Detailed Density Functional Theory Study of the Cationic Zirconocene Compound [Cp(C5H4CMe2C6H4F)ZrMe].

Detailed density functional theory studies at the B3LYP and PBE-D3 levels of theory were performed on the cationic compound [Cp(C5H4CMe2C6H4F)ZrMe]+, with the F atom occupying either the ortho, meta, or para positions of the arene ring. In all cases, the arene ring coordinates with the cationic zirconium metal. The nature of this coordination is such that for the meta- or para-substituted arene ring, it is predominantly the ortho carbon atom of the C-H bond which interacts with the metal, as evident from noncovalent interaction studies. This is further corroborated by the natural bond orbital and quantum theory of atoms in molecular studies. In the case of the F atom being in the ortho position, we obtained clear-cut evidence for a Zr-F coordination.

Computational study of glucosepane-water and hydrogen bond formation: an electron topology and orbital analysis.

The collagen protein provides tensile strength to the extracellular matrix in addition to localising cells, proteins and protein cofactors. Collagen is susceptible to a build up of glycation modifications as a result of an exceptionally long half-life. Glucosepane is a collagen cross-linking advanced glycation end product; the structural and mechanical effects of glucosepane are still the subjects of much debate. With the prospect of an ageing population, the management and treatment of age-related diseases is becoming a pressing concern. One area of interest is the isolation of hydrated glucosepane, which has yet to be reported at an atomistic level. This study presents a series of glucosepane-water complexes within an implicit aqueous environment. Electronic structure calculations were performed using density functional theory and a high level basis set. Hydrogen bonds between glucosepane and explicit water were identified by monitoring changes to covalent bonds, calculating levels of electron donation from Natural Bonding Orbital analysis and the detection of bond critical points. Hydrogen bond strength was calculated using second-order perturbation calculations. The combined results suggest that glucosepane is very hydrophilic, with the imidazole feature being energetically more attractive to water than either hydroxyl group, although all hydrogen bonds, regardless of bond strength, were electrostatic in nature. Our results are in growing support of an earlier hypothesis that cross-links may result in an increase in interstitial water retention, which would permit the collagen fibril to swell, thereby potentially affecting the tensile and compression properties and biological function of connective tissues.

Computational studies of the unusual water adduct [Cp2TiMe(OH2)](+): the roles of the solvent and the counterion.

The recently reported cationic titanocene complex [Cp2TiMe(OH2)](+) was subjected to detailed computational studies using density functional theory (DFT). The calculated NMR spectra revealed the importance of including the anion and the solvent (CD2Cl2) in order to calculate spectra which were in good agreement with the experimental data. Specifically, two organic solvent molecules were required to coordinate to the two hydrogens of the bound OH2 in order to achieve such agreement. Further elaboration of the role of the solvent led to Bader's QTAIM and natural bond order calculations. The zirconocene complex [Cp2ZrMe(OH2)](+) was simulated for comparison.

Quo vadis, agostic bonding?

The role of agostic bonds in chemistry is highlighted. In particular, attention is given to the origin of the term, methods of investigations and their function in chemistry.

Main group multiple C-H/N-H bond activation of a diamine and isolation of a molecular dilithium zincate hydride: experimental and DFT evidence for alkali metal-zinc synergistic effects.

The surprising transformation of the saturated diamine (iPr)NHCH(2)CH(2)NH(iPr) to the unsaturated diazaethene [(iPr)NCH═CHN(iPr)](2-) via the synergic mixture nBuM, (tBu)(2)Zn and TMEDA (where M = Li, Na; TMEDA = N,N,N',N'-tetramethylethylenediamine) has been investigated by multinuclear NMR spectroscopic studies and DFT calculations. Several pertinent intermediary and related compounds (TMEDA)Li[(iPr)NCH(2)CH(2)NH(iPr)]Zn(tBu)(2) (3), (TMEDA)Li[(iPr)NCH(2)CH(2)CH(2)N(iPr)]Zn(tBu) (5), {(THF)Li[(iPr)NCH(2)CH(2)N(iPr)]Zn(tBu)}(2) (6), and {(TMEDA)Na[(iPr)NCH(2)CH(2)N(iPr)]Zn(tBu)}(2) (11), characterized by single-crystal X-ray diffraction, are discussed in relation to their role in the formation of (TMEDA)M[(iPr)NCH═CHN(iPr)]Zn(tBu) (M = Li, 1; Na, 10). In addition, the dilithio zincate molecular hydride [(TMEDA)Li](2)[(iPr)NCH(2)CH(2)N(iPr)]Zn(tBu)H 7 has been synthesized from the reaction of (TMEDA)Li[(iPr)NCH(2)CH(2)NH(iPr)]Zn(tBu)(2)3 with nBuLi(TMEDA) and also characterized by both X-ray crystallographic and NMR spectroscopic studies. The retention of the Li-H bond of 7 in solution was confirmed by (7)Li-(1)H HSQC experiments. Also, the (7)Li NMR spectrum of 7 in C(6)D(6) solution allowed for the rare observation of a scalar (1)J(Li-H) coupling constant of 13.3 Hz. Possible mechanisms for the transformation from diamine to diazaethene, a process involving the formal breakage of four bonds, have been determined computationally using density functional theory. The dominant mechanism, starting from (TMEDA)Li[(iPr)NCH(2)CH(2)N(iPr)]Zn(tBu) (4), involves the formation of a hydride intermediate and leads directly to the observed diazaethene product. In addition the existence of 7 in equilibrium with 4 through the dynamic association and dissociation of a (TMEDA)LiH ligand, also provides a secondary mechanism for the formation of the diazaethene. The two reaction pathways (i.e., starting from 4 or 7) are quite distinct and provide excellent examples in which the two distinct metals in the system are able to interact synergically to catalyze this otherwise challenging transformation.

