Acid directed in situ oxidation and decarboxylation of 4,4′,6, 6′-tetra-methyl-2,2′-bipyridine: Synthesis and structural characterisation of 4,4′,6-tri-carboxy-2,2′-bipyridine and its copper(II) coordination polymer

Article abstract of DOI:10.1016/j.ica.2012.12.033

The reaction of either 4,4′,6,6′-tetramethyl-2,2′- bipyridine, L, or 4,4′,6,6′-tetracarboxy-2,2′-bipyridine, H₄L, with Cu(OAc)₂·H₂O under acidic hydrothermal conditions (50:1 H₂O/HNO₃; 160 °C) led to the formation of crystalline {[Cu(HL′)(H₂O)]·H ₂O}, 1, which was structurally characterised to identify H ₃L′ as 4,4′,6-tricarboxy-2,2′-bipyridine. Clearly, in situ mono-decarboxylation of a tetracarboxylic acid ligand gave the tricarboxy-analogue, H₃L′. The structure of 1 consists of a 1D coordination polymer that cross-links through hydrogen-bonding between adjacent carboxylic acid and carboxylate groups, as well as through an aqua ligand and lattice water molecule, to form a densely interconnected 3D network. Regioselective mono-decarboxylation of L or H₄L at the 6′-carboxylic acid position may also be affected by heating L or H ₄L in acidic solution under hydrothermal conditions (2:1 H ₂O/HNO₃; 160°C) to yield crystalline 4,4′,6-tricarboxy-2,2′-bipyridinium nitrate hydrate {[H ₄L′][NO₃]·H₂O}, 2, which was also structural characterised. The structure of 2 features an array of hydrogen-bonding interactions that generate a 3D network. More forceful heating of the acidic solution at 180°C led to double decarboxylation and the formation of 4,4′-dicarboxy-2,2′-bipyridine, whereas heating at 200°C led to total decarboxylation and the formation of 2,2′- bipyridine. Details of the structures of 1 and 2 along with their synthesis are discussed.

Full text of DOI:10.1016/j.ica.2012.12.033

Inorganica Chimica Acta 403 (2013) 102–109
Contents lists available at SciVerse ScienceDirect
Inorganica Chimica Acta
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Acid directed in situ oxidation and decarboxylation of
4,40,6,60-tetra-methyl-2,20-bipyridine: Synthesis and structural
characterisation of 4,40,6-tri-carboxy-2,20-bipyridine and its
copper(II) coordination polymer
Niamh R. Kelly a, Sandrine Goetz a, Chris S. Hawes b,1, Paul E. Kruger b,
⇑
a School of Chemistry, Trinity College Dublin, Dublin 2, Ireland
b Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch 8041, New Zealand
a r t i c l e i n f o
a b s t r a c t
The reaction of either 4,40,6,60-tetramethyl-2,20-bipyridine, L, or 4,40,6,60-tetracarboxy-2,20-bipyridine,
H4L, with Cu(OAc)2ꢀH2O under acidic hydrothermal conditions (50:1 H2O/HNO3; 160 °C) led to the forma-
tion of crystalline {[Cu(HL0)(H2O)]ꢀH2O}, 1, which was structurally characterised to identify H3L0 as 4,40,6-
tricarboxy-2,20-bipyridine. Clearly, in situ mono-decarboxylation of a tetracarboxylic acid ligand gave the
tricarboxy-analogue, H3L0. The structure of 1 consists of a 1D coordination polymer that cross-links
through hydrogen-bonding between adjacent carboxylic acid and carboxylate groups, as well as through
an aqua ligand and lattice water molecule, to form a densely interconnected 3D network. Regioselective
mono-decarboxylation of L or H4L at the 60-carboxylic acid position may also be affected by heating L or
H4L in acidic solution under hydrothermal conditions (2:1 H2O/HNO3; 160 °C) to yield crystalline 4,40,6-
tricarboxy-2,20-bipyridinium nitrate hydrate {[H4L0][NO3]ꢀH2O}, 2, which was also structural character-
ised. The structure of 2 features an array of hydrogen-bonding interactions that generate a 3D network.
