60th Annual Report on Research 2015

Overview: Progress in the first grant year has focused on the synthesis and characterization of iron(II) dibromide complexes supported by conjugated a-diimine ligands. We have investigated two related ligand classes, the aryl bis(imino)acenaphthene (Ar-BIAN) ligands and the pendant donor-modified aryl bis(imino)acenapthene tridentate ligands. The following sections highlight the progress we have made in both areas.

A. Synthesis, Characterization, and Electronic Structure Studies of Iron Dibromide Complexes Supported by Ar-BIAN Ligands

New anhydrous iron dibromide complexes (4a and 4b) bearing ^dppBIAN and ^MesBIAN ligands (dpp = 2,6-diisopropylphenyl; Mes = 2,4,6-trimethylphenyl; BIAN = bis(imino)acenaphthene) were prepared in two steps from commercially available starting materials (1 and 2, Figure 1) and were characterized by ¹H NMR and IR spectroscopy. The purity of both ^dppBIANFeBr₂ (4a) and ^MesBIANFeBr₂ (4b) were confirmed by elemental analysis and melting point measurements. The structures of both iron complexes were investigated by single crystal X-ray diffraction. The spin state for both complexes was investigated by Mössbauer spectroscopy and the magnetism was investigated by Evans method (NMR). Quantum chemical density functional theory computations were performed at the B3LYP level to provide insights into the electronic structure of these compounds; the computational results were found to be in acceptable agreement with the experimental metrical and Mössbauer parameters. Qualitative molecular orbitals and spin density plots were generated to describe the high spin Fe(II) electronic structure of the metal centers supported by a redox-innocent Ar-BIAN chelate for both 4a and 4b, providing a baseline for comparison of Ar-BIANFe complexes bearing redox non-innocent chelates. Subsequent in situ­ reduction of these iron complexes promoted the hydrosilylation of 1-hexene with phenylsilane at ambient temperature.

Figure 1. Synthesis of Ar-BIANFeBr2 complexes 4a/4b from commercial starting materials.

B. Preparation and Characterization of Iron Complexes Supported by Conjugated Tridentate Ligands

The synthesis of two pendant donor N-modified α-diimine bis(imino)acenaphthene (BIAN) ligands (^MesNNN or 7a and ^dppNNN or 7b), as well as iron(II) bromide complexes supported by these ligands (^MesNNNFeBr₂ or 6a and ^dppNNNFeBr­₂ or 6b) is described in Figure 2. The pendant donor is a pyridine ring connected to the conjugated BIAN ligand through a single-carbon bridge to make tridentate NNN ligands. The free ligands were successfully isolated as a tautomer of the bound ligand. Characterization methods included Nuclear Magnetic Resonance, Infrared spectroscopy, melting point, elemental analysis, X-ray diffraction, Mössbauer spectroscopy, and magnetic measurements by the Evans (NMR) method. Preliminary experiments indicate that in situ­ reduction of complex 6a promotes the hydrosilylation of hydrocarbons with phenylsilane at ambient temperature.

Figure 2. Synthesis of NNNFeBr₂ complexes 6a/6b and tautomeric ligands 7a/7b from commercial starting materials.

Impact: In the first grant year, we have prepared and characterized two different classes of iron complexes bearing conjugated a-diimine ligands that may behave in a redox-active fashion in catalytic hydrosilylation reactions. Our current focus is on exploring the catalytic activity of these complexes.

Four undergraduate research students were primarily supported by this PRF grant in the first 2014-2015 grant year which encompassed Summer 2014 and Summer 2015, and one additional undergraduate research student was primarily supported by the Ronald E. McNair Program with supplies provided by this PRF grant. This experience has afforded the students an opportunity to learn important experimental techniques and skills in inorganic chemistry, including the synthesis and handling of air-sensitive compounds, interdisciplinary characterization techniques with the physics department at Knox College, and communicating their research results through written reports and poster sessions at professional meetings. These experiences are extremely beneficial for the two students currently enrolled at Knox College as they apply this Fall to graduate programs. The three students supported fully or in part by this PRF grant in the Summer of 2014 all graduated in 2015 and are currently enrolled in PhD programs in chemistry at either Cornell University, Purdue University, or Washington University in St. Louis.

The results of this project have been disseminated through presentations at regional and national meetings. Both the PI and the students have presented at meetings, including the Midstates Consortium for Mathematics and Science 2014 Undergraduate Research Symposium at Washington University in St. Louis, MO, the 2015 Joint Great Lakes and Central Regional American Chemical Society Meeting in Grand Rapids, MI, and the 249th National Meeting of the American Chemical Society in Denver, CO.