Linear bilateral extended 2,2′:6′,2′′-terpyridine ligands, their coordination complexes and heterometallic supramolecular networks

Article abstract of DOI:10.1039/c4sc01025f

Octahedral metal complexes of tridentate 2,2′:6′,2′′-terpyridine (terpy) fused with five-membered furan rings mimic the topology of tetrahedral metal complexes of bidentate 5,5′-functionalized 2,2′-bipyridine (bipy). Herein, we report the robust synthesis of 2,6-bis(2-substituted-furo[2,3-c]pyridine-5-yl)pyridine based ligands to access a series of linear bilateral extended terpy derivatives. This molecular design of alternating five- and six-membered rings has been applied to extend the applicability of terpy as a building block in supramolecular chemistry. The complexation of 2,6-bis(2-substituted-furo[2,3-c]pyridine-5-yl)pyridine derivatives with metal ions preferring octahedral geometry (Fe²⁺, Ru²⁺, and Zn²⁺) gives molecular "crossings" and "corners". Such design elements, functionalized with 4-pyridyl groups, allowed the construction of 3D and 2D heterometallic supramolecular networks containing Fe²⁺, Ag⁺or Fe²⁺, Cu⁺metal centers.

Full text of DOI:10.1039/c4sc01025f

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Linear bilateral extended 2,20:60,200-terpyridine
ligands, their coordination complexes and
heterometallic supramolecular networks†
Cite this: Chem. Sci., 2014, 5, 4317
Janis Veliks,a Jui-Chang Tseng,b Karla I. Arias,a Florian Weisshar,a Anthony Lindena
ac
*
and Jay S. Siegel
Octahedral metal complexes of tridentate 2,20:60,200-terpyridine (terpy) fused with five-membered furan
rings mimic the topology of tetrahedral metal complexes of bidentate 5,50-functionalized 2,20-bipyridine
(bipy). Herein, we report the robust synthesis of 2,6-bis(2-substituted-furo[2,3-c]pyridine-5-yl)pyridine
based ligands to access a series of linear bilateral extended terpy derivatives. This molecular design of
alternating five- and six-membered rings has been applied to extend the applicability of terpy as a
building block in supramolecular chemistry. The complexation of 2,6-bis(2-substituted-furo[2,3-c]
Received 8th April 2014
Accepted 30th June 2014
pyridine-5-yl)pyridine derivatives with metal ions preferring octahedral geometry (Fe2+, Ru2+, and Zn2+
)
gives molecular “crossings” and “corners”. Such design elements, functionalized with 4-pyridyl groups,
allowed the construction of 3D and 2D heterometallic supramolecular networks containing Fe2+, Ag+ or
Fe2+, Cu+ metal centers.
DOI: 10.1039/c4sc01025f
www.rsc.org/chemicalscience
building blocks17 and motivate the synthesis of ligands with
new geometries. Specically, this study addresses the synthesis
Introduction
and characterization of linear bilateral extended terpyridines
(Fig. 1) mimicking the linear geometry of 5,50-functionalized
2,20-bipyridine (bipy in which sites for skeletal substitution are
perpendicular to the metal coordination vector).18,19 Such
ligands would allow the introduction of terpy into “linear”
molecular assemblies – a complement to the “stub”, “W”, “V or
De novo design and synthesis of functional supramolecular
architectures benet from ready access to components with
well-dened assembly geometries.1,2 A mainstay component3 of
supramolecular and materials chemistry, 2,20:60,200-terpyridine4
(terpy) ligands form coordination complexes with various
metals and have photophysical and electrochemical properties
suitable for supramolecular chemistry,5 nanotechnology,6 solar
cells,7 catalysis,8 antitumor,9 and antibacterial10 research. A vast
range of accessible terpy derivatives incorporate into geomet-
rically well-dened supramolecular assemblies.4,11 Examples
include the synthesis of Borromean link precursors,12 metal–
organic dendrimers,13 and molecular grids.14
Bifunctional molecular strings of terpy and bipy structural
units, used in rotoxane-based molecular machines, switches
and muscles,15 and multi-component assemblies, such as
coordination polymers,16 and metal–organic frameworks
(MOFs),16c reveal system properties dependent on the geometry
aDepartment of Chemistry, University of Zurich, Winterthurerstrasse 190, 8057 Zurich,
Switzerland. E-mail: jss@oci.uzh.ch
bChaoyang University of Technology, Department of Applied Chemistry, 168, Jifeng
E. Rd., Wufeng District, Taichung, 41349 Taiwan, Republic of China
cSchool of Pharmaceutical Science and Technology, Tianjin University, 92 Weijin Road,
Nankai District, Tianjin, 300072, People’s Republic of China
† Electronic supplementary information (ESI) available: Full experimental
synthesis procedures and characterization data, including 1H NMR and 13C
NMR spectra of all new compounds and CIF les. CCDC 995858–995866. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4sc01025f
Fig. 1 (A) The “stub”, “W”, “V or L”, and “C” motifs based on terpy. (B)
Fused five-membered rings to terpy mimicking 5,50-functionalized
bipy.
