4′-(o-toluyl)-2,2′ : 6′,2″-terpyridine: Synthesis, bromination, complexation, and X-ray crystallographic characterization

Article abstract of DOI:10.1080/00958972.2010.546397

The ligands 4′-(o-toluyl)-2,2′ : 6′,2″-terpyridine and 4′-[2-(bromomethyl)phenyl]-2,2′ : 6′,2″-terpyridine were synthesized. X-ray crystallographic studies have been performed on complexes that were prepared using these ligands. In one case, a halogen exc

Full text of DOI:10.1080/00958972.2010.546397

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4′-(o-Toluyl)-2,2′ꢀ:ꢀ6′,2′′-terpyridine:
synthesis, bromination, complexation,
and X-ray crystallographic
characterization
Paul A. Thornley a , Jocelyn C. Starkey a , Ramin Zibaseresht a ,
Matthew I.J. Polson a , Jan L. Wikaira a & Richard M. Hartshorn a
a Department of Chemistry , University of Canterbury , Private
Bag 4800 , Christchurch , New Zealand
Published online: 17 Jan 2011.
To cite this article: Paul A. Thornley , Jocelyn C. Starkey , Ramin Zibaseresht , Matthew I.J.
Polson , Jan L. Wikaira & Richard M. Hartshorn (2011) 4′-(o-Toluyl)-2,2′ꢀ:ꢀ6′,2′′-terpyridine:
synthesis, bromination, complexation, and X-ray crystallographic characterization, Journal of
Coordination Chemistry, 64:1, 145-158, DOI: 10.1080/00958972.2010.546397
To link to this article: http://dx.doi.org/10.1080/00958972.2010.546397
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Journal of Coordination Chemistry
Vol. 64, No. 1, 10 January 2011, 145–158
49-(o-Toluyl)-2,29 : 69,299 -terpyridine: synthesis, bromination,
complexation, and X-ray crystallographic characterization
PAUL A. THORNLEY, JOCELYN C. STARKEY, RAMIN ZIBASERESHT,
MATTHEW I.J. POLSON, JAN L. WIKAIRA and RICHARD M. HARTSHORN*
Department of Chemistry, University of Canterbury, Private Bag 4800,
Christchurch, New Zealand
(Received 12 August 2010; in final form 3 December 2010)
The ligands 40-(o-toluyl)-2,20 : 60,200-terpyridine and 40-[2-(bromomethyl)phenyl]-2,20 : 60,200-
terpyridine were synthesized. X-ray crystallographic studies have been performed on complexes
that were prepared using these ligands. In one case, a halogen exchange reaction occurs,
resulting in a dinuclear copper complex in which the bromine in the terpyridine derivative is
replaced by chlorine and is ultimately found as a disordered bridging bromide.
Keywords: Terpyridine; Crystallography; Halogen exchange; Cobalt; Copper
1. Introduction
Morgan and Burstall [1] were the first to isolate terpyridine (tpy) in the 1930s. Since this
discovery, terpyridines have been widely studied. The extent of this study can be
attributed to the wide, varied, and practical applications [2] of terpyridines. Some of
these applications include photovoltaic cells [3], electronic memory storage [4], light
emitting diodes [5], and plastic solar cells [6]. Many of these applications depend on the
ability to tune the electronic and spectroscopic properties of tpy complexes, and this can
be achieved by preparing molecules that differ in the position(s) and electron donating/
withdrawing characteristics of the substituents [7].
There are many positions where the tpy ligand can be substituted, and these
substituents are usually introduced as part of tpy precursors [8]. By far, the most
common site of substitution is the 40 position of the central pyridine ring, principally
because of the ease of synthesis of the substituted terpyridine from the related aldehyde.
In these terpyridine molecules, the substituent is directed away from the meridional
coordination site of the ligand, and in the particular case of the p-toluylterpyridine (ttp)
ligand, even the methyl group is remote from the metal binding site. Numerous
complexes of ttp have been synthesized [9].
In this article, we investigate the synthesis and characterization of terpyridine
ligands that are a step towards enabling a 40 substituent to interact with the coordinated
*Corresponding author. Email: richard.hartshorn@canterbury.ac.nz
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2011 Taylor & Francis
DOI: 10.1080/00958972.2010.546397

