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 an d trimethylated tren (one methylation per primary amine group of tren) with the objective of correlating: the influence of geometry (trigona l bipyramidal, evident from UV/vis spectroscopy) and N-alkyl substitut ion in the ligand on these inherent properties. At 25.0 degrees C the protonation constants of Me(3)tren are not significantly different fro m those of tren and Me(6)tren, and the stability constant for the Cu(I I) complex is of the same order of magnitude as that for the [Cu(tren) (H2O)](2+) complex ion; The pK(a) for deprotonation of the coordinated water molecule of [Cu(Me(3)tren)(H2O)](2+) is intermediate between th e values for the complexes containing the unsubstituted and the fully substituted tren ligand. Substitution (pyridine for water) kinetics me asurements employing stopped-flow and temperature-jump methods reveale d different patterns of reactivity: pyridine replaces water in [Cu(Me( 3)tren)(H2O)](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)(H2O)](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 Delta H-double dagger = 60 /- 8 and 51.9 +/- 0.9 kJ mol(-1), Delta S-double dagger = 5 +/- 27 and -23 +/- 3 J mol(-1) K-1, and Delta V-double dagger = -8.7 +/- 4.6 and -6.2 +/- 1.1 cm(3) mol(-1), respectively, It is concluded that this r eaction follows an I-a mechanism, similar to that reported for the com parable reaction of [Cu(tren)(H2O)](2+). An X-ray structural determina tion on a crystal of [Cu-2(Me(3)tren)(2)(CN)] (ClO4)(3) . 2CH(3)CN dem onstrated trigonal bipyramidal geometry about each copper(II) center. As has been found in comparable complexes of tren and Me(6)tren, the a xial nitrogen to copper bond is shorter than the equatorial nitrogen-c opper bonds, and the angle made by N(axial)-Cu-N(equatorial) is less t han 90 degrees (84.6-85.4 degrees), signifying that each copper ion li es below the plane of the equatorial nitrogen atoms.