金属串配合物[Cr3(dpa)4LL'](dpa=dipyridylamide; L,L'=Cl,BF4, CCPh)的Cr-Cr成键特性

采用密度泛函理论UBP86方法计算了Cr₃(dpa)₄Cl₂ (1)、Cr₃(dpa)₄(BF₄)₂ (2)、Cr₃(dpa)₄Cl(BF₄) (3)、Cr₃(dpa)₄(CCPh)₂ (4)和Cr₃(dpa)₄Cl(CCPh) (5)金属串配合物的结构, 并对配合物的构型、Cr—Cr键的本质以及轴向配体对Cr—Cr键的影响进行了研究. 结果表明: (1) Cr—Cr平均键长较长的配合物趋于形成对称构型, 较短时趋于形成非对称构型. 最稳定的五重态的Cr—Cr平均键长最长, 故优化时趋于形成对称构型; 七重态的Cr—Cr平均键长最短, 趋于形成非对称构型; (2) 五重态的Cr₃⁶⁺金属链均存在三中心三电子σ键, 含弱σ给电子轴向配体BF₄^-的2和3的Cr—Cr短键还具有弱的π相互作用. 七重态下, 对称构型的4中仅有三中心三电子σ键, 而非对称构型的1-3、5的Cr—Cr短键为三重键, 非对称构型仍具有Cr₃⁶⁺链的σ离域作用, 仍具有分子导线的潜在应用; (3) 轴向配体L与Cr的作用主要表现为n_L→4_sCr或n_L→3_dz²Cr离域, 较强的σ给电子配体CCPh^-还存在σ_C—C→4_sCr离域. Cr与L的结合强度为2＜3＜1＜5＜4, CCPh^-与Cr的结合最强, 使Cr—Cr键减弱、Cr—Cr距离增长, 故4的各自旋重态均为对称构型.     

Electronic ground state of iron in octamethyltetrabenzporphyriniron(II), a new square planar ferrous porphyrin

Reaction of iron powder with 1,3,4,7-tetramethylisoindole at 350° affords the square planar complex octamethyl-tetrabenzporphyriniron(II), whose magnetic moment indicates a spin quintet ground state. Mössbauer measurements at 4.2°K in an applied magnetic field of 50 kG show that the electric field gradient at iron is positive and axially symmetric, consistent with a ⁵B_2g ground term. The bistetrahydrofuran adduct is also high spin, whereas the bispyridine adduct is diamagnetic with ¹A_1g ground state. Comparisons are made with data for the related tetraphenylporphiniron(II) and phthalocyanineiron(II) derivatives.     

Excited‐State Dynamics of the Metal String Complex Co3(dpa)4(NCS)2 from Femtosecond Transient Absorption Spectra

Transient absorption spectroscopy is used to study the excited‐state dynamics of Co₃(dpa)₄(NCS)₂, where dpa is the ligand di(2‐pyridyl)amido. The ππ*, charge‐transfer, and d–d transition states are excited upon irradiation at wavelengths of 330, 400 and 600 nm, respectively. Similar transient spectra are observed under the experimental temporal resolution and the transient species show weak absorption. We thus propose that a low‐lying metal‐centered d–d state is accessed immediately after excitation. Analyses of the experimental kinetic traces reveal rapid conversion from the ligand‐centered ππ* and the charge‐transfer states to this metal‐centered d‐d state within 100 fs. The excited molecule then crosses to a second d–d state within the ligand‐field manifold, with a time coefficient of 0.6–1.4 ps. Because the ground‐state bleaching band recovers with a time coefficient of 10–23 ps, we propose that an excited molecule crosses from the low‐lying d–d state either directly within the same spin system or with spin crossing via the state ²B to the ground state ²A₂ (symmetry group C₄). In this trimetal string complex, relaxation to the ground electronic surface after excitation is thus rapid.     

