4,4′-Bipyridazine: A new twist for the synthesis of coordination polymers

Article abstract of DOI:10.1039/b703911e

A new polydentate ligand 4,4′-bipyridazine (4,4′-bpdz) was prepared by employing inverse electron demand cycloaddition of 1,2,4,5-tetrazine. A unique combination of structural simplicity, ampolydentate character and efficient donor properties towards Cu(i), Cu(ii) and Zn(ii) provide wide new possibilities for the synthesis of coordination polymers incorporating the 4,4′-bpdz module either as a bi-, tri- or tetradentate connector between the metal ions. 1D coordination polymers Cu ₂(4,4′-bpdz)(CH₃CO₂)₄· 4H₂O and Zn(4,4′-bpdz)(NO₃)₂, and interpenetrated (4,4)-nets in [Cu(4,4′-bpdz)₂(H ₂O)₂]S₂O₆ were closely related to 4,4′-bipyridine compounds. 1D "ladder-like" polymer Cu ₂(4,4′-bpdz)₃(CF₃CO₂) ₄ and the unprecedented 3D binodal net ({8⁶6 ³;8³) in [Cu₃(4,4′-bpdz) ₆(H₂O)₄](BF₄)₆· 6H₂O were based upon a combination of linear and angular organic bridges. Complex [Cu₃(OH)₂(4,4′-bpdz) ₃(H₂O)₂CF₃CO_{2 2}](CF₃CO₂)₂·2H₂O has a "NbO-like" 3D topology incorporating discrete dihydroxotricopper(ii) clusters linked by tri- and tetradentate ligands. The tetradentate function of the 4,4′-bpdz ligand was especially relevant for copper(i) complexes, which adopt layered Cu₂X₂(4,4′- bpdz) (X = Cl, Br) or 3D chiral framework (X = I) structures based upon infinite (CuX)_n chains. The electron deficient character of the ligand was manifested by short anion-π interactions (O-π 3.02-3.20; Cl-π 3.35 A), which may be involved as a factor for controlling the supramolecular structure. The Royal Society of Chemistry.

Full text of DOI:10.1039/b703911e

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4,4ꢀ-Bipyridazine: a new twist for the synthesis of coordination polymers†‡
Konstantin V. Domasevitch,*a Il’ya A. Gural’skiy,a Pavlo V. Solntsev,a Eduard B. Rusanov,b
Harald Krautscheid,c Judith A. K. Howardd and Alexander N. Chernegab
Received 15th March 2007, Accepted 20th April 2007
First published as an Advance Article on the web 9th May 2007
DOI: 10.1039/b703911e
A new polydentate ligand 4,4ꢀ-bipyridazine (4,4ꢀ-bpdz) was prepared by employing inverse electron
demand cycloaddition of 1,2,4,5-tetrazine. A unique combination of structural simplicity,
ampolydentate character and efficient donor properties towards Cu(I), Cu(II) and Zn(II) provide wide
new possibilities for the synthesis of coordination polymers incorporating the 4,4ꢀ-bpdz module either
as a bi-, tri- or tetradentate connector between the metal ions. 1D coordination polymers
Cu2(4,4ꢀ-bpdz)(CH3CO2)4·4H2O and Zn(4,4ꢀ-bpdz)(NO3)2, and interpenetrated (4,4)-nets in
[Cu(4,4ꢀ-bpdz)2(H2O)2]S2O6 were closely related to 4,4ꢀ-bipyridine compounds. 1D “ladder-like”
polymer Cu2(4,4ꢀ-bpdz)3(CF3CO2)4 and the unprecedented 3D binodal net ({86}{63;83}) in
[Cu3(4,4ꢀ-bpdz)6(H2O)4](BF4)6·6H2O were based upon a combination of linear and angular organic
bridges. Complex [Cu3(OH)2(4,4ꢀ-bpdz)3(H2O)2{CF3CO2}2](CF3CO2)2·2H2O has a “NbO-like” 3D
topology incorporating discrete dihydroxotricopper(II) clusters linked by tri- and tetradentate ligands.
The tetradentate function of the 4,4ꢀ-bpdz ligand was especially relevant for copper(I) complexes, which
adopt layered Cu2X2(4,4ꢀ-bpdz) (X = Cl, Br) or 3D chiral framework (X = I) structures based upon
infinite (CuX)n chains. The electron deficient character of the ligand was manifested by short anion–p
˚
interactions (O–p 3.02–3.20; Cl–p 3.35 A), which may be involved as a factor for controlling the
supramolecular structure.
prototypes for supramolecular synthesis of coordination arrays
involving polyfunctional pyridazine ligands.
Introduction
The design of novel bridging ligands attracts significant attention
in view of their potential applications in framework coordination
polymers.1,2 Inherent functional and conformational properties of
the ligands are commonly relevant for the connectivity within the
entire structure and they could assist in the development of metal–
organic frameworks in a rational fashion,3,4 including synthesis
of polynuclear systems, hybrid organic/inorganic materials and
highly-connected frameworks incorporating metal cluster motifs.