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L”, and “C” motifs stemming from substituents at positions 40,
4/400, 5/500, and 6/600 with angles relative to the coordination
vector of 180ꢀ, 120ꢀ, 60ꢀ, and 0ꢀ, respectively (Fig. 1A).
Fusion of a ve-membered ring to the six-membered ank-
ing rings of terpy specically addresses the linear bilateral
geometry mentioned above (Fig. 1B), and focuses this work on
the practical and versatile synthesis of 2,6-bis(2-substituted-
furo[2,3-c]pyridine-5-yl)pyridines (Fig. 2). Complexation of these
ligands with octahedrally coordinating metal ions gives access
to molecular “crossings” and “corners” suitable for the design
of heterometallic supramolecular networks.
Scheme 1 Retrosynthesis of linear terpyridine 1.
Results and discussion
Retrosynthesis
resulted in 5,500-dihydroxyterpyridine 12 in excellent yield.28
These reactions were performed routinely on a 40 g to 90 g scale
and have the potential for further scale-up. Subsequent depro-
tonation of hydroxy groups with NaH and treatment with
MOMCl (prepared in situ)29 afforded terpy 13, with ortho-
directing groups at the 5,500-positions. Regioselective ortho-
lithiation30 with n-BuLi in the presence of TMEDA and subse-
quent quenching with iodine gave the desired diiodo-terpy 14.
During iodine addition, a thick precipitate formed complicating
both stirring and appropriate cooling. Therefore, an optimal
yield was obtained when the reaction was run on a 1–3 g scale
per batch. Straightforward trituration of crude product with hot
ethanol afforded pure 14.
Retrosynthetic analysis of linear terpyridine 1 leads to the
opening of fused ve-membered rings at a heteroatom Y, giving
a disubstituted ethynyl derivative 2, which could be prepared by
Sonogashira coupling20 between dihalide 3 and acetylene 4
(Scheme 1). Terpyridine 5 stems from 6 with ortho-directing
groups at the 5- and 500- positions to enable ortho-metalation,21
and subsequent addition of a halide electrophile would lead to
compound 3. For Y ¼ O, 6 is 5,500-dihydroxy-2,20:60,200-terpyr-
idine and the target ligands are 2,6-bis(2-substituted-furo[2,3-c]-
pyridine-5-yl)pyridines.
Synthesis of the terpy core
The pursuit of 2,6-bis(2-substituted-furo[2,3-c]pyridine-5-yl)-
pyridine-based ligands motivated the development of an effi-
cient chromatography-free synthesis (Scheme 2) of the key
intermediate 4,400-diiodo-5,500-bis(methoxy-methoxy)-2,20:60,200-
terpyridine (14). The route to 14 follows a Stille22 cross-
coupling strategy via 8 and 10. Regioselective lithiation23 of
commercially available 2,5-dibromopyridine (7) with n-butyl-
lithium and subsequent addition of 2-methoxy-4,4,5,5-tetra-
methyl-1,3,2-dioxaborolane gave 5-borylated pyridine 824 in
good yield.
Synthesis of acetylenes
The modular synthesis of linear bilateral terpy ligands requires
functionalized acetylenes to incorporate desired moieties in the
anking positions. The introduction of manisyl groups31
The precursor of the central ring, 2,6-bis(trimethylstannyl)-
pyridine (10), was synthesized by nucleophilic stannylation of
2,6-dichloropyridine (9) with freshly prepared NaSnMe3 in good
yield.25,26 Terpy 11 was prepared by Stille coupling between
bromide 8 and bisstannane 10 according to the procedure
24
reported by Schluter. Oxidation/hydroxydeboronation27 of 11
¨
Scheme
2
Synthesis of 4,400-diiodo-5,500-bis(methoxymethoxy)-
2,20:60,200-terpyridine (14). (a) n-BuLi, Et2O, ꢁ78 ꢀC, 3 h, then B-
methoxypinacolborane, ꢁ78 ꢀC to rt, 12 h, 83%; (b) Na, Me3SnCl, DME,
ꢁ15 ꢀC, then 9, ꢁ15 ꢀC to rt, 18 h, 84%; (c) 5 mol% Pd(PPh3)4, toluene,
reflux, 24 h, 56%; (d) 30% H2O2, aq. NaOH, THF, rt, 18 h, 96%; (e) 60%
NaH, THF, DMF, 0 ꢀC, then MOMCl/MeOAc,31 0 ꢀC to rt, 12 h, 96%; (f)
2.2 eq. n-BuLi, TMEDA, THF, ꢁ78 ꢀC, 1 h, then 2.2 eq. I2, ꢁ78 ꢀC to rt,
50%.
