4,4″-disubstituted terpyridines and their homoleptic FeII complexes

Article abstract of DOI:10.1002/ejic.201300231

A novel synthetic route to 4,4″-disubstituted-2,2′:6′, 2″-terpyridine ligands by Suzuki-Miyaura cross-coupling was elaborated by synthesizing compounds 4a-5c. The considerable stability of 4-substituted lithium triisopropyl 2-pyridylborates 2a-c, which are less prone to protodeboronation than similarly functionalized neutral boronic acid derivatives, enabled this synthetic route. The terpyridine core structure was further functionalized by exposing 4,4″-dichloroterpyridine (4b) to Suzuki coupling conditions to yield 4,4″-diarylterpyridines 5a-c. Homoleptic Fe^II complexes 8a-f of the reported terpyridine ligands were formed quantitatively, which demonstrates the lack of steric repulsion of substituents at the 4- and 4″-positions during complexation. The solid-state structures of particular ligands and Fe^II complexes were analyzed by single-crystal X-ray crystallography. UV/Vis absorption data for the Fe ^II complexes are also provided to complement the results reported here. A novel synthetic route to terpyridine ligands is reported. Pyridine building blocks are interlinked by Suzuki-Miyaura cross-coupling reactions. The potential of the method is demonstrated by assembling the 4,4″- disubstituted terpyridine ligands shown, which are subsequently converted into their homoleptic Fe^II complexes. Copyright

Full text of DOI:10.1002/ejic.201300231

FULL PAPER
DOI:10.1002/ejic.201300231
4,4ЈЈ-Disubstituted Terpyridines and Their Homoleptic FeII
Complexes
Gero D. Harzmann,[a] Markus Neuburger,[a] and Marcel Mayor*[a,b]
Keywords: Cross-coupling / Borates / Heterocycles / Iron / Tridentate ligands
A novel synthetic route to 4,4ЈЈ-disubstituted-2,2Ј:6Ј,2ЈЈ-ter- terpyridines 5a–c. Homoleptic FeII complexes 8a–f of the re-
pyridine ligands by Suzuki–Miyaura cross-coupling was ported terpyridine ligands were formed quantitatively, which
elaborated by synthesizing compounds 4a–5c. The consider- demonstrates the lack of steric repulsion of substituents at
able stability of 4-substituted lithium triisopropyl 2-pyr- the 4- and 4ЈЈ-positions during complexation. The solid-state
idylborates 2a–c, which are less prone to protodeboronation
than similarly functionalized neutral boronic acid derivatives,
enabled this synthetic route. The terpyridine core structure
was further functionalized by exposing 4,4ЈЈ-dichloroterpyr-
idine (4b) to Suzuki coupling conditions to yield 4,4ЈЈ-diaryl-
structures of particular ligands and FeII complexes were ana-
lyzed by single-crystal X-ray crystallography. UV/Vis absorp-
tion data for the FeII complexes are also provided to comple-
ment the results reported here.
fluorometric chemosensors. Complexes of tpy also show a
high potential in clinical applications;[22–24] examples are
their ability to intercalate with DNA and their cytotoxicity,
which has been studied, for instance, on Ru(tpy)Cl3 that
exhibits activity against certain leukaemia cells well com-
parable to the activity of cisplatin.[22,23] Terpyridines have
also been widely introduced into nanostructures[25–27] and
show interesting self-assembly structures on Au sur-
faces.[28,29] Because of their extraordinary optical and elec-
trochemical properties, in particular their high absorption
coefficients, tpy complexes were implemented into lumines-
cent devices.[30] For example, their implementation into dye-
sensitized solar cells (DSSC) in 1991[31] provided cells with
increased incident photon-to-current conversion efficiency
(IPCE)[31,32] with respect to the parent Grätzel cells.[33,34]
However, the substitution patterns of the tpy subunits
within the molecules of the previously described functional
materials reveal only little variation, since nearly all substit-
uents are attached to the 4Ј-position of the middle ring and
occasionally to the 5- and 5ЈЈ-positions of the exterior rings,
pointing at restricted synthetic accessibility of other substi-
tution patterns. An additional handicap of the majority of
the coupling reactions affecting the exterior tpy rings is that
most of them rely on quite toxic organotin reagents.[35,36]
We therefore became interested in environmentally less
harmful synthetic procedures to make alternative substitu-
tion patterns of the tpy parent structure available. In par-
ticular, 4,4ЈЈ-disubstituted terpyridines moved into our fo-
cus of interest because of the almost linear arrangement
of the tpy subunit in molecular architectures comprising a
complexed tpy and the strong electronic coupling between
the peripheral substituents and the coordinating nitrogen
atoms.
Introduction
Terpyridine (tpy) subunits and their metal complexes are
important building blocks in modern chemistry.[1] Their
highly versatile coordination chemistry, together with their
interesting photophysical and electrochemical properties,
make them central subunits in the preparation of a broad
range of molecular devices as well as functional materials.
Due to the overwhelming variety of functional materials
comprising terpyridines, only a small selection of almost
randomly chosen examples can be discussed here. For ex-
ample a plethora of metallo-supramolecular architec-
tures[2–6] or dendritic molecular networks[7,8] based on tpy
complexes have been reported. Self-assembly processes re-
versibly switchable by external triggers have been shown for
metallo-macrocycles induced by bases[9] or for redox-state-
dependant helicates.[10–12] Furthermore, molecular muscles
consisting of mechanically interlocked ligands comprising
tpy units were developed[13–16] as well as functional poly-
mers containing tpy metal complexes[17–19] like, for example,
fully π-conjugated polymers with pendant tpy substituents
showing pronounced ionochromism as well as distinct
metal-dependant
photoluminescence quenching
ef-
fects.[20,21] These might act as highly sensitive and specific
[a] Department of Chemistry, University of Basel,
St. Johanns-Ring 19, 4056 Basel, Switzerland
Fax: +41-61-267-1016
E-mail: marcel.mayor@unibas.ch
Homepage: http://www.chemie.unibas.ch/~mayor/
[b] Institute of Nanotechnology (INT), Karlsruhe Institute of
Technology (KIT),
P. O. Box 3640, 76021 Karlsruhe, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300231.
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After tpy groups were described for the first time in the stituted tpy units by using a C–H arylation of pyridine N-
literature in 1932,[37] it took almost 60 years until their po- oxides with subsequent reduction has recently been re-
tential as building blocks in supramolecular assemblies was ported;[54] however, it depends on electron-withdrawing
investigated, which, together with their increasing role in groups at the 4Ј-position. The synthetically interesting
coordination chemistry and materials science, has been re- building block 4,4ЈЈ-dichloro-2,2Ј:6Ј,2ЈЈ-terpyridine was re-
viewed repeatedly over the last ten years.[27,38–43] Today cently assembled by using a Hiyama cross-coupling[55] pro-
there are mainly two fundamentally different strategies for tocol, which is, from a synthetic strategy viewpoint, a little
the assembly of the tpy core. The most broadly applied is bit limited in versatility because of the unavailability of
the ring assembly strategy, including the well-established structurally diverse starting materials.
Kröhnke condensation[44,45] and its advancements,[46] which
Here we present a new, efficient and versatile synthesis of
are by far the most common reaction pathways. The second 4,4ЈЈ-disubstituted terpyridine derivatives using a Suzuki–
approach is the assembly of the tpy moiety starting from Miyaura coupling protocol. In particular 4,4ЈЈ-dichloro and
intact pyridine rings and utilizing palladium-catalyzed 4,4ЈЈ-dimethoxy terpyridine were assembled, and, in a sub-
cross-coupling procedures. So far, mainly Stille type reac- sequent coupling reaction, the chlorine atoms of the first
tions have been reported, mostly with 2,6-dihalopyridines one are substituted by differently protected para-thiophenyl
as the central building blocks and 2-(trialkylstannyl)pyr- groups. Iron(II) complexes of these tpy ligands were synthe-
idines as the coupling agents.[35,47–51] The considerably less sized and fully characterized. Solid-state structures provide
toxic Suzuki–Miyaura cross-coupling reaction has not been insight into the spatial arrangement of the peripheral sub-
considered so far for the assembly of tpy structures, pro- stituents.
bably because of the known lability of pyridin-2-ylboronic
acid derivatives, which are prone to protodeboron-
ation.[52,53] However, a few examples of 4,4ЈЈ-disubstituted
tpy groups have already been reported. A 4,4ЈЈ-dimethyl-
2,2Ј:6Ј,2ЈЈ-terpyridine was synthesized by applying a Stille
cross-coupling protocol.[35] A 4,4ЈЈ-distannylated tpy was
Results and Discussion
Molecular Design and Synthetic Strategy
Inspired by a recent article by Billingsley and Buchwald
obtained by an inverse-type Diels–Alder reaction followed reporting the use of some 2-pyridylboron derivatives in par-
by a tin–halogen exchange.[36] The assembly of 4,4ЈЈ-disub- ticularly optimized Suzuki–Miyaura coupling reactions,[56]
Scheme 1. Retrosynthetic analysis of the route towards 4,4ЈЈ-disubstituted terpyridine ligands and the resulting homoleptic [Fe(tpy)2]2+
complexes.
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we considered adapting this procedure for the assembly of required
a
2-boronic acid-4-chloropyridine derivative.
rarely precedented 4,4ЈЈ-disubstituted terpyridines.[36,55,57] Treatment of the commercially available 2-bromo-4-chloro-
Starting from accessible α-brominated 2,4-disubstituted pyridine with different borylation reagents like B(O-TMS)3,
pyridines, the desired 2-pyridylboron derivatives should be B(OMe)3 and bis(pinacolato)diboron did not provide even
readily available. Compared to neutral α-pyridine boronic traces of the desired reagents for the cross-coupling reac-
acid derivatives,[52,53] the lithium triisopropyl 2-pyridyl- tion. The starting material was consumed in all attempts,
boronate seems to be considerably less prone to protode- and thus we hypothesized that the different pyridine bo-
boronation and can be prepared on a large scale.[56] De- ronic acid derivatives were formed but they probably de-
pending on the reactivity of the substituents at the 2- and composed during the course of the reaction, the workup or
4-positions of the pyridine starting material, two different the isolation procedures. Treating the 2-bromo-4-substi-
assembly routes can be considered. With the halide more tuted pyridine precursors 1a–c with a slight excess of nBuLi
strongly prone to metal halide exchange reactions at the 2- in a toluene/THF mixture at –78 °C to promote the bro-
position, the terpyridine core is assembled first (Scheme 1: mine–lithium exchange, followed by quenching with
A–C, red arrows), while with the more reactive halide at the B(OiPr)3 and allowing to warm up to room temperature
4-position, the desired substituents at the 4- and 4ЈЈ-posi- provided the corresponding lithium triisopropyl 2-pyr-
tions of the terpyridine target structures are introduced first idylborates[56] 2a–c in good to excellent yields (92–100%;
(Scheme 1: D–F, blue arrows). For both routes, the key step see Scheme 2). All three lithium salts, 2a–c, exhibit high
is the assembly of the terpyridine core by forming the corre- benchtop stabilities and are available in multigram quanti-
sponding lithium triisopropyl 2-pyridylborates, which react ties. 4-Methoxy precursor 1c was synthesized from the
subsequently in a Suzuki-type coupling reaction with 2,6- phenolic derivative 1d by following a literature pro-
dihalopyridine (A–B or E–F, respectively). In order to im- cedure.[58]
prove the yield in this twofold cross-coupling reaction, the
transformation of the commercial 2,6-dibromopyridine into
the more reactive 2,6-diiodopyridine might be considered
(G in Scheme 1). After formation of the terpyridine core by
the red reaction pathway, the halide atoms at the 4 and 4ЈЈ
positions can be substituted in a subsequent cross-coupling
reaction. This introduction of the peripheral substituents at
a later stage of the reaction sequence provides the modular-
ity required to make a large variety of different 4,4ЈЈ-disub-
stituted terpyridines available. On the basis of these
thoughts, the red reaction pathway was favoured and was
mainly followed during the investigations reported here.
