(L)][PF6]
2.228(1)
[ZnCl2(L)]
M–Cl1
M–Cl2
M–N1
M–N2
M–N3
M–N4
2.486(1)
2.391(1)
2.413(4)
2.303(5)
2.300(4)
2.322(4)
2.476(1)
2.328(1)
2.360(4)
2.223(4)
2.232(4)
2.235(4)
2.356(1)
2.492(1)
2.167(4)
2.056(4)
2.094(4)
2.070(4)
2.234(1)
2.244(1)
2.131(4)
2.077(4)
2.108(4)
2.187(3)
2.509(3)
1.984(3)
1.977(3)
2.051(3)
2.193(2)
2.220(2)
2.215(2)
Cl1–M–Cl2
Cl1–M–N1
Cl1–M–N2
Cl1–M–N3
Cl1–M–N4
Cl2–M–N1
Cl2–M–N2
Cl2–M–N3
Cl2–M–N4
N1–M–N2
N1–M–N3
N1–M–N4
N2–M–N3
N2–M–N4
N3–M–N4
104.58(6)
87.9(1)
159.5(1)
88.4(1)
90.0(1)
120.0(1)
94.0(1)
93.7(1)
159.1(1)
75.2(2)
145.9(1)
74.7(1)
99.0(2)
74.5(2)
71.4(2)
102.55(5)
87.3(1)
160.6(1)
87.6(1)
89.7(1)
117.1(1)
94.2(1)
93.7(1)
161.9(1)
76.3(2)
149.2(1)
76.3(1)
101.2(2)
76.6(2)
73.3(2)
84.41(4)
105.4(1)
99.3(1)
94.9(1)
175.6(1)
90.8(1)
172.8(1)
90.1(1)
92.6(1)
94.1(2)
159.7(2)
77.7(2)
83.4(2)
83.4(2)
82.0(2)
115.59(4)
112.1(1)
103.1(1)
95.8(1)
105.70(8)
96.1(1)
97.1(1)
100.25(7)
101.09(7)
114.59(7)
170.3(1)
174.2(1)
97.09(7)
95.79(7)
129.80(7)
115.4(2)
113.8(1)
76.3(1)
114.2(2)
76.4(1)
75.9(1)
86.5(1)
96.7(1)
80.1(1)
165.0(2)
83.4(1)
82.8(1)
147.22(9)
75.01(9)
73.57(9)
a For [Ni2Cl2(L)2]Cl2, Cl2 = Cli.
pyridylcarboxyaldehyde (A) in 82% overall yield. The first route
to L was based on traditional syntheses of tris(pyridylmethyl)-
amines and required conversion of A to 6-(2Ј,5Ј-dimethoxy-
phenyl)-2-pyridylmethylbromide (B), which was achieved by
borohydride reduction of A and treatment of the intermediary
alcohol with phosphorous tribromide. Reaction of B and N,N-
bis(2-pyridylmethyl)amine (bpma) in the presence of triethyl-
amine gave L, isolated in 74% yield after chromatography (silica
support, dichloromethane eluent). The second route was direct
coupling of A and bpma using sodium cyanoborohydride
and afforded L in near to quantitative yield. Clearly the latter
reductive amination is the more convenient route, and it should
be readily adaptable to the preparations of other mono-
substituted tpa-derivatives.
Metal complexes. The complexes [MCl2(L)] (M = Mn, Fe and
Zn), [CoCl(L)]2[CoCl4], [Ni2Cl2(L)2]Cl2 and [CuCl(L)]Cl were
prepared in good yield (70–90%) by simply mixing the
appropriate metal dichloride and L (1.0–1.05 equivalents) in
methanol or ethanol solution. The Fe() complex was air-
sensitive and was always handled under a nitrogen atmosphere;
the product(s) from its oxidation were not investigated. The
crystals of [CoCl(L)]2[CoCl4] were not of sufficient quality for
an X-ray structural analysis and, therefore, were treated with Na-
[BPh4] to give [CoCl(L)][BPh4]. A similar metathesis reaction of
[CuCl(L)]Cl with K[PF6] afforded [CuCl(L)][PF6]. The prepar-
ation of the Cu() complex [Cu(L)][BF4] from L and [Cu(MeC-
N)4][BF4] under rigorously anaerobic conditions is described
elsewhere.18 When a clear yellow solution of [Cu(L)][BF4]
was treated with dioxygen in acetone at Ϫ40 ЊC, the solution
immediately turned green. When no solid formed on equilibra-
tion of this solution with diethyl ether under dinitrogen, the
solution was left to evaporate in air. A few crystals of the green
tetramer [Cu4(L)4(CO3)2][BF4]4ؒ5.2H2O were obtained.
Fig. 1 View of L showing the 10% thermal ellipsoids at 294 K.
splayed outward from the approximate C3 symmetric axis that
the molecule would have if the dimethoxyphenyl substituent
were ignored. The geometry minimises interactions between the
nitrogen lone pairs. Likewise, the methoxy groups of the PY*
leg are oriented away from the tpa-core and the twist about the
dimethoxyphenyl–pyridine inter-annular bond is 24Њ.
Crystal structures
The following labelling scheme for the nitrogen donors of L is
used throughout the following discussion: Npy* for the nitrogen
(N1) of the 6-dimethoxyphenyl-2-pyridylmethyl (PY*) leg; Npy
for the nitrogen (N2 or N3) of a 2-pyridyl (PY) leg; Nam for the
amine nitrogen (N4). Key bond length and angle data from the
structures of the complexes are presented in Table 1.
[MCl2(L)]ؒMeOH (M ؍ Mn, Fe). The Mn() and Fe() com-
plexes are isomorphous, co-crystallising with methanol in the
orthorhombic space group Pna21. In the crystal structures, the
only significant intermolecular interaction is the hydrogen bond
between the lattice methanol and a chloride ancillary ligand
[Mn: Cl2ؒ ؒ ؒO1Me 3.16(4) Å; Fe: Cl2ؒ ؒ ؒO1Me 3.158(6) Å] of
the [MCl2(L)] (M = Mn, Fe) complex, e.g. Fig. 2. The metal()
centres are in distorted octahedral environments bound by the
Ligand L. Fig. 1 shows the molecular structure of L. The
three pyridylmethyl legs are anti- with respect to Nam and
J. Chem. Soc., Dalton Trans., 2002, 4224–4235
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Fig. 2 View of [FeCl2(L)] ؒMeOH showing the 10% thermal ellipsoids
at 294 K.
four nitrogens of L and two chloride co-ligands. The distortion
from ideal octahedral coordination geometry is imposed
by L, for example the average Nam–M–Npy/py* angle is 73.5Њ for
[MnCl2(L)] and 75.4Њ for [FeCl2(L)], and results in acute
Cl1–M–N2, Cl2–M–N4 and N1–M–N3 angles (along the octa-
hedral axes) of 159.5(1)Њ, 159.1(1)Њ and 145.9(1)Њ for [MnCl2(L)]
and 160.6(1)Њ, 161.9(1)Њ and 149.2(1)Њ for [FeCl2(L)]. The M–N
bond lengths for both complexes, Table 1, are indicative of
Fig. 3 View of the [CoCl(L)]ϩ ion showing the 10% thermal ellipsoids
at 294 K.
