Manifestation of stereoelectronic effects on the calculated carbon-hydrogen bond lengths and one bond (1)J(C-H) NMR coupling constants in cyclohexane,six-membered heterocycles,and cyclohexanone derivatives

Cyclohexane (1), oxygen-, sulfur-, and/or nitrogen-containing six-membered heterocycles 2-5, cyclohexanone (6), and cyclohexanone derivatives 7-16 were studied theoretically [B3LYP/6-31G(d,p) and PP/IGLO-III//B3LYP/6-31G(d,p) methods] to determine the structural (in particular C-H bond distances) and spectroscopic (specifically, one bond (1)J(C-H) NMR coupling constants) consequences of stereoelectronic hyperconjugative effects. The results confirm the importance of n(X) --> sigma*(C-H)(app) (where X = O, N), sigma(C-H)(ax) --> pi*(C=O), sigma(S-C) --> sigma*(C-H)(app), sigma(C-S)-->sigma*(C-H)(app), beta-n(O) --> sigma*(C-H), and sigma(C-H) --> sigma*(C-H)(app) hyperconjugation, as advanced in previous theoretical models. Calculated r(C-H) bond lengths and (1)J(C-H) coupling constants for C-H bonds participating in more than one hyperconjugative interaction show additivity of the effects.     

Density functional study of the sigma and pi bond activation at the Pd=X (X = Sn, Si, C) bonds of the (H2PC2H4PH2)Pd=XH2 complexes. Is the bond cleavage homolytic or heterolytic?

The mechanism for the activation of the sigma bonds, the O-H of H2O, C-H of CH4, and the H-H of H2, and the pi bonds, the C[triple bond]C of C2H2, C=C of C2H4, and the C=O of HCHO, at the Pd=X (X = Sn, Si, C) bonds of the model complexes (H2PC2H4PH2)Pd=XH2 5 has been theoretically investigated using a density functional method (B3LYP). The reaction is significantly affected by the electronic nature of the Pd=X bond, and the mechanism is changed depending on the atom X. The activation of the O-H bond with the lone pair electron is heterolytic at the Pd=X (X = Sn, Si) bonds, while it is homolytic at the Pd=C bond. The C-H and H-H bonds without the lone pair electron are also heterolytically activated at the Pd=X bonds independent of the atom X, where the hydrogen is extracted as a proton by the Pd atom in the case of X = Sn, Si and by the C atom in the case of X=C because the nucleophile is switched between the Pd and X atoms depending on the atom X. In contrast, the pi bond activation of C[triple bond]C and C=C at the Pd=Sn bond proceeds homolytically, and is accompanied by the rotation of the (H2PC2H4PH2)Pd group around the Pd-Sn axis to successfully complete the reaction by both the electron donation from the pi orbital to Sn p orbital and the back-donation from the Pd dpi orbital to the pi orbital. On the other hand, the activation of the C=O pi bond with the lone pair electron at the Pd=Sn bond has two reaction pathways: one is homolytic with the rotation of the (H2PC2H4PH2)Pd group and the other is heterolytic without the rotation. The role of the ligands controlling the activation mechanism, which is heterolytic or homolytic, is discussed.     

Structures, photoluminescence, and reversible vapoluminescence properties of neutral platinum(II) complexes containing extended pi-conjugated cyclometalated ligands

