In situ pyrazolylborate ligand synthesis and coordination behaviours in aluminum oxo clusters

Abstract

Herein we describe the discovery of the in situ synthesis of pyrazolylborate ligands and their coordination behaviours towards Al(III) ions. The in situ synthesis of pyrazolylborate is facile, inexpensive and the resistant ligands of varying steric demand are readily accessible. The occurrence of such an in situ synthesis is related to the presence of Al(ⁱOPr)₃ Lewis acids. Compared with the classical N,N′,N′′ tridentate cis-chelating pyrazolylborate scorpionate ligands, the in situ version herein having two pyrazolyls located at the opposite sides of the phenylboronic acid plane forms N,O,N′ trans-chelating ligands. We then studied the advantageous and coordination behaviours of the trans-coordinating ligands with Al(III) ions. The substituents on pyrazolyl and benzoyl rings have an important effect on the size of molecular clusters. The structures obtained by changing the functional groups (e.g. 3-F, 4-F, and 3-Cl) on the benzene ring are all dimeric molecules as shown in the AlOC-126 series. When changing both the substituents on pyrazole and benzene rings, higher nuclearity tetrahedral AlOC-127 and hexanuclear AlOC-128 were isolated. The in situ formation of the ligand represents a potential new direction for the development of trans-chelating scorpionate analogues.