Spectroscopic determination of magnetic exchange parameters and structuralgeometry for trinuclear compounds: (CuL)(2)Mn center dot xB (L = N-(4-methyl-6-oxo-3-azahept-4-enyl)oxamato and B = (CH3)(2)SO (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 4-30 0 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 li nked to two CuL complex ligands and two (CH3)(2)SO molecules in a cis confi guration. The Cu-Mn-Cu linkage is bent with an angle of 127 degrees. The sp ectra for 1 and 2 exhibit narrow and intense Mn-II spin-forbidden transitio ns 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 prop osed first by Tanabe and co-workers and adapted to the CuMnCu topology, a t heoretical expression for the temperature dependence of the intensity of th e transition has been derived and compared to the experimental data. The pa rameters of this expression are the angle alpha (2 alpha - 180 degrees - Cu MnCu) 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 st ructural data, and J has been found as -38 cm(-1). For 2, J has been estima ted 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 sta te 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 c onfirmed 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 measurement s.