Spectroscopic Determination of Magnetic Exchange Parameters and Structural Geometry for Trinuclear Compounds: (CuL)2Mn·xB (L = N-(4-Methyl-6-oxo-3-azahept-4-enyl)oxamato and B = (CH3)2SO (x = 2) or H2O (x = 5))

Abstract

The optical absorption spectroscopy for the trinuclear compounds Mn(CuL)2·xB with L standing for N-(4-methyl-6-oxo-3-azahept-4-enyl)oxamato and B = (CH3)2SO (x = 2) (1) or H2O (x = 5) (2) has been investigated in the 4−300 K temperature range. The crystal structure of 1 was known; it consists of isolated and neutral trinuclear units in which the central MnII ion is linked to two CuL complex ligands and two (CH3)2SO molecules in a cis configuration. The Cu−Mn−Cu linkage is bent with an angle of 127°. The spectra for 1 and 2 exhibit narrow and intense MnII spin-forbidden transitions in the 24000−28000 cm-1 range, activated by an exchange mechanism. The temperature dependences of the main feature corresponding to the 6A1 → 4A1, 4E(G) MnII transitions have been recorded. Using a model proposed first by Tanabe and co-workers and adapted to the CuMnCu topology, a theoretical expression for the temperature dependence of the intensity of the transition has been derived and compared to the experimental data. The parameters of this expression are the angle α (2α = 180° − CuMnCu) and the interaction parameter between the local ground states, J (H = −JSMn·(SCu1 + SCu2)). For 1, α = 26.5° was known from the structural data, and J has been found as −38 cm-1. For 2, J has been estimated as −32.6 cm-1 with α = 0°. The spectra for 2 exhibit cold and hot bands; the energy difference between these bands depends on both J and the interaction parameter J* between the CuII ions in their ground state and the MnII ion in its spin flip excited state. J* has been estimated as +35 cm-1. The linear geometry of the Cu−Mn−Cu linkage for 2 has been confirmed by a WAXS (wide-angle X-ray scattering) study. The intramolecular Cu···Cu distance has been estimated as 10.2 Å. These results have been discussed in relation to the information deduced from magnetic measurements.