Two-step spin crossover by guest-disorder induced local symmetry breaking within a 3D Hofmann-like framework.

A 3D Hofmann-like metal-organic framework has been prepared which contains a 2,1,3-benzothiadiazole-based pillaring ligand. Encapsulation of a polycyclic aromatic hydrocarbon, chrysene, within the pore structure leads to a new pathway to multi-step spin crossover behaviour in which the observed two-step spin transition arises due to the presence of multiple site environments associated with local guest positional effects within the host lattice.

Guest-Induced Multistep to Single-Step Spin-Crossover Switching in a 2-D Hofmann-Like Framework with an Amide-Appended Ligand.

A detailed study of the two-dimensional (2-D) Hofmann-like framework [Fe(furpy)Pd(CN)]·G (furpy: -(pyridin-4-yl)furan-2-carboxamide, G = HO,EtOH (·HO,Et), and HO (·HO)) is presented, including the structural and spin-crossover (SCO) implications of subtle guest modification. This 2-D framework is characterized by undulating Hofmann layers and an array of interlayer spacing environments─this is a strategic approach that we achieve by the inclusion of a ligand with multiple host-host and host-guest interaction sites. Variable-temperature magnetic susceptibility studies reveal an asymmetric multistep SCO for ·HO,Et and an abrupt single-step SCO for ·HO with an upshift in transition temperature of ∼75 K.

Co-existence of five- and six-coordinate iron(II) species captured in a geometrically strained spin-crossover Hofmann framework.

Inclusion of an angular bridging ligand, 4,2':6',4''-terpyridine (TPy), into a Hofmann-type framework produces an irregular network in which six- and five-coordinate Fe species co-exist. The octahedral sites show thermally-induced spin-crossover (SCO) and the rare five-coordinate Fe sites are high-spin.

Regulation of Multistep Spin Crossover Across Multiple Stimuli in a 2-D Framework Material.

We investigate the effects of a broad array of external stimuli on the structural, spin-crossover (SCO) properties and nature of the elastic interaction within the two-dimensional Hofmann framework material [Fe(cintrz)Pd(CN)]·guest (cintrz = -cinnamalidene 4-amino-1,2,4-triazole; ·guest; guest = 3HO, 2HO, and Ø). This framework exhibits a delicate balance between ferro- and antiferro-elastic interaction characters; we show that manipulation of the pore contents across guests = 3HO, 2HO, and Ø can be exploited to regulate this balance. In ·3HO, the dominant antiferroelastic interaction character between neighboring Fe sites sees the low-temperature persistence of the mixed spin-state species {HS-LS} for {Fe1-Fe2} (HS = high spin, LS = low spin).

Three Distinct Spin-Crossover Pathways in Halogen-Appended 2D Hofmann Frameworks.

We probe, here, a family of 2D Hofmann-type frameworks, [Fe(Pd(CN))(bztrzX)]·HO [·HO; X = F, Cl, Br; = 1 (X = Cl, Br) and 3 (X = F); bztrzX = ()-1-(2-Xphen-1-yl)--(4-1,2,4-triazol-4-yl)methanimine], with halogen-appended ligands. In all cases, there are two crystallographically distinct Fe sites, ({Fe1-Fe2}), driven by the presence of a range of host-host and host-guest interactions. We find that lattice modification through X variation influences the elastic coupling between the Fe sites, the emergence of ferroelastic or antiferroelastic interactions between these sites, and the relative spin-state stabilization/destabilization at each site.

Hierarchical Spin-Crossover Cooperativity in Hybrid 1D Chains of Fe -1,2,4-Triazole Trimers Linked by [Au(CN) ] Bridges.

Foremost, practical applications of spin-crossover (SCO) materials require control of the nature of the spin-state coupling. In existing SCO materials, there is a single, well-defined dimensionality relevant to the switching behavior. A new material, consisting of 1,2,4-triazole-based trimers coordinated into 1D chains by [Au(CN) ] and spaced by anions and exchangeable guests, underwent SCO defined by elastic coupling across multiple dimensional hierarchies.

Dual-supramolecular contacts induce extreme Hofmann framework distortion and multi-stepped spin-crossover.

