Forward Donation Research Articles

Impact of ligand fields on Kubas interaction of open copper sites in MOFs with hydrogen molecules: an electronic structural insight.

We investigate hydrogen sorption on open copper sites in various ligand coordinations of metal-organic frameworks (MOFs), including the triangular T(CuL3) in MFU-4l, the linear L(CuL2) in NU2100, and the paddlewheel P(CuL4)2 in HKUST-1 from an electronic structure perspective using DFT calculations. The ligand-field-induced splitting of d states and spd hybridizations in copper are thoroughly examined. The hybridization between Cu s, p, and d orbitals occurs in various forms to optimize the Coulomb repulsion of different ligand fields. Despite the Cu+ oxidation state, which is typically conducive to strong Kubas interactions with hydrogen molecules, the vacant spd z 2 hybrid orbitals of the open copper site in the L(CuL2) coordination are unsuitable for facilitating electron forward donation, thereby preventing effective hydrogen adsorption. In contrast, the vacant spd z 2 hybrid orbitals in the T(CuL3) and P(CuL4)2 coordinations can engage in electron forward donations, forming bonding states between the Cu spd z 2 and H2 σ bonding orbitals. The forward donation in the T(CuL3) configuration is significantly stronger than in the P(CuL4)2 configuration due to both the lower energy of the vacant orbitals and the larger contributions of p and d z 2 characters to the hybrid orbital. Additionally, the occupied Cu pd xz/yz and pd x 2-y 2 hybrid orbitals in the T(CuL3) configuration promote electron back donation to the H2 σ* antibonding orbital, leading to the formation of π bonding states. In the P(CuL4)2 coordination, the repulsion from the electron density distributed over the surrounding ligands prevents the H2 molecule from approaching the copper center closely enough for the back donation to occur. The complete Kubas interaction, involving both forward and back electron donations, results in a large dihydrogen-copper binding energy of 37.6 kJ mol-1 in the T(CuL3) coordination. In contrast, the binding energy of 10.6 kJ mol-1 in the P(CuL4)2 coordination is primarily driven by electrostatic interactions with a minor contribution of the Kubas-like forward donation interaction. This analysis highlights the pivotal role of coordination environments in determining the hydrogen sorption properties of MOFs.

Structural and electronic properties of the adsorption of nitric oxide molecule on copper clusters CuN(N = 1–7): A DFT study

The interconversion between different isomers of copper clusters has been investigated using density functional theory. Seven distinct transition states have been located for the isomerization of the Cu3, Cu4, Cu5, Cu6 and Cu7 clusters. The chemisorption of nitric oxide on copper clusters has been studied and the approach of nitric oxide to produce either the nitrosyl (–NO) or the isonitrosyl (–ON) complexes has been compared. The interaction of NO with the copper cluster complexes leads to significant odd–even oscillations of the spin density differences (ΔSD), mean polarizability (α¯) and HOMO-LUMO gaps. The computed binding energies and ionization potentials of the copper clusters are in excellent agreement with the experimental values. The binding energy, charge transfer, forward donation, υ(NO) and bond ellipticity values are higher in the nitrosyl complexes than their isonitrosyl counterparts. The relationship between the ΔSD and the NO stretching frequencies is approximately linear with high correlation coefficients of R2 = 0.94 and 0.96 for nitrosyl and isonitrosyl complexes, respectively. The stable copper clusters have been reoptimized at couple cluster singles and doubles method with Dunning double-ζ basis sets (CCSD/cc-pVDZ level).

Variational Forward-Backward Charge Transfer Analysis Based on Absolutely Localized Molecular Orbitals: Energetics and Molecular Properties.

To facilitate the understanding of charge-transfer (CT) effects in dative complexes, we propose a variational forward-backward (VFB) approach to decompose the overall CT stabilization energy into contributions from forward and backward donation in the framework of energy decomposition analysis based on absolutely localized molecular orbitals (ALMO-EDA). Such a decomposition is achieved by introducing two additional constrained intermediate states in which only one direction of CT is permitted. These two "one-way" CT states are variationally relaxed such that the associated nuclear forces can be readily obtained. This allows for a facile integration into the previously developed adiabatic EDA scheme, so that the molecular property changes arising from forward and back donation can be separately assigned. Using ALMO-EDA augmented by this VFB model, we investigate the energetic, geometric, and vibrational features of complexes composed of CO and main group Lewis acids (BH3, BeO/BeCO3) and complexes of the N2, CO, and BF isoelectronic series with [Ru(II)(NH3)5]2+. We identify that the shift in the stretching frequency of a diatomic π-acidic ligand (XY), such as CO, results from a superposition of the shifts induced by permanent electrostatics and backward CT: permanent electrostatics can cause an either red or blue shift depending on the alignment of the XY dipole in the dative complex, and this effect becomes more pronounced with a more polar XY ligand; the back-donation to the antibonding π orbital of XY always lowers the X-Y bond order and thus red-shifts its stretching frequency, and the strength of this interaction decays rapidly with the intermolecular distance. We also reveal that while σ forward donation contributes significantly to energetic stabilization, it affects the vibrational feature of XY mainly by shortening the intermolecular distance, which enhances both the electrostatic interaction and backward CT but in different rates. The synergistic effect of the forward and backward donations appears to be more significant in the transition-metal complexes, where the forward CT plays an essential role in overcoming the strong Pauli repulsion. These findings highlight that the shift in the XY stretching frequency is not a reliable metric for the strength of π back-donation. Overall, the VFB-augmented EDA scheme that we propose and apply in this work provides a useful tool to characterize the role played by each physical component that all together lead to the frequency shift observed.

