EPR of trigonal bipyramidal copper(II) complexes with tripod ligands

Abstract

Very recent studies in one of our laboratories (KDK) have emphasized the usefulness of some tripod ligands in studying and modeling certain aspects of the coordination chemistry and structure properties of 5-coordinate copper(I) and copper(II) complexes [1]. Furthermore, the general features of the EPR spectra of 5-coordinate copper(II) complexes covering the full range from trigonal bipyramidal to square pyramidal are also now reasonably well understood [2]. In this paper we report the results of a single crystal and polycrystalline epr study of a well characterized complex: [Cu(tmpa)X]PF 6 with tmpa = tris (2-pyridyl)-methylamine and X = Cl − ( 1) and a polycrystalline multifrequency (9–60 GHz) study of a related complex: [Cu(tmpa)X]PF 6 with X = N − 3 ( 2). The former material has a nearly trigonal bipyramidal geometry about a Cu(II) center and, of the 14Cu(II) centers which lie within a radius 12 Å of one placed at an origin, 7 have their Z axes (the line perpendicular to the N1CuCl ‘plane’, see ref. 1) parallel to the one at origin and the others are nearly parallel (within 7.5°). The second material has not yet been obtainable in suitable single crystal form for X-ray or EPR studies. Two aspects of the epr studies are of importance in non-diluted studies: (A) the intramolecular magnetic parameters and (B) the intermolecular interactions. The former reflects a growing awareness of the need for correlation of epr data with other properties on structurally well-characterized materials [3,4], and the latter (B) is of importance here largely because of the potential influence on the former (A) [5]. The room temperature X-band epr of 1 produced an axially symmetrically g tensor with g z = g ‖ = 2.00 and g ⊥ = 2.17 in sensible agreement with the frozen solution measurements [1]. The lineshape was Lorentzian and the width was nearly isotropic with a peak-to-peak linewidth of about 30 gauss. Since the X-ray results suggest that g averaging should be minimal, the g factors as measured in this pure material should reflect the molecular parameters. Some small measurement frequency dependent effects were noted which are consistent with exchange averaging in which the exchange and observation frequencies are comparable. These have been discussed elsewhere [5]. In 2, the polycrystalline epr spectrum could be analyzed, for the most part, with standard methods. The parameters g ‖ = 2.00 and g ⊥ = 2.16 were obtained directly from the undiluted material and are in good agreement with solution results. The computer fitting of the spectrum indicates that the line is Lorentzian and somewhat narrower than in 1. Some extra-absorption appeared between the parallel and perpendicular spectra which could be fitted with appropriate intermolecular dipolar interactions. Furthermore, the epr lineshape, calculated linewidth and effective g-values were independent of measurement frequency. These results suggest that the effective exchange field in 2 is higher than 1 and that it is higher than the highest measurement frequencies (60 GHz). In addition, to a good first approximation, the measured magnetic parameters in the pure material should reflect those for the molecular complex [4]. Finally, we conclude that the ligand arrangement of 2 is similar to 1 and that tmpa effectively constrains the copper(II) site to a trigonal bipyramidal geometry.