Synthesis, Structure, and Bonding of the 3-S)2(μ3-NCMe3)2>n Series (n = 0, -1) Containing a Cubane-Like Fe4S2N2 Core: Stereochemical Influence due to One-Electron Reduction of a Completely Bonging Tetrahedral Metal System

Article abstract of DOI:10.1021/ja00367a017

The preparations, spectral properties, structures, and resulting bonding implications of the cubane-like Fe4(NO)4((μ₃-S)2(μ₃-NCMe3)2 molecule and its monoanion as the bis(triphenylphosphine) iminium salt are reported.The neutral tetrairon cluster was obtained accidentally in an attempt to prepare the (as yet) unknouwn iron analogue of the previously characterized cubane-like Co4(NO)4(μ₃-NCMe3)4 molecule by the reaction of Hg2 with (Me3CN)2S in refluxing benzene.An X-ray diffraction study of this molecular compound containing an unprecendented cubane-like Fe4S2N2 core has allowed an assessment of the stereochemical effect of the electronically equivalent S- and RN-bridged ligands on a completely bonding iron tetrahedron within the same molecule.A subsequent structural determination of its monoanion, generated by the reduction of the neutral parent with Na/Hg amalgam in THF and isolated after an exchange of the PPN⁺ cation for the Na⁺ ion in solution, has provided an operational test from a comparative geometrical analysis in experimentally differentiating between two possible limiting configurations (suggested by a qualitative MO metal cluster model) upon the addition of an electron to a completely bonding tetrahedral metal system.The neutral tetramer experimentally conforms to C_2v-2mm symmetry with one mirror plane crystallographically required; no variations in the resulting two sets of either the Fe-S or Fe-N bond lengths are observed (i.e., both sets have the same means of 2.22 and 1.91 Angstroem, respectively).The geometry of the Fe2S2 face (possessing an Fe-Fe single-bond length of 2.642 (1) Angstroem) of the Fe2S2N2 core is virtually identical (within 0.01 Angstroem and 0.5<*>) with that of the six equivalent Fe2S2 faces of the Fe4S4 core in Fe4(NO)4(μ₃-S)4; the geometry of the Fe2N2 face, for which the Fe-Fe single-bond length of 2.496 (1) Angstroem is expectedly shorter by 0.15 Angstroem, likewise compares favorably with that of the two (Fe-Fe)-bonded Fe2N2 fragments in Fe3(CO)9(μ₃NMe)2 (which contains an isosceles iron triangle with one nonbonding edge).The intermediate Fe-Fe bond lengths of mean 2.562 Angstroem in the four chemically equivalent Fe2SN faces correspond to the calculated mean (2.57 Angstroem) of the Fe-Fe bond lengths for the Fe2S2 and Fe2N2 faces.The monoanion, which retains the idealized C_2v-2mm architecture, exhibits relatively large Fe-Fe bond-length increases of 0.059 Angstroem in the Fe2S2 face and 0.056 Angstroem in the Fe2N2 face in contrast to a smaller average increase of 0.012 Angstroem in each of the four chemically equivalent Fe2SN faces.A concomitant lengthening of the four equivalent Fe-S bonds in the Fe2S2 face and two equivalent Fe-S bonds in the Fe2SN faces by 0.025 and 0.038 Angstroem, respectively, is counterbalanced by a shortening of the four equivalent Fe-N bonds in the Fe2N2 face and two equivalent Fe-N bonds in the Fe2NS faces...