More forceful heating of the acidic solution at 180 °C led to double decarboxylation and the formation of
4,40-dicarboxy-2,20-bipyridine, whereas heating at 200 °C led to total decarboxylation and the formation
of 2,20-bipyridine. Details of the structures of 1 and 2 along with their synthesis are discussed.
Ó 2012 Elsevier B.V. All rights reserved.
Article history:
Available online 28 December 2012
Coordination Polymers Special Issue
Keywords:
Coordination polymer
4,40,6,60-Tetracarboxy-2,20-bipyridine
Decarboxylation
Crystal engineering
Supramolecular chemistry
2,20-Bipyridine
MOF
In situ ligand synthesia
1. Introduction
quence of such forcing conditions is the occurrence of unexpected
side reactions during solvothermal treatment although these can
The study of coordination polymer species has been an increas-
ing endeavour in modern inorganic and materials chemistry over
the last two decades due in part to the promise such species hold
in a wide range of applications: from magnetic and luminescent
materials; through gas separation and storage; to catalysis and
sensing [1–13]. While the examples reported to date have provided
a better understanding of the fundamental physical and chemical
properties of these materials [14–20]; the development of the next
generations of framework species will be driven by the discovery
of new ligand types from which they are prepared [21–31].
Solvothermal synthesis has become a standard method for the
preparation of coordination polymers as it provides access to high
liquid temperatures, due to the autogenous pressure produced
within the sealed reaction vessel, which helps to promote self-
assembly processes [32]. However, in some instances a conse-
provide unique routes to useful ligands or engender decomposition
pathways within ligand species [33–36]. We have taken advantage
of the solvothermal method in the synthesis of coordination poly-
mers derived from polypyridine poly-carboxylic acid ligands, such
as 4,40-dicarboxy-2,20-bipyridine and 4,40,6,60-tetracarboxy-2,20-
bipyridine [37–43]. Our interest in these ligands is driven by the
recognition that the 2,20-bipyridine and carboxylate functional
groups of may potentially coordinate to each metal ion within
the periodic table so the scope for the preparation of innumerable
coordination polymers is enormous. Our research effort has used
these ligand types in conjunction with metal ions from across
the main group, transition and lanthanide series in the synthesis
of coordination polymers. We have also shown that these ligands
may hydrogen bond through either functionality so the prepara-
tion of hybrid networks combining the strength of coordination
bonding with the flexibility of hydrogen bonding is also feasible.
In recent work we developed a solvothermal oxidation protocol
that employs dilute aqueous nitric acid solutions in the synthesis
of the polypyridine poly-carboxylic acid ligands from their
methyl-precursors [44]. Encouraged by these results, we wished
⇑
Corresponding author. Tel.: +64 33642438; fax: +64 3 364 2110.
E-mail address: paul.kruger@canterbury.ac.nz (P.E. Kruger).
Present address: School of Chemistry, Monash University, Clayton, Victoria 3800,
1
Australia.
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.12.033

N.R. Kelly et al. / Inorganica Chimica Acta 403 (2013) 102–109
103
to modify the approach by incorporating a metal ion in the oxida-
tion solution to see whether a coordination polymer might be
formed following (or during) the oxidation of the ligand precursor.
In the current study we treated an aqueous nitric acid solution of
either 4,40,6,60-tetramethyl-2,20-bipyridine or 4,40,6,60-tetracarb-
oxy-2,20-bipyridine with Cu(II) ions under solvothermal conditions
and report here the successful isolation of an oxidised ligand along
with the formation of a Cu(II) coordination polymer, 1, which was
structurally characterised to identify a novel ligand species H3L0,
where H3L0 is 4,40,6-tricarboxy-2,20-bipyridine, Scheme 1. Clearly,
in situ mono-decarboxylation of a tetracarboxylic acid ligand gave
the tricarboxy-analogue. Details of the structures of 1 along with
the metal ‘free’ ligand {[H4L0][NO3]ꢀH2O}, 2, and their synthesis
are discussed.
(10 ml) and concentrated nitric acid (0.2 ml). Large green block-
shaped crystals were obtained directly and return crystallographic,
analytical and spectroscopic data identical to above. Yield: 0.077 g,
83%.