In this way, polydentate nitrogen donor ligands offer very efficient
approaches for the design and fine-tuning of the structure and
properties of coordination polymers, as revealed by examining
metal complexes with different bifunctional azole ligands: 1,2,4-
triazoles,5 4,4ꢀ-bipyrazole,6 4-pyridylpyrazole7 and 4,4ꢀ-bitriazole.8
Pyridazines, the most typical connectors for bridging between
closely situated metal ions, may be expected to be especially rich
and potentially versatile for the synthesis of coordination poly-
mers. A variety of characteristic metal–pyridazine chains, helices9
and polynuclear motifs10 may be viewed as attractive structural
In this context, a paradigmatic prototype for the tectons bearing
multiple pyridazine donor-N sites is provided by 4,4ꢀ-bipyridazine
(Scheme 1), a structurally simple molecule combining the ability
for interconnection of either closely separated or distal metal
ions, efficient bridging of many metal centers and the ability
to sustain different binding directions depending on the molec-
ular conformations. From the crystal design perspective, 4,4ꢀ-
bipyridazine may unite the potential of the parent monofunctional
heterocycle, linear rod-like connectors of the 4,4ꢀ-bipyridine type,
and many of the angular bridging ligands. Herein, we report very
simple synthesis of this hitherto unknown ligand, coordination
preferences of 4,4ꢀ-bipyridazine and its utility for generation of
extended coordination polymers.
Scheme 1 4,4ꢀ-Bipyridazine ligand explored in the present study.
aInorganic Chemistry Department, Kiev University, Volodimirska Str.64,
Kiev, 01033, Ukraine. E-mail: dk@univ.kiev.ua
bInstitute of Organic Chemistry, Murmanskaya Str.4, Kiev, 253660, Ukraine
Results and discussion
cInstitut fu¨r Anorganische Chemie, Universita¨t Leipzig, Linne´straße 3, D-
04103, Leipzig, Deutschland
In combination with transition metal ions 4,4ꢀ-bipyridazine acts
as an efficient polydentate N-donor readily producing a variety
of polymeric coordination architectures. The ligand revealed
quite diverse and versatile structural functions, originating in
the availability of two pairs of non-equivalent donor-N atoms
dDept. of Chemistry, University of Durham, Durham, UK DH1 3LE
† The HTML version of this article has been enhanced with colour images.
‡ Electronic supplementary information (ESI) available: Detailed pro-
cedure for preparation of unsubstituted 1,2,4,5-tetrazine. See DOI:
10.1039/b703911e
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and therefore 4,4ꢀ-bipyridazine can support the integrity of the
coordination patterns either as a bi-, tri- or tetra-connected
module (Scheme 2).
◦
˚
Table 1 Selected bond distances (A) and angles ( ) for complexes 1–3
Cu2(4,4ꢀ-bpdz)(CH3CO2)4·4H2O 1 (a: −x, 1−y, 1−z)
Cu1–O1
Cu1–O3
Cu1–O2a
Cu1–O4a
Cu1–N1
Cu1–Cu1a
1.986(2)
1.963(1)
1.978(2)
1.975(1)
2.179(2)
2.6294(6)
O3–Cu1–O1
O3–Cu1–N1
O1–Cu1–N1
O2a–Cu1–O1
N1–Cu1–Cu1a
89.19(7)
98.91(6)
96.52(7)
168.40(6)
173.45(5)
Zn(4,4ꢀ-bpdz)(NO3)2 2 (a: 0.5−x, 0.5+y, 1.5−z)
Zn1–N1
Zn1–N3a
Zn1–O1
Zn1–O2
Zn1–O4
Zn1–O5
2.042(2)
2.044(2)
2.206(2)
2.102(2)
2.175(2)
2.329(2)
N1–Zn1–N3a
N1–Zn1–O2
N1–Zn1–O1
O4–Zn1–O1
O4–Zn1–O5
O2–Zn1–O1
103.51(6)
97.44(6)
155.52(6)
84.97(8)
55.52(6)
58.95(6)
[Cu(4,4ꢀ-bpdz)2(H2O)2]S2O6 3 (a: 0.5−x, −0.5 + y,
−0.5 + z; b: −0.5−x, 0.5 + y, 0.5 + z).
Cu1–N1
Cu1–N5
Cu1–N3b
Cu1–N7a
Cu1–O1
Cu1–O2
2.067(3)
2.049(3)
2.047(3)
2.041(3)
2.272(3)
2.417(3)
N5–Cu1–N1
N7a–Cu1–N3b
O1–Cu1–O2
N5–Cu1–O1
N1–Cu1–O1
N1–Cu1–O2
175.1(1)
179.3(1)
177.8(1)
91.3(1)
93.6(1)
86.5(1)
Scheme 2 Ampolydentate properties of 4,4ꢀ-bipyridazine in metal com-
plexes: (a) “4,4ꢀ-bipyridine like” coordination in structures 1–5; (b)
“3,3ꢀ-bipyridine like” coordination in 4, 5; (c) tridentate mode in 6; (d)
tetradentate mode in 6–9.
