Clarifying notes on the bonding analysis adopted by the energy decomposition analysis.

We discuss the fundamental aspects of the EDA-NOCV method and address some critical comments that have been made recently. The EDA-NOCV method unlike most other methods focuses on the process of bond formation between the interacting species and not just only on the analysis of the finally formed bond. This is demonstrated using LiF as an example. There is a difference between the interactions between the initial species which form the bond and are also the final product of bond cleavage, and the interactions between the fragments in the eventually formed molecule. The flexibility of the method allows the choice of the interacting fragments which helps to identify the charge and electron configuration of the fragments which describe the bond. This is very helpful in cases where the bond may be described with several Lewis structures. We reject the idea that it would be a disadvantage to have "bond path functions" as the energy components in the EDA, which actually indicate the variability of the method. The bonding analysis in a different sequence of the bond formation gives important results for the various questions that can be asked. This is demonstrated by using CH2, CO2 and the formation of a guanine quartet as examples. The fact that a bond is always defined by the bound molecule, the fragments, and their states is universal and deeply physical, as we show here again for various examples. The results of the EDA-NOCV method are in full accordance with the physical mechanism of the chemical bond as revealed by Ruedenberg.

Theoretical study on preference of open polymer vs. cyclic products in CO2/epoxide copolymerization with cobalt(III)-salen bifunctional catalysts.

The preference of open chain of growing macromolecule vs. possible cyclic form was examined for the bifunctional cobalt(III)-salen catalyst for the copolymerization of CO2 with epoxides. A variety of possible isomers was considered (resulting from trans/cis-ß salen arrangement, different mutual orientation of quaternary ammonium-chains, and possible binding modes). To explore the conformational space, a combined approach was applied, utilizing semiempirical (PM7) MD and the DFT calculations. The preference of the open and cyclic macromolecules attached to the metal center was compared with the corresponding results for isolated model macromolecules, and the systems built of the macromolecule interacting with the tetra-butyl ammonium cation. Result shows that the cyclic structures are strongly preferred for isolated ions, with relatively low cyclization barriers. In the field of positive point charge, the open structures are strongly preferred. For the ions interacting with tetrabutyl ammonium cation, the cyclic structures are preferred, due to delocalization of the positive charge in the cation. For the complexes involving model and "real" Co(III)-salen catalysts, the open structures are strongly preferred. The possible cyclization by dissociation of alkoxide and its transfer to the neighborhood of quaternary ammonium cation is characterized by high activation barriers. Further, the transfer of alkoxide from the metal center to the cation is less likely than the transfer of carbonate, since the metal-alkoxide bond-energy energy is much stronger than energy of metal-carbonate bonding, as shown by ETS-NOCV results. The conclusions are in qualitative agreement with experimental data showing high selectivity towards copolymer formation in the copolymerization processes catalyzed by bifunctional Co(III) salen-complexes. Graphical abstract.

Exploring the conformational space of cobalt(III)-salen catalyst for CO2 /epoxide copolymerization: Effect of quaternary ammonium salts on preference of alternative isomers.

Model catalysts for CO2 /epoxide copolymerization based on Co(III) complexes were studied, with focus on the preference of their alternative isomers, cisß vs. trans. The systems range from model structures without the co-catalyst, as derived from crystallographic data, to complex models with two (CH2 )4 N+ R3 co-catalyst chains (R = Me, Bu) grafted onto a Co(III)-salcy core. To explore the conformational space of the latter complexes, a computational protocol was developed, combining a systematic model-building approach with static and molecular dynamics calculations, and multilevel energy assessment (PM7 and DFT). Results demonstrate an influence of the co-catalyst on the relative stability of the isomers. The cisß isomer is preferred for complexes without N+ -chains and the cisß â†” trans isomerization is feasible. Five-coordinate species and open-shell electronic configurations are energetically disfavored. The cisß preference decreases with the introduction and enlargement of (CH2 )4 N+ R3 : both isomers can be populated for R = Me, while the trans isomer is visibly preferred for R = Bu. © 2018 Wiley Periodicals, Inc.

