7826 Chem. Commun., 2012, 48, 7826–7828 This journal is c The Royal Society of Chemistry 2012 Cite this: Chem. Commun., 2012, 48, 7826–7828 Nonclassical oxygen atom transfer reactions of oxomolybdenum(VI) bis(catecholate)w Travis Marshall-Roth, Sean C. Liebscher, Karl Rickert, Nicholas J. Seewald, Allen G. Oliver and Seth N. Brown* Received 16th May 2012, Accepted 15th June 2012 DOI: 10.1039/c2cc33523a Mechanistic studies indicate that the oxomolybdenum(VI) bis(3,5-di-tert-butylcatecholate) fragment deoxygenates pyridine- N-oxides in a reaction where the oxygen is delivered to molyb- denum but the electrons for substrate reduction are drawn from the bound catecholate ligands, forming 3,5-di-tert-butyl-1,2- benzoquinone. Inner-sphere redox reactions involve both changes in oxida- tion state and changes in bonding. Classically, in reactions such as the oxygen atom transfer (OAT) 1 reaction depicted in eqn (1a), the changes in oxidation state and those in bonding are co-localized: molybdenum is both oxidized and forms a new bond to oxygen, while nitrogen is reduced and the N–O bond is broken. Co-localization is not, however, obligatory. For example, in complexes with redox-active ligands, bonding changes may take place at a redox-inert metal center while the corresponding changes in oxidation state take place at the coordinated ligand (eqn (1b)). The most thoroughly studied example of such a ‘‘non-classical’’ inner-sphere redox reaction is proton-coupled electron transfer (PCET), where the motion of the hydrogen nucleus may be quite separated from the motion of the electron. 2 In contrast to PCET, nonclassical OAT (eqn (1b)) would be a two-electron redox process. ð1aÞ ð1bÞ Nonclassical OAT has been postulated for simple oxygen atom donors 3 but only observed using dioxygen as oxidant, 4 though nonclassical transfer reactions of the isoelectronic nitrene group from aryl azides to early metal complexes have been described. 5 Redox participation of catecholate in oxygen atom transfer to rhenium(V) has been proposed, but net oxida- tion takes place at the metal, not the ligands. 6 Reduced bipyr- idine complexes of uranium 7 and thorium 8 have been shown to abstract oxygen from pyridine-N-oxides, but it has not been established whether the electrons flow directly from the reduced bipyridine to the N–O bond while both ligands are coordinated, or whether neutral bipyridine dissociates from a reduced metal center prior to OAT. Here we describe the deoxygenation of pyridine- N-oxides by the bis(3,5-di-tert -butylcatecholato)oxomolyb- denum( VI) fragment. Mechanistic data indicate that this reaction takes place by nonclassical oxygen atom transfer. Air- and moisture-stable dark purple six-coordinate adducts MoO(3,5-DBCat) 2 (L) containing pyridine- or 4-picoline-N-oxide are readily prepared by mixing 2 equiv. of 3,5-di-tert-butyl- catechol with MoO 2 (acac) 2 in the presence of the appropriate N-oxide. The same adducts also form immediately on addi- tion of pyridine-N-oxide to the catecholate-bridged dimer Mo 2 O 2 (3,5-DBCat) 4 , 9 as judged by NMR spectroscopy. The molecular structure of MoO(3,5-DBCat) 2 (Opic) (Fig. 1) confirms that the molecule adopts a cis geometry, like its 3,6- di-tert-butylcatecholate analogue 3 and the bis(amidophenoxide) complex ( t BuClip)MoO(3,5-lutidine). 10 This geometry has been rationalized on the basis of maximizing p-donation by allowing the catecholate trans to the oxo to donate into the lone dp orbital not involved in p bonding to the oxo group. The importance of p bonding is confirmed by structural features in the trans catecholate that show significant transfer of electron density to Mo (metrical oxidation state (MOS) 11 = 1.56(9)), which is not observed in the cis catecholate (MOS = 2.16(17)). Department of Chemistry and Biochemistry, 251 Nieuwland Science Hall, University of Notre Dame, Notre Dame, IN 46556-5670, USA. E-mail: Seth.N.Brown.114@nd.edu; Fax: +1 574 631 6652; Tel: +1 574 631 4659 w Electronic supplementary information (ESI) available: Details of syntheses, variable-temperature NMR, and kinetics experiments. CCDC 882420. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2cc33523a ChemComm Dynamic Article Links www.rsc.org/chemcomm COMMUNICATION Downloaded by University of Notre Dame on 17 July 2012 Published on 19 June 2012 on http://pubs.rsc.org | doi:10.1039/C2CC33523A View Online / Journal Homepage / Table of Contents for this issue