4-Cyanopyridine and amide-N and amide-o linkage isomers of 4-pyridinecarboxamide on Trans-Chloro(1,4,8,11 tetraazacyclotetradecane)ruthenium(II/III)

Abstract

The synthesis, UV−vis spectra, and electrochemical behavior of the nitrile-bonded trans-[RuIICl(cyclam)(4-NCpyH+)](BF4)2 (4-Ncpy = 4-cyanopyridine; cyclam = 1,4,8,11-tetraazacyclotetradecane) and of trans-[RuIIICl(cyclam)(NHC(O)-4-pyH+)]2+ are described. The UV−vis spectrum of the Ru(II) nitrile complex shows a MLCT band at 548 nm at pH 1, which is shifted to 440 nm at pH 6, for the unprotonated species. trans-[RuIICl(cyclam)(4-NCpyH+)]2+ was electrolytically oxidized (+600 mV vs Ag/AgCl) at pH 1 to Ru(III), followed by hydrolysis (k = 0.25 s-1) of the coordinated nitrile to give trans-[RuIIICl(cyclam)(NHC(O)-4-pyH+)]2+, in which the amide is deprotonated and coordinated through nitrogen. The identity of the species is pH dependent, the nitrogen-bonded amide prevailing at low pH (<7), but the oxygen-bonded amide is formed through linkage isomerization at higher pH (>8). Reduction of trans-[RuIIICl(cyclam)(NHC(O)-4-pyH)]2+ in acidic media does not result in fast aquation (k = 2.4 × 10-5 s-1) as for other amides on ruthenium(II) pentaammine, but instead linkage isomerization occurs, resulting in the oxygen-bonded species, with an estimated rate constant of 2 × 10-2 s-1, smaller than in the pentaammine analogues.