Reacting K2PtCl4 with the tridentate R-C(wedge)N(wedge)C-H2 ligands 2,6-di-(2'-naphthyl)-4-R-pyridine (R = H, 1a; Ph, 1b; 4-BrC6H4, 1c; 3,5-F2C6H3, 1d) in glacial acetic acid, followed by heating in dimethyl sulfoxide (DMSO), gave complexes [(R-C(wedge)N(wedge)C)Pt(DMSO)] (2a-d). In the crystal structures of 2a-c, the molecules are paired in a head-to-tail orientation with Pt...Pt separations >6.3 A, and there are extensive close C-H...pi (d = 2.656-2.891 A), pi...pi (d = 3.322-3.399 A), and C-H...O=S (d = 2.265-2.643 A) contacts. [(Ph-C(wedge)N(wedge)C)Pt(PPh3)] (3) was prepared by reacting 2b with PPh3. Reactions of 2a-d with bis(diphenylphosphino)methane (dppm) gave [(R-C(wedge)N(wedge)C)2Pt2(mu-dppm)] (4a-d). Both head-to-head (syn) and head-to-tail (anti) conformations were found for 4a.6CHCl3.C5H12, whereas only one conformation was observed for 4b.2CHCl3 (syn), 4c.3CH2Cl2 (syn), and 4d.2CHCl3 (anti). In the crystal structures of 4a-d, there are close intramolecular Pt...Pt contacts of 3.272-3.441 A in the syn conformers, and long intramolecular Pt...Pt separations of 5.681-5.714 A in the anti conformers. There are weak C-H...X (d = 2.497-3.134 A) and X...X (X = Cl or Br; d = 2.973-3.655 A) interactions between molecules 4a-d and occluded CHCl3/CH2Cl2 molecules, and their solvent channels are of varying diameters (approximately 9-28 A). Complexes 2a-d, 3, and 4a-d are photoluminescent in the solid state, with emission maxima at 602-643 nm. Upon exposure to volatile organic compounds, 4a shows a fast and reversible vapoluminescent response, which is most intense with volatile halogenated solvents (except CCl4). Powder X-ray diffraction analysis of desolvated 4a revealed a more condensed molecular packing of syn and anti complexes than crystal 4a.6CHCl3.C5H12.     

Cationic rhodium mono-phosphine fragments partnered with carborane monoanions [closo-CB11H6X6]- (X = H, Br). Synthesis, structures and reactivity with alkenes

Addition of the new phosphonium carborane salts [HPR(3)][closo-CB(11)H(6)X(6)] (R = (i)Pr, Cy, Cyp; X = H 1a-c, X = Br 2a-c; Cy = C(6)H(11), Cyp = C(5)H(9)) to [Rh(nbd)(mu-OMe)](2) under a H(2) atmosphere gives the complexes Rh(PR(3))H(2)(closo-CB(11)H(12)) 3 (R = (i)Pr 3a, Cy 3b, Cyp 3c) and Rh(PR(3))H(2)(closo-CB(11)H(6)Br(6)) 4 (R = (i)Pr 4a, Cy 4b, Cyp 4c). These complexes have been characterised spectroscopically, and for 4b by single crystal X-ray crystallography. These data show that the {Rh(PR(3))H(2)}(+) fragment is interacting with the lower hemisphere of the [closo-CB(11)H(6)X(6)](-) anion on the NMR timescale, through three Rh-H-B or Rh-Br interactions for complexes 3 and 4 respectively. The metal fragment is fluxional over the lower surface of the cage anion, and mechanisms for this process are discussed. Complexes 3a-c are only stable under an atmosphere of H(2). Removing this, or placing under a vacuum, results in H(2) loss and the formation of the dimer species Rh(2)(PR(3))(2)(closo-CB(11)H(12))(2) 5a (R = (i)Pr), 5b (R = Cy), 5c (R = Cyp). These dimers have been characterised spectroscopically and for 5b by X-ray diffraction. The solid state structure shows a dimer with two closely associated carborane monoanions surrounding a [Rh(2)(PCy(3))(2)](2+) core. One carborane interacts with the metal core through three Rh-H-B bonds, while the other interacts through two Rh-H-B bonds and a direct Rh-B link. The electronic structure of this molecule is best described as having a dative Rh(I) --> Rh(III), d(8)--> d(6), interaction and a formal electron count of 16 and 18 electrons for the two rhodium centres respectively. Addition of H(2) to complexes 5a-c regenerate 3a-c. Addition of alkene (ethene or 1-hexene) to 5a-c or 3a-c results in dehydrogenative borylation, with 1, 2, and 3-B-vinyl substituted cages observed by ESI-MS: [closo-(RHC[double bond, length as m-dash]CH)(x)CB(11)H(12-x)](-)x = 1-3, R = H, C(4)H(9). Addition of H(2) to this mixture converts the B-vinyl groups to B-ethyl; while sequential addition of 4 cycles of ethene (excess) and H(2) to CH(2)Cl(2) solutions of 5a-c results in multiple substitution of the cage (as measured by ESI-MS), with an approximately Gaussian distribution between 3 and 9 substitutions. Compositionally pure material was not obtained. Complexes 4a-c do not lose H(2). Addition of tert-butylethene (tbe) to 4a gives the new complex Rh(P(i)Pr(3))(eta(2)-H(2)C=CH(t)Bu)(closo-CB(11)H(6)Br(6)) 6, characterised spectroscopically and by X-ray diffraction, which show coordination of the alkene ligand and bidentate coordination of the [closo-CB(11)H(6)Br(6)](-) anion. By contrast, addition of tbe to 4b or 4c results in transfer dehydrogenation to give the rhodium complexes Rh{PCy(2)(eta(2)-C(6)H(9))}(closo-CB(11)H(6)Br(6)) 7 and Rh{PCyp(2)(eta(2)-C(5)H(7))}(closo-CB(11)H(6)Br(6)) 9, which contain phosphine-alkene ligands. Complex has been characterised crystallographically.     