146
P.A. Thornley et al.
metal ion. Thus we describe the preparation and X-ray crystallographic characteriza-
tion of 40-(o-toluyl)-2,20 : 60,200-terpyridine (ottp), and bromination to give its ꢀ-bromo
derivative, 40-[2-(bromomethyl)phenyl]-2,20 : 60,200-terpyridine. We also report the results
of several crystallographic studies of complexes that resulted from attempts to
coordinate these ligands to copper(II) and cobalt(II).
2. Experimental
All reagents and solvents were of reagent grade or better and were used as received
unless otherwise stated. Carbon tetrachloride (CCl4) (ꢀ500 mL) was stored over
phosphorus pentoxide (P2O5) for initial drying for at least 4 days and refluxed under N2
for 4 h before distillation. All deuterated NMR solvents were supplied by Cambridge
Isotope Laboratories.
1H, COSY, NOESY, and HSQC experiments were recorded on a Varian INOVA 500
spectrometer at 23ꢁC, operating at 500 MHz. The INOVA was equipped with a variable
temperature, inverse-detection 5 mm probe or a triple-resonance indirect detection PFG
probe. The 13C-NMR spectra were recorded on either a Varian UNITY 300 NMR
spectrometer equipped with a variable temperature direct broadband 5 mm probe, at
23ꢁC, operating at 75 MHz or on a Varian INOVA 500 spectrometer at 23ꢁC, operating
at 125 MHz, using a 5 mm variable temperature switchable PFG probe. Chemical shifts
are expressed in parts per million (ppm) on the ꢁ scale and were referenced to the
appropriate solvent peaks: CDCl3 referenced to CHCl3 at ꢁH 7.25 (1H) and CHCl3 at ꢁC
77.0 (13C); CD3OD referenced to CHD2OD at ꢁH 3.31 (1H) and CD3OD at ꢁC 49.3
(13C); and DMSO-d6 referenced to CD3(CHD2)SO at ꢁH 2.50 (1H) and (CD3)2SO at ꢁC
39.6 (13C). The peaks are described as singlets (s), doublets (d), triplets (t), or
multiplets (m).
2.1. Ligand synthesis
2.1.1. 40-(o-Toluyl)-2,20 : 60,200-terpyridine (ottp) (1). o-Tolualdehyde (2.4 g, 20 mmol)
was added to stirred propane-2-ol (100 mL). To this solution, 2-acetylpyridine (4.84 g,
40 mmol), KOH pellets (3.08 g, 40 mmol), and concentrated ammonia solution (58 mL,
50 mmol) were added. The solution was then heated at reflux for 4 h during which a
white precipitate was formed. The solution was cooled to room temperature and filtered
under vacuum. The ppt was washed with 50% ethanol and then recrystallized from
ethanol.
1
Yield ¼ 3.54 g (51.2%). M.p. (170–173ꢁC). H-NMR (500 MHz CDCl3): ꢁ ¼ 8.72 (d,
0
2H, H6,600), 8.71 (d, 2H, H3,300), 8.49 (s, 2H, H3 ,50), 7.90 (t, 2H, H4,4 ) 7.30–7.36 (m, 6H,
00
00
H
5,5 ,toluyl), and 2.38 (s, 3H, CH3). 13C-NMR (75 MHz, CDCl3) 156.5, 155.6, 152.2,
149.4, 139.9, 137.1, 135.4, 130.7, 129.7, 128.5, 126.2, 124.1, 121.9, 121.6, 20.7 (CH3).
MS(ES) m/z: 324.1383 ([M þ Hþ], 100%).
2.1.2. 40-[2-(Bromomethyl)phenyl]-2,20 : 60,200-terpyridine (2). CCl4 (50 mL) was
transferred from the bulk sample using a dry syringe under N2. It was added to a
dry three-necked round bottom flask that had been flushed with N2. While stirring

