Hypervalent radical formation probed by electron transfer dissociation of zwitterionic tryptophan and tryptophan‐containing dipeptides complexed with Ca2+ and 18‐crown‐6 in the gas phase

The relationship between peptide structure and electron transfer dissociation (ETD) is important for structural analysis by mass spectrometry. In the present study, the formation, structure and reactivity of the reaction intermediate in the ETD process were examined using a quadrupole ion trap mass spectrometer equipped with an electrospray ionization source. ETD product ions of zwitterionic tryptophan (Trp) and Trp‐containing dipeptides (Trp‐Gly and Gly‐Trp) were detected without reionization using non‐covalent analyte complexes with Ca²⁺ and 18‐crown‐6 (18C6). In the collision‐induced dissociation, NH₃ loss was the main dissociation pathway, and loss related to the dissociation of the carboxyl group was not observed. This indicated that Trp and its dipeptides on Ca²⁺(18C6) adopted a zwitterionic structure with an NH₃⁺ group and bonded to Ca²⁺(18C6) through the COO^? group. Hydrogen atom loss observed in the ETD spectra indicated that intermolecular electron transfer from a molecular anion to the NH₃⁺ group formed a hypervalent ammonium radical, R‐NH₃, as a reaction intermediate, which was unstable and dissociated rapidly through N–H bond cleavage. In addition, N–C_α bond cleavage forming the z₁ ion was observed in the ETD spectra of Trp‐GlyCa²⁺(18C6) and Gly‐TrpCa²⁺(18C6). This dissociation was induced by transfer of a hydrogen atom in the cluster formed via an N–H bond cleavage of the hypervalent ammonium radical and was in competition with the hydrogen atom loss. The results showed that a hypervalent radical intermediate, forming a delocalized hydrogen atom, contributes to the backbone cleavages of peptides in ETD. Copyright © 2015 John Wiley & Sons, Ltd.     

Nicotinamide complex of silver(III) with expanded coordination number

Abstract

In strongly alkaline media ([OH^?]?≥?0.12 M), nicotinamide (nica) forms a complex with square-planar Ag(OH)₄^? [nica]?≥?0.05 M. The complex decomposes in seconds to nicotinamide N-oxide. The correlation of maximum absorbance of the complex with concentrations of nicotinamide and hydroxide requires that the complex is either the five-coordinate Ag(OH)₄(H_-1nica)^2? or the six-coordinate Ag(OH)₅(nica)^2?. Comparison with the reactions of Ag(OH)₄^? with nicotinate ion (nic^?) and acetamide under similar conditions indicates that nicotinamide coordinates with Ag(OH)₄^? by the amido group rather than the nitrogen on the pyridine ring or the amido oxygen. Kinetics of the Ag(III)-nica redox reaction are consistent with direct reaction between nicotinamide and uncoordinated Ag(OH₄)^?. Oxidation takes place at the pyridine ring, yielding nicotinamide N-oxide. Silver(III) is reduced to monovalent silver.     

Ab initio study of the insertion reactions of Sc+(1D) with HF,HCl, H2O,H2S,NH3, PH3, CH4, and SiH4

The mechanism of scandium cation Sc⁺(¹D) insertion into HF, HCl, H₂O, H₂S, NH₃, PH₃, CH₄, and SiH₄ has been investigated by ab initio molecular theory. All these reactions involve the initial formation of intermediate complexes followed by an H‐atom migration process via a transition state to insertion products. The Sc⁺(¹D) insertion into eight compound reactions indicate that (i) the reaction with hydride of the right‐hand group is more exothermic than that of the left‐hand group and has a lower barrier, and (ii) the reaction with the second‐row hydride has a lower overall barrier and is less exothermic than with the first‐row hydride. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

DNA binding,cleavage, and cytotoxicity of binuclear phenolate nickel(II) complexes

