Aminoalkyl‐1,1‐bis(phosphinic acids): Stability, Acid–Base, and Coordination Properties

Four geminal bis(phosphinic acids), namely, aminomethylbis( H ‐phosphinic acid) (H 2 L 1 ) and 4‐aminobutyl‐1‐hydroxy‐1,1‐bis(R‐phosphinic acid) with R = H (H 2 L 2 ), Me (H 2 L 3 ) and CH 2 CH 2 COOH (H 4 L 4 ), were studied. Their acid–base properties and coordination ability towards Cu 2+ , Ni 2+ and Zn 2+ ions were studied by potentiometry, UV/Vis spectroscopy and NMR spectroscopy. The amine group in H 2 L 1 has a lower protonation constant (log  K a = 6.79) than those found for other studied bisphosphinates (log  K a = 10.75–11.05) with distant amine groups. The structure of [Ca(H 2 L 2 ‐ O , O′ )(H L 2 ‐ O , O′ )]Cl revealed an octahedral arrangement of the metal coordination sphere and a linear polymeric structure, which forms through eight‐membered Ca(–O–P–O–) 2 Ca rings. The structure of [Cu(H L 3 ‐ O , O′ ) 2 (H 2 O)] · 5H 2 O shows two chelating bisphosphinate groups in an equatorial O 4 environment. The structure of [Cu(H 0.5 L 3 ‐ O , O′ )(NO 3 ) 0.5 ] · 2.25H 2 O shows two different coordination environments, one is an elongated tetragonal pyramid, and the other is a trigonal bipyramid with a bidentate nitrate ion.