Nature of cyanoargentate bridges defining spin crossover in new 2D Hofmann clathrate analogues

Abstract

Chemical composition is leading among the numerous factors that determine the spin transition properties of coordination compounds. Classic dicyanometallic bridges {M(CN)₂}⁻ are commonly used to build Hofmann-like spin-crossover frameworks, but some extended bridges are also synthetically available. In this paper, we describe a successful synthesis of two very similar spin-crossover frameworks that differ in the cyanometallic bridges involved, namely [Fe(etpz)₂{Ag(CN)₂}₂] (1) and {Fe(etpz)₂[Ag₂(CN)₃][Ag(CN)₂]} (2) (where etpz = 2-ethylpyrazine). Magnetic and Mössbauer studies demonstrated the occurrence of abrupt one-step high-spin (HS) ↔ low-spin (LS) transitions for both complexes. The spin transition temperatures are T_1/2 ↓ = 233 K and T_1/2 ↑ = 243 K for 1 and T_1/2 ↓ = 188 K and T_1/2 ↑ = 191 K for 2 with thermal hysteresis loops of 10 K for 1 and 3 K for 2. The bridging mononuclear [Ag(CN)₂]⁻ units and Fe^II cations assemble to form infinite 2D layers in the structure of 1. Interestingly, compound 2 forms 2D layers of Fe^II cations bridged by both binuclear [Ag₂(CN)₃]⁻ and mononuclear [Ag(CN)₂]⁻ units. The structures of 1 and 2 comprise different types of intermolecular interactions including Ag⋯Ag and Ag⋯N_etpz, which induce the creation of supramolecular 3D frameworks. The synergy between metallophilic interactions and the spin transition is also confirmed by the variation of Ag⋯Ag distances during spin crossover. The characterization of such analogues allowed us to analyze in detail the effect of the cyanometallic bridge on the structure of new frameworks and on the bistability in Hofmann-like complexes.