First Report on Crystal Engineering of Hg(II) Halides with Fully Substituted 3,4-Pyridinedicarboxamides: Generation of Two-Dimensional Coordination Polymers and Linear Zig-Zag Chains of Mercury Metal Ions

Article abstract of DOI:10.1021/acs.cgd.5b00989

Two new ligands N,N,N′,N′-tetraisopropyl/tetraisobutyl-3,4-pyridinedicarboxamide L1-L2 and six of their Hg(II)X₂ complexes (where X = Cl^-, Br^-, and I^-) have been synthesized and characterized. Single crystal X-ray structures of three complexes of L1 (1-3) with HgCl₂/Br₂/I₂ and two complexes of L2 (4-5) with HgCl₂/Br₂ show that these are two-dimensional (2D) coordination polymers, with three different (one new) topologies and five coordinated Hg(II) ions, in square pyramidal coordination. Complex 6, of L2 with HgI₂, is a dimer with a four coordinated tetrahedral Hg(II) ion. The ligands behave as two- or three-connecting linkers for forming coordination polymers (CPs) but only one-connector in the dimer. Various types of hydrogen bonding and other noncovalent interactions have been calculated, analyzed, and discussed for all the complexes and the ligand L1. Although the large size and soft character of iodide coupled with the steric effects of large isobutyl groups are mainly responsible for a change in the primary structure of 6, a significant role of the semilocalized LP···π and C-H···O noncovalent interactions into this has also been found. The latter transform this dimer into a stable 2D hydrogen-bonded network instead of a coordination polymer. The unique halide bridged 2D structure of 1 forms one-dimensional zig-zag chains of metal ions owing to the mercurophilic interactions. Weaker interactions of the same kind further extend along the basic skeleton of 2D CP and are facilitated by LP···π, strong C-H···Cl and C-H···N(py) interactions.

Full text of DOI:10.1021/acs.cgd.5b00989

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Article
‘
First Report on Crystal Engineering of Hg(II)halides with Fully
Substituted 3,4-Pyridinedicarboxamides: Generation of 2D Coordination
Polymers And Linear Zig-zag Chains of Mercury Metal Ions’
Love Karan Rana, Sanyog Sharma, and Geeta Hundal
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b00989 • Publication Date (Web): 07 Oct 2015
Downloaded from http://pubs.acs.org on October 8, 2015
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‘
First Report on Crystal Engineering of Hg(II)halides with Fully
Substituted 3,4ꢀPyridinedicarboxamides: Generation of 2D
Coordination Polymers And Linear Zigꢀzag Chains of Mercury
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Metal Ions’
Love Karan Rana, Sanyog Sharma, Geeta Hundal*
Department of Chemistry, UGC sponsored centre of advance studiesꢀI, Guru Nanak Dev
University, Amritsarꢀ143005, Punjab, India.
lovekaranrana@yahoo.com ; sharma.sanyog@yahoo.com; geetahundal@yahoo.com
dedicated toꢀ Our mentor (late) Prof. (Dr.) M.S. Hundal
Abstractꢀ
Two new ligands N,N,N′,N′ꢀtetraisopropyl/tetraisobutylꢀ3,4ꢀ pyridinedicarboxamide L1ꢀL2
ꢀ
ꢀ
ꢀ
and six of their Hg(II)X
characterized. Single crystal Xꢀray structures of three complexes of L1 (1ꢀ3) with HgCl
and two complexes of L2 (4ꢀ5) with HgCl /Br , show that these are 2D coordination
polymers, with three different (one new) topologies and five coordinated Hg(II) ions, in
square pyramidal coordination. Complex 6, of L2 with HgI , is a dimer with a four
2
complexes (where X= Cl , Br and I ) have been synthesized and
2
2
/Br /
I
2
2
2
2
coordinated tetrahedral Hg(II) ion. The ligands behave as 2ꢀ or 3ꢀconnecting linkers for
forming CPs but only 1ꢀconnector in the dimer. Various types of Hꢀbonding and other nonꢀ
covalent interactions have been calculated, analyzed and discussed for all the complexes and
the ligand L1. Though large size and soft character of iodide coupled with the steric effects
of large isobutyl groups, are mainly responsible for a change in the primary structure of 6, but
a significant role of the semilocalized LP···π and CꢀH···O nonꢀcovalent interactions into this
has also been found. The latter transform this dimer to a stable 2D Hꢀbonded network instead
of a coordination polymer. The unique halide bridged 2D structure of 1 forms 1D zigꢀzag
chains of metal ions owing to the mercurophilic interactions. Weaker interactions of the
same kind further extend along the basic skeleton of 2D CP and are facilitated by LP···π,
strong CꢀH···Cl and CꢀH···N(py) interactions.
Introductionꢀ
Coordination network solids, of which coordination polymers (CPs) and metalꢀorganic
1
frameworks (MOFs) form a class and subclass respectively, are compounds relentlessly
getting hold of scientific attention for the past few decades. They are ideal examples of
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inorganic crystal engineering for having utilized various nonꢀcovalent interactions to obtain
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technologically useful materials with various dimensionalities and topologies and provide an
important interface between synthetic and material chemistry. Nonꢀporous coordination
polymers though may have lesser applications than MOFs but are structurally equally
interesting and the simplicity of their selfꢀassembly coupled with ease of structural
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modifications, makes them ubiquitous in nature, especially the 1D CPs. Moreover, various
interchain/interlayer, metal domain (M···M, M―H···X ), ligand domain (D―H···A,
anion···π, halogen bonding, halogen···halogen) and periphery domain(CꢀH···X, CꢀH···π,
11,12
lone pair···π etc.) Hꢀbonding and other nonꢀcovalent interactions
supplement the
coordination bonding to generate compounds with higher dimensionalites. The strength,
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1,12
directionality and distance dependence
of most of these intermolecular interactions not
but may guide the secondary structure as well, by
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only dictates the primary structure
influencing molecular assembly and crystal packing. Therefore a detailed study of various
nonꢀcovalent interactions, shaping the final outcome of a selfꢀassembly process is the soul of
crystal engineering.
The dimensionality and topology resulting from coordination polymerization and/or
hydrogen bonding is dependent on many factors like the type of ligand, metal ion, anion,
solvent, temperature etc. The use of divergent polydentate ligands in metal coordination
instead of the traditional convergent chelating ligands can be very helpful in extending the
2
0
coordination network through space in 1,2 or 3D architectures and be a good innovation for
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crystal engineering of coordination networks. The spherical d metal ions, due to the absence
of crystal field stabilization energy, have flexible coordination geometries and are amenable
to linear and other distorted geometries which facilitate the formation of 1D CPs. On the
other hand, general lability of these ions renders the coordinate bonds reversible, permitting
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rearrangement of metal and ligands to acquire highly ordered 2D and 3D networks also.
Literature shows that a lot of quality work has been done regarding the selfꢀassembly
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of pyridine based carboxamides
derived from primary amines and various acids like
nicotinic acid, isonicotinic acid and 2,6ꢀpyridinedicarboxylic acid. A closer look however,
shows that fully (alkyl/phenyl) substituted amide derivatives of various pyridine carboxylic
acids also provide the necessary rigidity, binding sites and intermolecular interactions to
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allow the creation of CPs. However not much has been reported
on the coordination
behavior of such amides. Therefore a few years back one of us and Kapoor et al., undertook
work on N,Nꢀdialkyl/diphenylꢀnicotinamide, ꢀisonicotinamide, ꢀpicolinamide and N,N,N′,N′ꢀ
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tetraalkylꢀ2,6ꢀpyridinemono/dicarboxamides. Whereas nicotinamides and isonicotinamide
derivatives(divergent polydentate ligands) generated many interesting 1D, 2D and 3D
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coordination/Hꢀbonded architectures
and showed remarkable magnetic properties for
cobalt(II) and copper(II) complexes, those of picolinamides and 2,6ꢀpyridinedicarboxamides
(convergent polydentate ligands) generally yielded zero dimensional coordination
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compounds.