Fig. 2 Molecular design of linear bilateral extended terpyridines.
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Scheme 3 Synthesis of acetylene 18. (g) n-BuLi, THF, ꢁ78 ꢀC, 30 min,
then I2, ꢁ78 ꢀC to rt, overnight, 81%; (h) trimethylsilylacetylene, 5 mol%
Pd(PPh3)2Cl2, 10 mol% CuI, toluene, Et3N, reflux, 18 h, 95%; (i) KF,
MeOH, 40 ꢀC, 36 h, 96%.
Scheme 4 Synthesis of 5-ethynyl-2,20-bipyridines 22. (j) 1. n-BuLi,
THF, ꢁ78 ꢀC, 1 h; 2. ZnCl2, THF, ꢁ78 to 0 ꢀC, 1 h; 3. 7, 1.5 mol%
Pd(PPh3)4, THF, reflux, 18 h, 20a: 31%, 20b: 48%;34 (k) trimethylsilyla-
cetylene, 10 mol% Pd(PPh3)2Cl2, 5 mol% CuI, Et3N, reflux, 18 h; (l) KF,
MeOH, rt, 18 h, over 2 steps 22a: 72%, 22b: 76%.
improves the solubility of polypyridine ligands and facilitates
later homologation; therefore, several acetylenes containing
this group were prepared. The synthesis of 1-ethynyl-4-methoxy-
2,6-dimethylbenzene (18)32 was accomplished according to
Scheme 3. The bromide 1533 was converted to the iodide 16 by
lithiation and subsequent quenching with iodine. Under opti-
mized conditions for Sonogashira coupling, 16 reacted
smoothly with trimethylsilylacetylene to produce silyl-protected
acetylene 17. Deprotection with KF in methanol gave 18 in high
yield (Scheme 3).
Table 1 Synthesis of 23 and 24 by Sonogashira couplinga
Yield (%)
Entry
R ¼
eq.e
Time (h)
Disubst.
Monosubst.
14f
1
2
2.5
2.5
20
20
23a
23b
95
92
—
—
—
—
—
—
3
5.0
4.5b/18c
23c
86
—
—
—
4
2.1d
20
23d
88
—
—
—
5
6
7.8
20
20
23e
23f
85
90
—
—
—
—
—
—
2.08
7
8
1.05
1.05
6
6
23f
18
17
24a
24b
40
39
24
22
23g
9
1.05
15
23h
15
24c
38
28
a
b
Reaction conditions: (m) acetylene 4, 5 mol% Pd(PPh3)2Cl2, 10 mol% CuI, THF, Et3N, reux; (n) Trimethylsilylacetylene, 5 mol% Pd(PPh3)2Cl2,
10 mol% CuI, THF, Et3N, reux. c Then the crude mixture was subjected to TMS deprotection with KF, MeOH; (o) 1.0 eq. 23c. d 4-Iodopyridine (25),
10 mol% Pd(PPh3)2Cl2, 20 mol% CuI, THF, Et3N, reux. Unless otherwise stated, equivalents indicated for acetylenes 4. f Recovered 14.
e
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excess amount (>2 eq.) of corresponding alkynes in good to high
yields (Table 1, entries 1, 2, 5, and 6). Due to the instability of 4-
ethynylpyridine,36 23d was prepared in a stepwise fashion. First,
bisethynyl-terpy 23c was synthesized in two steps by Sonoga-
shira coupling between 14 and trimethylsilylacetylene following
one-pot deprotection of silyl groups in the presence of KF (Table
1, entry 3). A subsequent coupling reaction with 4-iodopyridine
(25) under standard conditions (Table 1, entry 4) gave 23d in
good yield.
To incorporate two different substituents into the 4- and 400-
positions, Sonogashira coupling was performed with 1.05
equivalents of acetylene (Table 1, entries 7–9), giving a statis-
tical mixture of mono- and bis-coupled products 24a–c and 23f–
h, respectively, as well as unreacted diiodo terpyridine 14. These
mixtures were easily separated by column chromatography, and
pure monosubstituted intermediates 24a–c were obtained. The
non-symmetric terpyridine/bipyridine conjugates 24a and 24b
were further subjected to coupling with manisyl acetylene 18 to
give products 26a and 26b in good yields (Scheme 5).
With the library of 4,400-disubstituted terpyridines 23a–h, 26a
and 26b in hand, developing general cyclization conditions to
access the 2-substituted-furo[2,3-c]pyridine motif became the
focus.
