Compressed and Expanded Lattices - Barriers to Spin-State Switching in Mn3+ Complexes.

We report the structural and magnetic properties of two new Mn3+ complex cations in the spin crossover (SCO) [Mn(R-sal2323)]+ series, in lattices with seven different counterions in each case. We investigate the effect on the Mn3+ spin state of appending electron-withdrawing and electron-donating groups on the phenolate donors of the ligand. This was achieved by substitution of the ortho and para positions on the phenolate donors with nitro and methoxy substituents in both possible geometric isomeric forms. Using this design paradigm, the [MnL1]+ (a) and [MnL2]+ (b) complex cations were prepared by complexation of Mn3+ to the hexadentate Schiff base ligands with 3-nitro-5-methoxy-phenolate or 3-methoxy-5-nitro-phenolate substituents, respectively. A clear trend emerges with adoption of the spin triplet form in complexes 1a-7a, with the 3-nitro-5-methoxy-phenolate donors, and spin triplet, spin quintet and thermal SCO in complexes 1b-7b with the 3-methoxy-5-nitro-phenolate ligand isomer. The outcomes are discussed in terms of geometric and steric factors in the 14 new compounds and by a wider analysis of electronic choices of Mn3+ with related ligands by comparison of bond length and angular distortion data of previously reported analogues in the [Mn(R-sal2323)]+ family. The structural and magnetic data published to date suggest a barrier to switching may exist for high spin forms of Mn3+ in those complexes with the longest bond lengths and highest distortion parameters. A barrier to switching from low spin to high spin is less clear but may operate in the seven [Mn(3-NO2-5-OMe-sal2323)]+ complexes 1a-7a reported here which were all low spin in the solid state at room temperature.

Spontaneous Chiral Resolution of a MnIII Spin-Crossover Complex with High Temperature 80†K Hysteresis.

Non-centrosymmetric spin-switchable systems are of interest for their prospective applications as magnetically active non-linear optical materials and in multiferroic devices. Chiral resolution of simple spin-crossover chelate complexes into the Δ and Λ forms offers a facile route to homochiral magnetic switches, which could be easily enantiomerically enriched. Here, we report the spontaneous resolution of a new hysteretic spin-crossover complex, [MnIII (sal2 323)]SCN ⋅ EtOH (1), into Δ and Λ forms, without the use of chiral reagents, where sal2 323 is a Schiff base resulting from condensation of 1,2-bis(3-aminopropylamino)ethane with 2-hydroxybenzaldehyde. The enantiopurity of the Δ and Λ isomers was confirmed by single crystal X-ray diffraction and circular dichroism. Quantum chemistry calculations were used to investigate the electronic structure. The opening of a wide 80 K thermal hysteresis window at high temperature highlights the potential for good magneto-optical function at ambient temperature for materials of this type.

Proton-Induced Spin State Switching in an FeIII Complex.

Reversible proton-induced spin state switching of an FeIII complex in solution is observed at room temperature. A reversible magnetic response was detected in the complex, [FeIII (sal2 323)]ClO4 (1), using Evans' method 1 H NMR spectroscopy which indicated cumulative switching from low-spin to high-spin upon addition of one and two equivalents of acid. Infrared spectroscopy suggests a coordination-induced spin state switching (CISSS) effect, whereby protonation displaces the metal-phenoxo donors. The analogous complex, [FeIII (4-NEt2 -sal2 323)]ClO4 (2), with a diethylamino group on the ligand, was used to combine the magnetic change with a colorimetric response. Comparison of the protonation responses of 1 and 2 reveals that the magnetic switching is caused by perturbation of the immediate coordination sphere of the complex. These complexes constitute a new class of analyte sensor which operate by magneto-modulation, and in the case of 2, also yield a colorimetric response.

Crystallographic Detection of the Spin State in FeIII Complexes.

