PCM study of the solvent and substituent effects on bond dissociation energies of the C?NO bond

Quantum chemical calculations are used to estimate the equilibrium C? NO bond dissociation energies (BDEs) for eight X? NO molecule (X = CCl₃, C₆F₅, CH₃, CH₃CH₂, iC₃H₇, tC₄H₉, CH₂CHCH₂, and C₆H₅CH₂). These compounds are studied by employing the hybrid density functional theory (B3LYP, B3PW91, B3P86) methods together with 6‐31G** and 6‐311G** basis sets and the complete basis set (CBS‐QB3) method. The obtained results are compared with the available experimental results. It is demonstrated that B3P86/6‐31G** and CBS‐QB3 methods are accurate for computing the reliable BDEs for the X? NO molecule. Considering the inevitably computational cost of CBS‐QB3 method and the reliability of the B3P86 calculations, B3P86 method with 6‐31G** basis set may be more suitable to calculate the BDEs of the C? NO bond. The solvent effects on the BDEs of the C? NO bond are analyzed and it is shown that the C? NO BDEs in a vacuum computed by using B3PW91/6‐311G** method are the closest to the computed values in acetontrile and the average solvent effect is 1.48 kcal/mol. Subsequently, the substituent effects of the BDEs of the C? NO bond are further analyzed and it is found that electron denoting group stabilizes the radical and as a result BDE decreases; whereas electron withdrawing group stabilizes the group state of the molecule and thus increases the BDE from the parent molecule. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Factors affecting the lability of the σ(M-X) bond in cycloplatinated and cyclopalladated complexes containing [C(sp, ferrocene),N,X] or [C(sp, phenyl),N,X] (X = S, N) terdentate ligands

The reactions of the cyclometallated complexes [M{[(η⁵-C₅H₃)-CHN-(C₆H₄-2-SMe)]Fe(η⁵-C₅H₅)}Cl] [with M = Pt (5a) or Pd (5b)] with PPh₃ under different experimental conditions are reported. These studies have allowed the isolation of [M{[(η⁵-C₅H₃)-CHN-C₆H₄-2-SMe]Fe(η⁵-C₅H₅)}(PPh₃)]X [M = Pt and X⁻ = Cl⁻ (6a) or (7a) or M = Pd and X⁻ = Cl⁻ (6b) or (7b)] and the neutral complex [Pd{[(η⁵-C₅H₃)-CHN-(C₆H₄-2-SMe)]Fe(η⁵-C₅H₅)}Cl(PPh₃)] (8b). In 6-7a,b the ferrocenyl Schiff base behaves as a [C(sp², ferrocene),N,S]⁻ group while in 8b it acts as a [C(sp², ferrocene),N]⁻ ligand. The X-ray crystal structure of 7b confirms the mode of binding of the ferrocenyl ligand. The comparison of the results obtained and those reported for [M{(C₆H₄)-CHN-(CH₂-CH₂-2-SEt)}Cl] and [M{(C₆H₄)-CHN-(C₆H₄-2-SMe)}Cl] {with a [C(sp², phenyl),N,S]⁻ terdentate ligand} or [M{[(η⁵-C₅H₃)-CHN-(CH₂)₃-NMe₂]Fe(η⁵-C₅H₅)}Cl] {in which the ligand acts as a [C(sp², ferrocene),N,N′]⁻ group} have allowed the elucidation of the relative importance of the factors affecting the lability of the M-X (X = S or N′) and M-Cl bonds in cyclometallated compounds with [C,N,S]⁻ and [C(sp², ferrocene),N,X]⁻ ligands.     

