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.  In the second year of the granting cycle, my lab has published one peer-reviewed RSC journal article in Dalton Transactions, given four presentations at local and national conferences (including one presentation at the 243rd and one presentation at the 244th ACS National Meetings with undergraduate coauthors), and given three invited talks.  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 hydroxy-substituted bipyridine ligands.

This past year, Richard Bognanno (BS, 2012) continued the work begun by Michelle Fuentes (BS, 2011) in studying the two metal complexes, [Ru(4bpy(OH)2)3]2+ and [Ru(4bpy(OMe)2)3]2+ (4bpy(OH)2 = 4,4'-dihydroxy-2,2'-bipyridine; 4bpy(OMe)2 = 4,4'-dimethoxy-2,2'-bipyridine)  Both complexes are highly symmetrical containing all ligands of the same type.  The methoxy-substituted complexes lack the ability to be deprotonated unlike the hydroxy-substituted complexes, however the ligands share similar electronic properties when the hydroxy-groups are protonated.  The work was submitted and accepted for publication to Dalton Transactions during this granting cycle.  We have performed detailed experimental and theoretical work on these complexes and compared them to the less symmetrical [Ru(bpy)2(4bpy(OH)2)]2+ (bpy = 2,2'-bipyridine) reported in an earlier Inorganic Chemistry manuscript (also supported by ACS PRF).  Most notably, we have noticed that all of the hydroxyl-substituted complexes change their electronic properties when deprotonated with a shift of the visible absorbance bands to lower energy upon deprotonation.  For the mixed-ligand [Ru(bpy)2(4bpy(OH)2)]2+ complex, the deprotonated 4bpy(O-)2 ligand molecular orbitals mix with the metal center resulting in orbitals of the mixed metal-ligand type.  As a result, there are electronic transitions assigned as mixed Metal-Ligand to Ligand Charge Transfer.  In the more highly symmetrical complex, [Ru(4bpy(OH)2)3]2+, there is no metal-ligand molecular orbital mixing upon deprotonation of the ligands.  This result becomes apparent in the structure of the visible spectrum of the complex as well as electrochemical studies whereby a completely reversible redox wave is observed.  The oxidation of the mixed metal-ligand molecular orbitals observed in [Ru(bpy)2(4bpy(O-)2)] are not reversible.

In addition, we have begun a collaboration with Dr. Elizabeth Papish at Drexel University.  She has supplied us with another bipyridine ligand where hydroxyl substitution is ortho to the pyridyl nitrogens, 6bpy(OH)2 (6bpy(OH)2 = 6,6'-dihydroxy-2,2'-bipyridine).  I have synthesized and Kyle Hufziger (BS, 2012) carried out studies on the complex, [Ru(bpy)2(6bpy(OH)2)]2+.  We have presented this work at the 244th ACS National Meeting in Philadelphia, PA.  Having the substitution in the 6 and 6' position orients the hydroxyl group closer to the metal center than in [Ru(bpy)2(4bpy(OH)2)]2+, however, similar electron-donating properties can be observed through resonance upon deprotonation.  Notably, the RuIII/II reduction potential for [Ru(bpy)2(6bpy(OH)2)]2+ is 1.12 V vs. SCE compared to 1.16 V vs. SCE for [Ru(bpy)2(4bpy(OH)2)]2+, confirming the similar redox potentials of the complex.  We have obtained crystal structures of both the protonated and deprotonated forms of the complex.  The Ru-N distances for the hydroxy-bipyridine ligands are approximately 0.03 to 0.04 Å longer for [Ru(bpy)2(6bpy(OH)2)]2+ compared to [Ru(bpy)2(4bpy(OH)2)]2+.  We have also carried out several UV/visible absorption and emission studies on the complex as a function of protonation state.  We are currently preparing a manuscript on this work and are anticipating a submission of this manuscript by the end of the year.

Matthew Kasher (BS, 2012) prepared and carried out initial studies on [Ru(phen)2(4bpy(OH)2)]2+ (phen = 1,10-phenanthroline).  This work was presented at the 243rd ACS National Meeting in San Diego, CA.  David Charboneau (BS, 2015) continued work on this project during this past summer and was funded by the ACS PRF grant.  David improved upon the synthetic and purification procedure for the complex and also has worked towards the preparation of the 6bpy(OH)2 analog.  He is continuing work into this semester on both of these complexes.

Amanda Rasbach (MS, 2013) has made progress in the synthesis of an unsymmetrical bipyridine ligand, 4-methoxy-2,2'-bipyridine, 4bpy(OMe).  The goal behind this project is to synthesize a bipyridine-containing complex with only one deprotonatable group, which will allow for us to study only one distinct proton equilibria, rather than sampling multiple potential protonation states.  Amanda has successfully synthesized and characterized [Ru(bpy)2(4bpy(OMe)]2+ and is currently working on the synthesis of the corresponding mono-hydroxy-bipyridine complex, [Ru(bpy)2(4bpy(OH)]2+ (4bpy(OH) = 4-hydroxy-2,2'-bipyridine).

Ruthenium terpyridyl complexes with hydroxyl-substituted polypyridyl ligands.

Kent Maghacut (BS, 2011; MS, 2012) continued work on [Ru(tpy)(tpyOH)]2+ and [Ru(tpyOH)2]2+ (tpy = 2,2':6',2''-terpyridine; tpyOH = 4'-hydroxy-2,2':6',2''-terpyridine).  Kent carried out a series of spectroelectrochemical studies on these complexes and we are currently carrying out computational studies to analyze the spectroelectrochemical data in collaboration with Dr. Timothy Dudley at the University of Minnesota – Crookston.  We are continuing to work on a manuscript on this work.

Benjamin Freeman (BS, 2013) was funded this past summer and worked towards the synthesis of several complexes containing hydroxyl groups, [Ru(tpy)(bpy)(H2O)]2+, [Ru(tpy)(4bpy(OH)2)(H2O)]2+, [Ru(tpyOH)(bpy)(H2O)]2+, and [Ru(tpyOH)(4bpy(OH)2)(H2O)]2+.  The goal is to determine if the quantity and arrangement of hydroxyl groups affect the water oxidation properties of these complexes.  This summer, Ben successfully made the chloro-substituted complexes and is currently in the process of carrying out ligand substitution to replace the chloride with an aqua ligand.  We have arranged to collaborate with Dr. Elizabeth Papish on water oxidation studies when Ben has successfully prepared the aqua complexes.

Ruthenium bimetallic complexes.

Benjamin Freeman initially began work on synthesizing bimetallic complexes containing the bridging 2,3-diphenyl-di(pyridin-2-yl)pyrazine (dpdpz) ligand.  Ben has successfully synthesized the ligand, however, attempts to synthesize the bridging complex [(tpyOH)Ru(dpdpz)Ru(tpyOH)]4+ have not been as fruitful to this point.

While the ACS-PRF grant has helped our lab to obtain presentable and publishable results, the grant has also provided the opportunity to train several students.  Students trained from this grant have gained employment in industry or are currently enrolled in graduate school for chemistry.  During the upcoming year we will continue to push projects towards their completion with the ultimate goals of publication and applying for other additional funding.