Electronic and Steric Effects on L-Lactide Ring-Opening Polymerization with NSSN-type Zr(IV) Complexes

In this work the mechanism of L-lactide polymerization promoted by NSSN zirconium complexes was investigated through DFT methods with the aim to understand as the electronic and steric features of the ligand affect the energy reaction. It was observed that the rate determining step of the process is the opening of the L-lactide ring and that by increasing the steric hindrance, evaluated by changing geometric parameters and topographic steric maps, or the electron-withdrawing properties of the ligand, the corresponding energy barrier increases. On the other hand, calculations foresee that a small and electron-releasing substituent on the nitrogen atom of the ligand, such as the methyl group, is desirable in order to obtain NSSN zirconium based catalysts with improved activity in the ROP of the L-lactide.     

Ligand effects on rates and regioselectivities of Rh(I)-catalyzed (5 + 2) cycloadditions: a computational study of cyclooctadiene and dinaphthocyclooctatetraene as ligands

The first theoretical study on the effects of ligands on the mechanism, reactivities, and regioselectivities of Rh(I)-catalyzed (5 + 2) cycloadditions of vinylcyclopropanes (VCPs) and alkynes has been performed using density functional theory (DFT) calculations. Highly efficient and selective intermolecular (5 + 2) cycloadditions of VCPs and alkynes have been achieved recently using two novel rhodium catalysts, [Rh(dnCOT)](+)SbF(6)(-) and [Rh(COD)](+)SbF(6)(-), which provide superior reactivities and regioselectivities relative to that of the previously reported [Rh(CO)(2)Cl](2) catalyst. Computationally, the high reactivities of the dnCOT and COD ligands are attributed to the steric repulsions that destabilize the Rh-product complex, the catalyst resting state in the catalytic cycle. The regioselectivities of reactions with various alkynes and different Rh catalysts are investigated, and a predictive model is provided that describes substrate-substrate and ligand-substrate steric repulsions, electronic effects, and noncovalent π/π and C-H/π interactions. In the reactions with dnCOT or COD ligands, the first new C-C bond is formed proximal to the bulky substituent on the alkyne to avoid ligand-substrate steric repulsions. This regioselectivity is reversed either by employing the smaller [Rh(CO)(2)Cl](2) catalyst to diminish the ligand-substrate repulsions or by using aryl alkynes, for which the ligand-substrate interactions become stabilizing due to π/π and C-H/π dispersion interactions. Electron-withdrawing groups on the alkyne prefer to be proximal to the first new C-C bond to maximize metal-substrate back-bonding interactions. These steric, electronic, and dispersion effects can all be utilized in designing new ligands to provide regiochemical control over product formation with high selectivities. The computational studies reveal the potential of employing the dnCOT family of ligands to achieve unique regiochemical control due to the steric influences and dispersion interactions associated with the rigid aryl substituents on the ligand.     

Studies on 4,7-di-substitution effects of one ligand in [Ru(phen)3]2+ with DFT method

Studies on the complex [Ru(phen)3]2+ (phen = 1,10-phenanthroline) and its derivatives with 4,7-di-substitution on one ligand(phen) were carried out using the DFT method at the B3LYP/LanL2DZ level of theory. The trends in the substituent effects caused by the electron-pushing group (OH) and the electron-withdrawing group (F), on the electronic structures and the related properties, for example, the energies and the components of some frontier molecular orbitals, the spectroscopy properties, and the net charge populations of some main atoms of the complexes, etc., have been investigated. The computational results show that the substituents have some interesting effects on the electronic structures and the related properties of the complexes. First, according to the analysis of components of LUMO of the complexes, the electron-withdrawing group (F) can activate the main ligand (the substituted ligand, i.e., 2R-phen) and passivate the coligands, on the contrary, the electron-pushing group (OH) can activate the coligands and passivate the main ligand in the first electronic excited states of complexes. Second, both the electron-pushing group (OH) and the electron-withdrawing group (F) can cause a red shift in the electronic ground bands. Third, the characteristics of the atomic net charge populations on the main ligand can also be analyzed in detail by means of a schematic map expressed by several series of arrowheads based on the law of polarity alternation and the idea of polarity interference. The most negative charges are populated on N1, the next most net negative charges are populated on C3 among the skeleton atoms for the three complexes, etc. The computational results can be better used to explain some experimental phenomena and trends.     

