Rotational order-disorder and spin crossover behaviour in a neutral iron(II) complex based on asymmetrically substituted large planar ionogenic ligand

Octahedrally coordinated spin crossover (SCO) FeII complexes represent an important class of switchable molecular materials. This study presents the synthesis and characterisation of a novel complex, [FeII(ppt-2Fph)2]0·2MeOH, where ppt-2Fph is a new asymmetric ionogenic tridentate planar ligand 2-(5-(2-fluorophenyl)-4H-1,2,4-triazol-3-yl)-6-(1H-pyrazol-1-yl)pyridine. The complex exhibits a hysteretic thermally induced SCO transition at 285 K on cooling and at 293 K on heating, as well as light induced excited spin state trapping (LIESST) at lower temperatures with a relaxation T(LIESST) temperature of 73 K. Single crystal analysis in both spin states shows that the compound undergoes an unusual partial (25%) reversible order-disorder of the asymmetrically substituted phenyl group coupled to the thermal SCO. The highly cooperative SCO transition, analysed by structural energy framework analysis at the B3LYP/6-31G(d,p) theory level, revealed the co-existence of stabilising and destabilising energy variations in the lattice. The observed antagonism of intermolecular interactions and synchronous rotational disorder, which contributes to the overall entropy change, is suggested to be at the origin of the cooperative SCO transition.

Nature of cyanoargentate bridges defining spin crossover in new 2D Hofmann clathrate analogues

Chemical composition is leading among the numerous factors that determine the spin transition properties of coordination compounds. Classic dicyanometallic bridges {M(CN)2}- are commonly used to build Hofmann-like spin-crossover frameworks, but some extended bridges are also synthetically available. In this paper, we describe a successful synthesis of two very similar spin-crossover frameworks that differ in the cyanometallic bridges involved, namely [Fe(etpz)2{Ag(CN)2}2] (1) and {Fe(etpz)2[Ag2(CN)3][Ag(CN)2]} (2) (where etpz = 2-ethylpyrazine). Magnetic and Mössbauer studies demonstrated the occurrence of abrupt one-step high-spin (HS) ↔ low-spin (LS) transitions for both complexes. The spin transition temperatures are T1/2 ↓ = 233 K and T1/2 ↑ = 243 K for 1 and T1/2 ↓ = 188 K and T1/2 ↑ = 191 K for 2 with thermal hysteresis loops of 10 K for 1 and 3 K for 2. The bridging mononuclear [Ag(CN)2]- units and FeII cations assemble to form infinite 2D layers in the structure of 1. Interestingly, compound 2 forms 2D layers of FeII cations bridged by both binuclear [Ag2(CN)3]- and mononuclear [Ag(CN)2]- units. The structures of 1 and 2 comprise different types of intermolecular interactions including Ag⋯Ag and Ag⋯Netpz, which induce the creation of supramolecular 3D frameworks. The synergy between metallophilic interactions and the spin transition is also confirmed by the variation of Ag⋯Ag distances during spin crossover. The characterization of such analogues allowed us to analyze in detail the effect of the cyanometallic bridge on the structure of new frameworks and on the bistability in Hofmann-like complexes.

Crystal structure of a water oxidation catalyst solvate with composition (NH4)2[FeIV(L-6H)]·3CH3COOH (L = clathrochelate ligand)

The synthetic availability of mol-ecular water oxidation catalysts containing high-valent ions of 3d metals in the active site is a prerequisite to enabling photo- and electrochemical water splitting on a large scale. Herein, the synthesis and crystal structure of di-ammonium {μ-1,3,4,7,8,10,12,13,16,17,19,22-dodeca-aza-tetra-cyclo-[8.8.4.13,17.18,12]tetra-cosane-5,6,14,15,20,21-hexa-onato}ferrate(IV) acetic acid tris-olvate, (NH4)2[FeIV(C12H12N12O6)]·3CH3COOH or (NH4)2[FeIV(L-6H)]·3CH3COOH is reported. The FeIV ion is encapsulated by the macropolycyclic ligand, which can be described as a dodeca-aza-quadricyclic cage with two capping tri-aza-cyclo-hexane fragments making three five- and six six-membered alternating chelate rings with the central FeIV ion. The local coord-ination environment of FeIV is formed by six deprotonated hydrazide nitro-gen atoms, which stabilize the unusual oxidation state. The FeIV ion lies on a twofold rotation axis (multiplicity 4, Wyckoff letter e) of the space group C2/c. Its coordination geometry is inter-mediate between a trigonal prism (distortion angle φ = 0°) and an anti-prism (φ = 60°) with φ = 31.1°. The Fe-N bond lengths lie in the range 1.9376 (13)-1.9617 (13) Å, as expected for tetra-valent iron. Structure analysis revealed that three acetic acid mol-ecules additionally co-crystallize per one iron(IV) complex, and one of them is positionally disordered over four positions. In the crystal structure, the ammonium cations, complex dianions and acetic acid mol-ecules are inter-connected by an intricate system of hydrogen bonds, mainly via the oxamide oxygen atoms acting as acceptors.

Crystal structure of polymeric bis-(3-amino-1H-pyrazole)-cadmium dibromide

The reaction of cadmium bromide tetra-hydrate with 3-amino-pyrazole (3-apz) in ethano-lic solution leads to tautomerization of the ligand and the formation of crystals of the title compound, catena-poly[[di-bromido-cadmium(II)]-bis-(μ-3-amino-1H-pyrazole)-κ2 N 3:N 2;κ2 N 2:N 3], [CdBr2(C3H5N3)2]n or [CdBr2(3-apz)2]n. Its asymmetric unit consists of a half of a Cd2+ cation, a bromide anion and a 3-apz mol-ecule. The Cd2+ cations are coordinated by two bromide anions and two 3-apz ligands, generating trans-CdN4Br2 octa-hedra, which are linked into chains by pairs of the bridging ligands. In the crystal, the ligand mol-ecules and bromide anions of neighboring chains are linked through inter-chain hydrogen bonds into a two-dimensional network. The inter-molecular contacts were qu-anti-fied using Hirshfeld surface analysis and two-dimensional fingerprint plots, revealing the relative qu-anti-tative contributions of the weak inter-molecular contacts.