C-H...N and C-H...S hydrogen bonds--influence of hybridization on their strength

Simple complexes connected through C-H...S and C-H...N interactions are investigated: CH4...NH3, C2H4...NH3, C2H2...NH3, CH4...SH2, C2H4...SH2, and C2H2...SH2. Ab initio and DFT calculations are performed (SCF, MP2, B3LYP) using different basis sets up to the MP2/aug-cc-pVQZ//MP2/aug-cc-pVDZ level of approximation. The Bader theory is applied since MP2/6-311++G(d,p) wave functions are used to find and to characterize bond critical points in terms of electron densities and their Laplacians. The influence of hybridization on the properties of C-H...S and C-H...N systems is also studied showing that the strength of such interactions increases in the following order: C(sp3)-H...Y, C(sp2)-H...Y, C(sp)-H...Y, where Y = S, N--it is in line with the previous findings on C-H...O hydrogen bonds. The results also show that CH4...SH2 and C2H4...SH2 complexes should be rather classified as van der Waals interactions and not as hydrogen bonds. The frequency associated with the C-H stretch of C(sp3)-H...S is blue-shifted.     

Dynamic behavior of chiral sulfonimidoyl-substituted allyl and alkyl (dimethylamino)titanium(IV) complexes: metallotropic shift, reversible beta-hydride elimination/reinsertion, and ab initio calculations of allyl and alkyl aminosulfoxonium ylides

Variable-temperature (1)H and (13)C NMR spectroscopy of the sulfonimidoyl-substituted allyltitanium(IV) complexes E-1a-c and Z-1a-c, which carry diethylamino groups at the Ti atom, revealed a fast 1,3-C,N-shift of the Ti atom, leading to an equilibrium between the epimeric Calpha-titanium allyl complexes A and C and the N-titanium allyl aminosulfoxonium ylide B. Based on these findings a model for the reactions of E-1a-c and Z-1a-c with aldehydes is proposed, which features regio- and diastereoselective reactions of the N-titanium ylide B at the alpha-position and the Calpha-titanium complex A at the gamma-position. Model ab initio calculations of the methylene and allyl (dimethylamino)sulfoxonium ylides 10 and 14, respectively, revealed short Calpha-S bonds, a stabilization by both electrostatic interaction and negative hyperconjugation, and a low Calpha-S rotational barrier. The ylides preferentially adopt Calpha-S and Calpha-N conformations in which the lone pair orbital at the Calpha atom is periplanar to the S=O bond and that at the N atom periplanar to the Calpha-Ph bond. Variable-temperature NMR spectroscopy of the sulfonimidoyl-substituted alkyltitanium(IV) complex 16, which carries diethylamino groups at the Ti atom, revealed a dynamic behavior leading to a complete topomerization of all four methylene hydrogens of the Calpha-ethyl groups. Two fast processes are held responsible for the topomerization of the hydrogens of 16. The first one is a reversible intramolecular beta-hydride elimination/alkene-Ti-H insertion with the intermediate formation of a complex between (Et(2)N)TiH and a 1-alkenyl sulfoximine, and the second one consists of a reversible 1,3-C,N-shift of the Ti atom in combination with a Calpha-S bond rotation. Interestingly, the room-temperature NMR spectra of the corresponding sulfonimidoyl-substituted alkyltitanium(IV) complex 17, which carries isopropoxy groups at the Ti atom, give no indication of a similar dynamic behavior of this complex.     