Studies on the mechanism of allylic coupling reactions: a hammett analysis of the coupling of aryl silicate derivatives

Hammett analysis of the palladium-catalyzed allyl-aryl coupling reaction has demonstrated that the rate of the coupling reaction is enhanced by electron-withdrawing groups on the aryl siloxane. The positive slope of the Hammett plot indicated involvement of a charged transition state in which negative charge on the aryl ring is stabilized inductively. This result is consistent with either transmetalation or reductive elimination being the rate-determining step in the coupling process. Furthermore, the influence of ligand on the metal site has been assessed from competition studies as a function of ligand type, cone angle, and electronic effects. From the relative ratios of coupling products produced in the Hammett study, it is possible to gather insight into the role of the electronic as well as the steric effects of ligands on the mechanism of the coupling reaction.     

Mechanisms of nucleophilic and electrophilic attack on carbon bonded palladium(II) and platinum(II) complexes

A systematic mechanistic study is reported for the formation of palladium(II) carbene complexes by nucleophilic attack of aromatic amines on isocyanide derivatives. The most prominent step of the reaction involves direct attack of the amine nitroge on the isocyanide carbon to give an intermediate which then is converted to the final carbene species by the agency of the entering amine itself which behaves as a bifunctional catalyst. The rate of the primary step is affected by the donor ability of the entering amine, by the electrophilic character of the isocyanide carbon, and by steric crowdiness around the reacting centers, with the solvent also playing an important role. The reaction system displays a high versatility through a proper choice of the substituents on the amine and isocyanide aromatic rings and of the ancillary ligands in the metal complex.A mechanistic study is also described of the cleavage of the platinum-carbon σ-bond by electrophilic attack by the proton on organoplatinum(II) complexes. The particular mechanism which is operative, viz. direct electrophilic attack at the metalcarbon bond or oxidative addition/reductive elimination, appears to be the result of many factors. These include electronic and steric properties of the cleaved group and of ancillary ligands, steric configuration of the substrate, nature of the electrophile and solvating ability of the medium.     

DFT studies of structural preference of coordinated ethylene in W(CO)3(PX3)2(CH2CH2) (X=H, CH3, F, Cl, Br, and I)

A set of phosphine complexes of the type W(CO)₃(PX₃)₂(CH₂CH₂) (X=H, CH₃, F, Cl, Br, and I) were investigated by density functional theory method (BP86) to examine the effect of the substituent X on the orientation of C-C vector of the ethylene ligand with respect to one of the metal-ligand bonds as well as the donation and the backdonation in the bonding ligands of phosphine and ethylene. When X=CH₃, H, F, and Cl, the ethylene C-C vector prefers to be coplanar with metal-phosphine bonds, while for the ethylene complexes containing PBr₃ and PI₃ ligands, the structural preference is coplanarity of the ethylene and the metal-carbonyl bonds. The molecular orbital calculations and natural bond orbital analysis were used to examine the structural consequences derived from these complexes. It can be concluded that the structural preferences in the complexes have a clear relation to electronic effects of phosphine ligands. Our calculations for halide phosphine complexes, particularly for PBr₃ and PI₃, allow us to conclude that in addition to electronic effects, steric factors can also affect the orientation of the ethylene ligand in complexes.     

Synthesis of mixed Ar,Ar'-BIAN ligands (Ar,Ar'-BIAN = bis(aryl)acenaphthenequinonediimine). Measurement of the coordination strength of hemilabile ligands with respect to their symmetric counterparts

