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The goal of this research is to extend metallosupramolecular chemistry to employ metal complexes with tridentate macrocyclic ligands as alternative vertices in the formation of molecular squares.  The focus of the Petroleum Research Fund Award is on the use of platinum group metal complexes as vertices in the preparation of molecular squares through self-assembly processes.   We have focused primarily on the trithiacrown ligand 1,4,7-trithiacyclononane (9S3) in Pt(II) and Pd(II) chemistry and the tetrathiacrowns, 1,4,7,10-tetrathiacyclododecane (12S4) and 1,5,9,13-tetrathiacyclohexadecane (164), with Ru(II) and Rh(III).

Since the starting metal thiacrown complexes are prepared with chloro ligands, dechlorination is an initial step that must be achieved.  We have tried unsuccessfully to remove chloride from Rh(III) for the two previous summers.  In 2010, we have discovered that preparation of an alternative complex, [Rh(12S4)Br₂]Br , yielded a precursor that underwent debromination.  A solid-state reaction with excess of silver nitrate results in full debromination to produce a complex, [Rh(12S4)(H₂O)₂](NO₃)₃.  Efforts are underway in our laboratories to use this as a corner in self-assembly processes to form a Rh(III)-containing molecular square.

We have also studied the properties of our reported molecular square, [{Pt([9]aneS₃)(bipy)}4)](OTf)8. Our initial report on the square described multinuclear NMR spectra obtained in CD₃NO₂, and we have observed no change in the molecular square over a multiday period in that solvent.  However, we reported that in CD₃CN, we observe the square forming a second coordination polymer which we postulated to be a molecular triangle.  Through collaborations with Professor Markus Germann at Georgia State University, we were able to obtain DOSY NOESY, and ROESY NMR data on the mixture.  Unfortunately, the rates of diffusion for both species in acetonitrile were too similar, and we were not able to distinguish each polygon.  Efforts are underway to repeat these studies in nitromethane which has a higher viscosity.  The higher viscosity should slow down the rates of diffusion and enable a better differentiation between the two polygons.  However, NOESY NMR data from Professor Germann clearly showed that the two species were comprised of a larger and smaller polygon, consistent with the presence of a square and triangle.  Also, through collaborations for high resolution electrospray mass spec data with Dr. Thomas Blake at the CDC in Atlanta, we were able to confirm the molar masses of the two components as consistent with a square and triangle. 

We have continued our studies involving equilibria between squares and triangles involving other systems.  Proton, ¹³C, and ¹⁹⁵Pt NMR spectroscopy have proven critical in this regard.  We have examined the variation among the three main components of the polygons.  Viewing the coordination polymer as a {M(T)(L)}_x species (x = 3 or 4), where M is the metal ion, T is the capping ligand, and L is the linker ligand, we have prepared a series of potential square/triangle species.  We have focused on Pt(II) or Pd(II) as the metal ions to direct the self-assembly processes.  We note that the Pd(II) polygons are much less stable as expected based upon the relative relates of ligand substitution for the two metal ions.  Most of our attention has been on the use of the trithiacrown, [9]aneS₃ , as the capping ligand.  However, we are expanding to include the cyclic triamine, [9]aneN₃, which may offer some possibilities in molecular and ion recognition using hydrogen bonding.  Lastly, we have explored a series of L or linker ligands to see how the organic structure of the linker ligand may affect the square/triangle equilibrium ratio.  To date, we have examined the following linkers: 4,4’-bipyridine (bipy), pyrazine, 2,7-diazapyrene (diaz), and 1,2-bis(pyridyl)ethylene (bpe).  Interestingly, only with the 4,4’-bipyridine ligand do we observe the square/triangle equilibrium being influenced by solvent.  The latter two ligands show solvent independent equilibria (we observe both polygons) whereas the pyrazine shows only the square ,  only in nitromethane, and only with Pt(II).  In MeCN, there is a total degradation of the pyrazine square.  We attribute the properties of the pyrazine linker to its steric rigidity as well as an electronic affect for having the second nitrogen donor within the same ring system.  For polygons formed with diaz and bpe, we note an enhanced amount of triangle relative to square compared to bipy.  Also, we have obtained a crystal structure of the molecular square, [{Pt([9]aneS₃)(bpe)}4)](OTf)8 .  In contrast to the reported bipy molecular square, all anionic triflates are excluded out of the square cavitiy.  Lastly, we are also attempting to measure quantitatively the square triangle equilibrium constants for these systems as well as other thermodynamic parameters by obtaining van’t Hoff plots from variable temperature NMR data.