Ligand Field Strength Research Articles

Influence of η1‐Acetylide Ligands on the Spin State and Catalytic Activity of Iron(II) PNP Pincer Complexes

Here, we report the preparation, structures and reactivity of iron(II) PNP pincer complexes bearing one, two or three η1‐acetylide ligands bound to the metal center. The described complexes are accessible through the reaction of either one, two or ≥ 3 equivalent of LiCCR (R = SiMe3, Ph) with the dibromide complex [Fe(PNP)Br2] (1). Through increasing the number of η1‐acetylides the overall ligand field strength can be modulated resulting in S = 2, S = 1 and S = 0 spin states for mono‐, bis‐ and tris‐acetylated complexes, respectively. These findings are supported by DFT calculations on the spin state spitting in these complexes and appear to affect their binding affinity towards additional co‐ligands. The Fe(II) mono‐, bis‐ and tris‐acetylides were tested as catalyst in the catalytic dimerization of terminal alkynes. These test reactions revealed that only the bis‐acetylated complexes are catalytically competent while mono‐acetylated species are inactive. The tris‐acetylated complexes were found to be off‐cycle intermediates in the presence of lithium acetylides.

Unraveling the Stability and Magnetic Properties of Bis-Hydrated Mn(II) Complexes via Tailored Ligand Design.

Exploring the electronic structure and dynamic behavior of Mn(II) complexes reveals fascinating magnetic properties and prospective biomedical applications. In this study, we investigate the solvent phase dynamics of heptacoordinated Mn(II) complexes through ab initio molecular dynamics simulations and density functional theory (DFT) calculations with effectively varying temperatures. We observed that the complex with high stability ([Mn(pmpa)(H2O)2]) remains relatively rigid as the temperature increases to 90 °C, with only a minor change in its radial distribution functions (RDFs), compared to the RDF peaks at 25 °C. To elucidate the impact of halogens on the magnetic anisotropy of seven-coordinated Mn(II) complexes, we performed both DFT and multireference calculations. This shows that the zero-field splitting (ZFS) parameter D follows the order D(I)> D(Br)> D(Cl). We observed a significant increase in the D-value following the substitution of soft Se-donors in the equatorial position and heavier halogens in the axial position. The D-value of halogen derivatives of Se-analogues varies in the order of D(Cl) < D(I) < D(Br), deviating from the regular spectrochemical series with the discrepancy between the covalency of the Mn(II)-Se bond and the ligand field strength. We anticipate that this study will enhance our understanding of the solvent phase dynamics and structural aspects of ZFS in various Mn(II) complexes with different electronic environments.

Influence of ligand field strength on the kinetics of transition metal-catalyzed reactions

Influence of ligand field strength on the kinetics of transition metal-catalyzed reactions

Impact of ligand substituents on the spiral arrangement in glutarohydrazide-based binuclear-iron(II) helicates

Coordination-driven self-assembly, particularly, helical metal complexes are quite interesting considering their distinctive architectures. Herein, we assemble new Fe(II) compounds namely, [Fe2(L1)2](BF4)4·2H2O, 1 and [Fe2(L2)2](BF4)4, 2 from flexible glutarohydazide ligands featuring helical-like di-nuclear arrays. Structurally, in all the compounds, the two chelating ligands were arranged differently, i.e. inverted U-shaped conformation for 1 and a twisted spiral arrangement for 2 were observed respectively. Interestingly, the observed structural variations of all compounds are a result of the ligand substituents in the six positions of the pyridyl moiety in which the relatively bulky methyl group substituted ligand resists the spiral arrangement which was observed in the Br substituted analog. Magnetic investigations revealed that compounds 1 and 2 are in the high spin state at all the measured temperature ranges. Further ligand fine-tunings in the pyridyl-aldehyde moiety for the presented helical-like arrays bearing relatively strong ligand field strength are suggested for achieving the spin crossover (SCO) event.

Phosphorescent PdII-PdII Emitter-Based Red OLEDs with an EQEmax of 20.52.