In order to gain structural insight into the spatial ar-
rangement of the complexed ligands, the corresponding
homoleptic [Fe(tpy)2](PF6)2 complexes were considered. We
focussed particularly on differently protected thiol deriva-
tives, as we are interested in their self-assembly features on
noble metal electrodes.
First attempts to assemble the terpyridine unit by a two-
fold Suzuki–Miyaura reaction of the corresponding lithium
borate salt were performed with the Cl-substituted deriva-
tive 2b and 2,6-dibromopyridine (3a). A dry Schlenk tube
was charged with 1 equiv. of 3a and 3 equiv. of 2b together
with the catalytic system consisting of 4 mol-% Pd2dba3 and
24 mol-% Ph2PHO, and 6 equiv. potassium fluoride (KF)
as base. Under an inert protecting atmosphere, the reaction
mixture was dissolved in dry and degassed 1,4-dioxane. The
sealed reaction vessel was heated to 75 °C for 60 hours. Af-
ter cooling to room temperature, ethyl acetate (EtOAc) was
added, and product 4b was isolated by filtration and flash
column chromatography (FCC). The reaction protocol
faced scaling and reproducibility problems, which were
probably due to both the air and moisture sensitivity of
the catalyst/ligand complex and the limited reactivity of the
bromine leaving groups of 3a. For several similar starting
conditions, the isolated yield of 4b varied between 10 and
50%.
In order to maintain the success of the catalytic system,
the bromine atoms of 3a were substituted by iodine atoms,
Synthesis
The red reaction pathway (Scheme 1), which was fol- which are known to be more reactive as leaving groups in
lowed on the basis of the considerations discussed above, Pd-catalyzed cross-coupling reactions.[59] Transhalogena-
Scheme 2. Synthesis of terpyridine derivatives 4a–c by a twofold Suzuki–Miyaura coupling reaction with lithium triisopropyl 2-pyridylbor-
ates 2a–c. Reagents and conditions: i) NaH, MeI, DMF, 0 °C to room temp., 3 d, 24%; ii) nBuLi, toluene/THF (4:1), B(OiPr)3, –78 °C
to room temp., 22 h; iii) Pd2dba3, Ph2PHO, KF, 1,4-dioxane, 75 °C, 60 h, 50%; iv) NaI, HI, reflux, 7 h, 76%; v) Pd2dba3, Ph2PHO, KF,
1,4-dioxane, 75°–85 °C, 68 h.
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tion of 2,6-dibromopyridine (3a) was achieved by heating it in a degassed mixture of toluene and water (5:1). The sealed
to reflux together with sodium iodide in concentrated reaction vessel was heated to reflux for 24 hours. After cool-
hydriodic acid, and the pure product, 2,6-diiodopyridine ing to room temperature and aqueous workup, the desired
(3b), was isolated by crystallization in a yield of 76%.[60–62] disubstituted product 5a was isolated by FCC in only 16%
Substitution of 2,6-dibromopyridine (3a) by 2,6-diiodopyr- together with 37% of the monosubstituted 4-chloro-4ЈЈ-[p-
idine (3b) in the reaction mixture described above improved (methylthio)phenyl]terpyridine (6a). The ratio between di-
both the reproducibility and the isolated yield. Under and monosubstituted products 5a/6a suggests further deac-
otherwise identical reaction conditions, 4,4ЈЈ-dichloroter- tivation of the second chlorine leaving group upon substitu-
pyridine (4b) was isolated in 66% yield after FCC. These tion with an electron-donating group. Both methylmer-
improved reaction conditions were applied for the assembly capto-functionalized terpyridines 5a and 6a displayed lim-
of two additional terpyridine derivatives. Unfunctionalized ited solubility features, which might also handicap their re-
terpyridine 4a was isolated in 55% yield under the same activity. Furthermore, rather harsh reaction conditions are
reaction conditions with use of lithium borate salt 2a and required to remove the methyl protection group of a meth-
diiodopyridine 3b. Use of the same reaction conditions with ylmercapto group.
lithium (methoxypyridyl)borate salt 2c provided 4,4ЈЈ-di-
To address both the solubility as well as the harsh depro-
methoxyterpyridine (4c) in only 32% yield together with 4- tection conditions, the methyl group was replaced by an
methoxybipyridine as a major side product isolated in 36% ethyl(trimethylsilyl) group in a second attempt. Fortunately,
yield. As this side product was mainly observed with the this rather small structural alteration turned out to have a
electron-donating methoxy substituent, our working hy- large impact on the yield obtained. When 1 equiv. of 4b and
pothesis is that the reaction intermediate formed after the a sixfold excess (2 equiv.) of 2,4,6-tris{4-[2-(trimethylsilyl)-
first coupling reaction, 2-iodo-4-methoxybipyridine, is ethyl]-thiophenyl}-1,3,5,2,4,6-trioxatriborinane (7b) were
prone to deiodination.
dissolved in a degassed toluene/water mixture (5:1) together
The reasonable availability of modular key intermediate with 10 mol-% of the Pd catalyst and 6 equiv. K2CO3, di-
4,4ЈЈ-dichloroterpyridine (4b) further favoured the red path- substituted product 5b was isolated in a yield of 99% after
way of the synthetic strategy (Scheme 1). The poor reac- aqueous workup and FCC. Borylation agent 7b was pre-
tivity of the chlorine substituents in palladium-catalyzed viously prepared in two steps starting from 4-bromothio-
cross-coupling reactions was an advantage during the as- phenol with a highly improved overall yield of 79% gen-
sembly of the terpyridine unit, as the large difference in re- erally by following literature protocols.[64–66] Acetyl is a par-
activity between the bromine and chlorine substituent pro- ticularly appealing thiol protection group preventing the
vided regioselectivity. For the further decoration of the formation of disulfides and requiring very mild conditions
dichloroterpyridine 4b, however, the inactivity of the pe- for removal.[67,68] However, the functionality should not be
ripheral leaving groups becomes a synthetic challenge. For- present during the Suzuki–Miyaura coupling because of the
tunately, the catalytic system developed by Guram and co- base lability of thiophenolacetylates. Thus acetyl-protected
workers,[63] consisting of PdCl2[PtBu2(p-NMe2-Ph)]2 as cat- terpyridine derivative 5c was obtained by transprotection of
alyst and K2CO3 as base, enabled the substitution of the 5b. First attempts using silver tetrafluoroborate in dichloro-
chlorine atoms of 4b.
methane (DCM) in the presence of an excess amount of
In first attempts, 4b was treated with 4-(methylthio)phen- acetyl chloride (AcCl) to obtain mild transprotection condi-
ylboronic acid. An oxygen-free Schlenk tube was charged tions failed, and mainly the ethyl(trimethylsilyl)-protected
with 1 equiv. of 4b and 3–7 equiv. of the boronic acid to- starting material was recovered by FCC. Transprotection
gether with 5 mol-% of the catalyst {PdCl2[PtBu2(p-NMe2- was accomplished by treating 5b with 10 equiv. tetrabutyl-
Ph)]2} and 4 equiv. of K2CO3 as base. Under a nitrogen ammonium fluoride (TBAF) in degassed tetrahydrofuran
protection atmosphere, the reaction mixture was dissolved (THF) at room temperature and subsequent cooling to
Scheme 3. Assembly of terminally protected thiol terpyridines 5a–c. Reagents and conditions: i) 4b (1 equiv.), 4-(methylthio)phenylboronic
acid (2.5 equiv.), PdCl2[PtBu2(p-NMe2-Ph)]2, K2CO3, toluene/H2O (10:1), reflux, 14 h; ii) 4b (1 equiv.), 2,4,6-tris{4-[2-(trimethylsilyl)ethyl]-
thiophenyl}-1,3,5,2,4,6-trioxatriborinane (7b) (2 equiv.), K2CO3, PdCl2[PtBu2(p-NMe2-Ph)]2, toluene/H2O (5:1), reflux, 24 h; iii) 5b
(1 equiv.), THF, TBAF (10 equiv.), AcCl (200 equiv.), room temp. to –10 °C, 3.5 h.
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Scheme 4. Synthesis of homoleptic FeII terpyridine complexes 8a–e. (a) Ethanol was used as co-solvent instead of DCM. (b) Acetone
was added as additional co-solvent.
–10 °C followed by the addition of 200 equiv. acetyl chlor- of the co-crystallization of solvent molecules into the mo-
ide. Rigorous exclusion of oxygen is crucial in order to lecular structures, no correct elemental analyses for the FeII
avoid polymer formation of the deprotected bifunctional terpyridine complexes could be obtained, and thus these
thiophenolic terpyridines by disulfide formation. Careful compounds were analyzed by HRMS. In addition, the
aqueous workup taking into consideration the base lability chemical identity of some compounds was corroborated by
of the acetyl group followed by FCC provided acetyl-pro- X-ray analysis, as suitable single crystals were obtained for
tected ligand 5c in 96% yield as an ivory solid. An overview terpyridine ligands 4b and 5a–c as well as for FeII terpyrid-
of the transformations of 4b into 4,4ЈЈ-disubstituted terpyr- ine complexes 8a and 8c. Single crystals were obtained
idine ligands 5a–c is given in Scheme 3.
either by slow evaporation of the solvent or by diffusion
The potential of these 4,4ЈЈ-disubstituted terpyridines as techniques, and details of the single-crystal growth pro-
chelating ligands was investigated by synthesizing the corre- cedure are given in the Experimental Section for each case
sponding homoleptic FeII complexes 8a–e (Scheme 4). The individually. The solid-state structures of ligands 4b and 5a–
terpyridine ligands were dissolved in mixtures of methanol c are displayed in Figure 1. In all four cases, the three pyr-
(MeOH) and DCM; in the case of 4b a MeOH/EtOH mix- idine rings are arranged almost in a plane with the nitrogen
ture was used and for 5a acetone was added as co-solvent atoms on opposite sites, enabling the formation of intramo-
to the MeOH/DCM mixture. To the dissolved ligand was lecular hydrogen bonds between neighbouring pyridine
added iron(II)chloride (0.5 equiv.), and the reaction mixture rings. Neither bond lengths nor angles between bonds dif-
immediately turned into a deep purple colour. An excess fered considerably from already reported solid-state struc-
amount of water was added, and the organic solvents were tures comprising comparable structural motives, and thus
–
removed at reduced pressure at 50 °C. The PF6 salts of these solid-state structures served mainly to prove the ident-
bis(terpyridine) iron complexes 8a–e were precipitated by ities of the ligands. The crystallographic data are collected
the addition of an excess amount of NH4PF6 as saturated
aqueous solution. The dark purple coloured precipitate was
collected by filtration and washed excessively with water.