High level of ab initio and density functional theory evaluation of the C?O bond dissociation energies in the dimethyl ether anion

The computational study of four possible first steps for the Wittig rearrangement of the dimethyl ether anion was investigated with a highly accurate complete basis set ab initio and density functional theory method. The initial step in all of these pathways is the C O bond breaking. The energies for these paths were computed and compared with the discussion of the mechanism of the Wittig [1, 2]‐rearrangement. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 73: 299–306, 1999     

Why is Re-Re bond formation/cleavage in [Re(bpy)(CO)3]2 different from that in [Re(CO)5]2? Experimental and theoretical studies on the dimers and fragments

The Re(NN)(CO)3(THF) (NN=bpy=2,2'-bipyridine or dmb=4,4'-dimethyl-2,2'-bipyridine) radical, produced by homolysis of [Re(NN)(CO)3]2 in THF solution by visible irradiation, dimerizes with rate constants kd=20 +/- 3 and 11 +/- 4 M(-1) s(-1) for NN=dmb and bpy, respectively. The dimerization processes are strikingly slow compared to those of typical metal radicals including Re(CO)5 (kd approximately 10(9) M(-1) s(-1)). In order to explain such slow reactions, we have performed B3LYP hybrid DFT and fully ab initio RHF and MP2 calculations on several conformations of [Re(bpy)(CO)3]2 (cis, trans, skewed cis, skewed trans) and [Re(CO)5]2 (staggered) and on their constituent monomer radicals and anions. The calculations show that the most stable geometry of [Re(bpy)(CO)3]2 is skewed cis, and the experimental infrared spectrum and photochemical properties of the [Re(bpy)(CO)3]2 dimer are best described by the calculated properties of the skewed cis conformer in which there is no low-lying unoccupied orbital that is predominantly sigma(MM) in character. The Re(bpy)(CO)3(THF) ligand radical is more stable than the 5-coordinate "17-electron" metal radical, Re(bpy)(CO)3, suggesting that the extremely slow dimerization rate most likely arises from the solvent blocking the binding site (i.e., the estimated fraction of the five-coordinate monomer is 1.6 x 10(-2)). Theoretical results are consistent with our experimental results that the dimerization process proceeds via the Re centered radical, which is involved in a pre-equilibrium favoring the ligand-centered radical. Furthermore, time-dependent DFT calculations on [Re(bpy)(CO)3]2 and [Re(bpy)(CO)3]- identify the origin of UV-vis absorption in THF.     

Novel route in the synthesis of ψ[CH2NH] amide bond surrogate

An alternative method for the synthesis of pseudopeptides containing a ψ[CH₂NH] amide bond surrogate is reported. The synthetic approach is based on a nucleophilic displacement of the chiral N-protected β-iodoamines with conveniently protected amino acid esters. The compatibility of this method with both conventional and microwave-assisted peptide synthesis should increase the potentiality of the ψ[CH₂NH] peptide bond isostere in peptide chemistry.     

The first decaruthenium hydrido cluster: synthesis and crystal structure of [N(PPh3)2]2[Ru10C(CO)24]·C6H14 and [N(PPh3)2][HRu10C(CO)24]

X-ray structural studies of new thermolysis products from the reaction of Ru₃(CO)₁₂ in heptane in the presence of 1,3,5-trimethylbenzene (mesitylene) confirm that they are the decaruthenium carbido-cluster dianion [Ru₁₀C(CO)₂₄]²⁻ (I) and the hydrido decaruthenium carbido-cluster monoanion [HRu₁₀C(CO)₂₄]⁻ (II). Both anions have the giant tetrahedron Ru₁₀ metal framework, and the monohydride provides the first example of a hydrido ligand in a tetrahedral Ru₄ cavity.     

Preparation and properties of poly-[4-4′-(p-biphenylene)amide]

Poly-[4-4′-(p-biphenylene)amide] was prepared by polycondensation of 4-amino-4′-carboxyl-biphenyl at 260–270°C in a nitrogen atmosphere. The thermal behaviour of the polyamide and the weight loss of the monomer, during the reaction of polycondensation, were investigated by Differential Thermal Analysis (DTA) and by Thermalgravimetric Analysis (TGA). Infrared spectrum and intrinsic viscosity of the polyamide were also studied.     