2.4. Synthesis of 4,40,6-tricarboxy-2,20-bipyridinium nitrate hydrate, 2
H4L (0.083 g; 0.25 mmol) was placed in a 45 ml TeflonÒ-lined
digestion bomb in water (10 ml) and concentrated nitric acid
(5 ml). This was stirred for 10 min., sealed, heated to 160 °C for
36 h and cooled to room temperature at a rate of 5 °C/h. The result-
ing solution was filtered and allowed to sit at room temperature
for 1 week in an uncovered vial, after which, colourless plate crys-
tals suitable for a single crystal X-ray diffraction study were ob-
tained. Yield: 0.041 g, 43%. Found: C, 42.1; H, 2.9; N, 11.1%.
C
13H11N3O10 requires: C, 42.3; H, 3.0; N, 11.4%. m
max/cmꢁ1: 3538
2. Experimental
w, br, 3089 w, 1954 w, br, 1742 m, 1717 m, 1603 m, 1571 w,
1518 w, 1480 w, 1422 br, m, 1393 br, m, 1269 s, 1216 s, 1150 m,
1036 m, 999 w, 929 w, 920 w, 844 m, 818 w, 801 w, 763 s, 690
m, 665 m, 654 m.
2.1. Materials and methods
Solvents and reagents were purchased from Aldrich and used as
received. Water was triply distilled in house prior to use. 4,40,6,60-
tetramethyl-2,20-bipyridine (L) and 4,40,6,60-tetracarboxy-2,20-
bipyridine (H4L) were prepared as detailed previously [37–43].
Solvothermal syntheses were carried out using a Parr Instrument
general-purpose digestion bomb employing a Teflon insert with a
45 ml capacity. IR spectra were recorded on a Perkin Elmer Spec-
trum One FTIR spectrometer using a universal ATR sampling acces-
sory in the 4000–650 cmꢁ1 region. Elemental analyses were
performed at the Micro-analytical laboratories, University College
Dublin. Thermogravimmetric analysis was carried out on a Mettler
TC 11 system under a flow of nitrogen at a heating rate of 10 °C/
min for all measurements.
2.5. Double decarboxylation of 4,40,6,60-tetracarboxy-2,20-bipyridine to
form 4,40-dicarboxy-2,20-bipyridine
4,40,6,60-Tetracarboxy-2,2’-bipyridine (0.083 g; 0.25 mmol) was
placed a 45 ml TeflonÒ-lined digestion bomb in H2O (8 ml) and
conc. HNO3 (1 ml). This was stirred for 10 min, sealed, heated to
180 °C for 36 h and cooled at a rate of 5 °C/h. Yield: 45 %. Anal. Calc.
for C12H8N2O4: C, 59.0; H, 3.3; N, 11.5. Found: C, 58.7; H, 3.6; N,
0
0
11.2%. dH 8.92 (d, 2H, J = 4.8 Hz, H6,6 ), 8.85 (br s, 2H, H3,3 ), 7.92
+
0
(d, 2H, J = 4.8 Hz, H5,5 ); HRMS(ES): [MH] requires: 245.0557;
Found: 245.0556. m
max/cmꢁ1: 3109 w, 2435 m, 1860 w, br, 1717
s, 1603 w, 1457 m, 1364 s, 1289 vs 1267 s, 1241 s, 1137 m, 1065
m, 1010 m, 914 w, 864 w, 819 w, 764 s, 679 s, 516 w.