It was then dissolved in acetonitrile (MeCN), dried with
magnesium sulfate and, after filtration, isolated by evapora-
–
tion of the solvents to provide the PF6 salts of FeII terpyr-
idine complexes 8a–e as dark purple powders in excellent
yields.
Even for terpyridine 8e comprising four acetyl-protected
peripheral thiophenol groups, which was assembled by the
longest linear synthetic sequence consisting of five steps, a
respectable overall yield of 63% was achieved starting from
2-bromo-4-chloropyridine.
Molecular Structures
All new compounds and the final FeII terpyridine com-
plexes were fully characterized by H- and 13C NMR spec-
troscopy, mass spectrometry and elemental analysis or high-
ligands 4b, 5a, 5b and 5c with ellipsoids plotted at the 50% prob-
resolution mass spectroscopy (HRMS). Probably because ability level. The hydrogen atoms were omitted for clarity.
1
Figure 1. ORTEP plots of the solid-state structures of terpyridine
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in Table S1, and some specific bond lengths are listed in the of the central pyridine ring of the second terpyridine ligand.
Supporting Information. The chemical shifts of the rest of the H atoms either remain
The solid-state structures of FeII terpyridine complexes rather unaffected upon complexation like in the case of the
8a and 8c are displayed in Figure 2, and their crystallo- proton signals at the 5- and 5ЈЈ-positions of the two outer
graphic data are summarized in Table S2. On the left, the pyridine moieties or the AB system of the para-methylsulf-
dicationic FeII complexes are displayed, and on the right, anylphenyl substituents of 8c, or face varyingly strong
the relative spatial arrangement of the two terpyridine li- downfield shifts. Those shifts are moderately pronounced
gands is highlighted by displaying the planes defined by the for the protons at the 3- and 3ЈЈ-positions with 0.34 ppm or
atoms of each terpyridine subunit. With interplane angles more severely for the signals of the protons at the 3Ј- and
of 89.82° for 8a and 88.70° for 8c, both ligand systems pro- 5Ј-positions (0.89 and 1.02 ppm, respectively) or at the cen-
vide an almost perfect octahedral ligand sphere resulting in tral 4Ј-position (0.97 and 1.02 ppm, respectively). Analo-
the expected low-spin FeII complexes. The observed bond gous observations were made for the other FeII terpyridine
lengths and atom positions of the terpyridine subunits and complexes 8b, 8d and 8e reported in this paper. The chemi-
1
the central metal ion of the complexes are, within small cal shifts found for the H NMR signals of the FeII com-
deviations, comparable to common [M(tpy)2] motifs.[38] As plexes corroborate the low-spin state of the complexed FeII
expected, the complex structure is hardly affected by sub- ions, going along with the octahedral geometries observed.
stituents at the remote 4 and 4ЈЈ positions.
Optical Properties
The UV/Vis spectra of the five homoleptic FeII com-
plexes 8a–e were recorded at several concentrations
(0.001 mm, 0.01 mm and 0.05 mm) in acetone at room tem-
perature, and their averaged absorbances were converted
into the corresponding extinction coefficients ε, which were
plotted against the respective wavelengths (Figure 4).
Characteristic bands can be unambiguously identified in
all spectra. In particular, the metal-to-ligand charge-trans-
fer (MLCT) bands found in the region between 550 and
570 nm are typical for FeII terpyridine complexes. Those
MLCT transitions arise from the charge transfer of a pri-
marily metal-localized electron from the respective 3d π-
orbital into the lowest unoccupied molecular orbital
(LUMO) of the terpyridine ligands (π*ǟπ transition).
Table 1 compares λmax values of the MLCT bands of 8a–e
with those of the parent [Fe(tpy)2]2+ complex 8f.[69] The
λmax. values of FeII complexes 8a and 8b display (λmax.
=
556 nm; ε = 13693 and 9167 Lcm–1 mol–1, respectively) only
a small bathochromic shift. Interestingly, both the chlorine
substituents of 8a with a Hammet σ parameter of +0.5 and
the methoxy substituents of 8b with a Hammet σ parameter
of –0.5 result in a similar reduction of the energy of the
Figure 2. Left: ORTEP plots of the crystal structures of the
[Fe(4b)2]2+ and the [Fe(5a)2]2+ cations (ellipsoids plotted at the 50%
probability level and H atoms, solvent molecules and counterions
omitted for clarity). Right: geometric representation of the virtual
planes comprising terpyridine ligands of 8a and 8c.
MLCT band as that of the parent complex 8f (λmax.
=
Complexation of the terpyridine ligands alters the chemi- 552 nm, ε = 11900 Lcm–1 mol–1). In analogy to the substitu-
cal environment of the coordinating nitrogen atoms con- ents at the 4Ј-positions of RuII terpyridine complexes,[70] it
siderably due to a structural change from an all-anti confor- seems that the electron-accepting chlorine substituents in
mation in the free ligand (see Figure 1) to an all-syn confor- 8a reduce the MLCT band by stabilizing the ligand-based
mation in the FeII complexes (see Figure 2). Naturally this LUMO π*-orbital more strongly than the iron-centred
structural transformation is also reflected in pronounced HOMO π-orbital, while the electron-donating methoxy
1
shifts of the H NMR signals of the terpyridines. Figure 3 substituents in 8b reduce the HOMO–LUMO gap by desta-
displays the 1H NMR spectra of free ligands 4b and 5a bilizing the metal-centred HOMO more strongly than the
together with those of the resulting FeII complexes 8a and ligand-centred LUMO. With a λmax. value of 568 nm for 8c
8c. Coloured dots guide the eye to the assignment of the 1H and 8d (ε = 20173 and 18067 Lcm–1 mol–1, respectively) and
NMR signals. Considerable shifts were observed for most 567 nm (ε = 11867 Lcm–1 mol–1) for 8e, more pronounced
signals of the terpyridine subunits. With values of 1.18 and bathochromic shifts of the MLCT bands were observed.
1.26 ppm, the H-atoms at the 6- and 6ЈЈ-positions, respec- According to previous reports in the literature, the re-
tively, displayed very distinct upfield shifts, which can be duction of the energy gap can be attributed in this case to
explained by their location right above the anisotropic cone the stabilizing effect of the additional para-phenyl substitu-
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Figure 3. Visualization of the chemical shifts of the signals for the H atoms of the ligands upon complexation for 4b (a) to 8a (b) at the
top and for 5a (c) to 8c (d) at the bottom. The positions of the H atoms are indicated by coloured dots.
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previously only hardly accessible model ligands with sub-
stituents at the 4- and 4ЈЈ-positions. The coordination sites
of these ligands are easily accessible, which was demon-
strated by the quantitative formation of the corresponding
homoleptic FeII complexes. The ligands and complexes were
fully characterized, and the identities of representative
structures were corroborated by X-ray analysis.
The method reported here is expected to increase the
structural diversity of terpyridine-based supramolecular
architectures and devices. We are particularly interested in
the substituents at the 4,4ЈЈ-positions of terpyridines be-
cause of their strong coupling to the coordinated metal
atom.
Figure 4. UV/Vis absorption spectra of FeII terpyridine complexes
8a–e.
Experimental Section
ents. The enlarged conjugated π-system stabilizes the li-
gand-based LUMO π*-orbital, making 8c–e better π-ac-
ceptors than the parent [Fe(tpy)2]2+ complex 8f.[1]
General Procedures: All commercially available compounds were
purchased and used as received unless explicitly stated otherwise.
Diphenylphosphane oxide, KF, 1,4-dioxane, 2,6-dibromopyridine,
Pd2(dba)3, B(OiPr)3, TBAF (1 m in THF), NH4PF6, tBu–O–O–
tBu, nBuLi (1.6 m in hexanes), FeCl2 and Pd(amphos)Cl2 were pur-
chased from Sigma–Aldrich. Acetyl chloride and 4-bromothi-
ophenol were purchased from Acros. 4-(Methylthio)phenylboronic
acid was purchased from Aldrich or Indofin. 2-Bromo-4-chloropyr-
idine was purchased from Fluorochem or Combi-Blocks. 2-Bromo-
4-hydroxypyridine was purchased from Synchem OHG. HI and
vinyltrimethylsilane were purchased from Alfa Aesar. 1H NMR,
13C NMR, 19F NMR, 31P NMR and 11B NMR spectra were re-
corded with a Bruker DPX NMR spectrometer operating at 400
and 101 MHz, a Bruker BZH NMR at 250 and 63 MHz or a
Bruker DRX-500 at 500 and 126 MHz. The chemical shifts are
reported in parts per million (ppm) relative to tetramethylsilane or
a residual solvent peak, and the J values are given in Hz (Ϯ0.1 Hz).
Table 1. Characteristic bands in the UV/Vis spectra of 8a–f.
Complex
λmax.
(LC/
MC)
[nm]
ε
λmax.
(MLCT)
ε
[Lcm–1 mol–1] [nm]
[Lcm–1 mol–1]
8a
8b
8c
8d
8e
364
365
355
361
7 773
3 557
556
556
568
568
567
13 693
9 167
20 173
18 067
11 867
94 420
86 567
81 333
14 600
38 000[71]
338
380
8f
318[71]
552
11 900
Furthermore, in the higher-energy spectral region several Mass spectra were recorded either with a Finnigan MAT 95Q in-
strument for EI measurements or with a Bruker Esquire 3000 plus
for ESI measurements. Elemental analyses were performed with an
Analysator 240 instrument from Perkin–Elmer. High-resolution
mass spectra (HRMS) were measured as HR-ESI-ToF-MS with a
Maxis 4G instrument from Bruker. The absorption spectra were
recorded at room temperature with an Agilent 8453 Diode Array
Spectrophotometer by using 1 cm diameter cuvettes. For column
chromatography, usually silica gel 60 (40–63 μm) from Fluka was
used, and TLC was performed on silica gel 60 F254 glass plates with
a thickness of 0.25 mm purchased from Merck.
absorbance maxima can be found between 335 and 380 nm,
which are summarized in Table 1. The weaker bands be-
tween 364 and 380 nm can be attributed to metal-centred
electronic transitions, whereas the more intense bands at
338 and 361 nm arise from ligand-centred transitions
(either π*ǟπ or π*ǟn).[1] However, those ligand-centred
transitions in the displayed region of the absorption spectra
(Ն 330 nm) can only be found for complexes 8c–e, which
consistently exhibit the extended conjugated π-systems
within their terpyridine ligands going along with a bathoch-
romic shift of the observed absorbance maxima.[1,69,72] In
the case of compounds 8c and 8d, the very intense LC
bands seem to overlap with the expected MC bands; the
LC and MC bands can be discretely resolved only in the
UV spectrum of 8e.