ETS-NOCV decomposition of the reaction force for double-proton transfer in formamide-derived systems.

The analysis of the electronic-structure changes along IRC paths for double-proton-transfer reactions in the formamide dimer (R1), formamide-thioformamide system (R2), and the thioformamide dimer (R3) was performed based on the extended-transition-state natural orbitals for chemical valence (ETS-NOCV) partitioning of the reaction force, considering the intra-fragments strain and the inter-fragments interaction terms, and further-the electrostatic, Pauli-repulsion and orbital interaction components, with the latter being decomposed into the NOCV components. Two methods of the system partitioning into the fragments were considered ('reactant perspective'/bond-formation, 'product perspective' / bond-breaking). In agreement with previous studies, the results indicate that the major changes in the electronic structure occur in the transition state region; the bond-breaking processes are, however, initiated already in the reactant region, prior to entering the TS region. The electrostatic contributions were identified as the main factor responsible for the increase in the activation barrier in the order R1 < R2 < R3.

The influence of the metal cations and microhydration on the reaction trajectory of the N3 ↔ O2 thymine proton transfer: Quantum mechanical study.

This study involves the intramolecular proton transfer (PT) process on a thymine nucleobase between N3 and O2 atoms. We explore a mechanism for the PT assisted by hexacoordinated divalent metals cations, namely Mg2+ , Zn2+ , and Hg2+ . Our results point out that this reaction corresponds to a two-stage process. The first involves the PT from one of the aqua ligands toward O2. The implications of this stage are the formation of a hydroxo anion bound to the metal center and a positively charged thymine. To proceed to the second stage, a structural change is needed to allow the negatively charged hydroxo ligand to abstract the N3 proton, which represents the final product of the PT reaction. In the presence of the selected hexaaqua cations, the activation barrier is at most 8 kcal/mol. © 2017 Wiley Periodicals, Inc.

ETS-NOCV Decomposition of the Reaction Force: The HCN/CNH Isomerization Reaction Assisted by Water.

The partitioning of the reaction force based on the extended-transition-state natural orbital for chemical valence (ETS-NOCV) scheme has been proposed. This approach, together with the analysis of reaction electronic flux (REF), has been applied in a description of the changes in the electronic structure along the IRC pathway for the HCN/CNH isomerization reaction assisted by water. Two complementary ways of partitioning the system into molecular fragments have been considered ("reactant perspective" and "product perspective"). The results show that the ETS-NOCV picture is fully consistent with REF and bond-order changes. In addition, proposed ETS-NOCV decomposition of the reaction force allows for the quantitative assessment of the influence of the observed bond-breaking and bond-formation processes, providing detailed information about the reaction-driving and reaction-retarding force components within the assumed partitioning scheme. © 2017 Wiley Periodicals, Inc.

Dehydrogenation of ammonia-borane by cationic Pd(II) and Ni(II) complexes in a nitromethane medium: hydrogen release and spent fuel characterization.

A highly electrophilic cationic Pd(II) complex, [Pd(MeCN)4][BF4]2 (1), brings about the preferential activation of the B-H bond in ammonia-borane (NH3·BH3, AB). At room temperature, the reaction between 1 in CH3NO2 and AB in tetraglyme leads to Pd nanoparticles and formation of spent fuels of the general formula MeNHxBOy as reaction byproducts, while 2 equiv. of H2 is efficiently released per AB equiv. at room temperature within 60 seconds. For a mechanistic understanding of dehydrogenation by 1, the chemical structures of spent fuels were intensely characterized by a series of analyses such as elemental analysis (EA), X-ray photoelectron spectroscopy (XPS), solid state magic-angle-spinning (MAS) NMR spectra ((2)H, (13)C, (15)N, and (11)B), and cross polarization (CP) MAS methods. During AB dehydrogenation, the involvement of MeNO2 in the spent fuels showed that the mechanism of dehydrogenation catalyzed by 1 is different from that found in the previously reported results. This AB dehydrogenation derived from MeNO2 is supported by a subsequent digestion experiment of the AB spent fuel: B(OMe)3 and N-methylhydroxylamine ([Me(OH)N]2CH2), which are formed by the methanolysis of the AB spent fuel (MeNHxBOy), were identified by means of (11)B NMR and single crystal structural analysis, respectively. A similar catalytic behavior was also observed in the AB dehydrogenation catalyzed by a nickel catalyst, [Ni(MeCN)6][BF4]2 (2).

