Electronic structure and reactivity of organofluorine compounds

It was established on the basis of MNDO and STO-3G quantum-chemical calculations of fluoroethylenes that the fluorine atom at the vinylic position greatly stabilizes all the-MOs of the molecule on account of the accepting induction effect. The absence of additional stabilization of the-MO is due to the antiphase overlap of the free pair of the fluorine atom and the-MO. The concept of the electronic depletion of the double bond in fluorinated olefins is not confirmed by the calculations.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1801–1805, August, 1989.     

Electronic structure and reactivity of organofluorine compounds

The basicity of isomeric bromoethylamines having a CF₃ substituent in the - and -positions with respect to the amino group was studied. According to potentiometric titration in H₂O, CH₃NO₂, and CH₃CN, the basicity of -trifluoromethylamines is 4–5 orders of magnitude lower than that of -trifluoromethyl isomers. Quantum-chemical calculations (AM1) showed that the decrease in the basicity of -trifluoromethylamines does not correlate with the change in the electron density at the nitrogen atom.For part 5, seeIzv. Akad. Nauk, Ser. Khim., 1992, 1334 [Bull. Russ. Acad. Sci., Div. Chem. Sci., 1992,41, 1041 (Engl. Transl.)].Deceased October 8, 1993.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 279–281, February, 1994.     

Electronic structure and reactivity of 3-hydroxyquinoline

The reactivity indexes of the neutral, dipolar, cationic, and anionic forms of 3-hydroxy-quinoline were calculated by the simple MO LCAO method using dynamic and statistical approximations. The predicted (on the basis of the localization energies) charge distributions, boundary densities, free valence indexes, and orientations of electrophilic substituents for the cationic and anionic forms of 3-hydroxyquinoline are in good agreement with the experimental data. The orientations of nucleophilic and radical substituents for the four forms of 3-hydroxyquinoline are predicted. The reactivity indexes of the neutral form of 3-hydroxyquinoline were calculated by means of the Pariser-Parr-Pople method.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 191–196, February, 1972.     

The structure and reactivity of iron nitride complexes

The structure and reactivity of discrete iron nitride complexes is described. Six-coordinate, four-fold symmetric nitrides are thermally unstable, and have been characterized at cryogenic temperatures by an arsenal of spectroscopic methods. By contrast, four-coordinate, three-fold symmetric iron nitrides can be prepared at room temperature. A range of diamagnetic iron(IV) nitrides have been reported and in some cases, isolated. Among these are the isolable, yet reactive, tris(carbene)borate iron(IV) nitrides. These complexes can effect two-electron nitrogen atom transfer to a range of substrates, in some cases with complete atom transfer occuring through Fe-N bond cleavage. These nitrides are also active in single electron pathways, including the synthesis of ammonia by a mechanism involving hydrogen atom transfer to the nitride ligand. One-electron oxidation of a tris(carbene)borate iron(IV) nitride leads to an isolable iron(V) complex that is unusually reactive for a metal nitride.     

Electronic structure and reactivity of cobalt oxide dimers and their hexacarbonyl complexes: a density functional study

The dimers of cobalt oxide (CoO)(2) with cyclic and open bent structure are studied with the B1LYP density functional; the ordering of states is validated by the CCSD(T) method. The D(2h)-symmetry rhombic dioxide Co(2)O(2) with antiferromagnetically ordered electrons on cobalt centers is the global minimum. The cyclic peroxide Co(2)(O(2)) with side-on-bonded dioxygen in (7)B(2) ground state is separated from the global minimum by an energy gap of 3.15 eV. The dioxide is highly reactive as indicated by the high value of proton affinity and chemical reactivity indices. The four-member ring structures are more stable than those with three-member ring or chain configuration. The thermodynamic stability toward dissociation to CoO increases upon carbonylation, whereas proton affinity and reactivity with release of molecular oxygen also increase. The global minimum of Co(2)O(2)(CO)(6) corresponds to a triplet state (3)A" with oxygen atoms shifted above the molecular plane of the rhombic dioxide Co(2)O(2). The SOMO-LUMO gap in the ground-state carbonylated dioxide is wider, compared to the same gap in the bare dicobalt dioxide. The peroxo-isomer Co(2)(O(2))(CO)(6) retains the planar Co(2)(O(2)) ring and is only stable in a high-spin state (7)A". The carbonylated clusters have increased reactivity in both redox and nucleophilic reactions, as a result of the increased electron density in the Co(2)O(2)-ring area.     