Synthesis, Analytical and Spectroscopic Data for 1c–8e
2-Bromo-4-methoxypyridine (1c): This compound was prepared by
adapting a related protocol.[58] 2-Bromo-4-hydroxypyridine 1d
(1.74 g, 10.0 mmol) and NaH (60% dispersion in mineral oil,
560 mg, 14.0 mmol) were dissolved in DMF (20 mL) and stirred
for 30 min. The black solution was cooled to 0 °C and MeI (2.87 g,
1.26 mL, 20.0 mmol) was added dropwise. Due to its high viscosity,
the mixture was warmed to ambient temperature and additional
DMF (20 mL) was added. Then, the reaction mixture was stirred
for 3 d and afterwards was quenched by the addition of water and
extracted with TBME (5ϫ 150 mL). The combined organic layers
were washed with brine, dried with MgSO4 and subsequently con-
centrated under reduced pressure. The crude product was purified
by FCC on silica using EtOAc/n-hexane (1:1) as eluent. Upon evap-
oration of the solvent, the product was obtained as a yellow oil.
Conclusions
A new synthesis of terpyridine units from suitably halo-
genated pyridine precursors is reported. The Suzuki–
Miyaura cross-coupling reaction used has considerably bet-
ter environmental features compared to Stille type cou-
plings, which are the most common alternative for the as-
sembly of such ligands by C–C bond forming reactions. The
1
Analytical Data for 1c: Yield 24% (433 mg, 2.35 mmol). H NMR
potential of the method was demonstrated by synthesizing (400 MHz, CDCl3, 25 °C): δH = 8.07 (d, 3JH,H = 5.8 Hz, 1 H, H6),
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4
3
4
6.91 (d, JH,H = 2.3 Hz, 1 H, H3), 6.70 (dd, JH,H = 5.8, JH,H
=
Analytical Data for 2c: Yield 100% (1.38 g, 4.54 mmol). M.p. Ͼ
2.3 Hz, 1 H, H5), 3.76 (s, 3 H, -OCH3) ppm. 13C NMR (101 MHz, 300 °C. H NMR (500 MHz, CD3OD, 25 °C): δH = 8.22 (d, JH,H
1
3
4
CDCl3, 25 °C): δC = 166.87 (1 C, Cq), 150.63 (1 C, Ct), 142.95 (1 = 6.0 Hz, 1 H, H6), 7.13 (d, JH,H = 2.7 Hz, 1 H, H3), 6.69 (dd,
C, Cq), 113.31 (1 C, Ct), 110.22 (1 C, Ct), 55.67 (1 C, Cmethoxy) ppm. 3JH,H = 6.0, JH,H = 2.8 Hz, 1 H, H5), 3.93 [hept, JH,H = 6.2 Hz,
4
3
MS (EI+, 70 eV): m/z (%) = 187.0 (45) [M+], 108.0 (100) [M+ – Br],
93.0 (17) [M+ – Br – CH3]. HRMS (ESI-ToF): calcd. for
[C6H6BrNO + H]+ 187.9706; found 187.9707.
3 H, -CH(CH3)2], 3.84 (s, 3 H, -OCH3), 1.15 [d, JH,H = 6.2 Hz,
18 H, -CH(CH3)2] ppm. 13C NMR (126 MHz, CD3OD, 25 °C): δC
= 167.01 (Cq, 1C), 148.90 (Ct, 1C), 113.28 (Ct, 1C), 109.18 (Ct, 1C),
64.89 (Cisopropyl, 3C), 55.49 (Cmethoxy, 1C), 25.42 (Cmethyl, 18C)
ppm. Note: The carbon atom bound to the boron ligand cannot
be seen in the 13C NMR spectrum. 11B NMR (128 MHz, CD3OD,
25 °C): δ = 2.94 (s, 1 B) ppm.
3
Representative Procedure A (Synthesis of 4-Substituted Lithium Tri-
isopropyl 2-Pyridylborates): 4-Substituted lithium triisopropyl 2-
pyridylborates 2a–c were prepared by adapting a procedure de-
scribed in the literature.[56] An oven-dried round-bottomed flask
was charged with the appropriate 2,4-disubstituted pyridine deriva-
tive and placed under an argon atmosphere. The reactant
(1.0 equiv.) was dissolved in a 4:1 solvent mixture of toluene and
THF, and triisopropylborate (1.1 equiv.) was added to the reaction
mixture. The reaction mixture was then cooled to –78 °C by using
a dry ice/acetone bath before nBuLi (1.6 m in hexanes, 1.1 equiv.)
was added dropwise with a syringe pump. After the addition was
completed, the reaction mixture was kept at –78 °C for an ad-
ditional 30 min and was then allowed to slowly warm up to room
temperature overnight (22 h) in the cooling bath. The crude mix-
ture was then concentrated under reduced pressure and dried rigor-
ously under high vacuum at 110 °C for 10 h to give the correspond-
ing products in very good to quantitative yields.
2,6-Diiodopyridine (3b): Compound 3b was prepared by slight
modification of a procedure described in the literature.[61,62,60] 2,6-
Dibromopyridine 3a (4.83 g, 19.8 mmol) and NaI (4.00 g,
27.7 mmol) were dissolved in HI (57 wt.-%, 15 mL, 199 mmol) and
heated to reflux for 7 h under an inert atmosphere. The mixture
was slowly poured on ice-cold aqueous sodium hydroxide (40%,
15 mL) and ice (20 g) and extracted with diethyl ether (3ϫ 15 mL).
The combined extracts were successively washed with water, aque-
ous Na2SO3 and once more with water, dried with MgSO4 and
filtered, and the solvent was evaporated under reduced pressure.
The crude product was recrystallized from a mixture of chloroform
and n-hexane to afford pure 3b as colourless needles.
Analytical Data for 3b: Yield 76% (4.49 g, 13.6 mmol). M.p. 177–
3
180 °C. 1H NMR (400 MHz, CDCl3, 25 °C): δH = 7.67 (d, JH,H
=
Lithium Triisopropyl 2-Pyridylborate (2a): Compound 2a was pre-
pared according to representative procedure A. 2-Bromopyridine
1a (3.32 g, 2.01 mL, 21.0 mmol), triisopropylborate (4.34 g,
5.34 mL, 23.1 mmol), nBuLi (1.6 m in hexanes, 14.4 mL,
23.1 mmol), toluene (60 mL) and THF (15 mL) were used to give
the desired product as an ivory solid in quantitative yield.
7.8 Hz, 2 H, H3/H5), 6.92 (t, JH,H = 7.7 Hz, 1 H, H4) ppm. 13C
NMR (101 MHz, CDCl3, 25 °C): δC = 138.60 (1 C, Ct), 134.41 (2
C, Ct), 116.45 (2 C, Cq) ppm. MS (EI+, 70 eV): m/z (%) = 330.8
(100) [M+], 203.9 (73) [M+ – I], 126.9 (7) [I+], 77.0 (25) [M+ – 2I].
HRMS (ESI-ToF): calcd. for [C5H3I2N + H]+ 331.8428; found
331.8424.
3
Analytical Data for 2a: Yield 100% (5.74 g, 21.0 mmol). M.p. Ͼ
1
300 °C. H NMR (400 MHz, CD3OD, 25 °C): δH = 8.39 (m, 1 H,
2,2Ј:6Ј2ЈЈ-Terpyridine (4a): Compound 4a was prepared by adapt-
ing a related protocol.[56] A Schlenk tube was charged with diiodo-
pyridine 3b (844 mg, 2.55 mmol) and mixed with a 1.5-fold excess
of lithium triisopropyl 2-pyridylborate 2a (2.11 g, 7.71 mmol) using
KF (804 mg, 13.7 mmol) as base, Pd2dba3 (83.7 mg, 90.0 μmol) as
the catalyst and diphenylphosphane oxide (114 mg, 550 μmol) as
the ligand. All those reagents were degassed and the flask was put
under an inert atmosphere three times. The mixture was then dis-
solved in 1,4-dioxane (30 mL), which had previously been degassed
for 30 min, and the Schlenk tube was sealed pressure-safe and kept
at 85 °C for 72 h. After being diluted with EtOAc, the reaction
mixture was filtered through a plug of silica gel (6 cm) to get rid
of insoluble side products. Concentration of the solution under re-
duced pressure yielded the crude product, which was subjected to
FCC on silica gel using EtOAc/n-hexane (15:85) as the eluent.
Upon evaporation of the solvent under reduced pressure, the de-
sired product was obtained as a white solid in good yield.
3
H6), 7.57 (d, 2 H, H3/H4), 7.09 (m, 1 H, H5), 3.93 [hept, JH,H
=
3
6.2 Hz, 3 H, -CH(CH3)2], 1.15 [d, JH,H = 6.2 Hz, 18 H, -
CH(CH3)2] ppm. 13C NMR (101 MHz, CD3OD, 25 °C): δC
=
149.64 (Ct, 1C), 144.87 (Cq, 1 C), 128.64 (Ct, 1 C), 122.16 (Ct, 1
C), 64.87 (Cisopropyl, 3 C), 25.43 (Cmethyl, 18 C) ppm. Note: The
carbon atom bound to the boron ligand cannot be seen in the 13C
NMR spectrum. 11B NMR (128 MHz, CD3OD, 25 °C): δ = 2.96
(s, 1 B) ppm.
Lithium Triisopropyl 2-(4-Chloropyridyl)borate (2b): Compound 2b
was prepared according to representative procedure A. 2-Bromo-4-
chloropyridine 1b (4.75 g, 24.7 mmol), triisopropylborate (5.20 g,
6.39 mL, 27.1 mmol), nBuLi (1.6 m in hexanes, 16.9 mL,
27.1 mmol), toluene (100 mL) and THF (25 mL) were used to give
the desired off-white product in quantitative yield.
