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46064-B3
Vanadium-Scorpionate Coordination Complexes: Syntheses, Characterization, and Reactivities

Craig C. McLauchlan, Illinois State University

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. 

            Until recently, our aims were exclusively to synthesize vanadium(III) phosph(on)ate cages with a “V₄P₄O₁₂”  core unit structures with tetrahedral rather than octahedral vanadium centers.  In a recent report, we employed the sterically bulky Mes₂nacnac ligand that has been shown to force V(III) in to a tetrahedral geometry.   We synthesized and structurally characterized the sterically bulky β-diiminate Mes₂nacnac ligand and synthesized several V(III) complexes with Mes₂nacnac in addition to a novel {LVO(m-OH)}₂ dimer.

            In our attempts to make cage complexes, we are also pursuing a metathesis reaction between a tridentate ligand, such as tris(pyrazol-1-yl)methane (Tpm, 3), tris(pyrazol-1-yl)methanesulfonate (Tpms, 4), and tris(4-imidazol-1-yl)carbinol (4-TIC, 5), and tris(2-methylimidazol-1-yl)carbinol (2^Me-TIC, 6), coordinated to vanadium and a phosphonate salt.  All previously existing vanadium(III) phosph(on)ates with a “V₄P₄O₁₂”  core unit have an octahedral geometry around the metal with the ligand being the relatively bulky tridentate scorpionate ligand hydrotris(pyrazol-1-yl)borate, Tp.  The chemistry of Tp with V (III, IV, and V) is rich, however, hydrolysis of Tp ligands in the presence of vanadium (and other metals!) is well known and the stability of Tp is lacking.  A more stable ligand system is desirable, but one that maintains the facial tridentate binding nature of Tp is preferred.  As such, we have been employing Tpm, Tpms, 2-TIC, and 4-TIC ligands, with the most success with the ionic Tpms ligand.  Results from the Tpms work have been submitted for publication.  We report there the synthesis of TpmsVCl₂(DMF) and an oxidized counterpart.  The binding of the Tpms shows equilibrium between a N,N,N- binding mode and an N,N,O- binding mode with the SO₃ binding instead of the N on pyrazole, a phenomenon that has been noted before with Cu complexes of Tpms.

A final ligand system that we have pursued this year under this proposal is 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.  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.  The synthesized vanadium-ligand complexes will ultimately be combined with phosphate or phosphonate salts in metathesis reactions to attempt to form cage structures.

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. 

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.

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