Aromatic vs aliphatic C-H bond activation by rhodium(I) as a function of agostic interactions: catalytic H/D exchange between olefins and methanol or water

The aryl-PC type ligand 3, benzyl(di-tert-butyl)phosphane, reacts with [Rh(coe)(2)(solv)(n)()]BF(4) (coe = cyclooctene, solv = solvent), producing the C-H activated complexes 4a-c (solv = (a). acetone, (b). THF, (c). methanol). Complexes 4a-c undergo reversible arene C-H activation (observed by NMR spin saturation transfer experiments, SST) and H/D exchange into the hydride and aryl ortho-H with ROD (R = D, Me). They also promote catalytic H/D exchange into the vinylic C-H bond of olefins, with deuterated methanol or water utilized as D-donors. Unexpectedly, complex 2, based on the benzyl-PC type ligand 1 (analogous to 3), di-tert-butyl(2,4,6-trimethylbenzyl)phosphane, shows a very different reversible C-H activation pattern as observed by SST. It is not active in H/D exchange with ROD and in catalytic H/D exchange with olefins. To clarify our observations regarding C-H activation/reductive elimination in both PC-Rh systems, density functional theory (DFT) calculations were performed. Both nucleophilic (oxidative addition) and electrophilic (H/D exchange) C-H activation proceed through eta(2)-C,H agostic intermediates. In the aryl-PC system the agostic interaction causes C-H bond acidity sufficient for the H/D exchange with water or methanol, which is not the case in the benzyl PC-Rh system. In the latter system the C-H coordination pattern of the methyl controls the reversible C-H oxidative addition leading to energetically different C-H activation processes, in accordance with the experimental observations.     

Substrate-triggered activation of a synthetic [Fe2(μ-O)2] diamond core for C-H bond cleavage

An [Fe(IV)(2)(μ-O)(2)] diamond core structure has been postulated for intermediate Q of soluble methane monooxygenase (sMMO-Q), the oxidant responsible for cleaving the strong C-H bond of methane and its hydroxylation. By extension, analogous species may be involved in the mechanisms of related diiron hydroxylases and desaturases. Because of the paucity of well-defined synthetic examples, there are few, if any, mechanistic studies on the oxidation of hydrocarbon substrates by complexes with high-valent [Fe(2)(μ-O)(2)] cores. We report here that water or alcohol substrates can activate synthetic [Fe(III)Fe(IV)(μ-O)(2)] complexes supported by tetradentate tris(pyridyl-2-methyl)amine ligands (1 and 2) by several orders of magnitude for C-H bond oxidation. On the basis of detailed kinetic studies, it is postulated that the activation results from Lewis base attack on the [Fe(III)Fe(IV)(μ-O)(2)] core, resulting in the formation of a more reactive species with a [X-Fe(III)-O-Fe(IV)═O] ring-opened structure (1-X, 2-X, X = OH(-) or OR(-)). Treatment of 2 with methoxide at -80 °C forms the 2-methoxide adduct in high yield, which is characterized by an S = 1/2 EPR signal indicative of an antiferromagnetically coupled [S = 5/2 Fe(III)/S = 2 Fe(IV)] pair. Even at this low temperature, the complex undergoes facile intramolecular C-H bond cleavage to generate formaldehyde, showing that the terminal high-spin Fe(IV)═O unit is capable of oxidizing a C-H bond as strong as 96 kcal mol(-1). This intramolecular oxidation of the methoxide ligand can in fact be competitive with intermolecular oxidation of triphenylmethane, which has a much weaker C-H bond (D(C-H) 81 kcal mol(-1)). The activation of the [Fe(III)Fe(IV)(μ-O)(2)] core is dramatically illustrated by the oxidation of 9,10-dihydroanthracene by 2-methoxide, which has a second-order rate constant that is 3.6 × 10(7)-fold larger than that for the parent diamond core complex 2. These observations provide strong support for the DFT-based notion that an S = 2 Fe(IV)═O unit is much more reactive at H-atom abstraction than its S = 1 counterpart and suggest that core isomerization could be a viable strategy for the [Fe(IV)(2)(μ-O)(2)] diamond core of sMMO-Q to selectively attack the strong C-H bond of methane in the presence of weaker C-H bonds of amino acid residues that define the diiron active site pocket.     

