Solution behavior of amidine complexes: an unexpected cis/trans isomerization and formation of di- and trinuclear platinum(III) and platinum(II) species.

Platinum bis-amidine complexes (both the cis and trans isomers) are stable in acetone and chlorinated solvents but are unstable in protic solvents such as methanol or water. In the latter solvents an initial cis/trans isomerization leads to formation of an equilibrium mixture with a cis/trans ratio of about 1:4; subsequently a dinuclear platinum(III) complex (1) is formed under aerobic conditions while, under anaerobic conditions, a trinuclear platinum(II) compound (2) is obtained. We hypothesize that the process of isomerization and formation of polynuclear compounds (1 and 2) have a common precursor: a dinuclear platinum(II) species supported by two bridging amidinato ligands (3), formed in small yield, which can either dissociate back to monomers of cis/trans configuration or evolve in two different polynuclear species depending upon the aerobic/anaerobic conditions. In aerobic conditions, oxidation of platinum(II) to platinum(III) together with formation of two additional amidinato bridges across the two platinum centers takes place leading to compound 1. In contrast, in anaerobic conditions, oxidation of platinum is prevented and the dinuclear platinum(II) precursor remains in solution until it reacts with an extra molecule of the starting mononuclear complex which loses its two amidine ligands and cross-links the two bridging amidinato ligands of 3 to yield compound 2. This latter features two triply bridging amidinato ligands linking the three platinum units to form a pocket. Complexes 1 and 2 have been characterized by means of IR and NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray crystallography.