Nature of the water/aromatic parallel alignment interactions.

The water/aromatic parallel alignment interactions are interactions where the water molecule or one of its O-H bonds is parallel to the aromatic ring plane. The calculated energies of the interactions are significant, up to ΔE(CCSD)(T)(limit) = -2.45 kcal mol(-1) at large horizontal displacement, out of benzene ring and CH bond region. These interactions are stronger than CH···O water/benzene interactions, but weaker than OH···π interactions. To investigate the nature of water/aromatic parallel alignment interactions, energy decomposition methods, symmetry-adapted perturbation theory, and extended transition state-natural orbitals for chemical valence (NOCV), were used. The calculations have shown that, for the complexes at large horizontal displacements, major contribution to interaction energy comes from electrostatic interactions between monomers, and for the complexes at small horizontal displacements, dispersion interactions are dominant binding force. The NOCV-based analysis has shown that in structures with strong interaction energies charge transfer of the type π → σ*(O-H) between the monomers also exists.

Application of 2-substituted benzyl groups in stereoselective glycosylation.

The use of 2-O-(2-nitrobenzyl) and 2-O-(2-cyanobenzyl) groups controls stereoselective formation of 1,2-trans-glycosidic linkages via the arming participation effect. The observed stereoselectivity likely arises from the intramolecular formation of cyclic intermediate between the electron-rich substituent and the donor oxacarbenium ion providing the expected facial selectivity for attack of the glycoside acceptor. The stereodirecting effect of the 2-nitro- and 2-cyanobenzyl groups attached at the remote position (C-3, C-4, and C-6) of the donor molecule have also been investigated. To prove the postulated mechanism based on the participation effect of 2-substituted benzyl groups in the glycosylation stereoselectivity we used DFT theoretical calculation methodology.

Analysis of the bonding between two M(µ-NAr(#)) monomers in the dimeric metal(II) imido complexes {M(µ-NAr(#))}2 [M = Si, Ge, Sn, Pb; Ar(#) = C6H3-2,6-(C6H2-2,4,6-R3)2]. The stabilizing role played by R = Me and iPr.

The nature of the bonding between the two M(µ-NAr(#)) imido monomers [M = Si, Ge, Sn, Pb; Ar(#) = C6H3-2,6-(C6H2-2,4,6-R3)2; R = Me, iPr] in the {M(µ-NAr(#))}2 dimer is investigated with the help of a newly developed energy and density decomposition scheme as well as molecular dynamics. The approach combines the extended transition state energy decomposition method with the natural orbitals for chemical valence density decomposition scheme within the same theoretical framework. The dimers are kept together by two σ bonds and two π bonds. The σ bonding has two major contributions. The first is a dative transfer of charge from nitrogen to M. It amounts to -188 kcal/mol for {Si(µ-NAr(#))}2, -152 kcal/mol for {Ge(µ-NAr(#))}2 with -105 kcal/mol for {Sn(µ-NAr(#))}2, and -79 kcal/mol for {Pb(µ-NAr(#))}2. The second is a charge buildup within the ring made up of the two dimers. It amounts to -82 kcal/mol for M = Si with -61 kcal/mol for M = Ge and ∼-50 kcal/mol for M = Sn and Pb. We finally have π bonding with a donation of charge from M to nitrogen. It has a modest contribution of ∼-30 kcal/mol. The presence of isopropyl (iPr) groups is further shown to stabilize{M(µ-NAr(#))}2 [M = Si, Ge, Sn, Pb; Ar(#) = C6H3-2,6-(C6H2-2,4,6-iPr3)2] compared to the methylated derivatives (R = Me) through attractive van der Waals dispersion interactions.