Crystal structure and Hirshfeld surface analysis of poly[[tetra-aqua-(μ-1,3,4,7,8,10,12,13,16,17,19,22-dodeca-aza-tetra-cyclo-[8.8.4.13,17.18,12]tetra-cosane-5,6,14,15,20,21-hexaonato)iron(IV)dilithium] tetra-hydrate]

The title compound, [FeLi2(C12H12N12O6)(H2O)4]·4H2O, consists of iron complex anions, lithium cations and water mol-ecules. The complex anion shows a clathrochelate topology. The coordination geometry of the FeIV centre is inter-mediate between a trigonal prism and a trigonal anti-prism. In the crystal, the complex anions are connected through two Li cations into dimers, which are connected by Li-O bonds, forming infinite chains along the b-axis direction.

Crystal structure of bis-{3-(3,4-di-meth-oxy-phen-yl)-5-[6-(pyrazol-1-yl)pyridin-2-yl]-1,2,4-triazol-3-ato}iron(II)-methanol-chloro-form (1/2/2)

The unit cell of the title compound, [Fe(C18H15N6O2)2]·2CH3OH·2CHCl3, consists of a charge-neutral complex mol-ecule, two methanol and two chloro-form mol-ecules. In the complex, the two tridentate 2-(5-(3,4-di-meth-oxy-phen-yl)-1,2,4-triazol-3-yl)-6-(pyrazol-1-yl)pyridine ligands coordinate to the central FeII ion through the N atoms of the pyrazole, pyridine and triazole groups, forming a pseudo-octa-hedral coordination sphere. Neighbouring tapered mol-ecules are linked through weak C-H(pz)⋯π(ph) inter-actions into one-dimensional chains, which are joined into two-dimensional layers through weak C-H⋯N/C/O inter-actions. Furthermore, the layers stack in a three-dimensional network linked by weak inter-layer C-H⋯π inter-actions of the meth-oxy and phenyl groups. 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 32.0%, H⋯C/C⋯H 26.3%, H⋯N/N⋯H 13.8%, and H⋯O/O⋯H 7.5%. The average Fe-N bond distance is 2.185 Å, indicating the high-spin state of the FeII ion. Energy framework analysis at the HF/3-21 G theory level was performed to qu-antify the inter-action energies in the crystal structure.

Structural diversity in proline-based lead bromide chiral perovskites

Lead halide hybrid perovskites incorporating chiral organic cations attract considerable attention due to their promising application in multifarious optoelectronic devices. However, the examples of chiral hybrid perovskites are still limited, which greatly impedes their further studies in various optoelectronic fields. Herein, we report on new low-dimensional lead-halide hybrid perovskites incorporating the enantiopure chiral α-amino acid L-proline. Two hybrid perovskites (L-proH)PbBr₃·H₂O (Pro-PbBr₃) and (L-proH)₄Pb₃Br₁₀·4H₂O (Pro-Pb₃Br₁₀) have been synthesized by employing different ratios of organic and inorganic precursors. According to structural analysis, the inorganic sublattice of compound Pro-PbBr₃ is built of one-dimensional (1D) [PbX₃]_∞^n- lead halide chains, whereas the inorganic sublattice of compound Pro-Pb₃Br₁₀ is built upon a rare two-dimensional (2D) [Pb₃Br₁₀]_∞^4n- honeycomb-type inorganic framework. Hirshfeld surface analysis revealed an important role of various hydrogen bonding interactions in providing the binding between organic and inorganic parts of these hybrid perovskites. The optical band gap values of new hybrid perovskites as estimated using the Tauc plot approach are 4.19 eV (Pro-PbBr₃) and 4.13 eV (Pro-Pb₃Br₁₀). Also, new compounds display low-temperature broadband photoluminescence which can be attributed to the self-trapped excitons. These results show the potential of α-proline for constructing novel and highly demanded chiral hybrid perovskites, which will hold great promise for further optoelectronic applications.

Order-Disorder, Symmetry Breaking, and Crystallographic Phase Transition in a Series of Bis(trans-thiocyanate)iron(II) Spin Crossover Complexes Based on Tetradentate Ligands Containing 1,2,3-Triazoles

We report herein a series of neutral trans-thiocyanate mononuclear spin crossover (SCO) complexes, [Fe^IIL(NCS)₂] (1-4), based on tetradentate ligands L obtained by reaction of N-substituted 1,2,3-triazolecarbaldehyde with 1,3-propanediamine or 2,2-dimethyl-1,3-diaminopropane [L = N¹,N³-bis((1,5-dimethyl-1H-1,2,3-triazol-4-yl)methylene)propane-1,3-diamine/-2,2-dimethylpropane-1,3-diamine, 1/2 and N¹,N³-bis((1-ethyl/1-propyl-1H-1,2,3-triazol-4-yl)methylene)-2,2-dimethylpropane-1,3-diamine, 3/4]. The thermal-induced SCO behavior is characterized by abrupt transitions with an average critical temperature (ΔT_1/2)/hysteresis loop width (ΔT_hyst) in the range 190-252/5-14 K, while the photo-generated metastable high-spin (HS) phases are characterized by T_LIESST temperatures in the range 44-59 K. Single crystal analysis shows that except 1, all compounds experience reversible symmetry breaking coupled with the thermal SCO. Furthermore, 4 experiences an additional phase transition at ca. 290 K responsible for the coexistence of two HS phases quenched at 10 K through LIESST and TIESST effects. The molecules form hexagonally packed arrays sustained by numerous weak CH···S and C···C/S···C/N···C bonds involving polar coordination cores, while non-polar pendant aliphatic substituents are segregated inside, occupying hexagonal channels. Energy framework analysis of complexes with one step SCO transition (1, 2, and 4) shows a correlation between the cooperativity and the amplitude of changes in the molecule-molecule interactions in the lattice at the SCO transition.

