Side‐On Bonded Beryllium Dinitrogen Complexes

The preparation and spectroscopic identification of the complexes NNBe(η²‐N₂) and (NN)₂Be(η²‐N₂) and the energetically higher lying isomers Be(NN)₂ and Be(NN)₃ are reported. NNBe(η²‐N₂) and (NN)₂Be(η²‐N₂) are the first examples of covalently side‐on bonded N₂ adducts of a main‐group element. The analysis of the electronic structure using modern methods of quantum chemistry suggests that NNBe(η²‐N₂) and (NN)₂Be(η²‐N₂) should be classified as π complexes rather than metalladiazirines.     

Generation and Identification of the Linear OCBNO and OBNCO Molecules with 24 Valence Electrons

Two structural isomers containing five second-row element atoms with 24 valence electrons were generated and identified by matrix-isolation IR spectroscopy and quantum chemical calculations. The OCBNO complex, which is produced by the reaction of boron atoms with mixtures of carbon monoxide and nitric oxide in solid neon, rearranges to the more stable OBNCO isomer on UV excitation. Bonding analysis indicates that the OCBNO complex is best described by the bonding interactions between a triplet-state boron cation with an electron configuration of (2s)⁰(2p_σ)⁰(2p_π)² and the CO/NO⁻ ligands in the triplet state forming two degenerate electron-sharing π bonds and two ligand-to-boron dative σ bonds.     

Two isomers of Pd2(S2NC7H4)4

The dichloromethane solvates of the isomers tetrakis(μ‐1,3‐benzothiazole‐2‐thiolato)‐κ⁴N:S;κ⁴S:N‐dipalladium(II)(Pd—Pd), (I), and tetrakis(μ‐1,3‐benzothiazole‐2‐thiolato)‐κ⁶N:S;κ²S:N‐dipalladium(II)(Pd—Pd), (II), both [Pd₂(C₇H₄NS₂)₄]·CH₂Cl₂, have been synthesized in the presence of (o‐isopropylphenyl)diphenylphosphane and (o‐methylphenyl)diphenylphosphane. Both isomers form a lantern‐type structure, where isomer (I) displays a regular and symmetric coordination and isomer (II) an asymmetric and distorted structure. In (I), sitting on an centre of inversion, two 1,3‐benzothiazole‐2‐thiolate units are bonded by a Pd—N bond to one Pd atom and by a Pd—S bond to the other Pd atom, and the other two benzothiazolethiolate units are bonded to the same Pd atoms by, respectively, a Pd—S and a Pd—N bond. In (II), three benzothiazolethiolate units are bonded by a Pd—N bond to one Pd atom and by a Pd—S bond to the other Pd atom, and the fourth benzothiazolethiolate unit is bonded to the same Pd atoms by, respectively, a Pd—S bond and a Pd—N bond.     

Striking Differences in Properties of Geometric Isomers of [Ir(tpy)(ppy)H]+: Experimental and Computational Studies of their Hydricities,Interaction with CO2, and Photochemistry

We prepared two geometric isomers of [Ir(tpy)(ppy)H]⁺, previously proposed as a key intermediate in the photochemical reduction of CO₂ to CO, and characterized their notably different ground‐ and excited‐state interactions with CO₂ and their hydricities using experimental and computational methods. Only one isomer, C‐trans‐[Ir(tpy)(ppy)H]⁺, reacts with CO₂ to generate the formato complex in the ground state, consistent with its calculated hydricity. Under photocatalytic conditions in CH₃CN/TEOA, a common reactive C‐trans‐[Ir(tpy)(ppy)]⁰ species, irrespective of the starting isomer or monodentate ligand (such as hydride or Cl), reacts with CO₂ and produces CO with the same catalytic efficiency.     