The synthesis of Ar,Ar'-BIAN ligands (Ar,Ar'-BIAN = bis(aryl)acenaphthenequinonediimine) having different aryl groups bound to the two nitrogen atoms is reported for the first time. The ligands were obtained by two different strategies: (i) by a transimination reaction starting from a symmetric Ar,Ar-BIAN ligand having aryl groups bearing strongly electron-withdrawing substituents or (ii) by a two-step-one-pot sequence. The ligands synthesized have been chosen so that the electronic difference between the two aryl groups is very large, but the steric difference is variable and, in one case, the ligand is almost sterically symmetric. The coordination strength of the new ligands towards three palladium complexes has been measured by a competition experiment following a protocol previously described by us. The coordination strength of the mixed ligands is the mean of those of the corresponding symmetric counterparts. The X-ray crystal structure of a palladium pi-allyl complex of the electronically asymmetric-sterically symmetric ligand (3,5-(CF(3))(2)C(6)H(3)),(3,5-Me(2)C(6)H(3))-BIAN has been solved, together with those of the two symmetric analogues to allow a comparison. Despite the fact that the dodecafluorinated ligand has a K(eq) value about three orders of magnitude lower than the non-fluorinated counterpart, no notable difference is observed in the N-Pd and Pd-C(allyl) distances in the three complexes. Calculations at the density functional level confirm that Pd-BIAN distances are not strictly correlated to the coordination energies, which are in qualitative agreement with the spectroscopic evidence. The bond length is thus not a good indication of the bond strength in these cases.     

Synthesis,Coordination Chemistry and Bonding of Strong N‐Donor Ligands Incorporating the 1H‐Pyridin‐(2E)‐Ylidene (PYE) Motif

A range of N‐donor ligands based on the 1H‐pyridin‐(2E)‐ylidene (PYE) motif have been prepared, including achiral and chiral examples. The ligands incorporate one to three PYE groups that coordinate to a metal through the exocyclic nitrogen atom of each PYE moiety, and the resulting metal complexes have been characterised by methods including single‐crystal X‐ray diffraction and NMR spectroscopy to examine metal–ligand bonding and ligand dynamics. Upon coordination of a PYE ligand to a proton or metal‐complex fragment, the solid‐state structures, NMR spectroscopy and DFT studies indicate that charge redistribution occurs within the PYE heterocyclic ring to give a contribution from a pyridinium–amido‐type resonance structure. Additional IR spectroscopy and computational studies suggest that PYE ligands are strong donor ligands. NMR spectroscopy shows that for metal complexes there is restricted motion about the exocyclic C? N bond, which projects the heterocyclic N‐substituent in the vicinity of the metal atom causing restricted motion in chelating‐ligand derivatives. Solid‐state structures and DFT calculations also show significant steric congestion and secondary metal–ligand interactions between the metal and ligand C? H bonds.     

Investigation of transition metal-imido bonding in M(NBut)2(dpma)

A complete series down group 6 of the formula M(NBu(t))(2)(dpma) has been synthesized, where dpma is N,N-di(pyrrolyl-alpha-methyl)-N-methylamine. A fourth complex, Mo(NAr)(2)(dpma) (4), was also prepared, where Ar is 2,6-diisopropylphenyl. All four of these complexes display geometries in the solid state best described as square pyramidal with one imido ligand occupying the axial position and the other an equatorial site. In all cases, the axial imido ligand has a significantly smaller M-N(imido)-C bond angle with respect to the equatorial multiple-bond substituent. From the (1)H, (13)C, and (14)N NMR spectra, the axial (bent) imido appears to be more electron-rich than the equatorial and linear imido, with the differences becoming less pronounced down the column. The angular deformation energies for the axial imido ligands were studied by DFT in order to discern if and to what extent imido bond angles were important energetically. The electronic energies associated with straightening the axial imido ligand, while holding the remainder of the molecule at the ground-state geometry, for the Cr, Mo, and W derivatives were calculated as 4.5, 2.7, and 2.0 kcal/mol, respectively. A straight-line plot is found for deformation energies versus estimated electronegativity of the group 6 metals in the +6 oxidation state. The study suggests that the electronic differences between metal imido ligands of different angles are quite small; however, the effects may be more pronounced for metal centers with higher electronegativity, e.g. Cr(VI) with electron-withdrawing ligands.     

