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

Abstract

[EN] 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 {Fe-II[Hg-II(SCN)(3)](2) mu-(4,4'-bipy)(2)}(n) (1) (4,4'-bipy = 4,4'-bipyridine). The structure of 1 consists of infinite linear [Fe(mu-4,4'-bipy)](n)(2n+) chains linked by in situ formed {[Hg-II(SCN)(3)](2)(mu-4,4'-bipy)}(2n-) anionic dimers. The thermal dependence of the high-spin fraction, his, features four magnetic phases defined by steps following the sequence gamma(HS) = 1 (phase 1) <-> gamma(HS) = 1/2 (phase 2) <-> gamma(HS) approximate to 1/3 (phase 3) <-> gamma(HS) = 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], <-> approximate to [HS:2LS] <-> [LS1] inferred from single crystal-analysis. Furthermore, two additional phases are generated at low temperature. One, LS2 (gamma(HS) = 0, phase 5); is due to spontaneous symmetry breaking of the : LS-1 state below 85 K The other results from irradiating the low-temperature-LS2 Phase at 15 K with red light to photogenerate a HS phase of low symmetry (HS*) (gamma(HS) = 1, phase 6). Detailed structural studies of the six phases-unravd the pivotal role-played by the internal dihedral angle of the 4,4'-bipy ligands in the microscopic. mechanism responsible for multistability and multistep behavior in 1.