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A family of five new bis-bidentate azole–triazole Rat ligands (1,3-bis(5-(azole)-4-isobutyl-4H-1,2,4-triazol-3-yl)benzene), varying in choice of azole (2-imidazole, 4-imidazole, 1-methyl-4-imidazole, 4-oxazole and 4-thiazole), and the corresponding family of spin-crossover (SCO) and redox active triply bridged dinuclear helicates, [FeII2L3]4+, has been prepared and characterised. X-ray crystal structures show all five Fe(II) helicates are low spin at 100 K. Importantly, DOSY NMR confirms the intactness of these SCO-active dinuclear helicates in D3-MeCN solution, regardless of HS fraction: γHS(298 K) = 0–0.81. Variable temperature 1H NMR Evans and UV-vis studies reveal that the helicates are SCO-active in MeCN solution. Indeed, the choice of azole in the Rat ligand used in [Fe2L3]4+ tunes: (a) solution SCO T1/2 from 247 to 471 K, and (b) reversible redox potential, Em(FeII/III), from 0.25 to 0.67 V for four helicates, whilst one has an irreversible redox process, Epa = 0.78 V, vs. 0.01 M AgNO3/Ag. For the four reversible redox systems, a strong correlation (R2 = 0.99) is observed between T1/2 and Epa. Finally, the analogous Ni(II) helicates have been prepared to obtain Δo, establishing: (a) the ligand field strength order of the ligands: 4-imidazole (11 420) ∼ 1-methyl-4-imidazole (11 430) < 2-imidazole (11 505) ∼ 4-oxazole (11 516) < 4-thiazole (11 804 cm−1), (b) that Δo ([NiII2L3]4+) strongly correlates (R2 = 0.87) with T1/2 ([FeII2L3]4+), and (c) interestingly that Δo strongly correlates (R2 = 0.98) with Epa for the four helicates with reversible redox, so the stronger the ligand field strength, the harder it is to oxidise the Fe(II) to Fe(III).

The Co center is active in electrochemical CO2 reduction (CO2RR), and its activity can be tuned by changing its coordination environment. However, the coordination number around the Co center cannot be readily changed in homogeneous systems owing to bimolecular decomposition of reduced low-coordinate Co species. Herein we report the systematic tuning of N atom numbers from 2 to 5 in the first coordination sphere around Co centers supported on two-dimensional metal-organic layers (MOLs) for the electrochemical CO2RR. The N atoms come from a combination of bipyridine, terpyridine, and phenylpyridine ligands. The Co centers are isolated and stabilized on the MOL to prevent bimolecular decomposition. All of the catalysts, denoted MOL-Co-Nx (x = 2-5), are active in reducing CO2 to CO electrochemically, but their activities are highly dependent on the number of coordinating N atoms. MOL-Co-N3 showed the highest current density of 2.3 A mg-1 with a CO Faradaic efficiency of 99% at an overpotential of only 380 mV. Density functional theory calculations attribute the high activity of the Co-N3 center to a balance of ligand field strength and open coordination site: the high ligand field strength promotes back-bonding, while the open coordination site allows HCO3- assistance, both of which accelerate C-O cleavage. MOLs thus provide a unique platform to systematically study the relationship between the coordination environment and the reactivity of open metal sites in electrocatalysis.

It is a crucial challenge to address both magnetic anisotropy and stability for single-molecule magnets (SMMs) used in next-generation nanodevices. Highly axial lanthanide SMMs with neutral charge ...

Double-hybrid (DH) approximations have entered into the limelight of density functional theory (DFT) computations of different properties; however, little is known regarding their accountability for spin-state energetics in transition metal complexes. In this work, taking high-level all-electron fixed-node diffusion Monte Carlo data as a reference, we present a survey of the applicability of parameterized and parameter-free DHs as well as their dispersion and non-local corrected versions for predicting the spin splitting energies of transition metal complexes collected from the literature and from our own proposals herein. Our proposed parameter-free DH based on the spin-opposite-scaled (SOS) scheme incorporating the Perdew-Burke-Ernzerhof (PBE) exchange and strongly constrained and appropriately normed (SCAN) correlation as well as high balanced fractions of nonlocal exchange and correlation without any additional correction, SOS0-PBESCAN0-2(a), is found to be superior for overall performance. This model not only surpasses other DFT approximations from various rungs of the "Jacob's Ladder" classification and recently reported DHs for the present purpose but also outperforms wave function-based approaches in most cases. Dissecting the roles played by the non-local exchange and correlation contributions as well as their interplay, it is shown that this good performance arises mainly from an appropriate compromise between energy- and density-driven errors. Furthermore, by employing the proposed model and a variety of modified versions thereof, we scrutinize the roles of various factors, such as the ligand field strength and oxidation state of the metal ions, in both qualitative and quantitative descriptions of spin-state energetics in other complexes with different metals and ligands.

We applied density functional theory calculations to ruthenium–nitrosyl complexes, which are known to exist in high-level radioactive waste generating during reprocessing of spent nuclear fuel, to give a theoretical correlation between 99Ru Mossbauer spectroscopic parameters and ligand field strength for the first time. The structures of the series of complexes, [Ru(NO)L5] (L = Br–, Cl–, NH3, CN–), were modeled based on the corresponding single-crystal X-ray coordinates. The comparisons of the geometries and total energies between the different spin states suggested that the singlet spin state of [Ru(II)(NO+)L5] complexes were the most stable. This result was supported by the benchmark calculations of the 99Ru Mossbauer isomer shift (δ) and quadrupole splitting (ΔEQ) values. The calculated results of both the δ and ΔEQ values reproduced the experimental results by reported previously and increased in the order of L = Br–, Cl–, NH3, CN–. Finally, we estimated the ligand field strength (Δo) based on molecul...