In complexes with Cu2+ and Zn2+ ions the ligand was typically
bidentate and thus it utilized only a half of the available coordina-
tion functionality. This is consistent with previous examinations of
pyridazine itself that is coordinated monodentately in a number
of Cu2+ complexes, many of which are closely related to their
pyridine analogs. Similarly, in some cases 4,4ꢀ-bipyridazine plays
the role of a simple rod-like bitopic linker between pairs of metal
ꢀ
˚
ions separated by ca. 11 A, as typically occurs for 4,4 -bipyridine
bridges.
Complexes related to 4,4ꢀ-bipyridine analogs
The “bipyridine-like function” of the ligand was best illustrated
by 1D polymeric structures of Cu2(4,4ꢀ-bpdz)(CH3CO2)4·4H2O
1 and Zn(4,4ꢀ-bpdz)(NO3)2 2. Structure 1 is based upon typ-
ical centrosymmetric copper(II)/acetate dimers linked into lin-
ear chains employing the axial coordination positions at two
metal ions (Fig. 1, Table 1). Such a pattern has an exact
precedent in the chemistry of 4,4ꢀ-bipyridine in the structure of
Cu2(4,4ꢀ-bipy)(CH3CO2)4·DMF.11 All Cu–O distances are uniform
Fig. 1 Structure of 1 involving linear coordination chains bridged by
tetraaqua ensembles.
˚
(1.963(1)–1.986(2) A), while Cu–N separations were appreciably
longer and actually the same as for the bipyridine analog (2.179(2)
a twisted configuration (dihedral angle between two pyridazine
cycles 44.54(7)◦), while in the copper complex 1 the molecule was
planar and centrosymmetric. Such a 1D chain shape bears a close
resemblance with a polymer Zn(4,4ꢀ-bipy)(NCS)2,13 however, the
structure of the nitrate analog is completely different since the
metal ions coordinate the additional water molecules: Zn(4,4ꢀ-
bipy)(H2O)2(NO3)2.14
It is likely that the organization of the structure 2 was greatly
influenced by a set of specific interactions, which occur between
the electronegative oxygen atoms and aromatic p-clouds (Fig. 2,
b). Such anion–p stacking itself has very few recent literature
precedents, all of which involve interactions with the most
electron deficient heteroaromatic systems: 1,2,4,5-tetrazines,15
˚
and 2.187(3) A, respectively). The chains aggregate with extensive
hydrogen bonding involving very rare discrete water tetramers (O–
12
˚
O 2.79, 2.82 A), and the interaction between the coordination
portion and the water assemblies occurs via bifurcated weak hy-
drogen bonds, which engage non-coordinated pyridazine nitrogen
atom as one of the hydrogen bond acceptors (O5–O3 3.056; O5–
˚
N2 3.278 A, Fig. 1).
In the zinc complex Zn(4,4ꢀ-bpdz)(NO3)2 2 the coordination
mode of the ligand is identical: the metal ions were connected
into zig-zag chains (N–Zn–N 103.51(6)◦) and the coordination
environment was completed with two bidentate nitrate groups
˚
(Zn–O 2.102(2)–2.329(2) A) (Fig. 2, Table 1). The ligand adopts
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Fig. 2 Two views of 1D zig-zag polymer 2: (a) coordination chain formed
by linear 4,4ꢀ-bipyridazine bridges, (b) the interchain interactions that
occur by means of close anion–p stacking (O1 · · · p 3.017(2); O4 · · · p
˚
2.948(2) A).
1,3,5-triazines16 and condensed pyridazino[4,5-d]pyridazine.17 The
nitrate oxygen atoms exhibit appreciably short contacts with
the heterocyclic ring and appear nearly above the centroids of
pyridazine cycles from the adjacent chains (O–centroid distances
˚
2.95 and 3.02 A; Table 2). Such an interaction of non-coordinated
−
NO3 anions (with the 1,3,5-triazine cycle) was even certainly
weaker (O–p distances 3.20 A). This is the first observation of a
Fig.
3
Structure of the square-grids coordination layer
16
[Cu(H2O2)(4,4ꢀ-bpdz)2]n in 3, (a) with non-coordination dithionate
anions situated inside the cages, (b) Diagonal–diagonal interpenetration
of the coordination nets generating the 3D supramolecular structure.
˚
pronounced anion–p binding with simple diazines, and it allows
the postulation of special properties of the 4,4ꢀ-bipyridazine ligand
compared with a 4,4ꢀ-bipyridine prototype.
Structure of the metal–organic framework in [Cu(4,4ꢀ-
bpdz)2(H2O)2]S2O6 3 was also closely related to the very char-
acteristic coordination archetypes provided by bitopic rod-like
connectors. Metal ions coordinate four pyridazine nitrogen atoms
u 78.9◦), and are shorter than O–p contacts in the 4,4ꢀ-bipyridine
21
˚
analog (3.23 A).
