Synthesis and Stereochemical Analysis of the n Series (n = 0, -1) Which Possesses a Cubanelike Fe4S4 Core: Direct Evidence for the Antibonding Tetrametal Character of the Unpaired Electron upon a One-Electron Reduction of a Completely Bonding Tetrahedral Metal Cluster

Article abstract of DOI:10.1021/ja00376a028

Presented herein are the preparations, properties, and structural determinations of the ^- monoanion and its neutral parent, which have provided a means of examination of the geometrical changes associated with the 0/-1 electron-transfer couple for a cubanelike Fe4S4 core coordinated to ?-acidic nitrosyl ligands.The sought-after black Fe4(NO)4(μ-S)4 tetramer was synthesized in 20percent yield by the reaction of Hg2 with sulfur under toluene reflux.Cyclic voltammetric measurements of Fe4(NO)4(μ-S)4 in CH2Cl2 solution revealed two reversible 0/-1 and -1/-2 couples.Its reduction by potassium benzophenone in toluene solution containing 2,2,2-cryptand afforded the corresponding black monoanion (as the ⁺ salt), which from room-temperature susceptibility measurements is presumed to contain one unpaired electron.Both compounds are reasonably air stable in crystalline form but decompose readily in solution.The most striking structural feature found upon the reduction of the 60-electron Fe4(NO)4(μ-S)4 cluster to its 61-electron monoanion is that the idealized cubic T_d-43m Fe4S4 core containing a completely bonding iron tetrahedron of average length 2.651 Angstroem in the neutral parent is slightly but significantly deformed in the monoanion toward tetragonal D_2d-42m symmetry, as evidenced by (1) a lengthening of the two Fe-Fe bonds normal to the resulting one S4-4 axis by 0.053 Angstroem ( to 2.703 (1) and 2.704 (1) Angstroem) relative to the average lengthening of the other four Fe-Fe bonds by 0.037 Angstroem (to a mean 2.688 Angstroem and range 2.682 (1)-2.695 (1) Angstroem) and (2) the six nonbonding S...S distances, which are equivalent under T_d symmetry, dividing into an inverse pattern of two shorter distances (of 3.493 (2) and 3.499 (2) Angstroem and four longer distances (of range 3.511 (2)-3.520 (2) Angstroem and mean 3.517 Angstroem).A concomitant average increase of only 0.014 Angstroem in the 12 Fe-S bond lengths of the Fe4S4 core is observed upon reduction to the monoanion, for which the partitioned 8 and 4 equivalent Fe-S distances under D_2d symmetry possess identical mean lengths of 2.231 Angstroem.These architectural variations can be rationalized via a qualitative metal cluster model that predicts that the unpaired electron in the monoanion occupies (under T_d symmetry) a triply degenerate t₁ level of largely tetrairon-antibonding character, which thereby produces the observed tetragonal distortion via a first-order Jahn-Teller vibronic effect; the relatively large bond-length increases in all six (instead of just two) Fe-Fe bonds suggest that the HOMO in the monoanion has considerable antibonding character among all six pairs of iron atoms.These collective findings provide a basic understanding of the geometrical differences imposed by one-electron redox couples on the Fe4S4 cores of the structurally determined ⁿ series (n = 0,1-), the 5-C5H5)4(μ₃-S)4>ⁿ series (n = 0, 1+, 2+), and ...