Syn- and anti-Conformation in oxodipyrromethenes: crystal and molecular structure of 3,4-dimethyl-2,2′-pyrromethen-5(1H)-one and its N-methyl derivative

Abstract

The structure of two oxodipyrromethenes have been determined from three-dimensional diffractometer data. One is 3,4-dimethyl-2,2′-pyrromethen-5(1H)-one, C11H12N2O (I), which crystallizes in the monoclinic space group P21/n with a= 13.597(1), b= 5.809(1), c= 12.558(1)Å, β= 101.29(1)°, Z= 4. The second is a derivative of this compound in which one of the pyrrole nitrogen atoms is methylated. This is 1′,3,4-trimethyl-2,2′-pyrromethen-5(1H)-one, C12H14N2O, (II). It also crystallizes in P21/n with a= 7.122(1), b= 22.873(1), c= 7.012(1)Å, β= 104.64(2)°, Z= 4. Both structures were solved by direct methods and refined by full-matrix least squares. The isomers obtained both had the Z-configuration, but in (I), the nitrogen atoms are syn, while in (II), they are anti. Both compounds show relatively small deviation from planarity, but (II) shows larger deviations than (I). Both form hydrogen-bonded dimers between the lactam oxygen atom and pyrrole nitrogen atoms. A comparison of the structures in the solid state and in solution indicates the difference in energy between a planar and twisted conformation is small.