Iron(II) spin crossover complexes of tetradentate Schiff-bases: tuning T1/2 by choice of formyl-heterocycle component

Four new tetradentate Schiff-base ligands were prepared in situ from the 1 : 2 condensation of 1,3-diaminopropane and either 2-thiazolecarboxaldehyde (L2thiazole), 4-thiazolecarboxaldehyde (L4thiazole), 4-oxazolecarboxaldehyde (L4oxazole), or 5-bromopyridine-2-aldehyde (L5Br-pyridine), and complexed with [Fe(NCS)2(pyridine)4] to give four monometallic FeII complexes, [Fe(Lheterocycle)(NCS)2]. Structural characterisation shows the expected octahedral FeII centres in all cases, with Lheterocycle occupying the equatorial plane and the two thiocyanate ligands trans to each other, resulting in an N6 coordination sphere. Solid state magnetic measurements showed that the two complexes with the thiazole-based ligands exhibit the beginning of a spin transition above 300 K, with T1/2 = 350 K for [Fe(L4thiazole)(NCS)2] and 400 K for [Fe(L2thiazole)(NCS)2], whereas the 4-oxazole-based ligand gives [Fe(L4oxazole)(NCS)2] which remains high spin at all measured temperatures (50-400 K). Interestingly, [Fe(L5Br-pyridine)(NCS)2] crystallised as two solvent-free polymorphs: magnetic measurements on samples with both polymorphs present showed a two step SCO with an abrupt transition at T1/2 = 245 K assigned to the transition in polymorph A (as this was also seen in a sample of pure polymorph A), and a gradual transition at T1/2 = 304 K assigned to polymorph B. These findings show that the order of increasing ligand field strength for these heterocycles is 4-oxazole ≪ 5Br-pyridine < 4-thiazole < 2-thiazole.

Solution Spin Crossover Versus Speciation Effects: A Cautionary Tale

Two acyclic tetradentate Schiff base ligands, HL^X-OH (X = H and Br), were synthesised by 2:1 condensation of either 2-pyridinecarboxaldehyde or 5-bromo-2-pyridinecarboxaldehyde and 1,3-diamino-2-propanol and then used to prepare six mononuclear complexes, [Fe^II(HL^X-OH)(NCE)₂], with three different NCE co-ligands (E = BH₃, Se, and S). The apparent solution spin crossover switching temperature, T_1/2, of these 6 complexes, determined by Evans method NMR studies, is tuned by several factors: (a) substituent X present at the 5 position of the pyridine ring of the ligand, (b) E present in the NCE co-ligand, (c) solvent employed (P'), and (d) potentially also by speciation effects. In CD₃CN, for the pair of NCE = NCBH₃ complexes, when X = H, the complex is practically LS (extrapolated T_1/2 ∼624 K), whereas when X = Br, it is far lower (373 K), which implies a higher field strength when X = H than when it is Br. The same trend, X = H results in a higher apparent T_1/2 than X = Br, is seen for the other two pairs of complexes, with E = Se (429 > 351 K, ΔT_1/2 = 78 K) or S (361 > 342 K, ΔT_1/2 = 19 K). For the family of three X = Br complexes, the change of E from BH₃ (373 K) to Se (351 K) to S (342 K) leads to an overall ΔT_1/2(apparent) = 31 K, whereas the decreases are far more pronounced in the X = H family (BH₃ ∼624 > Se 429 > S 361 K). Changing the solvent used from CD₃CN to (CD₃)₂CO and CD₃NO₂, for [Fe^II(HL^Br-OH)(NCE)₂] with either E = BH₃ or S, revealed excellent, and very similar, positive linear correlations (R² = 0.99) of increasing solvent polarity index P' (from 5 to 7) with increasing apparent T_1/2 of the complex (E = BH₃ gave T_1/2 300 < 373 < 451 K , ΔT_1/2 = 151 K; E = S gave T_1/2 288 < 342 < 427 K, ΔT_1/2 = 147 K). Several other solvent parameters were also correlated with the apparent T_1/2 of these complexes (R² = 0.74-0.96). Excellent linear correlations (R² = 0.99) are also obtained with the coordination ability (a^TM) of the three NCE co-ligands with the apparent T_1/2 in both families of compounds, [Fe^II(HL^X-OH)(NCE)₂] where X = H or Br. The ¹⁵N NMR chemical shifts of the nitrogen atom in the three NCE co-ligands (direct measurement) show modest correlations (R² = 0.74 for L^H-OH family and 0.80 for L^Br-OH family) with the apparent T_1/2 values of the corresponding complexes.

Spin transition and symmetry-breaking in new mononuclear FeII tren-complexes with up to 38 K hysteresis around room temperature

New FeII complexes based on the well-known tripodand ligand type undergo abrupt hysteretic spin transition due to the symmetry-breaking in the room temperature region.

