Generation of maghemite nanocrystals from iron–sulfur centres

Iron oxide nano-crystals 0.1–1.1 μm in diameter were generated on sulfur-doped amorphous carbon surfaces by electron beam irradiation of the novel 13e− high-spin complex [Fe(4-methyl-1,2-benzenedithiolate)2][NHEt3] encapsulated in a triblock copolymer. Possible relevance to iron nano-mineralization from Fe–S ferredoxin proteins and iron dysregulation in neurological disorders is discussed. Graphical abstract: Generation of maghemite nanocrystals from iron–sulfur centres Iron is an essential element for mammals and, amongst many other functions, plays an important role in the human brain.1 Recent research has indicated a strong association between iron dysregulation and Alzheimer's disease (AD), although it is unknown how the chemical and magnetic state of iron is linked to AD pathogenesis.2–4 Reports from Collingwood et al., and Dobson et al., for example, have shown the presence of iron oxide as the mixed oxidation state mineral, magnetite (Fe3O4) in AD tissue, a possible source of redox-active iron, but it remains unclear how this kind of iron mineral forms in the tissue.5,6 These unsolved and important questions have led us to consider how atomic resolution microscopy might provide new insight into nanoscale iron mineralization. Recently we reported methodology for studies of the nano-mineralisation of osmium, gold, ruthenium and iridium from their respective 1,2-dicarba-closo-dodecarborane-1,2-dithiolate complexes encapsulated in polymer micelles upon electron beam irradiation.7–9 Here we report the synthesis and characterization of the novel 13e− iron(iii) complex [Fe(4-methyl-1,2-benzenedithiolate)2][NHEt3] (1), containing Fe–S bonds analogous to those in the ubiquitous iron–sulfur ferredoxin proteins. Importantly, recent research has indicated a strong relationship between neurodegenerative disorders and defective Fe–S clusters.10,11 We have characterized complex 1 using Mössbauer, Raman and far-infra red spectroscopy, and investigated the generation of iron nanocrystals from 1 encapsulated in a poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) polymer (Scheme 1) by electron beam irradiation, and used electron energy-loss spectra (EELS) to identify the oxidation state of iron and its coordination environment in the nanocrystals

Kinetics and mechanism of sequential ring methyl C–H activation in cyclopentadienyl rhodium(iii) complexes

We have studied activation of the methyl C–H bonds in the cyclopentadienyl ligands of half-sandwich Rh(III) complexes [η5-CpXRh(N,N′)Cl]+ by observing the dependence of sequential H/D exchange on variations in CpX = Cp* (complexes 1 and 2), Me4PhCp (CpXPh, 3) or Me4PhPhCp (CpXPhPh, 4), and chelated ligand N,N′ (bpy, 1; phen, 2–4). H/D exchange was fastest in d4-MeOD (t1/2 = 10 min, 37 °C, complex 1), no H/D exchange was observed in DMSO/D2O, and d4-MeOD enhanced the rate in CD3CN. The proposed Rh(I)–fulvene intermediate was trapped by [4 + 2] Diels–Alder reactions with conjugated dienes and characterized. The Rh(I) oxidation state was confirmed by X-ray photoelectron spectroscopy (XPS). Influence of solvent on the mechanisms of activation and Diels–Alder adduct formation was modelled using DFT calculations with the CAM-B3LYP functional and CEP-31 g basis set, and influence on the reaction profile of the dimiine ligand and phenyl substituent using the larger qzvp basis set. The Rh(III)–OH intemediate is stabilised by H-bonding with methanol and a Cp* CH3 hydrogen. The Rh(I)(Me4fulvene) species, stabilised by interaction of methanol with a coordinated water, again by two H-bonds H2O–HOMe (1.49 Å) and fulvene CH2 (1.94 Å), arises from synchronous transfer of the methanol OH proton to a Rh(III)–OH ligand and Cp* methyl hydrogen to the methanol oxygen. Additionally, the observed trend in catalytic activity for complexes 1–4 was reproduced by DFT calculations. These complexes form a novel class of catalytic molecular motors with a tunable rate of operation that can be stalled in a given state. They provide a basis for elucidation of the effects of ligand design on the contributions of electronic, rotational and vibrational energies to each step in the reaction pathway at the atomic level, consideration of which will enhance the design principles for the next generation of molecular machines

