Supramolecular Chemistry of Halogens: Complementary Features of Inorganic (M−X) and Organic (C−X‘) Halogens Applied to M−X···X‘−C Halogen Bond Formation

Electronic differences between inorganic (M−X) and organic (C−X) halogens in conjunction with the anisotropic charge distribution associated with terminal halogens have been exploited in supramolecular synthesis based upon intermolecular M−X···X‘−C halogen bonds. The synthesis and crystal structures of a family of compounds trans-[MCl2(NC5H4X-3)2] (M = Pd(II), Pt(II); X = F, Cl, Br, I; NC5H4X-3 = 3-halopyridine) are reported. With the exception of the fluoropyridine compounds, network structures propagated by M−Cl···X−C halogen bonds are adopted and involve all M−Cl and all C−X groups. M−Cl···X−C interactions show Cl···X separations shorter than van der Waals values, shorter distances being observed for heavier halogens (X). Geometries with near linear Cl···X−C angles (155−172°) and markedly bent M−Cl···X angles (92−137°) are consistently observed. DFT calculations on the model dimers {trans-[MCl2(NH3)(NC5H4X-3)]}2 show association through M−Cl···X−C (X ≠ F) interactions with geometries similar to experimental values. DFT calculations of the electrostatic potential distributions for the compounds trans-[PdCl2(NC5H4X-3)2] (X = F, Cl, Br, I) demonstrate the effectiveness of the strategy to activate C−X groups toward halogen bond formation by enhancing their electrophilicity, and explain the absence of M−Cl···F−C interactions. The M−Cl···X−C halogen bonds described here can be viewed unambiguously as nucleophile−electrophile interactions that involve an attractive electrostatic contribution. This contrasts with some types of halogen−halogen interactions previously described and suggests that M−Cl···X−C halogen bonds could provide a valuable new synthon for supramolecular chemists

Supramolecular Chemistry of Halogens: Complementary Features of Inorganic (M−X) and Organic (C−X‘) Halogens Applied to M−X···X‘−C Halogen Bond Formation

Electronic differences between inorganic (M−X) and organic (C−X) halogens in conjunction with the anisotropic charge distribution associated with terminal halogens have been exploited in supramolecular synthesis based upon intermolecular M−X···X‘−C halogen bonds. The synthesis and crystal structures of a family of compounds trans-[MCl2(NC5H4X-3)2] (M = Pd(II), Pt(II); X = F, Cl, Br, I; NC5H4X-3 = 3-halopyridine) are reported. With the exception of the fluoropyridine compounds, network structures propagated by M−Cl···X−C halogen bonds are adopted and involve all M−Cl and all C−X groups. M−Cl···X−C interactions show Cl···X separations shorter than van der Waals values, shorter distances being observed for heavier halogens (X). Geometries with near linear Cl···X−C angles (155−172°) and markedly bent M−Cl···X angles (92−137°) are consistently observed. DFT calculations on the model dimers {trans-[MCl2(NH3)(NC5H4X-3)]}2 show association through M−Cl···X−C (X ≠ F) interactions with geometries similar to experimental values. DFT calculations of the electrostatic potential distributions for the compounds trans-[PdCl2(NC5H4X-3)2] (X = F, Cl, Br, I) demonstrate the effectiveness of the strategy to activate C−X groups toward halogen bond formation by enhancing their electrophilicity, and explain the absence of M−Cl···F−C interactions. The M−Cl···X−C halogen bonds described here can be viewed unambiguously as nucleophile−electrophile interactions that involve an attractive electrostatic contribution. This contrasts with some types of halogen−halogen interactions previously described and suggests that M−Cl···X−C halogen bonds could provide a valuable new synthon for supramolecular chemists

Understanding the Behavior of Halogens as Hydrogen Bond Acceptors

The similarities and differences between the behavior of carbon-bound and terminal metal-bound halogens and halide ions as potential hydrogen bond acceptors has been extensively investigated through examination of many thousands of interactions present in crystal structures. Halogens in each of these environments are found to engage in hydrogen bonding, and geometric preferences for these interactions have been established. Notably, typical H···X−M angles are markedly different for X = F than for X = Cl, Br, I. Furthermore, there are significant parallels between the behavior of moderately strong hydrogen bond acceptors X−M and the much weaker acceptors X−C. The underlying reasons for the observed geometric preferences have been established by ab initio molecular orbital calculations using suitable model systems. The results are presented within the context of their potential applications in crystal engineering and supramolecular chemistry, including relevance to nucleation in halogenated solvents. The broader implications of the results in areas such as halocarbon coordination chemistry, binary metal halide solid-state chemistry, and the study of weakly coordinating anions are also discussed