Electronic structure and reactivity of low-spin Fe(III)-hydroperoxo complexes: comparison to activated bleomycin

The spectroscopic properties, electronic structure, and reactivity of the low-spin Fe(III)-hydroperoxo complex [Fe(N4Py)(OOH)](2+) (1, N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) are investigated in comparison to those of activated bleomycin (ABLM). Complex 1 is characterized by Raman features at 632 (Fe-O stretch) and 790 cm(-1) (O-O stretch), corresponding to a strong Fe-O bond (force constant 3.62 mdyn/A) and a weak O-O bond (3.05 mdyn/A). The UV-vis spectrum of 1 shows a broad absorption band around 550 nm that is assigned to a charge-transfer transition from the hydroperoxo to a t(2g) d orbital of Fe(III) using resonance Raman and MCD spectroscopies and density functional (DFT) calculations. Compared to low-spin [Fe(TPA)(OH(x))(OO(t)Bu)](x+)(TPA = tris(2-pyridylmethyl)amine, x = 1 or 2), an overall similar Fe-OOR bonding results for low-spin Fe(III)-alkylperoxo and -hydroperoxo species. Correspondingly, both systems show similar reactivities and undergo homolytic cleavage of the O-O bond. From the DFT calculations, this reaction is more endothermic for 1 due to the reduced stabilization of the .OH radical compared to .O(t)Bu and the absence of the hydroxo ligand that helps to stabilize the resulting Fe(IV)=O species. In contrast, ABLM has a somewhat different electronic structure where no pi donor bond between the hydroperoxo ligand and iron(III) is present [Neese, F.; Zaleski, J. M.; Loeb-Zaleski, K.; Solomon, E. I. J. Am. Chem. Soc. 2000, 122, 11703]. Possible reaction pathways for ABLM are discussed in relation to known experimental results.     

Electronic structure and reactivity of S—S dications

The electronic structure and reactivity of some S—S dications were studied at the MP2/6-31G* level of theory. The results obtained indicate a stepwise electrophilic addition of disulfonium dication moiety to the double C=C bond to be the preferable mechanism.     

Electronic structure and electrophilic reactivity of discrete copper diphenylcarbenes

The β-diketiminato Cu(I) arene adduct {[Me₃NN]Cu}₂(μ-toluene) (3) is prepared in 62% isolated yield by addition of the neutral β-diketimine H[Me₃NN] to copper t-butoxide in toluene. An X-ray structure of 3 shows that the bridging toluene ligand exhibits η²-bonding to each Cu center via four contiguous C atoms. Reaction of the dicopper 3 with 1 equiv. N₂CPh₂ provides {[Me₃NN]Cu}₂(μ-CPh₂) (4) as purple crystals in 70% isolated yield. Dicopper carbene 4 possesses a Cu–Cu distance of 2.485(1) Å in the solid state and dissociates a [Me₃NN]Cu fragment in arene solvents to provide low concentrations of [Me₃NN]CuCPh₂ (2) and [Me₃NN]Cu(arene). DFT calculations performed on terminal carbene 2 and dicopper carbene 4 illustrate relationships between these two bonding modes and suggest electrophilic reactivity at the carbene carbon atom bound to Cu. Dicopper carbene 4 undergoes efficient carbene transfer to HCCPh and PPh₃ resulting in the formation of 1,3,3-triphenylcyclopropene and Ph₃PCPh₂ while reaction with the isocyanide CNAr (Ar = 2,6-Me₂C₆H₃) results in loss of the carbene as Ph₂CCPh₂. In each case, the [Me₃NN]Cu fragment is trapped by the incoming nucleophile as the three-coordinate [Me₃NN]Cu(L). Reaction of 4 with O₂ rapidly generates benzophenone and {[Me₃NN]Cu}₂(μ-OH)₂.     

Electronic structure of porphyrin complexes of cobalt

The electronic structure of Co(I) and Co(II) porphine has been investigated by the MO-LCAO-SCF method in the CNDO/2 approximation. It has been shown that as a result of the one-electron reduction of the original Co(II) porphine, the extra electron is localized to a considerable degree on the central ion of the complex. Consequently, the spatial structure of cobalt porphine may be altered, and the cobalt ion can deviate from the plane of the macrocyclic ligand. The interaction of molecular oxygen with Co(I) porphine has been investigated, and it has been established that the formation of a complex of the bent type, in which the O₂ molecule is at a 135° angle to the plane of the ligand, is most favorable.Translated from Teoreticheskaya i Éksperimental'naya Khimii, Vol. 21, No. 1, pp. 1–10, January–February, 1985.     

Electronic structure of transition metal dicarbollide complexes

This paper reports on the results of our electronic structure study of bisdicarbollide complexes of transition metals Fe, Co, Ni, and Cu (36 compounds) by X-ray photoelectron and X-ray emission spectroscopy. Translated fromZhurnal Struktumoi Khimii, Vol. 40, No. 2, pp. 358–371, March–April, 1999.     

Electronic structure and reactivity of N-cyclohexylthiophthaleviide,a vulcanization inhibitor

CNDO/2 has been applied to calculate reactivity indices for N-cyclohexylthiophthalimide CTP. A quantitative evaluation is provided for the interaction in an accelerator-sulfur-CTP system. Comparison with experiment indicates a parameter governing the inhibitive activity.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 31, No. 1, pp. 57–60, January–February, 1995.     

Electronic structure of high-spin iron(III)-alkylperoxo complexes and its relation to low-spin analogues: reaction coordinate of O-O bond homolysis.