[Ru(bpy)2(phen)]^2+主配体上双取代效应DFT法研究

对钌联吡啶菲咯啉配合物[Ru(bpy)2(phen)]^2+及其主配体(phen)上5,6-双取代衍生物,用密度泛函(DFT)法在B3LYP/LanL2DZ水平上进行理论计算研究。探讨供电子基团(OH)和拉电子基团(F)在主配体上的取代对配合物的电子结构及相关性质,如配合物前沿分子轨道的能量、组成、光谱性质、原子的净电荷布居及配位键长键角等的影响规律。计算结果表明,取代基对该系列取代衍生物的电子结构,特别是第一激发态的电子云分布影响较大,拉电子基团(F)能活化主配体,钝化辅助配体;而代电子基团(OH)则相反。无认是供电子基团(OH),还是拉电子基团(F)都导致取代衍生物的电子基谱带红移。此外,用基于极性交替规律及极性叠加概念的多系列箭头的图示方法对主配体上的原子净电荷布居的特征作了讨论。计算结果能较好地解释有关的实验现象与规律。     

Effect of CO on the Oxidative Addition of Arene C?H Bonds by Cationic Rhodium Complexes

Sequential addition of CO molecules to cationic aryl–hydrido Rh^III complexes of phosphine‐based (PCP) pincer ligands was found to lead first to C? H reductive elimination and then to C? H oxidative addition, thereby demonstrating a dual role of CO. DFT calculations indicate that the oxidative addition reaction is directly promoted by CO, in contrast to the commonly accepted view that CO hinders such reactions. This intriguing effect was traced to repulsive π interactions along the aryl‐Rh‐CO axis, which are augmented by the initially added CO ligand (due to antibonding interactions between occupied Rh d_π orbitals and occupied π orbitals of both CO and the arene moiety), but counteracted by the second CO ligand (due to significant π back‐donation). These repulsive interactions were themselves linked to significant weakening of the π‐acceptor character of CO in the positively charged rhodium complexes, which is concurrent with an enhanced σ‐donating capability. Replacement of the phosphine ligands by an analogous phosphinite‐based (POCOP) pincer ligand led to significant changes in reactivity, whereby addition of CO did not result in C? H reductive elimination, but yielded relatively stable mono‐ and dicarbonyl aryl–hydrido POCOP–Rh^III complexes. DFT calculations showed that the stability of these complexes arises from the higher electrophilicity of the POCOP ligand, relative to PCP, which leads to partial reduction of the excessive π‐electron density along the aryl‐Rh‐CO axis. Finally, comparison between the effects of CO and acetonitrile on C? H oxidative addition revealed that they exhibit similar reactivity, despite their markedly different electronic properties. However, DFT calculations indicate that the two ligands operate by different mechanisms.     

Hinge Distortion in Platinum(II) Dimers with a Pt(2)S(2) Ring. An ab Initio Molecular Orbital Study

Ab initio calculations have been carried out for Pt(II) dimers with S(2)(-) and/or RS(-) bridging ligands in order to elucidate the factors governing the structural preference, planar or hinged, of these complexes. Calculations have been performed for systems which model all structurally characterized Pt(II) binuclear complexes. All possible determinants of the hinge angle have been taken into account in the ab initio calculations. These show that electronic rather than steric effects govern the geometry of the central Pt(2)S(2) ring. Only one minimum of energy has been found for each complex, but the energetic cost of the hinge motion is low. Complexes with two S(2)(-), or with one S(2)(-) and one RS(-), bridging ligands are hinged; the decrease of the through-ring antibonding interaction between the in-plane sulfur p orbitals with folding appears to be the determinant for hinging. Complexes with two RS(-) bridging ligands can be planar or hinged depending on the relative orientation, syn or anti, of the R groups, on the nature, L(4) or L(2)L(2)', of the terminal ligands, and in the latter case on their disposition, cis or trans. However, the aromatic or aliphatic nature of bridging thiolates does not influence the geometry of the Pt(2)S(2) ring significantly. Conclusions drawn from platinum dimers can be extended to the palladium analogs.     

Regioselectivity of the Claisen rearrangement in meta-allyloxy aryl ketones: an experimental and computational study, and application in the synthesis of (R)-(-)-pestalotheol D

A study of the regioselectivity of the Claisen rearrangement of meta-allyloxy aryl ketones showed that the electron-withdrawing carbonyl group has a major influence and strongly directs rearrangement to the more hindered ortho position. However, when the ketone is part of a ring structure, its electronic effect can be negated by conversion into its triisopropylsilyl enol ether, which dramatically reverses the regiochemistry of the Claisen rearrangement. DFT calculations suggest that the effect is electronic although there is also a steric effect of the bulky silyl group. This strategy for influencing the regiochemical outcome of the Claisen rearrangement was then employed in a short synthesis of the furo[2,3-g]chromene, (-)-pestalotheol D, that confirms the absolute stereochemistry of the natural product.     