Striking Differences in Properties of Geometric Isomers of [Ir(tpy)(ppy)H]+: Experimental and Computational Studies of their Hydricities,Interaction with CO2, and Photochemistry

We prepared two geometric isomers of [Ir(tpy)(ppy)H]⁺, previously proposed as a key intermediate in the photochemical reduction of CO₂ to CO, and characterized their notably different ground‐ and excited‐state interactions with CO₂ and their hydricities using experimental and computational methods. Only one isomer, C‐trans‐[Ir(tpy)(ppy)H]⁺, reacts with CO₂ to generate the formato complex in the ground state, consistent with its calculated hydricity. Under photocatalytic conditions in CH₃CN/TEOA, a common reactive C‐trans‐[Ir(tpy)(ppy)]⁰ species, irrespective of the starting isomer or monodentate ligand (such as hydride or Cl), reacts with CO₂ and produces CO with the same catalytic efficiency.     

Formation and Characterization of a BeOBeC Multiple Radical Featuring a Quartet Carbyne Moiety

Through reaction of beryllium dimers with carbon monoxide, a carbonyl complex BeBeCO is formed in solid neon. Upon visible light excitation, the BeBeCO complex rearranges to a BeCOBe isomer, which further isomerizes to a low‐energy BeOBeC species under UV‐visible light excitation. These species are identified on the basis of infrared absorption spectroscopy with isotopic substitutions and quantum chemical studies. The BeOBeC molecule is characterized to be a multiple radical species having an electronic quintet ground state featuring an unusual quartet carbyne unit with three unpaired electrons on the carbon center. Bonding analysis indicates that the strong Pauli repulsion between carbon 2s lone pair electrons and the σ electrons of the BeOBe fragment significantly weakens the Be?C bonding and destabilizes the triplet state of the BeOBeC radical with a doublet carbyne unit. The three‐center π‐bonding of BeOBe is also found to play a role in stabilizing the quartet carbyne.     

The Oxygen‐Rich Beryllium Oxides BeO4 and BeO6

Two novel isomers of BeO₄ with the structures OBeOOO and OBe(O₃) in the electronic triplet state have been prepared as well as the known disuperoxide complex Be(O₂)₂ in solid noble‐gas matrices. We also report the synthesis of the oxygen‐rich bis(ozonide) complex Be(O₃)₂ in the triplet state which has a D_2d equilibrium geometry. The molecular structures were identified by infrared absorption spectroscopy with isotopic substitutions as well as quantum chemical calculations.     

Solvent Migration in Microhydrated Aromatic Aggregates: Ionization‐Induced Site Switching in the 4‐Aminobenzonitrile–Water Cluster

The dependence of the preferred microhydration sites of 4‐aminobenzonitrile (4ABN) on electronic excitation and ionization is determined through IR spectroscopy of its clusters with water (W) in a supersonic expansion and through quantum chemical calculations. IR spectra of neutral 4ABN and two isomers of its hydrogen‐bonded (H‐bonded) 4ABN–W complexes are obtained in the ground and first excited singlet states (S₀, S₁) through IR depletion spectroscopy associated with resonance‐enhanced multiphoton ionization. Spectral analysis reveals that electronic excitation does not change the H‐bonding motif of each isomer, that is, H₂O binding either to the CN or the NH site of 4ABN, denoted as 4ABN–W(CN) and 4ABN–W(NH), respectively. The IR spectra of 4ABN⁺–W in the doublet cation ground electronic state (D₀) are measured by generating them either in an electron ionization source (EI‐IR) or through resonant multiphoton ionization (REMPI‐IR). The EI‐IR spectrum shows only transitions of the most stable isomer of the cation, which is assigned to 4ABN⁺–W(NH). The REMPI‐IR spectrum obtained through isomer‐selective resonant photoionization of 4ABN–W(NH) is essentially the same as the EI‐IR spectrum. The REMPI‐IR spectrum obtained by ionizing 4ABN–W(CN) is also similar to that of the 4ABN⁺–W(NH) isomer, but differs from that calculated for 4ABN⁺–W(CN), indicating that the H₂O ligand migrates from the CN to the NH site upon ionization with a yield of 100 %. The mechanism of this CN→NH site‐switching reaction is discussed in the light of the calculated potential energy surface and the role of intracluster vibrational energy redistribution.     

