Iron(II), manganese(II) and cobalt(II) complexes containing tetradentate biphenyl-bridged ligands and their application in alkane oxidation catalysis

A series of manganese(II), iron(II) and cobalt(II) bis(triflate) complexes containing linear tetradentate bis(imine) and bis(amine) ligands with a biphenyl bridge have been synthesized. The twist in the ligand backbone due to the biphenyl unit leads in the case of the bis(imine) ligands (1 and 2) containing sp2 hybridised N donors, to a distorted cis-alpha coordination geometry, whereas in the case of the biphenyl- and biphenylether-bridged bis(amine) ligands (7 - 9 and 12), a trans coordination geometry is observed. The catalytic properties of the complexes for the oxidation of cyclohexane, using H2O2 as the oxidant, have been evaluated. Only the iron complexes show any catalytic activity under the conditions used, but the low conversions and selectivies observed indicate that these catalysts lead predominantly to free radical auto-oxidation.     

A role for confined water in chaperonin function

Chaperonins engulf other proteins and accelerate their folding by an unknown mechanism. Here, we combine all-atom molecular dynamics simulations with data from experimental assays of the activity of the bacterial chaperonin GroEL to demonstrate that a chaperonin's ability to facilitate folding is correlated with the affinity of its interior surface for water. Our results suggest a novel view of the behavior of confined water for models of in vivo protein folding scenarios.     

Theory for an order-driven disruption of the liquid state in water

Water is known to exhibit a number of peculiar physical properties because of the strong orientational dependence of the intermolecular hydrogen bonding interactions that dominate its liquid state. Recent full-atom simulations of water in a nanolayer between graphite plates submersed in an aqueous medium have raised the possibility of a new addition to this list of peculiarities: they show that application of a strong, uniform electric field normal to and between the plates can cause a pronounced decrease in particle density, rather than the increase expected from electrostriction theory for polarizable fluids [Vaitheeswaran et al., J. Phys. Chem. B 70, 6629 (2005)]. However, in seeming contradiction to this result, another study that simulated a range of similar systems has reported a less surprising electrostrictive increase in particle density upon application of the field [Bratko et al., J. Am. Chem. Soc. 129, 2504 (2007)]. In this work, we attempt to reconcile these conflicting simulation phenomena using a statistical mechanical lattice liquid model of water in an applied field. By solving the model using mean-field theory, we show that a field-induced transition to a markedly lower-density phase such as that observed in recent simulations is possible within a certain parameter regime, but that outside of this regime, the more conventional electrostrictive result should be obtained. Upon modifying the model to treat the case of bulk water under constant pressure in an applied field, we predict a density drop with rising field, and subsequently observe the predicted behavior in our own molecular dynamics simulations of liquid water. Our findings lead us to propose that the model considered here may be useful in a variety of contexts for describing the trade-off between orientational ordering of water molecules and their participation in the liquid phase.     

Detection of coherent oceanic structures via transfer operators

Coherent nondispersive structures are known to play a crucial role in explaining transport in nonautonomous dynamical systems such as ocean flows. These structures are difficult to extract from model output as they are Lagrangian by nature and not revealed by the underlying Eulerian velocity fields. In the last few years heuristic concepts such as finite-time Lyapunov exponents have been used in an attempt to detect barriers to oceanic transport and thus identify regions that trap material such as nutrients and phytoplankton. In this Letter we pursue a novel, more direct approach to uncover coherent regions in the surface ocean using high-resolution model velocity data. Our method is based upon numerically constructing a transfer operator that controls the surface transport of particles over a short period. We apply our technique to the polar latitudes of the Southern Ocean.     

A Synthetic High‐Spin Oxoiron(IV) Complex: Generation,Spectroscopic Characterization,and Reactivity

High versus low : The high‐yield generation of a synthetic high‐spin oxoiron(IV) complex, [Fe^IV(O)(TMG₃tren)]²⁺ (see picture, TMG₃tren = 1,1,1‐tris{2‐[N2‐(1,1,3,3‐tetramethylguanidino)]ethyl}amine), has been achieved by using the very bulky tetradentate TMG₃tren ligand, in order to both sterically protect the oxoiron(IV) moiety and enforce a trigonal bipyramidal geometry at the iron center, for which an S=2 ground state is favored.

     

Ligand topology variations and the importance of ligand field strength in non-heme iron catalyzed oxidations of alkanes

A series of iron(II)-bis(triflate) complexes [Fe(L)(OTf)2] containing linear tetradentate bis(quinolyl)-diamine and bis(quinolylmethyl)-diamine ligands with a range of ligand backbones has been prepared. The coordination geometries of these complexes have been investigated in the solid state by X-ray crystallography and in solution by 1H and 19F NMR spectroscopy. Because of the labile nature of high-spin iron(II) complexes in solution, dynamic equilibria of complexes with different coordination geometries (cis-alpha, cis-beta, and trans) are observed with certain ligand systems. In these cases, the geometry observed in the solid-state does not necessarily represent the only or even the major geometry present in solution. The ligand field strength in the various complexes has been investigated by variable-temperature (VT) magnetic moment measurements and by UV-vis spectroscopy. The strongest ligand field is observed with the most rigid ligand that generates [Fe(L)(OTf)2] complexes with a cis-alpha coordination geometry, and the corresponding [Fe(L)(CH3CN)2]2+ complex displays spin crossover behavior. The catalytic properties of the complexes for the oxidation of cyclohexane have been investigated using hydrogen peroxide as the oxidant. An increased flexibility in the ligand results in a weaker ligand field, which increases the lability of the complexes. The activity and selectivity of the catalysts appear to be related to the strength of the ligand field and the stability of the catalyst.     

Mechanistic studies of an unusual epoxide-forming elimination of a beta-hydroxyalkyl rhodium porphyrin

A new and remarkably facile sp3-C-O bond forming reaction of beta-hydroxyalkyl Rh porphyrins to form epoxides has been discovered and its mechanism investigated.