Competing Phases Involving Spin-State and Ligand Structural Orderings in a Multistable Two-Dimensional Spin Crossover Coordination Polymer

Abstract

Competition between spin-crossover and structural ligand ordering is identified as responsible for multistability and generation of six different phases in a rigid two-dimensional coordination polymer formulated {FeII[HgII(SCN)3]2μ-(4,4′-bipy)2}n (1) (4,4′-bipy = 4,4′-bipyridine). The structure of 1 consists of infinite linear [Fe(μ-4,4′-bipy)]n2n+ chains linked by in situ formed {[HgII(SCN)3]2(μ-4,4′-bipy)}2n− anionic dimers. The thermal dependence of the high-spin fraction, γHS, features four magnetic phases defined by steps following the sequence γHS = 1 (phase 1) ↔ γHS = 1/2 (phase 2) ↔ γHS ≈ 1/3 (phase 3) ↔ γHS = 0 (phase 4). These four magnetic states are consistent with structural ordering stemming from the different commensurate or incommensurate high- and low-spin populations [HS] ↔ [HS:LS] ↔ ≈ [HS:2LS] ↔ [LS1] inferred from single crystal analysis. Furthermore, two additional phases are generated at low temperature. One, LS2 (γHS = 0, phase 5), is due to spontaneous symmetry breaking of the LS1 ...