Spectroscopic and DFT studies of photoactivatable Mn(I) tricarbonyl complexes

A combined experimental study and density functional theory calculations of fac‐[MnBr (CO)₃L] complexes (L = 2‐(2′‐pyridyl)benzimidazole ligand, furnished with either morpholine (L^morph) or phthalimido (L^phth) side‐chain) were performed using different spectral and analytical tools. The synthesized complexes released carbon monoxide upon the exposure to LED source light at 468 nm. Illumination of fac‐[MnBr (CO)₃L] (10 μM) in the myoglobin solution (Mb) produced about 25 μM MbCO. The plateau of the CO release process is attained within 25 min. With the aid of time‐dependent density functional theory calculations, the observed lowest energy absorption transition at ~ 400 nm has a ground‐state composed of d (Mn)/π (pyridyl) and excited‐state of ligand π*‐orbitals forming MLCT/π‐π^*. Natural population analyses of fac‐[MnBr (CO)₃L] were carried out to get information about the strength of Mn–CO bonds, electronic arrangment and natural charge of manganese ion.     

Multiple Bonding Between Group 3 Metals and Fe(CO)3−

Heteronuclear Group 3 metal/iron carbonyl anion complexes ScFe(CO)₃^?, YFe(CO)₃^?, and LaFe(CO)₃^? are prepared in the gas phase and studied by mass‐selective infrared (IR) photodissociation spectroscopy as well as quantum‐chemical calculations. All three anion complexes are characterized to have a metal–metal‐bonded C_3v equilibrium geometry with all three carbonyl ligands bonded to the iron center and a closed‐shell singlet electronic ground state. Bonding analyses reveal that there are multiple bonding interactions between the bare group‐3 elements and the Fe(CO)₃^? fragment. Besides one covalent electron‐sharing metal–metal σ bond and two dative π bonds from Fe to the Group 3 metal, there is additional multicenter covalent bonding with the Group 3 atom bonded to Fe and the carbon atoms.     

Structure and Stability of Interstellar Molecule C_3S

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An ab initio study of the electronic structure of diimide

Ab initio calculations on the structure and geometry of the three isomers of N₂H₂ (trans-diimide, cis-diimide, and 1,1-dihydrodiazine) were performed both on HF and CI level using gaussian basis sets with polarization functions. The trans and cis isomers have singlet ground states; the trans isomer is found to be lower in energy than the cis isomer by 6.9 kcal/mol (HF) and 5.8 kcal/mol (CI), respectively. The barrier for the trans-cis isomerization is predicted to be 56 (HF) and 55 (CI) kcal/mol. H₂ N=N has a triplet ground state with a non-planar equilibrium geometry and a rather long NN bond of 1.34 Å. Its lowest singlet state, however, is planar with an NN double bond of 1.22 Å; it is found to lie about 3 kcal/mol above the triplet and 26 kcal/mol above the singlet ground state of trans-diimide.     

Synthesis and structures of ruthenium di‐ and tricarbonyl complexes derived from 4,5‐diazafluoren‐9‐one

Carbon monoxide (CO) has recently been shown to impart beneficial effects in mammalian physiology and considerable research attention is now being directed toward metal–carbonyl complexes as a means of delivering CO to biological targets. Two ruthenium carbonyl complexes, namely trans‐dicarbonyldichlorido(4,5‐diazafluoren‐9‐one‐κ²N,N′)ruthenium(II), [RuCl₂(C₁₁H₆N₂O)(CO)₂], (1), and fac‐tricarbonyldichlorido(4,5‐diazafluoren‐9‐one‐κN)ruthenium(II), [RuCl₂(C₁₁H₆N₂O)(CO)₃], (2), have been isolated and structurally characterized. In the case of complex (1), the trans‐directing effect of the CO ligands allows bidentate coordination of the 4,5‐diazafluoren‐9‐one (dafo) ligand despite a larger bite distance between the N‐donor atoms. In complex (2), the cis disposition of two chloride ligands restricts the ability of the dafo molecule to bind ruthenium in a bidentate fashion. Both complexes exhibit well defined ¹H NMR spectra confirming the diamagnetic ground state of Ru^II and display a strong absorption band around 300 nm in the UV.     