Terpyridine complexes
147
under N2, 40-(o-toluyl)-2,20 : 60,200-terpyridine (0.84 g, 2.6 mmol), purified N-bromosuc-
cinimide (NBS) [10] (0.46 g, 2.6 mmol), and a catalytic amount of purified dibenzoyl
peroxide [10] were added. The solution was irradiated with a tungsten lamp while being
heated at reflux, under N2, for 4 h. The solution was cooled to room temperature and
filtered. The filtrate contained the crude 40-[2-(bromomethyl)phenyl]-2,20 : 60,200-terpyr-
idine. Remaining CCl4 was removed under vacuum. The crude product could be
recrystallized from a 2 : 1 mixture of EtOH and acetone, but remained contaminated
with starting material. Further attempts to purify the brominated material resulted in
lower yields due to decomposition of the product. The pale yellow ppt was filtered and
dried under vacuum. The ppt was stored in the dark, in an airtight container.
1
Yield ¼ 2.60 g (64%); m.p. (138–140ꢁC); H-NMR (500 MHz CDCl3): ꢁ ¼ 8.72 (d,
2H), 8.71 (d, 2H), 8.58 (s, 2H), 7.91 (t, 2H), 7.58 (d, 1H), 7.35–7.44 (m, 5H), and 4.45 (s,
2H, CH2Br). 13C-NMR (75 MHz CDCl3) 156.2, 155.8, 150.5, 149.5, 140.1, 137.3, 135.3,
131.2, 130.4, 129.2, 129.0, 124.2, 121.8, 121.7, 31.8 (CH2Br). MS(ES) m/z: 402.0603,
403.0625 ([M þ Hþ]).
2.2. Crystalline samples of metal complexes
2.2.1. [Co(ottp)2]Cl2 2.25CH3OH (3). Cobalt(II) chloride (30.7 mg, 1.29 ꢂ 10ꢃ4 mol)
.
was dissolved in methanol (5 mL) and added to a solution of 40-(o-toluyl)-2,20 : 60,200-
terpyridine (83.4 mg, 2.58 ꢂ 10ꢃ4 mol) in CHCl3 (2 mL). The resulting solution turned
red/brown. The solution was exposed to diethyl ether vapor and the resulting ethanol/
CHCl3 solution formed a small number of red/brown cubic crystals after 2 days.
2.2.2. Cu(ottp)Cl2 CH3OH (4). Copper(II) chloride (11.3 mg, 6.648 ꢂ 10ꢃ4 mol) was
.
dissolved in methanol (5 mL) and added to a solution of 40-(o-toluyl)-2,20 : 60,200-
terpyridine (21.5 mg, 6.648 ꢂ 10ꢃ4 mol) in CHCl3 (2 mL). The resulting solution turned
blue. Vapor diffusion of diethyl ether into the ethanol/CHCl3 solution through a small
hole resulted in the formation of a small number of blue cubic crystals after a week.
2.2.3. [(Cl-ottp)Cu(l-Cl)(l-Br)Cu(Cl-ottp)][PF6]2 (5). Copper(II) chloride (15.6 mg,
9.15 ꢂ 10ꢃ5 mol) was dissolved in water (5 mL) and added to a solution of 40-[2-
(bromomethyl)phenyl]-2,20 : 60,200-terpyridine (36.8 mg, 9.15 ꢂ 10ꢃ5 mol) dissolved in
ethanol (5 mL). The resulting solution turned blue/green. Two drops of a saturated
methanolic ammonium hexafluorophosphate solution were added, resulting in a pale
blue/green precipitate. The solution was filtered and the precipitate washed with water
and CH2Cl2. The precipitate was dissolved in CH3CN (1 mL) and vapor diffusion of
petroleum ether into the CH3CN solution resulted in formation of a few blue/green
needle-like crystals over 1 week.
2.3. X-ray structure analysis
Crystals were mounted on a glass fiber using perfluorinated oil. Data were collected at
low temperature using an APEX II CCD area detector. The crystals were mounted
37.5 mm from the detector and irradiated with graphite monochromated Mo-Kꢀ

