Base-catalysed aquation of αβ-syn- and αβ-anti-chloro- and bromo-[1,9-bis(2′-pyridyl)-2,5,8-triazanonane]- and -[1,11-bis(2′-pyridyl)-2,6,10-triazaundecane]-cobalt(III) cations. An unusually base-sensitive halogenopentamine cobalt(III) system

Abstract

Complexes of the type [Co(picdien)X][ClO₄]₂ and [Co(picditn)X][ClO₄]₂[picdien = 1,9-bis(2′-pyridyl)-2,5,8-triazanonane, picditn = 1,11-bis(2′-pyridyl)-2,6,10-triazaundecane; X = Cl, Br, NO₂, NCS, N₃, MeCO₂, or H₂O] have been prepared. All complexes have the αβ configuration, those of picdien existing in either or both syn and anti forms, while only the anti forms of the picditn complexes have been isolated. Structures have been established by single-crystal X-ray diffraction and ¹H n.m.r. spectroscopy in dimethyl sulphoxide and D₂O. All complexes are unusually sensitive to base-catalysed hydrolysis over very wide ranges of pH. The pH-independent contribution to the solvolysis of the chloro- and bromo-picdien complexes is observed only at high temperatures and at high [H⁺] but there is a very important pH-independent contribution to the solvolysis of the corresponding picditn species, which are also somewhat more sensitive to base catalysis. Proton-exchange studies show that proton transfer is faster than substitution in even the most labile systems. The mechanism is discussed.