A New Inorganic Ring System: Planar Fe3(μ2-SR)3 in 3- ---Synthesis, Structurs, and Solution Conformation and Equilibrium

Article abstract of DOI:10.1021/ja00278a040

Anaerobic reaction of equimolar FeX2 and RS^- in acetonitrile affords the trinuclear complexes ^3- (with X=Cl and R=Ph (4a), p-tolyl (4b), 2,6-Me2C6H3 (4d), and X=Br, R=Ph (4c)) isolated as Et4N⁺ salts.These species constitute the fourth known type of discrete Fe(II) thiolates, the others being ^2-, ^2-, and ^2-.The following crystallographic results were obtained (crystal system; space group; cell parameters; Z; unique data (I>3?(I)); R(percent)). (Et4N)3 (4a): monclinic; C2/c; a=24.500(6) Angstroem, b=13.488(3) Angstroem, c=18.217(5) Angstroem, β=110.68(2) deg, 4; 2679; 4.9. (Et4N)3(4d)*3MeCN: monoclinic: P2₁/n; a=15.07(1) Angstroem, b=22.38(3) Angstroem, c=20.46(3) Angstroem, β=91.5(1) deg; 4, 3311; 9.7. (Et4N)3 (4c)*MeCN: hexagonal, P62c; a=b=13.970(4) Angstroem, c=18.274(3) Angstroem; 2; 890; 5.7.The reaction system FeBr2 + NaSCH2Ph in acetonitrile yields a different type of compound with the composition (Et4N)2 (5): triclinic; P1; a=12.874(2) Angstroem, b=13.949(3) Angstroem, c=43.272(8) Angstroem, α=94.85(2) deg, β=94.07(2) deg, γ=114.98(1) deg; 4; 7402; 6.7.Anions of type 4 contain a new type of inorganic ring system, Fe3(μ2-SR)3, distinguised by its nearly or exactly planar conformation and unusually large Fe-S-Fe angles of ca. 140 deg, the largest values observed for this angle type.The phenyl groups in 4a and 4c (which has imposed C_3h symmetry) are coplanar with the Fe3S3 rings.In 4d these groups are rotated by ca. 90 deg owing to the steric effects of the methyl groups.Distorted tetrahedral coordination at each Fe(II) atom is completed by two halide ligands.The planar structure is to be contrasted with the chair-type conformation of the rings in 5 and other species with adamantane-like sterochemistry.Ring conformations influence the extent of antiferromagnetic interactions in the solid state.Magnetic susceptibility data of planar 4a are fit well by two -J values at 19-22 cm^-1.For ^2-, containing adamantane-like Fe4(μ2-SR)6 cages with Fe3(μ2-SR)3 rings in the chair conformation, best fits of the magnetic data give one -J value at 32 or 36 cm^-1 and one or two others at 42-66 cm^-1 depending on the coupling symmetry.MO calculations at the extended Hueckel level on ^3- suggest that the inherently stable conformation of the ring is planar.This structure is set in large measure by the sterochemical preference of the S-Fe-S angle and by transannular nonbonded repulsions.The decrease in Cl***Cl distances from 6.6 to 3.9 Angstroem in changing from the planar to the chair conformation emphasizes the latter effect.In acetonitrile the equilibrium 2^3- = ^2- + 2^2- has been approached inboth directions and is shifted to the left with decreasing temperature.At ca.<240 K the formation of ^3- is essentially complete.In this temperature range the magnetic moment per Fe atom (μ_Fe) is 4.1-4.2 μ_B for 4a in solution and as its crystalline Et4N⁺ salt.This excellent . . .