Synthesis and Oxidation of Vanadyl Complexes Containing Bispidine Ligands

The reaction of 3‐methyl‐9‐oxo‐7‐picolyl‐2,4‐di(2‐pyridyl)‐3,7‐diazabicyclo[3.3.1]nonane‐1,5‐dicarboxylate (L 2 ) with vanadyl sulfate in methanol yields the purple complex[V IV =O(L 2′ )](ClO 4 ) 2 , where L 2 is hydrated to the corresponding 9‐diol L 2′ . The crystal structure of [V IV =O(L 2′ )](ClO 4 ) 2 has a saturated coordination sphere with a long bond to the tertiary amine N7 trans to the oxido group (2.36 Å). Oxidation of [V IV =O(L 2′ )](ClO 4 ) 2 with an excess of aqueous hydrogen peroxide yields the orange complex [V V =O(O 2 )(L 2′ )]ClO 4 . This can be further oxidized with Ce 4+ in MeCN at –40 °Cto the unstable vanadium(V)‐oxidosuperoxido complex [V V =O(O 2 )(L 2′ )] 2+ , as shown by EPR spectroscopy. The X‐ray crystal structure of [V V =O(O 2 )(L 2′ )]ClO 4 shows that the N7‐pendant pyridine donor is dissociated and the V–N7 bond is elongated (2.56 Å). The O–O distance of the side‐on bound peroxide in [V V =O(O 2 )(L 2′ )]ClO 4 is unexpectedly short (1.28 Å) and is therefore in the range which is typical for a superoxido group. However, the IR spectrum of [V V =O(O 2 )(L 2′ )]ClO 4 shows 18 O isotope shifts, which support the assignment as a peroxido complex, and this is also supported by EPR and UV/Vis spectroscopic analysis. The short O–O bond is attributed to a static disorder rather than to libration of the “non‐rigid” peroxido group.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)