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 [Fe2(NCX)4(R-trz)5]·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 ([Fe2(NCX)4(furtrz)5]·furtrz·MeOH; X = S (furtrz-S) and X = Se (furtrz-Se)) gradual and incomplete one-step SCO transitions are observed (furtrz-S (T1/2 = 172 K) and furtrz-Se (T1/2 = 205 K)) and a structural evolution from [HS-HS] to [HS-LS] per dinuclear species. Contrasting this, within the thtrz family ([Fe2(NCX)4(thtrz)5]·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 (T1/2(1,2) = 170, 200 K) in which the FeII 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. Detailed structure- function analyses have been conducted within this growing dinuclear family to rationalise these diverse spin-switching properties

Self-assembly of an octanuclear high-spin FeII molecular cage

A discrete octanuclear high-spin Fe cage [FeL](BF)·n(solvent) was synthesised via metal ion-directed self-assembly. The cage formation is facilitated by incorporating a relatively flexible ditopic ligand with chelating pyrazolyl-pyridine functional units. The synthesis, structure, and magnetic properties of this metallo-cage are presented

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. The magnetic properties are defined by this unusual topology such that only Fe1 is in the appropriate environment for a high-spin to low-spin transition to occur. Magnetic susceptibility data reveal a complete and abrupt hysteretic spin transition (T1/2↓ = 120 K and T1/2↑ = 130 K) of this iron(II) site; Fe2 remains high-spin. This material additionally exhibits a photomagnetic response (uncommon for Hofmann-related materials), showing light-induced excited spin-state trapping [LIESST; T(LIESST) = 61 K] with associated bistability evidenced in a hysteresis loop (T1/2↓ = 60 K and T1/2↑ = 66 K)

Investigation of the spin crossover properties of three dinulear Fe(II) triple helicates by variation of the steric nature of the ligand type

The investigation of new spin-crossover (SCO) compounds plays an important role in understanding the key design factors involved, informing the synthesis of materials for future applications in electronic and sensing devices. In this report, three bis-bidentate ligands were synthesized by Schiff base condensation of imidazole-4-carbaldehyde with 4,4-diaminodiphenylmethane (L1), 4,4′-diaminodiphenyl sulfide (L2) and 4,4′-diaminodiphenyl ether (L3) respectively. Their dinuclear Fe(II) triple helicates were obtained by complexation with Fe(BF4)2·6H2O in acetonitrile. The aim of this study was to examine the influence of the steric nature of the ligand central atom (–X–, where X = CH2, S or O) on the spin-crossover profile of the compound. The magnetic behaviours of these compounds were investigated and subsequently correlated to the structural information from single-crystal X-ray crystallographic experiments. All compounds [Fe2(L1)3](BF4)2 (1), [Fe2(L2)3](BF4)2 (2) and [Fe2(L3)3](BF4)2 (3), demonstrated approximately half-spin transitions, with T1/2↓ values of 155, 115 and 150 K respectively, corresponding to one high-spin (HS) and one low-spin (LS) Fe(II) centre in a [LS–HS] state at 50 K. This was also confirmed by crystallographic studies, for example, bond lengths and the octahedral distortion parameter (∑) at 100 K. The three-dimensional arrangement of the HS and LS Fe(II) centres throughout the crystal lattice was different for the three compounds, and differing extents of intermolecular interactions between BF4− counter ions and imidazole N–H were present. The three compounds displayed similar spin-transition profiles, with 2 (–S–) possessing the steepest nature. The shape of the spin transition can be altered in this manner, and this is likely due to the subtle effects that the steric nature of the central atom has on the crystal packing (and thus inter-helical Fe–Fe separation), intermolecular interactions and Fe–Fe intra-helical separations

Self-assembled Co(II) molecular squares incorporating the bridging ligand 4,7-phenanthrolino-5,6:5 ',6 '-pyrazine

Three high-spin tetranuclear cobalt(ii) complexes have been prepared with the bridging ligand 4,7-phenanthrolino-5,6:5′,6′-pyrazine (ppz) through metal-ion directed self-assembly. The complexes differ by the incorporation of three different coordinating anions: chloride, thiocyanide and selenocyanide. The physical properties of these complexes have been investigated in detail

Thermal spin crossover behaviour of two-dimensional Hofmann-type coordination polymers incorporating photoactive ligands

Two spin crossover (SCO)-active 2D Hofmann-type framework materials, [Fe(3-PAP)2Pd(CN)4] (A) and [Fe(4-PAP)2Pd(CN)4] (B) containing the photoactive azo-benzene-type ligands 3-phenylazo-pyridine (3-PAP) and 4-phenylazo-pyridine (4-PAP) were prepared. These materials form non-porous Hofmann-type structures whereby 2D [FeIIPd(CN)4] grids are separated by 3- or 4-PAP ligands. The iron(ii) sites of both materials (A and B) undergo abrupt and hysteretic spin transitions with characteristic transition temperatures T1/2↓,↑: 178, 190 K (ΔT: 12 K) and T1/2↓,↑: 233, 250 K (ΔT: 17 K), respectively. Photo-magnetic characterisations reveal light-induced excited spin state trapping (LIESST) activity in both A and B with characteristic T(LIESST) values of 45 and 40 K. Although both free ligands show trans- to-cis isomerisation in solution under UV-irradiation, as evidenced via absorption spectroscopy, such photo-activity was not observed in the ligands or complexes A and B in the solid state. Structural analysis of a further non-SCO active isomer to B, [Fe(4-PAP)2Pd(CN)4]·1/2(4-PAP) (B·(4-PAP)), which contains free ligand in the pore space is reported