Analytical Data for 2b: Yield 100% (7.59 g, 24.7 mmol). M.p. Ͼ
300 °C. 1H NMR (400 MHz, CD3OD, 25 °C): δH = 8.32 (dd, 3JH,H
= 5.5 Hz, H6), 7.56 (d, JH,H = 2.1 Hz, 1 H, H3), 7.15 (dd, JH,H
4
3
Analytical Data for 4a: Yield 55% (325 mg, 1.39 mmol). M.p. 88–
1
3
4
3
90 °C. H NMR (400 MHz, CDCl3, 25 °C): δH = 8.74 (ddd, JH,H
= 5.5, JH,H = 2.3 Hz, 1 H, H5), 3.93 [hept, JH,H = 6.2 Hz, 3 H,
4
5
3
-CH(CH3)2], 1.15 [d, 3JH,H = 6.2 Hz, 18 H, -CH(CH3)2] ppm. 13
C
= 4.8, JH,H = 1.7, JH,H = 0.9 Hz, 2 H, H6/H6ЈЈ), 8.66 (dt, JH,H
4
3
= 8.0, JH,H = 1.0 Hz, 2 H, H3/H3ЈЈ ), 8.51 (d, JH,H = 7.8 Hz, 2
NMR (101 MHz, CD3OD, 25 °C): δC = 149.67 (Ct, 1C), 144.85
(Cq, 1C), 128.61 (Ct, 1C), 122.15 (Ct, 1C), 64.87 (Cisopropyl, 3C),
25.44 (Cmethyl, 18C) ppm. Note: The carbon atom bound to the
boron ligand cannot be seen in the 13C NMR spectrum. 11B NMR
(128 MHz, CD3OD, 25 °C): δ = 2.98 (s, 1 B) ppm.
3
3
H, H3Ј/H5Ј), 7.99 (t, JH,H = 7.8 Hz, 1 H, H4Ј), 7.34 (ddd, JH,H
3
4
= 7.5, JH,H = 4.8, JH,H = 1.2 Hz, 2 H, H5/H5ЈЈ) ppm. 13C NMR
(101 MHz, CDCl3, 25 °C): δC = 156.22 (Cq, 2 C), 155.35 (2 C, Cq),
149.13 (2 C, Ct), 137.86 (1 C, Ct), 136.80 (2 C, Ct), 123.74 (Ct, 2
C), 121.13 (Ct, 2 C), 120.99 (2 C, Ct) ppm. HRMS (ESI-ToF):
calcd. for [C15H11N3 + H]+ 234.1026; found 234.1026. C15H11N3
(233.27): calcd. C 77.23, H 4.75, N 18.01; found C 77.41, H 4.85,
N 18.20.
Lithium Triisopropyl 2-(4-Methoxypyridyl)borate (2c): Compound
2c was prepared according to representative procedure A. Toluene
(32 mL), THF (8 mL), 2-bromo-4-methoxypyridine 1c (873 mg,
4.55 mmol), triisopropylborate (960 mg, 1.18 mL, 5.00 mmol) and
nBuLi (1.6 m in hexanes, 3.13 mL, 5.00 mmol) were used to yield
the desired product as a brown solid.
4,4ЈЈ-Dichloro-2,2Ј:6Ј2ЈЈ-terpyridine (4b): Compound 4b was pre-
pared by adapting a related protocol.[56] A Schlenk tube was
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charged with diiodopyridine 3b (331 mg, 1.00 mmol) and mixed
with a 1.5-fold excess of lithium triisopropyl 2-(4-chloropyridyl)-
borate 2b (971 mg, 3.00 mmol) using KF (352 mg, 6.00 mmol) as
base, Pd2dba3 (36.6 mg, 40.0 μmol) as the catalyst and diphenyl-
phosphane oxide (50.0 mg, 240 μmol) as the ligand. All those rea-
gents were degassed and the flask was put under an inert atmo-
sphere three times. The mixture was then dissolved in 1,4-diox-
ane (8 mL), which had previously been degassed for 30 min, and
the Schlenk tube was sealed pressure-safe and kept at 75 °C for
60 h. After being diluted with EtOAc, the reaction mixture was
filtered through a plug of silica gel (4 cm) to get rid of insoluble
side products. Concentration of the solution under reduced pres-
sure yielded the crude product, which was subjected to FCC on
silica gel using a gradient of EtOAc/n-hexane (15:85 to 25:75) as
the eluent. Upon evaporation of the solvent under reduced pres-
sure, the desired product was obtained as a white solid in good
yield.
3JH,H = 4.8, 4JH,H = 1.0 Hz, 1 H, H5Ј), 6.83 (dd, 3JH,H = 5.7, 4JH,H
= 2.6 Hz, 1 H, H5), 3.93 (s, 3 H, -OCH3) ppm. 13C NMR
(126 MHz, CDCl3, 25 °C): δC = 166.90 (1 C, Cq), 158.15 (1 C, Cq),
156.06 (Cq, 1 C), 150.37 (Ct, 1 C), 149.22 (Ct, 1 C), 137.10 (1 C,
Ct), 124.04 (1 C, Ct), 121.47 (1 C, Ct), 111.08 (1 C, Ct), 106.20 (1
C, Ct), 55.49 (1 C, Cmethoxy) ppm. MS (EI+, 70 eV): m/z (%) =
186.1 (85) [M+], 185.1 (100) [M+ – H], 156.1 (55) [M+ – OCH3
H], 155.1 (22) [M+
OCH3]. HRMS (ESI-ToF): calcd. for
[C11H10N2O + H]+ 187.0866; found 187.0869.
+
–
Representative Procedure B (Functionalization of 4,4ЈЈ-Disubstituted
2,2Ј:6Ј,2ЈЈ-Terpyridines by Applying Suzuki–Miyaura Reactions):
The described Suzuki–Miyaura cross-coupling reactions were per-
formed by modification of a known literature procedure,[63] which
was adapted according to the needs and findings of each individual
coupling reaction. Generally an oven-dried Schlenk tube was
charged with 4,4ЈЈ-dichloroterpyridine 4b or its corresponding de-
rivative and the appropriate amount of arylboronic acid depending
on the desired degree of substitution. Additionally, K2CO3 was
used as base (2–6 equiv.) and PdCl2{PtBu2(p-NMe2-Ph)}2 (0.01–
0.05 equiv.) as the catalyst. All the reagents were repeatedly de-
gassed and put under an inert atmosphere before they were sus-
pended in a previously degassed solvent mixture of H2O/toluene
(usually 1:6). Once the solvents were added, the Schlenk tube was
sealed and heated to reflux (approx. 125 °C oil bath temp.) for usu-
ally 24 h. The workup strongly varied depending on the respective
reagents and functional groups present and will be specified for
each corresponding reaction.
Analytical Data for 4b: Yield 66% (199 mg, 660 μmol). M.p. 171–
173 °C. 1H NMR (400 MHz, CDCl3, 25 °C): δH = 8.55 (m, 4 H,
3
H3/H6/H3ЈЈ/H6ЈЈ ), 8.43 (d, JH,H = 7.8 Hz, 2 H, H3Ј/H5Ј), 7.92
(t, 3JH,H = 7.8 Hz, 1 H, H4Ј), 7.31 (dd, 3JH,H = 5.4, 4JH,H = 1.9 Hz,
2 H, H5/H5ЈЈ) ppm. 13C NMR (101 MHz, CDCl3, 25 °C): δC
=
157.63 (Cq, 2 C), 154.46 (2 C, Cq), 150.20 (2 C, Ct), 145.38 (2 C,
Cq), 138.29 (1 C, Ct), 124.23 (Ct, 2 C), 122.10 (2 C, Ct), 121.63 (2
C, Ct) ppm. MS (EI+, 70 eV): m/z (%) = 301.0 (100) [M+], 266.0
(33) [M+ – Cl]. C15H9Cl2N3 (302.16): calcd. C 59.63, H 3.00, N
13.91; found C 59.59, H 3.04, N 13.70.
4,4ЈЈ-Bis[4-(methylthio)phenyl]-2,2Ј:6Ј,2ЈЈ-terpyridine (5a): Com-
pound 5a was prepared according to representative procedure B.
4,4ЈЈ-Dichloro-2,2Ј:6Ј2ЈЈ-terpyridine 4b (60.0 mg, 200 μmol), 4-
(methylthio)phenylboronic acid (84.0 mg, 500 μmol), K2CO3
(112 mg, 800 μmol) and PdCl2{PtBu2(p-NMe2-Ph)}2 (2.83 mg,
4.00 μmol) were added to a microwave vessel and suspended in pre-
viously degassed toluene (2.5 mL) and H2O (250 μL). The reaction
vessel was sealed and kept at 115 °C for 14 h. The bright yellow
mixture was allowed to cool down to room temperature and was
subsequently diluted with DCM and water. The reaction mixture
was then extracted with DCM (3ϫ) and with EtOAc (2ϫ) followed
by extraction of the individual organic phases with brine (3ϫ each).
The combined organic phases were dried with MgSO4, filtered and
concentrated under reduced pressure. The crude product was fur-
ther purified by FCC using a gradient of EtOAc/n-hexane (1:3 to
1:1) to yield the desired disubstituted product 5a in a modest yield
of 16%. Additionally, the monosubstituted product 4-chloro-4ЈЈ-[4-
(methylthio)-phenyl]terpyridine 6a was isolated as a side product
in a yield of 37%.
4,4ЈЈ-Dimethoxy-2,2Ј:6Ј2ЈЈ-terpyridine (4c): Compound 4c was pre-
pared in a similar way to compound 4b. A Schlenk tube was
charged with diiodopyridine 3b (331 mg, 1.00 mmol) and mixed
with a 1.5-fold excess of lithium triisopropyl 2-(4-methoxypyridyl)-
borate 2c (909 mg, 3.00 mmol) using KF (352 mg, 6.00 mmol) as
base, Pd2dba3 (36.6 mg, 40.0 μmol) as the catalyst and diphenyl-
phosphane oxide (50.0 mg, 240 μmol) as the ligand. All those rea-
gents were degassed and the flask was put under an inert atmo-
sphere three times. The mixture was dissolved in previously de-
gassed 1,4-dioxane (16 mL) before the reaction vessel was sealed.
The reaction mixture was kept at 85 °C for 68 h. After dilution
with EtOAc, the reaction mixture was filtered through a plug of
silica gel to get rid of insoluble side products. Concentration of the
solution under reduced pressure yielded the crude product, which
was subjected to FCC on silica gel using a gradient from EtOAc/
n-hexane (1:1) to pure EtOAc as the eluent. Evaporation of the
solvent under reduced pressure yielded the desired product 4c as
well as the deiodinated 4-methoxybipyridine 4d as a side product.
