Structures of hexakis(antipyrine)lead(II) complexes: the first example of the stereochemically active and inactive 6 s 2 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-1 H -pyrazol-3-one) were synthesized in the form of triflate (trifluoromethanesulfonate) and perchlorate salts, namely, hexakis(1,5-dimethyl-2-phenyl-2,3-dihydro-1 H -pyrazol-3-one-κ O )lead(II) bis(trifluoromethanesulfonate), [Pb(C 11 H 12 N 2 O) 6 ](CF 3 SO 3 ) 2 , and hexakis(1,5-dimethyl-2-phenyl-2,3-dihydro-1 H -pyrazol-3-one-κ O )lead(II) bis(perchlorate), [Pb(C 11 H 12 N 2 O) 6 ](ClO 4 ) 2 , abbreviated as [Pb(anti) 6 ] X 2 ( X = OTf and ClO 4 ). In the case of [Pb(anti) 6 ](OTf) 2 , two polymorphic forms crystallizing in the monoclinic I 2/ a ( 1a ) and triclinic P 1 ( 1b ) space groups were distinguished. [Pb(anti) 6 ](ClO 4 ) 2 ( 2 ) crystallized in the trigonal space group P 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 6 s 2 lone pair, respectively. The distribution of the Pb—O bond lengths for the homoleptic lead(II) complexes with a PbO 6 core was presented. Correlations between Pb—O bond lengths and cordination number (CN) for the selected lead(II) and lead(IV) complexes were analysed.