Das zweikernige Tri-μ-hydroxo-bis[(1,4,7-triazacyclononan)cobalt(III)]-Kation (1) wird dargestellt. In saurer Losung entsteht daraus das trans-Diaqua-di-μ-hydroxo-bis[(1,4,7-triazacyclononan)cobalt(III)]-Kation (2), dessen Struktur rontgenographisch bestimmt wurde. Die Kinetik der Gleichgewichtseinstellung der Spaltung der ersten OH-Brucke von 1 wurde gemessen. Es wird ein Mechanismus vorgeschlagen. Bei dem die cis-trans-Isomerisierung von 2 den geschwindigkeitsbestimmenden Schritt darstellt, und der alle(auch fruhere) experimentelle Daten der OH-Bruckenspaltung sowie die Bildung von Tri-μ-hydroxo-bis[amincobalt(III)]-Komplexen widerspruchsfrei deutet.



Novel μ-Hydroxo-Transition Metal Complexes, I. Preparation and Crystal Structure of the trans-Diaqua-di-μ-hydroxo-bis[(1,4,7-triazacyclononane)cobalt(III)]Cation; Kinetics and Mechanism of its Formation



The binuclear tri-μ-hydroxo-bis[(1,4,7-triazacyclononane)cobalt(III)] cation (1) is prepared. In acidic solutions 1 forms the trans-diaqua-μ-hydroxo-bis[(1,4,7-triazacyclononane)cobalt(III)] cation (2), the structure of which has been established by single crystal X-ray diffraction. The equilibration kinetics of the first hydroxo-bridge cleavage of the triply-bridged complex 1 have been studied. A mechanism is proposed involving cis-trans isomerization of 2 as the rate determining step. The mechanism is consistent with all (including previously published) experimental data for the OH-bridge cleavage processes of tri-μ-hydroxo-bis[aminecobalt(III)] complexes.

Neue μ-Hydroxo-Übergangsmetallkomplexe, I. Darstellung und Struktur des trans-Diaqua-di-μ-hydroxo-bis[(1,4,7-triazacyclononan)cobalt(III)]-Kations; Kinetik und Mechanismus seiner Bildung

Publisher Summary Polynuclear complexes with hydroxide (or oxide) as bridging ligands constitute an important class of complexes. They are formed by hydrolysis of mononuclear aqua complexes of most metal ions and, therefore, constitute an important aspect of the hydrolytic chemistry of metal ions. They display a chemistry which is interesting and is also relevant in relation to applied chemistry and to biochemistry. This chapter reviews the chemistry of hydroxo-bridged chromium(III), cobalt(III) complexes, and includes studies of the related rhodium(III) and iridium(III)systems. The review focuses the chemical properties of these complexes, and special emphasis has been made on reactions that involve the cleavage or formation of a hydroxo bridge. Reactions of bridging groups other than hydroxide and reactions of nonbridging ligands are also discussed in the chapter. A large number of X-ray crystal-structure determinations have been made during the past two decades. The hydroxo-bridged chromium(III) oligomers constitute a very interesting class of complexes from a spectroscopic and magnetochemical point of view. The most important methods for obtaining structural information about solutions are probably potentiometric pH measurements and visible/ultraviolet spectroscopy, but many other techniques have been applied, including bridge-cleavage experiments, magnetic measurements, and electron spin resonance (ESR) spectroscopy.

Hydroxo-Bridged Complexes of Chromium (III), Cobalt (III), Rhodium (III), and Iridium (III)

Hydrolysis of the Organometallic Aqua Ion fac-Triaquatricarbonylrhenium(I). Mechanism, pKa, and Formation Constants of the Polynuclear Hydrolysis Products

The phosphate monoester in 2-H is hydrolyzed an unprecedented 1011 times more rapidly than the corresponding unbound phosphate. Clean conversion of the bidentate monoester (2-H) to the tridentate inorganic phosphate (3), together with the crystal structure of an analog of 2-H, provides detailed mechanistic insight into the hydrolysis reaction.

Structure and Reactivity of a Dinuclear Cobalt(III) Complex with a Bridging Phosphate Monoester

Two new ligands, L1 and L2, have been prepared via N-functionalization of 1,4,7-triazacyclononane (tacn) with pairs of ethyl- or propyl-guanidine pendants, respectively. The X-ray crystal structure of [CuL1](ClO4)2 (C1) isolated from basic solution (pH 9) indicates that a secondary amine nitrogen from each guanidine pendants coordinates to the copper(II) center in addition to the nitrogen atoms in the tacn macrocycle, resulting in a five-coordinate complex with intermediate square-pyramidal/trigonal bipyramidal geometry. The guanidines adopt an unusual coordination mode in that their amine nitrogen nearest to the tacn macrocycle binds to the copper(II) center, forming very stable five-membered chelate rings. A spectrophotometric pH titration established the pKapp for the deprotonation and coordination of each guanidine group to be 3.98 and 5.72, and revealed that [CuL1]2+ is the only detectable species present in solution above pH ∼8. The solution speciation of the CuL2 complex (C2) is more complex, with ...

J. Boesch

Papers

The formation of dinucsear μ-hydroxo-cobalt(III) complexes

Bldg. und kinetische stabilitaet zweikerniger my-hydroxo-kobalt(iii)-komplexe mit nh3, tren, tach und 2,6-bis-(aminomethyl)-pyridin