TERMINAL AND NEW BRIDGING COORDINATION OF METHYLGUANIDINE, ARGININE, AND CANAVANINE TO PLATINUM(II) - THE 1ST CRYSTALLOGRAPHIC STUDY OF BONDING BETWEEN A TRANSITION-METAL AND A GUANIDINE LIGAND

The compound [Pt(trpy)Cl]Cl reacts with the guanidine-containing compounds methylguanidine, arginine, and canavanine to form the yellow monometallic complexes [Pt(trpy)MeGua]2+, [Pt(trpy)ArgH]2+, and [Pt(trpy)CanH]2+ and the red bimetallic complexes [{Pt(trpy)}2MeGua]4+, [{Pt(trpy)}2Arg]3+, and [{Pt(trpy)}2Can]3+. The yellow and the red complexes containing each of the three ligands are separated by cation-exchange chromatography, and the corresponding PF6−, ClO4−, and BPh4− salts are isolated by precipitation. The stability of the complexes upon heating and acidification and the relative difficulties of displacement of the guanidine ligands are studied semiquantitatively. The compositions of the new complexes are determined by mass, UV-vis, IR, and 1H, 13C, and 195Pt NMR spectroscopy. In the yellow complexes, each guanidine ligand probably binds to a Pt(trpy)2+ group through a trigonal (imino-type) nitrogen atom. In the red complexes, each guanidine ligand bridges two Pt(trpy)2+ groups, probably through a tetrahedral (amino-type) and a trigonal nitrogen atom. This unprecedented bridging coordination of a guanidine ligand to transition-metal atoms is confirmed by the crystallographic study of [{Pt(trpy)}2Can](ClO4)3•5.5H2O. It crystallizes in the space group P21 (No. 4) with the following lattice properties: a = 12.733 (3) Ǻ, b = 27.039 (4) Ǻ, c = 14.813 (3) Ǻ, β = 115.219 (8)°, V = 4614 (2) Ǻ3, and Z = 4. The structure was refined to a Rw value of 0.0572. In both types of the complex, which coexist in the solid state, the two Pt(trpy)2+ groups are nearly parallel and nearly eclipsed; the Pt-Pt distances of 2.9884 (7) and 2.9872 (8) Ǻ are virtually identical, but the bond distances involving the Pt atoms and the guanidine group are different. This and a previous report (Ratilla, E. M. A.; Kostic, N. M. J. Am. Chem. Soc. 1988, 110, 4427) show that the arginine side chain is a potential ligand in metalloproteins and in metal-dependent enzymes. © 1990, American Chemical Society. All rights reserved.