Highly cytotoxic vanadium(V) complexes of salan ligands; insights on the role of hydrolysis

Vanadium(V) oxo complexes with tetradentate diamine bis(phenolato) "salan" ligands of the type LVO(OiPr) (L is salan) with different steric and electronic substitutions at the ortho and para positions to the binding phenolato moiety were synthesized and their hydrolytic stability and cytotoxicity were analyzed. With one exception bearing large steric groups, all complexes examined displayed marked cytotoxic activity, comparable to, and often higher than, that of cisplatin. While the hydrolytic stability changed significantly depending on the substituent at the ortho position relative the O-donor with little effect of para substitutions, the cytotoxic activity largely was not affected, and high cytotoxicity was recorded also for relatively unstable complexes. Additional measurements revealed that the cytotoxicity is largely maintained following pre-incubation of up to 18 hours of the complexes in the biological medium prior to cell addition, suggesting that hydrolysis products might serve as the active species. In addition, appreciable cytotoxic activity was measured for an isolated hydrolysis product that was analyzed crystallographically to exhibit a dimeric structure with bridging oxo ligand where both metal centers are bound to the salan ligand, supporting the aforementioned conclusions.     

Trans or (Unusual) Cis Geometry in d(2) Octahedral Dioxo Complexes. A DFT Study

Geometry optimization of the cis and the trans isomers of several octahedral dioxo complexes of d(2) electronic configuration are performed using the gradient-corrected density functional theory (B3LYP and, for some key structures, BP86). With only monodentate sigma donor ligands (ReO(2)(NH(3))(4)(+), 7), the usual energy order is found (i.e., the trans isomer is the most stable). Complexes with a chelating bidentate ligand, OsO(2)(OCH(2)CH(2)O)(NH(3))(2) (10) and ReO(2)(HN=CHCH=NH)(NH(3))(2)(+) (11), are used as models for the experimental complexes 5 and 2 in which the arrangement of the O=M=O unit is trans and cis, respectively. Our calculations actually show an inversion of the relative energy of the two isomers in going from 10 to 11: while the trans isomer is found to be the most stable in 10, the unusual cis diamagnetic isomer is favored by about 29 kcal mol(-)(1) in 11. This result is traced to the geometric and electronic properties of the bidentate ligand, in particular an acute bite angle and good pi acceptor character. In complex 14 with a bipyridine chelating ligand (weaker pi acceptor than diaza-1,4-butadiene in 11), this energy difference is, however, reduced to 7.5 kcal mol(-)(1) (partial geometry optimization).     

Cationic terminal gallylene complexes by halide abstraction: coordination chemistry of a valence isoelectronic analogue of CO and N2

While N(2) and CO have played central roles in developing models of electronic structure, and their interactions with transition metals have been widely investigated, the valence isoelectronic diatomic molecules EX (E = group 13 element, X = group 17 element) have yet to be isolated under ambient conditions, either as the "free" molecule or as a ligand in a simple metal complex. As part of a program designed to address this deficiency, together with wider issues of the chemistry of cationic systems [L(n)M(ER)](+) (E = B, Al, Ga; R = aryl, amido, halide), we have targeted complexes of the type [L(n)M(GaX)](+). Halide abstraction is shown to be a viable method for the generation of mononuclear cationic complexes containing gallium donor ligands. The ability to isolate tractable two-coordinate products, however, is strongly dependent on the steric and electronic properties of the metal/ligand fragment. In the case of complexes containing ancillary pi-acceptor ligands such as CO, cationic complexes can only be isolated as base-trapped adducts, even with bulky aryl substituents at gallium. Base-free gallylene species such as [Cp*Fe(CO)(2)(GaMes)](+) can be identified only in the vapor phase by electrospray mass spectrometry experiments. With bis(phosphine) donor sets at the metal, the more favorable steric/electronic environment allows for the isolation of two-coordinate ligand systems, even with halide substituents at gallium. Thus, [Cp*Fe(dppe)(GaI)](+)[BAr(f)(4)](-) (9) can be synthesized and shown crystallographically to feature a terminally bound GaI ligand; 9 represents the first experimental realization of a complex containing a valence isoelectronic group 13/group 17 analogue of CO and N(2). DFT calculations reveal a relatively weakly bound GaI ligand, which is confirmed experimentally by the reaction of 9 with CO to give [Cp*Fe(dppe)(CO)](+)[BAr(f)(4)](-). In the absence of such reagents, 9 is stable for weeks in fluorobenzene solution, presumably reflecting (i) effective steric shielding of the gallium center by the ancillary phosphine and Cp* ligands; (ii) a net cationic charge which retards the tendency toward dimerization found for putative charge neutral systems; and (iii) (albeit relatively minor) population of the LUMOs of the GaI molecule through pi overlap with the HOMO and HOMO-2 of the [Cp*Fe(dppe)](+) fragment.     

