Methylene-bis[(aminomethyl)phosphinic acids]: synthesis, acid-base and coordination propertiesElectronic supplementary information (ESI) available: Overall protonation constants and stability constants; distribution diagrams of H2L2 and H2L3; HR-MS spectra of Cu-L1 and Cu-L2 systems; geometrical parameters of the bis(phosphinate) moiety and coordination spheres in the solid state. CCDC 898276, 898277, 898278 and 898279. For ESI and crystallographic data in CIF or other electronic format see DOI:

Three symmetrical methylene-bis[(aminomethyl)phosphinic acids] bearing different substituents on the central carbon atom, (NH 2 CH 2 )PO 2 H-C(R 1 )(R 2 )-PO 2 H(CH 2 NH 2 ) where R 1 = OH, R 2 = Me (H 2 L 1 ), R 1 = OH, R 2 = Ph (H 2 L 2 ) and R 1 ,R 2 = H (H 2 L 3 ), were synthesized. Acid-base and complexing properties of the ligands were studied in solution as well as in the solid state. The ligands show unusually high basicity of the nitrogen atoms (log K 1 = 9.5-10, log K 2 = 8.5-9) if compared with simple (aminomethyl)phosphinic acids and, consequently, high stability constants of the complexes with studied divalent metal ions. The study showed the important role of the hydroxo group attached to the central carbon atom of the geminal bis(phosphinate) moiety. Deprotonation of the hydroxo group yields the alcoholate anion which tends to play the role of a bridging ligand and induces formation of polynuclear complexes. Solid-state structures of complexes [H 2 N&z.dbd;C(NH 2 ) 2 ][Cu 2 (H −1 L 2 ) 2 ]CO 3 ·10H 2 O and Li 2 [Co 4 (H −1 L 1 ) 3 (OH)]·17.5H 2 O were determined by X-ray diffraction. The complexes show unexpected geometries forming dinuclear and cubane-like structures, respectively. The dinuclear copper( ii ) complex contains a bridging μ 2 -alcoholate group with the − O-P(&z.dbd;O)-CH 2 -NH 2 fragments of each ligand molecule chelated to the different central ion. In the cubane cobalt( ii ) complex, one μ 3 -hydroxide and three μ 3 -alcoholate anions are located in the cube vertices and both phosphinate groups of one ligand molecule are chelating the same cobalt( ii ) ion while each of its amino groups are bound to different neighbouring metal ions. All such three metal ions are bridged by the alcoholate group of a given ligand. Symmetrical methylene-bis[(aminomethyl)phosphinic acids] show high nitrogen basicity, high stability of complexes with divalent metal ions and tend to form polynuclear complexes.