A diamagnetic iron complex and its twisted sister - structural evidence on partial spin state change in a crystalline iron complex

We report here the syntheses of a diamagnetic Fe complex [Fe(HL)₂] (1), prepared by reacting a redox non-innocent ligand precursor N,N'-bis(3,5-di-tert-butyl-2-hydroxy-phenyl)-1,2-phenylenediamine (H₄L) with FeCl₃, and its phenoxazine derivative [Fe(L')₂] (2), which was obtained via intra-ligand cyclisation of the parent complex. Magnetic measurements, accompanied by spectroscopic, structural and computational analyses show that 1 can be viewed as a rather unusual Fe(III) complex with a diamagnetic ground state in the studied temperature range due to a strong antiferromagnetic coupling between the low-spin Fe(III) ion and a radical ligand. For a paramagnetic high-spin Fe(II) complex 2 it was found that, when crystalline, it undergoes a thermally induced process where 25% of the molecules in the material change to a diamagnetic low-spin ground state below 100 K. Single crystal X-ray studies conducted at 95 K afforded detailed structural evidence for this partial change of spin state of 2 showing the existence of crystallographically distinct molecules in a 3 : 1 ratio which exist in high- and low-spin states, respectively. Also, the magnetic behaviour of 2 was found to be related with the crystallinity of the material as demonstrated by near-IR radiation to unpaired electrons conversion ability of amorphous sample of 2.

A Dimeric Chromium(II) Pincer as an Electron Shuttle for N=N Bond Scission

Reduction of the bis-pyrazolyl pyridine complex [CrL]₂ with 4 KC₈ , followed by addition of one azobenzene (overall mole ratio 1:4:1), PhNNPh, transfers reducing equivalents to three azobenzenes, to form [K₃ Cr(PhNNPh)₃ ]. This has three κ² PhNNPh^2- ligands and K⁺ bound to nitrogen atoms of azobenzene. When the stoichiometry is modified to 1:4:3, the product is changed to [K₂ CrL(PhNNPh)₂ ], which has C₂ symmetry except for the intimate ion pairing of two K⁺ ions to reduced azobenzene nitrogen atoms, and to pyrazolate and phenyl rings. The origin of the observed delivery of reducing equivalents to several, not to a single N=N bond, is traced to the resistance of the one-electron-reduced substrate to receiving a second electron, and is thus a general phenomenon. [CrL]₂ alone is shown to be a two-electron reductant towards benzo[c]cinnoline (BCC) resulting in a product of formula [Cr₂ L₂ (BCC)], in which the reducing equivalents originate purely from Cr^II . An analogous study of the reaction of [CrL]₂ with azobenzene yields [Cr₂ L₂ (PhNNPh)(THF)], an adduct in which one THF has displaced one of four hydrazide nitrogen/Cr bonds. Together these illustrate different modes for the Cr₂ L₂ unit to bind and reduce the N=N bond. Collectively, these results show that two divalent Cr, without added K⁰ , have the ability to reduce the N=N bond. Further KC₈ reduction of preformed Cr₂ L₂ (RNNR) inevitably gives products in which K⁺ stabilizes the charge in the increasingly electron-rich nitrogen atoms, in a phenomenon which mimics proton coupled electron transfer: K⁺ performs the role of H⁺ . A least-squares fit of the two singly reduced DFT structures shows that the only major change is a re-orientation of one of the two phenyl rings in order to avoid repulsion with potassium but to still allow interaction of that phenyl π system with K⁺ . This shows both the impact of K⁺ , being modest to nitrogen/chromium interactions, but nevertheless accommodating some π donation of phenyl to potassium. Finally, delivering increasing equivalents of KC₈ leads to complete cleavage of the N=N bond, and both N bind to three Cr^II . The varied impacts of the K⁺ electrophile on NN multiple bond reduction is discussed.

[FeII(L•)2][TCNQF4•−]2: A Redox-Active Double Radical Salt

The reaction of [FeII(L•)2][BF4]2 with LiTCNQF4 results in the formation of [FeII(L•)2][TCNQF4•−]2·2CH3CN (1) (L• is the neutral aminoxyl radical ligand 4,4-dimethyl-2,2-di(2-pyridyl)oxazolidine-N-oxide; TCNQF4 is 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane). Single-crystal X-ray diffraction; Raman, Fourier-transform infrared (FTIR) and ultraviolet–visible spectroscopies; and electrochemical studies are all consistent with the presence of a low-spin FeII ion, the neutral radical form (L•) of the ligand, and the radical anion TCNQF4•−. 1 is largely diamagnetic and the electrochemistry shows five well-resolved, diffusion-controlled, reversible one-electron processes.