Three binuclear phenolate complexes, [Ni₂(L¹)₂(OAc)](BPh₄)·DMF (1), [Ni₂(L²)₂(OAc)](BPh₄) (2), and [Ni₂(L³)₂(OAc)](OH)·3H₂O (3), where L¹ = 2-{[bis-(2-hydroxy-ethyl)-amino]-methyl}-4-methyl-phenol, L² = 2-{[bis-(2-hydroxy-ethyl)-amino]-methyl}-4-methoxy-phenol, and L³ = 2-{[bis-(2-hydroxy-ethyl)-amino]-methyl}-4-tert-butyl-phenol), have been synthesized. Single-crystal diffraction reveals that all the metal atoms are in a distorted octahedral geometry. The interactions of the complexes with calf thymus DNA (CT-DNA) have been investigated by UV–vis absorption, fluorescence emission, and circular dichroism spectroscopy and viscosity measurements. Furthermore, DNA cleavage mechanism shows that the complexes may be capable to promote DNA cleavage through oxidative DNA damage pathway, which is indicative of the involvement of hydroxyl radical, singlet oxygen, or singlet oxygen-like entity in the cleavage process. Cytotoxicity studies on the Hela and MCF-7 cancer cell lines show that complexes 1–3 exhibit excellent activity toward the tested tumor cell lines with respect to the standard drug carboplatin, revealing that they have the potential to act as effective metal-based anticancer drugs.     

Theoretical study on the insertion reactions of Ti+(2F) with HF,HCl, H2O,H2S,NH3, PH3, CH4, and SiH4

The insertion reactions of the titanium atom cation Ti⁺(²F) into HF, HCl, H₂O, H₂S, NH₃, PH₃, CH₄, and SiH₄ have been studied by ab initio molecular orbital theory. All these reactions involve the initial formation of an intermediate complex followed by a hydrogen migration process via a transition state to inserting product. The Ti⁺(²F) insertion into eight compounds' reactions show that the reaction with hydride of the right‐hand group is more exothermic than that of the left‐hand group and has a lower overall barrier, and that the reaction with the second‐row hydride has a lower overall barrier and is less exothermic than with the first‐row hydride. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 47–54, 1999     

A new tetranuclear copper(II) complex containing decameric water clusters bridged by–OH–and pyridine-2,4, 6-tricarboxylate: synthesis and magnetic characterization

Using pyridine-2,4,6-tricarboxylic acid (H₃ptc) and 2,2-bipyridine (2,2-bipy), a tetranuclear copper(II) compound [Cu₄(2,2-bipy)₄(ptc)₂(H₂O)₂(OH)₂] · 12H₂O (1) has been isolated under hydrothermal conditions. Variable temperature magnetic susceptibility of 1 from 2–300 K indicates anti-ferromagnetic interactions. The magnetic exchange coupling constants of J = ?159.4 and J′ = ?18.66 cm^?1 for 1 can be obtained through fit of the magnetic data, corresponding to two kinds of bridges, hydroxyl anions (OH^?) and pyridine carboxylate oxygen of ptc^3?. Moreover, decameric water clusters can also be observed, which are located between these tetranuclear copper(II) entities, forming a series of intricate O-H ··· O hydrogen bonds and stabilizing the resulting three-dimensional (3-D) hydrogen-bonded framework structure.     

Crystal structure of complexes of pentavalent neptunium with 1,8-bis[2-(diphenylphosphinylmethyl)]phenoxy-3,6-dioxaoctane (L) and hexavalent actinides (U,Np, Pu) with 1,8-bis[2-(diphenylphosphinyl)]phenoxy-3,6-dioxaoctane (L<Superscript>1</Superscript>)

Complexes of pentaand hexavalent actinides with phosphoryl-containing podands incorporating a triethylene glycol fragment and 2-(diphenylphosphinyl)phenyl (L¹) or 2-(diphenylphosphinylmethyl)phenyl (L) end groups were synthesized and structurally characterized. The complexes can be described by the following formulas: [NpO₂L(C₂H₅OH)(NO₃)] (I) for pentavalent neptunium and [AnO₂(L¹)₂](OH)₂ · nH₂O (II), where An is U (IIa), Np (IIb), and Pu (IIc), for the isostructural compounds of hexavalent actinides. In I, L is a bidentate bridging ligand. The Np⁵⁺ coordination polyhedron is a pentagonal bipyramid. The bipyramid equatorial plane is formed by the oxygen atoms of two podands L, the bidentate nitrate ion, and the ethanol OH group. The oxygen atoms of the phosphoryl groups of the podand are involved in the coordination environment of two NpO₂⁺ cations where they connect the electrically neutral neptunoyl nitrate fragments to infinite chains along the [100] direction, which are in turn connected into ribbons by strong hydrogen bonds. The crystal of II consists of the complex cations [AnO₂(L¹)₂]²⁺, hydroxyl ions, and water molecules of crystallization. The environment of AnO₂²⁺ is formed by four ligands L¹ whose oxygen atoms form a tetragonal-bipyramidal coordination environment. Each of the two crystallographically independent ligands L¹ is connected to two AnO₂⁺ cations. This gives positively charged layers of actinyl cations perpendicular to the [010] direction connected by molecular ligands. The layers contain channels accommodating hydroxyl ions and crystallization water molecules.     