Synthesis, Characterization, Antimicrobial Studies and Corrosion Inhibition Potential of 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane: Experimental and Quantum Chemical Studies.

The macrocylic ligand, 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane (MHACD) was synthesized by the demetallation of its freshly synthesized Ni(II) complex (NiMHACD). Successful synthesis of NiMHACD and the free ligand (MHACD) was confirmed by various characterization techniques, including Fourier transform infra-red (FT-IR), proton nuclear magnetic resonance (1H-NMR), carbon-13 nuclear magnetic resonance (13C-NMR), ultraviolet-visible (UV-vis), and energy dispersive X-ray (EDX) spectroscopic techniques. The anti-bacteria activities of MHACD were investigated against Staphylococcus aureus and Enterococcus species and the results showed that MHACD possesses a spectrum of activity against the two bacteria. The electrochemical cyclic voltammetry study on MHACD revealed that it is a redox active compound with promising catalytic properties in electrochemical applications. The inhibition potential of MHACD for mild steel corrosion in 1 M HCl was investigated using potentiodynamic polarization method. The results showed that MHACD inhibits steel corrosion as a mixed-type inhibitor, and the inhibition efficiency increases with increasing concentration of MHACD. The adsorption of MHACD obeys the Langmuir adsorption isotherm; it is spontaneous and involves competitive physisorption and chemisorption mechanisms. Quantum chemical calculations revealed that the energy of the highest occupied molecular orbital (HOMO) of MHACD is high enough to favor forward donation of charges to the metal during adsorption and corrosion inhibition. Natural bond orbital (NBO) analysis revealed the presence of various orbitals in the MHACD that are capable of donating or accepting electrons under favorable conditions.

Adsorption states of typical intermediates on Ag/Al2O3 catalyst employed in the selective catalytic reduction of NOx by ethanol

The adsorption of ethanol and important intermediates onto Ag/Al2O3 catalyst employed in the selective catalytic reduction of NOx by ethanol was simulated by density functional theory. Considering the interaction between Ag metal and Al2O3 support, typical Ag–O–Al entities, i.e., Ag–O–Altetra and Ag–O–Alocta, (tetra = tetrahedral and octa = octahedral refer to the coordination sites of Al), were selected as potential adsorption sites on the surface of the catalyst. Ethanol, and enolic and isocyanate species were preferentially adsorbed and activated by Ag–O–Altetra entities rather than by Ag–O–Alocta entities. The strong Lewis acidity of Altetra in the Ag–O–Altetra entity was very important, enabling the entity to accept an electron via forward donation from either the C–O σ bond in ethanol or the N–C σ bond in the –NCO species. Moreover, the hybridization of the Ag and Al orbitals was critical for electron back donation from the Ag–O–Altetra entity to the C–C π bond in the enolic species or N–C π bond in the –NCO species. The significant activation of the N–C bond in –NCO on the Ag–O–Altetra sites facilitated cleavage of –NCO to form N2. Thus, we can conclude that the acidity of the Al site and the interaction between Ag and Al play key roles in the selective catalytic reduction of NOx by ethanol over Ag/Al2O3.

Room Temperature Hydrogen Physisorption on Exposed Metals in A Highly Cross‐Linked Organo‐Iron Complex

Room Temperature Hydrogen Physisorption on Exposed Metals in A Highly Cross‐Linked Organo‐Iron Complex

Selective Adsorption of Organosulfur Compounds from Transportation Fuels by π‐Complexation

Ab initio molecular orbital (MO) calculations were performed on the adsorption bond energies between the main sulfur compounds and the Cu+ on CuCl and CuY zeolite, for desulfurization of transportation fuels by π‐complexation sorbents. The relative adsorption bond energies of these compounds were measured by the elution order, based on the breakthrough curves of these compounds, from a column of CuY zeolite using commercial diesel and gasoline as the influents. The order from the elution followed: 4,6‐dimethyldibenzothiophene ≥ dibenzothiophene > benzothiophene ≥ 2‐methylthiophene > thiophene. The order is in agreement with that predicted from the calculations. The calculated values for benzene and thiophene were also in excellent agreement with the measured values that we reported earlier. The methyl and benzo groups have an electron‐donating effect on the aromatic rings that undergo π‐complexation. For the sulfur compounds, the thiophene ring was bonded to Cu+. For π‐complexation on both CuY and CuCl, the amount of electron forward donation was more than that of electron back donation for thiophenic adsorbates, but the reverse was true for benzene and toluene.

