Synthesis of new bitopic tetra(pyrazolyl)-ligands with neopentane and o-xylene backbones

Bitopic pyrazole-containing ligands attract interest because of their potential use as building blocks for crystal engineering and for synthesis of heterobimetallic complexes for catalysis. Several new bitopic pyrazole-containing ligands were prepared from the corresponding pyrazoles and tetrahalogen- or tetratosyloxy derivatives of o-xylene and neopentane in a superbasic medium (KOH-DMSO). The reaction of unsubstituted pyrazole with 1,3-dibromo-2,2-dibromomethylpropane yields a new tetrapyrazole ligand with a neopentane backbone, while no reaction occurs in case of 3,5-dimethylpyrazole. The product represents a bitopic ligand comprising two 1,3-bis(pyrazol-1-yl)propane units directly linked together. The use of tetratosyloxy- instead of tetrabromo-derivative gives previously unknown 1,1-bis(3,5-dimethylpyrazol-1-ylmethyl)oxetane, while the unsubstituted pyrazole in the same conditions yields a mixture tetrapyrazolyl-neopentane and dipyrazolyloxetane derivatives. A new bitopic ligand with aromatic o-xylene linker inaccessible by other routes was prepared from o-bis(dibromomethyl)benzene and pyrazole in a superbasic KOH-DMSO medium. In case of 3(5)-methylpyrazole and 3,5-dimethylpyrazole no products could be isolated, apparently due to steric hindrance.