Analytical Data for 4c: Yield 32% (92.4 mg, 320 μmol). M.p. 167–
Analytical Data for 5a: Yield 16% (15.0 mg, 30.0 μmol). M.p. (de-
1
3
169 °C. H NMR (500 MHz, CDCl3, 25 °C): δH = 8.50 (d, JH,H
=
composition) Ͼ 245 °C. 1H NMR (400 MHz, CDCl3, 25 °C): δH
=
5.6 Hz, 2 H, H6/H6ЈЈ), 8.41 (d, 3JH,H = 7.8 Hz, 2 H, H3Ј/H5Ј), 8.12
8.83 (d, 4JH,H = 1.2 Hz, 2 H, H3/H3ЈЈ), 8.72 (dd, 3JH,H = 5.1, 4JH,H
4
3
(d, JH,H = 2.5 Hz, 2 H, H3/H3ЈЈ), 7.91 (t, JH,H = 7.8 Hz, 1 H,
3
= 0.5 Hz,2 H, H6/H6ЈЈ), 8.47 (d, JH,H = 7.8 Hz, 2 H, H3Ј/H5Ј),
H4Ј), 6.84 (dd, JH,H = 5.7, 4JH,H = 2.6 Hz, 2 H, H5/H5ЈЈ), 3.94 (s,
3
3
3
7.99 (t, JH,H = 7.8 Hz, 1 H, H4Ј), 7.71 (d, JH,H = 8.5 Hz, 4 H,
6 H, -OCH3) ppm. 13C NMR (126 MHz, CDCl3, 25 °C): δC
=
3
4
Hphenyl), 7.53 (dd, JH,H = 5.1, JH,H = 1.9 Hz,2 H, H5/H5ЈЈ), 7.37
166.79 (Cq, 2 C), 158.14 (2 C, Cq), 155.24 (2 C, Cq), 150.61 (2 C,
Ct), 138.04 (1 C, Ct), 121.54 (2 C, Ct), 110.10 (2 C, Ct), 107.18
(2 C, Ct), 55.40 (2 C, Cmethoxy) ppm. MS (EI+, 70 eV): m/z (%) =
292.1 (100) [M+ – H], 278.1 (22) [M+ – CH3], 263.1 (44) [M+-
2CH3], 262.1 (19) [M+ – OCH3], 146.6 (5) [M2+]. HRMS (ESI-
ToF): calcd. for [C17H15N3O2 + H]+ 294.1237; found 294.1238.
(d, JH,H = 8.5 Hz, 4 H, Hphenyl), 2.55 (s, 6 H, -SCH3) ppm. 13C
3
NMR (101 MHz, CDCl3, 25 °C): δC = 157.08 (2 C, Cq), 155.63 (2
C, Cq), 149.95 (2 C, Ct), 148.70 (2 C, Cq), 140.55 (2 C, Cq), 138.24
(1 C, Ct), 135.19 (2 C, Cq), 127.62 (Cphenyl, 4 C), 126.88 (Cphenyl, 4
C), 121.55 (2 C, Ct), 121.44 (2 C, Ct), 119.04 (2 C, Ct), 15.73 (2 C,
Cthiomethyl) ppm. Note: Some quaternary carbon atoms cannot be
unambiguously identified in the 13C NMR spectrum. MS (EI+,
70 eV): m/z (%) = 477.1 (100) [M+], 462.1 (11) [M+ – CH3], 430.2
Analytical Data for 4d: Yield 36% (66.5 mg, 360 μmol). M.p. 65–
1
3
67 °C. H NMR (250 MHz, CDCl3, 25 °C): δH = 8.65 (d, JH,H
=
4.0 Hz, 1 H, H6Ј), 8.47 (d, 3JH,H = 5.7 Hz, 1 H, H6), 8.37 (d, 3JH,H (8) [M+ – SCH3], 415.0 (6) [M+ – SCH3–CH3], 238.6 (8) [M2+].
3
= 8.0 Hz, 1 H, H3Ј ), 7.95 (d, JH,H = 2.5 Hz, 1 H, H3), 7.79 (app. HRMS (ESI-ToF): calcd. for [C29H23N3S2 + H]+ 478.1406; found
3
4
3
td, JH,H = 7.8, JH,H = 1.8 Hz, 1 H, H4Ј), 7.29 (ddd, JH,H = 7.4,
478.1407.
Eur. J. Inorg. Chem. 2013, 3334–3347
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Analytical Data for 6a: Yield 37% (29.0 mg, 70.0 μmol). M.p. 133– yellow colour. After further 2 h in the cooling bath, the reaction
4
136 °C. 1H NMR (400 MHz, CDCl3, 25 °C): δH = 8.74 (d, JH,H
=
was cautiously quenched by the addition of saturated NaHCO3
solution (240 mL). The reaction mixture was extracted with DCM
(2ϫ) and with EtOAc (2ϫ), before the combined organic phases
were dried with MgSO4, filtered and concentrated under reduced
pressure. Finally the resulting crude product was further purified by
FCC using a gradient from EtOAc/n-hexane (1:1) to pure EtOAc to
yield the desired disubstituted and poorly soluble product 5c in a
3
1.2 Hz, 1 H, H3), 8.71 (d, JH,H = 5.1 Hz, 1 H, H6), 8.58 (m, 2
3
H, H3ЈЈ/H6ЈЈ ), 8.49 (d, JH,H = 7.8 Hz, 1 H, H3Ј or H5Ј), 8.44 (d,
3JH,H = 7.8 Hz, 1 H, H3Ј or H5Ј), 7.96 (t, JH,H = 7.8 Hz, 1 H,
3
3
3
H4Ј), 7.70 (d, JH,H = 8.4 Hz, 2 H, Hphenyl), 7.51 (dd, JH,H = 5.1,
4JH,H = 1.8 Hz, 1 H, H5), 7.39 (d, JH,H = 8.4 Hz, 2 H, Hphenyl),
3
3
4
7.32 (dd, JH,H = 5.2, JH,H = 2.1 Hz, 1 H, H5ЈЈ), 2.54 (s, 3 H,
-SCH3) ppm. 13C NMR (101 MHz, CDCl3, 25 °C): δC = 157.96 (1 good yield.
C, Cq) 156.75 (1 C, Cq), 155.85 (1 C, Cq), 154.39 (1 C, Cq), 150.22
Analytical Data for 5c: Yield 96% (514 mg, 964 μmol). M.p. 241–
(1 C, Ct), 149.90 (1 C, Ct), 148.96 (1 C, Cq), 145.32 (1 C, Cq), 140.56
(1 C, Cq), 138.25 (1 C, Ct), 135.14 (1 C, Cq), 127.67 (2 C, Cphenyl),
126.92 (2 C, Cphenyl), 124.13 (1 C, Ct), 122.12 (1 C, Ct), 121.72 (1
C, Ct), 121.68 (1 C, Ct), 121.67 (1 C, Ct), 118.88 (1 C, Ct), 15.70 (1
C, Cthiomethyl) ppm. MS (EI+, 70 eV): m/z (%) = 389.1 (100) [M+],
374.1 (15) [M+ – CH3], 342.1 (19) [M+ – SCH3], 194.6 (5) [M2+].
HRMS (ESI-ToF): calcd. for [C22H16ClN3S + H]+ 390.0826; found
390.0824.
4
244 °C. 1H NMR (400 MHz, CDCl3, 25 °C): δH = 8.82 (d, JH,H
=
3
1.2 Hz, 2 H, H3/H3ЈЈ), 8.77 (d, JH,H = 5.1 Hz, 2 H, H6Ј/H6ЈЈ),
3
3
8.50 (d, JH,H = 7.8 Hz, 2 H, H3Ј/H5Ј), 8.00 (t, JH,H = 7.8 Hz, 1
H, H4Ј), 7.80 (d, 3JH,H = 8.4 Hz, 4 H, Hphenyl), 7.55 (app. dd, 3JH,H
4
= 5.1, JH,H = 3.2 Hz, 6 H, H4/H4ЈЈ/Hphenyl), 2.45 (s, 3 H, Hmethyl
)
ppm. 13C NMR (101 MHz, CDCl3, 25 °C): δC = 193.77 2 C (CC=O),
156.88 (2 C, Cq), 149.76 (2 C, Ct), 148.79 (2 C, Cq), 139.69 (2 C,
Cq), 138.34 (1 C, Ct), 135.29 (4 C, Cphenyl), 129.40 (2 C, Cq), 128.12
(4 C, Cphenyl), 125.23 (2 C, Cq), 121.98 (2 C, Ct), 121.90 (2 C, Ct),
119.61 (2 C, Ct), 30.55 (2 C, Cmethyl) ppm. MS (FAB+): m/z (%) =
4,4ЈЈ-Bis{4-[2-(trimethylsilyl)ethyl]thiophenyl}-2,2Ј:6Ј,2ЈЈ-terpyridine
(5b): Compound 5b was prepared by applying representative
procedure B. 4,4ЈЈ-Dichloro-2,2Ј:6Ј2ЈЈ-terpyridine 4b (159 mg,
534.1 (100) [M+], 491.1 {17} [M+ – {(C=O)CH3}], 449.1 (17) [M+
–
–
2{(C=O)CH3}]. MS (EI+, 70 eV): m/z (%) = 491.1 (25) [M+
500 μmol),
2,4,6-tris{4-[2-(trimethylsilyl)ethyl]thiophenyl}-
{(C=O)CH3}], 449.1 (100) [M+ – 2{(C=O)CH3}]. HRMS (ESI-
ToF): calcd. for [C31H23N3O2S2 + H]+ 534.1304; found 534.1305.
1,3,5,2,4,6-trioxatriborinane 7b (709 mg, 1.00 mmol), K2CO3
(419 mg, 3.00 mmol) and PdCl2{PtBu2(p-NMe2-Ph)}2 (35.4 mg,
50.0 μmol) were added to a Schlenk tube and suspended in pre-
viously degassed toluene (25 mL) and H2O (5 mL). The reaction
vessel was sealed and kept at 125 °C for 24 h. The black suspension
was allowed to cool down to room temperature and successively
diluted with DCM and water. The reaction mixture was then ex-
tracted with DCM (3ϫ) and with EtOAc (3ϫ) followed by washing
of the individual organic layers with brine (3ϫ each). The com-
bined organic phases were dried with MgSO4, filtered and concen-
trated under reduced pressure. Finally the crude product was fur-
ther purified by FCC using EtOAc/n-hexane (1:3) to yield the de-
sired disubstituted product 5b in an excellent yield.
2-[(4-Bromophenyl)thio]ethyltrimethylsilane (7a): Compound 7a was
prepared according to the literature.[64–66] A microwave vessel was
charged with 4-bromothiophenol (11.1 g, 58.8 mmol), vinyltrimeth-
ylsilane (7.29 g, 10.7 mL, 70.6 mmol) and di-tert-butyl peroxide
(1.29 g, 1.61 mL, 8.82 mmol), and the reaction mixture was de-
gassed before the vessel was closed safely and kept at 100 °C for
5.5 h. After cooling down to room temperature, the reaction mix-
ture was diluted with n-hexane and washed twice with aqueous
NaOH solution (1 m). The combined aqueous phases were washed
with n-hexane (3ϫ), and the combined organic layers were dried
with MgSO4 and subsequently concentrated under reduced pres-
sure. Finally, the crude product was purified by vacuum distillation
(b.p.: 103 °C at 9.4ϫ10–2 mbar) to yield the desired product 7a as
a colourless liquid.