Scheme 5 Synthesis of non-symmetric ligand precursors 26. (p)
Acetylene 18, 5 mol% Pd(PPh3)2Cl2, 10 mol% CuI, THF, Et3N, reflux,
17–20 h.
Manisyl substituted 5-ethynylbipyridines 22a and 22b were
synthesized from known bromopyridines 19a31 and 19b34
(Scheme 4). Negishi coupling35 with 2,5-dibromopyridine (7)
afforded bromobipyridines 20a and 20b,34 which coupled well
with trimethylsilylacetylene to give 21a and 21b. Subsequent
deprotection with KF formed ethynylbipyridines 22a and 22b in
good yields over 2 steps.
Ligand synthesis
In order to obtain ligand L4, conditions for acidic depro-
tection of the MOM group,37 followed by base-assisted cyclo-
isomerization were tested on the mixed terpyridine/bipyridine
26a. Various reaction conditions, like varying the acid, base,
solvent, and reaction time, were tried (Table 2, entries 1–5). The
use of HCl, Cs2CO3, and DMF resulted in the isolation of L4 in
high yield (Table 2, entry 5). These conditions also led to the
isolation of L1–3, L5, and L6 (Table 2, entries 6–10) in good to
high yields.
Simple symmetric ligand precursors – bisethynyl-terpy deriva-
tives 23a, 23b, and 23e, as well as a mixed variation with
bipyridine 23f, were prepared by Sonogashira coupling with an
Table 2 One-pot MOM-deprotection/cycloisomerisation of 23 and
26a
Metal complexes
A linear bilateral extended conformation of ligands can be
acquired by formation of complexes with a 2 : 1 ligand-to-metal
ratio (2 : 1 complexes). Initially, complexation of simple
symmetric ligands L1–3 was tested with divalent octahedral
metals (Ru2+, Zn2+, and Fe2+) to form molecular “crossings”
(Table 3). Ruthenium(II) complexes [L2Ru](PF6)2 were prepared
by heating the corresponding ligands L1–3 with RuCl2(DMSO)4
Yield
in ethylene glycol at 120 C,4c resulting in high product yields
ꢀ
Entry SM HCl (eq.) Solvent Base
Eq. Time (h)
L
(%)
(Table 3, entries 1, 4, and 7). Simple phenyl-substituted ligand
L1 and n-hexyl-substituted ligand L2 formed 2 : 1 zinc(II)
complexes with zinc(II) triate in a mixture of tetrahydrofuran
and methanol at room temperature (Table 3, entries 2 and 5). As
1b
2b
3b
4b
5
6
7
8
9
26a
4
MeOH NaOMec
MeOH NaOMec 40 40
8
20
L4
L4
0
5
26a 20
26a 42
26a 42
26a 21
26b 21
23a
23b
23d
EtOH
THF
NaOMec 85 24
Cs2CO3 60 24
Cs2CO3 43 76
Cs2CO3 43 48
Cs2CO3 43 48
Cs2CO3 43 48
Cs2CO3 43 48
Cs2CO3 15 72
L4 11
L4 19
L4 97
L5 94
L1 92
L2 95
L3 90
L6 73
ꢀ
4-pyridyl-substituted ligand L3 is less soluble, heating at 50 C
DMF
DMF
DMF
DMF
DMF
DMF
was necessary to facilitate complexation, as shown in Table 3,
entry 8. Similarly, Fe2+ complexes were prepared by reacting
iron(II) tetrauoroborate with the corresponding ligand in a
mixture of tetrahydrofuran and water at room temperature
(Table 3, entries 3 and 6), but, in the case of L3, addition of
5
5
5
10
23f 10
a
32% aq. HCl was added to the starting material (SM) in the indicated acetonitrile to the reaction mixture was necessary to improve
solvent and heated to 80 ꢀC until the deprotection of MOM was
solubility and yield (Table 3, entry 9). All resulting metal
complexes were precipitated with aqueous KPF6 and were
obtained in good to high yield.
complete (monitored by LC-MS). Then the base was added, and the
reaction mixture was heated to 90 ꢀC for the corresponding time,
unless otherwise stated. b Heated to reux. c 5.4 M NaOMe in MeOH.