We report a single example of thermal spin crossover in a series of FeIII complexes, [FeIII(R-sal2323)]+, which typically stabilize the low-spin (S = 1/2) state. Single-crystal X-ray diffraction analysis of 53 such complexes with varying "R" groups, charge-balancing anions, and/or lattice solvation confirms bond lengths in line with an S = 1/2 ground state, with only the [FeIII(4-OMe-sal2323)]NO3 complex (1a) exhibiting longer bond lengths associated with a percentage of the spin sextet form at room temperature. Structural distortion parameters are investigated for the series. A magnetic susceptibility measurement of 1a reveals a gradual, incomplete transition, with T 1/2 = 265 K in the solid state, while Evans method NMR reveals that the sample persists in the low-spin form in solution at room temperature. Computational analysis of the spin state preferences for the cations [FeIII(4-OMe-sal2323)]+ and [FeIII(sal2323)]+ confirmed the energetic preference for the spin doublet form in both, and the thermal spin crossover in complex 1a is therefore attributed to perturbation of the crystal packing on warming.

Homochiral Mn3+ Spin-Crossover Complexes: A Structural and Spectroscopic Study.

Structural, magnetic, and spectroscopic data on a Mn3+ spin-crossover complex with Schiff base ligand 4-OMe-Sal2323, isolated in crystal lattices with five different counteranions, are reported. Complexes of [Mn(4-OMe-Sal2323)]X where X = ClO4- (1), BF4- (2), NO3- (3), Br- (4), and I- (5) crystallize isotypically in the chiral orthorhombic space group P21212 with a range of spin state preferences for the [Mn(4-OMe-Sal2323)]+ complex cation over the temperature range 5-300 K. Complexes 1 and 2 are high-spin, complex 4 undergoes a gradual and complete thermal spin crossover, while complexes 3 and 5 show stepped crossovers with different ratios of spin triplet and quintet forms in the intermediate temperature range. High-field electron paramagnetic resonance was used to measure the zero-field splitting parameters associated with the spin triplet and quintet states at temperatures below 10 K for complexes 4 and 2 with respective values: DS=1 = +23.38(1) cm-1, ES=1 = +2.79(1) cm-1, and DS=2 = +6.9(3) cm-1, with a distribution of E parameters for the S = 2 state. Solid-state circular dichroism (CD) spectra on high-spin complex 1 at room temperature reveal a 2:1 ratio of enantiomers in the chiral conglomerate, and solution CD measurements on the same sample in methanol show that it is stable toward racemization. Solid-state UV-vis absorption spectra on high-spin complex 1 and mixed S = 1/S = 2 sample 5 reveal different intensities at higher energies, in line with the different electronic composition. The statistical prevalence of homochiral crystallization of [Mn(4-OMe-Sal2323)]+ in five lattices with different achiral counterions suggests that the chirality may be directed by the 4-OMe-Sal2323 ligand.

Thermal and Magnetic Field Switching in a Two-Step Hysteretic MnIII Spin Crossover Compound Coupled to Symmetry Breakings.

A MnIII spin crossover complex with atypical two-step hysteretic thermal switching at 74 K and 84 K shows rich structural-magnetic interplay and magnetic-field-induced spin state switching below 14 T with an onset below 5 T. The spin states, structures, and the nature of the phase transitions are elucidated via X-ray and magnetization measurements. An unusual intermediate phase containing four individual sites, where 1 / 4 are in a pure low spin state, is observed. The splitting of equivalent sites in the high temperature phase into four inequivalent sites is due to a structural reorganization involving a primary and a secondary symmetry-breaking order parameter that induces a crystal system change from orthorhombic→monoclinic and a cell doubling. Further cooling leads to a reconstructive phase transition and a monoclinic low-temperature phase with two inequivalent low-spin sites. The coupling between the order parameters is identified in the framework of Landau theory.

Domain Wall Dynamics in a Ferroelastic Spin Crossover Complex with Giant Magnetoelectric Coupling.

Pinned and mobile ferroelastic domain walls are detected in response to mechanical stress in a Mn3+ complex with two-step thermal switching between the spin triplet and spin quintet forms. Single-crystal X-ray diffraction and resonant ultrasound spectroscopy on [MnIII(3,5-diCl-sal2(323))]BPh4 reveal three distinct symmetry-breaking phase transitions in the polar space group series Cc → Pc → P1 → P1(1/2). The transition mechanisms involve coupling between structural and spin state order parameters, and the three transitions are Landau tricritical, first order, and first order, respectively. The two first-order phase transitions also show changes in magnetic properties and spin state ordering in the Jahn-Teller-active Mn3+ complex. On the basis of the change in symmetry from that of the parent structure, Cc, the triclinic phases are also ferroelastic, which has been confirmed by resonant ultrasound spectroscopy. Measurements of magnetoelectric coupling revealed significant changes in electric polarization at both the Pc → P1 and P1 → P1(1/2) transitions, with opposite signs. All these phases are polar, while P1 is also chiral. Remanent electric polarization was detected when applying a pulsed magnetic field of 60 T in the P1→ P1(1/2) region of bistability at 90 K. Thus, we showcase here a rare example of multifunctionality in a spin crossover material where the strain and polarization tensors and structural and spin state order parameters are strongly coupled.

