Structural variations in the complexes of cadmium(II), hexamethylenetetramine, and 2-, 3- and 4-nitrobenzoates
Graphical abstract
Syntheses and structural characterization of four new cadmium(II)-)–hexamethylenetetramine (hmt) complexes with isomeric nitrobenzoates reveal that noncovalent interactions play a very important role in determining the final molecular structures of the self-assembled species.
Introduction
Metallo-supramolecular assemblies (both discrete and extended) have received a lot of attention recently as they often exhibit interesting molecular architectures and/or unusual properties [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. The supramolecular architecture is often constructed from small molecular and ion subunits by coordinate covalent bonds, hydrogen bonds, π-–π, C-–H/π and cation–π interactions [11], [12], [13], [14], [15], [16], [17], [18], [19]. The mode of assembly of such extended supramolecular architectures can be tuned and predicted by careful selection of the chemical structure of organic ligands. In spite of the humongous increase in the construction of diverse architectures, the control of dimensionality is still a major challenge as the network topologies based on molecular building blocks are usually controlled and modified by the selection of the coordination geometry of the central metal atom, the structure of organic ligands, the nature of solvent used, and the ratio of metal salt to organic ligand. It is obvious that by judicious choice of central metal atom and organic spacer, it is possible to create complexes with interesting architecture [20]. It has been observed that the polycyclic tertiary amine, hexamethylenetetramine (hmt), is a versatile ligand with the capability of adopting different coordination modes spanning from terminal monodentate to bridging bi-, tri- and tetradentate and the resulting coordination polymers exhibit enormous numbers of topologies including linear, zigzag, double, triple and quadruple one-dimensional chains as well as rectangular grids, flat and undulating two dimensional layers and cubic diamondoid complex three-dimensional networks [9], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. The metal ion Cd(II) is well suited for the construction of various coordination polymers, as its size and d10 electronic configuration permit a wide variety of coordination numbers and geometries specially with hmt [11], [31], [32], [33], [34], [35], [36], [37], [38].
It has been observed in many systems that weak forces combine to play a decisive role in supramolecular architecture and hence are quite important in crystal engineering for the design of functional materials [11], [39], [40], [41]. Among the various weak forces, cation–π interactions involving metal cations, particularly alkali metal cations, have been investigated most thoroughly [15], [16]. There are some examples of studies of cation–π interactions involving organic cations [15]. We would like to explore if an apparently innocent substitution in the phenyl ring of benzoate ion can bring about changes in the molecular architecture of their Cd(II) complexes with hexamethylenetetramine (hmt). Earlier we have investigated Cd(II)-)–benzoate system with hmt and found a wide variety of structures with μ2- and μ3-bridging modes of hmt [11]. Architectural changes viz. 1D chain to 3D polymer, are also observed for the analogous phenylacetate [11], with hmt in μ2 bridging mode and maleate [36] terephthalate [37] and malonate [38], where the hmt is in μ3 bridging mode.
Here we report the synthesis of four new cadmium complexes {[Cd(2-nbz)2(μ2-hmt)(OH2)]·2H2O}n (1), [Cd(3-nbz)2(hmt)2(OH2)2]·2(3-nbzH) (2), [Cd(3-nbz)4(μ2-hmt)]n (3) and {[Cd2(4-nbz)4(μ2-hmt)2(OH2)2]·H2O}n (4) with three isomeric nitrobenzoates, [2-nbz = 2-nitrobenzoate, 3-nbz = 3-nitrobenzoate and 4-nbz = 4-nitrobenzoate]. In both the complexes 2 and 3 the ligand is 3-nbz. 2 is a monomeric compound but 3 is polymeric in nature. The presence of coordinated water molecules along with the non-coordinated carboxylic acid, 3-nbzH in 2 may be responsible for this special topology. Various weak forces such as C-–H/π, π-–π, cation-–π interactions, along with H-bonds play an important role in stabilizing the different topologies.
Section snippets
Experimental
The nitrobenzoic acids, hexamethylenetetramine (hmt) and cadmium carbonate were purchased from Lancaster and were of reagent grade. They were used as received without further purification. Cd(2-nbz)2·2H2O and Cd(3-nbz)2·2H2O were prepared in our laboratory by reacting cadmium carbonate with 2-nitrobenzoic acid and 3-nitrobenzoic acid respectively.
{[Cd(2-nbz)2(μ2-hmt)(OH2)]·2H2O}n (1)
The structure of 1, a zigzag polymer, is shown in Fig. 1 together with the atomic numbering scheme in the metal coordination sphere. The cadmium atom is seven-co-ordinate with a distorted pentagonal bipyramidal structure being bonded to two bidentate ligands and a water molecule in the equatorial plane together with two nitrogen atoms from hmt ligands in axial positions.
