Quasi-Three-Coordinate Iron and Cobalt Terphenoxide Complexes {Ar^iPr₈OM(μ-O)}₂ (Ar^iPr₈ = C₆H-2,6-(C₆H₂-2,4,6-ⁱPr₃)₂-3,5-ⁱPr₂; M = Fe or Co) with M(III)₂(μ-O)₂ Core Structures and the Peroxide Dimer of 2-Oxepinoxy Relevant to Benzene Oxidation

Abstract

The bis(μ-oxo) dimeric complexes {Ar(iPr8)OM(μ-O)}2 (Ar(iPr8) = C6H-2,6-(C6H2-2,4,6-(i)Pr3)2-3,5-(i)Pr2; M = Fe (1), Co (2)) were prepared by oxidation of the M(I) half-sandwich complexes {Ar(iPr8)M(η(6)-arene)} (arene = benzene or toluene). Iron species 1 was prepared by reacting {Ar(iPr8)Fe(η(6)-benzene)} with N2O or O2, and cobalt species 2 was prepared by reacting {Ar(iPr8)Co(η(6)-toluene)} with O2. Both 1 and 2 were characterized by X-ray crystallography, UV-vis spectroscopy, magnetic measurements, and, in the case of 1, Mössbauer spectroscopy. The solid-state structures of both compounds reveal unique M2(μ-O)2 (M = Fe (1), Co(2)) cores with formally three-coordinate metal ions. The Fe···Fe separation in 1 bears a resemblance to that in the Fe2(μ-O)2 diamond core proposed for the methane monooxygenase intermediate Q. The structural differences between 1 and 2 are reflected in rather differing magnetic behavior. Compound 2 is thermally unstable, and its decomposition at room temperature resulted in the oxidation of the Ar(iPr8) ligand via oxygen insertion and addition to the central aryl ring of the terphenyl ligand to produce the 5,5'-peroxy-bis[4,6-(i)Pr2-3,7-bis(2,4,6-(i)Pr3-phenyl)oxepin-2(5H)-one] (3). The structure of the oxidized terphenyl species is closely related to that of a key intermediate proposed for the oxidation of benzene.

Highly Reactive CoIII,IV2(μ-O)2 Diamond Core Complex That Cleaves C–H Bonds

“…The improvement of fit quality by introducing the cobalt scatterer is also observed in the Fourier transform spectrum (Figure S10 and S11). Such a short Co···Co distance has previously been reported for the crystal structures of Co III 2 (μ-O) 2 complexes (2.67–2.74 Å) − and other high-valent M 2 (μ-O) 2 complexes with a diamond-core structure (Table S3) and is also confirmed in our DFT geometry optimization ( vide infra ). Notably for both complexes, we did not observe significant features at path-lengths longer than 3 Å in the Fourier transform characteristic of peroxo-bridged core structures such as Co III (μ-OH)­(μ-OO)­Co III and Cu II (μ-η 2 :η 2 (O 2 ))­Cu II where the M···M separation is significantly longer (Table S3).…”

“…The average distances of Co–O oxo bonds of DFT geometry optimized trans structures 2 and 3 are 1.86 and 1.83 Å, respectively. These Co–O oxo distances are consistent with those of several Co 4 O 4 cubane complexes with six-coordinated cobalt sites (values are from X-ray crystallography and DFT calculations), , and are slightly longer than those of diamond core Co 2 O 2 crystal structures with cobalt centers of lower coordination numbers (1.78–1.83 Å). − Furthermore, these geometry-optimized structures clearly showed that the six pyridines are in two distinct local environments with a ratio of 2:1, which is experimentally observed in our 1 H NMR results obtained for 2 ( vide supra , Figure S4). Similar results were obtained using other commonly used DFT functionals M06-L, B3LYP* (15% HF), and B3LYP (Table S7).…”

“…In contrast, the high-valent diamond core chemistry of cobalt has been much less investigated, likely due to the difficulty of generating high-valent oxocobalt­(IV) species for characterization. , There is evidence showing that high-valent oxocobalt­(IV) species are involved in a number of water oxidation , and C–H bond cleavage reactions . To date, six Co III 2 (μ-O) 2 complexes supported by tridentate or bidentate ligands have been structurally characterized. − However, no higher-valent derivative is available. In the present study, we report an unprecedented high-valent Co III,IV 2 (μ-O) 2 complex ( 3 ) supported by a tetradentate tris­(2-pyridylmethyl)­amine (TPA) ligand (Scheme ).…”

Intramolecular Hydrogen-Bond Interactions Tune Reactivity in Biomimetic Bis(μ-hydroxo)dicobalt Complexes

“…As opposed to the abundance of bis­(μ-hydroxo)­dicobalt­(III) examples, − relatively few bis­(μ-oxo)­dicobalt­(III) systems have been reported. With only one exception, , none of these complexes contain Co III in octahedral or tetragonal coordination geometries, and none are stable over long periods of time at room temperature. − Two groups have separately reported the preparation of [(TPA) 2 Co III 2 (μ-O) 2 ]­[ClO 4 ] 2 ( 4 ), which is the only example of a Co 2 (μ-O) 2 core within an octahedral coordination geometry. , Kojima et al provide only UV–vis absorption spectra in support of the formation of 4 , suggesting that it is stable at room temperature . In contrast, Wang et al provide a mass spectrum, and UV–vis, 1 H NMR, and X-ray absorption spectroscopic evidence for the preparation of 4 , reporting that it is only stable below −40 °C (decaying with t 1/2 ∼ 4 h) .…”

Self-assembly of an Organometallic Fe9O6 Cluster from Aerobic Oxidation of (tmeda)Fe(CH2tBu)2

“…For iron-specific systems, Power has shown that Fe-aryl complexes exhibit formal O=O scission and O-atom insertion into M-Caryl bonds. 38,39 In an extensive DFT study on the related O2 reaction with the twocoordinate compound FeMe2, 40 Cundari reported that the O-atom insertion is calculated to proceed via initial formation of open-shell Fe III -(O2 -) species and eventual generation of dioxo intermediates (as seen in Cr oxidation chemistry reported by Theopold) 41 prior to O-atom insertion into the Fe-C bonds. Additionally, Chirik has shown that ferrous dialkyls are susceptible to Fe-C homolysis upon addition of π-acidic redox-active ligands, 42 suggesting that coordination of O2 could lead to the generation of free alkyl radicals.…”