Nonclassical Isomers Research Articles

Dihydrogen to Dihydride Isomerization Mechanism in [(C5Me5)FeH2(Ph2PCH2CH2PPh2)]+ through the Experimental and Theoretical Analysis of Kinetic Isotope Effects

The isomerization of complex [Cp*Fe(dppe)(eta2-H2)]+, generated in situ by low-temperature protonation of Cp*Fe(dppe)H with either HBF4 or CF3COOH, to the dihydride tautomer trans-[Cp*Fe(dppe)(H)2]+ is irreversible and follows first-order kinetics in the -10 to +15 degrees C range with Delta H double dagger = 21.6 +/- 0.8 kcal mol(-1) and DeltaS double dagger = 5 +/- 3 eu. The isomerization rate constant is essentially independent of the nature and quantity of a strong acid. Density functional theory (DFT) calculations on various models, including the complete system at both the quantum mechanics/molecular mechanics (QM/MM) and full QM levels, probe the relative importance of steric and electronic effects for the relative stability of the nonclassical and classical isomers and identify two likely isomerization mechanisms: a "direct" pathway involving simultaneous H-H bond breaking and cis-trans isomerization and a "via Cp" pathway involving agostic C5Me5H intermediates. Both pathways are characterized by activation energies in close correspondence with the experimental value (21.3 and 22.2 kcal mol(-1), respectively). Further kinetic studies were carried out for the Cp*Fe(dppe)H + CF3COOD and Cp*Fe(dppe)D + CF3COOD systems at 273 K. The [Cp*Fe(dppe)(eta2-HD)]+ complex establishes a very rapid isotope redistribution equilibrium with the eta2-H2 and eta2-D2 analogues. The equilibrium constant value (K = 3.3 +/- 0.3) indicates a significant equilibrium isotope effect. Simulation of the rate data provides access to the individual isomerization rate constants kHH, kHD, and kDD for the three isotopomers, yielding kinetic isotope effects: kHH/kHD = 1.24 +/- 0.01 and kHD/kDD = 1.58 +/- 0.01 (and, consequently, kHH/kDD = 1.96 +/- 0.02). The analysis of the DFT-calculated frequencies, using the [Cp*Fe(dhpe)H2]+ model system, for the [Cp*Fe(dhpe)(eta2-XY)]+ isotopomers as well as transition states for the "direct" (TSdir) and "via Cp" (TSrot) pathways (X = H, D) allowed the computation of the expected isotope effects. A comparison with the experiment strongly suggests that the mechanism occurs via the "direct" pathway for the present system, although the small difference in the calculated energy barriers suggests that the "via Cp" pathway may be preferred in other cases.

Biological Evaluation and Structure—Activity Relationships of Bis‐(3‐aryl‐3‐oxo‐propyl)methylamine Hydrochlorides and 4‐Aryl‐3‐arylcarbonyl‐1‐methyl‐4‐piperidinol Hydrochlorides as Potential Cytotoxic Agents and Their Alkylating Ability Towards Cellular Glutathione in Human Leukemic T Cells.

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Classical 1,2,4‐Triboracyclopentanes and Their Rearrangement into Nonclassical 2‐Boryl‐1,3‐diboracyclobutanes: Intramolecular C–H Bond Activation by a B–B Moiety

Abstract1,2,4‐Triboracyclopentanes 1a–c in solution spontaneously transform into their nonclassical isomers 2a–c. These have been characterised by NMR spectroscopy and X‐ray structural analyses. Compounds 2a–c are strongly distorted when compared with the five‐membered ring of 1a, the structure of which was determined for comparison. In addition, the pattern of the substituents at the boron centres in 2a,b is different from that in 1a,b. This can be rationalised on the basis of a rapid topomerisation in 2c as determined by analysis of its temperature dependent 13C NMR spectra. The environment of the boron centres B1 and B3 is exchanged by transforming the two 3c–2e bonds which both involve B1 into a B2–H and a B1–C 2c–2e bond. This leads to the 2‐boryl‐1,3‐diboracyclobutane 4c and formation of corresponding new 3c–2e bonds employing B3 gives the topomer of 2c. The low barrier of this topomerisation reveals that 2a–c may also be regarded as nonclassical isomers of the 2‐boryl‐1,3‐diboracyclobutanes 4a–c. Ab initio computations of models at the MP4SDTQ/6‐311+G**// MP2/6‐311+G** levels support this interpretation. In addition, they show that the transformation of 1u to 2u occurs by an intramolecular C–H bond activation process initiated by the B–B moiety. Derivatives of the type 1 and 2 described here represent the first examples of 1,2‐diboron heterocycles which can be isolated as classical as well as nonclassical isomers. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Aromaticity of C 32 fullerene isomers and the 2( N+1) 2 rule