Three dinuclear Pd(II) complexes (1, 2, and 3) with intense red phosphorescence at room temperature are here synthesized using strong ligand field strength compounds. All three complexes are characterized by nuclear magnetic resonance, high-resolution mass spectrometry, and elemental analyses. Complexes 2 and 3 are characterized by single-crystal X-ray diffraction. The crystalline data of 2 and 3 reveal complex double-layer structures, with Pd-Pd distances of 2.8690(9) Å and 2.8584(17) Å, respectively. Furthermore, complexes 1, 2, and 3 show phosphorescence at room temperature in their solid states at the wavelengths of 678, 601, and 672nm, respectively. In addition, they show phosphorescence at 634, 635, and 582nm, respectively, in the 2wt.% (PMMA) films, and phosphorescence at 670, 675, and 589nm, respectively, in the deoxygenated CH2Cl2 solutions. Among three complexes, complex 1 shows red emission at 634nm with phosphorescent quantum yield Ф = 67% in the 2wt.% PMMA film. Furthermore, complex 1-based organic light-emitting diode is fabricated using a vapor-phase deposition process, and their maximum external quantum efficiency reaches 20.52%, which is the highest percentage obtained by using the dinuclear Pd(II) complex triplet emitters with the CIE coordinates of (0.62, 0.38).

Establishing the origin of Marcus-inverted-region behaviour in the excited-state dynamics of cobalt(III) polypyridyl complexes.

Growing interest in the use of first-row transition metal complexes in a number of applied contexts-including but not limited to photoredox catalysis and solar energy conversion-underscores the need for a detailed understanding of their photophysical properties. A recent focus on ligand-field photocatalysis using cobalt(III) polypyridyls in particular has unlocked unprecedented excited-state reactivities. Photophysical studies on Co(III) chromophores in general are relatively uncommon, and so here we carry out a systematic study of a series of Co(III) polypyridyl complexes in order to delineate their excited-state dynamics. Compounds with varying ligand-field strengths were prepared and studied using variable-temperature ultrafast transient absorption spectroscopy. Analysis of the data establishes that the ground-state recovery dynamics are operating in the Marcus inverted region, in stark contrast to what is typically observed in other first-row metal complexes. The analysis has further revealed the underlying reasons driving this excited-state behaviour, thereby enabling potential advancements in the targeted use of the Marcus inverted region for a variety of photolytic applications.

Geometrically isomeric [Ir(C^N)(C’^N’)(N’’^N’’’)]-tris-heteroleptic complexes versus [Ir(C^N)2(N’’^N’’’)]-bis-heteroleptic complex: Towards High-performance near-infrared (NIR) polymer light-emitting diodes

Despite efficient NIR-OLEDs/PLEDs fabricated from [Ir(C^N)2(O^O)]/[Ir(C^N)2(N^O)]-bis-heteroleptic and few [Ir(C^N)(C’^N’)(O^O)]/[Ir(C^N)(C’^N’)(N^O)]-tris-heteroleptic iridium(III)-complexes, the problems to dissociation of N^O/O^O-ancillary units upon acid-hydrolysis or pyrolysis, together with geometrically isomeric complexity, have to be faced. Herein, a molecular design strategy to C1-symmetrical [[Ir(C^N)(C’^N’)(N’’^N’’’)]-tris-heteroleptic Ir(III)-complex (HC^N = Hiqbt, HC’^N’ = Hppy and HN’’^N’’’ = Hftrpz) is developed. The improved stability of [Ir(iqbt)(ppy)(ftrpz)] (2) is arisen from the strong HN’’^N’’’-ligand-field strength as compared to the N^O/O^O-ancillary counterparts. Meanwhile, owing to the Hftrpz-induced asymmetry, two geometrical isomers (2a and 2b) of [Ir(iqbt)(ppy)(ftrpz)] are first isolated. Noticeably, using the two isomeric NIR-phosphors [Ir(iqbt)(ppy)(ftrpz)]-a/b (2a and 2b) as the dopants, their high-performance NIR-PLEDs than that with the [Ir(iqbt)2(ftrpz)] (1) counterpart, are realized. Especially the NIR-PLED-2b, exhibits both an attractively high efficiency (ηEQEmax = 5.94 %; λem = 692 nm) and almost negligible (< 5 %) efficiency roll-off. And thus, this finding engenders [Ir(C^N)(C’^N’)(N’’^N’’’)]-tris-heteroleptic Ir(III)-complexes a new platform to the realization of high-performance, low-cost, large-area and scalable NIR-PLEDs.

