Pyrazolylborate−Zinc Complexes of RNA Precursors and Analogues Thereof

The Tp* ligand tris(3-cumenyl-5-methylpyrazolyl)borate was found
to stabilize zinc complexes of nucleobases,
of their natural precursors, and of nucleoside and nucleotide
derivatives. Dihydroorotic acid and orotic acid are
bound as monodentate carboxylate ligands. Uracil is coordinated
via its deprotonated N1; 6-methylthiouracil
acts as a bidentate ligand via N1 and S. Analogously, xanthine is
a monodentate ligand bound by its deprotonated
N7, while 6-mercaptopurine seems to bind in a bidentate fashion via N7
and S. Spectroscopic evidence indicates
coordination of uridine and 2‘,3‘-O-isopropylideneuridine
via their deprotonated N3, as well as of xanthosine via
N7. The hydrolytic cleavage of
2‘,3‘-O-isopropylideneuridine
5‘-(bis(p-nitrophenyl) phosphate) by Tp*Zn−OH
is preceded by an attachment of one Tp*Zn unit to the deprotonated
uracil base, presumably via N3.