Large Spin Density Research Articles

Controlling Redox and Wirelike Charge-Delocalization Properties of Dinuclear Mixed-Valence Complexes with MCp*(dppe) (M = Fe, Ru) Termini Bridged by Metalloporphyrin Linkers.

Four dinuclear organometallic molecular wire complexes with diethynylmetalloporphyrin linkers 1 MM' , [5,15-bis{MCp*(dppe)ethynyl}-10,20-diarylporphinato]M' (Cp* = η5-C5Me5; dppe = 1,2-bis(diphenylphosphino)ethane; M/M' = Fe/Zn (1 FeZn ), Ru/Zn (1 RuZn ), Fe/Ni (1 FeNi ), Ru/Ni (1 RuNi ); aryl = 3,5-di-tert-butylphenyl), are synthesized and characterized by NMR, CV, UV-vis-NIR, and ESI-TOF mass spectrometry techniques. Electrochemical investigations combined with electronic absorption spectroscopic studies reveal strong interactions among the electron-donating, redox-active MCp*(dppe) termini and the metalloporphyrin moieties. The monocationic species of the four complexes obtained by chemical oxidation have been characterized as mixed-valence Class II/III or Class III compounds according to the Robin-Day classification despite the long molecular dimension (>1.5 nm), as demonstrated by their intense intervalence charge transfer bands (IVCT) in the near IR region. DFT calculations indicate large spin densities on the metalloporphyrin moieties. Furthermore, the wirelike performance can be finely tuned by coordination of appropriate nitrogen donors to the axial sites of the metalloporphyrin.

Dual Photochemistry of Benzimidazole.

Monomers of benzimidazole trapped in an argon matrix at 15 K were characterized by vibrational spectroscopy and identified as 1H-tautomers exclusively. The photochemistry of matrix-isolated 1H-benzimidazole was induced by excitations with a frequency-tunable narrowband UV light and followed spectroscopically. Hitherto unobserved photoproducts were identified as 4H- and 6H-tautomers. Simultaneously, a family of photoproducts bearing the isocyano moiety was identified. Thereby, the photochemistry of benzimidazole was hypothesized to follow two reaction pathways: the fixed-ring and the ring-opening isomerizations. The former reaction channel results in the cleavage of the NH bond and formation of a benzimidazolyl radical and an H-atom. The latter reaction channel involves the cleavage of the five-membered ring and concomitant shift of the H-atom from the CH bond of the imidazole moiety to the neighboring NH group, leading to 2-isocyanoaniline and subsequently to the isocyanoanilinyl radical. The mechanistic analysis of the observed photochemistry suggests that detached H-atoms, in both cases, recombine with the benzimidazolyl or isocyanoanilinyl radicals, predominantly at the positions with the largest spin density (revealed using the natural bond analysis computations). The photochemistry of benzimidazole therefore occupies an intermediate position between the earlier studied prototype cases of indole and benzoxazole, which exhibit exclusively the fixed-ring and the ring-opening photochemistries, respectively.

Structures and Stabilities of Carbon Chain Clusters Influenced by Atomic Antimony.

The C-C bond lengths of the linear magnetic neutral CnSb, CnSb+ cations and CnSb- anions are within 1.255-1.336 Å, which is typical for cumulene structures with moderately strong double-bonds. In this report, we found that the adiabatic ionization energy (IE) of CnSb decreased with n. When comparing the IE~n relationship of CnSb with that of pure Cn, we found that the latter exhibited a stair-step pattern (n ≥ 6), but the IE~n relationship of CnSb chains took the shape of a flat curve. The IEs of CnSb were lower than those of corresponding pure carbon chains. Different from pure carbon chains, the adiabatic electron affinity of CnSb does not exhibit a parity effect. There is an even-odd alternation for the incremental binding energies of the open chain CnSb (for n = 1-16) and CnSb+ (n = 1-10, when n > 10, the incremental binding energies of odd (n) chain of CnSb+ are larger than adjacent clusters). The difference in the incremental binding energies between the even and odd chains of both CnSb and pure Cn diminishes with the increase in n. The incremental binding energies for CnSb- anions do not exhibit a parity effect. For carbon chain clusters, the most favorable binding site of atomic antimony is the terminal carbon of the carbon cluster because the terminal carbon with a large spin density bonds in an unsaturated way. The C-Sb bond is a double bond with Wiberg bond index (WBI) between 1.41 and 2.13, which is obviously stronger for a carbon chain cluster with odd-number carbon atoms. The WBI of all C-C bonds was determined to be between 1.63 and 2.01, indicating the cumulene character of the carbon chain. Generally, the alteration of WBI and, in particular, the carbon chain cluster is consistent with the bond length alteration. However, the shorter C-C distance did not indicate a larger WBI. Rather than relying on the empirical comparison of bond distance, the WBI is a meaningful quantitative indicator for predicting the bonding strength in the carbon chain.