148
P.A. Thornley et al.
˚
(ꢂ ¼ 0.71073 A) radiation. The data reduction was performed using SAINTPLUS [11].
Intensities were corrected for Lorentzian polarization effects and for absorption effects
using multi-scan methods [12]. Space groups were determined from systematic absences
and checked for higher symmetry (table 1). Structures were solved by direct methods
using SHELXS-97 [13] and refined with full-matrix least squares on F2 using SHELXL-
97 [13] or with SHELXTL [14]. All non-hydrogen atoms were refined anisotropically,
unless specified otherwise. Hydrogen atoms were placed at ideal positions and refined
with a riding model (Supplementary material).
3. Results and discussion
3.1. Ligand synthesis and structural study
The preparation of 40-(o-toluyl)-2,20 : 060,200-terpyridine (1) was straightforward using the
1
method described. Complete assignments of H and 13C-NMR spectra were made and
were consistent with values given in the literature [15]. The melting point range
measured for the compound was somewhat higher than that reported (170–173ꢁC, cf
158ꢁC), but has proven reproducible over numerous preparations. The ottp ligand (1)
crystallizes in the monoclinic space group C2/c. The molecule (figure 1) has a very
ꢁ
˚
slight, 0.051 A, deviation from planarity, with the planar toluyl group twisted 78.4 with
respect to this plane. This is slightly larger than reported in the Pt complex of the same
˚
ligand [15]. The methyl group is slightly, 0.4 A, out of the plane of the toluyl ring.
The radical bromination to form 40-[2-(bromomethyl)phenyl]-2,20 : 60,200-terpyridine
was performed in carbon tetrachloride. The crude product of this reaction was used
without purification in complexation reactions because of its tendency to decompose.
The yields from this reaction were sometimes lower (in 30–40% range, by NMR), but
that mixture can be resubmitted to the reaction conditions in order to achieve better
outcomes. The crude 40-[2-(bromomethyl)phenyl]-2,20 : 60,200-terpyridine was character-
ized using 1H-NMR, COSY, HSQC, and 13C-NMR spectroscopy. The 1H-NMR
spectrum clearly shows a new peak, in comparison to the 1H-NMR spectrum for 40-(o-
toluyl)-2,20 : 60,200-terpyridine, at 4.45 ppm that can be assigned to the brominated
methylene group. There is also a small peak at 2.30 ppm in the spectrum which can be
assigned to the o-toluyl-methyl group of unreacted 40-(o-toluyl)-2,20 : 60,200-terpyridine.
The mass spectrum of 2 (figure 2) shows significant m/z peaks at 402.0603 and at
404.0605, which are in good agreement with the theoretical isotope distribution. This
two peak set, two units apart, is typical of mass spectra for molecules containing a
single bromine atom.
3.2. Structural studies of complexes
Crystals of the complexes have been prepared as described and then structurally
characterized. The small amounts isolated precluded full characterization of the solid
material, so that the following results are of necessity limited to the single crystal that
was examined in each case. However, with the exception of the halogen exchange
reaction observed for 5, the fact that such complexes are formed is unsurprising.

Terpyridine complexes
149

150
P.A. Thornley et al.
Figure 1. Molecular structure of 40-(o-toluyl)-2,20 : 60,200-terpyridine (1) with thermal ellipsoids at 30%
probability.
1: TOF MS ES+
404.05
402.06
3.94e12
100
%
403.06
405.06
406.06
0
1: TOF MS ES+
2.54e5
402.06
404.06
100
%
403.06
405.06
405.23
406.06
396 398 400 402 404 406 408 410 412 414 416 418 420 422
0
m/z
386 388
390
392 394
424 426
Figure 2. 40-[2-(Bromomethyl)phenyl]-2,20 : 60,200-terpyridine mass spectrum (bottom) and calculated isotope
pattern (top).
3.2.1. [Co(ottp)2]Cl2 2.25CH3OH. Two 40-(o-toluyl)-2,20 : 60,200-terpyridine ligands are
.
bound by cobalt, forming the distorted octahedral complex shown in figure 3. The
crystal system is monoclinic and the space group P21/n. As a consequence of
geometrical constraints enforced by the ligand structure, the two central pyridine