Crystal structure of bis{3-(3,5-dichlorophenyl)-5-[6-(1H-pyrazol-1-yl)pyridin-2-yl]-4H-1,2,4-triazol-4-ido}iron(II) methanol disolvate

The asymmetric unit of the title compound, [FeII(C16H9Cl2N6)2]·2CH3OH, consists of half of a charge-neutral complex molecule and a discrete methanol molecule. The planar anionic tridentate ligand 2-[5-(3,5-dichlorophenyl)-4H-1,2,4-triazol-3-ato]-6-(1H-pyrazol-1-yl)pyridine coordinates to the FeII ion through the N atoms of the pyrazole, pyridine and triazole groups, forming a coordination sphere of the central ion that deviates moderately from an octahedral geometry. The average Fe—N bond distance is 1.953 Å, indicating the low-spin state of the FeII ion. The cone-like-shaped molecules, nested into each other, are linked through double weak C—H(pz)...π(ph) interactions into mono-periodic columns, which are further linked through weak C—H...N′/C′ interactions into di-periodic layers. The layers interact through double weak C–H(ph)...Cl bonds with neighbouring molecules. Energy framework analysis at the B3LYP/6–31 G(d,p) theory level reproduces the strong interaction within the layers and the weaker interlayer interactions. Intermolecular contacts were quantified using Hirshfeld surface analysis and two-dimensional fingerprint plots, the relative contributions of the contacts to the crystal packing being H...H 26.1%, H...C/C...H 24.4%, H...Cl/Cl...H 18.9% and H...N/N...H 12.1%.

Crystal structure of bis{3-(3-bromo-4-methoxyphenyl)-5-[6-(1H-pyrazol-1-yl)pyridin-2-yl]-1,2,4-triazol-3-ato}iron(II) methanol disolvate

The title charge-neutral complex is a low-spin complex with a moderately distorted pseudo-octa­hedral coordination environment of the metal ion. As a result of their asymmetric shape, the mol­ecules stack into chains, which eventually pack into layers and, finally, into a three-dimensional network connected by weak C—H⋯N, C—H⋯C hydrogen bonds and C—H⋯π inter­actions.

The unit cell of the title compound, [Fe^II(C₁₇H₁₂BrN₆O)₂]·2MeOH, consists of a charge-neutral complex mol­ecule and two independent mol­ecules of methanol. In the complex mol­ecule, the two tridentate ligand mol­ecules 2-[5-(3-bromo-4-meth­oxy­phen­yl)-4H-1,2,4-triazol-3-yl]-6-(1H-pyrazol-1-yl)pyridine coordinate to the Fe^II ion through the N atoms of the pyrazole, pyridine and triazole groups, forming a pseudo-octa­hedral coordination sphere around the central ion. In the crystal, neighbouring asymmetric mol­ecules are linked through weak C—H(pz)⋯π(ph) inter­actions into chains, which are then linked into layers by weak C–H⋯N/C inter­actions. Finally, the layers stack into a three-dimensional network linked by weak inter­layer C—H⋯π inter­actions between the meth­oxy groups and the phenyl rings. The inter­molecular contacts were qu­anti­fied using Hirshfeld surface analysis and two-dimensional fingerprint plots, revealing the relative contributions of the contacts to the crystal packing to be H⋯H 34.2%, H⋯C/C⋯H 25.2%, H⋯Br/Br⋯H 13.2%, H⋯N/N⋯H 12.2% and H⋯O/O⋯H 4.0%. The average Fe—N bond distance is 1.949 Å, indicating the low-spin state of the Fe^II ion. Energy framework analysis at the HF/3–21 G theory level was performed to qu­antify the inter­action energies in the crystal structure.

Crystal structure of bis{3-(3,4-dimethylphenyl)-5-[6-(1H-pyrazol-1-yl)pyridin-2-yl]-4H-1,2,4-triazol-4-ido}iron(II) methanol disolvate

The title compound, a charge-neutral bis­{2-(3,4-di­methyl­phen­yl)-4H-1,2,4-triazol-3-ato)-6-(1H-πyrazol-1-yl)pyridine} iron(II) complex di­methanol solvate, is a low-spin complex with a moderately distorted pseudo­octa­hedral coordination environment of the metal ion. As a result of the cone shape, the mol­ecules are stacked in mono-periodic columns that are bound by weak hydrogen bonds into di-periodic layers, which, in turn, are arranged in a three-dimensional lattice bound by weak inter­layer inter­actions.

As a result of the high symmetry of the Aea2 structure, the asymmetric unit of the title compound, [Fe^II(C₁₈H₁₅N₆)₂]·2MeOH, consists of half of a charge-neutral complex mol­ecule and a discrete methanol mol­ecule. The planar anionic tridentate ligand 2-[5-(3,4-di­methyl­phen­yl)-4H-1,2,4-triazol-3-ato]-6-(1H-pyrazol-1-yl)pyridine coordinates the Fe^II ion meridionally through the N atoms of the pyrazole, pyridine and triazole groups, forming a pseudo-octa­hedral coordination sphere of the central ion. The average Fe—N bond distance is 1.955 Å, indicating a low-spin state of the Fe^II ion. Neighbouring cone-shaped mol­ecules, nested into each other, are linked through double weak C—H(pz)⋯π(ph’) inter­actions into mono-periodic columns, which are further linked through weak C—H⋯N′/C′ inter­actions into di-periodic layers. No inter­actions shorter than the sum of the van der Waals radii of the neighbouring layers are observed. Energy framework analysis at the B3LYP/6–31 G(d,p) theory level, performed to qu­antify the inter­molecular inter­action energies, reproduces the weak inter­layer inter­actions in contrast to the strong inter­action within the layers. Inter­molecular contacts were qu­anti­fied using Hirshfeld surface analysis and two-dimensional fingerprint plots, showing the relative contributions of the contacts to the crystal packing to be H⋯H 48.5%, H⋯C/C⋯H 28.9%, H⋯N/N⋯H 16.2% and C⋯C 2.4%.

