N,N,O ligands based on triazoles and transition metal complexes thereof

Article abstract of DOI:10.1002/ejic.201000391

The reaction of 2H-benzotriazole (1) with dichloroacetic acid leads to the symmetric bis(2H-benzotriazol-2-yl)methane (2c) in 40 % yield. Deprotonation of 2c at the bridging methylene group and subsequent carboxylation with CO ₂ yielded the new tripodal N,N,O ligand bis(2H-benzotriazol-2-yl) acetic acid Hbbta (3). The sterically less demanding sodium bis(1H- ,4-triazol-1-yl)propionate Na[btp] (5) was obtained by saponification of methyl -bis(1H- ,4-triazol-1-yl)propionate (4). The heteroscorpionate hgand 3 viras treated with [MnBr(CO)₅] and [RuCl₂(PPh₃J ₃] to form the manganese tricarbonyl complex [Mn(bbta)(CO) ₃] (6) and the air stable ruthenium complex [Ru(bbta)Cl(PPh ₃)₂] (7). DFT calculations and the IR spectra of the carbonyl complex 6 reveal ligand 3 to be a less electron-donating ligand compared to the Ms(1Hpyrazol-1-yl)acetic acid Hbpza. In case of the 1H- ,4-triazole-based ligand 5 formation of coordination polymers were observed as proven by the synthesis of [Zn(btp)2∞ (8) and [Mn(btp)₂]∞ (9). Polymer 8 crystallises in linear chains of zinc atoms bridged by two molecules of ligand 5 each. These polymer chains show an interesting π-stacking interaction between uncoordinated triazole residues of the ligands. Deposition of the coordination polymer on highly ordered pyrolytic graphite (HOPG) was successful and was analysed by scanning tunnelling microscopy (STM). Reaction of Na[btp] (5) with MnSO₄-H₂O resulted in a metal organic framework (MOF) [Mn(Mp)₂]∞ (9).

Full text of DOI:10.1002/ejic.201000391

FULL PAPER
DOI: 10.1002/ejic.201000391
N,N,O Ligands Based on Triazoles and Transition Metal Complexes Thereof
Eike Hübner,[a] Nina V. Fischer,[a] Frank W. Heinemann,[a] Utpal Mitra,[b]
Viacheslav Dremov,[b] Paul Müller,*[b] and Nicolai Burzlaff*[a]
Keywords: Scorpionate ligands / Tripod ligands / Coordination polymers / Zinc / Manganese / Ruthenium
The reaction of 2H-benzotriazole (1) with dichloroacetic acid
leads to the symmetric bis(2H-benzotriazol-2-yl)methane (2c)
in 40% yield. Deprotonation of 2c at the bridging methylene
group and subsequent carboxylation with CO2 yielded the
new tripodal N,N,O ligand bis(2H-benzotriazol-2-yl)acetic
acid Hbbta (3). The sterically less demanding sodium 3,3-
bis(1H-1,2,4-triazol-1-yl)propionate Na[btp] (5) was obtained
to be a less electron-donating ligand compared to the bis(1H-
pyrazol-1-yl)acetic acid Hbpza. In case of the 1H-1,2,4-tri-
azole-based ligand 5 formation of coordination polymers
were observed as proven by the synthesis of [Zn(btp)2]ϱ (8)
and [Mn(btp)2]ϱ (9). Polymer 8 crystallises in linear chains of
zinc atoms bridged by two molecules of ligand 5 each. These
polymer chains show an interesting π-stacking interaction
by saponification of methyl 3,3-bis(1H-1,2,4-triazol-1-yl)pro- between uncoordinated triazole residues of the ligands. De-
pionate (4). The heteroscorpionate ligand 3 was treated with
[MnBr(CO)5] and [RuCl2(PPh3)3] to form the manganese
tricarbonyl complex [Mn(bbta)(CO)3] (6) and the air stable
ruthenium complex [Ru(bbta)Cl(PPh3)2] (7). DFT calculations
and the IR spectra of the carbonyl complex 6 reveal ligand 3
position of the coordination polymer on highly ordered pyro-
lytic graphite (HOPG) was successful and was analysed by
scanning tunnelling microscopy (STM). Reaction of Na[btp]
(5) with MnSO4·H2O resulted in a metal organic framework
(MOF) [Mn(btp)2]ϱ (9).
Our recent work focuses on derivatives of such N,N,O
ligands, to broaden the field of available heteroscopionate
ligands with various electronic and steric properties. So far
examples of chiral bis(1H-pyrazol-1-yl)acetic acids,[10] 3,3-
bis(imidazol-2-yl)propionic acids[11] or 2-[bis(1H-pyrazol-1-
yl)methyl]phenolates[12] have been obtained. Moreover,
tricarbonylrhenium complexes are of particular interest as
model compounds for radiopharmaceutical applications.[13]
Introducing triazoles instead of pyrazoles or imidazoles
as N-donor seems to be one reasonable option to establish
a new field of N,N,O heteroscorpionate ligands. To the best
of our knowledge, so far mainly bis(1H-1,2,4-triazol-1-yl)-
acetic acids have been applied in coordination chemistry,
e.g. in cytotoxic copper(I) complexes or metal-organic
frameworks (MOFs).[14–16]
Introduction
N,N,O ligands such as bis(1H-pyrazol-1-yl)acetic acids
are a versatile class of ligands structurally related to the
well-known hydrotris(1H-pyrazol-1-yl)borate (Tp) ligand.
Efficient syntheses for achiral, chiral and enantiopure
bis(1H-pyrazol-1-yl)acetic acids have been developed over
the years since they have been introduced to coordination
chemistry in 1999 by A. Otero.[1] Various transition metal
complexes bearing these tripodal, κ3-N,N,O-binding ligands
indicate binding properties as weak electron-donating li-
gands, similar to the Tp ligands.[2,3] Examples for transition
metal complexes with manganese and ruthenium are
[Mn(bdmpza)(CO)3] and [Ru(bdmpza)(PPh3)3] {bdmpza:
bis(3,5-dimethyl-1H-pyrazol-1-yl)acetate}.[4–6]
Further-
2H-Benzotriazole, which is well known to form a chelat-
ing N,N-donor ligand by reaction with dichloromethane,[17]
offers rather large steric demand but electron deficiency.