Effect of axial anthracene ligands on the luminescence of trinickel molecular wires

Two nickel compounds having extended metal atom chains (EMACs) with the Ni₃(dpa)₄L₂ core, where dpa is the anion of 2,2′-dipyridylamine and L represents an anthracene derivative, have been synthesized in good yield and excellent purity by reaction of Ni₃(dpa)₄Cl₂ and the respective anthracene derivatives. For the compound Ni₃(dpa)₄(AnCC)₂ (1) AnCC is the anion of 9-ethynylanthracene and for Ni₃(dpa)₄(AnCOO)₂ (2) AnCOO is anthracene-9-carboxylate. The two compounds have been characterized by X-ray crystallography and other techniques. The cyclovoltammograms (CVs) of 1 and 2 display reversible redox processes at E_1/2 = 0.911 V and 1.020 V, respectively (vs. Ag/AgCl). The magnetic data suggest that the terminal Ni atoms are in a high spin state while the central Ni atom is diamagnetic, in agreement with other trinickel molecular wires. Although compound 1 is strongly luminescent, and represents the first such example in an EMAC, 2 does not display luminescence. The strong emission in 1 is related to donor–acceptor interactions between the Ni?Ni?Ni wire, the triply bonded CC linking unit, and the conjugated aromatic anthracene derivative, a process that is unavailable for 2 because the carboxylate group is almost perpendicular to the anthracene moiety.     

电场对金属串配合物M3(dpa)4Cl2 (M=Co,Rh, Ir; dpa=dipyridylamide)结构的影响

应用密度泛函理论PBE0 方法研究具有分子导线潜在应用的金属串配合物M₃(dpa)₄Cl₂ (1: M=Co, 2: M=Rh, 3: M=Ir; dpa=dipyridylamide)在电场作用下的几何和电子结构. 结果表明: 配合物基态均是二重态. 1和2的M₃⁶⁺金属链形成三中心三电子σ键, 3 中M₃⁶⁺形成三中心四电子σ键且存在弱的δ键. 随金属原子周期数增大其M―M键增强、LUMO与HOMO能隙减小、金属原子的反铁磁耦合减弱以至消失且自旋密度向配体的离域增强. 在Cl4→Cl5 电场作用下, 低电势端的M3－Cl5 键缩短, 高电势端的M2―Cl4 键增长, M―M平均键长略为缩短, M―M键增强, 有利于分子线的电子传递; 分子能量降低, 偶极矩线性增大. 低电势端Cl5的负电荷向高电势端Cl4 转移, 且3 中金属原子的正电荷由高电势端向低电势端的转移较明显, 自旋电子由低电势端向高电势端金属原子移动, 但桥联配体dpa^-与M和Cl 所在的分子轴间没有电荷转移. 电场使LUMO与HOMO能隙减小, 有利于分子的电子输运. 随金属原子周期数增大, 电场作用下M―M平均键长变化减小, LUMO、HOMO的能级交错现象减少.     

Molecular fragmentations: Part IV. The mass spectral fragmentation of carbon tetrachloride

MINDO/3 calculations have been made of the molecular structures and energies of seven isomeric forms of the molecular cation (CCl₄)⁺, of the mass spectral fragment pairs:

and also of a number of neutral fragment pairs. Reaction energy profiles have been calculated for two fragmentations of (CCl₄)⁺, into [(CCl₂)⁺ + Cl₂], and into [(CCl₃)⁺ + Cl^?], in the latter of which the reaction proceeds via a rather stable intermediate; for the fragmentation of three electronic states of (CCl₃)⁺ into [(CCl₂)⁺ + Cl^?], where the ground singlet state and first triplet state of (CCl₃⁺ yield the ground doublet state of (CCl₂)⁺, but the first excited singlet of (CCl₃)⁺ yields the first excited doublet of (CCl₂)⁺ ; and for the fragmentation of the ground state of (CCl₃)⁺ into [(CCl)⁺ + Cl₂].     

Structural and electronic properties of a Benzoin monomer

In the present work, we report structural and electronic properties of a benzoin monomer named as 2-oxo-1,2-diphenylethyl-2-bromopropanoate (C₁₇H₁₄BrO₃). The previously synthesized compound is characterized by single crystal X-ray diffraction. The optimized molecular geometry (bond lengths, and bond angles), HOMO-LUMO analysis and molecular electrostatic potential (MEP) are calculated by density functional theory (DFT) and Hartree-Fock (HF) methods with 6-311G(d) basis set in the neutral ground state and using DFT methods for singly oxidized doublet, singly reduced doublet, and neutral triplet state for the benzoin compound. The X-ray structure determination of the compound is compatible with the geometric parameters calculated at B3LYP/6-311G(d). In the triplet state the HOMO-LUMO energy gap of 2.39?eV which indicates semi-conductor property is recommended for the photovoltaic devices.     