Cooperative Spin Crossover above Room Temperature in the Iron(II) Cyanoborohydride-Pyrazine Complex

Hysteretic spin crossover in coordination complexes of 3d-metal ions represents one of the most spectacular phenomena of molecular bistability. In this paper we describe a self-assembly of pyrazine (pz) and Fe(BH₃CN)₂ that afforded the new 2D coordination polymer [Fe(pz)₂(BH₃CN)₂]_∞. It undergoes an abrupt, hysteretic spin crossover (SCO) with a T_1/2 of 338 K (heating) and 326 K (cooling) according to magnetic susceptibility measurements. Mössbauer spectroscopy revealed a complete transition between the low-spin (LS) and the high-spin (HS) states of the iron centers. This LS-to-HS transition induced an increase of the unit cell volume by 10.6%. Meanwhile, a modulation of multiple [C-H^δ+···H^δ--B] dihydrogen bonds stimulates a contraction in direction c (2.2%). The simplicity of the synthesis, mild temperatures of transition, a pronounced thermochromism, stability upon thermal cycling, a striking volume expansion upon SCO, and an easy processability to composite films make this new complex an attractive material for switchable components of diverse applications.

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.

Crystal structure of 9-aminoacridinium chloride N,N-dimethylformamide monosolvate

9-Aminoacridinium chloride N,N-dimethylformamide monosolvate, C13H11N2 +Cl−·C3H7NO, crystallizes in the monoclinic space group P21/c. The salt was crystallized from N,N-dimethylformamide. The asymmetric unit consists of two C13H11N2 +Cl− formula units. The 9-aminoacridinium (9-AA) molecules are protonated with the proton on the N atom of the central ring. This N atom is connected to an N,N-dimethylformamide molecule by a hydrogen bond. The H atoms of the amino groups create short contacts with two chloride ions. The 9-AA cations in adjacent layers are oriented in an antiparallel manner. The molecules are linked via a network of multidirectional π–π interactions between the 9-AA rings, and the whole lattice is additionally stabilized by electrostatic interactions between ions.

Expanding manganese(IV) aqueous chemistry: unusually stable water-soluble hexahydrazide clathrochelate complexes

Mn cage complexes are rare, and the ones successfully isolated in the solid state are not stable in water and organic solvents. Herein, we present the first report of mononuclear Mn clathrochelates, in which the encapsulated metal exists in the oxidation state +4. The complexes are extremely stable in the crystalline state and in solutions and show rich redox chemistry.

Thermodynamic Stability and Speciation of Ga(III) and Zr(IV) Complexes with High-Denticity Hydroxamate Chelators

Increasing attention has been recently devoted to ⁸⁹Zr(IV) and ⁶⁸Ga(III) radionuclides, due to their favorable decay characteristics for positron emission tomography (PET). In the present paper, a deep investigation is presented on Ga(III) and Zr(IV) complexes with a series of tri-(H₃L1, H₃L3, H₃L4 and desferrioxamine E, DFOE) and tetrahydroxamate (H₄L2) ligands. Herein, we describe the rational design and synthesis of two cyclic complexing agents (H₃L1 and H₄L2) bearing three and four hydroxamate chelating groups, respectively. The ligand structures allow us to take advantage of the macrocyclic effect; the H₄L2 chelator contains an additional side amino group available for a possible further conjugation with a biomolecule. The thermodynamic stability of Ga(III) and Zr(IV) complexes in solution has been measured using a combination of potentiometric and pH-dependent UV–vis titrations, on the basis of metal–metal competition. The Zr(IV)-H₄L2 complex is characterized by one of the highest formation constants reported to date for a tetrahydroxamate zirconium chelate (log β = 45.9, pZr = 37.0), although the complex-stability increase derived from the introduction of the fourth hydroxamate binding unit is lower than that predicted by theoretical calculations. Solution studies on Ga(III) complexes revealed that H₃L1 and H₄L2 are stronger chelators in comparison to DFOB. The complex stability obtained with the new ligands is also compared with that previously reported for other hydroxamate ligands. In addition to increasing the library of the thermodynamic stability data of Ga(III) and Zr(IV) complexes, the present work allows new insights into Ga(III) and Zr(IV) coordination chemistry and thermodynamics and broadens the selection of available chelators for ⁶⁸Ga(III) and ⁸⁹Zr(IV).

Solution equilibria studies on Ga(III) and Zr(IV) complexes with a series of tri- and tetrahydroxamate ligands are presented. For this purpose, the rational design and synthesis of two cyclic complexing agents bearing three and four hydroxamate chelating groups was performed. The thermodynamic and speciation studies allow a discussion of the structure−complex stability dependence. The Zr(IV)-tetrahydroxamate complex is characterized by one of the highest formation constants reported to date for a hydroxamate zirconium chelator.

Spin crossover in iron(II) Hofmann clathrates analogues with 1,2,3-triazole

Hofmann-like cyanometallic complexes represent one of the biggest and well-known classes of FeII spin-crossover compounds. In this paper, we report on the first FeII Hofmann clathrate analogues with unsubstituted 1,2,3-triazole, which exhibit temperature induced spin transition. Two new coordination polymers with the general formula [FeII(1,2,3-triazole)2MII(CN)4] (M = Pt, Pd) undergo abrupt hysteretic spin crossover in the range of 190-225 K as revealed by magnetic susceptibility measurements. Two compounds are isostructural and are built of infinite cyanometallic layers which are supported by 1,2,3-triazole ligands. The thermal hysteresis loop is very stable at different scan rates from 0.5 to 10 K min-1. The compounds display strong thermochromic effect, changing their colour from pink in the low-spin state to white in the high-spin state. Our findings show that 1,2,3-triazole is suitable for elaboration of spin-crossover Hofmann clathrate analogues, and its use instead of more classical azines can advantageously expand this family of complexes.