5
high-spin [Mn(): S = /2; Fe(): S = 2] species, as is confirmed
by their magnetic properties (see below). For each complex,
the dimethoxyphenyl–pyridine inter-annular twist is 48Њ and the
M–Npy* bond [Mn: 2.413(4) Å; Fe: 2.360(4) Å] is significantly
longer than the other M–N bonds (av. Mn–Npy/am: ∼2.31 Å;
av. Fe–Npy/am: ∼2.23 Å); the latter distances are comparable to
those previously found in octahedral high-spin M() complexes
of tpa and derivatives, e.g. 2.205–2.366 Å in [Mn2(tpa)2(ca)]-
[ClO4]2 (ca = dianion of chloranilic acid),19 2.18–2.25 Å in
[Fe(6-Me3tpa)(CH3CN)2]2ϩ,7 2.14–2.255 Å in [Fe2(O2CCH3)2-
(tpa)2F2]2ϩ 20 2.17–2.23 Å in [Fe(6-Me3tpa)(bf )]ϩ (bf = benzoyl-
,
formate)21 and average ∼2.165 Å in [Fe2Cl2(tpa)2][BPh4]2.22 The
structures of [MCl2(tpa)] (M = Mn, Fe) are not available for
comparison.†
[CoCl(L)][BPh4]. The crystal structure of [CoCl(L)][BPh4]
reveals a slightly distorted trigonal bipyramidal coordination
geometry for the Co() ion (τ33 = 0.92) with the three pyridyl
groups in the equatorial plane and the amine and a chloride co-
ligand in the apical positions, Fig. 3 and Table 1. The Co–Npy*
bond length [2.131(4) Å] is not significantly larger than the
Co–Npy bond lengths [av. 2.095 Å]; to achieve this the
dimethoxyphenyl–pyridyl inter-annular twist increases to ca.
65Њ. Constrained by the ligand L, the Nam–Co–Npy/py* angles
average 76.2Њ and consequently the Co() ion appears
“displaced” by ca. 0.45 Å from the equatorial plane toward
the chloride co-ligand; the Co–Nam distance is 2.187(3) Å. An
analogous (tpa)Co() complex is unknown; the reported
examples of Co() complexes of tpa are all octahedral.23–25
Fig. 4 View of the [Ni2Cl2(L)2]2ϩ ion showing the 10% thermal
ellipsoids at 294 K.
co-ligands. The Ni–Cl bond lengths are 2.356(1) and 2.492(2) Å
and the Ni–Cl–Ni bond angle is 95.6(1)Њ. That the Ni–Npy*
bond [2.167 Å] is notably longer than the other Ni–N bond
lengths [2.056–2.094 Å, Table 1] is directly attributable to the
steric interactions introduced by the dimethoxyphenyl sub-
stitution of the PY* leg. The bond distances and angles, with
the exception of those for the Ni–Npy* bond, are similar to
those recently reported for the corresponding Ni(tpa) dimer,
[Ni2(µ-Cl)2(tpa)2][ClO4]2.26 Notably, the dimethoxyphenyl
rings in [Ni2(µ-Cl)2(L)2]Cl2 are near parallel to the Ni2(µ-Cl)2
[Ni2(ꢀ-Cl)2(L)2]Cl2ؒMeOHؒ5H2O. Each Ni() ion in this
centrosymmetric dimer sits in a distorted octahedral N4Cl2-
coordination environment, Fig. 4. The two nickel ions are
separated by 3.59 Å, asymmetrically bridged by two chloride
plane which requires
a
large dimethoxyphenyl–pyridyl
inter-annular twist of 72Њ. This arrangement minimises inter-
actions between the dimethoxyphenyl and the pyridyl rings
lying in the plane of the Ni2(µ-Cl)2 core and maximises
stabilising dimethoxyphenyl edge C–H-to-pyridyl face inter-
actions between PY* and PY legs on adjacent Ni centres
(see Fig. 4); the dimethoxyphenyl edge carbon to pyridyl plane
† Note added at proof: a report of the ferrous chloride complexes of
four tpa derivatives, which includes crystal structures for octahedral
[FeCl2(κ4N-tpa)] and trigonal bipyramidal [FeCl2(κ3N-Ph2tpa)] and
data indicating the structure of each complex is conserved in
acetonitrile solution, has appeared: D. Mandon, A. Machkour, S.
Goetz and R. Welter, Inorg. Chem., 2002, 41, 5364.
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J. Chem. Soc., Dalton Trans., 2002, 4224–4235

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Table 2 Selected key bond length (Å) and bond angle (Њ) data for
[Cu4(L)2(CO3)2][BF4]4ؒ5.2H2O
distance is ∼3.7 Å. Space-filling models reveal the ligands L in
this structure to completely enclose and shield the Ni2(µ-Cl)2
core.
CuA–N2A
CuA–N3A
CuA–N4A
CuA–O1C
CuA–O1Ci
1.982(5)
CuB–N1B
CuB–N2B
CuB–N3B
CuB–N4B
CuB–O2C
2.251(6)
1.954(5)
1.973(5)
2.057(7)
1.948(5)
1.974(5)
2.017(7)
1.957(6)
2.411(6)
[CuCl(L)][PF6]. As the structures of [CuCl(L)][PF6] and
“CuCl2(L)” in dichloromethane solution differ (see below),
crystal structure analyses were completed for both of these
Cu() complexes. The crystal structure of [CuCl(L)]Clؒ3H2O is
already described elsewhere.18 Suitable crystals of [CuCl(L)]-
[PF6] were obtained by recrystallisation from dichloromethane–
methanol. In each cation, the Cu() ion is bound by the four
nitrogens of L and by a chloride ancillary ligand in a slightly
distorted square pyramidal arrangement (τ = 0.15). The two Npy
and the Nam donors of L and the ancillary chloride form the
basal plane of the pyramid, and the bulkier PY* leg adopts the
axial position with the Cu–N distance of 2.509(3) Å being sub-
stantially longer than the Cu–Nam [2.051(3) Å] and equatorial
Cu–Npy (av. 1.98 Å) distances. The average Nam–Cu–Npy angle
is 83.1Њ and the Nam–Cu–Npy* angle is 80.1(1)Њ and the inter-
annular twist between the rings of the PY* leg is 24Њ. In the
crystal structure, Fig. 5, cations of [CuCl(L)][PF6] are arrayed
N2A–CuA–N3A
N2A–CuA–N4A
N2A–CuA–O1C
N2A–CuA–O1Ci
N3A–CuA–N4A
N3A–CuA–O1C
N3A–CuA–O1Ci
N4A–CuA–O1C
N4A–CuA–O1Ci
O1C–CuA–O1Ci
164.2(3)
82.8(3)
98.5(3)
97.3(2)
82.4(3)
96.8(3)
89.0(2)
172.8(3)
92.7(2)
80.2(3)
N1B–CuB–N2B
N1B–CuB–N3B
N1B–CuB–N4B
N1B–CuB–O2C
N2B–CuB–N3B
N2B–CuB–N4B
N2B–CuB–O2C
N3B–CuB–N4B
N3B–CuB–O2C
N4B–CuB–O2C
87.0(2)
101.7(2)
81.6(3)
123.1(3)
162.9(3)
84.2(3)
94.3(3)
82.7(3)
93.1(3)
155.2(3)
(η1-O,η2-OЈ)-CO3}2][BF4]4ؒ5.2H2O crystallised from aceto-
nitrile–ether. The identity and structure of the complex was
established by X-ray crystallography, Fig. 6 and Table 2. The
tetramer is centrosymmetric with each of the four Cu() ions
best described as five-coordinate and close to square-pyramidal
in geometry. The outermost pair of copper ions, (CuB)2, each
have a N4O-donor set: L is tetradentate with the bulky PY* leg
in the weak-field, axial position (CuB–Npy* 2.25 Å) and with a
carbonato oxygen (O2C) and the Nam and two Npy nitrogens
equatorially bound at usual distances (CuB–Nam/py ∼ 2.0 Å). A
second carbonato oxygen (O1C) bridges the innermost pair of
copper ions, (CuA)2, each of which shows N3O2-donor set with
the PY* leg unbound and dangling away from the Cu4(µ-CO3)2
core. Equatorially bound to each CuA ion at normal distances
are O1C of one carbonato ligand, and the Nam and two Npy
nitrogen atoms. The bridging oxygen atom of the other carbon-
ato ligand takes up the axial position (CuA–O1Ci 2.41 Å). The
third oxygen atom of each carbonato ligand (O3C) lies between
a CuA–CuB pair and could be considered a second axial ligand
to these copper ions, albeit weakly bound given the distances
(CuAؒ ؒ ؒO3C 2.62, CuBؒ ؒ ؒO3C 2.62 Å). Each of the latter
carbonato oxygen atoms is also hydrogen bonded with a water
molecule (O3Cؒ ؒ ؒOW1 2.77 Å). The Cu4{µ3-(η1-O,η2-OЈ)-
CO3}2 core is not unique, having been recently discovered in
a copper() carbonato complex of a tetrapyrazolyldiamine
macrocycle.27 In contrast, (tpa)CuII forms a very different
carbonato complex, [{Cu(κ4N-tpa)}2{µ2-(η1-O,η1-OЈ)-CO3}]2ϩ
,
which has a single symmetrical, bridging-bidentate carbonato
ligand equatorially bound to two square pyramidal copper()
ions.28
Fig. 5 View of a symmetry-related pair of [CuCl(L)]ϩ ions (10%
thermal ellipsoids at 294 K); the Cuؒ ؒ ؒCli distance is 3.5 Å.
