Flexible dinucleating N,N,N-tridentate ligands based on a xanthene scaffold

Article abstract of DOI:10.1016/j.ica.2012.12.040

A versatile route to flexible ditopic ligands based on a 2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene scaffold, namely Williamson ether synthesis employing the 4,5-bis(chloromethyl) derivative of the xanthene and a phenol-substituted ligand donor, is demonstrated by the preparations of bis(terpyridine)xanthene L¹ and the bis(di(2-pyridylmethyl)amine) xanthene L² in good yields. The reaction of L¹ with [Pd(PhCN)₂Cl₂] afforded the coordination complexes [L ¹(PdCl)₂][PF₆]₂ (4[PF ₆]₂) and [L¹(PdCl)][PF₆] (5[PF ₆]). The X-ray crystal structures of L¹ and 5[PF ₆]·solvate were determined and reveal intra- and intermolecular offset face-to-face stacking between terpyridine rings and intramolecular C-H...π-phenyl ring interactions. Variable temperature ¹H NMR studies show temperature-dependent chemical shift changes suggestive for face-to-face interactions between the terpyridines in 4[PF ₆]₂ and of 5[PF₆] being sustained in solution.

Full text of DOI:10.1016/j.ica.2012.12.040

Inorganica Chimica Acta 399 (2013) 55–61
Contents lists available at SciVerse ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Flexible dinucleating N,N,N-tridentate ligands based on a xanthene scaffold
Tanya Tan a, Carolina Gimbert-Suriñach a, Mohan Bhadbhade b, Stephen B. Colbran a,
⇑
a School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia
b Mark Wainwright Analytical Centre, University of New South Wales, Sydney, NSW 2052, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 October 2012
Received in revised form 13 December 2012
Accepted 15 December 2012
Available online 31 January 2013
A versatile route to flexible ditopic ligands based on a 2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene scaf-
fold, namely Williamson ether synthesis employing the 4,5-bis(chloromethyl) derivative of the xanthene
and a phenol-substituted ligand donor, is demonstrated by the preparations of bis(terpyridine)xanthene
L1 and the bis(di(2-pyridylmethyl)amine)xanthene L2 in good yields. The reaction of L1 with [Pd(PhCN)2
Cl2] afforded the coordination complexes [L1(PdCl)2][PF6]2 (4[PF6]2) and [L1(PdCl)][PF6] (5[PF6]). The X-
ray crystal structures of L1 and 5[PF6]ꢀsolvate were determined and reveal intra- and intermolecular offset
Keywords:
Ditopic ligand
N,N,N-Chelate ligand
Terpyridine
Palladium complex
Offset face-to-face ring stacking
face-to-face stacking between terpyridine rings and intramolecular C–H. . .p-phenyl ring interactions.
Variable temperature 1H NMR studies show temperature-dependent chemical shift changes suggestive
for face-to-face interactions between the terpyridines in 4[PF6]2 and of 5[PF6] being sustained in solution.
Ó 2013 Published by Elsevier B.V.
1. Introduction
[3i,4,5], typically the two terpyridyl groups are separated by linear
spacers such as phenyl, biphenyl or bis(imide) derivatives; e.g. La–c
,
In this paper we report a straightforward route to flexible ditop-
ic ligands with terpyridine (L1) or di(2-pyridylmethyl)amine (L2)
metal-binding domains spanned by a xanthene bridge, and the
syntheses and characterisation of palladium chloride complexes
of L1.
Fig. 1. Ditopic ligands with proximal terpyridyl groups are more
rare and include examples with angular and U-shaped geometries
provided by more complex spacer scaffolds; e.g. Ld–h, Fig. 1. Ter-
pyridines such as those in Fig. 1 and their metal complexes have
application in a vast variety of fields including catalysis, sensors,
supramolecular chemistry and material science. For instance,
square-planar terpyridine–platinum(II) complexes have been
extensively exploited for their characteristic photophysical proper-
ties in molecular sensors and switches and, as well, they are DNA
intercalators [5a]. Although fewer studies of analogous palla-
dium(II) complexes have been reported, the complexes which are
analogous to those reported herein have shown remarkable molec-
ular recognition abilities [5a]. Metal complexes of bis(dipicolinyl-
amine)-spacer systems are increasingly studied, including as
artificial phosphatases [7], as phosphoprotein recognition and
inhibitor systems [8], and as fluorescent or electrochemical sensors
for various biologically-relevant cations and anions [9].