Single crystal-to-single crystal transformation - from two distinct to three distinct spin crossover centers in 2D coordination polymer [Fe(bbtr)3](CF3SO3)2

1,4-Di(1,2,3-triazol-1-yl)butane (bbtr) forms a two-dimensional (2D) coordination polymer (1) in a reaction with iron(II) triflate. In the crystal lattice there are two crystallographically unique iron(II) ions surrounded octahedrally by a 1,2,3-triazole ring coordinated through nitrogen atoms N3. Single crystal X-ray diffraction studies revealed that spin crossover for each crystallographically independent iron(II) ion proceeds at a different temperature (T_1/2(Fe1) = 201 K; T_1/2(Fe2) = 216 K), while the magnetic measurements showed that there is one step, complete thermally induced spin crossover (T_1/2 = 205 K). Complex 1 undergoes, with time, single crystal-to-single crystal transformation (SCSC) to the converted system (1c) from the R3̄ to the P6₃ space group, accompanied by significant changes in the lattice parameter c (a shortening of approximately one-third) and consequently unit cell volume. Structural transformation is associated with rebuilding of the polymeric layer as well as the anion network, which is reflected in the results of Mössbauer studies. In the polymorphic system (1c) there are three crystallographically independent iron(II) ions. The temperature dependence results for magnetic susceptibility indicated complete, one-step spin crossover very similar to that of 1; however, single-crystal X-ray diffraction studies of 1c revealed that spin crossover for each crystallographically independent iron(II) ion occurs in a different manner, revealing three elementary stages (T_1/2(Fe1) = 200 K; T_1/2(Fe2) = 212 K, T_1/2(Fe3) = 214 K).

Tunable Synchronicity of Molecular Valence Tautomerism with Macroscopic Solid-Liquid Transition by Molecular Lattice Engineering

The combination of a cobalt-dioxolene core that exhibits valence tautomerism (VT) with pyridine-3,5-dicarboxylic acid functionalized with chains bearing two, four, or six oxyethylene units led to new complexes ConEGEspy (n = 2, 4, and 6). These complexes commonly form violet crystals of the low-spin (ls)-[Co^III (nEGEspy)₂ (3,6-DTBSQ)(3,6-DTBCat)] (ls-[Co^III ], 3,6-DTBSQ = 3,6-di-tert-butyl semiquinonato, 3,6-DTBCat = 3,6-di-tert-butyl catecholato). Interestingly, violet crystals of Co2EGEspy in the ls-[Co^III ] transitioned into a green liquid, accompanied by an almost complete VT shift (94 %) to the high-spin (hs)-[Co^II (nEGEspy)₂ (3,6-DTBSQ)₂ ] (hs-[Co^II ]) upon melting. In contrast, violet crystals of Co4EGEspy and Co6EGEspy in the ls-[Co^III ] exhibited partial VT (33 %) and only a 9.3 % VT shift after melting, respectively. These data demonstrate the tunability of the synchronicity of the molecular VT and macroscopic solid-liquid transitions by optimizing the tethered chains, thus establishing a new strategy for coupling bistable molecules with the macroscopic world.

Crystal structure of (N 1,N 3-bis-{[1-(4-meth-oxy-benz-yl)-1H-1,2,3-triazol-4-yl]methyl-idene}-2,2-di-meth-yl-propane-1,3-di-amine)-bis-(thio-cyanato)-iron(II)

The unit cell of the title compound, [FeII(NCS)2(C29H32N8O2)], consists of eight charge-neutral complex mol-ecules. In the complex mol-ecule, the tetra-dentate ligand N 1,N 3-bis-{[1-(4-meth-oxy-benz-yl)-1H-1,2,3-triazol-4-yl]methyl-ene}-2,2-di-methyl-propane-1,3-di-amine coordinates to the FeII ion through the N atoms of the 1,2,3-triazole and aldimine groups. Two thio-cyanate anions, coordinated through their N atoms, complete the coordination sphere of the central Fe ion. In the crystal, neighbouring mol-ecules are linked through weak C⋯C, C⋯N and C⋯S inter-actions into a one-dimensional chain running parallel to [010]. The inter-molecular contacts were qu-anti-fied using Hirshfeld surface analysis and two-dimensional fingerprint plots, revealing the relative contributions of the contacts to the crystal packing to be H⋯H (37.5%), H⋯C/C⋯H (24.7%), H⋯S/S⋯H (15.7%) and H⋯N/N⋯H (11.7%). The average Fe-N bond distance is 2.167 Å, indicating the high-spin state of the FeII ion, which does not change upon cooling, as demonstrated by low-temperature magnetic susceptibility measurements.