Chiral organic–inorganic lead halide perovskites based on α-alanine

Novel hybrid organic–inorganic perovskites AlaH⁺PbHal₃⁻·H₂O incorporating chiral α-alanine were synthesized.

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.

Tunable microwave absorption of switchable complexes operating near room temperature

Materials that are able to switch microwave radiation are strongly desired for their potential applications in electronic devices. In this paper, we show the spin-dependant interaction of spin-crossover materials with microwave radiation, namely, the ability of coordination compounds [Fe(NH₂trz)₃]Br₂ and [Fe(NH₂trz)₃](NO₃)₂ that undergo a cooperative spin transition between low-spin and high-spin states to operate as thermoswitchable microwave absorbers. The characteristics of the microwave reflection and transmission of these spin-crossover complexes were investigated at variable temperatures. The evolution of both the transmission and reflection spectra in the 26–37 GHz frequency band within the temperature range of spin crossover showed significant differences in the interaction of microwave radiation with the high-spin and low-spin forms of the compounds. The microwave transmission coefficient shows a notable decrease upon transition to the high-spin state, while the reflection coefficient can be both increased or decreased on the characteristic frequencies during the spin transition. The different microwave absorbing properties of the low-spin and high-spin forms are found to be associated with a notable microwave permittivity change upon spin crossover. The switchable microwave reflection/transmission correlates well with the transition characteristics found in the optical and differential scanning calorimetry measurements. These results widen the spectroscopic range in which spin-crossover materials can be applied and contribute to the creation of a preliminary database of the microwave absorbing properties of spin-crossover complexes.

Iron(ii) spin-crossover complexes are shown to be effective microwave switches operating near room temprature.

Spin crossover in 2D iron(ii) phthalazine cyanometallic complexes

Two new spin-crossover analogues of Hofmann clathrates based on a bicyclic ligand phthalazine have been synthesized.

Crystal structure and Hirshfeld surface analysis of 4-{[(anthracen-9-yl)methyl]amino}benzoic acid

In the molecule of the title anthracene derivative, C22H17NO2, the benzene ring is inclined to the mean plane of the anthracene ring system (r.m.s. deviation = 0.024 Å) by 75.21 (9)°. In the crystal, molecules are linked by pairs of O—H...O hydrogen bonds, forming classical carboxylic acid inversion dimers with an R 2 2(8) ring motif. The dimers are linked by C—H...π interactions, forming a supramolecular framework.

(E)-3-{[(2-Bromo-3-methylphenyl)imino]methyl}benzene-1,2-diol: crystal structure and Hirshfeld surface analysis

In the title Schiff base derivative carrying a 2-bromo-3-methyl­phenyl group, the conformation about the C=N bond is E. In the crystal, O—H⋯O hydrogen-bond inter­actions consolidate the crystal packing. A Hirshfeld surface analysis and fingerprint plots were used to further investigate the inter­molecular inter­actions in the solid state.

The title compound, C₁₄H₁₂BrNO₂, was synthesized by the condensation reaction of 2,3-di­hydroxy­benzaldehyde and 2-bromo-3-methyl­aniline. It crystallizes in the centrosymmetric triclinic space group P. The configuration about the C=N bond is E. The dihedral angle between the planes of the 5-(2-bromo-3-methyl­phenyl ring and the catechol ring is 2.80 (17)°. In the crystal, O—H⋯O hydrogen-bond inter­actions consolidate the crystal packing.

Crystal structure and DFT study of a zinc xanthate complex

In the title compound, the Zn^II ion is coordinated by two N atoms of the N,N,N′,N′-tetra­methyl­ethylenedi­amine ligand and two S atoms from two 2-meth­oxy­ethyl xanthate ligands. Two C—H⋯O and two C—H⋯S intra­molecular inter­actions occur. In the crystal, mol­ecules are linked by C—H⋯O and C—H⋯S hydrogen bonds, forming a three-dimensional supra­molecular architecture.

In the title compound, bis­(2-meth­oxy­ethyl xanthato-κS)(N,N,N′,N′-tetra­methyl­ethylenedi­amine-κ²N,N′)zinc(II) acetone hemisolvate, [Zn(C₄H₇O₂S₂)₂(C₆H₁₆N₂)]·0.5C₃H₆O, the Zn^II ion is coordinated by two N atoms of the N,N,N′,N′-tetra­methyl­ethylenedi­amine ligand and two S atoms from two 2-meth­oxy­ethyl xanthate ligands. The amine ligand is disordered over two orientations and was modelled with refined occupancies of 0.538 (6) and 0.462 (6). The mol­ecular structure features two C—H⋯O and two C—H⋯S intra­molecular inter­actions. In the crystal, mol­ecules are linked by weak C—H⋯O and C—H⋯S hydrogen bonds, forming a three-dimensional supra­molecular architecture. The mol­ecular structure was optimized using density functional theory (DFT) at the B3LYP/6–311 G(d,p) level. The smallest HOMO–LUMO energy gap (3.19 eV) indicates the suitability of this crystal for optoelectronic applications. The mol­ecular electrostatic potential (MEP) further identifies the positive, negative and neutral electrostatic potential regions of the mol­ecules. Half a mol­ecule of disordered acetone was removed with the solvent-mask procedure in OLEX2 [Dolomanov et al. (2009 ▸). J. Appl. Cryst.42, 339–341] and this contribition is included in the formula.

Crystal structure and Hirshfeld surface analysis of 2,2′-{(1E,1′E)-[ethane-1,2-diylbis(azanylylidene)]bis(methanylylidene)}bis[4-(trifluoromethoxy)phenol]copper(II) hydroquinone hemisolvate

The title structure has a square-planar coordination sphere around the copper(II) ion. In the crystal, mol­ecules are linked by weak C—H⋯O and C—H⋯π hydrogen bonds and very weak π-stacking inter­actions, forming a three-dimensional supra­molecular architecture.