Atomic and molecular forms of oxygen on Ag(331). Theoretical analysis using the DFT method

Oxygen adsorption on Ag(331) is analyzed in a cluster approximation using the density functional theory (DFT) method. Adsorption centers (AC) for the bridge (S2) and three-center (S3) coordinations of oxygen are identified on the stepwise face Ag(331) and the Ag-O bond energies at these centers are calculated. For atomic adsorption, the Ag-O bond strength varies from 50 to 65 kcal/mole, depending on AC. The heat of molecular adsorption DH = 5 kcal/mole for S2(L1-L2) type AC. The molecule is oriented parallel to Ag(110) between the terraces with R(O-O) = 1.34 å Calculations showed that the ground state of the O₂Ag20(331) system is a triplet, but a part of spin density is delocalized on silver atoms, so that the spin density on oxygen ρ_s(O) = 0.46 (ρ_s = 1.0 for the free O₂ molecule). The energy of the singlet state is 9 kcal/mole greater than that of the ground state.     

Theory of chemical bonds in metalloenzymes. VII. Hybrid‐density functional theory studies on the electronic structures of P450

A first principle investigation has been carried out for intermediate states of the catalytic cycle of a cytochrome P450. To elucidate the whole catalytic cycle of P450, the electronic and geometrical structures are investigated not only at each ground state but also at low‐lying energy levels. Using the natural orbital analysis, the nature of chemical bonds and magnetic interactions are investigated. The ground state of the Compound 1 ( cpd1 ) is calculated to be a doublet state, which is generated by the antiferromagnetic coupling between a triplet Fe(IV)?O moiety and a doublet ligand radical. We found that an excited doublet state of the cpd1 is composed of a singlet Fe(IV)?O and a doublet ligand radical. This excited state lies 20.8 kcal mol^?1 above the ground spin state, which is a non‐negligible energy level as compared with the activation energy barrier of ΔE^# = 26.6 kcal mol^?1. The reaction path of the ground state of cpd1 is investigated on the basis of the model reaction: ³O(³p) + CH₄. The computational results suggest that the reactions of P450 at the ground and excited states proceed through abstraction (³O‐model) and insertion (¹O‐model) mechanisms, respectively. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Synthesis,Structure, and Cyclic Voltammetric Property of Metal String Complex [Ni3(dpa)4(ClO4)(Cl)]·CH2Cl2

The metal string complex [Ni₃(dpa)₄(ClO₄)(Cl)] · CH₂Cl₂ ( 1 ) [dpa = bis(2‐pyridyl)amine] with different axial ligands was synthesized and characterized by elemental analysis, IR, UV/Vis, and fluorescence spectroscopy and TG analysis. The molecular structure was determined by single‐crystal X‐ray analysis and its electrochemical properties were investigated. This metal string complex is the first example with different axial ligands, and in its structure a different structural packing relative to the metal string complex [Ni₃(dpa)₄(Cl)₂] ( 2 ) with two axial chloride ligands is generated. The intense C–H ··· π interactions observed for 1 provide additional stability. The axial mono‐substitution of Cl^– by ClO₄^– in 1 relative to 2 results in one obviously short Ni–Ni distance and a higher stability towards oxidation.     

