Structural, Spectroscopic, Thermodynamic and Kinetic Properties of Copper(II) Complexes with Tripodal Tetraamines

Spectroscopic, thermodynamic, and kinetic measurements have been made on aqueous solutions of copper(II) complexes of hexamethylated tren and trimethylated tren (one methylation per primary amine group of tren) with the objective of correlating the influence of geometry (trigonal bipyramidal, evident from UV/vis spectroscopy) and N-alkyl substitution in the ligand on these inherent properties. At 25.0 degrees C the protonation constants of Me(3)tren are not significantly different from those of tren and Me(6)tren, and the stability constant for the Cu(II) complex is of the same order of magnitude as that for the Cu(tren)(H(2)O)(2+) complex ion. The pK(a) for deprotonation of the coordinated water molecule of Cu(Me(3)tren)(H(2)O)(2+) is intermediate between the values for the complexes containing the unsubstituted and the fully substituted tren ligand. Substitution (pyridine for water) kinetics measurements employing stopped-flow and temperature-jump methods revealed different patterns of reactivity: pyridine replaces water in Cu(Me(3)tren)(H(2)O)(2+) with a second-order rate constant of (4.4 +/- 0.8) x 10(2) M(-)(1) s(-)(1) at 25.0 degrees C, whereas the corresponding process for Cu(Me(6)tren)(H(2)O)(2+) is relatively complex and is discussed in more detail. Substitution in the former complex ion is characterized in the forward and reverse directions, by DeltaH() = 60 +/- 8 and 51.9 +/- 0.9 kJ mol(-)(1), DeltaS() = 5 +/- 27 and -23 +/- 3 J mol(-)(1) K(-)(1), and DeltaV() = -8.7 +/- 4.6 and -6.2 +/- 1.1 cm(3) mol(-)(1), respectively. It is concluded that this reaction follows an I(a) mechanism, similar to that reported for the comparable reaction of Cu(tren)(H(2)O)(2+). An X-ray structural determination on a crystal of Cu(2)(Me(3)tren)(2)(CN)(ClO(4))(3).2CH(3)CN demonstrated trigonal bipyramidal geometry about each copper(II) center. As has been found in comparable complexes of tren and Me(6)tren, the axial nitrogen to copper bond is shorter than the equatorial nitrogen-copper bonds, and the angle made by N(axial)-Cu-N(equatorial) is less than 90 degrees (84.6-85.4 degrees ), signifying that each copper ion lies below the plane of the equatorial nitrogen atoms.