Synthesis and Characterization of Sulfur-Voided Cubanes. Structural Analogues for the MoFe₃S₃ Subunit in the Nitrogenase Cofactor

Abstract

A new class of Mo/Fe/S clusters with the MoFe(3)S(3) core has been synthesized in attempts to model the FeMo-cofactor in nitrogenase. These clusters are obtained in reactions of the (Cl(4)-cat)(2)Mo(2)Fe(6)S(8)(PR(3))(6) [R = Et (I), (n)Pr (II)] clusters with CO. The new clusters include those preliminarily reported: (Cl(4)-cat)MoFe(3)S(3)(PEt(3))(2)(CO)(6) (III), (Cl(4)-cat)(O)MoFe(3)S(3)(PEt(3))(3)(CO)(5) (IV), (Cl(4)-cat)(Pyr)MoFe(3)S(3)(PEt(3))(2)(CO)(6) (VI), and (Cl(4)-cat)(Pyr)MoFe(3)S(3)(P(n)Pr(3))(3)(CO)(4) (VIII). In addition the new (Cl(4)-cat)(O)MoFe(3)S(3)(P(n)Pr(3))(3)(CO)(5) cluster (IVa), the (Cl(4)-cat)(O)MoFe(3)S(3)(PEt(3))(2)(CO)(6)cluster (V), the (Cl(4)-cat)(O)MoFe(3)S(3)(P(n)Pr(3))(2)(CO)(6) cluster (Va), the (Cl(4)-cat)(Pyr)MoFe(3)S(3)(P(n)Pr(3))(2)(CO)(6) cluster (VIa), and the (Cl(4)-cat)(P(n)Pr(3))MoFe(3)S(3)(P(n)Pr(3))(2)(CO)(6) cluster (VII) also are reported. Clusters III-VIII have been structurally and spectroscopically characterized. EPR, zero-field (57)Fe-Mössbauer spectroscopic characterizations, and magnetic susceptibility measurements have been used for a tentative assignment of the electronic and oxidation states of the MoFe(3)S(3) sulfur-voided cuboidal clusters. A structural comparison of the clusters with the MoFe(3)S(3) subunit of the FeMo-cofactor has led to the suggestion that the storage of reducing equivalents into M-M bonds, and their use in the reduction of substrates, may occur with the FeMo-cofactor, which also appears to have M-M bonding. On the basis of this argument, a possible N(2)-binding and reduction mechanism on the FeMoco-cofactor is proposed.

A Possible Prebiotic Formation of Ammonia from Dinitrogen on Iron Sulfide Surfaces

“…[15] Coucouvanis et al reported structural analogues for the MoFe 3 S 3 subunit in the nitrogenase cofactor and they discuss the accommodation and activation of N 2 by insertion into an FeÀFe bond. [18] Our approach uses the moderate reduction potential according to reaction (1) in the synthesis of ammonia by the following overall reaction (2; Scheme 1). [19] Thermodynamic calculations show that the redox system FeS-H 2 S/FeS 2 is able to reduce dissolved molecular nitrogen to ammonia.…”

Competing HH, SS, and MM Bond Formation in the “Shape‐Shifting” Cluster [Ru4S3(arene)4]2+

“…The slurry was stirred at 50 8C for 1 h. The solid was collected by filtration and washed with THF (20 mL). The dried brown powder was extracted into H 2 O (100 mL) this solution was treated with KPF 6 (2.50 g 13.3 mmol) to precipitated a mixture of [(cym) 3 Crystallography: Data was collected at À78 8C on a Siemens Platform/CCD automated diffractometer. Data processing was performed with SAINT PLUS version 6.22.…”

“…A space-filling model indicates steric crowding between the arene substituents, which would lead to restricted rotation about the ruthenium-arene bonds. Indeed, the 1 H NMR spectrum for 1 2+ is simple at 70 8C but becomes complex at lower temperatures as arene rotation is slowed (Figure 2 3 , where up and down refer to the orientation relative to the S 2 and S faces of the Ru 4 square (see Scheme 2). The number of isomers implies that rotation of the S 2 ligand is slow on the NMR timescale, such that two cis-(up) 2 (down) 2 isomers are implicated, those where the cisoid Me groups are parallel (k ; Scheme 2) or perpendicular (? )…”

“…We have shown that [{(cym)RuCl 2 } 2 ] (cym = 4-iPrC 6 H 4 Me) reacts with (Me 3 Si) 2 S to produce trigonal bipyramidal [(cym) 3 . [6] We have found that this simple synthesis also affords the previously unrecognized tetrametallic species [(cym) 4 In MeCN solution, cluster 1 2+ is stable at 80 8C in the presence of PPh 3 , and it is stable to UV photolysis.…”

Superclusters: A Host–Guest Complex with a Cyclic Array of Three Bridged MoFe3S4 Clusters

“…[3] Nevertheless, partial analogues exist among the plethora of Fe/Mo/S clusters. [4][5][6] [9] Black crystals of 2 suitable for single crystal X-ray structure determination [10] can be obtained upon recrystalli- core) show a single reversible reduction wave with a potential that varies from 0.83 to 1.31 V. [12] The EPR spectrum of 2 is rather complex and does not resemble the S = 3 = 2 signal of the single MoFe 3 S 4 cubane clusters. This is probably due to intercubane coupling interactions.…”