Auxiliary alkyl chain modulated spin crossover behaviour of [Fe(H2Bpz2)2(Cn-bipy)] complexes.

Three new alkyl chain substituted complexes [Fe(H2Bpz2)2(Cn-bipy)] (pz = pyrazolyl, Cn-bipy = bipyridine alkyl chain diester, n = 3 (3), 4 (4) and 5 (5)) show versatile spin state switching behaviour with different "tail" lengths as revealed by structural and magnetic analyses. The most striking phenomenon is observed for 5 which undergoes an abrupt spin transition accompanied by thermal hysteresis of ca. 10 K, which is attributed to crystal packing effects derived from the competition between π⋯π and C-H⋯O interactions. Interestingly, each of the complexes exhibits similar gradual and complete spin crossover in methanol solution with a transition temperature around 249 K, as deduced from temperature-dependent UV-vis spectroscopy. This highlights the differences between the solid state (ligand field; crystal packing) and solution (ligand field; solvation) effects on spin crossover. This work demonstrates that the length of the complex's alkyl chain substituents on the complex can have a large impact on the transition temperature and profile of solid state spin crossover, offering a potential path to the fabrication of soft matter spin crossover materials.

Giant Magnetoelectric Coupling and Magnetic-Field-Induced Permanent Switching in a Spin Crossover Mn(III) Complex.

We investigate giant magnetoelectric coupling at a Mn3+ spin crossover in [MnIIIL]BPh4 (L = (3,5-diBr-sal)2323) with a field-induced permanent switching of the structural, electric, and magnetic properties. An applied magnetic field induces a first-order phase transition from a high spin/low spin (HS-LS) ordered phase to a HS-only phase at 87.5 K that remains after the field is removed. We observe this unusual effect for DC magnetic fields as low as 8.7 T. The spin-state switching driven by the magnetic field in the bistable molecular material is accompanied by a change in electric polarization amplitude and direction due to a symmetry-breaking phase transition between polar space groups. The magnetoelectric coupling occurs due to a γη2 coupling between the order parameter γ related to the spin-state bistability and the symmetry-breaking order parameter η responsible for the change of symmetry between polar structural phases. We also observe conductivity occurring during the spin crossover and evaluate the possibility that it results from conducting phase boundaries. We perform ab initio calculations to understand the origin of the electric polarization change as well as the conductivity during the spin crossover. Thus, we demonstrate a giant magnetoelectric effect with a field-induced electric polarization change that is 1/10 of the record for any material.

Modulation of Mn3+ Spin State by Guest Molecule Inclusion.

Spin state preferences for a cationic Mn3+ chelate complex in four different crystal lattices are investigated by crystallography and SQUID magnetometry. The [MnL1]+ complex cation was prepared by complexation of Mn3+ to the Schiff base chelate formed from condensation of 4-methoxysalicylaldehyde and 1,2-bis(3-aminopropylamino)ethane. The cation was crystallized separately with three polyatomic counterions and in one case was found to cocrystallize with a percentage of unreacted 4-methoxysalicylaldehyde starting material. The spin state preferences of the four resultant complexes [MnL1]CF3SO3·xH2O, (1), [MnL1]PF6·xH2O, (2), [MnL1]PF6·xsal·xH2O, (2b), and [MnL1]BPh4, (3), were dependent on their ability to form strong intermolecular interactions. Complexes (1) and (2), which formed hydrogen bonds between [MnL1]+, lattice water and in one case also with counterion, showed an incomplete thermal spin crossover over the temperature range 5-300 K. In contrast, complex (3) with the BPh4-, counterion and no lattice water, was locked into the high spin state over the same temperature range, as was complex (2b), where inclusion of the 4-methoxysalicylaldehyde guest blocked the H-bonding interaction.

Stress-Induced Domain Wall Motion in a Ferroelastic Mn3+ Spin Crossover Complex.