In the title com­plex, [Cu(C₁₈H₁₂F₆N₂O₄)]·0.5C₆H₆O₂, the Cu^II ion has a square-planar coordination geometry, being ligated by two N and two O atoms of the tetra­dentate open-chain Schiff base ligand 6,6′-{(1E,1′E)-[ethane-1,2-diylbis(aza­nylyl­idene)]bis­(methanylyl­idene)}bis­[2-(tri­fluoro­meth­oxy)phenol]. The crystal packing is stabilized by intra­molecular O—H⋯O and inter­molecular C—H⋯F, C—H⋯O and C—H⋯π hydrogen bonds. In addition, weak π–π inter­actions form a three-dimensional structure. Hirshfeld surface analysis and two-dimensional fingerprint plots were performed and created to analyze the inter­molecular inter­actions present in the crystal, indicating that the most important contributions for the crystal packing are from F⋯H/H⋯F (25.7%), H⋯H (23.5%) and C⋯H/H⋯C (12.6%) inter­actions.

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

The title compound, bis­[butyl­tris­(1H-pyrazol-1-yl)borato]iron(II), belongs to the class of neutral scorpionate complexes. The crystal structure at low temperature and the magnetic susceptibility measurement in the high-temperature region reveal the low-spin character of the iron(II) ion.

The asymmetric unit of the title compound, [Fe(C₁₃H₁₈BN₆)₂], contains two half independent complex mol­ecules. In each complex, the Fe^II atom is located on an inversion center and is surrounded by two scorpionate ligand butyl­tris­(1H-pyrazol-1-yl)borate mol­ecules that coordinate to the iron(II) ion through the N atoms of the pyrazole groups. The two independent complex mol­ecules differ essentially in the conformation of the butyl substituents. In the crystal, the complex mol­ecules are linked by a series of C—H⋯π inter­actions, which generate a supra­molecular 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.

Photoinduced hole transfer from tris(bipyridine)ruthenium dye to a high-valent iron-based water oxidation catalyst

An efficient water oxidation system is a prerequisite for developing solar energy conversion devices. Using advanced time-resolved spectroscopy, we study the initial catalytic relevant electron transfer events in the light-driven water oxidation system utilizing [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) as a light harvester, persulfate as a sacrificial electron acceptor, and a high-valent iron clathrochelate complex as a catalyst. Upon irradiation by visible light, the excited state of the ruthenium dye is quenched by persulfate to afford a [Ru(bpy)3]3+/SO4˙- pair, showing a cage escape yield up to 75%. This is followed by the subsequent fast hole transfer from [Ru(bpy)3]3+ to the FeIV catalyst to give the long-lived FeV intermediate in aqueous solution. In the presence of excess photosensitizer, this process exhibits pseudo-first order kinetics with respect to the catalyst with a rate constant of 3.2(1) × 1010 s-1. Consequently, efficient hole scavenging activity of the high-valent iron complex is proposed to explain its high catalytic performance for water oxidation.

Crystal structure, DFT and MEP study of (E)-2-[(2-hy-droxy-5-meth-oxy-benzyl-idene)amino]-benzo-nitrile

The asymmetric unit of the title compound, C15H12N2O2, contains two crystallographically independent mol-ecules in which the dihedral angles between the benzene rings in each are 13.26 (5) and 7.87 (5)°. An intra-molecular O-H⋯N hydrogen bonds results in the formation of an S(6) ring motif. In the crystal, mol-ecules are linked by weak C-H⋯O and C-H⋯N hydrogen bonds, forming layers parallel to (011). In addition, π-π stacking inter-actions with centroid-centroid distances in the range 3.693 (2)-3.931 (2) Å complete the three-dimensional network.

Crystal structure and Hirshfeld surface analysis of (Z)-6-[(2-hy-droxy-4-methyl-anilino)-methyl-idene]-4-methyl-cyclo-hexa-2,4-dien-1-one

The title compound, C15H15NO2, is a Schiff base that exists in the keto-enamine tautomeric form and adopts a Z configuration. The mol-ecule is almost planar, with the two phenyl rings twisted relative to each other by 9.60 (18)°. There is an intra-molecular N-H⋯O hydrogen bond present forming an S(6) ring motif. In the crystal, pairs of O-H⋯O hydrogen bonds link adjacent mol-ecules into inversion dimers with an R 2 2(18) ring motif. The dimers are linked by very weak π-π inter-actions, forming layers parallel to (01). Hirshfeld surface analysis, two-dimensional fingerprint plots and the mol-ecular electrostatic potential surfaces were used to analyse the inter-molecular inter-actions, indicating that the most important contributions for the crystal packing are from H⋯H (55.2%), C⋯H/H⋯C (22.3%) and O⋯H/H⋯O (13.6%) inter-actions.

Hirshfeld surface analysis and crystal structure of N-(2-meth-oxy-phen-yl)acetamide

The title compound, C9H11NO2, was obtained as unexpected product from the reaction of (4-{2-benz-yloxy-5-[(E)-(3-chloro-4-methyl-phen-yl)diazen-yl]benzyl-idene}-2-phenyl-oxazol-5(4H)-one) with 2-meth-oxy-aniline in the presence of acetic acid as solvent. The amide group is not coplanar with the benzene ring, as shown by the C-N-C-O and C-N-C-C torsion angles of -2.5 (3) and 176.54 (19)°, respectively. Hirshfeld surface analysis and two-dimensional fingerprint plots indicate that the most important contributions to the crystal packing are from H⋯H (53.9%), C⋯H/H⋯C (21.4%), O⋯H/H⋯O (21.4%) and N⋯H/H⋯N (1.7%) inter-actions.

