THE [2/2]-SITE DIFFERENTIATED CLUSTERS [FE(4)S(4)L(2)(RNC)(6)] CONTAINING 2 LOW-SPIN IRON(II) SITES

Whereas the chemistry of [1:3] site-differentiated Fe4S4 cubane-type clusters has been extensively explored, [2:2] clusters have not been similarly examined. A class of such clusters has been prepared and studied. Reaction of [Fe(4)S(4)X(4)](2-) (X = Cl-, Br-) with excess isonitrile in refluxing THF affords [Fe(4)S(4)X(2)(RNC)(6)](R = Me, t-Bu) in good yield. Substitution reactions readily occur at the halide sites to give [Fe(4)S(4)L(2)(t-BuNC)(6)] (L = RO(-), RS(-)) Thiolate clusters can also be prepared by reaction of [Fe4S4(SR)(4)](2-) With exdess RNC in the presence of Et(3)NH(+). Mossbauer spectroscopy indicates the presence of two tetrahedral Fe(III) and two low-spin Fe(II) sites, which was confirmed by X-ray structural studies of two clusters. The compounds [Fe(4)S(4)L(2)(t-BuNC)(6)] (L = p-MeC(6)H(4)O(-)(7), PhS(-) (8)) crystallize in monoclinic space, group P2(1)/n with Z = 4. For 7/8, a = 16.536(3)/21.900(6) Angstrom, b = 12.133(2)/11.994(7) Angstrom, c 31.872(6)/22.865(7) Angstrom, and beta = 95.62(4)/101.69(2)degrees. The clusters closely approach idealized C-2 symmetry. Opposite faces of the [Fe4S4](2+) cores contain two six-coordinate low-spin sites (FeS3)-S-II(t-BuNC)(3) and two tetrahedral sites Fe(III)S(2)L(2). The low-spin sites are isolated from each other by 3.46 Angstrom and are separated from the tetrahedral sites by 3.0-3.1 Angstrom. The two tetrahedral sites are separated by Fe-Fe distances of 2.766(2) Angstrom (7) and 2.730(4) Angstrom (8) and are implicated in antiferromagnetically coupled [Fe2S2](2+) fragments. Core structures of three [2:2] clusters are essentially congruent, thereby defining the structural motif for [Fe4S4](2+) clusters containing two tetrahedral Fe(III) and two low-spin six-coordinate Fe(II) sites. The redox characteristics of the clusters are reported. The set [Fe(4)S(4)L(2)(RNC)(6)] (L = halide, RO(-), RS(-)) comprises the largest and most thoroughly characterized class of [2:2] site-differentiated clusters.