2.2. Synthesis of {[Cu(HL0)(H2O)]ꢀH2O}, 1, from 4,40,6,60-tetramethyl-
2,20-bipyridine (L)
2.6. Crystallographic measurements
Cu(OAc)2.H2O (0.06 g; 0.3 mmol) and L (0.053 g; 0.25 mmol)
were placed in a 45 ml TeflonÒ-lined digestion bomb in water
(10 ml) and concentrated nitric acid (1 ml). This was stirred for
10 min., sealed, heated to 160 °C for 36 h and cooled to room tem-
perature at a rate of 5 °C/h. Large green block-shaped crystals suit-
able for a single crystal X-ray diffraction structural analysis were
obtained directly. Yield: 0.038 g, 41%. Found: C, 40.1; H, 2.5; N,
Single crystal data and experimental details for 1 and 2 are
summarised in Table 1. Single crystal analyses were performed at
153 K with a Bruker SMART APEX CCD diffractometer using graph-
ite mono-chromated Mo Ka radiation (k = 0.71073 Å). A full sphere
of data was obtained for each using the omega scan method. Data
were collected, processed and corrected for Lorentz and polariza-
tion effects using SMART [45] and SAINT-NT [46] software. Absorp-
tion corrections were applied using SADABS [47]. The structures
were solved using direct methods and refined by full matrix least
squares against F2 using the SHELXTL [48] programme package. The
data for 1 suggested the presence of a slight twinned domain but
this could not be indexed using the GEMINI [49] programme. Suc-
cessful refinement was achieved without making allowance for
the twin. For 1 and 2, all non-hydrogen atoms were refined aniso-
tropically. Aromatic hydrogen atoms were assigned to calculated
positions with isotropic thermal parameters fixed at 1.2 times that
of the attached carbon atom.
7.1%. CuC13H10N2O8 requires: C, 40.4; H, 2.6; N, 7.2%. m :
max/cmꢁ1
3335 w, br, 3059 w, 2922 w, br, 1721 m, 1644 m, 1593 s, 1569
m, 1432 w, 1415 w, 1380 w, 1348 s, 1295 m, 1249 m, 1219 s,
1105 w, 1087 w, 1045 w, 1015 w, 946 w, 876 m, 823 w, 788 m,
771 m, 738 s, 710 m, 682 m, 667 s.
2.3. Synthesis of {[Cu(HL0)(H2O)]ꢀH2O}, 1, from 4,40,6,60-tetracarboxy-
2,20-bipyridine, (H4L)
A very similar method to that above was followed except in this
instance H4L (0.083 g; 0.25 mmol) was used in place of L in water
OH
HO
O
O
O
Cu2+(aq) / HNO3
or
N
N
N
N
H2O / HNO3
(- CO2)
OH
4,4',6,6'-tetramethyl-2,2'-bipyridine,L
4,4',6-tricarboxy-2,2'-bipyridine,H3L'
Scheme 1. In situ oxidation and decarboxylation of 4,40,6,60-tetramethyl-2,20-bipyridine yields 4,40,6-tricarboxy-2,20-bipyridine.

104
N.R. Kelly et al. / Inorganica Chimica Acta 403 (2013) 102–109
Table 1
Crystallographic data for compounds 1 and 2.
Compound
1
2
Moiety formula
Formula weight
Crystal habit
Crystal size (mm)
Crystal system
Space group
a (Å)
C
13H10CuN2O8
C13H11N3O10
369.25
colourless plates
385.77
green prisms
0.31 ꢂ 0.18 ꢂ 0.1
0.24 ꢂ 0.14 ꢂ 0.04
orthorhombic
monoclinic
Pna21
17.3392(7)
P21/c
9.0411(7)
b (Å)
11.4434(5)
25.0890(19)
c (Å)
6.7896(3)
6.5439(5)
a
(°)
90.00
90.00
b (°)
90.00
95.672(2)
c
(°)
90.00
1347.19(10)
90.00
1477.1(2)
V (Å3)
Z
4
4
Dcalc (g cmꢁ3
F(000)
)
1.902
780
1.660
760
Absorption coefficient
1.673
0.146
h (°)
2.13–24.99
1.62–25.00
Reflections measured
Independent reflections
Index ranges
10059
2354
11608
2592
ꢁ20 6 h 6 20, ꢁ13 6 k 6 13, ꢁ8 6 l 6 8
ꢁ10 6 h 6 10, ꢁ29 6 k 6 29, ꢁ7 6 l 6 7
Rint
0.0194
0.0522
Data/restraints/parameters
2354/13/233
1.097
2592/11/253
1.214
Goodness-of-fit (GOF) on F2
R1, wR2 [I P 2
R1, wR2 (all data)
max, d )
min (e Åꢁ3
r
(I)]
0.0300, 0.0840
0.0301, 0.0841
1.08 and ꢁ0.58
0.0762, 0.1488
0.0962, 0.1568
0.63 and ꢁ0.50
d
q
q
To ascertain sample homogeneity of the samples, 6–10 single
laboratory solvents which thwarted any attempts at solution anal-
yses. The crystals of 2 are soluble in water and alcohol solution.