[ZnCl2(L)]. Colourless rhombs of [ZnCl2(L)] crystallised
from methanol–ether. The crystal structure reveals chains of
[ZnCl2(L)] molecules, weakly linked by edge-to-face inter-
molecular interactions between a pyridyl ring of each molecule
of [ZnCl2(L)] and the dimethoxyphenyl ring of an adjacent
molecule; the edge carbon to adjacent ring plane distances are
∼3.5 Å. Each chain has a partner chain running in the opposite
direction, with pairs of complex molecules in adjacent chains
related by crystallographic inversion. In [ZnCl2(L)] molecules,
Fig. 7, the Zn() ion exhibits a trigonally-distorted square
pyramidal coordination geometry (τ = 0.29) with the base com-
prised of Cl2 and Nam and the two Npy donors of L and with
Cl1 at the apex. The Zn–Cl bond lengths for the equatorial and
axial chlorides are similar [Zn–Cl1 2.234(1) Å; Zn–Cl2 2.244(1)
Å]. The PY* leg is unbound, ‘dangling’ away from the metal
centre; the interannular twist between the pyridine and dimeth-
oxyphenyl rings is 50Њ. In contrast, all crystallographically
characterised (tpa)zinc complexes exhibit tetradentate tpa lig-
ands: e.g., [ZnX(tpa)][ClO4] (X = Cl, Br),23 [Zn2(tpa)2(µ-OH)2]-
(ClO4)2 and [Zn3(tpa)3(µ-CO3)](ClO4)4ؒH2O,29 and [ZnX(tpa)]-
[BPh4] (X = Cl, PhCO2).30
in centrosymmetric pairs with the chloride of one occupying
the sixth “octahedral” position of the other; overall the
structure of the pair is akin to that of the Ni() dimer, but the
inter-cation Cuؒ ؒ ؒCli distance is 3.5 Å more consistent with
description as a pair of cations. Also of note is the different
positioning of the PY* legs in the Ni() and Cu() complexes
revealed by a comparison of Figs. 4 and 5. The structure of the
Cu() cation is very similar to those of the two crystallographi-
cally independent, square pyramidal cations (cation A: τ = 0.03;
cation B: τ = 0.003) within the structure of [CuCl(L)]Clؒ3H2O
in which there are no inter-cation Cuؒ ؒ ؒCl interactions.18
In contrast, [Cu(tpa)Cl][PF6] adopts a trigonal bipyramidal
coordination geometry (τ = 1.005) with the amine group and the
chloride co-ligand in the axial positions and with the three
pyridyl rings equatorial.10 Adoption of the square pyramidal
geometry by the [CuCl(L)]ϩ cation minimises intramolecular
steric interactions with the bulky PY* leg.
[{Cu(L)}4(CO3)2][BF4]4ؒ5.2H2O. Green crystals of the bis-
(carbonato)-bridged tetramer, [{Cu(κ3N-L)}2{Cu(κ4N-L)}2{µ3-
J. Chem. Soc., Dalton Trans., 2002, 4224–4235
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Fig. 6 Views of the [Cu4(L)4(CO3)2]4ϩ ion (10% thermal ellipsoids) showing: (a) the asymmetric unit and (b) the full tetramer (with H-atoms omitted
for clarity).
Table 3 Magnetic moment data at 295 K from Gouy balance meas-
urements and pyridine-ring deformation band data from FTIR spectra
along with the indicated number of bound N-donors
Solid-state measurements
Bulk samples of all complexes except [Cu4(L)4(CO3)2][BF4]4
gave correct partial elemental analyses; [Cu4(L)4(CO3)2][BF4]4
was not analysed due to lack of sample. FTIR spectra of the
bulk samples show characteristic pyridine-ring deformation
bands,31 Table 3, which indicate ligand coordination modes in
accord with the crystal structure analyses. The Mn(), Fe(),
Co(), Ni() and Cu() complexes are paramagnetic and mag-
netic moments of the bulk samples were obtained by Gouy
balance measurements, Table 3. The magnetic moment of the
Ni() complex is consistent with ferromagnetically coupled Ni
centres within the dimeric cations,32 those of the Cu() com-
plexes are as expected, and those of the Mn(), Fe() and Co()
complexes are indicative for high-spin species, all in keeping
with the deductions from the crystal structures.
