Rhenium(I)-Induced Cyclization of Thiosemicarbazones Derived from β-Keto Esters

The reactions of methylacetoacetate and ethyl 2-methylacetoacetate thiosemicarbazones (H2LA and H2LB, respectively)
with [ReX(CO)5] and [ReX(CO)3(CH3CN)2] (X = Cl, Br) were explored under various experimental conditions. Besides
the adducts fac-[ReX(CO)3(H2L)], in which the rhenium is coordinated to three carbonyl groups, the X anion, and
the N,S-bidentate thiosemicarbazone ligand, the following complexes were also isolated:  fac-[ReBr(CO)3(HpyzB)],
the tetrameric complexes fac-[Re(pyzA)(CO)3]4 and fac-[Re(pyzB)(CO)3]4, and fac-[Re(pyzB)(CO)3(H2O)] (where HpyzA
and HpyzB are pyrazolones derived by cyclization of H2LA and H2LB, respectively). The cyclization reactions were
monitored by 1H NMR spectroscopy and the complexes isolated were identified by elemental analysis, mass
spectrometry, IR and 1H NMR spectroscopy, and in some cases by X-ray diffractometry. The isolation and the full
structural identification of the rather unusual fac-[ReBr(CO)3(HpyzB)], which contains the enol form of the pyrazolone
ligand, affords new insight into the cyclization of thiosemicarbazones derived from β-keto esters.