The synthesis, structure and ethene polymerisation catalysis of mono(salicylaldiminato) titanium and zirconium complexes

The silyl ethers 3-But-2-(OSiMe3)C6H3CH=NR (2a-e) have been prepared by deprotonation of the known iminophenols (1a-e) and treatment with SiClMe3 (a, R = C6H5; b, R = 2,6-Pri2C6H3; c, R = 2,4,6-Me3C6H2; d, R = 2-C6H5C6H4; e, R = C6F5). 2a-c react with TiCl4 in hydrocarbon solvents to give the binuclear complexes [Ti{3-But-2-(O)C6H3CH=N(R)}Cl(mu-Cl3)TiCl3] (3a-c). The pentafluorophenyl species 2e reacts with TiCl4 to give the known complex Ti{3-But-2-(O)C6H3CH=N(R)}2Cl2. The mononuclear five-coordinate complex, Ti{3-But-2-(O)C6H3CH=N(2,4,6-Me3C6H2)}Cl3 (4c), was isolated after repeated recrystallisation of 3c. Performing the dehalosilylation reaction in the presence of tetrahydrofuran yields the octahedral, mononuclear complexes Ti{3-But-2-(O)C6H3CH=N(R)}Cl3(THF) (5a-e). The reaction with ZrCl4(THF)2 proceeds similarly to give complexes Zr{3-But-2-(O)C6H3CH=N(R)}Cl3(THF) (6b-e). The crystal structures of 3b, 4c, 5a, 5c, 5e, 6b, 6d, 6e and the salicylaldehyde titanium complex Ti{3-But-2-(O)C6H3CH=O}Cl3(THF) (7) have been determined. Activation of complexes 5a-e and 6b-e with MAO in an ethene saturated toluene solution gives polyethylene with at best high activity depending on the imine substituent.     

Infrared spectra of methylidynes formed in reactions of re atoms with methane, methyl halides, methylene halides, and ethane: methylidyne C-H stretching absorptions, bond lengths, and s character

Rhenium carbyne complexes (HC identical with ReH 3, HC identical with ReH 2X, HC identical with ReHX 2, [X = F, Cl, and Br] and CH 3C identical with ReH 3) are produced by reactions of laser-ablated Re atoms with methane, methyl halides, methylene halides, and ethane via oxidative C-H(X) insertion and alpha-hydrogen migration in favor of the carbon-metal triple bond. The stabilities of the carbyne complexes relative to other possible products are predicted by DFT calculations. The diagnostic methylidyne C-H stretching absorptions of HC identical with ReH 3 and its mono- and dihalo derivatives are observed on the blue sides of the precursor C-H stretching bands, and the frequency decreases and the bond length increases in the order of H, F, Cl, and Br, following the decreasing s character in hybridization for the C-H bond. The dihalo methylidynes have higher C-H stretching frequencies and s characters than the monohalo species. The rhenium methylidynes have C s structures, and as a result the HC identical with ReH 3 and CH 3C identical with ReH 3 complexes have two equivalent shorter and one longer Re-H bonds, as compared to the tungsten methylidyne HC identical with WH 3 with three equivalent W-H bonds.     