The bond lengths to the oxygen atoms in the equatorial plane vary considerably with the bond to the water molecule
Conclusions
Four complexes are synthesized by using Cd(II), hmt and o-,m- or p-nitrobenzoate in water and characterized structurally. For all three isomeric nitrobenzoates (1, 3 and 4), zigzag 1D coordination polymers are formed whereas compound 2 is a monomer. The presence of a non-coordinated 3-nbzH which forms hydrogen bonds with hmt restricts the hmt from bridging the metal ions in 2. In 3, two nitrogen atom of hmt coordinate with Cd(II) ion with very low N-–Cd-–N bond angle which is unique. The
Acknowledgements
We thank EPSRC and the University of Reading for funds for the X-Calibur system for solving the structures 1 and 2. Crystallography for 3 and 4 was performed at the DST-FIST, India-funded Single Crystal Diffractometer Facility at the Department of Chemistry, University of Calcutta. One of the authors (S.H.) would like to thank University Grants Commission, India for funds provided through Minor Research Project No. F. PSW-006/10-11 (ERO).
References (50)
Coord. Chem. Rev.
(2011)- et al.
Coord. Chem. Rev.
(2003) - et al.
Polyhedron
(2012) - et al.
Inorg. Chem. Commun.
(2011) - et al.
Int. J. Mass Spectrom.
(2007) - et al.
Inorg. Chim. Acta
(2011) - et al.
Inorg. Chim. Acta
(2010) - et al.
Polyhedron
(2010) - et al.
Inorg. Chem. Commun.
(2009) - et al.
Polyhedron
(2007)
J. Mol. Struct.
Polyhedron
Chem. Rev.
Chem. Rev.
Chem. Rev.
Chem. Rev.
Dalton Trans.
Chem. Soc. Rev.
Cryst. Growth Des.
Chem. Commun.
Cryst. Growth Des.
Org. Biomol. Chem.
Chem. Commun.
Chem. Eur. J.
CrystEngComm
Cited by (13)
Variations of structures on changing the ratios of metal ions in rare Ca(II)–Zn(II) hetero-metallic self-assembled coordination polymers of hexamethylenetetramine and benzoate
2021, Journal of the Indian Chemical SocietySingle crystal growth, structural characterization and magnetic properties study of an antiferromagnetic trinuclear iron(III) acetate complex with uncoordinated hexamine
2021, Inorganica Chimica ActaCitation Excerpt :In this system neutral hexamine molecule is inserted. Usually, the hexamine molecule acts as a ligand [28–34] although there are several examples of hexamine being uncoordinated in presence of a transition metal cation [35–38]. In compound 1, hexamine molecules are uncoordinated but with significant impact upon formation of a supramolecular network.
Structural variations in self-assembled coordination complexes of hexamethylenetetramine, zinc(II) and carboxylates (RCOO<sup>−</sup>, R = –CH<inf>3</inf>/−C<inf>6</inf>H<inf>5</inf>): Encapsulation of the water hexamer in benzoate assembly
2018, Inorganica Chimica ActaCitation Excerpt :In this regard, the choice of ligand system, metal ions, ancillary ligands, solvent system, template, temperature etc are important factors. Hexamethylenetetramine (hmt) is quite a ubiquitous ligand in the formation of numerous coordination complexes with disparate networks topologies and has been widely used in the formation of considerable number of hmt-driven metal-organic networks in the last few years [8–15]. Hmt, soluble in water as well as in polar organic solvents, is a versatile ligand with the ability of implementing diverse coordination modes extending over terminal monodentate to bridging bi-, tri- and tetradentate.
Six new metal(II) complexes with 3D network structures based on carboxylate and hexamethylenetetramine: Syntheses and structures
2015, PolyhedronCitation Excerpt :A large number of coordination polymers with hmt as a spacer are reported [13–22], displaying linear, zigzag, double, triple and quadruple one-dimensional chains, as well as rectangular grids, flat and undulating two dimensional layers and cubic diamondoid complex three-dimensional networks. Besides coordinating to metal ions, hmt, due to its good H-bond accepting behavior, has the capability to form diverse molecular adducts and supramolecular assemblies [23–32]. In order to get a better understanding of the influence of the carboxylate residue in the formation of new coordination compounds, we have studied the metal–hmt–carboxylate system, also aiming to investigate the role that the weak non-covalent interactions play in forming the final supramolecular frameworks.
Synthesis, crystal structures and luminescent properties of Cd(II)-arenesulfonate complexes containing N-heterocyclic auxiliaries
2015, PolyhedronCitation Excerpt :The design and synthesis of Cd(II) coordination complexes have attracted considerable attention owing to their fascinating structural diversity, potential applications in semiconductors, catalysis and photoluminescence, as well as the physiological toxicity of the Cd(II) cation [1–4]. As an important family of multidentate O-donor ligands, organic polycarboxylate anions as bridging ligands have become excellent building blocks, which have been widely used to construct Cd(II)-carboxylates with intriguing structures and versatile properties [5–9]. In contrast, Cd(II)-sulfonates are much less investigated owing to the perception that sulfonate is viewed as a poor ligand arising from its weak coordination ability [10,11].