Comparative studies of the photochemical and thermal reactivities of the molybdenum-molybdenum single bond in [Mo2(η5-C5H5)2(CO)6] and triple bond in [Mo2(η5-C5H5)2(CO)4] towards nitrite and nitrate

Irradiation at λ = 507 and 391 nm of [Mo₂(η⁵-C₅H₅)₂(CO)₆] in a degassed tetrahydrofuran (THF) or THF-MeOH solution containing nitrite gives [Mo(η⁵-C₅H₅)₂NO] and several oxo complexes including [{Mo(η⁵-C₅H₅)(O)₂}₂O] in good yields. The quantum yields for the disappearance of [Mo₂(η⁵-C₅H₅)₂(CO)₆] in the reaction with NO₂⁻ depend on the nitrite concentration, thus suggesting participation of the metal-radical intermediate in the reduction of nitrite. Reactions of [Mo₂(η⁵-C₅H₅)₂(CO)₄] with nitrite or nitrate in the dark give the same nitrosyl and oxo complexes as above. An oxygen atom in nitrite or nitrate is mainly transferred onto the molybdenum atom both in the photochemical and the dark reactions.     

Is Se-Se bond cleavage the most favourable process in electron attachment to diselenides? The importance of asymmetry

Accurate ab initio calculations reveal that, in contrast with what has been commonly assumed up to now, electron attachment to diselenides does not always result in a Se-Se bond fission and, in general, the mechanism behind the bond fission may involve the crossing of several states.     

Is the peptide bond formation activated by Cu(2+) interactions? Insights from density functional calculations

The catalytic role that Cu(2+) cations play in the peptide bond formation has been addressed by means of density functional calculations. First, the Cu(2+)-(glycine)2 --> Cu(2+)-(glycylglycine) + H2O reaction was investigated since mass spectrometry low collision activated dissociation (CAD) spectra of Cu(2+)-(glycine)2 led to the elimination of a water molecule, which suggested that an intracomplex peptide bond formation might have occurred. Results show that this intracomplex condensation is associated to a very high free energy barrier (97 kcal mol(-1)) and reaction free energy (66 kcal mol(-1)) because of the loss of metal coordination during the reaction. Second, on the basis of the salt-induced peptide formation theory, the condensation reaction between two glycines was studied in aqueous solution using discrete water molecules and the conductor polarized continuum model (CPCM) continuous method. It is found that the synergy between the interaction of glycines with Cu(2+) and the presence of water molecules acting as proton-transfer helpers significantly lower the activation barrier (from 55 kcal/mol for the uncatalyzed system to 20 kcal/mol for the Cu(2+) solvated system) which largely favors the formation of the peptide bond.     

Cα-Cβ and Cα-N bond cleavage in the dissociation of protonated N-benzyllactams: dissociative proton transfer and intramolecular proton-transport catalysis

In mass spectrometry of protonated N-benzylbutyrolactams, the added proton is initially localized on the carbonyl oxygen, which is the thermodynamically preferred protonation site. Upon collisional activation, dissociative proton transfer takes place leading to the occurrence of fragmentation reactions. The major fragmentations observed are the cleavages of C(α)-C(β) and C(α)-N bonds on the two sides of the methylene linker, which is different to the cleavage of the amide bond itself seen in most amide cases. Theoretical calculations and isotopic labeling experiments demonstrate that the phenyl ring regulates the proton transfer reactions. The proton directly migrates to the C(β) position via a 1,5-H shift leading to the efficient loss of benzene, while it stepwise migrates to the amide nitrogen resulting in the formation of a benzyl cation. The stepwise proton transfer is achieved via intramolecular proton-transport catalysis. The C(γ) position accepts the proton from the carbonyl oxygen via a 1,6-H shift, and then donates it to the amide nitrogen via a 1,4-H shift. The general 1,3-H shift from the carbonyl oxygen to the amide nitrogen can be excluded in this case due to its significant energy barrier. The substituent effects are also applied to explore the reaction mechanism, and it proves that both C(β) and C(γ) are involved in the dissociative proton transfer processes. For monosubstituted N-benzylbutyrolactams, the abundance ratios of the two competing product ions are well correlated with the nature of the substituents.     