Maximum Spin Polarization in Chromium Dimer Cations as Demonstrated by X‐ray Magnetic Circular Dichroism Spectroscopy

X‐ray magnetic circular dichroism spectroscopy has been used to characterize the electronic structure and magnetic moment of Cr₂⁺. Our results indicate that the removal of a single electron from the 4sσ_g bonding orbital of Cr₂ drastically changes the preferred coupling of the 3d electronic spins. While the neutral molecule has a zero‐spin ground state with a very short bond length, the molecular cation exhibits a ferromagnetically coupled ground state with the highest possible spin of S=11/2, and almost twice the bond length of the neutral molecule. This spin configuration can be interpreted as a result of indirect exchange coupling between the 3d electrons of the two atoms that is mediated by the single 4s electron through a strong intraatomic 3d‐4s exchange interaction. Our finding allows an estimate of the relative energies of two states that are often discussed as ground‐state candidates, the ferromagnetically coupled ¹²Σ and the low‐spin ²Σ state.     

Spin-inversion mechanisms in O2 binding to a model heme complex revisited by density function theory calculations

Spin-inversion mechanisms in O₂ binding to a model heme complex, consisting of Fe(II)-porphyrin and imidazole, were investigated using density-functional theory calculations. First, we applied the recently proposed mixed-spin Hamiltonian method to locate spin-inversion structures between different total spin multiplicities. Nine spin-inversion structures were successfully optimized for the singlet–triplet, singlet–quintet, triplet–quintet, and quintet–septet spin-inversion processes. We found that the singlet–triplet spin-inversion points are located around the potential energy surface region at short Fe–O distances, whereas the singlet–quintet and quintet–septet spin-inversion points are located at longer Fe–O distances. This suggests that both narrow and broad crossing models play roles in O₂ binding to the Fe-porphyrin complex. To further understand spin-inversion mechanisms, we performed on-the-fly Born-Oppenheimer molecular dynamics calculations. The reaction coordinates, which are correlated to the spin-inversion dynamics between different spin multiplicities, are also discussed.     

Simple versus composite nature of the magnetic coupling in copper and nickel-based metallic chains. Over- and underestimation of J

Extended metal atom chains (EMACs) are molecular linear arrangements of metal atoms featuring magnetic properties. By means of the density functional theory (DFT), we have studied the magnetic coupling constants for [Cu₃(dpa)₄Cl₂]⁺, Ni₃(dpa)₄Cl₂ and Ni₅(tpda)₄Cl₂ to understand which is the origin of the previously reported theoretical underestimation of J for nickel complexes. We have decomposed J = J_σ + J_δ, finding that the former contribution is underestimated and the latter part is overestimated at the DFT/B3LYP level of computation. Varying the amount of Hartree-Fock exchange, we show that the B3LYP functional fails to describe the σ interaction properly, whereas the δ coupling is exaggerated.     

Mössbauer spectra of the tetrakis-(1,8-naphthyridine) iron(II)perchlorate in external magnetic fields. Evidence of slow relaxation ln paramagnetic iron(II)

⁵⁷Fe Mössbauer spectra of [FeL₄] (ClO₄)₂ where L = 1,8-naphthyridine have been measured at 4.2°K in external magnetic fields up to 55 kG parallel to the direction of the γ-rays. The spectra have been fitted in the spin hamiltonian approximation assuming an orbital singlet ground state of the ⁵D multiplet of Fe²⁺. The fit of the spectra is not unique, yet the possible spin hamiltonian parameter sets found lead to a spin doublet ground state split by less than 1 cm^?1. The transition probabilities for spin-lattice relaxation have been calculated for those ground states. Orbach processes via excited spin hamiltonian states cannot be neglected. The results explain the fluctuations observed in the spectra in low external magnetic fields (10 kG).The spin hamiltonian parameters provide information on the orbital energy levels. Therefrom the reduction of the quadrupole splitting by spin—orbit coupling results to be small thus explaining the extremely large quadrupole splitting of 4.54 mm/sec.     

Structural diversity of homobinuclear transition metal complexes of the phenazine ligand: theoretical investigation

