Phosphine ligands as protecting groups for 3d complexes in oxidation by O2

A series of heteroleptic mixed phosphine/alkoxide 3d complexes was designed to evaluate PPh3 as a protecting group, stabilizing the metal coordination sphere prior to O-2 exposure, but oxidatively dissociating in the presence of O-2 to allow facile O-2 reduction by low coordinate metal centers. Complexes of the form [(Ph3P)(2)M(OC4F9)(2)] (M = Fe (1), Co (2), Ni (3), Zn (4)) and [(Ph3P)(2)M(pin(F))] (M = Co (8), Ni (9), Zn (10) were prepared, along with related complexes with non-reactive L-donors, [(DME)Fe(OC4F9)2] (5) and [(Ph3PO)(2)M(OC4F9)(2)] (M = Fe (6), Ni (7)). Complexes were characterized by UV-vis and NMR spectroscopies, elemental analysis, and single-crystal X-ray diffraction for 1, 3, 6, 7, 8, 9, and 10. The electronic structure of 3 is particularly notable, with similar to D-4h geometry at room temperature and similar to T-d at low temperature, as determined by temperature dependent UV-vis and NMR (H-1, P-31) spectroscopies. Complexes (M = Fe, Co, Ni) were screened for O-2 reactivity to assess the efficacy of PPh3 as a protecting group. Dimeric [Fe-2(mu(2)-O)(OPPh3)(2)(OC4F9)(4)] (13) was isolated after O-2 reactivity with 1 and characterized as described above. Related Fe complexes 5 and 6 were each combined with O-2 to generate intermediate species capable of both stoichiometric oxidase of hydroquinone to benzoquinone and sub-stoichiometric oxygen atom transfer of thioanisole (PhSMe) to methyl phenyl sulfoxide. Fluorinated alkoxide ligand choice influenced O-2 reactivity in Co-II complexes, as reactivity was only seen in the less sterically hindering pin(F)-containing complex 8. In the Ni-II complexes, on the other hand, the particular fluorinated ligand was not a factor in O-2 reduction, as 3 and 9 exhibited similar reactivity. Related dimeric compounds [Co-2(pin(F))(2)(THF)(4))] (11) and [Zn-2(pinF)(2)(THF)(2))] (12) were also isolated and characterized. (C)020 Elsevier Ltd. All rights reserved.