As part of ongoing research into the emergent properties result-
ing from synergistic interactions between two metal centres in
bimetallic complexes [1], we sought a simple route to flexible
ditopic ligands constructed from a spacer proximally connecting
two ligand donor domains. We required: (1) the synthetic method-
ology to be short with good yields and the versatility to allow
introduction of different ligand donor domains. (2) The new ditopic
ligands to be soluble, easily handled and to be sufficiently flexible
to allow the interactions between the proximate metal centres in
their metal complexes to optimize in response to changing condi-
tions such as the charge or co-ligands on one or both metal centres.
Therefore we developed, and report herein, a short route to flexible
dinucleating ligands based on Rebek’s 2,7-di-tert-butyl-9,9-
dimethyl-9H-xanthene scaffold [2]. Rebek’s xanthene bridge was
chosen for its ease of synthesis and derivatisation, and for the sol-
ubility and ease of handling it confers on its more complicated
derivatives [3].
2. Experimental
2.1. General procedures
The xanthene-bridged ligands described in this paper add to the
large and growing number of ditopic bis(ligand domain)-spacer
systems in the literature [3–9]. In bis(terpyridine)-spacer systems
Phenol-derivatized 1 [2b,3b,10] and 2 [11] (see Scheme 1), and
[Pd(PhCN)2Cl2] [12] were synthesized as described in the litera-
ture. Acetonitrile and methanol were taken from an Innovative
Technology Pure Solvent Dispenser prior to use. All other reagents
are commercially available and were used as received. 1H and 13C
NMR spectra were recorded on a Bruker DPX 300, Bruker Avance
⇑
Corresponding author. Tel.: +61 2 93854737; fax: +61 2 93856141.
E-mail address: s.colbran@unsw.edu.au (S.B. Colbran).
0020-1693/$ - see front matter Ó 2013 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.ica.2012.12.040

56
T. Tan et al. / Inorganica Chimica Acta 399 (2013) 55–61
N
N
N
N
SPACER
N
N
SPACER
O
O
N
N
O
La
O
O
Lc
Lg
Ar
O
O
Ar
O
N
S
O
Lf
Ld
O
N
O
N
O
Ar
O
Ar
Lb
N
Le
Lh
Fig. 1. Examples of bis(terpyridine) ligands described in the literature.
OH
2
3
(2.2 eq.)
(2.1 eq.)
O
K2CO3 (5 eq.)
CH3CN
K2CO3 (5 eq.)
CH3CN
96 %
Cl
66 %
Cl
reflux
reflux
1
N
N
N
2
O
O
OH
O
O
N
O
O
N
N
N
N
N
N
N
N
N
N
N
N
N
N
L1
L2
3
Scheme 1. Syntheses of dinucleating ligands L1 and L2.
III 500 or Bruker Avance III 600 spectrometers. High-resolution
positive-mode ESI mass spectra were acquired in an Orbitrap Velos
XL mass spectrometer in the Bioanalytical Mass Spectrometry
Facility of the University of New South Wales.
(18 mL) was heated to reflux for 3 days. The mixture was cooled
to room temperature and the solvent removed under vacuum.
The product was extracted into dichloromethane (20 mL) and
washed with water (3 x 30 mL). The organic layer was dried over
Na2SO4, filtered and the solvent removed to give L1 as a white solid
(503 mg, 0.50 mmol, 96%).
2.2. Synthesis of L1
L1: 1H NMR (300 MHz, CDCl3): d = 8.49 (4H, ddd, J = 4.8, 1.8 and
0.8 Hz, py), 8.43 (4H, s, py), 8.35 (4H, dt, J = 7.8 and 1.1 Hz, py),
7.71–7.65 (8H, m, py), 7.47 (2H, d, J = 2.3 Hz, Ph–xanthene), 7.40
(2H, d, J = 2.3 Hz, Ph–xanthene), 7.14 (4H, ddd, J = 7.3, 4.8 and
A
mixture of the bis(chloromethyl)xanthene 1 (220 mg,
0.52 mmol), the 4-terpyridylphenol 2 (359 mg, 1.10 mmol) and
potassium carbonate (0.73 g, 5.32 mmol) in dry acetonitrile

T. Tan et al. / Inorganica Chimica Acta 399 (2013) 55–61
57
1.1 Hz, py), 6.92 (4H, d, J = 8.9 Hz, Ph), 5.14 (4H, s, –O–CH2), 1.72
0.051 mmol, 47%). Yellow needles of the monometallic species
5[PF6]ꢀsolvate were formed after slow evaporation of the filtrate
of recrystallization (4.6 mg, <3%).