Photochromic and photoswitchable magnetic (photomagnetic) properties of spin-crossover (SCO) metal complexes in the solid state were observed almost at cryogenic temperature. Here we present a new crystalline complex [FeII(TMPT)6](ClO4)8 (TMPT = 1-(1,2,4-triazol-4-yl)-2,4,6-trimethyl-pyridinium) showing photochromism and reversible hysteresis shift, with one direction being triggered by thermal annealing and the other by irradiation at room temperature. IR/Raman and single-crystal X-ray diffraction data revealed that these behaviors are very probably induced by thermal disorder of ClO4– anions, which change intermolecular interactions and the ligand field strength of the FeII center. This discovery may inspire the design and synthesis of new photochromic and photomagnetic compounds through controlling structural disorder.

Two isostructural mononuclear CoII complexes, [Co(xantpo)(NCE)2 ] (E=S (1) and O (2); xantpo=9,9-dimethyl-4,5-bis(diphenylphosphoryl) xanthene), supported by a bidentate phosphine oxide ligand are reported. The cobalt complexes exhibit characteristic tetrahedral structures coordinated with two oxygen and two nitrogen atoms. Magnetic property measurements show their similar static magnetic behaviours but very different dynamic magnetic behaviours. Both complexes show field-induced slow magnetic relaxation behaviours, but the relaxation of 2 is much slower than that of 1. Fittings to the magnetic data and ab initio CASSCF calculations reveal significant changes in the zero field splitting (ZFS) parameters (D and E), which can be attributed to the small geometrical changes of the Co ions and the different ligand field strength of the two terminal ligands.

A new undergraduate inorganic chemistry laboratory experiment is presented. An introduction of the spectrochemical series and ligand exchange is explored using the coordination complex dirhodium tetraacetate, Rh2(OAc)4. Students have measured the absorption spectra of the Rh2 complex in the presence of various ligand environments. In dichloromethane solution, Rh2(OAc)4 displays an absorption feature at 661 nm (222 M–1 cm–1), which shifts to higher energy upon introduction of neutral axial ligands. The magnitude of the shift corresponds to the ligand’s placement within the spectrochemical series. Experimental techniques of synthetic coordination chemistry and UV–vis spectroscopy are emphasized in this experiment as well as concepts such as molecular orbital (MO) diagrams and the spectrochemical series.

For the purpose of further exploring the effect of nonperipherally attached substituents on single-ion magnet (SIMs) performance, tetrasubstituted bis[1,8,15,22-tetrakis(3-pentyloxy)phthalocyaninato]terbium double-deckers, in both the reduced form TbH[Pc(α-OC5H11)4]2 (1) and the neutral form Tb[Pc(α-OC5H11)4]2 (2), were prepared. Single-crystal X-ray diffraction analysis for 2 unambiguously demonstrates the pinwheellike molecular structure with C4 symmetry. Magnetic investigations of the two bis(phthalocyaninato)terbium double-deckers reveal their characteristic SIM nature. 2 exhibits SIM performance superior to that of 1, as revealed by the larger energy barrier of 466 K for the former species and 431 K for the latter species due to the presence of organic radical–f (radical–Tb) interactions. The enhanced SIM performance of 2 in comparison to 1 actually stems from the presence of radical–f interactions and an enhanced ligand field strength. The latter positive factor is indicated by the electrostatic pot...

The pentafluorophenylethynyl ligand, C2C6F5, is a commonly used electron withdrawing ligand. In the context of developing complexes with blue-shifted emission for organic light emitting diode applications, it is often necessary to have electron poor ligands that are also relatively high on the spectrochemical series. The trifluoropropynyl ligand, C2CF3, is also electron withdrawing and has never been directly compared to C2C6F5. Herein complexes of the type trans-[M(cyclam)(C2R)2]OTf, where M = CoIII and CrIII and C2R = C2C6F5, are prepared and characterized by X-ray crystallography, cyclic voltammetry, and UV-Vis, IR, and Raman spectroscopy. Emission spectroscopy for the CrIII complex is also reported. These C2C6F5 complexes are compared to the corresponding complexes where C2R = C2CF3, and C2C6H5. For trans-[Co(cyclam)(C2R)2]OTf, the reversible CoIII/II reduction for the C2C6F5 complex occurs 120 mV more cathodic than that for the C2CF3 complex, indicating that C2CF3 is more electron withdrawing, a conclusion that is supported by analysis of C C stretching frequencies. The lowest energy metal-centered (MC) 1A1g → 1T2g absorption band for the C2CF3 and C2C6F5 complexes in CH3CN appear at 425 and 438 nm, respectively, suggesting that C2CF3 is a stronger field ligand. In the region of the LMCT transitions, the absorption spectra for the trans-[Cr(cyclam)(C2R)2]OTf complexes, where C2R = C2C6H5 and C2C6F5, show significant vibronic structure that is also superimposed on the MC transitions, suggesting some degree of charge transfer character in the latter. The CrIII complexes are all emissive in room-temperature fluid solution with the C2CF3 complex showing significant fine structure and the C2C6H5 and C2C6F5 complex emission being broad, structureless, and redshifted by comparison. This suggests that emission in the C2CF3 complex is of 2Eg (Oh) origin and emission for the C2C6H5 and C2C6F5 complexes is of 2T1g (Oh) origin. Emission for all CrIII complexes in 77 K glass shows significant fine structure. The presence of a dominant 0-0′ emission band in the C2C6H5 and C2C6F5 complexes suggests a non-centrosymmetric ground state. This is supported by the crystal structure for trans-[Cr(cyclam)(C2C6F5)2]OTf that shows an angle of 77.8° between the planes defined by the aryl rings.