Terpyridine complexes
151
Figure 3. The molecular diagram of the bis[40-(o-toluyl)-2,20 : 60,200-terpyridine] cobalt complex (3) with
thermal ellipsoids at 30% probability.
˚
˚
nitrogen–cobalt bond lengths of 1.865(5) A (N2–Co1) and 1.946(5) A (N5–Co1) are
significantly shorter than the four outer pyridine nitrogen–cobalt bond lengths,
˚
1.999(5)–2.175(5) A. One of the terpyridine units sits further from the cobalt,
˚
approximately 0.15 A, than the other terpyridine unit. One methanol site containing
O101 is present at ¼ occupancy.
The presence of two chloride ions in the X-ray structure implies that it is a cobalt(II)
complex. Yu et al. [16] describe two bis(terpyridine) cobalt complexes in which the
cobalt is in either the 2þ or 3þ oxidation state, red and orange, respectively. Table 2
lists the Co–N bond lengths and the observed crystal colors for some cobalt terpyridine
complexes with cobalt in a variety of oxidation and spin states and includes data from
the complex [Co(ottp)2]Cl2 ꢅ 2.25CH3OH. The color and bond length comparisons are
consistent with a cobalt(II) oxidation state which agrees with the charge balance from
the X-ray structure.
The complex is chiral, due to the relative positioning of the methyl substituents on the
terpyridine ligands. The substituent on each ligand removes the mirror plane that would
otherwise be coincident with the plane of the other terpyridine group and relates the
two halves of the terpyridine ligand containing the substituent. Both enantiomers are
present in the crystal, and the packing diagram (figure 4) shows these isomers in the

152
P.A. Thornley et al.
˚
Table 2. Bond lengths (A) and colors of cobalt terpyridine complexes with cobalt in different oxidation and
spin states.
N atom
Co(II) LS [17]
Co(II) HS [17]
Co(III) [18]
[Co(ottp)2 ꢅ Cl2]ꢅ2.25CH3OH
1
2
3
4
1.950
1.856
1.955
1.944
1.862
1.948
Green
2.083
1.904
2.089
2.093
1.906
2.096
Brown
1.930
1.863
1.926
1.937
1.853
2.003
1.869
2.001
2.182
1.939
5
6
Crystal color
1.921
Pale yellow
2.162
Red/brown
LS, low spin and HS, high spin.
Figure 4. The hydrogen bonding and short contacts between the chlorides, methanol, and the bis[40-(o-
toluyl)-2,20 : 60,200-terpyridine] cobalt complex (3) are shown. Only hydrogens involved in these interactions
have been included and portions of the cations have also been excluded for clarity.
structure, and the intermolecular interactions that link them. Chloride ions 1 and 2
˚
˚
are hydrogen bonded to methanol (Cl1–O100 2.322 A and Cl2–O200 2.831 A) and to
˚
o-toluyl methyl hydrogens (Cl1–H44 A 2.776 A and Cl2–H43C 2.902 A). There are a
˚
˚
number of other short contacts to aromatic hydrogen atoms as shown in the diagram.
.
3.2.2. [Cu(ottp)Cl2] CH3OH. The X-ray crystal structure (figure 5) shows copper
bound to one 40-(o-toluyl)-2,20 : 60,200-terpyridine ligand and two chlorides. The
geometry around copper is distorted square pyramidal. The crystal system is triclinic

Terpyridine complexes
153
Figure 5. The molecular structure of the 40-(o-toluyl)-2,20 : 60,200-terpyridine copper complex (4) with thermal
ellipsoids at 30% probability. H atoms have been omitted for clarity.
ꢀ
and the space group P1. Although not shown in this diagram, there is hydrogen
˚
bonding between the chloride (Cl1) and the methanol at a distance of 2.381 A.
Figure 6 shows interactions between complexes in the crystal. The chloride at the base
of the square pyramid of each complex interacts over a longer distance with the copper
of a second complex, and vice versa, forming a kind of dimer. The copper–copper
˚
˚
distance in this dimer is 4.029 A, the copper–basal-chloride bond length is 2.509 A, and
˚
the copper–chloride interaction to an adjacent complex is 3.772 A. There are also
hydrogen bonds holding pairs of complexes to other pairs of complexes. This involves
hydrogen bonding between, for example, the 3,300 or 5,500 hydrogen atoms and the
chloride ions, Cl2A and Cl2F, respectively.
3.2.3. [(Cl-ottp)Cu(l-Cl)(l-Br)Cu(Cl-ottp)][PF6]2. The asymmetric unit is shown in
figure 7. Figure 8 views the structure along the plane of the pyridine rings showing the
structural relationships in the bridging halide–dicopper unit, and the packing diagram is
shown in figure 9. The coordination geometry around each copper center is
approximately square pyramidal. The dimer is bridged by one chloride and one
bromide. Each bridging halide position has an occupancy of 50% Br and 50% Cl, while
the bromine in the original ligand has been displaced by a chlorine.
The only source of bridging bromide is from 40-[2-(bromomethyl)phenyl]-2,20 : 60,200-
terpyridine starting material. On the basis of the X-ray crystallographic results, the
bromine atom in the starting material has exchanged with chloride from the copper salt.
This appears to be a facile process [17]. The preparation of heavier halides from lighter