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Table 3 Synthesis of metal complexes with simple symmetric ligands L1–3a
Entry
Ligand
R0 ¼ R00
¼
Metal source
Solvent
Time (h)
T (ꢀC)
Product
Yield (%)
1
2
3
RuCl2(DMSO)4
Zn(OTf)2
Fe(BF4)2$6H2O
Ethylene glycol
THF/MeOH
THF/H2O
18
18
18
120
rt
rt
[L12Ru](PF6)2
[L12Zn](PF6)2
[L12Fe](PF6)2
92
94
90
L1
4
5
6
RuCl2(DMSO)4
Zn(OTf)2
Fe(BF4)2$6H2O
Ethylene glycol
THF/MeOH
THF/H2O
18
18
18
120
rt
rt
[L22Ru](PF6)2
[L22Zn](PF6)2
[L22Fe](PF6)2
99
87
90
L2
L3
7
8
9
RuCl2(DMSO)4
Zn(OTf)2
Fe(BF4)2$6H2O
Ethylene glycol
THF/MeOH
THF/MeCN/H2O
18
48
18
120
50
rt
[L32Ru](PF6)2
[L32Zn](PF6)2
[L32Fe](PF6)2
98
94
80
a
A solution of metal source (0.5 eq.) was added to a solution of L1–3 (1.0 eq.) and stirred at the indicated temperature. Aer the stated reaction time,
sat. aq. KPF6 was added to induce precipitation and the solid was collected by ltration.
Analogous to simple ligands L1–3, mixed terpyridine/bipyr- forming either “corner” complexes with L4 and L5, or
idine ligands L4–6 reacted with Zn(OTf)2 and Fe(BF4)2$6H2O “crossing” complexes with L6, leaving the bipyridine coordi-
selectively at the terpyridine coordination site (Table 4), nation site unreacted. This selectivity can be explained by the
Table 4 Synthesis of metal complexes with mixed terpyridine/bipyridine ligands L4–6a
Entry
Ligand
R0, R00
Metal source
Solvent
Time (h)
T (ꢀC)
Product
Yield (%)
1
2
3
Zn(OTf)2
Fe(BF4)2$6H2O
RuCl2(DMSO)4
THF/MeOH
THF/H2O
Ethylene glycol
18
18
48
rt
rt
120
[L42Zn](PF6)2
[L42Fe](PF6)2
[L42Ru](PF6)2
98
97
—
L4
b
4
5
6
Zn(OTf)2
Fe(BF4)2$6H2O
RuCl2(DMSO)4
THF/H2O
THF/H2O
Ethylene glycol
6
6
48
rt
rt
120
[L52Zn](PF6)2
[L52Fe](PF6)2
[L52Ru](PF6)2
99
80
—
L5
b
7
8
9
Zn(OTf)2
Fe(BF4)2$6H2O
RuCl2(DMSO)4
THF/MeOH
THF/H2O
Ethylene glycol
20
18
48
rt
rt
120
[L62Zn](PF6)2
[L62Fe](PF6)2
[L62Ru](PF6)2
92
80
—
L6
b
a
A solution of metal source was added to a solution of L4–6 and stirred at the indicated temperature. Aer the stated reaction time, sat. aq. KPF6
was added to induce precipitation and the solid was collected by ltration. b Complicated mixture, product could not be isolated.
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Fig. 4 The molecular structures of (a) phenyl substituted ligand L1 (b)
mixed bipyridine/terpyridine ligand L4 (hydrogen atoms omitted for
clarity, displacement ellipsoids drawn at the 50% probability level).
kinetic lability of Zn(II) and Fe(II) complexes and the thermo-
dynamic stability of the highest order chelate. In contrast, the
kinetically inert character of Ru(II) led to a complicated mixture
of oligomers when L4–6 were used (Table 4, entries 3, 6, and 9),
and the isolation of the desired 2 : 1 complexes was not
successful. Therefore, an alternative approach towards ruth-
enium(II) complexes was considered for the mixed ligand
systems L4–6. By rst introducing ruthenium(II) and then
functionalizing the obtained complex with bipyridine moieties,
the desired “corner” and “crossing” complexes were obtained
(Schemes 6 and 7). Diiodoterpyridine 14 was subjected to
complexation with 0.5 equivalents of RuCl2(DMSO)4 by heating
in ethylene glycol (Scheme 6). Unexpectedly, during the reac-
tion, the MOM protecting groups were cleaved, leading to the
formation of compound 27. This nding obviated harsh acidic
conditions37 for MOM-deprotection.
Scheme 6 Indirect synthesis towards the ruthenium(II) complex with
mixed ligand L6. (q) 0.5 eq. RuCl2(DMSO)4, ethylene glycol, 120 ꢀC, 48
h, then aq. KPF6; (r) 10 mol% PdCl2(PPh3)2, 20 mol% CuI, 4.2 eq. 22a,
DMF, DIPEA, 80 ꢀC, 48 h, then aq. KPF6.
Crude complex 27 was used in the next step, where Sono-
gashira cross-coupling20b with acetylene 22a and in situ ring
closure38 afforded compound [L62Ru](PF6)2 directly, thus,
leading to the desired complex in just two steps starting from 14
with a 48% overall yield.
