Novel (cyanamide)ZnII complexes and zinc(ii)-mediated hydration of the cyanamide ligands

Abstract

The cyanamides NCNR₂ (R₂ = Me₂, Ph₂, C₅H₁₀) react with ZnX₂ (X = Cl, Br, I) in a 2 : 1 molar ratio at RT, giving a family of zinc(II) complexes [ZnX₂(NCNR₂)₂] (R₂ = Me₂, X = Cl 1, X = Br 2, X = I 3; R₂ = C₅H₁₀, X = Cl 4, X = Br 5; X = I 6; R₂ = Ph₂, X = Cl 7, X = Br 8, X = I 9; 75–92% yields). Complexes 7 and 8 undergo ligand redistribution in wet CH₂Cl₂ solutions giving the [Zn(NCNPh₂)₄(H₂O)₂][Zn₂(μ-X)₂X₄] (X = Cl 10, Br 11) species that were characterized by ¹H NMR, HRESI-MS, and X-ray diffraction. Halide abstraction from 1–3 by the action of AgCF₃SO₃ or treatment of Zn(CF₃SO₃)₂ with NCNR₂ (R₂ = Me₂, C₅H₁₀) leads to labile complexes [Zn(CF₃SO₃)₂(NCNR₂)₃] (R₂ = Me₂, 12; C₅H₁₀, 13). Crystallization of 12 from wet CH₂Cl₂ or from the reaction mixture gave [Zn(NCNMe₂)₃(H₂O)₂](SO₃CF₃)₂ (12a) or [Zn(CF₃SO₃)₂(NCNMe₂)₂]_∞ (12b), whose structures were determined by X-ray diffraction. The Zn^II-mediated hydration was observed for the systems comprising ZnX₂ (X = Cl, Br, I), 2 equiv. NCNR₂ (R₂ = Me₂, C₅H₁₀, Ph₂) and ca. 40-fold excess of water and conducted in acetone at 60 °C (R₂ = Me₂, C₅H₁₀) or 80 °C (R₂ = Ph₂) in closed vials, and it gives the urea complexes [ZnX₂{OC(NR₂)NH₂}] (R₂ = Me₂, X = Cl 13, X = Br 14, X = I 15; R = C₅H₁₀, X = Cl 16, X = Br 17; X = I 18; R₂ = Ph₂, X = Cl 19, X = Br 20, X = I 21; 57–81%). In contrast to the Zn^II-mediated hydration of conventional nitriles, which proceeds only in the presence of co-catalyzing oximes or carboxamides, the reaction with cyanamides does not require any co-catalyst. Complexes 1–9, 12–19 were characterized by ¹H, ¹³C{¹H} NMR, IR, HRESI-MS, and X-ray crystallography (for 1–3, 8, 9, 13–15, and 17), whereas 20 and 21 were characterized by HRESI⁺-MS and ¹H and ¹³C{¹H} NMR (for 20). The structural features of the cyanamide complexes 1, 2, 7, and 8 were interpreted by theoretical calculations at the DFT level.