154
P.A. Thornley et al.
Figure 6. The X-ray crystal structure packing diagram for 4 with chloride/copper interactions and the
chloride/hydrogen interactions.

Terpyridine complexes
155
Figure 7. The asymmetric unit of 5 with a view of the Br/Clꢀ50% occupancy. Thermal ellipsoids are shown
at 30% probability.
halides by first row transition metals was first reported in 1925 by Biltz and Keunecke
[18]. The enthalpy of a carbon–bromine bond is 276 kJ molꢃ1 and for a copper–bromide
bond it is 331 kJ molꢃ1 [19]. The enthalpy of a copper–chloride bond is 383 kJ mol–1 and
for a carbon–chlorine bond it is 397 kJ mol–1 [20]. It is therefore enthalpically favorable
for the bromide to be bonded to copper and the chlorine to be bonded to carbon, rather
than the other way around.
In principle, this result could represent a single complex containing one bridging
bromide and one bridging chloride, or from the mixture within the crystal of complexes
containing two bridging bromides, two bridging chlorides, or one of each halide. The
single complex is the best description as, if the refinement outcome were due to a
mixture of the three complexes being present, there would be a number of different Cu–
Cu distances (due to the different combinations of bridging ligands) present in the same
crystal. This would result in significantly larger thermal parameters along the Cu–Cu
vector for the coppers. There is no evidence of this (figure 8), hence our conclusion that
there is only one complex present with one bridging bromide and one bridging chloride
in the crystal analyzed. Searching the CCDC for a similar Cu–Cl/Br–Cu motif revealed
only one other structure with this unit [21]. In that report the occupancy of the Cl and
Br sites was varied by changing the initial stoichiometry of the halides, with the
preferential formation of mixed Cl/Br complexes. The pendant chloromethyl group has
some residual electron density nearby. This is due to some disorder which was not
considered significant enough to model.
The dinuclear structural element of this complex bears some similarity to that of
[Cu(ottp)Cl2] described above. In both structures, the tpy-based ligand and a halide
form an approximate square plane, and the halide in this plane is also associated with

156
P.A. Thornley et al.
Figure 8. The [(Cl-ottp)Cu(ꢆ-Cl)(ꢆ-Br)Cu(Cl-ottp)] complex (5) viewed along the plane of the pyridine rings.
Figure 9. The packing diagram of 5 showing the PF6 counter ions.

Terpyridine complexes
157
the second copper. The difference appears to be that the presence of the additional
chloride in the mononuclear complex leads to a weakening of the axial bridging
interaction.
4. Conclusion
40-(o-Toluyl)-2,20 : 60,200-terpyridine has been prepared and derivatized. This ligand and
its brominated derivative can be used to form metal complexes, although in the latter
case a halogen exchange reaction occurred. 40-[2-(Bromomethyl)phenyl]-2,20 : 60,200-
terpyridine promises to be a useful synthon to allow additional donors to be added to
the terpyridine unit. We plan to introduce groups that can interact with the metal ion
bound to the terpyridine, or that are capable of binding a second metal ion.
Supplementary material
CCDC 788100 (1), CCDC 788101 (4), CCDC 788102 (5), and CCDC 788103 (3)
contain the supplementary crystallographic data for this article. These data can be
obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/ci.
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