Nucleic Acid Quadruplexes Based on 8-Halo-9-deazaxanthines: Energetics and Noncovalent Interactions in Quadruplex Stems.

Structural and energetic features of artificial DNA quadruplexes consisting of base tetrads and their stacks with Na(+)/K(+) ion(s) inside the central pore and incorporating halogenated derivatives of xanthine, 8-fluoro-9-deazaxanthine (FdaX), 8-chloro-9-deazaxanthine (CldaX), 8-bromo-9-deazaxanthine (BrdaX), or 8-iodo-9-deazaxanthine (IdaX), have been investigated by modern state-of-the-art computational tools. The DNA (or RNA) quadruplex models based on 8-halo-9-deazaxanthines are predicted to be more stable relative to those with unmodified xanthine due to the increased stabilizing contributions coming from all three main types of weak interactions (H-bonding, stacking, and ion coordination). Methods for analyzing the electron density are used to understand the nature of forces determining the stability of the system and to gain a predictive potential. Quadruplex systems incorporating polarizable halogen atoms (chlorine, bromine, or iodine) benefit significantly from the stabilizing stacking between the individual tetrads due to an increased dispersion contribution as compared to xanthine and guanine, natural references used. Ion coordination induces a significant rearrangement of electron density in the quadruplex stem as visualized by electron deformation density (EDD) and analyzed by ETS-NOCV and Voronoi charges. Na(+) induces larger electron polarization from the quadruplex toward the ion, whereas K(+) has a higher propensity to electron sharing (identified by QTAIM delocalization index). We expect that our results will contribute to the development of novel strategies to further modify and analyze the natural G-quadruplex core.

Theoretical description of halogen bonding - an insight based on the natural orbitals for chemical valence combined with the extended-transition-state method (ETS-NOCV).

In the present study we have characterized the halogen bonding in selected molecules H3N-ICF3 (1-NH 3 ), (PH3)2C-ICF3 (1-CPH 3 ), C3H7Br-(IN2H2C3)2C6H4 (2-Br), H2-(IN2H2C3)2C6H4 (2-H 2 ) and Cl-(IC6F5)2C7H10N2O5 (3-Cl), containing from one halogen bond (1-NH 3 , 1-CPH 3 ) up to four connections in 3-Cl (the two Cl-HN and two Cl-I), based on recently proposed ETS-NOCV analysis. It was found based on the NOCV-deformation density components that the halogen bonding C-X (…) B (X-halogen atom, B-Lewis base), contains a large degree of covalent contribution (the charge transfer to X (…) B inter-atomic region) supported further by the electron donation from base atom B to the empty σ*(C-X) orbital. Such charge transfers can be of similar importance compared to the electrostatic stabilization. Further, the covalent part of halogen bonding is due to the presence of σ-hole at outer part of halogen atom (X). ETS-NOCV approach allowed to visualize formation of the σ-hole at iodine atom of CF3I molecule. It has also been demonstrated that strongly electrophilic halogen bond donor, [C6H4(C3H2N2I)2][OTf]2, can activate chemically inert isopropyl bromide (2-Br) moiety via formation of Br-I bonding and bind the hydrogen molecule (2-H 2 ). Finally, ETS-NOCV analysis performed for 3-Cl leads to the conclusion that, in terms of the orbital-interaction component, the strength of halogen (Cl-I) bond is roughly three times more important than the hydrogen bonding (Cl-HN).

ETS-NOCV description of σ-hole bonding.