Analytical Data for 5b: Yield 99% (320 mg, 490 μmol). M.p. 172–
4
173 °C. 1H NMR (400 MHz, CDCl3, 25 °C): δH = 8.80 (d, JH,H
=
3
1.2 Hz, 2 H, H3/H3ЈЈ), 8.70 (d, JH,H = 5.1 Hz, 2 H, H6Ј/H6ЈЈ),
3
3
Analytical Data for 7a: Yield 98% (16.7 g, 57.8 mmol). 1H NMR
8.46 (d, JH,H = 7.8 Hz, 2 H, H3Ј/H5Ј), 7.96 (t, JH,H = 7.8 Hz, 1
3
3
3
H, H4Ј), 7.68 (d, JH,H = 8.4 Hz, 4 H, Hphenyl), 7.50 (dd, JH,H
=
(400 MHz, CDCl3, 25 °C): δH = 7.37 (d, JH,H = 8.6 Hz, 2 H,
4
3
3
5.1, JH,H = 1.8 Hz, 2 H, H4/H4ЈЈ), 7.39 (d, JH,H = 8.4 Hz, 4 H,
Hphenyl), 3.03 (m, 4 H, Hethyl), 0.98 (m, 4 H, Hethyl), 0.06 (s, 18 H,
TMS) ppm. 13C NMR (101 MHz, CDCl3, 25 °C): δC = 157.03 (2
C, Cq), 155.66 (2 C, Cq), 149.86 (2 C, Ct), 148.60 (2 C, Cq), 139.55
(2 C, Cq), 138.07 (1 C, Ct), 135.50 (2 C, Cq), 128.73 (4 C, Cphenyl),
127.53 (4 C, Cphenyl), 121.55 (2 C, Ct), 121.38 (2 C, Ct), 118.89 (2
C, Ct), 29.19 (2 C, Cethyl), 16.89 (2 C, Cethyl), 1.49 (2 C, Ct) ppm.
MS (EI+, 70 eV): m/z (%) = 649.2 (28) [M+], 73.0 (100) [TMS+].
C37H43N3S2Si2 (650.06): calcd. C 68.36, H 6.67, N 6.46; found C
68.52, H 6.66, N 6.43.
Hphenyl), 7.14 (d, JH,H = 8.6 Hz, 2 H, Hphenyl), 2.91 (m, 2 H, -S-
CH2-CH2-TMS), 0.90 (m, 2 H, -S-CH2-CH2-TMS), 0.02 (s, 9 H,
TMS) ppm. 13C NMR (101 MHz, CDCl3, 25 °C): δC = 136.68 (1
C, Cq), 132.02 (2 C, Ct), 130.63 (2 C, Ct), 119.59 (1 C, Cq), 29.88
(1 C, Cethyl), 16.97 (1 C, Cethyl), –1.56 (CTMS, 3C) ppm. MS (EI+,
70 eV): m/z (%) = 290.0 (8) [M+], 73.0 (100) [TMS+]. C11H17BrSSi
(289.31): calcd. C 45.67, H 5.92, N 0.00; found C 45.70, H 5.95, N
0.00.
2,4,6-Tris{4-[2-(trimethylsilyl)ethyl]thiophenyl}-1,3,5,2,4,6-trioxatri-
borinane (7b): Compound 7b was prepared by following a literature
procedure.[62,63] 2-[(4-Bromophenyl)thio]ethyltrimethylsilane 7a
(9.55 g, 33.0 mmol) was dissolved in THF (90 mL) and degassed
for 30 min before it was cooled to –78 °C using a dry ice/acetone
bath. Subsequently nBuLi (2.5 m in hexanes, 21.8 mL, 54.4 mmol)
was added dropwise with a syringe pump over 2 h, which caused
the solution to turn yellow. After 30 min of further stirring at
–78 °C, triisopropylborate (22.5 g, 27.6 mL, 116 mmol) was added
dropwise over 1 h. The reaction was allowed to slowly warm up to
room temperature in the cooling bath for 66 h before it was
quenched by the addition of water. The reaction mixture was con-
centrated to the water phase, which was then extracted with Et2O
(3ϫ 100 mL). The combined organic layers were washed with
4,4ЈЈ-Bis(4-acetylthiophenyl)-2,2Ј:6Ј,2ЈЈ-terpyridine (5c): Compound
5c was prepared by transprotection of 4,4ЈЈ-bis{4-[2-(trimethylsil-
yl)ethyl]thiophenyl}-2,2Ј:6Ј,2ЈЈ-terpyridine 5b. An oven-dried
round-bottomed flask was charged with 5b (650 mg, 1.00 mmol)
and put under an inert atmosphere. Subsequently the terpyridine
ligand was dissolved in rigorously degassed THF (120 mL), before
TBAF (1 m in THF, 10.0 mL, 10.0 mmol) was added, which caused
the formerly colourless solution to turn orange immediately. After
1.5 h of stirring, the reaction mixture was cooled down to –10 °C,
and afterwards previously degassed acetyl chloride (15.7 g,
14.3 mL, 200 mmol) was added dropwise to the orange solution,
which subsequently turned colourless again followed by a bright
Eur. J. Inorg. Chem. 2013, 3334–3347
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FULL PAPER
3
water, brine and once more with water, dried with MgSO4 and con-
centrated under reduced pressure to yield the crude product as a
colourless solid. The pure product was obtained by recrystallization
from n-hexane as a colourless powder in good yield.
3JH,H = 8.1 Hz, 4 H, H3Ј/H5Ј), 8.79 (t, JH,H = 8.1 Hz, 2 H, H4Ј),
3
3
8.38 (d, JH,H = 2.7 Hz, 2 H, H3/H3ЈЈ), 7.07 (d, JH,H = 6.5 Hz, 4
3
4
H, H6/H6ЈЈ), 6.78 (dd, JH,H = 6.5, JH,H = 2.7 Hz, 4 H, H5/H5ЈЈ)
ppm. 13C NMR (126 MHz, [D6]acetone, 25 °C): δC = 169.08 (Cq,
4 C), 161.67 (4 C, Cq), 160.06 (4 C, Cq), 154.09 (4 C, Ct), 138.52
(2 C, Ct), 124.78 (4 C, Ct), 114.90 (4 C, Ct), 111.71 (4 C, Ct), 57.19
(4 C, Cmethoxy) ppm. 19F NMR (376 MHz, [D6]acetone, 25 °C): δF
Analytical Data for 7b: Yield 81% (6.29 g, 8.88 mmol). 1H NMR
3
(400 MHz, CDCl3, 25 °C): δH = 8.08 (d, JH,H = 8.2 Hz, 6 H,
3
Hphenyl), 7.34 (d, JH,H = 8.2 Hz, 6 H, Hphenyl), 3.04 (m, 6 H, -S-
= –72.44 (d, JP,F = 708.1 Hz, 12 F) ppm. 31P NMR (162 MHz,
1
CH2-CH2-TMS), 0.98 (m, 6 H, -S-CH2-CH2-TMS), 0.06 (s, 27 H,
TMS) ppm. 13C NMR (101 MHz, CDCl3, 25 °C): δC = 143.96 (3
C, Cq), 136.13 (6 C, Ct), 126.69 (6 C, Ct), 28.36 (3 C, Cethyl), 16.85
(3 C, Cethyl), 1.52 (CTMS, 9C) ppm. Note: The carbon atom bound
to the boron ligand cannot be seen in the 13C NMR spectrum. MS
(EI+, 70 eV): m/z (%) = 708.3 (9) [M+], 101.1 (20) [Et – TMS+],
73.1 (100) [TMS+].
[D6]acetone, 25 °C): δP = –144.25 (hept, 1JP,F = 707.9 Hz, 2 P) ppm.
MS (ESI+): m/z (%) = 786.9 (16) [M2+ – PF6 ], 321.1 (100) [M2+
–
–
2PF6 ]. HRMS (ESI-ToF): calcd. for [C34H30FeN6O4]2+ 321.0834;
found 321.0839. UV/Vis: λmax (ε, Lcm–1 mol–1) = 365 (3557), 556
(9167) nm.
–
{[Fe(5a)2]2+[(PF6)–]2} (8c): Compound 8c was prepared by adapting
representative procedure C. FeCl2 (3.60 mg, 30.0 μmol), 4,4ЈЈ-bis[4-
(methylthio)phenyl]-2,2Ј:6Ј,2ЈЈ-terpyridine 5a (30.0 mg, 60.0 μmol),
MeOH (10 mL), DCM (25 mL), acetone (15 mL), water (50 mL)
and NH4PF6 (205 mg, 1.26 mmol) were used. The product was iso-
lated as a dark purple solid in quantitative yield.
Representative Procedure C (Complexation of the Previously De-
scribed Terpyridine Ligands with FeCl2): The described complex-
ation was usually performed by dissolving the appropriate terpyrid-
ine ligand (2.2 equiv.) in organic solvent (MeOH/EtOH/DCM) be-
fore FeCl2 (1.0 equiv.) was added. Usually an instantaneous colour
change occurred. The solution was diluted with the fivefold amount
of H2O and afterwards it was stirred for some time before being
extracted with some organic solvent (e.g. diethyl ether) to get rid
of excess ligand. Subsequently, the reaction mixture was liberated
from any organic solvents under reduced pressure before excess sat-
urated NH4PF6 solution (15–50 equiv.) was added. The addition
of NH4PF6 resulted in the expected anion exchange and in the
precipitation of the desired homoleptic {[Fe(tpy)2]2+[(PF6)–]2} com-
plexes. This precipitation was usually completed by storing the re-
action mixture in the refrigerator overnight. The mixture was then
filtered through a fine glass frit (only by gravitation) and washed
excessively with H2O to get rid of excess NH4PF6. Finally the
strongly coloured residue was dissolved in acetone or MeCN, dried
with MgSO4, filtered and concentrated under reduced pressure and
high vacuum to yield the desired products in usually excellent
yields.