High-valent diphenylacetylene complexes of tungsten

The W(4f7//2) binding energy of [{WCl4(PhC2Ph)}2]1 obtained by X-ray photoelectron spectroscopy is similar to that of [{WCl4(NPh)}2] and is consistent with a do tungsten(VI) formulation. The reaction of complex 1 and [NEt4][WCl5(PhC2Ph)]2 with NaOH–EtOH gave cis-stilbene, indicating considerable electron transfer from the metal to the co-ordinated alkyne. Reduction of complex 1 with 2 equivalents of sodium–mercury amalgam in the presence of phosphines gave the complexes [WCl3(PhC2Ph)L2](L = PMe3, PMe2Ph or PMePh2) with magnetic moments and W(4f7//2) binding energies similar to those of the d1 tungsten(V) organoimido complex [WCl3(NPh)(PMe3)2]. Decomposition of the alkyne complexes with NaOH–EtOH again gave cis-stilbene. The crystal structure of [WCl3(PhC2Ph)(PMe3)2]3 has been determined. The W–Cl bond trans to the alkyne ligand is long [2.479(3)Å], and the W–C bond lengths [2.011(13) and 2.038(12)Å] indicate a four-electron-donor alkyne ligand. The geometry is similar to that of [WCl3(NPh)(PMe3)2]. Reduction of [{WCl4(PhC2Ph)}2] using 4 equivalents of sodium–mercury amalgam in the presence of phosphines gave the complexes [WCl2(PhC2Ph)L3](L = PMe3 or PMe2Ph) which again produced cis-stilbene on decomposition with NaOH–EtOH. The acetylenic carbon resonance at δ 223.15 in the 13C-{1H} NMR spectrum of [WCl2(PhC2Ph)(PMe3)3]6 is also indicative of a four-electron-donor alkyne ligand. Its W(4f7//2) binding energy is similar to [WCl2(NPh)(PMe3)3] and is consistent with tungsten(IV). A crystal structure of complex 6 shows a similar ligand geometry to [WCl2(NPh)(PMe3)3], and the W–C bond lengths [2.019(11) and 2.006(11)Å] indicate a four-electron-donor alkyne ligand. Hartree–Fock and scattered wave Xα calculations have been performed on the model complexes [WCl5(HC2H)]–8, [WCl3(HC2H)(PH3)2]9 and [WCl2(HC2H)(PH3)3]10. Molecular orbital and population analyses indicated that the acetylene–tungsten bond in each involves W(5dπ)→ HC2H(π*) back donation as well as HC2H(π)→ W(5dσ) and HC2H(π⊥)→ W(5dπ) forward donation, consistent with a four-electron-donor alkyne formalism. Electron withdrawal from the tungesten to the more electronegative Cl ligand in complexes 8 and 9 is minimised by rotation of the alkyne away from the meridional vectors. In complex 10 the HC2H(π⊥)→ W(5dπ) donation and phosphine contributions compensate and no rotation is observed. The total d atomic orbital population of complex 8 is close to that of WCl6, and populations of complexes 9 and 10 step up linearly from this. The computational results support the experimental evidence that [{WCl4(PhC2Ph)}2]1, [WCl3(PhC2Ph)(PMe3)2]3 and [WCl2(PhC2Ph)(PMe3)3]6 are complexes of tungsten-(VI), -(V) and -(IV) respectively.

Activation of Small Molecules by Transition Metal Ions and their Complexes

A QUALITATIVE, symmetry based view of the nature of the bonding in so-called π-complexes of unsaturated organic molecules with transition metals was put forward by Dewar1 and Chatt2. Simultaneous charge transfer from filled ligand orbitals to unfilled metal orbitals and from essentially non-bonding metal orbitals to antibonding ligand orbitals forms the basis of a model which successfully explains, for example, the change in the C—O stretching frequency of co-ordinated carbon monoxide compared with that of the free ligand. The relative importance of “forward donation” and “back-donation” is not a matter for generalization; a theory of the bonding in Zeise's salt suggests that they are comparable3 even for such a relatively poor acceptor as ethylene.