Structures with different coordination modes of the ligand
4,4ꢀ-Bipyridazine ligand offers even wider prospects for the
synthesis of coordination frameworks as may be compared with
the prototypal 4,4ꢀ-ꢀbipyridine. When coordinated via the pairs
of N2, N2ꢀ or N1N2 nitrogen atoms, the organic module could
be functionally equivalent to 3,3ꢀ- or 3,4ꢀ-bipyridines and the
different bridging modes may coexist within a single coordination
topology. In this respect the ligand can act as a “mixture of
isomeric bipyridines” and such a combination could support
unusual metal–organic connectivities.
˚
(Cu–N 2.041(3)–2.067(3) A) in the equatorial plane and two distal
˚
water molecules in the apical positions (Cu–O 2.272(3), 2.417(3) A)
and the coordination portion of the structure exists in the form
of 2D (4,4)–net (Fig. 3). The latter provides rectangular meshes
˚
with inner cavities of ca. 6 × 6 A size and interpenetrate with
the identical (4,4)-nets situated orthogonally.18 Similar ensembles
of (4,4)-nets interpenetrated in a diagonal–diagonal fashion19 was
observed for related [Cu(4,4ꢀ-bipy)2(H2O)2]X (X = SiF6, GeF6,20
S2O621) complexes. The dithionate counter anions are situated in
the channels provided by interpenetrated networks and held by a
set of OH–O hydrogen bonds and anion–p stackings, which are
Thus, complex Cu2(4,4ꢀ-bpdz)3(CF3CO2)4 4 (Fig. 4, Table 3)
adopts a ladder-like motif based upon T-shape CuN3 coordi-
nation.3 Within the ladder, two linear {Cu(4,4ꢀ-bpdz)(CF3CO2)2}n
˚
comparable with the ones observed in structure 2 (O–p 3.018(4) A,
Table 2 Geometry of the anion–p interactions in structure 2
u/◦
b
Xa
X–C(N) range/A
X · · · centroid distance/A
X · · · plane distance/A
˚
˚
˚
Group
N1N2C1C2C3C4
N3N4C5C6C7C8
O4a
O1b
3.00–3.45
2.99–3.56
2.948(2)
3.017(2)
2.894(2)
2.926(2)
79.0
75.9
a Symmetry codes: a: x−1, y, z; b: 1.5−x, −0.5+y, 1.5−z. b Angle of the X–p axis to the plane of the aromatic cycle.
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◦
˚
Table 3 Selected bond distances (A) and angles ( ) for complexes 4–6
Cu2(4,4ꢀ-bpdz)3(CF3CO2)4 4 (a: x, −1+y, z)
Cu1–O1
Cu1–O3
Cu1–N1
Cu1–N3
Cu1–N5a
1.937(2)
1.989(2)
2.323(2)
2.026(2)
2.010(2)
O1–Cu1–O3
O1–Cu1–N3
O3–Cu1–N3
N3–Cu1–N5a
N3–Cu1–N1
177.43(7)
91.80(8)
88.81(8)
171.62(8)
94.16(8)
[Cu3(H2O)4(4,4ꢀ-bpdz)6(H2O)4](BF4)6·6H2O 5 (a: 2−x, −y,
1−z)
Cu1–N1
Cu1–N3
Cu2–N5
Cu2–N7
Cu2–N9
Cu2–N11
Cu2–O2
2.000(3) × 2
2.025(3) × 2
2.019(3)
2.028(4)
2.051(4)
2.033(3)
2.347(4)
N1–Cu1–N3
N1–Cu1–N3a
N5–Cu2–N7
N5–Cu2–N11
N5–Cu2–N9
N5–Cu2–O2
N7–Cu2–O2
88.1(1)
91.9(1)
92.4(1)
172.4(1)
89.1(1)
93.4(1)
90.8(2)
[Cu3(OH)2(4,4ꢀ-bpdz)3(H2O)2{CF3CO2}2](CF3CO2)2·2H2O
6 (a: 0.5−x, 0.5−y, 1−z; b: −0.5+x, −0.5+y, z)
Cu1–O1
Cu1–N1
Cu1–N3
Cu2–O1
Cu2–O2
Cu2–N2a
Cu2–N4
Cu2–N5b
1.887(2) × 2
2.340(3) × 2
2.146(2) × 2
1.883(2)
1.957(2)
2.459(2)
O1–Cu1–N3
O1–Cu1–N1
N3–Cu1–N1
O1–Cu2–O2
N4–Cu2–N5b
N4–Cu2–N2a
O1–Cu2–N4
Cu2–O1–Cu1
84.91(7)
93.10(8)
91.30(8)
171.5(1)
176.37(9)
88.25(9)
87.54(8)
118.23(9)
2.069(2)
2.008(2)
Fig. 5 (a) Trinuclear coordination unit in structure 5 supported by two
weak aqua bridges, (b) Situation of BF4− anion inside the crystal cage and
its interaction with the ligand frames by CH–F hydrogen bonding and
˚
anion–p stackings (F9 · · · p 3.05, 3.14; F11 · · · p 3.27 A).
bipyridazine connectivity is an unprecedented 3D binodal four-
connected net based upon two kinds of the vertices, which differ
in the coordination sequences: 4, 12, 36, 78. . . for Cu1; and 4,
12, 33, 73. . . for Cu2 (total Schla¨fli symbol {86}{63;83}) (Fig. 6).