νpy ϩ py*/cmϪ1
No. bound N
µ/µB
[MnCl2(L)]
[FeCl2(L)]
1602, 1572
1601, 1571
1607, 1568
1608, 1572
1605, 1567
1608, 1573
1609, 1581
1605, 1585, 1572
4
4
4
4
4
4
4
3
5.94
5.32
a
[CoCl(L)]2[CoCl4]
[CoCl(L)][BPh4]
[Ni2Cl2(L)2]Cl2
[CuCl(L)]Cl
[CuCl(L)][PF6]
[ZnCl2(L)]
4.70
3.21
1.93
1.96
a
a Not measured.
aqueous acetonitrile as the feed solvent. Strong peaks for the
molecular ion were observed for [CuCl(L)]ϩ, [CoCl(L)]ϩ
and [Ni2Cl2(L)2]2ϩ, along with peaks for the ions [M(L)]2ϩ and
[M(L) ϩ OAc]ϩ which arise from chloride dissociation
and chloride–acetate exhange,23 respectively. In contrast, the
positive-ion ESI-MS spectra of [MCl2(L)] (M = Mn, Fe and Zn)
Solution-state measurements
Electrospray ionisation mass spectra. Table 4 presents results
from positive-ion ESI-MS spectra for the complexes which were
introduced into the spectrometer using 1% acetic acid in 1 : 1
4228
J. Chem. Soc., Dalton Trans., 2002, 4224–4235

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Table 4 Major ions (m/z) observed by positive-ion electrospray ionisation mass spectroscopya
[MCl2(L) ϩ H]ϩ
[M(L)(OAc)]ϩ
[MCl(L)]nnϩ
[M(L)]2ϩ
Other ions
[MnCl2(L)]
[FeCl2(L)]
553 (1)
540 (8)
541 (100)
544 (38)
517 (3)
241 (1)
427c (100)
311 (65)
263 (15)
571b (10)
517 (22)
520 (100)
520 (75)
519 (30)
526 (100)
524 (100)
241 (52)
243 (70)
243 (100)
242 (100)
245 (10)
245 (60)
[CoCl(L)]2[CoCl4]
[CoCl(L)][BPh4]
[Ni2(µ-Cl2)(L)2]Cl2
[CuCl(L)]Cl
[CuCl(L)][PF6]
[ZnCl2(L)]
543 (10)
548 (42)
562 (5)
427c (100)
a Relative peak heights (%) are indicated in parentheses. Only the most intense peak for each ion is quoted; the correct isotope pattern for each ion
was observed. The feed solvent employed was 1% acetic acid in 1 : 1 water–acetonitrile. b Peak for [FeCl2(L) ϩ H3O]ϩ. c Peak for [HL]ϩ.
Table 5 Molar electrical conductivity values ( 10%) for the com-
plexes (∼1 mM) in dichloromethane
ΛM/S cm2 molϪ1
[MnCl2(L)]
[FeCl2(L)]
[CoCl(L)][BPh4]
[NiCl2(L)]
2.9
4.1
21.1
3.6
[CuCl2(L)]
[CuCl(L)][PF6]
[ZnCl2(L)]
4.5
24.1
3.8
[NBu4][PF6]
26.3
Fig. 7 View of [ZnCl2(L)] showing the 10% thermal ellipsoids at
294 K.
were much harder to obtain, requiring higher receiver plate
gains and higher cone voltages, and are dominated by peaks for
[M(L) ϩ OAc]ϩ and HLϩ along with weaker peaks for [MCl2(L)
ϩ H]ϩ from protonation of the complex. The results indicates
the Co, Ni and Cu complexes to dissolve as ions in the polar
acetonitrile–water solution, whereas the Mn, Fe, Zn com-
plexes do not thus requiring fragmentation or exchange of two
chlorides for acetate to produce ions.
Fig. 8 Vis-NIR spectra for the metal() chloride complexes of L in
dichloromethane.
an energy of 10Dq), which is the only spin-allowed transition
for the high-spin octahedral d6 Fe2ϩ ion.34 The electronic spec-
tra of [CoCl(L)][BPh4] and [CoCl(L)]2[CoCl4] are comparable
to those of the prototypal trigonal-bipyramidal Co() com-
plexes [CoBr(Me3tren)]Br and [CoCl(tren)]Cl.34–36 The spec-
trum of [CoCl(L)][BPh4] displays peaks at 20 410 cmϪ1, 15 625
cmϪ1 and a shoulder at 12 225 cmϪ1 attributable to the charac-
Electrical conductivity. Table 5 presents salient data from
measurements on dilute (∼1 mM) solutions of the complexes
in dichloromethane. The dichloride complexes are all non-
electrolytes, [MCl2(L)], which is noteworthy for the Ni() and
Cu() complexes since the crystal structures of these show
discrete [Ni2(µ-Cl)2(L)2]2ϩ or [CuCl(L)]ϩ cations, respectively,
and chloride anions.
4
4
4
teristic EЉ(P)
4AЈ2(F), AЈ2(P)
4AЈ2(F) and EЉ(F)
4AЈ2(F) transitions, respectively, of a high-spin tbp-d7 Co()
complex.34–36 The spectrum of [CoCl(L)]2[CoCl4] shows these
d–d bands overlaid with the diagnostic peaks at 15 015–14 370
4
cmϪ1 for the T1(P)
4A2 transition of the [CoCl4]2Ϫ anion.
The electronic spectrum of the Ni() complex in dichloro-
methane, [Ni(L)Cl2], shows three d–d bands at 8 550 cmϪ1 (ν1),
14 665 cmϪ1 (ν2) and 23 800 cmϪ1 (ν3—this band appears as a
shoulder to a higher energy ligand-centred transition). The
spectrum is diagnostic for a five-coordinate complex species
having a N3Cl2 donor set.31,32,37 The electronic spectrum of
[CuCl(L)][PF6] exhibits a peak at 10 750 cmϪ1 with a higher
energy shoulder at 13 700 cmϪ1, a pattern indicative for a
five-coordinate copper() centre with trigonal pyramidal
geometry.10,11,18,38 Likewise the X-band EPR spectrum in
dichloromethane at 77 K shows an ‘inverse’ axial pattern with
g⊥ = 2.235 (A⊥ = 112 G) and g|| ≈ 2.0 (A|| ≈ 86 G), characteristic
Electronic spectra. Data for the ligand and the complexes
recorded in dichloromethane solution are presented in Table 6.
Two intense ligand-centred bands at 38 900–37 600 and 31 750–
30 600 cmϪ1 are found in all electronic spectra, and are the
only peaks (with extinction coefficients >1 dm3 molϪ1 cmϪ1
)
observed for the Mn() and Zn() complexes, consistent with
the high-spin d5 and the d10 electron configurations, respec-
tively. The visible–NIR bands of the other metal complexes,
Fig. 8, are assigned to d–d transitions and give information
about the stereochemistry in solution. For [FeCl2(L)], the weak
band at 8 810 cmϪ1 is assigned to the 5Eg
5T2g transition (at
J. Chem. Soc., Dalton Trans., 2002, 4224–4235
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Table 6 Electronic absorption spectral data for the complexes in dichloromethane solution
λmax/nm (ε/dm3 molϪ1 cmϪ1
)
L
260 (14 250), 318 (7040)
265 (7650), 328 (4900)
261 (10 000), 327 (5100), 421 (950), 1135 (25)
263 (14 000), 327 (3380), 490 (190), 621 (130), 640 (150), 818
(35)
[MnCl2(L)]
[FeCl2(L)]
[CoCl(L)][BPh4]
[CoCl(L)]2[CoCl4]
266 (9600), 325 (3150), 488 (175), 635 (270), 666 (270), 696
(270), 805 (36)
[NiCl2(L)]
262 (10 350), 326 (3500), 420 (180), 682 (65), 1170 (68)
257 (17 700), 320 (8000), 770 (200), 1050 (130)
265 (13 800), 315 (5050), 730 (106), 930 (130)
267 (9650), 325 (3200)
[CuCl2(L)]
[CuCl(L)][PF6]
[ZnCl2(L)]
2
for a trigonal bipyramidal species with a dz ground-state. In
contrast, [CuCl2(L)] displays a peak at 12 990 cmϪ1 with a lower
energy shoulder at 9 525 cmϪ1 and the X-band EPR spectrum is
axial with g|| = 2.236 (A|| = 176 G) and g⊥ = 2.027, data all
consistent with a square pyramidal Cu() species.10,11,18,38
Variable-temperature 1H NMR spectroscopy. Of the com-
plexes, only [ZnCl2(L)] is diamagnetic. In the ambient tem-
perature spectrum the peak for the diastereotopic methylene
protons was noticeably broad and therefore a variable-
1
temperature H NMR study was undertaken, Fig. 9. In the
Scheme 1 A possible mechanism for exchange of the diastereotopic
methylene protons in the [ZnCl2(L)].
dissociation of one pyridyl leg and the amine from the zinc ion.