A theoretical investigation on the N–N bond cleavage in Ta(IV) hydrazidium and Ta(V) hydrazido complexes

The reaction mechanism of the N–N bond cleavage in Ta(IV) hydrazido and hydrazidium complexes is studied using density functional theory. The N–N bond cleavage in Ta(IV) hydrazidium generates formal Ta(IV) nitridyl. The N–N bond cleavage in Ta(V) hydrazido gives terminal Ta(V) nitrido species. In the tetrahydrofuran solvent, terminal Ta(V) nitrido dimerizes through a one-step direct pathway leading to the [Ta(V),Ta(V)] bis(μ-nitrido) product. Two Ta–N bonds form simultaneously between the Ta center of one molecule and the terminal N atom of another. In the toluene solvent, there are two pathways of H atom abstraction and protonation producing mononuclear Ta(V) parent imide. The former consists of three steps originated from formal Ta(IV) nitridyl. The latter is unfavorable with terminal Ta(V) nitrido as the precursor.     

PCM study of the solvent and substituent effects on bond dissociation energies of the O?NO Bond—A DFT study

A PCM continuum model, at the B3LYP, B3P86, and B3PW91 three‐parameter hybrid DFT methods with 6‐311G** basis set, is used to study the bond dissociation energies (BDEs) of benzyl nitrites. Compared the computed results with the experimental values, it is noted that B3PW91 functional is the best method to compute the BDEs of benzyl nitrites. The solvent and substituent effects on the BDEs of the O? NO bond are analyzed, and it is shown that the BDE of the O? NO bond decreases with the increment of the Hammett constants of substituent groups on benzene for benzyl nitrites except C₆H₅CH₂O? NO. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Role of anions in the kinetics of the acid catalyzed dissociation of tris-(2,2′bipyridine) Fe(II) complex

Excess of Cl⁻ and NO₃⁻ ions bring about a significant increase in the rate of dissociation of the tris-(2,2′bipyridine) Fe(II) complex in HCl, HClO₄ and H₂SO₄ in a very narrow range of H⁺ concentration (∼0·1−0·3 M). Above and below this concentration range, these anions do not lead to significant variation in the rate of dissociation. This points to the involvement of these ions in a kinetic step which is only important in the rate expression in this narrow range. It is suggested that the anions occupy the coordination site left vacant by the partial dissociation of the complex leading to the half bonded intermediate. This would significantly lower the free energy of activation for the subsequent protonation step leading to complete dissociation and thus lead to acceleration. The lowering of the free energy of activation will be mainly due to an increase in entropy of this intermediate on account of charge reduction and lower rigidity due to lack of hydrogen bonding. The retarding effect of SO₄²⁻ concentrations and the unambiguously interpreted as its addition leads to simultaneous changes in H⁺ and HSO₄⁻ concentrations and the possibility of ion pair formation with the complex cannot be ruled out.     

A New [5]Ferrocenophane Containing a N–Si–O–Si–N Bridge. Molecular Structure in the Solid State and in Solution

The reaction of 1,1-diaminoferrocene 1 with 1,3-dichloro-1,1,3,3-tetramethyl-1,3-disiloxane in the presence of triethylamine gave the new 1,5,3,2,4-diazaoxadisila[5]ferrocenophane, which was characterized in the solid state by X-ray structural analysis, ¹³C and ²⁹Si MAS NMR spectroscopy, and in solution by [¹H], [¹³C], [¹⁵N], and [²⁹Si]NMR spectroscopy. The ideal ferrocene geometry is slightly distorted, and the cycle containing the heteroelements N, Si, and O is nonplanar. In solution, NMR spectra indicate dynamic processes which may involve both the cyclopentadienyl rings and ring inversion.     

Synthesis of benzoindoloquinolizines via a Cu(I)-mediated C–N bond formation

An effective synthesis of the multi ring-fused benzoindoloquinolizines has been accomplished by Cu(I)-mediated and MW-assisted C–N_amide bond formation of benzo[a]quinolizin-4-ones. The deamination of tetrahydro-2H-pyrido[2,1-a]isoquinolines was also studied and was found to give benzoquinolizines. The benzo[a]quinolizin-4-ones were prepared based on the annulations of C-1 substituted 3,4-dihydroisoquinolines and azlactones.     

