Isotopic exchange reactions between iron-sulphur-nitrosyl clusters and nitrite

The anion Fe₄S₃(NO)₇]^? undergoes slow exchange with labelled nitrite ¹⁵NO₂]^? to yield a product Fe₄S₃(¹⁴NO)(¹⁵NO)₆]^? in which complete isotopic exchange has occurred at the basal Fe(NO)₂ groups, but with no exchange at the apical Fe(NO) group. The neutral Fe₄S₄(NO)₄ reacts rapidly with ¹⁵NO₂^? to give fully exchanged Fe₄S₃(¹⁵NO)₇]^?, and it is proposed that the conversion proceeds by fragmentation, followed by complete isotopic exchange and rapid reassembly. The binuclear anion Fe₂S₂(NO)₄]^2? also yields, with ¹⁵NO₂]^2? in CD₂Cl₂ solution, the fully exchanged Fe₄S₃(¹⁵NO)₇]^?, and a mechanism involving successive fragmentation, exchange and reassembly steps is proposed; however in aqueous solution, a clean exchange reaction occurs to give Fe₂S₂(¹⁵NO)₄]^2?. Neutral binuclear esters Fe₂(SR)₂(NO)₄ (R = Me, Et, or Ph) with ¹⁴NO₂]^? yield the mononuclear paramagnetic Fe(¹⁴NO)₂(¹⁴NO₂)₂]^?, and with ¹⁵NO₂]^? the analogous Fe(¹⁵NO)₂(₁₅NO₂)₂]^?.