Therefore, synthesis of bis(2H-benzotriazol-2-yl)acetic acid
promises to result in an interesting ligand candidate. In con-
trast, 1H-1,2,4-triazole may act as small and sterically non-
demanding N-donor.
more, model complexes with biorelevant transition metals
such as iron(II) and zinc(II) support the concept of bis(1H-
pyrazol-1-yl)acetic acids to be good structural mimics for
the active site of metalloenzymes with an 2-His-1-carboxyl-
ate motif.[7–9]
[a] Inorganic Chemistry, Department of Chemistry and
Pharmacy & Interdisciplinary Center for Molecular Materials
(ICMM),
University of Erlangen-Nürnberg,
Egerlandstraße 1, 91058 Erlangen, Germany
Fax: +49-9131-85-27387
Ligand Design
Reaction of 2H-benzotriazole (1) with dichloromethane
leads to the three possible isomers bis(1H-benzotriazol-1-
yl)methane (2a), (1H-benzotriazol-1-yl)(2H-benzotriazol-2-
yl)methane (2b) and bis(2H-benzotriazol-2-yl)methane (2c)
E-mail: burzlaff@chemie.uni-erlangen.de
[b] Department of Physics & Interdisciplinary Center for
Molecular Materials (ICMM),
University of Erlangen-Nürnberg,
Erwin-Rommel-Strasse 1, 91058 Erlangen, Germany
View this journal online at
wileyonlinelibrary.com
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N,N,O Ligands and Transition Metal Complexes
(Scheme 1).[17,18] Almost independently from the reaction
conditions the synthesis yields a mixture of all three iso-
mers. A reaction with dichloromethane in the presence of
KOH and a phase-transfer catalyst for example results in
40% 2a, 31% 2b and 6% 2c.[18] Isomer 2c is the isomer
required for the synthesis of a symmetric and bulky hetero-
scorpionate tripodal N,N,O ligand. Therefore, higher
amounts of isomer 2c would be desirable.
Figure 1. Molecular structure of bis(2H-benzotriazol-2-yl)acetic
acid (3); thermal ellipsoids are drawn at the 50% probability level.
Most hydrogen atoms have been omitted for clarity. Selected
bond lengths [Å] and angles [°]: d(C2–O1) = 1.320(5), d(C2–O2)
=
1.179(5), d(C1–N12)
= 1.440(5), d(C1–N22) = 1.462(5),
Є(N12,C1,N22) = 112.2(3).
Due to the annelated benzene rings, compound 3 repre-
sents a ligand with relatively large steric demand. Díez-
Barra and co-workers recently published an interesting
pathway to various esters of N-heterocyclic 3,3-bis(hetero-
aryl)propionic acids, such as methyl 3,3-bis(1H-1,2,4-
triazol-1-yl)propionate (4).[19] The corresponding acid, 3,3-
bis(1H-1,2,4-triazol-1-yl)propionic acid or the salt sodium
3,3-bis(1H-1,2,4-triazol-1-yl)propionate (Nabtp) (5) are ob-
tained by saponification of ester 4 in presence of water and
NaOH in THF (Scheme 2). Due to the very good solubility
of 5 in water, purification of 5 in the presence of excess base
or other inorganic salts proved to be rather difficult.
Scheme 1. Synthesis of the Hbbta ligand 3.
Instead of reacting 1 with dichloromethane, the reaction
with dichloroacetic acid as starting material offers an alter-
native synthetic pathway. In the presence of K2CO3, KOH
and phase-transfer catalyst 2H-benzotriazole (1) reacts with
dichloroacetic acid to the desired isomer 2c in yields up to
40% (Scheme 1).
Reaction of 2c with nBuLi in THF leads to a dark blue
anion, due to its large delocalized π-electron system. In
analogy to the synthesis of Li[bdmpza], that is obtained
from the corresponding bis(3,5-dimethyl-1H-pyrazol-1-yl)-
methane,[1] further reaction with dry gaseous carbon diox-
ide affords the bis(2H-benzotriazol-2-yl)acetic acid (Hbbta)
Scheme 2. Sodium 3,3-bis(1H-1,2,4-triazol-1-yl)propionate (Nabtp)
(5) obtained by saponification of the ester methyl 3,3-bis(1H-1,2,4-
triazol-1-yl)propionate (4).
(3) after acidic workup. The presence of 3 is clearly indi-
However, saponification with a slight deficit of NaOH
1
cated by a shift of the bridging CH H NMR signal from δ yields a mixture of 4 and 5. Ester 4 was removed by washing
= 7.43 ppm in 2c to 8.25 ppm for 3, as well as the appropri- with chloroform to yield the sodium propionate 5 as a
ate mass spectrum. The IR resonance at 1763 cm–1 (THF) colourless solid. Extracted ester 4 can be recirculated in a
is assigned to the asymmetric carboxylate vibration. Finally, second saponification step. The sodium propionate 5 is
an X-ray analysis revealed the molecular structure of 3 clearly indicated by the absence of the resonance of the
1
(Figure 1).
methyl ester at δ = 3.71 ppm in the H-NMR spectra and
Although, decarboxylation occurs during the reaction of the asymmetric carboxylate vibration at 1612 cm–1 (meth-
2H-benzotriazole with dichloroacetic acid, ligand 3 was anol). Ligands 3 represents on one side a new and sterically
found to be stable at room temperature as crystalline pow- demanding bis(triazolyl)carboxylic acid whereas ligand 5
der as well as in solution.
on the other side is sterically less demanding.
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The orbital plots show the capability of 3 to act as π-ac-
ceptor (LUMO+1, Table 1, Ia) as well as the pyrazoles
(LUMO, Ic) do. Noticeable is the energetically low and
strongly delocalised LUMO+1. The orbital plot of the
HOMO (IIa) points out the π-donor character of the 1H-
benzotriazole ligands in accordance to the pyrozoles (IIc).
σ-Donor binding properties (HOMO–2, IVa) are provided
similar to pyrazoles (HOMO–2, IVc).
Solely the HOMO–1 (IIIa) does not seem to be involved
in the interaction between the frontier orbitals and the
metal centre. Obviously, all considered orbitals apart from
the HOMO-2 show conspicious delocalisation in the aro-
matic benzene ring. Over all, bis(2H-benzotriazol-2-yl)acet-
ic acid should be able to act as σ-donor, π-donor and π-
acceptor. The π-donor properties may be less distinctive due
to the delocalization into the aromatic system. The two pos-
sible nitrogen binding sites in 3 do not show up any differ-
ences in the orbital plots, due to the molecular symmetry.