t
(6H, s, CH3), 1.37 (18H, s, Bu). 13C{H} NMR (150.9 MHz, CDCl3):
d = 160.2, 156.4, 155.5, 149.5, 149.0 (CH, py), 146.6, 145.8, 136.6
(CH, py), 130.9, 129.8, 128.8 (CH, Ph), 125.3 (CH, Ph–xanthene),
123.6 (CH, py), 123.3 (CH, Ph–xanthene), 123.2, 121.3 (CH, py),
118.1 (CH, py), 115.0 (CH, Ph), 66.2 (O–CH2), 35.0, 34.9, 32.8
4[PF6]2: 1H NMR (600 MHz, dmso-d6): d = 8.65 (4H, d, J = 5.2 Hz,
py), 8.62 (4H, s, py), 8.59 (4H, d, J = 7.9 Hz, py), 8.16 (4H, t,
J = 7.3 Hz, py), 7.92 (4H, d, J = 8.1 Hz, Ph), 7.78 (4H, t, J = 6.6 Hz,
py), 7.63 (2H, d, J = 2.2 Hz, Ph–xanthene), 7.46 (2H, d, J = 2.2 Hz,
Ph–xanthene), 6.97 (4H, d, J = 8.1 Hz, Ph), 5.18 (4H, s, –O–CH2),
1.72 (6H, s, CH3), 1.35 (18H, s, tBu). 13C{H} NMR (100 MHz,
dmso-d6): d = 162.3, 157.6, 154.2, 153.2, 153.0 (CH, py), 146.4,
145.7, 142.5 (CH, py), 130.1(CH, Ph), 129.7, 129.4 (CH, py), 126.8
(CH, Ph–xanthene), 125.9 (CH, py), 125.8 (CH, py), 124.4 (CH,
Ph–xanthene), 122.5, 120.1 (CH, py), 115.1 (CH, Ph), 66.3 (–O–
(CH3),
31.9
(tBu).
(+)-HR-ESI-MS
Calcd.
for
[M+H]+
([C67H60N6O3]+): m/z 997.4710. Found: m/z 997.4777.
2.3. Synthesis of 4-(2-(bis(pyridine-2-ylmethyl)amino)ethyl)phenol, 3
Under a nitrogen atmosphere, a solution of tyramine (0.549 g,
4.00 mmol) and 2-pyridinecarboxaldehyde (0.857 g, 8.00 mmol)
in methanol (100 mL) was cooled using an ice bath. To this solution
glacial acetic acid (0.83 mL, 14.8 mmol) followed by a solution of
sodium cyanoborohydride (0.593 g, 9.43 mmol) in methanol
(20 mL) were added. The resulting mixture was stirred at room
temperature for four days, cooled in an ice bath and hydrochloric
acid 5 M added until pH 5 was reached. After stirring for 30 min,
a sodium bicarbonate aqueous solution was added until no further
gas evolved. The mixture was extracted with dichloromethane,
dried over sodium sulfate and the solvent was removed under vac-
uum to give an oily residue, which was purified by flash chroma-
tography through silica using dichloromethane/methanol 9:1.
Compound 3 was isolated as a brown oil (1.14 g, 3.57 mmol,
89%). 1H NMR (300 MHz, CDCl3): d = 8.51 (2H, d, J = 5.0 Hz, py),
7.61 (2H, t, J = 7.7 Hz, py), 7.41 (2H, d, J = 7.7, py), 7.13 (2H, dd,
J = 7.7 and 5.0 Hz, py), 6.92 (2H, d, J = 8.5 Hz, Ph), 6.70 (2H, d,
J = 8.5 Hz, Ph), 3.88 (4H, s, NCH2py), 2.76 (4H, s broad, NCH2CH2Ph).
13C{H} NMR (75 MHz, CDCl3): d = 159.7, 155.3, 148.5 (CH, py),
136.9 (CH, py), 131.2, 129.8 (CH, Ph), 123.3 (CH, py), 122.2 (CH,
py), 115.5 (CH, Ph), 60.1 (NCH2py), 57.0 (NCH2CH2Ph), 33.2 (NCH2-
CH2Ph). (+)-HR-ESI-MS Calcd. for [M + H]+ ([C20H22N3O]+): m/z
320.1757. Found: m/z 320.1752.
t
CH2), 34.8, 34.8, 33.0 (CH3), 31.8 (CH3, Bu). (+)-HR-ESI-MS Calcd.