Aromatic character of classical and non-classical fullerene isomers of C 32 was analyzed in terms of Hess–Schaad resonance energy (HSRE), topological resonance energy (TRE), bond resonance energy (BRE) and the 2( N+1) 2 rule of spherical aromaticity. According to the 2( N+1) 2 rule, C 32 fullerene isomers must be highly aromatic with a closed-shell electronic configuration. However, they are predicted to be antiaromatic with negative TREs, negative HSREs and large negative BREs. Two of the isomers have half-filled degenerate HOMOs, which is incompatible with the 2( N+1) 2 rule. Since they all are aromatized by acquiring two more electrons, it is obvious that the neutral C 32 species do not have a maximum degree of aromaticity. Thus, the aromaticity of a fullerene cage is strongly dependent on the connectivity of carbon atoms.

Fullerene growth and the role of nonclassical isomers

We report results of a survey of the energetics and stability of classical and 1-heptagon nonclassical fullerene clusters in the range of sizes ${\mathrm{C}}_{30}$ to ${\mathrm{C}}_{70},$ using both total-energy tight-binding and first-principles density functional theory methods. We conclude from this survey that 1-heptagon isomers of high stability exist at every nuclearity, and that these nonclassical isomers play an important part in the growth mechanism of fullerenes.

Ab initio study of rearrangements on the (CH)2(BR)2, R=H, and NH2 potential energy surfaces.

A comprehensive survey of the (CH)2(BH)2 potential energy surface was carried out at the [MP4/6-311 + G(d,p)]//MP2/6-31G(d) level. Many of the classical and nonclassical isomers of the carborane surface are separated by high activation barriers, which explains why derivatives of most isomers could be prepared as stable compounds at room temperature. The transition states are grouped into two types, hydrogen migration (terminal-to-bridge and bridge-to-terminal) and group migration (BH, CH, and CH2). The rearrangement of 1,3-diamino-1,3-diboretene (1-NH2) to 1,2-diamino-1,2-diboretene (2-NH2) was computed and compared to the rearrangement in the parent (1-->2). The effect of the amino group is to substantially increase the barrier height and stabilize the product, 2-NH2.

Classical and Nonclassical Isomers of Tropylium, Silatropylium, and Germatropylium Cations. Descending the Periodic Table Increases the Preference for Nonclassical Structures

Ab initio calculations at the G2(MP2,SVP) level predict that there is a striking transition from a preference for classical isomers on the C7H7+ surface to a preference for nonclassical isomers on the C6H7Ge+ surface. The situation for C6H7Si+ is intermediate between these extremes, with classical and nonclassical isomers tending to have comparable energies despite their significant geometric differences.

A tandem mass spectrometry and Fourier transform-ion cyclotron resonance study of ionized ethylated acetone and deuterated analogs

The unimolecular dissociation of (CH3)2C+OC2H5 ions (I) and their deuterated analogs, generated by ion-molecule reactions (IMR) in acetone-ethyl iodide mixtures was studied by tandem mass Spectrometry methods. Two significant processes that yielded I ions were identified. The Fourier transform ion cyclotron resonance study showed that the reaction between ionized ethyl iodide and neutral acetone was the principal source of I. This process involved the formation of the stable mixed ionized dimer, [C2H5I·O=C(CH3)2]+· (II), which dissociated by the loss of an I atom. Other important fragmentation pathways of II were the formation of C2H5I+·, (CH3)2CO+·; and (CH3)2COI+ and the loss of CH3CHI·. The major dissociation of I was the loss of C2H4. The activation energy for this reaction was determined by metastable ion appearance energy measurements to be ∼55 kJ mol−1 above the thermochemical minimum. The analysis of the metastable and collision-induced dissociation of D-labeled I showed an unusual time-energy effect on the degree of H/D mixing, with the highest selectivity for the ethene loss [β-H(D)-atom shift] being observed for ions with the lowest internal energies. Collisional excitation could not produce significant H/D mixing among dissociating ions. The results were rationalized by the existence of two species— the classical (2-ethoxypropyl) and nonclassical (proton-bound acetone-ethene pair) isomers of I. The classical structure was originally formed by IMR or from II. The energy barrier for the classical to nonclassical isomerization lay well above the thermochemical threshold for C2H4 loss, providing only limited H-atom mixing in nonclassical ions that were always formed in their dissociative state. The effect of the proton affinity of the carbonyl compound on the H/D mixing in RR′C+OC2H5 ions was studied. It was shown that the selectivity for the ethene loss (β-H-atom shift) generally increased with the increase of the proton affinity of RR′CO. Neutralization-reionization mass spectrometry was applied to a study of (CH3)2C+OR ions, where R = H, I, C2H5. The observation of a recovery signal for the ion I was attributed to the formation of the 2-ethoxypropyl radical. Neutral counterparts of (CH3)2COI+ ions were also generated, being the first example of IO-substituted alkyl radicals.