While decarboxylation reactions on bipyridine ring systems are
known [52] we were sufficiently intrigued by the formation of the
4,40,6-tricarboxy-2,20-bipyridine ligand so set about testing its
threshold of formation under these acidic conditions. To this end,
repetition of the hydrothermal treatment of 4,40,6,60-tetramethyl-
2,20-bipyridine or 4,40,6,60-tetra-carboxy-2,20-bipyridine in the
presence of nitric acid at a more forcing 180 °C led to the formation
of a crystalline products whose 1H NMR and IR spectroscopic prop-
erties were consistent with the formation of 4,40-dicarboxy-2,20-
crystals from each batch were randomly selected and separately
indexed on the diffractometer. In each case the cell parameters
were consistent with those found for the single crystals used in
the full data collection and ensure phase purity.
3. Results and discussion
3.1. Synthesis and characterisation 1 and 2
The synthesis of 1 and 2 employed solvothermal methods in an
H2O:HNO3 solvent mix with a 50:1 volumetric ratio for 1 and 2:1
volumetric ratio for 2, at 160 °C for 1.5 days followed by cooling
to room temperature at a rate of 5 °C/h. This led to the direct iso-
lation of large green single crystals for 1 whilst single crystals of
2 appeared after 7 days following slow evaporation of the filtrate
at RT in an open vial. The infrared spectrum of the copper complex,
1, contains characteristic carboxyl stretching bands at 1720 (
and 1348 (
s(CO)) cmꢁ1, respectively, that may be attributed to the
presence of a carboxylic acid group. Further stretches at 1644,
1593 ( as(CO)), 1295 and 1249 (
s(CO)) cmꢁ1, respectively, may
be assigned to unidentate binding of a carboxylate group as the
peak separation value ( ) is large [50,51]. The infrared spectrum
of 2 contains characteristic carboxyl stretches with peaks at 1717
and 1603 cmꢁ1 and 1269 and 1216 cmꢁ1 assigned to
as(CO) and
s(CO) stretching, respectively [50,51]. Indeed, broad stretches
mas(CO))
m
m
m
D
m
m
within the region 1450–1300 cmꢁ1 region may be assigned to the
presence of the nitrate anion. Microanalytical data for 1 were
consistent with 1:1 metal-to-ligand ratio and with retrospect
(subsequent to the crystallographic analysis) the data for 2 were
consistent with the formation of a mono-decarboxylated product
and the formation of a zwitterionic bis-bipyridinium nitrate spe-
cies of a the new ligand, 4,40,6-tricarboxy-2,20-bipyridine. Each
crystalline crop was indefinitely stable at RT when removed from
their mother liquors and those of 1 are insoluble in all common
Fig. 1. Molecular structure and atomic numbering scheme for 1 with symmetry
equivalent of Cu1 included showing further coordination of the carboxylate oxygen
atom O3. 2,20-Bipyridyl hydrogen atoms and lattice water molecule omitted for
clarity. Symmetry operators: a = x ꢁ 0.5, y + 0.5, z.

N.R. Kelly et al. / Inorganica Chimica Acta 403 (2013) 102–109
105
bipyridine, evidently through the double decarboxylation of the
tetracarboxylate. Employing a higher temperature of 200 °C led
to complete decarboxylation and the formation of 2,20-bipyridine.
Milder temperatures of ca. 140 °C led to lower isolated yield of
the tri-carboxylate whereas lower temperature again ca. 100 °C
simply retrieved either starting material. These outcomes are con-
sistent with those previously reported by us when investigating
the nitric acid mediated hydrothermal oxidation of methyl substi-
tuted polypyridines in the formation of a series of polypyridine-
polycarboxylic acids [44].
Fig. 2. The 1D polymer formed by coordination of adjacent Cu1 centres through carboxylate oxygen atom O3. Adjacent CuN2O2 chromophores within polymer, which extends
along the crystallographic a-axis, are rotated by ca. 60° with respect to each other.