Effect of B2O3 content on the structural and optical properties of (Ba0.85Ca0.15) (Ti0.9Zr0.1)O3 glass

This study explores the structural and optical properties of (Ba0.85Ca0.15) (Ti0.9Zr0.1)O3 system mixed with B2O3. The samples with the composition xB2O3 + (100-x) (Ba0.85Ca0.15) (Ti0.9Zr0.1)O3 (where x = 5, 10, 20, 30 and 40 mol. %) were prepared using the melting quenching technique to form borate-based glass containing BaO, CaO, ZrO2 and TiO2 (BBCZT). X-ray diffraction analysis confirmed the amorphous nature of the prepared samples. The macroscopic structure of the glasses was assessed from the density measurements determined through Archimedes' principle, while information regarding the atomic structural units and vibrational modes were obtained from Infrared measurements. Increasing the B2O3 content led to variation of the relative intensity of these modes together with a decrease (increase) in density (molar volume), indicating a less dense and open-structure for the glass network. The absorption spectra showed absorption bands associated with the optical transitions B22g→B21g and B22g→A21g of Ti3+ ions occupying distortion octahedral sites (TiOh3+) in the glass matrix. The ligand field strength (10Dq), optical energy gap (Eopt), Urbach's energy (ΔE), and nonlinear optical properties (NLO) were calculated in the study.

Active Learning of Ligands That Enhance Perovskite Nanocrystal Luminescence.

Ligands play a critical role in the optical properties and chemical stability of colloidal nanocrystals (NCs), but identifying ligands that can enhance NC properties is daunting, given the high dimensionality of chemical space. Here, we use machine learning (ML) and robotic screening to accelerate the discovery of ligands that enhance the photoluminescence quantum yield (PLQY) of CsPbBr3 perovskite NCs. We developed a ML model designed to predict the relative PL enhancement of perovskite NCs when coordinated with a ligand selected from a pool of 29,904 candidate molecules. Ligand candidates were selected using an active learning (AL) approach that accounted for uncertainty quantified by twin regressors. After eight experimental iterations of batch AL (corresponding to 21 initial and 72 model-recommended ligands), the uncertainty of the model decreased, demonstrating an increased confidence in the model predictions. Feature importance and counterfactual analyses of model predictions illustrate the potential use of ligand field strength in designing PL-enhancing ligands. Our versatile AL framework can be readily adapted to screen the effect of ligands on a wide range of colloidal nanomaterials.

Photophysical Studies of Helicate and Mesocate Double-Stranded Dinuclear Ru(II) Complexes.

The metal-ligand charge transfer (3MLCT) and phosphorescence-quenching metal-centered (3MC) states of the helicate and mesocate diastereoisomers of a double-stranded dinuclear polypyridylruthenium(II) complex have been investigated using ultrafast transient absorption spectroscopy. At 294 K, transient signals of the helicate decayed significantly slower than those of the mesocate, whereas at 77 K, no clear contrast in kinetics was observed. Contributions to excited-state decay from high-lying 3MLCT states were identified at both temperatures. Spectroscopic data (294 K) suggest that the 3MC state of the helicate lies above the 3MLCT and that the reverse is true for the mesocate; this was further validated by density functional theory calculations. The stabilization of the 3MC state relative to the 3MLCT state in the mesocate was explained by a reduction in ligand field strength due to distortion near the ligand bridge, which causes further deviation from octahedral geometry compared to the helicate. This work illustrates how minor structural differences can significantly influence excited state dynamics.

Ligand Field Sensitive Spin Acceleration in the Iron-Catalyzed [2 + 2] Cycloaddition of Unactivated Alkenes and Dienes.

Redox-active pyridine(diimine) (PDI) iron catalysts promote the reversible [2 + 2] cycloaddition of alkenes and dienes to cyclobutane derivatives that have applications ranging from fuels to chemically recyclable polymers. Metallacycles were identified as key intermediates, and spin crossover from the singlet to the triplet surface was calculated to facilitate the reductive coupling step responsible for the formation of the four-membered ring. In this work, a series of sterically and electronically differentiated PDI ligands was studied for the [2 + 2] cycloaddition of ethylene and butadiene to vinylcyclobutane. Kinetic studies revealed that the fastest and slowest turnover were observed with equally electron-deficient supporting ligands that either feature phenyl-substituted imine carbon atoms (MeBPDI) or a pyrazine core (MePZDI). While the oxidative cyclization was comparatively slow for both catalysts, the rate of reductive coupling─determined by stoichiometric 13C2H4 labeling studies─correlated with the turnover frequencies. Two-state density functional theory studies and the distinct electronic structures of related (iPrBPDI) and (iPrPZDI) iron methyl complexes revealed significantly different ligand field strengths due to either diminished ligand σ-donation (MeBPDI) or promoted metal π-backbonding (MePZDI). Spin acceleration, leading to fast reductive coupling and catalytic turnover, was promoted in the case of the weaker ligand field and depends on both the nature and position of the electron-withdrawing group. This study provides strong evidence for the role of two-state reactivity in C(sp3)-C(sp3) bond formation and insights on how ligand design either promotes or inhibits spin acceleration in earth-abundant metal catalysis.

