BINDING OF MOLYBDENUM IRON SULFUR CLUSTERS BY AMINO-ACID ESTERS

Reaction of both the double-cubane and monocubane clusters [NEt4]3[Mo2Fe7S8(mu-SEt)6(SEt)6] 1 and [NEt4]3[MoFe3S4(SEt)3{Fe(cat)3}] 2 (H-2cat = catechol) with L-cysteine ethyl ester hydrochloride (HCys-OEt.HCl) gives the common double-cubane product [NEt4]4[Mo2Fe7S8(mu-SEt)6(Cys-OEt.HCl)6] 3. The cysteinate-bound cluster has been characterised by H-1 NMR, UV/VIS, IR and Mossbauer spectroscopies. The conversion of the ethanethiolate-bound cluster 1 into the cysteinate cluster 3 proceeds through the chloro-intermediate [Mo2Fe7S8(mu-SEt)6Cl6]3- 4. Reaction of cluster 1 or 2 with L-tyrosine methyl ester hydrochloride forms only the chloro-substituted cluster 3 or [NEt4]3[MoFe3S4Cl3{Fe(cat)3}] 5, respectively. The monocubane chloro-substituted cluster 5 has been isolated by this and other routes, and fully characterised. On reaction of chloro-substituted dicubane 4 with 6 equivalents of tetraethylammoniUM L-tyrosinate methyl ester [NEt4][Tyr-OMe] the tyrosinate-bound cluster [Mo2Fe7S8(mu-SEt)6(Tyr-OMe)6]3- 6 is formed. Proton NMR, UV/VIS, IR and Mossbauer parameters are reported.