Self-emitting blue and red EuOX (X = F, Cl, Br, and I) were successfully synthesized and characterized. Far-infrared and Raman measurements revealed that the vibration modes prominently reflected the Eu-O and Eu-X bond characters of these materials. X-ray photoelectron spectroscopy (XPS) of the red-emitting EuOX compounds showed that Eu exclusively existed as Eu3+, while in the blue-emitting EuOX, a mixed Eu3+/Eu2+ state was observed. For the red-emitting EuOX (X = F, Cl, and Br), the maximum wavelengths of the charge-transfer (CT) bands were red-shifted: F → Cl → Br (282, 320, and 330 nm for F, Cl, and Br, respectively). Using one-electron spin-polarized band structure calculations, it was verified that the red-shift of the CT energy from F to Br in EuOX was mainly due to the relative positions of the halogen orbital energies being gradually increased, following the trend in their electronegativity. For the blue-emitting EuOX (X = Cl, Br, and I), the emission band maxima were red-shifted from Cl to I (409, 414, and 432 nm for Cl, Br, and I, respectively), which was quite opposite to the trend predicted based on the spectrochemical series in crystal field theory, which was in good agreement with the previous results of the calculated 5d → 4f transition energies of the Eu2+ activator based on the crystal field theory. Through photoluminescence, UV-visible absorbance, and XPS, it was elucidated that the red emission due to Eu3+ was strongly masked by the intensified blue emission associated with the small amount of Eu2+ in the blue-emitting EuOX (X = Cl, Br, and I). These materials may provide a platform for modeling new phosphors for application in solid-state lighting.

We report the synthesis, crystal structure, phonons and luminescence properties of three novel heterometallic metal organic frameworks (MOFs) with perovskite-like topology of the following formulas: [C3H5N2]Na0.5Cr0.5(HCOO)3 (ImNaCr), [C3H5N2]Na0.5Al0.5(HCOO)3 (ImNaAl) and [C3H5N2]Na0.5Al0.475Cr0.025(HCOO)3 (ImNaAlCr with 5 mol% of Cr3+). ImNaCr crystallizes in a monoclinic system (P2/n space group) with one imidazolium cation (Im+) in an asymmetric unit forming six N–H⋯O and four C–H⋯O hydrogen bonds. In contrast to other known heterometallic MOFs, the complete substitution of Cr3+ ions with smaller Al3+ ions leads to a change of the crystal symmetry. ImNaAl adopts a monoclinic P21/n space group with two distinct Im+ cations and different H-bonding patterns. The DSC measurements and XRD single-crystal studies show that the studied crystals do not undergo structural phase transitions in the 80–400 K range. The high-pressure Raman studies of ImNaCr reveal the presence of two reversible structural instabilities, first in the 0.4–1.1 GPa range and second near 4 GPa. The first pressure-induced phase transition involves weak distortion of the metal-formate framework, while the second one is associated with partial and reversible amorphization of the sample. We discuss the stability of heterometallic formate MOFs depending on their building blocks. The luminescence measurements show that both the fully concentrated crystal (ImNaCr) and the diluted one (ImNaAlCr) exhibit a Cr3+-based emission characteristic of intermediate ligand field strength. We also show that the spectroscopic properties of heterometallic MOFs depend strongly on the templated cation, i.e. the decreasing size of the organic cation leads to an increase in the crystal field.

The substitution of H2O in aquacyano-5,6-dioxo-5,6-seco-heptamethyl-cob(III)yrinate, (aquacyano-5-seco-cobester, [AC-5-seco-Cbs]+), in which the C5–C6 bond of the corrin ring of this vitamin B12 derivative is cleaved, by a variety of anionic ligands is reported. The pKa for ionization of coordinated water decreases from 9.8 ± 0.3 in aquacyanocobyrinic acid heptamethyl ester (aquacyanocobester, [ACCbs]+) to 7.28 at 25 °C (ΔH = –88 ± 17 kJ mol−1 and ΔS = –434 ± 56 J K−1 mol−1) in [AC-5-seco-Cbs]+. The pKa, confirmed by determining the pH-dependence of coordination of SO32– by [AC-5-seco-Cbs]+, shows Co(III) in this complex behaves much more like Co(III) in simple hexacoordinate complexes than in intact cobalt corrins. A comparison of log K values for coordination of CN–, SO32–, NO2–, N3– and S2O32– by [ACCbs]+ and [AC-5-seco-Cbs]+ demonstrates that cleavage of the corrin ring significantly decreases the affinity of Co(III) for the softer ligands CN–, SO32– and, more marginally, NO2–. However, [AC-5-seco-Cbs]+ has a higher affinity for N3– and S2O32– than [ACCbs]+. These trends correlate with the position of the ligands in the spectrochemical series (N3–