DFT/BP86 calculations have been carried out on a series of hypothetical binuclear compounds of general formula (L₃M)₂(C₁₂N₂H₈) (M?=?Sc–Ni, L₃?=?(CO)₃, (PH₃)₃ and Cp^?, and C₁₂N₂H₈?=?phenazine ligand-denoted Phn). The various structures with syn and anti configurations have been investigated, in order to determine the phenazine’s coordination to first-row transition metals of various spin states with syn and anti conformations. The lowest energy structures depend on the nature of the metal, the spin state, and the molecular symmetry. This study has shown that the electronic communication between the metal centers depends on their oxidation state and the attached ligands. The tricarbonyl and the triphosphine ligands gave rise to comparable results in terms of stability order of isomers, metal-metal bond distances, and the coordination modes. Metal-metal multiple bonding has been evidenced for Sc, Ti, and V complexes to compensate the electronic deficiency. The Cr, Mn, Fe, Co, and Ni-rich metals prefer the anti conformation due to the enhancement of the metal valence electron count. The spin density values calculated for the triplet and quintet spin structures point out that the unpaired electrons are localized generally on the metal centers. The Wiberg bond indices are used to evaluate the metal-metal bonding. Furthermore, calculations using the BP86-D functional which take into account the attractive part of the van der Waals type interaction potential between atoms and molecules that are not directly connected to each other gave comparable results to those obtained by BP86 functional in terms of coordination modes, HOMO-LUMO gaps, metal-metal bond orders, and the stability order between isomers, but with slight deviation of M–C, M–N, and M–M bond distances not exceeding 3%.     

Density functional study of structural and electronic properties of AlnN (1 ≤ n ≤ 12) clusters

Low‐lying equilibrium geometric structures of Al_nN (n = 1–12) clusters obtained by an all‐electron linear combination of atomic orbital approach, within spin‐polarized density functional theory, are reported. The binding energy, dissociation energy, and stability of these clusters are studied within the local spin density approximation (LSDA) and the three‐parameter hybrid generalized gradient approximation (GGA) due to Becke–Lee–Yang–Parr (B3LYP). Ionization potentials, electron affinities, hardness, and static dipole polarizabilities are calculated for the ground‐state structures within the GGA. It is observed that symmetric structures with the nitrogen atom occupying the internal position are lowest‐energy geometries. Generalized gradient approximation extends bond lengths as compared with the LSDA lengths. The odd–even oscillations in the dissociation energy, the second differences in energy, the highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) gaps, the ionization potential, the electron affinity, and the hardness are more pronounced within the GGA. The stability analysis based on the energies clearly shows the Al₇N cluster to be endowed with special stability. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Density functional and multiconfigurational ab initio study of the ground and excited states of Os2

Multiconfigurational ab initio methods predict that the ⁵Π_u state as the ground state instead of the ⁷Δ_u state. Although multiconfigurational perturbation theory correctly predicts the ground state, they overestimate the bond dissociation energy (BDE). Only multireference configuration interaction method can reasonably calculate the BDE. The spin‐orbit effect on the spectroscopic constants is not significant. The results calculated by density functional theory (DFT) vary significantly depending on the selection of a DFT functional. No DFT functional gives the same energy ordering as calculated by the second‐order multiconfigurational perturbation theory (CASPT2). The old generalized gradient approximations functionals are well suited for predicting the ground state and calculating the bond length and the vibrational frequency of Os₂. According to the CASPT2 calculation, the ground state of Os₂ has a quadruple bond. © 2014 Wiley Periodicals, Inc.     

IR,Raman, and Surface‐enhanced Raman Spectroscopic Study on Triruthenium Dipyridylamide Diruthenium Nickel Dipyridylamide Family: Metal‐metal Bonding and Structures

We report the infrared, Raman, and surface‐enhanced Raman scattering (SERS) spectra of triruthenium dipyridylamido complexes and of diruthenium mixed nickel metal‐string complexes. From the results of analysis on the vibrational modes, we assigned their vibrational frequencies and structures. The infrared band at 323–326 cm^?1 is assigned to the Ru₃ asymmetric stretching mode for [Ru₃(dpa)₄Cl₂]^0–2+. In these complexes we observed no Raman band corresponding to the Ru₃ symmetric stretching mode although this mode is expected to have substantial Raman intensity. There is no frequency shift in the Ru₃ asymmetric stretching modes for the complexes with varied oxidational states. No splitting in Raman spectra for the pyridyl breathing line indicates similar bonding environment for both pyridyls in dpa^– , thus a delocalized structure in the [Ru₃]^6–8+ unit is proposed. For Ru₃(dpa)₄(CN)₂ complex series, we assign the infrared band at 302 cm^?1 to the Ru₃ asymmetric stretching mode and the weak Raman line at 285 cm^?1 to the Ru₃ symmetric stretching. Coordination to the strong axial ligand CN^– weakens the Ru‐Ru bonding. For the diruthenium nickel complex [Ru₂Ni(dpa)₄Cl₂]^0–1+, the diruthenium stretching mode ν_Ru‐Ru is assigned to the intense band at 327 and 333 cm^?1 in the Raman spectra for the neutral and oxidized forms, respectively. This implies a strong Ru‐Ru metal‐metal bonding.     