Scheme 7 Indirect synthesis towards ruthenium(II) complexes with
mixed ligands L4 and L5. (s) 0.5 eq. RuCl2(DMSO)4, EtOH, reflux, 72 h,
then aq. KPF6; (t) 5 mol% PdCl2(PPh3)2, 10 mol% CuI, 22a or 22b, DMF,
Et3N, 90 ꢀC, 24–39 h, then aq. KPF6.
The corresponding non-symmetric Ru(II) complexes
[L42Ru](PF6)2 and [L52Ru](PF6)2 were prepared similarly
(Scheme 7). Complexation of manisyl-substituted iodoterpyr-
idine 24c with 0.5 eq. of RuCl2(DMSO)4 in EtOH in one step gave
complexed, deprotected, and partially cyclized product 28,
which was used without further purication in the subsequent
reaction. The Sonogashira coupling with ethynylbipyridine 22a
or 22b and one-pot cyclization gave the desired complexes with
43% or 40% isolated yields over two steps, respectively.
The 1H NMR spectra of [L42Zn](PF6)2 and [L42Fe](PF6)2 (Fig. 3,
spectra b and c) parallel that of [L42Ru](PF6)2 (Fig. 3, spectrum d).
Given that [L42Ru](PF6)2 is synthesized by graing bipy units
onto the preformed terpy ruthenium(II) complex 28, it seems
reasonable to conclude that Fe(II) and Zn(II) also bind at the terpy
unit of L4. Terpyridine transition metal complexes exhibit char-
acteristic strong upeld shis of the proton o and j signals (d–p*
back-donation) and downeld shi for the signal of proton l
(deshielding due to the conformational change of terpyridine)
compared to the shis observed for the free ligand (Fig. 3,
spectrum a). The proton signals corresponding to the bipyridine
moiety are less affected. These observations hold true for all Zn2+,
Fe2+, and Ru2+ complexes in the ligand series L4–6.
Fig. 3 1H NMR (400 MHz, CD2Cl2) for the aromatic regions in L4 and
its Zn2+, Fe2+, and Ru2+ complexes.
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Fig. 5 The switching of L1 conformation from “L-shaped” to the “linear bilateral” and theꢁ structures of the cations of (a) [L12Zn](PF6)2$Et2O$6MeCN;
(b) [L12Ru](PF6)2$2Et2O$3MeCN; (c) [L12Fe](PF6)2$2Et2O$MeCN (hydrogen atoms, PF6 and solvent molecules omitted for clarity, displacement
ellipsoids drawn at the 50% probability level).
Table 5 Selected bite angles and bond lengths of L1 metal complexes
[L12Fe](PF6)2
[L12Ru](PF6)2
[L12Zn](PF6)2
C(2)–N(2)–C(2)a (ꢀ)
N(2)–M–C(2)b (ꢀ)
142.1(3)
92.8(1)
1.971(3)
1.882(3)
146.2(4)
89.9(2)
2.061(5)
1.983(4)
150.1(2)
87.13(6)
2.183(2)
2.080(2)
b
˚
M–N(1) (A)
b
˚
M–N(2) (A)
a
Average of two such parameters in the molecule. b Average of four such
parameters in the molecule.
Structure
Single crystals of the free ligands L1 and L4 suitable for X-ray
crystallography were obtained by diethyl ether vapor diffusion
into a saturated dichloromethane solution. In Fig. 4, the
molecular structures clearly show the presence of furo[2,3-c]-
pyridine-5-yl motifs linked by the central 2,6-disubstituted
pyridine ring, thus forming the desired ligand architecture. The
usual conformation of uncoordinated terpyridine has
a
“L-shaped” structure with the anking substituents placed
almost perpendicular to each other, as seen for L1 (Fig. 4a). One
arm of the mixed ligand L4 consists of furo[2,3-c]pyridine-5-yl,
bipyridyl and manisyl groups in a linear disposition and the
second arm is the 2-manisyl-furo[2,3-c]pyridine-5-yl unit
(Fig. 4b). The manisyl rings in L4 adopt a clinal conformation to
the mean plane of the ligand, in contrast to the periplanar
conformation of the simple phenyl substituents of L1. This
deviation from planarity seen in the crystal structure of L4,
correlates with the much better solubility of the manisyl
substituted ligands L4–6 in common organic solvents.
Fig. 6 Emission and absorption spectra of (A) free ligands L1–6 and (B)
Zn(II), Fe(II) and Ru(II) complexes of L5 in CH2Cl2 solution.
illustrated by the molecular structures of the Zn2+, Ru2+, and
Fe2+ complexes with L1 depicted in Fig. 5. Crystals for
complexes [L12Zn](PF6)2, [L12Ru](PF6)2, and [L12Fe](PF6)2 were
obtained by diffusion of diethyl ether vapor into the corre-
sponding acetonitrile solutions.