Scheme 1 depicts a possible mechanism. The energy barrier
for pyramidal inversion of dibenzylmethylamine is 28 3 kJ
molϪ1 41
, which provides an estimate for the lower limit to the
barrier for inversion in L, considerably smaller than that for
[ZnCl2(L)] and consistent with complex formation constraining
the pyramidal inversion of the amine.
Discussion
Solid-state structures
The crystal structures presented here are the most complete
set of first d-series metal() halide complexes for a particular
“tpa-like” ligand. Fig. 10 diagrammatically presents the key
Fig. 9 Variable-temperature 300 MHz 1H-NMR spectra for [ZnCl2-
(L)] in CDCl3.
298 K spectrum, an exchange-broadened singlet is observed at
δH 4.40 for the diastereotopic methylene protons. As the tem-
perature is lowered, the broad methylene peak coalesces and
then emerges as a pair of AB doublets at δH 4.61 and 4.15,
which sharpen in the lower temperature spectra. The coales-
cence temperature (T c) is 290 3 K, which corresponds to a
free energy barrier (∆G‡290 K) of 56 0.5 kJ molϪ1 for exchange
of the diastereotopic methylene protons.39 The pyridyl and aryl
proton peaks remain sharp in all spectra and are consistent with
an unbound PY* and two bound PY legs for the complex in
solution. Separate sets of peaks for L and the complex were
observed when excess free L was deliberately added, implicating
an intramolecular fluxional process in the exchange of the
diastereotopic methylene protons in the complex. This must
involve pyramidal inversion of the amine, which is typically a
low energy process40,41 but for [ZnCl2(L)] requires at a minimum
Fig. 10 Plot showing a comparison of the metal-to-ligand bond
distances in the metal() chloride complexes of L (the average M–Cl
and M–Npy distance is indicated).
Ϫ
x)ϩ
metal-to-ligand bond length data for these [MIIClx(L)](2
(x = 1, 2) complexes. For the Mn2ϩ–Co2ϩ complexes, where
high- or low-spin dn-electron configurations are possible, the
metal-to-ligand bond distances are all indicative for a high-spin
4230
J. Chem. Soc., Dalton Trans., 2002, 4224–4235

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Table 7 Coordination properties of the divalent metal–chloride complexes of L
Mn2ϩ
Fe2ϩ
Co2ϩ
Ni2ϩ
Cu2ϩ
Zn2ϩ
Coordination number
Coordination sphere
Stereochemistry
6
6
5
6
5
5
N4Cl2
Oh
0.97
N4Cl2
Oh
0.92
N4Cl
N4Cl2
Oh
0.83 (0.77)
N4Cl
N3Cl2
a
a
a
tbpb
spc
d-spd
Metal ion radiuse/Å
0.89 (0.81)
0.87 (0.79)
0.88 (0.82)
a Octahedral. b Trigonal bipyramidal. c Square pyramidal. d Trigonally-distorted square pyramidal. e Values for high-spin six-coordinate (and high-
spin five-coordinate) metal ions taken from ref. 43.
Table 8 Ligand field stabilisation energies (in Dq) for ideal five- and six-coordinate complexes
Square pyramidala
Trigonal bipyramidala
Octahedral
Strong-field
Weak-field
Strong-field
Weak-field
Strong-field
Weak-field
Ϫ8.0
Co2ϩ (d7)
Ni2ϩ (d8)
Cu2ϩ (d9)
Ϫ19.14
Ϫ18.28
Ϫ9.14
Ϫ9.14
Ϫ10.00
Ϫ13.34
Ϫ14.16
Ϫ5.45
Ϫ6.27
Ϫ18.0
Ϫ12.0
Ϫ7.09
Ϫ6.0
a Pyramid base in the xy plane.
complex, which is anticipated given the π-donor chloride
ancillary ligands and the weakening of the ligand field caused
by the steric interactions introduced by the dimethoxyphenyl
substitution of the PY* leg (which in all cases is the more
weakly bound leg). The general trend of decreasing metal-to-
ligand bond lengths from Mn2ϩ to Cu2ϩ tracks the increasing
effective nuclear charge. The apparent “dip” in the metal-to-
ligand bond lengths for the Co() complex compared to its
Fe() and Ni() neighbours simply reflects the dependence of
the metal ionic radius upon coordination number: the Co()
complex is five-coordinate whereas its neighbours are six-
coordinate, Table 7. Similarly the six-coordinate Ni() complex
is flanked by five-coordinate Co() and Cu() neighbours.
Ligand-field arguments suffice to rationalise the other devi-
ations from the general decline in metal-to-ligand bond lengths.
For example, the marked increase in the M–Npy* bond length
in the square pyramidal [CuCl(L)]ϩ ion corresponds to the
Solution behaviour
The ESI-MS spectral results for the [MIIClx(L)](2Ϫx)ϩ (x = 1, 2)
complexes strongly suggest that each gives the same species
in polar acetonitrile–water solution as found in its crystal
structure (see above). However, in dichloromethane, which has
a comparatively low dielectric constant (εr ≈ 8.9 at 25 ЊC),
separation into ions is less favourable, and the molar conduc-
tivity values reveal that a second chloride, when available,
coordinates to the metal centres. For the dichloride complexes
of the smaller metal ions, the electronic [for Ni() and Cu()],
EPR [for Cu()] and NMR [for Zn()] spectral data are indica-
tive of five-coordinate [MCl2(κ3N-L)] structures in dichloro-
methane solution, consistent with displacement of the bulky
and more weakly bound PY* leg by the second chloride ligand.
In contrast, [CuCl(κ4N-tpa)]ϩ in dichloromethane is unreactive
toward extra chloride ion.18
filling of the dz orbital (the static Jahn-Teller effect for the d9
2
Cu2ϩ ion). Likewise for the d10 Zn() complex where the full
d-subshell results in the comparatively longer coordination
bonds.
Conclusions
The structures and/or physicochemical properties of the transi-
tion metal complexes of L differ considerably from those of
their tpa-analogues. A single 6-aryl substituent to the tpa skel-
eton is sufficient to alter the chemistries of the resulting metal
complexes: the intramolecular steric interactions introduced by
the single dimethoxyphenyl-substituent prevent Npy* from
approaching close to a metal centre and, consequently, weaken
the ligand field of L compared to tpa thereby favouring high
spin complexes. For the smaller metal ions, the steric effect of
the dimethoxyphenyl substituent is exacerbated to the point
where there is a fine balance between bound and unbound PY*
legs — hemilability — in the Ni() and Cu() systems and no
evidence for binding of the PY* leg in the Zn() dichloride
complex. Moreover, binding of the PY* leg in the Ni() and
Cu() systems can be conveniently modulated by (the avail-
ability of ) the ancillary ligands and (changes to) the solvent.