Hg(II)X
Working on these lines, we now report, for the first time a series of six
complexes of two new N,N,N′,N′ꢀtetraisopropyl/tetraisobutylꢀ3,4ꢀ
2
pyridinedicarboxamide ligands L1ꢀL2, five of which form 2D CPs. In contrast to N,N,N′,N′ꢀ
tetraalkylꢀ2,6ꢀpyridine dicarboxamides, these ligands offer three divergent coordination sites
and may bind in various coordination modes (Scheme 1).
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Scheme 1. Coordination modes shown by the ligands (i) (Ʋ
2
ꢀǪ ꢀǧ : ǧ ), (ii) (Ʋ ꢀǪ ꢀǧ : ǧ : ǧ ), (iii)
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monodentate.
Hg(II) is the metal ion of our choice (vide supra) and also for the reason that there are far
lesser number of its CPs known in comparison to its congeners. The large vdW radius of
mercury(II) enables it to have larger number of nonꢀcovalent interactions, resulting in better
opportunities for newer architectures. The majority of Hg(II) CPs are 1D in nature largely
because of its large size, lower coordination number and inclination of Hg(II) for tetrahedral
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coordination and only a few examples are known for 2D CPs. Our pick of halides as
counter anions is inspired by their known polarizability and anisotropy and by the proven role
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of anions, in general
and halides in particular, in the crystal engineering of Hg(II)
compounds where halide bridging is ubiquitous. Binding to a metal ion increases the polarity
and tunes the basicity of halide ions and amide oxygens, respectively. This, coupled with the
electron deficient pyridine ring and abundant peripheral alkyl groups, offers just the right
combination for generating a large number of CꢀH···X/O/π, π···π and lone pair···π
4
interactions. The accumulative “Gulliver effect” of these weak interactions is envisaged to
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support, direct or even dictate the dimensionality and topology of these coordination
compounds, leading to diverse supramolecular architectures.
Xꢀray crystal structure of N,N,N′,N′ꢀTetraisopropylꢀ3,4ꢀpyridinedicarboxamide ligand
(L1)ꢀ
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Fig. 1. (a) ORTEP of L1 shown at 30% probability. Hydrogens have been removed for
clarity (b) conformation of L1.
L1 crystallizes in an orthorhombic nonꢀcentrosymmetric Pca2 space group. ORTEP
1
representation (Fig. 1a) shows two crystallographically independent molecules (A & B) of
the ligand L1 in the asymmetric unit. The torsion angles (C4ꢀC3ꢀC6ꢀO1), (C1ꢀC2ꢀC13ꢀ
O2),(C21ꢀC22ꢀC25ꢀO3) and (C24ꢀC23ꢀC32ꢀO4) are ꢀ105(1), ꢀ105(1), 104(1) and 103(1)°,
respectively and the dihedral angle (β) between the respective pyridine rings and amide
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6
groups, (Table 2) indicate that the amide groups are considerably twisted with respect to the
pyridine rings and are almost perpendicular to each other. The carbonyl groups as well as the
alkyl groups on the two arms are in an antiꢀconformation with respect to each other (Fig. 1b).
This may be attributed to the steric factors due to the presence of isopropyl groups on the
fully substituted amide groups, placed ortho to each other. Both A and B molecules form
their own set of parallel helical chains, along the a axis due to C15ꢀH15C···O1, C16ꢀ
H16B···O1and C30ꢀH30B···O4, C31ꢀH31C···O4 intermolecular Hꢀbonding, respectively.
Since A & B are in a parallel but anti conformation w.r.t each other therefore these two sets
of helical chains are further joined to each other due to C21ꢀH21···N1 and C18ꢀH18C···N4
Hꢀbonding, to shape up helical tapes parallel to the a axis (Fig. S1a, Table S3). These tapes
are in turn linked through C9ꢀH9···O1, C27ꢀH27···N4 and C24ꢀH24···O3 interactions to
propagate the supramolecular arrangement along the b axis generating a 2D network (Fig.
S1b)
2 2
Crystal structure of [Hg(L1)Cl ][HgCl ](1)
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The structure of this double salt is unique in the sense that there are [HgCl ] and
[
HgL1Cl ] moieties in the asymmetric unit of the complex 1 (Fig. 2a). The complex is a 2D
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coordination polymer (Fig. 2b) whose stepwise formation may be explained as follows. First
the formation of a centrosymmetric dimer by joining of two [HgL1Cl ] units, through ꢀ ꢀCl3
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bridges between two Hg2 ions takes place while the second chloride ion Cl4 is bonded
terminally (Table 2). Each Hg2 ion in this dimer is five coordinated by three chlorides and
pyridine nitrogen and one amide oxygen (O2). The ligand L1 is a 2ꢀconnecting linker
between two successive dimers and results in forming 1D ladder like, polymeric chains of
edge sharing, slightly distorted square pyramids (Addison parameter τ = 0.29), parallel to
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the a axis. Now the second moiety [HgCl ] is forming a 1D, stair case type, polymeric chain
of edge sharing octahedrons, (Fig. 2c) parallel to the former by using ꢀ ꢀ Cl1 and Cl2 ions
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bridging between successive Hg1 ions. These two kinds of chains are in turn bonded to each
other due to bridging through Cl3 and Cl4 ligands in ꢀ
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ꢀ modes respectively,
ꢀ
forming a 2D coordination polymer in the ab plane. Thus in this 2D polymer all Cl ions are
in bridging mode. Topologically the structure is new, containing infinite layers of alternating
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stairs and ladders that form 4,5ꢀconnected 2ꢀnodal net with point symbol {3.5 }{3 .4 .5 .6}
Fig. 2d).
(
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Fig. 2. Showing (a) ORTEP diagram of the asymmetric unit for complex (1) (b) 2D coordination
network due to 2 connected L1 and halide bridging in ab plane (c) alternate chains of edge sharing
octahedrons and square pyramids (d) topology of the complex with alternating stairs and ladders.
Hg1(magenta), Hg2 (cyan) in Fig. bꢀd.
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The conformation of L1 ligand is slightly changed in the complex. The amide groups are still
perpendicular to the pyridine ring and in antiꢀconformation to each other (C5ꢀC4ꢀC6ꢀO1 and
C2ꢀC3ꢀC13ꢀO2 torsion angles are 93.8(10) and 93.9(10)° and β of 80.4(6) and 81.8(5)°), but
they are now rotated more with respect to pyridine rings. Various CꢀH···Cl interactions (vide
infra) support this 2D coordination polymer (Table 3).
2
Crystal structure of [Hg(L1)Br ] (2)
The asymmetric unit of the complex 2 is shown in Fig. 3a while a perspective view (Fig. 3b)
illustrates that Hg(II) ion is 5ꢀcoordinated through 2ꢀconnecting linker L1, a terminal Br2 and
a bridging Br1 ions (Table 2) forming a 2D CP of edge sharing regular square pyramids (τ =
0.05 ) in the bc plane. Each Hg(II) ion in this 2D CP is bridged by two L1 molecules which
are mutually twisted (~60° ), forming ‘step shaped’ 1D polymeric chains, running parallel to
c axis.
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Fig. 3. (a) ORTEP diagram of the asymmetric unit of complex 2 (b) polyhedral representation
of 2D CP showing edge sharing square pyramids in antiꢀparallel fashion (c) topology of the
complex showing 2ꢀ and 4ꢀconnected linker and metal nodes, respectively. Terminal
bromides are omitted.
