Versatility of the bis(iminopyrrolylmethyl)amine ligand: tautomerism, protonation, helical chirality, and the secondary coordination sphere with halogen bonds in the formation of copper(ii) and nickel(ii) complexes

Abstract

The reaction of N,N-di(2,6-bis(isopropyl)phenylimino-pyrrolyl-α-methyl)-N-methylamine H₂L1 with copper(I) sources such as CuX (X = Cl (1), Br (2), and I (3)) afforded bis(chelated) ionic copper(II) complexes of the type [CuL1H]X. A similar type of mononuclear structure was obtained with Cu(NO₃)₂·(H₂O)₃. Conversely, binuclear copper(II) complexes [Cu₂(μ-L1)(μ-OOCCH₃)(μ-OH)](4) and [Cu₂(μ-L1H)(μ-OOCPh)(μ-O)] (5) were obtained from the reaction of Cu(O₂CR)₂·H₂O with H₂L1. Notably, these reactions in the presence of a base yielded the neutral copper(II) complex [CuL1] (6). This product was also obtained from the reaction of complex 2 or 4 with NaOH in methanol. All structures feature a dianionic imino-pyrrole motif and a protonated central amine function except 4. The reaction of H₂L1 with NiCl₂·DME gave the mononuclear complex [NiCl₂(L1H₂)], 7. In contrast to this, the reaction of the newly synthesized sterically less encumbered ligand N,N-di(phenylimino-pyrrolyl-α-methyl)-N-methylamine H₂L2 with NiCl₂·DME gave the binuclear complex [NiCl(L2H₂)(HOMe)]₂[Cl]₂ (8). Both 7 and 8 show the amine-azafulvene ligand form and coordination of the central amine. The reaction of complex 7 with NaHBEt₃ yielded a neutral complex [NiL1] (8) containing the imino-pyrrole form. In the molecular structures, interesting secondary coordination spheres incorporating guest molecules such as CHCl₃ and MeOH in the crystal lattices and the presence of helical enantiomers were observed and analysed. In one case, CHCl₃ was found inside an unusual cage-like structure supported by halogen bonds. Preliminary DFT calculations on the geometry of the nickel complex with H₂L1 showed that the pentacoordinated tbp geometry is more stable than the square planar geometry.