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RESEARCH PRODUCT

Synthesis of N-methyl alkylaminomethane-1,1-diphosphonic acids and evaluation of their complex-formation abilities towards copper(II)

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Abstract A series of N -methyl alkylaminomethane-1,1-diphosphonic acids ( 3a – g ) with a common tertiary nitrogen atom (CH 3 –N–R) bearing linear or branched alkyl, cycloheptyl or phenylalkyl R substituents was synthesized in the reaction of diethylphosphonate with triethyl orthoformate and secondary amine followed by hydrolysis, and by the Eschweiler–Clarke methylation of the alkylaminomethane-1,1-diphosphonic acids with formic acid and formaldehyde. Complex-formation abilities of 3a – g towards copper(II) in solution were studied by means of pH-potentiometry, ESI-MS spectrometry, UV–Vis and EPR methods. Evaluation of stability constants for the Cu(II) complexes of 3a – g has revealed their dependence on a combination of steric and electronic effects imposed by R substituents. It has been demonstrated that compounds with linear or branched alkyl on the N atom ( 3a , 3d , 3e ) form the most stable complexes with Cu(II), the least stable are complexes of 3f , which can be attributed to the steric effect imposed by cycloheptyl ring attached directly to the N atom. In addition to the main [Cu(HL) 2 ] complexes, with well-known {O, O} chelating binding mode, [Cu(HL)L] and [CuL 2 ] species with the {O, N} donor set, protonated dinuclear [Cu 2 H x L 3 ] ( x  = 4, 5, 6) species have been detected in all studied systems. The nitrogen coordination in the [Cu(HL)L] and [CuL 2 ] complex has been confirmed by Vis and EPR spectra.