The use of control experiments as the sole route to correct the mechanistic interpretation of mercury poisoning test results: The case of P,C-palladacycle-catalysed reactions

In the study of the metallic mercury interaction with cyclopalladated derivatives of tris-ortho-tolylphosphine [{(κ2P,C-L)Pd(μ-X)}2], the redox-transmetallation of palladacycles by metallic mercury with the formation of P,C-chelated benzylmercurials [(κ2P,C-L)HgX] was revealed at moderate temperature in the presence of excess Hg(0). In contrast, the mercury(0) testing of the Suzuki reaction with the same chloride P,C-precatalyst gave a negative result, even at a higher temperature and with a greater excess of Hg(0). The structures of these mercurials have been convincingly confirmed spectrally (1Н, 13С, 31Р and 199Hg NMR), and via X-ray diffraction study and DFT calculations for the chloride mercurial. Our results and the analysis of known data on the mercury diagnostics of catalytic systems with classical and anomalous P,C-precatalysts enable us to assume that the intact state of palladacycle is present in the key anionic intermediate [(κ2P,C-L)Pd0]– of catalytic cycles of the Suzuki and Heck reactions and a lower reactivity of Hg(0) with palladium(0) complexes in comparison with that of palladium(II) analogues.