Iron(ii) spin crossover (SCO) materials have been widely studied as molecular switches with a wide variety of potential applications, including as displays, sensors, actuators or memory components. Most SCO materials have been either monometallic or polymeric, and it is only relatively recently that chemists have really started to focus on linking multiple metal centres together within the one, discrete, molecule in an effort to enhance the SCO properties, such as abrupt, hysteretic, and multistep switching, as well as the potential for quantum cellular automata, whilst still being readily amenable to characterisation. Here we present a review of the ligand designs of the last two decades that have led to self assembly of discrete di- to poly-nuclear iron(ii) complexes of helicate, cage, cube, and other supramolecular architectures with rich SCO activity, and to an increased focus on host-guest interactions. Analysis of selected octahedral distortion parameters (Σ, CShM) reveals interesting differences between these structural types, for example that the iron(ii) centres in grids are generally significantly more distorted than those in squares or cages, yet are still SCO-active. Of the 127 complexes reviewed (79 published 2012-Feb. 2018), 54% are dinuclear, 10% trinuclear, 31% tetranuclear, and the remaining 5% are penta, hexa and octanuclear. Of the 93 designer ligands utilised in these polynuclear architectures: 60 feature azoles; 55 provide all donors to the Fe(ii) centres (no co-ligands coordinated) and form exclusively 5-membered chelate rings via either bidentate azole-imine/pyridine or tridentate heterocycle-imine/amine/thioether/pyridine-heterocycle binding pockets.

Access to a new family of thioether-linked PSRT ligands, 4-substituted-3,5-bis{[(2-pyridylmethyl)sulfanyl]methyl}-4H-1,2,4-triazoles (analogues of the previously studied amino-linked PMRT ligands), has been established. Four such ligands have been prepared, PSPhT, PS(i)BuT, PS(t-Bu)PhT, and PS(Me)PhT, with R = Ph, (i)Bu, (t-Bu)Ph, and (Me)Ph, respectively. Three dinuclear colorless to pale green iron(II) complexes, [Fe(II)2(PSRT)2](BF4)4·solvent, featuring N4S2 donor sets, were prepared. Single-crystal structure determinations on [Fe(II)2(PSPhT)2](BF4)4·2MeCN·H2O, [Fe(II)2(PSPhT)2](BF4)4·2(1)/2MeCN·(1)/2H2O·THF, [Fe(II)2(PS(Me)PhT)2](BF4)4·2MeCN, and [Fe(II)2(PS(i)BuT)2](BF4)4·4MeCN reveal that all four are stabilized in the [HS-HS] state to 100 K and that both possible binding modes of the bis-terdentate ligands, cis- and trans-axial, are observed. Variable-temperature magnetic susceptibility studies of air-dried crystals (solvatomorphs of the single crystal samples) reveal the first examples of spin crossover (SCO) for a dinuclear iron(II) complex with N4S2 coordination. Specifically, [Fe(II)2(PSPhT)2](BF4)4·2(1)/2H2O undergoes a multistep but complete SCO from [HS-HS] to [LS-LS], whereas [Fe(II)2(PS(Me)PhT)2](BF4)4·1(1)/2MeCN·2H2O exhibits a half-SCO from [HS-HS] to [HS-LS]. In contrast, [Fe(II)2(PS(i)BuT)2](BF4)4·MeCN·H2O remains [HS-HS] down to 50 K. The reflectance spectrum of pale green [Fe(II)2(PSPhT)2](BF4)4·(1)/2CHCl3·2(1)/2H2O (solvatomorph A) reveals a trace of LS character (572 nm band (1)A1g → (1)T1g). Evans' (1)H NMR method and UV-vis spectroscopy studies revealed that on cooling dark green acetonitrile solutions of these complexes from 313 to 233 K, all three undergo SCO centered at or near room temperature. The tendency of the complexes to go LS in solution reflects the electronic impact of R on the σ-donor strength of the PSRT ligand, whereas the opposite trend in stabilization of the LS state is seen in the solid state, where crystal packing effects, of the R group and solvent content, dominate the SCO behavior.

A family of three new mononuclear complexes of the general form [Fe(L(pz))2(NCE)2] has been prepared (L(pz) = 4-p-tolyl-3-(2-pyrazinyl)-5-(2-pyridyl)-1,2,4-triazole; E = S, Se, BH3). All three exhibit spin crossover, in two cases with hysteresis, with T1/2 being predictably tuned by varying the coordinated anion.