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))

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)(2)SO (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 Mn-II ion is linked to two CuL complex ligands and two (CH3)(2)SO molecules in a cis configuration. The Cu-Mn-Cu linkage is bent with an angle of 127 degrees. The spectra for 1 and 2 exhibit narrow and intense Mn-II 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 (6)A(1) --> (4)A(1), E-4(G) Mn-II 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 alpha (2 alpha - 180 degrees - CuMnCu) and the interaction parameter between the local ground states, J (H = -JS(Mn).(S-Cu1 + S-Cu2)) For 1, alpha = 26.5 degrees 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 alpha = 0 degrees. 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 Cu-II ions in their ground state and the Mn-II 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 Angstrom. These results have been discussed in relation to the information deduced from magnetic measurements.