An extended nitro-functionalised 1,2,4-triazole ligand has been used to induce considerable lattice distortion in a 2-D Hofmann framework material via competing supramolecular interactions. Single crystal X-ray diffraction analyses on [Fe3(N-cintrz)6(Pd(CN)4)3]·6H2O (N-cintrz: (E)-3-(2-nitrophenyl)acrylaldehyde) reveal a substantial deviation from a regular Hofmann structure, in particular as the intra- and inter-layer contacts are dominated by hydrogen-bonding interactions rather than the typical π-stacking arrays. Also, the 2-D Hofmann layers show an assortment of ligand conformations and local FeII coordination environments driven by the optimisation of competing supramolecular contacts.

A new spin crossover Fe coordination environment in a two-fold interpenetrated 3-D Hofmann-type framework material.

A 3-D FeII Hofmann-type framework material has been prepared which contains a three-connecting pyridyl-donor ligand with amide functionality and [Au(CN)2]- metallo-ligands. The FeII sites display a rare FeII(py)3(N[triple bond, length as m-dash]C)3 coordination environment, which we show for the first time to be conducive to spin crossover (SCO).

Spin Crossover in Iron(III) Quinolylsalicylaldiminates: The Curious Case of [Fe(qsal-F)](Anion).

A family of iron(III) spin crossover complexes with different counteranions, [Fe(qsal-F)]A (qsal-F = 4-fluoro-2-[(8-quinolylimino)methyl]phenolate; A = PF , OTf , NO , ClO , BF , or NCS ) have been prepared. All compounds are isostructural and crystallize (triclinic 1̅ space group) with two independent iron(III) centers (Fe1 and Fe2) in the asymmetric unit. No solvent molecules are found in the crystal lattice, allowing us to directly probe the relative influence of anion variation on the spin crossover characteristics.

Spin crossover modulation in a coordination polymer with the redox-active bis-pyridyltetrathiafulvalene (pyTTF) ligand.

A one-dimensional Fe coordination polymer (CP) has been formed which includes the redox-active ligand bis-pyridyltetrathiafulvalene (pyTTF) and a Schiff base-like NO ligand. This CP is both spin crossover (SCO) and redox-active in the solid-state, and chemical oxidation results in SCO modification.

Heteroatom substitution effects in spin crossover dinuclear complexes.

We probe the effect of heteroatom substitution on the spin crossover (SCO) properties of dinuclear materials of the type [Fe(NCX)(R-trz)]·S (X = S, Se; S = solvent; R-trz = (E)-N-(furan-2-ylmethylene)-4H-1,2,4-triazol-4-amine (furtrz); (E)-N-(thiophen-2-ylmethylene)-4H-1,2,4-triazole-4-amine (thtrz)). For the furtrz family ([Fe(NCX)(furtrz)]·furtrz·MeOH; X = S (furtrz-S) and X = Se (furtrz-Se)) gradual and incomplete one-step SCO transitions are observed (furtrz-S (T = 172 K) and furtrz-Se (T = 205 K)) and a structural evolution from [HS-HS] to [HS-LS] per dinuclear species. Contrasting this, within the thtrz family ([Fe(NCX)(thtrz)]·4MeOH; X = S (thtrz-S) and X = Se (thtrz-Se)) more varied SCO transitions are observed, with thtrz-S being SCO-inactive (high spin) and thtrz-Se showing a rare complete two-step SCO transition (T = 170, 200 K) in which the Fe sites transition from [HS-HS] to [HS-LS] to [LS-LS] per dinuclear unit with no long range ordering of spin-states at the intermediate plateau.

Guest-Adaptable Spin Crossover Properties in a Dinuclear Species Underpinned by Supramolecular Interactions.