[C67H60Cl2N6O3Pd2]2+: m/z 640.1083. Found: m/z 640.1072.
5[PF6]: 1H NMR (400 MHz, dmso-d6): d = 8.55 (2H, d, J = 7.7 Hz,
py), 8.50 (2H, s, py), 8.46 (2H, d, J = 8.0 Hz, py), 8.38 (2H, d,
J = 5.7 Hz, py), 8.36 (2H, d, J = 4.9 Hz, py), 8.29 (2H, s, py), 8.06
(2H, td, J = 7.8 and 1.4 Hz, py), 7.89 (2H, d, J = 8.6 Hz, Ph), 7.85
(2H, td, J = 7.6 and 1.4 Hz, py),7.63 (1H, d, J = 1.9 Hz, Ph–xanthene),
7.59 (1H, d, J = 1.9 Hz, Ph–xanthene), 7.50–7.55 (4H, m, py and Ph),
7.48 (1H, d, J = 1.9 Hz, Ph–xanthene), 7.45 (1H, d, J = 1.9 Hz, Ph–
xanthene), 7.27 (2H, dd, J = 6.6 and 4.9 Hz, py), 7.07 (2H, d,
J = 8.6 Hz, Ph), 6.83 (2H, d, J = 8.5 Hz, Ph), 5.23 (2H, s, –O–CH2),
t
5.07 (2H, s, –O–CH2), 1.72 (6H, s, CH3), 1.36 (9H, s, Bu), 1.34 (9H,
t
s, Bu). 13C{H} NMR (100 MHz, dmso-d6): d = 162.1, 160.1, 157.6,
155.1, 154.3, 154.2, 153.0, 152.0 (CH, py), 149.3 (CH, py), 148.8,
146.7, 146.0, 145.7, 141.9 (CH, py), 137.9 (CH, py), 130.0, 129.9
(CH, Ph), 129.8, 129.7, 128.7, 128.6 (CH, py), 128.2 (CH, Ph),
126.9 (CH, Ph–xanthene), 126.3, 125.9 (CH, Ph–xanthene), 125.4
(CH, py), 124.8 (CH, py), 124.4 (CH, Ph–xanthene), 123.9 (CH,
Ph–xanthene), 122.9, 122.6, 121.4 (CH, py), 119.6 (CH, py), 116.5
(CH, py), 115.1 (CH, Ph), 115.0 (CH, Ph), 66.6 (–O–CH2), 65.5 (–O–
t
CH2), 34.8, 34.8, 32.9 (CH3), 31.8 (CH3, Bu). (+)-HR-ESI-MS Calcd.
[C67H60ClN6O3Pd]+: m/z 1137.3403. Found: m/z 1137.3445.
2.4. Synthesis of L2
Following the same procedure as for L1, compound 1 (56.5 mg,
0.135 mmol), 3 (103 mg, 0.322 mmol) and potassium carbonate
(224 mg, 1.62 mmol) were heated under reflux for four days. L2
was isolated as a dark brown oil (87.6 mg, 0.089 mmol, 66%).
L2: 1H NMR (300 MHz, dmso-d6): d = 8.46 (4H, d, J = 4.8 Hz, py),
7.64 (4H, td, J = 7.9 and 1.8 Hz, py), 7.50 (2H, d, J = 2.3 Hz, Ph–xan-
thene), 7.39 (2H, d, J = 2.3 Hz, Ph–xanthene), 7.35 (4H, d, J = 7.9 Hz,
py), 7.20 (4H, dd, J = 4.8 and 7.9 Hz, py), 6.91 (4H, d, J = 8.5 Hz, Ph),
6.71 (4H, d, J = 8.5 Hz, Ph), 5.03 (4H, s, –O–CH2), 3.78 (4H, s,
NCH2py), 2.67 (8H, m, NCH2CH2Ph), 1.64 (6H, s, CH3), 1.29 (18H,
s, tBu). 13C{H} NMR (75 MHz, dmso-d6): d = 159.4, 156.7, 148.7
(CH, py), 145.4, 145.0, 136.4 (CH, py), 132.2 (CH, Ph), 129.9 (CH,
Ph), 129.5 (CH, Ph), 124.5, 123.2, 122.7, 122.5 (CH, py), 121.3
(CH, py), 114.1 (CH, Ph), 64.8 (O–CH2), 59.5 (NCH2py), 55.6 (NCH2
CH2Ph), 34.9, 34.3 (NCH2CH2Ph), 34.2, 32.2 (CH3), 31. 3 (tBu). (+)-
HR-ESI-MS Calcd. for [M+H]+ ([C65H73N6O3]+): m/z 985.5739.
