Single- and Double-Cubane Clusters in the Multiple Oxidation States [VFe₃S₄]^3+,2+,1+

Abstract

A new series of cubane-type [VFe(3)S(4)](z)() clusters (z = 1+, 2+, 3+) has been prepared as possible precursor species for clusters related to those present in vanadium-containing nitrogenase. Treatment of [(HBpz(3))VFe(3)S(4)Cl(3)](2)(-) (2, z = 2+), protected from further reaction at the vanadium site by the tris(pyrazolyl)hydroborate ligand, with ferrocenium ion affords the oxidized cluster [(HBpz(3))VFe(3)S(4)Cl(3)](1)(-) (3, z = 3+). Reaction of 2 with Et(3)P results in chloride substitution to give [(HBpz(3))VFe(3)S(4)(PEt(3))(3)](1+) (4, z = 2+). Reaction of 4 with cobaltocene reduced the cluster with formation of the edge-bridged double-cubane [(HBpz(3))(2)V(2)Fe(6)S(8)(PEt(3))(4)] (5, z = 1+, 1+), which with excess chloride underwent ligand substitution to afford [(HBpz(3))(2)V(2)Fe(6)S(8)Cl(4)](4)(-) (6, z = 1+, 1+). X-ray structures of (Me(4)N)[3], [4](PF(6)), 5, and (Et(4)N)(4)[6] x 2MeCN are described. Cluster 5 is isostructural with previously reported [(Cl(4)cat)(2)(Et(3)P)(2)Mo(2)Fe(6)S(8)(PEt(3))(4)] and contains two VFe(3)S(4) cubanes connected across edges by a Fe(2)S(2) rhomb in which the bridging Fe-S distances are shorter than intracubane Fe-S distances. Mössbauer (2-5), magnetic (2-5), and EPR (2, 4) data are reported and demonstrate an S = 3/2 ground state for 2 and 4 and a diamagnetic ground state for 3. Analysis of (57)Fe isomer shifts based on an empirical correlation between shift and oxidation state and appropriate reference shifts results in two conclusions. (i) The oxidation 2 --> 3 + e(-) results in a change in electron density localized largely or completely on the Fe(3) subcluster and associated sulfur atoms. (ii) The most appropriate charge distributions are [V(3+)Fe(3+)Fe(2+)(2)S(4)](2+) (Fe(2.33+)) for 1, 2, and 4 and [V(3+)Fe(3+)(2)Fe(2+)S(4)](3+) (Fe(2.67+)) for 3 and [V(2)Fe(6)S(8)(SEt)(9)](3+). Conclusion i applies to every MFe(3)S(4) cubane-type cluster thus far examined in different redox states at parity of cluster ligation. The formalistic charge distributions are regarded as the best current approximations to electron distributions in these delocalized species. The isomer shifts require that iron atoms are mixed-valence in each cluster.

VFe3S4 Single and Double Cubane Clusters: Synthesis, Structures, and Dependence of Redox Potentials and Electron Distribution on Ligation and Heterometal

“…With the generalized clusters [(Tp)MFe 3 S 4 L 3 ] z (M = V, Mo), potentials can be determined at constant M and z and variable L and at constant L and z and variable M. Comparisons at constant z and variable M involve clusters differing by one electron but with the same (mean) iron oxidation state based on 57 Fe isomer shifts. 18,19,21,33 In the case of z = 2−, for example, the [MFe 3 S 4 ] 2+ core is formalized as M 3+ + 3Fe 2.33+ (2Fe 2+ + Fe 3+ ). For z = 2− and 1−, iron atoms are electronically delocalized and redox changes are effectively localized in the Fe 3 portion of the core.…”

Hydroxide-Promoted Core Conversions of Molybdenum−Iron−Sulfur Edge-Bridged Double Cubanes: Oxygen-Ligated Topological PN Clusters

“…Suitable crystals of a solvated form of (Et 4 N) 3 [4] were acquired by ether diffusion into a 50:1 acetonitrile/water (v/ v) solution at room temperature. Crystals of solvated (Et 4 N) 3 [5] were obtained by either diffusion into a 50:1 acetonitrile/methanol (v/v) solution at room tempareature.…”

“…Suitable crystals of a solvated form of (Et 4 N) 3 [4] were acquired by ether diffusion into a 50:1 acetonitrile/water (v/ v) solution at room temperature. Crystals of solvated (Et 4 N) 3 [5] were obtained by either diffusion into a 50:1 acetonitrile/methanol (v/v) solution at room tempareature. Crystals of (Et 4 N) 4 [6]·7MeCN were obtained by the cooling of a concentrated acetonitrile solution to 243 K. Crystals of (Et 4 N) 5 [7]·7MeCN were grown by ether diffusion into an acetonitrile solution at room temperature.…”

“…Crystals of (Et 4 N) 4 [6]·7MeCN were obtained by the cooling of a concentrated acetonitrile solution to 243 K. Crystals of (Et 4 N) 5 [7]·7MeCN were grown by ether diffusion into an acetonitrile solution at room temperature. Crystals were coated with paratone-N oil and mounted on a Bruker APEX CCD-based diffactometer equipped with an Oxford 700 low-temperature apparatus operating at 193 K for compounds (Et 4 N) 3 [5], (Et 4 N) 4 [6], and (Et 4 N) 5 [7] and 173 K for compound (Et 4 N) 3 [4]. Data were collected with scans of 0.3 s/frame for 45 s, so that 1271-1850 frames were collected for a hemisphere of data for (Et 4 N) 3 [5], (Et 4 N) 4 [6], and (Et 4 N) 5 [7]; data for (Et 4 N) 3 [4] were collected with scans of 0.3 s/frame for 45 s. The first 50 frames were recollected at the end of the data collection to monitor for decay; no significant decay was detected for any compound.…”

“…4,5,10 As such, P N -type clusters are pertinent to the synthetic problem of nitrogenase cluster synthesis, especially if the P N and FeMo-cofactor clusters are antecedental, one to the other. The core composition of FeMoco, 13 MoFe 7 S 9 X = MoFe 7 (μ 2 -S) 3 (μ 3 -S) 6 (μ 6 -X), includes an interstitial atom X detected by X-ray crystallography, which allows X = C, N, or O. 14 Any ultimately successful synthetic approach to FeMoco or a close variant thereof must include at some point introduction of atom X.…”