George Christou

Bio: George Christou is an academic researcher from Imperial College London. The author has contributed to research in topics: Paramagnetism & Magnetic susceptibility. The author has an hindex of 3, co-authored 4 publications receiving 90 citations.

Preparation and Physical Properties of Trinuclear Oxo-Centered Manganese Complexes of the General Formulation (Mn30(02CR)6L3)09+ (R = Me or Ph;

Abstract: The reaction of N-n-Bu4Mn04 with appropriate reagents in nonaqueous solvents leads to the high-yield formation of trinuclear oxo-centered Mn complexes of general formulation (Mn30(02CR)6L3)z+ (1, R = Me, L = pyr, z = 1; 2, R = Me, L = pyr, z = 0, monopyridine solvate; 3, R = Me, L = pyr, z = 0, unsolvated; 4, R = Ph, L3 = (pyr),(H20), z = 0; 5, R = Me, L = HIm, z = 1; pyr = pyridine, HIm = imidazole). The crystal structures of complexes 2 and 4 have been determined. Complex 2 crystallizes in rhombohedral space group R32 with (at -50 "C) a = b = 17.552 (6) A, c = 10.918 (3) A, y = 120.00 (1)O, and Z = 3. A total of 1546 unique data with F > 3u(F) were refined to conventional values of R and R, of 5.77 and 5.86%, respectively. Complex 4 crystallizes in monoclinic space group P2, with (at -156 "C) a = 15.058 (IO) A, b = 23.600 (17) 8, c = 14.959 (10) A, fl = 91.01 (3)", and Z = 2. A total of 7174 unique data with F > 3a (6 were refined to values of R and R, of 8.64 and 8.43%, respectively. Both 2 and 4 possess an oxo-centered Mn,O unit characteristic of "basic carboxylates" with peripheral ligation provided by bridging carboxylate and terminal pyr (or H20) groups. Each Mn is distorted octahedral, and consideration of overall charge of the trinuclear units necessitates a mixed-valence Mn"Mn2" description. In 2, the presence of a C3 axis through the central 0 and perpendicular to the Mn30 unit necessitates the Mn centers to be crystallographically equivalent, suggesting rapid intramolecular electron transfer or electronic delocalization. In contrast, 4 possesses no imposed symmetry elements and is in a trapped-valence situation in accord with its mixed-ligand nature, since two Mn centers have a terminal pyr group while the third Mn has a terminal H20 molecule. The latter metal center is assigned as the Mn" ion based on its longer metal-ligand distances. Variable temperature solid state magnetic susceptibility studies have been performed on 1-4 in the temperature range of -6 to -340 K. Satisfactory fits to the observed susceptibility data have been obtained by assuming isotropic magnetic exchange interactions and employing the appropriate spin Hamiltonians and derived susceptibility equations. All Mn centers are shown to be in high-spin electronic configurations and to be antiferromagnetically coupled. The derived exchange parameters are all relatively small in magnitude, IJ1 < 11 cm-'. In addition, it is found that the antiferromagnetic exchange interactions are smaller for the Mn,O units than those for isostructural Fe30 systems, and a rationalization is proposed. Since phase transitions had previously been characterized by heat capacity measurements on (Fe30(02CMe),(pyr),)(pyr), isostructural complex 2 was investigated by differential scanning calorimetry (DSC) in the temperature range 153-303 K. An exothermic thermal effect is clearly evident with a peak at 184.7 K in the cooling curve (the other mixed-valence Mn30 systems showed no thermal effects in their DSC thermograms). The origin of this thermal effect and comparisons with the corresponding behavior of the Fe complex are described.