Found: m/z 985.5716.
2.6. X-ray crystallography
Data for 5[PF6]ꢀsolvate were recorded on a Bruker Kappa-
Appex-II CCD diffractometer, and data for L1 were collected on
beamline MX2 at the Australian Synchrotron, Melbourne. The crys-
tals were mounted in inert oil on cryoloops and transferred to the
cold gas stream for the diffraction measurements. Data for 5[PF6]-
ꢀsolvate were collected by using monochromated Mo K
a radiation
(k = 0.71073 Å) in
x
-scans, and data for L1 were measured at
monochromated synchrotron radiation set to wavelength
k = 0.71085 Å. Absorption corrections based on multiple scans
were applied only to data from 5[PF6]ꢀsolvate by using SADABS
[13]. The structures were solved by direct methods and refined
on F2 against ALL reflections using the SHELXL-97 program [14].
3. Results and discussion
2.5. Synthesis of [L1(PdCl)2][PF6]2 (4[PF6]2)
3.1. Synthesis of ligands L1 and L2
[Pd(PhCN)2Cl2] (104 mg, 0.27 mmol) was dissolved in dichloro-
methane (3 mL), added dropwise to a solution of L1 (108 mg,
0.108 mmol) in dichloromethane (3 mL) and the resulting solution
was stirred at room temperature overnight. The fine yellow precip-
itate was filtered off and dissolved in the minimum amount of
The bis(terpyridine)xanthene derivative L1 was obtained in
excellent yield by treating bis(chloromethyl)xanthene 1 [2b,3b,10]
with 4-terpyridinylphenol 2 [11] under standard Williamson ether
synthesis conditions; Scheme 1. Ligand L1 is a white solid that is
highly soluble in chlorinated solvents, tetrahydrofuran, ethyl ace-
tate and toluene, and is sparingly soluble in diethyl ether. Following
the same methodology, ditopic ligand L2 containing two
bis(2-pyridylmethyl)amine groups, was obtained in moderate yield
from 1 and dipicolinyltyramine 3 (Scheme 1). Dipicolinyltyramine 3
dimethylsulfoxide. To this solution,
a concentrated aqueous
solution of [NH4][PF6] was added to afford 4[PF6]2 as a bright yel-
low solid. The product was recrystallized by adding diethyl ether to
a
concentrated solution of 4[PF6]2 in acetonitrile (72.4 mg,

58
T. Tan et al. / Inorganica Chimica Acta 399 (2013) 55–61
Table 1
is new and was readily available from the reductive amination of
pyridine-2-carboxaldehyde with tyramine (89% yield).
Crystallographic structure and refinement data for L1 and 5[PF6]ꢀsolvate.
The 1H and 13C NMR spectra of L1 and L2 are consistent with
their Cs-symmetric structures: the requisite peaks corresponding
to those anticipated for the ligand donor domains and the xan-
thene scaffold are observed in each case. For example, the 1H
NMR spectra of L1 and L2 in dmso-d6 show one characteristic O-
methylene singlet at d 5.11 and 5.03, respectively, only slightly
downfield relative to the methylene signal for the precursor xan-
thene 1 at d 4.94.