Domain wall motion is detected for the first time during the transition to a ferroelastic and spin state ordered phase of a spin crossover complex. Single-crystal X-ray diffraction and resonant ultrasound spectroscopy (RUS) revealed two distinct symmetry-breaking phase transitions in the mononuclear Mn3+ compound [Mn(3,5-diBr-sal2 (323))]BPh4 , 1. The first at 250 K, involves the space group change Cc→Pc and is thermodynamically continuous, while the second, Pc→P1 at 85 K, is discontinuous and related to spin crossover and spin state ordering. Stress-induced domain wall mobility was interpreted on the basis of a steep increase in acoustic loss immediately below the the Pc-P1 transition.

Modulation of Jahn-Teller distortion and electromechanical response in a Mn3+ spin crossover complex.

Structural, magnetic and electromechanical changes resulting from spin crossover between the spin quintet and spin triplet forms of a mononuclear Mn3+ complex embedded in six lattices with different charge balancing counterions are reported. Isostructural ClO4 - and BF4 - salts (1) and (2) each have two unique Mn3+ sites which follow different thermal evolution pathways resulting in a crossover from the spin quintet form at room temperature to a 1:1 spin triplet:quintet ratio below 150 K. The PF6 - (3) and NO3 - (4) salts which each have one unique Mn3+ site show a complete conversion from spin quintet to spin triplet over the same temperature range. A complete two step spin crossover is observed in the CF3SO3 - lattice (5) with a 1:1 ratio of spin quintet and spin triplet forms at intermediate temperature, while the BPh4 - lattice (6) stabilizes the spin triplet form over most of the temperature range with gradual and incomplete spin state switching above 250 K. An electromechanical piezoresponse was detected in NO3 - complex 4 despite crystallization in a centrosymmetric space group. The role of deformations associated with stress-induced spin triplet-spin quintet switching in breaking the local symmetry are discussed and computational analysis is used to estimate the energy gap between the two spin states.

Comparison of two field-induced ErIII single ion magnets.

We present the synthesis, magnetic and photophysical properties of four mononuclear LnIII complexes in two isostructural lattices containing GdIII and ErIII. A heptadentate Schiff base ligand and acetate versus trifluoroacetate were used to synthesise complexes 1-4, among which the two ErIII complexes 2 and 4 exhibit field-induced SIM behaviour with almost similar Ueff values (31.6 K for 2 and 32.7 K for 4). Ab initio calculations show the structure of the low-lying energy states and highlight that there is already significant tunnelling in the ground doublet state, but the application of a weak magnetic field of 0.05 T is sufficient for ac magnetic measurements to suppress tunnelling in the ground state. The calculated main magnetic axes (gZ) of the ground Kramers doublets show small differences between the two ErIII compounds 2 and 4 due to their different ligand fields.

Spin state solvomorphism in a series of rare S = 1 manganese(iii) complexes.

Structural, magnetic and spectroscopic data of four complex salts, [Mn(napsal2323)]NTf2, 1,[Mn(napsal2323)]ClO4, 2, [Mn(napsal2323)]BF4, 3 and [Mn(napsal2323)]NO3, 4, of the [Mn(napsal2323)]+ complex cation indicate that the Mn3+ ion is stabilized in the rare S = 1 spin triplet form in this ligand sphere. Zero-field splitting values of D = +19.6 cm-1 and |E| = 2.02 cm-1 for complex 1 were obtained by High Field Electron Paramagnetic Resonance (HFEPR) measurements conducted over a range of frequencies. Structural and magnetic data also indicate that co-crystallization of complexes 2 and 3 with 0.5 equivalents of ethanol yields the high spin S = 2 forms of the perchlorate and tetrafluoroborate solvates [Mn(napsal2323)]ClO4·0.5(C2H5OH), 2·0.5EtOH and [Mn(napsal2323)]BF4·0.5(C2H5OH), 3·0.5EtOH.

Directing self-assembly in solution towards improved cooperativity in Fe(iii) complexes with amphiphilic tridentate ligands.