Coordination towards the metal centre with one of the ni-
trogen donors will always lead to the same product.
DFT Calculations
To gather some information about the electronical and
coordination properties of the two ligands 3 and 5, DFT
calculations were performed. Comparison of the calculated
frontier orbitals of 2-methyl-2H-benzotriazole, 1-methyl-
1H-1,2,4-triazole and 1-methyl-1H-pyrazole (Table 1) gives
information about the donor and acceptor properties of the
binding nitrogen sites of 3 and 5. The frontier orbitals of 2-
methyl-2H-benzotriazole are assumed to be comparable to
those of the 2H-benzotriazol-2-yl donor in 3, the frontier
orbitals of 1-methyl-1H-1,2,4-triazole to those of the 1H-
1,2,4-triazol-1-yl donor in 5 and the frontier orbitals of 1-
methyl-1H-pyrazole to those of the 1H-pyrazol-1-yl donor
in bis(1H-pyrazol-1-yl)acetic acids. The binding behaviour
of histidine according to Holm and co-workers is domi-
nated by its frontier orbitals.[20] Recently, we and others suc-
cessfully discussed the binding of imidazole- and pyrazole-
based N,N,O ligands by means of their frontier orbitals.[11]
In contrast, ligand 5 shows some differences regarding
the two possible N donors. Both can act as π-acceptors
(LUMO, Ib) and σ-donors (HOMO, IIb) although coordi-
nation will not lead to the same product due to the broken
symmetry. N2 has a smaller orbital coefficient in one of the
two orbitals suitable for π-donors compared to N4 (IVb,
HOMO–2). On the other hand N4 has a total lack of π-
donors capabilities compared to N2 in IIIb (HOMO–1). In
total, due to missing delocalisation, the orbitals of 5 seem
to show some similarity to those of the pyrazole donor.
Especially N2 can be compared to the nitrogen of a pyr-
azole-based ligand. But 5 provides two different binding
sites, both able to act as as donor and acceptor.
Table 1. Comparison of the Khon–Sham orbitals of 2-methyl-1H-
benzotriazole, 1-methyl-1H-triazole and 1-methyl-1H-pyrazole.
Besides a discussion of the orbital plots, the HOMO–
LUMO energy gap allows to estimate further properties
based on the concept of chemical hardness. Molecules with
a large HOMO–LUMO energy gap are considered to be
hard, molecules with a small energy gap are considered to
be soft.[21] Following the calculated orbital energies in
Table 1, ligand 3 must clearly be accepted to be the softest
of the three ligands discussed here. The low HOMO–
LUMO energy gap can be explained by the strongly delo-
calized structure of 2H-benzotriazole. Due to the DFT cal-
culations performed here and well-known errors of the re-
sulting orbital energies, no further estimations on the reac-
tivity of these ligands towards metal fragments are ex-
tracted from these results.[22] Furthermore, the different co-
ordination properties of the two possible bindings sites N2
and N5 of 1-methyl-1H-1,2,4-triazole cannot be explained
by means of the HOMO–LUMO energy gap.
Metal Complexes
Deprotonation of the carboxylate donor allows ligand 3
to react as a monoanionic N,N,O ligand with organometal-
lic precursors such as [MnBr(CO)5] and [RuCl2(PPh3)3]
(Scheme 3).
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N,N,O Ligands and Transition Metal Complexes
Unfortunately, 6 turned out to be rather unstable
towards external influences such as heat and light. After
some hours at daylight, complex 6 already decomposes to
an orange solid. As known from the synthesis of 3, higher
temperatures lead to decarboxylation. Therefore, heating of
6 in solution above 40 °C leads to decomposition indicated
by the appearance of several signals in the IR carbonyl re-
gion (1900–2050 cm–1). Consequently, synthesis of 6 affords
temperatures of 30 °C or lower which causes a longer reac-
tion time compared to the synthesis of [Mn(bpza)(CO)3].[4]
Purification of 6 by means of column chromatography was
only successful with stabilized THF as solvent, leading to
residues of butylated hydroxytoluene (BHT) in the product.
Attempts of an analogous reaction of ligand 3 with
[ReBr(CO)5] instead of [MnBr(CO)5] failed, probably due
to a slower reaction, which leads to decomposition.
To further investigate the steric demand and coordina-
tion properties of ligand 3, the ruthenium complex
[Ru(bbta)Cl(PPh3)2] (7) was synthesized analogously to the
preparation of [Ru(bdmpza)Cl(PPh3)2].[5] Reaction of the
potassium salt of 3 with [RuCl2(PPh3)3] leads to [Ru(bbta)-
Cl(PPh3)2] (7), a light orange powder, as indicated by the
FD mass peak (Scheme 3). It is well known, that in case of
sterically less demanding ligands such as bpza, two dia-
stereomers are obtained, while sterically more demanding
ligands such as bdmpza lead to just one symmetrical iso-
mer.[5] The 31P NMR spectrum shows only one singlet for
the PPh3 ligands (δ = 33.3 ppm). This suggests that only
one isomer was formed by the reaction. This is backed by
the absence of more than one set of benzotriazole signals
in the 13C{1H} NMR spectrum. An X-ray structure deter-
mination doubtlessly reveals the symmetrical structure of 7
(Figure 2).
Scheme 3. Synthesis of [Mn(bbta)(CO)3] (6) and [Ru(bbta)Cl-
(PPh3)2] (7).
The facial N,N,O-binding behaviour of ligand 3 is clearly
indicated by the IR stretching frequencies of the complex
[Mn(bbta)(CO)3] (6), which is obtained by a reaction of the
potassium salt of 3 with [MnBr(CO)5]. The IR spectrum of
complex 3 exhibits a single AЈ and two close AЈЈ and AЈ
signals [6: ν (THF) = 2047, 1963, 1933 cm–1]. This is typical
˜
for an unsymmetric “piano stool” carbonyl complex.[4] An
IR signal observed at 1707 cm–1 is assigned to the asymmet-
ric carboxylate vibration. The formation of 6 is backed by
the appropriate MH+ signal of the FAB mass spectrum.