Six new mononuclear Fe(II) complexes of the [FeL(NCE)2] family have been synthesized: L = N,N‘-dimethyl-N,N‘-bis-(quinoline-2-ylmethyl)ethane-1,2-diamine (2MeLquinoline), E = S (1), Se (2), BH3 (3) and L = N,N‘-dibenzyl-bis-(pyridin-2-ylmethyl) ethane-1,2-diamine (DibenzylL), E = S (4a and 4b), Se (5), and BH3 (6). Introduction of quinolyl groups in 1–3 results in elongated Fe–N(quinolyl) distances and severe distortion of the coordination environment. The quinolyl and benzyl groups in the tetradentate ligand hamper a dense crystal packing and only weak van der Waals interactions were found for all these complexes. 1, 2, 3, 4b and 5 are all stabilized in a high spin (HS) state over all temperatures. 4a, a polymorph of 4b, undergoes a spin crossover (SCO) around 81 K. 6 is also SCO-active and the conversion curve is centred at 183 K. A comparison of 23 analogous [FeL(NCE)2] complexes reported so far shows that both the ligand field strength of the NCE− co-ligands and the Fe–NC(E) angle play a determinant role in T1/2. The replacement of the tetradentate ligand (confined to those of the same type as 2MeLquinoline and 2MeLquinoline) does not affect the first coordination sphere notably, except in the case of a highly steric ligand 2MeLquinoline, and hence has a slight effect on the temperature of SCO.

Designing efficient Fe(ii) chromophores requires optimization of numerous, at times conflicting, properties. It has been suggested that replacement of polypyridine ligands with cyclometalated analogs will be effective at destabilizing the quintet state and therefore extending the lifetime of photoactive metal-to-ligand charge transfer states. However, cyclometalated Fe(ii) complexes are not oxidatively stable due to the strong electron-donating nature of this ligand, which limits their applicability. Here we use density functional theory calculations to show how simple addition of nitro and carboxylic acid groups to these cyclometalated complexes can engender a less oxidizable Fe(ii) center while maintaining, or even improving, the favorable ligand field strength.

peer-reviewed A reversible temperature-dependent high-spin to low-spin transition with T1/2¼ 60 C has been observed in the resonance Raman spectra of the equilibrium reduced and photoreduced heme a3 of the thermophilic ba3 heme–copper oxidoreductase. The transition is based on the frequency shifts of the spin-state marker bands n2 (CbCb) and n10 (CaCm) and is attributed to the displacement of the heme iron along the heme normal as a consequence of the Fe–Np repulsion at temperature below 40 C which will increase the ligand field strength forcing the pairing of d electrons into the lower energy orbitals. PUBLISHED peer-reviewed

Through in situ, high-pressure X-ray diffraction experiments we have shown that the homoleptic perovskite-like coordination polymer [(CH3)2NH2]Cu(HCOO)3 undergoes a pressure-induced orbital reordering phase transition above 5.20 GPa. This transition is distinct from previously reported Jahn–Teller switching in coordination polymers, which required at least two different ligands that crystallize in a reverse spectrochemical series. We show that the orbital reordering phase transition in [(CH3)2NH2]Cu(HCOO)3 is instead primarily driven by unconventional octahedral tilts and shifts in the framework, and/or a reconfiguration of A-site cation ordering. These structural instabilities are unique to the coordination polymer perovskites, and may form the basis for undiscovered orbital reorientation phenomena in this broad family of materials., Unconventional octahedral tilts unique to the coordination polymer perovskites support a high-pressure structural phase transition in [(CH3)2NH2]Cu(HCOO)3, which results in Jahn–Teller axis directional switching and orbital reordering.

The [FeN6] chromophores found in [Fe(didentate)3]2+ complexes, where didentate is a non-symmetrical 2-(6-membered-heterocyclic ring)-benzimidazole ligand (Lk), exist as mixtures of two geometrical mer (C1-symmetry) and fac (C3-symmetry) isomers. Specific alkyl-substituted six-membered heterocyclic rings connected to the benzimidazole unit (pyridines in ligands L1-L3, pyrazines in L4-L5 and pyrimidines in L6-L7) control the ligand field strength and the electron delocalization so that [FeII(Lk)3]2+ display tunable thermally-induced spin transitions in solution. Thermodynamic, spectroscopic (UV-Vis, NMR) and magnetic studies in solution demonstrate that [Fe(L6)3]2+ (L6 = 1-methyl-2-(pyrimidin-2-yl)-1H-benzo[d]imidazole) exhibits a close to room temperature spin transition (T1/2 = 273(3) K) combined with a high stability formation constant ( in acetonitrile), which makes this complex suitable for the potential modulation of lanthanide-based luminescence in polymetallic helicates. A novel method is proposed for assigning specific thermodynamic spin crossover parameters to fac-[Fe(L6)3]2+ and mer-[Fe(L6)3]2+ isomers in solution. The observed difference relies mainly on the entropic content ΔS-ΔS = 11(1) J mol-1 K-1, which favors the spin transition for the meridional isomer. Intermolecular interactions occurring in the crystalline state largely overcome minor thermodynamic trends operating in diluted solutions and a single configurational isomer is usually observed in the solid state. Among the thirteen solved crystal structures 1-13 containing the [M(Lk)3]2+ cations (M = Fe, Ni, Zn, Lk = L6-L7), pure meridional isomers are observed six times, pure facial isomers also six times and a mixture (44% mer and 56% fac) is detected only once. Solid-state magnetic data recorded for the FeII complexes show the operation of slightly cooperative spin transitions in 7 (fac-[Fe(L6)3]2+) and 12 (mer-[Fe(L7)3]2+). For the meridional isomer in 6, a two-step spin state transition curve, associated with two phase transitions, is detected.