Analytical Data for 8c: Yield 100% (37.0 mg, 30.0 μmol). M.p. Ͼ
300 °C. 1H NMR (400 MHz, [D6]acetone, 25 °C): δH = 9.49 (d,
3JH,H = 8.1 Hz, 4 H, H3Ј/H5Ј), 9.17 (d, JH,H = 1.7 Hz, 4 H, H3/
4
3
3
H3ЈЈ), 8.93 (t, JH,H = 8.1 Hz, 2 H, H4Ј), 7.79 (d, JH,H = 8.6 Hz,
8 H, Hphenyl), 7.54 (dd, 3JH,H = 6.1, 4JH,H = 2.0 Hz, 4 H, H5/H5ЈЈ),
3
3
7.46 (d, JH,H = 6.0 Hz, 4 H, H6/H6ЈЈ), 7.37 (d, JH,H = 8.7 Hz, 8
H, Hphenyl), 2.52 (s, 12 H, Hmethyl) ppm. 13C NMR (101 MHz, [D6]
acetone, 25 °C): δC = 161.64 (Cq, 4C), 159.59 (Cq, 4C), 154.05 (Ct,
4C), 150.91 (Cq, 4C), 144.12 (Cq, 4C), 139.11 (Ct, 2C), 132.11 (Cq,
4C), 128.48 (Cphenyl, 8C), 127.05 (Cphenyl, 8C), 125.09 (Ct, 4C),
121.77 (Ct, 4C), 14.79 (Cmethyl, 4C) ppm. Note: Only 12 instead of
the expected 13 signals can be seen in the 13C NMR spectrum,
which is most likely because of the coincidence of two tertiary car-
bon signals at δ = 125.09 ppm. 19F NMR (376 MHz, [D6]acetone,
25 °C): δF = –72.20 (d, JP,F = 707.2 Hz, 12 F) ppm. 31P NMR
1
1
(162 MHz, [D6]acetone, 25 °C): δP = –144.21 (hept, JP,F
=
–
{[Fe(4b)2]2+[(PF6)–]2} (8a): Compound 8a was prepared by adapting
representative procedure C. FeCl2 (63.4 mg, 500 μmol), 4,4ЈЈ-
dichloro-2,2Ј:6Ј2ЈЈ-terpyridine 4b (350 mg, 1.10 mmol), MeOH
(10 mL), EtOH (10 mL), water (100 mL) and NH4PF6 (1.22 g,
7.50 mmol) were used. The product was isolated as a deep purple
powder in quantitative yield, whereas the remaining excess ligand
was recovered as well during the extraction.
707.9 Hz, 2 P) ppm. MS (FAB+): m/z (%) = 1155.4 (10) [M+
PF6 ], 1029.3 (9) [M+ – 2PF6 + F], 1010.3 (24) [M+ – 2PF6 ],
–
–
–
504.9 (31) [M2+-2PF6 ]. MS (ESI+): m/z (%) = 1155.2 (3) [M2+
–
–
–
–
PF6 ], 505.2 (100) [M2+-2PF6 ]. HRMS (ESI-ToF): calcd. for
[C58H46FeN6S4]2+ 505.1003; found 505.1014. UV/Vis (acetone):
λmax (ε, Lcm–1 mol–1) = 355 (94420), 568 (20173) nm.
{[Fe(5b)2]2+[(PF6)–]2} (8d): Compound 8d was prepared by adapting
representative procedure C. FeCl2 (6.30 mg, 50.0 μmol), 4,4ЈЈ-bis
{4-[2-(trimethylsilyl)ethyl]thiophenyl}-2,2Ј:6Ј,2ЈЈ-terpyridine
(75.3 mg, 110 μmol), MeOH (15 mL), DCM (15 mL), water
(75 mL) and NH4PF6 (359 mg, 2.20 mmol) were used. The product
was isolated as a dark purple solid in quantitative yield.
Analytical Data for 8a: Yield 100% (473 mg, 500 μmol). M.p. Ͼ
300 °C. 1H NMR (400 MHz, [D6]acetone, 25 °C): δH = 9.32 [d,
3J(H,H) = 8.1 Hz, 4 H, H3Ј/H5Ј], 8.89 (m, 6 H, H4Ј/H3/H3ЈЈ), 7.37
5b
3
3
4
(d, JH,H = 6.1 Hz, 4 H, H6/H6ЈЈ), 7.29 (dd, JH,H = 6.1, JH,H
=
2.2 Hz, 4 H, H5/H5ЈЈ) ppm. 13C NMR (101 MHz, [D6]acetone,
25 °C): δC = 161.21 (4 C, Cq), 160.55 (4 C, Cq), 155.66 (4 C, Ct),
148.27 (4 C, Cq), 140.27 (2 C, Ct), 129.04 (4 C, Ct), 126.52 (4 C,
Analytical Data for 8d: Yield 100% (82.5 mg, 50.0 μmol). M.p. Ͼ
300 °C. 1H NMR (400 MHz, [D6]acetone, 25 °C): δH = 9.46 (d,
Ct), 125.83 (4 C, Ct) ppm. MS (FAB+): m/z (%) = 805.0 (23) [M+
–
3JH,H = 8.1 Hz, 4 H, H3Ј/H5Ј), 9.13 (d, JH,H = 1.6 Hz, 4 H, H3/
4
PF6 ], 679.0 (42) [M+ – 2PF6 + F], 660.0 (42) [M+ – 2PF6 ].
–
–
–
3
3
H3ЈЈ), 8.88 (t, JH,H = 8.1 Hz, 2 H, H4Ј), 7.79 (d, JH,H = 8.6 Hz,
MS (ESI+): m/z (%) = 330.1 (100) [M2+ – 2PF6 ]. HRMS (ESI-
–
8 H, Hphenyl), 7.53 (dd, 3JH,H = 6.1, 4JH,H = 1.9 Hz, 4 H, H5/H5ЈЈ),
ToF): calcd. for [C30H18Cl4FeN6]2+ 328.9843; found 328.9847. UV/
3
3
7.45 (d, JH,H = 6.0 Hz, 4 H, H6/H6ЈЈ), 7.35 (d, JH,H = 8.6 Hz, 8
H, Hphenyl), 3.06 (m, 8 H, Hethyl), 0.92 (m, 8 H, Hethyl), 0.04 (s, 36
Vis (acetone): λmax (ε, Lcm–1 mol–1) = 364 (7773), 556 (13693) nm.
{[Fe(4c)2]2+[(PF6)–]2} (8b): Compound 8b was prepared by adapting
representative procedure C. FeCl2 (5.20 mg, 40.0 μmol), 4,4ЈЈ-di-
methoxy-2,2Ј:6Ј2ЈЈ-terpyridine 4c (25.0 mg, 80.0 μmol), MeOH
(10 mL), DCM (10 mL), water (50 mL) and NH4PF6 (326 mg,
2.00 mmol) were used. The product was isolated as a deep purple
powder in quantitative yield.
H, TMS) ppm. 13C NMR (101 MHz, [D6]acetone, 25 °C): δC
=
161.43 (Cq, 4C), 159.50 (Cq, 4C), 153.96 (Ct, 4C), 150.68 (Cq, 4C),
142.95 (Cq, 4C), 139.07 (Ct, 2C), 132.38 (Cq, 4C), 128.44 (Cphenyl
,
8C), 128.42 (Cphenyl, 8C), 125.07 (Ct, 4C), 124.99 (Ct, 4C), 121.63
(Ct, 4C), 28.53 (Cethyl, 4C), 17.07 (Cethyl, 4C), –1.69 (CTMS, 12C)
ppm. 19F NMR (376 MHz, [D6]acetone, 25 °C): δF = –72.26 (d,
Analytical Data for 8b: Yield 100% (38.2 mg, 40.0 μmol). M.p. Ͼ
1JP,F = 709.1 Hz, 12 F) ppm. MS (ESI+): m/z (%) = 677.2 (100)
300 °C. 1H NMR (500 MHz, [D6]acetone, 25 °C): δH = 9.24 (d, [M2+ – 2PF6 ]. HRMS (ESI-ToF): calcd. for [C74H86FeN6S4Si4]2+
–
Eur. J. Inorg. Chem. 2013, 3334–3347
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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677.2108; found 677.2126. UV/Vis (acetone): λmax (ε, Lcm–1 mol–1)
= 361 (86567), 568 (18067) nm.
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{[Fe(5c)2]2+[(PF6)–]2} (8e): Compound 8e was prepared by adapting
representative procedure C. FeCl2 (6.30 mg, 50.0 μmol), 4,4ЈЈ-bis(4-
acetylthiophenyl)-2,2Ј:6Ј,2ЈЈ-terpyridine 5c (56.2 mg, 100 μmol),
MeOH (15 mL), DCM (15 mL), water (75 mL) and NH4PF6
(408 mg, 2.50 mmol) were used. The product was isolated as a dark
purple solid in quantitative yield.
[4]
Analytical Data for 8e: Yield 99% (70.0 mg, 50.0 μmol). M.p. Ͼ
300 °C. 1H NMR (400 MHz, [D6]acetone, 25 °C): δH = 9.52 (d,
3JH,H = 7.9 Hz, 4 H, H3Ј/H5Ј), 9.22 (m, 4 H, H3/H3ЈЈ), 8.94 (t,
[5]
[6]
3JH,H = 7.7 Hz, 2 H, H4Ј), 7.89 (d, JH,H = 8.2 Hz, 8 H, Hphenyl),
3
3
7.58 (m, 8 H, H5/H5ЈЈ/H6/H6ЈЈ), 7.54 (d, JH,H = 8.3 Hz, 8 H,
Hphenyl), 2.43 (s, 12 H, Hmethyl) ppm. 13C NMR (101 MHz, [D6]-
acetone, 25 °C): δC = 192.99 (4 C, CC=O), 161.51 (4 C, Cq),
159.74 (4 C, Cq), 154.37 (4 C, Ct), 150.62 (4 C, Cq), 139.36 (2 C,
Ct), 137.07 (4 C, Cq), 135.96 (8 C, Cphenyl), 132.23 (4 C, Cq),
128.89 (8 C, Cphenyl), 125.82 (4 C, Ct), 125.37 (4 C, Ct), 122.52 (4
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[7]
[8]
[9]
1
25 °C): δF = –72.22 (d, JP,F = 707.5 Hz, 12 F) ppm. 31P NMR
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[11]
[12]
[13]
(162 MHz, [D6]acetone, 25 °C): δP = –144.20 (hept, 1JP,F = 708 Hz,
–
2 P) ppm. MS (ESI+): m/z (%) = 561.2 (100) [M2+ – 2PF6 ]. HRMS
(ESI-ToF): calcd. for [C62H46FeN6O4S4]2+ 561.0902; found
561.0912. UV/Vis (acetone): λmax (ε, Lcm–1 mol–1) = 338 (81333),
380 (14600), 567 (11867) nm.
Structure Determination by Single-Crystal X-ray Analysis: The in-
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with the Diamond program.[79] The X-ray crystallographic param-
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sented in Tables S1 and S2.
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CCDC-924273, -924274, -924275, -924276, -924277 and -924278
contain the supplementary crystallographic data for 4b, 5a, 5b, 5c,
8a and 8c. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Supporting Information (see footnote on the first page of this arti-
cle): Crystallographic data, NMR (1H-, 13C-, DEPT-135-, 11B-,
19F- and 31P-) and MS (EI-MS, FAB-MS, ESI-MS and HR-ESI-
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Received: February 18, 2013
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