Each “copper trimer” coordinates twelve bipyridazine molecules
and is linked to eight equivalent closest neighbors (half of the links
represented by pairs of the organic ligands), and when considering
the trimeric units as augmented net nodes, the overall 3D topology
may be regarded as an eight-connected primitive hexagonal net
(Schla¨fli symbol {36;418;53;6}) (Fig. 6). Non-coordinated anions
are involved in hydrogen bonding with the water molecules
and also in extensive CH–F and anion–p bonding with the
coordination framework. It allows the postulation of a certain
templating effect of tetrafluoroborate anions for stabilization of
small crystal cavities as depicted in Fig. 5(b). Surprisingly, atom
F9 interacts with two pyridazine cycles simultaneously, with short
Fig. 4 1D “ladder-like” polymer 4 involving square-pyramidal coordina-
tion at the copper ions and two types of the bipyridazine bridges.
ꢀ
˚
chains (Cu–Cu 10.98 A) were supported by rod-like 4,4 -bpdz
connectors, while cross-linking of the chains achieved by an
additional set of the ligands, which were coordinated in a “3,3ꢀ-
˚
bipy” fashion (Cu–Cu 9.97 A). This feature is essential for a
minimization of inner volume of the resulting rectangular meshes.
The structure of [Cu3(4,4ꢀ-bpdz)6(H2O)4](BF4)6·6H2O 5 is much
more complicated and the coexistence of the above linear and
angular bipyridazine bridges (in a 1 : 1 proportion) clearly
contributes to the very unusual framework topology. Two unique
copper(II) ions adopt a similar 4 + 2 octahedral coordination, with
four nitrogen atoms situated in the equatorial plane (Cu–N 2.00–
˚
F–p (centroid) separations (3.05 and 3.14 A).
Structures involving 1,2-pyridazine bridges
˚
2.05 A, Table 3) and two distal aqua ligands in the apical positions
(Fig. 5). One of the water molecules is bridging between the two
Partial utilization of the ligand donor-N functionality in com-
plexes 1–5 is a clear consequence of the inherent electron deficient
character of the pyridazine cycle, the factor mitigating against the
˚
copper ions, with very lengthy Cu–O distances of 2.48 and 2.58 A,
and this unites the coordination centers into “trimers”. Copper–
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Fig. 7 (a) Finite dihydroxotricopper(II) ensembles sustaining vertices
of the framework in 6. Note that the trinuclear unit accommodates
four 1,2-diazine bridges and two monodentate pyridazine cycles, (b)
Interconnection of the tricopper clusters by combination of three- and
tetradentate 4,4ꢀ-bpdz ligands leading to a 3D “NbO-like” four-connected
topology.
Fig. 6 (a) Fragment of the structure of 5, showing the scheme for
connection of the tricopper units to the six closest neighbors. Two
additional links occur in the axial directions leading to the generation
of a primitive hexagonal lattice (hex); (b) Overall 3D metal–bipyridazine
linkage in the form of a four-connected binodal {86}{63;83} net.
coordination of the ligand to many metal ions. However, higher
donor capacity of the 4,4ꢀ-bipyridazine module may be anticipated
for the systems involving small inorganic bridges (such as l-
chloro, l-hydroxo) that could assist in interconnection of closely
separated metal ions and formation of 1,2-diazine bridges.10, 17 In
this case 4,4ꢀ-bipyridazine may provide either tri- or tetradentate
coordination modes, which offer a special potential from the
design perspective.
Thus a partial hydrolysis in aqueous solution (see Experimental)
led to crystallization of the hydroxopolymer [Cu3(OH)2(4,4ꢀ-
bpdz)3(H2O)2{CF3CO2}2](CF3CO2)2·2H2O 6. The structure was
based upon finite dihydroxotricopper(II) units allowing accommo-
dation of four 1,2-diazine bridges and two additional pyridazine
groups were coordinated monodentately (Fig. 7, Table 3). Accord-
ingly, the polymeric structure was generated by a combination
of tri- and tetradentate 4,4ꢀ-bipyridazine molecules (in a 2 : 1
proportion), which connect each of the tricopper units to four
closest neighbors. In this way, pairs of antiparallel tridentate
ligands assemble the clusters into the chains and the tetradentate
bridges extend this motif in the orthogonal direction (Fig. 7). The
entire framework adopts a four-connected 3D topology, which
is commonly related to the NbO structure and incorporates the
tricopper units as an origin of the net connectivity.
This coordination motif has a precedent in the chemistry of
parent pyridazine10 and formation of the complicated framework
incorporating trinuclear nodes in 6 is itself very illustrative for
the general principles of crystal design. In the hydroxocluster, the
central copper ion has a distorted octahedral environment with
˚
appreciably short Cu–OH bonds (1.887(2) A) and two kinds of
˚
Cu–N bonds (2.146(2) and 2.340(3) A). Metal–hydroxo bonds
were also found to be the shortest separations within the typically
distorted 4 + 2 coordination octahedron of two outer copper ions,
while the axial positions were occupied by the distal aqua ligand
˚
˚
(2.669 A) and pyridazine-N donor (2.459 A).