Molecular crystals with guest-adaptable crystalline structures and properties are comparatively rare owing to their inherent reduced structural stability and malleability to support molecular variation. To overcome this intrinsic challenge, here we introduce structural stabilizing supramolecular interactions into a dinuclear material and henceforth demonstrate a dynamic structural and spin crossover property interchange between solvated (A·3MeOH) and desolvated (A·Ø) products (A = [Fe( o-NTrz)(NCS)]; 4-( o-nitrobenzyl)imino-1,2,4-triazole). Relatively uncommon for molecular species, the guest molecules in A·3MeOH are evolved (A·Ø) via a single-crystal to single-crystal transformation with affiliated phase transition resulting in a reversible transformation from one- to two-step spin crossover (SCO) transition character.

Guest-Responsive Elastic Frustration "On-Off" Switching in Flexible, Two-Dimensional Spin Crossover Frameworks.

In this study we exploit the flexible nature of porous coordination polymers (PCPs) with integrated spin crossover (SCO) properties to manipulate the multistability of spin-state switching profiles. We previously reported the two-dimensional Hofmann-type framework [Fe(thtrz)Pd(CN)]·EtOH,HO (1·EtOH,HO), N-thiophenylidene-4 H-1,2,4-triazol-4-amine), displaying a distinctive two-step SCO profile driven by extreme elastic frustration. Here, we reveal a reversible release mechanism for this elastic frustration via stepwise guest removal from the parent phase (1·EtOH,HO → 1·HO → 1·Ø).

Increasing spin crossover cooperativity in 2D Hofmann-type materials with guest molecule removal.

Molecule-based spin state switching materials that display ambient temperature transitions with accompanying wide thermal hysteresis offer an opportunity for electronic switching, data storage, and optical technologies but are rare in existence. Here, we present the first 2D Hofmann-type materials to exhibit the elusive combination of ambient temperature spin crossover with wide thermal hysteresis (Δ = 50 and 65 K). Combined structural, magnetic, spectroscopic, and theoretical analyses show that the highly cooperative transition behaviours of these layered materials arise due to strong host-host interactions in their interdigitated lattices, which optimises long-range communication pathways.

Spin-State Patterning in an Iron(II) Tripodal Spin-Crossover Complex.

A mononuclear iron(II) complex that displays a gradual two-step spin-crossover (SCO) transition is reported. The intermediate plateau (IP) occurs between HSLS and HSLS (HS = high spin; LS = low spin) ratios over the region of ca. 190-170 K.

Four-step iron(ii) spin state cascade driven by antagonistic solid state interactions.

A four-stepped cascade of Fe(ii) high spin (HS) to low spin (LS) states is demonstrated in a family of 2-D Hofmann materials, [Fe(saltrz)(M(CN))]·8(HO) (M = Pd ( ), Pt ( ); saltrz = ()-2-(((4-1,2,4-triazol-4-yl)imino)methyl)phenol). Alongside the fully HS and LS Fe(ii) states, fractional spin state stabilization occurs at HS/LS values of 5/6, 2/3, and 1/6. This unconventional spin state periodicity is driven by the presence of multiple spin crossover (SCO) active Fe(ii) sites which are in subtly distinct environments driven by a network of antagonistic host-host and host-guest interactions.

Guest Programmable Multistep Spin Crossover in a Porous 2-D Hofmann-Type Material.

The spin crossover (SCO) phenomenon defines an elegant class of switchable materials that can show cooperative transitions when long-range elastic interactions are present. Such materials can show multistepped transitions, targeted both fundamentally and for expanded data storage applications, when antagonistic interactions (i.e.

Hysteretic Four-Step Spin Crossover within a Three-Dimensional Porous Hofmann-like Material.

Materials that display multiple stepped spin crossover (SCO) transitions with accompanying hysteresis present the opportunity for ternary, quaternary, and quinary electronic switching and data storage but are rare in existence. Herein, we present the first report of a four-step hysteretic SCO framework. Single-crystal structure analysis of a porous 3D Hofmann-like material showed long-range ordering of spin states: HS, HS LS , HS LS , HS LS , and LS.

Exploiting Pressure To Induce a "Guest-Blocked" Spin Transition in a Framework Material.