Although the NMR spectra of 6 are broad, complex 6
shows a clear set of signals for the carbonyl residues at δ =
218.3 and 221.0 ppm in the 13C{1H} NMR spectrum. The
signal occurring at δ = 162.0 ppm is assigned to the carb-
oxylate donor. Table 2 shows a comparison of the charac-
teristic IR signals of 6 to analogous tricarbonyl complexes,
which allows to estimate the electronic properties of 3.[4]
Table 2. IR data of complexes [MnL(CO)3] with various tripodal
ligands.
Ligand L
bbta
bpza[4]
bdmpza[4]
Tp[4]
ν [MnL(CO) ] 2047, 1963, 2041, 1946, 2036, 1942, 2033, 1930
˜
3
1933
1925
1915
Figure 2. Molecular structure of [Ru(bbta)Cl(PPh3)2] (7); thermal
ellipsoids are drawn at the 50% probability level. Hydrogen atoms
The IR stretching vibrations at higher wavenumbers indi- have been omitted for clarity. Selected bond lengths [Å] and angles
[°]: d(C2–O1) = 1.272(6), d(C2–O2) = 1.211(6), d(Ru–N11) =
cate in total (σ- and π-bonding as well as π-back bonding)
2.118(4), d(Ru–N21) = 2.182(4), d(Ru–O1) = 2.106(3), d(Ru–Cl) =
that ligand 3 is a less electron-donating ligand compared
2.4070(16), d(Ru–P1)
=
2.3488(18), d(Ru–P2)
=
2.3415(15),
87.54(15),
170.34(10),
95.92(11),
173.10(12),
166.24(11),
85.11(11),
to the other ligands. This may be explained by the large
delocalized π-electron system of 2H-benzotriazole and elec-
tron density located at the benzene rings instead of then N-
donor as indicated by the DFT calculations. Therefore,
Hbbta may be the N,N,O ligand providing the most electron
deficiency at the metal center from this series of ligands,
meanwhile providing noticeable steric demand.
Є(N11,Ru,N21)
Є(N21,Ru,O1)
Є(O1,Ru,P1)
Є(N11,Ru,P1)
Є(N11,Ru,Cl)
Є(N21,Ru,P2)
=
=
78.59(16), Є(N11,Ru,O1)
86.53(14), Є(O1,Ru,Cl)
83.82(11), Є(O1,Ru,P2)
91.19(13), Є(N11,Ru,P2)
86.07(12), Є(N21,Ru,P1)
95.62(12), Є(N21,Ru,Cl)
=
=
=
=
=
=
=
=
=
=
Є(Cl,Ru,P1) = 103.53(5), Є(Cl,Ru,P2) = 89.72(5), Є(P1,Ru,P2) =
95.11(6).
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P. M üller, N. Burzlaff et al.
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Signals at δ = 83.3 ppm and 164.6 ppm are assigned to shows.[7] Two ligands act as bridging ligands towards a sec-
–
the bridging CH and the CO2 carboxylate donor. Due to ond metal centre at a time each. This results in a linear
the ambient temperature reaction conditions, no decompo- alignment of the zinc atomes (see Figures 3 and 4). In total,
sition by decarboxylation was observed. The pure com- every zinc atom is connected to two other zinc centres. Each
pound 7 itself, was found to be remarkably stable against bridge is build up by two ligands. This results in the total
air. No indication of oxidation of the ruthenium centre was formula ratio of Zn/btp = 1:2.
notified even after weeks. Due to the stability of complex
7, and the weak electron-donating properties of ligand 3,
compound 7 promises to be an interesting precursor for or-
ganometallic ruthenium complexes. Further work might fo-
cus on ligand exchange reactions with [Ru(bbta)Cl(PPh3)2],
e.g. the exchange of PPh3 or Cl by cumulynidene ligands.
In contrast to the potassium salt of 3, reaction of the
sodium salt 5 with [MnBr(CO)5] does not lead to the corre-
sponding manganese tricarbonyl complex. During the reac-
tion in THF a bright-yellow substance precipitates. IR spec-
tra of the reaction mixture shows three carbonyl absorption
bands at ν = 2032, 1936, 1908 cm–1. The lower signals may
˜
be in accordance with the expected more electron-donating
properties of 5 compared to 3 as shown by the DFT-calcu-
lations, but these results have to be interpreted with caution,
since the spectra where obtained from the crude suspension
of the reaction mixture. Any attempts to purify the ob-
Figure 3. Molecular structure of [Zn(btp)2]ϱ (8); a cutout of a linear
chain with three zinc atoms is shown; thermal ellipsoids are drawn
at the 50% probability level. Hydrogen atoms have been omitted
for clarity. Selected bond lengths [Å] and angles [°]: d(Zn–O1) =
tained insoluble substances failed. Similar formation of in- 1.9494(11), d(Zn–N13) = 2.0045(13), d(C1–O1) = 1.2745(19),
d(C1–O2)
Є(O1,Zn,N13B)
=
1.2301(19), Є(O1,Zn,O1A)
121.34(5), Є(O1,Zn,N13C)
=
100.41(7),
soluble and unisolable products was obsersed by the reac-
tion of 5 with [ReBr(CO)5] and [RuCl2(PPh3)3]. Reaction of
5 with Zn(ClO4)2 in methanole immediately precipitates a
white powder, too (Scheme 4). IR spectra of this zinc com-
plex 8 show an absorption band assigned to the asymmetric
=
= 104.48(5),
Є(N13B,Zn,N13C) = 106.02(8), d(Zn–Zn) = 6.773 Å, centroid–
centroid distance of the triazole rings: 5.043 Å.
