Intramolecularly alkylated Cosalen complexes: thermolysis and photolysis of coenzyme B12 models with a cobalt-to-ligand carbon bridge

Abstract

Product and kinetic studies on the anaerobic thermolysis and photolysis of the cobalt-carbon bond in intramolecularly bridged organoCosalen compounds are presented. NMR, UV-Vis and electron spin resonance spectroscopic investigations reveal the formation of paramagnetic Co(II) complexes upon thermolysis or photolysis in solution. Solid state thermolysis (studied by differential thermal analysis, thermogravimetry, mass spectrometry and magnetochemistry) also leads to cobalt (II) products. The fate of the carbon radical produced by homolytic cleavage of the CoC bond in solution depends on the reaction conditions used. In the presence of a radical trap (tempo, pbn), carbon radical spin trapping is the major pathway. On the other hand, a Co(II) salen complex carrying an ω-alkenyl side chain was shown to be the major product when Cosalen(CH2)4 (2) was subjected to prolonged photolysis in the absence of a radical trap. While the chemistries of intramolecularly bridged organoCosalen and open chain alkylCosalen complexes are qualitatively quite similar, kinetic studies using the radical trap method reveal large quantitative differences. The bridged complex Cosalen(CH2)3 (1) is quite unreactive towards thermolysis and photolysis, even at high trap concentration. Quantum yields (Φ) of Cosalen(CH2)4 (2) and butylCosalen (4) were determined in toluene as a function of the concentration of tempo. For 2, Φmax is reached at 0.5 M of tempo, while for 4 Φ is already maximal at a very small excess of tempo (≈0.0005 M); in both cases the complex concentration is ca. 104 M. These results are discussed in terms of the solvent-caged radical pair concept.