The combination of π‐donating amido with π‐accepting pyridine coordination units in a tridentate chelate ligand causes a strong nephelauxetic effect in a homoleptic CrIII complex, which shifts its luminescence to the NIR‐II spectral range. Previously explored CrIII polypyridine complexes typically emit between 727 and 778 nm (in the red to NIR‐I spectral region), and ligand design strategies have so far concentrated on optimizing the ligand field strength. The present work takes a fundamentally different approach and focusses on increasing metal–ligand bond covalence to shift the ruby‐like 2E emission of CrIII to 1067 nm at 77 K., Chromium(III) polypyridyl complexes typically emit in the red or NIR‐I spectral region. We report a chromium(III) complex, which emits at 1067 nm at 77 K upon excitation at 450 nm.

We present a unique investigation of a mononuclear cis-dicyanamido-cobalt(ii) complex, [Co(L)(N(CN)2)2] (1) (L = N,N'-di-tert-butyl-2,11-diaza[3,3](2,6)pyridinophane), which displays a reversible thermo-induced HS (S = 3/2) to LS (S = 1/2) Spin Crossover (SCO) with a T1/2 value of around 238 K. The appropriate ligand field strength around the CoN6 coordination environment is obtained using a tetradentate ligand and two dicyanamide [N(CN)2]- coligands at the cis position. Variable temperature XRD and several spectroscopic studies have been performed, which confirm the SCO behavior of 1. Interestingly, complex 1 pronounces redox stimulus responsive reversible spin-state switching from HS Co(ii) to LS Co(iii). These findings represent a rare example of the successful use of pseudohalide to design and synthesize a SCO cobalt(ii) complex.

Two types of asymmetric binuclear platinum(II) complexes (Pt-1 and Pt-3) bearing bridging ligands of 2-(2,4-difluorophenyl)-5-(pyridin-2-yl)pyridine and 2-(2,4-difluorophenyl)-4-(pyridin-2-yl)pyridine as well as their corresponding mononuclear counterparts (Pt-2, Pt-4, and Pt-5) were synthesized and characterized. Different chelating constructions of the second platinum(II) ions and the bridging ligands in Pt-1 and Pt-3 gave rise to two kinds of electron-transition pathway during their photophysical processes. The meta-/para-carbon of nitrogen on the center pyridyl segments set different levels of ligand field strength to the second platinum(II) ions, lowering their occupied d orbital to varying degrees. Pt-1 showed an enhanced spin–orbit coupling (SOC), caused by the additional metal component through direct orbital hybridization at higher states, where the fixed molecular skeleton induced by the additional metal–ligand bonding also helped to suppress molecular distortion in the excited state, ensuring a high quantum yield (Φ, 0.89 in toluene), which is among the best results in bimetallic complexes. While the second platinum(II) ion in Pt-3 seemed to make no contribution to the radiative transition, and only contributed to the HOMO, it provided a benefit by enlarging the conjugate system. Solution-processed organic lighting emitting devices (OLEDs) fabricated with the bimetallic Pt-1 emitter achieved superior efficiencies and up to 21% external quantum efficiency (EQE) in the Kelly-green region.

The reaction of Fe(NCS)3 prepared in situ in MeOH with 5-X-SalEen ligands (5-X-SalEen=condensation product of 5-substituted salicylaldehyde and N-ethylethylenediamine) provided three Fe(III) complexes, [Fe(5-X-SalEen)2 ]NCS; X=Me (1), X=Br (2), X=OMe (3). All the complexes reveal similar structural features but a very different magnetic profile. Complex 1 shows a gradual spin crossover while complexes 2 and 3 show a sharp spin transition. The T1/2 for complex 2 is 237 K while for complex 3 it is much higher with a value of 361 K. The spin transition temperature is shifted towards higher temperature with increasing electron-donation ability of the ligand substituents. This experimental observation has been rationalized with DFT calculations. UV-Vis and cyclic voltammetry studies support the fact that the electron density on the ligand increases from Me to Br to OMe substituents. To understand the change in spin states, temperature-dependent EPR spectra have been recorded. The spin state equilibrium in the liquid state has been probed with Evans NMR spectroscopic method, and thermodynamic parameters have been evaluated for all complexes.

The quintet-singlet energy difference (Δ EQ/S) in Fe(II) polypyridine complexes is often interpreted in terms of metal-ligand π interactions. DFT calculations on a series of substituted [Fe(bpy)3]2+ (bpy = 2,2'-bipyridine) complexes show the disparate magnitudes of substituent effects on tuning Δ EQ/S and reduction potentials ( E°). In this series, E° spans a much larger range than Δ EQ/S (2.07 vs 0.29 eV). While small changes in Δ EQ/S are controlled by metal-ligand π interactions, large changes in E° arise from modification of the electrostatic environment around the Fe center. Molecular orbital analysis reveals that, contrary to the typical description of bpy as a π-acceptor, bpy is better described as acting as both a π-donor and π-acceptor in [Fe(bpy)3]2+ complexes, even when it is substituted with highly electron withdrawing substituents. Overall, substituent modification is a useful strategy for fine-tuning the ligand field strength but not for significant reordering of the spin-state manifold, despite the large effect on metal-ligand electrostatic interactions.

