Geometric factors in the structural and thermodynamic properties of copper(II) complexes with tripodal tetraamines

The tripodal tetramine ligands N(CH2CH2CH2NH2)(3) (trpn) and N[(CH2CH2CH2NH2)(2)(CH2CH2NH2)] (332) react with Cu(NO3)(2) . 3H(2)O in water to give light blue copper(II) complexes. These were characterized by X-ray crystallography to be the square-pyramidal binuclear Cu(II) species [Cu(trpn)(NO3)](2)(NO3)(2) and [Cu(332)(NO3)]2(NO3)(2) . 2H(2)O. Selected crystallographic details are as follows, respectively: formula C18H18Cu2N12O12, C16H18Cu2N12O14; M = 751.74, 759.72 Da; both triclinic; both <P(1)over bar>; a = 8.4346(8), 8.446(4) Angstrom; b = 9.0785(9), 8.744(3) Angstrom; c = 11.9310(12), 12.007(3) Angstrom; alpha = 94.50(1), 102.68(2)degrees; beta = 103.56(1), 94.79(3)degrees; gamma = 117.42(1), 117.69(4)degrees; V = 769.7(5), 748.2(13) Angstrom(3); both Z = 1; R = 4.16, 4.00; R(w) = 11.34, 6.74 for 2887 (I greater than or equal to 2 sigma(I)), 2457 (F-o(2) greater than or equal to 3 sigma(F-o(2))) structure factors and 199, 209 refined parameters. The binuclear complex dications exhibit a square pyramidal coordination geometry around the copper atoms. Three amine functions (one tertiary and two primary) are coordinated to one copper atom and the remaining primary amine arm bridges to the second copper center. Potentiometric and visible spectrophotometric studies show that a protonated square-pyramidal [Cu(HL)(H2O)(2)](3+) cation (L = trpn, 332, 322 (322 = N[(CH2CH2CH2NH2)(CH2CH2NH2)(2)])) predominates in the intermediate pH region, in contrast to the established trigonal-bipyramidal structure of the tren (tren = tris(2-aminoethyl)amine)) complex of Cu(II). Each [Cu(HL)(H2O)(2)](3+) has one protonated uncoordinated ligand arm which explains the formation of the binuclear species at neutral pH.