Synthesis,spectroscopic characterization and DFT study of dinuclear ruthenium sawhorse-type complexes derived from the reaction of trinuclear aggregates and (Z)-5-arylidenerhodanines

The synthesis of dinuclear ruthenium sawhorse-type complexes [Ru₂(μ-ArCH:Rhod)₂(CO)₄]_n 12a–e and [Ru₂(ArCH:Rhod)₂(μ-ArCH:Rhod)₂(CO)₄] 13a–e through reaction of [Ru₃(CO)₁₀(NCMe)₂] and [Ru₃(CO)₁₂] and the corresponding (Z)-5-arylidenerhodanines (ArCH:Rhod) 10a–e, respectively, are reported. These complexes are arranged in a sawhorse structure in which two bridged (Z)-5-arylidenerhodanines coordinate to the metals using sulfur and nitrogen of the rhodanine ring. A Density Functional Theory method was used to gain insight into the polymerization process by calculating dimerization Gibbs energies (ΔG_dim). Values between ?10.7 and ?5.3 kcal mol^?1 indicate that dimerization is a spontaneous process. A reaction pathway for formation of the sawhorse compounds [Ru₂(μ-ArCH:Rhod)₂(CO)₄] was calculated and the rate-determining step for the mechanism is coordination of a second (Z)-5-arylidenerhodanine ligand with activation energies between 41.1 and 47.8 kcal mol^?1. In order to understand the apparent thermodynamic favorability of the fragmentation step, we calculated the fragmentation energy (ΔE_Frag) for the key intermediate and its energetic contributors, the interaction energy, ΔE_int and the reorganization energy, ΔE_reorg. Low values of ΔE_Frag imply that the fragmentation is thermodynamically facile. Large values of ΔE_int are countered by opposite and large values of ΔE_reorg which indicate that the cleavage of the trimetallic intermediate aggregate is determined by the nature of the ligand and the balance between its interaction with the metal and the extent of structural reorganization.     

Stereochemical effects during [M-H]− dissociations of epimeric 11-OH-17β-estradiols and distant electronic effects of substituents at C(11) position on gas phase acidity

The affinity of estradiol derivatives for the estrogen receptor (ER) depends strongly on nature and stereochemistry of substituents in C₍₁₁₎ position of the 17β-estradiol (I). In this work, the stereochemistry effects of the 11α-OH-17β-estradiol (III_α) and 11β-OH-17β-estradiol (III_β) were investigated using CID experiments and gas-phase acidity (ΔH_acid^∘) determination. The CID experiments showed that the steroids decompose via different pathways involving competitive dissociations with rate constants depending upon the α/β C₍₁₁₎ stereochemistry. It was shown that the fragmentations of both deprotonated [III_α-H]⁻ and [III_β-H]⁻ epimers were initiated by the deprotonation of the most acidic site, i.e. the phenolic hydroxyl at C₍₃₎. This view was confirmed by H/D exchange and double resonance experiments. Furthermore, the ΔH_acid^∘ of both epimers (III_α and III_β), 17β-estradiol (I), and 17-desoxyestradiol (II) was determined using the extended Cooks’ kinetic method. The resulting values allowed us to classify steroids as a function of their gas-phase acidity as follows: (III_β)≫(II)>(I)>(III_α). Interestingly, the α/β C₍₁₁₎ stereochemistry appeared to influence strongly the gas-phase acidity. This phenomenon could be explained through stereospecific proton interaction with π-orbital cloud of A ring, which was confirmed by theoretical calculation.     