A new functionalized 1,2,4-triazole ligand, 4-[(E)-2-(5-methyl-2-thienyl)vinyl]-1,2,4-triazole (thiome), was prepared to assess the broad applicability of strategically producing multistep spin transitions in two-dimensional Hofmann-type materials of the type [FePd(CN)(R-1,2,4-trz)]·nHO (R-1,2,4-trz = a 4-functionalized 1,2,4-triazole ligand). A variety of structural and magnetic investigations on the resultant framework material [FePd(CN)(thiome)]·2HO (A·2HO) reveal that a high-spin (HS) to low-spin (LS) transition is inhibited in A·2HO due to a combination of guest and ligand steric bulk effects. The water molecules can be reversibly removed with retention of the porous host framework and result in the emergence of an abrupt and hysteretic one-step spin transition due to the removal of guest internal pressure.

Perturbation of spin crossover behavior by covalent post-synthetic modification of a porous metal-organic framework.

Covalent post-synthetic modification is a versatile method for gaining high-level synthetic control over functionality within porous metal-organic frameworks and for generating new materials not accessible through one-step framework syntheses. Here we apply this topotactic synthetic approach to a porous spin crossover framework and show through detailed comparison of the structures and properties of the as-synthesised and covalently modified phases that the modification reaction proceeds quantitatively by a thermally activated single-crystal-to-single-crystal transformation to yield a material with lowered spin-switching temperature, decreased lattice cooperativity, and altered color. Structure-function relationships to emerge from this comparison show that the approach provides a new route for tuning spin crossover through control over both outer-sphere and steric interactions.

Thermal- and light-induced spin-crossover bistability in a disrupted Hofmann-type 3D framework.

The expected 3D and 2D topologies resulting from combining approximately linear bis- or monopyridyl ligands with [Fe(II)M(II)(CN)4] (M(II) = Pt, Pd, Ni) 4,4-grid sheets are well established. We show here the magnetic and structural consequences of incorporating a bent bispyridyl linker ligand in combination with [Fe(II)Pt(II)(CN)4] to form the material [Fe(H2O)2Fe(DPSe)2(Pt(CN)4)2]·3EtOH (DPSe = 4,4'-dipyridylselenide). Structural investigations reveal an unusual connectivity loosely resembling a 3D Hofmann topology where (1) there are two distinct local iron(II) environments, [Fe(II)N6] (Fe1) and [Fe(II)N4O2] (Fe2), (2) as a consequence of axial water coordination to Fe2, there are "holes" in the [Fe(II)Pt(II)(CN)4] 4,4 sheets because of some of the cyanido ligands being terminal rather than bridging, and (3) bridging of adjacent sheets occurs only through one in two DPSe ligands, with the other acting as a terminal ligand binding through only one pyridyl group.

An investigation of photo- and pressure-induced effects in a pair of isostructural two-dimensional spin-crossover framework materials.

Two new isostructural iron(II) spin-crossover (SCO) framework (SCOF) materials of the type [Fe(dpms)2(NCX)2] (dpms = 4,4'-dipyridylmethyl sulfide; X = S (SCOF-6(S)), X = Se (SCOF-6(Se))) have been synthesized. The 2D framework materials consist of undulating and interpenetrated rhomboid (4,4) nets. SCOF-6(S) displays an incomplete SCO transition with only approximately 30 % conversion of high-spin (HS) to low-spin iron(II) sites over the temperature range 300-4 K (T1/2 = 75 K).

Spin crossover intermediate plateau stabilization in a flexible 2-D Hofmann-type coordination polymer.

The abrupt and hysteretic two-step spin crossover in a new triazole-based 2-D Hofmann-type complex shows a record breaking 120 K intermediate plateau (IP) region stabilized by negative cooperative interactions.

A ferrocenyl-substituted 1,2,4-triazole ligand and its Fe(II), Ni(II) and Cu(II) 1D-chain complexes.

As part of a program aimed at making bifunctional iron(II) spin-crossover (SCO) materials, particularly those having redox/electron transfer as the second function, we have made the new ferrocene-triazole ligand ATF ([(4H-1,2,4-triazol-4-yl)amino]methylferrocene), (1), and a series of M(II) complexes of this ligand with emphasis on iron(II). Polynuclear 1D-chain complexes [Fe(ATF)3](Br)2·0.5(H2O) (2), [Fe(ATF)3](ClO4)2·0.