L1
5[PF6]ꢀsolvatea
Empirical formula
Crystal habit
C67H60N6O3
thin light yellow
plates
(C67H60ClN6O3Pd)ꢀ(F6P)ꢀsolvatea
yellow plates
Crystal size (mm)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
0.08 ꢁ 0.02 ꢁ 0.01 0.37 ꢁ 0.06 ꢁ 0.05
monoclinic
P21/c
monoclinic
P21/c
10.058(2)
27.711(6)
18.798(4)
90.00
21.035(6)
13.514(4)
25.839(7)
90.00
A crystal of L1 suitable for a single-crystal X-ray diffraction
study was grown from a concentrated chloroform/diethyl ether
solution at 278 K. Depicted in Fig. 2 are views of the molecular
structure of L1 and of the packing within the crystal. Table 1 pre-
sents the crystallographic structure and refinement data. The
xanthene scaffold is slightly puckered as expected with the angle
between the normal to the two phenyl rings being 11.8° (see
Figs. S1A and S2 in the Supporting Information). The two pheno-
lic rings are not parallel; the B phenolic ring lies approximately
in the xanthene plane whereas the A phenolic ring is orthogonal
a
(°)
b (°)
98.22(3)
90.00
105.904(15)
90.00
c
(°)
U (Å3)
Z
5185.4(18)
4
7064(3)
4
Dcalc (Mg mꢂ3
M
)
1.277
1.317
997.21
2112
1400.83
2872
F(000)
T (°C)
hmax (°)
k (Å)
100
155
2.5–22.5
0.71085
0.079
2.7–29.1
0.71073 (Mo K
0.0409
a)
l
(mmꢂ1
)
to it, facilitating C–H edge-to-p-face inter-aryl interaction (see
Absorpt. Tmin–Tmax
Reflections measured
Unique reflections
Observed reflections
–
0.864–0.981
49865
12345
Fig. 2A). The distances between C4A, C5A and C6A from phenolic
ring A and C7B, C6B from phenolic ring B are 3.65–4.02 Å (see
Fig. S1A in the Supporting Information). The two terpyridyl
groups are approximately parallel, and show significant offset
face-to-face stacking interactions, with distances between the
parallel terpyridyl rings varying 3.3–3.5 Å (see Fig. 2) [5b,5f,15].
In the crystal, pairs of L1 molecules that are aligned head-to-
head and related by crystallographic inversion pack into columns
that lie parallel to the x-axis; see Figs. 2B and S2, Supporting
Information. There are significant intermolecular offset face-to-
face stacking interactions in the range 3.3–3.6 Å between the
adjacent pairs in each column; see Figs. 2B and S2, Supporting
Information. The overall crystal structure shows that the L1 mol-
ecules form layers within the crystal lattice parallel to the x-axis
(see Figs. S3 and S4 in the Supporting Information).
67736
8769
6965
6121
[I > 2
r(I)]
Rint
0.061
0.050
0.134
8769
693
0.127
0.079
0.275
12345
923
R [F2 > 2
r
(F2)]
wR (F2)
Reflections used
Parameters
Restraints
S
0
98b
1.06
0.32, ꢂ0.27
1.05
1.01, ꢂ0.61
Max./min.
D
q
(e Åꢂ3
)
a
The voids in the crystal lattice are filled by molecules of solvent from the
1
crystallisation and refined to
dichloromethane + ½ acetonitrile + ½ diethyl
4
ether + 15
/ C-atoms for ill-defined, disordered, solvent molecules.
4
b
Restraints were only applied to the molecules of solvent in the crystal lattice.
[NH4][PF6] afforded the bimetallic complex [L1(PdCl)2][PF6]2
(4[PF6]2), Scheme 2. After 4[PF6]2 was collected by filtration, the
yellow acetonitrile–diethylether mother liquor was set aside. A
few crystals of the monometallic complex [L1(PdCl)][PF6] (5[PF6])
formed and were manually collected. Attempts to selectively
3.2. Palladium complexes 4[PF6]2 and 5[PF6]
Upon mixing ditopic ligand L1 with 2.5 equiv. of [Pd(PhCN)2Cl2]
at room temperature a bright yellow precipitate formed, which
upon dissolution in dimethylsulfoxide and anion metathesis with
Fig. 2. (A) View of the molecular structure of L1, showing the heteroatom labeling scheme and 50% thermal ellipsoids; (B) View of the crystal structure illustrating the face-to-
face stacking parallel to the x-axis between a pair of L1 molecules related by a crystallographic inversion and molecules of L1 in the adjacent pairs (s is a crystallographic
inversion centre).

T. Tan et al. / Inorganica Chimica Acta 399 (2013) 55–61
59
[PF6]2
[PF6]
j
j''
j'
i
i''
i'
O
O
i) Pd(PhCN)2Cl2 (2.5 eq.)