An amphiphilic iron(iii) complex with a tridentate Schiff-base ligand was prepared by condensation of a hexadecyloxy functionalised salycylaldehyde with a diamine followed by complexation with FeCl2 and anion methathesis with NaClO4. The complex shows spin crossover both in the solid state and solution. However in solution self-assembly and consequently aggregation of individual molecules form concentration dependent particles with sizes of 300 nm for higher concentrations, or 5 nm for lower concentrations. Aggregate formation was confirmed by NANO-flex 180° DLS Size, scan-rate dependent cyclic voltammetry and scanning electron microscopy. Molecular simulations were used to investigate the self-assembly of the complex in solution, including the role of residual water molecules. The simulations showed the self-assembly of reverse micelle-like structures when a small water cluster is inserted in solution, whereas no large aggregates formed in dehydrated environments. The perchlorate anions were found near the metal centres, stabilizing the aggregates around the water pool. Simulations of pre-assembled structures further showed the lack of stability of large aggregates in the absence of water. The larger aggregates promoted efficient communication between the iron(iii) centres and the compound displayed spin crossover in solution at around 220 K with a 10 K hysteresis window, as measured by NMR and SQUID magnetometry.

Spin Crossover Behavior in a Homologous Series of Iron(II) Complexes Based on Functionalized Bipyridyl Ligands.

A series of bulky substituted bipyridine-related iron(II) complexes [Fe(H2Bpz2)2(L)] (pz = pyrazolyl) were prepared, where L = 5,5'-dimethyl-2,2'-bipyridine (bipy-CH3, 1), L = dimethyl-2,2'-bipyridyl-5,5'-dicarboxylate (MeObpydc, 2), L = diethyl-2,2'-bipyridyl-5,5'-dicarboxylate (EtObpydc, 3), or L = diisopropyl-2,2'-bipyridine-5,5'-dicarboxylate ( i-PrObpydc, 4). The crystal structures of five new iron(II) complexes were determined by X-ray diffraction: those of 1, 3, and 4 and two modifications of 3 (3B) and 4 (4B). Complexes 1 and 3B display incomplete spin crossover (SCO) behavior because of a freezing-in effect, whereas 3 and 4B undergo gradual and incomplete SCO behaviors. Complexes 2 and 4 show a completely gradual and steep SCO, respectively. Such different SCO behaviors can be attributed to an electronic substituent effect in the bipyridyl ligand conformation and a crystal packing effect. Importantly, the electronic substituent effect of the isopropyl acetate group and C-H···O supramolecular interactions in 4 contribute to a highly cooperative behavior, which leads to an abrupt thermally induced spin transition.

A room temperature spin crossover ionic liquid.

Two new paramagnetic ionic liquids (ILs) comprising a mononuclear iron(III) or manganese(III) complex cation, charge balanced by a dicyanamide anion are reported which show a range of spin states. Both are liquids at room temperature and the Fe(III) based IL exhibits a spin crossover close to 300 K. The spin crossover profile is independent of the solvation, and is both air and moisture stable.

Correction: Electronic vs. structural ordering in a manganese(III) spin crossover complex.

Correction for 'Electronic vs. structural ordering in a manganese(III) spin crossover complex' by Anthony J. Fitzpatrick et al., Chem. Commun., 2015, DOI: 10.1039/c5cc05129k.

Electronic vs. structural ordering in a manganese(III) spin crossover complex.

A symmetry breaking spin transition in a Mn(III) complex is reported with three structural phases, a high symmetry high temperature S = 2 phase, an intermediate S = 1/S = 2 ordered phase and an aperiodic low temperature phase with S = 1 cations. The aperiodicity is interpreted as resulting from long-range ordering of the NTf2(-) anions.

Characterization and reactivity of a terminal nickel(III)-oxygen adduct.

High-valent terminal metal-oxygen adducts are hypothesized to be the potent oxidizing reactants in late transition metal oxidation catalysis. In particular, examples of high-valent terminal nickel-oxygen adducts are scarce, meaning there is a dearth in the understanding of such oxidants. A monoanionic Ni(II)-bicarbonate complex has been found to react in a 1:1 ratio with the one-electron oxidant tris(4-bromophenyl)ammoniumyl hexachloroantimonate, yielding a thermally unstable intermediate in high yield (ca. 95%). Electronic absorption, electronic paramagnetic resonance, and X-ray absorption spectroscopies and density functional theory calculations confirm its description as a low-spin (S = 1/2), square planar Ni(III)-oxygen adduct. This rare example of a high-valent terminal nickel-oxygen complex performs oxidations of organic substrates, including 2,6-di-tert-butylphenol and triphenylphosphine, which are indicative of hydrogen atom abstraction and oxygen atom transfer reactivity, respectively.