The complexation of linear terpy ligands with metal ions
preferring octahedral geometry transforms the overall ligand
conformation from “L-shaped” to a “linear bilateral” one, as
These crystal structures possess triclinic symmetry and
ꢀ
belong to space group P1. In eꢁach case, the asymmetric unit
consists of one cation, two PF6 anions, and a cavity situated
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about a center of inversion containing disordered solvent
molecules. The three structures are essentially isostructural
except that the contents of the solvent cavities may differ (for
the treatment and estimation of the solvent content, see the
deposited CIF les in the ESI†).
The C(2)–N(2)–C(2) bite angle in these complexes increases
in the order Fe2+ < Ru2+ < Zn2+ (atom numbers are dened in the
sketch in Table 5). For an ideally “linear” ligand topology
overall, this angle should be ꢂ160ꢀ. The ligand in the zinc
complex therefore has an almost “linear” topology, while in the
iron complex it has a slightly more bent character. This trend
correlates with the metal-to-nitrogen bond lengths, although
that is not necessarily intuitive. An increase in the M–N2
distance would tend to decrease the bite angle, whereas
increases in the M–N1 distances would open it.
Fig. 7 The structure of the symmetry-unique part of the polymeric
cation in L3FeAg. (Color code: Fe, orange; Ag, silver; N, light blue; O,
red; C, dark gray; hydrogen atoms and PF6 omitted for clarity,
ꢁ
displacement ellipsoids drawn at the 50% probability level.)
extended terpy ligands L1–6 and their corresponding Zn(II),
Fe(II), and Ru(II) complexes can be found in the ESI† (Tables S1,
S2 and Fig. S15–S17).
Photophysical properties
The UV/Vis spectra of the free ligands L1–6 display absorption
maxima in a range of 239 to 338 nm (Fig. 6A; see ESI† Table S1).
These absorptions can be attributed to p–p* transitions. The
ligands L1–6 are uorescent with emission maxima from 338 to
426 nm and quantum yields ranging from 0.14 to 0.62.
Heterometallic supramolecular networks
A solution of AgPF6 in ethanol was layered over a solution of the
The absorption spectra of all zinc(II), iron(II), and ruth- 4-pyridyl substituted [L32Fe](PF6)2 complex in acetonitrile. A
enium(II) complexes with L1–6 (Fig. 6B; see ESI† Table S2,
Fig. S16, and S17) display pronounced ligand-centered (LC)
transitions. However, the complexes with iron(II) and ruth-
enium(II) also show characteristic lower energy bands from
360 to 600 nm which can be attributed to MLCT transitions.39
The zinc(II) complexes exhibit a uorescence comparable to that
of the parent ligands with quantum yields ranging from 0.10 to
0.63.
The MLCT bands of the iron(II) and ruthenium(II) complexes
with L1–6 are non-luminescent at room temperature. However,
excitation at the LC bands shows weak but detectable
uorescence.
The free ligands and their zinc(II) complexes exhibit solid-
state emission with moderate to medium quantum yields (ESI†
Tables S1 and S2). A more detailed discussion about the solu-
tion and solid-state photophysical properties of the bilateral
Scheme 8 Synthesis of 3D L3FeAg and 2D L3FeCu heterometallic Fig.
8
The diamondoid net (dia-b) topology of L3FeAg and
supramolecular networks.
interpenetration.
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Similarly, a [Cu(CH3CN)4]PF6 solution was slowly diffused
into the acetonitrile solution of [L32Fe](PF6)2, using a small
amount of cyclohexane between the layers. Dark purple block-
like crystals were obtained over three weeks, and the resulting
product [L32FeCu(MeCN)]n(PF6)3n$4.5nMeCN (further referred
to as L3FeCu) was separated by ltration (Scheme 8).
The X-ray crystal structures of complexes L3FeAg and
L3FeCu revealed that they are coordination polymers. The
asymmetric unit of L3FeAg contains two repeats of the chemi-
cally unique portion of the polymeric cation (Fig. 7), six disor-
ꢁ
dered PF6 anions and a cavity situated about a center of
Fig. 9 The structure of the symmetry-unique part of the polymeric
cation in L3FeCu. (Color code: Fe, orange; Cu, purple; N, light blue; O,
red; C, dark gray; hydrogen atoms and PF6 omitted for clarity,
inversion containing disordered solvent molecules. The
cationic structure is
a three-dimensional doubly inter-
ꢁ
penetrating coordination framework with (6,4) diamondoid net
(dia-b) topology.41 The nodes of this framework are tetrahedral
silver(I) centers (Fig. 8, light blue balls) and octahedral iron(II)
centers (Fig. 8, magenta balls), where the four arms of the
molecular “crossing” act as linkers. The inversion related
network is interpenetrated in a manner characteristic of dia-
mondoid type networks.16a
displacement ellipsoids drawn at the 50% probability level.)
