The Use of Fluorinated Alkoxide Ligands in ∼D3h Compounds Increases Co Electrophilicity and Eases Co(III)/Co(II)
Mary M Bussiere1, James McNeely1, Linda H Doerrer1
1Boston University, Chemistry Department, 590 Commonwealth Avenue, Boston, Massachusetts 02215, United States.
Abstract:
Eleven Co(II) complexes with three perfluorotrityl-derived monodentate alkoxide ligands, termed the "Fox" ligands, were prepared and fully characterized. Complexes of "big Fox," (OC(C6F5)3)1- = (OFoxB)1-, and "medium Fox," (OC(CF3)(C6F5)2)1- = (OFoxM)1-, yielded the same structural motifs: [Co2(μ-OFox)2(OFox)2] (1B, 1M); [Co(THF)2(OFox)2] (2B, 2M); and [Co(OFox)3]1- ([3B]1-, [3M]1-). Complexes of "little Fox," (OCH(C6F5)2)1- = (OFoxL)1- yielded different structural motifs due to less ligand bulk, leading to π-π stacking of -C6F5 rings in the solid state: [Co2(μ-OFoxL)2(OFoxL)2(THF)2] (1L·2 THF), {K(Et2O)}2[Co2(μ-OFoxL)2(OFoxL)4] (K2[4L]), and {K(THF)2}2[Co(OFoxL)4] (K2[5L]). The electronic structures of ∼D3h compounds (1B, 1M, [3B]1-, and [3M]1-) were investigated using DFT and ab initio ligand field theory (AILFT). Cyclic voltammetry demonstrated a quasi-reversible Co(III)/Co(II) redox couple in {K(18c6)}[3M] and the pyridine adduct {K(18c6)}[3M-py]. Bulk oxidation generated Co(III) compounds 3M and 3M-py respectively, in situ, as confirmed by UV-vis spectroelectrochemistry (SEC). Compounds 3M and 3M-py were not isolable. 3M-py is reduced over time into [Co(py)2(OFoxM)2] (6), in addition to other unidentified products. These new Co(II) and Co(III) compounds have increased electrophilicity at Co and decreased HOMO energy compared to hydrogenated analogs, leading to one case of a 500 mV cathodic shift ([3M]1-) in the Co(III)/Co(II) reduction potential versus related fluorinated analogs.
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