Crystal structure and Hirshfeld surface analysis of two 5,11-methano-benzo[g][1,2,4]triazolo[1,5-c][1,3,5]oxa-diazo-cine derivatives

In the title compounds, 9-bromo-2,5-dimethyl-11,12-di-hydro-5H-5,11-methano-benzo[g][1,2,4]triazolo[1,5-c][1,3,5]oxa-diazo-cine, C13H13BrN4O (I), and 7-meth-oxy-5-methyl-2-(pyridin-4-yl)-11,12-di-hydro-5H-5,11-methano-benzo[g][1,2,4]tri-azolo[1,5-c][1,3,5]oxa-diazo-cine, C18H17N5O2 (II), the triazole ring is inclined to the benzene ring by 85.15 (9) and 76.98 (5)° in compounds I and II, respectively. In II, the pyridine ring is almost coplanar with the triazole ring, having a dihedral angle of 4.19 (8)°. In the crystal of I, pairs of N-H⋯N hydrogen bonds link the mol-ecules to form inversion dimers with an R 2 2(8) ring motif. The dimers are linked by C-H⋯π and C-Br⋯π inter-actions forming layers parallel to the bc plane. In the crystal of II, mol-ecules are linked by N-H⋯N and C-H⋯O hydrogen bonds forming chains propagating along the b-axis direction. The inter-molecular inter-actions were investigated using Hirshfeld surface analysis and two-dimensional fingerprint plots, and the mol-ecular electrostatic potential surface was also analysed. The Hirshfeld surface analysis of I suggests that the most significant contributions to the crystal packing are H⋯H (42.4%) and O⋯H/H⋯O (17.9%) contacts. For compound II, the H⋯H (48.5%), C⋯H/H⋯C (19.6%) and N⋯H/H⋯N (16.9%) inter-actions are the most important contributions.

Efficient visible light-driven water oxidation catalysed by an iron(iv) clathrochelate complex

A water-stable FeIV clathrochelate complex catalyses fast and homogeneous photochemical oxidation of water to dioxygen with a turnover frequency of 2.27 s-1 and a maximum turnover number of 365. An FeV intermediate generated under catalytic conditions is trapped and characterised using EPR and Mössbauer spectroscopy.

Crystal structure and magnetic properties of (tris{4-[1-(2-methoxyethyl)imidazol-2-yl]-3-azabut-3-enyl}amine)iron(II) bis(hexafluoridophosphate)

The title compound, [Fe(C₂₇H₄₁N₁₀O₃)](PF₆)₂, is an example of an iron(II) spin-crossover compound. In this compound, C⋯F and CH⋯F/O contacts, present between the cations and anions, extend the structure into a three-dimensional supra­molecular network.

In the complex cation of the title compound, [Fe(C₂₇H₄₁N₁₀O₃)](PF₆)₂, the tripodal tris­{4-[1-(2-meth­oxy­eth­yl)imidazol-2-yl]-3-aza­but-3-en­yl}amine ligand is coordinated to an Fe^II ion through the nitro­gen atoms of three imidazole and three imino groups. The Fe atom exhibits a distorted octa­hedral geometry. In the crystal, l and d anti­podes are arranged in layers in the bc plane. Weak C⋯F and C—H⋯F/O contacts exist between the ligands of the complex cation and the PF₆⁻ anions, generating a three-dimensional network. At 120 K, the Fe^II ion is in a low-spin state, with an average Fe—N bond distance of 1.970 (2) Å. On heating, the Fe^II ion converts to the high-spin state, as demonstrated by magnetic susceptibility measurements.

Crystal structure and Hirshfeld surfaces analysis of the nickel(II) complex of the Shiff base ligand 6,6'-{(1E,1'E)-[ethane-1,2-diylbis(aza-nylyl-idene)]bis-(methanylyl-idene)}bis-[2-(tri-fluoro-meth-oxy)phenol]

In the title complex, the nickel(II) ion has a square-planar coordination sphere, being ligated by two N and two O atoms of the tetra­dentate Schiff base ligand 6,6′-{(1E,1′E)-[ethane-1,2-diylbis(aza­nylyl­idene]bis­(methanylyl­idene)}bis­[2-(tri­fluoro­meth­oxy)phenol].

In the title complex, (6,6′-{(1E,1′E)-[ethane-1,2-diylbis(aza­nylyl­idene)]bis(methanylyl­idene)}bis­[2-(tri­fluoro­meth­oxy)phenol]-κ⁴O,N,N′,O′)nickel(II), [Ni(C₁₈H₁₂F₆N₂O₄)], the nickel(II) ion has a square-planar coordination sphere, being ligated by two N and two O atoms of the Schiff base ligand 6,6′-{(1E,1′E)-[ethane-1,2-diylbis(aza­nylyl­idene)]bis­(methanylyl­idene)}bis­[2-(tri­fluoro­meth­oxy)phenol] (L). Inversion-related mol­ecules are linked by a short Ni⋯Ni inter­action of 3.2945 (6) Å forming a dimer. In the crystal, the dimers stack up the a axis, with a closest Ni⋯Ni separation of ca 3.791 Å. There are no other significant inter­molecular inter­actions present. However, the Hirshfeld surface analysis and the two-dimensional fingerprint plots indicate that the packing is dominated by H⋯F/F⋯H, H⋯H, O⋯H/H⋯O and C⋯H/H⋯C contacts.

Crystal structure and Hirshfeld surface analysis of a Schiff base: (Z)-6-[(5-chloro-2-meth-oxy-anilino)methyl-idene]-2-hy-droxy-cyclo-hexa-2,4-dien-1-one

The title compound, C14H12ClNO3, is a Schiff base that exists in the keto-enamine tautomeric form and adopts a Z configuration. In the crystal, the dihedral angle between the planes of the benzene rings is 5.34 (15)°. The roughly planar geometry of the mol-ecule is stabilized by a strong intra-molecular N-H⋯O hydrogen bond. In the crystal, pairs of centrosymmetrically related mol-ecules are linked by O-H⋯O hydrogen bonds, forming R 2 2(10) rings. Besides this, the mol-ecules form stacks along the [001] direction with C-H⋯π and C-H⋯Cl contacts between the stacks. The inter-molecular inter-actions in the crystal were analysed using Hirshfeld surfaces. The most significant contribution to the crystal packing is from H⋯H contacts (30.8%).