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The centrosymmetrically related parallel chains then get linked to each other due to a HgꢀBr1
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bond generating (1 0 0) layers with 3ꢀconnected uninodal net having point symbol (6 ) and
hcb type topology (Fig. 3c). The net contains 32ꢀmembered metallacyclic rings. Amide
oxygen O2 remains free and is involved in Hꢀbonding interactions. The CP is strengthened
by C16ꢀ H16C···Br1 Hꢀbonding interactions(vide infra) (Table 3) as well as by CꢀH···π
interactions between alkyl group C12 and the pyridine (Table S1,ESI). The amide groups of
L1 are perpendicular to the pyridine ring (C2ꢀC3ꢀC6ꢀO1 and C5ꢀC4ꢀC13ꢀO2 torsion angles
are 75.8(7) and 83.3(7)° and β of 79.6(3) and 87.3(4)°) and in antiꢀconformation as in the free
L1, with the uncoordinated amide group rotated more with respect to the pyridine ring.
Crystal structure of [Hg(L1)I2]2 complex 3
The ORTEP presentation of asymmetric unit containing one L1 and HgI
shown in the Fig. 4a. The crystal structure of the 2D coordination polymer consists of centroꢀ
symmetric [Hg(L1)I dimeric unit (Fig. 4b). Each Hg(II) ion is five coordinated, in a
2
units, is
2 2
]
regular square pyramidal ( τ= 0.06) coordination by two bridging and two terminal I and two,
2ꢀconnected L1 ligands. The latter is in anti-conformation visꢀàꢀvis the amide oxygens and
coordinates through pyridine N and one amide oxygen. The 2D coordination polymer
contains linear antiꢀparallel chains of edge sharing polyhedra (Fig. 4c), linked by L1 ligands
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CP(c) polyhedral representation of 2D CP showing edge sharing square pyramids in antiꢀ
parallel fashion (c) topology of the complex showing 2ꢀ and 4ꢀconnected linker and metal
nodes, respectively. Terminal I are omitted.
in the bc plane and is topologically same as complex 2 (Fig. 4d) with 32 member
metallacycles. Intermolecular Hꢀbonding C11ꢀ H11B···O2 (ESI, Table S6) and other intraꢀ
molecular Hꢀbonding interactions further support this 2D array (vide infra). The amide
groups of L1 are perpendicular to the pyridine ring (C4ꢀC3ꢀC6ꢀO1 and C1ꢀC2ꢀC13ꢀO2
torsion angles are ꢀ83.3(7) and 89.4(7)° and β of 86.9(4) and 85.9(4)°) and in antiꢀ
conformation as in the free L1, but the uncoordinated amide group is rotated more with
respect to the pyridine ring.
2
Crystal structure of [Hg(L2)Cl ] (4)
ORTEP diagram of the asymmetric unit is shown in (Fig.5a). A perspective view (Fig. 5b,
inset) shows that each metal ion is in a slightly distorted square pyramidal geometry (τ =
0
.29) with two terminal chloride ions and one pyridine and two carbonyl oxygens from three
different L2 ligands. Thus L2 acts as a 3ꢀconnecting linker. There are two alternating Hg L2
and Hg L2 units forming 14 and 26 member metallacyclic rings, respectively in the CP.
2
2
4
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Complex forms a brick work type 2D polymeric sheet (Fig. 5c). Topologically the structure
composed of (1 0ꢀ1) layers, that form 5ꢀconnected uninodal net having SP 2ꢀperiodic net type
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topology, with point symbol {3 .4 .5 }. The conformation of the L2 is anti. Relative to L1
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the amide groups of L2 are less rotated w.r.t the pyridine ring ( β being 72.9(2) and 65.2(2)°)
but are in anti conformation as well.
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Fig. 5. (a) ORTEP diagram of the asymmetric unit of complex (4) (b) perspective view of the
2D CP, inset: shows coordination polyhedron (c) topological view of the 2D CP.
Crystal structure of [Hg(L2)Br ] (5)
2
ORTEP diagram of the asymmetric unit is shown in (Fig.6a). The complex is isoꢀstructural to
complex 4. A perspective view of the complex (Fig. 6b) shows that each metal ion is in a
slightly distorted square pyramidal geometry (τ 0.21) (Fig. 6b, inset), with two terminal
bromide ions and three different L2 ligands. Each ligand is acting as 3ꢀconnecting linker. The
amide groups of L2 are less rotated with respect to the pyridine ring (β being 68.0(3) and
69.8(3)°) but are in antiꢀconformation as well. The arrangement forms a 2D brick work type
coordination network with alternating Hg L2 and Hg L2 units. The ꢀ ꢀ bridging mode of
2
2
4
4
3
L2 makes it a 3ꢀconnected linker and Hg(II) is a 3ꢀconnected node (Fig. 6c) extending to
form 2Dꢀsheets running diagonally to the bcꢀplane. Topologically the structure is composed
of (1 0ꢀ1) layers, that form 5ꢀconnected uninodal net having SP 2ꢀperiodic net type topology,
3
3
4
with point symbol {3 .4 .5 }. The 2D sheet is assisted by various intraꢀ and intermolecular Cꢀ
H···X Hꢀbonding interactions with carbonyl oxygens and bromide ion (vide infra)
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Fig. 6. (a) The ORTEP diagram of the asymmetric unit of complex 5 (b) perspective view of
the 2D CP, inset: coordination polyhedron (c) Topological view of the complex with brick
work like structure
2 2
Crystal structures of [Hg(L2)(I )] (6)
ORTEP diagram (Fig. 7a) of the asymmetric unit and a perspective view of the complex (Fig.
b) show that it is a centroꢀsymmetric dimeric complex with [Hg(L2)(I )] unit. Each Hg(II)
7
2
2
has a near regular tetrahedral geometry with one terminal and two bridged iodide atoms and
a pyridine coordinating L2 ligand molecule in antiꢀconformation. The amide groups of L2
a
are in anti-conformation with one rotated less than the other. Hg1···Hg1 (a=ꢀx+1/2,ꢀ
y+1/2+1,ꢀz+1) distance 3.876(1) Å found here is the shortest among this series. In the crystal
structure C5ꢀH5···O2 and C21ꢀH21···O1 Hꢀbonding interactions and a strong LP···π
interaction connect these dimers to give 2Dꢀsheet structure running parallel to ab-plane (vide
infra, Fig. 9b).
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Fig. 7. (a) ORTEP diagram (b) Perspective view of the centrosymmetric dimeric unit of
complex 6 along with atom numbering scheme.
Discussionꢀ
(1) Bond distancesꢀ
Atomic and van der Waals radii of Mercury(II) are known to be 1.50 < r(Hg) < 1.73
48ꢀ50
Å.
The van der Waal radii of halogens have been taken as Cl (1.75Å), Br (1.85Å)
51
and I (1.98Å) and that of N(1.55) and O(1.52). All distances smaller than the sum of
van der Waals radii (< 3.48, 3.58, 3.71 Å for Cl, Br and I, respectively), have been
21
considered to define the actual coordination number with vdW of mercury taken as a
mean value, 1.73Å. A search of HgꢀX(bridging) bond distances in Cambridge
52,53
Structural Database (with the help of ConQuest version 1.14 )
reveal the reported
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ranges from 2.337 Å to 3.401 Å , 2.423 to 3.132 Å and 2.706 to 3.488 Å for
Cl, Br and I, respectively. As expected the terminally bonded halide atoms have
smaller distances than the bridging ones, in general and presently also. All bridging
halides here are found unsymmetrically bridged, with one bond significantly smaller
than the other.