Dihydrogen versus Dihydride: Relativistic Effects on the Relative Stabilities of Nonclassical and Classical Isomers of M(PH3)3H4 (M = Fe, Ru, Os)

Dihydrogen versus Dihydride: Relativistic Effects on the Relative Stabilities of Nonclassical and Classical Isomers of M(PH3)3H4 (M = Fe, Ru, Os)

Anti van't Hoff/Le Bel geometries of boron compounds. A theoretical study of classical and nonclassical isomers of B 2CH 4, B 2NH 3 and B 2OH 2

The potential energy surfaces of B 2CH 4, B 2NH 3 and B 2OH 2 have been studied using quantum chemical ab initio methods at the MP2/6-31G(d) level of theory. Improved energies are calculated at MP4/6-311G(2df,2pd). Four energy minimum forms are predicted for B 2CH 4. Six isomers are calculated as energy minima for B 2NH 3, while three equilibrium structures are predicted for B 2OH 2. The electronic structure of the molecules is investigated using the topological analysis of the electron density distribution and its associated gradients and Laplacian.

Transition metal polyhydride complexes: a theoretical view

The factors governing the stabilities of classical and non-classical isomers in transition metal polyhydride complexes are summarized in this review. Diagonal lines in the Periodic Table, which divide the classical (left-hand side of the line) and non-classical (right-hand side of the line) isomers, were defined for neutral (through Ru and Ir) and cationic (between Tc/Ru and Os/Ir) complexes with only phosphine ligands. The carbonyl ligand was found to stabilize significantly the non-classical isomers due to its strong π-accepting ability. The strong “α”- and “π”-donating ability of the Cp ligand makes most of its polyhydride complexes adopt classical structures. In this review, we also discuss some unique structural features in several types of polyhydride complex, such as the orientation of η 2-H 2; ligands in non-classical structures and the distortions in the “four-legged piano-stool” complexes (MH nL 4-nC p) and the pseudo-pentagonal-bipyramidal complexes (MH n L 6− n Cp).

Transition metal polyhydride complexes. 4. Highly stable nonclassical isomers with octahedral structures

Ab initio calculations at the MP2 level with effective core potentials have been used to study the relative stabilities of classical and nonclassical isomers of 14 second- and third-row transition-metal polyhydride complexes with the formula ML 8-n H n , where n=47 and L=PH 3 . Results show that the model complexes are divided into two groups. One group has as its most stable structure a classical one with the maximum coordination number, while the other has as its most stable structure a nonclassical one with an octahedral geometry

Transition-metal polyhydride complexes. 3. Relative stabilities of classical and nonclassical isomers

Ab initio calculations with effective core potentials have been used to study the relative stabilities of classic nonclassical isomers of 18-electron polyhydride transition-metal complexes. Systematic calculations on ML 7-n H n and ML 8-n H n (n=2-7), where M=Mo, W, Tc, Re, Ru, Os, Rh, and Ir, and L=PH 3 and CO, lead to the following conclusions. The trans influence of two H ligands is significantly destabilizing and influences the stability and structure of the isomers

Transition metal polyhydride complexes. 2. Theoretical methods for the determination of stabilities of classical and nonclassical isomers

Ab initio calculations with effective core potentials have been used to study the relative stability of classical nonclassical isomers of the model complexes, IrH 5 L 1 (L=Be, PH 3 , and PMe 3 ). The examination of different levels and techniques of electron correlation suggests that the second-order Moller-Plesset (MP2) method will provide a reliable prediction of the stability of one isomer over the other. The conventional substitution of a PR 3 ligand by PH 3 in quantum chemical calculations has also been examined