Crystal structures, single-molecule magnetic behavior, and ab initio calculations of four new phenoxo-bridged dinuclear dysprosium complexes and their gadolinium(III) analogues are explored. Complexes [Dy2 (DMOMP)2 (DBM)4 ]2 ⋅CHCl3 (1; DMOMP=1-methyl-3,5-dimethoxy-4-hydroxybenzene, DBM=1,3-diphenylpropane-1,3-dione); [Dy2 (DMOAP)2 (DBM)4 ]2 ⋅CHCl3 (2; DMOAP=syringaldehyde); Dy2 (DMOEP)2 (DBM)4 (3; DMOEP=methyl syringate); and solvent-free Dy2 (DMOMP)2 (DBM)4 (4), which is obtained by the transformation of single crystal into single crystal from 1, have nearly identical core structures and only differ in the substituents at the para position of the phenol moieties of the bridging ligand. In this system, the electronic effects are efficiently implemented to significantly modify the ligand field strength and exchange coupling by modulating the substituents on the phenol backbone. The effective energy barrier (Ueff ) of magnetization reversal is improved significantly to fivefold magnitude, at most, and the hysteresis temperature up to 3.5 K by deliberately using the electron-withdrawing substituent to replace the electron-donating one. The origin of the two relaxation processes in 1 is mostly attributed to the existence of two molecules in one unit, which is illuminated by means of the transformation of single crystal into single crystal.

Homoleptic complexes [Fe(Ln)]X2 (L1 = 1,1′-(α,α′-o-xylyl)-2,2′-biimidazole, L2 = 1,1′-(α,α′-3,4-dibromo-o-xylyl)-2,2′-biimidazole, L3 = 1,1′-(α,α′-2,5-dimethoxy-o-xylyl)-2,2′-biimidazole; X = BF4– or ClO4–) were synthesized by direct reactions of the Fe(II) precursor salts and bidentate ligands L1, L2, or L3. All mononuclear complexes undergo gradual temperature-driven spin-crossover (SCO) between the high-spin (HS, S = 2) and low-spin (LS, S = 0) states. Complexes with ligands L1 and L2 synthesized in methanol exhibit complete SCO with the midpoint of the LS↔HS conversion varying from 233 to 313 K, while complexes with ligand L3, crystallized from an ethanol/dichloromethane mixture, exhibit incomplete SCO with the residual HS/LS ratio of ∼1:4 for [Fe(L3)3](BF4)2 and ∼1:1 for [Fe(L3)3](ClO4)2. Complexes with L1 can also be recrystallized from ethanol/dichloromethane, in which case they exhibit very gradual and incomplete SCO, similar to those of the complexes with L3. The differences in magnetic behavior ...

The ligand field splitting Δt in a number of homoleptic tetrahedral complexes of Cu(I) and Ni(0) are determined from electronic structure calculations by DFT at the BP86/6-31+G(d,p) and BP86/LanL2DZ levels. Some ligands are monodentate (e.g. CO, NH3, Cl− etc.) and some are bidentate (e.g. ethylenediamine, 2,2′-bipyridine etc.). The Δt values obtained with the two basis sets for a given metal ion are comparable except for the phosphines. They correlate linearly with Shimura’s dCo parameters which are considered to be most effective for constructing a spectrochemical series. Earlier it was believed that the dCo parameters are applicable only to the octahedral complexes of cobalt(III). It is further shown that the dCo parameters bear a linear relationship with Chatt’s ligand constants PL. Implications are discussed.

Computational high-throughput screening is an essential tool for catalyst design, limited primarily by the efficiency with which accurate predictions can be made. In bulk heterogeneous catalysis, linear free energy relationships (LFERs) have been extensively developed to relate elementary step activation energies, and thus overall catalytic activity, back to the adsorption energies of key intermediates, dramatically reducing the computational cost of screening. The applicability of these LFERs to single-site catalysts remains unclear, owing to the directional, covalent metal–ligand bonds and the broader chemical space of accessible ligand scaffolds. Through a computational screen of nearly 500 model Fe(II) complexes for CH4 hydroxylation, we observe that (1) tuning ligand field strength yields LFERs by comparably shifting energetics of the metal 3d levels that govern the stability of different intermediates and (2) distortion of the metal coordination geometry breaks these LFERs by increasing the splittin...

A series of tetrahedral CoII complexes [CoLX2] (X = NCS (1), Cl (2), Br (3) and I (4); L = 9,9-dimethyl-4,5-bis(diphenylphosphino) xanthene) based on a P-donor ligand has been prepared to investigate the influence of terminal ligand field strength on the anisotropy of CoII single-ion magnets. It has been observed that heavier and softer terminal ligands are able to decrease the anisotropy of the tetrahedral CoII centers. Thorough analyses of experimental and theoretical studies show that all complexes have an easy-axis type magnetic anisotropy and slow relaxation behaviors of tetrahedral CoII centers. Detailed ab initio theory studies disclose that the changes in the ligand field strength imposed by the terminal ligands result in modifying the single ion anisotropy (D) of polyhedra 1-4. Furthermore, the isostructural ZnII analogue (5) has been prepared to examine the influence of dipolar interactions between adjacent CoII centres and magnetic dilution experiments were performed.