All chloride ions in complex 1 are bridging with their MꢀX bond lengths in the
acceptable range. (Table 2). Same is true for the complex 2 and 3. The longest Hgꢀ
Br(B) known in CSD is though shorter than 3.361(2)Å reported here for complex 2
60
but as CSD also reports the longest HgꢀBr(terminal) to be 3.487Å, therefore it has
been retained, more so as it is less than sum of vdW radii. However the maximum Hgꢀ
I(bridging) distance 2.986 Å in complex 6 is much smaller than the sum of vdW
distances which is conspicuous in comparison to 4ꢀ5 (with no bridging halide). A
higher covalent character of I increases its propensity to undergo bridging readily.
Moreover an almost symmetrical I bridging in 6 in comparison to 3 is attributed to
the lesser steric effects in case of the former due to four coordinated Hg(II).
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HgꢀN distances are in the range (2.501– 3.498Å, based on CSD search) found
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for mercury and pyridine/pyrazine complexes
except for complex 1 where HgꢀN
2
.174(6) Å is significantly and surprisingly shorter and is attributed to the linear C16ꢀ
H16C···N1(py) (Fig. S2, Table S4, ESI) along a axis. Since the coordination through
N1 is also propagating the chains along a axis, the Hꢀbonding contact acts
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cooperatively to bring N(py) closer to Hg(II). HgꢀO distances
found here are all
within the range 2.434ꢀ3.649Å but are invariably longer than the corresponding HgꢀN
distances, despite a smaller radius of O as compared to N. This has been observed
67
earlier also. In fact Khavasi et al. have mentioned more than 100 reports (based on
CSD search) having HgꢀO longer than 3.00Å. It is in accordance with a lesser affinity
of harder O towards class b Hg(II) metal ion, than N. In general HgꢀO distance here is
curiously and finely tuned by the ∠CꢀOꢀHg angle (θ) (Table 2) when larger θ values
are associated with smaller distances, bringing amide oxygen in a better overlapping
position with the metal ion.
(
2) Coordination mode of ligandsꢀ
The ligands can coordinate in three different modes using pyridine nitrogen and two
amide oxygens (Scheme 1). The former has consistently been found in all the
complexes, with or without the assistance from amide oxygens so that these may act
as one, two or three connecting linkers. The coordination modes I and II result in the
formation of 2D CPs while mode III forms dimers only. In mode I, invariably it is the
amide group placed at position 4 which partcipates in coordination, akin to
isonicotinamide analogues, accounting for the linearity of selfꢀassemblies. Extension
in the second dimension is accompalished by halide bridging. Mode II, on the other
hand extends the CP in both the dimensions using the organic moiety only thus has
the primary structure directing effect, with anions largely being responsible for
secondary effects. For modes I and III, anion has the privilage of directing the primary
structure with L guiding the secondary effects.
(3) Role of anionꢀ
Not only the role of counter ions in the complex structure is well documented but
their effect on the selfꢀassembly process via nonꢀcovalent interactions has also been
4
2ꢀ44
found to be significant.
Choice of halides as counter anions in the two series was
made to explore both the aspects i.e. direct influence of halide ion coordination to
metal ion as well as their participation in large number of weak CꢀH···X/ LP···π
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interactions, on the resulting dimensionality and topology of the complex. The nature,
size and mode of coordination of the halide ions have not only affected the binding
mode of the ligands and acted as the major factor in deciding the primary structure of
the complex formed, but also influenced the secondary structures by generating
various nonꢀcovalent interactions. While all three complexes,1ꢀ3 of propyl derivatives
have 2ꢀconnected ligand L1, those of butyl derivatives 4ꢀ5 are 3ꢀ and 6 has 1ꢀ
connected linker L2. Consequently 1ꢀ5 are 2D CPs containing five coordinated Hg(II)
ions while 6 is a dinuclear complex with a four coordinated Hg(II) ion. This may be
attributed to the propensity of iodine atom to lower the coordination number of the
Hg(II) by increasing the electron density at the metal center because of the softer
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character of iodine compared to other halogen atoms. Another factor is the large size
69
of the iodine atom relative to other halogen atoms. The large size of 5p orbitals of
iodine atom renders easy availability of its electrons to the metal atom. It also hinders
the approach of more L2 ligands closer to the metal ion due to steric factors.
2 2
Interestingly 3 also has [Hg(L)I ] repeat unit in the polymer, signifying the
importance and stability of a dinuclear moiety in both. A combination of the large
butyl groups and halide ion in 6 compromises the denticity of L2 due to
overwhelming steric factors. The loss is compensated by the more covalent nature of
the HgꢀI bond and CꢀH···O and LP···π, nonꢀcovalent interactions (vide infra).
Moreover the terminal or bridging mode of halide ions also affects the primary
structures. Although 1ꢀ5 are all 2D polymers with dimeric repeat units created due to
halide bridging between two closest Hg(II) ions in 1ꢀ3 but in 4ꢀ5 they are formed due
to bridging L2 groups (with halides being terminal only). The latter are the ones to
have 3ꢀconnected L2 so as to recompense for bridging halide ions.
(4) Nonꢀcovalent interactionsꢀ
The choice of ligands and counteranions in the present study is based upon
maximizing the weak Hꢀbonding interactions which do not contribute significantly
towards any crystal structure on an individual basis but gain importance on the ground
of sheer numbers. A plethora of weak interactions many a times offsets the effect of
fewer strong interactions. As put forth by Aakeroy “ CꢀH···X interactions can tilt the
balance between several options of stronger bonded networks thus acting as an
important steering force in solid state assembly”. Presently, the fully substituted
70ꢀ73
amide nitrogens lose the possibility of strong amideꢀamide Hꢀbonding
nonetheless
the presence of alkyl groups ensures gain in terms of a large number of weak Hꢀ
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bonding interactions, in the periphery domain. Further the pyridyl nitrogen is
invariably involved in connecting metal centres whereas the amide groups may or
may not coordinate and are important assets. Not only because of the Hꢀbonding
ability of amide functionality and the rotational flexibility of the uncoordinated group,
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leading to various conformation
but also due to the difference in Hꢀbond
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acceptance of the metal coordinated amide group due to its changed basicity, in the
11,12
ligand domain.
2
Additionaly the use of HgX increases the number of weak Hꢀ
δ+
δꢀ
bonding interactions manifolds as polarity of metalꢀhalogen bond (M ―X ) makes
halogen a good hydrogen bond acceptor than their carbonꢀbound counterparts.
Various weak Hꢀbonding interactions found in the present complexes and their effect
on the overall crystal structure and dimensionality is discussed below
(
a) CꢀH···X interactionsꢀ
The polar nature of the M―X (metalꢀhalogen) makes them good hydrogen bond
acceptors, because of the increased electrostatic component for D―H···X―M bond
instead of D―H···X―C bond. The strength of the D―H···X―M hydrogen bonds
follows the order X = F>>Cl ≥ Br>I for D=C,N,O. This trend is understood as the
polarity of M―X bond follows a reverse order. It is indeed this Hꢀbond acceptor
tendency of the M―X bond that has attracted attention on a number of fronts like in
77
metabolically important ion channels, in material chemistry, with different
78
79,80
applications including thermochromic and magnetic materials.
