Structures of hexakis(antipyrine)lead(II) complexes: the first example of the stereochemically active and inactive 6s2 lone pair for one lead(II) complex.

Homoleptic lead(II) complexes with the monodentate O-donor ligand antipyrine (anti or 1,5-dimethyl-2-phenyl-2,3-dihydro-1H-pyrazol-3-one) were synthesized in the form of triflate (trifluoromethanesulfonate) and perchlorate salts, namely, hexakis(1,5-dimethyl-2-phenyl-2,3-dihydro-1H-pyrazol-3-one-κO)lead(II) bis(trifluoromethanesulfonate), [Pb(C11H12N2O)6](CF3SO3)2, and hexakis(1,5-dimethyl-2-phenyl-2,3-dihydro-1H-pyrazol-3-one-κO)lead(II) bis(perchlorate), [Pb(C11H12N2O)6](ClO4)2, abbreviated as [Pb(anti)6]X2 (X = OTf and ClO4). In the case of [Pb(anti)6](OTf)2, two polymorphic forms crystallizing in the monoclinic I2/a (1a) and triclinic P\overline 1 (1b) space groups were distinguished. [Pb(anti)6](ClO4)2 (2) crystallized in the trigonal space group P\overline 3. The crystal structures were determined by single-crystal X-ray diffraction. In all three structures, the metal ion is surrounded by six O atoms from the antipyrine molecules. Compounds 1a and 2 display regular and holodirected octahedral geometry with three similar [2.496 (1), 2.523 (1) and 2.539 (1) Å] or six equivalent Pb-O bond lengths [2.474 (2) Å]. Complex 1b has lower hemidirected symmetry, forming a ψ-pentagonal bipyramid, with six different Pb-O bond distances in the range 2.327 (2)-2.852 (2) Å and one position occupied by a lone electron pair. Shorter C-H...O contacts in 1b [2.998 (4) and 3.141 (4) Å] compared to those in 1a [3.354 (2) and 3.365 (2) Å] and 2 [3.360 (3) and 3.384 (3) Å] between the antipyrine molecules and the counter-ions seem to stabilize the active lone pair. In all three structures, the [Pb(anti)6]2+ cations organize into a hexagonal columnar packing, while the ordered counter-ions occupy the free cavities between them. Density functional theory (DFT) calculations reveal that the isolated [Pb(anti)6]2+ cation favours a distorted hemidirected geometry. The calculated eccentricity values [0.14 (1b) and 0 (1a and 2)] confirm the active and inert behaviour of the 6s2 lone pair, respectively. The distribution of the Pb-O bond lengths for the homoleptic lead(II) complexes with a PbO6 core was presented. Correlations between Pb-O bond lengths and cordination number (CN) for the selected lead(II) and lead(IV) complexes were analysed.