A precise regulation of the temperature of spin crossover (SCO) is an essential task to envisage practical applications of switchable materials. Here, we present three isostructural complexes with the formula [Fe(LCl)(NCE)2], where LCl is the tetradentate ligand N,N′-bis[(4-chloro-2-pyridyl)methyl]ethane-1,2-diamine and E = S (1-S), Se (1-Se), and BH3 (1-BH3). Single-crystal X-ray diffraction, magnetic measurements, Mossbauer spectroscopy and DSC measurements have been carried out to study their SCO properties. 1-S remains in a high spin state over all temperatures due to the weak ligand field strength imposed by NCS− ligands. The replacement of NCS− by NCSe− allows the occurrence of SCO around 177 K. Using NCBH3− instead allows a further increase of the conversion temperature (T1/2) by ca. 30 K. A literature survey of complexes containing NCE− ligands was carried out to examine their influence on the SCO properties. A 50–90 K difference in T1/2 was found between NCS− and NCSe− derivatives and a 30–50 K difference was found between NCSe− and NCBH3− derivatives.

In this work, a general, user-friendly method – ab initio ligand field theory (AILFT), is described and illustrated. AILFT allows one to unambiguously extract all ligand field parameters (the ligand field one-electron matrix V LFT , the Racah parameters B and C , and the spin-orbit coupling parameter ζ ) from relatively straightforward multi-reference ab initio calculations. The method applies to mononuclear complexes in d n or f n configurations. The method is illustrated using complete active space self-consistent field (CASSCF) and N-electron valence perturbation theory (NEVPT2) calculations on a series of well documented octahedral complexes of Cr III with simple ligands such as F − , Cl − , Br − , I − , NH 3 and CN − . It is shown that all well-known trends for the value of 10 Dq (the spectrochemical series) are faithfully reproduced by AILFT. By comparison of B and ζ for Cr III in these complexes with the parameters calculated for the free ion Cr 3+ , the covalency of the Cr-ligand bond can be assessed quantitatively (the non-relativistic and relativistic nephelauxetic effects). The variation of ligand field parameters for complexes of 3d, 4d and 5d elements is studied using MCl 6 3− (M = Cr III , Mo III , W III ) as model examples. As reflected in variations of 10 Dq , B and ζ across this series, metal-ligand covalency increases from CrCl 6 3− to MoCl 6 3− to WCl 6 3− . Using the angular overlap model, the one-electron parameters of the ligand field matrix are decomposed into increments for σ- and π- metal-ligand interactions. This allows for the quantification of variations in σ- and π-ligand donor properties of these ligands. Using these results, the well documented two-dimensional spectroscopic series for complexes of Cr III is quantitatively reproduced. Comparison of the results obtained using CASSCF and NEVPT2 reveals the importance of dynamic electron correlation. Finally, the limitations of the AILFT method for complexes with increasing metal-ligand covalency are analyzed and discussed.

The local structures and electron paramagnetic resonance (EPR) parameters for Cu2+ in 59B2O3–10K2O–(30−x)ZnO–xBaO–1CuO (BKZBC) glasses are theoretically investigated with distinct modifier BaO concentrations x (= 0, 6, 12, 18, 24 and 30 mol %). The clusters are found to undergo the relative tetragonal elongations of about 13.5 and 5.0% at zero and higher BaO concentrations. The concentration dependences of the measured d–d transition bands, g factors and A// are suitably reproduced from the Fourier type functions or sign functions of the relevant quantities with x by using only six adjustable parameters. The features of the EPR parameters and the local structures of Cu2+ are analysed in a consistent way by considering the differences in the local ligand field strength and electronic cloud admixtures around Cu2+ under addition of Ba2+ with the highest ionicity and polarisability. The present theoretical studies would be helpful to the researches on the structures, optical and EPR properties for the...

In this paper, we describe the synthesis, characterization and stability studies of new FeII complexes prepared with ligand N,N'-dimethyl-N,N'-bis-(4-nitro-pyridin-2-ylmethyl)-ethane-1,2-diamine (L₄²NO₂). The properties of these complexes, probed by magnetic, spectroscopic and electrochemical techniques, are compared to those of the parent complexes prepared with the unsubstituted ligand N,N'-dimethyl-N,N'-bis-(pyridin-2-ylmethyl)-ethane-1,2-diamine (BPMEN or L₄²). The para-functionalization of the pyridine moieties with a π-accepting nitro group increases the ligand field strength and the Lewis acidity of the metal center at the same time, without altering the complex stability.

This review is aimed at the current research progression of a unique class of Pt(II) metal complexes bearing at least one azolate-containing bidentate chelate. The azole fragment can link to a neutral heteroaromatic entity or another azole and form bidentate chelates, such as monoanionic 3-pyridyl-1H-pyrazole and derivatives, dianionic 3,3′-bi-1H-pyrazole, 3,3′-(1-methylethylidene)-bis-1H-pyrazole, and their analogs. These azole-containing chelates readily react with a variety of Pt(II) reagents to afford the corresponding bis-bidentate Pt(II) complexes. Most of them were highly emissive in solution, doped polymer matrix, thin film, and even as crystal or powder, due to the high ligand field strength exerted by these chelates and their high propensity in forming the singular square-planar architecture and intermolecular aggregates with substantially strengthened Pt⋯Pt interaction, according to their structural design. Therefore, they hold bright prospects in academic research and future optoelectronic applications such as organic light-emitting diodes.

