46641-B3
Self-Assembly of Molecular Squares from Platinum Group Metal Complexes with Thiacrowns and Related Ligands
The goal of this research is to extend metallosupramolecular chemistry to employ platinum group metal complexes with tridentate and tetradentate thiacrown ligands as alternative vertices in the formation of molecular squares. In the first year of the Award, we have focused on the initial syntheses of the Pt(II), Pd(II), Ru(II), and Rh(III) thiacrown complexes that will subsequently form the corners in the coordination polymer. These synthetic steps have gone well. We have used the nine-membered ring 1,4,7-trithiacyclononane (9S3) as a capping ligand in the Pt(II) and Pd(II) complexes which form elongated square pyramidal structures. Due to the electronic requirements of the d6 metal ions, we have employed the tetradentate 1,4,7,10-tetrathiacyclododecane (12S4) to coordinate with Ru(II) and Rh(III). All of these complexes are formed with two cis chloro ligands. The removal of coordinated chloride ions is the next required step prior to self-assembly. We have now employed three methods in this capacity for the metal ions. These methods are: 1) the use of triflic acid to replace chloride with triflate ions forming hydrogen chloride 2) the removal of chloride using thallium salts (triflate, hexafluorophosphate) to form thallium chloride 3) the removal of chloride using silver triflate to form silver chloride. The triflic acid method was successful for the Pt(II) and Pd(II) thiacrown complexes. However, the setup is tedious and the generated hydrogen chloride does pose some disposal problems for the reaction. We would note that the method only gave partial dechlorination with the Rh(III) complexes (one chloride removed), probably due to its higher cationic charge. The thallium dechlorination of the Ru(II) complexes proved disappointing. Due to solubility issues with the starting reagents and products, acetonitrile was the required solvent. We discovered that this solvent ligand could not be removed once present. Given the problems of Tl toxicity, we have abandoned this procedure altogether. The removal of chloro ligands in Ru(II) complexes using silver triflate can be achieved, provided careful attention is paid to the solvent selection and reaction conditions. We are currently improving this method. We have also studied the conditions that favor self-assembly to form molecular squares. We have found that solvent plays a key role in the promotion of the desired product. Also, we have examined the properties of our previously reported molecular square containing platinum(II)/thiacrown vertices. We have found that there is evidence for equilibrium between the molecular square and a molecular triangle in the self-assembly process. Furthermore, our reported crystal structure showed half of the anions found in the complex to be contained inside the cavity of the molecular square. Such inclusion could have applications in the field of anion recognition. Our initial square was prepared using 4,4'-bipyridine as a linear linker between metal complex vertices. We have now tried several other related linkers in this capacity including pyrazine and 2,7-diazapyrene. We now have a series of Pt(II) (5 complexes total) and Pd(II) (3 complexes total) self-assembled products that are currently being analyzed. Proton NMR data are the first attempt at characterization, but the definitive proof of a molecular square remains an X-ray crystal structure. Attempts are underway to obtain these data.