Molecular basis for the differential sensitivity of rat and human α9α10 nAChRs to α-conotoxin RgIA

In this study we exploit the flexible nature of porous coordination polymers (PCPs) with integrated spin crossover (SCO) properties to manipulate the multi-stability of spin-state switching profiles. We previously reported the 2-D Hofmann-type framework [Fe(thtrz)2Pd(CN)4].EtOH,H2O (1.EtOH,H2O), N-thiophenylidene- 4H-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 step-wise guest removal from the parent phase (1.EtOH,H2O → 1.H2O → 1..). Parallel variable temperature structural and magnetic susceptibility measurements reveal a synergistic framework flexing and 'on-off' switching of multistep SCO character concomitant with the onset of guest evacuation. In particular, the two-step SCO properties in 1.EtOH,H2O are deactivated such that both the partially solvated (1.H2O) and desolvated (1..) phases show abrupt and hysteretic one-step SCO behaviors with differing transition temperatures (1.H2O: T.↓: 215 T.↑: 235 K; 1..: T.↓: 170 T.↑: 182 K). This 'on-off' elastic frustration switching is also reflected in the light-induced excited spin state trapping (LIESST) properties of 1.EtOH,H2O and 1.., with non-quantitative (ca. 50 %, i.e. LS ↔ 1:1 HS:LS) and quantitative (ca. 100 %, LS ↔ HS) photo-induced spin state conversion achieved under light irradiation (510 nm at 10 K), respectively. Conversely, the two-step SCO properties are retained in the water saturated phase 1.3H2O but with subtle shift in transition temperatures. Comparative analysis of this and related materials reveals the distinct roles that indirect and direct guest-interactions play in inducing, stabilizing and quantifying elastic frustration and the importance of lattice flexible in these porous framework architectures

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^IIM^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^IIPt^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^IIN₆] (<b>Fe1</b>) and [Fe^IIN₄O₂] (<b>Fe2</b>), (2) as a consequence of axial water coordination to <b>Fe2</b>, there are “holes” in the [Fe^IIPt^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. The magnetic properties are defined by this unusual topology such that only <b>Fe1</b> is in the appropriate environment for a high-spin to low-spin transition to occur. Magnetic susceptibility data reveal a complete and abrupt hysteretic spin transition (<i>T</i>_1/2↓ = 120 K and <i>T</i>_1/2↑ = 130 K) of this iron­(II) site; <b>Fe2</b> remains high-spin. This material additionally exhibits a photomagnetic response (uncommon for Hofmann-related materials), showing light-induced excited spin-state trapping [LIESST; <i>T</i>(LIESST) = 61 K] with associated bistability evidenced in a hysteresis loop (<i>T</i>_1/2↓ = 60 K and <i>T</i>_1/2↑ = 66 K)

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^IIM^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^IIPt^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^IIN₆] (<b>Fe1</b>) and [Fe^IIN₄O₂] (<b>Fe2</b>), (2) as a consequence of axial water coordination to <b>Fe2</b>, there are “holes” in the [Fe^IIPt^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. The magnetic properties are defined by this unusual topology such that only <b>Fe1</b> is in the appropriate environment for a high-spin to low-spin transition to occur. Magnetic susceptibility data reveal a complete and abrupt hysteretic spin transition (<i>T</i>_1/2↓ = 120 K and <i>T</i>_1/2↑ = 130 K) of this iron­(II) site; <b>Fe2</b> remains high-spin. This material additionally exhibits a photomagnetic response (uncommon for Hofmann-related materials), showing light-induced excited spin-state trapping [LIESST; <i>T</i>(LIESST) = 61 K] with associated bistability evidenced in a hysteresis loop (<i>T</i>_1/2↓ = 60 K and <i>T</i>_1/2↑ = 66 K)

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^IIM^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^IIPt^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^IIN₆] (<b>Fe1</b>) and [Fe^IIN₄O₂] (<b>Fe2</b>), (2) as a consequence of axial water coordination to <b>Fe2</b>, there are “holes” in the [Fe^IIPt^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. The magnetic properties are defined by this unusual topology such that only <b>Fe1</b> is in the appropriate environment for a high-spin to low-spin transition to occur. Magnetic susceptibility data reveal a complete and abrupt hysteretic spin transition (<i>T</i>_1/2↓ = 120 K and <i>T</i>_1/2↑ = 130 K) of this iron­(II) site; <b>Fe2</b> remains high-spin. This material additionally exhibits a photomagnetic response (uncommon for Hofmann-related materials), showing light-induced excited spin-state trapping [LIESST; <i>T</i>(LIESST) = 61 K] with associated bistability evidenced in a hysteresis loop (<i>T</i>_1/2↓ = 60 K and <i>T</i>_1/2↑ = 66 K)