Use of the TCNQF4 2- Dianion in the Spontaneous Redox Formation of [FeIII (L- )2 ][TCNQF4 ⋅- ]

The reaction of [Fe^II (L^. )₂ ](BF₄ )₂ with Li₂ TCNQF₄ results in the formation of [Fe^III (L^- )₂ ][TCNQF₄ ^{. -} ] (1) where L^. is the radical ligand, 4,4-dimethyl-2,2-di(2-pyridyl)oxazolidine-N-oxide and TCNQF₄ is 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane. This has been characterised by X-ray diffraction, Raman and Fourier transform infrared (FTIR) spectroscopy, variable-temperature magnetic susceptibility, Mössbauer spectroscopy and electrochemistry. X-ray diffraction studies, magnetic susceptibility measurements and Raman and FTIR spectroscopy suggest the presence of low-spin Fe^III ions, the anionic form (L^- ) of the ligand and the anionic radical form of TCNQF₄ ; viz. TCNQF₄ ^{. -} . Li₂ TCNQF₄ reduces the [Fe^II (L^. )₂ ]²⁺ dication, which undergoes a reductively induced oxidation to form the [Fe^III (L^- )₂ ]⁺ monocation resulting in the formation of [Fe^III (L^- )₂ ][TCNQF₄ ^{. -} ] (1), the electrochemistry of which revealed four well-separated, diffusion-controlled, one-electron, reversible processes. Mössbauer spectroscopy and electrochemical measurements suggest the presence of a minor second species, likely to be [Fe^II (L^. )₂ ][TCNQF₄ ^2- ].

Hexahalorhenate(iv) salts of metal oxazolidine nitroxides

Eight coordination compounds of formulae [Fe^II(L˙)₂][Re^IVCl₆] (1a), [Fe^II(L˙)₂][Re^IVBr₆] (1b), [Co^II(L˙)₂][Re^IVCl₆]·CH₃CN (2a), [Co^II(L˙)₂][Re^IVBr₆] (2b), [Ni^II(L˙)(CH₃CN)₃][Re^IVCl₆]·CH₃CN (3a), [Ni^II(L˙)(CH₃CN)₃][Re^IVBr₆]·3CH₃CN (3b), [Cu^II(L˙)₂][Re^IVCl₆] (4a) and [Cu^II(L˙)₂][Re^IVBr₆] (4b), where L˙ is the aminoxyl radical chelating ligand, 4,4'-dimethyl-2,2'-di(2-pyridyl)oxazolidine-N-oxide, have been synthesised. Structural and magnetic studies reveal metal-radical intramolecular antiferromagnetic interactions in the [M^II(L˙)₂]²⁺ cations in the iron, cobalt and copper based compounds (1a, 1b, 2a, 2b, 4a and 4b) with the central metal ion low-spin in the case of iron (1a and 1b) and a gradual, cobalt based, spin-crossover transition present in 2a and 2b. The nickel based compounds, 3a and 3b, were analysed in the dried form (3a(dried) and 3b(dried)) and directly in acetonitrile (3a(solvated) and 3b(solvated)). Microanalysis and IR spectroscopy on 3a(dried) and 3b(dried) suggest that the dried samples are best formulated as [Ni^II(L˙)(H₂O)₃][Re^IVX₆], where X = Cl (3a(dried)) and Br (3b(dried)). All forms of 3a and 3b exhibit cationic metal-radical ferromagnetic interactions resulting in S = 3/2 ground states. In addition, 3a(dried) exhibits spin-canting behaviour with an ordering temperature of 2.7 K, an open hysteresis loop with a coercive field H_c = 580 Oe, and a remanent magnetisation M_r = 0.21μ_B, resulting in a canting angle of ∼1.8°. In contrast, 3b(dried) shows no spin-canting behaviour; a maximum in χ_Mvs. T at T = 3 K suggesting long-range antiferromagnetic ordering. 3a(solvated) and 3b(solvated) show no indication of long-range magnetic ordering, unlike 4a and 4b where anomalies are evident in the low-temperature magnetic susceptibility measurements.

Computational Examination on the Active Site Structure of a (Peroxo)diiron(III) Intermediate in the Amine Oxygenase AurF

In this work, we report the first computational investigation on the structure and properties of the (peroxo)diiron(III) intermediate of the AurF enzyme. Our calculations predict that, in the oxidized state of the AurF enzyme, the peroxo ligand is depicted in a μ-1,1-coordination mode with a protonated bridging ligand and is not in a μ-η(2):η(2) or μ-1,2 mode. Computed spectral data for the μ-1,1-coordination mode correlate well with experimental observations and unravel the potential of the energetics-spectroscopic approach adapted here.

Oxidation of methane by an N-bridged high-valent diiron–oxo species: electronic structure implications on the reactivity

Methane activation by dinuclear high-valent iron–oxo species: do we need two metals to activate such inert bonds? Our theoretical study using DFT methods where electronic structure details and mechanistic aspects are established answers this intriguing question.