The energy separation between the classical and nonclassical isomers of protonated acetylene. An extensive study in one- and n-particle space

The energy separation between the classical and nonclassical forms of protonated acetylene has been reinvestigated in light of the recent experimentally deduced lower bound to this value of 6.0 kcal/mol. The objective of the present study is to use state-of-the-art ab initio quantum mechanical methods to establish this energy difference to within chemical accuracy (i.e., about 1 kcal/mol). The one-particle basis sets include up to g-type functions and the electron correlation methods include single and double excitation coupled-cluster (CCSD), the CCSD(T) extension, multireference configuration interaction, and the averaged coupled-pair functional methods. A correction for zero-point vibrational energies has also been included, yielding a best estimate for the energy difference between the classical and nonclassical forms of 3.7±1.3 kcal/mol.

Structure and relative energies of some nitrogen-containing radical cations by MNDO calculation

MNDO calculations have been performed on the energy surfaces of four nitrogen-containing radical cations: [CH3N]˙+, [CH5N]˙+, [N2H4]˙+, and [N2H2]˙+. In general, the calculated structures and relative energies compare well with ab initio calculations and experiment (when available). The MNDO method overestimates the stabilities of non-classical isomers and consequently underestimates the energy barriers of 1,2-hydrogen shifts connecting both classical and non-classical species. The energy barriers for the loss of H atoms from X atoms (H–X–N–) are overestimated while those from N atoms (–X–N–H) are underestimated. Specific fragmentations or isomerization within each radical cation are also commented on. The results on the whole suggest that MNDO is a suitable method in this area.

Deamination of bicyclo[2.2.2]octan-2-yl- and bicyclo[3.2.1]octan-2-yl-amines. Evidence for classical precursors of non-classical carbonium ions

Bicyclo[2.2.2]octan-2-yl- and exo-bicyclo[3.2.1]octan-2-yl-amines have been deaminated in acetic acid by nitrous acid and via their N-phenyltriazenes; their ethyl N-nitrosocarbamates have also been solvolysed in ethanol. Product distributions by a given method from the structurally Isomeric starting materials are similar to each other and to the common product distribution obtained from bicyclo[2.2.2]-octane-2-yl and exo-bicyclo[3.2.1]octan-2-yl toluene-p-sulphonates. Each amine gives, however, a small but unmistakable excess of the structurally unrearranged product compared (in the case substitution) with the distribution obtained from the solvolysis of the corresponding bicyclo-octyl toluene-p-sulphonates. Each amine gives, however, a small but unmistakable excess of the structurally unrearranged product compared (in the case of substitution)with the distribution obtained from the solovolysis of the corresponding bicyclo-octyl toluene -p-sulphonates. endo-Bicyclo[3.2.1]octan-2-ylamine has also been deaminated in acetic acid by nitrous acid and via its ethyl N-nitrosocarbamate in ethanol. The product ratios of these reactions are characteristically different from those of the isomeric amines but, as far as substitution is concerned, are similar to what is obtained from endo-bicyclo[3.2.1]octan-2-yl toluene-p-sulphonate. A common mechanism describe all the deaminative reactions. We propose that classicalcarbonium ions are the initial products of fragmentation of diazo-intermediates. These are intercepted to only a small extent to give products structurally and sterochemically characteristic of the original amines; to an even smaller extent they rearrange to isomeric classical carbonium ions is rearrangement to non-classical isomers. From both classic carbonium ion is produced as has been implicated in the solvolysis of the corresponding toluene-p-sulphonates. endo-Bicyclo[3.2.1]octan-2-ylamine deamination gives rise to an isomeric symmetrical non-classical carbonium ion, the same one that intervenes in the solvolysis of endo-bicyclo[3.2.1]octan-2-yl toluene-p-sulphonate. Symmetrical and unsymmetrical non-classical carbonium ions once formed give product ratio largely independent of their origins or modes of formation although the symmetrical one appears to undergo a small extent of isomerization to the (more stable) unsymmetrical species. These result are contrasted with those obtained from simple carbocyclic system (without branching at the β-carbon)in which deamination and toluene-p-sulphonate solovolysis give characteristically different and unrelated product distributions.