The photochemistry of cobalt(III)-aminoacidato complexes (1999)
AuthorsTelfer, Shane G.show all
The photodecarboxylation reaction of Co(III)-aminocarboxylato complexes has been examined from several angles. Firstly, the currently accepted mechanism for the formation of [Co(bpy)(CHâ‚‚NHâ‚‚)]2+, following the UV photolysis of [Co(bpy)2(gly)]2+ (bpy = 2,2'-bipyridine, gly = glycinate), has been tested. A Co(II)-bound aminocyclopropylmethyl radical, with a lifetime of around 10^-4s, has been proposed as a reaction intermediate. This assertion was tested with the use of a radical clock, derived from chelated cyclopropylglycine. If a Co(II)-bound aminocyclopropylmethyl radical is formed, it will ring-open with rate constant k >= 10^7 s^-1 (298 K). This rate constant has been estimated on the basis of transition-state theoretical calculations and published data for related radicals. The cyclopropyl group actually survived photolysis, and was found as cyclopropanecarboxaldehyde. This result implies that either the rate determining step has been wrongly assigned, or the proposed mechanism is incorrect. Carbonyl compounds were also detected following the photolysis of [Co(bpy)2(aa)]^2+complexes, where aa = alaninate, valinate, phenylglycinate, and aminoisobutyrate. It was proposed that a Co-C-N metallacycle is formed briefly, but decomposes to give a Co(I) complex and an iminium ion. Hydrolysis of the latter fragment would account for the carbonyl compounds. Secondly, some novel Co-C-N metallacycles have been prepared via the photodecarboxylation reaction. Cobalt(III) complexes with N,N-bis(2- pyridylmethyl)aminoacidate (amino acid = glycine, alanine, and cyclopropylglycine) ligands with 1,10-phenanthroline (phen) filling the remaining coordination sites, were prepared. Upon UV photolysis in aqueous solution, all three complexes yielded Co-CN metallacycles which were sufficiently stable to allow characterisation by conventional ^1H NMR, ^13C NMR, and UV-vis techniques. The solid state structure of the photolysis product of the glycinate derivative was determined by X-ray crystallography. These organometallic products eventually decompose, giving carbonyl compounds, free bis(2- pyridylmethyl)amine (bpa), free phen and the Co(II) ion. A peroxo-bridged Co(III) dimer, [Co(phen)(bpa)(02)Co(phen)(bpa)]^4+, crystallised from this mixture, and was characterised by X-ray crystallography. Thirdly, the UV photolysis reactions of a series of [Co(bpy)2(aa)]^2+in DMSO solution were investigated. Carbonyl compounds are also produced in this solvent. The formation of trans(N)-[Co(aa)2(bpy)]+ complexes was also observed. This was ascribed to secondary (thermal) chemistry between dissolved molecular oxygen and some of the photolysis products: free amino acid, free phen, and Co(II). This same mixture gave rise to the [Co(bpy)3]3+ ion in aqueous solution. This difference was rationalised on the basis of the equilibria of the various [CoII(aa)x(bpy)y]^(2-x)+ complexes in the different solvents, and the potentials at which they are oxidised.