Transition-metal Complexes of Pyrrole Pigments. X. Divalent and Trivalent Manganese Chelates of Dipyrromethenes

The solution spectra of two tris(dipyrromethenato)manganese(III) complexes and of one bis(dipyrromethenato) manganese (II) as well as the diffuse reflectance spectrum for the latter were examined. The high spin state was confirmed for all the manganese complexes by magnetic susceptibility measurements. For the tris-complexes, one broad low energy band (∼9500 cm−1) is assigned to the 5A1g←5B1g transition and one pair of higher energy bands (∼14000 cm−1) are attributed to the 5B2g←5B1g and 5Eg←5B1g transitions for a D4h symmetry as a consequence of Jahn-Teller effect. In the bis-complex, manganese(II) is placed in a tetrahedral ligand field and one group of ligand-field bands lying below 16000 cm−1 are assigned to: 4T1(4G)←6A1(6S), 4T2(4G)←6A1(6S), and 4E, 4A1(4G)←6A1(6S) in an increasing order of energy. The exceedingly high intensity of these bands is ascribed to the covalent nature of coordinate bonds. A strong IR band due to the skeletal stretching mode of the dipyrromethene moiety was observed in the 1600 cm−1 range and shifted slightly toward lower energy upon metalcoordination.