Synthesis, structure, and reactivity of fluorous phosphorus/carbon/phosphorus pincer ligands and metal complexes

Abstract

Reactions of the diphosphine 1,3-C₆H₄(CH₂PH₂)₂ and fluorous alkenes H₂CCHR_fn (R_fn = (CF₂)_n−1CF₃; n = 6, 8) at 75 °C in the presence of AIBN give the title ligands 1,3-C₆H₄(CH₂P(CH₂CH₂R_fn)₂)₂ (3-R_fn) and byproducts 1,3-C₆H₄(CH₃)(CH₂P(CH₂CH₂R_fn)₂) (4-R_fn) in 1 : 3 to 1 : 5 ratios. Workups give 3-R_fn in 4–17% yields. Similar results are obtained photochemically. Reaction of 1,3-C₆H₄(CH₂Br)₂ and HP(CH₂CH₂R_f8)₂ (5) at 80 °C (neat, 1 : 2 mol ratio) gives instead of simple substitution the metacyclophane [1,3-C₆H₄(CH₂P(CH₂CH₂R_f8)₂CH₂-1,3-C₆H₄CH₂P(CH₂CH₂R_f8)₂CH₂]²⁺2Br⁻, which upon treatment with LiAlH₄ yields 3-R_f8 (20%), 4-R_f8, and other products. Efforts to better access 3-R_f8, either by altering stoichiometry or using various combinations of the phosphine borane (H₃B)PH(CH₂CH₂R_f8)₂ and base, are unsuccessful. Reactions of 3-R_fn with Pd(O₂CCF₃)₂ and [IrCl(COE)₂]₂ (COE = cyclooctene) give the palladium and iridium pincer complexes (2,6,1-C₆H₃(CH₂P(CH₂CH₂R_fn)₂)₂)Pd(O₂CCF₃) (10-R_fn; 80–90%) and (2,6,1-C₆H₃(CH₂P(CH₂CH₂R_f8)₂)₂)Ir(Cl)(H) (11-R_f8; 29%), which exhibit CF₃C₆F₁₁/toluene partition coefficients of >96 : <4. The crystal structure of 10-R_f8 shows CH₂CH₂R_f8 groups with all-anti conformations that extend in parallel above and below the palladium square plane to create fluorous lattice domains. NMR monitoring shows a precursor to 11-R_f8 that is believed to be a COE adduct.