In the field of
crystal engineering the supramolecular synthons utilize this Hꢀbond acceptor tendency
along with the directional properties of metal halides in the formation of Hꢀbonds and
can be used to link molecular building blocks. Though the increased electronegativity
of Hg in comparison to metals of lower atomic weight, complicates the polarity of
Hg―X bond. However, an increased covalent character down the halogen group
substantially decreases the polarity of HgꢀBr/I bond compared to Hg―Cl bond. So a
stronger H⋅⋅⋅X bonding is expected in chlorides. To compare the relative hydrogen
bond acceptor tendency of metal bound halogens, three geometric parameters are used
in the analysis of hydrogen bonds, H···X distance, and angles (D―H⋅⋅⋅X) and
(H⋅⋅⋅X―M). The donorꢀacceptor pairs are put on a common scale by the use of
81
H X
normalized distances RHX = (dH····X) /( r + r ), which was given by Lommerse et al.
It allows a direct comparison of the bond acceptor capabilities of halogens and is
calculated on the basis of van der Waals radii of H, F, Cl, Br, I atoms. Normalized
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distance vs angle plots that utilize RHX vs 1ꢀcos T( T = 180ꢀ (D―H···X)), are used to
82
investigate the geometrical preferences for the different Hꢀbond types. The values of
ꢀcos T corresponds to the D―H···X angle for e.g. 1ꢀcos T = 0 gives D―H···X =
1
180° and so on with similar correspondence for (H···XꢀM) angles. From crystal
engineering point terminal metal halides (except for F which is mostly isotropic) are
highly directional Hꢀbond acceptors and have angular preferences, with H···X―M
lying in the close range of 90ꢀ130° due to a region with negative electrostatic
potential around the halogen where it accepts the hydrogen.
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The mean normalized hydrogen bond distances RHX for CꢀH···X―M have been
8
2
calculated (based on CSD) by Brammer et al. (given in Table 3 for ca. RHX < 1.05,
3
(R ) ≤ 1.15) and analysed to give the trend H···F<< H···Cl ≅ H···Br < H···I which is
HX
ꢀ
the same as found in CꢀH···X . Using the above defined criterion and cut off such
interactions found presently have been given in Table 3. As the RHX distances for two,
one and two interactions for complex 1, 2 and 5, respectively are less than the mean RHX
reported earlier therefore they are termed as relatively stronger interactions (Fig. 8). In
fact the shortest RHX distances in bromide complexes 2 and 5 are equal to mean RHX
distance found for H···Cl―M and that of chloride complex 1 is even less than mean
R
HX distance (0.943Å) for H···F―M interactions, indicating them to be relatively much
stronger. This is interesting in the sense that Brammer et al. in their study only
considered the terminal bound halide ions whereas in the present case we have both
terminal and bridged halide atoms, depending on the complex. Also that study did not
include bifurcated or trifurcated hydrogen bond donors. In complex 1 Cl3 is not only
triply bridging but is acting as a double Hꢀbond acceptor to H1 and H2, making a weak
2
and a strong contact. Similarly ꢀ ꢀCl4 accepts two Hꢀbonds, a weak and a strong one,
from H1 and H7, respectively. The bifurcated Hꢀbonds with H1 are both weak.
Similarly Br1 in complex 2 is bridging but makes a strong Hꢀbond with H16C. All Br
atoms in complex 5 are terminal though, with H5 making a bifurcated but comparable
Hꢀbond with Br2, albeit with smaller D―H···X angle. As there is no literature
available, including the bridged halides/multifurcated hydrogens to compare to,
therefore we give our results as mere observations since their number is not sufficient to
draw any generalizations. But it may be stated safely that the mode of coordination of
halide ion to the metal does not affect the RHX distance considerably however it may
influence the D―H···X angle making these interactions more directional. The data in
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Table 3 again endorse the same trend H···Cl < H···Br < H···I, as noted earlier. In case
of complex 3, the RHX distance for the strongest interaction is close to that found earlier
for H···X―C cases, which clearly reflect the diminished M―X bond polarity with
iodide. In all five complexes these interactions help reinforce the 2D coordination
polymers.
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Fig. 8. Showing C―H···X interactions in (a) 1 (b) 2 (c) 5.
(
b) LP···π interactionsꢀ These interactions are relatively rare but highly effective and of great
83
importance in the host guest chemistry of bioꢀmolecules. High directional property of these
interactions makes them very useful in crystal engineering of small molecular systems as well
84
and have been found to be energetically favourable. Reedijk et al., have done a systematic
study based on the CSD data base considering electron deficient six membered rings and
various electron rich atoms (O/N/halogens) and defined criterion to categorize them in terms
of strength. Accordingly such interactions are considered strong if distances (Table 4) from
LP to ring centre, closest bond of ring or closest atom of the ring are less than sum of the
8
5
vdW radii of halogen and C atom and they may be classified as delocalized, semiꢀlocalized
or localized, respectively. Apart from the distance, the angle subtended at the centroid (atom
86
LPꢀcentroidꢀplane, angle β in Table 4), lying between 75ꢀ90° also indicates significantly
strong interactions. According to these criterion, in our complexes 1 and 6, weak to
moderate, semiꢀlocalized (SL), LP···π interactions have been seen which further stabilize
their 2D/dimeric molecular structures, respectively. Though the distances observed, make
them fall under weak interactions but angle β lies close to 75° making them moderate enough
to have same significant effect on structure.
According to halogen bonding theory, the polarizability and the anisotropic character among
the halogen atoms increases from F to I. In accordance with this it is more convenient for I
than Br and Cl to have an electron rich site at its equator region on forming a lone pairꢀπ
interaction with electron deficient ring. In 1 a highly directional and effective Hg―Cl4··· π or
lone pair··· π interaction formed between ꢀ
2
ꢀCl4 atom and the electron deficient pyridine ring
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further sustains the polymeric chain running along aꢀaxis. (Fig. 9a). In 6, Hg ―I2··· π may be
considered as one of the factors to stabilize a dimeric structure for complex 6 instead of a 1D
CP which would have formed if I2 were bridging and coordinating to Hg(II). It also gives
additional support to the CꢀH···O bonded 1D chains running parallel to bꢀaxis (Fig. 9b). It is
clear from the above discussion that coordinated anions whether they are terminal or bridged
can participate in the selfꢀassembly process by utilizing favourable nonꢀcovalent interactions
that cooperatively control the topology of the final product.
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Fig. 9. (a) Lp···π and CꢀH···π interactions that endorse the coordination polymeric chain of 1 running
along a-axis (b) Lp···π interaction holds the successive dimers of 6 to form 1D chains(also supported
by C5ꢀH5···O2) running parallel to b axis.
(c) C(alkyl)ꢀH···π interactionsꢀ CꢀH···π though directional are considered even weaker than Oꢀ
H···π and CꢀH···O because of the poor polarization of the donor as well as acceptor
87ꢀ89
groups.
Nonetheless they participate in supramolecular assemblies especially when the
donor or acceptors are somewhat stronger. For the present study CꢀH···π interactions were
found in all four chloro and bromo complexes, (Table S1, ESI) between the alkyl groups and
pyridine rings(Fig. 10). In all of them their role is to reinforce the 2D CPs but they do not
further extend the dimensionality of the crystal system. The strongest CꢀH···π interaction is
found in complex 1(Fig. 9a).
(
d) π···π interactions – Interactions involving aromatic systems though lack
directionality and strength, can contribute to selfꢀassembly when extended structures
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contain building blocks with aromatic moieties.
orientation
Aromatic rings show different
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5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 10. Showing relevant CꢀH…π interactions in (a) complex (2) (b) complex (4) (c) complex (5).
π…π interactions have also been shown for complex (4) and (5).
preferences, like stacked arrangement and an edgeꢀ or pointꢀtoꢀface Tꢀshaped conformation.