Neutral Boryl Radicals in Mixed-Valent B(III) Br-B(II) Adducts.

The general strategies to stabilize a boryl radical involve single electron delocalization by π-system and the steric hinderance from bulky groups. Herein, a new class of boryl radicals is reported, with intramolecular mixed-valent B(III) Br-B(II) adducts ligated by a cyclic (alkyl)(amino)carbene (CAAC). The radicals feature a large spin density on the boron center, which is ascertained by EPR spectroscopy and DFT calculations. Structural and computational analyses revealed that the stability of radical species was assisted by the CAAC ligand and a weak but significant B(III)Br-B(II) interaction, suggesting a cooperative avenue for stabilization of boryl radicals. Two-electron reduction of these new boryl radicals provides C-H insertion products via a borylene intermediate.

Coherent spin transport in a natural metalloprotein molecule

Recently, the long-range spin-selective transport in chiral molecules has been investigated for bio-spintronics. The experimental results for a natural metalloprotein molecule suggested a high spin selectivity. I performed first-principle calculations of electron spin transport in a natural metalloprotein molecule based on the Landauer formula. A gold–metalloprotein–gold device model was used to confirm the high spin polarization. There was a relatively large spin density at some amide groups in the helical peptide structures. Furthermore, a large spin density of iron atoms enhanced the spin density of the neighboring coordinated atoms, resulting in spin polarization in the whole molecule.

Oxidative Intramolecular C–C Bond Formation Reactions of 1,2-Diarylbenzenes: Syntheses of Highly Conjugated Double-Bridged Polycyclic Aromatic Hydrocarbons

AbstractOxidation reactions of 1,2-diarylbenzenes induce intramolecular C–C bond formation. The substrates studied were prepared by the stepwise Suzuki–Miyaura coupling reaction that introduced 2-naphthyl, 2-anthranyl, and 2-pyrenyl groups on the ortho-positions of benzene. The subsequent oxidation reaction with FeCl3 induced an oxidative C–C bond formation reaction in the interior regions of the molecules. In marked contrast to our previous observations, two C–C bonds were formed. Theoretical calculations indicated that large spin densities at the reaction positions of the bis(cation radical) and/or cation radical species are needed for the C–C bond formation. The π-expanded molecules obtained showed bathochromic shifts in the absorption spectra.

Interplay of local moment and itinerant magnetism in cobalt-based Heusler ferromagnets: Co2TiSi, Co2MnSi and Co2FeSi

Heusler ferromagnets based on Co are important materials for spintronics. This is due to the exceptional combinations of high Curie temperature and strong spin polarization, including half-metallicity, found in some of these. We investigate the full Heusler compounds, ${\mathrm{Co}}_{2}\mathrm{TiSi}$, ${\mathrm{Co}}_{2}\mathrm{MnSi}$, and ${\mathrm{Co}}_{2}\mathrm{FeSi}$ using first principles calculations. ${\mathrm{Co}}_{2}\mathrm{TiSi}$ and ${\mathrm{Co}}_{2}\mathrm{MnSi}$ are half metals, while ${\mathrm{Co}}_{2}\mathrm{FeSi}$ is not. The trends in the Curie temperatures are reproduced by the calculated spin wave dispersions. Remarkably, ${\mathrm{Co}}_{2}\mathrm{TiSi}$ is a very itinerant magnet but ${\mathrm{Co}}_{2}\mathrm{FeSi}$ and ${\mathrm{Co}}_{2}\mathrm{MnSi}$ show local moment behavior regarding the Fe and Mn, while retaining the itinerancy of the Co magnetism. These materials can therefore be described as itinerant systems with embedded local moment atoms. This provides an explanation for their exceptional behavior. Our results do not support the half-metallic character proposed for ${\mathrm{Co}}_{2}\mathrm{FeSi}$, but they are consistent with a higher Curie temperature relative to the Mn compound. The density of states and transport spin polarizations ${\mathrm{Co}}_{2}\mathrm{FeSi}$ have opposite signs. Importantly, although there is a large minority spin density of states at the Fermi level, leading to a low density of states spin polarization, we find a very strong transport spin polarization in ${\mathrm{Co}}_{2}\mathrm{FeSi}$. This, combined with the large moment, cubic structure, and high Curie temperature supports the further investigation of ${\mathrm{Co}}_{2}\mathrm{FeSi}$ for spintronic applications that make use of the transport spin polarization.