This bridging coordination is possible, since every ligand
carboxylate vibration at ν = 1634 cm–1, indicating coordina- coordinates via the N-donor in 4-position of only one of the
˜
tion of ligand 5 via the carboxylate donor. NMR spectra in two triazole rings. The second triazole ring of each ligand is
D2O, in which the obtained zinc complex 8 slowly dissolves, not coordinated to any metal centre and remains free. The
show the resonances of the free ligand 5. Finally, an X-ray two coordinated triazole rings of the two bridging ligands
structure determination explains the unusual behaviour of are aligned coplanar to each other (Figure 3). The centroid-
8 (see Figures 3 and 4). Obviously, a linear coordination centroid distance of these triazole rings is rather large with
polymer [Zn(btp)2]ϱ (8) has been formed. Each zinc atom 5.043 Å. Each chain of the linear coordiation polymer ad-
is tetrahedrally coordinated by four ligands, each binding ditionally shows interaction with adjacent chains (Figure 4)
towards the metal centre with the carboxylate group as O- by a coplanar orientation of the uncoordinated triazole
donor or the nitrogen atom at 4-position of the triazole rings (centroid–centroid distance: 3.618 Å).
residue as N-donor (Figure 3). The bond lengths are in the
typical range for a tetrahedral coordinated zinc centre with π-stacking interaction since
The short distance of these triazole rings let us assume a
comparison with the
a
N,O-donor ligands, as comparison with [Zn(bdtbpza)Cl] centroid–centroid distance in hexagonal graphite shows
[d(Zn–O): 1.990(2) Å, d(Zn–N): 2.053(2), 2.067(3) Å] only a slightly smaller distance (d = 3.354 Å).[23]
The formation of linear coordination polymers explains
the poor solubility of the zinc complex 8. In water or D2O
the compound probably slowly dissociates to give the free
ligand and Znaq2+, which explains the identical NMR spec-
tra of 5 and 8, and crystallizes again as coordination poly-
mer. This coordination behaviour of 5 may also explain the
insoluble products obtained by reaction of 5 with other
metal salts although the products could not be crystallized
so far. It is noteworthy that recently Klein Gebbink and co-
workers obtained a linear coordination polymer with
the N,N,O ligand 3,3-bis(1-methyl-2H-imidazol-2-yl)-
propionate.[24]
Recent studies have shown that the real space structure
of metal coordination complexes can be investigated by
Scheme 4. Formation of a linear coordination polymer [Zn(btp)2]ϱ
(8).
scanning tunnelling microscopy (STM).[25] Therefore a
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N,N,O Ligands and Transition Metal Complexes
pected if one considers sticking to the surface with one ring
parallel and one perpendicular to the plane. Due to this
strong clustering high resolution topography and spec-
troscopy where not achieved, even while atomic resolution
of the graphite background was routinely observed.
Following a procedure similar to the successful synthesis
of [Zn(btp)2]ϱ (8), but employing MnSO4·H2O instead of
Zn(ClO4)2, we found the formation of the two-dimensional
coordination polymer [Mn(btp)2]ϱ (9). Instead of the split
ν(C=N) bands we observed in case of [Zn(btp)2]ϱ (8) [ν =
˜
1530 cm–1 and 1513 cm–1], now only one signal at
1521 cm–1 for ν(C=N) is observed in the IR spectrum (KBr)
of 9. This can be explained by a closer look at the molecular
structure of 9, gained by X-ray structure determination.
Three different metal centres coordinate the two triazole
donors and the carboxylate functionality each. Hence, all
carboxylate and triazol moieties have the same electronic
environment. A cutout of the coordination geometry of one
manganese atom is shown in Figure 6.
Figure 4. Molecular structure of [Zn(btp)2]ϱ (8); thermal ellipsoids
are drawn at the 50% probability level. Hydrogen atoms have been
omitted for better clarity. A cutout of three adjacent linear polymer
chains is shown. Centroid–centroid distance of the triazole rings in
π-stacking arrangement: 3.618 Å.
10–9  aqueous solution of 8 was drop coated on highly
oriented pyrolytic graphite (HOPG) and investigated by
STM in ambient conditions.
[Zn(btp)2]ϱ (8) was found to form stable, straight lines
(Figure 5, a). This can be attributed to a template effect of
the substrate, stabilizing the polymer in positions of mini-
mal energy along step edges and subsurface defect lines. As
seen in Figure 5 (b) the minimum observed width of the
structures is approx. 2.7 nm, roughly the diameter of a
double chain. Therefore we have to assume a strong ten-
dency to form clustered chains. This can be attributed to
the π-stacking interaction of the non coordinating triazole
rings as observed in the crystal structure. The measured pe-
riodicity along the chain is 1.5 nm. This is very close to the
distance between every 2nd triazole ring. This can be ex-
Figure 6. Molecular structure of [Mn(btp)2]ϱ (9); thermal ellipsoids
are drawn at the 50% probability level. Hydrogen atoms have been
omitted for better clarity. A cutout of the surrounding of one man-
ganese ion is shown. Selected bond lengths [Å] and angles [°]:
d(Mn–O1) = 2.1297(11), d(Mn–N13B) = 2.2806(13), d(Mn–N23C)
= 2.2678(13), d(C1–O1) = 1.2673(18), d(C1–O2) = 1.2371(19), Є
(O1,Mn,O1D)
=
180.00, Є(O1,Mn,N13B)
=
88.10(5),
Є
(O1,Mn,N23C) = 92.84(5), Є(N13B,Mn,N23C) = 90.56(5).
The manganese center is octahedrally coordinated by six
different ligands. Two ligands are coordinating through the
carboxylate functionalities in trans-position towards each
other. The left coordination sites are occupied by the N-
atoms of the 4-position of four triazole rings, belonging to
four different btp ligands. Recently, M. Du and S. R. Batten
and co-workers reported on the coordination properties of
bis(1H-1,2,4-triazol-1-yl)acetate (btza).[16] It is noteworthy,
that reaction of Mn(OAc)2 with Hbtza afforded a coordina-
tion polymer [{[Mn(btza)2(OH2)2]·2H2O}ϱ], in which the
manganese centers are coordinated by four independent li-
gands each, two coordinated by the carboxylates and two
by the triazole rings. Two further coordination sites have
been occupied by additional water molecules. Evidently, by
the elongation of the carbon backbone from acetate to pro-
Figure 5. STM topographies showing a) the aggregation of linear
polymer strands on the surface; and b) STM topography indicating
a double stranded polymer on the surface.
Eur. J. Inorg. Chem. 2010, 4100–4109
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4105

P. M üller, N. Burzlaff et al.
FULL PAPER
pionate, a full occupation of the manganese coordination
sites by ligands and vice versa becomes possible.
Conclusions
In conclusion, the syntheses of 3 and 5 lead to two new
ligands with interesting coordination properties. Ligand 3
shows κ3-N,N,O-coordination as it is well-known from
other tripodal monoanionic N,N,O ligands. Estimation of
the electronic properties of 3 via synthesis of the manganese
tricarbonyl complex [Mn(bbta)(CO)3] (7) and DFT calcula-
tions show 3 to be a rather weak electron-donating ligand.