Comparable hemilability in the Ni() and Cu() complexes of
alkyl-tpa derivatives is found only when the tpa-skeleton has
three 6-alkyl substituents. An advantage of mono(6-aryl) sub-
stitution of the tpa-skeleton is that the coordinative lability is
predicably restricted to the bulkier aryl-substituted leg (PY*) of
the Ni() and Cu() complexes, a useful feature which possibly
could be exploited in designs for new molecular devices based
upon, for example, pH- or redox-controlled switching of the
conformation and binding of the PY* leg.18,42
Table
7 summarises the coordination geometries and
environments found in the crystal structures and lists ionic radii
for the high-spin five- and six-coordinate metal ions. Six-
coordination is observed for the larger high-spin Mn2ϩ and
high-spin Fe2ϩ ions where the ionic radius of the six-coordinate
ion is greater than 0.9 Å, whereas five-coordination appears
preferred when the metal ion is smaller than this. The exception
is the Ni() complex — here the d8 Ni2ϩ ion is in a weak-field
environment and, although only a small component of the
overall bonding energy, the ligand field stabilisation energy
(LFSE: Ϫ12 Dq) favours the octahedral complex, Table 8.
From the LFSE values in Table 8 it appears that the d7 Co()
complex should adopt a square pyramidal geometry. However,
the listed LFSE values are for ideal geometries with Lapical
–
CoII–Lbasal angles of 90Њ, and if the Lapical–CoII–Lbasal angles
deviate by 15Њ from this, as they must for [CoCl(L)]ϩ due to the
constraints imposed by L, a trigonal bipyramidal geometry
becomes favoured.36 Also noteworthy is that only in [ZnCl2(L)]
is the PY* leg dangling, displaced by a second chloride — the
larger ligand-field splittings with π-acceptor pyridyl compared
to a π-donor chloride ligand favour coordination of the PY* leg
for all of the metal ions except the d10 Zn() ion (LFSE = 0 Dq).
Finally, although not one of the [MIIClx(L)](2Ϫx)ϩ series, the
structure of the [Cu4(L)4(CO3)2]4ϩ ion suggests the binding of
the PY* leg to Cu() becomes more finely balanced when the
ancillary ligand lies closer to pyridine in the spectrochemical
series.
Two final points: First, [Cu4(L)4(CO3)2]4ϩ is the first complex
species to exhibit both κ3N- and κ4N-bound tpa-related ligands
within the same molecule or ion. Second, the NMR spectra of
J. Chem. Soc., Dalton Trans., 2002, 4224–4235
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[ZnCl2(L)] exhibit fluxional behaviour attributable to pyramidal
inversion of the amine donor of L with ∆G‡(290 K) = 56 0.5 kJ
molϪ1; to our knowledge this is the first time this process has
been characterised in a transition metal complex of tpa or a
ligand based upon the tpa-skeleton.
evaporation until crystallisation commenced. The product
was collected by filtration. Additional product was obtained by
further concentration of the mother liquor and addition of
n-pentane to give 11.1 g (75%) of the product as white flakes,
mp 80–81.5 ЊC; m/z (EI-MS) 186 (Mϩ, 90%), 158 (50), 78 (100);
δH (CDCl3) 9.93 (s, 1H, CHO), 7.60–7.92 (3H, m, Py).
6-(2Ј,5Ј-Dimethoxyphenyl)-2-pyridylcarboxyaldehyde (A). 6-
Bromo-2-pyridylcarboxyaldehyde (1.67 g, 9.0 mmol) and Pd-
(PPh3)4 (0.50 g, 0.43 mmol) were dissolved in toluene (40 mL)
under a nitrogen atmosphere. Aqueous sodium carbonate solu-
tion (6 mL, 2 M) and 2,5-dimethoxyphenylboronic acid (1.86 g,
10 mmol) in methanol (10 mL) were added and the solution
heated at reflux for 8 h. After cooling, dichloromethane
(50 mL), an aqueous sodium carbonate solution (15 mL, 2 M)
and concentrated ammonia (2 mL) were added. The solution
was extracted with dichloromethane, the organic layer separ-
ated and dried over magnesium sulfate and the solvent removed
in vacuo. The residue was separated by flash chromatography on
silica (dichloromethane–toluene 5 : 1 eluent). The second frac-
tion was collected and recrystallisation from dichloromethane–
hexane gave a white crystalline solid (1.80 g, 82%), mp 55–56
ЊC; m/z (EI-MS) 243 (Mϩ, 95%), 225 (100), 210 (75), 197 (70);
δH (CDCl3) 10.16 (s, 1H, CHO), 8.12 (1H, dd, Py), 7.89 (2H, m,
Py), 7.52 (1H, s, Ph), 6.98 (2H, m, Ph), 3.86 (3H, s, CH3O), 3.83
(3H, s, CH3O); δC (CDCl3) 194.57, 156.87, 154.58, 153.11,
152.00, 137.18, 129.97, 129.00, 120.07, 116.64, 116.52, 113.65,
56.87, 56.40; ν/cmϪ1 (KBr disc) 2954w, 2828w, 1708s, 1584s,
1506s, 1463s, 1411s, 1353m, 1288m, 1257s, 1223s, 1185s.
6-(2Ј,5Ј-Dimethoxyphenyl)-2-pyridylmethanol. 6-(2Ј,5Ј-Di-
methoxyphenyl)-2-pyridylcarboxaldehyde (0.20 g, 0.82 mmol)
and sodium borohydride (0.04 g, 1.1 mmol) were heated at
reflux in methanol (20 mL) for 15 min. The solvent was
removed in vacuo, water (10 mL) and 1 M sodium hydroxide
(3 drops) were added and the reaction mixture extracted with
dichloromethane. Removal of the solvent in vacuo gave a
colourless oil (0.19 g, 95%). m/z (EI-MS) 245 (Mϩ); δH [(CD3)2-
SO] 7.84 (1H, t, Py), 7.74 (1H, d, Py), 7.43 (1H, d, Py), 7.36
(1H, d, Ph), 7.12 (1H, d, Py), 7.00 (1H, dd, Ph), 5.45 (1H, t,
OH), 4.65 (2H, d, CH2), 3.81 (3H, s, CH3O), 3.79 (3H, s,
CH3O).
6-(2Ј,5Ј-Dimethoxyphenyl)-2-pyridylmethylbromide (B). 6-
(2Ј,5Ј-Dimethoxyphenyl)-2-pyridylmethanol (0.66 g, 2.69
mmol) was dissolved in dichloromethane (50 mL), excess phos-
phorous tribromide (3 mL) added dropwise and the solution
stirred for 3 h at room temperature (during this time a white
precipitate formed) and then heated at reflux for 30 min. After
cooling, the mixture was washed with brine (50 mL), extracted
into dichloromethane, dried over magnesium sulfate and the
solvent removed in vacuo. Recrystallisation from diethyl ether
afforded a white crystalline solid (0.65 g, 78%), mp 104–105ЊC;
m/z (EI-MS) 307 (Mϩ, 10%), 278 (5), 228 (100), 198 (35);
δH (CDCl3) 7.77 (1H, d, Py), 7.72 (1H, t, Py), 7.41 (1H, d, Ph),
7.39 (1H, s, Py), 6.93 (2H, m, Ph), 4.63 (2H, s, CH2), 3.84 (3H, s,
CH3O), 3.81 (3H, s, CH3O).
