Electron exchange coupling in a naturally occurring tetramangano cluster in the mineral helvite, (Mn4S)(SiBeO4)3

Abstract

The mineral helvite, (Mn4S)(BeSiO4)3, contains discrete tetrahedral Mn4S+6 clusters in which the S-2 is tetrahedrally coordinated and each Mn(II) is in a distorted tetrahedron of one S-2 and three oxygens; the cluster is situated within an encompassing lattice of SiO4-4 and BeO4-6 tetrahedra. Mn4S+6 centers provide an interesting model for comparison to the polynuclear manganese center that is associated with photosynthetic water oxidation. Magnetic susceptibility data between 77 and 298 K have been measured for a natural helvite sample containing principally Mn4S+6 centers but with significant contamination from Mn3FeS+6 and Mn3CaS+6. The data exhibited Curie-Weiss behavior with [mu]eff = 5.969 B.M. and [theta] = 178.3 K. An analysis of the magnetic susceptibility, based on Van Vleck's formalism, demonstrated the presence of antiferromagnetic coupling, with a coupling constant J = -5.83 cm-1. Mossbauer spectra of Mn3FeS centers in helvite and of Fe4S centers in the related mineral danalite have also been recorded. Isomer shifts show little temperature dependence and lie in the range 1.23-1.43 mm/sec.. This range is typical of tetrahedrally coordinated Fe(II) in several ionic crystals but is significantly above that of Fe(II) in ferredoxins and below that in the [quinone-Fe(II)-quinone] complex of the photosynthetic bacterium,Rhodopseudomonas sphaeroides. Quadrupole splittings are highly temperature dependent, ranging from 2.4 mm/sec at 4.2 K to less than 0.5 mm/sec at 248 K.