Furthermore, synthesis of [Ru(bbta)Cl(PPh3)2] (6) might
enable the future synthesis of various organometallic com-
plexes. In contrast to 3, ligand 5 does not show tripodal
binding towards metal centres but prefers the formation of
coordination polymers as proven by the synthesis of
[Zn(btp)2]ϱ (8) and [Mn(btp)2]ϱ (9). Interestingly, 8 as-
sembles as linear chains of zinc ions, connected by two
bridging ligands 5. The chains show interactions via π-
stacking of the uncoordinated triazole residue. The coordi-
nation polymer [Mn(btp)2]ϱ (9) shows two-dimensional
polymer sheets.
This results in an array of coordination polymer sheets.
Within a sheet all the manganese atoms are situated in one
plain [d(Mn–Mn) = 8.064, 8.279 and 9.538 Å], shielded by
the ligands form above and from below. The top view on
such a sheet is given in Figure 7.
Experimental Section
Figure 7. Molecular structure of [Mn(btp)2]ϱ (9); hydrogen atoms
have been omitted for better clarity. Cutout of a coordination poly-
mer sheet is shown (top view).
General Remarks: All operations regarding manganese carbonyl
and ruthenium complexes were carried out under an inert gas at-
mosphere by using conventional Schlenk techniques. All solvents
(analytical-grade purity) were degassed and stored under nitrogen
atmosphere. The yields refer to analytically pure substances and
were not optimised. IR spectra were recorded by use of a Varian
Excalibur FTS-3500 FT-IR spectrometer in CaF2 cuvets (0.2 mm).
1H and 13C{1H} NMR spectra: Bruker AC 250 MHz, Bruker
DPX300, Bruker DPX400, δ values relative to TMS or the deuter-
ated solvent. Mass spectra were recorded on a modified Finnigan
MAT 312 using either EI or FAB technique or on a Jeol JMS-
700 using FD technique. Elemental analysis: Euro EA 3000 (Euro
Vector) and EA 1108 (Carlo Erba). Chemicals were used as pur-
chased without further purification. Methyl bis(1,2,4-triazol-1-yl)-
propionate, [MnBr(CO)5] and [RuCl2(PPh3)2] were synthesized ac-
cording to literature.[19,26–28]
Interestingly, no solvent molecules are intercalated into
the structures of 9 and 8, respectively, although incorpo-
rated water molecules were observed for the analogous co-
ordination polymers derived from btza.[16] In coordination
polymer [Mn(btp)2]ϱ (9), the stacked sheets are interlocked
into each other, leaving no larger spaces within the structure
[d(O2–N11{other sheet}) = 3.512 Å]. A short distance be-
tween the uncoordinated carboxylate oxygen atom O2 and
C3 of the neighbouring sheet [d(O2–C3{other sheet}) =
3.329 Å] might indicate hydrogen bonds in between the
sheets. Figure 8 shows the side view on the arrangement
of the polymer sheets in the crystal structure, showing the
coordination polyhedra of the manganese atoms.
Synthesis of Bis(2H-benzotriazol-2-yl)methane (2c): 2H-Benzotri-
azole (11.9 g, 100 mmol), K2CO3 (24.9 g, 180 mmol), KOH powder
(10.1 g, 180 mmol) and benzyltriethylammonium chloride (1.00 g,
4.39 mmol) are suspended in THF (500 mL). Dichloroacetic acid
(6.45 g, 50.0 mmol) is added and the white suspension heated under
reflux for 96 h. The solvent is removed in vacuo and the remaining
residue dissolved in water (250 mL). The aqueous solution is ex-
tracted with ethyl ether (5ϫ200 mL) and the combined organic
layers dried with Na2SO4. Evaporation of the ethyl ether at room
temperature to approx. 500 mL affords a first fraction of a mixture
containing large amounts of the undesirable isomers. Decanting of
the diethyl ether solution and evaporation of the remaining diethyl
ether affords 2c in colourless crystals; yield 5.01 g (20.0 mmol,
40%); 1H NMR (250 MHz, CDCl3, 25 °C): δ = 7.38 and 7.78
3
4
[AAЈBBЈ pattern, J(H,H) = 6.7, JH,H = 3.1 Hz, 8 H, Ar-H], 7.43
(s, 2 H, CHbridge) ppm. 13C{1H} NMR (62.9 MHz, CDCl3, 25 °C):
δ = 71.6 (Cbridge), 118.6 (Caryl), 127.6 (Caryl), 145.2 (CNaryl) ppm.
Bis(2H-benzotriazol-2-yl)acetic Acid (3): A solution of 2c (0.826 g,
3.30 mmol) in dry THF was treated with nBuLi (2.10 mL, 1.6  in
n-hexane, 3.36 mmol) at –80 °C. The dark blue solution is stirred
for 2 h and warmed up to –40 °C. Dry carbon dioxide ist passed
Figure 8. Molecular structure of [Mn(btp)2]ϱ (9); cutouts of three
adjacent coordination polymer sheets are shown (side view, poly-
hedral representation of the octahedral coordination of the manga-
nese atoms).