The structure of the iron(II) based molecular “crossings”
reveal that the furo[2,3-c]pyridine arms of the ligand are signif-
icantly tilted; therefore, the angle between the N atoms of both
anking 4-pyridiyl groups and the N atom of the central ring of
terpy, N(4)–N(2)–N(5), is 134.43(5)ꢀ. The shortest Ag(1)/Ag(2)
and Fe(1)/Fe(2) distances within a single net are 16.9095(6) and
˚
16.7399(8) A, respectively. This causes signicant voids and
channels within the structure. Each unit cell contains one
solvent-lled centrosymmetric cavity which comprises 31.5% of
the total volume (calculated using PLATON42).
The X-ray crystal structure of L3FeCu shows that Fe(II) based
molecular “crossings” in the presence of Cu(I) assemble into a
structure similar to that of L3FeAg. However, one coordination
site of the tetrahedral Cu(I) center is occupied by acetonitrile
(Fig. 9). Consequently, one of four 4-pyridyl groups, which is
disordered, is not coordinated to the copper atom and the
cationic structure of L3FeCu consists of stacked 2D polymeric
layers with a (6,3) honeycomb (hcb)41 net topology (Fig. 10).
Adjacent layers are related by inversion symmetry. The nodes of
this framework are tetrahedral copper(I) centers (Fig. 10, dark
red balls) and octahedral iron(II) centers (Fig. 10, magenta balls),
where only three arms of the molecular “crossing” act as linkers.
The shortest Fe(1)/Fe(1) and Cu(1)/Cu(1) distances within
Fig. 10 Ball and stick representation of one cationic layer of L3FeCu
viewed down the a axis. (Color code: Fe, magenta; Cu, dark red; N,
light blue; O, red; C, dark gray.)
small amount of cyclohexane between the layers was used to
slow down the diffusion process. In a few days ne needle-like
crystals40 formed which over two weeks transformed into dark
purple blocks that were separated by ltration to give coordi-
nation polymer [L32FeAg]n(PF6)3n$6.5nMeCN (further referred
to as L3FeAg) (Scheme 8).
˚
each net are 16.7466(6) and 18.3235(6) A, respectively. The angle
between the N atoms of anking 4-pyridiyl groups at the N atom
of the central ring of terpy, N(10)–N(7)–N(9), is 135.23(5)ꢀ.
While the L3FeAg and L3FeCu structures are similar
(Fig. 11), the 3D topology in L3FeAg is reduced to a 2D layer
network by replacing a pyridine connection to silver with a
competing MeCN ligand in L3FeCu. Interestingly, in L3FeAg,
the Ag–N(pyridyl) bond lengths at each unique Ag(I) cation are
grouped into two pairs of distinctly different distances, one with
˚
˚
an average distance of 2.25 A and the other pair at 2.41 A.
The 1H and 13C NMR spectra of vacuum dried and dissolved
crystals of L3FeAg and L3FeCu in acetonitrile-d3 are almost
identical to that of [L32Fe](PF6)2 suggesting that the solid state
structure disassociates in solution. The proton signals of the 4-
Fig. 11 Schematic representation of the 3D cationic structure of
L3FeAg and 2D layered cationic structure of L3FeCu.
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pyridyl groups are slightly shied due to the presence of Ag+ or tools in the hands of chemists and could inspire the creation of
Cu2+ ions.
new designed molecular architectures.
Conclusions
Acknowledgements
The concept of a linear bilateral extended terpyridine was
developed by fusing ve-membered rings to the anking pyri-
dine rings of the terpy ligand, thus mimicking the extended
geometry of 5,50-functionalized 2,20-bipyridine. It was realized
synthetically by developing a modular synthesis of 2,6-bis(2-
substituted-furo[2,3-c]pyridine-5-yl)pyridine based ligands. This
modular synthesis allowed for the introduction of alkyl, aryl,
and heteroaryl functionalities in the anking positions of these
ligands. The molecular “crossings” were synthesized by coor-
dinating simple symmetric ligands to divalent metal cations
(Fe2+, Ru2+and Zn2+) forming 2 : 1 complexes. In the case of
mixed bipyridine/terpyridine ligands, zinc(II) and iron(II) selec-
tively form complexes at the terpyridine coordination site. The
corresponding ruthenium(II) complexes were prepared through
an indirect methodology. First, unfunctionalized ruthenium(II)
terpyridine 2 : 1 complexes were prepared, then bipyridyl
This work was funded by the Swiss National Science Founda-
tion. We thank PD Dr Laurent Bigler and Urs Stadler for
measuring mass spectra, and Peter Uebelhart for his work on X-
ray crystal structures. J. S. S. acknowledges support from the
Collaborative Innovation Center of Chemical Science and
Engineering, Tianjin.
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