Borohydride, Azide, and Chloride Anions As Terminal Ligands on Fe/Mo/S Clusters. Synthesis, Structure and Characterization of [(Cl₄-cat)(PPr₃) MoFe₃S₄(X)₂]₂(Bu₄N)₄ and [(Cl₄-cat)(PPr₃)MoFe₃S₄ (PPr₃)(X)]₂(Bu₄N)₂ (X = N₃^-, BH₄^-, Cl^-) Double-Fused Cubanes. NMR Reactivity

Our explorations of the reactivity of Fe/Mo/S clusters of some relevance to the FeMoco nitrogenase have led to new double-fused cubane clusters with the Mo₂Fe₆S₈ core as derivatives of the known (Cl₄-cat)₂Mo₂Fe₆S₈(PPr₃)₆ (I) fused double cubane. The new clusters have been obtained by substitution reactions of the PPr₃ ligands with Cl^-, BH₄^-, and N₃^-. By careful control of the conditions of these reactions, the clusters [(Cl₄-cat)(PPr₃)MoFe₃S₄(BH₄)₂]₂(Bu₄N)₄ (II), [(Cl₄-cat)(PPr₃)MoFe₃S₄(PPr₃)(BH₄)]₂(Bu₄N)₂ (III), [(Cl₄-cat)(PPr₃)MoFe₃S₄(N₃)₂]₂(Bu₄N)₄ (IV), [(Cl₄-cat)(PPr₃)MoFe₃S₄(PPr₃)(N₃)]₂(Bu₄N)₂ (V), and [(Cl₄-cat)(PPr₃)MoFe₃S₄Cl₂]₂(Et₄N)₄ (VI) have been obtained and structurally characterized. A study of their electrochemistry shows that the reduction potentials for the derivatives of I are shifted to more positive values than those of I, suggesting a stabilization of the reduced clusters by the anionic ligands BH₄^- and N₃^-. Using ¹H NMR spectroscopy, we have explored the lability of the BH₄^- ligand in II in coordinating solvents and its hydridic character, which is apparent in its reactivity toward proton sources such as MeOH or PhOH.