Cyclometallation of amino-imines on palladium complexes. The effect of the solvent on the experimental and calculated mechanism

Abstract

The ortho-metallation reaction of chloro and acetato complexes of palladium(II) having 5- and 6-membered κ²-(N_amino,N_imino) chelate ligands, [Pd(Me₂NCH₂(CH₂)_nNCH(4-ClC₆H₄))(X)₂] (X = AcO, Cl; n = 1, 2), in methanol solution has been carried out. The results are a good indicator of the distinct nature of the transition state involved in the processes where acetic acid has been used as a solvent. The presence of Brønsted bases, contrary to what has been reported for Pt^II analogue compounds, does not affect this process, which indicates the “coordinated” nature of the leaving [H⁺] species. The characterization of the solution chemistry of the process in methanol has been fully achieved and the structure determination of one of the final cyclometallated complexes, [Pd(Me₂NCH₂(CH₂)_nNCH(2-C,4-ClC₅H₃))Cl], has been carried out. The experimental temperature and pressure dependent kinetico-mechanistic parameters involved in the process have been determined and the results agree with the previously established experimental trends where important differences were observed in the presence of acidic or protonatable ligands on the palladium centre. From a DFT calculation perspective, we have carried out a systematic mechanistic study of the reaction of the simpler system N-methylbenzylimine with palladium(II) acetate in acetic acid and acidic media to account for the experimental differences observed. The calculations indicate that neither the accepted classical 6-membered transition state mechanism nor an alternative 4-membered type justify the acceleration observed in strong acid. Only when coordination of three protonated acetato ligands on the Pd^II square-planar complex is considered, i.e.[Pd(HAcO)₃(κ¹-N_imino)]²⁺, can the observed acceleration of the Pd^II–C bond formation in protic media be accounted for. The results suggest an alternative and accessible way to accelerate the reaction in such acidic media, even though more complex paths may be plausible. Both the presence of a high concentration of Lewis base in the medium and a protonatable nature of the ligands used are key factors to be considered in order to account for the experimental results reported in the literature.