Experimental
Elemental analyses for C, H and N were determined by the
Australian National University Microanalytical Unit. Electro-
spray mass spectra (ESI-MS) were acquired on a VG Quattro
mass spectrometer employing capillary voltages of 3–5 kV
and cone voltages of 5–60 V. The solvent system was 50 : 50
acetonitrile–water, depending on the sample, with or without
1% acetic acid added. 1H and 13C{1H} NMR spectra were
recorded on a Bruker AC 300F (300 MHz) spectrometer.
Electronic spectra of solutions of the complexes in 1 cm
quartz cuvettes were recorded between 240 and 2000 nm on a
CARY 5 spectrometer in the dual beam mode. The solutions of
[FeCl2(L)] were prepared under nitrogen in a M. Braun glove-
box. X-Band EPR spectra of both frozen solution (at 77 K;
liquid nitrogen dewar) were recorded using a Bruker EMX 10
EPR spectrometer. Electrical molar conductivity measurements
were made using an in-house built conductivity bridge and a
YSI model 3403 conductivity cell thermostated at 25 ЊC. The
cell constant (K) was determined with a standard aqueous solu-
tion of KCl (0.001 M). The molar conductivity, ΛM, of a sample
solution was determined from ΛM = 1000K/cm, where cm is the
molar concentration of the complex (ca. 1 mM).
Preparation of [6-(2Ј,5Ј-dimethoxyphenyl)-2-pyridylmethyl)]-
bis(2-pyridylmethyl)amine (L)
I. Precursors. 2,5-Dimethoxyphenylboronic acid. n-Butyl-
lithium [14.5 mL (2.5 M in hexane), 36 mmol] was added to 1,4-
dimethoxybenzene (5.4 g, 36 mmol) in anhydrous diethyl ether
(100 mL). After 2 h the mixture was added dropwise to a solu-
tion of tri(iso-propyl)borate (16.9 mL, 72 mmol) in dry diethyl
ether (10 mL) cooled to Ϫ70 ЊC. After addition, the mixture
was stirred for 30 min at Ϫ70 ЊC, then warmed to ambient
temperature and stirred for 16 h. Diethyl ether (100 mL)
and 10% aqueous HCl (100 mL) were added and the organic
layer separated, washed with water (40 mL) and dried over mag-
nesium sulfate. The solvent was removed and the product(s)
separated by flash chromatography on silica (hexane–
dichloromethane 3 : 1 eluent). The second major fraction was
collected and recrystallisation from hexane–dichloromethane
gave a white crystalline solid (4.6 g, 72%), mp 91–93 ЊC (Found:
C, 52.38; H, 6.22. BC8H11O4 requires C, 52.80; H, 6.09%);
m/z (EI-MS) 182 (Mϩ); δH (CDCl3) 7.38 (1H, br d, Ph), 6.97
(1H, dd, Ph), 6.86 (1H, d, Ph), 5.89 (2H, s, OH), 3.88 (3H, s,
CH3O), 3.81 (3H, s, CH3O).
6-Bromo-2-pyridylcarboxyaldehyde. The method of Chuang
et al.14 was adapted as follows. To a slurry of 2,6-dibromo-
pyridine (19.0 g, 0.08 mol) in anhydrous diethyl ether (150 mL),
cooled to below Ϫ60 ЊC using an acetone-slush cooling bath,
n-butyllithium [32 mL (2.5 M in hexane), 0.08 mol] was added
at a rate such that the temperature did not exceed Ϫ60 ЊC. After
addition was complete, the reaction mixture was allowed to
warm to Ϫ40 ЊC for 15 min; a clear yellow solution resulted.
This solution was cooled to Ϫ80 ЊC and a solution of dimethyl-
formamide (6.4 g, 0.088 mol) in dry diethyl ether (10 mL) was
added keeping the temperature below Ϫ70 ЊC. Stirring was con-
tinued for 2 h at this temperature, producing a grey precipitate.
The mixture was then allowed to warm to Ϫ10 ЊC and hydro-
lysed with 6 M hydrochloric acid (30 mL). The aqueous phase
was separated and extracted with diethyl ether. The extracts and
the original diethyl ether phase were combined, washed with
water, dried over magnesium sulfate and concentrated by rotary
II. Method 1. 6-(2Ј,5Ј-Dimethoxyphenyl)-2-pyridylmethyl-
bromide (0.9 g, 3 mmol), bis(2-pyridylmethyl)amine (0.7 g,
3 mmol) and triethylamine (5 mmol) were stirred in tetrahydro-
furan (50 mL) at ambient temperature for 5 d. The solvent
was removed in vacuo and the product separated by flash
chromatography on silica. Initially, dichloromethane was used
as the eluent to remove unreacted 6-(2Ј,5Ј-dimethoxyphenyl)-
2-pyridylmethylbromide. Then a 95 : 5 dichloromethane–
methanol mixture was used to elute a yellow band. This band
was collected and the solvent removed to leave a yellow oil
which solidified upon drying in vacuo to give a cream powder
(0.95 g, 74%), mp 128–129 ЊC (Found: C, 71.96; H, 6.12; N,
12.93. C26H26N4O2ؒ0.5H2O requires C, 71.70; H, 6.25; N,
12.86%); m/z (EI-MS) 426 (Mϩ); λmax/nm (CH2Cl2) 260 (ε/dm3
molϪ1 cmϪ1 14 250), 318 (7040); δH (CDCl3) 8.53 (2H, d, Py),
4232
J. Chem. Soc., Dalton Trans., 2002, 4224–4235

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7.70 (6H, m, Py), 7.51 (1H, d, Py), 7.41 (1H, d, Ph), 7.13 (2H, q,
Py), 6.99 (2H, m, Ph), 3.96 (2H, s, CH2), 3.94 (4H, s, CH2), 3.81
(3H, s, CH3O), 3.78 (3H, s, CH3O).
72%). Recrystallisation from a chloroform–heptane–diethyl
ether mixture gave crystallographic quality crystals (72 mg,
90%), mp 132 ЊC (decomp.) (Found: C, 50.82; H, 5.43; N,
8.95. C26H26Cl2N4NiO2ؒ3H2O requires C, 51.18; H, 5.29;
N, 9.18%).
III. Method 2. Sodium cyanoborohydride (0.40 g, 6.35 mmol)
was added slowly to an ice-bath cooled mixture of 6-(2Ј,5Ј-
dimethoxyphenyl)-2-pyridylcarboxaldehyde (1.85 g, 7.5 mmol),
bis(2-pyridylmethyl)amine (1.33 g, 6.75 mmol) and acetic acid
(0.9 g, 15 mmol) in methanol (50 mL). The resulting solution
was stirred at room temperature for 3 d, then acidified by
addition of concentrated hydrochloric acid to quench the
excess sodium cyanoborohydride. The mixture was evaporated
under reduced pressure until almost dry, then water was added
and the resulting solution made alkaline by addition of
solid sodium carbonate. A dark brown oil separated and was
extracted with dichloromethane. The extracts were dried over
sodium sulfate and the solvent removed in vacuo to afford a
brownish oily product (2.7 g, ∼100%). Recrystallisation from
petroleum ether (bp 60–80 ЊC)–dichloromethane yielded a
white solid (1.96 g, 73%), mp 128–130 ЊC. The 1H NMR
spectral data were identical to those of the ligand prepared by
method 1.
