Highly stable cooperative distortion in a weak Jahn-Teller d 2 cation: Perovskite-type ScVO 3 obtained by high-pressure and high-temperature transformation from bixbyite

A novel ScVO 3 perovskite phase has been synthesized at 8 GPa and 1073 K from the cation-disordered bixbyite-type ScVO 3. The new perovskite has orthorhombic symmetry at room temperature, space group Pnma, and lattice parameters a = 5.4006(2) \uc5, b = 7.5011(2) \uc5, and c = 5.0706(1) \uc5 with Sc 3+ and V 3+ ions fully ordered on the A and B sites of the perovskite cell. The vanadium oxygen octahedra [V-O 6] display cooperative Jahn-Teller (JT) type distortions, with predominance of the tetragonal Q 3 over the orthorhombic Q 2 JT modes. The orthorhombic perovskite shows Arrhenius-type electrical conductivity and undergoes a transition to triclinic symmetry space group P-1 close to 90 K. Below 60 K, the magnetic moments of the 4 nonequivalent vanadium ions undergo magnetic long-range ordering, resulting in a magnetic superstructure of the perovskite cell with propagation vector (0.5, 0, 0.5). The magnetic moments are confined to the xz plane and establish a close to zigzag antiferromagnetic mode. \ua9 2011 American Chemical Society.Peer reviewed: YesNRC publication: Ye

Characterization of a tricationic trigonal bipyramidal iron(IV) cyanide complex, with a very high reduction potential, and its iron(II) and iron(III) congeners

Currently, there are only a handful of synthetic S = 2 oxoiron(IV) complexes. These serve as models for the high-spin (S = 2) oxoiron(IV) species that have been postulated, and confirmed in several cases, as key intermediates in the catalytic cycles of a variety of nonheme oxygen activating enzymes. The trigonal bipyramidal complex [FeIV(O)(TMG3tren)]2+ (1) was both the first S = 2 oxoiron(IV) model complex to be generated in high yield and the first to be crystallographically characterized. In this study, we demonstrate that the TMG3tren ligand is also capable of supporting a tricationic cyanoiron(IV) unit, [FeIV(CN)(TMG3tren)]3+ (4). This complex was generated by electrolytic oxidation of the high-spin (S = 2) iron(II) complex [FeII(CN)(TMG3tren)]+ (2), via the S = 5/2 complex [FeIII(CN)(TMG3tren)]2+ (3), the progress of which was conveniently monitored by using UV−vis spectroscopy to follow the growth of bathochromically shifting ligand-to-metal charge transfer (LMCT) bands. A combination of X-ray absorption spectroscopy (XAS), Mössbauer and NMR spectroscopies was used to establish that 4 has a S = 0 iron(IV) center. Consistent with its diamagnetic iron(IV) ground state, extended X-ray absorption fine structure (EXAFS) analysis of 4 indicated a significant contraction of the iron-donor atom bond lengths, relative to those of the crystallographically characterized complexes 2 and 3. Notably, 4 has an FeIV/III reduction potential of ∼1.4 V vs Fc+/o, the highest value yet observed for a monoiron complex. The relatively high stability of 4 (t1/2 in CD3CN solution containing 0.1 M KPF6 at 25 °C ≈ 15 min), as reflected by its high-yield accumulation via slow bulk electrolysis and amenability to 13C NMR at −40 °C, highlights the ability of the sterically protecting, highly basic peralkylguanidyl donors of the TMG3tren ligand to support highly charged high-valent complexes

Oxoiron(IV) Complex of the Ethylene-Bridged Dialkylcyclam Ligand Me2EBC

We report herein the first example of an oxoiron(IV) complex of an ethylene-bridged dialkylcyclam ligand, [FeIV(O)(Me2EBC)(NCMe)]2+ (2; Me2EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane). Complex 2 has been characterized by UV–vis, 1H NMR, resonance Raman, Mössbauer, and X-ray absorption spectroscopy as well as electrospray ionization mass spectrometry, and its properties have been compared with those of the closely related [FeIV(O)(TMC)(NCMe)]2+ (3; TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), the intensively studied prototypical oxoiron(IV) complex of the macrocyclic tetramethylcyclam ligand. Me2EBC has an N4 donor set nearly identical with that of TMC but possesses an ethylene bridge in place of the 1- and 8-methyl groups of TMC. As a consequence, Me2EBC is forced to deviate from the trans-I configuration typically found for FeIV(O)(TMC) complexes and instead adopts a folded cis-V stereochemistry that requires the MeCN ligand to coordinate cis to the FeIV═O unit in 2 rather than in the trans arrangement found in 3. However, switching from the trans geometry of 3 to the cis geometry of 2 did not significantly affect their ground-state electronic structures, although a decrease in ν(Fe═O) was observed for 2. Remarkably, despite having comparable FeIV/III reduction potentials, 2 was found to be significantly more reactive than 3 in both oxygen-atom-transfer (OAT) and hydrogen-atom-transfer (HAT) reactions. A careful analysis of density functional theory calculations on the HAT reactivity of 2 and 3 revealed the root cause to be the higher oxyl character of 2, leading to a stronger O---H bond specifically in the quintet transition state

A more reactive trigonal-bipyramidal high-spin oxoiron(IV) complex with a cis-labile site

The trigonal-bipyramidal high-spin (S = 2) oxoiron(IV) complex [FeIV(O)(TMG2dien)(CH3CN)]2+ (7) was synthesized and spectroscopically characterized. Substitution of the CH3CN ligand by anions, demonstrated here for X = N3– and Cl–, yielded additional S = 2 oxoiron(IV) complexes of general formulation [FeIV(O)(TMG2dien)(X)]+ (7-X). The reduced steric bulk of 7 relative to the published S = 2 complex [FeIV(O)(TMG3tren)]2+ (2) was reflected by enhanced rates of intermolecular substrate oxidation