Fig. 3. (a) Representation of one mode of hydrogen bonding interactions between adjacent chains in 1, showing both strong O–Hꢀ ꢀ ꢀO and weak C–Hꢀ ꢀ ꢀO hydrogen bonds
(dashed lines). Hydrogen atoms not participating in this mode of hydrogen bonding are omitted for clarity. (b) Part of the 3D network structure of 1 formed by hydrogen-
bonding between adjacent 1D polymers through carboxylic acid hydrogen atom H2 with carboxylate oxygen atom O5, O2–H2ꢀ ꢀ ꢀO5, as viewed approximately down the
crystallographic a-axis.

106
N.R. Kelly et al. / Inorganica Chimica Acta 403 (2013) 102–109
3.2. Structure of {[Cu(HL0)(H2O)]ꢀH2O}, 1
each [(H2O)Cu(HL0)] link is rotated from the previous one by ca.
60°. These 1D chains interact with one another, in the first in-
stance, through strong hydrogen-bond interactions between the
carboxylic acid oxygen atom, O2, and the carboxylate oxygen atom,
O5, (O2ꢀ ꢀ ꢀO5 2.584(4) Å) of an adjacent chain, which is recipro-
cated by a weak C–Hꢀ ꢀ ꢀO hydrogen bond originating from the pyr-
idine 5-position (C2ꢀ ꢀ ꢀO1 3.437(6) Å), Fig. 3. The chains are further
interconnected via the hydrogen bond donor/acceptor functional-
ity provided by the aqua ligand, which interacts with non-coordi-
nating carboxylate oxygen atom O5 from an adjacent chain, as
well as both donating and receiving hydrogen bonds from symme-
try related lattice water molecules. The lattice water molecule also
donates a hydrogen bond to non-coordinating carboxylate oxygen
atom O4. On the basis of these interactions, the structure is best
described as a series of one-dimensional coordination polymer
chains engaged in three-dimensional hydrogen bonding interac-
tions, in which each one-dimensional chain is directly linked to
four others through the hydrogen bond donors and acceptors pres-
ent on the [Cu(HL0)(OH2)] unit, and each lattice water molecule
links a further three chains, as represented in Fig. 4. Details of all
hydrogen bonding interactions are brought together within
Table 2.
The atomic numbering scheme and atom connectivity for 1 are
shown in Fig. 1. The structure was refined in the orthorhombic
Pna21 space group and crystallographic data are shown in Table 1.
As can be seen from Fig. 1, a 1:1 Cu(II):HL0 complex has formed. It is
immediately apparent that the 4,40,6,60-tetracarboxylic acid-2,20-
bipyridine ligand has been decarboxylated at the 60-position to
form a novel ligand: 4,40,6-tricarboxy-2,20-bipyridine, HL0. This
newly formed ligand is chelating to Cu1 in tridentate fashion via
the 2,20-bipyridyl-6-carboxylate core (N2O). Coordination about
the slightly distorted square pyramidal copper centre is completed
by a coordinated water molecule, O1w, and a symmetry related 4-
carboxylate oxygen atom, O3a, from a neighbouring unit. The com-
plex is neutral with the 40-carboxylic acid group remaining in its
acid form and non-coordinated. A lattice water molecule, O2w, is
also present in the asymmetric unit and partakes in several hydro-
gen bonding interactions, as described below. The 4-carboxylate
and 40-carboxylic acid groups are twisted by ca. 11° and 8°, respec-
tively, to the plane of the pyridyl ring to which they are attached,
whilst the 6-carboxylate group is essentially co-planar with the
pyridyl ring to which it is attached by virtue of its involvement
in chelation to Cu1. The pyridyl rings of the HL0 ligand are essen-
tially co-planar with a 3.1° twist between their mean-planes.