We report the synthesis, crystal structure, and dielectric, vibrational and emission spectra of two novel heterometallic perovskite-type metal–organic frameworks (MOFs) of the following formula: [(CH3)2NH2][K0.5Cr0.5(HCOO)3] (DMAKCr) and [C2H5NH3][K0.5Cr0.5(HCOO)3] (EtAKCr). DMAKCr crystallizes in a trigonal structure (R space group) and undergoes an order–disorder phase transition to the monoclinic system (P space group) at about 190 K. The dielectric studies confirm the presence of first-order relaxor-like structural transformation. In the high-temperature phase, the dimethylammonium cations are dynamically disordered over three equal positions and upon cooling the dynamical disorder evolves into a two-fold one. This partial ordering is accompanied by a small distortion of the metal–formate framework. EtAKCr crystallizes in a monoclinic structure (P21/n space group) with ordered EtA+ cations and does not experience any phase transition. The differences in the thermal behavior caused by the substitution of Na+ ions by larger K+ ions in the [cat]MIMIII (cat = DMA+, EtA+, MI = Na+, K+ and MIII = Cr3+ and Fe3+) heterometallic MOF family are discussed taking into account the impact of the hydrogen bond (HB) pattern and other factors affecting the stability of metal–formate frameworks. The optical studies show that DMANaCr and EtAKCr exhibit Cr3+-based emission characteristics for intermediate ligand field strength.

Temperature-dependent optical absorption spectra have been recorded for Cr3+ doped silicate glasses between 10 K and 773 K. Due to the intermediate ligand field strength of the Cr3+ coordination environment, overlap of the lowest-energy spin-allowed 4A2 → 4T2 band and the two spin-forbidden transitions to the 2E and 2T1 states at approximately 650 nm results in interference dips. An analytical model equation is fitted to the spectra and electronic structure parameters extracted. This represents the first time that variations of these parameters with temperature are obtained over a large temperature range, revealing characteristics of all three overlapping d-d transitions. Keywords: Chromium(III) doped glasses, Absorption spectra, Interference dips, Temperature, Spectra calculations

The original magnetism of a model spin crossover complex [Fe(H2Bpz2)2(bipy)] (H2Bpz2 = dihydrobis(1-pyrazolyl)borate, bipy = 2,2′-bipyridine) has been altered from spin state switching to paramagnetic behavior, through grafting a weak electronic-donating group: bromomethyl (in 1) and Br (in 2) onto the bipy ligand at positions C5 and C5´. The introduction of the electron-donating bromo groups into the bipy ligand reduce the π-acceptor character, leading to a weaker ligand field strength, thus stabilizing the high-spin state.

The polarographic technique gives a convenient method for studying redox reactions. The complexes studied give well defined, diffusion controlled one electron transfer reduction waves. The waves are irreversible in behavior. In case of P+(II) complexes, a regular cathodic shift in half wave potential of changing over from the trans structural macro cycle to the is observed. The redox properties of polyaza macrocyclic ligands have been of continued interest. As a result of these studies several macro cyclic complexes have been used as oxidant, reductants and electro catalysts. Earlier work on the redox behavior of macrocyclic complexes had mainly dealt with unsaturated systems. Later studies have dealt with many types of saturated systems also. It is important to study the effect of ligand structure, chelate ring size, donar unsaturation, substituent pattern and relative position of five and six membered chelate rings. It has been shown that a decrease in the ligand field strength of the macrocyclic ligands conclude in an anodic shift in the oxidant and reduction potentials. A change in the nature of the ring substituents changes the redox potential. The changes in the redox potential in these complexes are influence the substituents on the metal nitrogen interaction. In this we discuss the polarographic behavior of the complexes of (L1) Me2 (ET4) (14) diene (C20H40N4), Me2 (ET4) (14) ane (C20H44N4), (L3) Me6 (ET4) (14) diene (C16H35N4) and, (L4) Me2 (14) ane (C16H36N4). The polarograms of the complexes in aqueous solution were marked on a Metrohm Polarecord 50 usig a dropping mercury electrode (d.m.e.) as the working device. To choose a correct supporting electrolyte, polarographic wave of a few complexes were recorded in different electrolytes. It was observed that the best waves were prepared in 0.1 M sodium perchlorate. The one electron transfer irreversible waves related to the reduction of Pt (II) to Pt (I) and Pd (II) to Pd (I). It is reasonable to assume that the Pt – N distance is the significant factor. It has been seen that a change in the symmetry of the ligand result in change in the ligand field strength.

The introduce of halogen atoms (F, Cl, Br, I) to the triazole-based ligand N-benzyl-4-amino-1, 2, 4-triazole (L) have shown a significant effect on the spin-crossover (SCO) properties of the corresponding one-dimensional (1D) coordination Fe (II) polymers with chemical formula [Fe(4-X–L)3](BF4)2 (X=F, Cl, Br, and I). Compared with the [FeL3](BF4)2, the electronegative halogen atoms have increased the π-acceptor character of these ligands, which thus improved the ligand field strength and induced the occurrence of SCO for the Fe (II) polymers at higher temperatures. The SCO transition temperatures T1/2 have changed from 200 K to 249 (I), 250 (Br), 252 (Cl) and 275 (F) K, respectively. More importantly, the UV-Vis absorption spectrum and density functional theory (DFT) calculations indicated that the transition temperatures and the width of thermal hysteresis loops have shown a positive correlation with the electronegativity of the halogen atoms.

Two unique trinuclear terpyridine-ruthenium complexes having similar components placed at different angels were successfully synthesized and fully characterized for the purpose of determining the photophysical and electronic properties. The corresponding UV–vis absorptions and similar electrochemical properties showed characteristic MLCT transitions in both complexes. However, complex C2 displayed an intense emission at 77 K, as expected, where highly packed complex C1 is almost non-emissive. This result may due to that a distinct configuration adopted by the complex impacts ligand field strength.