Structural and spectroscopic insights on the coordination chemistry of the rigid heterotopic chelating ligand 1H-pyrazolo[4,3-h]quinoline

Abstract

In this work, we report the coordination chemistry of a rigid heterotopic ligand 1H-pyrazolo[4,3-h]quinoline HL1 in copper(II) and zinc(II) complexes and examine both the structural properties and solution-phase behavior of these species. On reaction with zinc(II) nitrate in acetonitrile, a mononuclear complex [Zn(HL1)2(OH2)2](NO3)2 1 is generated, where a size mismatch between the metal ion and the binding pocket leads to an anomalously long Zn–N bond to the pyrazole nitrogen. This is consistent with solution-state absorption and fluorescence measurements which indicate a relatively low stability constant logβ of 9.1(3) in acetonitrile. The reaction of HL1 with copper(II) nitrate in methanol gave four new crystalline phases, the major forms of which are two polymorphs of [Cu2(μ2-L1)2(NO3)2(MeOH)2] 2α and 2β which also constitutes the dominant solution-phase behavior of this mixture. Additionally, trace amounts of the nitric acid salt of HL1 were crystallized along with the slow formation of a tetranuclear species [Cu4(μ2-L1)6(OH2)2](NO3)2 · 2MeOH 3 which also features anomalously long bridging distances from L1 as a result of its backbone rigidity.