Synthesis, characterization and self-assembly of three dicyanamide bridged pol-ynuclear copper(II) complexes with N 2 O donor tridentate Schiff bases as blocking ligands

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Introduction
Coordination polymers with various architectures such as discrete square, linear chain, zigzag chain, square network, honeycomb, square grid, diamondoids etc have been developed in last few decades [1][2][3][4][5][6][7].The importance of the design and synthesis of specific architecture lies mainly in the fact that several properties of coordination polymers are directly linked with their structures and topology [8][9].The ability of pseudo-halides, especially azide, cyanate and thiocyanate, to bridge metal ions in end-on, end-to-end, a combination of both and many other modes, it is very common to use pseudohalides for the syntheses of such systems [10][11][12][13][14][15].
Focusing to copper(II), dicyanamide bridged polynuclear copper(II) complexes have received considerable attention for their potential applications in bioinorganic modeling chemistry [32], magnetic materials [33] and catalysis [34].They could also be used to explore interesting supramolecular interactions in them.Several well established non-covalent interactions; e.g.hydrogen bonding, π•••π stacking, C-H•••π forces etc., have widely been used to organize such supramolecular assemblies [35][36][37][38][39].  (3).In all three complexes, copper(II) centres are bridged by dicyanamide in end to end fashion, as established from the single crystal X-ray crystallographic studies.Complexes 1 and 2 are zigzag polymers, whereas complex 3 is a helical one.Herein, we would like to report the synthesis, spectroscopic characterizations, crystal structures and supramolecular assemblies of three new end-to-end dicyanamide bridged polymeric copper(II) complexes.

Experimental Section
All chemicals were of reagent grade and used as purchased from Sigma-Aldrich without further purification.

Preparations
A methanol solution (10 mL) of N,N-dimethyl-1,2-diaminoethane (0.10 mL, 1 mmol) and 1-hydroxy-2-acetonaphthone (186 mg, 1 mmol) was refluxed for ca. 1 h to form a tridentate Schiff base, HL 1 .The ligand was not isolated.A methanol solution (5 mL) of copper(II) acetate monohydrate (200 mg, 1 mmol) was added into the methanol solution of the protonated ligand HL 1 with constant stirring.A methanol:water solution (5 mL) of sodium dicyanamide (89 mg, 1 mmol) was added into the reaction mixture to get a dark green solution.The stirring was continued for ca. 2 additional h.Dark green single crystals, suitable for X-ray diffraction, were obtained after few days by slow evaporation of the solution in open atmosphere.

Synthesis of [Cu
It was prepared in similar method as that of complex 1, except that N-ethyl-1,2diaminoethane (0.10 ml, 1 mmol) was used instead of N,N-dimethyl-1,2-diaminoethane.Single crystals, suitable for X-ray diffraction, were obtained on slow evaporation of the solution in refrigerator.
Yield: 285 mg (74%  It was also prepared in a similar method as that of complex 1, except that N-methyl-1,2diaminoethane (0.10 ml, 1 mmol) was used instead of N,N-dimethyl-1,2-diaminoethane.Single crystals, suitable for X-ray diffraction, were obtained on slow evaporation of the solution.

Physical measurements
Elemental analyses (carbon, hydrogen and nitrogen) were performed using a PerkinElmer 240C elemental analyzer.IR spectra in KBr (4500-500 cm -1 ) were recorded with a PerkinElmer Spectrum Two spectrophotometer.Electronic spectra in acetonitrile were recorded on a PerkinElmer Lambda 35 UV-visible spectrophotometer.Powder X-ray diffraction was performed on a Bruker D8 instrument with Cu K α radiation.

X-ray crystallography
The structural analysis of complex 1 was performed on an Agilent SuperNova diffractometer with Atlas detector using mirror monochromatized Mo K α (λ = 0.71073 Å) radiation at 170 K. CrysAlis PRO program was used for data collection and processing [40].The intensities were corrected for absorption using the built-in absorption correction method [41].
The structure was solved with the program Superflip [42] and refined by full-matrix least squares on F 2 using the WinGX [43] software equipped with SHELXL-97 [44][45].All nonhydrogen atoms were refined with anisotropic thermal parameters.All hydrogen atoms were calculated to their optimal positions and treated as riding atoms using isotropic displacement parameters 1.2 larger than the respective host atoms.
Suitable single crystals of complexes 2 and 3 were used for data collection using a 'Bruker SMART APEX II' diffractometer equipped with graphite-monochromated Mo-K α radiation (λ = 0.71073 Å) at 170 K.The molecular structure was solved by direct method and refined by full-matrix least squares on F 2 using the SHELX-97 package [44][45].Non-hydrogen atoms were refined with anisotropic thermal parameters.The hydrogen atoms attached to nitrogen atoms were located by difference Fourier maps and were kept at fixed positions.All other hydrogen atoms were placed in their geometrically idealized positions and constrained to ride on their parent atoms.Multi-scan empirical absorption corrections were applied to the data using the program SADABS [46].Details of crystallographic data and refinements are given in Table 1.

