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 21:1 ratio afforded the new ligand bis3, 2, 1dihydroborate L]⁻a bis(pyrazolyl)borate in which each pyrazolyl ring is functionalised with a pyrazin-2-yl group at the C³ positionL]⁻ is therefore a potentially chelating tetradentate ligand with two externally-directed N atoms (the pyrazinyl N⁴ atoms) which are available for additional metal–ion bindingleading to eg coordination polymers The crystal structure of TlL] shows it to be a simple mononuclear complex with the Tl(I) ion coordinated in the N₄ binding pocket of the ligandand the externally-directed N atoms involved only in intermolecular N H–C hydrogen-bonding interactions The two Tl–N bonds to the pyrazolyl N² atoms (average length 270 Å) are much shorter than the bonds to the pyrazinyl N¹ atoms (average length 305 Å) also there is an obvious gap in the apical position of the metal–ion coordination sphere characteristic of a stereochemically active lone pair The crystal structure of PbL₂] Et₂O shows that the Pb(II) centre is nine-coordinate with two tetradentate chelating ligands and the ninth donor being a pyrazinyl N⁴ 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 capped square antiprismatic with no obvious gap in the coordination sphere suggesting that the lone pair is stereochemically inactive.