CH2Cl2, r.t.
h
O
N
O
O
O
h'
h''
L1
g
g''
f''
g'
f'
ii) [NH4][PF6]
f
e
e'
e''
d
d''
N
c'
N
a'
b'
c''
b''
N
N
N
c
N
N
N
N
N
N
d'
b
Pd
Cl
Pd
Cl
Pd
Cl
a
a''
[L1(PdCl)2][PF6]2
[L1(PdCl)][PF6]
4[PF6]2
5[PF6]
47 %
Scheme 2. Synthesis of palladium(II) complexes, 4[PF6]2 and 5[PF6].
prepare 5+ using slow dropwise addition of one equivalent of
[Pd(PhCN)2Cl2], shorter reaction times and various solvents were
unsuccessful. Mixtures containing 42+, 5+ and uncomplexed L1
immediately formed that quickly began to precipitate bimetallic
4
2+ as its chloride salt. The low solubility of 4(Cl)2 drives the forma-
tion of this product. Attempted crystallisations of 4(Cl)2 and
4[PF6]2 always produced fibrous crystalline needles that were
unsuitable for X-ray analysis.
Complexes 4[PF6]2 and 5[PF6] were characterized by 1H and 13
C
NMR spectroscopy, by high resolution mass spectrometry, and by
X-ray crystallography for 5[PF6]. The high resolution ESI mass spec-
trum of 4[PF6]2 showed an envelope of peaks for 42+ cation at m/z
640.1072 (calc. m/z 640.1083) and that of 5[PF6] exhibited an enve-
lope of peaks for the 5+ cation at m/z 1137.3445 (calc. m/z
1137.3403). The peaks for the 42+ and 5+ parent ions displayed
the anticipated isotope patterns.
The 1H and 13C NMR spectra for 4[PF6]2 and 5[PF6] are completely
characteristic for each complex cation. Variable temperature (VT-)
1H NMR spectra over the aromatic regions of complexes 4[PF6]2
and 5[PF6] in dmso-d6 are presented in Fig. 3. Proton assignments
were made with the aid of 1H–1H COSY and NOESY NMR experi-
ments. The spectra for 4[PF6]2, Fig. 3A, reveal the C2-symmetry of
the 42+ complex ion: the two terpyridyl domains, and both halves
of the complex ion, are equivalent. The spectra for 5[PF6] reveal
the loss of symmetry and exhibit separate peaks for the protons of
0
the uncomplexed (indicated by a ‘‘ ’’) and complexed (indicated
by a ‘‘ 00 ’’) terpyridyl domains. In the VT-1H NMR spectra, the peaks
for the terpyridyl protons also show notable temperature-depen-
dent chemical shift changes. For example, as the temperature of
the VT-1H NMR spectrum is raised the peaks corresponding to the
6,600-terpyridyl protons of the Pd(II)-coordinated terpyridine (la-
beled ‘‘e’’ for 42+ and ‘‘e00’’ for 5+) move upfield while the 30,50-terpyr-
idyl proton resonances (labeled ‘‘d’’ for 42+ and ‘‘d00’’ for 5+) move
downfield. The chemical shift changes are such that these reso-
nances switch relative position in the high and low temperature
spectra. It has been suggested that such chemical shift changes indi-
cate face-to-face p-stacking interactions between terpyridyl groups
in solution at the lower temperature(s) that weaken with increasing
molecular freedom as the temperature is raised [1d,16]. No evidence
for exchange of the PdCl centre between the occupied and unoccu-
pied terpyridyl domains is seen in the VT NMR spectra for 5[PF6],
Fig. 3B, over the temperatures surveyed (up to 380 K).
Crystals of 5[PF6]ꢀsolvate grew upon slow evaporation of an ace-
tonitrile–dichloromethane–diethyl ether solution. The molecular
structure of 5[PF6]ꢀsolvate was determined by single crystal
Fig. 3. Variable temperature 1H NMR spectra in dmso-d6: (A) 4[PF6]2 at 600 MHz;
(B) 5[PF6] at 500 MHz. The labeling schemes adopted for the protons of ligand L1 in
the complexes are presented in Scheme 2.