The stacking can also be an offset or slipped packing and need not be perfect faceꢀtoꢀface
arrangement of the rings. In nonꢀpolar πꢀarene system such as benzene, faceꢀtoꢀface
alignment is unstable because of the electrostatic repulsion between the negatively charged
clouds between the two rings. Here the Tꢀshaped motif is found to be more stable because π–
σ attraction is maximized in this geometry. Introduction of substituents or heteroatoms
disturbs the uniform charge distribution in benzene resulting in the development of partial
atomic charges and a permanent dipole which introduces electrostatic dipole–dipole and
dipole–induced dipole interactions. In addition to this a metal atom which is coꢀordinated to a
nitrogen heteroatom further enhances the electronꢀwithdrawing effect due to its positive
charge. Hence, aromatic nitrogen heterocycles increase the tendency of stack because of their
low πꢀelectron density. The stability order of the interaction involving two πꢀ systems
follows the order πꢀdeficient–πꢀdeficient > πꢀdeficient–πꢀrich > πꢀrich–πꢀrich.
For understanding πꢀπ interactions it is important to investigate the strength and
orientation preference of the aromatic rings. In order to explore the effect of πꢀπ interaction
on the crystal packing of the complex 4 and complex 5 containing ligand L
2
, we have
calculated the geometrical parameters that account for the amount of slippage of the two
9
1
pyridine rings involved in πꢀπ interactions. These geometric parameters were calculated for
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all complexes but only isostructural complexes, 4 and 5 showed these interactions at
90
reasonable CgꢀCg distances as shown in Fig. 10bꢀc and given in Table S2(ESI). It shows
that a centroid distance of ~4 Å an angle of ~27° between the ring normal and the centroid
vector, corresponds to a horizontal displacement of ~1.8 Å. It means that one py ring is
shifted somewhat less than the 1,3ꢀCꢀC distance in the ring from the faceꢀtoꢀface alignment
to give weak interactions. In the complexes 1,2 and 3 the oxygen atom (C=O group) lies
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
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56
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60
1
.272, 1.316, 1.247 Å above the plane of pyridine ring with angles centred at oxygen atom
(∠CꢀOꢀHg) as 124, 138, 127°, respectively. In comparision to this the oxygen atom in
complex 4 and 5 lies slightly towards the plane of pyridine ring, at 1.198, 1.203 Å, with
2 2
shorter angles of 121, 119° in the 14 membered Hg L2 units, bringing he two rings closer for
the interaction. (Fig. 10 bꢀc).The secondary effects of πꢀπ interactions support the 2D
network. A 3ꢀconnecting ligand L2, giving brick wall type crystal structure of these two
complexes with alternate 14 and 26 member metallacyclic rings, helps bringing two adjacent
pyridine rings face to face at reasonable distance.
(
e) CꢀH···O(amide) interactionsꢀ The CꢀH···O=C hydrogen bonding which forms a
significant part of organic crystal engineering, is also equally important in metalꢀ
organic systems because same functional groups are present as part of organic
ligands. Though flexible and directional in nature, the nonclassical CꢀH···O hydrogen
bonds are weak, because of smaller electronegativity difference between C and H and
weak Hꢀbond acceptor ability of ꢀC=O group. In contrast to van der Waals forces, the
CꢀH···O hydrogen bonds are directional in nature and also can have attractive
influence beyond the van der Waals contacts. Like strong hydrogen bonds D―H···A
where D,A= N or O, the hydrogen bonds containing CꢀH···O interactions can be the
part of structureꢀdirecting recognition motifs, so called ‘supramolecular synthons’
which involve nonꢀcovalent hydrogen bonding interactions that can lead to hydrogen
bonding frameworks of various dimensionality. The metal atom can guide the
formation of hydrogen bonds between neighbouring molecules by coordinating to a
rigid ligand, having hydrogen bonding groups present at the periphery domain.
Presently both coordinated and uncoordinated amide oxygens have been
involved in inter and intraꢀmolecular CꢀH···O Hꢀbonding interactions (Tables S3ꢀS9).
The –C=O group here may be considered in the ligand or the periphery domain
depending on its role in coordination. In complexes 1ꢀ5 the 2D coordination polymer
is supported by various CꢀH···O interactions involving CꢀH donors at the periphery
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1
1
1
1
1
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1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
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4
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4
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5
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5
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5
5
5
6
domain of the complex. However in complex 6 they have the significant role of
changing the dimensionality of the crystal structure. Both amide oxygens in the
dimeric coordination complex 6 are free and take part in Hꢀbonding. C5ꢀH5···O2 Hꢀ
bonding interactions(Table S7) connect these dimers to give 1D Hꢀbonded tapes along
b-axis, C21ꢀ H21···O1 Hꢀbonding interactions, in turn connect these tapes (Fig. 9b)
to give a 2Dꢀsheet structure running parallel to ab-plane. These interactions create
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
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5
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7
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9
0
1
2
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8
9
0
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2
3
4
5
6
7
8
9
0
4
2
4 2
R (24) and R (18) supramolecular synthons in the crystal structure. They become
the major force to extend a simple dimeric molecular structure to a 2D crystal
structure with the help from aforementioned LP···π interactions. Both amide oxygens
in 1 are involved in intramolecular Hꢀbonding while in 2 and 3 the free amide
oxygens are supporting 1D propagation by forming catemers synthons due to
intermolecular Hꢀbonding (Fig. S3). In complex 4 and 5, CꢀH···O intermolecular Hꢀ
2
bonding forms R (8) supramolecular synthons employing one of the coordinating
4
oxygens(Fig. S4, ESI) in the larger metalacycle. The intramolecular CꢀH···O
interactions in these complexes invariably have shorter H···O and C···O distances
and smaller CꢀH···O angles whereas the intermolecular interactions have longer
distances and wider angles. Collectively however, they are scattered over a wider
range of distances and angles which is typical of such ‘soft’ interactions, as they can
be easily deformed. There is no clear trend in the Hꢀbond distances/angles between
the coordinating and nonꢀcoordinating amide oxygens. This indicates that metal
coordination does not exert any significant electronic influence on the basicity of the
amide oxygen unlike the coordinating carbonyl group which becomes much better Hꢀ
bond acceptor on metal coordination due to the back bonding. This may be due to the
fact that MꢀO (amide) bonding is weak here due to steric factors and hardness of
oxygen and also any gain in the basicity of amide group is balanced by the loss
incurred due to lack of resonance stabilization of the amide group on metal binding.
f) Mercurophilic interactionsꢀ Metallophilic interactions have recently been employed
(
1
0
92ꢀ113
to prepare finite or infinite metal chains for closed shell d metal ions.
While
evidence for
strong mercurophilic interactions is rather debatable despite there being many reports
1
14ꢀ116
such interactions are well documented for Au(I), Ag(I) and Cu(I)
1
17ꢀ125
for short Hg(II)···Hg(II) distances.
reported/calculated with Hg···Hg distance ranging from 3.570 Å
The wide range may be partially attributed to the acceptable value of VDW radius of
Mercurophilic interactions have been
126ꢀ129
130
to 4.484 Å.
1
31,132
mercury which has been reported from 1.71 (vide supra) up to 2.2 Å
with the
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latter being thought as a better estimate than others. Though Hg···Hg distance as short
as 3.41Å has never been reported but many halide bridged Hg based structures, with
ubiquitous Hg
2
X
2
bridges are known to have Hg···Hg inetarctions within this
1
33ꢀ140
range.
However, compounds with these interactions leading to linear metal
1
41
chains are rather rare. Lu et al., have discussed them and described the formation of
linear and zigꢀzag chains of mercury metal ions with distances 3.77, 3.94 and 4.01 Å
for their own compounds. They also carefully analyzed that halide bridging alone is
not suffice for the formation of linear mercury chains, with shorter distances but nonꢀ
covalent interactions (π···π in their case) have significant effect on such architectures.