Ab initio study of ternary complexes X:(HCNH)(+):Z with X, Z = NCH, CNH, FH, ClH, and FCl: diminutive cooperative effects on structures, binding energies, and spin-spin coupling constants across hydrogen bonds

Ab initio calculations have been performed on a series of complexes in which (HCNH)(+) is the proton donor and CNH, NCH, FH, ClH, and FCl (molecules X and Z) are the proton acceptors in binary complexes X:HCNH(+) and HCNH(+):Z, and ternary complexes X:HCNH(+):Z. These complexes are stabilized by C-H(+)···A and N-H(+)···A hydrogen bonds, where A is the electron-pair donor atom of molecules X and Z. Binding energies of the ternary complexes are less than the sum of the binding energies of the corresponding binary complexes. In general, as the binding energy of the binary complex increases, the diminutive cooperative effect increases. The structures of these complexes, data from the AIM analyses, and coupling constants (1)J(N-H), (1h)J(H-A), and (2h)J(N-A) for the N-H(+)···A hydrogen bonds, and (1)J(C-H), (1h)J(H-A), and (2h)J(C-A) for the C-H(+)···A hydrogen bonds provide convincing evidence of diminutive cooperative effects in these ternary complexes. In particular, the symmetric N···H(+)···N hydrogen bond in HCNH(+):NCH looses proton-shared character in the ternary complexes X:HCNH(+):NCH, while the proton-shared character of the C···H(+)···C hydrogen bond in HNC:HCNH(+) decreases in the ternary complexes HNC:HCNH(+):Z and eventually becomes a traditional hydrogen bond as the strength of the HCNH(+)···Z interaction increases.     

Ring-opening polymerization of coordination complexes: silver(I) complexes with bis(amidopyridine) ligands derived from thiophene

The thiophene-based bis(N-methylamido-pyridine) ligand SC4H2-2,5-{C(=O)N(Me)-4-C5H4N}2 reacts with silver(I) salts AgX to give 1 : 1 complexes, which are characterized in the solid state as the macrocyclic complexes [Ag(2){SC4H2-2,5-(CONMe-4-C5H4N)2}2][X]2, which have the cis conformation of the C(=O)N(Me) group, when X = CF3CO2, NO3, or CF3SO3 but as the polymeric complex [Ag(n){SC4H2-2,5-(CONMe-4-C5H4N)2}n][X]n, with the unusual trans conformation of the C(=O)N(Me) group, when X = PF6. The bis(amido-pyridine) ligand SC4H2-2,5-{C(=O)NHCH2-3-C5H4N}2 reacts with silver(I) trifluoroacetate to give the polymeric complex [Ag(n){SC4H2-2,5-(CONHCH2-3-C5H4N)2}n][X]n, X = CF3CO2. The macrocyclic complexes contain transannular argentophilic secondary bonds. The polymers self assemble into sheet structures through interchain C=O...Ag and S...Ag bonds in [Ag(n){SC4H2-2,5-(CONMe-4-C5H4N)2}n][PF6]n and through Ag...Ag, C=O...Ag and Ag...O(trifluoroacetate)...HN secondary bonds in [Ag(n){SC4H2-2,5-(CONHCH2-3-C5H4N)2}n][CF3CO2]n.     

Geometric isotope effect of various intermolecular and intramolecular C-H...O hydrogen bonds, using the multicomponent molecular orbital method

The geometric isotope effect (GIE) of sp- (acetylene-water), sp(2)- (ethylene-water), and sp(3)- (methane-water) hybridized intermolecular C-H...O and C-D...O hydrogen bonds has been analyzed at the HF/6-31++G level by using the multicomponent molecular orbital method, which directly takes account of the quantum effect of proton/deuteron. In the acetylene-water case, the elongation of C-H length due to the formation of the hydrogen bond is found to be greater than that of C-D. In contrast to sp-type, the contraction of C-H length in methane-water is smaller than that of C-D. After the formation of hydrogen bonds, the C-H length itself in all complexes is longer than C-D and the H...O distance is shorter than D...O, similar to the GIE of conventional hydrogen bonds. Furthermore, the exponent (alpha) value is decreased with the formation of the hydrogen bond, which indicates the stabilization of intermolecular C-H...O hydrogen bonds as well as conventional hydrogen bonds. In addition, the geometric difference induced by the H/D isotope effect of the intramolecular C-H...O hydrogen bond shows the same tendency as that of intermolecular C-H...O. Our study clearly demonstrates that C-H...O hydrogen bonds can be categorized as typical hydrogen bonds from the viewpoint of GIE, irrespective of the hybridizing state of carbon and inter- or intramolecular hydrogen bond.     

