Extending the Application Scope of Organophosphorus(V) Compounds in Palladium(II) Pincer Chemistry

1-Dimethylthiocarbamoyloxy-3-diphenylphosphinobenzene was used as a key precursor for the synthesis of a whole series of organophosphorus(V) pincer ligands combining thiocarbamate donor group with P=X coordination arm, where X = S, Se, O, NR (R = Ph, 4-NO2C6H4, COOBu, CH2Ph) or CHR' (R' = COOEt, CN). Direct cyclopalladation of the ligands obtained (used either as individual compounds or generated in situ in the reaction mixture) afforded hybrid pincer complexes with five- and six-membered fused metallacycles. In the case of phosphine sulfide and phosphine selenide derivatives, the cyclopalladation can readily be accomplished in CH2Cl2 at room temperature, leading to the target pincer complexes in high yields. Their phosphine imide and phosphonium ylide counterparts can also be obtained under mild reaction conditions, but the yields strongly depend on the ligand stability (35-93%). Even in the case of the phosphoryl-functionalized ligand, the desired pincer-type complex was synthesized in ca. 15% yield, although the main direction of metalation was C(sp(3))-H bond activation of one of the methyl groups in OC(S)NMe2 moiety. Realization of K3S,C,X-coordination (X = S, Se, O, N, or C) in the resulting pincer complexes was unambiguously confirmed based on the multinuclear NMR (H-1, C-13, P-31, and Se-77) and IR spectroscopic data. Comparative single-crystal XRD analyses allowed for outlining the main structural features of the palladacycles obtained depending on the nature of ancillary P(V)-donor group. In addition, the possibility of solid-phase cyclopalladation was demonstrated by the examples of phosphine chalcogenide and stable phosphine imide ligands. In most cases, this approach afforded the desired complexes with the same or even higher efficiencies than the conventional solution-based method. Preliminary investigations on the cytotoxic activity of some of the complexes obtained against several human cancer cell lines revealed the high activity of phosphine imide-based palladacycles, rendering further studies in this field promising.