Reports: UR351546-UR3: Polyaromatic Metal Complexes: Pathways to Graphene Sandwiches

Stephanie Hurst, PhD, Northern Arizona University

ACS Report for July 2011 to August 2012 – Stephanie Hurst 51546-UR3

The objective of this research proposal (51546-UR3) was to explore and develop bonding geometries of platinum and palladium complexes to give functional, one- and two-dimensional organometallic materials with a range of "tunable" properties.

We introduced platinum atoms into the sandwich motif through the use of [Pt02(dba)3] (dba = dibenzylideneacetone) in combination with our previously optimized methodology for the synthesis of the analogous palladium sandwich complexes. The new complexes were of general formula [Pt3Tr2X3]- (Tr = tropylium, C7H7+) and were confirmed through multi-nuclear NMR, microanalysis and single-crystal X-ray diffraction. EI-MS studies also confirms this synthesis with a set of peaks with the correct position and splitting pattern characteristic of the [Pt3Tr2Cl3]-, [Pt3Tr2Br3]- and [Pt3Tr2I3]- complexes. In addition we carried out 195Pt NMR studies in collaboration with Dr. Brian Cherry of the Magnetic Resonance Research Centre (MRRC) at Arizona State University (ASU), and these results confirmed the presence of a single strong peak for all three platinum nuclei in the sandwich complex. A systematic up-field shift was observed with different halogen ligands as anticipated, however no 195Pt-195Pt coupling was observed. This lack of coupling may be due to either a lack of coupling between the metal centres, or an identical conformation of the three platinum atoms which gives rise to the observed singlet signal. We also reported the formation a range of mixed-metal compounds of formula characteristic of the expected complexes [Pd3Tr2X3]-, [Pd2PtTr2X3]-, [PdPt2Tr2X3]- and [Pt3Tr2X3]- (X = Cl, Br or I) upon mixing of the [Pd02(dba)3] and [Pt02(dba)3] precursor complexes. Although we made significant attempts it was not possible to fully separate these mixtures into individual complexes. Our results on this work were recently published in the Journal of Organometallic Chemistry.

Our ongoing work in this area is to continue to investigate the degree of interaction between the metal atoms by probing the degree of ligand-metal and metal-metal coupling through replacement of the halide ligands by silver abstraction for NMR active nuclei e.g. MeC15N and 31PPh3. This effort will be considerably enhanced by the purchase of a 500 MHz NMR for the Department of Chemistry and Biochemistry for dedicated research use. Prof. Hurst was a contributor to this grant which leveraged local and state based resources.

In addition to their great diversity of applications in catalysis, palladium complexes are also of great interest in the formation of large multi-dimensional coordination systems. These unique systems range from the macrocyclic ring systems, self-assembled polyhedra and pseudo-infinite arrays based upon metal-organic frameworks, and these systems can achieve great molecular diversity through the application of a variety of "node" and "linker" systems. New metal-containing systems make nodes by possessing properties of stability to environmental conditions, multiple ligand-binding sites and ability to coordinate a range of organic compounds. We have demonstrated that the [Pd3Tr2]2+ system preferentially forms one-dimensional polymers of general formula [Pd3Tr2(X)3] (X = halogen or other anion), and we are currently working on a variety of ligand systems to extend this to infinite two-dimensional molecular structures.

We have found that it is possible to bind a range of nitrogen based ligands systems that will be essential to making large macro-molecular systems. We have synthesized the systematic systems [Pd3Tr2(X)3][Y2] (X = MeCN, PhCN, pyridine and others) (Y = tetrafluoroborate and triflate) and demonstrated through a variety of techniques including single-crystal X-ray diffraction analysis that the nitrogen lone-pair coordinates to the available site on the palladium atom. A ligand of particular interest was 4-cyanopyridine which possesses two atoms with available lone pairs, in this example preliminary comparison and analysis suggests that the pyridine is in the active site, leaving the nitrile moiety available for additional subsequent interactions with other metal ions that may stabilize multi-dimensional frameworks. We are currently preparing an article on this work for publication in a peer-reviewed journal such as the ACS journal Organometallics.

Another direction that our group is advancing the proposed research is the development of dimetallic metal systems based on the [M2(dba)3] (M = Pd or Pt, dba = dibenzylideneacetone) system. This complex [Pd2(dba)3] is an essential catalyst used around the world and especially in organic chemistry and the petroleum industry. The pi-conjugated system of the dba ligand stabilizes two zero-valent palladium or platinum atoms via eta bonding, and our group has been optimizing the organic ligand backbone by systematic substitution with a range of alkyl and alkoxy groups. We have observed structure-property relationships in both 1H NMR and UV-Visible spectra as well as systematic changes in the metal-metal bond length via single-crystal X-ray diffraction analysis. This critical information is helping us design larger catalytic system utilizing new organic groups such as the dicannamylidene acetone (dca) ligand.

The work reported here has been disseminated at a range of events. Prof. Hurst presented a poster on this work at the Gordon Inorganic Research Conference in June 2012. In addition graduate student Nathan Fisher presented work on the catalytic activity of the palladium sandwich complexes at the ACS national meeting in Denver, CO. Numerous undergraduates from freshman to seniors have been involved with this project and our group expects to publish an additional two papers on this work in the next year. In summary we have made an auspicious beginning and expect to continue this positive trajectory in the coming two years of the grant.