Stabilization of high-valent transition-metal complexes with macrocyclic tertiary amines. Reinvestigation of the synthesis, electrochemistry, and spectroscopy of osmium (III) macrocyclic amine complexes and X-ray crystal structure of trans-[OsIII(16-TMC)Cl2]ClO4 (16-TMC = 1,5,9,13-Tetramethyl-1,5,9,13-tetraazacyclohexadecane)

Abstract

Dropwise addition of an ethanolic solution of Na2OsCl6 to a refluxing ethanolic suspension of L and tin plates yielded trans-[OsIIILCl2]+ [L = 14aneN4 (1,4,8,11-tetraazacyclotetradecane), 15aneN4 (1,4,8,12-tetraazacyclopentadecane), 16aneN4 (1,5,9,13-tetraazacyclohexadecane), teta (meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane), 16-TMC (1,5,9,13-tetramethyl-1,5,9,13-tetraazacyclohexadecane)]. The UV-vis spectra of trans-[OsIIILCl2]+ exhibited two ligand-to-metal charge-transfer (LMCT) pπ(Cl) → dπ(Os) transition bands in the 250-280- and 280-310-nm regions. The LMCT transition energy for trans-[OsIIILCl2]+ varies with L = (NH3)4 > (en)2 > 2,3,2-tet > 3,2,3-tet > 14aneN4 > 15aneN4 ∼ 16aneN4 > 16-TMC > teta (2,3,2-tet = 3,7-diazanonane-1,9-diamine; 3,2,3-tet = 4,7-diazadecane-1,10-diamine). In acetonitrile, reversible Os-(III)/Os(II) couples were observed at potentials -1.6 to -1.3 V vs. the Cp2Fe+/0 couple. The E1/2 value for trans-[OsIIILCl2]+ is relatively insensitive to the macrocyclic ring size and decreases from L = 14aneN4 to L = teta. For Os(III) secondary amine complexes, the electrochemical oxidation of Os(III) to Os(IV) was irreversible. For trans-[OsIII(16-TMC)Cl2]+, a reversible Os(IV)/Os(III) couple was found at 0.67 V vs. the Cp2Fe+/0 couple and 0.81 V vs. NHE in acetonitrile and 0.1 M HCl, respectively. trans-[OsIV(16-TMC)Cl2]2+ has been characterized spectroscopically with a pπ(Cl) → dπ[Os(IV)] transition at 365 nm. The structure of trans-[OsIII(16-TMC)Cl2]ClO4 has been determined by X-ray crystallography: orthorhombic; space group Pnma; a= 14.982(3), b = 11.036(3), c= 13.586 (4) Å; V= 2246.3 (9) Å3; Z = 4. The N-methyl groups of the 16-TMC ligand adopt a "two up" and "two down" configuration, and the two chloride ligands are trans to each other. The Os-N and Os-Cl distances are 2.263 (17) and 2.348 (1) Å, respectively. © 1987 American Chemical Society.