Crystal structure of {N 1,N 3-bis-[(1-tert-butyl-1H-1,2,3-triazol-4-yl)methyl-idene]-2,2-di-methyl-propane-1,3-di-amine}-bis-(thio-cyanato)-iron(II)

The unit cell of the title compound, [FeII(NCS)2(C19H32N8)], consists of two charge-neutral complex mol-ecules. In the complex mol-ecule, the tetra-dentate ligand N 1 ,N 3-bis-[(1-tert-butyl-1H-1,2,3-triazol-4-yl)methyl-ene]-2,2-di-methyl-propane-1,3-di-amine coordinates to the FeII ion through the N atoms of the 1,2,3-triazole and aldimine groups. Two thio-cyanate anions, also coordinated through their N atoms, complete the coordination sphere of the central Fe ion. In the crystal, neighbouring mol-ecules are linked through weak C-H⋯C/S/N inter-actions into a three-dimensional network. The inter-mol-ecular contacts were qu-anti-fied using Hirshfeld surface analysis and two-dimensional fingerprint plots, revealing the relative contributions of the contacts to the crystal packing to be H⋯H 50.8%, H⋯C/C⋯H 14.3%, H⋯S/S⋯H 20.5% and H⋯N/N⋯H 12.1%. The average Fe-N bond distance is 2.170 Å, indicating the high-spin state of the FeII ion, which does not change upon cooling, as demonstrated by low-temperature magnetic susceptibility measurements. DFT calculations of energy frameworks at the B3LYP/6-31 G(d,p) theory level were performed to account for the inter-actions involved in the crystal structure.

The flexibility of long chain substituents influences spin-crossover in isomorphous lipid bilayer crystals

These two compounds are isomorphous but show different spin-state behaviour on cooling. This may be influenced by the packing of alkyl vs. alkynyl chains in the solid state.

Crystal structure of {N 1,N 3-bis-[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl-idene]-2,2-di-methyl-propane-1,3-di-amine}bis-(thio-cyanato-κN)iron(II)

The unit cell of the title compound, [FeII(NCS)2(C25H28N8)], consists of two charge-neutral complex mol-ecules related by an inversion centre. In the complex mol-ecule, the tetra-dentate ligand N 1,N 3-bis-[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl-ene]-2,2-di-methyl-propane-1,3-di-amine coordinates to the FeII ion through the N atoms of the 1,2,3-triazole moieties and aldimine groups. Two thio-cyanate anions, coordinating through their N atoms, complete the coordination sphere of the central ion. In the crystal, neighbouring mol-ecules are linked through weak C-H⋯π, C-H⋯S and C-H⋯N inter-actions into a two-dimensional network extending parallel to (011). The inter-molecular contacts were qu-anti-fied using Hirshfeld surface analysis and two-dimensional fingerprint plots, revealing the relative contributions of the contacts to the crystal packing to be H⋯H (35.2%), H⋯C/C⋯H (26.4%), H⋯S/S⋯H (19.3%) and H⋯N/N⋯H (13.9%).

Crystal structure of the mixed methanol and ethanol solvate of bis-{3,4,5-trimeth-oxy-N'-[1-(pyridin-2-yl)ethyl-idene]benzohydrazidato}zinc(II)

The unit cell of the title compound, [Zn(C17H18N3O4)2]·CH4O·C2H6O, contains two complex mol-ecules related by an inversion centre, plus one methanol and one ethanol solvent molecule per complex molecule. In each complex, two deprotonated pyridine aroylhydrazone ligands {3,4,5-trimeth-oxy-N'-[1-(pyridin-2-yl)ethyl-idene]benzohydrazide} coordinate to the ZnII ion through the N atoms of the pyridine group and the ketamine, and, additionally, through the O atom of the enolate group. In the crystal, dimers are formed by π-π inter-actions between the planar ligand moieties, which are further connected by C⋯O and C⋯C inter-actions. The inter-molecular inter-actions were investigated using Hirshfeld surface analysis and two-dimensional fingerprint plots, revealing that the most important contributions for the crystal packing are from H⋯H (44.8%), H⋯C/C⋯H (22.2%), H⋯O/O⋯H (18.7%) and C⋯C (3.9%) inter-actions.

Hydrophobic tail length in spin crossover active iron(ii) complexes predictably tunes T½ in solution and enables surface immobilisation

Linear correlation of the hydrophobic alkyl tail length R employed in [Fe^II(LH-OR)(NCBH₃)₂] with the spin crossover switching temperature is a very convenient method of predictably tuning the iron(ii) spin state.

