SYNTHESIS AND CRYSTAL STRUCTURES OF LUMINESCENT MONONUCLEAR Ni(II) AND Cd(II) COMPLEXES WITH 1,10-PHENANTHROLINE

Two new mononuclear coordination complexes constructed by non-covalent interactions, [Ni(phen)2(H2O)2](ClO4)2 (1) and [Cd(ClO4)2(phen)2] (2), where phen=1,10-phenanthroline, were synthesized and characterized by single-crystal X-ray diffractometry. The structural determination revealed that the coordination geometry around the Ni/Cd centres is distorted octahedral for each complex. In the complexes 1 and 2 the metal atoms have N4O2 coordination cores. In 1 the intermolecular OH···O hydrogen bonds, which involve the coordinating water molecules as donors and the perchlorate O atoms as acceptors form zigzag-like hydrogen-bonding chains, which are further assembled via π-π stacking interactions between adjacent phen rings, thus forming a supramolecular network structure. In the crystal structure of 2, π–π stacking interactions are noted between neighbouring hydrophobic phen ligands, yielding 1D supramolecular chains. The luminescence studies show that compound 1 displays blue luminescence, while compound 2 – white-light emission upon excitation with ultraviolet light.


Introduction
Supramolecular aggregates based on small molecules are held together in the crystal by weak attractive non-covalent interactions, such as hydrogen bonding, ionic interactions, metal coordination, π-π stacking or hydrophobic interactions.Non-covalent interactions play an important role in chemistry, physics and especially in biodisciplines [1][2][3].They determine the structure of biomacromolecules such as DNA and proteins and are responsible for the molecular recognition process [4,5].
1,10-Phenanthroline (phen) is a bidentate ligand that entails several appealing structural and chemical properties such as rigidity, planarity, aromaticity, basicity, chelating capability that makes it a versatile starting material for supramolecular chemistry.The rigid aromatic phenyl and pyridine rings on phen have the potential to generate π-π and C-H-π stacking interactions, which have great significance in the crystal packing and influence the architecture of the resulting crystal structures.Recently we have reported the synthesis and crystal structure of Cr(III) supramolecular systems containing phenanthroline and acetylacetonate ligands constructed by combining two or three organizing forces (metal coordination, hydrogen bonds and π-π stacking interactions), which demonstrate metal-to-ligand charge transfer (MLCT) nature.The phen ligand is coordinated to Cr(III) ions to form additional five-member rings, which increase the π-π* conjugation length and the conformational coplanarity [6].

Experimental
All reagents and solvents were obtained from commercial sources and were used without further purification.Caution!Perchlorate salts are dangerous and should be handled with care and only in small quantities.Cadmium salts are carcinogenic and toxic [7].Emission spectra were recorded for single crystals at room temperature on an Excitation YAG:Nd 3+ laser, third harmonic generation, λ= 337 nm, duration 10 ns, time repetition 10 Hz.

X-ray structure determination
Diffraction measurements for 1 and 2 were carried out at room temperature on an Xcalibur "Oxford Diffraction" diffractometer equipped with CCD area detector and a graphite monochromator utilizing MoK radiation.Final unit cell dimensions were obtained and refined on an entire data set.All calculations to solve the structures and to refine the proposed models were carried out with the SHELXS97 and SHELXL2014 programs [8,9].For compound 1 the perchlorate anions are disordered over two positions (80/20% and 65/35% occupancy).Hydrogen atoms attached to carbon atoms were placed in geometrically idealized positions and refined by using a riding model.
The figures were produced using the Mercury program [10].Crystallographic data for new structures reported herein were deposited with the Cambridge Crystallographic Data Centre and allocated the deposition numbers CCDC 1563783-1563784.These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Crystal structure analysis
Both coordination compounds were obtained in the identical synthetic conditions by mixing the warm methanolic solutions of the corresponding starting perchlorate salts, 1,10-phenanthroline and acetylacetonate.The final acetylacetonate-free crystalline solids were precipitated from the slowly cooled solutions.Compound 1 is a blue-violet crystalline solid; in addition to 1 in the same reaction, pink crystals of the known compound [Ni(phen) 3 ](ClO 4 ) 2 •0.5H 2 O [11] were precipitated.The X-ray data and the details of the refinement for both compounds are summarized in Table 1.Selected geometric parameters for 1 and 2 are given in Tables 2 and 3.
Only one type of π-π stacking interactions was registered in 2, between neighbouring pyridine-pyridine rings of hydrophobic phen molecules of mononuclear compound (centroid-to-centroid distance equal to 3.753 Å), yielding one-dimensional supramolecular ladder-like chains along the c direction (Figure 3(b), Table 4).The Cd•••Cd separation through the stacked ligands between neighbouring complexes is 9.443 Å.

Luminescent properties
The luminescence properties of complexes 1 and 2 as well as pure phen ligand were studied in the solid state at room temperature, λ ex = 337 nm (Figure 4).The free phen ligand fluorescence spectrum exhibits two intensive bands at 2.95 eV (420 nm) and at 2.8 eV (443 nm), and a little one at 1.85 eV (670 nm) (Figure 4), the presence of which can be assigned to ligand chromophore moieties.The corresponding Ni(II) and Cd(II) complexes present different emission peak ratios that create different colours.Compound 1 emits blue-green fluorescence with characteristic sharp emission peaks at 3.25 eV (381 nm), 2.50 eV (495 nm) and 1.85 eV (670 nm), while monomeric complex 2 gives white-light emission with characteristic emission peaks at 3.20 eV (387 nm), 2.95 eV (420 nm) and 2.40 eV b c a (516 nm).The same blue-shift emission was registered for the coordination compound with a similar composition [Cd(Hpac)(phen) 2 ]ClO 4 (where Hpac=pyrazole-3-carboxylic acid) [17].The authors explain that the emission bands in their spectrum come from a mixed ligand charge transition combining with a perchlorate-phen charge transition.

Conclusions
In summary, we have synthesized two bis-phenanthroline mononuclear complexes of Ni(II) and Cd(II) along with perchlorate counter ions.The smaller nickel(II) ion forms a cis complex with outer-sphere perchlorates, while the cadmium(II) ion forms a trans complex involving inner-sphere perchlorate ligands.Crystal packing in 1 reveals supramolecular 3D architecture governed by combination of OH•••O hydrogen bonds and stacking interactions between the planar adjacent phen fragments, while in 2 the crystal packing is managed exclusively through π-π stacking interactions between neighbouring pyridine rings of hydrophobic phen molecules, yielding one-dimensional supramolecular ladderlike chains.Compounds 1 and 2 reveal intraligand-based luminescent properties.

Figure 2 .
Figure 2. (a) The fragment of 1D hydrogen-bonding chain in 1.(b) Tape of the cation complexes supported by π−π stacking interactions along the a axis.(c) Two modes of phen overlapping; view perpendicular to the plane of the overlapping phen ligands.

Figure 3 .
(a) ORTEP view of the molecular structure of mononuclear compound in 2 with partial atomic numbering scheme.Displacement ellipsoids are drawn at the 30% probability level.(b) Tape of the complexes supported by π−π stacking interactions; view perpendicular to the plane of the overlapping phen ligands.

Figure 4 .
Figure 4. Solid-state luminescence emission spectra of compounds 1 and 2, as well as free phen ligand.