Reports: UNI350346-UNI3: Ruthenium and Osmium Complexes with Ligands Capable of Proton Coupled Electron Transfer

Jared J. Paul, PhD , Villanova University

The overarching goal of this grant proposal is to synthesize and study the electronic properties of metal complexes containing ligands capable of proton coupled electron transfer.  During the past year, we have synthesized a series of ruthenium complexes containing hydroxyl-substituted-bipyridine and -terpyridine ligands.  In the first year of the granting cycle, my lab has published one peer-reviewed ACS journal article in Inorganic Chemistry and given six presentations at local, regional and national conferences (including two presentations at the 241st ACS National Meeting with undergraduate presenters).  In addition, during the past year, one graduate student, one BS/MS student and five undergraduate students have worked on projects supported by the ACS-PRF grant. 

Ruthenium complexes with the 4,4'-dihydroxy-2,2'-bipyridine (bpy(OH)2) ligand.

 We have synthesized two complexes containing the bpy(OH)2 ligand, specifically, [Ru(bpy)2(bpy(OH)2)]2+ and [Ru(bpy(OH)2)3]2+ (bpy = 2,2'-bipyridine) as well as the methoxy-substituted analogs to be used as control complexes.  The methoxy-substituted complexes lack the ability to be deprotonated unlike the hydroxy-substituted complexes, however the ligands share similar electronic properties. 

[Ru(bpy)2(bpy(OH)2)]2+ was synthesized by Samantha Klein (MS, 2011) who is also first author on the paper.  We obtained crystallographic data on both the protonated form and deprotonated form of the complex as well as performed electrochemical, spectroscopic and computational studies on the complexes under different conditions of solvent and pH.  Summarily, the metal complex’s electronic properties are sensitive to both the polarity and hydrogen bonding ability of the solvent.  Most notably, we have determined that once deprotonated, the bpy(O-)2 ligand mixes heavily with the metal center and leads to new mixed Metal-Ligand to Ligand Charge Transfer transitions to the unsubstituted-bipyridine ligand.  These transitions appear in the visible region of the absorbance spectrum.  Two additional undergraduate students have performed studies on the complex, Sarah Pattison (BS, 2011) and Kyle Hufziger (BS, 2012).  This past summer, Kyle was funded by this grant.  He has been carrying out steady-state emission studies on [Ru(bpy)2(bpy(OH)2)]2+ to determine quantum yields as a function of solvent and protonation state.  The complex becomes almost completely quenched when deprotonated.  As a result of this observation, we are working towards learning more about quenching mechanisms in the complex.  As a result, we have begun a collaboration with Thomas J. Meyer at the University of North Carolina at Chapel Hill.  Kyle was able to spend a week over the summer at UNC-Chapel Hill working with transient absorbance spectroscopy and helping to further our collaborative efforts.  Our collaboration consists of studying N-methyl-4,4'-bipyridinium (monoquat), an excited state quencher that is also capable of accepting a proton from the complex in hopes of studying proton coupled electron transfer processes between the complex and monoquat.

[Ru(bpy(OH)2)3]2+ was synthesized by Michelle Fuentes (BS, 2011) who presented on the work at the 241st ACS National Meeting.  The complex varies from the mixed-ligand [Ru(bpy)2(bpy(OH)2)]2+ complex in that it is completely symmetrical.  As a result, upon deprotonation, there are no Metal-Ligand to Ligand Charge Transfer transitions, although there is a red shift in the absorbance spectrum.  We are currently performing computational studies on the complex to better understand the nature of this shift.  Richard Bognanno (BS, 2012) took over the project from Michelle and was funded this past summer by the grant.  Rich has synthesized and crystallized the methoxy-substituted analog of the complex.  In addition, we have begun to use a technique called spectroelectrochemistry to determine the spectroscopic signature of the complex when oxidized from RuII to RuIII.  Upon oxidation, the Metal to Ligand Charge Transfer (MLCT) transition decreases and a red-shifted shoulder appears.  The unsubstituted [Ru(bpy)3]2+ complex shows only a decrease of the MLCT band with no apparent red shift.  Additional oxidation studies using CeIV as an oxidant also show a decrease in the MLCT band with a new absorbance band appearing at lower energy.  We hypothesize that this low energy transition might be a Ligand to Metal Charge Transfer (LMCT) from the hydroxy-substituted bipyridine ligand to the oxidized metal-centered orbital.  Rich is currently gathering more information and we hope to have a manuscript submitted in the coming months on this project where both Michelle and Rich will be co-authors on the paper.  In addition, Rich was able to spend a week over the summer at UNC-Chapel Hill learning transient absorbance with our studies on [Ru(bpy)2(bpy(OH)2)]2+.

Ruthenium complexes with the 4'-hydroxy-2,2':6',2''-terpyridine (tpyOH) ligand.

We have synthesized two complexes containing the tpyOH ligand, [Ru(tpy)(tpyOH)]2+ and [Ru(tpyOH)2]2+ (tpy = 2,2':6',2''-terpyridine).  Preliminary work was begun by Alessa Wood (BS, 2010) and Kent Maghacut (BS, 2011; MS, 2012) took over the project this past year.  He presented this project at the 241st ACS National Meeting.  A significant advantage of the tpyOH ligand is that there is only proton per ligand, giving greater control of the protonation state.  Like the bpy(OH)2 complexes, upon deprotonation, there is a noticeable red shift if the MLCT bands and the shift is highly dependent upon the polarity and hydrogen-bonding ability of the solvent.  A series of electrochemical studies have demonstrated that the RuII/III redox potential decreases as the number of tpyOH ligands increases.  The redox potentials occur at 1.29 V vs. SCE for [Ru(tpy)2]2+, 1.19 V vs. SCE for [Ru(tpy)(tpyOH)]2+, and 1.10 V vs. SCE for [Ru(tpyOH)2]2+.  These potentials scale with the electron-donating capability of the tpyOH ligand, stabilizing the RuIII oxidation state.  We anticipate submitting a manuscript detailing our work during the coming year and both Alessa and Kent will be co-authors on the paper.   

In addition to the above described projects, Matthew Kasher (BS, 2011) has begun preliminary work towards the synthesis of bimetallic complexes by synthesizing tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine (tpphz).  He is currently working towards optimizing the synthetic process.

The ACS-PRF grant has been of great benefit not only to lifting my research program off the ground, but also training a significant number of students.  During the upcoming year, I plan to push the projects described above towards completion, continue to collaborate with UNC-Chapel Hill on excited-state proton coupled electron transfer in these complexes, and work towards the synthesis of bimetallic metal complexes.

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