434 research outputs found
EPR spectroscopy of iron- and nickel-doped [ZnAl]-layered double hydroxides: modeling active sites in heterogeneous water oxidation catalysts
Iron-doped nickel layered double hydroxides (LDHs) are among the most active heterogeneous water oxidation catalysts. Due to inter-spin interactions, however, the high density of magnetic centers results in line-broadening in magnetic resonance spectra. As a result, gaining atomic-level insight into the catalytic mechanism via electron paramagnetic resonance (EPR) is not generally possible. To circumvent spin-spin broadening, iron and nickel atoms were doped into non-magnetic [ZnAl]-LDH materials and the coordination environments of the isolated Fe(III) and Ni(II) sites were characterized. Multifrequency EPR spectroscopy identified two distinct Fe(III) sites (S = 5/2) in [Fe:ZnAl]-LDH. Changes in zero field splitting (ZFS) were induced by dehydration of the material, revealing that one of the Fe(III) sites is solvent-exposed (i.e. at an edge, corner, or defect site). These solvent-exposed sites feature an axial ZFS of 0.21 cmā»Ā¹ when hydrated. The ZFS increases dramatically upon dehydration (to -1.5 cmā»Ā¹), owing to lower symmetry and a decrease in the coordination number of iron. The ZFS of the other (āinertā) Fe(III) site maintains an axial ZFS of 0.19-0.20 cmā»Ā¹ under both hydrated and dehydrated conditions. We observed a similar effect in [Ni:ZnAl]-LDH materials; notably, Ni(II) (S = 1) atoms displayed a single, small ZFS (Ā±0.30 cmā»Ā¹) in hydrated material, whereas two distinct Ni(II) ZFS values (Ā±0.30 and Ā±1.1 cmā»Ā¹) were observed in the dehydrated samples. Although the magnetically-dilute materials were not active catalysts, the identification of model sites in which the coordination environments of iron and nickel were particularly labile (e.g. by simple vacuum drying) is an important step towards identifying sites in which the coordination number may drop spontaneously in water, a probable mechanism of water oxidation in functional materials
MāM Bond-Stretching Energy Landscapes for M_2(dimen)_(4)^(2+) (M = Rh, Ir; dimen = 1,8-Diisocyanomenthane) Complexes
Isomers of Ir_2(dimen)_(4)^(2+) (dimen = 1,8-diisocyanomenthane) exhibit different IrāIr bond distances in a 2:1 MTHF/EtCN solution (MTHF = 2-methyltetrahydrofuran). Variable-temperature absorption data suggest that the isomer with the shorter IrāIr distance is favored at room temperature [K = ~8; ĪHĀ° = ā0.8 kcal/mol; ĪSĀ° = 1.44 cal mol^(ā1) K^(ā1)]. We report calculations that shed light on M_2(dimen)_(4)^(2+) (M = Rh, Ir) structural differences: (1) metalāmetal interaction favors short distances; (2) ligand deformational-strain energy favors long distances; (3) out-of-plane (A_(2u)) distortion promotes twisting of the ligand backbone at short metalāmetal separations. Calculated potential-energy surfaces reveal a double minimum for Ir_2(dimen)_(4)^(2+) (4.1 Ć
IrāIr with 0Ā° twist angle and ~3.6 Ć
IrāIr with Ā±12Ā° twist angle) but not for the rhodium analogue (4.5 Ć
RhāRh with no twisting). Because both the ligand strain and A_(2u) distortional energy are virtually identical for the two complexes, the strength of the metalāmetal interaction is the determining factor. On the basis of the magnitude of this interaction, we obtain the following results: (1) a single-minimum (along the IrāIr coordinate), harmonic potential-energy surface for the triplet electronic excited state of Ir_2(dimen)_(4)^(2+) (R_(e,IrāIr) = 2.87 Ć
; F_(IrāIr) = 0.99 mdyn Ć
^(ā1)); (2) a single-minimum, anharmonic surface for the ground state of Rh_2(dimen)_(4)^(2+) (R_(e,RhāRh) = 3.23 Ć
; F_(RhāRh) = 0.09 mdyn Ć
^(ā1)); (3) a double-minimum (along the IrāIr coordinate) surface for the ground state of Ir_2(dimen)_(4)^(2+) (R_(e,IrāIr) = 3.23 Ć
; F_(IrāIr) = 0.16 mdyn Ć
^(ā1))
Iron Is the Active Site in Nickel/Iron Water Oxidation Electrocatalysts
Efficient catalysis of the oxygen-evolution half-reaction (OER) is a pivotal requirement for the development of practical solar-driven water splitting devices. Heterogeneous OER electrocatalysts containing first-row transition metal oxides and hydroxides have attracted considerable recent interest, owing in part to the high abundance and low cost of starting materials. Among the best performing OER electrocatalysts are mixed Fe/Ni layered double hydroxides (LDH). A review of the available experimental data leads to the conclusion that iron is the active site for [NiFe]-LDH-catalyzed alkaline water oxidation
The Nature of the Mn(III) Color Centers in Elbaite Tourmalines