The spectroscopic properties of the high-spin Fe(III)-alkylperoxo model complex [Fe(6-Me(3)TPA)(OH(x))(OO(t)Bu)](x)(+) (1; TPA = tris(2-pyridylmethyl)amine, (t)Bu = tert-butyl, x = 1 or 2) are defined and related to density functional calculations of corresponding models in order to determine the electronic structure and reactivity of this system. The Raman spectra of 1 show four peaks at 876, 842, 637, and 469 cm(-1) that are assigned with the help of normal coordinate analysis, and corresponding force constants have been determined to be 3.55 mdyn/A for the O-O and 2.87 mdyn/A for the Fe-O bond. Complex 1 has a broad absorption feature around 560 nm that is assigned to a charge-transfer (CT) transition from the alkylperoxo to a t(2g) d orbital of Fe(III) with the help of resonance Raman profiles and MCD spectroscopy. An additional contribution to the Fe-O bond arises from a sigma interaction between and an e(g) d orbital of iron. The electronic structure of 1 is compared to the related low-spin model complex [Fe(TPA)(OH(x))(OO(t)Bu)](x)(+) and the reaction coordinate for O-O homolysis is explored for both the low-spin and the high-spin Fe(III)-alkylperoxo systems. Importantly, there is a barrier for homolytic cleavage of the O-O bond on the high-spin potential energy surface that is not present for the low-spin complex, which is therefore nicely set up for O-O homolysis. This is reflected by the electronic structure of the low-spin complex having a strong Fe-O and a weak O-O bond due to a strong Fe-O sigma interaction. In addition, the reaction coordinate of the Fe-O homolysis has been investigated, which is a possible decay pathway for the high-spin system, but which is thermodynamically unfavorable for the low-spin complex.     

Quadricyclanes. Part II: Electronic structure and chemical reactivity

The electronic structure of quadricyclane and 3-methylidenequadricyclane obtained by photoelectron spectroscopy, is used as a basis for the discussion of cycloadditions to these systems. The electronic structure of 3-heteroquadricyclanes, arrived at by theoretical calculations, agrees well with that expected from the above measured systems. A surprising outcome is that the orbital most responsible for the observed 2,4-cycloadditions to these heterosystems in not the HOMO but the third highest orbital which lies well below the former. This strongly suggests that these 2,4-cycloadditions proceed not in a concerted fashion but presumably involve as rate-determining step the formation of a resonance-stabilized zwitterionic intermediate. The nature of this intermeiate is discussed and the feasability of its formation investigated on the basis of thermochemical considerations.     

Electronic structure and properties of transition metal-benzene complexes.

A comprehensive theoretical study of the geometries, energetics, and electronic structure of neutral and charged 3d transition metal atoms (M) interacting with benzene molecules (Bz) is carried out using density functional theory and generalized gradient approximation for the exchange-correlation potential. The variation of the metal-benzene distances, dissociation energies, ionization potentials, electron affinities, and spin multiplicities across the 3d series in MBz complexes differs qualitatively from those in M(Bz)(2). For example, the stability of Cr(Bz)(2) is enhanced over that of CrBz by almost a factor of 30. On the other hand, the magnetic moment of Cr(Bz)(2) is completely quenched although CrBz has the highest magnetic moment, namely 6 mu(B), in the 3d metal-benzene series. In multidecker complexes involving V(2)(Bz)(3) and Fe(2)(Bz)(3), the metal atoms are found to couple antiferromagnetically. In addition, their dissociation energies and ionization potentials are reduced from those in corresponding M(Bz)(2) complexes. All of these results agree well with available experimental data and demonstrate the important role the organic support can play on the properties of metal atoms/clusters.     

A comparative study of high-spin manganese and iron complexes

Different metal complexes of the general form M(OH) _n (H₂O)_{6– n} have been studied for manganese and iron. Oxidation states considered for manganese are Mn(III), Mn(IV) and Mn(V) and for iron Fe(II), Fe(III) and Fe(IV). Oxygen containing ligands are used throughout with varying numbers of hydroxyl and water ligands. Some metal-oxo and some charged complexes were also studied. Large Jahn-Teller distortions were found for the Mn(III) and Fe(IV) complexes. Consequences of these distortions are that water ligands have to be placed along the weak JT-axis and that five-coordination by a loss of one of these water ligands is quite competitive with six-coordination in particular for manganese. For Fe(II) and Fe(III) lower coordinations than six are preferred due to the presence of two repulsive e _g electrons. For the metal-oxo complexes five-coordination is also preferred due to the strong trans effect from the oxo ligand. All complexes studied have high-spin ground states. An interesting effect is that the spin is much more delocalized on the ligands for the iron complexes than for the manganese complexes. This effect, which is chemically important for certain iron enzymes, is rationalized by the large number of 3d electrons on iron. For manganese with only five 3d electrons no spin delocalization is needed to obtain the proper high-spin states. Received: 4 February 1997 / Accepted: 24 February 1997