Effects of tris(pyrazolyl)borato ligand substituents on dioxygen activation and stabilization by copper compounds

Dinuclear [Cu2(mu-O)2(Tp(R,R')2] complexes, analogues of the active site of oxyhemocyanin, are theoretically studied, and the effect of the substituents of the tris(pyrazolyl)borate ligands, Tp(R,R'), is analyzed. Density functional theory calculations reveal that the type of bridging oxygen, peroxo, or bisoxo is strongly influenced by the nature and position of the R substituents because of variable substituent...bridging oxygen interactions, as well as electronic effects. The electronic effects of ligands at the 5 position are not significant, but peroxo complexes are favored by electron-withdrawing groups at the 3 position while bisoxo ones are strongly sterically disfavored.     

Electronic and steric control in regioselective addition reactions of organolithium reagents with enaldimines

A reaction mode of imines derived from naphthalene-1-carbaldehyde and acyclic alpha,beta-unsaturated aldehydes with organolitium reagents was dependent on the characteristic nature of a substituent on the imine nitrogen atom. An imine having an electron-withdrawing aryl group on the nitrogen atom behaves as a 1,2-directing imine toward organolithium reagents. In contrast, an imine bearing an alkyl or a bulky aryl group favors 1,4-addition of organolithium reagents. Electronic and steric tuning of a substituent on the imine nitrogen atom for a reaction mode was rationalized on the basis of molecular orbital calculations.     

Nucleophilic reactivity of a series of peroxomanganese(III) complexes supported by tetradentate aminopyridyl ligands

Peroxomanganese(iii) adducts have been postulated as important intermediates in manganese-containing enzymes and small molecule oxidation catalysts. Synthetic peroxomanganese(iii) complexes are known to be nucleophilic and facilitate aldehyde deformylation, offering a convenient way to compare relative reactivities of complexes supported by different ligands. In this work, tetradentate dipyridyldiazacycloalkane ligands with systematically perturbed steric and electronic properties were used to generate a series of manganese(ii) and peroxomanganese(iii) complexes. X-Ray crystal structures of five manganese(ii) complexes all show the ligands bound to give trans complexes. Treatment of these Mn(II) precursors with H(2)O(2) and Et(3)N in MeCN at -40 °C results in the formation of peroxomanganese(iii) complexes that differ only in the identity of the pyridine ring substituent and/or the number of carbons in the diazacycloalkane backbone. To determine the effects of small ligand perturbations on the reactivity of the peroxo group, the more thermally stable peroxomanganese(iii) complexes were reacted with cyclohexanecarboxaldehyde. For these complexes, the rate of deformylation does not correlate with the expected nucleophilicity of the peroxomanganese(iii) unit, as the inclusion of methyl substituents on the pyridines affords slower deformylation rates. It is proposed that adding methyl-substituents to the pyridines, or increasing the number of carbons on the diazacycloalkane, sterically hinders nucleophilic attack of the peroxo ligand on the carbonyl carbon of the aldehyde.     

Furanoside thioether–phosphinite ligands for Rh-catalyzed asymmetric hydrosilylation of ketones

A series of thioether–phosphinite ligands, easily prepared in a few steps from inexpensive d-(+)-xylose, were tested in the Rh-catalyzed hydrosilylation of ketones. Systematic variation of the electronic and steric properties of the thioether moiety provided useful information about the ligand parameters which control enantiodiscrimination. The results show that the enantiomeric excesses are strongly dependent on the steric properties of the substituent on the thioether moiety and on the steric properties of the substrate. High activities and good enantiomeric excesses (up to 90%) were obtained in the hydrosilylation of several aryl ketones.