SYNTHESIS AND SPECTROSCOPIC CHARACTERIZATION OF DIETHYLTIN(IV) DERIVATIVES OF DIPEPTIDES - CRYSTAL AND MOLECULAR-STRUCTURE OF DIETHYLTIN GLYCYLTYROSINATE

Dipeptide complexes of the diethyltin(IV) moiety, Et2SnL, have been synthesized, H2L being glycylglycine (H2GlyGly), glycylatanine (H2GlyAla), alanylalanine (H2AlaAla), glycylvaline (H2GlyVal), valylvaline (H2ValVal), glycylmethionine (H2GlyMet), glycyltyrosine (H2GlyTyr). The crystal and molecular structure of the complex Et2SnGlyTyr has been determined by single-crystal X-ray diffraction. It consists of monomeric units, with the tin atom having a considerably distorted trigonal bipyramidal environment. The dipeptide acts as a tridentate ligand bonding the tin of the C2Sn fragment (equatorial carbon atoms) with the peptide nitrogen atom (equatorial) and axial (monodentate) carboxyl oxygen and amino nitrogen atoms, into a monomeric unit. Bond lengths and angles are reported. Infrared spectroscopic data show the occurrence of monodentate carboxyl in all solid compounds, as well as in methanol solutions of some representative complexes. Sn-119 Mossbauer spectroscopic data, and their rationalization through point-charge model (literal version) calculations of the parameter nuclear quadrupole splitting (DELTA-E) confirm the general occurrence of trigonal bipyramidal structures of the Et2SnGlyTyr type, in the solid state, and give evidence of variations of the C-Sn-C angle in the individual Et2SnL species. Monomers occur in CH3OH solution as suggested by osmometric measurements. C-13 and Sn-119 NMR spectroscopic data in CD3OD show the persistence of the solid-state structures also in the solution phase, where the order of magnitude of the C-Sn-C angles, as estimated from the coupling constants \1J(Sn-119, C-13)\, corresponds to that shown by Et2SnGlyTyr in the solid state. Sn-119 Mossbauer parameters of Et2SnGlyGly in frozen CH3OH solution are consistent with the assumptions from the NMR studies.