The ETS-NOCV analysis was applied to describe the σ-hole in a systematic way in a series of halogen compounds, CF3-X (X = I, Br, Cl, F), CH3I, and C(CH3)nH3-n-I (n = 1,2,3), as well as for the example germanium-based systems. GeXH3, X = F, Cl, H. Further, the ETS-NOCV analysis was used to characterize bonding with ammonia for these systems. The results show that the dominating contribution to the deformation density, Δρ 1 , exhibits the negative-value area with a minimum, corresponding to σ-hole. The "size" (spatial extension of negative value) and "depth" (minium value) of the σ-hole varies for different X in CF3-X, and is influenced by the carbon substituents (fluorine atoms, hydrogen atoms, methyl groups). The size and depth of σ-hole decreases in the order: I, Br, Cl, F in CF3-X. In CH3-I and C(CH3)nH3-n-I, compared to CF3-I, introduction of hydrogen atoms and their subsequent replacements by methyl groups lead to the systematic decrease in the σ-hole size and depth. The ETS-NOCV σ-hole picture is consistent with the existence the positive MEP area at the extension of σ-hole generating bond. Finally, the NOCV deformation density contours as well as by the ETS orbital-interaction energy indicate that the σ-hole-based bond with ammonia contains a degree of covalent contribution. In all analyzed systems, it was found that the electrostatic energy is approximately two times larger than the orbital-interaction term, confirming the indisputable role of the electrostatic stabilization in halogen bonding and σ-hole bonding.

On the nature of Ni···Ni interaction in a model dimeric Ni complex.

A new dinuclear complex (NiC(5)H(4)SiMe(2)CHCH(2))(2) (2) was prepared by reacting nickelocene derivative [(C(5)H(4)SiMe(2)CH=CH(2))(2)Ni] (1) with methyllithium (MeLi). Good quality crystals were subjected to a high-resolution X-ray measurement. Subsequent multipole refinement yielded accurate description of electron density distribution. Detailed inspection of experimental electron density in Ni···Ni contact revealed that the nickel atoms are bonded and significant deformation of the metal valence shell is related to different populations of the d-orbitals. The existence of the Ni···Ni bond path explains the lack of unpaired electrons in the complex due to a possible exchange channel.

Spin ground state and magnetic properties of cobalt(II): relativistic DFT calculations guided by EPR measurements of bis(2,4-acetylacetonate)cobalt(II)-based complexes.

The spin ground state of the core ion and structure of the bis(2,4-acetylacetonate)cobalt(II) model complex and its synthetic aqua and ethanol derivatives, Co(acac)(2)L(n), (L = EtOH, H(2)O), were examined by means of density functional theory (DFT) calculations supported by electron paramagnetic resonance (EPR) measurements. Geometry optimizations were carried out for low-spin (doublet) and high-spin (quartet) states. For the Co(acac)(2) complex two possible conformations, a square-planar and a tetrahedral one, were taken into account. For all structures relative energies were calculated with both "pure" and hybrid functionals. The calculated data were complemented with the results of the EPR investigations carried out at liquid helium temperature, allowing for definite assignment of the high-spin state for the Co(acac)(2)(EtOH)(2) complex. However, because of the unresolved spectral features, only effective g-values could be assessed, whereas the zero-field splitting parameters (ZFS) were calculated by means of the spin-orbit mean field (SOMF) relativistic DFT method for which direct spin-spin (SS) and spin-orbit coupling (SOC) contributions were quantified.

Applications of the ETS-NOCV method in descriptions of chemical reactions.

The present study characterizes changes in the electronic structure of reactants during chemical reactions based on the combined charge and energy decomposition scheme, ETS-NOCV (extended transition state-natural orbitals for chemical valence). Decomposition of the activation barrier, ΔE (#), into stabilizing (orbital interaction, ΔE (orb), and electrostatic, ΔE (elstat)) and destabilizing (Pauli repulsion, ΔE (Pauli), and geometry distortion energy, ΔE (dist)) factors is discussed in detail for the following reactions: (I) hydrogen cyanide to hydrogen isocyanide, HCN → CNH isomerization; (II) Diels-Alder cycloaddition of ethene to 1,3-butadiene; and two catalytic processes, i.e., (III) insertion of ethylene into the metal-alkyl bond using half-titanocene with phenyl-phenoxy ligand catalyst; and (IV) B-H bond activation catalyzed by an Ir-containing catalyst. Various reference states for fragments were applied in ETS-NOCV analysis. We found that NOCV-based deformation densities (Δρ (i)) and the corresponding energies ΔE (orb)(i) obtained from the ETS-NOCV scheme provide a very useful picture, both qualitatively and quantitatively, of electronic density reorganization along the considered reaction pathways. Decomposition of the barrier ΔE(#) into stabilizing and destabilizing contributions allowed us to conclude that the main factor responsible for the existence of positive values of ΔE (#) for all processes (I, II, III and IV) is Pauli interaction, which is the origin of steric repulsion. In addition, in the case of reactions II, III and IV, a significant degree of structural deformation of the reactants, as measured by the geometry distortion energy, plays an important role. Depending on the reaction type, stabilization of the transition state (relatively to the reactants) originating either from the orbital interaction term or from electrostatic attraction can be of vital importance. Finally, use of the ETS-NOCV method to describe catalytic reactions allows extraction of information on the role of catalysts in determination of ΔE (#).