Hirshfeld surfaces
Hirshfeld surfaces [47][48][49] and the associated 2D-fingerprint [50][51][52] plots were calculated using Crystal Explorer [53] which accepted a structure input file in CIF format.Bond lengths to hydrogen atoms were set to standard values.For each point on the Hirshfeld isosurface, two distances d e , the distance from the point to the nearest nucleus external to the surface and d i , the distance to the nearest nucleus internal to the surface, were defined.The normalized contact distance (d norm ) based on d e and d i was given by where r i vdw and r e vdw were the van der Waals radii of the atoms.The value of d norm was negative or positive depending on intermolecular contacts, being shorter or longer than the van der Waals separations.The parameter d norm displayed a surface with a red-white-blue color scheme, where bright red spots highlighted shorter contacts, white areas represented contacts around the van der Waals separation, and blue regions were devoid of close contacts.For a given crystal structure and set of spherical atomic electron densities, the Hirshfeld surface was unique [54] and it was this property that suggested the possibility of gaining additional insight into the intermolecular interaction of molecular crystals.There is no significant hydrogen bonding interaction in complex 1.  4.

Result and discussion
The X-ray crystal structure determination reveals that copper(II) centers are bridged singly by end to end dicyanamide with the formation of both P and M helical chains (Figure 3).
Important bond lengths and bond angles are listed in Table 2.The asymmetric unit consists of a copper(II) centre, one deprotonated Schiff base ligand and a dicyanamide anion.Each copper(II) centre is coordinated equatorially by one amine nitrogen atom, N(1), one imine nitrogen atom, N  5.

IR and electronic spectra and X-ray powder diffraction pattern
In the IR spectra of complexes 1-3 strong and sharp bands at 1577, 1579 and 1581 cm -1   respectively are routinely noticed due to azomethine (C=N) groups of Schiff bases [55].Two bands at 3382 and 3387 cm -1 respectively in the IR spectra of complexes 2 and 3 are observed due to N-H stretching vibrations [61].For all complexes 1-3 three bands in the region 2170-2300 cm -1 indicate the presence of dicyanamides [62].
The electronic spectra of each complex in acetonitrile display a single broad absorption band due to d-d transitions around 600 nm [63].Copper(II), in square pyramidal environment, usually have three transitions in between of 2 A 1g ← 2 B 1g , 2 B 2g ← 2 B 1g , and 2 E g ← 2 B 1g states.The broad absorption band is due to two overlapping bands corresponding to 2 B 2g ← 2 B 1g , and 2 E g ← 2 B 1g states [64].The UV absorption bands around 310 nm may be assigned to intra ligand nπ * transitions of azomethine (C=N) function of Schiff base [65][66].The band around 385 nm may be attributed to LMCT transition from the N donor centres of Schiff base to copper(II).
The experimental PXRD patterns of the bulk products are in good agreement with the simulated XRD patterns from single-crystal X-ray diffraction, indicating consistency of the bulk sample.The simulated patterns of the complexes are calculated from the single crystal structural data (Cif files) using the CCDC Mercury software.

Hirshfeld surface analysis
The Hirshfeld surfaces of the complex, mapped over d norm (range of -0.

Concluding Remarks
In conclusion, we report here the synthesis and characterization of three end to end dicyanamide bridged polynuclear copper(II) complexes (1-3) with three very similar tridentate Schiff bases as blocking ligands.In each complex, one tridentate Schiff base ligand and dicyanamide anion occupy the equatorial positions of copper(II) in similar fashion.A nitrogen atom from a symmetry related bridging dca coordinates axially at a rather long distance furnishing an elongated square-pyramidal (4 + 1) geometry for each copper(II) center.
Complexes 1 and 2 form zigzag chains, whereas complex 3 forms both P (right handed) and M (left handed) helical ones.There is no significant hydrogen bonding interactions in complex 1, but complex 2 forms a 2D sheet by interchain hydrogen bonding interactions.The adjacent P and M helices of complex 3 are interconnected by hydrogen bonding interactions to generate a 2D supramolecular sheet structure.All three complexes show significant C-H•••π interactions.
Hirshfeld surface analysis was used for visually analyzing intermolecular interactions in the crystal structures.Surfaces mapped with d norm help to envisage hydrogen bonding interactions.
Fingerprint plots reveal the percentage of intermolecular contacts (O⋯H and N⋯H) in the complexes.Hydrogen atoms of ethyl groups attached to amine nitrogen atoms is not shown for clarity.

Figure 1 :
Figure 1: Perspective views of complexes 1 and 2 with selective atom numbering scheme.

Figure 2 :
Figure 2: Sheet structure formed by inter chain hydrogen bonding interactions in complex 2.

Figure 3 :
Figure 3: Perspective views of P and M helical chains of complex 3 with selective atom

Table 1 :
Crystal data and refinement details of complexes 1-3.