Agostic or not? Detailed Density Functional Theory studies of the compounds [LRh(CO)Cl], [LRh(COD)Cl] and [LRhCl] (L = cyclic (alkyl)(amino)carbene, COD = cyclooctadiene).

Detailed Density Functional Theory (DFT) studies were conducted on the rhodium compounds [LRh(CO)Cl] (1), [LRhCl] (2) and [LRh(COD)Cl] (3) (L = cyclic (alkyl)(amino)carbene, CAAC; COD = cyclooctadiene). Particular attention was paid to the two cyclohexyl hydrogens of the CAAC which are in close proximity to the metal centre. Bader and NBO analyses confirmed an agostic interaction, and NBO analysis revealed that in the case of 1, the Rh-CO antibonding orbital acts as an acceptor. Removal of the CO ligand (2) did not significantly change the agostic interaction of the two cyclohexyl hydrogens or the geometry of the cyclohexyl ligand. Replacement of the 2e(-) donor CO with the 4e(-) donor COD gives a different picture. Although both cyclohexyl hydrogens are still in close proximity to the metal centre, neither are agostically bound to it. In fact, the very weak interaction of one of them is of the same order as that present in the Cl-H bond. We thus suggest a revised description of agostic bonding.

Quantitative detection of human tumor necrosis factor α by a resonance raman enzyme-linked immunosorbent assay.

Tumor necrosis factor α is an inflammatory cytokine which has been linked with many infectious and inflammatory diseases. Detection and quantification of this key biomarker is commonly achieved by use of an enzyme-linked immunosorbent assay (ELISA). This fundamental technique uses the spectroscopic detection of a chromogen such as 3,3',5,5'-tetramethylbenzidine (TMB). Horseradish peroxidase (HRP), bound to the detection antibody, catalyzes the oxidation of TMB by hydrogen peroxide to generate colored products which may be measured spectrophotometrically. In this study we have used a conventional ELISA kit and shown that, by replacing the traditional colorimetric detection with resonance Raman spectroscopy, we can achieve 50 times lower detection limits and the potential for multiplexed analysis is increased. In this approach, the laser wavelength was tuned to be in resonance with an electronic transition of the oxidized TMB. The relative intensity of the enhanced Raman bands is proportional to the amount of TMB, thus providing a means of improved quantification. Furthermore, TMB is one of the most widely used chromogenic substrates for HRP-based detection and commercial ELISA test kits, indicating that this detection technique is applicable to a large number of target analytes.

Weinreb amides in carbene chemistry: a time-resolved IR investigation into a potential intramolecular stabilization mechanism.

A chloromethylhydroxamiccarbene was generated photochemically in an attempt to form an intramolecularly stabilized carbene. A rapidly formed intermediate at 1645 cm(-1) decayed with an observed rate of 1.99 × 10(6) s(-1). Other intermediates were also observed. These also decayed, albeit much more slowly (k(obs) = 3.47 × 10(3) and 1.98 × 10(4) s(-1)). Multiple intermediates are apparently a function of both the proximal N,O-dimethylhydroxamic ester and multiple conformers of both the carbene and precursor.

Multidentate ligands for the synthesis of multimetallic complexes. 2. Formation of a planar Cu4OH motif.

The reaction of the multidentate Schiff base species TrenSal, TrenBrSal, and TEtSal with copper acetate is reported. The heptadentate ligands generate a tetrametallic L(2)Cu(4)OH motif that contains an internalized hydroxide anion. In contrast, the hexadentate TEtSal ligand is found to form an open trimetallic motif. The importance of L(2)Cu(4)OH to the family of copper complexes with an endohedral hydroxide anion is discussed. [(TrenBrSal)(2)Cu(4)OH][OAc] is analyzed by temperature-dependent magnetic measurements.

DFT investigation of the 'quasi-living' propene polymerisation with Cp*TiMe3/B(C6F5)3: the 'naked cation' approach.