Binary group 15 polyazides. structural characterization of [Bi(N3)4]-, [Bi(N3)5]2-, [bipy·Bi(N3)5]2-, [Bi(N3)6]3-, bipy·As(N3)3, bipy·Sb(N3)3, and [(bipy)2·Bi(N3)3]2 and on the lone pair activation of valence electrons

The binary group 15 polyazides As(N(3))(3), Sb(N(3))(3), and Bi(N(3))(3) were stabilized by either anion or donor-acceptor adduct formation. Crystal structures are reported for [Bi(N(3))(4)](-), [Bi(N(3))(5)](2-), [bipy·Bi(N(3))(5)](2-), [Bi(N(3))(6)](3-), bipy·As(N(3))(3), bipy·Sb(N(3))(3), and [(bipy)(2)·Bi(N(3))(3)](2). The lone valence electron pair on the central atom of these pnictogen(+III) compounds can be either sterically active or inactive. The [Bi(N(3))(5)](2-) anion possesses a sterically active lone pair and a monomeric pseudo-octahedral structure with a coordination number of 6, whereas its 2,2'-bipyridine adduct exhibits a pseudo-monocapped trigonal prismatic structure with CN 7 and a sterically inactive lone pair. Because of the high oxidizing power of Bi(+V), reactions aimed at Bi(N(3))(5) and [Bi(N(3))(6)](-) resulted in the reduction to bismuth(+III) compounds by [N(3)](-). The powder X-ray diffraction pattern of Bi(N(3))(3) was recorded at 298 K and is distinct from that calculated for Sb(N(3))(3) from its single-crystal data at 223 K. The [(bipy)(2)·Bi(N(3))(3)](2) adduct is dimeric and derived from two BiN(8) square antiprisms sharing an edge consisting of two μ(1,1)-bridging N(3) ligands and with bismuth having CN 8 and a sterically inactive lone pair. The novel bipy·As(N(3))(3) and bipy·Sb(N(3))(3) adducts are monomeric and isostructural and contain a sterically active lone pair on their central atom and a CN of 6. A systematic quantum chemical analysis of the structures of these polyazides suggests that the M06-2X density functional is well suited for the prediction of the steric activity of lone pairs in main-group chemistry. Furthermore, it was found that the solid-state structures can strongly differ from those of the free gas-phase species or those in solutions and that lone pairs that are sterically inactive in a chemical surrounding can become activated in the free isolated species.     

N,N′-methylenedipyridinium Pt(II) and Pt(IV) hybrid salts: synthesis, crystal and molecular structures of [(C5H5N)2CH2] · [PtCl4] and [(C5H5N)2CH2] · [PtCl6]

The highly insoluble organic-inorganic hybrid ionic compounds N,N??-methylenedipyridinium tetrachloroplatinate(II) [(C₅H₅N)₂CH₂] · [PtCl₄] and N,N??-methylenedipyridinium hexachloroplatinate(IV) [(C₅H₅N)₂CH₂] · [PtCl₆] were obtained by the treatment of N,N??-methylenedipyridinium dichloride monohydrate [(C₅H₅N)₂CH₂]Cl₂ · H₂O with K₂[PtCl₄] or (NH₄)₂[PtCl₆], respectively, in an aqueous solution. Both complexes were isolated, purified, characterised by elemental analysis, and their molecular structures were confirmed by powder X-ray diffraction. The crystal structure of both compounds consists of separated discrete dications [(C₅H₅N)₂CH₂]²⁺ and anions [PtCl _n ]^2? (n = 4 or 6). As anticipated, the dications formed a butterfly shape consisting of two pyridine rings bound to the methylene group via their N atoms, while the Pt centre had a square planar geometry in [(C₅H₅N)₂CH₂] · [PtCl₄] and an octahedral coordination in [(C₅H₅N)₂CH₂] · [PtCl₆]. Interestingly, both crystal structures are stabilised by intermolecular C-H??Cl non-standard hydrogen bonds, ??-?? ring interactions between two pyridine rings of adjacent dications, and also by Cl-?? interactions.