FeN4 Environments upon Reduction: A Computational Analysis of Spin States, Spectroscopic Properties, and Active Species.

FeN4 motifs, found, for instance, in bioinorganic chemistry as heme-type cofactors, play a crucial role in man-made FeNC catalysts for the oxygen reduction reaction. Such single-atom catalysts are a potential alternative to platinum-based catalysts in fuel cells. Since FeNC catalysts are prepared via pyrolysis, the resulting materials are amorphous and contain side phases and impurities. Therefore, the geometric and electronic nature of the catalytically active FeN4 site remains to be clarified. To further understand the behavior of FeN4 centers in electrochemistry and their expected spectroscopic behavior upon reduction, we investigate two FeN4 environments (pyrrolic and pyridinic). These are represented by the model complexes [Fe(TPP)Cl] and [Fe(phen2N2)Cl], where TPP = tetraphenylporphyrin and phen = 1,10-phenanthroline. We predict their Mössbauer, UV-vis, and NRV spectral data using density functional theory as windows into their electronic structure differences. By varying the axial ligand, we further show how well small chemical changes in both complexes can be discerned. We find that the differences in ligand field strength in pyrrolic and pyridinic coordination result in different spin ground states, which in turn leads to distinct Mössbauer spectroscopic properties. As a result, pyrrolic nitrogen donors with a weaker ligand field are predicted to show more pronounced spectroscopic differences under in situ and operando conditions, while pyridinic nitrogen donors are expected to show less pronounced spectroscopic changes upon reduction and/or ligand loss. We therefore suggest that a weaker ligand field leads to better detectability of catalytic intermediates in in situ and operando experiments.

Controlling the Photophysical Properties of a Series of Isostructural d6 Complexes Based on Cr0, MnI, and FeII.

Development of first-row transition metal complexes with similar luminescence and photoredox properties as widely used RuII polypyridines is attractive because metals from the first transition series are comparatively abundant and inexpensive. The weaker ligand field experienced by the valence d-electrons of first-row transition metals challenges the installation of the same types of metal-to-ligand charge transfer (MLCT) excited states as in precious metal complexes, due to rapid population of energetically lower-lying metal-centered (MC) states. In a family of isostructural tris(diisocyanide) complexes of the 3d6 metals Cr0, MnI, and FeII, the increasing effective nuclear charge and ligand field strength allow us to control the energetic order between the 3MLCT and 3MC states, whereas pyrene decoration of the isocyanide ligand framework provides control over intraligand (ILPyr) states. The chromium(0) complex shows red 3MLCT phosphorescence because all other excited states are higher in energy. In the manganese(I) complex, a microsecond-lived dark 3ILPyr state, reminiscent of the types of electronic states encountered in many polyaromatic hydrocarbon compounds, is the lowest and becomes photoactive. In the iron(II) complex, the lowest MLCT state has shifted to so much higher energy that 1ILPyr fluorescence occurs, in parallel to other excited-state deactivation pathways. Our combined synthetic-spectroscopic-theoretical study provides unprecedented insights into how effective nuclear charge, ligand field strength, and ligand π-conjugation affect the energetic order between MLCT and ligand-based excited states, and under what circumstances these individual states become luminescent and exploitable in photochemistry. Such insights are the key to further developments of luminescent and photoredox-active first-row transition metal complexes.

Inducing Ferrimagnetic Exchange in 1D-FeSe2 Chains Using Heteroleptic Amine Complexes: [Fe(en)(tren)][FeSe2]2.

[Fe(en)(tren)][FeSe2]2 (en = ethylenediamine, C2H8N2, tren = tris(2-aminoethyl)amine, C6H18N4) has been synthesized by a mixed-ligand solvothermal method. Its crystal structure contains heteroleptic [Fe(en)(tren)]2+ complexes with distorted octahedral coordination, incorporated between 1D-FeSe2 chains composed of edge-sharing FeSe4 tetrahedra. The twisted octahedral coordination environment of the Fe-amine complex leads to partial dimerization of Fe-Fe distances in the FeSe2 chains so that the FeSe4 polyhedra deviate strongly from the regular tetrahedral geometry. 57Fe Mössbauer spectroscopy reveals oxidation states of +3 for the Fechain atoms and +2 for the Fecomplex atoms. The close proximity of Fe atoms in the chains promotes ferromagnetic nearest neighbor interactions, as indicated by a positive Weiss constant, θ = +53.8(6) K, derived from the Curie-Weiss fitting. Magnetometry and heat capacity reveal two consecutive magnetic transitions below 10 K. DFT calculations suggest that the ordering observed at 4 K is due to antiferromagnetic intrachain interactions in the 1D-FeSe2 chains. The combination of two different ligands creates an asymmetric coordination environment that induces changes in the structure of the Fe-Se fragments. This synthetic strategy opens new ways to explore the effects of ligand field strength on the structure of both Fe-amine complexes and surrounding Fe-Se chains.