Effects of paramagnetic (Fe(C 5 Me 5 ) 2 ) + cation on the anionic single-molecule magnet, (Mn 12 O 12 (O 2 CC 6 H 4 F(-o)) 16 (H 2 O) 4 ) −

Abstract: The preparation and physical characterization are reported for the single-molecule magnet salts A[Mn12O12(O2CC6H4F(-o))16(H2O)4 ]( A + =PPh4 + (2a), Fe(C5Me5)2 + (2b), and Co(C5Me5)2 + (2c)). The effects of the magnetic cation on the magnetization relaxation behavior of the [Mn12] − anions are investigated. All complexes exhibit out-of-phase ac magnetic susceptibility ( M) signals in the 4.8–5.1 K range at 1 kHz ac frequency. The temperature of the M peaks is frequency dependent, as expected for a single-molecule magnet. From Arrhenius plots of the frequency dependence of the temperature of the M peaks, the effective energy barriers Ueff for changing the magnetization direction from spin ‘‘up’’ to spin ‘‘down’’ were estimated to be in the 52–57 K range. Magnetization hysteresis loops were observed for all the complexes studied. They show clear hysteresis loops with steps, indicating the effect of the magnetic cation on the magnetization relaxation of the anionic [Mn12] − complex is rather small. The least-squares fittings of variable-field magnetization data show the ground state of complex 2a is best described as S= 21/2 with g=1.96 and D =− 0.56 K, while complexes 2b and 2c have S= 19/2 ground states. The fitting parameters are g= 1.96 and D =− 0.54 K for complex 2b and g=1.95 and D =− 0.57 K for complex 2c. These analyses show the magnetic cation has essentially no effect on the ground state spin or on the parameters g and D for the [Mn12] − anion. © 2001 Elsevier Science Ltd. All rights reserved.

Reexamination of lead(II) coordination preferences in sulfur-rich sites: implications for a critical mechanism of lead poisoning.

Abstract: Recent studies suggest that the developmental toxicity associated with childhood lead poisoning may be attributable to interactions of Pb(II) with proteins containing thiol-rich structural zinc-binding sites. Here, we report detailed structural studies of Pb(II) in such sites, providing critical insights into the mechanism by which lead alters the activity of these proteins. X-ray absorption spectroscopy of Pb(II) bound to structural zinc-binding peptides reveals that Pb(II) binds in a three-coordinate Pb(II)-S3 mode, while Zn(II) is known to bind in a four-coordinate mode in these proteins. This Pb(II)-S_3 coordination in peptides is consistent with a trigonal pyramidal Pb(II)-S_3 model compound previously reported by Bridgewater and Parkin, but it differs from many other reports in the small molecule literature which have suggested Pb(II)-S_4 as a preferred coordination mode for lead. Reexamination of the published structures of these “Pb(II)-S_4” compounds reveals that, in almost all cases, the coordination number of Pb is actually 5, 6, or 8. The results reported herein combined with this new review of published structures suggest that lead prefers to avoid four-coordination in sulfur-rich sites, binding instead as trigonal pyramidal Pb(II)-S_3 or as Pb(II)-S_(5-8). In the case of structural zinc-binding protein sites, the observation that lead binds in a three-coordinate mode, and in a geometry that is fundamentally different from the natural coordination of zinc in these sites, explains why lead disrupts the structure of these peptides and thus provides the first detailed molecular understanding of the developmental toxicity of lead.

The Mn Cluster in the S0 State of the Oxygen-Evolving Complex of Photosystem II Studied by EXAFS Spectroscopy: Are There Three Di-μ-oxo-bridged Mn2 Moieties in the Tetranuclear Mn Complex?

Abstract: A key component required for an understanding of the mechanism of the evolution of molecular oxygen by the photosynthetic oxygen-evolving complex (OEC) in photosystem II (PS II) is the knowledge of the structures of the Mn cluster in the OEC in each of its intermediate redox states, or S-states. In this paper, we report the first detailed structural characterization using Mn extended X-ray absorption fine structure (EXAFS) spectroscopy of the Mn cluster of the OEC in the S0 state, which exists immediately after the release of molecular oxygen. On the basis of the EXAFS spectroscopic results, the most likely interpretation is that one of the di-μ-oxo-bridged Mn−Mn moieties in the OEC has increased in distance from 2.7 A in the dark-stable S1 state to 2.85 A in the S0 state. Furthermore, curve fitting of the distance heterogeneity present in the EXAFS data from the S0 state leads to the intriguing possibility that three di-μ-oxo-bridged Mn−Mn moieties may exist in the OEC instead of the two di-μ-oxo-bridged...