Carbon-doped boron nitride nanosheets as highly sensitive materials for detection of toxic NO and NO2 gases: A DFT study

Using density functional theory calculations, we investigate sensing mechanism of C-doped hexagonal boron nitride (h-BN) nanosheets toward NO and NO2 molecules. The results indicate that C-doping induces a large spin density and electron density redistribution in h-BN, which leads to improved adsorption energy of NO and NO2. The adsorption of NO and NO2 is able to alter significantly electronic structure of C-doped h-BN nanosheets as evidenced by relatively large variation in the band gap values. Moreover, the application of an external electric field can avoid the strong adsorption of NO and NO2 molecules over C-doped h-BN sheets. Compared to NO and NO2, the adsorption of CO, CO2, H2O or NH3 on C-doped h-BN sheets is remarkably weaker, suggesting that the sensing of NO and NO2 can be performed selectively in the presence of these molecules. Therefore, C-doped h-BN nanosheets can be viewed as promising and sensitive room-temperature sensors for NO and NO2 molecules.

C59N Heterofullerene: A Promising Catalyst for NO Conversion into N2O

AbstractWe report, for the first time, the possibility of using N‐doped C60 fullerene (C59N) as a novel and highly active metal‐free catalyst for reduction of NO molecules to N2O. First‐principles density functional theory calculations are used to find the most energetically favorable adsorbed/coadsorbed configurations of NO molecules over C59N. Our results indicate that introducing a N impurity in C60 can induce a positive charge and large spin density over the carbon atoms nearest to the dopant atom. Consequently, these carbon atoms are identified as the most reactive sites to interact with the NO molecule. According to our results, the dissociation of NO molecule over C59N is almost impossible, due to its relatively high activation energy (1.35 eV). The reduction of NO over C59N starts with the coadsorption of two NO molecules to form (NO)2 species, followed by the dissociation of (NO)2 into N2O and an active atomic oxygen (Oads). The energy barrier to convert NO molecules into N2O ranges from 0.33 to 0.75 eV, which indicates this process is likely to proceed at normal temperature. Besides, by overcoming a negligible energy barrier (0.18 eV), the remaining Oads moiety can be easily eliminated by a CO molecule. To further understand the role of nitrogen atoms in the NO reduction process, the catalytic performance of high percentage N‐doped fullerene (C48N12) is also studied.

A Crystalline Diazadiborinine Radical Cation and Its Boron‐Centered Radical Reactivity

AbstractOne‐electron oxidation of 1,4,2,5‐diazadiborinine 1 has been studied. While the reaction of 1 a bearing phenyl groups on the B atoms with AgAl{OC(CF3)3}4 afforded a complex mixture, the same oxidation reaction with 1 b featuring bulky mesityl substituents on the B atoms rendered the corresponding cation radical 2 b as an isolable species. X‐ray diffraction analysis, EPR spectroscopy, and DFT calculations of 2 b revealed the delocalization of the unpaired electron over the entire π‐system of 2 b, as well as a large spin density (0.76 in total) on the two equivalent boron atoms. The chemical trapping reaction of 2 b with p‐benzoquinone and triphenyltin hydride afforded the dicationic species 3 containing two newly formed B−O bonds and the monocationic product 2b‐H containing a B−H bond, respectively, thus confirming the boron‐centered radical reactivity of 2 b.

A Crystalline Diazadiborinine Radical Cation and Its Boron-Centered Radical Reactivity.

One-electron oxidation of 1,4,2,5-diazadiborinine 1 has been studied. While the reaction of 1 a bearing phenyl groups on the B atoms with AgAl{OC(CF3 )3 }4 afforded a complex mixture, the same oxidation reaction with 1 b featuring bulky mesityl substituents on the B atoms rendered the corresponding cation radical 2 b as an isolable species. X-ray diffraction analysis, EPR spectroscopy, and DFT calculations of 2 b revealed the delocalization of the unpaired electron over the entire π-system of 2 b, as well as a large spin density (0.76 in total) on the two equivalent boron atoms. The chemical trapping reaction of 2 b with p-benzoquinone and triphenyltin hydride afforded the dicationic species 3 containing two newly formed B-O bonds and the monocationic product 2b-H containing a B-H bond, respectively, thus confirming the boron-centered radical reactivity of 2 b.