Multiple boron-boron bonds in neutral molecules: an insight from the extended transition state method and the natural orbitals for chemical valence scheme.

We have analyzed the character of B═B and B≡B bonds in the neutral molecules of general form: LHB═BHL (2-L) and LB≡BL (3-L), for various ancillary ligands L attached to the boron center, based on a recently developed method that combines the extended transition state scheme with the theory of natural orbitals for chemical valence (ETS-NOCV). In the case of molecules with the B═B bond, 2-L, we have included L = PMe(3), PF(3), PCl(3), PH(3), C(3)H(4)N(2)═C(NHCH)(2), whereas for molecules containing the B≡B connection, 3-L, the following ligands were considered L = CO, PMe(3), PCl(3), (Me(2)NCH(2)CH(2)O)(2)Ge. The results led us to conclude that use of phosphorus ligands leads to strengthening of the B═B bond by 6.4 kcal/mol (for 2-PMe(3)), by 4.4 (for 2-PF(3)) and by 9.2 (for 2-PH(3)), when compared to a molecule developed on the experimental basis, 2-C(3)H(4)N(2) (ΔE(total) = -118.3 kcal/mol). The ETS scheme has shown that all contributions, that is, (i) orbital interaction ΔE(orb), (ii) Pauli repulsion ΔE(Pauli), and (iii) electrostatic stabilization ΔE(elstat), are important in determining the trend in the B═B bond energies, ΔE(total). ETS-NOCV results revealed that both σ(B═B) and π(B═B) contributions are responsible for the changes in ΔE(orb) values. All considered molecules of the type LB≡BL, 3-L, exhibit a stronger B≡B bond when compared to a double B═B connection in 2-L (|ΔE(total)| is lower by 11.8-42.5 kcal/mol, depending on the molecule). The main reason is a lower Pauli repulsion contribution noted for 3-CO, 3-PMe(3), and 3-PCl(3) molecules. In addition, in the case of 3-PMe(3) and 3-PCl(3), the orbital interaction term is more stabilizing; however, the effect is less pronounced compared to the drop in the Pauli repulsion term. In all of the systems with double and triple boron-boron bonds, the electronic factor (ΔE(orb)) dominates over the electrostatic contribution (ΔE(elstat)). Finally, the strongest B≡B connection was found for 3-Ge [L = (Me(2)NCH(2)CH(2)O)(2)Ge], predominantly as a result of the strongest σ- and π-contributions, despite the highest destabilization originating from the sizable bulkiness of the germanium-containing ligand. The data on energetic stability of multiple boron-boron bonds (relatively high values of bond dissociation energies |ΔE(total)|), suggest that it should be possible to isolate experimentally the novel proposed systems with double B═B bonds, 2-PMe(3), 2-PF(3), 2-PCl(3), and 2-PH(3), and those with triple B≡B connections, 3-PMe(3), 3-Ge, and 3-PCl(3).

Palladium catalysts for dehydrogenation of ammonia borane with preferential B-H activation.

Cationic Pd(II) complexes catalyzed the dehydrogenation of ammonia borane in the most efficient manner with the release of 2.0 equiv of H(2) in less than 60 s at 25 degrees C. Most of the hydrogen atoms were obtained from the boron atom of the ammonia borane. The first step of the dehydrogenation reaction was elaborated using density functional theory calculations.