Crystal structure and magnetic properties of bis-[butyl-tris-(1H-pyrazol-1-yl)borato]iron(II)

The asymmetric unit of the title compound, [Fe(C13H18BN6)2], contains two half independent complex mol-ecules. In each complex, the FeII atom is located on an inversion center and is surrounded by two scorpionate ligand butyl-tris-(1H-pyrazol-1-yl)borate mol-ecules that coordinate to the iron(II) ion through the N atoms of the pyrazole groups. The two independent complex mol-ecules differ essentially in the conformation of the butyl substituents. In the crystal, the complex mol-ecules are linked by a series of C-H⋯π inter-actions, which generate a supra-molecular three-dimensional structure. At 120 K, the average Fe-N bond distance is 1.969 Å, indicating the low-spin state of the iron(II) atom, which does not change upon heating, as demonstrated by high-temperature magnetic susceptibility measurements.

Discrimination between two memory channels by molecular alloying in a doubly bistable spin crossover material

A multistable spin crossover (SCO) molecular alloy system [Fe1-x M x (nBu-im)3(tren)](P1-y As y F6)2 (M = ZnII, NiII; (nBu-im)3(tren) = tris(n-butyl-imidazol(2-ethylamino))amine) has been synthesized and characterized. By controlling the composition of this isomorphous series, two cooperative thermally induced SCO events featuring distinct critical temperatures (T c) and hysteresis widths (ΔT c, memory) can be selected at will. The pristine derivative 100As (x = 0, y = 1) displays a strong cooperative two-step SCO and two reversible structural phase transitions (PTs). The low temperature PTLT and the SCO occur synchronously involving conformational changes of the ligand's n-butyl arms and two different arrangements of the AsF6 - anions [T1c = 174 K (ΔT1c = 17 K), T2c = 191 K (ΔT2c = 23 K) (scan rate 2 K min-1)]. The high-temperature PTHT takes place in the high-spin state domain and essentially involves rearrangement of the AsF6 - anions [TPTc = 275 K (ΔTPTc = 16 K)]. This behavior strongly contrasts with that of the homologous 100P [x = 0, y = 0] derivative where two separate cooperative one-step SCO can be selected by controlling the kinetics of the coupled PTLT at ambient pressure: (i) one at low temperatures, T c = 122 K (ΔT c = 9 K), for temperature scan rates (>1 K min-1) (memory channel A) where the structural modifications associated with PTLS are inhibited; (ii) the other centered at T c = 155 K (ΔT c = 41 K) for slower temperature scan rates ≤0.1 K min-1 (memory channel B). These two SCO regimes of the 100P derivative transform reversibly into the two-step SCO of 100As upon application of hydrostatic pressure (ca. 0.1 GPa) denoting the subtle effect of internal chemical pressure on the SCO behavior. Precise control of AsF6 - ↔ PF6 - substitution, and hence of the PTLT kinetics, selectively selects the memory channel B of 100P when x = 0 and y ≈ 0.7. Meanwhile, substitution of FeII with ZnII or NiII [x ≈ 0.2, y = 0] favors the low temperature memory channel A at any scan rate. This intriguing interplay between PT, SCO and isomorphous substitution was monitored by single crystal and powder X-ray diffractometries, and magnetic and calorimetric measurements.

Substituents drive ligand rearrangements, giving dinuclear rather than mononuclear complexes, and tune CoII/III redox potential

Three new tetradentate imine ligands, HLHBr, HLClH and HLBrH (HLR1R2) were synthesised by 2 : 1 condensation of the appropriately n-halo substituted pyridine-2-carboxaldehyde (5-bromo-4a, 6-bromo-4b or 6-chloro-4c) with 1,3-diaminopropan-2-ol (5). Reactions of each of these three ligands with one equivalent of cobalt(ii) tetrafluoroborate resulted in the formation of three N4O2 coordinated cobalt(ii) complexes: the anticipated mononuclear complex [CoII(HLHBr)(MeOH)2](BF4)2 (1), and two unexpected dinuclear complexes, [CoII2(LBrH-BF2OMe)]2(BF4)2 (2) and [CoII2(LClH-BF2OMe)]2(BF4)2 (3). Dinuclear 2 and 3 result from complexation of cobalt(ii) to the ligands derived from the sterically demanding 6-halo substituted pyridine-2-carboxaldehydes (4b and 4c) undergoing rearrangement, reacting with MeOH and a BF4 anion, resulting in a pair of borate ester bridges between the two cobalt(ii) centres. A similar type of rearrangement is proposed for the PF6 analogues. Cyclic voltammetry in acetonitrile reveals that cobalt(ii) complexes 1-3 undergo a quasi-reversible oxidation: Em = 0.57, 0.38 and 0.29 V vs. 0.01 AgNO3/Ag, respectively. The observed Em value is tuned by the ligand, with the 6-chloro-substituent leading to the lowest Em value being observed for the corresponding cobalt complex, 3, rather than for either of the complexes of the n-bromo-substituted ligands (n = 6 or 5), 2 and 1.