Constructing high axiality mononuclear dysprosium molecular magnets via a regulation-of-co-ligands strategy.

Two lanthanide complexes with formulae [DyIII(LN5)(pentafluoro-PhO)3] (1) and [DyIII(LN5)(2,6-difluoro-PhO)2](BPh4) (2) (LN5 = 2,14-dimethyl-3,6,10,13,19-pentaazabicyclo[13.3.1]nonadecal (19),2,13,15,17-pentaene) were structurally and magnetically characterized. DyIII ions lie in the cavity of a five coordinate nitrogen macrocycle, and in combination with the introduction of multi-fluorinated monodentate phenoxyl coligands a high axiality coordination symmetry is built. Using the pentafluorophenol co-ligand, complex 1 with a D2d coordination environment, is obtained and displays moderate single-molecule magnets (SMMs) behavior. When difluorophenol co-ligands were used, a higher local axisymmetric pentagonal bipyramidal coordination geometry was observed in complex 2, which displays apparent slow magnetic relaxation behavior with a hysteresis temperature of up to 5 K. Further magnetic studies of diluted samples combined with ab initio calculations indicate that the high axiality plays a crucial role in suppressing quantum tunneling of magnetization (QTM) and consequently results in good slow magnetic relaxation behavior. Different fluoro-substituted phenoxyl co-ligands have phenoloxy oxygen atoms with different electrostatic potentials as well as a different number of phenoloxy coligands along the magnetic axis, resulting in different ligand field strengths and coordination symmetries.

Influence of barium substitution on the physical, thermal, optical and luminescence properties of Sm3+-doped metaphosphate glasses for reddish orange light applications.

Our present study focuses on examining the thermal, structural and luminescent characteristics of sodium barium metaphosphate glasses doped with Sm3+. Glass samples with molar compositions (100 - y)[(50P2O5)-(50-xNa2O)-(xBaO)]-ySm2O3, where x = 20, 25, 30, 35, 40 and y = 0.3 and 1% were first synthesized by conventional melt quenching and later dehydroxylated under a constant N2 flow to ensure final glasses with a very high degree of chemical and optical homogeneity and free of water. Upon the addition of BaO and Sm2O3, refractive index, molar mass, density, glass transition temperature and dilatometric softening temperature exhibited an increase, whereas the coefficient of thermal expansion showed a decrease. The FTIR spectra analysis reveals a network depolymerization that intensifies with rising BaO concentration, ultimately transitioning from a modifier oxide to a glass-forming element, at higher BaO concentrations. All doped samples exhibited prominent absorption bands in the visible (VIS) and near-infrared (NIR) regions, as revealed by the optical absorption spectra. The Na2O modifier demonstrated greater influence on Sm3+ emission compared to BaO, a phenomenon that can explained by the moderation of the local ligand field strength resulting from this substitution. With an increase in Sm2O3 concentration from 0.3 to 1 mol%, the experimental lifetimes of the 4G5/2 level decrease, primarily attributed to the presence of energy transfer mechanisms. A discussion of Judd-Ofelt parameter analysis and glass radiation properties will be presented.

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.

Binuclear methylphosphinidine complexes of cyclopentadienylruthenium carbonyls: effects of the higher ligand field strength of ruthenium derivatives relative to iron derivatives.

