Synthesis of the new ligand bis[3-(2-pyrazinyl)-pyrazol-1-yl]dihydroborate, and the crystal structures of its complexes with thallium(I) and lead(II)

Reaction of 3-(2-pyrazinyl)pyrazole with KBH4 in a 2.1:1 ratio afforded the new ligand bis[3-(2-pyrazinyl)-pyrazol-1-yl]dihydroborate, [L](-), a bis(p yrazolyl)borate in which each pyrazolyl ring is functionalised with a pyraz in-2-yl group at the C-3 position.[L](-) is therefore a potentially chelati ng tetradentate ligand with two externally-directed N atoms (the pyrazinyl N-4 atoms) which are available for additional metal-ion binding, leading to e.g. coordination polymers. The crystal structure of [TIL] shows it to be a simple mononuclear complex with the Tl(I) ion coordinated in the N-4 bind ing pocket of the ligand, and the externally-directed N atoms involved only in intermolecular N ... H-C hydrogen-bonding interactions. The two Tl-N bo nds to the pyrazolyl N-2 atoms (average length 2.70 Angstrom) are much shor ter than the bonds to the pyrazinyl Ni atoms (average length 3.05 Angstrom) ; also there is an obvious gap in the apical position of the metal-ion coor dination sphere characteristic of a stereochemically active lone pair. The crystal structure of [PbL2]. Et2O shows that the Pb(II) centre is nine-coor dinate, with two tetradentate chelating ligands and the ninth donor being a pyrazinyl N-4 atom from an adjacent complex unit. The molecules therefore form infinite one-dimensional chains in the crystal via bridging pyrazinyl groups. The coordination geometry about the Pb(II) ions is approximately ca pped square antiprismatic, with no obvious gap in the coordination sphere s uggesting that the lone pair is stereochemically inactive. (C) 1999 Elsevie r Science Ltd. All rights reserved.