Complex formation of copper(ii), nickel(ii) and zinc(ii) with ethylophosphonoacetohydroxamic acid: solution speciation, synthesis and structural characterization

We present herein the thermodynamic and X-ray characterisation of a novel ethyl phosphonohydroxamic acid-based Cu(ii) metallacrown, predominating in solution in a broad pH range.

Room temperature hysteretic spin crossover in a new cyanoheterometallic framework

A new iron(ii)-based spin-crossover compound with thermal hysteresis operating under ambient conditions is reported.

Strong Cooperative Spin Crossover in 2D and 3D FeII-MI,II Hofmann-Like Coordination Polymers Based on 2-Fluoropyrazine

Self-assembling iron(II), 2-fluoropyrazine (Fpz), and [M^II(CN)₄]^2- (M^II = Ni, Pd, Pt) or [Au^I(CN)₂]^- building blocks have afforded a new series of two- (2D) and three-dimensional (3D) Hofmann-like spin crossover (SCO) coordination polymers with strong cooperative magnetic, calorimetric, and optical properties. The iron(II) ions, lying on inversion centers, define elongated octahedrons equatorially surrounded by four equivalent centrosymmetric μ₄-[M^II(CN)₄]^2- groups. The axial positions are occupied by two terminal Fpz ligands affording significantly corrugated 2D layers {Fe(Fpz)₂([M^II(CN)₄]}. The Pt and Pd derivatives undergo thermal- and light-induced SCO characterized by T_1/2 temperatures centered at 155.5 and 116 K and hysteresis loops 22 K wide, while the Ni derivative is high spin at all temperatures, even at pressures of 0.7 GPa. The great stability of the high-spin state in the Ni derivative has tentatively been ascribed to the tight packing of the layers, which contrasts with that of Pt and Pd derivatives in the high- and low-spin states. The synthesis and structure of the 3D frameworks formulated {Fe(Fpz)[Pt(CN)₄]}·1/2H₂O and {Fe(Fpz)[Au(CN)₂]₂}, where Fpz acts as bridging ligand, which is also discussed. The former is high spin at all temperatures, while the latter displays very strong cooperative SCO centered at 243 K accompanied by a hysteresis loop 42.5 K wide. The crystal structures and SCO properties are compared with those of related complexes derived from pyrazine, 3-fluoropyridine, and pyridine.

Imparting hysteretic behavior to spin transition in neutral mononuclear complexes

The paper presents an experimental study of spin transition thiocyanate complexes of Fe(ii) with aliphatic substituents with focus on the interaction between spin- and phase-transitions.

High temperature spin crossover in [Fe(pyrazine){Ag(CN)2}2] and its solvate

Thermally induced spin transition at 370 K has been observed in a Hofmann-clathrate-like metal–organic framework.

Synthesis, Structure and Characterization of Trisodium Bis[N-(2-oximinopropionyl)glycinato(3 – )-O,N,N'] Cobaltate(III) Nonahydrate

A cobalt(III) complex with pyruvylglycine oxime (H3G) of composition Na3[CoG2] · 9 H2O is synthesized and investigated by means of X-ray analysis, infrared, electronic and NMR spectroscopy. The crystals are triclinic, space group P1̄, a = 15.477(5), b = 9.931(4), c = 7.773(4)Å; α = 75.60(3); β = 82.86(4); γ = 93.54(3)°; Ζ = 2. The structure was refined to final R = 0.036 for 3347 independent observed reflections. The structure consists of the complex anions, Na+ cations and water molecules. The ligands are coordinated via the deprotonated oxime and amide nitrogen and the carboxyl oxygen atoms in a meridional fashion (Co—O(carboxyl) = 1.971 and 1.947 Å; Co— N(oxime) = 1.891 and 1.909; Co— N(amide) = 1.876 and 1.866Å). The Co(III) coordination octahedron is slightly distorted; the Co—O and Co—N distances are in agreement with the data for oxime and peptide complexes of Co(III). The unit cell contains 2 enantiomeric complex anions related by a center of symmetry. The electronic, IR and NMR (1H and 13C) spectra of the complex are compared to those of the free ligand and of dipeptidato complexes of Co(III).

Preparation and Crystal Structure of a Mixed Metal Assembly [Ni(phen)3][Cu(H-1pap)]2(NO3) · 8 H2O Featuring Octahedral Cationic and Square-planar Anionic Modules

The new mixed metal assembly [Ni(phen)3][Cu(H-1pap)]2(NO3 ) · 8 H2O (2) (H2pap = CH3- C(=NO H)-C(O )-NH-(CH2)3-NH-C(O )-C(=NOH)-CH3) was obtained by co-crystallisation of [Li(H2O)4][Cu(H-1pap)] · 2 H2O (1) and tris(1,10-phenanthroline)nickel(II) nitrate and studied by means of X-ray crystallography (triclinic, space group P1, a = 13.471(3), b = 13.641(3), c = 15.401(3) Å, α = 108.21(3), β = 97.73(3), γ = 107.74(3)°, V = 2476.6(9) Å , Z = 2 ,R1 = 0.0677 for 4672 unique reflections with I > 2σ(I)). The assembly indicates a network structure and consists of isolated cationic and anionic modules (octahedral [Ni(phen)3]2+ and square-planar [Cu(H-1pap)]-), non-coordinated nitrate anions and solvating water molecules. The elements of the crystal structure are linked by interactions of different types: by an extended system of H bonds, stacking interactions between aromatic rings, long apical Cu-N contacts and specific π-π interaction between a deprotonated oxime group of the complex anion and a phenanthroline ligand