60
T. Tan et al. / Inorganica Chimica Acta 399 (2013) 55–61
Fig. 4. (A) View of the structure of 5+ in 5[PF6]ꢀsolvate showing the heteroatom labeling scheme and 50% thermal ellipsoids at 155 K; for clarity, H-atoms are omitted and only
the major (59%) orientation of the twofold rotationally disordered tert-butyl group on side A of the xanthene bridge is shown. (B) View orthogonal to the x-axis illustrating the
columnar packing of 5+ cations parallel to the y-axis. Key bond lengths (Å) and angles (°): N1B–Pd1 1.940(6), N2B–Pd1 2.048(8), N3B–Pd1 2.028(8), Pd1–Cl1 2.293(2); N1B–
Pd1–N2B 80.8(3), N2B–Pd1–Cl1 100.7(2), N3B–Pd1–Cl1 97.7(2), N3B–Pd1–N1B 80.9(3).
X-ray crystallography. The crystal packing of the 5+ ions creates
voids that are filled with well-defined [PF6]ꢂ counter-ions and with
low-occupancy and disordered molecules of the crystallisation sol-
vents (see Table 1, footnotes). The 5+ ions dominate the structure
solution and are excellently defined. The acute bond angles about
the palladium ion sum to 360.1° indicative for square planar geom-
etry (see Fig. 4). The bond distances and angles (Fig. 4) to palladium
are unremarkable being exactly as expected for a terpyridine–pal-
ladium(II) complex [5a]. The xanthene-bridge flattens slightly in 5+
with the angle between the normal to the two phenyl rings being
7.6° (cf. 11.8° in L1, see above). A head-to-head intermolecular
interaction between the metal-free terpyridine of one molecule
and the terpyridine–Pd–Cl group of the other leads to columns of
stacked antiparallel cations (Fig. 4B). Within each column, each
of the two terpyridyl groups of 5+ stack face-to-face with two par-
allel neighbouring terpyridines: the inter-plane distances between
the terpyridines are 3.3 Å within 5+ and 3.4 Å for the nearest inter-
ion interactions (see Fig. 4 and Fig. S5 in the Supporting Informa-
tion). Columnar stacking was also found for a platinum complex
of the bis(terpyridine) ligand Lg (shown in Fig. 1) [3i]. The intermo-
lecular interactions of [Lg(PtCl)]+ differ from that of [L1(PdCl)]+ (5+)
in that the contact is between two metal-free terpyridyl groups
and two Pt atoms of adjacent cations. The strength of the dz2 —dz2
bis(terpyridine) ligand L1 and the bis(di(2-pyridylmethyl)amine)
ligand L2. Extensive intra- and intermolecular face-to-face interac-
tions between the terpyridyl domains in L1 and in its palladium
complexes were observed by single crystal X-ray diffraction stud-
ies, and variable temperature NMR spectroscopy experiments sug-
gest these interactions may remain for the complexes in solution.
The straightforward syntheses of L1 and L2, and the wide versatility
of terpyridine [3i,4,5] and di(2-pyridylmethyl)amine [7–9,17] do-
mains as metal-ion binding groups, make these ditopic ligand
promising precursors to bimetallic complexes for exploitation in
the fields of catalysis, sensors and supramolecular chemistry.
Acknowledgments
The Australian Research Council (DP0988410) supported this
work. Analytical support from the Bioanalytical Mass Spectrometry
Facility (BMSF) and the Nuclear Magnetic Resonance Facility at
UNSW is gratefully acknowledged. We also thank the Australian
Synchrotron Facility, Melbourne, for the X-ray data for L1.
Appendix A. Supplementary material
r
-interaction may determine whether M. . .M or inter-terpyridine
CCDC 882845 (for L1) and CCDC 882846 (for 5[PF6]ꢀsolvate) con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Atom labeling and additional crystallographic figures of L1 and
5[PF6]ꢀsolvate. Supplementary data associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/
j.ica.2012.12.040.
face-to-face stacking interactions are observed. In [Lg(PtCl)]+ [3i],
the Pt. . .Pt interaction is strong and preferred over formation of
the inter-terpyridine face-to-face stacking interactions, while in
[L1(PdCl)]+ (5+) the Pd. . .Pd interaction would be weaker thus
favouring the observed inter-terpyridine face-to-face interactions.
Bimetallic palladium and platinum species derived from ligand Lh
depicted in Fig. 1 also presented similar columnar crystal packing
of terpyridyl groups [6c,6e,6f]. For example, [Lh(PdCl)2]2+ and [Lh
(PtCl)2]2+ both crystallise in columns of interpenetrated pairs of
cations where one terpyridine–Pd–Cl unit of one molecule lies in
the middle of the two terpyridine–Pd–Cl groups of the other. This
interlocked structure is possible for ligand Lh due to its wide span
and rigidity.
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