The tetradentate function of the ligand may be even more
characteristic and predictable for the extremely soft acids (such as
Cu+ or Ag+) favoring efficient back-bonding and coordination to
unsaturated N atoms. In this context we have examined copper(I)
chloride, bromide and iodide, as a system allowing the rational
design of coordination topology by hybrid organic/inorganic
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◦
˚
interconnection of the metal ions with l-halide and 1,2-diazine
bridges.
Table 4 Selected bond distances (A) and angles ( ) for complexes 7–9
Cu2Cl2(4,4ꢀ-bpdz) 7 (a: x, 1.5-y, −0.5+z; b: x, 1.5-y, 0.5+z)
Isostructural complexes Cu2X2(4,4ꢀ-bpdz) (X = Cl 7, Br 8)
adopt 2D polymeric arrays integrating infinite (CuX)n chains
(Fig. 8, Table 4). Along the chain, each pair of metal ions
are bridged by pyridazine and thus sets of tetradentate 4,4ꢀ-
bipyridazine ligands provide pairwise interconnection of the
inorganic chains into the layers. This pattern has no direct
analogy in the chemistry of the parent heterocycle itself. All
known copper(I) chloride and bromide complexes with pyridazine
involve Cu2X2 rhombes as basic elements of the structure,22 while
a very comparable chain-like motif was characteristic for bulky di-
azines such as 2,3-diazabicyclo(2.2.1)hept-2-ene23 and 1,2,3,6,7,8-
hexahydro-cinnolino[5,4,3-cde]cinnoline.24 However, orientation
of the pyridazine bridges along the coordination chains in 7 and 8
is very specific and it facilitates steric accessibility of the chlorine
(bromine) atoms, which are in close contact with the aromatic–
Cu1–N1
Cu1–N2a
Cu1–Cl1
Cu1–Cl1b
2.003(3)
2.087(3)
2.282(1)
2.375(1)
N1–Cu1–N2a
N1–Cu1–Cl1
N1–Cu1–Cl1b
Cl1–Cu1–Cl1b
Cu1–Cl1–Cu1a
111.2(1)
124.23(9)
101.83(9)
110.98(6)
92.80(4)
Cu2Br2(4,4ꢀ-bpdz) 8 (a: x, 1.5-y, −0.5+z; b: x, 1.5-y, 0.5+z)
Cu1–N1
2.033(5)
2.076(5)
2.425(1)
2.470(1)
N1–Cu1–N2a
N1–Cu1–Br1
N1–Cu1–Br1b
Br1–Cu1–Br1b
Cu1–Br1–Cu1a
114.7(2)
116.7(1)
102.8(1)
113.23(5)
88.94(4)
Cu1–N2a
Cu1–Br1
Cu1–Br1b
Cu2I2(4,4ꢀ-bpdz) 9 (a: 0.5+y, 0.5-x, 0.25+z; b: 0.5-y,
−0.5+x, −0.25+z)
˚
Cu1–N1
Cu1–N2a
Cu1–I1
2.059(2)
2.097(2)
2.6047(4)
2.6185(4)
N1–Cu1–N2a
N1–Cu1–I1
N2–Cu1–I1a
I1–Cu1–I1b
Cu1–I1–Cu1a
97.79(8)
112.82(5)
104.11(5)
118.26(1)
78.69(1)
p frames from the adjacent layer (Cl–p 3.347; Br–p 3.473 A)
(Fig. 8(b)). These parameters only slightly exceed separations for
the recently documented chloride–p stacking with the electron
Cu1–I1b
25
˚
deficient 1,3,5-triazine cycle (3.20 A). Such a type of interaction
could be regarded as very characteristic for the 4,4ꢀ-bipyridazine
ligand and it contributes to the controlling of the framework
structure.
In this context, it is should not be considered unusual that
the iodide analog Cu2I2(4,4ꢀ-bpdz) 9 adopts a different, very
remarkable 3D coordination pattern. There are no close iodine–p
interactions. The inorganic portion of the structure exists as a four-
˚
fold helix (CuI)n with a pitch at 11.98 A (parameter c of the unit
cell) and the tetradentate organic bridges interconnect the helices
into a 3D tetragonal framework (Fig. 9). Despite the significant
length of the bipyridazine connector, the structure is not porous: a
conformational flexibility of the ligand (dihedral angle between the
cycles 34.7◦) provides possibility for the “collapse” of the channels
along the c direction and facilitates dense packing (Fig. 10).
For the complexes 7–9, the higher electronegativity of the
halogen atoms and the higher polarization of the CuI–X bonds
favors effective r-donor Cu–N coordination, which is reflected by
a certain elongation of the Cu–N bonds in the bromide and the
iodide versus the chloride compound (average Cu–N 2.055, 2.079
˚
and 2.045 A respectively, Table 4).