Spin-Hall Voltage over a Large Length Scale in Bulk Germanium.

We exploit the spin-Hall effect to generate a uniform pure spin current in an epitaxial n-doped Ge channel, and we detect the electrically induced spin accumulation, transverse to the injected charge current density, with polar magneto-optical Kerr microscopy at a low temperature. We show that a large spin density up to 400 μm^{-3} can be achieved at the edges of the 100-μm-wide Ge channel for an applied electric field lower than 5 mV/μm. We find that the spin density linearly decreases toward the center of the Ge bar, due to the large spin diffusion length, and such a decay is much slower than the exponential one observed in III-V semiconductors, allowing very large spin accumulations over a length scale of tens of micrometers. This lays the foundation for multiterminal spintronic devices, where different spin voltages can be exploited as inputs for magnetologic gates on the same Ge platform.

Optomagnonics in magnetic solids

Coherent conversion of photons to magnons, and back, provides a natural mechanism for rapid control of interactions between stationary spins with long coherence times and high-speed photons. Despite the large frequency difference between optical photons and magnons, coherent conversion can be achieved through a three-particle interaction between one magnon and two photons whose frequency difference is resonant with the magnon frequency, as in optomechanics with two photons and a phonon. The large spin density of a transparent ferromagnetic insulator (such as the ferrite yttrium iron garnet) in an optical cavity provides an intrinsic photon-magnon coupling strength that we calculate to exceed reported optomechanical couplings. A large cavity photon number and properly selected cavity detuning produce a predicted effective coupling strength sufficient for observing electromagnetically induced transparency and the Purcell effect, and even to reach the ultra-strong coupling regime.

Emission property and DFT calculation for the 3MLCT luminescence of Ru(bpy)2(L)2+ complex

Electronic structures of ruthenium complexes [Ru(bpy)2(L)]2+ were studied by DFT calculations, where bpy is 2,2′-bipyridine and L's are its derivatives. They are parent (1) and five complexes with L = 4,4′-dimethyl-2,2′-bipyridine (2), 6,6′-dimethyl-2,2′-bipyridine (3), biquinoline (4), 5-nitro-1,10-phenanthroline (5) and two acetonitrile molecules (6). These complexes were characterized by ESI-MS, 1H NMR, UV–vis spectroscopy and elementary analysis. The crystal structures of 3·(PF6)2, 4·(PF6)2, 5·(PF6)2 and 6·(PF6)2 were also determined.Spin densities were analyzed to characterize the excited triplet states in those complexes. It was found that the complexes have three triplet (called here 3MLCT, 3MC1 and 3MC2) states. The 3MC1 and 3MC2 states have elongated RuN bonds and large spin densities on Ru2+. Those states are stable and would lead to the nonradiative relaxation to the ground state. In particular, those of 3, 5 and 6 are very stable, which results in their poor quantum yields (Φ < 0.01) of the emissions. In contrast, both 1 and 2 showed the 3MLCT states. Energy gaps between triplet and ground states were compared with energies corresponding to wavelengths of emission spectra of 1–5. Good agreement between the calculated and the experimental energies was found. Moreover, dual emission which depends on the excitation wavelength was observed in complex 4.

Synthesis, crystal structures, and magnetism of the binuclear radical complex [N-methylpyridinium]2[Ni(tdas)2

The novel binuclear radical complex [N-methylpyridinium]2[Ni(tdas)2]2 (tdas = 1,2,5-thiadiazole-3,4-dithiolate) has been prepared and its crystal structures determined by X-ray crystallography at three different temperatures in order to investigate the changes in coordination structure and magnetic properties with temperature. In the binuclear radical complex the two nickel ions assume a distorted pyramidal geometry and are bridged by two S atoms of different tdas anionic ligands. At room temperature, the ESR spectrum of the binuclear radical complex in polycrystalline powder and the theoretical calculations reveal a very strong antiferromagnetic interaction, leading to diamagnetic crystals. In contrast, a peak with g = 2.05 appears in the ESR spectrum of the title complex in acetonitrile solution, which indicates that a fraction of the binuclear radical complexes dissociate. The theoretical calculations also reveal that as temperature increases the antiferromagnetic coupling strength gradually decreases, yet at 150 °C the antiferromagnetic coupling strength remains as strong as 2J = −734.26 cm−1. The strong antiferromagnetic coupling strengths should be attributed to the large spin densities on the nickel atoms and the relevant bridging sulfur atoms. This study is the first to report the correlations between the structure, magnetism, and temperature of a binuclear radical nickel complex with tdas as ligand and this study is also the first report the magnetic coupling strength of radical binuclear nickel complex with tdas as bridging ligand and with N-methylpyridinium as counter cation.

