NMR-STUDY ON THE COORDINATION BEHAVIOR OF DISSYMMETRIC TERDENTATE TRINITROGEN LIGANDS ON METHYLPALLADIUM(II) COMPOUNDS

Neutral compounds of the type [(6-RC(5)H(3)N-2-C=NCH2CH2-2-C5H4N)Pd(CH3)(Cl)] (R = H (MAP), CH3 (6'-CH3-MAP)) have been synthesized starting from [(cyclooctadiene)Pd(CH3)(Cl)], while ionic compounds of the type [(6-RC(5)H(3)N-2-C=NCH2CH2-2-C5H4N)Pd(CH3)](+)[(Y)](-)(Y=Cl, CF3SO3,NO3;R=H,CH3) were formed by reacting the chlorides in a polar solvent with silver triflate and silver nitrate, respectively. An X-ray structure of the ionic compound [(C5H4N-2-C=NCH2CH2-2-C5H4N)Pd(CH3)(CF3SO3)] is presented. The crystals are of the triclinic spacegroup P-1(-), with a=7.690(1)Angstrom, b=8.653(2)Angstrom, c=13.888(1) Angstrom, alpha=98.01(1)degrees, beta=99.83(1)degrees, and gamma=95.55(1)degrees. The X-ray structure shows a square planar palladium complex in which the MAP ligand is terdentate coordinated to the palladium atom. The fourth coordination site is occupied by the methyl ligand. The triflate anion is not coordinating. The methyl-palladium distance is 2.024(8) Angstrom; the lengths of the palladium-nitrogen bonds are 2.039(7), 2.058(7),and 2.024(8) Angstrom for N(1), N(2), and N(3), respectively. The neutral compounds exist in various isomeric forms. In a noncoordinating solvent, a bidentate coordination of the alpha-diimine moiety is preferred at elevated temperatures. At lower temperatures, a terdentate coordination fashion of the ligands is preferred, resulting in five-coordinate compounds. For N-N-N = MAP, a square pyramidal five-coordinate configuration is observed, whereas for 6'-CH3-MAP an initially formed ionic square planar configuration at 223 K isomerizes to a trigonal bipyramidal five-coordinate configuration. These isomerization reactions proved to be reversible. In a polar solvent both neutral and ionic compounds were observed in a varying ratio for both ligands. The neutral and the ionic compounds could reversibly be converted into each other by interchanging the solvent. Synthesis in benzene or diethyl ether of [(6-CH3C5H3N-2-C=NCH2CH2-2-C5H4N)Pd(CH3)(Cl)] resulted in the formation of two kinetic products. These consist of a bidentate coordination of the ligand with the nitrogen atoms of the NCH2CH2C5H4N moiety, resulting in a six-membered ring, in a cis isomer, and in a trans isomer. The kinetic products could be converted into the mixture of thermodynamic isomers in which the ligand is bidentate coordinated with the alpha-diimine moiety, vide supra, upon heating. The mechanisms of the ligand rearrangements in the palladium complexes are discussed.