Abstract
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.
Highlights
We have reviewed all 79 of the new discrete polynuclear SCOactive Fe(II) complexes reported between 2012 to February 2018, as well as selected examples from before which brings the total to 127 complexes
By applying supramolecular self-assembly strategies,[43,58,155,156,157,158] and linking multiple donor pockets of the right ligand field strength and denticity into carefully structured ligands (Fig. 52), chemists have created a wealth of spin crossoverSpin crossover (SCO)-active discrete polynuclear Fe(II) complexes, ranging to date from dinuclear to octanuclear
Linking Fe(II) centres into polynuclear architectures has led to access to multistep SCO with stable intermediate spin states, e.g. [high spin (HS)–low spin (LS)], which opens up the possibility of multi-state molecular switches
Summary
Spin crossover (SCO) complexes are an interesting class of materials exhibiting molecular bistability with potential applications in nanotechnological devices such as memory storage units, sensors, actuators or displays.[1,2,3,4,5,6,7,8] The bistability arises from their ability to be switched between two electronic states – high spin (HS) and low spin (LS) – by external stimuli such as a change in temperature or pressure, or light irradiation or guest presence/absence, in a readily detectable and reversible way. The first of this family of SCO-active dinuclear Fe(II) triply triazole bridged complexes, phenol N4-substituent (L1, Chart 1), [Fe2L15(NCS)4]Á4MeOH in the low-spin and high-spin states, was reported in 2011 by Garcia and co-workers.[18] SQUID magnetometry revealed a sharp [HS–HS] to [LS–LS] transition with T1/2 = 150 K (Fig. 4a) and the complex was structurally. While most SCO-active Fe2L3 helicates exhibit [HS–HS] to 50% HS transitions (half SCO), a desolvated [Fe2(L12)3](ClO4)[4] complex based on the methylimidazole analogue of L11, L12 (Chart 2, bis-imine made from 1-methyl-2-imidazole-carboxaldehyde and 4,40-oxydianiline), was shown by Kruger and coworkers to undergo a complete SCO (Fig. 7b).[69] The wT value at 300 K was 6.6 cm[3] K molÀ1, consistent with two uncoupled HS Fe(II) ions. 341, 402 Not stated Not stated Not stated k155 m170 k115 m130 k150 m165 140 210–265 348
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