Catalysis design for ring-opening polymerization of cyclic esters: 1. Group 1 metal and thallium(I) trispyrazolylborate complexes with hemilabile ligands

The synthesis of 3-(2-methoxy-1,1-dimethylethyl)pyrazole, pz*H is described together with its reactions with the borohydrides MBH(4), where M = Li, Na, and K, under melt conditions. At 180 degrees C, this procedure leads to a mixture of products for M = Li, and at higher temperatures, a derivative LiTp'pz*H, 1, is isolated, wherein a B-H bond and a methyl group have been eliminated and a B-O bond has been formed. For M = Na, the reaction proceeds to give the tris-pyrazolylborate derivative NaTp*, 2, but at higher temperatures the tetra-pyrazolylborate complex NaB(pz*)(4), 3, is obtained. The reactions involving KBH4 and pz*H yield the dinuclear complex K(2)(Tp*)(2)pz*H, 4. The reaction between NaTp* and TlOAc in CH(2)Cl(2) at room temperature leads to the formation of TlTp*, 5, along with NaOAc. Thallium 5 reacts with methyllithium in diethylether to give LiTp*, 6, and thallium metal, and, similarly, 5 and KH react in tetrahydrofuran to give KTp*, 7 and Tl(0). 1-7 have been characterized by elemental analysis, NMR spectroscopy, and by single-crystal X-ray studies, the latter of which reveal the versatile modes of binding for this new ligand bearing hemilabile ether appendages.