Formation of boron, nickel(II) and iridium(III) complexes with an azophenine derivative: Isomerization, delocalization and extension of the π-conjugated system on coordination

Abstract

The azophenine N,N′,N″,N‴-tetra-4-ethylphenyl-2,5-diamino-1,4-benzoquinonediimine (4-EtApH2), its diprotonated species [4-EtApH4](ClO4)2 (1) and complexes, the bis(boron difluoride) complex [(BF2)2(μ-4-EtAp)] (2), the mono- and dinuclear nickel(II) complexes [Ni(4-EtApH2)3](ClO4)2 (3) and [{Ni(acac)}2(μ-4-EtAp)] (4), respectively, and the dinuclear iridium(III) complex [{Ir(cp∗)Cl}2(μ-4-EtAp)] (5), have been prepared. The molecular structures of these compounds were determined by X-ray analysis. The coordination environments around the boron center of 2 and the Ni centers of 3 and 4 were tetrahedral, octahedral and square-planar, respectively. Furthermore, each Ir center of 5 was coordinated by two nitrogen atoms from 4-EtAp2−, Cl− and η5-cp∗. Coordination of the ligand to Ni to form 3 caused an isomerization from the para-benzoquinonediimine of 4-EtApH2 to the ortho form, with a bond alternation at each of the two N–C–C–C–N π-systems. Diprotonation and dinuclear complex formation to construct 1, 2, 4 and 5 led to a delocalization of six π-electrons over each of the two N–C–C–C–N moieties connected by single bonds. The π–π∗ transition absorption gradually shifted to longer wavelengths (4-EtApH2≈3→2→1→4≈5), suggesting an extension of the π-conjugation range. Cyclic voltammograms of 3 and 4 showed one and two redox potential(s), respectively.