New activation mechanism for half-sandwich organometallic anticancer complexes

The Cpx C–H protons in certain organometallic RhIII half-sandwich anticancer complexes [(η5-Cpx)Rh(N,N′)Cl]+, where Cpx = Cp*, phenyl or biphenyl-Me4Cp, and N,N′ = bipyridine, dimethylbipyridine, or phenanthroline, can undergo rapid sequential deuteration of all 15 Cp* methyl protons in aqueous media at ambient temperature. DFT calculations suggest a mechanism involving abstraction of a Cp* proton by the Rh–hydroxido complex, followed by sequential H/D exchange, with the Cp* rings behaving like dynamic molecular ‘twisters’. The calculations reveal the crucial role of pπ orbitals of N,N′-chelated ligands in stabilizing deprotonated Cpx ligands, and also the accessibility of RhI–fulvene intermediates. They also provide insight into why biologically-inactive complexes such as [(Cp*)RhIII(en)Cl]+ and [(Cp*)IrIII(bpy)Cl]+ do not have activated Cp* rings. The thiol tripeptide glutathione (γ-L-Glu-L-Cys-Gly, GSH) and the activated dienophile N-methylmaleimide, (NMM) did not undergo addition reactions with the proposed RhI–fulvene, although they were able to control the extent of Cp* deuteration. We readily trapped and characterized RhI–fulvene intermediates by Diels–Alder [4+2] cyclo-addition reactions with the natural biological dienes isoprene and conjugated (9Z,11E)-linoleic acid in aqueous media, including cell culture medium, the first report of a Diels–Alder reaction of a metal-bound fulvene in aqueous solution. These findings will introduce new concepts into the design of organometallic Cp* anticancer complexes with novel mechanisms of action

Abrupt Spin Crossover Behavior in a Linear N1,N2-Triazole Bridged Trinuclear Fe(II) Complex

The synthesis, structures and magnetic properties of a new trinuclear spin crossover complex [FeII3(pyrtrz)6(TsO)6]·10H2O·2CH3OH (C2) and its analogue binuclear [FeII2(pyrtrz)5(SCN)4]·7H2O (C1), are reported here. These two compounds are synthesized based on the pyrrolyl functionalized Schiff base 1,2,4-triazole ligand 4-((1H-pyrrol-2-yl)methylene-amino)-4H-1,2,4-triazole (pyrtrz), which represent rare discrete multi-nuclear species, with µ2-N1,N2-triazole bridges linking the FeII centers. DC magnetic susceptibility measurements revealed an abrupt single-step spin crossover (SCO) behavior for compound 2 on the central FeII site and single-crystal X-ray diffraction (173 K) showed that this compound crystallizes in the monoclinic space group (P21/c), and multiple intramolecular interactions were found responsible for the abrupt transition. Compound 1 is a binuclear complex with thiocyanate as terminal ligands. This compound stays in high spin state over the whole temperature range and displays weak antiferromagnetic exchange coupling

Estimate of the vibrational contribution to the entropy change associated with the spin transition in the d4 systems [MnIII(pyrol)3tren] and [CrII(depe)2I2].

The vibrational contribution to DeltaS of the low-spin ((3)T(1)) to high-spin ((5)E) spin transition in two 3d(4) octahedral systems [Mn(III)(pyrol)(3)tren] and [Cr(depe)(2)I(2)] have been estimated by means of DFT calculations (B3LYP/CEP-31G) of the vibrational normal-modes frequencies. The obtained value at the transition temperature for the Mn(iii) complex is DeltaS(vib)(44 K) = 6.3 J K(-1) mol(-1), which is comparable with the proposed Jahn-Teller contribution of R ln3 = 9.1 J K(-1) mol(-1) and which is approximately half of the experimentally determined 13.8 J K(-1) mol(-1). The corresponding value for the Cr(ii) complex is DeltaS(vib)(171.45 K) = 46.5 J K(-1) mol(-1), as compared to the experimental value of 39.45 J K(-1) mol(-1). The analysis of the vibrational normal modes reveals that for the d(4) systems under study, contrary to Fe(ii) d(6) systems, not all metal-ligand stretching vibrations make a contribution. For the Mn(iii) complex, the only vibration that contributes to DeltaS(vib) involve the nitrogens occupying the Jahn-Teller axis, while in the case of Cr(ii) the contributing vibrations involve the Cr-I bonds. Low-frequency modes due to ring vibrations, metal-ligand bending and movement of the molecule as a whole also contribute to the vibrational entropy associated with the spin transition