Tuning the Magnetic Moment of [Ru2(DPhF)3(O2CMe)L]+ Complexes (DPhF=N,N′‐Diphenylformamidinate): A Theoretical Explanation of the Axial Ligand Influence

The magnetic behaviour of the compounds containing the [Ru₂(DPhF)₃(O₂CMe)]⁺ ion (DPhF^?=N,N′‐diphenylformamidinate) shows a strong dependence on the nature of the ligand bonded to the axial position. The new complexes [Ru₂(DPhF)₃(O₂CMe)(OPMe₃)][BF₄]?0.5 CH₂Cl₂ ( 1 ? 0.5 CH₂Cl₂) and [Ru₂(DPhF)₃(O₂CMe)(4‐pic)][BF₄] ( 2 ) (4‐pic=4‐methylpyridine) clearly display this influence. Complex 1 ?0.5 CH₂Cl₂ shows a magnetic moment corresponding to a S=3/2 system affected by the common zero‐field splitting (ZFS) and a weak antiferromagnetic interaction, whereas complex 2 displays an intermediate behaviour between S=3/2 and S=1/2 systems. The experimental data of complex 1 are fitted with a model that considers the ZFS effect using the Hamiltonian ?_D= S ? D ? S . The weak antiferromagnetic coupling is introduced as a perturbation, using the molecular field approximation. DFT calculations demonstrate that, in the [Ru₂(O₂CMe)(DPhF)₃(L)]⁺ complexes, the energy level of the metal–metal molecular orbitals is strongly dependent on the nature of the axial ligand (L). This study reveals that the increase in the π‐acceptor character of L leads to a greater split between the π* and δ* HOMO orbitals. The influence of the axial ligand in the relative energy between the doublet and quartet states in this type of complexes was also analysed. This study was performed on the new complexes 1 ?0.5 CH₂Cl₂ and 2 . The previously isolated [Ru₂(DPhF)₃(O₂CMe)(OH₂)][BF₄]?0.5 CH₂Cl₂ ( 3 ? 0.5 CH₂Cl₂) and [Ru₂(DPhF)₃(O₂CMe)(CO)][BF₄]?CH₂Cl₂ ( 4 ?CH₂Cl₂) complexes were also included in this study as representative examples of spin‐admixed and low‐spin configurations, respectively. The [Ru₂(DPhF)₃(O₂CMe)]⁺ ( 5 ) unit was used as a reference compound. These theoretical studies are in accordance with the different magnetic behaviour experimentally observed.     

Oxidative Addition of Dihydrogen to Divanadium in Solid Ne: Multiple‐Bonded Triplet HVVH and Singlet V2(μ‐H)2

Dinuclear compounds of early transition metals with a high metal–metal bond order are of fundamental interest due to their intriguing bonding situation and of practical interest because of their potential involvement in catalytic processes. In this work, two isomers of V₂H₂ have been generated in solid Ne by the reaction between V₂ and H₂ and detected by infrared spectroscopy: the linear HVVH molecule (³Σ_g^? ground state), which is the product of the spin‐allowed reaction between V₂ (³Σ_g^? ground state) and H₂, and a lower‐energy, folded V₂(μ‐H)₂ isomer (¹A₁ ground state) with two bridging hydrogen atoms. Both isomers are characterized by metal–metal bonding with a high bond order; the orbital occupations point to quadruple bonding. Irradiation with ultraviolet light induces the transformation of linear HVVH to folded V₂(μ‐H)₂, whereas irradiation with visible light initiates the reverse reaction.