Bulky bis-guanidine ligand-based neutral and cationic zinc alkyl, halide, and hydride complexes: synthesis, characterization, and catalytic application

Abstract

The tetra-aryl substituted bis-guanidine ligand L(3H) (L(3H) = {(ArHN)(ArNH)–CN–C=(NAr)(NHAr)}; Ar = 2,6-ⁱPr₂-C₆H₃) was used for the synthesis of mononuclear zinc(II) alkyl [L(2H)ZnR, R = Me (Zn-1), Et (Zn-2)], iodide [L(2H)ZnI, (Zn-3)], and hydride [L(2H)ZnH, (Zn-4)] complexes. Moreover, the reactions of Zn-1, Zn-2, and Zn-3 with [PhNMe₂H]⁺ [B(C₆F₅)₄]⁻ yielded the biguanide zinc alkyl [L(3H)ZnR]⁺ [B(C₆F₅)₄]⁻ (R = Me (Zn-5), Et (Zn-6)) and iodide [L(3H)ZnI]⁺ [B(C₆F₅)₄]⁻ (Zn-7) cationic compounds in good yields. All newly isolated and thermally stable zinc complexes (Zn-1–Zn-7) were well characterized by multinuclear NMR (¹H, ¹³C{¹H}, with additional ¹¹B and ¹⁹F{¹H} for Zn-5–Zn-7), HRMS, and single crystal X-ray diffraction studies. Moreover, the well-defined zinc iodide cation (Zn-7) [L(3H)ZnI]⁺ [B(C₆F₅)₄]⁻ was further used as a catalyst for the synthesis of cyanohydrin enol ether products (RCR′CNOSiMe₃) by TMSCN addition to carbonyl compounds (RCOR′, R′ = alkyl/aryl) under mild reaction conditions.