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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 Pt(II) or Pd(II) thiacrown complexes are prepared with chloro ligands, dechlorination is an initial step that must be achieved. We have now tried several methods including dechlorination with silver and thallium as well as reaction with triflic acid. Of these, the silver reaction is the most promising. The thallium reaction failed altogether. Although the triflic acid reaction showed initial promise, it fails to give full dechlorination and control of reaction stoichiometry was problematic. Our self-assembly process of the presumed fully dechlorinated species, [Pd(9S3)]2+, rather consisted only of [Pd(9S3)](Cl)+, so that its reaction with pyrazine produced the bridged binuclear complex, [{Pd(9S3)(Cl)}2(pyrazine)]2+. This binuclear complex is isolated as triflate salt and does contain an interesting extended structure which consists of chains of dimers running parallel to the b axis of the crystal. Intermolecular π-π interactions between the pyrazine rings account for the extended structure.

Silver dechlorination of the starting reagent [Pt(9S3)Cl2] in MeCN results in the formation of an unusual double salt which contains two [Pt(9S3)(MeCN)2]2+ cations along with a single [Pt(9S3)2]2+ cation. The presence of the coordinated acetonitrile in the structure importantly confirms its ability to bind to the Pt(II) in a cis bis fashion which is a key factor in the self-assembly process into a molecular square. We have also examined the self-assembly formation of our previously reported molecular square, [{Pt([9]aneS3)(bipy)}4)](OTf)8, focusing on conditions of solvent, time, temperature, and concentration in order to maximize square formation.

We have also studied the properties of our reported molecular square. Measurements using 19F NMR spectroscopy on the square, [{Pt([9]aneS3)(bipy)}4)](OTf)8, show that the triflate ions migrate out of the square cavity in solution, in contrast to its solid state solution which shows half of the triflates within the square cavity. 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 wish to report that in CD3CN, we observe the square forming a second coordination polymer, presumably a molecular triangle. Efforts are underway to confirm the identity of the coordination polymer using DOSY NMR and electrospray mass spec. We are also studying effects of temperature and concentration on the position of the equilibrium and investigating how different linker ligands affect the relative concentrations of square to triangle.

As noted above with the Pt(II and Pd(II) chemistry, the starting Ru(II) complex, [Ru(12S4)(dmso)(Cl)](PF6) must be dechlorinated prior to self-assembly. We now have a successful procedure for the removal of Cl in these complexes, based upon Ag(I) reactions. We are working on self-assembly of the Ru complexes into molecular squares and triangles. In contrast, the dechlorination of the related Rh(III) complex, [Rh(12S4)(Cl2)](PF6) has proven a challenge. To date, we have not found a method to yield full dechlorination of the Rh center. The higher cationic charge of the Rh versus the Ru is probably the source.

We have also explored complexes of the formula [M(9S3)(EPh3)2](PF6)2 (E = P, As, or Sb) as alternative corners for self-assembly. If M = Pd(II) or if less than two EPh3 ligands are present, the geometry of the complex is an elongated square pyramid. The geometry is now observed in over seventy complexes. However, if M = Pt(II) and two EPh3 ligands are present, a distorted trigonal bipyramidal geometry is seen. The distortion towards a trigonal bipyramidal shape is due to intramolecular π-π interactions between two phenyl rings on each of the EPh3 ligands. There is no ligand donor effect, and all three Group 15 donor complexes show the same degree of distortion. The two phenyl rings need to approach within 3.7-3.8 Å and need to be roughly parallel (the mean least squares planes between the phenyl rings needs to be less than 10°). This is an unusual case of π-π intramolecular interactions changing the overall geometry in a coordination complex.

In this series of complexes involving Group 15 donors, we have identified several periodic trends. As the Group 15 donor ligands becomes larger (and a poorer donor ligand), we see shorter M-S bond distances, weaker ligand fields, line broadening in their multinuclear NMR spectra, and a progressive upfield chemical shift in their 195Pt NMR resonances. There is also a noted difference in the reactivity between the Pt(II) and Pd(II) species. For example, we were able to prepare Pt(II) complexes with SbPh3, but could not with the Pd(II) analogs. The complex [Pt(9S3)(SbPh3)2](PF6)2 has the shortest Pt-S distances observed in approximately 60 crystal structures of 9S3 in Pt(II) complexes. This is a consequence of the poor donor qualities of the triphenylstibine.