Some time ago we reported the quasi-living polymerization of propene with the catalytic mixture of Cp*TiMe(3) and B(C(6)F(5))(3) (Cp* = C(5)Me(5)). Surprisingly, this mixture is extremely sensitive towards the nature of the anion and the presence of aluminium alkyl. This intriguing observation led us to the attempt to unearth the underlying reaction mechanism using a computational approach. In this communication, we are reporting the first results with the 'naked cation' approach. We obtained evidence, that the 1,2 insertion is the predominant reaction pathway. Whereas initial 1,2 and 2,1 insertion barriers are comparable, consequent second insertion is more discriminating between the two. Although we obtained evidence for the formation of beta-H agostic bonds, we found that beta-H elimination is a rare event due to the rather high activation barrier. We can conclude that the quasi-living polymerisation is at least partly an intrinsic property of the cation.

Cationic and dicationic zirconocene compounds as initiators of carbocationic isobutene polymerisation.

We report the polymerisation of isobutene with the intramolecular aryl stabilised zirconocene cations [eta(5)-Cp-(eta(5)-C(5)H(4)-CMe(2)C(6)H(4)Me)ZrMe](+) (3), [(eta(5)-C(5)H(4)-CMe(2)C(6)H(4)Me)(2)ZrMe](+) (4) and dication [(eta(5)-C(5)H(4)-CMe(2)C(6)H(4)Me)(2)Zr](2+) (5) (Cp = C(5)H(5)) which were generated in situ by the reaction of the corresponding dimethyl compound with the cation generating agent (CGA) B(C(6)F(5))(3). Whereas the cationic compounds gave polyisobutene in moderate yields, the dicationic compound gave polyisobutene in excellent yields and high molecular weight. Computational studies of the cationic and dicationic compounds were undertaken to ascertain the nature of the cationic initiator. Judging from natural charges and bond critical points, the coordinated monomer is more strongly polarised in the case of the dication 5, compared with the monocation 3. Furthermore, in the case of the cationic compound 3, decomposition into the allylic compound was computed. We found that the activation barrier of this process is 103.3 kJ mol(-1), which is too high for a direct C-H activation by the Zr-Me group.

Structural and reactivity studies of "pincer" pyridine dicarbene complexes of Fe0: experimental and computational comparison of the phosphine and NHC donors.

Reduction of [Fe(C-N-C)(Br)(2)] or [Fe(C-N(Me)-C)(I)(2)] with Na/Hg under N(2) gave the Fe(0) complexes [Fe(C-N-C)(N(2))(2)] and [Fe(C-N(Me)-C)(N(2))(2)] for which C-N-C = 2,6-bis(arylimidazol-2-ylidene)-pyridine, C-N(Me)-C = 2,6-bis(arylimidazol-2-ylidene)-3,5-dimethyl-pyridine and aryl = 2,6-iPr(2)C(6)H(3). Substitution of the coordinated N(2) by CO or CN(2,6-xyl), xyl = 2,6-dimethylphenyl, took place readily to afford [Fe(C-N-C)(L)(2)] and [Fe(C-N(Me)-C)(L)(2)], L = CO or CN(2,6-xyl). The electronic characteristics of the N-heterocyclic carbene and phosphine donors in the complexes [Fe(C-N-C)(CO)(2)], [Fe(C-N(Me)-C)(CO)(2)] and [Fe(P-N-P)(CO)(2)], P-N-P = 2,6-bis(di-tert-butylphosphinomethyl)-pyridine, have been evaluated by spectroscopic, structural and computational methods. The unexpected reduced Fe(0) to CO backbonding in [Fe(C-N-C)(CO)(2)] and [Fe(C-N(Me)-C)(CO)(2)] compared to [Fe(P-N-P)(CO)(2)] is accounted for by backbonding from the Fe-N(pyridine) bond in the imidazol-2-ylidene complexes. Reduction of [Fe(C-N-C)(Br)(2)] under Ar gave mixtures from which a complex with a metalated pyridine and a dangling imidazolium group was isolated. Photolysis of [Fe(C-N-C)(N(2))(2)] in the presence of benzaldehyde phenylimine gave an ortho-metalated benzaldehyde phenylimine. The Fe(II) complex [Fe(C-N-C)(CO)(Br)(2)] was prepared by the reaction of [Fe(C-N-C)(Br)(2)] with CO or reduction of CO(2) with [Fe(C-N-C)(N(2))(2)].

Tribenzyl{eta(5)-[2-(p-tolyl)prop-2-yl]cyclopentadienyl}zirconium.

The title compound, [Zr(C(7)H(7))(3)(C(15)H(17))], (I), crystallizes from light petroleum with two independent molecules in the asymmetric unit. Whereas in the parent molecule, Zr(eta(5)-C(5)H(5))(CH(2)Ph)(3), all three Zr-CH(2)Ph angles are equal, in (I), they differ significantly. In spite of their different environments, both independent molecules in (I) exhibit a small, an expected, and a large Zr-CH(2)Ph angle. The angles are similar to those of the closely related tribenzyl[eta(5)-(benzyldimethylsilyl)cyclopentadienyl]zirconium complex. The smallest Zr-CH(2)Ph angle and the consequently relatively short Zr.C(ipso) distance are indicative of eta(2)-bonding of the benzyl group.