Synthesis and characterization of the undecaosmium carbido carbonyl cluster: Crystal and molecular structures of [N(PPh3)2][Os11C(CO)27(μ-Cl)]

A chloro-derivative of undecaosmium carbido cluster [Os₁₁C(CO)₂₇(µ-Cl)]-1 anion has been prepared and fully characterized by spectroscopic and crystallographic methods. The structure1 is an important intermediate for the conversion of [Os₁₁C(CO)₂₇]² 2 dianion to [OS₁₀C(CO)₂₄]²-3 dianion.     

Synthesis,structure and antioxidant properties of manganese(II), zinc(II) and cobalt(II) complexes with bis(benzimidazol-2-ylmethyl)allylamine

Three new metal complexes, namely: [Mn(AIDB)Cl₂]·DMF (1), [Zn(AIDB)Br₂]·CH₃OH (2) and [Co(AIDB)Cl₂]·CH₃OH (3) having a ligand bis(benzimidazol-2-ylmethyl)allylamine (AIDB), have been synthesized in high yields and characterized by elemental analyses, molar conductivities, IR, UV–Vis spectra and single-crystal X-ray diffraction. The structural analysis revealed that all the three complexes 1–3 have five-coordinated trigonal bipyramid geometry where the degree of distorting is 1>3>2. In vitro antioxidant activity assay demonstrates that the complexes 1 and 3 display high scavenging activity against hydroxyl (OH·) and superoxide (O₂^−·) radicals.

     

Magnesium tetraorganyl derivatives of group 13 metals as intermediate products in the synthesis of group 13 metal alkyls and aryls

Reactions of organomagnesium halides with group 13 metal halides lead to the formation of R₃M type compounds (R = alkyl, aryl; M = Al, Ga, In) and are considered as the simplest methods of R₃M compound syntheses. These seemingly simple reactions reveal a much more complex chemistry involving mixed magnesium-group 13 metal compounds. To elucidate the reaction course of reactions of organomagnesium halides with group 13 metal halides, we have studied reactions of R₃M with organomagnesium halides. The interaction of Et₃M with R¹MgX led to the formation of following products being mixtures of crystalline ionic complexes with the general composition of [Et_4-nR¹_nM]⁻[XMg (thf)₅]⁺·(thf): [Et_2.2Al(CH=CH₂)_1.8]⁻[BrMg (thf)₅]⁺·(thf) ( 1 ), [Et₃Ga(CH=CH₂)]⁻[BrMg (thf)₅]⁺·(thf) ( 2 ), [Et₄Al]⁻[BrMg (thf)₅]⁺·(thf) ( 3 ), [Et₄Ga]⁻[BrMg (thf)₅]⁺·(thf) ( 4 ), [Et_2.9Al(C₆H₅)_1.1]⁻[BrMg (thf)₅]⁺·(thf) ( 5 ), [Et_2.9Ga(C₆H₅)_1.1]⁻[BrMg (thf)₅]⁺·(thf) ( 6 ), [Et_3.4GaMe_0.6]⁻[IMg (thf)₅]⁺·(thf) ( 7 ) and [Et₄In]⁻[BrMg (thf)₅]⁺·(thf) ( 8 ). A comparison of the production course of group 13 metal trialkyls R₃M with a thermal decomposition of 1–8 products showed that reactions of MX₃ with RMgX (X = Br, I; R = alkyl, aryl) yield initially intermediate ionic compounds, which must then be thermally decomposed to obtain pure R₃M compounds. If group 13 metal bromides and iodides, and alkyl (aryl)magnesium bromides and iodides in thf are used, only intermediate products with the [R₄M]⁻[XMg (thf)₅]⁺·(thf) structure are formed.     

Use of solvent isotope effect to identify an intermediate carbanion in the β-elimination reactions from N-[2-(4-pyridyl)ethyl]quinuclidinium and N-[2-(2-pyridyl)ethyl]quinuclidinium induced by acetohydroxamate/acetohydroxamic acid buffers

Solvent isotope effect is a useful technique for identifing and characterizing an intermediate carbanion in the base-induced -elimination reaction from N-[2-(4-pyridyl)ethyl]quinuclidinium, 1, and N-[2-(2-pyridyl)ethyl]quinuclidinium, 2. While at high [buffer]k _obs(D₂O) > k _obs(H₂O) due to the presence of a primary kinetic solvent isotope effect on the reprotonation of the intermediate carbanion by BD, at low [buffer] no solvent isotope effect is observed, and k _obs(D₂O) k _obs(H₂O). The data are in agreement with a reversible E1cb mechanism in which carbon deprotonation occurs from NH⁺, the substrate protonated at the nitrogen atom of the pyridine ring. In the absence of solvent isotope effect at low [buffer], and with the similarity of the results obtained with the two isomers, 1 and 2, the significance of an intramolecular proton transfer in the intermediate carbanion can be excluded in these processes.     

