George Christou

TL;DR: In this paper, a trigonal pyramidal coordinated PbS3 unit was solved using data collected at approx. − 160°C and refined to conventional R values of 5.7 and 4.6%, respectively for 1 and 2.

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.

TL;DR: In this paper, the effects of the magnetic cation on the magnetization relaxation behavior of the [Mn12O12(O2CC6H4F(-o))16(H2O)4 ] was investigated for single-molecule magnet salts.