Experimental
All chemicals were of analytical grade and were used as received
without further purification. The copper(II) salt CuS2O6·6H2O
was prepared by ion exchange starting with barium dithion-
ate. Hitherto unknown 4,4ꢀ-bipyridazine was easily accessi-
ble following the methodology of the “inverse electron de-
mand Diels–Alder cycloadditons” of 1,2,4,5-tetrazine26 and us-
ing inexpensive and available on a kilogram scale cis,trans-1,4-
bis(dimethylamino)butadiene27 as an extremely reactive “double
dienophile” (scheme 3).
4,4ꢀ-Bipyridazine
Solution of 12.45 g (0.15 mol) 1,2,4,5-tetrazine in 150 mL of
dry 1,4-dioxane was added dropwise for a period of 40 min
to a stirred solution of 10.11 g (0.072 mol) of cis,trans-1,4-
bis(dimethylamino)butadiene in 60 mL of dry 1,4-dioxane. The
immediate evolution of nitrogen gas and discoloration were
Fig. 8 (a) 2D layered structure of isomorphous Cu2X2(4,4ꢀ-bpdz) com-
plexes (X = Cl, Br) generated by tetradentate organic bridges and infinite
(CuX)n chains, (b) complementary anion–p interactions between the
successive layers.
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Scheme 3 One-pot synthesis of 4,4ꢀ-bipyridazine involving cycloaddition
and two-step elimination of nitrogen and dimethylamine.
(%): C, 60.75; H, 3.82; N, 35.43. Found (%): C, 60.44; H, 3.89; N,
35.17.
Coordination compounds
Preparation of Zn(4,4ꢀ-bpdz)(NO3)2 2. A solution of 0.030 g
(0.1 mmol) of Zn(NO3)2·6H2O in 2 mL methanol was layered over
a solution of 0.016 g (0.1 mmol) of 4,4ꢀ-bipyridazine in a mixture
of 2 mL methanol and 1 mL chloroform. Large colorless prisms
of the complex (0.024 g, 70%) grew on the walls of the tube as the
solutions slowly interdiffused in a 15 d period.
Anal. for 2, C16H26Cu2N4O12. Calc. (%): C, 32.38; H, 4.42; N,
9.44. Found (%): C, 32.12; H, 4.38; N, 9.16.
Fig. 9 Coordination motif in the form of 1D pyridazino/cuproidide helix,
which supports the structure of the 3D polymer 9 (a), interconnection of
the neighboring (CuI)n helices with the tetradentate organic bridges (b).
Coordination compounds Cu2(4,4ꢀ-bpdz)(CH3CO2)4·4H2O 1,
[Cu(4,4ꢀ-bpdz)2(H2O)2]S2O6 3, Cu2(4,4ꢀ-bpdz)3(CF3CO2)4 4, and
[Cu3(4,4ꢀ-bpdz)6(H2O)4](BF4)6·6H2O 5. These were prepared in
50–85% yields by slow evaporation of aqueous solutions of the
components at r.t., while prolonged heating of the solutions may
result in hydrolysis and crystallization of hydroxo compounds, as
is illustrated in the following examples.
Anal. for 1, C8H6N6O6Zn (large green-blue prisms). Calc. (%):
C, 27.64; H, 1.74; N, 24.19. Found (%): C, 27.48; H, 1.76; N,
24.03. For 3, C32H18Cu2F12N12O8 (green prisms). Calc. (%): C,
36.47; H, 1.72; N, 15.96. Found (%): C, 36.21; H, 1.76; N, 15.80.
For 4, C16H16CuN8O8S2 (deep-blue blocks). Calc. (%): C, 37.50;
H, 3.15; N, 21.87. Found (%): C, 37.29; H, 3.07; N, 22.04. For 5,
C48H46B6Cu3F24N24O10 (blue prisms). Calc. (%): C, 31.49; H, 2.53;
N, 18.37. Found (%): C, 31.62; H, 2.40; N, 18.54.
Preparation of Cu2(4,4ꢀ-bpdz)3(CF3CO2)4 4. 0.039 g (0.1 mmol)
of Cu(CF3CO2)2·6H2O and 0.016 g (0.1 mmol) of 4,4ꢀ-bpdz were
dissolved in 2 ml water. Evaporation of the solution at r.t. over
concentrated H2SO4 yields 0.030 g (85% based on the ligand) of
the complex Cu2(4,4ꢀ-bpdz)3(CF3CO2)4 3.
Fig. 10 View of the structure of 9 along the direction of the cuproiodide
helices illustrating “collapse” of the anticipated channels and formation
of the densest packing.
Preparation
of
[Cu3(OH)2(4,4ꢀ-bpdz)3(H2O)2{CF3CO2}2]
accompanied with a pronounced exothermic effect. After the
addition was completed, the red–brown reaction mixture was
(CF3CO2)2·2H2O 6. 0.039 g (0.1 mmol) of Cu(CF3CO2)2·6H2O
and 0.016 g (0.1 mmol) of 4,4ꢀ-bpdz were dissolved in 4 ml water
and the solution was stirred in a 10 ml vial at nearly reflux
temperature for 3 h. Several 1 ml portions of water were added
during this period as the solution evaporated. The mixture was
cooled and filtered. Slow evaporation at r.t. led to crystallization
of the hydroxo complex 6, as small grass-green prisms. The yield
was 0.024 g (60%).
