ISONITRILE BINDING TO A SITE-DIFFERENTIATED SYNTHETIC ANALOG OF BIOLOGICAL [4FE-4S] CLUSTERS - EQUILIBRIA, MAGNETIC-INTERACTIONS AND THE SPIN-ISOLATED [3FE-4S] CLUSTER FRAGMENT, AND THE STRUCTURE OF A LOW-SPIN IRON(II) SUBSITE

The cubane-type cluster [Fe4S4(LS3)Cl]2- [3, LS3 = tris[(4,6-dimethyl-3-mercaptophenyl)thio]-2,4,6-tris(ρ-tolylthio)benzene(3-)] contains iron subsites differentiated in the ratio 3:1. Cluster 3 undergoes substitution reactions at the unique subsite with stoichiometric cyanide and excess isonitrile in acetonitrile solution to afford [Fe4S4(LS3)(CN)]2- (4) and [Fe4S4(LS3)(RNC)3]1- [R = Me (5), Et, t-Bu (7), C6H11, and 2,6-Me2C6H3 (9)], respectively. The reaction products were identified by their characteristic isotropically shifted 1H NMR spectra, which are consistent with trigonal symmetry. Equilibrium constants for the binding of three RNC ligands range from 380 (5) to 4.9. cross. 105 M-2 (9) at 297 K. The reactions have large negative entropy changes and are moderately exothermic: ΔH = -23 (5) and -27 (7) kcal/mol. Mössbauer spectroscopic and magnetic susceptibility studies of (Ph4P)[7] demonstrate the existence of a low-spin Fe(II) subsite, assigned to the trigonal Fe(RNC)3 group, and a spin-isolated [Fe3S4]° cluster fragment. The latter is shown to have the same ground-state electronic structure as singly reduced 3Fe clusters of proteins: an S = 2 state derived from a high-spin Fe3+ subsite (S = 5/2) and a pair of ferromagnetically coupled, electronically delocalized Fe atoms (Fe2.5+, S = 9/2). Given the common spin ground state, the impressive similarity among zero-field-splitting parameters and magnetic hyperfine tensors, and the different structural environments, it is highly probable that the spin-coupled S = 2 ground state is an intrinsic property of the [Fe3S4]° core. This state explains the much larger 1H isotropic shifts of the isonitrile clusters compared to 4. The cyanide cluster binds only one strong-field ligand and retains the conventional S = 0 ground state of [Fe4S4]2+ clusters such as 3. The failure to detect clusters with one or two RNC ligands indicates that for the stepwise formation constants, K2-3 ≫ K1-2, K0-1. This situation is brought about by the ligand field stabilization energy of low-spin Fe(II), whose formation is induced by the binding of three strong-field ligands. The reaction of [Fe4S4Cl4]2- with excess t-BuNC in THF gives [Fe4S4Cl2(t-BuNC)6] in 60%yield. This compound crystallizes in monoclinic space group C2/c with a = 11.834 (2) Å, b = 21.195 (3) Å, c = 18.523 (3) Å, β = 93.64 (1)°, and Z = 4. It contains the cubane [Fe4S4]2+ core, which has two tetrahedral FeS3Cl subsites of standard dimensions [Fe-Fe = 2.741 (3) Å] and two trigonal-octahedral FeS3(t-BuNC)3 subsites whose Mössbauer properties are nearly identical with those of the unique subsite of 7. The six-coordinate subsites are separated from each other by 3.444 (3) Å and from the tetrahedral sites by 2.994 (3) Å. It is probable that the unique site in [Fe4S4(LS3)(RNC)3]1- has a closely similar stereochemistry and is separated from the Fe atoms of the spin-isolated Fe3S4 cluster fragment by > 3 Å. The structure of the cluster is analyzed and compared with that of the cubane [Fe4S4(CO)12], which contains low-spin FeIIIS3(CO)3 subsites. © 1990, American Chemical Society. All rights reserved.