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We proposed the studies of a new family of late first row transition metal complexes having oxygen donor ligands in low, medium and high oxidation states with fluorinated alkoxide oxygen donor ligands.  Our goals were (i) preparation of new transition metal complexes with convenient oxidation states, (ii) preparation of low and high oxidation species without additional ligands, (iii) preparation of high oxidation state complexes via atom transfer reactions, (iv) placement of oxygen donor ligands such as OC₄F₉, OCH(CF₃)₂ and related species in the ligand field series, and (v) to survey these new reactive systems for development into catalytic C-H oxidation systems under mild conditions.  Last year we accomplished goals (i) and (iv), and this year we have made progress further progress on (ii) and have some interesting observations with regard to objectives (iii) and (v).  

Although the starting materials [M(OC₄F₉)_n]^m- were readily prepared in crystalline form, the oxidized neutral products (M = Ni, Co) have not been isolated except as oils.  Therefore, a tertiary alcohol with extensive fluorination but fewer degrees of rotational freedom was required and investigations were begun on a related tertiary alkoxide ligand, -OC(C₆F₅)₃.  The parent alcohol was described in the literature but primarily in the studies of trityl cation analogs.  The compound is prepared by reaction of LiC₆F₅ with perfluorobenzophenone and hydrolysis of the resulting lithium alkoxide with water, to which we have added an X-ray crystallographic study. 

      Reactivity patterns of HOC(C₆F₅)₃ were not entirely as expected based on our published work with OC₄F₉ and fluorinated aryloxides.  The alcohol was readily converted to TlOC(C₆F₅)₃ via TlOEt, but use of Ag₂O as an oxide base source toward the putative AgOC(C₆F₅)₃, led to black powder (presumed Ag⁰) and the ketone from which the alcohol had been prepared.  Several attempted metathesis reactions of transition metal halides MX_n with TlOC(C₆F₅)₃  were consistent with precipitation of the highly insoluble TlX, but these reactions also formed copious quantities of perfluorobenzophenone as evidenced by ¹⁹F NMR and ESI-MS spectroscopies.  Undaunted, we pushed on and tested two alternative synthetic approaches to [M{OC(C₆F₅)₃}_n]^m- target compounds. 

Bonding of this ligand to transition metals has been pursued through two routes: (i) alcoholysis of strong internal bases in complexes such as [M{N(TMS)₂}₂] and {Cu(mes)}_n and (ii) reaction of metal halides MX_n with the alcohol in the presence of Et₃N.  The former route has been successful in the preparation of a microcrystalline purple powder whose elemental analysis is consistent with {Co(OC(C₆F₅)₃)₂}-0.5 C₆H₁₄.  Structural data have not been obtained but we posit a bridged dimeric structure of the form [(F₅C₆)₃CO-Co-(m₂-OC(C₆F₅)₃)-Co-OC(C₆F₅)₃] based on literature work with the bulky tri(cyclohexyl)methanol.  Preliminary work with this Co(II) species shows reactivity with both oxidizing and reducing agents, and work is underway vigorously to characterize the putative Co(III) and Co(I) products, as well as pursue two-electron oxidation chemistry of the Co(I) derivatives.

      Expanding our work to bidentate, chelating ligands, we have greatly increased the little chemistry that was known with the dodecafluoropinacolate (ddfp) ligand.  Preliminary work on combination of H₂ddfp with transition metal sulfates, with an aqueous solution of NaOH and the alcohol.  Prior to our work only the structure of [K₂Ni(ddfp)₂]-4H₂O was known in the late metals and two Cr(V)(ddfp) compounds had also been prepared.  The Ni(II) compound is square planar and diamagnetic, showing the relatively strong ligand field imposed by the ddfp ligand.  We have now expanded this series to include the paramagnetic Mn(III), Co(II) and Cu(II) in {K(THF)₂(18C6)}[Mn(THF)(ddfp), {K(DME)₂}₂[Co(ddfp)], [K₂Cu(ddfp)₂]-4H₂O, and {K(18C6)}₂[Cu(ddfp)₂]- 5 H₂O.  The monoacids (KHddfp), {K(18C6)}(Hddfp), and diamagnetic Cu(I) derivative [KCu(ddfp)(PPh₃)]₂-THF have also been prepared and characterized structurally.  The Cu(II) and Mn(III) compounds are air-stable, but the Co(II) and Cu(I) species must be handled in anoxic environments. 

      We began our studies of lower valent transition metal species with Cu(I), and have prepared several different derivatives including heterobimetallic K and Tl complexes of the form {MCu(OR)₂}, the related bis(alkoxides) with weakly coordinating cations {K(18C6)}⁺ and (Ph₄P)⁺, as well as the dimeric phosphine compounds {(Ph₃P)Cu(OR)}₂.  The synthetic pathways and highlights of the characterization data collected to data are summarized in the scheme below. 

      Arguably the most important and prevalent redox chemistry of Cu(I) is with O₂, whether it be oxidase, monooxygenase, or dioxygenase chemistry.  Therefore we began our pursuit of reactive and useful high oxidation states by mixing O₂ with the most simple Cu(I) derviatives.  These {KCu(OR)₂} compounds showed immediate reactivity with O₂ at -78°C.  Interestingly, the observed chromophores are dependent on the relative Cu(I) concentration in solutions that are saturated with O₂.  Manometry studies showed that for Cu(I) concentrations less than ~ 2.0 mM the Cu:O₂ ratio is 2:1 but for concentrations above 4.5 mM, the ratio is 3:1.

Furthermore, the {KCu(OR)₂} complexes do not behave as 1:1 electrolytes in either THF or CH₂Cl₂, the former being particularly surprising.  The complexes with weakly coordinating cations {K(18C6)}⁺ and (Ph₄P)⁺ in A⁺[Cu(OR)₂]^- do behave as 1:1 electrolytes, but do not activate O₂ under the same conditions.  This is a particularly exciting result and parsing out these differences is one of our primary goals in the future work. 

      In summary, our initial hypothesis that fluorinated oxygen-donor ligands would serve as medium field ligands has been proven spectroscopically and the reactivity studies of one particular family, the Cu(I)-O₂ system, shows significant promise toward C-H activation with earth abundant metals and potentially catalytic systems.