4106
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 4100–4109

N,N,O Ligands and Transition Metal Complexes
in for a period of 3 h, meanwhile the colour changes from dark
blue over dark red to light yellow. The solvent is removed in vacuo
and the remaining residue dissolved in water (100 mL) and acidi-
fied with diluted HCl to pH = 7. The aqueous phase is extracted
with ethyl ether (2ϫ100 mL) to remove impurities. The aqueous
solution is further acidified to pH = 3 and extracted with ethyl
ether (3ϫ200 mL) and the combined organic layers dried with
Na2SO4 and concentrated in vacuo. The yellowish residue is recrys-
tallised from acetone to yield 3 as colourless crystals; yield 0.243 g
30 min. To the resulting white suspension [MnBr(CO)5] (0.275 g,
1.00 mmol) was added, the reaction mixture heated to 30 °C and
controlled by IR on a regular basis. After 72 h the reaction was
completed and the solution filtered. The filtrate was concentrated
in vacuo and after washing with water (2ϫ5 mL) and little diethyl
ether dried in vacuo to yield 6 as a yellow, light and heat sensitive
solid; yield 0.186 g (0.430 mmol, 43%); m.p. Ͼ60 °C (slow dec.),
1
Ͼ200 °C (fast dec.). H NMR (300 MHz, CDCl3, 25 °C): δ = 7.6–
8.1 (m, broad, 8 H, Haryl), 7.83 (br., 1 H, CHbridge) ppm. 13C{1H}
NMR (75.5 MHz, CDCl3, 25 °C): δ = 80.4 (CHbridge), 115.8
(CHaryl), 120.3 (CHaryl), 129.3 (CHaryl), 131.5 (CHaryl), 144.7
(CNaryl), 146.3 (CNaryl), 162.0 (COOH), 218.3 (CO), 221.0 (CO)
1
(0.826 mmol, 25%); m.p. 151–153 °C (dec.). H NMR (300 MHz,
3
4
CDCl3, 25 °C): δ = 7.44, 7.90 (AAЈBBЈ pattern, JH,H = 3.6, JH,H
= 3.0 Hz, 8 H, Ar-H), 8.25 (s, 1 H, CHbridge) ppm. 13C{1H} NMR
(75.5 MHz, [D6]DMSO, 25 °C): δ = 80.3 (Cbridge), 118.3 (CHaryl), ppm. IR (THF): ν = 2047 [s (C=O)], 1963 [s (C=O)], 1933 [s
˜
–
127.8 (CHaryl), 143.9 (CNaryl), 163.3 (CO H) ppm. IR (THF): ν =
(C=O)], 1707 [s (CO2 )] cm–1. FAB-MS: m/z (%) = 433 (25) [MH+],
349 (13) [MH+ 3CO] 307 (25) [MH+
3CO CO2].
˜
2
1763 [s (CO2H)], 1563 [m (C=N)] cm–1. FD-MS: m/z (%) = 296
–
–
–
(100) [MH+], 250 (46) [M+ – CO2]. C14H10N6O2 (294.27 gmol–1): C17H9MnN6O5 (432.23 gmol–1): calcd. C 47.24, H 2.10, N 19.44;
calcd. C 57.14, H 3.43, N 28.56; found C 57.40, H 3.48, N 28.49.
found: Due to the instability of 6 against light and heat, no sample
free of a stabilizing agent (BHT) could be purified.
Synthesis of Sodium 3,3-Bis(1H-1,2,4-triazol-1-yl)propionate (5): To
methyl 3,3-bis(1H-1,2,4-triazol-1-yl)propionate (1.00 g, 4.50 mmol)
a solution of pure NaOH (0.090 g, 2.25 mmol, 0.5 equiv.) in water
(100 mL) is stirred for 24 h at ambient temperature. The solvent is
removed in vacuo at 40 °C and the residue is extracted five times
with chloroform to remove not reacted methyl ester and dried in
vacuo. The chloroform may be removed in vacuo to recover methyl
bis(1,2,4-triazol-1-yl)propionate.
Synthesis of [Zn(btp)2]ϱ (8): To a solution of Nabtp (5) (0.230 g,
1.00 mmol) in methanole (20 mL) a solution of zinc perchlorate
hexahydrate (0.186 g, 0.499 mmol) in methanole (10 mL) is slowly
added. Immediately, 8 precipitates as a fine white powder. The sus-
pension is stirred for 15 min at ambient temperature, the residue
filtered off, washed with methanol and dried in vacuo, to give 8 as
a colourless powder.
Crystallisation from water affords colourless blocks suitable for X-
ray structure determination.
5 was obtaineds as a white powder; yield 0.434 g (1.89 mmol, 84%);
m.p. Ͼ260 °C (dec.). 1H NMR (300 MHz, D2O, 25 °C): δ = 3.61
3
3
(d, JH,H = 7.1 Hz, 2 H, CH2), 7.27 (t, JH,H = 7.1 Hz, 1 H,
CHbridge), 8.10 (s, 2 H, Htriazole), 8.84 (s, 2 H, Htriazole) ppm.
13C{1H} NMR (75.5 MHz, D2O, 25 °C): δ = 40.7 (CH2), 69.8
Yield 0.187 g (0.390 mmol, 78%); m.p. Ͼ240 °C (dec.).
–
IR (KBr): ν = 1634 [s (CO )], 1530 (w) and 1513 [w (C=N)] cm–1.
˜
2
C14H14N12O4Zn (479.73 gmol–1): calcd. C 35.05, H 2.94, N 35.04;
(Cbridge), 145.2 (Ctriazole), 152.8 (Ctriazole), 175.1 (COO–) ppm. IR
found C 34.78, H 2.93, N 34.78.
–
(MeOH): ν = 1612 [s (CO )], 1511 [m (C=N)] cm–1. FD-MS (after
˜
2
addition of dil. HCl): m/z (%) = 210 (60) [MH+], 140 (10) [M+
–
Due to the coordination polymer properties of 8 further analystical
methods failed. NMR spectra in D2O show similar resonances as
in case of the free ligand 5.
tz]. C7H7N6NaO2 (230.16 gmol–1): calcd. C 36.53, H 3.07, N 36.51;
found C 36.86, H 3.16, N 36.86.
Synthesis of [Mn(btp)2]ϱ (9): A test tube was charged with btpNa
(0.101 g, 0.439 mmol) and MnSO4 ϫH2O (0.037 g, 0.219 mmol),
and water (5 mL) was added. The tube was heated at 70 °C for 4 d.
After cooling, pale yellow crystals had formed, that were collected,
triturated with water (5ϫ5 mL), and dried in vacuo at 80 °C to
Synthesis of [Ru(bbta)Cl(PPh3)2] (7): To Hbbta (3) (0.100 g,
0.340 mmol) in dry THF (50 mL) tBuOK (0.038 g, 0.339 mmol)
was added and the reaction mixture stirred at ambient temperature
for 30 min. To the resulting white suspension [RuCl2(PPh3)3]
(0.326 g, 0.34 mmol) was added and stirred for additional 2 h and
the dark-brown suspension changes colour to orange. The solvent
is removed in vacuo and the remaining residue washed with water
(2ϫ10 mL) and little ethyl ether, pentane (2ϫ10 mL) and meth-
anol (2ϫ5 mL) to yield 7 as bright orange powder.
yield 9 a pale yellow powder; yield 0.066 g, 0.141 mmol, 64%; m.p.
