Linkage Isomerism In Cobalt(Iii) Pentaammine Complexes Of 2-Pyridone

The reactions of the heterocyclic ligand 2-pyridone, coordinated to the cobalt(III)pentaammine moiety, have been investigated. The complex has been crystallized as the oxygen- and nitrogen-bonded linkage isomers in both protonated (neutral ligand) and deprotonated forms, a first for an ambidentate ligand-complex system, and these materials have been characterized by UV-visible, infrared, and H-1 and C-13 NMR spectroscopy. In aqueous solution (I = 1.0 M, NaCl, 25 degrees C) the pK(a) of the O-bonded isomer is 6.83 +/- 0.05, and that of the N-bonded form, 3.56 +/- 0.05; these values are surprisingly close and are even closer in Me(2)SO. In acid solution (water or Me(2)SO) the (2-pyridone-O)pentaammine complex reacts slowly (k(H) = 1.3 x 10(-5) s(-1), 0.5 M HClO4, 25 degrees C) and the products are the free ligand and the solvento complex; no N-bonded isomer is formed concurrently. In weakly basic solution the deprotonated O-bonded isomer rearranges slowly and completely to form the deprotonated N-bonded linkage isomer (k(ON(1)) = 1.2 X 10(-5) s(-1), 25 degrees C); the rate coincidentally is very much the same as in acid, but the reactions are different. The isomerization reaction has also been observed in Me(2)SO, but it is both slower (ca. 4-fold) and detectably reversible (90% N- and 10% O-bonded forms, commencing with either isomer). The complex undergoes a very mild base-catalyzed reaction (k(OH) = 9 x 10(-5) M(-1) s(-1); I = 1.0 M, NaClO4, 25 degrees C), but up to 0.1 M OH- (I = 1 M) no appreciable base-catalyzed hydrolysis was observed; decomposition is a problem above ca. 0.02 M OH-. In acid solution (water or Me(2)SO) the N-bonded isomer is protonated on oxygen, with essential retention of the imine-like bonded nitrogen. In aqueous acid the 2-pyridone-N complex rearranges with a small amount (18%) of parallel solvolysis to form the 2-pyridone-O complex, and the latter subsequently solvolyzes. The products of this reaction have been isolated by ion exchange chromatography and identified by visible spectrophotometry. The rates of rearrangement (k(NO(2)) = 3.8 X 10(-6) s(-1)) and direct solvolysis (k(s) = 8.2 X 10(-7) s(-1)) of the 2-pyridone-N complex have been determined. In Me(2)SO, the protonated N-bonded form also rearranges and solvolyses concurrently; more than 90% is the O-bonded linkage isomer. The O-bonded form in Me(2)SO initially gives significant amounts of protonated N-bonded isomer along with solvolysis. This apparent divergence from the aqueous chemistry is explained in terms of the position of the N/O equilibrium. The positions of the N/O equilibria for protonated and deprotonated isomers are related to the relative pK(a)s, and this matter and the observed solvent effects are discussed. The chemistry of these complexes is compared with that of the succinimide analogues, and the factors affecting O to N linkage isomerization reactions in coordinated amides are discussed.