The characteristic red color of many natural tourmalines is due to the presence of Mn(III) cations substituting for aluminum and lithium. These sites originate as Mn(II) and are oxidized by natural Ī³-irradiation over geologic time as they sit in the Earthās crust. Presented here is a thorough analysis of the spin-allowed and spin-forbidden transitions which give rise to the color of these gemstones. Ligand field analysis, supplemented by time-dependent density functional theory, was used to correct the historical assignments of the symmetry-allowed transitions in the polarized UVāvisible absorption spectrum. Heat-induced reduction of the oxidized manganese sites provided a probe of the relationship between the spin-allowed and spin-forbidden bands. Notably, the intensity of the spin-forbidden transition was highly dependent on the neighboring ions in the Y-site. Simulations and modeling showed that increased intensity was observed only when two Mn(III) ions occupied adjacent substitutions in the Y-site via a proposed exchange-coupling mechanism
The Nature of the Mn(III) Color Centers in Elbaite Tourmalines
The characteristic red color of many natural tourmalines is due to the presence of Mn(III) cations substituting for aluminum and lithium. These sites originate as Mn(II) and are oxidized by natural Ī³-irradiation over geologic time as they sit in the Earthās crust. Presented here is a thorough analysis of the spin-allowed and spin-forbidden transitions which give rise to the color of these gemstones. Ligand field analysis, supplemented by time-dependent density functional theory, was used to correct the historical assignments of the symmetry-allowed transitions in the polarized UVāvisible absorption spectrum. Heat-induced reduction of the oxidized manganese sites provided a probe of the relationship between the spin-allowed and spin-forbidden bands. Notably, the intensity of the spin-forbidden transition was highly dependent on the neighboring ions in the Y-site. Simulations and modeling showed that increased intensity was observed only when two Mn(III) ions occupied adjacent substitutions in the Y-site via a proposed exchange-coupling mechanism
Electronic Structures of Reduced and Superreduced Ir_2(1,8-diisocyanomenthane)_4^(n+) Complexes
Molecular and electronic structures of Ir_2(1,8-diisocyanomenthane)_4^(n+) (Ir(dimen)^(n+)) complexes have been investigated by DFT for n = 2, 1, 0 (abbreviated 2+, 1+, 0). Calculations reproduced the experimental structure of 2+, Ī½(Cā”N) IR, and visible absorption spectra of all three oxidation states, as well as the EPR spectrum of 1+. We have shown that the two reduction steps correspond to successive filling of the IrāIr pĻ orbital. Complexes 2+ and 1+ have very similar structures with 1+ having a shorter IrāIr distance. The unpaired electron density in 1+ is delocalized along the IrāIr axis and over N atoms of the eight Cā”Nā ligands. The second reduction step 1+ ā 0 changes the Ir(CNā)_4 coordination geometry at each Ir site from approximately planar to seesaw whereby one āNā”CāIrāCā”Nā moiety is linear and the other bent at the Ir (137Ā°) as well as N (146Ā°) atoms. Although complex 0 is another example of a rare (pĻ)2 dimetallic species (after [Pt_2(Ī¼-P_2O_5(BF_2)_2)_4]^(6ā), J. Am. Chem. Soc. 2016, 138, 5699), the redistribution of lower lying occupied molecular orbitals increases electron density predominantly at the bent Cā”Nā ligands whose N atoms are predicted to be nucleophilic reaction centers
EPR spectroscopy of iron- and nickel-doped [ZnAl]-layered double hydroxides: modeling active sites in heterogeneous water oxidation catalysts
Iron-doped nickel layered double hydroxides (LDHs) are among the most active heterogeneous water oxidation catalysts. Due to inter-spin interactions, however, the high density of magnetic centers results in line-broadening in magnetic resonance spectra. As a result, gaining atomic-level insight into the catalytic mechanism via electron paramagnetic resonance (EPR) is not generally possible. To circumvent spin-spin broadening, iron and nickel atoms were doped into non-magnetic [ZnAl]-LDH materials and the coordination environments of the isolated Fe(III) and Ni(II) sites were characterized. Multifrequency EPR spectroscopy identified two distinct Fe(III) sites (S = 5/2) in [Fe:ZnAl]-LDH. Changes in zero field splitting (ZFS) were induced by dehydration of the material, revealing that one of the Fe(III) sites is solvent-exposed (i.e. at an edge, corner, or defect site). These solvent-exposed sites feature an axial ZFS of 0.21 cmā»Ā¹ when hydrated. The ZFS increases dramatically upon dehydration (to -1.5 cmā»Ā¹), owing to lower symmetry and a decrease in the coordination number of iron. The ZFS of the other (āinertā) Fe(III) site maintains an axial ZFS of 0.19-0.20 cmā»Ā¹ under both hydrated and dehydrated conditions. We observed a similar effect in [Ni:ZnAl]-LDH materials; notably, Ni(II) (S = 1) atoms displayed a single, small ZFS (Ā±0.30 cmā»Ā¹) in hydrated material, whereas two distinct Ni(II) ZFS values (Ā±0.30 and Ā±1.1 cmā»Ā¹) were observed in the dehydrated samples. Although the magnetically-dilute materials were not active catalysts, the identification of model sites in which the coordination environments of iron and nickel were particularly labile (e.g. by simple vacuum drying) is an important step towards identifying sites in which the coordination number may drop spontaneously in water, a probable mechanism of water oxidation in functional materials