The structures and energetics of the binuclear methylphosphinidene complexes of cyclopentadienylruthenium carbonyls of the type [MePRu2(CO)nCp2] (n = 4, 3, 2, 1) have been investigated for comparison with their previously studied iron analogues. For the tetracarbonyls and tricarbonyls [MePM2(CO)nCp2] (n = 4, 3) substituting ruthenium for iron has relatively little effect on the energetically preferred structures. Thus such structures have two-electron donor bridging MeP groups with no metal-metal bond for the tetracarbonyls and a metal-metal single bond for the tricarbonyls. This leads to favored 18-electron configurations for both ruthenium atoms in all cases. However, the higher ligand field strengths of ruthenium complexes relative to analogous iron complexes have major effects on the energetically preferred structures for the dicarbonyls and monocarbonyls [MePM2(CO)nCp2] (M = Fe, Ru; n = 2, 1). Thus the 11 lowest energy structures for the dicarbonyl [MePFe2(CO)2Cp2] are triplet or quintet spin state structures whereas the 6 lowest energy structures for the ruthenium analogue [MePRu2(CO)2Cp2] are all singlet structures. These low-energy singlet [MePRu2(CO)2Cp2] structures include species in which both ruthenium atoms attain the favored 18-electron configurations in different ways: either by a Ru-Ru single bond and an agostic C-H-Ru interaction from the methyl group, a Ru-Ru single bond and a four-electron donor bridging MeP ligand with PRu double bonds, or a formal RuRu double bond with a two-electron donor bridging MeP ligand. The 8 lowest energy structures for the diiron monocarbonyl [MePFe2(CO)Cp2] are all triplet or quintet spin structures whereas the lowest energy structure for the diruthenium monocarbonyl [MePRu2(CO)Cp2] by more than 20 kcal mol-1 is a singlet structure with a formal RuRu double bond and bridging CO and four-electron donor MeP groups. Thermochemical information predicts such monocarbonyl derivatives to be the dominant binuclear decarbonylation products of the tricarbonyls [RPRu2(CO)3Cp2] by thermal or photochemical methods.

Two-dimensional spin-crossover coordination polymers based on the 1,1,2,2-tetra(pyridin-4-yl)ethene ligand.

A series of two-dimensional (2D) spin-crossover coordination polymers (SCO-CPs) [FeII(TPE)(NCX)2]·solv (1: X = BH3, solv = H2O·2CH3OH·DMF; 2: X = Se, solv = H2O·2CH3OH·0.5DMF; 3: X = S, solv = H2O·2CH3OH·0.5DMF) were synthesized by employing 1,1,2,2-tetra(pyridin-4-yl)ethene (TPE) and pseudohalide (NCX-) coligands. Magnetic measurements indicated that complexes 1-3 exhibited SCO behaviors with diminishing thermal hysteresis (7/4/0 K) upon decreasing the ligand-field strength. The critical temperatures (Tc) during spin transition were found to be inversely proportional to the coordination ability parameters (a™) with a linear correlation. The guest effect was also investigated in the solvent-exchanged phases 1-SE/2-SE/3-SE wherein the DMF molecules were replaced by methanol molecules. Compared with 1-3, 1-SE/2-SE/3-SE displayed more abrupt and complete single-step SCO behaviors but narrower thermal hysteretic loops. The results reported here demonstrate that the Tc values of these two families were dominated by the ligand-field strength of the NCX- anions (NCBH3 > NCSe > NCS), whereas the guest effect only modulated the kinetic factor of the SCO nature in this system.

Substituent effect on the spin transition of Mixed-Valence Cyanide-Bridged {FeIII2FeII} complexes

We synthesized three isostructural mixed-valence {FeIII2FeII} trinuclear complexes {[(Tp4-Me)FeIII(CN)3]2FeII(1-(4-R-phenyl)-1H-imidazole)}·Sol (1: R = Br, Sol = 10·H2O; 2: R = Cl, Sol = 2 CH3OH; 3: R = F, Sol = 2 CH3OH; Tp4-Me = tri(4-methyl-pyrazol-1-yl)borate) and characterized their structures and magnetic and spectroscopic properties. The linear trinuclear structures of complexes 1–––3 are isomorphic. Magnetic susceptibility measurements and variable-temperature IR analyses revealed that the magnetic behavior of the complexes can be finely tuned by the substituents, which significantly influence the ligand field strength. The complexes exhibit complete one-step spin-crossover (SCO) behavior with T1/2↑ = 146 K, T1/2↓ = 142 K for 1; T1/2↑ = 155 K, T1/2↓ = 150 K for 2; T1/2↑ = 147 K, T1/2↓ = 144 K for 3, respectively. Photomagnetic study indicates that all of them show the light-induced spin state transition from low-spin to high spin states by applying 946 nm light for 1 and 808 nm light for 2 and 3. Variable-temperature structural analysis demonstrates that the three complexes exhibit π···π interactions between the intermolecular benzene ring and the pyrazole ring. Our study demonstrates that the modification of substituent ligand can significantly affect the intermolecular interactions of these trinuclear complexes and the SCO behaviour of them.