Cytotoxicity,apoptosis, interaction with DNA,cellular uptake,and cell cycle arrest of ruthenium(II) polypyridyl complexes containing 4,4′-dimethyl-2,2′-bipyridine as ancillary ligand

Two new ruthenium(II) polypyridyl complexes, [Ru(dmb)₂(DNPIP)](ClO₄)₂ (1) (DNPIP?=?2-(2,4-dinitrophenyl)imidazo[4,5-f][1,10]phenanthroline, dmb?=?4,4′-dimethyl-2,2′-bipyridine) and [Ru(dmb)₂(DAPIP)](ClO₄)₂ (2) (DAPIP?=?2-(2,4-diaminophenyl)imidazo[4,5f][1,10]phenanthroline), were synthesized and characterized. The DNA-binding behaviors of these complexes have been studied by UV-Vis absorption titration, viscosity measurements, and photocleavage. The DNA-binding constants are 7.39 (±0.16)?×?10⁴ (s?=?2.68) and 2.73 (±0.16)?×?10^4?(mol?L^?1)^?1 (s?=?0.64) for 1 and 2, respectively. Their evaluation as cytotoxic agents on different cancer cell lines was investigated with IC₅₀ values of 59.5, 51.3, and 70.3?µmol?L^?1 for 1, >100, 87.9, and 77.9?µmol?L^?1 for 2 against BEL-7402, HepG-2, and MCF-7 cells, respectively. Complex 1 is more active than 2 against selected cancer cell lines. The apoptosis induced by these complexes was studied. Cellular uptake showed that these complexes could enter into the cytoplasm and accumulate in the nuclei. The cell cycle arrest and antioxidant activity against hydroxyl radicals were also investigated.     

Synthesis and antimicrobial evaluation of some new N-pyridinium,quinolinium, and isoquinolinium sulfonate derivatives

1,3-propane- and/or 1,4-butane sultone (sultones are internal esters of hydroxyl sulfonic acid) are used as a chemical intermediate to introduce sulfopropyl and/or sulfobutyl groups into heterocyclic molecules and to confer water solubility and anionic character to the molecules. Therefore, the synthesis of three novel functionalized N-sulfonates is described. These sulfonates contain pyridyl (2a–f), quinolyl (4a–m), and isoquinolyl (6a,b) functional groups with potential biological activity. The synthesized quaternary ammonium salts, 3-(2 or 4-arylpyridinium-1-yl)propane or butane-1-sulfonate (2a–f), 3-(alkylquinolinium-1-yl)propane or butane-1-sulfonate (4a–m), and 3-(alkylisoquinolinium-1-yl)propane or butane-1-sulfonate (6a,b), were screened for their antimicrobial and antifungal activities. Among all tested compounds, it was found that compound 4-(4-carboxypyridinium-1-yl)butane-1-sulfonate (2f) showed high activity against Gram-positive bacteria, Gram-negative bacteria, and tested fungi. Most of the compounds showed a moderate degree of antimicrobial activity. The structures of these compounds were confirmed on the basis of their analytical and spectral data (infrared, ¹H-NMR and ¹³C-NMR spectroscopy, and mass spectral data).     

Electrochemical Dissociative Oxidation of the Unsaturated Pd3(dppm)3(CO)2+ Cluster; An Astonishing Resemblance with the Photo-Oxidative Analogue

Cyclic voltammetry (CV), rotating disk electrode voltammetry (RDE) and bulk electrolysis were used to investigate the electrochemical oxidation of the title cluster in acetonitrile (CH₃CN). Two irreversible 2-electron oxidation processes occur at +0.95 V and +1.15 V vs. SCE. Bulk electrolysis demonstrates that the d⁹–d⁹ Pd₂(dppm)₂(NCCH₃) ₂ ²⁺ 4 complex is generated among the first intermediates, and the d⁸ Pd(dppm)(NCCH₃) ₂ ²⁺ 3 is formed as the final product. The intermediacy of “Pd₃(dppm)₃(CO)⁴⁺” and “Pd₃(dppm) ₃ ⁴⁺ ” is suspected but not confirmed. This oxidation process exhibits a close resemblance to the photo oxidative reactivity of the title cluster in the presence of chlorocarbons (R–Cl) for which the sole observed product is Pd(dppm)Cl₂.This paper is dedicated to Professor Brian Johnson on the occasion of his retirement.     

