A comparison of the gas, solution, and solid state coordination environments for the Cu(II) complexes of a series of linear aminopyridine ligands with varying ratios of 5- and 6-membered chelate rings

The synthesis and characterization of the copper(II) complexes of a series of tetradentate, pentadentate and hexadentate aminopyridine ligands that contain ethylenediamine and/or propylenediamine groups are described. The ligands include: 1,12-bis(2-pyridyl)-2,5,8,11-tetraazadodecane, TRIEN-pyr; 1,13-bis(2-pyridyl)-2,5,9,12-tetraazatridecane, DIEN-PN-pyr; 1,14-bis(2-pyridyl)-2,6,9,13-tetraazatetradecane, DIPN-EN-pyr; 1,15-bis(2-pyridyl)-2,6,l0,14-tetraazapentadecane, TRIPN-pyr; 1,9-bis(2-pyridyl)-2,5,8-triazanonane, DIEN-pyr; 1,11-bis(2-pyridyl)-2,6,10-triazaundecanenane, DIPN-pyr; 1,6-bis(2-pyridyl)-2,5-diazahexane, EN-pyr; and 1,7-bis(2-pyridyl)-2,6-diazaheptane, PN-pyr. The following methods were used to determine the binding geometries of the copper(II) complexes in the solid, solution, and gas phases: magnetic susceptibility measurements, absorption spectroscopy, EPR spectroscopy, electrochemistry, and electrospray ionization mass spectrometry. An X-ray structure was determined for the DIPN-pyr complex. The solid state structures were all found to be monomeric Cu(II) complexes with the coordination number set by the denticity of the ligand while the solution structures of all of the complexes except those with TRIPN-pyr and DIPN-pyr were found to be square pyramidal or elongated octahedral. The TRIPN-pyr and DIPN-pyr complexes showed considerable trigonal bipyramidal distortions. The gas phase data showed that the substitution of 6-membered for 5-membered chelate rings helped the ligand span more coordination sites. The TRIEN-pyr complex was 4- or 5-coordinate compared to the 5- or 6-coordination seen with the other three hexadentate ligands, and the DIPN-pyr complex was weakly 5-coordinate as compared to the 4-coordinate DIEN-pyr complex. The preferred structures of the ligands were consistent with their electrochemical behavior which showed the stability of the Cu(II) complex decreased in the order: DIPN-EN-pyr, TRIEN-pyr, DIEN-PN-pyr > DIEN-pyr > DIPN-pyr > TRIPN-pyr > PN-pyr > EN-pyr.