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Vanadium coordination chemistry lies at the heart of the research interests of the PI and his research group.  This proposed research outlines synthetic methods and characterization techniques to study new vanadium-organophosponates with relatively porous molecular structures prepared from vanadium(III) and (IV) precursors. 

The goal of the proposed work is to develop synthetic methodologies in the preparation of vanadium- organophosphonate complexes and to completely characterize these complexes.  A series of tridentate scorpionate ligands with O and/or N binding moieties are being employed to direct the stereochemistry involved and direct the formation of lower dimensional structures.  The long term goal is, ultimately, to study these complexes for catalytic activity.  The project can be roughly divided in to four main areas:

1)    Synthesis of molecular “cages” of V(III) complexes with organophosphonates

2)    Synthesis of a parallel series of V(IV) complexes

3)    Characterization of synthesized products.

4)    Studies on the catalytic activity of the synthesized products.

 Our initial studies have focused on goals 1-3.  Here we report the progress on each area.

1)   Synthesis of molecular “cages” of V(III) complexes with organophosphonates

During the past 12 months this project has been all but suspended as we focused our resources and time to pursue goal #2, where more significant headway was being achieved.  This area is still a desired goal of the project and a return to this area in the near future is expected.

2) Synthesis of a parallel series of V(IV) complexes

During the past year we have almost exclusively been making vanadium complexes with the cyclopentadienyltris(dialkylphosphito)cobaltate(III) ligand system, CpPCo (shown), often referred to as the “Kläui ligand” (R=Me,Et for us).  One of the advantages of the CpPCo ligand to this project is the sterically directing nature of the ligand – it is tripodal/tridentate as with the scorpionate ligands, but with oxygen as the binding atom.  The O,O,O-bonding nature of this ligand stabilizes the oxophilic vanadium that may allow us to make molecular structures.  Previously, we have reported a series of V(III) and V(IV) complexes with the CpPCo ligand with a number of co-ligands.   These complexes have proven surprisingly robust and the substitution chemistry of the CpPCoVCl₂(DMF) allows facile preparation of the series with co-ligands such as oxalate or another equivalent of CpPCo.  Of late we have been using the oxidized material, nominally CpPCoVOCl(DMF), with similar substitutions chemistry to generate a series of bridged phosphate, phosphate, and phosphonate dimers.  A generalized structure is shown below.  The stereochemistry of the vanadyl complexes makes formation of the dimers much more easily accomplished than tetramer formation and we have exploited that advantage.  Tetramer formation is still a long-term goal, but is more likely with V(III) complexes.

3)   Characterization of synthesized products

To help train students, we have been characterizing our products using many techniques, including spectroscopy (NMR (¹H, ¹³C,³¹P, ⁵¹V) EPR, IR, UV-vis), magnetic susceptibility, and cyclic voltammetry.   MALDI mass spectrometry has already proven useful in identifying our reaction products and will help in identifying future products.    The X-ray structures of those compounds that have formed single crystals have also been very interesting.  We have recently been contacted by several possible collaborators who are interested in studying our complexes by solid-state NMR and SQUID. The SQUID collaboration is fairly local and could involve being able to expose students to another technique. 

4)   Studies on the catalytic activity of the synthesized products

The ultimate goal is to create well-characterized relatively open molecular structures that could be studied for oxidative catalytic activity via ¹H and ¹³C NMR spectroscopy and by gas chromatography-mass spectrometry. Preliminary studies have been done on our simpler species using the catalyzed oxidation of 3,5-di-tert-butyl-1,2-dihdroxybenzene to the ortho-quinone species, because it is easily monitored via GC-MS. Because catalytic activity relies on easy conversion between oxidation states, it is not unexpected that, thus far, we have noted better efficiency with our V(IV) complexes as the V(IV)/V(V) couple might more easily facilitate good catalytic activity.  Although not in the scope of the original proposal, we have begun a preliminary set of studies (using GC) on our series of dimers (see goal 2) to examine their efficacy as oxidation catalysts on this simple catcehol/quinone system.

Students involved in this project have presented our work internally at ISU poster sessions such as the Undergraduate Research Symposium, Graduate Research Symposium, and an end-of-summer poster session.  This work has also been presented by a student at a regional ACS meeting and a manuscript was published in Acta Crystallographica, Sect. E.  A more in-depth manuscript that details the work reported here is in preparation.