甲醛与乙腈相互作用的理论研究

在HF/6-311G(d,p)、 MP2/6-311G(d,p)和B3LYP/6-311G(d,p)水平上,对H2CO和CH3CN以及设计的4种结构H2CO…CH3CN复合物等进行几何全优化和振动频率计算,排除振动频率为负值的非局域极小点结构,并对稳定的环状构型复合物结合能进行基组重叠误差校正和零点振动能校正.分子间相互作用的能量分解分析显示,静电能在H2CO...CH3CN相互作用能量中占主导地位,电荷转移能居第二位.     

Novel thiophene-based cycloruthenated compounds: synthesis, characterization, and reactivity

The reactions between a series of thiophene-based imines with [(η(6)-C(6)H(6))RuCl(μ-Cl)](2), in a basic medium, and in MeCN give a family of ruthenacycles of stoichiometry [Ru(C^N)(NCMe)(4)]PF(6) (C^N = orthometalated thiopheneimine). In these species, the C-H activation process is produced in most cases at the thiophene ring. When two C-H bonds are competing (thiophene vs aryl), the cyclometalation can be driven regioselectively to the thiophene unit or to the aryl ring as a function of the location of the iminic C=N bond. Cyclometalation can also be oriented to positions 2 or 3 of the thiophene depending on the situation of the imine in the heterocycle (3 or 2, respectively). In all studied cases, the η(6)-C(6)H(6) ligand was substituted by acetonitrile. The X-ray structures of two representative complexes have been determined. These thiophene-based metallacycles react with iodine under very mild conditions affording, after hydrolysis, substituted 3-iodo-2-formyl(benzo)thiophenes or substituted 2-iodo-3-formyl(benzo)thiophenes, as a function of the organometallic precursor.     

Modeling of hydrogen bonds in monohydrated 2,4-dithiothymine: an ab initio and AIM study

Twelve tautomers of 2,4-dithiothymine are calculated at the MP2/6-31+G(d) level, and the most stable one is referred to the di-keto form (P12). Then four H-bonded complexes between P12 and water are optimized at the MP2/6-31+G(d) level of theory. The calculation of vibrational frequencies and natural bond orbital analysis are also carried out at the same level to investigate the hydrogen bonds involved in all the systems. Within all the four complexes, three types of hydrogen bonds are formed, in which the O-H...S and N-H...O bonds are the normal bonds with the X-H bond elongation and red shift of the corresponding stretch frequencies, while the C-H...O interaction is an improper, blue-shifting hydrogen bond accompanied with the contraction of the C-H bond and a blue shift of the C-H stretch frequency. The topological properties are investigated with the atoms-in-molecules (AIM) theory. The NMR chemical shielding for the isolated and the four monohydrated 2,4-dithiothymine are calculated using the "gauge-including atomic orbital" (GIAO) method. The 1H chemical shifts are influenced by the formation of hydrogen bonds.     

Double bonds in motion: bis(oxazolinylmethyl)pyrroles and their metal-induced planarization to a new class of rigid chiral C2-symmetric complexes

