Syntheses and specific structures of higher-order mixed chelate lanthanide complexes containing terpyridine, acetylacetonate, and nitrate ligands

Novel higher-order mixed chelate complexes of lanthanide(III) have been synthesized, which contain 2,2′:6′,2-terpyridine (terpy, tridentate), 2,4-pentanedionate (acac, bidentate), and nitrate (NO3−, bidentate): general formula, [Ln(terpy)(acac)(NO3)2H2On]. Thirteen quaternary chelating complexes of the lanthanide series (from La3+ to Lu3+ with the exception of Ce and Pm) have been obtained. In the series, complexes obtained using Nd(III) to Lu(III) (as the anhydrous compounds, n = 0) have a coordination number of nine and the lighter lanthanide (with larger ionic size) complexes contain one water molecule (n = 1, for La(III) coordination number, ten; and for Pr(III) coordination number, nine). The structural and characteristic properties of these complexes are as follows. (1) The five donor atoms (N3O2) from the ligands (terpy and acac) form a distorted pentagonal plane (equatorial) around the central lanthanide ion. (2) Except for the larger lanthanide complexes (La and Pr), two nitrate ligands are located at axial positions as bidentate ligands. (3) The ionic size effect of the central lanthanide ion reflects the variation in Ln–N(terpy) bond lengths. (4) The splitting parameter, Δν, of the NO3− ligand (ν1 + ν4 combination band which is observed at 1700–1800 cm−1 and splits into two weak components) increases with decreasing ionic size across the lanthanide series.