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Research Progress.  This overall goal of this research is the preparation of bimetallic oxidation catalysts that mimic enzymes by employing tethered redox “cofactors” to accomplish multi-electron oxidations.  The original proposal called for the preparation of redox-active metallocene and Ruthenium-diimine derivatives of metal salen and porphyrin complexes and the characterization of their reactivity towards molecular oxygen.  In our last report, we described the synthesis of a ferrocene-containing pyridine derivative, 3, and preliminary studies of its ability to function as an axial ligand in conventional salen complexes. 

During the present grant period, we prepared an adduct between 3 with Co(salen), 5, and studied its reactivity towards molecular oxygen.  Our results are summarized in the scheme below:

UV-vis and IR studies of the reactivity of 5 towards molecular oxygen support the initial formation of a terminal dioxygen complex (6) followed by its eventual conversion to the bridging peroxide dimer (7).  Analogous reaction sequences are well-established reaction of Co(salen) derivatives with O₂ in pyridine.  The chief novelty of our system is that we can control the stability of 6 by manipulating 5's monomer-dimer equilibrium.  At low concentrations an appreciable quantity of 5 is present and both the formation of 6 and its conversion to 7 in occurs over a period of a few hours.  At high concentrations of 5, 6 is stable for several days.  We currently suspect that 6 is a Co^II-superoxide complex based on its UV-vis spectrum and its observed conversion to 7 in the presence of 5.  However, IR studies on air-exposed dichloromethane solutions of 5 show new bands at 1060 and 1027 cm^-1 consistent with a terminal superoxide as well as bands at 882, 818, and 792 cm^-1, which may indicate a weakened O-O bond.  We intend to resolve the issue of the O-O bond order (and the Fc oxidation state) during the next grant period using a combination of ¹⁸O labeling experiments, magnetic moment determinations, crystallography, and spectroelectrochemistry.

In addition to the above efforts, we pursued two parallel strategies for attachment of photoactive Ru-diimine complexes to Co(salen) complexes.  Our first strategy involved that preparation of 9, a [Ru(bpy)₂(phen)]²⁺-analogue of 3, by reaction of isonicotinoyl chloride with [Ru(bpy)₂(5-aminophen)]²⁺, as shown below.

The photochemical and redox properties of 9 were studied using UV-vis spectroscopy, emission spectroscopy, transient emission spectroscopy, and cyclic voltammetry and found to be comparable to other Ru-diimine complexes.  Unfortunately, we have so far been unable to study complex formation between 9 and Co(salen) due because the low overall yield (~3%) has prevented us from obtaining enough to study its ability to form adducts with M(salen) complexes and metalloporphyrins.  During the next grant period, we hope to obtain the desired quantities through a repeat of our current synthesis as well as increase the overall yield by exploring alternate conditions.

Our second strategy for the conjugation of photoactive Ru-diimine groups into M(salen) and related complexes involves EDC coupling between amine derivatives of [Ru(bpy)₂(phen)]²⁺ or [Ru(bpy)₃]²⁺  and carboxylic acid derivatives of metal salen complexes, as shown schematically below.

The synthesis of Co, Cu, and Ni variants of 10 was accomplished during the present grant period.  Initial attempts to couple [Ru(bpy)₂(5-aminophenanthroline)](NO₃)₂ to carboxylic acids gave low yields, presumably due to deactivation of the amine by the phen and Ru²⁺ center.  Consequently, we synthesized 11, an aliphatic amine derivatives of Ru(bpy)₂(4,4'-Me₂bpy)](NO₃)₂, using the synthesis shown below:

Our initial efforts focused on the synthesis of 11 from 4,4'-Me₂bpy via 13 and 14.  However, the yield was extremely low due to the poor yield for formation of 14 and significant deamination of 14 to give 4-ethenyl,5-Mebpy during the preparation of 11.  Because we surmise that deamination is occurring when the primary amine group coordinates to Ru, we are currently pursuing an alternate synthesis involving the metallation of 13 followed by deptrotection of the amine to give 11.

Career impact.   Since Westmont College chemistry department has been generous in providing funds for supplies and equipment, the majority of this grant provided summer support for three undergraduate researchers.  The work we accomplished during this grant period enabled us to developed valuable experience in the use of UV-vis, IR, ESI-MS, and cyclic voltammetry for characterizing metal-salen complexes and their oxygen reactivity.  In addition, we gained valuable experience with the synthesis of pyridine and aliphatic amine derivatives of Ru-diimine complexes.

In addition to my own professional development, the grant provided three different undergraduate students with research experience during the summer of 2010, all of whom are continuing work on the project this year.  These students, Bryan Brautigham, Lauren Bernau, and Ashley Greenawalt, will be completing their studies this year and are currently applying to graduate, medical, and dental schools, respectively.  In addition, Lauren is planning to present a paper on this work at the 2010 Southern California Conference for Undergraduate Research.  I anticipate using the remaining grant funds to purchase ¹⁸O₂ and for the support of 1 or 2 research students during the summer of 2011.