Presently the 2D CP, complex 1 with extensive chloride bridging is of particular
interest in this regard. The unique architecture of the complex offers infinite, linear
zigꢀzag metal chains (Hg1···Hg1···Hg2···Hg2···Hg1···Hg1) with Hg1···Hg2 3.81,
Hg2···Hg2 3.96 and Hg1···Hg1 4.01 Å, ∠Hg1···Hg1···Hg2 117°,
∠Hg1···Hg2···Hg2 92°, along b axis (Fig. 11). A longer Hg1···Hg1 interaction at
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
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33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
4.36Å with Hg1···Hg1···Hg1 119° and Hg2···Hg1···Hg1 117° joins these linear
chains to form a 2D, network of metal chains. Thus mercurophilic interactions support
the original 2D topology formed due to coordination bond. While extensive halide
bridging might be responsible for the propagation of 1D linear chains but their
extension in the other dimension is clearly supported by the LP···π interactions
between the Cl4 and Py ring, discussed above. The (ꢀ
3
2
ꢀCl3,ꢀ ꢀCl4ꢀHg2) moieties in
the Hg10 rectangular metallacycles (~11.5 x 7.2 Å, distances between the closest
Hg(II) ions) are held together by the organic ligand. The ensuing LP···π , strong Cꢀ
H···Cl (Fig. 8a and 9a) and linear C16ꢀH16···N1(Fig. S2, ESI) interactions are
responsible for constricting the metallacycles breadthwise, shortening the Hg1···Hg1
distance (Table 2) parallel to the a axis.
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2
3
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5
6
7
8
9
1
1
1
1
1
1
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1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
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3
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5
5
5
6
0
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2
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9
0
1
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7
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0
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2
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 11. Showing the formation of linear zigꢀzag chains( red contacts) and 2D
network (cyan contacts) due to mercurophilic interactions. Hg(II) (blue), Cl (green).
The organic part has been removed for clarity (vide supra Fig. 2b).
ꢀ
Complex 6 also shows Hg1···Hg1 interactions within the dimer at a distance of 3.88
Å which is within the conceivable range for such interactions. The other two
complexes, having dimeric structure due to bridging halides, are 2 and 3, with
Hg···Hg distance of 4.28 and 4.33 Å much longer than 6. The difference is clearly
due to a change in coordination number from five to four in the latter.
Conclusionsꢀ
In this paper we have described the selfꢀassembly of the HgX (X=Cl, Br, I) and divergent
2
ligands L1 and L2 based upon 3,4ꢀpyridinedicarboxamides. The dimensionality and topology
of the selfꢀassembled systems are shown to be an interesting outcome of the divergent nature
of ligands, coordination mode of anions, steric factors, Hꢀbonding and other nonꢀcovalent
2 2
interactions. A combination of labile Hg(II) metal ion having ubiquitous Hg X bridges and
divergent ligands has given the expected 2D CPs. A ligand vs anion competition for the metal
ion is imminent where steric congestion becomes overwhelming like in complexes (4ꢀ6) with
bigger butyl groups. The primary structure is guided by size, polarisability and covalency of
the anion with bridging I ion being preferred over tritopic ligand resulting in the only dimer
in the two series. The coordinated anions whether they are terminal or bridged can participate
in the selfꢀassembly process by utilizing favourable nonꢀcovalent interactions that
cooperatively control the topology of the final product. For e.g. the 2D Hꢀbonded network of
2
4
6
2 4
owing to the CꢀH…O interactions forming R (18) and R (24) synthons and a highly
effective, semilocalized LP···π nonꢀcovalent interaction. 2D CPs in 1ꢀ5 are strengthened by
various modest nonꢀcovalent interactions such as CꢀH···X (X=Cl,Br,I),CꢀH···O(amide), Cꢀ
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H(alkyl)···π and πꢀπ. The unique 2D metal ion architecture, seen in the 2D self assembled CP
of 1 is also formed and supported by various nonꢀcovalent interactions like mercurophilic,
LP···π, CꢀH···Cl, CꢀH···N(py). The strength of CꢀH···X interactions is found to be
independent of biꢀ or trifurcated participation of halide or hydrogen atoms in this Hꢀ
bonding.
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60
Experimental Section
All the commercially available chemicals were purchased from Aldrich and used without
further purification. All solvents were dried by standard methods. Elemental analyses (C, H,
1
N) were performed on a Perkin–Elmer model 2400 CHN analyzer. The H NMR spectra were
carried out in DMSOꢀd with TMS as an internal reference, on a JEOLꢀFT NMRꢀ300 MHz
6
spectrophotometer. The infrared spectra (KBr pellet) were recorded using PerkinꢀElmer FTꢀ
ꢀ
1
IR C92035 spectrophotometer in the range 400–4000 cm . HRMS spectra were recorded on
a Bruker’s microTOFꢀQII spectrophotometer. Single crystal structural Xꢀray diffraction
studies were carried out on a Bruker’s ApexꢀII CCD diffractometer using Mo Kα
(λ=0.71069Å) at room temperature.
Synthesis of ligands
N,N,N′,N′ꢀTetraisopropylꢀ3,4ꢀpyridinedicarboxamide(L1), N,N,N′,N′ꢀTetraisobutylꢀ3,4–
pyridinedicarboxamide (L2)
In a 250 ml RBF containing (5.9 mmol, 1.0 g) of 3,4ꢀpyridine dicarboxylic acid in DMF (2
ml), SOCl (30 ml) was added and the reaction was refluxed for 2hr. After the solution
2
2
became clear dry toluene was added and the excess of SOCl was removed by vacuum
distillation. The reaction mixture was cooled down subsequently to room temperature and
pyridine (17 mmol, 1.4 ml) and N,Nꢀdiisopropyl (15 mmol, 2.1 ml for L1) or N,Nꢀ
diisobutylꢀamine (15 mmol, 2.6 ml for L2) were added to the reaction mixture and stirred for
1
hour. The solution was left undisturbed overnight. The organic layer was washed with
distilled water followed by a saturated solution of NaHCO thrice and dried over Na SO . A
slow evaporation gave lightꢀbrown colored crystals for L1 while L2 remains a viscous liquid.
3
2
4
31 3 2
Anal. Calcd for L1, C19H N O (%): C, 68.43; H, 9.37; N, 12.60; Found: C, 67.99; H, 9.35;
1
N, 12.56. H NMR (DMSO, δ from TMS), (Fig. S5): 8.5845 (d, 1H, J = 5.1 Hz), 8.479 (s,
1H), 7.296 (d, 1H, J = 4.8 Hz), 3.480 (m, 2H), 3.265 (m, 2H), 1.378 (s, 12H), 1.096 (d, 12H,
ꢀ1
J = 6.0 Hz). FTꢀIR (KBr, cm ) (Fig. S6): 2971 (s), 1634 (s), 1445 (s), 1345 (s), 1171 (s),
+
1036 (s), 822 (s). HRMS m/z, (Fig. S7): 334.2561 [M+1] (calc. 334.2489).
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5
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Anal. Calcd for L2, C23
H
39
N
3
O
2
(%): C, 33.60; H, 1.21; N, 11.20; Found: C, 33.68; H, 1.18;
1
N, 11.54. H NMR (DMSO, δ from TMS), (Fig. S8): 8.6345 (d, 1H, J = 5.1 Hz), 8.510 (s,
1H), 7.3135 (d, 1H, J = 5.0 Hz), 3.233ꢀ3.195 (m, 4H), 2.947ꢀ2.879 (m, 4H), 2.013ꢀ1.940 (m,
2H), 1.846ꢀ1.767 (m, 2H), 0.8755 (d, 12H, J = 6.3 Hz), 0.7055 (d, 12H, J = 6.0 Hz). FTꢀIR
ꢀ
1
(
(
KBr, cm ) (Fig. S9): 2963 (s), 1626 (s), 1438 (s), 1257 (s), 1096 (s), 840 (s). HRMS m/z,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
+
Fig. S10): 412.2973 [M+Na] (calc. 412.2934).