Relevance of supramolecular interactions, texture and lattice occupancy in the designer iron(II) spin crossover complexes

New Fe-II complexes of formula [Fe(3-Br-phen)(2)(NCS)(2)] Solvent (Solvent = 0.5 CH3OH (1), 2 Ch(2)Cl(2) (2), desolvation of 2 (3), 0.5 CH3COCH3 (4) and 0 (5)) have been synthesized. Fe-57 Mossbauer and magnetic investigation reveal unique features atypical of classic [Fe(phen)(2)(NCS)(2)] polymorphs. Complex 1, prepared by precipitation in MeOH, undergoes upon cooling below room temperature an incomplete and gradual thermally induced spin conversion, while 4 prepared by an extraction method remains mostly in the low-spin state. The non solvated compounds 3 and 5, display a more abrupt spin crossover oil cooling around T-1/2 = 175 K and T-1/2 = 198 K, respectively. Defects/soft lattice inclusion due to different methods of material synthesis, extent of aging, reaction medium and associated solvent molecules have enormous influence oil the particle size and magnetic properties of these complexes. Scanning electron micrographs helps to establish a logical relationship among methods employed for synthesis, texture of materials and their effect oil magnetic properties. The crystal structure of 2 determined in the monoclinic space group P2/c (100 K) reveals a mononuclear complex consisting of a distorted FeN6 octahedron in the low-spin state, constructed from two 3-bromo-1, 10-phenanthroline and two isothiocyanato anions in cis position. Intermolecular interactions between mononuclear units of the S center dot center dot center dot Br, S center dot center dot center dot C(H) and pi-pi type afford a 2D supramolecular network. DFT calculations for the single molecule 2 reveals an energy difference between high-spin and low-spin isomers of 7 kJ/mol suggesting a slight destabilization of the low-spin state compared to [Fe(phen)(2)(NCS)(2)]. Normal co-ordinate analysis was also carried out for 3 and compared with experimental temperature dependent Raman spectra for 5. (C) 2009 Elsevier Inc. All rights reserved

Vibrational properties and cooperativity of the 3D spin crossover network [Fe(pyrazine)][Pt(CN)4]\mathrm{[Fe(pyrazine)][Pt(CN)_{4}]}

Nuclear inelastic scattering of synchrotron radiation has been used to determine the phonon density of vibrational states (pDOS) for the high-spin and low-spin phases of the hydrated and dehydrated isomer of the spin crossover polymer [Fe(pyrazine)][Pt(CN)4]. Density functional theory calculations have been performed for molecular models of the 3D polymeric system. The models contain 15 Fe(II)/Zn(II) centres and allowed the assignment of the observed bands to the corresponding vibrational modes. Thermodynamic parameters like the mean force constant and the vibrational entropy but also sound velocities of the molecular lattices in both spin states have been derived from the pDOS. Modelling of the low-spin and high-spin centres in the environment or matrix of different spins has revealed the enthalpic and entropic components of the intramolecular cooperativity. In contrast to the 1D spin crossover systems (Rackwitz, et al., Phys. Chem. Chem. Phys., 2013, 15, 15450) based on the rigid 1,2,4-triazole derivatives the distortion of the low-spin iron Fe(II) centre by the matrix of high-spin Fe(II) (modelled as Zn(II)) occurs only in two dimensions, defined by the [M(CN)4]2− sheets, rather than concerning all six Fe–N bonds, as in 1D systems. The enthalpic intramolecular cooperativity has been determined to be 15 kJ mol−1 which is lower than that in 1D systems (20–30 kJ mol−1). Yet, the entropic contribution stabilizes the low-spin state in a low-spin matrix, a behaviour which is opposite to what was found for the 1D systems