Bonding and correlation analysis of various Si2CO isomers on the potential energy surface

At various levels of theory, singlet and triplet potential energy surfaces (PESs) of Si₂CO, which has been studied using matrix isolation infrared spectroscopy, are investigated in detail. A total of 30 isomers and 38 interconversion transition states are obtained at the B3LYP/6‐311G(d) level. At the higher CCSD(T)/6‐311+G(2d)//QCISD/6‐311G(2d)+ZPVE level, the global minimum ¹1 (0.0 kcal/mol) corresponds to a three‐membered ring singlet O‐cCSiSi (¹A′). On the singlet PES, the species ¹2 (0.2 kcal/mol) is a bent SiCSiO structure with a ¹A′ electronic state, followed by a three‐membered ring isomer Si‐cCSiO (¹A′) ¹3 (23.1 kcal/mol) and a linear SiCOSi ¹4 (¹Σ⁺) (38.6 kcal/mol). The isomers ¹1, ¹2, ¹3 , and ¹4 possess not only high thermodynamic stabilities, but also high kinetic stabilities. On the triplet PES, two isomers ³1 (³B₂) (18.8 kcal/mol) and ³7 (³A″) (23.3 kcal/mol) also have high thermodynamic and kinetic stabilities. The bonding natures of the relevant species are analyzed. The similarities and differences between C₃O, C₃S, SiC₂O, and SiC₂S are discussed. The present results are also expected to be useful for understanding the initial growing step of the CO‐doped Si vaporization processes. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

1,1‐Dilithioethylene: Toward Spectroscopic Identification of the Definitive Singlet Ground Electronic State of a Peculiar Structure

1,1‐Dilithioethylene is a prototypical carbon–lithium compound that is not known experimentally. All low‐lying singlet and triplet structures of interest were investigated by using high‐level theoretical methods with correlation‐consistent basis sets up to pentuple ζ. The coupled cluster methods adopted included up to full triple excitations and perturbative quadruples. In contrast to earlier studies that predicted the twisted C_2v triplet to be the ground state, we found a peculiar planar C_s singlet ground state in the present research. The lowest excited electronic state of 1,1‐dilithioethylene, the twisted C_s triplet, was found to lie 9.0 kcal mol^?1 above the ground state by using energy extrapolation to the complete basis set limit. For the planar C_s singlet and twisted C_s triplet states of 1,1‐dilithioethylene, anharmonic vibrational frequencies were reported on the basis of second‐order vibrational perturbation theory. The remarkably low (2050 cm^?1) C?H stretching fundamental (the C?H bond near the bridging lithium) of the singlet state was found to have very strong infrared intensity. These highly reliable theoretical findings may assist in the long‐sought experimental identification of 1,1‐dilithioethylene. Using natural bond orbital analysis, we found that lithium bridging structures were strongly influenced by electrostatic effects. All carbon–carbon linkages corresponded to conventional double bonds.     

Theoretical study on electronic structure of (CNC)Fe_2N2 and its N2 elimination mechanism

The density function theory (DFT) is to elucidate the electronic structure of bis(dinitrogen) Fe(0) complex, (CNC)Fe_2N₂, and its N₂ elimination mechanism. (CNC)Fe_2N₂ has a low‐spin singlet (S = 0) ground state with a distorted square pyramidal structure. Fragment orbital interaction analysis yields total occupancy of π* orbitals (LUF₍₄₎O and LUF₍₄₎O−1) of apical N3 N4 is 0.188 while that of basal N1 N2 is 0.187 in ^S0(CNC)Fe_2N₂, suggesting nearly the same activation extent for both basal and apical N₂ ligands. The lowest‐lying triplet state T₁ (3‐A′) has a repulsive potential energy surface along the Fe N3 bong length by PBE functional, while a minimum on T₂ state (3‐A″) with higher energy is found by B3LYP functional. The nonadiabatic N₂ elimination mechanism of (CNC)Fe_2N₂ involves an S₀‐T₁ states crossing, which lowers the activation energy to 9.7 kcal/mol and produces high‐spin intermediate (CNC)Fe N₂. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Side-On Bonded Beryllium Dinitrogen Complexes

