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The electronic structure and related electronic properties of two novel isostructural octacyanometallates Cd2(H2O)4[MoIV(CN)8]·2H2O and Cd2(H2O)4[WIV(CN)8]·2H2O are described from a combined experimental and computational approach. The impact that the octacoordinated heavy metals W and Mo have on the electronic structure and optical response of isostructural materials whose electronic properties strongly depend on the crystal structure is discussed. It is found that the effect of the polarization power of the metal centers, combined with the ligand field of cyanos, produces considerable changes in the electronic structure and, consequently, in the band gap energy. The ab initio calculations, which were performed with generalized gradient PBE and hybrid HSE06 density functionals, accurately reproduce the electronic structure of [Mo(CN)8]4− and [W(CN)8]4− building units, revealing that the electronic transitions associated with the band gap energy have an origin in the charge transfer phenomena of metal to ligand nature. Moreover, the optical band gap transitions have an allowed indirect behavior in the Γ → M → D direction which is associated with the (x2 − y2) → π* transition. The allowed direct and indirect (experimental and theoretical) band gap energies together with the exciton effective masses are reported. [ABSTRACT FROM AUTHOR]

The electronic structure and derived optical properties of five synthesized metal-dicyanoaurate(I), (K)M[Au(CN)2], (M = Mn, Co, Ni, Zn and Cd), coordination polymers are described from a combined experimental analysis and theoretical study based on density functional theory. In this sense, the topological features that influence the electronic structure, which in turn give rise to electronic transitions associated with the band gap energy, are studied from first principles calculations (with hybrid HSE06 and GGA-PBE density functionals) and electronic spectroscopy. The impact of gold (through spin-orbit coupling) and aurophilic interactions on the electronic transitions that gives rise to optical properties is described. The calculated projected density of states and band dispersion diagrams shed light on the molecular orbital distribution and the role of a dicyanoaurate(I) molecular block as the origin of the optical properties. Infrared, Raman and ultraviolet-visible spectroscopic analyses reveal the effect that charge transfer interactions, of a metal → ligand and metal → metal nature, have on the electronic behavior within the solids through association with the polarizing power of transition metals and gold atoms. [ABSTRACT FROM AUTHOR]

Pillared ferrous nitroprussides form an interesting series of coordination polymers where the spin crossover (SCO) behavior has been observed for several pillar organic molecules. This contribution reports such a reversible spin transition for the titled composition. The high-to-low spin transition (HS → LS) involves an electron density redistribution in the iron atom and its coordination environment, which is probed by magnetic (SQUID) measurements, IR, Raman, Mössbauer spectra, and DSC curves. The Mössbauer spectra recorded at 300 and 5 K disclose why in pillared ferrous nitroprussides, the SCO is possible despite the repulsive NO–NC electrostatic interaction in the interlayer region and the low electron density found at the CN5σ orbital of the equatorial ligands. On the sample cooling and related unit cell contraction, the NO–NC repulsive interaction enhances, and electron density is returned from the NO group and axial CN to the iron atom in the nitroprusside ion. This effect is particularly reinforced during the HS → LS transition. The increase of the electron density at the iron atom results in a stronger π-back donation interaction with the equatorial CNs to increase the charge accumulated at their CN5σ orbitals. This makes possible the formation of a stronger Fe–NC coordination bond, increasing the 10Dq value, and contributing to stabilizing the LS state of the iron atom. Such concerted electron density redistribution mechanisms, supported by IR and Mössbauer spectra, make possible the SCO in pillared ferrous nitroprussides. No previous study has been reported in that sense. [ABSTRACT FROM AUTHOR]

This contribution reports the preparation and thermally induced spin crossover (SCO) in the Hofmann-like Fe(3-hydroxypyridine)2[M(CN)4] series, where M = Ni, Pd, and Pt. These three 2D coordination polymers crystallize with a monoclinic unit cell, in the C2/m space group (No. 12). Their crystal structures were solved and refined from powder XRD patterns. The thermally induced SCO in that series was studied from magnetic measurements (SQUID), DSC curves, and Mössbauer, IR, and Raman spectra. In Hofmann-like analog solids, pyridine and its analogs are the ligands of choice to observe SCO, particularly for 3-substituted pyridines. For 4-substituted derivatives, SCO in tetracyanide-based coordination polymers is rare or not observed. The nature of that regularity is discussed in this contribution, from NBO calculations, XPS, IR, Raman, UV-vis-NIR, and Mössbauer spectra. The mentioned regularity is related to the distortion of the iron atom coordination polyhedron. A significant distortion is unfavorable for the formation of a strong coordination bond of the axial organic ligand to the iron atom, and at the same time, it reduces the available energy gap to stabilize the low-spin electronic configuration for the iron atom, making the SCO not possible. XPS, IR, Raman, UV-vis, and Mössbauer spectra provided conclusive clues in that sense. [ABSTRACT FROM AUTHOR]