The synthesis of a new class of chiral C(2)-symmetric tridentate N-donor ligands, a series of 2,5-bis(2-oxazolinylmethyl)pyrroles, was achieved in four steps starting from the known 2,5-bis(trimethylammoniomethyl)pyrrole diiodide (1). Reaction of 1 with NaCN in dimethyl sulfoxide gave 2,5-bis(cyanomethyl)pyrrole (2) cleanly, which was then cyclized with amino alcohols to give the 2,5-bis(2-oxazolinylmethyl)pyrroles 3 a-c (3 a: bis[2-(4,4'-dimethyl-5-hydrooxazolyl)methyl]pyrrole; 3 b: (S,S)-bis[2-(4-isopropyl-4,5-dihydrooxazolyl)methyl]pyrrole; 3 c: (S,S)-bis[2-(4-tertiobutyl-4,5-dihydrooxazolyl)methyl]pyrrole). Metallation of 3 a-c with one molar equivalent of tBuLi and their subsequent reaction with a stoichiometric amount of [PdCl(2)(cod)] (cod=cyclooctadiene) gave the palladium(II) complexes 4 a-c. Whereas the arrangement of the N-donor atoms in the crystallographically characterized complex 4 a is almost ideally square planar, all three heterocycles in the ligand are twisted out of the coordination plane, leading to a chiral conformation of the complex. Attempts to freeze out these two conformers in solution at 200 K (NMR) failed, and this suggests that the activation barrier for conformational racemization is significantly below 10 kcal mol(-1). The palladium-induced shift of two double bonds as well as the porphyrinogen/porphyrin-type oxidation of the complexes 4 a-c led to the planarization of the 2,5-bis(oxazolinylmethyl)pyrrolide ligands in the palladium(II) complexes 5 a-c, 6 b, and 6 c, and to the formation of rigid chiral C(2)-symmetric systems as shown by X-ray diffraction studies. The formation of the conjugated system of double bonds in this transformation is accompanied by the emergence of an intra-ligand chromophore. This is evident in the absorption spectrum of 6 c which displays an intense band with a maximum at 485 nm attributable to an intra-ligand pi*<--pi transition and a characteristic vibrational progression of nu approximately 1350 cm(-1). Complexes 4 b and 4 c were tested in the catalytic asymmetric Michael addition of ethyl 2-cyanopropionate to methyl vinylketone (catalyst loading: 1 mol %) and were found to give maximum ee values of 43 % (4 b) and 21 % (4 c) at low conversions.     

Activation of carbon-hydrogen bonds via 1,2-addition across M-X (X = OH or NH(2)) bonds of d(6) transition metals as a potential key step in hydrocarbon functionalization: a computational study

Recent reports of 1,2-addition of C-H bonds across Ru-X (X = amido, hydroxo) bonds of TpRu(PMe3)X fragments {Tp = hydridotris(pyrazolyl)borate} suggest opportunities for the development of new catalytic cycles for hydrocarbon functionalization. In order to enhance understanding of these transformations, computational examinations of the efficacy of model d6 transition metal complexes of the form [(Tab)M(PH3)2X]q (Tab = tris-azo-borate; X = OH, NH2; q = -1 to +2; M = TcI, Re(I), Ru(II), Co(III), Ir(III), Ni(IV), Pt(IV)) for the activation of benzene C-H bonds, as well as the potential for their incorporation into catalytic functionalization cycles, are presented. For the benzene C-H activation reaction steps, kite-shaped transition states were located and found to have relatively little metal-hydrogen interaction. The C-H activation process is best described as a metal-mediated proton transfer in which the metal center and ligand X function as an activating electrophile and intramolecular base, respectively. While the metal plays a primary role in controlling the kinetics and thermodynamics of the reaction coordinate for C-H activation/functionalization, the ligand X also influences the energetics. On the basis of three thermodynamic criteria characterizing salient energetic aspects of the proposed catalytic cycle and the detailed computational studies reported herein, late transition metal complexes (e.g., Pt, Co, etc.) in the d6 electron configuration {especially the TabCo(PH3)2(OH)+ complex and related Co(III) systems} are predicted to be the most promising for further catalyst investigation.     

含杂三元环化合物键强变化规律

用键强参数f~i, ~A~-B, f~A~-~B可定量地表达含杂三元环化合物CH2-CH2-X(X=Be, BH, CH2, NH, O及S)中成键情况及键强变化规律: 1. 无论在三元环中, 还是在C-X-C中, X=Be→O时, f~c~-~x随X中重原子的核电荷数Z递增而增大。其原因: X中重原子实对于价电子的有效核电荷数随其核电荷数Z的增大而增大。C-X的"键电荷"也随X的Z值递增。2. 当X=Be→O, 在形成CH2-CH2-X时, 较强键被削弱, 较弱键则增强, 致环内各-键键强和键长都有平均化的趋势。其原因为: 三元环分子中形成σ共轭的三中心键, C-X与C-C键共享"键电荷"。这种"σ-共轭效应"与π-共轭效应有相似处。3.由于硫的价电子云平均半径较大, 可向c-C提供更多共享电荷, 故在含S三元环化合物CH2-CH2-S中C-S键受到更大程度的削弱, C-C则更被增强。