Expansion of the asymmetric unit reveals further coordination
about Cu1 and the formation of a 1D polymeric chain that extends
along the crystallographic a-axis, Fig. 2. An alternating polymeric
chain is formed through coordination of the carboxylate oxygen
atom, O3, to the Cu(II) centre of an adjacent molecule such that
3.3. Thermal behaviour of 1
The thermal behaviour of 1 was investigated by thermo-gravi-
metric analysis (TGA) and the results from this study are displayed
in Fig. 5. Analysis of the TGA curve for 1 reveals a gradual weight
loss of ca. 4.5% between 120 < T < 195 °C consistent with the loss
Fig. 4. Topological representation of the densely interconnected one-dimensional coordination polymer chains in the structure of 1, where linkages through coordination
bonds are represented as blue lines, direct hydrogen bonds between chains are represented as green dashes, and hydrogen bonds between chains and lattice water molecules
are represented as red dashes. Nodes selected as Cu1 (blue) and lattice water O2 W (red). (For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
Table 2
Hydrogen bonding parameters for compounds 1 and 2.
Compound
D–Hꢀ ꢀ ꢀA
d(D–H) (Å)
d(Hꢀ ꢀ ꢀA) (Å)
d(Dꢀ ꢀ ꢀA) (Å)
<(DHꢀ ꢀ ꢀA) (°)
1
O2–H2ꢀ ꢀ ꢀO5i
0.76(8)
1.85(8)
2.01(2)
1.960(17)
2.19(3)
2.17(3)
2.584(4)
2.825(5)
2.800(4)
2.988(5)
2.985(6)
164(9)
161(5)
172(4)
152(5)
154(5)
O1 W–H1 Wꢀ ꢀ ꢀO2 W
O1 W–H2 Wꢀ ꢀ ꢀO5ii
O2 W–H1W2ꢀ ꢀ ꢀO1Wiii
O2 W–H2W2ꢀ ꢀ ꢀO4iv
0.844(14)
0.846(14)
0.871(14)
0.878(14)
2
N1–H1Aꢀ ꢀ ꢀO9
0.821(15)
0.829(16)
0.837(16)
0.836(16)
0.844(15)
0.844(15)
0.844(15)
2.061(17)
1.946(18)
1.814(17)
1.698(17)
2.62(3)
2.874(4)
2.763(4)
2.645(3)
2.531(4)
3.273(4)
2.799(4)
3.001(4)
171(4)
168(4)
172(4)
175(5)
135(4)
170(4)
172(4)
N2–H2ꢀ ꢀ ꢀO9
O1–H1ꢀ ꢀ ꢀO3v
O4–H4ꢀ ꢀ ꢀO1 W
O1 W–H1W1ꢀ ꢀ ꢀO8vi
O1 W–H1W1ꢀ ꢀ ꢀO10vi
O1 W–H2W1ꢀ ꢀ ꢀO2vii
1.964(17)
2.163(17)
Symmetry codes: i = 3/2 ꢁ x, ꢁ½ + y, ꢁ3/2 + z; ii = x, y , ꢁ1 + z; iii = 2 ꢁ x, 2 ꢁ y, ½ + z; iv = 3/2 ꢁ x, ½ + y, ꢁ½ + z; v = 1 ꢁ x, 1 ꢁ y, 1 ꢁ z; vi = 2 ꢁ x, ꢁ½ + y, 3/2 ꢁ z; vii = 2 ꢁ x,
1 ꢁ y, 1 ꢁ z.

N.R. Kelly et al. / Inorganica Chimica Acta 403 (2013) 102–109
107
3.4. Structure of 4,40,6-tricarboxy-2,20-bipyridinium nitrate, 2
The atomic numbering scheme and atom connectivity for 2 are
shown in Fig. 6. The structure was refined in the monoclinic P21/c
space group and crystallographic data are given in Table 1. It is
immediately apparent from the structure that the tetra-carboxylic
acid precursor has mono-decarboxylated in situ at the 60-position
under these conditions to form 4,40,6-tri-carboxy-2,20-bipyridini-
um nitrate, where each bipyridine nitrogen atom is protonated.