[CuCl(L)]Cl. L (102 mg, 0.24 mmol) and CuCl2ؒ2H2O
(35 mg, 0.2 mmol) were dissolved in methanol (3 mL). After
10 min diethyl ether (15 mL) was added to dilute the solution. A
light blue microcrystalline solid formed which was collected by
filtration and washed with diethyl ether. The solid was recrystal-
lised from methanol–diethyl ether (115 mg, 95%), mp 194 ЊC
(decomp.) (Found: C, 55.19; H, 4.64; N, 9.91. C26H26Cl2CuN4O2
requires C, 55.67; H, 4.67; N, 9.99%); X-band EPR (CH2Cl2, 77
K) g|| 2.236 (A|| 176 G), g⊥ 2.027.
[CuCl(L)][PF6]. To [Cu(L)Cl]Cl (28 mg, 0.05 mmol) in
methanol (5 mL) was added K[PF6] (9.5 mg, 0.052 mmol)
in methanol (5 mL). The pale blue solid which immediately
precipitated was collected and recrystallised from dichloro-
methane–methanol to yield the product, a blue microcrystalline
solid (28.4 mg, 85%), mp 238 ЊC (decomp.) (Found: C, 46.19; H,
3.86; N, 8.30. C26H26N4ClCuF6O2P requires C, 46.58; H, 3.91;
N, 8.36%); X-band EPR (CH2Cl2, 77 K) g⊥ 2.235 (A|| 112 G), g||
2.095 (A|| 86 G);
Preparation of metal complexes
[MnCl2(L)]. L (102 mg, 0.24 mmol) and MnCl2ؒ4H2O
(40 mg, 0.2 mmol) were dissolved in methanol (3 mL) and
stirred for 10 min. Diethyl ether (15 mL) was added to the
solution. A white solid formed which was collected by filtration
and washed with diethyl ether. The solid was recrystallised from
methanol–diethyl ether (90 mg, 75%), mp 180 ЊC (decomp.)
(Found: C, 55.49; H, 5.11; N, 9.59. C26H26Cl2MnN4O2ؒCH3OH
requires C, 55.48; H, 5.18; N, 9.59%).
[Cu4(L)4(CO3)2][BF4]4. Under nitrogen in a Schlenk flask, L
(43 mg, 0.1 mmol) was added to [Cu(CH3CN)4][BF4] (32 mg,
0.1 mmol) in deoxygenated acetonitrile (10 mL). A clear yellow
solution of [Cu(L)][BF4]18 formed immediately. The yellow
solution was cooled to Ϫ40 ЊC and was bubbled with dry
oxygen for 5 min. The solution turned green and was warmed
to room temperature. At this point no attempt was made to
exclude air from the green solution, a portion of which (4 mL)
was layered with diethyl ether (16 mL) and set aside to crystal-
lise. Several green wedge-shaped crystals were obtained after
2 d. The formulation of the compound rests solely on the X-ray
crystal structure analysis.
[FeCl2(L)]. Under a nitrogen atmosphere in a Schlenk flask,
L (100 mg, 0.235 mmol) and FeCl2ؒ6H2O (29 mg, 0.23 mmol)
were dissolved in deoxygenated methanol (3 mL). The Schlenk
flask was connected to a second containing deoxygenated
diethyl ether (20 mL) and set aside. After 2 d, clear yellow crys-
tals of the product had formed and were collected in a nitrogen-
filled glovebox (60 mg, 46%), mp 165 ЊC (decomp.) (Found: C,
55.35; H, 4.91; N, 9.25. C26H26Cl2FeN4O2ؒCH3OH requires C,
55.38; H, 5.12; N, 9.57%).
[ZnCl2(L)]. L (46 mg, 0.11 mmol) and ZnCl2 (15 mg, 0.11
mmol) were dissolved in methanol (3 mL). After 10 min diethyl
ether (15 mL) was added and the white microcrystalline solid
which formed was filtered off and washed with diethyl ether.
Recrystallisation from methanol–diethyl ether afforded colour-
less crystals of the product (50 mg, 81%), mp 226 ЊC (decomp.)
(Found: C, 54.47; H, 4.56; N, 9.77. C26H26Cl2N4O2Znؒ0.5H2O
requires C, 54.61; H, 4.76; N, 9.80%); δH (300 MHz) (CDCl3,
298 K) 9.30 (dt, 2 H, H6py), 7.85 (td, 2 H, H4py), 7.82 (dd, 1 H,
H3/5py*), 7.61 (d, 1 H, H4py*), 7.48 (ddd, 2 H, H5py), 7.42 (td, 1 H,
H6Јpy*), 7.28 (dt, 2 H, H3py), 6.99 (dd, 1 H, H5/3py*), 6.98 (m,
2 H, H3Ј,4Јpy*), 4.40 (br s, 4 H, CH2), 3.94 (s, 2 H, CH2), 3.86
(s, 3H, CH3O), 3.84 (s, 3H, CH3O).
[CoCl(L)]2[CoCl4]. L (102 mg, 0.24 mmol) and CoCl2ؒ6H2O
(48 mg, 0.2 mmol) were dissolved in methanol (3 mL) and
stirred for 15 min. Diethyl ether (15 mL) was added to dilute the
solution. The light green powder that formed was filtered off
and washed with diethyl ether (80 mg, 72%). This was recrystal-
lised with methanol and diethyl ether to afford a pale green
microcrystalline solid (63 mg, 88%), mp 190 ЊC (decomp.)
(Found: C, 49.07; H, 4.50; N, 8.22 C52H52N8Cl6Co3O4ؒ2CH3OH
requires C 49.46; H 4.73; N, 8.47%).
[CoCl(L)][BPh4]. Excess Na[BPh4] (50 mg) in methanol
(5 mL) was added to [CoCl(L)]2[CoCl4] (50 mg, 0.04 mmol) in
methanol (5 mL). A grey solid precipitated. This was collected
by filtration, washed with methanol and then dissolved in
dichloromethane (2 mL). Methanol (5 mL) was slowly added
and the resulting pale green solution was set aside to slowly
evaporate for 3 d. Green hexagonal crystals formed that
were suitable for X-ray crystallographic analysis (24 mg,
73%), mp 190 ЊC (decomp.) (Found: C, 70.25; H, 5.40; N,
6.63. C49H45N4BClCoO2ؒH2O requires C, 69.98; H, 5.64;
N, 6.53%).
X-Ray crystallography
Relevant crystal and refinement data are collected in Table 9.
For [Ni2(µ-Cl)2(L)2]Cl2ؒMeOHؒ5H2O one of the lattice chloride
counter ions is disordered with the lattice waters. The asym-
metric unit contains one half of the dimeric cation (one Ni),
and in the lattice half a methanol and half a chloride which are
well ordered, along with the disordered half a chloride which
was refined distributed over three sites with 2.5 water oxygens.
CCDC reference numbers 190789–190796.
See http://www.rsc.org/suppdata/dt/b2/b207406k/ for crystal-
lographic data in CIF or other electronic format.
[Ni2Cl2(L)2]Cl2. L (102 mg, 0.24 mmol) and NiCl2ؒ6H2O
(90 mg, 0.2 mmol) were dissolved in methanol (3 mL) and
stirred for 10 min. Diethyl ether (15 mL) was added to dilute
the solution. A light green microcrystalline solid formed, which
was filtered off and washed with diethyl ether (80 mg,
Acknowledgements
This research was funded from an UNSW Vice-Chancellor’s
Goldstar Award.
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