◦
stirred at 80 C for 4 h (elimination of dimethylamine) and then
left overnight at r.t. The precipitate was filtered, washed with
5 mL CH2Cl2 and twice crystallized from methanol with charcoal
yielding a pure product (4.44 g, 39%) as thin colorless needles.
1H-NMR (400 MHz, dmso-D6, ppm) d: 9.77 (s, 2H, 3-CH), 9.37
(d, 2H, 6-CH), 8.23 (dd, 2CH, 5-CH). Anal. for C8H6N4. Calc.
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Anal. for 6, C32H26Cu3F12N12O13. Calc. (%): C, 31.89; H, 2.17;
N, 13.95. Found (%): C, 31.79; H, 2.23; N, 14.21.
Copper(I) complexes 7–9. These were prepared using a layering
technique. In a typical synthesis, a solution of 0.038 g (0.2 mmol)
of CuI in 3 ml acetonitrile was layered over a solution of 0.016 g
(0.1 mmol) of the ligand in 3 ml of acetonitrile–chloroform mixture
(2 : 1 v/v). Slow interdiffusion of the solutions led to a black
crystalline product Cu2I2(4,4ꢀ-bpdz) 9 in 15 d. The yield was 0.043 g
(80%). The chloride Cu2Cl2(4,4ꢀ-bpdz) 7 and bromide Cu2Br2(4,4ꢀ-
bpdz) 8 (deep-red blocks) were prepared similarly using anhydrous
acetonitrile and chloroform freshly distilled over P2O5.
Anal. for 7, C8H6Cl2Cu2N4. Calc. (%): C, 26.98; H, 1.70;
N, 15.74. Found (%): C, 26.71; H, 1.77; N, 15.52. For 8,
C8H6Br2Cu2N4. Calc. (%): C, 21.59; H, 1.36; N, 12.59. Found
(%): C, 21.42; H, 1.42; N, 12.28. For 9, C8H6Cu2I2N4. Calc. (%):
C, 17.82; H, 1.12; N, 10.40. Found (%): C, 17.69; H, 1.16; N, 10.31.
Crystallography
Crystallographic measurements were made using a Stoe Imaging
Plate Diffraction System for 1, 3, 5 and 6 (213 K, absorp-
tion corrections using DIFABS), Bruker APEX area-detector
diffractometer for 2, 4 and 9 (173 K, absorption corrections by
SADABS28) and at r.t. using a four-circle Enraf Nonius CAD-
4 diffractometer for 7 and 8 (absorption correction was based
˚
on psi-scans) (Mo-Ka radiation, k = 0.71073 A). The structures
were solved by direct methods using the program SHELXS-97.29
The refinement and all further calculations were carried out using
SHELXL-97.29 The non-H atoms were refined anisotropically,
using weighted full-matrix least-squares of F2 (Table 5). For 2
and 9, all hydrogen atoms were located and refined isotropically.
For other structures CH hydrogens were added geometrically, OH
hydrogen atoms were located and then fixed in structures 1, 4
and 6. For 5, the hydrogen atoms of the water molecules were not
located. In both trifluoroacetate structures 3 and 6, the CF3 groups
shows typical rotational disorder, which was resolved with fixed
geometry and with partial occupancies 0.5, 0.5 and 0.7, 0.3 (for two
unique carboxylate anions in 3) and 0.5, 0.5 in 6. The disordered
fluorine atoms were refined anisotropically for the sake of the
overall convergence. One of the non-coordinated water molecules
in structure 5 was equally disordered over two positions. Large
anisotropy for thermal motion was also suggestive for possible
−
disorder of one of the independent BF4 anions. However, it was
not possible to resolve the disordering scheme. No constraints
−
in BF4 geometry were applied. Graphical visualisation of the
structures was made using the program Diamond.30
CCDC reference numbers 640632–640640.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b703911e
Conclusions
The present study provides an efficient synthetic methodology
and a very simple chemical access to 4,4ꢀ-bipyridazine as a
novel polydentane N-donor for engineering network solids. The
functional and conformational features of the ligand may be
especially well suited for the development of highly connected
coordination polymers and open frameworks, the design of
polynuclear metal–organic clusters and their integration into the
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extended structure. Coordination preferences of the ligand and its
affinity towards “soft” Cu(I) cations allows rational approaches
towards novel hybrid organic/inorganic materials incorporating
typical inorganic motifs bridged by pyridazine. Our results suggest
that the weak anion–p interactions, which are known entirely for
the most electron deficient heterocycles, may also be relevant
for the pyridazine derivatives as a factor for controlling the
supramolecular structure.
10 T. Otieno, S. J. Rettig, R. C. Thompson and J. Trotter, Inorg. Chem.,
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Acknowledgements
The work was in part supported by a grant from Deutsche
Forschungsgemeinschaft UKR 17/1/06 (HK and KVD).
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