Spin crossover in discrete polynuclear iron(ii) complexes

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.

The Tyranny of Arm-Wrestling Methyls on Iron(II) Spin State in Pseudo-Octahedral [Fe(didentate)3] Complexes

“…Since then, a myriad of metal coordination complexes and polymeric materials have been shown to display spin transitions, often referred to as spin-crossover (SCO) materials. These have been studied in detail and extensively reviewed over the last two decades [11][12][13][14][15][16][17][18][19][20]. Due to the 'on-off' switching of the magnetic properties accompanying the spin transition from the low-spin diamagnetic configuration ( 1 A 1 label in octahedral symmetry) to the high-spin paramagnetic form ( 5 T 2 label in octahedral symmetry) for d 6 transition metals in pseudo-octahedral geometry (Scheme 1a), the 'magic' [Fe II N 6 ] chromophores, where N is a heterocyclic nitrogen donor atom, have been intensively investigated [11][12][13][14][15][16][17][18][19][20].…”

“…These have been studied in detail and extensively reviewed over the last two decades [11][12][13][14][15][16][17][18][19][20]. Due to the 'on-off' switching of the magnetic properties accompanying the spin transition from the low-spin diamagnetic configuration ( 1 A 1 label in octahedral symmetry) to the high-spin paramagnetic form ( 5 T 2 label in octahedral symmetry) for d 6 transition metals in pseudo-octahedral geometry (Scheme 1a), the 'magic' [Fe II N 6 ] chromophores, where N is a heterocyclic nitrogen donor atom, have been intensively investigated [11][12][13][14][15][16][17][18][19][20]. Various external stimulations such as changes in temperature [21,22], pressure [23,24], magnetic field [25] or light-irradiation [26,27] can be used for inducing the SCO processes, which makes these microscopic magneto-optical switches very attractive for their introduction into responsive macroscopic materials [12,13,16,[28][29][30][31].…”

“…Taking typical Fe-N bond distances of r LS = 2.0 Å and r HS = 2.2 Å [11][12][13][14][15][16][17][18][19][20] leads to ∆ LS oct /∆ HS oct ≈ 1.75 accompanying the spin transition, whereas P changes very little (P HS : P LS 19B), except for a faint reduction of the nephelauxetic effect with larger bond lengths [32].…”

Simultaneous Spin‐Crossover Transition and Conductivity Switching in a Dinuclear Iron(II) Coordination Compound Based on 7,7′,8,8′‐Tetracyano‐p‐quinodimethane

“…The Fe II SCO phenomenonc an be switched reversiblyf rom ap aramagnetic high spin (HS) to ad iamagneticl ow spin (LS) state exhibiting magnetic bistability,w hich can be induced by multiple externals timuli, such as temperature,p ressure, magnetic field, light irradiation, and guest exchange. [12][13][14][15][16][17][18][19][20][21][22][23][24] On the other hand, TCNQ radicals are known to act as am agnetic mediator through coordinatecovalentbonds, [25][26][27][28][29][30][31] as well as an electrical conductor through p-p stackingi nteractions. [32][33][34][35][36][37][38] Therefore,i f the Fe II SCO unit can be linked to TCNQ radicals through both coordinate covalent bonds and p-p stacking interactions, then such am aterialc ould lead to as witchable molecule-based crystalline material possessing strongly coupledm agnetism (from localized d-electron spins) and electrical conductivity (from delocalized p-electron spins), which is known as a p-d composite system.…”

A high-nuclearity complex containing a decanuclear iron(iii)/oxo cage in a football-like structure and rare (R-/S)-hemiacetalate ligands in a butterfly-like format

“…[1][2][3][4][5][6] There are numerous reasons for the interest in the synthesis and study of high nuclearity 3d molecular metal clusters at moderate-to-high oxidation states. 7,8 High nuclearity 3d metal clusters, 9,10 especially the ones that include Ni II , Co II , Mn III and Fe III , oen display fascinating structures, occasionally attractive properties, [11][12][13][14] including single molecular magnet (SMM) and coupled SMM behavior, [15][16][17] and promising applications for data storage, quantum computing and spintronics. 18,19 The high nuclearity iron(III)-oxo chemistry has being developed over many years by virtue of their important role in bioinorganic chemistry, molecular magnetism and so on.…”