Mono- and dinuclear nickel(ii) complexes of resolved Schiff-base ligands with extended quinoline substituents

Abstract

The coordination chemistry of four enantiopure tetradentate bis(iminoquinoline) ligands with nickel(II) salts is reported. The previously reported ligands CBQ, CPQ, BBQ, and BPQ result from the condensation of (1R,2R)-cyclohexyldiamine or (R)-BINAM with two equivalents of 2-formylbenzo[h]quinoline or 8-isopropyl-2-quinolinecarboxaldehyde {CBQ = (1R,2R)-cyclohexanediamine-N,N′-bis(benzo[h]quinoline-2-ylmethylene), CPQ = (1R,2R)-cyclohexanediamine-N,N′-bis[[(8-isopropyl)-2-quinolinyl]methylene], BBQ = [(R)-1,1′-binaphthalene]-2,2′-diamine-N,N′-bis(benzo[h]quinoline-2-ylmethylene), BPQ = [(R)-1,1′-binaphthalene]-2,2′-diamine-N,N′-bis[[(8-isopropyl)-2-quinolinyl]methylene]}. Reaction of NiI₂ with the (1R,2R)-cyclohexyl ligands gives the mononuclear distorted trigonal-bipyramidal (TBP) complexes [Ni(N₃-CBQ)I₂] and [Ni(N₃-CPQ)I₂]. Incomplete iodide abstraction from [Ni(N₃-CPQ)I₂] with AgOTf leads to partial replacement of the iodide with hydroxide from adventitious water to give [Ni(N₃-CPQ)I_1.6(OH)_0.4] (distorted TBP). The corresponding reaction with [Ni(N₃-CBQ)I₂] again fails to remove all of the iodide, resulting instead in conversion to the syn dinuclear [Ni₂(CBQ)(μ-X)₂I₂] (X = Cl_0.925I_0.075) complex, where chloride abstraction from the solvent (CH₂Cl₂) has resulted in a mixed halide system and the metal centers are square-pyramidal. Reaction of Ni(OTf)₂ with CBQ leads to the isolation of the octahedral cation [Ni(CMBQ)₂]²⁺, with CMBQ [(1R,2R)-cyclohexanediamine-mono-N-(benzo[h]quinoline-2-ylmethylene)] being the partial hydrolysis product of CBQ. [Ni(CMBQ)₂][OTf]₂ crystallizes as a 1 : 1 mixture of P and M helical diastereomers. The coordination of NiI₂ with the (R)-BINAM derived ligands yields the anti dinuclear P-helical complexes [Ni₂(BBQ)(μ-I)₂I₂] and [Ni₂(BPQ)(μ-I)₂I₂]: one nickel ion is coordinated in each bidentate iminoquinoline pocket and the geometry at the metal centers is distorted square-pyramidal. Characterisation by ¹H NMR, UV-Vis, electronic circular dichroism (ECD) spectroscopy, combustion analysis, and HRMS is reported in addition to structural and halide abstraction studies.