Engineering of Gold(I)-Zinc(II) bimetallic complexes for visible
light emitting materials: a structural – spectroscopic study.
E. Priola
*.
(1), A. Giordana.
(1)E. Bonometti,
(1)E. Diana,
(1)R. Rabezzana,
(1)et Lorenza Operti
(1)(1) Dipartimento di Chimica, Università di Torino, Via Pietro Giuria 7, Torino,
*contact : [email protected]
Introduction
2,2’-bipyridine family
2-(2′-pyridyl)-1,8-naphthyridine (pyNP)
1-Pyridyl-3-phenyl imidazo[1, 5-a]pyridine
derivatives
Luminescence properties in solid
state and solution behaviour.
Conclusions
In the solid state, the formation of the supramolecular network generate a clear visible shift respect to the free ligand and respect to pure Zinc(II) compound for all ligands. We try to analyze the same behavior in solution of different solvents and different counterions, but the results seem to demonstrate the formation of these compounds only in the solid state. This is, however, in contradiction with ESI mass spectra of the same solution, that seem to demonstrate the existence of this fragment at least in gas phase.
- The high lateral steric hindrance of this family of ligands make difficult to couple dicyanoaurates to form Au….Au contacts.
- The overall pattern can be
explained with quadrupolar
coupling and a mixture of weak C-H…N and π- π stacking.
- In this case, Zn(II) sites are
completely surrounded by six nitrogen donors, preventing the formation of strong hydrogen bonding that could support the aurophilicity.
(3) {Zn(impy)2[Au(CN)2]2}.DMSO
(4){Zn(pyNP)2[Au(CN)2]}[Au(CN)2] (5) {Zn(pyNP)[Au(CN)
2]2}.ACN
(2) {Zn(dmbipy)2[Au(CN)2](H2O)}[Au(CN)2] (1) {Zn(bipy)2[Au(CN)2](H2O)}[Au(CN)2]
This ligand has three possible coordinating sites, but the naphtiridine bite is too small to be occupied by a second Zn(II). The coordination is thus similar to an asymmetric bipy. However, its different electronic characteristics make the trigonal bipyramid more stable than an octahedral geometry completed by water, although the solvent of crystallization is always Ethanol. The absence of hydrogen bond, however, doesn’t prevent this compound to form strong trimers interacting through strong aurophilic interactions. From Acetonitrile has been obtained a mono chelated compound. The occurrence of this phase is probably due to higher steric hindrance of this molecule that make more difficult and not immediate a second chelation. In this case, only two vertex of the trigonal bipyramidal coordination polyhedron of zinc has been blocked by organic molecules, and three dicyanoaurates can interact. This form a wavy 1D chain that through aurophilic interaction became a 2D interacting layer.
Bimetallic compounds of gold(I) are of great interest for the multitude of properties they exhibits, from strong luminescence to magnetism, vapochromism or exotic high pressure and high temperature behavior. [1] Most of these properties has been connected with the appearance of aurophilic interactions, defined by Schmidbaur as “ the tendency of gold(I) centers to attract other gold(I) centers to distances often beyond that of metallic gold”. [2] This relativistic interaction, however, often need to be supported of engineered to be manifested, especially in molecular compounds. We synthetized and analyzed a series of Zn(II)-Au(I) formed with two chelating ligands bonded to zinc, that make only two or one site of the coordination sphere open to the attack of the linear bridging building block dicyanoaurate. By changing different families of chelating ligands with different electronic and steric behaviors, we obtained and characterized five new molecular materials in very pure and highly crystalline forms from simple self aggregations. The phenomena involved in crystal growth has been studied. The solid state luminescence, interesting for zinc materials, has been measured and analyzed.
2,2’-bipyridine coordinates, as expected from the thermodynamics, four sites of zinc centers, making possible the attack of a dicyanoaurate and of a very tightly bonded water molecule (it cannot be substituted from other coordinating solvents like alcohols, DMSO, ACN or DMFA, and). This last solvent molecule is very important in stabilizing the framework with strong hydrogen bonds to cyanide; this is demonstrated by the amorphous nature of the same product obtained in anhydrous media. These hydrogen bonds stabilize the strong charge assisted aurophilic interactions forming a 2D pattern.[3] This behavior is similar to that observed in the homologous complex of phenanthroline.[4]
Water molecules or other hydrogen bond donor coordinated to zinc are not always guaranty of stabilization of metallophilic interactions. In the case of this poorly substituted bipy, the formation of a charge assisted hydrogen bond chain with no strict Au…Au contacts is preferred
A series of supramolecular architecture based on a Zn(II)-Au(I) could be obtained by coordination compounds with general stoichiometry {Zn(L)2[Au(CN)2]}[Au(CN)2] ( L= chelating ligands (1) bipy, (2) dmbipy, (3) impy and (4) pyNP) has been obtained with
high purity and crystallinity although the presence of complex equilibria in solution. The pattern in the solid state is strictly dependent on electronic and steric properties of L, and a moltitude of weak forces like aurophilic interactions, hydrogen bonds or π – π stacking are foundamental in stabilyzing the framework. All these interaction has been analized in solutions and seem to appear during crystal growth process, although further studies are needed. All these materials have a strong orange/yellow luminescence in the solid state, respect to the UV/blue luminescence of ligands or zinc complexes alone.
Distance (A°)
[Zn(bipy)2(M(CN)2)(H2O)][M(CN)2]
(M = Ag, Au)
Distance (A°) for
[Zn(phen)2(M(CN)2)(H2O)][M(CN)2] (M = Ag, Au) Ag1-Ag2 3.271(2) 3.046(2) Ag1-Ag2’ 3.552(2) 3.057(2) Au1-Au2 3.267(1) 3.118(1) Au1-Au2’ 3.626(1) 3.206(1) ‘ = x, y, z-1
Crystal of (4) from Ethanol solution Crystal of (1) from Ethanol solution
Bibliography
[1] J. Lefebre, R.J. Batchelor, D.B. Leznoff, J. Am. Chem. Soc., 2008, 130, 10662-10673.
A.B. Cairns, J. Catafest, C. Levelut, J. Rouquette, A. van der Lee, L. Peters, A.L. Thompson, V.Dmitriev, J. Haines, A.L. Goodwin, Nature Mat., 2013, 12, 212-216.
[2] H. Schmidbaur, A. Schier, Chem. Soc. Rev., 2012, 41, 370-412. [3] E.R.T. Tiekink, Coord. Chem. Rev., 2014, 275, 130-153.