Ruthenium(II) complexes: synthesis,cytotoxicity in vitro,apoptosis, DNA-binding,photocleavage, and antioxidant activity studies

Two new ruthenium(II) complexes, [Ru(dmp)₂(APIP)](ClO₄)₂ (1) (APIP?=?2-(2-aminophenylimidazo[4,5-f][1,10]phenanthroline), dmp?=?2,9-dimethyl-1,10-phenanthroline) and [Ru(dmp)₂(HAPIP)](ClO₄)₂ (2) (HAPIP?=?2-(2-hydroxyl-5-aminophenyl)imidazo[4,5-f][1,10]phenanthroline), were synthesized and characterized. The DNA-binding properties of these complexes were investigated by absorption titration, viscosity measurements, and photoactivated cleavage. The DNA-binding constants for 1 and 2 have been determined to be 2.3 (±?0.3)?×?10⁴ (mol?L^?1)^?1 and 3.3 (±?0.4)?×?10⁴ (mol?L^?1)^?1. The results indicate that 1 and 2 interact with DNA through intercalative mode. The cytotoxicities of 1 and 2 were assessed against BEL-7402, HepG-2 and MCF-7 cell lines using standard MTT assay. The apoptosis induced by these complexes was studied with the acridine orange/ethidium bromide staining method. The antioxidant activity on hydroxyl radical was also investigated.     

Phosphorescent Cationic Iridium (III) Complexes with 1,3,4-Oxadiazole Cyclometalating Ligands:Solvent-Dependent Excited-State Dynamics

To elucidate the nature of low-lying triplet states and the effect of ligand modifications on the excited-state properties of functional cationic iridium complexes,the solventdependent excited-state dynamics of two phosphorescent cationic iridium (III) complexes,namely[Ir (dph-oxd)₂(bpy)]PF₆( 1 ) and[Ir (dph-oxd)₂(pzpy)]PF₆( 2 ),were investigated by femtosecond and nanosecond transient absorption spectroscopy.Upon photoexcitation to the metal-to-ligand charge-transfer (MLCT) states,the excited-state dynamics shows a rapid process (τ=0.7-3 ps) for the formation of solvent stabilized ³MLCT states,which significantly depends on the solvent polarity for both 1 and 2 .Sequentially,a relatively slow process assigned to the vibrational cooling/geometrical relaxation and a long-lived phosphorescent emissive state is identified.Due to the different excited-state electronic structures regulated by ancillary ligands,the solvation-induced stabilization of the ³MLCT state in 1 is faster than that in 2 .The present results provide a better sight of excited-state relaxation dynamics of ligand-related iridium (III) complexes and solvation effects on triplet manifolds.     

Synthesis and structural characterization of [(C5H4)SiMe2(N-t-Bu)]Ti[(o-C6H4)C(Ph)C(Ph)], generated via an alkyne-Ti benzyne coupling reaction

The thermolysis of [(C₅H₄)SiMe₂(N-t-Bu)]TiPh₂ in the presence of diphenylacetylene proceeds at 80 °C in cyclohexane solution with the sole formation of the titanacyclic complex [(C₅H₄)SiMe₂(N-t-Bu)]Ti[(o-C₆H₄)C(Ph)C(Ph)], which has been characterized by solution NMR measurements and X-ray crystallographic analysis. This reaction is accompanied by the elimination of benzene and presumably occurs via coupling of a titanium benzyne intermediate with diphenylacetylene. The two chemically inequivalent Ti-C bonds of 2.081(7) and 2.103(6) Å in [(C₅H₄)SiMe₂(N-t-Bu)]Ti[(o-C₆H₄)C(Ph)C(Ph)] reflect the increased electrophilicity of the d⁰ Ti(IV) center arising from the presence of the bifunctional ansa-cyclopentadienyldimethylsilylamido ligand.