Synthesis and characterization of Bis(imino)phosphine [NPN] ligands and their copper(I) halide complexes

A series of four new bis(imino)phosphine [NPN] ligands PhP(C₆H₄-2-HC=NAr)₂ (Ar = 4-OMeC₆H₄ (L¹, 1), 4-ClC₆H₄ (L², 2), 4-BrC₆H₄ (L³, 3), 2,6-iPr₂C₆H₃ (L⁴, 4)) were synthesized and structurally characterized. Their reactions with Cu(I) halides CuX (X = Cl, Br, I) were explored for the assembly of coordination complexes LⁿCuX, and the effects of substituent (Ar) and anion (X) were also investigated in a comparative manner. It was found that the reaction of methoxy substituted L¹ (Ar = 4-OMeC₆H₄) with CuX gave the mononuclear Cu(I) complexes L¹CuX (X = Cl (1a), Br (1b), I (1c)) with isostructural feature. In contrast, when halogen-substituted ligands L^2-3 (4-ClC₆H₄, 4-BrC₆H₄) were applied, their reactions with CuCl afforded the formation of tetranuclear dimer [Lⁿ(CuCl)₂]₂ (n = 2 (2a), 3 (3a)) bridged by the Cu₂Cl₂ square, regardless of the composition of the reactants. On the other hand, reactions of L^2-3 with CuBr and CuI generated the mononuclear Cu(I) complexes LⁿCuX (n = 2, X = Br (2b), I (2c); n = 3, X = Br (3b), I (3c)) again. In all these cases above, L^1-3 ligands played the tridentate role by biting the copper(I) centers through NPN sites. When the bulky L⁴ (Ar = 2,6-iPr₂C₆H₃) was involved, the binuclear dimer (L⁴CuI)₂ (4c) bridged by iodide anions was obtained, where each L⁴ ligand coordinated the copper center using the bidentate P^N mode with another imino arm pendant. In further exploration, 1,10-o-phenanthroline (denoted as phen) was introduced to produce the heteroleptic complex L¹CuCl(phen) (1d), where both imino arms in L¹ were pendent. The obtained ligand (L^1-4) and Cu(I) complexes were characterized by melting point, elemental analysis, FT-IR, UV–Vis, NMR, and single crystal X-ray diffraction.