Theoretical description of hydrogen bonding in oxalic acid dimer and trimer based on the combined extended-transition-state energy decomposition analysis and natural orbitals for chemical valence (ETS-NOCV).

In the present study we have analyzed hydrogen bonding in dimer and trimer of oxalic acid, based on a recently proposed charge and energy decomposition scheme (ETS-NOCV). In the case of a dimer, two conformations, α and ß, were considered. The deformation density contributions originating from NOCV's revealed that the formation of hydrogen bonding is associated with the electronic charge deformation in both the σ-(Δρ(σ)) and π-networks (Δρ(π)). It was demonstrated that σ-donation is realized by electron transfer from the lone pair of oxygen on one monomer into the empty P*(H-O) orbital of the second oxalic acid fragment. In addition, a covalent contribution is observed by the density transfer from hydrogen of H-O group in one oxalic acid monomer to the oxygen atom of the second fragment. The resonance assisted component (Δρ(π)), is based on the transfer of electron density from the π-orbital localized on the oxygen of OH on one oxalic acid monomer to the oxygen atom of the other fragment. ETS-NOCV allowed to conclude that the σ(O---HO) component is roughly eight times as important as π (RAHB) contribution in terms of energetic estimation. The electrostatic factor (ΔE(elstat)) is equally as important as orbital interaction term (ΔE(orb)). Finally, comparing ß-dimer of oxalic acid with trimer we found practically no difference concerning each of the O---HO bonds, neither qualitative nor quantitative.

Sterically less-hindered half-titanocene(IV) phenoxides: ancillary-ligand effect on mono-, bis-, and tris(2-alkyl-/arylphenoxy) titanium(IV) chloride complexes.

A series of mono-, bis-, and tris(phenoxy)-titanium(IV) chlorides of the type [Cp*Ti(2-R-PhO)(n)Cl(3-n)] (n=1-3; Cp*=pentamethylcyclopentadienyl) was prepared, in which R=Me, iPr, tBu, and Ph. The formation of each mono-, bis-, and tris(2-alkyl-/arylphenoxy) series was authenticated by structural studies on representative examples of the phenyl series including [Cp*Ti(2-Ph-PhO)Cl(2)] (1 PhCl2), [Cp*Ti(2-Ph-PhO)(2)Cl] (2 PhCl), and [Cp*Ti(2-Ph-PhO)(3)] (3 Ph). The metal-coordination geometry of each compound is best described as pseudotetrahedral with the Cp* ring and the 2-Ph-PhO and chloride ligands occupying three leg positions in a piano-stool geometry. The mean Ti-O distances, observed with an increasing number of 2-Ph-PhO groups, are 1.784(3), 1.802(4), and 1.799(3) A for 1 PhCl2, 2 PhCl, and 3 Ph, respectively. All four alkyl/aryl series with Me, iPr, tBu, and Ph substituents were tested for ethylene homopolymerization after activation with Ph(3)C(+)[B(C(6)F(5))(4)](-) and modified methyaluminoxane (7% aluminum in isopar E; mMAO-7) at 140 degrees C. The phenyl series showed much higher catalytic activity, which ranged from 43.2 and 65.4 kg (mmol of Ti x h)(-1), than the Me, iPr, and tBu series (19.2 and 36.6 kg (mmol of Ti x h)(-1)). Among the phenyl series, the bis(phenoxide) complex of 2 PhCl showed the highest activity of 65.4 kg (mmol of Ti x h)(-1). Therefore, the catalyst precursors of the phenyl series were examined by treating them with a variety of alkylating reagents, such as trimethylaluminum (TMA), triisobutylaluminum (TIBA), and methylaluminoxane (MAO). In all cases, 2 PhCl produced the most catalytically active alkylated species, [Cp*Ti(2-Ph--PhO)MeCl]. This enhancement was further supported by DFT calculations based on the simplified model with TMA.