Alternative Spin States in Synthetic Analogues of Biological Clusters: Spin-Quartet Ground States and Structures of [Fe4S4(Sph)4]3- and [Fe4Se4(Sph)4]3- As Theirtetramethylammonium Salts
The compounds (Me4N)3[Fe4X4(SPh)4]·2MeCN [X = S (1), Se (2)] were prepared in good yields by chemical reduction of the corresponding dianion cluster salts and were thoroughly characterized as part of an investigation of alternative spin states in cubane-type clusters having the [Fe(μ3-X)4]+ core. Clusters with this core unit are found in reduced ferredoxins and other Fe-S proteins and enzymes. The two compounds obey the Curie-Weiss law over the temperature ranges 2.1-15 K (1) and 7.0-50 K (2) with Curie constants of 1.891 (1) and 1.883 (2) emu K/G, consistent with a S = 3/2 ground state. Magnetization behaviors between 1.8 and 100 K and at fields up to 50 kOe were successfully analyzed in terms of a spin Hamiltonian with 5 = 3/2. Mössbauer spectra revealed isomer shifts similar to those of other reduced clusters and independent of chalcogenide atom X, very small magnetic splittings in large applied magnetic fields, and negative hyperfine fields at all 57Fe sites. Compound 1 crystallizesin monoclinic space group C2/c with a = 22.868 (6) Å,b= 11.211 (2) Å, c = 20.387 (4) Å, β = 90.08 (2)°, and Z = 4. Compound 2 was obtained in orthorhombic space group Fddl with a = 39.386 (8) Å, b = 23.991 (5) Å, c = 11.471 (2) Å, and Z = 8. Refinement of compounds 1/2 with use of 2839/1957 unique data converged at R = 4.44%/3.32%. Clusters 1 and 2 have very similar structures. Core Fe-X bond lengths may be organized into a set of four long plus eight short with mean values (Å) of 2.344 (6) + 2.287 (19) for 1 and 2.488 (4) + 2.403 (12) for 2, with the long bonds approximately perpendicular to the direction of a C2 axis crystallographically imposed in each case. The cores have tetragonally elongated (D2d) configurations with the elongation along the direction of an idealized 4 axis. The magnetic and spectroscopic results, together with crystallographic definition of the clusters, establish beyond question that the reduced clusters [Fe4X4(SPh)4]3-. (X = S, Se) can stabilize electronic configurations with spin-quartet ground states. Examples of “gnonstandard” biological clusters with S > 1/2 are cited, including the S = 3/2 cluster of the Fe protein of Azotobacter vinelandii nitrogenase. All aspects of the present work support the published spin-state assignment of this cluster. While it may be significant that 1 and 2 have elongated tetragonal cores and S = 3/2 ground states, examination of all results for reduced clusters does not yet afford a clear spin-state-structure correlation.
MSU Digital Commons Citation
Carney, M. J.; Whitener, Mark; Holm, Richard H.; Papaefthymiou, G. C.; Spartalian, K.; and Frankel, R. B., "Alternative Spin States in Synthetic Analogues of Biological Clusters: Spin-Quartet Ground States and Structures of [Fe4S4(Sph)4]3- and [Fe4Se4(Sph)4]3- As Theirtetramethylammonium Salts" (1988). Department of Chemistry and Biochemistry Faculty Scholarship and Creative Works. 312.