–
Ͼ311 °C (dec.). IR (KBr): ν = 3436 (br), 3114 (m), 1621 [s (CO )],
˜
2
1521 [s (C=N)], 1438 (w), 1395 (m), 1340 (w), 1266 (w), 1199 (w),
1131 (m), 1007 (w), 908 (w), 822 (w), 673 (m), 552 (w) cm–1.
C14H14MnN12O4 (469.28 gmol–1): calcd. C 35.83, H 3.01, N 35.82;
found C 35.85, H 3.05, N 35.95.
Crystallisation from chloroform yields orange crystals suitable for
X-ray structure determination.
Calculations
Yield 0.195 g (0.20 mmol, 59%); m.p. Ͼ140 °C (dec.). 1H NMR
(400 MHz, CDCl3, 25 °C): δ = 6.7–7.6 (m, 38 H, Haryl), 7.94 (s, 1
All DFT-calculations and full geometry optimizations were carried
out by using Jaguar 6.0012[29] running on Linux 2.4.18–14smp on
five Athlon MP 2800+ dual-processor workstations (Beowulf-clus-
ter) parallelized with MPICH 1.2.4. MM+ calculated structures
were used as starting geometries. Complete geometry optimizations
were carried out on the implemented N31G6* basis set and the
BP86 density functional. Orbital plots[22] were obtained using Mae-
stro 7.0.113, the graphical interface of Jaguar.[29]
H, CHbridge) ppm. 13C{1H} NMR (101 MHz, CDCl3, 25 °C): δ =
3
83.3 (CHbridge), 118.3 (CHaryl), 120.2 (CHaryl), 127.2 (vt, JC,P
=
2
5 Hz, m-PPh3), 127.7 (m, i-PPh3), 129.1 (p-PPh3), 134.9 (vt, JC,P
= 5 Hz, o-PPh3), 135.1 (d, JC,P = 2 Hz, CHaryl), 135.4 (CHaryl),
–
145.0 (CNaryl), 146.4 (CNaryl), 164.6 (CO2 ) ppm. 31P{1H} NMR
(121.5 MHz, CDCl , 25 °C): δ = 33.3 (PPh ) ppm. IR (THF): ν =
˜
3
3
1967 [s (CO2 )], 1690 [w (CN)] cm–1. FD-MS: m/z (%) = 954 (5)
[M+], 662 (44) [MH+ – bbta], 262 (50) [(PPh3)3]. C50H39ClN6O2-
P2Ru (954.37 gmol–1): calcd. C 62.93, H 4.12, N 8.81; found C
62.27, H 4.19, N 8.60.
–
STM Measurements
The STM imaging was carried out under ambient conditions using
a home-built, low-drift microscope and RHK100 control elec-
tronics. A drop of aqueous 10–9  sample solution was placed onto
Synthesis of [Mn(bbta)(CO)3] (6): To Hbbta (3) (0.294 g,
1.00 mmol) in dry THF (50 mL) tBuOK (0.112 g, 1.00 mmol) was a freshly cleaved HOPG surface and left to dry. Sections without
added and the reaction mixture stirred at ambient temperature for molecules clearly showed atomic resolution of the underlying
Eur. J. Inorg. Chem. 2010, 4100–4109
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4107

P. M üller, N. Burzlaff et al.
FULL PAPER
Table 3. Details of the structure determination for 3, 7, 8 and 9.
3
7
8
9
Empirical formula
Formula mass /gmol–1
Crystal colour/habit
Crystal system
Space group
a /Å
b /Å
c /Å
α /°
β /°
C14H10N6O2
294.28
colourless block
monoclinic
P21/c
9.836(3)
9.7389(15)
14.726(3)
90.00
99.220(17)
90.00
1392.4(6)
2.10–25.91
–12 to 12
–11 to 11
–18 to 18
608
C50H39ClN6O2P2Ru
954.33
red plate
monoclinic
P21/c
18.183(8)
10.654(5)
22.591(6)
90.00
93.08(3)
90.00
4370(3)
1.81–25.46
–21 to 21
–12 to 0
0 to 27
C14H14N12O4Zn
239.87
colourless block
monoclinic
C2/c
22.8880(11)
6.1072(2)
13.4816(7)
90.00
92.934(3)
90.00
1882.01(15)
3.43–29.52
–31 to 31
–8 to 8
–18 to 18
976
C14H14MnN12O4
234.66
colourless block
triclinic
¯
P1
7.7640(4)
8.0636(4)
8.2794(5)
108.605(3)
104.916(5)
92.918(4)
469.66(4)
2.74–28.99
–10 to 10
–10 to 10
–11 to 11
239
γ /°
V /Å3
θ /°
h
h
l
F(000)
1952
Z
4
4
4
1
µ (Mo-Kα) /mm–1
Crystal size /mm
Dcalcd. /gcm–3, T /K
Reflections collected
Indep. Reflections
Obs. refl. (Ͼ2σI)
Parameter
Weight parameter a
Weight parameter b
R1 (obsd.)
R1 (overall)
wR2 (obsd.)
wR2 (overall)
Diff. peak/hole /eÅ–3
0.101
0.542
1.359
0.757
0.3ϫ0.25ϫ0.15
1.404, 293(2)
5225
2675
840
203
0.0407
0
0.0545
0.2665
0.0975
0.1402
0.65ϫ0.2ϫ0.18
1.450, 293(2)
8312
8088
5080
560
0.0736
0.0813
0.0480
0.1248
0.1219
0.1484
0.725/–0.590
0.12ϫ0.09ϫ0.08
1.693, 150(2)
24456
2639
2310
141
0.0241
2.0966
0.0287
0.0371
0.0605
0.0628
0.400/–0.369
0.18ϫ0.18ϫ0.07
1.659, 150(2)
13616
2491
2224
142
0.0477
0.1519
0.0278
0.0340
0.0889
0.0864
0.414/–0.302
0.189/–0.195
graphite substrate. Typically, tunnelling currents between 10 and
200 pA were employed. The bias voltage was Ϯ72.9 mV. The scan
frequency was varied between 2 to 5 Hz. Resolution was 256ϫ256
points. Manually cut Pt/Ir(10%) tips were used.
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[8]
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nates were refined freely. All details and parameters of the measure-
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(for 9) contain the supplementary crystallographic data for
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Received: April 8, 2010
Published Online: August 4, 2010
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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