Large and small Fermi-surface spin density waves in the Kondo lattice model

We demonstrate the existence of metallic spin density waves (SDWs) in the Kondo lattice model on a square lattice for a wide range of parameters by means of real space dynamical mean field theory. In these SDWs, the spin polarization as well as the charge density depend on the lattice site and are modulated along one direction of the square lattice. We show that within this phase of metallic SDWs the Fermi surface changes from small to large, when the coupling strength is increased. Furthermore, the transition between the large Fermi-surface SDW phase and the paramagnetic phase is of second order, while the transition between the small Fermi-surface SDW phase and the paramagnetic phase is of first order. A local quantum critical point is thus avoided in our calculations by undergoing a first order phase transition.

Synthesis, crystal structure, and π–π stacking magnetism of the mononuclear radical complex [N-hydrogenpyridinium][Pd(mnt)2

The mononuclear radical anionic complex [N-hydrogenpyridinium][Pd(mnt)2] (mnt = malonitrile-2,3-dithiolate) with a new countercation has been prepared and its crystal structure determined by X-ray crystallography. In the mononuclear radical anionic complex the palladium ion assumes a slightly distorted square-planar geometry and is bridged by four S atoms of two mnt anionic ligands. There are two kinds of slipped π–π stacking interactions between adjacent mononuclear radical anionic complexes in the crystal (i.e., PS-1 and PS-2). The ESR spectra, the variable-temperature susceptibilities, and the theoretical calculations reveal a very strong antiferromagnetic interaction between the adjacent mononuclear radical anionic complexes, leading to diamagnetism of the present complex. Further, the theoretical calculations indicate that the strong antiferromagnetic strength should be attributed to the PS-1 slipped π–π stacking interaction; the large spin densities on the associated short contact atoms should be a major factor leading to the strong magnetic coupling strength. This study is the first to reveal the mechanism of the strong antiferromagnetic interaction of a mononuclear radical anionic palladium complex with mnt as ligand.

A Phosphine‐Coordinated Boron‐Centered Gomberg‐Type Radical

AbstractThe P‐coordinated boryl radical [Ph2P(naphthyl)BMes]. (Mes=mesityl) was prepared by (electro)chemical reduction of the corresponding borenium salt or bromoborane. Electron paramagnetic resonance (EPR) analysis in solution and DFT calculations indicate large spin density on boron (60–70 %) and strong P–B interactions (P→B σ donation and B→P negative hyperconjugation). The radical is persistent in solution and participates in a Gomberg‐type dimerization process. The associated quinoid‐type dimer has been characterized by single‐crystal X‐ray diffraction.

A Phosphine-Coordinated Boron-Centered Gomberg-Type Radical.

The P-coordinated boryl radical [Ph2P(naphthyl)BMes]˙ (Mes=mesityl) was prepared by (electro)chemical reduction of the corresponding borenium salt or bromoborane. Electron paramagnetic resonance (EPR) analysis in solution and DFT calculations indicate large spin density on boron (60-70%) and strong P-B interactions (P→B σ donation and B→P negative hyperconjugation). The radical is persistent in solution and participates in a Gomberg-type dimerization process. The associated quinoid-type dimer has been characterized by single-crystal X-ray diffraction.

Normal-Mode Splitting in the Coupled System of Hybridized Nuclear Magnons and Microwave Photons.

In the weak ferromagnetic MnCO_{3} system, a low-frequency collective spin excitation (magnon) is the hybridized oscillation of nuclear and electron spins coupled through the hyperfine interaction. By using a split-ring resonator, we performed transmission spectroscopy measurements of the MnCO_{3} system and observed avoided crossing between the hybridized nuclear magnon mode and the resonator mode in the NMR-frequency range. The splitting strength is quite large due to the large spin density of ^{55}Mn, and the cooperativity value C=0.2 (the magnon-photon coupling parameter) is close to the conditions of strong coupling. The results reveal a new class of spin systems, in which the coupling between nuclear spins and photons is mediated by electron spins via the hyperfine interaction.