The nitrate anion is present for the sake of charge balance as the
4- and 40-carboxylate groups remain in their acidic form whereas
the 6-carboxylic acid is deprotonated; i.e., 2 is zwitterionic. The
molecule adopts the cisoid-orientation with respect to the bipyrid-
inium core with a slight twist about the C5–C6 bond of ꢃ4°. The 40
and 4-carboxylic acid groups and the 6-carboxylate group are
twisted by ca. 18°, 4° and 10°, respectively, from the plane of the
bipyridinium ring to which they are attached, so the molecule is
essentially planar. The cisoid orientation is in contrast to previously
reported [2,20-bipyH2]2+ species which, due to a combination of
steric and electrostatic effects, are found in the transoid orientation
[53]. The orientation found here probably results from the stabilis-
ing influence of the numerous hydrogen-bond interactions involv-
ing each of the constituents, Figs. 6–8. Indeed, the nitrate anion
participates in three hydrogen-bond interactions: a bifurcated
hydrogen-bond is present between the nitrate oxygen atom O9
and the protonated bipyridinium nitrogen atoms N1 and N2
(O9ꢀ ꢀ ꢀN1 2.874(4) Å and O9ꢀ ꢀ ꢀN2 2.763(4) Å); and a third interac-
tion involves the lattice water molecule, O1w, and the nitrate oxy-
gen atom O10 (O1wꢀ ꢀ ꢀO10 2.800(4) Å). Furthermore, a hydrogen-
bond between O1w and the carboxylate oxygen atom O4
(O4ꢀ ꢀ ꢀO1W 2.531(4) Å), and the catemeric hydrogen-bond be-
tween the carboxylic acid oxygen atoms O3 and O1, from an adja-
cent molecule, (O3ꢀ ꢀ ꢀO1 2.645(4) Å), in combination with those
above leads to the formation of a 2D sheet, Fig. 7. When all hydro-
gen-bonds are considered it is clear that a 3D hydrogen-bonded
network results. Fig. 8 focuses on one section of this 3D network
in order to highlight the hydrogen-bond that links the 2D sheets
together. The sheets link to one another via a hydrogen-bond be-
tween water molecule, O1W, and the carboxylic acid oxygen atom,
O2, (O1WꢀꢀꢀO2 3.001(4) Å) in the sheet above it. Details of all the
hydrogen-bond interactions in 2 are brought together within
Table 2.
Fig. 5. Thermogravimetric analysis curve for 1.
Fig. 6. Molecular structure and atomic numbering scheme for 2. Hydrogen-bonding
shown as dashed lines. Only those hydrogen atoms involved in hydrogen-bonding
are shown for clarity.
of the lattice water molecule (calc. 4.6%), followed by a further
weight loss of ca. 4.7% between 210 < T < 250 °C, consistent with
loss of the metal-bound water molecule. The anhydrous phase is
stable from 250 < T < 330 °C where after complex decomposition
occurs.
4. Conclusions
We have demonstrated that the reaction of 4,40,6,60-tetra-
methyl-2,20-bipyridine, L, or 4,40,6,60-tetracarboxy-2,20-bipyridine,
Fig. 7. The 2D sheet structure of 2. For clarity only hydrogen atoms involved in hydrogen-bonding (dashed line) are shown.

108
N.R. Kelly et al. / Inorganica Chimica Acta 403 (2013) 102–109
Fig. 8. (a) Close-up view showing how the hydrogen-bonded 2D sheets link into 3D through additional hydrogen-bonding (dashed lines) involving the lattice water molecule.
(b) Side-on view of the 3D network in 2 showing the wave-like appearance of the 2D sheets that link via hydrogen-bonding (dashed lines) involving the lattice water
molecule. Nitrate anion omitted for clarity.
H4L, with Cu(OAc)2ꢀH2O under acidic hydrothermal conditions
leads to in situ oxidation and mono-decarboxylation in the forma-
tion of the novel ligand 4,40,6-tricarboxy-2,20-bipyridine, H3L0. The
H3L0 ligand on coordination to Cu(II) forms a 1D polymer that, by
virtue of the presence of carboxylic acid, carboxylate and water
functionalities, forms an ornately connected 3D hydrogen-bonded
network. Heating L or H4L under acidic hydrothermal conditions
also gives H3L0 as crystalline 4,40,6-tricarboxy-2,20-bipyridinium ni-
trate hydrate {[H4L0][NO3]ꢀH2O}, 2. More forcing conditions led to
double or total decarboxylation of the ligand and the formation
of 4,40-dicarboxy-2,20-bipyridine or simply 2,20-bipyridine. The
structure of 2 features an array of hydrogen-bonding interactions
that generate a 3D network.
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.ica.2012.12.033.
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