This contribution reports the thermally induced spin crossover (SCO) in the titled series of hybrid solids from the magnetic (SQUID), DSC, IR, Raman, and Mössbauer data. The onset temperature for the high-to-low spin transition follows the order Cl (217 K) > Br (210 K) > I (155 K). This order is inverse to the one known for the halogen-bond propensity, Cl < Br < I. Their crystal structure for the high-spin (HS) phase was solved and refined from the powder XRD data, complemented with the IR spectra and TG curves. For the low-spin (LS) phase, the crystal structure was obtained by periodic DFT calculations, relaxing the room temperature (HS) structure. At room temperature, the CN–ON distance for these dangling ligands in the interlayer region follows the order Cl (5.03 Å) < Br (5.08 Å) < I (5.70 Å). The repulsive electrostatic interaction between these charge centers is responsible for a certain kinetic effect observed in the recorded magnetic data. This interaction was also detected in the recorded Mössbauer spectra. For the 3X-pyridine analogs, the thermally induced spin-crossover was observed only for X = F. This study discusses the nature of that difference in the behavior between the two subseries from three probes for the stabilization energy (SE) of the LS phase. Notably, (1) the effect of the substituent position on the deformation of the iron atom coordination environment; (2) the Mössbauer quadrupole splitting (ΔQS) value; and (3) the onset of the a1g → eg(eg) band absorption in the UV-vis-NIR spectrum. For a non-distorted octahedron, the value of SE coincides with 10Dq (the energy splitting between eg and t2g orbitals for the iron atom). This study is part of an effort to rationalize those structural features affecting the SCO in 2D ferrous nitroprussides. [ABSTRACT FROM AUTHOR]

The titled material crystallizes with an orthorhombic unit cell, in the P21212 space group (Nr. 18). Its crystal structure was solved and refined from powder XRD patterns. The solid framework is formed by stacked undulated sheets of inorganic nature, Fe[Fe(CN)5NO], separated by bimolecular organic pillars, [4-(3-Phenylpropyl)pyridine]2, which remain coordinated to the axial coordination sites for the iron atom. The molecules forming these pillars remain coupled through C–H⋯π and dispersive interactions between neighboring molecules. When this solid is cooled and then warmed, a reversible spin transition, from high to low spin (HS → LS), and vice versa, is observed. This transition occurs in the temperature interval of 135–165 K, with a hysteresis between them of about 30 K. That hysteresis loop appears with a pronounced asymmetry when the slopes for the HS → LS and LS → HS transitions are compared. This effect is discussed in terms of the related structural changes in the solid structure during the spin transitions. The transition was also monitored from IR and Raman spectra recorded at 80 and 300 K. Relevant information on the electronic structure for both, the LS and HS phases of this material, was derived from the corresponding XPS spectra recorded at 114 and 270 K. This contribution emphasizes the role of the nitrosyl group (NO) as an electron buffer for tuning the bonding properties of the inorganic layer at the CN 5σ orbital to make possible the observed thermally-induced SCO behavior. The thermally-induced spin transition in this solid shows an asymmetric hysteresis loop, which was ascribed to the nature of the pillar molecule. [ABSTRACT FROM AUTHOR]

A series of 2D coordination polymers show thermally induced spin-crossover where the temperature for the spin transition, according to the SQUID magnetic data, follows the order Ni < Pd < Pt. A similar order is observed from the enthalpy change (ΔH) during the spin transition, obtained from DSC curves. Both the magnetic and DSC measurements show a similar hysteresis. The thermally induced spin transition was also monitored using IR and Raman spectra, with the ν(CN) frequency probing the spin transition in the iron atom. These three coordination polymers crystallize with an orthorhombic unit cell, in the P21212 space group (Nr. 18). For Ni and Pd, the crystal structure was solved and refined from powder XRD patterns and calculated using a periodic DFT algorithm implemented in the VASP software package for Pt. Information on the structure for the LS phase in the series was derived from analog calculations by relaxing the structure for the HS phase. The observed order for the spin transition temperature was rationalized in terms of electron density found at the CN 5σ orbital, which determines the Fe–NC bond strength. Such electron-density results from the M → CN π-back bonding interaction. This interaction subtracts charge from the inner metal (M) and accumulates it at that orbital. The N1s core-level binding energy, which is obtained using the corresponding XPS spectrum, probes that charge relocation. An excellent correlation was observed between the spin transition temperature and the N1s binding energy. [ABSTRACT FROM AUTHOR]

This study aims to evaluate the potential of incorporating fly ash (FA) and municipal solid waste incinerator bottom ash (MIBA) as a partial substitute of cement in the production of self-compacting concrete mixes through an experimental campaign in which four replacement levels (i.e., 10% FA + 20% MIBA, 20% FA + 10% MIBA, 20% FA + 40% MIBA and 40% FA + 20% MIBA, apart from the reference concrete) were considered. Compressive and tensile strengths, Young's modulus, ultra-sonic pulse velocity, shrinkage, water absorption by immersion, chloride diffusion coefficient and electrical resistivity were evaluated for all concrete mixes. The results showed a considerable decline in both mechanical and durability-related performances of self-compacting concrete with 60% of substitution by MIBA mainly due to the aluminium corrosion chemical reaction. However, workability properties were not significantly affected, exhibiting values similar to those of the control mix. [ABSTRACT FROM AUTHOR]