Ultrafast Wiggling and Jiggling: Ir_2(1,8-diisocyanomenthane)_4^(2+)
Binuclear complexes of d^8 metals (Pt^(II), Ir^I, Rh^I,) exhibit diverse photonic behavior, including dual emission from relatively long-lived singlet and triplet excited states, as well as photochemical energy, electron, and atom transfer. Time-resolved optical spectroscopic and X-ray studies have revealed the behavior of the dimetallic core, confirming that MāM bonding is strengthened upon dĻ* ā pĻ excitation. We report the bridging ligand dynamics of Ir2(1,8-diisocyanomenthane)_4^(2+)(Ir(dimen)), investigated by fsāns time-resolved IR spectroscopy (TRIR) in the region of Cā”N stretching vibrations, Ī½(Cā”N), 2000ā2300 cm^(ā1). The Ī½(Cā”N) IR band of the singlet and triplet dĻ*pĻ excited states is shifted by ā22 and ā16 cm^(ā1) relative to the ground state due to delocalization of the pĻ LUMO over the bridging ligands. Ultrafast relaxation dynamics of the ^1dĻ*pĻ state depend on the initially excited FranckāCondon molecular geometry, whereby the same relaxed singlet excited state is populated by two different pathways depending on the starting point at the excited-state potential energy surface. Exciting the long/eclipsed isomer triggers two-stage structural relaxation: 0.5 ps large-scale IrāIr contraction and 5 ps IrāIr contraction/intramolecular rotation. Exciting the short/twisted isomer induces a ā¼5 ps bond shortening combined with vibrational cooling. Intersystem crossing (70 ps) follows, populating a ^3dĻ*pĻ state that lives for hundreds of nanoseconds. During the first 2 ps, the Ī½(Cā”N) IR bandwidth oscillates with the frequency of the Ī½(IrāIr) wave packet, ca. 80 cm^(ā1), indicating that the dephasing time of the high-frequency (16 fs)^(ā1) Cā”N stretch responds to much slower (ā¼400 fs)^(ā1)IrāIr coherent oscillations. We conclude that the bonding and dynamics of bridging di-isocyanide ligands are coupled to the dynamics of the metalāmetal unit and that the coherent IrāIr motion induced by ultrafast excitation drives vibrational dephasing processes over the entire binuclear cation
Iron Is the Active Site in Nickel/Iron Water Oxidation Electrocatalysts
Efficient catalysis of the oxygen-evolution half-reaction (OER) is a pivotal requirement for the development of practical solar-driven water splitting devices. Heterogeneous OER electrocatalysts containing first-row transition metal oxides and hydroxides have attracted considerable recent interest, owing in part to the high abundance and low cost of starting materials. Among the best performing OER electrocatalysts are mixed Fe/Ni layered double hydroxides (LDH). A review of the available experimental data leads to the conclusion that iron is the active site for [NiFe]-LDH-catalyzed alkaline water oxidation
Electronic Structures of Reduced and Superreduced Ir_2(1,8-diisocyanomenthane)_4^(n+) Complexes
Molecular and electronic structures of Ir_2(1,8-diisocyanomenthane)_4^(n+) (Ir(dimen)^(n+)) complexes have been investigated by DFT for n = 2, 1, 0 (abbreviated 2+, 1+, 0). Calculations reproduced the experimental structure of 2+, Ī½(Cā”N) IR, and visible absorption spectra of all three oxidation states, as well as the EPR spectrum of 1+. We have shown that the two reduction steps correspond to successive filling of the IrāIr pĻ orbital. Complexes 2+ and 1+ have very similar structures with 1+ having a shorter IrāIr distance. The unpaired electron density in 1+ is delocalized along the IrāIr axis and over N atoms of the eight Cā”Nā ligands. The second reduction step 1+ ā 0 changes the Ir(CNā)_4 coordination geometry at each Ir site from approximately planar to seesaw whereby one āNā”CāIrāCā”Nā moiety is linear and the other bent at the Ir (137Ā°) as well as N (146Ā°) atoms. Although complex 0 is another example of a rare (pĻ)2 dimetallic species (after [Pt_2(Ī¼-P_2O_5(BF_2)_2)_4]^(6ā), J. Am. Chem. Soc. 2016, 138, 5699), the redistribution of lower lying occupied molecular orbitals increases electron density predominantly at the bent Cā”Nā ligands whose N atoms are predicted to be nucleophilic reaction centers
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