Synthesis of complexes of HgCl
2
2 2 2
with L1ꢀL2 [[Hg(L1)Cl ][HgCl ] (1), [Hg(L2)Cl ] (4)]
In a 50 ml RBF, L1 (0.30 mmol, 0.10g) and HgCl
2
(0.45 mmol, 0.12g) (for 1) or L2 (0.26
mmol, 0.10 g) and HgCl (0.38 mmol, 0.10g) (for 4) were dissolved in methanolꢀacetonitrile
2
mixture (1:1) and refluxed for half an hour. The reaction mixture was transferred to a conical
flask, where slow evaporation gave colorless crystals for (1) and (4), within a week.
Anal. Calcd for (1), C19
3.54; N, 4.70. FTꢀIR (KBr, cm ) (Fig. S11): 2970 (s), 1639 (s), 1445 (s), 1344 (s), 1210 (s),
042 (s), 847.
4 2 3 2
H31Cl Hg N O (%): C, 26.04; H, 3.57; N, 4.79; Found: C, 26.55; H,
ꢀ1
1
Anal. Calcd for (4), C23
5.99; N, 6.89. FTꢀIR (KBr, cm ) (Fig. S12): 2963 (s), 1619 (s), 1445 (s), 1264 (s), 1096 (s),
47 (s).
Synthesis of complex of HgBr
2 3 2
H39Cl HgN O (%): C, 41.79; H, 5.95; N, 6.36; Found: C, 41.06; H,
ꢀ1
8
2
2 2
with L1ꢀL2 [[Hg(L1)Br ] (2), [Hg(L2)Br ] (5)]
Solutions of L1 (0.30mmol, 0.10g) and HgBr
2
(0.45mmol, 0.16g) in ethanol (for 2) or L2
(0.26mmol, 0.1g) and HgBr (0.38mmol, 0.14g) in methanol (for 5), were put in 50 ml RBF.
2
The mixture was refluxed for 1 hr; the cooled solution was filtered and transferred to conical
flask where slow evaporation of the solvent gives colourless crystals of (2) and (5).
Anal. Calcd for (2), C H Br HgN O (%): C, 32.89; H, 4.50; N, 6.06; Found: C, 32.06; H,
19
31
2
3
2
ꢀ1
4
.55; N, 6.84. FTꢀIR (KBr, cm ) (Fig. S13): 2970 (s), 1633 (s), 1445 (s), 1344 (s), 1156 (s),
47 (s).
8
Anal. Calcd for (5), C H Br HgN O (%): C, 36.83; H, 5.24; N, 5.60; Found: C, 36.37; H,
23
39
2
3
2
ꢀ1
5
.65; N, 5.65. FTꢀIR (KBr, cm ) (Fig. S14): 2965 (s), 1646 (s), 1451 (s), 1261 (s), 1098 (s),
50 (s).
8
Synthesis of complex of HgI with L1ꢀL2 [[Hg(L1)I ] (3), [Hg(L2)I ] (6)]
2
2 2
2 2
Methanolic solution of HgI
2 2
(0.45mmol, 0.20g) and L1 (0.30mmol, 0.10g) for (3) or HgI
(0.38mmol, 0.17g) and L2 (0.26mmol, 0.10g) in acetonitrile, for (6), were added to a 50 ml
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RBF and the mixture was refluxed for 1 hr. The reaction mixture was transferred to a conical
flask, where slow evaporation gives colorless crystals within a week.
Anal. Calcd for (3), C38
Found: C, 28.84; H, 3.98; N, 5.30. FTꢀIR (KBr, cm ) (Fig. S15): 2971 (s), 1634 (s), 1445 (s),
345 (s), 1171 (s), 1036 (s), 822 (s).
62 4 2 6 4
H I Hg N O (%): Calculated: C, 28.96; H, 3.97; N, 5.33; (%)
ꢀ1
1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
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60
Anal. Calcd for (6), C46
H
78
I
4
Hg
2
N
6
O
4
(%): C, 32.73; H, 4.66; N, 4.98; Found: C, 32.75; H,
.61; N, 4.95. FTꢀIR (KBr, cm ) (Fig. S16): 2956 (s), 1639 (s), 1445 (s), 1264 (s), 1096 (s),
54 (s).
ꢀ1
4
8
SingleꢀCrystal Xꢀray Diffraction.
Xꢀray crystallography
Xꢀray quality crystals were obtained by slow evaporation from methanol or
methanol/acetonitrile mixture. Xꢀray data of L1 and (1ꢀ6) complexes were processed by
SAINT. Lorentz and polarization effects and empirical absorption corrections were applied
using SADABS from Bruker. The structures were solved by direct methods, using SHELXSꢀ
1
42
2
97
and refined by fullꢀmatrix least squares refinement methods based on F , using
143
SHELXLꢀ97.
All nonꢀhydrogen atoms were refined anisotropically. All hydrogen atoms
were fixed geometrically with their Uiso values 1.2 times of phenylene carbons. All
1
44
calculations were performed using Wingx package.
Crystal data and structure refinements
3
are given in Table 1. Refinement of 1 showed residual electron density of ~7 eÅ (at 1.93Å)
3
from Cl4 and a peak of ~4 eÅ very close (0.21Å ) from Cl2. Inclusion of the former as
C/N/O atoms gave chemically irrational structure. Attempts to refine it as disordered Cl4 by
splitting it at the two positions with variable sofs, did not help as Cl4 moved to its original
155
position with full site occupancy. A use of SQUEEZE routine of PLATON,
to this data
did not show any voids or SAVs (solvent accessible voids) which suggested that there is no
unaccounted electron density. Therefore these peaks were ignored as spurious peaks which
may be an outcome of any unresolved disorder in the corresponding chlorine atoms. However
the structure did not show any disorder except for two terminal methyl groups C18, C19.
3
Refinement of complex 5 showed a residual electron density peak of ~4 eÅ close to (0.77 Å)
from heavy atom Hg which may be due to series termination error because appropriate
absorption correction had been applied. Cifs of both the structures showed A level alerts due
3
to this. Residual peaks (~1ꢀ2.5 eÅ ) were also seen around Hg in all other complexes and in 3
and 6 around I as well for the same reason and resulted in some ‘C’ level alerts. Structures of
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L1, 1, 3, 4 and 6 showed small to moderate amount of disorder in alkyl groups in terms of
high thermal parameters. As this is typical of the terminal alkyl groups therefore this disorder
was not resolved. Only in L1 the refinement was converged using restraints over CꢀC
distances.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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0
Acknowledgements. GH and LKR are thankful to University Grants Commission (India) for
research grant and fellowship, respectively under UPE (University with potential for
excellence) scheme for GNDU. S. S. is thankful to UGC for senior research fellowship.
Supporting Information. Tables S3ꢀS9 showing data for CꢀH···O/N Hꢀbonding
1
interactions; H NMR, IR and Mass spectra for L1 and L2; IR spectra for complexes 1ꢀ6.
Figures showing TGA plots and their descripton. Figures showing powder patterns for all
compounds. The cif and structure factor data are available from Cambridge structure data
base, free of cost, CCDC number = 1411204 ꢀ1411210. This information is available free of
charge via the Internet at http://pubs.acs.org/.
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