The preparation and spectroscopic identification of the complexes NNBe(η²-N₂) and (NN)₂Be(η²-N₂) and the energetically higher lying isomers Be(NN)₂ and Be(NN)₃ are reported. NNBe(η²-N₂) and (NN)₂Be(η²-N₂) are the first examples of covalently side-on bonded N₂ adducts of a main-group element. The analysis of the electronic structure using modern methods of quantum chemistry suggests that NNBe(η²-N₂) and (NN)₂Be(η²-N₂) should be classified as π complexes rather than metalladiazirines.     

Square‐Planar Ruthenium(II) Complexes: Control of Spin State by Pincer Ligand Functionalization

Functionalization of the PNP pincer ligand backbone allows for a comparison of the dialkyl amido, vinyl alkyl amido, and divinyl amido ruthenium(II) pincer complex series [RuCl{N(CH₂CH₂PtBu₂)₂}], [RuCl{N(CHCHPtBu₂)(CH₂CH₂PtBu₂)}], and [RuCl{N(CHCHPtBu₂)₂}], in which the ruthenium(II) ions are in the extremely rare square‐planar coordination geometry. Whereas the dialkylamido complex adopts an electronic singlet (S=0) ground state and energetically low‐lying triplet (S=1) state, the vinyl alkyl amido and the divinyl amido complexes exhibit unusual triplet (S=1) ground states as confirmed by experimental and computational examination. However, essentially non‐magnetic ground states arise for the two intermediate‐spin complexes owing to unusually large zero‐field splitting (D>+200 cm^?1). The change in ground state electronic configuration is attributed to tailored pincer ligand‐to‐metal π‐donation within the PNP ligand series.     

Co‐crystallized cis and trans isomers of dichloro­bis(2‐picolylamine)iron(II)

The octa­hedral cis and trans isomers of dichloro­bis(2‐picolyl­amine)iron(II), [FeCl₂(C₆H₈N₂)₂], co‐crystallize in a 1:1 ratio. The cis isomer lies on a twofold axis, whereas the trans isomer lies on an inversion centre. The structure is fully ordered, with both Fe atoms in a pure high‐spin state. The Fe, Cl and N(H₂) atoms of both isomers lie in the same plane, allowing all Cl and amine H atoms to be engaged in extensive two‐dimensional hydrogen bonding. The hydrogen‐bonded layers are inter­connected through π–π inter­actions between the pyridine rings. Searches in the Cambridge Structural Database uncover very few examples of such isomer co‐existence.     

A TD‐DFT Study on the Effect of Intermolecular Hydrogen Bonding on the Photophysical Properties of N‐Methyl‐1,8‐naphthalimide

The effect of intermolecular hydrogen bonding on the photophysical properties of N‐methyl‐1,8‐naphthalimide ( 2 ) has been investigated by time‐dependent density functional theory (TD‐DFT) method. The UV and IR spectra of 2 monomer and its hydrogen‐bonded complexes formed with 2,2,2‐trifluoroethanol (TFE) 2 +TFE and 2 +2TFE have been calculated, which confirm the presence of intermolecular hydrogen bonding interactions between the carbonyl groups of the aromatic imide and the hydroxyl group of the polyfluorinated alcohol. The absorption and fluorescence intensities going from 2 monomer via hydrogen‐bonded complex 2 +TFE to 2 +2TFE were found to be gradually enhanced with the wavelength gradually red‐shifted. The enhancements of the fluorescence intensities from 2 monomer to hydrogen‐bonded complexes 2 +TFE and 2 +2TFE were attributed to the decrease of the intersystem crossing (ISC) efficiency from the first excited singlet state S₁ ¹(ππ*) to the second excited triplet state T₂ ³(nπ*), whose energy was increased relative to its ground state due to the intermolecular hydrogen bonding interactions.