Strengthening of N−H···Co Hydrogen Bonds upon Increasing the Basicity of the Hydrogen Bond Acceptor (Co)

Low temperature crystal structures of (DABCO)H+Co(CO)4- (1) and (DABCO)H+Co(CO)3PPh3- (2) (DABCO = 1,4-diazabicyclooctane) indicate that both salts exhibit N−H···Co hydrogen bonding. IR and NMR data indicate that these hydrogen bonded species persist in nonpolar solvents such as toluene, but exist as solvent separated ions in more polar solvents. Replacement of the axial CO ligand by PPh3 leads to a shortening of the N···Co separation in the solid state from 3.437(3) to 3.294(6) Å. This change is accompanied by an increase in the angle between the equatorial carbonyl ligands. Thus, the crystallographic results suggest a strengthening of the N−H···Co hydrogen bond upon increasing the basicity of the metal center, the first observation of this type in the solid state. This assertion is supported by variable-temperature 1H and 13C NMR data in toluene-d8 solution which, discussed in the light of ab initio calculations, indicate that the barrier to a fluxional process involving cleavage of the N−H···Co hydrogen bond is greater in 2 than in 1. The crystal structures of 1 and 2 have been determined by X-ray diffraction at 135(5) and 123(5) K, respectively [1 monoclinic, P21/n (No. 14), a = 8.728(2), b = 23.333(5), c = 12.146(2) Å, β = 95.74(2)°, V = 2461.1(9) Å3, Z = 8, R(F) = 0.043, Rw(F) = 0.043, S(F) = 1.21; 2 orthorhombic, Pca21 (No. 29), a = 16.084(8), b = 8.874(3), c = 17.312(3) Å, V = 2471(1) Å3, Z = 4, R(F) = 0.065, Rw(F) = 0.060, S(F) = 1.16]

Hybrid-DFT Modeling of Lattice and Surface Vacancies in MnO

We have investigated the properties of defects in MnO bulk and at (100) surfaces, as used in catalytic applications, using hybrid-level density functional theory (i.e., inclusion of exact exchange within the exchange-correlation evaluation) in a hybrid QM/MM embedded-cluster approach. Initially, we calculate the formation energy for bulk Mn and O vacancies, comparing charged-defect compensation with charge carriers at the Fermi Level (ϵf) and through Schottky defect formation. Oxygen vacancies were also investigated at the (100) surface, where the vacancy formation energy is very similar to the bulk. Defect levels associated with the most stable vacancies are calculated using the ΔSCF method: all are positioned mid band gap, with surface environments failing to alter strongly the overall nature of the defect relative to bulk. Chemical activity of the (100) MnO surface was considered through the adsorption of a probe CO2 molecule, which is considered the initial step in the transformation of CO2 into hydrocarbons. CO2 adsorption was strongest over a neutral oxygen vacancy, where the associated trapped electrons of the defect transfer to the adsorbate and thus activate it. However, we have shown with our embedded-cluster approach that the neutral oxygen vacancy is not necessarily the dominant species, which has implications when interpreting results for future catalytic applications

Limits to doping of wide band gap semiconductors

The role of defects in materials is one of the long-standing issues in solid-state chemistry and physics. On one hand, intrinsic ionic disorder involving stoichiometric amounts of lattice vacancies and interstitials is known to form in highly ionic crystals. There is a substantial literature on defect formation and the phenomenological limits of doping in this class of materials; in particular, involving the application of predictive quantum mechanical electronic structure computations. Most wide band gap materials conduct only electrons and few conduct holes, but rarely are both modes of conduction accessible in a single chemical system. The energies of electrons and holes are taken from the vertical ionization potentials and electron affinities; polaronic trapping of carriers is excluded. While the focus here is defect energetics, the atomic and electronic structures have been carefully examined in all cases to ensure physical results were obtained.</p

Limits to Doping of Wide Band Gap Semiconductors

Limits to Doping of Wide Band Gap Semiconductor

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