Reactivity and stability of platinum(ii) formyl complexes based on PCP-type ligands. The significance of sterics

Abstract

The synthesis and characterization of several Pt(II) complexes, including formyl complexes, based on the PCP-type pincer ligands C₆H₄[CH₂P(iPr)₂]₂ (^iPrPCP) and C₆H₄[CH₂P(tBu)₂]₂ (^tBuPCP) are described. The chloride complex (^iPrPCP)PtCl (6) and the unsaturated cationic complexes [(PCP)Pt]⁺X⁻ (X = OTf⁻, BF₄⁻) (1, 7), based on both PCP ligands, were prepared and the latter reacted with carbon monoxide to give the corresponding cationic carbonyl complexes [(PCP)Pt(CO)]⁺X⁻ (X = OTf⁻, BF₄⁻) (2, 8a). Hydride nucleophilic attack on both carbonyl complexes resulted in rare neutral platinum formyl complexes (^iPrPCP)Pt(CHO) (3) and (^tBuPCP)Pt(CHO) (9). Complex 3 undergoes decarbonylation to the corresponding hydride complex within hours at room temperature, while the bulkier complex 9 is more stable and undergoes complete decarbonylation only after 3–4 d. This observation demonstrates the very significant steric effect of the ligand on stabilization of the corresponding formyl complexes. Reaction of complex 9 with triflic acid resulted in the carbonyl complex [(^tBuPCP)Pt(CO)]⁺ OTf⁻ (8b) with liberation of H₂, an unusual transformation for a metal formyl. Reaction with methyl triflate resulted in the Fischer carbene-type complex, the methoxy-methylidene [(^tBuPCP)Pt(CHOCH₃)]⁺OTf⁻ (11). The X-ray structures of complexes 2, 6, 8a and 11 were determined.